Yttrium-substituted bismuth titanate (Bi4−x Y x Ti3O12) thin film for use in non-volatile memories

Fatigue-free Bi_3.2Y_0.8Ti₃O₁₂ (BYT) thin films were grown on Pt/TiO₂/SiO₂/Si substrates using direct liquid injection–metal organic chemical vapor deposition. The BYT film capacitor with top Au electrode showed higher remanent polarization (2P _r) and lower leakage current density compared with Bi_3.2La_0.8Ti₃O₁₂ (BLT) film capacitors. BYT films showed strong (1 1 7) orientation with smaller grain size, while BLT films showed strong c-axis orientation. The 2P _r value of the BYT capacitor was 15 C cm^–2 and remained essentially constant up to 1×10¹⁰ read/write switching cycles at a frequency of 1 MHz. The leakage current of the BYT film was 3.5×10^–7 A cm^–2 at an applied voltage of 2 V, which is about three orders lower than that of the BLT film.     

Effects of oriented growth on properties of Ag/Bi<Subscript>4</Subscript>Ti<Subscript>3</Subscript>O<Subscript>12</Subscript>/p-Si heterostructure prepared by Sol-Gel method with rapid thermal annealing techniques

Ag/Bi₄Ti₃O₁₂/p-Si heterostructures were fabricated by Sol-Gel method with rapid thermal annealing techniques. The effects of oriented growth on ferroelectric characteristics and electrical properties of Bi₄Ti₃O₁₂ film were investigated. Bi₄Ti₃O₁₂ films on bare p-Si exhibit preferred c-axis-orientation with the increase of annealing temperature, which would impair the ferroelectric properties but help to drop down the leakage current density of Bi₄Ti₃O₁₂ films. The Polarization-Voltage curves and the electrical characteristics curves show that the Bi₄Ti₃O₁₂ films annealed at 650 C for 5 min have good ferroelectric and electrical properties with a remanent polarization of 8.3 C/cm² and a leakage current density of < 5 × 10^–9 A/cm² at 6 V, which demonstrate that the Ag/Bi₄Ti₃O₁₂/p-Si heterostructure by Sol-Gel method with rapid thermal annealing techniques is a promising configuration for MFS-FETs applications.     

Diamond nanopatterns fabricated by room-temperature nanoimprinting technology with diamond molds using polysiloxane

Fabrication of diamond nanopatterns in room-temperature (RT) nanoimprint lithography (NIL) with chemical vapor deposited (CVD) diamond molds using polysiloxane as RT-imprint resist material was investigated. The diamond molds of a convex lattice with 1 μm line-width and 5 μm pitch, and a cylinder dot with 200 nm diameter and 1 μm pitch which has a height of 1 μm using RT-NIL process were fabricated with Bi₄Ti₃O₁₂ octylate (oxide) mask in electron beam lithography technology. The maximum radio frequency (RF) oxygen plasma-etching selectivity (diamond/polysiloxane) of 19 was obtained under the conditions of RF power of 100 W, oxygen gas flow rate of 10 sccm and background gas pressure of 30 Pa. It was found that the optimum imprinting pressure and its depth obtained after the press duration of 10 min were 0.8 MPa and about 0.5 μm, respectively. The resulting diamond nanopatterns of a concave lattice with 1 μm line-width and 5 μm pitch, and a concave cylinder dot with 200 nm diameter and 1 μm pitch which have a height of 1 μm after RF oxygen plasma-etching (100W, 10 sccm, 30 Pa, 40 min) were fabricated with diamond mold RT-NIL using polysiloxane.     

The high-temperature thermal expansion of BiPbSrCaCuO superconductor and the oxide components (Bi2O3, PbO,CaO, CuO)

The thermal expansion of superconducting Bi_1.6Pb_0.4Sr₂Ca₂Cu₃Ox (BiPbSrCaCuO) and its oxide components Bi₂O₃, PbO, CaO and CuO have been studied by high-temperature dilatometric measurements (30–800°C). The thermal expansion coefficient for the BiPbSrCaCuO superconductor in the range 150–830°C is =6.4×10^–6K^–1. The temperature dependences of L/L of pressed Bi₂O₃ reveals sharp changes of length on heating (T ₁=712°C), and on cooling (T ₂=637°C and T ₃=577°C), caused by the phase transition monoclinic-cubic (T ₁) and by reverse transitions via a metastable phase (T ₂ and T ₃). By thermal expansion measurements of melted Bi₂O₃ it is shown that hysteresis in the forward and the reverse phase transitions may be partly caused by grain boundary effect in pressed Bi₂O₃. The thermal expansion of red PbO reveals a sharp decrease in L/L, on heating (T ₁=490°C), related with the phase transition of tetragonal (red, a=0.3962 nm, c=0.5025 nm)-orthorhombic (yellow, a=0.5489 nm, b=0.4756 nm, c=0.5895 nm). The possible causes of irreversibility of the phase transition in PbO are discussed. In the range 50–740°C the coefficient of thermal expansion of pressed Bi₂O₃ (_m=3.6 × 10^–6 and _c=16.6×10^–6K^–1 for monoclinic and cubic Bi₂O₃ respectively), the melted Bi₂O₃ (_m=7.6×10^–6 and _c=11.5×10^–6K^–1), PbO (_t=9.4×10⁶ and _or=3.3×10^–6K^–1 for tetragonal and orthorhombic PbO respectively), CaO (=6.1×10^–6K^–1) and CuO (=4.3×10^–6K^–1) are presented.     

Diamond Crystallization in the System B4C–C

Superhard polycrystalline diamond material consisting of crystallites less than 20 m in size and containing less than 5 wt % B₄C is synthesized in the graphite–B₄C system at 2600–2800 K and 8–9 GPa. In the Raman spectrum of this material, the main band (1332 cm^–1) is shifted to lower frequencies by 40 cm^–1, typical of heavily boron-doped diamond films. Based on experimental data, a mechanism is proposed for the transformation of graphite into polycrystalline diamond at temperatures between the melting points of the B₄C–diamond and B₄C–graphite eutectics.     

Nucleation control of diamond synthesized by microwave plasma CVD on cemented carbide substrate

Diamond was coated on to cemented carbide substrate by microwave plasma CVD, in which nucleation control of diamond crystals was investigated under constant deposition conditions; total pressure 30 torr, CH₄ flow rate 1 ml min^–1, H₂ flow rate 199 ml min^–1 and microwave power 550 W. Nucleation tends to occur selectively on the edge part of WC grains of the cemented carbide substrate with coarse WC grain size of about 1 m, where the nucleation density was 9×10⁶ cm^–2. The density increased to about 5×10⁷ cm^–2 when using a finegrained substrate (WC grain size 0.5 m). A considerably enhanced nucleation was observed by introducing a number of fine microflaws on to the substrate surface. Microflawing treatment with diamond fine powder (grain size 0–1/4 m) suspended in an ultrasonic cleaner bath was effective for increasing the diamond nucleation density up to 5×10⁸ cm^–2. The grain size of grown diamond crystals decreased with increasing microflawing time.     

Reactions of MnO2, Mn2O3, α-Bi2O3, and Bi12Ti1 – x Mn x O20with Supercritical Isopropanol

Heterogeneous reactions between supercritical isopropanol and metal oxides were studied for the first time. The results demonstrate that Bi₂O₃ is reduced by isopropanol to metallic bismuth, while MnO₂ and Mn₂O₃ are reduced to Mn₃O₄ (hausmannite). The possibility of oxygen extraction from nonstoichiometric oxides is demonstrated by the example of the reaction Bi₁₂Ti_{1 – x} Mn⁵⁺ _x O_{20 +} Bi₁₂Ti_1–x Mn²⁺ _x O_{20 –} .     

Single Crystals of p-Type Solid Solutions between Bismuth and Antimony Chalcogenides for Cooling to T < 150 K

Sb₂Te₃–Bi₂Te₃ crystals (25–60 mol % Bi₂Te₃) doped with Bi₂Se₃ and excess Te were studied with the aim of identifying the optimal compositions for the p-legs of low-temperature coolers. The crystals were grown by the floating-crucible technique. Their transport properties were studied in the range 100–400 K. By measuring axial thermopower profiles, it was shown that increasing the Bi₂Te₃ and Bi₂Se₃ contents of the crystals has an adverse effect on their homogeneity. Crystals were prepared with a carrier concentration in the range (1–5) × 10¹⁹ cm^–3 and a thermoelectric power above 200 V/K at room temperature and the highest thermoelectric figure of merit at temperatures below 200 K. The maximum temperature drops and thermoelectric figures of merit were calculated for low-temperature stages of magnetothermoelectric coolers with hot-junction temperatures of 200 and 170 K and Bi–Sb n-legs.     

Diffusional penetration of silver from electrodes into PZT ceramics

Diffusion of silver was studied in a ceramic based on lead zirconate-titanate, Pb_0.95Sr_0.05(Zr_0.53Ti_0.47)O₃+1 wt%. Nb₂O₅ (PZT), by means of a radio-tracer method. Parameters of silver diffusion and silver content in PZT after continuous diffusion saturation of the ceramic by this impurity were determined in the temperature range 500–850 °C. Concentration-depth profiles and silver content in the ceramic were obtained as a result of metal diffusion from the electrode during ceramic silvering. Our results show that silver has a high diffusivity. No evaporation of silver during metallization (T _max=750 °C) was found, but part of it ( 0.1 mg cm^–2) penetrates into the ceramic from the electrode to a depth of more than 1000 m, and the silver concentration varies from 2×10¹⁹ to 2×10¹⁸ at cm^–3. The silver concentration does not exceed 0.2 at% at diffusion saturation of the ceramic during 100–120 h over the temperature range 650–850 °C.     

Nonlinear electrical properties of ZnO varistors fast-fired by using millimeter-wave sintering process

Fast firing of Bi₂O₃-based ZnO varistor materials, which includes zero minutes soaking at 1100°C with 120°C/min heating and 145°C/min cooling rate, was made possible using millimeter-wave sintering (mS) technique. The overall sintering time of the process is less than 18 minutes, and the varistor characteristics obtained are = 38, J _L = 5.55 × 10^–6 A/cm² and V _bk = 600 V/mm, whereas the intrinsic parameters of the materials are _b = 2.84 eV, N _d = 1.85 × 10²⁴ m^–3 and N _s = 7.02 × 10¹¹ cm^–2. By contrast, conventional sintering (cS) process needs higher sintering temperature (1200°C), longer soaking time (60 min) and slower ramping rate (30°C/min) to obtain ZnO materials with the same marvelous nonlinear properties as those prepared by mS-process. Moreover, millimeter-wave sintering (24 GHz, mS) process enhances the densification kinetics and grain growth behavior more efficiently than the microwave sintering (2.45 GHz, S) process, resulting in better varistor characteristics for ZnO materials. However, sintering by millimeter-wave for too long period induces overfiring of the samples, which results in a density reversion phenomenon. Such a phenomenon leads to the decrease in surface state (N _s) and the potential barrier height (_b), which are presumed to be the mechanism leading to the degradation of ZnO materials' nonlinear properties.     

Chemical vapour growth of HfC whiskers and their morphology

HfC whiskers were prepared from a gas mixture of HfCl₄ + CH₄ + H₂ + Ar in the presence of metal impurities, and the growth conditions and morphology were examined. The HfC whiskers preferentially grew at an H/Cl ratio of above 8, an HfCl₄ gas flow rate of 10–20 standard cm³ min^–1, a CH₄ flow rate of 10–20 standard cm³ min^–1, and at temperatures above 1050 °C. HfC whiskers, 60–170 m long, with a ball-like tip and periodically varying diameters, were obtained at 1250 °C using a cobalt impurity.     

Synthesis of new compounds of the pseudo-ternary system SrO-Bi2O3-TeO2 in air

The phase equilibria of the pseudo-ternary system SrO-Bi₂O₃-TeO₂ at 600 to 850 C were examined in air by solid-state reaction techniques and X-ray powder diffraction. Three pseudo-ternary compounds were found: a hexagonal solid solution Bi₂O₃ · 2xSrO · 5xTeO₂ (x = 0.02 to 0.20,a = 0.394(1) to 0.399(5),c = 1.89(6) to 1.86(7) nm), a rhombohedral phase Bi₄Sr₃Te₅O₁₉ (a = 0.405(2),c = 2.71 (5) nm in hexagonal terms) and a cubic phase Bi₂SrTeO₇ (a = 1.086(9) nm). The Mössbauer spectra of¹²⁵Te in the compounds indicated that the tellurium atoms were mostly in the state of Te⁺⁴ in the former two compounds but were exclusively Te⁺⁶ in Bi₂SrTeO₇. The electrical conductivity of the first two were about 10^–2–1 cm^–1 at 600 C. However, Bi₂SrTeO₇ was an electrical insulator.     

Formation of oxide phases in the system Fe2O3-NiO

Mixed metal oxides in the system Fe₂O₃-NiO were prepared by coprecipitation of Fe(OH)₃/Ni(OH)₂ and the thermal treatment of hydroxide coprecipitates up to 800 or 1100°C. X-ray diffraction showed the presence of -Fe₂O₃, NiO and NiFe₂O₄ in samples prepared at 800°C. The oxide phases -Fe₂O₃, NiO, NiFe₂O₄ and a phase with structure similar to NiFe₂O₄ were found in samples prepared at 1100°C. Fourier transform-infrared spectra of oxide phases formed in the system Fe₂O₃-NiO are discussed. Two very strong infrared bands at 553 and 475 cm^–1, a weak intensity infrared band at 383 cm^–1 and two shoulders at 626 and 441 cm^–1 were observed for -Fe₂O₃ prepared at 1100°C. NiFe₂O₄, prepared at the same temperature, showed two broad and very strong infrared bands at 602 and 411 cm^–1, while NiO showed a broad infrared band at 466 cm^–1. Fourier transform infrared spectroscopic results were in agreement with X-ray diffraction.     

Dependence of non-linearity coefficients on transition metal oxide concentration in simplified compositions of ZnO+Bi2O3+MO varistor ceramics (M=Co or Mn)

Ceramics with simplified compositions of ZnO + Bi₂O₃ + CoO or MnO show non-linearity coefficients () of 40–65 provided the concentration of transition metal ions is > 1.5mol.... Samples doped with Co have higher non-linearity coefficients than those with Mn. This is attributed to the stability of multiple oxidation states of Co(III) + Co(II) in contrast to Mn(II) as the stable species in sintered ZnO ceramics. Electron paramagnetic resonance and diffuse reflectance spectral studies establish this fact. Low-signal capacitance-voltage measurements show that donor density (N _d) in these ceramics ranges from 0.3 to 1.8 × 10¹⁹ cm^–3, which is comparatively larger than those of commercial varistors. The barrier height (_b) reaches up to 0.66 eV and the breakdown voltage is around 3.4–3.7 eV. Admittance spectroscopy and isothermal capacitance transient spectroscopy (ICTS) are used for characterizing the bulk traps originating from the transition metal dopants. Capacitance-voltage analyses above the breakdown voltages show negative capacitance, indicative of oscillatory charge redistribution involving multivalent states of Co and the shallower interface states. Multiple trapping relaxations are evident from the complex-plane capacitance studies.     

Fast Li+ ion conduction in Li2O–(Al2O3 Ga22O3)–TiO2–P2O5 glass–ceramics

Fast lithium ionic conducting glass-ceramics have been obtained by heat-treatment of glasses in the systems Li₂O–M₂O₃–TiO₂–P₂O₅ (M = Al and Ga). The glass–ceramics were mainly composed of LiTi₂(PO₄)₃ in which Ti⁴⁺ ions were partially replaced by M³⁺ ions. Considerable enhancement of the conductivity with the substitution of M³⁺ ions for Ti⁴⁺ ions was observed. The maximum conductivity obtained at room temperature was 1.3 × 10^–3 S cm^–1 for the aluminium system and 9 × 10^–4 S cm^–1 for the gallium system.     

High pressure-high temperature effect on melt-textured YBa2Cu3O7 ? δ high temperature superconductive material

High pressure-high temperature (HP-HT) treatment of melt-textured YBa₂Cu₃O_{7 –} at 2 GPa, and in the 800–950 °C pressure-temperature range for 15–30 min in contact with monoclinic pre-annealed zirconia induces: (1) the increase of the material density from 5.7 to 6.3 g/cm³ (by 9%), (2) the increase of critical current density in the direction of c-axis of YBa₂Cu₃O_{7 –} grains from 3–3.5 × 10³ up to 7 × 10³ A/cm² (in the self-field at 77 K) while in the ab-plane it remains unchanged (10⁴ A/cm²), (3) the increase of Vickers microhardness from 3.95 up to 5.3 GPa (estimated under the 4.91-N load). The increase in dislocation density in the (001) planes of HP-HT treated YBa₂Cu₃O_{7 –} grains from 10⁸ up to 10¹² cm^–2 may be one of the reasons of the increase in critical current density. The spaces between twin domains in YBa₂Cu₃O_{7 –} before treatment were 100–150 nm. Completely detwinned wide areas or regions, where each second twin domain was narrowed to approximately 20 nm or tapered down to zero thickness within the first domain and the disappearance of 1/6 301 stacking faults have been observed in the treated samples.     

Electrical and microstructural properties of Bi2−x Sb x Te and Bi2−x Sb x Te2 foils obtained by the ultrarapid quenching process

This paper is concerned with the feasibility and reproducibility of the ultrarapid quenching process used to fabricate Bi_2–x Sb _x Te and Bi_2–x Sb _x Te₂ alloys for thermoelectric applications. Microstructural properties of the materials, obtained in the shape of foils, were studied concerning the phase analysis, cell parameters, texture, and microstructure observations. The Bi_2–x Sb _x Te alloys were found to have the (2 0 3) texture. The (2 0 4) texture, with an additional (1 1 0) component for x values greater than 0.4, was predominant for Bi_2–x Sb _x Te₂ foils. The electrical properties of these materials were then characterized by measuring the Seebeck coefficient, Hall coefficient, and electrical resistivity. It was found that Bi_2–x Sb _x Te foils changed from n- to p-type for an x-value of about 1.2. A maximum Seebeck coefficient, ||, of 36×10^–6 V K^–1 was measured for Bi₂Te. In the case of Bi_2–x Sb _x Te₂ foils, the change from n- to p-type was observed for an x value of about 1. A maximum Seebeck coefficient, ||, of 32×10^–6 V K^–1, was measured for Bi_1.4Sb_0.6Te₂. Measurements of the temperature-dependent electrical resistivity, Hall and Seebeck coefficients of the foils were carried out and the analysis revealed a semi-metallic behavior.     

Effect of heat treatment on the infrared absorption spectra of Sr-Na borosilicate glass

Internal modes of vibrations are studied here at different temperatures (27–800°C) and in the frequency range 200–4000 cm^–1 through heat treatment. The baseline method was used. The strong bands of SiO₄ tetrahedra in this glass show an increase in absorbance at high temperature (600–800°C). The deformation of SiO₄ tetrahedra is investigated. This is found to depend on the ionic radius of the divalent metal oxide introduced, and the coordination number of the cation. Also from a study of the temperature dependence of the relative integrated intensity of the modes 600–800 and 850–1450 cm^–1, the relaxation time and rotational energy barrier of the glasses selected indicate that the glassy phases are transformed to crystalline phases at 500°C.     

Solid hydrogen and deuterium. II. Pressure and compressibility calculated by a lowest-order constrained-variation method

The pressure and the compressibility of solid H₂ and D₂ are obtained from ground-state energies calculated by means of a modified variational lowest-order constrained-variation (LOCV) method. Both fcc and hcp structures are considered, but results are given for the fcc structure only. The pressure and the compressibility are calculated or estimated from the dependence of the ground-state energy on density or molar volume, generally in a density region of 0.65^–3 to 1.3^–3, corresponding to a molar volume of 12–24 cm³/mole, where = 2.958 å, and the calculations are done for five different two-body potentials. Theoretical results for the pressure are 340–460 atm for solid H₂ at a particle density of 0.82^–3 or a molar volume of 19 cm³/mole, and 370–490 atm for solid ⁴He at a particle density of 0.92^–3 or a molar volume of 17 cm³/mole. The corresponding experimental results are 650 and 700 atm, respectively. Theoretical results for the compressibility are 210 × 10^–6 to 260 × 10^–6 atm^–1 for solid H₂ at a particle density of 0.82^–3 or a molar volume of 19 cm³/mole, and 150 × 10^–6 to 180 × 10^–6 atm^–1 for solid D₂ at a particle density of 0.92^–3 or a molar volume of 17 cm³/mole. The corresponding experimental results are 180 × 10^–6 and 140 × 10^–6 atm^–1, respectively. The agreement with experimental results is better for higher densities.     

The crystallization and electrical properties of lead-based ferroelectric thin films for uncooled pyroelectric infrared detector

Epitaxially grown and polycrystalline PT, PLT and PZT thin films with thickness from 1 to 2 m have been prepared on Pt/Ti/SiO₂/Si substrates by means of the modified sol–gel spin-coating technique. The ferroelectric thin films have good crystallization behavior, excellent dielectric and pyroelectric properties. The pyroelectric coefficients of PT and PLT thin films are 2.9×10^–8 C/cm² k and (3.37–5.25)×10^–8 C/cm² k, respectively. The figures of merit for voltage responsivity of PT and PLT thin films (F _I ) are 0.60×10^–10 Ccm/J and (0.79–1.13)×10^–8 Ccm/J, respectively. The figures of merit for current responsivity of these films are 9.0×10^–9 Ccm/J and (10.5–16.0)×10^–9 Ccm/J, and the figures of merit for detectivity of these films are 0.74×10^–8 Ccm/J and (0.79–1.13)×10^–8 Ccm/J, respectively.