The spectrums of properties exhibited by ferroelectric materials are dielectric, ferroelectric, piezoelectric and pyroelectric effect. This is the makes these materials to have a wide range of useful application. Infrared detectors are used pyroelectric effect of ferroelectric materials. It is used in nonvolatile memories due to have ferroelectric hysteresis. Its piezoelectric properties make them useful for actuator, radio frequency filter, sensor, and transducer. Ferroelectric capacitors are used, their good dielectric behavior. According to the necessity of the system they are available in different form such as single crystals, ceramics, thin film, and polymer, composite. The diversity of properties ferroelectric materials always attracted the attention of engineers and researchers. Size reduction of this material from micro to nanoscale established an enormous consideration to develop nanotechnology. Its vast use of different filed imposed the in detail research in adding to the development of processing and characterization method. This chapter will put some light on some fundamental principle of ferroelectricity, the list of perovskite materials and their application.
Like ferromagnetic materials, the functional properties of ferroelectric materials find wide range of applications, ranging from actuators and sensors to memory or optical devices. A ferroelctric class of materials cannot define in a single sentence. So before we define the ferroelectric materials, we should classify dielectric materials. Dielectric is belonging to a class of insulating materials that on the application of an electric field shows dielectric polarization . Here the center of symmetry plays a significant role for their properties. Crystal structure with a center of symmetry have such an arrangement of atoms around a point or center that by the inversion, we can get the same arrangement of atoms in the crystal. Dielectric materials belong to a group of non Centro symmetric crystal structure. In 432 point group the entire non Centro symmetric point group shows piezoelectric properties . The properties due to which voltage obtains form charge a separation in the face due to the mechanical stress and vice versa. Both direct and inverse piezoelectric effects have a wide range of application in electronic devices . Barium titanate is an example of non-centrosymmetric piezoelectric material used in microphone and transducer [2, 3, 4]. In non centrosymmetric crystals there is an axis of symmetry, called polarity. These piezoelectric polar crystals are shown pyroelectricity. With changes in temperature there is a charge separation. The cells of polar structure have efficient dielectric polarization, so often called a spontaneous polarization. Either by stress or by a change in temperature the dipole moment of these polar structures is change. The is a charge separation in the surface, results in the spontaneous polarization. So the polar dielectric spontaneous polarization direction and magnitude can be modified with the applied stress. Zinc oxide, which belongs to family of polar dielectric shows wurtzitecrystal structure . In this structure, between hexagonally packed oxygen ions layer, Zn2+ions are at the tetrahedral site.
Between these layers dielectric, whose spontaneous polarization depends on the direction and magnitude of the applied stress. Adequate amount of stress can change the direction of spontaneous polarization. But the exclusion of the stress does not bring back the original magnitude and direction of the spontaneous polarization. These families of polar dielectric are called ferroelectric. Figure 1 shows the way the Centro symmetric, acentric, polar and ferroelectric, dielectrics are related to each other. Many review articles shown the history of ferroelectric [6, 7, 8, 9, 10, 11, 12]. Many great scientist open the path of discovery of the ferroelectric.
Pyroelctricity was studied by Brewster. Piezoelectricity was discovered by J. P. Curie. Debye, Boltzmann, Pockelsetc helps in conceptualizing the polarization in the dielectric. It was E. Schoridgener, who coined the term ferroelectricity, but JoesphValasek known for the discovery of ferroelectric. In 1920 the Rochelle salt (sodium potassium tartrate) shows spontaneous polarization which can be switch with the magnitude and direction of the applied field. This is the first manifestation of the ferroelectricity in a crystal. That is the trademark of ferroelectricity. Logically effectively the term ferroelectricity is defined as the switchable polarization between two or more stable state by the application of electric field. There exist some exceptions. Some semiconductor materials show ferroelectric properties. They do not posses electric polarization. In some ferroelectrics materials the spontaneous polarization cannot be switched with the electric field. This is either due to they are too conducting or reach the electrical break down first. This ferroelectric property first observed in Rochelle salt. But later it is observed in oxides, polymer, ceramics, and liquid crystal. When it is about ferroelectric property, the perovskite structure materials have a special importance. So in this chapter, we will discuss some perovskite ferroelectrics which are used in various electronics devices. According to the structure, there are five types of structure. (i) Organicpolymer (ii) Charge ordered ferroelectrics (iii) Magnetic order ferroelectric (iv) Corner-sharing oxygen octahedral (v) Hydrogen bonded radical compound (v) Ceramic polymer composites . Among these ferroelectricgroup a mostly used ferroelectrics are the corner sharing oxygen octahedral oxides.
Basically, the corner sharing oxygen octahedral oxide structures is represented as Aa+Bb+O3 . Figure 2 shows the schematic figure of O2− ions corner sharing oxygen octahedral. The Bb+ cation is seated within every octahedron. The b of the cation has the value lies in between 3 to 6. Aa+ ions lie in the gap among the octahedral with its value of
2. Perovskite ferroelectrics
It is a family of a subgroup of corner sharing oxygen octahedral material’s exhibiting ABO3 structure. This family contains some mostly used piezoelectric and ferroelectric ceramics. Some member which is used in different field of Technology are strontium titanate (STO), barium titanate (BaTiO3), (SrTiO3), lead titanate (PbTiO3), barium strontium titanate (BST), PZT, potassium niobate (KN) (KNbO3) etc. Some of these perovskite are discussed in detail as follows.
2.1 Barium titanate ceramics
Barium titanate which a member of perovskite family exhibit good piezoelectric, ferroelectric and high dielectric constant. This FE ceramics are used in first piezoelectric transducer. But now it is mostly used in multilayer capacitors (MLCs) due to having a high dielectric constant. It is also used in positive temperature coefficient Resistance (PTCR), sensor, PTC Thermistors, IR detectors, RAM, sonars and in electro optic devices. By doping Barium titanate which is insulator in pure form can be changed into a semiconductor. Barium titanate basically white powder of inorganic compound. With a decrease in temperature the octahedral TiO6 undergoes distortion gives rise to five structural phase transition from hexagonal, cubic, tetragonal, orthorhombic, and rhombohedra. Very large spontaneous polarization and high dielectric constant are observed in this phase due to the distorted octahedral. At Curie point, i.e. above 120°C the distorted octahedral of TiO6 comes to equilibrium result in an isotropic cubic structure [13, 14] so only this phase does not exhibit FE properties.
Capacitor, MLC etc. used first BaTiO3 as the dielectric ceramic with large dielectric constant and dielectric loss for manufacturing. The factor that affects the dielectric properties of the materials is not only its structure but also its synthesis route which will reflect in its size of the grain, density, purity etc. . The Applied frequency of the electric field, temperature and do pants also affects the dielectric properties of the materials [16, 17, 18, 19, 20, 21, 22, 23].
The dielectric constant of Barium titanate prepared by any method increase depends on the grain size and distribution of grains [17, 24]. At room temperature the frequency dependent dielectric constant decreases at low frequency and then increases slightly and become constant at high frequency. Temperature dependent dielectric constant decreases at higher temperature . Ions of different size can be can be added to the perovskite structure. So doping at both A and B site of this perovskite structure is used to tolerate its electrical properties . At A site mono, die and trivalent acceptor do pants are substituted to produce P-type semiconductor, whereas N type semiconductor are obtained by donor dopant of tri, tetra and pentavalent ions at the B site of the perovskite. The concentration of the dopants also affects the electrical properties of Barium titanate. Increase in concentration of Donor dopant makes the semiconducting ceramics to an insulating one. Barium titanate ceramics have application in various engineering fields.
2.1.1 Multilayer capacitor (MLCs)
Mostly BaTiO3 is used in capacitor due to having a high dielectric constant. In thin dielectric form packed in a minimum space not only with high capacitance but also mechanically tough . It is used as passive component in the circuit for large scale integration (LSI). It is also cost effective one with the use of internal electrode made of, nickel (Ni) and copper (Cu) .
2.1.2 (s2) PTC thermistor
The high resistivity of BaTiO3 makes it as good candidates for PTC thermistor. Doping at A and B site of BaTiO3 convert them as a semiconducting material. Below the Curie temperature these semiconductor materials have low resistance. Above crystal particle boarder barrier layer persuades by the surface state. So FE characteristics with high dielectric constant exhibit by this crystal boundary of high resistance. So below Curie temperature the potential barrier is small. Low resistivity has been exhibited in these materials due to effortlessly penetration of electron. The height of potential barrier increases above the Curie temperature makes electron difficult to pass through it results in increasing the resistivity of the material. In various electronics circuits this semiconducting BaTiO3 materials are used as constant temperature heaters or switching devices. For temperature or parameter related to temperature can be detected, measured and control of temperature with the use of this PTC thermistor. Among all the available sensor materials PTC has the highest temperature coefficient of resistance.
2.1.3 Nanogenerator (NG)
High piezoelectric effect with biocompatibility make them use as Nanogenerator. Piezoelectric potential is induced in between the two electrodes of NG, this is due to mechanical stress. Commercial devices can be work using this generated electrical energy without an external energy source.
2.2 Strontium titanate (SrTiO3)
Another member of perovskite family is strontium ferrite STO (SrTiO3) is a complex oxide. It exhibits cubic structure at room temperature. O2− ions are bonded with six folded coordinate to Ti4+ ions and with twelve folded coordinate to Sr2+ ion. Each Sr2+ ion lies in between four TiO6 octahedral. To decrease in temperature, it undergoes a phase transition. The first transition below room occur at −168°C. The opposite rotation of adjacent oxygen octahedral at this term turns the cubic structure to tetragonal structure. To further decrease in temperature at −236°C the changeable phonon modes turns the tetragonal structure to orthorhombic structure. The orthorhombic structure is change to rhombohedra structure at −263°C . As Curie–Weiss law suggests the dielectric constant increases with the phase transition below room temperature. At this temperature due to quantum fluctuations leads to the quantum PE . The charge storage capacity is high, chemical stability, optical transparency in the visible region with good insulating properties makes it use in modern electronics applications such as phase shifters, high-voltage capacitors, delay lines, filters, tunable oscillators etc. [31, 32, 33]. It is used in cancer treatment and in thermo – electric generators due to they have insolubility and high melting point properties [34, 35]. It shows photoconductivity when exposed to light due to having an direct gap and indirect band gap of 3.75 eV and 3.25 eV respectively . Its conductivity increases with the contact of light to the crystal. Its conductivity persists for several days, with small decay [37, 38].
The first oxide, which is superconducting below 0.35 K is the strontium oxide (STO) . It can be used as a tremendous substrate for superconductors with a high operating temperature and for oxide-based thin films. It is used as single-crystal substrate due to the enhanced electrical conductivity of niobium doped STO for the growth of perovskite oxides. It is also used for other ferroelectric and magnetic oxide substrate due to its large lattice parameter. Variation of temperature and pressure parameter can lead to increase oxygen vacancies in both crystal and thin films of SrTiO3. So it becomes more conducting due to stimulate free electrons in the conduction band and also opaque. This used as gate-dielectric material due to the growth of high quality growth of epitaxial SrTiO3 layers on silicon without forming silicon oxide Furthermore, it allow the incorporation of other perovskite oxides based thin-film on top of silicon . It is also used for its piezoelectricity, ferroelectricity and Pyro electricity application in nanoscale [41, 42, 43, 44]. It is used for various technological applications such as in super capacitor, nonvolatile memory, tunable microwave capacitors, ultralow-temperature scanning microscopies, high-density dynamic random access memories, soft phonon devices, oxygen sensors [45, 46].
2.3 Barium strontium titanate (BST)
The model of BaTiO3 (BTO) has been used to develop Barium strontium titanate (Ba
Due to its high value of capacitance make it useful to construct high capacitance capacitor. It has uses in tunable microwave devices tunable capacitor, phase shifters, tunable transformers. BST varactors are a good replacement of the presently used semiconductor varactor and mechanical tuners. It is not only the drawback of large size with small tuning speed of mechanical tunners, but also small power handling capability of semiconductor varactor. BST also used in band pass and low pass tunable filter.
Semiconductor based phase shifter are used in fighter aircraft radar and cellular telephone base stations are associated with high loss at microwave frequencies with low power use ability. BST is the best replacement of these semiconductor based phase shifter associate with small loss, inexpensive and with better power handling properties. It is used in micro strip antenna. In tunable microwave application thin film of BST is used. The reduced size with small weight makes it compatible with microwave circuit. Dielectric constant not decreases sharply with the variation of the thickness of the film [47, 48, 49]. When it’s used in metal–insulator–metal capacitor shows high dielectric constant, low dielectric loss, high leakage current density, high charge storage density makes it’s used in dynamic random access memory (DRAM).
2.4 Lead titanate (PbTiO3)
Another member of perovskite family is the inorganic lead salt of titanic acid compound, i.e. Lead titanate (PbTiO3). Yellow powder of lead titanate is water insoluble. It shows a high Curie point of 490°C. It shows second order phase transition due to which it changes from cubic pyroelectric phase to ferroelectric tetragonal phase. At room temperature it shows the tetragonal structure belongs to
2.5 Lead zirconate titanate (PZT)
Lead zirconate titanate with perovskite structures is an inorganic intermetallic compound. It is a solid solution of lead zirconate and lead titanate represented as [Pb (ZrxTi1−x) O3, 0≤x≤1]. PZT has Pb2+ ions at A site with random occupation of B site by Ti4+ and Zr4+ ions. It undergoes phase change with composition, but also with temperature. Above its Curie temperature it shows the pyroelectric effect with cubic structure. It undergoes a structural change from PE cubic phase either to FE rhombohedral phase or tetragonal phase. The spontaneous polarization in these structures is oriented in< 100⟩ and ⟨111⟩ set of directions for tetragonal and rhombohedral phase respectively. The morphotropic phase boundary is around at 52/48 of Zr/Ti ratio separating FE tetragonal and orthorhombic phases. At this boundary PZT has shown maximum dielectric and piezoelectric constants. Rhombohedral phase with 8 possible domains and tetragonal phase with 6 domains with total 14 domains are equally favorable at this composition. Piezoelectric PZT ceramics can be tailored according to application with ions having valence diverse from the host ions in the lattice. Especially PZT at MPB is modified to form soft and hard PZT. Acceptor ions are used either by the use of Al3+, Fe3+ at B site and Na+ and K+ at A site of the perovskite to produce oxygen vacancies in the lattice [80, 81]. Donor ions PZT produced domains wall motion fromed by Nd3+, La3+ at A site where as Nb5+, Sb5+ at B site in the lattice [82, 83, 84, 85]. Hard PZT formed by donor doping shows low dielectric constant, small electrical losses, small piezoelectric co-efficient with high coercive field. So it is difficult to pole and dipole the sample and make them useful for rough applications. Whereas soft PZT has shown larger losses, high dielectric constant and piezoelectric co-efficient. So they can easily dope and dipole. They are used in FeRAM, actuator of STM/AFM type, ultrasound transducer, capacitor with high dielectric constant, IR sensor, etc. High value of piezoelectric coefficient of FE PZT make them useful for micro sensor, micro actuator used micro electromechanical system (MEMS) devices [86, 87, 88]. Nano rods, wire and hollow tube of PZT are now used for different application [89, 90, 91, 92, 93, 94, 95, 96, 97, 98]. PZT Nanogenerator are used for piezoelectric effect to produce piezoelectric energy in the microscale. The intrinsic polar crystal structure of PZT nanofibers shows high piezoelectric voltage constant. High aspect ratio of nanostructure PZT overcomes extremely brittle nature observed in bulk PZT and its thin films and used for generation of energy in alternating loads [89, 90].
2.6 Lead magnesium niobate (PMN)
Single crystal of Pb (Mg1/3Nb2/3) O3 (PMN) materials belongs to the FE relaxor materials and are used in , and the work in the electrostrictive actuators with high-strain and capacitor with high dielectric constant [100, 101, 102]. The Perovskite type structure of Pb (B1, B2) O3 formula with lower valence B1 such as Zn2+, Mg2+, Fe3+ and Ni2+ and higher valency B2 is such as Ta5+, Nb5+ and W5+. On cooling below the Curie temperature relaxor PMN ferroelectrics shows a wide dispersive and the diffused phase transition. Heterogeneity in composition on a microscopic scale leads to the diffuse phase transition in the relaxor ferroelectric.
In micoregion the stoichiometry is not obeyed by disorder B-site leads to change in FE transition temperatures leads to broadening dielectric peak. Strong frequency dependent dielectric constant has been observed in the relaxor ferroelectric. Curie temperature changes linearly with frequency with high dielectric loss below this temperature. Some relaxor shows second order phase transition. This relaxor remnant polarization is starting to decrease from Curie temperature and becomes zero with the increasing in temperature [103, 104, 105, 106].
2.7 Potassium niobate (KNbO3)
KN (KNbO3) is also exhibited perovskite structure. It shows interesting ferroelectric properties at low temperature. It shows three phase transitions simple displacive type at low temperature with different symmetry and PE phase at high-temperature . Oxygen behaves as rigid body in the octahedral and vibrates about Nb atoms . Liberation of oxygen octahedral leads to the irregular anisotropy exhibited by oxygen atom due to mean square displacements. Its structure shows two subshell obtained from the splitting of oxygen octahedron. Six niobium atoms are from the third nearest sub shell. 24 oxygen atoms form a fourth adjacent octahedral shell consists of four sub shell of six atoms. Fifth shell is made of 12 niobium atoms. Neutron diffraction study predicts the structural change with temperature KNbO3. It shows three phase transition from cubic to tetragonal, tetragonal–orthorhombic and orthorhombic–rhombohedral at
2.8 Sodium niobate (NaNbO3)
This is also the member of perovskite family, but with anti ferroelctric properties. Six phase transition in between −200 to 650°C range affects its structural, dielectric, and optical properties. Phase transition at 200,360 and 480°C has been observed due to off center displacement of Nb ion with tilting of oxygen octahedral. Its stable cubic structure of the
Several reports on ferroelectric materials and their use for different piezoelectric application has been studied in the last few years. As discussed in this chapter the effect of perovskite structure affect its ferroelectric properties. Doping in these perovskite structures also responsible for the enhanced of its properties by tailoring its crystal structure. These materials now investigated in the composite, nanowire and the nanorods form to make the device mechanically robust and more compact. This is a vital field for the research, as a key element in the digital world.