Contact Press / Media
Dr. Holger Friedrich
Fraunhofer-Center for High Temperature Materials and Design
95448 Bayreuth, Germany
Phone +49 921 78510-300
Fax +49 921 78510-001
Refractory materials form the crucial interface between the units of thermal process technology and the material to be heated. Their functionality influences the product quality as well as the cost of the heat processes. Refractory materials have to comply with the highest quality standards in critical application areas, such as metal, glass and ceramic production. Refractory materials must withstand very different thermal, chemical and mechanical loads. This results in a large number of refractory products: refractory and insulation bricks, dense shaped products, such as basic, fireclay bricks or silica bricks, kiln furniture, fiber materials, as well as unshaped masses for joining, repairs or coatings. Despite a strong price pressure, the consequential cost of quality problems of the refractory materials are very high. Therefore, the highest quality standards must be adhered to. The Fraunhofer-Center HTL offers numerous services for the development, manufacturing and testing of refractory materials.
Special databases and methods for material selection for the development of refractory materials are available at HTL. This makes it possible to carry out benchmarking on existing material solutions and to identify requirements profiles for new developments. For example, HTL is used to develop insulation materials for high-temperature applications using direct foaming methods or to develop kiln furniture aids with very low heat capacity. HTL has reliable computer simulation methods that can predict thermal or mechanical material properties from their microstructure. Thermodynamic databases are also available for the prediction of the high-temperature behavior of refractory materials. By using appropriate computer tools, the efficiency in the development of new refractory materials can be significantly increased. All major forming processes, such as dry and wet pressing, slip casting or extrusion, are available at HTL on a laboratory or technical scale. Ceramic fiber materials are produced at HTL with different spinning units as well as machines for textile processing. This allows prototypes to be produced by spinning, braiding, nonwoven production, winding or prepreg techniques. In addition to ceramic fiber materials, fiber-reinforced composites can also be developed. Coatings and solders are developed on the basis of dispersions and special organometallic and glass-ceramic precursors and are applied by means of dipping, spraying or brushing. For the heating processes, HTL uses oxidic and non-oxidic furnaces with application temperatures of up to 2400 °C and useful volumes up to 0.4 m³. Information on specific development possibilities for refractory materials in general can be found at: Refractory Materials. Information on kiln furniture in particular can be found at Kiln Furniture.
HTL can optimize the heat treatment process for the production of refractory materials. This in particular concerns shaped refractory products which are produced via the powder route. Starting from drying of the green bodies through binder burnout or pyrolysis up to the sintering process, efficient methods for the identification of optimal process parameters are available for all heat treatment steps. The basis for the process optimization are in-situ measuring methods, so-called ThermoOptical measurement systems (TOM) with which all relevant material data during the heat treatment are measured. The sample sizes are approximately 10 to 100 cm³, so that a sufficiently representative volume is recorded for many applications. The samples are heated under the same conditions as in the industry. This means that the furnace atmosphere of the industrial furnaces is reproduced as accurately as possible in the TOM furnace. The in-situ measurement data are parameterized using robust models and then used in specially developed FE models to simulate the respective process step and to optimize the relevant process parameters. Critical process steps such as quartz conversion, carbonate decomposition, dewatering of clay minerals or high-temperature phase transformations are investigated and optimized using special methods. The FE simulation particularly allows for the upscaling of samples to component scale, which is essential for refractory bricks in order to adequately take into account the gradients occurring in the components during the heat treatment. Primarily temperature cycles, atmospheres (composition and flow) and, if necessary, setting plans are optimized. The optimized process parameters are, as far as possible, verified in the laboratory furnaces and then transferred to the industrial units. The optimization of the heat treatment of refractory materials serves, on the one hand, to reduce production costs and, on the other hand, to increase or ensure product quality.
HTL develops special high-temperature measurement methods for the testing of refractory materials. Investigations with already established procedures are carried out as well. At the focus of the development are novel ThermoOptical measurement systems, with which high-temperature properties of refractory materials can be determined under defined and representative conditions. The focus is on measuring methods for the determination of thermal, mechanical and chemical properties. All material data of refractory materials relevant for the thermal management of high temperature processes are covered by the TOM methods: thermal conductivity, thermal expansion, heat capacity and emissivity. In addition, HTL measures mechanical and thermomechanical material properties at high temperatures, which are relevant for the service life of refractory components. These include elastic and creep behavior, high-temperature stability, thermal shock, temperature and load change characteristics. In addition to the TOM methods, tests are also carried out with commercially available high-temperature measurement methods. Where possible, the tests will be carried out according to the standards applicable to refractory materials. Through the use of FE methods, HTL is able to simulate the application behavior of refractory components and, if necessary, to optimize the component geometry. In addition, the FE simulation helps in the evaluation of test results generated on smaller samples or in shorter load cycles. Methods that are used at HTL for the assessment of component damage also serve this purpose. These include computer tomography and ultrasound transit time measurements. Together with lifetime models, these methods allow estimations of the maximum possible application times of refractory materials and serve to advise users in the selection of materials. High-temperature chemical interactions of refractory materials with other media can also be investigated at HTL. After structural tests in the special furnaces of HTL, structural analyzes are carried out, in which the crystalline phases, diffusion profiles and other changes of the microstructure are identified by state of the art analytical methods. Furthermore, weight changes, wetting and adhesion properties can be measured with the TOM systems at high temperatures under defined conditions, so that the application behavior of refractory materials can be investigated with respect to very many aspects. Further details on the measurement of the high-temperature properties of refractory materials can be found under: High temperature characterization. Further details on service life estimations can be found under: Life cycle analysis.