IIIHHHHHHHHHHHHHHH US005320990A United States Patent (19) 11 Patent Number: 5,320,990 Guiton et al. 45 Date of Patent: Jun. 14, 1994 54 PROCESS FOR SINTERING ALUMINUM gen Removal on the Thermal Conductivity of Alumi NITRDE TO A HIGH THERMAL num Nitride', J. Am. Ceram. so., 72(11) 2031-41 (1989). CONDUCTIVTTY AND RESULTANT Watari et al., "Sintering Chemical Reactions to Increase SINTERED BODES Thermal Conductivity of Aluminum Nitride", J. Mater. 75 Inventors: Theresa A. Guiton; Lynne K. Mills, Sci, 26 (1991) 4727-4732. both of Midland, Mich. Yagi et al., "Migration of Grain Boundary phases in AlN Ceramics by Heating in Reduced Atmosphere', 73 Assignee: The Dow Chemical Company, Nippon Ceramics Kyokai Gakujutsu Ronbunshi, 97(11) Midland, Mich. 1372-78 (1989). (21) Appl. No.: 39,657 Kuramoto et al. "Translucent AlN Ceramic Substrate', 22 Filed: Mar. 30, 1993 Proceedings, 36th ECC (1986). Kurokawa et al. "Highly Thermal Conductive AlN 51) Int. Cl...... C04B 35/58 Substrates', ISHM 1987 Proceedings. 52 U.S. C...... 501/98; 501/96; Mizutani et al., "Sintering of AlN', Bull of the Cer Soc. 501/153; 264/65; 264/66 of Japan. 58) Field of Search ...... 501/96, 98, 153; Watariet al., "Thermal Conduction Mechanism of Alu 264/65, 66 minum Nitride Ceramics", J. Mater. Sci 27 (1992) 56) References Cited 2627-2630. U.S. PATENT DOCUMENTS Geith, et al., "Thermal Conductivity of Calcium-doped Aluminum Nitride Ceramics', J. Mater Sci., 28 (1993) 4,778,778 10/1988 Mallia et al...... 501/96 865-869. 4,847,221 7/1989 Horiguchi et al. ... 50/98 4,897,372 1/1990 Huseby et al...... S01/96 Primary Examiner-Karl Group 4,952,535 8/1990 Merkel ...... 501/98 X Attorney, Agent, or Firm-D. R. Howard 5,034,357 7/1991 Yamakawa et al...... 501/98 X 57 ABSTRACT OTHER PUBLICATIONS Guiton et al., "Optimiazation of Aluminum Nitride Prepare sintered aluminum nitride bodies having a ther Thermal Condictivity Via Controlled Powder Process mal conductivity of at least 200 W/m-K by sintering ing", Mat. Res. Soc. Syump. Proc. vol. 271 (1992). under non-reducing conditions and controlling interre Miyashiro et al., "High Thermal Conductiviy Alumi lated parameters such as binder burnout atmosphere, nun Nitide Ceramic Substrates and Packages', IEEE heating rate, sintering temperature, time at sintering Trans. on Comp., Hybrids, and Manu. Tech. vol. 13, temperature, cooling rate and cooling temperature. No. 2, Jun. 1990. Sintered bodies having thermal conductivities in excess Buhr et al., "Phase composition, Oxygen Content, and of 200, especially in excess of 270, W/m-K result from Thermal Conductivity of AlNCY2O3) Ceramics', J. Am this process. Ceram. So., 74(4) 718-23 (1991). Virkar, "Thermodynamic and Kinetic Effects of Oxy 13 Clains, No Drawings 5,320,990 1. 2 from admixtures of AlN powder and Y2O3 powder and PROCESS FOR SINTERING ALUMNUM sinter the wrapped bodies attemperatures of 1773, 1873, NITRDE TO A HIGH THERMAL CONDUCTIVITY 1973, 2073 and 2173K for one hour in a 0.1 MPa nitro AND RESULTANTSNTERED BODES gen gas atmosphere. They also sinter at 2173K in the same atmosphere for two, three and five hours. They BACKGROUND OF THE INVENTION use a heating rate of 15K/minute and report thermal The present invention relates generally to a process conductivities as high as 220 W/m-K. for preparing sintered aluminum nitride bodies and to T. A. Guiton et al., in "Optimization of Aluminum bodies resulting from the process. The present invention Nitride Thermal Conductivity Via Controlled Powder relates more particularly to a process for preparing O Processing', Mat. Res. Soc. Symp. Proc, Vol 271, sintered aluminum nitride bodies having a thermal con 851-56 (1992), suggest that thermal conductivity is ductivity as high as about 285 watts/meter.'K (W/m- strongly dependent on oxygen chemistry and sintering K). parameters. They disclose two sets of sintering parame Aluminum nitride (AlN) is subject to increasing inter ters, denominated as "Cycle 1" and "Cycle 2" in Table est as a microelectronic substrate material. With a ther 15 II (page 852). Cycle 2 includes a heating rate of 2.5 nal conductivity approaching that of berylia (BeO) and C./min, a sintering temperature of 1850 C., a sintering a thermal expansion coefficient well matched to silicon, time of 3 hours, a cooling rate of 1 C/min and a cool AlN represents an attractive alternative in high power ing temperature of 1500' C. or multi-chip module applications. Y. Kurokawa et al., in "Highly Thermal Conductive At room temperature, single crystal AlN has a theo 20 Aluminum Nitride Substrates', ISHM Proceedings, retical thermal conductivity of 319 W/m-K. Polycrys 654-61 (1987), report thermal conductivity measure tailine ceramics tend to have lower thermal conductivi ments of 160 to 260 W/m-K for AlN substrates. They ties than single crystal AN due to a number of factors. prepare substrates by firing an admixture of AlN pow The factors include random orientation of AlN grains, der and a CaC2 powder reductant at 1900 C. crystalline lattice impurity levels and existence of crys 25 talline grain boundary phases with even lower thermal U.S. Pat. No. 4,847,221 discloses a process for prepar conductivities. ing sintered AlN bodies as well as the resultant bodies. F. Miyashiro et al., in “High Thermal Conductivity The process comprises firing an admixture of AlN pow Aluminum Nitride Ceramic Substrates and Packages', der and one or more rare earth compounds in an amount IEEE Transactions on Components, Hybrids, and Manu 30 of 0.01 to 15 wt.% in a reducing atmosphere at a tem facturing Technology, Vol 13, No. 2, 313-19 (June 1990), perature of 1550° C. to 2050 C. for four hours or more. suggest that three key technologies are very important The reducing atmosphere preferably contains at least if one is to realize the highest thermal conductivity by one of CO gas, H2 gas, and C (gaseous or solid phase). sintering. The technologies are: reducing or minimizing The resultant bodies have thermal conductivities as oxygen content of AlN powders; proper choice and 35 high as 272 W/m-K. quantity of additives; and sintering conditions in terms U.S. Pat. No. 4,778,778 reports, at column 2, lines of temperature, time and atmosphere. They show, in 10-26, a particular sintering cycle that is described in a FIGS. 8-10: sintering at 1800 Centigrade ("C.) and copending application. The cycle provides high thermal 1900 C. for two hours with three weight percent conductivities without using very high purity aluminum (wt-%) yttria (Y2O3); sintering at 1800° C. in the pres nitride powder. The cycle includes: increasing the tem ence of varying amounts of Y2O3; and sintering at 1800 perature of a compacted AlN body from room tempera C. for 24 hours with three wt-% Y2O3 with and without ture to a sintering temperature at a rate of no more than a reductive atmosphere. They suggest, in FIGS. 10 and 250 C. per hour (C./hr); sintering the body in an inert 1, that a reducing atmosphere yields the highest ther atmosphere at a temperature of 1600 C. to 1900 C.; mal conductivity 45 and cooling the sintered body at a rate of no more than H. Buhr et al., in "Phase Composition, Oxygen Con 300 C./hr. The '778 patent discloses an improvement tent, and Thermal Conductivity of AlNCY2O3) Ceram upon this cycle. The improvement includes introducing ics", J. Am, Ceram, Soc. 744), 718-723 (1991), disclose an amount of hydrogen gas along with the inert gas up sintering cold isostatically pressed cylindrical compacts to a temperature of 1200° C., after which pure inert gas under a pressure of 0.2 MPa nitrogen. They employ 50 is introduced. The heating rate is 10° C./hr to 200 heating rates of 16 to 30K per minute, sintering temper C./hr, preferably 20' C./hr to 80 C./hr. The cooling atures of 1750 to 1790 C., and sintering times of be rate is preferably between 100° C./hr and 275 C./hr. tween one and three hours. The sintering cycle used in the example is as follows: A. V. Virkar et al., in "Thermodynamic and Kinetic 25 C./hr to 800° C., 33 C./hr to 1000 C., 80 C./hr to Effects of Oxygen Removal on the Thermal Conductiv 55 1500 C., 300 C./hr to 1800° C., soak for six hours, and ity of Aluminum Nitride', J. Ann. Ceram. Soc. 721 1, cooldown at 140 C./hr. 2031-42 (1989), fabricate polycrystaline AlN ceramics with various rare earth oxides and alkaline earth oxides. SUMMARY OF THE INVENTION They sinter/anneal samples of the ceramics at 1850 C. One aspect of the present invention is a sintered poly for up to 1000 minutes. They obtain thermal conductivi crystalline aluminum nitride ceramic body having a ties as high as 200 W/m-K. thermal conductivity at room temperature (25 C.) of K. Watariet al., in "Sintering Chemical Reactions to greater than 270 W/m-K. Increase Thermal Conductivity of Aluminum Nitride', A second aspect of the present invention is an im J. Mater. Sci. 26, 4727-32 (1991), discuss chemical reac proved process for preparing a sintered polycrystalline tions to increase thermal conductivity by decreasing 65 aluminum nitride body having a thermal conductivity oxygen contents during AlN sintering with an Y2O3 of greater than about 200W/m-K by heating an admix additive in a reducing nitrogen atmosphere with car ture of aluminum nitride powder and at least one pow bon. They wrap cold isostatic pressed bodies formed dered sintering aid in the presence of nitrogen gas to a 5,320,990 3 4 sintering temperature, holding the admixture at that dures such as attritor milling and wet and dry ball mill temperature for a period of time sufficient to convert ing, Wet ball milling with an appropriate solvent and the admixture to a sintered body, and thereafter cooling suitable milling media provides satisfactory results. the body to ambient temperature, the improvement Milling media, usually in the form of cylinders or balls, comprising a combination of heating to the sintering should have no significant adverse effect upon admix temperature at a rate of from greater than O' C. per ture components or upon sintered bodies prepared from minute to about 6 C. per minute, maintaining that tem the admixture. A liquid milling or mixing medium such perature for a period of time sufficient to convert the as ethanol, heptane or another organic liquid may be admixture to a sintered body having a density of greater used. After mixing, the organic liquid may be removed than about 95 percent of theoretical density, and cool O by conventional procedures to yield an admixture suit ing the sintered body in the presence of a vacuum or an able for conversion to ceramic greenware. Oven drying inert gas from the sintering temperature to a tempera and spray drying produce satisfactory results. ture of about 1400° C. at a rate of from greater than 0 An organic binder may also be used during process C./minute to about 6' C./minute before cooling the ing of the admixture into a sintered body. Suitable bind sintered body further to ambient temperature (about 25° 15 ers are well known in the art and typically comprise C.). high molecular weight organic materials that are solu A third aspect of the present invention centers upon ble in organic solvents. Illustrative binders include the sintered body resulting from the process of the sec polyethyloxazoline, industrial waxes such as paraffin, ond aspect. highly viscous polyglycols, polymethylmethacrylate 20 and polyvimyl butyral. Polyethyloxazoline is particu DESCRIPTION OF PREFERRED larly suitable. The binder is suitably added to admixture EMBODIMENTS components prior to milling. AlN powder suitable for purposes of the present Ceramic greenware may be prepared by any one of invention may be of commercial or technical grade. It several conventional procedures such as extrusion, in should not contain any impurities that would have a 25 jection molding, die pressing, isostatic pressing, slip significant adverse effect upon desired properties of a casting, roll compaction or forming or tape casting to resulting sintered product. Although some level of im produce a desired shape. Particularly satisfactory re purities is present in commercial powders, that level sults are obtained by dry pressing a spray dried admix should be less than that which produces the aforemen ture or tape casting a slurry. tioned adverse effect. 30 The ceramic greenware is desirably subjected to con The AlN powder typically has a bound oxygen con ditions sufficient to remove the organic binder prior to tent of less than about 4 wt-%. The oxygen content is sintering. Binder removal, also known as binder burn desirably less than about 3 wt-% and preferably less out, typically occurs by heating the greenware to a than about 2 wt.%. temperature that ranges from about 50 C. to about The AIN powder also typically has a surface area, 35 1000° C. to pyrolyze, or thermally decompose, the measured by conventional B.E.T. absorption methods, binder. The temperature varies depending upon the of from about 1.5 to about 10 square meters per gram binder. Thermal decomposition may be carried out at or (m2/g). The powder surface area is desirably from near ambient pressure or in a vacuum. It may be carried about 2 to about 9 m2/g. out in the presence of atmospheric air or in a nonoxidiz AlN powder meeting these specifications are prefera ing atmosphere. The nonoxidizing atmosphere is desir bly prepared either by carbothermal reduction of alu ably established with an inert gas. The inert gas is suit mina (Al2O3) or direct nitridation of aluminum metal. ably nitrogen, a source of nitrogen such as ammonia, or AlN powders may also be prepared by other processes a noble gas such as argon. The inert gas is preferably using aluminum alkyls or aluminum halides. Preferred nitrogen. As a general rule, binder burn out in the pres carbothermal AlN powders are available from The 45 ence of an inert gas such as nitrogen yields a higher Dow Chemical Company under the trade designation residual carbon level than binder burn out in the pres XUS 35544 and XUS 35548 or Tokuyama Soda under ence of atmospheric air. Binder burnout in the presence the trade designations Grade F and Grade H. Mixtures of nitrogen is preferred for purposes of the present of these and other powders may also be used. invention. The AlN powder may be mixed with any of the art 50 Sintered polycrystalline aluminum nitride bodies hav recognized sintering aids such as, for example, oxides or ing a thermal conductivity of greater than about 200 fluorides of metals selected from the group of yttrium; W/m-K are suitably prepared under nonoxidizing con rare earth metals such as lanthanum, cerium, praseo ditions by combining several, interrelated sintering pa dymium, neodymium, samarium, gadolinium, and dys rameters. The parameters are heating rate, sintering prosium; and alkaline earth metals such as calcium, 55 temperature, time at sintering temperature, cooling rate, strontium and barium. A combination of sintering aids cooling temperature, cooling environment and type and may be used in place of a single sintering aid. Yttrium amount of sintering aid. compounds, particularly yttria, yield satisfactory re Sintering of the greenware occurs in the presence of sults. gaseous nitrogen or a source of gaseous nitrogen and is The sintering aid or combination of sintering aids is 60 followed by cooling in a nonreducing environment. The suitably admixed with AlN powder in an amount of latter may be established by using either an inert gas or from about 0.05 wt-% to about 10 wt-%, based upon a vacuum. The inert gases described as suitable for combined weight of sintering aid and AlN powder. The binder burn out are also suitable for use in this aspect of amount is desirably from about 0.05 wt-% to about 3 the process. One means of establishing a nonreducing wt-%. The sintering aid suitably has a surface area simi 65 environment, at least in part, includes placing the green lar to that of the AlN powder. ware into a crucible fabricated from a non-reducing Preparing an admixture of AlN powder and the sin material prior to sintering and cooling. The non-reduc tering aid(s) may be carried out by conventional proce ing material is desirably selected from the group con 5,320,990 5 6 sisting of boron nitride, aluminum nitride, molybdenum metal, and tungsten metal. Boron nitride and aluminum TABLE II-continued nitride are preferred non-reducing materials for a Sinter Sinter Heat Cool Cooling Condi- Temp Time Rate Rate Temp graphite furnace. Molybdenum metal or tungsten metal tion (C) (min.) (C./min) (C./min) (C) are preferred non-reducing materials for a tungsten 5 furnace. 2 1850-1875 80-180 0.6 s0.2 s1275 3 1850-1875 80-180 0.8 0. s1275 Because the parameters of heating rate, cooling rate, 4 1850-1875 80-180 s.4 0.1 1160 sintering temperature, time at sintering temperature, 5 1850-1875 80-180 is 1.7 0.1 1160 cooling temperature and amount and type of sintering 6 1850-875 80-80 0.6 s0.6 1160 aid are closely interrelated, several, but not all, parame 10 7 1850-1875 30-180 0.6 S0.4 60 ter combinations lead to thermal conductivities of 200 8 1850-1875 30-180 s1 0.4 1160 W/m-K or more. As an additional consideration, a 9 1863 165 s3 0. is 1400 given combination of parameters may produce such a thermal conductivity in one aluminum nitride powder The combinations shown in Tables I and II are partic but not in another. This disparity stems from variations 15 ularly suitable when the powder is XUS 35548 (The in powder properties, particularly impurity levels, and Dow Chemical Company). Some modification of the in methods of synthesizing powders as in carbothermal combinations may be needed depending upon factors synthesis versus direct nitridation. As a further consid such as oxygen content of the AlN powder, residual eration, differences in binder burn out atmosphere lead carbon content of the AlN greenware and amount and to modifications of desirable parameter combinations. 20 type of sintering aid present in the greenware. If, for Using a single AlN powder as an example, various example, the powder has an oxygen content at or near parameter combinations for nitrogen binder burn out the upper limit of the specification and the amount of (BBO) and air BBO that produce a thermal conductiv Y2O3 is 3 wt.% in Table I or 2 wt-% in Table II, modifi ity of 200 W/m-K or more using 3 wt-% Y2O3 as a cation may be necessary only for condition 9 of Table sintering aid are shown in Table I. The AlN powder is 25 II. As another example, modification of conditions 1,7, commercially available from The Dow Chemical Com 9, 11 and 14 of Table I may also be necessary when such pany under the trade designation XUS 35548 and has a powder is used in combination with a lesser amount, the following specification: an oxygen content of such as 2 wt-%, of Y2O3. The modification, particularly 0.8-0.2 wt.%; a carbon contents 0.08 wt.%; a silicon with respect to Table I, may be as simple as lowering contents 100 parts per million (ppm); a calcium con 30 tents.200 ppm; an iron contents 35 ppm; and a surface the upper limit of the cooling rate by less than 1" area of 2.80.2 m2/g. C./min. If the same powder has an oxygen content at or Parameter combinations for nitrogen BBO that pro near the lower limit of the specification, the lesser duce a thermal conductivity of 270 W/m-K or more amount of Y2O3 may be used with little or no modifica using the same AlN powder and either 2 wt.% or 3 35 tion of the combinations shown in Tables I and II. Fur wt-% Y2O3 as a sintering aid are shown in Table II. The ther modifications may be made without undue experi amount of Y2O3 varies in direct proportion to the AlN mentation. powder's oxygen content. In other words, with an oxy The combinations shown in Tables I and II generally gen content at or near 0.6 wt-%, 2 wt-% Y2O3 suffices suffice to yield a sintered body having a density of whereas an oxygen content at or near 1.0 wt-% requires greater than about 95 percent of theoretical density. as much as 3 wt.% Y2O3. The actual level of sintering The density is desirably297.5%, preferably 2.99% and aid is tied, at least in part to residual carbon levels, and more preferably 299.5% of theoretical density. may be readily determined without undue experimenta Sintered AlN bodies prepared under nonreducing tion. conditions in accordance with the process of the present 45 invention have a thermal conductivity in excess of TABLE I about 200 W/m-K. The thermal conductivity is desir BBO Sinter Sinter Heat Cool Condi- Atmos- Temp Time Rate Rate ably greater than about 240W/m-K, preferably greater tion phere ("C.) (min.) ("C./min) (C./min) than about 270 W/m-K. The thermal conductivity is 1 N2 1817 65 is 1.2 s5.4 50 also desirably about 319 W/m-K or less. A thermal 2 N2 1817 165 SO.6 s5.4 conductivity of about 285 W/m-K or less is readily 3 N2 1817 65 S5.4 0.1 attainable. 4. N2 1817 165 s5.4 s.1 5 N2 1817 s278 s5.4 0.1 Sintered AlN bodies prepared as described herein 6 N2 1862 165 s5.4 s5.5 also display color/translucency combinations that 7 N2 1908 238 s2.3 s5.5 55 range from light cream and translucent to dark gray or 8 N2 1908 238 is 1.6 s5.5 even black and opaque. Surface appearance, also known 9 N2 1908 233 1.6 s5.5 10 N2 1908 165 s5.4 s0.5 as mottling (marked with spots or blotches of different l N2 1908 65 3 is 1.1 color or shades of color as if stained), also varies from a 12 N2 1908 165 0.6 s5.5 high degree of mottling to an absence of visually detect 13 Air 1862 65 is .3 0.1 able mottling. Skilled artisans can attain a desired com 14 Air 1862 165 SO.8 s0.5 bination of color, degree of mottling and thermal con 5 Air 1862 165 s0.6 sO.5 ductivity without undue experimentation. The following example is solely for purposes of illus TABLE II tration and is not to be construed, by implication or Sinter Sinter Heat Cool Cooling 65 otherwise, as limiting the scope of the present invention. Condi- Temp Time Rate Rate Temp All parts and percentages are by weight and all temper tion (°C.) (min.) ("C./min) ("C./min) ("C.) atures are in degrees Centigrade (C) unless otherwise l i850-1875 80-80 0.6 0. is 650 specified. 7 S EXAMPLE TABLE V-continued Ceramic greenware is prepared from admixtures of Sintering Package Design 2000 g quantities of various AlN powders, either 2 or Soak soak Cooling 3% (41 g or 62 g) Yaos (Molycorp, 99.99% purity) and 5 Sinter Heating Temper- Time Cooling Temper Total 6.7% (134.2 g) of a binder composition. The binder g Rate ature (min- Rate ature Tine composition is a 33/67 weight ratio blend of polye- Run ("C./min) ("C.) utes) ("C.Amin) (C) (hours) thyloxazoline and polyethylene glycol 3350 (The Dow s 3 862.5 65 2.8 400 2.8 Chemical Company). The binder is dissolved in 3000 g 6 10 30 600 20.9 of ethanol after which the AlN and Y2O3 powders are 10 7 3. 862.5 329 2.8 400 16.7 s 8 S 900 30 0.5 200 30.9 added. The admixtures are ball milled for five hours. 9 5 825 30 0.5 600 13.7 The solvent is removed by spray drying. O 900 30 5 200 is TABLE III 11 3 862.5 2.8 400 3 2 3. 862.5 65 2.8 157 4.2 Powder Data s 15 13 3. 862.5 6S O. 400 88 Powder O. C Si Ca Fe S.A. 4. S 825 30 3 600 A. - P - two to Po Pm) Po re. 5 5 825 300 0.5 200 31.6 A. 0.63 0.03 77 58 17 2.75 6 S 900 300 0.5 600 21.2 04 0.05 4. O7 17 2.86 7 3 1862.5 E.65 2.8 63 1.3 5 . 88: 2. . 20 18 3 862.5 65 2.8 400 2.8 E 6 : : 220 22 9 5 3 200 3.3 20 825 30 0.5 200 35 2. 3 1817 65 2.8 400 13.4 The dried powders are dry pressed into greenware 22 0.6 862.S 65 2.3 400 26.8 using a 7/8 inch (2.2 cm) round die under uniaxial pres- 23 1825 300 5 1200 20.7 sure at 6.9 megapascals (MPa). The binder composition 25 24 1900 30 O.S 600 25.6 is removed from the greenware in the presence of eithero 2526 3 1825908 6530 2.8S 400600 14.84.5 air (air BBO) or nitrogen (N2 BBO). Binder removal 27 3 1862.5 165 5.5 400 114 employs a heating rate of 2 C./min up to 550 C., a one 28 3. 1862.5 65 2.8 i400 2.8 hour hold at that temperature and a cooling rate of 2 29 5.4 862.5 65 2.8 400 2 C./min down to room temperature (25 C.). 30 1 1900 300 0.5 1200 43.1 The AlN powders and their chemical composition are shown in Table III. AlN powders A and B are dif ferent lots of powder commercially available from The TABLE V Dow Chemical Company under the trade designation 35 Measured Thermal Conductivities (W/m-K) for Sintered XUS 35548. AN powder C is a powder commercially s Parts Prepared Fron Nitrogen Lebindered Greenware available from The Dow Chemical Company under the Sin trade designation XUS 35544. AlN powders D and E tet are commercially available from Tokuyana Soda Co., ing Ltd. as, respectively, grades F and H. Run A-2 B-2 B-3 C-3 O-3 E-3 A boron nitride (BN) box measuring 7.5 inch by 4.5 is is loos zoo to zoos inch by 3.5 inch (19.0 cm by 11.4 cm by 8.9 cm) is used 2 17-218 208-213 208-212 206-211 06 197-98 as a container to establish a non-reducing environment. 3 189-93 189-95, 192-196 183-84 89 180-182 The greenware resulting from binder removal is placed 4, 181-81 160-64 80-81 73-74 180 188-189 in boron nitride setters, one for air BBO and one for N2 S 218-223 25-29 213-27 217-21.8 22A 207-214 BBO. A one cubic foot (0.028 cubic meter) graphite 45 2, fi 25. i. 3. 2. furnace (Thermal Technology Model 121212G) is used 8 201-202 184-85 204-209 204-207 99 2.05-208 for sintering. s 9 208-20 214.215 208-210 210-23 214 204-206 The conditions used for sintering are shown in Table 10 213-225, 209-23 205-206. 209-22 214. 98-198 IV. Table V shows the results of sintering N2 BBO 50 11 82-186 65-67 81-8 i7-18, 182 92-95 greenware. Table VI shows the results of sintering air 2 233-233 26-223 28-29 214-222 230 216-29 BBO greenware. Column headings in Tables V and VI, 13 269-274. 258-267 274-277 260-270 285 256-263 such as A-3 or B-2 refer to the AlN powder type (Table 4 24-22, 206-206. 208-213 205-205 20 202-199 III) before the hyphen and the amount of sintering aid, 15 18-200 187-206. 212-22, 186-94 20, 208-208 in weight percent, after the hyphen. Two thermal con- 6 204-208 206-207 201-204 97-200 2 198-199 ductivity values are shown for most sintering runs in 55 17 213-217 215-25 209-23 206-209 216 202-204 Tables V and VI. This reflects measurements made on 18 217-220 221-223 22-24 207-209 226 205-205 .. 19 228-23 232-236 221-228 26-220 253 216-27 two different pieces of greenware made from a single 20 242-245 244-244. 245-247 236-236 240 229-229 admixture and sintered at the same time. 21 189-195 177-178 187-193 177-182 189 193-198

TABLE IV 2322 222-23223-223 217-227225-230 26-29222-224 220-22223-27 217223 24-215208-208 Sintering Package Design 24, 237-238 225-228 22-225 223-224, 225 24-222 Soak Soak Cooling 25 193-96 80-186 199-203 92-193 96 196-202 sing: Hsing Ther R cling Tip- 26 197-203 97-199 98-200 9-193 190 84-85 - Ren3 (S/min)ate ()ture isin KateAmin (S)o - (965 282, 217-220208-20 202-203 205-20,208-25 2.05-206.206-207 203205 202-20398-199 825 3OO 0.5 1600 26 29 28-220 200-204 2-21, 203-210 203 202-205 2 3 1862.5 65 2.8 1400 2.8 30 235-235 225-233 227-22.8 229-235 235 28-221 3 5 825 30 5 200 8.3 s Researansmitmarxta:8.88waxwomnica 5 1900 30 5 GOO 8.6 5,320,990 9 10 TABLE VI microscopy reveals the presence of crystalline AlN and Measured Thermal conductivities (W/m-K) For Sintered secondary boundary phases. The boundary phases con Parts Prepared From Air Debindered Greenware tain Y4Al2O9, YAlO3 or both. The boundary phase may Sin be along grain boundaries, at triple points or both. The ter analysis does not reveal the presence of yttrium nitride. ing Similar results are expected with these powders and Run A-2 B-2 B-3 C-3 D-3 E-3 the sintering conditions of Tables I and II. Although 1 78-179 58-166 180-185 179-87 182 82-83 2 160-163 153-15S 17-172 167-75 64 69-170 some changes in sintering conditions may be required, 3 153-53 34-135 61-164. 162-64 6 58-162 similar results are also expected with other AlN pow 4 138-14 125-125 161-161 147-148 148 160-161 O ders. 5 63-66 154-155 179-81 169-170 75 173177 What is claimed is: 6 82-187 169-177 186-89 88-89 183 182-185 7 57-160 44-145 175-179 0-163 167 173-73 1. An improved process for preparing a sintered poly 8 162-164. 152-15S 179-182 67-70. 68 8-185 crystalline aluminum nitride body having a thermal 9 182-183 159-166 185-194 184-89 82 78-179 conductivity of greater than about 200 watts/meter.' K 10 165-66 153-153 180-183 168-169 71 175-177 15 by heating an admixture of aluminum nitride powder 146-47 120-123 62-163 154-159 159 164-66 2 67-67 156-60 186-188 18-182 168 180-84 and at least one powdered sintering aid in the presence 13 207-21 198-200 217-27 24-214 28 206-207 of nitrogen gas to a sintering temperature, holding the 14 177-182 162-165 179-182 18-184 183 78-186 admixture at that temperature for a period of time suffi 15 179-85 169-70 85-189 172-77 175 188-194 cient to convert the admixture to a sintered body, and 16 138-14 125-134 49-152 133-140 13 135-147 20 7 S9-161 148-151 172-177 63-164. 162 162-173 thereafter cooling the body to ambient temperature, the 18 161-64. 154-156 175-76 64-168 164 171-76 improvement comprising a combination of heating to S 171-180 65-70 179-18 171-79 193 180-184 the sintering temperature at a rate of from greater than 20 177-189 178-184 192-193 186-92 180 192-199 21 65-68. 140-139 171-174 170-178 177 172-175 0 C. per minute to about 6' C. per minute, maintaining 22 172-73 160-162 186-87 175-178 175 179-183 that temperature for a period of time sufficient to con 23 176-176 166-172 185-190 75-177 189 82-190 25 vert the admixture to a sintered body having a density 24, 177-179 75-182 90-19 78-186 176 185-86 of greater than about 95 percent of theoretical density, 25 156-57 145-48 75-179 59-61. 163 172-175 26 167-167 49-150 169-175 174-176 177 170-71 and cooling the sintered body in the presence of a vac 27 61-62 53-154 174-182 165-96 173 174-75 uum or an inert gas from the sintering temperature to a 28 164-167 156-159 181-181 74-75 77 178-81 temperature of about 1400° C. at a rate of from greater 29 165-165 153-158 175-181 170-70 70 77-180 30 than 0° C./minute to about 6' C./minute before cooling 30, 188-196 18-85 200-200 187-194. 95 192-192 the sintered body further to ambient temperature, 2. The process of claim 1 wherein the admixture is The data in Table V show that a thermal conductivity converted to ceramic greenware prior to sintering. of 200 W/m-K or more is readily attainable under most 3. The process of claim 1 wherein cooling is accom of the sintering conditions of Table IV. In fact, sintering 35 plished in the presence of an inert gas. run 13 provides thermal conductivities of 270-285 4. The process of claim 3 wherein the inert gas is W/m-K for several powders. Sintering runs 3, 4, 11 and selected from the group consisting of nitrogen, argon, 21 are notable exceptions in that the sintering conditions and helium. fail to yield any sintered materials with a thermal con 5. The process of claim 1 wherein the rate of cooling ductivity of 200W/m-K or more. Sintering runs 14, 25 40 is from about 0.1 C./minute to about 5.5 C./minute. and 26, for example, show that some variability in ther 6. The process of claim 1 wherein the sintered body is mal conductivity is present even when two pieces of further present in a non-reducing environment during substantially identical greenware are subjected to the cooling in the presence of a vacuum or an inert gas. same sintering conditions. Sintering runs 2, 7, 8, 10, 15, 7. The process of claim 6 wherein the non-reducing 16 and 27 show that variations in powder properties 45 environment is established by placing the admixture lead to differing results under identical sintering condi into a crucible fabricated from a non-reducing material tions. Based upon the number of sintering run/powder prior to sintering and cooling. combinations that do provide a thermal conductivity of 8. The process of claim 7 wherein the non-reducing at least 200 W/m-K, combinations that do not are material is selected from the group consisting of boron readily modified to at least that level by skilled artisans SO nitride, aluminum nitride, molybdenum metal and tung without undue experimentation. sten metal. The data in Table VI show that a thermal conductiv 9. The process of claim 1 wherein the sintering tem ity of 200W/m-K or more is rarely attainable under the perature is within a range of from about 1817 to about conditions shown in Table IV. The success of sintering 1908 C. run 13 suggests that a skilled artisan might make modifi 55 10. A sintered polycrystalline aluminum nitride body cations such as an increase in amount of sintering aid or having a microstructure characterized by a crystalline a decrease in cooling rate, heating rate or both to aluminum nitride phase and secondary grain boundary achieve a thermal conductivity of at least 200 W/m-K. phases and a thermal conductivity of greater than about A comparison of run 13 from Table V with run 13 from 270 watts/meter.' K. Table VI suggests that a thermal conductivity in excess 11. The body of claim 10 wherein the thermal con of about 270 W/m-K may not be attainable when an air ductivity is less than about 319 watts/meter.K. BBO procedure is used with those combinations. How 12. The sintered body of claim 10 wherein the grain ever, with elevated levels of Y2O3 or some other appro boundary phases comprise at least one yttrium-alumi priate sintering additive, such thermal conductivities nate selected from Y4Al2O9 and YAlO3. may be possible. 65 13. The sintered body of claim 10 wherein the grain Analysis of all of the sintered bodies, including those boundary phases are located along grain boundaries, at having a thermal conductivity greater than about 270 triple points or both. W/m-K, by powder X-ray diffraction and electron s 28