DE4124119A1 - High purity, finely dispersed aluminium nitride powder prodn. - by mixing aluminium salt soln. with sodium carboxymethyl:cellulose soln. calcining the easily filtered prod., nitriding and igniting - Google Patents

Abstract

The prodn. involved the reaction of a homogeneous mixt. of Al203 powder and C (II) in a N2-contg. atmos., followed by pyrolysis to remove excess C. The reaction mixt. is prepd. by mixing aq. Al salt soln. with aq. Na carboxymethylcellulose (Na-CMC) soln. at pH 3-8, filtering off the sparingly soluble ppte. of Al cellulose-glycolate, washing, drying and calcining at 200-350 deg.C. then nitriding and igniting by known methods. Pref., the pH is adjusted by addn. of ammonia. USE/ADVANTAGE - (I) is useful for the prodn. of sintered polycrystalline ceramics with high thermal conductivity, for use as dielectrical insulators, high temp. dielectrics and substrates for high performance electronic circuits, etc.. The invention provides a simplified process for the prodn. of mixt. (II) in the form of very pure, highly reactive powder which is more suitable for nitriding than prior art mixts. (which require long mixing times and subsequent grinding operations)

Description

The invention relates to a carbothermal process for the production of highly pure, finely dispersed AlN powder.

This powder is used to manufacture densely sintered, polycrystalline Ceramics of high thermal conductivity, the preferred application in electrical and semiconductor technology as

• - electrical insulators
• - dielectric for high temperatures
• - Substrate for high performance electronics.

Further areas of application are chemically extraordinary resistant high temperature material application as crucible material for aluminum smelting or in single crystal growing, as Evaporator sources for modern surface coating processes, as a carrier material for laser diodes, as a covering material for Gas laser u. a.

In principle, there are three ways of producing AlN powder to reduce.

The main process is the direct nitriding of metallic aluminum powder different grain sizes in N₂ or NH₃ atmosphere at high temperatures [G. V. Samsonov i T. V. Dubovik, Tsvet. Metal. 35 (1962) 56] or in the arc [G. P. Vissokov and L. B. Brakalov, J. Mat. Sci. 18 (1983) 2011].

Since hard products are created in principle, there is always a subsequent one Grinding process inevitable. This only leads to once a limited fineness of the powder, for increased oxygen absorption, for the entry of impurities from grinding media and Grinding pot, and on the other hand, residues remain due to the process unconverted aluminum.

The second group of implementations goes from AlN compounds of Type R₃Al (R-halogen, alkyl groups), with exposure to NH₃ Nitrogen-containing interconnections arise that one Be subjected to pyrolysis.

Al-alkyl compounds are difficult to prepare and their handling  is not harmless because of the fire hazard, so that a method for AlN extraction in this way is more scientific Interested [L. V. Interrante, L.E. Carpentier II, C. Whitmarsh and W. Lee, Materials Research Society, Symposia Proceedings, Vol. 73, Palo Alto, California, April 15 - 19, 1986]. In the case of halogen substituents, fluorine and chlorine are used Commitment. The reactions of AlF₃ (made from (NH₄) AlF₆) with NH₃ at 800 ° C. . . 1000 ° C to AlN requires equipment very expensive Pt-Rh alloys because they are in equilibrium with the fluorine compounds free hydrogen fluoride occurs [I. Huseby, J. Amer. Ceram. Soc. 66 (1983) 217]. Reactions of AlCl₃ with NH₃ generally take place in the gas phase [I. Kimura, N. Hotta, H. Nukui, N. Saito and S. Yasukawa, J. Mater. Science Letters 7 (1988) 66]. Due to the extraordinarily high resulting Powder surfaces are difficult to absorb oxygen prevent. In addition, the preparation of larger quantities of AlN Difficulties because of a certain response time in the Pyrolysis zone is a prerequisite for AlN formation.

The third way are reduction processes where aluminum oxide in intimate mixing with fine carbon powder in N₂ or NH₃ atmosphere is converted to AlN. The finer the oxide and Carbon particles are the less the required Reaction temperature. At the same time determines the fineness of Al₂O₃ the AlN powder particle size. For this procedure, two Committed: Mechanical mixing of carbon (soot) with Al₂O₃ [O. Serpek, C.A. 14 (1920) 503] or mixing Al₂O₃ or a freshly precipitated aluminum hydroxide with a carbon dispenser in the wet phase and subsequent decomposition [N. Kuramoto, Jp 60-1 76 910 of Feb. 22, 84].

The mechanically produced mixtures can only be found at relative Convert high temperatures of <1600 ° C into nitridic powders. These high temperatures already lead to pre-compression (Sintering temperature of AlN 1800 ° C), so that grinding processes are inevitable will.

The mixture in the wet phase represents a significant improvement  In DD-WP C01B / 3 12 776 3 it is proposed to mix the Metal oxide and carbon by dissolving a water-soluble, coking organic compound and the corresponding metal hydroxide in water, followed by drying and subsequent coking to produce the organic compound.

Extensive homogenization in the wet phase also requires long mixing times and large amounts of liquid, which have to be removed by the drying process. This creates undesirable caking. Beyond that freshly precipitated aluminum hydroxide difficult to filter, which means the washing process is extended considerably. Also kick during the thermal conversion of the carbon donor (e.g. Sugar) melting processes that lead to the densification of the cake and thus lead to coarsening of the powder particles. Are here too Grinding operations inevitable.

The invention has for its object with a simplified method a powder mixture more suitable for the reduction nitridation to manufacture from aluminum oxide and carbon.

According to the invention the object is achieved in that the powder mixture is prepared by an aqueous aluminum salt solution and an aqueous Na carboxymethyl cellulose solution at pH Of 3 . . . 8 are mixed, being a sparingly soluble salt of aluminum cellulose glycolate, which then filters off, washed, dried and calcined at 200 ° C-350 ° C becomes.

The pH is adjusted with the addition of NH₃. The resulting highly active Al₂O₃-C mixture can already at temperatures fully carbonitrided from 1400 ° C to 1600 ° C will.

Depending on the pH value, the carbon content (depending on Insert) according to the following equations in the desired manner can be varied.

1. Cellulose-CH₂-OCH₂COONa + Al ³⁺ + 2 OH ^- →
Cellulose-CH₂-OCH₂COOAl (OH) ₂ + Na⁺
2. Cellulose-CH₂-OCH₂COONa + Al ³⁺ + OH ^- →
(Cellulose-CH₂-OCH₂COO) ₂AlOH + 2 Na⁺
3. 3 cellulose-CH₂-OCH₂COONa + Al ³⁺ →
(Cellulose-CH₂-OCH₂COO) ₃Al + 3 Na⁺

The Al-cellulose compounds are flaky and easy to filter so that a washing process is not difficult and a continuous process can be designed.

Drying takes place in the drying cabinet or by freeze drying (The latter preferred at higher pH, Eq. 1. and 2.). The The dried product is calcined in two stages with the exclusion of air: 200 ° C (3 h) and 350 ° C (4 h) and thereby in a homogeneous Al₂O₃-C mixture converted. Experience this highly active powder a conversion under oxygen and moisture-free nitrogen in aluminum nitride. The residual carbon is by annealing in air at 500 ° C. . . 600 ° C away, after which a light gray loose AlN powder remains.

As a result of the formation of a bond between the carbon donor and Al ³⁺ , there is an ideal homogeneous distribution of Al₂O₃- and carbon particles.

Unwanted additions of abrasion and contamination from unreacted Al metal are excluded so that the impurity content of the AlN powder is essentially determined by the purity of the materials used.

Embodiment

2 mol aluminum salt (AlCl₃ _* 6 H₂O: 483 g or Al (NO₃) ₃ _* 9 H₂O: 750 g) are in 4 l deion. Water and 1500 g Na carboxymethyl cellulose in 25 l deion. Dissolve water overnight and homogenize well by stirring.

Both solutions are mixed in 5 l deion. Water sprayed and a pH of 3. . . 5 adhered to (pH control with glass electrode). A yellowish-white flaky precipitate falls  that is easy to filter.

After washing several times with deion. Water (at least 3 times with the same Amount of water like the volume of precipitation) is freeze-dried, creating a loose scaly yellowish white product is obtained.

The Al-Carboximethylcellulose is calcined under Exclusion of air in two stages: 200 ° C (3 h) and 350 ° C (4 h).

The highly active alumina-carbon mixture obtained is under flowing oxygen and moisture-free nitrogen treated at 1400 ° C in a graphite crucible for 8 h and then in a corundum crucible at 500 ° C. . . 600 ° C in air annealed. Then a loose light gray AlN powder remains.

Claims (2)

1. A process for the production of high-purity, finely dispersed aluminum nitride powder by reduction nitriding, in which a homogeneous mixture of aluminum oxide powder and carbon is prepared, this mixture is converted into aluminum nitride in a nitrogen-containing atmosphere and excess carbon is subsequently burned out, characterized in that the homogeneous mixture is prepared, by an aqueous aluminum salt solution and an aqueous sodium carboxymethyl cellulose solution at a pH of 3. . . 8 are mixed, a sparingly soluble salt of aluminum cellulose glycolate precipitating, which is then filtered, washed, dried, calcined at 200 ° C. to 350 ° C. and nitrided and annealed in a known manner. 2. The method according to claim 1, characterized in that the pH is adjusted by adding NH₃.