arXiv subjects

K. B. R. Varma

Publications and source records attributed to K. B. R. Varma.

At least 19 recordsLinked to original sources

arXiv2022

Manipulation of ferroelectric and magnetic properties properties of single phase Bi4Ti3O12-3BiFeO3 solid solution through La substitution

In this work, the potential to exploit Aurivillius higher order composition (n=6) given by Bi4Ti3O12-3BiFeO3 as single phase and high temperature magnetoelectric materials are well explored by elaborately studying the concerned structure and physical properties. Bi4Ti3O12-3Bi1-xLaxFeO3 (x=0, 0.01, 0.05, 0.1, 0.2) was synthesized via solid-state reaction route and X-ray diffraction data revealed that the lattice parameters associated with orthorhombic cell (space group P21am) decrease with increase in La3+ content confirming the incorporation of La. In the frequency dependent dielectric measurements, the ferroelectric to paraelectric transition temperature (Tc) corresponding to the parent (x=0) had two phase transitions vis., 803 K (broader) and 983 K (sharper) whereas phase transitions are single that progressively becomes sharper with increase in La substitution. Ferroelectric switching behavior (polarization (P) versus electric field (E)) was demonstrated for Bi4Ti3O12-3BiFeO3 and remnant polarization decreased significantly for La substituted compositions reflecting weaker ferroelectric ordering. In detailed Zero Field-cooled and Field cooled temperature dependent magnetization measurements, Bi4Ti3O12-3BiFeO3 exhibited Tc around 296 K. Though a marginal increase in the magnetization values for La substituted compositions (x=0.1 and x=0.2), the overall temperature dependent magnetization curves indicate overall paramagnetic nature. The results are more reflective of the switching off of the possibilities of long range ferromagnetic ordering with A-site rare earth doping in these Aurivillius layered compounds.

cond-mat.mtrl-sci↗

arXiv2015

Growth Stress Induced Tunability of Dielectric Constant in Thin Films

It is demonstrated here that growth stress has a substantial effect on the dielectric constant of zirconia thin films. The correct combination of parameters - phase, texture and stress - is shown to yield films with high dielectric constant and best reported equivalent oxide thickness of 0.8 nm. The stress effect on dielectric constant is twofold, firstly, by the effect on phase transitions and secondly by the effect on interatomic distances. We discuss and explain the physical mechanisms involved in the interplay between the stress, phase changes and the dielectric constant in detail.

cond-mat.mtrl-sci↗

arXiv2013

Effect of TeO2 addition on the dielectric properties of CaCu3Ti4O12 ceramics derived from the oxalate precursor route

CaCu3Ti4O12 (CCTO) ceramics which has perovskite structure gained considerable attention due to its giant permittivity. But, it has high tan δ (0.1 at 1kHz) at room temperature, which needs to be minimised to the level of practical applications. Hence, TeO2 which is a good glass former has been deliberately added to CCTO nano ceramic (derived from the oxalate precursor route) to explore the possibility of reducing the dielectric loss while maintaining the high permittivity. The structural, morphological and dielectric properties of the pure CCTO and TeO2 added ceramics were studied using X-ray diffraction, Scanning Electron Microscope along with Energy Dispersive X-ray Analysis (EDX), spectroscopy and Impedance analyzer. For the 2.0 wt % TeO2 added ceramics, there is a remarkable difference in the microstructural features as compared to that of pure CCTO ceramics. This sample exhibited permittivity values as high as 7387 @ 10 KHz and low dielectric loss value of 0.037 @ 10 kHz, which can be exploited for the high frequency capacitors application.

cond-mat.mtrl-sci↗

arXiv2013

Structural and dielectric properties of glasses in the system TeO2-CaCu3Ti4O12

The glasses in the system (100-x)TeO2-xCaCu3Ti4O12, (x=0.25 to 3 mol %) were fabricated. The color varied from olive green to brown as the CaCu3Ti4O12 (CCTO) content increased in TeO2 matrix. The X-ray powder diffraction and differential scanning calorimetric analyses that were carried out on the as-quenched samples confirmed their amorphous and glassy nature respectively. The dielectric constant and loss in the 100 Hz-1MHz frequency range were monitored as a function of temperature (50-400oC). The dielectric constant and the loss (D) increased as the CCTO content increased in TeO2 at all the frequencies and temperatures under investigation. Further, the effective dielectric constant and D were found to be frequency independent in the 50-200oC temperature range. The value obtained for the loss at 1MHz was 0.0019 which was typical of low loss materials, and exhibited near constant loss (NCL) in the 100Hz-1MHz frequency range. The electrical relaxation was rationalized using the electric modulus formalism. These glasses may be of considerable interest as substrates for high frequency circuit elements in conventional semiconductor industries owing to their high thermal stability.

cond-mat.mtrl-sci↗

arXiv2013

Dielectric properties of Poly(methyl methacrylate) (PMMA)/CaCu3Ti4O12 Composites

Materials with high dielectric constant are in great demand for the miniaturization of electronic devices. More specifically, high dielectric constant polymer-ceramic composites are useful for embedded capacitor applications. A composite consisting of giant dielectric CaCu3Ti4O12 (CCTO) incorporated into the Poly(methyl methacrylate) (PMMA) polymer matrix has been fabricated by melt mixing followed by hot pressing. The composites thus fabricated has been characterised for structural, morphological and dielectric properties. The composites dielectric constants had increased when the CCTO content increased in the PMMA matrix. The dielectric constant of PMMA is around 4.9 @ 100Hz which has increased to 15.7 @ 100Hz when the ceramic content has increased to 40 Vol %. At low frequencies, space charge polarisation is dominant. This composite also exhibited remarkably low dielectric loss at high frequency, which makes this composite a suitable candidate for the capacitors in high frequency application.

cond-mat.mtrl-sci↗

arXiv2013

Fabrication and Characterization of Poly(methyl methacrylate)CaCu3Ti4O12 Composites

Composites comprising Poly(Methyl Methacrylate) (PMMA) and CaCu3Ti4O12 (CCTO) via melt mixing followed by hot pressing were fabricated. These were characterized using X-ray diffraction (XRD), thermo gravimetric (TGA), scanning electron microscopy (SEM) and Impedance analyser for their structural, morphology and dielectric properties. Composites were found to have better thermal stability than that of pure PMMA. The composite, with 38 Vol % of CCTO (in PMMA), exhibited remarkably low dielectric loss at high frequencies and the low frequency relaxation is attributed to the space charge polarization MWS effect. Theoretical models were employed to rationalise the dielectric behaviour of these composites. At higher temperatures, the relaxation peak shifts to higher frequencies, due to the merging of both {beta} and {alpha} relaxations into a single dielectric dispersion peak. The AC conductivity in the high frequency region was attributed to the electronic polarization

cond-mat.mtrl-sci↗

arXiv2013

Synthesis of Nanoparticles of the Giant Dielectric Material, CaCu3Ti4O12 from a Precursor Route

A complex oxalate precursor, CaCu3(TiO)4(C2O4)8.9H2O, was synthesized and the precipitate that obtained was confirmed to be monophasic by the wet chemical analyses, X-ray diffraction, FTIR absorption and TG, DTA analyses. The thermal decomposition of this oxalate precursor led to the formation of phase-pure calcium copper titanate, CaCu3Ti4O12, at 680oC. The bright field TEM micrographs revealed that the size of the as synthesized crystallites to be in the 30 to 80 nm range. The powders so obtained had excellent sinterability resulting in high density ceramics which exhibited giant dielectric constants upto 40000 (1 kHz) at 25oC accompanied by low dielectric losses, less than 0.07.

cond-mat.mtrl-sci↗

arXiv2013

Structural, Thermal and Electrical properties of Poly(methyl methacrylate)-CaCu3Ti4O12 composite sheets fabricated via melt mixing

Poly(Methyl Methacrylate) (PMMA) and CaCu3Ti4O12 (CCTO) composites were fabricated via melt mixing followed by hot pressing technique. These were characterized using X-ray diffraction (XRD), thermo gravimetric (TGA), Thermo Mechanical (TMA), Differential scanning calorimetry (DSC), Fourier transform infrared (FTIR), and Impedance analyser for their structural, thermal and dielectric properties. Composites were found to have better thermal stability than that of pure PMMA. However, there was no significant difference in the glass transition (Tg) temperature between the polymer and the composite. The appearance of additional vibrational frequencies in the range 400-600 cm-1 in FTIR spectra indicated a possible interaction between PMMA and CCTO. The composite, with 38 Vol % of CCTO (in PMMA), exhibited remarkably low dielectric loss at high frequencies and the low frequency relaxation is attributed to the space charge polarization/MWS effect. The origin of AC conductivity particularly in the high frequency region was attributed to the electronic polarization.

cond-mat.mtrl-sci↗

arXiv2013

Dielectric properties of Poly(vinylidene fluoride) / CaCu3Ti4O12 composites

The possibility of obtaining relatively high dielectric constant polymer-ceramic composite by incorporating the giant dielectric constant material, CaCu3Ti4O12 (CCTO) in a Poly(vinylidene fluoride) (PVDF) polymer matrix by melt mixing and hot pressing process was demonstrated. The structure, morphology and dielectric properties of the composites were characterized using X ray diffraction, Thermal analysis, Scanning Electron Microscope, and Impedance analyzer. The effective dielectric constant of the composite increased with increase in the volume fraction of CCTO at all the frequencies (100 Hz to 1MHz) under study. The dielectric loss did not show any variation upto 40 % loading of CCTO, but showed an increasing trend beyond 40%. The room temperature dielectric constant as high as 95 at 100Hz has been realized for the composite with 55 Vol % of CCTO, which has increased to about 190 at 150oC. Theoretical models like Maxwells Clausius Mossotti, Effective medium theory, logarithmic law and Yamada were employed to rationalize the dielectric behaviour of the composite and discussed. Keywords: A. Ceramic; A. Polymer matrix composite

cond-mat.mtrl-sci↗

arXiv2013

Effect of calcium stoichiometry on the dielectric response of CaCu3Ti4O12 Ceramics

CaxCu3Ti4O12 polycrystalline powders with variation in calcium content were prepared via the oxalate precursor route. The structural, morphological and dielectric properties of the ceramics fabricated using these powders were studied using x ray diffraction, Scanning Electron Microscope along with Energy Dispersive X ray Analysis, Transmission Electron Microscopy, Electron Spin Resonance (ESR) spectroscopy and Impedance analyzer. The X ray diffraction patterns obtained for the x = 0.97, 1.0 and 1.1 powdered ceramics could be indexed to a body centered cubic perovskite related structure associated with the space group Im3. The ESR studies confirmed the absence of oxygen vacancies in the ceramics that were prepared using the oxalate precursor route. The dielectric properties of these suggest that the calcium deficient sample (x= 0.97) has a reduced dielectric loss while retaining the high dielectric constant which is of significant industrial relevance.

cond-mat.mtrl-sci↗

arXiv2013

Nanoparticles of the giant dielectric material, CaCu3Ti4O12 from a precursor route

A method of preparing the nanoparticles of CaCu3Ti4O12 (CCTO) with the crystallite size varying from 30 to 200 nm is optimized at a temperature as low as 680 1C from the exothermic thermal decomposition of an oxalate precursor, CaCu3(TiO)4(C2O4)8 ? 9H2O. The phase singularity of the complex oxalate precursor is confirmed by the wet chemical analyses, X-ray diffraction, FT-IR and TGA,DTA analyses. The UV Vis reflectance and ESR spectra of CCTO powders indicate that the Cu(II) coordination changes from distorted octahedra to nearly flattened tetrahedra (squashed) to square-planar geometry with increasing annealing temperature. The HRTEM images have revealed that the evolution of the microstructure in nanoscale is related to the change in Cu(II) coordination around the surface regions for the chemically prepared powder specimens. The nearly flattened tetrahedral geometry prevails for CuO4 in the near surface regions of the particles, whereas square-planar CuO4 groups are dominant in the interior regions of the nanoparticles. The powders derived from the oxalate precursor have excellent sinterability, resulting in high-density ceramics which exhibited giant dielectric constants upto 40,000 (1 kHz) at 25 1C, accompanied by low dielectric loss 0.07.

cond-mat.mtrl-sci↗

arXiv2013

In-situ synthesis and characterization of polyaniline -CaCu3Ti4O12 nano crystal composites

High dielectric constant nano composite of Polyaniline (PANI) CaCu3Ti4O12 (CCTO) was synthesized using a simple procedure involving in-situ polymerization of aniline in dil. HCl. The PANI and the composite were subjected to X-ray diffraction, Fourier transform infrared, Thermo gravimetric, Scanning electron microscopy and Transmission electron microscopy analyses. The presence of the nano crystallites of CCTO embedded in the nanofibers of PANI matrix was established by TEM. Frequency dependent characteristics of the dielectric constant, dielectric loss and AC conductivity were studied for the PANI and the composites. The dielectric constant increased as the CCTO content increased in PANI but decreased with increasing frequency of measurement. The dielectric loss was two times less than the value obtained for pure PANI around 100Hz. The AC conductivity increased slightly upto 2kHz as the CCTO content increased in the PANI which was attributed to the polarization of the charge carriers.

cond-mat.mtrl-sci↗

arXiv2013

Dielectric properties of Poly(vinylidene fluoride)/ CaCu3Ti4O12 nanocrystal composite thick films

The Poly(vinylidene fluoride)/CaCu3Ti4O12 (CCTO) nanocrystal composite films (85nm) with relatively high dielectric permittivity (90 at 100Hz) were prepared by the solution casting followed by spin coating technique. The structural, the microstructural and the dielectric properties of the composites were studied using X-ray diffraction, Scanning Electron Microscope, and Impedance analyzer respectively. The effective dielectric permittivity of the composite increased with increase in the volume fraction of CCTO at all the frequencies (100 Hz to 1MHz) under investigation. The room temperature dielectric permittivity which is around 90 at 100Hz, has increased to about 290 at 125oC (100Hz). These results may be exploited in the development of high energy density capacitors.

cond-mat.mtrl-sci↗

arXiv2012

Synthesis dependent characteristics of Sr1-xMnxTiO3 (x=0.03, 0.05, 0.07 and 0.09)

Sr1-xMnxTiO3 (where x = 0.03, 0.05, 0.07 and 0.09) was synthesized via different routes that include solid-state, oxalate precipitation and freeze drying. In oxalate precipitation technique, compositions corresponding to 3 and 5 mol % doping of Mn were monophasic whereas the higher compositions revealed the presence of the secondary phases such as MnO, Mn3O4 etc., as confirmed by high resolution X-ray diffraction (XRD) studies. The decomposition behavior of the precursors prepared using oxalate precipitation method corresponding to the above mentioned compositions was studied. Nanopowders of compositions pertaining to 5 to 9 mol % of Mn doping were obtained using freeze-drying technique. The average crystallite size of these nanopowders was found to be in the 35 to 65 nm range. The microstructural studies carried out on the sintered ceramics, fabricated using powders synthesized by different routes established the fine grained nature (< 1 microm) of the one obtained by freeze drying method. Raman scattering studies were carried out in order to complement the observations made from XRD regarding the phase purity. The dielectric properties of the ceramics obtained by different synthesis routes were studied in the 80 - 300 K temperature range at 100 kHz and the effect of grain size has been discussed.

cond-mat.mtrl-sci↗

arXiv2011

Relaxor ferroelectric behavior and structural aspects of SrNaBi2Nb3O12 ceramics

SrNaBi2Nb3O12 powder was prepared via the conventional solid-state reaction method. X-ray structural studies confirmed the phase to be a three-layered member of the Aurivillius family of oxides. SrNaBi2Nb3O12 (SNBN) ceramics exhibited the typical characteristics of relaxor ferroelectrics, associated with broad and dispersive dielectric maxima. The variation of temperature of dielectric maxima (Tm) with frequency obeyed the Vogel-Fulcher relationship. Relaxor behavior was believed to be arising from the cationic disorder at A-site. Pinched ferroelectric hysteresis loops were observed well above Tm.

cond-mat.mtrl-sci↗

arXiv2010

Electrical Relaxation and Transport in 0.5Cs2O- 0.5Li2O-3B2O3 Glasses

The frequency and temperature dependence of the dielectric constant and the electrical conductivity of the transparent glasses in the composition 0.5Cs2O-0.5Li2O-3B2O3 (CLBO) were investigated in the 100 Hz - 10 MHz frequency range. The dielectric constant for the as-quenched glass increased with increasing temperature, exhibiting anomalies in the vicinity of the glass transition and crystallization temperatures. The temperature coefficient of dielectric constant was estimated (35 \pm 2 ppm.K-1) using Havinga's formula. The dielectric loss at 313 K is 0.005 \pm 0.0005 at all the frequencies understudy. The activation energy associated with the electrical relaxation determined from the electric modulus spectra was found to be 1.73 \pm 0.05 eV, close to that of the activation energy obtained for DC conductivity (1.6 \pm 0.06 eV). The frequency dependent electrical conductivity was analyzed using Jonscher's power law. The combination of these dielectric characteristics suggests that these are good candidates for electrical energy storage device applications.

cond-mat.mtrl-sci↗

arXiv2010

Thermal properties of Ba3Ti3B2O12 glasses

Transparent glasses in the system 3BaO-3TiO2-B2O3 (BTBO) were fabricated via the conventional melt-quenching technique. The as-quenched samples were confirmed to be glassy by differential thermal analysis (DTA) technique. The thermal parameters were evaluated using non-isothermal DTA experiments. Kauzmann temperature was found to be 759 K based on heating rate dependent glass transition and crystallization temperatures. Theoretical relation for temperature dependent viscosity was proposed for these glasses and glass-ceramics.

cond-mat.mtrl-sci↗

arXiv2010

Laser Induced Periodic Surface Crystalline Patterns on SrO-0.5Li2O-4.5B2O3 and BaO-0.5Na2O-4.5B2O3 Glasses and Optical Second Harmonic Generation

Transparent SrO-0.5Li2O-4.5B2O3 (SLBO) and BaO-0.5Na2O-4.5B2O3 (BNBO) glasses were fabricated via the conventional melt-quenching technique. The amorphous and glassy characteristics of the as-quenched samples were respectively confirmed by X-ray powder diffraction (XRD) and Differential Thermal Analysis (DTA) studies. Optically clear SLBO and BNBO glass nano/microcrystal composites were obtained by heat-treating the as-quenched glasses slightly above their respective glass transition temperatures. These glass nano/microcrystal composites exhibited second harmonic signals in transmission mode when exposed to infrared laser beam at λ=1064 nm. Interestingly second-harmonic waves were found to undergo optical diffraction and was attributed to the presence of self-organized microcrystalline patterns associated with 0.5-2μm periodicities.

cond-mat.mtrl-sci↗