Apparatus - Hydrothermal Technology - page 117

electrode potential, the electrolysis current, and the quantity of electricity passed
through the Ti electrode were recorded as functions of the electrolysis time to
investigate the effect of oxygen evolution at the Ti electrode on the microstructure
of the growing films in a series of experiments. The pressure of the electrolytic cell
was controlled at higher values than the saturated vapor pressure. Similarly, the
galvanostatic electrolysis can be used to control the rate of oxygen evolution.

Autoclave for Ammonothermal Synthesis
The first investigations on metal ammonia solutions were carried out by Weyl in 1864
[96] and Seely in 1871 [97] . It is well known that alkali and alkaline earth metals
(except Be), as well as lanthanides like europium and ytterbium, dissolve in liquid
ammonia under normal pressure. These interesting metal ammonia solutions have
inspired many preparative chemists to synthesize materials in ammonia solution under
hydrothermal conditions. The reaction products obtained in liquid ammonia under nor-
mal pressure are mostly microcrystalline or even amorphous. But this problem can be
solved by working with supercritical ammonia as both solvent and reactant at pressures
up to 6 kbar and temperatures up to 520 C. Thus, specially designed autoclaves are
needed. As many of the compounds prepared by this method are sensitive to moisture
and oxygen, special glove boxes have been constructed. Today, this ammonothermal
synthesis is very popular for the synthesis of gallium nitride [98] . Figure 3.33 shows

Figure 3.33 Schematic diagram of the
autoclave designed by Jacobs and
Schmidt [99] .

Ammonia gas inlet fitting

Silver disk

Draw plate
Plunger nut

Main nut

Plunger

Sealing ring

Ball joint