2010.10.06

1. M1 colloquium Phase boundary study of Pd-H(D) systems by means of electrical resistivity measurements H.Araki , Y.Sakamaki , S.Harada , M.Kubota , Journal of Alloys and Compounds 446–447 (2007) 436–438 Shimizu-group M1 Yasuhiro Asakura 2010.10.06

2. Contents ・Introduction 1)Palladium-Hydrogen system 2)Previous work － Superconductivity － The 55K anomaly ・Experiment ・Result and Discussion ・Summary

3. Introduction Hydrogen storing alloy ・Hydrogen has high chemical energy and can emit as clean water when using the energy. ・The formulation of “hydrogen energy system” lead the solution of the environmental and energy problems on a global scale. ・Hydrogen storing alloy is the material can use hydrogen energy in safety. ・First palladium (Pd) was proposed as the metal can heavily absorb hydrogen.

4. Introduction Palladium-Hydrogen system(1) palladium ・face-centered cubic (fcc) structure ・dissociate hydrogen molecules (H2/D2) ・absorb large amount of H(D) atoms up to H(D)/Pd=1 fcc structure The spaces occupied by hydrogen are the interstitial octahedral (O) sites of palladium. The interstitial octahedral (O) sites in the fcc lattice

5. Introduction Palladium-Hydrogen system(2) palladium hydride ・become superconductors ・H(D) atoms in Pd are known to cause a quantum diffusion due to tunneling effect . Peculiar temperature dependence , H-D Isotope effect ・Can hold hydrogen with higher density than liquid hydrogen It is expected that the property of more compressed hydrogen can be investigated under high pressure.

6. Introduction Previous work in PdH -Superconductivity- Tc vs concentration x ・In 1972, discovered by Skoskiewicz1 (Palladium didn’t become the superconductive phenomenon. ) ・In 1979, it was reported that the superconducting transition temperature (Tc) increases with increasing concentration of hydrogen. PdDx Relation of Tc and concentration x Tc = 150.8(x-x0)2.244 ( X0= 0.715 for PdHx, 0.668 for PdDx ) PdHx Above expression was estimated using this figure. R.W. Standly, M. Steinback, C.B. Satterthwaite, Solid State Commun. 31 (1979) 801. [1]T.Skoskiewicz, Phys. Solidi, all K123 (1972)

7. Introduction Previous work in PdH -The 55 K anomaly- These anomalies were caused by an order-disorder transition of hydrogen in palladium. In Fig.1, the anomaly temperature increases with increasing in the concentration of hydrogen. Like Fig.2, the phase diagram is not as complex as had been thought in the past2. Fig.2 Phase Diagram Fig.1The excess specific heat per mole of hydrogen CPdHx, CPd measured specific heat. CelectronPdHx,CelectronPd were evaluate by past measurement data. H. Araki, M. Nakamura, S. Harada, T. Obata, N. Mikhin, V. Syvokon, M. Kbota, J. Low Temp. Phys. 134 (2004) 1145. [2] O. Blaschko, J. Less-Common Met. 100 (1984) 307.

8. Introduction Motivation If the anomaly of specific heat is caused by an order-disorder transition of hydrogen with H(D) sites migrating, it is interesting to consider the possibility of a phase transition with the other quantum behavior of hydrogen, such as the quantum diffusion. The electrical resistivity of PdHx and PdDx at high concentration has been measured in order to compare the anomaly temperature at the phase boundary in PdHx to that in PdDx.

9. Experiment Sample : Palladium rod ( 99.95 % pure, 0.500 mmφ) － outgassed and annealed under vacuum ・Hydrogen was loaded into the palladium by electrolytic method. － The loading was carried out at 203 K with electrolytes of 5.25 mol/ℓH2SO4 in H2O or D2SO4 in D2O . ・After loading, the samples were preserved in liquid nitrogen. － To hinder the loss of hydrogen V- The electrical resistivity measurement use for the four-probe method Electrode Concentration x was determined by using above expression. PdH(D)x I- I+ PdHx : Tc = 5.12 K ⇒ x = 0.936 PdDx : Tc =7.15, 6.90, 6.75, 6.60 K ⇒x =0.925, 0.921, 0.918, 0.916 V+

10. Result and Discussion The results at low temperature region from 4.2 K to 140 K The resistivity ρfor lower temperature than the Debye temperature Using Matthiessen’s rule ρ = ρi + ρL(T) ρ i : Residual resistivity (not depend on temperature) ρL(T) : Resistivity depended on temperature ( lattice vibration) Isochronal temperature increase resistivity curves after quenching

11. Result and Discussion About the residual resistivity ρi ・The resistivity in the Pd sample due to impurities. ・The resistivity of PdHx and PdDx are in proportion to the hydrogen vacancy concentration (1-x). Hydrogen vacancies would behave as impurities in PdH1.0 or PdD1.0. ρi vs concentration x

12. Result and Discussion PdHx ・The resistivity after rapid cooling (quenching) is smaller than that the after slow cooling below 94 K for PdHx. ・This slight deference occurring at anomaly temperature (Ta). This slight deference is dependant on the cooling rate and exists in the vicinity of Ta . PdDx has same behavior. PdDx

13. Result and Discussion Dependence of resistivity on the annealing time at 77.4 K after quenching from 140 K to 77.4 K. The annealing time dependence of the resistivity The resistivity of short-time annealing is lower than that of long-time annealing.

14. Result and Discussion Previous work Is this difference that the phase of the PdH(D)x changed from α+βphase to β phase? Is further annealing required? or The difference is normalized by the total resistivity change between 20 and 70 K H. Araki, M. Nakamura, S. Harada, T. Obata, N. Mikhin, V. Syvokon, M. Kbota, J. Low Temp. Phys. 134 (2004) 1145. K. Yamakawa, J. Alloys Compd. 389 (2005) 113.

15. Summary ・Resistivity anomaly was observed PdH0.936 and PdD0.916～0.925. ・Slight differences of the resistivity, which were dependent on the cooling rate, were observed. Thus, the phase boundary tends toward the high hydrogen concentration region. ・The decrease in resistivity caused by hydrogen ordering was not observed during annealing at 77.4 K for a period of 100 hour. Thank you for your attention !!