WO1993014027A2 - Process for preparing ultrafine aluminum nitride powder - Google Patents

Process for preparing ultrafine aluminum nitride powder Download PDF

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Publication number
WO1993014027A2
WO1993014027A2 PCT/US1992/010574 US9210574W WO9314027A2 WO 1993014027 A2 WO1993014027 A2 WO 1993014027A2 US 9210574 W US9210574 W US 9210574W WO 9314027 A2 WO9314027 A2 WO 9314027A2
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WIPO (PCT)
Prior art keywords
aluminum nitride
temperature
surface area
particles
powdered
Prior art date
Application number
PCT/US1992/010574
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French (fr)
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WO1993014027A3 (en
Inventor
Alan W. Weimer
Gene A. Cochran
John P. Henley
Glenn A. Eisman
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The Dow Chemical Company
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Publication date
Application filed by The Dow Chemical Company filed Critical The Dow Chemical Company
Priority to JP5512439A priority Critical patent/JPH08508000A/en
Priority to EP93901411A priority patent/EP0620803B1/en
Priority to DE69228562T priority patent/DE69228562T2/en
Publication of WO1993014027A2 publication Critical patent/WO1993014027A2/en
Publication of WO1993014027A3 publication Critical patent/WO1993014027A3/en

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    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
    • C01B21/00—Nitrogen; Compounds thereof
    • C01B21/06—Binary compounds of nitrogen with metals, with silicon, or with boron, or with carbon, i.e. nitrides; Compounds of nitrogen with more than one metal, silicon or boron
    • C01B21/072—Binary compounds of nitrogen with metals, with silicon, or with boron, or with carbon, i.e. nitrides; Compounds of nitrogen with more than one metal, silicon or boron with aluminium
    • C01B21/0728—After-treatment, e.g. grinding, purification
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
    • C01B21/00—Nitrogen; Compounds thereof
    • C01B21/06—Binary compounds of nitrogen with metals, with silicon, or with boron, or with carbon, i.e. nitrides; Compounds of nitrogen with more than one metal, silicon or boron
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
    • C01B21/00—Nitrogen; Compounds thereof
    • C01B21/06—Binary compounds of nitrogen with metals, with silicon, or with boron, or with carbon, i.e. nitrides; Compounds of nitrogen with more than one metal, silicon or boron
    • C01B21/072—Binary compounds of nitrogen with metals, with silicon, or with boron, or with carbon, i.e. nitrides; Compounds of nitrogen with more than one metal, silicon or boron with aluminium
    • C01B21/0722—Preparation by direct nitridation of aluminium
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
    • C01B21/00—Nitrogen; Compounds thereof
    • C01B21/06—Binary compounds of nitrogen with metals, with silicon, or with boron, or with carbon, i.e. nitrides; Compounds of nitrogen with more than one metal, silicon or boron
    • C01B21/072—Binary compounds of nitrogen with metals, with silicon, or with boron, or with carbon, i.e. nitrides; Compounds of nitrogen with more than one metal, silicon or boron with aluminium
    • C01B21/0726—Preparation by carboreductive nitridation
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
    • C01B21/00—Nitrogen; Compounds thereof
    • C01B21/06—Binary compounds of nitrogen with metals, with silicon, or with boron, or with carbon, i.e. nitrides; Compounds of nitrogen with more than one metal, silicon or boron
    • C01B21/0615—Binary compounds of nitrogen with metals, with silicon, or with boron, or with carbon, i.e. nitrides; Compounds of nitrogen with more than one metal, silicon or boron with transition metals other than titanium, zirconium or hafnium
    • C01B21/0617—Binary compounds of nitrogen with metals, with silicon, or with boron, or with carbon, i.e. nitrides; Compounds of nitrogen with more than one metal, silicon or boron with transition metals other than titanium, zirconium or hafnium with vanadium, niobium or tantalum
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
    • C01B21/00—Nitrogen; Compounds thereof
    • C01B21/06—Binary compounds of nitrogen with metals, with silicon, or with boron, or with carbon, i.e. nitrides; Compounds of nitrogen with more than one metal, silicon or boron
    • C01B21/064—Binary compounds of nitrogen with metals, with silicon, or with boron, or with carbon, i.e. nitrides; Compounds of nitrogen with more than one metal, silicon or boron with boron
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
    • C01B21/00—Nitrogen; Compounds thereof
    • C01B21/06—Binary compounds of nitrogen with metals, with silicon, or with boron, or with carbon, i.e. nitrides; Compounds of nitrogen with more than one metal, silicon or boron
    • C01B21/068—Binary compounds of nitrogen with metals, with silicon, or with boron, or with carbon, i.e. nitrides; Compounds of nitrogen with more than one metal, silicon or boron with silicon
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
    • C01B21/00—Nitrogen; Compounds thereof
    • C01B21/06—Binary compounds of nitrogen with metals, with silicon, or with boron, or with carbon, i.e. nitrides; Compounds of nitrogen with more than one metal, silicon or boron
    • C01B21/076—Binary compounds of nitrogen with metals, with silicon, or with boron, or with carbon, i.e. nitrides; Compounds of nitrogen with more than one metal, silicon or boron with titanium or zirconium or hafnium
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01P—INDEXING SCHEME RELATING TO STRUCTURAL AND PHYSICAL ASPECTS OF SOLID INORGANIC COMPOUNDS
    • C01P2006/00—Physical properties of inorganic compounds
    • C01P2006/12—Surface area

Definitions

  • the present invention generally concerns a process for preparing aluminum
  • AIN powder 5 nitride (AIN) powder.
  • the present invention more particularly concerns preparing AIN powder that has a surface area greater than 10 square meters per gram (m 2 /g), desirably greater than 15 m2/g.
  • Aluminum nitride synthesis generally occurs via one of four known processes.
  • One process involves carbothermally reducing and nitriding alumina (AI2O3 + 3C + N 2 - * » 2AIN + 3CO).
  • a second process directly reacts aluminum metal with nitrogen (2AI + N 2 ⁇ * 2AIN).
  • a less common process reacts aluminum chloride and ammonia in a vapor phase (AICI3 + 4NH 3 -* ⁇ AIN + 3NH 4 CI).
  • U.S. -A 3J28J 53 discloses an even less common process wherein aluminum phosphide reacts with ammonia (AIP + NH3 ⁇ AIN + I/4P4 + 3 2H2).
  • the reaction is generally carried out at a temperature within a range of 1673 to 1973 K as disclosed by Kuramoto et al. in U.S.-A 4,618,592.
  • the resultant AIN powder is fine enough to allow consolidation to near theoretical density via pressureless sintering in the presence of sintering aids. Higher temperatures
  • the agglomerates are not amenable to densification by pressureless sintering.
  • the direct reaction of aluminum metal to AIN is exothermic and generates approximately 328 kilojoules per gram-mole of AIN at 1800 K.
  • Aluminum metal melts at about 933 K.
  • the reaction of aluminum and nitrogen starts at about 1073 K. The reaction, once
  • An uncontrolled reaction can reach AIN sintering temperatures and remain at these temperatures for extended periods of time.
  • the uncontrolled reaction typically yields sintered AI agglomerates having a surface area, as determined by Brunauer-Emmett-Teller (BET) analysis, of less than 2 r ⁇ * 2/g.
  • BET Brunauer-Emmett-Teller
  • One variation of the direct nitridation process employs plasma reactors to vaporize aluminum metal at temperatures approaching 10,000 K.
  • the vaporized metal then reacts with nitrogen, ammonia, or mixtures of nitrogen and ammonia or nitrogen and hydrogen.
  • the resultant AIN powder particles have an average particle size of less than 0J micrometer ( ⁇ m) and a surface area of approximately 30 m 2 /g. These particles can be pressureless sintered to near theoretical density at temperatures as low as 1873 K.
  • One aspect of the present invention is a process for preparing AIN powderthat comprises passing paniculate Al metal and a nitrogen source through a heated reaction zone that is maintained at a temperature sufficient to individually heat substantially all oftheAl metal particles at a heating rate of at least 100 K per second to a temperature within a temperature range of from 2473 K to 3073 K, at a rate of flow sufficient to maintain substantially all ofthe Al metal particleswithin said temperature range for a time period of from 0.2 to 10 seconds, the time period being sufficientto convert the particulate AI metal to a product having an AIN content of at least 75 percent by weight (wt-%), based upon product weight, the AIN having a surface area of greater than 10 m 2 /g.
  • a second aspect of the present invention is a process for preparing a composite or a mixture of AIN powder and a powdered compatible material that comprises passing a nitrogen source and an admixture of a powdered compatible solid material and particulate Al metal through a heated reaction zone that is maintained at a temperature sufficientto individually heat substantially all of the admixture particles at a heating rate of at least 100 K per second to a temperature within a temperature range of from 2473 K to 3073 K, at a rate of flow sufficientto maintain substantially all of the admixture particles within said temperature range for a time period of from 0.2 to 10 seconds, the compatible solid material being present in an amount sufficient to yield a material containing AIN and the compatible material, the time period being sufficientto convert at least 75 wt-% of the particulate AI metal to AI , the AIN having a surface area within a range of greater than 10 m 2 /g.
  • the product beneficially has an AIN content of at least 90 wt-%, based upon product weight.
  • Athird aspect of the present invention is a carbothermal process for preparing AIN powder that comprises passing a nitrogen source and a powdered admixture of AI2O3 and carbon through a heated reaction zone that is maintained at a temperature sufficientto individually heat substantially all of the powdered admixture particles at a heating rate of at least 100 per second to a temperature within a temperature range of from 2473 Kto 3073 K, ata rate of flow sufficient to maintain substantially all of the powdered admixture particles within said temperature range for a time period of from 0.2 to 20 seconds, the time period being sufficient to convert the powdered admixture to a product having an AIN content of at least 75 wt-%, based upon product weight, the AIN having a surface area of greater than 10 m 2 /g.
  • the product passes from the heated reaction zone into a cooling zone maintained at a temperature sufficient to individually cool substantially all product particles at a cooling rate of at least 100 K per second to a temperature below 1073 K.
  • the product must be cooled rapidly in order to minimize, if not eliminate, partial sintering or agglomeration of product particles. Partial sintering may interfere with subsequent pressureless sintering efforts. Agglomeration may lead to unacceptably low product surface areas.
  • the product can be passed through the heated reaction zone one or more additional times to increase the yield of AI N.
  • the product can be passed through the reaction zone either alone or in conjunction with an amount of the powdered admixture.
  • a fourth aspect of the present invention is a process for converting AIN having a surface area of less than 15 m 2 /g to AIN having a surface area greater than or equal to 15 m /g that comprises passing a nitrogen source and powdered AIN having a surface area of less than 15 m /g through a heated reaction zone maintained at a temperature sufficient to individually heat substantially all of the AIN particles at a heating rate of at least 100 K per second to a temperature within a temperature range of from greater than 2473 K to 3073 K, at a rate of flow sufficient to maintain substantially all of the AIN particles within said temperature range for a period of time sufficientto dissociate the AIN into AI and nitrogen as dissociation products and thereafter passing the dissociation products into a cooling zone maintained at a temperature sufficientto convert the dissociation products into AIN particles having a surface area of greater than 15 m 2 /g and individually cool substantially all of said particles at a cooling rate of at least 100 K per second to a temperature below 1073 K.
  • the process of the present invention is suitably carried out in an apparatus like that disclosed in U.S.-A 5,1 10,565.
  • the apparatus comprises four principal components: a cooled reactant transport member; a reactor chamber; a heating means; and a cooling chamber.
  • a purge gas may be introduced into spaces surrounding the reactor chamber.
  • the transport member may be likened to a conduit disposed within a gas flow space that is desirably annular.
  • the transport member is suitably maintained at a temperature below that at which powdered aluminum metal melts for the first and second aspects of the invention. Similar temperatures suffice forthe third aspect of the invention.
  • the temperature is beneficially sufficient to substantially preclude the powdered reactants, particularly Al metal and AI2O3, from melting and coalescing either within the transport member or proximate to its exit. Accordingly, the temperature is desirably sufficientto allow substantially all of the powdered reactants to enter the reactor chamber as discrete particles. A temperature below the melting point of Al (about 933 K) yields satisfactory results.
  • the range is beneficially from 275 to 373 K, desirably from 275 to 323 K, and preferably from 275 to 298 K.
  • Higher temperatures may be used in the fourth aspect as AI sublimes at 2273 K and dissociates at 2473 K. The highertemperatures beneficially do not exceed the sublimation temperature.
  • the powdered reactants are suitably fed into the transport section via a powderfeeding mechanism.
  • AIN having a surface area of less than 15 m 2 /g may be fed in the same manner.
  • the powderfeeding mechanism is not particularly critical so long as it provides a metered or controlled flow of powdered material to the transport section.
  • the feeding mechanism may be a single screw feeder, a twin screw feeder, a vibratory feeder, a rotary valve feeder or some other conventional feeder.
  • the powdered Al metal should have a purity greater than about 97 percent and a weighted mean particle size less than about 500 ⁇ m.
  • the compatible solid material is suitably AIN or a fine ceramic powder that, when mixed with the AIN product, forms a desirable powdered admixture or composite product powder.
  • Suitable ceramic powders include silicon carbide, boron nitride, boron carbide, titanium diboride, silicon nitride, titanium nitride, titanium carbide, tungsten carbide or tantalum nitride.
  • the fine ceramic powder used as a compatible material beneficially has a purity of greaterthan 98%, a surface area of from 10 to 30 ⁇ 7g and a particle size within a range of from 0.01 to 1 ⁇ m.
  • AIN when used as a compatible materialJs suitably a portion ofthe AIN product.
  • the compatible material content is desirably from 28 to 91 wt-% and preferably from 33 to 67 wt-%, based upon admixture weight.
  • the Al 2 0 3 used in the present invention suitably has a particle size within a range of from 0.05 to 20 ⁇ m and a purity greater than about 99.8 percent.
  • the range is beneficially from 0.2 to 2 ⁇ m.
  • the purity is desirably greaterthan about 99.97 percent. Purities of 99.99 percent or greater will produce satisfactory products, but a a greater cost.
  • the carbon is suitably a material selected from acetylene black, plant carbon, thermal black, coke, carbon black and graphite.
  • the material is beneficially carbon black or graphite.
  • the admixtures may be prepared by using a conventional mixing apparatus.
  • Illustrative apparatus include ribbon blenders, roller mills, vertical screw mixers, V-blenders, and fluidtzed zone mixers such as that sold under the trade designation FORBERG'".
  • the powder feed rate varies with reactor design and capacity as well as the powdered reactants.
  • an acceptable feed rate for powdered aluminum is from 0.02 to 0.5 kilograms per minute (kg/m) for a reactor having a reaction zone volume of 2J6 ft 3 (0.06 m 3 ). Acceptable feed rates for reactors having greater reaction zone volumes are readily determined without undue experimentation.
  • Gaseous nitrogen (N 2 ) is fed into the transport section in an amount and at a rate of flow sufficientto satisfy two requirements. First, the flow rate should be at least stoichiometric, or sufficient to satisfy the relevant equation.
  • the flow rate should be sufficient to entrain either the powdered reactants or the low (less than 15 m 2 /g) surface area Al N powder prior to the entry thereof into the reactor chamber.
  • the flow rate is suitably at least one and one-half times stoichiometric.
  • the flow rate is beneficially from 1.5 to 4 times stoichiometric, desirably from 1.5 to 3 times stoichiometric.
  • An excessively high flow rate decreases residence time of powdered material, eitherthe reactants orthe low surface area AIN powder, within the reaction zone and, in turn, reactor capacity.
  • the flow rate for the carbothermal reaction is typically greater than that for the direct nitridation reaction.
  • a suitable flow rate for the direct nitridation reaction might be 3 standard cubic feet per minute (SCFM) (85 standard liters per minute (SLM)) whereas a suitable flow rate forthe carbothermal reaction might be 4 SCFM (1 13 SLM).
  • Gaseous nitrogen ( 2(g)) is also fed into the gas flow space.
  • This gas flows from the gas flow space into the reactor chamber. In doing so, it acts to minimize, if not substantially eliminate, contact of powdered reactants with reactor chamber surfaces neartheirjuncture with the transport member exit. Such contact is undesirable because these surfaces tend to be at temperatures that promote coalescence of the powdered reactants, particularly Al metal and AI.O.. Coalescence leads, in turn, to cessation of operations due to reactor plugging.
  • the N2(g) should be as pure as possible. Moisture and residual oxygen impurities adversely affect AIN product quality.
  • the N 2 (g) dew point is beneficially less than 233 K and desirably less than 193 K. Oxygen impurity levels are beneficially less than 5 and desirably less than 1 part per million parts (ppm) of gas. If the moisture content is too highJt may be necessary to pass the gaseous nitrogen through a drying bed or desiccant.
  • the gas may also be purified by conventional means to reduce the residual oxygen content.
  • the N2(g) flow rates control residence time of the Al metal powder, the admixture of a powdered compatible solid material and particulate AI, or the powdered admixture of AI.O and carbon within the reaction zone.
  • the N2(g) flow rates also control residence time of the low surface area AIN powder within that zone.
  • AI metal powder either alone or in admixture with a powdered compatible solid material, is added to the reaction zone
  • the residence time is suitably within a range of from 0.2 second to 10 seconds, beneficially from 2 to 8 seconds and desirably from 4 to 6 seconds. Residence times of less than 0.2 second tend to yield an incompletely converted product containing unreacted metal. Residence times in excess of 10 seconds provide no great advantage in terms of conversion percentage.
  • the same residence times provide satisfactory results.
  • the residence time is suitably within a range of from 0.2 second to 20 seconds, beneficially from 2 to 16 seconds and desirably from 4 to 15 seconds. Residence times of less than 0.2 seconds do not provide a satisfactory Al N product yield. Residence times in excess of 20 seconds offer no substantial advantage in terms of conversion percentage.
  • Ammonia may be used instead of, or in addition to, N 2 (g). A mixture of N 2 (g) and hydrogen may also be used.
  • the entrained flow of powdered reactants or low surface area AIN powder enters the reaction zone in a form approximating that of a well dispersed dust cloud.
  • the powdered reactants or low surface area AIN powder particles are heated almost instantly by gas convective and conductive heat transfer and by thermal radiation from reactor walls that define the heated reaction zone.
  • the reaction zone is beneficially maintained at a temperature within a range of from 2473 to 3073 K.
  • the temperature range is desirably from 2473 to 2773 K.
  • Temperatures below 2473 K lead to AIN products with surface areas of 8 m /g or less.
  • the actual temperature within the reaction zone may be determined by optical pyrometry or other suitable means.
  • the reactor walls may be conventionally heated either indirectly by radiation from heating elements spaced around them, inductively via an inductive coil, or directly by electrical resistance.
  • the powdered reactants or low surface area Al N powder particles are heated at rates within a range of from 100 to 10,000,000 K per second. The range is beneficially from 1000 to 10,000,000 K per second, desirably from 10,000 to 1,000,000 K per second.
  • the rate at which an individual particle is heated varies in response to a number of factors. The factors include size, proximity to the source of heat, and density of the dust cloud. The rate should not, however, be so low that substantial coalescence of reactant particles can occur during a melting phase prior to reaching reaction temperatures. In the case of low surface area AIN particles, the rate should notbe so low that the AIN passes into the cooling zone without being substantially dissociated into N2(g) and Al as dissociation products.
  • the gaseous nitrogen flow that provides an entrained flow of powdered reactants into the reaction zone also provides an entrained flow of powdered Al N or, in the case of low surface area AIN feed material, dissociation products out ofthe reaction zone.
  • the entrained flow or dust cloud of AIN powder or the entrained flow of dissociation products beneficially exits the reaction zone and almost immediately enters a cooling zone. It is believed that the dissociation products recombine to form AIN powder having a surface area in excess of 15 m /g at or nearthe cooling zone entrance.
  • the cooling zone quenches or rapidly cools the AIN powder below its reaction temperature. Rapid cooling helps maintain the very fine particle size of AI particles that form upon cooling below the dissociation temperature (about 2473 K) of AIN.
  • Cooling rates within the cooling zone beneficially approximate the heating rates within the reaction zone.
  • the cooled walls of the cooling zone and cooled gas tend to rapidly deplete remaining amounts of sensible heat from the AIN particles. If cooling does not occur at a sufficiently rapid rate, AIN particles tend to fuse or sinter together, thereby resulting in formation of undesirable agglomerates or large grains of AIN product.
  • the fused particles tend to have surface areas much lower than 10 m 2 /g. Actual cooling times vary depending upon factors such as particle size, cooling zone configuration and gas flow rates.
  • the cooled AIN particles are suitably collected and processed by conventional technology.
  • the AIN product has a surface area that is beneficially greater than 10 m 2 /g when powdered reactants are used as a starting material and greater than 15 m /g when low surface area AIN powder is used as the starting material.
  • the surface area is desirably within a range of greater than 15 to 65 m /g. The range is preferably from 19 to 60 m /g.
  • a 6 inch (0J 52m) inside diameter by 1 1 feet (3.35 m) long heated zone vertical graphite tube furnace was brought to, and maintained at, a temperature of 2200 C C (2473 K) as measured by optical pyrometers.
  • High purity Al metal powder commercially available from Aluminum Company of America (Alcoa) under the trade designation 7123, was loaded into an overhead feed hopper that was purged with N 2 (g).
  • the Al powder nominally 99.97% pure, had a surface area of 0.449 m /g, an oxygen content of 0.223 %, a silicon content of 75 ppm, a calcium content of less than 10 ppm, a chromium content of less than 10 ppm, an iron content of 44 ppm and a mean particle size of 18 ⁇ m.
  • the N 2 (g) gas had a dew point of less than -80°C (193 K) and an oxygen content of less than one ppm.
  • the powdered Al was conveyed from the hopper to the top of the heated zone of the tube furnace via a loss-in-weight twin screw feeder connected to a reactant transport member like that described herein at a rate of 0.2 pounds (0.09 kg) per minute.
  • the reactant transport member was maintained at a temperature of 283 K.
  • Nitrogen gas flowed through the reactant transport member at a rate of 3 SCFM (85 SLM) thus sweeping the Al powder with it into the top of the heated zone.
  • An additional one SCFM (28.3 SLM) of N 2 (g) flowed through the gas flow space within which the transport member was disposed and into the top of the furnace.
  • the flow of gas was sufficient to provide the powder with an average residence time in the heated zone of about 4.3 seconds accounting forthe decrease in nitrogen flow due to reaction and the conversion to AIN.
  • the characteristic heating time for the feed Al particles equated to an estimated heating rate of approximately 10 5 K/second.
  • the submicron product powder was swept through the cooling zone by approximately 3 SCFM (85 SLM) of unreacted nitrogen gas exiting the reaction zone.
  • the calculated residence time in the cooling zone was approximately 3.2 minutes.
  • Product powder from the reactor was collected downstream from a cooling zone and analyzed.
  • the cooling zone had an inside diameter of 18 inches (45.7 cm), a length of six feet (1.8m) and a volume of 10.6 ft 3 (0.3 m 3 ).
  • Coolant maintained at a temperature of 283 K and flowing through a jacket surrounding the cooling zone cooled the product powder and gas to a measured temperature of approximately 303 K.
  • the rate of cooling approximated the rate of heating.
  • An x-ray diffraction pattern ofthe product indicated that the powderwas substantially AIN with some unreacted metal.
  • the oxygen and nitrogen contents of powder placed into a glove box with a N 2 (g) atmosphere immediately following synthesis were determined by a LECO analyzer to be, respectively, 0.16% and 27J %.
  • the nitrogen content equated to an AIN content of 79%.
  • the powder had an unmilled aggregate BET surface area of 15.1 m 2 /g.
  • the product powder was dry ball milled fortwo hours using AIN media to break up light agglomerates of product.
  • the term "light agglomerates" describes agglomerates that break apart readily when rubbed between thumb and fingers with minimal pressure.
  • the BET surface area after ball milling was 15.7 m 2 /g. Taking into account about 21 % of unreacted aluminum with an estimated surface area of 0.5 m /g, the AI had a calculated, weighted, average surface area of 19.7 m /g.
  • a typical surface area resulting from such temperatures is on the order of 5 m 2 /g.
  • Example 1 The apparatus of Example 1 was used to convert a powdered admixture of AI 2 0 3 and carbon black to AIN via a carbothermal reduction-nitridation reaction.
  • the powdered admixture was prepared from 25 pounds (lb) (1 1.4 kg) of acetylene carbon black and 62 lb (28.2 kg) of Al O .
  • the A!_0 3 commercially available from Aluminum company of America underthe trade designation A16-SG, had a surface area of 9.46 rtrVg.
  • the carbon commercially available from Chevron Chemical Company under the trade designation Shawiniga ⁇ " acetylene black, had a surface area of 67 nrVg.
  • the AI.O. and carbon black were blended by bail milling for4 hours.
  • the solution pH was adjusted to a pH of 3.5 by adding 20 ml of a 65 % nitric acid solution. Thirty five lbs (15.9 kg) of the ball milled admixture were added to the deionized water solution, then stirred for 30 minutes before adding 17.75 lb (8.06 kg) of a 20 % colloidal Al 2 0 3 solution with continued agitation. The solution was agitated for an additional two hours before it was spray dried while maintaining an outlet temperature of 403 K. The spray dried powder had a carbon content of 26.6% as determined by analysis using a LECO model IR-412 with a Model HF-400 furnace.
  • the spray dried powder was loaded into the feed hopper and purged with N (g).
  • the furnace was brought to, and maintained at, a temperature of 2173 K.
  • N 2 (g) flowed into the reaction zone at the same rate as in Example 1.
  • the powder was metered into the cooled reactanttransport member at a rate of 0.2 lb/minute (0.09 kg/mi n) as in Example 1.
  • the flow rate provided an approximate residence time of 3.2 seconds.
  • the product powder was collected downstream from the cooling zone as in Example 1.
  • the powder was reloaded into the hopper and passed through the reaction zone a second time while the furnace was at a temperature of 2123 K.
  • the reloading was replicated two additional times save for increasing the temperature to 2223 K to provide a total residence time of about 13 seconds.
  • the remaining product from Comparative Example A was loaded into the feed hopper and passed through the heated reaction zone.
  • the furnace was heated to, and maintained at, 2573 K for this pass.
  • the residence time in the reaction zone was about 3.0 seconds.
  • the powder, after heat treatment as in Comparative Example A contained 0.6 % carbon, 8.7% oxygen and 31.7% nitrogen, indicating an AIN content of about 80.9%.
  • the BET surface area was 52.6 m 2 /g.
  • Example 2 By contrasting Example 2 with Comparative Example A, the beneficial increase in surface area by operating above the dissociation temperature of AIN is readily apparent. Similar results are expected by eliminating the low temperature passes of Comparative Example A and using one or more passes at temperatures at or above the dissociation temperature. Suitable temperatures and other operating parameters are disclosed herein.

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Abstract

Rapidly heat powdered aluminum, an admixture of powdered aluminum and a compatible solid material, a powdered admixture of alumina and carbon, or aluminum nitride powder having a surface area lower than desired in the presence of a source of nitrogen at a temperature of 2473 to 3073 K to produce aluminum nitride, then promptly quench the aluminum nitride product. The product has a surface area of greater than 10 m2/g, preferably greater than 15 m¿2?/g.

Description

PROCESS FOR PREPARING ULTRAFINE ALUMINUM NITRIDE POWDER
Technical Field
The present invention generally concerns a process for preparing aluminum
5 nitride (AIN) powder. The present invention more particularly concerns preparing AIN powder that has a surface area greater than 10 square meters per gram (m2/g), desirably greater than 15 m2/g. Background Art
Aluminum nitride synthesis generally occurs via one of four known processes.
10 One process involves carbothermally reducing and nitriding alumina (AI2O3 + 3C + N2 -*» 2AIN + 3CO). A second process directly reacts aluminum metal with nitrogen (2AI + N2 ■* 2AIN). A less common process reacts aluminum chloride and ammonia in a vapor phase (AICI3 + 4NH3 -*■ AIN + 3NH4CI). U.S. -A 3J28J 53 discloses an even less common process wherein aluminum phosphide reacts with ammonia (AIP + NH3 → AIN + I/4P4 + 3 2H2).
15 The carbothermal reduction reaction is endothermic and requires approximately
335 kilojoules per gram-mole of AIN at 1873 K. The reaction is generally carried out at a temperature within a range of 1673 to 1973 K as disclosed by Kuramoto et al. in U.S.-A 4,618,592. The resultant AIN powder is fine enough to allow consolidation to near theoretical density via pressureless sintering in the presence of sintering aids. Higher temperatures
20 generally result in the formation of sintered agglomerates of AIN particles. The agglomerates are not amenable to densification by pressureless sintering.
The direct reaction of aluminum metal to AIN is exothermic and generates approximately 328 kilojoules per gram-mole of AIN at 1800 K. Aluminum metal melts at about 933 K. The reaction of aluminum and nitrogen starts at about 1073 K. The reaction, once
25 initiated, is self-propagating if not controlled. An uncontrolled reaction can reach AIN sintering temperatures and remain at these temperatures for extended periods of time. The uncontrolled reaction typically yields sintered AI agglomerates having a surface area, as determined by Brunauer-Emmett-Teller (BET) analysis, of less than 2 rτ*2/g. The agglomerates are not readily amenable to further sintering to densities approaching theoretical density via pressureless sintering techniques.
One variation of the direct nitridation process employs plasma reactors to vaporize aluminum metal at temperatures approaching 10,000 K. The vaporized metal then reacts with nitrogen, ammonia, or mixtures of nitrogen and ammonia or nitrogen and hydrogen. The resultant AIN powder particles have an average particle size of less than 0J micrometer (μm) and a surface area of approximately 30 m2/g. These particles can be pressureless sintered to near theoretical density at temperatures as low as 1873 K. Disclosure of Invention One aspect of the present invention is a process for preparing AIN powderthat comprises passing paniculate Al metal and a nitrogen source through a heated reaction zone that is maintained at a temperature sufficient to individually heat substantially all oftheAl metal particles at a heating rate of at least 100 K per second to a temperature within a temperature range of from 2473 K to 3073 K, at a rate of flow sufficient to maintain substantially all ofthe Al metal particleswithin said temperature range for a time period of from 0.2 to 10 seconds, the time period being sufficientto convert the particulate AI metal to a product having an AIN content of at least 75 percent by weight (wt-%), based upon product weight, the AIN having a surface area of greater than 10 m2/g.
A second aspect of the present invention is a process for preparing a composite or a mixture of AIN powder and a powdered compatible material that comprises passing a nitrogen source and an admixture of a powdered compatible solid material and particulate Al metal through a heated reaction zone that is maintained at a temperature sufficientto individually heat substantially all of the admixture particles at a heating rate of at least 100 K per second to a temperature within a temperature range of from 2473 K to 3073 K, at a rate of flow sufficientto maintain substantially all of the admixture particles within said temperature range for a time period of from 0.2 to 10 seconds, the compatible solid material being present in an amount sufficient to yield a material containing AIN and the compatible material, the time period being sufficientto convert at least 75 wt-% of the particulate AI metal to AI , the AIN having a surface area within a range of greater than 10 m2/g. The product beneficially has an AIN content of at least 90 wt-%, based upon product weight.
Athird aspect of the present invention isa carbothermal process for preparing AIN powder that comprises passing a nitrogen source and a powdered admixture of AI2O3 and carbon through a heated reaction zone that is maintained at a temperature sufficientto individually heat substantially all of the powdered admixture particles at a heating rate of at least 100 per second to a temperature within a temperature range of from 2473 Kto 3073 K, ata rate of flow sufficient to maintain substantially all of the powdered admixture particles within said temperature range for a time period of from 0.2 to 20 seconds, the time period being sufficient to convert the powdered admixture to a product having an AIN content of at least 75 wt-%, based upon product weight, the AIN having a surface area of greater than 10 m2/g.
In an aspect related to the first three aspects, the product passes from the heated reaction zone into a cooling zone maintained at a temperature sufficient to individually cool substantially all product particles at a cooling rate of at least 100 K per second to a temperature below 1073 K. The product must be cooled rapidly in order to minimize, if not eliminate, partial sintering or agglomeration of product particles. Partial sintering may interfere with subsequent pressureless sintering efforts. Agglomeration may lead to unacceptably low product surface areas. In a related aspect, the product can be passed through the heated reaction zone one or more additional times to increase the yield of AI N. The product can be passed through the reaction zone either alone or in conjunction with an amount of the powdered admixture.
A fourth aspect of the present invention is a process for converting AIN having a surface area of less than 15 m2/g to AIN having a surface area greater than or equal to 15 m /g that comprises passing a nitrogen source and powdered AIN having a surface area of less than 15 m /g through a heated reaction zone maintained at a temperature sufficient to individually heat substantially all of the AIN particles at a heating rate of at least 100 K per second to a temperature within a temperature range of from greater than 2473 K to 3073 K, at a rate of flow sufficient to maintain substantially all of the AIN particles within said temperature range for a period of time sufficientto dissociate the AIN into AI and nitrogen as dissociation products and thereafter passing the dissociation products into a cooling zone maintained at a temperature sufficientto convert the dissociation products into AIN particles having a surface area of greater than 15 m2/g and individually cool substantially all of said particles at a cooling rate of at least 100 K per second to a temperature below 1073 K. The process of the present invention is suitably carried out in an apparatus like that disclosed in U.S.-A 5,1 10,565. The apparatus comprises four principal components: a cooled reactant transport member; a reactor chamber; a heating means; and a cooling chamber. A purge gas may be introduced into spaces surrounding the reactor chamber.
The transport member may be likened to a conduit disposed within a gas flow space that is desirably annular. The transport member is suitably maintained at a temperature below that at which powdered aluminum metal melts for the first and second aspects of the invention. Similar temperatures suffice forthe third aspect of the invention. The temperature is beneficially sufficient to substantially preclude the powdered reactants, particularly Al metal and AI2O3, from melting and coalescing either within the transport member or proximate to its exit. Accordingly, the temperature is desirably sufficientto allow substantially all of the powdered reactants to enter the reactor chamber as discrete particles. A temperature below the melting point of Al (about 933 K) yields satisfactory results. The range is beneficially from 275 to 373 K, desirably from 275 to 323 K, and preferably from 275 to 298 K. Higher temperatures may be used in the fourth aspect as AI sublimes at 2273 K and dissociates at 2473 K. The highertemperatures beneficially do not exceed the sublimation temperature.
The powdered reactants, whetherthey be Al metal, an admixture of powdered Al metal and a powdered compatible material, or a powdered admixture of AI2O3 and carbon, are suitably fed into the transport section via a powderfeeding mechanism. AIN having a surface area of less than 15 m2/g may be fed in the same manner. The powderfeeding mechanism is not particularly critical so long as it provides a metered or controlled flow of powdered material to the transport section. As such, the feeding mechanism may be a single screw feeder, a twin screw feeder, a vibratory feeder, a rotary valve feeder or some other conventional feeder.
The powdered Al metal should have a purity greater than about 97 percent and a weighted mean particle size less than about 500 μm. The compatible solid material is suitably AIN or a fine ceramic powder that, when mixed with the AIN product, forms a desirable powdered admixture or composite product powder. Suitable ceramic powders include silicon carbide, boron nitride, boron carbide, titanium diboride, silicon nitride, titanium nitride, titanium carbide, tungsten carbide or tantalum nitride. The fine ceramic powder used as a compatible material beneficially has a purity of greaterthan 98%, a surface area of from 10 to 30 πτ7g and a particle size within a range of from 0.01 to 1 μm. AIN, when used as a compatible materialJs suitably a portion ofthe AIN product. Admixtures having a compatible material content of from 20 to 95 wt-%, based on admixture weight, yield beneficial results. The compatible material content is desirably from 28 to 91 wt-% and preferably from 33 to 67 wt-%, based upon admixture weight.
The Al203 used in the present invention suitably has a particle size within a range of from 0.05 to 20 μm and a purity greater than about 99.8 percent. The range is beneficially from 0.2 to 2 μm. The purity is desirably greaterthan about 99.97 percent. Purities of 99.99 percent or greater will produce satisfactory products, but a a greater cost.
The carbon is suitably a material selected from acetylene black, plant carbon, thermal black, coke, carbon black and graphite. The material is beneficially carbon black or graphite. The admixtures may be prepared by using a conventional mixing apparatus.
Illustrative apparatus include ribbon blenders, roller mills, vertical screw mixers, V-blenders, and fluidtzed zone mixers such as that sold under the trade designation FORBERG'".
The powder feed rate varies with reactor design and capacity as well as the powdered reactants. Byway of illustration, an acceptable feed rate for powdered aluminum is from 0.02 to 0.5 kilograms per minute (kg/m) for a reactor having a reaction zone volume of 2J6 ft3 (0.06 m3). Acceptable feed rates for reactors having greater reaction zone volumes are readily determined without undue experimentation. Gaseous nitrogen (N2) is fed into the transport section in an amount and at a rate of flow sufficientto satisfy two requirements. First, the flow rate should be at least stoichiometric, or sufficient to satisfy the relevant equation. The equation is 2 Al + N2 -> 2AIN for the direct nitridation reaction and AI2O3 + 3C + N → 2AIN + 3CO for the carbothermal reaction. Second, the flow rate should be sufficient to entrain either the powdered reactants or the low (less than 15 m2/g) surface area Al N powder prior to the entry thereof into the reactor chamber. The flow rate is suitably at least one and one-half times stoichiometric. The flow rate is beneficially from 1.5 to 4 times stoichiometric, desirably from 1.5 to 3 times stoichiometric. An excessively high flow rate decreases residence time of powdered material, eitherthe reactants orthe low surface area AIN powder, within the reaction zone and, in turn, reactor capacity. The flow rate for the carbothermal reaction is typically greater than that for the direct nitridation reaction. By way of illustration, a suitable flow rate for the direct nitridation reaction might be 3 standard cubic feet per minute (SCFM) (85 standard liters per minute (SLM)) whereas a suitable flow rate forthe carbothermal reaction might be 4 SCFM (1 13 SLM).
Gaseous nitrogen ( 2(g))is also fed into the gas flow space. This gas flows from the gas flow space into the reactor chamber. In doing so, it acts to minimize, if not substantially eliminate, contact of powdered reactants with reactor chamber surfaces neartheirjuncture with the transport member exit. Such contact is undesirable because these surfaces tend to be at temperatures that promote coalescence of the powdered reactants, particularly Al metal and AI.O.. Coalescence leads, in turn, to cessation of operations due to reactor plugging.
The N2(g) should be as pure as possible. Moisture and residual oxygen impurities adversely affect AIN product quality. The N2(g) dew point is beneficially less than 233 K and desirably less than 193 K. Oxygen impurity levels are beneficially less than 5 and desirably less than 1 part per million parts (ppm) of gas. If the moisture content is too highJt may be necessary to pass the gaseous nitrogen through a drying bed or desiccant. The gas may also be purified by conventional means to reduce the residual oxygen content.
The N2(g) flow rates control residence time of the Al metal powder, the admixture of a powdered compatible solid material and particulate AI, or the powdered admixture of AI.O and carbon within the reaction zone. The N2(g) flow rates also control residence time of the low surface area AIN powder within that zone. When AI metal powder, either alone or in admixture with a powdered compatible solid material, is added to the reaction zone, the residence time is suitably within a range of from 0.2 second to 10 seconds, beneficially from 2 to 8 seconds and desirably from 4 to 6 seconds. Residence times of less than 0.2 second tend to yield an incompletely converted product containing unreacted metal. Residence times in excess of 10 seconds provide no great advantage in terms of conversion percentage. When low surface area Al N is added to the heated reaction zone, the same residence times provide satisfactory results. When a powdered admixture of Al203 and carbon is added to the reaction zone, the residence time is suitably within a range of from 0.2 second to 20 seconds, beneficially from 2 to 16 seconds and desirably from 4 to 15 seconds. Residence times of less than 0.2 seconds do not provide a satisfactory Al N product yield. Residence times in excess of 20 seconds offer no substantial advantage in terms of conversion percentage. Ammonia may be used instead of, or in addition to, N2(g). A mixture of N2(g) and hydrogen may also be used.
The entrained flow of powdered reactants or low surface area AIN powder enters the reaction zone in a form approximating that of a well dispersed dust cloud. The powdered reactants or low surface area AIN powder particles are heated almost instantly by gas convective and conductive heat transfer and by thermal radiation from reactor walls that define the heated reaction zone. The reaction zone is beneficially maintained at a temperature within a range of from 2473 to 3073 K. The temperature range is desirably from 2473 to 2773 K. Temperatures below 2473 K lead to AIN products with surface areas of 8 m /g or less. The actual temperature within the reaction zone may be determined by optical pyrometry or other suitable means.
The reactor walls may be conventionally heated either indirectly by radiation from heating elements spaced around them, inductively via an inductive coil, or directly by electrical resistance. The powdered reactants or low surface area Al N powder particles are heated at rates within a range of from 100 to 10,000,000 K per second. The range is beneficially from 1000 to 10,000,000 K per second, desirably from 10,000 to 1,000,000 K per second. The rate at which an individual particle is heated varies in response to a number of factors. The factors include size, proximity to the source of heat, and density of the dust cloud. The rate should not, however, be so low that substantial coalescence of reactant particles can occur during a melting phase prior to reaching reaction temperatures. In the case of low surface area AIN particles, the rate should notbe so low that the AIN passes into the cooling zone without being substantially dissociated into N2(g) and Al as dissociation products.
The gaseous nitrogen flow that provides an entrained flow of powdered reactants into the reaction zone also provides an entrained flow of powdered Al N or, in the case of low surface area AIN feed material, dissociation products out ofthe reaction zone. The entrained flow or dust cloud of AIN powder or the entrained flow of dissociation products beneficially exits the reaction zone and almost immediately enters a cooling zone. It is believed that the dissociation products recombine to form AIN powder having a surface area in excess of 15 m /g at or nearthe cooling zone entrance. The cooling zone quenches or rapidly cools the AIN powder below its reaction temperature. Rapid cooling helps maintain the very fine particle size of AI particles that form upon cooling below the dissociation temperature (about 2473 K) of AIN. Cooling rates within the cooling zone beneficially approximate the heating rates within the reaction zone. The cooled walls of the cooling zone and cooled gas tend to rapidly deplete remaining amounts of sensible heat from the AIN particles. If cooling does not occur at a sufficiently rapid rate, AIN particles tend to fuse or sinter together, thereby resulting in formation of undesirable agglomerates or large grains of AIN product. The fused particles tend to have surface areas much lower than 10 m2/g. Actual cooling times vary depending upon factors such as particle size, cooling zone configuration and gas flow rates. The cooled AIN particles are suitably collected and processed by conventional technology.
The AIN product has a surface area that is beneficially greater than 10 m2/g when powdered reactants are used as a starting material and greater than 15 m /g when low surface area AIN powder is used as the starting material. The surface area is desirably within a range of greater than 15 to 65 m /g. The range is preferably from 19 to 60 m /g. The following examples illustrate, but do not limit, the scope of the invention. All parts and percentages are by weight unless otherwise stated. Example 1 - Direct Nitridation at 2473K
A 6 inch (0J 52m) inside diameter by 1 1 feet (3.35 m) long heated zone vertical graphite tube furnace was brought to, and maintained at, a temperature of 2200CC (2473 K) as measured by optical pyrometers.
High purity Al metal powder, commercially available from Aluminum Company of America (Alcoa) under the trade designation 7123, was loaded into an overhead feed hopper that was purged with N2(g). The Al powder, nominally 99.97% pure, had a surface area of 0.449 m /g, an oxygen content of 0.223 %, a silicon content of 75 ppm, a calcium content of less than 10 ppm, a chromium content of less than 10 ppm, an iron content of 44 ppm and a mean particle size of 18 μm. The N2(g) gas had a dew point of less than -80°C (193 K) and an oxygen content of less than one ppm.
The powdered Al was conveyed from the hopper to the top of the heated zone of the tube furnace via a loss-in-weight twin screw feeder connected to a reactant transport member like that described herein at a rate of 0.2 pounds (0.09 kg) per minute. The reactant transport member was maintained at a temperature of 283 K. Nitrogen gas flowed through the reactant transport member at a rate of 3 SCFM (85 SLM) thus sweeping the Al powder with it into the top of the heated zone. An additional one SCFM (28.3 SLM) of N2(g) flowed through the gas flow space within which the transport member was disposed and into the top of the furnace. The flow of gas was sufficient to provide the powder with an average residence time in the heated zone of about 4.3 seconds accounting forthe decrease in nitrogen flow due to reaction and the conversion to AIN. The characteristic heating time for the feed Al particles equated to an estimated heating rate of approximately 105 K/second.
The submicron product powder was swept through the cooling zone by approximately 3 SCFM (85 SLM) of unreacted nitrogen gas exiting the reaction zone. The calculated residence time in the cooling zone was approximately 3.2 minutes.
Product powder from the reactor was collected downstream from a cooling zone and analyzed. The cooling zone had an inside diameter of 18 inches (45.7 cm), a length of six feet (1.8m) and a volume of 10.6 ft3 (0.3 m3). Coolant maintained at a temperature of 283 K and flowing through a jacket surrounding the cooling zone cooled the product powder and gas to a measured temperature of approximately 303 K. The rate of cooling approximated the rate of heating. An x-ray diffraction pattern ofthe product indicated that the powderwas substantially AIN with some unreacted metal. The oxygen and nitrogen contents of powder placed into a glove box with a N2(g) atmosphere immediately following synthesis were determined by a LECO analyzer to be, respectively, 0.16% and 27J %. The nitrogen content equated to an AIN content of 79%. The powder had an unmilled aggregate BET surface area of 15.1 m2/g. The product powder was dry ball milled fortwo hours using AIN media to break up light agglomerates of product. The term "light agglomerates" describes agglomerates that break apart readily when rubbed between thumb and fingers with minimal pressure. The BET surface area after ball milling was 15.7 m2/g. Taking into account about 21 % of unreacted aluminum with an estimated surface area of 0.5 m /g, the AI had a calculated, weighted, average surface area of 19.7 m /g.
Similar results are expected attemperatures as high as 3073 K. The AIN surface area tends to increase as temperatures in the heated zone increase above 2473 K.
Replication ofthe process at heating zone temperatures below 2473 K, such as 1873 K or 2373 K, yields a product wherein the AIN has a surface area that is much smaller. A typical surface area resulting from such temperatures is on the order of 5 m2/g.
Comparative Example A - Carbothermal Process at 2223 K
The apparatus of Example 1 was used to convert a powdered admixture of AI203 and carbon black to AIN via a carbothermal reduction-nitridation reaction. The powdered admixture was prepared from 25 pounds (lb) (1 1.4 kg) of acetylene carbon black and 62 lb (28.2 kg) of Al O . The A!_03, commercially available from Aluminum company of America underthe trade designation A16-SG, had a surface area of 9.46 rtrVg. The carbon, commercially available from Chevron Chemical Company under the trade designation Shawinigaπ" acetylene black, had a surface area of 67 nrVg. The AI.O. and carbon black were blended by bail milling for4 hours.
Two hundred lbs (90.8 kg) of deionized waterwere loaded into a 55 gallon (208 1) plastic drum. The following components were added to the water and mixed for 5 minutes: 1.5Triton® X- 00, an alkylphenoxy(polyethoxy)ethanol, commercially available from Rohm & Haas Co.; 40 miililiters (ml) of Arquad® C-50, a coco-alkyltrimethyl quaternary ammonium chloride, commercially available from Akzo Chemicals, inc.; and 150 ml of Antifoam"* B, a silicone product, commercially available from Dow Corning Corporation. The solution pH was adjusted to a pH of 3.5 by adding 20 ml of a 65 % nitric acid solution. Thirty five lbs (15.9 kg) of the ball milled admixture were added to the deionized water solution, then stirred for 30 minutes before adding 17.75 lb (8.06 kg) of a 20 % colloidal Al203 solution with continued agitation. The solution was agitated for an additional two hours before it was spray dried while maintaining an outlet temperature of 403 K. The spray dried powder had a carbon content of 26.6% as determined by analysis using a LECO model IR-412 with a Model HF-400 furnace.
The spray dried powder was loaded into the feed hopper and purged with N (g). The furnace was brought to, and maintained at, a temperature of 2173 K. N2(g) flowed into the reaction zone at the same rate as in Example 1. The powder was metered into the cooled reactanttransport member at a rate of 0.2 lb/minute (0.09 kg/mi n) as in Example 1. The flow rate provided an approximate residence time of 3.2 seconds. The product powder was collected downstream from the cooling zone as in Example 1. The powder was reloaded into the hopper and passed through the reaction zone a second time while the furnace was at a temperature of 2123 K. The reloading was replicated two additional times save for increasing the temperature to 2223 K to provide a total residence time of about 13 seconds. X-ray diffraction analysis of the product showed that it was substantially AIN. A portion of the product was placed in a furnace and heated at 1 123 K for 2 hours in the presence of air to remove residual carbon. The heated product was analyzed to contain 1.1 % carbon, 8.0% oxygen and 32.0% nitrogen, indicating an AIN content of about 82%. The BET surface area was 6.0 m2/g. Example 2 Carbothermal Process at 2573 K
The remaining product from Comparative Example A was loaded into the feed hopper and passed through the heated reaction zone. The furnace was heated to, and maintained at, 2573 K for this pass. The residence time in the reaction zone was about 3.0 seconds. The powder, after heat treatment as in Comparative Example A, contained 0.6 % carbon, 8.7% oxygen and 31.7% nitrogen, indicating an AIN content of about 80.9%. The BET surface area was 52.6 m2/g.
By contrasting Example 2 with Comparative Example A, the beneficial increase in surface area by operating above the dissociation temperature of AIN is readily apparent. Similar results are expected by eliminating the low temperature passes of Comparative Example A and using one or more passes at temperatures at or above the dissociation temperature. Suitable temperatures and other operating parameters are disclosed herein.

Claims

Claims :
1. A process for preparing aluminum nitride powder that comprises passing particulate aluminum metal and a nitrogen source through a heated reaction zone that is maintained ata temperature sufficientto individually heat substantially all ofthe aluminum metal particles at a heating rate of at least 100 K per second to a temperature within a temperature range of from 2473 Kto 3073 K, at a rate of flow sufficientto maintain substantially all ofthe aluminum metal particles within said temperature range for a time period of from 0.2 to 10 seconds, the time period being sufficientto convert the particulate aluminum metal to a product having an aluminum nitride content of at least 75 percent by weight, based upon product weight, the aluminum nitride having a surface area of greater than 10 m /g.
2. A carbothermal process for preparing aluminum nitride powder comprises passing a nitrogen source and a powdered admixture of alumina and carbon through a heated reaction zone that is maintained at a temperature sufficientto individually heat substantially all ofthe powdered admixture particles at a heating rate of at least 100 K per second to a temperature within a temperature range of from 2473 Kto 3073 K, ata rate of flow sufficient to maintain substantially all ofthe powdered admixture particles within said temperature range for a time period of from 0.2 to 20 seconds, the time period being sufficientto convert the powdered admixture to a product having an aluminum nitride content of at least 75 percent by weight, based upon product weight, the aluminum nitride having a surface area of greaterthan 10 m /g.
3. A process for preparing a composite ormixture of aluminum nitride powder and a powdered compatible material comprises passing an admixture of a powdered compatible solid material and particulate aluminum metal and a nitrogen source through a heated reaction zone that is maintained at a temperature sufficientto individually heat substantially all ofthe admixture particles at a heating rate of at least 100 K persecond to a temperature within a temperature range of from 2473 K to 3073 K, at a rate of flow sufficient to maintain substantially all of the admixture particles within said temperature range for a time period of from 0.2 to 10 seconds, the compatible solid material being present in an amount sufficient to yield a composite material containing aluminum nitride and the compatible material, the time period being sufficient to convert at least 75 percent by weight of the particulate aluminum metal to aluminum nitride, the aluminum nitride having a surface area of greaterthan 10 m2/g.
4. A process as claimed in Claim 3 wherein the compatible material is selected 5 from aluminum nitride, silicon carbide, boron nitride, boron carbide, titanium diboride, silicon nitride, titanium nitride, titanium carbide, tungsten carbide or tantalum nitride.
5. A process as claimed in Claim 1 , Claim 2 or Claim 3 further comprising a sequential step wherein the particulate product is passed into a cooling zone after it exits the
10 heated reaction zone and quenched to a temperature below that at which particulate aluminum metal is converted to aluminum nitride.
6. A process as claimed in Claim 5 wherein the product is quenched at a cooling rate of at least 100 K per second within a time of from 0J second to about 4 minutes after said product exits the heated reaction zone.
15 7. A process for converting aluminum nitride having a surface area of less than 15 m2/g to aluminum nitride having a surface area greater than or equal to 15 m /g comprises passing a nitrogen source and powdered aluminum nitride having a surface area of less than 15 m2/g through a heated reaction zone maintained at a temperature sufficient to individually heat substantially all of the aluminum nitride particles at a heating rate of at least
20 100 K per second to a temperature within a temperature range of from greater than 2473 K to 3073 K, at a rate of flow sufficient to maintain substantially all of the aluminum nitride particles within said temperature range for a period of time sufficient to dissociate the aluminum nitride into aluminum and nitrogen as dissociation products and thereafter passing the dissociation products into a cooling zone maintained at a temperature sufficient to convert
25 the dissociation products into aluminum nitride particles having a surface area of greater than 15 m /g and individually cool substantially all of said particles at a cooling rate of at least 100 K per second to a temperature below 1073 K.
30
35
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