Growth and processing of single crystal superconducting YBa2Cu3O7−δ

A method for growing millimeter-sized crystals of YBa₂Cu₃O_7– is described. Processing conditions were developed to improve the superconducting transition temperature and sharpness, as observed by measurements of the dc magnetic susceptibility. Some of our crystals have been used in observations of the Raman Effect, specific heat, nuclear magnetic resonance, and nuclear quadrupole resonance, with results indicating that the samples are excellent.     

Growth,morphology and slip system of α-Si3N4 single crystal

Under the conditions of growth temperatures 1500 to 1700° C and total gas pressure 10 to 50 Torr, -Si₃N₄ single crystals have been grown by chemical vapour deposition from a mixture of NH₃, SiCl₄ and H₂. The crystals were transparent and brownish-red to colourless. The effects of the growth conditions on the crystal morphology, growth habit and growth direction have been investigated. On the basal and prismatic planes, the variation in Knoop hardness with orientation of the indenter long-axis has been measured at temperatures up to 1500° C; maximum hardness values were obtained along the 1 0 ¯1 0 direction for the basal plane and along the [0 0 0 1] directions for the prismatic planes. Hardness anisotropy analysis suggests that the active slip systems of -Si₃ N₄ are {1 0 ¯1 0} [0 0 0 1] from room temperature to 1500° C.     

Phase diagram, crystal growth and superconductivity of CaLaBaCu3O7−Y crystal

CaLaBaCu₃O_7–y (CLBCO) is a high T _c superconductor with unique tetragonal structure and cross-cationic substitution sites. In this paper, we present our study on the BaO—CuO—CLBCO phase diagram and its application to the crystal growth of CLBCO. The eutectic composition of BaO—CuO system is confirmed by the improved thermogravimetric analysis ITGA and is used for the crystal growth as a self-flux system. A pseudo-phase diagram of CLBCO—eutectic flux system is established by differential thermal analysis DTA which enables the selection of a suitable starting composition and temperature program for the crystal growth. We explore the relation between crystal growth, cell parameters and the main superconducting transition temperatures T _c. The T _c of CLBCO crystals depends strongly on the Ca/La ratio in the starting materials. As for YBa₂Cu₃O_7–y YBCO, a small amount of BaF₂ additive greatly increases the main transition temperature of CLBCO crystals. This can be understood by the cluster model we proposed previously for YBCO crystal growth. Based on the research on the phase diagram, crystal growth and the superconductivity, high T _c crystals of CLBCO with size up to 6.5 × 6.5 × 0.4³ have been grown from the eutectic self-flux system with BaF₂ as additive. T _c was determined by an a.c. susceptibility measurement, which shows the main transition temperature up to 60 K for the as-grown crystals. After annealing in flowing oxygen at 600 to 400 °C for two days, T _c increased to 73 K.     

Magnetic properties of (Dy1−xGdx)3Ga5O12 garnet single crystals

Magnetic properties of Dy_1–x Gd _x )₃Ga₅O₁₂ (DGGG) garnet single crystals were calculated using the Weiss molecular field theory and also measured using the vibrating sample magnetometer in the temperature range 4.2–40 K in the effective magnetic field from 0–7 T. The magnetic properties of DGGG single crystals are distributed between those of Dy₃Ga₅O₁₂ (DGG) and Gd₃Ga₅O₁₂ (GGG) single crystals, but are considerably closer to those of DGG. Based on the magnetic properties, the magnetic entropy change, S_M, was evaluated in the temperature range below 15 K. DGGG single crystals are a prospective material for magnetic refrigeration below 15 K.     

Growth and characteristics of ternary Zn1−x Mg x O films using magnetron co-sputtering

ZnMgO thin films with different Mg compositions were grown on Si (001) substrates by radio frequency magnetron co-sputtering, and their structural and optical properties have been investigated. As the Mg content was increased up to 10 at.%, the surface became smoother and the growth rate decreased. The strong peaks at 34.5–34.7^° corresponding to the (0002) diffraction peak of ZnMgO indicated the preferred growth along the c-axis. From the photoluminescence measurements, a shift of the emission peak and an increase of the activation energy were also observed with increasing Mg content.     

Growth characteristics and device properties of MOD derived β-Ga2O3 films

β-Ga₂O₃ films prepared by metal organic deposition (MOD) on (000l) sapphire substrates, have been developed for ultraviolet photodectors. The structural, surface, optical properties of MOD derived β-Ga₂O₃ films depending on growth temperatures were investigated. As growth temperature increased, the crystallinity of β-Ga₂O₃ films enhanced, crystallite size and surface roughness increased. The optical band gap of β-Ga₂O₃ films maintained within 4.8–4.9 eV at different growth temperatures. Metal–semiconductor–metal ultraviolet photodetectors based on MOD derived β-Ga₂O₃ films were successfully fabricated, demonstrating the responsivity of 0.76 A/W at 20 V and the upper limits of the rise and decay time of 50 and 30 ms, respectively, indicating a promising low-cost approach for Ga₂O₃-base photoelectronics applications.     

High temperature characteristics of Ir–Ta coated and aluminized Ni-base single crystal superalloys

High temperature oxidation and hot corrosion properties of Ir–Ta coated and aluminized Ni-base superalloys are presented. An Ir–Ta binary alloy, proposed as a novel metallic bond coat material, was coated on a Ni-base single crystal superalloy TMS-75 using electron beam physical vapor deposition, followed by a conventional low activity Al pack cementation process. Cyclic oxidation tests and hot corrosion tests revealed that these Ir–Ta coated and aluminized specimens showed reasonably good oxidation and hot corrosion resistance. In addition, it was found that the formation of TCP phases is suppressed by the presence of the Ir–Ta enriched layer. These results indicated that the Ir–Ta alloy system is promising as a new metallic bond coat material for high temperature structural materials.     

Structural and electronic characteristics of Fe-doped β-Ga2O3 single crystals and the annealing effects

Journal of Materials Science - As capture traps, Fe impurities were intentionally incorporated into beta-gallium oxide (β-Ga2O3) crystals to compensate for unintentional n-type conductivity...     

Interfacial reactions between titanium film and single crystal α-Al2O3

Titanium is commonly used to join metals and ceramics by active metal brazing methods. In this work, titanium was sputter deposited on to single-crystal -Al₂O₃ substrates and the interfacial reactions between the titanium film and the Al₂O₃ substrate were studied. Al₂O₃ was reduced by titanium when samples were annealed at 973 and 1173 K for 300 s in an argon gas flow. Metallic aluminium was produced at the interface, and this diffused from the interface into the titanium film. At 1173 K, the intermetallic compound Ti₃Al and the intermediate titanium oxides, such as Ti₂O and TiO, were formed. The Al⁰ diffusion is important in stimulating interfacial reactions.     

Structure and mechanical characteristics of ceramic Nb2O5 and Nb2(1 − y)Ta2y O5

The structure and mechanical characteristics of ceramic Nb₂O₅ and ceramic materials produced from coprecipitated Nb_{2(1 ? y)}Ta_2y O₅ pentoxides by a conventional ceramic processing technique and exposure to high-intensity light have been studied by scanning probe microscopy and Raman spectroscopy.     

Thermal conductivity in theab-plane of a large single crystal YBa2Cu3O7−x with several oxygen contents

The ab-plane thermal conductivity _ab of a large single crystal of YBa₂Cu₃O_7-x has been studied as a function of annealing time in the temperature range from 10 up to 150 K. An observed anomaly in _ab below T_c is enhanced for short time annealing in an oxygen flow but the superconducting enhancement of _ab is rather suppressed for long-time annealing. The superconducting enhancement in _ab is suppressed in the over-doped regime in comparison with that on the optimized boundary. It is considered that this suppression in _ab originates in the screening effect on the electron-electron interaction due to charge carrier doping.     

Effect of correlated and point-like disorder on transport properties of YBa2Cu3O7−δ single crystals

Pinning by point-like defects and twin boundaries in single-crystalline YBa₂Cu₃O_7– has been investigated by measuring resistance and current-voltage characteristics in fields up to 5 T. A number of untwinned and twinned samples with different densities of point-like defects were studied. The experimental results are compared within the framework of vortex-entanglement.     

High-field Magnetization and Magnetoresistance of CaCu3Ti4−x Ru x O12

We have investigated magnetization (M) and magnetoresistance (MR) of CaCu₃Ti_4?x Ru _x O₁₂ (x=0, 1.5, and 4). The magnetization in magnetic fields (H) up to 52 T decreases with increasing Ru content x. The M–H curves for x=0 and 4 are almost linear in field, but that for x=1.5 is non-linear and shows a convex-bending in high magnetic fields. Furthermore, a large negative MR about ?70% at 52 T is observed for x=1.5. These experimental results suggest that the magnetic field increases the itinerancy for x=1.5.     

The crystal structure,magnetic and magnetocaloric properties of Mn8−xCrxGa5

Mn₈Ga₅ alloy has certain significance in the study of materials for magnetic refrigeration applications due to its low coercivity. In this paper, the structure, magnetic properties and magnetocaloric effects (MCE) of Mn_8?xCr_xGa₅ (x?=?1.3, 1.7) polycrystalline alloys were first studied by using X-ray diffraction (XRD) technique, scanning electron microscope (SEM), energy-dispersion (EDS) analysis and magnetization measurement. The two samples crystallized in a cubic (Zn₈Cu₅)-type structure with space group of I-43 m (No. 217). The lattice constants and crystallite sizes of Mn_8?xCr_xGa₅ increase with the increase of Cr concentration. As the Cr content increases, the Curie temperature (T_c) of Mn_8?xCr_xGa₅ (x?=?1.3, 1.7) is increased from 145 to 187 K. The saturation magnetic moments of Mn_8?xCr_xGa₅ are 33.8 (x?=?1.3) and 36.6 (x?=?1.7) emu/g under the magnetic field of 16 kOe at 80 K, respectively. A second-order magnetic phase transition occurred in Mn_8?xCr_xGa₅ alloy samples. The maximum magnetic entropy changes (|?S_M|) of Mn_8?xCr_xGa₅ are 0.9 (x?=?1.3) and 0.8 J·kg^?1·K^?1 (x?=?1.7), and the relative cooling power (RCP) values are 25 (x?=?1.3) and 17 J/kg (x?=?1.7) under 15 kOe magnetic field.

     

Structure and optical properties of single crystal of fluorene–thiophene oligomers

Solid-state optical properties of polymorph 2,7-(2-thienyl)-9,9-dihexylfluorene [TFT(C6)₂] and 2,7-di(2-thienyl)-9,9-cyclopentanefluorene [TFT] related to their crystal structure are reported. 3D phase of the first molecule displays a very peculiar arrangement, tetragonal system, where adjacent molecules cannot face to each other, due to fourfold alternating axis, therefore preventing close intermolecular interactions. The 3D packing of the second molecule shows a herringbone arrangement, where dimers are alternatively shifted. Both exhibit significant photoluminescence (PL), which changes in position and shape as a function of the solid state organization. Such different arrangements are related to different PL quantum yield values preliminarily determined.     

Structural and dielectric properties of Ba5EuTi3−xZrxNb7O30 relaxor ferroelectrics

Polycrystalline samples of Ba₅EuTi_3−xZr_xNb₇O₃₀ [x=0, 1, 2, 3] of Tungsten–Bronze (TB) structural family have been prepared by a high-temperature solid-state reaction technique. X-ray diffraction (XRD) analysis as a function of temperature shows the orthorhombic structure of the compounds at room temperature. Dielectric studies at four different frequencies, 1, 10, 100 kHz and 1 MHz, show that the above compounds have ferroelectric phase transition of diffuse-type, and also have relaxor properties. With increasing Zr concentration (x), the transition temperature of the compounds decreases.     

Temperature dependence of the pseudogap in Y1−zPrzBa2Cu3O7−δ single crystals

We investigate the conductivity in the basis plane of Y_1?zPr_zBa₂Cu₃O_7?δ and YBa₂Cu₃O_7?δ single crystals. The excess conductivity, Δσ, is described by Δσ ~ (1 ? T/T*)exp(Δ_ab*/T) in a wide temperature range T_f < T < T*, where T* is the mean field temperature of the superconducting transition. We determine that the temperature dependence of the pseudogap (PG) is described in terms of the BCS-BEC crossover theoretical model. It is observed that a reduction in the praseodymium content results to the widening of the temperature interval that the PG region is realized and a narrowing of the area of existence of the linear dependence of the resistivity, ρ(T), in the ab plane.