Review of Thermal Boundary Resistance of High-Temperature Superconductors

Solid-solid thermal boundary resistance plays an important role in the thermal stability of many electronic circuits, microdevices, and superconducting devices. The thermal boundary resistance (R _b ) at any interface causes a temperature discontinuity, which can result in heat accumulation on one side of the boundary and raise the temperature much above the stable region, causing device failure. With the advent of high-critical-temperature (high-T _c ) superconductors, it is possible to make superconducting devices at practically achievable temperatures. As the current trend goes toward the development of more and more high-T_c superconducting devices, the need for a better understanding of the thermal boundary resistance of high-T_c superconductors becomes mandatory. This paper compiles all the theoretical and experimental work to date onR _b in high-T_c superconductors, both in thin-film and bulk forms, and provides a critical review of the cited works. This paper also describes the possible effect of the superconducting state onR _b for high-T _c superconductors, based on the experiments for both high-T_c and low-T_c bulk superconductors, and a possible explanation for these data based on the existing theory for low-T _c superconductors.     

Effects of hot pressing temperature on microstructure,hardness and corrosion resistance of Al2NbTi3V2Zr high-entropy alloy

A novel lightweight high-entropy alloy Al₂NbTi₃V₂Zr was fabricated by vacuum hot pressing. The effects of sintering temperature (1200–1550°C) on the microstructure, hardness and corrosion resistance of the alloy were investigated. Results showed that Al₂NbTi₃V₂Zr mainly consisted of simple cubic matrix and (Zr, Al)-based intermetallic phase (α-phase) at sintering temperatures of 1200–1350°C. Moreover, the matrix phase transformed from simple cubic to body-centred cubic phase, and (Ti, Zr, Al)-based intermetallic precipitated from the matrix at temperature of 1450°C. The fabricated Al₂NbTi₃V₂Zr alloy had low density of 5.05–5.23?g?cm^–3, high hardness of 510–728?HV and excellent corrosion resistance in 10?wt-% HNO₃ solution.     

Effect of rare-earth (Eu, Yb and Ag) substitutions on superconducting properties of the Bi1.7Pb0.3 Sr2Ca2−xRx(R = Eu, Yb, and Ag)Cu3OY system

The superconducting properties of Bi^1.7Pb_0.3Sr₂Ca^2–xR^x(R = Eu, Yb and Ag)Cu₃O_Y have been investigated by X-ray diffraction, electrical resistance and the peritectic transition of these superconductors was studied kinetically under different atmospheres and temperature gradients. X-ray diffraction results show that the volume fraction of the high-T_c (2 2 2 3) phase decreases and that of the low-T_c (2 2 1 2) phase increases as Eu, Yb and Ag concentrations increase. The resistivity measurements reveal that the T_c onset decreases down to 100 K for Eu, 85 K for Yb and 106 K for Ag concentrations. These results are explained on the basis of possible variations of hole concentration with trivalent rare earth ion substitutions. Activation energies and frequency factors for crystallisation were determined by non-isothermal differential thermal analysis (DTA), employing different models. It was found that both peritectic transition and reaction rate were dependent on the ambient atmosphere. Kinetic studies under different atmospheres revealed that the thermal stability of Bi-2 2 1 2 phase was greatly enhanced under oxygen atmosphere.     

The incommensurate solid solution phase Na5−4x Zr1+x(PO4)3:0.04 <x <0.15

The orthophosphate solid solution phase, Na_5?4x Zr_1+x(PO₄)₃:0.04 ? x ? 0.15 has trigonal symmetry with an apparent one dimensional incommensurate superstructure parallel to c_HEX. Using selected area electron diffraction patterns as a guide, an indexing scheme for the powder X-ray data has been devised. The parameter $\text{k = c}{}_{\mathrm{<mtext>supercell</mtext>}}\text{c}{}_{\mathrm{<mtext>subcell</mtext>}}$ varies smoothly with composition from ～ 10.4 at x = 0.04 to ～4.4 at x = 0.11 and is believed to originate in ordering of the extra interstitial Zr⁴⁺ ions. The Na⁺ ion conductivity increases gradually with x and for x = 0.108 varies from ～5×10^?8 ohm^?1 cm^?1 at 25°C to ～1×10^?3 ohm^?1 cm^?1 at 300°C.     

Magnetic field dependence of the critical current density for the bismuth-based bulk high-Tc superconductors

Three types of bismuth-based bulk samples were prepared through uniaxial pressing at room temperature, hot isostatic pressing (HIP) and drawing and rolling. Transport current properties were characterized in a steady field up to 1.12 T at 77 K (T/T _c=0.75). The Josephson weak-link decoupling fields have been found to be 5 mT for the cold-pressed pellet and 30 mT for the HIPed pellet and the rolled tape. At the decoupling field the transport critical current density,J _c, drops 80% from 124 (OT) to 29 A cm^–2 (5 mT) for the cold-pressed pellet, 80% from 582 (OT) to 126 A cm^–2 (30 mT) for the HIPed sample and 50% from6500 (0 T) to 2850 A cm^–2 (30 mT) for the rolled tape. In the flux flow regime, whereB is perpendicular to thec-axis a modified Kim's modelJ _c=(/B ₀)/[(1+B/B ₀)] ⁿ can be used to describe the field dependence of the critical current density, J_c, in the field range 0.2–1.12 T. The effective upper critical fields were estimated to be 0.98, 1.54 and 1.94 T for the three types of samples, respectively. An adjustable range ofB _c2 for bismuth-based bulk highT _c superconductors is given. Flux shear may operate in these materials. The prediction of this pinning mechanism is yielded from fitting the equation qualitatively. WhenB is parallel to thec-axis, the absence of strongly intragranular flux-pinning is emphasized by the poor flux flow regime for the rolled tape sample.     

Melt-textured Bi1.6Pb0.4Sr2Ca2Cu3O11 bulk superconductors fabricated in a simple tube furnace

Using the simplified version of the melt-textured growth (MTG) technology with a simple tube furnace, we have fabricated superconductors satisfying the nominal composition Bi_1.6Pb_0.4Sr₂Ca₂Cu₃O₁₁. We would note that the material used in the fabrication was prepared by mixing a precursor Bi_1.6Pb_0.4Sr₂Cu₃O _x and CaO powder. This two step technique was found to be superior to the single step solid state reaction method after many trials. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) morphologies show that the melt-textured bulk samples are made up of stacks of highly textured single crystal-like layers. X-ray analysis as well as d.c. magnetization measurements were carried out and theJ _c value was found to be 1.3×10³ A cm^–2 at 77 K using the Bean model. At this stage, thoughJ _c is not so high as that of the best samples obtained from other complicated methods involving special (hot) pressing and sintering techniques, we do not need to apply any mechanical treatment at or after the heating procedure.     

Role of diffusion-annealing temperature on the microstructural and superconducting properties of Cu-doped MgB2 superconductors

This study deals with not only investigate the effect of the copper diffusion on the microstructural and superconducting properties of MgB₂ superconducting samples employing dc resistivity as a function of temperature, scanning electron microscopy (SEM) and X-ray diffraction (XRD) measurements but also calculate the diffusion coefficient and the activation energy of copper for the first time. Electrical-resistivity measurements indicate that both the room-temperature resistivity value and zero resistivity transition temperatures (T _c ) increase with increasing the diffusion-annealing temperature from 650 to 850?°C. SEM measurements show that not only the surface morphology and grain connectivity improve but also the grain size of the samples increases with the increase in the diffusion-annealing temperature up to 850?°C. As for the XRD results, all the samples contain the MgB₂ phase only and exhibit the polycrystalline superconducting phase with more intensity of diffraction lines, leading to the increasement in the lattice parameter a and c. Additionally, the diffusion coefficient is observed to increase from 6.81?×?10^?8 to 4.69?×?10^?7?cm²?s^?1 as the diffusion-annealing temperature increases, confirming that the Cu diffusion at lower temperatures is much less significant. Temperature dependence of the Cu diffusion coefficient is described with the aid of the Arrhenius relation D?=?3.75?×?10^?3 exp (?1.15?±?0.10?eV/k _B T) and the corresponding activation energy of copper in MgB₂ system is found to be about 1.15?eV. The possible reasons for the observed improvement in microstructural and superconducting properties of the samples due to Cu diffusion are also discussed.     

Superconducting Properties and Microstructure of Nanocrystalline MgB2 Bulk Prepared by Sintering of Ball-Milled Powders

Ultrafine nanostructured MgB₂ bulks with an average grain size less than 10 nm have been fabricated by high-energy ball milling and subsequent high pressure sintering. Microstructural evolution in MgB₂ subjected to high-energy ball milling has been investigated by means of X-ray diffraction (XRD). The finer grain size of MgB₂ powders of about 7 nm has been estimated from Rietveld refinement analysis of XRD data, which is confirmed by a transmission electron microscope (TEM) observation. There is almost no grain growth in the subsequent sintering at low temperature of 600?°C under pressure of 3?C5 GPa for 10?C30 min. The nanocrystalline MgB₂ bulks exhibit the lower onset critical transition temperatures (T _{c onset}) of 32?C33?K. The relative wider width of the magnetic hysteresis loops at high external magnetic field and the higher critical current density (J _c ) are obtained in nanocrystalline bulks. J _c is as high as 10⁵?A/cm² in 8?T at 10?K and 2.7×10³?A/cm² in 4?T at 20?K.     

Preparation and electrical transport properties of In filled and Te-doped CoSb3 skutterudite

Nanopowders with nominal compositions of Co₄Sb_11.5Te_0.5 and In_0.5Co₄Sb_11.5Te_0.5 were prepared via hydrothermal synthesis at 180 °C for 48 h, then heat treated and finally hot pressed at 625 °C and 80 MPa for 1 h in vacuum to form bulk samples. The phase compositions of the samples were determined by X-ray diffraction. Hall Effect measurement of the samples was carried out at room temperature. The fracture surface of the samples was observed by field emission scanning electron microscopy. The electrical conductivity and the Seebeck coefficient of the samples were measured from room temperature to around 748 K. The In-filled and Te-doped CoSb₃ sample with longer time annealing before hot pressing had much better electrical transport properties with the highest power factor of 38.4 μWcm^?1 K^?2 around 573 K.     

Effect of HIPing on conductivity and impedance measurements of DyBi5Fe2Ti3O18 ceramics

X-ray diffraction, a.c. impedance and conductivity (a.c. and d.c.) have been used to characterize DyBi₅Fe₂Ti₃O₁₈. Samples were prepared by solid state double sintering method. A few samples were also subjected to hot isostatic pressing (HIP) at 800°C for 2 h at 100 MPa pressure. The data on XRD, impedance and conductivity of two sets of samples are compared to understand study of effect of HIPing on the properties of DyBi₅Fe₂Ti₃O₁₈     

Superconductivity of 2212 phase in Bi-based high-T c superconductors

In the Bi-based high-T _c superconductors, three superconducting transition points were observed above the liquid-N₂ temperature range. Allotropes of the 2212 phase were found. These allotropes were metastable and can interchange with the 2212 phase, and theirT _c's vary from ～85 to ～100 K.     

A method for preparation of a high-quality high temperature superconducting ceramics

An improved method for hot pressing of high temperature superconducting (HTSC) powders prepared by conventional solid phase synthesis of the initial BaCO₃, CuO and Ln₂O₃ allows one to obtain HTSC targets and magnetic shields possessing a high degree of homogeneity. By sputtering such targets HTSC films with critical current density of 3.3·10⁶ A/cm² have been deposited. Shields, prepared according to this method, show a shielding coefficient of 10⁵ andH _c of 79 Öe in the constant magnetic field and in the alternating magnetic field the amplitude is 90 Öe in the frequency range of 70–3000 Hz.     

Polymorphisme sous pression de Sm2Ge2O7 et identification de la nouvelle phase H de la serie des digermanates de terres rares

The pressure-temperature phase diagram of Sm₂Ge₂O₇ was determined up to 80 kbar and 1400°C. In this pressure-temperature range only two phases were found. The B″ triclinic phase synthetised at atmospheric pressure is stable up to 40 kbar and at higher pressures appears the H phase which is a new type of structure in the rare earth digermanate series. The lattice parameters of the H phase were determined on small single crystals which were grown with water as mineralizer. The symmetry of the H phase is hexagonal with a space group P 6/m m m. The cell parameters of H-Sm₂Ge₂O₇ are $\text{a}\text{= 11,26}\text{A}\text{?}$ and $\text{c}\text{= 31,80}\text{A}\text{?}$. The measured density is 7,01 g/cm³ and therefore the cell contains twenty seven Sm₂Ge₂O₇ units. The calculated density is 7,16 g/cm³. The X-ray diffraction pattern of H-Gd₂Ge₂O₇ previously obtained can be indexed with similar parameters $\text{a}\text{= 11,19}\text{A}\text{?}$ and $\text{c}\text{= 31,65}\text{A}\text{?}$.     

Reproducibility ofT c in a Bi2Sr2CaCu208 superconductor

The reproducibility ofT _c in superconductive Bi₂Sr₂CaCu₂O₈ is known to be very poor. In our study, the reproducibility was found to depend greatly on preparation conditions. DTA, TGA and powder X-ray diffraction methods were used to study the Bi₂Sr₂CaCu₂O₈ superconductors prepared by different routes. Resistivity and diamagnetic susceptibility were taken as the measure of superconductivity. It was found that those superconductors prepared from the oxides/carbonates (one-step) process resulted in scatteredT _c data which were less reproducible. The two-step synthesis from precalcined precursors of Bi-Sr-O and Sr-Ca-Cu-0 containing mixtures, resulted in a much improved reproducibility with the predominantT _c = 80 K superconducting phase (more than 95%). It was also found that the amount ofT _c = 115 K phase tends to decrease after repeated pulverization and sintering, leading to a single 80 K phase state. Compaction of the pulverized powder at a pressure > 4.5 ton cm^–2 was found to induce a preferred (00L) orientation during sintering.     

The Effects of Substitution of Sn for Cu in Bi1.75Pb0.25Sr2CaCu2.3−x Sn x Sn x O y

The influence of Sn doping on superconductivity in the Bi-based 2212 phase is studied in this paper. For the samples R–T relations and magnetic hysteresis loops were measured. X-ray powder diffraction analysis was also performed. For Bi_1.75Pb_0.25Sr₂CaCu_2.3?x Sn _x O _y , the experimental results show that by adding the proper amount of Sn the superconductivity of the samples can be improved. As x = 0.15, the critical temperature T _c, the critical current density J _c, and the magnetic pinning force density F reach a maximum. At T = 11 K, the critical state parameters H _c1, H _c2, κ, λ, and ξ are calculated and compared with the results reported by other researchers. The experimental results also show that the Sn doping is able to speed up the growth of the 2223 phase. In brief, Sn doping is an effective way of improving the superconductivity in Bi-based superconductors.     

Structural and electrical properties of V-doped ZnO prepared by the solid state reaction

This paper reports the synthesis, crystal structure and electrical conductivity properties of vanadium (V)-doped zinc oxide (ZnO) powders (i.e. Zn_1?2X V _X O binary system, x = 0, 0.0025, 0.005, 0.0075 and in the range 0.01 ≤ x ≤ 0.15). I-phase samples, which were indexed as single phase with a hexagonal (wurtzite) structure in the V-doped ZnO binary system, were determined by X-ray diffraction (XRD). The limit solubility of V in the ZnO lattice at this temperature is 3 mol % at 950 °C. The impurity phase at 950 °C was determined as ZnV₂O₆ when compared with standart XRD data. The research focused on single I-phase ZnO samples which were synthesized at 950 °C because of the limit of the solubility range is widest at this temperature. It was observed that the lattice parameters a and c decreased with V doping concentration. The electrical conductivity of the pure ZnO and single I-phase samples were studied using the four-point probe dc method at temperatures between 100 and 950 °C in an air atmosphere. The electrical conductivity values of pure ZnO and 3 mol % V-doped ZnO samples at 100 °C were 2.75 × 10^?6 and 7.94 × 10^?5 Ω^?1 cm^?1, and at 950 °C they were 3.4 and 54.95 Ω^?1 cm^?1, respectively. In other words, the electrical conductivity increased with V doping concentration.     

Effects of growth temperature on electrical and structural properties of sputtered GaN films with a cermet target

GaN films have been deposited at 100–400 °C substrate temperature on Si (100) and sapphire (0001) substrates by RF reactive sputtering in an (Ar + N₂) atmosphere. A (Ga + GaN) cermet target for sputtering was made by hot pressing the mixed powders of metallic Ga and ceramic GaN. The effects of substrate temperature on the GaN formation and its properties were investigated. The diffraction results showed that GaN films with a preferential (10–10) growth plane had a wurtzite crystalline structure. GaN films became smoother at higher substrate temperature. The Hall effect measurements showed the electron concentration and mobility were 1.04 × 10¹⁸ cm^?3 and 7.1 cm² V^?1 s^?1, respectively, for GaN deposited at 400 °C. GaN films were tested for its thermal stability at 900 °C in the N₂ atmosphere. Electrical properties slightly degraded after annealing. The smaller bandgap of ~3.0 eV is explained in terms of intrinsic defects and lattice distortion.     

Growth kinetics of the θ phase in AlCu thin film bilayers

The growth kinetics of the θ (Al₂Cu) phase in AlCu thin film bilayers were studied by MeV He⁺ backscattering and X-ray diffraction. In the temperature range 160–200°C the growth is diffusion limited with an activation energy of 1.02 eV and a pre-exponential factor of 7 × 10^?3cm²s^?1. Nucleation of the γ₂ phase occurs at 200°C with long annealing times. There is no evidence of the presence of other phases in the investigated temperature range.     

Characterization of ohmic contacts to InP

The results of a study of the electrical and metallurgical properties of thin metallic layers deposited on InP for use as ohmic contacts are presented. The layers were heat treated at temperatures up to 550°C and were examined with Auger electron spectroscopy. For contact to n-type InP three thin film systems were investigated: gold, nickel and a composite Ni/Au/Ge layer. Nickel was found to produce ohmic behavior in the Ni/Au/Ge/InP system with a minimum specific contact resistance r_c of 3×10^?5 Ω cm² for a net doping of 3×10¹⁶ cm^?3. For contact to p-type InP a film consisting of Au/Mg was investigated. For heat treatment of the Au/Mg/InP system above 350°C, r_c decreased as the temperature of the heat treatment increased and the surface morphology exhibited increasing signs of alloying at higher temperatures. The smoothest surface was obtained at 446°C for 50 min with r_c≈1×10^?4Ω cm² for a net doping of 6×10¹⁷ cm^?3.     

Superconductors of the BPSCCO system obtained from rapid cooling process

Bi-based (BPSCCO) superconductors have been extensively studied due to their interesting superconducting properties, especially those that present high transition temperature (T_c). In this work, superconductors of the BPSCCO system were prepared from rapid cooling process and studied under its structural and magnetic properties. Sample as-prepared shows an amorphous behavior, which is converted progressively into 2223 phase. This process permits the control of Pb or Bi loss and the crystallization of the desired phase using several heat annealing processes. The 2201 and 2212 phases were also observed as intermediate phases, before the crystallization of the 2223 phase. The superconductor obtained in this work presented a T_c around 77-K.