Geoscience Reference
In-Depth Information
mineralizer molalities (cf. shaded area in
Figure 4.16
). PbZr
0.6
Ti
0.4
O
3
precipitates
along with PbO in the small region shown on the top of the phase-purity region.
The stability area of PbO is relatively small and limited to a high concentration of
precursors. In highly alkaline solutions, lead oxide dissolves. When the ratio of
metals in precursor materials is greater than one, the area of the perovskite phase-
purity region widens. However, at some reaction conditions it causes contamination
of ceramic materials with additional phases.
Figure 4.17
shows the stability field diagram for CaO
H
2
O
system at 200
C as a function of precursor concentration and pH. The pH values
are calculated from the model for the temperatures shown in
Figure 4.17
. Initial
room temperature pH of the starting slurries have been recalculated to pH's at the
experimental temperatures. The diagram shows stability fields for HAp and mone-
tite (Mt). Experimental points represent phase assemblages according to the
legends. At the phosphorus concentrations at 200
C, the HAp stability field extends
to an equilibrium pH as low as 2.9. The latter pH corresponds to a room tempera-
ture pH of about 3.8. Powders run at 200
C and which were prepared from slurries
with room temperature pH's lower than about 3.8 yielded only monetite
[110]
.
These thermodynamic calculations help in obtaining advanced ceramic powders
with controlled size and shape. Hence, especially for the nanomaterials processing
it is highly useful. However, there are several other additional parameters which
influence the kinetics of the hydrothermal reactions like the stirring rate, external
fields like magnetic field, sonar, and mechanochemical grinding.
a
P
2
O
5
a
NH
4
NO
3
a
Figure 4.17 Calculated stability field
diagram for the HAp system at 200
C and
25 bar with Ca:P ratio at 1.24.
0
MT+HA
-1
MT
CaH
2
PO
4
+
(aq)
CaPO
4
-
(aq)
-2
Ca
2+
(aq)
200
C
25 bar
°
-3
Ca:P = 1.24
HA
-4
Synthesis products
MT only
MT + HA
HA only
-5
Calculated stability fields
Monetite (MT)
Hydroxyapatite (HA)
99% Ca precipated as HA
Dominant Ca ion species
-6
-7
2
4
6
8
10
pH
