Effect of Nb doping on structural and electric properties of PZT (65/35) ceramic

The aim of this study is to determine the effect of Nb⁵⁺ doping on PZT (65/35) based bulk materials in their structure, micro structure and electrical properties. The Nb content was chosen 0-9 mole%. These materials were prepared by conventional mixed oxide method. X ray diffraction studies suggest the compound to be of rhombohedral perovskite phase. Excess addition of Nb result in pyrochlore and fluorite phase develops in PZT (65/35) sample. Detailed studies of dielectric constant as a function of temperature (40degC to 500degC) and frequency (100 Hz to 1 MHz) suggest that the compounds undergo diffuse type of phase transition. Maximum dielectric constant and resistivity were found to be strongly dependent on doping and measuring frequencies. Using complex impedance analysis micro structural parameters such as bulk and grain boundary resistance, bulk charge carrier concentration and relaxation time were determined     

Growth and optical properties of Sn–Si nanocomposite thin films

The growth and optical properties of nanocomposite thin films comprising of nanocrystalline Sn and Si are reported. The nanocomposite films are produced by thermal annealing of bilayers of Sn and Si deposited on borosilicate glass substrates at various temperatures from 300 to 500 °C for 1 h in air. X-ray diffraction reveals that the as-deposited bilayers consist of nanocrystalline Sn films with a crystallite size of 30 nm, while the Si thin films are amorphous. There is onset of crystallinity in Si on annealing to 300 °C with the appearance of the (111) peak of the diamond cubic structure. The crystallite size of Si increases from 5 to 18 nm, whereas the Sn crystallite size decreases with increase in annealing temperature. Significantly, there is no evidence for any Sn–Si compound, and therefore it is concluded that the films are nanocomposites of Sn and Si. Measured spectral transmittance curves show that the films have high optical absorption in the as-deposited form which decreases on annealing to 300 °C. The films show almost 80 % transmission in the visible-near infrared region when the annealing temperature is increased to 500 °C. There is concomitant decrease in refractive index from 4.0, at 1750 nm, for the as-deposited film, to 1.88 for the film annealed at 500 °C. The optical band gap of the films increases on annealing (from 1.8 to ~2.9 eV at 500 °C). The Sn-Si nanocomposites have high refractive index, large band gap, and low optical absorption, and can therefore be used in many optical applications.     

Transmission electron microscopy study of Ni–Si nanocomposite films

Nickel induced crystallization of amorphous Si (a-Si) films is investigated using transmission electron microscopy. Metal-induced crystallization was achieved on layered films deposited onto thermally oxidized Si(3 1 1) substrates by electron beam evaporation of a-Si (400 nm) over Ni (50 nm). The multi-layer stack was subjected to post-deposition annealing at 200 and 600 °C for 1 h after the deposition. Microstructural studies reveal the formation of nanosized grains separated by dendritic channels of 5 nm width and 400 nm length. Electron diffraction on selected points within these nanostructured regions shows the presence of face centered cubic NiSi₂ and diamond cubic structured Si. Z-contrast scanning transmission electron microscopy images reveal that the crystallization of Si occurs at the interface between the grains of NiSi₂ and a-Si. X-ray absorption fine structure spectroscopy analysis has been carried out to understand the nature of Ni in the Ni–Si nanocomposite film. The results of the present study indicate that the metal induced crystallization is due to the diffusion of Ni into the a-Si matrix, which then reacts to form nickel silicide at temperatures of the order of 600 °C leading to crystallization of a-Si at the silicide–silicon interface.     

Nanocrystalline wurtzite Si–nickel silicide composite thin films with large band gap and high resistivity

Stabilization of wurtzite Si nanocrystals embedded in a metal/metal silicide matrix by the metal induced crystallization process is demonstrated. The process involves the growth of 50 nm thick Ni films on borosilicate glass (BSG) substrates followed by 700 nm thick amorphous Si films and annealing of this multilayered stack at 550 °C in furnace atmosphere for 1 h. The presence of wurtzite Si is established based on electron diffraction studies and is also confirmed by the Raman signature of wurtzite Si at 504 cm⁻¹. It is shown that the growth of wurtzite Si is mediated by the formation of Nickel Silicide, as evidenced by the Raman signal at 294 cm⁻¹. The films exhibit a band gap greater than 1.9 eV with dc resistances of the order of 10 GΩ. It is proposed that such high resistivities should make this form of Si ideal for PV and microwave device applications.     

Phenomenological understanding of flash sintering in MnCo2O4

The dual role of electric field in the flash sintering process of conducting MnCo₂O₄ is demonstrated. The flash and conventionally sintered MnCo₂O₄ samples produced at different temperatures are characterized using energy dispersive X-ray and micro-Raman spectroscopy to elucidate the micro-level spatial distribution of evolved phases. Raman signal mapping over the two ways sintered samples exposes differently grown areas of cobalt oxide based secondary phase. Electrical conductivity of conventionally sintered sample is recorded as a function of temperature and E-field and is utilized to discover the charge carrier activation mechanism during the flash effect. The conductivity before the flash-onset is shown to be comparable to that occurs by Poole-Frenkel effect and Phonon-assisted tunneling i.e. by the mechanism that occurs before the dielectric breakdown of semiconductors and insulators. The observed results, finally, confirm that catalyst like drift action of E-field on cobalt oxide formation is responsible for enhancement in the flash-sintering.     

Effect of Calcium Doping on Dielectric and Pyroelectric Properties of PLZT

Effect of calcium doping on structural, dielectric and pyroelectric properties of Pb_0.92-xLa_0.08Cax(Zr0.65Ti0.35)_0.98O₃ (PLCZT) composition derived by the solid state reaction method was studied. Rhombohedral perovskite phase was confirmed in all the PLCZT samples. Calcium doping did not show any structural change in PLCZT (8/65/35) up to 2.5 % doping, however grain size and crystallite size was found to increase up to 1%, but no systematic variation of transition temperature was found with Ca doping. P-E hysteresis loop parameters (E_c and P_r) and pyroelectric properties were found to decrease with Ca doping up to 1% and there after increased.