Influence of Titanium Dioxide on the Sintering and Microstructural Evolution of Mullite

Addition of TiO₂ under the solubility limit enhances the initial sintering and grain size of mullite; amounts in excess of that limit inhibit sintering and drastically increase the total porosity and mean pore size.     

Development of Low–Thermal Expansion Mullite Bodies

A series of ceramic compositions based on variations in the crystal–glass ratio of a mullite body were developed. The thermal expansion of these compositions varies from 3.7 to 5.0×10^–6/°C to 800°C. The materials are particularly useful for applications involving silicon, in that an identical thermal expansion is available. The high–temperature creep data for the lower–expansion compositions are inferior as a result of their higher glass contents. Raw material sources and fabrication procedures for specific compositions are given.     

Microstructural Study of Sintered Mullite Obtained from Premullite

The microstructural evolution of sintered mullite prepared from a premullite powder after thermal treatments at 1570°C was observwed by transmission electron microscopy and energy-dispersive X-ray microanalysis. The results show that there are grains with lower and higher contents of Al₂O₃ than that in stoichiometric mullite and small glassy-phase areas with the same composition as mullite; α-Al₂O₃ precipitates after samples are annealed.     

Microstructural Development of Silicon Carbide Containing Large Seed Grains

Fine (}0.1μm) β-SiC powders, with 3.3 wt% large (}0.44μm) α-SiC or β-SiC particles (seeds) added, were hot-pressed at 1750°C and then annealed at 1850°C to enhance grain growth. Microstructural development during annealing was investigated using image analysis. The introduction of larger seeds into β-SiC accelerated the grain growth of elongated large grains during annealing, in which no appreciable β→α phase transformation occurred. The growth of matrix grains in materials with β-SiC seeds was slower than that in materials with α-SiC seeds. The material with β-SiC seeds, which was annealed at 1850°C for 4 h, had a bimodal microstructure of small matrix grains and large elongated grains. In contrast, the material with α-SiC seeds, also annealed at 1850°C for 4 h, had a uniform microstructure consisting of elongated grains. The fracture toughnesses of the annealed materials with α-SiC and β-SiC seeds were 5.5 and 5.4 MPa·^1/2, respectively. Such results suggested that further optimization of microstructure should be possible with β-SiC seeds, because of the remnant driving force for grain growth caused by the bimodal microstructure.     

Influence of Kyanite Content on Microstructures and Properties of Castables Containing Spherical Lightweight Mullite Aggregates

Five lightweight mullite castables were prepared using the lightweight mullite aggregate(M45A) with narrow particle size distribution and spherical shape,mullite powder(M45P),kyanite powder,microsilica and Secar 71 cement as raw materials. After the specimens fired at 1100 ℃,1200 ℃ and 1300 ℃ for 3h,respectively,the effects of kyanite contents( 8%,10%,12%,14%,16% by mass) on the physical properties,phase composition and microstructures were investigated through an X-ray diffractometer( XRD),a scanning election microscopy( SEM) and a microscopy measurement method,etc. The results show that,for the specimens fired at 1300 ℃ with the kyanite content from 8%to16%, the bulk densities and apparent porosities change little,but the cold modulus of rupture and the crushing strengths decrease significantly,and the linear change rates increase from-0.87% to 0.64%. The most apposite mode is the specimen containing 12% kyanite fired at 1300 ℃, which has a high crushing strength of 29.3 MPa,a low linear change of 0.18%,an apparent porosity of 36.8% and a bulk density of 1.73 g/cm~3.     

Thermal Expansion Discontinuities of Mullite

Thermal expansion data of mullite were collected between 25° and 800°C with high accuracy by means of an X-ray high-temperature Guinier powder camera, and high-resolution single-crystal Bond techniques. 3/2-type mullites, synthesized from chemically coprecipitated precursors and 2/1-type fused-mullite single crystals, were used for the measurements. Mullite displays low and nonlinear thermal expansions along the crystallographic a, b , and c axes below about 200°C, and linear expansions between 200° and 400°C. Between about 400° and 500°C, expansion curves are discontinuous. Above 500°C, a, b , and c expansions are linear again. The temperature-induced expansion discontinuities are relatively strong parallel to a , but are much weaker parallel to b and c . The slopes of the a and volume expansion curves are higher above the expansion discontinuity than below, while there is no significant tendency for those of b and c . Thermal expansion coefficients and discontinuity effects are reversible without any hysteresis. The increase of the a and volume expansion curves above the expansion discontinuities may be due to possible deformations, rotations, and tiltings of the [AlO₆] octahedra. Another factor responsible for the higher a and volume expansion coefficients at high temperatures may be a geometrical deformation of the [Al^⋆O₄] tetrahedra. At present, no conclusive model exists to explain the discontinuous change of thermal expansion coefficients between 400° and 500°C.     

蓝晶石含量对球形45莫来石轻骨料浇注料显微结构及性能的影响

为改善含球形45莫来石轻骨料浇注料的体积稳定性和显微结构,引入了不同含量的蓝晶石(w=8%、10%、12%、14%、16%).采用X-射线衍射仪、扫描电子显微镜等对1300℃保温3 h煅烧后试样的物相组成、显微结构与性能进行了表征.研究发现:随着蓝晶石含量的增加,1300 ℃煅烧后试样的强度显著降低,线变化率由-0.87%逐渐增加至0.64%,骨料基质结合程度先变好后变差.综合考虑各项性能,含12%蓝晶石的试样综合性能最优:1.73 g/cm3的体积密度,36.8%的显气孔率,29.3 MPa的耐压强度,0.18%的线变化率,0.58 W/(m· K)的导热系数(800 ℃)和1339 ℃(T0.6)的荷重软化温度.     

粒度对蓝晶石制备莫来石晶体的影响研究

莫来石晶体主要应用于热障涂层,适量莫来石晶体可以降低涂层之间的失配应力,提高涂层的结合强度和抗热震性能.本论文主要研究粒度对蓝晶石高温下生成莫来石晶体的影响,将特定粒度蓝晶石和氧化铝混合样,在催化剂及一定温度制度下进行热处理,采用SEM和XRD对产物进行分析发现:-200～+325目蓝晶石分解温度较高,反应过程中没有形成莫来石晶体;-325～+400目之间的蓝晶石颗粒表面和内部的晶体晶形完整、均匀,反应体系表面晶体分布均匀,长径比为20～30;-400目蓝晶石体系经烧结后在体系表面有晶体分布,但是晶体分布不均匀,晶体长径比也不一致.     

莫来石/刚玉复相陶瓷热断裂特性研究

通过调整复相陶瓷中颗粒相的组成与级配,利用引入的热膨胀失配机制,控制莫来石/刚玉复相陶瓷显微结构,改善高温复相陶瓷热稳定性,着重研究了莫来石/刚玉复相陶瓷的热断裂特性.研究表明,全部采用莫来石颗粒相时,由于柱状莫来石颗粒极易发生热冲击穿晶断裂,且热膨胀失配导致的微裂纹较长,密度较小,故复相陶瓷热稳定性较差;全部采用刚玉颗粒相时,虽然基质相裂纹扩展至刚玉颗粒表面时穿晶断裂与沿晶的裂纹扩展同时发生,有利于改善复相陶瓷热稳定性,但由于热膨胀失配过度,未经热冲击时已产生宏观裂纹,故热稳定性最差.而采用莫来石/刚玉复合颗粒相,相比例为75/25制备的复相陶瓷试样,热膨胀失配形成的微裂纹较短,密度较大,其热稳定性最优.     

单翼迷宫式滴灌带抗热氧老化性能的影响研究

单翼迷宫式滴灌带产品新国家标准中增加了氧化诱导时间（OIT）,这对产品的抗热氧老化性能提出了要求。本文从滴灌带配方出发,讨论了产品原料与其OIT的料效关系,结果表明,防老化母料的加入比例对滴灌带的OIT影响最大,增加其比例可增大滴灌带的OIT;高密度聚乙烯（PE?HD）的加入比例对滴灌带的OIT影响大于线性低密度聚乙烯（PE?LLD）和低密度聚乙烯（PE?LD）的加入比例,增加PE?HD比例、减小PE?LLD和PE?LD比例可增大滴灌带的OIT;PE?LD的加入比例对滴灌带OIT的影响最小,在配方设计时应综合使用比例合理的PE?HD、PE?LLD、PE?LD以满足滴灌带良好的加工和其他性能要求;防老化母料中稳定剂比例不应随成本而减少,稳定剂比例增大时,滴灌带的OIT增大;通过对滴灌带的熔体质量流动速率（MFR）、炭黑含量及OIT指标测试,得出滴灌带MFR和炭黑含量与其OIT无明显的相互变化关系。     

氯化石蜡-70的热老化对膨胀型防火涂料性能的影响

研究了膨胀型防火涂料组分氯化石蜡-70的热老化对防火涂层防火性能的影响,并利用红外光谱分析了热老化后氯化石蜡-70对防火涂层的影响机理,研究结果表明:氯化石蜡-70的热老化是防火涂层热老化后防火性能衰减的重要影响因素,影响机理为氯化石蜡-70中Cl原子以HCl的形式提前放出,减少了发气量,更重要的是降低了阻燃能力,增加了烧蚀率,降低了碳化层的厚度。     

UV-791对PA6抗紫外光老化和热氧稳定性的影响

在己内酰胺水解开环聚合过程中加入一定量的光稳定剂(UV-791),合成改性聚己内酰胺(PA6)树脂.研究了UV-791对PA6光稳定性、热氧稳定性、末端氨基含量的影响.结果表明,UV-791的加入能明显提高PA6的耐紫外光及热氧稳定性,空气氛围下的起始分解温度提升26℃左右;在185℃空气氛围下老化30 min,改性PA6相对黏度下降百分比极低(UV-791含量0.2％时,相对黏度仅下降了1.3％),热氧稳定性明显得到改善.同时,加入量为0.05％时,端氨基含量提高了7.4 mmol/kg.而UV-791对PA6的熔融温度与结晶温度影响不大.     

Thermal Degradation of Fiber Coatings in Mullite-Fiber-Reinforced Mullite Composites

The thermal degradation behavior of single-layer BN and of double-layer BN/SiC chemically vapor-deposited fiber coatings in mullite-fiber-reinforced mullite composites was investigated by means of transmission electron microscopy after processing and heat treatment of the composites at 1000°, 1200°, and 1300°C for 6 h in air. The single-layer BN coatings were ˜0.7 mu m thick and consisted of turbostratic BN with (0001) basal planes lying parallel to the surfaces of the fibers plus nanosized areas that had no preferential orientation. This microstructure remained unchanged up to 1000°C; however, distinct coarsening of the randomly oriented BN crystallites occurred in the temperature range of 1000°-1200°C. The single-layer BN coatings were stable against oxidation, up to 1200°C. At higher temperatures, degradation of the coatings via oxidation occurred. Double-layer BN/SiC coating systems consisted of BN that was 0.08 mu m thick and SiC layers that were 0.16 mu m thick and deposited onto the mullite fibers. The turbostratic BN was highly anisotropic and did not undergo any microstructural change, up to 1300°C. The outer SiC layer of the double-layer coating system improved the oxidation resistance of BN in the 1200°-1300°C temperature range, despite a partial oxidation of SiC to SiO₂.     

莫来石物理性能研究进展

介绍了莫来石的晶体结构、力学性能、热学性能、光学性能和电学性能的研究现状,莫来石是一种重要的高温结构材料、绝热材料和功能材料,并对莫来石的研究方向进行了展望。     

UV-234对PC/ABS合金耐热氧老化性能的影响

采用熔融共混工艺方法,UV-234为耐热氧老化改性助剂,选用合适的聚碳酸酯(PC)和丙烯腈-丁二烯-苯乙烯共聚物(ABS)为合金基材,制备了耐老化PC/ABS合金。通过机械性能测试、色差测试及扫描电镜(SEM),对所得合金老化前后的性能、外观及微观结构进行了表征,并探讨了UV-234对合金耐热氧老化性能影响的机理。结果表明,在热老化初期,UV-234起到了类似酚类抗氧剂的作用,合金老化过程中自由基的生成得到了更好地抑制,从而有效阻止了合金的老化断链及极性发色基团的生成,改善了合金的耐热氧老化性能;而在热氧老化后期,由于UV-234的分子结构变化,逐渐失去了对合金老化过程中自由基的抑制作用,合金热氧老化速度加快,其力学性能急剧下降,黄变程度增大。     

溶胶-凝胶工艺对不锈钢涂覆莫来石涂层的影响

以九水合硝酸铝(Al(NO3)3·9H2O)和正硅酸乙酯(TEOS)为主要原料,氧化硼(B2O3)为助熔剂,采用溶胶-凝胶法和提拉浸渍法在不锈钢基片上制备莫来石涂层.分别研究了溶胶加热时间、溶胶浓度、热处理的升温速率、灼烧温度对涂层的影响.通过FT-IR和TG-DSC分析了莫来石前驱体的结构和热处理过程的成分变化,利用体视显微镜观察涂层形貌.实验结果表明:利用溶胶-凝胶法,当溶胶加热时间为7 h,溶胶浓度0.094×10-3 mol/mL和0.117×10-3 mol/mL之间,3 ℃/min升温速率下可得到完好、致密、无裂纹的莫来石前驱体陶瓷膜的涂层,热震性实验表明在950 ℃灼烧的涂层与基体结合强度较好.     

Formation of Mullite from Pyrophyllite by Mechanical and Thermal Treatments

When pyrophyllite is submitted to mechanical and thermal treatments, significant changes take place, according to XRD and DTA results. With grinding, the DTA endothermic peak is shifted to 540°C and becomes sharper and similar to kaolinite. A sharp exothermic peak is also produced at 1000°C, which is not present in the unground material. Increasing the mullite is associated with the formation of the latter peak. Thermal treatment causes the formation of mullite at higher temperatures.