Beat the Heat

Steinbeis team provides support with the development of a synthesis process for silicon carbide

Graphite is a popular material in construction. Its chemical resistance, density, and thermal stability in the absence of oxygen make it suitable for a variety of applications. In an oxygen atmosphere, however, graphite tends to burn at temperatures above 450°C, especially if it has no oxidation protection. This is where a collaborative project being carried out as part of the Central Innovation Program for SMEs (ZIM) comes in. The project is called evo-SiC (“new products made of gas-phase reacted silicon carbide for the semiconductor industry, components for thermal engineering, and chemical systems control”) and it involves Matworks (an enterprise in the Steinbeis Network), partners from industry, and Aalen University. The various parties have joined forces to research a new synthesis process for silicon carbide (SiC).

A reactor structure was developed specially for the project to provide a way to continuously produce silicon monoxide (SiO) to vary the concentration of reactive gas in a reaction chamber and thus control gas-phase reaction. This makes it possible to reinforce complex graphite geometries using silicon carbide, since the reaction on surfaces exposed to the reactive gas is channeled downward. The actual advantage of gas-phase synthesis is, that special silicon carbide structures can be produced which are not possible using conventional ceramic technology.

Compared to conventional graphite or SiC parts, the solution developed by the project team makes it possible to achieve significant cost savings, not to mention the functional advantages. KGT Graphit Technologie GmbH, the partner from industry, has several products in its range alone that should work better as (partly-)converted SiC-component. Just one example: SiC-reinforcement could be used to improve the oxidation resistance of PECVD boats. These are produced using plasma-enhanced chemical vapor deposition, making them much more durable.

For the experiments carried out for this project, the team working with the Steinbeis experts took a PECVD boat using the latest graphite technology as their starting point. The thickness of individual plates in the boat was 2.5mm so at a realistic conversion rate of 80μm/h, they could be completely converted into silicon carbide within around 16 hours. Graphite PECVD boats are subject to heavy wear during use due to oxygen plasma, but the project team expect the SiC convertion to significantly reduce this problem. Gas-phase synthesis appears to be a strong contender for the technology used on such delicate materials.

The team developed the process of SiC gas-phase synthesis by looking at three different carbon substrates that are important for this technology: an isostatic graphite (ISO-C), carbon fiber-reinforced carbon (CFC), and an amorphous carbon foam (ASC) of different densities and porosities. Scanning electron microscope analysis (SEM) carried out on the three carbon substrates showed clear SiC transformation on all three types of carbon. An SiC gradient layer formed inside the microstructure on the low-porosity ISO-C sample, with a thin but dense SiC layer formed on the surface. A thick SiC layer formed on the surface of the CFC sample showing some cracks. SiC areas where reactions took place can be seen in all of the images. The SEM images of the ASC samples show a carbon structure that nearly reacted to the silicon carbide in all areas. Due to its foam structure, the SiO-reactive gas has no difficulty penetrating the sample and this explains why there is sufficient reactive gas in all areas to convert the sample completely into silicon carbide.

Using the ASC samples as an example made it possible for the project partners to demonstrate the feasibility of completely converting materials into silicon carbide using gas-phase synthesis, without cracks and without destroying the mechanical structure. Depending on the porosity of the graphite substrate and how the process is carried out, the substrates can be completely converted or converted in gradations. As a result, for certain application areas or material requirements, a suitable graphite substrate can be specified. Given the current inevitability of the CFC samples forming cracks, the researchers believe a good option would be partial reaction, making it possible to combine “tough” CFC properties with the mechanical wear resistance of silicon carbide. Now that the project has come to a successful conclusion, the research and company partners working on the initiative agree that it provided an understanding of the mechanisms and reaction controls in a laboratory setting. A follow-on project has been started to upscale the procedure and carry out product development with backing from industry. The project team is already making good progress.