Reaction path in the magnesium thermite reaction to synthesize titanium diboride
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Reaction path in the magnesium thermite reaction to synthesize titanium diboride V. Sundaram, K. V. Logan, and R. F. Speyer School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332-0245 (Received 30 September 1996; accepted 3 July 1997)
TiB2 , along with MgO and Mg3 B2 O6 , was formed by a thermite reaction between Mg, amorphous B2 O3 , and TiO2 powders in argon. The mixture 5Mg–TiO2 –B2 O3 along with binary mixtures and single components were analyzed using differential thermal analysis (DTA) and x-ray diffraction (XRD). Large (25 g) specimens were also ignited in bulk using a resistance-heated nichrome wire. The reaction path in forming TiB2 in the three component mixture was deduced. Mg reduces TiO2 and B2 O3 to form Ti and MgB2 , respectively, which in turn react to form TiB2 . In an oxidizing atmosphere, the significant speed of the reaction permitted solid state reaction to form TiB2 before atmospheric oxygen could diffuse into the powder mass and react to form oxide phases. Thermite reactions in air have the advantage (over furnace heating in air) of not providing time at elevated temperatures for Mg and intermediate products to become consumed in the formation of oxides, nor time for oxidation degradation of TiB2 .
I. INTRODUCTION
Titanium diboride (TiB2 ) is a refractory material of extreme hardness (3400 kgymm2 KHN1 ) with potential applications in cutting tool inserts, wear resistant coatings, and electrode materials for electrolysis of corrosive media. It has demonstrated good thermal shock and wear resistance as well as chemical stability. The processing methods currently being used to produce TiB2 include reduction of a mixture of boron anhydride and titanium oxide with carbon,2 borothermic reduction of TiO2 , TiOx (OH)y , or Ti(O– n–Bu)4 -derived polymers,3 and electrolysis of fused mixtures of TiO2 , B2 O3 , and the oxides and fluorides of calcium, magnesium, or sodium.2 Recently, laser-induced chemical vapor deposition has been utilized to deposit TiB2 films on ceramic substrates.4 Thermite reactions utilize the thermal energy released in the reaction to become self-sustaining. The reaction is initiated at one end of a powder compact and propagates through its length. Use of thermite reactions in the formation of advanced ceramics have shown a number of advantages. Low cost raw materials, e.g, ceramic oxides, can be used. High reaction temperatures permit rapid firing without the use of furnaces. A variety of advanced ceramics and composites such as SiC, B4 C, Al2 O3 –SiC, and MgO–B4 C have been fabricated via thermite reactions.5 In previous work at Georgia Tech, thermite synthesis involving oxidation-reduction reactions has been used to produce TiB2 and an Al2 O3 –TiB2 composite.6,7 In a recent work on the self-propagating hightemperature synthesis (SHS) of TiB2 –xAl composites, J. Mater. Res., Vol. 12, No. 10, Oct 1997
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