A GIANT MAGNETOCALORIC EFFECT FOR CRYOGENIC REFRIGERATION

AN ENORMOUS MAGNETOCALORIC EFFECT HAS BEEN DISCOVERED IN FINE-GRAIN SPINEL POWDERS AT LIQUID HELIUM TEMPERATURES. FOR EXAMPLE, AN ADIABATIC-DEMAGNETIZATION TEMPERATURE DROP FROM 8.5 TO 5.0 K HAS BEEN MEASURED WITH A MAGNETIC FIELD OFONLY 3.8 GAUSS; THESE EFFECTS HAVE BEEN DUPLICATED ON FOUR POWDER SAMPLES AND APPEAR TO BE REVERSIBLE. TWO DIFFERENT SPINELS ARE INVOLVED. AN OPPORTUNITY EXISTS FOR PROVIDING REFRIGERATION IN THE 2 TO 15 K RANGE BY STAGING A SIMPLE MAGNETIC REFRIGERATOR ONTO A CLOSED-CYCLE, STIRLING CRYOCOOLER. THE MAGNETIC REFRIGERATOR WOULD USE THESE SPINEL POWDERS AND INVOLVE MAGNETIC FIELDS ACHIEVABLE WITH SIMPLE SOLENOIDS. TOWARD THIS END, PHASE I WILL MEASURE THEMAGNETOCALORIC EFFECTS AND MAGNETIC FIELD DEPENDENCE OF THE SPECIFIC HEATS OF THESE POWDERS AS A FUNCTION OF PARTICLE SIZE AND SPINEL TYPE IN THE 2 TO 15 K RANGE. DATA WILL BE ANALYZED ACCORDING TO THE TDS EQUATION USING A GINZBURG-LANDAU EXPANSION OF THE STATE FUNCTION, AND THESE ANALYSES WILL BE USED TO IDENTIFY THE MOST FAVORABLE POWDER FOR THIS APPLICATION. RESEARCH ALSO WILL BE DONE TO OPTIMIZE THE THERMAL EQUILIBRATION TIME WITHIN THE POWDER COMPACTS. FINALLY, CLOSED-CYCLE MODELING ESTIMATES WILL BE MADE BASED ON THESE RESULTS TO QUANTIFY THE ANTICIPATED PERFORMANCE OF THIS MAGNETIC REFRIGERATOR.