Method for the production of gallium oxide

A method for producing high purity gallium oxide by reacting gallium and water vapor at elevated temperatures and pressure in a sealed autoclave lined with a fluorinated hydrocarbon polymer such as polytetrafluoroethylene to form gallium hydroxide. The hydroxide thus formed is recovered and calcined above 600.degree. C to yield high purity gallium oxide.

The invention relates to a method for the production of gallium oxide. 
Methods for the production of gallium oxide are known. Thus this product is 
usually obtained by precipitation of the hydroxide with subsequent 
dehydration. For instance, gallium is dissolved in acid, and the formed 
salt solution is neutralized with NH.sub.3 or NaOH. The resulting 
hydroxide is filtered, dried, and calcined to Ga.sub.2 O.sub.3. 
An oxide thus produced contains most of the impurities of the acid or lye 
used. 
The purity demanded today in semiconductor technology cannot be attained 
with this procedure. 
The problem therefore existed of finding a method by which Ga.sub.2 O.sub.3 
could be obtained in the required purity. 
According to the invention, this problem is solved by reacting gallium with 
water vapor at elevated pressure and elevated temperature in an autoclave 
lined with polytetrafluoroethylene or an equivalent plastic-based 
material. The Ga hydroxide thus formed is transformed into Ga oxide by 
calcining above 600.degree.C. 
This is done by heating gallium with an excess of water which has been 
distilled twice in the autoclave to above 200.degree. C, Ga hydroxide 
being formed to begin with. It has been found in this connection that for 
this reaction the use of an appratus lined with polytetrafluoroethylene is 
indispensable. Polytetrafluoroethylene is indifferent to water vapor and 
also to gallium at the reaction temperature, so there is no possibility 
that the hydroxide will be contaminated by new extraneous ions. Although 
polytetrafluoroethylene (e.g. Teflon available from E.I. duPont de 
Nemours, or Fluon available from ICI, or Hostaflon available from Hoechst) 
is especially preferred for use in the invention, the process may also be 
carried out with other chemically stable synthetic materials which are not 
affected by the temperatures and chemicals that are used. Highly 
fluorinated hydrocarbons are especially suitable for use in the invention 
since they are not affected by the respective temperatures and chemicals 
employed in the process. As an example of the wide range of fluorinated 
hydrocarbons which may be employed are polychlortrifluorethylene and 
copolymers of vinylidenefluorides (available under the trandename 
fluorothon). During the heating of the reaction mixture to reaction 
temperature, the corresponding quantity of water vapor is initially formed 
according to the equation Ga + 3H.sub.2 O .dbd. Ga (OH).sub.3 + 3/2 
H.sub.2. The autoclave pressure is maintained between about 200-300 bar 
during the heating operation. 
In addition to the buildup in water vapor pressure during the temperature 
rise, there is also formed by the reaction a hydrogen pressure which also 
remains after the cooling of the reaction temperature. 
Subsequently, for the transformation of the hydroxide into oxide, the 
reaction mixture is heated to above 600.degree. C. 
It is possible in this manner, starting with a 99.9999% pure gallium, to 
obtain a gallium oxide of a purity of 99.999%.

EXAMPLE 
A 16 ml autoclave lined with polytetrafluorethylene is charged with 2.5 g 
gallium and 8 g water. The autoclave is closed and heated to 220.degree. 
C. After remaining at 220.degree. C for 5 hours, the product is cooled, 
and after pressure relief the autoclave opened. The formed Ga (OH).sub.3 
is heated to above 600.degree. C, Ga.sub.2 O.sub.3 being formed.