Apparatus - Hydrothermal Technology - page 81

pH 7 is less corrosive than slightly acidic water, especially for alloys containing Si.

Several conventional methods of studying corrosion are known. Usually low-carbon
steel is used in the fabrication of simple acid digestion bombs, Morey-autoclaves,
modified Bridgman autoclaves, and other related ones. Inconel, Udimet, and Stellite
are highly useful because of their high strength at high temperature. These alloys are
used in the fabrication of cold-cone-seal bombs employed in phase-equilibria studies
at pressures up to 5.4 kbar and temperatures up to 950 C. The recent TZM (Titanium
Zirconium Molybdenum) autoclaves are used at pressures up to 10 kbar.
Although the use of glass vessels began in the nineteenth century, for low-
temperature and low-pressure experiments thick glass vessels are still used. The
quartz glass pressure vessel for hydrothermal studies was first used in 1964 by
Speed and Filice [6] . Recently, in 1991, Popolitov [7] used a thick-walled silica
vessel with a Teflon capping for experiments up to 200 C and

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500 bar pressure.
The construction details of various autoclaves will be given separately in the forth-
coming section.

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3.3 Liners

In the majority of hydrothermal experiments, the mineralizer used is highly corro-
sive and it can attack the vessel, which is inimical to obtaining high-purity crystals
for devices. It requires a suitable lining for the inner wall of the autoclave or sepa-
rate liners placed in the autoclave. Even in the case of quartz growth, where an
acmite-protective layer is found all along the inner wall of the autoclave, the purity
of the substance is doubtful. Therefore, for the growth of high-purity quartz, a sil-
ver lining for the autoclave is provided. Similarly, other metal linings are provided
depending upon the solvent medium. For example, a copper lining is used in many
of the borate systems. Copper partly dissolves in dilute bases at 400 C, and copper
crystals are deposited in the cooler region of the autoclave. Similarly, nickel lining
can be used, but it is also readily attacked by the solvent. Hence, noble metal lin-
ing, liners, or capsules are used successfully for alkaline and neutral media.
Titanium is much more corrosion resistant, but it is difficult to handle. Litvin and
Tules [8] have summarized the applicability of various liners as listed in Table 3.5 .
The use of glass tube as a liner was first applied by De Senarmont as early as
AD 1851 for the growth of carbonates, sulfates, and sulfides. Figure 3.1 shows the
sealed glass tube used by De Senarmont [9] . The tubes were enclosed in water-
filled fused gun-barrels and heated up.
Daurree [10] first introduced the pressure balance arrangement between the
small glass tube containing the nutrient and the steel tube hosting this glass tube
liner. An improved version using pyrex glass and again water as a pressure trans-
mitter was reported by Allen et al. [11] . In fact, most of the earlier workers during
the nineteenth century, including von Chroustshoff, found that the glass tube they
were using was much attacked under hydrothermal conditions; however, they did
not mention anything about precautions taken to overcome this effect, as the