Preparation and photovoltaic properties of anodically grown Ag2O films

The semiconducting and photovoltaic properties of p-type Ag₂O films grown anodically on silver electrodes were studied, in view of possible applications in solar energy conversion. Films were grown in different alkaline solutions; the best results were obtained for 0.02M Ag₂SO₄ + 0.17M NH₄OH + 5.7 × 10^–3M Ba(OH)₂ saturated with Ag₂O powder, stirred mechanically at room temperature. Film thicknesses of up to 10m were thus obtained for the first time in anodically grown Ag₂O. Photovoltaic spectra taken at 300 K give a bandgap ofEg = 1.42 ± 0.04 eV. Evaporated gold on Ag₂O appears to be ohmic while aluminium and platinum are rectifying. The barrier height of Ag/Ag₂O is 0.90 ± 0.02 eV, that of Al/Ag₂O is 0.93 ± 0.02 eV, and that of platinum 0.94 ± 0.02 eV. The best cells give an open-circuit voltage,V _oc, of over 150 mV, and a short circuit current,I _sc = 100A cm^–2 under 50 mW cm^–2 illumination.     

Processing and microstructure of porous and dense PZT thick films on Al2O3

The processing of porous PZT thick-film ceramics on Al₂O₃ has been studied. The films were screen-printed from a thixotropic ink of PZT with a 58% solids content. The thick films were sintered between 1000 and 1150°C for 2 h. The sintered films show a 10 m thickness and an average pore diameter ranging from 1–2 m. The PZT forms a continuous skeleton that can be filled with the desired polymer. Dense and continuous PZT films were fabricated by screen-printing PZT ink on previously electroded Al₂O₃ substrates with Ag/Pd 70/30 paste. Densification of the PZT was obtained by sintering near the liquidus temperature of the Ag-Pd system.     

Preparation and characterization of copolymerized methylsilsesquioxane-benzylsilsesquioxane microparticles for electrophoretic sol-gel deposition

Copolymerized methylsilsesquioxane-benzylsilsesquioxane, (100 – x)MeSiO_3/2·xBnSiO_3/2 (mol%), microparticles of 0.3–0.4 m in average diameter have been prepared by the sol-gel method and applied to the formation of thick films on indium tin oxide (ITO)-coated glass substrates by the electrophoretic deposition. The microparticles obtained were considered to be hybrid containing both methyl and benzyl groups from thermal analyses and structural investigations. The microparticles became fusible at x 80 by a heat treatment. Thick film, ca. 5 m thick, consisting of electrophoretically deposited pure BnO_3/2 microparticles became transparent upon thermal sintering of the particles during heat treatment at temperatures higher than 100°C. On the other hand, the increase in optical transmittance during the heat treatment of the films consisting of 20MeSiO_3/2·80BnSiO_3/2 microparticles was as small as about 20% in visible range even after the heat treatment. The smaller increase in transmittance is probably due to the relatively high viscosity of the 20MeSiO_3/2·80BnSiO_3/2 upon sintering in comparison with pure BnO_3/2 for elimination of the voids in the films.     

Spherical particle preparation of Na+-ion conductive Na4Zr2Si3O12 by mist pyrolysis

Spherical polycrystalline particles of Na₄Zr₂Si₃O₁₂ (NZS) have been prepared by a mist pyrolysis technique with alkoxide ethanol solution. Firing the precursor particles, obtained by pyrolysing at 550 °C, yielded the NZS with the highest S/N ratio and the least second phase at 1000 °C. Each spherical particle with an average size of 0.5 m was composed of primary particles of 0.1 m diameter after firing. Comparing the particle formation process with the sol-gel, alkoxide pyrolysis and the aqueous solution processes, it would appear that the primary particle size of the mist-pyrolysed NZS depends on the size and residual alkyl group content of the precursor polymers.     

The Superconductive Critical Magnetic Field of Beryllium

We have measured the temperature dependence of the superconductive critical magnetic field of three samples of beryllium from three different sources. The irregular geometric shape of one sample prevented a complete analysis of the data, but the data for the other two samples were well represented by the BCS model of superconductivity. The values of T _c and H _c (0) are found to be sample dependent. For the purest sample studied, T _c =24.34±0.02 mK, H _c (0)=107.7±0.2 T and (/V)=32.9±0.3 J/cm³ K². By a very small extrapolation of the data as a function of purity, it is inferred that the critical parameters for pure Be are: T _c =24.38±0.02 mK, H _c (0)=107.9 T±0.2 T and (/V)=33.0±0.3 J/cm³ K².     

Effects of junction formation conditions on the photovoltaic properties of sintered CdS/CdTe solar cells

Microstructures and properties of sintered CdS films on glass substrates and sintered CdTe films on polycrystal CdS substrates have been investigated. The CdS films, which contained 9 wt % CdCl₂ as a sintering aid and were sintered at 650° C for 1 h in nitrogen, are transparent and have an average grain size of 15m and an electrical resistivity of 0.5cm. The CdTe films, which were coated on the sintered CdS substrate and were sintered above 610° C for 1 h in nitrogen, have a dense structure with an average grain size larger than 5m. All polycrystal CdS/CdTe solar cells were fabricated by this successive coating and sintering method. The sintering temperature of CdTe films on the sintered CdS films was varied from 585 to 700° C. Compositional interfaces and p-n juctions are formed during sintering. The highest solar efficiency (7.18%) was found in a solar cell made by sintering the composite layer of glass-CdS-CdTe at 625° C for 1 h. A fabrication temperature below 610° C resulted in poor solar cell efficiencies due to the porous structure of the CdTe films and above 650° C also resulted in poor efficiencies due to the formation of a CdS_1-x Te_x layer at the interface and a large p-n junction depth.     

Substrate/film interactions in thick films of YBaCuO on alumina

The substrate-film interaction in thick films (>10m) of YBa₂Cu₃O_7–x on alumina processed under normal conditions is investigated using electron-probe microanalysis. The formation of a mixture of barium aluminate and alumina over a thickness of about 2m in the interfacial region is established quantitatively using compositional mapping and point-count analysis across the substrate-film interface. Diffusion of aluminium into the film over severalm beyond the reaction layer is also observed. The variation of oxygen composition across the interface has been mapped. Leaching of oxygen from the 1-2-3 phase in the bulk is suggested as the reason for the observed decrease inT _c(0) and increase in T _c in films of YBa₂Cu₃O_7–x on alumina.     

Giant dielectrics from modified boundary layers in n-BaTiO<Subscript>3</Subscript> ceramics involving selective melting reactions of silver/glass composites at the grain boundaries

High permittivity ceramics with _eff > 10⁵ can be realized from semiconducting BaTiO₃ by the two-step processing namely, sintering the donor doped samples in static air followed by electroding with the fired-on silver/glass composites. Doping with Sb⁵⁺ and Bi³⁺ not only enhances the grain conductivity but also increases the grain size (10–60 m), when sintered at 1370 C in static air. The ceramic samples are electroded with the paste containing nanometer particles of silver dispersed in varied amounts of low melting (600–900 C) glass compositions PbO + Bi₂O₃ + B₂O₃ ± SiO₂ ± CuO. High permittivities are obtained for these capacitors stable over a wider range of temperature and over a broad frequency range. The grain boundary layer effect superimposed with the contributions from the barrier layers formed during electroding, related to ceramic microstructure is proposed to be responsible for the unusual high permittivity in semiconducting BaTiO₃. The energy dispersive X-ray analyses indicate selective melting reactions at the grain boundary layers with higher concentrations of the low melting oxides at the grain boundaries near to the electrodes. Impedance spectroscopy on BaTiO₃ ceramics demonstrates that they are electrically heterogeneous with insulating grain boundaries together with the ceramic/electrode interface acting as barrier layers. On the basis of the symmetrical Schottky-barrier model of the grain boundary region, the barrier height and donor concentration N_d of the grains were obtained by the modified 1/C²–V plot. These modified boundary layer capacitors having high field strength withstandability can be used in a wide range of frequencies.     

Preparation and properties of antimony-doped SnO2 films by thermal decomposition of tin 2-ethylhexanoate

Transparent Sb-doped SnO₂ films were prepared at 600° C on glass substrates by thermal decomposition of tin 2-ethylhexanoate and antimony tributoxide. The films 100 to 300 nm thick, which are composed of fine particles, were very smooth. The films showed no preferred orientation. The minimum resistivity (2.1×10^–2 cm) was attained at a concentration of 8 at% Sb on the substrate precoated with SiO₂. The transmission of these films was about 80% over a wavelength range from 0.4 to 2.0 m.     

Stress characterization of device layers and the underlying Si1−x Ge x virtual substrate with high-resolution micro-Raman spectroscopy

Silicon–germanium (Si–Ge) epitaxially grown mismatched heterostructures are becoming increasingly important for high-frequency microelectronics applications. One option under serious consideration is that of using Si–Ge virtual substrates, i.e., compositionally graded layers designed to accommodate the lattice mismatch between the underlying Si substrate and the overlying active epilayers(s). This assists in the prevention of misfit dislocations that can impact adversely on the active device regions. The stress in both device silicon cap layers and the underlying Si_1–x Ge _x virtual substrates is characterized with high-resolution micro-Raman spectroscopy (RS). The device layers of the samples studied composed of a 7-nm thick silicon channel, a 6-nm thick SiGe layer and were capped with a 7-nm thick silicon layer. The device layers are grown over a 1-m thick constant composition Si_0.70Ge_0.30 virtual substrate capping layer, and the Si-Ge virtual substrate is grown on a p⁺-type (0 0 1) silicon wafer with a thickness of about 500 m. RS measurement results with a 488-nm Ar⁺ visible laser source indicate that the Si_0.70Ge_0.30 capping layer at the virtual substrate is fully unstrained, while the top silicon cap layer is in extremely high tension. The use of a 325-nm HeCd UV laser for the RS measurements, which probes only a very small depth into the Si cap layer (approximately 9 nm) confirms this high tensile stress is in the top silicon cap layer. The tensile stress in the top silicon cap layer is estimated to be as large as 2.4 GPa by analyzing the shift of the Si Raman peak with respect to the standard strain-free silicon sample. The measured stress value is almost equal to the theoretically predicted tensile stress that should exist in the fully strained Si cap layer. This implies that the Si cap layer remains strained in samples with this structure.     

Surface resistance of large-area Tl2Ba2CaCu2O8 thin films at microwave and millimeter wave frequencies measured by three noncavity techniques

The surface resistanceR _s of Tl₂Ba₂CaCu₂O₈ films fabricated on LaAlO₃ wafers up to 3 inches (7.6 cm) in diameter through a post-deposition anneal process was measured over the frequency range 5.55–94.1 GHz by the following techniques: 5.55 and 27.5 GHz high-temperature superconductor (HTS)-sapphire resonators, 10 GHz parallel plate resonator, and 94.1 GHz scanning confocal resonator.R _s was found to exhibit a quadratic dependence on frequencyf at 77 K:R _s f ^2.0±0.1. The highest-quality films yieldR _s =145±15 at 10 GHz and 77 K. Scanning confocal resonator mapping ofR _s across a 2-inch (5.1 cm) diameter wafer yielded a base value forR _s of 16±1 m at 77 K and 94.1 GHz (equivalent to 180±10 at 10 GHz) and good uniformity inR _s across the wafer. HTS-sapphire resonator measurements ofR _s for fifteen 1.2 cm square parts cut from a 3-inch diameter wafer yieldedR _s values scaled to 10 GHz of 196±10 at 80 K. Similar values were measured for Tl₂Ba₂CaCu₂O₈ films prepared on both sides of a 2-inch diameter wafer.Rs values at 10 GHz and 80 K of 147–214 were maintained over the course of 40 independent and successive deposition runs and corresponding anneals under nominally identical film fabrication conditions. Surface resistance at 5.55 GHz remained below 80 for maximum rf magnetic fields up to 85 Oe at 4.2 K and 7 Oe at 80 K, respectively. Results are compared with predictions of the two-fluid model. The relative advantages and disadvantages of the different techniques for measuring surface resistance are discussed.     

Measurement of the Micrometer Diameters of Focused Laser-Beam Radiation with a Thin-Wire Bolometer

The profile distributions of a focused Gaussian beam of 63-m laser radiation for E- and H-polarizations were recorded by a thin-wire bolometer 3 m in diameter. The profiles were used to determine the beam diameters, which were D ^E = (8.16 ± 0.12) m and D ^H = (8.00 ± 0.12) m upon elimination of the dominant systematic errors.     

YBCO Thin Film Preparation on Unbuffered Ag Substrate by Resistive Evaporation

It is widely accepted that thin film formation of YBCO on conducting and flexible substrate is one of the keys for further development of advanced devices in the microelectronics. Various fabrication methods such as spray pyrolysis, powder-in-tube processing, dip-coating rolling-assisted biaxially textured substrate, etc., widely used for YBCO thick films (few tens of m) production. However in this work we report for the first time on the preparation of YBCO thin film on unbuffered silver substrate using a simple conventional vacuum system equipped with only one single resistively heated evaporation source. The subsequent heat treatment was carried out under a low oxygen partial pressure and temperature that never exceeded 740° C. A thin film of Ag was first deposited on MgO substrate. A pulverized stoichiometric mixture of Y, Cu, and BaF ₂ was then deposited on this film. The amorphous YBCO film of a 500-nm thickness was obtained after the heat treatment. The results of the film evaluation are presented and discussed.     

The effect of additives on the crystallization and sintering of 2MgO-2Al2O3-5SiO2 glass-ceramics

Crystallization and sintering behaviour of three cordierite (2MgO-2Al₂O₃-5SiO₂) glasses containing different amount of additives were investigated and compared by using differential thermal analysis (DTA), X-ray diffraction (XRD), scanning electron microscopy (SEM), and the Archimedes method. The stoichiometric 2MgO-2Al₂O₃-5SiO₂ (MAS) glass and the 2MgO-2Al₂O₃-5SiO₂ glass containing 3 wt% of B₂O₃ and 3 wt% of P₂O₅ (MASBP) showed two exotherms (one for -cordierite formation from a glass and the other for -cordierite formation from the -cordierite phase), whereas the 2MgO-2Al₂O₃-5SiO₂ glass containing 2 wt % of B₂O₃, 2 wt% of P₂O₅, and 2 wt % of TiO₂ (MASBPT) showed only a single exotherm representing -cordierite formation. By using Kissinger, Augis-Bennett, Ozawa, and modified Kissinger methods, the activation energy values for -cordierite formation in the MASBP and MASBPT glasses were determined as 310±6 and 326±13 kJ mol^–1, respectively, whereas that in the MAS glass was determined as 868±5 kJ mol^–1. The MASBPT glass showed the lowest peak temperature value for -cordierite formation (980 °C) amongst the three glasses. Both the MASBP and MASBPT glasses showed excellent sintering behaviour (> 99.7% of theoretical density).     

Neutron Diffraction Studies of Nuclear Magnetic Ordering in Silver

Nuclear antiferromagnetism in fcc silver metal, already investigated by NMR measurements, has been studied in a single crystal of ¹⁰⁹ Ag by neutron absorption and diffraction techniques. Below the Neel temperature T_N , a (001) Bragg reflection with a resolution limited width demonstrates long range order in a simple type-I AFM structure with the ordering vector k = (2/a)(001) (up-down structure). The entropy at the transition, ln 2 in zero magnetic field, corresponding to a critical polarization P_c = 0.75 and T_N = 700 ± 80 pK. Magnetic field B versus entropy S phase diagrams of the (001) structure have been constructed for two directions of B: [001] and The critical field extrapolated to S = 0 is 100 ± 10 T, and for both field directions S_c is highest around B = 30 T. The transition to the paramagnetic state is presumably of second order. The nuclear magnetization was measured by transmission of unpolarized neutrons, and the dimensionless static volume susceptibility in SI units was found to be 0.36 ± 0.01 in the ordered state independently of B and S. The ac susceptibility at 7.9 Hz showed a kink at the transition only when the sample was not exposed to neutrons.     

The mechanical properties of laminated microscale composites of Al/Al2O3

The tensile properties, at both room and elevated temperatures, of laminated thin films containing alternate layers of aluminium and aluminium oxide were investigated. At room temperature the strength of the films followed a Hall-Petch type relationship dependent on the interlamellar spacing, and the strength could be extrapolated from data for conventional grain size aluminium. At the finest interlayer spacing of 50 nm, the strength was equivalent to/70, where is the shear strength of aluminium and the samples exhibited very extensive ductility. At elevated temperatures, cavitation became an important deformation mechanism but it occurred preferentially at Al/Al rather than Al/Al₂O₃ boundaries. The microstructure of the films was probed using transmission electron microscopy and fractography was used to investigate deformation and fracture mechanisms.     

High-Purity As2S1.5Se1.5 Glass Optical Fibers

Core–clad optical fibers were fabricated from high-purity As₂S_1.5Se_1.5 glass, and their properties were studied. The arsenic sulfo-selenide was prepared by melting a mixture of high-purity arsenic monosulfide, arsenic, and selenium. Optical fibers with core/clad diameters of 300/400 and 200/400 m were fabricated by the double-crucible method. The minimum loss was found to be 60 ± 20 dB/km at 4.8 m and 200–300 dB/km between 4 and 6 m. The numerical aperture of the fibers was 0.28. A 1.5-m-long section of the fiber transmitted 6-W CO laser radiation. The average bending strength of the 400-m-diameter fibers was 0.8 GPa.     

Gas atomization of cobalt ferrite-phosphate melts

A rapidly solidified (Co, Fe)₃O₄ spinel was formed in a cobalt-iron-phosphate glass matrix by gas atomization of melts of composition 37.5% mol % CoO, 37.5%Fe₂O₃, 25% P₂O₅; and 40% CoO, 40% Fe₂O₃, 20% P₂O₅, and the material has been characterized using size analysis, X-ray diffraction, Mössbauer spectroscopy, and scanning electron microscopy with energy dispersive X-ray spectroscopy.In the 20 mol % P₂O₅ composition, atomized powder 50–100m in diameter contained 33 wt % (Co, Fe)₃O₄. The ferrite crystallized as randomly oriented, faceted dendrites parallel to 100, and the growth was apparently diffusion controlled. Unlike the ferrite, where iron is in both tetrahedral and octahedral coordination, the iron in the glassy matrix was predominantly in distorted octahedral coordination. Overall particle size distributions were broad (15–700m), with 50 wt % smaller than 200m. The maximum in the size distribution shifted from 150 to 80m on increasing the atomizing pressure from 7 to 20 bar (100 to 300 psi). Calculations have shown that the cooling rates obtained with oxide melts vary strongly with droplet size, and less strongly with melt temperature.     

Mechanical and histological evaluations of cobalt-chromium alloy and hydroxyapatite plasma-sprayed coatings in bone

This study evaluated the mechanical and histological behavior of cobalt-chromium (CoCr) alloy and hydroxyapatite (HA) plasma-sprayed coatings in canine cortical bone after 6 and 12 weeks of implantation, using CoCr alloy as the substrate. the substrate was bond-coated with microtextured CoCr alloy coating to ensure adherence between the substrate and top coats. A macrotextured CoCr alloy top coat with surface roughness R _a=34.25±5.50 m was produced to create suitable pores ranging from 25 m to 200 m for bone ingrowth. For HA top coat, a relatively smooth surface (R _a=15.14±3.21 m) was prepared for bone apposition. Shear testing of bone/implant interfaces showed that the CoCr alloy top coat exhibited significantly lower (p<0.01) mean shear strength than the HA top coat at each time interval. The maximum shear strength was 10.88±0.38 MPa for HA-coated implants 12 weeks post-implantation. After histological evaluations, substantial differences in the extent of new bone formation and the types of implant/bone contact were found between two kinds of implants. Direct bone-to-HA coating contact was consistently observed, while a layer of fibrous tissue intervening at the bone-CoCr alloy coating interface was found. Occasionally, partial dissolution of HA coating was seen after 12 weeks of implantation. The results of this study suggested that plasma-sprayed macrotextured CoCr coatings may not be an effective alternative for biological fixation.     

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.