A silicon carbide lollipop

Silicon carbide is a non-oxide ceramic that has attracted the interest of many researchers in recent decades as engineering material. It exhibits a range of properties that make it suitable for a myriad of advanced technological applications. It shows excellent thermal stability and thermal shock resistance, superb mechanical properties in terms of hardness and wear resistance, as well as high chemical stability. All these characteristics make it the best candidate for a large number of uses, such as filter and catalyst supports for elevated temperatures, seal pumps for automotive water pumps, heat exchangers, bearing and abrasion-resistant components or composite materials in armor protection. Biomedical applications have also been considered due to SiC biocompatibility. For example silicon carbide myocardial heart probes, bone prosthetics and coronary heart stents have been already used in medical surgeries.

For many of these technologies the development of lightweight, stiff and strong porous SiC structures will be extremely advantageous. One way to reduce weight is to use highly porous foams and microlattices, but porosity drastically reduces the strength of a ceramic material. However, there are many examples of porous natural materials (such as bone or wood) that are lightweight and strong. A common characteristic of these biological materials is the hierarchical arrangement of their structural constituents, from the macro down to the nano scale, which we are still far from replicating in synthetic structures. Another key factor is the consolidation of the walls or struts. In particular, it is important to ensure that they are free of microdefects that could compromise their strength [1]. It is therefore important to develop processing approaches that will address these two issues: structural control and wall “quality”.

In our study we employed two different processing approaches based on water-based suspensions to build ultra-light SiC structures: self-assembly of “responsive” functionalized SiC fibers able to respond to pH changes [2] and ice templating, based on directional solidification of water based suspensions to produce porous layered scaffolds. By using diverse additives to stabilize the suspensions we have been able to build SiC structures with porosities up to 98 vol%, tailoring their architectures and generating interesting functional properties. For example, the addition of graphene to the solution allows the fabrication of electrically conductive and hydrophobic SiC scaffolds.

An important challenge we face in trying to obtain ultralight but strong SiC scaffolds, is the development of an adequate sintering process to consolidate the structure. The extremely high chemical and mechanical stability of SiC is related to the covalent bonding between Si and C. In silicon carbide, the atoms are arranged in a tetrahedral structure with very short and strong bonds. But at the same time, this structure is also responsible for the limited sinterability of SiC due to the low self-diffusion coefficient of Si and C [3]. The use of sintering aids is often necessary when an external pressure is not applied. In our study we use liquid phase sintering by adding a specific amount of oxides such as Al2O3 or Y2O3 to the suspensions [4] and [5]. These additives tend to melt at lower temperatures producing a liquid phase between the SiC fibers, promoting atom diffusion and facilitating sintering.

The sintering of our SiC-fiber scaffolds has been performed in a graphite furnace under a flow of Argon and using a SiC/Al2O3 powder bed to control the atmosphere. The SiC lollipop in the photo was formed during sintering due to vapour deposition. During the firing process the SiC in the powder bed decomposes generating volatile silicon and carbon. The atmosphere in the furnace is rich in both elements creating an environment that enable deposition from the gas leading to the formation of SiC spheres. This SEM image on this issue's cover shows one of these SiC “lollipops” formed during the manufacturing of an ultralight SiC structure. We are optimizing the production of these hierarchically designed architectures for their use as strong and lightweight structural materials. These highly porous scaffolds can be also applied in thermal management, filters and catalyst supports designed to work at elevated temperatures or as ceramic reinforcement in the production of composites.

The authors would like to acknowledge the European Commission funding under the 7th Framework Programme (Marie Curie Initial Training Networks; grant number: 289958, Bioceramics for bone repair). EGT would like to thank the support of RFEC-ATL, ONRG and DARPA.

Further reading

1. L.R. Dongchan Jang, Nat. Mater., September (2013), pp. 893–898

2. E. García-Tuñon, et al., Angewandte Chem. Inter. Ed., 52 (30) (2013), pp. 7805–7808

3. L.S. Agnieszka Gubernat, J. Eur. Ceram. Soc. (2007), pp. 781–789

4. M.N.-i. Manabu Fukushima, J. Eur. Ceram. Soci. (2010), pp. 2889–2896

5. V.V.R.J. Pujar, J. Mater. Sci. Lett. (2000), pp. 1011–1014

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DOI: 10.1016/j.mattod.2015.01.020