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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.
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.