S. Chehab

Numerical study on transient convection in float zone induced by g-jitters

Abstract The transient convection in the float zone caused by axial g-jitters is studied. A numerical code based on the finite difference method is developed to solve time-dependent buoyancy and surface tension driven convections in the float zone with non-deformable surface. First, the effects of g-jitters on the float zone crystal growth of bismuth silicate (BSO) in microgravity environment are investigated. The ranges of frequency and amplitude which may cause a vigorous transient convection in the BSO float zone are predicted. Second, the numerical analysis is extended to a parametric study to investigate more general characteristics of transient convection induced by the g-jitter. The examined problem is defined and is characterized with a set of parameters of the Marangoni number, Prandtl number, amplitude and frequency of the g-jitter. The responses of both temperature and velocity to the g-jitters are examined for various combinations of these parameters.

Magnetic susceptibility data and exchange interaction parameters for some pseudo-ternary semimagnetic semiconductor alloys

Abstract Previously reported values of the spin-glass transition temperature T g from magnetic susceptibility measurements on the alloys Cd x Zn y Mn z Te, Cd x Zn y Mn z Se and Cd 1- z Mn z Te 1- y Se y have been analyzed in terms of an indirect antiferro magnetic exchange interaction having the form J = I 0 r −2 exp (− αr ). Values of I 0 and α so obtained were found to be characteristic of the two structures, zinc blende and wurtzite. Values of nearest and next nearest neighbour exchange obtained from I 0 and α are shown to be in very good agreement with values determined previously from other types of measurement. The values of I 0 and α have also been used to predict values of the Curie-Weiss θ for the alloys and these show very good agreement with the values determined from magnetic susceptibility measurements. The exchange is attributed to virtual transitions between the valence band and a non-localized 3d band, and the values of α are discussed in these terms.

Magnetic properties of Cd1-zMnzTe1-ySey alloys

Abstract Measurements of electron spin resonance (ESR) and low-field magnetic susceptibility have been made as a function of temperature in the range 4.2-500 K on polycrystalline samples of both the zinc blende and wurtzite phase fields of the alloy system Cd1-zMnzTe1-ySey. From the susceptibility measurements, values have been obtained for the spin-glass transition temperature Tg, the Curie-Weiss temperature θ and the Curie constant C. The curves of ESR linewidth ΔH versus temperature have been fitted to the relation ΔH = T exp(-T/T0)+B(1-θ/T){1-exp(-T/T0}, and hence values obtained for the parameters T, T0 and B. The variations of the six parameters Tg, θ, C, T, T0 and B with both composition and crystal structure are considered in some detail.

A hydrogen sensor based on bonded hydronium NASICON

Abstract A potentiometric hydrogen sensor which meets most of the gas-sensor criteria is discussed. The device consists of a bonded-hydronium-NASICON solid electrolyte and porous platinum electrodes. Electromotive-force, response-time and discharge measurements at 25°C are presented, providing knowledge of both scientific and technological aspects of this device. The effects of water vapour on the sensor performance are also discussed.

Float-zone crystal growth of bismuth germanate and numerical simulation

Abstract Bismuth germanium Oxide BGO (Bi12GeO20) single crystals are known to have great potential for technological applications in solid-state devices. Our work involves crystal growth of this material from the melt using a float-zone (FZ) technique in both terrestrial and microgravity environments. The ground-based (normal gravity) experiments are presently being carried out, and the ensuing results will be used to optimize the microgravity experiments which will be executed aboard the SPACEHAB Shuttle mission scheduled for April 1994. The crystal growth technique that is being developed requires BGO feed rods of high quality. The methods developed to produce these, and the important float zone parameters such as temperature, translation rate and uniformity of radial heat distribution of the furnace for the crystal growth are discussed. An analysis of the effect of shape and convection patterns on the materials, using numerical simulation to compare the theoretical predictions with the experimental results is also presented.

Chemical processing of modified lead titanate

A modified lead titanate composition has been synthesized by two different chemical techniques. The influence of the powder processing methods and the calcination temperatures on the physical characteristics, e.g. surface area and phase compositions of the powders, has been investigated. Powders of the same composition have also been prepared by attrition milling the raw materials. Results of the preliminary investigations indicate that the above physical characteristics are strongly dependent on the powder processing methods and calcination temperatures.

Interpretation of ESR results for some semimagnetic semiconductor alloys

Abstract Previously reported values of the ESR linewidth variation with temperature for the alloys (i) Cd x Zn y Mn z Se (ii) Cd x Zn y Mn z Te and (iii) Cd 1− z Mn z Te 1− y Se y have been analyzed in terms of the relation, δ H = Γ exp (- T / T 0 ) + B {1 - exp(- T / T 0 )}(1−θ/ T ), where the first term is attributed to spatial inhomogeneity effects and the second to paramagnetic behaviour. Values of the parameters, Γ, T 0 and B have been determined as a function of composition in each alloy system. The parameter T 0 , which measures the potential barrier separating two neighbouring ground states of the disordered spin system, is found to satisfy the relation T 0 = (α + β f ) z , where f = y /( x + y ) for (i) and (ii) and f = y for (iii). β is positive for (i) and (ii) and negative for (iii), consistent with previous suggestions of Kremer and Furdyna, and may be correlated with the exchange constants J sp-d discussed by Hass et al. The parameter Γ, which measures the width of the distribution of the local field seen by manganese ions in the lattice, is found to satisfy the relation Γ = Dz 2 (1 − z ), where D appears to be characteristic of the crystal structure. The parameter B which measures the linewidth δ H at high temperatures due to paramagnetic effects is found to vary as B = B 0 + γ z and the values of the coefficient γ appear to be related to the exchange parameters of the materials.

Protonic conductivity of HyceramTM, a bonded hydronium NASICON

Abstract The impedance method has been used to investigate the protonic conductivity of a bonded-hydronium NASICON electrolyte. Both the bulk and grain-boundary phases of the electrolyte are dramatically affected by the presence of water vapor in the hydrogen gas. The conductivity at 25°C exceeds that of many protonic materials and the material is stable to at least 200°C. Conductivity mechanisms are also discussed.

Exchange mechanisms in CdxHgyMnzTe alloys

Abstract Previously obtained data on the spin glass transition temperature T g and Curie-Weiss temperature θ for Cd x Hg y Mn z Te alloys have been analyzed in terms of a combination of superexchange and Bloembergen-Rowland (BR) exchange. The superexchange analysis has been made initially for samples having energy gap E g ⩾1.5 eV. From the exchange parameters obtained, values of the superexchange contribution T gs for alloys with E g ⩽1.5 eV have been calculated and the BR contribution obtained as T gb = T g - T gs . The resulting values of T gb have been analyzed in terms of both the BR and Abrikosov equations and good agreement obtained in the former case with other band data. The analysis shows that BR becomes the major component of the exchange when E g ⩽0.45 eV. Using the exchange parameters obtained from T g , values of θ are predicted and good agreement obtained between prediction and experimental values.

Impedance measurements of platinum electrodes on a solid protonic conductor

Abstract Non-blocking electrode processes in a protonic conductor, bonded hydronium-NASICON, have been investigated at room temperature by Impedance Spectroscopy. A comparison of the platinum-paste electrode morphology and reactions with those of ZrO 2 -based cells is given. The electrode impedance shows a dramatic change when varying the hydrogen partial pressure P H 2 . The frequency dispersion (impedance) data have been successfully analyzed using the Boukamp EQIVCT fitting program, and an equivalent circuit has been accordingly presented. The rise of diffusion-controlled effects at low P H 2 is also manifested.