**3.3 Glass-ceramic properties and applications**

Glass-ceramics can be containing a substantial glassy phase if the range is highly crystalline. As a result of that, glass–ceramics can contain grain boundaries as well as it can range from transparent to opaque. Therefore, the properties of glass– ceramics depending on the microstructure and the chemical composition, which can be tuned to meet demanding requirements. In general, the most important of glass–ceramics properties that can be exhibited is about zero thermal expansion with high toughness. as well as, have a high impact resistance with resistant to thermal shock [5, 8].

The potential applications of glass–ceramics in energy storage capacitors was investigated by Du et al. [11]. Here, the Na2O-PbO-Nb2O5-SiO2 glass–ceramics system achieved a highest relative permittivity of >600 after heated the sample at 850°C. The results given by Gorzkowski et al. [46] shown that the interfacial

**Figure 7.** *Schematic representation of glass–ceramic processing [5].*

**Figure 8.**

*TEM micrographs showing Nanoscale crystallites precipitated from a glass matrix of SiO2/Lu2Si2O7 heat-treated at 1100°C (a), 1200°C (b) and 1300°C (c) [44].*

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

**Figure 9.** *Schematic of glass processing in the formation of a glass ceramic [45].*

polarization due to the difference in the dielectric and conductivity of glass phase and ceramic phase large hysteresis loss, led to low energy density (Max. 0.9 J cm−3) and breakdown strength (Max. 800 kV cm−1) of the BST glass–ceramics that expected much higher energy density (≈ 3.5 J cm−3 in case assuming is it ideal linear dielectric behavior) [46].

As the energy crisis intensifies, the search for novel, high performance and environmentally friendly energy storage devices have attracted wide attention. For a material to possess high energy density capabilities, a high relative permittivity and dielectric breakdown strength are needed; this is a challenging task since theoretical and experimental studies on crystalline materials usually show a contrary relationship between these properties [47]. Compromises between relative permittivity and dielectric breakdown strength have encouraged the search for more efficient materials such as polymers and glasses. Recently, a high relative permittivity and a high dielectric breakdown strength were found in fluoropolymers, with energy density values up 25 J cm−3 being reported [48]. A high dielectric breakdown of ≈ 12 MV cm−1 (1.2 MV mm−1) was also reported [49] in a low relative permittivity (εr ≈ 6) alkali-free glass, showing the possibility to reach an energy density of 35 J cm−3. These results are very promising since the relationships between relative permittivity and dielectric breakdown strength in inorganic glasses are not well established.

Xue et al. [38] studied the effect of glass concentrations on the energy storage capabilities of niobate glass ceramics (100-x)(40BaO-10Na2O-50Nb2O5)-x(63SiO2- 12BaO-16B2O3-9Al2O3) prepared via controlled-crystallization. They achieved dielectric breakdown strength of about 130 kV mm−1 and a charge–discharge efficiency of 92.5%, with a relative permittivity of about 50, see **Figure 10** below.

Glass-ceramics are amongst the most important hosts for transition metal ions [51]. There are several benefits of adding glass to ceramics, such as improving their dielectric properties and reducing both the sintering temperature and the porosity [6]. In glass–ceramics, the ferroelectric and dielectric properties are highly dependent on the characteristics of the glass matrix as well as the crystal volume fraction and crystal size [52]. Glass-ceramics are crucial in the development of more efficiently produced and controlled energy. They are thought to be efficient cathodes or solid electrolyte materials when used in lithium batteries. Dielectric glass ceramics are also of interest due to their uses in high-power microwave systems, distributed power systems, power electronics, and pulsed power.

### **Figure 10.**

*Relative permittivity versus temperature curves of niobate glass–ceramics prepared via the controlled-crystallisation route, samples were annealed at 1100°C. When the glass content was 30–60%, the samples were labelled G30 to G60, respectively [50].*

To decrease the thickness of the dielectric layers in a capacitor and reduce the weight of portable pulsed power systems, it may be necessary to have high dielectric breakdown strength in the materials. In any case, improved control of the porosity, along with enhanced energy storage capabilities, are important aspects of improving the performance of glass–ceramics [53]. The significance of glass–ceramics in many applications lies in the possibility of utilizing key properties such as transparency, strength, resistance to abrasion, and the controlled coefficient of thermal expansion. All these properties can be manipulated by the controlling the composition, extent of crystallisation, crystal morphology, crystal size, and aspect ratio of the materials [54]. Glass-ceramics are used in a range of applications across different fields such as telecommunication, radar, and navigation. With respect to the use of high-density electron device packages, the materials which are used in LTCC applications need to have good dielectric properties and a large coefficient of thermal expansion (CTE). These characteristics are important in order to achieve thermal matching between the components of the package [55, 56].

### **3.4 Chemical compositions of glass-ceramic systems**

### *3.4.1 Alumina-silica systems*

Silicates are salts of silicic acid, H4SiO4. Most of the silicates contain SiO2, while in the case of SiO2 or quartz glass, only SiO2 is present. For all other types of glass, additional oxides, such as: Li2O, BaO, B2O3, Na2O, K2O, MgO, CaO, or Al2O3 are employed as network formers or modifiers. Alkaline and alkaline earth oxides are important in glass due to their effects on the network. Therefore these additives work as effective network modifiers [57]. The alumina-silica system, Al2O3-SiO2, is amongst the most important binary oxides and ceramic systems. **Figure 11** shows the phase diagram of the binary Al2O3-SiO2 system. The low solid solubility for SiO2 in Al2O3, and vice versa, can be seen in this diagram. At a high temperature below 1890°C, part of the Al2O3 component is transformed to a molten state, while the other part remains solid. This also occurs with SiO2 at a temperature below 1600°C. The effect of the SiO2 ratio in reducing the melting point of Al2O3-SiO2, can be seen [58].

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

**Figure 11.** *Phase diagram of the binary alumina-silica system [58].*

**Figure 12** shows the silica structure as a network. In **Figure 12-A** the basic units for silica are represented; these are Si4+ with O2-. The network of pure silica, also known as quartz, is presented in **Figure 12-B**, while **Figure 12-C** shows the structure of this 2D network with the addition of some alkaline additives as modifiers [57].

### *3.4.2 Alkaline and alkaline earth silicates*

Glass-ceramics can be modified by a formulation process. Alkaline and alkaline earth oxides are modifiers of glass networks and act to decrease the connectivity of the glass network, effecting changes in the properties of the glass. There are many oxides that are used to modify glass-ceramics, such as: Ba, Li, B, Ca, Na, etc. [57]. These glass-ceramic systems contain binary and ternary systems. Both types of systems contain oxides, such as: BaO, B2O3, MgO, CaO, PbO, ZnO, Al2O3, SiO2, and SrO [5]. Some of the oxides contained within the glass are popular as sintering aids for ceramics. This is because of the high stability of their structures and the low glass transition temperatures, in addition to their thermal and electrical properties. The systems of PbO-containing glasses (e.g. PbO-B2O3-SiO2 and PbO-B2O3-ZnO) are important due to the aforementioned reason [59, 60].

Other glass systems include Bi2O3-B2O3, BaO-CaO-Al2O3-B2O3-SiO2, CdO-Bi2O-PbO-B2O3, and BaO-B2O3-SiO2 which have all been used to reduce the sintering temperature [3, 61]. The structure of PbO is not easy to crystallise, even with a high percentage of it within the glass, because PbO4 is formed within the glass system. Recently, many glass systems have been developed as alternative lead-free materials. In this research, BaO, ZnO, and Bi2O3 are used instead of PbO [61]. Most glasses containing a high proportion of PbO are undesirable due to the toxicity of lead oxide causing environmental problems during or after production. Therefore, most researchers focus on decreasing the PbO content or replacing it with other oxides. This issue was addressed by Bobkova and Khot'ko [62], who were studied the ZnO-SrO-B2O3 system. They found that the optimum glass was produced when it contained a high ZnO and low B2O3 content. In addition, two ternary systems, B2O3-CaO-SiO2 and B2O3-CaO-Al2O3, have been studied by Vartanyan et al. [63]. They found that these two systems successfully produced lead-free materials for low-temperature simultaneously fired ceramics (LSCs).

Glass manufacturing processes can be generally complex due to the materials which undergo different changes during the melting and cooling. The ternary systems of B2O3-CaO-Al2O3, B2O3-CaO-SiO2, and B2O3-ZnO-SiO2–BaO were developed at low melting points below 1000°C then used as sintering aids with ceramics at low sintering temperatures [62, 63]. Consequently, these systems (BaO-B2O3- SiO2) and (BaO-B2O3-SiO2- Al2O3) possess their own particular thermal physical properties, where achieved a coefficient of thermal expansion (11–17 ppm °C−1) and low permittivity (≈7) [64]. B2O3-Bi2O3-SiO2-ZnO (BBSZ) is commonly used as an appropriate additive to reduce the sintering temperature of many dielectric materials and make them suitable for LTCC applications [65, 66]. Therefore in order to improve the electromagnetic properties of Ni–Cu–Zn ferrite (NCZF) and BaTiO3, the BBSZ glass was chosen as a melting agent [67]. Glass-ceramics based on the CaO–MgO–SiO2 system has been studied with the additives B2O3, P2O5, Na2O, and CaF2. The results showed that these systems required high melting points and the final crystallisation of the glass-ceramic occurred with temperatures of 900°C, producing high levels of density [68].

### *3.4.3 Barium borate system*

The binary system of BaO-B2O3 is used in a wide range of glass ceramic production, oxide cathodes and the coloured materials in paints. In addition, these systems are used as sintering aids for low temperature co-fired ceramic applications due to the low softening temperature of the B2O3 [69]. **Figure 13** shows the phase diagram for the binary system of BaO-B2O3 [70]. The procedures of melting and cooling the BaO-B2O3 systems lead to glass formation. In the phase diagram, it can be seen that the lowest melting point with high stability for these systems occurs with approximately 60 to 80% B2O3. In practise, the optimum ratio also depends upon the rate of cooling of the glass [70].

The binary system of BaO-B2O3 was modified by SiO2 or Al2O3 as a network modifier. Therefore, it was expected that the mineral phase with odd physical characteristic would be obtained. Therefore, glass ceramics are prepared with low sintering temperatures [64]. It should be noted that the melting point of B2O3 is around 450°C, which is much lower than SiO2, which has a melting point of around 1710°C. However, the crystalline glass of B2O3 is much more difficult to produce than that of SiO2 [70].

SiO2 added to borate glasses generally leads to an improvement in the density of the glass and an increase in its Tg and chemical durability [71]. The effect of the Al2O3 on the characteristics of BaO-B2O3-SiO2 was reported by Lim et al. [72].

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

**Figure 13.**

*Phase diagram of the BaO-B2O3 system [59].*

It was found that the crystallisation temperature, sintering temperature, and glass transition temperature increased with increased amounts of Al2O3 in the glass systems. The ternary glass system BaO-B2O3-SiO2 was studied by Lim et al. [61, 69]. The SiO2 ratio was fixed at 10% and the effects of the amount of BaO/B2O3 on the thermal and physical properties of the glass system were studied. Increasing B2O3 content gave rise to a clear increase in the dielectric properties. The electrical conductivity of solid silicates shows a complex dependence on glass formation processes, devitrification, and temperature. Here, a slight change in the composition of the glass can lead to marked differences in the electrical conductivity values. In spite of the fact that most studies are about the binary systems, however, there is a scarcity of experimental data on the electrical conductivity of the glasses. Accordingly, the study of the influence of the metal oxide on the electrical conductivity is important for the multicomponent silicate glasses for electrical purposes [73, 74].

It was reported that the electrical conductivity is decreased by the presence of the K+ ions in the alumina-silica glasses while the glass transition temperature increased [73]. Wang et al. noted that the addition of Na2CO3 into Ba0.4Sr0.6TiO3 (BST) ceramics caused a decrease in the room temperature dielectric constant of BST, while the highest Na + and K+ content produced an increase in the energy storage density [75].

### **4. Ferroelectric glass-ceramic systems**

The following sections summarise the results of previous research on the use of glass additives into ferroelectric ceramics, producing ferroelectric glass-ceramic and composites, and on the heat treatment of amorphous glasses to produce glass-ceramics containing nanocrystalline ferroelectric phases. The emphasis of the present research is on lead-free ferroelectric materials and therefore previous work on lead titanate and PZT-based glass-ceramics is not included here.

### **4.1 Barium titanate systems**

BaTiO3-based ceramics have been widely used in a number of electronics and electrical industries, due to their excellent dielectric properties [76]. Glass additives have been used during the sintering process of ceramic materials to form a liquid phase to improve the sintering behaviour of well-known ferroelectric ceramics such as BaTiO3, PZT and KNN. During this process, the reactive liquid formed by the glass additives wets the solid particles and facilitates their rearrangement. This rearrangement of particles gives more effective packing and higher densities, desirable for ferroelectric applications. Furthermore, the capillary pressure developed at each inter-particle space provides an additional driving force for particle rearrangement and re-shaping during sintering, inducing densification.

The pioneering work of Stookey in 1949 [77] on photosensitive glasses led to the discovery of methods for heterogeneous nucleation, where crystal growth takes place from many nuclei dispersed in the glass matrix after the glass has been formed [78]. Later on, Herczog [79] studied the properties and composition of glasses suitable for crystallization of BaTiO3 from glasses. Perovskite BaTiO3 with a minor amount of BaAl2Si2O8 was obtained by heat treating glasses of compositions corresponding to (x)(BaTiO3) + (100-x)(BaAl2Si2O8). The grain size was controlled in the range 0.01 to 1 μm by varying the heating rate and the final heat treatment temperature. When the volume content of BaTiO3 in the glass-ceramic increased from 30 to 60% and the grain size was about 1 μm, the dielectric constant increased from 100 to 1200. For grain sizes less than or equal to 0.2 μm, the relative permittivity was further decreased and found to be independent of temperature; this was attributed to the presence of surface defects in the fine grains. The highest relative permittivity of 1300 at room temperature was achieved at average grain sizes of about 1 μm, as can be observed in **Figure 14**; the breakdown strength and electrical resistivity of this ceramic were also reported to be high.

Nano-sized BaTiO3 (20–80 nm) was produced by McCauley et al. [80] following the same approach as Herczog [79] and using different heat treatments and compositions. They observed intrinsic size effects at crystal sizes lower than 80 nm and predicted a critical size of 17 nm, at which BaTiO3 cannot support a ferroelectric transition. Takahashi et al. [81] reported the glass-ceramic of 0.65(Ba1-xSrx) TiO3–0.27SiO2–0.08Al2O3 (BSTS). The heating temperature of glass-ceramics was 1000°C with a small amount of Sn or Zr; also the relative permittivity also decreased with the addition of both of the Sn and Zr. The εr–T relationship showed a broadening and shift of the Curie point towards room temperature, as shown in **Figure 15**.

The use of solid state reaction for the preparation of BaTiO3 ceramics has advantages where the products have good crystallinity and low cost, as well as a high level of accuracy of stoichiometric control [82]. However, many ferroelectric materials have high sintering temperatures (>1200°C), making them unsuitable for certain applications, such as LTCC. In this case, it is necessary to include some additives to reduce the sintering temperature. There have been many studies aimed at lowering the sintering temperature of BaTiO3. The typical additives used include ZnO, CuO, LiF, and CdO. Amongst these additives, LiF proved to be a very effective sintering aid. The effect of LiF in BaTiO3 was investigated and the results indicate that the sintering point is reduced to approximately 900°C [83, 84].

The use of glass additives in BaTiO3 was also shown to improve densification behaviour and reduce the required sintering temperature. Jeon et al. [85] studied the effects of BaO-B2O3-SiO2 (BBS) glass in BaTiO3 ceramics. It was found that the *Ferroelectric Glass-Ceramic Systems for Energy Storage Applications DOI: http://dx.doi.org/10.5772/intechopen.93855*

**Figure 14.** *Variation of dielectric constant and loss tangent with the temperature at 1 kHz [79].*

**Figure 15.** *The* ε*r–T relationship for BSTS bulk glass-ceramic derived from SnO2-containing glasses [81].*

sintering temperature required to reach a high density (≈ 93%) could be reduced to ≈1000°C. The influence of glasses on the sintering behavior and properties of ceramics is dependent upon the glass additive content as well as the chemical reaction between the glass and ceramic phases. Three different glass additives for BaTiO3 were studied by Hsianga; these were BaO-B2O3-SiO2 (BBS), PbO-B2O3- SiO2 (PBS), and ZnO-B2O3-SiO2 (ZBS). It was reported that the glasses containing PbO and BaO led to degradation of the dielectric properties and densification of the ceramic. In addition, adding both BBS and PBS caused the formation of a large number of secondary phases [3].

Lin et al. [86] added a manganese oxide-silica glass to pure BaTiO3 and reported the effect of the liquid phase on the dielectric and ferroelectric properties of the material. The addition of the Mn-Si-O glass enabled densification of the nanocrystalline powder at temperatures in the range 1175–1300°C. At high glass concentrations, they observed the formation of Ba2TiSi2O8 and a Mn solid solution in BaTiO3 grains growing at the grain boundaries and inhibiting grain growth. **Figure 16** shows the influence of glass content on the structural and dielectric parameters [86]. It can be observed, that the ceramics with grain sizes in the range of 0.7–1 μm have the higher relative permittivity. Meanwhile, for the glass doped samples, the higher tetragonality of BaTiO3 induced through the liquid phase sintering led to larger dielectric permittivity. However, the dielectric loss was also affected by the Mn solid solution in BaTiO3 and the Ba2TiSi2O8 phases.

More recently, Chen et al. [87] studied the addition of 50–90% of B2O3–Bi2O3– SiO2–ZnO (BBSZ) glass on the dielectric and ferroelectric properties of BaTiO3. By this means, the sintering temperature was reduced to 400–450°C. The second phase of Bi24Si2O40 was observed when samples were sintered at 450°C and the BBSZ concentration was higher than 60 wt%. Once the solubility limit of BBSZ on BaTiO3 was exceeded (BBSZ>60 wt%) and the formation of the Bi24Si2O40 and glass phases took place, some pores of about 1–2 μm appeared due to the capillary pressure, rearranging the particles and affecting the packing; this reduction in density led to a lower relative permittivity for the 60 wt% modified samples. When the amount of liquid increased (up to 70 wt%), the porosity decreased and relative permittivity values of 132 and 207 were achieved at 100 kHz and 100 MHz, respectively. The dielectric loss remained at the same level as that of the pure glass at 100 kHz (0.006). At higher BBSZ concentration (80 and 90 wt%), the overall dielectric properties of the samples decreased due to the lower relative permittivity of the glass phase.

Choi et al. [88] reported that the addition of 1 to 7 wt% of BBS into BaTiO3 (prepared using flame spray pyrolysis) reduced the sintering temperature at 1000°C, yielding an improvement in relative permittivity in comparison with pure BaTiO3 sintered at 1300°C. It was also found that the glass additives improved the density

### **Figure 16.**

*Effect of glass addition on the (a) grain size (b) tetragonality and (c) dielectric constant and loss of die-pressed BaTiO3 [86].*

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

of the material with an increase in the grain size by several microns. Wang et al. [26] found that the sintering temperature of BaTiO3 ceramics could be reduced to about 1100°C using a glass with the composition 27.68BaCO3–6.92SrCO3- 29TiO2–22SiO2-12Al2O2–2.4BaF2 (mol %). The effect of this additive was a reduction in the average grain size. It was also found that the energy storage density of the ceramics increased gradually with increasing glass concentration; the highest energy density value of 0.32 J cm−3 was obtained for the sample with 7 mol% of the glass. The influence of glass additives on modified barium titanate ceramics was reported by Puli et al. [35], who studied the dielectric and ferroelectric properties of glass-modified BCZT, specifically the composition 0.85(BaZr0.2Ti0.8O3)- 0.15(Ba0.7ZCa0.3TiO3). The addition of 15 wt% (0.1BaO + 0.4B2O3 + 0.5ZnO) and (0.3BaO + 0.6B2O3 + 0.1ZnO) glass powders yielded dielectric breakdown field strengths of 260 and 280 kV cm−1, with recoverable energy density values of about 1.12 and 0.50 J cm−3, respectively. The resulting samples were composites comprising a mixture of two phases, BCZT and the glass phase. They attributed the high breakdown strength to the presence of alkali free glass composition, and the low loss dielectric to the low dielectric loss of BZT–BCT ceramic composition.

The effect of up to 9 wt% of CaO–B2O3–SiO2 (CBS) glass additions on the microstructure and electrical properties of Ba0.85Ca0.15Zr0.1Ti0.9O3 (BCZT) was studied by Lai et al. [89]. Samples were sintered at 1300°C for 4 hours in air; in the CBS-free sample, they observed the coexistence of orthorhombic and tetragonal BCZT phases with a small amount of CaTiO3 secondary phase by means of XRD. The addition of the CBS glass induced a phase transformation to pseudocubic, a shift of the reflections towards lower 2θ values and the coexistence of the orthorhombic and tetragonal phases disappeared. These phase transformations were accompanied by the appearance of another secondary phase, reported to be Ba2TiSi2O8. The density of the BCZT samples increased with CBS content, reaching a maximum when the addition of CBS was 2 wt%, and then slowly decreasing at higher CBS concentrations. Regarding the electrical properties, the sample with 2 wt% CBS exhibited the best ferroelectric properties, a dielectric constant of 8874 (at 106 Hz), a Curie temperature of 116°C, Pr of about 3.18 μC cm−2, and d33 of 159 pC N−1 due to the improved density achieved by the addition of CBS.

### **4.2 Potassium sodium niobate systems**

KNN is one of the most promising candidates for energy storage properties, yet at present much research has focused on the development of, for example, (BaO,SrO)-TiO2 [90, 91], (BaO,SrO)-Nb2O5 [50] and (BaO,Na2O)-Nb2O5 [92] glass-ceramics. Therefore, the study of KNN glass-ceramics still needs more exploration and in-depth research [93]. Many studies have been conducted on sintering of KNN, but there are currently few reports of true glass-ceramics.

Some previous studies have reported the formation of KNN in a glass-ceramic form [56]. Vernacolota et al. [94] reported that glass-ceramics containing KN and KNN phases can be obtained using silicate glasses prepared with alkali metal and niobium additions. The effects of substituting K by Na on thermal and crystallisation behaviour of KNN-SiO2 glass-ceramics were reported by Aronne et al. [95]. Kioka et al. [96] and Kongputhon et al. [97] studied the control of the crystallisation behaviour in KNN-SiO2, by varying the K:Na ratio, as a means of modifying the dielectric properties. Alumina-silicate glasses have also been used to fabricate glass-ceramics containing ferroelectric KNN and their dielectric properties reported by Yongsiri et al. [54]. The influence of CeO2 as a nucleation agent in borosilicate glasses containing KNN have been reported for energy storage capacitor application by Hanyu et al. [93]. On the basis of their results, Yongsiri et al. [54] suggested that

KNN glass-ceramics could be favourable for use in electro-optical applications. The microstructure, crystallite sizes, and crystal quantity of the KNN glass-ceramics were studied as a function of the heat treatment conditions. The heat treated glassceramics were found to contain a KAlSiO2 phase at heat treatment temperatures between 600 and 675°C, while the perovskite KNN phase was observed at a higher temperature of 700°C, **Figure 17**. The glass-ceramic containing 23.75 mol% of SiO2 exhibited a relative permittivity value of ≈ 260 and loss tangent ≈ 0.02 at 10 kHz.

The use of a two-stage incorporation method, which involved the separate preparation of KNN and glass powders prior to melting, was also studied by Yongsiri et al. [98]. The calcined KNN powder was mixed with 25 mol% of SiO2 then melted at 1300°C using the conventional melt-quenching technique. The glass was heat treated at temperatures from 525 to 575°C for crystallisation. Increasing heat treatment temperatures were found to improve the crystal size and crystallinity, which in turn plays an important role in controlling the properties of the glass ceramics, including physical, optical, and dielectric properties. It is clear from the SEM results, **Figure 18** that the crystallisation of the KNN phase occurred at temperatures lower than the observed crystallisation temperature of 648°C from the DTA results. Furthermore, the amorphous XRD patterns were observed in the glass-ceramic samples heat treated at temperatures lower than 550°C. The highest relative permittivity value was 474 at a heat treatment temperature of 550°C, while the transparency decreased with increased temperatures.

KNN ceramics prepared by solid state reaction usually require reaction temperatures around 800°C or, sometimes, the double calculation in order to obtain a homogenous powder; sintering temperatures are usually in the range 1100–1200°C [99]. The three starting materials used in KNN are Nb2O5, with a high melting point of 1520°C, and two alkali metal carbonates, K2CO3 and Na2CO3, with melting points of 891°C and 851°C, respectively. Therefore, the alkali metal carbonates/ oxides become volatile during the calcination, making it difficult to achieve a chemically homogeneous material. Excess alkali carbonate is used in order to compensate the losses during calcination and to obtain a single phase perovskite product [100]. The studies reported in [96, 100] indicated that a 5 mol% excess of K2CO3 and Na2CO3 in the raw materials leads to increased stability during calcination at 800°C. The other effect of this excess an increase in particle size, which is probably due to the liquid phase which forms during the calcination [100]. By adding 3% and 1% excess alkali carbonate the density of KNN has improved and, at the same time, this excess leads to reduce the sintering temperature to 1000°C [101, 102].

### **Figure 17.**

*XRD patterns of the glass-ceramic sample with 5 mol% alumina and 23.75 mol% SiO2 subjected to different heat treatment temperatures [54].*

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

Sintering temperatures for KNN ceramics are usually >1000°C. This needs to be lowered in order to decrease the alkali evaporation during the periods of high temperature, as well as to make it suitable for many desired applications [103]. If lithium additives are added to KNN, the sintering temperature reduces to lower than 1000°C, there is a shift in TC to a higher temperature, and there is an increase in the dielectric constant [104]. Barium borosilicate-based frit (BaO-B2O3-SiO2-Na2O-K2O-CuO-CaO) (abbreviated as BBS) was one glassy additive used previously as a sintering aid to KNN. It has been shown to reduce the sintering temperature to 1000°C. In addition, the additive led to an increase in the mechanical properties and a decrease in the dielectric loss, whilst the TC remained high at 400°C; samples with 1.5 wt% glass frit showed optimal properties as follows: εr = 410, tanδ = 0.57 and d33 = 108 pC N−1, **Figure 19** [105].

*(a)* ε*r and tan*δ *(b) d33, kp and Qm of KNN + x wt% BBS ceramics as a function of the x value [105].*
