Solution Combustion Synthesis of Calcium …
185
(Taylor Hobson precision) and the average roughness (R a ) value of 5 such readings
is reported. The Young’s modulus of the substrate and coatings were obtained by
nanoindentation (CSEM nano-hardness tester) technique by employing a Berkovich
indenter at a load of 10 mN. The wear resistance of the coatings was measured using
a reciprocating wear tester and more details are reported elsewhere [48]. For in vitro
studies, HAp coating prepared from 600 °C calcined powders was used. More details
of in vitro studies are reported elsewhere [48].
4 Results and Discussion
4.1 Effect of Fuel and Fuel Mixtures
The combination of the mixture of fuels used for the synthesis of the powder had
affected the nature of the powder. The mixture of fuels (urea (U) + ammonium
acetate [AA]) was found to be advantageous in getting flowable powders with average
agglomerated particle size < 100 μm when compared to the use of a single fuel
(U/AA). The powders obtained from different fuel combinations were sized in the
range between 45 and 90 μm by sieving to get flowable powders. It was observed
that with an increase in ammonium acetate concentration as the fuel, the particles
became too fine (< 45 μm) and 100% urea gave very coarse particles (> 100 μm).
The crystallite size of the powder depends on the flame temperature as well as the
nature of the combustion reaction. It has been reported that when urea is used as the
fuel it leads to the flaming type combustion and in course of this if the flame persists
for a longer duration, the particles will have enough time and duration to sinter and
yield larger particles [49]. Thus, with the use of a single fuel such as urea, larger-sized
particles were obtained. To decrease the sinterability and reduce the exothermicity
of the combustion reaction, the amount of urea (U) was reduced. Therefore, the stoichiometric amount of urea calculated was not used, instead varying concentrations of
urea (0.75 U/0.5 U/0.25 U) were used and to compensate the fuel ratio, an extra fuel
ammonium acetate (AA) was used. The reaction with 0.75U + 0.25AA was flaming
and there was more frothing. With the increase in ammonium acetate concentration and decrease in the urea amount, the combustion reaction became smoldering
type. The crystallite size of the particles decreases with the increase in the ammonium acetate concentration and yields amorphous particles. The ammonium acetate
molecules decompose at lower temperatures and generate a large volume of gases
that dissipates heat quickly from the system thereby releasing a large volume of gases.
With a mixture of fuels (75% U + 25% AA), very less amount of fine powder was
obtained and a large fraction of powder was in the range 45–90 μm size and hence
this combination of the fuel was selected for the synthesis of flowable hydroxyapatite
powder.
185
(Taylor Hobson precision) and the average roughness (R a ) value of 5 such readings
is reported. The Young’s modulus of the substrate and coatings were obtained by
nanoindentation (CSEM nano-hardness tester) technique by employing a Berkovich
indenter at a load of 10 mN. The wear resistance of the coatings was measured using
a reciprocating wear tester and more details are reported elsewhere [48]. For in vitro
studies, HAp coating prepared from 600 °C calcined powders was used. More details
of in vitro studies are reported elsewhere [48].
4 Results and Discussion
4.1 Effect of Fuel and Fuel Mixtures
The combination of the mixture of fuels used for the synthesis of the powder had
affected the nature of the powder. The mixture of fuels (urea (U) + ammonium
acetate [AA]) was found to be advantageous in getting flowable powders with average
agglomerated particle size < 100 μm when compared to the use of a single fuel
(U/AA). The powders obtained from different fuel combinations were sized in the
range between 45 and 90 μm by sieving to get flowable powders. It was observed
that with an increase in ammonium acetate concentration as the fuel, the particles
became too fine (< 45 μm) and 100% urea gave very coarse particles (> 100 μm).
The crystallite size of the powder depends on the flame temperature as well as the
nature of the combustion reaction. It has been reported that when urea is used as the
fuel it leads to the flaming type combustion and in course of this if the flame persists
for a longer duration, the particles will have enough time and duration to sinter and
yield larger particles [49]. Thus, with the use of a single fuel such as urea, larger-sized
particles were obtained. To decrease the sinterability and reduce the exothermicity
of the combustion reaction, the amount of urea (U) was reduced. Therefore, the stoichiometric amount of urea calculated was not used, instead varying concentrations of
urea (0.75 U/0.5 U/0.25 U) were used and to compensate the fuel ratio, an extra fuel
ammonium acetate (AA) was used. The reaction with 0.75U + 0.25AA was flaming
and there was more frothing. With the increase in ammonium acetate concentration and decrease in the urea amount, the combustion reaction became smoldering
type. The crystallite size of the particles decreases with the increase in the ammonium acetate concentration and yields amorphous particles. The ammonium acetate
molecules decompose at lower temperatures and generate a large volume of gases
that dissipates heat quickly from the system thereby releasing a large volume of gases.
With a mixture of fuels (75% U + 25% AA), very less amount of fine powder was
obtained and a large fraction of powder was in the range 45–90 μm size and hence
this combination of the fuel was selected for the synthesis of flowable hydroxyapatite
powder.