Cu Precipitations at the Surface of Crystalline Cu — Zr Alloys after Annealing in Hydrogen*

-Cu

_{Precipitations}

at

the

Surface

of

_Crystalline

Cu

—

Zr

Alloys

after

_Annealing

in

_Hydrogen

*

Xin-Nan Yu, Laboratorium

fur Festkorperphysik,

ETH Zurich, CH-8093 Zurich and

L.Schlapbach,

Institut de

Physique,

Universite de

Fribourg.

CH-1700

Fribourg

Crystalline

Cu-Zr

alloys

of various

_composition

were annealed (200

-400C) in

ultrahigh

vacuum and in

_hydrogen,

and their surface

_composition

was

analyzed

by

X-ray

photoelectron

spectroscopy.

Contrary

to the

mostly

observed _oxygen induced surface _segregation of Zr we _{found strong Cu}

segregation

after

annealing

Cu-rich

samples

in

_hydrogen

_atmosphere.

Scanning

electron

_micrographs

show surface Cu

_{precipitates,}

_apparently

formed under the effect of the heat of

_{chemisorption}

of

_hydrogen

on Cu. Introduction

Amorphous

and also

_{crystalline alloys}

of Cu with thed-transition metals

Zr,

Ni

and Pd have been studied

_quite

_intensively

for_{the last} few_{years among other}

reasons because of their

_special

dynamical

and diffusional

_properties

in the bulk and at the surface which results in

_interesting

_catalytic

and

_hydrogen

sorption phenomena

[1-15].

Studies of theelectronicstructure of

_amorphous

_CuxZr-|-x

_alloys

reveal that in the middle of the

_{glass forming}

_range

₍₂₀

-75 at%

_Cu),

i.e. around 50 at%

_Cu,

the Fermi level

_Ef

is situated in the Zr 4d band and there is no

tendency

for

ordering.

At Cuconcentrations > 80 at%Cu

Ef

shiftstothe

edge

of the Zr 4d

band so that the

_density

ofstates

_N(Ef)

_gets

_drastically

_reduced to s-band

values,

in

agreement

with a

sign

reversal of the Hall coefficient

_[1]

and

increased

tendency

for local order. These drastic

_changes

of electronic

properties

are interrelated to total _energy

_changes

and structural instabilities

and relaxation effects

_[2].

Activation

_energies

for metal atom diffusion in

amorphous

Cu

-Zr

_alloys

are rather

small,

by

far smaller than for

recrystallization,

implying

that

_{recrystallization}

is limited

_by

nucleation rather than surface diffusion

_[3].

The

_enthalpy

variation _upon

_{crystallization}

amounts

to5 kJ/mol

_[4].

As many other intermetallic

compounds

also

_alloys

of _{Cu with d-transition}

metals show surface

_segregation

effects. The rather

_{high mobility}

of the metal

surface more

easily

than in other

alloys.

Indeed,

the solid solution

_system

CuxNh-x

is the best studied surface

_{segregation alloy [6-8].}

On the clean

surface

_segregation

of Cu is observed for

_compositions

0 < x < 0.75 and Ni

segregation

is observed at the Cu-rich end of the

_{compositional}

_range.

Electronic

theory

based on a

tight binding

Hartree

approximation

is

capable

of

explaining

this crossover

by preserving charge

_{neurality [6]}

in a

comparable

model

_{by Mukeherjee}

et al.

_[7],

in which crossover is absent. Cu enrichment was also observed in the

_topmost

surface

layers

of

single crystalline

Cu3f\li97

and

_Cu24Ni76

after

_annealing

at700 to 800 K and after

_{sputtering [8].}

Oxygen

chemisorption

leads to the "normal" surface

segregation by

selective

oxydation

of the more reactive

_component

(Zr).

Activated bimetallic Zr-Ni

catalysts

thus consist of active Ni metal clusters

dispersed

on Zroxide

_support,

both of which are

_supported

on the

_underlying

bulk intermetallic

_{alloy [9].}

At

temperatures

around

400°

surface enrichment ofNi in_theform of NiO and Ni is found after

_annealing

of

ZrNi3

in

_O2

and

H2,

resp.

[9].

Preferential oxidation

of Zr and Zr enrichment at the surface was _{also observed upon oxygen}

exposure of

amorphous

Cu -Zr

_alloys

of various

_{compositions [10].}

Whereas surface

_segregation

effects induced

_by

_oxygen are known to be

strong

and

_important

_e.g. for bimetallic

_catalyst,

activation of

_hydrogen

_storage

materials and

_getters

_[9,11-13],

surface effects induced

_by

the

_adsorption

or

absorption

of

hydrogen

are

usually

rather weak or absent

[12,13,14].

In the

caseof Cu-d transition metal

alloys,

however,

the results on

hydrogen

induced surface

_phenomena

are rather controversial

[14]. Hydrogen adsorption

reduces the Cu

segregation

of Cu-Ni

alloys

as

compared

toclean

_surfaces,

in

agreement

with

_{theory [15]. Strong}

surface enrichment of Cu and Pd was

found _upon

_hydrogen

_{exposure of}

_amorphous

Cu-Zr and

Pd-Zr,

_resp., around

200C,

but on the Pd and Cu rich side

only [14,16].

Itwas

argued recently

that

this unusual Cu

segregation

is characteristic of

amorphous alloys

and does not exist on

crystalline alloys.

In view of the

importance

of these

_segregation

effects and their

_{potential application,e..g.}

in the

_preparation

of bimetallic

catalyst,

we have studied

hydrogen adsorption phenomena

on

crystalline

Cu-Zr

_alloys.

Photoelectron

_spectroscopy

was used to

_investigate

the surface

_composition

after various treatments of the

_alloys

in an

ultrahigh

vacuum

_system.

The formation of Cu

_precipitates

was observed after

annealing

of Cu rich

samples

in

_hydrogen.

_Scanning

electron

_micrographs

show that the

_precipitates

reach

urn diameter.

Experimental

Cu forms with Zr many intermetallic

compounds.

The

_{congruently melting}

compounds

are

CuZr2

_(MoSi2

_type

structure

_[17])

and

_Cu3Zr

or

Cu5-|Zr-|4

(Ag5-|Gd-|4

type

structure

_{[17]) according}

to older

_[18]

and newer

[19] phase

Cu3ZrOn.04. Cu5lZr-|4

and

_CuZr2

_by

RF levitation

_melting

in a watercooled Cu crucible at 5

.

10"7

mbar the constituants Cu

_{(99.99%, Johnson-Matthey),}

Zr

_(99,9%,

_{Johnson-Matthey)}

and in the case of

_Cu3ZrOn.o4

some CuO

(99.9%, Merck).

Guinier

X-ray

patterns

of

_CuZr2

and

_Cu5-|Zr-|4

_agree with

those

_reported

in the literature

_[17],

the

_patterns

of

_Cu3Zr

and

_Cu3ZrOo.o4

show additional _lines

_probably

due to the _presence of Zr richer

_compounds.

The lines of

_Zr02

and ofCuareabsent.

Heat treatment and surface

_analysis

of the

_samples

were

_performed

on a VG Escalab 5

_spectrometer

at base

_pressurel

_.10~1u

mbar

_{using MgKa}

radiation

of 1254eV

(Au

4f at 84.0

_eV,

1.2 eV

_FWHM).

The

_spectrometer

is

_equipped

with a

high

_pressure

cell,

fracture

_stage,

_heating

and

_{sample cleaning}

(diamond

file,

Ar+

_{bombardement)}

_facilities. Thus all

_sample

treatment was

performed

inside the

_ultrahigh

vacuum

_(UHV)

_spectrometer

_avoiding

the

transfer of the

sample

atair betweentreatment and

_analysis.

Micrographs

ofsome

samples

weretaken

by

a

scanning

electron

microscope.

Results and Discussion

Typical precipitates

observed after 10 to

15n

_annealing

at200C and 1 bar

_H2

of

_previously

_UHV-cleaned

_samples

of

_Cu3Zr,

_Cu3ZrOrj.04

and

_Cu5-|Zr-|4

are

shown in

_Fig.

_{1. The}

_alloy

surface is coveredto a

high degree

with

precipitates

of 0.1 to 1 mm diameter.

Analysis

of the characteristic

_X-rays

_by

EDAX and

XPS data revealthat the

_precipitates

are of_{pure Cu.} Those

_samples

are Cu

coloured,

contrary

to the

_{grey-metallic}

colour of the fractured

_samples.

Fig.1: SEM micrographotaCu3Zr alloy(UHV_fractured, 10hoursannealedat200Cat1 bar

H2) showingthespherically shapedsurface Cuprecipitates.

It is

_questionable

whether

_{hydrogen chemisorption}

is

_responsible

for this

Cu

segregation

and it is

_interesting

to learn what

_happened

to Zr.

Surface

Typical

Zr3d

_{photoelectron spectra}

and Cu : Zr ratios are shown in

Fig.2.

The more bulk orsubsurface

_(B)

and the more

_top

surface

_(S)

sensitive ratios as

evaluated from

_Cu3p

and

_Cu2p

core level

peaks, respectively,

are indicated as well as the colour. Air

exposed samples

show

heavily

oxidized Zr and a rather

_strong

Cu enrichment at the surface

_(Cu

: Zr

(S)

= 6.22

:1).

The Zr3d

binding

energy

(BE)

of 182.5 eV is smaller than _{that of}

_ZrC>2

_pointing

to a suboxide.

UHV clean surfaces areZr rich.

Annealing

in UHV

(200°

for

10n)

causessome oxidation of Zr due to _{oxygen diffusion} from bulk to surface and due to the

reaction with _{residual gas.} A mixture of Zr suboxide and Zr metal is found. Furthermore

_{annealing brings along}

a

_strong

Zr enrichment in the nearsurface

region

and relativetothe latteroneaCuenrichment on

_top

of thesurface.

No Cu

_precipitates

and no Cu colour were found on the air

_exposed,

UHV

cleaned and UHV annealed

_{samples, respectively.}

I i i i _{i—i—i—i-1} XPS _* I_^Zrmetal' Zro>ide y UHV I .Oil B 200°C 1.8:1 S 10HOURS _l_I_I_I_I I _{l_} 188 184 180 176 eV — BINDING ENERGY

Fig

2: Photoelectronspectraof the Zr3dcorelevels of

crystalline CU51

Zri4aftervarious sampletreatments. The Cu:Zr atomic ratios S and Bwereevaluated

using

the

_largely

topsurface sensitive

_{Cu2p peak}

_(S)

and the

largely

bulksensitive

Cu3p

peak

(B),

respectively.

Annealing

in

_{hydrogen (1}

bar

_{H2, 200C,}

_10n)

causesfull oxidation of

all surface and nearsurface Zr into

Zr02

aand does not

_{significantly}

affect the Cu : Zr

ratio,

although

the

sample changes completely

to Cu colour in a

surface

_layer

of several tenth of a millimeter thickness. On some

samples

a

splitting

of the Cu core level

peaks

was observed

apparently

due to the

formation ofsome

electrically

isolated Cu

precipitates

which

_get

charged

_upon

photoelectron

emission.

Apparently

annealing

in UHV leads to the

_{normal, fast,}

_oxygen induced surface

_segregation

of Zr with the formation of

Zr02

at the

_surface,

whereas

annealing

in

_hydrogen

_{ends up} in asurface

disproportionation

of Cu-Zr

_alloys

into

_Zr02

and Cu

_{precipitates.}

In order to

_analyse

the influence of time and

temperature,

hydrogen

and _oxygen

_partial

_pressure as well as of the

alloy

composition

on the formation and size of the Cu

_precipitates

annealing

of

various

_samples

at

_temperatures

_upto400C at0..1 to 30 bar

_H2

_duringg

0.5to

50n

was studied. Detailed results will be

published

elsewhere;

they

can be

summarized as follows:

-nosurface Cu

_precipitates

areformed ontheZrrich

compound CuZr2.

-minordifferenceswere

_only

observed between

_Cu5-|Zr-|4

and

_Cu3Zr.

-size and number of

_particles

per unit surface area

depend

on the

_annealing

parameters time, temperature,

and_pressure.

-the unannealed air

_exposed

samples

show a rather

high

oxygen contentdown to 100 nm

depth.

-samples

with additional _oxygen

_(Cu3ZrOo.04)

show similar Cu

precipitates.

Based on the above resultswe

_suggest

the

_following

mechanism:

The Cu rich

_alloys

have a rather

_high

_oxygen content. The oxygen is dissolved

in the metal matrix or

_present

in a

_ternary

suboxide. Residual gas and

hydrogen

gasare furthersources of oxygen. Elevated

_temperature

favours the

formation of the

_{thermodynamically}

more stable

_Zr02

by

a

disproportionation

reaction of the

_type

Cu-Zr-Ox

+

O2

Zr02

+ Cu + _Cu-Zr

Upon annealing

in UHV Zr is

_selectively

oxidized and

_strongly

enrichedon

_top

of the

surface,

i.e. Cu is covered

_{by Zr02-}

The _presence of

_hydrogen

blocks

the Zr enrichment and the free surface Cu forms

_{precipitates, possibly}

favoured

_by

the heat of

hydrogen

chemisorption

on Cu. Similar

disproportionation

of FeTi with the formation of surface Fe

_precipitates

was observed earlier

_[20].

We

_{gratefully acknowledge P.Wagli,}

ETHZ,

for

_taking

the SEM

_micrographs,

S.

BOchler,

M. Erbudak and F. Vanini for discussions and the Swiss National

Energy

Research Foundation

_(NEFF)

for financial

_support.

1. _D.NguyenMann, D.Pavuna,F.Cyrot-Lackmann, D.Mayou,and A.Pasturel,

Phys.Rev. B33,5920 ₍₁₉₈₆₎

2. Alfredo_L.YeyatiandM.Weissmann,Phys. Rev.. B35,2714₍₁₉₈₇₎ 3. E.C.Stelterand D.Lazarus,Phys.Rev. B_36,9545(1987)

4. J.Chevrier,SolidState Commun.65, 1461 ₍₁₉₈₈₎

5. W.Wagner,R.Poerschke andH.Wollenberger, Europhys. Lett, 2,379₍₁₉₈₆₎ 6. Yi-Chen

Cheng,

Phys.Rev. B34, 7400(1986)

7. S.Mukherjee, J.L.Moran-Lopez,V.Kumar,andK.H.Bennemann, Phys.

Rev.B25, 730(1982)

8. _Th.Berghaus,Ch.Lunau,H.NeddermeyerandV.Rogge,Surface Science

182, 133 ₍₁₉₈₇₎

9. R.B.Wright, M.R.Hankins, M.S.Owensand D.LCocke.J.Vac.Sci.Technol.A5,593 (1987)

10. P.Sen, D.D.Sarma, R.C.Budhani,

_K.L.Chopra,

andON.R.Rao, J.Phys.

F:Met.Phys.

14, 565₍₁₉₈₄₎

11. L.Schlapbach,A.Seiler, F.Stucki, H.C.Siegmann,J.Less-Common Met.73,

145 (1980)

12. _{L.Schlapbach,}NATO ASI B136,397₍₁₉₈₆₎

13. _{L.Schlapbach,} in _Hydrogen In Intermetalllc Compounds II, ed.

L.Schlapbach, Springer

TAP 64. chapt.2,in_press

14. F.Vanini, St.Buchler, X.N.Yu, M.Erbudak,L.Schlapbach,andA.Baiker,

Surface Science189, 1117₍₁₉₈₇₎

15. S.ModakandB.C.Khanra,Phys.Rev.B,34,5909(1986)

16. P.Oelhafen,R.Lapka,U.Gubler,J.Krieg, A.DasGupta,H.J.Guntherodt,

T.Mizoguchi, C.Hague, J.Kubler and _{S.R.Nagel, Rapidly Quenched} Metals IV,eds. T.Masumoto andK.Suzuki(JapanInstitute ofMetals,

Sendai),1259 ₍₁₉₈₂₎

17. P.Villars andL.D.Calvert, Pearson's Handbook ofCrystallographicDate for IntermetallicPhases,Am.Soc.forMetals,1985

18. T.B.Massalski,ed.,Binary AlloyPhaseDiagramsVolI,American Soc. forMetals, Ohio,1986

19. E.Keller, Y.Khan, U.Gorres,Z.Metallkde.77,43₍₁₉₈₆₎

20. G.Kirch, R.Hampelmann,E.Schwab, H.Zuechner,Z._Physik.ChemieNF, 126,109 ₍₁₉₈₁₎