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Enhancement in pyroelectricity of polar Ba0.9Sr0.1TiO3 –TeO2 glass-ceramics
Journal of Non-Crystalline Solids 535 (2020) 119964
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Enhancement in pyroelectricity of polar Ba0.9Sr0.1TiO3 –TeO2 glass-ceramics a
a
b
a,⁎
Vandna Tomar , Pardeep K. Jha , A.S.K. Sinha , Priyanka A. Jha , Prabhakar Singh a b
a,⁎
T
Department of Physics, Indian Institute of Technology (Banaras Hindu University) Varanasi, Varanasi-221005 (India) Department of Chemical Engineering and Technology, Indian Institute of Technology (Banaras Hindu University) Varanasi, Varanasi-221005 (India)
A R T I C LE I N FO
A B S T R A C T
Keywords: Pyroelectricity Glass-ceramics BST TeO2
In the present work, the piezoelectric and pyroelectric properties of lead free ferroelectric Ba0.9Sr0.1TiO3 ceramic are altered with the addition of TeO2 glass ceramics. For this, crystalline phase of γ- TeO2 prepared by meltquench method is mixed with Ba0.9Sr0.1TiO3 prepared by solid state reaction method, to form compounds (1-x) Ba0.9Sr0.1TiO3 – (x) TeO2, x = 0 – 0.1 at the steps of 0.025. The structural and impedance analyses confirm formation of solid solution up to x ≤ 0.05 and thereafter they show composite formation. The dielectric permittivity is observed to be maximum for composition x = 0.075. The XPS and EDX analyses are showing highest oxygen vacancy at x = 0.05. The piezoelectricity is measured using Piezo Force Microscopy technique and c-axis domains are observed showing the formation of polar glass-ceramics. The pyroelectric current increases with the increase in TeO2 content, showing the suitability of samples for energy harvesting and pyroelectric detectors.
1. Introduction In recent times, the glass ceramics find wide applications in the electronics industry due to their pore- free nature, large and complex structures [1]. The glasses can be crystallized into polar glass-ceramic form and can be used as thermal and pressure sensing elements [2]. Most importantly, these glass ceramics are polar where crystallites are arranged in a polar array. This macroscopic polarity gives rise to a better piezoelectric or pyroelectric activity as compared to poled ferroelectric ceramics. Due to the ease in polarization, they are promising candidates for pyroelectric detectors, hydrophones and surface acoustic wave devices along with the energy harvesting applications [3,4]. The ferroelectric ceramics have the ability to reorient crystallites with the field to impart long polar order. In polar but non- ferroelectric ceramics, this is not possible; consequently such materials do not exhibit piezoelectricity and pyroelectricity. The polar glass ceramics showed the formation of non-ferroelectric ceramic with polar order [5]. Hence, this could be an interesting route to increase polar order with high breakdown strength in a ceramic [5]. The domains in an unpoled ferroelectric polycrystalline ceramic are randomly oriented. The applied field during poling process aligns most of the domains with their polarization vectors in a direction parallel to the field. The grain structure of grain-oriented glass-ceramic is similar to that of a poled ceramic. Hence, poled ferroelectric ceramic and grainoriented polar glass-ceramic belong to the same conical point group, ∞m [5]. On the basis of simultaneous presence of piezoelectric and
⁎
pyroelectric properties, there are four categories of glass ceramics. In class (a) both properties are present. While class (b) and (d) are suitable for piezoelectric and pyroelectric applications, respectively and class (c) don't contain any such properties [5]. TeO2 is a well known material to exhibit better piezoelectric coefficient, acoustical and electro-optical properties. This elastic anisotropy is attributed to the stretching of weak and strong Te-O bonds [6]. In addition, amongst various lead free ferroelectric ceramics, Ba1-xSrxTiO3 has received attention as energy storage capacitors due to high dielectric constant and low loss. The glass ceramics based on 26.88BaO–6.72SrO–29TiO2–22SiO2–12Al2O3–2.4BaF2–0.5La2O3 (mol %) [7] are reported to exhibit better ferroelectric, piezoelectric and pyroelectric properties. In addition, the glass-ceramic formed by Ba0.4Sr0.6TiO3 with Bi2O3-B2O3-SiO2 are found to possess high recoverable energy storage density of the order of 1.98 J/cm3 [8]. Hence, barium strontium titanate has been chosen as a ferroelectric material and Ba1-xSrxTiO3 with x = 0.1 lies in ferroelectric range with the phase transition Curie temperature (Tc) ~ 120 °C. To tailor the piezoelectric/pyroelectric properties of a ferroelectric ceramic, TeO2 glass ceramic having crystalline phase of γ- TeO2 is mixed with lead free ferroelectric ceramic, Ba1-xSrxTiO3 (a solid solution of two Perovskites BaTiO3 and SrTiO3) to form the compounds (1x)Ba0.9Sr0.1TiO3 – (x) TeO2 where x varies from 0 to 0.1 in the steps of 0.025. The phase development, microstructure, domains and oxygen content have been examined with the variation of TeO2 concentration. The transition temperatures are studied with differential scanning
Corresponding authors. E-mail addresses: [email protected] (P.A. Jha), [email protected] (P. Singh).
https://doi.org/10.1016/j.jnoncrysol.2020.119964 Received 9 December 2019; Received in revised form 1 February 2020; Accepted 7 February 2020 0022-3093/ © 2020 Elsevier B.V. All rights reserved.
Journal of Non-Crystalline Solids 535 (2020) 119964
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calorimetric, dielectric and pyroelectric studies. The energy is also calculated through pyroelectric current. 2. Experimental 2.1. Sample preparation The polycrystalline perovskite structured Ba0.9Sr0.1TiO3 ceramics were prepared by solid state reaction method using carbonates and oxides precursors BaCO3 (M/s Alfa Aesar with purity ~ 99%), SrCO3 (M/s CDH with purity ~ 99%) and TiO2 (M/s Alfa Aesar with purity ~ 99.8%) in stoichiometric ratios and were mixed and ground thoroughly in an agate mortar to get a fine and homogeneous composition. The calcination of this powdered mixture was done at 1150 °C for 2 h in an alumina crucible. The disc-shaped pellets Ba0.9Sr0.1TiO3 were sintered at 1200 °C, 1250 °C and 1300 °C for 2 h. We obtained the phase pure Ba0.9Sr0.1TiO3 sample for sintering at 1200 °C for 2 h. The calcined powder was then again ground. TeO2 raw powder (M/s Alfa Aesar with purity ~ 99.99%) was weighed and its melt was prepared at 900 °C in an alumina crucible, then molten tellurium oxide was quenched at aluminium mould which was maintained at 200 °C. After this, it was annealed at 150 °C in a hot air oven to remove the remaining moisture for further characterizations and was allowed to cool to room temperature naturally for removing internal residual stresses. This sample was obtained in the form of the solid disc shape. The glass ceramic samples were prepared by using calcined powder of Ba0.9Sr0.1TiO3 ceramic and TeO2 powder obtained using melt quench technique. The compositions were formed as (1-x) Ba0.9Sr0.1TiO3 – (x) TeO2 where x = 0 – 0.1 in the steps of 0.025. After this, pellets of the powders obtained were made by pressing it uniformly with the help of hydraulic press under 2 MPa pressure. PVA was used as a binder, which was burned off in a hot air oven at 300 °C for 2 h. These disc-shaped pellets were sintered at 1200 °C for 2 h (optimized for Ba0.9Sr0.1TiO3).
Fig. 1. X-ray diffractograms confirming the BST phase, γ- TeO2 phase and glassceramics.
for macro (average/global) and local (micro) structural behaviors’. Similarly, the dielectric, impedance and pyroelectric studies are being carried out to understand the average electric response and PFM is used for microanalysis of piezo-response. In addition, switching behaviour of the domains is studied using I-V characteristics.
3.1. Structural studies 3.1.1. XRD The XRD confirms the single phase formation of calcined Ba0.9Sr0.1TiO3 (BST) (PCPDF 81–2263) and amorphous phase with the crystalline peaks corresponding to γ- TeO2 (PCPDF 52–1005) (Fig. 1). With the incorporation of TeO2 into BST, the samples are observed in crystalline forms. At x = 0.025, the XRD resembles with pure phase of BST and a small peak at 2θ ≈ 24° is observed that is matching well with the PCPDF file of TeO3 (PCPDF 80–0570). While with the increase in x, the intensity of the peak corresponding to 2θ ≈ 24° observed to increase gradually. However, at x = 0.05, the peak at 2θ ≈ 30° corresponding to γ- TeO2 starts appearing, the intensity of this peak is increasing with the increase in x. Further, some more peaks have also been observed at x = 0.05 in addition to the above mentioned two peaks (indexed in Fig. 1). These peaks match well with Tellurium trioxide TeO3 (PCPDF 80–0570). Moreover, the XRD peaks are observed to split into two peaks with the increase in x showing the alteration in phase of BST [9] and shown in the right panel of Fig. 1. The splitting of these peaks correspond to TeO2 (PCPDF 74–1131) as TeO2 has entered the lattice of BST and confirming the structure alteration. This shows that the solubility limit for (1-x) Ba0.9Sr0.1TiO3 – (x) TeO2 is x ≤ 0.05. It is seen that TeO2 crystallizes in γ- TeO2 through melt-quench method and upon mixing the two phases of BST and γ- TeO2, TeO3 phase evolves along with the structure alteration showing the interaction of TeO2 and BST. In addition, the XRD peaks have been indexed with the tetragonal phase (P4mm symmetry) and the lattice parameters calculated are shown in Fig. 2 with the error bars (dash lines are a guide to eye). It is observed from Fig. 2 that lattice constant ‘a’ increases with the increase in TeO2 content up to x = 0.075 and thereafter decreases. While, lattice constant ‘c’ remains independent of increase in TeO2 content and increases after x = 0.05. The tetragonal strain is observed to follow the same trend with x as that of lattice constant ‘c’. The nearly constant behaviour of ‘a’ shows that the lattice alteration is due to TiO6 and TeO6 octahedron stretching.
2.2. Characterization techniques X-ray diffraction study was adopted to identify the crystal structure using Rigaku smart lab X-Ray diffracto0meter at RT with Cu-Kα1 radiation (λ = 1.540598 Å) in the 2θ range of 20º – 70º with the scanning rate of 2 o/min. The glass-ceramic phase was analyzed and indexed using JCPDS. The lattice parameters, microstrain and crystallite size are calculated using XRD data. The microstructural studies were done using Scanning Electron Microscopy (SEM) (Oxford instrument SEM EVO 18). The surfaces of both samples were coated with low-temperature silver paste and cured at 300 °C for 2 hrs for electroding on the both sides of the disc-shaped pellets for further electrical measurements. The permittivity and impedance parameters were measured using a high precision LCR metre (Wayne Kerr) at an oscillation amplitude of 1 V. The measurements were carried out in the temperature range of 35 °C to 400 °C at the step of 5 °C within the frequency range of 20 Hz to 1 MHz. The samples were poled using DC polling unit (Marine India) at biasing field below the breakdown voltage (1 KV) for 2 h for pyro-current measurements. The pyrocurrent measurements was recorded using Keithley electrometer 6517B in the temperature range of 30 °C to 180 °C with the heating rate of 1 °C/min. The switching behaviour of the samples have been studied using I-V measurements from Keithley 2450 series. Piezo Force Microscopic (PFM) measurements were carried out with contact tip, Golden Silicon probe tip CSG10/Pt at a resonance frequency of 64 kHz and force constant 0.01–0.5 N/m. The Differential Scanning Calorimetry (DSC) was measured from Shimadzu DSC-60 Plus from RT to 250 °C. 3. Results and discussions In order to inspect the phase formation of the synthesized glassceramic samples, the samples are characterized by XRD, SEM and AFM 2
Journal of Non-Crystalline Solids 535 (2020) 119964
V. Tomar, et al.
Fig. 2. Lattice parameters a, c and tetragonal strain, c/a, calculated manually for the studied compositions (dash lines are a guide to eye).
Fig. 4. Te content measured through EDX at grains and grain boundaries for the studied compositions (with error bars) inset shows the EDX profile confirming the existence of constituent elements.
trend of lattice parameter ‘a’ shown in Fig. 2. Simultaneously, the O content determined at grain –boundaries is less than that of grains as determined from EDX (graph not shown here). Also, the minimum oxygen content and highest oxygen vacancy is observed for x = 0.05. As O being light element to be predicted from EDX, this is further verified using XPS. 3.1.3. RAMAN studies To confirm the incorporation of TeO2 into the BST lattice, Raman spectra are studied and are shown in Fig. 5. This figure consists of Raman spectra of TeO2 and substituted samples (1-x)Ba0.9Sr0.1TiO3 – (x) TeO2. The major phase observed in Fig. 5 resembles to that of the BST. As BST is tetragonal with C4v symmetry at room temperature, simultaneously, the synthesized compositions show the tetragonal structure. The peak at ~ 303 cm−1 is the characteristic of tetragonal BST confirming the tetragonal phase of the substituted samples. The Raman active modes for tetragonal symmetry are 4E (TO +LO) + 3A1(TO+LO)+B1(TO+LO) [12]. The E (TO) modes occur at frequencies ~ 190, 280 and 516 cm−1 while E (LO) modes occur at frequencies 140, 303, 640 and 720 cm−1. The intensity of longitudinal mode obtained at ~ 303 cm−1 (assigned to 3E(TO)+2E(LO)+B1 overlapping) is observed to decrease after x = 0.075 [13]. The addition of TeO2 into the matrix is visible through the Raman intensity at ~ 700
Fig. 3. SEM micrographs showing two types of grains and water mark and lamellar features for the studied compositions.
3.1.2. SEM micrographs The microstructure difference due to TeO2 addition on the grains can primarily be seen in SEM images. For this, SEM micrographs of the studied samples are shown in Fig. 3. The grain and grain boundary can be clearly distinguished in the studied samples. The formation of hexagonal like microstructure for highly developed grains with the angle between two sides of microstructures ~ 109° is observed. In addition, some rod like structures have also been started to develop at x = 0.05 (shown in inset). The grain size is observed to increase with the increase in x. The water-marks and lamellar features are also observed in the SEM images (marked by circles A, B, C and D in Fig. 3). A small channel between two grains is also appearing beyond the hexagonal grain. These features appear inside the grains and for a few instances across the grains too [10,11]. The rod shaped grains also appear to be connected. To further analyze the composite nature of the X-ray Diffractograms, EDX measurements have been done for the studied samples at the grain and grain boundaries shown in Fig. 4. Inset of this figure depicts the elemental constituents of the x = 0.075 composition. The EDX studies confirm the existence of the constituent elements within error limits. But, the atomic concentration of Te varied in the grain and grain boundary regions. It is seen that at x = 0.05 and 0.075, Te is nearly absent at the grain-boundaries and concentration is nearly constant in the grain region for x = 0.05 and x = 0.075 confirming the
Fig. 5. Raman spectra of TeO2 and substituted samples (1-x)Ba0.9Sr0.1TiO3 – (x) TeO2 where x = 0 – 0.1 in the steps of 0.025 (inset) Variation of Raman intensity with error bars corresponding to longitudinal mode (~ 700 cm−1) with x (lines are a guide to eye). 3
Journal of Non-Crystalline Solids 535 (2020) 119964
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Fig. 6. Deconvoluted XPS spectra of Sr, Ti, Te and O for the substituted samples (1-x) Ba0.9Sr0.1TiO3 – (x) TeO2 where x = 0.025 – 0.1 in the steps of 0.025.
cm−1 and is varying with the lattice constant trend (Fig. 5 inset). Thus longitudinal modes are governing the lattice constant ‘a’ as observed in Fig. 2. The presence of TeO2 is visible through the FWHM of the LO mode corresponding to the peak at ~ 702 cm−1 [6]. With the increase in TeO2 content, FWHM is observed to increase as there is overlapping of BST and TeO2 modes. This overlapping of Raman modes and splitting of the peaks in XRD pattern are caused due to TeO2 and TiO2 octahedron stretching.
Fig. 7. Variation of Z'' and M'' with frequency at Room temperature (40 °C) for the studied compositions.
x = 0.025, 0.05, 0.075 and 0.1, respectively). This shows the increase in oxygen vacancy concentration with x up to x = 0.05 and thereafter decrease in the concentration. 3.2. Electrical studies
3.1.4. XPS studies For investigation of the elemental constituents of the substituted samples, wide XPS spectra is analyzed and matched with the standard look up table and is shown in supplementary (Fig. S1). The XPS spectra corresponding to the constituent elements are deconvoluted and are shown in Fig. 6. It is observed that Sr 3d peaks have spin orbit energy 1.76 eV and a shifting in 3d3/2 component is observed with x showing the change in oxidation states. This shifting is showing the presence of multiple oxidation states of Sr, as Te has affected the A- site of lattice (observed through lattice constant ‘a’ and Raman spectra); it is showing the presence of 3d5/2 sub-oxide along with the shifting of the peaks. The sub-oxide peak is observed to decrease with x indicating the formation of defects at x = 0.1. With the deconvolution of Ti and O XPS spectra, it is observed that multiple oxidation states do exist in Ti along with the satellite peaks. This satellite peak has altered the Auger peak of O along with the existence of multiple oxidation states and oxygen vacancies [14]. The satellite peak accounts the transfer of e− from p-orbital of O to D- orbital of Ti [14]. The FWHM of 2p1/2 peak is broader than 2p3/2 peak in the studied samples. This shows that the auger peak of O and satellite peak of Ti are filling the oxygen vacancy through electrons. In addition, the O and Sr peaks are observed to merge with the increase in x showing the multiple oxidation states. Hence, A- site (Sr-O) is affecting the Ti-O-Ti angle and illustrating the solid solution/composite formation with x. In addition, O 1 s peak at ~ 529 eV is ascribed to the metal and O bond (Ba-O, Sr-O, Ti-O and Sr-O) while, 2nd peak of O belongs to metalhydroxide bond. The ratio of area of peaks corresponding to metaloxygen and metal-hydroxide corresponds to the oxygen vacancy concentration [15]. The ratio of area of peaks is observed to increase up to x = 0.05 and thereafter, it decreases (0.91, 1.40, 1.24 and 1.05 for
3.2.1. Impedance Studies For the solid solution/composite behavior, impedance spectroscopy is applied (Fig. 7). It is observed that in the log impedance plots, grain and grain boundary contributions starts appearing at x = 0.075. While, in the modulus plots, there is one contribution up to x = 0.05 and thereafter, two contributions appear at x = 0.075 and finally merged at x = 0.1. Thus, impedance behavior and appearance of different regimes indicates the possibility of solid solution up to x = 0.05 and thereafter, composite formation takes place [16]. The merging of M'' peak at x = 0.1 indicates the Maxwell –Wagner polarization at x = 0.1. The structural and micro-structural studies (XRD, Raman and SEMEDX) indicate that Te content in the grains is affecting the intensity of longitudinal modes and lattice parameter ‘a’. Now the question arises, how the content of Te at the grains affect the capacitive/resistive behaviour of the grains. Hence, impedance formalism has been adopted to study the dielectric relaxation mechanism for the studied samples. Further, Nyquist plots of impedance are studied and their equivalent circuits are also realized using Z-view software. Fig. 8 depicts the Nyquist plots and their equivalent circuit with the goodness of fit factor for the synthesized compositions. The equivalent circuit consisting of one resistance (R1) and one parallel R-CPE (R2eCPE) circuit connected in series is used to interpret the nature of impedance Nyquist plots (shown in Fig. 8). Here R and CPE are the resistance and constant phase element for the semi-circular contributions. This suggests non-ideal behaviour of capacitance and occurrence of more than one relaxation processes with similar relaxation time. The R1, R2, CPE-P and CPE-T parameters obtained from Z-view software are listed in Table 1. The nearly unity value of CPE-P is suggesting pure capacitive behaviour for 4
Journal of Non-Crystalline Solids 535 (2020) 119964
V. Tomar, et al.
Fig. 8. Impedance Nyquist plots and their equivalent circuit along with the goodness of fit parameter at room temperature (40 °C) for the studied compositions.
with the increase in x. In addition, the phase transition temperature is observed to decrease with the increase in x and becomes constant for x ≥ 0.075. At x = 0.0, the maxima in real part of permittivity matches well with the maxima in loss curve. In addition, the maxima in permittivity peak matches with the loss peak with the increase in x. Moreover, a gradual increase in permittivity is observed and exhibits maxima. Thereafter, a sharp decrease in the value of permittivity is observed with the increase in temperature showing the sharp phase transition. The diffusivity constant has been estimated using Curie Weiss law and the value of diffusivity constant is found to be 1. The phase transition temperature is observed to be inversely proportional to the tetragonal strain. This can be attributed to the alteration in A-site of the lattice with Te ions [10]. The high dielectric constant can be attributed to the increase in grain size with the Te content. The increase in grain size leads to the lesser number of grain boundaries which are more resistive than that of the grains leading to more polarization. Hence, the charge carriers create a layer around the interface, leading to the interfacial and orientational polarization. The orientational
the studied compositions. However, the value of CPE-P at x = 0.075 also accounts for the formation of gradient of charge carriers as observed in EDX studies. To analyze the capacitive behaviour, phase transition temperature alteration and solid solution/composite behavior, dielectric permittivity and loss are studied. 3.2.2. Dielectric permittivity and loss Fig. 9 depicts the variation of real part of dielectric permittivity and tan δ with temperature at 1 kHz frequency while the variation of real part of dielectric permittivity and tan δ with temperature at different frequencies is shown as supplementary (Fig. S2). It is observed that the value of dielectric permittivity at the transition temperature is higher at x = 0.075 (4188.10) but lower than that of x = 0.0 (4490.16) at 1 kHz frequency. But the value of dielectric loss is lower for x = 0.025. It is observed that the value of real part of dielectric permittivity observed to increase with x and approaches a maximum at x = 0.075 and thereafter decreases. Whereas the dielectric loss is observed to increase Table 1 Value of resistance, CPE and errors calculated from Z-view software. Sample
Rs
Error
CPE-T
Error
CPE-P
Error
Rp
Error
x x x x x
−49.71 −157.8 −53.04 −619.2 −58.78
5.679 31.973 16.229 76.896 12.724
2.53E-09 8.89E-10 5.98E-10 2.60E-09 9.47E-10
2.90E-11 2.02E-11 7.37E-12 1.70E-10 1.50E-11
0.93563 0.93278 0.96855 0.82471 0.95579
0.001 0.002 0.0011 0.005 0.00114
2.05E+07 1.58E+07 2.87E+07 3.63E+05 4.86E+06
1.61E+06 6.77E+05 9.76E+05 7.92E+03 1.28E+05
= = = = =
0.0 0.025 0.05 0.075 0.1
5
Journal of Non-Crystalline Solids 535 (2020) 119964
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Fig. 9. Variation of real part of dielectric permittivity and tan δ with temperature at 1 kHz frequency.
polarization may be originated due to the formation of vacancy dipoles around the interface [10].
Fig. 10. Variation of heat obtained from DSC and derivative of heat (Q′) to analyze the Tc and Tg, glass transition temperature for the studied samples.
3.2.3. DSC analysis To analyze the transition temperature, Tc and glass transition temperature, Tg, differential heat and heat obtained from DSC are plotted (Fig. 10). From the graphs, it can be observed that Tc is diminishing with the increasing TeO2 content. Moreover, a kink at ~ 220 °C is also observed in all the samples. This feature is converting the change in slope after 200 °C with the further addition of TeO2, indicating that Tg lies above 200 °C, a feature for glass. Thus, with the addition of TeO2 glass- ceramics, a composite feature is prevailing. 3.2.3. Pyroelectric studies The value of dielectric permittivity is observed to decrease for x = 0.1 and hence this composition is discarded from the piezoelectric/ pyroelectric measurements. Fig. 11 shows the pseudo - color Piezo Force Microscopy image of (1-x) BST-x TeO2 sample with x = 0.0 – 0.075. It is observed that the piezoresponse decreases with x in comparison to the parent BST sample. Moreover, the formation of columnar domains corresponding to polar glass ceramics is revealed with the substitution of TeO2. The pyroelectric current is measured with the technique developed P by Chynoweth [17] using the relation p = A (dTI/ dt ) with the heating rate of 1 °C/min where, p is pyroelectric coefficient, PI is pyroelectric current, A is area of the electrodes and dT/dt is heating rate (Fig. 12(a)). The pyroelectric current at the transition temperature is observed to increase with the increase in x (Fig. 12(b)). The hump observed in the pyroelectric current is showing the ferroelectric-paraelectric transition temperature which is in good agreement with the transition observed in dielectric permittivity and loss curves [18]. Hence, the energy calculated by taking the pyroelectric current at transition temperature using
Fig. 11. PFM image (pseudo - colored) of (1-x) BST-x TeO2 sample with x = 0.0 – 0.075. With the substitution of TeO2, the formation of columnar domains corresponding to polar glass ceramic is revealed (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.).
p2
relation E = ε′ε [19] where p is pyroelectric coefficient, ε' is real part of o dielectric permittivity and ε0 is permittivity in free vacuum, comes out to be 13.8 Jm−3K−2 for x = 0.075 sample (highest one). As seen
earlier, the dielectric constant and piezoelectricity are observed to decrease with the increase in x. The increase in pyroelectricity in
6
Journal of Non-Crystalline Solids 535 (2020) 119964
V. Tomar, et al.
Fig. 12. (a) Area normalized Pyroelectric current of (1-x) BST-x TeO2 sample with x = 0.0 – 0.075 (b) Area normalized Pyroelectric current (with errors bars and lines are a guide to eye) with composition at Tc.
changes in Ti-O octahedra as the intensity of LO modes are observed to decrease (intensity is maximum at x = 0.05). This has led to the reduction in Tc and the formation of c-axis domains. This is also felicitated through the formation of columnar grains and also confirms the composite formation of the studied samples.
comparison to the decrease in piezoelectricity can be understood as follows. The pyroelectric contribution due to the domain walls plays a major role in the enhancement of pyroelectric figure of merit. This increase in pyroelectric current might be due to the formation of c-axis domains formed with the gradient of Te (TeO2 with orthorhombic structure) and then the dipoles are getting sufficient energy to change the polarization. There are several relations to obtain pyroelectric figure of merit (FOM) and generally, these FOMs are used for selection of materials for IR sensors as these are indirect performance evaluators. Also, the conversion of thermal energy into electrical energy is an important application of pyroelectric ceramics. In the present case, energy harvesting
4. Conclusions In the present study, we have demonstrated that the TeO2 addition has affected A-site of the lattice leading to more defects at x = 0.1. There is decrement in the value of real part of dielectric permittivity and piezoelectric constant while an increment in pyroelectric activity is observed with the increase in x (E = 13.8 Jm−3K−2 for x = 0.075 sample (highest one)). This is attributed to the increase in the domain wall contribution with the increase in TeO2 content. This is also observed through the increase in area of I-V hysteresis with the TeO2 content as Te creates a concentration difference between bulk and grain- boundary interface and found to be responsible for the conversion from doubly to singly ionized oxygen vacancies at x = 0.1.
p2
FOM E = ε′ε is used and it is being employed to calculate energy for o harvesting applications. Earlier work states that BaTiO3 substituted with Sn showed a value of 17.1 J/m3K2 energy [19]. In our case, the energy observed is slightly lower than the reported results. Also, a comparative of the previous and this work is also presented in detail in Table 2. To analyze the domain dynamics of the studied samples, Fig. 13 shows the hysteresis I-V plots for (1-x) BST-x TeO2 samples for x = 0.0 to 0.075 with application of forward and reverse electric field. With the positive field, the hysteresis area is small however in negative electric field difference between forward and reverse field is very large. However, with substitution of TeO2, this difference between curves in negative electric field reduces significantly. But in positive electric field, it is observed that with the increase in x, the hysteresis area increases. This fact is in further correlation with the dielectric loss observed in the dielectric study (Fig. 9). This indicates that the polar domain induces more hysteresis with the domain movement and the increase in the domain wall conductivity as well as loss. This can be attributed to the concentration gradient of Te in bulk and grain –boundary as visible through EDX (Fig. 4) and XPS studies. In addition the oxygen vacancy is illustrated through the Arrhenius plots of conductivity. It is observed in Fig. 14 that activation energy is observed to increase up to x = 0.05 and thereafter decreases (Ea = 0.71 eV, 1.04 eV, 0.86 eV and 0.21 eV for x = 0.025, 0.05, 0.075 and 0.1, respectively). This activation is in correlation with the oxygen vacancy concentration observed from XPS analysis. Also, change in activation energy from 0.86 eV to 0.21 eV corresponds to the conversion of doubly ionized oxygen vacancies to singly ionized oxygen vacancies at x = 0.1 [15]. This change in oxygen vacancy felicitates the increase in pyroelectric coefficient in x = 0.075 sample. The TeO2 glass ceramics have altered the Sr-O bond, leading to the
CRediT authorship contribution statement Vandna Tomar: Methodology. Pardeep K. Jha: Formal analysis, Data curation. A.S.K. Sinha: Resources. Priyanka A. Jha: Conceptualization, Methodology, Writing - original draft, Visualization, Writing - review & editing. Prabhakar Singh: Supervision, Resources, Funding acquisition, Writing - review & editing. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper Acknowledgments This work is supported by CSIR Project No. 03(1402)/17/EMR-II. The Department is thankful to DST FIST (SR/FST/PSI-203/215(C)) for Departmental Funding. The authors are thankful to Design and Innovation centre, IIT(BHU) (DIC –IIT(BHU)/2018-19/225) for the funding The authors are thankful to Dr. Saral K Gupta, Banasthali Vidyapith for Raman characterisation. 7
BaO–SrO–TiO2–SiO2–Al2O3 glass system doped with La2O3 26.88BaO-6.72SrO-29TiO2-22SiO2-12Al2O3-2.4BaF2-1La2O3 (mol%) Ba0.6Sr0.4TiO3 ceramics doped B2O3-SiO2 glass Ba0.9Sr0.1TiO3 ceramics doped TeO2 glass X = 0.075
Ba0.4Sr0.6TiO3 + SrO-B2O3-SiO2
Ba0.3Sr0.7TiO3 with 0–8 mass% BaO-Al2O3-B2O3-SiO2 glass additive
Ba0.4Sr0.6TiO3 (BST) ceramics with Bi2O3-B2O3-ZnO (BBZ) glass additive
Ba0.3Sr0.7TiO3 (BST)
Studied system
1200
0.44 J/cm3 27 100,000 12,000 ~ 3000
440
×
3.18 J/cm × × 13.8 Jm−3K−2
~30,000 ~7500 ~1200
1.13 J/cm3 0.57 J/cm3 0.62 J/cm3
Spark Plasma Sintering Conventional Sintering Ceramic by solid state reaction route and glass by melt quench method then sintering of their mixture Glass by melt quench then mixing with raw form of constituents of ceramic and then sintering Ceramic powder of Ba0.4Sr0.6TiO3 by solid state reaction route and glass by melt quench method then sintering of their mixture Melt quench Melt-quenching technique. Sol-gel process Ceramic powder of Ba0.9Sr0.1TiO3 by solid state reaction route and TeO2 glass by melt quench method then sintering of their mixture 3
Dielectric constant
Energy storage density
Synthesis route
Table 2 A comparative of the energy storage density, dielectric constant and loss of the previously reported works and our work.
× ~10 ~0.07 ~0.1
[24] [25] [26] This work
[23]
[22]
3.5 × 10−3 .07
[21]
[20]
References
.0025 .0220 ~0.08
Dielectric loss
V. Tomar, et al.
Journal of Non-Crystalline Solids 535 (2020) 119964
Fig. 13. The hysteric I-V plot for (1-x) BST-x TeO2 sample x = 0.0 – 0.075.
Fig. 14. Arrhenius plots with the goodness of fit factor and activation energy for (1-x) BST-x TeO2 samples x = 0.0 – 0.075.
Supplementary materials
Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.jnoncrysol.2020.119964.
References
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Journal of Non-Crystalline Solids 535 (2020) 119964
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14701. [15] Z. Zhao, X. Song, T. Zhang, K. Hu, X. Liang, S. Li, Y. Zhang, I. Baturin, V. Shur, Influence of lanthanum substitution on microstructure and impedance behavior of barium strontium titanate glass-ceramics, J. Appl. Phys. 126 (2019) 074101. [16] P.K. Jha, P.A. Jha, P. Kumar, K. Asokan, R.K. Dwivedi, Gradient core-shell microstructure in mixed valence multiferroic: TM (Ti, Nb, W) substituted bismuth ferrite, J. Alloys Compd. 667 (2016) 178–183. [17] A.G. Chynoweth, Dynamic method for measuring the pyroelectric effect with special reference to barium titanate, J. Appl. Phys. 27 (1956) 78–84. [18] P.A. Jha, A.K. Jha, Effect of yttrium substitution on structural and electrical properties of barium zirconate titanate ferroelectric ceramics, Curr. Appl. Phys. 13 (2013) 1413–1419. [19] K.S. Srikanth, S. Patel, R. Vaish, Pyroelectric performance of BaTi1-x SnxO3, Int. J. Appl. Ceram. Tech. 15 (2018) 546–553. [20] Y. Hu, M. Sen, Effects of phase constitution and microstructre on energy storage properties of barium strontium titanate ceramics, J. Eur. Ceram. Soc. 37 (2017) (2016) 1–6. [21] H. Yang, F. Yan, Y. Lin, T. Wang, Enhanced energy storage properties of Ba0.4Sr0.6TiO3 lead-free ceramics with Bi2O3-B2O3-SiO2 glass addition, J. Eur. Ceram. Soc. 38 (4) (2018) 1367–1373. [22] Z.H. Wu, et al., Effect of BaO-Al2O3-B2O3-SiO2 glass additive on densification and dielectric properties of Ba0.3Sr0.7TiO3 ceramics, J. Ceram. Soc. Jpn. 116 (1350) (2008) 345–349. [23] K. Chen, Y. Pu, N. Xu, X. Luo, Effects of SrO–B2O3-SiO2 glass additive on densification and energy storage properties of Ba0.4Sr0.6TiO3 ceramics, J. Mater. Sci. Mater. Electron. 23 (8) (Aug. 2012) 1599–1603. [24] W. Zhang, J. Wang, S. Xue, S. Liu, B. Shen, J. Zhai, Effect of La2O3 additive on the dielectric properties of barium strontium titanate glass–ceramics, J. Mater. Sci. Mater. Electron. 25 (9) (Sep. 2014) 4145–4149. [25] Y. Zhang, T. Ma, X. Wang, Z. Yuan, Q. Zhang, Two dielectric relaxation mechanisms observed in lanthanum doped barium strontium titanate glass ceramics, J. Appl. Phys. 109 (8) (2011) 84115. [26] Z. Jiwei, Y.A.O. Xi, C. Xiaogang, Z. Liangying, H. Chen, Direct-current field dependence of dielectric properties in B2O3-SiO2 glass doped Ba0.4Sr0.6TiO3 ceramics, J. Mater. Sci. 37 (17) (Sep. 2002) 3739–3745.
6569–6580. [2] A. Bhalla, Polar glass ceramics, Ferroelectrics 94 (1) (1989) 479. [3] A. Halliyal, A.S. Bhalla, R.E. Newnham, L.E. Cross, Ba2TiGe2O8 and Ba2TiSi2O8 pyroelectric glass-ceramics, J. Mater. Sci. 16 (4) (1981) 1023–1028. [4] A. Halliyal, A.S. Bhalla, S.A. Markgraf, L.E. Cross, R.E. Newnham, Unusual pyroelectric and piezoelectric properties of fresnoite (ba2tisi 208) single crystal and polar glass-ceramics, Ferroelectrics 62 (1) (1985) 27–38. [5] K. Uchino, 1 - The development of piezoelectric materials and the new perspective, in: K. Uchino (Ed.), Advanced Piezoelectric Materials, Woodhead Publishing, 2010, pp. 1–85. [6] M. Ceriotti, F. Pietrucci, M. Bernasconi, Ab initio study of the vibrational properties of crystalline TeO2: the α, β, and γ phases, Phys. Rev. B - Condens. Matter Mater. Phys. 73 (10) (2006) 1–17. [7] X. Song, et al., Relaxation processes in barium strontium titanate glass-ceramics by thermally simulated depolarization current, J. Am. Ceram. Soc. 102 (3) (2019) 901–906. [8] H. Yang, F. Yan, Y. Lin, T. Wang, Enhanced energy storage properties of Ba0.4Sr0.6TiO3 lead-free ceramics with Bi2O3-B2O3-SiO2 glass addition, J. Eur. Ceram. Soc. 38 (4) (2018) 1367–1373. [9] P.A. Jha, P.K. Jha, A.K. Jha, R.K. Kotnala, R.K. Dwivedi, “Phase transformation and two-mode phonon behavior of (1 - X) [Ba Zr 0.025Ti0.975O3]-(x) [BiFeO3] solid solutions, J. Alloys Compd. 600 (2014) 186–192. [10] P.A. Jha, A.K. Jha, Influence of processing conditions on the grain growth and electrical properties of barium zirconate titanate ferroelectric ceramics, J. Alloys Compd. 513 (2012) 580–585. [11] W.D. Callister, D.G. Rethwisch, Materials Science and Engineering: An Introduction, John Wiley & Sons, 1940. [12] M.I. Aroyo, J.M. Perez-Mato, D. Orobengoa, E. Tasci, G. De La Flor, A. Kirov, Crystallography online: Bilbao crystallographic server, Bulg. Chem. Commun. 43 (2) (2011) 183–197. [13] R. Pirc, R. Blinc, Off-center $\mathrm{Ti}$ model of barium titanate, Phys. Rev. B 70 (13) (Oct. 2004) 134107. [14] A.S. Bangwal, P.K. Jha, P.K. Dubey, M.K. Singh, A.S.K. Sinha, V. Sathe, P.A. Jha, P. Singh, Porous and highly conducting cathode material PrBaCo 2 O 6−δ : bulk and surface studies of synthesis anomalies, Phys. Chem. Chem. Phys. 21 (2019)
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Enhancement in pyroelectricity of polar Ba0.9Sr0.1TiO3 –TeO2 glass-ceramics a
a
b
a,⁎
Vandna Tomar , Pardeep K. Jha , A.S.K. Sinha , Priyanka A. Jha , Prabhakar Singh a b
a,⁎
T
Department of Physics, Indian Institute of Technology (Banaras Hindu University) Varanasi, Varanasi-221005 (India) Department of Chemical Engineering and Technology, Indian Institute of Technology (Banaras Hindu University) Varanasi, Varanasi-221005 (India)
A R T I C LE I N FO
A B S T R A C T
Keywords: Pyroelectricity Glass-ceramics BST TeO2
In the present work, the piezoelectric and pyroelectric properties of lead free ferroelectric Ba0.9Sr0.1TiO3 ceramic are altered with the addition of TeO2 glass ceramics. For this, crystalline phase of γ- TeO2 prepared by meltquench method is mixed with Ba0.9Sr0.1TiO3 prepared by solid state reaction method, to form compounds (1-x) Ba0.9Sr0.1TiO3 – (x) TeO2, x = 0 – 0.1 at the steps of 0.025. The structural and impedance analyses confirm formation of solid solution up to x ≤ 0.05 and thereafter they show composite formation. The dielectric permittivity is observed to be maximum for composition x = 0.075. The XPS and EDX analyses are showing highest oxygen vacancy at x = 0.05. The piezoelectricity is measured using Piezo Force Microscopy technique and c-axis domains are observed showing the formation of polar glass-ceramics. The pyroelectric current increases with the increase in TeO2 content, showing the suitability of samples for energy harvesting and pyroelectric detectors.
1. Introduction In recent times, the glass ceramics find wide applications in the electronics industry due to their pore- free nature, large and complex structures [1]. The glasses can be crystallized into polar glass-ceramic form and can be used as thermal and pressure sensing elements [2]. Most importantly, these glass ceramics are polar where crystallites are arranged in a polar array. This macroscopic polarity gives rise to a better piezoelectric or pyroelectric activity as compared to poled ferroelectric ceramics. Due to the ease in polarization, they are promising candidates for pyroelectric detectors, hydrophones and surface acoustic wave devices along with the energy harvesting applications [3,4]. The ferroelectric ceramics have the ability to reorient crystallites with the field to impart long polar order. In polar but non- ferroelectric ceramics, this is not possible; consequently such materials do not exhibit piezoelectricity and pyroelectricity. The polar glass ceramics showed the formation of non-ferroelectric ceramic with polar order [5]. Hence, this could be an interesting route to increase polar order with high breakdown strength in a ceramic [5]. The domains in an unpoled ferroelectric polycrystalline ceramic are randomly oriented. The applied field during poling process aligns most of the domains with their polarization vectors in a direction parallel to the field. The grain structure of grain-oriented glass-ceramic is similar to that of a poled ceramic. Hence, poled ferroelectric ceramic and grainoriented polar glass-ceramic belong to the same conical point group, ∞m [5]. On the basis of simultaneous presence of piezoelectric and
⁎
pyroelectric properties, there are four categories of glass ceramics. In class (a) both properties are present. While class (b) and (d) are suitable for piezoelectric and pyroelectric applications, respectively and class (c) don't contain any such properties [5]. TeO2 is a well known material to exhibit better piezoelectric coefficient, acoustical and electro-optical properties. This elastic anisotropy is attributed to the stretching of weak and strong Te-O bonds [6]. In addition, amongst various lead free ferroelectric ceramics, Ba1-xSrxTiO3 has received attention as energy storage capacitors due to high dielectric constant and low loss. The glass ceramics based on 26.88BaO–6.72SrO–29TiO2–22SiO2–12Al2O3–2.4BaF2–0.5La2O3 (mol %) [7] are reported to exhibit better ferroelectric, piezoelectric and pyroelectric properties. In addition, the glass-ceramic formed by Ba0.4Sr0.6TiO3 with Bi2O3-B2O3-SiO2 are found to possess high recoverable energy storage density of the order of 1.98 J/cm3 [8]. Hence, barium strontium titanate has been chosen as a ferroelectric material and Ba1-xSrxTiO3 with x = 0.1 lies in ferroelectric range with the phase transition Curie temperature (Tc) ~ 120 °C. To tailor the piezoelectric/pyroelectric properties of a ferroelectric ceramic, TeO2 glass ceramic having crystalline phase of γ- TeO2 is mixed with lead free ferroelectric ceramic, Ba1-xSrxTiO3 (a solid solution of two Perovskites BaTiO3 and SrTiO3) to form the compounds (1x)Ba0.9Sr0.1TiO3 – (x) TeO2 where x varies from 0 to 0.1 in the steps of 0.025. The phase development, microstructure, domains and oxygen content have been examined with the variation of TeO2 concentration. The transition temperatures are studied with differential scanning
Corresponding authors. E-mail addresses: [email protected] (P.A. Jha), [email protected] (P. Singh).
https://doi.org/10.1016/j.jnoncrysol.2020.119964 Received 9 December 2019; Received in revised form 1 February 2020; Accepted 7 February 2020 0022-3093/ © 2020 Elsevier B.V. All rights reserved.
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calorimetric, dielectric and pyroelectric studies. The energy is also calculated through pyroelectric current. 2. Experimental 2.1. Sample preparation The polycrystalline perovskite structured Ba0.9Sr0.1TiO3 ceramics were prepared by solid state reaction method using carbonates and oxides precursors BaCO3 (M/s Alfa Aesar with purity ~ 99%), SrCO3 (M/s CDH with purity ~ 99%) and TiO2 (M/s Alfa Aesar with purity ~ 99.8%) in stoichiometric ratios and were mixed and ground thoroughly in an agate mortar to get a fine and homogeneous composition. The calcination of this powdered mixture was done at 1150 °C for 2 h in an alumina crucible. The disc-shaped pellets Ba0.9Sr0.1TiO3 were sintered at 1200 °C, 1250 °C and 1300 °C for 2 h. We obtained the phase pure Ba0.9Sr0.1TiO3 sample for sintering at 1200 °C for 2 h. The calcined powder was then again ground. TeO2 raw powder (M/s Alfa Aesar with purity ~ 99.99%) was weighed and its melt was prepared at 900 °C in an alumina crucible, then molten tellurium oxide was quenched at aluminium mould which was maintained at 200 °C. After this, it was annealed at 150 °C in a hot air oven to remove the remaining moisture for further characterizations and was allowed to cool to room temperature naturally for removing internal residual stresses. This sample was obtained in the form of the solid disc shape. The glass ceramic samples were prepared by using calcined powder of Ba0.9Sr0.1TiO3 ceramic and TeO2 powder obtained using melt quench technique. The compositions were formed as (1-x) Ba0.9Sr0.1TiO3 – (x) TeO2 where x = 0 – 0.1 in the steps of 0.025. After this, pellets of the powders obtained were made by pressing it uniformly with the help of hydraulic press under 2 MPa pressure. PVA was used as a binder, which was burned off in a hot air oven at 300 °C for 2 h. These disc-shaped pellets were sintered at 1200 °C for 2 h (optimized for Ba0.9Sr0.1TiO3).
Fig. 1. X-ray diffractograms confirming the BST phase, γ- TeO2 phase and glassceramics.
for macro (average/global) and local (micro) structural behaviors’. Similarly, the dielectric, impedance and pyroelectric studies are being carried out to understand the average electric response and PFM is used for microanalysis of piezo-response. In addition, switching behaviour of the domains is studied using I-V characteristics.
3.1. Structural studies 3.1.1. XRD The XRD confirms the single phase formation of calcined Ba0.9Sr0.1TiO3 (BST) (PCPDF 81–2263) and amorphous phase with the crystalline peaks corresponding to γ- TeO2 (PCPDF 52–1005) (Fig. 1). With the incorporation of TeO2 into BST, the samples are observed in crystalline forms. At x = 0.025, the XRD resembles with pure phase of BST and a small peak at 2θ ≈ 24° is observed that is matching well with the PCPDF file of TeO3 (PCPDF 80–0570). While with the increase in x, the intensity of the peak corresponding to 2θ ≈ 24° observed to increase gradually. However, at x = 0.05, the peak at 2θ ≈ 30° corresponding to γ- TeO2 starts appearing, the intensity of this peak is increasing with the increase in x. Further, some more peaks have also been observed at x = 0.05 in addition to the above mentioned two peaks (indexed in Fig. 1). These peaks match well with Tellurium trioxide TeO3 (PCPDF 80–0570). Moreover, the XRD peaks are observed to split into two peaks with the increase in x showing the alteration in phase of BST [9] and shown in the right panel of Fig. 1. The splitting of these peaks correspond to TeO2 (PCPDF 74–1131) as TeO2 has entered the lattice of BST and confirming the structure alteration. This shows that the solubility limit for (1-x) Ba0.9Sr0.1TiO3 – (x) TeO2 is x ≤ 0.05. It is seen that TeO2 crystallizes in γ- TeO2 through melt-quench method and upon mixing the two phases of BST and γ- TeO2, TeO3 phase evolves along with the structure alteration showing the interaction of TeO2 and BST. In addition, the XRD peaks have been indexed with the tetragonal phase (P4mm symmetry) and the lattice parameters calculated are shown in Fig. 2 with the error bars (dash lines are a guide to eye). It is observed from Fig. 2 that lattice constant ‘a’ increases with the increase in TeO2 content up to x = 0.075 and thereafter decreases. While, lattice constant ‘c’ remains independent of increase in TeO2 content and increases after x = 0.05. The tetragonal strain is observed to follow the same trend with x as that of lattice constant ‘c’. The nearly constant behaviour of ‘a’ shows that the lattice alteration is due to TiO6 and TeO6 octahedron stretching.
2.2. Characterization techniques X-ray diffraction study was adopted to identify the crystal structure using Rigaku smart lab X-Ray diffracto0meter at RT with Cu-Kα1 radiation (λ = 1.540598 Å) in the 2θ range of 20º – 70º with the scanning rate of 2 o/min. The glass-ceramic phase was analyzed and indexed using JCPDS. The lattice parameters, microstrain and crystallite size are calculated using XRD data. The microstructural studies were done using Scanning Electron Microscopy (SEM) (Oxford instrument SEM EVO 18). The surfaces of both samples were coated with low-temperature silver paste and cured at 300 °C for 2 hrs for electroding on the both sides of the disc-shaped pellets for further electrical measurements. The permittivity and impedance parameters were measured using a high precision LCR metre (Wayne Kerr) at an oscillation amplitude of 1 V. The measurements were carried out in the temperature range of 35 °C to 400 °C at the step of 5 °C within the frequency range of 20 Hz to 1 MHz. The samples were poled using DC polling unit (Marine India) at biasing field below the breakdown voltage (1 KV) for 2 h for pyro-current measurements. The pyrocurrent measurements was recorded using Keithley electrometer 6517B in the temperature range of 30 °C to 180 °C with the heating rate of 1 °C/min. The switching behaviour of the samples have been studied using I-V measurements from Keithley 2450 series. Piezo Force Microscopic (PFM) measurements were carried out with contact tip, Golden Silicon probe tip CSG10/Pt at a resonance frequency of 64 kHz and force constant 0.01–0.5 N/m. The Differential Scanning Calorimetry (DSC) was measured from Shimadzu DSC-60 Plus from RT to 250 °C. 3. Results and discussions In order to inspect the phase formation of the synthesized glassceramic samples, the samples are characterized by XRD, SEM and AFM 2
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Fig. 2. Lattice parameters a, c and tetragonal strain, c/a, calculated manually for the studied compositions (dash lines are a guide to eye).
Fig. 4. Te content measured through EDX at grains and grain boundaries for the studied compositions (with error bars) inset shows the EDX profile confirming the existence of constituent elements.
trend of lattice parameter ‘a’ shown in Fig. 2. Simultaneously, the O content determined at grain –boundaries is less than that of grains as determined from EDX (graph not shown here). Also, the minimum oxygen content and highest oxygen vacancy is observed for x = 0.05. As O being light element to be predicted from EDX, this is further verified using XPS. 3.1.3. RAMAN studies To confirm the incorporation of TeO2 into the BST lattice, Raman spectra are studied and are shown in Fig. 5. This figure consists of Raman spectra of TeO2 and substituted samples (1-x)Ba0.9Sr0.1TiO3 – (x) TeO2. The major phase observed in Fig. 5 resembles to that of the BST. As BST is tetragonal with C4v symmetry at room temperature, simultaneously, the synthesized compositions show the tetragonal structure. The peak at ~ 303 cm−1 is the characteristic of tetragonal BST confirming the tetragonal phase of the substituted samples. The Raman active modes for tetragonal symmetry are 4E (TO +LO) + 3A1(TO+LO)+B1(TO+LO) [12]. The E (TO) modes occur at frequencies ~ 190, 280 and 516 cm−1 while E (LO) modes occur at frequencies 140, 303, 640 and 720 cm−1. The intensity of longitudinal mode obtained at ~ 303 cm−1 (assigned to 3E(TO)+2E(LO)+B1 overlapping) is observed to decrease after x = 0.075 [13]. The addition of TeO2 into the matrix is visible through the Raman intensity at ~ 700
Fig. 3. SEM micrographs showing two types of grains and water mark and lamellar features for the studied compositions.
3.1.2. SEM micrographs The microstructure difference due to TeO2 addition on the grains can primarily be seen in SEM images. For this, SEM micrographs of the studied samples are shown in Fig. 3. The grain and grain boundary can be clearly distinguished in the studied samples. The formation of hexagonal like microstructure for highly developed grains with the angle between two sides of microstructures ~ 109° is observed. In addition, some rod like structures have also been started to develop at x = 0.05 (shown in inset). The grain size is observed to increase with the increase in x. The water-marks and lamellar features are also observed in the SEM images (marked by circles A, B, C and D in Fig. 3). A small channel between two grains is also appearing beyond the hexagonal grain. These features appear inside the grains and for a few instances across the grains too [10,11]. The rod shaped grains also appear to be connected. To further analyze the composite nature of the X-ray Diffractograms, EDX measurements have been done for the studied samples at the grain and grain boundaries shown in Fig. 4. Inset of this figure depicts the elemental constituents of the x = 0.075 composition. The EDX studies confirm the existence of the constituent elements within error limits. But, the atomic concentration of Te varied in the grain and grain boundary regions. It is seen that at x = 0.05 and 0.075, Te is nearly absent at the grain-boundaries and concentration is nearly constant in the grain region for x = 0.05 and x = 0.075 confirming the
Fig. 5. Raman spectra of TeO2 and substituted samples (1-x)Ba0.9Sr0.1TiO3 – (x) TeO2 where x = 0 – 0.1 in the steps of 0.025 (inset) Variation of Raman intensity with error bars corresponding to longitudinal mode (~ 700 cm−1) with x (lines are a guide to eye). 3
Journal of Non-Crystalline Solids 535 (2020) 119964
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Fig. 6. Deconvoluted XPS spectra of Sr, Ti, Te and O for the substituted samples (1-x) Ba0.9Sr0.1TiO3 – (x) TeO2 where x = 0.025 – 0.1 in the steps of 0.025.
cm−1 and is varying with the lattice constant trend (Fig. 5 inset). Thus longitudinal modes are governing the lattice constant ‘a’ as observed in Fig. 2. The presence of TeO2 is visible through the FWHM of the LO mode corresponding to the peak at ~ 702 cm−1 [6]. With the increase in TeO2 content, FWHM is observed to increase as there is overlapping of BST and TeO2 modes. This overlapping of Raman modes and splitting of the peaks in XRD pattern are caused due to TeO2 and TiO2 octahedron stretching.
Fig. 7. Variation of Z'' and M'' with frequency at Room temperature (40 °C) for the studied compositions.
x = 0.025, 0.05, 0.075 and 0.1, respectively). This shows the increase in oxygen vacancy concentration with x up to x = 0.05 and thereafter decrease in the concentration. 3.2. Electrical studies
3.1.4. XPS studies For investigation of the elemental constituents of the substituted samples, wide XPS spectra is analyzed and matched with the standard look up table and is shown in supplementary (Fig. S1). The XPS spectra corresponding to the constituent elements are deconvoluted and are shown in Fig. 6. It is observed that Sr 3d peaks have spin orbit energy 1.76 eV and a shifting in 3d3/2 component is observed with x showing the change in oxidation states. This shifting is showing the presence of multiple oxidation states of Sr, as Te has affected the A- site of lattice (observed through lattice constant ‘a’ and Raman spectra); it is showing the presence of 3d5/2 sub-oxide along with the shifting of the peaks. The sub-oxide peak is observed to decrease with x indicating the formation of defects at x = 0.1. With the deconvolution of Ti and O XPS spectra, it is observed that multiple oxidation states do exist in Ti along with the satellite peaks. This satellite peak has altered the Auger peak of O along with the existence of multiple oxidation states and oxygen vacancies [14]. The satellite peak accounts the transfer of e− from p-orbital of O to D- orbital of Ti [14]. The FWHM of 2p1/2 peak is broader than 2p3/2 peak in the studied samples. This shows that the auger peak of O and satellite peak of Ti are filling the oxygen vacancy through electrons. In addition, the O and Sr peaks are observed to merge with the increase in x showing the multiple oxidation states. Hence, A- site (Sr-O) is affecting the Ti-O-Ti angle and illustrating the solid solution/composite formation with x. In addition, O 1 s peak at ~ 529 eV is ascribed to the metal and O bond (Ba-O, Sr-O, Ti-O and Sr-O) while, 2nd peak of O belongs to metalhydroxide bond. The ratio of area of peaks corresponding to metaloxygen and metal-hydroxide corresponds to the oxygen vacancy concentration [15]. The ratio of area of peaks is observed to increase up to x = 0.05 and thereafter, it decreases (0.91, 1.40, 1.24 and 1.05 for
3.2.1. Impedance Studies For the solid solution/composite behavior, impedance spectroscopy is applied (Fig. 7). It is observed that in the log impedance plots, grain and grain boundary contributions starts appearing at x = 0.075. While, in the modulus plots, there is one contribution up to x = 0.05 and thereafter, two contributions appear at x = 0.075 and finally merged at x = 0.1. Thus, impedance behavior and appearance of different regimes indicates the possibility of solid solution up to x = 0.05 and thereafter, composite formation takes place [16]. The merging of M'' peak at x = 0.1 indicates the Maxwell –Wagner polarization at x = 0.1. The structural and micro-structural studies (XRD, Raman and SEMEDX) indicate that Te content in the grains is affecting the intensity of longitudinal modes and lattice parameter ‘a’. Now the question arises, how the content of Te at the grains affect the capacitive/resistive behaviour of the grains. Hence, impedance formalism has been adopted to study the dielectric relaxation mechanism for the studied samples. Further, Nyquist plots of impedance are studied and their equivalent circuits are also realized using Z-view software. Fig. 8 depicts the Nyquist plots and their equivalent circuit with the goodness of fit factor for the synthesized compositions. The equivalent circuit consisting of one resistance (R1) and one parallel R-CPE (R2eCPE) circuit connected in series is used to interpret the nature of impedance Nyquist plots (shown in Fig. 8). Here R and CPE are the resistance and constant phase element for the semi-circular contributions. This suggests non-ideal behaviour of capacitance and occurrence of more than one relaxation processes with similar relaxation time. The R1, R2, CPE-P and CPE-T parameters obtained from Z-view software are listed in Table 1. The nearly unity value of CPE-P is suggesting pure capacitive behaviour for 4
Journal of Non-Crystalline Solids 535 (2020) 119964
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Fig. 8. Impedance Nyquist plots and their equivalent circuit along with the goodness of fit parameter at room temperature (40 °C) for the studied compositions.
with the increase in x. In addition, the phase transition temperature is observed to decrease with the increase in x and becomes constant for x ≥ 0.075. At x = 0.0, the maxima in real part of permittivity matches well with the maxima in loss curve. In addition, the maxima in permittivity peak matches with the loss peak with the increase in x. Moreover, a gradual increase in permittivity is observed and exhibits maxima. Thereafter, a sharp decrease in the value of permittivity is observed with the increase in temperature showing the sharp phase transition. The diffusivity constant has been estimated using Curie Weiss law and the value of diffusivity constant is found to be 1. The phase transition temperature is observed to be inversely proportional to the tetragonal strain. This can be attributed to the alteration in A-site of the lattice with Te ions [10]. The high dielectric constant can be attributed to the increase in grain size with the Te content. The increase in grain size leads to the lesser number of grain boundaries which are more resistive than that of the grains leading to more polarization. Hence, the charge carriers create a layer around the interface, leading to the interfacial and orientational polarization. The orientational
the studied compositions. However, the value of CPE-P at x = 0.075 also accounts for the formation of gradient of charge carriers as observed in EDX studies. To analyze the capacitive behaviour, phase transition temperature alteration and solid solution/composite behavior, dielectric permittivity and loss are studied. 3.2.2. Dielectric permittivity and loss Fig. 9 depicts the variation of real part of dielectric permittivity and tan δ with temperature at 1 kHz frequency while the variation of real part of dielectric permittivity and tan δ with temperature at different frequencies is shown as supplementary (Fig. S2). It is observed that the value of dielectric permittivity at the transition temperature is higher at x = 0.075 (4188.10) but lower than that of x = 0.0 (4490.16) at 1 kHz frequency. But the value of dielectric loss is lower for x = 0.025. It is observed that the value of real part of dielectric permittivity observed to increase with x and approaches a maximum at x = 0.075 and thereafter decreases. Whereas the dielectric loss is observed to increase Table 1 Value of resistance, CPE and errors calculated from Z-view software. Sample
Rs
Error
CPE-T
Error
CPE-P
Error
Rp
Error
x x x x x
−49.71 −157.8 −53.04 −619.2 −58.78
5.679 31.973 16.229 76.896 12.724
2.53E-09 8.89E-10 5.98E-10 2.60E-09 9.47E-10
2.90E-11 2.02E-11 7.37E-12 1.70E-10 1.50E-11
0.93563 0.93278 0.96855 0.82471 0.95579
0.001 0.002 0.0011 0.005 0.00114
2.05E+07 1.58E+07 2.87E+07 3.63E+05 4.86E+06
1.61E+06 6.77E+05 9.76E+05 7.92E+03 1.28E+05
= = = = =
0.0 0.025 0.05 0.075 0.1
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Journal of Non-Crystalline Solids 535 (2020) 119964
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Fig. 9. Variation of real part of dielectric permittivity and tan δ with temperature at 1 kHz frequency.
polarization may be originated due to the formation of vacancy dipoles around the interface [10].
Fig. 10. Variation of heat obtained from DSC and derivative of heat (Q′) to analyze the Tc and Tg, glass transition temperature for the studied samples.
3.2.3. DSC analysis To analyze the transition temperature, Tc and glass transition temperature, Tg, differential heat and heat obtained from DSC are plotted (Fig. 10). From the graphs, it can be observed that Tc is diminishing with the increasing TeO2 content. Moreover, a kink at ~ 220 °C is also observed in all the samples. This feature is converting the change in slope after 200 °C with the further addition of TeO2, indicating that Tg lies above 200 °C, a feature for glass. Thus, with the addition of TeO2 glass- ceramics, a composite feature is prevailing. 3.2.3. Pyroelectric studies The value of dielectric permittivity is observed to decrease for x = 0.1 and hence this composition is discarded from the piezoelectric/ pyroelectric measurements. Fig. 11 shows the pseudo - color Piezo Force Microscopy image of (1-x) BST-x TeO2 sample with x = 0.0 – 0.075. It is observed that the piezoresponse decreases with x in comparison to the parent BST sample. Moreover, the formation of columnar domains corresponding to polar glass ceramics is revealed with the substitution of TeO2. The pyroelectric current is measured with the technique developed P by Chynoweth [17] using the relation p = A (dTI/ dt ) with the heating rate of 1 °C/min where, p is pyroelectric coefficient, PI is pyroelectric current, A is area of the electrodes and dT/dt is heating rate (Fig. 12(a)). The pyroelectric current at the transition temperature is observed to increase with the increase in x (Fig. 12(b)). The hump observed in the pyroelectric current is showing the ferroelectric-paraelectric transition temperature which is in good agreement with the transition observed in dielectric permittivity and loss curves [18]. Hence, the energy calculated by taking the pyroelectric current at transition temperature using
Fig. 11. PFM image (pseudo - colored) of (1-x) BST-x TeO2 sample with x = 0.0 – 0.075. With the substitution of TeO2, the formation of columnar domains corresponding to polar glass ceramic is revealed (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.).
p2
relation E = ε′ε [19] where p is pyroelectric coefficient, ε' is real part of o dielectric permittivity and ε0 is permittivity in free vacuum, comes out to be 13.8 Jm−3K−2 for x = 0.075 sample (highest one). As seen
earlier, the dielectric constant and piezoelectricity are observed to decrease with the increase in x. The increase in pyroelectricity in
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Journal of Non-Crystalline Solids 535 (2020) 119964
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Fig. 12. (a) Area normalized Pyroelectric current of (1-x) BST-x TeO2 sample with x = 0.0 – 0.075 (b) Area normalized Pyroelectric current (with errors bars and lines are a guide to eye) with composition at Tc.
changes in Ti-O octahedra as the intensity of LO modes are observed to decrease (intensity is maximum at x = 0.05). This has led to the reduction in Tc and the formation of c-axis domains. This is also felicitated through the formation of columnar grains and also confirms the composite formation of the studied samples.
comparison to the decrease in piezoelectricity can be understood as follows. The pyroelectric contribution due to the domain walls plays a major role in the enhancement of pyroelectric figure of merit. This increase in pyroelectric current might be due to the formation of c-axis domains formed with the gradient of Te (TeO2 with orthorhombic structure) and then the dipoles are getting sufficient energy to change the polarization. There are several relations to obtain pyroelectric figure of merit (FOM) and generally, these FOMs are used for selection of materials for IR sensors as these are indirect performance evaluators. Also, the conversion of thermal energy into electrical energy is an important application of pyroelectric ceramics. In the present case, energy harvesting
4. Conclusions In the present study, we have demonstrated that the TeO2 addition has affected A-site of the lattice leading to more defects at x = 0.1. There is decrement in the value of real part of dielectric permittivity and piezoelectric constant while an increment in pyroelectric activity is observed with the increase in x (E = 13.8 Jm−3K−2 for x = 0.075 sample (highest one)). This is attributed to the increase in the domain wall contribution with the increase in TeO2 content. This is also observed through the increase in area of I-V hysteresis with the TeO2 content as Te creates a concentration difference between bulk and grain- boundary interface and found to be responsible for the conversion from doubly to singly ionized oxygen vacancies at x = 0.1.
p2
FOM E = ε′ε is used and it is being employed to calculate energy for o harvesting applications. Earlier work states that BaTiO3 substituted with Sn showed a value of 17.1 J/m3K2 energy [19]. In our case, the energy observed is slightly lower than the reported results. Also, a comparative of the previous and this work is also presented in detail in Table 2. To analyze the domain dynamics of the studied samples, Fig. 13 shows the hysteresis I-V plots for (1-x) BST-x TeO2 samples for x = 0.0 to 0.075 with application of forward and reverse electric field. With the positive field, the hysteresis area is small however in negative electric field difference between forward and reverse field is very large. However, with substitution of TeO2, this difference between curves in negative electric field reduces significantly. But in positive electric field, it is observed that with the increase in x, the hysteresis area increases. This fact is in further correlation with the dielectric loss observed in the dielectric study (Fig. 9). This indicates that the polar domain induces more hysteresis with the domain movement and the increase in the domain wall conductivity as well as loss. This can be attributed to the concentration gradient of Te in bulk and grain –boundary as visible through EDX (Fig. 4) and XPS studies. In addition the oxygen vacancy is illustrated through the Arrhenius plots of conductivity. It is observed in Fig. 14 that activation energy is observed to increase up to x = 0.05 and thereafter decreases (Ea = 0.71 eV, 1.04 eV, 0.86 eV and 0.21 eV for x = 0.025, 0.05, 0.075 and 0.1, respectively). This activation is in correlation with the oxygen vacancy concentration observed from XPS analysis. Also, change in activation energy from 0.86 eV to 0.21 eV corresponds to the conversion of doubly ionized oxygen vacancies to singly ionized oxygen vacancies at x = 0.1 [15]. This change in oxygen vacancy felicitates the increase in pyroelectric coefficient in x = 0.075 sample. The TeO2 glass ceramics have altered the Sr-O bond, leading to the
CRediT authorship contribution statement Vandna Tomar: Methodology. Pardeep K. Jha: Formal analysis, Data curation. A.S.K. Sinha: Resources. Priyanka A. Jha: Conceptualization, Methodology, Writing - original draft, Visualization, Writing - review & editing. Prabhakar Singh: Supervision, Resources, Funding acquisition, Writing - review & editing. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper Acknowledgments This work is supported by CSIR Project No. 03(1402)/17/EMR-II. The Department is thankful to DST FIST (SR/FST/PSI-203/215(C)) for Departmental Funding. The authors are thankful to Design and Innovation centre, IIT(BHU) (DIC –IIT(BHU)/2018-19/225) for the funding The authors are thankful to Dr. Saral K Gupta, Banasthali Vidyapith for Raman characterisation. 7
BaO–SrO–TiO2–SiO2–Al2O3 glass system doped with La2O3 26.88BaO-6.72SrO-29TiO2-22SiO2-12Al2O3-2.4BaF2-1La2O3 (mol%) Ba0.6Sr0.4TiO3 ceramics doped B2O3-SiO2 glass Ba0.9Sr0.1TiO3 ceramics doped TeO2 glass X = 0.075
Ba0.4Sr0.6TiO3 + SrO-B2O3-SiO2
Ba0.3Sr0.7TiO3 with 0–8 mass% BaO-Al2O3-B2O3-SiO2 glass additive
Ba0.4Sr0.6TiO3 (BST) ceramics with Bi2O3-B2O3-ZnO (BBZ) glass additive
Ba0.3Sr0.7TiO3 (BST)
Studied system
1200
0.44 J/cm3 27 100,000 12,000 ~ 3000
440
×
3.18 J/cm × × 13.8 Jm−3K−2
~30,000 ~7500 ~1200
1.13 J/cm3 0.57 J/cm3 0.62 J/cm3
Spark Plasma Sintering Conventional Sintering Ceramic by solid state reaction route and glass by melt quench method then sintering of their mixture Glass by melt quench then mixing with raw form of constituents of ceramic and then sintering Ceramic powder of Ba0.4Sr0.6TiO3 by solid state reaction route and glass by melt quench method then sintering of their mixture Melt quench Melt-quenching technique. Sol-gel process Ceramic powder of Ba0.9Sr0.1TiO3 by solid state reaction route and TeO2 glass by melt quench method then sintering of their mixture 3
Dielectric constant
Energy storage density
Synthesis route
Table 2 A comparative of the energy storage density, dielectric constant and loss of the previously reported works and our work.
× ~10 ~0.07 ~0.1
[24] [25] [26] This work
[23]
[22]
3.5 × 10−3 .07
[21]
[20]
References
.0025 .0220 ~0.08
Dielectric loss
V. Tomar, et al.
Journal of Non-Crystalline Solids 535 (2020) 119964
Fig. 13. The hysteric I-V plot for (1-x) BST-x TeO2 sample x = 0.0 – 0.075.
Fig. 14. Arrhenius plots with the goodness of fit factor and activation energy for (1-x) BST-x TeO2 samples x = 0.0 – 0.075.
Supplementary materials
Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.jnoncrysol.2020.119964.
References
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