Silicon carbide: stability for nanometers and lightyears

In December 2013, European Space Agency ESA launched the Gaia satellite, that had to map the location and speed of about one billion stars over five years. Gaia is so accurate that it can detect a human hair – with a thickness of a few dozen microns – at a distance of about 1000 kilometers. Silicon carbide plays an important role in this satellite – the mirrors and the supporting structure for other optical parts are made of this advanced ceramic material.

Space mirror
Due to the right hardness, strength and stiffness – approximately twice as stiff as steel – and also the relatively low density and the low thermal expansion coefficient, silicon carbide became the most optimal material. These material properties make sure that silicon carbide hardly deforms. Indeed, for a correct measurement you would like to know for sure that a distant star lightyears away is really moving, and that a so-called movement is not due to an instability or inaccuracy from within the satellite itself. The mirrors are made from sintered silicon carbide powder, polished and provided with a thin layer of silicon carbide that was applied by chemical vapour deposition (CVD). This top layer was polished to an accuracy of 10 nm – to a mirror-like surface. Reflective surfaces have to be flat and smooth on a microscopical scale. The angle of the incident light beam with the mirror surface has to be equal to the angle of the reflected light beam. You can imagine that an incident light beam gets easily scattered – so reflected in several different directions, which is undesirable here – if the surface is rough and consists of a large number of small mirrors, so to speak, each with its own reflective direction.

Gaia satellite
Gaia satellite (image by ESA)

Silicon carbide
Silicon carbide as a material hardly occurs in nature, and has to be produced artificially. Large-scale industrial production makes use of a graphite electrode with an enormous electrical voltage applied over it, causing the temperature to rise to over 2000 degrees Celsius. A mixture of quartz sand (SiO2) and carbon (C) in the form of (petroleum) coke that is positioned next to this electrode reacts in a batch process over several days to form silicon carbide, with carbon monoxide as a by-product. Silicon carbide is a hard and wear-resistant material, and the form produced here is mainly used as a grinding and cutting tool.
There are a few methods for manufacturing high performance ceramic components from silicon carbide, such as are present in the Gaia. For example, pressureless sintered silicon carbide (SSiC) is produced by shaping pure silicon carbide powder with powder particles in the submicron range and additives as sintering aids using conventional ceramic shaping processes and sintering just above 2000 degrees Celsius in an inert atmosphere. The component must be given its final shape during the sintering process because silicon carbide is so hard that it can only be (post) processed using diamond. You would like to avoid this post processing as much as possible by doing the ceramic processing steps as carefully as possible, and already take the shrinkage during sintering into account in the design. In a slightly more exotic manufacturing method, a porous form of about 90% silicon carbide grains and the rest carbon is infiltrated with liquid silicon at high temperature, where the carbon reacts with silicon to result in a silicon carbide bond between the grains. Hardly any shrinkage occurs in this process, so you can accurately make large components out of this reaction-bonded or silicon-infiltrated silicon carbide (SiSiC).
Due to its high thermal conductivity and low thermal expansion coefficient, silicon carbide is well suited to withstand sudden temperature changes (thermoshocks).

Electrical properties
A property of silicon carbide that is not directly relevant for structural components is the electrical conductivity of the material. However, this also applies to a structural metal such as steel, which also conducts electricity, but where this property does not play a role in the mechanically supporting role of the steel either. Pure silicon carbide is an intrinsic semiconductor that, in terms of electrical conductivity, has a position between insulators and ‘real conductors’ such as metals. By doping silicon carbide – as is the case with silicon – with elements such as phosphorus or boron, it becomes an extrinsic semiconductor with a higher electrical conductivity. As a semiconductor, silicon carbide is, for example, used as a substrate for indium-gallium-nitride LEDs due to its small lattice mismatch. And perhaps in the future as a replacement for silicon wafers to make computer chips. The production of these wafers is somewhat similar: with silicon, a small seed crystal grows into a large silicon single crystal by growing from liquid silicon, whereas with silicon carbide, SiC powder sublimates (i.e. evaporates from the solid state) and subsequently precipitates on a seed crystal, which grows into a large single crystal as a result. In both cases, you get the wafers by cutting the single crystals into thin slices.

The material properties of silicon carbide play an important role in the immense universe, but also on a very small scale: at the ‘square nanometer’, as in the manufacture of chips in wafer steppers. For example, silicon wafers are positioned precisely and almost immobile on extremely flat substrates (‘wafer chucks’) made of silicon carbide, in order to be able to undergo the lithographic process steps properly.