Thermocouples have become established as standard temperature measuring devices in thermal analysis: they feature simple set-up and operation, and they are multifunctional, robust and compact.
The most frequently used thermocouple material for operation above 800°C is platinum-platinum /rhodium (10% ) – with regard to its chemical composition, also designated Pt-Pt 10% Rh,- or also referred to as type S. The main advantages of this thermocouple, developed by Le Chetalier more than 100 years ago, are high reproducibility, good corrosion and oxidative stability.
Compact platinum-platinum/rhodium features a virtually unlimited resistance at room temperature. This, however, changes under regular operation at high temperatures. Interdiffusion, selective evaporation, recrystallization and environmental influences are the main reasons for changes in thermal tension or failure of the thermocouple.
At temperatures above 1000°C, evaporation of rhodium and also diffusion of rhodium from the positive Pt 10% Rh side to the negative Pt side occur. Both effects result in impurities and increased wear and tear of the platinum wire. In order to minimize the risk of alloy formation over the gas phase, the majority of the thermocouple wire for DSC/DTA sample carriers is protected by a capillary of high-purity Al2O3.
In the temperature range above 1100°C, platinum recrystallizes into a coarse-grained structure. The described grain growth not only occurs within the metal or metal alloy, but also leads to “coalescence”of different platinum parts which are in contact with each other, such as DSC/TG sensor type S and Pt/Rh DSC crucibles. Only conditioning of new sample carriers and crucibles by means of a special thermal treatment lowers the “sticking tendency”.
Using unconditioned sample carriers and crucibles in the temperature range above 1000°C immediately leads to welding of the crucible onto the sensor and thus, destruction of the sample carrier.
Please pay attention to the instruction leaflet of your sample carrier in this regard. We would ask you to heat new Pt/Rh crucibles prior to using them in a separate furnace to the required end temperature of the measurement, lift the crucibles from the sensor after each measurement as a precaution, and go up to temperatures above 1100°C only stepwise in the beginning.
In our experience, using dispersion-hardened (so-called FKS) materials for the sensor surfaces and crucibles does not result in a long-term significant improvement.
One way of avoiding the described phenomenon is to underlay thin disks (between sensor surface and crucible). The risk of sticking is minimized and the sensitivity of the sample carrier is only slightly decreased.
In practice, the biggest influence on the service life of thermocouples is due to interactions with the environment. Diffused impurities, released from the samples, change the thermal tension or may even cause initial cracking of the thermocouple wire. In the table, you will find details on the chemical compatibility of platinum with other sample materials and gas atmospheres.
- Halogens (Cl2, F2, Br2), aqua regia
- Li2CO3, prior to emission of CO2 (decomposition)
- PbO, FeCl2
- Be alloys (evaporation)
- HCl with oxidants (e.g., chromic acid, manganates, iron (III) and molten salts); reducing atmospheres
- Metals and metal vapors (e.g., B, Pb, Zn, Sn, Ag, Au, Li, Na, K, Sb, Bi, Ni, Fe, etc.;m Se > 320°C (evaporation)
- Metals and metal oxides with reducing sub- stances such as C, organic compounds or H2
- Oxides in an inert gas atmosphere at higher temperatures (reduction)
- Sulfur (roughening of the surface, embrittlement)
- Alkali hydroxides, -carbonates, -sulfates, -cyanides and rhodanides at higher temperatures
- KHSO4 at higher temperatures
- Carbon black or free carbon >1000°C
- SiO2 under reducing conditions
- SiC and Si3N4 >1000°C (release of elementary Si)
- HBr, KCl solution at high temperatures
(No claim of completeness)