Arc-assisted electric discharges in high-pressure carbon monoxide gas mixtures

Continuous arc-assisted electric discharges have been obtained in CO-Ar and CO-N2-Ar gas mixtures at pressures of about 0.1 atm and with maximum discharge loadings of 7.9 and 23 kW/ mole/s, respectively. The discharge technique appears suitable for adaptation to CW, high-power CO lasers.     

A model of adhesive wear in the presence of a magnetic field — I

Adhesive wear was studied by considering the interaction between two sliding asperities. A simple model for the agglomeration of dislocations at the asperity junction is proposed and the enhancement of dislocation mobility by the application of a magnetic field is incorporated. Various experiments on rubbing and machining were performed in the presence of and in the absence of a magnetic field. The experimental observations were in fair agreement with the theoretical predictions.     

A physico-chemical investigation on the selective reduction of iron oxides in chromite

Equilibrium oxygen potential over mixtures of metallic iron and synthetically prepared spinel solid solutions of FeCr₂O₄ and Fe₃O₄ have been determined by equilibrating with atmosphere of hydrogen and water vapor in the overall temperature range of 1129 to 1351 K. The data are internally consistent. For the terminal chromite (FeCr₂O₄), the ΔG⁰ of the reaction: Fe(S)+Cr₂O₃(S)+1/2O₂(g)=FeCr₂O₄(S) is in good agreement with the acceptable literature values. The activity of Fe_1.5O₂ exhibits a strong negative departure from the Raoult's Law line in the composition range investigated. The reduction of dense and porous pellets of FeCr₂O₄ in hydrogen at 1050°C was found to be basically topochemical in nature with intermediate reaction zones. The reduction was much slower compared to those of iron oxides. The rates of reduction of porous pellets were an order of magnitude higher compared to those of the dense pellets. The product consisted of Fe and Cr₂O₃ in porous pellets. But simultaneous reduction of Cr₂O₃ was observed in dense pellets resulting in the formation of Fe?Cr alloy. The behaviors in this respect could be broadly explained by thermodynamic reasoning. Mass transfer rates were theoretically calculated for porous pellet reduction assuming τ=2 and 4, and were found to be about an order of magnitude higher compared to the actual reduction rates. Therefore it is claimed that the porous pellet reduction is not primarily controlled by rates of mass transfer. Such analysis was not attempted for dense pellet reduction due to its more complex nature.     

Electrical properties of Fe-Fe3O4-SiO2 gel nanocomposites

Nanocomposites containing Fe3O4 and alpha-Fe, respectively, in a SiO2 gel were prepared by subjecting a suitably chosen gel with iron ions to a reduction treatment at 923 K, followed by wet oxidation at the same temperature for 1 hour. The particle sizes of the two phases were estimated to have values in the range of 18 to 25 nm. The dc conductivity of the composites was found to arise due to a variable range hopping mechanism with a density of states calculated as approximately 10(18) eV(-1) cc(-1). The nanoparticles of alpha-Fe are believed to contribute to the latter. The ac conductivity variation as a function of frequency and temperature could be explained because of an overlapping small polaron tunneling mechanism in the Fe3O4 nanoparticles. The density of states estimated in the latter case was approximately 10(18) eV(-1) cc(-1). From the dielectric modulus spectra of the nanocomposites, a Kohlrausch-Williams-Watts (KWW) exponent of approximately 0.30 was extracted. This indicated the presence of a wide distribution of relaxation times in the system.     

Dielectric and ferroelectric properties of Ba3M3Ti5Nb5O30 (M=Sm or Y) ceramics

(M=Sm or Y) compounds were synthesized by the conventional powder compaction and high temperature solid state reaction technique. The calcined compounds were pelletized and sintered in the range 1275–1325 °C for 4 h. The dielectric and ferroelectric properties of the sintered pellets were measured and further characterised by X-ray diffraction and scanning electron microscopy (SEM). Sm and Y substituted compounds were found to have tetragonal structure and shown dielectric constants of 145 and 49 respectively at 1 MHz. The dielectric loss increases with frequency. Samarium compound had undergone a diffuse phase transition in the temperature range 195–210 °C whereas the same with Yttrium had shown the transition at around 25 °C.     

Processing, microstructure, and mechanical properties of cast In-Situ Al(Mg, Ti)-Al2O3(TiO2) composite

In-situ particle-reinforced aluminum alloy-based cast composites have been synthesized by solidification of the slurry obtained by dispersion of externally added titanium dioxide (TiO₂) particles in molten aluminum at different processing temperatures. Alumina particles (Al₂O₃) form in situ through chemical reaction of TiO₂ particles with molten aluminum. Simultaneously, the chemical reaction also releases titanium, which dissolves into molten aluminum and results in the formation of intermetallic phase Ti(Al_1−x ,Fe _x )₃ during solidification. Increasing the processing temperature increases (1) the amount of elongated as well as blocky intermetallic phase Ti(Al_1−x ,Fe _x )₃, (2) the proportion of alumina particles in the reinforcing oxides, and (3) the porosity content in the resulting cast in-situ composite. The difference in particle content and porosity between the top and the bottom of the cast ingot increases with increasing processing temperature. The hardness of the cast in-situ composite is significantly more than that of the matrix alloy due to the presence of reinforcing particles, but the hardness is greatly impaired by the presence of porosity at the top of the cast ingot. The percent elongation of the cast in-situ composite decreases with increasing processing temperature possibly due to increasing porosity as well as an increasing amount of elongated intermetallic phase, which affects the percent elongation of the matrix alloy. The tensile and yield stresses of the cast in-situ composite decreases with increasing processing temperature again due to increasing porosity, which affects the ultimate tensile stress more than the yield stress. In the cast in-situ composite containing 3.31 ± 0.77 vol pct of porosity, the Brinell hardness is about 6 times its yield stress. The estimated yield stress of the cast in-situ composite at zero porosity as given by the linear least-squares fit appears to increase with particle content at a significantly higher rate than that predicted by the shear-lag model.     

Synthesis and thermo-mechanical properties of mullite–alumina composite derived from sillimanite beach sand: effect of ZrO2

A mullite–alumina composite was developed by reaction sintering of sillimanite beach sand and calcined alumina. ZrO₂ (2–6 wt.%) was added as additive. The raw materials and additive were mixed, attrition milled and sintered in compacted form at 1400–1600°C with 2 h soaking. The effect of ZrO₂ on the densification behaviour, thermo-mechanical properties and microstructure was studied. It was found that addition of ZrO₂ slightly retards the densification process. All the samples achieved their highest bulk density at 1600°C. Thermo-mechanical properties of the sintered samples are not effectively altered by the presence of ZrO₂. ZrO₂ containing samples always show better resistance to thermal shock than the ZrO₂ free samples. Scanning electron micrography shows that ZrO₂ occupies both an intergranular and intragranular position in the mullite matrix. The mullite formed at 1600°C is mostly equiaxed in nature that suggests densification mainly occurs through solid state sintering.     

Failure analysis of high temperature studs

Studs in the interceptor valve of a 110 MW unit failed after a service life of 148,700 h. The studs were operated under a steam pressure of 35 kg/cm² and a temperature of 535°C. The studs were fractured at one end of the threaded end. Various techniques were employed to analyse the failure of the studs. It has been concluded that the failure of the studs was due to reverse temper embrittlement. The failure was delayed due to the presence of Mo and V. To reduce the tendency to this kind of failure, the following steps were recommended: (a) reduce the phosphorus content in the steel to a low level or (b) reduce the grain size to about 10 μm.     

Recent developments in spectroscopy of quantum structures

Two spectroscopic techniques, modulated reflectance spectroscopy and surface photovoltage spectroscopy, are being used increasingly to probe the electronic structure of low-dimensional semiconductors. We have found improved versions of these techniques: soft contact electroreflectance and soft contact surface photovoltage which offer operational advantages as well as extend the range of these spectroscopies. We also provide analytic formulation for extracting the transition parameters from the measured surface photovoltage spectrum of a quantum structure.