Production of Nanometric Bi₄Ti₃O ₁₂ Powders: from Synthesis to Optical and Dielectric Properties

Abstract

This paper aims to evaluate the synthesis and annealing parameters for production of nanometric Bi₄Ti₃O₁₂ and its properties. The powders were obtained through the solution combustion route and the impacts of annealing temperature on the materials’ physicochemical features as well as their optical and electrical properties were investigated. Thus, the prepared powders were annealed at 600ºC, 700ºC and 800ºC and then characterized by several techniques. The results demonstrated that the combustion method was effective for production of nanocrystalline powders with high levels of purity. A trend for particle and crystallite growth was observed as the calcination temperature increased. X-Ray, HRTEM and Raman spectroscopy confirmed the crystalline nature of the powders, whereas impedance spectroscopy demonstrated a reduction of electrical resistance according to the calcination temperature applied. Optical properties were not highly influenced by annealing. The temperature of 600ºC was appropriate to produce crystalline particles with desirable low sizes for application.

Keywords:
nanotechnology; spectroscopy; annealing; synthesis; thermal etching

1. Introduction

Nowadays, many efforts have been made in order to develop new lead-free optoelectronic materials^1,2. Among them, bismuth titanates have demonstrated promising optoelectronic properties³, such as ferroelectricity, photoconductivity and piezoelectricity^1,4-6. These characteristics make them an alternative to lead-containing materials, being useful for several devices including optical displays, capacitors, catalysts, sensors and transducers, among other applications^7-15.

The Bi₄Ti₃O₁₂ compound belongs to the Aurivillius family^5,9,16,17 and it has been studied due to its promising piezoelectric and dielectric properties¹. Based on this material, piezoelectric and pyroelectric devices have been produced to be utilized in a broad range of temperatures⁵. A highly anisotropic layered structure is characteristic of that material, in which the (Bi₂O₂)²⁺ fluorite-sheets are periodically arranged with the (Bi₂Ti₃O₁₀)^2‒ pseudoperovskite-sheets^{3,5,6,9,10,12,18-21} along the c-axis²². This phase presents a significant thermal stability, which melts around 1200ºC^4,23.

Different synthesis routes have been proposed for production of bismuth titanates and related materials. Methods such as sol-gel³, polymeric precursors²⁴, high energy milling^25,26, hydrothermal^10,18,27 and coprecipitation^6,9 have been assessed for their production. In addition, the use of the solution combustion route has been reported in some studies with promising results^20,22. In this chemical synthesis method, a self-induced high temperature is attained by an exothermic reaction based on a homogeneous mixture of metal salts and fuel. In addition to the relative ease and short time required for the synthesis, the properties of the synthesized powders are commonly quite superior to the ones produced by conventional routes²⁰.

Among existing techniques of Bi₄Ti₃O₁₂ synthesis, traditional procedures as the mixing of powders have been avoided due to the bismuth volatilization^1,8. The long periods of time at high temperatures can cause significant volatilization of this element, impacting on the material’s final properties. Moreover, repetitive stages of grinding and calcination are necessary to attain a satisfactory chemical homogeneity of the powders, which can be considered onerous compared to the other chemical routes of synthesis.

Besides these considerations, the production of Bi₄Ti₃O₁₂ compound in nanometric scale is highly desired²⁸ in order to increase the powders’ surface area and related properties. It has become crucial to applications involving direct use of the powder, such as in case of catalysts⁸. Moreover, some properties can be greatly improved when the material is composed by nanocrystals²⁹. The nanometric scale can also allow greater ease of sinterization, promoting the ceramic body densification even at low firing temperatures¹.

Despite the remarkable importance of controlling the particle sizes and crystallinity of nanometric Bi₄Ti₃O₁₂ powders, a systematic study correlating these features with annealing temperature; optical and electrical properties has not been reported yet. In this context, this work aims at evaluating the parameters of synthesis and calcination temperatures for production of nanometric Bi₄Ti₃O₁₂ powders by means of solution combustion route, analyzing their physicochemical features and properties.

2. Methodology

The powders were obtained through solution combustion route and the annealing temperature subsequent to the ignition was assessed in order to produce nanometric powders with optimal physicochemical characteristics. The conditions of synthesis and characterizations utilized in this work are better described in the following topics.

2.1. Synthesis

The synthesis of Bi₄Ti₃O₁₂ powders was performed by the solution combustion route. Stoichiometric amounts of titanium (IV) bis (ammonium lactato) dihydroxide (Sigma-Aldrich, 50% wt aqueous solution) and bismuth (III) nitrate pentahydrate (Sigma-Aldrich, 98%) were mixed and disposed in porcelain crucible. Urea (Synth, 98%) was chosen as the fuel, based on the good relationship between its heat of combustion (10.2 kJ.g^-1) and low decomposition temperature (135ºC) when compared to other ones such as glycine and citric acid³⁰. These characteristics allow the flame generation and also prevent non-reacted fuel remains after the synthesis.

The quantity of urea could be estimated by means of the ratio^31,32:

$\phi =\frac{\mathrm{total}\mathrm{valence}\mathrm{of}\mathrm{fuel}}{\mathrm{total}\mathrm{valence}\mathrm{of}\mathrm{oxidizer}}$

When φ equals unity, the quantity of fuel is in stoichiometric proportion. It can provide a large generation of gases and high flame temperature, which increases the powders crystallinity and surface area. Values of φ less than unity commonly culminate in a lower combustion temperature since the quantity of fuel is not enough for the complete reaction. On the other hand, when φ values are much higher than the unity, the reaction becomes incomplete due to the insufficient quantity of oxygen in the atmosphere³¹. It also reduces the temperature of the flame and facilitates the excess of reactants remaining on the powders surfaces after synthesis.

Therefore, a stoichiometric quantity of urea was mixed to the precursor salts and they were vigorously stirred in aqueous medium for 30 min at about 80ºC. Afterwards, water excess was eliminated at 100ºC also under magnetic stirring; and the viscous solution was then introduced in a partially opened EDG 3000 oven at 600ºC for ignition.

After the end of reaction (extinction of the flame), the remaining powders were maintained at 600ºC for 15 minutes in order to eliminate excessive amounts of residual organic matter and precursors salts partially decomposed. Then, the prepared powders were annealed at different temperatures (600ºC, 700ºC and 800ºC) for one hour. This procedure aimed for the determination of the optimal calcination temperature to eliminate residual organic matter, as well as for a systematic evaluation of its impacts on the powders’ crystallinity and particle sizes. The samples calcinated at 600ºC, 700ºC and 800ºC were named 600‒1, 700‒1 and 800‒1 respectively. It is noteworthy that an additional sample, 600‒0, also was evaluated in this study. It refers to the powder synthesized (600ºC for 15 minutes) without subsequent additional annealing.

2.2. Characterization

The powders' crystallographic properties were evaluated by the X-Ray diffraction technique (XRD). The analyses were performed in the Shimadzu XRD6000 diffractometer with Cu Kα radiation between 5º and 80º, 1º.min^‒1. Rietveld refinement was used to acquire the structural parameters of the synthesized phase. The GSAS‒EXPGUI software (ICDD card nº. 73‒2181) was used. For these analyses, micrometric yttrium oxide (Sigma Aldrich, 99.99%) was used as the pattern; instrumental parameters were acquired by the Le Bail method. The crystallite sizes were estimated by means of the Williamson-Hall methodology.

The surface area was determined by nitrogen physisorption (BET) in the Micromeritics ASAP 2020 equipment at 77 K. Purity of the samples was assessed by X-Ray fluorescence utilizing a Shimadzu EDX-720 equipment. The morphology and particle sizes were observed by means of Scanning Electron Microscopy (SEM). A JEOL JSM 6701F microscope was used under magnifications of 200 thousand times.

The presence of residual organic matter and phase transition were evaluated by Differential Scanning Calorimetry and Thermogravimetric Analysis (DSC/TGA). The same was done simultaneously in a Netzsch STA 449F3 equipment under argon atmosphere. The samples were disposed into the Pt-Rh crucibles and a sapphire disk was used as a reference.

Furthermore, as a complementary analysis, infrared absorption technique (FTIR) was performed by an Agilent Technologies Cary 630. The analyses were performed between 4000 cm^-1 and 650 cm^-1.

High Resolution Transmission Electron Microscopy (HRTEM) was carried out by a TECNAI G2F20 microscope. Electron diffraction patterns were acquired by the same equipment.

The optical properties were evaluated by Diffuse Reflectance Spectroscopy (DRS). The analyses were carried out in a Varian Cary 5G spectrophotometer between 250 nm and 800 nm.

Micro-Raman spectra were obtained by means of a Horiba Labram HR spectrophotometer. Magnifications of 50 times were used with a focusing area of 100 µm². A He-Ne laser (632.8 nm) without filter (17 µW) was applied. Silicon was used as the pattern. The analyses were performed in triplicate, between 100 cm^-1 and 1000 cm^-1.

In order to observe the ceramics microstructure after sinterization, fired pellets were produced. First, the powders were uniaxially pressed in a steel mold (5 kPa, 10 mm x 2 mm approximately) and heated at 1000ºC for one hour. The heating and cooling rates applied were 1ºC.min^‒1. To avoid bismuth volatilization, the green pellets were covered with sacrifice powders with the same composition.

To reveal the grains’ morphologies, the ceramics were polished and thermally etched at 900ºC for 15 min, also under sacrifice powders. The ceramic surfaces were then covered with gold by sputtering. After that, their microstructures were analyzed by SEM using a FEI Magellan 400L under magnifications of 100 thousand times.

The electrical responses of the ceramic pellets were acquired by Impedance Spectroscopy technique (IS) in a Solartron SI 1260 impedanciometer coupled with a Solartron 1296 dielectric interface. The measurements were performed between 1 MHz and 500 mHz; alternate current amplitude of 100 mV and at temperature range of 25ºC-350ºC. Gold was used as an electrode deposited by sputtering.

3. Calculation Procedures

To evaluate the electrical and optical performance of the materials, some physicochemical models are required. These equations and their meanings are described in the sequence below.

3.1. Electrical properties

The real Z’ and imaginary Z” parts of impedance measurements can be represented by the Nyquist complex-plane for several values of frequency. Based on the vector nature of impedance ‒ Z(ω) = Z’ + jZ”, in which ω is the angular frequency and j the complex number ‒ the electrical responses can also be plotted under polar coordinates (Bode diagram). For such, the following transformations are useful³³:

$\mathit{ɸ}={\tan }^{-1}\frac{{\mathit{Z}}^{″}}{{\mathit{Z}}^{\prime }}$

$\left|\mathit{Z}\right|=\sqrt{{\left({\mathit{Z}}^{\prime }\right)}^2+{\left({\mathit{Z}}^{″}\right)}^2}$

Where ɸ is the phase angle and |Z| the impedance modulus.

Immitance functions can be used to emphasize electrical phenomena which were not clearly defined only by Nyquist or Bode diagrams^33,34. Among them, the modulus M = M’ + jM” can be defined by the following equation^33-35:

$\mathit{M}=\mathit{j}\mathit{\omega }{\mathit{C}}_0\mathit{Z}$

Were C₀ is the capacitance of the empty cell (C₀ = ε₀ A/l, in which A is the area of electrodes and l the distance between them³⁴). The parameter ε₀ is the dielectric permittivity of the free space, 8.85. 10^-12 C²N^-1m^-2.

The dielectric permittivity ε = ε’ + j ε” is another derived quantity of impedance that is quite important to characterization of dielectrics³⁶. The immitance function for ε is defined by means of the ratio^33,34:

$\epsilon =\frac{1}{\mathit{j}\mathit{\omega }{\mathit{C}}_0\mathit{Z}}$

Therefore, the complex dielectric constant can be estimated by inversing the modulus, i.e., ε = M^-1.

3.2. Optical properties

The optical band gap can be estimated by the Tauc’s plot³⁷_,³⁸ and McLean analysis of absorption edge³⁹. It can be achieved by following the equation^40-42:

$\mathit{\alpha }\mathit{h}\mathit{\upsilon }=\mathit{B}{\left(\mathit{h}\mathit{\upsilon }-\mathit{Eg}\right)}^{\mathit{n}}$

Where α is the absorption coefficient, h is the Plank constant and ν the photon frequency. The parameter B is a constant dependent on the material’s physicochemical characteristics and E_g is the optical band gap⁴⁰. The parameter n can assume different values according to the nature of transition. Usually, n = 2 is applied in indirect transitions and n = 0.5 in direct ones^40,41.

Thus, the parameter E_g can be estimated by plotting (αhν)² versus the photon energy and extrapolating the straight line to α = 0⁴², known as Tauc plot. Diffuse reflectance measurements R_∞ can be converted into a magnitude proportional to absorption F(R_∞) by applying the Kubelka-Munk function^43,44:

$\mathit{F}\left({\mathit{R}}_{\infty }\right)=\frac{\mathit{\alpha }}{\mathit{S}}=\frac{{\left(1-{\mathit{R}}_{\infty }\right)}^2}{2{\mathit{R}}_{\infty }}$

The scattering factor S is almost independent from radiation wavelength. Hence, it can be neglected for this analysis. Therefore, this transformation allows the use of diffuse reflectance for measurement of the optical band gap of powders.

4. Results and Discussion

X-ray diffraction patterns of the samples are presented on the Figure 1. Even though the XRD diffractograms for different compounds into the TiO₂-Bi₂O₃ system seem to be quite similar, all the peaks were indexed to the Fmmm structure according to the crystallographic card previously cited.

Figure 1
XRD patterns of Bi₄Ti₃O₁₂ powders synthesized by combustion reaction and thermally treated at different temperatures

It can be observed that the Bi₄Ti₃O₁₂ compound was formed even in absence of annealing subsequent to the synthesis. In comparison, Subohi et al⁸ reported the production of bismuth titanate (selenite structure) achieved also by the combustion route using TiO₂ as the titanium source. After ignition at 450ºC, the remaining powder was non-crystalline. The same characteristic was obtained for their posterior study¹, where the synthesized powders presented amorphous structure after the ignition. Therefore, the crystalline powder obtained for the prepared powder at this study indicates that the metal precursors and fuel quantity were satisfactory in order to produce crystalline powders even in the absence of subsequent annealing.

The indexed phase has a highly anisotropic orthorhombic structure, space group Fmmm. The profiles adjusted by Rietveld refinement, as well as the parameters χ² and R_wp are presented on Figure 2. Satisfactory adjusts were obtained taking into account the low values attained for the parameter R_wp, and χ² close to the unity. Moreover, no remarkable difference between experimental data and adjusted profiles was observed.

Figure 2
Graphical results of Rietveld refinement for the samples according to the temperature of annealing

The broadening of the diffraction peaks decreases with the increment of the annealing temperature. It is related to the growing of the crystallites, which can also occur in the particle sizes. In order to assess this phenomenon, Table 1 indicates the crystallite sizes estimated for the samples, as well as the lattice parameters estimated by Rietveld refinement and surface area.

Table 1
Crystallite size, lattice parameters and surface area of the synthesized powders.

	D (nm)	a (Å)	b (Å)	c (Å)	Surface area (m2.g‒1)
600‒0	18	5.4281(5)	5.4358(2)	32.688(3)	10.41
600‒1	24	5.4267(4)	5.4326(2)	32.704(2)	9.37
700‒1	49	5.4240(2)	5.4302(2)	32.741(1)	6.21
800‒1	70	5.4252(2)	5.4306(1)	32.750(1)	5.73

The lattice parameters were close to those reported by the crystallographic card previously cited (5.41 Å, 5.45 Å and 32.84 Å for a, b and c respectively), which emphasizes that the structure utilized for the Rietveld refinement was suitable. Moreover, according to the Table 1 the powders presented nanometric crystallites sizes for all the conditions of annealing, which may be an indicative of small particle sizes. A tendency for increment of the crystallites according to the annealing temperature was observed. The sample treated at 800ºC, for instance, presented average crystallite size of 70 nm, which is close to three times larger than the samples treated at 600ºC (600‒0 and 600‒1).

Regarding the materials annealed at 600ºC, the sample 600‒1 showed crystallite size greater than the powder without subsequent thermal treatment (600‒0). Therefore, even in relative low values of temperature (600ºC), annealing for one hour was enough time to promote crystallite growth.

These results are relevant due to the fact that crystallites in nanometric scale can be considered as an indicative of small particle size. This characteristic can facilitate the sintering process and it enables the obtainment of fine grains after the processing. The sizes obtained for the crystallites are slightly larger than the ones related to High Energy Milling process (10 - 17 nm)²⁵ and considerably smaller than those obtained in hydrothermal synthesis when utilizing raw oxide as precursors (197 - 242 nm)¹⁰.

Concerning the surface area, this parameter was gradually reduced according to increment of temperature. The value regarding the sample 600‒0 is 10.41 m².g^‒1, which is much higher than the value obtained for the 800‒1 (5.73 m².g^‒1). This fact is probably a result of the particle growth promoted by the thermal treatment.

To evaluate the powders purity, the quantitative results of X-Ray fluorescence are indicated on Table 2. It is noteworthy that the powders are mainly composed by bismuth and titanium cations as expected for the Bi₄Ti₃O₁₂ phase. The measured weight percentage of these ions was also in good agreement with those values expected for Bi₄Ti₃O₁₂ compound (85.34% and 14.66% for Bi³⁺ and Ti⁴⁺ respectively, taking into account only the cations from the structure). Inorganic impurities arising from the precursor salts such as thorium and iron ions might be found in the samples, however in low quantity (lower than 1% wt).

Table 2
Quantitative results of X-Ray fluorescence (% wt) for the samples.

Sample	Bi	Ti	Th	Fe
600-0	83.72	15.50	0.68	0.10
600-1	83.77	15.56	0.57	0.10
700-1	83.82	15.54	0.54	0.10
800-1	84.24	15.11	0.57	0.08

Figure 3 shows the SEM micrographs under magnifications of 200 thousand times. Nanometric particles were observed for all conditions of annealing, arranged in soft-agglomerates. The tendency of particle growth according to the temperature was observed, while the same behavior was observed for the crystallite sizes. The average values of particle size observed by SEM micrographs were 39; 61; 89 e 132 nm for the samples 600‒0, 600‒1, 700‒1 e 800‒1, respectively. These results confirm that the powders are nanometric and the temperature of annealing highly impacts particles and crystallite sizes.

Figure 3
SEM micrographs of Bi₄Ti₃O₁₂ powders synthesized by combustion reaction and thermally treated at different temperatures: A) 600‒0; B) 600‒1; C) 700‒1; and D) 800‒1. Magnification of 200 thousand times

The nanometric particles observed in the powders are justified by the characteristics of the solution combustion method: the energy provided by the flame is rapidly dissipated after its extinction. Thus, the particle coalescence and growth is interrupted by the reduction of the atomic mobility.

Concerning the morphological characteristics, the particles are elongated arising from the anisotropic structure; and most of them present plate‒like morphology, which is characteristic of Bi₄Ti₃O₁₂ powders^18,20.

The thermal behavior of the samples was assessed by simultaneous DSC/TGA analyses. The results are shown in Figure 4. Regarding the DSC profiles, two endothermic phenomena were identified. The first occurred around 150ºC and it is usually related to elimination of adsorbed water. Nevertheless, the amount of adsorbed water is probably quite low due to the fact that this event is not strongly detected in the TGA profiles.

Figure 4
Simultaneous DSC/TGA analyses performed with Bi₄Ti₃O₁₂ powders synthesized by combustion reaction and thermally treated at different temperatures: A) 600‒0; B) 600‒1; C) 700‒1; and D) 800‒1

Regarding the second phenomenon, it can be visualized at around 750ºC (DSC) without mass loss. This event is attributed to crystalline conversion from tetragonal to orthorhombic structure, which was previously reported by Martinez et al⁴ and Thongtem et al⁹. However, this result is only observed for the samples heat-treated at 600ºC. Therefore, it is possible that small quantities of tetragonal‒Bi₄Ti₃O₁₂ could have remained in these samples but were not clearly detected by XRD.

With respect to TGA profiles, no significative mass loss could be visualized for all the samples. This fact indicates that basically the whole amount of volatile components were eliminated during the synthesis even in short periods of time (15 min, sample 600‒0) and at relative low temperatures.

To complement the results acquired by thermal analyses, the powders were analyzed by FTIR technique (Figure 5). Bands can be visualized at around 3325 cm^‒1, which are more intense for sample 600‒0 and are related to water adsorbed on the particles surfaces. Furthermore, two bands common for all samples occurred approximately in 815 cm^‒1 and 660 cm^‒1. In fact, infrared active modes are expected taking into account the elements and structure of the Bi₄Ti₃O₁₂ material⁴⁵. They correspond to the Bi‒O and Ti‒O stretching vibration respectively^46,47. These bands are therefore characteristic of the Bi₄Ti₃O₁₂ material.

Figure 5
FTIR spectra of the synthesized samples

Furthermore, low intense bands can be visualized for the sample 600‒0 between 1640 and 1000 cm^-1. These bands are related to water and residual molecules from the partially decomposed precursors, which remained adsorbed on the surface of particles. The probable infrared interactions and their respective groups present on the samples are the O‒H stretching, arising from the free adsorbed water (close to 1500 cm^‒1); NO₂, arising from the nitrate precursors (1200 cm^‒1 ‒ 1000 cm^‒1); and C‒H stretching, arising from the organic subproducts of the titanium precursor (1200 cm^‒1 ‒ 800 cm^‒1)⁴⁸.

These bands disappeared for the sample annealed for one hour at 600ºC, as well as for samples annealed at higher temperatures. These results corroborate that, even in low quantity (undetectable by thermogravimetric analysis), there are traces of residual volatiles arising from the precursors for the sample 600‒0.

The absence of bands in that range of wavenumbers (1640 cm^‒1 ‒1000 cm^‒1) for the samples 600‒1, 700‒1 and 800‒1 indicates that residual volatile matter was continuously being eliminated by the annealing treatments subsequent to synthesis. Thus, the temperature of 600ºC for one hour was efficient in completely eliminating the residual precursor molecules of the material.

The presence of residues on the powders surface can be insignificant for applications that require subsequent thermal treatment − production of sintered ceramic devices such as capacitors, for instance ‒ or can be crucial to applications that require direct use of the powders, such as some types of catalysts⁴⁹ and sensors. For the second situation, the active sites of the particles may be blocked by the organic molecules, limiting their efficiency⁵⁰.

The optical properties were evaluated by means of DRS technique. The results are indicated on Figure 6. It is noteworthy that two distinct regions define the diffuse reflectance spectra. The first one occurs in wavelengths lower than 360 nm, approximately. In this case, the diffuse reflection is low, indicating that radiation absorption has been occurring intensively. On the other hand, on the second region related to wavelengths higher than 400 nm the reflection seems to be very high, indicating that the photons absorption follows the opposite behavior. Hence, the samples are not able to significantly absorb photons from the visible and near-infrared spectra.

Figure 6
DRS spectra for the samples. Inset shows the Tauc’s plots and the samples’ band gap

For wavelengths between 360 and 400 nm, approximately, a transition of low-to-high diffuse reflectance can be observed, indicating that the optical band gap has been reached. To better evaluate this phenomenon, inset of Figure 6 presents the Tauc’s plot for the samples. It is noticed that the tangent for the (F(R_∞ )hν)² function intercepted similar values of energy, around 3.2 eV, in good agreement with recent reports⁵¹.

The band gap energy for dielectrics usually corresponds to the electron transference from the completely occupied anion valence subshell to the unoccupied cation ones⁵². Specifically for Bi₄Ti₃O₁₂, it is attributed to the electron transference from a combination of Bi‒6s and O‒2p levels to Ti‒3d ones^53,54. Therefore, these results indicated that the annealing subsequent to the synthesis was not significant as to impact the materials’ band gap.

The structural orders at short and medium ranges were evaluated by micro-Raman technique; the results are shown in Figure 7. Several characteristic modes for the Bi₄Ti₃O₁₂ can be observed between 100 cm^‒1 and 1000 cm^-1. The spectra may be divided in two distinct regions: above 227 cm^‒1, related to internal phonon modes of TiO₆ octahedrons from the pseudoperovskite-type sheets (A_1g, E_g and F_2u normal octahedron modes are Raman actives⁴⁵); and below 186 cm^-1, attributed to the translation modes of Bi and Ti sites^55,56. Table 3 relates these modes with their respective structural assignment.

Figure 7
Micro‒Raman spectra of the synthesized samples. Inset focuses samples annealed at 600ºC

Table 3
Raman phonon modes and their respective structural assignments.

Wavenumbers (cm‒1)	Structural assignment
116; 146; 186	Bi into the fluorite–type sheets38 ; translational modes of Bi and Ti39.
227	TiO6 bending and tilting39,40.
269	O‒Ti‒O bending vibration38.
328	TiO6 octahedron bending–stretching and tilting39,40.
537	TiO6 octahedron stretching39–41.
615	O‒Ti‒O into (Bi2Ti3O10)2- blocks40.
848	Symmetric Ti‒O vibration38,41.

As shown on the inset of Figure 7, the samples treated at 600ºC presented the same phonon modes than the others. This indicates that crystalline Bi₄Ti₃O₁₂ was formed even at this temperature, which corroborates with XRD and FTIR results. However, the intensity of the modes significantly increased according to the annealing temperature. It can be due to the increasing crystallinity promoted by the thermal treatment, making the signal from the structural assignment more intense. The increment in phase crystallinity with the annealing temperature is also in agreement with the results observed at XRD (Figure 1).

Based on results of particle sizes and crystallinity, the temperature of 600ºC has demonstrated the best conditions for bismuth titanate production. To better evaluate the powders’ crystallinity and morphology at this temperature, Figure 8 shows the HRTEM for the sample 600‒1. Nanometric crystals with sizes at about 50 nm were observed, confirming the nanocrystalline nature of the samples.

Figure 8
High Resolution Transmission Electrons Microscopy (HRTEM) and Electron Diffraction pattern for sample 600‒1

Electron diffraction demonstrated distinct spots, which is a remarkable characteristic of agglomerates of nanocrystals. The spots were indexed to Fmmm-Bi₄Ti₃O₁₂ structure, confirming that this layered compound was produced. Interplanar distances of 2.93 and 3.86 Å related to (171) and (111) planes, respectively, were measured. They are quite close to the ones reported by the ICDD card previously cited (2.97 and 3.81 Å).

Fired pellets were produced in order to evaluate the materials’ microstructure after sintering. Coverage with sacrificing powder is quite important to avoid bismuth volatilization at high temperatures (1000ºC). The SEM micrographs at magnification of 100 thousand times are presented on Figure 9. It is observed that fine grains, in sub-micrometric scale, are found in all samples. No evident alteration in the grain sizes after sintering according to the annealing temperature utilized for the powders preparation was observed (spherical equivalent diameter around 0.5 µm).

Figure 9
SEM micrographs of polished and thermally etched surface of sintered samples at 1000ºC/1h from Bi₄Ti₃O₁₂ powders: A) 600‒0; B) 600‒1; C) 700‒1; and D) 800‒1. Magnification of 100 thousand times

Only a few numbers of pores were observed, indicating satisfactory densification even under low values of temperature and time of firing process (1000ºC during 1 h). This result is probably a consequence of the nanometric sizes of the powders. The grains presented anisotropic morphology as a result of their highly anisotropic structure ((117)-plane preferentially oriented). The plate-like shape was observed for all samples after sintering, which is characteristic of the Bi₄Ti₃O₁₂ material.

To evaluate the electrical performance of the ceramic pellets, the electrical properties were assessed by IS. The results are shown in Figure 10. The samples have presented high electrical resistivity at room temperature (above 10 GΩ). Complete semicircles could be observed in Nyquist plots only for temperatures above 250ºC (Figure 10-A). It is noteworthy that the temperature utilized for the annealing affected the electrical performance after sintering significantly. The sample 600-0 has demonstrated the highest resistance at 250ºC, at about 1.8x10⁸ Ω, which decreased to 1.2x10⁸ Ω.cm for 600-1 and 1.3x10⁷ Ω.cm for 700-1. The sample 800-1 demonstrated the lowest electrical resistivity, 9x10⁵ Ω.cm, which could only be visualized on inset of Figure 10-A. The same trend was observed for other temperatures applied for analysis.

Figure 10
Electrical responses of Bi₄Ti₃O₁₂ ceramics: A) Nyquist plot comparing the samples at 250ºC; B) Bode diagram of sample 600-0 (logarithmic scale). Inset displays the phase angle ɸ according to frequency

Therefore, these results suggest that the electrical resistance is highly affected by the temperature and time of annealing applied in the stage of powders preparation. Since the microstructure is very similar for all the samples (see Figure 9), this phenomenon may be explained by the creation of point defects. Bismuth vacancies V_Bi ^’’’ generated by the volatilization of this element impose the creation of oxygen vacancies V_O ^•• to maintain the charge neutrality^22,36,52. These charged defects can reduce the electrical resistivity, which may culminate in leakage current and domain pinning²⁴.

To better evaluate the materials’ electrical performance according to temperature of measurements, Figure 10-B shows the Bode diagram for the most resistive sample (600-0). A continuous reduction of the electrical resistance with increment of temperature is evident, displaying a non-metallic conduction behavior that is characteristic of dielectrics³⁶. It is noteworthy that the phase angle ɸ (shown on inset of Figure 10-B) does not vary from 90 to 0º, indicating the presence of more than one arc contributing to the global resistance³³. Hence, some distortions in the impedance modulus together with non-defined plateaus at phase angle can be observed.

High resistivity phenomena in the Nyquist diagram can overlap the impedance semicircles, limiting data interpretation. In this case, electrical modulus may help understand the electrical measurements. Whereas the Z (ω) focuses on higher resistivity phenomena, M (ω) focuses on the lower capacitive ones³⁴. This way, Figure 11 shows the modulus formalism for sample 600-0 according to increment of temperature. It is noteworthy that even at room temperature (25ºC), two distinct arcs can be visualized. The first at high frequency (incomplete, occurring at frequencies above 2 kHz) and the second, better defined, with relaxation frequency of 2.5 Hz.

Figure 11
Results of modulus immitance for sample 600–0 according to temperature of measurements.

When temperature increases to 100ºC, the whole profile is displaced to higher values of frequency and a third semicircle appears at lower values of frequency. Literature studies have demonstrated that impedance data related to bismuth titanates are quite complex; therefore, several semicircles are indeed expected. According to recent reports^57,58, the first semicircle at higher frequencies can be attributed to crystalline plate, followed by plate boundary and grain boundary, respectively. Moreover, when temperature achieves 200ºC, a fourth arc appears in the modulus diagram for lower values of frequency. Based on its high capacitance (greater than 10^-8 F), this semicircle is attributed to the electrode⁵⁷.

Once the main phenomena that contribute to electrical properties are known, the dielectric characterization could be then evaluated by means of dielectric permittivity. The results for sample 600-0 are shown in Figure 12. It is meaningful that the dielectric permittivity changes significantly with the electric field frequency. That is attributed to the types of events that contribute to the polarization. At higher frequencies (close to 1 MHz, focused in Figure 12-A), electronic, ionic and even orientation polarization contribute to the dielectric constant³⁶. For this condition, ε’ seems to be close to 115 (25ºC, 1 MHz), which agrees with previous reports from literature⁵⁹. Since the temperature does not significantly affect the ionic and electronic polarization³⁶, ε’ does not considerably increase with temperature at high values of frequency.

Figure 12
Dielectric permittivity for sample 600–0: A) high frequency phenomena; B) low frequency phenomena.

The dielectric constant considerably increases by reducing the frequency of electric field (Figure 12-B). At 25ºC for instance, ε’ increases from 115 (1 MHz) to 360 (500 mHz). It can be explained by the space charge creation, attributed to the movement and accumulation of charged carriers in the interfaces. Since the concentration and movement of point defects decisively increase with temperature, the dielectric constant containing space charge contribution follows the same trend³⁶. The results have revealed, for example, that ε’ at 500 mHz increased from 360 (25ºC) to 2.4x10³ (150ºC).

It is possible to notice a low inflexion in the ε’ profile at low frequency (marked in Figure 12-B with an asterisk), followed by an indefinitely growth of the dielectric constant. This phenomenon was more evident at higher temperatures. Comparing to the modulus results and recent reports from literature⁵⁷, it can be attributed to the electrode.

Therefore, several properties were evaluated in this work. In summary, the optical properties don’t seem to be very influenced by the temperature of annealing, whereas the electrical resistivity, particle size, powder’s crystallinity and surface area are highly impacted. It occurs because temperature provides energy for atom mobility, which culminates in particles growth and consequent reduction of surface area, accompanied by an increment of the materials crystallinity. However, point defects are concomitantly created arising from bismuth volatilization, which reduces the electrical resistance of the final product. Higher values of temperature also provide energy to eliminate volatile residues and to convert small quantity of tetragonal phases to orthorhombic, which can also be formed at the synthesis stage.

Thus, Figure 13 presents a summary of the main physicochemical characteristics and their tendencies evaluated. The choice of the best condition of synthesis and calcination temperature can be made taking into account the specific requirements from the final application.

Figure 13
Summary of the main properties and their tendencies according to calcination temperature utilized in the synthesis stage.

5. Conclusions

In this work, the synthesis parameters and annealing temperature to produce nanometric Bi₄Ti₃O₁₂ powders by the solution combustion route were evaluated. Beyond that, the samples structure; and optical and dielectric properties were analyzed. It was observed that the combustion synthesis route was efficient in producing crystalline and nanometric Bi₄Ti₃O₁₂ powders. An evident tendency for particle size growth was observed by increasing the calcination temperature after the combustion reaction.

Powders were crystalline even in absence of subsequent annealing. In this case, traces of volatile matter arising from the precursor salts and fuel, which were completely eliminated by the annealing (600ºC for 1 h was enough for this), remained in the prepared powder. Moreover, at this temperature (600ºC) it is possible that a low quantity of tetragonal phase remains in the samples, which is completely converted to orthorhombic at higher temperatures. The crystalline nature of the powders could be confirmed by characteristic Raman modes and HRTEM technique. The optical properties were not significantly influenced by the heat treatment and the fired ceramics presented sub-micrometric grain sizes. The ceramic pellets have presented high electrical resistance, which is gradually reduced according to the temperature of annealing. For further work, anisotropy in these properties may be considered.

Therefore, the temperature of 600ºC has presented the best results for production of nanometric Bi₄Ti₃O₁₂, under the experimental conditions, when low particle sizes are required. Depending on the application, the annealing procedure can be employed as to modify the powders characteristics required for the application.

6. Acknowledgments

The authors thank CAPES for financial support; UNIFAL‒MG, UNESP, Embrapa - Instrumentation (Dr. Elaine Cristina Paris; MSc. Viviane Faria Soares and MSc. Silviane Hubinger) and DEMa (Prof. Dr. Ana Candida Martins Rodrigues and Dr. Rosário Suman Bretas) for technical assistance. Moreover, the authors would like to thank the Laboratory of Structural Characterization- LCE/DEMa/UFSCar for the use of general facilities. The authors declare that this research has no conflict of interest.

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Publication Dates

• Publication in this collection
  16 July 2018
• Date of issue
  2018

History

• Received
  15 Feb 2018
• Reviewed
  28 Apr 2018
• Accepted
  08 June 2018