Advances in Science and Technology Vol. 88

Abstract: Albeit widely established in plastic and metal industry, additive manufacturing technologies are still a rare sight in the field of ceramic manufacturing. This is mainly due to the requirements for high performance ceramic parts, which no additive manufacturing process was able to meet to date.The Lithography-based Ceramic Manufacturing (LCM)-technology which enables the production of dense and precise ceramic parts by using a photocurable ceramic suspension that is hardened via a photolithographic process. This new technology not only provides very high accuracy, it also reaches high densities for the sintered parts. In the case of alumina a relative density of over 99.4 % and a 4-point-bending strength of almost 430 MPa were realized. Thus, the achievable properties are similar to conventional manufacturing methods, making the LCM-technology an interesting complement for the ceramic industry.

Abstract: Three-dimensional printing has been used as a rapid freeform fabrication process to fabricate a wider range of green ceramic components with complex structures difficult to obtain using traditional ceramic fabrication process. In this study, calcia-based ceramic core composites were fabricated by three dimensional printing and sintering operation. The green bodies were printed using a CaO/TiO₂ powder mixture as a precursor material and ethylene glycol as a binder. They were sintered at 1400-1500 °C for 2 h. The phases and microstructures of these samples were characterized by X-ray diffraction and scanning electron microscopy. The effect of TiO₂ content and the sintering temperature on the density, hydration resistance and bending strength of the sintered bodies was investigated. It was found that increment of TiO₂ content and sintering temperature would result in an increase of density of the sintered bodies and then increase of hydration resistance and bending strength.

Abstract: Strong gravitational field causes the displacement or/and sedimentation of atoms in solids, by which we can changes the crystalline state or/and composition in multicomponent condensed matter. Perovskite-type doped manganite, La_1-xSr_xMnO₃ (LSMO) has unique magnetoresistance effect which is called “colossal magnetoresistance (CMR)”. In this study, the strong gravity experiment (0.40x10⁶G, 400°C, 20h) was performed on the LSMO oriented crystal to examine the change in composition or structure. The LSMO crystal whose growing crystal direction is normal to (214) plane was prepared by the floating zone method. The EPMA and XRD results of the gravity sample revealed that the La compositions decrease in the crystal grain, while the structure did not change much. The SQUID analysis showed that the magnetic property of the gravity sample had changed.

Abstract: The effect of directed laser treatment on the compacted binary mixtures Al₂O₃–Y₂O_3, Y₂O₃–TiO₂, Al₂O₃–TiO₂ and ternary mixtures Al₂O₃−Y₂O₃−TiO₂ was investigated. It was shown that phase formation occurs within the framework of binary mixtures Al₂O₃–Y₂O_3, Y₂O₃–TiO₂, Al₂O₃–TiO₂ and is accompanied by the formation of Y₃Al₅O₁₂, Y₂Ti₂O₇ and Al₂TiO₅ compounds. The set of phases and microstructures of generated ceramics depend on Al₂O₃ content and TiO₂/Y₂O₃ ratio in initial mixtures. Basing in the obtained results the tentative phase diagram for the Al₂O₃–Y₂O₃−TiO₂ system was constructed.

Abstract: Controlling the formation of condensed products during combustion of heterogeneous systems is one of the key problems in structural macrokinetics [1]. Modeling of this process at the macroscopic level should describe the change of macrostructural variables: porosity, size and shape of specimens. This paper provides the results of research in macrostructural transformations obtained using the model of a viscous compressible fluid [2].

Abstract: SHS process in the multilayer nanofoils possesses many unique properties, such as unexpectedly high propagating rate (up to 10² m/s) and extremely short time of reaction and product phase formation (10^-7 – 10^-6 s). Understanding of the mechanism of this process has critical significance not only for the theory of SHS, but also for various applications of the reactive nanofilms, e.g., joining of dissimilar materials and items. An overview of new experimental results shows existence of complex structure of the reaction waves in the multilayer foils and reveals some new “solid-flame” combustion phenomena at macroscopic level. Comparison of these data with the study of the micro- and nano-scale processes by different experimental methods sheds new light on the intrinsic mechanism of the process. The roles of the melts, reactive exothermic dissolution and methastable phases are discussed.

Abstract: This study contains results of carbonaceous SHS-refractory materials application for binding of the graphite products and melting of metals in the induction furnaces. The opportunity of producing strong graphite-graphite bond up to 5 MPa by means of the carbonaceous refractory material that demonstrated high chemical stability in the aggressive liquid metals and alloys environment has been shown. The results of the industrial tests of melting crucibles made of carbonaceous SHS-refractory materials have been presented in the case of aluminium melting. It has been shown that such crucibles stability is 5-6 times higher than that of standard graphite crucibles in aluminium melting conditions. The obtained research results testify that developed carbonaceous material is applied for lining of the induction furnace of melting unit is allow to increase the number of nonferrous metals (bronze) melting cycles from 5 to 6 times in comparison with the traditional graphite crucible melting. High chemical stability of the material to oxidizing environment as well as to metal melts is provided by formation of high-melting compounds in the carbonaceous exothermic systems during SHS-process.

Abstract: A combustion synthesis method has been developed for synthesis of Eu²⁺-doped CaAlSiN₃ phosphor and its photoluminescence properties were investigated. Ca, Al, Si, and Eu₂O₃ powders were used as the Ca, Al, Si and Eu sources. NaN₃ was added as both solid-state nitrogen source and reducing agent and NH₄Cl was added as catalytic agent. These powders were mixed and pressed into a compact which was then wrapped up with an igniting agent (Mg+Fe₃O₄). The wrapped reactant compact was ignited by electrical heating under a N₂ pressure of 0.2-1.0 MPa. Effects of experimental parameters on product yield and photoluminescence properties were investigated. The synthesized CaAlSiN₃:Eu²⁺ phosphor absorbs light in the region of 200-600 nm and shows a broad band emission in the region of 500-800 nm due to the 4f⁶5d¹→4f⁷ transition of Eu²⁺. The peak emission intensity is ~106 % of a commercially available phosphor, YAG:Ce³⁺(P46-Y3), and the peak emission wavelength ranges from 610 to 680 nm depending on the Eu²⁺ concentration upon excitation at 460nm. The synthesized phosphor also shows excellent thermal and chemical stability and thus has a potential application as a red phosphor for white LED lighting.

Abstract: The consolidation of refractory ceramic powders at relatively milder conditions with respect to conventional methods represents an important target to achieve. Based on results recently reported in the literature, it is possible to state that the combination of the Self-propagating High-temperature Synthesis (SHS) with the Spark Plasma Sintering (SPS) technologies provides a useful contribution in this direction. Specifically, the two-steps processing route consisting in the synthesis of the ceramic powders by SHS and their subsequent densification by SPS is successfully utilized to obtain various dense MB₂-based materials (M= Zr, Hf, Ta). In this regard, an important role is played by the SHS process, particularly for the synthesis of composite powders. Indeed, stronger interfaces are established among the different phases formed in-situ, so that diffusion phenomena are promoted during SPS. Additional benefits are produced by the use of the latter technology, due to the direct passage of the electric current through the powders undergoing sintering and the die containing them.

Abstract: As examples of the most typical methods to determine the shear strength of SiC/SiC composite joints, the asymmetrical four point bending test of a butt-joined composite, the tensile test of a lap-joined composite, and the compression test of a double-notched composite joint were analyzed by using a finite element method with the interface element. From the results, it was found that the shear strength in the asymmetrical bending test was controlled by both the surface energy and the shear strength at the interface regardless of their combination while the strength in the tensile test or the compression test was governed by the surface energy when both the surface energy and the shear strength were large. In addition, the interface element was employed in order to examine the influence of the specimen geometry on the microstructural fracture morphology in nanoSiC/SiC composite during a miniaturized Double Notch Shear (DNS) test. From the serial computations, it is revealed that a relationship between the inter-laminar shear strength and the yield stress seems to be very important for selecting appropriate specimen geometry of the miniaturized DNS test.