EXPERIMENTAL LIQUID METAL SLIP RING PROJECT by R. B. Clark HUGHES AIRCRAFT COMPANY Prepared for NATIONAL AERONAUTICS and SPACE A
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NASA CR-.72780 EXPERIMENTAL LIQUID METAL SLIP RING PROJECT by R. B. Clark HUGHES AIRCRAFT COMPANY prepared for NATIONAL AERONAUTICS AND SPACE ADMINISTRATION NASA Lewis Research Center Contract NAS 3-1 1537 Robert R. Lovell, Project Manager I NOTICE I This report was prepared as an account of Government-sponsored work, Neither the United States, nor the National Aeronautics and Space Administration (NASA), nor any person acting on behalf of NASA: A. ) Makes any warranty or representation, expressed or implied, with respect to the accuracy, completeness, or usefulness of the information contained in this report, or that the use of any information, apparatus, method, or process disclosed in this report may not infringe privately-owned rights; or B. ) Assumes any liabilities with respect to the use of, or for damages resulting from the use of, any information, apparatus, method or process disclosed in this report. As used above, "person acting on behalf of NASA" includes any employee or contractor of NASA, or employee of such contractor, to the extent that such employee or contractor of NASA or employee of such contractor prepares, disseminates, or provides access to any information pursuant to his employment or contract with NASA, or his employment with such contractor. Requests for copies of this report should be referred to National Aeronautics and Space Administration Scientific and Technical Information Facility P. 0. Box 33 College Park, Md. 20740 NASA CR-72780 FINAL REPORT EXPERIMENTAL LIQUID METAL SLIP RING PROJECT R. B. Clark HUGHES AIRCRAFT COMPANY Space Systems Division Los Angeles, California 90009 prepared for NATIONAL AERONAUTICS AND SPACE ADMINISTRATION June 22, 1970 Contract NAS 3-1 1537 NASA Lewis Research Center Cleveland, Ohio Robert R. Lovell, Project Manager Spacecraft Technology Division FORWARD The work described herein was done at the Space Systems Division of Hughes Air craft Company under NASA Contract No. NAS 3-1 1537 with Mr. Robert R. Lovell, Spacecraft Technology Division, NASA-Lewis Research Center, as Project Manager. TABLE OF CONTENTS SUMMARY INTRODUCTION Background Purpose and Scope Design Requirements Design Approach Conditions and Controls Significance of the Work MATERIALS SELECTION AND EXPERIMENTS Literature Search Expe riment s Apparatus and Procedures Screening Test Data Selection of Materials Long-Term Evaluation Test ENGINEERING TEST MODEL DESIGN AND FABRICATION Design Objectives Design Description Fabrication and Trial Assembly ENGINEERING TEST MODEL TESTING AND EVALUATION Introduction Assembly and Preliminary Measurements Thermal-Vacuum Performance Test Post- Test Examination TABLE OF CONTENTS (continued) DISCUSSION OF RESULTS Mate rial Selection Electrical Characteristics Slip Ring Configurations Gallium Residue Specifications, Procedures and Controls SUMMARY OF RESULTS APPENDICES A. Bibliography B. X-Ray Diffraction Analysis of Surface Contamination Products from Liquid Gallium Slip Rings C. New Technology REFERENCES DISTRIBUTION LIST LIST OF TABLES Gallium vs Mercury Low Melting Point Alloys of Gallium Candidate Materials for Electrodes Other Materials Considered for Electrodes Barrier Film Candidates Insulator Material Candidates Repeatability of Electrode Weights Screening Test Data, Beryllium Electrodes Screening Test Data, Beryllium- Copper Electrodes Screening Test Data, Nickel-Plated Copper Electrodes Screening Test Data, Stainless Steel Electrodes Screening Test Data, Tungsten Electrodes Screening Test Data, Averages Interface Resistances, Gallium to Electrode Materials Screening Test Results vs Selection Criteria, Electrode Materials Rating of Electrode Materials Long-Term Evaluation Test Data, Electrode Materials Spectroscopic Analysis of Long-Term Evaluation Test Samples Weight Changes of Insulator and Barrier Film Samples During Long-Term Evaluation Test Slip Ring Arrangement for Testing Capacitance Between Slip Rings Voltage and Temperature Data, Vacuum Test of ETM LIST OF TABLES (continued) 2 3. Sinusoidal Component of Re sistance vs Shaft Angle Data 24. Contaminating Elements Found in Engineering Test Model after Thermal-Vacuum Tests 25. Comparison of Slip Ring Characteristics LIST OF FIGURES Liquid Metal Slip Ring Assembly Electrode Configuration Proposed Contact Angle Matrix for Electrode Material Evaluation, Screening Test Material Evaluation Fixture Drawing of Sample Electrode Barrier Film and Insulator Samples Application of Barrier Films Argon Supply to Glove Box Setup for Machining Electrodes in Gallium Thermal Chamber, 44OC Thermal Clamber, 84OC Identification and Map of Material Evaluation Fixtures Enlarged View, ~eltin~/~reezin~Determination Measurement of Surface Tension by Ring Removal Electrical Noise Photographs, 43O~Chamber, 20 A LIST OF FIGURES (continued) Electrical Noise Photographs, Screening Test Beryllium Electrode 41 Negative Before and After Screening Test Beryllium-Copper Electrode Before and After Screening Test Nickel-Plated Copper Electrode Before and After Screening Test Stainless Steel Electrode Before and After Screening Test Tungsten Electrode Before and After Screening Test Electrodes of MEF 55 After Screening Test Insulator Samples After Screening Test Barrier Film Samples After Screening Test Location of Barrier Film-Electrode Material Samples in Figure 25 Barrier Film and Insulator Samples Before Long-Term Evaluation Test Samples in Vacuum Chamber for Long-Term Evaluation Test Gallium Wrinkles After Pumpdown Stainless Steel and Nickel-Plated Copper Material Evaluation Fixtures After Long-Term Evaluation Test Stainless Steel and Nickel-Plated Copper Electrodes After Long- Term Evaluation Test Barrier Film and Insulator Samples After Long-Term Evaluation Test Barrier Film and Insulator Samples with Gallium It Poured" Off viii LIST OF FIGURES (continued) Engineering Test Model with Thermal Shroud Removed Engineering Test Model Liquid Metal Slip Ring Assembly Slip Ring Configurations for ETM Dimensions of Radial Gap Ring Dimensions of Cup Gap Ring Capillary Containment of Liquid in a Gap Shaft Cross-Section of Slip Ring Assembly with Thermal Shroud Setup for Casting the Shaft Slip Rings Before Wetting with Gallium Initial Wetting of Cup Gap Inner Ring Assembling and Filling the Slip Rings Rewetting of Ring 2 Vacuum Test Setup Electrical Schematic, Vacuum Test Variation of Slip Ring Resistance with Shaft Angular Position Discoloration and Tracking Due to Electrical Breakdown of Lamp Circuit Gallium Sludge on Ring 1 at Disassembly Ring 8 Prior to Disassembly Shaft and Posts After Disassembly of ETM Cup Gap Outer Rings After Thermal-Vacuum Testing LIST OF FIGURES (continued) Cup Gap Inner Rings After Thermal-Vacuum Testing Cup Gap Rings with Gallium Removed Disassembly of Slip Ring 6 Radial Gap Inner Rings, Gallium Removed Ring 10, Gallium Frozen Insulators of Radial Gap Rings, Showing Black Powder Residue Radial Gap Slip Rings at Disassembly X-Ray Diffraction Film Prints from Sample 6, Grey Residue from Ring 6 X-Ray Diffraction Film Prints from Sample 10, Grey Residue from Ring 10 X-Ray Diffraction Film Prints from Sample 11, Gallium Drops from Below Ring 10 X-Ray Diffraction Film Print from Sample 13, Dark Powder Residue from Below Ring 10 (on adhesive tape) ABSTRACT Slip rings employing the liquid metal gallium to transfer electrical power across rotating joints were experimentally studied with respect to application in satellites. Candidate materials for electrodes, insulators and barrier films were tested. A ten ring assembly was operated in high vacuum for 60 days carrying 100 A. Data from electrical, physical, chemical and visual examination is preseni ed. Contact resistance was less than one microhm cm for nickel- plated electrodes wetted with gallium. Surface film on the gallium caused debris during rotation. Gallium was retained in the rings by surface tension. SUMMARY Liquid metal slip rings are sets of two concentric metal rings separated by a narrow annular gap, the gap being filled with a liquid metal, Electric current will flow from one ring to another with very low resistance, thus providing a means of transferring electrical power between two members of a machine which can rotate with respect to one another. Liquid metal slip rings may be expected to transfer electrical power and signals across rotating joints with extremely low power 10s s and electrical noise combined with long life, low mechanical friction and the elimination of the brush wear residue from conventional brush on ring assemblies. This program experi- mentally studied the feasibility of using the liquid metal gallium to transfer high power across rotating joints in satellites. Gallium was chosen primarily because of its very low rate of evaporation. Candidate materials for slip ring electrodes, insulators and non-wetting barrier films for use with the very corrosive gallium were selected after a literature search. The approach selected was to use common engineering materials and wet the electrodes with the gallium to obtain the lowest electrical interface resistance. Exotic techniques were avoided, but stringent controls were exercised, including doing much of the work in an atmosphere of inert argon gas. A screening test utilized 65 non-rotating material evaluation fixtures. Electrode samples in contact with gallium were subjected to various currents and temperatures for one month in an atmosphere of argon gas. All of the electrode samples were wetted with gallium by machining the active surface in a pool of liquid gallium. Electrical, physical, chemical