Advances in Science and Technology Vol. 63

Abstract: The starch consolidation technique is commonly used for obtaining porous ceramics due to bonding and porous-maker starch characteristic during gelling process. The method adopted here improved the water drainage by using a plaster porous base (70 and 90 consistency values) improving the water drainage by action of gravity and the capillary effect. It used slip with 50 vol% solids and as precursors oxide A-1000SG and commercial cornstarch with a mass concentration varying from 10 to 40%. For comparison between the present method and the common one, slip was put in impermeable and permeable base moulds. The gelling occurred at 70°C for 2 hours and the drying at 110°C. Pre-sintering was carried out at 1000°C and the sintering at 1600°C with a plateau of 1 hour. Results showed that the plaster consistency of the mould bases was preponderant on ceramics porosity. Porosity and apparent density measurements using light and electron microscopy revealed variation of 5% of porosity from the top to the bottom of the samples.

Abstract: The purpose of this research was to design a processing route for the conversion of hazardous Municipal Solid Waste (MSW) fly ash into a glass-ceramic material, stable and safe for further industrial, structural, or ornamental applications. The process firstly consists in the transformation of a hazardous MDW fly ash into a vitrous material. Thermal analyses information was used to design the controlled heat treatment that further transformed this vitrous material into an environmentally safe composite. The final product obtained was a glass-ceramic material with randomly oriented crystals embedded in its residual amorphous matrix. The crystalline phases were identified as Nepheline and Diopside. CLSM (Confocal Laser Scanning Microscopy) was used for real-time in-situ observation of the microstructure under simulated heat treatment conditions.

Abstract: Solution–combustion is an attractive approach to synthesis of nanomaterials for a variety of applications, including catalysts, fuel cells, and biotechnology. In this paper, several novel methods based on the combustion of a reactive solution are presented. These methods include selfpropagating sol-gel combustion and combustion of impregnated inert and active supports. It was demonstrated that, based on the fundamental understanding of the considered combustion processes, a variety of extremely high surface area materials could be synthesized. The controlling process parameters are defined and discussed. Examples of materials synthesized by the above methods are presented. A continuous technology for production of nanopowders by using the solution combustion approach is also discussed.

Abstract: Microwaves (MW) at the ISM frequency of 2.45 GHz have been used to ignite the Combustion Synthesis (CS) of pressed ceramic and metallic powders mixtures, using a single-mode applicator (TE103). This experimental apparatus allows to study the separate effect of prevalent electric or magnetic field on the selective heating of the reactive powders compacts, while transferring energy and not heat before and after CS. Microwave activated combustion syntheses (MACS) of TiC-40 wt%Fe and FeAl intermetallic compounds were performed in the two different field configurations (maximum of the electric field (E) and maximum of magnetic field (H)) in order to investigate any possible difference in the reaction mechanism and in the final microstructure of the products. MACS in the maximum of E has also been applied to high temperature materials joining, with the application of a moderate pressure during the irradiation with electromagnetic waves, to promote adhesion and to reduce the newly formed compounds porosity.

Abstract: Due to the lack of specialty kinetic methods and instruments, the kinetics of fast hightemperature reactions SHS-ceramic systems has not been adequately studied. Recently, we have developed a number of methods of so-called non-isothermal kinetics (NIK) and designed instruments allowing one to obtain information about reactions of ceramic systems in a wide range of practically important temperatures and rates. The use of one of the NIK-methods (called electrothermal analysis based on the phenomena of electro thermal explosion) allows one to study kinetics of SHS of some ceramic materials characterized by the total reaction time ~ 10 μs. In ETE, both samples pressed from reagents powders or cylindrical samples made from tightly rolled foils were studied. The joule heating was accompanied by high-speed scanning of the non-stationary temperature field on its surface. Description and technical characteristics of the specialty device electrothermoanalyzer ETA-100 manufactured by ALOFT are given. Kinetic parameters of fast EM reactions for the temperatures up to 3600 K can be measured by ETE method using ETA-100. New kinetic data for fast high-temperature gasless SHS yielding individual and composite materials (including refractory carbides and borides of transition metals, silicon and boron carbides, some refractory oxides and hard alloys) are presented. At high-speed impact of the samples, the reaction rate constants were found to exceed the combustion rate constants (measured by ETA-100) by many orders of magnitude. It was concluded that the kinetic mechanisms of the corresponding fast reactions in the static conditions and under the impact are dramatically different. It was shown that SHS in ETE mode has a significant potential as a modern practical method to be used for welding of refractory and dissimilar materials, production of coarse superabrasives, etc.

Abstract: The direct 3D method of numerical simulation of gasless combustion of mechanically activated solid powder mixtures is developed. The method under consideration falls into three stages. On the first stage, a simulation of mixture structure is performed. An analysis of the structure obtained in simulations is carried out. On the second stage, the thermal conductivity of solid powder mixture is calculated. On the third stage of simulation, the ignition and combustion of each particle of the mixture is considered with taking into account of heat exchange between contacting particles. The results of numerical simulations are represented dynamically and compared with the experimental data, obtained by high-speed digital recording of mechanically activated SHS systems combustion.

Abstract: A heterogeneous model for ignition and combustion of cylindrical free-gas samples with a gas-permeable and impermeable surface, including the description of structural and phase transformations is formulated. The effective method for numerical model research, taking into account a zonal structure of combustion wave is offered. Dynamics of porous structure forming of products from a stage of ignition up to the steady mode of exothermal reaction front propagation is considered. The calculated stationary combustion rate and elongation of a burned specimen versus its diameter, particle size of a fusible component, initial porosity and pressure of inert gas are received. Experimental data are qualitatively compared with calculated ones. The change in characteristics of combustion wave in a non-stationary mode is analyzed. The structural oscillations resulting in lamination of a porous specimen in a zone of synthesis products in a self-oscillatory combustion mode are found out. The factors, which are the reason for structural oscillations occurrence are determined. Modeling for mechanical compression of a sample shows that a stabilizing effect on the process of combustion consists of additional compensation of loosening forces.

Abstract: A comprehensive review of both past and current world-wide accomplishments in the area of combustion synthesis, with special emphasis on the production of oxide and nonoxide ceramic powders, is presented. This review focuses on scale-up and production initiatives undertaken by international companies and institutions. A significant part of this review is dedicated to manufacturing of dense and porous structures involving combination of self-propagating high-temperature technique with other processing methods. Finally, new initiatives and challenges associated with this novel synthesis technique are discussed.

Abstract: The rapidly growing market demand for nanoparticles calls for cost-effective and environmentally friendly technologies for their large-scale production. We developed a new, simple, economical and energy efficient synthesis of nanostructured complex oxides from inexpensive reactant mixtures, referred to as Carbon Combustion Synthesis of Oxides (CCSO). In CCSO the exothermic oxidation of carbon nanoparticles generates a steep thermal reaction wave that propagates through the solid reactant mixture converting it to the desired complex oxide product. CCSO is a modified form of SHS that maintains the advantages of SHS and circumvents some of its disadvantages. Specifically, it enables a more economical synthesis of complex oxides. The carbon used in CCSO is not incorporated in the product and is emitted from the sample as a CO2. A large gas emission provides a convective cooling mechanism trough the sample that significant reduces local temperature and prevents product partial melting and particles growth. CCSO does not require use of expensive fuel reactants and complex equipment and can be scaled up for continuous mass production. This paper reviewed the recent progress in the synthesis of advanced complex oxides nanoparticles by using carbon combustion process.

Abstract: A synthesis study of W2B5 powder was carried out by double self-propagating hightemperature synthesis (D-SHS) from CaWO4, Mg and B2O3 mixture and followed by HCl leaching methods. In the first stage of SHS reaction, molar composition ratio of CaWO4:B2O3:Mg mixture was selected as 1:1:8 (SHS-1) and 1:2.5:8 (SHS-2). The both SHS products were then acid leached (HCl) at optimum conditions for 1 hour and 80 °C to eliminate MgO, Mg3B2O6 and Ca3(BO3)2 phases. The first leached products were consisted of W, WB, W2B5, and W2B phases. In the second stage of SHS process, the both leached products were reacted with the same amount of Mg and B2O3 mixture keeping previous initial ratio values (1:8 and 2.5:8). A successful SHS reaction was achieved only by using the mixture with leached SHS-2 product. After following acid leaching, the chemical composition of the leached D-SHS product was found to be 98.2 wt. % W2B5, 1.2 wt. % WB4, and 0.6 wt. % WB.