High-Temperature Superconducting Materials Science and Engineering. New Concepts and Technology

481 Pages·1995·17.471 MB·English

by  Donglu Shi (Eds.)

Preface
We intend this book to provide an up-to-date introduction to the fascinating
field of high-temperature superconductivity. The focus of the book is on the
basic concepts and recent developments in the field, particularly in the areas
of vortex-state properties, structure, synthesis and processing, phase
equilibrium, defects characterization, thin films, conductor development, and
applications. Most of the theoretical issues are addressed in a straightforward
manner so that technical nonspecialists and university students can benefit
from the information. Furthermore, many physical concepts in
superconductivity are explained in light of current theories.
This book is written for a large readership including university students
and researchers from diverse backgrounds such as physics, materials science,
engineering and chemistry. Both undergraduate and graduate students will
find the book a valuable reference not only on superconductivity, but also on
materials-related topics, including structure and phase diagrams of complex
systems, novel ceramic and composite processing methods, thin film
fabrication, and new materials-characterization techniques. In particular, this
book gives a detailed introduction to experimental methods at low
temperatures and high magnetic fields. Thus, it can serve as a comprehensive
introduction to researchers in electromagnetic ceramics in general, and can
also be used as a graduate-level text in superconductivity.
The book devotes two chapters (Chapters 2 and 6) to cryogenic systems
and low-temperature measurements. Detailed experimental procedures for
both transport and magnetic measurements are presented at a level suitable
for people with no previous training in these areas. The book systematically
introduces cryogenics and low-temperature techniques specificallv for
characterizing superconducting properties. Furthermore, the book inciudes
numerous developments in magnetic property measurements of high-Tc
superconductors by using SQUIDs, ac susceptometers and vibrating sample
magnetometers. Recent developments in determining important
superconducting parameters such as To Hc2, and cJ are presented in detail,
providing essential information for researchers in the field. Chapters 2 and 6
are also valuable to beginners and technical nonspecialists in the study of
superconductivity and in low-temperature measurements.
Chapter 4 deals with vortex properties such as flux pinning, the Bean critical
state, and flux creep. Although these concepts were introduced long ago, new
vortex phenomena have been recently observed in high-Tc superconductors,
requiring new and modified physical models to interpret them. For example,
dynamic models including collective creep and Josephson coupling of bilayers
of CuO plates have been developed to solve the flux motion problem. More
fundamentally, flux avalanches observed near the critical state cannot be
explained by traditional flux-creep models: Chapter 4 gives a detailed
introduction to a new theory--self-organized criticality--for treating such
vortex motion problems.
Chapters ,6 7 and 8 should be of most interest to researchers in materials
xi
xii Preface
processing. These chapters give detailed descrip.tions of many traditional and
novel methods in synthesis and manufacture of ceramic materials with tailored
microstructures and desired forms for practical applications. Recent research
results in phase reactions, phase diagrams and heat treatment are presented
for several high-Tc systems. In addition, Chapter 5 presents new
characterization techniques using transmission electron microscopy, including
the electron energy-loss spectrum method. With these highly advanced
techniques, defect structures including tweeds, twin boundaries, grain
boundaries, stacking faults and dislocations in high-To ceramics are studied
in detail.
In Chapter ,3 the crystal structure of various perovskites is introduced for
most of the high-To superconductors. The effects of structural changes on
superconductivity and oxygen-ordering behavior are analyzed and illustrated
by means of various experiments.
The last chapter concerns one of the most exciting aspects of
superconductivity: applications. Although industrial and scientific applications
of conventional superconductors have been well established for several
decades, it is a challenging task to use high-Tc oxides at liquid nitrogen
temperature. Microstructure control, reproducibility and grain boundary weak
links are still challenges to be overcome. Nevertheless, the potential
applications of high-Tc materials are certainly recognized in many areas such
as power transmission lines, magnetic levitation and nuclear magnetic
resonance. The field is young, offering researchers exciting opportunities to
make a major contribution to science and technology.
stnemgdelwonkcA
eW thank all the authors for their enthusiasm, effort, cooperation and excellent
contribution in their area of expertise. In particular, we acknowledge the
superb technical editing work by Dr Gail Pieper of Argonne National
Laboratory. It is impossible to even imagine the completion of this book without
her participation in copyediting, proofreading, computer formatting and
organizing this book. eW are also grateful to Dr Roger Poeppel and Dr Harold
Myron of Argonne National Laboratory for their valuable suggestions during
the production of this book.
smynorcA
A-H model Ambegaokar-Halperin model
SCB theory Bardeen, Cooper and Schrieffer theory
CBED Convergent beam electron diffraction
CEBAF Continuous Electron BeamA ccelerator Facility
CFC Continuous-flow cryostat
CGR Carbon glass resistance
CSL mode Coincidence site lattice model
CCSL model Constrained coincidence site lattice model
CMOS Complementary metal oxide semiconductor
CSL Coincidence site lattice
CVD Chemical vapor disposition
CSD lattice
vectors Displacement-shift-complete lattice vectors
XDE Energy dispersive x-ray spectroscopy
SLEE Electron energy-loss spectroscopy
SFAXE Extended x-ray absorption fine structure
FENIX Fusion Engineering International Experimental Magnet
Facility
FLL Flux line lattice
GBD Grain boundary dislocation
G-L Ginzburg-Landau
grp Glass-reinforced plastic
HAGB High-angle grain boundary
HEMT High-electron mobility transistors
HREM High-resolution electron microscopy
HRTEM High-resolution transmission electron microscopy
HTF High-temperature ferroelectrics
HTS High-temperature superconductor
IR Irreversibility line
ITER International Thermonuclear Experimental Reactor
LAGB Low-angle grain boundary
LCT Large Coil Task
LHC Large Hadron Collider
STL Low-temperature superconductor
xiii
xiv smynorcA
MHD Magnetohydrodynamics
MOCVD Metal-organic chemical vapor deposition
MOD Metal-organic deposition
MPMG Melt-powder-melt-growth
MRI Magnetic resonance imaging
MTG Melt-texture growth
NMR Nuclear magnetic resonance
OPIT Oxide powder-in-tube
OSHA Occupational Safety and Health Administration
PDMG Platinum-doped melt-growth
PIT Powder-in-tube
PLD Pulsed laser deposition
PMP Powder-melt process
TRP Platinum resistance thermometer
QMG Quench-and-melt-growth
RPC Resonant pinning center
SBGP model Sommerfeld-Bloch-Gruneisen-Peierls model
TFFS Superconducting flux flow transistor
SIS Superconductor-insulator-superconductor
SLMG Solid-liquid-melt-growth
SEMS Superconducting Magnetic Energy Storage
SNS Superconductor-normal metal-superconductor
SOC Self-organized criticality
SQUID Superconducting quantum interference device
CSS Superconducting Super Collider
SUM Structural unit model
FFAT Thermally assisted flux flow
TEM Transmission electron microscopy
J-T Tinkham-Josephson
ISLV Very large scale integration
MSV Vibrating sample magnetometry
ITV Variable-temperature insert
ZSY Yttria-stabilized zirconia
High-Temperature Superconductivity
ni the Layered Cuprates:
An Overview
J. C. Phillips
1.1 Crystal ChemistrymStructure and Function: Why era Layered Cuprates
so Special?
In the past seven years, more than 35,000 research papers have appeared
on the subject of high-temperature superconductivity, with special emphasis
on the layered cuprates. The latter have evolved as layered multinary oxides
with pseudoperovskite structures from the cubic (Ba, Pb, Bi) oxide perovskite
superconductors, which in turn evolved from the cubic Chevrel chalcogenide
cluster compounds. Their crystal chemistry is just as different from that of
intermetallic compounds (such as Nb3Sn) as their transition temperatures (about
ten times larger), and it is clear that their novel crystal chemistry is the origin
of their novel superconductive properties. One of the central themes of this
chapter (and indeed of other chapters as well) is this close relationship, which
has many ramifications, not only conceptually, but also technologically, for
processing these materials to obtain desirable properties.
The simplest and most general way to analyze crystal chemistry is to discuss
size and electronegativity differences together with average valences. The latter
present no problem for any number of elements, but there are many different
definitions of the former, even for binary cases, and until quite recently all
seemed to have virtues in some cases and weaknesses in others. It was,
moreover, not clear that the binary definitions of differences could be extended
to multinary cases. Rabe et al., however, have recently shown that a recipe
does exist that organizes the entire crystallographic data base with a success
level higher than 95% for simple, common binary compounds. When this recipe
is applied to complex novel compounds, diagrams of the sort shown in Fig-
ure 1.1 emerge [1].
When we examine Figure 1.1, we see that three kinds of complex, margin-
ally stable materials form small islands on the diagram. The three kinds are
stable quasi-crystals, high-Tc (> 500 K) ferroelectrics (HTF), and high-Tc (> 10
K) superconductors (HTS). Because so many ferroelectrics have the BaTiO3
cubic perovskite structure, we are not surprised to see that the HTS oxide
island is adjacent to (but does not overlap) the HTF island. On the opposite
side of the HTF oxide island is the HTS chalcogenide (Chevrel) island. Thus,
while the average electronegative difference AXI I between cations and an-
2 erutarepmeT-hgiH Superconductivity ni eht dereyaL setarpuC
Fig. 1.1. A schematic summary of the domains of marginally unstable lattices with
special physical properties, adopted from ]1[ in terms of average multinary size ~R and
electronegativity XA differences. The dashed boundary denotes the region spanned by
binary alloys, the dotted boundary that spanned by known ternary and multinary
alloys. Here Q denotes stable quasi-crystals, and F denotes stable ferroelectrics with > c T
005 .K There are four superconductive domains ,S with T > .0K1 Domain 1 S includes
primarily compounds such as Nb3Sn with the 51A structure. Domain 2 S includes binary
compounds, primarily NbC and NbN (rock salt structure). Domain is 3 S the Chevrel
compounds, while is 4 S primarily the layered cuprates. Note how F straddles 3 S and S .4
ions is muchlarger in the oxides than in the chalcogenides, ee_ht average size
difference, AR is much the same. This average size difference AR is about half
that found in binary HTS such as NbN. This may be the reason why the anionic
(oxide and chalcogenide) HTS can form in spite of the packing problems
characteristic of perovskite and related structures.
What do we mean when we say that there are packing problems in the
perovskite structure ABO3? We mean that there is only one lattice parameter
(the cubic lattice constant [a]) which can be adjusted to reduce strain ener-
gies, but there are two bond lengths, A-O and B-O, which would like to have
certain "natural" (or binary) values. (In the crystal chemical literature these
"natural" lattice constants <ai> are sometimes called prototypical.) Similarly,
.J C. Phillips 3
for the layered pseudoperovskites one can speak of prototypical lattice con-
stants <bi> for each layer. The condition for the formation of layered cuprates
is that the prototypical planar square lattice constants <bi> be nearly equal to
b for CuO2 planes for all layers i, even in parent compounds that contain no
CuO2 planes, such as Bi4Ti3012 and Bi2Mo(W)O6. In fact, through knowledge
of b in the parent compounds, one would be led to discover the Bi- and
Tl-based cuprates 1.
The actual values of a or b would appear to be a compromise between
the values <ai> or <hi>, but this is not the whole story. Interplanar strain energies
can be reduced by the presence of a certain concentration of native defects in
each plane i, such as anionic vacancies and interstitials, which may be partially
aggregated or ordered. The concentration and spatial distribution of such defects
can be optimized for superconductive properties by suitable processing, as
described elsewhere in this book. In the case of the cuprates, this processing
is especially easy because of the high oxygen mobilities, second only to a few
materials (such as ZrO2) used commercially as oxygen getters in
high-temperature glass processing (T > 103~ The high oxygen mobilities
suggest small activation energies for diffusion, and these are related to the
high densities of oxygen vacancies and/or interstitials, together with the
resonant states associated with these electronic defects. We thus believe that
it is no accident that the normal-state and superconductive properties of the
layered cuprates are so sensitive to processing. Any theory of these properties
should recognize and discuss the role of such defects if it is to be considered
more than a logical tautology.
We pause at this point to discuss the extent to which electrically active
defects can be observed in complex crystals. There have been a great many
diffraction studies of cuprates, and many defects have been observed, either
directly or indirectly, from the static broadening of diffraction patterns and
the large R values which imply a high defect concentration. In such complex
materials one can expect to find many defects, only a few of which are elec-
trically active and which are associated directly with superconductivity. For
this reason, only a few diffraction experiments have succeeded in obtaining
direct evidence relating defects to superconductivity. These have involved
very carefully prepared samples where phase separation into ordered and
disordered regions has taken place 2,3. As shown in Figure 1.2, channeling
experiments have also revealed structural changes at cT not observable by
neutron diffraction 4,5. In any case, it is important to recognize that although
direct structural evidence for superconductive coupling to defects is difficult
to obtain, this problem arises because in such complex materials the back-
ground "noise" level is high (large R values). There have been some cases in
which this problem has been overcome, and these carry much more weight
than the many routine structural studies that have failed to identify such
coupling as a result of poor sample quality or inadequate resolution.
We see, then, that part of the practical answer to what makes the cuprates
so special is prosaic high oxygen mobilities, not mysterious and magical
many-electron interactions. In many respects the high oxygen mobilities of
the cuprates represent one of nature's felicitous coincidences. (Another felici-
4 erutarepmeT-hgiH Superconductivity ni eht dereyaL setarpuC
tous coincidence, crack-free SiO2 on Si, is the basis of integrated circuits and
the microelectronics industry.) There is, however, yet another felicitous coin-
cidence, and this is the nature of the resonant states that pin FE and enor-
mously enhance .cT We discuss this microscopic problem later. First, how-
ever, we discuss the normal-state transport properties of the cuprates and
explain microstructurally why these are so different from normal metals. The
same microstructural model enables us to understand many aspects of p(T,H)
for T near .cT After discussing the chemistry of resonant pinning centers, we
collect all these novel concepts to discuss the fundamental issue, which is
why cT is so high in the cuprates compared with normal metals.
1.2 Normal-State Percolative Transport in Ferroelastic Anionic Metals
Some ten years ago most condensed matter physicists believed that no
large qualitative differences in electronic transport existed between simple
nearly free electron s-p metallic crystals with one or a few atoms per unit cell
and complex multinary metals involving three or more elements and many
atoms per unit cell. In all cases it was supposed (Sommerfeld model) that
electrical currents were carried by electronic wave packets moving ballisti-
cally with Fermi group velocities vF, mean scattering times, z, and mean free
paths, l - .ZFV The temperature dependence of the electrical conductivity could
. . . . . . .
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11
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TEMPERATURE (K)
Fig. 1.2. Comparison fo atomic vibrational amplitudes ni OCBY measured by
conventional neutron diffraction Debye-Waller factors and by channeling, which si
sensitive ot out-of-plane displacements. A distinctive anomaly at T c ~ 09 K si observed
by channeling, but not by diffraction. ehT figure si adapted from .4
. C. Phillips 5
be explained (Bloch-Grtineisen), as in simple s-p metals, primarily by phase
space considerations, augmented in the case of transition metals by increased
scattering due to Peierls Umklapp (lattice recoil) processes 6. The only dif-
ferences to be expected for complex metals would be those associated with
larger unit cells and more complex energy band structures.
Before proceeding further, we should mention that there were already hints
of anomalous metallic behavior in low-dimensional organic polymers (such
as TTF-TCNQ) and layer compounds (such as NbSe2) which were extensively
studied 7 in the 1970s. Most of these anomalies were associated with collec-
tive electronic phases (charge density waves) which characteristically appear
in conjunction with one- or two-dimensional structures. What makes the
anomalies in the quasi-crystal and superconductive high-Tc novel metals so
striking is that they do not seem to be collective. The conductivities, for ex-
ample, are linear either in temperature T or in T ,1- a property that is strongly
suggestive of some phase-space mechanisms that affect single particles. Or-
dinary phase space mechanisms in d dimensions, however, give rise either to
T 2- or T n behavior, with n ~ 9( - d, so that linearity in T seems almost impos-
sible to achieve. This is because the usual arguments are based on Fermi liq-
uid theory, which has worked well for elemental and binary metals. We will
see, however, that an explanation of electronic transport in novel ternary metals
requires a fundamentally different approach.
The Fermi liquid description of the broad trends in the temperature de-
pendence of the electrical resistivities p(T) of elemental metals (such as the
alkalies and alkaline earths, the noble metals, AI and Pb) is remarkably suc-
cessful, as shown 8 by de Haas and his coworkers at Leiden in the 1930s.
The electronic momentum and energy are dissipated by electron-phonon
scattering. The rate at which this occurs is closely related to the lattice ther-
mal conductivity and specific heat, as described by the Bloch-Gr6neisen for-
mula, which contains the lattice Debye 0 as a characteristic temperature. In
this formula p(T) is linear for T >_ 0, freezes out for T -< 0 with a linear region
0.2 < T/0 _< 0.5, and finally goes to zero like T ~ with x( = 3(5) with (without)
Umklapp scattering. As the Dutch workers noted, the theory fails only to account
for a resistance minimum at T = Tm which shifts to lower temperatures for
purer metals and which occurs at Tm>10 K for the purest materials. The
minimum was ascribed to internal (magnetic) degrees of freedom of the
impurities, and a full perturbative theory of this effect was developed by .J
Kondo 9 and others some three decades later. The correctness of the
wave-packet description is demonstrated by good quantitative agreement in
the elemental metals between the values of 0 derived by fitting p(T) and the
lattice specific heat 6.
The temperature dependencies of the resistivities of quasi-crystals and HTS
do not resemble at all those of normal metals. Annealing at constant compo-
sition has almost no effect on p(T) in normal metals, but small changes in
processing can drastically affect the functional form of p(T) in these ternary
materials. In what follows we quote examples of what we believe to be the
"ideal" behavior of p(T) in novel metals. We will justify our choices both
phenomenologically and theoretically, but it is important to recognize that
6 High-Temperature Superconductivity in the Layered Cuprates
x = 58.0
~ ~ 59.0
r
....
: i i
o
0 5 01 51 02
! 1 1 I I 1 l
0 001 002 003
T(K)
Fig. 1.3. The planar resistivity p,,(T) ni iB( rSx_: d.~OuC:) as a function of ,x from the
semiconductive to the linear regime, from ,J01 as x is varied.
the range of behavior in novel metals is far greater than in normal metals.
This point by itself already tells us, without detailed knowledge of the func-
tional forms of p(T) that the usual normal-metal picture of electronic wave
packets being scattered by vibrational wave packets and isolated, randomly
distributed impurities, is inadequate for novel metals.
The most famous feature of p(T) in ternary novel metals is its linear be-
havior in the HTS cuprates. This behavior was observed initially in the planar
resistivity of polycrystalline samples of (La,Sr)2CuO 4 and YBa2Cu307_x, where
the conduction takes place primarily in CuO2 planes. There this behavior is
masked at low temperatures by the superconductive phase transitions, but in
(Bi 2-xSr 5~+6OuC2)x (where Tc is low because the CuO2 planes are widely
separated by semiconductive layers) the planar resistivity P ll is linear 10 in
T from 7 K to 700 K, a result (Figure 1.3) that cannot be explained in the normal
metal context by any reasonable vibrational spectrum.
A close correlation exists between the linearity of P ll (T) and the super-
conductive transition temperature .cT At the maximum Tc one has n = 1 in the
relation P l t(T) = oP + aTn, but as Tc decreases n increases to n = 2, for example
11 in T12Ba2CuO6+~ as 8 is increased from ~0.0 to ~0.1, as shown in Figure
1.4. Some researchers have suggested that this effect is associated for n = 1
with Fermi surface nesting, but this correlation seems extremely unlikely. Such
nesting would give rise to charge density waves which would reduce N(EF)