3.1 Raw material and batch characterization
The microstructure investigation of the raw talc samples shows that they contain mainly variable proportions of talc (Mg2Si4O10(OH)2), tremolite (Ca2Si8O22(OH)2), serpentine (Mg3Si2O5(OH)4), magnesite (MgCO3), dolomite (CaMg(CO3)2) and iron oxide (Fe2O3) (Fig. 1). Talc, serpentine and tremolite are represented as fine, fibrous, and micro-fibrous minerals (Figs. 1a-e). Magnesite exists as scattered crystals in the talc/tremolite/serpentine fibrous groundmass (Fig. 1c). Dolomite occurs as crystal clusters showing perfect twinkling (Fig. 1e). Iron oxides were mainly anhedral crystals filling the scattered fractures in places (Fig. 1d). The SEM-BSE images of the talc samples show that the major mineral is talc which appears as flat tabular and platy crystals (Fig. 1f). The microchemistry of the talc plates is showing their average contents of Si (27.90-34.71), Mg (20.13-20.44), Al (0.85-0.77), Fe (0.61-1.46), and Na (0.99-1.85wt.%) (Fig. 1f, Points 1, 2).
The detailed mineralogy of the talc samples matched by XRD confirms the existence of talc (pdf no. 19-0770 and 83-1768), tremolite (pdf no. 75-0861), serpentine (antigorite) (pdf no. 25-0645) and clinochlore ((Mg, Fe)5AlSi3O10(OH)8) (pdf no. 29-0701 and 83-1381) (Fig. 2, Table 1). In addition, kaolin sample shows major peaks of kaolinite (Al₂Si₂O₅(OH)₄) (pdf no. 80-0885) and minor peaks of anatase (TiO2) (pdf no. 71-1167), and quartz (SiO2) (pdf no. 05-0490) (Fig. 3, Table 1).
The XRF analyses show that MgO (21.84-32.54) and SiO2 (47.94-61.06wt.%) are the major oxides as talc is the main mineral in all samples (Table 2). The summation of Al2O3 (1.01-6.64), Fe2O3 (0.34-9.16) and CaO (0.05-7.47wt.%) are greater than 5wt.% in samples (T1-T5) (Table 2). The enrichment of these oxides is attributed to the presence of clinochlore (0.20-21.20), tremolite (12.50-53.30) and dolomite (6.10wt.%) at the expense of talc (11.60-94.80wt.%) (Table 1). The kaolin sample mainly contains of Al2O3 (37.34), SiO2 (44.56), and LOI (14.10wt.%) reflecting the high-grade quality of the sample (Table 2).
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, 43–47]. 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 (Al2Si2O7) and (H2O) (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 figure 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 [50–52]. Therefore, BT3, BT4 and BT5 samples would be densified and had higher liquid phase at lower temperatures compared to other samples.
3.2 Post-sintering characterization
3.2.1 Phase composition
The calcined-based chemical composition of BT1-BT6 batches was plotted on the phase diagram (MgO-Al2O3-SiO2) (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). BT3, BT4 and BT5 batches have relatively higher contents of melt (72.92, 81.39), (74.00, 81.11) and (71.56, 79.34wt.%) at 1355 and 1400ºC, respectively. While BT1, BT2 and BT6 batches show liquid phase contents in the range (69.09, 76.00), (68.52, 75.51) and (69.91, 76.69wt.%) at 1355 and 1400ºC, respectively.
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 silicate-rich 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 TiO2, Fe2O3, CaO, Na2O and K2O), 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 (21.34-73.41wt.%) at 1000-1300ºC (Fig. 7, Table 3). The crystalline phases identified by XRD are enstatite (Mg2Si2O6, pdf no. 84-0653), ferroan-enstatite ((Mg,Fe)2Si2O6, pdf no. 76-0545), diopside (CaMgSi2O6, pdf no. 96-900-5707), mullite (Al18Si6Si39, pdf no. 88-2049), Mg-Al spinel (MgAl2O4, pdf no. 77-0438), cristobalite (SiO2, pdf no. 82-0512) and cordierite (Mg2Al4Si5O18, pdf no. 82-1884 and 84-1220) in different contents (Fig. 7, 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].
Enstatite appears in all samples at the temperature range 1000-1300ºC, then disappears (Fig. 7, Table 3). Its crystallization is attributed mainly to the decomposition of talc. The enstatite content decreases with the temperature increase (84.90-7.90wt.%, Table 3) [21, 55].
Diopside appears only in BT3 and BT4 samples. It would be initiated during the decomposition of tremolite and decreasing with temperature rise (40.00-7.40wt.%, Table 3) at (1000-1200ºC) (Fig. 2) [44, 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).
Cordierite begins to exist with low concentrations at 1200ºC, however, it is recognized as the main phase in all BT samples at 1300, 1350 and 1375ºC (6.20-47.00), (56.60-87.40), (47.90-86.50), and (84.10-92.50wt.%), respectively (Table 3). Cordierite is crystallized beginning from 1200ºC at the expense of enstatite, mullite, cristobalite and diopside (Table 3) [21, 22, 24, 55, 57]. This is aided by the high contents of TIOs of the batches (2.03-11.44wt.%, Table 2) which accelerates the solid-state formation of orthorhombic cordierite by lowering its crystallization temperature [20, 24, 27, 58–60]. In addition, the TIOs initiate the early formation of silicate-rich melt (68.52-81.39wt.%) at 1355 and 1400ºC, respectively, which is aided by the partial dissolution of mullite, enstatite and cristobalite. Neogenic hexagonal cordierite crystals would be directly crystallized on cooling the silicate-rich melt [20, 23, 27, 61].
3.2.2 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). The microchemistry of groundmass massive cordierite is showing the average Si, Al, Mg, Ca and Fe to be bracketed between (36.40-42.57), (16.28-18.79), (8.48-9.08), (2.21) and (1.50wt.%), respectively (Figs. 9a, b, Points 1, 2, Table 4). The micro-sized anhedral massive cordierite crystals grow-up to larger ones with sub-hederal crystals in places at 1350ºC (Figs. 9c, d). These crystals have almost orthorhombic perpendicular faces, i.e., orthorhombic cordierite (Figs. 9c, d) which could refer to their solid-state crystallization during sintering [62, 63]. The elemental concentration of the latter cordierite crystals shows the average Si, Al and Mg to be (36.33-43.53), (18.43-15.61) and (5.39-7.68wt.%), respectively (Figs. 9c, d, Points 3, 4, Table 4). The cordierite microchemistry of BT1 and BT6 at 1300 and 1350ºC reflects the purity of the starting-up raw talc, which contain (58.83, 61.06), (2.34, 0.33), (31.54, 32.54), and (0.34, 0.14), (0.05, 0.20wt.%) for SiO2, Al2O3, MgO, Fe2O3 and CaO, respectively (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]. The microchemistry of both massive and neogenic cordierite is showing the average Si, Al, Mg, Ca, Fe and Ti to be (33.12-34.40), (21.04-22.70), (9.03-10.19), (4.09-1.41), (2.09-1.19) and (2.02-0.66wt.%), respectively (Figs. 9e, f, Points 5, 6, Table 4). This composition reflects the impurity of the starting-up raw talc-carbonate (Table 2).
BT3 and BT4 samples show well-developed neogenic holohedral hexagonal cordierite crystals that surrounded by fine agglomerated enstatite at 1300ºC (Figs. 10a, b). The enrichment of raw talc with Fe2O3, TiO2 and CaO (8.62, 9.16), (0.38, 0.05) and (7.47, 5.61wt.%), respectively (Table 2), would motivate the crystallization rate of the neogenic hexagonal cordierite prisms (Figs. 10a, b) from the developed melt (72.92, 74.00wt.% at 1355ºC). This could be interpreted as the existence of vacant spaces in the newly formed crystals to motivate the intake of both Mg and Al within the cordierite lattice from the surrounded melt [27, 66, 67]. These vacant spaces developed because of the atomic radii differences between Fe (2.26Å) and Mg (2.40Å) [68]. The microchemistry of the neogenic hexagonal cordierite crystals is showing their average content of Si, Al, Mg, Ca and Fe (22.45-22.50), (16.47-17.75), (8.40-9.50), (0.38) and (1.86wt.%), respectively (Figs. 10a, b, Points 7, 8, Table 4) which proves the neogenic cordierite near composition to that of stoichiometric cordierite (Si=24.01, Al=18.43, Mg=8.31wt.%) [69, 70]. Meanwhile, the agglomerated enstatite crystals show the average content of Si, Al, Mg, Ca, Fe and Cr to be 9.12, 7.42, 6.21, 2.20, 52.70 and 7.74wt.% (Fig. 10b, Point 9, Table 4).
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 (Figs. 10e, f). The glassy groundmass could be vitrified from the developed melt (~71.56wt.%). The microchemistry of massive cordierite of BT5 at 1300 and 1350ºC is showing the content of Si, Al, Mg, Ca and Fe to be (31.89-34.93), (11.64-20.27), (7.33-9.51), (8.34) and (1.85-8.66wt.%), respectively (Figs. 10c, d, Points 10, 11, Table 4). Meanwhile, microchemistry of primary mullite crystals is showing the content of Si, Al, Mg, Ca and Fe to be (30.05-28.57), (22.80-23.34), (1.66-1.60), (5.64-6.27) and (3.13-3.31wt.%), respectively (Fig. 10f, Points 12, 13, Table 4).
3.2.3 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 (1.70-5.80%) 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 (Fe2O3 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 Fe2O3 (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 Fe2O3 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/cm3) 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/cm3) 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 as enstatite (3.20), mullite (3.05), Mg-Al spinel (3.64) and cristobalite (3.58g/cm3) would promote the BD at 1200ºC [20, 23, 25].
Above 1200ºC, the BD of BT3 and BT4 samples drop to minimum values (1.80 and 1.89g/cm3, 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 O2 gas [71, 72]. Therefore, the physical measurements of both BT3 and BT4 samples above 1300ºC have been excluded.
Both the AP (3.30-36.20%) and WA (1.40-19.20%) values of the sintered samples are decreasing with the temperature increase (Figs. 11 c, d) up to 1300 and 1350ºC where slightly increase is notified. This could be attributed to the formation of microcracks among the crystalline phases at 1375ºC (Figs. 9, 10). These developed microcracks would appear due to the differences between the coefficient of thermal expansion between cordierite (1.50×10-6) (84.10-92.50wt.%, Table 3) and the minor Mg-Al spinel (7.6×10-6) (7.50-12.60wt.%, Table 3) [13, 27, 69, 70, 73].
3.2.4 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 figure 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 [74]. In contrast, at higher frequencies, the polarization decreases and thus ε' considerably decreases and reaches much lower values, i.e., ~15 at f ≥ 104 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 [75]. Further, at high frequencies, the space charge polarization releases, i.e., total polarization decreases [76]. 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 [77]. They generally describe a part of a molecule (functional groups etc.) or a molecule as a whole [78, 79]. 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 [80]. 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 103 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) [81, 82]. 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 modulated by using the Cole-Cole function [83]. 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 (σdc). 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 [84]. Correspondingly, GB and G can be represented by a parallel arrangement of (Rgb, Cgb) and (Rg, Cg), respectively. Hence, the capacitive contribution of GB and G to the present ceramics can be represented by two elements; RgCg and RgbCgb 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 (RcCc), corresponding to the DC conductivity effect connected in series with the equivalent circuit shown in figure 14. The deformed semicircle in the plot of BT5 is due to the coexistence of σdc 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, 73, 85, 86]. For properly understanding these factors, the dielectric properties (ε', tanδ at 106Hz) 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 [87], 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 [81]. The dielectric properties of BT6 seem to be dependent on the dielectric properties of their individual phases [24, 88, 89]. 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) [88].