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The experiments were carried out to show that it was possible to raise the chemical purity of the electrically fused alumina powders by means of the high-temperature (about 1750 °C) vacuum (about 10–5 mm Hg) roasting. Three samples of the commercial fused corundum were heat-treated, the samples having different size grading and impurity composition, after the treatment the impurity level turned out to be ten times as less: the samples were almost completely cleared off Na, Mg, K, Mn, Cu and Zn impurities, the iron content decreased by the factor of 8‒10. It was determined that the samples were built up of the porous pellets of the size of 16‒60 microns, formed out of the solid micro-particles of the size of 0,4‒0,7 microns. The pellets' specific surface area was within the 2,8 and 3,3 cm/gram.

The kinetics and mechanism of stepwise transformations underlying the isothermal bulk crystallization of As2Se3Sn x (x = 0.26, 0.40, 0.55) semiconductor glasses in the temperature range 210–310°C have been studied using 119Sn Mossbauer spectroscopy, X-ray diffraction, density and electrical conductivity measurements, and microhardness tests. The results demonstrate that small particles of the primary phase SnSe initiate heterogeneous nucleation and two-dimensional growth of crystals of the secondary, major phase As2Se3.

The process of advanced aluminum-alumina powders production for selective laser melting was studied. The economically effective method of obtaining aluminum&ndash, alumina powdery composites for further selective laser melting was comprehensively studied. The aluminum powders with 10&ndash, 20 wt. % alumina content were obtained by oxidation of aluminum in water. Aluminum oxidation was carried out at &le, 200 &deg, C. The oxidized powders were further dried at 120 &deg, C and calcined at 600 &deg, C. Four oxidation modes with different process temperatures (120&ndash, C) and pressures (0.15&ndash, 1.80 MPa) were investigated. Parameters of aluminum powders oxidation to obtain composites with 10.0, 14.5, 17.4, and 20.0 wt. % alumina have been determined. The alumina content, particle morphology, and particle size distribution for the obtained aluminum&ndash, alumina powdery composites were studied by XRD, SEM, laser diffraction, and volumetric methods. According to the obtained characteristics of aluminum&ndash, alumina powdery composites, they are suitable for the SLM process.

Simple and potentially energy efficient way for synthesis of Al/Al2O3 composites is described. Synthesis method is based on aluminum powder oxidation in water at 100-200 °C and following drying and calcination at 600 °C. Aluminum powder with average size of about 40 urn that is currently applied in additive manufacturing was used. Present work was devoted to the study of oxidation degree of aluminum and so the propagation depth and structure of Al2O3 shell in composite’s matrix. AI/Al2O3 composites were produced in different regimes with different maximum temperature and duration of the process of aluminium oxidation in water. Aluminum oxidation degree and aluminium content in obtained Al/Al2O3 were changed in the range from 5.5 % and 90.1 % to 21.3 % and 66.2 % respectively. The size of single crystals of Al2O3 in shell was changed from about 6 nm to 200 nm and the thickness of Al2O3 shell was changed from about 400 nm to 1000 nm by changing the regime for Al/Al2O3 composite production. Experimental results showed that Al2O3 shell was well clutched with aluminum core. Shell structure had cracks aroused by calcination process. Size distribution for obtained composites was very close for that of initial aluminum powder. All of these characteristics recommend the obtained Al/Al2O3 composites for additive manufacturing.

Porous multilayer graphenes have been synthesized by decomposition of hydrocarbons in a thermal plasma jet. Products of synthesis were characterized by electron microscopy, thermogravimetry, Raman spectroscopy, and X-ray diffraction. Possibility of producing a wide range of graphene materials with different morphology and structure has been shown. Influence of the experimental conditions on mesopores structure of the synthesis products has been investigated using the method of “limited evaporation.”

Glasses were synthesized by the vacuum melting method from highly pure elemental substances at a temperature of 700 °C followed by quenching of the ampoules with melts in air ranging from 500 °C, and from Tl2X and As2X3 compounds at 500 °C for 5 hours with quenching in air. X-ray amorphism, the lack of crystalline microinclusions when thin sections are viewed in the MIM-8and MIK-1 microscopes showed the glassy state and homogeneity of the obtained glasses. According to the molecular model, the correct trigonal pyramid AsX3has C3v symmetry with four vibrational modes: two symmetrical modes of stretching ν1(А1) and bond bending ν2(А1), as well as two asymmetric double-degenerate modes of stretching ν3(Е) and bond bending ν4(Е). The improper pyramid XtAsX2 with two identical bridge As–X bonds and one unbridged (terminal) short bond has lower symmetry Cs. Moreover,the degeneration of vibrations ν3(Е) and ν4(Е) is removed, and all six vibrational modes become active in the infrared spectra. Then, it can be expected that due to the lowering of symmetry С3v → Cs during the transition from the trigonal-pyramidal AsX3/2 structural units (s.u.) with the three bridge As–X bonds in the As2X3 crystals to the chained s.u. Tlq+Xq–AsX2/2(X = S, Se) with two bridge bonds and the short unbridged As–Xt bond, the absorption bands ν3(Е) and ν4(Е) in the vibrational IR spectrum of the TlAsX2 crystals are split into two bands: ν2(А', νs) and ν5(А'', νa) of the corresponding symmetric and asymmetric stretching vibrations of the As–X bonds inside theXtAsX2 pyramids with symmetry Сs,ν4(А', δs), and ν6(А'', δa) of the symmetric X–As–X and asymmetrical X–As–Xt bending vibrations(for single crystal TlAsS2 ν1 = 334, ν2 = 312, ν3 = 141, ν4 = 190, ν5 = 287, ν6 = 178 cm–1). The high frequency bands νcb and νca in the spectra of the crystals and broad absorption bands at 370–395 and 240–270 cm–1 in the spectra of glass of the corresponding TlAsSe2 and TlAsS2 can be related to the stretching vibrations of the As–Х–As bridges between the two АsХ3 pyramids, which arise during the removal of the degeneration of the intermolecular coupling mode (c) and differ in frequency(a ,b). In the IR vibrational spectra of TlAsX2 glasses, due to the disappearance of the long-range order and the presence of the dispersion of the lengths of the As–X chemical bonds, characteristic also for monoclinic crystals of TlAsX2, there are only four broad absorption bands in the wave number range of 60–400 cm–1.These bands are a superposition of 2–3 intense absorption bands in the spectra of the crystals and indicate the persistence of chained s.u. Tlq+Xq–AsX2/2 (X = S, Se) in the glass. Based on the results of DTA and thermodynamic calculations, the soft mode of synthesis of TlAsS2 and TlAsSe2 glasses uniform in composition and structure is proposed from thallium and arsenic chalcogenides. The vibrational IR spectra of polymer-chain glasses and TlAsX2 crystals are studied, the probable assignment of the observed absorption bands is performed, and the presence of a pyramidal structural unit Tlq+Xq–AsX2/2 in glasses and crystals with two bridge bonds and a shorter As–X unbridged bond is established., Стекла синтезировали методом вакуумной плавки из особо чистых элементных веществ при температуре 700 °С с последующей закалкой ампул с расплавами в воздухе от 500 °С, а также из соединений Tl2X и As2X3 при 500 °С в течение 5 ч с закалкой в воздухе. Рентгеноаморфность, отсутствие кристаллических микровключений при просмотре шлифов в микроскопах МИМ-8 и МИК-1 свидетельствовали о стеклообразном состоянии и однородности полученных стекол. В соответствии с молекулярной моделью правильная тригональная пирамида AsX3 имеет симметрию C3v с 4 колебательными модами: две симметричные моды растяжения ν1(А1) и изгиба связи ν2(А1), две асимметричные, дважды вырожденные моды растяжения ν3(Е) и изгиба связи ν4(Е). Неправильная пирамида XtAsX2 с двумя одинаковыми мостиковыми связями As−X и одной немостиковой короткой связью имеет более низкую симметрию Cs. При этом вырождение колебаний ν3(Е) и ν4(Е) снимается, и все 6 колебательных мод становятся активными в инфракрасных спектрах. С учетом вышеизложенного можно ожидать, что из-за понижения симметрии С3v → Cs при переходе от тригонально-пирамидальных структурных единиц (с.е.) AsX3/2 с тремя мостиковыми связями As−X в кристалле As2X3 к цепочечным с.е. Tlq+Xq−AsX2/2(X− S,Se) с двумя мостиковыми и короткой немостиковой связью As−Xt полосы поглощения ν3(Е) и ν4(Е) в колебательном ИК-спектре кристаллов TlAsX2 расщепляются на две полосы соответственно ν2(А', νs) и ν5(А'', νa) симметричных и асимметричных валентных колебаний связей As−X внутри пирамид XtAsX2 с симметрией Сs, ν4(А', s) и ν6(А'', a) симметричных X−As−X и асимметричных X−As−Xt деформационных колебаний (для монокристалла TlAsS2 ν1=334, ν2=312, ν3=141, ν4=190, ν5=287, ν6= 178 см−1). Высокочастотные полосы νcb и νca в спектрах кристаллов TlAsS2 (403 и 383 см−1) и TlAsSе2 (277 и 260 см−1) и широкие полосы поглощения при 370−395 и 240−270 см−1 в спектрах стекол соответственно TlAsS2 и TlAsSе2 можно отнести к колебаниям растяжения мостиков As−Х−As между двумя пирамидами АsХ3, которые возникают при снятии вырождения моды межмолекулярного сцепления (с) и различаются по частоте (а, b). В ИК колебательных спектрах стекол TlAsХ2 из-за исчезновения дальнего порядка и наличия дисперсии длины химических связей As−X, характерной также для моноклинных кристаллов TlAsX, наблюдаются лишь четыре широкие полосы поглощения в диапазоне волновых чисел 60−400 см−1. Эти полосы являются наложением 2–3 интенсивных полос поглощения в спектрах кристаллов и свидетельствуют о сохранении в стеклах цепочечных с.е. Tlq+Xq−AsX2/2(X−S,Se). На основании результатов ДТА и термодинамических расчетов предложен мягкий режим синтеза однородных по составу и структуре стекол TlAsS2 и TlAsSe2 из халькогенидов таллия и мышьяка. Исследованы колебательные ИК-спектры полимерно-цепочечных стекол и кристаллов TlAsХ2, впервые выполнено вероятное отнесение наблюдаемых полос поглощения и установлено наличие в стеклах и кристаллах пирамидальной структурной единицы Tlq+Xq−AsX2/2 с двумя мостиковыми и одной концевой более короткой связью As−X., Известия СПбЛТА, №219 (2017)

The aim of present study was the investigation of purification efficiency of industrial white fused alumina at high-temperature calcinations, in vacuum. Three powder samples of alumina with different initial purity and particle size distribution were studied. The impurities amount in the white fused alumina powder decreased by factor 10 after vacuum calcinations. White fused alumina samples were almost completely cleared of Na, Mg, K, Mn, Cu and Zn. The iron content in the studied samples decreased by factor 8–10. Comparison of the granulometric composition and specific surface area of samples, showed that samples look like sets of porous grains with the average sizes between 16–60 μm. In turn, porous grains are composed of dense micro-particles with a characteristic size of 0.4–0.7 μm. The specific surface area of grains lies in the range 2.8–3.3 cm2/g. Relatively high rate of purification takes place due to porous structure and high surface area of the samples.

A recently developed dynamic desorption technique is used for obtaining vapor isotherms on porous materials. This gravimetric technique does not require any preliminary calibration and is based on analyzing the kinetics of liquid evaporation from a porous sample under quasi-steady state conditions. The crucial feature of the technique is concerned with the fact that no vapor pressure measurements are necessary. The technique is illustrated by desorption of benzene vapors from mesoporous silica MCM-41. To calculate the pore size distribution, the Derjaguin–Broekhoff–de Boer theory in its combination with the Wheeler model for capillary condensation is used. In the calculations, the reference data on benzene adsorption on a nonporous silica gel from two different sources (published by different authors) are applied. The mean mesopore sizes estimated from desorption isotherms are shown to be in a fair agreement with the calculations through the geometrical method based on the X-ray diffraction data. The dynamic desorption technique can serve as an additional tool for the characterization of a porous media.

In this study a simple ecologically benign technology of high purity alumina production is presented. The synthesis process consists of three steps) oxidation of aluminum in water at temperature of 90 °C) calcinations of Al hydroxide in atmosphere at 1100 °C) high temperature vacuum processing of aluminum alpha oxide at 1750 °C. Oxidation of aluminum scrap was carried out under intensive mixing in water with small addition of KOH as a catalyst. It was shown that under implemented experimental conditions alkali was continuously regenerated during oxidation reaction and synergistic effect of low content alkali aqueous solution and intensive mixing worked. The product of oxidation of aluminum scrap is the powder of Al(OH)3. Then it can be preliminary granulated or directly subjected to thermal treatment deleting the impurities from the product (aluminum oxide). It was shown the possibility to produce the high-purity aluminum oxide of 5N grade (99.999 %). Aluminum oxide, synthesized by means of the proposed method, meets the requirements of industrial manufacturers of synthetic sapphire (aluminum oxide monocrystals). Obtained high pure aluminum oxide can be also used for the manufacture of implants, artificial joints, microscalpels, high-purity ceramics and other refractory shapes for manufacture of ultra-pure products.

A method for preparation of core-shell Al/Al2O3 composites with adjusted structure was proposed. It was based on two steps: fast hydrothermal partial oxidation of aluminum in continuous reactor and subsequent thermal treatment in muffle furnace to convert aluminum hydroxide to γ-Al2O3. It was shown the possibility to create metal-ceramic composites with adjustable content of metallic core by temperature of hydrothermal process. During experimental study it was established that oxidation degree for aluminum powder with average size of 22 μm was changed from 40 to almost 100 % when temperature in hydrothermal reactor changed from 280 to 350 °C. The reaction time was no more than 10 min. Obtained products were studied by scanning electron microscopy, pore structure analysis and XRD analysis. Prepared core-shell Al/γ-Al2O3 composites can be used as catalyst materials as well as for additive technologies.