4
76
B.K. Maji et al. / Journal of Alloys and Compounds 640 (2015) 475–479
value of MgO was interpolated at 2 deg. interval and plotted against temperature.
(where, M = Ca, Sr and Ba).
The % thermal expansion was fitted to a polynomial equation of 3rd order. The %
expansion obtained at various temperatures by using literature data and experi-
mental data are compared within the measured temperature range. The difference
in % thermal expansion reported in the literature (fitted value of equation reported
in the literature) and experimentally measured for MgO is added to the experimen-
The chemical composition of the phase pure compounds was analyzed by ICP-
OES and AAS technique and is found to maintain M/Mo mole ratio: 1:1, where
M = Ca, Sr, Ba.
2
.2. Electrical conductivity measurements of CaMoO
4
, SrMoO
4
4
and BaMoO pellets
tally measured value of ThO
ThO literature value by Belle and Berman [20]. The following equations (Eqs.
3)–(5)) are used to find out the corrected value of ThO using MgO as the standard.
2 2
. The corrected value of ThO is compared with the
2
The electrical conductivity measurements on CaMoO
4
, SrMoO and BaMoO
4
4
(
2
were carried out by AC impedance technique. The pellets were prepared in a
hydraulic pellet press (Ms. Kimaya Engineers, India) by using a tungsten carbide
die and plunger and then the pellets were sintered at 1023–1273 K for 10 h s in
air ambience. These sintered pellets (around 10 mm dia and 2–3 mm thickness)
were used for electrical resistivity measurements. The bottom and top flat surfaces
of the pellets were metalized using Ag-paste. The metalized pellets were contacted
with Pt-disk shaped electrodes as shown in Fig. 1. The sample, electrodes and
MgO correction value ¼ % expansion of MgO using literature equation
% expansion experimental value of MgO using interpolation
ꢁ
ð3Þ
MgO corrected % expansion of ThO2 ¼ Experimental % expansion of ThO2
þMgO correction value as given in Eq: ð3Þ
ð4Þ
thermo-couple assembly is enclosed inside
a one end closed alumina tube
[
15,16]. In this experiment, sample is sandwiched between the two electrodes
and firm contact is ensured by tightening the alumina disks with stainless steel
nut bolt on both side of the pellet as shown in Fig. 1. The Pt-wires spot welded to
the platinum disks were used as leads to measure electrical resistance of the sample
at various temperatures. The sample assembly was placed inside the cell and the
cell was put inside the furnace well. The temperature of the furnace was controlled
by a programmable PID temperature controller with ±1 K accuracy. The sample
temperature was measured with a K-type (chromel–alumel) thermocouple placed
at about 2 mm from the sample in the conductivity measurement cell.
Resistances of the sample were measured at each 50 K interval. The impedance
The MgO corrected ThO value is compared with literature value given by
2
Belle and Berman
ð5Þ
The literature value and corrected values of ThO
2
are found to be in good agreement.
Same procedure is applied to cross check the % thermal expansion of MgO by adding
difference of ThO (fitted) and ThO experimental and compared with MgO (litera-
2
2
ture value/fitted). In this study, the difference in fitted value and experimentally
measured value of standard MgO single crystal is taken as the correction factor for
the samples studied. The densities and dimensions of the pellets were once again
measured after the thermal expansion measurements were complete. The dimen-
sions were found to be unchanged. The percentage of average/mean linear thermal
expansion is calculated using Eq. (6).
(Z) measurements were carried out using an Autolab Frequency Response
Analyser (FRA) in the frequency range of 100 Hz–1 MHz. The resistance of the sam-
0
0
ples at various temperatures was determined by fitting the data of ꢁZ (Zimaginary
)
0
vs. Z (Zreal) Nyquist plot using fit and simulate functions available in the Autolab
00
0
FRA system. The real part of the semicircle (ꢁZ vs. Z ) is taken as the resistance
of the sample at a particular temperature. The fitting of the semicircle was done
by trial and error method on assuming various equivalent circuit models available
with the software provided by Autolab. The equivalent circuit that fits all the points
on the semicircle with minimum error is taken as the accepted model and the val-
ues of R and C calculated by the model for a particular temperature were taken as
the accepted value. The conductivity of the samples was calculated using the for-
mula given in the Eq. (2).
% TE ¼ ð
D
L=LÞ ꢀ 100
ð6Þ
The coefficient of thermal expansion (average CTE =
calculated by using the formula given in Eq. (7).
a
m
=
aav) of the compounds was
Average or mean CTE ¼
a
m
¼ ðD
L=LÞ ꢀ ð1=
D
TÞ
ð7Þ
where
in K.
D
L = change in length, L is the length of the pellet,
D
T = change in temperature
r
¼ ðL=AÞ ꢀ ð1=RÞ
ð2Þ
3
. Results and discussions
.1. Phase identification by powder-XRD
The powder-XRD patterns (Fig. 2) of the compounds confirmed
where L = the length of the pellet (cm), A = cross-sectional surface area of the cylin-
drical pellet (cm ), R = resistance of the pellet, thereby we got
fic impedance of the samples is calculated by multiplying A/L ratio of the pellet with
impedance values (Zim = Z or Zre = Z ) at all frequencies and at all temperatures then
plotted as (Zim/ohm cm) vs. (Zre/ohm cm).
2
ꢁ1
r in S cm . The speci-
3
0
0
0
the formation of crystalline single phase alkaline earth metal
molybdates (CaMoO , SrMoO and BaMoO ). There are no reactant
phases detected in the XRD pattern of the samples. The XRD pat-
terns of the compounds were indexed by using X-Pert’ Pro soft-
ware and found to stabilize in the tetragonal crystal system
2
.3. Thermal expansion measurements of CaMoO
4
, SrMoO
4
and BaMoO
4
pellets
pellets of
4
4
4
Thermal expansion measurements of CaMoO
4
, SrMoO
4
and BaMoO
4
around 10 mm dia. and 10 mm height were carried out by dilatometry in the tem-
perature range 298–800 K in air, by using a home-built apparatus [17,18]. Linear
Variable Differential Transformer (LVDT) was used as the displacement sensor.
The instrument was calibrated by measuring % thermal expansion of known stan-
1 4
(Space Group: I4 /a. (88) Scheelite type structure (ABO )). The lat-
tice constants of these compounds were calculated and shown in
Table 1. The values determined on indexing the XRD pattern were
compared with the literature values [21] and found to agree well
dard single crystal of MgO [19] and ThO
2
pellets [20]. The experimentally measured
Thermocouple
Pt-wire
5
00
BaMoO4
SrMoO4
CaMoO4
250
0
6
00
00
0
3
Alumina disc
Pt-disc
Sample
6
00
00
0
Alumina disc
3
2
0
30
40
50
60
2
-theta (deg.)
Fig. 1. Schematic of sample pellet and electrode assembly in the high temperature
electrical conductivity cell.
Fig. 2. XRD patterns of CaMoO
4
, SrMoO
4
4
and BaMoO .