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Beta-alumina
The new area of research emerged from industrial research
in the Ford Motor Company at Dearborn, Michigan where three researchers
Yung-Fang YuYao, J. T. Kummer and Neill Weber reinvented
b-alumina. Two articles revealed that b-alumina's structural peculiarities allowed
a high ionic conductivity and that consequently b-alumina
could be used in some applications as a substitute to traditional liquid
electrolyte. In addition to a high ionic conductivity, b-alumina presented other attractive properties
such as small electronic transfer, a mandatory characteristic for a
substance to be used as an electrolyte, as well as chemical passivity.
Moreover, it could be easily shaped (Yao Y.F.Y, and Kummer, J.T. Ion
Exchange properties of and rates of ionic diffusion in Beta-Alumina
J. Inorg Nucl. Chem29 (1967)
2453.-2475.Weber N., Kummer J.T. ,Sodium-Sulfur Secondary batteries,
Proceedings of the 21st
Annual. Power Sources Conference . 21 (1967) 37-39). These two publications
sparkled a booming activity of research all around the world, in the
early 1970s when the oil crisis raised the concern with energy. Two distinctive features characterize the science of
b-alumina. First, there was an immediate shift from the industrial
impetus to theoretical issues. Between 1967 and 1970 most of the research
on b-alumina was devoted
toward a sodium-sulfur cell for load-leveling and electrical vehicles.
Then b-alumina, together with another famous solid electrolyte,
silver iodide (AgI), known for a much longer time, but having less interesting
perspectives of applications, became a model-material for developing
methods of investigation and understanding the physical laws of high
ionic conductivity. In the series of 8 conferences on Fast Ion
Transport in Solids starting in Belgirate 1972 and ending in Belgirate
in 1992, one can see that in the theoretical section most of the papers
dealt with b-alumina in the 1970s. Practical problems lead
to fundamental research. It soon became clear that two characteristics
of the material, namely non-stoichiometry and structural disorder, play
predominant roles in the explanation of its exceptional ion transport
properties. Non-stoichiometry is characterized by the fact that the
material composition is not set to a unique value, as can be seen from
the wide range of values that the x parameter can take in the formula
11 Al2O3 x Na2O. In 1976, William
L. Roth proposed a new structural model that accounted for non-stoichiometry.
Basically, he showed that many cristallographic sites are available
for sodium ions, in contrast with former descriptions which had identified
only one possible site for each sodium ion. In other words, each sodium
"has the choice" between various sites, and, when located
on a given site, it "sees" several empty sites in its neighborhood.
This was an important step, but it remained to understand why jumping
to an empty site requires such a small amount of thermal energy that
it can occur at room temperature. Two years later, Colomban and coworkers,
the first chemists to accomplish the synthesis of the stoichiometric
compound (i.e. with x equal to 1), showed that this material has poor
ionic transport properties. This crucial experimental result gave strong
support to the idea of cooperative jumps involving two or more sodium
ions simultaneously, which was instigated by Whittingham
and Huggins in 1971 and developed by Wang and co-workers in 1975. Put simply, non-stoichiometry allows
for the formation of sodium ion pairs or small clusters that move as
a whole. The energy per ion required for an elementary process is thus
smaller than if each individual ions had to move for its own. The key
principles of fast ion transport in b-alumina
were thus settled by the end of the 70s, but a lot of work remained
to be done in order to get a
comprehensive view of the detailed structural and physical properties.
In the early 1980s when the journal Solid
State Ionics was created there were as many research activities
conducted in industrial laboratories as in academic settings. The second distinctive feature of b-alumina research was the collaboration between
physicists and chemists. Physicists were crucial for the in-depth characterization
of the b-alumina
structure and for the understanding of its dynamics. The problems that
solid state physicists had to tackle with were new, because they were
trained to study the dynamics of electrons more than those of ions.
The role played by b-alumina and other superionic conductors in the
development of new experimental methods and techniques is very important
for the history of solid state science. Most of the nuclear magnetic
resonance (NMR) and relaxation techniques, of the methods of X-ray and
neutron diffraction and diffusion, and of the instruments able to measure
the electrical response of a solid in a very wide frequency range from
10-6 to 1015 Hz have been designed for research
on superionic conductors. They are now widely used in various sectors
of materials science. Chemists were crucial to grow the pure and large
single crystals of b-alumina needed for the close investigation of
their structure. Two chemists were recognized as experts in this art
: Gregory C. Farrington from the General Electric Laboratory (then at
Penn University) and Philippe Colomban in the R. Collongues's Solid
State Chemistry Laboratory in Paris. By contrast to a number of research
fields in materials science, the community of b-alumina researchers
presented a good balance between U.S American and West European laboratories,
while Japan played a minor role in this domain. Chemists from many countries
searched inspiration in the peculiar structure of b-alumina to invent new materials with better performance.
Industrial chemists were in charge of sintering and shaping by injection
molding or other techniques. After a peak of publications in the mid 1980s the interest
in b-alumina declined for
several reasons. A number of industrial companies reduced their research
investments while the concern for energy storage no longer appeared
as a priority. Indeed, other applications were suggested for b-alumina such as in non-linear optics. But, the
material did not bring really new insights in this field. Moreover,
from a theoretical point of view, the intense exploratory activity of
the previous decade came to an end and only minor advances could be
expected. Physicists retrieved from the field while chemists went hunting
for other families of ionic solid conductors. Materials from the NASICON
family raised a lot of attention in the late 1980s, but they proved
less rewarding as b-aluminas. Finally, b-alumina
may not have the great commercial future that was expected in the 1970s.
It is a model material in itself but its association with electrode
materials in a system raised many difficulties. In the mid-seventies
sodium-sulfur and lithium iron sulphide batteries were enjoying world-wide
attention. In spite of interfacial problems between the electrolyte
and the sulphur electrode, the system has been continuously improved
over the past decades. Pilot systems have been developed and tested
by Chloride Silent Power Ltd in a joint venture with RWE in England
in order to equip electrical vehicles and by ABB Hochenergiebatterie
GmbH in Germany (I.W. Jones, S.N. Heavens and F.M. Stackpool Current
trends in sodium-sulphur battery development at CSPL and W. Fischer
Status and prospects of High Energy Na-S Batteries at ABB in Beta-aluminas and Beta-Batteries (Key Engineering
materials vols 59-60 (1991) pp. 305-314 and 315-326). A promising alternative
is the sodium-chloride battery, the Zebra battery developed by AEG Anglo-American
batteries (Johan Coetzel and Jim Sudworth Out of Africa, The Story
of the Zebra battery, reference missing). Whatever its commercial
applications, b-alumina proved to be a main pillar in the construction
of materials science. |
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