Electrical Conductivity of Silver Vanadium Tellurite Glasses

We have investigated mixed electronic-ionic conduction in 0.5[ x Ag₂O·(1 − x )V₂O₅]–0.5TeO₂ glasses, where the Ag₂O amount varies over a range of 5–40 mol%. The glass samples have been prepared by quenching the melt. The ac conductivity has been measured at frequencies from 10 Hz to 100 kHz and temperatures of 300–425 K. The data indicate that the conduction mechanism changed from being predominantly electronic to ionic for Ag₂O contents of >27.5 mol%. This transition is due to the change in glass structure, which affects both electronic- and ionic-transport properties. The electronic dc conductivity results have been analyzed in terms of a small polaron-hopping model.     

Constrained Sintering of Silver Circuit Paste

Densification kinetics and stress development during constrained sintering of a silver film on a rigid silicon substrate have been studied. Compared with free sintering, the sintering of constrained silver film exhibits a much lower densification and slower densification kinetics. The densification-controlled mechanism changes from fast grain-boundary diffusion kinetics for free sintering to slow lattice diffusion kinetics for constrained sintering. The in-plane tensile stress developed during constrained sintering of silver film, measured using a noncontact laser-scanning optical system, increases rapidly to a maximum level of 1.0–1.5 MPa initially, gradually decreases, and then becomes constant at 0.8–1.0 MPa. The maximum stress observed increases with increasing sintering temperature as a result of the faster densification rate. It is believed that the retardation of densification kinetics of constrained silver film is caused by a change in densification mechanism and the existence of in-plane tensile stress.     

Synthesis and Characterization of Nanocrystalline Niobium Nitride Powders

Nanocrystalline NbN powder was synthesized by the direct nitridation of amorphous Nb₂O₅ powder with high BET surface area. X-ray diffractometry analysis indicated that the cubic-phase NbN powder could be obtained by nitridation at 650°–800°C for 3–8 h. Transmission electron microscopy images showed that the particle sizes were in the range of 15–40 nm. The effect of the nitridation temperature and holding time on the powder properties was discussed.     

Processing and Properties of Strontium Bismuth Vanadate Niobate Ferroelectric Ceramics

The processing conditions, microstructure, and dielectric properties of strontium bismuth niobate vanadate ceramics, SrBi₂(V _x Nb_{1− x} )₂O₉ (SBVN, with 0 ≤ x ≤ 0.3), were systematically studied. A relative density of >90% was obtained for all the samples using a two-step sintering process. XRD showed that a single phase with the layered perovskite structure of SrBi₂Nb₂O₉ (SBN) was formed with a vanadium content of up to 30 at.%. SEM revealed that the average grain size decreased gradually with an increase in vanadium content. The Curie point was found to gradually increase from ∼418°C for SBN to ∼459°C for SBVN with 30 at.% vanadium. Dielectric constants at room temperature and their respective Curie points were found to peak at a composition with 10–15 at.%; vanadium. Moreover, a high concentration of vanadium (>5 at.%) resulted in a significant increase in tangent loss at low frequencies (<1000 Hz). The relationships between chemical composition, processing condition, microstructure, and dielectric properties of SBVN ferroelectric ceramics are discussed.     

Crystallization of Lead Niobate Glass by Mechanical Activation

Mechanical activation-triggered crystallization in PbNb₂O₆-based glass was dependent on the initial presence of nuclei. The crystallization cannot be initiated by mechanical activation in a highly amorphous glass composition quenched from 1350°C where PbNb₂O₆ nuclei did not exist. The steady growth of nanocrystallites of PbNb₂O₆ was observed with an increasing degree of mechanical activation in the glass quenched from 1300°C, where a density of PbNb₂O₆ nuclei existed before mechanical activation. The inability to nucleate in the highly amorphous oxide glass by mechanical activation is consistent with the much higher structural stability as compared with that of metallic glasses, such as Fe-Si-B. The mechanical activation-grown PbNb₂O₆ nanocrystals were 10–15 nm in size as observed using HRTEM and their crystallinities were further improved by thermal aging at an elevated temperature in the range of 550° to 650°C.     

Zinc Vanadates in Vanadium Oxide-Doped Zinc Oxide Varistors

Convergent-beam electron diffraction has been used to determine the space groups of β- and γ-Zn₃(VO₄)₂ particles in vanadium oxide-doped zinc oxide varistors. The crystal structure of β-Zn₃(VO₄)₂ has been determined to be monoclinic with space group P 2₁ and lattice parameters of a = 9.80 Å, b = 8.34 Å, c = 10.27 Å, and β= 115.8°, whereas that of γ-Zn₃(VO₄)₂ is monoclinic with space group Cm and a = 10.40 Å, b = 8.59 Å, c = 9.44 Å, and β= 98.8°. Energy-dispersive X-ray microanalysis of these two phases shows significant deviations from their expected stoichiometry. It is apparent that the β-phase is, in fact, the metastable Zn₄V₂O₉ phase, whereas the γ-phase either is a new oxide that consists of zinc, vanadium, and manganese or, more likely, is a zinc vanadate phase with a Zn:V atomic ratio of 1:1 that has the ability to go into solid solution with manganese.     

钒氮共掺杂纳米二氧化钛光触媒乳液的制备及性能研究

以四氯化钛、有机羧酸、稳定剂、表面活性剂、氨水、偏钒酸铵等简单易得的试剂为主要原料,采用常温络合-控制水解新技术,制备出了钒氮共掺杂的纳米二氧化钛透明乳液。采用X射线衍射(XRD)、纳米激光粒度分析仪、紫外-可见分光光度计等对样品进行了表征。结果表明:样品平均粒度在7nm左右,晶型为锐钛矿,样品最大吸收边拓展到了可见光区域。当钒氮掺杂量为0.1%,回流时间为30min时,得到钒氮共掺杂纳米TiO2水溶胶的光催化性能最好,经太阳光照射50min后,对浓度为50mg/L的酸性红3R染料溶液的降解率达到98%以上。     

Effect of NiO-to-Ni Transformation on Conductivity and Structure of Yttria-Stabilized ZrO2

The conductivity and structure of 8-mol%-yttria-stabilized zirconia with additions of nickel oxide has been investigated, both in the as-prepared state and after reduction of NiO in hydrogen. During reduction at 1000°C, the conductivity decreases by 40%–50% and most of the NiO dissolved in the zirconia is exsolved in the form of nickel particles. This is accompanied by the formation of tetragonal zirconia precipitates of size 10–40 nm and by an increase in the lattice parameter. On reoxidation in air at 1000°C, the change in lattice parameter is partly reversed, but the conductivity shows no further changes.     

Microstructure and Dielectric Properties of Barium Strontium Magnesium Niobate

Dielectric properties and their related microstructural characteristics in solid solutions of (1 – x )Ba(Mg_1/3Nb_2/3)O₃– x Sr(Mg_1/3Nb_2/3)O₃ (BMN–SMN, or BSMN) were investigated by measuring the relative permittivity (ɛ_r), Q values, and temperature coefficient of resonator frequency (τ_f), and by observing microstructure using transmission electron microscopy. When the tolerance factor ( t ) was >0.99 in BSMN with composition 0 < x < 0.5, where the tilting of oxygen octahedra was not involved, the microstructure included only 1:2 ordered phase. In the region where 0.99 > t > 0.97 with 0.7 < x < 1.0, the phase due to the antiphase tilting of oxygen octahedral, the disordered phase, and the 1:2 ordered phase were also present. In a few of the grains, core–shell-type structures, whose main components were dislocations and stacking faults, were found in the solid solution of BSMN.     

Influence of Copper(II) Oxide Additions to Zinc Niobate Microwave Ceramics on Sintering Temperature and Dielectric Properties

The effect of CuO additions on the firing temperature of ZnNb₂O₆ ceramics was investigated using dilatometry, transmission electron microscopy, and X-ray diffractometry. A 5 wt% CuO addition to ZnNb₂O₆ ceramics significantly lowered the firing temperature from 1150° to ∼900°C. The presence of a CuO-rich intergranular phase in the specimen was observed and was evidence of the formation of a liquid phase during sintering. The composition of the liquid phase was (ZnCu₂)Nb₂O₈. In particular, the low-fired ZnNb₂O₆ ceramics had good microwave dielectric characteristics— Q × f = 59 500, ɛ_r= 22.1, τ_f=–66 ppm/^oC. These properties were correlated with the formation of a second phase, (ZnCu₂)Nb₂O₈.     

Microstructure of Lanthanum Magnesium Niobate at Elevated Temperature

Microstructural studies of the complex perovskite compound La(Mg_2/3Nb_1/3)O₃ (LMN) were conducted using transmission electron microscopy (TEM) and X-ray diffractometry (XRD) at elevated temperatures. 1:1 chemical ordering of B-site cations and tilting of oxygen octahedra were observed in LMN. Three types of superlattice reflections, [1—2]{111}, [1—2]{110}, and [1—2]{100} were observed at room temperature and at 800°C in electron diffraction patterns. In the XRD experiments, the [1—2]{210} and [1—2]{300} extra peaks disappeared at temperatures >1200°C. However, the intensity of the superlattice [1—2]{111} peak did not change with increased temperature up to 1400°C. These results strongly indicated that the origin of superlattice reflection [1—2]{111} was different from that of the other superlattice reflections. It was mainly caused by the 1:1 chemical ordering of magnesium and niobium atoms. The TEM image observed at 800°C showed the ordered domain structures separated by the antiphase boundaries.     

Potassium Lithium Niobate Films Derived from Aqueous Precursor Solution

Potassium lithium niobate (KLN) films, which were prepared using an aqueous precursor solution, were characterized in terms of crystallinity, morphology, and electrical properties; 1.0 μm thick KLN films free from cracks and pores could be prepared on MgO(100), Pt(100)/MgO(100), and Pt(111)/Ti/SiO₂/Si substrates by multiple coating and heating at 700°C. The films were composed of compact grains and had sintered textures. The films crystallized with (310) and (540) preferred orientations on MgO(100) and Pt(100)/MgO(100) substrates. The KLN film prepared on Pt(100)/MgO(100) substrate had ɛ'= 136 and tan δ= 0.026 at 1 kHz.     

Effects of Water Amount and pH on the Crystal Behavior of a TiO2 Nanocrystalline Derived from a Sol–Gel Process at a Low Temperature

TiO₂ nanocrystallines with a size of 3–7 nm have been synthesized by the sol–gel method with excessive water at a low temperature of about 50°C. The effects of amount of water and pH on the formation of sols and powders containing TiO₂ nanocrystallines were investigated by X-ray powder diffraction (XRD), transmission electron microscopy, and high-resolution transmission electron microscopy. The results showed that a pure anatase phase was formed at pH 1, in contrast to the formation of particles with a mixture of anatase and brookite phases at pH 2–7. Moreover, at pH 1, the intensity of the XRD peaks and the crystal size increased with increasing water content when the water ratio increased from 40 to 100. Whereas the crystal intensity remained invariant, the crystal size still increased when the water ratio increased from 100 to 400. The photocatalytic activity was also investigated. It was shown that when the water ratio was below 100, the photocatalytic activity increased with increasing water content. Further increase of the water content decreased the photocatalytic activity. The photocatalytic activity decreased with increasing pH. All samples with pH=1 showed better photocatalytic activity than Degussa P25. Mechanisms for the formation of the TiO₂ nanocrystalline particles at such a low temperature were also discussed.     

Effect of Grain Shape on Abnormal Grain Growth in Liquid-Phase-Sintered Nb1−xTixC–Co Alloys

The effect of titanium substitution for niobium on the grain shape change, grain growth inhibition, and abnormal grain growth during liquid-phase sintering of Nb_{1− x} Ti_xC–Co alloy was studied. With increased titanium substitution, the shape of the Nb_{1− x} Ti _x C grains in the liquid cobalt matrix was changed from a cube with round corners to a cube with angular corners, which implied increased edge energy. As the grain corners became more angular, the grain growth became markedly inhibited, and abnormal grain growth occurred. The results could be best explained by the increased edge energy of the interface of the Nb_{1− x} Ti _x C grains, which increased the barrier for the growth by two-dimensional nucleation.     

Structural and Opto-electronic Study of a Novel Nanostructure Nb–S Co-doped Titania

A series of Nb–S co-doped titania (NST) nanoparticles was successfully produced in a single step by sol–gel method using titanium (IV) chloride, niobium (V) chloride and thiourea as precursors followed by annealing. The co-doping of sulfur and niobium caused a dramatic red-shift in absorption, improvement in surface properties and enhancement in thermal stability of the anatase phase. The reflectance spectra showed that the band gap of TiO₂ shifted when the Nb content was low (1 % Nb). Annealing at higher temperature showed an increase in particle size, decrease in absorbance and decrease in band gap energy. This observation was understood in terms of band bending with potential application in solar cells. The NST was further characterized by X-ray diffraction (XRD), energy-dispersive X-ray spectroscopy, scanning electron microscope (SEM) and thermal analysis. The results indicated that Nb and S are homogeneously incorporated into the anatase lattice by substitution of Ti and O, respectively. The XRD showed that Nb–S co-doped titania nanoparticles included the anatase phase at 350 °C calcination temperature. The SEM analysis revealed that the doping of Nb and S did not cause a change in morphology of the catalyst surface. The average crystallite size of the NST with semi-spherical shape was 21 nm.     

Phase Relations and Thermal Expansion Studies in the Ceria–Yttria System

The synthesis, characterization, and bulk and lattice thermal expansions of a series of compounds with general composition Ce_{1– x} Y _x O_{2– x /2} (0.0 ≤ x ≤ 1.0) are reported. The XRD pattern of each product was refined to learn the solid solubility limit and the homogeneity range. The solid solubility limit of YO_1.5 in CeO₂ lattice, under the conditions of slow cooling from 1400°C, is represented as Ce_0.55Y_0.45O_1.775 (i.e., 45 mol% of YO_1.5). The subsequent compositions were biphase. There was no solubility of CeO₂ into the lattice of YO_1.5. The bulk thermal expansion measurements from ambient to 1123 K, as investigated using a dilatometer, revealed that the α_l (293–1123 K) values, within the homogeneity range, decreased on increased Y³⁺ content. A similar trend was observed for average lattice thermal expansion coefficient, α_a (293–1473 K), as investigated using high-temperature XRD. No ordered phases were obtained in this system under the used conditions. These studies on Ce_{1– x} Y _x O_{2– x /2} (0.0 ≤ x ≤ 1.0) system can be used to simulate the phase relation and thermal expansion behavior of Pu_{1– x} Y _x O_{2– x /2} (0.0 ≤ x ≤ 1.0), because CeO₂ is widely used as a surrogate material for PuO₂.     

Scandium-Doped Anatase (TiO2) Nanoparticles Directly Formed by Hydrothermal Crystallization

Anatase-type TiO₂ solid solutions doped with 0–10 mol% scandium were formed by hydrothermal crystallization under weak basic conditions above 180°C for 5 h from amorphous co-precipitates that were obtained from the aqueous precursor solutions of TiOSO₄ and Sc(NO₃)₃ using aqueous ammonia. The anatase particles were spindle-like and consisted of nanosized-crystallites (23–25 nm). The lattice parameter c ₀ of anatase and the length and width of the spindle-like anatase gradually increased when the scandium content was increased. The diffuse reflectance spectra of the as-prepared TiO₂ doped with scandium showed that the onset of absorption slightly shifted to longer wavelengths with increasing scandium content. The band gap of anatase was slightly increased by making solid solutions with scandium oxide.     

Dielectric Properties of the Fluorite-like Bi2O3–Nb2O5 Solid Solution and the Tetragonal Bi3NbO7

Our analysis of the microwave dielectric properties of the δ-Bi₂O₃–Nb₂O₅ solid solution (δ-BN_ss) showed a continuous increase in permittivity and dielectric losses with an increasing concentration of Nb₂O₅. The only discontinuity was found for the temperature coefficient of resonant frequency, which is negative throughout the entire homogeneity range but reaches a minimum value for the sample with 20 mol% Nb₂O₅. At the same composition there is a discontinuity in the grain size of the δ-BN_ss ceramics. For the sample containing 25 mol% Nb₂O₅ two structural modifications were observed. A single-phase tetragonal Bi₃NbO₇, in the literature referred to as a Type-III phase, is formed in a very narrow temperature range from 850° to 880°C. A synthesis performed below or above this temperature range resulted in the formation of the end member of the δ-BN_ss homogeneity range. Compared with the δ-BN_ss the Bi₃NbO₇ ceramics exhibit lower microwave dielectric losses, an increased conductivity, and a positive temperature coefficient of resonant frequency.     

Sintering of a Crystallizable CaO-B2O3-SiO2 Glass with Silver

Effects of Ag addition on sintering of a crystallizable CaO-B₂O₃-SiO₂ glass have been investigated at 700°–900°C in different atmospheres. With Ag content increasing in the range of 1–10 vol%, the softening point, the densification, the onset crystallization temperature, and the total amount of crystalline phase formed of the crystallizable glass are reduced when fired in air. A bloating phenomenon is observed when the crystallizable CaO-B₂O₃-SiO₂ glass doped with 1–10 vol% Ag is fired at 700°–900°C for 1–4 h. Fired in N₂ or N₂+ 1% H₂, however, the above phenomena disappear completely. It is thus believed that the diffusion of Ag into the crystallizable glass, which is caused by the oxidation of Ag in air, is the root cause for the above results observed.     

Titanium Diboride–Tungsten Diboride Solid Solutions Formed by Induction-Field-Activated Combustion Synthesis

Solid solutions of titanium diboride–tungsten diboride (TiB₂–WB₂) were synthesized by induction-field-activated combustion synthesis (IFACS) using elemental reactants. In sharp contrast to conventional methods, solid solutions could be formed by the IFACS method within a very short time, ∼2 min. Solutions with compositions ranging from 40–60 mol% WB₂ were synthesized with a stoichiometric ratio (Ti + W)/B =½; however, samples with excess boron were also made to counter the loss of boron by evaporation. The dependence of the lattice constants of the resulting solid solutions on composition was determined. The "a" parameter decreased only slightly with an increase in the WB₂ content, whereas the "c" parameter exhibited a significant decrease over the range 40–60 mol% WB₂. Solid-solution powders formed by the IFACS method were subsequently sintered in a spark plasma sintering (SPS) apparatus. After 10 min at 1800°C, the samples densified to relative density 86%. XRD analysis showed the presence of only the solid-solution phase.