The low-temperature sintering and microwave dielectric properties of (Zn_(0.7)Mg_(0.3))TiO_3 ceramics with H_3BO_3

The effects of the addition of H 3 BO 3 on the microstructure, phase formation, and microwave dielectric properties of (Zn 0.7 Mg 0.3 )TiO 3 ceramics sintered at temperatures ranging from 890 ℃ to 950 ℃ are investigated. H 3 BO 3 as a sintering agent can effectively lower the sintering temperature of ZMT ceramics below 950 ℃ due to the liquid-phase effect. The microwave dielectric properties are found to strongly correlate with the amount of H 3 BO 3 . With the increase in H 3 BO 3 content, the dielectric constant (ε r ) monotonically increases, but the quality factor (Q × f ) reaches a maximum at 1 wt% H 3 BO 3 , and the apparent density of ZMT ceramics with H 3 BO 3 ≥ 1 wt% gradually decreases. At 950 ℃, the ZMT ceramics with 1% H 3 BO 3 exhibit excellent microwave dielectric properties: ε r = 19.8, and Q × f = 43800 GHz (8.94 GHz).     

微波混合加热技术及应用前景

报道了氧化锆和氧化硅陶瓷材料的微波烧结实验结果,并详细讨论了微波混合加热技术在陶瓷材料微波烧结中的应用。     

Susceptor-Assisted Enhanced Microwave Processing of Ceramics - A Review

Susceptor-assisted microwave processing is a rapidly growing technology due to its superiority over the conventional processing. In contrast to the conventional heating from the surface, the microwave heating occurs volumetrically via direct interaction with the material. Correspondingly, the microwave heating rates are in general much faster than the heating rates in the conventional furnaces, where heat has to be transferred from the heat sources to the material via conduction, convection and radiation. The need for the susceptor stems from the fact that the majority of the ceramics are low lossy materials and they cannot couple well with the microwave at room temperatures. The susceptor provides an easy and non-invasive technique to exploit the rapid microwave processing even for the highly microwave transparent ceramics, such as alumina, silicon nitride, quartz, etc. This article critically evaluates the susceptor-assisted microwave sintering and solid state synthesis of ceramics which have been reported over the last two decades. A wide range of ceramics has been considered and each case has been analyzed in terms of the enhancement of the processing rates and product qualities (grain structure, material properties, etc.) compared to the conventional processing. It has been shown that the susceptor-assisted microwave processing can greatly reduce the processing time while providing an easy pathway to achieve the desired product qualities. The use of the appropriate susceptor is the key to achieve the fast, smooth, and reliable microwave processing of ceramics and this article provides the required database for the appropriate design of the susceptor based on the process requirement.     

Enhancing magnetoelectric properties of 0–3 particulate composite ceramics by introducing nano-sized sintering aids via self-combustion method

The 0.55Pb(Ni_1/3Nb_2/3)O₃-0.05PbHfO₃-0.4PbTiO₃/Ni_0.875Zn_0.125Fe₂O₄ (PNNHT/NZF) 0–3 particulate composite ceramics were prepared by the conventional solid-state reaction method via introducing the nano-sized WO₃ and CuO sintering aids by the self-combustion method. Due to the introducing of nano-sized sintering aids, densified PNNHT/NZF composite ceramics are synthesized sintered at low sintering temperature. Furthermore, the nano-sized sintering aids act effectively as blocking layers, leading to compact composite ceramics with homogenously dispersed and isolated microstructure morphology, inhibiting inter-phase diffusion, avoiding interfacial chemical reaction, and achieving strong inter-phase coupling. Therefore, the synthesized PNNHT/NZF composite ceramics exhibit enhanced magnetoelectric properties.     

Effect of sintering temperature on the elemental diffusion and electrical conductivity of SrTiO<Subscript>3</Subscript>/YSZ composite ceramic

The 50 vol% SrTiO₃/yttria-stabilized zirconia (YSZ) composite ceramic was prepared through powder sintering route in 1400～1500 °C. Only the cubic YSZ and SrTiO₃ phases are detected in all the sintered ceramics, and the typical XRD peak positions of both phases have varied dramatically. The grain sizes and relative densities of all specimens increase evidently with the sintering temperature. The width of the SrTiO₃/YSZ interfacial region increases from 100.4 to 468.8 nm as the sintering temperature rises from 1400 to 1500 °C. The total electrical conductivities of the sample sintered at 1500 °C are remarkably higher than those at 1400 and 1450 °C, while the ion transference numbers drop from 0.837 to 0.731 with sintering temperature from 1400 to 1500 °C. The variations in the electrical properties above can be interpreted based on the effects of sintering temperature on the elemental diffusions during the sintering process.     

Stabilization of Nanoscale Quasi-Liquid Interfacial Films in Inorganic Materials: A Review and Critical Assessment

Recent observations of three classes of nanometer-thick, disordered, interfacial films in multicomponent inorganic materials are reviewed and critically assessed. The three classes of films are equilibrium-thickness intergranular films (IGFs) in ceramics, their free-surface counterparts, that is, surficial amorphous films (SAFs), and their metallic counterparts. Also briefly reviewed are several related wetting and adsorption phenomena in simpler systems, including premelting in unary systems, prewetting in binary liquids or vapor adsorption on inert walls, and frustrated-complete wetting. Analogous diffuse-interface and force-balance models are discussed with the goal of exploring a unifying thermodynamic framework. In general, the stability of these nanometer-thick interfacial films does not follow bulk phase diagrams. Stabilization of quasi-liquid interfacial films at subeutectic or undersaturation conditions in multicomponent materials can be understood from coupled interfacial premelting and prewetting transitions. More realistic models should include additional interfacial interactions, for example, dispersion and electrostatic forces, and consider the possibility for metastable equilibration. It is suggested that quasi-liquid grain boundary films in binary metallic systems can be used to validate a basic thermodynamic model. These nanoscale interfacial films are technologically important. For example, the short-circuit diffusion that occurs in interface-stabilized, subeutectic, quasi-liquid films explains the long-standing mystery of the solid-state activated sintering mechanism in ceramics, refractory metals, and ice.     

Microwave dielectric properties of the 5.5Li<Subscript>2</Subscript>O–Nb<Subscript>2</Subscript>O<Subscript>5</Subscript>–7TiO<Subscript>2</Subscript> ceramics

A new Li₂O–Nb₂O₅–TiO₂ (LNT) ceramic with the Li₂O:Nb₂O₅:TiO₂ mole ratio of 5.5:1:7 was prepared by solid state reaction route. The phase and structure of the ceramic were characterized by X-ray diffraction and scanning electron microscopy (SEM). The microwave dielectric properties of the ceramics were studied using a network analyzer. The microwave dielectric ceramic has low sintering temperature (∼1075°C) and good microwave dielectric properties of ε _r=42, Q×f=16900 GHz (5.75 GHz), and τ _f =63.7 ppm/°C. The addition of B₂O₃ can effectively lower the sintering temperature from 1075 to 875°C and does not induce degradation of the microwave dielectric properties. Obviously, the LNT ceramics can be applied to microwave low temperature-cofired ceramics (LTCC) devices.     

Preparation and characterization of CaTiO 3 –(Li 1/2 Nd 1/4 Sm 1/4 )TiO 3 microwave ceramics by a sol–gel combustion process

A novel sol–gel combustion process was adopted to synthesize reactive ceramic powder with composition of 0.25 CaTiO₃–0.75 (Li_1/2Nd_1/4Sm_1/4)TiO₃, and microwave dielectric ceramics were prepared at different sintering temperatures using the synthesized powder. The combustion behavior of citrate gel and the sintering feature of the synthesized powder were evaluated by using differential thermal analysis–thermogravimetric analysis and thermo-mechanical analysis techniques, respectively. The citrate gel exhibits a self-propagating behavior after being ignited in air at room temperature. The as-burnt powder is so highly reactive that it can be transformed into single-phase perovskite at 900 °C and it can be sintered at 1100 °C. The effects of sintering temperature on the density, microstructure, and dielectric properties of the sintered ceramics were investigated. The maximum values of density, dielectric constant, and Q×f were achieved after sintering at 1200 °C. At 1200 °C, very dense ceramics with uniform grains and good dielectric properties with a dielectric constant of 123.8, a Q×f value of 5110 (at 3.7 GHz), and a τ_f value of +12.5 ppm/°C were achieved via the sol–gel combustion route. PACS 77.84.Dy; 81.20.FW; 77.22.-d     

A Review of Research Trends in Microwave Processing of Metal-Based Materials and Opportunities in Microwave Metal Casting

Microwave processing of materials has emerged as a new method for processing of a variety of materials in the recent years. Microwaves have been used effectively with significant advantages, particularly in food processing and chemical synthesis. They are also found to be efficient for processing polymers, ceramics, polymeric composites, and ceramic composites. The physics of interaction of microwaves with characteristically different materials is not yet explored well; consequently, there are challenges in microwave processing of metal-based materials. Industrial processing of bulk metal is yet to be popular in spite of the fact that the feasibility of metal powder sintering was demonstrated a few decades ago. This article provides a summary of fundamental aspects of microwave processing of metal-based materials and their interaction with metallic materials. The processing challenges have been surveyed; developments in terms of techniques and tooling have been analyzed. Possible effects of microwave processing on metallic materials, in particular metal powders, bulk metals, bulk metal-metal powder systems, and sheet metals have been presented. Future research aspects of microwave processing of metallic materials with reference to metal casting have been identified.     

高温原位拉曼光谱研究BiFeO3陶瓷反应烧结相变过程

多铁性材料BiFeO3(BFO)由于具有潜在的磁电耦合效应而备受关注,但纯相陶瓷的制备始终是一个难点,部分原因在于对其反应烧结相变规律的认识尚不充分。高温原位拉曼光谱技术(HT-Raman)是表征复杂的固体相变及反应的有力手段。首次利用HT-Raman,研究了不同配比(1∶1, 1.03∶1和1.05∶1)的Bi2O3-Fe2O3在不同升降温速率(10和100℃·min-1)下的反应烧结相变过程,以及降温时反应产物的收缩效应。结果表明:Bi2O3-Fe2O3反应烧结生成BiFeO3的过程中,会产生中间过渡相Bi2Fe4O9和Bi25FeO39∶Bi2O3-Fe2O3配比为1.03∶1、升降温速率较快时,产物中杂相含量最少,可见Bi过量及较快的升降温速率能有效抑制杂相的生成。降温过程中,发现BFO的A1-1峰位随着温度降低发生蓝移,且二者呈良好的线性关系,这说明降温过程中BFO仅因温度变化产生晶格收缩,并没有结构相变。此外,还利用二维X射线衍射(2D-XRD)及背散射电子衍射(EBSD),表征了烧结产物的相组成及形貌。XRD结果也显示Bi过量时杂相含量较少,与拉曼结果一致。结合2D-XRD和EBSD的结果可知, Bi过量时烧结产物晶粒尺寸较大且均一,可见快速升降温有利于晶粒的成核与生长。研究结果可帮助进一步认清反应烧结规律,并指导纯相BiFeO3基陶瓷的制备。     

Constitutive modeling of the densification of PZT ceramics

In the present work, sintering constitutive models were employed to investigate the high-temperature densification behavior of undoped stoichiometric lead zirconate titanate (PZT) ceramics with perovskite structure. The constitutive models used were based on classical grain-boundary diffusion and interface reaction, and also the coupled mechanism of the two processes. PZT ceramics were fabricated and the calcination and sintering of the materials were discussed. Using the constitutive models employed and the experimental results obtained, the activation energy for densification of the PZT ceramics investigated was determined as 390±30 KJ/mol. It was noted that the coupled mechanism of grain-boundary diffusion and interface reaction was the controlling process during the densification of the materials investigated.     

Effects of sintering condition on phase formation,microstructure and dielectric properties of lead titanate ceramics

Lead titanate ceramics have been prepared by two different processing methods: conventional (or single-stage) and two-stage sintering. Effects of designed sintering conditions on phase formation, densification, microstructure and dielectric properties of the ceramics were characterized via X-ray diffraction, Archimedes density measurement, scanning electron microscopy and dielectric measurement, respectively. The potentiality of a two-stage sintering technique as a simple ceramic fabrication method to obtain highly dense and pure lead titanate ceramics was demonstrated. It has been found that, under suitable two-stage sintering conditions, dense perovskite lead titanate ceramics can be successfully achieved with better dielectric properties than those of ceramics from a single-stage sintering technique. PACS 77.22.-d; 77.84.-s; 77.84.Dy     

Effect of low-temperature high-pressure sintering on BiFeO3 density,electrical magnetic and structural properties

Single-phase multiferroic BiFeO₃ (BFO) powders were prepared by hydrothermal microwave synthesis and dense BiFeO₃ ceramics were fabricated for the first time by the low-temperature high-pressure (LTHP) sintering technique. Effect of sintering temperature ranging from 400 to 800 °C (3 min and 10 min) and pressure of 3–8 GPa on structural, microstructural, electric and magnetic properties were investigated through X-ray diffraction, scanning electron microscope (SEM), electrochemical impedance spectroscopy (EIS), density and magnetic measurements. The results highlighted that LTHP sintering method, thanks to the high pressure involved, requires lower temperature and shorter time than other techniques, avoiding BiFeO₃ phase degradation. SEM images show that for short experimental time (t = 3 min) the average grain size of the sintered samples was approximately the same size of raw powder. Extending the sintering time up to 10 min the grain growth phenomena occurred. Moreover the results indicate that the best obtained density value was around 98% of theoretical density. The dielectric behavior of BiFeO₃ ceramics was not significantly influenced by the LTHP sintering conditions. Magnetic measurements showed that ceramic BiFeO₃ is weakly ferromagnetic at room temperature.     

烧结压力对碳化钽陶瓷维氏硬度的影响

为了探究烧结压力对不同晶粒尺寸碳化钽(TaC)力学性能的影响,通过高温高压技术对纳米、微米尺寸TaC粉末进行高温高压烧结,制备不同烧结条件下的块状TaC陶瓷。利用X射线衍射等表征方法对烧结样品的物相、元素分布、压痕形态进行表征,结果表明:TaC在烧结过程中物相稳定,且无杂质渗入。利用维氏硬度计对不同烧结压力(3.0、4.0和5.5 GPa)条件下的3种陶瓷样品进行维氏硬度测试,并进行微观结构分析,结果表明:随着烧结压力由3.0 GPa提升到5.5 GPa,微米尺寸TaC的维氏硬度(21.0 GPa)优于3.0、4.0 GPa下的纳米尺寸TaC维氏硬度(17.5、19.2 GPa)。此外,研究发现,测试维氏硬度时,3.0 kg应用载荷对测试TaC维氏硬度更加精确。研究结果对结构陶瓷烧结和超高温陶瓷硬度研究具有指导意义。     

Multifractal analysis of grain pattern formation in LZT ceramics

LZT ceramics with medium ferrostiffness were obtained. Multifractal parameterization of the ceramic grain pattern at different stages of preparation was performed. The effects of pattern formation and sintering conditions on ferrostiffness are discussed. Possibilities for optimizing the production of ceramics and preventing destructive processes are shown.     

Effect of sintering temperature on phase formation,microstructure and dielectric properties of nanogold modified barium titanate ceramics

In this work, BaTiO₃ ceramics modified with 0.5 mol% Au nanoparticles were fabricated by using a combination of the solid-state reaction and pressureless-sintering techniques. By employing a combination of X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive X-ray (EDX), Archimedes principle and dielectric measurement techniques, it was found that no phases other than tetragonal BaTiO₃ were observed in all ceramics. In contrast to the tetragonality, the relative density, grain size and maximum dielectric constant at Curie temperature of the ceramics were found to increase with sintering temperature. In addition, it has been found that, under suitable sintering temperature, dense perovskite nanogold modified BaTiO₃ ceramics with fine-grained microstructure (∼1 μm) and better dielectric properties than those of gold-free ceramics can be produced.     

Laser-quality oxide Y<Subscript>3</Subscript>Al<Subscript>5</Subscript>O<Subscript>12</Subscript> ceramics. Comparative studies of its basic characteristics and laser ceramics of a known manufacturer

The effect of sintering aids of SiO₂, ZrO₂, B₂O₃, and MgO oxides on the optimum sintering temperature, ceramics grain growth, total volume of residual pores, and optical quality of obtained ceramics is studied. The best combinations of sintering aids are found; as a result, YAG:Nd (1 at%) samples of ceramics of high optical quality are obtained. An original method for characterizing laser properties of ceramics is developed. Comparative measurements of main laser characteristics of the obtained ceramics and ceramics of the Konoshima Chemical Corp. Ltd wellknown in the world practice, are performed.     

Nonuniform Distribution of Electric Properties in Ferrite Ceramics

Electric and magnetic properties of Li–Ti ferrite ceramics depending on sintering modes and controlled by the diffusion-based specimen-to-environment oxygen exchange are considered. The relationship between the properties of ceramics and the character of nonuniform distribution of oxygen in the bulk of a specimen is determined. A physical model of the processes giving rise to nonuniform distribution of oxygen in polycrystalline ferrite ceramics is proposed. The empirical dependences of specific conductivity on sintering time at 1373 K are obtained.     

Influence of sintering conditions on structural,thermal, electric and ferroelectric properties of Na0.5Bi0.5TiO3 ceramics

Na_0.5Bi_0.5TiO₃ ceramics were prepared by a conventional solid-state reaction method and by a hot-pressing route. The influence of sintering conditions on structural, thermal, dielectric and ferroelectric properties of these ceramics was investigated. All obtained samples exhibited a single perovskite phase. It was shown that the sintering conditions significantly influence the properties under investigation. This includes changes in the value of the electric permittivity ? and dielectric loss tanδ, a shift of T_m and T_d and change of the ferroelectric properties. These effects are mainly related to volatility of the Na and Bi components during the sintering process along with formation of their compensating charge defects, which leads to local structure change.     

Relations between physical properties of the biferroic Pb(Fe<Subscript>1-x</Subscript>Nb<Subscript>x</Subscript>)O<Subscript>3</Subscript>ceramics and their composition change

The work shows basic features of ferromagnetic ceramics Pb(Fe_{1 - x}Nb_x)O₃ with various contents of Fe/Nb (x=1/3, 1/2, 2/3). The synthesis were carried out by powders sintering and calcination whereas densification by hot uniaxial pressing and conventional sintering methods. Optimum conditions of PFN obtention with various contents were based on X-ray, microstructural, dielectrical and conductivity tests together with increase of iron contents in Pb(Fe_{1 - x}Nb_x)O₃ usage parameters of those ceramics worsen. Optimum parameters are shown by the ceramics with contents Pb(Fe_{1 / 2}Nb_{1 / 2})O₃ synthetized using calcination method and densified by hot uniaxial pressing method.