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Customized Tube Furnaces Offer Thermoelectric Material Researchers a Versatile Processing Solution

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Carbolite, has supplied the Fraunhofer Institute for Physical Measurement Techniques (Fraunhofer IPM) in Freiburg, Germany with a test rig that comprises two specially customized wire wound horizontal three-zone tube furnaces.

The customized furnaces based on a system that was developed by the Fraunhofer IPM in 1981 will provide the Institute’s research teams with a versatile processing system for studying and improving the thermo-electrical properties of advanced thermoelectric materials and systems.

The Institute will use the two separately controlled, rail-mounted three-zone tube furnaces to play a key role in optimizing compound semiconductors. The systems are designed to maximize operational flexibility by combining the two, three-zone furnaces to form a single large multi-zone furnace or separating them to give two differently heated regions with a gradient of 200°C between the two.

Custom built by Carbolite engineers, the three-zone tube furnaces feature a specially developed desk and rail system that provides vibration-free movement of the furnaces and is constructed to support a very long fixed quartz tube through both furnaces.

Fraunhofer IPM is engaged in research on new materials, particularly nanocomposites and the Institute's scope of activity includes materials research, development of thermoelectric modules, thermoelectric simulation, measuring technology and systems. Thermoelectrical energy harvesting will increasingly provide a significant contribution to more efficient use of energy in the future and will be featured in a wide range of power plants, industrial processes or automobiles.

Fraunhofer IPM has comprehensive experience in the entire range of thermoelectric technologies which include studying and developing processes for production of nanoscale thin layer systems using different coating and epitaxial processes; production and processing of solid materials; assembly and packaging technologies as well as thermoelectric measuring technology.

A double horizontal tube furnace was supplied by Carbolite to maximize the temperature gradient capabilities of the system. The two three-zone furnaces have different sizes. The larger has a heated length of 600mm, the smaller 400mm each with an integral recrystalised alumina work tube. The three zone design of the furnaces uses three controllers to apply separate control to the central zone and each of the end zones in each furnace.

These end zone controllers with their own thermocouples compensate for the tendency for tube ends to be cooler and ensure that the tubes furnaces have a larger uniform zone than can be achieved with only one control loop.

Each of the two furnaces is equipped to accept an accessory tube with a maximum outer diameter of 65mm. The customized system also includes smaller end insulation disks between both furnaces to enable a more linear temperature gradient from the one end of the furnace to the other. Power to the furnace’s end zones is automatically adjusted to compensate for heat loss.

To provide a more user-friendly, ergonomic working environment the furnace systems feature separate controller boxes that have been specially positioned under the worktable. The furnaces are specified to operate at a maximum temperature of 1200°C but by using modified control algorithms they are able to operate between 150°C and 600°C.

Before the systems were delivered to Fraunhofer IPM, Carbolite’s engineers made test measurements between 500°C and 700°C to ensure good uniformity in each middle zone and a sharp linear gradient in the middle of both furnaces with a small thermal wall of 15°C in both outer end zones.