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HIGH-TEMPERATURE HALL MEASUREMENT STATION

High temperature HALL measure system for material characterization
up to 900K.
Developed by Fraunhofer- IPM

Fraunhofer IPM is one of the leading research institutes in the field of thermoelectrics with know-how in advanced materials research, simulation, metrology, and systems. As a specialist in thermoelectric metrology we offer an extensive support for in-house thermoelectric characterization and for turn-key, tailor-made systems development.
Frauenhofer
   
Diagramm    
Measuring the Hall effect.

Edwin Hall

Edwin Hall was a pioneer of modern science in the 19th century.
In 1879, in his work on the interaction between electrical current and magnetism he made a discovery which massively increased our understanding of the subject.
He noticed that current in a conductor can be influenced by the application of a magnetic field.
This effect can be measured using the voltage which is picked up on the conductor perpendicular to the current and also the magnetic field.



 
High temperature hall High temperature hall
»IPM-HT-Hall 900 K«: Characterization and optimization of semiconductor materials at high temperatures up to 600°C and over. Full ceramic measurement head for measurement
in temperatures up to 600°C and over.
   

MATERIAL CHARACTERIZATION AT 900 K

Measurement of charge carrier loads in semi-conductors and thermoelectric materials

The efficiency of thermoelectric modules depends to a large extent on the material and its properties – including the number, type and mobility of the charge carriers.
Fraunhofer IPM developed the »HT- Hall 900 K« high-temperature Hall measurement station which characterizes the material at temperatures of around 20 to 600°C to determine these material parameters.
This is because modern thermoelectric materials are now used in very high ambient temperatures.
The measurement principle is based on the Hall effect which was discovered by the American physicist Edwin Herbert Hall in 1879.
He noticed that an electric current in a conductor can be influenced by applying a magnetic field. The voltage generated hereby provides deep insights into the conductor material being tested.
In addition to the identification of the charge carrier carrying the current, its mobility and concentration can also be determined.

Increasing the Efficiency of Thermoelectric Materials

The benefit of the »HT-Hall 900 K« is that the measurement is highly sensitive and identifies even minimal changes in the charge carrier concentration – including the influence of doping.
»Doping« is the term used to describe the process of the specific addition of foreign atoms to a semi-conductor to affect the number, mobility and type of charge carriers. Increasing the doping level of thermoelectric semi-conductors leads to larger charge carrier concentration and thus also to better electrical conductivity. But at the same time, this results in a decrease of the Seebeck coefficient which denotes the electrical voltage in the thermoelectric material generated by a temperature difference.
Therefore, both parameters, Seebeck coefficient and electrical conductivity, must be determined and optimised simultaneously in order to achieve thermoelectric material with an optimal efficiency. In general, there is less than one doping atom to 1,000,000 thermoelectric atoms. Therefore it is important to check the precise level of doping using a Hall measurement system. High-temperature measurement.

It is not just the doping, but also the ambient temperature which affects the number of charge carriers. The higher the temperature, the more charge carriers are activated. Modern thermoelectric materials are now used in motor vehicles, for example, at temperature of 600°C or higher.
Previous commercially available Hall measurement equipment was based on cryostat systems and is therefore only designed for low temperatures from around 4 K to 400 K. To date, there have not been any commercial systems available in the range from room temperature to above 600°C which is of interest for modern thermoelectric materials.
Fraunhofer IPM therefore developed a new measurement system specially for Hall measurements at 20 to 650°C, which allows fast, reliable measurements using the Hall principle. The new HT-Hall measurement station has been tested successfully at temperatures of up to 650°C. Work is currently being carried out to extend the measurement range up to 800°C
.

Charge Carrier Concentration Using the Example of Germanium Samples

Figure 6 shows temperature-dependent measurements of the charge carrier concentration on three different germanium samples. Pure germanium, also known as intrinsic germanium, produces a linear increase in the charge carriers as the temperature rises. The charge carriers are thermally activated by the increase in temperature.
Mildly doped material has more charge carriers at room temperature than pure material, but at higher temperatures it produces the same linear increase in charge carriers since more and more charger carriers are thermally activated in this case, too.
Very heavily doped material constantly shows the highest charge carrier concentration at temperatures of around 600°C.
            
Diagramm Diagramm
Charge carrier concentration in
germanium samples.
The various thermoelectric parameters in different materials: The common maximum of the three parameters, summarized by the quality index for thermoelectric efficiency ZT, is at the transition from semi-conductors to semimetals.
   
Diagramm