Enthalpies of formation of refractory borides by high-temperature direct synthesis calorimetry: RuB 1.1 and RhB 1.1
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Enthalpies of Formation of Refractory Borides by HighTemperature Direct Synthesis Calorimetry: RUB1.1 and RhBI.1 S.V. MESCHEL and O.J. KLEPPA During the past decade, we have in this laboratory pursued experimental work on the heats of formation of refractory borides. At first, these studies were based on solution calorimetry in liquid copper at 1100 ~ about 10 years ago, this method was used to obtain the enthalpies of formation of the borides of Mn, Fe, Co, and Ni. [1'21 However, we found that this approach is very time consuming; it also proved unsuitable for borides of the early transition metals. Attempts to use other liquid metal solvents for borides in solution calorimetry at 1100 ~ also failed. While trying to find a calorimetric solvent for LAB6, it was discovered that solid LaB6 (melting point 2700 ~ and solid Pt (melting point 1769 ~ in the right proportions, react smoothly and quantitatively to form a homogeneous liquid alloy at 1100 ~ TM This discovery provided the basis for the first application of a new calorimetric method, solute-solvent drop calorimetry. This method involves the drop of small capsules, which contain a solid mixture of solute and solvent from room temperature into a high-temperature calorimeter, where they generate a homogeneous liquid mixture. The "solute" is either the refractory compound which is studied or a mechanical mixture of its constituent elements. In our work on borides and other refractory compounds, the solvent usually has been a mixture of Pt, Pd, or Ni with B, Si, or Ge. During the past several years, we have used this method in calorimetric studies of a wide range of intermetallic and related compounds. Among the borides studied a r e CrB2, [4] ScB2, T i B 2 , V B 2 , [51 and YB4. [6] However the solute-solvent calorimetric method is still quite cumbersome compared to the direct synthesis method. In direct synthesis calorimetry, a powdered compact of the two elements in the right proportion is prepared first; this compact is then brought to chemical reaction either in a very small furnace completely enclosed in a room-temperature calorimeter or by dropping it from room temperature into a suitable high-temperature calorimeter. During the past several years, direct synthesis calorimetry has been applied very successfully to transition metal intermetallic compounds by Gachon. t7] However, to the best of our knowledge, the method has not previously been used for refractory borides. The phase diagram of Ru + B t8~ shows the existence of the compound RuBl. 1 which melts congruently near 1500 ~ the Rh + B diagram tsl similarly shows RhBL~ with a melting point of 1260 ~ We inferred from these facts that both of these compounds might be very
S.V. MESCHEL, Chemist, James Franck Institute, and O.J. Kleppa, Emeritus Professor, James Franck Institute and Departments of Chemistry and Geophysical Sciences, are with The University of Chicago, Chicago, IL 60637. Manuscript submitted January 22, 1991. 1680--VOLUME 22A, JULY 1991
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