DTA measurements of normal-incommensurate phase transition in (NH4)2ZnCl4 and K2ZnCl4 crystals

Differential thermal analysis (DTA) was applied to determine the changes in enthalpy and entropy of (NH₄)₂ZnCl₄ and K₂ZnCl₄ crystals at their phase transition from the orthorhombic normal phase to the incommensurate phase. The temperature of this transition, T _i , is 406 K for (NH₄)₂ZnCl₄ and 555 K for K₂ZnCl₄ and the entropy changes (S/R) are 0.053 and 0.035, respectively. The low value obtained for S/R is characteristic of incommensurate phase transitions. The results were compared with the data reported for other crystals of the A₂BX₄ family. Thermal properties of the crystals of the A₂ZnCl₄ subgroup were found to the correlated with the length of A-Cl bonds.     

Phase diagram of the Co-VC-NbC system

Conclusions The eutectic in the Co-VC system corresponds to the composition with 14% VC and a temperature of 1340 ± 20°C and the maximum solubility of vanadium carbide in cobalt is 6%. The Co-VC section is not a quasibinary section of the Co-V-C phase diagram.The eutectic in the Co-NbC system melts at 1360°C and contains 11–12% NbC.The as-cast alloys of the Co-VC-NbC system are in the metastable two-phase condition and the only carbide phase is a carbide of complex composition which is a three-component solid solution of vanadium and niobium monocarbides.The fusibility diagram of the equilibrium phase diagram of the Co-VC-NbC system is characterized by the four-phase eutectic equilibrium L Co + VC + NbC with the point of the ternary eutectic at 1330 ± 20°C and a composition of 11% VC, 4% NbC, and 85% Co. The Co-VC-NbC equilibrium phase diagram does not contain ternary compounds and the equilibrium phases are cobalt- and vanadium- and niobium carbide-base solid solutions. The total volume share of the carbide constituents of the ternary eutectic somewhat exceeds the share of carbide phases in the boundary eutectics Co-VC and Co-NbC.The total solubility of the carbides in cobalt does not exceed 2–3%.Translated from Poroshkovaya Metallurgiya. No. 3(315), pp. 80–87, March, 1989.     

A novel technique to measure the group velocity of sound in dispersive media

A technique is proposed for measuring the group velocity of sound by a frequency-modulated (FM) continuous wave (CW) technique. Detailed calculations are presented, showing that V(g)=(deltaomega(m)/deltaphi)d , where V(g), deltaomega(m), deltaphi, and d are, respectively, the group velocity, change of modulation frequency, phase change of the received signal detected by a biphase lock-in amplifier, and the distance traveled by the sound. The approach has the advantage of high resolution compared to the conventional time-of-flight measurement technique.     

A new mechanism of ultrasonic absorption in aqueous solutions of rigid macromolecules

The temperature- and frequency-dependences of the ultrasonic absorption in lyotropic liquid crystals of rod-like FD virus particles have been measured with the ultrasonic beam being parallel or perpendicular to the rod axis. The experimental results show a strongly anisotropic ultrasonic absorption in the lyotropic liquid crystals, which varies with temperature exactly as the shear viscosity of water. Experimental results are in good agreement with the following model for the absorption process. The authors believe that the absorption arises from differential flow of water between the relatively long and rigid rod-like macromolecules. This mechanism of ultrasonic absorption becomes dominant for frequencies of 1-100 MHz. The validity of this mechanism is, however, not restricted to the special lyotropic crystals discussed here. Instead, this absorption process is expected to be important also for other macromolecular solutions whenever the dissolved macromolecular objects have a sufficiently large linear dimension in the direction parallel to the sound beam.     

The Valve Metal Character of Zirconium as inferred from anode potential measurements in mineral acid solutions

A study on the value metal character of Zr in 0.1 M solutions of H₂SO₄, HNO₃, and H₃PO₄ has been performed using the anode potential as the primary variable in galvanostatic, potentiostatic, and capacity measurements. A method of surface pre-treatment, which suppresses both O₂ evolution and metal dissolution, has been described. Kinetic parameters of oxide growth have been calculated. The results indicate that:

• (i) the high field approximation is applicable following an exponential law, and
• (ii) the height and activation distance of the energy barrier for ion transport through the oxide phase (Verwey model) are the same three acids.

Measurements have been also made on the dielectric breakdown of oxide, and this occurs at potentials above 200 V. Direct capacity measurements give similar results as those based on reciprocal capacity calculated from galvanostatic experiments. It is concluded that the dominant anodic oxide species is ZrO₂ having a dielectric constant of 25. Open circuit potential measurements show that Zr is spontaneously oxidized in the three acids.     

Preparation of heavily dopedn-type Ge-Si thermoelectric alloys

Germanium-silicon alloys doped with phosphorus were prepared in vacuum by rapidly pouring the molten alloy into cooled copper moulds containing the phosphorus dopant. Without any further treatment, the ingots are milled into powder of grain size (L 5 µm) and hot pressed. It is shown that a high degree of homogenization of the alloy constituents can be obtained in the final compacts, thus eliminating the need for zone-levelled starting material. It is also shown that this work is in agreement with the plastic flow model of sintering known as the Mackenzie-Shuttleworth-McClelland model. Electrical measurements indicate that the dopant is effectively and uniformly incorporated into the alloy.     

Stress waves in the coupling zones of composite shells of revolution

The problem of the axisymmetric pulsed deformation of a composite shell of revolution and complex geometric shape is solved in the two-dimensional statement. The problem was solved numerically using the Wilkins explicit finite-difference scheme. The results obtained from the one-dimensional and two-dimensional theories, on the basis of which it is concluded that for ultrashort-term loadings with a duration of approximately 10^–1–10^–2 sec, the influence of stress risers is manifested only in the immediate proximity of points of abrupt change in the body's geometry (corner points), are compared. Here, the stress calculation must be performed in accordance with two-dimensional equations, since the onedimensional equations yield values on the low side in certain cases. In sections that are 1 mm and more distant from the corner points, it is possible to use the one-dimensional theory, since the error in stresses, which is obtained in this case, goes into the factor of safety.Institute of Strength Problems, Academy of Sciences of the Ukrainian SSR, Kiev. Moscow Institute of Thermotechnics. Translated from Problemy Prochnosti, No. 10, pp. 68–71, October 1989.