- Email: [email protected]
Effect of K2O addition on glass structure, complex impedance and energy storage density of NaNbO3 based glass-ceramics
Journal of Alloys and Compounds 785 (2019) 350e355
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Effect of K2O addition on glass structure, complex impedance and energy storage density of NaNbO3 based glass-ceramics Xin Peng, Yongping Pu*, Xinyi Du School of Material Science and Engineering, Shaanxi University of Science and Technology, Xi'an 710021, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 7 November 2018 Received in revised form 9 January 2019 Accepted 16 January 2019 Available online 19 January 2019
(40-x)Na2O-xK2O-40Nb2O5-20SiO2 (x ¼ 0, 5, 10, 15 mol%) glass-ceramics are synthesized by traditional melts method. The glass-ceramics are tested by X-ray diffraction (XRD) techniques, and NaNbO3 as major phase led a high permittivity. A microstructure with nanoscale grains enclosed by glass phase is observed by scanning electron microscope (SEM). With the increase of content of K2O, a relaxed glass network structure is obtained, and more kinds of phase are formed. Permittivity comes to 174 approximately when x ¼ 5 mol%. In addition, the activation energy (Ea) of residual glass phase for Na2O-K2O-Nb2O5-SiO2 glass-ceramics firstly increase then decrease. Breakdown strength (BDS) of all samples increase and then decrease with the increase of content of K2O, and maximum BDS is obtained when x ¼ 10 mol%. And maximum theoretical energy density is 1.43J/cm3 when x ¼ 5 mol%. © 2019 Elsevier B.V. All rights reserved.
Keywords: Na2O-K2O-Nb2O5-SiO2 glass-ceramics Breakdown strength Glass network structure
1. Introduction In recent years, pulse power devices have been widely used in military and auto industry. So the material, possessed of high energy storage density, which can charge and discharge in short time is urgently demanded [1,2]. Energy storage materials can be divided into four categories in energy-storage behaviors, antiferroelectrics, dielectric glass-ceramics, relaxor ferroelectrics and polymer-based ferroelectrics [3,4]. Antiferroelectrics possess high permittivity because a phase transformation is achieved between antiferroelectrics and ferroelectrics when a high external electric field is applied. Thus, a high energy storage can achieve for antiferroelectrics if a high breakdown strength (BDS) is obtained [5,6]. Dielectric glass-ceramics, a composite, possess high BDS because it has low porosity and residual glass phase has high compact glass network structure [7]. And dielectric phase, encircled by residual glass phase, has a high permittivity in dielectric glass-ceramics. The energy storage density of dielectric glass-ceramics can be calculated by the formula (ε0εrE2b)/2, which is proportional to dielectric constant and the square of BDS [2]. So it is more efficient to obtain a high energy storage that BDS is improved. Dielectric glass-ceramics are mainly divided into niobate-based and titanate-based glass-ceramics. Vacancy, leading to lower BDS,
* Corresponding author. E-mail address: [email protected] (Y. Pu). https://doi.org/10.1016/j.jallcom.2019.01.201 0925-8388/© 2019 Elsevier B.V. All rights reserved.
can be formed due to Ti4þ during the heat treatment in titanatebased glass-ceramics. So Many researcher are attracted by the niobate-based glass-ceramics [8]. PbO-SrO-Na2O-Nb2O5-SiO2 glassceramics system was investigated, and permittivity came to 600 approximately [9]. But lead-free material is demanded for ecological security. NaNbO3 was reported in several glasseceramics systems [7,10,11], which leads a high permittivity. In addition, interfacial polarization which is caused by different conductivity between glass and crystal phase, reported by the US Naval Research Laboratory [12], reduces the energy storage density seriously. What is more, BDS is mattered by glass network structure. Wang et al. [13] had the view that K2O can break the Si-O bond of glass network and create non-bridging oxygen ions in K2O-BaO-Nb2O5-SiO2 glassceramics, which changed the structure of glass network. The structure of glass network can be changed by alkali metal oxide as glass network modifier, which affect electrical properties. In recent investigations by researchers, the relation between BDS and interfacial polarization was studied in many paper. However, few work was investigated to the relation between the structure of glass network and BDS. In this study, Na2O-K2O-Nb2O5-SiO2 glasseceramics system was investigated. To obtain high permittivity and BDS, K2O was added to Na2O-K2O-Nb2O5-SiO2 glasseceramics. The structure of glassceramics was determined by Raman and differential thermal analysis. Dielectric properties and BDS was tested and theoretical energy density were calculated.
X. Peng et al. / Journal of Alloys and Compounds 785 (2019) 350e355
351
2. Experimental procedure Na2O-K2O-Nb2O5-SiO2 glass-ceramics were prepared through a traditional meltquenching method by melting raw material. Analytical purity powders of Na2CO3, K2CO3, Nb2O5, SiO2, were ball mixed for 8 h with distilled water as milling media for homogenous mixing. The prepared nominal composition of samples are given in Table 1 with corresponding sample names. The well-mixed reagent powders were put into an alumina crucible at 1100 C and melted for 1 h at 1450 C to form molten glass, which was cast into a preheated copper mold to form transparent glass and quickly placed in an annealing oven at 600 C for 10 h to reduce the residual stress. All samples with various composition was crystallized at different temperature for 4 h are shown in Table 1. All samples were cut and polished into sheets of 0.16e0.26 mm in thickness and were coated with silver paste electrode on both sides. The structure of glass-ceramics were confirmed by Raman spectroscopy (Renishaw-invia, Renishaw, U. K.) in the region of 1003500 cm1 at room temperature. The glass crystallization temperature was determined by using DSC (Model STA 409Pc, Netzsch, Germany, with a rate of 10 C min-1). The phase structure were analyzed by X-ray diffraction (D-MAX 2200pc, Rigaku Co, Tokyo, Japan) at room temperature to investigate the phase evolution. The microstructure observation of those crystallized samples were carried out using a scanning electron microscope (SEM; Model: JSM-5610LV, JEOL). For electrical measurements, these samples were polished to achieve parallel, smooth faces, and silver electrodes were coated on both faces. Then painting samples with silver pasted on both sides were treated at 600 C for 10 min to prepare electrodes for dielectric measurements. The measurements of permittivity of samples were performed by using a precision multifunction LCR meter (E4980A, Agilent Tech, CA, U.S.) with the measuring frequency of 100 Hz to 1 MHz. The samples were used to measure the dielectric breakdown strength with a high-voltage source (Model 610E) using a voltage ramping rate of about 1 kV/s at room temperature until dielectric breakdown, and at least 10 samples were used for each composition during BDS testing. The polarization-electric (P-E) loops were measured using a ferroelectric tester (TF Analyzer 2000, aixACCT, Aachen, Germany) at room temperature. Theoretical energy density of the glass-ceramics were calculated by (ε0εrE2b)/2. 3. Results and discussion 3.1. The analysis of vitrified structure 3.1.1. Thermal analysis of vitrified bonds In Fig. 1, DSC curves of all samples are showed. It is obvious that glass transition temperatures (Tg) reduce with the increase of content of Kþ from all the curves. The field strength parameter (Fs) defined by Dietzel of Naþ is larger than Kþ. The equation, Fs ¼ Z/d2, represented the ability to free O2, where Z stands for the cation charge, d denotes the mean M-O distance of oxide MaOb (M is glass network modifier, a and b are different constants) [14]. With the increase of content of Kþ, as a result, the degree of polymerization (DOP) of glass reduced gradually. Therefore, a higher concentration
Table 1 Samples with different content of K2O were crystallized at 950 C for 4 h. Sample
Crystallization system
Nominal composition(mol%)
40N 35N5K 30N10K 25N15K
950 C,4 h / / /
40Na2O-40Nb2O5-20SiO2 35Na2O-5K2O-40Nb2O5-20SiO2 30Na2O-10K2O-40Nb2O5-20SiO2 25Na2O-15K2O-40Nb2O5-20SiO2
Fig. 1. DSC curves of glass powders with different content of Kþ.
of K2O lead a more relaxed glass structure, which is caused to more free oxygen, and a smaller glass transition temperature (Tg) is obtained. In addition, the onset crystalline temperatures (Tc) show an obvious change with the increase of content of Kþ. Firstly, Tc have a slightly shift to the low temperature, which is attributed to the decreasing degree of polymerization of the glass network structure. Furthermore, more exothermic peaks (Tp) around 900 C appeared when x ¼ 15 mol, which indicated that more types of crystalline phases are generated. 3.1.2. Raman analysis of vitrified bonds As seen in Fig. 2(a), Raman spectrum is used to study the changing structure of the glass-ceramics. The signals are at three peaks of 257 cm1, 610 cm1, 770 cm1 and 940 cm1. The peak at 257 cm1 is attributed to the triple degenerate symmetric stretching vibration of OeNbeO bonds in NbO6 octahedron. The peak position at 257 cm1 shifts towards lower wavenumber at 265 cm1 with the increase of content of K2O. The peak at 610 cm1 is attributed to the double degenerate symmetric stretching vibration of OeNbeO bonds in NbO6 octahedron [21]. Meanwhile, the peaks in the range of 500e700 cm1 is due to the symmetric stretching vibration of Si]O. And the peaks around 770 and 940 cm1 are attributed to Qn. Generally, the peaks of Qn are in the 760-1200 cm1, and Q0, Q1, Q2, Q3, Q4 are at 760e850 cm1, 880e960 cm1, 940e1090 cm1, 1000e1190 cm1, 1080e1200 cm1, respectively [13,15]. The DOP is decreased due to increasing Q0 with the increase of content of Kþ. Free oxygen and non-bridging oxygen could be generated due to cations with low electric fields, and the DOP decreased [17e19]. Q0 increases, so Tg reduced. In Fig. 2(b), a simulated glass structure for K2O-Na2ONb2O5-SiO2 glass-ceramics is shown. 3.2. Phase structure and microstructure 3.2.1. Phase structure The XRD spectra of all glass ceramics samples are shown in Fig. 3(a). From the patterns, crystalline phase K0.1Na0.9NbO3 (PDF# 77-2037) appears with the increase of content of Kþ, and major phase NaNbO3 (PDF# 75-2102) appears for all samples. The phase fraction can be calculated by XRD peak intensity defined by Rietveld analysis in some studies [7,16]. The values of phase fractional P P P P P ratio are calculated by FA ¼ IA/( IAþ IBþ IC…þ IN), where FA
352
X. Peng et al. / Journal of Alloys and Compounds 785 (2019) 350e355
Fig. 2. (a) Raman spectra of all samples and (b) simulated glass structure for K2O-Na2O-Nb2O5-SiO2 glass-ceramics.
Fig. 3. (a) XRD patterns and (b) the ratio of each phase peak intensity for K2O-Na2O-Nb2O5-SiO2 glass-ceramics.
P represent phase fractional ratio, I denotes the total of XRD peak intensity of relevant phase. So the fractional ratio of relevant phases are illustrated in Fig. 3(b). The results show that the content of K0.1Na0.9NbO3 phase increases gradually while NaNbO3 phase decreases with the increasing Kþ, and non-perovskite phase appears. In Fig. 1, three peaks appeared around 900 C, which were attributed to non-perovskite phase. 3.2.2. Microstructure In Fig. 4, all glass-ceramics samples are investigated for microstructure at 950 C for 4 h with the varying Kþ. All images show that all the samples possesses nanoscale grains and lower porosity. In Fig. 4(a), the microstructure of tiny grains are uniform, which are caused by more pure phase NaNbO3. With the increase of content of Kþ, heterogeneous microstructure is formed by generated K0.1Na0.9NbO3 and non-perovskite phase for Fig. 4(b)-(c). In Fig. 4(d), microstructure is different with the former, and the increase of non-perovskite phase lead to a nonuniform microstructure. 3.3. Dielectric and ferroelectric properties Fig. 5 shows the frequency dependence of permittivity and dielectric loss characteristics of the glass-ceramics samples with varied K2O content over the frequency range of 20 Hz to 2 MHz at room temperature. The permittivity of all samples show a stable value with increase of frequency, which showed excellent frequency stability. The permittivity firstly increases and then
decreases with the increase of content of Kþ, which is attributed to the increase of intensity of crystallinity of the glass-ceramics and the generation of non-perovskite phase. And tiny peak at low frequency reveals that interfacial polarization is produced. The movement of glass network modifier (alkali metal cation) produces energy loss when AC is applied, which causes dielectric loss. A relaxed glass network facilitates the movement of glass network modifier, and an alkali metal cation with smaller radius is easily to move. So a high dielectric loss is obtained when x ¼ 0,15 mol% at low frequency. But there is not enough time for alkali metal cation to move at high frequency. Dielectric loss of all samples are approximative because of the same content of alkali metal cation at high frequency. P-E loops of glass-ceramics with varied K2O content is shown in Fig. 6. From the curves, a linear relationship between P and E is shown in all samples, which testifies that all the glass-ceramics samples are linear dielectric. Maximum polarization firstly increases then decreases when an equal external electric field is applied. A high polarization is obtained because of generating of phase NaNbO3 and K0.1Na0.9NbO3, lower polarization is attributed to non-perovskite phase. 3.4. Complex impedance spectroscopy The complex impedance spectroscopy and equivalent circuit of glass-ceramics with varied K2O content are shown in Fig. 7. Generally, glass-ceramics could be thought as a composite that numerous grains generated in host glass. All nanoscale grains are
X. Peng et al. / Journal of Alloys and Compounds 785 (2019) 350e355
353
Fig. 4. SEM images of K2O-Na2O-Nb2O5-SiO2 glass-ceramics with different content of Kþ: (a) 40N (b) 35N5K (c) 30N10K (d) 25N15K.
Fig. 6. PeE hysteresis loops of K2O-Na2O-Nb2O5-SiO2 glass-ceramics. Fig. 5. Permittivity and dielectric loss of K2O-Na2O-Nb2O5-SiO2 glass-ceramics with different content of Kþ.
surrounded by the residual glass phase in Fig. 4. So large different electric properties appear between the residual glass and ceramic phase. Complex impedance analysis is employed to study the effect of varied content of K2O on electric properties of glass-ceramics. ZView software is employed to calculate the resistance of grain and glass phase based on R-C equivalent. In Fig. 7(a), complex impedance spectra is measured at 500 C for all samples. The resistance of glass-ceramics increase with the increase of content of K2O. Fig. 7(b) shows the complex impedance spectra of 40N from 420 C to 500 C. And the intercept of each circular arc on the real axis decreases, which illustrates that the negative temperature coefficient of resistance behavior for glass-ceramics. The resistance of the residual glass phase, generally, is much larger than ceramic phase, which is fitted for 30N10 K at 500 C in Fig. 7(c). And an equivalent circuit is simulated to glass-ceramics in inset of Fig. 7(c). Rc is
resistance of ceramic phase, and Rr is resistance of residual glass phase. From low frequency to high frequency, residual glass phase is reacted first because of its large resistance. And ceramic phase is reacted at high frequency. In Fig. 7(d), ceramic phase is fitted in high frequency for 30N10K. To study electrical transport behaviors, the activation energy(Ea) of the residual glass and ceramic phase is calculated. The temperature dependence of the resistivity r can be expressed by an Arrhenius equation: r ¼ r0exp(Ea/kBT), where r0 is the pre-exponential factor, Ea is the activation energy, kB is the Boltzmann constant, and T is the absolute temperature. The Ea of the residual glass and ceramic phase of glass-ceramics are calculated from the slope of the function between resistivity and temperature, which are shown in Fig. 8(a) and (b). The Ea of the residual glass phase are larger than ceramic phase, which indicates that carrier migration needed more energy in the residual glass phase. To study the effect on carrier migration for different content of K2O,
354
X. Peng et al. / Journal of Alloys and Compounds 785 (2019) 350e355
Fig. 7. Complex impedance spectra of (a) all samples measured at 500 C (b) 40N measured at different temperature (The inset displays the data of 40N measured at high frequency) (c) 30N10K measured and fitted @500 C (The inset displays the equivalent circuit) (d) 30N10K measured and fitted @500 C at high frequency.
Fig. 8. Arrhenius fit to resistence data of (a) residual glass phase and (b) ceramic phase of all the K2O-Na2O-Nb2O5-SiO2 glass-ceramics samples. The inset displays activation energy of (a) residual glass phase and (b) ceramic phase.
the difference of activation energy(DEa) of the residual glass and ceramic phase are calculated. In Na2O-K2O-Nb2O5-SiO2 glassceramics system, high DEa is obtained when x ¼ 10 mol, which revealed that a difficult migration exist in glass-ceramics for carriers [20]. Generally, resistivity is increased when an alkali metal cation with a smaller radius is replaced by one with a larger radius in glass network, which is called by mixed alkali effect. After heat treatment, the residual glass phase in glass ceramics retains this effect. In Na2O-K2O-Nb2O5-SiO2 glass-ceramics system, uncrystallized Kþ with a larger radius can impede charge carrier movement in the residual glass phase, which cause a high BDS. However, BDS decreases when the content of Kþ comes to 15 mol%, which may be caused by high dielectric loss and a nonuniform microstructure. 3.5. Energy storage properties Energy storage of glass-ceramics are calculated, and the relation
of permittivity, BDS and energy storage are shown in Fig. 9. In Na2O-K2O-Nb2O5-SiO2 glass-ceramics system, a high permittivity is produced by antiferroelectric phase NaNbO3 and ferroelectric phase K0.1Na0.9NbO3. What is more, a relaxed glass structure is induced by Kþ, which leads to devitrification easily. So a maximal permittivity is achieved when x ¼ 5 mol%. However, non-perovskite phase, meanwhile, is formed due to relaxed glass structure, which reduces permittivity. In addition, BDS is influenced by the content of K2O. In Fig. 9(a), DEa have maximum value when x ¼ 10 mol%, and BDS had a similar trend. The difficulty level of migration for carriers could be according to DEa in the residual glass phase. Kþ with a larger radius impedes charge carrier movement, and a high BDS is obtained. However, high dielectric loss and a nonuniform microstructure cause a decrease in BDS when the content of Kþ comes to 15 mol%. For energy storage properties, a maximum value is obtained when x ¼ 5 mol%, which is attributed to high permittivity and BDS simultaneously. In Na2O-K2O-Nb2O5-SiO2 glass-ceramics
X. Peng et al. / Journal of Alloys and Compounds 785 (2019) 350e355
355
Fig. 9. (a) DEa and breakdown strength (b) permittivity, maximum breakdown strength and theoretical energy density for K2O-Na2O-Nb2O5-SiO2 glass-ceramics.
system, energy storage decreases when the content of K2O is too high. 4. Conclusions With the increase of content of K2O, Tg is decreased, which reveals that a relaxed glass network structure is obtained, and DOP is reduced due to the formation of non-bridging oxygen. So more kinds of phase can form because of a relaxed glass network structure, and permittivity is increased when x ¼ 5 mol%. But nonperovskite phase leads a decrease in permittivity when x ¼ 15 mol %. Meanwhile, with the increase of content of K2O, DEa increases, so BDS increases. However, when the content of Kþ comes to 15 mol%, high dielectric loss and a nonuniform microstructure cause a decrease in BDS. Acknowledgments This work was financed by the National Natural Science Foundation of China (51872175), the International Cooperation Projects of Shaanxi Province (2018 KW-027). References [1] C. Liu, F. Li, et al., Advanced materials for energy storage, Adv. Mater. 22 (8) (2010) E28eE62. [2] E.P. Gorzkowski, M.J. Pan, et al., Glass-ceramics of barium strontium titanate for high energy density capacitors, J. Electroceram. 18 (3e4) (2007) 269e276. [3] X. Hao, A review on the dielectric materials for high energy-storage application, J. Adv. Dielectr. 03 (01) (2013) 1330001. [4] Lei Zhang, Yongping Pu, Min Chen, Influence of BaZrO3 additive on the energy-storage properties of 0.775Na0.5Bi0.5TiO3-0.225BaSnO3 relaxor ferroelectrics, J. Alloys Compd. 775 (2019) 342e347. [5] Z. Liu, T. Lu, et al., Antiferroelectrics for energy storage applications: a review, Adv. Mater. Technol. 3 (9) (2018) 1800111. [6] K. Yang, J. Liu, et al., Effects of TiO2 addition on dielectric and energy storage properties of BaO-K2O-Nb2O5-SiO2 glass ceramics, Ceram. Int. 44 (6) (2018)
6181e6185. [7] Y. Zhou, Y. Qiao, et al., Improvement in structural, dielectric and energystorage properties of lead-free niobate glass-ceramic with Sm2O3, J. Eur. Ceram. Soc. 37 (3) (2017) 995e999. [8] H. Wang, J. Liu, et al., Ultra high energy-storage density in the barium potassium niobate-based glass-ceramics for energy-storage applications, J. Am. Ceram. Soc. 99 (9) (2016) 2909e2912. [9] D.F. Han, Q.M. Zhang, et al., Optimization of energy storage density in ANb2O6-NaNbO3-SiO2 (A¼[(1x)Pb, xSr]) nanostructured glasseceramic dielectrics, Ceram. Int. 38 (8) (2012) 6903e6906. [10] Y. Zhou, Q. Zhang, et al., Structural and dielectric characterization of Gd2O3added BaOeNa2OeNb2O5eSiO2 glasseceramic composites, Scripta Mater. 65 (4) (2011) 296e299. [11] C. Li, Q. Zhang, et al., Dielectric and energy storage properties of BaO-SrONa2O-Nb2O5-SiO2 glasseceramics with different crystallization times, J. Electron. Mater. 45 (6) (2016) 3025e3029. [12] M.J. Pan, E.P. Gorzkowski, et al., The effect of interfacial polarization on the energy density of ferroelectric glass-ceramics, IEEE Int. Symp. Appl. Ferroelectrics (2006), https://doi.org/10.1109/isaf.2006.4387824. [13] H. Wang, J. Liu, et al., Effect of K2O content on breakdown strength and energy-storage density in K2O-BaO-Nb2O5-SiO2 glass-ceramics, Ceram. Int. 43 (5) (2017) 4183e4187. [14] J. Shi, F. He, et al., Effects of Na2O/BaO ratio on the structure and the physical properties of low-temperature glass-ceramic vitrified bonds, Ceram. Int. 44 (9) (2018) 10871e10877. [15] Z. Wang, S. Cai, et al., Structural investigation of phosphorus in CaO-SiO2P2O5 ternary glass, Metall. Mater. Trans. B 48 (2) (2017) 1139e1148. [16] C.-W. Ahn, C.-S. Park, et al., Correlation between phase transitions and piezoelectric properties in lead-free (K, Na, Li)NbO3eBaTiO3Ceramics, Jpn. J. Appl. Phys. 47 (12) (2008) 8880e8883. [17] Y. Gao, Y. Hu, et al., Effect of glass network modifier R2O (R¼Li, Na and K) on upconversion luminescence in Er3þ/Yb3þ co-doped NaYF4 oxyfluoride glassceramics, J. Rare Earths 33 (8) (2015) 830e836. [18] A. Cesaratto, P. Sichel, et al., Characterization of archeological glasses by micro-Raman spectroscopy, J. Raman Spectrosc. 41 (12) (2010) 1682e1687. [19] K. Kioka, T. Honma, et al., Fabrication of (K, Na)NbO3 glasseceramics and crystal line patterning on glass surface, Opt. Mater. 33 (8) (2011) 1203e1209. [20] Q. Yuan, et al., Microstructure and dielectric properties of Ti0.995(In0.5Nb0.5) 0.005O2/SrO-B2B3-SiO2 glass-ceramics for energy storage, IEEE Trans. Dielectr. Electr. Insul. 24 (2) (2016) 712e719. [21] S. Dwivedi, T. Pareek, S. Kumar, Structure, dielectric, and piezoelectric properties of K0.5Na0.5NbO3-based lead-free ceramics, RSC Adv. 8 (43) (2018) 24286e24296.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Effect of K2O addition on glass structure, complex impedance and energy storage density of NaNbO3 based glass-ceramics Xin Peng, Yongping Pu*, Xinyi Du School of Material Science and Engineering, Shaanxi University of Science and Technology, Xi'an 710021, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 7 November 2018 Received in revised form 9 January 2019 Accepted 16 January 2019 Available online 19 January 2019
(40-x)Na2O-xK2O-40Nb2O5-20SiO2 (x ¼ 0, 5, 10, 15 mol%) glass-ceramics are synthesized by traditional melts method. The glass-ceramics are tested by X-ray diffraction (XRD) techniques, and NaNbO3 as major phase led a high permittivity. A microstructure with nanoscale grains enclosed by glass phase is observed by scanning electron microscope (SEM). With the increase of content of K2O, a relaxed glass network structure is obtained, and more kinds of phase are formed. Permittivity comes to 174 approximately when x ¼ 5 mol%. In addition, the activation energy (Ea) of residual glass phase for Na2O-K2O-Nb2O5-SiO2 glass-ceramics firstly increase then decrease. Breakdown strength (BDS) of all samples increase and then decrease with the increase of content of K2O, and maximum BDS is obtained when x ¼ 10 mol%. And maximum theoretical energy density is 1.43J/cm3 when x ¼ 5 mol%. © 2019 Elsevier B.V. All rights reserved.
Keywords: Na2O-K2O-Nb2O5-SiO2 glass-ceramics Breakdown strength Glass network structure
1. Introduction In recent years, pulse power devices have been widely used in military and auto industry. So the material, possessed of high energy storage density, which can charge and discharge in short time is urgently demanded [1,2]. Energy storage materials can be divided into four categories in energy-storage behaviors, antiferroelectrics, dielectric glass-ceramics, relaxor ferroelectrics and polymer-based ferroelectrics [3,4]. Antiferroelectrics possess high permittivity because a phase transformation is achieved between antiferroelectrics and ferroelectrics when a high external electric field is applied. Thus, a high energy storage can achieve for antiferroelectrics if a high breakdown strength (BDS) is obtained [5,6]. Dielectric glass-ceramics, a composite, possess high BDS because it has low porosity and residual glass phase has high compact glass network structure [7]. And dielectric phase, encircled by residual glass phase, has a high permittivity in dielectric glass-ceramics. The energy storage density of dielectric glass-ceramics can be calculated by the formula (ε0εrE2b)/2, which is proportional to dielectric constant and the square of BDS [2]. So it is more efficient to obtain a high energy storage that BDS is improved. Dielectric glass-ceramics are mainly divided into niobate-based and titanate-based glass-ceramics. Vacancy, leading to lower BDS,
* Corresponding author. E-mail address: [email protected] (Y. Pu). https://doi.org/10.1016/j.jallcom.2019.01.201 0925-8388/© 2019 Elsevier B.V. All rights reserved.
can be formed due to Ti4þ during the heat treatment in titanatebased glass-ceramics. So Many researcher are attracted by the niobate-based glass-ceramics [8]. PbO-SrO-Na2O-Nb2O5-SiO2 glassceramics system was investigated, and permittivity came to 600 approximately [9]. But lead-free material is demanded for ecological security. NaNbO3 was reported in several glasseceramics systems [7,10,11], which leads a high permittivity. In addition, interfacial polarization which is caused by different conductivity between glass and crystal phase, reported by the US Naval Research Laboratory [12], reduces the energy storage density seriously. What is more, BDS is mattered by glass network structure. Wang et al. [13] had the view that K2O can break the Si-O bond of glass network and create non-bridging oxygen ions in K2O-BaO-Nb2O5-SiO2 glassceramics, which changed the structure of glass network. The structure of glass network can be changed by alkali metal oxide as glass network modifier, which affect electrical properties. In recent investigations by researchers, the relation between BDS and interfacial polarization was studied in many paper. However, few work was investigated to the relation between the structure of glass network and BDS. In this study, Na2O-K2O-Nb2O5-SiO2 glasseceramics system was investigated. To obtain high permittivity and BDS, K2O was added to Na2O-K2O-Nb2O5-SiO2 glasseceramics. The structure of glassceramics was determined by Raman and differential thermal analysis. Dielectric properties and BDS was tested and theoretical energy density were calculated.
X. Peng et al. / Journal of Alloys and Compounds 785 (2019) 350e355
351
2. Experimental procedure Na2O-K2O-Nb2O5-SiO2 glass-ceramics were prepared through a traditional meltquenching method by melting raw material. Analytical purity powders of Na2CO3, K2CO3, Nb2O5, SiO2, were ball mixed for 8 h with distilled water as milling media for homogenous mixing. The prepared nominal composition of samples are given in Table 1 with corresponding sample names. The well-mixed reagent powders were put into an alumina crucible at 1100 C and melted for 1 h at 1450 C to form molten glass, which was cast into a preheated copper mold to form transparent glass and quickly placed in an annealing oven at 600 C for 10 h to reduce the residual stress. All samples with various composition was crystallized at different temperature for 4 h are shown in Table 1. All samples were cut and polished into sheets of 0.16e0.26 mm in thickness and were coated with silver paste electrode on both sides. The structure of glass-ceramics were confirmed by Raman spectroscopy (Renishaw-invia, Renishaw, U. K.) in the region of 1003500 cm1 at room temperature. The glass crystallization temperature was determined by using DSC (Model STA 409Pc, Netzsch, Germany, with a rate of 10 C min-1). The phase structure were analyzed by X-ray diffraction (D-MAX 2200pc, Rigaku Co, Tokyo, Japan) at room temperature to investigate the phase evolution. The microstructure observation of those crystallized samples were carried out using a scanning electron microscope (SEM; Model: JSM-5610LV, JEOL). For electrical measurements, these samples were polished to achieve parallel, smooth faces, and silver electrodes were coated on both faces. Then painting samples with silver pasted on both sides were treated at 600 C for 10 min to prepare electrodes for dielectric measurements. The measurements of permittivity of samples were performed by using a precision multifunction LCR meter (E4980A, Agilent Tech, CA, U.S.) with the measuring frequency of 100 Hz to 1 MHz. The samples were used to measure the dielectric breakdown strength with a high-voltage source (Model 610E) using a voltage ramping rate of about 1 kV/s at room temperature until dielectric breakdown, and at least 10 samples were used for each composition during BDS testing. The polarization-electric (P-E) loops were measured using a ferroelectric tester (TF Analyzer 2000, aixACCT, Aachen, Germany) at room temperature. Theoretical energy density of the glass-ceramics were calculated by (ε0εrE2b)/2. 3. Results and discussion 3.1. The analysis of vitrified structure 3.1.1. Thermal analysis of vitrified bonds In Fig. 1, DSC curves of all samples are showed. It is obvious that glass transition temperatures (Tg) reduce with the increase of content of Kþ from all the curves. The field strength parameter (Fs) defined by Dietzel of Naþ is larger than Kþ. The equation, Fs ¼ Z/d2, represented the ability to free O2, where Z stands for the cation charge, d denotes the mean M-O distance of oxide MaOb (M is glass network modifier, a and b are different constants) [14]. With the increase of content of Kþ, as a result, the degree of polymerization (DOP) of glass reduced gradually. Therefore, a higher concentration
Table 1 Samples with different content of K2O were crystallized at 950 C for 4 h. Sample
Crystallization system
Nominal composition(mol%)
40N 35N5K 30N10K 25N15K
950 C,4 h / / /
40Na2O-40Nb2O5-20SiO2 35Na2O-5K2O-40Nb2O5-20SiO2 30Na2O-10K2O-40Nb2O5-20SiO2 25Na2O-15K2O-40Nb2O5-20SiO2
Fig. 1. DSC curves of glass powders with different content of Kþ.
of K2O lead a more relaxed glass structure, which is caused to more free oxygen, and a smaller glass transition temperature (Tg) is obtained. In addition, the onset crystalline temperatures (Tc) show an obvious change with the increase of content of Kþ. Firstly, Tc have a slightly shift to the low temperature, which is attributed to the decreasing degree of polymerization of the glass network structure. Furthermore, more exothermic peaks (Tp) around 900 C appeared when x ¼ 15 mol, which indicated that more types of crystalline phases are generated. 3.1.2. Raman analysis of vitrified bonds As seen in Fig. 2(a), Raman spectrum is used to study the changing structure of the glass-ceramics. The signals are at three peaks of 257 cm1, 610 cm1, 770 cm1 and 940 cm1. The peak at 257 cm1 is attributed to the triple degenerate symmetric stretching vibration of OeNbeO bonds in NbO6 octahedron. The peak position at 257 cm1 shifts towards lower wavenumber at 265 cm1 with the increase of content of K2O. The peak at 610 cm1 is attributed to the double degenerate symmetric stretching vibration of OeNbeO bonds in NbO6 octahedron [21]. Meanwhile, the peaks in the range of 500e700 cm1 is due to the symmetric stretching vibration of Si]O. And the peaks around 770 and 940 cm1 are attributed to Qn. Generally, the peaks of Qn are in the 760-1200 cm1, and Q0, Q1, Q2, Q3, Q4 are at 760e850 cm1, 880e960 cm1, 940e1090 cm1, 1000e1190 cm1, 1080e1200 cm1, respectively [13,15]. The DOP is decreased due to increasing Q0 with the increase of content of Kþ. Free oxygen and non-bridging oxygen could be generated due to cations with low electric fields, and the DOP decreased [17e19]. Q0 increases, so Tg reduced. In Fig. 2(b), a simulated glass structure for K2O-Na2ONb2O5-SiO2 glass-ceramics is shown. 3.2. Phase structure and microstructure 3.2.1. Phase structure The XRD spectra of all glass ceramics samples are shown in Fig. 3(a). From the patterns, crystalline phase K0.1Na0.9NbO3 (PDF# 77-2037) appears with the increase of content of Kþ, and major phase NaNbO3 (PDF# 75-2102) appears for all samples. The phase fraction can be calculated by XRD peak intensity defined by Rietveld analysis in some studies [7,16]. The values of phase fractional P P P P P ratio are calculated by FA ¼ IA/( IAþ IBþ IC…þ IN), where FA
352
X. Peng et al. / Journal of Alloys and Compounds 785 (2019) 350e355
Fig. 2. (a) Raman spectra of all samples and (b) simulated glass structure for K2O-Na2O-Nb2O5-SiO2 glass-ceramics.
Fig. 3. (a) XRD patterns and (b) the ratio of each phase peak intensity for K2O-Na2O-Nb2O5-SiO2 glass-ceramics.
P represent phase fractional ratio, I denotes the total of XRD peak intensity of relevant phase. So the fractional ratio of relevant phases are illustrated in Fig. 3(b). The results show that the content of K0.1Na0.9NbO3 phase increases gradually while NaNbO3 phase decreases with the increasing Kþ, and non-perovskite phase appears. In Fig. 1, three peaks appeared around 900 C, which were attributed to non-perovskite phase. 3.2.2. Microstructure In Fig. 4, all glass-ceramics samples are investigated for microstructure at 950 C for 4 h with the varying Kþ. All images show that all the samples possesses nanoscale grains and lower porosity. In Fig. 4(a), the microstructure of tiny grains are uniform, which are caused by more pure phase NaNbO3. With the increase of content of Kþ, heterogeneous microstructure is formed by generated K0.1Na0.9NbO3 and non-perovskite phase for Fig. 4(b)-(c). In Fig. 4(d), microstructure is different with the former, and the increase of non-perovskite phase lead to a nonuniform microstructure. 3.3. Dielectric and ferroelectric properties Fig. 5 shows the frequency dependence of permittivity and dielectric loss characteristics of the glass-ceramics samples with varied K2O content over the frequency range of 20 Hz to 2 MHz at room temperature. The permittivity of all samples show a stable value with increase of frequency, which showed excellent frequency stability. The permittivity firstly increases and then
decreases with the increase of content of Kþ, which is attributed to the increase of intensity of crystallinity of the glass-ceramics and the generation of non-perovskite phase. And tiny peak at low frequency reveals that interfacial polarization is produced. The movement of glass network modifier (alkali metal cation) produces energy loss when AC is applied, which causes dielectric loss. A relaxed glass network facilitates the movement of glass network modifier, and an alkali metal cation with smaller radius is easily to move. So a high dielectric loss is obtained when x ¼ 0,15 mol% at low frequency. But there is not enough time for alkali metal cation to move at high frequency. Dielectric loss of all samples are approximative because of the same content of alkali metal cation at high frequency. P-E loops of glass-ceramics with varied K2O content is shown in Fig. 6. From the curves, a linear relationship between P and E is shown in all samples, which testifies that all the glass-ceramics samples are linear dielectric. Maximum polarization firstly increases then decreases when an equal external electric field is applied. A high polarization is obtained because of generating of phase NaNbO3 and K0.1Na0.9NbO3, lower polarization is attributed to non-perovskite phase. 3.4. Complex impedance spectroscopy The complex impedance spectroscopy and equivalent circuit of glass-ceramics with varied K2O content are shown in Fig. 7. Generally, glass-ceramics could be thought as a composite that numerous grains generated in host glass. All nanoscale grains are
X. Peng et al. / Journal of Alloys and Compounds 785 (2019) 350e355
353
Fig. 4. SEM images of K2O-Na2O-Nb2O5-SiO2 glass-ceramics with different content of Kþ: (a) 40N (b) 35N5K (c) 30N10K (d) 25N15K.
Fig. 6. PeE hysteresis loops of K2O-Na2O-Nb2O5-SiO2 glass-ceramics. Fig. 5. Permittivity and dielectric loss of K2O-Na2O-Nb2O5-SiO2 glass-ceramics with different content of Kþ.
surrounded by the residual glass phase in Fig. 4. So large different electric properties appear between the residual glass and ceramic phase. Complex impedance analysis is employed to study the effect of varied content of K2O on electric properties of glass-ceramics. ZView software is employed to calculate the resistance of grain and glass phase based on R-C equivalent. In Fig. 7(a), complex impedance spectra is measured at 500 C for all samples. The resistance of glass-ceramics increase with the increase of content of K2O. Fig. 7(b) shows the complex impedance spectra of 40N from 420 C to 500 C. And the intercept of each circular arc on the real axis decreases, which illustrates that the negative temperature coefficient of resistance behavior for glass-ceramics. The resistance of the residual glass phase, generally, is much larger than ceramic phase, which is fitted for 30N10 K at 500 C in Fig. 7(c). And an equivalent circuit is simulated to glass-ceramics in inset of Fig. 7(c). Rc is
resistance of ceramic phase, and Rr is resistance of residual glass phase. From low frequency to high frequency, residual glass phase is reacted first because of its large resistance. And ceramic phase is reacted at high frequency. In Fig. 7(d), ceramic phase is fitted in high frequency for 30N10K. To study electrical transport behaviors, the activation energy(Ea) of the residual glass and ceramic phase is calculated. The temperature dependence of the resistivity r can be expressed by an Arrhenius equation: r ¼ r0exp(Ea/kBT), where r0 is the pre-exponential factor, Ea is the activation energy, kB is the Boltzmann constant, and T is the absolute temperature. The Ea of the residual glass and ceramic phase of glass-ceramics are calculated from the slope of the function between resistivity and temperature, which are shown in Fig. 8(a) and (b). The Ea of the residual glass phase are larger than ceramic phase, which indicates that carrier migration needed more energy in the residual glass phase. To study the effect on carrier migration for different content of K2O,
354
X. Peng et al. / Journal of Alloys and Compounds 785 (2019) 350e355
Fig. 7. Complex impedance spectra of (a) all samples measured at 500 C (b) 40N measured at different temperature (The inset displays the data of 40N measured at high frequency) (c) 30N10K measured and fitted @500 C (The inset displays the equivalent circuit) (d) 30N10K measured and fitted @500 C at high frequency.
Fig. 8. Arrhenius fit to resistence data of (a) residual glass phase and (b) ceramic phase of all the K2O-Na2O-Nb2O5-SiO2 glass-ceramics samples. The inset displays activation energy of (a) residual glass phase and (b) ceramic phase.
the difference of activation energy(DEa) of the residual glass and ceramic phase are calculated. In Na2O-K2O-Nb2O5-SiO2 glassceramics system, high DEa is obtained when x ¼ 10 mol, which revealed that a difficult migration exist in glass-ceramics for carriers [20]. Generally, resistivity is increased when an alkali metal cation with a smaller radius is replaced by one with a larger radius in glass network, which is called by mixed alkali effect. After heat treatment, the residual glass phase in glass ceramics retains this effect. In Na2O-K2O-Nb2O5-SiO2 glass-ceramics system, uncrystallized Kþ with a larger radius can impede charge carrier movement in the residual glass phase, which cause a high BDS. However, BDS decreases when the content of Kþ comes to 15 mol%, which may be caused by high dielectric loss and a nonuniform microstructure. 3.5. Energy storage properties Energy storage of glass-ceramics are calculated, and the relation
of permittivity, BDS and energy storage are shown in Fig. 9. In Na2O-K2O-Nb2O5-SiO2 glass-ceramics system, a high permittivity is produced by antiferroelectric phase NaNbO3 and ferroelectric phase K0.1Na0.9NbO3. What is more, a relaxed glass structure is induced by Kþ, which leads to devitrification easily. So a maximal permittivity is achieved when x ¼ 5 mol%. However, non-perovskite phase, meanwhile, is formed due to relaxed glass structure, which reduces permittivity. In addition, BDS is influenced by the content of K2O. In Fig. 9(a), DEa have maximum value when x ¼ 10 mol%, and BDS had a similar trend. The difficulty level of migration for carriers could be according to DEa in the residual glass phase. Kþ with a larger radius impedes charge carrier movement, and a high BDS is obtained. However, high dielectric loss and a nonuniform microstructure cause a decrease in BDS when the content of Kþ comes to 15 mol%. For energy storage properties, a maximum value is obtained when x ¼ 5 mol%, which is attributed to high permittivity and BDS simultaneously. In Na2O-K2O-Nb2O5-SiO2 glass-ceramics
X. Peng et al. / Journal of Alloys and Compounds 785 (2019) 350e355
355
Fig. 9. (a) DEa and breakdown strength (b) permittivity, maximum breakdown strength and theoretical energy density for K2O-Na2O-Nb2O5-SiO2 glass-ceramics.
system, energy storage decreases when the content of K2O is too high. 4. Conclusions With the increase of content of K2O, Tg is decreased, which reveals that a relaxed glass network structure is obtained, and DOP is reduced due to the formation of non-bridging oxygen. So more kinds of phase can form because of a relaxed glass network structure, and permittivity is increased when x ¼ 5 mol%. But nonperovskite phase leads a decrease in permittivity when x ¼ 15 mol %. Meanwhile, with the increase of content of K2O, DEa increases, so BDS increases. However, when the content of Kþ comes to 15 mol%, high dielectric loss and a nonuniform microstructure cause a decrease in BDS. Acknowledgments This work was financed by the National Natural Science Foundation of China (51872175), the International Cooperation Projects of Shaanxi Province (2018 KW-027). References [1] C. Liu, F. Li, et al., Advanced materials for energy storage, Adv. Mater. 22 (8) (2010) E28eE62. [2] E.P. Gorzkowski, M.J. Pan, et al., Glass-ceramics of barium strontium titanate for high energy density capacitors, J. Electroceram. 18 (3e4) (2007) 269e276. [3] X. Hao, A review on the dielectric materials for high energy-storage application, J. Adv. Dielectr. 03 (01) (2013) 1330001. [4] Lei Zhang, Yongping Pu, Min Chen, Influence of BaZrO3 additive on the energy-storage properties of 0.775Na0.5Bi0.5TiO3-0.225BaSnO3 relaxor ferroelectrics, J. Alloys Compd. 775 (2019) 342e347. [5] Z. Liu, T. Lu, et al., Antiferroelectrics for energy storage applications: a review, Adv. Mater. Technol. 3 (9) (2018) 1800111. [6] K. Yang, J. Liu, et al., Effects of TiO2 addition on dielectric and energy storage properties of BaO-K2O-Nb2O5-SiO2 glass ceramics, Ceram. Int. 44 (6) (2018)
6181e6185. [7] Y. Zhou, Y. Qiao, et al., Improvement in structural, dielectric and energystorage properties of lead-free niobate glass-ceramic with Sm2O3, J. Eur. Ceram. Soc. 37 (3) (2017) 995e999. [8] H. Wang, J. Liu, et al., Ultra high energy-storage density in the barium potassium niobate-based glass-ceramics for energy-storage applications, J. Am. Ceram. Soc. 99 (9) (2016) 2909e2912. [9] D.F. Han, Q.M. Zhang, et al., Optimization of energy storage density in ANb2O6-NaNbO3-SiO2 (A¼[(1x)Pb, xSr]) nanostructured glasseceramic dielectrics, Ceram. Int. 38 (8) (2012) 6903e6906. [10] Y. Zhou, Q. Zhang, et al., Structural and dielectric characterization of Gd2O3added BaOeNa2OeNb2O5eSiO2 glasseceramic composites, Scripta Mater. 65 (4) (2011) 296e299. [11] C. Li, Q. Zhang, et al., Dielectric and energy storage properties of BaO-SrONa2O-Nb2O5-SiO2 glasseceramics with different crystallization times, J. Electron. Mater. 45 (6) (2016) 3025e3029. [12] M.J. Pan, E.P. Gorzkowski, et al., The effect of interfacial polarization on the energy density of ferroelectric glass-ceramics, IEEE Int. Symp. Appl. Ferroelectrics (2006), https://doi.org/10.1109/isaf.2006.4387824. [13] H. Wang, J. Liu, et al., Effect of K2O content on breakdown strength and energy-storage density in K2O-BaO-Nb2O5-SiO2 glass-ceramics, Ceram. Int. 43 (5) (2017) 4183e4187. [14] J. Shi, F. He, et al., Effects of Na2O/BaO ratio on the structure and the physical properties of low-temperature glass-ceramic vitrified bonds, Ceram. Int. 44 (9) (2018) 10871e10877. [15] Z. Wang, S. Cai, et al., Structural investigation of phosphorus in CaO-SiO2P2O5 ternary glass, Metall. Mater. Trans. B 48 (2) (2017) 1139e1148. [16] C.-W. Ahn, C.-S. Park, et al., Correlation between phase transitions and piezoelectric properties in lead-free (K, Na, Li)NbO3eBaTiO3Ceramics, Jpn. J. Appl. Phys. 47 (12) (2008) 8880e8883. [17] Y. Gao, Y. Hu, et al., Effect of glass network modifier R2O (R¼Li, Na and K) on upconversion luminescence in Er3þ/Yb3þ co-doped NaYF4 oxyfluoride glassceramics, J. Rare Earths 33 (8) (2015) 830e836. [18] A. Cesaratto, P. Sichel, et al., Characterization of archeological glasses by micro-Raman spectroscopy, J. Raman Spectrosc. 41 (12) (2010) 1682e1687. [19] K. Kioka, T. Honma, et al., Fabrication of (K, Na)NbO3 glasseceramics and crystal line patterning on glass surface, Opt. Mater. 33 (8) (2011) 1203e1209. [20] Q. Yuan, et al., Microstructure and dielectric properties of Ti0.995(In0.5Nb0.5) 0.005O2/SrO-B2B3-SiO2 glass-ceramics for energy storage, IEEE Trans. Dielectr. Electr. Insul. 24 (2) (2016) 712e719. [21] S. Dwivedi, T. Pareek, S. Kumar, Structure, dielectric, and piezoelectric properties of K0.5Na0.5NbO3-based lead-free ceramics, RSC Adv. 8 (43) (2018) 24286e24296.