P. Mahesh

Bio: P. Mahesh is an academic researcher from Indian Institute of Technology Guwahati. The author has contributed to research in topics: Dielectric & Variable-range hopping. The author has an hindex of 2, co-authored 2 publications receiving 27 citations.

Dielectric response and ac-conductivity studies of Gd 2 O 3 - contained K 0.5 Na 0.5 NbO 3 piezoelectric ceramics

Abstract: We report the role of Gd2O3 substitution on the dielectric properties of alkali niobate K0.5Na0.5NbO3 (KNN) ceramics. This system exhibits relaxor behaviour for 0.5 wt% of Gd2O3 in KNN matrix and shows enhanced dielectric permittivity (er = 4147 at T = 628 K) and low loss (Tanδ ~ 0.008) at frequency f = 1 MHz. The position of crystallographic phase transitions of Orthorhombic to Tetragonal (T1) and Tetragonal to Cubic (T2) shifts significantly from 480 K to 413 K and 668 K to 582 K, respectively. The temperature and the frequency dependence of the position of maximum dielectric permittivity Tm obeys the Vogel–Fulcher law, with the attempt frequency f0 = 5.32×1011 Hz, freezing temperature Tf = 598 K and activation energy EA= 0.038 eV for KNN + 0.5 wt% Gd2O3. The temperature dependence of AC-conductivity σac (T) analysis in the range 300 K < T < 600 K reveals the existence of variable range hopping of charge carriers with average hopping length RH in the range 7.93-29.24 nm and 5.55-26.41 nm for ξ = 0 and ξ = 0.5, respectively. The diffuseness exponent ‘γ’ (~1.74) estimated from the high temperature slopes of the diffused dielectric permittivity data reveals the weakly coupled relaxor behavior.

The dielectric behavior of Zn1−xNixO/NiO two-phase composites

Abstract: The effect of nickel content on the dielectric permittivity 'er' and the ac-electrical conductivity of Zn1−xNixO/NiO (0 ≤ x ≤ 0.55) two-phase composites were investigated. The antiferro to the paramagnetic Neel temperature TN (~ 523 K) of the NiO associated with the structural phase transition from the rhombohedral to the cubic phase has been exploited to realize a dielectric anomaly across 523–541 K in the Zn1−xNixO/NiO composite system. Also, a giant dielectric peak across 410 °C in pure NiO was observed together with an anomaly across TN. The formation of tiny polar clusters due to the compositional heterogeneity for the samples with x ≥ 0.16 drove the system to exhibit a weakly coupled relaxor-like behavior with a locally varying maximum temperature of T* (~ 530 K at 106 Hz), obeying the Vogel–Fulcher law and the Uchino–Nomura criteria. The values of the diffuseness-exponent 'γ' (1.91) and the shape-parameter 'δ' (88 °C) were determined by using the empirical scaling relation (eA/er = 1 + 0.5 (T − TA)2/ δ2), which is often used to describe relaxor-like behavior. Our results provide strong evidence for the variable-range-hopping of charge carriers between the localized states. The effects of non-ohmic sample-electrode contact impedance and negative-capacitance on the global dielectric behavior of a Zn1−xNixO/NiO composite system are discussed.

Insulation condition diagnosis of oil-immersed paper insulation based on non-linear frequency-domain dielectric response

Abstract: In this work, samples of oil-immersed paper with different moisture levels and aging condition were studied. Frequency domain spectroscopy (FDS) of the samples was examined at 40 °C under different excitation voltages. A non-linear phenomenon in frequency domain dielectric response with different moisture contents and aging condition was observed. The non-linear phenomenon appears to be related to the hindrance effect of cellulose on the movement of ions and the Poole-Frenkel effect on the concentration of ions. With aging, the nonlinear loss peaks at different frequencies caused by the effect of nonlinear conductance and interface polarization appear in the FDS results. The non-linear phenomenon was modeled and verified experimentally. The characteristic parameters and quantitative method to estimate moisture content and the degree of aging degree of oil-immersed paper can be obtained by the proposed method.

Excellent energy-storage performances in La2O3 doped (Na,K)NbO3-based lead-free relaxor ferroelectrics

Abstract: Relaxor ferroelectric (FE) materials have received increasing attention owing to their great potentials for energy-storage applications, especially for the ones with high energy-storage density, efficiency and thermal stability simultaneously. A novel lead-free [(Na0.5K0.5)0.97-xLi0.03](Nb0.94-xSb0.06)O3-xBi(Zn1/2Zr1/2)O3 (NKLNS-xBZZ) ceramics was developed by a solid-state reaction method. The addition of BZZ has induced obvious dielectric relaxation behavior, as well as improved thermal stability of dielectric response. Furthermore, 0.4 wt.% La2O3 was added into the NKLNS-0.06BZZ ceramic, leading to an increased breakdown strength as a result of the reduction of grain size, improvement of bulk resistivity and decrease of dielectric loss. A large recoverable energy-storage density (∼4.85 J/cm3) and a high efficiency (∼88.2 %) as well as an excellent thermal stability (±12 %, 25–140 °C) were simultaneously obtained, together with a fast discharge rate (t0.9∼112 ns). These results suggest that La2O3 doped NKLNS-0.06BZZ ceramic could become an attractive dielectric material for temperature-stable energy-storage capacitors.

Peculiarities of the temperature dependence of electron spin resonance and Raman studies of Zn1−xNixO/NiO two-phase nanocomposites

Abstract: A meticulous investigation of electron-spin-resonance (ESR) and Raman spectroscopy of the two-phase nanocomposites of Zn1−xNixO/NiO is reported. The temperature variation of X-band ESR parameters viz., resonance field HR(T) and line-width ΔHPP(T) follows the power-law variation (δHR = (ΔHPP)n) of Nagata and Ishihara model, which was used to understand the orientation of statistical ensemble of particles with respect to a given direction of the anisotropy axis. This analysis yields the exponent “n” ≃ 2.13 and 2.85 for the composite system Zn1−xNixO/NiO and pure NiO suggesting the presence of partial and randomly oriented ellipsoidal nanocrystallites, respectively. The Raikher and Stepanov model has been employed to probe the role of amorphous Ni3+ clusters on the observed ESR spectra. Interestingly, after Ni substitution, a new zone boundary phonon mode was noticed at 129 cm−1 for all the samples, which is usually forbidden in the first-order Raman scattering for wurtzite ZnO. In addition to the 2M magnon ...