For the first time, researchers have created a nanocomposite of ceramics and a two-dimensional material, opening the door for new designs of nanocomposites with such applications as solid-state batteries, thermoelectrics, varistors, catalysts, chemical sensors and much more.
Sintering uses high heat to compact powder materials into a solid form. Widely used in industry, ceramic powders are typically compacted at temperatures of 1472 degrees Fahrenheit or higher. Many low-dimensional materials cannot survive at those temperatures.
But a sintering process developed by a team of researchers at Penn State, called the cold sintering process (CSP), can sinter ceramics at much lower temperatures, less than 572 degrees F, saving energy and enabling a new form of material with high commercial potential.
“We have industry people who are already very interested in this work,” said Jing Guo, a post-doctoral scholar working in the group of Clive Randall, professor of materials science and engineering, Penn State. “They are interested in developing some new material applications with this system and, in general, using CSP to sinter nanocomposites.” Guo is first coauthor on the paper appearing online in Advanced Materials.
Using non-conventional methods, Christina Birkel and her colleagues in the Department of Chemistry of the TU Darmstadt produce metallic ceramics and new materials for the energy supply of the future.
The microwave oven in the laboratory of Christina Birkel, junior research group leader at the TU Darmstadt, is not only larger and significantly more expensive than the usual household device, but also more powerful and fire and explosion-proof. Birkel had the turntable and its plastic support removed. “That would have melted anyway,” she says. The chemist uses the oven for the synthesis of substances that experts call MAX phases. M stands for a transition metal, for example for titanium or vanadium, A for a main group element – usually aluminium – and X for carbon, and more rarely also nitrogen. Thus far, approximately 70 members of this family are known.
“Around the turn of the millennium, research efforts in the field of MAX phases have increased significantly,” explains Birkel. No wonder, because the materials are scratch-resistant, high-temperature stable and in many cases oxidation-resistant like a ceramic, but they also conduct electricity and sometimes have extraordinary magnetic proper ties. They are therefore also referred to as metallic ceramics. Similarly to clay minerals, MAX phases have a lamellar structure of alternating A and M-X-M layers.
Classified as a simple binary compound, magnesium diboride (MgB2) is also sometimes known as an intermetallic superconductor. It has a hexagonal crystal structure, which forms two-dimensional layers of boron in a graphite-like structure between the triangular layers of magnesium (as seen in the two images above).
MgB2‘s biggest claim to fame is its status as a relatively inexpensive superconductor. Among conventional superconductors (those that are phonon-mediated), it has one of the highest critical temperatures, at around 39 K. Though it had been known as a material to scientists for some time, its superconductivity was not discovered until 2001. Aside from being a high-temperature superconductor, MgB2 also has more than one superconducting energy gap, something theorized but rarely seen experimentally.
The bulk, polycrystalline form of the material can be easily made by exposing solid boron to magnesium vapor at high temperatures. Single crystals of the material are more difficult to form, but can be done so under high pressure. Thin films are often made through hybrid physical-chemical vapor deposition.
Applications (or possible applications) of MgB2 often take advantage of its superconductivity, such as in MRIs and tokamaks, but the material does have other suggested uses. The compound burns completely when ignited in oxygen, and so has been proposed for usage as a fuel in ramjets, or in blast-enhanced explosives and propellants.
