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Effect of BaO–Bi2O3–P2O5 glass additive on structural, dielectric and energy storage properties of BaTiO3 ceramics
Journal Pre-proof Effect of BaO-Bi2O3-P2O5 glass additive on structural, dielectric and energy storage properties of BaTiO3 ceramics
E. Haily, L. Bih, A. Elbouari, A. Lahmar, M. Elmarssi, B. Manoun PII:
S0254-0584(19)31249-0
DOI:
https://doi.org/10.1016/j.matchemphys.2019.122434
Reference:
MAC 122434
To appear in:
Materials Chemistry and Physics
Received Date:
26 September 2019
Accepted Date:
07 November 2019
Please cite this article as: E. Haily, L. Bih, A. Elbouari, A. Lahmar, M. Elmarssi, B. Manoun, Effect of BaO-Bi2O3-P2O5 glass additive on structural, dielectric and energy storage properties of BaTiO3 ceramics, Materials Chemistry and Physics (2019), https://doi.org/10.1016/j.matchemphys. 2019.122434
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Journal Pre-proof Effect of BaO-Bi2O3-P2O5 glass additive on structural, dielectric and energy storage properties of BaTiO3 ceramics E. Haily a, L. Bih a, b, A. Elbouari c, A. Lahmar d, M. Elmarssi d, B. Manoun e,f a Equipe
Physico-Chimie la Matière Condensée (PCMC), Faculté des Sciences de Meknès, Maroc.
b Département
Matériaux et Procédés, ENSAM Meknès, Université Moulay Ismail, Meknès, Maroc.
Laboratoire Physico-Chimie des Matériaux Appliquées (LPCMA), Faculté des Sciences Ben M’sik Casablanca, Maroc. c
Laboratory of Physics of Condensed Matter (LPMC), University of Picardie Jules Verne, Amiens, France. d
Université Hassan 1er, Laboratoire des Sciences des Matériaux, des Milieux et de la modélisation (LS3M), FST Settat, Morocco. e
Materials Science and Nano-engineering, Mohammed VI Polytechnic University, Lot 660 Hay Moulay Rachid, Ben Guerir, Morocco. f
Abstract: A phosphate Bi2O3-BaO-P2O5 (BBP) glass is added to BaTiO3 ceramics (BT) to investigate its influence on densification, rearrangement of structural units, and dielectric properties of the elaborated composites. The BT ceramic is elaborated by the solid-state method while the glasses BaO-Bi2O3-P2O5 (BBP) are synthesized by the melting-quench process. The synthesized composites are labeled BT-xBBP (x = 2.5, 5, and 7.5 wt %) where x stands for the glass content in weight percent. The density was measured to determine the appropriate sintering temperature of the composites, it was found that the glass addition induces a decrease in their densification and helps their sintering at lower temperatures, the suitable sintering temperature (SST) obtained for all the composites is 900°C. Raman spectroscopy and X-ray Diffraction are performed to study the structural approach of the BBP glass addition to BT ceramics. The scanning electron microscopy was used to examine the morphology of pure BT and sintered composites. It was observed that the BT-(x=5) composite had a uniform small grains microstructure. The composition dependence of the dielectric properties shows that a material BT-(x= 5) has the highest dielectric constant at
Journal Pre-proof room temperature. The P-E plots were carried out and the energy storage parameters (density and efficiency) of the composites are determined. These parameters are affected by the porosity and the remnant polarization of the composites.
Keywords: Phosphate, glasses, composite, dielectric properties, energy storage.
Introduction: Several materials with the perovskite-type structure are applied as dielectrics for highdensity energy storage capacitors. Barium titanate ceramic (BaTiO3) constitutes the archetype dielectric owing to its high permittivity and relatively low dielectric loss (Tanδ) [1,2]. BaTiO3 ceramics elaborated by the conventional solid-state method have a relatively high maximum polarization and a significant remnant polarization [3,4]. Unfortunately, the presence of defects as grain boundaries and pores in these ceramics limit their dielectric breakdown strength to (100 kV/cm) [5]. In order to improve the global energy storage of BT ceramics, one has to reduce defects and improve the dielectric permittivity of these materials. In such approach, many researchers have added several additives either oxides or glasses to BT [6-9]. The most investigated glass additives to reduce the sintering temperature and increase the microstructure quality are silicates and borates. Burn [10] reported that several borosilicate glasses-containing different modifiers, such as ZnO, PbO, BaO, and Bi2O3 have been used to improve the densification of BaTiO3 ceramics. By using a ZnO–B2O3–SiO2 [9], the sintering temperature of BaTiO3 is reduced from 1300 °C to 900 °C. The addition of 1mol% of BaOB2O3-SiO2 glass to BaTiO3 ceramics has allowed for low-temperature sintering and led to high-performance dielectrics [11]. The positive effect of a silicate BaO–TiO2–SiO2–Al2O3 glass additive to BT is also highlighted [8] by the fact that the dielectric breakdown strength
Journal Pre-proof (Eb) is increased to 140kV/cm. Such an increase of Eb could allow an improvement of the energy storage of the BT ceramics since the energy density (W) for a linear dielectric corresponds to W=1/20rEb2, where 0=8.85x1012 Fm-1 is the vacuum permittivity, and r is the relative dielectric permittivity. Therefore, it is possible to increase both constant dielectric and breakdown strength of BT ceramics by glass additives. However, it is worth noting that for a nonlinear dielectric material the charge and discharge processes lead to a loss of energy. Therefore, to achieve high energy storage of BT-based material it is necessary to consider its charge-discharge efficiency (η). The development of BT material with high ability energy storage demands to enhance both r, Eb and η parameters. In this perspective, different BTglass composites [6,9-12] are developed. These glass additives are within the B2O3–SiO2, BaO–B2O3–SiO2, Bi2O3–SiO2–B2O3-ZnO and BaO–Bi2O3–B2O3 systems. Effectively, the glass additives had improved the energy storage efficiency of BT ceramics along with the breakdown strength. However, in an opposite trend, the glass additive reduced the dielectric permittivity of the composites whatever the nature of the glass former oxide SiO2 or B2O3. According to our knowledge, a few scientific works have been doing on BT-phosphate composites [13,14]. Thus, the actual work is devoted to the investigation of new BT phosphate composites. The dielectric properties of the BT composites depend strongly on the nature and electrical behavior of the glasses. So, a selection strategy to develop phosphate glasses that should maintain as high as possible the dielectric constant of the phosphate-BT composites is performed. Firstly, we have investigated the glasses containing the same chemical elements as BT within the ternary BaO-TiO2-P2O5 system. Once it is done, we have studied their crystallization under heat treatments. Secondly, we have focused our interest to identify the chemical compositions permitting the formation of BaTiO3 phase in the glass-ceramics using XRD technique. We have found that the glass 20BaO-20TiO2-60P2O5 induces the formation
Journal Pre-proof of BT embedded in the glass-matrix. However, the amount of BT phase in this glass-ceramic is very low and thereby its dielectric properties are low too. In addition, this glass-ceramic is an electronic conductor which was the origin of the high dielectric loss in this material. To overcome this issue, we have substituted TiO2 by Bi2O3 and elaborated on new glasses inside the BaO-Bi2O3-P2O5 system. Among the glasses, it is found that the vitreous composition 10Bi2O3-10BaO-80P2O5 (BBP) exhibits higher dielectric properties than that of titaniumcontaining glasses. The present study deals with the investigation of the effects of this BBP glass on the structure, dielectric and energy storage properties of BT composites.
2. Experimental procedure BaTiO3 powder is prepared using the conventional solid-state method. Barium carbonate BaCO3 (99.9%, Aldrich) and titanium dioxide TiO2 (99.9%, Aldrich), are used as the starting materials. The precursors were weighed in a micro-analytical balance and mixed thoroughly according to the appropriate molar compositions in ethanol medium. The dried powders were then calcined at 1300°C in the air for 4 h. Phosphate glasses xBi2O3-(20-x)BaO-80P2O5, (x = 0, 10, 20 mol %), are prepared by the quenching-melting route. The raw powders containing different amounts of high-purity Barium carbonate BaCO3, bismuth trioxide Bi2O3, and di-hydrogen ammonium phosphate NH4H2PO4 purchased from Aldrich, were weighed, mixed, and melted in an Al2O3 crucible at 1050°C for crystallized h. The molten glasses are quenched in air and then milled to fine powders. According to their dielectric properties, we will consider only the glass (x=10) (BBP) in this study. The BT ceramics or composites are elaborated by mixing BT with the BBP glass powder. The ratios of BT and BBP in each sample are fixed by the chemical compositions: (100-x)BTxBBP (x=0, 2.5, 5, 7.5 wt%). In the following these ceramics are labeled (BT), (BT-x=2.5),
Journal Pre-proof (BT-x=5), and (BT-x=7.5). The (100-x)BT-xBBP powders are firstly granulated using the polyvinyl alcohol (PVA) as a binder (4 wt% PVA/96 wt% of powder). Then, pressed into cylindrical pellets (15 mm diameter and 1mm in thickness). Afterward, the pellets are heated at 450°C for 4 h for binder removal and then sintered at different temperatures for 2h in air. Density measurements are performed using the standard Archimedean method with water as a medium liquid. The structural approach of the sintered ceramic samples is studied by a DXR2 Raman spectrometer using laser excitation at 633 nm. The spectra were obtained in a back-scattering geometry in the wavenumbers 200-1400 cm-1 range. A Phillips D5000 X-ray diffractometer with Cu K radiation is used to identify the crystalline phases of the sintered ceramics. The XRD patterns are carried out with a step size of 0.04° within the range of 2θ from 15° to 70°. The microstructure of the sintered samples is evaluated by a scanning electron microscopy (MiniSEM Hirox SH-4000M). The dielectric properties of the composites are recorded by using a Solartron Modulab Xm MTS in the temperature range 300-573 K and at various frequencies from 100 Hz to 1 MHz. The P-E loops at room temperature of the composite samples are measured by a ferroelectric test system (aixACCT TF Analyzer 3000) at 1 kHz.
3. Results and discussions 3.1 Density measurements The density measurements are realized on BT-(x=2.5), BT-(x=5), and BT-(x=7.5) samples which have already submitted to sintering temperatures in the range 850–1000°C. The experimental density (or specific weight) of the pristine BaTiO3 ceramic is found to be 5.67 g/cm3 while that of BBP glass is 2.52 g/cm3. Fig. 1 shows the variation of the bulk density as a function of sintering temperature for all these BT-x ceramics. It is shown that the density values depend both on the glass content in the BT ceramics and the sintering
Journal Pre-proof temperatures. For a fixed sintering temperature, it decreases by increasing the glass ratio in the ceramics. This can be understood by taking into account that BaTiO3 (ρ=5.67 g/cm3) is denser than BBP glass (2.52 g/cm3). The higher glass content is the lower density of the composites. Moreover, this decrease may be attributed to the large difference in molecular weight between BaTiO3 (233.195 g/mol) and BBP glass (175.481 g/mol). Thus, the introduction of a lighter molecule in the structure instead of heavier decreases the density. From the analysis of Fig.1, it is also showed that whatever fixed glass content, the density varies non-linearly with temperature. A maximum of the density is obtained for a sintering temperature of 900°C. This latter could be considered as the suitable sintering temperature (SST) of the BT ceramics. It seems that at this temperature the glass behaves as a viscous medium and ensures good wetting of the BT grains [6], [13]. Such behavior of the glass at SST 900°C diminishes the porosity and thereby increases the density of the composites. This assumption will be confirmed by SEM micrographs in the 3.4 Section.
3.2 Raman spectroscopy Raman spectroscopy is used to study the network of the (1-x)BT-xBBP samples. This technique allows getting information about the structural units existing inside the BT-x ceramics and the collected spectra are shown in Fig. 2. The Raman spectra are given in the wavenumber range (200–1400 cm-1) where the vibration modes of barium titanate (BT) and phosphate glass BBP are active. Since most of the bands are broad in the BBP glass spectrum, with also some shoulders, we have performed its deconvolution spectrum using Peakfit V4.12 program, with a Gaussian function. This allowed us to better determine the sub-peaks. The BBP glass contains high content of the former P2O5 oxide and shows an O/P ratio equal to 2.75. Thus, one can consider it as an ultraphosphate glass. The Raman spectrum of BBP glass exhibits many broad peaks located at 1227 cm-1, 1164 cm-1, 1040 cm-1, 751 cm-1,
Journal Pre-proof 688 cm-1 and 440 cm-1. A Raman peak at 1227 cm-1 is attributed to the symmetric stretching of P=O in Q3 units. A peak near 1164 cm-1 is assigned to the symmetric stretching of (PO2)groups in Q2 units. A Raman peak around 1040 cm-1 is associated with the symmetric stretching of (PO3)2- in Q1 groups. Peaks around 751 cm-1 and 688 cm-1 are ascribed to the symmetric stretching of P-O-P in Q1 and Q2 structural units, respectively. A broad peak laying under 550 cm-1 is associated with the bending modes of the phosphate network [15-18]. A shoulder around 280 cm-1 is attributed to vibration mode of Bi-O bond in BiO3 pyramidal structural unit [20]. The appearance of this weak peak constitutes a fingerprint that bismuth oxide plays a glass former role in the vitreous material under study. The Raman spectrum of BT sample shows vibrational modes typical of BaTiO3 [1921]. The dominant vibrational modes are a peak at 254 cm-1 [A1 (TO)], a sharp peak at 301 cm-1 (B1 mode), a broad peak around 511 cm-1 [A1, E (TO)] and a weak peak at around 711 cm−1 [E (LO)] [24], In connection to titanium-oxygen linkages, this latter vibration mode is assigned to the asymmetric stretching of the Ti-O bonds. The mode at 511 cm-1 is due to the symmetric bending of the O-Ti-O bond. The mode observed at 301 cm-1 is due to Ti-O bonds in an off centric TiO6 octahedra as in the BaTiO3 ferroelectric phase [25]. The vibration mode observed at 254 cm-1 is assigned to the rotation of the TiO6 octahedra [24]. From all analysis of the BT Raman spectrum, one could associate the structure of BT compound to that of BaTiO3 with tetragonal type. Now after analyzing the Raman spectra of the BBP and BT components one can be able to extract from Fig. 2 relative structural information of the BT-x composites. It is evidenced that the addition of 2.5 to 7.5% of BBP glass to BT doesn’t induce any changes, either in the intensity or peak position, of the characteristic bands of the BT phase. Therefore, we can state that the tetragonal structure of BT is well-kept-up in the composites. This result does indicate that the chemical composition of the BBP glass prevents the degradation of the
Journal Pre-proof BT grains. However, it is worth noting that the structure of the BBP component undergoes a little transformation that is argued by the entrance of a peak at 940 cm-1 during the addition of BBP to BT composites. The intensity of this peak grows as the content of the glass goes up. It seems that this peak evolves when some phosphate groups are transformed into (Q1) or (Q0) structural units inside the network of BBP glass. The presence of some orthophosphate (Q0) units along with the BT phase in the structure of the composites is evidenced from the XRD study as will be seen in the next section.
3.3 XRD analysis Fig. 3 shows the XRD patterns of the BT-x ceramics sintered at 900°C. The XRD pattern of the sample BT corresponds to a pure BaTiO3 perovskite phase. All the XRD peaks are associated with BaTiO3 with a JCPDS file number 89-1428. The XRD pattern of the glass BBP indicates that is structureless. From the analysis of Fig. 3, it is obvious that the BT phase exists in the BT-x composites. All the XRD patterns are dominated by the BT phase since the content of the BBP glass is very low. However, when the BBP content increases from 2.5 to 7.5 wt%, one can note the emergence of a new crystalline Ba3(PO4)2 phase relatives to a JCPDS file number 80-1615. Owing to the fact that the peaks intensity of the BT phase remains unchanged in all the BT ceramics, we believe that the orthophosphate phase is formed essentially from structural rearrangements inside the BBP glass. In other words, the appearance of the Ba3(PO4)2 phase is due to the crystallization of the BBP glass during sintering of the BT-x ceramics. The formation of this barium phosphate phase suggests that barium and bismuth ions interact dissimilarly with phosphate P-O bonds. While the chemical interaction between Ba-P forms crystalline phase its homologous Bi-P results in the formation of amorphous phases.
Journal Pre-proof This is in agreement with the fact that the rest of the glass is rich in bismuth and phosphate since the crystallized phase doesn’t contain bismuth, and that the bismuth oxide is an incipient glass former which forms easily stable vitreous materials with P2O5 former. As discussed above in the Raman section, high polarizing Bi3+ cation exists in the network as BiO3 pyramidal structural unit suggesting the glass former role of Bi2O3. This glassy matrix accumulates at the grain boundaries of the BT phase and reduces the porosity of the BT-x composites as will be experienced from the following microstructure analysis. In addition, from the broadening of the diffraction peaks, the size of the crystallites of the BT phase was estimated, using the Scherrer formula (Equation 1): [26] 𝐾.𝜆
Dp = 𝐵 .cos 𝜃
(1)
Where Dp is the average crystallite size, K is the Scherrer constant, λ is the X-ray wavelength (1.5406 Å) and B is the Full Width at Half Maximum of XRD peak. The estimated crystallite size of pure BT sample was 33.02 nm, and it decreases slightly with the BBP glass addition to 32.25nm in BT-(x=2.5), 30.96nm in BT-(x=5) and 31.97nm in BT-(x=7.5). The estimated values of the average crystallite size are comparable to those found in the literature data [26,27]. From that, we can assume that the average crystallite size of the BT phase is reduced with the addition of BBP glass up to 5 mol%.
3.4. Microstructure analysis The scanning electron micrographs (SEM) of pure BT and sintered composites at 900°C, are shown in Figure 4(a)–(d). The SEM image of pure BT specimen reveals the presence of pores between the grains (Fig. 4a). This is because the grain boundaries of the BT particles usually migrate and lead to the growth of the grains. The beneficial influence of the BBP glass component on the microstructure of the composites is evidenced from Fig. 4b–c. Generally, one can see that the addition of BBP decreases the amount and the size of the
Journal Pre-proof pores. For instance, the addition of 2.5 wt% of the BBP glass component (Fig. 4b) reduces the interspace between the grains. A possible reason for this is the reduction of grain growth by the development of a thin barrier layer among the grains in agreement with the literature data [28]. However, some pores still exist since 2.5 wt% of BBP cannot fully fill the space between the grains. The composite with 5 wt% BBP (Fig. 4c) shows a uniform small grains microstructure that is sufficient to meet the requirements of dense fine-grained ceramics. The pores are almost completely disappeared and the BT composite is visibly homogeneous compared to the BT-(x=2.5) material. However, when the glass addition reaches 7.5 wt %, a reverse trend is observed since this glass amount raises the voids and cracks (Fig. 4d). From the above results, we ascertain that the BBP glass addition up to 5% has two beneficial effects on the microstructure of BT ceramics. Firstly, the grain size of the BT phase is reduced and distributed uniformly. Secondly, the size of pores is decreased. These effects could enhance the dielectric properties of the BT-x composites.
3.5. Dielectric properties Fig. 5 shows the frequency dependence of the relative permittivity εr (dielectric constant) and the dielectric losses Tan δ at room temperature within the frequency range from 100Hz to 1MHz of the composite (100-x)BT-xBBP. The dielectric constant of the pure BT ceramic is about 1450 while its dielectric loss is 0.04 at high frequencies; these values increase fastly when the frequencies decrease less than 1 kHz. The dielectric properties at low frequency are associated with many factors mainly the sample-electrode polarization, the defaults and the fluctuation of the composition [29]. At high frequencies, it is observed that the addition of BBP glass to pure BT has a positive effect on the dielectric properties. The addition of 2.5 mol % of BBP induces a slight increase and decrease in permittivity and dielectric loss, respectively. The same behavior is observed with the addition of 5 mol% of
Journal Pre-proof BBP. However, with a further addition of BBP glass (7.5 mol %) to pure BT the dielectric constant decreases. Therefore, the dielectric properties of the composites are compositiondependent and the material BT-(x=5) shows the greatest permittivity 1700 and relatively low loss of the order 0.012 over a large frequency domain. The observed decrease of the permittivity when the content of the glass BBP is high (7.5 mol %) could be due to the microstructure of the composite (Fig. 4d) which shows an excess of the glassy phase and an increase of the porosity. Fig. 6 presents the temperature dependence of εr and Tan δ at 1MHz in the temperature range from 30 to 300 °C of the composites (100-x)BT-xBBP. The values of the main dielectric parameters of the studied specimens are listed in Table 1. At room temperature, the permittivity of the composite depends on the composition and varies from 1021 to 1563; the dielectric loss is very low and varies from 0.011 to 0.037. For a fixed temperature, the permittivity increases first and then decreases with increasing BBP glass content. The BT(x=5) composite shows a high dielectric constant and reaches a maximum value εm 2236 at 136 °C. According to SEM images (Fig. 4), we can assume that the addition of BBP glass up to 5% improves the dielectric properties by clamping the BT particles, blocking the easy conduction channels and reducing the leakage current at grain boundaries. On the other hand, by increasing the BBP glass ratio to 7.5%, the dielectric constant decreases rapidly. This may be due either to the porous microstructure of BT-(x=7.5) (Fig. 4d) or to the appearance of a significant content of the secondary phase Ba3(PO4)2 (Fig. 3). Furthermore, one can note that the temperature dependence of the permittivity shows a large peak around 120-140 °C for all the composites. This peak is related to the BT phase in agreement with the fact that it exhibited a phase transition from ferroelectric to paraelectric phase [30]. The dielectric constant increases with increasing the temperature until the transition temperature and then decreases above 140 °C because of the disappearance of the
Journal Pre-proof ferroelectric domains. No diffusion or displacement of the Curie temperature Tc is observed for all the composites suggesting that the BaTiO3 phase keeps its normal structure despite the addition of glass.
3.6 Energy storage performance: The polarization hysteresis loop was measured by an Aixacct TF analyzer 3000. Unfortunately, the voltage amplitude is limited in this device, and the maximum electrical field which can be applied to the material is 15 kV/cm. This limited voltage prevents us from reaching the electrical breakdown field, which implies that the measured energy density is much lower than the real one. Moreover, the energy storage density is also underestimated by the fact that the hysteresis loop is measured at high frequency 5 kHz, which excludes partially Maxwell-Wagner-Sillars polarization detected at low frequency. However, the measured energy under these experimental conditions is very useful for many applications in high frequency especially in electronic. The P–E hysteresis loops of the composite (100-x)BT-xBBP are plotted in Fig. 7, which are measured at 15 kV/cm and 5 kHz. Unfortunately due to the limitation in the voltage amplitude in the measuring device, the hysteresis loops of the composites show a lossy behavior which is due to the leakage current present in the samples. The origin of this leakage current is coming from defects, which caused a hopping phenomenon [31]. Moreover, it can be seen that pure BT shows a decent hysteresis loop, giving a maximum polarization (Pm) of 0.53μC/cm2 and a remnant polarization (Pr) of 0.06μC/cm2 at 15 kV/cm. One can note that the shape of the P-E loops depends on the glass content in the composite. For instance, with increasing the content of BBP glass, the P-E loops became slimmer and the maximum polarization of the composites BT-(x=2.5) and BT-(x=5) are larger than that of pure BT. Also,
Journal Pre-proof it is observed that the remnant polarization of the material BT-(x=5) is slightly lower than that of pure BT whereas that of the BT-(x=7.5) is increased. Therefore, it is noticed that the composite BT-(x=2.5) exhibits the largest Pmax, while the higher dielectric constant is observed in BT-(x=5), this is maybe due either to a change in the polarization mechanism upon the composition and/or to the different nature of excitation between Solartron and aixACCT. Generally, the energy storage properties of the dielectric materials are characterized by the four following parameters: energy storage density, recoverable energy density (Wrec), energy loss density (Wloss), and energy storage efficiency (η) [32], [33]. The energy storage density is extracted from the P-E loops and obtained according to the following equation: (2)
𝑊 = ∫𝐸𝑑𝑃
The recoverable energy storage density (Wrec, equation 3) is associated with the integrated area between the polarization axis and the discharged curve; by integrating the area between charge and discharge curves one can obtain the loss energy (Wloss, equation 4). The energy storage efficiency (η) is evaluated through equation 5. These parameters are computed by using following equation [29,30]: 𝑃𝑚𝑎𝑥
𝑊𝑟𝑒𝑐 = ∫𝑃𝑟
(3)
𝐸𝑑𝑃
𝑃
(4)
𝑊𝑙𝑜𝑠𝑠 = ∫0 𝑚𝑎𝑥𝐸𝑑𝑃 ― 𝑊𝑟𝑒𝑐 Wrec
𝜂 = Wrec + Wloss x 100
(5)
Pr is the remnant polarization, Pmax is the maximum polarization of the maximum electrical field (the saturation polarization) and E is the applied electric field. The calculated parameters from P-E loops are listed in Table 2, and the glass content dependence of the recoverable energy (Wrec), the energy loss density (Wloss) and the efficiency (η) is shown in Fig. 8.
Journal Pre-proof From the P-E hysteresis loops, it is seen that the BT-(x=2.5) composite exhibits the highest recoverable energy storage density (Wrec) suggesting that the addition of the BBP glass increases the recovery energy of BT ceramic. Moreover, the energy loss of the BT(x=2.5) is almost comparable to that of pure BT so that there is a small improvement in energy storage efficiency of BT-(x=2.5). With the further addition of BBP up to 7.5%, one can note that the energy density decreases while the efficiency of storage reached its maximum value of 80.2% for the material BT-(x=5). The improvement of the energy density for the composite BT-(x=2.5) can be explained by taking into account the microstructure and SEM image (Fig. 4) that revealed reducing of grain size and decreasing amount pores. This reduces the free-charge trapping by the grain boundary and pore, this free charge is responsible for screening the ferroelectric polarization and the increasing of the fatigue phenomena in the materials. On the other hand, when the content of the glass is high the polarization hysteresis loop is affected, reducing, by the way, the value of the remnant and saturation polarization causing the decrease the energy storage density. Moreover, too much BBP coating also would destroy the core-shell structure in the process of sintering, leading to grain growth and the formation of secondary phases. For that, it is important to control the composition in optimizing the overall properties.
Conclusion: Bi2O3-BaO-P2O5 glass was prepared and used as a sintering aid and a coating agent for BaTiO3 ceramics. It had a considerable influence on the sintering temperature, microstructure, dielectric and energy storage properties of BT ceramics. Among all the studied specimens, the composite with a 5% addition of BBP glass had the highest permittivity, with relatively low dielectric losses, less than 0.02, and it showed an improvement in energy storage efficiency compared to pure BT ceramics. These results are mainly due to the positive effect of the glass
Journal Pre-proof on the microstructure since 5% of BBP glass reduces the porosity and the existed defects on the pure BT ceramic. It can, therefore, be concluded that this composite is a promising dielectric candidate for energy storage capacitor ceramics. Another benefit influence of the glass in the BT-glass composite is its ability to prevent the growth of the BT ceramics. Thus, the existing small grain size of BT presented the synergetic effect with the vitreous network of BBP to enhance the dielectric properties of the glass-ceramics.
Acknowledgments: The authors are grateful to the CNRST, OCP foundation for their financial support in the framework of around phosphates project.
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DECLARATION OF INTEREST STATEMENT
We state that: 1. The article is original. 2. The article has been written by the stated authors who are ALL aware of its content and approve its submission. 3.The article has not been published previously 4. the article is not unde consideration for publication elsewhere 5. No conflict of interest exists, or if such conflict exists , the exact nature must be declared. 6. If accepted, the article will not be published elsewhere in the same form, in any language, without the written concent of the publisher. All the best regards, Dr Lahcen BIH
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Fig. 1. Bulk density of the (100-x)BT-xBBP ceramics as a function of the sintering temperature.
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Fig. 2. Raman spectra of the BBP glass, the pristine BT and the composites (100-x)BT-xBBP.
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Fig. 3. XRD patterns of the composites (100-x)BT-xBBP.
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Fig. 4. SEM micrographs of the polished surface of the composites (100-x)BT-xBBP; (a) 0 wt%, (b) 2.5 wt%, (c) 5 wt% and (d) 7.5 wt%.
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Fig. 5. Frequency dependence of (a) the permittivity (εr) and (b) the dielectric loss (Tanδ) at RT of the composites (100-x)BT-xBBP.
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Fig. 6. Temperature dependence of (a) the permittivity (εr) and (b) the dielectric loss (Tanδ) at 1MHz of the composites (100-x)BT-xBBP.
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Fig. 7. P–E hysteresis loops of the composites (100-x)BT-xBBP at an electric field of 15 kV/cm.
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Fig. 8. Variation of Wrec, Wloss, and η of the composites (100-x)BT-xBBP at electric field of 15kV/cm.
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Highlights 1- Synthesis of the glass-ceramics BaO-Bi2O3-P2O5/BaTiO3 2- Investigation of the structure by Raman spectroscopy 3- Studies of the dielectric properties. 4- Investigation of their P-E loops and energy storage ability.
Journal Pre-proof Table 1: Main properties dielectric of the composites (100-x)BT-xBBP at 1MHz.
εr (RT)
Tan δ (RT)
εm
BT
1425.48
0.037
1800.58
BT-(x=2.5)
1490.05
0.0142
1895.38
BT-(x=5)
1563.30
0.0126
2236.37
BT-(x=7.5)
1021.67
0.0112
1504.16
Table 2: Polarization and energy storage properties of the composites (100-x)BT-xBBP at electric field of 15 kV/cm. Pmax μC/cm2
Pr μC/cm2
W rec
(mJ/cm )
(mJ/cm )
BT
0.534
0.069
3.6
1.6
69.21
BT-(x=2.5)
1.075
0.108
6.9
2.6
72.58
BT-(x=5)
0.651
0.050
4.8
1.2
80.20
BT-(x=7.5)
0.505
0.158
2.3
3.1
42.27
3
W loss
η (%) 3
E. Haily, L. Bih, A. Elbouari, A. Lahmar, M. Elmarssi, B. Manoun PII:
S0254-0584(19)31249-0
DOI:
https://doi.org/10.1016/j.matchemphys.2019.122434
Reference:
MAC 122434
To appear in:
Materials Chemistry and Physics
Received Date:
26 September 2019
Accepted Date:
07 November 2019
Please cite this article as: E. Haily, L. Bih, A. Elbouari, A. Lahmar, M. Elmarssi, B. Manoun, Effect of BaO-Bi2O3-P2O5 glass additive on structural, dielectric and energy storage properties of BaTiO3 ceramics, Materials Chemistry and Physics (2019), https://doi.org/10.1016/j.matchemphys. 2019.122434
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Journal Pre-proof Effect of BaO-Bi2O3-P2O5 glass additive on structural, dielectric and energy storage properties of BaTiO3 ceramics E. Haily a, L. Bih a, b, A. Elbouari c, A. Lahmar d, M. Elmarssi d, B. Manoun e,f a Equipe
Physico-Chimie la Matière Condensée (PCMC), Faculté des Sciences de Meknès, Maroc.
b Département
Matériaux et Procédés, ENSAM Meknès, Université Moulay Ismail, Meknès, Maroc.
Laboratoire Physico-Chimie des Matériaux Appliquées (LPCMA), Faculté des Sciences Ben M’sik Casablanca, Maroc. c
Laboratory of Physics of Condensed Matter (LPMC), University of Picardie Jules Verne, Amiens, France. d
Université Hassan 1er, Laboratoire des Sciences des Matériaux, des Milieux et de la modélisation (LS3M), FST Settat, Morocco. e
Materials Science and Nano-engineering, Mohammed VI Polytechnic University, Lot 660 Hay Moulay Rachid, Ben Guerir, Morocco. f
Abstract: A phosphate Bi2O3-BaO-P2O5 (BBP) glass is added to BaTiO3 ceramics (BT) to investigate its influence on densification, rearrangement of structural units, and dielectric properties of the elaborated composites. The BT ceramic is elaborated by the solid-state method while the glasses BaO-Bi2O3-P2O5 (BBP) are synthesized by the melting-quench process. The synthesized composites are labeled BT-xBBP (x = 2.5, 5, and 7.5 wt %) where x stands for the glass content in weight percent. The density was measured to determine the appropriate sintering temperature of the composites, it was found that the glass addition induces a decrease in their densification and helps their sintering at lower temperatures, the suitable sintering temperature (SST) obtained for all the composites is 900°C. Raman spectroscopy and X-ray Diffraction are performed to study the structural approach of the BBP glass addition to BT ceramics. The scanning electron microscopy was used to examine the morphology of pure BT and sintered composites. It was observed that the BT-(x=5) composite had a uniform small grains microstructure. The composition dependence of the dielectric properties shows that a material BT-(x= 5) has the highest dielectric constant at
Journal Pre-proof room temperature. The P-E plots were carried out and the energy storage parameters (density and efficiency) of the composites are determined. These parameters are affected by the porosity and the remnant polarization of the composites.
Keywords: Phosphate, glasses, composite, dielectric properties, energy storage.
Introduction: Several materials with the perovskite-type structure are applied as dielectrics for highdensity energy storage capacitors. Barium titanate ceramic (BaTiO3) constitutes the archetype dielectric owing to its high permittivity and relatively low dielectric loss (Tanδ) [1,2]. BaTiO3 ceramics elaborated by the conventional solid-state method have a relatively high maximum polarization and a significant remnant polarization [3,4]. Unfortunately, the presence of defects as grain boundaries and pores in these ceramics limit their dielectric breakdown strength to (100 kV/cm) [5]. In order to improve the global energy storage of BT ceramics, one has to reduce defects and improve the dielectric permittivity of these materials. In such approach, many researchers have added several additives either oxides or glasses to BT [6-9]. The most investigated glass additives to reduce the sintering temperature and increase the microstructure quality are silicates and borates. Burn [10] reported that several borosilicate glasses-containing different modifiers, such as ZnO, PbO, BaO, and Bi2O3 have been used to improve the densification of BaTiO3 ceramics. By using a ZnO–B2O3–SiO2 [9], the sintering temperature of BaTiO3 is reduced from 1300 °C to 900 °C. The addition of 1mol% of BaOB2O3-SiO2 glass to BaTiO3 ceramics has allowed for low-temperature sintering and led to high-performance dielectrics [11]. The positive effect of a silicate BaO–TiO2–SiO2–Al2O3 glass additive to BT is also highlighted [8] by the fact that the dielectric breakdown strength
Journal Pre-proof (Eb) is increased to 140kV/cm. Such an increase of Eb could allow an improvement of the energy storage of the BT ceramics since the energy density (W) for a linear dielectric corresponds to W=1/20rEb2, where 0=8.85x1012 Fm-1 is the vacuum permittivity, and r is the relative dielectric permittivity. Therefore, it is possible to increase both constant dielectric and breakdown strength of BT ceramics by glass additives. However, it is worth noting that for a nonlinear dielectric material the charge and discharge processes lead to a loss of energy. Therefore, to achieve high energy storage of BT-based material it is necessary to consider its charge-discharge efficiency (η). The development of BT material with high ability energy storage demands to enhance both r, Eb and η parameters. In this perspective, different BTglass composites [6,9-12] are developed. These glass additives are within the B2O3–SiO2, BaO–B2O3–SiO2, Bi2O3–SiO2–B2O3-ZnO and BaO–Bi2O3–B2O3 systems. Effectively, the glass additives had improved the energy storage efficiency of BT ceramics along with the breakdown strength. However, in an opposite trend, the glass additive reduced the dielectric permittivity of the composites whatever the nature of the glass former oxide SiO2 or B2O3. According to our knowledge, a few scientific works have been doing on BT-phosphate composites [13,14]. Thus, the actual work is devoted to the investigation of new BT phosphate composites. The dielectric properties of the BT composites depend strongly on the nature and electrical behavior of the glasses. So, a selection strategy to develop phosphate glasses that should maintain as high as possible the dielectric constant of the phosphate-BT composites is performed. Firstly, we have investigated the glasses containing the same chemical elements as BT within the ternary BaO-TiO2-P2O5 system. Once it is done, we have studied their crystallization under heat treatments. Secondly, we have focused our interest to identify the chemical compositions permitting the formation of BaTiO3 phase in the glass-ceramics using XRD technique. We have found that the glass 20BaO-20TiO2-60P2O5 induces the formation
Journal Pre-proof of BT embedded in the glass-matrix. However, the amount of BT phase in this glass-ceramic is very low and thereby its dielectric properties are low too. In addition, this glass-ceramic is an electronic conductor which was the origin of the high dielectric loss in this material. To overcome this issue, we have substituted TiO2 by Bi2O3 and elaborated on new glasses inside the BaO-Bi2O3-P2O5 system. Among the glasses, it is found that the vitreous composition 10Bi2O3-10BaO-80P2O5 (BBP) exhibits higher dielectric properties than that of titaniumcontaining glasses. The present study deals with the investigation of the effects of this BBP glass on the structure, dielectric and energy storage properties of BT composites.
2. Experimental procedure BaTiO3 powder is prepared using the conventional solid-state method. Barium carbonate BaCO3 (99.9%, Aldrich) and titanium dioxide TiO2 (99.9%, Aldrich), are used as the starting materials. The precursors were weighed in a micro-analytical balance and mixed thoroughly according to the appropriate molar compositions in ethanol medium. The dried powders were then calcined at 1300°C in the air for 4 h. Phosphate glasses xBi2O3-(20-x)BaO-80P2O5, (x = 0, 10, 20 mol %), are prepared by the quenching-melting route. The raw powders containing different amounts of high-purity Barium carbonate BaCO3, bismuth trioxide Bi2O3, and di-hydrogen ammonium phosphate NH4H2PO4 purchased from Aldrich, were weighed, mixed, and melted in an Al2O3 crucible at 1050°C for crystallized h. The molten glasses are quenched in air and then milled to fine powders. According to their dielectric properties, we will consider only the glass (x=10) (BBP) in this study. The BT ceramics or composites are elaborated by mixing BT with the BBP glass powder. The ratios of BT and BBP in each sample are fixed by the chemical compositions: (100-x)BTxBBP (x=0, 2.5, 5, 7.5 wt%). In the following these ceramics are labeled (BT), (BT-x=2.5),
Journal Pre-proof (BT-x=5), and (BT-x=7.5). The (100-x)BT-xBBP powders are firstly granulated using the polyvinyl alcohol (PVA) as a binder (4 wt% PVA/96 wt% of powder). Then, pressed into cylindrical pellets (15 mm diameter and 1mm in thickness). Afterward, the pellets are heated at 450°C for 4 h for binder removal and then sintered at different temperatures for 2h in air. Density measurements are performed using the standard Archimedean method with water as a medium liquid. The structural approach of the sintered ceramic samples is studied by a DXR2 Raman spectrometer using laser excitation at 633 nm. The spectra were obtained in a back-scattering geometry in the wavenumbers 200-1400 cm-1 range. A Phillips D5000 X-ray diffractometer with Cu K radiation is used to identify the crystalline phases of the sintered ceramics. The XRD patterns are carried out with a step size of 0.04° within the range of 2θ from 15° to 70°. The microstructure of the sintered samples is evaluated by a scanning electron microscopy (MiniSEM Hirox SH-4000M). The dielectric properties of the composites are recorded by using a Solartron Modulab Xm MTS in the temperature range 300-573 K and at various frequencies from 100 Hz to 1 MHz. The P-E loops at room temperature of the composite samples are measured by a ferroelectric test system (aixACCT TF Analyzer 3000) at 1 kHz.
3. Results and discussions 3.1 Density measurements The density measurements are realized on BT-(x=2.5), BT-(x=5), and BT-(x=7.5) samples which have already submitted to sintering temperatures in the range 850–1000°C. The experimental density (or specific weight) of the pristine BaTiO3 ceramic is found to be 5.67 g/cm3 while that of BBP glass is 2.52 g/cm3. Fig. 1 shows the variation of the bulk density as a function of sintering temperature for all these BT-x ceramics. It is shown that the density values depend both on the glass content in the BT ceramics and the sintering
Journal Pre-proof temperatures. For a fixed sintering temperature, it decreases by increasing the glass ratio in the ceramics. This can be understood by taking into account that BaTiO3 (ρ=5.67 g/cm3) is denser than BBP glass (2.52 g/cm3). The higher glass content is the lower density of the composites. Moreover, this decrease may be attributed to the large difference in molecular weight between BaTiO3 (233.195 g/mol) and BBP glass (175.481 g/mol). Thus, the introduction of a lighter molecule in the structure instead of heavier decreases the density. From the analysis of Fig.1, it is also showed that whatever fixed glass content, the density varies non-linearly with temperature. A maximum of the density is obtained for a sintering temperature of 900°C. This latter could be considered as the suitable sintering temperature (SST) of the BT ceramics. It seems that at this temperature the glass behaves as a viscous medium and ensures good wetting of the BT grains [6], [13]. Such behavior of the glass at SST 900°C diminishes the porosity and thereby increases the density of the composites. This assumption will be confirmed by SEM micrographs in the 3.4 Section.
3.2 Raman spectroscopy Raman spectroscopy is used to study the network of the (1-x)BT-xBBP samples. This technique allows getting information about the structural units existing inside the BT-x ceramics and the collected spectra are shown in Fig. 2. The Raman spectra are given in the wavenumber range (200–1400 cm-1) where the vibration modes of barium titanate (BT) and phosphate glass BBP are active. Since most of the bands are broad in the BBP glass spectrum, with also some shoulders, we have performed its deconvolution spectrum using Peakfit V4.12 program, with a Gaussian function. This allowed us to better determine the sub-peaks. The BBP glass contains high content of the former P2O5 oxide and shows an O/P ratio equal to 2.75. Thus, one can consider it as an ultraphosphate glass. The Raman spectrum of BBP glass exhibits many broad peaks located at 1227 cm-1, 1164 cm-1, 1040 cm-1, 751 cm-1,
Journal Pre-proof 688 cm-1 and 440 cm-1. A Raman peak at 1227 cm-1 is attributed to the symmetric stretching of P=O in Q3 units. A peak near 1164 cm-1 is assigned to the symmetric stretching of (PO2)groups in Q2 units. A Raman peak around 1040 cm-1 is associated with the symmetric stretching of (PO3)2- in Q1 groups. Peaks around 751 cm-1 and 688 cm-1 are ascribed to the symmetric stretching of P-O-P in Q1 and Q2 structural units, respectively. A broad peak laying under 550 cm-1 is associated with the bending modes of the phosphate network [15-18]. A shoulder around 280 cm-1 is attributed to vibration mode of Bi-O bond in BiO3 pyramidal structural unit [20]. The appearance of this weak peak constitutes a fingerprint that bismuth oxide plays a glass former role in the vitreous material under study. The Raman spectrum of BT sample shows vibrational modes typical of BaTiO3 [1921]. The dominant vibrational modes are a peak at 254 cm-1 [A1 (TO)], a sharp peak at 301 cm-1 (B1 mode), a broad peak around 511 cm-1 [A1, E (TO)] and a weak peak at around 711 cm−1 [E (LO)] [24], In connection to titanium-oxygen linkages, this latter vibration mode is assigned to the asymmetric stretching of the Ti-O bonds. The mode at 511 cm-1 is due to the symmetric bending of the O-Ti-O bond. The mode observed at 301 cm-1 is due to Ti-O bonds in an off centric TiO6 octahedra as in the BaTiO3 ferroelectric phase [25]. The vibration mode observed at 254 cm-1 is assigned to the rotation of the TiO6 octahedra [24]. From all analysis of the BT Raman spectrum, one could associate the structure of BT compound to that of BaTiO3 with tetragonal type. Now after analyzing the Raman spectra of the BBP and BT components one can be able to extract from Fig. 2 relative structural information of the BT-x composites. It is evidenced that the addition of 2.5 to 7.5% of BBP glass to BT doesn’t induce any changes, either in the intensity or peak position, of the characteristic bands of the BT phase. Therefore, we can state that the tetragonal structure of BT is well-kept-up in the composites. This result does indicate that the chemical composition of the BBP glass prevents the degradation of the
Journal Pre-proof BT grains. However, it is worth noting that the structure of the BBP component undergoes a little transformation that is argued by the entrance of a peak at 940 cm-1 during the addition of BBP to BT composites. The intensity of this peak grows as the content of the glass goes up. It seems that this peak evolves when some phosphate groups are transformed into (Q1) or (Q0) structural units inside the network of BBP glass. The presence of some orthophosphate (Q0) units along with the BT phase in the structure of the composites is evidenced from the XRD study as will be seen in the next section.
3.3 XRD analysis Fig. 3 shows the XRD patterns of the BT-x ceramics sintered at 900°C. The XRD pattern of the sample BT corresponds to a pure BaTiO3 perovskite phase. All the XRD peaks are associated with BaTiO3 with a JCPDS file number 89-1428. The XRD pattern of the glass BBP indicates that is structureless. From the analysis of Fig. 3, it is obvious that the BT phase exists in the BT-x composites. All the XRD patterns are dominated by the BT phase since the content of the BBP glass is very low. However, when the BBP content increases from 2.5 to 7.5 wt%, one can note the emergence of a new crystalline Ba3(PO4)2 phase relatives to a JCPDS file number 80-1615. Owing to the fact that the peaks intensity of the BT phase remains unchanged in all the BT ceramics, we believe that the orthophosphate phase is formed essentially from structural rearrangements inside the BBP glass. In other words, the appearance of the Ba3(PO4)2 phase is due to the crystallization of the BBP glass during sintering of the BT-x ceramics. The formation of this barium phosphate phase suggests that barium and bismuth ions interact dissimilarly with phosphate P-O bonds. While the chemical interaction between Ba-P forms crystalline phase its homologous Bi-P results in the formation of amorphous phases.
Journal Pre-proof This is in agreement with the fact that the rest of the glass is rich in bismuth and phosphate since the crystallized phase doesn’t contain bismuth, and that the bismuth oxide is an incipient glass former which forms easily stable vitreous materials with P2O5 former. As discussed above in the Raman section, high polarizing Bi3+ cation exists in the network as BiO3 pyramidal structural unit suggesting the glass former role of Bi2O3. This glassy matrix accumulates at the grain boundaries of the BT phase and reduces the porosity of the BT-x composites as will be experienced from the following microstructure analysis. In addition, from the broadening of the diffraction peaks, the size of the crystallites of the BT phase was estimated, using the Scherrer formula (Equation 1): [26] 𝐾.𝜆
Dp = 𝐵 .cos 𝜃
(1)
Where Dp is the average crystallite size, K is the Scherrer constant, λ is the X-ray wavelength (1.5406 Å) and B is the Full Width at Half Maximum of XRD peak. The estimated crystallite size of pure BT sample was 33.02 nm, and it decreases slightly with the BBP glass addition to 32.25nm in BT-(x=2.5), 30.96nm in BT-(x=5) and 31.97nm in BT-(x=7.5). The estimated values of the average crystallite size are comparable to those found in the literature data [26,27]. From that, we can assume that the average crystallite size of the BT phase is reduced with the addition of BBP glass up to 5 mol%.
3.4. Microstructure analysis The scanning electron micrographs (SEM) of pure BT and sintered composites at 900°C, are shown in Figure 4(a)–(d). The SEM image of pure BT specimen reveals the presence of pores between the grains (Fig. 4a). This is because the grain boundaries of the BT particles usually migrate and lead to the growth of the grains. The beneficial influence of the BBP glass component on the microstructure of the composites is evidenced from Fig. 4b–c. Generally, one can see that the addition of BBP decreases the amount and the size of the
Journal Pre-proof pores. For instance, the addition of 2.5 wt% of the BBP glass component (Fig. 4b) reduces the interspace between the grains. A possible reason for this is the reduction of grain growth by the development of a thin barrier layer among the grains in agreement with the literature data [28]. However, some pores still exist since 2.5 wt% of BBP cannot fully fill the space between the grains. The composite with 5 wt% BBP (Fig. 4c) shows a uniform small grains microstructure that is sufficient to meet the requirements of dense fine-grained ceramics. The pores are almost completely disappeared and the BT composite is visibly homogeneous compared to the BT-(x=2.5) material. However, when the glass addition reaches 7.5 wt %, a reverse trend is observed since this glass amount raises the voids and cracks (Fig. 4d). From the above results, we ascertain that the BBP glass addition up to 5% has two beneficial effects on the microstructure of BT ceramics. Firstly, the grain size of the BT phase is reduced and distributed uniformly. Secondly, the size of pores is decreased. These effects could enhance the dielectric properties of the BT-x composites.
3.5. Dielectric properties Fig. 5 shows the frequency dependence of the relative permittivity εr (dielectric constant) and the dielectric losses Tan δ at room temperature within the frequency range from 100Hz to 1MHz of the composite (100-x)BT-xBBP. The dielectric constant of the pure BT ceramic is about 1450 while its dielectric loss is 0.04 at high frequencies; these values increase fastly when the frequencies decrease less than 1 kHz. The dielectric properties at low frequency are associated with many factors mainly the sample-electrode polarization, the defaults and the fluctuation of the composition [29]. At high frequencies, it is observed that the addition of BBP glass to pure BT has a positive effect on the dielectric properties. The addition of 2.5 mol % of BBP induces a slight increase and decrease in permittivity and dielectric loss, respectively. The same behavior is observed with the addition of 5 mol% of
Journal Pre-proof BBP. However, with a further addition of BBP glass (7.5 mol %) to pure BT the dielectric constant decreases. Therefore, the dielectric properties of the composites are compositiondependent and the material BT-(x=5) shows the greatest permittivity 1700 and relatively low loss of the order 0.012 over a large frequency domain. The observed decrease of the permittivity when the content of the glass BBP is high (7.5 mol %) could be due to the microstructure of the composite (Fig. 4d) which shows an excess of the glassy phase and an increase of the porosity. Fig. 6 presents the temperature dependence of εr and Tan δ at 1MHz in the temperature range from 30 to 300 °C of the composites (100-x)BT-xBBP. The values of the main dielectric parameters of the studied specimens are listed in Table 1. At room temperature, the permittivity of the composite depends on the composition and varies from 1021 to 1563; the dielectric loss is very low and varies from 0.011 to 0.037. For a fixed temperature, the permittivity increases first and then decreases with increasing BBP glass content. The BT(x=5) composite shows a high dielectric constant and reaches a maximum value εm 2236 at 136 °C. According to SEM images (Fig. 4), we can assume that the addition of BBP glass up to 5% improves the dielectric properties by clamping the BT particles, blocking the easy conduction channels and reducing the leakage current at grain boundaries. On the other hand, by increasing the BBP glass ratio to 7.5%, the dielectric constant decreases rapidly. This may be due either to the porous microstructure of BT-(x=7.5) (Fig. 4d) or to the appearance of a significant content of the secondary phase Ba3(PO4)2 (Fig. 3). Furthermore, one can note that the temperature dependence of the permittivity shows a large peak around 120-140 °C for all the composites. This peak is related to the BT phase in agreement with the fact that it exhibited a phase transition from ferroelectric to paraelectric phase [30]. The dielectric constant increases with increasing the temperature until the transition temperature and then decreases above 140 °C because of the disappearance of the
Journal Pre-proof ferroelectric domains. No diffusion or displacement of the Curie temperature Tc is observed for all the composites suggesting that the BaTiO3 phase keeps its normal structure despite the addition of glass.
3.6 Energy storage performance: The polarization hysteresis loop was measured by an Aixacct TF analyzer 3000. Unfortunately, the voltage amplitude is limited in this device, and the maximum electrical field which can be applied to the material is 15 kV/cm. This limited voltage prevents us from reaching the electrical breakdown field, which implies that the measured energy density is much lower than the real one. Moreover, the energy storage density is also underestimated by the fact that the hysteresis loop is measured at high frequency 5 kHz, which excludes partially Maxwell-Wagner-Sillars polarization detected at low frequency. However, the measured energy under these experimental conditions is very useful for many applications in high frequency especially in electronic. The P–E hysteresis loops of the composite (100-x)BT-xBBP are plotted in Fig. 7, which are measured at 15 kV/cm and 5 kHz. Unfortunately due to the limitation in the voltage amplitude in the measuring device, the hysteresis loops of the composites show a lossy behavior which is due to the leakage current present in the samples. The origin of this leakage current is coming from defects, which caused a hopping phenomenon [31]. Moreover, it can be seen that pure BT shows a decent hysteresis loop, giving a maximum polarization (Pm) of 0.53μC/cm2 and a remnant polarization (Pr) of 0.06μC/cm2 at 15 kV/cm. One can note that the shape of the P-E loops depends on the glass content in the composite. For instance, with increasing the content of BBP glass, the P-E loops became slimmer and the maximum polarization of the composites BT-(x=2.5) and BT-(x=5) are larger than that of pure BT. Also,
Journal Pre-proof it is observed that the remnant polarization of the material BT-(x=5) is slightly lower than that of pure BT whereas that of the BT-(x=7.5) is increased. Therefore, it is noticed that the composite BT-(x=2.5) exhibits the largest Pmax, while the higher dielectric constant is observed in BT-(x=5), this is maybe due either to a change in the polarization mechanism upon the composition and/or to the different nature of excitation between Solartron and aixACCT. Generally, the energy storage properties of the dielectric materials are characterized by the four following parameters: energy storage density, recoverable energy density (Wrec), energy loss density (Wloss), and energy storage efficiency (η) [32], [33]. The energy storage density is extracted from the P-E loops and obtained according to the following equation: (2)
𝑊 = ∫𝐸𝑑𝑃
The recoverable energy storage density (Wrec, equation 3) is associated with the integrated area between the polarization axis and the discharged curve; by integrating the area between charge and discharge curves one can obtain the loss energy (Wloss, equation 4). The energy storage efficiency (η) is evaluated through equation 5. These parameters are computed by using following equation [29,30]: 𝑃𝑚𝑎𝑥
𝑊𝑟𝑒𝑐 = ∫𝑃𝑟
(3)
𝐸𝑑𝑃
𝑃
(4)
𝑊𝑙𝑜𝑠𝑠 = ∫0 𝑚𝑎𝑥𝐸𝑑𝑃 ― 𝑊𝑟𝑒𝑐 Wrec
𝜂 = Wrec + Wloss x 100
(5)
Pr is the remnant polarization, Pmax is the maximum polarization of the maximum electrical field (the saturation polarization) and E is the applied electric field. The calculated parameters from P-E loops are listed in Table 2, and the glass content dependence of the recoverable energy (Wrec), the energy loss density (Wloss) and the efficiency (η) is shown in Fig. 8.
Journal Pre-proof From the P-E hysteresis loops, it is seen that the BT-(x=2.5) composite exhibits the highest recoverable energy storage density (Wrec) suggesting that the addition of the BBP glass increases the recovery energy of BT ceramic. Moreover, the energy loss of the BT(x=2.5) is almost comparable to that of pure BT so that there is a small improvement in energy storage efficiency of BT-(x=2.5). With the further addition of BBP up to 7.5%, one can note that the energy density decreases while the efficiency of storage reached its maximum value of 80.2% for the material BT-(x=5). The improvement of the energy density for the composite BT-(x=2.5) can be explained by taking into account the microstructure and SEM image (Fig. 4) that revealed reducing of grain size and decreasing amount pores. This reduces the free-charge trapping by the grain boundary and pore, this free charge is responsible for screening the ferroelectric polarization and the increasing of the fatigue phenomena in the materials. On the other hand, when the content of the glass is high the polarization hysteresis loop is affected, reducing, by the way, the value of the remnant and saturation polarization causing the decrease the energy storage density. Moreover, too much BBP coating also would destroy the core-shell structure in the process of sintering, leading to grain growth and the formation of secondary phases. For that, it is important to control the composition in optimizing the overall properties.
Conclusion: Bi2O3-BaO-P2O5 glass was prepared and used as a sintering aid and a coating agent for BaTiO3 ceramics. It had a considerable influence on the sintering temperature, microstructure, dielectric and energy storage properties of BT ceramics. Among all the studied specimens, the composite with a 5% addition of BBP glass had the highest permittivity, with relatively low dielectric losses, less than 0.02, and it showed an improvement in energy storage efficiency compared to pure BT ceramics. These results are mainly due to the positive effect of the glass
Journal Pre-proof on the microstructure since 5% of BBP glass reduces the porosity and the existed defects on the pure BT ceramic. It can, therefore, be concluded that this composite is a promising dielectric candidate for energy storage capacitor ceramics. Another benefit influence of the glass in the BT-glass composite is its ability to prevent the growth of the BT ceramics. Thus, the existing small grain size of BT presented the synergetic effect with the vitreous network of BBP to enhance the dielectric properties of the glass-ceramics.
Acknowledgments: The authors are grateful to the CNRST, OCP foundation for their financial support in the framework of around phosphates project.
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DECLARATION OF INTEREST STATEMENT
We state that: 1. The article is original. 2. The article has been written by the stated authors who are ALL aware of its content and approve its submission. 3.The article has not been published previously 4. the article is not unde consideration for publication elsewhere 5. No conflict of interest exists, or if such conflict exists , the exact nature must be declared. 6. If accepted, the article will not be published elsewhere in the same form, in any language, without the written concent of the publisher. All the best regards, Dr Lahcen BIH
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Fig. 1. Bulk density of the (100-x)BT-xBBP ceramics as a function of the sintering temperature.
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Fig. 2. Raman spectra of the BBP glass, the pristine BT and the composites (100-x)BT-xBBP.
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Fig. 3. XRD patterns of the composites (100-x)BT-xBBP.
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Fig. 4. SEM micrographs of the polished surface of the composites (100-x)BT-xBBP; (a) 0 wt%, (b) 2.5 wt%, (c) 5 wt% and (d) 7.5 wt%.
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Fig. 5. Frequency dependence of (a) the permittivity (εr) and (b) the dielectric loss (Tanδ) at RT of the composites (100-x)BT-xBBP.
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Fig. 6. Temperature dependence of (a) the permittivity (εr) and (b) the dielectric loss (Tanδ) at 1MHz of the composites (100-x)BT-xBBP.
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Fig. 7. P–E hysteresis loops of the composites (100-x)BT-xBBP at an electric field of 15 kV/cm.
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Fig. 8. Variation of Wrec, Wloss, and η of the composites (100-x)BT-xBBP at electric field of 15kV/cm.
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Highlights 1- Synthesis of the glass-ceramics BaO-Bi2O3-P2O5/BaTiO3 2- Investigation of the structure by Raman spectroscopy 3- Studies of the dielectric properties. 4- Investigation of their P-E loops and energy storage ability.
Journal Pre-proof Table 1: Main properties dielectric of the composites (100-x)BT-xBBP at 1MHz.
εr (RT)
Tan δ (RT)
εm
BT
1425.48
0.037
1800.58
BT-(x=2.5)
1490.05
0.0142
1895.38
BT-(x=5)
1563.30
0.0126
2236.37
BT-(x=7.5)
1021.67
0.0112
1504.16
Table 2: Polarization and energy storage properties of the composites (100-x)BT-xBBP at electric field of 15 kV/cm. Pmax μC/cm2
Pr μC/cm2
W rec
(mJ/cm )
(mJ/cm )
BT
0.534
0.069
3.6
1.6
69.21
BT-(x=2.5)
1.075
0.108
6.9
2.6
72.58
BT-(x=5)
0.651
0.050
4.8
1.2
80.20
BT-(x=7.5)
0.505
0.158
2.3
3.1
42.27
3
W loss
η (%) 3