Attempts were made to directionally solidify lanthanum hexaboride as uniformly spaced rods, less than one micron in diameter, in a boron matrix. Lanthanum hexaboride and boron powders were combined in a near eutectic mixture, formed into right circular cylinders and melted by RF induction heating in a pure hydrogen atmosphere using a modified internal zone melting technique.
Ball milling, uniaxial cold pressing, isostatic pressing and hot pressing were investigated as possible means of increasing the sample density before melting in the RF induction furnace. RF coupling was achieved directly in the hot pressed pellet, and indirectly through the use of a molybdenum preheater in the uniaxially cold pressed pellets. Internal zone melting and conventional crucible melting in various materials were tried.
Directional growth was observed in the pellets that were uniaxially cold pressed and preheated with a molybdenum tube sleeve. Lanthanum hexaboride fibers with an ℓ/d ratio of about ten to one and other forms of aligned microstructures were observed in selected areas of the pellet.