Středoevropský technologický institut VUT

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    Porous pseudo-substrates for InGaN quantum well growth: Morphology, structure, and strain relaxation
    (AIP Publishing, 2023-10-10) Ji, Yihong; Frentrup, Martin; Zhang, Xiaotian; Pongrácz, Jakub; Fairclough, Simon M.; Liu, Yingjun; Zhu, Tongtong; Oliver, Rachel A.
    Strain-related piezoelectric polarization is detrimental to the radiative recombination efficiency for InGaN-based long wavelength micro-LEDs. In this paper, partial strain relaxation of InGaN multiple quantum wells (MQWs) on the wafer scale has been demonstrated by adopting a partially relaxed InGaN superlattice (SL) as the pseudo-substrate. Such a pseudo-substrate was obtained through an electro-chemical etching method, in which a sub-surface InGaN/InGaN superlattice was etched via threading dislocations acting as etching channels. The degree of strain relaxation in MQWs was studied by x-ray reciprocal space mapping, which shows an increase of the in-plane lattice constant with the increase of etching voltage used in fabricating the pseudo-substrate. The reduced strain in the InGaN SL pseudo-substrate was demonstrated to be transferable to InGaN MQWs grown on top of it, and the engineering of the degree of strain relaxation via porosification was achieved. The highest relaxation degree of 44.7% was achieved in the sample with the porous InGaN SL template etched under the highest etching voltage. Morphological and structural properties of partially relaxed InGaN MQWs samples were investigated with the combination of atomic force and transmission electron microscopy. The increased porosity of the InGaN SL template and the newly formed small V-pits during QW growth are suggested as possible origins for the increased strain relaxation of InGaN MQWs.(c) 2023 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/)
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    Mechanisms of plastic deformation and fracture in coarse grained Fe-10Al-4Cr-4Y2O3 ODS nanocomposite at 20-1300°C
    (Elsevier, 2023-05-01) Gamanov, Štěpán; Luptáková, Natália; Bořil, Petr; Jarý, Milan; Mašek, Bohuslav; Dymáček, Petr; Svoboda, Jiří
    The coarse-grained Fe-10Al-4Cr-4Y2O3ODS nanocomposite (denoted as FeAlOY) has been developed by the authors and shows promising potential for high-temperature structural applications at 1000-1300 & DEG;C. Compared to classical ODS alloys, the FeAlOY contains ten times higher volume fraction of the stable Y2O3 nanodispersion, which gives the alloy its high-temperature strength. Furthermore, the high content of Al in the matrix guarantees excellent oxidation resistance. In practice, one can expect that the FeAlOY is loaded in the temperature range of 20-1300 & DEG;C due to intermittent device operation. To ensure a safe operation, it is necessary to determine the tensile strength and ductility of the FeAlOY in the whole temperature range and detect the dominant mechanisms of strengthening, plastic deformation, and fracture in the characteristic temperature ranges. Above 1100 & DEG;C the FeAlOY reaches ultimate tensile strength of 100 MPa and plasticity of 1%. However, in the temperature range of 400-600 & DEG;C, the plasticity can climb above 40%. The achieved results can also be utilized for the design of the FeAlOY pieces shaping by hot pressing. & COPY; 2023 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
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    Comparison of fatigue performance of polyethylene pipe grades in the form of extruded and compression molded specimens
    (Elsevier, 2023-01-13) Trávníček, Lukáš; Poduška, Jan; Kučera, Jaroslav; Náhlík, Luboš; Hutař, Pavel
    Due to the very long operational lifetimes (up to 100 years) of polyethylene pipes, there is a strong demand for accelerated testing of pipe materials to experimentally prove the ability of a certain material to achieve such lifetime. Among the accelerated tests of performance, the cracked round bar (CRB) test is the most convenient. The CRB test takes advantage of cyclic loading that is applied on cylindrical specimens with a circumferential notch in the middle. The specimens are usually manufactured from plates produced by compression molding from the investigated material. However, for polyethylene pipe producers, the possibility of making the specimens by extrusion would save time and lower the costs. This study is dealing with the comparison of CRB test, and a new test carried out on notched pipe samples produced by extrusion. The goal is to investigate the possibility of measuring the performance directly on extruded pipe specimens. The comparison is illustrated on experimental results of several polyethylene pipe grades of PE80, PE100 and PE100RC often used for pipe manufacture.
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    A brief introduction and current state of polyvinylidene fluoride as an energy harvester
    (MDPI, 2022-09-29) Papež, Nikola; Pisarenko, Tatiana; Ščasnovič, Erik; Sobola, Dinara; Ţălu, Ştefan; Dallaev, Rashid; Částková, Klára; Sedlák, Petr
    This review summarizes the current trends and developments in the field of polyvinylidene fluoride (PVDF) for use mainly as a nanogenerator. The text covers PVDF from the first steps of solution mixing, through production, to material utilization, demonstration of results, and future perspective. Specific solvents and ratios must be selected when choosing and mixing the solution. It is necessary to set exact parameters during the fabrication and define whether the material will be flexible nanofibers or a solid layer. Based on these selections, the subsequent use of PVDF and its piezoelectric properties are determined. The most common degradation phenomena and how PVDF behaves are described in the paper. This review is therefore intended to provide a basic overview not only for those who plan to start producing PVDF as energy nanogenerators, active filters, or sensors but also for those who are already knowledgeable in the production of this material and want to expand their existing expertise and current overview of the subject.
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    Experimental Assessment of the Elastic Properties of Exocarp-Mesocarp and Beans of Coffea arabica L. var. Castillo Using Indentation Tests
    (MDPI, 2022-04-01) Tinoco Navarro, Hector Andres; Buitrago-Osorio, Jaime; Perdomo-Hurtado, Luis; López-Guzmán, Juliana; Ibarra, Carlos A.; Rincon-Jimenez, Alexander; Ocampo, Olga; Berrio, Lina V.
    The development of selective coffee harvesting technologies requires detailed knowledge of the geometrical, physical, and mechanical properties of the subsystems of the coffee bush, including the elastic properties of the substructures of fruit and the coffee bean, which are directly related to the selectivity problem. The elastic properties of the mesocarp-exocarp and bean are not described in the literature due to the difficulty of characterizing these locally, since measuring each component of the coffee fruit structure (mesocarp-exocarp and bean) is not an easy task. However, determining the elastic properties (of the mesocarp-exocarp and bean) could help create realistic simulations as an initial estimation for selective coffee harvesting studies. The present work aims to bridge the gap in the mechanical characterization of the sub-structures of the coffee fruits by assessing the elastic properties of the mesocarp-exocarp and bean. Indentation tests were performed on eighty Coffee arabica L. var. Castillo fruits and beans, which were previously classified into four ripening stages using fruit color data in the CIELab color space. Young's modulus and indentation hardness of the mesocarp-exocarp structure and beans were calculated, applying the Oliver and Pharr indentation model and Hertz contact theory.