A Novel Sample-handling Approach for Xrd Analysis with Minimal Sample Preparation

Powder diffraction ideally requires a very large number of crystallites in random orientations to be exposed to the X-ray beam. This is obtained in laboratory instruments by grinding the sample to a very fine grain size (< 10 µm) or by using fine-grained polycrystalline solids. When non-ideal powders are being characterized, one must find a means of increasing the number of crystallite orientations explored during analysis. This is typically done by moving the specimen in the beam to either analyze a larger number of crystallites or to explore more orientations of a number of crystallites. We are developing an alternate approach based on random motions generated within powdered samples to increase the effective number and orientations of grains being analyzed. Random motions are induced by sonic or ultrasonic vibrations applied to the sample holder to fluidize the powder. Depending on the design of the sample holder, one can obtain an overall flow of material through the system or internal motions within the system by granular convection. This provides greatly improved orientation statistics when samples are too coarse grained to be analyzed in a conve ntional manner. The sample holder is easily filled and emptied using vibration coupled with a directional force such as gravity. Real-time adjustment of the thickness of the sample is possible to optimize the diffracted signal in transmission geometries. This technique was demonstrated in a laboratory XRD instrument (INEL CPS120) as well as NASA's miniature XRD/XRF instrument for planetary exploration (CheMin). Dry powders and suspensions in water were loaded in flat samples and capillaries and subjected to vibration by piezoelectric actuators. A dramatic improvement in data quality for coarse-grained materials was observed. For example, powders with grain sizes as large as 150 µm were analyzed utilizing collimated X-ray beams less than 100 µm in diameter. The vibrating sample holder will be implemented in the CheMin XRD/XRF instrument proposed for the Mars '09 Mars Science Laboratory mission. The vibrating holder will enable characterization of powders delivered by an on-board rock-crusher, with no need for further grinding. This technique could also find a range of applications for high-quality analyses of difficult samples, including very small amounts of sample, as-received or grinding-intolerant material, process control, field analysis, etc.