D. K. Mishra

Defect induced room temperature ferromagnetism in single crystal, poly-crystal, and nanorod ZnO: A comparative study

A comparative study has been made for the defect induced room temperature ferromagnetism of single crystal, poly-crystal, and nanorod zinc oxide (ZnO), based on the magnetic properties and electronic properties by means of X-ray absorption near edge structure spectroscopy (XANES), X-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy (UPS), valence band photoemission spectroscopy (VB-PES), and SQUID-type magnetometry. Magnetic measurement demonstrates the defect-induced ferromagnetic nature at room temperature in different ZnO films and a strong correlation between their electronic properties and magnetic responses. The higher ferromagnetic behaviour in polycrystalline ZnO is attributed to the increasing number of surface defects and native defect sites (oxygen vacancies and zinc interstitials) present in ZnO. XANES studies reveal that the number of unoccupied p states in polycrystalline ZnO is higher than single crystal ZnO as well as nanorod ZnO. The more amount of oxygen vacancy caus...

Utilization of Chemically Synthesized Fine Powders of SiC/Al2O3 Composites for Sintering

Fine powders of (Al2O3)100–x(SiC)x (0 ≤ x ≤ 50) composites were prepared by chemical route (named as pyrophoric technique) to achieve a uniform mixture of SiC in an alumina matrix. The chemically synthesized fine SiC/Al2O3 composite powders were sintered to form composites at 1450°C which is well below the sintering temperature of SiC. Sintering was performed in an argon atmosphere. Highly dense SiC/Al2O3 microstructures were achieved. An improvement in bulk density and hardness has been achieved for SiC/Al2O3 composites with 20 wt% of SiC. Hexagonal-shaped grains have been obtained in (Al2O3)50(SiC)50 composite with well-connected grain boundaries. The peak position of alumina in SiC/Al2O3 composites shifts toward lower wavenumbers in Fourier transform infrared spectroscopy and higher wavenumbers in Raman spectroscopy due to the incorporation of SiC in the composites. The optical band gap decreases with the addition of SiC and the composite behaves more like a semiconductor rather than an insulator. These properties make SiC/Al2O3 composites attractive for various industrial applications.

A study on sintered TiO2 and TiO2/SiC composites synthesized through chemical reaction based solution method

TiO2 and TiO2/SiC composites are synthesized through chemical route followed by solid-state sintering at 1450℃. The X-ray diffraction pattern shows the peaks of TiO2 and SiC for the compositional ratio up to 60:40 whereas no peak corresponding to TiO2 has been observed for the SiC percentage of 50 and 60. It is confirmed from the micro-Raman and Fourier transform infrared spectra that TiO2 has got diffused into the SiC surfaces and interfaces in the composites having higher percentages of SiC. Maximum hardness has been observed for the TiO2 and SiC composition of 50:50. No optical transition has been observed for the composites with TiO2: SiC ratio equal to or below 1.5. The increase in absorption percentages with increasing concentration of SiC indicates that these composites can be utilized as photo-catalytic active material in visible light. The electrical resistivity measurements clearly show the increase in conductivity with increase in SiC percentage up to 50% in TiO2/SiC composites. All these TiO2/SiC composites are found to have carrier concentration in the order of 1016–1018/cm3 at room temperature and hence can be proven as good ceramic semiconductors. It is observed that the insulating nature of TiO2 gets transformed to the semiconducting nature by the addition of SiC. The electrical, mechanical and optical properties are also found to be strongly dependent on the SiC percentages.

Experimental and simulated study of electrical behaviour of ZnO film deposited on Al substrate for device applications

AbstractnIn this paper, zinc oxide (ZnO) film has been deposited on Al substrate by chemical wet and dry technique which is just a simple modified version of the dip coating method. In this method, it is possible to precisely control the immersion and withdrawal speed, with drying as well as annealing at the same time. The polycrystalline nature of ZnO has been confirmed by X-ray diffraction (XRD) analysis. The XRD analysis clearly indicates that some percentage of Al diffuses into the ZnO matrix at its interface region; hence it affects mobility of the sample. Hall measurement indicates the ZnO semiconductor as n-type. I–V characteristic of the sample shows that the contact is Ohmic and it can be used as a sensor at low potential value. The mobility decreases with increase in temperature. The simulation study carried out for I–V and mobility through simulation using ATLAS (SILVACO) software confirms that the experimental and simulation results are in close agreement with respect to the I–V characteristic and the mobility.

Extended arc thermal plasma assisted sintering of Al-Zr oxide composites

An inexpensive and simple sintering technique using an extended arc thermal plasma assisted heating (EATPAH) source has been designed and fabricated for rapid sintering of Al x -Zr100-x (X = 0, 10, 20,…, 100) oxide composites. High heat and mass transfer kinetics of thermal plasma assisted heating allows quick densification within a few minutes when compared with conventional heating source that takes tens of hours for identical densification. Sintering parameters such as sintering time, plasma power and plasmagen gas flowrate, and initial green density of composites are critically optimized. The green powder compacts sintered at a higher heating rate produced a sample with higher density and fine grain microstructure, whereas abnormal grain growth and lower density result at lower heating rate. This reveals that rapid sintering by EATPAH is effective for promoting the physical densification of the sintered end product. Hardness of the sintered products is studied to understand the density and shrinkage behaviour during plasma sintering. It is observed that highly dense sintered product with greater hardness and homogeneous phase distribution can be prepared within a few minutes by low cost EATPAH technique.

UV-Vis studies of 800 keV Ar ion irradiated NiO thin films

Abstract We report the evolution of optical absorption properties of 800 keV Ar ion irradiated NiO thin films through UV-Vis characterization. Our results indicate the existence of both Mott-Hubbard (d → d transition) and charge-transfer (p → d transition) characteristic of NiO. The optical band gap of NiO increases from 3.58 to 3.75 eV when irradiated at the fluence of 5 × 1014 ions cm-2 but it does not show any remarkable variation upon 800 keV Ar ion irradiation at higher fluences. The refractive index and electron polarizability at different ion fluences have been determined from the optical band gap. Both refractive index and electron polarizability follow an opposite trend to that of the energy gap as a function of ion fluence.

Role of Y2O3, CaO, MgO additives on structural and microstructural behavior of zirconia/mullite aggregates

Zirconia mullite (MUZ), Y2O3-MUZ, CaO-MUZ and MgO-MUZ composites, synthesized through plasma fusion technique, are becoming important due to their commercial scale of production within five minutes of plasma treatment from sillimanite, zircon and alumina mixture. The X-ray diffraction studies reveal the monoclinic zirconia phase in MUZ composite whereas mixed monoclinic, tetragonal and cubic phases of zirconia have been observed in Y2O3, CaO, MgO added MUZ composites. The Y2O3, CaO and MgO additives act as sintering aids to favour the transformation and stabilisation of tetragonal and cubic zirconia phases at room temperature. These additives also play a key role in the development of various forms of microstructure to achieve dense MUZ composites.