De Jonghe, L.C.

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Lawrence Berkeley National LaboratoryRecent WorkTitleMICROSTRUCTURE, DEFECTS, AND SODIUM TRANSPORT IN SODIUM BETA-ALUMINA SOLID ELECTROLYTESPermalinkhttps://escholarship.org/uc/item/43w554dhAuthorDe Jonghe, L.C.Publication Date1979-09-01eScholarship.org Powered by the California Digital LibraryUniversity of California~J ,. . • LBL-9802 C. ;;...... Lawrence Berkeley Laboratory UNIVERSITY OF CALIFORNIA Materials & Molecular Research Division To be presented at the U. S.-Japan Joint Seminar on Defects and Diffusion in Solids, Honolulu, HI, October 10-12, 1979 MICROSTRUCTURE, DEFECTS, AND SODIUM TRANSPORT IN SODIUM BETA-ALUMINA SOLID ELECTROLYTES Lutgard C. De Jonghe September 1979 TWO-WEEK LOAN COpy This is a Library. Circulating Copy which may be borrowed for two weeks. For a personal retention copy~ call Tech. Info. Division~ Ext. 6782. -Z::ECEIV5:D Lf.\.WRi:;'\!CE amm.E!.EY LABORATORY NUV 161979 LIBRARY AND DOCUME.N.TS SEC·flO N Prepared for the U.S. Department of Energy under Contract W-7405-ENG-48 DISCLAIMER This document was prepared as an account of work sponsored by the United States Government. While this document is believed to contain correct information, neither the United States Government nor any agency thereof, nor the Regents of the University of California, nor any of their employees, makes any warranty, express or implied, or assumes any legal responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by its trade name, trademark, manufacturer, or otherwise, does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof, or the Regents of the University of California. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof or the Regents of the University of California. -""" Presented at the U.S.-Japan Joint Seminar on Defects and Diffusion in Solids, October 10-12·, 1979, Honolulu, Hawaii, U. S. MICROSTRUCTURE, DEFECTS, AND SODIUM TRANSPORT IN SODIUM BETA-ALUMINA SOLID ELECTROLYTES Lutgard C. De Jonghe September, 1979 Materials and Molecular Research Division Lawrence Berkeley Laboratory University of California Berkeley, California 94720 MICp..oS'l'HUCl'UHE , DEFEC'l'S, l\ND SODIUM 'l'Hl\NSPOI{'r IN SODIUM HE'rl\-l\r.tJMIN/\ SOLID ELECTROLYTES Lutgard C. De Jonghe Materials and. Molecular Research Division Lawrence Berkeley Laboratory , and. Department of Materials Science University of California Berkeley, California 94720 ABSTRACT In this paper t some imperfections are discussed, including point defects, non-stoichiometry due to Na2 0 loss, planar defects and grain boundaries. Their relationship to fast ion transport is indicated. For most cases studied to date, only qualitative relationships between defects and ionic conductivity can be given. Introduction Sodium beta-alu.ninas are rapidly becoming some of the most studied materials. In spite of the attention that these solid electrolytes have received from the material science, the solid state physics, and the electrochemical community, many problems remain unanswered. In particular, the nature of the charge compensating defects for excess sodiums, the diffusion of ions in highly defective sy'stems, and the initiation of electro-chemical degradation need to be clarified further. 'For charge compensation, so far only the oxygen interstitial with two associated aluminum ion Frenkel pairl has been clearly shown to br] present. It l." I however, I'0ssible that other charge compensating defects in sodium beta-2 alumina are of importance as well. Other microstructural features and - 1 -defects such as planar disorder in 00.1 c1irection Llnd .:mtipha~;e type 'b d . 3 1 f 1 4 . ' . 5, 6 oun arl.es or comp ex au ts, graJ.n boundarl.es, etc., have been studied in beta-alumina, but to date, only qualitative relationships between those imperfections and sodium ion transport can be given. Of addi tional importance in sodium ion transport through solid electrolytes' are the polarization effects that can occur in the electrolyte at the electrode/ electrolyte interfaces. Accumulation at these interfaces of impurities originating from cell corrosion processes may be expected to make the electrode/electrolyte interface less and less reversible as time proceeds. Therefore, increasing polarization phenomenon may occur that eventually could lead to initiation of electrolyte breakdown. In this paper, we will discuss some aspects of non-stoichiometry involving Na2 ° loss from th: electro-lyte during fabrication. Further, microstructural defects including complex \ faults, 00.1 disorder and heterophase intergrowth will be reviewed, and role of grain boundaries in the conduction process will be discussed. 1) Non-stoichiometry, Soda Loss It has now been well established that sodium beta-alumina can exist over an appreciable range of non-stoichiometry. Recently, it was reported that a nearly stoichiometric sodium beta alumina, Na AlII 017' could be prepared by a de compos i tion and exchange process starting from ammonia beta-alumina. 7 The sodium rich limit of sodium beta-alumina has not been established with great accuracy, but is generally believed to be around x = 0.3 for the chemical formula (1 + x) Na 20.1l A12 03. It should be emphasized that the problem is complicated by the fact that sodium beta-alumina can have non-stoichiometry thatcis not thermodynamically stable, and that the presence of crystalographically closely related aluminateswi th different sodium content cannot always be readily detected. - 2 -Once a material has been prepared, sodium can be lost from i't. This involves Na2 0 loss from the surface at high temperatures. The process of'soda loss appears to have slightly different crystalographic consequences for beta and for beta"-alumina. In beta"-alumina, soda loss leads to the elimination of nearly all oxygens in the conduction plane, which leads to the collapse and subsequent shear of the lattice to a spinel-like structure.8 A lattice image of such a shear fault that was seen to form in the transmission electron microscope due to Na2 0 loss to the microscope atmosphere is shawn in Figure 1. This type of soda loss can give rise to very significant stresses that can readily cause f oi 1 fracture, Figure 2. It is interesting to note that in such foil fractures, the material behaves as a micro-composite in which the spinel layers are the hard members. Indeed, slip along the basal plane is easy but slip through the spinel blocks, at least at ambient temperature, is nearly impossible. The fracture in Figure 2 was caused by Na2 0 loss from the thin foil of beta"-alumina which led to very large tense stresses normal to the conduction planes of that part of the specimen that was being electron irradiated. The deterioration due to soda loss from beta-alumina foils seems to proceed in a somewhat different fashion, and the shear faults that lead to the formation of spinel-like intergrowthg do not seem to be found in beta-alumina. The reason is that when soda is lost 'om sodium beta-alumina, a large number of the oxygens in the conduction plane remain, therefore preventing the collapse of the crystal. An example of a. degraded beta-alumina foil is shown in Figure 3. Note that significant strain is not observed, as can be deduced from the la.ttice fringes that h "1"<" not been rlisplaced measurably next to the degraded region. Soda loss . \ I ' ,,.; - 3 -during high temperature fabrication from the surface of sodium beta- and especially sodiUm beta"-alumina may therefore lead to significant surface stresses, and the formation of a low conductivity surface layer. Evidence of the existence of surface or near surface stresses in.a sodium beta"-alumina tube may be seen in Figure 4 where bent grains were found near the surface of the tube wall but not in the center of it. We attribute the bending of the grains to high temperature deformation accommodating the stresses generated as a result of soda loss. Homogehization treatments at temperatures lower than the fabrication temperature of this material may, of course, reconstitute to some degree the surface layer. Such soda loss is not to be expected, however, as a result of electrolysis at low temperatures contrary to what has been asserted by Bovin.9 Indeed, bulk breakdown or bulk damage of beta-aluminas away from the electrode/ 10 electrolyte interfaces. due to current passage was. found not to occur. 2) Microstructural Defects A number of microstructural imperfections have been observed in beta-alumina: dis10cations,ll complex planar faults4 or antiphase boundaries,3 00.1 disorder and various hetero-intergrowths.12 . The existence of the complex faults or the antiphase boundaries could be attributed either to profuse climb of partial glide dislocations during high temperature fabri-cation,4 or to an order-disorder transformation at high temperatures. 3 The order-disorder transformation in beta-alumina must occur above 15000 C since the complex faults, as shown in Figure 5, decrease in density upon 3 annealing at those temperatures. As pointed out by Le Cars et aI, only X-ray studies near the melting point of beta~alumina could conceivably elucidate the origin of these complex faults. Another possible origin of domains may be the mode of grain growth during sintering rather than an order-disorder transformation or climb of partial glide dislocations. - 4 -When grain growth occurs, spinel ledges are nucleated on the basal planes and then propagate along them. 13 This kind of process, described .by De Jonghe, does indeed involve in the first place epitaxy of the oxygen sublattice. However, it is conceivable that the cation sublattice might be displaced with respect to that of the matrix by a partial dislocation such as the one that is terminating a complex fault or an antiphase boundary. In that way, disorder in the cation sublattice could be introduced during grain growth without the need of an order disorder transformation. It should be noted that where this complex fault or antiphase boundary intersects the conduction planes close oxygen pairs are created that are somewhat remiriescent of the close oxygen pairs that are produced when an oxygen interstitial is formed to compensate for the sodium excess. 14 Wolf has found that the binding energy between a sodium ion and an oxygen inter-stitial is between .04 and .07 eV. While the close pairs that are Produced by the complex fault or the antiphase boundary are not really oxygen interstitials and hence cannot act as charge compensators or affect stoichiometry of the crystal,i t is still likely that some interaction exists between this kind of disturbance in the oxygen sublattice and the sodium ions. It is therefore expected that the presence of many of the complex faults or antiphase boundaries would, in fact, reduce the ionic conductivity somewhat without affecting the stoichiometry of sodium-beta alumina. No quanti tati ve data, however, on this type of interaction are available. 12 15 00.1 disorder has been discussed by De Jonghe, stevens, and Sato and Hirotsu.16 From this it follows that not all of the defects that were observed by Bevan et all17 could be interpreted as being beta" intergrowth into beta-alumina and, therefore, the density of planar faults and the beta" intergrowth defect is far too small to account for the sodium -.. - 5 ~ excess. Rather, such planar faults are introduced during the growth of the beta-alumina crystals, and since their density is low it is unlikely that they are of much significance in the over-all conductivity of the electrolyte even though the local conducti vi ty may be significantly affected. No significant local soda excess is expected to be associated with the planar faults, since preferential local decomposition, as observed in beta"-alumina and shown in Figure 1, did not occur. 3. Grain Boundaries The effect of grain boundaries on the ionic conducti vi ty of poly-crystalline electrolyte has been studied by some workers and examined in . 5 some detail by De Jonghe. The most important crystallographic phenomena occurring at the grain boundaries of sodium beta-alumina is the attempt of adjacent grains to match the spinel blocks so that, as a consequence, the conduction planes can be continuous except in those places where the dislocations at the grain boundary accorilnlodate the orientational mismatch. For all practical purposes, the spinel blocks could be treated as structure-less units that attempt to be in registery. This kind of phenomenom leads to the existence of a preponderance of grain boundary dislocations with a -)-Burgers vector b = I spinel block. In the extreme case, complete blocking is present as shown in Figure 6. In the other extreme, almost complete continuity occurs of the conduction planes across the grain boundaries as is shown in Figure 7. There is then possible the ~hole range of inter-mediate grain boundary structures. This variety in grain boundary structures with their associated range of conductive properties will then contribute to the wide spread in relaxation times that is observed in lI.C measurements of the complex impedance. Clearly,J, grain boundaries signifi-cantly affect the ionic conductivity of a polycrystalline electrolyte. The detailed interpretation of dispersive measurement~3, however, ·~;(:oms - 6 -at present not possible since it would have to rely on the capability of constructing an absolutely equivalent electrical network of which each element has a physical significance. This difficulty clouds the interpre-tation that various investigators have given to the elements of simple equivalent circuits. Acknowledgments This work was supported by the Electric Power Research Institute through the Division of Materials Sciences, Office of Basic Energy Sciences, U. S. Department of Energy, under contract No. W 7405-Eng-48. References 1. W. L. Roth, F. Reidinger, and S. LaPlaca in "Superionic .Conductors," G. D. Mahan and W. L. Roth, Ed., Plenum Press, (1976), pg. 223 ff. 2. A. R. West, Mat. Res. Bull., 14, 441 (1979) 3. Y. LeCars, D. Gratias, R. Portier, and J. Thery, j. Solid State Chern., £, 218 (1975) 4. L. C. De Jonghe, J. Mat. Sci., 10, 2173 (1975) 5. L. C. De Jonghe, J. Mat. Sci. 14, 33 (1979) 6. E. Lilley and J. E. strutt, Int. Conf. Defects in Insulating Crystals, Gatlinburg, Tennessee, 9-14 October, 1979, Conf. 771002 (abstract) pg. 266. 7. R. Collongues, Mat. Res. Bull., li, 1281 (1978) 8. L. C. De Jonghe, Mat. Res. Bull., 12, 667 (1977) 9. J. O. Bovin, Nature, 273, 136 (1978) 10. L. C. De Jonghe, L. Feldman and P. Millett, Mat. Res. Bull., 14, 589 (1979) 11. R. Stevens, J. Mat. Sci., ~, 801 (1974) • ,... 7 -12. L. C. De Jonghe, J. Mat. Sci., 12, 497 (1977) 13. L. C. De Jonghe, in "Processing of Crystalline Ceramics," H. Palmour, III, R. F. Davis and T. M. Hare, Ed., Plenum Press, 1978, pp 433 ff. 14. D. Wolf, to be published, J. Phys. Chern. Solids, (1979) 15~ R. Stevens and L. J. Miles, J. Mat. Sci., 11, 1911 (1976) '- ' • 16. H. Sato and Y.,.Hirotsu, Mat. Res. Bull. 11, 1307(1976) 17. D., J. M. Bevan, B. Hudson, andP. T. Moseley., J. Mat. ScL,~, 1073 (1974) '. - 8 -16. R. Stevens and L. J. Miles, J. Mat. Sci., 11, 1911 (1976) 17. H. Sato and Y. Hirotsu, Mat. Res. Bull. 11, 1307 (1976) 18. D. J. M. Bevan, B. HudSon, and P. T. Moseley. , J. Mat. Sci. , 2., .. 1073 (1974) " 19. L. c. De Jonghe, J. Mat. Sci. , 14, 33 (1979) , - 9 -XBB 799-11598 XBB 799-11597 Figure 1. Lattice image of perfect beta" (top), and faulted beta" (bottom). TOe 00.3 and the 01.3 lattice fringes haye been imaged. The fault in the beta" resulted from soda loss during electron irradiation in the transmission electron microscope. It is a shear type fault that has resulted from the collapse of the lattice due to removal of most of the oxygen ions in the conduction plane. The shear displacement of the spinel blocks above and below the faults removes the charge problem that would result from the double occupancy by A1 3+ of the . octahedral interstices following the collapse of the oxygen bridges. - 10 -(XBB 799-11593) Figure 2. Foil fracture caused by soda loss to the electron microscope atmosphere during observation . The tensile stresses that result from the high s hear fault density that has been produced causes both tra..'1S- and intergranular fracture. Note that the material behaves as a rrd.crocomposi te in which the spinel blocks are the hard members. S':'nc." soda is not lost as a consequence of electrolysis, such degradation i s not found as a c onsequence of cell operation. 11-(XBB 799-11599) Figure 3. Lattice image of degraded beta-alumina. The degradation resulted from the los~ of soda to the electron microscope atmosphere. The soda loss in beta-alumina does not remove the majority of the oxygen ions in the conduction plane so tilat the crystallographic collapse and shear, as was the case in beta" -alumina, (Figure 1), does not occur" Only minor displacements of the 00.2 fringes (11. 3R) are observed, and large foil stresses leading to fracture, as was the case for beta" (Figure 2), are absent. - 12 -10,u-(XBB 793- 32 34) Figure 4. Bent grains are found near the surface of a beta"-alumina solid electrolyte tube (bottom), but not in the center of the tube (top) . The bent grains are believed to be formed during sintering when stresses, generated by soda loss to the furnace atmos-phere, are accommodated by climb at high temperature. ,/ , - 13 -500 R 1100 OF (0 (XBB-_ 799-11596) Figure 5 . Complex fault network in sodium beta- alumina. The faults ar~ predominantly on {2110} planes. These faults.. are cation faults in the spinel blocks , but are also anion faults in the conduction planes. The fault vector is ia<1210>. Where these faults intersect conduction planes a partial ionic blocking effect may occur. The fault density was found to decre ase upon annealing around 1500oC. - 1 4 -(XBB 799-1159 1) Figure 6. Lattice image of a completely blocking grain botmdary in sodium beta-alumina. Such blocking grain botmdaries are frequently fotmd in polycrystalline beta or beta" solid electrolytes due to the strong tendency of faceting on basal planes. 'r , - 15 -(XBB 799-11589) Figure 7. 60 simple tilt boundary in beta"-alumina. The tendency of the spinel blocks to match with each other is evident in this picture . An inclined edge dislocation runs from left to right through this boundary i it has a Burgers vector of 11. 3 ~. Before and a,fter the dislocation the. spinel biocks are seen to attempt to match y regardless of their detailed structure . .... : .. , .l , This report was done with support from the Department of Energy. Any conclusions or opinions expressed in this report represent solely those of the author(s) and not necessarily those of The Regents of the University of California. the Lawrence Berkeley Laboratory or the Department of Energy. Reference to a company or product name does not imply approval or recommendation of the product by the University of California or the U.S. Department of Energy to the exclusion of others that may be suitable. '"' TECHNICAL INFORMATION DEPARTMENT LAWRENCE BERKELEY LAB ORA TOR Y UNIVERSITY OF CALIFORNIA BERKELEY, CALIFORNIA 94720 r...-.. _ 

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MICROSTRUCTURE, DEFECTS, AND SODIUM TRANSPORT IN SODIUM BETA-ALUMINA SOLID ELECTROLYTES

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MICROSTRUCTURE, DEFECTS, AND SODIUM TRANSPORT IN SODIUM BETA-ALUMINA SOLID ELECTROLYTES

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