One-step preparation by planar broad beam ion milling for quantification of microporous microstructures and comparison to cross-section polishing

As functional materials with microscale and nanoscale porosity gain interest, the ability to rapidly characterize that porosity is increasingly important. The challenge with such small pores is that plastic damage during sample preparation by mechanical polishing may obscure important microstructural features. Alternative methods such as focused or broad beam ion cross-section milling are relatively costly and time consuming. An alternative approach to pore quantification is proposed using planar broad beam ion milling of a free surface with no other preparation. This eliminates the time and effort of sectioning and mounting for mechanical polishing, and it reduces the cost and effort compared to cross-section milling while providing a larger area of analysis. Microporous copper produced through oxide reduction was used to determine the accuracy of the approach. Cross-section broad beam ion milling was used as the control and compared to planar milling and vibratory polishing. It was found by incremental examinations during polishing that all three methods resulted in nearly equivalent pore size and morphology. Under the conditions applied, the planar ion milling approach reached a steady pore size within approximately 40 min and vibratory polishing required approximately 300 min. Additionally, planar and cross-section broad beam ion milling provide comparable crystallographic results with vibratory polishing reporting a somewhat lower result due to the lack of signal quality compared to the ion milling techniques. Planar surface milling, then, has potential as a nondestructive method to significantly reduce time and effort in preparation for microstructural analysis of porous materials.