Experimental investigation on constitutive behaviour of superconducting powder BSCCO

As a superconducting material, Bi 2233/Ag tape needs high-critical transport current density J _c, which is influenced by the uniform deformation and density of BSCCO powder in filaments during the forming process. The aim of this paper is to investigate the constitutive behaviour of BSCCO powder. The modified Drucker–Prager/Cap model is introduced to describe the constitutive behaviour of BSCCO powder. A series of cyclic loading experiments for BSCCO powder in a cylinder die were carried out. Based on the experiments, the relationships between the radial stress and the axial stress were obtained, and the parameters in the constitutive model were calculated. By pushing the compact powder from the die, the coefficient of friction between the BSCCO powder and the cylinder die was determined. Finally, the modified Drucker–Prager/Cap model is proposed and used to simulate the confined compression test of BSCCO powder.     

Gas pressure combustion sintering and hot isostatic pressing in the Ti-Si-C system

The processing of materials in the Ti-Si-C system produced by gas pressure combustion sintering (GPCS) and hot isostatic pressing (HIP) was studied. By variation of the raw material composition it was possible to produce different materials containing Ti₅Si₃, Ti₃SiC₂, TiSi₂, TiC, SiC and titanium. The different phase compositions of samples with the same reactants obtained by GPCS and HIP are due to the duration of each of the processes; in GPCS the equilibrium state was not achieved. The properties of the specimen are strongly related to their chemical behaviour during processing at high temperatures. Not all phases built up at the high temperatures during the GPCS and HIP processes are stable at lower temperatures, and some reactions occur during cooling, which influence densification, microstructure and properties of the bodies finally obtained.     

Effects of sintering temperature and time on the electrical properties of pellets made from beneficiated galena ore

The effects of sintering temperature and time of PbS pellets made by cold pressing of beneficiated galena ore on the room temperature resistivity and room temperature thermoelectric power are reported. The increased resistivity of such pellets with increasing sintering time has been explained in terms of a possible reaction between PbS and PbO leading to compensation of p-type conductivity. The increase in thermoelectric power with sintering temperature and time has been explained on the basis of inhomogeneity in the sample following Kwok's approach.     

Structural modifications of the superconducting phases on Bi system by electron beam irradiation

Superconducting samples of the Bi(Pb)-Sr-Ca-Cu-O system have been irradiated in transmission electron microscopy (TEM) to investigate the effect of a relative high fluence of electrons with 75 and 100 keV energy on the microstructure of the material. The diffraction pattern images show a dramatic change from very uniform lattice spots atab crystalline planes to a circular pattern corresponding to damage and breaking of the materials in very small crystallites.     

Cathodoluminescence microscopy of high T c superconducting YBCO and BSCCO materials

Spectroscopic and imaging cathodoluminescence (CL) microscopy has been used to study high T _c YBCO and BSCCO superconducting thin films and pellets in the scanning electron microscope. The effects of beam parameters, such as voltage, current, and diameter, have been investigated with a view to optimizing the CL signal intensity whilst preventing sample damage. Limiting the CL signal generation volume to within the thin film is important in eliminating any substrate contribution. Areas of strong luminescence have been observed in YBCO and BSCCO pellets as well as BSCCO thin films. At low beam energies, there is some correlation between the CL and secondary electron images. The CL spectra of the strongly luminescent spots differed from those of the poorly luminescencing superconducting phases. CL was able to identify a copper-rich impurity phase in the BSCCO material, and a barium cuprate phase in the YBCO material. It is proposed that the quality of a thin film, with respect to impurities, can be monitored using CL.     

球磨时间对热压烧结制备TiC-CoCrFeNi复合材料微观组织及力学性能的影响

采用机械合金化-热压烧结法,制备TiC-CoCrFeNi复合材料,研究球磨时间对材料微观组织及力学性能的影响。结果表明:Co,Cr,Fe和Ni粉体在球磨10h后形成fcc结构的单相固溶体。经1200℃/1h热压烧结后,烧结体中生成TiC和Cr7C3结构的碳化物,并弥散分布于CoCrFeNi固溶体中。球磨时间显著改变了烧结体中碳化物的数量和尺寸,进而影响材料的力学性能。在球磨10h时,烧结体中纳米级TiC相急剧增多,此时复合材料的硬度(671HV)和屈服强度(1440MPa)达到最大值。     

Influence of sintering time and quenching on the formation of highT c phase

Influence of sintering time and quenching in Bi_{2 −x} Pb _x Ca₂Sr₂Cu₃O _y (x=0.0, 0.1, 0.2, 0.25, 0.3 and 0.4) samples have been studied by resistance and XRD measurements. In samples sintered at 850°C for 4 days,T _c(0) increases with Pb concentration.T _c(0) increased from 81 K forx=0.0 to 109 K inx=0.30 sample and then decreased. Increasing the sintering time to 10 days decreased theT _c Quenching further decreased theT _c(0). From X-ray diffraction patterns, the intensity peaks of low and highT _c phases have been measured. The addition of Pb promotes highT _c-phase. Sintering time, slow cooling and rapid quenching studies show that there is an optimum sintering time and cooling rate to produce a highT _c-phase.     

Role of sintering time,crystalline phases and symmetry in the piezoelectric properties of lead-free KNN-modified ceramics

Lead-free KNN-modified piezoceramics of the system (Li,Na,K)(Nb,Ta,Sb)O₃ were prepared by conventional solid-state sintering. The X-ray diffraction patterns revealed a perovskite phase, together with some minor secondary phase, which was assigned to K₃LiNb₆O₁₇, tetragonal tungsten–bronze (TTB). A structural evolution toward a pure tetragonal structure with the increasing sintering time was observed, associated with the decrease of TTB phase. A correlation between higher tetragonality and higher piezoelectric response was clearly evidenced. Contrary to the case of the LiTaO₃ modified KNN, very large abnormal grains with TTB structure were not detected. As a consequence, the simultaneous modification by tantalum and antimony seems to induce during sintering a different behaviour from the one of LiTaO₃ modified KNN.     

Effects of Sr/Ca ratio and CuO content of the superconducting phases in the Bi-Pb-Sr-Ca-Cu-O system

The Bi-Pb-Sr-Ca-Cu-O superconductor was prepared by the three-step reaction process; the effects of Sr/Ca ratio and CuO content on the formation of the superconducting phases were studied. The Sr-short and Cu-rich nominal compositions are not advantageous for the formation of the 2223 phase but we are favored for the formation of the 2201 phase. The differences among the previous reports on the effect of composition on the formation of the superconducting phase is discussed. The best J _c value of the Ag-sheathed tape filled with the kind of powder is 13,000 A/cm² (77 K, OT).     

Structure reinforcement of porous hydroxyapatite pellets using sodium carbonate as sintering aid: Microstructure,secondary phases and mechanical properties

Porous HAP pellets suitable for loading therapeutic agents were prepared using microcrystalline cellulose (MCC) as pore former and sodium carbonate as sintering aid (SAID). The effect of sintering temperature on the microstructure, mechanical properties and disintegration of pellets prepared at different SAID content was studied. Pellets were characterized by SEM, image analysis, porosimetry and surface area. Secondary phases were identified by PXRD, ATR-FTIR and Raman spectroscopy. Increasing the sintering temperature decreased the diameter, porosity, surface area and friability of the pellets but increased the pore size, tensile strength and disintegration time. The effect of SAID was dependent on sintering temperature. With 5% SAID, a secondary β-tricalcium phosphate (β-TCP) phase was formed, indicated by FTIR peak at 980 cm^?1 and characteristic PXRD reflections, whereas with 10%, a secondary B-type carbonated hydroxyapatite phase (CHA) formed, indicated by FTIR peaks at 878 and 1450 cm^?1, a broad Raman peak in the region 1020 to 1050 cm^?1 and PXRD reflections. Pellets prepared with SAID showed high strength and also porosity. The biphasic HAP/β-TCP pellets exhibited remarkably great strength (4.39 MPa) at the high sintering temperature, while still retaining 43.9% porosity. Relationships were established between the mechanical properties or disintegration time of the porous pellets and the microstructural parameters.     

热压烧结Ti2AlC粉体的研究

以自蔓延高温合成（又称燃烧合成）的Ti2AlC粉体为原料，研究了不同热压温度对Ti2AlC粉体的烧结影响。实验结果表明，热压烧结Ti2AlC粉料可得到致密Ti2AlC陶瓷，在压力25MPa，保温2h的条件下，理想热压烧结温度为1400℃，热压温度〉1450℃时Ti2AlC会发生分解，并出现Ti2AlC2相；烧结温度为1400℃时Ti2AlC烧结体理论相对密度为98．1％，维氏硬度4．14GPa，断裂韧性7.86MPa·m^1/2；烧结样品的密度和断裂韧性随烧结温度升高而增大，其微观晶粒片状尺寸随烧结温度的升高而增大。     

A theory for the superconducting phases of UPt3

I discuss a phenomenological theory for the multiple superconducting phases of UPt₃ that is based on an order parameter belonging to an orbital 2D representation of the hexagonal point group which is coupled to a weak symmetry breaking field. I show that (1) the existing H-T and P-T phase diagrams (including an apparent tetracritical point in the H-T plane for all field orientations), (2) the anisotropy of the upper critical field over the full temperature range, (3) the correlation between superconductivity and basal plane antiferromagnetism and (4) low-temperature power laws in the transport and thermodynamic properties can be explained qualitatively, and in many respects quantitatively, by an odd-parity, E_2u order parameter with a pair spin projection of zero along the -axis. AFM ordering in the basal plane, which couples to the superconducting order parameter, acts as the symmetry breaking field that is responsible for both the apparent tetracritical point and the zero-field double transition.     

A novel equal channel angular pressing design to eliminate punch deflection and to reduce the pressing force

In the present study a new design of the equal channel angular pressing die and punch is introduced. The new design eliminates the deflection of the punch and at the same time lowers the frictional forces by reducing the contact area. In order to show the capability of the new design, the Al-7075 alloy samples were processed in two equal channel angular pressing dies. The measured data for the novel and conventional equal channel angular pressing processes were compared. The microstructure of the processed samples was examined with scanning electron microscopy and transmission electron microscopy. The results of the experimental study revealed that the novel die can fully eliminate the risk of the punch deflection and simultaneously reduce the maximum required pressing force by about 3.1 % to 9.8 % for 16 % to 40 % reduction in frictional contact surfaces. The analysis of experimental results showed that there is non-linear relation between the frictional contact area and the pressing load. In addition, it is shown that despites high contact pressures and high temperatures the use of graphite as a lubricant between sliding surfaces can considerably reduce the pressing load.     

Thermal conductivity and gap structure of the superconducting phases of UPt3

We present new measurements of the thermal conductivity (κ) of UPt₃ down to very low temperatures (16 mK) and under magnetic fields (up to 4 T) which cover all the superconducting phases of UPt₃. The measurements in zero field are compared with recent theoretical predictions for the thermal conductivity, which is dominated by impurity states at the lowest temperatures studied. The measurements under magnetic field at low temperatures are surprising since they don’t show the expected low field square root dependence, . The discontinuity ofdκ/dT atT _c changes drastically when passing from the high field low temperatureC phase to the low field high temperatureA phase: this is related to the change of the symmetry of the superconducting order parameter when crossing theA→C phase transition.     

Effect of powder particle morphology on the sintering behaviour of polymers

The sintering behaviour of several amorphous and semicrystalline polymeric powders was studied. The coalescence of particles of polystyrene, PMMA and polyethylene of various molecular weights was photographically followed. The coalescence of the amorphous materials was found to depend on the common parameters affecting sintering including particle size and polymer viscosity. However, unexpectedly fast sintering was observed for the crystalline polyethylenes. The dominating factor in their coalescence, unaccounted for by the Frankel expression, is the internal particle morphology which increases the total particle surface energy. The polyethylene particles were actually found to be aggregates of small nodules, less than 1m in diameter, interconnected by a very fine fibrillar network. It is concluded that some semicrystalline high viscosity polymers, known to be unprocessable by common methods, do sinter due to their highly developed internal particle morphology.     

On the threshold pinning force in type II superconducting films

The maximum pinning force of a two-dimensional vortex lattice in a random potential is calculated. A connection is established between this threshold pinning force and the potential energy discontinuities due to elastic and plastic instabilities of the vortex lattice. Inspired by recent computer simulations, we assume that the fluctuations in the commensurability between the random potential and the vortex potential breaks the vortex system up into a set of flowing channels in between trapped regions. Two instability mechanisms and their contribution to the threshold force are discussed within this channel-flow picture. We find that three different regimes exist depending on, w, the width of the channels;w=,a ₀w=a ₀ , wherea ₀ is the vortex lattice spacing. Weak pinning superconductors can pass through all three regimes as the reduced magnetic field is varied from 0 to 1, whereas strong pinning compounds can remain in the saturated region (w=a ₀ ) for all values of the field. We compare the expression for the threshold force with experimental results for both strong and weak pinning samples. A satisfactory qualitative agreement is obtained between theory and experiment.