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.     

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.     

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.     

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.     

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.     

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。     

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.     

片层状贝氏体铁素体长大动力学研究

为研究合金元素含量对钢中贝氏体铁素体长大动力学的影响,采用Zener-Hillert和Bosze-Trivedi动力学模型,通过选取热力学和动力学参数,计算了合金成分不同的钢的片层状贝氏体铁素体长大速度.研究表明:Fe-0.59C、Fe-0.81C和Fe-0.478C-4.87Ni合金在贝氏体相变时,贝氏体铁素体长大速度可以用无分配局部平衡条件下的扩散模型很好地描述,Fe-0.69C-1.8Ni-0.8Mo合金贝氏体铁素体长大速度略慢于理论值,Fe-C-8.7Ni合金贝氏体铁素体的长大速度比理论值约慢2个数量级;合金元素含量高的钢的贝氏体铁素体长大速度无法用扩散控制模型很好地描述;结合对贝氏体相变机制的讨论,提出贝氏体相变机制可能与相变温度和钢的成分相关.     

多孔阳极氧化铝模板的动力学研究及发展趋势

最近多孔阳极氧化铝膜以其有序的孔排列、可控孔径和优良的结构形态在纳米材料研究中引起了广泛的关注。尽管关于AAO膜在纳米材料制备方面的报道很多，但氧化参数如何影响膜的形态和几何结构还未被系统的论述。主要是要寻找氧化参数与膜形态间的关系从而提高AAO膜的可控性和重现性。同时总结了AAO膜的基本特点和形成机理，并对AAO模板在纳米技术领域中的现状进行分析。     

Phase Formation and Grain Growth Kinetics of High-Tc Superconducting Tl-1223 Ceramics

The phase formation and grain growth of (Tl,Pb,Bi)(Sr,Ba)₂Ca₂Cu₃O_9– ceramics were investigated in the temperature range between 860°C and 1000°C. It was found that partial melting seems to improve the texture of the samples. The phase purity, however, is limited due to an incomplete re-precipitation process due to the formation of a liquid phase, which is already present above 885°C. The resulting grain growth exponent n, however, appears to differ from the classical exponent of 1/3 for the coexistence of a liquid phase in the temperature range between 885°C and 920°C. For the first time, the activation energy for the high-T_c Tl-1223 phase has been determined showing faster grain growth kinetics than for Y-123 and Bi-2223.     

New insight to the oxidation kinetics of silicon-containing steel at high temperature

This study investigated the oxidation kinetics of silicon-containing steel at 1150–1300 °C using a Simultaneous Thermal Analyzer under atmospheric conditions similar to that of an industry reheating furnace. There is a critical time point for the oxidation kinetics at an oxygen concentration of 4·0 vol.-%., following which the oxidation rate constant increases with the increasing oxidation temperature. The model coefficient A in the kinetic oxidation equation was found to be constant. However, before the critical time point, the oxidation rate constant remained unchanged; the model coefficient A decreased with the increasing temperature. Therefore, the kinetic model of silicon-containing steel for isothermal oxidation was observed to be a modified one on the basis of the experimental data. In addition, the critical time point was prolonged with the increasing isothermal oxidation temperature. Moreover, the oxidation activation energy of the tested silicon-containing steel was 366·16 kJ mol^?1.     

On determination of heating conditions for the electrical sintering of a diamond-containing layer to a metal tool body through assessment of diffusion processes

The process variables (heating rate and duration) of pressure-assisted electrical sintering of a diamond-containing layer to a tool body are proposed to be determined based on the assessment of thickness of the diffusion layers that arise in the course of diffusion in the tool body-powder composition contact zones. A previously developed numerical model for electrical heating of an assembly unit (a macromodel) and a model for the diffusion process (a micromodel), which allows for the temperature dependence of the diffusion coefficient, are employed for that purpose. An example of choosing optimum heating conditions for the pressure-assisted sintering is presented. The results obtained can be used for the optimization of process variables for sintering powder-composition products.     

Electrical Properties and Grain Growth Kinetics of PZN-based Ceramics Using Microwave Sintering

Effectiveness of microwave sintering process through investigation of microstructural characteristics and electricalrproperties of x（0.94PbZn1/3Nb2/3O3 ＋ 0.06BaTiO3 ） ＋ （1 - x）PbZryTi1-xO3 （PBZNZT） ceramics with x = 0.6 and y = 0.52 was evaluated. The relative density of 95% was achieved with sintering at 800℃for 2 h. The small grain growth exponents indicate how easy the grain growth in these materials sintered using microwave radiation. Grain growth rate increases abruptly and is higher than that of conventional sintering at a temperature higher than 1050℃. This is attributed to the lower activation energy and higher grain boundary mobility. The activation energy required for the grain growth is found to be 132kJ/mol. Higher remanent polarization （Pr = 50. ltLC/cm2） and increase in remanent polarization with sintering temperature are observed in microwave sintering process when compared to that of conventional sintering process, due to fast increase in grain growth rate and homogeneity in the specimen. The results indicate lower sintering energy and reduction of PbO pollution in the working environment by microwave sintering process.     

Potential use of only Yb2O3 in producing dense Si3N4 ceramics with high thermal conductivity by gas pressure sintering

Abstract

Yb₂O₃ is an efficient sintering additive for enhancing not only thermal conductivity but also the high-temperature mechanical properties of Si₃N₄ ceramics. Here we report the fabrication of dense Si₃N₄ ceramics with high thermal conductivity by the gas pressure sintering of α-Si₃N₄ powder compacts, using only Yb₂O₃ as an additive, at 1900 °C under a nitrogen pressure of 1 MPa. The effects of Yb₂O₃ content, sample packing condition and sintering time on the densification, microstructure and thermal conductivity were investigated. Curves of the density plotted against the Yb₂O₃ content exhibited a characteristic ‘N’ shape with a local minimum at 3 mol% Yb₂O₃ and nearly complete densification below and above this concentration. The effects of the sample packing condition on the densification, microstructure and thermal conductivity strongly depended on the Yb₂O₃ content. The embedded condition led to more complete densification but also to a decrease in thermal conductivity from 119 to 94 W m^-1 K^?1 upon 1 mol% Yb₂O₃ addition. The sample packing condition had little effect on the density and thermal conductivity (102–106 W m^?1 K^?1) at 7 mol% Yb₂O₃. The thermal conductivity value was strongly related to the microstructure.