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Magnetocaloric effects in intermetallic compounds

Magnetocaloric effects in intermetallic compounds. • Introduction • Experimental results & discussion • Conclusions. Magnetic phase transitions Magnetocaloric effects & Magnetic refrigeration - Magnetic-refrigerant materials. 2 nd order phase transition & MCE

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Magnetocaloric effects in intermetallic compounds

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  1. Magnetocaloric effects in intermetallic compounds • Introduction • Experimental results & discussion • Conclusions • Magnetic phase transitions • Magnetocaloric effects & Magnetic refrigeration • - Magnetic-refrigerant materials • 2nd order phase transition & MCE • 1st order phase transition & MCE

  2. Introduction Magnetic phase transitions PM FM Tc

  3. TN TN

  4. Magnetic field-induced transition

  5. First-order phase transition Magnetization M Entropy Volume

  6. Second-order phase transition TC

  7. Magneto-caloric effect & Magnetic refrigeration Adiabatic ΔTad Isothermal ΔSm T T+ΔT S N ΔQ ΔQ Absorb heat T T-ΔT S N Cooling effect

  8. Thermodynamics Large ΔB Large Small CB,p

  9. Metal Gd sphere 3 kg Energy efficiency 20%-60% Cooling power 200 W-600 W C.O.P 2-9 ΔT = 4.5 K for 1.5 T ΔT = 11K for 5 T Magnetic field Superconducting magnet Gd

  10. Permanent magnetic field Space: 114 x 128 x 12.7 mm3 Field strength:  2 T Nd2Fe14B magnet Lee et al. JAP (2002)

  11. Magnetic refrigerant materials

  12. Adiabatic temperature change

  13. Ordering T:TC= 295 K Field change:ΔB = 5T FWHM : δTFWHM = 65 K MAX entropy change: -ΔSm(max) = 8.5 J/kgK Relative cooling power RCP(S) = -ΔSm(max)*δTFWHM =552 J/kg Cooling power What are important for MR?

  14. Experimental results & discussion Second order magnetic phase transition & MCE Gd Sth(max)= RLn(2J+1)=17.3 J/molK; Sth(max) = 110 J/kgK <10%

  15. TC = 298 K ΔB = 2 T ΔTad = 1.7 K Hashimoto et al (1982)

  16. First-order magnetic phase transition & MCE Orthorhombic Orthorhombic Pecharsky et al (1997)

  17. What makes Gd5Ge4-xSix have giant MCE? Single crystal Gd5Si1.7Ge2.3 Monoclinic (P1121/a) a = 7.585 Å b = 14.800 Å c = 7.777 Å β = 93.290 TC=240.4±1 K 0.05 T

  18. B-T phase diagram

  19. Magnetization Field-induced magnetic phase transition PM FM Field hysteresis 1 T

  20. Magnetic entropy changes TC = 240 K ΔB = 5 T ΔS(max) = 30.5J/kgK δTFWHM = 18K RCP(S) = 549 J/kgK Effect of magnetic anisotropy is small

  21. Specific heat capacity Gd5Si1.7Ge2.3 at TC ΔS = 11.0 ± 0.5 J/molK Latent heat L = 2.63 ± 0.12 kJ/mol

  22. ΔTad = Tc•ΔSm/Cp > 15 K

  23. Thermal expansion ΔL/L = (L(T)-L(T = 5 K))/L(T = 5 K) Transition at TC = 240.0 ±1.0 K T’C = 236.0 ±1.0 K Thermal hysteresis ΔT = 4 K ΔLa/La = 6.8x10-3 >0 ΔLb/Lb = -2.0x10-3 <0 ΔLc/Lc = -2.1x10-3 <0 Relative volume change ΔV/V = 2.7x10-3 Clausius-Clapeyron relation dTC/dp = 3.2 ± 0.2 K/kbar M. Nazih et al. 2002

  24. Transition-metal based compound: MnFeP1-xAsx Crystal structure (0.15  x  0.65) Fe2P-type; Hexagonal Space group P-62m At transition Δc/c > 0 Δa/a < 0 ΔV/V < 0 There is no crystallographic symmetry change. Magnetic moment 4 µB/f.u. Fe-layer Mn-layer Fe-layer 3g 1b/2c 3f

  25. X-T phase diagram Composition dependence of TC H PM AF FM T O X Bacmann et al. JMMM(1994)

  26. Magnetization 160-330 K Field hysteresis 0.5 T Thermal hysteresis 3.4 K

  27. B – T phase diagram of MnFeP0.45As0.55 Ordering T: TC = 306 K T’C = 302.2 K Thermal hysteresis: 3.8 K ΔTC/ΔB = 4.2 K/T First order phase transition

  28. Specific heat capacity Tp= 296 K Latent heat : L = 526 J/mol Cp = 550 J/kgK (T > 300 K)

  29. Magnetic entropy changes TC = 306 K ΔB = 5 T -ΔS(max)= 18.3 J/kgK δT = 21.3 K RCP(S) = 390 J/kg ΔTad =Tc•ΔSM/Cp ΔTad = 10 K (ΔB=5 T)

  30. Magnetic entropy change in different compositions MnFeP1-xAsx Isothermal magnetic entropy changes:

  31. Conclusions • MCE is closely related to the critical behavior of magnetic • phase transition. • Second order transition gives broad MCE peak. MCE is small. • First order transition gives sharp MCE peak. MCE can be large. • 2. Gd5Si1.7Ge2.3 has a simultaneous structural and magnetic phase • transition at 239 K. This transition is a first order transition with • thermal hysteresis  7.4 K and with field hysteresis 1 T. • The MCE related with first order phase transition is quite large. • Effect of magnetic anisotropy on MCE in this material is negligible. • MnFeP1-xAsx (0.25<x<0.65) has a first order phase transition • with thermal hysteresis  3.4 K and field hysteresis  0.5 T. • The MCE related with this transition is also quite large.

  32. 4. Advantages of MnFeP1-xAsx as a magnetic refrigerant 1. Large MCE 2. Tunable ordering temperature( between 168 and 332 K) 3. Small hysteresis 4. Lower cost : MnFe(P,As): Mn,Fe,P,As(99%, 150$/kg) Gd-Si-Ge Gd: Gd(4N): 4000 $/kg. Fe-Rh: Rh: 12000$/kg

  33. Acknowledgment This work is supervised by E. Brück, J.H.K. Buschow, F.R. de Boer. Collaborators: L. Zhang, W. Dagula, X.W. Li Financially supported by the STW.

  34. Bean-Rodbell model Gibbs free energy G = Gex + GH + Gdist + Gentr + Gpress Volume change is due to the effect of magnetization. Tc: Curie temperature T0: Curie temperature (not compressible) V : volume V0 : volume(absent of exchange interaction) N: number of atoms/V0 K: compressibility σ: relative magnetization (J =1/2)

  35. Bean et al. PR(1962) 2 1 η = 0 σ J=1/2 Set P = 0 η = 0; σ = 0 TC = T0 η < 1 corresponds to 2nd order phase transition η > 1 corresponds to 1st order phase transition For MnFeP0.5As0.5η = 1.62, J = 2, T0 =250 K R. Zach et al. JAP (1998)

  36. Heat capacity in field Adiabatic T change

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