Our customers have been seeking a high strength platinum thermocouple wire for harsh conditions, having found the pure platinum limb of type R and S thermocouples fails earlier than the alloyed limb, with a bamboo structure forming.
The bamboo structure is the result of grain growth. When individual grains in the wire occupy the complete wire diameter, the wire becomes a chain of grains linked by transverse grain boundaries. These boundaries are points of weakness. They are easily displaced or separated, causing a circuit break and stopping the temperature reading.
We know our customers need the wire to have two key properties: working at high temperatures, but without significantly changing the electrical properties of pure platinum. So we put our knowledge to the test.
Through extensive research and development, we trialled several routes to restrict the grain growth. We found a solution in microalloying zirconium with platinum, which oxidises in situ when processed. These zirconium oxide particles enhance the wire by:
- Increasing the yield stress.
- Restricting grain growth by grain boundary pinning at high temperatures.
- Being electrically neutral so having no effect on the EMF.