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The thermal conductivity l and electrical resistivity r were measured on two samples of the cuprate superconductor YBa2Cu4O8 ~Y124!. The l results pass through a maximum at about half of the superconducting transition temperature Tc . The peak l value is much higher than the peak value for polycrystalline YBa2Cu3O72d , Y123, and is about equal to ab plane values for good melt processed Y123. In the normal state most of the l is due to phonon transport l p and estimates of l p were obtained from the data by making a correction for electronic transport. Phonon-phonon and phonon-electron scattering limit the l p of both Y124 and Y123. In the two compounds, the phonon-phonon scattering is about equally strong, but electron-phonon scattering is considerably weaker in Y124. In the superconducting state, below the l maximum at Tc/2, l varies as T n (n’1/2). This behavior has also been observed in Y123, but is not consistent with phonon scattering theory. The implications of this observation are discussed. @S0163-1829~98!08117-X#

At low oxygen potentials, YBa2Cu3O7−x decomposes by chemical reactions involving reduction. The results of this study show that the compound also decomposes at higher oxygen potentials. The initial decomposition products were found to be Y2BaCuO5 and a Ba‐Cu oxide phase. The Ba‐Cu phase was found to be a peroxide‐type and the decomposition can be described by a chemical reaction involving oxidation: 4(YBa2Cu3O7−x)+(1/2 − 3/2δ +2x)O2⇄2Y2BaCuO5+3Ba2Cu3O6−δ+CuO. At 800 °C the equilibrium pressure for this reaction is slightly

Dense polycrystalline samples of the compound YBa2Cu4O∼8, Y124, were prepared by a three‐step procedure. In the first step a mixture of Y2O3, CuO and BaO2 was reacted at 960 °C under an oxygen pressure of 55 bar. The compound did not sinter under these conditions but electrical resistivity and susceptibility measurements showed that the oxide mixture converted to Y124. In the second step densification was produced by hot isostatic pressing at 875 °C. This degraded the superconducting properties severely and a final high‐temperature anneal in pressurized oxygen was required to restore optimum properties. This anneal consisted of a one hour hold at 1010 °C in 40‐bar O2 followed by a 24‐h anneal at 950 °C in 55‐bar O2. After this treatment the samples were ∼90% dense, phase pure Y124. The maximum grain dimensions were about 10 μm, and Auger spectroscopy data were obtained on a fracture surface. These data showed that the grain boundary composition does not differ significantly from the bulk values.

Hot isostatic pressing and heat treatments in high‐pressure oxygen were used to study the effect of pressure on the stability of YBa2Cu3O7−x. At 875 °C the compound decomposes at an oxygen pressure of about 7 bars. The decomposition involves formation of Y2BaCu3O5 and a Ba‐Cu oxide phase. Formation of the latter phase involves additional oxidation because this phase can contain as many as 1.5 oxygen atoms per barium atom. The decomposition can most simply be visualized by considering the chemical reaction 2(YBa2Cu3O7−x)+( (1)/(4) +x−y/2)O2→Y2BaCuO5+Ba3Cu5O9.5−y. Encapsulated samples also decompose during hot isostatic pressing, but the process appears to be more complex. The compound YBa2Cu4O8 was detected in addition to the two products observed in oxygen‐treated samples. The reaction above, with YBa2Cu3O7−x serving as an oxygen source, aids in understanding the microstructural observations for encapsulated samples. Decomposition can be completely reversed by annealing in air for 20 h at 875 °C.

A comparative method was used to measure the thermal conductivity λ of a YBa2Cu3O7−x sample over the temperature range 305–360 K. Using one sample, data were obtained at two different x levels. Surprisingly, the smaller x value (higher Tc) produced a lower thermal conductivity. This observation is discussed in terms of scattering of phonons by point defects and electrons.

The amorphous metallic glass ${({\mathrm{Mo}}_{0.6}{\mathrm{Ru}}_{0.4})}_{82}$${\mathrm{B}}_{18}$ was annealed for various lengths of time at temperatures up to 650\ifmmode^\circ\else\textdegree\fi{}C. The changes in structure due to these annealing treatments were determined by measurements of superconducting properties (${T}_{c}$,${B}_{c2}$,${J}_{c}$), electrical resistivity (${\ensuremath{\rho}}_{n}$), low-temperature specific heat (electronic coefficient $\ensuremath{\gamma}$), small-angle x-ray scattering (SAXS), and transmission electron microscopy (TEM). At lower annealing temperatures (\ensuremath{\le}450\ifmmode^\circ\else\textdegree\fi{}C) only small changes occurred in the composition-dependent parameters (${T}_{c}$,$\ensuremath{\gamma}$) but large degradations were exhibited in those quantities (${J}_{c}$, fracture strain) sensitive to the defect structure. No evidence of microstructural changes was revealed by SAXS or TEM in this regime. At higher annealing temperatures (or longer times) evidence for the development of, first, phase separation, then, the beginnings of crystallization was found by SAXS and TEM. In this regime larger degradations of ${T}_{c}$ and $\ensuremath{\gamma}$ occurred along with a change in the form of the ${F}_{p}(b)$ curve (i.e., a change in the fluxoid pinning mechanism). The above observations imply annealing-induced long-range compositional variations. The activation energy for the onset of long-range compositional inhomogeneity was determined to be 309 kJ ${\mathrm{mol}}^{\ensuremath{-}1}$ (3.2 eV).

The effects of microadditions of cerium on the rate of annealing embrittlement, stress relief, and as-quenched magnetic domain structure in three iron-based metallic glasses have been investigated. Ribbons melt-spun from optimally doped castings of composition Fe 80 B 16 Si 2 C 2 and Fe 80 B 20 did not embrittle during annealing until the onset of crystallization, whereas Fe 78 B 13 Si 9 ribbons were unaffected by cerium additions. In the first two glasses optimal doping resulted in the disappearance of quenched-in stresses in the ribbons, as evidenced by an absence of maze domains, and, for the Fe 80 B 20 alloy, a slight enhancement of stress relief rate for small times and temperatures. The effects of doping were qualitatively different in the Fe 78 B 13 Si 9 glass. We conclude that annealing embrittlement in the first two glasses is an impurity effect, and that the embrittling impurities are oxygen and/or sulfur, the dissolved concentrations of which are strongly reduced by cerium additions. That low concentrations (50–100 at. ppm) of these impurities are sufficient to cause annealing embrittlement suggests that impurity segregation may be involved in the embrittlement process.

A model is presented of the simultaneous spreading and cooling of the liquid drop in a hammer and anvil apparatus for rapid quenching of liquid metals. The viscosity of the melt is permitted to vary with temperature, and to avoid mathematical complications which would be associated with spatial variation of the viscosity, Newtonian cooling is assumed. From an expression for the force required to spread the specimen, coupled equations for the mechanical energy balance for the system and the heat transfer from the sample to the hearth and hammer were obtained, and solved numerically. The sample reaches its final thickness when the force required to deform it becomes greater than the force exerted on it by the decelerating hammer. The model was fit to measurements of sample thickness versus hammer speed, using the interface heat transfer coefficient, h, as an adjustable parameter. The values of h so obtained vary somewhat with the melt alloy/substrate metal combination. From predicted cooling curves, the effects of hammer speed, sample size, and initial melt temperature on the cooling rate and the efficiency of glass formation can be assessed. Addition of sample superheat shifts the cooling curve relative to the expected position of the time‐temperature‐transformation curve for formation of crystalline material from the melt, and thus is an effective means of increasing the probability of glass formation in this type of apparatus.

The reversible thermal relaxation found in various amorphous materials has been observed in the glassy metal-metal alloy Zr/sub 76/Ni/sub 24/. The superconducting transition temperature, T/sub c/, can increase reversibly in this alloy with increasing temperature of anneal T/sub a/ for T/sub a/ or =300 /sup 0/C an irreversible decrease in T/sub c/ occurs. The effect of plastic deformation on T/sub c/ appears to be identical to the effect of heat treatment. It is speculated that high temperatures need not be produced during deformation, but rather that stress fields encourage the same atomic-scale relaxation processes which are active in thermal annealing.

The superconducting transition temperature ${T}_{c}$ has been measured for amorphous ${T}_{5}{T}_{9}$ alloys (where ${T}_{5}=\mathrm{Nb}\mathrm{or}\mathrm{Ta}$, ${T}_{9}=\mathrm{Rb}\mathrm{or}\mathrm{Ir}$) as a function of composition. Nb and Ta dominated the ${T}_{c}$ behavior with ${T}_{c}$ for Nb alloys \ensuremath{\approx} 4.8-5.1 K and ${T}_{c}$ for Ta alloys \ensuremath{\approx} 3.1-3.3 K. Changing the average group number by varying the $\frac{{T}_{5}}{{T}_{9}}$ ratio had little effect on ${T}_{c}$ within the \ensuremath{\sim} 10 at.% range of composition in which the amorphous alloys form. ${T}_{c}$ decreased approximately linearly with $x$ for ${({\mathrm{Nb}}_{1\ensuremath{-}x}{\mathrm{Ta}}_{x})}_{55}{\mathrm{Rh}}_{45}$ and ${({\mathrm{Nb}}_{1\ensuremath{-}x}{\mathrm{Ta}}_{x})}_{55}{\mathrm{Ir}}_{45}$. The electrical resistivity ${\ensuremath{\rho}}_{n}$ and the upper critical field ${B}_{c2}$ were measured as functions of temperature. The Sommerfeld constant $\ensuremath{\gamma}$ was calculated from ${\ensuremath{\rho}}_{n}$ and ${\frac{d{B}_{c2}}{\mathrm{dT}}|}_{{T}_{c}}$ using the Ginzburg-Landau-Abrikosov-Gor'kov theory. The Debye temperature ${\ensuremath{\Theta}}_{D}$ was calculated from measurements of Young's modulus. The above parameters allowed calculations to be made of $N({E}_{F})$, the density of electronic states at the Fermi level, and $\ensuremath{\lambda}$, the electron-phonon coupling constant. It was found that $\ensuremath{\lambda}$ more accurately reflected the ${T}_{c}$ behavior than did $N({E}_{F})$ and that both electronic and phonon contributions had to be considered in the determination of $\ensuremath{\lambda}$.

Measurements of the low-temperature specific heat ${C}_{p}$ of liquid-quenched Zr-Ni glasses for a large number of compositions in the range from 55 to 74 at.% Zr revealed an unusual composition dependence of the density of states at the Fermi level, $N({E}_{F})$. Furthermore, for some compositions the variation of ${C}_{p}$ near the superconducting transition temperature ${T}_{c}$ indicated the presence of two superconducting phases, i.e., two superconducting transitions were detected. Comparison of the individual ${T}_{c}$'s in phase-separated samples to the composition dependence of ${T}_{c}$ for all of the samples suggests that amorphous phases with compositions near 60 and 66.7 at.% Zr occur. We discuss these results in terms of an "association model" for liquid alloys (due to Sommer), in which associations of unlike atoms with definite stoichiometries are assumed to exist in equilibrium with unassociated atoms. We conclude that in the composition range studied, associate clusters with the compositions ${\mathrm{Zr}}_{3}$${\mathrm{Ni}}_{2}$ and ${\mathrm{Zr}}_{2}$Ni occur. In only a few cases are the clusters sufficiently large, compared with the superconducting coherence length, for separate superconducting transitions to be observed. The variation of $N({E}_{F})$ with composition is discussed, as well as the effects of this chemical short-range ordering on the crystallization behavior and glass-forming tendency.

Fluxoid pinning has been re-examined in the model system Nb-38 at.% Hf which contains small coherent or semi-coherent normal precipitates as the major pinning centres. Since the growth of the precipitates has been found to be a coarsening reaction, the pinning strength of the system could be continuously increased by increasing the precipitate size without accompanying changes in the fundamental superconductivity parameters T c and HC2. Peaks in volume pinning force, F p, developed on ageing. The temperature dependence of these peaks in F p indicated that the pinning mechanism was not a simple matching of fluxoid lattice and precipitate spacing. The scaling laws were not observed. Large magnetic history effects were observed, the occurrence of which appears to be related to the relative ‘softness’ of the fluxoid lattice. The results of the superconducting and microstructural data are compared with summation models. Plots of the specific pinning force, Q, versus the elementary interaction force, f...

Previous measurements of low temperature specific heat (ref. 1) demonstrated the presence of phase separation and chemical short range ordering in Zr-Ni glasses with zirconium contents from 60 to 66 at. % Zr. This ordering is found to play a significant role in the crystallization process.

Superconducting transition temperatures (T c) were measured for a series of Mo-P-B alloys in the Mo-rich corner of the ternary system. The alloys were studied in two forms: (a) slowly cooled after induction melting, and (b) rapidly quenched in an arc hammer apparatus. The rapid quenching produced an amorphous structure, but most of the alloys also contained some crystalline material, indicating that this system is not conducive to easy glass formation. Ternary slowly cooled crystalline alloys in the concentration range 20 to 30 at % phosphorus and 10 to 20 at % boron had relatively highT c values of 9.6 to 9.9 K; higher than for either the Mo-P or Mo-B binary systems.T c values up to 9.02 K were observed for completely amorphous foils, consistent with the previously reported maximumT c values for amorphous alloys in this system. This remains the highestT c reported for a metallic glass. All of the rapidly quenched foils were brittle.

Amorphous Fe90−xAlxZr10 alloys were prepared by rapid solidification in either an arc hammer apparatus or by melt-spinning for the composition range 0 25. An annealing embrittlement phenomenon was observed for the alloys with x > 5 in the ductile melt-spun ribbons. The thicker foils produced in the arc hammer apparatus were generally brittle as-cast even though the structure was completely amorphous. The annealing embrittlement was observed to correlate with increases in microhardness. The composition dependences of the crystallization behavior and the annealing embrittlement are consistent with recent suggestions that viscosity increases caused by structural relaxation during annealing are involved in embrittlement.

Superconductivity in niobium-oxygen body-centered-cubic solid-solution alloys (oxygen content 0.024-3.50 at.%) was studied by calorimetric, magnetic, and resistive measurement techniques. These measurements included low-temperature-specific-heat capacity, superconducting-normal transition temperature ${T}_{c}$, direct-current magnetization, and electrical resistivity, as well as x-ray lattice parameter, microhardness, and optical metallography to characterize the samples. Oxygen in solid solution lowers the ${T}_{c}$ of niobium. In contrast to the prediction of DeSorbo, we found that $\ensuremath{\gamma}$, the electronic coefficient of low-temperature-specific-heat capacity, also decreases with oxygen concentration. Our data indicated that the "band-structure" electronic density of states at the Fermi level ${N}_{\mathrm{bs}}(0)$ and the electron-phonon coupling constant $\ensuremath{\lambda}$ both decrease with oxygen content. Therefore, both the density of electronic states and the phonon spectrum may be controlling the magnitude of ${T}_{c}$ in the niobium-oxygen system. Additional superconducting parameters were calculated for the niobium-oxygen alloys from our calorimetric, magnetic, and resistive data. The Ginzburg-Landau parameter ${\ensuremath{\kappa}}_{\mathrm{GL}}$ was found to increase from less than 1.0 for essentially pure Nb to about 10 for the Nb-3.5-at.%-O alloy. Calculated values of ${H}_{c2}(4.2 \mathrm{K})$ versus atomic-percent oxygen exhibit a maximum at 2-at.% oxygen which was observed experimentally.

‘‘Amorphous’’ Ni60Nb40 has been prepared by mechanical alloying of elemental nickel and niobium powders in a laboratory ball mill in controlled environments. X‐ray diffraction was used to follow the progress of the mechanical alloying which eventually produced ‘‘amorphous’’ diffraction patterns similar to those for liquid quenched amorphous Ni60Nb40. Crystallization behavior was measured by differential scanning calorimetry for the mechanically alloyed and liquid quenched material. The differences that were observed in this behavior, and in the products of crystallization, may be attributed to impurities (especially oxygen) introduced during mechanical alloying.

Results exhibiting the influence of cold rolling fluxoid pinning in a metalic glass superconductor (Mo0.6Ru0.4)82B18 are reported. Inhomogeneous deformation up to 20% reduction in thickness does not change either the superconducting transition temperature Tc or upper critical field Hc2 but does degrade the critical current density by as much as 34% from the value in the the as‐cast condition. Possible mechanisms for this degradation of Jc are suggested. Pinning by inhomogeneities in composition or density rather than by the simple surface or unresolved second‐phase precipitates appears to be likely.

Comparison of Auger electron spectra for intergranular and transgranular areas on fracture surfaces of sintered YBa2Cu3O7-x indicates the presence of a grain-boundary layer which is copper-rich and oxygen-poor compared to the bulk. This layer is believed to be an important factor limiting the critical current density in sintered material. The composition of the grain-boundary layer is strongly affected by small changes in sample composition, as might be expected for a line compound. These small deviations from the stoichiometric composition also affect the critical current density and the electrical resistivity above the transition temperature.

The semiempirical scaling law for flux pinning, Fp=ABnc2bl (1−b)m has been tested for bronze process Nb3Sn conductors under uniaxial tension. The dependences of the bulk pinning force Fp on the upper critical field Bc2 and the reduced field, b=B/Bc2, were weakly affected by strain. However, the factor A, which depends on the number and strength of the pinning centers, and on the Ginzburg‐Landau parameter κ decreased by more than a factor of 20 when the compressive prestrain on the Nb3Sn was removed by application of external stress. The variations of Tc and Bc2 with strain suggest that the change in κ is not sufficient to account for the change in A. Therefore, it is probable that strain induces microstructural changes which affect the number and/or strength of the pinning centers.