Key Engineering Materials Vol. 799

Abstract: Zirconium carbide based materials were consolidated via spark plasma sintering and high-pressure high-temperature (HPHT) sintering methods. Fully dense ZrC-TiC compacts were produced by HTHP SPS technique at 1600-1700°C and were characterized by 370 GPa and 23.7 GPa values of Young’s modulus and Vickers microhardness, respectively. By the addition of 2 wt.% silicon nitride, the compacts of a full density were obtained at 1600°C; however, the further addition of Si₃N₄ resulted in a decrease in both density and hardness. Oxidation behavior in air of the ZrC-TiC and ZrC-TiC-Si₃N₄ compacts was explored by high temperature XRD method showing intensive oxidation at ~1000°C. Microstructural analysis certified that the addition of certain amount of Si₃N₄ increases the phase distribution homogeneity and contributes to the microstructure refinement.

Abstract: Reactive sintering is a process where synthesis reaction of the ceramic phases is combined with sintering (densification) of the composite. Dense lightweight titanium oxycarbide-aluminium oxide ceramic-ceramic composites were produced from titanium dioxide, carbon black as graphite source and aluminium precursors by high energy attritor milling, followed by reactive sintering. Titanium oxycarbide and aluminium oxide phases were synthesized during reactive sintering in situ. To investigate the microstructure evolution and phase formation, the specimens were sintered at different temperatures (600-1725 °C) in vacuum. Scanning electron microscopy and X-ray diffraction were used to analyze the microstructure and phase formation. Mechanical performance (hardness and fracture toughness) was evaluated.

Abstract: During production of cemented carbides hard and brittle tungsten carbide (WC) and ductile metal powders (mainly from Fe-group) are milled together. Complete milling results in a Gaussian distribution and narrow particle size range of the milled powder which promote the homogeneity and improve the properties of sintered composites. Cobalt, conventional metal employed in cemented carbides, possesses good comminution characteristics with WC powder. However, its toxicity and fluctuating price pushes researchers to find suitable alternatives and Fe-based alloys have shown most promising results. Cemented carbides with the Fe-Cr system as metal binder phase have potential to perform better than regular WC-Co composites in corrosive and oxidative environments. The goal of this paper was to prepare uniform cemented carbides powders with relatively high fraction of stainless Fe-Cr steel. To achieve a uniform powder mixture is a challenge at high ductile steel fraction. High energy milling (HEM) is a powerful technique for achieving (ultra) fine powder mixtures with narrow powder size range. HEM was carried out in a novel high energy ball mill RETSCH Emax. Milling in tumbling ball mill, which is the most widely used method, was employed for reference. Prepared powder mixtures were characterised in terms of particle size, size distribution and shape. In addition, powder mixtures were consolidated via spark plasma sintering to evaluate the effect of the milling method and the duration on the microstructure of final cemented carbide.

Abstract: Growing economy is a basis of public well-being, and its rapid growth is implemented on the expense of environmental quality; therefore, the harmonization of different aims and interests is one of the most important aspects of sustainable development concept. One of the priorities of sustainable development in Lithuania – the reduction of building and building materials industries’ impact on environment by increasing their eco and energy efficiency; consequently, the use of waste from renewable resources for the synthesis of chemical materials and production of building materials may reduce the usage of limited sources and emission of greenhouse gases. Such waste open up the opportunities to partially or fully replace traditional materials such as petroleum based polymers or cements which production emits many pollutants, uses lots of energy and natural resources. Nowadays, wood and agricultural waste are used for the production of biofuel; however, such heat source provides with short-term benefit. The use of the above mentioned waste materials for the production of bio-based thermal insulating composites would provide with long-term benefit and contribute to the improvement of currently targeted energy performance of buildings. During the study, it was obtained that the ratios of wood bark/rapeseed binder changes the main parameters of biocomposites. The obtained products are characterized by the density ranging from 329 kg/m³ to 378 kg/m³ and thermal conductivity – from 0.0665 W/(m·K) to 0.0781 W/(m·K). The compressive strength and short-term water absorption have shown that biocomposites can be used as a self-bearing structural materials due its sufficient strength which is approx. 2 MPa and water uptake varies from 20 vol.% to 40 vol.%.

Abstract: A proper recycling and secondary reuse of end-of-life car tires (ELTs) remain an important issue for many regions. Devulcanised crumb rubber as a product of ELT recycling could be used for very different applications, including site remediation by spilled oil products. In current paper a development of new bio-based composite material containing devulcanised crumb rubber and cenospheres is suggested. Designed bio-based composite can be used as a oil spill adsorbent, material for civil engineering, and as a raw material for further modification by introduction of ferromagnetic components. Homogenised peat with peat content 14.0 ± 1.5 wt%, particle size 0.01 – 0.5 mm used as a natural bio-based binder. Devulcanised crumb rubber particle size 1.0 – 4.0 mm was used as a porous substrate. Designed compositions contained 5, 10, 15, 20 wt% (before drying) of devulcanised crumb rubber. Cenospheres have duplex function for designed composite: lightweight additive that makes final product floatable, and at the same time minimises shrinkage of dried product. It was found that devulcanised crumb rubber reduces density of final composite and adding open porosity. Developed bio-based composites could be manufactured in granules by drum granulation technique for further environmental applications. Formability and apparent density of bio-based composite are studied.

Abstract: The main objective of this study was to carry out more detailed research on the effects of the amount of cement paste on the physical and mechanical properties of porous fine aggregate concrete (PFAC). Fine foamed glass aggregate (prepared of local glass breaks) and crushed expanded polystyrene aggregate (prepared of local packing tare of household equipment), ordinary Portland cement (OPC), plasticizing and air entraining admixtures, as well as pozzolanic additive – metakaolin-based waste (local waste in production process of foamed glass aggregate), were used for the preparation of forming mixtures. Fine aggregates were coated by an extremely thin layer of porous cement paste in the samples with the lowest amount of OPC (70 kg/m³), and the aggregates contact with each other mainly at the points (empty spaces between the aggregates are interconnected between each other). There were no empty spaces between the aggregates observed, and porous cement paste seems to be monolithic in the samples with the highest amount of OPC (370 kg/m³). Increased amount of OPC (from 70 to 370 kg/m³) results in denser structure, increased dry density, compressive strength, thermal conductivity coefficient and decreased water absorption of the samples.

Abstract: The change in the elastic-strength and sorption properties of epoxy polymers in the process of natural aging, taking into account the sequence of different climatic seasons effects, was studied. We studied polymers obtained on the basis of low-viscosity epoxy resins and a hardener, which is a mixture of aromatic and aliphatic di-and polyamines, modified by salicylic acid. Differences in the processes of structural relaxation and elimination of the initial nonequilibrium depending on the season of the exposure start are revealed. Mathematical models that describe changes in the elastic-strength properties of epoxy polymers during climatic aging, depending on the accumulated quantitative values ​​of the acting environmental factors, have been developed. For the first time, it was proposed to use as a variable parameter the total duration of time intervals for the studied period the surface temperature of the samples at which exceeded the glass transition temperature.

Abstract: The concept of manufacturing of near-net-shaped three-dimensional ceramic-based parts with the required geometry is motivated by design and development of advanced ceramics for high-performance applications. It is an incontestable fact, that additive manufacturing (AM), as an intensively developing smart technology, owns a unique position in modern-day product development, proposing new waste-less, cost effective and environmentally friendly pathways for manufacturing of arbitrary geometries. This work takes a new step for the synthesis of TiC/TiN lattice with geometrically defined porosity by three stage strategy (i) preparation of a TiC/Ti composite by energy saving self-propagating high-temperature synthesis (SHS) technique. (ii) preparation of designed 3D TiC/Ti shape by selective laser melting (SLM); (iii) nitridation of the consolidated samples in nitrogen environment. The influence of synthesis and nitridation conditions on the characteristics of both the powder and produced parts was investigated; set of parameters has been adjusted and optimized. Phase composition and microstructure features were examined after each procedure to reveal the physicochemical transformations and morphology evolution of the composites.

Abstract: Hydrogen is expected to be a viable solution for green-energy investment in future. However, hydrogen storage is a big challenge for stationary and mobile applications. Severe Plastic Deformation (SPD) techniques are well-known to be effective in enhancement of hydrogenation in metals hydrides. This paper shows the effect of a novel SPD technique named “High Pressure Torsion Extrusion-HPTE” on the hydrogenation of metal hydrides and compare it with the conventional method of ECAP. Results of mechanical testing and X-ray diffraction patterns showed significant enhancement in hardness and microstructural refinement in materials after HPTE. Accordingly, hydrogenation kinetics improved dramatically. This achievement could be an initiative to implement HPTE in synthesis of metal hydrides for clean energy applications.

Abstract: Sm₂O₃/ZnO nanoparticles were prepared via microwave-assisted hydrothermal and sol-gel combustion synthesis. Characteristics of obtained samples were compared in dependence of Sm₂O₃ content and calcination temperature. Prepared nanostructures were characterized with scanning electron microscopy and X-ray diffraction. Nanoparticles prepared via microwave-assisted hydrothermal and sol-gel method have flower-like and spherical shape respectively. The photocatalytic activity of samples under solar light simulated illumination was found to be affected by content of Sm₂O₃, calcination temperature and preparation method. The first-order rate constant of MB solution degradation of samples prepared via microwave-assisted hydrothermal method approximately three times exceeds that of sol-gel samples.