Water formation on oxygen exposure of some hydrides of intermetallic compounds

It is found that a large number of metal hydrides such as hydrides of materials in Ca−Mg−Ni can form water when exposed to O₂ largely without signs for concomitant substrate oxidation. One can speak of a catalytic reaction in this connection as the H-depleted metal matrix can in most cases be rehydrided and water synthesis repeated on exposure to O₂. In some cases a considerable portion of the H combustion reaction takes place in a matter of seconds. Some of the metal hydrides such as hydrides of CaNi₄B, CeNi₃, and YbNi₂, are relatively stable concerning decomposition of the alloy by the product water, while others, for example hydrides of CaNi₃, and CaNi₂, are rapidly decomposed by the product water forming the hydroxide of the electropositive metal. They also produce fine ferromagnetic Ni-particles which could be interesting for other catalytic purposes. Guest professor, work also performed at University of California at San Diego where support by a grant from DOE, Basic Energy Sciences, is acknowledged.     

Mechanism of solid-state eutectoid transformation in an aluminium-zinc alloy

The growth rate of pearlite during isothermal transformation of an aluminium-zinc eutectoid has been determined experimentally. Different theoretical models, assuming either volume or boundary diffusion of zinc to be a rate controlling mechanism for the eutectoid transformation, have been applied. With volume diffusion models, the calculated growth rates were lower than the experimental growth rates by a factor of three orders of magnitude. Reasonable agreement between the calculated and experimental growth rates has been obtained on applying the boundary diffusion models. The activation energy for boundary diffusion of zinc in the aluminium-zinc alloy was estimated to be ≅11.6 kcals/mole.     

Effect of doping and thermal vacancies on positron annihilation in semiconductors

The effect of lattice defects on positron annihilation in semiconductors was studied. In silicon, any detectable doping effect could not be found. In germanium, the thermal equilibrium measurements of annihilation lineshapes showed no vacancy effect. From these experimental facts, the interaction of positrons with lattice defects was discussed.     

Equation of state for solids under high pressure,with application to alkali halides

A new equation of state has been derived from an analysis of the relations between energy, pressure and volume combined with a Padé approximation for the pressure. This modified equation of state is first tested by comparison with the exact expressions that are obtained for four-potential models, the Lennard-Jones, the Morse, the Born-Mayer, and the Varshni-Shukla potentials, with excellent results. It is also compared with experimental high-pressure data for four alkali halide crystals with quite good results.     

Positron study of the martensitic transformation in Fe-29.5at.%Ni

Precision measurements of positron lifetime spectra in Fe-29.5at.%Ni alloy specimens were performed to clarify the nature of the martensitic transformation in this alloy. The forward transformation did not introduce any defects which behave as strong trapping sites for positrons. On the other hand, the reverse transformation accompanied the production of defects, to which the positron lifetime is sensitive, maybe dislocations. From the present results, the nature of the transformation was discussed.     

PAC measurements of In-Pt intermetallic compounds

The quadrupole interaction parameters for¹¹¹Cd in In₇Pt₃ and In₃Pt₂ compounds were measured by TDPAC in the temperature range 80–1000 K. The EFG temperature behavior follows theT ^3/2 relation.     

Hall effect of Ce intermetallic compounds

Hall effect measurements between 300 K and 4.2 K for CeAl₂, CeSn₃, CeCu₂Si₂, CeBe₁₃, CePd₃, Ce(Pd_0.88Ag_0.12)₃ and CeRh₃ are discussed in detail. The skew scattering contribution to the Hall coefficient observed in some of these compounds is discussed in terms of the existing theories.     

Pressure effects on intermetallic actinide compounds

Abstract

Effects of volume compressions on the electronic structure have been studied theoretically for a number of different actinide systems. For the elemental metals the dhcp→fcc crystallographic transformation for Am, Cm, Bk and Cf is calculated and compared with experiment. The magnetism of actinide compounds has also been studied. The spin and orbital 5f moments in UFe₂ are found to be almost equal in magnitude but of opposite directions, which now has been confirmed by neutron experiments. Calculation of the 5f spin polarization in UW, suggests that the 5f localization remains stable even at high pressures. The orbital and spin composition of the magnetic moment in NpOs₂ is studied as a function of volume.     

X-Ray isochromats of some chromium intermetallic compounds

A systematic investigation of the fine structure near the short wavelength limit of the continuous X-ray spectrum for the chromium compounds CrB, CrSb, and Cr₂Te₃ has been made by means of a vacuum spectrometer with a curved (radius one meter) topaz crystal. Information about the density of states in the conduction band and the characteristic energy losses in these compounds is given.     

Magnetic properties of some Cr-Ga intermetallic compounds

Magnetic susceptibility measurements in the range 10 – 300 K on the compounds Cr₃Ga, CrGa, Cr₅Ga₆ and CrGa₄ are reported here. All except CrGa₄ appeared to be Pauli paramagnetic, but CrGa₄ appeared to be basically diamagnetic with a parasitic weak ferromagnetic component.     

Characterization of Zn−Fe intermetallic compounds

Zn?Fe intermediate phases have been investigated. The distribution of quadrupole splittings P(QS) was determined from the Mössbauer spectrum: the P(QS) gives evidence for two different iron sites in the γ phase. The analysis of Mössbauer spectrum taken in an applied magnetic field shows the negative sign of electrical field gradients is present.     

Au-Sn金属间化合物的第一性原理研究

采用基于密度泛函理论的第一性原理平面波赝势法,计算研究了Au-Sn二元系金属间化合物的生成焓、结合能、电子结构、弹性性质和结构稳定性. 计算结果表明：Au₅Sn合金的生成焓最小,说明Au₅Sn较容易生成,但Au₅Sn在热力学和力学上是不稳定的;AuSn₂和AuSn₄的键合作用较强,弹性模量、剪切模量均大于AuSn和Au₅Sn;从电子结构的角度,AuSn₂和AuSn₄ 的成键主要来自于Au原子d 轨道与Sn原子p轨道的杂化;而AuSn以Sn–Sn键的相互作用为主,Au₅Sn相中Au 的占比较大,导致Au–Au共价键发挥作用,抑制了Sn导带p电子的成键. 关键词： 电子结构 弹性性质 第一性原理 Au-Sn金属间化合物     

Magnetic phase transitions in layered intermetallic compounds

Magnetic, magnetoelastic, and magnetotransport properties have been studied for the RMn₂Si₂ and RMn₆Sn₆ (R is a rare earth metal) intermetallic compounds with natural layered structure. The compounds exhibit wide variety of magnetic structures and magnetic phase transitions. Substitution of different R atoms allows us to modify the interatomic distances and interlayer exchange interactions thus providing the transition from antiferromagnetic to ferromagnetic state. Near the boundary of this transition the magnetic structures are very sensitive to the external field, temperature and pressure. The field-induced transitions are accompanied by considerable change in the sample size and resistivity. It has been shown that various magnetic structures and magnetic phase transitions observed in the layered compounds arise as a result of competition of the Mn–Mn and Mn–R exchange interactions.     

Magnetism and superconductivity in intermetallic uranium compounds

Superconductivity, spin-fluctuation effects and magnetic order are reviewed for intermetallic compounds of uranium with d-transition elements. Large values for the effective mass of the pairing electrons are deduced from critical field studies on the superconducting compounds. Magnetism in the 5f-electron compounds is strongly anisotropic even for paramagnetic materials. Magnetic order does not frequently occur in these intermetallics, spin fluctuations are often observed. A unique combination of spin-fluctuation phenomena and superconductivity is met in UPt₃.     

Hall effect of Yb intermediate valent intermetallic compounds

Hall effect measurements between 300 K and 4.2 K for the intermediate valent intermetallics YbCuAl, YbCu₂Si₂, YbInAu₂, YbPd, YbAl₃ and YbAl₂ are presented and discussed in detail. Correlations found between the Hall coefficient, the electrical resistivity and the magnetic susceptibility for some of these compounds suggest an anomalous contribution due to skew scattering.     

Rod melting of intermetallic compounds by rf heating

With the help of a special combination of inductor and water-cooled copper boat it is possible to obtain compact rods of V₃Si of up to 10 cm length with only 10 kW oscillator power.     

Magnetic order in TbNiAl and TbCuAl intermetallic compounds

Polycrystalline samples of the ternary intermetallics TbNiAl and TbCuAl are investigated by means of magnetization and neutron diffraction measurements. When Cu replaces Ni in the crystal structure, the magnetic behaviour is shown to be totally different, although both elements are non-magnetic. Analysis of the neutron spectra by the Rietveld refinement method gives evidence that TbNiAl orders antiferromagnetically whereas TbCuAl becomes ferromagnetic. Frustrated spins at the Tb atoms are found only in the TbNiAl compound. Their alignment changes at a second magnetic transition. The magnetic structures and the transition temperatures are discussed.     

Radiation induced microstructural evolution and amorphization of intermetallic compounds

Abstract

A theoretical model is developed to study the influence of radiation-induced microstructural evolution on the amorphization kinetics of intermetallic compounds. The amorphization mechanism is assumed to be the buildup to a critical level of defect complexes. A complex consists of a coupled interstitial-vacancy pair. It is shown that the process of amorphization under particle bombardment is obstructed in alloy systems in which interstitials exhibit a tendency to cluster. In these systems, interstitial clustering delays the buildup of complexes. Under electron irradiation, the complex concentration attains a very low level after high doses, and the crystalline-to-amorphous transition is inhibited down to fairly low temperatures. During heavy ion bombardment, cascade damage produces an enhancement of complex formation and the transition takes place. It is shown that the kinetics of amorphization under ion bombardment depends on temperature at low temperatures, where amorphization is mostly due to complex accumulation. On the other hand, the present analyses indicate that direct in-cascade amorphization becomes more important as the bombardment temperature increases. Zr₃Al is used as a model system. The theoretical calculations yield good agreement with experimental results.     

Anomalous hall effect in some uranium intermetallic compounds

We report Hall effect measurements on the uranium intermetallic compounds UAl₂, U_0.97La_0.03Al₂, U_0.93La_0.07Al₂, U_0.85La_0.15Al₂, UPt₃ and UPd₃. We find in all of these substances, a large positive Hall effect at high temperatures and maxima or minima at low temperatures. We have compared our data with the results of magnetic skew scattering theories and found qualitative agreement at high temperatures in the cases of U_1-xLa_xAl₂ and UPt₃, but not in the case of UPd₃.