**1. Introduction**

Nanocomposite materials can be obtained through the crystallisation of the grainboundary glass phase in a ceramic matrix; the electrical and structural properties are improved with glass additives [1]. Over the last few decades, the field of electronic ceramics applications has been progressing. Some newer applications, such as in low temperature co-fired ceramics (LTCCs) and dynamic random access memories (DRAMs), utilise the material's dielectric properties. LTCC applications require the sintering temperature to be below the melting point of the electrode materials [2]. The chemical processes of adding glass and using starting materials with ultra-fine particle sizes improve the characteristics of ceramics at low sintering temperatures [3]. Glass additives can have useful effects on the dielectric constant due to their effect in broadening the diffusive phase transition at the Curie temperature, something that is desirable in the application of multilayer ceramic capacitors [4].

Glass–ceramics are ceramic materials that are produced through the controlled nucleation and crystallisation of glass through thermal treatment. Depending upon the chemical composition and microstructure of glass–ceramics, they can exhibit useful thermal, optical, chemical, mechanical, electrical, and magnetic properties. Useful composite materials can be produced by combining glass–ceramics and other materials, such as metals [5]. Low sintering temperatures and high relative permittivity are of primary importance in the manufacture of ferroelectric ceramics. Typical

dielectric ceramic materials, such as barium titanate (BaTiO3) and lead titanate zirconate (PZT), have found many applications in the electronics industry. Certain additives for BaTiO3 and PZT, such as LiF and PbO-B2O3-SiO2, can reduce their sintering temperatures to around 900°C and improve their ferroelectric properties, making them suitable for a range of different electronic applications [6, 7].

It is known that the functional properties of ceramic materials are strongly dependent on microstructure, which provides opportunities to develop new or improved ceramic materials through microstructural engineering. One of these approaches involves combining ferroelectric perovskite with glass-forming oxides in order to form ferroelectric glass–ceramics [5, 8]. The microstructure of such materials comprises ferroelectric nanocrystals dispersed within a glass matrix [9], giving rise to novel materials having pore-free, fine-grained microstructures, low thermal expansion coefficients, high mechanical strength, high chemical stability and good dielectric properties [10]. Such materials have potential applications in high energy density capacitors [11], as well as piezoelectric [12] and electro-optic devices [10, 13]. The glass–ceramic processing route can provide well-controlled microstructure, formed by the crystallisation of chemically and microstructurally homogeneous glasses, at relatively low cost [5, 8].

## **2. Fundamentals of Ferroelectrics**

Ferroelectrics are insulating solids that have spontaneous polarisation. This means that they contain a permanent polarisation at the unit cell level, even in the absence of external electric fields. Additionally, ferroelectric materials exhibit the ability to alter the orientation of their polarisation between two or more directions when under the influence of external electric fields. In order to exhibit spontaneous electric polarisation, there must be a noncentrosymmetric arrangement of the ions and their electrons in these materials. Many ferroelectric materials have perovskite structures with a general chemical formula of ABO3 ABO3-type oxides are known to stabilise with a wide range of A (Pb, Ba, Ca, Sr) and B (Ti, Zr, Sn) ions, with A ions having larger ionic radii than B ions.

Ferroelectrics have typical properties which are essential for their use in electronic devices. High relative permittivity and low-loss dielectric characteristics are most important in multilayer ceramic capacitors (MLCC), which are widely used in electronic devices. There have been progressive developments in the manufacture of MLCCs to increase both the relative permittivity and the number of layers, as well as decreasing the layer thickness, t, according to the equation below [14, 15].

$$\mathbf{C} = \frac{\mathbf{c}\_o \mathbf{c}\_r \mathbf{A}}{\mathbf{t}} \tag{1}$$

where ԑr is the relative permittivity or relative dielectric constant. The capacitance itself is dependent upon ԑr, the area of the parallel plates, A, and the thickness of the dielectric material, t.

Ferroelectrics are polar crystals with the ability to alter their polarisation direction upon the application of an external electric field. They exhibit spontaneous polarisation, even in the absence of external electric fields. In the unit cell, net permanent dipole moments are present in ferroelectric materials. In polycrystalline ceramics, the orientation of the dipole moments are random and therefore a net polarisation is not normally present after cooling through TC in the absence of an external electric field. The overall orientation of the dipole moments in

*Ferroelectric Glass-Ceramic Systems for Energy Storage Applications DOI: http://dx.doi.org/10.5772/intechopen.93855*

polycrystalline and single crystal ferroelectrics are not completely random at the scale of the unit cell, since they form ordered groups, referred to as domains. Within the domains, there is a uniform alignment of dipoles, with neighboring domains being separated by boundaries known as domain walls.

The direction of spontaneous polarisation in ferroelectrics can be altered through an applied electric field, as shown in **Figure 1**. With the increase of the electric field, the domains begin to align, giving rise to an increase and saturation in the polarisation at high field. In the absence of an external electric field, some of the domains remain aligned. Thus, the crystal displays remnant polarisation. If the field is reversed, the domains change direction. The direction of polarisation flips and produces a hysteresis loop when the external electric field alternates between negative and positive [16, 17].

### **2.1 Energy storage in capacitors**

Significant improvements over the last couple of decades in both the energy storage density and reliability of capacitors have been achieved through a combination of novel materials, diagnostic methods, and manufacturing techniques. Capacitors, inductors, and batteries are means through which electrical energy is stored. **Figure 2** depicts a graph of the specific energy for different energy conversion and storage devices plotted against their specific powers [18].

The characteristics of energy-storage in four types of the most highly studied dielectric materials, namely, relaxor ferroelectrics, polymer-based ferroelectrics, antiferroelectric, and dielectric glass–ceramics were reviewed by Hao [19].

The changes in polarisation upon the application of an electric field are a critical aspect of energy storage dielectrics. This response can be used to estimate the stored energy, which should exclude hysteresis losses. Dielectrics may be grouped into being either linear or non-linear, according to the relationship between the applied electric field and the polarisation. A simple equation (below) may be used to describe their behavior [20].

$$\mathbf{D} = \mathbf{c}\_o \mathbf{E} + \mathbf{P} = \mathbf{c} \mathbf{E} \tag{2}$$

**Figure 1.** *Illustration of the polarisation-electric field relation, P-E hysteresis loop, for a typical ferroelectric crystal [16].*

Therefore:

$$\mathbf{P} = \mathbf{e}\_o \left(\mathbf{e}\_\mathbf{r} - \mathbf{1}\right) \mathbf{E} = \mathbf{x}\_o \mathbf{e}\_o \mathbf{E} \tag{3}$$

where χ is dielectric susceptibility and D is the dielectric displacement. Energy density, U, is a measure of the energy stored per unit volume. For dielectrics, this can be obtained by the following relationship:

$$\mathbf{U} = \bigcap\_{\mathbf{0}}^{\mathrm{E}\_{\mathrm{max}}} \mathbf{P} \mathbf{d} \mathbf{E} \tag{4}$$

Using formula above (Eq. (4)), the U values of the dielectrics can be obtained through the numerical integration of the area between the polarisation and curves for the electric-field polarisation (P-E) loops. **Figure 3**, shows that upon reaching the maximum electric field strength (Emax), the polarisation approaches its

### **Figure 3.**

*The typical dependence of (a) polarisation and (b) relative permittivity on the electric field of ferroelectrics in the first quarter shows the charge–discharge cycle. The area I (green shaded area) corresponds to the discharged or recoverable, energy density and area II (red shaded area) correspond to the energy density loss [19].*

maximum (Pmax) and the capacitor holds the electrical energy (Ustore), as illustrated by the red and green areas.

The recoverable electrical energy density (Urec) is released during the discharge process when the electrical field reduces from Emax to zero. This is represented by the green area in **Figure 3**. Therefore, an amount of the stored energy (the red segment surrounded by the loops) is dissipated during the process of depolarisation, denoted the hysteresis loss, Uloss [19, 21].

The above analysis indicates that there are three prerequisites to designing an effective dielectric material for practical use with high efficiency and high recoverable energy-storage density. These three requirements need to be satisfied simultaneously and are small remnant polarisation, large saturation polarisation, and a high electric breakdown field [22].

**Figure 4(a)**-**(d)** depicts typical P-E loops and an illustration of the energystorage of four types of dielectrics: (a) linear dielectric with constant permittivity (e.g. Al2O3, glass), (b) antiferroelectric with zero net remnant polarisation (e.g. PbZrO3), (c) ferroelectric with spontaneous polarisation (e.g. PbTiO3, BaTiO3), and (d) relaxor ferroelectrics with nanosized domains, e.g. (Pb,La)(Zr,Ti)O3.

Even though linear dielectrics often have lower energy losses and higher breakdown fields, small polarisation values resulting from the use of low-permittivity dielectrics can reduce their effectiveness for high-energy storage purposes, unless very high breakdown fields can be achieved. Ferroelectrics generally have moderate electric field endurances and larger saturated polarisations, however, due to their larger remnant polarisations, they are often less efficient and have smaller energy-storage densities. **Figure 4** demonstrates that antiferroelectrics and relaxor ferroelectrics are more attractive for high energy storage due to their relatively moderate breakdown fields, smaller remnant polarisations, and larger saturated polarisations.

### **Figure 4.**

*Schematic description of the energy storage characteristics of (a) linear dielectrics, (b) antiferroelectrics, (c) ferroelectrics, and (d) relaxor ferroelectric ceramics [23].*

Novel manufacturing processes, such as the use of composite technology and glass-crystallisation techniques, have allowed for the production of ceramicpolymer composites and glass–ceramics. These materials could potentially combine the larger polarisations of ferroelectrics and the higher breakdown fields of linear dielectrics. Therefore, amongst the aforementioned four groups of dielectrics, namely, relaxor ferroelectrics, ceramic-polymer composites, glass–ceramics, and antiferroelectrics, the former two are generally thought to be the most useful for high energy storage purposes and therefore much research has been conducted on these two types of material [19, 23].

Pb(Zr,Ti)O3 (PZT) based materials have been widely used in energy storage applications because of their high dielectric constant. However, the environmental issues derived from the use of lead have encouraged many searches for more environmentally friendly materials.

The perovskite structure of BaTiO3, capable of high dielectric constant values, spontaneous polarization, low dielectric loss and ferroelectricity offers an alternative for lead-based capacitors. As mentioned earlier on, for energy storage applications a high dielectric breakdown strength is required to allow device miniaturization. It is well known that the energy storage properties of BaTiO3 based ceramics can be improved by reducing the porosity [24], tuning the grain size [25], the addition of glass additives [26], presence of secondary phases, etc. For example, the relative permittivity of BaTiO3 increases as the grain size decreases [27], reaching a maximum of 5000 at grain sizes of about 0.8 to 1.1 μm [28]. This was attributed to domain size and stress effects. Further reductions in the grain size resulted in a rapidly decreased permittivity. Furthermore, the dielectric breakdown strength increases with decreasing grain size [29], being about 8.5 kV mm−1 when the grain size is 3.5 μm [30].

The addition of glass additives to induce liquid phase sintering is a widely used technique to improve the energy storage capabilities of BaTiO3 based ceramics. During the liquid phase sintering, a thin layer of the fluxing agent coats the BaTiO3 grains leading to improved relative densities and reduced sintering temperatures. Until now, the use of several glass additives in BaTiO3 ceramics has been proved to show promising results for energy storage applications. For example, Sarkar and Sharma [31] demonstrated that the addition of B2O3 and PbB2O4 to BaTiO3 significantly reduced the sintering temperature to about 800°C, which is suitable for commercial applications as multilayer capacitors. Moreover, they doubled the dielectric breakdown strength of BaTiO3 by the addition of 10 mol% of PbB2O4 [31]. However, this improvement in the dielectric breakdown strength was accompanied by a small decrease in the dielectric constant.

The aliovalent substitution at the Ba2+ and/or Ti4+ sites in the perovskite structure of BaTiO3 has been demonstrated [32] to be an effective approach to tailor the energy storage properties of BaTiO3 to meet industrial application requirements. Recently, Puli et al. [33] investigated the dielectric, ferroelectric and energy density properties of (1-x){BaZr0.2Ti0.8O3}˝-(x){Ba0.7ZCa0.3TiO3} where x = 0.1, 0.15 and 0.20, hereinafter denoted BCZT. They reported a dielectric of the permittivity of 8400 when x = 0.15 and a low loss (tanδ ) of 0.014 in samples sintered at 1600°C. **Figure 5** shows the discharge energy density, charge energy density, and energy storage efficiency reported by Puli and co-workers, measured at a maximum electric field of 80 kV cm−1 [33]. They achieved an energy storage efficiency of about 70% when x = 0.15.

Wang et al. [34] achieved an energy density of 0.52 J cm−3 in a (Ba0.85Ca0.15) (Ti0.9Zr0.1)O3 ceramic prepared by the sol–gel method. They attributed it to the improved microstructure compared to that obtained by the conventional solidstate reaction method. In order to simultaneously attain high dielectric breakdown *Ferroelectric Glass-Ceramic Systems for Energy Storage Applications DOI: http://dx.doi.org/10.5772/intechopen.93855*

**Figure 5.**

*(a) Composition dependence of recoverable energy density (Urec), stored energy density (Ustor), and energy storage efficiency (*ƞ *%) of (1-x){BaZr0.2Ti0.8O3}-(x){Ba0.7ZCa0.3TiO3} where x = 0.1, 0.15 and 0.20 (b) Weibull plots of the breakdown strength of BCZT ceramics sintered at 1600°C [33].*

strength, high energy density and a high dielectric constant in a material, the glass–ceramic concept has been devised. Here, the high dielectric breakdown of the linear dielectric (glass) and the high dielectric constant/large polarization typical of ferroelectric ceramics are combined in a nanostructured composite-type material. Puli et al. [35] followed the glass–ceramic approach to improve the energy storage properties of BCZT ceramics. They added 15 wt% of two different alkali-free glass compositions, namely 0.1BaO + 0.4B2O3 + 0.5ZnO and 0.3BaO + 0.6B2O3 + 0.1ZnO, to BCZT, they reported a slight improvement in the dielectric breakdown field to about 28 kV mm−1 but a lower energy density compared to glass-free BCZT. The low energy density values reported were attributed to the low relative permittivity values (≈ 270) for glass–ceramic composition.

Another lead-free perovskite material that exhibits useful ferroelectric properties is the solid solution system potassium-sodium niobate (KNN). The solid

solution in the binary system KNbO3-NaNbO3 crystallises as an orthorhombic perovskite, [36], with the composition around K0.5Na0.5NbO3 being the most popular due to its closeness to the morphotropic phase boundary (MPB) which occurs at about 52.5% Na [37]. The solid solution (K,Na)NnO3 exhibits ferroelectric behaviour which diminishes at high sodium additions until it completely disappears due to the nonpolar, antiferroelectric end-member NaNabO3 [38, 39]. The dielectric constant of K0.5Na0.5NbO3 at room temperature is about 290 [38] and reaches 990 at 473 K. The use of additives to reduce the grain size and to improve the energy storage abilities of KNN ceramics has shown promising results. Qu et al. [40] achieved an energy storage density of 2.48 J cm−3 and a breakdown strength of 29.5 kV mm−1 by reducing the grain size of KNN to 0.5 μm through the addition of Sr.(Sc0.5Nb0.5)O3 (SSN), although they reported the presence of porosity at the grain boundaries. Highly dense KNN-SNN samples were achieved through the addition of 0.5 mol% ZnO, leading to a breakdown strength of 40 kV mm−1 and an energy storage density of 2.6 J cm−3 [41].
