Ravi Kumar N V

The dielectric behavior of spark plasma sintered SiC(O)/HfCxN1-x nanocomposites synthesized through polymer derived ceramic route was investigated in the frequency range of 1 kHz to 1 MHz at room temperature. The nanostructural features revealed HfCxN1-x nanocrystals encapsulated in a nanometric thin layer of carbon dispersed uniformly in a SiC(O) matrix with segregated free carbon. The nanocomposites exhibited colossal permittivity values in the order of 103 at 1 kHz which reduced to 646 at 1 MHz. The interfacial polarization mechanism existing between complex nanostructural interfaces and the percolation of HfCxN1-x nanocrystals are believed to be responsible for the high permittivity values observed in the measured frequency range. The AC conductivity exemplified a frequency independent behavior at lower frequencies while at higher frequencies, the conductivity exhibited frequency dependence, indicating the existence of hopping type mechanism.

Different polymorphs of niobium pentoxide (Nb2O5) were synthesized using niobium ethoxide as a precursor by varying the pyrolyzing temperature. The room temperature X-ray diffractograms revealed the irreversible phase evolution from amorphous to pseudohexagonal (823 K) to orthorhombic (1023 K) and to monoclinic crystal structure (1223 K). While phase evolution was also confirmed by thermogravimetry and dilatometry, Raman spectroscopy clearly suggested complete elimination of free carbon in the pyrolysed ceramics. The sintering conditions were optimized to produce a highly dense (>95%) thermodynamically stable monoclinic Nb2O5. The electrical properties of stable monoclinic Nb2O5 sample were thoroughly studied. The monoclinic Nb2O5 was found to have a dielectric constant of around 28 with a dielectric loss of 0.008 at room temperature and at 100 kHz. At low measurement frequencies, an anomalous increase in the effective dielectric permittivity with increasing temperature was observed. Large values of the ε’ are associated with polarization due to the accumulation of free electrons at the grain boundaries. An analysis of the dispersion curves of Nb2O5 revealed that two relaxation processes are responsible for the observed anomalies, and the temperature dependencies of their parameters (dielectric strength, relaxation time and spectrum broadening parameter) were determined. The low-frequency process (relaxation time τ0 ~ 0.45 s), which makes the largest contribution to the dielectric constant, was apparently due to the inhomogeneous conductivity of ceramics. It was revealed that the DC conductivity of ceramics has thermoactivation character with activation energy of about 660 meV and was determined by the oxygen vacancies.

The low-frequency dielectric behavior of in situ crystallized and stabilized nanocrystalline t-NbO2 in amorphous silicon oxycarbide (SiOC) ceramic nanocomposites derived from niobium-modified poly(hydridomethylsiloxane) (PHMS) is reported. Commercially available PHMS is modified using Nb(OC2H5)5 via chemical and mechanical mixing route. It is observed that the synthesis route and heat-treatment temperatures significantly affect the crystallization of the ceramic phases, where c-NbC and t-NbO2 in amorphous SiOC are formed through chemical and mechanical modification route, respectively. The variation in dielectric permittivity and loss at room temperature with respect to ceramic phases is comprehensively investigated. In contrast to amorphous SiOC ceramics derived from PHMS (ε′ = 100.0 at 1 Hz), t-NbO2 in amorphous SiOC ceramic exhibits colossal dielectric permittivity (ε′ = 5.0 × 105 at 1 Hz) with low loss (tan δ < 3.0). This is attributed to an interfacial charge polarization and in situ growth of insulator/semiconductor/insulator ceramic phases in amorphous SiOC ceramic nanocomposites.