陶陶墙地砖坯体强度分析和强度影响因素及坏体压制特征探讨

根据力学理论综合分析了墙地砖坯体强度的形成机理及影响坯体强度的因素,如层裂、含湿度、致密度等。并对坯体强度与致密度;致密度与压制力及坯料之间的关系以及层裂对强度的影响进行了简要的分析。最后对满足坯体强度压制特征作了初步探讨。     

影响特种陶瓷干压成形坯体致密度的因素

从干压工艺和粉体性能两个方面探讨了影响坯体致密度的因素。分析了999／5亚微米氧化铝造粒粉颗粒在加压过程中的坯体密度变化,全面分析了引起层裂的原因。重点讨论了粘结剂及水分含量对坯体致密度的影响。     

地砖坯体压制时裂断的力学分析

在生产地砖坯体过程中,由于压力的设定或粉料的级配、湿度变化往往产生坯体一压制出来就有裂断现象出现。应如何设定压机的压制力呢? 坯体是由瘠性材料压制而成,是一种脆性材料,本文运用材料力学中适用于脆性材料的第二强度理论来分析坯体出现断裂的力学机理,作一个粗略的探讨。 一、坯体的受力及基本假设 1.坯体的受力 坯体的受力情况如图1(a)所示。坯体受压制力P_Y及背压,摩擦力Pm,模框的反作用力Px和Pz。 2.基本假设 坯体一般较薄,Px、Pz可视为沿高度方     

粉体表面改性对α-Al2O3陶瓷干压成型性能及制品强度的影响

利用有机羧酸与氧化铝等粉体表面的羟基的酯化反应, 将引起团聚的强极性多羟基表面结构改变成为由碳氢长链覆盖的非极性有机表层结构.这种粉体改性消除了粉体间的硬团聚, 降低了压制成型过程中内摩擦力; 大幅度提高了陶瓷坯体和制品的均匀性及致密度; 显著提高了制品的强度.     

涂抹蜂蜜水增强卫生陶瓷坯体强度的研究

建立了一种提高卫生陶瓷坯体强度的实验方法。在卫生陶瓷坯体表面涂抹一定量蜂蜜水,干燥后采用三点弯曲法测试其抗弯强度。采用正交试验法优化了增强坯体强度的蜂蜜与水的配比,讨论了蜂蜜种类、实验温度及坯体干湿等对坯体强度的影响。结果表明:在卫生陶瓷坯体表面涂抹蜂蜜水可显著提高坯体的抗折强度,有效减少卫生陶瓷生坯破损及脚边、水箱口等位置开裂缺陷,当蜂蜜与水质量比为40:60时,对坯体增强效果最佳。     

陶瓷坯体压制裂纹的生成机理

根据采用钢模压制法成型的陶瓷坯体的裂纹形成特点,指出其本质是坯体在压制力作用下破坏的结果。利用材料力学的第二强度理论推导出裂纹生成条件的力学解析式:σ_y=σ_u/(1-μ)。即裂纹生成与压制力、坯体强度和坯体的泊松比有关。该表达式与实际情况吻合良好,对防止陶瓷压坯出现裂纹有重要指导意义。     

冷等静压成型压制工艺对坯体性能的影响

将Si粉(粒度≤0.044mm)和聚乙烯醇水溶液(PVA,其中聚乙烯醇质量份数为5%)按85:15的质量比混匀,过筛造粒,振动装料后进行冷等静压成型,研究了成型压力、升压速度和保压时间对硅粉坯体强度和密度的影响。研究表明:对于硅粉来说,随成型压力的增加,坯体的密度和强度随着增大,但压力增大到275MPa后,坯体密度和强度趋于恒定,密度约为1.61g/cm~3,强度约为2.0MPa。保压时间对截面尺寸小的坯体的密度和强度影响不大,密度最大值与最小值相差约0.03g/cm~3,变化率仅为1.96%;强度最大值与最小值相差约0.1MPa,变化率约为6.71%。升压速度对截面尺寸小的坯体的密度和强度影响不大。     

喷雾造粒ZrO2粉料坯体性能的研究

以喷雾造粒ＺｒＯ２粉料为研究对象,通过对粉料的压力－密度曲线及坯体的ＳＥＭ显微结构分析,发现喷雾造粒粉料的粒度分布对坯体密度的影响较小;颗粒强度的大小是获得均匀的坯体结构的关键,颗粒的强度与其大小及环境湿度有关,因此颗粒适当的增塑及去除大颗粒均有利于坯体显微结构的改善。     

粉体表面改性对α—AI2O3陶瓷干压成型性能及制品强度的影响

利用有机羟酸与氧化铝等粉体表面的羟基的酯化反应，将引起团聚的强极性多羟基表面结构改变成为由碳氢长链覆盖的非极性有机表层结构，这种粉体改性消除了粉体间的硬团聚，降低了压制成型过程中内磨擦力；大幅度提高了陶瓷坯体和制品的均匀性及致密度；显著提高了制品的强度。     

涂抹蜂蜜水增强卫生陶瓷坯体强度实验

工艺实验发现,在卫生陶瓷坯体表面涂抹一定浓度的蜂蜜水,可显著提高坯体的抗折强度,能减少卫生陶瓷生坯破损及脚边、水箱口等位置开裂缺陷。笔者以抗折强度试条为测试对象,研究了蜂蜜和水的比例对抗折强度的影响。结果表明,蜂蜜∶水=40∶60组合效果最佳,生坯抗折强度可提高50%以上;同时研究发现,蜂蜜类型、试条温度也对坯体的抗折强度有较大影响。     

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.     

固相烧结—Ⅰ气孔显微结构模型及其热力学稳定性,致密化方程

讨论了二维及三维闭口气孔的稳定性，发现二维状态时气孔的稳定性问题可以用数学方法根据气孔的颗粒配位数和二面角的大小解析；而三维状态时气孔则可借助球形气孔模型近似地确定。在这一模型的基础上，建立了烧结中期和后期的气孔显微结构模型，并由此推导了因相烧结中，后期作用于气孔的烧结应力和固相烧结中斯和后期的致密化方程。     

Powder Processing for the Fabrication of Si3N4 Ceramics: I, Influence of Spray-Dried Granule Strength on Pore Size Distribution in Green Compacts

The effect of spray-dried granule strength on the micro-structure of green compacts obtained by isostatic pressing was quantitatively analyzed. The fracture strength of single granules of Si₃N₄ powder made with ultrafine A1₂O₃ and Y₂O₃ powders was measured directly by diametral compression. It was found that fracture strength increased notably with the increasing relative density of the granule and the decreasing size of agglomerates in suspension before spray-drying. Even when green bodies were prepared at an isostatic pressure of 200 MPa, intergranular pores, which negatively affected densification of the sintered bodies, occurred between unfractured granules. The volume and size of these pores in the green compacts increased with the increasing fracture strength of the granules. In the case of closely packed granules, an isostatic pressure of 800 MPa was required to completely collapse the intergranular pores. A simple equation was derived to calculate the isostatic pressure necessary for complete collapse of intergranular pores in the green compacts, and it was determined that granule strength must be kept as low as possible to obtain uniform green compacts.     

Effect of Glass-Transition Temperature of Polyethylene Glycol-Plasticized Polyvinyl Alcohol on Granule Compaction

The glass-transition temperature ( T _g) is a fundamental property of all organic binders, It is illustrated with polyethylene glycol (PEG)-plasticized polyvinyl alcohol (PVA) binder systems that densification is enhanced when the binder T _g, is below the pressing temperature. This is a result of the increased deformability of the binder system and the decrease in the granule strength. Because of the reduction in the binder film strength with increasing PEG content, increasing the PEG content to lower the binder T _g, results in reductions in the compact green strength.     

Effect of surface-active substances on moldability and elastic after-effect of certain refractory mixtures

Conclusions The addition of surface-active additives to refractory mixtures makes for better densification of the green material and fired parts by reducing the elastic after-effect during pressing.     

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。     

Factors Affecting the Compaction and Densification Behavior of Ti(C,N)‐Based Cermet Powders

In this study, the effects of the major factors on compaction and densification behavior are investigated for Ti(C, N)‐based cermet powders. The relative density equation for green compact of composite powders is modified to predict the green density of Ti(C, N)‐based cermet powders prepared under different degrees of pressing pressures, and the theoretical values are found to be in good agreement with experimental results. It has been found that the composite powders with micron‐sized particles have a better compatibility than those with nano‐sized particles by analyzing the effects of the particle size and purity on the starting mono‐powders. It has also been found that the volume shrinkage and porosity of the former are lower than that of the latter. In addition, it shows that high oxygen content has a negative impact on both the compatibility of composite powders and the uniformity of pore size distribution of sintered cermets. It has also been discussed in this study how the pressing parameters such as pressing pressure, pressing temperature, and dwell time influence the resulting cermets. The results indicate that a better compatibity is reached at a pressing rate of 100 mm/min or a pressing temperature of 100°C.     

Effect of monomers content in enhancing solid-state densification of silicon carbide ceramics by aqueous gelcasting and pressureless sintering

The possibility of obtaining solid-state sintered silicon carbide (SiC) through aqueous gelcasting using commercial SiC powders was demonstrated. Green bodies were prepared from thixotropic SiC slurries in aqueous medium with optimized pH and solid-loading. The monomer system in gelcasting provides strength to the green bodies through formation of a gel network by polymerization and the carbon from polymeric gel enhances the densification of SiC, thereby avoiding addition of carbon externally to the gelcasting batches. Maximum bulk density of 3.16 g/cm³ (98.4% of relative density) was achieved in gelcast SiC on sintering at 2150 °C in argon atmosphere. The effect of carbon on SiC densification is evinced from the changes in microstructure of sintered SiC with increase in carbon content. The density and microstructure of gelcast and sintered SiC was comparable to that obtained from dry pressing and sintering of additive mixed SiC powders.     

The effect of liquid phase chemistry on the densification and strength of cold sintered ZnO

Cold sintering is a chemo-mechanical densification process which allows densification of ceramics at low temperatures below 300 °C. This substantial reduction in the sintering temperature is enabled by an externally applied pressure and a compatible transient liquid phase. In this paper, ZnO is cold sintered using various commercial organic acids: formic, acetic and citric acid. The effect of these different transient phases on densification, microstructural evolution and mechanical response is investigated. Fourier transform infrared spectroscopy, thermogravimetric analyses and transmission electron microscopy were conducted to explain the chemical interactions in the cold sintering process. High relative densities (～ 96 %) were achieved by formic and acetic acid, whereas poor densification was obtained for citric acid (< 80 %), despite the higher expected solubility of zinc oxide. The higher biaxial strength found in samples sintered with formic acid compared to acetic acid (i.e. ～90 MPa vs. ～40 MPa) is discussed supported by fractographic analyses.     

Sintering Behavior and Optical Properties of Yttria

Sintering in air of a pure yttria powder was investigated on green samples shaped by slip casting. The "relative density/grain size" trajectory has been drawn and hypotheses concerning the mechanisms controlling grain growth and densification were formulated. Samples were fully densified by an additional hot isostatic pressing step on pre-sintered samples. After optimal polishing, optical properties were measured in the UV, visible, and infrared ranges.