Sintering high purity,magnesia with additions of hafnium dioxide

Conclusions The introduction of hafnium dioxide into high-purity active magnesia accelerates its sintering and with 0.1–0.3 mol % HfO₂ a ceramic with an apparent density of 99–100% of the theoretical (3.56–3.63 g/cm³) can be obtained after firing at 1300–1400°C. The optimum quantity of HfO₂ additive is close to 0.25 mol %.The calcination temperature of the mixture of magnesium hydroxide and additive obtained by precipitation from a solution of MgCl₂ and Hf(SO₄)₂ with ammonia, and the fabrication pressure do not greatly affect the final density of the ceramics.Sintering of spectrally pure MgO containing 0.25 mol. % HfO₂ begins at 950°C, then the apparent density grows rapidly with rise in firing temperature, approaching 3.40 g/cm² at 1100°C. Selective recrystallization at these low temperatures is slow and sintering is not accompanied by a substantial grain growth. At 1300°C and higher firing temperatures densification of the ceramics approaches the limit in several minutes.The mechanism of the transfer of substance during the sintering of these specimens is volume self-diffusion from the grain boundaries to the surfaces of the bridges formed between them. The energy of activation of this process [3.9 eV (6.2 × 10^–16 J) in the 1000–1300°C range] and the coefficient of self-diffusion for MgO calculated in accordance with this (6 × 10^–14 cm²/sec at 1100°C) correspond to existing data on the diffusion of magnesium into MgO.With the incorporation of 0.25 mol. % HfO₂ in less pure magnesia obtained from chemically pure MgCl₂, sintering is little different from the sintering of spectrally pure MgO, but the limiting apparent density of the ceramics in this case is somewhat lower-of the order of 99% of the theoretical.     

Sintering of magnesium hydroxide obtained from magnesium chloride solutions

Conclusions We studied two batches of magnesium hydroxide obtained by precipitating out (using dolomite milk) from magnesium chloride solution formed when processing potassium ores.The effect of the compaction pressure, the firing temperature, and prior heat treatment on the sintering process of magnesium hydroxide was investigated.The experimental samples of magnesium hydroxide exhibit high sinterability and facilitate the production of periclase powders (powder bodies) having a porosity of 6.8–9.9% at a firing temperature of 1600°C. The degree of sintering of magnesium hydroxide increases with increasing compaction pressure and firing temperature. Prior heat treatment of the material at 800–1000°C intensifies the sintering process with simultaneous reduction of shrinkage.The studies conducted on the specimens prepared from a fired briquette established that the periclase (magnesite) powder obtained from magnesium hydroxide is suitable for the production of magnesia refractories.A. V. Kushchenko and G. G. Eliseeva (UkrNIIO) participated in this investigation.Translated from Ogneupory, No. 2, pp. 7–10, February, 1988.     

Synthesis of Spinel from Caustic Magnesite and Alumina Dust

The sintering of a mixture of a caustic dust and an alumina dust collected from electric filters taken in MgO/Al₂O₃ ratios of 0.1, 0.28, 0.5, and 0.75 at 1650°C is studied. Materials with superior physicomechanical properties are obtained: open porosity, 1.2 – 8.4%; density, more than 3.5 g/cm³, and compressive strength, 160 – 410 MPa. High-density pellets (free of additions) are prepared at a MgO/Al₂O₃ ratio of 0.75, with compressive strength as high as 160 MPa.     

Some thermomechanical properties of pure oxide ceramics

Conclusions The proposed vacuum furnace makes it possible to determine the deformation temperature under load and the coefficient of thermal expansion to high temperatures.Ceramics of pure oxides, Al₂O₃, ZrO₂, MgO, and BeO, have high temperatures of softening under load. The temperature of initial softening of Al₂O₃ ceramics containing additives lies in the range 1860–1930°C. The magnesia and beryllia specimens show high softening temperatures under load, but in vacuum at high temperatures they are very volatile. The initial softening temperature of ZrO₂ is about 2250°C.The linear expansion of pure-oxide ceramics reaches 2–3% at 1800–2000°C. Values obtained for the average coefficients of expansion for Al₂,O₃, ZrO₂, MgO, and BeO are little different from those in the literature.The compressive strength and bending strength of pure oxides at high temperatures are relatively low. The highest o_bend at high temperatures is possessed by specially pure zirconia, stabilized with MgO. Magnesia and beryllia in compressive strength at high temperatures exceed the other oxides.The highest spalling resistance is shown by beryllia ceramics. The combined addition to alumina of 1% TiO₂ and 5% ZrO₂ leads to a reduction in sintering temperature and an increase in thermal shock resistance. Ceramics based on specially pure zirconia stabilized with an optimum amount of CaO and MgO show a high thermal shock resistance.     

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.     

Hot press compaction of magnesium oxide obtained by decomposing certain compounds

Conclusions A study was made of the densification during hot pressing at 1300–1600°C of magnesium oxide activated by decomposing the hydroxide and the basic magnesium hydrocarbonate. During decomposition of these compounds at 500–700°C with a soak of 15 min, magnesium oxide forms that is actively compacted almost to the theoretical density (98.5–99.5%) at relatively low temperatures (1500–1600°C) and pressures of 150 kg/cm².We investigated the influence of the time and temperature of heat processing of the hydroxide and the basic hydrocarbonate of magnesium on the fineness of the grains and the defects of the crystalline lattices of periclase thus formed, and also on the capacity for subsequent compaction during hot pressing.The reduction in the degree of compaction during hot pressing of the materials, heat processed at temperatures below 500°C, is due to the increase in the content in them of undecomposed residue, which hinders the diffusion sintering in subsequent stages of pressing.A reduction in the degree of compaction with rise in temperature of heat processing above 500°C or with an increase in the heat-processing time with the optimum temperature, is connected not with a reduction in the defects of the crystalline lattice of the periclase formed, but with the sizes and physical state of its particles.We also studied the effect of additions of magnesium oxide obtained by heat processing the hydroxide or the basic hydrocarbonate of magnesium on the compaction during hot pressing of industrial magnesia. The introduction of 10–20% of this additive ensures a reduction in the optimum pressing temperature of 100–300°C and an increase in the density of the specimens almost to theoretical.Translated from Ogneupory, No.2, pp.46–53, February, 1967.     

Sintering and characterization of flyash-based mullite with MgO addition

Mullite ceramics were fabricated at relatively low sintering temperatures (1500-1550 °C) from recycled flyash and bauxite with MgO addition as raw materials. The densification behavior was investigated as function of magnesia content and sintering temperature. The results of thermal analysis, bulk density and pore structure indicate that MgO addition effectively promoted sintering, especially above 1450 °C. Due to the presence of large interlocked elongated mullite crystals above 1450 °C, associated with enhanced densification, an improvement in mechanical strength was obtained for the samples containing magnesia. The addition of magnesia slightly decreases the LTEC at 1300 °C due to the formation of low-expansion α-cordierite, but slightly increases the LTEC above 1400 °C due to the formation of high expansion corundum and MgAl₂O₄ spinel.     

Stability of silicon nitride heated in air and in carbon monoxide

Conclusions Silicon nitride oxidizes in air at 1100–1500°C mainly with the formation of silica. An addition of NaF contributes to oxidation of the silicon nitride to silicon oxynitride.In a coke filling silicon nitride decomposes at 1450–1550°C into oxynitride and cubic silicon carbide.Additions of CaF₂ and MgO contribute to the conversion of the silicon nitride in a carbon monoxide atmosphere into the silicon oxynitride at 1550°C.Translated from Ogneupory, No.6, pp. 33–39, June, 1967.     

The question of the stabilization and sintering of high purity zirconium dioxide

Conclusions The results of an investigation of the stabilization and sintering of test bodies based on zirconia of high purity (with 99.5% base oxide) and with the addition of 4–15 mol.% CaO or MgO established the following.In conditions of oxidizing fire at 1710°C with a 5-h soak, complete sintering occurs in the body containing 10 mol.% of stabilizing oxide (zero open porosity), there is adequate stabilization, the material assumes great strength and spalling resistance compared with the other bodies investigated.With an increase in the amount of stabilizing oxide to 12–15% although a sintered and fully stabilized product results there is a reduction in density which is especially strong with CaO additions. Furthermore, there is a sharp reduction in the strength and spalling resistance.The relatively low density of the fired bodies containing 10 mol.% stabilizing addition (for CaO 5.20, for MgO 5.28 g/cm³) is determined chiefly by the presence of pores both inside and on the boundaries of the crystals in the material.An increase in the firing temperature at 2200°C only slightly affects the density of the material. An increase in the density of the material containing 10 mol.% CaO is attained by:changing the form of the anion added with the stabilizer from CO₃ to F with this there is a sharp deterioration in spalling resistance of the material;by precalcining the original ZrO₂;by sintering the preliminary stabilized product as a result of which specimens are obtained with a bulk density of 5.54 g/cm³.     

Effect of phosphoric acid concentration on some properties of finely milled refractory materials

Conclusions The concentration of phosphoric acid seriously affects the setting, weakening, and sintering of the finely milled refractory materials.Increasing the acid concentration boosts the setting rate of alumina, MgO·Al₂O₃, chromite, electrocorundum, dunite, and dinas, but hardly affects the setting of MgO, Cr₂O₃, magnesite, zircon, and quartzite.The materials that were studied are almost unaffected at 800°C. Increasing the concentration of H₃PO₄ improves the sintering of alumina, MgO·Al₂O₃, chromite, zircon, and dinas, but impairs the sintering of MgO, Cr₂O₃, magnesite, electrocorundum, dunite, and quartzite.Translated from Ogneupory, No.3, pp. 40–43, March, 1968.     

Kinetics of Sintering of a Low-Cement Corundum Castable Modified by an Eutectoid Phase

The effect of an eutectoid additive on the kinetics of sintering of a low-cement corundum castable is studied. An eutectic in the CaO – Al₂O₃ system produced at 1395°C is shown to accelerate the sintering.     

The sintering of flotation-concentrated magnesite

Conclusions An investigation was carried out of the sintering of flotation-concentrated magnesite containing (in terms of the calcined substance) 95.3% MgO, 0.52% SiO₂, and 2.0% CaO.The process parameters for the production of a high-density (open porosity less than 11%) sintered powder from concentrated magnesite proved to be as follows: 15% caustic magnesite added as binder to the concentrated magnesite, grinding the mix of concentrated and caustic magnesites in tube mills to a powder of a specific surface greater than 6000 cm²/g (i.e., containing 98% particles smaller than 60), wetting with water to a moisture content of 6–8%, forming 10–12-mm-thick briquets on a smooth-roller press at a pressure of 1000–1200 kgf/cm², and firing the briquets in a rotary kiln at a flame cone temperature of 1760°C.This technology for the mass production of a high-density, good-quality powder from concentrated magnesite precludes the use of a sintering additive and thermal activation both of which lower the refractoriness and increase the net cost of the finished product.Translated from Ogneupory, No. 2, pp. 3–7, February, 1977.     

Sintering mullite — corundum briquette in oxidizing and reducing conditions

Conclusions Rapid sintering of mullite-corundum briquette occurs in the range 1400–1600°. The rate of sintering of mullite-corundum briquette in reducing atmospheres at 1200–1400° is higher than in oxidizing conditions, owing to the formation in these conditions of a large quantity of liquid phase. At temperatures above 1400° the process of reduction of the SiO₂ is accelerated, and volatile SiO is formed preventing sintering of the briquette.Translated from Ogneupory, No. 3, pp. 24–27, March, 1973.     

Change in the phase composition and properties of zirconia refractories under the prolonged effect of high temperatures

Conclusions A systematic study has been made of the behavior of ZrO₂ stabilized with various oxides under the prolonged action of temperatures up to 2300°C.The optimal concentrations of Y₂O₃, Nd₂O₃, CaO, and MgO in the zirconia refractories have been established; this ensures the successful service life of the articles under conditions of multiple prolonged action of high temperatures.The ultimately permissible temperature at which is it possible to operate the zirconia articles has been determined: with the stabilization of ZrO₂ by MgO it is 1900°C; with CaO, 2000°C; and with oxides of rare-earth elements, more than 2300°C.Translated from Ogneupory, No. 2, pp. 13–19, February, 1985.     

Dynamic in situ observation of automotive catalysts for emission control using X-ray absorption fine structure

Two main pivotal subjects of research in automotive catalysts were studied by modern X-ray absorption analysis techniques. One is oxygen storage/release behaviour, and the other is sintering inhibition of Pt particles. First, three types of CeO₂–ZrO₂ (Ce:Zr = 1:1 molar ratio) compounds with different oxygen storage/release capacities and different structural properties were prepared, and the valence change of Ce as a function of temperature during oxygen release/storage processes was investigated. The reduction of surface Ce mainly occurred in the range 100–170 °C, and the reduction of bulk Ce progressed at high temperatures of 170 °C and above. The Ce reduction behaviour depended not only on the homogeneity of the Ce and Zr for bulk reduction at high temperatures but also on the particle size of the CeO₂–ZrO₂ samples for surface reduction at low temperatures. Secondly, sintering inhibition of Pt in Pt/Al₂O₃, Pt/MgO and Pt/ceria-based catalysts after 800 °C ageing in air was studied. We found that the Pt–O–M (M = Mg, Ce) bond acted as an anchor and inhibited the sintering of Pt particles on MgO or ceria-based oxide. Especially, it was noteworthy that the Pt–O–Ce⁴⁺ bond on the ceria-based support breaks easily through the reduction of Ce (Ce⁴⁺ → Ce³⁺) during the usual stoichiometric and reducing conditions.     

Investigation of the sintering of zirconia

Conclusions We investigated the sintering of zirconium dioxide as a function of the activity of the starting material. It was shown that preliminary firing or stabilization of zirconium dioxide reduces its capacity for densification at temperatures of up to 1500°C, while stabilizing it directly during sintering intensifies this capacity. The prestabilized zirconia has the lowest sintering rate.Introducing monoclinic unfired zirconia into the prestabilized material intensifies sintering; the optimal addition is 30%.It is established that the zirconia is sintered by volume diffusion of vacancies.We investigated the sintering of active zirconia obtained by decomposing zirconium nitrate. The greatest degree of densification is obtained with a preliminary short heat processing of the nitrate at 400°C. Activation is connected with the formation of metastable tetragonal and cubic modifications with defect crystal lattices. Increasing the heat-process temperature of the nitrate or prolonging the soak at 400°C, leading to the formation of stable monoclinic ZrO₂ with an ordered crystalline lattice, impairs sintering.Incorporating small additions of active ZrO₂ in the industrial material, and providing rapid firing in an oxidizing atmosphere, greatly increases the degree of sintering. Introducing stabilizing additives intensifies sintering. The maximum densification is obtained by the formation of 60–70% solid solution. Further increase in these additions reduces the shrinkage and densification of the specimens.Translated from Ogneupory, No. 6, pp. 33–40, June, 1968.     

Raman studies of peroxide formation,decomposition, and reduction on Ba/MgO

Ba/MgO is an active catalyst for the oxidative coupling of methane to form ethane and ethylene. It has been proposed that activation of methane occurs via reaction with peroxide species present at the surface of the catalyst. In the present work, Raman spectroscopy has been used to investigate the formation, decomposition, and reduction of BaO₂ on 4 mol% Ba/MgO. The presence of BaO₂ is evidenced by the presence of a band at 842 cm^–1. The peroxide forms above 300°C but is stable to decomposition at temperatures up to 500°C. Reduction of BaO₂ to BaO proceeds via Ba(OH)₂. BaCO₃ forms when either BaO or BaO₂ is exposed to CO₂. Once formed, BaCO₃ is stable to decomposition in He or O₂ at temperatures up to 500°C. Only BaCO₃ is observed when a mixture of CH₄ and O₂ is passed over the catalyst at 500°C.     

Thermal transformations during the interaction of chamotte and orthophosphoric acid

Conclusions Up to 300°C chamotte does not react with orthophosphoric acid. In the range 300–800°C crystallization of the type SiO₂·P₂O₅ occurs. With further increase in temperature the silicophosphate dissolves, and at 1030°C is converted from the low- to the high-temperature form, and at 1200°C it changes completely into the melt.Starting from 500°C the mullite decomposes. The intensity of the mullite line at 1400°C is slight.In the range 700–1000°C a large quantity of liquid phase, formed during decomposition of the silicophosphates and mullite, sharply depresses the refractoriness-under-load and increases their shrinkage.At 1200°C AlPO₄ crystallizes in the cristobalite form. An increase in the quantity of AlPO₄ increases the refractoriness-under-load of the specimens.Mixtures of chamotte and orthophosphoric acid after firing at 1400°C contain (in reducing order) AlPO₄ (cristobalite form), cristobalite, mullite, and quartz; they may be recommended as low sintering mortars.Translated from Ogneupory, No.2, pp. 39–43, February, 1970.     

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.     

The effect of long-term firing at 2100–2200°C on the concentration and microstructure of solid solutions in the systems ZrO2-Y2O3-CaO and ZrO2-Y2O3-MgO

Conclusions Solid solutions in the system ZrO₂-MgO fired at a high temperature (above 2000°C) in vacuo (5 · 10^–4mm Hg) for 5 h decompose as a result of the complete vaporization of the MgO. Solid solutions of ZrO₂-CaO undergo partial decomposition when fired in these conditions. The addition of 2 mole % or more yttrium oxide to the solid solutions ZrO₂-CaO and ZrO₂-MgO results in significantly lower CaO and MgO vaporization.The long-term exposure of solid solutions in the system ZrO₂-CaO and ZrO₂-MgO to 2200°C in a helium atmosphere results in the formation of an intercrystalline layer in which not only the stabilizing oxide but also the impurities are concentrated.In three-component solid solutions which contain yttrium oxide the degree of vaporization is lower and the intergranular secondary phase less developed so that the degree of collective recrystallization is lower.Translated from Ogneupory, No. 8, pp. 44–48, August, 1976.