ReB2 ve Re7B3 : Sentezi, karakterizasyonu ve teorik hesaplamaları

The investigation of superhard materials has attracted much interest due to their applications in industry. Diamond is the hardest and most incompressible single-phase material known up to now. Several researchers have devoted themselves to produce new superhard materials, as hard as or even harder than diamond. One of those promising material is rhenium diboride (ReB2). In addition to ReB2, Re7B3 is also considered from quantum chemical calculations as another possible superhard compound. The present thesis consists of two parts: mechanical characterization of arc melted ReB2 and synthesis and comprehensive characterization of Re7B3. The controversial Vickers hardness range of ReB2 has not been cleared out by experimentalists and theoreticians. This large range, reported as 20 GPa to 48 GPa, causes an argument on whether ReB2 is a superhard material or not. In order to clarify this point and to understand the actual hardness, we have examined how hardness of ReB2 changed in correlation with the excess boron amount in the compound. Arc melting synthesis of ReB2 is only possible when surplus of boron is employed. Until now, there is no clear information how much excess should be applied. To investigate the effect of excess boron in the final product, a set of ReB2 compound was synthesized with two different boron contents, Re:B=1:4 and 1:7. By the Vickers micro-indentation hardness test, average hardness of ReB2 was measured 24 GPa. As the free boron content increased from ReB3.5 to ReB5.5, the average hardness decreased from 24.89 ±0.66 GPa to 23.88 ±0.34 GPa. It is the first result ever that compares the hardness values of ReB2 depending on the boron excess in the ReB2 structure. Hence, the inverse relationship between excess boron amount and the hardness of ReB2 was successfully proved for the first time ever. The obtained micro-hardness values are lower than the corresponding average values of 35 GPa reported by Chung et. al. 16 This may be due to the fact that the limitation effect of excess amorphous boron in the structure. Different experiments have been conduct to reduce/remove the rest (unreacted) boron from ReB2. Hereby, HNO3 treatment and cracking and re-melting methods were most successful ones. As reported by ICP-OES results, ReB3.5 decreased to ReB3.3 by nitric acid treatment, while ReB3.6 decreased to ReB2.9 by cracking and re-melting method. In addition, crystalline boron was used as a reactant instead of the amorphous powder to investigate the effect of allotropic forms and purity of boron on the ReB2 synthesis. There is no need to employ surplus of boron for the synthesis from the crystal boron. Among the Re-B system, Re7B3 has not been studied as extensively as ReB2. Recently the superconductivity of Re7B3 was discovered with the transition temperature of 3.3 K for the first time.1 Besides the superconductivity of Re7B3, there is an interest in checking the hardness properties of the Re-B compounds. Örmeci et. al. also investigated the hardness of Re7B3 by using the crystal structure and the tabulated atomic radii by the FPLO package.2 There was a basic conflict either to use B-B interaction as a bond in the calculations -14 GPa- or to not use it -20 GPa- . For deeper understanding of this phenomenon -in other words- the hardness of Re7B3, we have decided to synthesize Re7B3 as a pure powder and characterize its chemical, physical and mechanical properties. Experimental results proved that the hardness of Re7B3 is 19 GPa. So, the B-B interaction should be considered in the hardness calculations. The compound Re7B3 has not been synthesized as a pure phase so far. Usually, Re3B or ReB2 are the byproduct as a minor phase. Therefore, it is a big challenge to synthesize a single phase of Re7B3 not only because of the narrow phase region of Re7B3, but also due to the incalculable boron loss during the synthesis. In this thesis, we investigated the synthesis of Re7B3 with three different methods; synthesis by the direct reaction of elemental component at high temperature, synthesis in a metal flux, and arc melting synthesis. The preparation of Re7B3 was accomplished by the lead flux method for the first time ever. By applying arc melting method and by tuning the starting composition, we obtained Re7B3 as the main product (ca. 95 wt. %). In the second part of the thesis, the detailed synthesis method was reported with the chemical, physical and mechanical characterization results done by XRD, ICP-OES, EDXS, SEM, Vickers micro-indentation, wear test, magnetic susceptibility, and specific heat capacity measurements.