High temperature superconductivity


Sohd hydrogen is known to undergo phase transitions as the pressure is increased. One phase of sohd hydrogen that is postulated to exist under conditions of extreme pressure is metallic hydrogen (64—66). Metallization of hydrogen at extremely high pressures was first predicted from theory in 1935. MetaUic hydrogen, predicted to have unusual properties, including very high temperature superconductivity, could store 100 times more energy compared with the same mass of Hquid hydrogen.  [c.414]

The stereochemical preference of the copper(II) ion is square planar or distorted octahedral because of the ligand field stabilization that arises from the electronic configuration. This perturbation ia an octahedral symmetry is known as Jahn-TeUer distortion. Other configurations that occur for copper(II) iaclude distorted tetrahedrons as well as a variety of five coordinate stmctures. Most copper(II) compounds are blue or green ia color and exhibit a variety of magnetic phenomena (1). The majority of copper(II) compounds exhibit paramagnetic behavior as a result of the single unpaired 2d electron. There are, however, a significant number of polynuclear copper compounds that are sufficiendy condensed to show spia—spia coupling of the unpaired electrons. This spia pairing may be so weak as to be observed only at near absolute zero temperatures or it may be strong enough to render the compound diamagnetic at room temperature or above. There have been reports of ferromagnetic polynuclear compounds as well. Probably the most significant, has been the high temperature superconductivity of copper oxide-containing materials (2,3).  [c.253]

The intimate mix of basic and technological approaches to the study of high-temperature superconductors, indeed their inseparable nature, was analysed recently by a group of historians of science, led by the renowned scholar Gerald Holton (Holton et al. 1996). Holton et al. conclude that historical study of cases of successful modern research has repeatedly shown that the interplay between initially unrelated basic knowledge, technology and products is so intense that, far from being separate and distinct, they are all portions of a single, tightly woven fabric. This paper belongs to a growing literature of analysis of the backgrounds to major technical advances (e g.. Suits and Bueche 1967, TRACES 1968). Holton s analysis is timely in view of the extreme difficulties of applying high-temperature superconductivity to practical tasks (see a group of papers introduced by Goyal 1995).  [c.280]

The metal has unusual superconductive properties. As little as 1 percent gadolinium improves the workability and resistance of iron, chromium, and related alloys to high temperatures and oxidation.  [c.188]

Rhenium hexafluoride is a cosdy (ca 3000/kg) material and is often used as a small percentage composite with tungsten or molybdenum. The addition of rhenium to tungsten metal improves the ductility and high temperature properties of metal films or parts (11). Tungsten—rhenium alloys produced by CVD processes exhibit higher superconducting transition temperatures than those alloys produced by arc-melt processes (12).  [c.233]

Finally, glass-ceramics will play a key role in the growing arsenal of advanced materials, both alone and in combinations with other materials. For example, glass-ceramic composites incorporating ceramic fibers yield high temperature strengths superior to metal alloys, and also exhibit gradual failure behavior similar to that of metals and plastics, as opposed to the normal catastrophic behavior common to brittie ceramics (5) (see Composite materials, CERAMiC-MATRix). Low dielectric-constant glass-ceramics are projected as the best candidates for high performance multilayer packaging materials. Other potential products include superconducting glass-ceramics, bioceramics for bone implants and prostheses, durable glass-ceramics for waste disposal, and refractory, corrosion-resistant glass-ceramic coatings for superalloys.  [c.326]

Other Alloys. Other alloying ranges include the aluminides, TiAl [12003-96-2] and Ti Al the superconducting alloys, Ti—Nb type the shape-memory alloys, Ni—Ti type and the hydrogen storage alloys, Fe—Ti (see Shape-MEMORY alloys). The aluminides TiAl and Ti Al have excellent high temperature strengths, comparable to those of nickel- and cobalt-base alloys, but having less than half the density. The Ti Al-type alloys exhibit ultimate strengths of 1 GPa (145,000 psi), 800 MPa (116,000 psi) yield, 4—5% elongation, and 7% reduction in area. The TiAl type alloys have lower ductihty and toughness but maintain thek strengths to 800—900°C. The modulus of elasticity is high, at 125—165 GPa ((18-24) x 10 psi), and oxidation resistance is good (51). The aluminides are intended for both static and rotating parts in the turbine section of newer gas-turbine akcraft engines.  [c.108]

Superconductivity is pardy typified by a perfect metallic conductor that has no resistance to current dow below a critical transformation temperature (T). Besides the disappearance of electrical resistance, there is an expulsion of magnetic dux described as the Meissner effect. The BCS theory explains traditional superconductivity based on the concept of Cooper pairs. The electrons interact with phonons and attract each other, resulting in a lower combined energy less than the Fermi energy of the normal conduction electrons. The Cooper electron pairs move in the metal in such a way that at equihbtium the combined momenta are unchanged. Normal scattering effects, therefore, cause no effect on the forward momentum of the electron population accelerating in an electrical field. In spite of the success of the BCS theory for low temperature superconductors, the high temperature oxide superconductors do not conform well to the prediction of this model. Specifically, the BCS theory does not predict a T greater than 30 K or the existence of weak or no isotope effects.  [c.360]

Cesium is ideally suited for use in magnetohydro dynamic (MHD) power generation. The metal can be used as the plasma seeding agent in closed-cycle MHD generators using high temperature nuclear reactors (qv) as the primary heat source. However, open-cycle MHD offers considerable potential for increasing the efficiency of fossil fuel fired power plants from 30—35% to 45—50%. Hot combustion gases are seeded using cesium oxide or cesium carbonate, potassium carbonate, or a mixture to form a highly conductive plasma that is accelerated through a magnetic field channel, ideally a superconducting magnet, and an electric current is generated at right angles to both the flow of plasma and the magnetic field. The off-gases thereafter pass to a conventional power generator. One of the significant potential side benefits of this process is the scmbbing of sulfur from the off-gases by the seeding material. Potassium carbonate is considerably cheaper but also much less effective than the cesium compounds the use of a mixture of the salts has been proposed to be the best choice (56) (see Plasma technology).  [c.378]

In spite of its long history, it was not until 1957 that Bardeen, Cooper and Schrieffer ) provided a satisfactory explanation of superconductivity. This BSC theory suggests that pairs of electrons (Cooper pairs) move together through the lattice, the first electron polarizing the lattice in such a way that the second one can more easily follow it. The stronger the interaction of the two electrons the higher Tc, but it turns out as a consequence of this model that should have an upper limit 35 K. The advent of high-temperature superconductors therefore necessitated a new, or at least modified, explanation for the pairing mechanism. Various suggestions have been made but none has yet gained universal acceptance.  [c.1183]

The optimism about inexpensive superconductivity was stimulated by two notable discoveries. In 1986 Alex Muller and Georg Bednorz discovered a new class of superconductors, ceramic in form, having a critical temperature of 35 K. In 1987 Paul Chu produced a compound that became superconducting at 94 K. While 94 K is still a vei-y low temperature, it is easily and inexpensively attainable with liquid nitrogen (77 K). Materials with critical temperatures as high as 135 K have been found and there is speculation that someday a material may be discovered that is superconducting at room temperature. Because the critical temperatures of these superconductors are considerably higher than that of their metallic counterparts, the phenomenon has been labeled Ingh-temperature superconductivity or HTS.  [c.1100]

Oxygen is the low-Z diatomic which is known to transfonn to a metal and, at about 1 K, a superconductor at high pressures. The transition pressure is slightly greater than 100 GPa [7]. The conductive phase consists of O2 molecules that is, it is not an atomic phase. Optical, infrared and visual spectral, and x-ray diffraction data show that the relevant e phase of oxygen is very anisotropic, and it is reasonable to conjecture that the electrical conductivity also depends upon crystallographic orientation. The other group VI elements also have metallic, superconductive phases at high pressures and low temperatures.  [c.1960]

Magnets in high field nmr spectrometers are cryostats having niobium alloy wound solenoids, which are superconducting at Hquid helium temperatures (4 K) (see Magnetic MATERIALS Niobiumand niobium compounds Superconductingmaterials). Because the wire has no resistance at this temperature, the magnet does not require additional energy to maintain constant current and hence constant field strength. In modem nmr spectrometers the magnetic field strength, B, ranges from 1.4—17.6 T (14—176 kG), corresponding to observation frequencies for of 60—750 MH2. Three critical considerations for these magnets are field strength, field stabiUty (drift), and field homogeneity. The advantages of higher field strength are better sensitivity and resolving power. However, in selecting a new instmment these properties must be balanced against cost. As of this writing (1994) a good rule of thumb for the cost of an nmr spectrometer up to 500 MH2 is approximately 1000/MHz for a basic spectrometer. The 6OO-MH2 and 750-MH2 spectrometers typically cost about 1.0 and 2.5 x 10 , respectively.  [c.401]

Ideally a standard cell is constmcted simply and is characterized by a high constancy of emf, a low temperature coefficient of emf, and an emf close to one volt. The Weston cell, which uses a standard cadmium sulfate electrolyte and electrodes of cadmium amalgam and a paste of mercury and mercurous sulfate, essentially meets these conditions. The voltage of the cell is 1.0183 V at 20°C. The a-c Josephson effect, which relates the frequency of a superconducting oscillator to the potential difference between two superconducting components, is used by NIST to maintain the unit of emf. The definition of the volt, however, remains as the Q/A derivation described.  [c.20]

The Bq field is created by a large-diameter solenoidal-shaped superconducting magnet. The gradient fields are created by room temperature gradient coils located within the bore of the magnet. These coils are driven by high current audio frequency amplifiers. The B field is introduced into the patient by means of a large LC circuit which surrounds the anatomy to be imaged (58). The same or a separate LC circuit is used to detect the signals from the precessing spins in the body.  [c.55]

Properties. Pure thorium metal is a dense, bright silvery metal having a very high melting point. The metal exists in two allotropic modifications. Thorium is a reactive, soft, and ductile metal which tarnishes slowly on exposure to air (12). Having poor mechanical properties, the metal has no direct stmctural appHcations. A survey of the physical properties of thorium is summarized in Table 1. Thorium metal is diamagnetic at room temperature, but becomes superconducting below 1.3—1.4 K.  [c.36]

Pure barium is a silvery-white metal, although contamination with nitrogen produces a yellowish color. The metal is relatively soft and ductile and may be worked readily. It is fairly volatile (though less so than magnesium), and this property is used to advantage in commercial production. Barium has a bcc crystal stmcture at atmospheric pressure, but undergoes soHd-state phase transformations at high pressures (2,3). Because of such transformations, barium exhibits pressure-induced superconductivity at sufftciendy low temperatures (4,5).  [c.471]

Tavrin, Y, Glyantsev, VN, Siegel, M Determination of the Critical Mass of Ferrous Inclusions in Nickel Base Material Using a New High-Temperature Superconducting Gradiometer, Fall Conference of the American Society of Nondestructive Testing, Oct. 20-24, 1997, Pittsburgh, USA  [c.992]

Fenni level is, however, nearly zero. These considerations are consistent with the absence of high temperature superconductivity in [60]fullerene, A Cgg and AgCgg. In conclusion, the superconducting behaviour strongly depends on the concentration of conduction band electrons, on the lattice constant and the degree of orientational order, yielding composites which display values between 2 and 40 K. The highest values that are reported are those of K- (33 K), Rb- (33 K) and Cs- (40 K, stabilized under hydrostatic pressure) doped A Cgg composites (figure Cl.2.7). Their properties may be best understood on the basis of a high average phonon frequency in combination with weak intennolecular interactions and strongly scattering intramolecular modes.  [c.2416]

Magnetic resonance imaging (MRI) uses cryogenics to cool high conductivity magnets for nonintmsive body diagnostics. Low temperature infrared detectors are utilized in astronomical telescopes. Cryogenic technology is being used to increase the speed of computers. Cryogenic refrigerators have been appHed industrially for cryopumping to yield high pumping speeds and ultrahigh vacuum. With the recent advent of high temperature superconductivity, it is anticipated that appHcations of superconductivity at near Hquid nitrogen temperature have great potential for electric power transmission, magnetic transportation systems, and magnets for energy generation in fusion processes.  [c.326]

In the fall of 1990, a new crystalline form of carbon, based on Cgo, was synthesized for the first time by Kratschmer, Huffman and co-workers [4]. Their discovery of a simple method usmg a carbon arc for preparing gram quantities of Ceo nd C70 represented a major advance to the field because previous synthesis techniques could only supply trace quantities [1, 5]. The availability of large quantities of Cgo and C70 fullerenes provided a great stimulus to this research field. It was soon found [6, 7] that the intercalation of alkali metals into solid Ceo to a stoichiometry MaCeo (where M = K, Rb) could greatly modify the electronic properties of the host fullerene lattice, yielding not only metallic conduction, but also relatively high transition temperature (18 [c.36]

The antecedents and circumstances of this research program are spelled out in some detail by Suits and Bueche (1967), two former research directors of GE, and much more recently in a popular book by Fleischer (1998). Both publications analyse why a hard-headed industrial laboratory saw fit to finance such apparently blue-sky research. Suits and Bueche say ...the research did not arise from any direct or specific need of GE s businesses and was related to them only in a general way. Why, then, was the research condoned, supported and encouraged in an industrial laboratory The answer is that a large company and a large laboratory can invest a small fraction of its funds in speculative ventures in research these ventures promise, however tentatively, departures into entirely new businesses. This research met no recognised pre-existent need indeed, to adopt my preferred word, it was a pure parepisteme. A recent historical study of a number of recent practical inventions, with a focus on high-temperature superconduction (Holton el al. 1996) concludes ... above all, historical study of cases of successful modern research has repeatedly shown that the interplay between initially unrelated basic knowledge, technology and products is so intense that, far from being separate and distinct, they are all portions of a single, tightly woven fabric .  [c.402]

Off-Axis M g netron Sputtering. When the substrate faces the target, the technique is referred to as on-axis sputtering. This technique gives the fastest film growth rate. However, this arrangement and low pressures generally yield poor quahty films, particularly in terms of stoichiometry and defects for multicomponent materials. The off-axis geometry, schematically shown on Figure 3a, overcomes these problems and has been used to make high quahty high temperature ternary superconducting oxide films (see Superconductingmaterials). The substrates ate also mounted on a heater—holder which is outside the region of direct on-axis ion flux but still within the outer edge of the plasma region. The atoms that deposit on the substrate are generally low energy thermalized neutral atoms and the film stoichiometry matches the target. The off-axis arrangement convenient for the preparation of multilayet magnetic stmetures is shown in Figure 3b.  [c.390]

Many improvements have occurred in magnetic separators, especially in the high intensity and high gradient units. Many powerfiil magnet materials have also been developed, eg, neodymium—iron—boron. Magnetic separators using a superconducting magnet have been used (Fig. 16c), eg, processing of kaolin clays (2,51). The industrial units operate at temperatures near absolute zero, achieve magnetic fields of 5 T, and are characterized by greatly reduced power costs (up to 90% over conventional units), high capacities (20—40 t/h), and superior performance. These types of separators are expected to gain in importance as advances in high temperature superconductors are made. Progress has also been made in the use of magnetohydrodynamic and magnetohydrostatic methods in minerals processing (2,6,25). These methods can separate minerals by density, magnetic susceptibihty, and electrical conductivity simultaneously (see Magnetohydrodynamics).  [c.410]

A phase diagram, with notation for the titanium—hydrogen system, is available, as is a review of kinetic data and physical properties (16). The phases may be conveniendy described by reference to pure metallic titanium, which at room temperature and pressure has a hexagonal close-packed (hep) stmeture (a-titanium). At high temperatures, this converts to a body-centered cubic (bcc) stmeture ( P-titanium). At high pressures, the hep a-phase converts to a hexagonaHy distorted bcc stmeture, CO. In the titanium—hydrogen system, the a-phase, ie, the soUd solution of H in hep titanium, exists up to ca 0.12 atom % at 25°C and up to 7.9 atom % (TiHqq ) between 300 and 600°C. Its formation has negligible effect on the titanium s lattice parameters, but whether the hydrogen atoms (radius 41 pm) occupy tetragonal (34 pm) or octahedral (62 pm) interstices within the lattice remains unclear. The region of stabihty of the P-phase H solution in bcc titanium extends from 0 atom % H at 882°C to ca 50 atom %, ie, a stoichiometry corresponding to TiH, at ca 300°C. In this case, hydrogen solubiUty expands the P-titanium lattice and the hydrogens have been shown to occupy tetrahedral sites. An fee 5-phase exists as a mixture with the a or P soUd solutions over a wide range of hydrogen concentrations, but only as a single phase region between 57 and 64 atom % hydrogen (TiH 94) at room temperature. Above 64 atom %, and at relatively low temperatures, variously reported as 20—41°C, transformation from this fee phase to a tetragonaHy distorted fee stmeture, the S-phase, takes place in neat stoichiometric T1H2. In the S-phase, the H atoms are symmetrically located in tetrahedral interstices. In addition, a metastable y-phase with a stoichiometry TiH has been identified and is formed when the a-phase containing 1—3 atom % H is cooled. A phase transition from a to the hexagonaHy distorted bcc CO stmeture occurs under high pressures of hydrogen, eg, 4 GPa (40 kbar) for TiHq 23- By quenching this phase to 90 K, a metastable superconducting phase (T = 4.3 K) is formed.  [c.117]

Other uses of CdS take advantage of its semiconducting properties. It is an n-ty e semiconductor with a band gap (wurt2ite phase) at 2.58 eV (480 nm). It is used as a thin-film cell to convert solar energy to electrical power, is a photoconductor, and is electroluminescent (40). These properties have found use in phosphors, photomultipHers, radiation detectors, thin-film transistors, diodes and rectifiers, electron-beam pumped lasers, and smoke detectors (35). CdS, when shock fractured by pressure release from the cubic phase, has been reported to be a high temperature superconductor having a critical temperature for conversion to the superconductive state in excess of 190 K (43). Finally the nonlinear optical properties of coUoidal suspensions of CdS in glass or polymer matrices have been explored as possible light transistors for optical computing appHcations (32).  [c.396]

High Temperature Ceramic Superconductors. In 1986, it was reported that the compound (La, Ba)2Cu04 displayed the transition to the superconducting state at a temperature of - 35 K. To be superconducting, a material must display two effects. First, it must display the Meissner effect at temperatures below T, the superconducting transition temperature. That is, the material must become diamagnetic and expel an externally appHed magnetic field. Second, the electrical resistivity must decrease to 2ero at temperatures below T.  [c.346]

Superconductivity The physical state in which all resistance to the flow of direct-current electricity disappears is defined as superconductivity. The Bardeen-Cooper-Schriefer (BCS) theoiy has been reasonably successful in accounting for most of the basic features observed of the superconducting state for low-temperature superconductors (LTS) operating below 23 K. The advent of the ceramic high-temperature superconductors (HTS) by Bednorz and Miller (Z. Phys. B64, 189, 1989) has called for modifications to existing theories which have not been finahzed to date. The massive interest in the new superconductors that can be cooled with liquid nitrogen is just now beginning to make its way into new applications.  [c.1127]

There are presently four famihes of high-temperature superconductors under investigation for practical magnet appheations. Table 11-25 shows that all HTS are copper oxide ceramics even though the oxygen content may vary. However, this variation generally has little effect on the phvsical properties of importance to superconductivity.  [c.1127]

The electronic theory of metallic superconduction was established by Bardeen, Cooper and Schrieffer in 1957, but the basis of superconduction in the oxides remains a battleground for rival interpretations. The technology of the oxide ( high-temperature ) superconductors is currently receiving a great deal of attention the central problem is to make windable wires or tapes from an intensely brittle material. It is in no way a negative judgment on the importance and interest of these materials that they do not receive a detailed discussion here it is simply that they do not lend themselves to a superficial account, and there is no space here for a discussion in the detail that they intrinsically deserve.  [c.280]

Niobium finds use in the production of numerous stainless steels for use at high temperatures, and Nb/Zr wires are used in superconducting magnets. The extreme corrosion-resistance of tantalum at normal temperatures (due to the presence of an exceptionally tenacious film of oxide) leads to its application in the construction of chemical plant, especially where it can be used as a liner inside cheaper metals. Its complete inertness to body fluids makes it the ideal material for surgical use in bone repair and internal suturing.  [c.978]

A schematic coalfired MHD generator in Figure 2 is shown using a combustion plasma at up to 3,000 K, seeded with alkali salts for high conductivity, fed directly into the generator with a superconducting magnet providing up to 7 tesla. The combustion products, still at about 2,000 K, then pass through an air preheater, where the hot gas exhausted from the MHD generator preheats the air for the combustor leading to the high temperatures required to create the plasma.  [c.745]

It is used extensively by the chemical industry where corrosive agents are employed. Zirconium is used as a getter in vacuum tubes, as an alloying agent in steel, in surgical appliances, photoflash bulbs, explosive primers, rayon spinnerets, lamp filaments, etc. It is used in poison ivy lotions in the form of the carbonate as it combines with urushiol. With niobium, zirconium is superconductive at low temperatures and is used to make superconductive magnets, which offer hope of direct large-scale generation of electric power. Zirconium oxide (zircon) has a high index of refraction and is used as a gem material. The impure oxide, zirconia, is used for laboratory crucibles that will withstand heat shock, for linings of metallurgical furnaces, and by the glass and ceramic industries as a refractory material. Its use as a refractory material accounts for a large share of all zirconium consumed.  [c.56]

Electrical. Unlike their crystalline counterparts, amorphous metals generally have high electrical resistivity not only at room temperature, but also, because of a very small temperature coefficient, near absolute zero. This can be understood by the atomic nature of the glass that is, the randomness of the stmcture which efficiently scatters electrons. Figure 14 illustrates the difference between amorphous and crystalline FePC (97). The resistivity of the glass has Httle temperature dependence until it crystallizes at 675 K. Certain metallic glasses, eg, LagQAu2Q (98), do show superconductivity. The critical temperature is as high as 8.7 K. MetaUic glass superconductors are also relatively insensitive to composition (99) and have very short electron mean-free paths.  [c.342]

Yttrium—barium—copper oxide, YBa2Cu202 is a newly developed high T material which has been found to be fully superconductive at temperatures above 90 K, a temperature that can be maintained during practical operation. The foremost challenge is to be able to fabricate these materials into a flexible form to prepare wines, fibers, and bulk shapes. Ultrapure powders of yttrium—barium—copper oxide that are sinterable into single-phase superconducting  [c.482]


See pages that mention the term High temperature superconductivity : [c.657]    [c.644]    [c.360]    [c.359]    [c.59]    [c.1127]    [c.280]    [c.927]    [c.253]    [c.1183]    [c.1474]    [c.1558]    [c.239]    [c.26]   
Chemistry of the elements (1998) -- [ c.945 , c.1232 ]