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.     

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.     

Investigating sintering of magnesia obtained by chemical beneficiation

Conclusions A study was made of the sintering behavior of samples of magnesium oxide obtained by chemical purification. The magnesia obtained by the nitrate method is a low sinter-active material. The density of the material after firing at up to 2400°C equals about 3.0 g/cm³. Partial hydration of the material improves the sintering results. The magnesia obtained by the ammonium-sulfate technology is an active material which when fired yields a densely sintered product with a density of 3.3–3.4 g/cm³.Translated from 0gneupory, No. 5, pp. 25–33, May, 1991.     

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     

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。     

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.     

The sintering kinetics of ultrafine tungsten carbide powders

The sintering kinetics of nano grained tungsten carbide (n-WC) powders has been analyzed by non isothermal and isothermal sintering. Non isothermal sintering experiments reveal a multi staged sintering process in which at least three major sub-stages can be distinguished. The isothermal shrinkage strain also exhibits an asymptotic behavior with time indicating an end point density phenomenon in most of the temperature ranges. Combined microstructural and kinetic data analyses suggest that differences in the sinterability of inter and intra agglomerate pore phases introduce sub-stages in the sintering process which manifest as stagnant density regions in both the isothermal and non isothermal experiments. Kinetic analysis of the data reveals very low activation energies for sintering suggesting that particle rearrangement and agglomeration at low temperatures may be brought about by surface diffusion leading to neck growth and grain rotation. At higher temperatures rapid grain boundary diffusion by overheating along inter particle boundaries induced by sparking may be a dominant sintering mechanism. Although grain growth and densification in conventional WC powders generally obey an inverse relation to each other, in n-WC powders both can act synergistically to increase the net densification rate. In fact, complete densification cannot be achieved in n-WC powders without grain growth as one abets the other.     

Roof bricks from well-brine magnfsia

Conclusions Existing production equipment at the Nikitov Dolomite Plant was used for establishing the production parameters for magnesite-chromite roof brick from well-brine magnesia, and two large experimental batches of brick were produced. The bricks possessed good physicoceramic properties and increased durability in the roof of high-productivity open-hearth furnaces.Translated from Ogneupory, No. 5, pp. 10–15, May, 1976.     

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。     

Effect of sintering temperature on mechanical properties of magnesia partially stabilized zirconia refractory

The optimized sintering conditions for a 3.5 wt% magnesia partially stabilized zirconia (Mg-PSZ) refractory were proposed in our recent research. The influence of the sintering temperature on the development of phase composition, microstructure, densification, thermal expansion and mechanical strength was studied in detail by X-ray diffraction (XRD), scanning electron microscope (SEM), He-pycnometer, high temperature dilatometry and three-point bending test. The samples sintered at 1670 °C had the highest bend strength, the maximum densification, the lowest thermal expansion coefficient (CTE), a homogeneous microstructure and a linear change in thermal expansion.     

Constrained sintering kinetics of 3YSZ films

3YSZ green layers approximately 10 μm thick were screen-printed onto 3YSZ substrates and their constrained sintering kinetics were measured at 1100-1350 °C using an optical dilatometer. The densification rates of the same powder in the form of pellets and free-standing films were also measured. The constrained densification rate was greatly retarded compared with the free densification rate at a given temperature and density. The retardation increased with increasing density and temperature and could not be properly accounted for by existing theories of constrained sintering. As a result the apparent activation energy is much lower for constrained sintering (135 ± 20 kJ mol⁻¹) than for free sintering (660 ± 30 kJ mol⁻¹). It is proposed that this is because the constrained microstructure exhibits larger and more widely separated pores at the higher temperatures.     

Effect of magnesia-alumina spinel precursor sol on the sintering property of fused magnesia refractory

The sintering property of fused magnesia refractory was investigated by the fused magnesia powder as matrix and the synthetic magnesia-alumina spinel precursor sol as binder. The spinel precursor sol was prepared by co-precipitation method and was characterized by its particle size, thermal analysis, microstructure, phase development with temperatures, and so on. The effect of spinel precursor sol on the sintering property of fused magnesia refractory were studied after heat treatment at 1450?°C and 1550?°C. The results showed that the bulk density, flexural strength and linear shrinkage of the sintered samples firstly increase and then decrease with increasing spinel precursor sol. The bulk density and flexural strength (cold modulus of rupture) of the sample sintered at 1550?°C, introduced with 1?wt% spinel precursor sol, reached 3.10?g/cm³ and 47.25?MPa, respectively. From the experimental results, magnesia-alumina spinel precursor sol can replace the traditional binders and improve the sintering property of fused magnesia products.     

The production and service testing of magnesia wares from Rapna magnesia

Conclusions Magnesite refractories from Rapna magnesia are of high quality.Tests on periclase — spinel wares from Rapna magnesia in the crowns of Marten's furnaces working under intensified conditions showed that their wear was 15 to 17% lower than that of regular periclase — spinel bricks.It was shown that, by the use of periclase — spinel bricks from Rapna magnesia in the crown of a Marten's furnace working with an oxygen jet, it will be possible to lengthen the run of the furnace by 16–21%.Translated from Ogneupory, No. 7, pp. 1–6, July, 1969.