WO1993001147A1

WO1993001147A1 - Aluminum nitride densification with minimal grain growth

Info

Publication number: WO1993001147A1
Application number: PCT/US1992/005816
Authority: WO
Inventors: Yi-Hung Chiao
Original assignee: The Dow Chemical Company
Priority date: 1991-07-08
Filing date: 1992-07-08
Publication date: 1993-01-21
Also published as: NO940066D0; IL104859A0; FI940066A; CA2103048A1; EP0593679A1; FI940066A0; JPH06508815A; NO940066L

Abstract

A method is disclosed of forming a dense, high thermal conductivity aluminum nitride article. A powder compact includes aluminum nitride powder and a densifying composition. The densifying composition includes YF3, Al2O3 and either Y2O3 or CaF2. The powder compact is heated to a temperature of less than 1650 °C, in a suitable atmosphere and for a time sufficient, to cause densification of the powder compact without a significant increase of the grain size of the powder compact, thereby forming a dense, high thermal conductivity aluminum nitride article.

Description

ALUMINUM NITRIDE DENSIFICATION WITH MINIMAL GRAIN GROWTH

Aluminum nitride has many structural and electrical applications. Aluminum nitride articles that have high density and high thermal conductivity are generally produced by densifying a powder compact that includes aluminum nitride powder and at least one densifying additive.

One method of densifying aluminum nitride powder includes forming a powder compact of aluminum nitride and a densifying additive that includes one or more oxides of a Group Ha or a Group Ilia element. However, such compacts typically must be heated to a temperature above 1800°C in order to form a dense, high thermal conductivity aluminum nitride article. Aluminum nitride densified at temperatures above 1800°C often has a grain size that is significantly larger than that of aluminum nitride in the powder compact. Grain size influences physical properties of dense aluminum nitride articles. A relatively large grain size can adversely affect properties such as thermal shock resistance, strength and fracture toughness.

In another method, the densifying additive includes at least one fluoride of a Group Ha or a Group Ilia element. These densifying additives generally have lower melting points than do Group Ha and Group Ilia oxides. However, they often volatilize at temperatures which are lower than are required to obtain sufficient densification of aluminum nitride. Also, Group Ha and Group Ilia fluorides typically exhibit poor wetting of aluminum nitride powder and are often relatively hygroscopic, thereby limiting their usage in densification of aluminum nitride powder.

Thus, a need exists for an improved method for densifying aluminum nitride which overcomes or minimizes the aforementioned problems.

One aspect of the present invention is a method of forming a sintered body of aluminum nitride that comprises heating, following removal of binder in an atmosphere that favors subsequent densification by sintering to at least 90 percent of theoretical density, a powder compact formed from a binder and a homogeneous powdered admixture of AIN, AI2O3, YF3 and Y2O3 to a sintering temperature within a range of from 1500°Cto 1650^cC ata heating rate sufficient to form a densifying amount of a high temperature yttrium-aluminum oxyfiuoride sintering liquid and maintaining that temperature for a period of time sufficient yield a sintered body having a grain size that is less than one order of magnitude larger than that of the aluminum nitride powder, a density greater than or equal to 90 percent of theoretical density and a thermal conductivity greater than or equal to 100 W/m-K and containing, when cooled, aluminum nitride and a grain boundary phase that includes yttrium oxyfiuoride and yttrium aluminate, the admixture containing, based upon admixture weight, AI in an amount within a range of from 88 to^"99 percent by weight and, as sintering additive a combined total amount of AI2O3, YF3 and Y2O3 within a range of from 1 to 12 percent by weight, the sintering additives having a composition defined and encompassed by areas I and of Figure 1 wherein, based upon total weight of sintering additives with all weight fractions totaling 1.0, the AI2O3 weight fraction ranges from 0.04 to 0.36 for area I and from 0.72 to 0.8 for area II, the Y2O3 weight fraction ranges from greater than 0 to 0.68 for area I and from greaterthan O to 0.06 for area II, and the YF3 weight fraction ranges from 0.18 to 0.88 for area and from 0.16 to 0.28 for area II.

Asecond, related aspect of the invention is a method of forming a sintered body of aluminum nitride that comprises heating, following removal of binder in an atmosphere that favors subsequent densification by sintering to at least 90 percent of theoretical density, powder compact formed from a binder and a homogeneous powdered admixture of AIN, AI2O3, YF3 and CaF2 to a sintering temperature within a range of from 1500°Cto 1650°C at a heating rate sufficient to form a densifying amount of a high temperature yttπum-calcium-aluminum oxyfiuoride sintering liquid and maintaining that temperature for period of time sufficient to yield a sintered body having a grain size that is less than one order of magnitude larger than that of the AIN powder, a density greater than or equal to 90 percen of theoretical density and a thermal conductivity greater than or equal to 100 W/m K and containing, when cooled, aluminum nitride and a grain boundary phase that includes yttrium oxyfiuoride and yttrium aluminate or calcium aluminate or both, the admixture containing, based upon admixture weight, AIN in an amount within a range of from 88 to 99 percent by weight and, as sintering additives, a combined total amount of AI2O3, YF3 and CaF₂ within a range of from 12 to 1 percent by weight, the sintering additives having a composition defined and encompassed by areas I and II of Figure 2 wherein, based upon total weight of sintering additives with all weight fractions totaling 1.0, the AI2O3 weight fraction ranges from 0.08 to 0.30 for area I and 0.08 to 0.68 for area II, the CaF2 weight fraction ranges from greater than 0 to 0.24 for area i and from greaterthan O to 0.68 for area II, and the YF3 weight fraction ranges from 0.57 to 0.92 for area I and from 0.01 to 0.56 for area II.

This invention has many advantages. The invention allows densification of the powder compact without significantly increasing the aluminum nitride grain size. The rate of densification can also be increased at a given sintering temperature. In addition, the sintering temperatures suitable for use in the present invention allow sintering in conventional alumina sintering furnaces. Also the combination of high thermal conductivity and relatively small grain size increases the performance of aluminum nitride articles in many applications such as in high-wattage, high-circuit density electronic packaging.

Figure 1 is a ternary composition graph showing combinations of YF3, AI2O3 and Y2O3 that, when used as sintering additives for aluminum nitride, provide a density greater than or equal to 90 percent of theoretical density and a thermal conductivity of greater than or equal to 100W/m K at sintering temperatures within a range of from 1500°Cto 1650°C, preferably 1525°C to 1625°C. The combinations are identified as regions or domains I and II. Figure 2 is a ternary composition graph showing combinations of YF₃ AI2O3 and CaF2 that, when used as sintering additives for aluminum nitride, provide a density greater than or equal to 90 percent of theoretical density and a thermal conductivity of greater than or equal to 100W/m K at sintering temperatures within a range of from 1500°Cto 1650°C, preferably 1525°C to 1625°C. The combinations are identified as regions or domains I and II. The above features and other details of the invention will now be more particularly described and pointed out in the claims. The particular embodiments of the invention are shown by way of illustration and not as limitations of the invention. The principle features of this invention may be employed in various embodiments without departing from the scope of the invention.

A powdered admixture suitable for purposes of the present invention is homogeneous and comprises AIN powder and a densifying composition or combination of sintering additives. Suitable AIN powder includes AIN powder that, when combined with a suitable densifying composition and heated to a temperature, and for a period of time, sufficient to densify the Al N powder, will densify without a significant increase of the average grain size of the powder compact. "Significant increase", as used herein, means less than one order of magnitude.

The AIN powder desirably has a particle size within a range of from 0.1 to 2.5 μm, preferably from 0.1 to 1.5 μm, a specific surface area within a range of from 1.5 to 7 m2/gm, preferably 2.0 to 5.0 m2/gm, most preferably 3.0 to 5.0 m2/gm and an oxygen content within a range of from 0.2 to 2.5% by weight, preferably 0.8 to 1.8% by weight. Oxygen can be present in the AIN powder as impurities within the crystal lattice of AIN powder or in a metal oxide, such as aluminum oxide (AI2O3) or aluminum oxynitride. at the surface of AIN powder grains. Examples of suitable aluminum nitride (AIN) powder include: Electronic Grade AIN powder, commercially available from The Dow Chemical Company; and Grade F AIN powder, commercially available from Tokuyama Soda Company.

Densifying compositions suitable for use with the present invention include combinations of three sintering aids. The sintering aids are AI2O3, YF3 and either CaF₂ or Y2O3. The relative amounts of each sintering aid are shown in regions I and II Figures 1 and 2 wherei the third sintering aid is, respectively, calcium fluoride and yttrium oxide. The amounts are expressed in weight fractions so that the sum of all weight fractions equals 1.0. The combinations allow the aluminum nitride powder to densify and form a densified aluminum nitride article or body without significantly increasing aluminum nitride grain size.

Substitutions of another alkaline earth fluoride for CaF₂ or a rare earth oxide or rare earth fluoride for, respectively, Y2O3 and YF3 should provide some combinations that yie satisfactory results in terms of thermal conductivity and density. The substitutions would, however, lead to development of ternary composition diagrams with regions or domains of satisfactory thermal conductivity and density that differ from those of Figures 1 and 2. In addition, the use of aluminum nitride powder with a substantially greater oxygen content would lead to modification of domains I and II in Figures 1 and 2.

The densifying compositions designated as regions or domains I and II in Figures and 2 typically provide a density of at least 95 percent of theoretical density and a thermal conductivity of at least 100 W/m°K (watts/(meter) (degrees Kelvin)) when they are combined with aluminum nitride and heated to 1625°C at a rate of 2.5°C/minute and maintained at that temperature for 6 hours. As the temperature decreases to 1550°C, or even 1500°C, thermal conductivity values of at least 100 W/m°K may be attained, but the density decreases to at leas 90 percent of theoretical density. Further decreases in temperature lead to reductions in both thermal conductivity and density and yield materials that may be suitable for uses that do not require high density, high thermal conductivity or both. Conversely, an increase in sintering temperature may broaden the regions wherein thermal conductivity values of at least 150 W/m°K may be attained. Similar effects are observed with variations in heating rate and sintering time. An example of a suitable calcium fluoride (CaF₂) is reagent grade CaF₂, commercially available from Fisher Scientific Company. An example of a suitable aluminum oxide (Al2θ3)is reagent grade AI2O3, commercially available from Johnson Matthey Alfa Products. An example of a suitable yttrium fluoride (YF3) is 99.9% pure powder, commercially available from Aldrich Chemical Co.. An example of a suitable yttrium oxide (Y2O3) is 99.99% pure powder, commercially available from Unocal-Molycorp.

The AI2O3 portion of the sintering aid combination is in addition to any oxygen- containing species that may be present on aluminum nitride powder surfaces. High resolution transmission electron microscopy of such surfaces shows they are typically amorphous. Energy dispersive spectroscopy in an analytical transmission electron microscope using a focused electron beam that is localized within the coating shows they typically contain Al-O-N or Al-0 species or both.

The powdered admixture of AIN, AI2O3, YF₃ and either Y2O3 or CaF2 is suitably prepared by conventional procedures. The admixture contains, based upon admixture weight, AIN in an amount within a range of from 88 to 99 percent by weight. The range is preferably from 91 to 97 percent by weight. On the same basis, a combined total amount of AI2O3, YF3 and either Y2O3 or CaF₂ within a range of from about 12 to about 1 percent by weight. The range is preferably from 9 to 3 percent by weight. A binder can be combined with the powdered admixture to help form a powder compact. An example of a suitable binder is a mixture of Cimarec® binder (XUS 40303.00), commercially available from The Dow Chemical Company, and polyethylene glycol. The binder can be mixed with a suitable solvent to form a binder solution that is then combined with the powder mixture. Examples of suitable solvents include ethanol and trichloroethane. More than one binder or solvent can be used to form the binder solution. The amount of binder is preferably within a range of from 3 to 10% by weight of binder solution. A suitable amount of binder solution is within a range of from 1.0 ml/gm to 2 ml/gm of powder mixture.

The binder solution can also include a dispersantthat is suitable for reducing agglomeration of the powdered admixture. Examples of suitable dispersants include natural fish oil and fatty acids. The amount of dispersant is preferably in a range of from 0.1 to 0.005 gm/ml of binder solution.

Conventional grinding or milling media may be used to blend the powdered admixture and binder solution. High density aluminum nitride grinding media, commercially available from U.S. Stoneware Corp., provides satisfactory results. Suitable amounts of grinding media are within a range of from 1 to 4 grams/gram of the combined powder mixture and binder solution.

The powdered admixture and binder solution can be blended and milled using conventional apparatus and procedures. The milling apparatus is suitably made from, or lined with, a material, such as polyethylene, that will not react with powdered admixture components or the binder solution. The milling apparatus, such as a bottle, is typically rotated for a period of 1 to 12 hours at a rate of from 10 to 100 revolutions per minute to convert the powdered admixture and binder solution into a milled slurry. The milled slurry is then recovered from the milling media, dried and converted to a powder by conventional procedures, such as spray drying. The milled dry powder is then formed into a precursor pellet having a suitable shape by conventional technology. Suitable shapes include disks, right cylinders, spheres and briquettes.

If desired, milled dry powder can be sifted through a suitable screen to form a dried, milled mixture of substantially uniform consistency before it is formed into a shape. Suitable screen mesh sizes are in a range of from 40 to 120 (425μm to 125μm screen opening). The mesh size is preferably 60 (250μm screen opening). Suitable screen materials include stainless steel, copper, and synthetics, such as nylon. Precursor pellets or greenware may be formed by die pressing, injection moldin tape casting, slip casting or by some other conventional method. The pellets are exposed to conditions sufficient to remove the binder and dispersant therefrom. In one conventional procedure, precursor pellets are loaded into a suitable furnace for binder removal. The temperature in the furnace is then raised to between 500°C and 700°C for a period of 1 to 6 hours, or as long as is needed to remove at least a substantial portion of the binder. The precursor pellets are preferably exposed to a temperature of about 550°C for about one hour i air or for about 5 hours in a nitrogen atmosphere. Removal of the binder and dispersant converts each precursor pellet to a powder compact. Removal of the binder, also known as binder burn-out, may occur either in air or in a nitrogen atmosphere when the weight fraction of AI2O3 is intermediate between its uppe and lower limits respectively for Figures 1 and 2. As the weight fraction of AI2O3 approaches i lower limit, air is a preferred binder burn-out atmosphere. At or nearthe lower AI2O3 limit, a nitrogen atmosphere may lead to a thermal conductivity or density or both that is lower than desired. Conversely, asthe AI2O3 weight fraction approaches its upper limit, nitrogen is a preferred binder burn-out atmosphere. At or nearthe upper limit, air may yield the same results as nitrogen at the lower limit.

When using sintering aid compositions shown in Figure 1, the burn-out atmosphere can be either air or nitrogen when the AI₂O3 weight fraction is from 0.08 to 0.32 for area I and from 0.76 to 0.80 for area II. The atmosphere is air when the AI₂O3 weight fraction is from 0.04 to less than 0.08 for area I and from 0.72 to less than 0.76 for area II. The atmosphere is nitrogen when the AI₂O3 weight fraction is from greater than 0.32 to 0.36 for area I and from greater than 0.80 to 0.84 for area II. When using sintering aid compositions shown in Figure 2, the burn-out atmosphere can be either air or nitrogen when the AI2O3 weight fraction is from 0.12 to 0.26 for area I and from 0.12 to 0.64 for area II The atmosphere is airwhen the AI2O3 weight fraction is from 0.08 to less than 0.12 for areas I and II. The atmosphere is nitrogen when the AI2O3 weight fraction is from greater than 0.26 to 0.30 for area I and from greater than 0.64 to 0.68 for area II.

A quantity of powder compacts is loaded onto a suitable tray which is loaded int a suitable crucible. Examples of suitable trays and crucibles are those formed of boron nitride. The crucible is desirably capped with a lid, preferably formed of boron nitride. The capped crucible and its contents are loaded into a furnace, such as a refractory-type furnace with graphite or tungsten heating elements.

The capped crucible and its contents are exposed to conditions sufficient to convertthe powder compacts into a densified or sintered aluminum nitride article without substantially increasing the grain size of aluminum nitride over that of the aluminum nitride powder. "Without a significant increase of grain size," as that term is used herein, means that, during densification to about 90% theoretical density, the average grain size of AIN in the powder compact increases by no more than one order of magnitude. The average AIN grain size in the sintered article is in a range of from 1 to 5 μm.

The sintered aluminum nitride article has a density that is desirably at least 90%, preferably at least 95%, of the theoretical density of aluminum nitride. The sintered article has a thermal conductivity that is desirably at least 100, preferably at least 150, W/m°K.

An example of conditions sufficient to densify the aluminum nitride in the powder compact includes heating the powder compact at a rate between 2 and 30°C per minute from ambient temperature to a sintering temperature. Heating preferably occurs at a rate of 2.5 to 25°C per minute beginning at 1200^CC when the sintering additives are YF₃, AI2O3 and Y2O3, and at 1000°C when the sintering additives are YF3, AI2O3 and CaF₂. A satisfactory heating schedule begins at 24°C per minute from 20°C to 1000°C. This is followed by heating at 20°C per minute up to 1575°C. If highertemperatures are used, the heating rate is reduced to 15°C per minute. Heating desirably occurs at atmospheric pressure.

The powder compact is heated to a temperature sufficient to cause the AIN to densify to at least ninety percent of the theoretical density of aluminum nitride without a substantial increase of the grain size of the powder compact. The AIN is preferably densified to at least 95 percent, more preferably to 98 percent or more, of theoretical density.

The sintering temperature is desirably within a range of from 1500°C to 1650°C, preferably from 1530°C to 1625°C, more preferably from 1550°C to 1600°C. Temperatures less than 1500°C typically do not lead to densities of at least 90 percent of theoretical density in the absence of uneconomical sintering times. Temperatures in excess of 1650°C may be used, but produce no substantial advantage otherthan a possible greater latitude in sintering aid combinations that may allow one or more components to be eliminated. Such higher temperatures may, however, require the use of specially designed sintering furnaces rather than conventional alumina sintering furnaces.

The sintering temperature is maintained for a period of time sufficient to attain a density of at least 90 percent of theoretical density and a thermal conductivity of at least 100W/m°K. Suitable times range from 2 to 16 hours, desirably from 4 to 12 hours, preferably from 6 to 8 hours. After maintaining the sintering temperature for such a period of time, the temperature within the furnace is desirably lowered at a rate from 1 to 50°C per minute to a temperature of 1200°C. The temperature is preferably lowered at a rate from 5 to 30°C per minute.

After cooling to ambient, a sintered body containing aluminum nitride and a grain boundary phase results. When the sintering additives are YF3, Y2O3 and AI2O3, the grain boundary phase includes yttrium oxyfiuoride and yttrium aluminate. When the sintering additives are YF3, CaF2 and AI2O3, the grain boundary phase includes yttrium oxyfiuoride and yttrium aluminate or calcium aluminate or both. Although the mechanism of the invention is not completely understood, it is believed that the components of the densifying composition react to form a sintering liquid, such as a eutectic or a peritectic liquid. When the sintering additives are YF3, Y2O3 and AI2O3, the sintering liquid is a yttrium-aluminum oxyfiuoride. When the sintering additives are YF3, CaF2 and AI2O3, the sintering liquid is a yttrium-calcium-aluminum oxyfiuoride.

The invention will now be further and specifically described by the following examples. All parts are by weight (PBW) and all percentages are by weight unless otherwise stated. Example I A series of admixtures were prepared that contained AIN powder and a densifying composition that included Y2O3, YF3, and AI2O3. Three different electronic grade aluminum nitride powders were used. The powders were: AIN-1, a material commercially available from The Dow Chemical Company under the designation XUS 35544 and having an oxygen content of 1.07% and a surface area of 3.42 m²/g; AIN-2, an experimental material developed by The Dow Chemical Company with an oxygen content of 1.0% and a surface area of 4.00 m²/g; and AIN-3, an experimental material developed by The Dow Chemical Company with an oxygen content of 1.7% and a surface area of 4.35 m²/g. The aluminum nitride powde and the densifying composition contained in each admixture are shown in Table I together with the burn-out atmosphere, either air or nitrogen (N₂), the percent of theoretical density and the thermal conductivity.

Additional components were combined with the mixtures to form slurries All of the slurries included 100 parts of a combination of AIN and the densifying combination, 0.5 parts of fish oil as a dispersant, 2.3 parts Cimarec^® binder, 4.2 parts polyethylene glycol, and 12 parts of a 50/50 blend of ethanol and chlorothene. The slurries were milled for two hours using 1.5 parts of high density AIN milling media, commercially available from U.S. Stoneware Corp, per part of aluminum nitride powde The milled slurries were recovered from the milling media and dried to yield a milled dry powder. The powder was then dry-pressed into pellets using a die set and a hydraulic press with a pressure of 15,000 psi (103.4 MPa). The binder used in forming the pellets was removed by firing or heating the pellets in an oven under a stream of compressed air or nitrogen at a temperature of 550°C for a period of one hour. The ramp rate during cooling and heating was 2 °C/minute.

The debindered pellets were disposed in refractory crucibles and loaded in a high temperature sintering furnace. The pellets were densified by heating the crucibles and their contents in the furnace at a rate of 30°C/minute to a temperature of 1200^CC, followed by heating at a rate of 2.5°C/minute to 1625°C and maintained at that temperature for a period of time of about six hours under a stream of nitrogen gas. The densified pellets were then cooled to ambient temperature at a rate of about 5°C/minute. Table I

I id

= Not an example of the invention = Not Measured

= Not an example of the invention Not Measured

Example II

A second series of samples was prepared by the method described above in Example I, with the exception that CaF2 was used in place of Y2O3 and the cooling rate was increased to 50°C/minute. The composition, theoretical density and thermal conductivity are shown in Table II.

Table II

Not an example of the invention = Not Measured

= Not an example of the invention = Not Measured

The data in Tables I and II illustrate two points. First, the burn-out atmosphere can make a substantial difference in density as shown i n samples 1 1 , 12, and 17-20 of Table I and 3, 4, 7, 8, 15 and 16 of Table II. Second, it illustrates the significance of the upper and lower limits for AI2O3. Example III

A third series of samples were prepared by a modification of the method described in Example I using the sintering aids of Example II. The modifications were a decrease in sintering temperature to 1535°C, an increase in the heating rate to 25°C/min. and an increase in the cooling rate to 50°C/min. The results are shown in Table III in the same manner as in Tables I and II.

Table III

Burn¬

Sample Sample

AIN out Al₂θ₃ YF₃ CaF₂ % Thermal

Number Number onductivity nat^ion Wt-% Atmos¬ (Weight (Weight (Weight Theoret- C phere Fraction) Fraction) Fraction) ical (W/m°K) Density 95

105 111

119

123

= Notan exampleofthe invention -- = NotMeasured

The data in Table III demonstrate that a density of 90 % of theoretical and a thermal conductivity of 100W/m°K are attainable within the composition ranges specified for AI2O3, CaF2 and YF₃. If a density of 95% of theoretical density with the same thermal conductivity is desired, the composition limits must be narrowed. Suitable results should be attainable with an AI2O3 weight fraction of 0.20 to 0.45, a CaF₂ weight fraction of 0.06 to 0.38 and a YF₃ weight fraction of 0.40 to 0.60. Example IV

A series of samples were prepared using the compositions of samples 1 1 -20 from Table I, but two different modifications of the sintering conditions. One modification maintained the same sintering temperature, but increased the heating rate from 2.5°C/min to 15°Omin. The other modification maintained the same heating rate, but lowered the sintering temperature to 1550°C. The cooling rate was 50°C/minute as in Examples II and III. Thermal conductivity and percent theoretical density values are presented in Table IV together with the values from Table I.

= Not an example of the invention = Not Measured

Example V

A series of samples were prepared using the compositions of samples 7-12 from Table II, but two different modifications of the sintering conditions. One modification maintained the same sintering temperature, but increased the heating rate from 2.5°C/min to 15°Cmin. The other modification increased the heating rate from 2.5°Cmin to 15°C min and lowered the sintering temperature to 1600°C. Thermal conductivity and percent theoretical density values are presented in Table V together with the values from Table II.

= Not an example of the invention = Not Measured

The data presented in Tables IV and V demonstrate that some sintering aid combinations yield acceptable thermal conductivity values and densities at temperatures as low as 1550°C. The data also show that a faster heating rate to a sintering temperature of 1625°C improves thermal conductivity, density or both. A similar effect is expected at 1550°C. As such, samples 5, 7, 9 and, perhaps, 2 and 4 would have a density of 90 percent of theoretical density or greater at an increased heating rate such as 15°C/rrtinute or higher. Example VI

A sample was prepared using a modification of the procedure of Example I with a composition that contained 100 parts AIN (90 wt %) and 1 1 parts of sintering aids. The sintering aids and their weight fractions were: O.27 AI2O3; 0.55 YF3; and 0.18 Y2O3. The modified procedure altered the binder formulation to form a tape cast sample. The binder removal procedure by using nitrogen ratherthan air, a time of 4 hours ratherthan 2 hours and a temperature of 600°C ratherthan 550°C. Sintering occurred at a set temperature of 1650°C ratherthan 1625°C. The sintered material had a density of 3.26 g/cc (98.5 % theoretical density) and a thermal conductivity of 145 W/m°K. Analysis of the phase chemistry by x-ray powder diffraction revealed the presence of Y₃AI5O₁₂ and YOF.

In the absence of AI2O3, yttrium aluminate does not form and the density and thermal conductivity values are lower. Similar results are expected with other compositions disclosed herein. Equivalents

Those skilled in the art will recognize, or be able to ascertain using no more than routine experimentation, many equivalents to the specific embodiments of the invention described specifically herein. Such equivalents are intended to be encompassed in the scope of the following claims.

Claims

1. A method of forming a sintered body of aluminum nitride that comprises heating, following removal of binder in an atmosphere that favors subsequent densification b sintering to at least90 percent of theoretical density, a powder compact formed from a binder and a homogeneous powdered admixture of AIN, AI2O3, YF3 and Y2O3 to a sintering temperature within a range of from 1500°Cto 1650°C at a heating rate sufficient to form a densifying amount of a high temperature yttrium-aluminum oxyfiuoride sintering liquid and maintaining that temperature for a period of time sufficient to yield a sintered body having a grain size that is less than one order of magnitude larger than that of the aluminum nitride powder, a density greater than or equal to 90 percent of theoretical density and a thermal conductivity greater than or equal to 100 W/m K and containing, when cooled, aluminum nitride and a grain boundary phase that includes yttrium oxyfiuoride and yttrium aluminate, the admixture containing, based upon admixture weight, AIN in an amount within a range of from 88 to 99 percent by weight and, as sintering additives, a combined total amount of AI2O3, YF3, and Y2θ3"within a range of from 1 to 12 percent by weight, the sintering additives having a composition defined and encompassed by areas I and II of Figure 1 wherein, based upon total weight of sintering additives with all weight fractions totaling 1.0, the AI2O3 weight fraction ranges from 0.04 to 0.36 for area I and from 0.72 to 0.84 for area ll, the Y2O3 weight fraction ranges from greater han O to 0.68 for area I and from greaterthan O to 0.06 for area II, and the YF3 weight fraction ranges from 0.18 to 0.88 for area l and from 0.16 to 0.28 for area II.

2. The method of Claim 1 wherein the thermal conductivity is greater than or equal to 150 W/m-K and the sintering additives have a composition defined and encompassed by area I of Figure 1.

3. The method of Claim 5 wherein the burn-out atmosphere is air when the AI2O3 weight fraction is from 0.04 to less than 0.08 for area I and from 0.72 to less than 0.76 for area II, nitrogen when the AI2O3 weight fraction is from greater than 0.32 to 0.36 for area I and from greater than 0.80 to 0.84 for area II and either air or nitrogen when the AI2O3 weight fraction is from 0.08 to 0.32 for area I and from 0.76 to 0.80 for area II.

4. A method of forming a sintered body of aluminum nitride that comprises heating, following removal of binder in an atmosphere that favors subsequent densification by sintering to at le'ast 90 percent of theoretical density, a powder compact formed from a binder and a homogeneous powdered admixture of AIN, AI2O3, YF3 and CaF to a sintering temperature within a range of from 1500°Cto 1650°C at a heating rate sufficient to form a densifying amount of a high temperature yttrium-calcium-aluminum oxyfiuoride sintering

5 liquid and maintaining that temperature for a period of time sufficient to yield a sintered body having a grain size that is less than one order of magnitude larger than that of the aluminum nitride powder, a density greater than or equal to 90 percent of theoretical density and a thermal conductivity greater than or equal to 100 W/m K and containing, when cooled, aluminum nitride and a grain boundary phase that includes yttrium oxyfiuoride and yttrium •

10 aluminate or calcium aluminate or both, the admixture containing, based upon admixture weight, AIN in an amount within a range of from 88 to 99 percent by weight and, as sintering additives, a combined total amount of AI2O3, YF3 and CaF₂ within a range of from 12 to 1 percent by weight, the sintering additives having a composition defined and encompassed by areas I and II of Figure 2 wherein, based upon total weight of sintering additives with all weight

15 fractions totaling 1.0, the AI2O3 weight fraction ranges from 0.08 to 0.30 for area I and 0.08 to 0.68 for area II, the CaF2 weight fraction ranges from greater than 0 to 0.24 for area I and from greater than 0 to 0.68 for area II, and the YF3 weight fraction ranges from 0.57 to 0.92 for area I and from 0.01 to 0.56 for area II.

5. The method of Claim 4 wherein the thermal conductivity is greater than or 20 equal to 150 WVm-K and the sintering additives have a composition defined and encompassed by area I of Figure 2.

6. The method of Claim 4 wherein the burn-out atmosphere is air when the AI2O3 weight fraction is from 0.08 to less than 0.12 for areas I and II, nitrogen when the AI2O3 weight fraction is from greater than 0.26 to 0.30 for area I and from greater than 0.64 to 0.68

25 for area II and either air or nitrogen when the AI2O3 weight fraction is from 0.12 to 0.26 for area I and from 0.12 to 0.64 for area II.

7. The method of Claim 1 or Claim 4 wherein the heating rate is from 2 to 30°C/minute over a temperature range of from ambient to the sintering temperature.

8. The method of Claim 1 or Claim 4 wherein the heating rate is from 2.5 to 30 25°C/minute over a temperature range of from 1200 to the sintering temperature.

9. The method of Claim 1 or Claim 4 further comprising a step wherein the sintered body is cooled at a rate of 1 to 50°Ominute to a temperature of 1200°C.

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