E.S. Windsor, D.E. Newbury and J.D. Kessler

Objective: To develop a new substrate for the chemical analysis of carbonaceous particles with the Electron Probe Microanalyzer.

Problem: Particles need to be supported on substrates before they can be analyzed microscopically. In electron beam instruments, the chemical composition of the substrate is important because the substrate material itself will become excited during an analysis and contribute x-rays to spectra collected from the particles of interest. Therefore, the substrate should be composed of a material that will not interfere with the analysis of the supported particles. Traditionally, carbon planchets have been the substrates used for the majority of particle analyses in the Scanning Electron Microscope and the Electron Probe Microanalyzer. Although these carbon planchets are satisfactory for most particle applications, they will interfere with the analysis of carbon-containing particles and should not be used for such analyses. Beryllium planchets are available commercially as an alternative to carbon for analyzing carbonaceous particles. However, beryllium is undesirable because it is extremely toxic. Currently, there is no other commercially available bulk substrate for the analysis of carbonaceous particles.

Approach: Working under the Chemical Characterization of Materials Program within CSTL, we have developed a procedure to prepare substrates of elemental boron. Boron is an ideal substrate material for particle analysis. It is an elemental semiconductor of low atomic number. As such, the substrate will not charge under the electron beam and x-rays produced by electron excitation of the substrate itself will have low energy and minimal spectral interference with the particles of interest. We anticipate that these boron substrates will be useful to laboratories performing air pollution studies and other atmospheric particle analyses. Most importantly, they will provide a non-toxic alternative to beryllium where a low atomic number, non-carbonaceous substrate is required. The procedure we use to prepare these substrates will be submitted for forthcoming publication. Materials-oriented electron microscopy laboratories should easily be able to adopt our procedure that uses only basic sample preparation equipment.

Figure (A) Boron crystalline pieces. (B) Polished boron substrated mounted in epoxy

Results and Future Plans: Boron, being a very hard material (9.5 Moh's scale), polishes to a very flat specular surface. Particles are easily observed and analyzed. We have found that the sample preparation process introduces very little contamination onto the surface of the substrate. The minor carbon contamination observed is easily removed by oxygen plasma etching. Future work may involve a more thorough investigation of surface contamination by Auger electron spectroscopy. We may also investigate the possibility of preparing boron thin films for use as substrates for particle analysis.

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