Effectively Pin-hole Collimated USAXS as a Premier Tool to Quantitatively Analyze Anisotropic Hierarchical Porous Microstructures*

The 2D collimated Bonse-Hart USAXS instrument was used to study the anisotropic and hierarchical pore microstructures in freeze-cast lithium titanate (Li₄Ti₅O₁₂). USAXS data with scattering vector q=0.0001Å^-1 to 0.02Å^-1 were collected at sample orientations from -10^o to 190^o. I vs. azimuthal angle polar data at q values of 0.0003Å^-1, 0.0006Å^-1, 0.001Å^-1, and 0.005Å^-1 were also collected. Six Li₄Ti₅O₁₂ samples differing by salt and gel additives and lithium titanate particle size were measured. Samples are strongly anisotropic at low q (aspect ratio of 0.3 to 0.4) for intercolumnar porosity and mildly anisotropic at high q (aspect ratio of 0.8) for interparticle pores. Models suggests the pore microstructures are comprised of two large anisotropic populations: 1μm (Φ=0.77; SA=2.88m²/cm³) and 0.5μm (Φ=0.05; SA=0.35m²/cm³) and two small, relatively isotropic populations: 38nm (Φ=0.02; SA=1.38m²/cm³) and 14nm (Φ=0.16; SA=35.3m²/cm³).