Segregation phenomena at surfaces and at grain boundaries in oxides and carbides

Solid State Ionics 12 (1984) 299-307

North-Holland, Amsterdam

SEGREGATION PHENOMENA AT SURFACES AND AT GRAIN BOUNDARIES IN OXIDES AND CARBIDES

W.D. KINGERY

Ceramics Division, Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139, USA

Impurities and dopants have been found to segregate to the surface and to grain boundaries in oxides such as MgO and in carbides such as Sic. In addition, there is a charge accumulation which corresponds to a nonstoichiometric

boundary or to local electron energy levels or both. This charge leads to a space charge layer adjacent to the boundary or

surface. In addition, some boundaries show a dopant accumulation several hundred &igstriims in width. It seems unlikely

that boundary and surface transport data can be properly interpreted without appropriate microchemical information.

On a trouvt que les impure& et les dopants segregeaient a la surface et aux joints de grains des oxides tel que

MgO et des carbures tel que Sic. En plus il se produit une accumulation de charges qui correspond a un interface non

stoechiometrique ou a des niveaux d’energie localises pour les electrons ou aux deux. Cette charge induit une zone de

charge d’espace adjacente au plan de joint ou a la surface. De plus quelques interfaces presentent une accumulation de dopant de plusieurs centaines d’angstriim d’tpaisseur. 11 semble improbable que les don&es concernant le transport aux

interfaces et aux surfaces soient interpr&ttes sans information appropriee sur leur microchimie.

1. Introduction

Segregation phenomena at surfaces and at grain

boundaries in oxides and in carbides has been studied by several methods, and from time to time the question of grain boundary width has been discussed. In recent years, direct observation of grain boundaries by transmission electron microscopy, with lattice imaging, and with related techniques along with model calculations indicate that the boundary thickness of material diverging from the normal crystal structure is only a few lngstriims wide, and that the coincidence site lattice model with boundary dislocations seems appropriate for describing boundary struc- tures. A strain field extends a few tens of &rgstrijms from the boundary, but the boundary itself should be considered as a region of only a few atoms wide. Some of the older interpretations of diffusion data in terms of very wide boundaries seem to be measuring something else. This is in concord with Atkinson and Taylor’s [l] result of 7 A width for nickel oxide. These models also lead us to expect anisotropic boun-

dary properties, and, as for surfaces, one must expect accumulation of surface-active constituents at the boundary.

Boundary segregation directly affects the usual measurement of boundary diffusion coefficients for solutes, which gives us the product of a boundary diffusion coefficient, the boundary width and the segregation coefficient [2]. It may also affect the mobility of other constituents in the boundary. I pro- pose to discuss the relatively small number of data now available for segregation at grain boundaries in oxides and silicon carbide, and then to speculate on boundary diffusion phenomena and how they might be influenced.

2. Experimental measurements

Experimental determinations of surface and boundary segregation have been done in part with SIMS measurements, and in part with Auger spectroscopy, but mostly using polycrystalline samples, which have

300 W. D. Kingery / Segregation phenomena

0.20 t

c* Fe 0 .

I ii I

-60 40 20 0 20 40 60

DISTANCE FROM BOUNDARY (nm)

Fig. 1. Boundary segregation of scandium, chromium and iron in MgO [3].

been ion-milled to a thickness of about 1000 A or so, and then measured with a dedicated (Vacuum Gen- erator HB5) scanning transmission electron micro- scope. With a beam diameter of 10 A, a bright-field image of clean boundaries having an apparent width of 20 A, and an uncertainty of positioning the beam of perhaps 20 A, we obtain a chemical analysis having an effective spatial resolution of about 50 A.

3. Experimental results

A typical result for segregation in an oxide material is illustrated for scandium, chromium and iron, each present at a concentration of about 1000 atom ppm in MgO in fig. 1 [3]. Other con-

-100 -60 -20 0 20 DISTANCE (“In)

Fig. 2. Boundary segregation of aluminum, silicon, calcium and titanium in MgO [4].

l Slow cooled from 1500°C

0 Atr quenched from1 IOOY

0 Air quenched from 15009

0.005 0.010 ( Nommol CF~

15

Fig. 3. Influence of concentration of iron in MgO, and of the cooling rate, on observed boundary segregation [5].

stituents are also segregated in MgO, as is shown for aluminum, silicon, calcium and titanium in fig. 2 [4].

-For dilute solutions,the relationship between boundary concentration and bulk concentration is approxi- mately linear, as shown in fig. 3 [5]. Since the equili- brium concentration is increased as the temperature is lowered, cooling rate can have an influence, and moderately rapid quenching is necessary to freeze in the high-temperature state. The surface concentration of iron versus temperature for different quenching methods is illustrated in fig. 4, determined by SIMS [6]. These results lead to a concentration at the boundary versus temperature such as illustrated in fig. 5. In this case, the iron content corresponds to a condi- tion where complex ions such as iron-vacancy and iron-vacancy-iron associates are present so that the interpretation depends on details of the defect model. A similar case for smaller solute concentrations is illustrated in fig. 6; Johnson et al. [7] have measured calcium segregation in aluminum oxide versus temperature and determined an enthalpy of segregation of about 30 kcal per mole.

W. D. Kingery / Segregation phenomena 301

lntermedlate cool

o- o- 0 1000 0 1000

Fig. 4. Iron concentration adjacent to the surface of MgO

and samples subjected to different quenching techniques [6].

In magnesium oxide containing about 500 parts per million scandium, SC&, the scandium added as a solute is accompanied by the introduction of charge- compensating excess vacant magnesium ion sites, and these lead to the formation of a negative boundary charge. As is illustrated in fig. 7, we find two sorts of segregation [8]. Silicon and calcium, present as impurities, are concentrated on the boundary plane which is quite narrow, while scandium (or some fraction of the scandium) in solution as SC& contributes

Temperature ( “C 1 1700 I600 1400 1200 1000

1 I I I I

10”s e

l/T(K~l0-~)

Fig. 5. Temperature dependence of iron segregation of the surface of MgO in air [6].

-301 ‘4.4 4.6 4.0 5.0 5.2

f x104(K)

Fig. 6. Temperature dependence of CaO segregation in a grain boundary in aluminum oxide (after ref. [13]).

to the space charge, balancing the negative charge on the boundary itself. As a result, scandium distribution extends further from the boundary, and it can be characterized by extensions of standard electro- chemical space charge analyses.

For iron, chromium and scandium, we find different relationships as a function of composition in that the increase in boundary segregation of chromium is not as strong as iron and scandium (fig. 8). This is attributed to the higher energy of the association of chromium to form complex ions which do not contribute to the positive space charge [3].

0 0 05 I.0 15 20 2.5 3.0

Sputtered depth (nm)

Fig. 7. Change in concentration of silicon, calcium and scandium adjacent to a grain boundary iu MgO [8].

302 W. D. Kingery / Segregation phenomena

k Fe A

d Cr q d

f

0

23

iii z l0,000

a

% I /-

0 500 1.000 lb00 CONCENTRATION IN BULK (CATION ppm)

Fig. 8. Concentration dependence of boundary segregation

of iron, scandium and chromium in MgO [3].

The results described thus far are for magnesium oxide, but we find similar results for aluminum oxide, in which substantial segregation occurs for Y3+, Ca’+, Si4+, Ti4+, Zr4+, but not for Mg’+, Ti3+, or Cr3+ [9].

A more nonstoichiometric oxide is manganese- zinc-ferrite in which calcium additions and silicon segregation are correlated with magnetic properties. We have investigated a sample prepared in the. Bell Laboratories from which two layers have been sampled [lo]. One layer was taken about 50 pm,

0.03 o h/Fe


Fig. 9. Boundary segregation of calcium and silicon in a

manganese-ferrite [lo].

some Sgrain diameters below the surface, while the other sample was taken towards the center of the sample, about 4000 grn from the surface. In each case, we find calcium and silicon segregation at the boundary (fig. 9). In this sample, prepared by milling powders, there were quite severe local variations in concentrations of constituents and concentrations on the grain boundaries. As illustrated in fig. 10, there were precipitates containing calcium silicate dis- tributed throughout the structure. All of the reported data for boundaries are ones which appear clean at a magnification of 500000X and which the apparent grain boundary width is less than 30 A. Although there is considerable scatter, the data clearly show that segregation is a function of depth from the surface (fig. 11). Calcium apparently diffuses along the boundary as a function of oxidation, as has already been reported by Paulus. We observe that at some boundaries, zinc and manganese are depleted at the boundary relative to iron (fig. 12), a result that is attributed to the increase of iron at the boundary, rather than an actual depletion of zinc and manganese.

In a sample containing 500 parts per million Ca, determination of oxygen concentration across a boundary near the surface illustrates an enhancement of oxygen concentration as shown in fig. 13. No similar concentration enhancement was found at the center of the sample, and this oxygen concentration seems to correspond to oxygen diffusion into the sample. As shown in fig. 14, a sample containing 2180 parts per

Fig. 10. Microstructure of a manganese-zinc-ferrite illustrat-

ing calcium silicate precipitates and the non-uniformity of

the structure [lo].


SAMPLE

INTERIOR

I I NEAR 1

SURFACE

11

!i,L 2 0 500 1000 1500 2000 2500

+ Co CONTENT (ppm)

Fig. 11. Change in boundary segregation and behavior with calcium concentration near the surface and far from the surface of a sample subjected to reduction and oxidation [lo].

million calcium subjected to an identical heat treat- ment does not show oxygen diffusion. This suggests that for oxygen, the segregation of calcium and silicon on the boundary decreases the boundary diffusion coefficient for oxygen.

These effects of boundary segregation are not restricted to oxides. Similar results are found for aluminum dissolved in silicon carbide; aluminum is added as a constituent to enhance sintering or hot- pressing behavior. As shown in fig. 15, aluminum

15

160 120 60 0 60 120 180

DISTANCE FROM BOUNDARY (IVY0

Fig. 12. Apparent decrease in zinc and manganese adjacent to a boundary in manganese-zinc-ferrite [lo].

500ppm Ca

.25 SAMPLE INTERIOR

.20 ?k In

g .15 ‘F

2 5 .30. NEAR-SURFACE .

ti

G 0

.25

l

.20 1 I -120 -80 -60 -40 -20 0 20 40 60 80 120

DISTANCE FROM BOUNDARY (w

Fig. 13. Increase in oxygen concentration adjacent to a near surface boundary in manganese-zinc-ferrite sample containing 500 ppm calcium [lo].

2180 ppm Co

5 301 j .30 NEAR-SURFACE

,201 , , / , , . , 1

-120 -80 -60 -40 -20 0 20 40 60 80 120


Fig. 14. Oxygen distribution across boundaries in manganese-zinc-ferrite containing 2180 ppm calcium. No boundary segregation is observed [lo].

1600°C. 3h

A 2000°C. 15h

2200°C. 15h

0 I I 800 400 0 400 800

Distance from grain boundary (K,

Fig. 15. Segregation of aluminum at a grain boundary in silicon carbide [l 11.

304 W.D. Kingery / Segregation phenomena

T (“C)

2200 2000 1800

2-

\z

=1- i( Es= 116 KJ/mol

3.8 4.2 46 50

IO’/ T C K-’ 1 Fig. 16. Temperature dependence of aluminum boundary

segregation in silicon carbide [ 111.

segregation at the boundary increases at lower tem- peratures, and the influence of temperature is shown in fig. 16 [ 111. Depending on some assumptions as to the analytical effectiveness, an energy of boundary segregation of 1.5 f 0.3 eV is determined. If a sample is heated for a long time, aluminum evaporates from the surface. The rate of disappearance of aluminum from the grain boundary under these conditions is more rapid than can be accounted for by bulk diffusion, and our calculations indicate that the boundary diffusion coefficient for aluminum is about lo3 times larger than the lattice diffusion coefficient. The enhanced diffusion of aluminum along boundaries or dislocations or stacking faults influences the concentration distribution in a sample of silicon carbide which has been ion-implanted with aluminum and then heat-treated at high temperature to diffuse the

aluminum into a wide Gaussian distribution as illustrated in fig. 17 [12]. On the side of this distribution towards the interior of the crystal (which is not affec- ted by the implantation process), we find a tail corre- sponding to the enhanced diffusion along dislocations or stacking faults or perhaps sub-grain boundaries.

4. Discussion

The experimental results in hand clearly indicate that boundary segregation is significant in several

0011 0 1000 2000 3000

Depth from surface ctir

Fig. 17. Concentration profile of aluminum ion implanted in

silicon and then annealed to develop a Gaussian concentra-

tion distribution by lattice diffusion at 1350” C. The tail

toward the interior of the crystal indicates enhanced special diffusion [ 121.

oxide and carbide systems and seems to arise from several different sources:

(a) Strain energy, which leads to a high heat of solution in the crystal lattice can give rise to lower energy for the system if segregation of the constituent occurs in the more open structure of the grain boundary. This seems to be clearly the case for calcium in MgO, Y203 in A1203, and other systems.

(b) Solid-state ion configurations seem to be important in silicon carbide where the strain energy for aluminum segregation can contribute but a small fraction of the observed enthalpy of segregation. In this covalent crystal, we believe that the strong ten- dency of silicon for sp3 orbital configuration and four- fold coordination makes it energetically preferable for aluminum to occupy the more random sites in the more open boundary structure. This clearly is an area in which theoretical calculations are called for.

(c) Space-charge effects arise when aliovalent sol- vents are dissolved in a host lattice to increase the vacancy concentration and give rise to a substantial boundary charge, according to well-known theory, which had its origin with Frenkel [13] and Lehovec [14]. Calculations indicate that either added grain boundary electron energy levels in the band gap or substantial concentrations of negative magnesium ion vacancies must be present in the boundary plane. This is a subject theorists are beginning to treat for semi- conducting oxides such as zinc oxide, and is of basic


importance for thoroughly understanding the nature of grain boundary segregation. In either event, the boundary plane clearly fits within the nonstoichiometry theme of this conference, and the nature of defects present in the boundary plane which is characterized by nonstoichiometry and a considerable variation in chemistry from the bulk is just beginning to be appreciated. We see no fundamental reason not to think about the defect structure of the boundary plane in the same sorts of terms as the bulk volume, and can anticipate this being a subject for active development.

The influence of segregation on diffusion is well- known from classical results for the alkali halides in which “wet” material has greatly enhanced boundary diffusion as compared with “dry” samples [15] for nickel and cobalt diffusion in MgO containing calcium and silicon [ 161 and for a few other systems. Results presented earlier suggest that with manganese-zinc- ferrite, there may be an impurity effect on oxygen diffusion. In this meeting, newer results are to be reported by Atkinson [17] and by Stubican [18]. At present, it seems fair to say that several conjectures as to models of grain boundary segregation affecting different systems can be imagined which might either increase or decrease diffusion coefficients. Systematic experiments seem required.

However, I would like to suggest some cautions. Frequently, one finds submicroscopic precipitation on boundaries and dislocations as shown in fig. 18 [19]. Such observations urge that detailed careful sample characterization accompany any diffusion results.

A similar concern is related to orientation effects and, particularly, differences in diffusion parallel to a

Fig. 18. Precipitates of fine particle size spine1 are nucleated and grow at boundaries in magnesium oxide [19].

e o.o~‘\/-“a,,_ A -0 u e B c 0 ’ ’ ’ ’ ‘I I+ ’ ’ ’ ’ -

-lco -60 -20 0 20 60 loo Olstonce from qroln boundary ( nm )

Fig. 19. Grain segregation of bismuth and distribution of cobalt across a grain boundary zinc oxide [20].

boundary and across a boundary. The bismuth and cobalt boundary segregation in a sample of the zinc- oxide which has reasonably good varistor properties is illustrated in fig. 19. When such a sample is degraded in an applied dc field, interstitial positive ions migrate in the direction of the field and pile up at the boundary. This is influenced by the local field at the boundary, but the net result is that boundaries act as bar- riers for diffusion across the boundary (fig. 20). We have seen what may be similar results in manganese ferrite, in which the dopant concentration sometimes varies sharply across a grain boundary, as illustrated in fig. 21. There have been interpretations of boundary migration in terms of a boundary diffusion coefficient and width along a boundary. If it is assumed that the boundary diffusion is independent of

0.03 , / ( , , , 0 , ] , ’ , S g 0.07- (+)

E, (-1

5 0.06- . SC

. BI

0 co

r=l 0.05- Ii

rFe - \

I o.w- ‘0

3

F 0.03- 1

$ o.oz- &O 1

g O.O~‘_-~--o--o l\

L,O_;;~~~,~““o” _

- 0 q -‘ P 5

‘/j.& ______&d.__ ___ f ,+___-_ 0 ’ ’ ’ ’ ‘1 I’ ’ ’

’ ; .-..- -- . . . .

-1cQ -60 -20 0 20 60 100 Ikstonce from qroln boundary ( nm 1

Fig. 20. Distributibn of bismuth and cobalt adjacent to a boundary in zinc oxide after long-time application of a dc field in the sense indicated [20].

306 W.D. Kingery / Segregation phenomena

,22k-,+--J i MnlFe 1

.14 1 \ i

-120 -80 -60 -40 -20 0 20 40 60 80 120


Fig. 21. Sharp change in concentration of zinc and

manganese across a grain boundary in a manganese-zinc-

ferrite [lo].

direction, one can calculate the apparent boundary width. Results of 75 8, obtained for aluminum oxide seem rather high [21]. If we assume that the boundary has a thickness of 7.5 A, then we calculate that diffusion along the boundary is about 100 times more rapid than diffusion across the boundary, an equally rational result.

5. Summary

As we develop increasing data for boundary segregation in oxides and carbides, we find that it is very common indeed. For systems in which lattice properties are influenced by solute concentration, we may expect grain boundary properties to be equally or more strongly influenced.

It seems plausible that the sources of segregation are related to strain effects, to chemical-bonding orbital configuration or boundary electron energy levels, and to the development of space charges adjacent to a boundary as the main factors contributing to the reduction of boundary-free energy and development of “boundary-active” constituents.

The influence of boundary segregation on properties such as diffusion1 remains a virtually untouched field, but it seems that we now have the tools available to carry out systematic studies on a reasonably effective basis.

References

[l] A. Atkinson and RI. Taylor, AERE Harwell Report

HL 80/632 (C.14), February (1980). [2] P. Gas and J. Bernardini, Surface Sci. 72 (1978) 365.

[31

[41 r51

[61

[71

PI

[91 [lOI

[ill

[I21

[I31

[I41

u51

N. Mitzutani, A.J. Garrett-Reed and W.D. Kingery,

Ceram. Inter., to be published.

Y. Chiang and W.D. Kingery, work in progress.

T. Mitamura, E.L. Hall, W.D. Kingery and J.B. Vander

Sande, Ceram. Inter. 5 (1979) 131.

J.R.H. Black and W.D. Kingery, J. Am. Ceram. Sot.

62 (1979) 176.

W.D. Johnson, D.F. Stein and R.W Rice, in: Grain boundaries in engineering materials, eds. J.L. Walter,

J.H. Westbrook and D.A. Wordford (Claitors Publ.

Div., Baton Rouge, La., 1975).

Y. Chiang, A.F. Henriksen, W.D. Kingery and D.

Fineho, J. Am. Ceram. Sot. 64 (1981) 385.

C.-W. Li and W.D. Kingery, work in progress.

Y. Chiang and W.D. Kingery, in: Advances in ceramics,

to be published.

Y. Tajima and W.D. Kingery, J. Mater. Sci., to be published.

Y. Tajima, K. Kijima and W.D. Kingery, J. Chem.

Phys., to be published.

J. Frenkel, Kinetic theory of liquids (Oxford University

Press, Oxford, 1946).

K. Lehovec, J. Chem. Phys. 21 (1953) 1123.

J.F. Laurent and J. Benard, J. Phys. Chem. Solids 7

(1958) 218;

J. Cabane, J. Chem. Phys. Physicochem. Biol. 59 (1962)

1123, 1135.

[16] B.J. Wuensch and T. Vasilos, J. Am. Ceram. Sot. 49

(1966) 433. [17] A. Atkinson, Solid State Ionics 12 (1984), this issue.

[18] V.S. Stubican, Solid State Ionics 12 (1984), this issue.

[19] C. Berthelet, W.D. Kingery and J.B. Vander Sande, Ceram. Inter. 2 (1976) 62.

[20] Y. Chiang and W.D. Kingery, J. Appl. Phys. 53 (1982) 1765.

[21] R.E. Mistler, Sc.D. Thesis, M.I.T., Cambridge, MA. (1967).

Comment

Hj. Matzke: In connection with the question of impurity segre-

gation at pore surfaces, it seems worthwhile to men- tion the experience accumulated with irradiated nuclear fuels, e.g. UO;?. Here, during irradiation,


there is a constant in-growth of impurities by fission- ing U-atoms. The fission products comprise about 30 different elements. In particular, those with a low solubility accumulate at pore surfaces, as well as on grain boundaries and on new surfaces formed in fission-gas containing bubbles. With the technique described in your lecture, i.e. STEM, thin films of segregated impurities at inner surfaces can easily be

detected, and precipitated impurities at grain boundaries are equally well seen.

J.A. Kilner: We have measured ia0 self-diffusion coefficient in

several ferrites and found in all cases fast diffusion along grain boundaries. We also determined that these boundaries looked clean by TEM.

Documents

Segregation phenomena at surfaces and at grain boundaries in oxides and carbides