Title of Invention: Process for producing dense ceramic product
Abstract: A process for the production of dense polycrystalline silicon carbide shaped articles, includes the steps: (a) heating a powder compact containing silicon carbide and alumina or a precursor thereof together with a secondary sintering assist, to an intermediate temperature for an extended dwell, and then (b) heating the product of step (a) to a higher temperature for sufficient time to produce a dense polycrystalline silicon carbide product. The secondary sintering assist includes one or more rare earths or their precursors, for example scandia, yttria or dysprosia.
Detailed Description: Part PAC: EXAMPLES 1 AND 2
Part PAR: The raw materials used were silicon .alpha. carbide from Lonza known as grade UF10; alumina from Alcoa known as grade A16SG; yttria from Starck and dysprosia from Cerac (see table 1). The powders were weighted and ball milled using silicon carbide milling media. The starting composition are shown in table 2 and the conditions for milling are shown in table 3.
Part TBL: TABLE 1 ______________________________________ Raw Materials Raw Material Source ______________________________________ .alpha. SiC Lonza UF10 .alpha. Al.sub.2 O.sub.3 Alcoa A16SG Y.sub.2 O.sub.3 Starck Lot P 1/87 Dy.sub.2 O.sub.3 Cerac D1016 Lot 4291 ______________________________________
Part TBL_2: TABLE 2 ______________________________________ Starting Composition (weight percent) Raw Material Example SiC Al.sub.2 O.sub.3 Y.sub.2 O.sub.3 Dy.sub.2 O.sub.3 ______________________________________ 1. 88.0 10.0 2.0 0.0 2. 88.0 10.0 0.0 2.0 ______________________________________
Part TBL_3: TABLE 3 ______________________________________ Conditions used for Ball Milling Operation ______________________________________ Time 16 hours Powder 300 g Milling Media 1500 g Fluid 600 ml iso-propanol Binder 2.degree. wt % ______________________________________
Part PAR_2: After milling, the milling media was separated from the slurry which was subsequently spray dried. The powder was uniaxially pressed and cold isostatically pressed at a pressure of 150 MPa into a 70 mm square tile and a 65 mm diameter disc for examples 1 and 2 respectively. The samples were then heated in air to 400.degree. C. for 5 hours to remove the binder. Each sample was placed in a graphite work box with a loose fitting lid. The work box was then heated in a graphite resistance furnace. Each firing cycle consisted of heating the specimen initially in an atmosphere of argon. At 1600.degree. C. the furnace was evacuated and backfilled with carbon monoxide. The details of the firing cycle are given in table 4. The specimens after firing had a smooth surface finish. The results are given in table 5.
Part TBL_4: TABLE 4 ______________________________________ Firing Cycle ______________________________________ Heat to 1400.degree. C. at 10.degree. C. min.sup.-1 Heat to 1600.degree. C. at 5.degree. C. min .sup.-1 antonio78@example.org. C. for 30 minutes Heat to 1900.degree. C. at 5.degree. C. min.sup.-1 Heat to 2000.degree. C. at 2.5.degree. C. min.sup.-1 antonio78@example.org. C. 60 minutes antonio78@example.org. C. min.sup.-1 ______________________________________
Part TBL_5: TABLE 5 ______________________________________ Example Details Example 1 2 ______________________________________ Atmosphere Ar/CO Ar/CO Mass (g) 101 118 GBD (g .multidot. cc.sup.-1) 1.70 1.71 FBD (g .multidot. cc.sup.-1) 3.24 3.25 Wt Change % -5.0 -4.3 Comments 1 1 ______________________________________ Comments 1 = Good surface finish GBD = Green bulk density FBD = Fired bulk density
Part PAC_2: EXAMPLES 3 to 7
Part PAR_3: To confirm that silica and excess free carbon have an adverse effect on the densification of silicon carbide, free carbon was added to react with the silica. The same source of silicon carbide and yttria was used as for Example 1 to produce the samples. The alumina used obtained from Alcoa was known as grade A1000 and the carbon source was a phenolic resin. The powders were batched and ball milled using silicon carbide milling media (see table 6).
Part TBL_6: TABLE 6 ______________________________________ Milling Conditions ______________________________________ Time 16 hours Powder 400 g Balls 2000 g SiC Fluid 800 ml iso propanol ______________________________________
Part PAR_4: The samples were prepared as outlined for Example 1 with the exception that for examples 4 to 7 a phenolic resin was used as the source of free carbon. The resulting powder slurries were spray dried and green discs (65 mm diameter) were uniaxially pressed at 35 MPa and wet bag CIP at 150 MPa. The samples were heated in nitrogen to pyrolyse the resin and form free carbon (see table 7). The weight loss after pyrolysis for the phenolic resin was found to be 45.6%. The firing cycle used is given in table 8.
Part TBL_7: TABLE 7 ______________________________________ Curing and Pyrolysis and Conditions in Nitrogen ______________________________________ Heat to 100.degree. C. at 30.degree. C. h.sup.-1 Dwell for 1 h Heat to 150.degree. C. at 30.degree. C. h.sup.-1 Dwell for 1 h Heat to 600.degree. antonio78@example.org. C. h.sup.-1 Dwell for 1 h antonio78@example.org. C. h.sup.-1 ______________________________________
Part TBL_8: TABLE 8 ______________________________________ Summary of Firing Cycles ______________________________________ Heat to 600.degree. C. at 5.degree. C. min.sup.-1 in Ar gas Heat 600.degree. C. to 1400.degree. C. at 10.degree. C. min.sup.-1 Heat 1400.degree. C. to 1550.degree. C. at 5.degree. C. min.sup.-1 antonio78@example.org. C. for 30 minutes Heat 1550.degree. C. to 1650.degree. C. at 5.degree. C. min.sup.-1 antonio78@example.org. C. for 30 minutes Evacuate to 1 mm Hg and fill CO gas Heat 1650.degree. C. to 1900.degree. C. at 5.degree. C. min.sup.-1 antonio78@example.org. C. for 60 minutes Heat 1900.degree. C. to 2030.degree. C. at 2.5.degree. C. min.sup.-1 antonio78@example.org. C. for 60 minutes Cool to room antonio78@example.org. C. min.sup.-1 until the natural cooling rates takes over. ______________________________________
Part PAR_5: The results of the firing are listed in table 9.
Part TBL_9: TABLE 9 ______________________________________ Effect of free carbon content on densification using a two stage firing Example 3 4 5 6 7 ______________________________________ SiC 88.0 88.0 88.0 88.0 88.0 Al.sub.2 O.sub.3 10.0 10.0 10.0 10.0 10.0 Y.sub.2 O.sub.3 2.0 2.0 2.0 2.0 2.0 Mass (g) 249 251 251 254 261 GBD (g .multidot. cc.sup.-1) 1.68 1.72 1.73 1.71 1.73 Carbon content (calc) 0 0.5 1.0 2.0 3.0 FBD (g .multidot. cc.sup.-1) 3.20 3.25 3.25 3.18 3.09 Wt Change % -582.797.7268 ______________________________________
Part PAR_6: From the results in table 9, it can be seen that the addition of small amounts of carbon enhances the level of fired bulk density obtained (Examples 4 and 5). For higher levels of carbon addition, there is a decrease in fired bulk density (Examples 6 and 7). It is believed that this behaviour is the result of the unfavourable reaction of the carbon at high temperatures. Thus there exists an optimum range for the carbon content. It is important to note that for thinner samples the addition of carbon was not found to be necessary in order to produce high bulk density fired products. It is believed that for these samples the silica layer is able to decompose and can be removed from the powder compact. In thick samples, this becomes more difficult as it requires considerable time to decompose and remove the unwanted oxide in the powder compact. Thus the removal of the said unwanted phase is required to produce dense uniform thicker samples.
Part PAC_3: COMPARATIVE EXAMPLE 8
Part PAR_7: The effect of firing cycle on cracking and the formation of lower fired bulk density cores was investigated. The composition was the same as example 1 and the sample preparation was the same as example 2. The firing cycle used is shown in table 10.
Part TBL_10: TABLE 10 ______________________________________ Firing cycle ______________________________________ Heat to 600.degree. C. at 5.degree. C. min.sup.-1 in Ar gas Heat 600.degree. C. to 1400.degree. C. at 10.degree. C. min.sup.-1 Heat 1400.degree. C. to 1650.degree. C. at 5.degree. C. min.sup.-1 antonio78@example.org. C. for 30 minutes Evacuate to 1 mm Hg and fill CO gas Heat 1650.degree. C. to 1900.degree. C. at 5.degree. C. min.sup.-1 Heat 1900.degree. C. to 2030.degree. C. at 2.5.degree. C. min.sup.-1 Hold at temp for 60 minutes Cool to room antonio78@example.org. C. min.sup.-1 until the natural cooling rate takes over. ______________________________________
Part PAR_8: The results of the firing are listed in table 11.
Part TBL_11: TABLE 11 ______________________________________ Effect of the firing cycle on cracking Example 8 ______________________________________ SiC 88.0 Al.sub.2 O.sub.3 10.0 Y.sub.2 O.sub.3 2.0 Mass (g) 249 GBD (g .multidot. cc.sup.-1) 1.68 FBD (g .multidot. cc.sup.-1) 3.22 Wt Change % -5.0 Comments Shattered ______________________________________
Part PAR_9: The results show that larger samples (example 8) are subject to high stresses during the firing cycle. These stresses can become large enough to overcome the cohesive strength of the samples leading to its failure. Comparison of examples 8 and 3 show that for larger samples the elimination of intermediate holds for these specimens leads to cracking. Comparison with Example 1, shows that lower sample masses do not require holds, however, the use of larger sample masses has an adverse effect on the densification of samples. Similar effects could also be observed with increased furnace charges even if the component mass were small. It is believed that the presence of the silica impurities, in the form of silica, a silicate or related phase has an adverse effect on the densification behaviour of bodies. For smaller sample masses, it is possible to reduce the effects of this phase as a result of volatilisation without resorting to techniques such as intermediate temperature dwells. However, as the sample size is increased the density is impaired even with a controlled heating.
Part PAC_4: EXAMPLES 9 TO 12
Part PAR_10: The effect of composition was also investigated. The same sources of silicon carbide, alumina, yttria and phenolic resin as used for example 3 were used to produce the examples 9 to 11. For example 12, the same sources of silicon carbide, alumina and phenolic resin were used together with ceria from Cerac. The sample preparation was the same as example 3. All the samples were fired together in the one work box. The firing cycle used is given in table 12.
Part TBL_12: TABLE 12 ______________________________________ Firing cycles ______________________________________ Heat to 600.degree. antonio78@example.org. C. min.sup.-1 in Ar gas Heat 600.degree. C. to 1400.degree. C. at 10.degree. C. min.sup.-1 Heat 1400.degree. C. to 1550.degree. C. at 5.degree. C. min.sup.-1 antonio78@example.org. C. for 30 minutes Heat 1550.degree. C. to 1650.degree. C. at 5.degree. C. min.sup.-1 antonio78@example.org. C. for 30 minutes Evacuate to 1 mm Hg and fill CO gas Heat 1650.degree. C. to 1900.degree. C. at 5.degree. C. min.sup.-1 antonio78@example.org. C. for 90 minutes Heat 1900.degree. C. to 2030.degree. C. at 2.5.degree. C. min.sup.-1 antonio78@example.org. C. for 60 minutes Cool to room antonio78@example.org. C. min.sup.-1 until the natural cooling rate takes over. ______________________________________
Part PAR_11: The results of the firing cycle are listed in table 13.
Part TBL_13: TABLE 13 ______________________________________ Effect of composition densification using a two stage firing Example 9 10 11 12 ______________________________________ SiC 93.0 79.0 63.0 88.0 Al.sub.2 O.sub.3 5.0 20.0 35.00 10.0 Y.sub.2 O.sub.3 2.0 1.0 2.0 0 CeO.sub.2 0 0 0 2.0 Mass (g) 250 248 248 250 GBD (g .multidot. cc.sup.-1) 1.67 1.74 1.82 1.72 Carbon content (calc) 0.6 0.9 0.7 1.0 FBD (G .multidot. CC.sup.-1) 3.19 3.31 3.26 3.19 WT Change % -2.2 -4.9 -5.6 -3.7 ______________________________________
Part PAR_12: From the results it can be seen that a wide range of alumina contents can be used to produce high density samples. At the lower and upper limits of alumina addition, the fired bulk density decreases.
Part PAR_13: It will be clearly understood that the invention in its general aspects is not limited to the specific details given hereinbefore.


{'bibliographic_information': "{'Patent Number': '058558411', 'Series Code': '8', 'Application Number': '7159161', 'Application Type': '1', 'Art unit': '191', 'Application Filing Date': '19960919', 'Title of Invention': 'Process for producing dense ceramic product', 'Issue Date': '19990105', 'Number of Claims': '29', 'Exemplary Claim Number(s)': '1', 'Primary Examiner': 'Fiorilla; Christopher A.'}", 'source_file': './grant/1999/pftaps19990105_wk01.txt', 'source_data': 'USPTO-Grant'}