Synthesis, sintering and optical properties of CaMoO4: A promising scheelite LTCC and photoluminescent material

Article abstract of DOI:10.1002/pssa.201127620

The synthesis of nanocrystalline calcium molybdate (CaMoO₄) through an autoigniting combustion technique is reported in this paper. The structural characterization of the as-prepared nanocrystallites were done by X-ray diffraction (XRD), Fourier transform Raman, and Fourier transform infrared (IR) spectroscopy and the morphological studies using transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The studies reveal that the as-prepared powder itself was phase pure with tetragonal structure and of particle size 25 nm. The sample was sintered at a relatively low temperature of 775 °C to a high density of ～95% for the first time, without the use of any sintering aid. The optical bandgap energy calculated from the ultraviolet-visible absorption spectrum for the as-prepared and annealed sample was 3.72 and 3.99 eV, respectively. The photoluminescence spectra of the sample showed an intense emission in the green region (528 nm). The dielectric constant and loss factor of the sample at 5 MHz was found to be 11.00 and 6.40 × 10^-3 at room temperature. The temperature coefficient of dielectric constant was -95.04 pp/°C. These observations reveal that nanostructured CaMoO₄ is a promising scheelite low-temperature co-fired ceramic (LTCC) and also an excellent luminescent material. Copyright

Full text of DOI:10.1002/pssa.201127620

Phys. Status Solidi A, 1–8 (2012) / DOI 10.1002/pssa.201127620
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applications and materials science
Synthesis, sintering and optical
properties of CaMoO4: A promising scheelite
LTCC and photoluminescent material
,1
S. Vidya1, S. Solomon2, and J. K. Thomas*
1 Electronic Materials Research Laboratory, Department of Physics, Mar Ivanios College, Thiruvananthapuram-695 015, Kerala, India
2 Department of Physics, St. John’s College, Anchal, Kollam District 691306, Kerala, India
Received 23 October 2011, revised 4 January 2012, accepted 30 January 2012
Published online 23 February 2012
Keywords CaMoO4, combustion synthesis, dielectric response, sintering
* Corresponding author: e-mail jkthomasemrl@yahoo.com, Phone: þ91 471 2530887, Fax: þ91 471 2532445
The synthesis of nanocrystalline calcium molybdate (CaMoO4) the use of any sintering aid. The optical bandgap energy
through an autoigniting combustion technique is reported in calculated from the ultraviolet–visible absorption spectrum for
this paper. The structural characterization of the as-prepared the as-prepared and annealed sample was 3.72 and 3.99 eV,
nanocrystallites were done by X-ray diffraction (XRD), Fourier respectively. The photoluminescence spectra of the sample
transform Raman, and Fourier transform infrared (IR) spectro- showed an intense emission in the green region (528 nm). The
scopy and the morphological studies using transmission dielectric constant and loss factor of the sample at 5 MHz was
electron microscopy (TEM) and scanning electron microscopy found to be 11.00 and 6.40 ꢁ 10ꢂ3 at room temperature. The
(SEM). The studies reveal that the as-prepared powder itself temperature coefficient of dielectric constant was ꢂ95.04 pp/8C.
was phase pure with tetragonal structure and of particle size These observations reveal that nanostructured CaMoO4 is a
25 nm. The sample was sintered at a relatively low temperature promising scheelite low-temperature co-fired ceramic (LTCC)
of 775 8C to a high density of ꢀ95% for the first time, without and also an excellent luminescent material.
ß 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
1 Introduction Synthesis of advanced ceramics an acousto-optic filter [9], an efficient mixed electron–hole
materials as nanocrystals is one of the major fields in ion conductor [10], and also as a dielectric material for
materials processing technology. The importance of nano- microwave applications [11].
materials is that its properties can be tuned by controlling the
A variety of techniques have been reported in the
particle size. Molybdates with scheelite structure have literature on the synthesis of CaMoO4 such as solid-state
received much attention owing to their remarkable proper- reaction [12], coprecipitation [13], combustion method [14],
ties. In the scheeelite structure [MoO4]2ꢂ group adopts a citrate complex method [6], and Czochralski method [15]. A
tetrahedral co ordination in space group I41/a and the cation low-temperature synthesis was reported by Wang et al. [16]
is bonded to eight oxygen atoms forming an octahedron. in which pure CaMoO4 nanoparticles were obtained after
Calcium molybdate (CaMoO4) is a well-known scheelite calcination for 3–7 h. Thongtem et al. [17] reported a cyclic
class of compound that possesses a wide range of microwave radiation method for the synthesis of CaMoO4
applications. CaMoO4 finds applications in various fields with different morphology. CaMoO4 powders prepared
as laser host materials [1], solid-state scintillators [2, 3], through all these methods are relatively large in particle
phosphor microparticles [4] luminescence material [5], size with irregular morphology and of inhomogeneous
solid-state laser material [6], and optical filters [7]. It composition due to the tendency of MoO3 to vaporize at
has good thermal and chemical stability that makes it an high temperatures [18]. Also, in most of these methods a
excellent host for luminescent materials under UV and prolonged processing temperature is needed to obtain the
X-ray excitation due to its luminescent MoO4 tetrahedron required phase.
unit [8]. CaMoO4 crystal has been reported as a dispersive In the present work a single-step modified combustion
element in electronically tunable lasers, playing the role of technique is reported for the preparation of nanostructured
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S. Vidya et al.: Synthesis, sintering and optical properties of CaMoO4
CaMoO4, in which the as-prepared powder obtained is phase fed to a photomultiplier detector. The variation of intensity
pure with perfect scheelite structure. The structural, optical, was recorded as a function of wavelength. The UV–Vis
and dielectric characterization of the CaMoO4 is also pre- absorption spectrum was obtained using a Jasco spectropho-
sented. The sample is sintered at a relatively low temperature tometer in the range 200–800 nm.
of 775 8C for the first time without the use of any sintering aid.
The sintering behavior of the nanoparticles obtained by
The low sintering temperature, good dielectric and optical the present combustion method, was studied by mixing 5%
properties makes CaMoO4 prepared through the present polyvinyl alcohol with the as-prepared CaMoO4 nanoparti-
method, a promising candidate for low-temperature cofired cles and pressed in the form of a cylindrical pellet of 12 mm
ceramic (LTCC) and luminescent applications.
diameter and ꢀ2 mm thickness at a pressure about 350 MPa
using a hydraulic press. The sintering temperature of the
2 Experimental CaMoO4 nanopowder was synthes- pellet was then optimized at 775 8C for 3 h in a furnace. The
ized by a modified combustion process where ammonia surface morphology of the sintered sample was examined
and citric acid were used as fuel and complexing agent, using scanning electron microscopy (SEM) using a JEOL
respectively. Aqueous solutions containing Ca and Mo ions JSM 5610 LV microscope. The dielectric response of the
were prepared by dissolving stoichiometric amounts of pellets in the low-frequency range was studied using an LCR
Ca(NO3)2 in double-distilled water and MoO3 in 10% meter (Hioki-3532-50 LCR HiTester) in the frequency range
ammonia solution. Citric acid was then added to the solution 100 Hz–5 MHz at temperatures ranging from 30 to 250 8C.
as complexing agent. The required amount of citric acid was The pellets were made in the form of a disk capacitor with
calculated based on the total valence of the oxidizing and the specimen as the dielectric medium for the dielectric
reducing agents for the maximum release of energy during measurements. Both the flat surfaces of the sintered pellets
combustion [19]. The oxidant-to-fuel ratio of the system were polished and then silver paste is applied as electrodes.
was adjusted to ꢀ1 by adding concentrated nitric acid and
ammonium hydroxide solution. This solution was stirred
well using a magnetic stirrer for uniform mixing and a clear
3 Result and discussions
3.1 Structural characterization The XRD pattern
solution with no precipitate or sedimentation was obtained. of as-prepared CaMoO4 nanopowder is shown in Fig. 1.
All the peaks including the minor ones are indexed
The solution containing the precursor mixture of pH of ꢀ7.0
was heated on a hot plate at ꢀ250 8C kept in a ventilated for a tetragonal structure with space group 141/a. The lattice
˚
fume hood. The solution boils on heating and undergoes constants calculated from the XRD data are a ¼ 5.540 A
˚
dehydration accompanied by foaming. On persistent heating and c ¼ 12.795 A and it agreed very well with the reported
the foam gets autoignited, giving a voluminous fluffy powder XRD data (JCPDS card No. 85-1267). This indicates that
of nano CaMoO4.
the formation of CaMoO4 phase was complete during the
The structure of the as-prepared powder was examined combustion process itself without any calcination step.
by a powder X-ray diffraction (XRD) technique using a The crystallite size of the combustion product calculated
Bruker D-8 X-ray diffractometer with nickel-filtered Cu Ka from the full width at half-maximum (FWHM), using the
radiation. The Fourier transform Raman spectrum of the Scherrer formula for the major (112) plane is ꢀ25 nm.
nanocrystalline CaMoO4 was obtained at room temperature
The broadening of XRD peaks is mainly due to three
in the wave number range 50–1200 cmꢂ1 using a Bruker phenomena: particle-size effect, instrumental broadening,
RFS/100S spectrometer at a power level of 150 mW and at a and strain broadening. Instrumental broadening has little
resolution of 4 cmꢂ1. The samples were excited with an effect on particle size but the effect of strain broadening is
Nd:YAG laser lasing at 1064 nm and the scattered radiations noticeable. This can be resolved with Williamsons–Hall
were detected using Ge detector. The infrared (IR) spectra method [20] using Eq. (1)
of the samples were recorded in the range 400–4000 cmꢂ1
bcosu ¼ Kl=t þ 4esinu;
(1)
on a Thermo-Nicolet Avatar 370 Fourier Transform
Infrared (FTIR) Spectrometer using the KBr pellet method.
Particulate properties of the combustion product were
examined using transmission electron microscope using a
Hitachi H-600 electron microscope operating at 200 kV. The
samples for transmission electron microscopy (TEM) were
prepared by ultrasonically dispersing the powder in metha-
nol and allowing a drop to dry on a carbon-coated copper
grid. The photoluminescent spectra of the samples were
measured using Perkin-Elmer fluorescence spectrometer
(Model:LS55) with a 40-W xenon lamp as the excitation
source. The photons from the source were filtered by an
excitation spectrometer. The monochromatic radiation was
then allowed to fall on the disk samples and the resulting
radiation was filtered by an emission spectrometer and then Figure 1 XRD pattern of as-prepared nano CaMoO4 powder.
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Phys. Status Solidi A (2012)
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Figure 2 Williamsons–Hall plot of nano CaMoO4.
Figure 3 Raman spectra of nano CaMoO4 obtained by modified
combustion technique.
where b is the FWHM, u the corresponding diffraction
˚
angle, l the wavelength of CuKa radiation (1.5406 A), e the
strain and t is the particle size. This equation represents a
straight line with slope equal to the microstrain and the
reciprocal of the Y-intercept gives the particle size.
The Williamsons–Hall plot is given in Fig. 2. The
microstrain obtained from the plot is ꢂ0.1545. The negative
value of the slope means that the effect of strain broadening is
least [21, 22] and the broadening is mainly due to the
particle-size effect. The corrected particle size calculated
from the inverse of the Y-intercept is 25 nm.
The structure is further examined by means of
vibrational analysis. The scheelite-type crystal belongs to
the I41/a space group with two formula units per primitive
cell. The compound CaMoO4 has [MoO4]2ꢂ molecular ionic
units with strong Mo–O covalent bonds, which have weak
coupling with Ca2þ cations. The Td symmetry of the five
tetrahedral MoO4 ion is reduced to S4 in the crystal lattice
and the presence of two MoO4 groups in each primitive cell
further reduces it to C4h. Group theoretical calculation shows
that the vibrational degrees of freedom of CaMoO4,
excluding the two acoustical modes (1Au þ 1Eu) can be
written as
Figure 4 FTIR spectra of nano CaMoO4 obtained by a modified
combustion technique.
Bg modes are active at 404 and 206 cmꢂ1. The band at
324 cmꢂ1 is the superposition of n2Ag and n2Eg modes of
vibrations. The three Eg internal modes are observed at 794,
144 and 113 cmꢂ1. The external modes involve the move-
ment of MoO4 ions as a whole, which are only weakly
G ¼ 3Ag þ 4Au þ 5Bg þ 3Bu þ 5Eg þ 4Eu:
(2)
All the even vibrations 3Ag, 5Bg, and 5Eg are Raman coupled with Ca2þ cations. Hence, the external modes 2Bg
active and the odd modes 4Au and 4Eu are IR active. The 3Bu and 1Eg are assigned to the lowest-frequency band, 85 cmꢂ1
modes are neither Raman nor IR active.
The IR spectrum shows a very strong absorption band at
.
Both Raman and IR spectra of the as-prepared sample 832 cmꢂ1 and a very weak one at 407 cmꢂ1. The former is
obtained in the present study matches exactly with the earlier due to the Mo–O stretching vibration mode and the latter is
reported ones [17, 21]. The 13 remaining Raman active due to the weak Mo–O bending vibration mode. These
modes 3Ag, 5Bg, and 5Eg can be divided into two groups. The modes correspond to the IR active n3Eu and n4Eu vibrations.
internal modes are observed in the region 887–180 cmꢂ1 and The medium intense band at 434 cmꢂ1 is due to the Au mode
the external modes in the region 155–95 cmꢂ1. The internal of the antisymmetric bending vibrations involved in the
modes include the vibrational and rotational modes of the O–Mo–O bond vibrations. Also, the respective bands at 3425
tetrahedron, with the latter occurring toward the 4 lower and 1600 cmꢂ1 can be ascribed to O–H stretching vibration
frequency regions of internal modes. and H–O–H bending vibration of the physically adsorbed
The Raman and IR spectra of CaMoO4 obtained are water molecules on the sample surface.
shown in Figs. 3 and 4, respectively.
It may be noted that a single-phase CaMoO4 could be
The strong intense bands at 879, 848, and 794 cmꢂ1 are obtained through solid-state reaction route only after
attributed to the Ag, Bg and Eg modes, respectively. The other prolonged calcination of the reaction mixture with multiple
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S. Vidya et al.: Synthesis, sintering and optical properties of CaMoO4
Figure 5 TEM image of the as-prepared CaMoO4 nanopowder.
Figure 6 SEM image of sintered CaMoO4.
intermediate grindings [11]. In the combustion processes several trials. The sintered sample showed a maximum
reported for the preparation of nanocrystals of some other density of ꢀ95% of the theoretical value. It may be noted that
ceramic oxides, EDTA, polyvinyl alcohol, urea, etc. were for CaMoO4 powder prepared through solid-state reaction
used as chelating agents and fuels [19]. However, in these method had to be heated at a temperature of 1100–1300 8C to
cases prolonged calcination of the as-prepared powder at obtain a well-sintered pellet [11, 23]. The reduction of
high temperature was required to obtain a phase-pure sintering temperatures for the nanomaterials is mainly due to
compound. In the present autoigniting combustion method, its high surface to volume ratio and the sintering process is
by using citric acid as complexing agent and ammonia as initiated by the motion of the increased number of atoms on
fuel, we were able to prepare phase-pure nanopowders of the surface. The sintering temperature for CaMoO4 at 775 8C
CaMoO4 in a single-step combustion without any calcination reported in the present work is the lowest value so far,
step. Thus, the current method offers an easy, cost-effective without the use of any sintering aid.
and time-saving method for the preparation of high-quality
nanopowders of CaMoO4.
The surface morphology of the well-sintered sample is
examined by the scanning electron microscopic (SEM)
technique and the micrograph is shown in Fig. 6. It is
3.2 Morphology The particle morphology, size observed from the micrograph that the size of the grains have
distribution and crystallinity were investigated using TEM, increased to ꢀ2 mm due to the sintering process, with good
as shown in Fig. 5.
Particles are nearly spherical in shape with narrow size
distribution. It is found that most of the nanoparticles have
intergrain connectivity and negligible porosity.
3.3 Optical properties The absorption spectrum of
homogeneous morphology with uniform diameter of 20– the sample measured in the range 200–800 nm is given in
30 nm. During the measurement, noindividual particlelarger Fig. 7.
than 30 nm in size was found. The absence of agglomeration
From the absorption spectra it is clear that the sample
of the particles indicates the good nanocrystalline nature absorbs heavily in the UV region. The absorption spectrum
of the sample. Selected-area electron diffraction (SAED) of CaMoO4 exhibits typical optical behavior of a wide-
patterns revealed that the CaMoO4 nanoparticles were bandgap semiconducting oxide, having an intense absorp-
crystallized with bright polycrystalline diffraction rings tion band around 255 nm with a steep edge. It reveals that
(inset of Fig. 5). SAED patterns are composed of a number of
bright spots arranged in concentric rings. The electrons
reflected and diffracted from crystallographic planes of the
unit cells of the CaMoO4 form bright spots in the SAED
pattern. The rings are diffused as well as hollow, which
indicates the presence of nanocrystals of different orien-
tations. The SAED pattern also indicate the polycrystalline
nature of the crystallites, but the spotty nature of the pattern is
due to the fact that the finer crystallites having related
orientations are agglomerated together, resulting in a limited
set of orientations.
The sintering behavior of the nanocrystals of CaMoO4
powder synthesized through the present combustion
route was studied systematically. The green density of the
specimen used for the sintering study was ꢀ55%. The
sintering temperature was optimized at 775 8C for 3 h after Figure 7 UV–Vis spectrum of CaMoO4 nanopowders.
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CaMoO4 nanomaterials can readily respond to ultraviolet
radiation. Such materials find applications in filters and
sensors for UV radiation. The optical absorption in the
shorter-wavelength region, less than 400 nm, is mainly due
to the electron transition from the top of the valence band to
the bottom of the conduction band [24].
A semiconducting material is characterized by its
electronic band structure in which the bandgap energy (Eg)
is determined from the energy difference between the
highest occupied molecular orbital (HOMO) and the lowest
unoccupied molecular orbital (LUMO). In both crystalline
and amorphous materials, the absorption coefficient near the
fundamental absorption edge is dependent on photon energy.
In the high-absorption region, the dependence of the
absorption coefficient on photon energy is expressed by
Tauc’s equation, as in Eq. (3) [25]
Figure 9 UV–Vis spectrum of CaMoO4 nanopowders heated at
700 8C for 1 h.
À
Á
m
ðahnÞ ¼ b hn ꢂ Eg
;
(3)
where b is an energy-independent constant, a the optical
absorption coefficient, h the Planck constant, n the
frequency of incident photon, Eg the optical bandgap, and
m is a constant that characterizes the nature of the band
transition. The values for m ¼ 1/2 and 3/2 corresponds to
direct allowed and direct forbidden transitions, respectively,
whereas m ¼ 2 and 3 corresponds to indirect allowed and
indirect forbidden transitions, respectively. The bandgap
can also be obtained by the extrapolation of the straight-line
portion of the (ahn)1/m versus hn plot to hn ¼ 0.
CaMoO4 is a direct-bandgap material and its optical
bandgap determined using Tauc’s plot is 3.725 eV, shown in
Fig. 8. There is a slight reduction in the experimental value
when compared with the theoretical one [22, 26]. The
exponential optical absorption edge and the optical bandgap
energy are influenced by the degree of structural disorder in
the lattice, the electro negativity of transition-metal ions, the
connectivity of the polyhedrons, the deviation in the O–X–O
bonds, oxygen vacancies, the distortion of the [XO4]2ꢂ
tetrahedra and intrinsic surface states [27–30]. As there is
no evidence of structural disorders from our XRD and
Figure 10 Tauc’splotoftheopticalabsorptionspectrumof700 8C
annealed CaMoO4.
vibrational spectra, the significant reduction in the bandgap
energy of CaMoO4 is attributed to surface and interface
intrinsic defects and quantum confinement effects that
cause the formation of intermediate energy levels between
the valence and conduction band. The presence of these
intermediate levels resulted in the reduction of the optical
bandgap energy.
The UV–Visible spectrum of the sample annealed at
700 8C for 1 h was also recorded and is shown in Fig. 9. The
corresponding Tauc plot to determine the optical bandgap is
given in Fig. 10.
A blue shift (225 nm) in the UV absorption peak of the
annealed sample relative to the as-prepared sample is
observed. Accordingly, the bandgap energy calculated
from Tauc’s plot has increased to 3.990 eV. This value is
comparable with the earlier reported one by Longo et al. [31]
for a well-ordered CaMoO4 crystal. This indicates that on
annealing the sample, its structure becomes more ordered
and the defect levels vanish. The increase in bandgap for
the annealed sample points out that the elimination of
intermediate levels occurs due to heating.
Figure 11 shows PL spectra of CaMoO4 nanopowder
excited at 370 nm, at room temperature. The recorded
Figure 8 Tauc’s plot of the optical absorption spectrum of as-
prepared CaMoO4 nanopowder.
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S. Vidya et al.: Synthesis, sintering and optical properties of CaMoO4
trapped electrons present in the well-ordered powders,
resulting in green luminescence. Thus, we can say that the
nanopowder prepared through the combustion technique
possesses high uniformity and is homogeneously oriented,
resulting in an order arrangement. The phase purity and
structure was already confirmed by XRD and vibrational
analysis. Thus, the PL result exactly corroborates the XRD
and vibrational analysis.
3.4 Dielectric properties The values of dielectric
constant (er) and loss factor (tand) of sintered pellets of
CaMoO4 nanopowder, synthesized through the combustion
process were studied in the frequency range 100 Hz to 5 MHz
at different temperatures with silver electrodes on both sides
of the circular disk, as given in Fig. 12.
Figure 11 Photoluminescent spectra of as-prepared nano CaMoO4.
It can be clearly noted that the loss factor decreases as
frequency increases while the dielectric constant remains
almost unaltered in the frequency region above 1 MHz. The
dielectric constant er and loss factor tand values of the
CaMoO4 pellets at 5 MHz in room temperature were 11.00
and 6.40 ꢁ 10ꢂ3, respectively.
spectrum shows a broad band in the visible region ranging
from 400 to 650 nm. Many explanations regarding the origin
and the mechanisms responsible for the PL emissions of
molybdates are there [1, 31, 32]. For instance, Marques et al.
[33] and Spassky et al. [34] attributed the green PL emission
to structural disorder in the CaO8–MoO4 clusters. The great
dependence of the PL properties on the morphology and
crystallinity was investigated by Ryu et al. [35]. Liu et al.
[36] made the conclusion that the charge-transfer transitions
into the [MoO4]2– complex can be considered the main
reason responsible for the green PL emissions and by
theoretical calculations, Campos et al. [37] stated that the
PL emission processes of CaMoO4 powders can be related to
the existence of MoO3 and distorted MoO4 complex clusters
into the lattice. Thus it can be stated that the green
PL emission at room temperature in CaMoO4 may be linked
with several factors, such as: distortions on the [MoO4]
tetrahedron groups caused by the different angles between
O–Mo–O particle sizes, crystalline degree, morphology and
surface defects. In this case, these factors promote the
formation of visible-light emission centers responsible for
the PL property of this material.
The variation of er and tand at different temperatures
ranging from 30 to 250 8C is shown in Fig. 13. It is clear from
The PL pattern for the as-prepared powder shows a
greenish luminescence, which indicates that the sample is
structurally disordered at short range as observed by XRD
and Raman spectroscopy. This disordered nanopowder has
resulted in a wide-band emission in the visible region. Thus,
each wavelength emission is attributed to different types of
electronic transition and structural arrangement. The blue
and green emissions are thus attributed to shallow defects in
the bandgap and to a more ordered structure [37], whereas
the yellow and red emissions are linked to defects deeply
inserted in the bandgap and to a greater disorder in the lattice
[30]. As our sample is concerned, the measured PL emission
spectrum may be due to the charge-transfertransitions within
the [MoO4]2ꢂ complex, in the ordered system. This features
can be further explained as the generation of localized states
in the bandgap and nonhomogeneous charge distribution in
the cell by the oxygen complex clusters that result in the
trapping of electrons [31]. When excited with 370 nm
Figure 12 Variation of er and loss factor with frequency.
Figure 13 Variation of er and tand at different temperature
radiation more energetic wavelength could excite more self- ranging from 30 to 250 8C at frequency 5 MHz.
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Phys. Status Solidi A (2012)
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the graph that the temperature dependence of the dielectric material with strong green emission which is attributed to the
constant is very minimal in the measured temperature range. perfect ordering of MoO4 tetrahedron. These nanocrystals
The loss factor decreases with increasing temperature and is could be sintered at a relatively low temperature of
of the order of 10ꢂ4 at temperatures above 100 8C. The er 775 8C to a high density. The SEM image of the sintered
and tand values of the CaMoO4 at 5 MHz at 250 8C were material indicates high densification of the material. The
10.77 and 2.20 ꢁ 10ꢂ4, respectively.
room-temperature dielectric constant (er) and loss factor of
The value of dielectric constant for the sintered CaMoO4 the sintered pellet at 5 MHz was 11.00 and 6.40 ꢁ 10ꢂ3
,
pellet in the present study is comparable with the values respectively. The temperature coefficient of dielectric
reported by Choi et al. [11] but the value of loss factor in constant is ꢂ95.04 ppm/8C. The decrease in sintering
our study is found to be much lower. Hence, the CaMoO4 temperature, low dielectric constant and low loss makes
pellets prepared from nanopowder have a low dielectric nano CaMoO4 an excellent low-temperature cofired
loss when compared with pellets from micrometer-sized ceramic (LTCC) in addition to it being a good a luminescent
powder.
The temperature coefficient of dielectric constant (TCK
is determined using Eq. (4) [38, 39] given below, between
material.
)
Acknowledgements The authors acknowledge the Centre
for Earth Science Studies (CESS), Trivandrum and Sophisticated
Test and Instrumentation Centre (STIC), Cochin University of
Science and Technology, Cochin for granting the experimental
facilities.
temperatures 30 and 250 8C at 5 MHz.
Â
Ã
TCK ¼ fððK250 ꢂ K30Þ=220Þ=K30g ꢁ 106 ppm=ꢃC; (4)
where K30 and K250 are the dielectric constants at 30
and 250 8C, respectively, and 220 8C is the temperature
difference. The obtained TCK is ꢂ95.04 ppm/8C which is a
negative value. This indicates that the CaMoO4 derived
from nanoparticles possesses a low value of temperature
coefficient of dielectric constant, making it ideal for
temperature-sensitive applications.
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Nanostructured CaMoO4 prepared by the present
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the sintering temperature is as low as 775 8C. Usually, glass
and polymer composite materials are used for LTCC
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4 Conclusions Nanocrystalline semiconducting CaMoO4
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The XRD studies have shown that the nanopowder was
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powder itself is phase pure. TEM analysis confirms the
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