2118
L. Singh et al. / Materials Research Bulletin 48 (2013) 2117–2122
Merck, India having purity of 99.5% or better were used as
starting materials. Standard aqueous solutions of Ca2+, Cu2+
2.3. Characterization
,
Zn2+, citric acid and glycine were prepared in distilled water.
Solutions of metal ions and stoichiometric amount of solid TiO2
along with aqueous solution of citric acid/glycine equivalent to
metal ions were mixed. While in the solution route Ti was taken
in the form of titanium oxynitrate, TiO(NO3)2, prepared in our
lab, in both the semi wet routes Ti was taken as solid TiO2. The
solutions were heated on a hot plate using a magnetic stirrer at
70–90 8C to evaporate water to yield the viscous gel and then
dried at 110 8C in hot air oven for 12 h for its complete removal.
The highly viscous mass thus obtained was converted to solid
precursor which was then calcined in air at 800 8C for 6 h in a
muffle furnace. The resultant mixtures were ground into fine
powders using a pestle and mortar, and cylindrical pellets were
made using a hydraulic press. The pellets were sintered at
950 8C for 12 h in air produced nanocrystalline CaCu2.90Zn0.10-
Ti4O12 ceramic. The sintered pellet was polished using SiC
abrasive paper and diamond paste until there was a mirror
finish. In order to perform the electrical measurements, silver
paste was applied to both sides of the circular faces of the
ceramic pellet, then dried at 600 8C for 20 min, and cooled
naturally to room temperature. The synthetic parameters affect
the particle size, the microstructure and dielectric properties of
CCZTO ceramic obtained by diverse routes.
The crystalline phases of the sintered samples were identified
using an X-ray Diffractometer (Rich-Siefert, ID-3000) employing
˚
Cu-Ka radiation (1.5406 A) in the 2u scan range of 208–808. The
XRD data are also used to determine the average crystallite sizes
(D) of the ceramics using the Debye–Scherrer formula [19] as
shown in Eq. (4),
k
l
D ¼
(4)
b
cos u
˚
l (1.5406 A) is the wavelength of the X-ray, k is the Scherrer
where
constant taken as 0.89 assuming that the crystallites are spherical,
is the diffraction angle and is the full width at half maxima
u
b
(FWHM). The microstructures of the fractured surfaces were
examined using a Scanning Electron Microscope (SEM, Model JEOL
JSM5410). The Energy Dispersive X-ray Analyzer (EDX, Model
Kevex, Sigma KS3) was used for elemental analysis in the sintered
samples of the above system. Transmission electron microscopic
(TEM Technai 12 G2, FEI) studies of the sintered samples were
carried out by placing the sample on carbon-coated copper grid
operated at an accelerated voltage of 120 kV. The dielectric data on
Zn doped CCTO ceramics of the above system were collected using
the LCR meter (PSM 1735, Newton 4th Ltd., UK) with variation in
temperature at few selected frequencies.
2.1. Semi-wet method
3. Results and discussion
In this method, the mixing process was performed in solution
state as nitrate solutions of metal ions along with solid TiO2. The
technique involves mixing of solutions of a metal precursor and an
organic polyfunctional acid possessing at least one hydroxyl and
one carboxylic acid group such as citric, glycine, tartaric and
glycerol which results in complexation of the metal by the
polycarboxylic acid.
The ideal temperature for the decomposition of precursor gel
was determined by the thermo gravimetric analysis (TGA) and
differential thermal analysis (DTA). Simultaneous TGA/DTA
characterizations, carried out on SWR1, SWR2 and SR ceramics
of dried precursor powder with a heating rate of 10 8C minꢀ1 in
static air from room temperature to 1000 8C, are shown in Fig. 1a–
c. The TGA curve of the SWR1 (Fig. 1a) shows three stages of weight
loss. While the first one is in the temperature range of room
temperature to 300 8C, the second one is from 300 8C to 500 8C and
third one is from 500 8C to 780 8C. In the TGA curve, no further
weight loss was observed in the temperature range 800–1000 8C.
The first weight loss as confirmed by DTA, is due to an exothermic
reaction, in which the elimination of residual water and the
dehydration of hydrated water occurred. The second weight loss in
the temperature range 300–500 8C is due to the combustion of gel,
excess citric acid, and formation of an intermediate compound
CaCu2.9Zn0.10O4 [18], which is supported by the main exothermic
peak observed at 400 8C in the DTA curve. The third peak is due to
an endothermic addition reaction of an intermediate compound
CaCu2.9Zn0.10O4 with TiO2 to crystallize final product CaCu2.9Zn0.10-
Ti4O12 (CCZTO). The formation of intermediate and final product
CCZTO ceramic are shown by chemical Eqs. (5) and (6) given
below:
2.1.1. Semi-wet route using citric acid
In this route mixing process was performed in solution state as
Ca(NO3)2, Cu(NO3)2 and Zn(NO3)2 along with solid TiO2. Calculated
amount of citric acid equivalent to metal ions was dissolved in
distilled water which was then added to the metal solution to form
the complex.
2.1.2. Semi-wet route using glycine
In this route glycine was used in place of citric acid. Glycine is
a complexing agent and it can complex a cation at both the
carboxylic end and at amino group end and provides a fuel for
the ignition step. The ignition step raises temperature to form
very fine size crystalline precursor powder at very low
temperature [16].
2.2. Solution route
CaCO3 þ 2:9CuO þ 0:10ZnO ! CaCu2:9Zn0:10O4 þ CO2
(5)
(6)
In this route aqueous solutions of nitrates of calcium, copper
and zinc were prepared. In order to avoid the use of water insoluble
TiO2 and cost-bearing titanium isopropoxide, Ti(OR)4, the former
was dissolved in H2SO4 and converted to titanium oxysulphate
(Eq. (1)) which was then changed to titanium oxyhydrate solid
(Eq. (2)) and then titanium oxynitrate (Eq. (3)) solution which is as
good as Ti(OR)4 [17] for the synthesis of CCZTO.
CaCu2:9Zn0:10O4 þ 4TiO2 ! CaCu2:9Zn0:10Ti4O12
The absence of peak in the DTA curve beyond 800 8C supports
the findings of TGA and confirms that the final product forms
around 800 8C. However, the thermogram for SWR2 (Fig. 1b) where
glycine is used as fuel shows the decomposition of dry precursor in
two steps. In the first stage elimination of residual water and the
dehydration of hydrated water occurred and in the second stage
weight loss observed at 400 8C in which the intermediate
compound directly combined to TiO2 to form CaCu2.9Zn0.10Ti4O12
The absence of peak in DTA curve beyond 400 8C supports the
findings of TG analysis and confirms the formation of the ceramic
TiO2 þ H2SO4 ! TiOðSO4Þ þ H2O
(1)
(2)
(3)
TiOðSO4Þ þ 2NH4OH ! TiOðOHÞ2 þ ðNH4Þ2SO4
TiOðOHÞ2 þ 2HNO3 ! TiOðNO3Þ2 þ 2H2O
.