Some properties of highly refractory ceramics in the system MgO-MgAl2O4

Conclusions A region for compositions in the system MgO-MgAl₂O₄was detected which has great interest for practical use.Compositions containing up to 15% Al₂O₃ permit a big improvement to be obtained in the spalling resistance of periclase ceramics with a simultaneous reduction in the sintering temperature.Spinel and compositions close to it are distinguished from periclase by a lower volatilization capacity. This makes it possible to use the corresponding ceramic materials at temperatures that are much higher than with the ceramics containing a predominant content of periclase, despite the fact that in terms of deformation the spinel is much inferior to the periclase. The limiting service temperature of periclase ceramics (especially in vacuum) is limited not by the thermomechanical and refractory properties but by the high volatilization of the magnesia.It should be mentioned that there is a reduction in the sintering temperature and that bodies close in composition to spinel have high strengths.Bodies of intermediate composition containing from 20 to 50% Al₂O₃ apparently are of no practical interest as highly refractory ceramics.     

钢包渣线用超低碳MgO-MA-C砖的研制与应用

以高纯电熔镁砂、镁铝尖晶石微粉和纳米炭素为主要原料,分别加入总量为4%(质量分数)的硅粉、铝粉和硅粉-铝粉复合粉为防氧化剂,加入清洁的自来水混练均匀后,在500 t 摩擦压砖机上成型为碳质量分数小于1%的超低碳MgO-MA-C 标型砖试样.在对比各试样(包括普通MgO-C砖试样)的抗氧化性能、高温抗折强度、体积稳定性和抗渣性的基础上,对配方和生产工艺进行了优化,研制出了性能较好的超低碳(碳质量分数小于1%)MgO-MA-C砖.所研制砖在武钢200 t转炉钢包渣线区使用,平均使用次数达75次.     

Electric conductivity of sintered high-temperature ceramics of the YCrO3-MgCr2O4 system

Conclusions The alloying of yttrium chromite with magnesium chromite results in the formation of limited solid solutions. The second phase (MgCr₂O₄) becomes identifiable starting with 10 mole % of the additive.The addition of the alloying component results in an increase in the electric conductivity and an improvement in the strength properties and sinterability of the material of the YCrO₃-MgCr₂O₄ system.The materials investigated here should be fired in a medium with a reduced partial oxygen pressure.The electric conductivity of materials of the YCrO₃-MgCr₂O₄ system is higher in oxidizing conditions than in an inert medium so that the fired material should be subjected to a follow-up heat treatment in an oxidizing medium.Translated from Ogneupory, No. 6, pp. 64–67, June, 1978.     

氧化镁基MgAl_2O_4复相陶瓷的制备与性能

研究了Mg O-Mg Al2O4复相陶瓷的组分变化及添加剂对烧结性能、力学性能以及热性能的影响。结果表明:当Al2O3含量为20%时,Mg O-Mg Al2O4复相陶瓷与固体氧化物燃料电池阳极支撑材料Ni O/YSZ的热膨胀系数一致;复合添加剂Ca O/Si O2可提高材料的烧结性能、力学性能以及热性能,但其添加量对各性能的影响有所不同:当Ca O/Si O2含量为2%时,气孔率最低,抗弯强度最高;当Ca O/Si O2含量为8%时,抗热震指数达到最高值;添加剂的含量对材料的热膨胀系数没有明显影响。选取Al2O3含量为20%、Ca O/Si O2添加量为6%的复相陶瓷与Ni O/YSZ粘接成实验组合构件,该组合件经600℃热震温差20次空冷循环,结合状态和强度无明显变化。     

MgO-MgAl2O4-ZrO2系材料的抗渣性研究

以菱镁矿粉(d<,50>=6.62 μm)、m-ZrO<,2>微粉(d<,50>=3.99μm)、α-Al<,2>O<,3>微粉(d<,50>=1.89μm)为原料,采用半干法成型,于1 600 ℃保温3 h烧成后制成MgO-MgAl<,2>O<,4>、MgO-ZrO<,2>和MgO-MgAl<,2>O<,4>-ZrO<,2>质试样,检测试样的加热永久线变化率、显气孔率、体积密度、抗渣侵蚀性、抗渣渗透性,并利用Factsage6.1热力学软件对试样的抗渣性进行了分析.结果表明:1.MgO-ZrO<,2>材料的烧结性能优于MgO-MgAl<,2>O<,4>材料及MgO-MgAl<,2>O<,4>-ZrO<,2>材料;2.在MgO-MgAI<,2>O<,4>材料中引入ZrO<,2>,有利于提高其抗渣侵蚀性,但抗渣渗透性较差,主要同试样的组织结构相对疏松及熔渣中的Fe<'3+>和Mg<'2+>的渗透有关;3)试验结果与热力学模拟结果吻合较好.     

水泥窑用MgO-MgAl2O4-ZrO2-La2O3复合耐火材料的研究

根据耐火材料和水泥熟料的反应机理,找到一种既可阻止窑皮中C2S相变,又不损害镁铝质耐火材料的高温性能的添加物,开发了一种基于MgO-MgAl2O4-ZrO2-La2O3体系的新型复合耐火材料。该材料具有优异的耐火性能、抗热震性能、挂窑皮性能、抗侵蚀性能和较高的机械强度,是一种替代镁铬砖的理想材料,适用于水泥窑烧成带。     

SiC加入量对MgO-MgAl2O4-SiC材料性能的影响

以电熔镁砂颗粒(3~1 mm,≤1 mm)、电熔镁砂粉、镁铝尖晶石粉和SiC粉为原料,研究了SiC加入量(w)分别为5%、10%、15%、20%时对MgO-MgAl2O4材料经1 100℃和1 500℃保温3 h后的物理性能、抗氧化性能以及1 600℃保温3 h后的抗渣性能的影响。结果表明:加入SiC有利于提高材料的烧后强度。材料经1 100℃处理后,SiC几乎完全氧化;1 500℃烧后表明,当SiC加入量不超过10%时,抗氧化性能随着SiC加入量增加而增强;超过10%时,反而降低;1 600℃烧后表明,随着SiC加入量的增加,试样抗渣性能有所下降。对1 500℃烧后试样进行XRD和SEM分析发现,在试样氧化层中生成大量的镁橄榄石。     

Medium infrared transparency of MgO-MgAl2O4 directionally solidified eutectics

MgAl₂O₄-MgO eutectic ceramics have been fabricated by the laser floating zone method. Increasing growth rates from 10 to 50 mm/h, the microstructure transformed from irregular MgO rod-to-lamellae phase and it approached to almost homogeneous rod morphology. At the highest used velocity of 750 mm/h, the cell structure was completely dominant and the samples were free from transversal cracks. Although the highest flexure strength was found at 750 mm/h growth rate, the maximum optical transmittance in the medium-infrared range was obtained for 50 mm/h growth rate and for 1 mm thick samples reached values higher than 75% in the wavelengths between 4 and 5.3 μm. The enhanced transmittance for the sample with 50 mm/h growth rate can be explained in terms of the close refraction indexes of the component phases and the characteristic lengths of the resulting microstructure showing fully dense ceramics with the finest and almost homogeneous microstructure.     

Computer modeling of the local structure and mixing properties of a solid solution of MgAl2O4-MgCr2O4

An original method of the atomistic computer modeling of substitutional solid solutions is applied to the noble spinel MgAl₂O₄-magnesia chromite MgCr₂O₄ binary system. The parameters of the interatomic potentials in the partially ionic approximation are optimized using the experimental values of the structural, elastic, and thermodynamic characteristics of these minerals. The point defect formation energy is estimated. The modeling of Mg(Al _x Cr_{1 ? x} )₂O₄ solid solutions is carried out in a supercell of the 4 × 4 × 4 structural type of spinel that contains 3584 atoms; the GULP 3.4.9 program, which is implemented in the SKIF Chebyshev supercomputer (Moscow State University), is used. The mixing properties of the solid solution such as the mixing enthalpy, the interaction parameters, the mixing volumes, the deviations of the compression moduli from additivity, and the oscillatory entropy were determined. Calculations of the Gibbs energy have yielded the assessments of the stability fields of the MgAl₂O₄-MgCr₂O₄ solid solution and the comparisons of these assessments with the experimental data. Histograms of interatomic spacings are plotted and the values of the relaxation parameters of the Cr-O and Al-O bonds are estimated; these values agree well with the experimental values obtained by measuring the optical absorption spectra on a Cr³⁺ ion.     

Corrosion of chemical-mineralogical gradient submicron-porous Al2O3-MgAl2O4 composite in contact with molten converter slag

Penetration resistance and corrosion mechanism of a novel chemical-mineralogical gradient submicron-porous corundum-spinel composite (A90) to converter slag were investigated and compared to its dense corundum counterpart. Slag-penetration resistance of A90, due to its special submicron-pore structure, is superior to that of the corundum counterpart. Gradient Al₂O₃-rich spinels pre-embedded on the inner-surfaces of the pores are essential for making the slag deficient in Fe²⁺ and Fe³⁺ and CaO-rich. The resultant CaO-rich slag reacted with the corundum matrix of A90, building up a continuous dense layer composed of tightly interlocked highly refractory tabular CaA1₁₂O₁₉ and thus suppressing the further slag penetration and corrosion. Due to the lack of MgO in the corundum/molten-slag reaction system, large amounts of low melting iron oxides still remained in the liquid slag, making it much less viscous, so it could penetrate readily the corundum substrate via the grain boundaries in it at the test temperature.     

Growth-microstructure relationship in MgO-MgAl2O4 eutectic fabricated by micro-pulling down method with MgAl2O4 seed crystals

The MgO-MgAl₂O₄ eutectic was directionally solidified via micro-pulling down method in the form of rods with 2–3 mm diameter. MgAl₂O₄ single crystals (with <111> orientation) were used as crystallization seeds. At low pulling rates, especially 0.15 mm/min triangle-like cross-section was observed, which was linked to the eutectic MgAl₂O₄ following the crystallographic direction of the seed. MgO precipitates in the form of lamellae and rods with median equivalent diameter ranging from 0.19 to 0.85 μm, depending on the pulling rate. The preferred crystallization direction was <111> for both phases, however notable traces of other directions, e.g. <100>, <110> and <331> were found as well.     

KCl-LiCl-KF系熔盐介质中MgAl_2O_4粉体的合成制备

以工业氧化铝和轻质氧化镁(分析纯)为主要原料,在500~1 000℃范围内,研究了KCl、LiCl及KCl-LiCl熔盐介质中MgAl2O4粉体的合成制备以及KF的作用和影响。采用X射线衍射仪、激光粒度分析仪及扫描电子显微镜对产物粉体进行了分析表征,并探讨了MgAl2O4的合成反应机制。结果表明:1)复合熔盐(KCl-LiCl)较之单一熔盐(LiCl、KCl)、单一熔盐LiCl较之KCl更有利于MgAl2O4粉体的合成;2)KF的引入能促进熔盐介质中MgAl2O4的形成,并明显降低MgAl2O4的合成温度;3)在未引入KF的熔盐介质中,MgAl2O4合成反应机制为模板机制;在引入KF的熔盐介质中,MgAl2O4合成反应机制包括模板机制和溶解-沉淀机制。     

Synthesis and characterization of a MgO-MgAl2O4-ZrO2 composite with a continuous network microstructure

Using analytically pure MgO, analytically pure Al₂O₃ and analytically pure ZrO₂ as raw materials, Mg_4.68Al_2.64Zr_1.68O₁₂ was prepared at 1993 K for 10 h, and then, a MgO-MgAl₂O₄-ZrO₂ composite with a continuous network was successfully obtained by controlling the cooling rate based on the in-situ decomposition reaction of Mg_4.68Al_2.64Zr_1.68O₁₂ at temperatures below 1887 K. The three phases of MgO, MgAl₂O₄ and ZrO₂ are highly dispersed in this continuous network microstructure, with ZrO₂ intertwined by MgO and MgAl₂O₄ and micropores with a size of less than 2 µm. Furthermore, the synthesis mechanism of Mg_4.68Al_2.64Zr_1.68O₁₂ is given as follows: first, MgAl₂O₄ is synthesized using the reaction: MgO(s)+Al₂O₃(s)=MgAl₂O₄(s) at temperatures below 1894 K; and then, Mg_4.68Al_2.64Zr_1.68O₁₂ is further prepared through MgO and ZrO₂ diffusion and dissolution into MgAl₂O₄ at temperatures above 1894 K, for example, at 1923 K or 1993 K in this work.     

Effect of the composition of phosphorus-containing raw materials on glass formation in the Li2O-Li2SO4-P2O5 system

Calculation and experimental methods showed that the use of lithium metaphosphate for making up batches of Li₂O-Li₂SO₄-P₂O₅ multilithium glass systems reduces the founding temperature-time parameters while simultaneously increasing the homogeneity of the melts.