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The recent discovery of superconducting oxide ceramics with critical temperatures (Tc) near 100°K has stimulated research at an unprecedented pace. Single phase YBa2 Cu30x (x = 6.9), the subject of most of the interest, is an oxygen deficient 1:1:3 stacked perovskite derivative, (The structure of this materials has been described by a number of authors, many of whom are referenced by Jorgensen et al.) At room temperture it has an orthorhombic structure with lattice parameters near: a = 3.82Å, b = 3.88Å, and c - 11.68 Å. At lower oxygen values (x≤6.5) the structure is tetragonal with lattice parameters near: a = 3.86 and c = 11.80. It has been suggested that tetragonal YBa2 Cu30x is not superconducting. YBa2 Cu30x, regardless of oxygen content, is referred to here as 123.
Superconducting thin films (0.3-0.7μm) in the TI-Ca-Ba-Cu-0 system have been prepared on various single crystal substrates by sequential electron beam evaporation followed by appropriate sintering and annealing. Oxygen-annealed films show Tc as high as 110K and critical current densities to 600,000 A/cm2. X-ray diffraction analyses of these films show predominantly the Tl2Ca2Ba2Cu2O10 phase (c-parameter near 36Å), but some also contain up to 50 at% of the Tl2CaBa2Cu2O8 phase (c-parameter near 30Å). The complete absence of hkl reflections other than 00I demonstrates the highly oriented nature of the films as well as the absence of other Tl phases. The diffraction peaks are noticeably broader for the 36Å phase than for the 30Å phase. For a 0.7μm film such broadening is consistent with coherent sizes along the c-axis of 1200 - 1400Å and 500Å, respectively, for the 30Å and 36Å phases, and of strain values near 1.4-1.8 x 10-3 for both phases.
Silver nanowire-based contacts represent one of the major new directions in transparent contacts for opto-electronic devices with the added advantage that they can have Indium-Tin-Oxide-like properties at substantially reduced processing temperatures and without the use of vacuum-based processing. However, nanowires alone often do not adhere well to the substrate or other film interfaces; even after a relatively high-temperature anneal and unencapsulated nanowires show environmental degradation at high temperature and humidity. Here we report on the development of ZnO/Ag-nanowire composites that have sheet resistance below 10 Ω/sq and >90% transmittance from a solution-based process with process temperatures below 200 °C. These films have significant applications potential in photovoltaics and displays.
Beta-gallium oxide (β-Ga2O3) is of increasing interest to the optoelectronic community for transparent conductor and power electronic applications. Considerable variability exists in the literature on the growth and doping of Ga2O3 films, especially as a function of growth approach, temperature, and oxygen partial pressure. Here pulsed laser deposition (PLD) was used to grow high-quality β-Ga2O3 films on (0001) sapphire and (−201) Ga2O3 single crystals and to explore the growth, stability, and dopability of these films as function of temperature and oxygen partial pressure. There is a strong temperature dependence to the phase formation, morphology, and electronic properties of β-Ga2O3 from 350 to 550 °C.
Depending on their application temperature thermoelectric (TE) materials are classified in three main categories; as low (up to 250°C), intermediate (up to 550°C) and high (above 600°C) temperature. Currently, Skutterudites (CoSb3) based materials have shown promising results in the intermediate temperature range (300-500°C). This family of material is highly suitable for automotive, marine transportation and industrial power generation applications to recover the waste heat from the exhaust and generate electricity. Conventional TE modules need p- and n-type semiconductor materials and for the skutterudite family, iron (Fe) has proven to be among the best candidates for the substitution of cobalt sites. Additionally, rare earths are introduced as rattlers in the crystal cages of the skutterudite to decrease the thermal conductivity, thus improving the figure of merit ZT of the TE material. For practical application for device fabrication, stability of these materials is of great importance. Compositional stability is being addressed as the material decomposes above certain temperature. Temperature dependent x-ray diffraction study was performed on Fe substituted, Yb-filled skutterudites, using Beam Line I711 at MAX LAB, to observe the crystal structure as a function of temperature. Diffraction patterns were collected from room temperature up to 500°C by utilizing Huber furnace. The results show success in filling process showing almost 80% reduction of the thermal conductivity from bulk. Additionally the thermal expansion coefficient value was within the average value for skutterudites which proves practical application of this powder for industrial applications.
Concerning a materials ability to convert heat to electrical energy, the electrical power factor S2/ρ as well as the thermal conductivity at elevated temperatures are of special interest. Since Flash experiments measure the thermal diffusivity and standard steady-state heat-flow experiments are inaccurate at elevated temperatures due to radiation errors inherent to this technique, direct and accurate thermal conductivity data on type-I clathrate single crystals at elevated temperatures are scarce in literature. Here we report 3ω thermal conductivity data on single crystalline Ba8Cu5.09Ge40.91 (BCG), La1.23Ba6.99Au5.91Si39.87, and Ce1.06Ba6.91Au5.56Si40.47 in the temperature range between 80 and 330 K, and specific heat data on BCG between 2 and 300 K. The comparison of our room temperature phonon thermal conductivity data (κph) to results on transition metal (TM) free type-I clathrates in terms of the guest free space (Rfree) suggests a stronger dependence of κph on Rfree for the clathrates containing TM elements.
Composites of 8 mol% yttria-stabilized zirconia (YSZ) and titanium nitride (TiN) were obtained by mechanical mixing of commercial powders. High-density samples of (1-x) YSZ / x TiN, with x = 0, 25, 50, and 75 wt.%, were obtained by spark plasma sintering (SPS) at 1450 °C for 5 min. Surface contamination with carbon from the SPS was eliminated by diamond sawing of parallel surfaces. X-rays diffraction analyses showed that samples are composed by a mixture of the initial phases, without appreciable reaction as inferred from calculated lattice parameters. dc 4-probe electrical measurements in the 100-850°C under showed that samples have a metallic behavior, indicating that the percolation threshold was attained for the sample with the lowest content of the TiN (x=25 wt.%), which corresponds to ∼27 vol.%.
The use of nanoparticle precursors for electronic materials including sulfides, selenides, oxides and the elements has potentially wide ranging implications for improving device properties and substantially reducing the deposition costs. To realize this goal the complex interfacial chemistry of these small particles must be controlled. In this paper we present a number of cases demonstrating the complexity of this chemistry. These include CuInSe2 where the kinetics of phase formation dominate the sintering process; CdTe where sintering proceeds with and without the sintering enhancement of CdCl2, but produces materials different electronically than bulk materials; and the use of compound and elemental nanoparticles ( Ag, Al, Hg-Cu-Te and Sb-Te) for contacts to elemental and compound semiconductors (Si and CdTe).
Our team has been investigating the use of particle-based contacts in CdTe solar cell technologies. Toward this end, particles of Cu-doped HgTe (Hg-Cu-Te) and Sb-Te have been applied as contacts to CdTe/CdS/SnO2 heterostructures. These metal telluride materials were characterized by standard methods. Hg-Cu-Te particles in graphite electrodag contacts produced CdTe solar cells with efficiencies above 12% and series resistance (Rse) of 6 Ω or less. Metathesis preparation of Cu(I) and Cu(II) tellurides (i.e., Cu2Te and CuTe, respectively) were attempted as a means of characterizing the valence state of Cu in the Hg-Cu-Te ink. For Sb-Te contacts to CdTe, open circuit voltages (Vocs) in excess of 800 mV were observed, however, efficiencies were limited to 9%; perhaps a consequence of the marked increase in the Rse (i.e., >20 Ω) in these non-graphite containing contacts. Acetylene black was mixed into the methanolic Sb-Te colloid as a means of reducing Rse, however, no improvement in device properties was observed.
Metal-organic and hybrid metal-organic/metal nanoparticle inkswere evaluated for use in the inkjet printing of copper and silver conducting lines. Pure, smooth, dense, highly conductive coatings were produced by spray printing with (hexafluoroacetylacetonato)copper(I)-vinyltrimethylsilane Cu(hfa)·VTMS) and (hexafluoroacetylacetonato)silver(I)(1,5-cyclooctadiene) (Ag(hfa)COD) metal-organic precursors on heated substrates. Good adhesion to the substrates tested, glass, Kapton tape and Si, has been achieved without use of adhesion promoters. The silver metal-organic ink has also beenused to print metal lines and patterns with a commercial inkjet printer. Hybrid inks comprised of metal nanoparticles mixed with the metal-organic complexes above have also been used to deposit Cu and Ag films by spray printing.This approach gives dense, adherent films that are much thicker than those obtained using the metal-organic inks alone. The conductivities of the silvercoatings obtained by both approaches are near that of bulk silver (2 μΩ·cm). The copper coatings had conductivities at least an order ofmagnitude less than bulk.
Yttrium oxide and barium strontium titanate (BST) thin films were grown directly on Si substrates by the pulsed laser deposition (PLD) technique. Because the optimum oxygen pressure during PLD process is of the order of 10 mTorr, some of the oxygen atoms are trapped inside the grown films and contribute to the growth of a silicon oxide interfacial layer. The use of an UV source during the growth resulted in the reduction of the optimum oxygen pressure and, as a consequence, the amount of trapped oxygen and thickness of the interfacial layer. In addition to that, UV radiation influenced the film morphologies and electrical properties. A further reduction of the interfacial layer was obtained on substrates that were exposed prior to deposition to NH3 for short periods of time under UV radiation.
This paper reports the properties of undoped CdO films and CdO films doped with the group VII element F and the group IV element Sn. The CdO films are made by low-pressure chemical-vapor deposition. We observe that undoped CdO films can achieve a carrier concentration of 1021 cm−3, apparently by controlling the intrinsic defect. However, the electron mobility of these films is only around 2 cm2 V−1 s−1. With fluorine doping, an electron mobility of ∼260 cm2 V−1 s−1 has been achieved. However, low carrier concentration results because of the low solubility of F in CdO film. CdO films doped with both Sn and F demonstrate carrier concentrations of 1021 cm−3 and reasonable electron mobilities of around 20 cm2 V−1 s−1. Due to the small effective electron mass of CdO, a large Burstein-Moss shift is observed for films with high carrier concentration. The shift enables the fundamental absorption edges of undoped CdO films to reach 3.0 eV and 3.3 eV for films doped with both Sn and F.
Composite thin films of 60 wt% Ba0.6Sr0.4TiO3 and 40 wt% MgO were produced by Pulsed Laser Deposition. The biaxial texture of the BST component on the MgO substrate has been established with XRD. All as-deposited films had an enlarged BST out-of-plane lattice parameter. A more relaxed lattice constant as well as higher degree of texture has been obtained in the films deposited at higher temperature and lower deposition rate. Post-deposition annealing in flowing oxygen results in a further relaxation and alignment of the BST lattice. The as-deposited films were not tunable at room temperature. The greatest dielectric tuning was achieved in films annealed at 1200 °C. The observed difference in tunability for the films annealed at different temperatures may result from a spatial redistribution of BST material on the substrate surface during annealing.
Air sintering and subsequent oxygen annealing at high temperatures are integral to the production of ceramic superconducting YBa2Cu3O7-δ. We present results directed toward optimizing the temperature for these two process steps, using static magnetization to determine the intragranular properties. Surprisingly, pure oxygen stabilizes the material against decomposition at temperatures near the melting point, allowing the production of significantly higher quality single phase material.
The effects of oxygen and helium ion irradiation on the superconducting properties of Tl2Ca2Ba2Cu3010 thin films were investigated. The transition temperature and width were monitored as a function of ion fluence using both magnetization and resistivity measurements. These data suggest that superconductivity is completely suppressed at 0.020 dpa for both He and 0 ion irradiation. Further, the rate of decrease in Tc as a function of deposited energy showed that the dominant mechanism causing damage-induced suppression of Tc in these films was from atomic collisions.
There is considerable interest in the interplay, of superconductivity and magnetism in La2CuO4. Several groups [1–3] have noted signs of superconductivity in nominal La2CuO4, and antiferromagnetism has been reported in La2CuO4-y with y>0. The superconductivity previously reported was of a filamentary nature because although resistance drops to near zero were observed, only small Meissner effects (∼10-3 to 10-4 of perfect diamagnetism) were observed. In this paper we describe our studies of. ceramic samples of La2CuO4-y, loaded with oxygen by high pressure. This technique results in substantial incorporation of O2 in the material (of the order of 0.3 to 0.4% by weight) and in gross changes in the bulk resistivity and magnetization as well as discernable changes in the lattice parameters of the material.
Thick (20–300 /im) freestanding films of ceramic YBa2Cu3O0–9 have been obtained by resintering YBa2Cu3O0–9 powder in air after screen printing onto fused silica substrates. An interfacial reaction occurs between the powder and the silica substrate which efficiently debonds the resintered film upon cooling. The magnetization and transport properties of these films are comparable to those for high quality bulk ceramic samples.
We have examined several crystals belonging to the Tl‐2122 structure type (Tl2CaBa2Cu2O8 with c=29Å), and have shown that cation solid solution occurs. Such cation disorder appears to be responsible for the observed small differences in lattice parameters reported by various investigators and to contribute towards the substantial variation in the superconducting transition temperatures.
The structural, magnetic and electrical properties of EuBa2Cu3O7-δ have been investigated both for bulk ceramics and screen printed thick films. The bulk ceramic reaches zero resistance at 92 K, and has large intragranular critical current density inferred from magnetization loops; the directly measured critical current density is substantially lower because of the weak links (Josephson junctions) between grains. Screen printed thick films on strontium titanate have poor magnetic and resistive transitions with evidence for two phases. High temperature oxygen anneals produce a bulk ceramic with significantly enhanced magnetic properties.
Magnetization and resistivity data are presented which show a substantial difference in superconducting transitions for nominal Tl2Ca2Ba2Cu3Ox plates with identical crystal structures grown from two different “melt” compositions. A Tl-O and CuO-rich flux yielded approximately stoichiometric plates which have sharp transitions beginning near 111 K, while a second Tl-O-rich flux produced plates containing more Tl and less Ba with very broad transitions starting near 96 K. These data demonstrate the extreme sensitivity of superconductivity to cation site disorder in the Tl-Ca-Ba-Cu-O system.