Equal-channel angular pressing of an Al-6061 metal matrix composite

An Al-6061 metal matrix composite, reinforced with 10 vol % Al₂O₃ particulates, was subjected to equal-channel angular (ECA) pressing at room temperature to a total strain of 5. It is shown that the intense plastic straining introduced by ECA pressing reduces the grain size from 35 m to 1 m and this leads to an increase in the microhardness measured at room temperature. Inspection revealed some limit cracking of the larger Al₂O₃ particulates as a consequence of the ECA pressing. Tensile testing after ECA pressing gave a maximum ductility of 235% at a temperature of 853 K when testing at strain rates from 10^–4 to 10^–3 s^–1. It is suggested that high strain rate superplasticity is not achieved in this material after ECA pressing due to the presence of relatively large Al₂O₃ particulates.     

Limits on Superconductivity in Nanoparticles and Nanowires

Recent experimental results are consistent with the prediction that superconductivity is suppressed in aluminum nanoparticles which are so small (10 nm diameter) that the electronic energy level spacing exceeds the superconducting energy gap in bulk material. Very recent experiments on nanowires (diameter 5–10 nm, length 150 nm) of MoGe indicate that a dissipative phase transition between superconducting and normal behavior occurs when the normal resistance of the wire equals the superconducting quantum resistance h/4e² 6.5 k.     

Stability of nickel coated sapphire whiskers

The effect of high temperature annealing (1100 to 1300° C) on the stability of nickel coated purified sapphire whiskers has been investigated. It was found that the initially coherent coating spheroidized to form a series of partially adherent nickel particles, which increased in size with continued time at temperature. The time for complete adhesion of the particles, which was established from the product of the number and volume of particles per unit area of surface, decreased from 72 h at 1100° C to 3 h at 1300° C, giving an activation energy of 70 kcal/mol. In addition at 1300° C, after 8 h, there was evidence for whisker breakdown from a sapphire-nickel reaction.     

Temperature and randomness dependence of the pair-breaking parameter in superconducting aluminum films

Magnetoconductance and excess conductance due to superconducting fluctuations in aluminum films are measured in order to study the temperature dependence of the pair-breaking parameter at temperatures nearT _c . The parameter _M is estimated from the relation =/8k _B T_in, where _in is the inelastic scattering time deduced from the analysis of the magnetoconductance. The parameter _F is determined by fitting theories to data on the excess conductance at zero magnetic field. It is shown that: (1) For films with a wide range of the sheet resistanceR , 12R 200 /, the temperature dependence of _M nearT _c agrees well with the theory of Brenig et al. (2) For clean films withR 100 /, the value of _F analyzed with theories including the correction term to the Maki-Thompson contribution shows almost the same temperature dependence as _M . In a film withR 200 /, however, a discrepancy between _M and _F remains.On leave from College of General Education, Kyushu University, Ropponmatsu, Fukuoka, Japan.     

Microstructure and mechanical properties of rapidly quenched L20 and L20+L12 alloys in Ni-Al-Fe and Ni-Al-Co systems

Ductile L2₀-type wires and+L1₂-type duplex wires with high strengths and large elongation in the Ni-Al-Fe and Ni-Al-Co ternary systems have been manufactured directly from the liquid state by an in-rotating-water spinning method. The wire diameter was in the range 80 to 180m and the average grain size was 2 to 4m for the wires and 0.2 to 1.0m for the+ wires. _y, _f and _p of the wires were found to be about 360 to 760 MPa, 560 to 960 MPa, and 0.2 to 5.5%, respectively, for the Ni-Al-Fe system, those of the+ wires were about 395 to 660 MPa, 670 to 1285 MPa, and 3.5 to 17%, respectively, for the Ni-Al-Fe system, and about 260 to 365 MPa, 600 to 870 MPa, and 4.0 to 7.0%, respectively, for the Ni-Al-Co system. Cold-drawing caused a significant increase in _y and _f and the values attained were about 1850 and 2500 MPa, respectively, for Ni-20Al-30Fe and Ni-25Al-30Co wires drawn to about 90% reduction in area. The high strengths, large elongation and good cold-workability of the melt-quenched and+ compound wires have been inferred to be due to the structural change into a low-degree ordered state containing a high density of phase boundaries, suppression of grain-boundary segregation and refinement of grain size.     

Correlation between Superconducting Gap and Pseudogap in High-T c Cuprates

We studied the low-temperature energy gap 2 ₀ on Bi ₂ Sr ₂ CaCu ₂ o ₈₊ (Bi2212) and La _2–x Sr _x CuCO ₄ (La214) systematically over a wide range of doping level p using STS, break junction tunneling spectroscopy, Raman scattering and low-T electronic specific heat data. We have also studied the electronic specific heat of La214 in the normal state at T > T _c , and confirmed that pseudogap behavior appears at around T*, below which the in-plane resistivity and magnetic susceptibility tend to be slightly suppressed. Similar suppression appears in and of Bi2212 below the onset temperature of pseudogap T*. It is pointed out in the present study that 2 ₀ is closely related to T* in both Bi2212 and La214 systems; T* 2 ₀ /4.3k _B . It is also pointed out that 2 ₀ is in almost linear proportion to k _B T _max ( T*), where T _max is the temperature exhibiting a broad peak in –T curves and k _B T _max can be considered to give a measure of the effective antiferromagnetic exchange energy J _eff. The factors in 2 ₀ k _B T _max J _eff are 1 for La214 and 2 for Bi2212, respectively. We also report that in both Bi22l2 and La214 systems T _c roughly scales with p ₀ except in highly doped samples, where T _c 2 ₀ .     

Supercarrier Effective Mass Isotope Effect by Interband Scattering

Supercarrier effective mass isotope effect (exponent ) is investigated using a two-band model with interband pair scattering. The corresponding repulsive interaction incorporates besides the dominating electronic (Coulomb) part an electron-phonon contribution inversely proportional to the ionic mass factor. Calculations illustrating the behaviour of T _c , its isotope exponent , and with doping in La _2–x Sr _x CuO ₄ type underdoped system reflect the observed tendencies. ² Both and diminish with doping, the sign of is opposite to . A typical estimation gives || 0.2.     

Low temperature internal friction on γ-irradiated polyvinylidene fluoride (PVDF)

A least-squares fitting of the below room temperature part of the internal friction spectra, obtained by the torsion pendulum technique on as-received and-irradiated (up to 1 Grad) strips and fibres of polyvinylidene fluoride [-(CH₂-CF₂-) _n -.; PVDF] by a superposition of single Debye functions, reveals that the spectral component features are determined not only by purely amorphous chain characteristics but also by the dosedependence of crystallinity. A careful analysis of the relaxation spectra confirms that at least one relaxation effect (236 K) is created upon irradiation. The analysis of the dose dependence of the characteristics of the (glass transition; 220 K) and _u (apparent upper glass transition; 270 K) relaxations, suggests the probable influence of crystallinity on the molecular motion in the amorphous phase. The increase of the intensity of the relaxation (190 K) is related to the irradiation-induced crystallite degradation.Fellow of the Interuniversitair Instituut voor Kernwetenschappen     

Chebyshev approximation for the roots of the equation BiWГ(μ)=μVГ(μ)

A minimax approximation, uniform for Bi [0, ), is developed for the roots of the equation BiW()=V(), by means of Chebyshev polynomials.Translated from Inzhenero-Fizicheskii Zhurnal, Vol. 28, No. 4, pp. 710–715, April, 1975.     

First principles study of effect of lattice misfit on the bonding strength of Ni/Ni3Al interface

By using a discrete variational X (DV-X) method, the electronic structures and bonding strengths of Ni/Ni₃Al (or /) interface with different lattice misfits () were calculated in the framework of the nonrelativistic first-principles theory. In order to describe the effect of on the interfacial binding strength and the structural stability of coherent / interface, we calculated the interfacial binding covalent bond density (CBD) and the local environmental total bond overlap population (LTBOP). Very obvious effects of lattice misfits on the electronic structures of coherent / interface were found. On one hand, less than –0.6% negative lattice misfit can increase the binding strength of the /' interface. On the other hand, the local environmental total bonding strength of the /' interface decreases with increasing magnitude of . Therefore, the magnitude and sign of lattice misfit must be carefully controlled to balance the high-temperature creep strength of Ni-base single crystal superalloy and the structural stability of the /' interface when one designs new alloys.     

Hydrazine method of synthesis of γ-Fe2O3 useful in ferrites preparation. Part IV – preparation and characterization of magnesium ferrite, MgFe2O4 from γ-Fe2O3 obtained from hydrazinated iron oxyhydroxides and iron (II) carboxylato-hydrazinates

Electro-magnetic properties and microstructural characterization of MgFe₂O₄ synthesized by a ceramic technique at 1000°C from iron oxides, consisting of mainly -Fe₂O₃ and traces of alpha-Fe₂O₃, prepared from iron ore rejects, are compared with the ferrite obtained from commercial alpha-Fe₂O₃. The sources of -Fe₂O₃ are hydrazinated iron (II) carboxylates and iron oxyhydroxides which autocatalytically decompose giving mainly -Fe₂O₃ of uniform particles of 10–30 nm (by scanning electron microscopy (SEM) studies) having high surface area. The ferrite synthesized from such nanoparticle size -Fe₂O₃ gave a porosity of 25% with grains ranging from 0–3 m. On the other hand, MgFe₂O₄ obtained from commercial alpha-Fe₂O₃ grains (of 1–2 m size) gave particles of 0–6 m with a porosity 42%. Saturation magnetization values 922–1168 G are found for MgFe₂O₄ from -Fe₂O₃ source while the alpha-Fe₂O₃ source gave the lowest value, 609. The Curie temperature, T_c, from magnetic susceptibility, initial permeability and resistivity measurements indicated a highest T_c of 737 K for MgFe₂O₄ from alpha-Fe₂O₃, while lower values are found for the ferrite prepared from -Fe₂O₃.     

CO/pH2: A Molecular Thermometer

We utilize reversible temperature dependent changes in the IR absorption spectrum of CO molecules isolated in solid parahydrogen (pH₂) to probe bulk temperature changes during rapid vapor deposition. The intensity of a well resolved feature near 2135 cm^–1 increases monotonically with temperature over the 2 to 5 K range. The thermally populated initial state of this transition lies 12 K above the CO/pH₂ ground state. During the deposition of 100 ppm CO/pH₂ samples, we detect temperature gradients 10 K/cm in 0.1 cm-thick samples subjected to heat loads 10 mW/cm². The resulting estimated thermal conductivity (TC) is 3(±2) mW/cm-K, averaged over the 2 to 5 K region. This value is 1000 times lower than the TC of single crystal solid pH₂, and 10 times lower than previously measured for pH₂ solids doped with 100 ppm concentrations of heavy impurities [Manzhelii, Gorodilov, and Krivchikov, Low Temp. Phys. 22, 131 (1996)]. We attribute this abnormally low TC to the known mixed fcc/hcp structure of the rapid vapor deposited solids.     

Preparation and analysis of high-T c Nb-Ge films

The dependence of T_c on sputtering conditions, chemical composition, and phase structure for Nb-Ge films has been studied. It was found that T_c varied with composition, but was not dependent on exact stoichiometry of the film; the Nb/Ge ratio was <3 for very high T_c 23 K films and 4 for films with T_c 20 K. X-ray results showed that the films with T_c 23 K contained a certain amount of tetragonal Nb₅Ge₃ with a lattice parameter of A15 phase 5.14 Å. The depth profile of very high T_c films showed no increase of oxygen concentration at the film-substrate interface; no correlation of high T_c and impurities was found.     

Effects of aluminium on the electrical and mechanical properties of PTCR BaTiO3 ceramics as a function of the sintering temperature

The effect of AI additions on the electrical behaviour of positive temperature coefficient of resistance (PTCR) BaTiO₃ ceramic sintered in air at temperatures ranging between 1220 and 1400° C have been investigated. Two batches of material, both showing a PTCR effect, were prepared identically except that additions of AI₂O₃ (0.55 mol %) were made to one of them. It has been confirmed that the presence of aluminium results in an increase in the temperature at which the maximum resistivity, _max, occurs as well as reducing the sintering temperature, in the presence of silicon, to 1240° C. Additionally, direct comparisons between the two materials have demonstrated that such additions result in an increase of 100% in the minimum resistivity, _min, at sintering temperatures beyond 1280° C. A similar increase in _max for sintering temperatures below 1360° C and a five-fold reduction in the ratio of _max/_min in samples sintered above 1320° C have also been attributed to the presence of aluminium. It was further found that aluminium increases the average grain size by 30% and promotes the formation of a liquid phase.     

Hall-Effect in LuNi2B2C and YNi2B2C Borocarbides in Normal and Superconducting Mixed States

It was shown that the Hall resistivity _xy for LuNi ₂ B ₂ C and YNi ₂ B ₂ C is negative in the normal and mixed states and has no sign reversal below T _c . In the mixed state the scaling relation _{xy xx} (_xx is the longitudinal resistivity) was found for both compounds with 2.0. In the normal state a distinct nonlinearity in the _xy(H) dependence, accompanied by a large magnetoresistance, was found below 40 K only for LuNi ₂ B ₂ C. The difference in the behaviour of Lu- and Y-based borocarbides seems to be connected with the difference in the Fermi surfaces of these compounds.     

Equation of motion approach to the Hubbard model in infinite dimensions

We consider the Hubbard model on the Bethe lattice with infinite Coordination number and construct (i) a systematic series of self-consistent approximations to the one-particle Green's function, G⁽ⁿ⁾(), n = 2,3,... and (ii) conduct an exact diagonalization study of the Hubbard star and the star of the stars. We present analytic and numerical results for the Mott-Hubbard transition at half filling. We find consistently (i) a critical U_c 2.5 and (ii) that the gap opens like (U-U_c)^3/2.     

Sol-gel prepared Ni-alumina composite materials

The sol-gel method has been utilized for the preparation of dense, homogeneous ceramic-metal composites with up to 50% Ni in Al₂O₂. Examination by SEM and TEM shows that the materials consist of micrometre-size Al₂O₃ with metallic Ni in isolated regions from 50 m down to nanometre size. The density ranges from 97% (10% Ni) to 74% (50% Ni) of the theoretical number. The hardness decreases from 18 GPa for pure alumina to 10 GPa for alumina containing 50% Ni. The fracture toughness increases significantly from K _1c=3–4 MPa m^1/2 to K _1c=8.5 MPa m^1/2. The elastic and shear moduli decrease from E=400 GPa and G=160 GPa for pure alumina to E=320 GPa and G=135 GPa when containing 50% Ni. The electrical resistivity is 10⁶m with 10 to 40% Ni but decreases drastically at 50% Ni content.     

Thermoluminescence study of semiconductor materials SixTe60−x As30Ge10

The effect of gamma irradiation on Si_xTe_60–xAs₃₀Ge₁₀, where x= 5, 12 and 20, has been studied using thermoluminescence (TL). As expected in semiconductor materials, both x=5 and 20 chalcogenides showed a wide TL peak ranging from 80–300 °C. However, these two materials also exhibited a sharp peak at 360 and 380 °C for x = 5 and 20, respectively. On the other hand, the material with x=12 showed very little response to gamma radiation, but if the sample was exposed to ultraviolet light (after being glowed of any TL up to 500°C) and then glowed (called phototransfer-thermoluminescence), several peaks appeared at 80, 180, 300, and 350°C. The x= 5 and 20 samples did not show any response to ultraviolet light. Because the TL response depended on the ratio of Te/Si, it can be concluded that the TL technique can also be used to characterize semiconductor materials, and it would complement other techniques such as electrical conductivity and differential thermal analysis.     

Transport results from untwinned YBCO: Washboard frequency of the vortex lattice,and the quasiparticle mean free path

The washboard frequency of the moving vortex lattice in untwinned YBa₂ Cu₃ O_6.93 may be observed through mode-locking to an externally applied ac current of frequency _ext. The interference between and _ext results in jumps in the dc current-voltage characteristics when and _ext are harmonically related¹. The interference effect disappears in the vortex liquid state. The Hall conductivity _xy below T_c in YBCO contains contributions² from a positive quasiparticle (qp) term (H) and a negative vortex term (1/H). The qp term is surprisingly large well below T_c and implies a large gap anisotropy and a long qp mean free path (mfp). The thermal Hall effect³ _xy is closely related to the qp _xy; _xy is produced by asymmetric scattering of qp by pinned vortices. The qp mfp at H = 0, extracted from _xy and extended to low T by _xy, increases remarkably from 90 Å at T_c to more than 0.5m at 22 K.     

Investigation of the excitonic insulator phase in bismuth-antimony alloys

Pressure-induced metal-semiconductor transitions in bismuth-antimony alloys in a strong magnetic field (up to 70 kOe) at helium temperatures have been investigated. It is found that for values of the overlap-gap |G|1 meV the alloy forms an excitonic insulator (EI) in magnetic fields above a certain threshold (30–40 kOe). It is inferred that the EI energy gap increases with the magnetic field. The maximum gap observed in fields of 70 kOe turns out to be ₀₀7.5 K. An analysis of the results shows that transitions to the EI phase are observed from both the semimetal and the semiconducting states. The critical transition temperatureT _c is related to the EI gap by the expressionT _c0.7. Arguments are advanced in support of the fact that the formation of the EI phase involves the pairing of electrons at theL point with holes at theT point.