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research briefs Glassy carbon microlattice structures go smaller, stronger than ever before Two years ago, scientists at Karlsruhe Institute of Technology reported that they had used 3-D laser lithography to build ceramic microlattice structures that were surprisingly strong. An organized hierarchy that consists of nanoscale building blocks gives those porous structures their strength, lead author Jens Bauer said at the time. “Because the size of the building blocks is that small, the material is much more flaw tolerant and, therefore, has a higher strength,” Bauer said. So what if the size of the building blocks was even smaller—would the material The smallest lattice in the world is visible by micrograph only. Struts and braces are 0.2 µm in diameter. Total size of the lattice is ~10 µm. be that much more flaw tolerant and that much stronger? KIT scientists set out to answer that question by shrinking their previous creation with the fabrication of smaller-yet nanolattice structures. Bauer and KIT scientists again turned to 3-D laser lithography to harden a polymer photoresist with a computer controlled laser beam. Although this method is great at fabricating intricate, precise, and tiny structures, it has just one small problem—it cannot go small enough. The KIT team wanted to build nanolattice structures so small that they fall below the resolution limit of laser lithography, which can build structures with struts as small as ~5–10 μm long and 1 μm wide. So the team devised a new technique. After fabricating microlattice structures with laser lithography, the scientists added a pyrolysis step that shrinks the lattice by 80%, resulting in über-small vitrified structures with struts shorter than 1 μm long and just 200 nm wide. During pyrolysis, firing to ~900°C in a vacuum furnace initiates a game of chemical bond rearrangement, leaving behind only glassy carbon nanolattices that are five-times smaller than comparable metamaterials. Credit: J. Bauer; Karlsruhe Institute of Technology So they are small, but are they comparably strong? “According to the results, load-bearing capacity of the lattice is very close to the theoretical limit and far above that of unstructured glassy carbon,” coauthor Oliver Kraft says in a KIT press release. “Diamond is the only solid having a higher specific stability.” The paper, published in Nature Materials, is “Approaching theoretical strength in glassy carbon nanolattices” (DOI: 10.1038/nmat4561). n Sick of the brick? Piezoelectric transformers poised to shrink power converters by A. Erkan Gurdal, S. Tuncdemir, and C.A. Randall Even though electronic devices have become significantly smaller in past decades, adapters for charging and supplying power to those devices have remained largely the same. Adapters—technically known as power converters—have not changed simply because of limitations of electromagnetic transformers. Piezoelectric materials can convert mechanical to electrical energy and vice versa. Given the solid-state nature of piezoelectric transformers, they offer a major advantage over electromagnetic transformers in their high efficiency in ultracompact volumes. Therefore, piezoelectric transformers have tremendous potential in electronic applications where size and weight matter, such as avionics and portable electronics. On the other hand, piezoelectric transformers can cost significantly more and have driving circuits that are more complicated than electromagnetic transformers. Piezoelectric transformers require the use of hard piezoelectric ceramic materials, or hard-piezoceramics, which require high temperatures (>1,200˚C) to form the ceramic structure. The most notable piezoelectric materials—Pb(Zr,Ti)O3 (PZT) and BaTiO3 (BT)—have ceramic perovskite structures. Designing devices with multilayer forms, where piezoceramic and metal layers are stacked alternatively on top of each other, can maximize power (capacitance) in compact volumes. However, multilayer structures require large amounts of metal, which is mostly limited to platinum because of the high processing temperatures required for hard piezoceramics. Because power is proportional to layer count and, therefore, the amount of electrode used, use of large amounts of precious metals can significantly increase production costs. Hence, hard-piezoceramic compositions need to be modified to lower processing (sintering) temperatures, which would allow use of cheaper and electrically and thermally competent precious metals or alloys—preferably base-metal electrodes, such as silver/silver–palladium and copper and nickel. Hard PZT is one of the most, if not the most, utilized hard piezoceramic. Unfortunately, nickel is not chemically compatible with hard PZT. However, copper seems promising considering previous results with copper cofired multilayer piezoceramics with soft PZT. 18 www.ceramics.org | American Ceramic Society Bulletin, Vol. 95, No. 3


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