Characterization of vacancy-like defects in H2 cycled Mg

and of ordered-nanochannels in Siby combined PAS techniques

Roberto S. BrusaDepartment 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 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 defectsin 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.

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 thePPC8 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

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 H2 desorption requires phase transformation MgH2 Mg at T 573 - 673 K,and exhibits very slow kinetics.

residual O (< 10-4 at-1 )

Self supporting sample were activated and then subjected to sorption cycle

SORPTION CYCLE at 623 K:

i) At 1.5 Mpa H2 -20 h (ABSORPTION STEP)ii) Chamber evacuated (DESORPTION STEP)

Fig. (a) : Desorption rate Q(t)/(mMg + mH2) ( wt. % H2/ s)

Fig. b: H amount desorbed (wt. % H2)(time integral of the Desorption rate)

With Sievert’s type techniques H desorption flow Q (t) [mass hydrogen/s] from MgH2

was monitored .

4th

9th

H sorption cycles in pure Mg

Checchetto Brusa et al 2011 Phys. Rev. B 84 054115

4th

9th

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 desorptionobeys 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)=ktb) H diffusionc) Bulk – NG

H sorption cycles in pure Mg-analysis

PLEPS at NEPOMUC - FRMII

SURF-beam at TRENTO

e+ beams

BaF2 + PT detector

e+ pulsed beam START SIGNAL

STOP SIGNAL

n

i ii

i tItF

1exp)(

Lifetime spectroscopy

2° cycle , 16 keV

e+ beam 0.05-25 keV0.15 nm - 3 m

Ge detector

Ge detector

e+ beam 0.05-25 keV0.15 nm - 3 m

511 E keV

cEpL

2

Doppler broadening spectrosvopy: CDB -DBS

505 510 515 520 525 530 535 540

101

102

103

104

105 low-momentum electrons(conduction and valence bands)

high-momentum electrons(core electrons)

background in asingle-detector system

background ina coincidence system

Si coincidence Si non-coincidence

Cou

nts

Energy [keV]

0 10 20 30 40 50 60 70pL [10-3 m0c]

500 505 510 515 520 525100

101

102

103

104

coun

ts

energy ( keV )

500 505 510 515 520 525100

101

102

103

104

coun

ts

energy ( keV )

A

T

annihilation with low momentum electrons

areapeakareacentral

TAS

ii d

idMgOMgOMgMg SEfSEfSEfES )(

fMg= probability to annihilate in MgfMgO= probability to annihilate at MgO fd= probability to annihilate in a defect

SMg = characteristic S value of MgSMgO=characteristic S value of MgOSd =characteristic S value of the defect

Doppler broadening spectroscopy: DB

GMg () =characteristic G() spectrum of Mg

GMgO()=characteristic G ()spectrumof MgO

Gd ()=characteristic G() spectrumof the defect

Doppler broadening spectroscopy : CDB

Mg= 218±2 ps and GMg()

In Mg single crystal (99.99% purity)

In Mg single crystal (99.99% purity) cold worked at RT

v-Mg= 245±5 ps and Gv-Mg ()

Mg

Vacancy in Mg

In Mg single crystal (99.99% purity) at the surface

MgO

GMgO ()

Reference measurements for G() and

0 5 10 15 20 25 30

-0.5

0.0

0.5

1.0

1.5

re

lative

diffe

renc

e to

ann

ealed

M

g sin

gle cr

ysta

l

pL ( x 10-3 m0c )

vacancy vacancy cluster MgO pure Nb

GoMgO () Go

v-Mg ()

measurement of Gc-Mg ()

0,1 1 100,84

0,88

0,92

0,96

1,00

1,04

1 10 100 1000mean positron implantation depth ( nm )

Mg foil, polished

Mg foil, 10h @ 420oC

Mg foil, 1h @ 500oC

norm

aliz

ed S

par

amet

er

positron implantation energy ( keV )

ii d

idMgOMgOMgMg SEfSEfSEfES )(

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

0),(),()(),(2

2

EzPEznzCzEznD b

20

2)(

zz

eAzC

dzEznf bMg 0 ),(

Fractions f are related to the positron density n(z, E):

dzEznzCfdi 0 ),()(

0

),(

zMgO dz

EzdnDf

Guess defect profile

e+ implantation profile

1 10 100 1000

1E-4

1E-3

0.01E= 3 keV

E= 5 keV

E= 10 keV

E= 18 keV

P(z

)

depth z [nm]

E= 1keV

Analysis with the stationary positron diffusion equation

0,1 1 100,84

0,88

0,92

0,96

1,00

1,04

1 10 100 1000mean positron implantation depth ( nm )

Mg foil, polished

Mg foil, 10h @ 420oC

Mg foil, 1h @ 500oC

norm

aliz

ed S

par

amet

er

positron implantation energy ( keV )

0.1 1 10

0.0

0.2

0.4

0.6

0.8

1.0

1 10 100 1000mean positron implantation depth ( nm )

f

positron implantation energy ( keV )

surface defect #1 defect #2 bulk

Ef Mgv Ef Mgc

EfMg EfMgO

41 2 3

Coincidence DBS measurementsIn point 1, 2, 3, 4

EfEGEfEGEfEGEfEGEEG MgcMgcvvMgMgMgOMgO ,

with four measurements we construct a system, f are known, G are the unknown

0 5 10 15 20 25 30

-0.5

0.0

0.5

1.0

1.5

re

lative

diffe

renc

e to

ann

ealed

M

g sin

gle cr

ysta

l

pL ( x 10-3 m0c )

vacancy vacancy cluster MgO pure Nb

Goc-Mg ()

Samples[ps] [ps] [ps]

I1

%I2

%I3

%

#0 80±4 241±4 380 12 67 21#1 125±2 263±2 380 26 29 44#2 122±2 260±2 380 25 25 49#4 165±3 - 380 42 - 57#8 177±3 - 380 50 - 49

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 , 1.5-2 m)

Lifetime results

The fraction are consistent with the lifetime intensities.

#0

#2

#4

#8

#1

CDB resultsSample CDB fractions fi (%)

fMg fc-Mg fv-Mg fo

Mg

#0 55 14 18 13

#1 42 23 17 18

#2 43 29 14 14

#4 45 41 - 14

#8 55 35 - 10

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.

After 4° cyclepeak at 1800 s disappearance of V and saturation of I3 coming from e+ in clusters

4th cycle peak at 3103 s Mg nucleation at grain boundaries which act as nucleation centers

From 5th to 9th cyclesacceleration 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)

H kinetics and role of vacancy-like defects

In the free energy of formation of the critical Mg nucleus ΔG = ΔGvolume + ΔGinterface + ΔGstrain,ΔGvolume the volume free E ΔGinterface the interface free E ΔGstrain, 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 ΔGstrain 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, 184103 (2009). α = specific positron trapping rate at grain

boundaries

lower limit value for the Cc

in the frame of the extreme diffusion-limited regime

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

Evaluation of vacancy-like defects Concentrationsamples Cv

at-1

α

m/s

Cc

at-1

As deposited 1.2x10-5 30 -

After 1st cycle 3.5x10-6 45 -

After 2nd cycle 3x10-6 55 -

After 4th and 8th 0 - 2x10-6

0 2 4 6 8 10

0.20.40.60.81.0

0.2

0.4

0.6

no Nb low Nb high Nb

I 2

number of cycles

I 3#0

#1

#4

#8

#2 Samples[ps] [ps] [ps]

I1

%I2

%I3

%#0HC - 296±1 - - 99.5 -#1HC - 263±2 415 - 57 42#2HC - 204±1 415 - 41 58#4HC - 207±5 415 - 48 51#8HC - 229±3 415 - 60 39

Nb ( 5 at %) doped Mg ̴

Mg+Nb (5%) CDB fractions fi

(%)

#0HC 25 34 28 13 - #1HC 30 40 19 11 - #2HC 39 57 - < 1 3 < f < 4 #4HC 43 40 - < 5 12 <f < 17 #8HC 33 39 13 < 2 13 < f < 15

Studying porous materials withPs

2 4 6 8 10 12 14

10-3

10-2

10-1

100

0GPa 2GPa 4GPa 6GPa 8GPa

Cou

nts

[arb

.uni

ts]

Time [ns] 0 2 4 6 80.00

0.05

0.10

0.15

0.20

0.25

0.30

0

20

40

60

80

100

Pore radius Pore radius at 8 GPaPo

re ra

dius

[nm

]

Intensity of o-Ps lifetime in pores

Pressure [GPa]

Intensity [%]

RRRn

nRRRP AAop

2sin2

11..With the Tao-Eldrup model 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 rapidcooling down.

2GPa (2.21g/cm3), 4GPa (2.25g/cm3), 6GPa (2.41g/cm3), 8GPa (2.67g/cm

Shrinking of voids in silica Spectrosil (fused Quartz)

< 1nm

> 1nmPs

e+

Ps

Ps

Ps

e+

e+

Ps probes:1. Connected porosity (if not capped)-3-PAS, TOF2. Size of pores in a wide range- PALS, 3-PAS3. 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 samplewith nanochannels

Orderen nanochannels in Silicon

Ps

Positronium converter

Positron beam

Ps

Ps

Vacuum

Ps

Ps

QUANTUM CONFINEMENT

222 / amEg

gB EkT )3/2(

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

Si p-type 0.15-0.21 Ohm/cmcurrent from 4-18 mA/cm2 , 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

470 480 490 500 510 520100

101

102

103

104

coun

ts

energy ( keV )

2γ rayspeak area

o-Ps 3γ raysvalley area

Annealed1h 300°C

Annealed2h 100°C#0

10 nm

a) b)

0.1 1 10

0.0

0.1

0.2

0.3

0.4

0.5

Mean positron implantation depth [nm]

not oxidized 0.5h 100°C 2h 100°C (#0) 4h 100°C 1h 300°C 1h 400°C

F 3 (o

-Ps

fract

ion)

Positron implantation energy [keV]

10 1000

Optimum oxidation for the Ps yield

0.1 1 10

0.1

0.2

0.3

0.4

0.5

Mean positron implantation depth [nm]

F 3 (o

-Ps

fract

ion)

Positron implantation energy [keV]

#3#4#5

0.1

0.2

0.3

0.4

0.5

#0#1#2

10 1000

0.1 1 100

50

100

150

200

250

300

350

400

450

420 440 460 480 500 520

#0 at 1 keV Silicon at 1 keV

coun

ts [a

rbitr

ary

units

]

Annihilation -ray energy (E)

Cou

nts

in P

eak+

Val

ley

(P+V

)

Positron implantation energy [keV]

Silicon #0

ENENPEP escapedSi 32

ENVEV Si

32

W

DetectorSample3cm

4cm

z

Ps yield with the size of the nano-channels

ENVEV Si

32

ENENPEP escapedSi 32

0.1 1 10

0.1

0.2

0.3

0.4

0.5

Mean positron implantation depth [nm]

F 3 (o

-Ps

fract

ion)

Positron implantation energy [keV]

#3#4#5

0.1

0.2

0.3

0.4

0.5

#0#1#2

10 1000

Corrected o-Ps fraction due to Detector solid angle

0.1 1 10

0.1

0.2

0.3

0.4

0.5

Mean positron implantation depth [nm]

F 3 (o

-Ps

fract

ion)

Positron implantation energy [keV]

#3#4#5

0.1

0.2

0.3

0.4

0.5

#0#1#2

10 1000

0.1 1 10

0.1

0.2

0.3

0.4

0.5

Mean positron implantation depth [nm]

F 3 (o

-Ps

fract

ion)

Positron implantation energy [keV]

#3#4#5

0.1

0.2

0.3

0.4

0.5

#0#1#2

10 1000

ECEBEAEF )(3

121

121EE

eFFFEA

6.1

0

1

1

EE

EB

2

2

21

1

C

EE

CCKEC

0 2 4 6 8 10 12 14 16 180

10

20

30

40

50

60

70

Inte

nsity

[%]

Positron implantation energy [keV]

I1 I2 I3

0.2

0.4

0.6

20

30405060

1

2

3

Life

time

[ns]

o-Ps formation o-Ps out diffusion probability o-Ps annihilation

via 3γ into pores

PALS in #1

Fitting with the diffusion equation

200

400

600

800

1000

1200

1400

1600

#0 #1 #2 #3 #4 #5

o-P

s di

ffusi

on le

ngth

[nm

]

Sample

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

LPs

0.1 1 10

0.1

0.2

0.3

0.4

0.5

Mean positron implantation depth [nm]

F 3 (o

-Ps

fract

ion)

Positron implantation energy [keV]

#3#4#5

0.1

0.2

0.3

0.4

0.5

#0#1#2

10 1000

2 channeltrons

target position

5 NaI scintillators

TOF ApparatusBEAMPrompt peak 16 ns

zo

zo

o-Ps Time of Flight measurements

where

tf

tp

z0

If tp ˂˂ tf tm tf

Mariazzi S, Brusa R S et al., Phys. Rev. Lett. 104 243401 (2010)

0 50 100 150 200 250

0.068 eV

coun

ts [a

rbitr

ary

units

]

o-Ps time of flight [ns]

7 keV, T = 150 K

2

0.076 eV

7 keV, T = 200 K

0 100 200 300 400

FWHM = 23 ns

coun

ts [a

rb. u

nits

]

Time of flight [ns]

Prompt

0.096 eV

7 keV, T = 300 K3

4

8

0.140 eV 4 keV, T = 300 K

After smoothing, subtraction of the background, and correction by multiplying by 142exp1 t

t

140 160 180 200 220 240 260 280 300 3200.065

0.070

0.075

0.080

0.085

0.090

0.095

0.100

aver

age

o-Ps

em

issio

n en

ergy

[eV]

Sample temperature [K]

Ps cooling - 5-8 nm channels

0.0 0.1 0.2 0.3

T=1515±15K

T=305±10K

T=1425±25K

T=195±10K

T=1260±15K

T=145±10K

7 KeV, T = 300 K 7 KeV, T = 200 K 7 KeV, T = 150 K

log(

dN/d

E) [a

rbitr

ary

units

]o-Ps kinetic energy [eV]

dEENdttN )()(

3)()( ttNEN

0 50 100 150 200 250

0.068 eV

coun

ts [a

rbitr

ary

units

]

o-Ps time of flight [ns]

7 keV, T = 150 K

2

0.076 eV

7 keV, T = 200 K

0 100 200 300 400

FWHM = 23 ns

coun

ts [a

rb. u

nits

]

Time of flight [ns]

Prompt

0.096 eV

7 keV, T = 300 K3

4

8

0.140 eV 4 keV, T = 300 K

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

Thermalized Ps

1 100.0

0.1

0.2

0.3

0.4

Positron implantation depth [nm]

T = 300 K T = 200 K T = 150 K

F 3 o

-Ps

fract

ion/

impl

ante

d po

sitro

ns

Positron implantation energy [keV]

100 1000

0.0 0.1 0.2 0.3

T=1515±15K

T=305±10K

T=1425±25K

T=195±10K

T=1260±15K

T=145±10K

7 KeV, T = 300 K 7 KeV, T = 200 K 7 KeV, T = 150 K

log(

dN/d

E) [a

rbitr

ary

units

]

o-Ps kinetic energy [eV]

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

quantum confinement and thermalization

Crivelli et al., Phys. Rev. A 81, 052703 (2010)

Cassidy et al., Phys. Rev. A 81, 012715 (2010)

Similarsamples

42 meV in pores of 2.7 nm

Ps

Positronium converter

Ps

Ps

Vacuum

Ps

Ps

Permanence time of Ps in nano-channels before escaping into vacuum

tm= tp+tf

tf

tp

z0

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

t p thermal=19±9 ns tp cooled = 5±3 nsvthermal = 4.9x104±2x103 m/s

T=310±20 K

13.4± 0.9 meV

vcooled = 1.0x105±1x104 m/s

T=1370±300 K

59.4 ± 13.0 meV

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

Experimental Pick off lifetime of 44±4 ns is less than expectedby 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 (1100±300K ) we find 51±8 ns .

t exp = tp = 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 detectorsStrong need of friendly programs of analysis

based on diffusion equationStudy 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 NOTOG. NEBBIA

positron Group, Università di Trentopositron Group, Università di TrentoINFN, Padova-Trento

S. MARIAZZI L. RAVELLI and W. EGGER C. MACCHI , A. SOMOZAR. CHECCHETTO, A. MIOTELLOUniversità di Trento

positron Group, Università di Trento

INFIMAT, Tandil, Buenos AiresUniversität der Bunderswehr