Structural Elucidation, Electronic and Microwave Dielectric Properties of Ca(Sn x Ti1-x )O3, (0 ≤ x ≤ 0.8) Lead-Free Ceramics.

The lead-free Ca(Sn x Ti1-x )O3, (0 ≤ x ≤ 0.8) sample has been successfully prepared through the ball milling process, sintered at 1200 °C for 3 h. The structural, morphological, vibrational, and microwave dielectric properties of synthesized samples were analyzed by X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FT-IR), and impedance analysis. All the samples have an orthorhombic phase structure with a space group of Pbnm formation, and the crystalline size and strain changes with respect to Sn4+ doping were observed in the XRD analysis. From a morphological point of view, on increasing the content "x", the grain size reduces from 3.29 to 1.37 μm. The existence of vibrations and the bridging stretching mode of Ti-O-Ti and Ti-O-Sn both are associated with the broadband in the region below 800 cm-1 verified by FT-IR. The variation in electrons hopping off the host compound with respect to Sn4+ ions was analyzed in AC conductivity. The changes of dielectric properties such as complex permittivity, modulus spectroscopy, and dielectric loss at room temperature with a different frequency range of 1.00-2.00 GHz are discussed.

Introduction

The ceramic microwave dielectric plays a significance role in the development of the Global Positioning Systems, modern transport system, and satellite for broadcasting.[1] Titanate-based compounds with a perovskite structure have been determined as one of the notable diverse categories of the materials, having novel and tremendous applications such as photocatalysis, thermoelectric, ferroelectrics, and batteries.[2,3] Ferroelectric materials are utilized in the industry as a ferroelectric memory gear and dynamic random-access memory, which were mostly made from lead. With the increasing demand of nanotechnologies, nanoelectronics, and microelectronics, it was necessary to develop new lead-free ferroelectrics. Among those, the oxide, that belongs to the ABO3 family, was the most suitable candidate because of its ferroelectric and reputed piezoelectric properties.[4−7]

Calcium titanate is remarkably known as a chemical-resistive n-type semiconductor and thermal-resistive element and exhibits optical, electrical, and thermal properties. These optimum dielectric properties of calcium titanate reveal a wide range of applications in the sensors, capacitor, and microwave communication system.[8] CaTiO3 can occur in a crystalline or may be in amorphous form; the stable cubic phase is above 1370 °C, while the stable tetragonal and orthorhombic phase is above 1250 and 1213 °C, respectively.[9,10]

The sintering temperature can influence the phase of CaTiO3, the average crystallite size, morphology, and particle size, and the dielectric constant of CaTiO3 (CTO) escalated remarkably with raising the sintering temperature.[11] Low-loss microwave-based (MW) ceramics such as CaTiO3 have been recommended as a dielectric resonator in MW ICs (integrated circuits).[12] The researcher has examined to enhance the effectiveness of this ceramic material by coupling Fe and by doping Mn.[13,14]

Calcium titanate shows a high dielectric constant (εr) of 170 and an average quality factor of 3600 GHz and is usually recommended for stimulating material for microwave applications.[15] The εr of CaTiO3 is considered to be the function of temperature, and it has been investigated that the εr is increases as the temperature drops and saturates at low temperature, as can be noticed. This response could be considered as a “fingerprint” of emerging ferroelectricity. The dielectric constant of CaTiO3 is higher than that of TiO2 at low temperature.[16] Many researchers already have publicized the different synthesis routes to prepare CaTiO3 materials, for example, the co-precipitation method, Hydro thermal technique, Sol–gel auto combustion method, and so on.[17] The most high temperature ball milling synthesis of CaTiO3 has been conducted using mistreatment mixtures of calcium carbonate (CaCO3), calcium oxide (CaO), and titanium dioxide (TiO2).[18]

In the present study, an effort is made to synthesize the Ca(SnTi1–)O3 lead-free ceramic (0 ≤ x ≤ 0.8) samples at frequencies 1.00–2.00 GHz via a ball milling process. The ball milling is a mechanical technique that is broadly used to grind powders into fine particles, and its effect on the crystallite size, dielectric properties, and microstructure development of milled powders is studied. Due to this importance, we studied the effect of Sn4+ on the relationship between the structural, microstructural, vibrational, and MW dielectric properties (dielectric constant and tangent loss) of the CaTiO3 materials which by varying of the frequency are improved obviously.

Results and Discussion

Phase Formation Analysis

The XRD patterns of the pure CaTiO3 and doped Ca(SnTi1–)O3, (0 ≤ x ≤ 0.8) ceramics sintered at 1200 °C for 3 h in air are shown in Figure . The XRD pattern revealed that the specimens have a single-phase base composition of CaTiO3 that corresponded to pdf card # 82-232, suggesting an orthorhombic structure with a Pbnm space group.[19,20] The most intense peak is observed at 33.2 °C and indexed as 2 0 0. The peak (2 0 0) was sharper in the sample with a Sn4+ concentration of x = 0, and no peak splitting was seen to specify existence of the superstructure. Similarly, the presence of the superstructure would be confirmed if peak splitting was identified for high-Sn4+ concentration (x ≥ 0.2) samples. Moreover, with the increasing of the Sn4+ content (x), the main diffraction peak (2 0 0) had an obvious shifting to a lower angle as shown in Figure b. This shifting may be due to inhomogeneity or accredited to the substitution of relatively larger ionic radii of Sn4+ (RSn = 0.69 Å) than Ti4+ (RTi = 0.64 Å) at the six fold coordinated B site of the perovskite structure succeeding the Braggs diffraction law, whose mathematical expression is 2dsin θ = nλ (Table ).[21,22]

Figure 1

(a) XRD patterns of Ca(SnTi1–)O3, (0 ≤ x ≤ 0.8) ceramics sintered at 1200 °C for 3 h and (b) shifting of the (2 0 0) peak to lower 2θ° values with increasing (x).

Table 1

Indexed Pattern for the Ca(SnTi1–)O3 at λ = 0.15418 nm

2θexp	2θcalc	Iexp.	h	k	l	dexp.	dcalc.
23.25	23.30	582.84	0	0	2	3.82134	3.81377
29.85	29.92	593.44	1	1	1	2.98913	2.98334
33.15	33.19	1110.92	2	0	0	2.69891	2.69608
42.75	42.97	627.95	1	1	3	2.11248	2.10210
47.55	47.61	918.72	2	2	0	1.90972	1.90783
59.11	59.32	747.24	3	1	2	1.56092	1.55587
69.45	69.55	674.18	0	4	0	1.35183	1.34993

Lattice parameters were calculated using the least-square refinement method such that ΣΔd (Δd = dobs – dcal), which should be minimum. It can also be seen that the lattice parameters and lattice volume increased with increasing the Sn content at the B site of Ca(SnTi1–)O3, (0 ≤ x ≤ 0.8), solid solution as shown in Table . The increase in the lattice parameters and lattice volume should be attributed to the fact that the ionic radius of the substituted Sn4+ ion (0.69 Å) is larger than that of the Ti4+ ion (0.64 Å).[22]

Table 2

Lattice Parameters of Orthorhombic Structure of Ca(SnTi1–)O3 Solid Solution Sintered at 1200 °C

contents (x)	Structure	space group	ρexp (g cm–3)	ρth (g cm–3)	a (Å)	b (Å)	c (Å)	volume (Å3)
0.0	orthorhombic	Pbnm	3.48	4.04	5.37730	5.38595	7.51729	217.7145
0.2	orthorhombic	Pbnm	3.94	4.85	5.38013	5.39254	7.72184	224.0308
0.4	orthorhombic	Pbnm	4.10	5.08	5.63068	5.43169	7.97118	243.7914
0.6	orthorhombic	Pbnm	4.31	5.46	5.60671	5.54661	8.29084	257.8309
0.8	orthorhombic	Pbnm	4.19	5.59	5.57172	5.63664	8.38005	263.1817

The crystallite size (D) of Ca(SnTi1–)O3, (0 ≤ x ≤ 0.8) ceramics was calculated using a well-known Debye–Scherrer equation[23]where D, K, λ, β, and θ denoted the crystal size, constant (0.89), X-ray wavelength, full width at half maximum (fwhm) of the most concentrated peak, and Bragg angle, respectively. This technique is used to study XRD data, where the crystal size is associated with the expansion of the strong diffraction peak.

Mathematically, the dislocation density (δ) and micro strain (s) were calculated by using these equations[24]

The lattice strain (η) was calculated through the given equation[25]

The deviation in the calculated lattice strain and crystallite sizes of all prepared Ca(SnTi1–)O3, (0 ≤ x ≤ 0.8) ceramic samples with compositions is given in Table . The estimated crystallite sizes were found to be in the 331–440 nm range.

Table 3

Calculated Average Crystallite Size (D), Dislocation density(δ), Micro Strain (s), and Lattice Strain (η) of Ca(SnTi1–)O3, (0 ≤ x ≤ 0.8)

composition	D (nm)	porosity (%)	δ (nm–2)	s (×10–2)	η (×10–3)
0.0	440.389	13.75	5.1562	2.1055	0.7784
0.2	378.469	18.66	8.2651	2.4703	0.9057
0.4	331.456	19.29	9.1022	2.8207	1.0342
0.6	343.345	21.25	8.4828	2.7229	0.9984
0.8	413.925	24.87	5.8365	2.2587	0.8281

The behavior of the crystallite size is presented in Figure . The crystallite size depends on the lattice strain and radius of the substituted ions. It has been observed that the average crystallite size generally decreases with the increasing Sn4+ content (x); this may be attributed toward growth of the crystal restricted by the substitution element with ionic radii RSn greater than RTi.[26]

Figure 2

Crystallite size of Ca(SnTi1–)O3, (0 ≤ x ≤ 0.8) ceramics at 1200 °C .

The particle size and lattice strain of Tin-doped calcium titanate samples were determined using the Williamson–Hall technique from the broadening of the XRD peaks.[27]where β is the fwhm in radian by fitting the prominent peaks, θ is the diffraction angle in radian, k is the shape factor value 0.94, “λ” is the XRD wavelength (λ = 0.15418  nm), and “D” is the effective crystalline size. Figure shows that the Tin-doped calcium titanate samples, the slope of the linear data-plotted fit against 4sin θ versus β cos θ, contribute the information about the inverse of intercept, and the lattice strain yields the value of the crystalline size (DW–H).[28] The points are noted to be widely spaced around the fitted line. It has been observed that certain additional parameters of the analyzed sample were not taken into consideration, or alternative techniques should be used.

Figure 3

Williamson–Hall (W–H) plots of Ca(SnTi1–)O3, (0 ≤ x ≤ 0.8).

Microstructural Analysis

The grain morphology of Ca(SnTi1–)O3, (0 ≤ x ≤ 0.8) ceramics was studied through the SEM micrographs which are recoded at a magnification of 10k sintered at 1200 °C (0 ≤ x ≤ 0.8) for 3 h in air as evident in Figure . Through this analysis, the grains of the specimens were revealed to be faceted, spherical, and plate-like shaped micro-size connected grains with different contents “x”. Furthermore, the distribution of grain size is less uniform. Additional increase in the content “x” influences the growth of the grain which is complemented by a significant decrease in terms of residual porosity. As a result, these microstructures become favorable terms of the dielectric properties. However, the average grain size, Davg, for different compositions was determined using ImageJ software. The average grain size is found to be 3.29, 2.82, 2.39, 1.98, and 1.37 μm for the compositions x = 0.0, 0.2, 0.4, 0.6, and 0.8, respectively, as shown in Figure f. In Figure a, sintered at 1200 °C, the existence of a few larger grains can be seen, which might be attributed to calcium titanate attempting to minimize internal energy by reducing the total space of the grain boundary, resulting in the subsequent grain growth.[29] This means that substituting Sn4+ for Ti4+ in the perovskite lattice can demote the grain growth as shown in Figure a–e. This nature of the morphology has been previously described for CaTiO3 ceramics.[30] The porosity of the samples was determined by using eq and is shown in Table .[31]where ρexp and ρth are the experimental and theoretical densities (calculated using Archimedes’ principle), as shown in Table , respectively.

Figure 4

SEM micrograph of (a) CaTiO3, (b) Ca(Sn0.2Ti0.8)O3, (c) Ca(Sn0.4Ti0.6)O3, (d) Ca(Sn0.6Ti0.4)O3, and (e) Ca(Sn0.8Ti0.2)O3 and (f) average grain size of CST.

Electronic and Dielectric Properties

FT-IR Spectrum

The vibrational modes of pure CaTiO3 and Sn-doped CaTiO3 were examined through Fourier transform infrared spectroscopy (FTIR) as depicted in Figure . The spectra of both pure and Sn-doped CaTiO3 represent a common peak at 3440 cm–1, which can be estimated as the stretching mode of the water/hydroxyl group presumably adsorbed at the surface. In addition, the bending mode of the hydroxyl group could be ascribed to the small peak positioned at 1437 cm–1. In the wavenumber range of 500–600 cm–1, stretching and bridging vibrational modes of Ti–O and Ti–O–Ti are particularly evident.[32] The stretching mode of vibrations and the bridging stretching mode of Ti–O–Ti and Ti–O–Sn both are associated with the broadband in the region below 800 cm–1. In the FTIR spectra of Sn-doped CaTiO3 ceramics, no extra peak for Sn–O/Sn–Ti–O vibrational modes is detected, which could be explained by trace substitution of Sn with Ti.

Figure 5

FTIR spectrum of Ca(SnTi1–)O3, (0 ≤ x ≤ 0.8) ceramics at 1200 °C.

Dielectric Spectroscopy

The dielectric reaction in a solid material can be described by expressing the relative dielectric permittivity as a complex quantity made up of real and imaginary componentswhere εr′ and εr″ denote the real and imaginary part of the dielectric permittivity, respectively. It represents the quantity of stored energy in dielectric materials as polarization and energy loss occurs when the external field is applied.[33] The real and imaginary part of the dielectric permittivity can be calculated from complex impedance data by using these equations

Figure a illustrates the frequency dependence of the real component of dielectric permittivity, measured at 1.00–2.00 GHz for CST ceramics sintered at 1200 °C. It is clearly observed that εr′ increases slowly with a rise in the frequency and the increasing of the content (x), and it reaches a saturation limit at f ∼ 1.8 GHz. On further increasing the frequency, there is a rapid decrease in the real permittivity. As a result, the dipoles fail to maintain the accurate oscillatory field when the permittivity decreases with the increasing frequency.[34]

Figure 6

(a) Frequency dependence plot of the real part of dielectric permittivity. (b) Frequency dependence plot of the imaginary part of dielectric permittivity of Ca(SnTi1–)O3 ceramics.

The imaginary part of dielectric permittivity, as a function of the frequency range from 1.00 to 2.00 GHz for CST (x = 0.0, 0.2, 0.4, 0.6, 0.8) ceramics sintered at 1200 °C, is represented graphically in Figure b. The εr″ values increase with increasing the frequency upto 1.8 GHz and the decrease in permittivity at the higher-frequency region (f > 1.8 GHz). These developments of the curves are usual conducts for most of the dielectric ceramics due to the existence of strong dielectric relaxation as the rotation of the dipoles becomes deficient to align with the oscillatory AC electric field with the growing frequency.[35]

Complex Impedance Spectroscopy

Currently, the complex impedance spectroscopy (CIS) mechanism is generally used to investigate the structural properties and bonding of the various types of materials, comprising the ionic insulator and ferroelectric and linked ceramics under various experimental conditions.[36]where , Co is the geometrical capacitance, ε* is the complex permittivity, and ω = 2πf is the angular frequency.

The deviation in impedance Z′ of Ca(SnTi1–)O3, (0 ≤ x ≤ 0.8) ceramics as a function of the frequency (1.00–2.00 GHz) is shown in Figure a. Initially, it has been investigated that the magnitude of Z′ increases with increasing the frequency and concentration of Sn4+ (0 ≤ x ≤ 0.8) in the frequency ranges 1.00–1.6 GHz; thereafter, it appears to slightly decrease in the high-frequency region (f > 1.6 GHz. It is simply possible because rising temperatures and frequencies cause the release of space charge polarization.[37] From this behavior, we can also conclude that the conduction mechanism is directly related to frequency. As the frequencies increase from 1.00 to 1.6 GHz, the magnitude of Z′ decreases with the increases in concentration Sn (0 ≤ x ≤ 0.8).

Figure 7

(a) Real (Z′) vs frequency and (b) Imaginary (Z″) vs frequency of the Ca(SnTi1–)O3 ceramics.

Figure b demonstrate the reciprocal dependence of the frequency of the hypothetical component Z″ (also called loss spectrum) of Ca(SnTi1–)O3, (x = 0.0, 0.2, 0.4, 0.6, 0.8). The magnitude of Z″ decreases by increasing the frequencies (1.00–1.8 GHz) and also the concentration of Sn contents at room temperature. When the frequencies increase from 1.8–2.00 GHz, the Z″ spectrum sharply increases at concentration Sn (x = 0.0, 0.2, 0.4, 0.6, 0.8). It demonstrates that with the addition of the Sn concentration, the magnitude of Z″ decreases, and all the peaks move toward the higher-frequency region. At higher frequencies, the contribution from the grain predominates attributable to the absence of the space charge effects of the various compositions.[38]

Analysis of Complex Modulus

The complex modulus formalism is a technique which plays a significant role in studying the electrical relaxation process in ionic conducting materials.[39] This particular technique is used for the easy suppressing of electrode polarization effects.

The complex modulus can be stated quantitatively using the following formula[40]

The following formulae were used to determine the real and imaginary components of the complex electrical modulus

Figure a shows the real part of the modulus M′ as a function of frequencies (1.00–2.00 GHz) of Ca(SnTi1–)O3, (0 ≤ x ≤ 0.8). It was observed that for all samples, the real part of the electrical modulus M′ decreases, with the increasing of the concentration Sn4+ and frequencies at room temperature. The mobility of long-distance charges is demonstrated in the low-frequency range. Similarly, the high-frequency region indicates the mobility of short-distance charges because of the potential well limitation. It could be described through a conduction phenomenon resulted by charge carrier’s long-range mobility.[41]

Figure 8

(a) Variation of the real modulus (M′) vs frequency and (b) imaginary modulus (M″) vs frequency of the Ca(SnTi1–)O3 ceramics.

Figure b illustrate the frequency dependence of the imaginary part of the modulus M″ of Ca(SnTi1–)O3, (0 ≤ x ≤ 0.8) at room temperature. It has been reported that the hypothetical modulus M″ decreases with the increasing frequency, but it reaches the maximum and then increases along with concentration Sn4+. This may be explained as the frequency region below the maximum peak classifies within the range charge carriers owing to extended range hopping. However, because charge carriers are mobile over short distances, they are restricted to potential wells in the frequency region above the peak maximum.[42] The specific region wherever the exact maximum peak arises is basically the sign of the transition from long-range to short-range mobility as frequency increases.[43]

Tangent Loss

The power dissipation (dielectric loss) of commercial capacitors could be calculated through the given expressions[44]

In Figure , the dielectric loss (tan δ) values for all the sintered samples of Ca(SnTi1–)O3, (x = 0.0, 0.2, 0.4, 0.6, 0.8) ceramics are the highest at 2.00 GHz and the lowest at 1.00 GHz. The tan (δ) values are found to increase with the increasing frequency (upto 1.7 × 109 Hz) and the increase in the concentration Sn (0.0 ≤ X ≤ 0.8). The frequency further increases (f > 1.7 GHz), and the dielectric loss abruptly decreases. The tan δ value decreases with an increase in the frequency because the charge carrier growth does not align with the frequency of the applied field beyond a definite frequency range. It is important to note that DC conductivity and electrode polarization of the samples primarily cause the low values of tan (δ) at a lower-frequency range.[34,45] On the other hand, the large value of tan δ characterizes decent microwave (MW)-absorbing characteristics of the material. This particular arrangement of frequency dependency with the losses of “tan δ” is generally interrelated with conduction losses.[46] Therefore, the curves presented in Figure actually signify the main dielectric relaxation related to the dipolar polarization.

Figure 9

Variation of tangent loss of Ca(SnTi1–)O3, (x = 0.0, 0.2, 0.4, 0.6, 0.8) ceramics as a function of the frequency at room temperatures.

AC Conductivity

Figure shows the mechanism of electrical conduction and helps in understanding the activities of charge carriers, the field effect of the mobile charges, and actual functions of the domain structure. The variation of AC conductivity with the frequency (1.00–2.00 GHz) could clarify the frequency dependence of AC conductivity, which can be describe through the given equationwhere εr  corresponds to the dielectric constant, εo  is the permittivity of the free space (8.85 × 10–14 F/cm), tan δ = dielectric loss, and ω2 is angular frequency (i.e., ω = 2πf).

Figure 10

Frequency response curves of AC conductivity of Ca(SnTi1–)O3, (0 ≤ x ≤ 0.8) ceramics sintered at 1200 °C.

Figure clearly shows that at the lower-frequency region, the magnitude of conductivity gets to zero. When the concentration of the Sn content (0.0 ≤ x ≤ 0.8) and the frequency (upto 1.8 GHz) increase, then the AC conductivity value grows gradually until it reaches a definite value. However, when the frequencies are greater (f > 1.8 GHz), than the conductivity abruptly decreases. At higher frequencies, the curves merge with each other. Consequently, the conduction mechanism in the material increases with the frequency. It is because of the increase in the motion of charge carriers which are thermally activated due to the rise in temperature.[47] In every sample, we can also note the shifting of the peak toward the high-frequency region. The value of conductivity is high for the (x = 0.6) sample as compared to others. Conversely, due to large impudence, the values of AC conductivity are relatively low for the (x = 0) sample.[48]

Conclusions

In the present study, the Ca(SnTi1–)O3, (0 ≤ x ≤ 0.8) lead-free ceramics were successfully prepared via a ball milling process sintered at 1200 °C. When Sn4+ ions are doped in CaTiO3 ceramics, the average crystallite size, dislocation density, lattice parameter, unit cell volume, lattice strain, and micro-strain value change. The phase analysis confirmed the formation of the orthorhombic structure with Pbnm symmetry. The average crystallite size also decreased from 440.389 to 331.456 nm (CST). The surface morphology reveals the formation of grains with different sizes (small and large) and shapes (spherical, oval, and irregular with low porosity). The grain was sub-microns in size that decreased from 3.29 to 1.37 μm. We obtained excellent microwave dielectric properties in this study for the application of the microwave wireless communication system. The dielectric loss (tan δ) increases with increasing of the Sn4+ concentration. These lead-free ceramic materials are a suitable candidate for the application in Global Positioning Systems and communication antennas.

Materials and Methods

The ceramic samples with an ostensible composition Ca(SnTi1–)O3 were prepared through the ball milling method. Therefore, as to prepare the samples of Ca(SnTi1–)O3, suitable quantities of chemical-agent grade raw materials of CaCO3 (SIGMA-ALDRICH) with purity ≥99.9%, TiO2 (SIGMA-ALDRICH) with purity ≥99.5%, and SnO2 (UNI-CHEM) with purity ≥99.9% were computed to the relevant stoichiometric molar ratios. As subjected to milling, the powder was ball milled horizontally in a polymer container for a maximum of 24 h using distilled water and zirconia balls. Once the milling step was completed, the samples were then put to the drying method at 90 °C for 24 h, and then, they were grinded. After grinding, the powder was calcined at a temperature of 950 °C for the compositions with 0 ≤ x ≤ 0.8 for 3 h, by keeping cooling and heating effects constant at a rate of 5 °C/min. Then, the weighing range from 0.5 to 0.7 g, diameter of about 13 mm, and also the thickness of 4–5 mm pellets of the calcined fine powders were determined, with the 80 MPa pressure using a stainless-steel dye. Sintering was carried out at 1200 °C for 3 h after the calcined pellets were put on a ceramic foil, with the cooling/heating temperatures kept at the same rate of 5 °C/min.

Characterization of CaTiO3 and Sn-Doped CaTiO3

The phase formation of sintering and milling samples was performed through an X-ray diffractometer (JDX-3532, JEOL, Japan) with Cu-Kα radiations of wavelength λ = 0.1540598 nm, functioned at 45 kV and 40 mA used to identify the phases. Primarily, the step size 0.05 (°C), the scan rate 0.5 (°C/min), and the scan range 10.020–70.020 °C were assumed. A scanning electron microscopy instrument (SEM) (JSM-5910, JEOL Japan) was used to analyze the microstructures. The samples were refined and thermally etched for 1 h at temperatures of 10% less than their sintering temperatures as a special condition for SEM. Microwave (MW) dielectric properties of the sintered sample were measured using an impedance analyzer (Agilent-E4991A, from 1 MHz to 3 GHz).