烧结法制备微晶玻璃过程中晶化和致密化两种典型的关系

以钢渣和粉煤灰为主要原料,采用烧结工艺,制得以透辉石为主晶相的微晶玻璃;通过热分析、X射线衍射、收缩率、扫描电镜等分析方法,阐述了烧结过程中晶化和致密化的关系;详细说明了由于在微晶玻璃热处理过程中二者发生的先后顺序不同,而使微晶玻璃结构和性质不同。     

高岭土尾矿微晶玻璃的烧结与晶化

本研究对高岭土尾矿为主要原料制备的CaO－Al2O3－SiO2系统玻璃进行了烧结收缩试验,借助X射线衍射分析了影响烧结致密化过程的因素;探讨了玻璃组成对烧结和晶化的影响,采用合适的玻璃组成,用烧结工艺获得了主晶相为β-CaO·SiO2的高岭土尾矿微晶玻璃。     

CaO-B2O3-SiO2系低温共烧陶瓷的致密化行为及性能

以CaO-B2O3SiO2(CBS)玻璃粉末为原料.采用流延成形工艺制备了CBS系生料带.生料带在排完胶后可以在775～950℃内烧结.研究了生料带在烧成过程中致密化和晶化行为以及烧成温度对其介电性能和烧结性能的影响.结果表明:烧成样品中的主晶相为β-CaSiO3,850～900℃烧成样品中有少量CaB2O4.在样品烧成过程中,排胶后的CBS玻璃粉末首先烧结致密化,然后才开始晶化,即致密化过程要早于晶化过程,CBS玻璃的析晶倾向于整体析晶, 这有利于CBS玻璃粉末的烧结.由于玻璃中析出晶相与残余玻璃相存在密度差,烧成样品的体积密度随着烧成温度的升高而降低.烧成温度的升高可促使玻璃中晶体析出和长大,且析出晶相具有比CBS玻璃低的相对介电常数(岛),样品的εr随烧成温度的升高呈下降趋势.     

晶界非晶晶化法制备SiC／晶化玻璃纳米复合材料

以纳米ＳｉＣ颗粒为主晶相，锂铝镁硅玻璃为晶界相，用晶界非晶晶化法制备了ＳｉＣ％３０％晶化玻璃纳米晶复合材料。玻璃相含量，热压温度和热压压力是影响致密化的主要因素，适当的热处理使晶界玻璃析出纳米晶。     

液相烧结Al2O3/3Y-TZP复相材料的致密化与力学性能

以CaO-MgO-SiO2玻璃为烧结助剂,用液相烧结法制备了氧化铝和3%氧化钇稳定四方氧化锆复相材料.研究了烧结助剂对材料致密化、显微结构及力学性能的影响.结果表明:由于CaO-MgO-SiO2玻璃具有较小的液相粘度,因而对材料的致密化有较大促进作用,可使材料在1 500℃获得致密.烧成温度和材料组成均对Al2O3和ZrO2的平均晶粒尺寸产生影响.显微结构中少量的Al2O3和ZrO2晶粒为晶内分布.引入烧结助剂降低了材料的烧结温度,使材料具有细晶结构,因而具有良好的力学性能,在最佳条件下,样品的抗弯强度可达到778 MPa.     

反应析晶烧结法制备可加工氟闪石玻璃陶瓷致密化研究

采用两种加热方式对氟云母晶体和普通钠钙玻璃混合粉末进行烧结,用扫描电镜观察烧结过程中玻璃陶瓷的组织演变,结合样品收缩率和密度的变化,研究了玻璃陶瓷的致密化行为.研究结果表明,致密化是通过玻璃的粘性流动实现的,氟云母加入对玻璃粘性流动有阻碍,降低了烧结驱动力.加热至850 ℃,氟云母与玻璃发生反应析晶形成氟闪石,对致密化影响很大.反应析晶发生前,玻璃陶瓷的密度随温度升高和等温时间延长而提高;反应析晶发生后,逐渐形成密集的相互交织的氟闪石晶体骨架对玻璃粘性流动产生严重阻碍,玻璃陶瓷的密度随温度升高略有下降.     

燃烧反应加压法制备细晶氧化铝陶瓷的致密化机理

采用燃烧反应加压法制备了致密的细晶氧化铝陶瓷。用透射电镜和场发射扫描电镜观察了致密样品,研究了快速升温条件下晶粒尺寸和致密度随烧结时间的变化规律。基于对氧化铝陶瓷致密化过程中晶粒生长动力学及结构形成过程的分析,探讨了燃烧反应加机械压力法制备细晶氧化铝的致密化机理。结果表明:烧结时间在2min以内的时,氧化铝晶粒尺寸没有明显变化,由表面扩散导致的晶粒颈部生长可以忽略。晶粒长大的抑制是由快速的升温速率以及短的烧结时间控制。样品中通过基面位错的滑移和攀移过程在(1120)面上生成具有Burgers矢量为1/3[1010]的层错,致密化过程为晶粒的高温塑性流动及晶界滑移共同作用结果。     

晶化温度对CaO-Al2O3-SiO2-Fe2O3系粉煤灰微晶玻璃析晶及性能的影响

以粉煤灰、石灰石和无水碳酸钠为原料,通过烧结法制备了以钙长石、钙铁辉石和霞石为晶相的复合晶相微晶玻璃.借助差热分析、X射线衍射及扫描电子显微镜研究了晶化温度(850～1100℃)对微晶玻璃析晶行为、显微形貌和性能的影响.结果表明:随晶化温度的升高钙长石和霞石晶相的含量先增加后降低,钙铁辉石晶相含量逐渐增加,同时微晶玻璃中晶体形态逐渐由球形微晶发育成柱状,最后长成片状;晶化温度的升高有利于微晶玻璃的烧结致密化,1050℃时微晶玻璃的线收缩率和体积密度达到最大,分别为17.04％和2.76 g/cm3,吸水率最小,为0.01％.过高的晶化温度(1100℃)会降低其致密程度.     

液相烧结Al_2O_3/3Y–TZP复相材料的致密化与力学性能

以CaO–MgO–SiO2玻璃为烧结助剂,用液相烧结法制备了氧化铝和3%氧化钇稳定四方氧化锆复相材料。研究了烧结助剂对材料致密化、显微结构及力学性能的影响。结果表明:由于CaO–MgO–SiO2玻璃具有较小的液相粘度,因而对材料的致密化有较大促进作用,可使材料在1500℃获得致密。烧成温度和材料组成均对Al2O3和ZrO2的平均晶粒尺寸产生影响。显微结构中少量的Al2O3和ZrO2晶粒为晶内分布。引入烧结助剂降低了材料的烧结温度,使材料具有细晶结构,因而具有良好的力学性能,在最佳条件下,样品的抗弯强度可达到778MPa。     

硼酸对熔融石英烧结及其晶化的影响

本文通过XRD、SEM等分析,从显微结构方面研究了硼酸对熔融石英烧结及其晶化的影响.结果表明硼酸在本实验范围内促进烧结,而且当硼酸含量小于0.5wt%时,随硼酸含量的增加,效果越明显;当硼酸含量为0.5wt%至1.0wt%时,变化不大;当硼酸含量大于1.0wt%时,烧结的驱动力大大提高,促进烧结的效果也更加明显.研究还发现,添加硼酸同时也促进了熔融石英的晶化,而且随硼酸含量的增加,晶化越严重,其结晶度成指数增长.当硼酸含量小于1.0wt%时,结晶相为低温方石英相;当硼酸含量高于1.0wt%时,结晶相以低温方石英相为主,同时还生成新的结晶相(BSi)O_2和Si_(30.72)B_(1.28)O_(64).     

Modelling of elimination of strength-limiting defects by pressure-assisted sintering at low stress levels

The structural reliability of sintered products depends on large defects introduced during powder processing, which cannot be removed by pressureless sintering. Here, we present a model how a large single ellipsoidal void is deformed, and finally disappears by pressure-assisted sintering. Taya-Seidel’s model is applied to predict the shrinkage of a large void in a compressible linear viscous material by using bulk viscosity, shear viscosity, and sintering stress that are determined experimentally for sintering of alumina powder at low stress levels. The application of mechanical stress promotes the densification rate. Its effect is maximum for hot isostatic pressing (HIP) and minimum for sinter forging. The effect is intermediate for hot pressing (HP) and spark plasma sintering (SPS), because the hydrostatic component of stress varies with densification. While a crack-like defect can be removed during densification, a spherical void must be eliminated by shear deformation in the final stage during dwell time.     

Hot‐Pressing Kinetics and Densification Mechanisms of Boron Carbide

The hot‐pressing kinetics of boron carbide at different stages in the hot‐pressing process was investigated. Based general densification equation and pore‐dragged creep model, the densification and grain growth kinetics were analyzed as a function of various parameters such as sintering temperature, sintering pressure and dwell time. Stress exponent of n ≈ 3 at the initial dwell stage suggests the plastic deformation may dominates the densification. The further TEM observations and the calculation based on effective stress and plastic yield stress also indicate that plastic deformation may occur and account for the large increase in density at the initial stage of sintering. Calculated grain size exponent of m ≈ 3 suggests that the grain‐boundary diffusion dominates the densification at the final stage. During the final stage of sintering, grain growth may be determined by evaporation/condensation and grain‐boundary migration.     

Influence of processing atmosphere on the microstructural evolution of submicron alumina powder during sintering

Investigations into the sintering of submicron oxide powders have revealed interesting behavior, particularly insofar as it concerns their microstructural evolution in the early, low temperature transformations during heating. In this work, experiments were conducted on a submicron alumina powder, whose microstructural evolution and densification were characterized after sintering from 900 °C to 1400 °C in air, dry air and high vacuum (10⁻⁸ atm). The results indicated that the processing atmosphere strongly influences the particle size distribution at low temperatures before shrinkage occurs. Shrinkage began concomitantly with grain growth and the sintering atmosphere influenced the sintering kinetics. This factor, which is associated with previous narrowing of the particle size distribution, may affect grain growth and densification during the final stage of sintering.     

Transient liquid phase sintering of high-purity mullite for high-temperature structural ceramics

High-purity mullite ceramics, promising engineering ceramics for high-temperature applications, were fabricated using transient liquid phase sintering to improve their high-temperature mechanical properties. Small amounts of ultrafine alumina or silica powders were uniformly mixed with the mullite precursor depending on the silica-alumina ratio of the resulting ceramics to allow for the formation of a transient liquid phase during sintering, thus, enhancing densification at the early stage of sintering and mullite formation by the reaction between additional alumina and the residual glassy phase (mullitization) at the final stage of sintering. The addition of alumina powder to the silica-rich mullite precursor resulted in a reaction between the glassy silica and alumina phases during sintering, thereby forming a mullite phase without inhibiting densification. The addition of fine silica powder to the mullite single-phase precursor led to densification with an abnormal grain growth of mullite, whereas some of the added silica remained as a glassy phase after sintering. The resulting mullite ceramics prepared using different powder compositions showed different sintering behaviors, depending on the amount of alumina added. Upon selecting an optimum process and the amount of alumina to be added, the pure mullite ceramics obtained via transient liquid phase sintering exhibited high density (approximately 99%) and excellent high-temperature flexural strength (approximately 320 MPa) at 1500 °C in air. These results clearly demonstrate that pure mullite ceramics fabricated via transient liquid phase sintering with compositions close to those of stoichiometric mullite could be a promising process for the fabrication of high-temperature structural ceramics used in an ambient atmosphere. The transient liquid phase sintering process proposed in this study could be a powerful processing tool that allows for the preparation of superior high-temperature structural ceramics used in the ambient processing atmosphere.     

Liquid Phase Sintering of Alumina, I. Microstructure Evolution and Densification

The microstructure evolution and densification of alumina containing 10 vol% calcium aluminosilicate glass and 0.5 wt% magnesium oxide sintered at 1600°C were quantified by measuring the evolution of pore-size distribution, the redistribution of liquid phase, and the fraction of closed and open pores. The densification stopped at a limiting relative density during the final stage of sintering, and the small and large pores were filled simultaneously by glass during sintering. In addition, the results indicate that the pressure build-up of the trapped gases in pores causes a significantly negative contribution to the driving force, and consequently the observed reduction in densification during the final stage of liquid phase sintering.     

A review of two-step sintering for ceramics

The development of high-density ceramic materials with fine-grained microstructures has been studied to considerably improve their properties for high-performance applications. Many alternatives have been searched to refine their microstructure by changing their composition and/or processing. Among such alternatives, the densification of ceramic materials by sintering curve control is an effective, simple and economical microstructure refinement method. Thus, different thermal treatment techniques such as spark plasma sintering and microstructural forms of control such as the control of sintering conditions have been used to obtain nanostructured materials. One of the techniques widely used in recent years is two-step sintering. Two-step sintering (TSS) is a promising method used to obtain high-density bodies and smaller grain sizes. Two TSS methodologies are known: sintering with thermal pretreatment at a low sintering temperature, followed by a second stage at elevated temperature, and the more recent approach presented by Chen and Wang, which has been the most widely used. In addition to the sintering conditions (temperature, heating rate and sintering holding times) that must be suitable for each composition type, the starting materials, particle size and processing method may influence the obtained microstructure, especially the reduced grain size and increased densification. The current review of two-step sintering presents the effect of this technique on the grain density and sizes of different ceramic materials. The influence of the addition of doping agents and its effect on the mechanical properties in different systems is also presented in the current study.     

Constrained-Film Sintering of a Borosilicate Glass: In Situ Measurement of Film Stresses

We have measured in-plane stresses developed in a borosilicate glass (BSG) film during its constrained sintering on a rigid substrate. Samples were prepared by casting BSG slurries on a silicon substrate and sintered inside a hot stage at 715°C just above the glass-softening temperature. Inplane stresses from the constrained-film sintering were determined by wafer-curvature measurements using an optical system. The measured stresses were tensile and rose rapidly from zero to a maximum level of 20 kPa during the initial stage of sintering and gradually decreased to zero at the final stage; these stresses were considerably smaller than those calculated from available microstructural models. We also measured the densification profiles of the free and constrained films. The stresses had no apparent effect on the densification profile of the constrained film up to 90% relative density; but beyond that, the densification kinetics were reduced in the constrained film. We believe that the stresses could have prevented a few large pores from shrinking during the initial stage of sintering, which then leads to an overall lower density and larger pores in the constrained film.     

Sintering and densification of nanocrystalline ceramic oxide powders: a review

Abstract

Observation of the unconventional properties and material behaviour expected in the nanometre grain size range necessitates the fabrication of fully dense bulk nanostructured ceramics. This is achieved by the application of ceramic nanoparticles and suitable densification conditions, both for the green and sintered compacts. Various sintering and densification strategies were adopted, including pressureless sintering, hot pressing, hot isostatic pressing, microwave sintering, sinter forging, and spark plasma sintering. The theoretical aspects and characteristics of these processing techniques, in conjunction with densification mechanisms in the nanocrystalline oxides, were discussed. Spherical nanoparticles with narrow size distribution are crucial to obtain homogeneous density and low pore-to-particle-size ratio in the green compacts, and to preserve the nanograin size at full densification. High applied pressure is beneficial via the densification mechanisms of nanoparticle rearrangement and sliding, plastic deformation, and pore shrinkage. Low temperature mass transport by surface diffusion during the spark plasma sintering of nanoparticles can lead to rapid densification kinetics with negligible grain growth.     

Densification behavior and mechanical properties of nano-alumina ceramics prepared by Spark Plasma Sintering with pressure applied at different sintering stages

High densification and fine grain size are the key to achieve excellent mechanical properties of ceramic materials. Pressure-assisted sintering is an effective approach to achieve this goal. However, the pressure at different sintering stages has different effects on the densification behavior of nano-ceramics. In this work, it is found that adjusting the pressure applying regime during Spark Plasma Sintering of nano-alumina ceramics can effectively increase the densification rate and balance the relationship between the densification behaviors of particle coarsening, grain growth and vapor migration. When the pressure is applied at the beginning of the second sintering stage, the high densification and fine grain size microstructures can be both obtained at lower temperatures, leading to the best mechanical properties. This result is of great significance for the preparation of nano-ceramics with excellent mechanical properties.