Oliver Hunold Synthesis, electronic structure, elastic properties, and interfacial behavior of icosahedral boron-rich solids Band 2017,27 ISBN: 978-3-8440-5144-5 Price: 45,80 € / 57,30 SFR |
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Boron-rich solids are commonly characterized by icosahedral clusters, where 12 B atoms form an icosahedron, giving rise to outstanding mechanical and transport properties. However, broader applications are limited due to the high synthesis temperature required to obtain the icosahedra-based crystalline structure. Utilizing high power pulsed magnetron sputtering (HPPMS), the deposition temperature may be lowered as compared to direct current magnetron sputtering by enhanced surface diffusion. Therefore, HPPMS was utilized to investigate the influence of the substrate temperature on the structural evolution of B-rich Al-Y-B thin films. The formation of the intended AlYB_1_4 phase together with the (Y,Al)B_6 impurity phase, containing 1.8 at.% less B than AlYB_1_4, was observed at a growth temperature of 800 C and hence 600 C below the bulk synthesis temperature. Based on density functional theory (DFT) calculations it is inferred that minute compositional variations may lead to formation of competing phases, such as (Y,Al)B_6. Furthermore, 800 C still limits the usage significantly. Therefore, quantum mechanical material design was applied to identify phases with even higher phase stabilities compared to AlYB14. Phase stability of T´_0.75Y_0.75B_14 (T= Sc, Ti, V, Y, Zr, Nb, Si) critically depends on the exact magnitude of charge transferred by T and Y to the B icosahedra. The highest phase stabilities have been identified for Sc_0_._7_5Y_0_._7_5B_1_4, Ti_0_._7_5Y_0_._7_5B_1_4, and Zr_0_._7_5Y_0_._7_5B_1_4. ln combination with Young´s modulus values up to 517 GPa these phases are very interesting from a wear-resistance point of view. Still high synthesis temperatures limit the use of such systems onto technologically relevant substrate materials. However, amorphous B-rich solids, which can be synthesized without additional heating, exhibit attractive mechanical and electrical properties. Within these solids a dense B network, characterized by icosahedra-like bonding, provides the required electronic structure. Young´s modulus values of amorphous T"´_0_._7_5Y_0_._7_5B_1_4 (a- T"´_0_._7_5Y_0_._7_5B_1_4, T"´ = Sc, Ti, V, Y, Zr, Nb) reach up to 393 GPa for a-Nb_0_._7_5Y_0_._7_5B_1_4. To critically appraise these predicted data experimentally, a-TiYB_1_4 and a-ZrYB_1_4 thin films were synthesized. Chemical composition analysis revealed the presence of high oxygen impurities. The measured Young´s modulus values for a-TiYB_1_4 (301±8 GPa) and a-ZrYB_1_4 (306±9 GPa) were more than 20% smaller than the predicted ones but still comparable to other hard coatings. These deviations can be rationalized based on theoretical data revealing that the presence of oxygen impurities disrupts the dense 8 network causing a concomitant decrease in molar density and Young´s modulus. This in turn highlights the possible usage of amorphous transition metal borides, which can even be synthesized at room temperature, exhibiting stiffness values of up to 82% compared to TiB_2. Therefore, as a first step towards applications, the suitability of a-AlYB_1_4 as a coating for polymer forming was investigated. The influence of surface oxidation on the interaction between CH_4 and a-AlYB_1_4 has been studied theoretically by using OFT and experimentally by ultra-high vacuum - atomic force microscopy. Theory and experiments show the same trend as interaction increases for the oxidized scenario. Oxygen chemisorption induces changes in surface bonding leading to the higher interaction for the latter case. The data serve as proof of concept for the here implemented research strategy for exploring polymer - hard coating interactions in varying atmospheres based on cerrelative experimental and theoretical methods. The results of this work contribute to the understanding of the electronic structure-elasticity relationship of icosahedral B-rich solids as weil as comprise important aspects for applications, such as the influence of impurity incorporations on the elastic properties, the formation of competing phases during synthesis, and the effect of atmosphere exposure on the interaction with a workpiece. |
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Source: INIS International Nuclear Information System DE17F4812 | |
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