ThermoOptical Measuring Methods (TOM)

The TOM systems can be used for in-situ characterization and analysis of very different material properties in the temperature range from room temperature to over 2000°C:

1. Thermal properties
2. Mechanical properties
3. Thermomechanical properties
4. Chemical properties
5. Electrical and optical properties

Accurate measurements of high-temperature properties require a precisely defined ambient temperature and an atmosphere adapted to the sample material, e.g., inert gas atmosphere for oxidation-sensitive samples. Additionally, a sample volume representative of the material must be measured. For highly heterogeneous materials such as coarse ceramics, refractory materials, or composite materials, several 10 to 100 cm³ are required. These requirements are met with the TOM systems.

At the HTL, there are eight TOM systems available, specifically designed for in-situ measurement and optimization of thermal processes. The systems are used for the following thermal treatment processes:

• Drying
• Binder burnout
• Pyrolysis
• Dehydration
• Sintering
• Liquid metal infiltration

In addition, the quality of the shaping can also be checked with the TOM systems. Using a highly precise shadowing method, dimensional changes of the samples during the thermal treatment are measured in situ. Weight changes can also be measured in situ. Microphones register the acoustic emission when cracks form in the samples. Furthermore, with special load application devices, uniaxial loads can be applied, and the creep behavior of the sample can be studied.

At the HTL, five new TOM systems have been developed that can measure material properties in a controlled atmosphere. The following high-temperature properties are measured:

• Strength / stiffness / elongation at break
• Fatigue strength
• Compressive deformation / creep
• Thermal shock / thermal cycle resistance
• Dynamic modulus of elasticity
• Temperature / thermal conductivity
• Thermal capacity / thermal expansion
• Wetting behavior of melts
• Emissivity integral / spectral
• Corrosion behavior against gases and dusts
• Electrical impedance

For mechanical characterization, force sensors with maximum forces between 3 N and 5 kN are used. Fatigue tests are designed for frequencies up to 300 Hz. Thermal shock and thermal cycling tests can be carried out with defined laser heating. The characterization and analysis of thermal material properties is also performed non-contact with a laser flash method specially developed for large samples.

The HTL also develops TOM systems according to customer requirements. For this purpose, measuring furnaces with the desired maximum temperature, size, and atmosphere are offered. In addition to optical in-situ dimensional measurement, mechanical parameters such as modulus of elasticity, creep strength, wedge splitting strength, or viscosity are available as measured quantities. Special systems for high-temperature measurement of thermal properties, such as thermal conductivity, thermal expansion, thermal shock resistance, etc., are also developed. The construction of TOM systems takes place in the certified Center of Device Development (CeDeD) of Fraunhofer ISC.

• TOM_ac for controlled atmospheres, graphite-heated up to 2200 °C: Dimensional changes, load tests, and gravimetry
• TOM_air for operation in air up to 1750°C: Dimensional changes, load tests, gravimetry, and acoustic emission measurements
• TOM_metal for measurements in hydrogen atmosphere or overpressure, graphite-heated up to 1800°C: Dimensional measurements
• TOM_gas for measurements in gas burner atmosphere up to 1500 °C: Gravimetry
• TOM_II for measurements in combustion atmospheres up to 1500 °C: Dimensional measurements
• TOM_wave for measurements of thermomechanical properties up to 1750 °C: Thermal shock, thermal conductivity, emissivity, etc.
• TOM_I for measurements of thermal conductivity on small samples up to 2000 °C: Thermal conductivity, shrinkage
• TOM_imp for measurements of electrical impedance in air up to 1000 °C
• TOM_mech for measurements of mechanical properties up to 1500 °C: Strength, stiffness, elongation at break, fatigue, creep, etc.
• TOM_fiber for measurements of mechanical properties of fibers and minicomposites up to 1500 °C: Strength, stiffness, elongation at break, creep
• TOM_chem for measurements of corrosion in particle and gas streams up to 1500 °C and 40 m/s: Gravimetry
• TOM_dry for measurements of weight changes during drying in controlled atmosphere in laminar gas flow: Gravimetry, shrinkage and surface temperature