Surface alteration of LnBa2Cu3O7−x following exposure to moisture, O2 and CO2

Physica C 165 North-Holland

( 1990) 270-278

SURFACE ALTERATION AND CO2

OF LnBa,Cu@_,

S. MYHRA *, P.R. CHALKER, Materials

Development

Received

27 October

Division,

Hawell

P.T. MOSELEY Laboratory,

Oxon,

FOLLOWING

EXPOSURE

TO MOISTURE,

O2

and J.C. RIVIERE L/K

1989

The effects of exposing surface of the LnBa$Iu,O,_, (Ln=Y, Yb, Gd) high temperature superconductor to 0,. CO2 and morsture have been studied by post hoc XPS and SEM. Evidence is presented for incipient hydration and carbonate formation. Some factors likely to affect the susceptibility of the material to this type of “weathering” process are suggested.

1. Introduction

The existence of high temperature superconductivity (HTSC) for a range of perovskite-related layer compounds is now well established, e.g. [ l-41. While the initial emphasis was on the search for compounds with yet higher superconducting transition temperatures, most of the scientific and technological resources are now being deployed in the materials science of the presently known HTSCs with a for particular view to producing materials applications. There are many obstacles on the road to commercialization of the HTSC discovery. One of these is the well-established susceptibility of the HTSCs towards degradation from exposure to the ambient atmosphere. Several studies of the initial stages of the degradation process have employed surface analytical techniques [ 5,6,7] while others have used thermogravimetric methods [ 81 or transmission electron microscopy [ 91. These investigations have established that it is the moisture in the ambient atmosphere that is of particular significance, that complete break-down of the lattice will occur, and that in the presence of COZ the degradation products include carbonates. It has been established that the degradation is dramatic on the microstructural scale * Permanent address: Division of Science and Technology, Grifftth University, Nathan, Queensland, Australia. 0921-4534/90/$03.50 ( North-Holland )

0 Elsevier Science Publishers

B.V

and can have significant effects on the macroscopic properties. The aim of the present investigation is to pursue these studies with the aid of X-ray Photoelectron Spectroscopy (XPS) and in the context of the type of chemical attack which is commonly described as “weathering”. A range of specimens of the type LnBa&u@_, (Ln=Y. Gd and Yb) has been investigated. The results of these experiments will be discussed within the framework of recent advances [ 10,l 1 ] in the understanding of the chemical durability of the ATi03 perovskites when they are exposed to aqueous attack at epithermal temperatures.

2. Experimental details 2.1. Specimen

preparation

and characterization

The specimens (the formulations are abbreviated Ln-l-2-3 henceforth) were produced by the normal bulk-sintering route, e.g. ref. [ 121, with final heattreatment at 950°C in oxygen for 16 h. The final shapes were discs with diameters of 12-14 mm and thicknesses 2-3 mm. All specimens exhibited the “metallic” conductivity at temperacharacteristic tures above the transition point, T,, with an abrupt drop to zero resistivity (within the experimental resolution). The values for 7,, at the point of the knee, were 78 K, 82 K and 85 K for Yb, Gd and Y. respectively. The resistivity curves are shown in fig. 1.

S. Myhra et al. /Surface alteration oJLnBaZCu30,_,

-1

Gd

Ba2Cu30,_3c

Yb

Bo~CU~O,_~

271

r- r-

20

Lo

60

80

100

120

140

160

Temperature/

180

200

220

240

260

280

300

Kelvin

Fig. 1.The resistivity vs. temperature curves for the three Ln-1-2-3 formulations.

The discs were analyzed by XRD and found to be of single phase, having the characteristic orthorhombic structure. The specimens were also analyzed by routine SEM. The grain sizes ranged from sub-pm upwards to about 30 pm. The porosity was variable although Gd- l-2-3 specimen appeared to have less open porosity than the other two compositions.

carrier gas. Following exposure the specimen was outgassed and returned to the spectrometer as described above. Final outgassing usually required pumping in the fast-entry lock for approximately 15 min.

2.2. Exposure procedures

The XPS analysis was carried out in a VG ESCALAB Mk I instrument. The excitation was by Al Kcr radiation ( 1486.6 eV) at a source power of 300 W. Survey scans over the kinetic energy range 4001500 eV were recorded with a constant analyzer energy (CAE) of 100 eV, increments of 1 eV and counting times of 1 s per increment. Detailed scans were carried out over the major XPS peaks for the constituent species as well as over the carbon 1s line. In addition, some X-ray induced Auger electron (XAES) lines were monitored. All the latter scans were recorded with a CAE of 20 eV, increments of 0.2 eV and counting times of 1 s per increment. Multiple scans were averaged in order to improve the signal-to-noise ratios. The base vacuum ranged from 1O-8 to < 10e9 Torr, depending on the outgassing from the specimen surface and on the length of time after specimen entry from the fast-entry lock. Binding and kinetic energies were checked against the energy of the C 1s excitation for adventitious carbon (285.0 eV). Since these specimens are good con-

Fresh and clean surfaces were prepared by abrasion with an alumina rod in an argon glove box. The specimens were mounted on a standard stub also in this environment, and then transferred under argon in a purpose-built transfer-flask from the glove box to the spectrometer. Exposure to high-purity O2 and COa was carried out by transporting the specimens from the analysis position to the fast-entry lock, which was then backfilled and maintained at 0.5 atm. for 30 s by differential pumping. The steam exposure took place in the glove box. Freshly distilled HZ0 was further purged by vigorous boiling in order to remove dissolved gases such as O2 and COZ, and then sealed in a flask and transferred into the glove box. Steam was generated by heating the flask until steam escaped through a narrow bore tube protruding from the neck of the flask. Freshly prepared specimen surfaces were then exposed for 30 s to steam transported in the Ar

2.3. Surface analytical procedures

272

S. Myhra

et al. /Surface

alteration

qfLnBa_S’u307_

\

ductors charge shifting was found to be insignificant. However. it is possible that differential charging may have occurred for the insulating reaction products on the surfaces. The SEM analyses were carried out in the standard secondary electron mode.

3. Results and discussion

The relatively mild from of aqueous attack by exposure to moisture in an argon carrier gas for a period of less than a minute had a surprisingly dramatic effect in the sense that the surface alteration was visible to the naked eye as a grey-white layer. The extent of visual alteration was greatest for the Gd-l-2-3 specimen, somewhat less for the Yb-I-2-3 and barely visible for the Y-l-2-3 specimens. The specimens were examined by SEM after exposure to moisture (and exposure to atmospheric ambient conditions). The results are illustrated in fig. 2 which shows secondary electron images for the Y-l-2-3 surface. The results for the other two l-2-3 formulations were similar. The fields of view were analyzed by EDS. The only significant difference was that Ba was enhanced vis a vis Y and Cu in the case of the fracture face, and in comparison with the compositions of the other two types of surfaces. Several microstructural features in the field of view of fig. 2c were also analyzed by EDS. The large surface crystallites tended to be either rich in Cu (but with some Ba) or rich in Ba (and generally without any contribution from the other cations). Other large crystallites were found to have compositions similar to that of the l-2-3 phase.

The freshly prepared surfaces of the l-2-3 specimens exhibited the same general surface analytical features which have been established by a number of other studies [ 12-141. Representative XPS data. mainly for the Cd- I-2-3 specimen, are shown in fig. 3. Similar features were observed for the other two formulations.

Fig. 2. Secondary electron images of an as-received Y-I-2-3 surface. (a). a fresh fracture face (b) and a fracture face after cxposure to moisture in the glove box (c).

S. Myhra et al. /Surface alteration of LnBaJCuJOr_,

I

1

1

265

290 285 Binding Energy (eV)

280

Fig. 3. (a) The C 1s envelope for an as-received surface (i), and after exposure to O2 (ii), CO2 (iii) and Hz0 (iv). The positions of the graphitic (285.0 eV) and carbonate (289.0 eV) components are indicated. 0

273

1s:

This envelope tended to be dominated by a large contribution at 53 1.5 eV and a shoulder of lower intensity at a binding energy near 529 eV. In the cases of the Cd and Yb formulations there were also significant high binding energy shoulders at z 534.8 eV. The latter might have been due to incomplete dissociation of the carbonate precursor materials, or to the presence of carbonate reaction products from environmental degradation resulting from exposure to laboratory ambient conditions during the two months of storage. It is possible that carbonates may have formed at interior surfaces, which were then subsequently exposed by abrasion in the glove box. Ba 3d:

The major contribution was the “normal” component with a binding energy of 780.2 eV which is consistent with a divalent oxide state. In addition the unusual (but typical for the metallic oxides [ 13 ] )

Bindng Energy (eVJ Fig. 3 (b) The 0 Is envelope for the same surface conditions as in (a). The positions of the characteristic HTSC (LBE) = 529.0 eV), the typical oxide (“oxides”=530.8 eV), and the carbonate and/or hydroxyl (HBE= 534.8 eV) components are indicated.

shoulder at lower binding energy (778.2 eV) was observed. In the cases of the Cd and Yb formulations there were also high binding energy shoulders at 783.0 eV. This binding energy was presumably due to differential charge shifting and may be consistent with the high binding energy 0 1s contributions being associated with the presence of carbonates. The Ba 3d,, z binding energy has been measured as 779.6 eV, but BaC03 is a good electrical insulator. CM 2p:

The characteristic features in the Cu 2p envelopes were consistent with predominant divalency in the sense that the binding energy of the 2p,,, level was 933.5 eV; there was considerable broadening of the main peaks, and the characteristic satellite structure was centred at approximately 942.5 eV. The area of the satellite relative to that of the 2p,,, main peak was smaller than one would expect for a divalent Cu compound. Thus one might infer the presence of

S. Mvhra el al. / Su$ace alteration ofLnBa2C‘u107_

274

I

I

I

1

Ba 365/z

960 Fig. 3. (c) The Ba 3d,,z envelope for the same surface conditions asin (a). The positionsofthecharacteristicHTSC (LBE=778.5 eV). the typical oxide (“oxide”=780.2 eV), and the carbonate and/or hydroxyl (HBE= 784.0 eV) components are indicated.

Cu species either in the surface generally or in specific minor phase regions. These HTSC compounds are relatively susceptible to loss of oxygen, which may have occurred during storage in the reducing glove box environment. Exposure to UHV at laboratory ambient temperatures may also promote oxygen loss. monovalent

Ln Spwies.

The main 3d excitations were monitored. The 3d5, 2 binding energies were essentially in agreement with those for tri-valent oxide model compounds: 157.0 eV ( l56.8), I 187.2 eV ( 1187.5) and 278.7 eV for measurements on Ln,O, for Y, Gd and Yb, respectively. The envelopes tended to be smeared out due to the presence of a second contribution of unknown origin at higher binding energy, which has been observed in several other studies of Y-1-2-3 [ 151. We confirmed that the Gd- I-2-3 compound is similarly af-

950

940

Binding Energy (eV)

Binding Enetgy (eV)

Fig. 3. (d) The Cu 2p envelope for the same surface conditions as in (a). The binding energy for typical divalent copper compounds is indicated (“divalent”=933.8 eV).

flitted by comparing the envelope of the Gd 3d5,1 level for a pure Gd203 oxide powder to that of the Gd- l-2-3 specimen. Carbon

In spite of precautions taken to ensure cleanliness during preparation and transport of the specimens. there was typically 20 at% carbon present during analysis. Most of this (half to two-thirds) was found to be a species of low binding energy (285.0 eV) which can be ascribed either to adventitious contamination or to the presence of carbon in the grain boundaries. Surface analytical investigations of fracture faces have shown that carbon is found in grain boundaries and on pore surfaces [ l6,17 1. However. some of the carbon was also present as a high binding energy species (289.0 eV). which is likely to rcfleet the extent of carbonate minor phases. Binding energies and other relevant parameters from the literature for the Y-1-2-3 compound and other com-

S. Myhra et al. /Surface

alteration of LnBaZCu307_,

275

~~~

I5 I I

I 905

910

915 920 9% Kinetic Energy(eVj

930

of interest

are summarised

1190

Binding

Fig. 3. (e) The lineshape for the Cu LMM transitions for the same surface conditions as in (a). The position for a typical HTSC peak position (9 16.5 eV) is indicated.

pounds

11%

in table I.

O2 E_~posurc: The effects of exposure to oxygen were insignificant, except for the Gd-1-2-3 specimen for which there was a considerable enhancement in the high binding energy shoulder of the Ba 3d envelope. CO2 E.xposure: Again, there were no clear trends after exposure to COz. There were some changes in the relative contributions of high and low binding energy components to the C 1s envelopes, but this was most likely a result of inevitable non-reproducibility in the repeated preparation procedures for fresh surfaces. E,uposure to Moisture: There were a number of significant changes in the XPS spectra from the surfaces. The features which were characteristic of the freshly prepared surfaces

11% Energy(eVJ

1180

Fig. 3. (f) The Gd 3dS12 envelope for the same surface conditions as in (a), as well as for GdzOs (v). The measured binding energy for the trivalent oxide is indicated ( 1087.8 eV).

(low binding energy contributions to the 0 1s and Ba 3d envelopes, mixed valence character of the Ln 3d envelopes and relatively small satellite contributions to the total Cu 2p envelopes) tended to be suppressed by exposure to moisture. However, even in the case of the most heavily affected surface (the Gd-1-2-3 formulation) these features were still resolvable after exposure. In addition to these general trends there was substantial broadening of the 0 1s and Ba 3d envelope towards the high binding energy sides. The effect on the Cu 2p envelope was to increase the relative contributions of the satellites towards those of a typical divalent copper compound. Also, the Cu LMM spectrum exhibited additional structure. The Ln 3d (Ln=Y, Gd and Yb) envelopes tended to become narrower and more well-defined suggesting a trend towards a predominantly single chemical environment. The Gd-1-2-3 specimen was exposed to COz following treatment with moisture. but without inter-

276

S. Mvhra

er al. / Swface

alteration

qf‘LnBa2Cu,0,_,

Table I Literature values for binding and kinetic energies of some relevant compounds. The modificatton some compounds. The low binding energy components to the 0 Is and Ba 3d lines for the Y-l-2-3 Compound

YBazCu@_,

XPS Binding Energies 0

cu

IS

2P,,z

531.2

933.8

528.7

(eV

)

Ba 3d 5,2 779.6 777.8

Auger parameter, o*, is also listed for compound are shown.

XAES and cr * Kinetic Energies Ln

Ba

3d,,z

MNN

157.0

597.8 597.8

‘Y*

530.3

Ba metal BaO

528.4

YLO?

529.8

156.8

Gd>O,

530.3

1187.5

779.1 778.9

602.2 598.6

vening re-polishing of the surface. There were no significant changes to the XPS features as a result of this procedure.

The XPS survey spectra were used for quantification of all species in the surface layers. It should be noted that while XPS is surface-sensitive (the interaction volume is confined to the first few monolayers). it is a broad-beam technique in the sense that the signal. in the present case, was generated across an area of 1 cm?. Thus the concentrations of species in the surface. as inferred from XPS results will be averages over the lateral extent of the interaction volume. Also. in this particular version of the “weathering” experiment the surface species are not permanently lost to the solution, even if the effect of the aqueous film is to cause complete dissolution of the surface layers. since the surface is dehydrated primarily by sublimination, after the termination of exposure. There were only minor changes in the surface compositions, within the experimental uncertainties. after exposure to O2 and CO> in comparison with the original surface. After exposure to H20, and H,O+COz, the Ba concentration was enhanced by 40 to 50 at% while that of Cu was lower, by 20 to 50 ato/o. The carbon concentration was also reduced by 50 ato/ after this treatment. The plausible interpretation of these results is that significant lateral dif-

(B *

Ref.

916.5

1850.3

[I51

918.6 916.9 918.0

1851.3 1849.4 1851.6

181

(‘u

LMM 1377.4 1375.6

932.7 932.5 933.6

Cu metal cuzo cue

(eV)

1381.3 1377.5

191 201

ferentiation occurred as a result of the exposure, with Cu being present in a relatively small number of large precipitates and Ba accounting for a much larger surface coverage in smaller and more numerous crystals. Likewise, the results suggest that carbon had been concentrated over a smaller area of the post-exposure surface in comparison with the original distribution.

The processes whereby materials are degraded (e.g., exposure to environmental attack) are governed by kinetic and thermodynamic factors. The mechanisms fall into three broad categories: i) degradation may occur due to ion-exchange whereby atomic species are selectively extracted from a solid. ii) the solid may be dissolved in toto as a result of chemical attack on the network. iii) modification may occur (usually initiated by one or both of the mechanisms above) by reconstruction at free surfaces, by phase transition, by precipitation and by nucleation and growth of new phases. The present results suggest that exposure to O2 and CO2 has little, if any, effect on these materials, except for physi- and chemi-sorption at free surfaces. Kinetic factors are clearly important as shown by the different rates of degradation for the three l-2-3 formulations. It seems unlikely that substitution of the lanthanide species will have any significant effect on

S. Myhra et al. /Surface

these rates. It is more probable, and in accord with studies of the ATi03 (A=Ca, Ba, Sr) titanate perovskites, that microstructural variations govern the kinetics [ 10,l 1 1. These variations may be due to the changes in fabrication parameters from one batch to another. Significant factors may be the presence and extent of minor phases, the range of grain sizes, the properties of the grain boundaries, the presence of pores and cracks, the connectivity of routes of ingress from the surface to the bulk, etc. The present experiments in which the surface is exposed to moisture in a carrier gas are akin to those which fall under the heading of “weathering”. The essential feature of these experiments is that the ratio of specimen surface area to “solution” volume (SA/V) is very high resulting in thermodynamic constraints being dominant. In the present context the “solution” constitutes the thin film of moisture on the exposed surface. Thus it is likely that the solution limits will be reached in the very early stages of the evolution of the system, which will promote saturation, precipitation, and nucleation and growth of secondary phases. Recent studies of the chemical durability of the ATi03 perovskites in the epithermal temperature range have identified the mechanisms which are operative at the solid/liquid interface [ 10,111. It has been found that species are released into solution as a result of attack by base-catalyzed hydrolysis on the TiOh octahedra with the following reaction paths (the Ti-0 bonds of the octahedral network are shown as g>Ti
break-down

)Ti<“)Ti<+OH-+~Ti
occurs as OH-

/

The OH- radical is regenerated drolysis reaction )Tii”+H20+>Ti
attaches

1’

(1)

by a further

hy-

(2)

The kinetics of this reaction are controlled by the local pH, which in the present case of the very high value for the SA/V ratio may be driven to high value by H+ (or H30+) ion exchange reactions in the first one or two monolayers. The hydrolysis reaction leads to complete dissolution of the matrix and the formation of hydroxyl compounds. In the presence of

alteration

of LnBatCu30,_,

217

CO2 in the solution or in the ambient atmosphere further reactions will occur with the formation of carbonate phases. Under conditions of high SA/V these phases will precipitate on the dissolving substrate. In the present case the XPS observations suggest that the formation of BaCO, and one or more of the many hydroxylated and/or hydrated Cu-carbonates takes place. The formation of BaCO, reaction products has been observed in many studies (e.g. [ 6,9,2 1 ] ). The l-2-3 HTSC compounds are oxygen deficient perovskites in the sense that the oxygen sites in the Ln-plane are unoccupied. Likewise, only half the sites in the plane between the Ba atoms are occupied. For purposes of surface reactivity, those unoccupied sites may effectively be defect sites which will promote chemical attack and increase the kinetics of the interface reactions. Thus one might expect the l-2-3 compounds to be intrinsically more reactive than the perovskites. Further investigations are clearly required, in particular on good single crystal and/or thin film specimens, in order to more fully understand the thermodynamic and kinetic constraints on the interface processes. It is likely that a more complete description will point the way towards techniques for passivating and protecting the surfaces of HTSC compounds [ 22,231.

Acknowledgements The research was supported in part by the Harwell Underlying Research Programme. One of the investigators (S.M.) is appreciative of the support and hospitality extended during a period of attachment to the Hat-well Laboratory. Special thanks go to Dr. A. Crossley, Mr. R. Bartram for assistance with the data acquisition and Mr. P. Topart for specimen preparation.

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