Search

The paper presents the development of unique processing methods for large alumina windows, domes, and thin sheets with a fine-grained structure and high optical transmission. Gelcasting combined with post-hot isostatic pressing proved to be a suitable processing method for windows and domes. This method provided alumina ceramics with a real in-line transmission of up to 72 % at a wavelength of 633 nm and with a theoretical in-line transmission in the infrared region between 1500 and 4000 nm for 0.8 mm thick bodies. Complex-shaped bodies, domes, were prepared with minimum warping during sintering. Thin transparent alumina sheets were prepared by gel-tape casting. This novel tape casting method utilized the advantages of the gelcasting method. The real in-line transmission of the tapes was lower compared with the windows. Nevertheless, the thin tapes with a thickness below 350 μm exhibited reasonable transparency even after pressureless sintering and without a polishing of sintered samples.

Heterophase TiO2 nanoparticles formed in the presence of a very low concentration of a substance with two donor groups were studied and the mechanism of their formation was proposed. The TiO2 nanoparticles were prepared by low-temperature hydrolysis and polycondensation of Ti (IV) n-propoxide in the presence of lactic acid (LA). The phase composition of the nanoparticles was controlled by the LA/Ti molar ratio ranging from 0.02 to 3.0. At low LA/Ti molar ratios, biphasic anatase-brookite was formed, at average LA/Ti molar ratios, anatase-brookite-rutile particles were formed, and at high molar ratios, pure anatase was formed. By using the low LA/Ti molar ratios, the low-temperature colloidal synthesis made it possible to control the composition of heterophase mesoporous TiO2 nanoparticles with a high specific surface area and high photocatalytic activity in the production of hydrogen by water splitting. The highest photocatalytic activity was exhibited by anatase-brookite nanoparticles containing 46–65% brookite, synthesized at LA/Ti molar ratios in the range of 0. 033 to 0.1.

Funding number: CEITEC 2020,LQ1601 The physicochemical properties of rare-earth zirconates can be tuned by the rational modification of their structures and phase compositions. In the present work, La3+-, Nd3+-, Gd3+-, and Dy3+-zirconate nanostructured materials were prepared by different synthetic protocols, leading to powders, xerogels, and, for the first time, monolithic aerogels. Powders were synthesized by the co-precipitation method, while xerogels and aerogels were synthesized by the sol-gel technique, followed by ambient and supercritical drying, respectively. Their microstructures, thermogravimetric profiles, textural properties, and crystallographic structures are reported. The co-precipitation method led to dense powders (SBET < 1 m2 g-1), while the sol-gel technique resulted in large surface area xerogels (SBET = 144 m2 g-1) and aerogels (SBET = 168 m2 g-1). In addition, the incorporation of lanthanide ions into the zirconia lattice altered the crystal structures of the powders, xerogels, and aerogels. Single-phase pyrochlores were obtained for La2Zr2O7 and Nd2Zr2O7 powders and xerogels, while defect fluorite structures formed in the case of Gd2Zr2O7 and Dy2Zr2O7. All aerogels contain a mixture of cubic and tetragonal ZrO2 phases. Thus, a direct effect is shown between the drying conditions and the resulting crystalline phases of the nanostructured rare-earth zirconates. College of Environmental Science and Forestry, State University of New York Office of Research, Innovation and Economic Development, California State Polytechnic University, Pomona European Commission Ministerstvo Å kolství, Mládeže a TÄ lovýchovy Hungarian Scientific Research Fund European Regional Development Fund Hungarian Science Foundation

Mixtures of polymeric siloxane binder and titania were subjected to ball milling for variable time. After milling, they were spin-coated and inkjet printed onto soda-lime glass plates producing a titania-polysiloxane hybrid layers which were converted into titania-amorphous silica hybrid layers by thermal curing. Various experimental methods indicated that the ball milling process induced significant changes in the textural properties of the layer forming material. While some material properties and layer parameters show monotonous systematic decrease with milling time (particle size, surface roughness, layer thickness and haze), others show a maximum (specific surface area, photocurrent density). Thus by a careful control of the milling process we can adjust and optimize the performance of both powder and immobilized photocatalysts.

Translucent zirconia represents a favourite material for monolithic ceramic dental restorations. However, materials approaches employed so far to improve the translucency of zirconia ceramics are accompanied by a significant decline in strength. Thus, we aimed to develop dental 3Y-TZP ceramics that can provide excellent strength and, simultaneously, enhanced translucency. In this investigation, machinable tetragonal zirconia ceramics based on fine mesostructured zirconia particles stabilized with 3 mol% of yttria and prepared by the gelcasting processing method were developed. Properties of sintered samples were characterised, namely: shrinkage, density, structure, surface roughness, hardness, biaxial strength, and total forward transmittance. Zirconia ceramics with an average biaxial strength of 1184 MPa and a total forward transmittance of 46.7% for a 0.5 mm thick sample at a wavelength of 600 nm were obtained. These ceramics exhibited homogeneous structure with grains sizes up to 620 nm and purely tetragonal phase composition. The developed ceramics provided a favourable combination of high translucency comparable even with the mixed cubic/tetragonal structure of a common 4Y-TZP, and very high strength that is achievable only in the pure tetragonal 3Y-TZP.

As widely-available silicon solar cells, the development of GaAs-based solar cells has been ongoing for many years. Although cells on the gallium arsenide basis today achieve the highest efficiency of all, they are not very widespread. They have particular specifications that make them attractive, especially for certain areas. Thanks to their durability under challenging conditions, it is possible to operate them in places where other solar cells have already undergone significant degradation. This review summarizes past, present, and future uses of GaAs photovoltaic cells. It examines advances in their development, performance, and various current implementations and modifications.

This paper deals with a review of potential technical applications of piezoelectric polymer systems for Industry 4.0 applications. Polyvinylidene difluoride (PVDF) is a highly non-reactive thermoplastic fluoropolymer produced by the polymerization of vinylidene difluoride. Copolymers of PVDF with beta phase could be used in piezoelectric and triboelectric applications which are presented in this paper. PVDF is a candidate for harvesters and sensors that cannot be realized with piezoceramics or single crystals. Both technical applications for sensing and energy harvesting are presented in this paper. These technologies could be useful in mechanical engineering applications like a strain sensor, strain energy harvesting, vibration energy harvester, load sensing and triboelectric sensors/generators. The opportunity for future usage in Industry 4.0 devices is outlined and experimental results of individual systems are presented.

Electrospinning as a versatile technique producing nanofibers was employed to study the influence of the processing parameters and chemical and physical parameters of solutions on poly(vinylidene fluoride) (PVDF) fibers&rsquo, morphology, crystallinity, phase composition and dielectric and piezoelectric characteristics. PVDF fibrous layers with nano- and micro-sized fiber diameters were prepared by a controlled and reliable electrospinning process. The fibers with diameters from 276 nm to 1392 nm were spun at a voltage of 25 kV&ndash, 50 kV from the pure PVDF solutions or in the presence of a surfactant&mdash, Hexadecyltrimethylammonium bromide (CTAB). Although the presence of the CTAB decreased the fibers&rsquo, diameter and increased the electroactive phase content, the piezoelectric performance of the PVDF material was evidently deteriorated. The maximum piezoelectric activity was achieved in the fibrous PVDF material without the use of the surfactant, when a piezoelectric charge of 33 pC N&minus, 1 was measured in the transversal direction on a mean fiber diameter of 649 nm. In this direction, the material showed a higher piezoelectric activity than in the longitudinal direction.

A new piezophotocatalytically active membrane based on polyvinylidene fluoride (PVDF) nanofibers with α-Fe2O3 nanoparticles incorporated into them has been synthesized by electrospinning. The composition, morphology and structure of α-Fe2O3 containing PVDF nanofibers mats were investigated by SEM, XRD, Raman, FTIR, XPS and DSC. Studies have shown that the inclusion of α-Fe2O3 nanoparticles promotes additional crystallization and self-polarization of PVDF into an electroactive β-phase. With the help of XPS, the mechanism of interfacial ion-dipole electrostatic interaction, leading to self-polishing and crystallization of the structure, is shown. Using measurements of VB XPS, UPS and the frequency dependence of the dielectric constant, the role of polarization effects on the formation of long-lived metastable energy states in the electronic structure of PVDF near the Fermi level is shown. Piezocatalytic and piezophotocatalytic activity was assessed by the decomposition of Methylene Blue (MB) under the influence of ultrasound and UV-Vis radiation. The efficiency of piezocatalysis and piezophotocatalysis was 60.4% and 99.5%, respectively. With the help of experiments on the capture of active redox forms, it was found that ∙OH radicals are the main component of the MB degradation. Two different mechanisms of generating ∙OH radicals for piezocatalysis and piezophotocatalysis have been reasonably proposed. The stability of the catalyst was shown after five successive cycles of piezophotocatalysis.

A method for manufacturing bi-layered zirconia rods of core-shell geometry with a porous core and a dense shell has been developed. Core-shell rods were successfully prepared by thermoplastic co-extrusion of assembled feed rods composed of core and shell zirconia feedstocks. Rheological analysis and adjustment of the feedstock viscosities enabled co-extrusion of regular core-shell rods of uniform shell thickness. Tapioca starch was used as a pore-forming agent in the core feedstock. Binder removal and high temperature treatment had to be modified in order to safely remove the starch particles. Sintering analysis revealed constrained sintering of porous cores in the core-shell rods due to the rigid dense shell, which resulted in an increased porosity in the core. Defect-free core-shell rods with a core porosity of up to 40% and different thicknesses of the dense shell were prepared.

The paper specifies the electrostatic spinning process of specific polymeric materials, such as polyvinylidene fluoride (PVDF), polyamide-6 (PA6, Nylon-6) and their combination PVDF/PA6. By combining nanofibers from two different materials during the spinning process, new structures with different mechanical, chemical, and physical properties can be created. The materials and their combinations were subjected to several measurements: scanning electron microscopy (SEM) to capture topography, contact angle of the liquid wettability on the sample surface to observe hydrophobicity and hydrophilicity, crystallization events were determined by differential scanning calorimetry (DSC), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and Fourier-transform infrared spectroscopy (FT-IR) to describe properties and their changes at the chemical level. Furthermore, for the electrical properties of the sample, the dielectric characteristics and the piezoelectric coefficient were measured. The advantage of the addition of co-polymers was to control the properties of PVDF samples and understand the reasons for the changed functionality. The innovation point of this work is the complex analysis of PVDF modification caused by mixing with nylon PA6. Here we emphasize that the application of nylon during the spin influences the properties and structure (polarization, crystallization) of PVDF.

Bi-layered zirconia rods of core-shell geometry with a porous core of different core porosities and a dense shell of various shell thicknesses were investigated. Core-shell structures were successfully prepared by thermoplastic co-extrusion of assembled feedrods. For comparison, non-layered rods with different porosities and tubes were also prepared. Mechanical properties of sintered core-shell rods were determined and compared with the properties of non-layered rods and tubes. Increasing porosity in the core of the core-shell rods decreased Young’s modulus and the dense shell improved the fracture resistance of the core-shell rods against bending loading. The fracture force of core-shell rods was in all cases considerably higher than the fracture force of non-layered porous rods or tubes with the same Young’s modulus. The fracture behaviour of core-shell rods and tubes was analysed and correlated with the calculated stress distribution in these structures. The principle of the core-shell concept was described and discussed.

The method of inclusion of various additives into a polymer depends highly on the material in question and the desired effect. In the case of this paper, nitride salts were introduced into polyvinylidene fluoride fibers prepared by electrospinning. The resulting changes in the structural, chemical and electrical properties of the samples were observed and compared using SEM-EDX, DSC, XPS, FTIR, Raman spectroscopy and electrical measurements. The observed results displayed a grouping of parameters by electronegativity and possibly the molecular mass of the additive salts. We virtually demonstrated elimination of the presence of the γ-phase by addition of Mg(NO3)2, Ca(NO3)2, and Zn(NO3)2 salts. The trend of electrical properties to follow the electronegativity of the nitrate salt cation is demonstrated. The performed measurements of nitrate salt inclusions into PVDF offer a new insight into effects of previously unstudied structures of PVDF composites, opening new potential possibilities of crystalline phase control of the composite and use in further research and component design.

Bi-layer rods of core–shell geometry were prepared by thermoplastic co-extrusion of assembled feedrods. The zirconia-toughened alumina was used for the tough core and alumina for the hard shell surrounding the core. Core–shell rods with three different shell thicknesses were successfully co-extruded and regular interface between the core and the shell was achieved. The benefits of using the shell feedstock with a lower viscosity compared to the viscosity of the core feedstock were described and discussed in detail. The perfect joining between the core and the shell components achieved during co-extrusion was retained also after binder removal and sintering. The component interface was sharp and relatively smooth. The core–shell structure resulted in a modification of the mechanical properties of bi-layer rods in comparison with monolithic rods. Flexural strength tests of monolithic and bi-layer sintered rods were performed and the advantage of the core–shell concept was presented and explained.