United States Patent (19) 11 Patent Number: 4,578,233 Huseby et al. 45 Date of Patent: Mar. 25, 1986 (54 PRESSURELESS SINTERING PROCESS TO native Ceramic Substrate for High Power Applications PRODUCE HIGH THERMAL in Micro Circuits'-pp. 402–406 IEEE, May 1984. CONDUCTIVITY CERAMIC BODY OF K. Shinozaki, et al., "Sintering Behavior and Thermal ALUMNUMNTRIDE Characteristics of Pressureless Sintered AlN with Y203 Addition,' 22nd Symposium on Basic Science of Ce (75) Inventors: Irvin C. Huseby, Schenectady; Carl ramics, Yogyo-Kyokai, Jan. 1984, p. 43. F. Bobik, Burnt Hills, both of N.Y. Bulletin of American Ceramic Society, vol. 63, No. 8 73) Assignee: General Electric Company, (1984), p. 1009. Schenectady, N.Y. Heraeus PS-B-80, 4 pages. Komeya, et al. "Effects of Various Additives on Sinter 21 Appl. No.: 728,624 ing of Aluminum Nitride', Yogyo-Kyokai-shi, 89, 6, 22 Filed: Apr. 29, 1985 (pp. 58-64), 1981. (Abstract continued on next page.) Related U.S. Application Data Primary Examiner-Helen M. McCarthy 63 Continuation-in-part of Ser. No. 667,516, Nov. 1, 1984, Attorney, Agent, or Firm-Jane M. Binkowski; James C. abandoned. Davis, Jr.; James Magee, Jr. 51) Int. Cl." ...... C04B 35/58; F27D 7/06 57 ABSTRACT 52 U.S. C...... 264/65; 264/61; A process for producing an aluminum nitride ceramic 264/63, 264/66; 501/96; 501/98; 501/152 body having a composition defined and encompassed 58) Field of Search ...... 264/61, 63, 65, 66; by polygon FJDSR but not including line RF of FIG. 4, 501/96, 98, 152 a porosity of less than about 10% by volume, and a thermal conductivity greater than 1.00 W/cm-K at 25° 56) References Cited C. which comprises forming a mixture comprised of U.S. PATENT DOCUMENTS aluminum nitride powder containing oxygen, yttrium 3,833,389 9/1974 Komeya et al...... 501/96 oxide, and free carbon, shaping said mixture into a con 3,930,875 1/1976 Ochiai et al...... 501/98 pact, said mixture and said compact having a composi 4,097,293 6/1978 Komeya et al...... 501/98 tion wherein the equivalent % of yttrium and aluminum 4,203,733 5/1980 Tanaka et al...... 51/295 ranges from point D up to point F of FIG. 4, said com 4,435,513 3/1984 Komeya et al...... 501/96 pact having an equivalent % composition of Y, Al, O 4,478,785 10/1984 Huseby et al...... 264/65 4,519,966 5/1985 Aldinger et al...... 501/96 and N outside the composition defined and encom 4,533,645 8/1985 Huseby et al...... 501/96 passed by polygon FJDSR and FIG._4, heating said 4,540,673 9/1985 Takeda et al. . 501/96 compact up to a temperature at which its pores remain 4,547,471 10/1985 Huseby et al...... 264/65 open reacting said free carbon with oxygen contained in said aluminum nitride producing a deoxidized compact, FOREIGN PATENT DOCUMENTS said deoxidized compact having a composition wherein 3313836 10/1984 Fed. Rep. of Germany . the equivalent % of Al, Y, O and N is defined and 48-32110 4/1973 Japan ...... 501/92 encompassed by polygon FJDSR but not including line 49-19686 3/1974 Japan. RF of FIG. 4, and sintering said deoxidized compact at 54-12488 5/1979 Japan ...... 501/97 59-207882 11/1984 Japan ...... 501/96 a temperature of at least about 1840 C. producing said 2132911 7/1984 United Kingdom . ceramic body. OTHER PUBLICATIONS Werdecker, W. et al.-"Aluminum Nitride-An Alter 24 Claims, 4 Drawing Figures 4,578,233 Page 2

THER PUBLICATIONS Litvimenko, V. F. et al. "Thermophysical Properties of "Science of Ceramics", vol. 6, 1973, pp. XX/3-XX13. Aluminum Nitride-Yttria Materials', translated from Komeya et al., Trans. & J. Brit. Ceram. Soc., 70 (3), pp. Poroshkovaya Metallurgiya, No. 6(246), pp. 77-79, 107-113 (1971) "The Influence of Fibrous Aluminum Jun., 1983, pp. 490-492. Nitride on the Strength of Sintered AlN-Y2O3. Schwetz, K. A. et al. "Sintering of Aluminum Nitride Advanced Optical Ceramics, Phase III, Final Report, with Low Oxide Addition', Progress in Nitrogen Ce DIN:82SDR2006, Feb. 1982, pp. 4-68 to 4-80 and pp. ramics (1983) pp. 245-252. 4-127 to 4-137, esp. pp. 4-128 and 4-129. Iwase, N. et al. “Development of a High Thermal Con Slack, G. A. "Nonmetallic Crystals with High Thermal ductive AlN Ceramic Substrate Technology', the In Conductivity", J. Phys. Chem. Solids, 1973, vol. 34, pp. ternational Journal for Hybrid Microelectronics, vol. 7, 321-335. #4, Dec. 1984. U.S. Patent Mar. 25, 1986 Sheet 1 of 4 4,578,233

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(SN 4,578,233 1. 2 largely on the composition desired in the resulting sin PRESSURELESS SINTERNG PROCESS TO tered body. PRODUCE HIGH THERMAL CONDUCTIVITY According to the present invention, aluminum nitride CERAMC BODY OF A LUMINUM NITRIDE powder can be processed in air and still produce a ce ramic body having a thermal conductivity greater than This application is a continuation-in-part of copend 1.00 W/cm-K at 25 C., and preferably greater than ing application Ser. No. 667,516 filed on Nov. 1, 1984 in 1.25 W/cm-K at 25' C. the names of Irvin Charles Huseby and Carl Francis In one embodiment of the present invention, the alu Bobik, now abandoned. minum nitride in a compact comprised of particulate The present invention relates to the production of a 10 aluminum nitride of known oxygen content, free carbon liquid phase sintered polycrystalline aluminum nitride and yttrium oxide, is deoxidized by carbon to produce a body having a thermal conductivity higher than 1.00 desired equivalent composition of A1, N, Y and O, and W/cm-K at 25 C., and preferably higher than 1.25 the deoxidized compact is sintered by means of a liquid W/cm-K at 25°C. In one aspect of the present process, phase containing mostly Y and O and a smaller amount aluminum nitride is deoxidized by carbon to a certain 15 of A1 and N. extent, and then it is further deoxidized and/or sintered Those skilled in the art will gain a further and better by utilizing yttrium oxide to produce the present ce understanding of the present invention from the de ramic. tailed description set forth below, considered in con A suitably pure aluminum nitride single crystal, con junction with the figures accompanying and forming a taining 300 ppm dissolved oxygen, has been measured 20 part of the specification in which: to have a room temperature thermal conductivity of 2.8 FIG. 1 is a composition diagram (also shown as FIG. 1 in copending Ser. No. 553,213, filed on Nov. 18, 1983) W/cm-K, which is almost as high as that of BeO single showing the subsolidus phase equilibria in the recipro crystal, which is 3.7W/cm-K, and much higher than cal ternary system comprised of A1N, YN, Y2O3 and that of al-Al2O3 single crystal, which is 0.44 W/cm-K. 25 Al2O3. FIG. 1 is plotted in equivalent % and along each The thermal conductivity of an aluminum nitride single axis of ordinates the equivalent % of oxygen is shown crystal is a strong function of dissolved oxygen and (the equivalent % of nitrogen is 100% minus the equiva decreases with an increase in dissolved oxygen content. lent % of oxygen). Along the axis of abscissas, the For example, the thermal conductivity of aluminum equivalent % of yttrium is shown (the equivalent % of nitride single crystal having 0.8 wt % dissolved oxygen, 30 aluminum is 100% minus the equivalent % of yttrium). is about 0.8 W/cm.K. In FIG. 1, line ABCDEF but not lines CD and EF Aluminum nitride powder has an affinity for oxygen, encompasses and defines the composition of the sintered especially when its surface is not covered by an oxide. body of Ser. No. 553,213. FIG. 1 also shows an example The introduction of oxygen into the aluminum nitride of an ordinates-joining straight line ZZ joining the lattice in aluminum nitride powder results in the forma 35 oxygen contents of an YN additive and an aluminum tion of A1 vacancies via the equation: nitride powder. From the given equivalent % of yt trium and A1 at any point on an ordinates-joining line 3N-3 - 30-2 -- V (1) passing through the polygon ABCDEF, the required (N-3) (N-3) (Alt 3) amounts of yttrium additive and A1N for producing the 40 composition of that point on the ordinates-joining line Thus, the insertion of 3 oxygen atoms on 3 nitrogen sites can be calculated; will form one vacancy on an aluminum site. The pres FIG. 2 is an enlarged view of the section of FIG. 1 ence of oxygen atoms on nitrogen sites will probably showing the composition of the polycrystalline body of have a negligible influence on the thermal conductivity Ser. No. 553,213; of A1N. However, due to the large difference in mass 45 FIG. 3 is a composition diagram showing the sub between an aluminum atom and a vacancy, the presence solidus phase equilibria in the reciprocal ternary system of vacancies on aluminum sites has a strong influence on comprised of A1N, YN, Y2O3 and Al2O3 FIG. 3 is the thermal conductivity of A1N and, for all practical plotted in equivalent % and along each axis of ordinates purposes, is probably responsible for all of the decrease the equivalent % of oxygen is shown (the equivalent % in the thermal conductivity of A1N. 50 of nitrogen is 100% minus the equivalent % of oxygen). There are usually three different sources of oxygen in Along the axis of abscissas, the equivalent % of yttrium nominally pure A1N powder. Source #1 is discrete is shown (the equivalent % of aluminum is 100% minus particles of Al2O3. Source #2 is an oxide coating, per the equivalent % of yttrium). In FIG. 3, line, i.e. poly haps as Al2O3, coating the A1N powder particles. gon, FJDSR but not including line RF encompasses and 55 defines the composition of the sintered body produced Source #3 is oxygen in solution in the A1N lattice. The by the present process; and amount of oxygen present in the A1N lattice in A1N FIG. 4 is an enlarged view of the section of FIG. 3 powder will depend on the method of preparing the showing polygon FJDSR. A1N powder. Additional oxygen can be introduced FIGS. 1 and 3 show the same composition diagram into the A1N lattice by heating the A1N powder at 60 showing the subsolidus phase equilibria in the recipro elevated temperatures. Measurements indicate that at cal ternary system comprised of A1N, YN, Y2O3 and - 1900 C. the A1N lattice can dissolve - 1.2 wt % Al2O3 and differ only in that FIG. 1 shows the polygon oxygen. In the present invention, by oxygen content of ABCDEF of Ser. No. 553,213 and the line ZZ", A1N powder, it is meant to include oxygen present as whereas FIG. 3 shows the polygon FJDSR. The com sources #1, #2 and #3. Also, in the present invention, 65 position defined by polygon ABCDEF includes the the oxygen present with A1N powder as sources #1, #2 composition defined by polygon FJDSR. and #3 can be removed by utilizing free carbon, and the FIGS. 1 and 2 were developed algebraically on the extent of the removal of oxygen by carbon depends basis of data produced by forming a particulate mixture 4,578,233 3 4. of YN of predetermined oxygen content and A1N pow EF in FIG. 1, shaping said mixture into a compact, and der of predetermined oxygen content, and in a few sintering said compact at a temperature ranging from instances a mixture of A1N, YN and Y2O3 powders, about 1850° C. to about 2170° C. in an atmosphere se under nitrogen gas, shaping the mixture into a compact lected from the group consisting of nitrogen, argon, under nitrogen gas and sintering the compact for time hydrogen and mixtures thereof to produce said poly periods ranging from 1 to 1.5 hours at sintering temper crystalline body. atures ranging from about 1860 C. to about 2050° C. in Copending Ser. No. 553,213 also discloses a polycrys nitrogen gas at ambient pressure. More specifically, the talline body having a composition comprised of from entire procedure ranging from mixing of the powders to greater than about 1.6 equivalent % yttrium to about sintering the compact formed therefrom was carried out 10 19.75 equivalent % yttrium, from about 80.25 equiva in a nonoxidizing atmosphere of nitrogen. lent % aluminum up to about 98.4 equivalent % alumi Polygon FJDSR of FIGS. 3 and 4 also was devel num, from greater than about 4.0 equivalent % oxygen oped algebraically on the basis of data produced by the to about 15.25 equivalent % oxygen and from about examples set forth herein as well as other experiments 84.75 equivalent % nitrogen up to about 96 equivalent which included runs carried out in a manner similar to 15 % nitrogen. that of the present examples. Copending Ser. No. 553,213 also discloses a polycrys The best method to plot phase equilibria that involve talline body having a phase composition comprised of oxynitrides and two different metal atoms, where the A1N and a second phase containing Y and O wherein metal atoms do not change valence, is to plot the com the total amount of said second phase ranges from positions as a reciprocal ternary system as is done in 20 greater than about 4.2% by volume to about 27.3% by FIGS. 1 and 3. In the particular system of FIGS. 1 and volume of the total volume of said body, said body 3 there are two types of non-metal atoms (oxygen and having a porosity of less than about 10% by volume of nitrogen) and two types of metal atoms (yttrium and said body and a thermal conductivity greater than 1.0 aluminum). The A1, Y, oxygen and nitrogen are as W/cm-K at 22 C. sumed to have a valence of +3, +3, -2, and -3, re 25 Briefly stated, the present process for producing a spectively. All of the A1, Y, oxygen and nitrogen are sintered polycrystalline aluminum nitride ceramic body assumed to be present as oxides, nitrides or oxynitrides, having a composition defined and encompassed by line, and to act as if they have the aforementioned valences. i.e. polygon, FJDSR but not including line RF of FIG. The phase diagrams of FIGS. 1 to 4 are plotted in 3 or 4, a porosity of less than about 10% by volume, and equivalent percent. The number of equivalents of each 30 preferably less than about 4% by volume of said body of these elements is equal to the number of moles of the and a thermal conductivity greater than 1.00 W/cm-K particular element multiplied by its valence. Along the at 25 C., and preferably greater than 1.25W/cm K at ordinate is plotted the number of oxygen equivalents 25 C. comprises the steps: multiplied by 100% and divided by the sum of the oxy (a) forming a mixture comprised of aluminum nitride gen equivalents and the nitrogen equivalents. Along the 35 powder containing oxygen, yttrium oxide or a precur abscissa is plotted the number of yttrium equivalents sor therefor, and a carbonaceous additive selected from multiplied by 100% and divided by the sum of the yt the group consisting of free carbon, a carbonaceous trium equivalents and the aluminum equivalents. All organic material and mixtures thereof, said carbona "compositions of FIGS. 1 to 4 are plotted in this manner. ceous organic material thermally decomposing at a Compositions on the phase diagrams of FIGS. 1 to 4 temperature ranging from about 50° C. to about 1000 can also be used to determine the weight percent and C. to free carbon and gaseous product of decomposition the volume percent of the various phases. For example, which vaporizes away, shaping said mixture into a com a particular point in the polygon FJDSR in FIG. 3 or 4 pact, said mixture and said compact having a composi can be used to determine the phase composition of the tion wherein the equivalent % of yttrium and aluminum polycrystalline body at that point. 45 ranges from point D up to point F of FIG. 3 or 4, i.e. FIGS. 1 to 4 show the composition and the phase from greater than about 1.6 equivalent % to about 5.5 equilibria of the polycrystalline body in the solid state. equivalent % yttrium and from about 94.5 equivalent % In copending U.S. patent application Ser. No. 553,213 to less than about 98.4 equivalent % aluminum, said entitled "High Thermal Conductivity Aluminum Ni compact having an equivalent % composition of Y, A1, tride Ceramic Body' filed on Nov. 18, 1983, in the 50 O and N outside the composition defined and encom names of Irvin Charles Huseby and Carl Francis Bobik passed by polygon FJDSR of FIGS. 3 or 4, and assigned to the assignee hereof and incorporated (b) heating said compact in a nonoxidizing atmo herein by reference, there is disclosed the process for sphere at a temperature up to about 1200° C. thereby producing a polycrystalline aluminum nitride ceramic providing yttrium oxide and free carbon, body having a composition defined and encompassed 55 (c) heating said compact in a nitrogen-containing by line ABCDEF but not including lines CD and EF of nonoxidizing atmosphere at a temperature ranging from FIG. 1 therein (also shown as prior art FIG. 1 herein), about 1350° C. to a temperature sufficient to deoxidize a porosity of less than about 10% by volume of said the compact but below its pore closing temperature body and a thermal conductivity greater than 1.0 reacting said free carbon with oxygen contained in said W/cm-K at 22°C. which comprises forming a mixture 60 aluminum nitride producing a deoxidized compact, said comprised of aluminum nitride powder and an yttrium deoxidized compact having a composition wherein the additive selected from the group consisting of yttrium, equivalent % of A1, Y, O and N is defined and encom yttrium hydride, yttrium nitride and mixtures thereof, passed by polygon FJDSR but not including line RF of said aluminum nitride and yttrium additive having a FIG. 3 or 4, said free carbon being in an amount which predetermined oxygen content, said mixture having a 65 produces said deoxidized compact, and composition wherein the equivalent % of yttrium, alu (d) sintering said deoxidized compact in a nitrogen minum, nitrogen and oxygen is defined and encom containing nonoxidizing atmosphere at a temperature of passed by line ABCDEF but not including lines CD and at least about 1840' C., and generally ranging from 4,578,233 5 6 about 1840 C. to about 2050 C., and preferably from in said compact before said deoxidation by said carbon about 1880 C. to about 1950° C., and more preferably having an oxygen content ranging from greater than from about 1890 C. to about 1950° C., producing said about 1.95% by weight to less than about 5.1% by polycrystalline body, said sintering temperature being a weight of said aluminum nitride, said free carbon being sintering temperature for said composition of said deox in an amount which produces said deoxidized compact, idized compact. and In the present process, the composition of the deoxi (d) sintering said deoxidized compact at ambient pres dized compact in equivalent % is the same as or does sure in a nitrogen-containing nonoxidizing atmosphere not differ significantly from that of the resulting sin containing at least about 25% by volume nitrogen at a tered body in equivalent %. 10 temperature ranging from about 1880° C. to about 2050 In the present invention, oxygen content is determin C., preferably from about 1890 C. to about 1950° C., able by neutron activation analysis. producing said polycrystalline body. By weight% or % by weight of a component herein, Briefly stated, in another embodiment, the present it is meant that the total weight % of all the components process for producing a sintered polycrystalline alumi is 100%. 15 num nitride ceramic body having a composition defined By ambient pressure herein, it is meant atmospheric and encompassed by line, i.e. polygon, FJDSR but not or about atmospheric pressure. including line RF of FIG.3 or 4, a porosity of less than By specific surface area or surface area of a powder about 10% by volume, and preferably less than about herein, it is meant the specific surface area according to 4% by volume of said body and a thermal conductivity BET surface area measurement. 20 greater than 1.00 W/cm-K at 25' C., and preferably Briefly stated, in one embodiment, the present pro greater than 1.25W/cm-K at 25 C. comprises the steps: cess for producing a sintered polycrystalline aluminum (a) processing an aluminum nitride powder into a nitride ceramic body having a composition defined and compact for deoxidation by free carbon by providing an encompassed by line, i.e. polygon, FJMW but not in oxygen-containing aluminum nitride powder having an cluding line WF of FIG. 3 or 4, a porosity of less than 25 oxygen content up to about 5.0% by weight of said about 10% by volume, and preferably less than about aluminum nitride powder, forming a mixture comprised 2% by volume of said body and a thermal conductivity of said aluminum nitride powder, yttrium oxide or a greater than 1.00 W/cm-L at 25 C., and preferably precursor therefor, and a carbonaceous additive se greater than 1.35 W/cm-K at 25 C. comprises the steps: lected from the group consisting of free carbon, a carbo (a) forming a mixture comprised of aluminum nitride 30 naceous organic material and mixtures thereof, said powder containing oxygen, yttrium oxide or a precur carbonaceous organic material thermally decomposing sor therefor, and a carbonaceous additive selected from at a temperature ranging from about 50° C. to about the group consisting of free carbon, a carbonaceous 1000 C. to free carbon and gaseous product of decom organic material and mixtures thereof, said carbona position which vaporizes away, shaping said mixture ceous organic material thermally decomposing at a 35 into a compact, said mixture and said compact having a temperature ranging from about 50° C. to about 1000 composition wherein the equivalent % of yttrium and C. to free carbon and gaseous product of decomposition aluminum ranges from point D up to point F of FIG. 3 which vaporizes away, said free carbon having a spe or 4, i.e. from greater than about 1.6 equivalent % to cific surface area greater than about 100 m2/g, the alu about 5.5 equivalent % yttrium and from about 94.5 minum nitride powder in said mixture having a specific equivalent % to less than about 98.4 equivalent % alu surface area ranging from about 3.5 m2/g to about 6 minum, said compact having an equivalent % composi m2/g, shaping said mixture into a compact, said mixture tion of Y, A1, O and N outside the composition defined and said compact having a composition wherein the and encompassed by polygon FJDSR of FIG. 3 or 4, equivalent % of yttrium and aluminum ranges from during said processing said aluminum nitride picking up point M up to point F of FIG. 3 or 4, i.e. from greater 45 oxygen, the oxygen content of said aluminum nitride in than about 1.6 equivalent % to about 4.0 equivalent % said compact before said deoxidation by carbon ranging yttrium and from about 96.0 equivalent % to less than from greater than about 1.50% by weight, and prefera about 98.4 equivalent % aluminum, said compact hav bly greater than about 1.95% by weight up to about ing an equivalent % composition of Y, A1, O and N 5.1% by weight of said aluminum nitride, outside the composition defined and encompassed by 50 (b) heating said compact in a nonoxidizing atmo polygon FJDSR of FIG.3 or 4, the aluminum nitride in sphere at a temperature up to about 1200° C. thereby said compact containing oxygen in an amount ranging providing yttrium oxide and free carbon, from greater than about 1.95% by weight to less than (c) heating said compact in a nitrogen-containing about 5.1% by weight of said aluminum nitride, nonoxidizing atmosphere at a temperature ranging from (b) heating said compact in a nonoxidizing atmo 55 about 1350° C. to a temperature sufficient to deoxidize sphere at a temperature up to about 1200° C. thereby the compact but below its pore closing temperature providing yttrium oxide and free carbon, reacting said free carbon with oxygen contained in said (c) heating said compact at ambient pressure in a aluminum nitride producing a deoxidized compact, said nitrogen-containing nonoxidizing atmosphere contain deoxidized compact having a composition wherein the ing at least about 25% by volume nitrogen at a tempera 60 equivalent % of A1, Y, O and N is defined and encom ture ranging from about 1350° C. to a temperature suffi passed by polygon FJDSR but not including line RF of cient to deoxidize the compact but below its pore clos FIG.3 or 4, said free carbon being in an amount which ing temperature reacting said free carbon with oxygen produces said deoxidized compact, and contained in said aluminum nitride producing a deoxi (d) sintering said deoxidized compact in a nitrogen dized compact, said deoxidized compact having a com 65 containing nonoxidizing atmosphere at a temperature of position wherein the equivalent % of A1, Y, O and N is at least about 1840 C., and generally ranging from defined and encompassed by polygon FJMW but not about 1840 C. to about 2050 C., and preferably from including line WF of FIG. 3 or 4, the aluminum nitride about 1880 C. to about 1950° C., and more preferably 4,578,233 7 8 from about 1890 C. to about 1950 C., producing said In another embodiment of the present process, said polycrystalline body. mixture and said compact have a composition wherein Briefly stated, in another embodiment, the present the equivalent % of yttrium and aluminum ranges be process for producing a sintered polycrystalline alumi tween points D and F but does not include points D and num nitride ceramic body having a composition defined F of FIG. 4, said yttrium in said compact ranging from and encompassed by polygon FJMW but not including greater than about 1.6 equivalent % to less than about line WF of FIG. 3 or 4, a porosity of less than about 5.5 equivalent %, said aluminum in said compact rang 10% by volume, and preferably less than about 2% by ing from greater than about 94.5 equivalent % to less volume, of said body and a thermal conductivity greater than about 98.4 equivalent %, and said sintered body than 1.00 W/cm-K at 25° C., and preferably greater 10 and said deoxidized compact are comprised of a compo than 1.35 W/cm-K at 25° C. comprises the steps: sition wherein the equivalent percent of A1, Y, O and N (a) processing an aluminum nitride powder into a is defined and encompassed by polygon FJDSR but compact for deoxidation by free carbon by providing an does not include lines DJ and RF of FIG. 4. aluminum nitride powder having an oxygen content In yet another embodiment of the present process, ranging from greater than about 1.0% by weight to less 15 said mixture and said compact have a composition than about 4.5% by weight of said aluminum nitride wherein the equivalent % of yttrium and aluminum powder, forming a mixture comprised of said aluminum ranges from point D to point J of FIG.4, said yttrium in nitride powder, yttrium oxide or precursor therefor, said compact ranges from about 5.5 equivalent % to and a carbonaceous additive selected from the group about 2.5 equivalent %, said aluminum in said compact consisting of free carbon, a carbonaceous organic mate 20 ranges from about 94.5 equivalent % to about 97.5 rial and mixtures thereof, said carbonaceous organic equivalent %, and said sintered body and said deoxi material thermally decomposing at a temperature rang dized compact are comprised of a composition wherein ing from about 50 C. to about 1000 C. to free carbon the equivalent percent of A1, Y, O and N is defined by and gaseous product of decomposition which vaporizes line DJ of FIG. 4. away, said free carbon having a specific surface area 25 The calculated compositions of particular points in greater than about 100 m2/g, the aluminum nitride pow FIGS. 3 or 4 in the polygon FJDSR are shown in Table der in said mixture having a specific surface area rang I as follows: ing from about 3.5 m2/g to about 6 m/g, shaping said TABLE I mixture into a compact, said mixture and said compact Composition ... having a composition wherein the equivalent % of yt 30 (Equivalent trium and aluminum ranges from point Mup to point F %) Vol % and (Wt %) of Phases ... of FIG. 3 or 4, i.e. from greater than about 1.6 equiva Point Y Oxygen AlN Y4Al2O9 YAlO3 ... lent % to about 4.0 equivalent % yttrium and from F 1.6 4.0 95.8(93.8) m 4.2(6.2) about 96.0 equivalent % to less than about 98.4 equiva J 2.5 4.1 94.0(91.9) 6.0(8.1) --- lent % aluminum, said compact having an equivalent % 35 D 5.5 8.5 87.3(83.2) 12.7(16.8) - R 2.5 5.8 93.4(90.4) r- 6.6(9.6) composition of Y, A1, O and N outside the composition S 5.3 8.5 87.7(83.6) 11.4(15.1) -0.9(1.3) defined and encompassed by polygon FJDSR of FIG. 3 M 4.0 6.3 90.6(87.4) 9.4(12.6) - ... or 4, during said processing said aluminum nitride pick W 2.1 5.0 944(91.9) m 5.6(8.1) ... ing up oxygen, the oxygen content of said aluminum "Wt % is given in parentheses, nitride in said compact before said deoxidation by car 40 Vol % is given without parentheses bon ranging from greater than about 1.95% by weight up to about 5.1% by weight of said aluminum nitride The polycrystalline aluminum nitride body produced and being greater than said oxygen content of said start by one embodiment of the present process has a compo ing aluminum nitride powder by an amount ranging sition defined and encompassed by polygon, i.e. line, from greater than about 0.03% by weight up to about 45 FJDSR but not including line RF of FIGS. 3 or 4. The 3% by weight of said aluminum nitride, sintered polycrystalline body of polygon FJDSR but (b) heating said compact in a nonoxidizing atmo not including line RF of FIGS. 3 or 4 produced by the sphere at a temperature up to about 1200° C. thereby present process has a composition comprised of from providing yttrium oxide and free carbon, greater than about 1.6 equivalent % yttrium to about 5.5 (c) heating said compact at ambient pressure in a 50 equivalent % yttrium, from about 94.5 equivalent % nitrogen-containing nonoxidizing atmosphere contain aluminum up to about 98.4 equivalent % aluminum, ing at least about 25% by volume nitrogen at a tempera from greater than about 4.0 equivalent % oxygen to ture ranging from about 1350° C. to a temperature suffi about 8.5 equivalent % oxygen and from about 91.5 cient to deoxidize the compact but below its pore clos equivalent % nitrogen up to about 96.0 equivalent % ing temperature reacting said free carbon with oxygen 55 nitrogen. contained in said aluminum nitride producing a deoxi Also, the polycrystalline body defined and encom dized compact, said deoxidized compact having compo passed by polygon FJDSR but not including line RF of sition wherein the equivalent % of A1, Y, O and N is FIG.3 or 4 is comprised of an A1N phase and a second defined and encompassed by polygon FJMW but not phase which ranges in amount from greater than about including line WF of FIG.3 or 4, said free carbon being 60 4.2% by volume to about 12.7% by volume of the total in an amount which produces said deoxidized compact, volume of the sintered body, and such second phase can and be comprised of Y4Al2O9 or a mixture of Y4Al2O9 and (d) sintering said deoxidized compact at ambient pres YA103. Specifically, when the second phase is com sure in a nitrogen-containing nonoxidizing atmosphere prised of Y4Al2O9, it ranges in amount from about 6.0% containing at least about 25% by volume nitrogen at a 65 by volume to about 12.7% by volume of the sintered temperature ranging from about 1880 C. to about 2050” body. However, when the second phase is a mixture of C., preferably from about 1890° C. to about 1950° C., second phases comprised of YA103 and Y4Al2O9, both producing said polycrystalline body. of these second phases are always present in at least a 4,578,233 10 trace amount, i.e. at least an amount detectable by X-ray and YA103 at least in a trace amount, i.e. at least in an diffraction analysis, and in such mixture, the YA103 amount detectable by X-ray diffraction analysis. Specif. phase can range to less than about 6.6% by volume of ically, in this embodiment, the amount of YA103 phase the sintered body, and the Y4Al2O9 phase can range to can range to less than about 6.6% by volume of the less than about 12.7% by volume of the total volume of sintered body, and the amount of Y4Al2O9 phase can the sintered body. More specifically, when a mixture of range to less than about 12.7% by volume of the sin Y4Al2O9 and YA103 phases is present, the amount of tered body. YA103 phase decreases and the amount of Y4Al2O9 phase increases as the composition moves away from In another embodiment, the present process produces line RF toward line DJ in F.G. 4. 10 a sintered body defined by line DJ of FIG. 4 which has As can be seen from Table I, the polycrystalline body a phase composition comprised of A1N and Y4Al2Og at point D composition would have the largest amount wherein the Y4A12O9phase ranges from about 6.0% by of second phase present which at point D would be volume to about 12.7% by volume of the body. Line DJ Y4Al2O9. of FIG. 4 has a composition comprised of from about In another embodiment, the polycrystalline alumi 15 2.5 equivalent % to about 5.5 equivalent % yttrium, num nitride body produced by the present process has a from about 94.5 equivalent % to about 97.5 equivalent composition defined and encompassed by polygon, i.e. % aluminum, from about 4.1 equivalent % to about 8.5 line, FJMW but not including line WF of FIGS. 3 or 4. equivalent % oxygen and from about 95.9 equivalent % The sintered polycrystalline body of polygon FJMW to about 91.5 equivalent % nitrogen. but not including line WF of FIG. 3 or 4 produced by 20 In another embodiment, the present process produces the present process has a composition comprised of a sintered body defined by line MJ of FIG. 4 which has from greater than about 1.6 equivalent % yttrium to a phase composition comprised of A1N and Y wherein about 4.0 equivalent % yttrium, from about 96.0 equiva the Y4Al2O9 phase ranges from about 6.0% by volume lent % aluminum up to about 98.4 equivalent % alumi to about 9.4% by volume of the body. Line MJ of FIG. num, from greater than about 4.0 equivalent % oxygen 25 4 has a composition comprised of from about 2.5 equiv to about 6.3 equivalent % oxygen and from about 93.7 alent % to about 4.0 equivalent % yttrium, from about equivalent % nitrogen to less than about 96.0 equivalent 97.5 equivalent % to about 96.0 equivalent % alumi % nitrogen. num, from about 4.1 equivalent % to about 6.3 equiva Also, the polycrystalline body defined and encom lent % oxygen and from about 95.9 equivalent % to passed by polygon FJMW but not including line WF of 30 about 93.7 equivalent % nitrogen. FIG. 3 or 4 is comprised of an A1N phase and a second In the present process, the aluminum nitride powder phase which ranges in amount from greater than about can be of commercial or technical grade. Specifically, it 4.2% by volume to about 9.4% by volume of the total should not contain any impurities which would have a volume of the sintered body, and such second phase can significantly deleterious effect on the desired properties be comprised of Y4Al2O9 or a mixture of Y4Al2O9 and 35 of the resulting sintered product, and preferably it is YA103. Specifically, when the second phase is com greater than about 99% pure A1N excluding oxygen. prised of Y4Al2O9, it ranges in amount from about 6.0% The starting aluminum nitride powder used in the pres by volume to about 9.4% by volume of the sintered ent process contains oxygen generally ranging in body. However, when the second phase is a mixture of amount up to about 5.0% by weight and usually ranging second phases comprised of YA103 and Y4Al2O9, both 40 from greater than about 1.0% by weight to less than of these second phases are always present in at least a about 4.0% weight, i.e. up to about 4% by weight. trace amount, i.e. at least an amount detectable by X-ray Typically, commercially available aluminum nitride diffraction analysis, and in such mixture, the YA103 powder contains from about 1.5 weight % (2.6 equiva phase can range up to about 5.6% by volume of the lent %) to about 3 weight % (5.2 equivalent %) of sintered body, and the Y4Al2O9 phase can range up to 45 oxygen and such powders are most preferred on the about 9.4% by volume of the total volume of the sin basis of their substantially lower cost. tered body. More specifically, when a mixture of Y4Al The oxygen content of aluminum nitride is determin 2O9 and YA103 phases is present, the amount of able by neutron activation analysis. YA103 phase decreases and the amount of Y4Al2O9 Generally, the present starting aluminum nitride phase increases as the composition moves away from 50 powder has a specific surface area which can range line WF toward line MJ in FIG. 4. widely, and generally it ranges up to about 10 m2/g. In one embodiment, the present polycrystalline body Frequently, it has a specific surface area greater than has a composition defined and encompassed by polygon about 1.0 m2/g, and more frequently of at least about 3.0 FJDSR but not including lines RF or DJ of FIGS. 3 or m2/g, usually greater than about 3.2 m2/g, and prefera 4, i.e. it has a composition comprised of from greater 55 bly at least about 3.4 m/g. than about 1.6 equivalent % yttrium to less than about Generally, the present aluminum nitride powder in 5.5 equivalent % yttrium, from greater than about 94.5 the present mixture, i.e. after the components have been equivalent % aluminum to less than about 98.4 equiva mixed, usually by milling, has a specific surface area lent % aluminum, from greater than about 4.0 equiva which can range widely, and generally it ranges to lent % oxygen to less than about 8.5 equivalent % oxy 60 about 10 m2/g. Frequently, it ranges from greater than gen and from greater than about 91.5 equivalent % about 1.0 m2/g to about 10 m2/g, and more frequently nitrogen to less than about 96.0 equivalent % nitrogen. from about 3.3 m2/g to about 10 m2/g, and preferably In this embodiment, the phase composition of the sin from about 3.5 m2/g to about 6 m2/g, more preferably tered body is comprised of A1N and a mixture of sec from about 3.6 m2/g to about 6.0 m2/g and most prefer ond phases comprised of Y4Al2O9 and YA1O3. This 65 ably from about 3.6 m2/g to about 5.2 m2/g according second phase mixture ranges in amount from greater to BET surface area measurement. Generally, for a than about 4.2% by volume to less than about 12.7% by given composition of a deoxidized compact, the higher volume of the body and always contains both Y4Al2O9 the surface area of the aluminum nitride, the lower is the 4,578,233 11 12 sintering temperature required to produce a sintered dation by free carbon is at least partly carried out in air, body of a given porosity. and during such processing of the aluminum nitride Generally, the yttrium oxide (Y2O3) additive in the powder, it picks up oxygen from air usually in an present mixture has a specific surface area which can amount greater than about 0.03% by weight of the range widely. Generally, it is greater than about 0.4 5 aluminum nitride, and any such pick up of oxygen is m/g and generally it ranges from greater than about controllable and reproducible or does not differ signifi 0.4 m2/g to about 6.0 m2/g, usually from about 0.6 m2/g cantly if carried out under the same conditions. If de to about 5.0 m2/g, more usually from about 1.0 m2/g to sired, the processing of the aluminum nitride powder about 5.0 m2/g, and in one embodiment it is greater than into a compact for deoxidation by free carbon can be 2.0 m2/g. 10 carried out in air. In the practice of this invention, carbon for deoxida In the present processing of aluminum nitride, the tion of aluminum nitride powder is provided in the form oxygen it picks up can be in any form, i.e. it initially may of free carbon which can be added to the mixture as be oxygen, or initially it may be in some other form, elemental carbon, or in the form of a carbonaceous such as, for example, water. The total amount of oxygen additive, for example, an organic compound which can 15 picked up by aluminum nitride from air or other media thermally decompose to provide free carbon. generally is less than about 3.0% by weight, and gener The present carbonaceous additive is selected from ally ranges from greater than about 0.03% by weight to the group consisting of free carbon, a carbonaceous less than about 3.0% by weight, and usually it ranges organic material and mixtures thereof. The carbona from about 0.1% by weight to about 1% by weight, of ceous organic material pyrolyzes, i.e. thermally decom 20 the total weight of the aluminum nitride. Generally, the poses, completely at a temperature ranging from about aluminum nitride in the present mixture and compact 50° C. to about 1000 C. to free carbon and gaseous prior to deoxidation of the compact has an oxygen con product of decomposition which vaporizes away. In a tent of less than about 5.1% by weight, and generally it preferred embodiment, the carbonaceous additive is ranges from greater than about 1.50% by weight, usu free carbon, and preferably, it is graphite. 25 ally greater than about 1.95% by weight to less than High molecular weight aromatic compounds or ma about 5.1% by weight, and more usually it ranges from terials are the preferred carbonaceous organic materials about 2.0% by weight to about 4.5% by weight, of the for making the present free carbon addition since they total weight of aluminum nitride. ordinarily give on pyrolysis the required yield of partic The oxygen content of the starting aluminum nitride ulate free carbon of submicron size. Examples of such 30 powder and that of the aluminum nitride in the compact aromatic materials are a phenolformaldehyde conden prior to deoxidation is determinable by neutron activa sate resin known as Novolak which is soluble in acetone tion analysis. or higher alcohols, such as butyl alcohol, as well as In a compact, an aluminum nitride containing oxygen many of the related condensation polymers or resins in an amount of about 5.1% by weight or more gener such as those of resorcinol-formaldehyde, aniline-for 35 ally is not desirable. maldehyde, and cresolformaldehyde. Another satisfac In carrying out the present process, a uniform or at ... tory group of materials are derivatives of polynuclear least a significantly uniform mixture or dispersion of the aromatic hydrocarbons contained in coal tar, such as aluminum nitride powder, yttrium oxide or precursor dibenzanthracene and chrysene. A preferred group are therefor powder and carbonaceous additive, generally polymers of aromatic hydrocarbons such as polyphen 40 in the form of free carbon powder, is formed and such Tylene or polymethylphenylene which are soluble in mixture can be formed by a number of techniques. Pref. aromatic hydrocarbons. erably, the powders are ball milled in a liquid medium at The present free carbon has a specific surface area ambient pressure and temperature to produce a uniform which can range widely and need only be at least suffi or significantly uniform dispersion. The milling media, cient to carry out the present deoxidation. Generally, it 45 which usually are in the form of cylinders or balls, has a specific surface area greater than about 10 m2/g, should have no significant deleterious effect on the preferably greater than 20 m2/g, preferably greater than powders, and preferably, they are comprised of poly about 100 m2/g, and still more preferably greater than crystalline aluminum nitride or steel. Generally, the 150 m2/g, according to BET surface area measurement milling media has a diameter of at least about inch and to insure intimate contact with the A1N powder for 50 usually ranges from about : inch to about inch in carrying out its deoxidation. diameter. The liquid medium should have no signifi Most preferably, the present free carbon has as high a cantly deleterious effect on the powders and preferably surface area as possible. Also, the finer the particle size it is non-aqueous. Preferably, the liquid mixing or mill of the free carbon, i.e. the higher its surface area, the ing medium can be evaporated away completely at a smaller are the holes or pores it leaves behind in the 55 temperature ranging from above room or ambient tem deoxidized compact. Generally, the smaller the pores of perature to below 300° C. leaving the present mixture. a given deoxidized compact, the lower is the amount of Preferably, the liquid mixing medium is an organic liq liquid phase which need be generated at sintering tem uid such as heptane or hexane. Also, preferably, the perature to produce a sintered body having a porosity liquid milling medium contains a dispersant for the alu of less than about 1% by volume of the body. 60 minum nitride powder thereby producing a uniform or By processing of the aluminum nitride powder into a significantly uniform mixture in a significantly shorter compact for deoxidation by free carbon, it is meant period of milling time. Such dispersant should be used herein to include all mixing of the aluminum nitride in a dispersing amount and it should evaporate or de powder to produce the present mixture, all shaping of compose and evaporate away completely or leave no the resulting mixture to produce the compact, as well as 65 significant residue, i.e. no residue which has a signifi handling and storing of the compact before it is deoxi cant effect in the present process, at an elevated temper dized by carbon. In the present process, processing of ature below 1000 C. Generally, the amount of such the aluminum nitride powder into a compact for deoxi dispersant ranges from about 0.1% by weight to less 4,578,233 13 14 than about 3% by weight of the aluminum nitride pow determined by pyrolyzing the organic material alone der, and generally it is an organic liquid, preferably and determining weight loss. Preferably, thermal de oleic acid. composition of the organic material in the present com In using steel milling media, a residue of steel or iron pact is done in the sintering furnace as the temperature is left in the dried dispersion or mixture which can range is being raised to deoxidizing temperature, i.e. the tem from a detectable amount up to about 3.0% by weight perature at which the resulting free carbon reacts with of the mixture. This residue of steel or iron in the mix the oxygen content of the AlN. ture has no significant effect in the present process or on Alternately, in the present process, yttrium oxide can the thermal conductivity of the resulting sintered body. be provided by means of an yttrium oxide precursor. The liquid dispersion can be dried by a number of 10 The term yttrium oxide precursor means any organic or conventional techniques to remove or evaporate away inorganic compound which decomposes completely at the liquid and produce the present particulate mixture. a temperature below about 1200° C. to form yttrium If desired, drying can be carried out in air. Drying of a oxide and by-product gas which vaporizes away leav milled liquid dispersion in air causes the aluminum ni ing no contaminants in the sintered body which would tride to pick up oxygen and, when carried out under the 15 be detrimental to the thermal conductivity. Representa same conditions, such oxygen pick up is reproducible or tive of the precursors of yttrium oxide useful in the does not differ significantly. Also, if desired, the disper present process is yttrium acetate, yttrium carbonate, sion can be spray dried. yttrium oxalate, yttrium nitrate, yttrium sulfate and A solid carbonaceous organic material is preferably yttrium hydroxide. admixed in the form of a solution to coat the aluminum 20 If the compact contains a precursor for yttrium oxide, nitride particles. The solvent preferably is non-aqueous. it is heated to a temperature up to about 1200° C. to The wet mixture can then be treated to remove the thermally decompose the precursor thereby providing solvent producing the present mixture. The solvent can yttrium oxide. Such thermal decomposition is carried be removed by a number of techniques such as by evap out in a nonoxidizing atmosphere, preferably in a vac oration or by freeze drying, i.e. subliming off the sol 25 uum or at ambient pressure, and preferably the atmo vent in vacuum from the frozen dispersion. In this way, sphere is selected from the group consisting of nitrogen, a substantially uniform coating of the organic material hydrogen, a noble gas such as argon and mixtures on the aluminum nitride powder is obtained which on thereof. Preferably, it is nitrogen, or a mixture of at least pyrolysis produces a substantially uniform distribution about 25% by volume nitrogen and a gas selected from of free carbon. 30 the group consisting of hydrogen, a noble gas such as The present mixture is shaped into a compact in air, argon and mixtures thereof. In one embodiment, it is a or includes exposing the aluminum nitride in the mix mixture of nitrogen and from about 1% by volume to ture to air. Shaping of the present mixture into a com about 5% by volume hydrogen. pact can be carried out by a number of techniques such The present deoxidation of aluminum nitride with as extrusion, injection molding, die pressing, isostatic 35 carbon, i.e. carbon-deoxidation, comprises heating the pressing, slip casting, roll compaction or forming or compact comprised of aluminum nitride, free carbon tape casting to produce the compact of desired shape. and yttrium oxide at deoxidation temperature to react Any lubricants, binders or similar shaping aid materials the free carbon with at least a sufficient amount of the used to aid shaping of the mixture should have no signif oxygen contained in the aluminum nitride to produce a icant deteriorating effect on the compact or the present deoxidized compact having a composition defined and resulting sintered body. Such shaping-aid materials are encompassed by polygon FJDSR but not including line preferably of the type which evaporate away on heating RF of FIGS. 3 or 4. This deoxidation with carbon is at relatively low temperatures, preferably below 400' carried out at a temperature ranging from about 1350 C., leaving no significant residue. Preferably, after re C. to a temperature at which the pores of the compact moval of the shaping aid materials, the compact has a 45 remain open, i.e. a temperature which is sufficient to porosity of less than 60% and more preferably less than deoxidize the compact but below the pore closing tem 50% to promote densification during sintering. perature, generally up to about 1800° C., and prefera If the compact contains carbonaceous organic mate bly, it is carried out at from about 1600 C. to 1650 C. rial as a source of free carbon, it is heated at a tempera The carbon-deoxidation is carried out, preferably at ture ranging from about 50° C. to about 1000 C. to 50 ambient pressure, in a gaseous nitrogen-containing non pyrolyze, i.e. thermally decompose, the organic mate oxidizing atmosphere which contains sufficient nitrogen rial completely producing the present free carbon and to facilitate the deoxidation of the aluminum nitride. In gaseous product of decomposition which vaporizes accordance with the present invention, nitrogen is a away. Thermal decomposition of the carbonaceous required component for carrying out the deoxidation of organic material is carried out preferably in a vacuum 55 the compact. Preferably, the nitrogen-containing atmo or at ambient pressure, in a nonoxidizing atmosphere. sphere is nitrogen, or it is a mixture of at least about Preferably, the nonoxidizing atmosphere in which ther 25% by volume of nitrogen and a gas selected from the mal decomposition is carried out is selected from the group consisting of hydrogen, a noble gas such as ar group consisting of nitrogen, hydrogen, a noble gas gon, and mixtures thereof. Also, preferably, the nitro such as argon and mixtures thereof, and more prefera 60 gen-containing atmosphere is comprised of a mixture of bly it is nitrogen, or a mixture of at least about 25% by nitrogen and hydrogen, especially a mixture containing volume nitrogen and a gas selected from the group up to about 5% by volume hydrogen. consisting of hydrogen, a noble gas such as argon and The time required to carry out the present carbon mixtures thereof. In one embodiment, it is a mixture of deoxidation of the compact is determinable empirically nitrogen and from about 1% by volume to about 5% by 65 and depends largely on the thickness of the compact as volume hydrogen. well as the amount of free carbon it contains, i.e. the The actual amount of free carbon introduced by py carbon-deoxidation time increases with increasing rolysis of the carbonaceous organic material can be thickness of the compact and with increasing amounts 4,578,233 15 16 of free carbon contained in the compact. Carbon-deoxi that its composition is not defined or encompassed by dation can be carried out as the compact is being heated the polygon FJDSR not including line RF of FIG. 4. to sintering temperature provided that the heating rate The amount of free carbon used to carry out the allows the deoxidation to be completed while the pores present deoxidation should produce the present deoxi of the compact are open and such heating rate is deter 5 dized compact leaving no significant amount of carbon minable empirically. Also, to some extent, carbon deox in any form, i.e. no amount of carbon in any form which idation time depends on deoxidation temperature, parti would have a significantly deleterious effect on the cle size and uniformity of the particulate mixture of the sintered body. More specifically, no amount of carbon compact i.e. the higher the deoxidation temperature, the in any form should be left in the deoxidized compact 10 which would prevent production of the present sintered smaller the particle size and the more uniform the mix body, i.e. any carbon content in the sintered body ture, the shorter is deoxidation time. Also, to some should be low enough so that the sintered body has a extent, deoxidation time depends on its final position on thermal conductivity greater than 1.00 W/cm-K at 25 the phase diagram, i.e. as line DJ is approached, deoxi C. Generally, the present sintered body may contain dation time increases. Typically, the carbon-deoxida 15 carbon in some form in a trace amount, i.e. generally tion time ranges from about hour to about 1.5 hours. less than about 0.08% by weight, preferably in an Preferably, the compact is deoxidized in the sintering amount of less than about 0.065% by weight, more furnace by holding the compact at deoxidation tempera preferably less than about 0.04% by weight, and most ture for the required time and then raising the tempera preferably less than 0.03% by weight of the total weight ture to sintering temperature. The deoxidation of the 20 of the sintered body. compact must be completed before sintering closes off A significant amount of carbon in any form remaining pores in the compact preventing gaseous product from in the sintered body significantly reduces its thermal vaporizing away and thereby preventing production of conductivity. An amount of carbon in any form greater the present sintered body. than about 0.065% by weight of the sintered body is In the present deoxidation with carbon, the free car 25 likely to significantly decrease its thermal conductivity. bon reacts with the oxygen of the aluminum nitride The present deoxidized compact is densified, i.e. liq producing carbon monoxide gas which vaporizes away. uid-phase sintered, at a temperature which is a sintering It is believed that the following deoxidation reaction temperature for the composition of the deoxidized com occurs wherein the oxygen content of the aluminum pact to produce the present polycrystalline body having nitride is given as Al2O3: 30 a porosity of less than about 10% by volume, and pref. erably less than about 4% by volume of the sintered body. For the present compositions defined and encom passed by polygon FJDSR of FIG. 4 excluding line RF, In the deoxidation effected by carbon, gaseous car this sintering temperature generally is at least about bon-containing product is produced which vaporizes 35 1840' C. and generally ranges from about 1840 C. to away thereby removing free carbon. about 2050° C. with the minimum sintering temperature If the compact before deoxidation is heated at too fast increasing generally from about 1840 C. for a composi a rate through the carbon-deoxidation temperature to tion represented by a point next or nearest to point R to sa sintering temperature, and such too fast rate would generally about 1855 C. for a composition at point J of -depend largely on the composition of the compact and 40 FIG. 4. the amount of carbon it contains, the present carbon More specifically, in the present invention, for the * deoxidation does not occur, i.e. an insufficient amount present deoxidized compact having a constant particle of deoxidation occurs, and a significant amount of car size, the minimum sintering temperature occurs at a bon is lost by reactions (3) and/or (3A). composition represented by a point next to point R 45 within the polygon FJDSR and such temperature in C+AlN-AlCN) (3) creases as the composition moves away from point R toward any point on line JD. C+ N2-CHg) (3A) Specifically, the minimum sintering temperature is dependent largely on the composition (i.e. position in The specific amount of free carbon required to pro 50 the FIG. 4 phase diagram), the green density of the duce the present deoxidized compact can be determined compact, i.e. the porosity of the compact after removal by a number of techniques. It can be determined empiri of shaping aid materials but before deoxidation, the cally. Preferably, an initial approximate amount of car particle size of aluminum nitride, and to a much lesser bon is calculated from Equation (2), that is the stoichio extent the particle size of yttrium oxide and carbon. The metric amount for carbon set forth in Equation (2), and 55 minimum sintering temperature increases within the using such approximate amount, the amount of carbon polygon FJDSR as the composition moves from next or required in the present process to produce the present nearest to point R to point J, as the green density of the sintered body would require one or a few runs to deter compact decreases, and as the particle size of aluminum mine if too much or too little carbon had been added. nitride, and to a much lesser extent, yttrium oxide and Specifically, this can be done by determining the poros 60 carbon increases. ity of the sintered body and by analyzing it for carbon To carry out the present liquid phase sintering, the and by X-ray diffraction analysis. If the compact con present deoxidized compact contains sufficient equiva tains too much carbon, the resulting deoxidized com lent percent of Y and O to form a sufficient amount of pact will be difficult to sinter and will not produce the liquid phase at sintering temperature to densify the present sintered body, or the sintered body will contain 65 carbon-deoxidized compact to produce the present sin carbon in an excessive amount. If the compact contains tered body. The present minimum densification, i.e. too little carbon, X-ray diffraction analysis of the result sintering, temperature depends on the composition of ing sintered body will not show any Y4Al2O9 phase and the deoxidized compact, i.e. the amount of liquid phase 4,578,233 17 18 it generates. Specifically, for a sintering temperature to m2/g. In this embodiment, the aluminum nitride in the be operable in the present invention, it must generate at compact before deoxidation has an oxygen content least sufficient liquid phase in the particular composi ranging from greater than about 1.95% by weight to tion of the deoxidized compact to carry out the present less than about 5.1% by weight, and usually from about liquid phase sintering to produce the present product. 5 2.0% by weight to about 4.5% by weight, of such alu For a given composition, the lower the sintering tem minum nitride, the deoxidized compact as well as the perature, the smaller is the amount of liquid phase gen resulting sintered body is comprised of from greater erated, i.e. densification becomes more difficult with than about 1.6 equivalent % to about 4.0 equivalent % decreasing sintering temperature. However, a sintering yttrium, from about 96.0 equivalent % to less than about temperature higher than about 2050 C. provides no 10 98.4 equivalent % aluminum, from greater than about significant advantage. 4.0 equivalent % to about 6.3 equivalent % oxygen and The present deoxidized compact is densified, i.e. liq from about 93.7 equivalent % to less than about 96.0 uid-phase sintered, preferably at a temperature ranging equivalent % nitrogen, the sintering temperature ranges from about 1840 C. to about 2050 C., more preferably from about 1880 C. to about 1920 C., and the sintering from about 1880 C. to about 1950' C., and still more 15 atmosphere is nitrogen. The resulting sintered body has preferably from about 1890° C. to about 1950 C., to a porosity of less than about 2% by volume of the body produce the present polycrystalling body. and a thermal conductivity greater than 1.35 W/cm-K The deoxidized compact is sintered, preferably at at 25 C. The sintered body of this embodiment has a ambient pressure, in a gaseous nitrogen-containing non phase composition comprised of AlN and a second oxidizing atmosphere which contains at least sufficient 20 phase comprised of Y4Al2O9 or a mixture of YAlO3 and nitrogen to prevent significant weight loss of aluminum YaAl2O9. When the sintered body is comprised of AlN nitride. In accordance with the present invention, nitro and Y4Al2O9, the amount of Y4Al2O9 ranges from about gen is a necessary component of the sintering atmo 6% to about 9.4%. When the sintered body is com sphere to prevent any significant weight loss of AlN prised of AlN and a mixture of YAlO3 and Y4Al2O9, the during sintering, and also to optimize the deoxidation 25 total amount of such second phase mixture ranges from treatment and to remove carbon. Significant weight loss greater than about 4.2% by volume to about 9.4% by of the aluminum nitride can vary depending on its sur volume of the body and contains YAlO3 and Y4Al2O9 in face area to volume ratio, i.e. depending on the form of at least an amount detectable by X-ray diffraction analy the body, for example, whether it is in the form of a thin SS. or thick tape. As a result, generally, significant weight 30 In another embodiment of the present invention for loss of aluminum nitride ranges from in excess of about producing a sintered body having a composition de 5% by weight to in excess of about 10% by weight of fined and encompassed by polygon FJMW but not the aluminum nitride. Preferably, the nitrogen-contain including line WF of FIG.4, the aluminum nitride pow ing atmosphere is nitrogen, or it is a mixture at least der in the present mixture has a specific surface area about 25% by volume nitrogen and a gas selected from 35 ranging from about 3.6 m2/g to about 5.2 m2/g, the free the group consisting of hydrogen, a noble gas such as carbon has a specific surface area greater than about 100 argon and mixtures thereof. Also, preferably, the nitro m/g, the sintering atmosphere is nitrogen, the sintering gen-containing atmosphere is comprised of a mixture of temperature ranges from about 1890 C. to about 1950 nitrogen and hydrogen, especially a mixture containing C., the sintered body has a porosity of less than about from about 1% by volume to about 5% by volume 40 1% by volume and contains carbon in an amount of less hydrogen. than about 0.04% by weight of the sintered body and Sintering time is determinable empirically. Typically, has a thermal conductivity greater than 1.41 W/cm-Kat sintering time ranges from about 40 minutes to about 90 25° C. minutes. The present sintered polycrystalline body is a pres In one embodiment, i.e. the composition defined by 45 sureless sintered ceramic body. By pressureless sinter polygon FJDSR but not including lines DJ and RF of ing herein it is meant the densification or consolidation FIG. 4, where the aluminum nitride in the carbon-deox of the deoxidized compact without the application of idized compact contains oxygen, the yttrium oxide fur mechanical pressure into a ceramic body having a po ther deoxidizes the aluminum nitride by reacting with rosity of less than about 10% by volume. the oxygen to form Y4Al2O9 and YAlO3, thus decreas- 50 The polycrystalline body of the present invention has ing the amount of oxygen in the AlN lattice to produce the appearance of having been liquid-phase sintered. the present sintered body having a phase composition Substantially all of the AlN grains are rounded or signif comprised of AlN and a second phase mixture com icantly or substantially rounded and have a smooth prised of YAlO3 and Y4Al2O9. surface, i.e. they have the appearance of a liquid-phase In another embodiment, i.e. line DJ of FIG. 4, where 55 sintered ceramic. The AlN grains have about the same the aluminum nitride in the carbon-deoxidized compact dimensions in all directions and are not elongated or contains oxygen in an amount significantly smaller than disc shaped. Generally, the AlN phase has an average that of polygon FJDSR but not including lines DJ and grain size ranging from about 1 micron to about 20 RF of FIG. 4, the resulting sintered body has a phase microns. An intergranular second phase comprised of composition comprised of AlN and Y4Al2O9. 60 Y4Al2O9 or a mixture of Y4Al2O9 and YAlO3 is present In one embodiment of the present process for produc along some of the AlN grain boundaries. The morphol ing the sintered body having a composition defined and ogy of the microstructure indicates that this intergranu encompassed by polygon FJMW but not including line lar second phase was a liquid at sintering temperature. WF of FIG. 4, the aluminum nitride powder in the The present sintered body has a porosity of less than present mixture, i.e. the dried mixture after milling to 65 about 10% by volume, and generally less than about 4% form such mixture, has a specific surface area ranging by volume, of the sintered body. Preferably, the present from about 3.5 m2/g to about 6 m2/g. Also, the free sintered body has a porosity of less than about 2% and carbon has a specific surface area greater than about 100 most preferably less than about 1% by volume of the 4,578,233 19 20 sintered body. Any pores in the sintered body are fine After reaction (5) has gone to completion, the deoxi sized, and generally they are less than about 1 micron in dized compact now is comprised of the following com diameter. Porosity can be determined by standard me position which was calculated on the basis of Reaction tallographic procedures and by standard density mea (5): SutementS. 5 88.78 grams AlN powder containing 1.71 weight % The present process is a control process for produc Oxygen (85.55 grams AlN+3.22 grams Al2O3) and ing a sintered body of aluminum nitride having a ther 6.7 grams Y2O3. mal conductivity greater than 1.00 W/cm-K at 25 C., From this weight composition, the composition in and preferably greater than 1.25 W/cm-K at 25 C. equivalent % can be calculated as follows: Generally, the thermal conductivity of the present poly O crystalline body is less than that of a high purity single crystal of aluminum nitride which is about 2.8 W/cm-K Wt (g) Moles Equivalents at 25°C. If the same procedure and conditions are used AlN 85.55 2,087 6.262 throughout the present process, the resulting sintered Al2O3 3.22 3.160 x 10-2 0.90 body has a thermal conductivity and composition 15 Y2O3 6.7 2.967 x 10-2 0.78 TOTAL EQUIVALENTS = 6.630 which is reproducible or does not differ significantly. W = Valence Generally, thermal conductivity increases with a de Wt M = Moles s- Y crease in volume 9% of second phase, and for a given MW = molecular weight composition with increase in sintering temperature. Eq = Equivalents In the present process, aluminum nitride picks up 20 Eq = Mix V Valences: oxygen in a controllable or substantially controllable A -- 3 manner. Specifically, if the same procedure and condi Y - 3 tions are used in the present process, the amount of N - 3 oxygen picked up by aluminum nitride is reproducible O - 2 or does not differ significantly. Also, in contrast to 25 Eq% Y in deoxidized compact = (6) yttrium, yttrium nitride and yttrium hydride, yttrium no. Y equivalents oxide does not pick up oxygen, or does not pick up any no. Y equivalents -- no. All equivalents x 100% significant amount of oxygen, from air or other media in the present process. More specifically, in the present =- -i-0.178 x 100% =- 2.69% process, yttrium oxide or the present precursor therefor 30 does not pick up any amount of oxygen in any form Eq% O in deoxidized compact = (7) from the air or other media which would have any - no. Oequivalents . significant effect on the controllability or reproducibil no. O equivalents -- no. N equivalents x 100% ity of the present process. Any oxygen which yttrium 35 oxide might pick up in the present process is so small as -- 0.1906.630 E 0.178 X 100% is-- 5.55% (8) to have no effect or no significant effect on the thermal body.conductivity or composition of the resulting sintered This deoxidized compact as well as the sintered body Examples of calculations for equivalent % are as contains about 2.69 equivalent % Y and about 5.55 ...follows: 40 equivalent % Oxygen. For a starting AlN powder weighing 89.0 grams To produce the present sintered body containing 2.7 ... measured as having 2.3 weight % oxygen, it is assumed equivalent % Y and 5.2 equivalent % O, i.e. comprised that all of the oxygen is bound to AlN as Al2O3, and of 2.7 equivalent % Y, 97.3 equivalent % Al, 5.2 equiva that the measured 2.3 weight % of oxygen is present as lent % O and 94.8 equivalent % N, using an AlN pow 4.89 weight % Al2O3 so that the AlN powder is as 45 der measured as having 2.3 weight % Oxygen (4.89 sumed to be comprised of 84.65 grams AlN and 4.35 weight % Al2O3), the following calculations for weight grams A2O3. % from equivalent % can be made: A mixture is formed comprised of 89.0 grams of the 100 grams= weight of AlN powder starting AlN powder, 6.7 grams of Y2O3 and 0.60 grams x grams=weight of Y2O3 powder free carbon. 50 z grams= weight of Carbon powder. During processing, this AlN powder picks up addi Assume that during processing, the AlN powder tional oxygen by reactions similar to (4) and now con picks up additional oxygen by reaction similar to (9) and tains 2.6 weight % oxygen. in the compact before deoxidation now contains 2.6 weight % oxygen (5.52 weight % Al2O3) and weighs 2AlN-3H2O-Al2O3-2NH3 (4) 55 100.12 grams The resulting compact now is comprised of the foll 2AIN-3H2O-Al2O3-2NH3 (9) lowing composition: 89.11 grams AlN powder containing 2.6 weight % oxy After processing, the compact can be considered as gen, (84.19 g AlN-4.92 g Al2O3), 6.7 grams Y2O3 60 having the following composition: and 0.60 grams Carbon. During deoxidation of the compact, all the carbon is assumed to react with Al2O3 via reaction (5) Weight (g) Moles Equivalents AN 94.59 2.308 6.923 Al2O3 +3C+N2-2AlN+3CO(g) (5) 65 Al2O3 5.53 0.0542 0.325 Y2O3 X 4.429 x 10-3x 0.02657x In the present invention, the carbon will not reduce C 2. .0833z Y2O3, but instead, reduces Al2O3. 4,578,233 21 22 During deoxidation, 3 moles of carbon reduce 1 mole determinable empirically and depends largely on the of Al2O3 and in the presence of N2 form 2 moles of AlN particular sintered body, the particular flattening tem by the reaction: perature and flattening time period. The flattening treat ment should have no significant deleterious effect on Al2O3-3C--N2-2AlN-3CO (10) 5 the sintered body. A decrease in flattening temperature After deoxidation, all the carbon will have reacted requires an increase in flattening pressure or flattening and the compact can be considered as having the fol time. Generally, at a temperature ranging from about lowing composition: 1840° C. or from about 1880 C. to about 2050° C., the applied flattening pressure ranges from about 0.03 psi to 10 about 1.0 psi, preferably from about 0.06 psi to about Weight (g) Moles Equivalents 0.50 psi, and more preferably from about 0.10 psi to AN 94.59 - 2.275z 2.308 - 0.05551z 6.923 -- 0.16652 Al2O3 5.53 - 2.830Z. 0.0542 - 0.02775Z 0.325 - 0.6652. about 0.30psi. Typically, for example, heating the sand Y2O3 X 4.429 x 10-3x 0.02657x wiched sintered body at the sintering temperature T = Total Equivalents = 7,248 + 0.02657x 15 under a pressure of from about 0.03 psi to about 0.5 psi for 1 hour in nitrogen produces a flat body useful as a Equivalent Fraction of Y = 0.027 = 0087 (11) substrate, especially as a supporting substrate for a semi conductor such as a silicon chip. Equivalent Fraction of O = 0.052 = (12) The present invention makes it possible to fabricate 0.325 - 0.16g 0.02657, 20 simple, complex and/or hollow shaped polycrystalline T aluminum nitride ceramic articles directly. Specifically, Solving Equations (11) and (12) for x and z: the present sintered body can be produced in the form x=7.57 grams of Y2O3 powder of a useful complex shaped article without machining or z=0.833 grams of free carbon. without any significant machining such as an impervi A body in a form or shape useful as a substrate, i.e. in 25 ous crucible, a thin walled tube, a long rod, a spherical the form of a flat thin piece of uniform thickness, or body, a tape or a hollow shaped article. The dimensions having no significant difference in its thickness, usually of the present sintered body differ from those of the referred to as a substrate or tape, may become non-flat, unsintered body, by the extent of shrinkage, i.e. densifi for example, warp, during sintering and the resulting cation, which occurs during sintering. sintered body may require a heat treatment after sinter 30 The present ceramic body has a number of uses. In ing to flatten it out and make it useful as a substrate. the form of a thin flat piece of uniform thickness, or This non-flatness or warping is likely to occur in the having no significant difference in its thickness, i.e. in sintering of a body in the form of a substrate or tape the form of a substrate or tape, it is especially useful as having a thickness of less than about 0.070 inch and can packaging for integrated circuits and as a supporting be eliminated by a flattening treatment, i.e. by heating 35 substrate for an integrated circuit, particularly as a sub the sintered body, i.e. substrate or tape, under a suffi strate for a semiconducting Sichip for use in computers. cient applied pressure at a temperature in the present The present ceramic body also is useful as a sheath for sintering temperature range of from about 1840 C. to temperature sensors. about 2050 C., for a period of time determinable empir The invention is further illustrated by the following ically, and allowing the sandwiched body to cool to 40 examples wherein the procedure was as follows, unless below its sintering temperature, preferably to ambient otherwise stated: or room temperature, before recovering the resulting The starting aluminum nitride powder contained flat substrate or tape. oxygen in an amount of less than 4% by weight. Specifically, in one embodiment of this flattening process, the non-flat substrate or tape is sandwiched 45 The starting aluminum nitride powder was greater between two plates and is separated from such plates by than 99% pure AlN exclusive of oxygen. a thin layer of AlN powder, the sandwiched body is In Examples 5A and 5B of Table II, the starting alu heated to its sintering temperature, i.e. a temperature minum nitride powder had a surface area of 3.4 m2/g which is a sintering temperature for the sandwiched (0.541 micron) and based on a series of deoxidations sintered body, preferably in the same atmosphere used 50 carried out with carbon powder, it was determined to for sintering, under an applied pressure at least suffi have contained about 2.4 weight % oxygen. cient to flatten the body, generally at least about 0.03 In Examples 7A, 7B and 8-11 of Table II and 16A psi, for a time period sufficient to flatten the sandwiched and B of Table III, the starting aluminum nitride pow body, and then the sandwiched body is allowed to cool der had a surface area of 3.84 m2/g (0.479 micron) and to below its sintering temperature before it is recovered. 55 contained 2.10 wt % oxygen as determined by neutron One embodiment for carrying out this flattening activation analysis. treatment of a sintered thin body or substrate tape com In the remaining examples of Table II, the starting prises sandwiching the sintered non-flat substrate or aluminum nitride powder had a surface area of 4.96 tape between two plates of a material which has no m2/g (0.371 micron) and contained 2.25 wt % oxygen as significant deleterious effect thereon such as molybde- 60 determined by neutron activation analysis. num.or tungsten, or an alloy containing at least about In all of the examples of Table II and Examples 16A 80% by weight of tungsten or molybdenum. The sand and B of Table III, the Y2O3 powder, before any mix wiched substrate or tape is separated from the plates by ing, i.e. as received, had a surface area of about 2.75 a thin layer, preferably a discontinuous coating, prefera m2/g. bly a discontinuous monolayer, of aluminum nitride 65 The carbon used in all of the examples of Tables II powder preferably just sufficient to prevent the body and III was graphite and it had, before any mixing, a from sticking to the surfaces of the plates during the specific surface area of 200 m2/g (0.017 micron) as listed flattening heat treatment. The flattening pressure is by the vendor. 4,578,233 23 24 Non-aqueous heptane was used to carry out the mix bered with an A or B may be referred to herein by their ing, i.e. milling, of the powders in all of the examples of number only. - Tables II and III. In all of the examples of Tables II and III, the same In all of the examples of Tables II and III the milling atmosphere was used to carry out the deoxidation of the media was hot pressed aluminum nitride in the approxi 5 compacts as was used to carry out the sintering of the mate form of cubes or rectangles having a density of deoxidized compact except that the atmosphere to about 100%. carry out the deoxidization was fed into the furnace at In Examples 6A, 6B, and 12 to 15 of Table II, the a rate of 1 SCFH to promote removal of the gases pro AlN, Y2O3 and carbon powders were immersed in non duced by deoxidation, and the flow rate during sinter aqueous heptane in a plastic jar and vibratory milled in 10 ing was less than about 0.1 SCFH. the closed jar at room temperature for about 68 hours The atmosphere during all of the heat treatment in all producing the given powder mixture. In the remaining of the examples in Tables II and III was at ambient examples of Table II and all of the examples of Table pressure which was atmospheric or about atmospheric III, the AlN, Y2O3, and carbon powders were immersed pressure. in non-aqueous heptane containing oleic acid in an 15 The furnace was a molybdenum heat element fur amount of about 0.7% by weight of the aluminum ni aCC. tride powder in a plastic jar and vibratory milled in the The compacts were heated in the furnace to the given closed jar at room temperature for a period of time deoxidation temperature at the rate of about 100 C. per which varied from about 15 hours to about 21 hours minute and then to the given sintering temperature at producing the given powder mixture. 20 the rate of about 50° C. per minute. In all of the Examples of Tables II and III, the milled The sintering atmosphere was at ambient pressure, liquid dispersion of the given powder mixture was dried i.e. atmospheric or about atmospheric pressure. in air at ambient pressure under a heat lamp for about 20 At the completion of heat treatment, the samples minutes and during such drying, the mixture picked up were furnace-cooled to about room temperature. oxygen from the air. 25 All of the examples of Tables II and III were carried In all of the Examples of Tables II and III, the dried out in substantially the same manner except as indicated milled powder mixture was die pressed at 5 Kipsi in air in Tables II and III and except as indicated herein. at room temperature under a pressure to produce a Carbon content of the sintered body was determined compact having a density of roughly 55% of its theoret by a standard chemical analysis technique. ical density. 30 Based on the predetermined oxygen content of the In those examples of Tables II and III wherein the starting AlN powders and the measured compositions sintered body is given as being of A size or of B size, the of the resulting sintered bodies, as well as other experi compacts were in the form of a disk, in those examples ments, it was calculated or estimated that in every ex wherein the sintered body is given as being of C size, ample in Tables II and III, the aluminum nitride in the the compacts were in the form of a bar, and in those 35 compact before deoxidation had an oxygen content of examples wherein the sintered body is given as being of about 0.3% by weight higher than that of the starting D size, the compacts were in the form of a substrate aluminum nitride powder. - which was a thin flat piece, like a tape, of uniform thick Measured oxygen content was determined by neu ness, or of a thickness which did not differ significantly. tron activation analysis and is given in wt %, which is In Table II the composition of the mixture of pow 40 % by weight of the sintered body. ders is shown as Powder Mixture whereas in Table III In Tables II and III, in those examples where the it is shown as Powders Added. oxygen content of the sintered body was measured, the In all of the examples of Tables II and III, the given equivalent % composition of the sintered body was powder mixture as well as the compact formed there calculated from the starting powder composition and from had a composition wherein the equivalent % of 45 from the given measured oxygen content of the sintered yttrium and aluminum ranged from point D up to point body. The Y, Al, N and oxygen are assumed to have F of FIG. 4. their conventional valences of +3, +3, -3, -2, re The equivalent % composition of Y, Al, O and N of spectively. In the sintered bodies, the amount of Y and the compacts of all of the Examples of Tables II and III, Al was assumed to be the same as that in the starting i.e. before deoxidation, was outside the composition 50 powder. During processing, the amount of oxygen gain defined and encompassed by polygon FJDSR of FIG. and nitrogen loss was assumed to have occurred by the 4. overall reaction: In all of the examples of Tables II and III, the alumi num nitride in the compact before deoxidation con tained oxygen in an amount ranging from greater than 55 about 1.95% by weight to less than about 5.1% by During deoxidation, the amount of oxygen loss and weight of the aluminum nitride. nitrogen gain was assumed to have occurred by the In each of the examples of Tables II and III, one overall reaction: compact was formed from the given powder mixture and was given the heat treatment shown in Table II. 60 Al2O3-3C--N2-2AlN-3CO (14) Also, the examples in Table II having the same number but including the letters A or B indicate that they were The nitrogen content of the sintered body was deter carried out in an identical manner, i.e. the powder mix mined by knowing the initial oxygen content of the tures was prepared and formed into two compacts in the starting aluminum nitride powder and measuring the same manner and the two compacts were heat treated 65 oxygen content of the sintered body and assuming that under identical conditions, i.e. the two compacts were reactions 13 and 14 have occurred. placed side by side in the furnace and given the same In Tables II and III, an approximation sign is used in heat treatment simultaneously, and these examples num front of the equivalent percent oxygen for sintered bod 4,578,233 25 26 ies whose oxygen content was not measured but calcu absolute accuracy is about 3% and the repeatability is lated, and these calculations were based on the compo about it l%. As a comparison, the thermal conductivity sition of the powder mixture as well as that of the result of an Al2O3 single crystal was measured with a similar ing sintered body and were carried out as follows: apparatus to be 0.44 W/cm-K at about 22 C. Examples 2A, 2B, 3 and 4 (i.e. 113C, 113C1, 113D, In Tables II and III, the size of the resulting sintered and 113E) are assumed to have the same equivalent body is given as A, B, C or D. The body of A size was percent oxygen as Example 1B (113A2). The equivalent in the form of a disk about 0.17 inch in thickness and percent oxygen of Examples 5A, 7A, 8, and 11 (i.e. about 0.32 inch in diameter. The body of B size was also 134C, 150A, 150B and 150E) was calculated from the in the form of a disk with a thickness of about 0.27 inch X-ray diffraction analysis data. Examples 9 and 10, (i.e. 10 150C and 150D) are assumed to have the same equiva and a diameter of about 0.50 inch. The body of C size lent percent oxygen as Example 7A (150A). Examples was in the shape of a bar measuring about 0.16 13, 14 and 15 (i.e. 90B1, 90D2 and 90K) are assumed to inchx0.16 inch x 1.7 inches. The body of D size was in have the same equivalent percent oxygen as Example 12 the form of a substrate, i.e. a thin piece of uniform thick (84F). Example 5B (134C2) is assumed to have the same ness, or of no significant difference in thickness, having equivalent percent oxygen as Example 5A (134C). 15 a diameter of about 1.5 inch and a thickness of 0.042 The equivalent % oxygen content of the sintered inch. body of Example 16B (131D1) was calculated from the In all of the examples of Tables II and III, the con equation: pacts were placed on a molybdenum plate and then 20 given the heat treatment shown in Tables II and III. - Y - In all of the Examples of Tables II and III wherein O = (2.91R -- 3.82) 3.86 the sintered body was of C size or of D size, the starting compact was separated from the molybdenum plate by where a thin discontinuous layer of AlN powder. O=equivalent percent oxygen The sintered body of Example 10 exhibited some Y=equivalent percent yttrium 25 non-flatness, i.e. exhibited some warping, and was sub jected to a flattening treatment. Specifically, the sin v/o Y4Al2Og tered body produced in Example 10 was sandwiched R = W, YAO / Yo, between a pair of molybdenum plates. The sandwiched 30 sintered body was separated from the molybdenum Weight loss in Tables II and III is the difference plates by a thin discontinuous coating or monolayer of between the weight of the compact after die pressing aluminum nitride powder which was just sufficient to and the resulting sintered body. prevent sticking of the sintered body to the plates dur Density of the sintered body was determined by the ing the flattening treatment period. The top molybde Archimedes method. 35 num plate exerted a pressure of about 0.11 psi on the Porosity in % by volume of the sintered body was sintered body. The sandwiched sintered body was determined by knowing the theoretical density of the heated in nitrogen, i.e. the same atmosphere used to sintered body on the basis of its composition and com sinter it, to about 1900 C. where it was held for about paring that to the density measured using the following 1 hour and then furnace cooled to about room tempera equation: ture. The resulting sintered body was flat and was of uniform thickness, i.e. its thickness did not differ signifi (15) cantly. This flat sintered body would be useful as a porosity = ( T theoreticalmeasured densitydensity ) 100% substrate for a semiconductor such as a silicon chip. EXAMPLE 1. 45 Phase composition of the sintered body was deter 2,154 grams of Y2O3 powder and 0.035 grams of mined by optical microscopy and X-ray diffraction graphite powder were added to 14.4 grams of aluminum analysis, and each sintered body was comprised in % by nitride powder and the mixture, along with aluminum volume of the sintered body of aluminum nitride phase nitride milling media, was immersed in non-aqueous and the given volume % of the given second phases. 50 heptane containing oleic acid in an amount of about The X-ray diffraction analysis for volume 2% of each 0.7% by weight of the aluminum nitride in a plastic jar second phase is accurate to about 20% of the given and vibratory milled in the closed jar at room tempera value. tire for about 17 hours. The resulting dispersion was The thermal conductivity of the sintered body of dried in air under a heat lamp for about 20 minutes and Example 13 (90B1), Example 14 (90D2) and Example 15 55 during such drying, the aluminum nitride picked up (90K) was measured by laser flash at about 25 C. oxygen from the air. During milling, the mixture picked The thermal conductivity of the sintered body of all up 0.725 gram AlN due to wear of the AlN milling of the remaining examples was measured at 25 C. by a media. steady state heat-flow method using a rodshaped sample Equivalent portions of the resulting dried mixture -0.4 cm x 0.4 cm x 2.2 cm sectioned from the sintered 60 were die pressed producing compacts. body. This method was originally devised by A. Berget Two of the compacts were placed side by side on a in 1888 and is described in an article by G. A. Slack in molybdenum plate. the "Encyclopaedic Dictionary of Physics', Ed. by J. The compacts were heated in nitrogen to 1600 C. Thewlis, Pergamon, Oxford, 1961. In this technique the where they were held for 1 hour and then the tempera sample is placed inside a high-vacuum chamber, heat is 65 ture was raised to 1965 C. where it was held for 1 hour. supplied at one end by an electrical heater, and the This example is shown as Examples 1A and 1B, in temperatures are measured with fine-wire thermocou Table II. Specifically, one of the sintered bodies, Exam ples. The sample is surrounded by a guard cylinder. The ple 1B, had a measured oxygen content of 4.66% by 4,578,233 27 28 weight of the body. Also, it had a phase composition Examples 3, 4, 6A and B, 15, 16A and B were carried comprised of AlN, 11.3% by volume Y4Al2O9 and 1.6% out in substantially the same manner as Examples 2A by volume YAlO3. Also, it had an equivalent % compo and B except as indicated herein and except as shown in sition comprised of 8.72% O, (100%-8.72%) or 91.28% Tables II and III. N, 4.96% Y and (100%-4.96%) or 95.04% Al. This 5 In Examples 5A and 5B, the two compacts were composition lies outside the polygon FJDSR of FIG. 4. heated to sintering temperature at a rate of about 190 The compacts used in Examples 2A, 2B, 3 and 4 were C. per minute. produced in Example 1. In Examples 2A and 2B, the Examples 7A and B, 8 to 11, and 14 were carried out compacts were first heated to a deoxidation tempera- in the same manner as Examples 1A and B except as ture of 1500° C. where it was held for hour, then the 10 indicated herein and except as shown in Table II. temperature was raised to 1600 C, where it was held In Examples 12 and 13, the compacts were heated to for 1 hour and then it was raised to the sintering temper- sintering temperature at a rate of about 100° C. per ature of 1900' C. where it was held for 1 hour. minute. TABLE II Properties of Sintered Body Powder Mixture Heat Treatment Measured - (wt %) - - Deoxidation - F - Sintering - Atmos- Oxygen Carbon Ex. Sample AlN Y2O3 C Temp (°C) - Time (Hr) + Temp (°C) - Time (Hr) - phere (wt %) (wt %) 1A 113A1 87.36 2.44 0.20 1600 1 -- 1965 l N2 -- m 1B 113A2 -- p n f 4.66 -- 2A 113C FF A 1500 -- 1900 1 N2 - 0.006 1600 l 2B 113C1 t -- t A - m 3 113D 1500 -- 1870 - 1 N2 - 0.014 1600 - 1 4. 113E t 1500 -- 1950 1 N2 -- m 1600 1 5A 134C 89.28 9.19 153 1900 l N2 - 0.063 5B 134C2 F. - 0.040 6A 98B. 94.19 4.75 1.06 1500 -- 1870 1 N2 2.41 0.026 1600 1. 6B 98B2 f - m 7A 150A 92.35 7.00 0.65 1600 - l -- 1900 1. N2 - -- 7B 150A -- - m 8 50B f r 1600 . l -- 1870 N2 - Mr. 9 150C rt 1600 - -- 1950 1 N - m O 150D F r 1600 -- 1900 l N2 - m 1. 150E t t 1600 - 1 -- 1840 - N2 - m 12 84F 88.98 9.75 1.27 1900 Ar - 3.88 0.431 13 90B 88.98 9.69 1.32 1900 l H2 - 0.658 14 90D2 t 1600 -- 1900 -- H2 --- 0.418 15 90K f r 1500 -- 1900 - Air w- m 1600 l - Properties of Sintered Body Weight Vol. Thermal Equivalent % Loss Density Porosity % Second Phases Conductivity Ex. Oxygen Yttrium (%) (g/cc) (vol%) Y4Al2Og YAlO3 (W/cm . K Q 25° C) Size

iA -8.72 4.96 3.5 ------al- - A B 8.72 4.96 3.5 3.41 <1 11.3 .6 - A. 2A -8.7 4.96 - 3.39 1 -- 1.34 C 2B -8.7 4.96 2.3 - - - - A 3 -8.7 4.96 - 3.42 < - - 1.34 C 4 - 8.7 4.96 - 3.41

TABLE III Heat Treatment Powders Added (wt % Deoxidation -- Sinterin Ex. Sample AlN Y2O3 C Temp "C. Time Hr -- Temp "C. - Time Hr Atmosphere 16A 131D 89.42 9.39 1.19 1500 -- 1900 - H2 + 25% N2 4,578,233 29 30 TABLE III-continued 16B 13D1 AF -- FA Properties of Sintered Body Measured Wt Approximate Volume Thermal Oxygen Carbon Equivalent % Loss Density Porosity % Second Phases Conductivity Ex. (wt %) (wt %). Oxygen Yttrium % (g/cc) (%) Y2O3 Y4Al2O9 YAO3 W/cm. KG 25' C. Size 16A - -- - 6.0 3.70 - 3.35 - - M- 1.52 C 16B - 0.014 - 6.0 3.70 2.8 - - 1.2 5.8 m - A

tion which was defined and encompassed by polygon Examples 5A, 5B, 6A, 6B, 7A, 7B, 8, 9, 10 and 11 FUMW but not line WF Of FIG. 4. illustrate the present invention. The sintered body pro Example 12 illustrates that the use of an argon atmo duced in Examples 5A, 5B, 6A, 6B, 7A, 7B, 8, 9, 10 and sphere resulted in a large amount of carbon being left in 11 would be useful for packaging of integrated circuits the sintered body. as well as for use as a substrate for semiconductors such Example 13 illustrates that a lack of the deoxidation as a silicon chip. step and the use of a hydrogen atmosphere results in a Examples 1A and B, 2A and B, 3 and 4 did not pro sintered body which had a low thermal conductivity duce a sintered body of the present composition. Specif 20 and which contained a large amount of carbon. ically, in these examples not enough carbon was used Example 14 illustrates that even though there was a and therefore there was insufficient deoxidation of the deoxidation step carried out, the use of the hydrogen aluminum nitride as illustrated by the equivalent per atmosphere resulted in a sintered body which had a low cent composition of the sintered bodies of these exam thermal conductivity and which contained a large ples and as illustrated by the measured oxygen content 25 amount of carbon. and the phase composition of the sintered body of Ex Both Examples 13 and 14 illustrate the deleterious ample 1B. effect of carbon on the thermal conductivity of the Examples 5A and 5B illustrate the present invention. sintered body and that sintering in hydrogen produces Since the sintered bodies of Examples 5A and B were substantial weight loss. produced in an identical manner simultaneously, it is 30 Example 15 illustrates that the use of an argon atmo known from other work and a comparison of Examples sphere results in a sintered body having a low thermal 5A and 5B, that the sintered body of Example 5B would conductivity. necessarily have a thermal conductivity of about 1.38 In the examples of Table III, too much carbon was W/cm-K at 25° C. and a porosity of less than 1% by added to the powder mixture thereby resulting in a volume of the body. The measured carbon content of 35 deoxidized compact and a sintered body having a com the sintered bodies of Examples 5A and 5B shows that position outside the polygon FJDSR of FIG. 4. How it was insufficient to have a significant effect on the ever, Examples 16A and B illustrate the operability of thermal conductivity of the sintered bodies. The sin an atmosphere comprised of a mixture of hydrogen and tered bodies of Examples 5A and 5B have an equivalent 25% by volume nitrogen. percent composition and a phase composition which is 40 In U.S. Pat. Nos. 4,478,785, and 4,533,645, entitled defined and encompassed by polygon FJDSR but not HIGH THERMAL CONDUCTIVITY ALUMINUM line RF of FIG. 4. NITRIDE CERAMICBODY, incorporated herein by Examples 6A and 6B illustrate the present invention. reference, there is disclosed the process comprising Since the sintered bodies of Examples 6A and B were forming a mixture comprised of aluminum nitride pow produced in the same manner simultaneously, it is 45 der and free carbon wherein the aluminum nitride has a known from other work and a comparison of Examples predetermined oxygen content higher than about 0.8% 6A and B that the sintered body of Example 6A had a by weight and wherein the amount of free carbon reacts porosity of less than 1% by volume of the body, and with such oxygen content to produce a deoxidized pow that the sintered body of Example 6B had a composition der or compact having an oxygen content ranging from which was the same or did not differ significantly from 50 greater than about 0.35% by weight to about 1.1% by that of Example 6A. The sintered bodies of Examples weight and which is at least 20% by weight lower than 6A and 6B had an equivalent percent composition and a the predetermined oxygen content, heating the mixture phase composition defined and encompassed by poly or a compact thereof to react the carbon and oxygen gon FJMW but not line WF of FIG. 4. Also, based on producing the deoxidized aluminum nitride, and sinter other work, and a comparison of Examples 6A and 6B 55 ing a compact of the deoxidized aluminum nitride pro with Examples 5A and 8, it is known that the sintered ducing a ceramic body having a density greater than bodies of Examples 6A and 6B had athermal conductiv 85% of theoretical and a thermal conductivity greater ity greater than 1.35W/cm-K at 25° C. than 0.5 W/cm-K at 22 C. Examples 7A and B and 8-11 illustrate the present In copending U.S. patent application Ser. No. invention. Based on other work and a comparison of 60 656,636, entitled HIGH THERMAL CONDUCTIV Examples 7-11, it is known that the sintered body of ITY CERAMIC BODY, filed on Oct. 1, 1984, in the Example 10 had a porosity of less than 1% by volume names of Irvin Charles Huseby and Carl Francis Bobik and that all of the sintered bodies of Examples 7-10 had and assigned to the assignee hereof and incorporated athermal conductivity greater than 1.35 W/cm-K at 25 herein by reference, there is disclosed the process for C. and that the sintered body of Example 11 had a ther 65 producing an aluminum nitride ceramic body having a mal conductivity greater than 1.00 W/cm-K at 25 C. composition defined and encompassed by polygon Also, all of the sintered bodies of Examples 7-11 had an JKLM but not including line MJ of FIG. 4 of Ser. No. equivalent percent composition and a phase composi 656,636 and a thermal conductivity greater than 1.42 4,578,233 31 32 W/cm-K at 25°C. which comprises forming a mixture 679,414, a porosity of less than about 4% by volume, comprised of aluminum nitride powder containing oxy and a minimum thermal conductivity of 1.42 W/cm-K gen, yttrium oxide, and free carbon, shaping said mix at 25 C. which comprises forming a mixture comprised ture into a compact, said mixture and said compact of aluminum nitride powder containing oxygen, yttrium having a composition wherein the equivalent % of yt 5 oxide and free carbon, shaping said mixture into a com trium and aluminum ranges from point L to less than pact, said mixture and said compact having a composi point J of FIG. 4 of Ser. No. 656,636, said compact tion wherein the equivalent % of yttrium and aluminum having an equivalent % composition of Y, Al, O and N ranges between points J and Al of FIG. 4 of Ser. No. outside the composition defined and encompassed by 679,414, said compact having an equivalent % composi polygon JKLM of FIG. 4 of Ser. No. 656,636, the alu 10 tion of Y, Al, O and N outside the composition defined minum nitride in said compact containing oxygen in an and encompassed by polygon PJFAl of FIG. 4 of Ser. amount ranging from greater than about 1.4% by No. 679,414, the aluminum nitride in said compact con weight to less than about 4.5% by weight of the alumi taining oxygen in an amount ranging from greater than num nitride, heating said compact up to a temperature about 1.42% by weight to less than about 4.70% by at which its pores remain open reacting said free carbon 15 weight of the aluminum nitride, heating said compact with oxygen contained in said aluminum nitride produc up to a temperature at which its pores remain open ing a deoxidized compact, said deoxidized compact reacting said free carbon with oxygen contained in said having a composition wherein the equivalent % of Al, aluminum nitride producing a deoxidized compact, said Y, O and N is defined and encompassed by polygon deoxidized compact having a composition wherein the JKLM but not including line MJ of FIG. 4 of Ser. No. 20 equivalent % of Al, Y, O and N is defined and encom 656,636, and sintering said deoxidized compact at a passed by polygon PJFAl but not including lines JF and temperature ranging from about 1890° C. to about 2050 AlF of FIG. 4 of Ser. No. 679,414, and sintering said C. producing said ceramic body. deoxidized compact at a temperature ranging from In copending U.S. patent application Ser. No. 675,048 about 1880 C. to about 2050 C. wherein the minimum entitled HIGH THERMAL CONDUCTIVITY CE 25 sintering temperature increases from about 1880 C. for RAMIC BODY filed Nov. 26, 1984, in the name of a composition defined and encompassed by polygon Irvin Charles Huseby and Carl Francis Bobik and as A3JFA2 excluding lines A3J, JF and A2F to about signed to the assignee hereof and incorporated herein 1925 C. for a composition at point P of FIG. 4 of Ser. by reference, there is disclosed the process for produc No. 679,414 producing said ceramic-body, said sintering ing an aluminum nitride ceramic body having a compo 30 sition defined and encompassed by polygon PONKJ but temperature being a sintering temperature for said com not including lines KJ and PJ of FIG. 4 of Ser. No. position of said deoxidized compact. 675,048, a porosity of less than about 4% by volume, In copending U.S. patent application Ser. No. 682,468 and a minimum thermal conductivity of 1.50 W/cm-K entitled HIGH THERMAL CONDUCTIVITY CE at 25 C. which comprises forming a mixture comprised 35 RAMIC BODY, filed on Dec. 17, 1984, in the names of of aluminum nitride powder containing oxygen, yttrium Irvin Charles Huseby and Carl Francis Bobik and as oxide and free carbon, shaping said mixture into a com signed to the assignee herein and incorporated herein by pact, said mixture and said compact having a composi reference, there is disclosed a process for producing an tion wherein the equivalent % of yttrium and aluminum aluminum nitride ceramic body having a composition ranges between points K and P of FIG. 4 of Ser. No. 40 defined and encompassed by polygon LTIDM but not ... 675,048, said compact having an equivalent % composi including lines LM and DM of FIG. 4 of Ser. No. tion of Y, Al, O and N outside the composition defined 682,468, a porosity of less than about 4% by volume, and encompassed by polygon PONKJ of FIG. 4 of Ser. and a minimum thermal conductivity of 1.27 W/cm-K No. 675,048, the aluminum nitride in said compact con at 25 C. which comprises forming a mixture comprised taining oxygen in an amount ranging from greater than 45 of aluminum nitride powder containing oxygen, yttrium about 1.40% by weight to less than about 4.50% by oxide and free carbon, shaping said mixture into a com weight of the aluminum nitride, heating said compact pact, said mixture and said compact having a composi up to a temperature at which its pores remain open tion wherein the equivalent % of yttrium and aluminum reacting said free carbon with oxygen contained in said ranges from point Tl up to point M of FIG. 4 of Ser. No. aluminum nitride producing a deoxidized compact, said 50 682,468, said compact having an equivalent % composi deoxidized compact having a composition wherein the tion of Y, Al, O and N outside the composition defined equivalent % of Al, Y, O and N is defined and encom and encompassed by polygon LTlDM of FIG. 4 of Ser. passed by polygon PONKJ but not including lines KJ No. 682,468, the aluminum nitride in said compact con and PJ of FIG. 4 of Ser. No. 675,048 and sintering said taining oxygen in an amount ranging from greater than deoxidized compact at a temperature ranging from 55 about 1.85% by weight to less than about 4.50% by about 1900 C. to about 2050 C. producing said ce weight of the aluminum nitride, heating said compact to ramic body, said sintering temperature being a sintering a temperature at which its pores remain open reacting temperature for said composition of said deoxidized said free carbon with oxygen contained in said alumi compact. num nitride producing a deoxidized compact, said deox In copending U.S. patent application Ser. No. 679,414 60 idized compact having a composition wherein the entitled HIGH THERMAL CONDUCTIVITY CE equivalent % of Al, Y, O and N is defined and encom RAMICBODY filed Dec. 7, 1984, in the names of Irvin passed by polygon LTIDM but not including lines LM Charles Huseby and Carl Francis Bobik and assigned to and DM of FIG. 4 of Ser, No. 862,468, and sintering the same assignee hereof and incorporated herein by said deoxidized compact at a temperature ranging from reference, there is disclosed the process for producing 65 about 1890 C. to about 2050 C. wherein the minimum an aluminum nitride ceramic body having a composi sintering temperature increases from about 1890 C. for tion defined and encompassed by polygon PJFAl but a composition adjacent to line DM to about 1970° C. for not including lines JF and AlF of FIG. 4 of Ser. No. a composition on line TIL producing said ceramic body, 4,578,233 33 34 said sintering temperature being a sintering temperature point J of FIG. 4, said yttrium in said compact ranging for said composition of said deoxidized compact. from about 5.5 equivalent % to about 2.5 equivalent %, What is claimed is: said aluminum in said compact ranging from about 94.5 1. A process for producing a sintered polycrystalline equivalent % to about 97.5 equivalent %, and wherein aluminum nitride ceramic body having a composition said sintered body and said deoxidized compact are defined and encompassed by polygon FJDSR but not comprised of a composition wherein the equivalent including line RF of FIG. 4, a porosity of less than percent of Al, Y, O and N is defined by line DJ of FIG. about 10% by volume of said body and a thermal con 4, and said sintering temperature is at least about 1855 ductivity greater than 1.00 W/cm-K at 25 C. which C. comprises the steps: 10 5. The process according to claim 1 wherein the (a) forming a mixture comprised of an oxygen-con sintering temperature ranges from about 1890° C. to taining aluminum nitride powder, yttrium oxide, about 1950 C., said aluminum nitride powder in said and free carbon, shaping said mixture into a con mixture has a specific surface area ranging from about pact, said mixture and said compact having a com 3.6 m2/g to about 6.0 m2/g, said carbon has a specific position wherein the equivalent % of yttrium and 15 surface area greater than about 100 m2/g, said sintering aluminum ranges from point D up to point F of atmosphere is nitrogen and said sintered body has a FIG. 4, said yttrium in said compact ranging from porosity of less than about 1% by volume of said body. greater than about 1.6 equivalent % to about 5.5 6. The process according to claim 1 wherein said equivalent %, said aluminum in said compact rang nitrogen-containing atmosphere in step (b) contains ing from about 94.5 equivalent % to less than about 20 sufficient nitrogen to facilitate deoxidation of the alumi 98.4 equivalent said compact having an equivalent num nitride to produce said sintered body. % composition of Y, Al, %, O and N outside the 7. The process according to claim 1 wherein said composition defined and encompassed by polygon nitrogen-containing atmosphere in step (c) contains FJDSR of FIG. 4, sufficient nitrogen to prevent significant weight loss of (b) heating said compact in a nitrogen-containing 25 said aluminum nitride. nonoxidizing atmosphere at a temperature ranging 8. The process according to claim 1 wherein said from about 1350° C. to a temperature sufficient to process is carried out at ambient pressure. deoxidize the compact but below its pore closing 9. The process according to claim 1 wherein the temperature reacting said free carbon with oxygen aluminum nitride in said compact in step (a) before said contained in said aluminum nitride producing a 30 deoxidation of step (b) contains oxygen in an amount deoxidized compact, said deoxidized compact hav ranging from greater than about 1.5% by weight to less ing a composition wherein the equivalent % of Al, than about 5.1% by weight of said aluminum nitride. Y, O and N is defined and encompassed by polygon 10. The process according to claim 1 wherein said FJDSR but not including line RF of FIG. 4, said aluminum nitride in step (a) has a specific surface area free carbon being in an amount which produces 35 ranging up to about 10 m2/g and said free carbon has a said deoxidized compact, and specific surface area greater than about 10 m2/g. (c) sintering said deoxidized compact in a nitrogen 11. A process for producing a sintered polycrystalline containing monoxidizing atmosphere at a tempera aluminum nitride ceramic body having a composition ture of at least about 1840 C. producing said poly defined and encompassed by polygon FJMW but not crystalline body. including line WF of FIG. 4, a porosity of less than 2. The process according to claim 1 wherein said about 10% by volume of said body and a thermal con mixture and said compact have a composition wherein ductivity greater than 1.00 W/cm-K at 25° C. which the equivalent % of yttrium and aluminum ranges from comprises the steps: point M up to point F of FIG. 4, said yttrium in said (a) forming a mixture comprised of aluminum nitride compact ranging from greater than about 1.6 equivalent 45 powder, yttrium oxide, and free carbon, said free % to about 4.0 equivalent %, said aluminum in said carbon having a specific surface area greater than compact ranging from about 96.0 equivalent % to less about 100 m2/g, the aluminum nitride powder in than about 98.4 equivalent %, and wherein said sintered said mixture having a specific surface area ranging body and said deoxidized compact are comprised of a from about 3.5 m2/g to about 6.0 m2/g, shaping said composition wherein the equivalent percent of Al, Y, O 50 mixture into a compact, said mixture and said com and N is defined and encompassed by polygon FJMW pact having a composition wherein the equivalent but does not include line WF of FIG. 4. % of yttrium and aluminum ranges from point M 3. The process according to claim 1 wherein said up to point F of FIG. 4, said yttrium in said com mixture and said compact have a composition wherein pact ranging from greater than about 1.6 equivalent the equivalent % of yttrium and aluminum ranges be 55 % to about 4.0 equivalent %, said aluminum in said tween points D and F but does not include points D and compact ranging from about 96.0 equivalent % to F of FIG. 4 said yttrium in said compact ranging from less than about 98.4 equivalent %, said compact greater than about 1.6 equivalent % to less than about having an equivalent % composition of Y, Al, O 5.5 equivalent %, said aluminum in said compact rang and N outside the composition defined and encom ing from greater than about 94.5 equivalent % to less passed by polygon FJDSR of FIG. 4, the alumi than about 98.4 equivalent %, and wherein said sintered num nitride in said compact containing oxygen in body and said deoxidized compact are comprised of a an amount ranging from greater than about 1.95% composition wherein the equivalent percent of Al, Y, O by weight to less than about 5.1% by weight of said and N is defined and encompassed by polygon FJDSR aluminum nitride, but does not include lines DJ and RF of FIG. 4. 65 (b) heating said compact at ambient pressure in a 4. The process according to claim 1 said mixture and nitrogen-containing nonoxidizing atmosphere con said compact have a composition wherein the equiva taining at least about 25% by volume nitrogen at a lent % of yttrium and aluminum ranges from point D to temperature ranging from about 1350 C. to a tem 4,578,233 35 36 perature sufficient to deoxidize the compact but (d) sintering said deoxidized compact in a nitrogen below its pore closing temperature reacting said containing nonoxidizing atmosphere at a tempera free carbon with oxygen contained in said alumi ture of at least about 1840 C. producing said poly num nitride producing a deoxidized compact, said crystalline body. deoxidized compact having a composition wherein 5 14. The process according to claim 13 wherein said the equivalent % of Al, Y, O and N is defined and mixture and said compact have a composition wherein encompassed by polygon FJMW but not including the equivalent % of yttrium and aluminum ranges from line WF of FIG. 4, the aluminum nitride in said point M up to point F of FIG. 4, said yttrium in said compact before said deoxidation by said carbon compact ranging from greater than about 1.6 equivalent having an oxygen content ranging from greater 10 % to about 4.0 equivalent %, said aluminum in said than about 1.95% by weight to less than about compact ranging from about 96.0 equivalent % to less 5.1% by weight of said aluminum nitride, said free than about 98.4 equivalent %, and wherein said sintered carbon being in an amount which produces said body and said deoxidized compact are comprised of a deoxidized compact, and composition wherein the equivalent percent of Al, Y, O (c) sintering said deoxidized compact at ambient pres 15 and N is defined and encompassed by polygon FJMW sure in a nitrogen-containing nonoxidizing atmo but does not include line WF of FIG. 4. sphere containing at least about 25% by volume 15. The process according to claim 13 wherein said nitrogen at a temperature ranging from about 1880 mixture and said compact have a composition wherein C. to about 2050° C. producing said polycrystalline the equivalent % of yttrium and aluminum ranges be body. 20 tween points D and F but does not include points D and 12. The process according to claim 11 wherein the F of FIG. 4, said yttrium in said compact ranging from sintering temperature ranges from about 1890 C. to greater than about 1.6 equivalent % to less than about about 1950° C., said aluminum nitride powder in said 5.5 equivalent %, said aluminum in said compact rang mixture has a specific surface area ranging from about ing from greater than about 94.5 equivalent % to less 3.6 m2/g to about 5.2 m2/g, and said sintered body has a 25 than about 98.4 equivalent %, and wherein said sintered porosity of less than about 1% by volume of said body. body and said deoxidized compact are comprised of a 13. A process for producing a sintered polycrystalline composition wherein the equivalent percent of Al, Y, O aluminum nitride ceramic body having a composition and N is defined and encompassed by polygon FJDSR defined and encompassed by polygon FJDSR but not but does not include lines DJ and RF of FIG. 4. including line RF of FIG. 4, a porosity of less than 30 about 10% by volume of said body and a thermal con 16. The process according to claim 13 wherein said ductivity greater than 1.00 W/cm-K at 25 C. which mixture and said compact have a composition wherein comprises the steps: the equivalent % of yttrium and aluminum ranges from (a) forming a mixture comprised of aluminum nitride point D to point J of FIG. 4, said yttrium in said com powder, yttrium oxide or precursor therefor, and a 35 pact ranging from about 5.5 equivalent % to about 2.5 carbonaceous additive selected from the group equivalent %, said aluminum in said compact ranging consisting of free carbon, a carbonaceous organic from about 94.5 equivalent % to about 97.5 equivalent material and mixtures thereof, said carbonaceous %, and wherein said sintered body and said deoxidized organic material thermally decomposing at a tem compact are comprised of a composition wherein the perature ranging from about 50° C. to about 1000 40 equivalent percent of Al, Y, O and N is defined by line C. to free carbon and gaseous product of decompo DJ of FIG. 4, and said sintering temperature is at least sition which vaporizes away, shaping said mixture about 1855 C. into a compact, said mixture and said compact 17. The process according to claim 13 wherein the having a composition wherein the equivalent % of minimum sintering temperature ranges from about yttrium and aluminum ranges from point D up to 45 1890 C. to about 1950 C., said aluminum nitride pow point F of FIG. 4, said yttrium in said compact der in said mixture has a specific surface area ranging ranging from greater than about 1.6 equivalent % from about 3.6 m2/g to about 6.0 m2/g, said carbon has to about 5.5 equivalent %, said aluminum in said a specific surface area greater than about 100 m2/g, said compact ranging from about 94.5 equivalent % to sintering atmosphere is nitrogen and said sintered body less than about 98.4 equivalent % aluminum, said 50 has a porosity of less than about 1% by volume of said compact having an equivalent % composition of Y, body. Al, O and N outside the composition defined and 18. The process according to claim 13 wherein said encompassed by polygon FJDSR of FIG. 4, nitrogen-containing atmosphere in step (c) contains (b) heating said compact in a nonoxidizing atmo sufficient nitrogen to facilitate deoxidation of the alumi sphere at a temperature up to about 1200° C. 55 num nitride to produce said sintered body. thereby providing yttrium oxide and free carbon, 19. The process according to claim 13 wherein said (c) heating said compact in a nitrogen-containing nitrogen-containing atmosphere in step (d) contains nonoxidizing atmosphere at a temperature ranging sufficient nitrogen to prevent significant weight loss of from about 1350° C. to a temperature sufficient to said aluminum nitride. deoxidize the compact but below its pore closing 60 20. The process according to claim 13 wherein said temperature reacting said free carbon with oxygen process is carried out at ambient pressure. contained in said aluminum nitride producing a 21. The process according to claim 13 wherein the deoxidized compact, said deoxidized compact hav aluminum nitride in said compact in step (a) before said ing a composition wherein the equivalent % of Al, deoxidation of step (c) contains oxygen in an amount Y, O and N is defined and encompassed by poly 65 ranging from greater than about 1.5% by weight to less gon FJDSR but not including line RF of FIG. 4, than about 5.1% by weight of said aluminum nitride. said free carbon being in an amount which pro 22. The process according to claim 13 wherein said duces said deoxidized compact, and aluminum nitride in step (a) has a specific surface area 4,578,233 37 38 ranging up to about 10 m2/g and said free carbon has a by weight to less than about 5.1% by weight of said specific surface area greater than about 10 m2/g. aluminum nitride, 23. A process for producing a sintered polycrystalline (b) heating said compact in a nonoxidizing atmo aluminum nitride ceramic body having a composition sphere at a temperature up to about 1200° C. defined and encompassed by polygon FJMW but not 5 thereby providing yttrium oxide and free carbon, including line WF of FIG. 4, a porosity of less than (c) heating said compact at ambient pressure in a about 10% by volume of said body and a thermal con nitrogen-containing nonoxidizing atmosphere con ductivity greater than 1.00 W/cm-K at 25° C. which taining at least about 25% by volume nitrogen at a comprises the steps: temperature ranging from about 1350 C. to a tem (a) forming a mixture comprised of aluminum nitride 10 perature sufficient to deoxidize the compact but powder, yttrium oxide or precursor thereof, and a below its pore closing temperature reacting said carbonaceous additive selected from the group free carbon with oxygen contained in said alumi consisting of free carbon, a carbonaceous organic num nitride producing a deoxidized compact, said material and mixtures thereof, said carbonaceous deoxidized compact having a composition wherein organic material thermally decomposing at a tem 15 the equivalent % of Al, Y, O and N is defined and perature ranging from about 50° C. to about 1000 encompassed by polygon FJMW but not including C. to free carbon and gaseous product of decompo line WF of FIG. 4, the aluminum nitride in said sition which vaporizes away, said free carbon hav compact before said deoxidation by said carbon ing a specific surface area greater than about 100 2 having an oxygen content ranging from greater O than about 1.95% by weight to less than about m2/g, the aluminum nitride powder in said mixture 5.1% by weight of said aluminum nitride, said free having a specific surface area ranging from about carbon being in an amount which produces said 3.5 m2/g to about 6.0 m2/g, shaping said mixture deoxidized compact, and into a compact, said mixture and said compact (d) sintering said deoxidized compact at ambient pres having a composition wherein the equivalent % of 25 sure in a nitrogen-containing nonoxidizing atmo yttrium and aluminum ranges from point M up to sphere containing at least about 25% by volume point F of FIG. 4, said yttrium in said compact nitrogen at a temperature ranging from about 1880” ranging from greater than about 1.6 equivalent % C. to about 2050° C. producing said polycrystalline to about 4.0 equivalent %, said aluminum in said body. compact ranging from about 96.0 equivalent % to 30 24. The process according to claim 23 wherein the less than about 98.4 equivalent %, said compact sintering temperature ranges from about 1890 C. to having an equivalent % composition of Y, Al, O about 1950 C., said aluminum nitride powder in said and N outside the composition defined and encom mixture has a specific surface area ranging from about passed by polygon FJDSR of FIG. 4, the alumi 3.6 m2/g to about 5.2 m2/g, and said sintered body has a num nitride in said compact containing oxygen in 35 porosity of less than about 1% by volume of said body. an amount ranging from greater than about 1.95% sk k k sk k

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