A111D5

NATL INST OF STANDARDS & TECH R.I.C.

All 102146462

Miller, G. R. (Geral/Englneerlng propert QC100 .U573 V61;3;1979 C.2 NBS-PUB-C 197 c*E» f" NSRDS-NBS 61, Part III

U.S. DEPARTMENT OF COMMERCE /

National Bureau of Standards
NATIONAL BUREAU OF STANDARDS

The National Bureau of Standards 1 was established by an act of Congress March 3, 1901. The

Bureau’s overall goal is to strengthen and advance the Nation’s science and technology and

facilitate their effective application for public benefit. To this end, the Bureau conducts

research and provides: (1) a basis for the Nation’s physical measurement system, (2) scientific and technological services for industry and government, (3) a technical basis for equity in trade, and (4) technical services to promote public safety. The Bureau’s technical work is performed by the National Measurement Laboratory, the National Engineering Laboratory,

and the Institute for Computer Sciences and Technology.

THE NATIONAL MEASUREMENT LABORATORY provides the national system of

physical and chemical and materials measurement; coordinates the system with measurement

systems of other nations and furnishes essential services leading to accurate and uniform

physical and chemical measurement throughout the Nation’s scientific community, industry,

and commerce; conducts materials research leading to improved methods of measurement,

standards, and data on the properties of materials needed by industry, commerce, educational

institutions, and Government; provides advisory and research services to other Government

Agencies; develops, produces, and distributes Standard Reference Materials; and provides

calibration services. The Laboratory consists of the following centers:

Absolute Physical Quantities2 — Radiation Research — Thermodynamics and

Molecular Science — Analytical Chemistry — Materials Science.

THE NATIONAL ENGINEERING LABORATORY provides technology and technical

services to users in the public and private sectors to address national needs and to solve

national problems in the public interest; conducts research in engineering and applied science

in support of objectives in these efforts; builds and maintains competence in the necessary

disciplines required to carry out this research and technical service; develops engineering data

and measurement capabilities; provides engineering measurement traceability services; develops test methods and proposes ‘engineering standards and code changes; develops and

proposes new engineering practices; and develops and improves mechanisms to transfer

results of its research to the utlimate user. The Laboratory consists of the following centers:

Applied Mathematics — Electronics and Electrical Engineering2 — Mechanical

Engineering and Process Technology2 — Building Technology — Fire Research —

Consumer Product Technology — Field Methods.

THE INSTITUTE FOR COMPUTER SCIENCES AND TECHNOLOGY conducts

research and provides scientific and technical services to aid Federal Agencies in the selection,

acquisition, application, and use of computer technology to improve effectiveness and

economy in Government operations in accordance with Public Law 89-306 (40 U.S.C. 759),

relevant Executive Orders, and other directives; carries out this mission by managing the

Federal Information Processing Standards Program, developing Federal ADP standards

guidelines, and managing Federal participation in ADP voluntary standardization activities; provides scientific and technological advisory services and assistance to Federal Agencies; and

provides the technical foundation for computer-related policies of the Federal Government.

The Institute consists of the following divisions:

Systems and Software — Computer Systems Engineering — Information Technology.

'Headquarters and Laboratories at Gaithersburg, Maryland, unless otherwise noted; mailing address Washington,D.C. 20234.

2 Some divisions within the center are located at Boulder, Colorado, 80303.

The National Bureau of Standards was reorganized, effective April 9, 1978.
Hsttentl Boreas of Standards

jul 13 rm

act

Physical Properties Data Compilations

Relevant to Energy Storage.

III. Engineering Properties of Single and Polycrystalline

Sodium Beta and Beta -Alumina

G. R. Miller and D. £r. Paquette*

Department of Materials Science and Engineering

University of Utah

Salt Lake City, Utah 84112

*Present address: Ford Aerospace and Communications Corporation,

Aeroneutronic Division, Newport Beach, CA 92660.

t

U.S. DEPARTMENT OF COMMERCE, Juanita M. Kreps, Secretary

Jordan J. Baruch, Assistant Secretary for Science and Technology

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

Issued June 1979
Library of Congress Cataloging in Publication Data (Revised)

Main entry under title:

Physical properties data compilations relevant to energy storage.

(Nat. Stand, ref. data ser. ; NSRDS-NBS 61, pt. 1- ) “National Standard Reference Data System.”

Bibliography: v. 1, p. Includes indexes. CONTENTS: 1. Molten Salts Data Center, Cogswell

Laboratory, Rensselaer Polytechnic Institute, Molten salts: eutectic data.

—3. Miller, G. R. and Paquette, D. G. Engineering properties of

single and polycrystalline sodium beta and beta"-alumina.

Supt. of Docs, no.: C 13.48:61

1. Energy storage. 2. Fused salts. 3. Eutectics. I. Janz, George J.

II. Rensselaer Polytechnic Institute, Troy, N.Y. Molten Salts Data

Center. III. Series: United States. National Bureau of Standards. National standard reference data series : NSRDS-NBS 61, pt. 1- QC100.U 573 no. 61, pt. 1, etc. [TJ165] 602’. Is [621]

77-10824

NSRDS-NBS 61, Part III

Nat. Stand. Ref. Data Ser., Nat. Bur. Stand. (U.S.), 61, Part III, 19 pages (June 1979)

CODEN: NSRDAP

(C) 1979 by the Secretary of Commerce on Behalf of the United States Government

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 1979

For sale by the Superintendent of Documents, U.S. Government Printing Office Washington, D.C. 20402 - Price $1.25

Stock Number 003-003-02079-6
Foreword

The National Standard Reference Data System provides access to the quantitative data of physical science, critically evaluated and compiled for convenience and readily accessible through a

variety of distribution channels. The System was established in 1963 by action of the President’s

Office of Science and Technology and the Federal Council for Science and Technology, and

responsibility to administer it was assigned to the National Bureau of Standards.

NSRDS receives advice and planning assistance from a Review Committee of the National

Research Council of the National Academy of Sciences-National Academy of Engineering. A number of Advisory Panels, each concerned with a single technical area, meet regularly to examine

major portions of the program, assign relative priorities, and identify specific key problems in

need of further attention. For selected specific topics, the Advisory Panels sponsor subpanels

which make detailed studies of users’ needs, the present state of knowledge, and existing data re- sources as a basis for recommending one or more data compilation activities. This assembly of

advisory services contributes greatly to the guidance of NSRDS activities. The System now includes a complex of data centers and other activities in academic insti- tutions and other laboratories. Components of the NSRDS produce compilations of critically

evaluated data, reviews of the state of quantitative knowledge in specialized areas, and computations of useful functions derived from standard reference data. The centers and projects also

establish criteria for evaluation and compilation of data and recommend improvements in ex- perimental techniques. They are normally associated with research in the relevant field. The technical scope of NSRDS is indicated by the categories of projects active or being

planned : nuclear properties, atomic and molecular properties, solid state properties, thermodynamic and transport properties, chemical kinetics, and colloid and surface properties.

Reliable data on the properties of matter and materials are a major foundation of scientific and technical progress. Such important activities as basic scientific research, industrial quality control, development of new materials for building and other technologies, measuring and correcting

environmental pollution depend on quality reference data. In NSRDS, the Bureau’s responsibility

to support American science, industry, and commerce is vitally fulfilled.

Ernest Ambler, Director

m
Preface

This series of publications is aimed at providing physical properties data on materials used in energy

storage systems. It was inspired by a requirement in the Department of Energy’s Division of Energy Storage

Systems for materials property data needed by its contractors in the timely development of energy storage

devices. As prime contractor for this program, the Lawrence Livermore Laboratory (LLL) has requested the

Office of Standard Reference Data (OSRD) to manage the task of gathering the data, using its established

network of data centers and other identified sources of expertise. The OSRD monitors the progress of work,

reviews the results, and conveys the numerical data to LLL where the data are converted for entry into an automated data storage and retrieval system. Every effort is made to supply data which have been critically

examined in light of the latest knowledge concerning theory and experiment. However it must be recognized

that in a rapidly moving technology some of the data will be superseded rather quickly as new materials and

techniques are introduced. Thus access to the data via computer terminal as well as publication in this series

should help provide the practitioner with timely and useful data which he requires to solve his problems in

energy storage. Funding for this series of projects from the Department of Energy, Division of Energy

Storage, through the Lawrence Livermore Laboratory, is gratefully acknowledged.

Previous publications in the series "Physical Properties Data Compilations Relevant to Energy Storage”:

Janz, George J., Allen, Carolyn B., Downey, Joseph R., Jr., and

Tomkins, R. P. T., Physical Properties Data Compilations Relevant

to Energy Storage. I. Molten Salts Eutectic Data, Nat. Stand. Ref. Data Ser., Nat. Bur. Stand. (U.S.), 61, Part 1, 244pp. (March 1978).

Janz, G. J., Allen, C. B., Bansal, N. P., Murphy, R. M., and Tomkins,

R. P. T., Physical Properties Data Compilations Relevant to Energy

Storage. II. Molten Salts: Data on Single and Multi-Component Salt Systems, Nat. Stand. Ref. Data Ser., Nat. Bur. Stand. (U.S.), 61, Part II, 442 pp. (Apr. 1979).

IV
Physical Properties Data Compilations

Relevant to Energy Storage

III. Engineering Properties of Single and

Polycrystalline Sodium Beta and Beta" -Alumina

G. R. Miller and D. G. Paquette*

Department of Materials Science and Engineering; University of Utah; Salt Lake City, Utah 84112

A review of the properties of the two dimensional superionic conductors of the sodium beta alumina family is presented, with emphasis on the variability of properties with composition and processing. Processing methods,

including methods of distributing additives, may strongly influence engineering properties of these substances through changes in microstructures. Described are common methods of measurement of properties with

generally accepted ranges of property values. The need to compromise in the design of devices requiring several coincident optimal properties is, at this time, a necessary ingredient in the use of these materials as device components.

Key words: Battery application; engineering properties data; evaluated data; polycrystalline; single crystal; sodium beta alumina; sodium beta double prime alumina.

1. Introduction

1.1. General Background

The superionic conductor field has been recently

pursued with vigor in an attempt to produce a detailed

understanding of conduction mechanisms in the

apparently large class of materials which exhibit very

rapid motion of a constituent cation or anion. Of the

many interesting materials having "superionic” conductivities, some are three dimensional conductors, others

two and a few only one. There are two substances in the

class of two dimensional conductors which are of

extreme interest from a device construction point of

view, the so-called sodium beta (/3) and sodium beta

double prime (/3") -aluminas. These materials are of

interest in electrolytic device applications where a sodium ion conductor with virtually no electronic

conductivity is required. Several devices with this

requirement are in various stages of development in

university, government and industrial laboratories. The

sodium-sulfur [1 ]

1 and sodium-antimony-alumino chloride [2] high energy and power density storage cells are

in such a stage as to be considered very promising, the

sodium heat engine [3] (thermoelectric device) is under

*Present Address: Ford Aerospace and Communications Corporation, Aeroneutronic

Division, Newport Beach, CA 92660.

'Figures in brackets indicate literature references at the end of this paper.

strong consideration as a potentially erficient direct

energy conservation device and attempts could be made

to use a sodium ion superionic conductor for sodium

purification via electrolytic separation. The use of /3/f3"

mixtures as an electrolyte in gaseous concentration cells

is also of interest [4]. The sodium beta alumina family of materials seems to be the most promising solid electrolyte for these

applications because it is produced from cheap, available

materials and because it is relatively stable to degradation by liquid sodium.

The so-called sodium /

-alumina is characterized by a hexagonal lattice structure with aluminum and oxygen

ions forming spinel blocks and with sodium ions

occupying positions in two dimensional channels which

separate the spinel blocks [5]. In the c-direction

(perpendicular to the sodium ions channels) periodicity

of the crystal structure is found to occur within a distance equal to two spinel blocks and two channels,

thus the term "two block” beta alumina. The/3

crystal structure is trigonal [6,7] with many of the local characteristics of the /3 form except that periodicity in

the axial (or c-direction) requires traversing three spinel

blocks and thus the term three block beta alumina is often used. Compositionally, both forms may exist over

relatively wide ranges of stoichiometry, the (3 form

having the widest range [8,9]. Nominal composition of

the j8 form is commonly given as Na20 11Al 203 although wide deviations from this formula are observed

1
and all forms are rich in Na20 as compared to the

Na20- 11A1 2 03 formula.

The (3 " form is given nominally as Na2 0-5A1 203 but

the structure is unstable at high temperatures (above

about 1200 °C) [9], and so certain compounds are added

to stabilize the structure at high temperatures and allow

high temperature ceramic processing to be carried out. Common additives are Li

2 0 and MgO although other

additives have been used for stabilization [9]. From the

compensation models of Kummer [10] and Bettman and

Peters [7] , (3 "-alumina with Li

20 and MgO additives is expected to have higher conductivity than that of the

nominal composition. This is due to its ability to allow more sodium ions to be inserted into the structure. The/3

form has been produced in relatively pure form as well as with additions of Li

20 and MgO and other additives

although the positioning of the impurity ion may be

different in the (3 form than in the (3 " form [11]. In the

two block form, the Li ion is normally found in the

conducting channels whereas in the three block form the

Li ion is probably on the Al

+3 site near the channel.

These substances are most commonly available in

polycrystalline ceramic forms and are being developed

for use in devices requiring relatively high sodium ion

current densities. However, electrical properties of both

of these materials are usually measured with perturbative methods (low current densities). The high current

density requirement of actual devices (^>1000 Am-2

) may

lead to several other problems involving the otherwise

insignificant electronic conductivity, local joule heating,

local thermal expansion mismatch and slow crack

propagation driven by electrical forces, etc. For the latter two problems, it is obvious that high mechanical strength

may be a desirable characteristic and thus the production

of strong polycrystalline ceramic in industrially useful

shapes becomes important. Polycrystalline material also

is required from the point of view of economics in that

low cost forms will be required for all of the high volume

applications. Much of the engineering data now available

is gathered from polycrystalline samples whose microstructure (grain size, grain size distribution, grain

morphology, etc.) can vary widely and which affects all of the other engineering properties—strength, ionic

conductance, current carrying capability, thermal shock

resistance, storability in non-vacuum conditions, etc. When one combines the desire for property optimization

with the economics problems, one finds materials

available with widely varying characteristics depending on the processor, the composition and the intended

application.

1.2. Conductivity-Microstructure Effects

Processing of these ceramics is difficult primarily due

to their inherent chemical instability at high temperatures where soda loss is rapid. Methods of manufacture

must always account for rapid soda loss at temperatures

which are required to attain other optimal properties and

is often accomplished by encapsulating the material in

platinum or packing in powders containing soda so that a high soda vapor pressure is maintained about the sample.

Because of the extreme anisotropy of mechanical [11]

and electrical properties, even the methods for green

forming of ceramic bodies must be carefully considered.

Tubular shapes may have significantly different conductance as measured radially (through tube walls) and as measured axially (parallel to the walls) due to preferred

crystal orientations arising during ceramic forming

operations [12,13]. Byckalo et al. [14], have produced

slipcast two block sodium beta alumina samples in which

the crystals exhibit a high preferred orientation whereby

the poor conductivity directions of the crystals are aligned in a nearly parallel fashion. Since the single

crystal resistivity anisotropy is very large [10] , one must

expect marked directional dependencies in the apparent

resistivity in polycrystalline materials with strong

preferred orientation of the crystals. Byckalo et al., for

instance, report the preferred orientation anisotropy for

their samples of two block beta alumina to be about 40.6.

That is, the resistivity perpendicular to the preferred

orientation plane is about 40.6 times greater than in any

direction removed by 90° from the perpendicular.

However, these effects are dependent on processing

method and can be minimized.

Since these conductors are two dimensional in nature,

(the sodium ions are constrained to move in a plane)

there will be a different geometrical path traversed by the

ions in a polycrystalline material than if a single crystal were oriented with the applied electric field parallel to a conducting plane. Attempts have been made to calculate

the geometrical path effects on polycrystalline two

dimensional conductors so that the tendency toward

randomness can be accounted for in constructing a polycrystalline conductor from many single crystals.

The fact that sodium ions must follow a tortuous path in passing through a polycrystalline sample has lead to the

universally applied term "tortuosity factor," a purely

geometrical factor describing the ratio of the effective

path length to that which is found in an oriented single

crystal. The best calculations of this geometrical term

have been carried out by Mitoff [15]. He has found that

for the case of random equiaxed (cubic) crystals in a 3

dimensional array, the tortuosity factor should be about

1.363 for a two dimensional conductor. This calculated

result was found for uniform grain size and not for the

commonly encountered duplex microstructure wherein

a few very large grains (say up to 300 jum in length) are observed in a fine grain matrix where the grains are less

2
than 3 jU m mean diameter. It is expected that the

tortuosity factor will increase for such microstructures.

Although the tortuosity factor is relatively small, one

finds that the measured dc or low frequency conductivity

of the polycrystalline substance cannot be accounted for

by simply multiplying the single crystal resistivity by the

calculated tortuosity factor. The measured resistivity is always a factor in the range 3-5 times higher than

predicted [16,17,18,19]. The reasons for this failure are presently unclear except that grain boundary effects

certainly play a large part in increasing the low frequency

and dc resistivity, especially at low temperatures.

The relative contributions to the total dc resistivity

from the grains and the grain boundaries can be obtained

by measuring the frequency dependence of the impedance of polycrystalline samples. The combination of

grains in series with grain boundaries is often modeled as an electrical circuit consisting of a resistor (grain) in

series with the parallel combination of a resistor (grain

boundary resistance) and a capacitor (grain boundary

capacitance). Analysis of the total specific resistivity or

the complex impedance of a sample as a function of

frequency can give approximate values for the two

resistances and the capacitance provided that the

frequency range of the measuring apparatus extends

high enough to include the dispersive or "capacitive

shorting” effects associated with the grain boundaries.

Reports by Powers and Mitoff [16] and Hooper [19] have

given values for the apparent relative contribution of

grain boundaries to the total dc resistivity. In the case of

impurity doped two block sodium beta alumina doped

with Y2 03

, CaO, and Si0 2 [20,21,22], these impurities

migrate to grain boundaries and produce a high

resistivity layer of such thickness and dielectric constant

that shorting of the equivalent capacitive element is easily performed (at least at room temperature or below), and at frequencies often less than 100 kHz. In undoped two block beta alumina or two block beta

alumina with ZnO added as a sintering aid [20],

capacitive shorting is normally not observed at frequencies as high as 500 kHz to 3 MHz nor at temperatures as low as room temperature. For these relatively pure materials the grain boundary resistances have been

estimated by making measurements on a series of

samples with different grain sizes, and hence a variation

of the relative area fraction of grain boundary material

from sample to sample can be determined. This work has

shown that in two block /? alumina the grain boundary

contribution to the dc resistivity can be minimized to a small fraction of the total resistivity even for fine grain

ceramics.

The conductivity-microstructure relations for the

three block form are in much less mature condition than

for the two block form. In the three block form, an

impurity is always present as a stabilizing additive so that

processing can take place at elevated temperatures and

which will result in the existence of the three block form

at the temperature of processing. Three stabilizing

element systems are most common: (1) MgO in

quantities up to 5 weight percent, (2) Li

2 0 in quantities

up to about 1.1 weight percent and a mixture of the two

(Li

2 0-MgO) up to about 2.0 weight percent MgO plus

about 1.0 weight percent Li,0. Providing that the

resulting crystalline phase is essentially all three block

sodium beta alumina, (greater than 95 percent by

volume) one can attain similar resistivities for temperatures from about 25 °C and higher no matter what the

stabilizing element.

For an average composition, which at true equilibrium

would give three block sodium beta alumina, the

resistivity can be widely varied because of lack of

conversion to the three block form. By this is meant the

following: in the processing of polycrystalline three

block beta alumina samples, chemical powders which

provide the alumina, soda and stabilizing additive are mixed together and sintered. Thus a reaction between

several components must take place and during this

process several phases may be present depending on the

state of completion of the reaction. From phase diagrams

[8,9] one expects, for example, to encounter a stage

whereby three block and two block sodium beta alumina

coexist (not at equilibrium) with sodium aluminate

(NaA10

2 ). Since the conversion reaction to the three

block form is slow and many times must be accomplished

through a long, post sintering anneal treatment, it is easy

to produce a system with the two block and three block

form coexisting and occupying most of the volume. A

third phase (such as, possibly, NaAlO^) would occupy a small volume fraction. Such a mixed system, which may

be promoted by poor distribution of the stabilizing

additive [23], will exhibit conductivities lower than for

the single phase material by amounts which depend

directly upon the amount of nonequilibrium .phases

present. For instance, one can expect the resistivity for a sample containing ~ 70 volume percent three block

(equilibrium phase) and ~ 30 percent two block phase

(with a small amount of third phase) to be in the range of

0.06 to 0.12 ohm-meters at 573 K in material whose

average stabilizing element composition is 0. < 5 percent

by weight Li

20 (the exact value of the resistivity also

depends on grain size) while samples with the same mean

composition which are completely converted to the three

block form show resistivities in the range of 0.02 to 0.06

ohm-meters at the same temperature and for the same

average composition. Again the exact value will depend

upon grain size. The effect of grain boundary resistance and capacitance on the frequency dependence of sample imped3
ances is expected to be similar to results for the two-block

form. No engineering type /3 "-alumina has been pro- duced for which grain boundary shorting occurs at frequencies as low as 100 kHz at room temperatures.

Measurements must be carried out at higher frequencies.

At 300 K grain boundary shorting begins by about 50

MHz [22] . This capacitive shorting occurs over a very

wide range of frequencies and makes identification of the

single crystal resistivity very difficult. However, recent

measurements by Rasmussen [24] to frequencies of 300

MHz have allowed identification of the magnitude of this

shortable resistive contribution. Much work remains to be done in the area of electrical conductivitymicrostructure relationships in these materials.

For the /J" form doped with Li

20 as a stabilizing

additive, one can develop some qualitative relations

between impurity concentration and resistivity. In the

case of Li2 0 stabilized three block material, at constant

Na2 0 content, the resistivity is relatively independent of

Li2 0 concentration up to about 1.1 weight percent Li 20

whereupon it is claimed [25] that the Li ion enters the

conducting channels rather than on an A1

+3 site near the

channel. Apparently above about 1.1 weight Li2 0, ion

exchange with molten lithium salts is easily observed.

An interesting feature recently reported by Doremus

[26] is the appearance that in the three block form,

potassium ions which pass through the sodium ion

channels in the grains (single crystals) have great

difficulty in traversing the grain boundaries; in the case

of the two block form, the K~*~ions appear to easily pass

through grain boundaries as well as the grains. If this

observation is valid, an extension of this argument would

lead one to suppose that in the /?" form, the K^ions

entering only the surface grains will cause stress buildup

at the surface. This property could be useful for

constructing sodium purification apparatus but may also

enhance mechanical degradation.

1.3. Mixed Systems

Many experiments have been carried out to produce

mixed cation conductors or new cation conductors with

the two and three block beta alumina structure. These

ion exchange experiments have been reported in the

earlier review article by Kummer [10] and in a more

recent article by Kummer and Whittingham [27]. Also,

with soda concentrations in the range of about 6.8 to about 8.6 weight percent and with the addition of one of

the previously mentioned additives one can produce

equilibrium material in the polycrystalline form and

which consists of a mixture of /3 and/3" phases [28,29].

1.4. Mechanical Behavior

The degradation of the ceramic via mechanical

separation (cracking) can be carried out in several ways,

each of which may be encountered in common device

preparation and operation. The four common ways may

be described as:

(1) Rapid crack propagation as experienced in most

common, short time experimental tests of fracture

strength.

(2) Slow crack propagation in the presence of an ambient that corrodes the material, especially at high

stress regions, that is stress corrosion.

(3) Slow crack propagation under the influence of

electric fields, especially at high sodium ion current

densities.

(4) Cracking under the influence of rapidly changing

temperatures, that is thermal shock.

Each of these modes of inducing fracture must be

considered for the case of engineering design with these

materials as when, for instance, rigid seals to mating

components are formed, or when the material is used in a situation where high sodium ion current densities or when rapid thermal cycling is experienced in the

presence of an ambient in which good thermal transfer is effected (such as liquid sodium) . For each of these cases, one normally may decrease the degradation effects with a substance of high fracture toughness or high fracture

strength. In both the two and three block forms of sodium beta

alumina in polycrystalline form, the microstructure

attained after processing is quite variable and the

fracture strength as measured in rapid four point

bending tests is a strong function of both grain size and

grain size distribution. For instance in/?” alumina with

relatively uniform microstructure of mean size less than

about 5jum, one finds the fracture strength to be as high

as 2.9 108 N m~2 (newtons per square meter) at room

temperature [20,31] whereas with relatively uniform

grains of mean size greater than 200 /zm in a sample of the same nominal composition the fracture strength can

easily be as low as about l.Ox 108 N nr2 [32]. With the

common duplex microstructure (containing large

elongated grains whose rapid growth direction is parallel to the crystallographic basal plane) , the fracture strength

lies intermediate between these two values. Samples with

large volume fraction of large elongated grains in the fine

grain matrix tend to exhibit lower fracture strength than

do samples with a large volume fraction of fine grains.

Experimental data on the Li2 0 stabilized three block

form show a weak dependence of strength on grain size when the mean grain size is less than about 100 /um.

Above this grain size, the strength drops rapidly with

increasing grain size. Qualitatively the two materials

(two and three block sodium /3 -aluminas) behave in a similar fashion for similar microstructures if treated in

4
the same fashion. However the strength of fine grain /3 is generally higher than the fine grain (3" material.

Much data on the fracture strength of these materials

is obtained on sections of tubes mechanically fractured

in a diametral test. This test can give inflated values for

strength (as compared to three or four point bending

where the maximum tensile stress is applied over a larger

volume fraction of the sample than for the case of the

diametral test) and, although this diametral test is useful

for grading samples of tubular shape, the most common

geometrical form for devices, it is not the most recom- mended test. Three or four point bending tests are preferred methods. For fracture tests in four point

bending or a diametral tube section test, the samples are

typically cut by a diamond wafering blade, and are annealed to relieve stresses and "heal” surface damage in

order that the effect of damage introduced upon cutting

is minimized.

An important feature of these materials with regard to their mechanical and electrical behavior is their ability to take up water in the conduction planes [33,34]. In long

term storage in high humidity conditions the effects are

easily measured on even large, high density samples

where the surface area to volume ratio is comparatively

small. Typically in the three block material, the electrical

resistivity increases by up to about twenty percent and

the density of the substance increases [35] . Little change

in the fracture strength is noticed and the effects are

easily reversed by heating in a dry atmosphere. Of

greater importance is the effect of water on the Li 20 or MgO stabilized three block material which has been so processed such as to not be totally converted to the three

block form even though the nominal composition would

require such conversion at equilibrium. Without

conversion other phases can exist, one of which is the

two block form, and from the phase diagram of Weber

[6] , one must presume the existence of a small amount of

something such as sodium aluminate which is soluble in water. Samples of Li2 0 or MgO stabilized /3 "-alumina

neither of which are totally converted to/3 ", exposed to water vapor over several hours up to many days, will swell and fracture. It is also suggested that MgO

stabilized /? "-alumina with more than about 2 weight

percent MgO is especially unstable to water—probably more so than the Li2 0 stabilized substances [36]. Virkar

[37] has found that samples of Li2 0 stabilized /3"- alumina (but not totally converted to the three block

form) show time dependent crack propagation; that is, the fracture strength decreases with increasing time of

load application. This phenomenon could be interpreted

as stress corrosion cracking from atmospheric humidity.

Virkar [38] has also investigated the mechanical

behavior of completely converted Li 2 0 stabilized three

block beta-alumina in search for evidence for stress

corrosion cracking when the material is immersed in

liquid sodium, the corrosive media most commonly

encountered by these materials. He found that the

material is not susceptible to stress corrosion failure in

the presence of liquid sodium for times of a few days or

less, his interpretation being based on the fact that their

data did not fit any reasonably acceptable stress corrosion phenomenology (the stress intensity exponent was far too large.) Thus short term stability to stress corrosion in liquid sodium is established for at least the

compositions of three block material used by Virkar and

thus allows greater confidence in ceramic seal designs via

conventional elastic design criteria. Even though fracture strengths may be found to be

acceptably high, and the material is found stable to degradation by stress corrosion in liquid sodium and

complete conversion to the three block form helps ensure stability to water vapor attack, it is found that

these materials (two and three block) degrade in another

fashion. As Na ion current is passed, a mechanical failure

mechanism is engendered which is correctable with the

sodium ion current density. Thus one tends to assign the

terms "electromechanical degradation” or "degradation

by electrolysis” to the problem.

A critical sodium ion current density appears to exist

for these materials which depends upon composition and

microstructure [39]. In the Li2 0 stabilized three block

system on which most work has been performed, the

critical current density beyond which one observes short

time failure (within a few hours) is a function of the Li2 0

concentration. This failure is manifested by a rapid de- crease in fracture strength, presumably because hypercritical cracks have been introduced into the electrolyte

material.

The final comment on the mechanical behavior of

these substances concerns their resistance to thermal

shock. Rapid air quenching of the three block materials

has shown that mechanical integrity is maintained for a AT (the difference between the initial specimen

temperature and the ambient air) of as great as 1250 °C.

With better heat transfer as in the case of a water

quench, no significant change in elastic modulus or strength has been observed for a A Tof as much as 160 °C

[40]. For larger AT, the modulus and strength drop

abruptly indicating the formation of microcracks of

greater than critical length.

2. Properties: Mass Density, Lattice

Parameters, and Thermal Expansion

2.1. Mass Density

In each of these substances, one expects the density to vary strongly with composition and many compositions

5
are used. In the two block of /3 form, the range of Na2 0

compositions is rather wide while in the three block of /3"

form the Na2 0 variations are smaller but the nature and

concentration of stabilizing additives varies widely.

Further, the three block form has been available in only

polycrystalline microscopic forms and thus porosity

must be accounted for in any mass density determination. X-ray lattice parameter measurements can, of course, be used along with compositional analysis to fix the density.

Peter et al. [5] have measured the mass density of two

block sodium beta alumina of composition Na2.58 A1 21.81 O34 via room temperature lattice parameter measure- ments and have found it to be 3240±20 kgnT3

. The three

block material at room temperature, based on x-ray

lattice parameters of Bettman and Peters [7] and the

nominal chemical formula Na20 MgO- 5A12 0 3 has a density of 3300 ±50 kg m

3

. The actual sample used by

Bettman and Peters was nearer (Na 20) 0 80 (MgO) 0>64 •5A1 20 3 but without a detailed knowledge of the defect

structure, the calculation of a mass density to greater

accuracy cannot be made.

For polycrystalline Li2 0 stabilized three block (/3")

alumina containing 0.7 weight percent Li2 0 and 8.75

weight percent Na2 0 (the balance A12 03 ), hot pressed

samples showing no observable porosity give a density of

3280 ±20 kg nr"3 [13].

2.2. Thermal Expansion

Radzilowski [40] has measured the lattice parameters

of samples of MgO stabilized sodium /?" alumina of

chemical formula (Na2 0) o.so (MgO) o.64-5Al 20 3 as a function of temperature from 25 °C to 900 °C. Table la

gives interpolated values of and a and c lattice parameters

(based on a pseudo-hexagonal lattice rather than the

trigonal lattice) . For sodium /3 -alumina the only reported

data on thermal expansion exists in an NBS note of 1934

and was measured on single crystal samples of unknown

composition. For completeness the data are reproduced on table lb. Apparently the anisotropy in the thermal

coefficient of expansion is much greater in the two block

form than for the three block form.

3. Properties: Electrical Behavior

3.1. Electrical Contacts to Sodium Beta Aluminas

Reversible contacts for sodium ion transport measure- ments have been constructed from liquid sodium

reservoirs, [42], from sodium tungstates [43] and from

sodium amalgams [44] although only the first of these

contacts has been accepted with few reservations.

However, even in the case of liquid sodium contacts, one

finds articles [45,46,47,48] which question the wetting

TABLE la. Basal and axial lattice parameters of sodium /3"-alumina of chemical formula (Na

20)0 .go(MgO)o.64, 5Al203

. Measurements were made at atmospheric pressure. Units: Angstroms (10" u

'm). Reference [40].

Temperature °C

Lattice Parameter

25 100 300 600 900

Basal (a) 5.623 5.625 5.631 5.644 5.657

Axial (c) 33.510 33.528 33.569 33.645 33.717

TABLE lb. Thermal coefficient of expansion for sodium |3 -alumina of unknown chemical composition. Reference [41]. Units: 10

6 K1

Temperature °C

Coefficient

of Expansion

Room temp

to 200 °C

200 °C to 300 °C

300 °C to 400 °C400 °C to 500 °C

500 °C to 600 °C

600 °C to 700 °C

Basal (a) 5.1 6.2 6.0 5.8 5.7 6.0

Axial (cl 5.7 7.0 7.2 7.5 7.7 7.6

6
ability of liquid sodium on polycrystalline material. The

wetting characteristics are evidently a strong function of

surface roughness and the time the samples are held at high temperatures (>~620 K). Very rough surfaces

tend to be difficult for the liquid sodium to wet. Nevertheless it is also apparent that for samples with

large length to cross-sectional area ratios, the partial

wetting of the material is not worrisome in data

gathering, especially for true, four probe dc conductivity

measurements. For thin wall tubes where radial

conductance is of paramount importance, where length

to cross-sectional area ratios are very small and where

high average current densities are encountered, this

wetting problem can be more serious. This can result in

low apparent conductance and/or very high local

current densities. Typically one has to raise the

temperature of the liquid sodium/sodium beta alumina

interface to about 620 K or higher to promote the wetting

of the electrolyte. Subsequently the temperature can be

lowered to the temperature region where measurement is to be made (but, of course, still greater than 370 K, the

melting point of sodium). One must always consider the

possibility that measured resistivities may be current

density dependent. All data reported here are measured

with current densities of the order of only mA-cm-2

. In spite of the fact that data analysis is more complex

than for the dc case, low frequency ac conductivity

measurement techniques have commonly been used for

characterization of the beta aluminas because of the fact that experimental procedures are simpler and because more microscopic information can be obtained than from

simple dc bulk conductivity data. Blocking electrodes

may be used in ac measurements, and for polycrystalline

materials it is possible to determine the average

resistances of both the grains and the grain boundaries.

Details will be discussed below.

Two classes of irreversible or "blocking” electrodes

have been used: (1) those which improve contact

between a metal electrode and the sample by using an

ionic electrolyte containing sodium ions and (2) those

which use evaporated metals, [49] metal paste electrodes

or graphite. In both cases, the frequency which must be

used in the measurement must be high enough so that

the capacitive impedance of any blocking interface is smaller than or comparable to the sample impedance.

Ionic electrolyte and evaporated rpetal electrodes can be

used at frequencies as low as 10 kHz. For metal

electrodes making less intimate contact, frequencies up

to 100 MHz or even higher may be required. Graphite

electrodes have been used at frequencies as low as 20 kHz

[50]. Among the ionic electrodes which have been used

in various temperature ranges are aqueous solutions of

2N NaOH [14], molten salts such as NaN02 -NaN03 eutectic and organic solution such as 0.6M Nal and 0.6M

iodine in propylene carbonate [16,52,53]. One must

realize that these sodium superionic conductors have

relatively low resistivities and that the molten salt or aqueous solution contacts may well have comparable

resistivities. Thus, with thick electrodes, four probe

measurements are advised for accuracy, using voltage

probes which are intimately wetted to the sample by a molten salt or aqueous solution.

Electrodeless techniques have been used for sodium /3- alumina single crystal conductivity measurements at high frequencies. Measurements in a coil at 50 MHz have

been made [54] as well as microwave measurements in

cavities [54,55] and into the infrared [54,56]. These

measurement techniques have the advantage that

conductivity artifacts arising from electrodes are nonexistent. Electrodeless techniques have not been used

for conductivity measurements on industrially useful,

small grain, polycrystalline beta-alumina ceramics. The

reason is quite simple. No adequate theories exist which

will relate cavity dielectric loss to the bulk conductivity

of inhomogeneous conductors. The most relevant

existing theory is one which relates dielectric loss in a

• capacitor geometry to the conductivity of well separated

ellipsoids in an insulating dielectric matrix. The required

theory would be one which would relate dielectric loss for samples which were themselves ellipsoids (or other

convenient geometry) and include partially conducting

grain boundaries.

3.2. Temperature Dependence of the dc Sodium

Ion Conductivity in Single Crystals

3. 2. 1 . Sodium/3 -alumina

Direct current and relatively low frequency ac meas- urements of the resistivity of sodium beta alumina single

crystals grown by flux or fusion methods or obtained

from cutting fusion cast bricks have shown that over a wide temperature regime the resistivity may be accurately described by the relation: [10,43,49]

P =A T exp [E/hT\ (1)

where, for a sample containing 16 % excess sodium ions,

,4 is (4.5±0.2) 10-6 Om K,-1 the activation energy, E, is 15.44±0.75 kj mol

-

*, Tthe absolute temperature and k is Boltzmann s constant [43]. In terms of material

composition, it is generally assumed that the crystal

compositions are near the low soda phase boundary,

(near the nominal composition Na2 0 • 1 1 A12 03

) although

most crystals, when analyzed contain excess soda to place the composition in the range Na20- (9-10Al2 O3 ). Table 2 shows the resistivity as a function of temperature (after Whittingham and Huggins 143]. 1 hese data

7
TABLE 2. Sodium ion electrical resistivity of single crystal sodium

beta alumina at atmospheric pressure. Material contained 16 percent excess sodium ions from 1:11 formula. Reference [43].

Temperature °C -150 -100 25 200 800

Resistivity

(ohm-meters) 2045 3.97 0.72 0.109 0.0312

TABLE 3. Sodium ion electrical resistivity of single crystal MgO

stabilized sodium /8"-alumina. Reference [10].

Temperature °C 20 60 125 225 440

Resistivity

(ohm-meters) 0.21 0.07 0.025 0.011 0.007

were extracted from their graph; electrodes were Na0 . 4WO4 and the field frequency was 10 kHz.

3.2.2. Sodium ft" -Alumina

Since large single crystals of the three block form have

not been routinely available, only limited data on single

crystal conductivity is available. The data of Weber [10] on very small, flux-grown single crystals of the/3" form

stabilized by the addition of magnesia (MgO) of unknown

concentration provide the only low frequency direct

measurements known. Data were taken in the temperature range of 350 K to 870 K. The general behavior of the

temperature dependence of conductivity is roughly

corroborated by indirect measurements on polycrystalline samples where the polycrystalline effects (grain

boundaries) have been subtracted out [57]. The two

main features of the data on the )3 " single crystals in

comparison with that on single crystals of the two block

(/?) form are (1) the resistivity is generally lower and (2)

the diffusional form of equation (1) is not obeyed. The

material apparently exhibits a conductivity which

increases with increasing temperature but at a decreasing

rate. As temperature is increased one finds, for instance,

resistivity activation energies ranging from 30±5 kj

mol near room temperature to 11±2 kj mol” ’near 300

K. The resistivity, taken from graphical data by Weber is found in table 3. Electrodes were platinum and the field frequency was 1 MHz.

One should note that there is some indirect evidence

[57] which supports Weber’s data on the "curvature” of

the Arrhenius plot for sodium ft "-alumina resistivity

which has been gathered from polycrystalline lithia (Li2 0) stabilized material instead of MgO stabilized material. These data, corrected by a reasonable tortuosity

factor of 1-2, would tend to point to the suggestion that

this particular lithia stabilized material may have

intrinsic sodium ion conductivity as little as one third

that of the conductivity of the MgO stabilized material

used by Weber. Since the stoichiometry range for the

sodium P" alumina is apparently very narrow, the

sodium ion concentrations for both the lithia and

magnesia stabilized material are likely similar, the

possible difference in sodium ion concentrations is not

able to account for the difference in conductivities.

3.3. Frequency Dependence of the Impedance of

Single Crystal Sodium ft and /3" -Alumina

Since single crystal samples of only the two block form

have been available, all data on the frequency dependence of the real and imaginary parts of the crystal impedance have been carried out on the two block material. Frequency ranges covered, usually in steps of only a few decades by a single measuring technique, include dc

and extend into the infrared. At relatively high

frequencies near 1800 GHz at 4.2 K to 350 K a large die- lectric resonance is observed in the conductance or imaginary portions of the dielectric constant [54,55].

The magnitude and position of this conductance peak is relatively independent of temperature. However, at 300

K a second conductance peak has been observed at ~ 500

GHz [54] which is not present at low temperatures and

has not yet been explained. It is logical to interpret the

results of these high frequency measurements of

conductivity in terms of a collective response of the

mobile sodium ions rather than in terms of a single

particle model.

Below about 10 GHz the conductivity of single crystal

sodium beta alumina is found to be almost frequency

independent from temperatures as low as 4.2 K up to 300

K.

3.4. Sodium Ion Resistivity as a Function of

Temperature in Polycrystalline Sodium Beta and

Beta"-Alumina

3.4.1. Two Block Sodium Beta Alumina

Since the two block form may be prepared as a stable

substance without additives, dc and low frequency (<10

MHz) sodium ion resistivities characteristic of the nearly

pure (undoped) substances are available. However, since

processing variables can strongly influence the conductivity results, one must take great care in specifying: (1)

the average soda concentration, (2) the concentration

profile of soda, especially near surfaces where soda loss

8
may have occurred during preparation, and (3) the

microstructure of polycrystalline bodies both in the bulk

and near surfaces. Resistivities of the two block

substances at 573 K may vary from 0.025 to 0.11 fl m,

depending on the methods of sample preparation. Table 4

gives low frequency values for the resistivity of

polycrystalline sodium beta aluminas over the narrow

temperature range of 526 K to 667 K [58]

Almost universally one finds that the resistivities for

polycrystalline beta aluminas do not obey the relation

given in equation (1) over wide temperature ranges. The

apparent conductivity activation energies decrease with

increasing temperature in a manner similar to the single

crystal three block material and range from about 30 kj

mol -1 at 25 °C to about 14.6 kj mol-1 at 300 °C (±4 kj

moF 1 )- The apparent temperature dependent activation

energy can be attributed to the effect of grain boundary

regions which are electrically in series with the grains.

The activation energy associated with the grain

boundaries is usually significantly higher than that

associated with the grains and single crystals. Thus, the

higher the temperature the less the grain boundaries

affect the total conductivity, while at low enough

temperatures the grain boundaries can dominate the

conductivity.

3.4.2. Three Block Sodium Beta Alumina

Assuming that the ceramic processing steps used in

making polycrystalline three block sodium beta alumina

cause essentially complete conversion to three block

form (> 95% j3" ) , the resulting resistivities may be made

reasonably reproducible and almost independent of the

concentration of stablilizing additive (Li

20 or MgO or combinations of the two). The assumption is made that

reasonably high purity material has been used to prepare

samples so that stray impurity effects, which often show

up as enchanced grain boundary resistance, are not

present.

Unfortunately, because the present primary use of

these materials is in high temperature energy storage and

conversion devices, reported resistivity data are most

prevalent for temperatures of about 250 °C to 400 °C

where the resistivity is low enough to allow the design of

practical devices. Within this range of temperatures, the

TABLE 4. Sodium ion electrical resistivity of polycrystalline tubular forms of sodium j3 -alumina containing 8. 1 1 weight percent

Na20. Reference [581.

Temperature °C 253 300 350 394

Resistivity

(ohm-meters) 0.220 0.166 0.137 0.116

TABLE 5. Sodium ion electrical resistivity of completely converted

Li20 stabilized sodium /?"-alumina. Electrodes were Nal or

I in propylene carbonate for low temperature measure- ments and NaN03-NaN0 2 eutectic electrodes for the 300

°C measurements. The field frequency was about 30 kHz

and the composition was 0.75 wt. % Li 20, 8.75 wt. % Na 20,

90.5 wt. % ALO3 . Reference [38],

Temperature °C - l 22 79 r

. -i

138 202 300

Resistivity

(ohm-meters) 6.7

1

1.0 0.3 0.10 0.045

1

resistivity, excluding any preferred orientation effects can be approximated by an Arrhenius relation. Resistivity activation energies lie in a range of 13 to 30 kj mol \ Resistivities at 573 K, the most common reporting

temperature, lie in a range of 0.02 to 0.06 ohm-meters

with most materials (production types) showing resistivities in the range of 0.03 to 0.05 ohm-meters at 573 K.

(Table 5 gives resistivity data measured on polycrystalline Li 20 stabilized sodium y3" aluminas, the electrodes

being NaN02 -NaN03 electrodes for measurements near 300 °C and Nal or I in propylene carbonate for low

temperature measurements.) The low resistivity of 0.02

ohm-meters is many times associated with a coarse grained microstructure or at least a large volume fraction

of very large elongated grains within a fine grain matrix

(duplex microstructure). For the sake of optimum mechanical integrity, this microstructure condition is generally to be avoided although avoidance is often

difficult to achieve. Ormrod and Kirk [59] have meas- ured the conductivity of sodium yS” alumina stabilized

with either Li2 0 alone or with mixtures of Li 2 0 and MgO

at temperatures between about 226 °C and 400 °C and

have developed relations for the resistivity.

Mixed dopants (Li 2 0-MgO):

p=3.9- 10 4 exp [(23.84±0.19) k]/kT\ ohm-meters

Li 2 0 dopants:

p=5.8 • 10”4 exp [(21.81±0.19) k]/kT\ ohm-meters

Data for development of these equations was taken at 15

kHz with molten sodium salts. Silver blocking electrodes were also used by Ormrod

and Kirk to determine the resistivity as a function of

temperature between 25 °C and 800 °C at a field frequency of 500 kHz. The relation developed was highly

consistent with that obtained at 15 kHz with sodium salt electrodes.

Mixed Dopants (Li

2 0-MgO):

p= 5.0 -10 4 exp [23.64±0.19) k]/kT) ohm-meters

9
One should note that these authors experienced some

serious polarization losses in using the sodium tungsten

bronzes as suggested by Whittingham and Huggins, [43]

giving much higher resistivities (an order of magnitude)

than when the silver electrodes were used.

3.4.3. Equilibrium Mixed and Three Block Sodium Beta Alumina

With soda concentrations in the range of about 6.8 to about 8.6 weight percent and usually with the addition of one of the previously mentioned additives one can pro- duce material in the polycrystalline form which consists

of a mixture of ft and /6" phases. Generally one can consider the electrical conductivity characteristics in the dc

or low frequency regions to be intermediate between the

cases, of the pure /3 or pure p" phases [28].

3.5. Electronic Conduction in the Sodium (3 and

/T'-Aluminas

The contributions to conductivity by electronic

transference are not well known for these materials.

Generally, one can deduce that the electronic transfer- ence number is less than about 10“2 at 573 K to 673 K

because the Faradaic efficiency for sodium ion transfer

in sodium cells is nearly 100 percent. The only

reasonably accurate values thus far reported are those by

Weber [3] and relate to the three block material in

polycrystalline form with duplex microstructure and

containing Li2 0 as a stabilizing element. Weber has

calculated the electronic resistivity from sodium/sodium thermoelectric generator cells in which total electrical currents were measured and ionic evaporation

currents were subtracted. His values for electronic

resistivity vary from about 1000 ohm-meters at 752 K to about 10 ohm-meters at 979 K (table 6). The electrodes

placed upon the electrolyte tubes were composed of

porous molybdenum.

One expects that the magnitude of electronic

conductivity will be dependent on impurity additions

and temperatures as well as such other variables such as oxygen pressure, etc., but little data are reported. The

lack of data in this area and especially the lack of data as a function of common impurity contents and microstrucTABLE 6. Electronic resistivity of L^O stabilized sodium ^"-alumina

as calculated from thermoelectric generator cell data. Reference [3].

Temperature °C 479 578 706

Resistivity

(ohm-meters) 1000 100 10

tural change makes research on electronic conduction in

these normally ionic conductors imperative. Conduction

by electrons or holes will be of extreme interest in the use

of these materials for two primary reasons. The first, of course, involves electronic shorting of the ion conductor. Reasonable design limits on the allowable electronic

conductance can be calculated for any given device.

Second, for polycrystalline forms formed by sintering

processes, one always encounters the existence of

defects, especially pores. Small but non-zero electronic

conductance can, over a long period of time, allow the

formation of a metal (sodium in this instance) at free

surfaces such as the surface of pores or pre-existing

cracks. Electrically this can result in effectively shorting

the ion current in local regions if the porosity is connected or in pressure buildup in the pores over a period of time.

4. Properties: Mechanical Behavior

4.1. Young’s Modulus

The elastic moduli of these materials have as yet not

been carefully measured on single crystals and only

limited data are available on polycrystalline samples.

Since the modulus is expected to decrease rapidly with

porosity, care must be taken to use dense material for

characterization. A lone value for Young’s modulus of

sodium beta alumina was used in the work of Stevens

[60] and given as 1.6 • 10 1

N m-2. The source of this value

is, however, unknown. Measurements of Young’s

modulus on samples of L^O stabilized sodium ft "- alumina of density greater than 3200 kg m“3 using

longitudinal wave resonance and electromagnetic

excitation [61] show a value of 1.96 • 10 ± 0.14'10 n N

m-2. These values for both the two and three block forms were measured at 25 °C.

4.2. Fracture Strength

The fracture strength of both forms is a strong

function of grain size, grain size distribution and the

method of measurement. Since ceramics fail at lowest

stress under tension and pre-existing flaws in the ceramic

often determine the stress for fracture, one must take

care to sample a volume under tension which contains

the average flaw density. As a result, diametral testing

ususally gives average fracture stresses (over many

samples) which are higher than in, say, three or four

point bending where the volume of material under ten- sion is large. For either two or three block material in

four point bending, composed of uniform, fine grained

material with mean grain size less than 2-3 ju m, the

fracture strength at room temperature is often as high as 2.9 *10 8 N m-2

; for large grain material (mean grain size

10
200 fJL m) the fracture strength is as low as 1.0 • 10 8 Nm-2

760,62,63).

4.3. Fracture Strength-Electromechanical

Degradation

The effect ot electric current flow on the fracture

strength of Li2 0 stabilized sodium (3 "-alumina has been

studied via the following test [39] : Samples of material were contacted with NaN02 -NaN03 eutectic salt electrodes at 300 °C and a predetermined current density was passed for a period of 10 minutes, after which the

fracture strength was determined. As shown in table 7,

the average fracture strength decreases with sodium ion

current density, dropping to less than half its preelectrolysis strength when a current of 1.75 ’lO4 A nr2 has been passed. One should note that this effect is a function of microstructure and the composition of the

material. Demott and Redfern [64] have found that the

three block form stabilized with Li20 appears to be far more stable than that made with the mixed additives

(Li 2 0-MgO) of similar microstructures. As much as 3-104 Am-2 can be passed through tubes of their material over long periods of time (corresponding to charge

transfer of 107 ampere hours per m2

) without degradation. The critical current density, above which degradation occurs, decreases drastically as temperature is lowered. Very high Li

2 0 concentrations cause degradation at lower current densities. This behavior is not

unique to the three block from but also occurs in the two

block material [65]. It is also evident that the phenomenon of cracking proceeds from the electrolyte-electrode

interface, dejonghe [66] has prepared samples of "dog

bone” shape which allowed as much asl(T 5Am-2 to be

passed through the small area neck of the sample and no

mechanical degradation has been observed. To date

several models have arisen to explain this electromechanical degradation [64,67,68,69].

TABLE 7. Fracture strength of polycrystal LhO stabilized sodium (3

"- alumina after electrolysis for 10 minutes at given current

density in NaN02-NaN03 salt electrodes at 300 °C. Reference [39].

Current Density

A m

-2 0 M04 1.25 -104 1. 5-104 1.75- 104

Fracture Strength

N m-2 1.7M08 1.3 • 108 1. MO8 0.80 -108 0.72 -108

4. Summary

It is quite clear that these materials show great

promise as economical fast sodium ion conductors but

the potential user must be wary of the pitfalls that are encountered in incomplete specification of material

characteristics. Application of these materials to devices

requires several properties that are simultaneously

optimized, the two that are most important being

electrical conductance for sodium ions and good mechanical properties. It is somewhat unfortunate that

insofar as presently available data is concerned, the mechanical characteristics of the polycrystalline three

block material are far better understood than those of the

two block form whereas the relative knowledge of the

electrical characteristics is in an inverted state. Presumably this state of affairs is traceable to the recent commercial exploitation of the three block form and the

availability of single crystals of the two block form which

do not require the addition of a

' complicating” additive.

Fortunately significant progress has been made at several

laboratories to make available polycrystalline forms of

both high strength and low electrical resistivity.

Six major areas in which data are lacking for both

substances are: (1) the electronic transference number

and its dependence on temperature and oxygen partial

pressure, (2) the microstructure-conductivity relations,

(3) the mechanism for electromechanical degradation

under high current densities (4) electrical and mechanical data on single crystal (3 "-alumina, (5) thermal

coefficient of expansion of single crystal (3 -alumina and

(6) thermal conductivity and thermal diffusivity on both

types of material. Although data are beginning to appear

regarding electromechanical failure, precise modeling of

the phennomena is required before lifetime extrapolations will be possible and even then the ability to model

and make lifetime extrapolations may depend strongly on

the nature of the electrodes with which these materials

are in contact. A need still exists for careful, systematic studies of the

properties of these materials. Characterization of samples, careful calibration of equipment, running of standards, etc., are too often neglected. As a result, the data

reported here must be considered approximate and

tentative, to be replaced by data taken in future studies.

References

[ 1] Kummer, J.T., and Weber, N., Trans S.A.E. 76, 1003 (1968).

[ 2] Weber, N., Energy Conversion 14, 1 (1974).

[ 3] Hunt,T.K„ Weber, N., and Cole, T., Tenth IECEC Proceedings

(1976), p. 23.

[ 4] Lundsgaard, J.S., J. Mater. Sci. 9, 2061 (1974).

[ 5] Peters, C.R., Bettmann, M., Moore, J.W., and Glick, M.D., Acta

Cryst. B27, 1826 (1971).

[ 6] Yamaguchi, G., and Suzuki, K., Bull. Chem. Soc. Jap. 41, 93

(1968).

[ 7] Bettman, M., and Peters C.R., J. Phys. Chem. 73, 1774 (1969).

[ 8] Weber, N., and Venero, A., unpublished work. Ford Motor Company.

11
[ 9] Weber, N., and Venero, A., unpublished work, Ford Motor Com- [44] pany.

[10] Kummer, J.T., Prog. Sol. State Chem. 7, 141 (1972). [45]

[11] Roth, W.L., General Electric Technical Information Series No. 74CR0054 (1974).

[12] Ohta, T., Harata, M., and Imai, A., preprint of article to be submitted for publication. [46]

[13] Youngblood, G.E., Miller, G.R., and Gordon, R.S., J. Am. Ceram.

Soc., 61,86 (1978). [47]

[14] Byckalo, W., Rosenblatt, G., Lam, J., and Nicholson, P.S., Ceram. Bull. 55, 288 (1976).

[15] "Development of Sodium Sulfur Batteries for Utility Application,” Section E-3, EPRI Project Report EPRI EM-266 (1976). [48]

[16] Powers, R., and Mitoff, S.P., J. Electrochem. Soc. 122, 266

(1975). [49]

[17] Virkar, A.V., Miller, G.R., and Gordon, R.S., to be published in J. [50] Am. Ceram. Soc. (1978).

[18] "Sodium-Sulfur Battery Development,” EPRI Project Report, [51] EPRI 128-2, Sept. (1975).

[19] Hooper, A., J. Phys. D: Appl. Phys. 10, 1487 (1977).

[20] dejonghe, L.C., "Substructure and Properties of Sodium Beta- [52] Alumina Solid Electrolytes,” EPRI Report EPRI EM-494 [53] (1977).

[21] "Sodium-Sulfur Battery Development for Bulk Power Storage,”

EPRI Report 128-2, Sept. (1975).

[22] Imai, A., and Harata, M., Jap. J. Appl. Phys. 11, 80 (1972).

[23] Youngblood, G.E., Virkar, A.V., Cannon, W.R., and Gordon,

R.S., Ceram. Bull. 56, 206 (1977).

[24] Rasmussen, J.R., M.S. Thesis, University of Utah (1978).

[25] Tennenhouse, G.J. (private communication). [54]

[26] Yasui, I., and Doremus, R., J. Am. Ceram. Soc. 60,296 (1977).

[27] Kummer, J.T., and Whittingham, M.S., Unpublished report. [55]

[28] Haar, W., Fischer, W., Kleinschmager, H., and Weddigen, G., preprint of arpcle to be submitted for publication. [56]

[29] "Development of .Sodium-Sulfur Batteries for Utility Application,” Section C, EPRI Report EPRI EM-266 (1976). [57]

[30] Stevens, R., J. Mater. Sci., 9, 934 (1974).

[31] Tan, S.R., and May, G.J., J. Mater. Sci., 12, 1058 (1977). [58]

[32] Virkar, A.V., and Gordon, R.S., J. Am. Ceram. Soc., 60, 59

> (1977).

[33] Kline, D., Story, H.S., and Roth, W.L., J. Chem. Phys. 57, 5180

(1972). [59]

[34] Will, F.G., J. Electrochem. Soc., 123, 834 (1976).

[35] "Research on Electrodes and Electrolyte for the Ford Sodium- [60] Sulfur Battery,” Annual Report to NSF-RANN under contract [61] NSF-C805, July (1976).

[36] Although formally unreported, this observation has apparently been made by several workers in the field. [62]

[37] "Fabrication and Characterization of /3 "-Alumina Electrolytes [63] and Electronic-Ceramic Containers for the Sodium-Sulfur

Battery,” Semiannual Report to Ford Motor Co. (1976) —Uni- [64] versity of Utah.

[38] "Report on Development of )3 "-Alumina Electrolytes for the [65] Sodium-Sulfur Battery,” Report to Ford Motor Company,

under ERDA Contract #EY-76-C-02-2566, University of Utah,

July 1977. [66]

[39] "Research on Electrodes and Electrolyte for the Ford SodiumSulfur Battery,” Annual Report to NSF-RANN contract NSF [67] C805 (July 1974).

[40] Radzilowski, R.H., J. Am. Ceram. Soc. 53, 699 (1970).

[41] National Bureau of Standards Notes, J. Franklin Inst. 217, 623

(1934). [68]

[42] Imai, A., and Harata, M., Jap. J. Appl. Phys. 11, 180 (1972).

[43] Whittingham, M.S. , and Huggins, R. A., J. Chem. Phys. 54, 414 [69] (1971).

12

Armstrong, R.D., Dickinson, T., and Turner J., J. Electrochem.

Soc., 1135 (1971). Gibson, A., "A Study of the Electrode Interface Between Sodium

Metal and Beta Alumina Solid Electrolyte,” Proceedings of the Tenth International Power Sources Symposium, Brighton

(1976), paper no. 43. Miles, L.J., and Jones, I.W., "Na Transport Through /3 -A^Oj

Membranes,” Proc. Brit. Ceram. Soc. 19, 179 (1971). Sudworth, J.L., Tilley, A.R., and South, K.D., "Polarization Losses in Sodium/Sulfur Cells,” Fast Ion Transport in Solids—

Solid State Battery Devices, W. Van Gool, North Holland Publishing Co., Amsterdam (1973) p. 581. Armstrong, R.D., Dickinson, T., and Turner, J., Electroanal. Chem. and Interfacial Electrochem., 44, 157 (1973). Hooper, A., J. Phys. D 10, 1487 (1977). Baukal, W., and Knoedler, R., J. Appl. Electrochem., 5, 105

(1975). "Research on Electrodes and Electrolyte for the Ford SodiumSulfur Battery,” Annual Report to NSF-RANN for contract NSF C805 (July 1975;. Farrington, G.C., J. Electrochem. Soc. 121, 120 (1974). Rasmussen, J.R., Virkar, A.V., Miller, G.R. and Gordon, R.S., "Effect of Grain Shape on the Conductivity of Polycrystalline

Sodium (3 "-Alumina” . To be submitted to J. Electrochem. Soc.

(1978). This paper is also included in "Fabrication and

Characterization of P "-Alumina Electrolytes and Electronic Ceramic Containers and/or Current Collectors for the SodiumSulfur Battery”, Annual Report to the Ford Motor Company

for NSF-RANN C805, February 1978. Strom, U., Taylor, P.C., Bishop, S.G., Reinecke, T.L., and Ngai,

K.L., Phys. Rev. B13, 3329 (1976). Barker, A.S., Ditzenberger, J.A., and Remeika, J.P., Phys. Rev. B14, 4254 (1976L

Barker, A.S., Ditzenberger, J.A., and Remeika, J.P., Phys. Rev. B14, 386 (1976). Virkar, A.V., Miller, G.R., and Gordon, R.S., to be published in J. Am. Ceram. Soc. (1978). "Improvements of Electrolyte and Seal Technology for SodiumSulfur and Sodium Antimony Trichloride Load Leveling

Batteries,” Yvon Lazennec. Interim Research Project Report

to the Electric Power Research Institute 726-1 (1976). Ormrod, S.E., and Kirk, D.L., J. Phys. D.: Appl. Phys. 10, 1769

(1973). Stevens, R., J. Mater. Sci., 9, 934 (1974). "Report on Development of P "-alumina Electrolytes for the Sodium-Sulfur Battery,” Annual Report for Phase II to Ford Motor Co. from Univ. of Utah (1977). Tan, S.R., and May, G.J., J. Mater Sci., 12, 1061 (1977).

Virkar, A.V., and Gordon, R.S., J. Am. Ceram. Soc., 60, 58

(1977). Demott, D.S., and Redfern, B.A.W., J. de Physique 37, C7-423

(1976). dejonghe, L.C., reported at the International Conference on the Science of Ceramics—Ceramic Processing, North Carolina

State University, Raleigh, November (1977). Lazennec, Y., Lasne, C., Margotin, P., and Fally, J., J. Electrochem. Soc. 122, 734 (1975). Shetty, D.K., Virkar, A.V., and Gordon, R.S., "Electrolytic

Degradation of Lithia Stabilized Polycrystalline )3"-alumina,”

Fracture Mechanics of Ceramics, Vol. 4, Plenum Publishing

Corp., New York (1978), p. 651. Richman, R.H., and Tennenhouse, G.J., J< Am. Ceram. Soc. 58, 63 (1975). Tennenhouse, G.J., Ku, R.C., Richman, R.H., and Whalen, T.J., Ceram. Bull. 54, 523 (1975).

* u. GOVERNMENT PRINTING OFFICE: 979 O- 278-287
NBS-114A (REV. 9-78)

U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO. NSRDS-NBS 61, Part 32, Gov't Accession No.- 3. Recipient s Accession Ho.

4. TITLE AND SUBTITLE

Physical Properties Data Compilations Relevant

to Energy Storage

III. Engineering Properties of Single and Polycrystalline Sodium Beta and Beta"- Alumina

5. Publication Date June 1979

S. Performing Organization Code

7. AUTHOR(S) 8. Performing Organ. Report No. G. R. Miller and D. G. Paquette

9. PERFORMING ORGANIZATION NAME AND ADDRESS Project/Task/Work Unit No.

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, DC 20234

11. Contract/Grant No.

12. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street, City, state, ZIP)

Same as Item 9

13. Type of Report & Period Covered N/A

14. Sponsor!

15. SUPPLEMENTARY NOTES

Library of Congress Catalog Card Number: 77-10824

| | Document describes a computer program; SF-185, FIPS Software Summary, is attached.

16. ABSTRACT (A 200-word or ieas factual summary of most significant information. If document includes a significant bibliography or

literature survey, mention it here.) A review of the properties of the two dimensional superionic conductors of

the sodium beta alumina family is presented, with emphasis on the variability

of properties with composition and processing. Processing methods , including methods of distributing additives, may strongly influence engineering

properties of these substances through changes in microstructures. Described

are common methods of measurement of properties with generally accepted

ranges of property values. The need to compromise in the design of devices

requiring several coincident optimal properties is, at this time, a necessary

ingredient in the use of these materials as device components.

17. KEY WORDS (six to twelve entries ; alphabetical order; capitalize only the first letter of the first key word unless a proper name;

separated by ^em, colons)

ga^ery application; engineering properties data; evaluated data;

polycrystalline; single crystal; sodium beta alumina; sodium beta double

prime alumina.

18. AVAILABILITY [xl Unlimited 19. SECURITY CLASS 21. NO. OF

(THIS REPORT) PRINTED PAGES

1 1 For Official Distribution. Do Not Release to NTIS UNCLASSIFIED

19

[xl Order From Sup. of Doc., U.S. Government Printing Office, Washington, DC

20402, SD Stock No. '003-003 -0207 9 -6

20. SECURITY CLASS 22. Price (THIS PAGE)

$1.25

1 1 Order From National Technical Information Service (NTIS), Springfield,

V A. 22161 UNCLASSIFIED

USCOMM* DC
Where can you find

all the reference data

you need?

Right in the

Journal of Physical

and Chemical

Reference Data!

Now in its sixth year, this valuable publication has proved that it fills the important gaps for you in the literature of the

physical sciences. Published by the American Institute of Physics and the American Chemical

Society for the National Bureau of Standards, this quarterly gives you quantitative numerical data, with recommended

values and uncertainty limits chosen by

experts in the field.

Critical commentary on methods of measurement and sources of error, as well as full references to the original litera- ture, is an integral part of each of your

four issues a year. Can you afford to be without this prime

source of reliable data on physical and

chemical properties? To start receiving your copies, just fill in the order form and

drop into the mail. If you do use a pur- chase order, please attach the printed form

as this will help us to expedite your order. Send for complete list of reprints!

Journal of Physical and Chemical Reference Data American Chemical Society 1155 Sixteenth Street, N.W., Washington, D.C. 20036

Yes. I would like to receive the JOURNAL OF PHYSICAL AND CHEMICAL REFERENCE DATA at the one-year rate checked below:

Name

Home

„ Business

Street —

City State Zip

Members Nonmembers

U.S., Canada. Mexico

Other Countries

$25.00 $100.00

$29.00 $104.00

Bill me Bill company or school Payment enclosed Please Attach This Order Form To Purchase Order.
Announcement of New Publications in

National Standard Reference Data Series

Superintendent of Documents,

Government Printing Office,

Washington, D.C. 20402

Dear Sir:

Please add my name to the announcement list of new publications to be issued

in the series: National Standard Reference Data Series — National Bureau of

Standards.

Name

Company

Address

City State Zip Code

(Notification Key N-519)

NBS TECHNICAL PUBLICATIONS

PERIODICALS

JOURNAL OF RESEARCH—The Journal of Research

of the National Bureau of Standards reports NBS research

and development in those disciplines of the physical and

engineering sciences in which the Bureau is active. These

include physics, chemistry, engineering, mathematics, and

computer sciences. Papers cover a broad range of subjects,

with major emphasis on measurement methodology, and

the basic technology underlying standardization. Also in- cluded from time to time are survey articles on topics closely

related to the Bureau’s technical and scientific programs. As

a special service to subscribers each issue contains complete

citations to all recent NBS publications in NBS and non- NBS media. Issued six times a year. Annual subscription:

domestic $17.00; foreign $21.25. Single copy, $3.00 domestic;

$3.75 foreign. Note: The Journal was formerly published in two sections: Section A “Physics and Chemistry” and Section B “Mathematical Sciences.”

DIMENSIONS/NBS

This monthly magazine is published to inform scientists, engineers, businessmen, industry, teachers, students, and

consumers of the latest advances in science and technology,

with primary emphasis on the work at NBS. The magazine

highlights and reviews such issues as energy research, fire protection, building technology, metric conversion, pollution

abatement, health and safety, and consumer product per- formance. In addition, it reports the results of Bureau pro- grams in measurement standards and techniques, properties

of matter and materials, engineering standards and services,

instrumentation, and automatic data processing.

Annual subscription: Domestic, $11.00; Foreign $13.75

NONPERIODICALS

Monographs—Major contributions to the technical liter- ature on various subjects related to the Bureau’s scientific and technical activities. Handbooks—Recommended codes of engineering and industrial practice (including safety codes) developed in cooperation with interested industries, professional organizations,

and regulatory bodies. Special Publications—Include proceedings of conferences

sponsored by NBS, NBS annual reports, and other special

publications appropriate to this grouping such as wall charts,

pocket cards, and bibliographies.

Applied Mathematics Series—Mathematical tables, manuals, and studies of special interest to physicists, engineers,

chemists, biologists, mathematicians, computer programmers,

and others engaged in scientific and technical work.

National Standard Reference Data Series—Provides quantitative data on the physical and chemical properties of

materials, compiled from the world’s literature and critically evaluated. Developed under a world-wide program co- ordinated by NBS. Program under authority of National

Standard Data Act (Public Law 90-396).

NOTE: At present the principal publication outlet for these data is the Journal of Physical and Chemical Reference

Data (JPCRD) published quarterly for NBS by the American Chemical Society (ACS) and the American Institute of

Physics (AIP). Subscriptions, reprints, and supplements

available from ACS, 1155 Sixteenth St. N.W., Wash., D.C.

20056.

Building Science Series—Disseminates technical information

developed at the Bureau on building materials, components,

systems, and whole structures. The series presents research

results, test methods, and performance criteria related to the

structural and environmental functions and the durability

and safety characteristics of building elements and systems.

Technical Notes—Studies or reports which are complete in themselves but restrictive in their treatment of a subject.

Analogous to monographs but not so comprehensive in scope or definitive in treatment of the subject area. Often

serve as a vehicle for final reports of work performed at NBS under the sponsorship of other government agencies.

Voluntary Product Standards—Developed under procedures

published by the Department of Commerce in Part 10, Title 15, of the Code of Federal Regulations. The purpose

of the standards is to establish nationally recognized requirements for products, and to provide all concerned interests with a basis for common understanding of the characteristics of the products. NBS administers this program as a supplement to the activities of the private sector standardizing

organizations.

Consumer Information Series—Practical information, based on NBS research and experience, covering areas of interest to the consumer. Easily understandable language and

illustrations provide useful background knowledge for shopping in today’s technological marketplace.

Order above NBS publications from: Superintendent of

Documents, Government Printing Office, Washington, D.C.

20402.

Order following NBS publications—NBSIR’s and FIPS from

the National Technical Information Services, Springfield, Va. 22161.

Federal Information Processing Standards Publications

(FIPS PUB)—Publications in this series collectively consti- tute the Federal Information Processing Standards Register.

Register serves as the official source of information in the

Federal Government regarding standards issued by NBS

pursuant to the Federal Property and Administrative Services Act of 1949 as amended. Public Law 89-306 (79 Stat. 1127), and as implemented by Executive Order 11717

(38 FR 12315, dated May 11, 1973) and Part 6 of Title 15

CFR (Code of Federal Regulations).

NBS Interagency Reports (NBSIR)—A special series of

interim or final reports on work performed by NBS for outside sponsors (both government and non-government).

In general, initial distribution is handled by the sponsor;

public distribution is by the National Technical Information

Services (Springfield, Va. 22161) in paper copy or microfiche

form.

BIBLIOGRAPHIC SUBSCRIPTION SERVICES

The following current-awareness and literature-survey bibli- ographies are issued periodically by the Bureau:

Cryogenic Data Center Current Awareness Service. A litera- ture survey issued biweekly. Annual subscription: Domestic, $25.00; Foreign, $30.00.

Liquified Natural Gas. A literature survey issued quarterly.

Annual subscription: $20.00.

Superconducting Devices and Materials. A literature survey

issued quarterly. Annual subscription: $30.00. Send subscription orders and remittances for the preceding bibliographic

services to National Bureau of Standards, Cryogenic Data

Center (275.02) Boulder, Colorado 80302.
U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Washington, D.C. 20234

OFFICIAL BUSINESS

Penalty for Private Use, $300

POSTAGE AND FEES PAID

U.S. DEPARTMENT OF COMMERCE

COM-2 1 5

SPECIAL FOURTH-CLASS RATE

BOOK