Test and Post-Test Analysis of a Thermacore, Inc. Nickel Powder Wick Heat Pipe Solar Reciever

This report is a cradle-to-grave fabrication and postmortem analysis of a sodium-filled heat pipe solar receiver. The Stirling Thermal Motors Gen. H engine was tested with the Thermacore, Inc. heat pipe receiver on Sandia's Test Bed Concentrator II in the fall of 1996. Although engine performance was significantly increased relative to a direct insolation version of the receiver, hot spots did develop on the heat pipe receiver dome. Over the course of a couple of weeks, after tests were completed, the sodium was distilled out of this receiver, and the front dome was removed. Several failure spots and/or cracks (dubbed "volcanoes") were present on the surface of the wick. Postmortem analysis indicates that the cracks in the wick of the heat pipe are not a product of corrosive oxide action. Voids formed within the wick (created either by mechanical or thermal means) serve to concentrate phosphorous from the electroless plating into the liquid sodium. The presence of phosphorous has an apparently harmful effect on the wick. Examination of a virgin piece of the nickel wick material treated in the same manner as the bulk, prior to the introduction of sodium, would be the best baseline sample for comparison. This samplemore » could be analyzed for phosphorous migration into the wick and determine if there is any initial crack formation from the sintering process. Utiortunately a sample of this material was not available during the preparation of this report. Continued work to determine the mechanism of crack formation could significantly increase the hours of available lifetime testing for future solar thermal heat pipe receivers« less