Effects of Strontium on the Structural, Optical, and Microwave Dielectric Properties of Ba2Ti9O20 Ceramics Synthesized by a Mixed Oxide Route.

Solid solutions of Sr-doped barium nonatitanate (Ba2Ti9O20) ceramics were synthesized by a mixed oxide route, studied their structural, microstructural, optical, and microwave dielectric properties were studied. X-ray diffraction (XRD) has been used to reveal the structure and crystallite size of doped Ba2Ti9O20 ceramics. Rietveld refinements of XRD patterns revealed that all of the ceramics have a tetragonal structure with space group I4/m. The surface morphologies of all the samples were characterized by using scanning electron microscopy (SEM). Fourier transform infrared (FT-IR) studies gave the O-H and Ti-O modes of vibrations. The stretching modes of Ti-O and O-H were noted at 1400, 2960, 3700, and near to 440 cm-1, respectively. The band gap energy decreases with increasing Sr2+ contents, and a high value (2.12 eV) was observed in the base sample. The microwave dielectric properties of the ceramic samples were studied at different frequencies ranging from 1 to 2 GHz by using impedance spectroscopy. The obtained results showed the suitability of these samples for microwave dielectric resonator (antenna) applications.

Introduction

Due to their outstanding structural and dielectrical properties, barium-based titanate ceramics such as BaTi4O9, BaTi5O11, and Ba2Ti9O20 have been traditionally observed to be emerging materials. The Ba2Ti9O20 (barium nonatitanate) ceramic has attracted interest due to the importance of its many applications in the field of modern telecom industries: i.e. radio stations (software base), satellites for monitoring environmental parameters, and GPS (global positioning systems). In addition, this ceramic has good-quality microwave dielectric resonator components for high-speed communications devices, including high-capacity data storage devices, rechargeable batteries, and dielectric resonators. To make improvements to the barium-based compound, to give it excellent dielectric qualities at a reasonable cost, most enterprises and research have provided microlevel oscillators and low-frequency microwave analyzers or filters for these purposes.[1,2] Ba2Ti9O20, BaTi4O9, and BaTi5O11 exhibit high values of relative permittivity and a low dielectric loss in the radio frequency range.[3] Furthermore, a complex perovskite A2B9O20 type barium nonatitanate (Ba2Ti9O20) has been studied where the A and B sites generally represent cations while the O site represents an anion.[4−7] Meanwhile, doping divalent metals (Ca2+, Sr2+, Sn2+) in Ba2Ti9O20 modified the microwave properties, i.e. the dielectric constant (εr) of the ceramic pellets, the quality factor (Qf), and the temperature coefficient at the resonant frequency (τf), which are very necessary for making microwave wireless communication devices. In addition, the Sr2+ concentrations modified the microwave dielectric properties of the base ceramic material. Ba2Ti9O20 has good properties, i.e. a high value of the relative permittivity (εr = 38.4), a good temperature coefficient of the resonant frequency (τf = +20 ppm °C–1), a high quality factor (Qf = 3300 GHz), and low dielectric loss (tan σ = 0.00056), and solid solutions of doped concentrations of Ca, Sr, Zr, or Sn in Ba2Ti9O20 ceramics are very suitable for making good dielectric resonator antennas.[8−11] For Ba2Ti9O20 sintered ceramics, it is necessary to get maximum values of the quality factor (Qf × f) and low values of the temperature coefficient at the resonant frequency (τf). BaSnO3 and BaWO4 have negative τf values, which when they are added to the aforementioned dopants give positive τf values to give a nearly zero τf value, but the adjustment of τf and Qf × f is very difficult.[12−14] Many researchers have studied the effects of different dopants on the structural and microwave dielectric properties of Ba2Ti9O20 sintered ceramics.[15]

The objective of the present work is to study the effects of strontium carbonate on the microstructure development, crystal structure, and dielectric properties of solid solutions of (Ba1–Sr)2Ti9O20 (0.00 ≤ x ≤ 0.06) ceramics. The results of the work show that the influence of Sr2+ contents modifies the phase, microstructure, stretching vibration, and microwave properties (i.e relative permittivity and quality factor) of the Ba2Ti9O20 materials.

Results and Discussion

Phase Analysis

The XRD profile of (Ba1–Sr)2Ti9O20 (0.00 ≤ x ≤ 0.06) ceramics with various Sr contents are shown in Figure a,b. The tetragonal structure of I4/m (space group) basic contents of Ba2Ti9O20 ceramics matched with that of JCPD card number 01-080-0920, which represents the different diffraction peaks along with the lattice parameters a = 10.1479 Å, b = 10.1479 Å, and c = 2.9730 Å. The (130) peak shifted toward lower 2θ values was due to the increase in Sr2+ concentration in (Ba1–Sr)2Ti9O20 sintered ceramics; this may be due to the different ionic radii of Ba2+ and Sr2+ cations, as shown in Figure b. This might be ascribed to microstrain, inhomogeneity in the samples, or the replacement of the Ba2+ (RBa = 1.42 Å) cation by the comparatively larger radius of the Sr2+ (RSr = 1.44 Å) cation.[16]

Figure 1

(a) XRD patterns of (Ba1–Sr)2Ti9O20 (0.00 ≤ x ≤ 0.06) sintered ceramics. (b) Observed shift of the lower 2θ peak at (130).

The effect of Sr concentrations on the crystallite size when Sr was integrated in (Ba1–Sr)2Ti9O20 structure was evaluated using Scherrer’s formula[17]where λ represents the wavelength of the X-rays used (λ = 1.5406 Å), while β gives full width at half-maximum (fwhm) in radians, D is the average crystallite size, and θ is half of the angle between the incident and reflected X-ray beams.

The performances of various crystallite sizes are provided in Table . It was seen that the crystallite size decreased with an increase in strontium content. The crystallite size relies upon two factors: the radius of the substituted ions and also the lattice strain.

Table 1

Properties of (Ba1–Sr)2Ti9O20 (0.00 ≤ x ≤ 0.06) Ceramicsa

content (x)	structure	space group	ρexp (g cm–3)	ρth (g cm–3)	ρr (%)	D (nm)	δ (nm–2)	η (× 10-3)
0.00	tetragonal	I4/m	4.54	4.72	95.98	7.0513	0.02011	0.49159
0.02	tetragonal	I4/m	4.30	4.64	94.82	2.4559	0.16579	1.41143
0.04	tetragonal	I4/m	4.27	4.62	92.42	1.3979	0.51176	2.47973
0.06	tetragonal	I4/m	4.18	4.56	91.66	2.2393	0.19943	1.54798

Definitions: ρexp, experimental density; ρth, theoretical density; ρr, relative density; D, average crystallite size; δ, dislocation density; η, lattice strain.

The lattice strain is determined from the equation.[18]The calculated values of the lattice strain are given in Table . The lattice strain increases with an increase in strontium content up to x = 0.04; above this value the lattice strain decreases. The increase of lattice strain decreases the crystallite size up to x = 0.04, and at higher Sr doping levels, the ionic radius affects the crystallite size because the ionic radius of strontium is greater than that of barium.

The dislocation density can be determined with[19]where δ is the dislocation density while D is the crystallite size.

The crystalline phase (tetragonal structure) of the synthesized compound with high-intensity, sharp diffraction peaks (Figure ) may be due to an increase in the Sr contents and the phase formation as well. The relative density also affected the structural properties of the ceramic samples. The relative density can be calculated from the experimental and theoretical densities of sample as shown in Table . The theoretical density (ρx) of a sample can be calculated by using the formulawhere Z represents the number of atoms per unit cell while M and NA are the molecular weight and Avogadro’s number, respectively. Experimental density was measured by using the Archimedes principle.[20]

A Rietveld refinement was performed to confirm the exact the phase and lattice parameters of these ceramics using FullProf software. The refiment was carried out using the I4/m space group. The observed, calculated, and difference X-ray diffraction patterns for (Ba1–Sr)2Ti9O20 at x = 0.0, 0.02, 0.04, 0.06 are displayed in Figure a–d, respectively.

Figure 2

Rietveld refined XRD pattern of (Ba1–Sr)2Ti9O20 for (a) x = 0.0, (b) x = 0.02, (c) x = 0.04, and (d) x = 0.06 ceramics.

Microstructural Analysis

The SEM images of gold-coated and thermally etched samples of (Ba1–Sr)2Ti9O20 (0.0 ≤ x ≤ 0.06) sintered pellets are shown in Figure . The grain size variations and surface morphologies of the samples have been investigated. The SEM images of the sintered Ba2Ti9O20 ceramic without and with glass or Mn additions at varius sintering temperatures have been revealed by many researchers. The pure surfaces of Ba2Ti9O20 ceramics was observed to have pores and no grain growth. A decrease in porosity and the production of new grains was observed in Ba2Ti9O20 ceramics with glass dopants sintered at 1020 °C.[14] The effects of glass additions to Ba2Ti9O20 ceramics were modified grain growth and a low sintering temperature.[15,20] SEM images showed the surface morphalogy and porosity of all prepared pellets. The porosity and grain size increases with increasing Sr concentrations in Ba2Ti9O20 sintered ceramics (Figure ). These factors may affect the phase and microwave dielectric properties. For this purpose several researchers adopted and controlled the synthesis parameters during the preparation of ceramic dielectrics, which modified the microstructural dielectric properties.[16−22]

Figure 3

SEM images of gold-coated (Ba1–Sr)2Ti9O20 sintered ceramics: (a) x = 0.00; (b) x = 0.02; (c) x = 0.04; (d) x = 0.06.

Fourier Transform Infrared (FTIR) Spectroscopy

The FTIR spectra of (Ba1–Sr)2Ti9O20 (0.00 ≤ x ≤ 0.06) sintered ceramics are shown in Figure . To study the procedure of the chemical reaction for the preparation of a ceramic material, FTIR spectroscopy plays a vital role in analyzing the stretching and vibrational modes.[23,24] The O–H stretching mode was reported at different wavenumbers: i.e. 1400, 2960, and 37000 cm–1. This mode may occur due to the presence of humidity in the characterized samples. Only one asymmetric mode (near the wavenumber 1050 cm–1) has been reported, which showed that the carboxylate group bonded along with Ba entities.[15,17] In a given spectrum, a normal stretching vibrational mode (Ti–O) appears near the wavenumber 440 cm–1.

Figure 4

FTIR spectra of (Ba1–Sr)2Ti9O20 (0.00 ≤ x ≤ 0.06) ceramics.

Optical Studies

Figure shows the UV–vis optical absorption spectra of of (Ba1–Sr)2Ti9O20 (0.00 ≤ x ≤ 0.06) ceramics. It can be seen that the base sample exhibits an optical response at wavelengths shorter than 426 nm. This shows that the base sample is active under UV light and a narrow region of visible light. The light absorption edges in the UV optical absorption spectra of doped samples were extended to the visible light region (450–600 nm). The optical absorption band edge can be expressed by the Tauc equation (eq )where Eg is the band-gap energy and A is the proportionality constant while hν is the energy of a photon. By using a Tauc plot, the optical absorptions and band-gap energies (Eg) for all of the samples were calculated, as shown in the Figure . The natural transitions (electronic) of the samples have been studies by this method.

Figure 5

Absorption spectra and plots of the band gap energies of (Ba1–Sr)2Ti9O20 ceramics: (a) x = 0.0; (b) x = 0.02; (c) x = 0.04; (d) x = 0.06.

Many researchers have reported that the Ba2Ti9O20 sample is translucent for white light. It is very important to note that Ba2Ti9O20 ceramics are translucent in the visible spectrum.[25] The structural and transitional bands have been analyzed with the help of the photon energy.[26] To obtain an optical band gap energy, the electron should undergo inner-shell transitions. This band depends upon the absorption coefficient (α), which has been calculated by using eq .

AC Conductivity

AC conductivity measurements were made at room temperature for all the samples in the frequency range 1–2 GHz using an impedance analyzer. A frequency-dependent AC electrical conductivity study was carried out. The frequency-dependent AC conductivity of the (Ba1–Sr)2Ti9O20 ceramics are shown in Figure . The AC conductivity (frequency dependent) was determined by eq (27)where εr is the ielectric constant, ε0 is the permitivity of free space (8.85 × 10–14 F/cm), tan δ is the loss tangent, and ω2 is the angular frequency (i.e., ω = 2πf). The AC conductivity increases slowly at low frequency but abruptly increases at high frequency. These changes in AC conductivity may occur due to the hopping charge carriers and structure of the grains as well.[28,29] It is observed that the conductivity (σAC) varies with an increase in frequency. The trapping and fluctuations of conductivity may be due to the increase in temperature of the ceramic samples from the thermal activation energy. Due to the thermal activation energy, the movement of charge carriers increases inside the dielectric resonators.

Figure 6

Variation of AC conductivity with frequency of the (Ba1–Sr)2Ti9O20 ceramics.

Dielectric Properties

In the current research work, the dielectric properties, i.e. the relative permittivity and quality factor, has been measured in the frequency range 1–2 GHz. The dielectric constant value of the (Ba1–Sr)2Ti9O20 sintered ceramics changes with an increase in the Sr concentrations. The obtained dielectric constant values for Ba2Ti9O20, (Ba0.98Sr0.02)2Ti9O20, (Ba0.96Sr0.04)2Ti9O20, and (Ba0.94Sr0.06)2Ti9O20 were 28.87, 31.21, 32.78, and 36.93, respectively, at a frequency of 1 GHz. This shows that a small increase in the Sr concentration increased the relative permittivity values. Figure shows that the value of the dielectric constant decreases with an increase in the operating frequency. The value of dielectric constant can be explained easily according to the relative dipole moments and lattice structures of the (Ba1–Sr)2Ti9O20 samples. In the cubical structure of (Ba1–Sr)2Ti9O20 ceramics, the Ti atom coordinated octahedrally with 6 oxygen atoms. To obtain a permanent dipole moment, Ti atoms took centrosymmetric positions on the c axis, which enhanced the values of the relative permittivities. It was concluded that tetragonality depends upon the c/a ratio and the higher this ratio is, the relative permittivity maximum will be.[30,31]

Figure 7

Frequency-dependent relative permittivity of (Ba1–Sr)2Ti9O20 (0.00 ≤ x ≤ 0.06) ceramics.

The quality factor and relative permittivity both follow same the tendency with a variation in frequency: i.e. these values are the minimum and the maximum in high- and low-frequency regions, respectively. The quality factor variation along with frequency of (Ba1–Sr)2Ti9O20 ceramics is shown in Figure . In the low-frequency region, the values of the quality factor strongly depend upon the frequency while in the high frequency region there appears to be a saturation phase. The quality factor of the base sample (x = 0.00) decreases with Sr concentration and nearly vanishes at the content x = 0.02; due to space charge polarization the values of the quality factor may be affected. The quality factor is the ratio of energy stored per cycle to the energy dissipated per cycle of the operating AC source.[32] It was observed that the quality factor values decrease with an increase in the Sr concentrations.

Figure 8

Variation of quality factors of (Ba1–Sr)2Ti9O20 (0.00 ≤ x ≤ 0.06) ceramics with frequency.

Conclusion

(Ba1–Sr)2Ti9O20 (0.00 ≤ x ≤ 0.06) ceramic samples was prepared by a conventional method. A variation of crystallite size, lattice strain, and dislocation density was observed in Ba2Ti9O20 ceramics due to the Sr2+ contents. Their structural properties was investigated by using the X-ray diffraction (XRD) technique. The structural analysis determined the construction ofa tetragonal structure with space group I4/m. The average sizes of crystallites decrease from 7.0513 to 1.3979 nm (BST). A scanning electron microscopy (SEM) analysis showed spherical-shaped crystallites. The optical properties were studied by the UV–visible spectra and indicated that the band gap decreases from 2.12 to 1.61 eV with an increase in the Sr2+ concentration. The relative permittivities and quality factors of the samples were calculated at different frequencies. The overall findings in this research study may help in the applications of dielelctric resonators.

Materials and Experimental Methods

Solid solutions of (Ba1–Sr)2Ti9O20 (0.00 ≤ x ≤ 0.06) ceramics were prepared by a conventional (mixed oxide) route. High-grade raw materials were obtainedfrom Aldrich and Sigma: BaCO3 (99.9%, Aldrich), TiO2 (99.9%, Sigma), and SrCO3 (99.9%, Aldrich) were weighed accordingly and blended as per their stoichiometric ratios. The raw material mixture was dry-ground and mixed by horizontal ball milling for 1 day. After the milling process, the mixure was placed in an aluminia cruicible and then calcined at 850 °C in a furnace at a heating rate of 5 °C/min for 4 h. The prepared calcined powder was ground manually by using a pestle and mortar for 2.0 h to avoid agglomerations. The soft and fine powder was uniaxially pressed at 90 MPa pressure into a cylindrical pellet with 5.0 mm thickness and 10.0 mm diameter by using a manual pellet press machine (CARVER, USA). Then the green pellet was placed in a high-energy furnace for sintering at a temperature of 1020 °C for 4 h in an open atmosphere at a 5 °C/min cooling/heating rate. The Archimedes principle was used to estimate the approximate bulk densities of the samples. The crystalline phases of the sintered (Ba1–Sr)2Ti9O20 (0.00 ≤ x ≤ 0.06) ceramics was studied by using an X-ray diffractometer (XRD; JDX-03532, JEOL, Japan) with Cu Kα (λ = 0.15406 nm) radiation operated at 40.0 mA and 40.0 kV in 20° < 2θ < 80° range of Bragg angles at a scanning rate of 2°/min. The surface morphologies of all the samples have been analyzed by using scanning electron microscopy (SEM, JSM-5910, JEOL Japan). The Fourier transform infrared radiation (FTIR) absorption spectra were recorded on a PerkinElmer GX FTIR system at a 10 cm–1 resolution spectrum in the range of 400 to 4000 cm–1. The dielectric properties of the prepared sample were measured by using impedance spectroscopy (Agilent E4991A, 1 × 106 to 3 × 109 Hz).