Al2O3微粉和SiO2微粉对刚玉-莫来石材料性能的影响

在板状刚玉细粉中添加粘土或Al2O3微粉和不同种类的SiO2微粉制成刚玉-莫来石试样,分别于1200℃、1400℃和1600℃保温5 h烧成后,测定试样的体积密度、显气孔率和烧后线收缩率,利用XRD分析了试样在不同温度段的莫来石生成量,研究了添加Al2O3微粉和不同种类的SiO2微粉对材料的烧结性能和莫来石化的影响,并利用SEM观察了试样的显微结构.结果表明(1)加入Al2O3微粉和SiO2微粉均有利于材料的烧结;加入SiO2微粉的纯度和晶形不同,对试样莫来石化的影响也不同,无定形态SiO2微粉(即硅灰)的纯度越高,试样的莫来石生成量也越高.(2)加粘土的试样,其显微结构中柱状莫来石的晶体特征比较明显;而加入SiO2微粉的试样,其莫来石晶体和刚玉晶体相互交错,晶粒较小.     

烧成温度和保温时间对红柱石基材料烧结性能的影响

以高纯度南非红柱石(有4～1mm、<1mm和<0.088mm三种粒级,且4～1mm的颗粒料为1500℃3h煅烧过的熟料)和活性氧化铝细粉(粒度<0.044mm)为原料,经配料、混练、成型、干燥后,将一部分试样分别在1300℃、1400℃、1500℃和1600℃下保温3h煅烧;另一部分试样在1600℃下分别保温1h、3h、6h和9h煅烧,待试样随炉冷却至常温后,检测其体积密度、显气孔率、耐压强度和烧成线变化率等,并采用XRD、SEM等手段分析各试样的物相组成和显微结构,以研究烧成温度及保温时间对高纯度红柱石的莫来石化程度及对红柱石基材料烧结性能的影响。结果表明:提高烧成温度对促进高纯红柱石的莫来石化十分有效,延长保温时间对于促进制品的烧结作用很大;高纯红柱石基材料需在1600℃下保温3h烧成,才能使红柱石完全莫来石化;欲使材料烧结良好且玻璃相含量低,物相较纯,则保温时间要达到6h左右。     

南非红柱石细粉的烧结行为和莫来石化研究

采用南非红柱石细粉(粒度≤0.088mm)为原料,粉体在200MPa下成型为36mm×10mm试样,选择煅烧温度为1200～1600℃(温度间隔50℃),研究了南非红柱石细粉的烧结行为。通过对试样烧后线变化率、体积密度、显气孔率及XRD分析数据进行处理,分析了红柱石分解和莫来石化反应的行为特征。结果表明:1200℃左右已有莫来石生成;1500℃红柱石分解完全。随着温度的升高,红柱石和莫来石两者的消长主要出现在1300～1400℃,而反应过程有如下先后顺序:材料膨胀,红柱石分解,莫来石生成。温度低于1400℃,莫来石化反应膨胀和基质烧结同时存在,出现局部膨胀和局部烧结排除气孔共存的现象,导致显气孔率的变化趋势与烧后线变化率及体积密度的变化趋势不对应。高于1450℃,液相促进烧结起主导作用,材料大幅度收缩。1400℃煅烧后试样的线收缩率最小。     

Development of Anti—Creep Andalusite—Based Bricks for Blast Furnace Stove

Secondary mullitization and mineral phase compositions of reacting andalusite of different gain sizes with fine alumina powders are studies. By adopting reasonable process technology to control mullitization behavior during firing, good anti-creep andalusite bricks have been developed.     

Microstructure and thermal cycle resistance of plasma sprayed mullite coatings made from secondary mullitized natural andalusite powder

Natural andalusite powder was calcinated at a high temperature in air to realize secondary mullitization. The resultant secondary mullitized powder was spray-dried and heat-treated to improve sprayable capability. The heat-treated spherical powder was then plasma sprayed onto Ni-based high-temperature alloy (Hastelloy C-276) to form mullite coatings. The chemical composition and phase structure of the as-sprayed and thermally cycled mullite coatings were determined by means of energy dispersive X-ray fluorescence (ED-XRF) and X-ray diffraction. The microstructure of the as-sprayed coatings was analyzed by using a scanning electron microscope; and their porosity, microhardness and bonding strength were measured. Moreover, the phase transition temperature and enthalpy of the coatings were determined by means of differential scanning calorimetry; and their thermal shock resistance was evaluated as well. Results show that the spray-dried and heat-treated powder consists of mullite and a small amount of Al₂O₃; while the as-sprayed mullite coatings are composed of crystalline mullite as the major phase and a small amount of amorphous glass phase. During thermal cycle test, the amorphous glass phase is partially transformed to crystalline mullite, finally leading to failure of the coatings. Whether before or after thermal cycle, the mullite coatings experience phase transition around 980 °C, and the enthalpy of crystallization is determined to be − 141.9 × 10^− 3 J/kg and − 95.48 × 10^− 3 J/kg, respectively. The as-sprayed mullite coatings have a porosity of about 6.0 ± 0.2% and possess good thermal cycle resistance, showing promising prospect in a high-temperature application.     

Investigation on textural and structural evolution of the novel crack-free equimolar Al2O3-SiO2-TiO2 ternary aerogel during thermal treatment

Crack-free mesoporous equimolar Al₂O₃-SiO₂-TiO₂ ternary nanocomposite aerogel has been synthesized using an ethanol supercritical drying technique. The effects of heat treatment temperatures on its textural and structural evolution during thermal treatment are investigated in this study. XRD results reveal that only anatase phase is detected in the as-dried ternary aerogel, whereas peaks corresponding to silica and alumina phase are not shown due to its much faster polymerization rate of titania precursor. Structural transition from boehmite to γ-Al₂O₃ begins to occur at 450 °C within the ternary aerogel, and this process is completed at nearly 615 °C. The needle-like reticulated γ-Al₂O₃ grows along the anatase backbone, however, it is not evident in the XRD patterns. The morphologies of the ternary aerogel become more homogeneous after the structural transition, as indicated by the SEM analysis, which is also consistent with the BET results. With the increase of heat temperature up to 1050 °C, the γ-Al₂O₃ phase disappears and no separate SiO₂ is detected. At the same time, the silica-alumina network originates in a structure of Al-O-Si, and the silicon atoms incorporate into the alumina phase in the γ-Al₂O₃ structure, disordering the alumina primary particles. When the heat treatment temperature increases to 1200 °C, mullitization begins to occur along the titania backbone, whereas silica crystallization happens at 1300 °C. The 600 °C calcinated ternary aerogel is typically mesoporous, showing high specific surface area (255.37 m²/g), suitable average pore diameter (22.83 nm) and large pore volume (1.34 cm³/g). Moreover, the ternary aerogels show high surface acid activity at temperatures below 1000 °C, which have future applications for ideal catalysts and catalyst supports at elevated temperatures.     

Andalusite transformation and properties of andalusite-bearing refractories fired in different atmospheres

We found in our research that andalusite aggregate fired in a reducing atmosphere exhibits a lower mullitization rate than that fired in an air atmosphere. For investigating the effect of atmosphere on the transformation of andalusite and the properties of andalusite-containing refractories, andalusite powder (≤0.074?mm) and refractories containing andalusite aggregate (3–1?mm) were fired in air and carbon embedding, respectively. The phases and microstructure of the andalusite fired in both atmospheres were characterized by X-ray diffraction and scanning electron microscopy, respectively. The correlations of the properties of the andalusite-bearing refractories with the firing atmospheres were investigated in terms of volume stability, mechanical strength, and thermal shock resistance. The difference in the properties of the refractories was discussed with respect to the varied transformation rates of andalusite, and in terms of the different viscosities of the silica-rich glass caused by the different atmospheres.     

Mullitization process of andalusite concentrates – Role of natural inclusions

In the article the results of investigations into the mullitization process of two andalusite concentrates are presented. XRD, XRF and ICP investigations revealed nearly identical phase and chemical composition of both concentrates. However, the andalusites considerably differ in the rate of the mullitization process. XPS and LM investigations revealed that the factors responsible for mullitization include not only the content of mineral inclusions and grain size distribution but also impurities dispersion and the content of naturally occurring carbon in andalusite grains.     

In-situ synthesis and textural evolution of the novel carbonaceous SiC/mullite aerogel via polymer-derived ceramics route

A novel carbonaceous SiC/mullite composite aerogel is derived from catechol-formaldehyde/silica/alumina hybrid aerogel (CF/SiO₂/AlOOH) via polymer-derived ceramics route (PDCR). The effects of the reactants concentrations on the physicochemical properties of the carbonaceous SiO₂/Al₂O₃ aerogel and SiC/mullite aerogel are investigated. The mechanism of the textural and structural evolution for the novel carbonaceous SiC/mullite is further discussed based on the experimental results. Smaller reactants concentration is favorable to formation of mullite. Reactants concentration of 25% is selected as the optimal condition in considering of the mullite formation and bulk densities of the preceramic aerogels. Spherical large silica particles are also produced during heat treatment, and amorphous silica is remained after this reaction. With further heat treatment at 1400 °C, silicon carbide and mullite coexist in the aerogel matrix. The mullite addition decreases the temperature of SiC formation, when compared with the conventional methods. However, after heat treatment at 1450 °C, the amount of mullite begins to decrease due to the further reaction between carbon and mullite, forming more silicon carbide and alumina. The carbonaceous SiC/mullite can be transferred to SiC/mullite binary aerogel after carbon combustion under air atmosphere. The carbonaceous SiC/mullite has a composition of SiC (31%), mullite (19.1%), SiO₂ (14.4%), and carbon (35%). It also possesses a 6.531 nm average pore diameter, high surface area (69.61 m²/g), and BJH desorption pore volume (0.1744 cm³/g). The oxidation resistance of the carbonaceous SiC/mullite is improved for 85 °C when compared with the carbon based aerogel.     

高温烧结条件下硅线石的莫来石化

以黑龙江鸡西硅线石为基本原料,引入不同数量的煅烧工业氧化铝（αＡｌ２Ｏ３）,并分别于１６５０℃和１７５０℃下烧结,研究讨论所得试样的烧结性能及矿物组成。试验结果表明：随着工业氧化铝含量的增加,试样中玻璃相含量呈指数关系减少,莫来石生成量增加,少量未参加反应的氧化铝以刚玉相存在。在无矿化剂作用的情况下,试样在１７５０℃下的烧结性能比１６５０℃的好。