Antosik, A.

Dobrowolska, A.

Griessen, R.

Koeman, N.J.

Molen, S. J. van der

Remhof, A.

DSpace at VU

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PHYSICAL REVIEW B 66, 020101~R! ~2002!Switchable mirrors for visualization and control of hydrogen diffusion in transition metals
A. Remhof, S. J. van der Molen, A. Antosik, A. Dobrowolska, N. J. Koeman, and R. Griessen
Faculty of Sciences, Division of Physics and Astronomy, Vrije Universiteit, De Boelelaan 1081, 1081 HV Amsterdam, The Netherlands
~Received 13 March 2002; revised manuscript received 8 May 2002; published 24 June 2002!
We show that the switchable mirror material YHx can be used both as an indicator to monitor and as an agent
to control hydrogen diffusion in thin films. The applicability of the optical-indicator technique is demonstrated
for VHx thin films. The diffusion coefficient is typically 1025 cm2/s at concentrations around 0.7 H/V at
temperatures between 373 and 473 K. Deposition of a layer of Y on V makes it also possible to tune the
effective hydrogen mobility via the V/Y thickness ratio. This can be used for investigation of hydrogen
diffusion waves in laterally structured objects.
DOI: 10.1103/PhysRevB.66.020101 PACS number~s!: 66.30.2h, 68.55.Ln, 68.37.2d, 68.60.2pOne of the striking properties of hydrogen in metals is its
large mobility. Already at room temperature the H diffusion
coefficient can be as high as 1025 cm2/s, i.e., a value almost
comparable to diffusion in liquids. A review of experimental
data and techiques used so far to measure hydrogen diffusiv-
ity in bulk samples is given by Alefeld and Vo¨lkl.1 Most of
these methods are not applicable to thin films as they are
either hampered by the influence of the substrate ~e.g., in
Gorsky effect! or by the rather small volume of the film ~e.g.
in quasielastic neutron scattering!. Consequently, relatively
little is known about hydrogen diffusion in thin metallic
films and multilayers. The understanding and manipulation
of hydrogen transport through films is, however, important
for the control and optimization of coatings and thin-film
devices such as hydrogen detectors,2,3 metal-hydride switch-
able mirrors4,5 or tunable magnetical elements.6,7 Recently
den Broeder et al.8 presented an optical method to monitor
the lateral migration of hydrogen in Y, exploiting the intrin-
sic concentration dependent optical properties of the Y-H
system.9 Especially the progression of the boundaries sepa-
rating the various stable hydride phases can easily be de-
tected as discontinuities in the optical contrast.
The main purpose of this communication is to demon-
strate that visualization of H diffusion is also possible for
hydrogen in opaque transition-metal films. More specifically
~i! we demonstrate the feasibility to use a thin layer of Y as
an optical indicator to visualize the lateral H migration in
thin films of vanadium, ~ii! we show that the mobility of the
phase boundaries in a composite film ~e.g., V/Y! can be
tuned through the sample/indicator thickness ratio, and ~iii!
we determine quantitatively the H-diffusion coefficient in a
vanadium film by means of our optical indicator method.
The samples are prepared by means of e-gun evaporation
in an ultrahigh vacuum system ~background pressure
,1029 mbar). A typical sample consists of a V stripe of
length L510 mm, width b51 mm, and thickness 25 nm
,dS,250 nm. Usually 11 stripes of various thickness dS are
deposited onto one, polished amorphous quartz substrate.
The V stripes are covered with a thin layer of yttrium as an
optical indicator for hydrogen diffusion. Indicator thick-
nesses 10 nm ,dI,50 nm are examined. One end of each of
the combined V/Y stripes is covered in situ with a 10-nm-
thick Pd cap layer as indicated in Fig. 1 The thicknesses of
the metallic layers are monitored in situ by means of a water-0163-1829/2002/66~2!/020101~4!/$20.00 66 0201cooled quartz microbalance during deposition and are
checked ex situ by profilometry and by Rutherford back-
scattering ~RBS!. Furthermore, RBS is used to characterize
the chemical composition of the metallic stripes and to detect
eventual alloy formation at the interfaces. The RBS spectra
show well-defined layers, no intermixing at the interfaces,
and no chemical impurities. The thickness of the natural ox-
ide layer that forms in ambient air on the uncovered part of
the yttrium layer is determined to be 15 nm.
The gas loading cell and the experimental setup have been
described earlier.8,10 At t50 a sample assembly ~consisting
here of 11 individual V/Y stripes! is exposed to hydrogen. As
described by Den Broeder et al.,8 the Y underneath the Pd
immediately starts absorbing hydrogen, and forms the trans-
parent YH3-d phase. Further hydrogen uptake is achieved by
lateral hydrogen diffusion mainly through vanadium, since H
uptake cannot occur via the superficially oxidized Y. The
lateral migration of hydrogen in vanadium away from the
Pd-covered region can easily be monitored optically in the Y
indicator layer as the various yttrium hydride phases formed
at different hydrogen concentrations, exhibit characteristic
optical properties.9,11 For example, the front corresponding
to the coexisting a and b phases is clearly identified as a
discontinuous change in transmission and reflection.8 In Y,
the phase boundary of the hcp a-phase is ca ,max50.2 H/Y
and that of the fcc b-phase is cb ,min51.9 H/Y, thus DcI
FIG. 1. Schematic sample design. Yttrium-covered vanadium
stripes (1310 mm2) of thickness 25–125 nm are deposited onto an
a-SiO2 substrate (1531030.53 mm3). The stripes are partially
covered with a Pd caplayer to enable gas phase H loading. In the
actual sample series there are 11 stripes.©2002 The American Physical Society01-1
RAPID COMMUNICATIONS
A. REMHOF et al. PHYSICAL REVIEW B 66, 020101~R! ~2002!5cb,min2ca,max51.7 H/Y.11,12 In the following we call this
optical feature the ‘‘b-front’’ or simply the front and its po-
sition x f .
Figure 2 depicts a snapshot at t5104 s after the 11-stripe-
sample assembly was exposed to a hydrogen atmosphere of 1
mbar at T5473 K. The Pd-covered parts, where the hydro-
gen enters the samples and the lateral diffusion starts, is at
the top of the photo. The optical discontinuity within the
indicator corresponds to the boundary between the silvery
gray low concentration a-YH0.2 phase and the blue ~in re-
flection! dihydride phase b-YH1.9 . The results in Fig. 2 do
not only show that the progression of H in V is easily fol-
lowed through the optical appearence of the indicator layer,
but also that the front mobility can be tuned through the ratio
of the sample ~V! and the indicator ~Y! thicknesses. Note the
pronounced thickness dependence of the front mobility K
5x f
2/t . The thickness of the stripes in Fig. 2 decreases from
left to right from 125 nm to 25 nm. All the stripes are cov-
ered with 50-nm Y. Oxidation reduces the effectice Y thick-
ness down to 35 nm. The thickness of the Pd cover in the
upper part of the stripes is 10 nm for all the 11 stripes.
Systematic measurements, in which the temperature, the
sample thickness, and the indicator thickness are varied,
show that the mobilities exhibit an Arrhenius behavior ~for a
given sample thickness!. At a given temperature the mobility
increases with increasing dS /dI , i.e., in a thick sample cov-
ered by a thin indicator the front progresses faster than in a
thin sample covered by a thick indicator.
We show now that the hydrogen-diffusion coefficient can
be determined from experimental results of the type shown
in Fig. 2. For this we note that below the Pd caplayer the
hydrogen concentration cS0 in the sample is in thermody-
namical equilibrium with the surrounding H2 atmosphere.
We choose the coordinate system such that the direction of
FIG. 2. Photography of an 11 stripe sample loaded in a hydro-
gen atmosphere (pH251 mbar, T5473 K) for 104 s. The 11 com-
posite V/Y stripes take up H via the Pd layer, which is located at the
upper part of the photo. Lateral H migration occurs along the
stripes, away from the Pd-covered part. Within the indicator layer
~Y layer!, the presence of H leads to the formation of the YH2
phase, which appears blue in reflection. Note the influence of the V
thickness on the mobility of the YH2 front in the indicator. The
thickness of the V stripes decreases from 125 nm to 25 nm from left
to right. The 2nd, 7th, and the 9th stripe have a nonconstant V
thickness.02010diffusion is in the positive x direction. The reservoir-sample
interface lies at x50 and cS(x50)5cS0. The concentration
cS(x ,t) is determined by the diffusion equation
]c
]t
5
]
]x S D ]c]x D . ~1!
Since ~i! the enthalpy of hydrogen solution in the indicator
~here Y! is much more negative than in the sample ~here V!
and ~ii! the diffusion constant D in the sample ~V! is orders
of magnitude larger than in the indicator ~Y!, all the hydro-
gen flows via the sample into the indicator. This creates a
diffusion front at x5x f that is experimentally found to move
as x f
25Kt . The H atoms crossing in the time interval dt , the
cross section dSb at x f are used ~i! to move the front by dx f
and ~ii! to fill the overlaying indicator up to a concentration
of 1.9 H/Y. In this way c fdSbdx f hydrogen atoms are acco-
modated in the sample and DcIdIbdx f in the indicator. As
long as the indicator is still in the dilute YHx a-phase it takes
up all the hydrogen from the underlying sample. There is
essentially no difference in position between the position of
the front in the indicator and the sharp drop of concentration
in the sample at x f . Conservation of the number of particles
leads to
j fbdSdt5b~DcIdI1c fdS!dx f , ~2!
where j f52D f(dcS /dx)u f is the flux of H atoms at the front
through the sample. Assuming a concentration independent
D, the solution of Eq. ~1! with cS(0,t)5cS0 and the bound-
ary condition Eq. ~2! is
cS~x ,t !5cS02a erfS x2ADt D . ~3!
As the concentration c f at the front is given by
c f[cS~x f ,t !5cS02a erf~AK/2AD !, ~4!
we obtain from Eqs. ~2! and ~3!
dI
dS
5
2aAD
DcIApKeK/4D
1
a
DcI
erfS AK2AD D 2 cS0DcI . ~5!
In an experiment dI , dS , DcI , and K are known. The diffu-
sion coefficient D, the parameter a , and the equilibrium con-
centration cS0 can be extracted from measurements of K as a
function of dI /dS . The experimental data for two different
sets of experiments ~each of them for five values of dI /dS!,
are represented in Fig. 3 together with their respective fits.
We obtain a hydrogen-diffusion coefficient in vanadium DV
51.231025 cm2/s and an equilibrium concentration cS0
50.62 H/V at 473 K and at pH251 mbar. At T5373 K
and pH251 bar the measured values are DV50.67
31025 cm2/s and cS050.83 H/V.
For a discussion of these results we assume first that the
phase diagram of H in V films is the same as for bulk VHx .
Then our results at T5473 K pertain to the a-VHx phase in
which H occupies randomly tetrahedral sites in the bcc V lost
lattice. Our results at T5373 K, on the other hand, corre-1-2
RAPID COMMUNICATIONS
SWITCHABLE MIRRORS FOR VISUALIZATION AND . . . PHYSICAL REVIEW B 66, 020101~R! ~2002!spond to the b2-phase that exists for temperatures between
293 and 463 K at hydrogen concentrations between 0.45 and
0.83 H/V. The b2-phase is separated from the cubic
CaF2-like VH2 phase by a large miscibility gap.13
We consider now our data at 473 K. Recently, solubility
isotherms of thin-film V-H samples within the a-phase have
been studied by Andersson et al.14 Within the investigated
thickness range 50,dS,100 nm, no pronounced thickness
dependence of the isotherms could be observed. The equilib-
rium hydrogen concentrations depend strongly on pH2 and T
in the pressure range of pH250.5–50 mbar and at tempera-
tures between 450 and 500 K. In the case of our measure-
ment at T5473 K and pH251 mbar, this makes a direct
comparison difficult. However, within experimental uncer-
tainties, the measured value of cS050.62 H/V is consistent
with the data published by Andersson et al.
Apart from the equilibrium concentration at given hydro-
gen pressures, the slope of a solubility isotherm yields the
derivative of the chemical potential with respect to the hy-
drogen concentration ]m/]cS . The macroscopic diffusion
coefficient D, investigated here, is related to the tracer diffu-
sion coefficient Dt via D/Dt5(cS /kBT)(]m/]cS). Nuclear
magnetic resonance ~NMR! experiments on bulk vanadium
show a monotonic decrease of Dt with increasing hydrogen
concentration cS from Dt5231024 cm2/s at cS50.02 H/V
down to Dt5231025 cm2/s at cS50.68 H/V.15 The slope
of the 473 K solubility isotherm14 at cS50.62 H/V yields
(cS /kBT)(]m/]cS)>3, leading to a diffusion coefficient of
D5631025 cm2/s, comparable with our value of D51.2
31025 cm2/s. Again, a comparison between samples pre-
pared by different techniques, leading to different grain sizes,
stress states, impurities etc., hampers a direct comparison.
FIG. 3. Hydrogen front mobility K in the yttrium indicator
(AK5x f /At) versus sample thickness ratio (dI/dS) together with
the respective fits according to Eq. ~5!. The fits yield a hydrogen-
diffusion coefficient in V of DV51.231025 cm2/s, and a H con-
centration cS050.62 H/V at T5473 K and pH251 mbar ~upper
curve!. At T5373 K and pH251 bar the corresponding values are
DV50.6731025 cm2/s and cS050.83 H/V.02010Let us now focus on the results at T5373 K and pH2
51 bar. Even though the phase diagram for bulk V-H may
deviate markedly from that for thin films and nanocrystalline
samples,14,16 under the condition of 1 bar and 373 K we
expect our V samples to be in the saturated b2-phase. For
bulk samples this phase boundary lies at 0.82 H/V and is
essentially temperature independent between room tempera-
ture and 373 K. The H-rich g-phase cannot be reached at 373
K without applying high pressures ~ca. 50 bar!, so we ex-
clude the presence of this phase in our samples.13
Orimo et al.16 report a modification of the b2-g phase
boundary in the V-H system within nanometer-scale V
grains. They conclude that the b2-g phase boundary de-
creases from 0.82 H/V for 80-nm grains down to 0.67 H/V
for 10-nm grains. By using NMR techniques, the same au-
thors measured a hydrogen jump rate of n300 K5109 s21 at
300 K in 80-nm-sized V grains with cS50.82 H/V. Using an
activation energy of 240 meV,16,17 we obtain a jump rate
n373 K563109 s21 at 373 K. Together with a jump length of
0.15 nm, the distance between two interstitial sites, this re-
sults in a tracer diffusion coefficient of Dt52.3
31027 cm2/s. Compared with our measured D50.67
31025 cm2/s this implies D/Dt5(cS /kBT)(]m/]cS)529.
This value is consistent with the steepness of the 373 K
solubility isotherm near the b2-g phase boundary.13
In conclusion, we have demonstrated that thin layers
of YHx can be used as indicators to visualize hydrogen mi-
gration in opaque transition metals such as vanadium. In ad-
dition, our optical method allows measuring the equilibrium
concentration in the material under investigation and its
macroscopic diffusion coefficient. In principle, our technique
is applicable to any material that can be coated with a swit-
chable metal-hydride mirror material as long as ~i! its
H-diffusion coefficient is higher than the one of the indicator
and ~ii! its hydrogen affinity is low compared to the hydro-
gen affinity of the indicator. Furthermore, by varying the
sample/indicator thickness ratio the effective mobility of
the hydrogen front in the indicator layer can be tuned over a
wide range. In principle, D can be varied at room tempera-
ture by four orders of magnitude between the extreme values
of H in Y ~typically 1029 cm2/s) and H in V ~typically
1025 cm2/s). In a patterned thin film this opens the op-
portunity to locally vary the mobility of the front and to
investigate hydrogen diffusion waves.18 Future investigations
will include refractionlike and reflectionlike phenomena at
interfaces separating areas of different effective diffu-
sivities.19
We would like to thank A. Pundt, G. Majer, B. Hjo¨rvars-
son, and R. J. Wijngaarden for stimulating discussions, as
well as J. N. Rector for the performance of the RBS mea-
surements. This work was supported by the EU-TMR project
‘‘Metal-hydride films with switchable physical properties,’’
Project No. ERB FMRX-CT98-187. It is also part of the
research program of the Stichting voor Fundamenteel Onder-
zoek ~FOM! which is financially supported by the Dutch
NWO.1-3
RAPID COMMUNICATIONS
A. REMHOF et al. PHYSICAL REVIEW B 66, 020101~R! ~2002!1 Hydrogen in Metals, edited by G. Alefeld and J. Vo¨lkl ~Springer-
Verlag, Berlin, 1978!, Vols. 1 and 2.
2 Advanced Sensors and Monitors for Process Industries and the
Environment, edited by W. A. de Groot, SPIE Proceedings Vol.
3535 ~SPIE, Bellingham, 1999!.
3 J.A. Garcia and A. Mandelis, Rev. Sci. Instrum. 67, 3981 ~1996!.
4 J.N. Huiberts, R. Griessen, J.H. Rector, R.J. Wijngaarden, J.P.
Dekker, D.G. de Groot, and N.J. Koeman, Nature ~London! 380,
231 ~1996!.
5 P. van der Sluis, M. Ouwerkerk, and P.A. Duine, Appl. Phys. Lett.
78, 1315 ~1997!.
6 F. Klose, Ch. Rehm, D. Nagengast, H. Maletta, and A. Weidinger,
Phys. Rev. Lett. 78, 1150 ~1997!.
7 B. Hjo¨rvarsson, J.A. Dura, P. Isberg, T. Watanabe, T.J. Udovic, G.
Andersson, and C.F. Majkrzak, Phys. Rev. Lett. 79, 901 ~1997!.
8 F.J.A. den Broeder, S.J. van der Molen, M. Kremers, J.N. Huib-
erts, D.G. Nagengast, A.T.M. van Gogh, W.H. Huisman, N.J.
Koeman, B. Dam, J.H. Rector, S. Plota, M. Haaksma, R.M.N.
Hanzen, R.M. Jungblut, P.A. Duine, and R. Griessen, Nature
~London! 394, 656 ~1998!.
9 M. Kremers, N.J. Koeman, R. Griessen, P.H.L. Notten, R. Tol-
boom, P.J. Kelly, and P.A. Duine, Phys. Rev. B 57, 4943 ~1998!.0201010 A. Remhof, J.W.J. Kerssemakers, S.J. van der Molen, R. Gries-
sen, and E.S. Kooij, Phys. Rev. B 65, 054110 ~2002!.
11 P. Vadja, in Handbook on the Physics and Chemistry of Rare
Earth, edited by K. A. Gschneidner and L. Eyring ~Elsevier,
Amsterdam, 1995!, Vol. 20.
12 E.S. Kooij, A.T.M. van Gogh, D.G. Nagengast, N.J. Koeman, and
R. Griessen, Phys. Rev. B 62, 10 088 ~2000!.
13 C.-H. Fagerstroem, F. D. Manchester, and J. M. Pitre, in Phase
Diagrams of Binary Hydrogen Alloys, edited by F. D. Manches-
ter ~ASM International, Materials Park, 2000!.
14 G. Andersson, K. Aits, and B. Hjo¨rvarsson, J. Alloys Compd. 334,
14 ~2002!.
15 J.E. Kleiner, E.H. Sevilla, and R.M. Cotts, Phys. Rev. B 33, 6662
~1986!.
16 S. Orimo, F. Kimmerle, and G. Majer, Phys. Rev. B 63, 094307
~2001!.
17 Y. Fukai and S. Kazama, Acta Metall. 25, 59 ~1977!.
18 A. Mandelis, Phys. Today 53 ~8!, 29 ~2000!, and references
therein.
19 M.A. O’Leary, D.A. Boas, B. Chance, and A.G. Yodh, Phys. Rev.
Lett. 69, 2658 ~1992!.1-4


Switchable mirrors for visualization and control of hydrogen diffusion in transition metals

van der Molen, S. J.

NARCIS 

VU Research Portal

VU Research PortalSwitchable mirrors for visualization and control of hydrogen diffusion in transitionmetalsAntosik, A.; Dobrowolska, A.; Griessen, R.; Koeman, N.J.; Remhof, A.; van der Molen, S.J.published inPhysical Review B. Condensed Matter and Materials Physics2002document versionPublisher's PDF, also known as Version of recordLink to publication in VU Research Portalcitation for published version (APA)Antosik, A., Dobrowolska, A., Griessen, R., Koeman, N. J., Remhof, A., & van der Molen, S. J. (2002).Switchable mirrors for visualization and control of hydrogen diffusion in transition metals. Physical Review B.Condensed Matter and Materials Physics, 66(2), [020101].General rightsCopyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright ownersand it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights.            • Users may download and print one copy of any publication from the public portal for the purpose of private study or research.            • You may not further distribute the material or use it for any profit-making activity or commercial gain            • You may freely distribute the URL identifying the publication in the public portal ?Take down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.E-mail address:vuresearchportal.ub@vu.nlDownload date: 22. May. 2021rlandsRAPID COMMUNICATIONSPHYSICAL REVIEW B 66, 020101~R! ~2002!Switchable mirrors for visualization and control of hydrogen diffusion in transition metalsA. Remhof, S. J. van der Molen, A. Antosik, A. Dobrowolska, N. J. Koeman, and R. GriessenFaculty of Sciences, Division of Physics and Astronomy, Vrije Universiteit, De Boelelaan 1081, 1081 HV Amsterdam, The Nethe~Received 13 March 2002; revised manuscript received 8 May 2002; published 24 June 2002!We show that the switchable mirror material YHx can be used both as an indicator to monitor and as an agentto control hydrogen diffusion in thin films. The applicability of the optical-indicator technique is demonstratedfor VHx thin films. The diffusion coefficient is typically 1025 cm2/s at concentrations around 0.7 H/V attemperatures between 373 and 473 K. Deposition of a layer of Y on V makes it also possible to tune theeffective hydrogen mobility via the V/Y thickness ratio. This can be used for investigation of hydrogendiffusion waves in laterally structured objects.DOI: 10.1103/PhysRevB.66.020101 PACS number~s!: 66.30.2h, 68.55.Ln, 68.37.2d, 68.60.2pitsntalsacionmor--Hpadnorllainaateofraan-o-are-zetecttraes,ox-ofeensns-byHheheYedticgaumantheOne of the striking properties of hydrogen in metals islarge mobility. Already at room temperature the H diffusiocoefficient can be as high as 1025 cm2/s, i.e., a value almoscomparable to diffusion in liquids. A review of experimentdata and techiques used so far to measure hydrogen diffuity in bulk samples is given by Alefeld and Vo¨lkl.1 Most ofthese methods are not applicable to thin films as theyeither hampered by the influence of the substrate~e.g., inGorsky effect! or by the rather small volume of the film~e.g.in quasielastic neutron scattering!. Consequently, relativelylittle is known about hydrogen diffusion in thin metallifilms and multilayers. The understanding and manipulatof hydrogen transport through films is, however, importafor the control and optimization of coatings and thin-fildevices such as hydrogen detectors,2,3 metal-hydride switch-able mirrors4,5 or tunable magnetical elements.6,7 Recentlyden Broederet al.8 presented an optical method to monitthe lateral migration of hydrogen in Y, exploiting the intrinsic concentration dependent optical properties of the Ysystem.9 Especially the progression of the boundaries serating the various stable hydride phases can easily betected as discontinuities in the optical contrast.The main purpose of this communication is to demostrate that visualization of H diffusion is also possible fhydrogen in opaque transition-metal films. More specifica~i! we demonstrate the feasibility to use a thin layer of Yan optical indicator to visualize the lateral H migrationthin films of vanadium,~ii ! we show that the mobility of thephase boundaries in a composite film~e.g., V/Y! can betuned through the sample/indicator thickness ratio, and~iii !we determine quantitatively the H-diffusion coefficient invanadium film by means of our optical indicator method.The samples are prepared by means of e-gun evaporin an ultrahigh vacuum system~background pressur,1029 mbar). A typical sample consists of a V stripelength L510 mm, width b51 mm, and thickness 25 nm,dS,250 nm. Usually 11 stripes of various thicknessdS aredeposited onto one, polished amorphous quartz substThe V stripes are covered with a thin layer of yttrium asoptical indicator for hydrogen diffusion. Indicator thicknesses 10 nm,dI,50 nm are examined. One end of eachthe combined V/Y stripes is coveredin situ with a 10-nm-thick Pd cap layer as indicated in Fig. 1 The thicknessesthe metallic layers are monitoredin situ by means of a water0163-1829/2002/66~2!/020101~4!/$20.00 66 0201iv-rent-e--ysionte.fofcooled quartz microbalance during deposition andcheckedex situ by profilometry and by Rutherford backscattering~RBS!. Furthermore, RBS is used to characterithe chemical composition of the metallic stripes and to deeventual alloy formation at the interfaces. The RBS specshow well-defined layers, no intermixing at the interfacand no chemical impurities. The thickness of the naturalide layer that forms in ambient air on the uncovered partthe yttrium layer is determined to be 15 nm.The gas loading cell and the experimental setup have bdescribed earlier.8,10 At t50 a sample assembly~consistinghere of 11 individual V/Y stripes! is exposed to hydrogen. Adescribed by Den Broederet al.,8 the Y underneath the Pdimmediately starts absorbing hydrogen, and forms the traparent YH3-d phase. Further hydrogen uptake is achievedlateral hydrogen diffusion mainly through vanadium, sinceuptake cannot occur via the superficially oxidized Y. Tlateral migration of hydrogen in vanadium away from tPd-covered region can easily be monitored optically in theindicator layer as the various yttrium hydride phases format different hydrogen concentrations, exhibit characterisoptical properties.9,11 For example, the front correspondinto the coexistinga and b phases is clearly identified asdiscontinuous change in transmission and reflection.8 In Y,the phase boundary of the hcpa-phase isca,max50.2 H/Yand that of the fccb-phase iscb,min51.9 H/Y, thusDcIFIG. 1. Schematic sample design. Yttrium-covered vanadistripes (1310 mm2) of thickness 25–125 nm are deposited ontoa-SiO2 substrate (1531030.53 mm3). The stripes are partiallycovered with a Pd caplayer to enable gas phase H loading. Inactual sample series there are 11 stripes.©2002 The American Physical Society01-1oroiseel-yetiomv-thestiey-inanwth-neors aen-isofionstenttfits.theost-roththyeHeletRAPID COMMUNICATIONSA. REMHOF et al. PHYSICAL REVIEW B 66, 020101~R! ~2002!5cb,min2ca,max51.7 H/Y.11,12 In the following we call thisoptical feature the ‘‘b-front’’ or simply the frontand its po-sition xf .Figure 2 depicts a snapshot att5104 s after the 11-stripe-sample assembly was exposed to a hydrogen atmospherembar atT5473 K. The Pd-covered parts, where the hydgen enters the samples and the lateral diffusion starts,the top of the photo. The optical discontinuity within thindicator corresponds to the boundary between the silvgray low concentrationa-YH0.2 phase and the blue~in re-flection! dihydride phaseb-YH1.9. The results in Fig. 2 donot only show that the progression of H in V is easily folowed through the optical appearence of the indicator labut also that the front mobility can be tuned through the raof the sample~V! and the indicator~Y! thicknesses. Note thepronounced thickness dependence of the front mobilityK5xf2/t. The thickness of the stripes in Fig. 2 decreases frleft to right from 125 nm to 25 nm. All the stripes are coered with 50-nm Y. Oxidation reduces theeffecticeY thick-ness down to 35 nm. The thickness of the Pd cover inupper part of the stripes is 10 nm for all the 11 stripSystematic measurements, in which the temperature,sample thickness, and the indicator thickness are varshow that the mobilities exhibit an Arrhenius behavior~f r agiven sample thickness!. At a given temperature the mobilitincreases with increasingdS /dI , i.e., in a thick sample covered by a thin indicator the front progresses faster thanthin sample covered by a thick indicator.We show now that the hydrogen-diffusion coefficient cbe determined from experimental results of the type shoin Fig. 2. For this we note that below the Pd caplayerhydrogen concentrationcS0 in the sample is in thermodynamical equilibrium with the surrounding H2 atmosphere.We choose the coordinate system such that the directioFIG. 2. Photography of an 11 stripe sample loaded in a hydgen atmosphere (pH251 mbar,T5473 K) for 104 s. The 11 com-posite V/Y stripes take up H via the Pd layer, which is located atupper part of the photo. Lateral H migration occurs alongstripes, away from the Pd-covered part. Within the indicator la~Y layer!, the presence of H leads to the formation of the Y2phase, which appears blue in reflection. Note the influence of ththickness on the mobility of the YH2 front in the indicator. Thethickness of the V stripes decreases from 125 nm to 25 nm fromto right. The 2nd, 7th, and the 9th stripe have a nonconstanthickness.02010f 1-atryr,oe.hed,aneofdiffusion is in the positivex direction. The reservoir-samplinterface lies atx50 andcS(x50)5cS0. The concentrationcS(x,t) is determined by the diffusion equation]c]t5]]x S D ]c]xD . ~1!Since~i! the enthalpy of hydrogen solution in the indicat~here Y! is much more negative than in the sample~h re V!and ~ii ! the diffusion constantD in the sample~V! is ordersof magnitude larger than in the indicator~Y!, all the hydro-gen flows via the sample into the indicator. This creatediffusion front atx5xf that is experimentally found to movasxf25Kt. The H atoms crossing in the time intervaldt, thecross sectiondSb at xf are used~i! to move the front bydxfand ~ii ! to fill the overlaying indicator up to a concentratioof 1.9 H/Y. In this waycfdSbdxf hydrogen atoms are accomodated in the sample andDcIdIbdxf in the indicator. Aslong as the indicator is still in the dilute YHx a-phase it takesup all the hydrogen from the underlying sample. Thereessentially no difference in position between the positionthe front in the indicator and the sharp drop of concentratin the sample atxf . Conservation of the number of particleleads toj fbdSdt5b~DcIdI1cfdS!dxf , ~2!wherej f52D f(dcS /dx)u f is the flux of H atoms at the fronthrough the sample. Assuming a concentration independD, the solution of Eq.~1! with cS(0,t)5cS0 and the bound-ary condition Eq.~2! iscS~x,t !5cS02a erfS x2ADt D . ~3!As the concentrationcf at the front is given bycf[cS~xf ,t !5cS02a erf~AK/2AD !, ~4!we obtain from Eqs.~2! and ~3!dIdS52aADDcIApKeK/4D1aDcIerfS AK2AD D 2 cS0DcI . ~5!In an experimentdI , dS , DcI , andK are known. The diffu-sion coefficientD, the parametera, and the equilibrium con-centrationcS0 can be extracted from measurements ofK as afunction of dI /dS . The experimental data for two differensets of experiments~each of them for five values ofdI /dS!,are represented in Fig. 3 together with their respectiveWe obtain a hydrogen-diffusion coefficient in vanadiumDV51.231025 cm2/s and an equilibrium concentrationcS050.62 H/V at 473 K and atpH251 mbar. At T5373 Kand pH251 bar the measured values areDV50.6731025 cm2/s andcS050.83 H/V.For a discussion of these results we assume first thatphase diagram of H in V films is the same as for bulk VHx .Then our results atT5473 K pertain to thea-VHx phase inwhich H occupies randomly tetrahedral sites in the bcc V llattice. Our results atT5373 K, on the other hand, corre-eerVftV1-2eaicitysil-urtr-o-yfreenayneeisera-73V-/Vau-fre-Kersmi-ad-mitsueit-or-eofr a-luesp-atfu-asea-ect’’eder-hreRAPID COMMUNICATIONSSWITCHABLE MIRRORS FOR VISUALIZATION AND . . . PHYSICAL REVIEW B66, 020101~R! ~2002!spond to theb2-phase that exists for temperatures betwe293 and 463 K at hydrogen concentrations between 0.450.83 H/V. The b2-phase is separated from the cubCaF2-like VH2 phase by a large miscibility gap.13We consider now our data at 473 K. Recently, solubilisotherms of thin-film V-H samples within thea-phase havebeen studied by Anderssonet al.14 Within the investigatedthickness range 50,dS,100 nm, no pronounced thicknesdependence of the isotherms could be observed. The equrium hydrogen concentrations depend strongly onpH2 andTin the pressure range ofpH250.5–50 mbar and at temperatures between 450 and 500 K. In the case of our measment at T5473 K and pH251 mbar, this makes a direccomparison difficult. However, within experimental uncetainties, the measured value ofcS050.62 H/V is consistentwith the data published by Anderssonet al.Apart from the equilibrium concentration at given hydrgen pressures, theslopeof a solubility isotherm yields thederivative of the chemical potential with respect to the hdrogen concentration]m/]cS . The macroscopic diffusioncoefficient D, investigated here, is related to thetracer diffu-sion coefficient Dt via D/Dt5(cS /kBT)(]m/]cS). Nuclearmagnetic resonance~NMR! experiments on bulk vanadiumshow a monotonic decrease ofDt with increasing hydrogenconcentrationcS from Dt5231024 cm2/s atcS50.02 H/Vdown to Dt5231025 cm2/s at cS50.68 H/V.15 The slopeof the 473 K solubility isotherm14 at cS50.62 H/V yields(cS /kBT)(]m/]cS)>3, leading to a diffusion coefficient oD5631025 cm2/s, comparable with our value ofD51.231025 cm2/s. Again, a comparison between samples ppared by different techniques, leading to different grain sizstress states, impurities etc., hampers a direct comparisoFIG. 3. Hydrogen front mobilityK in the yttrium indicator(AK5xf /At) versus sample thickness ratio (dI /dS) together withthe respective fits according to Eq.~5!. The fits yield a hydrogen-diffusion coefficient in V ofDV51.231025 cm2/s, and a H con-centrationcS050.62 H/V at T5473 K and pH251 mbar ~uppercurve!. At T5373 K andpH251 bar the corresponding values aDV50.6731025 cm2/s andcS050.83 H/V.02010nndib-e---s,.Let us now focus on the results atT5373 K and pH251 bar. Even though the phase diagram for bulk V-H mdeviate markedly from that for thin films and nanocrystallisamples,14,16 under the condition of 1 bar and 373 K wexpect our V samples to be in the saturatedb2-phase. Forbulk samples this phase boundary lies at 0.82 H/V andessentially temperature independent between room tempture and 373 K. The H-richg-phase cannot be reached at 3K without applying high pressures~ca. 50 bar!, so we ex-clude the presence of this phase in our samples.13Orimo et al.16 report a modification of theb2-g phaseboundary in the V-H system within nanometer-scalegrains. They conclude that theb2-g phase boundary decreases from 0.82 H/V for 80-nm grains down to 0.67 Hfor 10-nm grains. By using NMR techniques, the samethors measured a hydrogen jump rate ofn300 K5109 s21 at300 K in 80-nm-sized V grains withcS50.82 H/V. Using anactivation energy of 240 meV,16,17 we obtain a jump raten373 K563109 s21 at 373 K. Together with a jump length o0.15 nm, the distance between two interstitial sites, thissults in a tracer diffusion coefficient ofDt52.331027 cm2/s. Compared with our measuredD50.6731025 cm2/s this impliesD/Dt5(cS /kBT)(]m/]cS)529.This value is consistent with the steepness of the 373solubility isotherm near theb2-g phase boundary.13In conclusion, we have demonstrated that thin layof YHx can be used as indicators to visualize hydrogengration in opaque transition metals such as vanadium. Indition, our optical method allows measuring the equilibriuconcentration in the material under investigation andmacroscopic diffusion coefficient. In principle, our techniqis applicable to any material that can be coated with a swchable metal-hydride mirror material as long as~i! itsH-diffusion coefficient is higher than the one of the indicatand ~ii ! its hydrogen affinity is low compared to the hydrogen affinity of the indicator. Furthermore, by varying thsample/indicator thickness ratio the effective mobilitythe hydrogen front in the indicator layer can be tuned ovewide range. In principle,D can be varied at room temperature by four orders of magnitude between the extreme vaof H in Y ~typically 1029 cm2/s) and H in V ~typically1025 cm2/s). In a patterned thin film this opens the oportunity to locally vary the mobility of the front and toinvestigate hydrogen diffusion waves.18 Future investigationswill include refractionlike and reflectionlike phenomenainterfaces separating areas of different effective difsivities.19We would like to thank A. Pundt, G. Majer, B. Hjo¨rvars-son, and R. J. Wijngaarden for stimulating discussions,well as J. N. Rector for the performance of the RBS msurements. This work was supported by the EU-TMR proj‘‘Metal-hydride films with switchable physical properties,Project No. ERB FMRX-CT98-187. It is also part of thresearch program of the Stichting voor Fundamenteel Onzoek ~FOM! which is financially supported by the DutcNWO.1-3thl.J.ette, Gib.J.Nurolies-rendv.RAPID COMMUNICATIONSA. REMHOF et al. PHYSICAL REVIEW B 66, 020101~R! ~2002!1Hydrogen in Metals, edited by G. Alefeld and J. Vo¨lkl ~Springer-Verlag, Berlin, 1978!, Vols. 1 and 2.2Advanced Sensors and Monitors for Process Industries andEnvironment, edited by W. A. de Groot, SPIE Proceedings Vo3535 ~SPIE, Bellingham, 1999!.3J.A. Garcia and A. Mandelis, Rev. Sci. Instrum.67, 3981~1996!.4J.N. Huiberts, R. Griessen, J.H. Rector, R.J. Wijngaarden,Dekker, D.G. de Groot, and N.J. Koeman, Nature~London! 380,231 ~1996!.5P. van der Sluis, M. Ouwerkerk, and P.A. Duine, Appl. Phys. L78, 1315~1997!.6F. Klose, Ch. Rehm, D. Nagengast, H. Maletta, and A. WeidingPhys. Rev. Lett.78, 1150~1997!.7B. Hjörvarsson, J.A. Dura, P. Isberg, T. Watanabe, T.J. UdovicAndersson, and C.F. Majkrzak, Phys. Rev. Lett.79, 901 ~1997!.8F.J.A. den Broeder, S.J. van der Molen, M. Kremers, J.N. Huerts, D.G. Nagengast, A.T.M. van Gogh, W.H. Huisman, NKoeman, B. Dam, J.H. Rector, S. Plota, M. Haaksma, R.MHanzen, R.M. Jungblut, P.A. Duine, and R. Griessen, Nat~London! 394, 656 ~1998!.9M. Kremers, N.J. Koeman, R. Griessen, P.H.L. Notten, R. Tboom, P.J. Kelly, and P.A. Duine, Phys. Rev. B57, 4943~1998!.02010eP..r,.-..e-10A. Remhof, J.W.J. Kerssemakers, S.J. van der Molen, R. Grsen, and E.S. Kooij, Phys. Rev. B65, 054110~2002!.11P. Vadja, in Handbook on the Physics and Chemistry of RaEarth, edited by K. A. Gschneidner and L. Eyring~Elsevier,Amsterdam, 1995!, Vol. 20.12E.S. Kooij, A.T.M. van Gogh, D.G. Nagengast, N.J. Koeman, aR. Griessen, Phys. Rev. B62, 10 088~2000!.13C.-H. Fagerstroem, F. D. Manchester, and J. M. Pitre, inPhaseDiagrams of Binary Hydrogen Alloys, edited by F. D. Manches-ter ~ASM International, Materials Park, 2000!.14G. Andersson, K. Aits, and B. Hjo¨rvarsson, J. Alloys Compd.334,14 ~2002!.15J.E. Kleiner, E.H. Sevilla, and R.M. Cotts, Phys. Rev. B33, 6662~1986!.16S. Orimo, F. Kimmerle, and G. Majer, Phys. Rev. B63, 094307~2001!.17Y. Fukai and S. Kazama, Acta Metall.25, 59 ~1977!.18A. Mandelis, Phys. Today53 ~8!, 29 ~2000!, and referencestherein.19M.A. O’Leary, D.A. Boas, B. Chance, and A.G. Yodh, Phys. ReLett. 69, 2658~1992!.1-4

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Switchable mirrors for visualization and control of hydrogen diffusion in transition metals

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