High Gd Concentration GaGdN Grown at Low Temperatures

GaGdN layers were grown at temperatures below 300°C by radio-frequency plasma-assisted molecular beam epitaxy on sapphire substrates. GaGdN samples with high Gd concentration as high as 12.5% were obtained by lowering the growth temperature. X-ray diffraction results showed no obvious secondary phase, which means that the phase separation can be suppressed by the growth at low temperatures. All samples, including those grown at room temperature, showed ferromagnetic characteristics. Photoluminescence emission was observed, though spectra exhibit broad and sharp luminescence bands related to many kinds of defects. It is suggested that electrons coming from defects, especially, nitrogen vacancy, stabilize ferromagnetism, and that the carrier-induced ferromagnetism occurs in the low-temperature-growth GaGdN.     

Reduction kinetics of Fe<Subscript>2</Subscript>MoO<Subscript>4</Subscript> fine powder by hydrogen in a fluidized bed

Iron molybdate (Fe₂MoO₄) powders with an average particle size of 100 μm were reduced by hydrogen using a fluidized-bed batch reactor in the temperature range of 923 to 1173 K. The extent of the reaction was followed as a function of time by gas chromatography. The fluidizing-gas velocity was set at about 1.5 times the minimum fluidization velocity. The ratio of the height of the static bed to its diameter is about 1. Under the prevailing experimental conditions, it was found that the chemical reaction was the rate-controlling factor. The activation energy for this process was 158±17 kJ/mol. The crystal size of the Fe₂Mo powder produced at lower temperatures was in the nanometer range, indicating the possibility of mass production of alloys and intermetallics in the nanorange, using a fluidized bed.     

Calciothermic reduction of titanium oxide in molten CaCl<Subscript>2</Subscript>

Titanium oxides were reduced to metallic titanium using the liquid calcium floating on the molten CaCl₂. A part of Ca dissolved into CaCl₂ and reacted with TiO₂ settled below CaCl₂. The by-product CaO also dissolved by about 20 mol pct into CaCl₂, which was effective in reducing the oxygen concentration in the obtained Ti particles. The compositional region in the Ca-CaCl₂-CaO system was examined for the less oxygen contamination in Ti and the better handling in leaching. A large amount of the residual calcium oxidized the titanium powder in leaching. The metallic Ti powder less than 1000 mass ppm oxygen could be obtained only for 3.6 ks using 5 to 7 mol pct Ca-CaCl₂ at 1173 K. The powder was slightly sintered like sponge, and contained approximately 1500 ppm Ca. The anatase phae, the intermediate product in the refining process of TiO₂, could be also supplied as raw material as well as rutile.     

The electrochemical behaviour of nitrogen-containing austenitic stainless steel in methanolic solution

The corrosion behaviour of nitrogen-containing austenitic stainless steel in methanol containing different concentrations of H₂SO₄, HCl, LiCl and H₂SO₄ + HCl has been investigated using a potentiostatic polarization method. The cathodic reaction in the H₂SO₄, HCl and H₂SO₄ + HCl solutions was proton reduction whereas in the neutral LiCl solution, oxygen reduction was the predominant cathodic reaction. Active, passive and transpassive behaviours were observed only for higher concentrations of H₂SO₄ (0.01–2.0 M) due to the inherent water content. A cathodic loop, characterized by measured negative current in the anodic region, was also observed in solutions, in which the concentration of H₂SO₄ was 1.0 M or higher. The relative stability of the passive films decreased as the H₂SO₄ concentration increased, and thus the steel suffered from mild pitting corrosion. In the chloride environment, the rate of corrosion increased as the Cl⁻ ion concentration increased. The presence of acid along with Cl⁻ ions enhanced corrosion, and the corrosion rate increased significantly. The steel suffered from mild intergranular corrosion in acidic chloride solutions of methanol. In the H₂SO₄ + HCl solutions, passive films were only formed when the H₂SO₄ to HCl concentration ratio was greater than ∼10:1.     

Nanocrystalline Ni coatings strengthened with ultrafine particles

In this study, nanocrystalline Ni powders and thermally sprayed coatings, containing ultrafine AlN particles, were synthesized and characterized. The results indicated that the presence of AlN particles in the powders drastically decreased the dimension of agglomerates formed by cryomilling and increased the surface roughness of the agglomerates. The AlN phase was broken down into ultrafine particles of approximately 30 nm in size. These particles were dispersed in the Ni matrix and enhanced the development of a nanocrystalline structure in the Ni matrix during cryomilling. Selected-area diffraction patterns, obtained from transmission electron microscopy (TEM) and X-ray mapping with scanning electron microscopy (SEM), confirmed the presence of AlN particles in the coatings. The presence of AlN particles also led to an increase in the amount of NiO phase that was distributed in the coating, in the form of ultrafine, round particles. AlN particles increased the microhardness of the Ni coating by approximately 60 pct. Indentation-fracture results also indicated that the fine, dispersed AlN particles raised the apparent toughness of the Ni coating. The synthesized Ni coatings containing ultrafine AlN particles were characterized as equiaxed nanocrystalline grains with an average size of 24 nm, in which twins were observed. The increase in microhardness resulted from both grain refinement and the presence of ultrafine particles. The latter played the primary role in strengthening.     

Time-dependent deformation in an enhanced SiC/SiC composite

Time-dependent deformation in an enhanced SiC/SiC composite has been studied under constant load at high temperatures of 1200 °C, 1300 °C, and 1400 °C. Creep damage evolution was evaluated by a Young’s-modulus change of partial unloading and microscopic observation. The addition of the glassy phase in the matrix is very effective for protecting the composite from oxidation. The transient creep is dominant in creep life at all the temperatures. An empirical equation is proposed to describe creep behavior of the composite. It is found that creep activation energy increases with creep time at stresses lower than matrix cracking stress, but the activation energy remains constant at stresses higher than the matrix cracking stress. The creep strain rate of the composite is considered to be controlled by creep of fibers based on examining the time, strain, stress, and temperature dependencies of creep strain rates.     

PERFORMANCE OF THREE STAGE TURBO－EQUALIZATION－DECODING

An increasing demand for high data rate transmission and protection over bandlimited channels with severe inter-symbol interference has resulted in a flurry of activity to improve channel equalization. In conjunction with equalization, channel coding-decoding can be employed to improve system performance. In this letter, the performance of the three stageturbo-equalization-decoding employing log maximum a posteriori probability is experimentally evaluated by a fading simulator. The BER is evaluated using various information sequence and interleaver sizes taking into account that the communication medium is a noisy inter symbol interference channel.     

POWER OPTIMIZATION OF FINITE STATE MACHINE BASED ON GENETIC ALGORITHM

Using state assignment to minimize power dissipation and area for finite state machines is computationally hard.Most of published results show that the reduction of switching activity often trades with area penalty.In this paper,a new approach is proposed.Experimental results show a significant reduction of switching activity without area penalty compared with previous publications.     

A combination of differential equations and convolution in understanding the spread of an epidemic

Nonlinear dynamical method of projecting the transmission of an epidemic is accurate if the input parameters and initial value variables are reliable. Here, such a model is proposed for predicting an epidemic. A method to supplement two variables and two parameters for this proposed model is demonstrated through a robust statistical approach. The method described here worked well in case of three continuous distributions. Model predictions could be lower estimates due to under-reporting of disease cases. Anad hoc procedure with a technical note is provided in the appendix An earlier version of this paper was presented at the annual conference of the Indian Society for Mathematical Modelling & Computer Simulation, Bangalore, November 14–15, 2002