PNNL

Abstract

In this project, the scanning electron microscopy (SEM) sampling method used during the statistical design study (SDS) was investigated to determine if any sampling biases were present in the analyzed data. Using NIST standard particle size distribution powders (NIST 1984 SRM) with the origin wet dispersion method it was determined that a bias to smaller particles was present. This was supported by theoretical calculations using Stoke’s law to determine the settling rate of a spherical particles of roughly the same size and mass as those found in the SDS. Based on the theoretical calculations, it was determined that the settling rate for each of the 76 powder sets in the SDS could be unique based on specific particle shape and mass distributions, making a universal correction factor/formula not applicable. Therefore, priority shifted to the development of an improved wet dispersion method that significantly reduced the particle settling rate for all particle size and shapes. This was achieved by replacing the original solvent (IPA) with a heavy liquid (Lithium heteropolytungstates), which drastically slowed the settling rate and allowed for the capture of a suitable homogeneous aliquot. SEM images and MAMA analysis were conducted on the NIST Standard and comparisons between the SEM/MAMA data and that captured by a dynamic image analysis (DIA) particle size analyzer were performed. The resulting data confirmed that the new method does indeed deliver an improved representative aliquot to the SEM stub. For instance, in the NIST certificate, the average particle size is ~17.1 µm ± 2.2 µm with a normal distribution, the initial wet dispersion method delivered an average particle size of 6.1 µm with a heavy bi-modal distribution and the improved method resulting in an average particle size of 12.7 µm with a normal distribution. Although the improved method was still short of the NIST certificate average, it was determined by AFM analysis that the resulting ~20-25% reduction in size was due to particles sinking into the carbon sticky tape used for SEM imaging. It is believed that that this bias can be calibrated for in a much more predicable manner than the original settling rate bias. In addition, the matching normal distribution curves between the NIST certificate and the heavy liquid method indicate a much-improved representative aliquot has been sampled and imaged. A surrogate CeO2 powder was used to reflect PuO2 more accurately and aid in implement radiological controls and shielding. The resulting data sets from the SEM/MAMA method and the particle size analyzer give almost identical average particle size’s as well as particle distribution statistics. Future work will re-analyze several select runs from the SDS to determine if morphological signatures can be found with the improved sampling method.