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03 September 2010 ETH Zurich

More efficient thermal insulation through ceramic foams

Ceramic foams developed, intended for use as energy-efficient thermal insulations in blast furnaces, as bone substitutes and for the controlled release of active ingredients in medicine

A scanning electron microscope photograph of a porous ceramic foam. Thousands of micro-particles form walls for air inclusions. The properties of the material differ depending on the particle size and type
A scanning electron microscope photograph of a porous ceramic foam. Thousands of micro-particles form walls for air inclusions. The properties of the material differ depending on the particle size and type.
Image Credit: Urs T. Gonzenbach, de Cavis AG.

For many people ceramics are synonymous with old crockery. However, only a few know of their importance in industry, e.g. for lining high-temperature furnaces to produce steel, cement or glass. Urs Gonzenbach, Senior Researcher at the Insititute for Nonmetallic Inorganic Materials, has worked with ceramics for eight years, especially with ceramic foams. This research topic was found rather by accident while working with very dense, non-porous ceramics: one morning in the laboratory, he found a suspension that had been aerated over night during the ballmilling process. At that time, this was an unwantedresult for the research project since pores are undesirable in dense ceramics. However, his team recognised thisproduct’s potential and asked Gonzenbach to further study this phenomenon.

Better material properties thanks to air inclusions

The highly porous ceramic foams that he has developed during this time have significant advantages over conventional porous ceramics: they are up to 50 percent lighter, have a thermal conductivity that is about thirty percent lower and at the same time are twice as strong. The material owes these properties mainly to its porous microstructure which consists of tiny air inclusions. Gonzenbach achieves this by foaming an aqueous ceramic suspension with air or another gas that forms bubbles like bath foam in a bathtub. However, whereas bath foam collapses relatively quickly, Gonzenbach keeps the ceramic foams stable for several days. He does this by mixing tiny particles into the suspension, e.g. aluminium or silicon oxide from ten nanometres to ten microns in size. The particles adsorb to the air bubbles and stabilise them by forming a kind of protective jacket around the air inclusion. As a result, the wet foam remains stable for a long time and can undergo further processing with no problems.

To make the hydrophilic micro-particles adsorb to the water/air interface in the first place, the researchers mix additives into the suspension before foaming. These additivies ensure that the particles become partly hydrophobic and thus attach themselves at the surfaces of air inclusions. The resulting particle-stabilised foam consists of up to 93 percent air. After controlled drying, it is fired – or “sintered” as the experts say – at 1600°C. This expels the last of the water from the foam, and the gaps between the micro-particles are closed. The product is a light-weight, highly porous ceramic foam that remains stable up to 1700°C.

Gonzenbach and his colleagues patented the research successes three years ago and founded the ” spin-off “de Cavis in early 2009 to market the ceramic foams. Gonzenbach says “We have developed a very promising novel technology that is currently looking for suitable applications.” Although the porous ceramic foams are still at the prototype stage, evaluations for industrial applications are underway. Most of the enquiries are from producers of thermal insulation materials which supply furnace manufacturers in the areas of steel, cement or glass production. Gonzenbach says, “We can now make foam from practically any ceramic raw material.” His team can match the properties to the customer’s needs by selecting the type and size of the micro-particles and by the additive used. The porosity of the stable foams has two big advantages for companies: less material is needed to line the furnaces, and they use less energy as a result of the improved thermal insulation, thus reducing operation costs and the environmental burden.

Foam as a support to build new bone

Gonzenbach also has his sights on possible medical applications: when treating serious illnesses, e.g. cancer, it is often necessary to remove a piece of bone. For large bone defects, a scaffold is needed to stimulate bone growth in order to fill this gap. The lightweight, porous ceramic foam could be suitable for this purpose. Therefore, ETH Zurich materials scientists are collaborating with chemists at EPF Lausanne and biologists at Lausanne University Hospital as part of a Swiss National Science Foundation project on possible therapeutic applications of this material. For example, the researchers are currently trying to modify the foam so that it self-degrades in the body as soon as enough bone has re-grown.

Applications of ceramic foams in cosmetics or pharmaceuticals are still at the basic research stage. If the foams are diluted sufficiently, millions of tiny microcapsules are formed. Active ingredients could be released into the body in a controlled way over a prolonged time through their porous inorganic shells. However, it will probably be some time before the micro-foams can also be used for pharmaceutical purposes, as the conditions imposed by the regulations for this use are particularly stringent.

Source: ETH Zurich /...

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