Composites are useful since they make it possible to combine very different properties within a single material. Nature discovered the value of composites long before us, for example the combination of hard calcium phosphate and flexible collagen in our own bones.
Composites are widely used for applications like catalysis, where catalyst particles are dispersed over a support to maximize activity or restrict sintering. If catalyst particles can be prepared at room temperature in solution then decorating a support can be fairly straightforward. But what if your catalyst and your support are both ceramic materials that require high temperature processing?
We’ve just published a new method for synthesizing composites of metal carbides or nitrides with metal oxides. It’s a simple ‘sol-gel‘ type method that starts with a homogeneous mixture of metal salts dispersed in a biopolymer. The technique relies on the different stabilities of metal oxides. If you heat a mixture of metal salts (e.g. nitrates) in an organic biopolymer under nitrogen, the first thing that happens is that the biopolymer starts to decompose, forming a carbon-rich matrix. The metal salts also decompose, normally forming nanoparticles of metal oxides. Many metal oxides will be reduced to carbides if they are heated high enough. This is a simple carbothermal reduction of the oxide by the carbon matrix. This happens at quite different temperatures though. For example Fe3O4 will transform to Fe3C at around 700 °C whereas the TiO2 to TiC transition happens much higher (>900 °C). So in theory a mixture of Ti and Fe heated to 800 °C would result in TiO2 and Fe3C.
Actually when we first tried this it seemed too good to be true that it worked so well! In the natural world, Ti and Fe exist alongside each other quite commonly as ilmenite (FeTiO3). However we’ve now shown this to work in a whole range of oxide/carbide and oxide/nitride combinations. Of course, characterizing the composites presents a whole new challenge since the particle sizes are extremely small (2-10 nm). This is where synchrotron X-ray diffraction (SPring-8) and also Small-Angle X-ray Scattering have come in handy! Our small crystallites mean that the X-ray diffraction peaks are substantially broadened and the peaks for the different crystalline phases overlap to create some challenging patterns to characterize. Synchrotron XRD gives us the resolution we need to be able to resolve the different phases and also investigate doping of phases into each other.