Geoscience Reference
In-Depth Information
gadolinium gallium garnet substrates
[118,119]
. It did not capture the attention of
specialists owing to the higher experimental pressure and temperature conditions
involved (T
500
C, P
1 kb). However, from 1990 onwards, the work in this
direction has been revived. With the use of electrochemical reactions under hydro-
thermal conditions, it has become possible to grow highly pure and dislocation-free
crystalline thin films of various perovskite-type oxides like BaTiO
3
, SrTiO
3
,
CaTiO
3
, and so on, under much lower pressure (
.
.
100 bar) and temperature
,
200
C) conditions over a variety of substrates
[120
(100
122]
. The method offers
a high success rate and there are efforts to obtain functionally gradient materials
through hydrothermal electrochemistry. With an increase in the demand for the
composites, the hydrothermal technique offers an excellent facility for the coating
of various compounds with metals and ceramics. For example, the coating of
hydroxyapatite (HAp) or bonelike apatite layers on chemically treated Ti metals,
organic polymers and other metals has emerged out as a promising field
[123
125]
. This is discussed in detail in Chapter 10.
The hydrothermal technique has become very useful for the production of whis-
kers, which are finding extensive applications in modern technology. For example,
whiskers of HAp are being produced under mild hydrothermal conditions.
Although, the crystal growth of whiskers with controlled morphology is not a new
field, it was already on the verge of extinction during the last decade. A few years
ago, some reports on the growth of whiskers which are biocompatible under mild
hydrothermal conditions appeared, giving a fresh lease on life to this field of
research concerning whiskers crystal growth
[126,127]
. Among the popular whis-
kers crystal growth, the most popular one is the biocompatible HAp whiskers of
nanometer size and this can be prepared at temperatures
200
C and pressure
,
2 MPa within a few hours. Whiskers prepared by the hydrothermal method have
a more controlled and better morphology, uniformity in size and purity in composi-
tion compared to the HAp whiskers produced by other methods. The whiskers
obtained by other methods usually contained contaminants like carbonates and het-
erogeneity in shape and size. The hydrothermally prepared whiskers are finding
promising applications mostly as reinforcements in composites and thermal insula-
tion. They are highly biocompatible, as they contain nontoxic species, such as Ca,
P, and OH
2
, and are noncarcinogenic, chemically inert both outside and inside the
living bodies. This is discussed in greater detail in Chapter 10.
The hydrothermal hot pressing (HHT) technique is becoming a very simple and
most effective processing technique for the preparation of solid bodies from inor-
ganic powders under mild hydrothermal conditions
[128
,
130]
. This technique is
especially useful for the solidification of thermally decomposable powders such as
carbonates, sulfates, and nitrates at high temperatures. The solidification process is
thought to be similar to liquid-phase sintering under pressure. In this technique, the
starting powder containing water is continuously compressed from the outside of
an autoclave under hydrothermal conditions. The powder is then treated at autoge-
neous pressure, while it is compressed at much higher pressure than the vapor pres-
sure inside the autoclave. The water still remaining in the compacts after the HHT
can be easily drained out by drying in air. By this means, porous ceramics can be
