无压烧结制备Al2O3/SiC纳米复合陶瓷

用沉淀法包裹微米级SiC颗粒，通过常压、埋烧制备Al2O3／SiC纳米复合陶瓷。通过XRD、TG和SEM等分析了煅烧和烧结过程中相组成的变化、烧成收缩和显微结构。结果表明：利用SiC粉埋烧及碳粉制造还原气氛，含8wt％SiC(平均粒径为5mm)的复合粉末经800℃煅烧、成型，试样于1550℃，2h烧结，可制备Al2O3／SiC纳米复合陶瓷，其相对体积密度达95．2％，在烧结过程中由SiC氧化形成的无定形SiO2及与基质氧化铝反应形成的莫来石前躯体可大大促进烧结。     

由回收材料成型的MgO—Al2O3-SiO2耐火陶瓷

采用化学方式对回收的沉淀物进行制备，评价MgO-Al2O3-SiO2耐火陶瓷，即尖晶石、莫来石和堇青石及其特性。这些沉淀物包括纯的和细的氧化镁、氧化铝以及用水处理过的硅烟。由这些沉淀物共同沉淀出了相应批量的氧化物陶瓷，然后采用适当的技术进行加工烧成。对于加工后的耐火陶瓷体的化学与相组成、形态、显微结构以及物理性能进行了研究。得出的结论是：将沉淀的批量料烧成到1700℃，可以得到致密、直接结合而且高度耐火的尖晶石和莫来石-刚玉料体。另一方面，在煅烧到1350℃之后，由其批量料可以加工出致密的、有气孔的耐火堇青石-尖晶石料体。所有这些料体均是具有非常大范围的热化学应用、物理应用和机械应用的耐火氧化物陶瓷。     

莫来石的低温合成及结构组成变化

以九水硝酸铝和正硅酸乙酯为原料,以无水乙醇为溶剂,用溶胶-凝胶法制备出分子级均匀混合的莫来石前驱体.热重-差示扫描量热、X射线衍射和红外分析结果表明:前驱体在985℃直接由非晶态转化为富铝莫来石晶体,没有经历尖晶石相变过程,随着热处理温度的提高,富铝莫来石逐渐转化为结构为3Al2O3·2SiO2的稳定莫来石.前驱体粉末经1 000℃煅烧4h后,用放电等离子烧结技术在1 450℃烧结10min能制成透明莫来石陶瓷.     

Sol-Gel法制备莫来石粉末的研究

本文研究了以正硅酸乙酯（TEOS）和硝酸铝为原料，用溶胶-凝胶法制备莫来石粉末的过程。探讨了温度和pH值对Al2O3-SiO2凝胶时间的影响；用IR和XRD研究了凝胶在热处理过程中的晶相变化。研究结果表明：温度升高，凝胶时间缩短；pH=3时，溶胶的聚集速度减慢，胶凝时间较长；莫来石相是经铝硅尖晶石转化而成．铝硅尖晶石在1000℃开始转化为莫来石，至1250℃全部转化；制备的莫来石粉末晶粒尺寸为100-200nm。     

纳米晶莫来石粉体的高压致密化研究

利用高压烧结法研究纳米晶莫来石粉体。以100℃的升温步幅从1 100℃加热到1 500℃,使用高压凹陷砧型装置在4GPa压力下合成晶粒大小为51nm的莫来石粉体。根据致密化温度函数分析致密试样的物相组成和晶体结构参数。经XRD分析表明,新相(蓝晶石和刚玉)的生成改变了烧后试样的密度。采用高压法可以获得相对致密的烧结体。由于莫来石晶粒生长的各向异性,其微观结构呈针状。在1 400℃煅烧的细长莫来石晶粒的长度达到5μm左右,升高到1 500℃后晶粒变粗并保持原来的针状结构。莫来石粉体的维氏硬度随着烧成温度的升高而增大(直到1 400℃),然而在1 500℃由于晶粒粗化试样的维氏硬度呈下降趋势。     

Sm2O3掺杂对纳米莫来石前驱粉体微观结构和烧结性能的影响

为了改善纳米莫来石粉体烧结性能,以硫酸铝和硅酸钠为主要合成原料,添加不同含量Sm2O3,采用共沉淀工艺制备莫来石前驱粉体,经过煅烧得到莫来石纳米粉体,研究了Sm2O3掺杂量对莫来石粉体微观结构和烧结性能的影响.研究表明:当Sm2O3的加入量为4wt%时,合成温度可由传统的1300 ℃左右降低至1000 ℃,晶粒尺寸约为39 nm,比表面积达到95.265 m2/g.说明适当掺杂Sm3+对于合成纳米莫来石具有改善微观结构,促进烧结,促进莫来石晶相形成的作用.     

锆莫来石材料反应烧结过程的研究

本文利用XRD、SEM和EDAX等方法系统研究了含ZrO_2约33%的ZrO_2-Al_2O_3-SiO_2系耐火材料,从生坯煅烧至1600℃所发生的结晶相变化,显微结构变化及烧结过程中ZrSiO_4的分解温度.结果表胆:ZrSiO_4的分解比莫来石的形成要快,莫来石是由ZrSiO_4分解的非晶SiO_2与Al_2O_3反应形成的.1450～1550℃是锆莫来石耐火材料煅烧过程中物相变化最激烈的温度范围,该反应对致密化产生不利影响.要使结构致密,烧成温度必须大干1600℃.     

刚玉-莫来石-锌铝尖晶石复相材料的合成与烧结

热力学计算表明:在Al2O3SiO2ZnO三元体系中,硅酸锌(Zn2SiO4)和锌铝尖晶石(ZnAl2O4)生成Gibbs自由能在温度为1326.85℃以下是负值,莫来石(3Al2O3·2SiO2)的Gibbs生成自由能在温度为426.85℃以上是负值。系统中Al2O3富存的条件下,Al2O3可与Zn2SiO4反应生成锌铝尖晶石。以ZnO,SiO2,Al(OH)3为原料,通过固相反应合成刚玉莫来石锌铝尖晶石复相材料。研究烧结温度和气氛对刚玉莫来石锌铝尖晶石复相材料的合成与烧结性能的影响。用X射线衍射分析复相材料中的物相成分。用扫描电子显微镜观察复相材料的显微结构。结果发现:Al2O3SiO2ZnO三元体系在所选择的配料点,1300℃及1600℃时均能生成刚玉、莫来石、锌铝尖晶石3种物相,不会出现低熔点的硅酸盐相,这明显区别于Al2O3MgOSiO2三元体系,该体系在1500℃左右会出现低熔点相。试样在900℃烧成后出现锌铝尖晶石相。随着温度升高,试样的致密化过程加快,部分抵消了1300℃左右莫来石形成所产生的体积膨胀。当烧结温度高于1300℃时,试样的致密化过程大大加快。还原性气氛不利于刚玉莫来石锌铝尖晶石复相材料的合成与烧结。     

纳米莫来石的制备及其对氧化铝陶瓷性能的影响

以硝酸铝和硅溶胶为原料,采用溶胶-凝胶法合成了纳米莫来石粉体,进而探讨了纳米莫来石对氧化铝陶瓷烧结性能、抗弯强度以及抗热震性能的影响.结果发现,在1100 ℃煅烧硝酸铝与硅溶胶先驱体时仅有少量Al2O3生成,当将煅烧温度升高到1200 ℃时,获得了单一的莫来石晶相,粉体的平均粒径在50～60 nm之间;在氧化铝中添加2wt%～10wt%的纳米莫来石,可以有效促进陶瓷体的致密烧结,并获得良好的抗弯强度与抗热震性能;纳米莫来石含量为5wt%的陶瓷,在1650 ℃烧结后的抗弯强度为247.49 MPa,经过1200 ℃热震后的抗弯强度为218.52 MPa;当纳米莫来石的添加量超过10wt%时,将降低陶瓷的饱和体积密度,并恶化陶瓷的抗弯强度与抗热震性能.     

溶胶-凝胶法低温合成莫来石的研究

以去离子水和无水乙醇为分散介质,以六水合氯化铝为铝源、正硅酸乙酯为硅源,通过1,2-环氧丙烷调凝,采用溶胶-凝胶法合成了莫来石前驱体凝胶。考察了环氧丙烷用量以及水浴温度对凝胶时间及莫来石合成的影响。研究表明,水浴温度65℃、1,2-环氧丙烷与铝的摩尔比为3:1时可以较快的制备出均匀稳定的莫来石前驱体凝胶。利用TG-DTA、XRD、FT-IR对干凝胶热处理过程中的结构变化及相演变进行了分析,结果表明,前驱体凝胶为第一类莫来石凝胶,在热处理过程中没有形成γ-Al_2O_3、铝硅尖晶石等中间相,在990℃左右凝胶晶化放热直接转变成单相莫来石。干凝胶经1000℃煅烧1h后可形成莫来石,1200℃煅烧后莫来石晶相发育良好。在热处理之前凝胶中已经形成Si-O-Al结构,这是莫来石低温合成的最主要原因。     

Decomposition of Mullite

Free surfaces of 2:1 mullite (2Al₂O₃·SiO₂) specimens decomposed with the evolution of SiO and O₂ when they were heated at high temperatures under low partial pressures of O₂; this reaction was analyzed thermodynamically. In addition, bubbles were observed at internal interfaces between mullite and fused-SiO₂ diffusion couples. These bubbles, when formed at 1 atm ambient pressure between 1650° and 1800°C, resulted from reaction of Si particles and residual SiO₂-rich glass in the fused cast mullite.     

莫来石晶须前驱体原位增强海胆状莫来石材料的性能研究

为了提高粉煤灰的高附加值利用,分别以氢氧化铝和粉煤灰漂珠作为海胆状莫来石前驱体的铝源和硅源,氢氧化铝和粉煤灰为莫来石晶须前驱体增强剂的铝源和硅源,AlF₃和V₂O₅为晶须促进剂和烧结助剂,采用固相法原位制备了莫来石陶瓷材料。主要研究了增强剂与海胆状莫来石前驱体的配比(质量比分别为3:7、4:6、5:5、6:4、7:3)对制备的莫来石陶瓷材料性能、物相组成和显微结构的影响。结果表明,随着增强剂与海胆状莫来石前驱体的配比从3:7增大至7:3,试样中形成的晶相全为莫来石,材料内部更加致密,体积密度和抗压强度逐渐增大,总气孔率逐渐减小,增强剂的引入提高了材料的线变化率和抗热震性能,但降低了材料的重烧线变化率,材料的残余抗压强度及残余强度比先增加后减小。当增强剂与海胆状莫来石前驱体的质量比为6:4时综合性能较佳。     

Tape Casting and Properties of Mullite and Zirconia–Mullite Ceramics

Mullite and ZrO₂-mullite ceramics have been prepared by tape casting mixtures of Al₂O₃, quartz, and ZrO₂ powders and subsequent reaction sintering. Tape casting leads to homogeneous, high-density green materials with good sinterability. The design of a thermal cycle which favors densification with respect to mullitization allows the preparation of nearly dense, nearly fully reacted materials at sintering temperatures below 1600°C. ZrO₂ additions limit grain growth, but the ZrO₂ content must not be too high when a high tetragonal:monoclinic ratio is required.     

Structural Parameters of Mullite Formed During Heating of Diphasic Mullite Gels

Diphasic gels of four different compositions were synthesized. Lattice parameters, crystallite size, and strain values of mullite formed at different stages of annealing of diphasic gels have been calculated by quantitative standard less analysis using X-ray diffraction. Results show that all three parameters change on progressive heating. Changes in the microstrain of mullite show a reciprocal relationship with crystallite size. Silica-rich gel shows that the relative changes in the calculated values of three structural parameters of mullite formed during its formation process are because of an increase in its crystallinity rather than changes in composition. However, gels of composition in the solid solution range show that the relationship between lattice constants and chemical composition of mullite formed at high temperature agrees with Cameron data.     

Desiliconization of Mullite Felt

High-temperature evaporation from 80% porous, rigid mullite (3Al₂O₃·2SiO₂) whisker felt was studied under vacuum and at various helium pressures using gravimetry, X-ray diffractometry, SEM, and EDS. Heat treatments at 1350° to 1550°C resulted in evaporation of SiO₂ from mullite with a rate strongly dependent on temperature and pressure. A concentration gradient of SiO₂ was observed through the cross section of samples after heating under vacuum, indicating that SiO₂ preferentially evaporated from whiskers on the periphery of the samples. The SiO₂ concentration gradient was accompanied by sharp microstructural changes across the specimens. The gradient was decreased by raising the ambient helium pressure during heat treatment. A mathematical model was developed to predict the SiO₂ concentration profile. The model agrees well with experimental results and demonstrates that the diffusivity of SiO₂ in the vapor phase controls the gradient of SiO₂ through the cross section of mullite felt.     

Infrared-Transparent Mullite Ceramic

Mullite ceramic, transparent in the infrared, was prepared by hot-pressing and hot-isostatically pressing starting materials derived from alkyloxides. A composition with 72.3 wt% Al₂O₃ yielded transparent, submicrometer grain size bodies at 1630°C, whereas higher temperatures produced glass-containing microstructures. A composition with 76 wt% A1₂O₃ formed precipitates of α-Al₂O₃ at the consolidation temperature, which could be removed by subsequent annealing between 1800° and 1850°C. Spectral transmittance and absorption coefficients of the bodies are reported. The formation of the second phases was linked to phase equilibria and grain growth that promoted compositional equilibration of the mullite phase. The results suggest adjustments to phase boundaries in the high-temperature segment of the SiO₂-Al₂O₃ phase diagram.     

Synthesis of Mullite Whiskers

Mullite whiskers were synthesized by heating a mixture of SiO₂ and silicon in an alumina tube reactor under a flow of H₂ and CF₄. The length and diameter of the whiskers were several hundred micrometers and >15 μm, respectively. It was postulated that a vapor phase reaction between SiF₄ and AlF₃ made possible the synthesis of the large mullite whiskers.     

High-Temperature Hydroxylation of Mullite

A plane-parallel, polished, 0.9 mm thick, single-crystal (001) plate of 2:1 mullite was treated for 6 h at 1600°C in an Ar/H₂O (90/10) gas mixture at 100 kPa. Optical microscopy studies and infrared (IR) reflection spectroscopy studies of the lattice vibrations yielded no evidence for change with respect to the untreated reference crystal. However, IR absorption spectroscopy showed that structurally bound OH groups were formed by the heat treatment in the Ar/H₂O gas mixture. IR absorption depth profile analysis showed a rather homogeneous OH distribution through the crystal. Five different hydroxyl groups were separated according to dipole orientations and peak positions: E ‖ a , ω _{a 1}= 3447 cm⁻¹, ω _{a 2}= 3579 cm⁻¹; E ‖ b , ω _{b 1}= 3456 cm⁻¹, ω _{b 2}= 3544 cm⁻¹; and E ‖ c , ω _{c 1}= 3498 cm⁻¹. All IR peaks were strongly broadened (between 90 and 150 cm⁻¹) because of a distribution in O-H binding distances caused by the real structure of mullite.     

Elastic Properties of Mullite

Using ultrasonic methods, we determined the ambient-temperature elastic constants of dense (99.7%) hot-pressed polycrystalline 3:2 mullite (3Al₂O₃2SiO₂). We report the usual polycrystal elastic constants: Young's, shear, and bulk moduli, and the Poisson's ratio (nu). The Poisson's ratio (nu = 0.280) suggests interatomic bonding that is very different from that of either alumina or silica. Temperature dependences of the elastic constants show a strong irregularity; for example, nu decreases as the temperature increases. This irregularity suggests an internal-state change that is perhaps related to the incommensurate structural modulation.