Combining diffraction and imaging for the study of structural phase transitions

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1 Wir schaffen Wissen heute für morgen Combining diffraction and imaging for the study of structural phase transitions Marisa Medarde Laboratory for Developments and Methods, Paul Scherrer Institut Kürzel, Datum

2 Purpose - Example of combined neutron diffraction and imaging using standard (medium) resolution combination of several options needs compromises - Phase transitions very common in diffraction, rare in imaging broad topic - of interest in both fundamental and applied studies - - Lead-gold eutectic: very good Pb-Au attenuation contrast (1/16) potentially interesting for ESS. Alternative liquid target material candidate,. Res. funded by the FP7 ESS-Preparatory Phase ( )

3 Why a liquid target? Main request for a spallation target: To produce lots of neutrons! Ideally: materials with high density and high atomic number: W, Pb H 2 O H 2 O P + P + Problem in high power n sources: - Density should be decreased to ensure an efficient cooling H 2 O P + Solution I : liquid circulating metal - Feasibility demostrated (PSI 2006) - Operated by SNS-I and J-Park He P + Solution II : solid rotating target - Being considered for SNS-II and ESS - Under development

4 Why lead-gold eutectic? Hg (liquid at RT) - Toxic + volatile + use restricted in Europe - Should be solidified before disposal expensive Lead-bismuth eutectic - (Tm = 125 ºC) Expands by solidifying Fractional release of Po Ar/7%-H 2 water-saturated Ar Temperature [K] - Polonium as a decay product of Bi (strong -emiter, volatile at T > 700ºC) H. Glasbrenner et al., JNM 343, 341 (2005) J. Neuhausen et al., Radiochimica Acta 92, 917 (2004)

5 PbAu has some advantages over Hg and PbBi T m = ºC lowest melting point after Hg (-35ºC) and LBE (125º C) Solid at RT eases decommissioning and disposal No Bi reduced Po production Expected neutronic performance similar to Hg and LBE.but it is unfortunately not perfect! Expensive Very little information available!

6 What is known about LGE? Eutectic composition 84.1% Pb 15.9% Au Liquid Eutectic melting temperature ºC Last phase diagram update Okamoto, J. Phase Equilibria 5, 14 (1993) Phase transition? 52 ºC Pb + AuPb 3 2Pb + AuPb 2

7 Combining diffraction and imaging Diffraction Crystallographic phases present in the solid eutectic vs T Phase transition at 52ºC? Imaging Size & sign of the volume anomaly at the phase transition, if any (strong preferred orientations prevent accurate estimations of the weight fractions of the different phases and hence the density of the eutectic) Size & sign of the volume anomaly at the melting point

8 DMC Cold neutron powder diffractometer ICON Cold neutron radiography d/d d (Å) DMC (SINQ), 2.46 Å HRPT (SINQ), Å HRPD (IPNS), 90º bank = 2.45 Å Time / pattern: ~2m L = 7.1 m; D = 2 cm Lithium based ZnS scintillator Exposure time: 57s Pixel readout time: 2µm / pixel Pixel size: ~ 30µm Heating rate: 1K/min

9 Temperature (ºC) 230ºC 20ºC Diffraction at DMC (SINQ) Liquid 212ºC AuPb 3 Pb AuPb 3 100ºC AuPb 2 AuPb 2 Crystallograhic phases in the solid eutectic Fast-cooled samples (quenched) Phase transition at ~100ºC AuPb 2 + 2Pb AuPb 3 + Pb 230ºC Liquid 212ºC Temperature (ºC) 20ºC AuPb 3 Pb AuPb Slowly-cooled samples (10 deg/h) No phase transition! AuPb 3 + Pb M. Medarde et al., J. Nuc. Mat. 441, (2011)

10 Density/volume anomalies at the phase transition and at the melting point Imaging at ICON (SINQ) 230C 130C RT Solid-Solid (100º C): Less than ~ 1% change Confirms estimate from diffraction M. Medarde et al., J. Nuc. Mat. 441, (2011) Solid-Liquid (212º C): ~ +3.5% volume increase RT 130C 230C No danger for the target station! R. Simons, M. Medarde, E. Lehmann et al., in preparation

11 Concluding remarks Combination of diffraction and imaging, even at medium resolution, has a clear potential for the study of structural phase transitions Important for a dedicated instrument: Variable resolution for diffraction High flux Possibility of simultaneous diffraction and imaging Adapted sample environment and instrument control Adapted software

12 Many thanks to Collaborators at the Paul Scherrer Institut Solid State Chemistry K. Conder, E. Pomjakushina, R. Frison, Neutron scattering L. Keller, D. Sheptyakov, V. Pomjakushin Neutron Imaging R. Simons, A. Kaesner, E. Lehmann Sample environment and cryogenics M. Bartkowiak, M. Zolliker ESS-Preparatory Phase Phase Project