University of Babylon · Web viewregion with a concomitant substantial reduction of the electrical...
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Lecture 7: Electro-ceramic Materials and Superconducting Ceramics Electro-ceramic Materials: Introduction The complexity of electroceramic materials cannot be covered exhaustively in the present short chapter; instead, only basic information on the various dielectric effects will be provided. Examples of ceramic and single crystal materials, and their properties and applications. Many oxide ceramic materials display properties that are conducive to important modern technical applications as sensors, actuators, capacitors, thermistors, varistors, solid electrolytes, ionic conductors, superconductors, permanent magnets with soft and hard characteristics, optoelectronic shutters, and many others. Frequently, such ceramics have distorted perovskite or spinel structures that impart ferro - , piezo - or pyroelectric “ smart ” properties. “ Ferroics ” are oxide ceramics with moveable domain walls that can be shifted in response to electrical, magnetic, temperature, and stress field gradients. They include ferrimagnetics (Y 3 Fe 5 O 12 , YIG), ferroelectrics
University of Babylon · Web viewregion with a concomitant substantial reduction of the electrical quality ( “ Q ” ) factor. Much later, it was found that barium titanates with
Electro-ceramic Materials:
Introduction
The complexity of electroceramic materials cannot be covered
exhaustively in the present short chapter; instead, only basic
information on the various dielectric effects will be provided.
Examples of ceramic and single crystal materials, and their
properties and applications.
Many oxide ceramic materials display properties that are conducive
to important modern technical applications as sensors, actuators,
capacitors, thermistors, varistors, solid electrolytes, ionic
conductors, superconductors, permanent magnets with soft and hard
characteristics, optoelectronic shutters, and many others.
Frequently, such ceramics have distorted perovskite or spinel
structures that impart ferro - , piezo - or pyroelectric “ smart ”
properties. “ Ferroics ” are oxide ceramics with moveable domain
walls that can be shifted in response to electrical, magnetic,
temperature, and stress field gradients. They include
ferrimagnetics (Y3Fe5O12, YIG), ferroelectrics (BaTiO3),
superconductors (YBa2Cu3O7-δ), piezoelectrics (Pb(Zr,Ti)O3, PZT),
PTC thermistors ((Ba,La)TiO3), hard (permanent) ferrimagnetic
materials (BaFe12O19 , magnetoplumbite), soft ferromagnetic
(transformer) magnets ((Mn,Zn)Fe2O4), nonlinear electro – optics
[(Pb,La)(Zr,Ti)O3; PLZT], electrostrictive ceramics [(Pb(Mg,Nb)O3;
PMN], and many others.
In contrast to polycrystalline ceramic materials with ferroic
properties, there exist nonferroic single crystal piezoelectrics
such as α - quartz or materials with a calcium gallium germanate (
CGG ) structure, as well as single crystal pyroelectrics with
perovskite structure such as lithium tantalate (LiTaO3).
Historical Development of Dielectric Ceramics
The historical development of dielectric ceramics, and in
particular for applications as efficient capacitors, is shown in
Figure 8.1. The first dielectric capacitors based on titania were
invented during the 1920s by Siemens in Germany, and further
developed during the 1930s. Concurrently, capacitors based on
magnesium titanate and silicate (steatite) were investigated and
utilized until the 1940s, when the development of ferroelectrics
such as barium titanate was begun. Pure stoichiometric BaTiO3 is an
excellent material for the construction of capacitors, owing to its
very high dielectric constant ( ε > 7000). At this time, BaTiO3
was the epitome of a dielectric, and therefore in Germany was given
the tradename “ Epsilon ”. Unfortunately, BaTiO3 is not applicable
to fashion electronic devices that require temperature stability,
as the temperature coefficient of the resonance frequency (τf ) has
a large negative value. Moreover, since its polarization mode is
based on spontaneous dipole polarization produced by distortion of
the oxygen coordination octahedra surrounding the titanium ion, the
dispersion of ε occurs in the microwave region with a concomitant
substantial reduction of the electrical quality ( “ Q ” )
factor.
Much later, it was found that barium titanates with a large surplus
of TiO2 in the lattice, such as BaTi4O9 and Ba2Ti9O20, have good
properties as microwave dielectrics, even though they are no longer
ferroelectrics. To date, barium titanate doped with oxides of
strontium, bismuth, neodymium, samarium, and tungsten, as well as
complex barium – zinc - tantalum oxide perovskites, fulfill the
dielectric requirements in terms of permittivity, Q - factor, and
temperature coefficients of the resonance frequency and
permittivity to a large degree. Giant dielectric permittivity was
rather unexpectedly discovered in CaCu3Ti4O12 (CCTO), with values
of ε exceeding 104 at low frequency. There is some evidence,
however, that this effect may not be related to true
ferroelectricity, but may instead involve the existence of highly
polarizable relaxational modes with a characteristic gap energy of
28 meV. The Internal Barrier Layer Capacitance ( IBLC ) model was
also invoked to explain the experimentally observed fact that ε
increases with the sintering time of CCTO, due to the incorporation
of an intergranular CuO phase into the structure of CCTO.
Characteristic Dielectric Parameters
The useful properties of a dielectric ceramic material – that is,
their figures ofmerit, and in particular for microwave applications
(see Section 8.4 ) – can be described by:
• The dielectric permittivity, expressed by the dielectric constant
ε ;
• The angle of dielectric loss ô , expressed by the quality ( “ Q ”
) factor.
• The temperature coefficient of the resonance frequency τf.
• A high dielectric constant exceeding ε = 100, since the required
size of a resonator is proportional to 1/(ε)1/2; that is, a high
value of ε leads to a miniaturization of the device.
• A high Q - factor; that is, a low dielectric loss; and
• A zero temperature coefficient of the resonance frequency to
attain temperature stability of the device.
Superconducting Ceramics
Introduction
The research and development of ceramic superconductors is of great
strategic importance for a variety of emerging energy technologies.
It is not only the insatiable curiosity of scientists but also the
urgent quest for a sustained development of future energy
technologies that is pushing research groups in universities,
government research organizations and key industries towards
exciting new results. Nevertheless, despite much effort a complete
understanding of the theory of the superconducting quantum state,
as well as the development of room - temperature superconductors
and efficient processing technologies, remain challenges for the
near future. Likewise, despite high expectations, the large – scale
application of ceramic superconductors for electrical high -
tension energy transmission cables, electric motors and generators,
as well as microcircuit switch components, are still missing.
Definitions
Certain materials exhibit a more or less abrupt drop in their
electric resistivity to zero and a strong diamagnetism (expulsion
of magnetic flux lines) when cooled to below a critical temperature
(Tc). If both physical properties exist simultaneously, the
material is termed a superconductor, characterized by a super –
current flowing through the crystal lattice without any dissipation
of energy. As the most interesting materials for industrial
applications with high Tcs are not ductile but rather are very
brittle, their fabrication processes from precursor powders are
similar to those of ceramics. Therefore, these materials are
classified as ceramic superconductors.
Other materials with potentially wide ranges of use in the near
future are known as ultraconductors ; examples include polymers
such as atactic polypropylene with properties similar to
superconductors – that is, high electric conductivity and current
densities over a wide temperature range, even though their
resistivity does not reach zero, their electric conductivity at
room temperature is orders of magnitude higher than that of copper.
Moreover, owing to the predominantly one - dimensional (1-D)
structure of polymers compared to two - dimensional (2-D) ceramic
superconductors.
Material Classification
Superconducting materials can be classified in different ways. One
common classify cation is based on their response to a high
magnetic field, whereby pure metals with an almost perfect crystal
lattice (except for V, Tc, and Nb) belong to type I
superconductors. The magnetic flux lines are unable to penetrate
the material, and above a critical magnetic field, Bc, the
superconductivity suddenly disappears; type I superconductors are,
therefore, not suited for high – magnetic field applications.
Because of the large distance allowed for electrons in a perfect
lattice to be coupled. In contrast, superconducting alloys and
compounds such as cuprates belong to type II superconductors. These
behave differently, in that the magnetic flux lines can penetrate
the material and allow, besides a higher critical temperature, far
higher current densities and a greater tolerance towards stronger
magnetic fields, if the magnetic flux lattice is pinned.
One different way of considering superconductors is related to
their compliance with the classic BCS theory. Hence conventional
(i.e., BCS - compatible) and unconventional (i.e., BCS -
incompatible) materials exist. The common scientific language
therefore distinguishes also between LTS ( low - temperature
superconductor s) and HTS ( high - temperature superconductor s).
In general, LTS are electron - doped (n - type), while HTS are hole
- doped (p - type) phases.
1) Nb - Bearing Low - Temperature Superconductors
Commercially used type II LT - superconductors, when applied for
the construction of magnets, are ductile Nb and NbTi alloys with
excellent mechanical properties and deformability, as well as the
brittle Nb3Sn alloy. Apart from this, Nb and Al are the most common
materials applied for superconducting tunnel junctions. The ductile
NbTi alloy carries maximum critical currents around a composition
of 47 mass% Ti; this alloy and Nb itself belong to the body –
centered cubic crystal structures. The Nb3Sn alloy crystallizes in
the cubic structure type (S.G. P m3n), as shown in Figure 9.2 a. An
important point for practical applications is the stability range,
from 18 to 25 atom% Sn, and the dependence of superconductivity on
the critical magnetic field. The body - centered Sn lattice with
lattice parameter a = 0.5293 nm contains Nb chains that
alternatively bisect the cube faces (Nb – Nb distance of the chain
cluster dNb-Nb =0.264nm). Tetragonal lattice distortion is observed
at low temperature, showing a value of about a/c = 1.0026 at 10 K.
As a rule, superconductivity is more resident in low - dimensional
than in three – dimensional (3-D) structures, and extended clusters
or at least layered structures favor a high transition
temperature.
2- Superconducting MgB2
This metallic compound is classified as an unusual high -
temperature - superconducting ( UHTS ) compound, because Tc = 39 K.
MgB2 crystallizes in a hexagonal layered structure, space group
P6/mmm with lattice parameters of a = 0.3086 nm and c = 0.3524 nm,
as shown in Figure 9.2 b, with the honeycombed boron layers
alternating with Mg layers. The strong interlayer σ-bonds are only
partly situated in the boron layer, since fewer valence electrons
are available compared to the carbon atoms of the similar graphite
structure, and weak intralayer σ-bonds are important for
understanding the exceptional superconducting properties.
Materials Processing
The processing of ceramic superconductors is a highly challenging
enterprise which has, in fact, limited their large - scale
industrial applications to date. In general, brittle alloys such as
Nb3Sn or ceramic high - Tc superconductors require confinement by a
conducting metal matrix for thermal and shape stability reasons,
and to lessen hazardous short - circuits and the release of
potentially toxic substances.
Hence, the production of wires requires ductile metal tubing into
which fine powders of superconducting materials will be filled,
heated under pressure, and hammered and/or drawn to the desired
dimensions. Single wires may be combined to multi - filamentary
strands, tapes, and cables. The tube materials are copper for
Nb3Sn, copper or silver for YBCO, silver for BSCCO, and stainless
steel for MgB2. The filled tubes are drawn successively from
millimeter - diameter tubes down to μm - size filaments. The
performance is sensitive not only to composition but also to
compositional uniformity.
Applications of Ceramic Superconductors
The range of applications of superconducting materials is
potentially enormous, and includes specialized uses as thin layers
and single crystals, apart from bulk ceramic materials. from which
it is clear that LTS, at least for the immediate future, will
command a market share far exceeding that of HTS.