DIFFUSION AND REACTIVITYImpurity precipitation and grain boundary diffusion in NaCl

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DIFFUSION AND REACTIVITYImpurity precipitation and grain boundary diffusion in NaCl

L. Harris

To cite this version:

L. Harris. DIFFUSION AND REACTIVITYImpurity precipitation and grain boundary diffusion in NaCl. Journal de Physique Colloques, 1980, 41 (C6), pp.C6-285-C6-288. �10.1051/jphyscol:1980672�.

�jpa-00220110�

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JOURNAL DE PHYSIQUE Colloque C6, supplément au n° 7, Tome 41, Juillet 1980, page C6-285

DIFFUSION AND REACTIVITY.

Impurity precipitation and grain boundary diffusion in NaCl

L. B. Harris

Department of Applied Physics, University of New South Wales, P.O. Box 1, Kensington, N.S.W. 2033, Australia

Résumé. — Dans ce laboratoire, les mesures de diffusion de cation dans les bicristaux, par injection d'argent dans la région de joint de grain, et par diffusion de la lumière et observations au microscope des électrons dans la structure du joint de grain, en addition avec d'autres observations, montrent que le mécanisme de transport le long des joints de grain n'est pas par diffusion au cœur des dislocations. Tout indique que des impuretés rési- duelles ségrégées donnent naissance à une structure de croissance macroscopique dans le joint de grain qui est l'agent essentiel de l'accroissement de la diffusion de cations et d'anions dans ce joint.

Abstract. — Measurements in this laboratory of cation diffusion in bicrystals, of silver injection in the grain boundary region, and of light-scattering and electron microscope observations of grain boundary structure, com- bined with other observations, show that the boundary transport mechanism is not dislocation pipe diffusion.

All evidence indicates that segregated residual impurities give rise to a macroscopic growth structure in the boundary that is the primary agent for enhanced cation and anion boundary diffusion.

Present understanding of how grain boundaries affect diffusion in the alkali halides is that the influence on anions at low temperatures is strong but that the relatively weaker effect on cations is often below the limit of detection. This brief summary is correct but misleading, since it overlooks the fact that cation grain boundary diffusion, when it is detected, is invariably larger than the corresponding anion diffusion. This is shown in figure 1, where the circles give measured values of D' 3, where D is grain boun- dary diffusion coefficient and S is grain boundary width [1], for penetration of N a⁺ and CI" tracer ions into nominally pure NaCl bicrystals. The value of boundary diffusion that can be detected depends on the magnitude of the single-crystal lattice diffusion from which the boundary component must be separated. Since lattice transport of anions is much less than that of cations, the small anion boundary component is not swamped by the flux of matter through the specimen bulk, as it is in the case of cations.

For the bicrystals of figure 1 the ratio D' <5/£>, where D is lattice diffusion coefficient, is of the order of 10~4 m near 720 K for both anions and cations.

Other data can be fitted onto figure 1. Grain boundaries in hot-pressed NaCl compacts, with an average grain size of 200 urn, increased the extrinsic conductivity by an order of magnitude over that in single crystals [3], while a similar increase in low-temperature anion diffusion was caused by dislocations and inter- crystalline regions in crushed NaCl, where crystallite and subgrain size was approximately 250 \im [4]. In each case the total measured transport consisted almost entirely of ion flow along internal boundaries, so values of D' 8 may be calculated from the equation,

C a t i o n s

Anions

Fig. 1. — Anion and cation boundary transport in NaCl. O diffusion of 22Na along 20° tilt boundary (this work); • diffusion of

36C1 along 45° twist, boundary [2]; cations, conductivity experiments [3]; anions, diffusion experiments [4]; x injection mobility [9].

D (observed) = 3 D' 5/a, where a is grain size and 3 5/a is the fraction of specimen occupied by boun- daries. The D (observed) corresponding to extrinsic conductivity is obtained from the Nernst-Einstein relation [5]. Although the results, plotted as the broken lines in figure 1, are only approximate, they indicate

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1980672

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C6-286 L. B. HARRIS

that the results for cations are distinctly larger than those for anions.

Within either the cation or the anion group in figure 1, there is considerable variation between measurements on boundaries produced by different methods, but a feature common to several quite different measurements is that grain boundary width 6 is large (- 1 pm) [I, 4, 6, 71. For this reason it is difficult to apply the model of grain boundary diffusion found valid for metals, that ions move rapidly down the cores of boundary dislocations, since this requires a very narrow boundary width (- 1 nm). It also leads to doubt as to whether dislocation pipe diffusion occurs in the alkali halides. Claims for the existence of dislocation pipe diffusion are based on the extra conductivity produced by Na+ ions injected by an electric field into LiF in the vicinity of the pits [8, 91. From experiments in which silver ions are similarly injected by an electric field into the grain boundary region of nominally pure KBr bicrystals, however, an entirely different conclusion can be reached [lo]. Since the injected silver transforms into colloids inside the specimen, its position within the boundary can be observed directly, and it is imme- diately apparent from figure 2, which gives views at different focus of the same region of a surface cleaved a t right angles to the tilt axis, that the silver has not been injected into boundary dislocations. The fact that the boundary shown in figure 2 migrated laterally

across specimens 0.1 mm thick for 10 min. produced maximum increase in conductivity, and hence that these conditions transported the Na+ ions through the specimen from anode to cathode - a Na' mobility of 2 x 10-l3 m2/Vs is calculated. Assuming 6 = 1 pm, this corresponds to the cross in figure 1, which indicates that the transport mechanism is similar in magnitude to that in NaCl boundaries.

Grain boundary diffusion is more readily detected at low temperatures where transport is controlled by charge-compensating vacancies introduced by divalent impurity, but the extrinsic defect processes that operate in single crystals are significantly less impor- tant within the boundaries. This is shown by many experiments in which doping that produces a large change in lattice transport has only a minor effect on boundary transport. Thus, the values given by the open circles in figure 1 were little changed by calcium doping which gave a sixfold increase in DN, in the lattice. This is matched by the changes observed in extrinsic conductivity, in which impurities gave a fifteenfold increase in single crystals but only a twofold increase in hot-pressed compacts [3]. Simi- larly, for the anion data of figure 1 there was stated to be no systematic variation with calcium doping of boundary diffusion in bicrystals [2], and no signi- ficant dependence of diffusion in crushed NaCl with concentration of added cation and anion impurities [4]. It is therefore not surprising that impurities in the space-charge layer along side the boundary are unable to account either for enhanced boundary diffusion or a large boundary width [7]. The controllink mechanism for boundary diffusion is different from that in the single crystal latticc, the major conse- quence of doping being to increase lattice diffusion and hence to make boundary diffusion more difficult to detect, as is shown in figure 3.

It is further found that variations in heat treatment

Fig. 2. -Top, focussed below surface : silver colloids (black spots) injected into lo0 tilt boundary. Bottom, focussed on surface : position of boundary dislocations (wavy horizontal etch line).

(downwards in figure) during growth reveals that the gross defects occupied by colloids include ghost sites formed by precipitates along a previous position of the boundary. From data on Naf ions injected into LiF tilt boundaries [9] - that 90 V applied at 600 K

(quenching as against dow cooling) has little effect on 0 ' 6 for cations, that D' 6 for anions does not vary with brain boundary angle or grain boundary type (tilt or twist) [2], and that the conductivity enhancement due to boundaries in hot-pressed compacts is permanent, in spite of high-temperature anneals [3]. Even Nat transport in LiF tilt boundaries is not sensitive to tilt angle [9]. Thus, to first order, boundary transport is insensitive to both the physical structure (such as dislocations) produced by lattice misorientation and to local point-defect concentration.

The positive information left to us is : at fixed extrinsic temperature a grain boundary produces approximately constant diffusion over a large boundary width. Large width is a feature of impurity segregation, which is inevitably accompanied by grain boundary precipitation. Such precipitation is common even when impurity concentration is of the order of l o p 6 mole fraction and, in the case of non-equilibrium segregation following cooling from high temperature, spreads over a boundary width of

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IMPURII'Y PRECIPITATION A N D GRAIN BOUNDARY DIFFUSION IN NaCl C6-287

Fig 3 Penetration profiles at 475 OC for 22Na dlHus~on Into NaCl bicrystals contaming lo0 t ~ l t boundary. Mole fraction of calcium : 0 10-

'.

shows gram boundary tall ; n o boundary tail.

many pm [6]. Mcasurements of the intensity of light scattered from the precipitates that form in the grain boundary region reveal that scattering centres are not eliminated by prolonged annealing at tempera- turcs well above those at which the precipitates dis- solve, while the return to low temperature causes many precipitates to reform at the sites they occupied previously. Thus, precipitation frequently appears in regions of the lattice imbued with semi-permanent disorder.' If one uses very slow cooling in order to avoid the large width associated with non-equilibrium segregation, then the electron microscope gold deco- ration technique shows that S is indeed reduced, but that local impurity concentration is larger, and that the boundary region is permeated by a distribution of irregular segregates, small precipitates and tiny voids.

Precipitation is unavoidably present in most boundaries. The methods of spccimen preparation - bi-

crystal growth from the melt or hot-pressmg of' powder - favour accumulation of impurities in the boundary, a process accentuated by impurity segregation to the mismatch plane. If cooling is too rapid for equilibrium segregation to be obtained, decreasing solid solubility with reduction in temperature causes precipitates to nuclcate away from the boundary, while slower cooling produces a non-uniform distribution of separated precipitates along the boundary plane. Volume change on solidification and precipitation, subsequent differential thermal expansion between precipitate and matrix, plus the incorporation of the tiny voids or bubbles found in melt-grown or hot- pressed specimens, establishes a macroscopic growth structure within the boundary region. Apart from attack by ambient moisture on hygroscopic precipitates, such an established structure is difficult to alter, dissolution and reprecipitation taking place within the structure without producing any essential change to its configuration. Rapid transport within such a system of interlinked cavities and interfaces, such as that depicted by silver colloids in figure 2, will be insensitive both to the amount of impurity that remains in solid solution and to that which comes out as second phase. Transport coeficients are then primarily determined by ions moving with a temperature-dependent mobility along a stable system of rapid transit paths.

Precipitates do not themselves contribute to transport, but the interface between precipitate and matrix is a region of continuously changing disorder. Since a low-temperature diffusion measurement rarely takes place under equilibrium conditions, the precipitate- matrix interface is changing while the measurement proceeds, which has the effect of preferentially increasing transport over the interface. Slow cooling, which reduces boundary width 6, increases the local concentration of interfaces, leading to an increased value of D', resulting in only a small change in the product D' 6, as is observed. Opposite variation in D' and 6 may also explain why extrinsic conductivity in hot-pressed compacts was relatively insensitive to grain size [3].

It is concluded that it is essential to specify the nature of the boundary structure on which grain boundary measurements are being carried out. This structure is much more than boundary dislocation con tent.

DISCUSSION

Question. - A. ATKINSON. each must be treated separately. Further, different

we

I , ~ , , ~ measured grain boundary and dislocation impurities have different segregation behaviour. What d i ~of ~nickel ~ in i nickel oxide and find grain ~ ~ is essential is to know the impurity content of the boundary widths

-

10-7 cm. This is in contrast to boundary, and the type of structure formed by speci- the value of 1 pm for NaCI. men preparation. Nickel oxide boundaries have pre-

viously been reported to have large widths [I].

Reply. - L. B. HARRIS. _{[ I ]}O S U U R N , C. M . and VEST, R. W., J . Phys. Chem.

Different systems behave in different ways, and Solids 32 (1971) 1343.

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C6-288 I.. B. HARRIS

Question.

-

P . L. PRATT.

The properties of grain boundaries in ionic crystals depend sensitively upon their structure, and this in turn depends upon the way in which they were made.

I believe that the different behaviours reported by Dr. Harris and by Dr. Atkinson arise from differences in the method of manufacture of their boundaries, and that dislocation-dominated behaviour can be found in suitably prepared NaCl grain boundaries.

Reply. - L. B. HARRIS.

I fully agree that grain boundary properties depend strongly on preparation methods. However, the production of NaCl grain boundaries that show dislocation-dominated behaviour requires elimination of segregation of divalent cation impurities, which means that it will be necessary to observe enhanced grain boundary diffusion in the intrinsic region.

U n t ~ l the intrinsic region is extended to lower temperatures than has been possible with the crystals currently available, this is unlikely to occur.

Quesriorz. - W . C . MACKRODT.

While one must be careful in comparing theory and experiment in this area of grain boundary effects, perhaps I could mention our recent calculations on defect stability at surfaces and in the bulk. These suggest that the relative stability of bulk to surface defects is very definitely system specific so that one might quite well expect differences between NaCl and X i 0 as mentioned. Furthermore. our calculations on NaC1 : C a + are exactly in line with Dr. Harris's experimental finding.

Reply. - L. B. HARRIS.

No comment required.

References

[I] HAI<RIS. L. R., J. Physicpe Collnq. 37 (1976) C7-365. 161 M I S I L ~ R , R. E. and COBLE, R L., J. Appl. Phys. 45 (1974) (21 RIGGS, K. R., Ph. D. Thesis, Un~versity of Missouri-Rolla. ¹⁵⁰⁷

1969. 171 Y A N , M. F., CANNON, R. M., BOWEN, H. K. and Con1 r.

R. L., J. Am. Crram. Soc. 60 (19771 120.

[3] PRA.IT, P. L., J. Physique Collnq. 34 (1 973) C9-213. [8] TUCKER, R., LASKAR, A . and THOMSOV. R., J. Appl. P/~.bs

[4] BARR, L. W.. H(X)DI.FJS, I. M., MOKRISOS. J A. and R L ~ H A H , 34 (1963) 445.

R., Trnns Furuday Soc. 56 (1960) 697. [9] MOMENT, R. L. and GOUWN, R. B., J Appl. Ph-vs. 35 (1964) [5] HARRIS. L. 9. and SCHLWERER. J. L.. J. Physrqrie Colloq. 2489.

34 (1973) C9-425 (101 HARRIS, L. B., Pl~ilm. Mag. 8 4 0 (1979) 17.