微波烧结氧化锌压敏电阻的致密化和晶粒生长

研究了微波烧结的ZnO压敏电阻的致密化和生长动力学, 微波烧结温度从900～1200℃, 保温时间从20min～2h. 研究表明, 微波烧结ZnO压敏电阻的物相组成和传统烧结的样品没有区别; 微波烧结有助于样品的致密化, 并降低致密化温度. 随着烧结温度的升高, 致密化和反致密化作用共同影响样品的密度, 其中Bi的挥发是主要影响因素. 微波烧结ZnO压敏电阻的晶粒生长动力学指数为2.9～3.4, 生长激活能为225kJ/mol, 传统烧结的ZnO压敏电阻的晶粒生长动力学指数为3.6～4.2, 生长激活能为363kJ/mol. 液相Bi₂O₃、尖晶石相和微波的“非热效应”是影响微波烧结ZnO压敏电阻陶瓷晶粒生长的主要因素.     

陶瓷材料烧结致密化过程的元胞自动机模拟

为研究陶瓷材料烧结致密化过程,以晶界能和晶界曲率生长驱动力理论为基础,建立了含有气孔的二相晶粒生长的元胞自动机模型,对陶瓷材料烧结致密化过程进行了模拟,并与制备的Al2O3/TiN陶瓷材料进行对比.结果表明,模型可有效地模拟陶瓷材料烧结时晶粒的生长及气孔的湮灭情况,能较好地再现烧结致密化过程,模拟结果与制备的陶瓷材料微观形貌组织十分接近.     

纳米Cu粉末的SPS烧结与晶粒长大行为的研究

为了研究金属材料在放电等离子烧结(SPS)过程中晶粒的长大行为和激活能的变化情况,利用SEM、FESEM、TEM等技术分析测定了纳米Cu粉坯体在SPS过程中组织形貌和晶粒尺寸的变化情况.研究表明:特定的工艺和烧结制度下,应用SPS技术可以得到均匀、致密的组织;脉冲电流的作用使晶粒表面大大活化,晶粒长大激活能大大降低,材料在迅速烧结的同时,晶粒也迅速长大.     

Kinetics and sintering mechanisms of hydro-thermally obtained hydroxyapatite

In this paper, the kinetics and sintering mechanism of hydro-thermally synthesized hydroxyapatite in the temperature range 900–1100 °C are presented. It was found that at 900 °C, the sintering process is very slow, whereas at 1000, 1050, and 1100 °C is much faster, caused by activated sintering with liquid phase/phase of solution on spots with concentrated material impurity and the formation of reaction/polyeutectic mixtures dominates. The activation energies of sintering were also determined. At all the investigated sintering temperatures, an approximate value of 90 kJ mol⁻¹ was found.     

升温速率对氧化铝粉末烧结行为的影响

采用不同的升温速率,在膨胀计中对脱脂后的氧化铝粉末射成形坯件进行一系列的烧结试验.结果表明,烧结致密化过程主要发生在升温阶段,快速升温有利于致密化的进行和抑止晶粒长大,但由于烧结时间较短和烧结炉最高温度的限制,产品的最终致密化程度不高.在低温时快速升温,高温时缓慢加热,可以获得较好的致密化效果和微观结构.试验和分析结果将为建立非等温烧结模型和烧结工艺参数的优化方法提供依据.     

Diffusion kinetics of borided AISI 52100 and AISI 440C steels

In this study, the case properties and diffusion kinetics of AISI 440C and AISI 52100 steels borided in Ekabor-II powder were investigated by conducting a series of experiments at temperatures of 1123, 1173 and 1223 K for 2, 4 and 8 h.The boride layer was characterized by optical microscopy, X-ray diffraction technique and micro-Vickers hardness tester. X-ray diffraction analysis of boride layers on the surface of the steels revealed the existence of FeB, Fe₂B and CrB compounds.The thickness of boride layer increases by increasing boriding time and temperature for all steels. The hardness of the boride compounds formed on the surface of steels AISI 52100 and AISI 440C ranged from 1530 to 2170 HV_0.05 and 1620 to 1989 HV_0.05, respectively whereas Vickers hardness values of untreated steels AISI 440C and AISI 52100 were 400 HV_0.05 and 311 HV_0.05, respectively. The activation energies (Q) of borided steels were 340.426 kJ/mol for AISI 440C and 269.638 kJ/mol for AISI 52100. The growth kinetics of the boride layers forming on the AISI 440C and AISI 52100 steels and thickness of boride layers were also investigated.     

Precipitation kinetics of the hardening phase in two 6061 aluminium alloys

The purpose of this work is to study the kinetics of the precipitation of the hardening phase in two Al-Mg-Si alloys. A review of the theoretical framework for solid-state reaction kinetics and the determination of kinetic parameters from DSC curves are provided, then used to quantify β″ precipitation in the two alloys studied. Despite failure to achieve high levels of accuracy, due, in particular, to the dilution of the alloys, the procedure to analyse this precipitation reaction yields valuable results. It is established that β″ precipitates homogenously as needles which grow through an enhanced-diffusion mechanism.     

金红石型二氧化钛粒子成长及动力学

利用XRD技术研究了以TiCl₄为原料、用前驱体方法制备的金红石型TiO₂的粒子成长及其动力学．结果表明：随着热处理温度的不断升高,TiO₂纳米粒子的粒径也逐渐增大,在773K以下,粒子生长缓慢,在773K以上,粒子生长迅速．温度一定,TiO₂粒子随热处理时间增加而长大,且在前2h内增长较快,2h后粒径趋于稳定,粒子的成长主要受温度控制,热处理时间对粒子成长影响较小;粒子成长的表观活化能以773K为界分为两部分,温度高于773K时,粒子生长表观活化能为54．62±373kJ／mol;温度低于773K时,粒子生长表观活化能为6．43±3.39kJ／mol．     

Effect of microwave sintering on the microstructure and electrical properties of low-voltage ZnO varistors

Coarse-grained ZnO varistors for low-voltage applications were prepared by microwave sintering technique under different soaking times of 5–150?min. For comparison, a low-voltage ZnO varistor was also prepared through a conventional sintering process. Microwave sintering remarkably enhanced the grain growth rate of ZnO varistors. Average grain size of the sample prepared by microwave sintering in 15?min was about 20?µm, which is similar to the grain size of sample prepared conventionally in 150?min time. In addition to grain growth, an increase in microwave sintering time led to precipitation of zinc titanate (Zn₂TiO₄) on the top surface of samples which sintered for long dwell times. X-ray diffraction and scanning electron microscopy results from different points of the samples declared that precipitation of Zn₂TiO₄ phase is due to the high rate of bismuth evaporation of Bi-rich liquid from top surface and the reaction between remaining titanium ions on the surface with ZnO. The results showed that increasing sintering time from 5 to 150?min increased the grain size from 14 to 33?µm, consequently, the breakdown field decreased from 90 to 27?V/mm, respectively. These changes led to a switch in the varistor application, from low to very low voltage.     

纳米Y-TZP材料烧结过程晶粒生长的分析

分析了无压烧结、热压烧结及SPS烧结过程中晶粒生长的行为及表现活化能．结果表明：在1100～1300℃之间,纳米Y-TZP材料在以上几种烧结条件下的晶粒生长行为不同．无压烧结时晶粒生长较慢,而热压烧结和SPS烧结时晶粒生长较快．对晶粒生长的活化能分析可在一定程度上解释以上现象．分析结果显示：无压烧结的表观活化能为281kJ／mol与纳米Y－TZP材料的晶界扩散活化能相近;热压烧结过程中,由于外压对扩散的促进作用,活化能比无压烧结时略有降低;在SPS烧结过程中,由于外加的脉冲电流能使晶粒表面大大活化,所以活化能与无压烧结相比大幅度下降．     

ZrO2-Y2O3(CaO)固体电解质的致密化烧结及电性能研究

研究了Ｙ２Ｏ３、ＣａＯ稳定剂及Ａｌ２Ｏ３、ＳｉＯ２（ＡＳ）添加剂对ＺｒＯ２固体电解质的烧结性能和电特性的影响。结果表明,加入ＡＳ的样品微观结构致密,其相对密度达到９７％。ＡＳ添加剂导致ＺｒＯ２固体电解质的粒界偏析和电导率下降,但是在ＡＳ含量很少时,对ＺｒＯ２的电特性影响不大。     

晶粒生长的控制及对铅钙铁铌陶瓷体系微波介质性能影响的研究

研究了晶粒生长的固有机制,并针对铅钙铁铌陶瓷体系的掺杂改性设计一组实验,实验结果表明工艺因素特别是烧结温度及保温时间对晶粒形貌有显著影响,随烧结温度及保温时间的增加晶粒尺寸不断增加,但超过一定值后,不断减小的气孔率又会增加。本文提出一种晶粒生长模型,将烧结分成烧结前期、烧结中后期以及临界点后烧结期3个阶段,并证明每种材料由于组成不同和基本晶粒的不同会有最佳的烧结临界点,把握该临界点是控制材料性能的关键。     

用差示扫描量热法研究Mg65Cu25Gd10块体非晶合金的晶化动力学

通过铜模吸铸法得到Mg65Cu25Gd10块体非晶舍金，用差示扫描量热仪（DSC）研究其晶化动力学和玻璃转变行为，玻璃转变温度Tg，晶化起始温度Tx，晶化峰值温度Tp都与加热速率有关，通过Kissinger方程可以得到表面激活能，发现晶化初始激活能Ex小于峰值激活能Ep，表明形核过程比生长过程容易，讨论了此非晶舍金的玻璃形成能力，根据JMA方程非等温模型研究了晶化动力学，Avrami参数表明在不同温度下的晶化机制是不同的．     

蒙特卡罗方法模拟陶瓷晶粒生长研究进展

概述了蒙特卡罗模拟方法的基本原理和思想,介绍了陶瓷烧结过程中,蒙特卡罗分别模拟单相系统和二相系统中晶粒生长的模型,综述了国内外蒙特卡罗方法模拟陶瓷烧结过程中晶粒生长的研究进展,指出了当前该研究领域中存在的问题,提出了今后发展的主要方向:将模型与具体材料和实际工艺联系起来,模拟定量化,并向三维模拟发展,以解决实际问题.     

Experimental and Numerical Studies of Densification and Grain Growth of 8YSZ during Flash Sintering

As a promising sintering technique, flash sintering utilizes high electric fields to achieve rapid densification at low furnace temperatures. Various factors can influence the densification rate during flash sintering, such as ultrahigh heating rates, extra-high sample temperatures, and electric field. However, the determining factor of the densification rate and the key mechanism during densification are still under debate. Herein, the densification and grain growth kinetic during flash sintering of 8 mol% Y₂O₃-stabilized ZrO₂ (8YSZ) is studied experimentally and numerically using finite element method (FEM). The roles of Joule heating and heating rate on the densification are investigated by comparing flash sintering with conventional sintering. An apparently smaller activation energy for the material transport resulting in densification is obtained by flash sintering ($Q_{\mathrm{d}}$ =424 kJ mol⁻¹) compared to the conventional sintering ($Q_{\mathrm{d}}$ = 691 kJ mol⁻¹). In addition, a constitutive model is implemented to study both the densification and the grain growth during flash and conventional sintering. Furthermore, the effect of electrical polarity on the density and the grain size evolution during flash sintering of 8YSZ is also investigated. The simulation results of average density and grain size inhomogeneity agree well with the experimental data.     

TiO2掺杂的NiO-NiFe2O4电导率及合成动力学研究

采用固相合成法,在1173～1213K温度范围内合成了NiO-NiFe2O4和0.5%TiO2掺杂的NiO-NiFe2O4。用X射线衍射表征了合成产物。在合成过程中将试样置于两个铂片之间,通过连续测量加热过程中试样的电阻,获得了试样电导率随时间的变化规律,建立了稳态扩散控制的NiFe2O4合成动力学方程,并讨论了掺杂0.5%TiO2对NiFe2O4合成反应速率的影响,获得了0.5%TiO2掺杂的NiFe2O4合成反应的速率系数、活化能、Ni 2＋和Fe3＋离子的平均扩散系数。     

氧化物添加剂对TiO2相变和晶粒生长的影响机制

添加氧化物可抑制或促进锐钛矿相变,并有效抑制TiO2晶粒生长,从而对TiO2的功能性质产生重要影响。本文归纳了氧化物影响TiO2相变和晶粒生长的机制。对于一种特定的氧化物添加剂,其对相变的影响效果取决于氧化物阴离子的离子半径、价态、配位情况、氧化物的添加工艺、添加量以及材料的处理温度和时间等多种因素的共同作用。氧化物添加剂抑制TiO2晶粒生长的机制与其在显微结构中的分布状况密切相关。     

Impact of AC and DC Electric Fields on the Microstructure Evolution in Strontium Titanate

Herein, the impact of AC and DC electric fields on microstructure evolution in strontium titanate is investigated. The focus is on nonthermal effects by using current-blocking electrodes. The seeded polycrystal technique allows investigating the impact of a DC electric field on grain growth for different grain-boundary orientations and the impact of the surrounding atmosphere. As in previous studies, faster grain growth is observed at the negative electrode. This effect is stronger for the (100) orientation and in reducing atmosphere. In AC electric field at 1450 °C, a low-enough frequency results in faster grain growth at both electrodes. These findings agree well with previous studies, where an electromigration of oxygen vacancies is found to cause a local reduction at the negative electrode, resulting in less space charge, less cationic segregation, and a higher grain-boundary mobility. At 1500 °C, AC electric fields are found to cause a complete grain growth stagnation at very small grain sizes. This behavior is unexpected; the physical reasons are not clear. Herein, a brief study of sintering in DC electric field reveals slightly faster sintering if a field is applied.     

On the control of the liquid-phase distribution in multi-material assemblies processed by liquid-phase sintering

Proper control of composition gradients in components processed by liquid-phase sintering requires the control of the migration of the liquid phase during sintering. A model considering isotropic interface energies has been developed for expressing the variation of the solid–liquid interface area with liquid volume fraction during the final stage of liquid-phase sintering (i.e. when residual porosity has disappeared). The model allows the computation of the driving forces for shape accommodation as a function of the dihedral angle ψ, liquid volume fraction, u, particle volume, V_p, and average particle coordination, n_c. It particularly enlightens the coupling between the effects of ψ and of n_c. The model allows a new insight into the control of microstructural evolution during liquid-phase sintering of assemblies of dual-phase materials with different average particle size. Taking ψ = 0° for the WC–Co system, the model was found to predict with a good accuracy the distribution of Co at equilibrium in assemblies made by heat-treating at 1400 °C under protective atmosphere stacks of cylinders of WC–Co cermets differing in mean WC particle size and cobalt content.