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.     

Effect of high-intensity light on the properties and structure of ceramic Nb<Subscript>2</Subscript>O<Subscript>5</Subscript>

This paper examines the effect of exposure to high-intensity light on the structure, thermal expansion, mechanical properties, and Raman spectrum of ceramic Nb₂O₅. We demonstrate that such exposure creates fractal micro- and nanostructures in ceramic Nb₂O₅, has a significant effect on its thermal expansion, increases its elastic moduli and the anisotropy in its mechanical properties, and produces isolated polyhedra in its crystal structure.     

Effect of Nb<Subscript>2</Subscript>O<Subscript>5</Subscript> additions on the power loss of NiZn ferrites

Nickel–zinc ferrite system, Ni_0.65Zn_0.35Fe₂O₄ + x Nb₂O₅ where x varies from 0.0 wt% to 1.5 wt% in steps of 0.3 wt%, has been prepared by conventional ceramic technique. The samples were sintered at 1250 °C for 4 h in air atmosphere followed by natural cooling. The power loss and microstructures of these materials are examined. Microstructures reveal that niobium oxide additions promoted grain growth with an increase in grain size from 4 μm to 13.2 μm with the increase in niobium concentration. The measured power loss at frequencies from 100 kHz to 10 MHz under different exciting flux densities from 5 mT to 30 mT was found to be low up to 3 MHz, thus making the materials suitable for power applications up to this frequency. In the total power loss, hysteresis loss is predominant below 500 kHz and eddy current loss component is much higher at higher frequencies.     

Synthesis of anisometric KSr<Subscript>2</Subscript>Nb<Subscript>5</Subscript>O<Subscript>15</Subscript> particles in the SrNb<Subscript>2</Subscript>O<Subscript>6</Subscript>–Nb<Subscript>2</Subscript>O<Subscript>5</Subscript>–KCl system

Anisometric and agglomerate-free template particles are important for fabrication of grain-oriented ceramics. In the present work, preparation of acicular KSr₂Nb₅O₁₅ (KSN) particles was firstly explored in the SrNb₂O₆–Nb₂O₅–KCl system by molten salt synthesis (MSS) method. It was found that the molar ratio of SrNb₂O₆ to Nb₂O₅, the amount of KCl salt and synthesis time could significantly affect the phase structure and morphology of KSN particles. When calcined at 1,150 °C for 6 h with the molar ratio of SrNb₂O₆ to Nb₂O₅ was 1 and the weight ratio of salt to oxide source was 1.50, pure KSN particles with well-developed acicular morphology were successfully obtained in this system. They were agglomerate-free and with proper scale in the size range of 5–30 μm, which made them the ideal templates for fabricating textured ceramics. In addition, some new reaction and growth mechanisms were proposed in this work.     

Electrical Properties of the Nb<Subscript>2</Subscript>O<Subscript>5</Subscript>-NiO System

The electrical resistivity of Nb₂O₅-NiO materials was measured in the range 20-500°C. The resistivity of the samples was found to be sensitive to the presence of ammonia or sulfur dioxide in air. Some of the oxides in the Nb₂O₅-NiO system are potentially attractive as materials for temperature and gas sensors.     

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).     

Crystal structure and electrical properties of CaNaBi<Subscript>2</Subscript>Nb<Subscript>3</Subscript>O<Subscript>12</Subscript> ceramics

Monophasic CaNaBi₂Nb₃O₁₂ powders were synthesized via the conventional solid-state reaction route. Rietveld refinement of the X-ray powder diffraction (XRD) data and selected area electron diffraction (SAED) studies confirmed the phase to be a three-layer Aurivillius oxide associated with an orthorhombic B2cb space group. The dielectric properties of the ceramics have been studied in the 300–800 K temperature range at various frequencies (1 kHz to 1 MHz). A dielectric anomaly was observed at 676 K for all the frequencies corresponding to the ferroelectric to paraelectric phase transition as it was also corroborated by the high temperature X-ray diffraction studies. The incidence of the polarization–electric field (P vs. E) hysteresis loop demonstrated CaNaBi₂Nb₃O₁₂ to be ferroelectric.     

Antireflecting coating from Ta<Subscript>2</Subscript>O<Subscript>5</Subscript> and SiO<Subscript>2</Subscript> multilayer films

Sol-gel method is important for depositing antireflective coating that allows control over thickness as well as the index of refraction. Antireflective coatings which are produced from Ta₂O₅ and SiO₂ multi-layer thin films using sol-gel spin coating method are presented. The refractive index and the thickness are controlled by the composition and the concentration of the solution respectively. The thickness, refractive index and extinction coefficient of the films were calculated through transmission and reflection measurement by an NKD analyser. Mechanical properties of the films were checked by the cross tape test and dry sun test at 760 W/m². The result shows that the sample heat treated at 450^∘C for 15 min approaches a reflectance with less than 0.5% at around 840 nm.     

Effects of Nb<Subscript>2</Subscript>O<Subscript>5</Subscript> addition on the microstructure,electrical, and mechanical properties of PZT/ZnO nanowhisker piezoelectric composites

Nb₂O₅-modified PZT/ZnO nanowhisker (denoted as PZT/ZnO_w–Nb₂O₅) piezoelectric composites were prepared by a solid state sintering technique. Effects of Nb₂O₅ addition on the microstructure, electrical, and mechanical properties of the PZT/ZnO_w composites were investigated. With increasing Nb₂O₅ content, the grain size of the composites was reduced and the fracture mode changed from intergranular to intragranular gradually. Compared with the PZT/ZnO_w composites, the dielectric, piezoelectric, and ferroelectric properties of the PZT/ZnO_w–Nb₂O₅ composites were improved significantly, while mechanical properties were enhanced slightly. The optimum electrical and mechanical properties were achieved for the PZT/ZnO_w composites modified with 0.75 wt% Nb₂O₅ sintered at 1150 °C, with dielectric permittivity ε_r, piezoelectric coefficient d ₃₃, planar electromechanical coupling k _p, remnant polarization P _r, fracture toughness K _IC, and flexural strength σ_f being on the order of 4930, 600 pC/N, 0.63, 29.2 μC/cm², 1.56 MPa m^1/2 and 130 MPa, respectively. The Nb₂O₅-modified PZT/ZnO_w piezoelectric composites, with comparable electrical properties and improved mechanical properties than those of commercial PZT-5H ceramics, are promising candidates for further applications.     

Study of surface morphology and optical properties of Nb<Subscript>2</Subscript>O<Subscript>5</Subscript> thin films with annealing

Nb₂O₅ films have been deposited on variety of substrates using the sol-gel dip coating technique. As-deposited films on all substrates are amorphous. Films were annealed under controlled ambience at 300, 400 and 600°C for 5 h. As-deposited films on glass substrate show uniform surface structure. The crystal structure and surface topography are found to depend strongly on the annealing temperature and nature of the substrates. The average grain size of 40 nm is observed in films annealed at 300°C. On annealing at 400°C increasing grain size and resulting fusing of them, enhanced surface roughness. Films deposited on NaCl substrates crystallized into a stable monoclinic phase and those deposited on single crystal Si substrates crystallized into hexagonal phase after annealing at 600°C. The as-deposited films show very high transmittance (>90%) in the visible region. The optical band gap is observed to increase from 4.35 eV when the films are in amorphous state to 4.87 eV on crystallization.     

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 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.     

Effect of secondary heat treatment on the spectroscopic properties of Na<Subscript>2</Subscript>O-Nb<Subscript>2</Subscript>O<Subscript>5</Subscript>-P<Subscript>2</Subscript>O<Subscript>5</Subscript> glasses

We have studied the optical absorption and luminescence spectra of 45Na₂O · xNb₂O₅ · (55 − x)P₂O₅ glasses containing 5, 10, 20, 25, 30, and 35 mol % Nb₂O₅. The results indicate that the absorption band around 26000 cm⁻¹, responsible for the yellow color of the glasses, is due to the [Nb^(5+)--O⁻] center and disappears upon secondary heat treatment. Heat treatment of europium-doped glasses increases the concentration of Eu³⁺ centers in an asymmetric environment, which is accompanied by an increase in luminescence efficiency. The reason for this is that the Eu³⁺ ions are located outside the niobate subsystem of the glass matrix. The europium in the glasses studied acts as a protector ion.     

Effect of nonstoichiometry on the structure and microwave dielectric properties of Ba(Co<Subscript>1/3</Subscript>Nb<Subscript>2/3</Subscript>)O<Subscript>3</Subscript>

Polycrystalline nonstoichiometric Ba(Co_1/3Nb_2/3)O₃ (BCN) materials have been synthesized and investigated. Deviations from stoichiometry have been shown to lead to the formation of crystalline Ba₆CoNb₉O₃₀ (barium deficiency) and Ba₈CoNb₆O₂₄ (cobalt deficiency). The effect of phase composition on the microwave dielectric properties of BCN has been studied. The results demonstrate that the dielectric properties of BCN-based materials can be tuned by varying cobalt content. The materials obtained are high-Q microwave dielectrics with temperature-stable properties.     

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.     

Study of structure and properties of ZnO–Bi<Subscript>2</Subscript>O<Subscript>3</Subscript>–P<Subscript>2</Subscript>O<Subscript>5</Subscript> glasses

Glasses of the ternary system ZnO–Bi₂O₃–P₂O₅ were prepared and studied in two compositional series 50ZnO–xBi₂O₃–(50 − x)P₂O₅ and (50 − y)ZnO–yBi₂O₃–50P₂O₅. Two distinct glass-forming regions were found in the 50ZnO–xBi₂O₃–(50 − x)P₂O₅ glass series with x = 0–10 and 20–35 mol.% Bi₂O₃. All prepared Bi₂O₃-containing glasses reveal a high chemical durability. Small additions of Bi₂O₃ (∼5 mol.%) improve thermal stability of glasses. All glasses crystallize on heating within the temperature range of 505–583 °C. Structural studies by Raman and ³¹P MAS NMR spectroscopies showed the rapid depolymerisation of phosphate chains within the first region with x = 0–15 and the presence of isolated Q⁰ phosphate units within the second region with x = 20–35. Raman studies showed that bismuth is incorporated in the glass structure in BiO₆ units and their vibrational bands were observed within the spectral region of 350–700 cm⁻¹. The evolution of properties and the spectroscopic data are both in accordance with a network former effect of Bi₂O₃.     

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.     

Structural and electrical properties of Ba(Fe<Subscript>1/3</Subscript>Nb<Subscript>1/3</Subscript>Ta<Subscript>1/3</Subscript>)O<Subscript>3</Subscript> perovskite

Ba(Fe_1/3Nb_1/3Ta_1/3)O₃ (BFNT) perovskite compound (phase purity>99%) was synthesized by conventional ceramic preparation method. XRD, microstructure, impedance spectroscopy and ac conductivity properties were analyzed in this study. BFNT compound has a cubic crystal structure, having grain size of 0.31 μm. This compound has shown normal ferroelectric behaviour but not obeying Curie-Weiss law. The impedance and electrical studies have been performed as a function of frequency and temperature. Impedance as a function of frequency revealed single relaxation process. The impedance spectroscopic plots exhibit the major response due to grains with partial contribution from the grain boundary and negligible electrode effect. Complex impedance plot showed data points lying on a single semicircle, implying the response originated from a single capacitive element corresponding to the bulk grains. Also, the centre of semicircle lies below the real axis indicating non-Debye type relaxation. Relaxation time was calculated from Z″_max of Cole–Cole plots. It is observed that conduction is due to hopping of charge carriers. Activation energies were computed from the Arrhenius plots of the sample.     

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.