Some Insights Into the Process of E-beam Generation of Metal Nanoparticles From Binary Metal Hydride and Azide Precursor
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PATRICK J. HERLEY* AND WILLIAM JONES** *Department of MaterialS Science and Engineering, State University of New York, Stony Brook, N. Y. 11794-2275, USA. "**Department of Chemistry, University of Cambridge, Cambridge, CB2 1EW, UK. ABSTRACT
Exposure of micron-sized particles of inorganic metal hydrides and azides to an intense electron beam within a transmission electron microscope results in melting followed by accelerated decomposition. The energy density localized in the specimen is considerable - arguably from one of the most intense energy deposition sources available. The induced effects have been evaluated using electron beam theory and it is concluded that, to a first approximation, the phenomena are attributable to extremely rapid heating. Criteria for the use of this technique to other possible systems are identified and future experiments applying this unique technique to other materials systems are proposed. INTRODUCTION Recently we have used the high electron beam flux of a transmission electron microscope (TEM) to decompose in situ binary metal hydrides and azides [1-7]. This technique [21 produces, in one decomposition event from a single micronsized particle, thousands of nanosized particles of the metal which are radially
distributed over the holey carbon support.
The purpose of this paper is to define the criteria governing the e-beam induced decomposition event and to provide guidelines whereby this technique can be used productively in synthesizing more advanced material systems. Specifically we ask: i) Is e-beam energy alone sufficient to induce melting and decomposition i.e. is decomposition to the metal only a thermal decomposition event? If so, can the process be duplicated simply by reaction in a furnace? ii) If the decomposition is not a simple thermal event to what extent does the electron beam induce radiation damage in these materials in their molten or solid state? iii) What is the mechanism of the process whereby metals which have extremely high melting points (>1900°C) and which cannot be melted directly in the beam, can be evaporated from a molten hydride or azide and form metallic nanoparticles?
705 Mat. Res. Soc. Symp. Proc. Vol. 354 01995 Materials Research Society
EXPERIMENTAL
Briefly, the experiment consists of encompassing a suitably isolated and pristine cluster of the material with the electron beam at low magnification. A typical cluster ranges in diameter from -2.0 to 5.0 gm. The beam intensity is then slowly increased and, as the specimen vibrates and begins to shrink, the beam is focused and the flux density continuously increased. In most cases, the sample eventually melts and forms a small bead on the holey carbon surface. All apertures are removed at this stage and the full, saturated e-beam focused on the bead. The bead often "spins" and moves over the substrate, rapidly spalling out a "rain" of nanosized particles at a distance of -1-10 pm from the melt. After cooling the residual bead cannot be induced to regenerate any further particles i.e. only bulk metal
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