Investigation of Possible Superconductivity in Titanium Borides
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Investigation of Possible Superconductivity in Titanium Borides Brian S. Ahern, Stephen Bachowski, and Jane Horrigan; Rome Air Development Center, Hanscom AFB, MA 01731
Interest in the titanium boride system stems from a patent disclosure by Vahldiek in 1982. In that disclosure TiBx(x=l.6) crystals were subjected to pressures of 300,000 psi and found at room temperature. In to have vanishingly low resistivities this study we processed TiBX crystals by a variety of techniques and pressurized the samples up to 350,000 psi. No high temperature superconductivity was observed. Introduction The claims made by Vahldiek in 1982 resurfaced as an abstract submitted to the September, 1987 ACS meeting in New Orleans. Conversations with Vahldiek combined with a literature review of his past work in processing and analysis of TiBx compounds prompted our attempt to verify the claims of Crystals were vanishingly low electrical resistivities. processed by three techniques including the Vacuum Arc Verneuil technique reported in the 1982 patent disclosure. Titanium and niobium boride compounds have surprisingly low electrical resistivities. In fact they are the best electrical conductors of all metallic compounds of borides, nitrides, carbides, silicides or oxides. The electrical properties of these borides are not well described by the classical theories of conduction and further theoretical examination is warranted. Experimental Analysis of the titanium-boron phase diagram gives no evidence of an equilibrium phase compound with the stoichiometry predicted by Vahldiek [1]. The only way to obtain a bulk composition of TiB1 .6 is to mix Ti with TiB2 . Achieving samples with this stoichiometry was attempted by several processing techniques. A mixture of these powders was processed by the Vacuum Arc Verneuil technique 12] in a sealed chamber with an argon overpressure of 200 psi. The same powder mixtures 0 were hot pressed at 10,000 psi and 1700 C for 4 hours. In addition, single crystals of TiB2 8mm in diameter by 50mm long were grown by RF float zone. X-ray analysis of the Verneuil samples revealed the presence of TiB and TiB2 only in a 100% densified polycrystalline matrix. The hot pressed samples, however contained at least 30Atomic% Ti 3 B4 in addition to TiB and TiB2 . The float zone samples were highly strained single crystals of TiB2 with no included phases. The single crystals displayed the expected (1000) growth direction .
Mat. Res. Soc. Symp. Proc. Vol. 169.
1990 Materials Research Society
1114
Room temperature resistivities of 8-10 x 10-6 Ohm-cm for the float zoned single crystal samples of TiB2 compared well with reported values of 6 - 20 xl0-6 Ohm-cm [3]. Representative samples were subjected to pressures up to 350,000 psi in a belt press apparatus. The electrical resistivities were not affected by the pressurization procedure. It was suggested that perhaps only a minority phase of the material would be altered by the pressurization and the electrical measurements would be insensitive to detect the presence of a sma
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