Highest quality standards for refractories

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.