Magnetic wood-based biomorphic Sr_3Co_2Fe_(24)O_(41) Z-type hexaferrite ecoceramics made from cork templates

摘要

Ecoceramics (environmentally conscious ceramics) are biomimetic/biomorphic ceramics, which use a naturally occurring and sustainable material as a template for their unique morphology and structure. Usually woods (or lignocellulosics) are used, due to the inherent cellular nature of their microstructures. The wood is pyrolised and the resulting carbon skeleton impregnated with a fluid, and this is then heated to combust the carbon template and convert the fluid precursor into a ceramic, while maintaining the structure of the original natural template. For the first time, ecoceramics have been made from cork, a totally sustainable wood that is harvested without harming the tree. Also for the first time, ecoceramics have been made of soft magnetic Z-type hexaferrites, in this case the room temperature multiferroic strontium Z ferrite Sr_3Co_2Fe_(24)O_(41) (SrZ). Cork powder was pyrolised at 1000 ℃, infiltrated with an aqueous sol-gel SrZ precursor, and then heated at 1200 ℃/2 h to produce the ecoceramic. The cellular structure of the cork was maintained, with a small reduction in the hexagonal cell dimension to 10 μm diameter, but the cell walls remained 1-2 μm thick, of a similar magnitude to the hexaferrite grain size. Both magnetic and XRD data agreed that there was a small portion of the SrW phase present in these ecoceramics as well, and the magnetic loop showed a magnetically soft ecoceramic with M_s = 59.5 A m~2 kg~(-1) (at 3 T), and a low H_c of 16 kA m~(-1).