THE AVAILABILITY OF EFFICIENT HEAT SWITCHES WHICH FACILITATE THE DISSIPATION OF HEAT TO THE ENVIRONMENT AT HIGH TEMPERATURES (>15 C) AND PREVENT HEAT EXCHANGE AT LOW TEMPERATURES (<-15 C) WOULD ALLOW TO MAINTAIN ALL VEHICLE COMPONENTS WITHIN AN APPROPRIATE TEMPERATURE RANGE THROUGHOUT THE MANY MISSION PHASES DESPITE CHANGING HEAT LOADS AND ENVIRONMENTAL TEMPERATURES. CURRENT STATE-OF- THE-ART TECHNOLOGIES FOR PASSIVE HEAT SWITCHES CAN BE DIVIDED INTO THREE MAIN CATEGORIES: PHASE-CHANGE BASED GAS-GAP AND THERMOMECHANICAL SWITCHES. HOWEVER MOST OF THESE SYSTEMS ARE EITHER BULKY OR NOT SCALABLE (E.G. PARAFFIN OR GAS-GAP SWITCHES) OR HAVE MODERATE SWITCHING RATIOS (E.G. HEAT PIPE VARIABLE EMISSIVITY OR THERMAL EXPANSION SWITCHES). FEW PROMISING TECHNOLOGIES INCLUDE SHAPE MEMORY ALLOY SWITCHES AND LIQUID-VAPOR PHASE CHANGE DIODES OR SWITCHES. ELECTROSTATIC OR ELECTROWETTING-ON-DIELECTRIC-BASED SWITCHES HAVE THE POTENTIAL TO ACHIEVE SWITCHING RATIOS UP TO 1000: 1 BUT REQUIRE ACTIVE ACTUATION. THE HEREIN PROPOSED TECHNOLOGY WOULD BE CAPABLE OF ACHIEVING CONDUCTANCE RATIOS OF AT LEAST 500 : 1 (UP TO 1000 : 1) OVER A LARGE AREA AT A FULL ON PERFORMANCE (HIGH CONDUCTANCE)>6 W/K AND AN OFF CONDUCTANCE ~ 1 MW/K. FURTHERMORE THE SWITCH IS COMPACT LIGHT-WEIGHT AND IS PASSIVELY TEMPERATURE-ACTUATED. SUPPORTED BY PRELIMINARY RESULTS WHICH YIELDED A CONDUCTANCE RATIO OF 72:1 WE PROPOSE TO DESIGN FABRICATE AND CHARACTERIZE A ROBUST HEAT SWITCH BASED ON ROOM-TEMPERATURE LIQUID METAL. THE PROPOSED HEAT SWITCH WILL BE BASED ON THE MANIPULATION OF A LIQUID METAL WITHIN A 9 CM LONG 4 CM WIDE AND 1 MM TALL CHANNEL. THE PRESENCE OR THE ABSENCE OF THE LIQUID METAL NEAR A HEAT SOURCE AFFECTS HEAT FLOW FROM THE SOURCE TO THE SINK BY A FACTOR OF 500 OR MORE. GALLIUM-BASED ALLOYS HAVE THE HIGHEST THERMAL CONDUCTIVITIES OF ALL LIQUIDS (17-40 W/M-K VS. 0.02 (AIR) TO 0.6 (WATER) W/M-K) AND ALLOW FOR EASY MANIPULATION ARE THERMALLY AND CHEMICALLY STABLE AND ADAPT EASILY TO DIFFERENT TOPOLOGIES. THE SWITCH WILL BE PASSIVELY CONTROLLED BASED ON THERMAL EXPANSION MISMATCH. AT HIGH TEMPERATURES I.E. IN THE ON STATE THE METAL WILL BE IN ITS LIQUID STATE AND CONNECT THE HEAT SOURCE TO THE HEAT SINK. AT LOW TEMPERATURES I.E. IN THE OFF STATE THE METAL WILL DISCONNECT THE INNER COMPONENTS OF THE VEHICLE WITH THE SHELL AND SOLIDIFY IN THE OFF POSITION. THE SWITCH WILL ENABLE CONTROL OF HEAT FLOWS OVER MULTIPLE TIME SCALES AND DIFFERENT LEVELS OF HEAT LOAD. OUR PASSIVE LIQUID METAL HEAT SWITCH NOT ONLY UNIQUELY MEETS ALL OF THE CRITERIA DESCRIBED IN THE NASA RESEARCH ANNOUNCEMENT BUT IT ALSO POTENTIALLY ALLOWS THE INTEGRATION INTO FLEXIBLE THERMAL MANAGEMENT SYSTEMS. TO SUCCESSFULLY ACHIEVE THE TARGETED PERFORMANCE GOALS (500+ : 1 SWITCHING RATIO >3 W/K ON CONDUCTANCE) WE BUILD UPON THE PI S EXPERTISE IN THE THERMAL MANAGEMENT OF HIGH-POWER ELECTRONICS PHASE CHANGE HEAT TRANSFER AND DROPLET WETTING DYNAMICS. TO EVALUATE THE PROGRESS OF THE PROPOSED EFFORT WE DEFINE THE FOLLOWING MILESTONES: YEAR 1 WILL FOCUS ON THE THERMO-CHEMICAL CHARACTERIZATION OF THE LIQUID MENTAL AND THERMAL PROTOTYPE DESIGN FABRICATION ANALYSIS AND OPTIMIZATION. YEAR 2 WILL FOCUS ON THE DEVELOPMENT AND CHARACTERIZATION OF THE ACTUATION MECHANISM FOR ON/OFF SWITCHING AND DURABILITY TESTING. FINALLY YEAR 3 WILL EXPLORE THE INTEGRATION OF THE HEAT SWITCH WITH FLEXIBLE MATERIALS. WE ENVISION THAT THIS PROJECT WILL UNLOCK FUNDAMENTAL DESIGN GUIDELINES FOR THE FABRICATION OF THERMAL SWITCHES CAPACITORS AND RECTIFIERS (DIODES). AS THE ONLY HEAT SWITCH TO SATISFY ALL OF THE REQUIRED AND DESIRED FEATURES OF PASSIVE TUNABLE THERMAL MANAGEMENT SYSTEMS THIS PROPOSED SWITCH WILL BE UNIQUELY APPLICABLE TO MANNED AND UN-MANNED LUNAR AND DEEPSPACE EXPLORATIONS AND COULD THEREFORE SERVE AS A DISRUPTIVE ADVANCE IN THE TUNABLE THERMAL MANAGEMENT OF SPACECRAFT SYSTEMS.

Washington University, The

Investigators