hydrogen trapping interactions with dislocations in pd at low temperature
DESCRIPTION
Dislocation in Pd + trapped H. Experimental Incoherent Inelastic Neutron Scattering Small-Angle Neutron Scattering Magnetic Susceptibility. Ab Initio Computations Relaxed Dislocation + H Binding Energetics Vibrational DOS. Low Temp. Hydride Formation?. - PowerPoint PPT PresentationTRANSCRIPT
Hydrogen Trapping Interactions with Dislocations in Pd at Low Temperature
Supported by the NSF, the ACS-PRF, and the Univ. Illinois
Brent J. Heuser University of Illinois at Urbana-Champaign
Goal: Characterization of hydrogen in perturbed environment of a dislocation
Dislocation in Pd + trapped H
ExperimentalIncoherent Inelastic Neutron Scattering
Small-Angle Neutron ScatteringMagnetic Susceptibility
Ab Initio ComputationsRelaxed Dislocation + H
Binding EnergeticsVibrational DOS
Hydride Formation?
Low Temp.
Collaborators and Facilities
J. King (U. Michigan), G. Summerfield (U. Michigan)E. Epperson (ANL), F. Boue (CEA Saclay)W.C. Chen (U. Illinois, NIST), H. Ju (U. Illinois)T. Udovic (NIST), J. Barker (NIST), C. Glinka (NIST)D. Trinkle (U. Illinois), A. Lipson (U. Illinois, Russian Acad. Sci.)
NCNR at NIST, Lujan Center at LANL, LLB at Saclay, IPNS at ANLMRL at U. Illinois
Background
Relevant Properties of H (D) in Pd:
• Pd good catalyst for H2 ↔ H + H.• H is an octahedral interstitial in fcc lattice.• elastic response due to 1s-4d hybridization.• V/V=0.11 (100% ’ at RT).• hydride formation accompanied by dislocation generation.• 0.2 eV activation energy for diffusion.• ~0.7 eV trapping energy at dislocation cores.• stoichiometric hydride phase difficult.• -Pd superconducting (Tc~1-8 K); Pd is paramagnetic.
Pd-H phase diagrams
Neutron Scattering Instruments
Small-angle neutron scattering:•no neutron energy loss•measurements in Q domain•length scales ~10 to 2000 Å•good for H (D) in metals•“clean” single crystals
Incoherent inelastic neutron scattering:•neutron energy loss or gain•measurements in time domain•vibrational density of states•good for H in metals
SANS at NIST
d/d vs QQ=(4/)sin/2
Energy Window1.2 ± 1.1 meV
FANS at NIST
Inelastic NeutronScattering
Rowe et al., PRL 33 (1974) 1297.
vs. k Phonon Dispersion Curves in
Single CrystalPdD0.63
(Coherent INS)
Hydrogen Vibrational DOS in Polycrystalline PdH0.7
(Incoherent INS)
Hunt and RossJ. Less-C. Metals (1976) 169.
TO
LO
Flat TOModes
Dispersion
Incoherent INS Measurements (21g Pd sheet):
Deformed PdH0.0008 (0.15 mg H)
-PdH0.63
Well-annealed -PdH0.015
PdH0.63
PdH0.0008
4K
PdH0.015
PdH0.0008
295K
Comparisions:
4 K: PdH0.0008 similar to -PdH0.63
295 K: PdH0.0008 similar to -PdH0.015
Conclusion:
→ phase transformation at dislocations
upon cooling from 295 to 4 K.
Lossof
Degeneracy?
Vibrational Density of States
Peak Shift at 295 K
Near-core trapping sites more open:
• softer optic modes and shift to lower energy.• lack of symmetry of trapping sites should result loss of degeneracy and broader peak.
H trapped further away for core (T=295 K):
• strain perturbation weaker and sites still degenerate.• peak shift due to local expansion due to presence of H atom.
OR
E0.015-E0.0008
68 meV – 59 meV0.63 = 0.14 [H]/[Pd]X
SANS Measurements of Deformed Single Crystal PdD0.0055
w/D
w/o D
Deformed PdD0.0055 at RT(trapped D in equil. w/bulk D)
Cross Section Model:Cylinder of trapped solutew/radius Ro and length Lo
local trapped concentration ~0.15 [D]/[Pd]
J. Alloys Compd. 261 (1997) 225.
DFT Relaxation of an Edge Dislocation in PdPd: ao=3.8528 Å (3.8718 Å exp.) H-H=0.766 Å (0.74 Å) C11=324 GPa (315 GPa) C12=196 GPA (257 GPa) C44=86 GPa (71 GPa)
Circles are relaxed Pd positions
w/1 H
w/o HPd site volumetric strain Oct. site volumetric strain
Pd site volumetric strain Oct. site volumetric strain
compressive
tensile
compressive
tensile
tensiletensile
compressivecompressive Dislocation Core:V/V = 0.089V/V = 0.045 (1st NN oct. site)Bulk (not shown):V/V = 0.046V/V = 0.025 (1st NN oct. site)
Local Volumetric Dilatation
Magnetic Susceptibility Measurements in Deformed PdH0.0004Pd is paramagnetic—low T tail due to Fe
impurities:fit of M(H)@2 K to paramagnetic Langevin function
yields CFe<10appm.
Deformed PdH0.0004 has a diamagnetic behaviorbelow 50 K and exhibits irreversible M(H) behavior
at 2 K indicative of a Type II superconductor.
w/o H
w/H
net=w/H – w/o H
Net M(H)@2 K
ZFC M(T)@0.5 Oe
Diamagneticresponse—looks
like Type II SC
Curie-Weiss:= 29 K
Diamagnetic contribution below 50 K
Phys. Lett. A, 339 (2005) 414.
Conclusions
• → phase transformation upon cooling 295→4 K based on incoherent INS.
• peak shape and peak location can serve as a probe of local disorder of trapping site.
• RT local concentration from SANS sufficient for phase transformation upon cooling.
• DFT calculations demonstrate large local dilatation with addition of one H atom.
Elastic Neutron Scattering
No H/D
w/H
w/D
x
Filling dislocation with H or D
bulk Pd
bulk Pd
bulk Pd
Scattering response: I 2
Scattering length density: = Natomb