BIBECHANA Vol. 20, No. 3, December 2023, 297-308 ISSN 2091-0762 (Print), 2382-5340 (Online) Journal homepage: http://nepjol.info/index.php/BIBECHANA Publisher: Dept. of Phys., Mahendra Morang A. M. Campus (Tribhuvan University) Biratnagar Effects of calcination and sintering temperatures on structural profile parameters of modified BNKT-ceramics Ram Prasad Aryal∗, Binod Kumar Bhattarai, Bhadra Pd. Pokhrel Department of Applied Sciences and Chemical Eng., Pulchwok campus, T.U, Pulchwok, Lalitpur, Nepal. ∗Corresponding author. Email: aryalram@ioe.edu.np Abstract We have carried out a detailed study of synthesis and variations of structural profile parameters: lattice constants, unit cell volume, densities, porosity, crystallite size, dislo- cation density, and micro-strain of BMZ modified BNKT (1 − x)Bi0.5(Na0.70K0.30)0.5TiO3- xBiMg0.5Zr0.5O3 for x = 0, 0.03, 0.06, 0.09 and 0.12 using X-ray diffraction. The conventional solid-state reaction method was used to synthesize BNKT samples. The BNKT powders were calcined at an optimized temperature of 850◦ C for 2 hours. Keeping the calcination tem- perature fixed, optimized sintering temperature and time were also determined to be 1140◦ C for 2 hours, based on the sharpness of the peaks and densification. The nature of the XRD patterns is qualitatively similar to that observed for calcined powders; however, the reflections are sharper in sintered samples presumably due to an increase in particle size. The XRD pro- file reveals an almost pure perovskite structure with cubic symmetries for all samples in both calcined powders as well as sintered samples. The crystallite size, dislocation densities, and micro-strain were determined by using William-Hall plots as well as the Scherrer formula. It is observed that the maximum bulk density was found to be 5.88 gm/cm3 for x = 0.03 which is 96.39% of the theoretical density. The crystallite size varies from 19.84 nm to 44.15 nm with compositions for calcined powders and from 57.01 nm to 62.69 nm for sintered samples. The very low value of crystallite sizes of calcined powder indicates that the particle size is compara- bly very low compared to that of sintered samples. The dislocation densities and micro-strain for calcined and sintered powders were observed in the range (5.1 − 25.4) × 10−4 nm−2, and (2.54− 3.1)× 10−4 nm−2 and (1.3− 4.8)× 10−3 and (8.93− 9.90)× 10−4 respectively which are determined using the William-Hall plot method. These results are also confirmed by the results obtained from the Scherrer method. Both results show improved profile parameters in sintered compositions than in calcination. This verifies that sintered BMZ-doped BNKT powders are promising candidates for many dielectric-based energy storage applications. Keywords Ferroelectrics, Micro-strain, Crystallites size, Williamson -Hall plot Article information Manuscript received: October 31, 2023; Accepted: November 2, 2023 DOI https://doi.org/10.3126/bibechana.v20i3.58594 This work is licensed under the Creative Commons CC BY-NC License. https://creativecommons. org/licenses/by-nc/4.0/ 297 http://nepjol.info/index.php/BIBECHANA aryalram@ioe.edu.np https://doi.org/10.3126/bibechana.v20i3.58594 https://creativecommons.org/licenses/by-nc/4.0/ https://creativecommons.org/licenses/by-nc/4.0/ Ram Prasad Aryal et al./ BIBECHANA 20 (2023) 297-308 298 1 Introduction Lead-based ceramics materials such as Lead Zir- conium Titanate (PZT) [1] and its derivatives ob- tained by doping the materials such as Nb, Sn, La, and Ba-modified (PZT) [2–5], Lead Titanate (PT), Lead Magnesium Niobate-Lead Titanate (PMN- PT) [6], etc., have ruled the commercial market of advanced ceramics for more than half a cen- tury due to their outstanding superior properties in piezoelectric devices (sensors, actuators, trans- ducers) and in energy harvesting devices (dielectric and supercapacitors). However, during the synthe- sis process at high temperatures, the volatile na- ture of Lead caused serious environmental pollu- tion [7] which induced immediate research to de- velop many lead-free advanced ceramics for vari- ous application purposes. So much research work has been done and is ongoing to enhance the prop- erties of conventional eco-friendly ceramics such as BaTiO3 [8], Bi0.5Na0.5TiO3 [9], K0.5Na0.5NbO3 [10], BiFeO3 [11], SrTiO3 [12], Bi0.5K0.5TiO3 [13] that could have efficiency comparable to that of lead-based ceramics [1]. Though there are no- table advancements in this area, no lead-free ma- terials could meet the efficiency similar to lead- based materials to serve as a practical substitute for electro-ceramics applications [14–18]. Among the various lead-free ceramics, Bi0.5(Na K)0.5TiO3 (BNKT) ceramics have gradually been recognized as one of the best potential candidates for electro- ceramics and have been extensively studied [19,20]. One of the major problems associated with BNKT ceramics is densification because of the volatile nature of Bi, Na, and K, small sintering tem- perature range, and potassium sodium ion seg- regation [10, 21]. Further, in material science, there should be a symbiotic relationship among the five elements i.e. composition-processing-structure- properties-applications for a long time [10]. How- ever, researchers have not paid much attention to the processing parameters as they should be in the BNKT system. So before analyzing the mi- crostructure profiles and their impacts on di-/piezo- /ferroelectric properties, it is important to the op- timization of calcination and sintering tempera- tures in the BNKT ceramics system to obtain high- quality ceramics with enhanced electrical proper- ties. Polycrystalline solid ceramics are recognized structurally by small segments of high crystallinity. Randomly oriented or textured crystallites are re- sponsible for the formation of anisotropy of the polycrystalline particles. Similarly, the random dis- tribution of atoms in the grain boundaries causes lattice strain and imperfection sources in the lat- tice that can affect the structural uniformity and overall performance of the material [22, 23]. The analysis of X-ray diffraction peak profiles is a ro- bust technique for characterizing the microstruc- tural attributes of ultrafine-grained materials. Mi- crostructural properties of the material affect their applications. In XRD measurements, photons are dispersed assuming that the crystal is elastic so there is no manifold scattering, no variations in the wavelength, and absorption of energy. According to the kinematical theory, intrinsic factors such as planar defects, dislocations or crystallite size in the microstructure of the material are responsible for the broadening of the diffraction peaks or bands. Examining the distinctive diffraction order depen- dence makes it possible to distinguish the influences of crystallite size and lattice strain on peak broad- ening. Standard X-ray diffraction profile analy- sis procedures, such as assessing the full width at half maximum (FWHM), integral breadths, and Fourier coefficient of the profiles yield information about apparent crystallite size and lattice strain [24–26]. Crystallite size measures the size of co- herently diffracting domains. Commonly employed techniques for quantitative analysis include Scher- rer’s equation, the Williamson-Hall (W-H) method, the Warren-Averbuch analysis, Rietveld refinement, and the pseudo-Voigt function. Nevertheless, the W-H method and Scherrer’s equation analyses are widely employed for determining the crystallite size and strain [27]. This paper aims to find an appropriate synthe- sis temperature and time to have a pure phase of (1 − x)BNKT-xBMZ (x = 0.00 − 0.12) polycrys- talline perovskite ceramic material. As reported in the literature, Bi, Na, and K are volatile at high temperatures, so it is important to determine the exact synthesis temperature to get a pure BNKT- based perovskite structure. Next, to carry out a comparative study of the microstructural proper- ties based on the compositions and temperatures. The contribution of crystallite size and microstrain to X-ray diffraction line broadening is analyzed by Scherrer’s formula and Williamson–Hall plot meth- ods. The porosities of the resulting samples are also calculated. 2 Materials and Methods 2.1 Materials Highly pure raw materials of metal oxide-based powders such as Bi2O3 (99%), Na2CO3 (99.5%), K2CO3 (99%), TiO2 (99%), MgO (99%), and ZrO2 (99%) all from Sigma–Aldrich were used as starting raw materials. Ram Prasad Aryal et al./ BIBECHANA 20 (2023) 297-308 299 2.2 Synthesis of (1-x) BNKT-xBMZ pow- ders Stoichiometrically measured powders were mixed thoroughly for 12 hours at 300 rpm using ethanol and Zirconia balls as ball milling media in a high- energy ball mill (PM400MA, Retsch, Germany). The dried slurry was calcined in the temperature range of 7000C to 9000C for 2 hours in a cov- ered crucible. The disk-shaped green pellets were made (approximately 10mm in diameter and 1.2mm in thickness) by mixing 2% PVA as binders into the powders and applying a uniaxial pressure of 10 tons. The green pellets of all the samples were kept at 5000C for 4 hours to burn off the binder and then sintered in the temperature range of 10900C to 11400C for 2 hours with a heating rate of 50C/min in closed alumina crucibles. Dur- ing this period, the pellets were embedded with the calcined powders of the same composition to re- duce the loss of Bi and Na. The perovskite crystal structure of the sintered powder sample annealed at 5000C for 8 hours was analyzed by the X-ray diffraction technique using a Powder diffractometer (MiniFlex600, Rigaku, Tokyo, Japan) with CuKα radiation (λ = 1.5406 Å) in the 2θ range from 100 to 800 with a step size of 0.020 and a 20/min scanning rate. By using Archimedes’ principle, the bulk density of the sintered pellets was measured. The (1-x)Bi0.5(Na0.7K0.3)0.5TiO3-xBiMg0.5Zr0.5O3 powder was obtained through ball milling and solid- state reaction by the following equation: (1− x)[0.25Bi2O3 + 0.175Na2CO3 + 0.075K2CO3+ TiO2] + x 2 [Bi2O3 + MgO + ZrO2] → (1− x)Bi0.5 (Na0.7K0.3)0.5TiO3 − xBiMg0.5Zr0.5O3 + 0.25CO2 (1) 3 Results and Discussion In this section, we have presented the results and discussion obtained by the X-ray diffraction study. We mainly focused on XRD profile parameters and their analysis. 3.1 Optimization of the operating temper- atures Fig. 1 (a-d) shows the XRD diffraction pattern of the undoped BNKT samples calcined at various temperatures (7000C-9000C) for 2 hrs. to optimize the calcination temperature. Based on peak inten- sities, phase formation, and impurities phases, the calcination temperature was optimized to 8500C for 2 hrs. as some sorts of small impurity phases were recorded at 8000C9000C, which may be due to the volatile nature of Bi and Na at high temperatures or incomplete phase formation in their high composi- tion. In addition, low-intensity peaks were recorded at 7000C. Similarly, green pellets prepared at the opti- mized calcination temperature were sintered at var- ious temperatures (10900C-11600C) with a soaking time of 2 hrs. at a heating/cooling rate of 50C/min to obtain the optimized sintering temperature. Fig. 2 (a-c) shows the corresponding XRD profile pat- terns. The sample shows some sort of known impu- rities (to be explained below) at all measured tem- peratures. Based on densification [28], which is a crucial parameter of sintered ceramics for further characterization and for high device performance, temperature 11400C for 2hrs. is set to be an opti- mized sintering temperature with bulk density (5.78 gm/cm3). In this way, the working temperature is set up. Figure 1: (a-d) XRD patterns of pure BNKT sample calcined at 9000, 8000, 8500 and 7000C respectively. Ram Prasad Aryal et al./ BIBECHANA 20 (2023) 297-308 300 Figure 2: (a-c) XRD patterns of pure BNKT ceramics Sintered at 11400, 11600, and 10900 C.. 3.2 Physical, and Structural Analysis Figures 3 (a, e) and 5 (a, e) show the XRD peak profiles of all calcined and sintered compositions (x = 0.00− 0.12) separately with enlarged versions of (110) fitted peaks in the inset, respectively. No splitting of (111) and (200) of the peaks recorded for the sintered systems shown in Fig. 7 (a, b) are in good agreement with that of the former reported for other BNT-based ceramics [29, 30], which pro- posed a structure consisting of cubic phases [31,32] throughout the entire compositional range. A small fraction of a known impurity peak called a pyrochlore phase Bi2Ti2O7 [28], besides (110) and (200), were seen in some compositions (indicated by ♠) shown in Fig. 5 (a). It has two reasons. First, it is the intermediate phase that can’t be transformed into the perovskite structure completely during the nucleation and crystal growth from the amorphous phase. Secondly, because of the volatile nature of Bi and Na at high sintering temperatures, non- stoichiometric structural defects resulted in the for- mation of a pyrochlore structure [33–37]. Figure 3: (a-e) XRD patterns of all the calcined samples (x=0.00-0.12) at 8500C along with enlarged view of (111) peak in the inset image respectively. Ram Prasad Aryal et al./ BIBECHANA 20 (2023) 297-308 301 Figure 4: (a-e) W-H plots of all the calcined samples (x=0.00-0.12) at 8500C. Figure 5: (a-e) XRD patterns of all the Sintered samples (x=0.00-0.12) at 11400C along with enlarged view of (111) peak in the inset image respectively. Ram Prasad Aryal et al./ BIBECHANA 20 (2023) 297-308 302 Figure 6: (a-e) W-H plots of all the Sintered samples (x=0.00-0.12) at 11400C. Figure 7: (a-b) XRD pattern of all calcined and sintered samples with enlarged peaks (110) of calcined, (111), and (200) for sintered samples respectively. Figure 8: (a-b) Compositional variation of Lattice constant and Theoretical density of calcined and sintered samples respectively. Ram Prasad Aryal et al./ BIBECHANA 20 (2023) 297-308 303 Table 1: Calculated physical and structural parameters of (1-x) BNKT-xBMZ ceramics at optimized calcination and sintering temperature. Calcined Samples Sintered Samples Compositions (x%) Lattice Con- stant (Å) Volume (Å3) Th. Density (gm/cm3) Lattice Con- stant (Å) Volume (Å3) Th. Density (gm/cm3) Bulk Density (gm/cm3) Rel. Den- sity (%) Porosity (%) 0 3.8908 58.90 6.04 3.8754 58.20 6.11 5.78 94.5 5.5 3 3.8773 58.29 6.19 3.8945 59.07 6.10 5.88 96.39 3.61 6 3.8915 58.93 6.20 3.8992 59.28 6.17 5.91 95.78 4.22 9 3.9241 60.43 6.13 3.9018 59.58 6.22 5.89 94.69 5.31 12 3.9308 60.74 6.21 3.9165 60.08 6.25 5.83 93.28 6.72 The lattice constant of sintered ceramics grad- ually increased and reached 3.9165 Å (x=0.12), whereas it decreased at first (x=0.03) and increased in calcined ceramics with BMZ contents, as shown in Fig. 8 (a). The variations of theoretical and rel- ative density (only for sintered ceramics) were plot- ted and shown in Fig. 8 (b) and Fig. 9 (c), respec- tively. The maximum relative density of 96.39% was recorded for the sintered composition x=0.03. The porosities of the composition vary from 0.04 - 0.55%. The calculated values of lattice constants, theoretical densities, relative densities, and porosi- ties for all the compositions were also tabulated in Table 1 for a comparative study. The slightly higher value of lattice parameters of calcined parameters than that of sintered samples may be due to the already complete phase formation of all samples. 3.3 Microstructure Analysis In 1918, Paul Scherrer discovered the first method to compute the crystallite size by employing his equation known as the Scherrer equation [28]. This equation characterizes the broadening of the XRD pattern with crystallite size [38]. The equation is written as Figure 9: FWHM of Broadened X-ray line (b) Schematic plot of Eqn. (6) Figure 10: (a,b) Compositional variation of Crystallite size of calcined and sintered samples via W-H plot and Scherer’s equation method respectively.(c) Compositional variation of Relative density and porosity of sintered samples, Ram Prasad Aryal et al./ BIBECHANA 20 (2023) 297-308 304 Figure 11: (a, b) Compositional variation of Dislocation density of calcined and sintered samples via W-H plot and Scherer’s equation method respectively. Figure 12: (a, b) Compositional variation of Macrostrain of calcined and sintered samples via W-H plot and Scherer’s equation method respectively. β = Kλ D cos(θ) (2) Where β is the broadening of diffraction peaks (FWHM), K is the Scherrer constant or shape fac- tor (0.94 for FWHM of spherical crystals with cubic symmetry [39]), D is the average crystallite size, θ is Bragg’s diffraction angle in degrees, and λ is the X-ray wavelength (CuKα = 1.5405 Å). Next, the Williamson-Hall (W-H) plot is an- other method to compute the broadening of X-ray diffraction peaks contributed by crystallite size and lattice strain as they are independent of each other. According to the X-ray line profile analysis, the broadening of the X-ray line results from the mi- crostructure of the material, which is caused by the random distribution of lattice defects within the crystal. The equations involved in the study of peak broadening are [40]. β = βcrystalline + βstrain (3) βcrystalline = Kλ D cos θ (4) βstrain = 4ϵ tan θ (5) Using equations (2), (3) and (4) it yields, β cos θ = Kλ D + 4ϵ sin θ (6) Equation (6) is also termed the expression of the Uniform Deformation Model (UDM) [11]. Where ϵ is the micro-strain. Equation (6) is in the form of y = mx+ c, where the slope (m = ϵ) measures the micro-strain (ϵ), and the intercept (c = Kλ D ) gives the crystallite size (D) of the W-H plot as shown in Fig. 9 (b). During the grain growth of the mi- cro/nanoparticles, they experience a lot of com- pression and relaxation in the lattice, resulting in deviation in the lattice constant as well as an over- all deviation in the lattice volume. On the other hand, the displacements of the atoms with respect to the neighboring lattice position result in strain broadening [25]. Micro-strains of all samples are calculated using the following formula based on Scherrer’s equation [40]. ϵ = β cos θ 4 (7) Dislocation is another crucial parameter affect- ing the nanocrystalline materials’ microstructure and performance. It is an imperfection or error in a Ram Prasad Aryal et al./ BIBECHANA 20 (2023) 297-308 305 crystal associated with one side of the lattice with respect to another part of the lattice within the same crystal. Imperfections in crystals such as dis- locations, different vacancies, and interstitials are not uniformly flawed. The dislocation densities (δ) (length of the dislocation lines/volume) are deter- mined by using the X-ray line profile analysis of the samples by using the following relation [40]. δ = 1 D2 (8) Where D is the crystallite size obtained from both Scherrer’s equation and W-H plot. The Structural parameters (Crystallite Size, Dislocation density, and Micro-strain) derived from the XRD pattern by using both methods are tabulated in Table 2. Table 2: Microstructural parameters of calcined and sintered (1-x) BNKT-xBMZ ceramics via Williamson-Hall plot method and Scherrer’s Formula method. Williamson-Hall Plot Method Compositions (x%) Crystallite size (nm) Dislocation density (nm−2) Microstrain Crystallite size (nm) Dislocation density (nm−2) Microstrain 0 23.74 1.77× 10−3 1.30× 10−3 60.34 2.75× 10−4 9.76× 10−4 3 44.15 5.13× 10−4 3.80× 10−3 57.01 3.08× 10−4 9.34× 10−4 6 19.84 2.54× 10−3 1.40× 10−3 60.34 2.75× 10−4 8.93× 10−4 9 25.86 1.50× 10−3 3.50× 10−3 62.69 2.54× 10−4 9.69× 10−4 12 36.66 7.44× 10−4 4.80× 10−3 59.6 2.82× 10−4 9.90× 10−4 Scherrer’s Formula Method Compositions (x%) Crystallite size (nm) Dislocation density (nm−2) Microstrain Crystallite size (nm) Dislocation density (nm−2) Microstrain 0 17.99 3.09× 10−3 2.04× 10−3 36.49 7.51× 10−4 1.01× 10−3 3 16.73 3.57× 10−3 2.22× 10−3 36.32 7.58× 10−4 1.02× 10−3 6 14.47 4.78× 10−3 2.59× 10−3 38.2 6.85× 10−4 9.67× 10−4 9 13.99 5.11× 10−3 2.67× 10−3 37.84 6.98× 10−4 9.77× 10−4 12 14.67 4.65× 10−3 2.63× 10−3 36.45 7.53× 10−4 1.01× 10−3 The values of crystallite size, microstrain, and dislocation density obtained from W-H Plot and Scherrer’s formula methods at two processing tem- peratures were tabulated in Table 2. Fig. 10 (a, b) shows that crystallite size varies almost in the same manner in both methods but the magnitude of crystallite sizes was quite different between the two methods. Generally, the magnitude of crys- tallite size calculated from Scherrer’s formula for non-zero residual stress is lower than the value ob- tained from the W-H plot. This is because the as- sumptions of Scherrer’s formula and W-H plot were quite different as mentioned above. It was also seen that the variations of crystallite sizes in cal- cined and sintered temperatures were in reversed order. The calculated values are in the order of previously reported values in modified BNKT ce- ramics [41–43]. In comparison, there is a large vari- ation in the average crystallite size obtained from the W-H plot method. This variation occurred as a result of the inequalities in the method of aver- aging the particle distribution. Based on the data variations in the two methods, Scherrer’s formula is more suitable for the determination of crystal- lite size in this study. Furthermore, the larger the crystallite size, the more crystallinity in the pow- der. The better crystalline quality corresponds to the lower Urbach energy (an indicator of the elec- tronic quality of thin absorbing materials used in solar cells) [44,45]; so, the sintered sample will show better performance [27]. Figures 11 (a, b) and 12(a, b) show the dis- location density and micro-strain variations com- puted via both methods, with calcined and sintered temperatures of all compositions. Both parameters were varied in the same manner for both methods. Normally, dislocation in a crystal indicates the de- fects or imperfections that affect the physical and chemical properties of the crystal. Dislocation den- sities influence many properties of the materials and it increases with the plastic deformation inside the crystals. Also, it was found that there were com- positional variations in the micro-strain values of calcined and sintered samples in both cases. This strain variation could be due to the variations in size and microstructure of the particle [27]. Smaller micro-strain values are seen in sintered composition compared to calcined samples in both cases. On the other hand, by increasing the micro-strain dis- location density increases but grain size decreases, and finally, these parameters reach saturation val- ues [46, 47]. Finally, the result shows the sintered compositions are the best/most applicable sample that holds the best crystalline quality (the least lat- tice strain, dislocation density, and leads to lower Urbach energy) which enables them to the favorable candidates for optoelectronic devices. 4 Conclusion The XRD results imply that the (1-x) BNKT- xBMZ bulk ceramic powder was successfully pre- pared at optimized calcination (8500 C/2hrs) and sintering (11400 C/2hrs) temperatures. Negligi- ble impurity phases were detected in a few com- positions. Further, from the literature as well as Ram Prasad Aryal et al./ BIBECHANA 20 (2023) 297-308 306 peak position and shapes, both calcined and sin- tered samples are in a cubic phase. Lattice pa- rameters of sintered samples were increased grad- ually with doping concentration. Micro-structure parameters (crystallite size, dislocation density, and micro-strain) were calculated using the X-ray line profile analysis technique for calcined and sintered samples separately via the W-H plot method and Scherrer’s formula. The calculation of most of the crystalline parameters is associated with the corre- sponding crystallite size and strain. Both meth- ods imply that sintered ceramics’ microstructure parameters were better than the calcined samples from the material’s application viewpoint. 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