Practical applications of high temperature superconductors

The author summarises the state-of-the-art in the use of high temperature superconductors in electrical engineering applications. Low power applications include antennas, resonators, delay lines, solid state devices for fast switching, and digital logic. High power applications include fault current limiters, machines, and power distribution (power cables). The author also discusses magnetic field production     

Intrinsic Josephson junctions in high temperature superconductors

The high temperature superconductors form natural superconducting multilayers where adjacent superconducting layers are weakly coupled by the Josephson effect. As a consequence, single crystals act intrinsically as vertical stacks of Josephson junctions. At present, intrinsic Josephson junctions are fabricated and investigated by several groups. There has been considerable progress both in production of high quality junctions and in theoretical understanding of the intrinsic Josephson effect. In this paper the basic physics of intrinsic Josephson junctions is outlined. Some examples of the present status of research are given.     

Cryogenic heat pipe for cooling high temperature superconductors

The research in this paper investigates a consumable-free method of operating a high temperature superconducting (HTS) coil in space. The HTS wire resides inside a cryogenic heat pipe which is used for isothermalization. This paper presents the design, implementation, and testing of a cryogenic heat pipe for cooling high temperature superconductors. As a proof-of-concept, an 86 cm long straight heat pipe was constructed and enclosed two straight lengths of HTS wire. The working fluid, at saturation condition, maintains a constant temperature below the HTS wire critical temperature. Testing of the heat pipe in a vacuum chamber was conducted to verify the drop in HTS resistance correlating to the wire operating in a superconducting state.     

High temperature (Tc) superconductors and their possible applications

The background of the recent discovery of high temperature oxide superconductors is given. The new class of materials poses many challenges to researchers and raises many exciting possibilities in technological and industrial applications. To physicists and chemists there are the questions of why this particular structure and type of compounds and what is the mechanism for superconductivity; to material scientists and electronic engineers there remain many questions concerning the preparation and the stability of the materials in forms suitable for thin film as well as heavy current uses.     

Specific-heat anomaly near superconducting transition in high temperature superconductors

A brief appraisal of the current state of understanding on the specific heat anomaly nearT _c in the oxide superconductors is presented. Spectacular specific heat double transitions are explained with plausible models.     

Role of apex oxygen and redox elements in high temperature superconductors

The presence of valence fluctuating redox ions such as Pb, Tl, Sb, etc can enhance the transition temperature of the new oxide superconductors. We propose the possibility of the existence of negative Hubbard parameter (U) on redox ion which results in effective negativeU on apex oxygen bridging redox ion and the CuO₂ planes. This enhancesT _c .     

Progress in research and development for high temperature and lowtemperature superconductors

It is noted that the most significant result in the development of Nb-Ti conductors in the last few years is the attainment of the critical current density J_c(5 T, 4.2 K)=(3-4)×10⁹ A/m² in industrial lots of wires. This value is close to the upper limit for traditional thermomechanical treatments with low-temperature annealing. New results on Nb₃Sn and other A-15 compounds have been obtained using methods that overcome some weak points of the bronze process. However, the bronze technology will remain the most suitable for large-scale production. Active work on the first stage of HTSC (high-temperature superconductor) development has produced some promising results. Critical current density values at 4.2 K and 77.3 K have been obtained in bulk samples such that practical applications of HTSC in high current technologies can be seriously considered     

Shock wave synthesis and processing of high temperature superconductors with defect microstructures

The tetragonal (T _c=55 K) and orthorhombic (non-superconducting) Tl₂Ba₂CuO₆ (Tl 2201) phases have been synthesized by aca 20 GPa planar shock wave in a microsecond time frame. Because of local rapid quench rates (up to 10⁶ K/s) defects related to metastable phases are frozen in the sample. In the 2201 phase the predominant defect identified by high resolution lattice imaging corresponds to a Tl-Ba-Cu-Tl-Cu (Tl 1212) phase which is metastable but can be synthesized via a low temperature reaction. Defects of this type may account for the enhanced flux trapping observed in the material using field modulated microwave absorption. Attempts at shock synthesizing the more complex Tl₂Ba₂CaCu₂O₈ and Tl₂Ba₂Ca₂Cu₃O₁₀ phases are also discussed. Shock processing pre-synthesized YBa₂Cu₃O₇ powder in the radial geometry followed by an O₂ anneal at 890 C produces near-theoretical density cylinders that sustain inter-grain critical currents at zero field of 1350 and 750 amps/cm² at 60 and 77 K respectively. The magnetic field dependence of the critical current and the magnitude of flux-pinning in this material relative to sintered pellets are enhanced by a factor of 2–3. Preliminary investigations of the defect microstructure of this material are discussed.     

Electron microscopic observations of Bi and Tl bearing cuprate high temperature superconductors

The second generation of high temperature superconductors typified by Bi₂Sr₂Ca _n−1Cu _n O_2n+4 and Tl₂Ba₂Ca _n−1Cu _n O_2n+4 exhibits curious structural properties which have direct relevance to the superconducting behaviour particularly transition temperature (T _c ). The present paper reports on investigations of structural properties at microlevel in Bi-bearing HTCS. We have found curious structural characteristics which manifests itself in the form of transformation froma _p ×a _p ×c to (2)^1/2 a _p ×(2)^1/2 a _p ×c through the loss of calcium atoms and the formation of five-fold modulated phase alongb through the loss of Ca and Sr atoms. We have also found the evidence of high periodicities (n=4) Bi₂Sr₂Ca₃Cu₄O₁₂. The microstructural characteristics of HTCS showing the higherT _c (R=0) ∼ 120 K exhibits unusual characteristics.     

Nickel based hardfacing alloys for high temperature applications

Abstract

Conventional nickel based hardfacing alloys deposited by arc welding usually have a nominally single phase microstructure and derive their hot strength primarily from solid solution strengthening. The present work is an attempt at designing improved alloys containing large volume fractions of ordered precipitates or intermetallic compounds. The alloy design has been carried out using a computer model capable of estimating microstructure and strength as a function of many variables. The results are tested experimentally against cast samples which simulate welding conditions during manual metal arc welding.

MST/1035     

Evaluation of high temperature dielectric films for high voltage power electronic applications

Three high temperature films, polyimide, Teflon perfluoroalkoxy and poly-P-xylene, were evaluated for possible use in high voltage power electronic applications, such as in high energy density capacitors, cables and microelectronic circuits. The dielectric properties, including permittivity and dielectric loss, were obtained in the frequency range of 50 Hz to 100 kHz at temperatures up to 200°C. The dielectric strengths at 60 Hz were determined as a function of temperature to 250°C. Confocal laser microscopy was performed to diagnose for voids and microimperfections within the film structure. The results obtained indicate that all films evaluated are capable of maintaining their high voltage properties, with minimal degradation, at temperatures up to 200°C. However, above 200°C, they lose some of their electrical properties. These films may therefore become viable candidates for high voltage power electronic applications at high temperatures.     

AC-measured shifts inT c of high temperature superconductors: The effect of interlayer structure

The apparentT _c of some high temperature superconductors such as BiSrCaCuO and TlBaCaCuO measured by low (10–20,000 Hz) a.c. inductance techniques is shifted downward by up to 30K in the presence of modest (400 Oe) d.c. magnetic fields. Dc magnetization (SQUID) measurements of the same samples show no such shifts in the same magnetic fields. Other materials like YBaCuO and TlPbSrCaCuO do not show such shifts ofT _c in a.c. inductance measurements in d.c. magnetic fields. While BiSrCaCuO and TlBaCaCuO are known to exhibit flux creep/flux flow phenomena resulting in low irreversibility line behaviour as measured by both a.c. and SQUID techniques, the aboveT _c shift is apparently a new phenomenon. Materials exhibiting this property contain two or more layers of metal ions between the CuO conductance layers (2212, 2223 structure). Materials with only one metal ion layer between the CuO layers (123, 1223 structure) do not show the apparentT _c shift, suggesting that the phenomenon is a result of the properties of magnetically-isolated CuO layers. Implications of the latter will be discussed.     

Silicon carbide and diamond for high temperature device applications

The physical and chemical properties of wide bandgap semiconductors silicon carbide and diamond make these materials an ideal choice for device fabrication for applications in many different areas, e.g. light emitters, high temperature and high power electronics, high power microwave devices, micro-electromechanical system (MEMS) technology, and substrates. These semiconductors have been recognized for several decades as being suitable for these applications, but until recently the low material quality has not allowed the fabrication of high quality devices. Silicon carbide and diamond based electronics are at different stages of their development. An overview of the status of silicon carbide's and diamond's application for high temperature electronics is presented. Silicon carbide electronics is advancing from the research stage to commercial production. The most suitable and established SiC polytype for high temperature power electronics is the hexagonal 4H polytype. The main advantages related to material properties are: its wide bandgap, high electric field strength and high thermal conductivity. Almost all different types of electronic devices have been successfully fabricated and characterized. The most promising devices for high temperature applications are pn-diodes, junction field effect transistors and thyristors. MOSFET is another important candidate, but is still under development due to some hidden problems causing low channel mobility. For microwave applications, 4H-SiC is competing with Si and GaAs for frequency below 10 GHz and for systems requiring cooling like power amplifiers. The unavailability of high quality defect and dislocation free SiC substrates has been slowing down the pace of transition from research and development to production of SiC devices, but recently new method for growth of ultrahigh quality SiC, which could promote the development of high power devices, was reported. Diamond is the superior material for high power and high temperature electronics. Fabrication of diamond electronic devices has reached important results, but high temperature data are still scarce. PN-junctions have been formed and investigated up to 400 ^∘C. Schottky diodes operating up to 1000 ^∘C have been fabricated. BJTs have been fabricated functioning in the dc mode up to 200 ^∘C. The largest advance, concerning development of devices for RF application, has been done in fabrication of different types of FETs. For FETs with gate length 0.2 μ m frequencies f_T = 24.6 GHz, f_{max (MAG)} = 63 GHz and f_{max (U)} = 80 GHz were reported. Further, capacitors and switches, working up to 450 ^∘C and 650 ^∘C, respectively, have also been fabricated. Low resistant thermostable resistors have been investigated up to 800 ^∘C. Temperature dependence of field emission from diamond films has been measured up to 950 ^∘C. However, the diamond based electronics is still regarded to be in its infancy. The prerequisite for a successful application of diamond for the fabrication of electronic devices is availability of wafer diamond, i.e. large area, high quality, inexpensive, diamond single crystal substrates. A step forward in this direction has been made recently. Diamond films grown on multilayer substrate Ir/YSZ/Si(001) having qualities close those of homoepitaxial diamond have been reported recently.     

Cobalt oxide-iron oxide selective coatings for high temperature applications

Cobalt oxide-iron oxide coatings on stainless steel have been prepared by spray pyrolysis technique. These coatings have absorptance (α) = 0·94 and emittance (ε ₁₀₀) = 0·20 which are much better values than for cobalt oxide or iron oxide alone on stainless steel substrates. The coatings have been found to be stable for temperatures upto 400°C.