An electromotive force difference method was used in research conducted by ACT’s R&D group to select four halides that were believed to be most compatible with superalloy envelopes: AlBr3, GaCl3, SnCl4, and TiCl4.
Heat pipes with several different superalloy envelopes were fabricated and placed on life test. During the life tests, the temperature of the evaporator and condenser for each heat pipe were monitored, to detect any problems. It is possible that oxygen can affect the outside of the titanium pipes during the test; therefore, the life tests are conducted inside a box that is purged with argon to prevent this.
Possible problems with incompatible fluid/envelope pairs include Non-Condensable Gas (NCG) generation, corrosion, and materials transport.
Three different outcomes were experienced during these exploratory life tests:
- Non-Condensable Gas Generation
Fig. 7: High ΔT between the evaporator and the condenser after 2000 hours of life testing, indicating the rapid formation of NCG.
The temperature difference between the evaporator and condenser were monitored, to detect non-condensable gas; see Figure 20. The SnCl4/superalloy, GaCl3/titanium pipes, and Therminol/titanium pipes all have a high ΔT, indicating that these envelope/working fluid pairs are incompatible.
Fig. 8: Backscatter Electron Image of a CP Ti/GaCl3 heat pipe shows a Ga-Ti reaction layer, confirming the pair is incompatible.
The envelope/working fluid incompatibilities were verified by sectioning and examining the heat pipes. Figure 21 shows that the heat pipe with a CP Ti envelope and GaCl3 working fluid, underwent extensive corrosion. Some of the corrosion layer was observed to crack and chip during polishing. The fracture surfaces were indicative of a brittle failure mode. In combination with the extensive cracking of the corrosion layer, this seems to indicate that the corrosion layer is quite brittle.
- Fig. 9: GaCl3 leak developed at the pinch-off tubes due to incompatibility Corrosion
All of the heat pipes with large amounts of gas generation showed corrosion when examined internally. The GaCl3/superalloy pipes were so incompatible, that they all leaked at the pinch-off weld after roughly one week of operation at 360°C (633K); see Figure 9, at right.
- Successful Long Term Life Tests
Fig. 10: Secondary Electron Image of C-2000 Envelope/ TiCl4 fluid shows a small reaction layer in this compatible heat pipe.
Finally, two envelope/fluid pairs (superalloy/TiCl4 at 400°C and superalloy/AlBr3 at 300°C) were found to be compatible and ran without any problems. These pipes have currently been running for 57,000 hours (6.7 years). The AlBr3 pipe is of particular interest, since it is running at 673 K (400°C). This is close to the temperature at which cesium starts to work. For example, sectioning and analysis confirmed the compatibility of these envelope/fluid pairs. Figure 10 revealed a 1 to 2-micrometer-thick corrosion layer on the surface of a heat pipe that paired Hastelloy C-2000 with TiCl4 as the working fluid.
