Structure and magnetic properties of sulfides of the type CdRE2S4 and Mg(GdxYb1−x)2S4

CdRE₂S₄ (RE = Gd, Tb, Dy, Ho, Er, Tm, and Yb) and Mg(Gd_xYb_1?x)₂S₄ were prepared by solid-state reactions. All the cadmium-containing compounds are cubic, i.e., the Th₃P₄ structure for Gd, Tb, and Dy and the spinel type for all the others. The first three compounds were deficient in CdS. In the case of the Mg system, for x = 1 the system is cubic Th₃P₄, for x = 0 cubic spinel, and for 0 < x < 1 orthorhombic MnY₂S₄ (Cmc2₁). All the materials studied are paramagnetic above 77 K. Below 77 K in the magnesium family both cubic materials are paramagnetic down to 4.2 K and the orthorhombic materials show magnetic ordering. In the cadmium family all but CdTm₂S₄ show exchange coupling.     

Solid-liquid equilibrium diagrams of common ion binary salt hydrate mixtures involving nitrates and chlorides of magnesium, cobalt, nickel, manganese, and iron(III)

The solid-liquid equilibrium diagrams of binary mixtures involving magnesium nitrate hexahydrate with cobalt nitrate hexahydrate, nickel nitrate hexahydrate (partly), manganese nitrate tetrahydrate, and iron(III) nitrate nonahydrate and of magnesium chloride hexahydrate with cobalt and nickel chlorides hexahydrates and manganese chloride tetrahydrate, and the of two manganese salts were determined. Those diagrams that showed a simple eutectic were fitted by the Ott equation and where the required BET parameters were available, the magnesium salt rich parts of the liquidus were modeled by means of this method.     

Preparation of a continuous series of solid solutions in the Ta2O5Nb2O5 system

A continuous series of Ta₂O₅Nb₂O₅ solid solutions was obtained by anodically dissolving calculated amounts of tantalum and niobium in a saturated solution of NH₄Cl in methanol (at room temperature), evaporation of the solvent, and thermal decomposition of the product at temperatures not exceeding 750–800°C. X-ray diffraction analysis showed that at all Ta:Nb ratios the product was isomorphic with β-Ta₂O₅ (γ-Nb₂O₅); all reflections could be indexed in an orthorhombic cell, with a increasing linearly from 6.170 to 6.192 Å and c decreasing linearly from 3.935 to 3.885 Å as Ta was substituted for Nb. The changes in parameter b as a function of composition were less pronounced and its values were between 3.657 and 3.662 Å. The method may be used for the preparation of various mixed oxides that are difficult to prepare by other techniques.     

Magnetic behavior in solid solution systems: I. (MnMg)Tb2S4

The solid solutions in the system Mn_xMg_1?xTb₂S₄ for 0 ≤ x ≤ 1 all have the orthorhombic MnY₂S₄ structure, space group Cmc2₁. In the temperature range 77–300 K the materials are paramagnetic and the Curie-Weiss law is obeyed. At low temperatures, ca. 20 K, there is a deviation from linearity of the curve of χ^?1 vs T. The curves of magnetization as a function of the magnetic field at 4.2 K are reminiscent of saturation curves, especially for low values of x. The magnetic interactions between the metal ions are discussed.     

Head-on collision of normal shock waves in dusty gases

The head-on collision of normal shock waves in dusty gases has been investigated numerically, using the modified random-choice method. The results concerning the various flow field properties as well as the waves configuration were compared with those of a pure gas case.     

The influence of phosphate on the hydration of cement minerals studied by DTA and TG

A thermal analysis study of the effect of phosphate, in the concentration range 0.7–4.5%, on the hydration reaction of 3 CaO · SiO₂(C₃S) is reported. The results are compared with strength development of phosphate cement reported by Nurse.     

Some new results and interpretation concerning the system FeYb2S4

Stoichiometric powdered FeYb₂S₄ having a cubic (special) spinel structure was prepared. The structure was based on X-ray diffraction and Mössbauer spectroscopy. The space group is Fd3m (No. 227) and the lattice constant a₀ = 10.828Å. All the iron is divalent but only ca. 80% fills the tetrahedral 8a site while ca. 20% fills the octahedral 16d site. The ytterbium is trivalent, most of it occupying the normal octahedral 16d spinel site, but ca. 16% occupies the normally empty octahedral 16c site, having been displaced by the iron.     

Study of the membrane effect on turbulent mixing measurements in shock tubes

The effect of the thin membrane on the time evolution of the shock wave induced turbulent mixing between the two gases initially separated by it is investigated using two different sets of experiments. In the first set, in which a single-mode large-amplitude initial perturbation was studied, two gas combinations (air/SF and air/air) and two membrane thicknesses were used. The main conclusion of these experiments was that the tested membrane has a negligible effect on the evolution of the mixing zone, which evolves as predicted theoretically. In the second set, in which similar gas combinations and membrane thicknesses were used, small amplitude random-mode initial perturbation, caused by the membrane rupture, rather than the large amplitude single-mode initial perturbation used in the first set, was studied. The conclusions of these experiments were: (1) The membrane has a significant effect on the mixing zone during the initial stages of its growth. This has also been observed in the air/air experiment where theoretically no growth should exist. (2) The membrane effect on the late time evolution, where the mixing zone width has reached a relatively large-amplitude, was relatively small and in good agreement with full numerical simulations. The main conclusion from the present experiments is that the effect of the membrane is important only during the initial stages of the evolution (before the re-shock), when the perturbations have very small amplitudes, and is negligible when the perturbations reach relatively large amplitudes. Received 29 August 1998 / Accepted 25 December 1998     

Shock waves in saturated thermoelastic porous media

The objective of this paper is to develop and present the macroscopic motion equations for the fluid and the solid matrix, in the case of a saturated porous medium, in the form of coupled, nonlinear wave equations for the fluid and solid velocities. The nonlinearity in the equations enables the generation of shock waves. The complete set of equations required for determining phase velocities in the case of a thermoelastic solid matrix, includes also the energy balance equation for the porous medium as a whole, as well as mass balance equations for the two phase. In the special case of a rigid solid matrix, the wave after an abrupt change in pressure propagates only through the fluid.