Characterization of vacancy-like defects in H 2 cycled Mg and of ordered-nanochannels in Si by...
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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,
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
Slide 2
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
Slide 3
Study of nanostructured materials for hydrogen storage vaulted
ceiling at Sumela monastery (Trabzon-Turkey)
Slide 4
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
Slide 5
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 )
Slide 6
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
Slide 7
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
Slide 28
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
Slide 29
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
Slide 30
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
Slide 31
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
Slide 32
W DetectorSample 3cm 4cm z Ps yield with the size of the
nano-channels
Slide 33
Slide 34
Corrected o-Ps fraction due to Detector solid angle
Slide 35
o-Ps formation o-Ps out diffusion probability o-Ps annihilation
via 3 into pores PALS in #1 Fitting with the diffusion
equation
Slide 36
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
Slide 37
2 channeltrons target position 5 NaI scintillators TOF
Apparatus TOF Apparatus BEAM Prompt peak 16 ns zozo
Slide 38
zozo o-Ps Time of Flight measurements where tftf tptp z0z0 If t
p t f t m t f
Slide 39
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
Slide 40
The two lines in log-lin graph correspond to two
beam-Maxwellian at two different T. Thermalized Ps Thermalized
Ps
Slide 41
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
Slide 42
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
Slide 43
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
Slide 44
At 7 keV e + implantation energy a thermalized o-Ps fraction is
found Measurements at three different distance z were done
Slide 45
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
Slide 46
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
Slide 47
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
Slide 48
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
Slide 49
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