Intermediate Temperature Heat Pipe Life Tests

William G. Anderson, Sanjida Tamanna, Calin Tarau, and John R. Hartenstine
Advanced Cooling Technologies, Inc.
1045 New Holland Ave., Lancaster, PA 17601 U.S.A.
717-295-6061, 717-295-6064 Fax, Bill.Anderson@1-act.com, Sanjida.Tamanna@1-act.com, Calin.Tarau@1-act.com, John.Hartenstine@1-act.com
David L. Ellis
NASA Glenn Research Center
21000 Brookpark Road, Cleveland, OH 44135 U.S.A.
216-433-8736, 216-977-7132 Fax, David.L.Ellis@nasa.gov

ABSTRACT
There are a number of different applications that could use heat pipes or loop heat pipes (LHPs) in the intermediate temperature range of 450 to 750 K, including space nuclear power system radiators, fuel cells, geothermal power, waste heat recovery systems, and high temperature electronics cooling. Titanium/water and Monel/water heat pipes are suitable for temperatures up to 550 K, based on life tests that have been running for over 54,000 hours (6.1 years). At higher temperatures, organic or halide working fluids can be used. Long term life tests (currently 50,000 hours or 5.7 years) show that Titanium/TiBr4 at 653 K, and Superalloys/AlBr3 at 673 K are compatible. These results are confirmed by optical and electron microscopy, and working fluids analysis on heat pipes chosen for destructive examination.

KEY WORDS: Intermediate Temperature Heat Pipe Life Tests, Halide Life Tests, Water Life Tests
1. INTRODUCTION
There are a number of different applications that could use heat pipes or loop heat pipes (LHPs) in the intermediate temperature range of 450 to 750 K, including space nuclear power system radiators, fuel cells, geothermal power, waste heat recovery systems, and high temperature electronics cooling.

The intermediate temperature region is generally defined as the temperature range between 450 and 750 K. At temperatures above 700-725 K, alkali metal (cesium) heat pipes start to become effective. Below about 725 K, the vapor density for cesium is so low that the vapor sonic velocity limits the heat transfer. Historically, water was used at temperatures up to about 425 K. More recently, it has been shown that water can be used with titanium or Monel envelopes at temperatures up to 550 K (Anderson, Dussinger, Bonner, and Sarraf, 2006). A survey of previous life tests on intermediate temperature working fluids can be found in Anderson (2007) and Anderson et al. (2010).

2. EXPERIMENTAL PROCEDURE
Since 2004, we have been conducting life tests at temperatures up to 550 K with water and Commercially Pure Titanium Grade 2 (CP-Ti) Titanium, titanium alloys, Monel 400, and Monel K500 heat pipes. The test setup for the titanium/water and Monel/water life tests are discussed in Anderson et al. (2006). As of April 2012, several of the titanium and Monel heat pipes have been on test for over 54,000 hours (6.1 years). Periodically, the temperature is lowered to 343 K, and the pipes are tested for Non-Condensable Gas (NCG). A small amount of NCG has been noted in these pipes at 70°C, but the gas cannot be detected at the operating temperature of 550 K.

At temperatures above 550 K, the surface tension of water is so low, and the vapor pressure is so high, that water is no longer an attractive fluid. At higher temperatures, three sets of organic fluids stand out as good intermediate temperature fluids:

1. Diphenyl, Diphenyl Oxide, and Eutectic Diphenyl/Diphenyl Oxide (Dowtherm A, Therminol VP, Diphyl),
2. Naphthalene
3. Toluene.

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In some cases, a non-organic working fluid is desirable, particularly for nuclear fission space power, where the radioactivity with organic working fluids can generate gas. Since 2006, we have been conducting life tests at temperatures up to 673 K with titanium and three corrosion resistant superalloys (Hastelloy B-3, Hastelloy C-22, and Hastelloy C-2000), and five different halides working fluids: AlBr3, GaCl3, SnCl4, TiCl4, TiBr4. The selection criteria were discussed in Anderson et al. (2007). Based on these life tests, two of the halides appear to be suitable for temperatures up to 673 K, and possibly at higher temperatures. Long term life tests are ongoing with TiBr4/titanium at 653 K, and with AlBr3/Superalloys at 673 K. As of April 2012, the AlBr3 and TiBr4 tests have been running for 50,000 hours (5.7 years).

In late 2010, several of the heat pipes were selected for destructive investigation (Table 1). One of each pair of water life test pipes was selected. The GaCl3 and SnCl4 pipes were known to be non-compatible, since they generated large amounts of NCG. The heat pipes containing halides were neutralized using an appropriate liquid. The neutralized fluid and water from the heat pipes using water as a working fluid were collected for chemical analysis.

To examine the cross-sections to determine the type and amount of corrosion in the wicks and heat pipes, the heat pipes were cut in half, pressure infiltrated with epoxy and sectioned at a location approximately one-third of the way above the bottom of the heat pipe. The sections were polished through 0.05-micron silica and examined using optical and scanning electron (SEM) microscopes.

3. RESULTS AND DISCUSSION
3.1 Microscopic Examination of Cross-sections
3.1.1 Titanium-Water Heat Pipes
Analysis of the cross-sections using optical and electron microscopy revealed little if any corrosion even when observed at high magnifications. When any evidence of corrosion was observed, the layer was typically around 1 micron thick. There were some indications in the backscattered electron (BSE) SEM images of changes in the structure of the Ti and Ti alloys as shown by the lighter bands near the surface in Figure 1. Energy dispersive spectroscopy (EDS) did not indicate that the layers changed composition, and they may be related to either a change of the Ti from the α phase to the β phase or changes in grain structure from pickup of interstitial elements such as O.

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The two titanium halide heat pipes examined had very different responses. CP Ti with a TiBr4 working fluid had minimal corrosion. There was evidence similar to the layers shown in Figure 1 of some potential change in the outer 10 microns.

CP Ti with GaCl3 working fluid underwent extensive corrosion as seen in Figure 2. EDS analysis indicated that the corrosion layer was a Ga-29.7 wt.% Ti alloy. Examination of the Ga-Ti phase diagram (NPL, 2012) led to the conclusion that a mixture of Ga2Ti, Ga3Ti and Ga(l) was probably formed. Given the extensive nature of the voids, particularly on the wires, it was hypothesized that considerable Ga(l) was present in the voids and lost during neutralization. The remaining Ga2Ti reaction layer also exhibited evidence of brittle fracture during polishing.

3.1.3 Monel-Water Heat Pipes
Figure 3 shows an optical micrograph of the envelope and wick for Heat Pipe 136, one of the Monel-water heat pipes that underwent the most change. The formation of a dark subsurface layer and bright nodules were observed in the Monel 400 wick using BSE imaging. This was typical of all Monel 400 surfaces examined to varying but large degrees. EDS spot analysis revealed that the surface nodules were nearly pure Cu, and the dark layer was a Cu-depleted zone. Most likely, there was a phase change from α to α1+ α2 (ASMI, 1992) followed by diffusion of the Cu to the surface during exposure.

The Monel K500 does not show similar changes. Close examination of the envelope reveals, at most, a very thin corrosion layer. Most likely, the layer was an oxide, but it was sufficiently thin to prevent definitive identification through EDS. Apparently, the composition of the Monel K500 stabilizes the α phase.

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3.1.4 Hastelloy Superalloy-Halide Heat Pipes
Table 1 lists the five corrosion-resistant Hastelloy C-series superalloy heat pipes that were examined. As shown in Figure 4, the SnCl4 working fluid caused considerable roughening of the C-22 envelope, up to 20-micron deep cracks in the substrate, a porous 10-micron thick corrosion layer and a thin, discontinuous Mo-W-Sn reaction layer beneath the corrosion product.

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Hastelloy C-2000 also underwent extensive reaction with the SnCl4 working fluid as shown in Figure 5. A 200-micron thick reaction layer identified as Ni3Sn2 with about 9 wt.% Cl was observed. In addition to the Ni-Sn-Cl reaction layer, Mo-Cl particles were observed at the reaction layer/substrate interface during X-ray mapping.

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Heat Pipes 153 and 157 showed good promise. Hastelloy C-2000 underwent little corrosion when used with TiCl4 working fluid. A 1 to 2 micron thick Ni-33 wt.% Ti-18 wt.% Mo-18 wt.%Cr-4 wt.% Cu-2 wt.% Cl corrosion layer was observed on the surface. BSE images showed that there might be a 0.5-micron thick region beneath the corrosion layer that was depleted in heavy elements.

Hastelloy C-22 exhibited a dual corrosion layer when tested with AlBr3 working fluid as shown in Figure 6. The total thickness was 5 to 10 microns. EDS analysis of the two layers showed that the outer layer composition was Ni-11.5 wt.% Cr-11.9 wt.% Mo-3.6 wt.% Fe-9.4 wt.% W-0.6 wt.% Mn-1.7 wt.% Co-0.3 wt.% V-0.8 wt.% Si-9.5 wt.% Br. The inner corrosion layer composition was Ni-12.8 wt.% Cr-12.4 wt.% Mo-3.2 wt.% Fe-6.4 wt.% W-0.2 wt.% Mn-1.3 wt.% Co-0.3 wt.% V-21.9 wt.% Br. Based upon these analyses, it appears that AlBr3 can slowly react with the C-22 over thousands of hours to form a relatively thin corrosion layer.

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3.2.5 Analysis of Working Fluids
Table 2 contains the results of the chemical analysis of the working fluids. As a standard, deionized water exposed to Ti and Monel 500 (DI Water Std.) was analyzed as well. Only the elements present in the envelope and wick alloys are listed. Minimal pickup of metals was observed for the heat pipes using water as a working fluid.

The heat pipes that used halides as a working fluid showed more contamination of the working fluids. Heat pipes 153 and 157, which appeared to form a protective layer, exhibited low total metal contents between 300 and 350 ppm. Heat Pipes 7 and 8, which used SnCl4, suffered more working fluid contamination with Cr being the major metal present. The relative amounts seem to be consistent with the qualitative levels of attack observed. GaCl3 was clearly the most aggressive working fluid when paired with Ti, which is consistent with the microscopic observations. 1.2 wt.% Ti was present in the working fluid of Heat Pipes 9 and 10. Since titanium was the only metal in both the envelope and fluid, no fluid analysis was made for Heat Pipe 4 (CP-Ti/TiBr3), which had little evidence of attack. The high level of Ti in the GaCl3 for Heat Pipe 10 is consistent with the large amount of corrosion and possible Ti-containing particles in the working fluid. Note that this pipe developed a leak in the first few hours after it was put on life test.

3. CONCLUSIONS
Titanium/water and Monel/water heat pipes are compatible at temperatures up to 550 K, based on ongoing life tests that have been running for over 54,000 hours (6.1 years) as of April 2012. Analysis of titanium/water heat pipe cross-sections using optical and electron microscopy revealed little if any corrosion even when observed at high magnifications. When any evidence of corrosion was observed, the layer was typically around 1 micron thick. Copper depleted zones, as well as copper surface nodules formed on the Monel 400 screen wick. This was not observed on the Monel K500 envelopes. An analysis of the water working fluids showed minimal pickup of metals.

A non-organic working fluid is desirable for nuclear fission space power and other applications where radioactivity can generate gas with organic working fluids. Long term life tests show that Superalloys/TiCl44 at 573 K (300°C), and Superalloys/AlBr3 at 673K (400°C) are compatible. As of April 2012, the AlBr3 and TiCl4 tests have been running for 50,000 hours (5.7 years).

Hastelloy C-2000 underwent little corrosion when used with TiCl4 working fluid, with the formation of only a 1-2 micron thick corrosion layer. Hastelloy C-22 exhibited a 5-10 micron thick dual corrosion layer when tested with AlBr3 working fluid. The working fluids of these two heat pipes exhibited total metal contents between 300 and 350 ppm. The results indicate that the tested envelope materials and working fluids can form viable material/working fluid combinations.

ACKNOWLEDGEMENT
The water and halide life tests were sponsored by NASA Glenn Research Center under Contracts NNC05TA36T, and NNC06CA74C. We would like to thank Cheryl Bowman and Ivan Locci of NASA Glenn Research Center for helpful discussions about the fluids and materials. The authors would like to acknowledge the metallographic sample preparation by Joy Buehler of NASA.
REFERENCES
Anderson, W. G., Hartenstine, J. R., Sarraf, D. B., and Tarau, C., “Intermediate Temperature Fluids for Heat Pipes and Loop Heat Pipes,” 15th International Heat Pipe Conference (15th IHPC). Clemson, USA, April 25-30, 2010.
Anderson, W. G., “Intermediate Temperature Fluids for Heat Pipes and LHPs,” Proceedings of the 2007 IECEC, AIAA, St. Louis, MO, June 25-27, 2007.
Anderson, W.G., Bonner, R.W., Dussinger, P.M., Hartenstine, J.R., Sarraf, D.B., and Locci, I.E., “Intermediate Temperature Fluids Life Tests – Experiments” Proceedings of the 2007 IECEC, AIAA, St. Louis, MO, June 25-27, 2007.
Anderson, W.G., Dussinger, P.M., Bonner, R.W., and Sarraf, D.B., “High Temperature Titanium-Water and Monel-Water Heat Pipes,” Proceedings of the 2006 IECEC, AIAA, San Diego, CA, June 26-29, 2006.
ASM International (ASMI), Cu-Ni Phase Diagram, ASM Handbook, Vol. 3, Alloy Phase Diagrams, ASM International, Materials Park, OH (1992), p. 2-173.
National Physical Laboratory (NPL), Calculated Ga-Ti Phase Diagram, London, UK, http://resource.npl.co.uk/mtdata/phdiagrams/gati.htm, retrieved March 12, 2012.