Title of Invention

"A PROCESS FOR PREPARATION OF ALUMINIUM NITRIDE POWDER"

Abstract

The invention provides a process for synthesis of aluminum nitride powder by a novel means of nitriding a shell of aluminum bearing species on the core of carbon particles at overpressures of nitrogen. The process is accomplished by depositing aluminum salt on nano-sized carbon particles followed by calcination of the aluminum salt and subsequent nitridation and decarbonisation of the product. It produces highly pure aluminium nitride material in the form of whiskers and particulates.

Full Text

The present invention relates to a process for the manufacture of aluminum nitride powder. Dense aluminum nitride is used as ceramic components for use in electronics, structural and high temperature materials due to its interesting properties such as high thermal conductivity, interesting mechanical, thermal and electrical properties. Thermal conductivity depends on the purity of the feedstock and consequently on that of aluminum nitride powder. AIN powder is also used for making ceramic composites and alloys with other ceramics, metals, polymers etc. which find varied uses as structural, electronic and refractory materials. The present day method of making aluminum nitride powder consists of carbothermal nitridation of alumina, direct nitridation of aluminum metal, fluoride decomposition, striking a DC arc between high purity alumina electrodes in nitrogen gas, etc. The first two processes mentioned above are mainly accomplished commercially. Careful control of composition of starting raw materials and of reaction atmosphere are necessary for obtaining high purity aluminum nitride powder, otherwise, the reaction often results in unreacted carbon and AI2O3, either of which impacts dramatically on the thermal conductivity of the sintered product. For the present day method of synthesis of aluminum nitride powder reference may be made to J. D. Boh in "Aluminum nitride articles by carbothermal nitridation", US Pat. No. 4857246, wherein precursor fiber or films containing alumina have been densified followed by its carbonization at 1000-1200'C and then carbothermally nitrided at 1550-1800°C to produce aluminum nitride. The drawbacks of this process are (i) the starting material is AI2O3, (ii) aluminum nitride fibers are produced, not particles, and (iii) chances of unconverted alumina remaining in the final product. References may also be made to H.Inoue, A.Tsuge and K.Komeya in "Process for preparing aluminum nitride", US Pat. No. 4680278 wherein aluminum hydroxide powder is mixed first either with carbon or such a substance which is capable of forming carbon powder by heating. An additive, selected from the group consisting of aluminum nitride powder, siUcon carbide powder or powders of substances capable of forming powder corresponding to these powders, was added to mix with the above mixture. This was then baked under nitrogen atmosphere at 1300-1850'C for 2-20 hours followed by heating the product in air at 750'C for 5-7 hours to produce aluminum nitride powder. The drawbacks are (i) excessive steps of mixing, and (ii) increase of level of impurity on introduction of additives. References may also be made to J.J.Kim, V.Venkateswaran, J.D.Jatz and A.Y.Sanein "Process for the continuous production of high purity, ultrafine, aluminum nitride powder by carbonitridation of alumina", US Pat. No. 5154907 wherein AI2O3 is mixed with three times its weight of carbon and 0.5% w/w CaF2, followed by dry mixing for five minutes at high speed setting. It was wet mixed for further four minutes after addition of a binder at high speed setting followed by overnight drying at 1200C, sieving and feeding at a fluid bed reactor. The product undergone milling to obtain micron, ultrafine aluminum nitride powder. The . drawbacks of this process are (i) chances of remaining carbon and calcuium impurities in the product, and (ii) involvement of costly processing steps such as grinding and milling in inert atmosphere. References may also be made to P.Ravel and R.Bachelard in "Carbonitriding of alumina to produce aluminum nitride", US Pat. No. 5525321 wherein finely ground alumina and acetylene black (equivalent C:Al203 = 3:1 w/w) were mixed with the help of a dispersant followed by attrition milling. A paste is thus obtained which is dried m a ventilated oven followed by feeding in a moving bed reactor at a rate of 4.05 kg/h and the feedmg of nitrogen at the lower part of the reactor at a low rate of 24 kg/h. The soaking temperature is 1450- ITOO^'C and the duration of soaking is 7 hours. The product was then decarbonized to obtain aluminum nitride powder. The drawbacks of this process are (i) starting with alumina, (ii) involvement of costly step of mixing by attrition milling, and (ii) The ratio of carbon and alumina is high (CiAhOs = 3:1 w/w). References may also be made to D.A.Dunn, M.S.Paquette, H.Easter and R.K.Pihlaja in "Continuous carbothermal reactor", US Pat. No. 4983553 wherein palletized mixture of aluminum oxide, carbon and optionally CaO is taken in a container through a heated reaction zone of temperature 1500-1900*'C, supplying gaseous nitrogen for 0.25-6.00 hours. The residual carbon is removed fi-om the product, by heating it in air to get aluminum nitride powder. The drawbacks of this process are (i) The carbon impurity in the final product was not examined, and (ii) There are chances of remaining calcium as impurity in the product. References may also be made to M.Kumagai, Y.Yoshu and R.Ucliimura in "Method for preparing aluminum nitride powder", US Pat. No. 4780299 wherein boehmite or pseudoboehmite is added to water at pH 1.2-4.5 to obtain its sol followed by addition of fine carbon powder and a solution of a dispersant. After having kneaded this, a dispersion of carbon and boehmite was obtained which was then solidified keeping the uniform state of matter via gelation. The product was then sintered using yttrium nitrate as a sintering agent followed by nitridation at 1350-1700°C. The product was then decarbonized to obtain aluminimi nitride powder. The drawbacks of this process are (i) starting with boehmite and preparation of its sol is a costly process, (ii) involvement of costly step of grinding the boehmite powder to 500A particle size, and (iii) chances of unreacted alpha alumina remaining in the product. References may ftirther be made to Y.Shintaku in "Method of making aluminum nitride powder", US Pat. No. 4612045 wherein metallic aluminum was first melted and atomized at SOO^C in a nitrogen atmosphere to get atomized particle of aluminum nitride. Its cooling yields solid aluminum nitride powder. The drawbacks of this process are (i) metallic aluminum is not available naturally and hence the raw material is costly, and (ii) chances of remaining unconverted metallic aluminum as impurity in the final product. The main drawbacks of the above processes are: 1. The step of mixing aluminum oxide or aluminum hydroxide or boehmite powder with carbon by attrition milling is time consuming, costly and does not ensure a homogenous mixture at the end. 2. As a result unconverted AI2O3 and C remain present at the end. Removal of residual carbon becomes very difficult. 3. Aluminum metal remains in the final product as impurity in the case of direct nitridation of aluminum metal process. 4. Use of an additive or a sintering agent other than aluminum nitride may increase the level of impurity in the product. The main object of the present invention is to provide a process for synthesis of aluminum nitride powder which obviates the above drawbacks. Yet another object of the present invention is to ensure the absence of A12O3 and C in the final product. Still another object of the present invention is to avoid metallic contaminants in the final product. In the present invention, that relates to a process for synthesis of aluminum nitride powder by carbothermal reduction of α-A12O3 particles in presence of nitrogen atmosphere, α- A12O3 is prepared by decomposition of A1(NO3)3 by pyrolysis in presence of carbon particles having an average size of 11nm at a temperature in the range of 350-400C. To achieve this the carbon particles are taken in a sol prepared by suspension in an aqueous solution of a non-ionic lyophillic agent such as polyoxyethylene(20)sorbitanmonooleate in conductivity grade water with constant stirring at room temperature. The mixture was then evaporated slowly at a temperature in the range of 100-150°C for a period in the range of 120-600 minutes and then pyrolysed under ambient atmosphere at a temperature in the range of 350-400°C for a period in the range of 30-60 minutes. The pyrolysed product then becomes a homogeneous mixture of γ-A12O3 and carbon by formation of a shell of A12O3 on a core of carbon particles. The pyrolysed product was then heated at a temperature in the range of 1600-1800°C for a period in the range of 1 -4 hours under nitrogen atmosphere having a pressure in the range of 50-70 psig and water vapour less than lOppm. The nitrided product was then decarbonized by heating in ambient atmosphere at a temperature in the range of 600-800°C for a period in the range of 2-4 hours. The product was characterized by XRD, SEM and C,N analyzer. The novelty of the present invention is the easier method of preparation of A1N in the form of whiskers and particulates with less impurities by the inventive step of nitridation of a calcined solid mass produced by crystallization of A1(NO3)3,9H2O on carbon particles of a carbon sol. Accordingly the present invention relates to a process for synthesis of aluminum nitride powder, wherein the said process comprises: a. adding carbon particles in the proportion in the range of 6.9-23.3wt% to an aqueous solution of polyoxyethylene(20)sorbitanmonooleate of concentration in the range of 0.7-2.4wt% under constant mechanical stirring; b. adding aqueous solution of aluminum salt to the said carbon sol as obtained in step (a) under constant mechanical stirring so as to obtain an ultimate ratio of Carbon : A12O3 in the range of 2:1 to 3:2 in the mixed sol; c. sonicating the resultant mixed sol as obtained in step (b) for a period in the range of 8-20 minutes; d. evaporating the sonicated sol as obtained in step (c) under constant stirring at a temperature in the range of 100-`50°C for a period in the range of 120-600 minutes to obtain a gel; e. pyrolysing the said gel as obtained in step (d) at a temperature in the range of 350- 400°C under ambient or nitrogen atmosphere in a furnace wherein the extracted gas may be dissolved in water for a period in the range of 30-60 minutes to obtain a pyrolysed product; f. characterized in nitridation of the pyrolysed product as obtained in step (e) at a temperature in the range of 1600-1800°C for a period in the range of 1-4 hrs under a nitrogen atmosphere with a pressure in the range of 50-70 psig at the hold temperature and in such a manner so as to reduce the nitrogen gas pressure of the order of 10 psig in an interval in the range of 15-30 minutes. g. finally decarbonizing the heat treated products as obtained in step (f) by holding at a temperature in the range of 600-800°C for a period in the range of 2-4 hours under ambient atmosphere to obtain aluminum nitride powder. In another embodiment of the present invention a sol of carbon particle in water may be prepared by addition of carbon particles to a solution of lyophillic non-ionic agent such as polyoxyethylene(20)sorbitanmonooleate which can modify the surfaces properties of carbon particles. In an embodiment of the present invention the salt of aluminum used may be such as A1(NO3)3.9H2O, Al2(SO4)3.12H2O, AIC13.6H2O. In yet another embodiment of the present invention the water used for dispersion medium may be deionized and of conductivity grade. In still another embodiment of the present invention, the gel may be pyrolised at a temperature in the range of 350-450°C imder ambient or nitrogen atmosphere in a fiimace and the extracted gas may be dissolved in water for safety of the environment. In still yet another embodiment of the present invention the nitridation may be effected at a temperature in the range of 1600-18000C for a period in the range of 1-4 hrs under a nitrogen atmosphere with a pressure in the range of 50-70 psig at the hold temperature and in such a manner so as to reduce the nitrogen gas pressure in steps of the order of 10 psig in an interval in the range of 15-30 minutes. The present invention, thus, provides a process for the manufacture of aluminimi nitride powder which comprises: 1. Preparation of a carbon sol in water by addition of carbon particles in the proportion range of 6.9-23.3wt% to an aqueous solution of polyoxyethylene(20)sorbitanmonooleate (0.7-2.4wt% with respect to water) under constant mechanical stirring. 2. Addition of an aqueous solution of aluminum nitrate to the said carbon sol imder constant mechanical stirring so as to obtain an ultimate ratio of C: AI2O3 in the range of 2:1 to 3:2 by weight. 3. Sonication of the suspension for a period in the range of 8-20 minutes. 4. Drying of the product of step 3 under constant stirring at a temperature in the range of 100-150°C for a period in the range of 120-600 minutes. 5. Pyrolysis of the product of step 4 at a temperature in the range of 350-400°C for a period in the range of 30-60 minutes. 6. Nitridation of the pyrolysed product at a temperature in the range of 1600-1800°C under nitrogen atmosphere having pressure in the range of 50-70 psig and H2O vapour below 10 ppm for a period in the range of 1-4 hours with stepwise reduction of the nitrogen gas pressure in steps of 10 psig by releasing the gas from the furnace at an interval in the range of 15-30 minutes. 7. Decarbonization of the product obtained in step 6 by heating in air at a temperature in the range of 600-8000'C for a period in the range of 2-4 hours. The following examples are given as illustration and should not be construed to limit the scope of the present invention. EXAMPLE 1 1.12 gm of polyoxyethylene(20)sorbitanmonooleate was dissolved in 161 gm of distilled water. 11.2 gm of carbon black was added to the above solution with mechanical stirring till a sol of carbon particles was obtained. 41.18 gm of A1(N03)3.9H20 (equivalent to 5.6 gm of AI2O3) was dissolved in 21.2 gm of distilled water to prepare an aqueous aluminum nitrate solution. The aluminum nitrate solution was then added dropwise to the above dispersion of carbon with constant stirring. The dispersion was then sonicated by an ultra soimd frequency of 120 kHz for 8 minutes. An unstable sol was thus obtained. This sol was then evaporated slowly with stirring on a hot plate at 1000C within a fume cupboard chamber for 600 minutes. The product was then pyrolysed at 350°C for 60 minutes to obtain a dry mass. The dried product was then heated in an uncovered graphite crucible under nitrogen atmosphere at 16000C for 2h. After attaining a pressure of 50 psig the pressure was reduced by a step of 10 psig every 30 minutes. The product was then cooled and decarbonised in a muffle furnace at 700°C under ambient atmosphere for 2 hours. A gray-white product was obtained. This product was then characterized by XRD and SEM, which shows that both whiskers and particles of AIN having ~2-6µ. size. Hot gas extraction analysis shows 33.33% nitrogen and 0.37% carbon content in the powder. EXAMPLE 2 1.12 gm of polyoxyethylene(20)sorbitanmonooleate was dissolved in 161 gm of distilled water. 11.2 gm of carbon black was added to the above solution with mechanical stirring till a sol of carbon particles was obtained. 41.18 gm of A1(N03)3.9H20 (equivalent to 5.6 gm of AI2O3) was dissolved in 21.2 gm of distilled water to prepare an aqueous aluminum nitrate solution. The aluminum nitrate solution was then added dropwise to the above dispersion of carbon with constant stirring. The dispersion was then sonicated by an ultra sound frequency of 120 kHz for 8 minutes. An unstable sol was thus obtained. This sol was then evaporated slowly with stirring on a hot plate at 1000C within a fume cupboard chamber for 600 minutes. The product was then pyrolysed at 2500C for 60 minutes to obtain a dry mass. The dried product was then heated in an uncovered graphite crucible under nitrogen atmosphere at 16000C for 2h. After attaining a pressure of 50 psig the pressure was reduced by a step of 10 psig every 30 minutes. The resultant product was then cooled and decarbonised in a muffle furnace at 600°C under ambient atmosphere for 4 hours. A gray-white product was obtained. This product was then characterized by XRD and SEM, which shows that both whiskers and particles of AIN having ~2-6µ. size and 0.34% carbon and 32.22% nitrogen content have been obtained. EXAMPLE 3 2.6 gm of polyoxyethylene(20)sorbitanmonooleate was dissolved in 109 gm of distilled water. 26 gm of carbon black was added to the above solution with mechanical stirring till a sol of carbon particles was obtained. 95.59 gm of A1(N03)3.9H20 (equivalent to 13 gm of AI2O3) was dissolved in 49.8 gm of distilled water to prepare an aqueous alimiinum nitrate solution. The aluminum nitrate solution was then added dropwise to the above dispersion of carbon with constant stirring. The dispersion was then sonicated by an ultra soimd frequency of 120 kHz for 20 minutes. A stable sol was thus obtained. This sol was then evaporated slowly with stirring on a hot plate at 1500C within a fume cupboard chamber for 120 minutes. The product was then pyrolysed at 400°C for 30 minutes to obtain a dry mass. The dried product was then heated in an uncovered graphite crucible under a positive nitrogen atmosphere at 1650'C for 2h. After attaining a pressure of 50 psig the pressure was reduced by a step of 10 psig every 30 minutes. The resultant product was then cooled and decarbonized in a muffle furnace at 700°C under ambient atmosphere for 2 hours. A gray-white product was obtained. This product was then characterized by XRD and SEM, which shows that both whiskers and particles of AIN having ~2-6ja. size and 0.42% carbon and 31.22% nitrogen content have been obtained. EXAMPLE 4 1.12 gm of polyoxyethylene(20)sorbitanmonooleate was dissolved in 161 gm of distilled water. 11.2 gm of carbon black was added to the above solution with mechanical stirring till a sol of carbon particles was obtained. 41.18 gm of A1(N03)3.9H20 (equivalent to 5.6 gm of AI2O3 and having impurities like chloride (CI) 0.01%, sulphates(S04) 0.01%, iron (Fe) 0.02%, potassium (K) 0.025% and sodium (Na) 0.025%) was dissolved in 21.2 gm of distilled water to prepare an aqueous aluminum nitrate solution. The aluminum nitrate solution was then added dropwise to the above dispersion of carbon with constant stirring. The dispersion was then sonicated by an ultra sound frequency of 120 kHz for 8 minutes. An unstable sol was thus obtained. This sol was then evaporated slowly with stirring on a hot plate at 1000C within a fume cupboard chamber for 600 minutes. The product was then pyrolysed at 3500C for 60 minutes to obtain a dry mass. The dried product was then heated in an uncovered graphite crucible under nitrogen atmosphere at 1650°C for Ih. After attaining a pressure of 50 psig the pressure was reduced by a step of 10 psig every 15 minutes. The resultant product was then cooled and decarbonised in a muffle furnace at 700°C under ambient atmosphere for 2 hours. A gray-white product was obtained. This product was characterized by XRD, which shows that a little amount of a-aiumina was also present alongwith AIN. EXAMPLE 5 2.6 gm of polyoxyethylene(20)sorbitanmonooleate was dissolved in 109 gm of distilled water. 26 gm of carbon black was added to the above solution with mechanical stirring till a sol of carbon particles was obtained. 95.59 gm of A1(N03)3.9H20 (equivalent to 13 gm of AI2O3) was dissolved in 49.8 gm of distilled water to prepare an aqueous aluminum nitrate solution. The aluminum nitrate solution was then added dropwise to the above dispersion of carbon with constant stirring. The dispersion was then sonicated by an ultra sound frequency of 120 kHz for 20 minutes. A stable sol was thus obtained. This sol was then evaporated slowly with stirring on a hot plate at 1500C within a fiime cupboard chamber for 120 minutes. The product was then pyrolysed at 4000C for 30 minutes to obtain a dry mass. The dried product was then heated in an uncovered graphite crucible under a positive nitrogen atmosphere at 1800°C for 2h. After attaining a pressure of 70 psig the pressure was reduced by a step of 10 psig every 20 minutes. The resultant product was then cooled and decarbonised in a muffle furnace at 800°C under ambient atmosphere for 2 hours. A gray-white product was obtained. This product was then characterized by XRD and SEM, which shows that both whiskers and particles of AIN having ~ 2-6µ, size and 0.35% carbon and 33.27% nitrogen content have been obtained. EXAMPLE 6 1.12 gm of polyoxyethylene(20)sorbitanmonooleate was dissolved in 148 gm of distilled water. 11.2 gm of carbon black was added to the above solution with mechanical stirring till a sol of carbon particles was obtained. 54.90 gm of A1(N03)3.9H20 (equivalent to 7.47 gm of AI2O3 and having impurities like chloride (CI) 0.01%, sulphates(S04) 0.01%, iron (Fe) 0.02%, potassium (K) 0.025% and sodium (Na) 0.025%) was dissolved in 28.3 gm of distilled water to prepare an aqueous alimiinum nitrate solution. The aluminum nitrate solution was then added dropwise to the above dispersion of carbon with constant stirring. The dispersion was then sonicated by an ultra sound frequency of 120 kHz for 8 minutes. An unstable sol was thus obtained. This sol was then evaporated slowly with stirring on a hot plate at 150°C within a fume cupboard chamber for 120 minutes. The product was then pyrolysed at 4000'C for 30 minutes to obtain a dry mass. The dried product was then heated in an uncovered graphite crucible under a positive nitrogen atmosphere at 16500C for 2h. After attaining a pressure of 50 psig the pressure was reduced by a step of 10 psig every 30 minutes. The resultant product was then cooled and decarbonized in a muffle furnace at 700°C under ambient atmosphere for 2 hours. A gray-white product was obtained. This product was then characterized by XRD and SEM, which shows that both whiskers and particles of AIN having ~2-6|i size and 0.30% carbon and 30.22% nitrogen content have been obtained. The main advantages of the process lies in producing fine aluminimi nitride whiskers and /or particulates having less imconverted reactants such as AI2O3 and C. The starting water soluble precursors of α-Al2O3 may be obtained from an intermediate in the conmiercial manufacture of AI2O3 or aluminium. The metallic impurities in the AIN powder so produced will be minimum. Yet another advantage of the process is the accomplishment of maximum homogenization of the composition of the starting reactants such as a -AI2O3 and C by deposition of Aluminum Nitrate on the nano sized substrate carbon particles. The cost of the starting carbon particles is also lower than other raw materials such as boehmite sols or sub-micron a- AI2O3 particles. WE CLAIM 1. A process for synthesis of aluminum nitride powder, wherein the said process comprises: a. adding carbon particles in the proportion in the range of 6.9-23.3wt% to an aqueous solution of polyoxyethylene(20)sorbitanmonooleate of concentration in the range of 0.7-2.4wt% under constant mechanical stirring; b. adding aqueous solution of aluminum salt to the said carbon sol as obtained in step (a) under constant mechanical stirring so as to obtain an ultimate ratio of Carbon : A12O3 in the range of 2:1 to 3:2 in the mixed sol; c. sonicating the resultant mixed sol as obtained in step (b) for a period in the range of 8-20 minutes; d. evaporating the sonicated sol as obtained in step (c) under constant stirring at a temperature in the range of 100-150°C for a period in the range of 120-600 minutes to obtain a gel; e. pyrolysing the said gel as obtained in step (d) at a temperature in the range of 350-400°C under ambient or nitrogen atmosphere in a furnace wherein the extracted gas may be dissolved in water for a period in the range of 30-60 minutes to obtain a pyrolysed product; f characterized in nitridation of the pyrolysed product as obtained in step (e) at a temperature in the range of 1600-1800°C for a period in the range of 1-4 hrs under a nitrogen atmosphere with a pressure in the range of 50-70 psig at the hold temperature and in such a manner so as to reduce the nitrogen gas pressure of the order of 10 psig in an interval in the range of 15-30 minutes. g. finally decarbonizing the heat treated products as obtained in step (f) by holding at a temperature in the range of 600-800°C for a period in the range of 2-4 hours under ambient atmosphere to obtain aluminum nitride powder. 2. A process as claimed in Claim 1 above in which the salt of aluminum used is chosen from the group A1(NO3)3.9H2O, Al2(SO4)3.12H2O, A1C13.6H2O. 3. A process as claimed in Claim 1-2 wherein the water used for dispersion is deionized and of conductivity grade. 4. A process for synthesis of aluminum nitride powder substantially as herein described with reference to the examples accompanying this specification.

Documents:

353-DEL-2002-Abstract-(19-03-2009).pdf

353-del-2002-abstract.pdf

353-DEL-2002-Claims-(19-03-2009).pdf

353-del-2002-claims.pdf

353-DEL-2002-Correspondence-Others-(19-03-2009).pdf

353-del-2002-correspondence-others.pdf

353-del-2002-correspondence-po.pdf

353-DEL-2002-Description (Complete)-(19-03-2009).pdf

353-del-2002-description (complete).pdf

353-del-2002-form-1.pdf

353-del-2002-form-18.pdf

353-del-2002-form-2.pdf

353-del-2002-form-3.pdf