The present invention relates to a new and distinct variety of apple tree named ‘G.814’. ‘G.814’ is useful in that it can be propagated clonally and used as a rootstock or root system for apple trees as well as for interstems of apple trees. When used as a rootstock, ‘G.814’, is: dwarfing, induces scion precocity, is cold hardy, induces scion precocity, has a high yield efficiency, is tolerant to replant disease, and is resistant to: crown and root rot (Phytophthora cactorum), fire-blight (Erwinia amylovora), and powdery mildew. ‘G.814’ is susceptible to Apple Stem Grooving Virus (ASGV) and wooly apple aphid.

I. Field and Utility Summary
The invention described herein is a new variety of apple tree, Malus domestica×Malus robusta hybrid, hereinafter referred to as 'G.814'. The new variety is useful as a dwarfing rootstock and is resistant to fire blight and crown rot. It is precocious and highly productive, and can be used as a rootstock and for interstems of apple trees.
II. Cultivation Summary
'G.814' originated from a planned cross in 1976 in Geneva, N.Y.
In the spring of 1976, pollen from a Malus robusta 'Robusta 5' apple tree (not patented) was applied to emasculated flowers of a Malus domestica 'Ottawa 3' apple tree (not patented) in Geneva, N.Y. In the fall of 1976, seeds resulting from this pollination were extracted from mature fruit derived from this cross. In the winter of 1976-77, the seeds were stratified and planted in large flats under conditions effective to germinate seeds and obtain seedlings. When germinated seedlings were about 2.5 cm tall they were inoculated with a mixture of isolates of the fungus Phytophthora cactorum (the causal agent of crown and root rots). The flats were flooded to mid-hypocotyl level and kept at 23° C. for one week. Surviving seedlings were transplanted into individual pots.
In the summer of 1977, each of the transplanted seedlings was inoculated with approximately 106 colony forming units of the Ea 273 strain of the fire blight bacterium Erwinia amylovora by inserting a 26-gauge hypodermic syringe needle into the shoot tip. The seedling designated as #814 was one of the survivors of this battery of inoculations from the same cross. All the surviving plants were transplanted to a field in Geneva, N.Y. in the fall of 1977. The surviving plants were then allowed to grow side shoots for propagation and evaluation. In 1980, 'G.814' was evaluated for rooting ability, lack of spine production, and low root brittleness in a layering bed (stool bed).
In 1984, three finished trees with 'G.814' rootstock were planted in a first test orchard in Geneva, N.Y. with Malus domestica cv. 'Northern Spy' (not patented) grafted onto this rootstock as the scion cultivar. This rootstock performed well (top 20% of many rootstocks tested) in these first test trials. In 1991, this rootstock was advanced to elite status based on field performance and four trees were planted with Malus domestica cv 'Empire' (not patented) for advanced evaluation. This planting was evaluated for twelve years resulting in the classification of 'G.814' in the dwarfing category similar to 'Malling 26' (not patented) apple rootstock and the cumulative productivity in the top 10% of the planting.
In 1994, an additional experimental orchard was planted in Geneva, N.Y. It featured several trees of 'G.814' as the rootstocks with scions 'Mutsu' (not patented), 'Haralson' (not patented), 'Royalty' (not patented), and 'Radiant' (not patented) to test the productivity with different scions. The tests resulted in acceptable performance with these scions.
In the spring of 2002, 'G.814' and other experimental rootstocks were planted as finished trees with 'Golden Delicious' (not patented) scion in Geneva, N.Y. In 2003 and 2004 the orchard was spray inoculated, while in bloom, with fire blight. All trees of 'G.814' survived, indicating good resistance of the rootstock in an orchard environment.
In February, 2002, several rootstock liners of 'G.814' were budded with Apple Stem Grooving Virus (ASGV) (Capillovirus spp.) infected wood and after the buds were healed the rootstocks were planted in a nursery in Geneva, N.Y. and monitored for survival and stunting. Measurements in August, 2002, showed that 50% of the plants had died and the surviving ones were stunted. These results indicated that this rootstock is susceptible to ASGV.
In summer, 2007, twenty rootstock liners were inoculated with two different strains of fire blight (ten liners per strain) in a greenhouse in Geneva, N.Y. As a result, the apple rootstock 'G.814' was classified as resistant to all of the strains tested. Furthermore, 'G.814' was observed to be susceptible to wooly apple aphid, resistant to powdery mildew, cold hardy, and tolerant of replant disease.
In the fall of 2005, the tree architecture of nursery trees was measured on 'G.814' and found that it promotes higher number of feathers and medium spreading habit on 'Gala' (U.S. Plant Pat. No. 03,637) scions.
III. Asexual Reproduction
Asexual reproduction of the 'G.814' apple rootstock has been achieved using the traditional method of clonally propagating apple rootstocks. In particular, the original seedling of the 'G.814' apple rootstock was planted in Geneva, N.Y., and allowed to develop into a "mother plant." The 'G.814' mother plant was then used to obtain rooted liners using conventional layering procedures. The resulting liners were then planted in a row to generate a layering stool bed (also referred to as the "mother stool bed"). The living tissues (i.e. leaves, stems, roots, buds, and spines) of the mother stool bed were observed to be identical to secondary and tertiary stool bed plants. In addition to conventional layering, the 'G.814' variety has been asexually reproduced in Geneva, N.Y., by root cuttings, by budding and grafting onto seedling and clonal rootstocks.
IV. Stability
Observations of trees from propagations in Geneva, N.Y. indicate that all clonally propagated trees have proven true to type and identical in all appearances to the original tree.

The United States of America, as Represented by the Secretary of Agriculture


An assembly is disclosed for protecting and supporting a fragile thermometer having a column and an integral flange base. The assembly includes a support base defining a threaded bore for receiving the integral flange base of the thermometer. A bottom cushioning pad is also provided for placement in the bottom of the bore. In addition, a ring-shaped upper cushioning pad is provided for being fitted over the column of the thermometer and against the top surface of the thermometer""s flange base. The thermometer is secured to the protective support base with a threaded collar member which receives the thermometer column and is threaded into the threaded bore of the support base which secures the thermometer to the base. An adapter bushing may also be used to attach thermometers having smaller diameter columns to the base. The bushing is received in the threaded collar member and defines an axially aligned hole having a predetermined diameter for closely receiving the column of a thermometer having a small diameter.

The invention relates generally to support bases for elongated members and, more particularly, to support base assemblies for fragile elongated members having integral flange bases such as Galileo thermometers.
Despite their expense, glass column Galileo style thermometers have become extremely popular with the public because of their unique shape, interesting visual effects and of course, their functionality. However, they are extremely fragile not just because they are made from glass but also because they have an integral small diameter flange base that offers very little support and allows the thermometer to tip over and break very easily.
The present invention addresses the aforementioned problem by providing a protective base assembly for glass column Galileo style thermometers which protects and supports the thermometer or any similar type elongated member having a column and an integral flange base in which the base has a bottom surface and a top surface with the column projecting upwardly therefrom. The protective base assembly includes a protective support base defining a threaded bore for receiving the integral flange base of the thermometer or fragile elongated member. A bottom cushioning pad is also provided which is placed in the bottom of the threaded bore. The bottom pad cushions and supports the flange base of the thermometer when it is placed in the threaded bore with its bottom surface positioned against the bottom pad. A ring-shaped upper cushioning pad is also provided which is placed on the top surface of the thermometer""s flange base by fitting it over the thermometer""s column and positioning it against the top surface of the thermometer""s flange so that the column located in the ring hole of the pad. In addition, a threaded collar member for being threaded into the threaded bore of the protective support base is provided. The threaded collar member defines an axially aligned hole for receiving the thermometer column. The collar member secures the thermometer to the protective support base when it is fitted over the thermometer""s column, i.e. with the column received in the collar""s axially aligned hole, and then is threaded into the support base""s threaded bore.
The assembly is also preferably provided with a plurality of differently sized adapter bushings for protecting and accommodating thermometers having smaller diameter columns. Each adapter bushing has a flange portion and a cylindrical portion. The cylindrical portion is received in the axially aligned hole of the threaded collar member so that its flange portion is positioned between the collar member and the upper ring-shaped cushioning pad placed on the top surface of the thermometer""s flange base. Each bushing also defines an axially aligned hole having a predetermined diameter for closely receiving the column of a thermometer having a diameter slightly less than the bushing""s predetermined diameter. This close receiving or fitting is desirable because it enhances securement of the thermometer to the protective support base.
The bushing""s flange portion is also provided with a thickness that serves to slightly compress the upper and bottom cushioning pads against the thermometer""s flange base when the collar is threaded into the threaded bore of the protective base. Pad compression is desirable because it cushions the thermometer""s flange base and prevents the flange base from moving or sliding around in the threaded bore of the protective base. This enhances securement of the thermometer to the protective base and also protects the thermometer should it happen to be bumped or knocked over.
The present invention also provides a unique wall mounting bracket for mounting the protective base and supported thermometer on a wall.


Gas Diffusion Electrodes (GDEs) play a pivotal role in clean energy production as well as in electrochemical processes and sensors. These gas-consuming electrodes are typically designed for liquid electrolyte systems, and are commercially manufactured by hand or in a batch process. However, CDEs using the new electrolytes demand improved electrode structures. This invention pertains to GDEs and gas diffusion media with new structures for systems using membrane electrode assemblies (MEAs), and automated methods of manufacture that lend themselves to continuous mass production. Unexpected improvements in gas and vapor transport through the electrode are realized by incorporating a new dispersion process in the construction, reformulating the applied mix with solution additives, and creating a novel coating structure on a conductive web. Furthermore, combining these changes with a judicious choice in coating methodology allows one to produce these materials in a continuous, automated fashion.

A gas diffusion electrode (GDE) consumes or is depolarized by a gas feed while allowing direct electronic transfer between the solid and gas phase. Together with the electrolyte, the GDE provides a path for ionic transfer, which is just as critical. GDEs are typically constructed from a conductive support, such as a metal mesh, carbon cloth, or carbon paper. This support is often called a web. The web is coated with hydrophobic wet-proofing layers, and finally, a catalytic layer is applied most commonly to one face. While the catalytic layer can consist of very fine particles of a precious metal mixed with a binder, many employ the methods similar to that in Petrow, et al., U.S. Pat. No. 4,082,699. This patent teaches the use of using finely divided carbon particles such as carbon black as the substrate for small (tens of angstroms) particles of the nobel metal. Thus called a xe2x80x9csupportedxe2x80x9d catalyst, this methodology has shown superior performance and utilization of the catalyst in electrochemical applications. However, the application of this supported catalyst as well as wet proofing layers to the web engages the need for a well-dispersed mix.
Often, GDEs are cited as key components in Fuel Cells. Here, the anode is typically depolarized with hydrogen while the cathode is depolarized with oxygen or air. The resulting products are energy in the form of electricity, some heat, and water. Examples of acid or alkaline fuel cells are well known. However, some have also realized that the energy-producing quality of a fuel cell can be adapted to industrial electrochemical processes and thus save energy and hence reduce operating costs.
GDEs also may allow the creation of a commodity directly from a gaseous feedstock. For example, Foller, et al. (The Fifth International Forum on Electrolysis in the Chemical Industry, Nov. 10-14, 1991, Fort Lauderdale, Fla., Sponsored by the Electrosynthesis Co., Inc.) describe the use of a GDE to create a 5 wt. % hydrogen peroxide in caustic. In this case, oxygen is the feedstock and a specific carbon black (without noble metals) is the catalyst. A typical chlor-alkali cell uses two solid electrodes to produce sodium hydroxide and chlorine. In this case, both the anode and cathode expend energy to evolve gas, and special measures are taken to keep the resulting hydrogen away from the chlorine due to a potentially explosive mixture. The typical chlor-alkali cathode can be replaced with an oxygen-depolarized cathode, as has been shown by Miles et al. in U.S. Pat. No. 4,578,159 and others. A cell run in such a manner saves approximately one volt, and the hydrogen/chlorine problem is eliminated. Aqueous hydrochloric acid is an abundant chemical by-product. One can recover the high-value chlorine by oxidizing solutions of HCl, and thus recycle the chlorine as a feedstock to the chemical plant.
Electrolysis becomes extremely attractive when the standard hydrogen-evolving cathode is substituted with an oxygen-consuming gas diffusion electrode due to the significant drop in energy consumption. The ability of the gas diffusion electrode to operate successfully in this and the preceding examples is acutely dependent on the structure of the gas diffusion electrode: for in all these cases, the electrode serves as a zone for liquid-gas-solid contact, as a current distributor, and most importantly, as a liquid barrier. The use of solid polymer electrolytes has greatly expanded the field of electrochemistry. As summarized above, electrochemical processes depend on the transfer of ionic and electronic charge through the use of an anode, cathode, and an ionic liquid electrolyte. However, with the advent of the solid polymer electrolyte fuel cell, the traditional liquid phase has been replaced with a membrane composed of a polymer electrolyte that transfers ionic charge under typical electrolytic conditions. One can deposit a catalyst layer directly on the membrane, or attach a gas diffusion electrode to one or both faces of the conducting membrane. Such an assembly can be called a membrane electrode assembly (MEA), or for fuel cell applications, a PEMFC (proton exchange membrane fuel cell).
These solid polymer electrolytes are often composed of ion-conducting membranes that are commercially available. For example, in addition to the previously mentioned Nafion (a cation exchange membrane), Asahi Chemical and Asahi Glass make perfluorinated cation exchange membranes whereby the ion exchange group(s) are carboxylic acid/sulfonic acid or carboxylic acid. These companies produce cation exchange membranes with only the immobilized sulfonic acid group as well. Non-perfluorinated ion exchange membranes are available through Raipore (Hauppauge, N.Y.) and other distributors such as The Electrosynthesis Co., Inc. (Lancaster, N.Y.). Anion exchange membranes typically employ a quaternary amine on a polymeric support and are commercially available as well.
Nafion is typically employed in some fuel cells. For the hydrogen/air(O2) fuel cell, hydrogen and oxygen are fed directly to the anode and cathode respectively, and electricity is generated. For these xe2x80x9cgas breathingxe2x80x9d electrodes to perform, the gas diffusion electrode structure must be highly porous to allow three phase contact between the solid electrode, the gaseous reactant, and the liquid or near liquid electrolyte. In addition to providing a zone for three-phase contact, the gas diffusion electrode structure aids in making electrical contact to the catalyst, enhances transport of reactant gasses into the zone, and provides for facile transport of product away from the zone (e.g. water vapor).
In addition to a gaseous hydrogen fuel and gaseous air (O2) oxidant, others employ a mixed phase system such as the methanol/air(O2) fuel cell. Here, liquid methanol is oxidized at the anode while oxygen is reduced at the cathode. Another utilization for ion-conducting membranes and gas diffusion electrodes includes the electrochemical generation of pure gasses [for example see Fujita et al. in Journal of Applied Electrocnemistry, vol. 16, page 935, (1986), electro-organic systhesis [for example see Fejita et al. in Journal of the Electrochemical Society, vol. 137, no. 5, page 1451 (1990)], or as transducers in gas sensors [for example see Mayo et al. in Analytical Chimica Acta, vol. 310, page 139, (1995)].
Typically, these electrode/ion-conducting membrane systems are constructed by forcing the electrode against the ion conducting membrane U.S. Pat. Nos. 4,272,353; 3,134,697; and 4,364,813 all disclose mechanical methods of holding electrodes against the conducting membrane. However, the effectiveness of a mechanical method for intimately contacting the electrode to the polymer membrane electrolyte may be limited since the conducting membrane can frequently change dimensions with alterations in hydration and temperature. Swelling or shrinking can alter the degree of mechanical contact.
Thus, an alternative method of contacting the electrodes with the polymer membrane electrolyte involves direct deposition of a thin electrode onto one or both sides of the conducting polymer substrate. Nagel et al. in U.S. Pat. No. 4,326,930 disclose a method for electrochemically depositing platinum onto Nafion. Others have employed chemical methods whereby a metal salt is reduced within the polymer membrane [for example see Fedkiw et al. in Journal of the Electrochemical Society, vol. 139, no. 1, page 15 (1192)].
In both the chemical and electrochemical methods, one essentially precipitates the metal onto the ion conducting membrane. This precipitation can be difficult to control due to the nature of the ion-conducting polymer membrane, the form of the metal salt, and the specific method employed to precipitate the metal. As the goal of a thin, porous, and uniform metal layer is often not met via precipitation, practitioners have turned to other deposition methods. For example, ion beam assisted deposition techniques are disclosed in co-pending provisional patent application by Allen et al. (Ser. No. 60/035,999); a method for coating the membrane with an ink composed of the supported catalyst and solvent is disclosed by Wilson and Gottesfeld in the Journal of the Electrochemical Society, volume 139, page L28, 1992; and a method of using a decal to deposit a thin layer of catalyst onto the ion-conducting membrane is summarized by Wilson et al. in Electrochimica Acta, volume 40, page 355, 1995. Thus, these approaches differ from the previous strategy by the catalyst layer being deposited onto the ion conducting membrane, and a gas diffusion structure is subsequently placed against this catalyst layer.
Regardless of whether the catalyst is fixed to the membrane, or coated onto an uncatalyzed gas diffusion electrode and then bonded to the membrane via mechanical, and/or thermal means, the structure and composition of the component contacting the catalyst contributes to the overall MEA performance. This component is variously called a xe2x80x9cdiffuserxe2x80x9d, an electrode xe2x80x9cbackingxe2x80x9d, xe2x80x9cgas diffusion mediaxe2x80x9d, a xe2x80x9cgas diffusion layerxe2x80x9d, or an xe2x80x9cuncatalyzed gas diffusion electrodexe2x80x9d and can predominate MEA performance during operation at high current density. We will use the term diffuser to encompass all these synomyms. A diffuser is a material that: 1) provides electrical contact between the catalyst and the electrochemical cell current collector, 2) distributes and facilitates efficient transport of feed gas or gasses to the electrode, and 3) becomes a conduit for rapid transport of product(s) from the electrode. Thus the electrode is the catalytic layer or zone characterized by a three-phase interface of solid, liquid, and gas whereas the diffuser is a two-phase structure for gaseous (or liquid) transport and electrical contact.
There are a few commercial providers for diffusers. Gore Associates (Elkton, Md.) offer Carbel(trademark), a conductive, microporous polymer. E-TEK, Inc. (Natick, Mass.) offers uncatalyzed versions of the gas diffusion electrodes found in their catalog. Of these, the uncatalyzed ELAT(trademark) is listed as the best material for MEA applications. The gas diffusion electrode structure designed for providing a three-phase zone, current contact, and a liquid barrier is being adopted for MEA applications.
Typical ELAT construction is detailed in U.S. Pat. No. 4,293,396 by Allen et al.. Here, a carbon cloth serves as the web. Carbon black is prepared for application to the carbon web by using techniques listed in U.S. Pat. No. 4,166,143 whereby solutions of Vulcan XC-72 or Shawinigan acetylene black (SAB) are mixed with water, ultrasonically dispersed with a sonic horn, mixed with Teflon(copyright) (TFE), and filtered. Layers of SAB mix serve as the wetproofing layer on each side of the web. Finally, layers of (catalyzed) Vulcan mix are coated onto one side of the assembly. Although the importance of mix penetration into the web is discussed, the actual coating method is not disclosed. The reported products were of limited lot size, so may have been individually prepared. After the final coat, the assembly may be sintered in air at a temperature sufficient to cause the Teflon to flow, typically 300-350xc2x0 C. This double sided structure was designed with the intent to create an electrode that both achieves good gas distribution and contact with the catalyst while providing a hydrophobic barrier to prevent electrolyte transport completely through the electrode. Regardless, no information is relayed as to how this structure could be produced with economical means.
Similarly, a typical ink application is described by Ralph et al. in The Journal of the Electrochemical Society, Vol. 144, page 3845, 1997 and references therein. Here, the goal is to minimize platinum usage. A gas diffusion electrode is constructed by using silk screen technology to coat a carbon paper web. The ink is comprised of catalyzed carbon black and binders including Teflon. The authors claim a resulting electrode structure comparable to that described by wilson and Gottesfeld or Wilson et al. cited previously above. If GDEs and ion conducting membranes are to be used in large volume, commercial processes such as power generation in electric vehicles, then one must meet a significant reduction in component cost. Thus, while the authors endorse the need for inexpensive manufacturing processes, they describe a batch coating design, which inherently limits product throughput.
In both the ink and mix preparation methods, it is generally accepted that the ultrasonic horn serves an important role in dispersing the carbon in solvent. Since the carbons are high surface area substances, it is important to prepare a uniform and stable suspension. Carbon blacks do not xe2x80x9cwet-outxe2x80x9d without a significant input of energy or shear into the solution. Some also modify the solution with additives as well to induce high shear. The ultrasonic horn performs this function of wetting-out by way of high frequency electrical energy directed from a stainless steal tip immersed in the solution. The action of the horn generates pressure waves through the vessel and produces high shear through cavitation. Although suitable for limited production runs or RandD sized samples, there are several limitations to ultrasound. First, since the energy is projected from a single source, i.e., the horn, the power is a function of the distance from the horn, and will diminish significantly as one moves away. Second, as the action of the carbon black on the horn leads to abrasion and accelerated corrosion, the projected power spectrum emanating from the horn changes in time. For these reasons, ultrasound may not be appropriate for production of large quantities of diffusers.
With the rise of PEMFCs as suitable clean power sources, and the parallel increase in the use of MEAs in industrial and sensor applications, there is a need for a diffuser tailored for these materials. The current diffuser technology employs structures that were originally designed for liquid electrolyte systems. In addition, the current routine use of the sonic horn produces carbon black dispersions for coating that may be non-uniform and difficult to control for production of large batches of diffuser. Furthermore, the current manufacturing methodology is limited in its applicability to continuously coating a webxe2x80x94a step believed to be crucial in producing an inexpensive product.
It is an object of this invention to provide improved diffuser structures with transport properties for MEA type electrodes.
It is a further object of this invention to introduce a method of manufacture that is compatible with continuous automation.
It is a still further object of the invention to introduce a dispersion methodology that provides an unexpected increase in performance from diffusers and gas diffusion electrodes fabricated from carbon blacks preparing using this technique.
These and other objects and advantages of the invention will become obvious from the following detailed description.
The novel gas diffusion electrode of the invention comprises an electrically conductive web provided on at least one side with a wet-proofing layer of a suitable polymer provided with an electrocatalyst thereon. The electrically conductive web is preferably a carbon cloth web or carbon paper or a metal mesh. The wet-proofing layer may also contain a dispersion of carbon black such as SAB.
The construction of the standard ELAT grew out of many refinements, geared to producing a general-purpose gas diffusion electrode that would work under numerous electrolytic conditions. Lindstrom et al. (U.S. Pat. No. 4,248,682) and the previously cited U.S. Pat. No. 4,293,396 document the progress of the ELAT development. The final structure of the ELAT electrode is determined by the underlying support web, the quantity and kind of carbon black coated onto the web, and the quantity of binder (often Teflon) mixed with the carbon black.
Typically, a final layer of liquid Nafion or ionomer is applied to the face or front of the GDE diffuser to aid in making contact to the electrode (MEA). Such solutions are readily available and come as a 5-10% wt ionomer with an equivalent weight of 1100 or less. Typical levels of Teflon in the Vulcan mix are 5-80% by weight, more preferably 30-70% by weight. The total weight of solid varies by electrode type, but ranges from 0.5 to 25 mg/cm2. The weight of solids is determined by the number of coats applied to the web, and obviously, the weight delivered per pass by the coating device. While any number of the conducting carbon blacks can be employed, for example Shawinigan Acetylene Black, Vulcan XC-72, Black Pearls 2000, or Ketjen Black, in general, the carbon black selected for wet-proofing is hydrophobic while the carbon black selected as the catalyst or electrode layer is more hydrophilic. The Nafion ionomer coated on the face can vary from 0.1 to 2 mg/cm2 is preferred. FIG. 1 is a schematic to delineate these various layers that comprise the structure of the ELAT gas diffusion electrode.
We have changed the structure of the ELAT to accomodate the different reactant and product transport and electronic contact requirements of MEAs. FIG. 1 also shows a comparison of the standard ELAT structure to two embodiments of a new gas diffuser structure. In comparing diffuser type xe2x80x9cAxe2x80x9d of FIG. 1 to the standard ELAT, one notes both a reduction in the number of coated coating layers being placed on only one side of the carbon cloth web. The uncoated side of the web is now oriented toward the gas feedstream while the coated layers are placed against the electrode (i.e, the face of the membrane electrode assembly). As will be shown in the Examples, these reduced layers and single-sided coatings allow for a reduced number of fabrication steps, and a thinner, more open structure amendable to high gas flux rates.
For diffuser type xe2x80x9cAxe2x80x9d, there are still two or more types of carbon black employed in the architecture of the structure. These are selected so as to create a gradient of hydrophobicity throughout the structure, as well as to provide a layer that can be more easily wetted at the catalyst interface. However, there are applications where a single kind of carbon black is appropriate, and diffuser type xe2x80x9cBxe2x80x9d in FIG. 1 illustrates this alternative structure. For diffuser type xe2x80x9cBxe2x80x9d, one or more coats of carbon black and binder are applied on one side of the web. This diffuser would be orientated as type xe2x80x9cAxe2x80x9d, that is, the uncoated side is towards the feedgas plenum while the coated side is against the electrode face of the MEA. Diffuser type xe2x80x9cBxe2x80x9d is easier to fabricate, and is the least expensive to manufacture.
While much focus has been made on the structure and performance of gas diffusion electrodes, little contribution has been made in the nature and effect of carbon black preparation methods for gas diffusion electrodes. While the sonic horn is frequently cited, we show here surprising enhancements in diffuser and gas diffusion electrode performance through other dispersion methods. For example, one preferred method introduces a pressurized flowing stream of solvent and carbon black into a xe2x80x9cYxe2x80x9d shaped chamber that divides the flow into two streams, which are recombined downstream using another xe2x80x9cYxe2x80x9d. The effect of splitting and recombining the stream introduces high shear and pressure differences on the solvent and carbon black, and effectively wets out the particles in a uniform and consistent manner. A commercial device is available through such companies as Microfluidics (Newton, Mass.). Other methodologies use rotor/stator methodology whereby one set of blades is fixed while the another set is spun at high rates around the fixed set. Such action creates high shear on the sample. Rotor/stator operations are often performed in batch mode. Another device is a mill where a spinning barrel with plates performs the function of delivering shear energy to the solution. Kady Company (Scarborough, Me.) provides a range of these machines. These and similar devices are called xe2x80x9chomogenizersxe2x80x9d and perform the vital function of dispersing solids into solvent in a uniform and consistent manner. The following Example section describes such a preparation and reports results for diffusers and gas diffusion electrodes unanticipated by simple homogenization of the carbon black solution.
While the placement and number of carbon black layers can control structure, and the method used to disperse the carbon black also determines performance, the technique employed to coat a web with mix determines the final structure as well. The previously cited ELAT patents describe a successful coating on the carbon cloth web results from physically penetrating into the woven structure to encase the fiber bundles with mix. Thus, the coating methodology most appropriate for this function is slot-die, knife-over-blade, or spraying followed by a knife operation. Slot-die coating is the preferred method as the slot acts as a control mechanism that meters out a fixed amount of mix. The weight of solids placed on the web is determined by the line speed, pump rate through the slot die, and mix composition (% solids). Furthermore, since the slot-die acts through creating a constant mass of mix between the slot-die head and the moving web, this coating action serves to both give some penetration into the cloth and compensate for surface roughness inherent in the cloth.
While slot-die has been used to coat various solid and porous substrates, using the slot-die to create gas diffusion electrodes and diffusers is a novel application. Typical widths of a slot-die range from 5-250 mm, but larger dies can be constructed. The gap of the slot die can be controlled via shims, but a typical range is between 4 and 100 mils, and more preferably 15-30 mils. Both the coating of the mix and the size of the drying section of the coating machine determine the line speed, as the freshly coated web is next run into a heated chamber. Typical line speeds range from 0.1 to 5 m/min. Multiple coats can be applied by a series of slot-die stations, or re-running a freshly-coated web through the machine. Other attachments to a manufacturing line would include a continuous sintering oven and a slitting machine to cut the final product into the desired dimensions.
For mixes consisting of carbon black or catalyzed carbon black and Teflon, a Gravure style coating method can be employed as well. Gravure coating employs a spinning rod that is dipped in mix at the lower half and then contacted with the moving web at the other upper segment. Typically the gravure-coating head spins in a direction opposite the direction of the moving web, allowing some penetration of the mix into the web. The quantity of the mix applied to the web per pass is controlled by the mix rheology, line speed, gravure rotation speed and gravure imprint pattern, and the area of the web contacting the head. Gravure coating works best with low viscosity mixes.
The selection of a coating method such as slot-die, gravure, knife-over-plate, or spraying is dependent on the fluid dynamics of the mix, mix stability during the coating process, and the electrode and/or diffuser structure desired on the web. One is not limited to one coating method. Typically, more than one coating station can be applied to the moving web to build up a multi-layer structure if so desired, whereupon the selection of coating station is dependent on the requirements of the mix.
In some cases, the composition of the dispersed carbon black mix is modified by adding additives such as iso-propyl alcohol (from 0.1 to 100%, more commonly between 5 to 30%, and preferably 25%), Fluorinert FC 75 or similar, Neoflon AD-2CR, polyvinyl alcohol, polyox, or similar stabilizers.
In some operations it is preferable to avoid iso-propyl alcohol, for example due to the constraints and costs of handling organic vapors, and a water-based mix is employed. For this type of mix, one or more of the following stabilizers and thickeners could be employed: Fluorinert FC 75 or similar; Neoflon AD-2CR; polyvinyl alcohol, ethylene glycol, polyethylene glycol alkyl ether; Polyox(copyright); Triton(copyright) X100; Tween(copyright); Joncryl 61J, Rhoplex AC-61, Acrysol GS (acrylic polymer solutions); and naphthalene formaldehyde condensate sulfonates.
The electrocatalyst may be any of those conventionally used such as platinum or a rhodiumxe2x80x94rhodium oxide catalyst described in U.S. patent application Ser. No. 013,080 filed Jul. 26, 1998. The specific coating method and stabilizer is dependent on the structure of diffuser desired.
In the following examples, there are described several preferred embodiments to illustrate the invention. However, it should be understood that the invention is not intended to be limited to the specific embodiments.
A standard ELAT is constructed for comparison with diffuser or gas diffusion electrode structures of type xe2x80x9cAxe2x80x9d or xe2x80x9cBxe2x80x9d. A web consisting of carbon cloth with a warp-to-fill ratio of unity, with approximately 25 to 50 yarns per inch, and a carbon content of 97-99% was selected from an available thickness of 5-50 mils, preferably around 10 mils. An appropriate weight of SAB or Vulcan XC-72 was dispersed with an ultrasonic horn. A solution of fluorinated hydrocarbon, otherwise called Teflon, was added to the mix to form a 50% wt (solids) component. To the web, a first mixture of dispersed SAB was coated onto each side, until coverage of approximately 3.5-7 mg/cm2 was obtained. This layer was considered the wet-proofing layer. The electrode was air dried at room temperature in between each coat. To this dried assembly, a second but similar mix of dispersed platinum catalyst on Vulcan XC-72 Teflon was coated on one side. Between one and eight coats were provided to achieve the desired metal loading, typically 0.2 to 52 mg catalyst/cm2. After the final application, the coated fabric heated to 340xc2x0 C. for about 20 minutes. As described, this would be a gas diffusion electrode. To make a diffuser, similar steps are performed except uncatalyzed Vulcan XC 72 is employed.
To construct a gas diffusion electrode or diffuser of type xe2x80x9cAxe2x80x9d structure of the invention, an identical procedure as outlined for Example 1 was followed, except the SAB/Teflon wetproofing layer was applied to one side of the web at approximately half the total carbon black loading, i.e. 1.5-3 mg/cm2. The catalyst coat and final treatment followed that as detailed above. To make a diffuser, similar steps were performed except uncatalyzed Vulcan XC 72 with a loading range of 0.5-1.5 mg/cm2 carbon black was employed.
To construct a gas diffusion electrode or diffuser of type xe2x80x9cBxe2x80x9d structure of the invention, an identical procedure as outlined for Example 2 was followed. However, only the SAB/Teflon wetproofing layer or platinum catalyzed Vulcan XC-72 was applied to one side of the web at at total loading of approximately 0.5-5 mg/cm2. Similar drying and heating steps as Example 1 followed. A diffuser was constructed in an identical manner except either SAB or Vulcan XC-72 without catalyst was employed.
A type xe2x80x9cBxe2x80x9d gas diffusion electrode similar to that of Example 3 was constructed through an automated coater. For this example, a knife-over-plate coater was used and the coater employed a 255 mm perpendicular stainless steel blade with a 45xc2x0 C. beveled edge. The blade was positioned over the cloth with a fixed gap of approximately 10 mils. The line speed was 2 meters/min., and mix, prepared as in Example 3, was fed at continuous rate to a reservoir in front of the blade. Samples thus prepared were subjected to the same heating and drying steps of Example 1.
Homogenized mixes of carbon black were created through the use of Microfluidic""s microfluidizer. A suspension of water and appropriate weights of either SAB or Vulcan XC-72 was fed to the machine, which was pneumatically operated. A single chamber configuration was employed using the 100 micron chamber, although other chamber sizes could be used as well. After a single pass through the homogenizer, Teflon was added to the mix in the same proportion as established in Example 1. The mix was filtered, and coated onto a carbon web as detailed in Example 1 or Example 2.
To prepare a diffuser similar to Type xe2x80x9cAxe2x80x9d with the slot die coating methodology, a mix similar to that described in EXAMPLE 5 is prepared, except prior to filtering a finite amount of Triton X100 is added to the carbon black solutions to make up approximately 1% weight Triton X to weight of carbon black. A typical range for this additive is 0-5% based on weight of carbon black. Furthermore, some dissolved Polyox is added to the solution in the amount of 10% weight based on carbon black. The typical range for this additive is 0-20% based on the weight of carbon black. Excess solution is removed.
A mix of Shawinigan Acetylene Black (SAB) or Vulcan XC-72 as prepared above is placed in a pressurized vessel that is connected to the slot die. A pressure of 10-15 psi is applied to the vessel to deliver mix to the slot-die head at an appropriate rate. The 250 mm long slot die is oriented to ride on the carbon cloth web, a gap of 18 mils is set for the slot die. For both the SAB and Vulcan mixes, the web passes the coating head at 2 m/min. Multiple coats of SAB and Vulcan are applied to the web until an appropriate weight of carbon is distributed. The coated cloth is dried prior to each additional coat. The final assembly is sintered at 340xc2x0 C. for 20 minutes prior to testing.
To prepare a catalyzed gas diffusion electrode similar to Type xe2x80x9cAxe2x80x9d with the gravure coating methodology, a mix similar to that described in EXAMPLE 5 is prepared, except now less water is removed and a less viscous consistency is attained. Also, the Vulcan XC-72 is catalyzed with a 30% wt of platinum. A carbon cloth web is rolled past a 12.7 mm diameter, 250 mm long gravure head that is rotating at 100 rpm. This gravure head has a 5.3 cell/cm2 pattern across the surface to aid in pick-up and distribution of the mix. The web is first coated with SAB at the rate of 2 m/min. Several coats are applied with air drying in-between coats. Next several layers of 30% Pt on Vulcan XC-72 are applied at 1 m/min, with drying in-between coats. The final assembly is sintered at 340xc2x0 C. for 20 minutes prior to testing.
In order to illustrate various aspects of these new electrode/diffuser structures, a series of diffusers is prepared and compared to the commercially available ELATE(trademark) diffuser. Several Type xe2x80x9cAxe2x80x9d diffusers of varying thicknesses are prepared according to the methods described in EXAMPLE 2 except the total weight of solids is increased or decreased to adjust the final assembly""s thickness. A Type xe2x80x9cBxe2x80x9d diffuser of SAB was prepared according to the methods described in EXAMPLE 3. For both these Type xe2x80x9cAxe2x80x9d and xe2x80x9cBxe2x80x9d diffusers, the methodology of dispersion of EXAMPLE 5 is employed. Table 1 summarizes the key differences among the backings. The thickness of each diffuser is taken with nine measurements across the entire (100 cm2) sample. A representative thickness of each type of diffuser is the total average of these nine measurements and number of backing samples.
For porous and/or fibrous gas filtration media, resistance to flow is often used as a characteristic measure for quality control and performance. This measurement is standardized and so widely performed that a commercially produced instrument is employed, called a xe2x80x9cGurleyxe2x80x9d device. The Gurley number is the time it takes to move a fixed pressure through a fixed area of sample, and thus the Gurley number indicates the resistance to gas flow. As the diffuser permeability is an important parameter for optimized fuel cell operation, the Gurley number is a good method to quantitatively characterize diffusers.
To determine the xe2x80x98Gurleyxe2x80x99 number of the various diffusers, an apparatus for measuring resistance to flow was constructed employing two water-filled xe2x80x9cUxe2x80x9d tubes, one 80 and the other 40 cm long, a nitrogen flow meter (0-20 LPM), and a back-pressure valve. Samples of electrode backing (10xc3x9710 cm.) are cut and fitted into a manifold with gaskets (5xc3x975 cm. exposed area), such that the uncoated side of the backing is oriented towards the nitrogen inlet. Prior to evaluation of a backing sample, the system""s inherent resistance to gas flow is evaluated by measuring back-pressure in the U-tubes over a range of flow rates. This system xe2x80x9cresistancexe2x80x9d is used as a correction in subsequent diffuser measurements.
In order to first establish an appropriate evaluation range, five samples of standard Type xe2x80x9cAxe2x80x9d diffusers were subjected to a series of output flow rates. These output rates are fixed by progressively increasing the input flow through the output back-pressure valve and/or input flow rate, and noting the output flow and input and output U-tube pressure. The output flow was varied from 1 to 7 LPM. Pressure differentials, corrected for the system resistance, are calculated, and the output flow rate in LPM is divided by the differential pressure in units of cm. of H2O, which is then normalized to the exposed backing area or 25 cm2. The resulting value is the characteristic constant for resistance to flow and is similar to the Gurley number. A plot of output flow versus the calculated resistance to flow shows that non-uniform flow is realized at the low and high flow rates, and an output ranges of 2-4 LPM is best for these measurements on diffuser materials. Based on this result, an output flow of 3.0 LPM was selected for subsequent comparative measurements.
An additional effect of the diffusers on PEM Fuel Cell performance is related to its role in helping to maintain water balance within the cell. Water balance in the fuel cell entails a delicate balance of hydrophobic and hydrophilic character within the backing layer. This balance depends critically on operating parameters such as current density of operation (which determines how much water is generated), humidification conditions and the flow rate of gases into the cell. Thus, depending on how the fuel cell is run with regards to current density, hydration, type of ion exchange membrane, and flow characteristics to the diffuser the hydrophilicity of the backing is also crucial as well as the structure of the diffuser. Thus, to illustrate how different diffuser structures effect transport under a constant set of operating conditions, these same samples were evaluated in a single cell of a PEM Fuel Cell testing apparatus.
Samples of ELAT or type xe2x80x9cAxe2x80x9d gas diffusion electrode were subjected to small scale tests in an apparatus designed to remove system influences from the experiment. That is, typical operation of a fuel cell or electrochemical process may be dependent on cell design, assembly, and system control parameters. This test used an electrolyte solution and catalyzed electrode to eliminate contact variance between the typical diffuser and MEA. Thus, the catalyst in this system was xe2x80x9cwettedxe2x80x9d and the results reflect electrode structural differences when the same catalyst and and catalyst loading were employed.
A gas diffusion electrode holder (1 cm dia.) was constructed whereby the catalyzed face was positioned in a solution containing 0.5MH2SO4 while the backside was subjected to an open gas plenum of approximately 20 cc. A potentiostat and three electrode set-up were employed to precisely control the applied potential to the test sample. A sheet (2.5xc3x972.5 cm) of platinum served as the counter electrode. Standard ELAT samples containing 30%Pt/C, 1 mg/cm2 were prepared using the method of Example 1. Type xe2x80x9cAxe2x80x9d electrodes were fabricated according to the steps of Example 2, whereupon the catalyst and loading was the same as the ELAT. Each electrode was sprayed with Nafion resulting in a coverage of 0.5 mg/cm2. After mounting in the holder, samples were immersed in the acid solution, which had been heated to approximately 55xc2x0 C. The electrodes were first fed oxygen at a stoichiometric excess (greater than 10 fold) under a very slight pressure xcx9c2 mm H2O) for conditioning as negatives potentials were applied vs. saturated calomel electrode (SCE) to reduce oxygen. After the exposure to oxygen, the cell was disconnected, flushed with nitrogen, and hydrogen was fed to the electrode under the same slight pressure and stoichiometric excess. Positive potentials were applied and the current was recorded. No accomodation for IR was made in the measurements, although 0.5-1 ohm had been measured through the electrode holder. Multiple samples from each ELAT or type xe2x80x9cAxe2x80x9d gas diffusion electrode were tested and averaged and the reported error bars were one standard deviation.
FIG. 2 is a plot of these tests. The applied potential is listed on the abscissa, while the resulting current due to hydrogen oxidation is displayed on the ordinate axis. Since the catalyst and wetting characteristics were identical for these structures, one concludes that the increase in current for type xe2x80x9cAxe2x80x9d over the ELAT gas diffusion electrode was due to the improved structure of the type xe2x80x9cAxe2x80x9d electrode.
The surprising effect of homogenized carbon is shown in FIGS. 3 and 4. In this test, standard ELATs were prepared as outlined in Example 1 but now used homogenized SAB and Vulcan XC 72 as described in Example 5. The three different gas diffusion electrodes (standard ELAT, ELAT with homogenized wet proofing layer, and ELAT with both SAB and Vulcan layers homogenized) were tested using the same testing procedure as detailed above, except both the oxygen and hydrogen curves were recorded. FIG. 3 is the oxygen reduction curve. Although there was some scatter in the data, attributed to uncompensated IR, it can be seen that as the layers were progressively homogenized, greater reductive (or negative) currents were generated for a fixed potential. A significant and unexpected improvement in ELAT performance was realized through homogenization of the carbon blacks. FIG. 4 is a similar plot except the electrode was used as an anode in ambient hydrogen and a significant and unexpected improvement was shown.
The same electrodes were next assembled in a fuel system to confirm the improvement. For this test, a standard fuel cell test station (Fuel cell Technologies, Inc NM) was used to control and humidify feed gasses, provide an electronic load, and record data from a cell 16 cm2. For these tests, electrodes were mechanically compressed against Nafion 115 to form the MEA. An ELAT with homogenized SAB and Vulcan XC 72 layers was used as a cathode, and the test was performed in air and oxygen. FIG. 5 shows the average results of five replicate standard ELATs compared to a typical homogenized carbon black ELAT when using air as the oxidant. For these plots, the fixed cell voltage is listed on the ordinate, while the recorded current is on the abscissa. FIG. 6 is similar to FIG. 5 except pure oxygen was the oxidant. In both FIG. 5 and FIG. 6, an improvement was seen for these electrodes in an actual system. It is surprising that a simple processing step, i.e. homogenization, produces such an increase in current for a fixed voltage (greater power).
The next example combined the improvements of carbon black homogenization with the new structures. A type xe2x80x9cAxe2x80x9d diffuser was constructed according to the homogenization procedure of Example 5. Since this was a diffuser, it was assembled as part of a MEA whereby the catalyst layer has been deposited directly on the ion conducting membrane. In this test, comparison was made with a standard ELAT diffuser and the type xe2x80x9cAxe2x80x9d structure in a fuel cell set up incorporating a cell 50 cm2. The cell was operated with hydrogen and air, and the load was systematically varied. FIG. 7 shows data that revealed an improvement unanticipated by either structural or homogenization changes alone. The trace labeled xe2x80x9coldxe2x80x9d is the standard ELAT diffuser and the trace labeled xe2x80x9cnewxe2x80x9d is the new diffuser structure with homogenized carbons. At the extreme loads, there was a test of a diffuser""s ability to transport oxygen and water vapor, and it shows a remarkable 100% improvement in current density for a cell voltage of around 0.4 volts. This example clearly shows a remarkable synergistic effect between the carbon dispersion and new diffuser structures.
Although the above examples have used fuel cell tests, diffusers and MEAs can be used in industrial electrochemical processes as well. A type xe2x80x9cBxe2x80x9d gas diffusion electrode composed of platinum catalyzed Vulcan XC-72 pressed against Nafion 430 was constructed and this same style of electrode was then manufactured through the knife-on-plate method described in Example 4. The performances of these cathodes operating as oxygen consuming electrodes in a concentrated HCl solution were compared. FIG. 8 summarizes the current potential curves derived from 6.25 cm2 samples of the type xe2x80x9cBxe2x80x9d assemblies. As shown here, the automated coating process did not introduce any significant changes in the structure of the electrode, and no difference in current was obtained.
A type xe2x80x9cBxe2x80x9d gas diffusion electrode composed by hand as illustrated in EXAMPLE 2 and then compared to a gravure method machine-coated Type xe2x80x9cBxe2x80x9d gas diffusion electrode as illustrated in EXAMPLE 7, whereby the platinum content is within 10% of each other. The two electrodes are tested whereby 16 cm2 samples of each are used as anodes and cathodes and evaluated PEM Fuel Cell elements in a hydrogen air mix at 70xc2x0 C. FIG. 9 summarizes the current potential curves derived from these samples. As shown here, the automated coating process did not introduce any significant changes in the structure of the electrode, and no difference in current was obtained.
A series of diffusers are constructed to illustrate the range of structures available for gas diffusion electrode or electrode backing applications. The gas permeation rate through the series was compared using the modified Gurley apparatus. A summary of resistance to flow measurements for the three types of electrode backing are shown as a bar chart in FIG. 10. For the Type xe2x80x9cAxe2x80x9d, three different thicknesses of carbon black were tested. One would anticipate that the resistance to flow should increase as the diffuser thickness increases. This trend is followed by the data. The standard double sided ELAT and thick Type xe2x80x9cAxe2x80x9d diffuser show a greater resistance to flow than the standard Type xe2x80x9cAxe2x80x9d. It is interesting to note that the thin Type xe2x80x9cAxe2x80x9d diffuser shows the greatest relative standard deviation across the average of measurements, indicating that the microporous coating of carbon black may have random pin-holes and thus show lower resistance to flow than the standard Type xe2x80x9cAxe2x80x9d. These data demonstrate that one can adjust the porosity and tortuosity in the diffuser structure, and that a suitable range of resistance to flow constants is over the range of 0.06 to 0.005 LPM/cm H2O/cm2, and more preferably from 0.05 to 0.008 LPM/cm H2O/cm2.
While porosity and tortuosity (as measured on a macroscopic scale by the Gurley measurements) do contribute to the performance of electrode backings (as we will show here), other factors such as catalyst layer permeability and ionic conductivity, backing hydrophobicity and water transport through the backing may contribute more significantly. We subjected this series to evaluation in a PEM Fuel Cell test apparatus.
A collection of polarization curves obtained for cells with different diffusers operating on oxygen, and 13.5% O2 in N2 are shown in FIG. 11. The results indicate that backings Type xe2x80x9cAxe2x80x9d thick and the standard ELAT exhibit limiting current behavior at ca. 20% lower current densities than for the other cases. This suggests that the thicker backings, with their lower gas permeation rates, exhibit more sensitivity to the particularly dilute portions of the cathode flow stream.
Although the uniformly hydrophobic backing Type xe2x80x9cAxe2x80x9d exhibits slightly lower performance relative to the thin and standard Type xe2x80x9cBxe2x80x9ds at the lower current densities, this diffuser has similar performance at high current density. Thus, there is some merit in creating a diffuser of completely uniform hydrophobicity. These results here indicate that a wide variety of structures are suitable for PEM Fuel Cells, and that the structure of the diffuser (or gas diffusion electrode) has to be matched to the specific operating conditions as well as cell design. All of these structures show an improvement over the standard ELAT design under dilute cathode feeds.
These examples demonstrate that new and unexpected advances in performance are obtained when combining homogenization in the preparation steps with the new gas diffusion electrode structures. These are fabricated into MEAs by either assembling a diffuser with a catalyst-coated membrane, or by incorporating a gas diffusion electrode with the membrane. The step of homogenization can be used to prepare mixes for automated coating, and the new structures are capable of being produced in an automated fashion.

Structures and methods of manufacture for gas diffusion electrodes and electrode components


An analytical apparatus, such as a quartz crystal microbalance, comprises a piezoelectric sensor and an oscillator circuit, coupled to the sensor, to oscillate at a frequency substantially determined by a resonant frequency of the sensor, and to provide an output signal at the oscillator frequency at an output. The oscillator circuit maintains a substantially constant drive signal to the piezoelectric sensor by an AGC (33) within a feedback loop of the oscillator. The gain control signal is used as an indication of the Q of the piezoelectric sensor. The drive signal to the sensor is made substantially sinusoidal by ensuring that all the elements in the feedback loop providing signal gain and attenuation are configured to operate in a substantially linear mode.

A quartz crystal microbalance is a device for detecting and measuring very small changes in mass. Its primary components are a quartz crystal and an oscillator circuit coupled to the quartz crystal to produce an output at a resonant frequency of the crystal. The output frequency, which is typically around 10 MHz, is measured to a high degree of accuracy, for example, with a frequency counter. The quartz crystal is, unlike the crystals normally used in electronic circuits, unencapsulated, so that it can interact with its environment. The deposition of small quantities of material onto the crystal changes its resonant frequency and allows the determination of the mass of material deposited. Typically, frequency changes are of the order of a few Hz to a few tens of Hz and changes of the order of nanograms in the mass of material deposited can be detected.
The piezoelectric sensor in research analytical apparatus is typically a thin slice of quartz from an artificially grown crystal, although other piezoelectric materials, such as tourmaline, ethylene diamine tartrate, ADP, KDP, and Rochelle salt, can also be used. Quartz has the advantages of being chemically unreactive and insoluble in water, as well as being relatively temperature insensitive. A typical quartz sensor 10 for a microbalance is shown in FIG. 1. The quartz crystal 16 is typically a circular "AT-cut" with metal electrodes 18 on opposing faces. The electrodes are typically sputtered thin (200 nm) films of gold, silver, or titanium, possibly with a sub-layer for improved adhesion. Lead wires 14 attach to the electrodes and also provide mechanical support for the crystal as well as some degree of isolation from base 12 of the sensor and lead out wires 15. The crystal is typically around 1 cm in diameter. The change in resonant frequency, ΔF of an AT-cut quartz crystal of area A, vibrating in air at fundamental frequency F, when the mass of the crystal is changed by ΔM, is given approximately by:Δf=−2.3×106F2ΔM/A. The quartz crystal microbalance is most frequently used to measure mass, but can also be used to detect changes in the viscosity and/or density of a liquid, since when vibrating in a liquid all these factors affect the vibrational frequency. Thus, the shift ΔF in the frequency of a quartz crystal on immersion in a liquid is given by:ΔF=F03/2(ηLρL/πμ0ρ0)1/2 Where: ΔF=Change in Frequency                F0=Resonance Frequency        ηLρL=Liquid absolute density and viscosity        μQρQ=Quartz elastic modulus and density
A quartz crystal microbalance can be used as a bio-sensor, that is as a device which uses as part of the sensor, or is sensitive to, material of biological origin. Typically, part of one or both electrodes of the sensor is coated with material which is capable of binding with a target bio-molecule or cell. When such a receptor is exposed to the target ("ligand") compound, the ligand is bound to the substrate causing a change in mass ΔM of the sensor, and/or viscosity/density changes in the local microenvironment and a consequent change in its vibrational frequency.
There is a general need for improvements in the sensitivity and effectiveness of known quartz crystal microbalances. More particularly, the oscillator drive circuits of conventional microbalances lack stability and precision, particularly when the sensor is immersed in a liquid. When a crystal surface is immersed in a liquid the "Q" of the crystal (a measure of the sharpness of the resonance or, equivalently, the energy dissipated per cycle) will drop substantially and the resonant frequency will shift slightly. The Q drop is due to the damping effect caused by the absorption of energy by the liquid; the change in resonant frequency is due to the dynamic mass of the liquid on the crystal face. When a crystal resonates its impedance drops from almost infinity to about 50-200 Ω, when immersed the impedance at resonance can be 100 KΩ or higher. In order that the apparatus can operate with the sensor both in and out of a liquid, it is necessary to provide a system with a wide dynamic range to accurately determine the sensors resonant frequency over this wide range of impedances. Although quartz crystal micro balances have been able to operate in either liquid or air, a difficulty arises when the system is required to operate with the sensor in either medium. Furthermore when operated in a liquid prior devices often exhibit a lack of stability, sensitivity, and precision, possibly related to the broader resonance and greater energy dissipation of the immersed sensor. Efforts to counteract some of this variability have included using dual matched oscillators, comparing the frequencies of working and reference crystals (Dunham G. C., Benson N. H., Petalenz D. and Janata J., Anal. Chem., 67 (1995) 267272).
There is also a need for improved screening processes for rapid evaluation of compounds of potentially therapeutic benefit. Pharmaceutical companies typically synthesize thousands of compounds, which are then screened in order to identify those which interact with a target molecule. Typically, one or other of the target and ligand are labelled with a radioactive or fluorescent tag, but such techniques are slow, expensive and require the handling of dangerous materials, and the labelling may interfere with the receptor-ligand interaction. A further problem is therefore the need for improved screening processes and quartz crystal microbalances promise some advantages in this field. However, in this context it is desirable to have a sensor-oscillator system which is stable even in a flowing liquid and which, in this configuration, is sensitive enough to allow bio-molecular interactions on the surface of the crystal electrode to be monitored in real time.
FIG. 2 illustrates an oscillator circuit 20 for a quartz crystal microbalance, known from Barnes C., Sensors and Actuators A., 29 (1991) 59-69. Quartz crystal 24 is series connected in a feedback path connecting the output of CMOS inverter 22 to its input, thereby forming an oscillator. A one-transistor buffer circuit 26 is coupled to the oscillator and provides an output at 28 to a frequency counter. The oscillator will operate in the range 1.7 to 20 MHz, but in the prior art was used by Barnes at 14 MHz. A quartz crystal microbalance using this circuit exhibits the disadvantages outlined above. The present invention seeks to alleviate the problems of conventional systems by providing an improved oscillator circuit.
Accordingly, we will describe analytical apparatus comprising a piezoelectric sensor and an oscillator circuit, coupled to the sensor, to oscillate at a frequency substantially determined by a resonant frequency of the sensor, and to provide an output signal at the oscillator frequency at an output and characterized in that the oscillator circuit incorporates means to maintain a substantially constant drive signal to the piezoelectric sensor. In a preferred embodiment, the analytical apparatus is a quartz crystal microbalance. Preferably, the substantially constant drive signal is maintained by AGC means within a feedback loop of the oscillator, the gain control signal from which can be used as an indication of the Q of the piezoelectric sensor. It is desirable that the drive signal to the sensor is substantially sinusoidal since this provides greater accuracy, sensitivity and stability for the apparatus. This can be achieved by ensuring that all the elements in the feedback loop providing signal gain and attenuation are configured to operate in a substantially linear mode.
We will further describe a method of measuring a characteristic of an oscillating piezoelectric sensor in a fluid medium comprising providing an oscillating drive signal to the sensor and characterized in that the method further comprises controlling the drive signal such that it is maintained at an approximately constant level despite changes in the fluid medium.
We will also describe analytical apparatus comprising a piezoelectric sensor; and an oscillator circuit, coupled to the sensor to oscillate at a frequency substantially determined by a resonant frequency of the sensor, and to provide a first output signal at the oscillator frequency; characterized in that the oscillator circuit comprises means to provide a second output signal correlated to the 'Q' of the sensor.
We will further describe an oscillator circuit comprising a piezoelectric sensor, to oscillate at a frequency substantially determined by a resonant frequency of the sensor and to provide an output signal at the oscillator frequency at an output, the circuit comprising an amplifier with feedback coupling to the output to an input of the amplifier to form a feedback loop through the amplifier; and characterized in that the feedback loop incorporates automatic gain control (AGC) means to maintain a substantially constant drive signal to the piezoelectric sensor.
An oscillator is generally constructed by providing an amplifier with feedback. The conditions for oscillation are that the loop gain should be equal to or greater than unity and that the loop gain phase shift is zero (or an integral multiple of 2π). The latter condition expresses a requirement that a signal propagating through the amplifier and the feedback network must experience a phase delay of one or more whole cycles at the frequency of oscillation. A quartz crystal has a series resonant frequency at which its impedance is zero and a slightly higher parallel resonant frequency at which its impedance is infinite; typically, a circuit incorporating a quartz crystal will oscillate close to the parallel resonant frequency of the crystal.
In FIG. 3, the conceptual block diagram of an improved oscillator is shown. In this circuit gain is provided by amplifier 32 and automatic gain control (AGC) means 33, 34. The AGC means comprises a variable gain means (amplifier or attenuator) 33 with gain (amplification/attenuation) controllable by a signal on control input 35. It further comprises means 34 to sense the output signal level from the circuit and to control the variable gain means in response to the sensed level. Feedback in the circuit is provided by line 38 which connects the output of the gain/attenuation element 33 to the input of amplifier 32. Quartz crystal 31, which determines the resonant frequency of the oscillator is connected between the output of amplifier 32 and an earth point, which may be a virtual earth. Alternatively, the circuit can be arranged so that the crystal is series connected. In the parallel-connected configuration shown, in order for crystal 31 to properly determine the resonant frequency of the circuit, the output impedance of the driving circuitry must not be too dissimilar to that of the crystal. Thus, if the amplifier has a low output impedance, the crystal can be driven via a resistor; alternatively, amplifier 32 may be a transconductance amplifier with a high output impedance. A conventional oscillator will generally include further capacitive and/or inductive tuning elements in series and/or in parallel with the crystal and/or in the feedback path 38. However, in the circuits described herein it is preferable to reduce such additional elements to a minimum so that the frequency of oscillation is as much as possible determined by the resonant frequency of the crystal alone. It will be seen later that in a preferred embodiment the passive elements of the oscillator drive circuit are, disregarding stray capacitance, almost entirely resistive.
Loop gain is provided by the combination of amplifier 32 and AGC means 33; one or other of these elements may provide some or all of the gain. In operation, the output of amplifier 32 drives crystal 31 at the resonant frequency and a signal at this frequency appears at output 36. The signal level at output 36 is monitored by level sensing means 34 which is coupled to the gain control input 35 of AGC means 33 to control the loop gain. Since the output of AGC means 33 is fed back to the input of amplifier 32, a substantially constant drive at the frequency of oscillation is maintained to crystal 31. A substantially constant signal level is also maintained at output 36. When used with a quartz crystal microbalance or similar apparatus, maintaining the level of drive signal to quartz crystal, which in the general case is any form of piezoelectric sensor, provides a number of advantages. The improved drive circuit is capable of maintaining sensor oscillation whilst the sensor is in a range of different or varying environments—for example, air, liquid, and flowing liquid. In a liquid, as described above the additional damping decreases the Q of the sensor and in the system of FIG. 3 the oscillator automatically compensates for this Q reduction.
It has also been found that it is important to provide a substantially sinusoidal drive signal to the crystal. When the crystal is damped, for example by a liquid, if a significantly non-sinusoidal or square wave is driving it, there is a tendency for the crystal to hop to an overtone frequency. This tendency is reduced with a sinusoidal drive. The oscillator drive and feedback circuitry (in FIG. 3, amplifier 32 and AGC means 33) is therefore preferably linear. In a preferred embodiment described below, all the amplifiers in the relevant part of the oscillator, that is those involved in providing a high frequency drive to the crystal, are linear, and preferably have low distortion.
The features described above assist the oscillator in locking more precisely onto the resonant frequency of the sensor, especially in "difficult" or liquid environments, thus providing a quartz crystal microbalance with improved sensitivity and robustness of use. Furthermore, this oscillator design can more easily follow frequency changes such as occur when, for example, detecting the binding of a ligand to the crystal and a substrate thereon.
It has been found that the level of the AGC control signal can be correlated with the 'Q' of the sensor and hence this control signal is also provided at a separate output 37. In the detailed circuit described with reference to FIG. 6, a DC level proportional to the sensor Q value is provided at this output. It has also been found that the sensor Q value can empirically be related to the viscosity of a fluid in which the sensor is immersed and the Q output can therefore be used to provide a direct indication of this viscosity. Accordingly in a preferred embodiment of the invention, changes in frequency and Q value are monitored to obtain useful information concerning material subjected to analysis using the sensor.
In FIG. 3, the crystal 31 has been shown connected in parallel with the amplifier output, but the skilled person will understand that the crystal could also be connected at other positions in the circuit and could, for example, be series connected.
Referring now to FIG. 4a, a block diagram of a preferred embodiment of an oscillator 40 is shown, based on the conceptual arrangement of FIG. 3. Like the conceptual system, the oscillator of FIG. 4a includes a piezoelectric sensor 42 driven by an amplifier 46, although in a preferred system this amplifier has unity gain and serves as a buffer. In the embodiment shown, amplifier/buffer 46 has two outputs, one driving the sensor 42, the second forming one input to a two-quadrant multiplier 48 which is voltage controlled by input 50 and operates as a voltage controlled attenuator, In a specific embodiment, amplifier 46 provides a current drive to a quartz crystal 42 and this current is mirrored to provide an input to the two-quadrant multiplier. In the arrangement of FIG. 4a, a second terminal of the sensor 42 is also driven by amplifier 44, which similarly to amplifier 46, provides an output to a second input of the two-quadrant multiplier 48. Again, amplifier 44 is preferably a unity gain buffer. The output of multiplier 48 is further amplified by amplifier 52, preferably an operational amplifier, and, as in the conceptual arrangement of FIG. 3, a positive feedback path 64 is provided to the input of amplifier/buffer 46. The output of amplifier 52 is made available at an output terminal 62; this provides a signal at the frequency of oscillation for input to a frequency counter.
The output of amplifier 52 is also coupled to rectifier means 54 which detects the signal level and integrator/comparator means 56 which, inter alia, smooths variations of signal level and integrates over a number of cycles. The signal level at the output 62 is compared with a signal level amplitude set on adjustment means 58, which is preferably a potentiometer. The output of integrator 56 is coupled to the "gain" control input 50 (in a preferred embodiment, an attenuation control input) to maintain an approximately constant level of output signal at output 62, and to maintain the drive signal to piezoelectric sensor 42. This control signal is, in a preferred embodiment, a DC level, made available at output 60 and proportional to the Q of the sensor.
The elements shown enclosed by dashed line 66 are available in a single integrated circuit package, which improves the matching between circuit elements and provides additional advantages of reduced circuit size and power consumption. An integrated circuit, such as the Comlinear CLC 520 or CLC 522 available from National Semiconductor Corporation, can be used. When the oscillator output is rectified and compared with the amplitude reference voltage and the result applied to what can be termed "cell gain control input", a substantially pure sinusoidal oscillation around the crystal resonance is generated with a tight control over loop gain. When the sensor Q falls, the output reduces, thus increasing the input to the gain cell and increasing the loop gain to compensate, so keeping the resonance constant. In the circuit of FIG. 4a, the two-quadrant multiplier 48 functions as a "gain cell". The feedback path 64 applies positive feedback and the circuit therefore resonates at a resonant frequency of the piezoelectric sensor
FIG. 4b illustrates the connections to input amplifier 46. This amplifier is unusual in that it has a single input, 46A, but two outputs, 46B and 46C. The functioning of this amplifier was outlined above; but, it is helpful to consider another equivalent way of looking at its function. The output 46B reflects the reactance of a load imposed on the second (intermediate) output 46C, so that the impedance placed on output 46C controls the gain between input 46A and output 46B. In the preferred embodiment, there is unity gain between 46A and 46C, but variable gain between 46A and 46B. Amplifier/buffer 44 is similar.
FIG. 5 shows a detailed circuit diagram of a basic oscillator circuit constructed in accordance with the block diagram of FIG. 4a. In FIGS. 4 to 7, like elements are indicated by the same reference numerals. Thus, the main elements of FIG. 4a can be identified in FIG. 5, that is crystal 42, amplifiers 44 and 46, multiplier 48 with attenuation/gain control input 50, amplifier 52, positive feedback path 64, rectifying means 54, integrator/comparator 56, and output level adjust means 58.
The two input buffers 44 and 46 convert the input voltage to a current that is a function of the differential input voltage and the value of the gain-setting impedance of crystal 42. This current is then mirrored to a gain stage (not shown) and attenuated by voltage controlled two-quadrant multiplier 48, after which it is converted to a voltage by output amplifier 52 with gain set by feedback resistor R4. Thus, the input, output, and gain control signals are all voltages.
An output at the oscillator frequency is provided at 62 and the components within the dashed line 66 can be obtained within a single integrated circuit. In the detailed circuit shown, it can be seen that amplifier 46 has an inverting input whilst amplifier 44 has a non-inverting input, and that amplifier 52 is an operational amplifier. Rectifier means 54 can at its simplest, be a single diode and the integrator/comparator 56 can again be constructed from a single operational amplifier. The skilled person will be aware that many variations on these basic circuit elements are possible. For example, other rectifying elements or more sophisticated full wave rectifiers could be used for the rectifier means. Many conventional variations are possible on integrator means 56 and tuned elements could be incorporated in series or in parallel with the crystal and/or in feedback loop 64, or alternatively, additional reactive (inductive or capacitive or resistive) components could be included in these positions.
FIG. 6 shows a detailed circuit of an embodiment incorporating various additions desirable in a practical implementation. It can be seen that this circuit closely corresponds to that of FIG. 5, with the addition of amplifiers/buffers 70 and 72 for the Q output 60 and the frequency output 62 respectively. Additional (low pass) filtering 68 has also been incorporated in a second feedback path from the output of amplifier 52 back to its inverting ("−") input. Positive and negative 5 volt power supplies 74 are also shown. In FIG. 6, connections to the sensor 42 are made via terminal 76 and connecting wires 78. Wires 78 may be, for example, a screened cable or a balanced twisted-pair type connection. Connectors 76, which may be a single connector, are chosen in accordance with conventional practice, to be suitable for relatively low-level high frequency (1-100 MHz) signals.
FIG. 7 shows one example of a complete quartz crystal microbalance including oscillator 40 coupled via connection 76 and wires 78 to crystal sensor 42 in a flow cell 80, with fluid inlet and outlet connections 81. The flow cell is preferably arranged so that fluid flowing through the cell contacts only a single face of the sensor, to improve the system performance when used with ionic, and hence conducting fluids, although the sensor may also be configured to operate in a fluid bath.
The frequency output 62 is coupled to a digital frequency counter 84 with a resolution of the order of 1 Hz; the Q output 60 is coupled to digital voltmeter 86. FIG. 8 shows an alternative arrangement of a sensor with quartz crystal 88 with lead wires 90 sandwiched between O-rings 94 and 96 and resting on a cover slip 98. This arrangement provides a well 92 which can be partially filled with liquid; a simpler configuration can dispense with cover slip 98 and lower O-ring 96. Generally speaking, gold sensor electrodes are preferred because of their reduced tendency to oxidize.
A quartz crystal microbalance such as that shown in FIG. 7, can be used for a variety of purposes. The apparatus is sensitive, among other things, to the density and viscosity of a liquid in which the sensor is immersed and can detect small mass changes even where a large tare is necessary to take account of initially deposited material. One actually constructed circuit was found to be stable to ¼ Hz/day, obviating a need for a reference crystal. The apparatus can also detect the binding of ligands of much smaller molecular weight than is possible with conventional quartz crystal microbalances, allowing direct screening of potential drug candidates. For example, the circuits described herein allow the detection of binding of ligands of molecular weight less than 600, for example ligands of molecular weight circa 180 or less, such as glucose.
The apparatus can be used to provide improved immunoassay apparatus. A typical immunoassay involves detecting the binding of an antigen to an antibody, or of a receptor to a ligand. One immunoassay process, known as the Bayston test (Bayston R., J. Clin. Path., 25 (1972) 718-720), relies on the antibody-antigen binding process in solution to form aggregates or clumps producing a visible precipitate. This process can be monitored using a quartz crystal microbalance, which is thought to respond to changes in the viscosity and density of the medium supporting the antibody and antigen. These changes cause a drop in the resonant frequency of the sensor, the largest drop occurring at the point of full agglutination and hence a quartz crystal microbalance simplifies detection of the titre of agglutination.
The improved quartz crystal microbalance can be used to monitor the characteristics of bacteria in solution. For example, one face of the piezoelectric sensor is coated with fibronectin to which bacteria such as Staphylococcus Epidermidis will bind. The microbalance also has the required sensitivity and robustness for a real time monitoring system to allow continuous assay of a sample, for example, to determine the number of cells in a solution. In such a configuration, the shift in resonant frequency of the sensor has been found to be related to the bacterial concentration, with a roughly linear relationship between the frequency shift and a logarithm of the bacterial concentration. In both this and the previous example, local changes in solution viscosity and density in the region of the electrode surface are believed to influence the resonant frequency of the sensor. It is believed that this effect is present even when there is no actual binding of a species directly to the crystal surface, but merely a local modification to the liquid around the vibrating sensor.
The apparatus can also be used to investigate binding and similar processes as they occur in complete or substantially complete cells. The cells can be grown onto the sensor following which the small additional mass changes due to binding and transport mechanisms at a cell surface can be monitored if desired, in vivo. This is possible because of the ability of the oscillator circuit to follow large changes in the resonant frequency of the crystal whilst maintaining oscillation and maintaining the sensitivity of the system to very small mass changes. Similarly, relatively thick multi-layer structures—for example twenty layers of protein—can be built up on the sensor before binding events are monitored.
Although some examples of possible applications of the invention have been given, many other alternatives will occur to those skilled in the relevant arts, and it should be understood that the present invention is not limited to the illustrated embodiments. Likewise, many other effective alternative configurations of the described oscillator circuit can be envisaged, and again it should be understood that the design of the circuit is not limited to those particular versions described and illustrated.

This patent is subject to a terminal disclaimer.

Quartz crystal microbalance with feedback loop for automatic gain control