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

The underlying theme of this presentation is the combined use of different PAS Techniques for the characterization of “open spaces” with dimension in the to 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

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

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, (2000) Appl. Phys. Lett. 79, 1492 (2001) Phys. Rev. B 71, (2005) Appl. Phys. Lett. 88, (2006). Phys. Rev. B 74, (2006) Background... Combined PALS, CDB, DBS for studying vacancy-like and cavities in He and H Implanted crystalline Si

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. Mg Mg hydride contains 7.6 wt. % of H H 2 desorption requires phase transformation MgH 2  Mg at T  K, and exhibits very slow kinetics. residual O (< at -1 )

Self supporting sample were activated and then subjected to sorption cycle SORPTION CYCLE at 623 K: i)At 1.5 Mpa H 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 Sievert’s 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 th 9 th

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 Processes limiting desorption: a)Surface, H –H2 recombination, linear equation  (t)=kt b) H diffusion c) Bulk – NG H sorption cycles in pure Mg- analysis

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

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

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

A T annihilation with low momentum electrons f Mg = probability to annihilate in Mg f MgO = probability to annihilate at MgO f d = probability to annihilate in a defect S Mg = characteristic S value of Mg S MgO =characteristic S value of MgO S d =characteristic S value of the defect Doppler broadening spectroscopy: DB

G Mg (  ) =characteristic G(  ) spectrum of Mg G MgO (  )=characteristic G (  )spectrumof MgO G d (  )=characteristic G(  ) spectrumof the defect Doppler broadening spectroscopy : CDB

 Mg = 218±2 ps and G Mg (  ) In Mg single crystal (99.99% purity) I n Mg single crystal (99.99% purity) cold worked at RT  v-Mg = 245±5 ps and G v-Mg (  ) Mg Vacancy in Mg I n Mg single crystal (99.99% purity) at the surface MgO G MgO (  ) Reference measurements for G(  ) and 

G o MgO (  ) G o v-Mg (  )

measurement of G c-Mg (  ) Vacancys were introduced in Mg by polishing a Mg film Annealing at 420°C produced vacancy clustering Vacancy clusters were removed after annealing at 500 °C

Fractions f are related to the positron density n(z, E): Guess defect profile e+ implantation profile Analysis with the stationary positron diffusion equation

Coincidence DBS measurements In point 1, 2, 3, 4 with four measurements we construct a system, f are known, G are the unknown

G o c-Mg (  )

Samples [ps] I1%I1% I2%I2% I3%I3% #080±4241± #1125±2263± #2122±2260± #4165± #8177±  1 is the reduced bulk lifetime and increase with the number of cycles pointing out a decrease of intragranular defects  2 is due to trapping into mono- and di-vacancies, these defects disappear after the forth cycle. They are mainly intragranular  3 is due to trapping into vacancy clusters ( size of about 8 vacancies). Their number increases with cycling. They are inferred to form mainly at grain boudaries. Measuerement in Mg bulk (16-18 keV,  m) Lifetime results

The fraction are consistent with the lifetime intensities. #0 #2 #4 #8 #1 CDB results SampleCDB fractions f i (%) f Mg f c-Mg f v-Mg fofo Mg # # # # #

The phase transition (MgH 2  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. After 4° cycle peak at 1800 s  disappearance of V and saturation of I 3 coming from e+ in clusters 4 th cycle peak at 3  10 3 s  Mg nucleation at grain boundaries which act as nucleation centers From 5 th to 9 th cycles acceleration of the desorption kinetics and of the H 2 desorbed amount  faster grow of the Mg phase into the MgH 2 matrix  increase of the crystalline quality of the Mg nano-grains, ( increase of  1 and its intensity I 1 ) H kinetics and role of vacancy-like defects

In the free energy of formation of the critical Mg nucleus ΔG = ΔG volume + ΔG interface + ΔG strain, ΔG volume the volume free E ΔG interface the interface free E ΔG strain, strain E due to the volumetric misfit between the critical nucleus of Mg and the matrix. 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 ΔG strain term

e + diffusion trapping model with a competitive e + trapping at intragranular point defects and at grain boundaries in polycrystalline materials. (Analytical Model of B. Oberdorfer, R. Würschum, PRB 79, (2009). α = specific positron trapping rate at grain boundaries lower limit value for the C c in the frame of the extreme diffusion-limited regime Having about 2x10 15 grains/cm 3 and considering that there are 4x10 22 Mg atoms/cm 3, we estimated that about 40 vacancy clusters decorate the boundary of each Mg grain Evaluation of vacancy-like defects Concentration samples C v at -1 α m/s C c at -1 As deposited1.2x After 1 st cycle3.5x After 2 nd cycle3x After 4 th and 8 th 0-2x10 -6

#0 #1 #4 #8 #2 Samples [ps] I1%I1% I2%I2% I3%I3% #0HC-296± #1HC-263± #2HC-204± #4HC-207± #8HC-229± Nb ( ̴ 5 at %) doped Mg Mg+Nb (5%) CDB fractions f i (%) #0HC #1HC #2HC3957-< 13 < f < 4 #4HC4340-< 512 <f < 17 #8HC333913< 213 < f < 15

Studying porous materials with Ps

With the Tao-Eldrup model R [nm] delta R=0.18 nm Commercial grade Spectrosil (density 2.20g/cm 3 ) was permanently densified applying at 500 °C a pressure and then realising the pressure and a rapid cooling down. 2GPa (2.21g/cm 3 ), 4GPa (2.25g/cm 3 ), 6GPa (2.41g/cm 3 ), 8GPa (2.67g/cm Shrinking of voids in silica Spectrosil (fused Quartz)

< 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

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

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 #0 (4-7 nm) mini T is K #1 (8-12 nm) min T is K 160 K Nano-size and Ps thermalization

Si p-type 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= nm #4= nm #5= nm Mariazzi S, Salemi A and Brusa R S 2008 Phys. Rev. B Tuning the size of nanochannels

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

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

Corrected o-Ps fraction due to Detector solid angle

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

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

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

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

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

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

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

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

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

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

t p thermal =19±9 ns t p cooled = 5±3 ns v thermal = 4.9x10 4 ±2x10 3 m/s T=310±20 K 13.4± 0.9 meV v cooled = 1.0x10 5 ±1x10 4 m/s T=1370±300 K 59.4 ± 13.0 meV

The measured t p =t p thermal can be compared with the value obtained by a diffusion model (Cassidy et al. PRB A82, (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 44±4 ns is less than expected by Tao-Eldrup RTE model at 300 K, ie 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 (1100±300K ) we find 51±8 ns. t exp = t p = 19 ns

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

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

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, Università di Trento INFN, Padova-Trento S. MARIAZZI L. RAVELLI and W. EGGER C. MACCHI, A. SOMOZA R. CHECCHETTO, A. MIOTELLO Università di Trento positron Group, Università di Trento INFIMAT, Tandil, Buenos Aires Universität der Bunderswehr