Reactions of V<Subscript>2</Subscript>O<Subscript>5</Subscript>, Nb<Subscript>2</Subscript>O<Subscript>5</Subscript>, and Ta<Subscript>2</Subscript>O<Subscript>5</Subscript> with AlN

Reactions of vanadium, niobium, and tantalum pentoxides with aluminum nitride have been studied using X-ray diffraction. At temperatures from 1000 to 1600°C, we have identified various V, Nb, and Ta nitrides. The composition of the niobium and tantalum nitrides depends on the reaction temperature. The tendency toward nitride formation becomes stronger in the order V₂O₅ < Ta₂O₅ < Nb₂O₅.     

Photoluminescence of Ta<Subscript>2</Subscript>O<Subscript>5</Subscript> films formed by the molecular layer deposition method

Ta₂O₅ films of different thicknesses (20–100 nm) synthesized by the molecular layer deposition method on p-type silicon substrates and thermally oxidized silicon substrates have been studied by the methods of high-frequency capacitance–voltage characteristics and photoluminescence. A hole-conduction channel is found to form in the Si–Ta₂O₅–field electrode system. A model of the electronic structure of Ta₂O₅ films is proposed based on an analysis of the measured PL spectra and performed electrical investigations.     

Effect of ultrarapid quenching on the structure and mechanical properties of Nb<Subscript>2</Subscript>O<Subscript>5</Subscript> and Ta<Subscript>2</Subscript>O<Subscript>5</Subscript>

We have studied the detailed structure and mechanical properties of the Nb₂O₅ and Ta₂O₅ pentoxides after ultrarapid quenching in comparison with the properties of Nb₂O₅ and Ta₂O₅ ceramics prepared by a conventional ceramic processing technique and using high-intensity light (HIL) in an optical furnace. The results demonstrate that high-energy processing (HIL and ultrarapid quenching) improves the hardness and strength of Nb₂O₅ and Ta₂O₅. At the same time, HIL processing and quenching lead to structural disordering of the Nb₂O₅ and Ta₂O₅ pentoxides.     

Memory characteristics of Al<Subscript>2</Subscript>O<Subscript>3</Subscript>/LaAlO<Subscript>3</Subscript>/SiO<Subscript>2</Subscript> multilayer structures with tunnel oxide thickness variation

Charge trap flash (CTF) memory devices are candidates to replace NAND flash devices. In this study, Pt/Al₂O₃/LaAlO₃/SiO₂/Si multilayer structures with lanthanum aluminate charge traps were fabricated for nonvolatile memory device applications. An aluminum oxide film was used as a blocking oxide for low power consumption in the program/erase operation and to minimize charge transport through the blocking oxide layer. The thickness of SiO₂ as tunnel oxide layer was varied from 30 to 50 Å. Thicknesses of oxide layers were confirmed by high resolution transmission electron microscopy (HRTEM) and all the samples showed amorphous structure. From the C–V measurement, a maximum memory window of 3.4 V was observed when tunnel oxide thickness was 40 Å. In the cycling test for reliability, the 30 Å tunnel oxide sample showed a relatively large memory window reduction by repeated program/erase operations due to the high electric field of ~10 MV/cm through tunnel oxide. The other samples showed less than 10% loss of memory window during 10⁴ cycles.     

Crystallization from high-temperature solutions in the K<Subscript>2</Subscript>O-P<Subscript>2</Subscript>O<Subscript>5</Subscript>-V<Subscript>2</Subscript>O<Subscript>5</Subscript>-Bi<Subscript>2</Subscript>O<Subscript>3</Subscript> system

We have studied general trends of crystallization from high-temperature solutions in the K₂O-P₂O₅-V₂O₅-Bi₂O₃ system at P/V = 0.5?2.0, K/(P + V) = 0.7?1.4, and Bi₂O₃ contents from 25 to 50 wt % and identified the stability regions of BiPO₄, K₃Bi₅(PO₄)₆, K₂Bi₃O(PO₄)₃, and K₃Bi₂(PO₄)_{3 ? x} (VO₄) _x (x = 0?3) solid solutions. The synthesized compounds have been characterized by X-ray powder diffraction and IR spectroscopy, and the structure of two solid solutions has been determined by single-crystal X-ray diffraction (sp. gr. C ₂/c): K₃Bi₂(PO₄)₂(VO₄), a = 13.8857(8), b = 13.5432(5), c = 6.8679(4) Å, β = 114.031(7)°; K₃Bi₂(PO₄)_1.25(VO₄)_1.75, a = 13.907(4), b = 13.615(2), c = 6.956(2) Å, β = 113.52(4)°.     

Electrical conductivity of Bi<Subscript>2</Subscript>O<Subscript>3</Subscript>-Tm<Subscript>2</Subscript>O<Subscript>3</Subscript>-Nb<Subscript>2</Subscript>O<Subscript>5</Subscript> solid solutions

New solid solutions, Bi_2?x?y Tm _x Nb _y O_3+δ, with tetragonal and cubic structures have been synthesized in the Bi₂O₃-Tm₂O₃-Nb₂O₅ system, and their electrical conductivity has been measured at temperatures from 670 to 1020 K. The 1020-K conductivity of the tetragonal solid solution Bi_1.8Tm_0.15Nb_0.05O_3+δ is comparable to that of Bi_1.75Tm_0.25O₃, the best conductor in the Bi₂O₃-Tm₂O₃ system.     

Effect of Y<Subscript>2</Subscript>O<Subscript>3</Subscript> on the crystallization behavior of SiO<Subscript>2</Subscript>–MgO–B<Subscript>2</Subscript>O<Subscript>3</Subscript>–Al<Subscript>2</Subscript>O<Subscript>3</Subscript> glasses

A series of glass comprising of SiO₂–MgO–B₂O₃–Y₂O₃–Al₂O₃ in different mole ratio has been synthesized. The crystallization kinetics of these glasses was investigated using various characterization techniques such as differential thermal analysis (DTA), thermo gravimetric analysis (TGA), X-ray diffraction (XRD), and scanning electron microscopy (SEM). Crystallization behavior of these glasses was markedly influenced by the addition of Y₂O₃ instead of Al₂O₃. Addition of Y₂O₃ increases the transition temperature, T _g, crystallization temperature, T _c and stability of the glasses. Also, it suppresses the formation of cordierite phase, which is very prominent and detrimental in MgO-based glasses. The results are discussed on the basis of the structural and chemical role of Y³⁺ and Al³⁺ ions in the present glasses.     

Effect of O<Subscript>2</Subscript> partial pressure and thickness on the gasochromic properties of sputtered V<Subscript>2</Subscript>O<Subscript>5</Subscript> films

V₂O₅ thin films were deposited by reactive DC-diode sputtering technique in a mixed atmosphere of O₂/Ar gas at room temperature from a high purity target of 99.99% vanadium. For the investigation, the thickness of the films and the O₂/Ar ratio during the sputtering process were the parameters. The sputtering rate of the V₂O₅ films dramatically decreases with increasing the O₂/Ar ratio. By X-ray diffraction it was found that films sputtered with 1% O₂/Ar ratio grow preferentially in two orientations: the 200 and the 001 orientation. The increase of the O₂/Ar ratio enhances the growth preferentially in the c-axis (001) and strongly decreases the growth in the a-axis (200) direction. The scanning electron microscope pictures confirm these results. In the visible region the optical transmittance is increased with increasing the O₂/Ar ratio in the sputter gas. Additionally, the optical band gap is slightly larger for the films sputtered with an O₂/Ar ratio higher than 5%. Beyond a thickness of about 220 nm and an O₂/Ar ratio of 10% the electrical sheet resistance of the films increases dramatically. During the insertion/extraction of hydrogen ions, the change in the optical transmission was investigated. The gasochromism of the V₂O₅ films was explained by use of the Infra Red (IR) measurements during the insertion/extraction of hydrogen ions.     

SiO<Subscript>2</Subscript>/TiO<Subscript>2</Subscript> fibers from titanium-modified polycarbosilane

We developed a process for preparing SiO₂/TiO₂ fibers by means of precursor transformation method. After mixing PCS and titanium alkoxide, continuous SiO₂/TiO₂ fibers were fabricated by the thermal decomposition of titanium-modified PCS (PTC) precursor. The tensile strength and diameter of SiO₂/TiO₂ fibers are 2.0 GPa, 13 μm, respectively. Based on X-ray diffraction (XRD), scanning electron microscopy (SEM), and high resolution transmission electron microscopy (HRTEM) measurements, the microstructure of the SiO₂/TiO₂ fibers is described as anatase–TiO₂ nanocrystallites with the mean size of ~10 nm embedded in an amorphous silica continuous phase.     

Growth of ultrathin Nb<Subscript>2</Subscript>O<Subscript>5</Subscript> films on quartz substrates

Niobium oxide films have been grown by reactive rf sputtering in a vacuum system and characterized by absorption spectroscopy and X-ray diffraction. The thickness of the (optically transparent) films has been determined as a function of sputtering time by examining interference effects in a plane-parallel layer. The average deposition rate is determined to be 7.4 ± 0.3 Å/min (95% confidence interval).     

Phase transitions and piezoelectric properties of SrBi<Subscript>2</Subscript>Ta<Subscript>2</Subscript>O<Subscript>9</Subscript> by molecular dynamics simulations

The phase transition sequence of SrBi₂Ta₂O₉ (SBT) and the local microscopic dynamics near the ferroelectric transition are investigated using a shell model with parameters fitted to first-principle calculations. We show that the complex interplay between polar and nonpolar instabilities leads to the presence of two phase transitions. In this way the existence of an intermediate orthorhombic paraelectric phase, characterized by the rotation of the TaO₆ octahedra, is demonstrated without using any explicit experimental data as input. The local polarization dynamics does not provide any indication of a relaxation process near the ferroelectric transition. Finally, dielectric and piezoelectric coefficients along crystallographic directions are investigated.     

Magnesiothermic Reduction of Mg<Subscript>4</Subscript>Ta<Subscript>2</Subscript>O<Subscript>9</Subscript> in the Combustion Regime

We have studied the magnesium reduction of the Mg₄Ta₂O₉ tantalate in the combustion regime and assessed the influence of starting mixture parameters on the combustion speed and temperature, the degree of reduction, and characteristics of the resultant tantalum powders. The use of Mg₄Ta₂O₉ as a precursor for the reduction process has made it possible to increase the degree of reduction and the specific surface area of tantalum powders. We have obtained powders with a specific surface area of up to 21 m²/g, which is a factor of 4–5 larger than in the case of the reduction of Ta₂O₅ under the same conditions.     

Anisotropy of dielectric losses in Al<Subscript>2</Subscript>O<Subscript>3</Subscript> and SiO<Subscript>2</Subscript> single crystals

The procedure and results of measurements of the dielectric loss tangent using the dielectric resonator technique on azimuthal modes of the HE (quasi-E) and EH (quasi-H) types are considered. The measurements were performed for uniaxial anisotropic single crystals of Al₂O₃ (at a frequency of 11 GHz) and SiO₂ (at 39 GHz) in a temperature range of 80–373 K and for an isotropic single crystal of Y₃Al₅O₂ (YAG) at room temperature in a frequency range of 9–15 GHz. The proposed method revealed the anisotropy of dielectric losses in Al₂O₃ and SiO₂ single crystals in the temperature range studied. According to this, losses along the optical axis of these crystals are lower than in the transverse plane. In the YAG crystal, the Q values for modes of the two types with the same frequency are close, which corresponds to isotropic losses. The dielectric losses in YAG increase in proportion to the frequency.     

Temperature-stable and low loss Fe-containing dielectrics in BaO-Ln<Subscript>2</Subscript>O<Subscript>3</Subscript>-Fe<Subscript>2</Subscript>O<Subscript>3</Subscript>-Ta<Subscript>2</Subscript>O<Subscript>5</Subscript> system

Polycrystalline samples of Ba₄Ln₂Fe₂Ta₈O₃₀ (Ln = La and Nd) were prepared by a high temperature solid-state reaction technique. The formation, structure, dielectric and ferroelectric properties of the compounds were studied. Both compounds are found to be paraelectrics with filled tetragonal tungsten bronze (TB) structure at room temperature. Dielectric measurements revealed that the present ceramics have exceptional temperature stability, a relatively small temperature coefficient of dielectric constant (τ _ε ) of −25 and −58 ppm/°C, with a high dielectric constant of 118 and 96 together with a low dielectric loss of 1.2 × 10⁻³ and 2.8 × 10⁻³ (at 1 MHz) for Ba₄La₂Fe₂Ta₈O₃₀ and Ba₄Nd₂Fe₂Ta₈O₃₀, respectively. The measured dielectric properties indicate that both materials are possible candidates for the fabrication of discrete multilayer capacitors in microelectronic technology.     

Investigation on the stability of Li<Subscript>5</Subscript>La<Subscript>3</Subscript>Ta<Subscript>2</Subscript>O<Subscript>12</Subscript> lithium ionic conductors in humid environment

In this paper, the stability in humid air of Li₅La₃Ta₂O₁₂ lithium ionic conductors synthesized by conventional solid-state reaction was investigated by internal friction, conductivity, weight variation, X-ray diffraction, and thermogravimetric analysis methods. It was found that when the Li₅La₃Ta₂O₁₂ samples were aged in open air at room temperature, the internal friction peaks associated with the short-distance diffusion of lithium vacancies gradually shift toward higher temperature and increase in height, while the weight of the sample increases and impurity phases of LiOH·H₂O appear. These results reveal that the Li₅La₃Ta₂O₁₂ compounds are unstable against moisture in open air at room temperature. It was suggested that the protons from the moisture substitute the lithium ions in Li₅La₃Ta₂O₁₂ samples to form Li₂O and new protonic derivatives, Li_5?x La₃Ta₂O_12?x (OH) _x (0<x<2.15), and the resultant Li₂O may react further with water to form LiOH·H₂O.     

Ultrasound velocity and absorption in BeO,Al<Subscript>2</Subscript>O<Subscript>3</Subscript>, ZrO<Subscript>2</Subscript>, and SiO<Subscript>2</Subscript> ceramics

We have measured the ultrasound velocity and absorption in BeO, Al₂O₃, ZrO₂, and SiO₂ ceramics. The results indicate that the ultrasound velocity in oxide ceramics depends on the nature of the basic oxide component, the density of the material, and the preferential alignment of the grains. The ultrasound velocity in ceramics is shown to correlate with their thermal conductivity: with increasing thermal conductivity, the ultrasound velocity increases. The ultrasound absorption in oxide ceramics decreases with decreasing temperature, and vice versa, with increasing temperature, the ultrasound attenuation coefficient increases.     

Structural and electrical properties of SiO<Subscript>2</Subscript>–Li<Subscript>2</Subscript>O–Nb<Subscript>2</Subscript>O<Subscript>5</Subscript> glass and glass-ceramics obtained by thermoelectric treatments

Glass and glass-ceramics with the molar composition of 60SiO₂–30Li₂O–10Nb₂O₅ (mole %) were studied. Ferroelectric lithium niobate (LiNbO₃) nanocrystals were precipitated in the glass matrix trough a thermal treatment, with and without the simultaneous application of an external electric field. The as-prepared sample, yellow and transparent, was heat-treated (HT) at 600 and 650 °C and thermoelectric treated (TET) at 600 °C. The applied electric fields were the following ones: (i) 5 × 10⁴ V/m; (ii) 1 × 10⁵ V/m. Differential thermal analysis (DTA), X-ray diffraction (XRD), scanning electron microscopy (SEM), Raman and dielectric spectroscopies were used to investigate the glass samples properties. The LiNbO₃ crystalline phase was detected in the 650 °C HT sample and in the 600 °C TET samples. The presence of an external electric field, during the heating process, promotes the glass crystallization at lower temperatures. In the TET samples, the surface crystallization of the cathode and the anode are different. The number and size of the crystallites, in the glass network, dominate the electrical dc behavior while the ac conductivity process is more dependent of the glass matrix structure. The obtained results reflect the important role carried out by the temperature and the applied electric field in the glass-ceramic structures.     

Electrical properties of V<Subscript>2</Subscript>O<Subscript>5</Subscript>-CaO-P<Subscript>2</Subscript>O<Subscript>5</Subscript> glasses exhibiting majority charge carrier reversal

The thermoelectric power and d.c electrical conductivity of x V₂O₅⋅40CaO⋅(60−x)P₂O₅ (10 ≤ x ≤ 30) glasses were measured. The Seebeck coefficient (Q) varied from +88 μ V K⁻¹ to −93 μV K⁻¹ as a function of V₂O₅ mol%. Glasses with 10 and 15 mol% V₂O₅ exhibited p-type conduction and glasses with 25 and 30 mol% V₂O₅ exhibited n-type conduction. The majority charge carrier reversal occurred at x = 20 mol% V₂O₅. The variation of Q was interpreted in terms of the variation in vanadium ion ratio (V^{5 +}/V^{4 +}). d.c electrical conduction in x V₂O₅⋅40CaO⋅(60−x)P₂O₅ (10 ≤ x ≤ 30) glasses was studied in the temperature range of 150 to 480 K. All the glass compositions exhibited a cross over from small polaron hopping (SPH) to variable range hopping (VRH) conduction mechanism. Mott parameter analysis of the low temperature data gave values for the density of states at Fermi level N (E_F) between 1.7 × 10²⁶ and 3.9 × 10²⁶ m⁻³ eV⁻¹ at 230 K and hopping distance for VRH (R_VRH) between 3.8 × 10⁻⁹m to 3.4 × 10⁻⁹ m. The disorder energy was found to vary between 0.02 and 0.03 eV. N (E_F) and R_VRH exhibit an interesting composition dependence.