No abstract

No abstract

No abstract

The development of the electronic and semiconductor branches of industry --the creation of various design elements for the new technology --has extended the field of use of single leucosapphire crystals, which have a wide range of transmission in the UV, visible, and IR regions of the spectrum combined with excellent thermomechanical properties.The large crystals were zone-grown from a melt of aluminum oxide blanks in special boats [i, 2].In order to obtain large single crystals of high purity, the aluminum oxide blanks must have a sufficiently high density and a minimum amount of impurity.The concentration of A1203 in the blanks should be not less than 99%.* The use for this purpose of sintered corundum plates made from microlite (TsM-332) measuring 7•215 mm, whose production method has been described in [8], does not meet the increasing demand from industry; it requires a significant amount of manual labor to stack them in a container and in several cases leads to the boiling of the melt when the crystals are grown.Therefore, the aim of the present study was to develop the manufacturing technology for corundum plates from 7 to 35-40 mm thick as the blanks for growing single crystals of leucosapphire of a size equal to that of the container.Corundum, deep-purified from alkalis, was used to produce the high-purity corundum [3].For this purpose, technical alumina, grade G-00, containing 99.64% of A1203, 0.02% of SiO2, 0.01% of TiO2, 0.01% of CaO, 0.01% of MgO, 0.03% of Fe203, and 0.28-0.33% of Na20, were mixed together in a ball mill with an addition of MgCI2"6H20 calculated as 0.2% of MgO [4]; the materials were fired in a batch or tunnel furnace under pilot-plant and industrial conditions. During the firing the density of the alumina increased from 3.665 to 3.95-3.99 g/cm 3 while the concentration of A1203 grew from 99.74-99.75% with a reduction in the amount of impurity Fe203 to 0.02% and of Na20 to 0.007-0.04%; the concentration of MgO in the corundum varied between 0.14-0.16%.Since the deep-purified corundum in the form of aqueous suspensions has been used up to the present time after milling and washing in HCI with seven decantations for products cast in gypsum molds [5], we studied the milling kinetics of corundum; the hammering on of iron; and the effect of these on the sintering of pressed products (no washing) and also on the concentration of impurity iron oxide in the products.The study of the milling kinetics of the deep purified corundum (dry method) in a laboratory vibromill (capacity i0 liter) showed that intense dispersion continues for ~ 4 h and the number of fractions of < 1 or 1-2 Dm increases significantly) the concentration of fractions of 2-5, 5-10, and 10-20 ~m is reduced (Fig. i). With prolonged milling for > 4 h, the concentration of all grains and, correspondingly, the specific surface and average diameter do not change (Fig. 2).Here and elsewhere mass fractions are given.

The refractories industry is generally supplied with alumina grade G-O, and less often G-O0, according to GOST 6912-74 containing not less than 98% A1203.* Recently, there has been an increase in the demand for corundum refractories containing more than 98% Al=Os and a minimum amount of silica-containing and alkaline impurities. Such articles are required in the lining and checkers of reactors in the chemical industry, high-temperature carbon reactors, the linings of high-temperature furnaces with controlled hydrogen-containing atmospheres for growing single crystals of leucosapphire, and also for making sintered chemically pure corundum articles for power engineering.The technology for laminated corundum chamotte for various high-duty goods, including gate valves for ladles used in steel casting, places very high demands on the alumina in terms of purity and phase composition [i, 2]. Milling and sintering of corundum articles is speeded up with the use of alumina containing more than 95% a-Al=03 [3, 4]. But such grades are not supplied in sufficient amounts. Moreover, the production of alumina of one and the same grade from different factories leads to irregular crystallization, and requires differences in the technology for retreatment in refractories factories.According to GOST 6912-74 the content of a-alumina in G-O0 and G-O aluminas should be not less than 25%, while in the GK alumina it equals not less than 85-95%. Alumina GK is more highly contaminated with impurities of silica, whose content is 0.15%; and in G-O alumina the amount of Si02 does not exceed 0.03%.According to TU 48-5-200-79 alumina grade GLMK is being produced with not less than 99.5% AI=03 and not less than 93% s-alumina and with reduced amounts of alkalis. It is obtained by calcining aluminum hydroxide, treating the product with sulfuric acid, followed by high-temperature calcination together with magnesium compounds in the presence of fluorinecontaining mineralizing additives. This alumina contains 0.2-0.4% magnesium oxide.Using the specifications TU 48-2853-310-81 a large batch of alumina grade GKOI was prepared for the refractories industry, containing a-Al=Os in amounts of 85-95%, but with alkali contents reaching 0.4%. A range of experimental grades is also being used containing from 25-30 to 40-45% a-alumina (grades 1-6).A study of various grades of alumina showed that the content of impurities in them is much lower than permitted by GOST and TU, and the content of alumina is 99.4 and even more than 99.5% (Table i), i.e., according to the specifications the alumina is "chemically pure$" All grades are close in composition to each other, with the exception of alumina 1 which has an increased content of SiO~, and aluminaGK, 5, and 6 with a high concentration of Na=O.The high losses upon ignition of almost all grades should be noted, as a result of the adsorption of water during storage. The highest content of AI=03 and the minimum amount of impurities are exhibited by alumina G-O and G-O0.In terms of the mineral composition the alumina...