K. C. James Raju

Large three-photon absorption in Ba0.5Sr0.5TiO3 films studied using Z-scan technique

Large picosecond nonlinearities in Ba0.5Sr0.5TiO3 thin films, grown at different temperatures in situ on (100) MgO substrates using rf magnetron sputtering technique, were studied using the Z-scan technique. The nonlinear absorption mechanism, studied near 800 nm using ∼2 and 25 ps pulses, switched from reverse saturable absorption type in the films deposited at temperature 600 °C. The magnitude of the 3PA coefficient was estimated to be ∼10−21 cm3/W2. Two-photon absorption (2PA) was the dominant mechanism recorded with ∼6 ns pulses. The observed behavior is correlated with morphological and crystallographic texture of the films. The linear refractive index and optical band gap of the films have also been calculated and these show a strong dependence on the substrate temperature.

Microwave dielectric and Raman scattering studies on bismuth zinc niobate thin films

Bismuth zinc niobate (Bi1.5Zn1.0Nb1.5O7) thin films were deposited on fused silica substrates by pulsed laser deposition. Its phase composition and microstructure were characterized by x-ray diffraction and atomic force microscopy. The as deposited films were amorphous and all of them were crystallized after a postdeposition annealing at 600 °C for 30 min in air. The crystallite size of the films was found to be varying between 31 and 60 nm with respect to the oxygen pressure in the deposition chamber. The films were found to be less oriented with the decreasing crystallite size. The films deposited under different oxygen pressures showed a microwave dielectric permittivity in between 98–124 and a loss tangent in between 0.005 and 0.007 over the X-band frequency range. From the Raman scattering data of the bulk and thin films, it was found that the Raman modes in thin films are relatively more intense and additional Raman modes were appearing in the films in the low frequency range, which reveal that the ...

Effect of misfit strain and surface roughness on the tunable dielectric behavior of Ba0.5Sr0.5TiO3 thin films

The effect of substrate temperature on the crystallographic texture and surface morphology of Ba0.5Sr0.5TiO3 deposited by rf magnetron sputtering on (111) oriented Pt (150 nm)/Ti (10 nm)/SiO2 (300 nm)/Si (100) substrates is reported. The onset of crystallinity is observed above 500 °C, beyond which all films are oriented along the (111) plane up to a temperature of 800 °C. Over this temperature range, the misfit strain and root mean square roughness (rmsroughness) peak at 600 °C, decreasing beyond this peak value. The tunability of the dielectric constant at 100 kHz is highest (60% for a field strength of 300 kV/cm) at the lowest values of rmsroughness and misfit strain and decreases as these values increase. It is also shown that the morphology of the Pt underlayers is strongly affected by the processing conditions. This in turn influences the tunable behavior of the BST films as much as the substrate temperature during growth and the consequent microstructural variations.

Effect of the amorphous-to-crystalline transition in Ba0.5Sr0.5TiO3 thin films on optical and microwave dielectric properties

Thin films of Ba0.5Sr0.5TiO3 were deposited at ambient temperature on fused silica substrates by RF magnetron sputtering followed by post-deposition annealing at temperatures between 400 and 1000 °C. The post-deposition annealing resulted in an amorphous–crystalline phase transition in the films. The amorphous–crystalline transition is accompanied by an increase in the refractive index, a decrease in the optical band gap and an increase in the microwave dielectric constant. At 800 °C the films exhibit a refractive index of 2.21 at 550 nm, an optical band gap of 3.49 eV and a microwave dielectric constant of 265, all of which indicate the crystalline nature of the films. The films lose oxygen when annealed at high temperatures as evidenced by the optical and microwave dielectric behaviour. The observed behaviour is explained in terms of defect centres created in the form of oxygen vacancies due to the annealing process and the structural phase transition.

Structural investigations on (BaxSr1−x)(ZryTi1−(y+δ)Taδ)O3 dielectric resonator compounds used for microwave applications

Structural investigations on ABO3 type perovskite dielectricresonator (DR) compounds, namely (Ba0.29Sr0.71)(Zr_0.973Ti_0.027)O_3 [BSZT] and (Ba_0.29Sr_0.71)(Zr_0.951Ti_0.039Ta_0.01)O_3 [BSZTTa] were carried out using X-ray Rietveld method. The structures were found to be isostructural with SrZrO_3 and belong to the space group Pbnm. In both the structures, the tilt angles of the BO_6 octahedra were lower than that in SrZrO_3. The volume of the octahedron was comparable for all the three. When Ta was substituted in BSZT, dielectric resonator properties of the resulting product BSZTTa improved substantially.

Flexible few-layered graphene/poly vinyl alcohol composite sheets: synthesis, characterization and EMI shielding in X-band through the absorption mechanism

1 mm thick flexible few-layered graphene (FLG)/poly vinyl alcohol (PVA) composite sheets containing 0.1 and 0.5 vol.% of FLG are prepared using an easy solution mixing process followed by a simple casting process. As-synthesized FLG is used as the filler. The morphology, structure and phase characteristics clearly showed the formation of a composite. A maximum electromagnetic interference (EMI) shielding effectiveness (SE) of ∼19.5 dB (in the X-band, 8.2–12.4 GHz) was obtained in the case of the composite with 0.5 vol.% of FLG. Absorption is found to be the dominant mechanism for EMI shielding. The high EMI SE is attributed to the network-like features formed by FLG in the PVA matrix. EMI SE and absorption are understood by analyzing the dielectric behaviour, ac-conductivity and transparency to visible light of the composite sheets.

Synthesis and Characterization of Co-Doped Ceria Ceramics by Sol-Gel Method

Doped and co-doped ceria electrolyte materials are very useful in solid oxide fuel cells. The Ce0.9Sm1−xSrxO2 (x = 0–0.09) compositions were synthesized through the sol-gel method. Dense Ce0.9Sm1−xSrxO2 ceramics were obtained through sintering the pellets at 1300°C for 8 h. XRD measurements indicate that all synthesized materials crystallized in cubic fluorite-type structure. Average crystallite size of the samples was in the range 21–27 nm. The relative density of Ce0.9Sm1−xSrxO2 samples was over 94% of the theoretical density. The lattice parameter increased linearly with increasing Sr concentration in Ce0.9Sm1−xSrxO2 following Vegards rule. Surface morphology was analyzed using SEM. It was observed that the thermal expansion increased linearly with increasing temperature. The two-probe a.c. impedance spectroscopy was used to study the grain, grain boundary and total ionic conductivity of doped and co-doped ceria in the temperature range 250°–500°C. The Ce0.9Sm0.07Sr0.03O2 composition showed higher grain ionic conductivity and minimum activation energy at 500°C.

Effect of CuO on the sintering and cryogenic microwave characteristics of (Zr0.8Sn0.2)TiO4 ceramics

Abstract The effect of CuO on the sintering temperature, microstructure and microwave dielectric properties of (Zr0.8Sn0.2)TiO4 (ZST) modified with 1 wt% of ZnO has been investigated. Microwave dielectric properties of ZST ceramics are measured from cryogenic to room temperatures (15–290 K). Crystallite sizes of sintered ZST ceramics as derived from XRD are in the 30–50 nm range. The addition of CuO effectively reduced the sintering temperature to 1300 1C, possibly due to liquid-phase effects. Addition of CuO did not cause any secondary phases up to 1.5 wt% of CuO. The dielectric constant (εr) and temperature coefficient of resonant frequency (εf) of ZST ceramics do not significantly vary with temperature, whereas the unloaded quality factor (Qu) changes noticeably. It is found that the Qu factor of the sample without CuO decreased with increase in temperature, whereas the samples with addition of CuO up to 1.0 wt% showed less dependence on temperature. The Qu factor of CuO-free ZST is 15,000 and that of ZST with 0.5 wt% of CuO is 11,800 at 15 K. The Qu factor while measured at room temperature ranged between 2900 and 7000. Efforts were made to understand whether the increase in Qu factor at both cryogenic and room temperatures is the result of intrinsic or extrinsic factors.

Effect of CeO2 and Nd2O3 on the Microstructure and Microwave Dielectric Properties of (Zr0.8,Sn0.2)TiO4 Ceramics

(Zr0.8,Sn0.2)TiO4 (ZST) ceramics were prepared by solid-state reaction method with 1 wt% ZnO and 0.5–1.5 wt% CeO2 or Nd2O3 as sintering aids. The effect of processing parameters and additive concentration on the structure, microstructure and microwave dielectric properties of ZST ceramics were investigated. The dielectric constant (ϵr) and temperature coefficient of the resonance frequency (τf) were not significantly affected by the addition of these additives. The unloaded quality factors (Qu) were effectively promoted by CeO2 and Nd2O3 additions. ϵr values of 40 and 38.3, Qxfo values of 57,500 and 59,300 were obtained for the samples sintered with 1.5 and 0.5 wt% of CeO2 and Nd2O3 respectively. The improvement in Qxfo value is primarily attributed to the increase in uniform grain size and density.

Crystallographic texture, morphology, optical, and microwave dielectric properties of dc magnetron sputtered nanostructured zirconia thin films

Nanocrystalline zirconia thin films have been deposited at ambient temperature by dc magnetron sputtering on glass and quartz substrates. The crystallite size as calculated from the x-ray diffraction patterns in the films varies between 10 and 25nm and is dependent on oxygen percentage in the sputtering gas. Interestingly, the presence of monoclinic and cubic phase is observed for the films deposited on glass at 40%, 60%, and 80% of oxygen in the sputtering gas, while those deposited on quartz showed only the monoclinic phase. Refractive index decreased with increase in percentage of oxygen in the sputter gas. Significantly, even at 100% oxygen in the sputtering gas, films of thickness of the order of 500nm have been grown starting from the metallic Zr target. The dielectric constants were measured using the extended cavity perturbation technique at X-band frequency (8–12GHz). The dielectric constant and loss tangent showed a very small decrease with increase in frequency but exhibited a stronger dependen...