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phosphor nanoparticles, because, the supercritical hydrothermal synthesis of these
phosphor nanomaterials is growing fast in recent years. Much of the literature data
on these materials deal with the other conventional methods like sol
gel, solid-state
reactions, melt, hydrolysis, and other wet chemical methods. As per the literature
survey data, the flow reactor is being used only by the authors' group to synthesize
nanoparticles of these materials.
10.6.11 Rare Earth Vanadates
YVO
4
:R
3
1
and GdVO
4
:R
3
1
(where R
Nd, Er, Eu) are known as efficient laser
host materials, excellent polarizer, and as a phosphor in its powder form
[210]
.
These compounds show high melting and no phase transitions. The synthesis of the
rare earth vanadates is popular by the conventional methods. The materials prepared
by these conventional methods encountered several problems like oxygen vacancies
resulting in major structural defects, presence of mixed phases in the products, and
loss of vanadium during crystallization leading to the loss of stoichiometry of the
product. In order to overcome these problems, the supercritical or near-critical
hydrothermal technique was proposed by Byrappa et al.
[211]
. There is only one
recent conference proceeding so far on the use of SCF technology (continuous flow
reactor) to synthesize rare earth vanadates on the whole in the literature. Both acidic
and basic solvents are used in the synthesis of yttrium and vanadium compounds,
which result in macro- and microcrystals. The addition of EDTA, or ammonium
metavanadate, into the system stunts the growth of larger particles and produces
nanoparticles of rare earth vanadates. The synthesis is normally carried out in the
temperature range 220
5
400
C.
Figure 10.45
shows what the rare earth vanadates
produced under subcritical conditions. The use of SCF technology with a residence
time of 2.08 s yielded highly dispersed nanoparticles of GdVO
4
:Eu
3
1
with
10
15 nm size. NH
4
VO
3
, KOH, Gd(NO
3
)
3
, and Eu(NO
3
)
3
were used as the starting
materials with gluconic acid as a modifier to inhibit the particle aggregation.
Figure 10.46
shows the nanoparticles and nanostructures prepared using SCF tech-
nology. These results have shown a great potential for the synthesis of these materi-
als using SCF technology.
Figure 10.45 YVO
4
:Nd
3
1
nanostructures and nanoparticles.
Source: Photographs courtesy of K. Byrappa.
