Preparation and Thermoelectric Properties of Polycrystalline Si<Subscript>0.4</Subscript>Ge<Subscript>0.6</Subscript>

Polycrystalline Si_0.4Ge_0.6 was prepared by electron-beam float-zone melting, and its thermoelectric properties were studied in the intrinsic region. The samples were found to be p-type, with a room-temperature resistivity from 5 to 20 Ωcm. The thermoelectric figure of merit of the alloy in the intrinsic region (500–1000 K) is lower than that of heavily doped alloys. Above 1000 K, a sharp rise in its thermoelectric figure of merit would be expected.     

Photoluminescence of Si<Subscript>0.9</Subscript>Ge<Subscript>0.1</Subscript>O<Subscript>2</Subscript> and GeO<Subscript>2</Subscript> films irradiated with silicon ions

We have studied the photoluminescence (PL) of GeO₂ and 90 mol % SiO₂-10 mol % GeO₂ films synthesized by method of RF magnetron sputtering and then irradiated with silicon ions and annealed. The PL of silicon-implanted GeO₂ films, related to the presence of Si nanocrystals (nc-Si), was observed for the first time. It is established that the transformation of the defect centers responsible for the PL in the spectral range 350–600 nm, as well as the formation of nc-Si emitting in the region of 700–800 nm, significantly depend on the matrix type. In particular, the PL intensity at 700–800 nm in 90 mol % SiO₂-10 mol % GeO₂ films is weak. The role of the isovalent substitution of Si and Ge atoms in the transformation of defect centers and the formation of nc-Si is discussed.     

Electret effect in Pb<Subscript>5</Subscript>Ge<Subscript>3</Subscript>O<Subscript>11</Subscript> crystals

The short-circuit thermally stimulated depolarization current (TSDC) through lead germanate single crystals has been measured as a function of temperature (100–800 K) and time. The results indicate that poling of the crystals at elevated temperatures (t ≥ 150°C) produces an electret state related to the spacecharge mechanism of polarization. Relatively mild poling conditions (E = 0.75 kV/cm, t = 150°C) produce a well-defined, long-lived electret with an abnormally large electret charge (1400 μC/cm²). The electret discharge leads to heating of the crystal, in the form of a peak in the range 520–570 K, whose height exceeds that of the pyroelectric peak near T _C by two to three orders of magnitude. From the TSDC data, we evaluated the stored electret charge, its density, and the activation energy of traps for electret charges, and made preliminary conclusions as to the nature of the traps. We demonstrate that neglect of the electret discharge current (which depends on a number of factors) when determining the pyroelectric coefficient p ^σ above 280 K leads to a significant scatter in data.     

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.     

Synthesis and High-Temperature Heat Capacity of Dy<Subscript>2</Subscript>Ge<Subscript>2</Subscript>O<Subscript>7</Subscript> and Ho<Subscript>2</Subscript>Ge<Subscript>2</Subscript>O<Subscript>7</Subscript>

The Dy₂Ge₂O₇ and Ho₂Ge₂O₇ pyrogermanates have been prepared by solid-state reactions in several sequential firing steps in the temperature range 1237–1473 K using stoichiometric mixtures of Dy₂O₃ (or Ho₂O₃) and GeO₂. The heat capacity of the synthesized germanates has been determined as a function of temperature by differential scanning calorimetry in the range 350–1000 K. The experimentally determined C _p (T) curves of the dysprosium and holmium germanates have no anomalies and are well represented by the Maier–Kelley equation. The experimental C _p (T) data have been used to evaluate the thermodynamic functions of the Dy₂Ge₂O₇ and Ho₂Ge₂O₇ pyrogermanates: enthalpy increment H°(T)–H°(350 K), entropy change S°(T)–S°(350 K), and reduced Gibbs energy Ф°(T).     

Synthesis and High-Temperature Heat Capacity of Sm<Subscript>2</Subscript>Ge<Subscript>2</Subscript>O<Subscript>7</Subscript> and Eu<Subscript>2</Subscript>Ge<Subscript>2</Subscript>O<Subscript>7</Subscript>

The Sm₂Ge₂O₇ and Eu₂Ge₂O₇ germanates have been prepared by solid-state reactions via multistep firing of stoichiometric mixtures of Sm₂O₃ (Eu₂O₃) and GeO₂ in air at temperatures from 1273 to 1473 K. The molar heat capacity of the samarium and europium germanates has been determined by differential scanning calorimetry in the range 350–1000 K and the C _p (T) data have been used to evaluate their thermodynamic properties.     

Acoustophotovoltaic effect in TlIn<Subscript>0.98</Subscript>Gd<Subscript>0.02</Subscript>Se<Subscript>2</Subscript> single crystals

An acoustophotovoltaic effect in TlIn_0.98Gd_0.02Se₂ single crystals has been detected and investigated.     

Effect of Gd<Subscript>2</Subscript>O<Subscript>3</Subscript> concentration in Bi-containing high-temperature solutions on the luminescence of epitaxial Gd<Subscript>3</Subscript>Ga<Subscript>5</Subscript>O<Subscript>12</Subscript> films

Single-crystal gadolinium gallium garnet films have been grown by liquid-phase epitaxy on (111) Gd₃Ga₅O₁₂ substrates from supercooled Bi₂O₃-B₂O₃ fluxed melts at different Gd₂O₃ concentrations. The luminescence spectra of the films have been measured at 10 and 300 K under unmonochromatized synchrotron X-ray excitation and selective UV synchrotron excitation. The Bi³⁺ luminescence is discussed.     

The response of open-volume defects in Si<Subscript>0.92</Subscript>Ge<Subscript>0.08</Subscript> to annealing in nitrogen or oxygen ambient

While the dynamics of thermal oxidation of Si has been adequately described for over three decades, details of SiGe oxidation are not entirely clear. In particular, the injection into the Si substrate of a super-saturation of interstitial defects during the formation of SiO₂ may not be replicated in the SiGe system. Here we describe the response of relatively stable open-volume defects in a Si₀₉₂Ge_0.08 substrate to annealing at 900°C for 30 min in either an inert (nitrogen) or wet oxidizing ambient. The defects are initialized by He⁺ implantation at an energy and dose of 60 keV and 1 × 10¹⁶ cm⁻² respectively, followed by annealing in an inert ambient at 800°C for 10 min. Confirmation of the creation and thermal evolution of the open-volume defects was provided by beam-based variable-energy positron annihilation spectroscopy. The positron measurements suggest that the defects are considerably reduced in size following the secondary nitrogen annealing step. In comparison, the average size of the open-volume defects is reduced to a significantly smaller degree following secondary oxygen annealing. This result is consistent with a suppression of interstitial formation during oxidation, in contrast with the case for the thermal oxidation of silicon, but further work is currently underway to test this interpretation.     

Anisotropic physical properties of Mn<Subscript>5</Subscript>Ge<Subscript>3</Subscript> crystals

We have prepared chemically homogeneous Mn₅Ge₃ single crystals and evaluated the anisotropy in their electrical resistivity and linear expansion coefficient. These properties have been shown to vary anomalously with temperature near the ferromagnetic transition of Mn₅Ge₃. In the temperature range 320–900 K, its resistivity remains constant.     

Effect of heating rate on the microstructural and magnetic properties of nanocrystalline Fe<Subscript>81</Subscript>Si<Subscript>4</Subscript>B<Subscript>12</Subscript>P<Subscript>2</Subscript>Cu<Subscript>1</Subscript> alloys

The effect of heating rate on the structural and magnetic properties of the nanocrystalline Fe₈₁Si₄B₁₂P₂Cu₁ alloy has been investigated. Amorphous Fe₈₁Si₄B₁₂P₂Cu₁ alloy was annealed at 753 K for 180 s at different heating rates ranging from 0.05 to 5 K/s in protective argon atmosphere. The structural and magnetic properties of the as-quenched and annealed alloys were studied using X-ray diffractometer (XRD), differential scanning calorimeter (DSC), vibrating sample magnetometer (VSM), and B–H loop tracer, respectively. Amorphous precursor prepared by industry-grade raw materials is obtained. The increase of heating rate is found to be significantly effective in decreasing the grain size of α-Fe(Si) phase, but the grain size increases at higher heating rate. The volume fraction of α-Fe(Si) phase shows a monotonic decrease with the increase of the heating rate. The coercivity H _c markedly decreases with increasing heating rate and exhibits a minimum at the heating rate of 0.5 K/s, while the saturation magnetization, M _s, shows a slight decrease. These results suggest that the effect of heating rate on H _c and M _s is originated from the changes of grain size and the volume fraction of α-Fe(Si) phase.     

Si<Subscript>3</Subscript>N<Subscript>4</Subscript>/TiN composites produced from TiO<Subscript>2</Subscript>-Modified Si<Subscript>3</Subscript>N<Subscript>4</Subscript> powders

Si₃N₄/TiN composites have been produced by hot pressing at temperatures from 1600 to 1800°C in a nitrogen atmosphere, using silicon nitride powders prepared by self-propagating high-temperature synthesis and surface-modified with titanium dioxide nanoparticles. We examined the effect of TiO₂ content on the microstructure, phase composition, and mechanical strength of the ceramics. It is shown that titanium nitride can be formed by the reaction Si₃N₄ + TiO₂ → TiN + NO + N₂O + 3Si. The Si₃N₄/TiN composites containing 5–20% TiN have a low density, high porosity, and a bending strength of 60 MPa or lower. In Si₃N₄/TiN ceramics produced using calcium aluminates as sintering aids, the silicon nitride grains are densely packed, which ensures an increase in strength to 650 MPa.     

Stress reduction during phase change in Ge<Subscript>2</Subscript>Sb<Subscript>2</Subscript>Te<Subscript>5</Subscript> by capping TiN film

It has been one of the most important issues to minimize the stress reduction during phase change in GST (Ge₂Sb₂Te₅) alloy for PRAM (Phase-change Random Access Memory) applications, because the alloy has been reported to face the significant stress during the phase change. We fabricated GST/oxide/substrate as a basic structure, and then added two more structures by capping an adhesion layer (Ti) or a barrier metal (TiN) on GST layer, respectively. We report that TiN-capped structure shows about 40% stress reduction during the phase change compared with that of the basic structure. The stress reduction is considered to be due to the intrinsic compressive stress in TiN film itself.     

Twinning modes of Er<Subscript>2</Subscript>Si<Subscript>2</Subscript>O<Subscript>7</Subscript>

Theory of deformation twinning by Bilby and Crocker is applied to calculate the twinning elements for all possible twinning modes in monoclinic Er₂Si₂O₇ TYPE C and TYPE D. The magnitude of shear strain was also calculated for each twinning mode. The criteria of small shear strain and minimum shuffling is applied to predict the operative twinning mode for monoclinic Er₂Si₂O₇ TYPE C and TYPE D. These predications of the theory are compared with the available experimental information and may act as guideline for future experimental work.     

CaAl<Subscript>2</Subscript>Si<Subscript>2</Subscript>O<Subscript>8</Subscript>-B<Subscript>2</Subscript>Al<Subscript>2</Subscript>Si<Subscript>2</Subscript>O<Subscript>8</Subscript>-H<Subscript>2</Subscript>O (B = Na,K) zeolites

The structures of gismondine and amicite are analyzed in comparison with one another and with those of the zeolites whose compositions lie in the hypothetical plane “CaAl₂Si₃O₁₀”-Na₂Al₂Si₃O₁₀-H₂O. It is shown that the structures are similar to each other and may undergo mutual transformations.     

Catalytic effect of Ti<Subscript>5</Subscript>Si<Subscript>3</Subscript> on thermal decomposition of Li<Subscript>3</Subscript>AlH<Subscript>6</Subscript>

Fine Ti₅Si₃ powder has been mechanochemically synthesized from a mixture of elemental Ti and Si powders. When Ti₅Si₃ is added as a catalyst into Li₃AlH₆, it shows a good catalytic ability by reducing the decomposition temperature and improving the decomposition kinetics as well. Although its catalytic effect is not as good as well-known TiCl₃, the use of Ti₅Si₃ has a benefit of releasing more hydrogen than TiCl₃ during dehydrogenation. This can be explained by that Ti₅Si₃, unlike TiCl₃, does not incur any chemical reactions with Li₃AlH₆ and thus remains inert during milling for dispersion.     

Microstructure characterization of in situ synthesized porous Si<Subscript>3</Subscript>N<Subscript>4</Subscript>–Si<Subscript>2</Subscript>N<Subscript>2</Subscript>O composites using feldspar additive

Porous Si₃N₄–Si₂N₂O bodies fabricated by multi-pass extrusion process were investigated depending on the feldspar addition content (4–8 wt% Si) in the raw silicon powder. The diameter of the continuous pores was about 250 μm. The polycrystalline Si₂N₂O fibers observed in the continuous pores as well as in the matrix regions of the nitrided bodies can increase the filtration efficiency. In the 4 wt% feldspar addition, the diameter of the Si₂N₂O fibers in the continuous pores of the nitrided bodies was about 90–150 nm. A few number of rope typed Si₂N₂O fibers (∼4 μm) was found in the case of 8 wt% feldspar addition. However, in the 8 wt% feldspar addition, the matrix showed highly porous structure composed of large number of the Si₂N₂O fibers (∼60 nm). The relative densities of the Si₃N₄–Si₂N₂O bodies with 4 wt% and 8 wt% feldspar additions were about 65% and 61%, respectively.     

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.     

Synthesis and high-temperature heat capacity of Gd<Subscript>2</Subscript>Sn<Subscript>2</Subscript>O<Subscript>7</Subscript>

Gd₂Sn₂O₇ gadolinium stannate with the pyrochlore structure has been prepared by solid-state reaction and its high-temperature heat capacity has been determined by differential scanning calorimetry in the temperature range 350–1020 K. The C_p(T) data are shown to be well represented by the classic Maier–Kelley equation. The experimental C_p(T) data have been used to evaluate the thermodynamic functions of gadolinium stannate: enthalpy increment H°(T)–H°(339 K), entropy change S°(T)–S°(339 K), and reduced Gibbs energy Ф°(Т).