Key Engineering Materials Vols. 510-511

Abstract: Micro-Brillouin light scattering experiments were performed on flux grown D-type (monoclinic, P2₁/c) Ho₂Si₂O₇ single crystal from room temperature to 850 K. Both the hypersonic velocity (V_LA) and related elastic constant (c₃₃) of the longitudinal acoustic phonons propagating along 001-direction showed a small dispersion at T565 K whereas the same could not be observed clearly in attenuation/damping. The overall temperature dependent behavior of elastic constant and hypersonic velocity was attributed to the lattice anharmonicity of the crystal in the investigated temperature range.

Abstract: The output considerations: velocity, pressure, density, internal energy, temperature and L/D ratio of explosively formed projectiles (EFPs) were investigated. The internal energy of the EFPs was inversely proportional to the density of the liner material. The shock pressure generated and its fluctuations were highest for Fe and Ta liner materials respectively. The least pressure variations were observed for Cu along its axial direction with highest length to diameter (L/D) ratio to supports deeper penetration. The maximum difference between numerical simulation and experiment was 9.7 % for Cu EFP.

Abstract: Polycrystalline thin films of Cadmium Sulfide (CdS) have been extensively studied for application as a window layer in CdTe/CdS and CIGS/CdS thin film solar cells. Higher efficiency of solar cells is possible by a better conductivity of a window layer, which can be achieved by doping these films with suitable elements. CdS thin films were deposited on properly cleaned glass substrate by thermal evaporation technique under vacuum2×10-5mbar. Films were structurally characterized by using X-ray diffraction. The X-ray diffraction spectra showed that the thin films were polycrystalline in nature. Aluminum was doped chemically in as deposited and annealed thin films by immersing films in AlNO33.9H2O solutions respectively. Comparison between the effects of different doping ratios on the structural and optical properties of the films was investigated. Higher doping ratios have improved the electrical properties by decreasing the resistivity of the films and slightly changed the bandgap energy Eg. The grain size, strain, and dislocation density were calculated for as-deposited and annealed films.

Abstract: Mn-Zn ferrite powders (Mn_0.5Zn_0.5Fe₂O₄) were prepared by the chemical co-precipitation method. The effect of sintering temperature on the crystalline phase formation and dielectric properties were investigated by X-ray diffraction and impedance analyzer respectively. The TGA/DTA analysis was carried out to know decomposition mechanism. Ferrites decomposed to Fe₃O₄ above 873 K sintering temperature. Crystallite size increased with increasing sintering temperature between 7-13 nm. The resistivity decreased with increase in temperature showing semiconducting like behaviour. Activation energy was in the range of 0.70 to 0.77eV for these samples. Dielectric constant decreased with increasing frequency. Relaxation peak occurred for loss tangent versus frequency curves. The ac conductivity explained using small polaron tunnelling (SPT) model.

Abstract: Samples of Cr doped cobalt ferrite were prepared by co-precipitation route. These particles were characterized by X-ray diffraction (XRD) at room temperature. The structural properties were observed before and after sintering. The FCC spinel structure was confirmed by XRD patterns of the samples. The crystallite sizes lie in the range of 37-60 nm. DC electrical properties as a function composition were measured. Scanning electron microscopy was used in order to investigate the surface morphology of the prepared samples. The system for thermoelectric power measurement was designed, developed and calibrated in the laboratory. The room temperature thermoelectric power was measured for the prepared samples. The magnitude of Seebeck coefficient depends on the composition and resistivity of the samples. The obtained values of Seebeck coefficient for CoFe₂O₄ are in good agreement to the reported values. Determined values of Seebeck coefficient for other studied compositions are an addition to the literature.

Abstract: The substrate temperature in depositions of thin films plays a vital role in the characteristics of deposited films. We studied few characteristics of cadmium sulphide thin film deposited at different temperature (150°C-300°C) on corning 7059 glass substrate. We measured transmittance, absorbance, band gap and reflectance via UV spectroscopy. It was found that the transmittance for 300nm to 1100nm was greater than 80%. The resistivity and mobility was calculated by Vander Pauw method which were 10-80 cm and 2-60 cm²V^-1S^-1 respectively. The thermoelectric properties of the film were measured by hot and cold probe method which shows the N-type nature of the film.

Abstract: ZnO thin films were prepared by sol-gel method. Prepared thin films were then characterized by SEM, XRD, EDX and Hall effect measurements. SEM confirmed the morphological studies of ZnO thin films. Crystallite size is calculated using the Scherrer formula. Crystallite and grain sizes are obtained through XRD and SEM. EDS analysis confirms mass percentage of ZnO deposited. Decreasing trend of magneto resistance with temperature is observed. The optical transmission spectra of the solgel deposited ZnO thin films showed high transmittance (>70%) in the visible region and indicates that the transmittance of ZnO films gradually decreased as the thickness increased. Decreasing trend of resistivity and sheet resistance with thickness are also observed. The IV characterization of ZnO thin films under influence of UV and dark conditions are reported. The dc electrical resistivity data follow the hoping model.

Abstract: The compound Dy₂Si₂O₇ exists in two polymorphs, the low temperature triclinic phase (type B) and a high temperature orthorhombic phase (type E).The dc and ac electrical conductivities of E-Dy₂Si₂O₇ are measured in the temperature range 290-510 K and frequency range 1 kHz to 1 MHz . The dc electrical transport data are analyzed according to Motts variable-range hopping model. The disorder parameter (T_o) and density of states at fermi level are obtained. The ac conductivity σ_ac (ω) is obtained through the dielectric parameters. The ac conductivity can be expressed as σ_ac (ω) =B ω ^s , where s is slope and it decreases with increase in temperature. The conduction mechanism in the compound is discussed in low and high temperature regions in the light of theoretical models.

Abstract: Phase purity, particle size and its distribution contributes a lot to the physical properties of M-type hexa-ferrites. These parameters are strongly influenced by the variation in synthesis parameters. In the present work, effect of synthesis parameters such as molar ratio (Fe/Sr) and volume rate of addition of precipitating agent on M-type hexa-ferrite (SrFe₁₂O₁₉) prepared by co-precipitation method have been investigated systematically. The molar ratio (Fe/Sr) in SrFe₁₂O₁₉ was varied as 12, 11, 10, 09, and 08. X-ray diffraction analysis revealed that molar ratio does not affect the phase purity. X-ray diffraction analysis of the samples prepared with different volume rate of addition of precipitating agent indicated that phase purity and micro-structural properties of SrFe₁₂O₁₉ are greatly influenced by the above synthesis parameter. High volume rate of addition of precipitating agent resulted in high phase purity, smaller particle size, and narrow particle size distribution.

Abstract: The pressure bulkhead of a passenger aircraft ruptured when the aircraft was cruising at a height of 28,000 ft. Because of the sudden rupture, the rear toilets disintegrated and things like toilet rolls, tissue papers, towels, were sucked into the stabilizer compartment, where the entire tail control system was located. The debris damaged the green hydraulic system, ruptured fuel line and jammed the control cables. The damage resulted in the aircraft going into pitch-up mode (take-off position) and almost approached stalling position. The failed part was located near the toilets. Water from the toilets reached in this region due to leakage and penetrated inside the sheets and corrosion started in the presence of stagnant water. This was supplemented by the stresses present on the structure during flight. Stress corrosion started from inner sheet and led to initiation of fatigue. The combined effect of stress corrosion cracking and fatigue resulted in the failure of this sheet. No third stage (catastrophic) failure was observed in the fracture. The absence of catastrophic failure mode in the fractured sheet showed that it stood up to last stage.