Electrical, Microstructural and Physical Characteristics of Talc-based Cordierite Ceramics

The aim of this work is to study the effect of various talc rocks for the preparation of talc-based cordierite ceramics. Raw talc and sintered cordierite-based ceramic samples (1000-1375ºC for 2 h) were characterized using XRD, XRF, TGA-DTG, laser PSDs, Archimedes method, SEM-EDAX and dielectric relaxation spectrometer (DRS). Results show that impurity oxide contents, particle size and mineralogical changes of green batches influenced the microstructure densification and crystallization of orthorhombic and hexagonal cordierite. The complex electric modulus plot shows the existence of two relaxation processes associated with the capacitive contribution grain boundaries and grain at low and high frequencies, respectively. The dielectric loss reached much lower values (0.0004–0.0007) for the ceramics composed of higher cordierite phase composition (87.00 to 92.00wt.%) that sintered at 1350 and 1375ºC. Such ceramics could be promising in electronic applications like capacitors, microwave devices and wireless communication.

Cordierite ceramics could be classified into: (i) traditional cordierite ceramics that mainly formulated from (natural raw materials, such as ball clays, kaolin, feldspars, silica, dolomite and talc) and (ii) advanced cordierite ceramics that are formulated from pure oxides, carbides, nitrides and nonsilicate glasses [16].
Cordierite ceramics can be prepared by several techniques, such as solid-state, co-precipitation, sol-gel and hydrothermal treatment [17,[25][26][27]. The solid-state reaction is the applied method for preparing cordierite ceramics, according to the stoichiometric composition of cordierite, at high temperature using natural starting materials including MgO, Al 2 O 3 and SiO 2 [20,27,28]. The natural raw materials as talc, kaolin and feldspars are usually used in industrial/traditional production of cordierite ceramics because the chemical contents of these materials, in terms of SiO 2 , Al 2 O 3 , and MgO, are close to those of cordierite composition [27,[29][30][31].
The talc-kaolin batches have been chosen as starting materials in the preparation of cordierite, which mentioned to the narrow firing range, usually with 50ºC [30]. Difficulties in preparing cordierite are over-sintering and sintering without fluxing agents [30,32,33]. Oxide impurities of raw materials such as CaO, TiO 2 , Fe 2 O 3 , Na 2 O and K 2 O play a key role to decrease the solid phases crystallization temperature as well as the melt initiation which accelerate sintering at the applied temperature [34,35].
The solid-state preparation of cordierite ceramics using raw material would result in transient intermediate phases such as: mullite, spinel, cristobalite and glass within the end product [22,36]. The latter phases would increase the dielectric constant as well as the thermal expansion of the sintered cordierite ceramics [18,21].
The permittivity and loss factor of cordierite ceramics are variable based on the applied sintering temperature, phase composition and microstructure. Cordierite ceramics that prepared at 1200ºC would have a permittivity of 9.50 at 1MHz and thermal conductivity of 1.12W/m.K [17]. The primary phases, such as mullite, quartz, anorthite, periclase and spinel are consumed on the expense of crystallize cordierite at 1200ºC.
Zhang et al. [35] found that cordierite ceramics sintered at 1250ºC exhibit permittivity of (5-10); loss factor of (0.02-0.04) (at 10 3 -10 6 Hz) with enhanced thermal shock resistance (▲T~350ºC). The effect of sintering temperature and holding time show well developed cordierite at 1250ºC, then continue to disappear above 1425ºC due to melting and mullite crystallization. In addition, cordierite ceramics prepared at 1300-1350ºC exhibit dielectric constants in the range (15)(16)(17)(18)(19)(20) at 10MHz, which would permit their uses in the electronic applications [37]. High purity cordierite ceramics sintered at 1400ºC have dielectric constants in the range between (5.18-3.78) at 1 MHz [38]. However, cordierite ceramics prepared at 1420°C have dielectric constant of 4.60 at 100Hz with an accompanied low thermal expansion of 1.41×10 -6 ºC -1 . The latter values make these cordierite ceramics proper for high temperature low-dielectric engineering/high density integrated circuitry applications [19]. The sintering process of cordierite ceramics include transformation of primary minerals such as kaolin and talc into cordierite between 1080-1420ºC. Microstructure of sintered samples shows cordierite crystals with tetragyric-pillared at 1360ºC and cuboid-shaped crystals at 1400ºC [19]. The connection between grains is distinct, indicating the growth of these crystals with rising temperature.
Cordierite ceramics can be used in many applications such as a refractory material in electrical heaters, resistant porcelain, capacitors, radoms, wireless communications, etc [17,19,23,27,35]. Nowadays, because of their low permittivity, low loss tangent and high-quality factor, cordierite ceramics are useful for low loss dielectric materials such as wireless communications and electronic packaging. Cordierite ceramics have been studied for use in wireless communication devices [39,40] and in radio-frequency range applications [37,41].
The present work suggests the formulation of different cordierite-based ceramics based on natural raw talc and kaolin. In addition, the sintered samples will be investigated to interpret their dielectric properties in terms of mineralogical, microstructural, physical and dielectric characteristics of these samples with recommended applications.

Material and Methods
The raw materials used in this work are mainly different grades of talc samples quarried for variable industrial applications from Wadi Antr (T1), Wadi Nukharia (T2), Wadi Mubarak (T3), Wadi Abu Fannani (T4), Um Salatit (T5) and Wadi Allaqi (T6) at the Eastern Desert, Egypt. In addition, a high-quality kaolin sample (K) was collected from Masabh Slama, Southern Sinai, Egypt. The microstructure of the talc samples was determined using transmitted light microscope (TLM) equipped with camera (Nikon, United States). The chemical and mineralogical composition of the raw material samples were determined by X-ray fluorescence (XRF) (Axios advanced, Sequential WD-XRF Spectrometer, Panalytical 2005) and X-ray diffraction (XRD) (Bruker AXS X-ray diffractometer with Cu-target, λ = 1.542 Å, at 40 kV, 30mA potential and scanning speed of 0.02º per second). The semi-quantification of crystalline phases was calculated by Match software. The decomposition behavior of raw materials was determined with TG/DTG analyses at a temperature rise of 10º/minute using TGA PT1000 (Nitrogen), Linseis, Germany.
Based on the raw material characteristics, six powder batches (BT1-BT6) were designed to achieve the stoichiometric composition of cordierite (MgO: Al 2 O 3 : SiO 2 = 2:2:5) by adding commercial alumina oxide (S.d. Fine-chem LTD, Chennai-India) when necessary. The batch powders were thoroughly mixed using isopropanol in Fritsch planetary ball mill (Mono-Ball Mill, Germany) then sieved to pass 250-mesh sieve (i.e., <63μm). The particle size distributions (PSDs) of the powder batches were determined by Analysette 22, NanoTec plus, Fritsch, Germany. The powder batches were dried overnight in an electrical oven at 100ºC and pressed using hydraulic press into cylindrical disc samples of 0.90cm diameter at 5-ton pressure.
The green discs were dried in an electrical oven heated at 100ºC for 24h, then thermally sintered in Nabertherm electrical muffle, Germany, applying the heating ramps 550ºC/2h; 1000ºC to 1375ºC/2h and one cooling ramp at a rate of 5ºC/ minute.
The physical characteristics of the sintered discs (1000-1375ºC) including bulk density (BD), apparent porosity (AP) and water absorption (WA) were measured by Archimedes' method (ASTM C373 88 (2006)) [42] using electronic balance, Sartorius, Practum 224-1S, Germany. Linear shrinkage (LS) was determined by a RS PRO Electronic Digital Caliper 150mm/6, UK. Fractured surfaces of selected sintered discs were microstructurally investigated by using SEM (Philips-XL30, Netherlands) attached with Edax (Energy Dispersion X-ray) module. The dielectric measurements for the sintered ceramics (BT1, BT2, BT5, and BT6), were carried out over a wide frequency range (10 -1 -10 6 Hz) at room temperature. This has been achieved by placing each sample between the two electrodes of the measuring cell that is connected to a high-resolution impedance analyzer spectrometer (Schlumberger Solartron 1260), Germany. The dielectric properties we are concerned here are the permittivity (ε'), and the loss tangent (tanδ).
DTG thermograms show that there are two main mass loss ramps (Fig. 4). The first ramp ranges from 0.08 to 6.12wt.% that recorded between 593 and 767ºC representing the decomposition of clinochlore, serpentine, dolomite and magnesite. The second mass loss ramp (0. 65-3.08wt.%) appears between 873 and 962ºC referring to the decomposition of tremolite and talc ( Fig. 4) [21,[43][44][45][46][47]. In addition, the kaolin sample shows that a distinguished mass loss of 6.04 wt.% at the temperature range 86-128ºC is due to the evaporation of adsorbed water on the kaolin grains. The second mass loss is 5.60wt.% at the temperature range 430-556ºC due to the de-hydroxylation of kaolinite into metakaolin (Al 2 Si 2 O 7 ) and (H 2 O) (figure not shown) [48,49].
Based on the characterization of the raw talc (T1-T6) and kaolin samples, six batches (BT1-BT6) were designed with the addition of corrective alumina, when needed, to adjust the stoichiometric composition of cordierite. The detailed chemical composition of the six batches is shown in Table 2.
The granulometric distribution of the six batches (BT1-BT6) is shown in Fig. 5. The cumulative 10, 50 and 90wt.% of the samples have PSD less than the ranges (0.70-1.60), (3.30-13.60) and (20.20-43.10μm); respectively. The coarsest PSD is represented in BT1, BT2 and BT6 samples where the cumulative 90wt.% is below 43.10, 31.40 and 35.30μm, respectively. However, the finest PSD is represented in BT3, BT4 and BT5 samples (90wt.% of the PSD is below 20.20, 23.20 and 26.80μm, respectively). For solid-state sintering, the fine particle grain size enhances the densification and the development of higher liquid phases at low sintering temperatures [50][51][52]. Therefore, BT3, BT4 and BT5 samples would be densified and had higher liquid phase at lower temperatures compared to other samples.

Phase Composition
The calcined-based chemical composition of BT1-BT6 batches was plotted on the phase diagram (MgO-Al 2 O 3 -SiO 2 ) ( Fig. 6) [53]. Applying the lever rule, the anticipated solid phases ranged between (31.48-18.61 wt.%) with accompanied melt at the temperatures 1355 and 1400ºC, respectively (Fig. 3) The BT1-BT6 batches have been pressed into cylindrical discs then sintered in the temperature range (1000-1375ºC) for 2h soaking time. The sintered samples have been cooled down to room temperature inside the muffle furnace then investigated by XRD for phase identification (Fig. 7) and semi-quantification ( Table 3). The glassy phase is decreasing with the temperature rise as indicated from the up-convex baseline of the XRD patterns at 1000-1200ºC in all samples (Fig. 7). These glassy phases exist in the range (26.59-78.66wt.%) ( Table 3) where they would reflect the partial decomposition behavior of the green batches into silicaterich melt. The latter would be vitrified as glassy phases as well as crystallized during sample cooling to room temperature. The initiation of the silicate-rich melt is encouraged by the effect of the total impurity oxides (TIO: summation of TiO 2 , Fe 2 O 3 , CaO, Na 2 O and K 2 O), as fluxing oxides, bracketed between 2.03 and 11.44wt.% (Table 2) [20,34,54].
On the contrary, the crystalline phases contents of the sintered samples increase with the temperature rise  Table 3). These crystalline phases are due mainly to the solid-state phase transformation as well as the possible crystallization of neogenic phases from the developed melt upon sample thermal treatment [21,24,27,45].
The kaolin loses the crystalline water to produce metakaolin during phase transformation at lower temperatures (550ºC), and then further breaks down into primary mullite [34,56]. The appearance of primary mullite and amorphous silica at 1000ºC could be attributed to the decomposition of kaolin content in all batches (Fig. 7, Table 3). Mullite is still appearing up to 1200ºC in some samples (BT2-BT4) and up to 1300ºC in others (BT1, BT5 and BT6). There is a fluctuation of the mullite contents at the variable sintering temperatures (58.10-4.50 wt.%, Table 3), however, a general decreasing trend with the temperature increase is notified (Table 3).
Cristobalite could result from the decomposition of the silicate minerals in the batches at all sintering temperatures (1000-1350ºC) ( Table 3) in addition to its possible crystallization from the developed silicate melts [55,57]. Cristobalite appears with variable concentrations (0.10-43.30wt.%, Table 3) with no general trends in most samples. Mg-Al spinel has the same trend of cristobalite where it appears at most sintering temperatures of all samples (Fig. 7, Table 3).

Microstructure
Megascopically, all the sintered samples are compact with rough surfaces (Fig. 8a) except for samples BT3 and BT4 which have glazed smooth bloated surfaces at 1200 and 1300ºC, respectively (Figs. 8b, c). The fractured surface microstructure of selected sintered samples is revealed by SEM-EDAX. Connected elongated pores of variable sizes appear in most samples at 1300ºC (Fig. 8d), however, closed pores are shown at 1350ºC (Fig. 8e). On the other hand, large pores filled with hexagonal, neogenic, euhedral crystals of cordierite are distributed in places (Fig. 8f). BT1 and BT6 samples show micro-sized anhedral massive cordierite crystals distributed in the samples groundmass at 1300ºC (Figs. 9a, b) Table 4). The cordierite microchemistry of BT1 and BT6 at 1300 and 1350ºC reflects the purity of the   Table 2). Microcracks are recorded in many places in the samples groundmass. These microcracks could play a role as pore connectors (Figs. 9, 10). The developed micro-cracks may result from the thermal expansion mismatch between the glassy and crystalline phases [20,27,64,65]. BT2 sample shows massive cordierite crystals as groundmass at 1300ºC (Fig. 9e), which still appear at 1350ºC. In places, neogenic-subhedral hexagonal cordierite prisms are shown at 1350ºC (Fig. 9f) in association with the still dominant massive cordierite. The former neogenic hexagonal cordierite would refer to its crystallization from the developed melt (~68.52wt.%) at 1350ºC during sample cooling [39].  Table 4). This composition reflects the impurity of the starting-up raw talc-carbonate (Table 2).
BT5 samples show massive cordierite in the groundmass at 1300 and 1350ºC (Figs. 10c, d). In addition, primary mullite crystals are detected as acicular crystals which are merged within the glassy groundmass in places at 1350ºC  Table 4).

Physical Characteristics
The physical properties of the sintered samples such as linear shrinkage (LS), bulk density (BD), apparent porosity (AP) and water absorption (WA) are affected mainly by the phase transformation and microstructure in the temperature range 1000-1375ºC (Fig. 11).
LS values were calculated (1. .80%) at the temperature range 1000-1375ºC (Fig. 11a). The LS increases with  the temperature rise to 1200ºC then slightly drops for BT1 and BT6, however, slightly increases for BT2 and BT5 samples. It is considered that the total flux oxides (Fe 2 O 3 and CaO) of BT2 and BT5 samples (0.98, 2.55) and (5.63, 4.48wt.%), respectively, ( Table 2), enhance the development of melt that formed at higher temperatures (75.51, 81.39wt.% at 1400ºC). The increase of the melt contents at higher temperatures that motivated by the rise of the fluxing oxides would encourage the advance of the LS of the samples [59].
The BT3 and BT4 samples with higher Fe 2 O 3 (5.63, 5.49wt.%) have the highest LS (5.80, 5.60%), respectively, at 1200ºC. This could be interpreted in terms of the role played by the Fe 2 O 3 contents which enhance the boost of melt produced by the sintering process (~72.92, 74.00wt.% at 1355ºC) [59].
The BD values are showing a general increase with the temperature up to 1350ºC (1.90-2.18g/cm 3 ) then slightly decrease above this temperature. The higher rate of the BD increases for both BT3 and BT4 up to 1200ºC (1.90-2.35g/ cm 3 ) is due to development of silicate melts that would vitrify into glassy phase filling the internal pores and glaze the outer surface of the samples (Fig. 8b) [60]. In addition, a possible crystallization of the high-density phases such  [20,23,25].
Above 1200ºC, the BD of BT3 and BT4 samples drop to minimum values (1.80 and 1.89g/cm 3 , respectively, Fig. 11) due mainly to the samples bloating. The latter is caused by the reduction of the samples Fe-contents in the talc raw materials (5.63, 5.49wt.%, respectively, Table 2) that associated the evolution of O 2 gas [70,71]. Therefore, the physical measurements of both BT3 and BT4 samples above 1300ºC have been excluded.

Dielectric Study
The cordierite-based ceramics (BT1, BT2, BT5 and BT6) sintered at different temperatures (1300, 1350, and 1375ºC) have been electrically investigated over a wide frequency range, at room temperature. Here, the frequency dependence of permittivity (ε') and loss tangent (tanδ), has been investigated at constant sintering temperature (1300ºC) as illustrated in Fig. 12. As clear, ε' (12a) shows much higher values at low frequencies (~2900 at f = 0.1 Hz) compared to those measured at higher frequencies due to the contribution of all possible polarization components; space charge (Ps), dipolar (Pd) or orientation, ionic (Pi) and electronic (Pe). So, at low frequencies, each polarization component has enough time to follow the external electric field, resulting in an overall polarization increase and thus the permittivity increases [73]. In contrast, at higher frequencies, the polarization decreases and thus ε' considerably decreases and reaches much lower values, i.e., ~15 at f ≥ 10 4 Hz (Table 5). So, at high frequencies, the polarization cannot instantaneously responds to the rapid changes in the electric field, i.e., it takes delay time as charges possess inertia [74]. Further, at high frequencies, the space charge polarization releases, i.e., total polarization decreases [75]. It can also be noticed that BT2 and BT5 have identical permittivity values and behavior. Further, they show much lower values than BT1 and BT6.
The frequency dependence of loss tangent (tanδ) shows pronounced relaxation peaks; one for BT2 and BT5 whereas two for BT1 and BT6 (Fig. 12b). The peaks correspond to a step like a decrease in the ε'-f representation, reporting the Kramers-Kronig relation [76]. They generally describe a  [77,78]. Because of ceramic polycrystalline and inhomogeneous, the low frequency relaxation peak (PL) has been explained on the basis of separation of charges at interfaces, i.e., interfacial or Maxwell-Wagner-Sillars (MWS) polarization [79]. For such a polarization, the ceramic structure is imagined to have a fairly conducting grain (G) separated by a poorly conducting grain boundary (GB). The electrons reach the phase boundary through hopping and if the resistance of GB is high enough, the charges are built-up at these boundaries, giving rise to MWS polarization that increases the permittivity (ε') greatly at low frequencies. Accordingly, such a polarization is associated with the capacitive contribution of GB. When the frequency is increased above 10 3 Hz, the mobile charges reverse their direction more often and become unable to build-up at the interfaces and thus a faster polarization mechanism, revealing a high frequency relaxation peak (PH) corresponds to the capacitive contribution of bulk grain (G). For properly understanding the possible capacitive contribution of GBs and G in the dielectric behavior of cordierite-based ceramics, the complex electric modulus plot (M'' vs M') is essentially suggested to resolve the corresponding relaxation processes, because it highlights the smallest capacitance and suppresses the electrode polarization effects (Fig. 13) [80,81]. The plots show small and big semicircles confirming the capacitive contribution of the grain boundary (GB) and grain (G), respectively. Both small and big semicircles seemed to be non-ideal (depressed semicircle) or deviation from Debye behavior and therefore, they are   [82]. The radius of each semicircle is reciprocally proportional to the resistance contribution, so that the larger semicircle radius implies the smaller resistance. Based on this, the bulk grain resistances of BT1, BT2, and BT6 sintered at 1300ºC show the lowest values as compared to those sintered at higher temperatures. It can also be noticed that the capacitive contribution of GBs is unseen for BT1 and BT6 (sintered at 1350ºC), BT5 (sintered at 1300 and 1350ºC), and BT2 (sintered at all temperatures) due to the predominance of DC conductivity (σ dc ). This means that the GB and G resistances are highly affected by sintering temperature. So that sintering temperature results in the development of new crystalline phases such as cristobalite and Mg-Al spinel affect the grain features of this ceramic. To analyze the complex plot, data are usually modeled by an ideal equivalent circuit consisting of a resistor R and a capacitor C according to a brick layer model [83]. Correspondingly, GB and G can be represented by a parallel arrangement of (R gb , C gb ) and (R g , C g ), respectively. Hence, the capacitive contribution of GB and G to the present ceramics can be represented by two elements; R g C g and R gb C gb connected in series by the equivalent circuit (Fig. 14). The complex modulus plot for BT1 sintered at 1375ºC exhibits small and large semicircle with a tendency to a third one at higher frequency. It can be represented by an additional third element (R c C c ), corresponding to the DC conductivity effect connected in series with the equivalent circuit shown in Fig. 14. The deformed semicircle in the plot of BT5 is due to the coexistence of σ dc and GBs effects. The dielectric response of these ceramics to application of the electric field is mainly dominated by the phase composition, microstructure, porosity, etc. [24, 35, 60, 1 3 72, 84, 85]. For properly understanding these factors, the dielectric properties (ε', tanδ at 10 6 Hz) together with the phase composition, porosity and sintering temperature, are summarized in Table 6. Both ε', and tanδ show a decrease with a porosity decrease, displaying a good correlation between the dielectric properties and microstructure of samples. As the porous regions contain air of the lowest permittivity (~1.00006) and loss (0) values and therefore, they reduce the interfacial polarization/surface [24], i.e., dielectric properties decrease. However, BT6 shows an inverse relationship to porosity. The possible reasons for such a case are not understood. It can also be noticed that BT1 sintered at 1300ºC has a relatively higher permittivity value (~15, Table 5) due to the presence of the enstatite phase of high permittivity, i.e., ~6.7 [86], as reported in literature and listed in Table 6. Once the sintering temperature increases, the enstatite phase transforms into the liquid phase, i.e., crystallinity decreases. Based on this transition, tanδ reached much lower values with increasing sintering temperature while ε' seems to be nearly the same for all the ceramics examined except for BT6, which again shows an inverse relationship. As a result, the dielectric properties of BT1, BT2, and BT5 can be highly enhanced upon sintering at temperatures higher than 1300ºC. As clear from Table 5, one of the best findings is the lowest loss tangent value of BT2 (0.0004) and BT5 (0.0007) compared to other ceramic samples over all sintering temperatures. This can be attributed to the high content of CaO (2.5% for BT2 and 1.25% for BT5, Table 2) that formation of the low-loss neogenic hexagonal cordierite, i.e., melt derived cordierite on the expense of the high-loss orthorhombic cordierite, i.e., solid-state derived cordierite  (Fig. 9f). This explains why the dielectric properties of BT2 are better than those of BT5. Basically, CaO with high content is required for minimizing the loss tangent of cordierite-based ceramics, i.e., dielectric properties are improved. The low values of ε' and tanδ, are required for high-speed signal transmission with minimum attenuation. Based on the frequency dependent dielectric properties, the measured properties expected to attain much lower values within a microwave frequency range (300 MHz-300 GHz). From this expectation, BT1, BT2, and BT5 would be promising in millimeter -wave and substrate applications as well as microwave devices and wireless communications applications [80]. The dielectric properties of BT6 seem to be dependent on the dielectric properties of their individual phases [24,87,88]. So, its properties show the highest values compared to those for the remaining ceramic samples due to the presence of Mg-Al spinel of high permittivity (7.5, Table 6) [87].

Conclusion
Talc rocks were used to synthesize cordierite-based ceramics for electrical applications by the reaction sintering method. The effects of talc rocks characteristics on the phase composition, microstructure, physical and electrical properties of cordierite-based ceramics were studied. It can be concluded that: 1. The talc CaO content would promote the microstructure densification and cordierite crystallization at the sintering temperature 1350-1375ºC. On the other hand, CaO and Fe 2 O 3 impurities together negatively affect the microstructure densification at 1300ºC due mainly to the melt formation and bloating. 2. The mineral variation of the raw talc rocks that composed of talc, tremolite, magnesite, dolomite, clinochlore, serpentine and iron oxides, contributed to the microstructure densification of cordierite-based ceramics at a relatively low temperature. 3. The microstructural and physical properties of sintered cordierite-based samples were determined. Massive, neogenic subhedral orthorhombic and neogenic hexagonal cordierite crystals were developed depending on impurity oxides and particle size of green batches. Neogenic hexagonal cordierite is crystallized from Fig. 12 The frequency dependence of the permittivity (ε') and loss tangent (tanδ) for BT1, BT2, BT5, and BT6 sintered at 1300ºC  .20%) are generally decreasing within the sintering temperature range (1000-1375ºC). 5. One of the best findings is the lowest loss tangent values of BT2 (0.0004) and BT5 (0.0007) compared to other ceramic samples over all sintering temperatures due to the high content of CaO (2.5wt.% for BT2 and 1.25wt.% for BT5) that facilitates formation of the lowloss neogenic hexagonal cordierite, i.e., melt derived cordierite on the expense of the high-loss orthorhombic cordierite, i.e., solid-state derived cordierite. These features make BT2 and BT5 promising in many applications, i.e., electrical capacitors, microwave devices, wireless communication, etc. The interesting features of these ceramics make them technologically important and competitive against other alternative materials, not only due to their electrical behavior, but also due to their cost-effectiveness.