WO1999012850A1

WO1999012850A1 - PROCESS FOR THE PROTECTION OF AlN POWDER AGAINST HYDROLYSIS

Info

Publication number: WO1999012850A1
Application number: PCT/SI1998/000018
Authority: WO
Inventors: Tomaz^¿ KOSMAC^¿; Kristoffer Krnel; Kristina Kos
Original assignee: Ins^¿Titut 'joz^¿Ef Stefan'
Priority date: 1997-09-05
Filing date: 1998-08-26
Publication date: 1999-03-18
Also published as: SI9700229A; AU8822098A

Abstract

The invention relates to a process for protecting aluminium nitride AlN powder against decomposition, i.e. hydrolysis in moist atmosphere or in an aqueous medium. The process for the protection of AlN powder against hydrolysis is characterized in that in a solution of orthosilicic acid or other silicic acid or water-soluble salt thereof with a concentration of from 1 x 10-3 to 5 x 10-1 mole dm-3, AlN powder is dispersed up to a mass fraction of 1 to 50 %, its particles having an average size of from 0.1 νm to 1 mm, and during dispersion this suspension is not heated above the temperature of 85 °C. In the case of dispersion by milling, an organic or inorganic oxidizing agent, typically H¿2?O2, is added to this suspension in the quantity of from 0.01 to 0.1 ml per gramme of AlN powder.

Description

PROCESS FOR THE PROTECTION OF AIN POWDER AGAINST HYDROLYSIS

The invention relates to a process for protecting aluminium nitride AIN powder against decomposition, i e hydrolysis in moist atmosphere or in an aqueous medium.

The invention provides a solution to the technical problem of protecting, i e passivating AIN powder against decomposition or hydrolysis in moist atmosphere or in an aqueous medium, the protection process being simpler, cheaper and ecologically more acceptable as compared to the hitherto known processes, whereas the protected, i e passivated powder is water-resistant to higher temperatures than most of the hitherto known passivated AIN powders and considerably more water-resistant during wet milling in an aqueous medium than the hitherto known passivated AIN powders .

Aluminium nitride ceramics are industrially important due to some of their extraordinary physical properties, such as good thermal conductivity, low thermal expansion coefficient, good wear- resistance, transparency etc. AIN powder is also used as an additive in the production of certain kinds of structural ceramics, such as Si₃N₄ or SiC. The greatest difficulty in the production of sintered AIN ceramics and ceramics containing AIN powder in the starting mixture is represented by the instability of the AIN powder in humidity or in an aqueous medium. Namely, the AIN powder reacts with water (hydrolyses) to form ammonia and aluminium hydroxide, as schematically represented by the following reactions: AIN + 2H₂0 - AIOOH + NH₃ (1)

NH₃ + H₂0 - NH₄ ⁺OH^" (2)

AIOOH + H₂0 - Al(OH)₃ (3)

Since ammonia is released in the reaction, dissociating in water according to reaction (2) , the simplest way to follow the rate of the hydrolysis and to verify the efficacy of the protection of the AIN powder against hydrolysis is by measuring the pH value of a diluted aqueous suspension of AIN powder.

Owing to the high reactivity of the AIN powder, its storage alone is a problem, not to mention the initial stages in the production of ceramics, such as powder deagglomeration, homogenization with organic and inorganic additives, granulation and shaping of green bodies, requiring operation in non-aqueous media, which is undesirable for economic, safety and ecological reasons. Therefore endeavours are made to protect AIN powder against hydrolysis, in order to facilitate storage and to make it possible to process and shape green bodies in an aqueous medium.

There are a variety of known processes for protecting AIN powder against hydrolysis in moist atmosphere or in an aqueous medium. One such process, used quite frequently, consists in protecting the surface of the AIN powder particles by long-chain organic molecules, such as carboxylic acids, particularly stearic acid, though use of cetyl alcohol, n-decanoic acid, dodecylamine and others is reported as well. These organic substances are characteristically hydrophobic and thus prevent water from coming into contact with the surface of the protected particles. Hydrophobization of ceramic powders having long been known, it is traditionally used in paint production, for example, and can also be used to protect AIN powder against hydrolysis. First, stearic acid or another of the said organic substances is dissolved in a non-polar organic solvent, generally cyclohexane or benzene, subsequently by intense stirring AIN powder is dispersed in the solution, and then the suspension is heated beyond the boiling point of the organic solvent by reflux. To remove the surplus carboxylic acid, the suspension is filtered, whereupon the cake is washed with an organic solvent and dried in air. The disadvantage with this process is that it involves the use of organic solvents that are flammable and dangerous to health, so that protecting AIN powder by this process requires special safety measures. To some extent this disadvantage is overcome by the process of dry milling of AIN powder with stearic or another carboxylic acid under a protective atmosphere of inert gas, which however, is technologically a fairly demanding process and, moreover, the carboxylic acid is not quite homogeneously distributed on the surface of the powder particles and is generally in excess. Another disadvantage with both of these processes for protecting AIN powder by a carboxylic acid is that, the protected powder being hydrophobic (water-repellent) , it cannot be dispersed in water without adding organic or inorganic wetting agents, which, as a rule, cause the suspension to foam. Besides the wetting agents, therefore, a third additive must be added to the suspension in order to make it free from air bubbles .

Another known process for the protection of AIN powder against hydrolysis consists in the adsorption of phosphoric acid or its organic or inorganic compounds on the surface of the powder particles. In the US Pat. 4,923,689 a process for protecting AIN powder by phosphoric acid, orthophosphoric acid and a variety of organic phosphorus acids and their compounds is described. According to this process, AIN powder is first dispersed in a heated organic solvent, usually ethyl alcohol, and while this suspension is being agitated, a solution of one of the mentioned compounds is added to it. After being dried, the protected powder is heated to a temperature of from 150° to 550°C. Thus protected AIN powder is more water-resistant at room temperature than powder protected by carboxylic acids, and, according to the patent, it is resistant even at temperatures of up to 80°C. The disadvantage with this process, however, is that AIN powder protected by inorganic phosphoric acids yields a very acid suspension, which is a drawback to the subsequent technological stages in ceramics production from AIN, particularly if as sintering additive Y₂0₃ is used, which dissolves at low pH values of the aqueous suspension. Yet a further disadvantage with this process is that the preparation of so protected powder involves the use of organic solvents, inconvenient from the economic and ecological viewpoints.

All these protected AIN powders, furthermore, have the disadvantage that they must not be exposed to intense shearing forces, e g during milling, otherwise the hydrolysis takes place.

The object of this invention, therefore, is to effectively protect AIN powder, so that it will not hydrolyse when stored in moist atmosphere over longer periods, nor will it hydrolyse when agitated or milled moderately in an aqueous medium, nor will the protected AIN powder possess the weaknesses of most powders, protected by hitherto known processes, while the protection process itself will be simpler, cheaper and ecologically more acceptable as compared to the hitherto known processes.

According to the invention, the said technical problem is solved by a process for protecting AIN powder by adsorption of silicic acid or its salt on the surface of the particles of AIN powder, which is being agitated or milled in water or in a water- containing liquid, the protection process according to the invention being characterized in that in water or in a solution, containing between 1*10^~3 and 5^χl0^_1 mole dm^-3 of dissolved silicic or orthosilicic acid, or other silicic acid or water-soluble salt thereof, AIN powder is dispersed up to a mass fraction of 1 to 60%, its particles having an average size of from 0.1 μm to 1 mm, the suspension of AIN powder is not heated above 90°C during dispersion, to this suspension, in the case of dispersion by milling yet another 0.01 to 0.1 ml of an organic or inorganic oxidizing agent, typically H₂0₂, per gramme of AIN powder is added, after dispersion the suspension of AIN powder is dried, or the AIN powder is separated from the liquid by filtering, centrifuging, or decantation, and subsequently dried or washed and dried, or, after dispersion and homogenizing with organic and inorganic additives, needed for the production of AIN ceramics, the suspension is used for shaping ceramic green bodies.

The process for the protection of AIN powder against hydrolysis in moist atmosphere or in an aqueous medium according to the invention comprises the following steps:

A water solution of silicic acid, orthosilicic acid, or a water- soluble salt of a silicic acid, e g sodium silicate, is prepared by introducing into water any commercially available or laboratory-prepared silicic acid, or salt thereof, or substance that reacting with water yields a silicic acid. The purity of these substances is not of crucial importance, as impurities, within boundaries customary for technical raw materials, do not affect the process for protecting AIN powder as such. Nevertheless, it is desirable that such substances be as pure as possible, since impurities degrade the properties of sintered AIN ceramics. A greater problem than the purity of the silicic acid or its compounds is its solubility in water, which depends on the type of substance, the type of solvent, and temperature. In water or a water solution, containing between l^χl0^~3 and l^χl0^_1 mole/1 of a silicic acid and usually having a pH of from 3 to 10, AIN powder is dispersed by agitation in a vessel or by wet milling. If dispersion is done by agitation in a vessel, there is no need to add a water-soluble oxidizing agent, such as H₂0₂, whereas in the case of dispersion by intense milling, another 0.01 to 0.1 ml of H₂0₂, or other water-soluble oxidizing agent that will liberate oxygen during agitation or milling, per gramme of AIN powder must be added before milling. Dispersing AIN powder in a suspension by agitation or milling requires no protective atmosphere of inert gas. The dispersion may take course at room temperature or at a higher temperature, which, however, must not exceed 85°C. The dispersion of AIN powder in a suspension has no time-limit, and although ordinarily it goes on for an hour, it can nevertheless be prolonged without diminishing the protection effect on AIN powder. After agitation or milling, the suspension of AIN powder may be dried, or the surplus solution of silicic acid or its salt may be removed by filtering, centrifuging, or decantation. The wet cake may either be washed with distilled or deionized water, or be left unwashed, and dried in air at a temperature that may exceed 100°C. If the protection of AIN powder is not needed for storage purposes, the homogenization of AIN powder may be performed with inorganic additives for sintering and organic additives for shaping in a solution of silicic acid, orthosilicic acid, or a water-soluble salt of a silicic acid, and followed by wet shaping, drying, and sintering.

The invention will be further described with reference to examples of execution. Exampl e 1

1000 ml of silicic acid solution with the concentration of 5^χl0^~3 mole dm^-3 are prepared by dissolving silicic acid (producer: Koch-Light Laboratories, chromatographic grade) in boiling deionized water. While this solution, cooled to room temperature and having a pH value of 6, is being agitated by a magnetic stirrer, 25 g of AIN powder (producer: H. C. Starck, Grade C) are dispersed in it. The agitation proceeds in air, at room temperature. Once the agitation, lasting one hour, is completed, the suspension is centrifuged. The wet cake is dried in air for 6 hours at a temperature of 85°C, and then for another 2 hours at 110°C. The concentration of Si0₂ on the protected AIN powder is 0.09%. Besides AIN, the dried powder contains no other crystalline phase, detectable by X-ray analysis. The resistance of thus protected AIN powder was tested by redispersing the powder in deionized water, specifically 5 g of powder per 100 ml of water, respectively at room temperature and at 70°C, and then observing the pH variation in time by means of a glass electrode and a pH-meter, over a period of 24 hours. For comparison purposes, powders protected by other inorganic acids were also prepared. The results are set out in Table 1 and Table 2. The results clearly show that the process for protecting AIN powder is effective at both room temperature and higher temperatures, specifically at 70°C, as opposed to powder protection by phosphoric acid, effective at room temperature, but not at higher temperatures, as the increase in the pH value shows.

Example 2

The protection process is the same as in Example 1, except that after dissolving the silicic acid in boiling water, the solution is cooled down to 85°C, whereupon the AIN powder is dispersed by agitation. The dispersion goes on for 10 minutes at 85°C, and then the suspension is cooled, filtered and dried. After redispersmg the protected powder in water, here 5 g per 100 ml, the suspension was heated to 90°C at a rate of 10°C/min. During heating to this temperature, there was no variation of the suspension pH value.

Example 3

The process for protecting AIN powder is the same as in Example 1, except that instead of the silicic acid solution a 10% solution of sodium silicate with a pH value of 10 is used. The dispersion proceeds at room temperature, dispersing 100 g of AIN per 250 ml of solution. The resistance of so protected AIN powder was tested by redispersing the powder in deionized water, specifically 5 g of powder per 100 ml of water, respectively at room temperature and at 70°C, and then observing the time variation of the pH over a 24-hour period. During the test, there was no variation of the pH.

Example 4

Into a planetary mill having a volume of 750 ml and fitted with 170 g of aluminate balls with a diameter of 20 mm, 50 g of AIN powder are weighed out. To these, 50 ml of silicic acid solution with a concentration of 5 mmole/1, prepared according to Example 1, and 5 ml of hydrogen peroxide are added. The AIN powder is dispersed in the solution of silicic acid and H₂0₂ by milling for 1 hour at 100 rpm. There is no variation of the suspension pH during milling, and, in contrast to the case when, under the same circumstances, AIN powder is milled in water or in a solution of silicic acid without adding H₂0₂, no characteristic smell of ammonia is perceived. After milling, the milling media are removed and the suspension is dried in air. During dispersion by wet milling the average size of the particles of AIN powder drops from the initial 1.5 μm to 1.3 μm, X-ray analysis shows the dried powder to be AIN, and, after being redispersed in deionized water, in this case 5 g/100 ml, it does not hydrolyse if kept 24 hours at room temperature. Seeing that the AIN powder was dispersed in the presence of H₂0₂, the oxygen content in AIN rose from the initial 2.6% to 3.1%.

Example 5

In an attritor mill having a volume of 750 ml and fitted with 1000 g of Zr0₂ balls with a diameter of 2 mm, a suspension of 100 g of AIN powder in 43 ml of absolute ethyl alcohol is prepared. The suspension is milled for 1/2 h at 800 rpm. After this thirty- minute milling, another 57 ml of silicic acid solution with a concentration of 5 mmole/1 are added. The suspension is milled for another 10 min, after which the milling media are removed and the suspension is dried at a temperature of 110^CC. During milling in the attritor mill the average size of the particles of AIN powder drops from the initial 1.5 μm to 1.0 μm. Thus prepared AIN powder, 5 g in this case, is dispersed again in 100 ml of deionized water in order to find out whether the powder is effectively protected against hydrolysis. After a 24-hour agitation at room temperature, the pH of the suspension shows no variation.

Example 6

Into a planetary mill having a volume of 750 ml, fitted with 170 g of aluminate balls with a diameter of 20 mm, 47 g of AIN powder and 3 g of Y₂0₃ powder are weighed out. To these, 50 ml of silicic acid solution with a concentration of 5 mmole/1, prepared according to Example 1, and 5 ml of H₂0₂ are added. After being milled for 1 hour at 100 rpm, the suspension is separated from the milling media and poured into a plaster mould, in which it dries. During milling and homogenization by Y₂0₃, AIN does not hydrolyse and can be sintered to a high density.

The solution according to the invention has the following advantages: by adsorption of silicic or orthosilicic acid, or other silicic acid or salt thereof, on the surface of the particles, an effective protection of AIN powder against hydrolysis in moist atmosphere or in an aqueous medium up to a temperature of 90°C is achieved, which has not been the case with most of hitherto known processes. Moreover, the AIN powder, protected according to the invention, is hydrolysis-resistant in an aqueous medium at higher pH values for a fairly long time, which also has not been the case with some of the hitherto known processes. The protected AIN powder according to the invention is not hydrophobic when dried, and can be well dispersed in water without wetting agents. By adding an organic or inorganic oxidizing agent it is also possible to mill the AIN powder in an aqueous medium, or to homogenize it with additives for shaping and sintering by wet milling or ultrasonic agitation at temperatures of up to 90°C. The process for protecting AIN powder according to the invention is neither expensive nor technologically demanding, nor does it require any special safety measures, seeing that the substances used are not flammable, and handling them is not dangerous to health or to the environment.

Claims

Patent Claims

1. Process for the protection of AIN powder against hydrolysis, characterized in that in a solution of orthosilicic acid or other silicic acid or water-soluble salt thereof with a concentration of between l^╧çl0^~3 and 5^╧çl0^_1 mole dm^-3, AIN powder is dispersed up to a mass fraction of 1 to 50%, its particles having an average size of from 0.1 ╬╝m to 1 mm, and during dispersion this suspension is not heated above a temperature of 85┬░C.

2. Process according to Claim 1, characterized in that, in the case of dispersion by milling, an organic or inorganic oxidizing agent, typically H₂0₂, is added to this suspension in the quantity of from 0.01 to 0.1 cm³ per gramme of AIN powder.

3. Process for the protection of AIN powder against hydrolysis according to Claims 1 or 2, characterized in that after dispersion the suspension of AIN powder is dried, or the AIN powder is separated from the liquid by filtering, centrifuging, or decantation, and subsequently dried or washed and dried.

4. Process for the protection of AIN powder against hydrolysis according to Claims 1 or 2, characterized in that to the suspension of AIN powder, additives for sintering and shaping are added during dispersion.