Resistance characteristics to oxidation of the metal-catalyzer for Cat-CVD

In order to obtain oxide films using catalytic chemical vapor deposition (Cat-CVD), oxidation experiments for catalyzers of Alumel, Al Chrom-O, Chromel, Hastelloy C-276, Kanthal, Kovar, Inconel-600, Inconel-601, Inconel X-750, Iron Chrome 30, Moleculoy, Monel, Ni, Nickel Chrome, Pt, SUS-304, SUS-316, Super Invar, and Ti have been carried out. The electric resistance measured for each metal heated at 900 °C, exposed for 30 min in O₂ atmosphere, have revealed that Alumel, Al Chrom-O, Chromel, Hastelloy C-276, Kanthal, Inconel-600, Inconel-601, Inconel X-750, Iron Chrome 30, Moleculoy, Ni, Nickel Chrome, Pt, SUS-304, and SUS-316 are in resistance to oxidation.     

Composition of alumina films grown on Si at low temperature with catalytic CVD

The X-ray photoelectron spectroscopy (XPS) measurements have been used to reveal the compositions of alumina (Al₂O₃) films formed on Si wafers using tri-methyl aluminium (TMA) and molecular oxygen (O₂) with catalytic chemical vapour deposition (Cat-CVD). The atomic ratio (O/Al) for Al₂O₃ samples formed at substrate temperature of 200-400 °C has been obtained to be 1.4 which is close to stoichiometry. The increase of growth rate at substrate temperatures below 200 °C and above 400 °C can be attributed to formation of aluminum oxides with non-stoichiometry and metallic aluminum incorporated in the films resulting from deficient oxygen. Angle resolved XPS measurements have revealed that the alumina/Si interface with no SiO₂ film has been obtained at substrate temperatures below 200 °C.     

Structural and optical studies of nanocrystalline V2O5 thin films

We report the structural and optical properties of nanocrystalline thin films of vanadium oxide prepared via evaporation technique on amorphous glass substrates. The crystallinity of the films was studied using X-ray diffraction and surface morphology of the films was studied using scanning electron microscopy and atomic force microscopy. Deposition temperature was found to have a great impact on the optical and structural properties of these films. The films deposited at room temperature show homogeneous, uniform and smooth texture but were amorphous in nature. These films remain amorphous even after postannealing at 300 °C. On the other hand the films deposited at substrate temperature T_S > 200 °C were well textured and c-axis oriented with good crystalline properties. Moreover colour of the films changes from pale yellow to light brown to black corresponding to deposition at room temperature, 300 °C and 500 °C respectively. The investigation revealed that nanocrystalline V₂O₅ films with preferred 001 orientation and with crystalline size of 17.67 nm can be grown with a layered structure onto amorphous glass substrates at temperature as low as 300 °C. The photograph of V₂O₅ films deposited at room temperature taken by scanning electron microscopy shows regular dot like features of nm size.     

Synthesis and photocatalytic activity of cobalt oxide doped ZnFe2O4-Fe2O3-ZnO mixed oxides

Novel cobalt oxide doped ZnFe₂O₄-Fe₂O₃-ZnO mixed oxides with the Zn/Fe molar ratio of 1/2 were synthesized with a citric acid complex method. The effects of cobalt oxide and calcination temperature on phase composition and photocatalytic activity of the mixed oxides were investigated. X-ray diffraction (XRD) analysis revealed that there were mainly ZnFe₂O₄, α-Fe₂O₃, amorphous ZnO and Fe₂O₃ in the 6 mol% cobalt oxide doped products calcined at 500 °C. 5-10 mol% cobalt oxide doping could significantly enhance the formation of ZnFe₂O₄ and altered the phase composition of the mixed oxides. Experimental results showed that cobalt oxide doping could remarkably improve the photocatalytic activity of the mixed oxides for phenol degradation. The 6 mol% cobalt oxide doped mixed oxides calcined at 500 °C exhibited better photocatalytic activity as compared with other samples.     

Investigation of chlorination process in aluminum production by carbothermic-chlorination reduction of Al2O3 under vacuum

The chlorination process in aluminum production by carbothermic-chlorination reduction of Al₂O₃ under vacuum has been investigated by XRD, SEM and thermodynamic analysis. The results of thermodynamic calculations indicated that the chlorination of Al₄C₃ or Al₄O₄C possibly proceeded at temperature 1300 °C under 5-50 Pa. The experiment results showed that aluminum could be produced by the chlorination of Al₄C₃ or Al₄O₄C and the decomposition of AlCl(g). According to the analysis of experiment results, Al₄C₃ and Al₄O₄C were the main reactants participated in chlorination process in aluminum production by carbothermic-chlorination reduction of Al₂O₃ under vacuum. AlCl(g) generated in chlorination of Al₄C₃ or Al₄O₄C would decompose into Al and AlCl₃(g), and then aluminum product would condense in condensation tower.     

Synthesis of LSMO at low temperature by novel hydroxide precursor technique

A novel technique of mixing individual hydroxide is employed to prepare La_0.65Sr_0.35MnO₃ (LSMO) at low temperature. Freshly prepared lanthanum and manganese hydroxides are mixed thoroughly with strontium hydroxide in stoichiometric ratio and heated at different temperatures ranging from 100 to 500 °C for 6 h. At 500 °C, formation of La_0.65Sr_0.35MnO₃ was confirmed by the X-ray diffraction studies (XRD). This is the lowest temperature so far reported in the literature. The particle size and morphology were investigated by scanning electron microscopy (SEM).     

Properties of screen-printed Ho-Ba-Cu-O films on YSZ substrates

High-T_c screen-printed Ho-Ba-Cu-O films were prepared on YSZ substrates by a melt processing method. The films were fired at T_s = 1000-1050 °C for 5 min and cooled to 450 °C by two steps in flowing O₂. The maximum critical current density J_c (77 K, 0 T) of 2.0 × 10³ A cm^− 2 was only attained under much limited firing conditions; T_s = 1020 °C and cooled to 800 °C at a cooling rate of 400 °C h^− 1.     

Influence of synthesis procedure on the morphology of bismuth oxide particles

A modified sol-gel procedure, based on the esterification reaction, was used to prepare the Bi₂O₃ precursor, which was then heated to 400 or 500 °C. β-Bi₂O₃ obtained at 400 °C showed well-shaped plate-like particles. The mixture β-Bi₂O₃/α-Bi₂O₃, obtained by prolonged heating at 400 °C, yielded pseudospherical particles having about 100 nm in size and much larger particles, as found by FE SEM. α-Bi₂O₃ obtained at 500 °C consisted of particles of varying shapes and sizes. Vitrification of α-Bi₂O₃ was also observed. XRD showed a small fraction of unidentified phase(s) in these samples. Different microstructures were obtained when the precipitation from aqueous Bi(NO₃)₃ solution with tetramethylammonium hydroxide at pH ∼ 14 was used. The precipitation at pH ∼ 3.5 yielded cloverleaf-like particles of good uniformity, which were assigned to BiOOH (isomorphous with (La_0.26Bi_0.24)Bi_0.5OOH. It was found that these particles were made up of much smaller primary BiOOH particles.     

Characterization of polycrystalline SiC films grown by HW-CVD using silicon tetrafluoride

SiC films were synthesized by hot-wire chemical vapor deposition using a tungsten filament and a gas mixture of SiF₄ and CH₄. The etching of the substrate instead of the film growth occurred on the samples prepared using only source gases without H₂ dilution. The atomic or molecular hydrogen was believed to control the density of radicals containing F in a gas phase or on a growth surface. Polycrystalline 3C-SiC(111) films were successfully obtained at substrate temperatures lower than 500 °C by using H₂ dilution. The growth mode limited by source-gas supply was found to be important to obtain polycrystalline SiC films. The SiC film grown at higher deposition pressure was amorphous and contained no Si-H_x bonds but 6% fluorine. In SiF₄/CH₄/H₂ system, the radicals containing F are considered to play very important roles in the reactions both on a growth surface and in a gas-phase.     

Facile synthesis of MnV2O6 nanostructures with controlled morphologies

MnV₂O₆ nanostructures including nanorods, nanobelts, and nanosheets, have been synthesized by a facile hydrothermal reaction between Mn(CH₃COO)₂·4H₂O and commercial V₂O₅. The synthesized products are characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The influences of synthetic parameters, such as, reaction time, temperature and medium, on the morphologies of the resulting products have been investigated. As the reaction temperatures increase from 120 °C to 180 °C, MnV₂O₆ nanorods and nanobelts are obtained, respectively. The time-dependent experimental results at 180 °C reveal that the sizes of MnV₂O₆ nanobelts increase gradually with the reaction proceeding. Interestingly, as the reaction is carried out with the aid of H₂O₂ solution, flower-like MnV₂O₆ nanosheets are formed.     

Characterization of nano-crystalline LiNiVO4 synthesized by hydrothermal process

LiOH·H₂O, Ni(CH₃COO)₂·4H₂O and NH₄VO₃ were used to synthesize nano-crystalline LiNiVO₄ by hydrothermal process in deionized water at 150 °C for 2 h and subsequent calcination at 300-600 °C for 6 h. By using an X-ray diffractometer (XRD), a transmission electron microscope (TEM) and a selected area electron diffraction (SAED) method, nano-crystalline LiNiVO₄ with inverse spinel structure was detected. The stretching vibration of VO₄ tetrahedrons analyzed by a Fourier transform infrared spectrometer (FTIR) was split into three bands at 661, 746 and 835 cm^− 1, and that analyzed by a Raman spectrometer was detected at 823.9 and 787.7 cm^− 1. The thermogravimetric and differential thermal analyses (TGA and DTA) show two discrete weight losses at 25-117 °C and 117-600 °C and four endothermic peaks at 84, 145, 202 and 372 °C, corresponding to the evaporation of water and the decomposition of inorganic and organic compounds.     

Thermoluminescence properties of Ce-doped LiSr4(BO3)3 phosphor

Borates LiSr₄(BO₃)₃ were synthesized by high-temperature solid-state reaction. The thermoluminescence (TL) and some of the dosimetric characteristics of Ce³⁺-activated LiSr₄(BO₃)₃ were reported. The TL glow curve is composed of only one peak located at about 209 °C between room temperature and 500 °C. The optimum Ce³⁺ concentration is 1 mol% to obtain the highest TL intensity. The TL kinetic parameters of LiSr₄(BO₃)₃:0.01Ce³⁺ were studied by the peak shape method. The TL dose response is linear in the protection dose ranging from 1 mGy to 1 Gy. The three-dimensional thermoluminescence emission spectra were also studied, peaking at 441 and 474 nm due to the characteristic transition of Ce³⁺.     

Nanoporous TiO2 thin film based conductometric H2 sensor

Nanoporous titanium dioxide (TiO₂) based conductometric sensors have been fabricated and their sensitivity to hydrogen (H₂) gas has been investigated. A filtered cathodic vacuum arc (FCVA) system was used to deposit ultra-smooth Ti thin films on a transducer having patterned inter-digital gold electrodes (IDTs). Nanoporous TiO₂ films were obtained by anodization of the titanium (Ti) thin films using a neutral 0.5% (wt) NH₄F in ethylene glycol solution at 5 V for 1 h. After anodization, the films were annealed at 600 °C for 8 h to convert the remaining Ti into TiO₂. The scanning electron microscopy (SEM) images revealed that the average diameters of the nanopores are in the range of 20 to 25 nm. The sensor was exposed to different concentrations of H₂ in synthetic air at operating temperatures between 100 °C and 300 °C. The sensor responded with a highest sensitivity of 1.24 to 1% of H₂ gas at 225 °C.     

Preparation and characterization of Nb2O5-Al2O3 composite oxide formed by cathodic electroplating and anodizing

Al foil was coated with niobium oxide by cathodic electroplating and anodized in a neutral boric acid solution to achieve high capacitance in a thin film capacitor. X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) revealed the niobium oxide layer on Al to be a hydroxide-rich amorphous phase. The film was crystalline and had stoichiometric stability after annealing at temperatures up to 600 °C followed by anodizing at 500 V, and the specific capacitance of the Nb₂O₅-Al₂O₃ composite oxide was approximately 27% higher than that of Al₂O₃ without a Nb₂O₅ layer. The capacitance was quite stable to the resonance frequency. Overall, the Nb₂O₅-Al₂O₃ composite oxide film is a suitable material for thin film capacitors.     

Synthesis, characterization and magnetoresistance properties study of magnetite thin films by electroless plating in aqueous solution

Polycrystalline half-metallic Fe₃O₄ films with 1 μm in thickness were synthesized on glass substrates directly by electroless plating in aqueous solution at 90 °C without heat treatment. The films have single pure spinal phase structure and the well-crystallized columnar grains grow perpendicularly to the substrates, as revealed by XRD, XPS and SEM. At room temperature, the films exhibit negative magnetoresistance (MR) ratio of about −5.1%, which is ascribed to intergranular tunneling of spin polarized electrons of Fe₃O₄. The resistivity R of the films with 1 μm in thickness at room temperature is about 5.2 × 10⁻¹ Ω cm. The cation distribution and the arrangement of the magnetic moments of the plated Fe₃O₄ ferrite thin films are different from that of the bulk materials, which is likely to be one of the reasons for the modification of R and MR properties.     

Effects of post-deposition surface treatment on the optical, structural and hydrogen sensing properties of TiO2 thin films

TiO₂ thin films were prepared by DC reactive magnetron sputtering in a mixture of oxygen and argon on glass and oxidized silicon substrates. The effect of post-deposition annealing (300 °C, 500 °C and 700 °C for 8 h in air) on the structural and morphological properties of TiO₂ thin films is presented. In addition, the effect of Pt surface modification (1, 3 and 5 nm) on hydrogen sensing was studied. XRD patterns have shown that in the range of annealing temperatures from 300 °C to 500 °C crystallization starts and the thin film structure changes from amorphous to polycrystalline (anatase phase). In the case of samples on glass substrate, optical transmittance spectra were recorded. TiO₂ thin films were tested as sensors of hydrogen at concentrations 10,000-1000 ppm and operating temperatures within the 180-200 °C range. The samples with 1 nm and in particular with 3 nm of Pt on the surface responded to hydrogen fast and with high sensitivity.     

A novel anode material of Fe2VO4 for high power Lithium ion battery

Spinel Fe₂VO₄ was synthesized by solid state method and its properties were characterized using XRD, SEM, TG-DSC and specific surface area measurements. The average size of the particles was 500 nm. TG-DSC test demonstrated that the Fe₂VO₄ was thermally stable in nitrogen atmosphere within 300 °C. Lithium insertion into the sample at room temperature and 55 °C has been investigated and the highest discharge capacity approached 250 mAh/g at an average voltage of 0.6 V vs Li⁺/Li. Capacity retention was unexpectedly good at 1C discharge rate at room temperature and 55 °C. At 5C discharge rate, the specific capacity was still 136 mAh/g. The results show that the Fe₂VO₄ is a promising anode material due to its high specific capacity, thermal stability and rate performances.     

Sputter deposited nanocrystalline Ni-25Al alloy thin films and Ni/Ni3Al multilayers

Thin films of nominal composition Ni-25at%Al have been sputter deposited from a target of the intermetallic compound Ni₃Al at different substrate deposition temperatures. The film deposited on an unheated substrate exhibited a strongly textured columnar growth morphology and consisted of a mixture of metastable phases. Nanoindentation studies carried out on this film exhibited a strong strain hardening tendency. In contrast, the film deposited at 200 °C exhibited a recrystallized non-textured microstructure consisting of grains of a partially ordered Ni₃Al phase. At higher deposition temperatures (∼400 °C), larger grains of the bulk equilibrium, long-range ordered, Ll₂ Ni₃Al phase were observed in the film. Unlike the film deposited on an unheated substrate, the films deposited at elevated temperatures did not exhibit any dependence of the hardness on the indentation depth and, consequently no strain hardening. The average hardness of the film deposited at 200 °C was higher than the one deposited at 400 °C. In addition to monolithic Ni-25Al thin films, multilayered Ni/Ni3Al thin films were also deposited. Multilayers deposited non-epitaxially on unheated substrates exhibited a strong {111} fiber texture while those deposited epitaxially on (001) NaCl exhibited a {001} texture. Free-standing multilayers of both types of preferred orientations as well as of different layer thicknesses were deformed in tension untill fracture. Interestingly, the {111} oriented multilayers failed primarily by a brittle fracture while the {001} multilayers exhibited features of ductile fracture.     

Electrical properties of alumina films grown on Si at low temperature using catalytic CVD

The electrical properties of alumina films formed at substrate temperatures as low as 27 °C using tri-methyl aluminum (TMA) and molecular oxygen (O₂) by catalytic chemical vapor deposition (Cat-CVD) have been investigated by capacitance-voltage (C-V), current-voltage (I-V) measurements and X-ray photoelectron spectroscopy (XPS). Substrate temperature dependence of dielectric constant and leakage current of the films has been explained on the basis of deficiency in oxygen. Interface trapping density of the order of 10⁹ ev^− 1cm^− 2 has been obtained. Angle resolved XPS measurements have revealed that the direct bonding of alumina and Si was realized with very small interface trapping density.     

Effect of thermal exposure on the stability of carbides in Ni-Cr-W based superalloy

The microstructure evolutions of Ni-Cr-W based superalloy during thermal exposure have been investigated systematically. M₆C carbides in the alloy decompose into M₂₃C₆ carbides at temperatures from 650 to 1000 °C due to its high content of Cr. The M₆C carbides decompose dramatically from 800 to 900 °C. At temperatures up to 1000 °C, a few M₂₃C₆ carbides form on the surface of M₆C carbides. The decomposition behavior of primary M₆C can be explained by the following reaction: M₆C → M₂₃C₆ + Me (W, Ni, Cr, Mo). At temperatures below 900 °C, coarse lamellar M₂₃C₆ carbides precipitate at the grain boundaries. The carbide lamellae line almost perpendicular to the grain boundaries. While the temperature is above 1000 °C, discrete M₂₃C₆ carbides precipitate at the grain boundaries. Moreover, there are lots of small M₂₃C₆ particles precipitated around M₆C carbides from 650 to 1000 °C.