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Characterization of vacancy-like defects in H 2 cycled Mg and of ordered- nanochannels in Si

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. Overview.

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Characterization of vacancy-like defects in H 2 cycled Mg and of ordered- nanochannels in Si

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  1. Characterization of vacancy-like defects in H2 cycled Mg and of ordered-nanochannels in Si by combined PAS techniques Roberto S. Brusa DepartmentofPhysics, Universityof Trento, Italy 5-9 September, Smolenice, Slovakia

  2. Overview 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 H2 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.

  3. Study of nanostructured materials for hydrogen storage vaultedceiling at Sumelamonastery (Trabzon-Turkey)

  4. Combined PALS, CDB, DBS for studying vacancy-like and cavities in He and H Implanted crystalline Si Background... 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) Some of the resultswere reviewed in a talk at the PPC8 in Coimbra (2005)

  5. Mg Mg hydride contains 7.6 wt. % of H H2 desorption requires phase transformation MgH2Mg at T 573 - 673 K, and exhibits very slow kinetics. • Sample : • 10 μm 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 μm. • grain size : 100 ± 5 nm by the Scherrer eq. on the (0002) XRD reflection peak • grain sizes do not change with H sorption cycles. residual O (< 10-4 at-1 )

  6. H sorption cycles in pure Mg Selfsupporting sample wereactivated and thensubjectedtosorptioncycle 9th 4th SORPTION CYCLE at 623 K: At 1.5 Mpa H2 -20 h (ABSORPTION STEP) Chamberevacuated (DESORPTION STEP) With Sievert’s type techniques H desorption flow Q (t) [mass hydrogen/s] from MgH2 wasmonitored . 9th Fig. (a) : Desorption rate Q(t)/(mMg + mH2) ( wt. % H2/ s) Fig. b: H amount desorbed (wt. % H2) (time integral of the Desorption rate) 4th ChecchettoBrusa et al 2011 Phys. Rev. B84 054115

  7. Processes limiting desorption: Surface , H –H2 recombination, linear equation (t)=kt b) H diffusion c) Bulk – NG H sorption cycles in pure Mg-analysis 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<T <623 K indicated that the desorption obeys to a Nucleation and Growth mechanism with a reaction order n = 2 and an activation energy  130 kJ/mol

  8. e+ beams PLEPS at NEPOMUC - FRMII SURF-beam at TRENTO

  9. e+ pulsed beam START SIGNAL BaF2 + PT detector   STOP SIGNAL Lifetime spectroscopy 2° cycle , 16 keV

  10. Doppler broadening spectrosvopy: CDB -DBS   Ge detector Ge detector e+ beam 0.05-25 keV 0.15 nm - 3 m e+ beam 0.05-25 keV 0.15 nm - 3 m 511  E keV

  11. annihilation with low momentum electrons Doppler broadening spectroscopy: DB A T SMg= characteristic S valueof Mg SMgO=characteristic S valueofMgO Sd=characteristic S valueof the defect fMg=probabilitytoannihilatein Mg fMgO=probabilitytoannihilate at MgO fd=probabilitytoannihilate in a defect

  12. Doppler broadening spectroscopy : CDB GMg()=characteristic G()spectrumof Mg GMgO()=characteristic G ()spectrumofMgO Gd ()=characteristic G()spectrumof the defect

  13. Reference measurements for G() and  Mg Mg= 218±2 ps and GMg() • In Mg single crystal • (99.99% purity) Vacancy in Mg • InMg single crystal (99.99% purity) coldworked at RT v-Mg= 245±5 ps and Gv-Mg () MgO • InMg single crystal (99.99% purity) at the surface GMgO()

  14. GoMgO()Gov-Mg()

  15. measurementofGc-Mg() Vacancyswereintroduced in Mg bypolishing a Mg film Annealing at 420°C produced vacancyclustering Vacancyclusterswereremoved afterannealing at 500 °C

  16. Analysis with the stationary positron diffusion equation e+implantationprofile Guessdefectprofile Fractionsf are relatedto the positron density n(z, E):

  17. 1 4 2 3 Coincidence DBS measurements In point 1, 2, 3, 4 withfourmeasurementsweconstruct a system, f are known, Gare the unknown

  18. Goc-Mg()

  19. Lifetime results Measuerement in Mg bulk (16-18 keV , 1.5-2 m) 1 is the reduced bulk lifetime and increasewith the numberofcycles pointing out a decreaseof intragranular defects 2 is due totrappinginto mono- and di-vacancies, thesedefectsdisappear after the forthcycle. They are mainly intragranular 3is due totrappingintovacancyclusters ( sizeofabout 8 vacancies). Theirnumberincreaseswith cycling. They are inferredtoform mainly at grainboudaries.

  20. CDB results #0 #1 #2 The fraction are consistentwith the lifetimeintensities. #4 #8

  21. H kinetics and role of vacancy-like defects The phase transition (MgH2Mg) is controlled by the nucleation and growth (NG) of Mg in the hydride phase. The NG mechanism progressive change and it is correlated to the change in difettology. 4th cycle peak at 3103 s  Mg nucleation at grain boundaries which act as nucleation centers After 4° cycle peak at 1800 s disappearanceofV and saturation of I3 coming from e+ in clusters • From 5th to 9th cycles • acceleration of the desorption kinetics and of the H2 desorbed amount • faster grow of the Mg phase into the MgH2 matrix • increase of the crystalline quality of the Mg nano-grains , • ( increase of 1 and its intensity I1)

  22. It can be inferred that vacancy clusters at grain boundaries could assists the nucleation process counteracting the volume change of the crytical Mg nucleus by reducing the ΔGstrain term In the free energy of formation of the critical Mg nucleus ΔG = ΔGvolume + ΔGinterface + ΔGstrain, ΔGvolume the volume free E ΔGinterfacethe interface free E ΔGstrain, strain E due to the volumetric misfit between the critical nucleus of Mg and the matrix.

  23. Evaluation of vacancy-like defects Concentration lower limit value for the Cc in the frame of the extreme diffusion-limited regime e+ diffusion trapping model witha competitive e+ trapping at intragranular point defects and at grain boundaries in polycrystalline materials. (Analytical Model of B. Oberdorfer, R. Würschum, PRB 79, 184103 (2009). • α = specific positron trapping rate at grain boundaries Having about 2x1015 grains/cm3 and considering that there are 4x1022 Mg atoms/cm3, we estimated that about 40 vacancy clusters decorate the boundary of each Mg grain

  24. Nb ( ̴ 5 at %) doped Mg #0 #1 #2 #4 #8

  25. Studying porous materials with Ps

  26. Shrinking of voids in silica Spectrosil (fused Quartz) With the Tao-Eldrupmodel R [nm] delta R=0.18 nm Commercial grade Spectrosil (density 2.20g/cm3) was permanently densified applying at 500 °C a pressure and then realising the pressure and a rapid cooling down. 2GPa (2.21g/cm3), 4GPa (2.25g/cm3), 6GPa (2.41g/cm3), 8GPa (2.67g/cm

  27. e+ < 1nm e+ > 1nm Ps e+ Ps Ps Ps Probing nano-pores • Ps probes: • Connectedporosity (ifnotcapped)-3-PAS, TOF • Sizeofpores in a wide range- PALS, 3-PAS • Distribution: DBS, PALS, 3-PAS But annihilation and diffusion of Ps depend from: • size of pores • shape of pores • chemical environment of pores • Ps thermalization and cooling

  28. Orderennanochannels in Silicon 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 (Antimatterexperiment: Gravity, Interferometry, Spectroscopy) experiement Top view of the silicon sample with nanochannels

  29. Nano-size and Ps thermalization Vacuum Ps Ps Ps Ps Ps QUANTUM CONFINEMENT Positron beam the minimum temperature is: 160 K #0 (4-7 nm) mini T is 180-60 K #1 (8-12 nm) min T is 45-20 K Positronium converter Mariazzi S, Salemi A and Brusa R S 2008 Phys. Rev. B78 085428

  30. Tuning the size of nanochannels 10 nm #0 #1 #2 #3 #4 #5 100 nm produced by electrochemical etching, as for porous silicon but adapting times and current for obtaining nano- structures 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 Si p-type 0.15-0.21 Ohm/cm currentfrom 4-18 mA/cm2, 15’ Mariazzi S, Salemi A and Brusa R S 2008 Phys. Rev. B78 085428

  31. 10 nm Annealed 2h 100°C #0 Annealed 1h 300°C 2γ rays peak area o-Ps 3γ rays valley area a) b) Optimum oxidation for the Ps yield

  32. Ps yield with the size of the nano-channels z W 4cm Sample Detector 3cm

  33. Corrected o-Ps fraction due to Detector solid angle

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

  35. Up to 42 % ofimplantedpositrons at 1 keV emittedas o-Ps 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. LPs

  36. TOF Apparatus zo BEAM Prompt peak 16 ns 2 channeltrons target position 5 NaIscintillators

  37. zo o-Ps Time of Flight measurements where z0 tf tp If tp ˂˂ tf tm tf

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

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

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

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

  42. Permanence time of Ps in nano-channels before escaping into vacuum Vacuum Ps Ps Ps Ps Ps z0 tf tp tm= tp+tf Positronium converter

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

  44. t p thermal=19±9 ns tp cooled = 5±3 ns vthermal = 4.9x104±2x103 m/s vcooled = 1.0x105±1x104 m/s T=310±20 K T=1370±300 K 13.4± 0.9 meV 59.4 ± 13.0 meV

  45. The measured tp=tp 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 = tp = 17 ns t exp = tp = 19 ns Experimental Pick off lifetime of 44±4 nsis less than expected by Tao-Eldrup RTE model at 300 K , ie. 77-97 nsfor 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 (1100±300K ) we find 51±8 ns .

  46. TOF apparatus will be set up at NEPOMUC • 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

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

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

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