2010.10.06

1. M1 colloquium Phase Diagram of Palladium-Hydrogen System H. Araki , M. Nakamura, S. Harada, T. Obata, N. Mikhin, V. Syvokon, M. Kubota , Journal of Low Temperature Physics, 134 (2004) 1145 Shimizu-group M1 Yasuhiro Asakura 2010.10.06

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

3. Introduction Hydrogen-storage 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 palladium ・face-centered cubic (fcc) structure ・dissociate hydrogen molecules (H2/D2) andabsorb large amount of H(D) atoms up to H(D)/Pd=1 The spaces occupied by hydrogen are the interstitial octahedral (O) sites of palladium. PdH(D)x (x:hydrogen concentration H(D)/Pd) fcc structure

5. Introduction Previous work in PdH(D)x -The 55 K anomaly- These anomalies were caused by an order-disorder transition of hydrogen in palladium. Anomaly in the resistivity of PdD0.634 I41/amd Phase Diagram of PdDx I4/m Long-range-ordered structures Journal of the Less-Common Metals, 100 (1984) 307 - 320

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 Palladium-Hydrogen system palladium hydride ・become superconductors ・H(D) atoms in Palladium at low temperature has a large diffusion coefficient due to quantum tunneling. Peculiar temperature dependence (for example,The 55 K anomaly) ・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.

8. Introduction Motivation Like previous figure, the phase diagram of the Pd-H(D) system around 55 K was rather complicated because of the long-range-ordering of hydrogen in Pd with a long relaxation time. The specific heat of PdHx at various concentrations has been measured in order to clarify the anomaly of that caused by the phase transition and in order to check the migration of hydrogen in palladium.

9. Experiment Sample : Palladium rod ( 99.99 % pure, 3.0 mmφ × 25 mm) － outgassed and annealed under vacuum (above 873 K for 24 hours) ・Hydrogen was loaded into the palladium by electrolytic method. － at 213 K with electrolytes of 5.25 mol/ℓH2SO4 in H2O. (current density of about 0.2 A/cm2 for 48 hours) Concentration x ⇒the change of the mass of the sample before and after hydrogen loading. H2SO4 PdHx : x =0 and 0.1777 to 0.8753 P. Tripodi et al. / Physics Letters A 276 (2000) 122–126

10. Result and Discussion The specific heat for PdHx was measured by the quasi-adiabatic heat pulse method (準断熱ヒートパルス法). Between 0.5 K to 100K (Hydrogen in palladium) CPdHx, CPd measured specific heat. CelectronPdHx,CelectronPd were evaluate by past measurement data. Cexcess caused by the hydrogen atoms dissolved in the palladium.

11. Result and Discussion Measurement raw data for PdHx Measurement raw data for pure Pd The excess specific heat due to hydrogenation Electronic specific heat for pure Pd Electronic specific heat for PdHx Hydrogen Einstein mode specific heat ・ ・ Above 75 K is due to optical mode vibrations of the hydrogen The specific heat capacity vs Temperature

12. Result and Discussion The excess specific heat converted for each hydrogen concentration. PdHx In the upper concentration range from x = 0.6572 to 0.8753 ・The height of the peak decreases with increasing x ―because of vacancies of hydrogen, 1-x At the same time ・The hydrogen concentration x dependence of the sharp peak temperature exist. New phase, only β-phase !! CexcessvsTemperatue

13. Result and Discussion This phase diagram show the peak of the specific heat by previous data and past data. The 55 K anomaly had been considered to be caused by an order-disorder transition. In β-phase, the sharp peak temperature moves to higher temperature linearly with increasing x. Extra anomalies of specific heat, however, were not recognized in this experiment. It was described that the phase diagram different of previous work. This simple phase diagram suggests several possibilities caused by hydrogen atoms and/or their vacancies in the sublattice. but

14. Summary ・The specific heat of PdHx over temperature range (0.5 K ～100 K) in the high and large concentration region (0≤x≤0.8753) was measured. ・Therefore, the new phase diagram was described. It is interesting that the pressure dependence of the anomaly temperature. Thank you for your attention !!