Key Engineering Materials Vol. 850

Abstract: We studied possibility of silylation of lithiated carbohydrates using chlorosilanes. We observed that after the lithiation, the silylation is concurrent with a protonation reaction. In the case of trimethyl chlorosilane, we achieved 1:1 ratio with 36% isolated yield, while triphenyl chlorosilane gave 1:2 (silylated:reduced) ratio and 4% isolated yield. In the case of benzyl protected carbohydrate, silylation at benzylic position was observed.

Abstract: The high surface area per unit volume and core-shell configuration of electrospun nanofiber mat has a potential for drug delivery applications. Co-axial electrospinning technique resolves the limitation in the traditional transdermal drug delivery materials allowing to load a larger concentration of therapeutic agents per unit mass of the polymer. Encapsulating the drug in the fiber core provides protection and prolonged drug release. In the present study mats of core-shell PVA/PVP-FeOOH nanofibers were successfully formed by the coaxial electrospinning process for the first time. The visible light responsive FeOOH drug carrier material was incorporated in fiber core to provide non-invasive drug release on demand. The prepared materials posses visible light triggered release of organic methylene blue molecules.

Abstract: Due to its magnetic, electrical, absorption, and emission properties, nanoscale gadolinium oxide is widely used in various fields. In this research, nanocrystalline Gd₂O₃ powders and films on glass substrates have been produced by the extraction-pyrolytic method. X-ray diffraction analysis revealed the formation of single phase Gd₂O₃ with cubic crystal structure and the mean crystallite size from 9 to 25 nm in all produced materials. The morphology of samples has been characterized by scanning electron microscopy and transmission electron microscopy.

Abstract: For sintering Al₂O₃ porous ceramic were used plasma-chemical synthesed powders with a specific surface area of 30 and 50 m²/g. Sintering were carried out in various conditions: sintering in nitrogen atmosphere, in air or vacuum at 1200-1700 °C. The most suitable sintering conditions are sintering in air at 1200-1500 °C using Al₂O₃ nanopowder with a specific surface area of 50 m²/g. Samples with open porosity of 35-45% and average pore size of 100-200 nm were obtained under these conditions.

Abstract: In the present study, seven refractory compounds - TiC, NbC, TaC, WC, Ti (C,N), (Ti,Nb)(C,N) and TiN - have been produced by plasma-chemical synthesis and carbothermal reduction process of a precursor prepared by sol-gel method. The phase composition, crystallite size, morphology and specific surface area of the synthesized powders are investigated by XRD, TEM, SEM and BET respectively. Spark plasma sintering (SPS) method (up to 1850 °C, heating rate of 100 °C/min and dwelling time of 5 min.) are used for investigation of compacting of these compounds. Characteristics of the obtained ceramic materials are provided.

Abstract: Holographic grating recording and relaxation is studied in different azobenzene molecular glassy films by circularly orthogonally polarized 532 nm laser beams L and R. The readout was made by circularly polarized (R or L) 632.8 nm laser beam. Sandwich-type samples (glass-film-glass) were also studied. Maximum diffraction efficiency of 81% has been achieved in sandwich-type AR-173 film. The following relaxation features have been found: after reaching diffraction efficiency (DE) maximum no DE decay took place; DE read out by R-polarized beam was always higher than that by L-polarized beam; in sandwich-type samples DE decayed until zero when read out by R-polarization whereas DE was zero when read out by L-polarization. 50% relaxation times varied from 4 to 44 minutes, and they mainly decreased when grating period was increased. The observed relaxation peculiarities can be understood if one assumes that volume birefringence grating (VBG) is recorded followed by volume density grating (VDG) and surface relief grating (SRG) recording. R-polarization "feels" all gratings whereas L-polarization only VDG and SRG. At large exposures VDG and SRG dominate. These results confirm the conclusion made by J.Mikelsone in her 2018 PhD thesis that birefringence grating recording in azobenzene materials is a neccessary condition for SRG appearance.

Abstract: This paper presents the results of the investigation of stress relaxation at the Si-SiO₂ interface using electron paramagnetic resonance (EPR) spectra, scanning electron microscopy (SEM) image technique, deflection analysis, X-ray photoelectron spectroscopy (XPS) and C-V characterisation of metal oxide semiconductor (MOS) structures. It has been shown, on the basis of EPR, XPS, C-V, and deflection data, that the mechanism of stress relaxation depends on the oxidation conditions: temperature, cooling rate and oxide thickness. In the Si-SiO₂-Si₃N₄ system the stress relaxation occurs due to the difference in the thermal expansion coefficient of SiO₂ and Si₃N₄ material. With an appropriate choice of oxidation conditions compressive stresses in SiO₂ and tensile stresses in Si are almost equal and stress can be reduced considerably at the interface.

Abstract: In this paper has shown that the three-dimensional hollow weaving technique enables to produce a textile pressure sensor in one continuous process. Based on the multilayer fabric principle, the hollow woven fabrics can be created by connecting adjacent layers of the fabrics according to certain rules. The appropriate fabric structure has been selected and the three-layer weaving technique was used to make the textile pressure switch. The fabric structure is selected to ensure that the top and bottom layers are kept at a distance from each other. The electrically conductive tracks were embedded in the hollow structure of the fabric in bottom and middle layers. Three conditions must be fulfilled in order to create the textile switch: a) the fabric in normal condition keeps the shape required, i.e. the conductive elements are physically separated from each other; b) when the fabric is pressed, conductive elements are in contact, i.e. the switch is now in an electrically on state; c) after the pressure has been removed the fabric returns to its original position, i.e. switch is in an electrically off state. The behavior of the electrically conductive yarn and conductive tracks were tested in various ways. The stainless steel yarn woven in particular way can be used to create woven conductive tracks. Example of application: the pressure sensitive woven rug, the whole area or part of which acts as the pressure sensor or simple switch. The pressure switch in the floor coverings can turn on alarm systems or indicator lamps in the floor or wall coverings for guidance systems in public buildings.

Abstract: The experimental research work is dedicated to magnesium-based binders. Two types of magnesium oxide were added in the compositions of binders – caustic magnesia and raw materials and calcinated products. Dolomite waste material (dolomite powder) from three quarries (two in Latvia, one in Russia) was tested. Magnesium-based binders are described as low-CO₂ emission materials, helping to reduce the energy consumption in building sector. The aim of this study is to investigate the possibility to obtain eco-friendly, low-CO₂ emission binding material from local dolomite waste materials and research the physical, mechanical and thermal properties of obtained samples.

Abstract: Nowadays, the type of foamed concrete performs a group of cemented composite materials that can compete with conventionally used autoclaved aerated concrete. Improving microstructure of the foamed concrete by inventive mixing technology allows to homogenizate the mix of foamed concrete. This original research is applied to inspection on mixing technology of foamed concrete by using a planetary ball mill. The objective of this paper is to clarify the correlation between physical-mechanical properties and intensive mixing time.