莫来石和莫来石-氧化锆复合材料的晶相转变和烧结性能

莫来石是高温领域中最有前途的工程材料之一,但是在室温下,具有较差的机械性能。因此,经常用一些颗粒、纤维或晶须来进行增强,提高它的性能。加入到莫来石的颗粒之一是ZrO2颗粒,通过ZrO2颗粒的晶相转变工艺,从四方晶系变为单斜晶系,从而可以提高莫来石的断裂韧性。本研究的目的是揭示利用Al-gerian高岭土、α-Al2O3和ZrO2通过烧成反应合成莫来石-ZrO2复合物,并研究了复合物的晶相转变和烧成性能。原料通过行星式球磨机进行混合,随后再进行研磨。压制的试样在1 100~1 600℃之间历时2h进行烧成。通过水浸方法测量其体积密度。用X-射线衍射(Rietveld方法)测出烧成试样中呈现的晶相。本文发现,当添加超过16%的氧化锆时,剩余的氧化锆晶相为四方晶相结构。所有的试样在1 250℃时都完全形成原生莫来石。在莫来石试样中,方英石是在1 150℃开始形成,在1 300℃消失,当有ZrO2加入时是在1 250℃消失。锆英石复合ZrSiO4是在1 250℃开始形成,在1 400℃完全消失。增加ZrO2比率可以促进球形颗粒的形成。     

氧化铝/莫来石复合陶瓷断裂行为研究

用X射线衍射方法测定了氧化铝/莫来石复合陶瓷的残余应力,并通过计算加以验证。在莫来石含量比较小的情况下,氧化铝/莫来石复合陶瓷基体拉应力与莫来石体积含量成线性关系。通过模型分析了氧化铝基体和莫来石颗粒的应力状态及其对裂纹扩展的影响。由莫来石颗粒引入的基体拉应力使裂纹倾向于向晶内扩展。观察了氧化铝/莫来石复合陶瓷断裂方式的转变,计算了穿晶断裂比率与基体应力的关系,进而建立了莫来石含量、基体应力、穿晶断裂比率三者的对应关系。这为复合陶瓷材料的制备和性能分析提供了可靠基础。     

莫来石晶种对反应烧结ZrO2/莫来石复相陶瓷显微结构的影响

通过显微结构观察研究了莫来石晶种对反应烧结ZrO2/莫来石复相陶瓷显微结构的影响。研究结果表明：与不含晶种试样相比，添加Ma(d50=1.87μm)和Mb(d50=0.83μm)晶种试样的显微结构比较均匀，莫来石晶粒多呈等轴状，且大小均一，晶内型ZrO2和封闭气孔较少。添加晶种对莫来石晶粒有明显细化作用。但晶种添加量和晶种颗粒尺寸对反应生成莫来石的晶粒尺寸无明显影响。采用溶胶－凝胶法制备的莫来石晶种（Mc)使试样中长出了一定量的长柱状的莫来石晶粒。     

添加MgO及烧结参数对莫来石形成的影响

对添加MgO及烧结参数对莫来石形成和致密化影响进行了研究。采用扫描电子显微镜,对粉末形态和烧结试样的显微结构特征进,亍描述。采用X射线衍射对烧结试样中相的特征进行观测。依据阿基米德原理采用密度和量化,测定烧结试样的密度。结果发现,MgO的添加不仅影响莫来石的形成,而且也促进了颗粒的生长。对于含有MgO0％、1％、2％的试佯仅有莫来石形成。而除莫来石之外,Al2O3存在于含MgO3％的试样之中。更高的MgO含量（4％、5％和6％）,形成三种相,即莫来石、Al2O3和尖晶石。与更高的烧结温度相对应,对于所有烧结时间和更高的密度,添加MgO1％所有试样的密度增加。在更高的MgO含量状态下,几乎在所有烧结温度下．更高的温度导致更低的密度并且更低的温度导致更高的密摩。     

氧化铝/莫来石复合陶瓷的耐磨性

利用SiC和Al2O3纳米粉末在空气中通过反应烧结法制备了氧化铝陶瓷和氧化铝/0.96～8.7 2vol.%莫来石复合陶瓷。通过磨粒磨损试验测定了样品的耐磨性,观察了样品的磨损表面,测量了磨损表面的剥落面积比率。磨损率可以用磨损表面的剥落面积比率定量表示。相对于氧化铝陶瓷,氧化铝/莫来石复合陶瓷的耐磨性有很大提高,主要是由于减小了剥落面积比率。     

加入片状晶种的氧化铝粉体的烧结研究

以熔盐法制备的片状α-AI2O3单晶颗粒作为晶种,CaO-AI2O3-SiO2体系为烧结助剂,采用无压烧结工艺制备了氧化铝陶瓷.通过对陶瓷密度的测定和显微结构的分析,研究了烧结温度和晶种加入量对坯体烧结致密化和显微结构的影响.研究结果表明,在低温区烧结体的密度随着晶种量的增加而增加,当烧结温度大于1500℃后,晶种的引入对烧结体的密度影响不大,在1575℃达到致密化,且陶瓷中出现了明显的柱状晶,柱状晶的长径比随着晶种量的增加而减小.加入10%晶种时柱状晶的数量和长径比都达到较大值.     

晶粒对以柠檬酸盐溶胶-凝胶法制备的莫来石粉末合成的影响

采用Al(NO3)3和四乙硅酸盐作为原始材料,并使用柠檬酸盐溶胶-凝胶方法制备3Al2O3·2SiO2粉末。加晶种的凝胶采用含有少量结晶的莫来石颗粒放入单相的Al2O3-SiO2凝胶中制做而成,在末加入晶种和加入晶种的单相凝胶中相的形成和热处理温度的关系通过差热分析和X-射线衍射来说明。人们发现加入晶种能够提高莫来石晶体的形成并且降低结晶形成的温度。加入晶种的凝胶中的3Al2O3·2SiO2的最初晶体形成温度约1100℃,甚至含1％晶种的凝胶能够使莫来石非常好的晶化,并且在1200℃下煅烧5h没有任何残余的相。然而,温度即使升高到1250℃,没有晶种的凝胶显示不能完成晶体形成。     

莫来石-氧化铝涂层增强氧化铝陶瓷基板制备及热导率/介电常数

莫来石-氧化铝涂层可以显著提高氧化铝陶瓷基板的表面硬度和耐磨性，从而更好地保护基板表面。此外，该涂层还可以改善氧化铝陶瓷基板的耐高温性能，提高其在高温环境下的应用场景。本次研究中，相关工作人员详细探究莫来石-氧化铝涂层增强氧化铝陶瓷基板的制备方式，并利用测试确定其热导率以及介电常数，通过这种方式，以期为增强氧化铝陶瓷基板的物理、机械以及化学性能提供技术支持。     

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.     

锆莫来石-碳化硅材料反应烧结过程的研究

研究了用锆英石、氧化铝和炭黑的混合物制备原位SiC颗料复合锆莫来石材料的反应烧结进程和显微结构特征。结果表明：反应烧过程中,SiC、莫来石的生成反应滞后于锆莫来石的分解反应;反应前期,SiC的生成占主导作用。反应后期,莫来石的形成及致密化进程占主导作用;材料中存在大量气孔,ZrO3以均匀分布和聚集体两种形式分布于莫来石、SiC及玻璃相构成的基质中。     

合成莫来石烧结工艺对其抗弯强度的影响

系统研究了合成莫来石烧结工艺对其抗弯强度的影响。结果表明经1300℃,3h+1500℃,4h处理的试样的常温抗弯强度最高;在800℃以前,其高温抗弯强度基本不变,在接近于1000℃时突然上升,而后又快速下降。借助于SEM照片,对实验条件下的断裂机理进行了深入分析,指出了断裂方式,并从微观上解释了抗弯强度的测定结果。     

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.     

莫来石超细粉末的制备

以硝酸铝和正硅酸乙酯为主要原料 ,利用Sol-Gel方法制备了粒径分布范围窄的高纯莫来石超细粉末。运用DSC、TG、IR、XRD和激光粒度分析等技术对Sol-Gel工艺条件 ,以及莫来石超细粉末进行了分析研究。研究表明 ,在干凝胶的热处理过程中 ,非晶态的干凝胶首先转化为硅铝尖晶石 ,再由硅铝尖晶石转化为莫来石相 ;在 1 30 0℃下热处理可获得纯莫来石相的超细粉末 ,其粒径分布在 0 .4～ 1 .0 μm之间 ,平均粒径为 0 .5 4μm。结果表明 ,采用Sol -Gel方法可以制备出高纯、粒径分布范围窄的超细莫来石粉末。     

Effects of alumina sols on the sintering of α-alumina ceramics

The effects of two kinds of alumina sols on the densification behavior of sub-micron grain sized α-alumina ceramics have been investigated. Composition of the sol-derived gels was investigated by energy dispersive spectra and Fourier transform infrared spectra. Structural evolution of the gels at different temperatures was characterized by a combination of X-ray diffraction and ²⁷Al magic angle spinning nuclear magnetic resonance spectroscopy. Results showed that the gel containing chlorine and carbon transformed to α-alumina at about 950 °C, significantly lower than the other gel which transformed at about 1050 °C. Density measurements and scanning electron microscopy analyses were used to investigate the sintering of alumina ceramics with or without alumina sols. It was found that the alumina sols had profound effects on the densification of alumina ceramics. The ceramic displayed the best densification behavior when the sol containing chlorine and carbon was added.     

烧结点火炉用莫来石质浇注料的研制与应用

针对烧结点火炉的使用工况要求,保持特级矾土骨料占70%(质量分数,下同)、结合剂纯铝酸钙水泥占5%、活性α-Al2O3微粉占4%不变,其余21%的细粉为不同组成的莫来石细粉、SiO2微粉、添加剂(分别为蓝晶石粉、碳化硅粉、硅线石粉),同时外加0.15%的三聚磷酸钠研制了莫来石质浇注料,并研究了SiO2微粉加入量(分别为0、1%、2%、3%、4%)和各添加剂加入量(分别为1%、2%、3%、4%、5%)对浇注料烘干或烧后(1 350℃3 h)性能的影响.结果发现,加入3%的SiO2微粉,同时加入3%的蓝晶石和4%的碳化硅时,研制的莫来石质浇注料性能较优良.该浇注料在几家钢厂的烧结点火炉上实际使用寿命目前已超过4年,证明使用效果较好.     

CeO2 和MgO助烧剂对矾土基莫来石合成料烧结的影响

以高铝矾土碎矿(w(Al2O3)为75%~84%)和煤矸石(w(Al2O3)约45%)为原料,按矾土基莫来石合成料的设计成分要求(w(Al2O3)为68%~72%)进行配比,于1500~1700℃分别保温3h煅烧合成了矾土基莫来石,并分别研究了助烧剂CeO2和MgO单独加入,CeO2 MgO复合加入,复合加入时CeO2与MgO的比例以及复合加入量对矾土基莫来石合成料烧结性能的影响。结果表明:加入CeO2 MgO复合助烧剂比单独加入CeO2或MgO的促烧结效果要好,当CeO2 MgO外加量为0.75%,CeO2与MgO质量比为1/1时,可使试样的烧结温度降至1600℃(不加助烧剂的为1700℃),得到显气孔率0.9%,体积密度为2.84g·cm-3,荷重软化开始温度为1570℃的合成莫来石;显微结构分析表明,此合成料荷重软化温度较高的原因是发育良好的柱状莫来石构成了交错连锁的网络结构,同时TiO2大部分固溶在莫来石晶体中。     

Low-temperature sintering of coarse alumina powder compact with sufficient mechanical strength

Coarse alumina powder compacts doped with various amounts of titania and copper oxide were pressurelessly sintered from 900 °C to 1600 °C. Their phase assemblages and microstructural evolution, as well as their properties, were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), differential scanning calorimetry/thermogravimetric (DSC/TG) analysis, and three-point bending and wetting test. The role of TiO₂ and CuO during the sintering is discussed in detail. The experimental results show that the liquid phase from the copper oxide appeared at approximately 1200 °C, so the solid-state reaction between alumina and titania took place at a lower temperature. Such solid state-reaction sintering had a strong impact on the grain growth and greatly promoted the densification of the alumina compact. In addition, the liquid phase inhibited the abnormal grain growth and microcracking. As a result, the coarse alumina powder compacts doped with 5 wt% TiO₂–CuO were fully densified and exhibited sufficient flexural strength (342±21 MPa) when sintered at a temperature of 1450 °C for 2 h.     

Spark plasma sintering to restrict sintering reactions and enhance properties of hydroxyapatite–mullite biocomposites

Herein, we demonstrate how spark plasma sintering (SPS) can be useful in restricting the sintering reactions and faster densification in Hydroxyapatite–Mullite system, which otherwise shows extensive sintering reactions during conventional pressureless sintering, as reported in a recent study [Nath et al. J. Am. Ceram. Soc. 93 (2010) 1639–1649]. The microstructure of SPSed Hydroxyapatite (HAp)-20 wt% mullite composites was characterized by submicron sized HAp and equiaxed mullite grains. Another important result has been the achievement of higher hardness of 7 GPa, which is much higher than pressureless sintered composites. The cell culture study including cellular viability using MTT analysis establishes good cytocompatibility of SPSed composites.     

Effect of fluorine-containing additive on the synthesis and sintering of compositions from natural raw materials in the “cordierite –mullite” system

The influence of the small addition of topaz on the processes of mineral formation in the “mullite–cordierite” system with a variable ratio of cordierite and mullite has been investigated. For this purpose, a series of experiments with different compositions of the initial mixtures, that is, with predominance of cordierite over mullite (KM mixtures) and predominance of mullite over cordierite (MK mixtures), has been performed. The addition of topaz in the form of topaz concentrate has been introduced in the amount of 1% by the weight of the investigated mixtures. The samples have been fired at various temperatures, that is, samples from KM mixtures (at a temperature of 1100–1300?°C) and samples from MK mixtures (at a temperature of 1400–1550?°C. The activating effect of topaz on mineral formation and sintering processes is determined by complex influence of fluoride products of topaz thermal decomposition. These reactively gaseous fluorides partially activate the solid-phase mass transfer processes and then reduce the viscosity of high-temperature melt formed during firing that intensifies the processes of ceramic matrix consolidation.Porous polycrystalline cordierite–mullitе–corundum ceramics from the KM mixtures, which contains 70%–87% cordierite, 3%–12% mullite, and 4%–11% corundum with water absorption of 3.5%–8% and bulk density of 2.10%–2.12?g/cm³ at the firing temperature of 1300?°C, has been developed. Mullite ceramics with notably corundum content (not more than 8%) from the MK mixtures with various density degrees at the firing temperature of 1500–1550?°C, porous ceramics with water absorption of 3%–11%, and dense ceramics with water absorption of less than 1% have been produced.     

Synthesis of beta-SiC powder from alkoxide precursor and study of its sintering behaviour

Abstract

Abstract

In this study, β-SiC powder was prepared using a pyrolysed spherical precursor derived from the hydrolysis mixture of phenyltrimethoxysilane and tetraethyl orthosilicate. Before the pyrolysed experiment, an alkoxide precursor was characterised using ²⁹Si solid nuclear magnetic resonance, Fourier transform infrared spectroscopy and thermogravimetric analysis. The alkoxide precursor was heated at 1800°C for 4 h under an Ar atmosphere. To examine the pyrolysed residue after heat treatment, the sample was collected and analysed with X-ray diffraction. The X-ray diffraction results for the sample show diffraction peaks at ～35, 60 and 73°, which correspond to the β-SiC phase. According to the results of chemical analysis, the SiC content of the powder that was prepared at 1800°C was determined to be 99·4%. The sintering behaviour of the prepared β-SiC powder was examined using B₄C and C as sintering additives in the temperature range of 1900–2200°C.