RELATED APPLICATIONS 
     The present application is based on, and claims priority from, France Application No. 06 04051, filed May 5, 2006, the disclosure of which is hereby incorporated by reference herein in its entirety. 
     FIELD OF THE INVENTION 
     The invention relates to guiding devices for electromagnetic waves with a frequency of less than 10 terahertz. 
     BACKGROUND OF THE INVENTION 
     The term “guiding device” is understood to mean any device intended to control the propagation of electromagnetic waves. These devices cover in particular: waveguides, electromagnetic cavities, reflectors, diffusers, antennas, filters and attenuators. 
     Some of these guiding devices are used not only to control the propagation of electromagnetic waves, but they may also employ electron beams or beams of other particles that may or may not be provided with an electric charge. This is the case in particular for all electron tubes and nearly all particle accelerators. 
     In the rest of this text, for more succinct expression, and to differ from the usually accepted meaning of the term “waveguide”, we will simply call any guiding device within the meaning defined above a “waveguide”. 
     One particular example of a waveguide within our intended meaning is that of cavities for high-precision atomic clocks. In this example, the cavity consists of a single body, of complex shape, which includes several holes. 
       FIGS. 1   a  and  1   b  show one particular example of a cavity employed for producing an atomic clock. A microwave is introduced via an access port  4 . This microwave interacts with a cesium beam (J c ) that passes through the cavity and is introduced via an aperture  6 . 
     In all waveguides, the waves are confined by the positioning, in space, of physical objects called “bodies”. Like any physical object, a body occupies a volume that is bounded by one or more closed surfaces. The vicinity of such a closed surface is called the “wall” of the body. 
     The particular feature of the body of a waveguide is that at least part of the surface of its walls interacts directly with the guided or confined electromagnetic waves and consequently must be endowed with controlled electromagnetic properties. 
     That part of a wall which interacts directly with the guided or confined electromagnetic waves, and which must be endowed with controlled electromagnetic properties, is called the “active” part of the wall. In the rest of the description, the term “active wall” will refer to an “active” part of a wall of a waveguide body. 
     It is the geometric and electromagnetic properties of the active walls that determine the electromagnetic properties of the waveguide. 
     Two types of characteristics of these active walls directly determine the electromagnetic behavior of the waveguide: 
     (1) their geometric shape; and 
     (2) their reflectivity with respect to electromagnetic waves. 
     In the most demanding applications, the aim is to achieve very precise control of the electromagnetic wave propagation, which means that the geometric shape of the active walls of the waveguide must be controlled very precisely. 
     Depending on the application, the aim is to have different reflectivities on the active walls. 
     For example, for an attenuator, the aim is to absorb the waves in the active wall. 
     However, for most applications, in particular for a waveguide in the usual meaning of the term, for an electromagnetic cavity or for a reflector, the aim is usually for the active wall to be as reflective as possible with respect to the waves, without absorbing the energy of the wave. This means that the electrical conductivity of the body near the wall must be as high as possible at the frequencies corresponding to the waves present in the waveguide in operation. 
     More precisely, for these types of waveguide, which will be called “low-absorption” waveguides, it is necessary to ensure that the conducting material constituting the active wall, in direct contact with the electromagnetic waves, has the optimum electrical conductivity over a thickness equal to a few “skin depths” of the most penetrating components (with respect to the walls) of the wave that should reside in or travel through the waveguide. 
     For example, for a waveguide intended to be used at ambient temperature and at frequencies close to 10 GHz, the walls of the waveguide being made of copper, the skin depth is a fraction of one micron and it is sufficient for there to be less than 10 microns of copper on the wall in order to approach to better than 99% the quality factor of a cavity made of solid copper. 
     In specific waveguide applications, the main functionality of controlling the electromagnetic wave propagation is not the only one involved in the specification and design of the waveguide. Many other contingencies must also be considered. 
     The most common additional criteria relate to the following points:
         the volume and total mass of the waveguide;   its resistance to mechanical attack, particularly accelerations, vibrations, impacts and stresses;   its resistance to thermal attack, particularly temperature rises during heat treatments and temperature cycling during operation;   its resistance to chemical attack, particularly to corrosive atmospheres;   the electrical conductivity of the volume or certain regions of the inactive walls of the bodies;   the manufacturability and manufacturing cost of the waveguide;   its functional endurance in the intended application environment; and   its ability to discharge the dissipated heat, very often essentially in the active walls.       

     DESCRIPTION OF THE PRIOR ART 
     One usual solution for producing a waveguide lies in the use of homogeneous metal bodies of high electrical conductivity. 
     Waveguides for radiofrequency waves or microwaves often use either a molded solid or recessed metal body, or a body consisting of a metal foil, the internal face of which defines the “activated wall” or “hot wall” of the cavity. 
     The most conventional solution consists in producing the body or bodies in a homogeneous metal of high electrical conductivity, such as copper, silver, gold or aluminum, and even in some cases to make use of superconducting materials. 
     There are two main drawbacks with this solution:
         if the metal is a solid metal, the body is heavy;   if the metal is thin, the body is easily deformable since metals having a high electrical conductivity are, without exception, particularly soft. It is therefore necessary to fit a special device for controlling the change in geometry of the active walls under the operating conditions of the waveguide.       

     Other drawbacks are the fact that gold and silver are very expensive, while aluminum easily oxidizes. 
     All these metals are easily deformable. This may pose problems if the waveguide is subjected to large accelerations or mechanical stress, for example during the take-off or landing of an aircraft, or rocket in the case of a waveguide intended to be used in a satellite. Very strong bodies must be made so that the active walls deform as little as possible. Metals having a high electrical conductivity also have, almost in all cases, a high thermal expansion coefficient, which effect may distort the shape of the waveguide volume in the operational environment in which the waveguide is used, if the waveguide is exposed to an inhomogeneous heat flux. As mentioned above, this distortion may be detrimental. 
     This solution also has additional drawbacks:
         since the volume of the body is electrically conducting, if it is subjected to a temperature gradient, permanent thermoelectric currents may be generated that may induce magnetic fields, these fields possibly disturbing the motion of charged particles in the waveguide.       

     However, these metals are all good thermal conductors. 
     As regards superconducting materials, these need to be permanently cooled in order to operate, which cooling requires a bulky, expensive and complex infrastructure. 
     In the example of the cavity for an atomic clock, shown in  FIG. 1   a , when this type of cavity is made conventionally, the single body is made of solid copper. 
     For reasons of convenience, the body of the cavity in  FIG. 1   a  is manufactured by assembling two half-bodies  10 ,  12 . The two half-bodies are assembled in a known manner using a thermal or mechanical effect. 
       FIG. 1   b  shows one of the two half-bodies  12  of the cavity of  FIG. 1   a.    
     The conventional process for producing the cavity of  FIG. 1   a  includes, in particular, steps for manufacturing two half-bodies  10 ,  12 , made of a copper alloy, which are symmetrical with respect to an assembly plane P, each half-body having a half-recess  16 ,  18 . Joining the two half-bodies together forms the recess  20 , the boundary of which is the “active wall” of the cavity, in direct contact with the electromagnetic waves. 
     A second standard solution consists in using a body most of the volume of which is made in a first material, which body includes a layer of a second material, having a high electrical conductivity, which is attached to or deposited on all or part of the surface of the body or bodies, on the active wall or active walls of the waveguide. 
     An advantageous variant of this second approach for producing a body consists in using, as first material for producing the volume of a body, a metal, insulator or semiconductor material having favorable thermomechanical properties, superior to those of bulk metals, with respect to the additional quality criteria mentioned above. In this case, a layer of a second material, that having a high electrical conductivity, may be attached to or deposited on the active walls of the cavity. 
     The thickness of this layer of the second material must be at least equal to a few “skin depths” of the most penetrating components (with respect to the walls) of the waves that should reside in or travel through the waveguide. 
     This second solution may allow some of the problems to be solved by a judicious choice of the first material used to produce a body. This may in particular be:
         either a metal or semiconductor or insulator material which has a lower density than metals that are good electrical conductors;   or a metal or semiconductor or insulator material which has a lower expansion coefficient than metals that are good electrical conductors;   or a metal or semiconductor or insulator material which has a lower thermoelectric coefficient than metals that are good electrical conductors;   or a metal or semiconductor or insulator material which has a higher mechanical strength than metals that are good electrical conductors.       

     The ideal would be to find a material that combines all these properties. 
     To find a metal that meets all these conditions seems very difficult, if not impossible, especially if, as is often the case, additional properties are also required of the metal. 
     Moreover, the insulator materials that could be selected for producing such a cavity body are often very hard materials which are difficult to form. 
     SUMMARY OF THE INVENTION 
     To alleviate the drawbacks of the waveguides of the prior art, the invention proposes a novel type of electromagnetic waveguide comprising at least one body supporting at least one active wall of predetermined geometric shape, 
     wherein the body or bodies of the waveguide, or the parts assembled to form the body or bodies of the waveguide, are produced from a volume of a ceramic selected from the following : silicon carbide, aluminum nitride, boron nitride, and especially 3C cubic and 2H hexagonal varieties of boron nitride, diamond, beryllium oxide, solid solutions of said materials or assemblies thereof. 
     The ceramics of the body according to the invention exhibit a high thermal conductivity and, for the most part, a low electrical conductivity. 
     For some applications, there are advantages in using for the body a ceramic that is electrically insulating or semi-insulating. 
     These ceramics for the bodies of the cavity may be employed in various forms:
         single crystals;   polycrystals, textured to a greater or lesser extent;   formed composites, the matrix of which differs in nature from that of the aggregates that are embedded therein;   laminated materials; and   assemblies of parts using known methods for assembling ceramics.       

     Compared to existing waveguides, with active walls of geometrically similar shape, the waveguides according to the invention offer improved thermomechanical characteristics for the same or similar electromagnetic characteristics. 
     Advantageously, a body of the waveguide according to the invention has, near the active wall(s) a coating (for example in layer form) made of an electrically conducting material. The electrically conducting material of the active wall(s) is made of a metal selected from the following: gold, silver, copper, aluminum. 
     In a preferred embodiment, the body has, near the active walls, one or more intermediate layers inserted between the coating of electrically conducting material and the ceramic volume. The function of the layer directly in contact with the ceramic can be to promote tying to the ceramic. In that case, such a layer is called a “tie layer”. This single layer or another layer of the stack of intermediate layers may serve as a diffusion barrier and thus prevent any inopportune chemical reaction between the external metal coating and the ceramic of the body. This single layer, or else one, two or more other layers of the stack, may again be used to accommodate the difference in expansion coefficient between the material of the electrically conducting coating and the ceramic of the body. 
     The intermediate layer(s) may be made of a metal selected from the following metals: aluminum, titanium, zirconium, hafnium, vanadium, niobium, tantalum, chrome, molybdenum, tungsten, or produced in an alloy of these metals, or else a carbide, silicide, nitride or boride compound of one or more of these metals, a metal, semiconductor or insulator compound, or else a ternary, quaternary or multiple solid solution of such compounds. 
     In one family of particular embodiments of waveguides according to the invention, the coating layer made of electrically conducting material, on the active walls of the body or bodies of the waveguide, is made of copper and the ceramic is silicon carbide. 
     The advantages of this type of waveguide according to the invention are:
         low bulk density;   very high mechanical strength;   very low thermal expansion coefficient;   good heat conduction;   compatibility with ultrahigh vacuum;   use of very high temperatures for producing or operating said waveguide, without impairing its performance; and   in certain cases, the electrical insulation properties of the cavity body are advantageously used for functions other than those that use “active walls” of the cavity.       

     One of the main applications of this invention is the production of microwave waveguides, particularly electromagnetic cavities, reflectors and antennas, of low weight and very high mechanical strength. 
     Other advantages associated with the waveguides according to the invention lie in the fact that their bodies have a very low thermal expansion coefficient and good heat conduction. Furthermore, the bodies of certain waveguides according to the invention may exhibit good compatibility with ultrahigh vacuum and allow the use of very high temperatures for producing or operating them, without impairing their performance. 
     The invention also relates to a process for manufacturing an electromagnetic waveguide comprising at least one body supporting at least one active wall of predetermined geometric shape, which process comprises at least the following steps:
         production of at least one body of the waveguide from a volume of a ceramic selected from the following : silicon carbide, aluminum nitride, boron nitride, and especially 3C cubic and 2H hexagonal varieties of boron nitride, diamond, beryllium oxide, solid solutions of said materials or assemblies thereof;   possible deposition of one or more intermediate layers on all or parts of the active walls of the body; and   deposition of a metal coating having a high electrical conductivity, either directly on the ceramic or on the intermediate layers, at least over the entire surface of the active walls of the body or bodies.       

     In a process for manufacturing a waveguide according to the invention, at least one of the bodies of the waveguide is obtained by assembling two half-bodies. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be better understood from the description of a first exemplary embodiment of a waveguide according to the invention with the aid of referenced drawings in which: 
         FIGS. 1   a  and  1   b , already described, show one particular embodiment of a cavity of the prior art; 
         FIGS. 2   a  and  2   b  show the steps of a process for manufacturing a body of a waveguide according to the invention; 
         FIGS. 2   c  and  2   d  show sectional views in a plane P of the cross sections of the half-bodies of  FIGS. 2   a  and  2   b  before assembly; and 
         FIG. 2   e  shows a cross section of the body of  FIGS. 2   a  and  2   b  before assembly. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A body  30  of a waveguide according to the invention, shown in  FIGS. 2   a  and  2   b , includes two microwave ports S 1  and S 2  and apertures  32  in the waveguide walls intended for passage of an electron beam EB. More precisely, this is a waveguide in the usual meaning of the term, comprising two outputs S 1  and S 2  for the microwave signals produced, in the waveguide, by the passage of the electron beam EB through the waveguide, via the apertures  32  made in the body of the waveguide. 
     In this embodiment, the body  30  of the cavity is obtained by assembling two half-bodies  34 ,  36  (see  FIG. 2   a ). 
       FIGS. 2   c  and  2   d  show sectional views in a plane P of the cross sections of the half-bodies of  FIGS. 2   a  and  2   b  before assembly.  FIG. 2   e  shows a cross section of the waveguide body  30  resulting from assembling the two half-bodies shown in  FIGS. 2   c  and  2   d.    
     The manufacturing process comprises the following main steps:
         production of the volume of the two half-bodies  34 ,  36  made of a silicon-carbide-based ceramic. In this particular embodiment, the sections C 1  and C 2  of each half-body  34 ,  36  are in the form of a half-tube with a rectangular cross section of the same shape, comprising an active wall  40 , inactive walls  42 , called closure walls of the waveguide, that are intended to be brought into contact with each other to assemble the body of the waveguide, and external walls  44  of the waveguide. Among these external walls may be distinguished adjacent walls  46  that join the closure walls  42 ;   deposition of one or more intermediate layers  50  on the active walls  40 , the closure walls  42  and the adjacent external walls  46  of the two half-bodies  34 ,  36  that join the closure walls  42 ; and   deposition of a copper coating  52  on the intermediate layers, on the active walls  40 , closure walls  42  and optionally also the adjacent walls  46 .       

     The intermediate layers  50  are inserted between the copper coating  52  and the surfaces of the active walls  40 , the closure walls  42  and possibly the adjacent external walls  46  of the ceramic body, on the one hand in order to obtain good adhesion of the metal coating to the surfaces of the walls of the body and, on the other hand, optionally, to act as a diffusion barrier and thus prevent any inopportune chemical reaction between the copper coating and the ceramic of the silicon-carbide-based body, and also, possibly for accommodating the difference in thermal expansion coefficient between the material of the electrically conducting coating  52  and the ceramic of the body  30 . 
     The composition of the intermediate layers depends on the heat treatments that the body will have to undergo during assembly of the waveguide, or during the subsequent life of the waveguide. Depending on the manufacturing temperatures or operating temperatures of the cavity, it is possible to use either a single layer, or two or more layers. In the simplest cases, it is possible to use a single layer, of sufficient thickness, of a material that reacts neither with the copper nor with the ceramic. 
     The intermediate layer(s)  50  may be made of a metal selected from the following metals: aluminum, titanium, zirconium, hafnium, vanadium, niobium, tantalum, chrome, molybdenum, tungsten, or produced in an alloy of these metals, or else a carbide, silicide, nitride or boride compound of one or more of these metals, a metal, semiconductor or insulator compound, or else a ternary, quaternary or multiple solid solution of such compounds. 
     The copper coating  52  forms the metal coating on the active walls of the two half-bodies and is deposited at least over the entire surface of the active walls  40  of the waveguide and also over all or part of the surface of the closure walls  42  and possibly also over all or part of the surface of the adjacent walls  46 . 
     For a copper coating thickness of a few microns, it is possible to obtain a level of absorption of microwaves in the X-band region (at a frequency of around 10 GHz) comparable to that of a solid copper waveguide, for the same geometry of the active walls; and
         assembly of the two half-bodies  34 ,  36  to form the waveguide body  30 , by brazing, welding or thermocompression bonding, on the closure walls  42  of the copper-coated half-bodies using known copper-to-copper assembly methods.       

     The two half-bodies may also be assembled by any other assembly method that allows the parts to be held together in intimate contact. 
     In the embodiment of the waveguide shown in  FIG. 2   b , the ceramic volumes of the two half-bodies  34 ,  36  are obtained by sintering a small-grain silicon carbide powder to which, according to known techniques, sintering-promoting additives, often based on boron and/or silicon, are usually added. 
     Each half-body  34 ,  36  is formed cold, before sintering, and is then ground after sintering. 
     The manufacturing process described for producing the waveguide of  FIG. 2   b  is of course applicable to waveguides (within the usual meaning of the term) or cavities for electron tubes, for example of the klystron type. In this case, the shapes of the half-bodies change according to the application. 
     A second embodiment of a waveguide according to the invention is that of a variant of the cavity shown in  FIG. 1   a , already described above:
           FIG. 1   a  shows a body of this cavity formed from two half-bodies; and     FIG. 1   b  shows one of the two half-bodies of the cavity of  FIG. 1   a  before the two half-bodies are assembled.       

     Each half-body may be produced according to the invention using the specified materials according to the invention, that is to say one, two or more ceramic volumes covered with one or more layers according to the invention. 
     The body of the cavity may be assembled as in the case of the first embodiment described above. 
     The invention applies to many fields covering, in particular, the following applications of “waveguides” produced according to the principles described in the invention:
         atomic clocks, for example cesium-beam or rubidium-beam atomic clocks;   microwave cavities and waveguides having metallic or superconducting “active walls”;   electronic devices: amplifiers, switches, limiters, which employ electrons or other charged particles, in a vacuum or in a controlled gaseous atmosphere, or else within a plasma; and   particle, particularly electron, proton or positron, accelerators, in which the particles may or may not have an electric charge or an electric or magnetic dipole or quadripole.
The invention relates to electromagnetic wave guiding devices or waveguides (f<10 THz) and to processes for manufacturing these waveguides, which comprise at least one body ( 30) supporting at least one active wall ( 40). The body ( 30) of the waveguide is made from a volume of a ceramic selected from the following: silicon carbides, aluminum nitride, boron nitrides, and especially 3C cubic and 2H hexagonal varieties of boron nitride, diamond, beryllium oxide or assemblies of said materials. Applications: waveguides, filter cavities, reflectors and antennas for radiofrequency waves and microwaves, atomic clocks and particle accelerators.