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Thomas J.K.,Electronic Materials Research Laboratory | Kumar H.P.,Wmo Arts And Science College | Prasad V.S.,Indian National Institute for Interdisciplinary Science and Technology | Solomon S.,St Johns College
Ceramics International | Year: 2011

High quality nanoparticles of barium hafnate have been synthesized using an auto-ignition modified combustion technique. The nanoparticles thus obtained have been characterized by powder X-ray diffraction, thermo gravimetric analysis, differential thermal analysis, Fourier transform infrared spectroscopy and transmission electron microscopy. The XRD studies have shown that the as-prepared BaHfO3 powders are phase pure. The particle size of the as-prepared powder was in the range 20-50 nm. The nanopowder could be sintered to 95% of the theoretical density at 1650 °C for 2 h. The ultrafine cuboidal nature of nanopowders with small degree of agglomeration improved the sinterability at relatively lower temperature and time. The microstructure of the sintered surface was examined using scanning electron microscopy. The dielectric constant (εr) of 30.8 and loss factor (tan δ) of 2.3 × 10-3 were obtained at 1 MHz. © 2010 Elsevier Ltd and Techna Group S.r.l.


Pazhani R.,Electronic Materials Research Laboratory | Padma Kumar H.,Wmo Arts And Science College | Moses Ezhil Raj A.,Scott Christian College | Solomon S.,St Johns College | Thomas J.K.,Electronic Materials Research Laboratory
Journal of Alloys and Compounds | Year: 2011

Phase pure zirconium oxide powders have been synthesized using the single step auto-ignition combustion method, the particles were nanometer sized (20 nm) and the size distribution was very narrow (3.4 nm). Systematic structural characterization revealed the t-ZrO 2 and indexed for its tetragonal structure (a = 3.5975 and c = 5.1649 ). Calculated microstrain in most of the plane indicated the presence of compressive stress (65-288 MPa) along various planes of the particles. Observed space group (P4 2/nmc) revealed the presence of cations in the 8e positions (0.75, 0.25, 0.75) and the anions in the 16 h positions (0.25, 0.25, 0.4534). The metal-oxide (Zr-O) band observed at the low wavenumber region further confirmed the phase purity of the as-prepared ZrO 2 nanopowders. Peaks at the binding energy positions 2.042 and 0.525 keV in the energy dispersive X-ray spectrum revealed oxygen deficient zirconia. The particle size estimated by TEM was in good agreement with the results obtained through X-ray line broadening (20.81 nm) measurements. The nanopowders were sintered to above 98% of the theoretical density by using vacuum sintering technique at a relatively low temperature of 1300 °C. Stable tetragonal ZrO 2 experimentally yield the permittivity value of about 28 at 10 MHz. © 2011 Elsevier B.V. All rights reserved.


Vidya S.,Electronic Materials Research Laboratory | Solomon S.,St Johns College | Thomas J.K.,Electronic Materials Research Laboratory
Journal of Materials Science: Materials in Electronics | Year: 2014

Nanocrystalline strontium tungstate (SrWO4) is synthesized through a single step modified combustion process. The X-ray diffraction, Fourier transform Raman and Infrared spectroscopy studies reveal that the as-prepared powder is single phase and possess tetragonal structure. The transmission electron microscopic investigations have shown that the particle size of the as prepared powder is in the range 18-22 nm. The optical constants are estimated from the UV-Visible studies and calculated optical band gap is 4.28 eV. The sample showed maximum transmission in the visible regions but poor transmittance in the ultraviolet region. The photoluminescence spectra recorded at different temperatures showed intense blue emission. The nanocrystalline SrWO4 obtained by the present combustion method was sintered to 95 % density at a relatively lower temperature of 810 C for 3 h. The dielectric constant (εr) and loss factor (tan δ) of the sintered SrWO4 pellets at 5 MHz measured at room temperature were 9.9 and 6.29 × 10-3 respectively. The experimental results obtained in this work demonstrate the application of SrWO4 as UV filters, transparent films for window layers on solar cells, anti-reflection coatings, scintillators, detectors and for low-temperature co-fired ceramic applications. © 2013 Springer Science+Business Media New York.


Vidya S.,Electronic Materials Research Laboratory | Solomon S.,St Johns College | Thomas J.K.,Electronic Materials Research Laboratory
Journal of Electronic Materials | Year: 2013

Nanocrystalline scheelite CaWO4, a promising material for low-temperature co-fired ceramic (LTCC) applications, has been successfully synthesized through a single-step autoignition combustion route. Structural analysis of the sample was performed by powder x-ray diffraction (XRD), Fourier-transform infrared spectroscopy, and Raman spectroscopy. The XRD analysis revealed that the as-prepared sample was single phase with scheelite tetragonal structure. The basic optical properties and optical constants of the CaWO4 nanopowder were studied using ultraviolet (UV)-visible absorption spectroscopy, which showed that the material was a wide-bandgap semiconductor with bandgap of 4.7 eV at room temperature. The sample showed poor transmittance in the ultraviolet region but maximum transmission in the visible/near-infrared regions. The photoluminescence spectra recorded at different temperatures showed intense emission in the green region. The particle size estimated from transmission electron microscopy was 23 nm. The feasibility of CaWO4 for LTCC applications was studied from its sintering behavior. The sample was sintered at a relatively low temperature of 810 C to high density, without using any sintering aid. The surface morphology of the sintered sample was analyzed by scanning electron microscopy. The dielectric constant and loss factor of the sample measured at 5 MHz were found to be 10.50 and 1.56 × 10-3 at room temperature. The temperature coefficient of the dielectric constant was -88.71 ppm/ C. The experimental results obtained in this work demonstrate the potential of nano-CaWO4 as a low-temperature co-fired ceramic as well as an excellent luminescent material. © 2012 TMS.


Vidya S.,Electronic Materials Research Laboratory | Solomon S.,St Johns College | Thomas J.K.,Electronic Materials Research Laboratory
Physica Status Solidi (A) Applications and Materials Science | Year: 2012

The synthesis of nanocrystalline calcium molybdate (CaMoO 4) 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 CaMoO 4 is a promising scheelite low-temperature co-fired ceramic (LTCC) and also an excellent luminescent material. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

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