FRACTURING OF POROUS ALUMINA INTO THIN PLATELETS

A new technique is developed, involving introduction of lithium into porous alumina and subsequent immersion of the powder in water, to fracture a porous material into angstrom-size particles. The process leads to wafer-like platelets that tend to be hexagonal in shape. The hexagonal-shaped crystallites may be a distorted alpha alumina. The X-ray powder diffraction pattern is different from known types of transition alumina. The lattice spacing of the crystallites is 7.572 Å in the c- direction and 5.537 Å in the a-direction. The platelets are stable in shape up to 1150°C and above this temperature tend to crumble to yet smaller size particles. The platelets are estimated to be less than 50 Å in thickness and from 1 to 3 μm across. In the lithiated alumina, before immersion in water, Li5AlO4, LiOH and LiAlO2 are present. We suggest at this stage a Li5AlO4 coating on the incipient platelets protects them from reaction with the lithium metal. During immersion in water, the aluminates apparently dissolve in the water (at low pH), convert to Al(OH)3 at medium pH, or form in part a Li-containing insoluble aluminate that coats the platelets at pH > 10. Thus the final composition of the platelets depends on the pH of the aqueous solution during immersion of the lithiated alumina. If the pH is 4 to 7, the platelets are lithium-free. © 1990.