Papers by Keyword: Corundum

Abstract: This study focused on the production of bioactive pure cordierite ceramic and the evaluation of its bioactivity by immersing it in simulated bodily fluid (SBF) and coating it on CP-Ti utilizing the dip coating Procedure. Cordierite is created by chemical coprecipitation, which involves combining Cordierite gel with a water-based sol-gel composed of Alumina Al2O3, magnesium oxide MgO, and silicon oxide SiO2. They used a hydraulic press to form cordierite into a cylindrical shape with a pressure of 4 bar and a holding time of 30 minutes at this pressure. Firing presses reach temperatures of 1250 C^o and have a holding time of 2 hours. A Field Emission Scanning Electron Microscope (FESEM) was used to examine the microstructure, which revealed a homogenous distribution of the compounds with negligible porosities. The phase transitions that cur during the sintering Process is studied using X-ray diffraction (XRD). The structural investigation of specimens revealed the formation of the cordierite phase at 1250 C^o, as well as the development of cristobalite, spinel, protoestatite, and corundum with cordierite phase. The dip coating Procedure was utilized to cover commercial pure titanium (CP-Ti) samples with cordierite powder. CP and Tafel exploration tests show that coated CP-Ti has superior passivation than uncoated Substrate. By comparing the findings of uncoated and coated CP-Ti, this paper concludes that the cordierite system acts as a good passivation layer.

Abstract: Pure niobium substrates were coated using laser cladding method. Pure molybdenum, Yttria Stabilized Zirkonia (YSZ) and corundum (Al₂O₃) powders were used as coating materials. Coatings were deposited on specimens as seperate paths with 3÷10mm width and 40mm of length. Two different laser power 3kW and 4kW were tested during deposition. In order to assess the quality of the Mo-YSZ and Mo-Al₂O₃ coatings, the light microscopy, Scanning Electron Microscopy (SEM), chemical analysis (EDS) and Vickers hardness test investigation were performed. The surface roughness and wear volume were also measured. As a result of YSZ-Mo powder cladding on the Nb substrate the composite layers were obtained without cracks and porosity not exceeding 1 μm. In addition, an increase in hardness of about 450 HV0.5 was revealed. As a result of Al₂O₃-Mo powder cladding on the Nb substrate the composite layers with many voids and cracks were obtained for each of the cladding variants.

Abstract: The main aim of this work is to obtain a compact spherical titanium powder by the metallo-thermic reduction. The feedstock is TiO₂ (rutile), and aluminum shavings are used as the reducing metal. In the reduction process granules of titanium powder and corundum were obtained. For cleaning titanium powder from corundum, various alkaline and acid processing methods were used. When using the alkaline treatment method of the powder, obtained after reduction of sand and floury rutile by aluminum shavings, its weight decrease was 36-37%.

Abstract: This document deals with study of nanoand micro particles of artificial corundum in cement mixtures. This inert material with high hardness is usually used as fine-grained additions of concrete mixture. The most important characteristic of corundum is very high hardness (number 9 on Mohr scale). That is the reason, why is mostly added to the cement mixture primary intended for high load flooring. In this paper were studied several type of recycled corundum classified due to its origin. Because it is a recycled material, it was necessary to first examined basic physical properties such as chemical composition or granulometry. We have applied this fine grain material to cement mixes and compared their mechanical properties. For comparison measurement were used a mixture without this admixture and mixture with a corundum with similar granulometry but non-recycled origin.

Abstract: The paper describes the theory of developing the heat-resistance criterion for disposable ceramic molds exposed to poured metal, creating a computational and experimental method for computing the heat resistance by rapidly heating the mold by an electric arc, as well as analysis of comparative data on quartz, dinas, and electric-corundum molds from the standpoint of the developed criterial-evaluation theory. In this case, the heat resistance is of one-time character, as this mold sustains only a single casting. Computational and experimental data indicate a low heat resistance of β-quartz, making them non-recommendable for annealing and metal pouring if there is not supporting filler. Molds based on electric-corundum or dinas fillers are free of such drawbacks, with the latter being considerably cheaper. The use of tridymited dinas powders is effective after grinding the spent dinas lining of electric furnaces.

Abstract: The red color of ruby is caused by Cr³⁺ impurities substituting Al³⁺ in Al₂O₃ structure. The complete substitution of Al³⁺ by Cr³⁺ produces Cr₂O₃ that has the green color. In this study, we focused on the phase transition of solid solution between corundum (Al₂O₃) and eskolaite (Cr₂O₃) mineral series. A series of Al₂O₃-Cr₂O₃ samples were prepared by mixing fine powder of Al₂O₃ and Cr₂O₃ in various ratios. They were ground together and then recrystallized. The obtained crystals were ground again in powder form for X-ray absorption near edge structure (XANES) measurements to avoid polarization dependence. The measurements were carried out at the Cr K-edge absorption energy using a 13-channel array Ge detector in the fluorescence mode. The XANES spectra showed line-shape transitions for 0.125 to 100% variation of Cr₂O₃. Significant variations in transitions were found when the Cr₂O₃ content was between 20% and 80%. With Cr₂O₃ concentration increasing, the sample colors apparently turned from pink to dark green. CIELAB color index measurements were performed. The variations of line-shape spectra of the Al₂O₃-Cr₂O₃ samples were found to be consistent with the CIELAB color index results. This Cr K-edge XANES study could be useful for the research in mineral sciences.

Abstract: Polymers rank among a significant group of technical materials. One of requirements for this constructional material is to have enough resistance to a dynamic loading. Especially in the area of connecting the materials reactoplastics are used, which are distinguished for a low resistance to the dynamic loading. One solution for increasing an impact strength is filling the reactoplastics, i.e. a formation of polymer composites. The aim of the research was to evaluate an impact strength of various polymer materials (composites filled with different types of a filler, structural two-component epoxies, mixed recycled plastics, PPO – polyphenyle oxide and PS – polystyrene). The Dynstat method CSN 64 0611 was chosen for the evaluation of the impact strength. Reactoplastics matrix on the basis of two-component epoxy adhesive with high liquidity and an increased speed of a hardening GER reaches low values of the impact strength (2.54 ± 0.30 kJ·m^-2). It is evident from the experimental results the positive effect of aluminium microparticles GER MFAI (3.26 ± 0.35 kJ·m^-2), glass beads GR B134 (2.83 ± 0.29 kJ·m^-2) and microfibres of polyamide from the process of tyre recovery GER MPA (3.92 ± 0.25 kJ·m^-2) on the impact strength. The results of this experiments show the possibility of the modification of the relative fragile epoxy resin with the filler, which increases the impact strength.

Abstract: Ceramic corundum molds on hydrolysed ethyl silicate solution which have insufficient inertness to poured alloys are widely used in the production of castings from reactive metals alloys. This is due to the presence of free silica, which is a strong oxidant of such alloy components as aluminium and titanium in vacuum conditions, in the form of a high content. To solve the indicated problem using aluminium-borophosphate concentrate, chemically cured with periclase, as a silica free binder of ceramic corundum molds was suggested. The use of the combination of the indicated mold materials allows not only enabling chemical inertness of the molds, but also operating the process of mold forming. The developed technology provides the acceleration of the mold manufacturing cycle in 4...6 times, the increase the strength of the mold covers, the reduction in the heat-resistant alloy casting cost. It enables significant improvement of the quality and increase of the economic efficiency of manufacturing of precision casting from reactive metals and alloys.

Abstract: This paper studied the high temperature creep properties of high-alumina bauxite (the mass fraction of Al₂O₃ in the new ore is about 78, the following abbreviations for Al₂O₃~78). The results indicated that the Al₂O₃~78 high-alumina bauxite mainly are corundum phase after high temperature sintered.When the temperature is 1100°C, corundum exists as crystal phase and the connections between grains are directly. The creep resistance of samples is very good at this temperature and the creep rate of 50 hours heat preservation is-0.266%. When the temperature is 1200°C, liquid phase starts to produce in a large number and the creep rate in 50 hours heat preservation is-1.589%. When the temperature is 1300°C, because of the further increase on the amount of liquid phase and wetting coated corundum particle, the direct connections between corundum particles are broken and the creep resistance is greatly reduced, the creep rate in 50 hours heat preservation is-4.088%. The creep curve fitting after 25 hours indicated that the creep property shows linear relations in three different temperatures after 25 hours. When the temperature is 1200°C and 1300°C, the creep variables arise rapidly in linear which declare the creep resistance of corundum is poor and increasing with temperature go up, more corundum phase is covered by glass phase and the creep resistance reduces dramatically.

Abstract: It is found out that activation of the sintering process for refractory clay-containing raw materials (rusk kaolin and wocheinite) is provided using additives of 3d-transition elements oxides (Fe₂O₃ and MnO₂) in the amount of 2 - 5 wt. % due to formation of defective solid solutions of mullite by isovalent substitution mechanism as well as fluxing additives of alkaline and alkaline-earth oxides (Na₂O, MgO) in the amount of 2 wt. % by regulating rheological properties of silicate melts.