The art of engineering nothing (or what we can do to ‘design’ porous ceramics)
by Kevin Plucknett and Brian Lilley, 19 March 2012
Ceramics have a wide range of performance advantages when compared with other materials. They invariably exhibit higher hardness and stiffness than metals or polymers, have significantly better resistance to wear and corrosion, and generally are of lower density. As a consequence, ceramics are employed in a diverse variety of applications, from simple building materials through to space shuttle thermal tiles, wear resistant coatings and automotive turbocharger rotors.
Fig. 1 – Scanning electron microscope image of the microstructure of a porous, high strength silicon nitride ceramic. The elongated whisker morphology develops in-situ during high temperature sintering (courtesy Mr. D. Gould, Dalhousie University)
In spite of these excellent properties, ceramics tend to be brittle in nature and are therefore highly susceptible to internal and surface flaws, including voids or pores. As a consequence, when working with brittle engineering ceramics we are invariably trying to eliminate or at least minimise any residual porosity in the material. The reason for this is simple; the pores can act in a manner to concentrate applied stress, thereby lowering the component’s strength. Engineering ceramics therefore tend to be expensive to manufacture, as processing to eliminate porosity can be challenging.
Some applications however may require controlled porosity to be retained, such as filtration of liquids and particulates, catalysis or bio-implants, and even to have a designed structure. Why could the design of ceramic gradients, porosity, and directionality be of importance to architecture? Considering ceramic materials as passive environmental modulators of temperature and humidity, porosity allows for absorption, directionality allows for transportation, and a gradient allows for a dynamic combination of heat energy and moisture content. The design of the material can be seen as a combination of these functions, together with lightweight structural capabilities.
Fig. 2- Simple schematic of functional grading in a laminated ceramic, showing changing fugitive pore filler content.
So how can we 'engineer' porosity into materials? Perhaps as importantly, how do we do this without influencing the strength too catastrophically? As a first step we can look at how conventional engineering ceramics are toughened, so any residual porosity has a less detrimental effect. Mechanisms to achieve this include incorporation of whiskers or fibres, much like straw is added to clay for building purposes, which promotes crack deflection and bridging (both of which increase the amount of energy needed to continue crack propagation). However, adding whiskers is problematic, largely due to health concerns (i.e. inhalation into the lungs).
This challenge can be avoided for some materials, as it is possible to grow whiskers in-situ, such as silicon nitride. Through careful control of the processing conditions (i.e. sintering additives and temperature), these ceramics can be prepared with moderately high porosity (e.g. ~30 vol. %) while still retaining reasonable strengths (up to ~400 MPa) due to the in-situ grown whiskers (Fig. 1). This approach is based on ‘partial sintering’, so that porosity is retained because the material is not processed to full density; in this instance the porosity is fine scale (i.e. nm to µm range) and does not degrade the strength too significantly.
The ‘design’ of porosity can be taken much further than this simple illustrative example. ‘Fugitive’ fillers can be employed that are removed at some stage of processing, leaving remnant pores that resemble the original filler material in their shape. Removal, or ‘burn out’, of the fillers can be typically achieved by heating in air up to ~400°C, and generally leaves no residue. Ideally the fugitive component can be removed at a temperature that is sufficiently low so as to not degrade the actual material being processed. Examples of common fillers include carbon (e.g. graphite) or biomaterials, such as starches. In the second case, the starch is effectively a renewable resource, and contains only ~25 atomic percent carbon, which is more environmentally acceptable; starches also have the benefit of being available in a variety of sizes, depending upon the original plant source. Removal, or ‘burn out’, of the fillers can be typically achieved by heating in air up to ~400°C, and generally leaves no residue.
Fig. 3a Schematic of directional ice crystal growth away from a cold substrate.
Fig. 3b Scanning electron microscope image of directional porosity formed in an aluminium oxide ceramic using freeze casting (courtesy Dr. R. Chen, Dalhousie University).
It is possible to vary the amount of pore-forming filler through the material, creating a functional grading (shown schematically in Fig. 2), which can be achieved using techniques such as ‘tape casting’; this approach involves processing of fluid suspensions into thin layers that can then be laminated together. Another interesting approach for producing ‘oriented’ porosity is based on freezing of aqueous suspensions. If cooled sufficiently, ice crystals can form in a suspension of fine particles, which tend to grow away from the cooled surface (Fig. 3). The ice growth pattern is dependent upon a variety of parameters, including the suspension characteristics (e.g. viscosity, solids loading, particle size, etc.) and the temperature (i.e. the degree of ‘under-cooling’ below the freezing point of the liquid). The ice is then removed by sublimation, leaving porosity that is a replica of the shapes of the ice crystals that were initially formed.
While the field of porous ceramics is not new, many advances have been made in recent years, in particular when relating structure and function. Novel approaches to form oriented and graded porosity have been developed, while moderately porous materials have been generated with mechanical properties approaching their dense equivalents. This evolving class of materials has many potential applications, and interest in porous ceramics can be expected to increase in the future. We have truly reached the age of ‘engineering nothing’!