Optimization of the calcination and two-step sintering temperatures of (K_0.48Na_0.48Li_0.04)(Nb_0.95Sb_0.05)O₃ ceramics

We considered the differential thermal analysis (DTA) and thermogravimetric analysis (TGA) data recorded for the (K_0.48Na_0.48Li_0.04)(Nb_0.95Sb_0.05)O₃ powder to determine the calcination temperature (figure 1). The DTA curve presents the early-stage heat absorption peaks at 107.2 and 179 °C, and the weight losses at these temperatures are attributed to the simultaneous losses of H₂O and CO₂ [16]. The total mass in the sample is reduced by 2.05% under these conditions. Rubio-Marcos et al [17] suggested that the decomposition of AHCO₃ to A₂CO₃, where A is Na⁺, K⁺, or Li⁺, occurs in the temperature range of 100 °C–180 °C. AHCO₃ is formed when the ceramic powder is mixed in an ethanol environment. The second transition region appears at approximately 315.9 °C, and the weight loss in this region is attributed to the release of an intermediate reaction layer of (K, Na, Li)₂(Nb, Sb)₄O₁₁ on the surface of Nb₂O₅ particles [16]. Approximately 3.33% of the weight is lost under these conditions. The endothermic peaks at 315.9 °C reflect the polymorphic transition of A₂CO₃ [17]. The major loss in weight occurred in a narrow temperature range spanning 415 °C–670 °C. The weight loss occurring in this region corresponded to the loss in CO₂ [17]. In the final stage, Further development of the reaction at 803.6 °C leads to the formation of a stoichiometric (K_0.48Na_0.48Li_0.04)(Nb_0.95Sb_0.05)O₃ phase at the surface of the reaction layer. Approximately 2.53% of the weight is lost at this stage (figure 1). Rubio-Marcos et al [17] reported that the decomposition of carbonates was completed at temperatures above 700 °C. This indicates that the KNN-based ceramics present the perovskite phase at a temperature higher than 700 °C. However, for a large sample volume, a single-phase structure cannot be formed at a calcination temperature of 700 °C. Above 700 °C, the decomposition of Na₂CO₃, K₂CO₃, and Li₂CO₃ carbonates results in the formation of a lubricating liquid phase. The samples were mechanically mixed with K₂CO₃, Na₂CO₃, Sb₂O₃, Li₂CO₃, and Nb₂O₅ to form KNLNS at approximately 803.6 °C (figure 1). Based on the thermal stability of the (K_0.48Na_0.48Li_0.04)(Nb_0.95Sb_0.05)O₃ powder, calcination was conducted at 800, 850, and 900 °C to study the structures and phases of the materials.

Zoom In Zoom Out Reset image size

XRD patterns of calcined KNLNS powders were recorded at 800, 850, and 900 °C for 2 h (figure 2(a)). Analysis of figure 2(a) revealed that the crystal structure of the KNLNS powders calcined at 800 °C presented a cubic symmetry, and the results agreed well with the results reported by Verma et al [18]. The perovskite phase occupied a large volume, and the presence of a second phase (Na₂Nb₄O₁₁) was observed. The second phase was formed in small amounts, and the peaks between 20 and 30 reflected the formation of this phase. Similar results were reported by Zhang et al [19]. The results reveal that the temperature is not high enough to drive the reaction and form a single-phase structure. When powdered KNLNS was calcined at 850 °C or 900 °C, a pure perovskite phase was formed (figure 2(a)). The calculated intensity ratio (I₀₂₂/I₂₀₀) for the (022)_O/(200)_O peaks in the 2θ range of 44°–48° were recorded to be 1.12 and 1.13 when the calcination temperatures were 850 or 900 °C, respectively (insert of figure 2(a)). The results confirmed the formation of the KNLNS materials presenting the single-phase orthorhombic crystal structure. Figure 2(b) presents the SEM images of the powdered KNLNS samples calcined at 850 °C. Analysis of figure 2(b) reveals that the grain size of the powder calcined at 850 °C is in the range of 0.1–0.4 μm, and the average grain distribution peak is approximately 0.18 μm (190 nm) (insert of figure 2(b)). Based on these experimental results, the temperature of 850 °C was chosen as the optimized calcination temperature for the KNLNS powder. These results agreed well with the results reported by Nandini et al [16]. However, for pure KNN ceramics, the calcination temperature can be as high as 1050 °C, as reported by Verma et al [18] for undoped KNN powder.

Zoom In Zoom Out Reset image size

XRD patterns recorded for the KNLNS samples sintered at 950, 1000, 1050, and 1100 °C for 4 h were analyzed to study the effect of the two-step sintering temperature on the phase structure of the materials (figure 3(a)). The presence of a pure perovskite phase is observed in figure 3(a), and the origin of this phase was attributed to the good crystallization property of ceramics. A significant extent of peak splitting was observed for the ceramics sintered at 1050 and 1100 °C, indicating a high crystalline degree, as detailed in the structural changes investigated at 2θ = 44°–47°. The results were obtained based on data simulation based on the Lorentz fitting function (figure 3(b)). It was observed that the 2θ of KNLNS patterns shifted from the left to the right with an increase in the sintering temperature from 950 to 1100 °C. Gaur et al [20] reported that KNLNS ceramic samples sintered at different temperatures in a very narrow range from 1050 °C to 1090 °C, which no evident shifting in the peaks at 2θ = 44°–47°. Figure 3(b) also reveals that the orthorhombic phase (O-phase) of the samples is characterized by the (022)_O and (200)_O peaks and their intensity tends to increase with an increase in the sintering temperature (T₂), similar to that described by [20]. It means that the crystallinity of the ceramic samples is better as the T₂ increases. Good crystallinity and single-phase structure can be attributed to the fact that the radius of the Sb⁵⁺ (0.62 Å) ions were comparable to the radius of the Nb⁵⁺ (0.64 Å) ions in the B-site and the radius of Li⁺ (0.92 Å) was comparable to that of K⁺ (1.38 Å) and Na⁺ (1.32 Å) lead to diffused well into the KNN lattice to produce a homogeneous solid solution, consistent with the report of Gaur et al [20].

Zoom In Zoom Out Reset image size

The elements in the (K_0.48Na_0.48Li_0.04)(Nb_0.95Sb_0.05)O₃ ceramic materials were identified by analyzing the EDS spectra (figure 4(a)). Analysis of the EDS spectra revealed the presence of Na, K, Nb, Sb, and O in the ceramics sintered at 1050 °C. The results agreed well with the results reported by Wu et al [21]. The absence of Li was observed by analyzing the DES spectra, and the absence of this element was attributed to the low atomic mass of the material. Figure 4(a) reveals that Sb was introduced at positions B in the KNN ceramic presenting the ABO₃ structure. The results revealed the formation of a homogeneous solid solution, and the results were consistent with the results obtained by studying the structure of the materials. The FTIR profiles recorded in the range of 400–3500 cm^–1 for the (K_0.48Na_0.48Li_0.04)(Nb_0.95Sb_0.05)O₃ ceramic materials sintered at 1050 °C were analyzed to affirm the results (figure 4(b)).

Zoom In Zoom Out Reset image size

The FTIR profiles reveal the presence of characteristic peaks of both KNLNS materials. Absorption bands at 423, 685, 810, 927, 1058, 1203, 1332, 1630, 2325, 3414, and 3512 cm⁻¹ were present in the profiles (figure 4). Analysis of the profiles reveals the presence of strong absorption peaks at approximately 423 and 3512 cm⁻¹. These peaks corresponded to the O–H stretching bond. The presence of these peaks revealed the absorbance of water vapor. The band at 1203 cm⁻¹ can be assigned to the asymmetrical stretching of the C–O group, and the group of bands at 810 and 927 cm⁻¹ can be attributed to the out-of-plane deformation of the carbonate group. Similar results were reported by Supriya et al [22]. The bands appearing at 685, 1332, 1630, 2325, and 3414 cm^–1 could be ascribed to the characteristic vibrations of B–O (where B is Nb⁺⁵ and Sb⁺⁵) [22] and A–O (where A is Na⁺, K⁺, or Li⁺) [22] that participated in the formation of a perovskite phase in KNLNS ceramics. According to Rubio-Marcos et al [17, 23], the FTIR profiles recorded for the (K_0.44Na_0.52Li_0.04)(Nb_0.86Ta_0.10Sb_0.04)O₃ ceramics present absorption peaks localized at 1640, 880, and 630 cm⁻¹. The presence of the octahedral BO₆ groups is confirmed by the presence of peaks at 1386, 1627, 2391, and 3441 cm^–1.

Figure 5 presents the microstructure of the (K_0.48Na_0.48Li_0.04)(Nb_0.95Sb_0.05)O₃ ceramics sintered at T₂ = 950 °C–1100 °C for 4 h. The sintering temperature T₂ strongly influenced the microstructure of the KNLNS ceramics. The average grain size of the ceramics increased with an increase in T₂ and reached the maximum value of 0.94 μm at 1050 °C. A dense microstructure and a clear grain boundary were observed under these conditions. The density decreased beyond 1050 °C. The increase in the ceramic grain size can be explained by the fact that the addition of Li⁺ (4% mol) and Sb⁵⁺ (5% mol) to KNLNS ceramics in the presence of Li⁺ and Sb⁵⁺ can improve the migration rate of grain boundaries and accelerate the process of grain growth in KNN-based ceramics by forming a liquid phase. According to Jiang et al [24], the grain size of the K_0.5Na_0.5NbO₃ ceramics increases from 3 to 10 μm when the one-step sintering temperature increases from 1060 to 1100 °C. This indicates that high sintering temperature effectively induces grain growth via the migration of grain boundaries. The diffusion rate increases under these conditions [25]. Interestingly, although the size of the ceramic grains increases with T₂, the size of the ceramic particles is smaller than the size of the particles formed following traditional sintering methods. This is explained by the schematic diagram in figure 6. When the traditional sintering process (insert of figure 6) is used, the size of the ceramic grains increases sharply with an increase in the sintering temperature. Particle size larger than 5 μm was recorded when the sintering temperature was above 1150 °C. Similar results were reported by Jiang et al [24]. The temperature of the samples was raised to 1150 °C during the two-step sintering process, and this temperature was maintained for 5 min to form a Li-liquid phase that improves the diffusion rate. Following this, the temperature was rapidly reduced to 1050 °C, which was maintained for 4 h (insert of figure 6). The use of this technique helped decrease the grain size, and the completion of the microstructure was recorded. This limited the evaporation of alkaline elements, generating fewer lattice defects.

Zoom In Zoom Out Reset image size

Zoom In Zoom Out Reset image size

It can be explained based on the model in the figure 6 that the first step of the two-step sintering process could be related to grain growth [26]. When the rapid cooling from a higher sintering temperature (T₁ = 1150 °C) to a lower sintering temperature (T₂ = 1050 °C) and holding for a suitable duration (t₂ = 4 h), which may freeze the microstructure of the ceramics, which lead to Inhibition of ceramic particle size growth in the second step of sintering because the prohibition of grain boundary migration.

Figure 7(a) shows the density versus sintering temperature plots for the KNLNS ceramics. As the T₂ increased, the density of the ceramics increased from 4.17 to 4.35 g cm⁻³, and the relative density increased in the range of 95.3%–98.1%. Similar results were reported by Jiang et al, who studied K_0.5Na_0.5NbO₃ ceramics [24]. They reported that the density of the K_0.5Na_0.5NbO₃ ceramic sintered at 1100 °C was 4.39 g cm⁻³ (relative density of 97.4%). The increase in density is closely related to the microstructure of the materials. A uniform distribution of grains and tight grain packing were observed at T₂ = 1050 °C. This results in an increase in ceramic density (4.35 g cm⁻³; relative density: 97.8%). The closed–porosity was presumed to be approximately 3.2%. However, the ceramic density decreases when T₂ = 1100 °C (figure 7(a)), and this can be attributed to the volatilization of alkali components during sintering. This causes a deviation from the original calculated component, and similar results were reported by Wang et al [27]. The increase in the relative density and the formation of grains of appropriate sizes can be attributed to the improvement in the electrical properties of the KNLNS ceramics. The details are reported in the following section.

Zoom In Zoom Out Reset image size

The trend in the change in the dielectric constant (ε_r ) of the ceramic samples at different temperatures is similar to the trend observed for the change in density (figure 7(b)). The value of ε_r increases with T₂ and reaches the maximum value (ε_r = 849) at T₂ = 1050 °C. However, when T₂ > 1050 °C, the value of ε_r decreases. Conversely, when T₂ increases, the value of the dielectric loss (tanδ) decreases, and the minimum value (tanδ = 0.073) is recorded at T₂ = 1050 °C. Beyond this temperature, the value increases. The observations can be attributed to the high density, large grain size, and improved crystalline quality of the ceramics, as reported by Tho et al [6]. In addition, the replacement of K⁺ (1.38 Å) ions and Na⁺ (1.32 Å) ions occupying the A sites of the perovskite lattice by Li⁺ (0.92 Å) ions and Nb⁵⁺ (0.64 Å) ions occupying the B-site by Sb (0.62 Å) lead to good crystallization at low sintering temperatures, which contributes to improve the dielectric constant, and a decrease in the dielectric loss of the materials.

Figure 8(a) presents the ε(T) curves for the (K_0.48Na_0.48Li_0.04)(Nb_0.95Sb_0.05)O₃ ceramics generated in the range of 30 °C–420 °C at 1 kHz. The ε(T) curves generated for the ceramics present two clear peaks which correspond to T_O−T (low temperature) and T_C (higher temperature). Figure 8(b) reveals that T₂ has a negligible influence on the T_O-T and T_C phase transition temperatures of the (K_0.48Na_0.48Li_0.04)(Nb_0.95Sb_0.05)O₃ ceramics. It shows that the value of T_C was obtained in the range of 373 °C–381 °C. While, the T_O-T of the ceramics ranges from 79 °C−115 °C, and it shifted to lower temperatures at optimum T₂. This can be explained by the substitution of Li⁺ and Sb⁵⁺ ions into the A and B sites in the ABO₃ structure of the KNN ceramic, respectively [20]. The maximum dielectric constant (ε_m) of the ceramics increases with T₂, and the maximum value (ε_max = 6659) was recorded at T₂ = 1050 °C. Beyond this temperature, the value decreases. As discussed above, the ceramic sample corresponding to the T₂ is 1050 °C, and the ceramic sample has a homogeneous microstructure, large grains, high density, and small grain boundaries of the ceramics. This result makes it easier to reverse ferroelastic domain walls leading to improved dielectric properties of ceramic samples [8].

Zoom In Zoom Out Reset image size

The electromechanical coupling coefficients (k_p, k_t), the piezoelectric coefficients (d₃₃) and mechanical quality coefficient Q_m of the (K_0.48Na_0.48Li_0.04)(Nb_0.95Sb_0.05)O₃ ceramics have been plotted in figures 9(a) and (b), respectively. The k_p, k_t, d₃₃, and Q_m values present similar behavior. Initially, the values tend to increase with an increase in T₂, and the values decrease when T₂ exceeds 1050 °C. The maximum values for k_p (0.33), k_t (0.35), d₃₃ (195 pC N⁻¹), and Q_m (83) were recorded at T₂ = 1050 °C. This result is consistent with the results obtained by studying the microstructure and ceramic density as well as good crystallization of the structure at the optimum sintering temperature. As reported by Tu et al [8], the large grain size, small grain boundaries and high density of KNLNS ceramic are favorable for getting enhanced piezoelectric properties. Wang et al [27] reported that the maximum density and the largest coefficient d₃₃ value were recorded when the 0.92(Na_0.535K_0.48)NbO₃ − 0.083LiNbO₃ samples were sintered at 950 °C.

Zoom In Zoom Out Reset image size

Furthermore, the increase in the Q_m value can be attributed to the improvement in the dielectric loss and the microstructure of the ceramics. It is explained that the replacement of K⁺ (1.38 Å) ions and Na⁺ (1.32 Å) ions occupying the A sites of the perovskite lattice by Li⁺ (0.92 Å) ions and the Nb⁵⁺ (0.64 Å) ions occupying the B-site by Sb (0.62 Å) leading to good crystallization, which contributes to a decrease in the dielectric loss of the materials and the result is improved the mechanical quality coefficient [8].

The hysteresis loops were generated at RT and analyzed to confirm the effects of different T₂ on the ferroelectric properties of KNLNS ceramics (figure 10(a)). Good saturated S–E hysteresis loops were obtained for the ceramics. Figure 10(b) presents the variations in the remnant polarization (P_r) and the coercive field (E_c) as a function of T₂. The value of P_r increases with an increase in T₂ and reaches the maximum value of 16.1 μC cm⁻² at T₂ = 1050 °C. The value decreases beyond this temperature. The sample sintered at T₂ = 950 °C is characterized by the E_c value of 9.3 kV cm⁻¹. The E_c values decrease with an increase in T₂ and reach the smallest value of 5.9 kV cm⁻¹ at T₂ = 1100 °C. The improvement in the ferroelectric properties is attributed to the increase in the particle size results in the improvement in the orientation directions of the domains. In other words, the perfect microstructure, small grain boundaries, and high ceramic density ease the process of polarization rotation. This improves the polarization conditions of the KNLNS ceramics [28, 29]. Similar results have been previously reported [30–32]. The results agree with the studied dielectric and piezoelectric characteristics of the ceramics. The ferroelectric properties of the ceramics deteriorate due to the evaporation of volatile alkali metal oxides (Na⁺, K⁺) at T₂ = 1100 °C [3].

Zoom In Zoom Out Reset image size

Based on the schematic diagram presented in figure 10(c), the energy storage density (W_rec, marked by the green area in figure 10(c); calculated by equation (1)) and the energy storage efficiency (η) were calculated using equation (2) as follows [3, 4]:

$$\begin{eqnarray}&&{W}_{{\rm{rec}}}={\int }_{{P}_{r}}^{{P}_{{\rm{\max }}}}E{\rm{d}}P\end{eqnarray} \tag{ 1 }$$

$$\begin{eqnarray}&&{\eta }=\frac{{W}_{{rec}}}{{W}_{{rec}}+{W}_{{loss}}}{\rm{\times }}100 \% \end{eqnarray} \tag{ 2 }$$

Figure 10(d) reveals that the trend in the changes in the energy storage density (W_rec) of the ceramics at different T₂ is similar to the trend in the change in the energy storage efficiency (η). W_rec and η decreased, and the minimum values of 0.21 J cm⁻³ and 18.3%, respectively, were recorded at T₂ = 1000 °C. However, when T₂ exceeds 1000 °C, W_rec and η increase linearly with an increase in T₂, and the maximum values of 0.36 J cm⁻³ and 48.1%, respectively, are recorded at T₂ = 1100 °C. The energy storage properties of the KNLNS ceramics can be significantly improved while optimizing the two-step sintering temperature. Similar results have been previously reported [3]. The optimal sintering conditions for the KNLNS ceramics can be determined. The values of W_rec (200%) and η (51.8%) increased under optimized conditions. The results indicate that optimization of preheating and two-step sintering temperatures helps increase the relative density of the materials. The grain size of the materials affects the domain size and configuration of the materials, which are critical factors that affect the electrical properties of the KNLNS ceramics.

Table 1 lists the electrical parameters of the (K_0.48Na_0.48Li_0.04)(Nb_0.95Sb_0.05)O₃ ceramics and other reported materials such as (K_0.48Na_0.48Li_0.04)(Nb_0.96Sb_0.04)O₃ [20], [Na_0.5K_0.5]_0.95(Li)_0.05(Sb)_0.05(Nb)_0.95O₃ (KNLNS) [33], (K_0.41Na_0.59)(Nb_0.79Sb_0.06Ta_0.15)O₃ (KNNST) [34], (K_0.535Na_0.485Li_0.05)(Nb_0.8Ta_0.2)O₃ (KNLNT) [35], [K_0.5Na_0.5]_{1 − x} (Li)_x (Sb)_x (Nb)_{1 − x} O₃ (KNLSN) [33], and [(K_0.5Na_0.5)_0.94Li_0.06]_0.97La_0.01(Nb_0.9Ta_0.1)O₃ (KNLLNT) [36]. Analysis of the electrical parameters T_s, k_p, ρ, d₃₃, P_r, T_C, and ε_r reveals that the fabricated materials are promising candidates that can be used to develop piezoelectrics devices.