New Synthesis for Lanthanum Hexaboride Nanocrystals; Final technical rept. 15 May 2006-14 Nov 2007

摘要

Although lanthanum hexaboride has been used for many years as a cathode material, new applications exploiting its unusual electron emission properties are now being investigated. Nanocrystalline LaB6 materials have been predicted to provide important advantages for IR absorbers as well as for nanoelectronics in general because of their low work function. Lanthanum hexaboride is usually made by mixing La and B with Al in a furnace at 1500K. The molten flux method precipitates large crystals. This method, however, is not suitable for nanocrystal production. The authors have synthesized LaB6 nanocrystals for the first time using a low temperature sonochemical synthesis. Several different types of crystals were synthesized. First, 20-200 nm diameter crystals were synthesized using lithium borohydride and lanthanum carbonate sonicated in a tetrahydrofuran solvent. These showed a good XRD pattern, confirming the lanthanum hexaboride structure. They also showed that they can control the thickness of the crystals by adding HCl during the synthesis. A small amount of HCl acid in the synthesis leads to thinner/smaller crystalline domains and reduces the amorphous material; adding more HCl results in further thinning as observed by further significant broadening of the peaks in XRD. A different synthesis strategy using Lanthanum chloride as the La precursor resulted in smaller (several nm diameter) boron/lanthanum/oxygen particles which could likely be annealed at high T to give LaB6. Another strategy using the same precursor with water as the solvent resulted in a purple powder containing amorphous boron/lanthanum particles 3-5 nm in diameter. Thus several different routes to varied structure LaB6 nanocrystals have been demonstrated.