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Uniaxial hot pressing

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Powder-based compaction of thermoelectric materials – Uniaxial hot pressing 

Powdermetallurgical preparation routes offer specific options for quality improvement of thermoelectric materials. Furthermore they can link formation of desired electrical and thermal material properties within one technological step to near-net shape forming for prototype fabrication of thermoelectric modules.
Hot pressing as a powder technical standard technology for the fabrication of components and sub-systems, which have proven to be suitable and cost- saving for thermoelectric functional materials

• allows processing of commercial powders
• is applicable from laboratory scales up to industrial fabrication
• supplies dense, mechanically stable material
• supplies shaped parts of defined geometry suitable for the fabrication of thermoelectric devices
• allows for fabrication of large-volume blocks of good homogeneity of thermoelectric materials, as well as  
• fabrication of segmented structures by pressing of powder stacks.

Hot pressing allows for defined control of electrical and thermal material properties by the technological parameters of the process (pressing temperature, magnitude of mechanical pressure, and duration of compaction or temperature-time-regime) as well as by suitable additives or preliminary treatment of powders. The obtained material properties are influenced by microstructure, especially by the density of compacts, by the content of electrically active doping and inactive additions that influence the formation of a micro- or nano-crystalline structure and grain boundary properties. Two hydraulic uniaxial hot pressing machines are available at DLR on which specimens of thermoelectric materials of diameters up to 150 mm (thickness up to about 25 mm) can be make. Pressing dies of graphite and ceramics are available for operation under vacuum (up to 950 °C), inert gas, or air (up to 1500 °C), respectively. Automatic control allows for programming of temperature/compression regimes.
Based on studies on the material preparation of doped iron disilicide a laboratory route for the preparation of thermogenerator elements by hot pressing of gas-atomized powders was developed at DLR. Doped silicide powders (p-type by Al or Mn doping; n-type by Co doping) are compacted in a double-layer pellet containing a p-type and n-type segments. P/n segmented blocks are obtained by cutting, are bridge-shaped to thermogenerator elements by a separating slit between the p-type and n-type segment and by attaching metallic contacting pads.