Structural and dielectric properties of Li2O–ZnO–BaO–B2O3–CuO glasses

Glass samples of the system (15Li₂O–30ZnO–10BaO–(45 − x)B₂O₃–xCuO where x = 0, 5, 10 and 15 mol%) were prepared by using the melt quenching technique. A number of studies, viz. density, differential thermal analysis, FT-IR spectra, a.c. conductivity and dielectric properties (constant εφ, loss tan δ, a.c. conductivity, σ_ac, over a wide range of frequency and temperature) of these glasses were carried out as a function of copper ion concentration. The analysis of the results indicate that the density increases while molar volume decreases with increasing of copper content indicates structural changes of the glass matrix. The glass transition temperature, T _g, and crystallization temperature, T _c, increase with the variation of concentration of CuO referred to the growth in the network connectivity in this concentration range, while glass-forming ability parameter (T _c − T _g) decreases with increasing CuO content, indicates an increasing concentration of copper ions that take part in the network-modifying positions. The FT-IR spectra evidenced that the main structural units are BO₃, BO₄, and ZnO₄. The structural changes observed by varying the CuO content in these glasses and evidenced by FTIR investigation suggest that the CuO plays a network modifier role in these glasses while ZnO plays the role of network formers. The dielectric constant decreased with increase in temperature and CuO content. The variation of a.c. conductivity with the concentration of CuO passes through a maximum at 5 mol%. In the high temperature region, the a.c. conduction seems to be connected with the mixed conduction viz., electronic conduction and ionic conduction.     

Characterisation of thermo-mechanical properties of MgO–Al2O3–SiO2 glass ceramic with different heat treatment temperatures

The effects of heat treatment temperature on crystallisation behaviour, precipitated phases and thermo-mechanical properties of some MgO–Al₂O₃–SiO₂ (MAS) glass-ceramics were investigated. Crystallisation behaviour of MgO–Al₂O₃–SiO₂ glasses in the presence of TiO₂ as a nucleation agent was studied. The crystalline phases present in the heat treated samples were identified by X-ray diffraction (XRD). It was observed from XRD studies that magnesium aluminium titanate initially precipitated and when the heat treatment temperature was increased to 1140 °C, depending on the thermal history, either magnesium silicate, aluminium titanate and quartz or magnesium aluminium titanate, magnesium aluminate and quartz were precipitated. SEM observation revealed that the heat treatment led to phase separation of droplet-shaped crystals before the needle-shaped crystals formed at 1140 °C. The effect of annealing temperature on the density and mechanical properties of these glass-ceramic were characterised by nanoindentation and the results revealed a significant increase in hardness of the fully crystallised system.     

Effects of copper and Al2O3 particles on characteristics of Cu–Al2O3 composites

High-energy milling was used for production of Cu–Al₂O₃ composites. The inert gas-atomized prealloyed copper powder containing 2 wt.%Al and the mixture of the different sized electrolytic copper powders with 4 wt.% commercial Al₂O₃ powders served as starting materials. Milling of prealloyed copper powders promotes formation of nano-sized Al₂O₃ particles by internal oxidation with oxygen from air. Hot-pressed compacts of composites obtained from 5 and 20 h milled powders were additionally subjected to the high-temperature exposure in argon at 800 °C for 1 and 5 h. Characterization of processed material was performed by optical and scanning electron microscopy (SEM), X-ray diffraction analysis (XRD), microhardness, as well as density and electrical conductivity measurements. Due to nano-sized Al₂O₃ particles microhardness and thermal stability of composite processed from milled prealloyed powders are higher than corresponding properties of composites processed from the milled powder mixtures. The results were discussed in terms of the effects of different size of starting copper powders and Al₂O₃ particles on the structure, strengthening of copper matrix, thermal stability and electrical conductivity of Cu–Al₂O₃ composites.     

Sol–gel derived CaO–B2O3–SiO2 glass/CaSiO3 ceramic composites: processing and electrical properties

The CaO–B₂O₃–SiO₂ glass/CaSiO₃ ceramic (CBS/CS) composites were fabricated via sol–gel processing routes. Their densification behavior, structures and dielectric properties were investigated. The precursors of CBS glass and CS ceramic filler were firstly obtained via individual soft chemical route and then mixed together in various proportions. The results indicated that the structures of CBS/CS composites are characteristic of CS and CaB₂O₄ (CB) ceramic phases distributed in the matrix of glass phase at 800–950 °C. The CS ceramic phase not only acts as fillers, but nuclei for the crystallization of CBS glass as well such that the CS content exhibits an effect on the densification and dielectric properties of the composites. The CBS/CS composites with 10% CS sintered at 850 °C own dielectric properties of ε_r < 5 and tanδ = 6.4 × 10⁻⁴ at 1 MHz.     

Synthesis and properties of magnetic solid superacid: SO4/ZrO2–B2O3–Fe3O4

A magnetic SO₄²⁻/ZrO₂–B₂O₃–Fe₃O₄ solid superacid catalyst is prepared via a simple chemical co-precipitation approach. The obtained materials were characterized in detailed by X-ray powder diffraction, thermogravimetric analysis–different scanning calorimetry, Fourier transform infrared spectroscopy (FTIR), electron microscopy (SEM and TEM), and Mossbauer spectra. Powder X-ray diffraction patterns show that in this composite oxide the transformation temperature of ZrO₂ from tetragonal to monoclinic phase is higher compared to the pristine SO₄²⁻/ZrO₂ material. The introduction of Fe₃O₄ endows the superacid with a super-paramagnetic property while in a ferromagnetic state after calcination. The superacid exhibits high catalytic activity in forming ethyl acetate by esterification.     

Optical properties and structure of R2O–Ga2O3–SiO2 and RO–Ga2O3–SiO2 glasses

Refractive index and molar refraction of Li₂O–, Na₂O–, CaO–, and BaO–Ga₂O₃–SiO₂ glasses have been used to test the validity of a structural model of silicate glasses containing Ga₂O₃ glasses. Ga₂O₃ enters these types of glass in a similar manner as Al₂O₃. It is assumed that, for (SiO₂/Ga₂O₃) >1 and (Ga₂O₃/R₂O) ≤1, Ga₂O₃ associates primarily with modifier oxides to form GaO₄ units. The rest of modifier oxide forms silicate units with non-bridging oxygen ions. Silicate structural units have the same factors as found for binary alkali- and alkaline earth silicate glasses. Differences between experimental and model values suggest another structure for (Ga₂O₃/SiO₂) ≥1.     

多孔Al2O3f/Al2O3复合材料研究进展

作为20世纪90年代兴起的一类连续陶瓷纤维增强陶瓷基复合材料,连续氧化铝纤维增韧氧化铝(Al₂O_3f/Al₂O₃)复合材料已经发展为与C_f/SiC、SiC_f/SiC等非氧化物复合材料并列的陶瓷基复合材料。以多孔基体实现基体裂纹偏转成为Al₂O_3f/Al₂O₃复合材料主要的增韧设计方法,形成的多孔Al₂O_3f/Al₂O₃复合材料具有优异的抗氧化性能和高温力学性能,可在高温富氧、富含水汽的中等载荷工况中长时服役,是未来重要的热结构材料。经过近30年的发展,多孔Al₂O_3f/Al₂O₃复合材料已被应用于航空发动机、燃气轮机等热端部件。本文综述了多孔Al₂O_3f...     

B2O3和Al2O3共同掺杂ZnO压敏陶瓷的性能

研究了B₂O₃(B)和Al₂O₃(Al)共掺杂对ZnO压敏陶瓷电学性能和微观结构的影响。结果表明,共掺杂B和Al的ZnO压敏陶瓷,具有低泄漏电流、高非线性和低剩余电压等优良电性能。B和Al的掺杂率为3.0%(摩尔分数)和0.015%(摩尔分数)的ZnO压敏陶瓷,其最佳样品的电参数为：击穿电压E_{1 mA}＝475 V/mm;泄漏电流J_L=0.16 μA/cm²;非线性系数α=106;剩余电压比K = 1.57。     

Tribological properties of Al6061–Al2O3 nanocomposite prepared by milling and hot pressing

Tribological properties of bulk Al6061–Al₂O₃ nanocomposite prepared by mechanical milling and hot pressing were investigated. Al6061 chips were milled for 30 h to achieve a homogenous nanostructured powder. A 3 vol.% Al₂O₃ nanoparticles (∼30 nm) were added to the Al6061 after 15 and 30 h from the beginning of milling. The milling times with Al₂O₃ in these two samples were then 15 h and 30 min, respectively. Additionally, 3 vol.% Al₂O₃ (1 μm and 60 μm) was added to the Al6061 after 15 h of milling; where, the micron size Al₂O₃ in these two samples, was milled 15 h with the matrix. Hot pressing of milled samples was executed at 400 °C under 128 MPa pressure in a uniaxial die. The hot pressed samples were characterized by micro-hardness test, bulk density measurements, pin on disc wear test, and finally scanning electron microscopy observations. Fifteen hour-milled nanocomposite with nanoscale Al₂O₃, showed improvement in wear resistance and bulk density compared with that of 30 min-milled nanocomposites due to better dispersion of Al₂O₃ nanoparticles, improved surface quality of nanocomposite particles before pressing and more grain refinement of Al matrix. Moreover, increasing the reinforcement size increased the wear rate because of reduction in relative density, hardness and inter-particle spacing.     

Structure and electrical conductivity relaxation studies of Nb2O5-doped B2O3–Bi2O3–LiF glasses

Glasses with composition (70 − x) B₂O₃·15Bi₂O₃·15LiF·xNb₂O₅ with x = 0–1.0 mol% were prepared by conventional glass-melting technique. The molar volume V _m values decrease and the glass transition temperatures T _g increase with increase of Nb₂O₅ content up to 0.2 mol%, which indicates that Nb⁵⁺ ions act as a glass former. Beyond 0.2 mol% Nb₂O₅ the V _m increases and the T _g decreases, which suggests that Nb⁵⁺ ions act as a glass modifier. The FTIR spectra suggest that Nb⁵⁺ ions are incorporated into the glass network as NbO₆ octahedra, substituting BO₄ groups. The temperature dependence of the dc conductivity follows the Greaves variable range hopping model below 454 K, and follows the small polaron hopping model at temperatures >454 K. σ _dc, σ _ac conductivity and dielectric constant (ε) decrease and activation energy for dc conduction ΔE _dc which increases with increasing Nb₂O₅ content up to 0.2 mol%, whereas σ _dc, σ _ac and (ε) increase and ΔE _dc decreases with increasing Nb₂O₅ content beyond 0.2 mol%. The impedance spectroscopy shows a single semicircle or arcs which indicate only the ionic conduction mechanism. The electric modulus formalism indicates that the conductivity relaxation is occurring at different frequencies exhibit temperature-independent dynamical process. The (FWHM) of the normalized modulus increases with increase in Nb₂O₅ content suggesting that the distribution of relaxation times is associated with the charge carriers Li⁺ or F⁻ ions in the glass network.     

Dielectric properties and microstructure of TiO2 modified (ZnMg)TiO3 microwave ceramics with CaO–B2O3–SiO2

The low-fired (ZnMg)TiO₃–TiO₂ (ZMT–TiO₂) microwave ceramics using low melting point CaO–B₂O₃–SiO₂ as sintering aids have been developed. The influences of Mg substituted fraction on the crystal structure and microwave properties of (Zn_1−x Mg _x )TiO₃ were investigated. The result reveals that the sufficient amount of Mg (x ≥ 0.3) could inhibit the decomposition of ZnTiO₃ effectively, and form the single-phase (ZnMg)TiO₃ at higher sintering temperatures. Due to the compensating effect of rutile TiO₂ (τ_f = 450 ppm/°C), the temperature coefficient of resonant frequency (τ_f) for (Zn_0.65Mg_0.35)TiO₃–0.15TiO₂ with biphasic structure was adjusted to near zero value. Further, CaO–B₂O₃–SiO₂ addition could reduce the sintering temperature from 1150 to 950 °C, and significantly improve the sinterability and microwave properties of ZMT–TiO₂ ceramics, which is attributed to the formation of liquid phases during the sintering process observed by SEM. The (Zn_0.65Mg_0.35)TiO₃–0.15TiO₂ dielectrics with 1 wt% CaO–B₂O₃–SiO₂ sintered at 950 °C exhibited the optimal microwave properties: ε ≈ 25, Q × f ≈ 47,000 GHz, and τ_f ≈ ± 10 ppm/°C.     

Microstructures formed during devitrification of Na2O·Al2O3·B2O3·SiO2·Fe2O3 glasses

Soda alumina borosilicate glasses of composition (24-y)Na₂O·yAl₂O₃·14B₂O₃·37SiO₂·25Fe₂O₃, y = 8, 12, 14, 16 mol%, were melted using Fe₂O₃ as raw material. Besides, samples with y = 12 and Fe₂O₃ concentrations of 14.32, 17.8, and 25.0 mol% were prepared from FeC₂O₄·2H₂O as raw material. The X-ray diffraction analyses showed the presence of magnetite for the samples from all the investigated compositions. Transmission electron microscopy (TEM) evidenced that all the samples are phase separated and droplets in the diameter range 100–1000 nm, enriched in iron, are formed. Inside these droplets, numerous small magnetite particles, with size in the 25–40 nm interval, are crystallized.     

Low-Magnetoresistance RuO2–Al2O3 Thin-Film Thermometer and its Application

Thermometers consisting of RuO₂–Al₂O₃ composite thin films were prepared by RF sputtering. It was found that different electrode-patterning techniques have dissimilar effects on the magnetoresistance (MR) and the temperature coefficient of resistance (TCR). In general, the thermometers with electrodes fabricated by photo-resist lithography exhibit superior performance compared to those with electrodes prepared using a metal mask. By adjusting the relative compositions of RuO₂ and Al₂O₃, the thermometers can be applied to a wide temperature range from 60 mK to 500 K. In a pulsed magnetic field up to 55 T, the MR at 4.2 K of a typical thermometer for the temperature range from 1.4 K to 300 K increases linearly with magnetic field to a maximum of ~15 %, corresponding to a temperature deviation of ~−4 %. As frequency increases from dc to 1.9 MHz, the MR decreases from  −13 % to ~ − 0.5 % at T = 1.3 K and H = 55 T. By integrating the thermometer with a heater on a sapphire chip, a micro-calorimeter can be developed and successfully used to measure the heat capacity of small mg-sized sample. The RuO₂–Al₂O₃ composite film can also be employed as an infrared bolometer operated at room temperature.     

Thermal properties and crystallization of PbO–MoO3–P2O5 glasses

Thermal properties and crystallization of glasses from PbO–MoO₃–P₂O₅ ternary system were studied in three compositional series (100 − x)[0.5PbO–0.5P₂O₅]–xMoO₃ (A), 50PbO–yMoO₃–(50 − y)P₂O₅ (B), and (50 − z)PbO–zMoO₃–50P₂O₅ (C). Glass transition temperature, crystallization temperature, coefficient of thermal expansion, and dilatation softening temperature of the studied glasses were determined by differential thermal analysis and dilatometry. Crystallization products of annealed glass samples were investigated by X-ray diffraction and Raman spectroscopy. X-ray diffraction analysis of crystallized glasses revealed the formation of PbP₂O₆, Pb₃P₄O₁₃, and PbMoO₄ in the samples of the B series. In the series A and C in the samples with a high MoO₃ content, crystalline compounds of Pb(MoO₂)₂(PO₄)₂ and (MoO₂)(PO₃)₂, respectively, were identified. Raman spectra of crystalline samples confirmed the results of X-ray diffraction measurements and provided also information on thermal stability of glasses and formation of glass-crystalline phases in the studied glass series.     

片状Al2O3对Al2O3/FEP复合材料热导率的影响

通过混炼工艺制备了片状Al₂O₃填充聚全氟乙丙烯(FEP)复合材料,以颗粒状Al₂O₃为对比样品,研究了片状Al₂O₃形状和尺寸对 FEP基复合材料热导率的影响,利用SEM观察了FEP基复合材料的微观形貌。结果表明：在低填充量下,Al₂O₃颗粒在FEP基体中呈“海岛”状分布,没有形成连续的导热网链,但其热导率明显提高;复合材料拉伸强度与断裂伸长率随Al₂O₃含量的增加而减小;低填充量时复合材料热导率的提高主要来自Al₂O₃的微细片状结构,这种微细片状结构一方面提高了有效导热路径,另一方面增加了颗粒与基体之间接触面积,因此有利于热导率的提高。     

Synthesis and properties of β-Ga2O3 nanostructures

A novel method was utilized to synthesize one-dimensional β-Ga₂O₃ nanostructures. In this method, β-Ga₂O₃ nanostructures have been successfully synthesized on Si(111) substrates through annealing sputtered Ga₂O₃/Mo films under flowing ammonia in a quartz tube. The as-obtained samples were analyzed in detail using the methods of X-ray diffraction (XRD), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM) and energy dispersive X-ray spectroscopy (EDX) attached to the HRTEM instrument. The results show that the formed nanostructures are single-crystalline Ga₂O₃. The annealing temperature has an evident influence on the morphology of the β-Ga₂O₃ nanostructures. The growth mechanism of the β-Ga₂O₃ nanostructures is also discussed by conventional vapor-solid (VS) mechanism.     

Microwave dielectric properties of MgAl2O4–CoAl2O4 spinel compounds prepared by reaction-sintering process

MgAl₂O₄ spinel exhibits fascinating microwave dielectric properties, but the synthesis of dense MgAl₂O₄ ceramics requires high firing temperatures. In this study, Co is introduced into MgAl₂O₄ ceramics to improve their sinterability and microwave dielectric properties. An Mg_1−xCo_xAl₂O₄ solid solution of a spinel structure was observed in the MgAl₂O₄–CoAl₂O₄ system, and dense Mg_1−xCo_xAl₂O₄ ceramics were obtained by sintering at 1475–1500 °C in air for 2–6 h. Co addition is effective in lowering the sintering temperature to 1475 °C. Q × f of Mg_1−xCo_xAl₂O₄ ceramics was increased to 49,300 GHz with an increase in Co content to 0.2, but degraded with a further increase in Co content. The temperature coefficient of resonant frequency of Mg_1−xCo_xAl₂O₄ ceramics was sustained at between −73 and −23 ppm/°C to the variation of Co content.     

Low-temperature sintering and microwave dielectric properties of 5Li2O–0.58Nb2O5–3.23TiO2 ceramics

The effect of B₂O₃ addition on the sintering, microstructure and the microwave dielectric properties of the 5Li₂O–0.58Nb₂O₅–3.23TiO₂ (LNT) ceramics have been investigated. It is found that the LNT ceramics could be sintered well at ∼880 °C with low-level doping of B₂O₃ (≤2 wt.%). Only Li₂TiO₃ solid solution (Li₂TiO₃ss) crystal structure could be detected for all the ceramics with various amounts of B₂O₃ addition from the X-ray diffraction (XRD) results. And interestingly, two phases with different color in SEM images are observed in B₂O₃-doped LNT ceramics. EDS results suggest that the two different phases are two Li₂TiO₃ss phases with different amount of Nb. In addition, there is no much degradation in the microwave dielectric properties with the B₂O₃ adding. In the case of 0.5 wt.% B₂O₃-doped samples sintered at 880 °C, good microwave dielectric properties of ?_r = 22, Q × f = 32,000 GHz, τ_f = 9.5 ppm °C⁻¹ are obtained.