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Refractory Materials

Sheets made of ceramic foams
© Faunhofer-Center HTL

Sheets made of ceramic foams

Refractory materials include dense shaped refractory products, refractory bricks and insulating bricks, fiber-based refractory products as well as unshaped products. They primarily have a load-bearing and insulating function. Refractory products must withstand high temperatures and corrosive media, e.g., as a furnace lining in high temperature furnaces. Depending on the application, they are subjected to mechanical, thermal and chemical loading. Refractory materials used as furnace linings require a certain minimum strength and creep resistance to ensure that they do not rupture or creep under their own weight. A high thermal resistance or refractoriness under load are further prerequisites for the required dimensional stability, i.e. creep and subsequent sintering of the refractory materials are not permissible. The planned temperature cycles in use also require a high resistance to changes in temperature and thermal shock resistance. A high slagging resistance is also an important prerequisite for applications, where the refractory materials have direct or indirect contact with corrosive gases or melt material. On the other hand, for insulating components, e.g., in permanent linings, low thermal conductivities and heat capacities are decisive to ensure that the refractory linings are compact and energy-efficient in design.

The above mentioned properties cannot all be realized in a refractory material and are even partially contrary to one another. A high strength generally requires a high relative density, which leads, however, to an increased thermal conductivity and volumetric heat capacity. A reduction of the relative density by the generation of additional porosity results on the one hand in a lower thermal conductivity, however, it generally leads to a degradation of the mechanical properties and the abrasion resistance. Dense materials with low thermal conductivity are frequently susceptible to thermal shock. For optimization of the properties in this conflict of objectives, systematic evaluation criteria are necessary.

At the Fraunhofer-Center HTL, refractory materials with optimized property profiles are developed to identify and deliver resource and energy-efficient solutions for various fire-proof applications as well as to provide end users with technological and economic benefits. The developed fire-proof materials include acidic, alkaline as well as contact-neutral materials made of oxide and non-oxide components as well as combinations thereof. The HTL with its test facilities and computer programs covers the entire manufacturing chain for the development of refractory components: From the selection of the materials[1] and the component design to the preparation and molding as well as the heat treatment right up to testing. Thus, for example, in addition to molding, other common procedures such as pressing, stamping and casting processes as well as special in-house HTL developments are available.

An example of the HTL's developed refractory materials are the highly-porous ceramic foams, which are manufactured using an in-house developed direct foaming procedure. The manufacturing process starts with a slurry consisting of water, a ceramic powder as well as foaming agents, binders and stabilizers. The slurry is mechanically whipped using simple equipment engineering, until a highly porous slurry foam is created. The latter is then filled into a near-final mold and then hardens and cures. After hardening and demolding, the green bodies feature a high stability and can be mechanically processed, if required. Subsequently, the organic components are fired, and the molded parts are sintered. Thanks to the direct foaming procedure, using cost-effective non-toxic organic raw materials, a wide range of ceramic powders (e.g., aluminum oxide, cordierite, mullite) can be processed to near net shape highly porous foam molded bodies. The refractory components manufactured in this way offer potential for applications such as refractory bricks and insulating bricks.

A further HTL-field is the development of ceramic fibers for items such as insulating elements. The HTL also offers the entire process chain associated with the manufacture of ceramic fibers. This includes the synthesis and working of spun materials, spinning, thermal processing of the fibers as well as the application of diverse types of fiber coatings. The diameter of the individual fibers can be controlled directly via the process parameters. Consequently, it is possible to manufacture ceramic fibers with a diameter considerably outside the respirable range as defined by the WHO. Furthermore, a manufacturing process that is as environmentally friendly as possible is established for the oxide ceramic fibers, which offers benefits in respect to the industrial emission guidelines of the EU. Short fibers, long fibers or endless fibers made from different material systems, such as aluminum oxide, mullite or silicon carbide - which are polycrystalline or amorphous, dense or porous, or even hollow - can be manufactured at the HTL via various spinning methods. These fibers are also used as reinforcement fibers for load-bearing refractory elements or as a material for insulating elements, such as vacuum formed components.

In order to increase the damage tolerance of refractory materials, HTL has developed a patented woven fiber fabric reinforcement. The method of action of the woven fiber is based on the defined connection between the fiber and the matrix material. In this way, the cracks on the interface area between the matrix and fiber material are stopped and diverted. Fracture energy is dissipated, and a quasi-ductile damage tolerant behavior results. The woven fabric can consist of carbon, glass, basalt, silicon carbide or ceramic oxide fibers depending on the application conditions.
These are introduced to the respective forms during the manufacturing process and transferred with the matrix material (oxide ceramics, non-oxide ceramics or graphite-based materials). Thermal treatment to stabilize the matrix is undertaken at the conclusion. The reinforced refractory materials produced in this manner offer large potential for applications, in which damage tolerant, load-bearing high-temperature components are in demand.
For the development of prototypes with complicated geometries, 3d print processes are used at HTL. Furthermore, for refractory materials subject to corrosion, ceramic protective coatingsare developed at HTL. They are applied using wet-chemical coating processes such as dipping, spraying or painting and are subsequently fired.

[1] Raether, F.:Ceramics Facing Competition with other Materials, Ceramic Applications, 4/2016, pp. 57-61

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Joachim Vogt

Contact Press / Media

Joachim Vogt

Fraunhofer-Center for High Temperature Materials and Design
Gottlieb-Keim-Str. 62
95448 Bayreuth, Germany

Phone +49 921 78510-417

Fax +49 921 78510-001

Holger Friedrich

Contact Press / Media

Dr. Holger Friedrich

Fraunhofer-Center for High Temperature Materials and Design
Gottlieb-Keim-Str. 62
95448 Bayreuth, Germany

Phone +49 921 78510-300

Fax +49 921 78510-001