Surface Oxidation of Ge33As12Se55 Films

Ge₃₃As₁₂Se₃₃ films deposited by ultrafast pulse laser deposition were annealed at various temperatures and pressures, and the dependence of the surface oxidation on the processing conditions was investigated by X-ray photoelectron spectroscopy (XPS). We found that even as-grown film contains a thin oxidized surface and the oxygen distribution exponentially decays along the normal direction of the films regardless of different processing conditions. The critical thicknesses of the oxidized layers were 6.5, 11, 48, and 98 nm, respectively, for the as-grown, 250°C annealing sample for 4 h under 1 × 10⁻⁶ Torr, and 150° and 250°C annealing samples for 15 h under 20 mTorr.     

Tensile Strengths of Se, As2S3, As2Se3, and Ge30As15Se55 Glass Fibers

The strengths of as-drawn, abraded, and pristine Se, As₂Se₃, As₂S₃, and Ge₃₀As₁₅Se₅₅ fibers are reported. Pristine strength values range from an average of 80 MPa for Se and Ge₃₀As₁₅Se₅₅ to 100 and 180 MPa for As₂Se₃ and As₂S₃, respectively. Although these values are considerably higher than those reported previously for bulk samples, in no case does the maximum observed strength reach more than about 6% of that expected from simple theoretial calculations. Several factors that can control the strength are discussed. The overall behavior is interpreted in terms of the glass structures which permit localized stress relaxation effects in regions of weak, nondirectional bonds. These effects occur at temperatures well below the glass-transition region.     

Template-Free Self-Assembly of a Nanoporous TiO2 Thin Film

We report a nanoporous TiO₂ thin film prepared using a supersaturated aqueous solution containing peroxotitanium complex ions. The film morphology can be regulated by chemical kinetics, which was partially controlled by solution conditions such as the concentrations of starting materials, pH values, and the temperatures of the solutions. Porous films with various morphologies from particulate to curved sheet shaped were prepared on different substrates including Si, polymers, and glass. Porous microstructures of films permitted us to prepare a crack-free film 2–3 μm in thickness. These films were still amorphous under the present treatment conditions. Dye-sensitized solar cells using annealed (anatase) films of different morphologies as electrodes yielded conversion efficiency ranging from 1.3% to 3.1%. Improvement in performance may be achieved by either increasing the film thickness or inducing crystallization in solutions.     

Crystallization of As2Se3-As2Te3 Glasses

Differential scanning calorimetry studies of As₄₀Se_xTe_60-x glasses with 0≤x≤60 show a 2-stage crystallization when 5≤x≤35, followed by a double-peaked melting endotherm. The enthalpy of crystallization for the low-temperature species is independent of composition and equals 11.6±0.9 cal/g for x≤25, whereas that for the high-temperature species decreases from 2.2±0.2 cal/g for x = 5 to 1.1±0.3 cal/g for x =15. The total enthalpy of melting also is independent of composition and equals 18.8±1.2 cal/g for x ≤25. No crystallization was observed for xe40. The high-temperature phase has the mono-clinic As₂Te₃ structure; that of the low-temperature phase was not identified. Annealing experiments for compositions with x = 15 or 20 gave time-temperature transformation maps. A kinetic analysis of the data was inconclusive.     

In Situ TEM Heating of Nanosized ZrO2

In situ TEM heating experiments at temperatures between 820° and 860°C provide significant insights into the atomistics of neck formation during the sintering of nanoscale faceted, single-crystal particles of ZrO₂. High-resolution TEM was used to observe the crystallography and the relative orientation in a variety of original particle-pairs and the relative orientation of their evolving necks. Previous studies have determined that unconstrained particles can rotate during neck growth. When there is a significant amount of misalignment, the particles are not able to align completely. In these instances, the neck that forms is epitaxial with one of the particles. Additionally, in particle-pairs with significant misalignment, the neck stops growing when it forms a crystallographic facet which is continuous with the preexisting facet of the template particle. When the two particles have close to the same orientation, the particles rotate into complete alignment and the neck that forms is consequently epitaxial with both particles. When the neck-particle system is in complete registry, the growing neck is rounded: there is an aperiodic arrangement of steps, even on the atomic resolution scale. The experimental results are discussed in terms of the random starting state of the two particles, the equilibrium crystal shape of the particle-neck system, and the operative kinetic mechanisms.     

In Situ Synthesis of ZrO2/ZrW2O8 Composites With Near-Zero Thermal Expansion

In this paper, ZrO₂ and WO₃ were used as the raw materials to prepare ZrO₂/ZrW₂O₈ composites by in situ reaction method and the thermal expansion property of the composites was studied. This novel method included a heating step up to 1473 K for 24 h, which combines the synthesizing and sintering of ZrW₂O₈. The result indicates that ZrO₂/ZrW₂O₈ composite shows near-zero thermal expansion when the weight ratio of ZrO₂ and WO₃ is 2.5:1. Compared with composites prepared previously by non-reactive sintering of ZrO₂ and ZrW₂O₈, the composites show higher relative density and lower porosity.     

Titanium Diffusion into (K0.5Na0.5)NbO3 Thin Films Deposited on Pt/Ti/SiO2/Si Substrates and Corresponding Effects

Lead-free (K_0.5Na_0.5)NbO₃ (KNN) thin films were prepared on Pt/Ti/SiO₂/Si substrates by a sol–gel processing method, and titanium diffusion from the substrates into the KNN films under different thermal treatment conditions were investigated by the secondary ion mass spectroscopy depth profile and X-ray photoelectron spectroscopy surface analysis. Titanium diffusion was evident in all the KNN thin films, which was further aggravated not only by increasing the annealing temperature, but also surprisingly by higher ramping rate attributed to the resulting larger grain boundaries. The pronounced effects of the titanium diffusion and the resulting substitution of Ti⁴⁺ for Nb⁵⁺ with different valence states on the composition, structure, and electrical properties of the KNN thin films are analyzed and discussed. The results showed that the Ti diffusion from the substrate played a crucial role in affecting the structure and electrical properties of the ferroelectric KNN thin films deposited on Pt/Ti/SiO₂/Si substrate.     

Effective Doping in Cubic Si3N4 and Ge3N4: A First-Principles Study

First-principles calculations have been conducted to investigate impurities in cubic Si₃N₄ and Ge₃N₄. Impurity species suitable for n - and p -type doping are suggested, in terms of the formation and ionization energies. The suggested species are P and O as n -type dopants and Al as a p -type dopant for c -Si₃N₄, and Sb and O as n -type dopants and Al as a p -type dopant for c -Ge₃N₄. The dependence of the formation energies on the chemical potentials indicates that a proper choice of growth conditions is mandatory for suppressing the incorporation of these impurities into anti and interstitial sites, where the impurities can be charged to compensate carriers.     

Crystallographic Domain Structure of an Epitaxial (Pr0.7Ca0.3)MnO3 Thin Film Grown on a SrTiO3 Single Crystal Substrate

We characterized the structure of the crystallographic domain of a (Pr_0.7Ca_0.3)MnO₃ (PCMO ( x =0.3)) thin film epitaxially grown on a (001)SrTiO₃ (STO) single-crystal substrate. We found that the domain structure exhibited an atomically smooth coherent interface with the STO substrate with no misfit dislocations. The crystallographic relationships between the domains and the substrate in their planes were interpreted to be PCMO ( x =0.3)[110][001]//STO[100][010] and PCMO ( x =0.3)[001][1-10]//STO[100][010]. The domain structure of PCMO ( x =0.3)[110][1-10]//STO[100][010] has less possibility of having the larger anisotropic strain energy found in a monoclinically distorted pseudo-cubic perovskite unit cell of PCMO ( x =0.3). This epitaxial growth structure is totally different from the previously reported PCMO ( x =0.5) epitaxial thin-film structure on STO substrate. Our observations suggested that an x value change strongly influences the structure of epitaxial PCMO thin films.     

Formation of Small-Sized CdSxSe1-x Crystals in Sol-Gel-Derived SiO2 Glasses

Silica glasses doped with small-sized CdS_x-Se_1-x crystals were prepared by the sol-gel method. Gels synthesized by the hydrolysis of Si(OC₂H₅)₄, in the presence of CdSeO₄ with NH₄SCN dissolved in HNO₃ or NH₄OH, were heated in H₂-N₂ atmosphere. The pH value of solutions for CdSeO⁴ and NH⁴SCN primarily determined the fraction of anions in CdS^xSe^1-x crystals. The anion content in crystals was dependent on the pH value of the solvent and/or heat-treatment temperature, and the sulfur fraction changed from 0.1 to 0.6. The optical absorption spectra were red-shifted as the selenium content and the crystal size increased, and the emission spectra showed a sharp band near the absorption edge position and a broad tail extending into the long wavelength. The optical band gap energies increased reciprocally proportional to the square of the crystal size.     

Heat Capacity and Structural Relaxation of Enthalpy in As2Se3 Glass

The evolution of enthalpy of As₂Se₃ glass during structural relaxation in the glass transition region was measured via differential scanning calorimetry for two types of time-temperature programs: rate-heating at 10 K/min following a cool at a constant rate (-20 to -0.31 K/min) and isothermal annealing following a temperature step from an equilibrium state. The rate-heating data yield kinetic parameters for the structural relaxation which predict accurately the evolution of the enthalpy during isothermal annealing. The glass heat capacities were independent of cooling rate within experimental precision (≤0.2%). In this respect, As₂Se₃ is unlike previously studied glasses whose heat capacities are more dependent on thermal history.     

Microstructure and Mechanical Properties of In Situ Produced TiC/TiB2/MoSi2 Composites

A novel microstructure of in situ produced TiC/TiB₂/MoSi₂ composite and its mechanical properties were investigated. The results indicate that TiC/TiB₂/MoSi₂ composites can be fabricated by reactive hot pressing the mixed powders of MoSi₂, B₄C, and Ti. A novel microstructure consisting of hollow particles of TiC and TiB₂ grains in an MoSi₂ matrix was obtained. Grains of in situ produced TiC and TiB₂ were much finer, from 100 to 400 nm. During the fracture process, hollow particles relieved crack tip stress, encouraging crack branching and changing the original direction of the main crack. The highest bending strength of this composite achieved was 480 MPa, twice that of monolithic MoSi₂, and the greatest fracture toughness of the composite reached 5.2 MPa·m^1/2.     

Chemical Diffusion in Polycrystalline Al2O3 as Determined from Electrical Conductivity Measurements

By analyzing the changes with time of the electrical conductivity of polycrystalline Al₂O₃ after the O₂ pressure was changed, a defect diffusion coefficient was obtained which was assigned to the Al or O ion, whichever is the faster-diffusing species. Both decreased grain size and MgO addition increase the defect diffusion coefficient. The grain-boundary defect diffusion coefficient for the undoped material was estimated to be: and that for the MgO-doped material was over the range 1100° to 1350°C (δ is the effective thickness of the boundary and s the coefficient of segregation of defects to the boundary region). The mechanism of grain-boundary diffusion is discussed in terms of defect mobility.     

Multiferroic BiFeO3 Thin Films Buffered by a SrRuO3 Layer

Multiferroic BiFeO₃ thin films of huge polarization have been successfully realized by using SrRuO₃ as a buffer layer on a Pt/TiO₂/SiO₂/Si substrate. They consist of a single perovskite phase and are nearly randomly orientated, where the SrRuO₃ buffer layer lowers the crystallization temperature and improves the crystallinity of BiFeO₃. With increasing deposition temperature during magnetron sputtering, they undergo an apparent grain growth and reduction in surface roughness. The multiferroic thin films deposited on the SrRuO₃-buffered Pt/TiO₂/SiO₂/Si substrate at higher temperatures show much improved polarization and reduced coercive field, together with a lowered leakage current. A huge remnant polarization (2 P _r) of 150 μC/cm² and a coercive field (2 E _c) of 780 kV/cm were measured for the BiFeO₃ film deposited at 650°C.     

In Situ Synthesis of TiCp/Ni3Al Composites under High Pressure

In this paper, TiC_p/Ni₃Al composites are synthesized in situ at near theoretical density under high-pressure (1.56.5 GPa), high-temperature (1073–1473 K) conditions. The grain size of TiC-reinforced particles is nanometer scale, which influences the Vickers microhardness of the composites. The effect of pressure on the grain size of TiC is discussed.     

PbTiO3–PbO–SiO2 Glass-Ceramic Thin Film by a Sol–Gel Process

PbTiO₃(PT)-PbO-SiO₂ glass-ceramic thin films were pro-duced by a sol-gel process. The crystallization of PT oc-curred at ∼700°C and was higher than that in PT-PbO-B₂ O₃ sol-gel glass-ceramics. A pinhole-free thin film was obtained by a rapid thermal annealing process when the designed glass-forming phase content in the thin film was >24 vol%. The measured dielectric constants of the films fairly agreed with the predicted values, based on a parallel mixing model. The dielectric constant was 219 and the di-electric loss was 0.04 in the 0.6PT-0.4(PbO-SiO₂) film that was fired at 700°C.     

In Situ Reaction of B2O3 with AlN and/or Si3N4 to Form BN-Toughened Composites

BN-toughened oxide matrix composites were formed by the in situ reaction of B₂O₃ with Si₃N₄ and/or AlN. A lowtemperature transient liquid phase aids densification at <1000°C, and the process tends to produce an intrinsically homogeneous microstructure. Mechanical properties and microstructure of the composites formed in situ were compared to those of composites prepared by conventional means from oxide and BN powders. Fracture toughness and flexural strength of the nearly isotropic in situ formed composites ranged from 2.82 to 3.66 MPa · m^1/2 and 130 to 320 MPa, respectively, with Young's moduli of 100 to 110 GPa. Densities achieved ranged from 90% to 97% of estimated theoretical densities. The strength and toughness values are intermediate to the extreme values for the anisotropic composites formed by hot-pressing mixed powders.     

In Situ Laser Raman Determination of Electrochemical Reactions of Y2O3-Stabilized ZrO2 and Molten Na2SO4

When used as components of advanced energy conversion systems, oxide ion conductors and molten electrolytes may react or decompose according to various mechanisms. Understanding these mechanisms is important in improving system performance. In situ laser Raman spectroscopy was used successfully to study the reactions between an oxide ion conductor and a molten salt. Changes in the Raman spectrum of molten Na₂SO₄ were observed near Y₂O₃-stabilized ZrO₂ electrodes when they were polarized anodically in the molten salt. These changes were interpreted to indicate the formation of yttrium sulfate complexes.     

In Situ Preparation of Si3N4/SiC Nanocomposites for Cutting Tools Application

Silicon nitride–silicon carbide nanocomposite has been prepared by an in situ method that utilizes the formation of SiC nanograins by carbothermal reduction of intentionally added fine SiO₂ during the sintering process. The mean value of room-temperature four-point bending strength is 675 MPa with the Weibull modulus of 6.4 and an indentation fracture toughness of 7.4 MPa·m^1/2. A significantly enhanced creep resistance was achieved by the incorporation of SiC nanoparticles into the matrix up to 1400°C. The tribological properties of the material were tested using a ball-on-disk configuration and showed a friction coefficient of about 0.7. The cutting inserts machined from this composite had three times longer lifetime compared with those available on the market. On the other hand, the scatter of results is much larger compared with those measured for the commercial inserts.     

Preparation and Characterization of Fine-Grained 3Y-TZP/BaFe12O19 In Situ Composites

Composites of BaFe₁₂O₁₉ particles dispersed throughout a 3-mol%-yttria-doped zirconia (3Y-TZP) matrix have been prepared using the pressureless reactive sintering of 3Y-TZP, BaCO₃, and γ-Fe₂O₃ powders. The reaction behavior of the mixed powder was studied with an in situ , high-temperature powder X-ray diffraction technique. The composite that was sintered at 1300°C consisted of submicrometer-sized 3Y-TZP grains and BaFe₁₂O₁₉ particles; the size of the 3Y-TZP grains was ∼100-300 nm, and the size of the BaFe₁₂O₁₉ particles was ∼200-500 nm. Based on the grain size, most of the BaFe₁₂O₁₉ grains presumably had a single-magnetic-domain structure. The 3Y-TZP/20-wt%-BaFe₁₂O₁₉ composite showed high magnetization and coercivity values, despite the low concentration of ferromagnetic phase. Preliminary mechanical tests revealed that the composite possessed moderately good mechanical properties.