Process of Making High Purity Aluminum Nitride

This invention relates to a method of making aluminum nitride. More particularly, it is concerned with a method of making high purity aluminum nitride using metalorganic precursors.

Aluminum nitride (AIN) has a hexagonal wurtzite crystal structure and a theoretical density of 3.26 g/cm. It has a low thermal expansion coefficient, a high thermal conductivity, and a high melting point. When fully densified, AlN is resistant to water and mineral acids at ambient temperatures and stable in air up to 1450.degree. C. The high temperature stability in air qualifies AlN as an excellent candidate for high temperature materials. However, the commercial AlN is usually contaminated with a large amount of oxygen in the form of Al.sub.2 O.sub.3 which adversely affects the desireable properties of the AlN.

It is an object of the present invention to provide a method of making a high purity aluminum nitride using a metalorganic precursor.

It is a further object of the present invention to provide a method of making an air stable high purity aluminum nitride at a high yield.

Further and other objects of the present invention will become apparent from the description contained herein.

In accordance with one aspect of the present invention, a new and improved method of making high purity aluminum nitride comprises the following steps:

Step 1--Trimethyl aluminum is reacted with high purity anhydrous ammonia forming a white crystalline (CH.sub.3).sub.3 Al:NH.sub.3.

Step 2--The product formed in step 1 is pyrolyzed in an inert atmosphere to form high purity aluminum nitride.

In accordance with another aspect of the present invention, a new and improved method of making a high purity densified aluminum nitride article comprises the following steps:

Step 1--Trimethyl aluminum is reacted with high purity anhydrous ammonia forming a white crystalline (CH.sub.3).sub.3 Al:NH.sub.3.

Step 2--The product formed in step 1 is pyrolyzed in an inert atmosphere to form high purity aluminum nitride.

Step 3--The produce from step 2 is pressed to form a pressed article.

Step 4--The product from step 3 is sintered at a temperature of about 1850.degree. C. to form a densified aluminum nitride article having a theoretical density of at least 91.3%.

In accordance with another aspect of the present invention, a new and improved method of making high purity aluminum nitride comprises the following steps:

Step 1--A hydrocarbon solution of Trimethyl aluminum is contacted with high purity anhydrous NH.sub.3(g) forming a precipitate generating heat.

Step 2--The heat generated in step 1 is removed by cooling the hydrocarbon solution.

Step 3--The precipitate formed from step 1 is partially redissolved in the hydrocarbon solution.

Step 4--The hydrocarbon solution from step 3 is evaporated with a nitrogen stream to form a white crystalline material.

Step 5--The white crystalline material is pyrolyzed in an inert atmosphere to form high purity aluminum nitride.

In accordance with another aspect of the present invention, a new and improved method of making high purity aluminum nitride comprises the following steps:

Step 1--Al(CH.sub.3).sub.3 (g) and NH.sub.3 (g) are contacted with a hydrocarbon solution forming a precipitate and generating heat.

Step 2--The heat generated in step 1 is removed by cooling the hydrocarbon solution.

Step 3--The precipitate formed from step 1 is partially redissolved in the hydrocarbon solution.

Step 4--The hydrocarbon solution from step 3 is evaporated with a nitrogen stream to form a white crystalline material.

Step 5--The white crystalline material is pyrolyzed in an inert atmosphere to form high purity aluminum nitride.

In accordance with another aspect of the present invention, a new and improved method of making high purity aluminum nitride comprises the following steps:

Step 1--Al(CH.sub.3).sub.3(g) is contacted with NH.sub.3(1) forming a precipitate and generating heat.

Step 2--The heat generated in step 1 is removed by cooling the NH.sub.3(1).

Step 3--The precipitate formed from step 1 is partially redissolved.

Step 4--The NH.sub.3(1) is evaporated with a nitrogen stream to form a white crystalline material.

Step 5--The white crystalline material is pyrolyzed in an inert atmosphere to form high purity aluminum nitride.

2M Al(CH.sub.3).sub.3 in a hydrocarbon solution such as hexane was bought from Aldrich Chemicals. The hydrocarbon solution was dried and deaerated by distilling over calcium hydride under N.sub.2 atmosphere.

High purity anhydrous NH.sub.3(g) was further dried by passing it through a column packed with KOH pellets. (3/4" in diameter, 6" length).

This reaction was carried out in a dry box. All of the glassware was dried in an oven at 110.degree. C. A three neck, 100 ml round bottom flask was equipped with a gas inlet having a stopcock and a gas outlet, a septum and a stir bar. 40 ml of Al(CH.sub.3).sub.3 in hexane (0.08 mole of Al(CH.sub.3).sub.3) was transferred into the flask with a syringe through the septum. An excess of gaseous ammonia (Matheson, high purity lecture bottle ammonia), dried by passing it through a KOH column, was added, with stirring, to the solution through a gas inlet stopcock at a rate of 0.57 ml/sec for (approx.) 2 hrs. A total of 0.186 moles of NH.sub.3 was added. The reaction is exothermic and a cooling of the flask is required to control the temperature increase. A white crystalline precipitate was formed at the interface of the hexane layer at the beginning of the reaction, and was then partially redissolved as it progressed. The solvent was removed by evaporation at room temperature using a N.sub.2 stream promulgating a precipitation of a white crystalline material as the evaporation proceeded. Alternatively the hexane can be removed by vacuum distillation at 10.sup.-1 to 10.sup.-3 mm-Hg at room temperature. The white crystalline material was washed with dried hexane twice before characterization or the white crystalline material can be purified by recrystallization from hydrocarbon solvents, e.g. hexane, toluene or by sublimation of the white crystalline material at 10.sup.-1 to 10.sup.-3 mm-Hg. The white crystalling material was characterized as (CH.sub.3).sub.3 Al:NH.sub.3 having a yield of 6.05 gr. of (CH.sub.3).sub.3 Al:NH.sub.3 (0.068 mole) which calculates to a 85% yield based on Al(CH.sub.3).sub.3.

(CH.sub.3).sub.3 Al:NH.sub.3 white crystalline material, fairly stable in air, which sublimes readily at 55.degree. to 70.degree. C. to release one mole of CH.sub.4 to yield 3(CH.sub.3).sub.2 Al:NH.sub.2 but is easily hydrolyzed by moisture and a further loss of one mole of CH.sub.4 occurs at 120.degree.-170.degree. C. to produce CH.sub.3 AlNH. Hexagonal AlN is formed at 900.degree. C. under helium, argon or under vacuum. Fourier Transform Infrared spectroscopic analysis (FTIR) of the (CH.sub.3).sub.3 Al:NH.sub.3 in KBr reveals the following absorption bonds:

3500 cm.sup.-1 N-H stretching;

2926 cm.sup.-1, 2885 cm.sup.-1, and 2891 cm.sup.-1 C-H stretching;

1515 cm.sup.-1 N-H bending;

800-850 cm.sup.-1 broad, Al-N.

The pyrolysis reaction was conducted under an inert atmosphere such as argon in a tube furnace with a heating rate of 10.degree. C./min, up to 900.degree. C. and held for 1 hr. then cooled at a cooling rate of 5.0.degree. C./min to room temperature. The net loss is 59.25 wt. % as compared to a 53.9 wt. % theoretical value. The x-ray diffraction indicates that the residue after pyrolysis has a major phase of hexagonal AlN and a weak phase of cubic AlN.

One gram of the pyrolysis product, (CH.sub.3).sub.3 Al:NH.sub.3, was die pressed to form a pellet. The pellet was sintered at 1850.degree. C., in a 50 psi. N.sub.2 overpressure to 91.3% of theoretical density (T.D.) as compared to 89.0% T.D. obtained from pellet made from a commercial AlN powder, a 2.3% improvement.

Other routes to produce air stable (CH.sub.3).sub.3 Al:NH.sub.3 include:

1. In Step 1, Al(CH.sub.3).sub.3(g) and NH.sub.3(g) are contacted with a deaerated, dried hexane or pentane solution to form a precipitate.

2. In Step 1, Al(CH.sub.3).sub.3(g) is dissolved in a stoichiometric amount of liquid NH.sub.3 to form a precipitate or Al(CH.sub.3).sub.3 in hexane solution is contacted with liquid NH.sub.3 to form a precipitate. The reactions are exothermic and careful removal of the heat generated is required. This can be done by cooling the reaction flask in a cold bath such as acetone/dry ice slush, isopropanol/dry ice slush.

A thermogravimetric analysis showed a weight loss of 59% from 50.degree. C. to 170.degree. C. under N.sub.2 or Ar. The expected weight loss was 54% as calculated from the pyrolysis reaction.

Mass spectral analysis of gases evolved during the pyrolysis showed 98% of the gas to be methane, as expected from the pyrolysis reaction. The adduct can be sublimed under vacuum from 300.degree. C. to 500.degree. C. to form AlN. Fourier Transform Infrared spectroscopic analysis (FTIR) of the precursor indicated that the compound was (CH.sub.3).sub.3 Al:NH.sub.3. Consistent with this structure was a broad N-H stretching frequency at 3500 cm.sup.-1, C-H stretching frequencies at 2926, 2885, 2819 cm.sup.-1, as well as N-H bending frequency at 1515 cm.sup.-1 characteristic of a free amine.

Hexagonal AlN was formed after the pyrolysis of (CH.sub.3).sub.3 Al:NH.sub.3 from about 900.degree. C. to about 1400.degree. C. under an inert gas such as argon, helium, or nitrogen. A diepressed AlN pellet prepared from the pyrolysis product of (CH.sub.3).sub.3 Al:NH.sub.3 was sintered at 1850.degree. C. in a 50 psi N.sub.2 overpressure to 91.3% of its theoretical density as compared to 89% T.D. obtained from a commercial AlN powder.

The metalorganic precursor of this invention is airstable, has a low melting point, a high AlN yield upon pyrolysis, and the gaseous species evolved during pyrolysis is nontoxic.

While there has been shown and described what is at present considered the preferred embodiment of the invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the scope of the invention as defined by the appended claims.