X-ray,electrical conductivity and infrared studies of the system Zn1−x CO x Mn1−x Fe x CrO4

The system Zn_1–x Co _x Mn_1–x Fe _x CrO₄ is tetragonal in the range 0.0x0.4 and cubic in the range 0.5x1. Electrical resistivity temperature behaviour obeys Wilson's law for all the compounds and thermoelectric coefficient values vary between 135 to 226V K^–1. All compounds exhibit P-type semiconductivity and possess low mobility values (10^–9 cm² V^–1 sec^–1). The infrared spectra show the presence of two strong absorption bands around 500 and 600 cm^–1. The probable ionic configuration for the system is suggested to be Zn _1–x ²⁺ Fe _x ^–y3+ -Co _x ²⁺ [Mn _1–x ³⁺ Fe _y ³⁺ CO _x ^–y2+ Cr³⁺]O₄.     

Characterizations of InzGa1?z As1?x?yN xSbyP-i-N structures grown on GaAs by molecular beam epitaxy

The GaAs-based double-heterojunction P-i-N structures using In_zGa_1–zAs_1–x–yN_xSb_y as the i-layer is investigated for the first time using solid source molecular beam epitaxy. High quality coherent bulk InGaAsN_3.45%Sb (0.5 m) with a lattice-mismatch of 2.6 × 10^–3 can be achieved due to the surfactant properties of antimony, while the bulk InGaAsN_2% at 0.5 m with 1.06× 10^–3 mismatch is fully relaxed. Rapid thermal annealing (RTA) resulted in 25 meV low temperature (LT) photoluminescence (PL) full-width half-maximums (FWHM) for the bulk InGaAsNSb layers. InGaAsNSb experiences 30% less blueshifting with subjected under the same annealing conditions as InGaAsN with similar N content. Room temperature (RT) absorption measurements are in the range of 0.9–1.4 eV. The absorption edge of 1.41 m is achieved for sample D4.     

High pressure synthesis and electrical properties of BaTiO3−xFx

The substitution of fluorine for oxygen in BaTiO₃ was investigated by the reaction of BaO₂, BaF₂, TiO₂ and Ti under the conditions of 3.0 GPa and 1300° C for 2 H. Batio_3–x F _x solid solutions were obtained as a single phase in the region of 0 x0.1. The X-ray diffraction data showed that the symmetrical change from tetragonal to cubic form occurred atx=0.08 at room temperature. Also, the solid solutions were characterized by TG-DTA analysis, ESCA spectroscopy and diffusion reflectance spectroscopy. As a result, the existence of Ti³⁺ was verified. The electrical resistivities of BaTiO_3–x F _x were in the range from 4cm for x=0.05 to 3Ocm forx=0.1 at 300 K and the relationship between In and 1000 T^–1 was linear. The thermoelectric powers at 300 K were –250V K^–1 forx=0.05 and –130VK^–1 forx=0.1. The electrical property of the solid solution was discussed on the basis of a conventional band model which involved localized donor levels associated with oxygen vacancies.     

Effect of Crystal Structure on the Electrical Properties of CaTi1 – xFexO3 – δ

The electrical conductivity of CaTi_{1 – x} Fe _x O_{3 –} (x= 0–0.5) was measured as a function of temperature and oxygen partial pressure. At 1000°C, the highest conductivity was observed at x= 0.2. The crystal structure of the materials with x= 0, 0.2, 0.25, and 0.3 was studied by x-ray powder diffraction and refined by the full-profile analysis method. The results were used to elucidate the mechanisms of the high-temperature (1000°C) formation, ordering, and transport of oxygen vacancies in CaTiO₃upon substitution of Fe³⁺for Ti⁴⁺. The composition dependences of ionic conductivity calculated for CaTi_{1 – x} Fe _x O_{3 –} agree well with experiment.     

Preparation of Thick LaCu1 – x Ni x O2.5 + δ Films Exhibiting a High-Temperature Metal–Semiconductor Transition

The conditions for producing thick LaCu_{1 – x} Ni _x O_{2.5 +} films on different substrates were optimized. The effects of the heat-treatment conditions, substrate material, and the nature of the liquid organic binder on the composition, structure, and properties of the films were studied. Single-phase coatings obtained on MgO, ZrO₂, BaSO₄, and Ni substrates 50 to 200 m in thickness were close in properties to bulk LaCu_{1 – x} Ni _x O_{2.5 +} and exhibited a metal–semiconductor transition at about 500°C.     

TiCxNy and TiCx-TiN films obtained by CVD in an ultrasonic field

TiC _x N _y mono- and TiC_x-TiN double-layer films with a thickness of 30 to 100 m were prepared on a carbon steel (C: 0.6 to 0.7%) substrate by CVD in an ultrasonic field (ultrasound frequency: 19kHz; power: 10 to 20Wcm^–2). The moderate deposition conditions for obtaining an adherent and thick film of TiC _x N _y were: substrate temperature: 1050° C; H₂, N₂, TiCl₄ and CH₄ flow rates: 6.2, 4.0, 0.9 and 0.26 to 2.0 ml sec^–1, respectively. The growth rate, grain size and degree of 2 2 0 preferred orientation were found to decrease with increase in CH₄ concentration. TiC _x N _y film on carbon steel had a Vickers microhardness of 1800 to 2600 and an adhesion strength to the substrate of more than 120 kg cm^–2. A TiC _x -TiN (x0.5) double-layer film was obtained at 1050° C by a controlled alternative deposition of TiC _x or TiN. Quasiepitaxial growth of crystallites in the double layers was found to prevail in both coatings of TiC _x (220)/TiN (220)/steel and TiN (200)/TiC_x (200)/steel.     

Non-equilibrium crystalline superconductors in Zr-Si binary alloys rapidly quenched from melts

The new non-equilibrium superconductor with a b c c structure has been found in rapidly quenched Zr-Si alloys. The silicon content in the b c c alloys was limited to the narrow range between 8 and 11 at%. The b c c alloys showed a superconducting transition whose temperature increased from 3.20 to 3.84 K with decreasing silicon content. The upper critical magnetic field and the critical current density for Zr₉₂Si₈ alloy were of the order of 3.58 × 10⁶ Am^–1 at 2.0 K and 3.3 × 10^–2 Am^–2 at 2.42 K in the absence of an applied field. The upper critical field gradient at the transition temperature and the electrical resistivity at 4.2 K were about — 1.82 × 10^–1 Am^–1 K^–1 and about 150 cm. The Ginzburg-Landau parameter and coherence length _GL (0) were estimated to be about 65 and 6.3 nm, respectively, from these experimental values by using the Ginzburg-Landau-Abrikosov-Gorkov theory and hence it is concluded that the present b c c alloys are extremely soft type-II superconductors with a high degree of dirtiness.     

Properties of LT-AlGaN films and HT-GaN films using LT-AlGaN buffer layers grown on (0001) sapphire substrates

Low-temperature (LT) Al_xGa_1–xN (0.1 < x < 0.8) films, 0.4 m in thickness, were prepared on (0001) sapphire substrates at 500,C by alternate supply of Ga and Al alkyls and ammonia (NH₃). Al composition in the solid phase was identified based on the shift of the (0002) Bragg angle of X-ray diffraction. A series of high temperature (HT) GaN films, 1.0 m in thickness, were also grown at 1000,C on the LT-Al_xGa_1–xN coated (0001) sapphire substrates with buffer layer thickness ranging from 7.5 to 20 nm. It was found that the optimized LT-Al_xGa_1–xN buffer layer thickness decreases linearly with the Al-content. As-grown HT-GaN films having LT-LT-Al_0.43Ga_0.57N buffer layers show smooth surface based on optical microscopic (OM) observations. Transmission electron microscopy (TEM) confirms the mono-crystalline nature of the HT-GaN films. The quenched near band-edge photoluminescence (PL) emissions and an apparent yellow luminescence of the HT-GaN films are attributed to the LT-Al_xGa_1–xN buffer layer induced mosaic microstructure and bonding defects in the films.     

Thermocapillary stability during gravity flow of a liquid film

Thermocapillary rupture of a film under conditions of turbulent undulatory flow is associated with the buildup of wave motion on its surface. Here an approximate solution to the problem and criterial relations are obtained for determining the limits of stable film flow.Notation _min, kg/m·sec minimum irrigation intensity at which no film rupture occurs - ₁, kg/m· sec irrigation intensity at which the first dry spot appears - q, W/m² thermal flux density - _D, °C temperature at the rupture section - x, m space coordinate along the warm surface in the direction of flow - y, m coordinate in the direction normal to the warm surface - _o, m mean thickness of the film between large waves - _c, m thickness of the continuous layer - _cr, m critical film thickness - _o=/_o andl _o=l _o/_o dimensionless initial amplitude and length of a wave - , sec^–1 recurrence frequency of large waves - t_cr, sec time till thermocapillary rupture of a film - t_p, sec time of penetration of a thermal perturbation through the film thickness - u, m/sec velocity of thermocapillary flow of the liquid - , W/m·°C thermal conductivity - c_p, kJ/kg·°C specific heat - , kg/m linear density - , N·sec/m² dynamic viscosity - a, m²/sec thermal diffusivity - , N/m surface tension - , N/m² tangential stress at the film surface - L, m length of the warm pipe segment - L_o, m distance from the inlet to the section where wave motion at the film surface occurs - ¯w, m/sec mean velocity of downward flow of liquid in the film - , m mean thickness of the laminar layer - g, m²/sec free-fall acceleration due to gravity Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 39, No. 4, pp. 581–591, October, 1980.     

Phase Composition of Zn2TiO4–Zn2ZrO4Materials Prepared by Low-Temperature Plasma Synthesis and Ceramic Processing

Zn₂Ti_{1 – x} Zr _x O₄solid solutions were prepared by low-temperature plasma synthesis and solid-state reactions, and their properties were compared. The Zn₂TiO₄–Zn₂ZrO₄pseudobinary system was found to contain two broad solid-solution ranges with inverted spinel structures: phase at 0 < x< 0.3, witha= 8.474–8.555 Å, and phase at 0.5 < x< 1.0, with a= 8.615–8.740 Å and c= 8.733–9.120 Å. The conductivity of the solid solutions notably decreases upon substitution of Zr⁴⁺for Ti⁴⁺and varies exponentially with temperature.     

High pressure consolidation of B6O-diamond mixtures

Sintered composites in the B₆O-xdiamond (x= 0–80 vol%) system were prepared under high pressure and high temperature conditions (3–5 GPa, 1400–1800°C) from the mixture of in-laboratory synthesized B₆O powder and commercially available diamond powder with various grain sizes (<0.25, 0.5–3, and 5–10 m). Relationship among the formed phases, microstructures, and mechanical properties of the sintered composites was investigated as a function of sintering conditions, added diamond content, and grain size of diamond. Sintered composites were obtained as the B₆O-diamond mixed phases when using diamond with grain sizes greater than 0.5 m, while the partial formation of the diamond-like carbon was observed when using diamond grain sizes less than 0.25 m. Microhardness of the sintered composite was found to increase with treatment temperature and pressure, and the fracture toughness slightly decreased. A maximum microhardness of H _v57 GPa was measured in the B₆O-60 vol% diamond (grain size < 0.25 m) sintered composite under the sintering conditions of 5 GPa, 1700°C and 20 min.     

Synthesis of ZnFe2O4–Zn2ZrO4Solid Solutions

Low-temperature plasma synthesis was used to prepare solid solutions ( and ) in the ZnFe₂O₄–Zn₂ZrO₄pseudobinary system. The Zn_{2 – x} Zr_{1 – x} Fe_2x O₄solid solutions were found to have a tetragonal spinel structure (a= 8.607–8.740 Å, c= 8.798–9.120 Å) in the composition range x= 0–0.55 and a cubic spinel structure (a= 8.447–8.539 Å) at x= 0.75–1.0. The tetragonal lattice distortion is attributed to a pseudo-Jahn–Teller effect. The electrical conductivity of the solid solutions shows semiconducting behavior and rises by a few orders of magnitude with increasing Fe³⁺content.     

Preparation and characterization of in1−xGaxAsyP1−y epilayers by liquid-phase epitaxy

In_1–x Ga _x As _y P_1–y epilayers with three different solid compositions of ln_0.73Ga_0.27As_0.60P_1.40, In_0.59Ga_0.41As_0.87P_0.13 and ln_0.53Ga_0.47As were grown on (1 0 0) InP substrate at 623° C by the step cooling technique of liquid-phase epitaxy. From the optical transmission measurements, the corresponding wavelengths of the InGaAsP epilayers were 1.30, 1.55 and 1.69 m, respectively, which are in good agreement with those obtained from the calculations using Vegard's law. The full widths at half maximum of the photoluminescent spectra at 14 K of these layers were as low as 18.6, 22.5 and 7.9meV, respectively. The electron mobility of the InGaAsP epilayers is a function of the solid composition with the ln_0.53Ga_0.47As epilayer having the highest electron mobility. The mobility and concentration of this layer are 8,873cm²V^–1 sec^–1, 9.7×10¹⁵cm^–3 and 22,900 cm²V^–1 sec^–1, 8.5×10¹⁵cm^–3 at 300 and 77 K, respectively. The compensation ratio is between 2 and 5.     

AlGaN/GaN HEMTS: material, processing, and characterization

The growth of AlGaN/GaN high electron mobility transistor (HEMT) structures on sapphire by metal organic vapor phase epitaxy (MOVPE) is described, with special emphasis on procedures to reduce dislocation density. All the processing steps involved in the fabrication of nitride-based HEMTs have been optimized, including dry etching by ion beam milling, evaporation of Pt/Ti/Au gate contacts, and SiN _x surface passivation. Devices with several gate lengths and different geometries have been fabricated by standard photo- and e-beam lithography. d.c. drain current and transconductance increase when gate length is reduced, up to 950 mA mm^–1 and 230 mS mm^–1, respectively, at V _GS=0 V, in HEMTs with a gate length L _G=0.2 m. A maximum output power higher than 5 W mm^–1 is estimated. Finally, small-signal measurements yield f _T=12 GHz and f _max=25 GHz for HEMTs with L _G=0.5 m, which increase up to 20 and 35 GHz for L _G=0.2 m, respectively. Limitation of high-frequency performance by parasitics is discussed.     

The effect of MgAl2O4 on the formation kinetics of Al2TiO5 from Al2O3 and TiO2 fine powders

The formation of Al_2(1–x)Mg _x Ti_(1+x)O₅ solid solutions from Al₂O₃-TiO₂-MgAl₂O₄ powder mixtures of 1 m particle size and moderate purity has been studied at 1300°C for different final composition values: x=0 (pure Al₂TiO₅), 10^–3, 10^–2 and 10^–1. Analysis of the kinetic data and microstructural observation indicates that MgAl₂O₄ affects the mechanism of Al₂TiO₅ formation by providing active nuclei for the growth of the new phase. These nuclei are probably constituted by Mg_0.5AlTi_1.5O₅, i.e. the equimolar Al₂TiO₅-MgTi₂O₅ solid solution, and are formed by reaction between MgAl₂O₄ and TiO₂ at temperatures above 1150 °C. As the value of x increases, the number of titanate particles per unit volume accordingly increases and the conversion of the original oxides is faster. At values of x10^–2, the prevailing mechanism is the nucleation and growth of Al₂TiO₅ nodules for fractional conversion up to 0.8. Further conversion of the residual Al₂O₃ and TiO₂ particles dispersed into the titanate nodules is slower and controlled by solid-state diffusion through Al₂TiO₅. At x=0.1, a large number of nucleation sites is present, and solid-state diffusion through Al₂TiO₅ becomes important even in the initial stage of reaction, as the diffusion distances are strongly reduced. The study of Al₂TiO₅ formation under non-isothermal conditions in the temperature range 1250–1550°C shows that reaction proceeds between 1300 and 1350 °C for x=0.01 and between 1250 and 1300 °C for x=0.1. Densification of the titanate becomes important at temperatures above 1300°C for x=0.1, but only above 1450 °C for x=0.01.     

Structural and electrical properties of cadmium-sulpho-selenide solid solutions

Complete solid solutions of CdS _x Se_1–x (0x1) were synthesized by vacuum fusion of stoichiometric proportions of CdS and CdSe. X-ray diffraction data revealed that they possess the hexagonal wurtzite structure. The unit cell lattice constants vary linearly with the composition parameter,x, following Vegard's law.Thin films of CdS _x Se_1–x (0x1) solid solutions could be deposited onto glass substrates by thermal evaporation of the bulk material in 10^–4 Pa vacuum. Structural investigation showed that the films are polycrystalline with predominant appearance of the (002) reflecting plane. On annealing at 250°C in 10^–2 Pa vacuum atmosphere, aggregation and rearrangement of the as-deposited tiny crystallite occurred, preserving the same crystal structure.The dark electrical resistivity of the films is independent on the film thickness, but it varies appreciably with the composition parameter,x, showing a minimum resistivity of 0.02 cm atx=0.55. The temperature dependence of the resistivity follows the semiconducting behaviour with an extrinsic and an intrinsic conduction below and above 70°C, respectively. The determined activation energies 0.2 eV and 0.8 eV, respectively, correspond to shallow and deep trap levels, respectively.     

Fabrication of bulk superconductors YBa2Cu3?xTexO7?δ with highly oriented layer structure using the melt-texture growth process

We report on the successful fabrication of bulk samples satisfying the nominal composition YBa₂Cu_3–x Te _x O_7– and containing highly single-crystal-like texture. While this type of crystal growing process is not new, our process requires only the existence of spatial temperature gradient in some parts of the common tube furnace. The samples with tellurium were found to be much more stable and better superconductors than the Y-123 specimen. Large layer textures with cross-section 50×500m² were observed to grow inside the usual bulk sample. The critical currents were measured by the d.c. magnetization method and found to be easily greater than 6000 A cm^–2 for the samples with Te.     

Kinetics and thermodynamics of intracrystalline Fe-Ga distribution in Y3(Fe,Ga)5O12

Garnets of composition Y₃Fe_5–x Ga _x O₁₂, withx=0–5, were synthesized from oxides. Samples with various Ga content were annealed at temperatures between 700–1290° C; the heating duration varied between 90 s and 1350 h. Cation distribution was measured by Mössbauer spectroscopy at room temperature. The standard free energy change for the exchange reaction Fe³⁺ (tet)+ Ga³⁺ (oct)Fe³⁺ (oct)+Ga³⁺ (tet) is about 20 kJ mol^–1, and decreases slightly with increasing Fe content. The specific rate constants for the ordering process were determined according to the Mueller model for order-disorder kinetics. The activation energies for the ordering process between 200–250 kJ mol^–1 were calculated from the temperature dependence of the specific rate constants.     

Dielectric properties and microstructural behaviour of B-site calcium-doped barium titanate ceramics

Dielectric properties and microstructural behaviour of Ba_1–x Sr _x Ti_1–y Ca _y O_3–y ceramics, where strontium and calcium were doped on the barium and titanium sites, respectively, within the range 0x0.24 and 0y0.05, were investigated. Calcium addition decreased the tetragonality,c/a, increased the unit cell volume, and lowered the Curie temperature, which were all attributed to the occupancy of Ca²⁺ ions on titanium sites. When sintered at a low oxygen partial pressure of 10^–9 MPa, a resistivity higher than 10¹¹ cm was maintained for the formulations containing B-site calcium substitution more than 0.5 mol %. With increasing the amount of calcium addition, the Curie peak was depressed and completely broadened for the compositions with calcium addition more than 3 mol %, where the average grain size was smaller than 1 m. Co-firing with nickel electrodes in a reducing atmosphere also depressed the Curie peak and inhibited the grain growth due to the diffusion of nickel into the dielectrics.