Ecoceramics: cork-based biomimetic ceramic 3-DOM foams

Abstract

Ecoceramics (environmentally conscious ceramics) [1] are a new class of biomimetic/biomorphic material that can be manufactured from renewable resources, such as wood from sustainable sources or wood wastes. The idea is to manufacture ceramics with the microstructure of wood. To this end, the wood is pyrolysed to convert it into carbon, which is nanoporous but maintains the wood's microstructure and morphology. It is possible to infiltrate this carbon matrix with a ceramic precursor, and then heat it to burn out the carbon and form the ceramic. The end ceramic product also has the microstructure of the wood template. The first such biomorphic ceramics produced were silicon carbide ‘SiC wood’ in the late 1990s [2], [3], made by impregnating the wood with TEOS (tetra-ethyl-ortho-silicate) and then heating in under nitrogen to make the carbide. A few oxide ecoceramics such as TiO2, Al2O3, ZrO2, SiO2, CeO2 and ferrites have also been reported, by heating the impregnated template in air [4], [5], [6]. Plant-based materials such as wood and vegetable fibres are natural bio-organic composites. They have a very porous cellular microstructure, which the cells use to transport water and gasses in the plant, forming a system of aligned channels or pores. Many different soft and hard woods have been use as pyrolysed templates for ecoceramics, as well as cellulose fibres and paper, charcoal, sawdust and MDF (medium density fibreboard). However, prior to the work by Pullar et al., cork had never previously been used as a precursor to make ecoceramics, despite its obvious suitability [7]. Cork is the bark of a slow growing, evergreen oak (Quercus suber L.), and Portugal is the major global cork producer (>50% of world output). The bark is harvested every 9–13 years without harming the tree, which continues to live on as a carbon sink for up to 300 years. Therefore, cork is an exceptionally sustainable and renewable resource, and furthermore, cork forests are one of the best examples of balanced conservation and development anywhere in the world. They play a key role in ecological processes such as water retention, soil conservation and carbon storage, and as cork oak trees sequester carbon in order to regenerate their bark, a harvested cork oak tree absorbs up to five times more CO2 than one that is left alone – a rare example where mankind's intervention actually helps. The cork oak forests of Portugal are also considered to be ‘Europe's Amazon forests’, supporting the greatest bio-diversity anywhere in Europe [8]. Cork has a more porous microstructure than other wood, consisting of a regular 3-DOM (three dimensionally ordered material) structure of hollow hexagonal honeycomb cells which are ∼20 μm wide, with up to 200 million cells per cm3 [9]. As such, it is an ideal natural template to form sustainable ecoceramics. Cork powder is a low cost and environmentally friendly by-product of the cork industry, it being estimated in 1997 that ∼35,000 tonnes were produced annually in Portugal, and 50,000 tonnes globally [10]. Cork powder was pyrolysed under argon in a graphite furnace at 900°C, infiltrated with a precursor salt solution or sol, and then calcined between 1000 and 1200°C in air to form the oxide ceramic. The authors have reported hexagonal ferrite [7] and CeO2 [11] cork-based ecoceramics, and the infiltration and calcination process can be found in more detail in the authors’ papers. This issue's cover of Materials Today shows a scanning electron microscope (SEM) image of cork-derived ecoceramics of Sr3Co2Fe24O41 Z-type hexagonal ferrite. The hexaferrites are hugely important magnetic materials commercially and technologically, being used in a multitude of applications, for example permanent magnets, electrical motors and transformers, actuators and sensors, information storage, mobile communications, transport, security, defence and aerospace [12]. They can absorb energy at microwave (GHz) frequencies, and therefore have applications as EM wave shielding, as well as in stealth and RAM (radar absorbing materials) technology. The material shown here is entirely ceramic, but with the very light and porous structure of cork – a lightweight ‘magnetic ceramic foam’ – with the cell dimensions and cell wall widths being retained. These ferrite ecoceramics also have excellent magnetic properties. We are also currently investigating a wide range of ceria, titania and zirconia cork-based ecoceramics for environmental and energy applications under the H2CORK project.