M. F. Garbauskas

The preparation of “1223” Tl-Ca-Ba-Cu-oxide superconducting films via the reaction of silver-containing spray deposited Ca-Ba-Cu-Oxide with thallium oxide vapor

Abstract A process is described for the preparation of superconducting films of “1223” TlxCa2Ba2Cu3Oy (0.65 10 000 A/cm2 was measured at 60 K-2 T with the magnetic field applied parallel to the crystallographic c-axis of the film.

Transport critical currents in spray pyrolyzed films of TlBa2Ca2Cu3Oz on polycrystalline zirconia substrates

Large critical currents with a relatively weak magnetic field dependence are obtained in thick films of TlBa2Ca2Cu3Oz. Transport measurements indicate Jc ≳105 A/cm2 at 77 K, zero field, and Jc≳104 at 60 K in a 2 T field applied along the c‐axis. The observed behavior is attributed to a large degree of uniaxial alignment of platelike grains, and to superior intragranular flux pinning. These results are consistent with recent theories concerning the nature of vortices in highly anisotropic (layered) superconductors and ‘‘brick wall’’models of intergranular current transport.

Studies of Organometallic Precursors to Aluminum Nitride

Abstract : The reaction of trialkylaluminum compounds with ammonia has been examined as a potential route to high purity AlN powder and to AlN thins films. This reaction proceeds in stages in which the initially formed Lewis acid/base adduct undergoes thermal decomposition to a series of intermediate alkylaluminum-amide and -imide species with increasing Al-N bonding, i.e., R3A1 + NH3 yields R3Al:NH3 yields AlN + 3RH (where R = CH3, C2H5, C4H9, etc.). The structure and properties of several of these species have been studied using various physical and chemical methods, leading to a better understanding of the chemistry of this novel A1N precursor system. The structure of the intermediate organoaluminum amide, (CH3)2AlNH2, has been determined by single crystal X-ray diffraction methods and found to contain molecular trimer units with a six- membered Al-N ring structure similar to those which make up the wurzite structure of AlN. This compound is readily volatile and has been used to deposit AlN thin films on Si surfaces by a low-pressure CVD process. This approach has also been used to prepare AlN as a high surface area, high purity powder. Keywords: Chemical vapor deposition; Organoaluminum amide.

Epitaxial growth and critical current density of thin films of YBa2Cu3O7−x on LaAlO3 substrates

Thin films of the high‐temperature superconductor YBa2 Cu3 O7−x have been produced on (100) LaAlO3 substrates by coevaporation and furnace annealing. A 14‐μm‐wide and 400‐μm‐long constriction patterned on a 0.8‐μm‐thick film had a zero resistance transition temperature of 90 K, a transition width of 1.5 K, and a critical current density of 8×104 A cm−2 at 77 K. Although x‐ray diffraction shows a definite c‐axis alignment normal to the substrate plane, further analysis reveals that c‐axis alignment in the substrate plane is also present. The detailed microstructural picture is revealed by transmission electron microscopy: a continuous layer, about 0.2 μm thick adjacent to the substrate, with c axis normal to the substrate plane, and the remaining top portion of the film, with the c axis in the film plane. In spite of the bilayer structure, the film remains epitaxial (the axes of the superconductor are parallel to the 〈100〉 directions of the substrate).

The crystal structure of SiB6

Abstract The accurate and detailed structure of the compound SiB 6 has been determined by single-crystal X-ray diffraction. The final R value was 6.1% for 4225 reflections. The cell is orthorhombic with space group Pnnm and a = 14.397(7) A, b = 18.318(9) A, c = 9.911(7)A, and from the electron density appears to contain 43 silicon atoms and 238 boron atoms. The structure contains many features found in other structures of boron-rich phases, and obeys the crystal chemistry rules established for them. It contains interconnected icosahedra, icosihexahedra, as well as several isolated boron and silicon atoms. An unusual feature of this structure is the presence of icosihexahedra containing silicon atoms similar to those found previously in BeB 3 .The accurate and detailed structure of the compound SiB/sub 6/ has been determined by single-crystal X-ray diffraction. The final R value was 6.1% for 4225 reflections. The cell is orthorhombic with space group Pnnm and a = 14.397(7) A, b = 18.318(9) A, c = 9.911(7) A, and from the electron density appears to contain 43 silicon atoms and 238 boron atoms. The structure contains many features found in other structures of boron-rich phases, and obeys the crystal chemistry rules established for them. It contains interconnected icosahedra, icosihexahedra, as well as several isolated boron and silicon atoms. An unusual feature of this structure is the presence of icosihexahedra containing silicon atoms similar to those found previously in BeB/sub 3/.

X-ray study of transition-metal dopants in β-boron☆

Abstract The atomic locations and concentrations of various transition element dopants in single crystals of β-boron have been studied by X-ray methods. The elements include Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zr, Nb, Hf, and Ta. These dopants do not enter normal boron positions in the framework at the concentrations studied; rather they occupy previously identified A1, A2, D, or E sites. The percentage occupancies of the several sites are correlated with the atomic sizes of the dopant atoms and their concentrations. Some differences between samples are related to their thermal histories. A number of competing interactions between six different, partially occupied boron sites and the dopant sites have been found. The doping mechanism is “displacive” rather than interstitial or substitutional.

Thermal expansion measurements using neutron diffraction of Bi2CaSr2Cu2Ox

Abstract The differential thermal expansion of Bi 2 CaSr 2 Cu 2 O x , Bi-(2122), has been measured using high-temperature neutron powder diffraction in air, 0.980% oxygen in nitrogen, and pure argon. Thermal expansion in the c -axis direction is greater by a factor of about 1.4 than that in either the a - or b -axis direction for all sample environments. The neutron results for Bi-(2122) in air are compared with results from similar X-ray powder diffraction measurements.

The incorporation of vanadium in β-rhombohedral boron as determined by single-crystal diffractometry☆

Abstract The solubility of vanadium in β-rhombohedral boron has been studied by single crystal diffractometry for two crystals of composition VB∼65 and VB∼165. The space group is R 3 m and the hexagonal cell dimensions are a = 10.972 A, c = 23.908 A for VB∼65 and a = 10.949 A, c = 23.840 A for VB∼165. At the higher concentration of vanadium, the same two sets of holes are occupied as in CrB∼41 and FeB∼49. The boron framework is essentially the same as in undoped β-rhombohedral boron. Vanadium is found to expand the unit cell slightly.

The fabrication of high critical current capability bismuth superconductor tape

A processing protocol for the preparation of Ag sheathed tapes of the bismuth superconductor composition Bi 1.7 Pb 0.3 Ca 2.05 Sr 2 Cu 3.05 O z , Bi-(2223), with critical currents densities, J c , exceeding 10,000 A cm −2 at 77 K and 0 T, is presented. The protocol is based on the combined in-situ reaction and sintering of a mixture of Bi 2 CaSr 2 Cu 2 O z , Bi-(2122), with the additional constituents needed to form the Bi-(2223), within the pre-fabricated tape. The conversion to a dense Bi-(2223) ceramic core was accomplished with a series of anneals at ∼830°C, for periods of up to 100 h, separated by uniaxial cold-pressings of the tape between the anneal. The effects on J c of variations in the anneal temperature, annealing time, and the number of pressings are presented.

Superconducting joints formed between powder-in-tube Bi/sub 2/Sr/sub 2/Ca/sub 2/Cu/sub 3/O/sub z//Ag tapes

Superconducting joints between Ag-clad, Bi/sub 2/Sr/sub 2/Ca/sub 2/Cu/sub 3/O/sub z/ tapes have been obtained with approximately 1/2 the current capacity of the tapes themselves. The Ag sheath is removed from one side of each tape without significantly disturbing the superconducting core. The exposed superconducting core of the two tapes is brought into contact and pressed so as to again seal the superconductor in a Ag sheath. A reaction anneal is performed to join the two cores together and repair damage associated with the removal of Ag and the pressing. Transport measurements using multiple voltage taps have been found useful in characterizing the critical current variation across the joint.<<ETX>>