Geoscience Reference
In-Depth Information
precursors at high concentrations does not require the addition of a mineralizer
because the necessary concentration of OH
2
groups is readily provided by the
hydroxide precursor. To the contrary, at dilute concentrations identical amounts of
mineralizers are needed, irrespective of whether a nitrate or a hydroxide is used as
precursor. However, the required mineralizer concentration differs substantially for
the three metals. This may be caused by the strong, specific effects of the chemical
identity of cations on activity coefficients due to their high concentration to form
alkaline earth titanates.
Eckert et al.
[169]
have proposed an in situ reaction mechanism and
dissolution
precipitation reaction mechanism (
Figure 10.40a and b
). As evident
from these figures, the in situ transformation model assumes that TiO
2
reacts ini-
tially with dissolved barium. This produces a continuous layer of BaTiO
3
through
which additional barium must diffuse in order to continue reaction until the TiO
2
supply is exhausted. The product layer may be either a dense or a porous layer or
of monocrystalline or polycrystalline nature. The dissolution
precipitation model
involves multiple steps. For an anhydrous TiO
2
precursor, Ti
O bonds must be
broken via hydrolytic attack, to form hydroxy-titanium complexes
4
2
x
x
Þ
capable of dissolution and reaction with barium ions or complexes (Ba
2
1
or
BaOH
1
) in solution to precipitate BaTiO
3
. In contrast, use of a hydrous TiO
2
reac-
tant bypasses some, or most, of the hydroxylation steps. BaTiO
3
nuclei may either
originate at the TiO
2
substrate (heterogeneous nucleation) or form directly in the
bulk solution (homogeneous nucleation). When hetereogeneous nucleation occurs,
the dissolving TiO
2
particle can be encapsulated, thereby limiting the supply of
soluble TiO
2
species available to react with the barium species. As with the in situ
transformation model, this diffusional barrier serves to slow if not to halt the hydro-
thermal reaction. Such mechanisms of formation of BaTiO
3
can also be applied to
other perovskite-type titanates.
Figure 10.41
shows the perovskite-type alkaline
earth titanates prepared under hydrothermal conditions
[163,172]
. The pH of the
medium, precursor, and the ratio of Me/Ti play an important role in determining
the morphology of these particles.
The preparation of alkaline earth titanates has been carried out using nonaque-
ous solutions, which come under solvothermal synthesis. This has some advantage,
as the solvothermal synthesis might allow the product to be free from foreign atoms
because the organic solution, having a low relative permittivity, is free of ionic spe-
cies. Chen and Xu
[176]
have synthesized PbTiO
3
powder under solvothermal con-
ditions. The starting materials involved are amorphous xerogels consisting of a
mixture of equivalent molar amounts of PbO and TiO
2
(1:1 ratio), prepared by
using lead acetylacetate and tetrabutyl titanate. The precursor xerogel was poured
into the solvent to form a suspension solution and 30.0 cm
3
of the suspension was
fed into a 40 ml capacity stainless steel autoclave with a Teflon liner. The required
amount of methanol was poured into the autoclave, and the autoclave was held at
240
C for 10
ð
Ti
ð
OH
Þ
60 h. The crystallinity increased with the increasing reaction time.
Although the crystallization of PbTiO
3
in an alcohol solution required higher
temperatures for longer times, the nanometer-sized particles, in comparison with
the micrometer-sized ones derived from an aqueous solution, exhibited a lower
