Effect of the basic properties of platelike corundum on its metal resistance

Conclusions It has been established that the wetting angle of sintered corundum by steel and the metal resistance of the corundum increase significantly with the concentration of Al₂O₃ and with its density.The size of the sintered corundum crystals, other things being virtually equal, does not have a significant effect on the angle of wetting by steel and on the metal resistance.An increase in the Al₂O₃ concentration in the initial aggregations of sintered corundum grains and an increase in their density help to improve the metal resistance of the corundum specimens made from that material.Translated from Ogneupory, No. 2, pp. 41–46, February, 1981.     

Effect of additives on the thermal-shock resistance of corundum-zircon refractories

Conclusions It is established that the addition of 0.1–1.0% of Bi₂O₃ increases 2–7 and 1.5-fold, respectively, the resistance of a material to the nucleation and spread of destructive cracks; CoO₂ in amounts up to 0.1% has virtually no effect on the total thermal-shock resistance but increases threefold the resistance to the nucleation of destructive cracks.The addition of SnO₂ produces a slight decrease in both the number of heat cycles withstood before the appearance of the first visible crack and in the total thermal-shock resistance of the specimens before total destruction.It is established that the favorable effect of the Bi₂O₃ additive can be reduced to the fact that it melts, forming a thin film of melt on the surface of the corundum and zircon grains; this facilitates the formation of denser structures and, after evaporation, the formation of fine discontinuous microcracks.Translated from Ogneupory, No. 6, pp. 39–42, June, 1981.     

Effect of MgO–TiO2–SiO2 additions on in-situ anisotropic grains growth and mechanical properties of corundum abrasive using pseudo-boehmite as raw material

Microcrystalline corundum abrasives with a duplex microstructure consisting of elongated anisotropic grains and fine equiaxed grains were fabricated via sol-gel processes using pseudo-boehmite as raw material. Elongated anisotropic grain, as a second phase reinforcement, significantly improved the fracture toughness and strength of samples. When the sample was doped 2 wt% MgO–TiO₂–SiO₂ additions with the molar ratio of 3:5:11, elongated anisotropic grains had the highest aspect ratio and were uniformly distributed among the fine equiaxed grains matrix. In addition, the sample obtained a maximum density above 3.92 g cm⁻³, single particle compressive strength of 56.4 N and fracture toughness of 5.93 MPa m^1/2. The relationship between the microstructure and the mechanical properties of the abrasives was also discussed in detail. Moreover, toughening mechanism of elongated anisotropic grains was clearly explained by charactering crack propagation and fracture surfaces of samples.     

Heat resistance of alumina ceramics

A method for evaluating the heat resistance of structural ceramics according to which the thermally stressed state is created by blowing a directed air flow into the tip of a notch in a heated prismatic specimen is presented. For this purpose a special complexly shaped notch is formed in order to provide free inflow of the air to its tip. The radius of curvature of the notch in alumina ceramics is 5 Μm. In blowing, the heat is removed predominantly from a local volume at the tip of the notch, thus providing a “local” thermal shock. The heat resistance of alumina ceramics obtained by sintering and reaction bonding is studied. The mechanical properties of Al₂O₃ tend to improve after a local thermal shock. The tendency is proved by testing a statistically reliable sample of unnotched specimens by the conventional method for determining the heat resistance. This tendency can be explained by “curing” of some of the defects (commensurable with the elements of the substructure) in densely sintered ceramics under the effect of thermal stresses. This was established due to the low scattering of the values of the mechanical properties measured in testing a sample of specimens with a special notch. It cannot be detected in tests of unnotched specimens within the same sample. A heat cycle of “850‡C-water” worsens the mechanical properties of notched and unnotched specimens due to the initiated microfracture. Translated from Ogneupory i Tekhnicheskaya Keramika, Nos. 1–2, pp. 14–19, January–February, 1999.     

Preparation and characterization of corundum ceramics doped with Fe2O3 and TiO2 for high temperature thermal storage

High-temperature thermal storage materials have received urgent attention for efficient thermal transfer in solar thermal power generation. Corundum ceramics doped with Fe₂O₃ and TiO₂ were prepared via a pressureless sintering. A Fe₂O₃–TiO₂ system with different Fe₂O₃/TiO₂ ratios was applied to corundum ceramics. Phase composition, microstructural evolution, sintering properties, high temperature resistance and thermophysical properties were evaluated. The results indicated that Fe₂O₃ and TiO₂ rendered the grains highly active and enhanced the bonding between grains due to existing stably in the lattice of corundum. In addition, decrease in the Fe₂O₃/TiO₂ ratio led to a new phase of FeAlTiO₅, which refined the grains. These effects gave the samples good sintering properties and thermal shock resistance, but the thermal expansion coefficient mismatch between FeAlTiO₅ and corundum deteriorated the high-temperature (1300 °C) stability. Formula C1 (Fe₂O₃/TiO₂ ratio of 9:1) sintered at 1600 °C had the optimum comprehensive properties, possessing a bending strength loss rate of 1.54% after 30 cycles of thermal shock (1100 °C-room temperature, air cooling) and a constant strength retention rate of approximately 71.34% after 90 h high-temperature cycle. The corresponding thermal conductivity and specific heat capacity were 18.81 W/(m·K) and 1.02 J/(g·K) at 25 °C, which was suitable as a high-temperature thermal storage material.     

Prospects of the use of aluminosilicate refractories for aluminum electrolyzers. Part 3. The role of the glass phase formed in the operation of aluminum electrolyzers

The compositions of glasses formed in the process of operation of an aluminum electrolyzer, the lowest content of aluminum at which the glass is separated into aluminosilicate and oxyfluoride components, and the temperatures of softening and of formation of drops of titanium-bearing glasses composition close to the glass in ShPD-45 refractory (with an additive of mullite-corundum chamotte) are determined. It is shown that the oxyfluoride glasses consist of both a silica skeleton and a corundum skeleton. The titanium-bearing glass phase can hinder penetration of aggressive gaseous byproducts of electrolysis. __________ Translated from Novye Ogneupory, No. 12, pp. 10–12, December, 2007. Parts 1 and 2 of the paper appear in “Novye Ogneupory” Nos. 9 and 10 of 2007.     

A lanxide ceramic composite material

A SiC-Al₂O₃ composite material was synthesized by controlled reaction impregnation known as the Lanxide process. Freely pouring SiC powder with particles 0.5 to 2.0 mm across was impregnated with an aluminum melt doped with Mg, Si, and Zn. The microhardness of the specimen was measured in various parts. The microhardness of the crystals obtained from the corundum melt was higher than that of grains of silicon carbide and other phases. Translated from Steklo i Keramika, No. 7, pp. 15–17, July, 1997.     

Synthesis and Properties of Fusion-Cast Refractories in the MgO – Al2O3 System

Results of a study of fusion-cast refractories in the MgO – Al₂O₃ high-alumina system are reported. A casting technology for spinelide refractories is proposed. The materials synthesized are analyzed for phase composition by x-ray diffractometry and petrography and are shown to consist of corundum, MgAl₂O₄ spinel and their solid solutions. The materials are tested for corrosion resistance in industrial glass melts, and a corundum spinelide material with 5% Mg is recognized as the most promising for practical use.     

Preparation and thermal shock resistance of corundum-mullite composite ceramics from andalusite

Corundum-mullite composite ceramics have high hardness, small plastic deformation and other excellent performances at high temperature. Corundum-mullite composite ceramics were fabricated from andalusite and α-Al₂O₃ by in-situ synthesis technology. Effects of mullite/corundum ratio and sintering temperatures on the water absorption, apparent porosity, bulk density, bending strength, thermal shock resistance, phase composition and microstructure of the sample were investigated. Results indicated that the in-situ synthesized mullite from andalusite combined with corundum satisfactorily, which significantly improved the thermal shock resistance as no crack formed after 30 cycles of thermal shock (1100 °C-room temperature, air cooling). Formula A4 (andalusite: 37.31 wt%, α-Al₂O₃: 62.69 wt%, TiO₂ in addition: 1 wt%, mullite: corundum=6:4 in wt%) achieved the optimum properties when sintering at 1650 °C, which were listed as follows: water absorption of 0.15%, apparent porosity of 0.42%, and bulk density of 3.21 g⋅cm⁻³, bending strength of 117.32 MPa. The phase composition of the sintered samples before and after thermal shock tests were mullite and corundum constantly. The fracture modes of the crystals were transgranular and intergranular fractures, which could endow the samples with high thermal shock resistance.     

Slag Resistance Mechanism of Lightweight Microporous Corundum Aggregate

The microstructures of the reaction interfaces between slag and corundum aggregates, microporous corundum produced in the laboratory and tabular corundum were observed after slag resistance experiments, and their associated slag resistance mechanisms were investigated. A continuous isolation layer was observed around the microporous corundum, which showed a significantly better slag resistance than tabular corundum. The formation of columnar crystals of CaAl₁₂O₁₉ (CA₆) and CaAl₄O₇ (CA₂) in the isolation layer was the main reason for the difference in slag resistance. With respect to the nucleation and growth of second phase, the slag resistance mechanism of lightweight microporous corundum was explored by thermodynamic and kinetic analysis. Due to its smaller pore size, the second phase is more likely to achieve supersaturation, and large quantities of crystal nuclei are generated for microporous corundum. The critical dissolved depths of the microporous and tabular corundum in saturated slag were calculated to be 0.14 and 0.27 μm, respectively. Additionally, the small pore sizes lead to an increase in the Ostwald ripening rate of the second phase, and the Ostwald ripening rate of microporous corundum was 12 times that of the tabular corundum based on Ardell's theory.     

Part 3. Phase Transformations within the Layer of a Dry Barrier Mixture for Aluminum Electrolysis Cells

It has been established that transformations within the layer of a dry barrier mixture (DBM) are selective in character and are mainly confined to the upper zones of the innermost part (core) of DBM. The DBM layer undergoes a solid-liquid sintering in these zones. The glass phase thus formed prevents the electrolysis products from penetration into the DBM layer. Modifier admixtures — titanium and iron oxides, products of electrolysis and degradation of the refractory layer AlF, SiF₄, and sodium oxide — decrease the viscosity of aluminosilicate melt. The grains of a disthene-sillimanite concentrate undergo cracking along the cleavage plane. Imperfections of the crystal lattice promote transformations in the grains of normal electrofused corundum. Oxyfluoride AlOF and an oxyfluoride of variable composition AlOF_{1 − x} are formed in the cathode lining of the electrolysis cell. For Parts 1 and 2, see Novye Ogneupory, Nos. 3 and 5, 2005. __________ Translated from Novye Ogneupory, No. 9, pp. 7 – 13, September, 2005.     

Formation of a liquid film of AgNO3 on a silver surface

The behavior of a AgNO₃/Ag₂O/Ag “sandwich” upon heating in vacuum was studied by in situ X‐ray photoelectron spectroscopy (XPS) and ex situ scanning electron microscopy (SEM). The AgNO₃/Ag₂O/Ag “sandwich” was prepared by exposure of a silver foil to a NO : O₂ mixture. The upper layer of the “sandwich” consists of AgNO₃ crystals of a mean size between 0.1 and 0.4 μm. Heating at 550 K in vacuum results in melting of the AgNO₃ crystals. A liquid film of AgNO₃, readily wetting the silver, covers the surface. Cooling below the melting point of AgNO₃ leads to the agglomeration of silver nitrate to long islands with a size reaching a few tens of micrometers (μm). The possible effects of AgNO₃ liquid‐phase formation on surface processes are discussed. This revised version was published online in July 2006 with corrections to the Cover Date.     

Critierional evaluation of thermal destruction of corundum concretes

Conclusions The resistance of corundum concretes to thermal destruction was studied using the experimentally plotted deformation diagrams and the standard thermal cycling method. The correlation between the values of thermal shock resistance obtained according to these methods was established for the corundum concretes.It was established that as compared to the concrete produced using a binder based on the Talyum cement, the concretes incorporating a hydraulically hardening (water-setting) binder prepared from the VTs-70 grade high-alumina cement exhibit better resistance to thermal stresses. With increasing strength and density of the concretes, their thermal destruction occurs catastrophically (abruptly) since the elements of a stronger structure (the concretes produced using the Talyum cement-based binder) are incapable of effectively hindering the growth of a potential crack.Modification of the structure of the concrete by introducing Cr₂O₃ additive leads not only to an increased thermal shock resistance in the high-temperature range due to an increased critical crack length, but also to an increased resistance ot crack displacement (opening).Translated from Ogneupory, No. 5, pp. 3–8, May, 1988.     

Easy one-pot method to control the morphology of polyethylene/carbon nanotube nanocomposites using metallocene catalysts

An easy one pot method is demonstrated for the controlled periodical surface coating of polyethylene over multiwalled carbon nanotubes (MWCNT) by insitu polymerization of ethylene using highly active metallocene catalysts (Cp₂ZrCl₂ and Cp₂TiCl₂) in combination with methylalumoxane. The crystallinity of the nanocomposite was increased and its morphology could be tuned from “sausage” like to “shish-kebab” in the presence of CNT depending on the experimental condition and choice of metal atom.     

Structural interpretation on the concentration dependence of molar volume in ternary aluminosilicate glasses containing oxides of the first and second groups of the periodic system

The concentration dependence of the molar volume of ternary aluminosilicate glasses, in which the content of R₂O (or RO) exceeds the Al₂O₃ content, is determined by the degree of depolymerization of the glass structural lattice. The molar volume in glasses with an equimolar content of R₂O (or RO) and Al₂O₃ is additive and depends on the partial values of the molar volume of the “silica” and “aluminate” components in the structural lattice. Translated from Steklo i Keramika, No. 5, pp. 8–11, May, 2000.     

Kinetic characteristics of extraction of the polymer component from a porous composite material

In a polychronic model of washing of LDPE out of a synthetic leather (SL) base with a selective solvent, the “kinetic nonequivalence” parameter of the polymer gel particles is the reflection of the characteristics of the material's pore structure and is not a function of the thermodynamic quality of the extractant. A decrease in the thermodynamic quality of the extractant is accompanied by an increase in the “strength” of the LDPE gel formed in the pores of the SL base. In extraction of LDPE from a SL base according to the mechanism of “capillary flow of gel,” the gel should be moved over the pores of the material in a sufficiently efficient regime of “hydrodynamic entrainment” by the stream of extractant. Translated fromKhimicheskie Volokna, No. 2, pp. 36–39, March–April, 2000.     

Various interactions in chocolate flavor

More than 300 volatile compounds have been identified in roasted cocoa beans and its products, making chocolate one of the most complicated natural flavors. Most beans, after harvesting, are subjected to a fermentation that is an important step in the formation of flavor precursors. Roasting is essential to the development of chocolate flavor both with respect to the loss of undesirable volatiles and the generating of key aroma compounds. Flavor is modified to meet demand using blends of beans and through variation in roasting conditions and the mechanical treatments employed to process beans into chocolate liquor and coating. The effect of fermentation and roasting on certain chemical properties related to flavor in chocolate is reviewed. Particular attention is given to monocarbonyls, headspace volatiles, pyrrole aldehydes and alkylpyrazines. One of 13 papers presented in the symposium “Flavor Research in Fats and Fat Bearing Foods,” AOCS Meeting, Atlantic City, October 1971. Presented in part in the symposium “Thermally Produced Flavor Components,” American Chemical Society Meeting, Washington, D.C., September 1971. Paper No. 4135 in the Journal Series of the Pennsylvania Agricultural Experiment Station.     

Features of phase transitions in electrolyzer hearth block material between joints

Phase transitions are considered for silicon oxycarbide material between joints in hearth blocks under aluminum electrolyzer operating conditions. It is established that in the electrolyzer there is formation of aluminum subcarbide, and also there is conversion of silicon carbide polytypes as a result of impurities of aluminum, sodium, magnesium, copper and iron. Due to phase transitions there is a reduction in joint strength, and therefore it is undesirable to use material based on corundum and black silicon carbide in hearth joints between blocks for aluminum electrolyzers. Use of pure α-SiO₂ free from impurities is recommended for this purpose. Translated from Novye Ogneupory, No. 4, pp. 47–52, April 2008.     

Supported gold clusters and cluster-based nanomaterials: characterization, stability and growth studies by in situ GISAXS under vacuum conditions and in the presence of hydrogen

This contribution is devoted to study of the thermal stability and growth of gold nanoparticles supported on SiO₂/Si(111) and Al₂O₃/SiO₂/Si(111) as a function of initial cluster size, support material and level of surface coverage. Experimental evidence for “flipping” of two dimensional cluster structures from vertical orientation to horizontal on the support is presented.     

Methods for determining hydration resistance of refractories

The “SPb SEC” Association has developed a standard of the organization (STO) “Refractories. Method for Determining Hydration Resistance.” The standard covers powders and articles based on magnesium oxide. The hydration resistance of powders is evaluated in terms of the change in the mass or in the grain composition of a sample after treating it with steam under pressure; the hydration resistance of articles is estimated from the change in their appearance. __________ Translated from Novye Ogneupory, No. 4, pp. 62–66, April, 2007.