GB2233969A

GB2233969A - Improvements in or relating to the manufacture of aluminium nitride.

Info

Publication number: GB2233969A
Application number: GB8915465A
Authority: GB
Inventors: James Anthony Charles; Young Wan Cho
Original assignee: Carborundum Co
Current assignee: Unifrax 1 LLC
Priority date: 1989-07-06
Filing date: 1989-07-06
Publication date: 1991-01-23
Also published as: AU5938090A; IE902433A1; WO1991000842A1; GB8915465D0

Abstract

A process is disclosed for manufacturing aluminium nitride in which a substantially homogeneous mixture of carbon and aluminium trihydroxide (Al(OH))3) or its dehydration product, gamma aluminium oxide is reacted with nitrogen by heating to a temperature of not less than 1200 DEG C and preferably not more than 1600 DEG C. The mixture is heated rapidly from ambient temperature to the reaction temperature in the presence of nitrogen in not more than five minutes. The mixture could be a bed fluidised by the nitrogen flow in a continuous production process.

Description

IMPROVEMENTS IN OR RELATING TO THE MANUFACTURE OF ALUMINIUM NITRIDE This invention relates to improvements in aluminium nitride.

Several processes are known for the manufacture of aluminium nitride in powder form. Direct nitridation of aluminium with nitrogen is believed to be the most common method. The product powders are, however, generally too coarse to use as a starting powder for sintering without comminution. High milling costs and a relatively high level of impurities in the final products are the main disadvantages of this method.

Carbothermic reduction and nitridation of aluminium oxide (A1203) can also be used to produce a relatively fine powder of aluminium nitride but a high reaction 0 temperature of 1580 C or more is required depending upon the reaction time and the final oxygen content being sought.

Another known process for the manufacture of aluminium nitride is to react halides of aluminium (e.g. All13) and 3 ammonia to produce a relatively pure and fine aluminium nitride powder. This process has the disadvantages of high materials costs and low productivity as well as presenting problems of corrosion.

It is an object of the present invention to provide an improved process for the manufacture of aluminium nitride in which the disadvantages referred to above shall be materially reduced if not substantially obviated.

According to the present invention the-re is provided a method of manufacturing aluminium nitride which comprises the steps of producing a substantially homogeneous mixture of carbon and aluminium trihydroxide (A1(OH)3) and reacting the mixture with nitrogen by heating to a temperature of not less than 12000cm It is preferred that the temperature be not more than 16000C and preferably in the range of 14500C to 15500C.

The reaction time is related both to the temperature and to the particle size but, in general, should not be less than 30 minutes and preferably not more than 6 hours.

It is highly desirable that the mixture be heated rapidly to the reaction temperature selected in the presence of nitrogen and that it should be heated from ambient temperature to the reaction temperature in not more than 5 minutes.

The nitrogen gas is advantageously supplied to the reaction at a rate of 100 to 2000 ml/min, preferably 500 to 1000 ml/min.

Conceivably, the mixture could be a bed fluidised by the nitrogen flow in a continuous production process.

The particle size of the starting material, i.e. aluminium trihydroxide is preferably in the range of 0.5 to 1.5 ,u, preferably less than 1 U and desirably between 0.6 to 0.8 p. The particle size of the aluminium nitride product is substantially the same as that of the starting material and, if the particle size is too low, there is an increase in oxygen impurities and handling problems. In fact an average particle size of 0.7 p has given optimum results.

In order to ensure intimate mixing of the aluminium trihydroxide and carbon it is preferred to produce the mixture as a porous compact using an organic solvent. The porosity allows the nitrogen to access the components of the mixture and participate in the reaction to produce aluminium nitride. Carbon black is preferably used to provide the carbon component and some carbon blacks are to be preferred to others, namely carbon blacks having a high specific surface area, a fine particle size and a good porosity. These qualities enhance the lower reaction times and/or lower reaction temperatures which can be achieved by the present invention.

For example, a carbon black having a specific surface area 2 of 250 m2/g, a particle size in a narrow range about 20 x 10 9 m and a porosity defined by an oil absorption value of DBP 165 ml/100 g was homogeneously mixed with aluminium trihydroxide having a particle size in a narrow range about 0.7 r in a molar ratio of AL(OH)3/C of 1:3. The mixture was stirred with the addition of a non-polar solvent (absolute alcohol) to give a slurry which was dried while stirring to give a thick paste which was extruded into cylindrical pellets of about 2 mm diameter and 5 to 10 mm in length. The pellets were then dried in an oven overnight at 1250C. A known amount of the pellets was subsequently heated in a graphite container in a vertical tube furnace under flowing nitrogen supplied at a rate of 1000 ml/min.A reaction temperature of 15000 C was emDloyed and the pellets were brought to this temperature from room temperature in under 5 minutes. The reaction time was found to be 30 minutes.

The product shape was the same as that of the starting material but fragile in the sense that it broke up into powder when shaken, the product particle size being of the order of 0.7 r. It will be appreciated that the lower temperatures made possible by the present invention and the related lesser reaction times reduces the risk of the particles sintering during the reaction and also the risk of grain growth.

It will be appreciated that the aluminium nitride product of the present invention is very suitable for use as electronic substrate material. It can be used as a sintered body to carry integrated circuits and it has other uses including in high temperature electrical insulators as its electrical resistance remains high at elevated temperatures.

A number of tests were carried out using aluminium trihydroxide of a particle size in the range of 0.5 to 1 u homogeneously mixed with carbon black in a non-polar solvent (absolute alcohol) by ball milling for 2 hours.

The slurry was dried while stirring to give a thick paste and then extruded to obtain cylindrical pellets of about 2 mm diameter and 5 to 10 mm length. The pellets were dried at 1250C.

A known amount of pellets was heated in a graphite container in a vertical tube furnace under flowing nitrogen. The molar ratio of Al(OH)3 to C was between 1:1.5 and 1:9. The reaction temperature was between 1200 and 15000C and was achieved in less than 5 minutes. The reaction time was between 0.5 to 6 hours and the nitrogen flow was between 100 and 2000 ml/min. After the reaction was completed, residual carbon was burnt out in air at 7000C for 4 hours.

In one such test in which the pellets were heated to a 0 temperature of 1500 C for a period of 120 minutes with a linear nitrogen flow rate of 2 metres/min, the product contained 1.7% oxygen and 0.1% carbon.

A level of 0.1% wt. carbon is readily obtainable using a material known as Conductex 975. Decarburisation depends upon the form of the carbon, the decarburisation temperature and the time.

The minimum nitrogen requirement is determined by the mass being treated but varies as the reaction proceeds with the evolution of CO. There will be a minimum theoretical nitrogen flow rate for the process but the reaction rate will increase substantially hyperbolically with less to be gained as the flow rate is increased.

The tests carried out all produced good quality single phase aluminium nitride.

However optimum conditions were found to be: 0 Reaction temperature: 1500 C Reaction time: 120 minutes Al(OH)3/C molar ratio: 1:3 Nitrogen flow rate: 1000 ml/mins.

Decarburization temperature: 7000C.

The best reaction product powder had the following characteristics: Average particle size: less than 1 Crystalline phase: single phase AlN Colour: white.

Without wishing to be bound by any theory it is believed that the relatively high temperatures required for the carbothermic reduction and nitridation of aluminium oxide result from the fact that such oxide is generally in the form of stable crystalline cC alumina. When heated, aluminium trihydroxide dehydrates to the oxide and water and this starts at about 250 to 3000 C. However, the trihydroxide initially becomes Y alumina and two or three other metastable phases of alumina and these phases react more readily with the carbon and nitrogen to initiate the desired reaction. Consequently, by starting with the trihydroxide and bringing the mixture rapidly to the reaction temperature lower reaction temperatures are possible as are lesser reaction times.As will be understood, the higher the reaction temperature employed, the lesser the reaction time required.

There are many applications where it is required that the product should have a low oxygen content. The present invention enables a lower oxygen content in the product to be achieved in a process using aluminium trihydroxide compared with equivalent processes using aluminium oxide at a lower reaction temperature and/or time. An oxygen content of approximately 1.7 wt. % can be achieved at a reaction temperature of 15000C for a period of 60 mins.

The oxygen content can, however, be materially reduced using a reaction temperature of 15500C and a time of 120 mins.

Thus by selecting appropriate reaction temperatures and times, the oxygen content in the final product can be reduced to about 1.0 wt. % or less. The invention, more specifically, provides a method of manufacturing aluminium nitride with an oxygen content of 1.0 wt. % or less which comprises the steps of producing a substantially homogeneous mixture of carbon and aluminium trihydroxide (Al(OH)3) and reacting the mixture with nitrogen by heating the mixture to a reaction temperature in the range of 14500C to 15500C inclusive in a period of not more than 5 mins. and maintaining the reaction temperature for a period of time of not less than 30 mins.

and not more than 6 hours until the reaction is substantially complete and the oxygen content of the product is not more than 1.0 wt. %.

Claims

CLAIMS:

1. A method of manufacturing aluminium nitride which comprises the steps of producing a substantially homogeneous mixture of carbon and aluminium trihydroxide (Al(OH)3)- and reacting the mixture with nitrogen by 0 heating to a temperature of not less than 1200 C.

2. A method according to claim 1 in which the temperature is not more than 16000C.

3. A method according to claim 1 or 2 in which the temperature is in the range of 1450 to 15500C.

4. A method according to claim 1, 2 or 3 in which the reaction time is not less than 30 minutes.

5. A method according to any one of the preceding claims in which the reaction time is not more than 6 hours.

6. A method according to any one of the preceding claims in which the mixture is heated from ambient temperature to the reaction temperature in the presence of nitrogen in not more than 5 minutes.

7. A method according to any one of the preceding claims in which the nitrogen gas is supplied to the reaction at a rate of 100 to 2000 ml/min.

8. A method according to any one of the preceding claims in which the nitrogen gas is supplied to the reaction at a rate of 500 to 1000 ml/min.

9. A method according to any one of the preceding claims in which the particle size of the aluminium trihydroxide is in the range of 0.5 to 1.5 )x.

10. A method according to claim 9 in which the particle size of the aluminium trihydroxide is less than 1

11. A method according to claim 10 in which the particle size of the aluminium trihydroxide is in the range of 0.6 to 0.8 .

12. A method according to any one of the preceding claims in which the carbon in the form of carbon black and the aluminium trihydroxide are mixed in the presence of a nonpolar solvent to form a slurry which is dried to a thick paste while stirring and the thick paste is extruded to obtain pellets which are then completely dried.

13. A method according to claim 12 in which the pellets are cylindrical with a diameter of about 2 mm and a length of 5 to 10 mm and are heated to the reaction temperature in a graphite container in a vertical tube furnace under flowing nitrogen.

14. A method of manufacturing aluminium nitride substantially as herein described.