Sintering magnesium oxide obtained by the chemical beneficiation of magnesite

Conclusions A quantitative relationship was established for sintering chemically beneficiated magnesite and a method of obtaining the MgO; we also established the parameters of hydrolysis of the magnesium chloride, the quantity of residual volatiles, the temperature and rate of thermal activation of residual volatiles, the temperature and rate of thermal activation of the magnesium oxide, not washed, and washed to get rid of the calcium impurities, and also the completeness of the hydration of the magnesium oxide during leaching.A production flow line is proposed for obtaining from the enriched material sintered periclase clinkers differing in density and degree of purity.Translated from Ogneupory, No. 7, pp. 43–50, July, 1982.     

Investigating the kinetics and mechanism of deformation of magnesia during sintering compacts

Conclusions We investigated the kinetics of deformation of specimens of different magnesium oxide samples and their mixtures. It was found that high deformation during heating is connected with the dispersion of the particles, which largely determines the pronounced defectiveness in the system in the form of the extensive surfaces of the grain boundaries.Deformation of the specimens of magnesia may be reduced by preliminary heat processing of the material or products which reduces the total number of defects (regardless of their nature).Deformation of the articles made from magnesia by sintering is a result of diffusion-viscous flow of the material under gravity forces and is accomplished by volume diffusion of the vacancies.Translated from Ogneupory, No.4, pp. 30–35, April, 1967.     

反应烧结法合成镁铝尖晶石耐火材料

以合成镁铝尖晶石(分为<350μm、<177μm和<74μm三种粒级)、镁砂(粒度<88μm)和刚玉粉(粒度<44μm)为原料,配制成尖晶石粒度不同,n尖晶石:n镁砂:n刚玉粉分别为80:10:10、60:20:20、40:30:30、30:35:35、20:40:40和10:45:45的试料,分别以100MPa、150MPa和200MPa的压力压制成50mm×20mm的柱状试样,在120℃干燥24h后于1600℃2h烧成,通过反应烧结法合成镁铝尖晶石耐火材料。测量烧后试样的线变化率、显气孔率和体积密度,并利用XRD和SEM分析烧后试样的相组成和显微结构。试验结果表明:随着镁砂和刚玉粉含量的增加以及尖晶石原料粒度的增大,烧后试样的线收缩率和体积密度减小,显气孔率增大;随着成型压力的增大,试样的线收缩率和显气孔率减小,体积密度增大;采用粒度<177μm的尖晶石原料,按n尖晶石:n镁砂:n刚玉粉=20:40:40的比例配料,在200MPa压力下成型,经1600℃2h烧成,可获得显气孔率为16.3%,烧成线收缩率为0.42%的镁铝尖晶石耐火材料。     

The sintering kinetics of well-brine magnesia

Conclusions The sintering of well-brine magnesium hydroxide proceeds in accordance with the laws governing solid-phase sintering, regardless of its degree of purity (within an MgO content of 96–98%) and of the type of precipitant used.The contraction kinetics of the magnesium hydroxide under isothermal conditions is characterized by the fact that l/lis approximately proportional to t^1/2. The contraction and compaction rates are at a maximum at 1000–1300°C and decrease significantly at higher temperatures.The contraction rate of specimens from calcined magnesium hydroxide was found to be 2–3 times lower than that of dried magnesium hydroxide.The specimens are compacted while contracting; in the elimination of the open pores P/P is approximately proportional to t^1/2 and in that of the closed pores to t^1/3.With an increase in the temperature from 1000 to 1700°C the compaction of the material is accompanied by periclase recrystallization. The periclase grains begin to grow rapidly after 1500C at an open porosity of 10–12%.At a temperature above 1500°C the recrystallization rate is so high that some open pores are entrapped in the growing crystals, resulting in closed porosity, the elimination of which is difficult. The sintering rate increases sharply at the same time.Given these general regularities, which support earlier findings, it is possible that the contraction will vary slightly with the chemical composition and heat treatment conditions of the specimens of well-brine magnesium hydroxide.Translated from Ogneupory, No. 6, pp. 39–44, June, 1974.     

烧结合成致密尖晶石的试验研究

采用粒度≤0.058mm、MgO质量分数为96.28%的轻烧镁砂和粒度≤0.043mm的αAl2O3为原料,分别在50MPa、100MPa、150MPa和200MPa的成型压力以及1600℃、1650℃、1700℃、1750℃和1800℃的烧成温度下进行合成尖晶石的试验研究。结果表明:通过控制合理的工艺参数,特别是m(MgO):m(Al2O3)的比约为1:1,成型压力约100MPa,烧成温度>1700℃时,可一步煅烧制得体积密度>3.30g·cm-3、尖晶石含量较高的致密镁铝尖晶石。     

Kinetics of sintering and recrystallization of magnesia

Conclusions We studied the sintering of magnesia having differences in the size and macrostructure of the original grains. It was found that sintering is sharply reduced with an increase in the grain sizes of the original magnesia from 0.03 to 0.3 and densification of their macrostructure. Further increase in the grain sizes has little effect on the shrinkage.The size and macrostructure of the crystals of magnesia also determine the temperature of initial shrinkage which sharply increases with increase in the size of the crystal to 0.3 and with improvement in their macrostructure.An addition of finely dispersed magnesia with a more defective crystalline lattice, obtained by heat processing basic hydrocarbonate, to the coarser magnesia with the ordered crystalline lattice leads to a substantial increase in shrinkage during sintering. However, during cooling we get embrittlement of the specimens, and as a result their densification is reduced.Specimens of magnesia, owing to the plate-like form of the particles, possess substantial anisotropy of shrinkage, which increases with a reduction in the thickness of the platelets of magnesia.Translated from Ogneupory, No.3, pp.41–48, March, 1967.     

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.     

Effect of rare-earth chromites on sintering of magnesia

The effect of neodymium, cerium, lanthanum, and yttrium chromites, added in amounts of not more than 8%, on the sintering of magnesia in air and in vacuum has been studied. On addition of rare-earth chromites, the initial magnesia is sintered to a density of 0.98. By their sintering action, these chromites fall into the following sequence: neodymium chromite — lanthanum chromite — yttrium chromite — cerium chromite. The highest density (0.98 – 0.99) and the lowest porosity (1–2%) are obtained on addition of 2% of neodymium chromite to magnesia. The process of sintering of magnesia is accompanied by an increase in the size of periclase crystals from 12 to 38–56 µm.Translated from Ogneupory, No. 1, pp. 2–4, January, 1994.     

两种镁源反应烧结合成镁铝尖晶石

分别以轻烧菱镁矿粉和分析纯MgO粉为镁源,以工业α-Al2O3粉为铝源,按n(MgO):n(Al2O3)=1:1的比例配料,经球磨混合、机压成型后,分别在1 550、1 600和1 650℃保温3 h合成了两种不同镁源的镁铝尖晶石试样,对比检测了两种试样的致密度、强度、物相组成和显微结构的差异。结果表明:1)利用两种不同镁源合成试样的致密度和强度均随合成温度的升高而增大,在合成温度相同时,以轻烧菱镁矿为镁源的试样的致密度和强度较高。2)以轻烧菱镁矿为镁源、1 650℃合成的试样的主晶相为尖晶石,还有少量CA2和CaSiO3,其尖晶石晶体发育较好,八面体形态可辨;以分析纯MgO为镁源、1 650℃合成的试样中只有单相镁铝尖晶石,但尖晶石晶体发育较差,只有少数形成较完整的八面体形态。3)轻烧菱镁矿中SiO2、CaO杂质的存在有利于试样中形成CaO-Al2O3-MgO-SiO2系低熔点相,促进镁铝尖晶石晶体的发育及试样的烧结。     

Investigating slip casting of ceramics based on stabilized ZnO2 obtained by the chemical coprecipitation method

Conclusions We studied the influence of a number of technological parameters (heat-process temperature, dispersion of solid phase, pH, and concentration of suspension) on the rheological properties of the water suspensions and certain technical properties of castings and materials made from yttria-stabilized ZrO₂, obtained by the method of coprecipitation from salt solutions.We demonstrated the essential influence of the structure of the original powders, the content of Y₂O₃, and the bonded chlorides on the properties of the castings and the sintered material.Translated from Ogneupory, No. 1, pp. 24–28, January, 1986.     

锆英石对镁砂烧结性及其制品性能的影响

探讨了锆英石对镁砂烧结性能的影响 ,并研究了由这种合成镁锆砂制成的镁砖的性能。结果表明 :锆英石能显著提高镁砂的烧结性能 ;利用合成镁锆砂制成的镁砖以高熔点的第二固相为结合相 ,性能明显优于普通镁砖     

Liquid phase flash sintering in magnesia silicate glass-containing alumina

Magnesia silicate glass-containing alumina was flash sintered using an E-Field in the 500–1500 V/cm range. The addition of glass allows to reduce the current needed for densification and improves the shrinkage obtained during field-assisted sintering process. This behaviour is related to the different sintering mechanisms involved in the two materials, i.e. solid state sintering for pure alumina and liquid phase sintering for glass-containing alumina.The estimated activation energy for conduction during flash sintering is compatible with ionic diffusion in silicate melt. Moreover, evidence of magnesium diffusion toward the cathode is recorded. The estimated sample temperature is in almost all cases lower than 1355 °C, which is the temperature at which the first liquid is formed in the ternary system MgO-SiO₂-Al₂O₃. Finally, it is shown that the application of an E-Field accounts for efficient liquid phase sintering at temperatures at which it cannot be reproduced conventionally.     

Effect of vibromilling on the sintering and properties of magnesia

Conclusions The vibrogrinding of magnesia reduces the temperature at which complete sintering occurs from 1950 to 1850°C, and increases the density and strength of the specimens.The vibrogrinding of magnesium oxide makes it possible to obtain homogeneous thermoplastic slip for the hot pressing of refractories under pressure, and greatly reduces the concentration of temporary bond in the material.The material made from pure magnesium oxide, vibroground for 15 min, has a very high specific volume electrical resistance, electrical strength, vacuum sealing values with a vacuum of 10^–6 mm Hg, and a low tangent of dielectric loss.Translated from Ogneupory, No. 3, pp. 57–60, March, 1971.     

Ceramics based on boron nitride obtained by reaction-based sintering

Reaction-based sintering of ceramics based on hexagonal boron nitride is investigated. It is found that the use of presintered aluminum nitride milled to the particle size of 45 – 125 μm is significantly more efficient than using finely disperse boron nitride or coarsely disperse aluminum nitride as the inert filling agent. Translated from Steklo i Keramika, No. 1, pp. 14–17, January, 1999.     

Tin oxide-based ceramics of high density obtained by pressureless sintering

Dense tin oxide-based ceramic semiconductors have a high potential as electrodes for aluminum production, glass industry, sputtering targets for transparent conducting thin films, varistors, and thermoelectrics due to their specific electrical properties, high corrosion resistance and ability to withstand high temperatures. The application of these ceramics is still limited because of the necessity to reach density of 99% of TD or greater and high electrical conductivity, and because of the manufacturing difficulties to produce components of different shapes and sizes with low cost. The paper reviews the results of the works conducted towards obtaining tin oxide-based ceramics with density up to 99.5+% of TD through low-temperature pressureless sintering. The selected sintering aids/dopants and firing conditions promoted both liquid phase sintering and electrical conductivity. The uniform microcrystalline structure is obtained.     

Micromechanical properties and microstructure evolution of magnesia partially stabilized zirconia prepared by spark plasma sintering

Magnesia partially stabilized zirconia (Mg-PSZ) is a widely used engineering ceramic owing to its high hardness and exceptional toughness. It is usually processed by conventional firing followed by subeutectoid aging. In this work, Mg-PSZ was prepared by spark plasma sintering (SPS) followed by sequential subeutectoid aging to fine-tune its mechanical properties. Mg-PSZ prepared by SPS with the rapid heating capability presents much smaller grains than conventionally prepared counterparts. After aging, a significant fraction of the matrix cubic phase transforms into tetragonal, orthorhombic, and monoclinic zirconia. Microindentation and in-situ microcompression tests reveal that aging Mg-PSZ for 4 h leads to maximum fracture toughness and fracture strain due to the tetragonal-to-monoclinic transformation toughening. Post compression TEM analyses show dominant monoclinic ZrO₂ decorated by a high density of twin boundaries and stacking faults formed to accommodate the shear deformation. Preparation of Mg-PSZ by SPS offers rapid and effective approaches in finetuning the phases and mechanical properties.     

Investigating the sintering of seawater magnesium hydroxide

Conclusions To obtain densely sintered seawater magnesia powders it is necessary to ensure a molar ratio of CaO/SiO₂ of not more than 1 in the slurry.Incorporating B₂O₃ in amounts of not less than 0.5% on the calcined weight improves the sintering of the seawater magnesium hydroxide: the porosity of the fired (rotary furnace) powder varies from 8 to 12%.Preliminary drying or heating of the slurry, followed by briquetting of the material, produces a seawater magnesia with a porosity of 7–11% and a content of 95–97% MgO, and 2.3-0.5% CaO. Preference should be given to predrying of the slurry, since in this case no fuel is needed for removing the chemically bonded water.Translated from Ogneupory, No. 2, pp. 16–21, February, 1969.