Microstructure and gas-sensing properties of nanocrystalline NiFe<Subscript>2</Subscript>O<Subscript>4</Subscript> prepared by spray pyrolysis

Nanocrystalline nickel ferrite with a crystallite size from 3 to 40 nm has been prepared by spray pyrolysis. The ⁵⁷Fe Mössbauer spectrum of NiFe₂O₄ samples has been found to vary systematically with crystallite size. The sensing response of the nanocrystalline nickel ferrite to 50 ppm NH₃ has been studied using in situ conductance measurements. NiFe₂O₄ offers a strong sensing response to ammonia at the level of its maximum concentration limit. The optimum nickel ferrite crystallite size and temperature for ammonia detection are determined.     

Peculiarities of the electric conductivity of As<Subscript>2</Subscript>S<Subscript>3</Subscript>〈Au〉 and As<Subscript>2</Subscript>S<Subscript>5</Subscript>〈Au〉 glasses

The temperature dependences of the dc conductivity in glasses of the (As₂S₃)_1−x Au_x and (As₂S₅)_1−x Au_x systems (0.04 at. % ≥x≥0) are reported for the first time. The curves show an anomalous behavior in the temperature interval of 360 K<T<300 K and exhibit a break for glass compositions with a low gold content. A model explaining the existence of impurity conductivity is proposed.     

Effects of Pressure-Induced Phase Transition in Model Hg<Subscript>2</Subscript>Br<Subscript>2</Subscript> Ferroelastics

Raman spectra of model improper ferroelastics (Hg₂Br₂ crystals) have been analyzed in a wide range of hydrostatic pressures. The baric dependences of the phonon frequencies are obtained. The revealing and anomalous behavior of the soft mode, which is genetically related to the acoustic phonon (ТА₁) at the Brillouin zone boundary (point X) of the tetragonal phase, are most interesting. The buildup of the second acoustic phonon (ТА₂) from the same point has also been found in the ferroelastic-phase spectra, and its baric behavior has been investigated. The splitting of doubly degenerate phonons of the E_g symmetry has been revealed at fairly high pressures and explained.     

Structural transformations in thin Ge<Subscript>2</Subscript>Sb<Subscript>2</Subscript>Te<Subscript>5</Subscript> films

Structural transformations in thin Ge₂Sb₂Te₅ films for phase-change memory applications have been studied by differential scanning calorimetry. As-grown, amorphous films have been shown to undergo structural transitions to a cubic and then to a hexagonal phase. A reproducible endothermic peak has been detected, which had not been reported earlier. A mechanism for the underlying process has been proposed.     

Phase equilibria and properties of solid solutions in the Tl<Subscript>5</Subscript>Te<Subscript>3</Subscript>-Tl<Subscript>9</Subscript>BiTe<Subscript>6</Subscript>-Tl<Subscript>5</Subscript>Te<Subscript>2</Subscript>I system

Phase equilibria in the Tl₅Te₃-Tl₉BiTe₆-Tl₅Te₂I system, containing ternary structural analogs of Tl₅Te₃, have been studied by differential thermal analysis, X-ray diffraction, microhardness tests, and emf measurements using concentration cells with a thallium electrode. A number of T-x sections, the 760- and 800-K sections, and the liquidus and solidus projections of the phase diagram are constructed, and the composition dependences of lattice parameters, microhardness, and emf are obtained. The results demonstrate that, despite the presence of a liquid-liquid miscibility gap, the system contains a continuous series of solid solutions isostructural with Tl₅Te₃.     

Electroluminescence of (Pb<Subscript>0.91</Subscript>La<Subscript>0.09</Subscript>)(Zr<Subscript>0.65</Subscript>Ti<Subscript>0.35</Subscript>)O<Subscript>3</Subscript> ceramics with nanopolar structure

Lanthanum-doped lead zirconate titanate (PLZT) relaxor ceramics with (Pb_0.91La_0.09)(Zr_0.65Ti_0.35)O₃ composition exhibits a repolarization-induced electroluminescence (EL) with a pronounced discrete character of emission. It is established that this behavior is related to the reorientation of nanodimensional polar regions in a strong pulsed electric field in the vicinity of a smeared phase transition. The temporal and temperature dependences of the EL intensity have been studied.     

Formation and structural phase transitions of mesoporous Al<Subscript>2</Subscript>O<Subscript>3</Subscript> and TiO<Subscript>2</Subscript>/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> xerogels under hydrothermal conditions

We have studied the effect of the hydrothermal synthesis temperature on Al₂O₃ structure formation and examined the role of the phase composition of the precursor gel and surfactant in the formation of the pore structure of Al₂O₃. A technique has been proposed for the synthesis of TiO₂/Al₂O₃ binary xerogels, and the effect of TiO₂ content on the pore structure parameters and adsorption properties of TiO₂/Al₂O₃ has been investigated.     

Transparency and temperature-induced variations of birefringence in LiKB<Subscript>4</Subscript>O<Subscript>7</Subscript> crystals

Temperature-induced variations in the birefringence in nonlinear LiKB₄O₇ (LKBO) single crystals grown by the Czochralski method have been studied in a 100–350 K range. The temperature dependences of the birefringence reveal deviations from monotonic behavior in the vicinity of 140 and 230 K. The range of transparency of LKBO single crystals has been determined.     

Temperature-Dependent Thermal Boundary Conductance at Al/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> and Pt/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> interfaces

With the ever-decreasing size of microelectronic devices, growing applications of superlattices, and development of nanotechnology, thermal resistances of interfaces are becoming increasingly central to thermal management. Although there has been much success in understanding thermal boundary conductance at low temperatures, the current models applied at temperatures more common in device operation are not adequate due to our current limited understanding of phonon transport channels. In this study, the scattering processes in Al and Pt films on Al₂O₃ substrates are examined by transient thermoreflectance testing at high temperatures. At high temperatures, traditional models predict the thermal boundary conductance to be relatively constant in these systems due to assumptions about phonon elastic scattering. Experiments, however, show an increase in the conductance indicating potential inelastic phonon processes.     

Properties of Mg(Fe<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Ga<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>)<Subscript>2</Subscript>O<Subscript>4 + δ</Subscript> solid solutions in stable and metastable states

Using pyrohydrolytic synthesis, we have obtained a continuous series of Mg(Fe_{1 − x} Ga _x )₂O_{4 + δ} solid solutions, as checked by X-ray diffraction. The magnetization and coercivity of the samples have been determined from field dependences in applied magnetic fields of ±5 T, and their conductivity has been assessed using current-voltage measurements at 300 K in electric fields from −200 to +200 V. The effective band gap of the solid solutions has been evaluated from their absorption spectra obtained by diffuse reflectance measurements. The optimal composition for spintronic applications is Mg(Fe_0.8Ga_0.2)₂O_{4 + δ}.     

Influence of C<Subscript>4</Subscript>F<Subscript>8</Subscript>/Ar/O<Subscript>2</Subscript> plasma etching on SiO<Subscript>2</Subscript> surface chemistry

The plasma assisted etching of SiO₂ in a commercial RF reactor with a variety of C₄F₈/Ar/O₂ chemistries has been studied by XPS, SIMS and FTIR. A simple model of surface reactions is proposed. In particular, the role of oxygen in the etch process has been investigated. According to our experiments, oxygen inhibits the formation of CFH based polymeric films on the surface. As the etching process is due to an exchange reaction between the oxygen in the SiO₂ and the gaseous fluorine species in the plasma, the presence of oxygen in the etch hinders this process by occupying adsorption sites on the surface. The results would confirm that argon does not participate in chemical reactions with the SiO₂ substrate but provides energy for reactions in which F, C and O are involved. The results also indicate that a thick fluorocarbon layer only forms on the surface in the absence of oxygen, regardless of the oxygen source. Consequently, only when the SiO₂ layer has been substantially removed does this film form.     

Two-Band Calculations on the Upper Critical Field of Sc<Subscript>2</Subscript>Fe<Subscript>3</Subscript>Si<Subscript>5</Subscript>

The ternary compound Sc₂Fe₃Si₅ has attracted much attention because of the various anomalous physical properties. The specific heat experiment and energy band structure calculation suggest that Sc₂Fe₃Si₅ is a two-gap superconductor. Based on this, we analyze the upper critical field for superconducting Sc₂Fe₃Si₅ crystals using the two-band Ginzburg-Landau theory. A two-parameter variational approach is adopted to obtain the upper critical field in arbitrary direction. The temperature and angular dependences of the upper critical field are plotted. The results reproduce the experimental data in a very broad temperature range and strongly support previous specific heat data and theoretical calculation, pointing to the existence of two energy gaps in Sc₂Fe₃Si₅. The anisotropy of the upper critical field is also studied and is about 2, in accordance with the experimental result. Moreover our calculations indicate that Sc₂Fe₃Si₅ has rather a three-dimension character, in agreement with the energy band calculation.     

Optical properties of undoped and Yb<Superscript>3+</Superscript>-doped YAl<Subscript>3</Subscript>(BO<Subscript>3</Subscript>)<Subscript>4</Subscript> crystals

The ordinary and extraordinary refractive indices of nominally undoped and Yb³⁺-doped (7 at %) YAl₃(BO₃)₄ crystals have been measured in the visible range at temperatures from 20 to 400°C. The refractive indices are found to increase with temperature. The thermo-optic coefficients are on the order of ～10^?6. The temperature dependences of the refractive indices show a number of anomalies, suggestive of an incommensurate system exhibiting devil’s staircase behavior.     

Lithium ion conductivity of LiLaO<Subscript>2</Subscript>-Li<Subscript>2</Subscript>ZrO<Subscript>3</Subscript> solid solutions

The limits of the LiLaO₂-and Li₂ZrO₃-based solid solutions in the LiLaO₂-Li₂ZrO₃ system have been determined: 0–10 mol % Li₂ZrO₃ and 0–5 mol % LiLaO₂, respectively. We have studied the transport properties (electronic conductivity, temperature and composition dependences of conductivity and activation energy) of lithium lanthanate and the solid solutions in the LiLaO₂-Li₂ZrO₃ system. Conduction in LiLaO₂ is likely due to lithium ion transport through a polyhedral network.     

Photosensitivity of thin-film structures based on CuIn<Subscript>3</Subscript>Se<Subscript>5</Subscript> and CuIn<Subscript>5</Subscript>Se<Subscript>8</Subscript> ternary semiconductor compounds

Polycrystalline n-type CuIn₃Se₅ and CuIn₅Se₈ films with thicknesses from 0.4 to 1 μm have been grown by pulsed laser ablation of bulk p-CuIn₃Se₅ and n-CuIn₅Se₈ crystals in vacuum. The temperature dependences of the resistivities of these crystals are determined by deep donor levels with energies E _D ? 0.2–0.3 eV. Photosensitive thin-film structures based on these films have been created for the first time and their photosensitivity spectra have been measured. The possibility of using thin CuIn₃Se₅ and CuIn₅Se₈ films in broadband photoconverters is demonstrated.     

Densification Peculiarities of Transparent MgAl<Subscript>2</Subscript>O<Subscript>4</Subscript> Ceramics—Effect of LiF Sintering Additive

A new method of doping of spinel nanopowders with LiF sintering additive has been proposed, consisting in its introduction via solution technology, which leads to improved sinterability of the ceramic. The effect of LiF doping on densification behavior, microstructure, and optical and mechanical properties of hot-pressed MgAl₂O₄ ceramics has been studied. Samples of MgAl₂O₄ optical ceramics with density close to that theoretically achievable have been produced by hot pressing of 0.5 wt % LiF-doped nanopowders at 1600°C for 1 h. It has been shown that addition of LiF changes crack propagation in spinel ceramic from transgranular to intergranular.     

Synthesis and luminescence properties of a Li<Subscript>3</Subscript>BaCaY<Subscript>3</Subscript>(MoO<Subscript>4</Subscript>)<Subscript>8</Subscript>:Er<Superscript>3+</Superscript> phosphor with a layered scheelite-like structure

A Li₃BaCaY₃(MoO₄)₈:Er³⁺ phosphor with a scheelite-like structure (sp. gr. C2/c) has been synthesized and its luminescence properties have been studied. The phosphor has been characterized by X-ray diffraction, differential thermal analysis, and IR spectroscopy.     

Nanocrystalline PbIn<Subscript>0.5</Subscript>Nb<Subscript>0.5</Subscript>O<Subscript>3</Subscript> ceramics and its properties

A ferroelectric relaxor PbIn_0.5Nb_0.5O₃ (PIN) ceramics has been obtained using a modified ceramic technology, with the sintering stage preceded by compression and shear straining of the synthesized charge in Bridgman anvils. The dimensions of ceramic grains after this pretreatment are spread over a range from 100 to 1250 nm. A comparative investigation of the properties of PIN ceramics obtained using the standard and modified technology showed that the proposed mechanical action at the charge preparation stage can be used for controlled modification of the properties of ferroelectric ceramics.     

Subsolidus phase relations in the system Tm<Subscript>2</Subscript>O<Subscript>3</Subscript>-Rh<Subscript>2</Subscript>O<Subscript>3</Subscript>

The Tm₂O₃-Rh₂O₃ system has been studied by x-ray diffraction, thermal analysis, and chemical analysis of annealed and quenched samples. The results have been used to construct a schematic subsolidus phase diagram of the system. Only one double oxide, TmRhO₃, has been identified. Some of its physicochemical properties are reported.     

Structural and dielectric properties of PbMg<Subscript>1/3</Subscript>Nb<Subscript>2/3</Subscript>O<Subscript>3</Subscript>-PbZrO<Subscript>3</Subscript> solid solutions synthesized at high pressures and temperatures

(1 ? x)PbMg_1/3Nb_2/3O₃ · xPbZrO₃ (1 ? x)PMN · xPZ) solid solutions have been synthesized at a pressure of 5 GPa and temperatures from 1300 to 1700 K, and their structural and dielectric properties have been studied. The composition dependences of the average unit-cell parameter and dielectric permittivity for the solid solutions indicate that the PMN-PZ system has a morphotropic phase boundary near x = 0.65. The solid solutions have a cubic structure for x < 0.65, a rhombohedral structure in the range 0.65 < x < 0.9, and an orthorhombic structure (similar to that of PbZrO₃) for x > 0.9. The temperature and frequency dependences of dielectric permittivity suggest that the (1 ? x)PMN · xPZ samples with x < 0.65 consist of two ferroelectric phases: a relaxor with antipolar dipole order and a normal ferroelectric with a diffuse phase transition. The effect of annealing temperature on the ferroelectric state of the samples with x < 0.65 is examined. In the composition range 0.65 < x < 0.9, the samples have normal ferroelectric properties, independent of annealing temperature.