Beitrag zur inneren Oxydation von Platin-(10% Rh-)1% Zr-Legierungen

Reinacher, Gerhard

doi:10.1515/ijmr-1971-621115

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“…The interface between the particles and the matrix can be important [23] and can itself influence the diffusion rate of oxygen [24,25]. In the 1970s Reinacher [26], and later Fischer [27] described a preferred oxidation of zirconium along the grain boundaries of the material. The low solubility of oxygen in the platinum matrix prevents the formation of zirconium oxide particles inside the grains themselves.…”

Section: Grain Growthmentioning

confidence: 99%

Stability assessment of a new high strength, high temperature Type R and Type S thermocouple

Pearce

Tucker

Rawal

et al. 2020

Meas. Sci. Technol.

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Platinum-rhodium Type R and S thermocouples fail in service because of two main reasons: excessive grain growth in the platinum limb at high temperatures (causing wire breakage) and temperature drift, i.e. deviation of electromotive force (emf) generated over time because of deposition of rhodium oxide on the platinum limb, out of accepted industry tolerances. Finding a solution to the problem is challenging because of the contradictory requirements of strength and narrow permissible emf range: emf is extremely sensitive to any dopants or alloying elements that can be used to improve strength at high temperatures. Johnson Matthey has developed a new thermocouple wire (platinum limb) called 'HTX ™ ' by doping platinum with a small quantity of zirconium which is oxidised during processing to electrically neutral zirconium oxide. Zirconium oxide improves the high temperature strength of platinum wire by restricting the grain growth but does not significantly affect the emf. The result is a wire which has the potential to last many times longer at high temperatures (>1200 °C) than standard platinum wire, whilst still achieving Class 1 tolerance (i.e. ±1 °C at 1000 °C). It also improves the thermoelectric drift characteristics of the thermocouple by counteracting any reduction in emf due to rhodium contamination by an increase in emf as any remaining zirconium is converted to zirconium oxide. We describe an impartial long-term assessment (up to 1500 h) of the thermoelectric stability of the HTX ™ wire by exposure to high temperature with periodic in situ calibration using a Co-C (1324 °C) high temperature fixed point. This yields stability measurements with very high precision.

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