Yeni üçlü geçiş metal borürlerinin sentez ve karakterizasyonu ve süper sert renyum diborür ve Tip-I klatratları için yeni sentez metotları

The present thesis consists of three parts: synthesis and characterization of ternary transition metal borides, superhard rhenium diboride (ReB2) and clathrate of type-I. The first part describes the synthesis and characterization of ternary transition metal borides within the systems of Ta-Co-B, Ta-Ni-B, Ta-Re-B, Nb-Re-B, Hf-Ni-B, Ta-La-B and Re-Ni-B. The employed preparation technique was the solid state high temperate reactions performed in a home-made arc melting furnace. Characterization and identification of the obtained phases was done by X-ray powder diffraction. In case of Nb-Re-B system, niobium successfully substituted the tantalum atoms of Ta3Re3B4 and a novel ternary compound was synthesized with the composition of Nb3Re3B4. Except for the Re-Ni-B system, all products were phase mixtures of ternaries with already well-known binaries. A novel ternary compound was isolated with the nominal composition Re:Ni:B = 1:1:1. Optical micrographs confirmed that new ?phase? comprised two different ternary borides, the shiny one being Re and the dark one Ni-rich, respectively. The analysis of the rhenium rich phase was performed with WDXS yielding an approximate composition of Re20Ni3B11. Based on this, the empirical formula of the Ni-rich dark phase was calculated to ReNi6.67B4. Re:Ni:B = 1:1:1 was also synthesized using the lead flux method.The second part of the study is dedicated to rhenium diboride, the second hardest material known after diamond. The preparation was done for the first time in a lead flux at 1073 K which is the lowest synthesis temperature recorded for ReB2, yet. In addition, both the reaction time (from 12 h to 5 h) and the molar ratios of the reactants (from Re/B/Flux Metal = 1:2:50 to 1:2:10) was reduced dramatically. Excess lead was removed in boiling HCl solution. Characterizations of the products were performed using X-ray powder diffraction method and ICP-OES. Lead free inhesion of final product was audited with ICP-OES and EDXS yielding the chemical composition 1:1.98 which deviates only marginally from the ideal 1:2 ratio. The thermal stability of the compound was investigated up to 1373 K by DTA. The average crystallite size of the synthesized powder was measured as 933 nm by the Hall-Williamson method and could successfully be reduced to 344 nm by ball-milling. The second synthesis technique employed on ReB2 was microwave induced plasma. In this case the elements were exposed to hydrogen plasma (1200 W) for 30 minutes, resulting in formation of ReB2 and a foreign phase. To exclude that the formation of the foreign phase was initialized by the contaminations in the used amorphous B, boron powder free of Mg was employed. Experiments showed that the formation of the byproduct is ?intrinsic? and not affected by the impurities. DTA measurements confirmed that foreign phase decomposes at 948 K to ReB2 and elemental boron. Annealing process and high temperatures inhibited the formation of the unknown phase indicating that its formation under microwave conditions is the result of the moderate temperatures governing in the hydrogen plasma. Synthesis of ReB2 from stoichiometric mixture of the elements by arc melting is the most common preparation technique. In the present study, it was proved that the start from the stoichiometric amounts of elements was not a successful route, effecting the final composition of the product due to the evaporation of amorphous boron during the arc melting process. Our results reveal that starting from molar ratios of Re:B = 1:4 to 1:7 yield the best result with respect to phase purity of ReB2In the last part of the thesis, Clathrate-I compound, K8Ge44?2 was prepared via two novel methods: synthesis in liquid ammonia and by microwave induced plasma. In both cases, the formation of the final product was accompanied by the presence of elemental germanium. Hydrogen plasma was the ?method of choice? giving the highest yield of clathrate compound and the lowest value for the byproduct elemental germanium.