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Characterization of vacancy-like defects in H 2 cycled Mg and of ordered-nanochannels in Si by combined PAS techniques Roberto S. Brusa Department of Physics, University of Trento, Italy 5-9 September, Smolenice, Slovakia

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The underlying theme of this presentation is the combined use of different PAS Techniques for the characterization of open spaces with dimension in the 10 -12 to 10 -8 m range. The Lecture will be divided into two parts: 1. PAS Techniques for the study of the role of vacancy-like defects in the H 2 sorption processes in Mg and Nb doped Mg materials 2. Ps formation and cooling in oxidized ordered nanochannels specially made in Si. This system is allowing to retrieve fundamental information which will be useful for characterizing open porosities. Overview

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Study of nanostructured materials for hydrogen storage vaulted ceiling at Sumela monastery (Trabzon-Turkey)

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Some of the results were reviewed in a talk at the PPC8 in Coimbra (2005) Phys. Rev. B 49, 7271 (1994) J. Appl. Phys. 85, 2390 (1999) J. Appl. Phys. 85,1401(1999) Phys Rev. B 61, 10154 (2000) Appl. Phys. Lett. 79, 1492 (2001) Phys. Rev. B 71, 245320 (2005) Appl. Phys. Lett. 88, 011920 (2006). Phys. Rev. B 74, 174120 (2006) Background... Combined PALS, CDB, DBS for studying vacancy-like and cavities in He and H Implanted crystalline Si

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Sample : 10 m Mg deposited by r.f. magnetron sputtering coated with a 10 nm thick Pd capping layer to prevent oxidation Morphology: columnar structure. Lateral dimension of the columns : 0.5 m. grain size : 100 5 nm by the Scherrer eq. on the (0002) XRD reflection peak grain sizes do not change with H sorption cycles. Mg Mg hydride contains 7.6 wt. % of H H 2 desorption requires phase transformation MgH 2 Mg at T 573 - 673 K, and exhibits very slow kinetics. residual O (< 10 -4 at -1 )

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Self supporting sample were activated and then subjected to sorption cycle SORPTION CYCLE at 623 K: i)At 1.5 Mpa H 2 -20 h (ABSORPTION STEP) ii)Chamber evacuated (DESORPTION STEP) Fig. (a) : Desorption rate Q(t)/(m Mg + m H2 ) ( wt. % H 2 / s) Fig. b: H amount desorbed (wt. % H 2 ) (time integral of the Desorption rate) With Sieverts type techniques H desorption flow Q (t) [mass hydrogen/s] from MgH 2 was monitored. 4 th 9 th H sorption cycles in pure Mg Checchetto Brusa et al 2011 Phys. Rev. B 84 054115 4 th 9 th

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Johnson-Mehl-Avramy eq. (t)=1-exp[-(kt) n (t) the fraction of transformed material k rate constant n reaction order. The phase transformation is limited only by bulk processes. analysis in stationary conditions at 583 K

< 1nm > 1nm Ps e+e+ e+e+ e+e+ Ps probes: 1.Connected porosity (if not capped)-3 -PAS, TOF 2.Size of pores in a wide range- PALS, 3 -PAS 3.Distribution: DBS, PALS, 3 -PAS size of pores shape of pores chemical environment of pores Ps thermalization and cooling But annihilation and diffusion of Ps depend from: Probing nano-pores

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Searching for a porous materials with an high yield of Ps emitted in vacuum to be used as e+ Ps converter for anti hydrogen formation, we have synthesized nanochannel in silicon AEGIS (Antimatter experiment: Gravity, Interferometry, Spectroscopy) experiement Top view of the silicon sample with nanochannels Orderen nanochannels in Silicon

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Ps Positronium converter Positron beam Ps Vacuum Ps QUANTUM CONFINEMENT the minimum temperature is: Mariazzi S, Salemi A and Brusa R S 2008 Phys. Rev. B 78 085428 #0 (4-7 nm) mini T is 180-60 K #1 (8-12 nm) min T is 45-20 K 160 K Nano-size and Ps thermalization

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Si p-type 0.15-0.21 Ohm/cm current from 4-18 mA/cm 2, 15 produced by electrochemical etching, as for porous silicon but adapting times and current for obtaining nano- structures 10 nm #0 #1 #2 #3 #4 100 nm #5 Possibility of tuning: #0 = 4-7 nm #1=8-12 nm #2= 8-14 nm # 3= 10-16 nm #4= 14-20 nm #5= 80-120 nm Mariazzi S, Salemi A and Brusa R S 2008 Phys. Rev. B 78 085428 Tuning the size of nanochannels

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2 rays peak area o-Ps 3 rays valley area Annealed 1h 300C Annealed 2h 100C #0 10 nm a)b) Optimum oxidation for the Ps yield

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W DetectorSample 3cm 4cm z Ps yield with the size of the nano-channels

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Corrected o-Ps fraction due to Detector solid angle

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o-Ps formation o-Ps out diffusion probability o-Ps annihilation via 3 into pores PALS in #1 Fitting with the diffusion equation

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The o-Ps fraction out-diffusing at 10 keV positron implantation energy is still 10 % in #0, 17 % in #1 23-25 % in #2, #3, #4 and #5. Up to 42 % of implanted positrons at 1 keV emitted as o- Ps L Ps

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2 channeltrons target position 5 NaI scintillators TOF Apparatus TOF Apparatus BEAM Prompt peak 16 ns zozo

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zozo o-Ps Time of Flight measurements where tftf tptp z0z0 If t p t f t m t f

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Mariazzi S, Brusa R S et al., Phys. Rev. Lett. 104 243401 (2010) After smoothing, subtraction of the background, and correction by multiplying by Ps cooling - 5-8 nm channels Ps cooling - 5-8 nm channels

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The two lines in log-lin graph correspond to two beam-Maxwellian at two different T. Thermalized Ps Thermalized Ps

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Fraction of o-Ps emitted thermalized : RT ~19 % 5% implanted e + 200 K ~15 % 4 % implanted e + 150 K ~9 % 2.5 % implanted e + Fraction of thermalized Ps Fraction of thermalized Ps

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quantum confinement and thermalization Crivelli et al., Phys. Rev. A 81, 052703 (2010) Cassidy et al., Phys. Rev. A 81, 012715 (2010) Similar samples 42 meV in pores of 2.7 nm

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Ps Positronium converter Ps Vacuum Ps Permanence time of Ps in nano-channels before escaping into vacuum Permanence time of Ps in nano-channels before escaping into vacuum t m = t p +t f tftf tptp z0z0

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At 7 keV e + implantation energy a thermalized o-Ps fraction is found Measurements at three different distance z were done

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t p thermal =199 ns t p cooled = 53 ns v thermal = 4.9x10 4 2x10 3 m/s T=31020 K 13.4 0.9 meV v cooled = 1.0x10 5 1x10 4 m/s T=1370300 K 59.4 13.0 meV

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The measured t p =t p thermal can be compared with the value obtained by a diffusion model (Cassidy et al. PRB A82, 052511 (2010)) the rate of the Ps emission from the sample is retrieved solving the diffusion equation t theory = t p = 17 ns Experimental Pick off lifetime of 444 ns is less than expected by Tao-Eldrup RTE model at 300 K, ie. 77-97 ns for 5-8 nm nanochannels sizes. Inferring that a Ps fraction annihilate hot and using as a first approximation the average T of thermal and cooled distributions (1100300K ) we find 518 ns. t exp = t p = 19 ns

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Tunable nanochannels will allow to study: Cooling and thermalization at tempertaure < 150 K Cooling and thermalization in presence of decorated surfaces Relations between diffusion and tortuosity TOF apparatus will be set up at NEPOMUC

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Concluding remarks Pas techniques can be improved with new arrays and faster detectors Strong need of friendly programs of analysis based on diffusion equation based on diffusion equation Study at low temperature can bring to a new Ps tools for porosity characterization

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THE WORK on Mg was DONE in COLLABORATION WITH : THE WORK on Ps was DONE in COLLABORATION WITH: S. MARIAZZI L. DI NOTO G. NEBBIA positron Group, Universit di Trento INFN, Padova-Trento S. MARIAZZI L. RAVELLI and W. EGGER C. MACCHI, A. SOMOZA R. CHECCHETTO, A. MIOTELLO Universit di Trento positron Group, Universit di Trento INFIMAT, Tandil, Buenos Aires Universitt der Bunderswehr