由尖晶石凝胶体形成的纳米级显微结构

在高铝浇注料中,由溶胶-凝胶体获得的尖晶石,其使用效果好于加入由水化尖晶石共沉淀方法制备的尖晶石.通过改进先质后制备的粉料证实了这种趋向.虽然用镁砂细粉合成的原位尖晶石-高铝浇注料的抗炉渣性能很好,但是与该浇注料相比,在耐火材料基质中采用凝胶法制备的纳米级尖晶石(MgAl2O4)改善了浇注料的性能.     

气氛和碳对MgO-C耐火材料基质中原位尖晶石形成的影响

碳、空气和还原气氛对MgO—C耐火材料基质中原位MgAl2O4尖晶石的形成在显微结构和相分析方面的影响,可由X射线衍射分析和扫描电子显微镜分析／能谱分析得到研究。尖晶石形成于1000℃以下的空气和还原气氛中。原位尖晶石的形态和它的形成机理的不同受气氛的影响。一般认为在氧化气氛中固态反应是尖晶石形成的主要机理,发现气-固反应在还原气氛中发挥关键作用。在还原气氛中MgO和C反应而生成Mg（气）,在还原气氛下烧成形成原位尖晶石。重点解释了其在MgO—C耐火材料中的应用。     

不同气氛下TiO_2对镁铝尖晶石的相演变和显微结构的影响

研究了在1 600℃和还原气氛下TiO2对MgAl2O4尖晶石的形成及其显微结构的影响。在含TiO2的MgAl2O4试样中检测到镁铝钛氧化物以次要相存在。在还原气氛下,含TiO2的试样比无TiO2试样的XRD峰移动的角度小。在含有矾土的试样中检测到MgAl2O4尖晶石、刚玉、镁钛尖晶石（Mg2TiO4）和非晶相。Mg2TiO4尖晶石与MgAl2O4尖晶石形成完整的固溶体,但是由于矾土系试样的非均匀显微结构,Mg2TiO4作为独立相而存在。而含TiO2的试样在空气中烧成后形成致密的显微结构。     

含有纳米氧化铝颗粒的铝尖晶石自流耐火浇注料的显微结构及物相演化

通过观察加入了纳米氧化铝颗粒的氧化铝尖晶石自流耐火浇注料在不同温度下的显微结构和物相组成,研究其物理和机械性能。结果表明,纳米氧化铝材料的加入对自流耐火浇注料的物理和机械性能有显著影响。随着浇注料中纳米氧化铝颗粒成分的增加,其在不同温度下的机械强度得到了极大地提高。这表明纳米氧化铝颗粒能影响烧制后的物相形成。在1 500℃下烧制普通样品时,发现板状氧化铝和尖晶石颗粒边界上形成了片状CA6晶体。而加入纳米氧化铝颗粒的样品,可在较低的温度(1 300℃)下形成CA6物相。     

MgO-MgAl2O4复合材料的制备及在耐火材料中的应用

采用烧结海水镁砂和α-Al2O4为原料,以固相反应法制备富镁尖晶石。原料Al2O,和MgO分别按比例进行混合配料。煅烧生成的镁铝尖晶石晶种量随MgO含量和温度的变化而变化,其变化范围为5％～50％。等量配比的试样,经1000℃煅烧2h,随后于1600℃烧结4h,可观察到煅烧粉末中尖晶石相占70％。初期煅烧温度和原生的尖晶石量对烧结试样的致密性和显微结构有着重要影响。探讨了尖晶石含量与结晶化效应和显微结构的关系。     

原料性能对镁碳耐火材料的显微结构和性能的影响

ＭｇＯ－Ｃ耐火材料的性能主要取决于ＭｇＯ，石墨和抗氧化剂如金属铝的性能，在本文中，论述了用不同粒度的ＭｇＯ，石墨和Ａｌ，并按重量配比制成不同的耐火材料，研究如强度，抗氧化性等与显微结构有关耐火材料的性能。     

微粉对凝胶注模氧化铝耐火材料性能和结构的影响

为了研究采用凝胶注模法制备的氧化铝耐火材料的烧后性能与氧化铝微粉加入量的关系,以粒径的0.178-0.200 mm氧化铝为骨料,添加17％、33％、50％、67％比例的粒径0.04-0.10 mm的氧化铝微粉混合成固体混合粉,加入单体聚丙烯酰胺(AM),交联剂N,N-亚甲基双丙烯酰胺(MBAM),催化剂N,N,N',N...     

尖晶石含量对氧化镁-尖晶石复合耐火材料性能的影响

在1400℃下通过常规的固体氧化物反应合成镁铝尖晶石,然后将重量百分数在0％-30％范围内的镁铝尖晶石加入到镁质耐火砖复合材料中。氧化镁一尖晶石耐火复合材料的最佳烧成温度由致密化研究和相分析决定。最终耐火制品用热膨胀、荷重软化温度、常温强度、经热震后常温强度的保持率、不同温度的高温强度和显微结构分析来表征。含有氧化镁成分的尖晶石含量为20％时,材料具有优越的性能。     

氮气气氛下Al对SiC-MgAl2O4复合材料物相和结构演变的影响

以SiC、MgAl₂O₄细粉为主要原料,分别添加质量分数为1%、3%、5%、7%、9%和12%的金属Al,置于流动氮气中,在1 500 ℃下保温5 h烧成得到SiC-MgAl₂O₄复合材料,对烧后试样进行XRD、SEM及EDS分析。结果表明,1 500 ℃高温烧结后,材料体系发生一系列复杂反应,试样中物相均以β-SiC、MgAl₂O₄、氮化物和sialon相为主。部分金属Al在高温下氮化形成氮化铝,并参与sialon相和MgAlON形成的反应。尖晶石则转变为富铝尖晶石和MgAlON两相共存。随着Al含量的增加,尖晶石中固溶铝含量达到极限时,析出α-Al₂O₃。Al含量的增加使得结合相Si-Al-O-N的形貌发生变化,由板带状向板柱状过渡,最终发育成形貌较为清晰的板层状。经SEM-EDS分析,sialon相中固溶一定量的Mg元素,为sialon多型体Mg-sialon相。     

烧结条件对β-SiAlON结合氧化铝耐火材料的影响

对相同组成的β-SiAlON结合Al2O3耐火材料在不同的温度和气氛下烧结进行了6种测试,从而确定了一种合理的相组成。用两种不同的电加热炉进行烧成,一种以石墨作为加热元件(炉G),另一种以MoSi2作为加热元件(炉M)。试样在氮气气氛中进行烧结,烧结温度1 400～1 650℃。电炉M中的氧分压通过使用炭床来控制。实验结果显示,在烧结温度为1 400℃时,不能合成β-SiAlON;在1 650℃,不同的气氛导致合成不同的相,除了主要成分Al2O3外,这些相有β-SiAlON、X-SiAlON、莫来石、玻璃相、SiC和C。     

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.     

Defect and microstructural evolution during drying of soapnut-based alumina foams

The present study demonstrates the use of soapnut, a naturally occurring surfactant for producing alumina ceramic foams. A range of slurry compositions with soapnut amounts ranging from 2 to 20 wt% in water, alumina loading of 35–55 vol% were studied. Though all slurry compositions foamed when subjected to mechanical agitation the formation of green ceramic foams free of macroscopic defects was found to be strongly dependent on conditions during drying of foamed slurries. Addition of guar gum to the slurries was shown to enhance foam stability and thus produce defect-free foams from compositions that otherwise either collapsed or resulted in other macroscopic defects during drying. Drying conditions also had a strong effect on microstructural parameters such as cell size and cell connectivity. Soapnut-based foams appear to have a greater connectivity between cells than foams produced by other comparable processes.     

Influence of magnesia on sintering stress of alumina

The sintering stress and the densification of MgO-doped Al₂O₃ were measured with a self-loading apparatus and a thermomechanical analyzer, respectively. The densification started at 950 °C and finished at about 1450 °C. The measured surface tensions were 0.7–0.8 N m⁻¹ in the intermediate sintering stage but drastically decreased to 0.2 N m⁻¹ in the final stage of sintering.     

Densification mechanisms and microstructural evolution during spark plasma sintering of boron carbide powders

Micron-sized boron carbide (B₄C) powders were subjected to spark plasma sintering (SPS) under temperature ranging from 1700 °C to 2100 °C for a soaking time of 5, 10 and 20 min and their densification kinetics was determined using a creep deformation model. The densification mechanism was interpreted on the basis of the stress exponent n and the apparent activation energy Q_d from Harrenius plots. Results showed that within the temperature range 1700–2000 °C, creep deformation which was controlled by grain-boundary sliding or by interface reaction contributed to the densification mechanism at low effective stress regime (n = 2,Q_d = 459.36 kJ/mol). While at temperature higher than 2000 °C or at high stress regime, the dominant mechanism appears to be the dislocation climb (n = 6.11).     

Grain growth during the sintering of compacted submicron powders

The kinetics of grain growth in compacts made of submicron titanium powders obtained by controlled hydrolysis of titanium alcoholates was studied. It was established that inhomogeneous packing of the particles in a compact made of agglomerated powder stimulates intense grain growth even in the early stage of sintering. It is shown that a material obtained by sintering homogeneously packed compacts composed of coarse non-agglomerated particles possesses, despite a low density, a homogeneous highly disperse structure and exhibits a much higher strength than a material obtained upon sintering a finely divided but agglomerated powder to maximum density. Translated from Ogneupory i Tekhnicheskaya Keramika, No. 4, pp. 18–22, April, 1997.     

Si粉和Al粉对N2保护热处理铝碳材料性能和显微结构的影响

为改善含碳材料埋炭保护热处理时的操作条件,并为提高含碳材料性能开辟新的途径,对添加Si粉或Al粉的铝碳材料在N2气氛中1200℃热处理5 h,测定试样热处理前后的质量变化率、抗折强度、体积密度、显气孔率、抗热震性、热膨胀系数和抗氧化指数,并用扫描电镜和XRD分析试样的显微结构和物相组成.结果发现,采用N2保护热处理工艺同埋炭热处理工艺一样可防止碳氧化,使铝碳材料形成碳结合并具有较好的性能指标.与添加Si粉的试样相比,添加Al粉的试样在热处理后具有较高的高温强度和较低的热膨胀系数;反应生成的Al2OC和AlN有益于材料抗热震性能和强度的提高,并减轻了其在埋炭保护热处理后由于生成Al4C3而产生的严重水化现象.部分Si粉与气氛反应生成纤维状的β-SiC、SixN和粒状的Si2N2O,它们对铝碳材料具有明显的增强作用.     

Influence of thickness on the constrained sintering of alumina films

Alumina films prepared by tape casting were sintered freely and under geometrical constraint at 1350 °C. The effect of film thickness on sintering kinetics and microstructure development was investigated. A decrease in film thickness in the constrained case leads to enhanced retardation of densification and increased orientation of anisometric pores.     

Sintering behavior,microstructural evolution,and mechanical properties of ultra-fine grained alumina synthesized via in-situ spark plasma sintering

Ultra-fine grained Al₂O₃ was fabricated by in-situ spark plasma sintering (SPS) process directly from amorphous powders. During in-situ sintering, phase transformation from amorphous to stable α-phase was completed by 1100 °C. High relative density over 99% of in-situ sintered Al₂O₃ was obtained in the sintering condition of 1400 °C under 65 MPa pressure without holding time. The grain size of in-situ sintered Al₂O₃ body was much finer (~400 nm) than that of Al₂O₃ sintered from the crystalline α-Al₂O₃ powders. For in-situ sintered Al₂O₃ from amorphous powders, we observed a characteristic microstructural feature of highly elongated grains in the ultra-fine grained matrix due to abnormal grain growth. Moreover, the properties of abnormally grown grains were controllable. Fracture toughness of in-situ sintered Al₂O₃ with the elongated grains was significantly enhanced due to the self-reinforcing effect via the crack deflection and bridging phenomena.     

Transparent YAG obtained by spark plasma sintering of co-precipitated powder. Influence of dispersion route and sintering parameters on optical and microstructural characteristics

A fine-grained (330 nm) yttrium aluminium garnet (YAG) ceramic, presenting a non-negligible transparency (66% RIT at 600 nm), was obtained by spark plasma sintering. The YAG powder was manufactured by co-precipitation, starting from a yttrium and aluminium chlorides solution. A soft precursor was obtained, whose phase evolution was studied by X-ray diffraction. Calcined powders were dispersed by either ball milling or by ultrasonication and then subjected to spark plasma sintering at several temperatures (1200–1400 °C) and for a reduced time (15 min). It is shown that the dispersion method plays a key role in enhancing the optical characteristics of YAG ceramics, in order to obtain a material with a small grain size, transparent in both the visible and the infrared range.     

A model for initial-stage sintering thermodynamics of an alumina powder

A model was proposed to calculate several thermodynamic parameters for the initial-stage sintering of an alumina powder obtained after calcinations at 900 °C for 2 h of a precursor. The precursor was synthesized by an alumina sulphate-excess urea reaction in boiling aqueous solution. The cylindrical compacts of the powder with a diameter of 14 mm were prepared under 32 MPa by uniaxial pressing using oleic acid (12% by mass) as binder. The compacts were fired at various temperatures between 900 and 1400 °C for 2 h. The diameter (D) of the compacts before and after firing was measured by a micrometer. The D value after firing was taken as a sintering equilibrium parameter. An arbitrary sintering equilibrium constant (K_a) was calculated for each firing temperature by assuming K_a = (D_i − D) / (D − D_f), where D_i is the largest value before sintering and D_f is the smallest value after firing at 1400 °C. Also, an arbitrary change in Gibbs energy (ΔG _a°) was calculated for each temperature using the K_a value. The graphs of ln K_avs. 1 / T and ΔG _a° vs. T were plotted, and the real change in enthalpy (ΔH°) and the real change in entropy (ΔS°) were calculated from the slopes of the obtained straight lines, respectively. Inversely, real ΔG° and K values were calculated using the real ΔH° and ΔS° values in the ΔG° = − RT ln K = ΔH° − TΔS° relation. The best fitting ΔH° and ΔS° values satisfying this relation were found to be 157,301 J mol^− 1 and 107.6 J K^− 1 mol^− 1, respectively.