Study of lanthanum-deficient compositions in the systems La-Cu-O and La-Sr-Cu-O

The effect of varying lanthanum composition upon the formation of the nonsuperconducting La_1.67Sr_0.33Cu₂O_5– and the stoichiometry variation tolerance of the ternary system La_2–xSr_xCuO₄ (with x=0 and 0.15) were investigated using a variety of techniques. Results indicate that total conversion to La₂CuO₄ from freeze-dried precursors with an LaCu = 2.001 ratio is achieved after sintering (1253 K, 12 h) and annealing in oxygen (773 K, 6 h). Small variations in the stoichiometry of solids La_2–xSr_xCuO₄ with x=0 and 0.15 were detected. Lanthanum-deficient composition in the ternary system induces the formation of the non-superconducting phase whose stoichiometry is better represented by La_1.60Sr_0.40Cu₂O_5–. The temperature-programmed reduction profiles of the cuprates can be associated with the formation of more or less stable intermediates.     

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.     

Formation processes of the superconducting phases withT cs over 100 K in the Tl-Ba-Ca-Cu-O,Tl-Pb-Sr-Ca-Cu-O and Bi-Pb-Sr-Ca-Cu-O systems

Journal of Materials Science Letters -     

Optimization of current leads for superconducting systems

This paper is a report of a thesis by M. Rauh² carried out at the Institut für Verfahrens und Kältetechnik. A copy of this dissertation is available from the Institute.     

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.     

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.     

Influence of interstitial oxygen on the superconducting transition temperature of vanadium

The critical temperatureT _c and residual resistivity ₀ were measured in homogeneously oxidized thin vanadium foils. The per at % oxygen induced variations are respectively T _c =–1.2K and ₀=53 n-m. TheT _c data are used to calculate the electron-phonon coupling constant, which decreases 5% per at % of oxygen.     

Refrigerator-recirculator systems for large forced-cooled superconducting magnets

Forced-cooled superconductors are viewed as a promising alternative in the development of high field superconducting magnets for future fusion devices. The high current density cable superconductor is protected against thermal instabilities by forcing (single phase) supercritical helium through the cable.The cryogenic cooling system for a forced-cooled superconducting magnet works as a refrigerator and a reciculator at the same time. The paper discusses the conceptual design of the cooling systems for forced-cooled superconducting magnets with the overall objective of reducing the refrigeration costs. The general conclusion of this article is that economic cooling systems must employ efficient cold pump recirculators in which the large flow demanded by the forced-cooled superconducting magnet is confined to the cold end of the refrigerating column. If the liquid helium pump efficiency is less than 40%, systems employing elevated temperature compressors are more economic.     

Conductivity measurements on ceria at high oxygen pressures

The a.c. conductivity of high purity ceria has been measured at oxygen pressures and temperatures in the ranges 0.01 to 400 atm and 700 to 1100° C respectively. Similar measurements have been made on impure ceria over the same temperature range for oxygen pressures up to 100 atm. The conductivity isotherms obtained exhibit flattish minima where the ionic contribution to the conductivity is a maximum, a region at low oxygen pressures, where n-type conduction occurs and increases with diminishing pressure, and a high pressure region where p-type conduction occurs and increases with pressure. Activation energies derived for the conductivity minima are 1.55 and 0.85 eV for the high purity and impure ceria respectively; the higher value for the high purity material implies that a significant electronic contribution is present.     

Application of sensing components in spiral slow-wave systems for monitoring the formation of solid phases in electrolyte solutions

Features of monitoring solid phase formation in electrolyte solutions using sensing components in spiral slow-wave systems are considered. It is shown that insoluble compounds of nonferrous metals in solution may absorb radio frequency or microwave radiation due to resonance interaction of electromagnetic waves with dipole formations.Translated from Izmeritelnaya Tekhnika, No. 9, pp. 60–62, September, 2004.     

The recovery of superconducting systems

Superconducting systems, as proposed for fusion reactors and alternators, must if possible be encouraged to return to operating conditions after a transient fault. Electric ac power systems de-energise a faulty section of line, then re-close. The analogy is useful. The paper describes measurements and calculations for superconductors under fault and recovery conditions. In the laboratory, as in the final installation, recovery may be initiated in a number of ways; for example the removal of a continuous heat input could be considered as equivalent to the removal of external heat flux in a fusion reactor toroidal field winding. More realistic would be the reduction of conductor current, simulating the effect of protective action in a large installation. Laboratory observations are correlated with computer calculations in which detailed thermo-dynamic properties of superconductors are used. The conditions under which the recovery of a large system is possible are discussed.     

Optimization of helium refrigerators and liquefiers for large superconducting systems

A general method for optimizing ideal Claude cycles with multiple expansion engines is presented for two types of engine arrangements. In one case, the inlet temperature of one engine is equal to the outlet temperature of another engine. In the second case, inlet and outlet temperatures of adjacent engines are independent of each other, the more general case which is sometimes used in practice. The optimizing technique is especially adapted to superconductive systems in which some of the helium vapour is used to cool electrical leads and mechanical support systems. The helium fraction to be returned to the refrigerator-liquefier vs the fraction to be used for other purposes is conventionally entered as a parameter which becomes part of the optimization technique.     

Large-area patterned formation of intermetallic phases on Ti/Al multilayer systems by laser interference metallurgy

Titanium and aluminium multilayer thin films with thicknesses of about 300 nm were heat treated using Laser Interference Metallurgy. By the interference of laser beams at the sample surface, a local periodic arrangement of reacted and unaffected regions at the micron scale was achieved. The intention of this treatment was the production of a thin film composite material which benefits from the combination of reacted and unaffected microstructures. In the reacted zones, a mixture of solidified titanium and various aluminide intermetallic phases has formed, whereas the pristine multilayer persisted in the not heat treated regions. Instead of a surface limited reaction as in the former experiments, a complete reaction along the thickness dimension could be obtained using Laser interference metallurgy. The focus of this research lies on the structural and chemical analysis of the laser interference irradiated multilayer system. The existence of the aforementioned intermetallic phases was studied by grazing incidence X-ray diffraction and transmission electron microscopy. A local chemical analysis of the different heat affected regions was carried out by 3D atom probe tomography. The phase formation present in this case was compared to theoretical models related to this topic.     

Structural transformations in the HP-HT formation of monophase superhard materials based on carbon and boron nitride dense phases

The known data on the mechanisms of structural transformations in the formation of bulk monophase materials based on diamond and cubic boron nitride at high pressures and temperatures have been briefly summarized. Two groups of materials have been defined: those based on the initial micron powders of diamond, cBN, and graphite-like BN as powder and pyrolytic deposits. The structure formation of the first group materials has been shown to be governed by the development of the lattice plastic deformations (by translational slip and twinning) and rotational deformation, while the structure formation of the cBN-based materials is governed by the primary recrystallization occurring at T > 1900°C and p = 7.7 GPa. The rotational deformation governs a plastic fragmentation of grains in materials. The formation of materials based on the initial graphite-like BN has been found to depend on the crystal-oriented phase transformation into BN dense (wurtzitic and sphaleritic) phases and recrystallization of the forming cBN phase. The mechanisms of the cBN primary recrystallization have been discussed.     

A cold box for the forced cooling systems of superconducting magnets

A cold box, which does not have temperature expanders, designed for a forced cooling system is described. The cold box can be used to maintain superconducting magnets at 4.5 ? + 5 K for a long period of time.Tests on the cold box have shown that the mass flow rate of forced cooling flow is 6 + ? 10 kg h^?1. For this flow rate, 1.1 – 1.3 kg h^?1 of liquid He from the transport dewar is required.     

On the testing of large superconducting magnet systems

A new approach to the testing of large superconducting magnet system is proposed.     

The development and investigation of superconducting magnetic systems for physical experiments

Superconducting solenoids designed for different physical experiments are described. The solenoids are made of multifilament niobium titanium wires and are connected in series. The magnetic induction achieved at 4.2 K is 9.5 T and with the use of an iron yoke with magnetic flux concentrators an additional 1.5–2 T could be added to the operating volume of about 50 cm³. Various modified cryostats which allow operation for several days without liquid helium refilling and which permit a wide range of physical experiments are described.