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Preparation, structure and dielectric properties of (Ba1−xSrx)2NaNb5O15 ceramics
Journal of Alloys and Compounds 453 (2008) 401–406
Preparation, structure and dielectric properties of (Ba1−xSrx)2NaNb5O15 ceramics Yifei Yang a,b , Yayan Liu a,∗ , Jian Meng a,∗ , Yan Huan a,b , Yanan Wu a a
Key Laboratory of Rare Earth Chemistry and Physics, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun, 130022 Jilin, PR China b Graduate School of Chinese Academy of Sciences, Beijing 100049, PR China
Received 31 August 2006; received in revised form 8 October 2006; accepted 17 November 2006 Available online 9 January 2007
Abstract In this study the tungsten–bronze type tetragonal (Ba1−x Srx )2 NaNb5 O15 system as a kind of lead-free ferroelectric ceramics has been synthesized by low-temperature combustion method. Microstructure and dielectric properties of (Ba1−x Srx )2 NaNb5 O15 system were also investigated. X-ray diffraction (XRD) study confirms the formation of single-phase tetragonal compounds in the crystal system at room temperature. The TEM photograph shows that the particles synthesized by low-temperature combustion method are uniform with an average particle size of 30 nm in diameter. Microstructure analysis of the surface of the compounds by scanning electron microscopy (SEM) suggests that small grains distribute uniformly on the surface of the samples. The dielectric constant (ε) and tangent loss (tan δ) of the samples have been measured as a function of both frequency (0–1 GHz) and temperature (0–500 ◦ C). Detailed studies of the dielectric properties suggests that they have undergone a phase transition well above the room temperature and it is also found that the compositions with x = 0.6 have the lowest phase transition temperature (Tc ) and the highest dielectric constant at Tc under 100 kHz. Moreover, the (Ba1−x Srx )2 NaNb5 O15 synthesized by low-temperature combustion method have better dielectric properties than those prepared by the solid-state reaction. © 2006 Elsevier B.V. All rights reserved. Keywords: Ferroelectric; Low-temperature combustion synthesis; Tungsten–bronze
1. Introduction Today, the research of lead-free ferroelectric ceramics and their applications are extremely important as a result of implementing the strategy for the sustainable development of the world, and strengthening to the consciousness of environmental protection. Therefore, a large number of lead-free ferroelectric oxides have been developed [1–3]. One of the lead-free ferroelectric oxides, mixed niobates with tungsten–bronze (TB) structure have received considerable attention owing to their potential applications as ferroelectric, pyroelectric, piezoelectric and nonlinear optic materials because they have large spontaneous polarization, large piezoelectric properties and high dielectric constants [4–8]. The TB structure has a general formula of (A1)4 (A2)2 (C)4 (B1)2 (B2)8 O30 , where A types of cations occupy A1, A2 and C sites while B types of
∗
Corresponding authors. Tel.: +86 431 5262221; fax: +86 431 5262221. E-mail address: [email protected] (Y. Liu).
0925-8388/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2006.11.130
cations occupy the B1 and B2 octahedral sites [9]. In the formula, A1, A2, C, and B are 15-, 12-, 9- and 6-fold coordinated sites in the crystal lattice structure. Generally, A1 and A2 sites can be filled by Na+ , K+ , Ca2+ , Sr2+ , Ba2+ , Pb2+ , Bi3+ and some rare earth cations, whereas B1 and B2 sites can be filled by W6+ , Nb5+ , Ta5+ , the smallest interstice C is often empty, and hence a formula is A6 B10 O30 for the filled tungsten–bronze structure. It is well known that the metal cations distribution in the different sites of the tungsten–bronze structure plays a crucial role in tailoring the ferroelectric and other physical properties. Moreover, the properties of the tungsten–bronze structure could be modified in a wide scale, and are complicated and interesting for the complex crystal structure [10,11]. Among these tungsten–bronze niobates, the compound of sodium-modified strontium barium niobate (Ba1−x Srx )2 NaNb5 O15 (BSNN), which can be considered as a binary system of Sr2 NaNb5 O15 (SNN) and Ba2 NaNb5 O15 (BNN), have received increasing interests since their relatively large electro-optic effect and second harmonic generation [12]. On the other hand, the system is one of the most attractive
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ones for practical applications, which have the potential morphotropic phase boundary (MPB), since the piezoelectric and dielectric properties peak at the MPB and most of the functional properties will be enhanced near the MPB. The single crystals of BSNN have already been synthesized and investigated by Neurgaonkar et al. [13] and Jiang et al. [14]. In recent years, the polycrystalline ceramics of the BSNN, which have already synthesized by the conventional solid-state reaction [15], have gained immense attention as their possible substitutes in many technological applications because of their low cost, easy fabrication into larger sizes and more complex shapes comparing to the single crystals. Since the physical properties of the ceramics largely depend on the size and morphology of the small particles according to the method of the preparation, the synthesis of ceramics powders with good stoichiometry, homogeneity and sinterability is necessary for the development of the BSNN ceramics. Therefore, the low-temperature combustion synthesis (LCS) developed from self-propagating high-temperature synthesis (SHS) combining with wet chemical techniques for the synthesis of metal oxide based ceramic powders with good stoichiometry and homogeneity. The process combining the advantages of wet chemical routes, which can produce compositionally homogeneous mixtures and uniform sized small crystalline particles, has made both the ignition and calcining temperature lower than those of traditional solid-state reaction [16,17]. In this paper, a series of BSNN ceramics have been synthesized by the combustion synthesis method. In addition, the microstructure, dielectric properties of BSNN ceramics with x = 0.6, 0.65, 0.75 nearer to the MPB and that with x = 0.4 were investigated. It is found that this synthesis method not only obtains the superior homogeneity and sub-micron sized powders but also improves the dielectric constant of the compounds. 2. Experimental Polycrystalline samples of BSNN were prepared by a low-temperature combustion synthesis process (LCS) using pure starting materials: NbCl5 , Ba(NO3 )2 , Sr(NO3 )2 , NaNO3 , NH4 NO3 , citric acid, ammonia solution and H2 O2 solution (30%) in a suitable stoichiometry. The schematic flow chart of the current LCS process is shown in Fig. 1. The hydrated niobium oxide (Nb2 O5 ·nH2 O) was prepared from NbCl5 and H2 O, which was added with citric acid and H2 O2 solution to get the Nb–citric solution used as the source of niobium. H2 O2 solution was added as it facilitated the breaking of polymer chain of the hydrated oxide of Nb2 O5 and the dissolution of Nb2 O5 ·nH2 O in citric acid by complex formation. Ba(NO3 )2 , Sr(NO3 )2 , NH4 NO3 , NaNO3 and citric acid were dissolved in deion water and mixed with the Nb–citric solution made in our laboratory. Citric acid was used as an organic fuel because it could not only form stable watersoluble complexes with metal ions but also act as a rich fuel. Herein Ba(NO3 )2 , Sr(NO3 )2 , NaNO3 and NH4 NO3 containing NO3 − were regarded as oxidizers, as well as the source of barium, strontium, sodium. The required amount of ammonia solution was added to achieve pH < 7 to form transparent mixture solution. This mixture solution was evaporated and then ignited at 300 ◦ C at ambient atmosphere, and then the mixtures were calcined at 800 ◦ C for 1 h. The process utilized the in-built exothermicity of combustion of the reaction system to synthesize the required materials directly, with the advantages of wet chemical routes which could produce a compositionally homogeneous mixture. The calcined powder was compacted into pellets at 200 MPa with 10 mm diameter and around 1 mm thickness, which were sintered at 1250 ◦ C for 12 h.
Fig. 1. Schematic flow chart of low-temperature combustion synthesis process. The formation and quality of the compounds were checked at room temperature using D/max-IIB X-ray Theta-Theta diffractometer with Cu K␣ radiation (λ = 0.15406 nm) in a wide range of Bragg’s angles 2θ (20◦ ≤ θ ≤ 80◦ ) with a scanning rate of 2◦ /min. The microstructure, the particle size and particle distribution studies were performed on the transmission electron microscope (TEM, JEM-1011, JEOL, Japan). The fracture morphology of the sintered pellets was studied at room temperature by a scanning electron microscopy (SEM, XL30, ESEM, FEG, FEI). Ag-electrode was screened on both flat surfaces of the pellets to measure the electrical properties of the compounds. Temperature-dependence dielectric measurement was made using the Agilent 4294A impedance analyzer equipped with a thermostat from room temperature (23 ◦ C) to 500 ◦ C at 1 kHz, 10 kHz, 100 kHz and 1 MHz, respectively.
3. Results and discussion 3.1. XRD analysis of BSNN The XRD analysis of the ceramics powders at room temperature in Fig. 2 show that BSNN are sure of single phase with tungsten–bronze structure and form solid solution. The XRD
Fig. 2. X-ray diffraction pattern of BSNN (x = 0.4–0.75) at room temperature.
Y. Yang et al. / Journal of Alloys and Compounds 453 (2008) 401–406
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Table 1 Crystal structure parameters of (Ba1−x Srx )2 NaNb5 O15 Composition
Ba1.2 Sr0.8 NaNb5 O15
Ba0.8 Sr1.2 NaNb5 O15
Ba0.7 Sr1.3 NaNb5 O15
Ba0.5 Sr1.5 NaNb5 O15
˚ a, b (A) ˚ c (A) ˚ 3) V (A c/a
12.451(5) 3.942(7) 611.2(7) 0.317
12.447(1) 3.944(2) 610.3(6) 0.317
12.438(7) 3.932(0) 608.3(6) 0.316
12.420(0) 3.915(5) 603.9(9) 0.315
analysis reveals that all the BSNN solid solution can be indexed by typical tetragonal structure. However the orthorhombic distortions of ceramics are difficult to confirm. It is obvious that the relative intensities and peak positions of diffraction peaks vary with increasing the content of Sr2 NaNb5 O15 , since it is expected that Sr2+ could substituted for Ba2+ in the system. These substitutions affect not only the structure but also the lattice parameters. The unit cell parameters along with cell volume ˚ 3 ) of the ceramics are shown in Table 1. (V, A The values of lattice parameters obtained in BSNN are summarized in Table 1 and agree well with those reported in the literature [7,15]. The maximum lattice parameter ‘c’ and ‘c/a’ is obtained in the composition in Ba0.8 Sr1.2 NaNb5 O15 , which is probably due to the MPB region. The value of lattice parameter and ‘c/a’ decrease with increasing the content of Sr2 NaNb5 O15 . The decrease in cell volume may be due to the substitution of Sr to Ba as Sr has less ionic radii compared with Ba. 3.2. Microstructure analysis of BSNN
Fig. 3. TEM micrograph of Ba1.2 Sr0.8 NaNb5 O15 powder obtained at 800 ◦ C.
TEM examination of the powders of Ba1.2 Sr0.8 NaNb5 O15 heated for 1 h at 800 ◦ C is shown in Fig. 3, which indicates that the particles have average sizes of about 30–50 nm according
Fig. 4. SEM micrographs of BSNN (x = 0.4–0.75) at different magnification.
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Fig. 5. Frequency dependence of ε and tan δ of BSNN (x = 0.4–0.75).
with the results from the XRD patterns. The other compounds have similar sizes with Ba1.2 Sr0.8 NaNb5 O15 which indicate that the combustion reaction can benefit the formation of small crystalline particles. The BSNN compounds were sintered into dense ceramics without using any additive. To study the surface morphology of the sintered pellets, all the surfaces were made flat and parallel. On the flat and clean surface gold coating was done by a sputtering method to increase the resolution of micrograph. The average grain size was determined by a linear intercept method. The SEM micrographs of the fracture surfaces of the compounds with different magnifications at room temperature are shown in Fig. 4. It is found that the grains distribute homogeneously and uniformly over the entire surface of the samples. The grain size evaluated from the micrograph is found to be in the range of 2–10 m. A similar type of microstructure has been found in many materials of this family [18].
smaller ionic radius than Ba2+ results in the variation in the tungsten–bronze structure. As expected, the substitution of Sr2+ ions induces the significant changes in the dielectric properties. At 100 kHz BSNN exhibit the high dielectric constant and the low dielectric loss from Figs. 6 and 7. The dielectric constants of the solid solution vary slightly with the composition. It is observed that the compounds have dielectric peaks and hence have ferroelectric–paraelectric phase transition of diffuse-type. The phase transition temperatures (Tc ) of the compounds BSNN with x = 0.4–0.75 are 289, 262, 264 and 264 ◦ C, respectively. The trend of Tc coincides well with the literature reported by Oliver [15]. The room temperature dielectric constants of the compounds increase from 195 to 277 with x increasing from 0.4 to 0.75 at 100 kHz. On the other hand, the dielectric constants at Tc decrease from 2693 to 2386 as x increases from 0.6 to 0.75
3.3. Dielectric properties of BSNN Fig. 5 shows the dependence of dielectric constants (ε) and dielectric losses (tan δ) with the composition of the SBNN ceramic samples around room temperature and under different applied frequencies. The value of ε decreases initially with increasing frequency then becomes almost frequency independent in dielectric constant in the range of 10 kHz–1 MHz, which is due to the reduction of active polarization mechanism. However the value of tan δ increases with increasing frequency in the high frequency region. Similar behavior of tan δ with frequency has also been observed in the similar groups [19]. The temperature dependence of ε at 100 kHz for the BSNN ceramics with 0.4 ≤ x ≤ 0.75 is shown in Fig. 6. Sr2+ with
Fig. 6. Temperature dependence of dielectric constant (ε) of the BSNN ceramics at 100 kHz.
Y. Yang et al. / Journal of Alloys and Compounds 453 (2008) 401–406
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Fig. 7. Temperature dependence of dielectric constant (ε) and dielectric loss (tan δ) for the BSNN ceramics at 10 kHz, 100 kHz and 1 MHz with x = 0.4–0.75.
at 100 kHz. The value of dielectric constants for BSNN (x = 0.4) at Tc is 1726. This low value may be due to the composition being farthest from the MPB region. The values of dielectric constant at room temperature and Tc in the present compositions are higher than the reported values [15] which may be due to the compounds synthesized by LCS having the smaller size particles. Temperature-dependence of dielectric constants (ε) and dielectric losses (tan δ) for all the compounds at select frequencies are shown in Fig. 7, respectively. The values of ε for these compounds increase with increasing temperature then decrease after attaining a maximum value. The variation of tan δ with temperature of all the compounds shows similar trend, which is considerably less at low temperature and increases significantly at high temperature. Such variations are strongly dependent on the frequencies. It is observed that the values of tan δ at high frequencies are much lower comparing to those at low frequencies. This kind of dependence of ε and tan δ on frequency is typically associated with losses by conduction [20]. The increase in tan δ at higher temperature may be due to enhanced conductivity in these compounds and transport of ions with higher thermal energy. On increasing the temperature, the electrical conductivity increases due to the increase in thermally activated drift mobility of electric charge carriers according to the hopping conduction mechanism. Therefore, the dielectric polarization increases causing a marked increase in tan δ as the temperature increases at higher temperature. This type of anomaly was also observed in some other type of compounds [21]. It is found that the dielectric constant of BSNN (x = 0.4–0.75) follow the Curie–Weiss law at temperatures much higher than Tc . The degree of disorder or diffusivity (γ) in the BSNN compound has been calculated using a modified empirical relation proposed by Uchino et al, to describe the diffuseness of the ferroelectric
phase transition [22]. 1 1 = ln K + γ ln(T − Tc ) − ln ε εmax
(1)
εmax is the maximum value of ε at Tc and K is an arbitrary constant. A linear relationship is observed for all samples. The value of γ for all the compounds at 100 kHz was obtained from the slope of the curve of ln(1/ε − 1/εmax ) versus ln(T − Tc ) in Fig. 8. The limiting values γ = 1 and 2 reduce the equation to Curie–Weiss law valid for the case of normal ferroelectric and to the quadratic valid for an ideal relaxor ferroelectric, respectively. The values of γ for x = 0.4–0.75 are found to be 1.18, 1.16, 1.17 and 1.34, respectively, which are deviation from Curie–Weiss behavior (γ = 1) as a result of the occurrence of disordering in the system. The γ values of BSNN (x = 0.6, 0.65) are lower
Fig. 8. Variation of ln(1/ε − 1/εmax ) with ln(T − Tc ) at 100 kHz in BSNN (x = 0.4–0.75).
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than those of BSNN (x = 0.4, 0.75) which may be due to the compounds with x = 0.6, 0.65 near to the MPB region. 4. Conclusions The (Ba1−x Srx )2 NaNb5 O15 (0.4 ≤ x ≤ 0.75) solid-solutions were prepared by the low-temperature combustion method, and their dielectric characteristics were investigated. The remarkable advantages of this simple preparation method are the synthesis of uniform sized small crystalline particles. It is concluded that the compounds have typical tetragonal structure at room temperature. This compound shows diffuse type ferroelectric phase transition with transition temperature well above Tc . The compositions in MPB region with x = 0.6 have the lowest Tc (Tc = 262 ◦ C) and the highest dielectric constant εmax (εmax = 2693) at Tc under 100 kHz. Moreover, the dielectric constants of the compounds are higher than those prepared by the solid-state reaction because the superior homogeneity and submicron sized powders were prepared without grinding steps. The improvement of the dielectric properties will further expand the potential application of the material for device fabrication. References [1] X. Wang, H. Chan, Solid State Commun. 125 (2003) 395–399. [2] V. Bobnar, B. Malic, J. Jolc, M. Kosec, J. Appl. Phys. 98 (2005) 0241131–024113-4. [3] H. Nagata, T. Takenaka, J. Eur. Ceram. Soc. 21 (2001) 1299–1302. [4] B. Yang, F. Li, J.P. Han, X.J. Yi, H.L.W. Chan, H.C. Chen, W. Cao, J. Phys. D 37 (2004) 921–924.
[5] A.M. Glass, J. Appl. Phys. 40 (1969) 4699–4713. [6] M. Kimura, T. Minamikawa, A. Ando, Y. Sakabe, Jpn. J. Appl. Phys. 36 (1997) 6051–6054. [7] Z.X. Cheng, S.J. Zhang, G.Y. Zhou, J.H. Liu, J.R. Han, H.C. Chen, Mater. Res. Bull. 35 (2000) 1107–1112. [8] B. Tribotte, J.M. Haussonne, G. Desgardin, J. Eur. Ceram. Soc. 19 (1999) 1105–1109. [9] B. Jasmieson, S.C. Abrahams, J.L. Bernstein, J. Chem. Phys. 48 (1968) 5048–5057. [10] A.Q. Jiang, G.H. Li, L.D. Zhang, J. Appl. Phys. 83 (1998) 4878–4883. [11] Y. Wu, M.J. Forbess, S. Seraji, S.J. Limmer, T.P. Chou, G. Cao, Mater. Sci. Eng. B 86 (2001) 70–78. [12] F.R. Nash, E.H. Turner, P.M. Brindenbaugh, J.M. Dziedzic, J. Appl. Phys. 43 (1972) 1–9. [13] R.R. Neurgaonkar, W.K. Cory, J.R. Oliver, J. Cryst. Growth 84 (1987) 629–637. [14] W.H. Jiang, W.W. Cao, X.J. Yi, H.C. Chen, J. Appl. Phys. 97 (2005) 0941061–094106-4. [15] J.R. Oliver, R.R. Neurgaonkar, L.E. Cross, J. Am. Ceram. Soc. 72 (1989) 202–211. [16] T.Y. Peng, X. Liu, K. Dai, J.R. Xiao, H.B. Song, Mater. Res. Bull. 41 (2006) 1638–1645. [17] C.R. Lin, Y.M. Chu, S.C. Wang, Mater. Lett. 60 (2006) 2270– 2273. [18] M. Khachane, A. Moure, M. Elaatmani, A. Zegzouti, M. Daoud, A. Castro, Mater. Res. Bull. (2006) 1–8. [19] X.M. Chen, Y. Yuan, Y.H. Sun, J. Appl. Phys. 97 (2005) 074108-1–0741084. [20] M.A.L. Nobrea, S. Lanfredi, Mater. Lett. 47 (2001) 362–366. [21] J. P´erez, H. Amor´ın, J.C. M’Peko, J. Portelles, J.A. P´erez, F. Guerrero, Mater. Sci. Eng. B 126 (2006) 22–27. [22] S.M. Pilgrim, A.E. Sutherland, S.R. Winzer, J. Am. Ceram. Soc. 73 (1990) 3122–3125.
Preparation, structure and dielectric properties of (Ba1−xSrx)2NaNb5O15 ceramics Yifei Yang a,b , Yayan Liu a,∗ , Jian Meng a,∗ , Yan Huan a,b , Yanan Wu a a
Key Laboratory of Rare Earth Chemistry and Physics, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun, 130022 Jilin, PR China b Graduate School of Chinese Academy of Sciences, Beijing 100049, PR China
Received 31 August 2006; received in revised form 8 October 2006; accepted 17 November 2006 Available online 9 January 2007
Abstract In this study the tungsten–bronze type tetragonal (Ba1−x Srx )2 NaNb5 O15 system as a kind of lead-free ferroelectric ceramics has been synthesized by low-temperature combustion method. Microstructure and dielectric properties of (Ba1−x Srx )2 NaNb5 O15 system were also investigated. X-ray diffraction (XRD) study confirms the formation of single-phase tetragonal compounds in the crystal system at room temperature. The TEM photograph shows that the particles synthesized by low-temperature combustion method are uniform with an average particle size of 30 nm in diameter. Microstructure analysis of the surface of the compounds by scanning electron microscopy (SEM) suggests that small grains distribute uniformly on the surface of the samples. The dielectric constant (ε) and tangent loss (tan δ) of the samples have been measured as a function of both frequency (0–1 GHz) and temperature (0–500 ◦ C). Detailed studies of the dielectric properties suggests that they have undergone a phase transition well above the room temperature and it is also found that the compositions with x = 0.6 have the lowest phase transition temperature (Tc ) and the highest dielectric constant at Tc under 100 kHz. Moreover, the (Ba1−x Srx )2 NaNb5 O15 synthesized by low-temperature combustion method have better dielectric properties than those prepared by the solid-state reaction. © 2006 Elsevier B.V. All rights reserved. Keywords: Ferroelectric; Low-temperature combustion synthesis; Tungsten–bronze
1. Introduction Today, the research of lead-free ferroelectric ceramics and their applications are extremely important as a result of implementing the strategy for the sustainable development of the world, and strengthening to the consciousness of environmental protection. Therefore, a large number of lead-free ferroelectric oxides have been developed [1–3]. One of the lead-free ferroelectric oxides, mixed niobates with tungsten–bronze (TB) structure have received considerable attention owing to their potential applications as ferroelectric, pyroelectric, piezoelectric and nonlinear optic materials because they have large spontaneous polarization, large piezoelectric properties and high dielectric constants [4–8]. The TB structure has a general formula of (A1)4 (A2)2 (C)4 (B1)2 (B2)8 O30 , where A types of cations occupy A1, A2 and C sites while B types of
∗
Corresponding authors. Tel.: +86 431 5262221; fax: +86 431 5262221. E-mail address: [email protected] (Y. Liu).
0925-8388/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2006.11.130
cations occupy the B1 and B2 octahedral sites [9]. In the formula, A1, A2, C, and B are 15-, 12-, 9- and 6-fold coordinated sites in the crystal lattice structure. Generally, A1 and A2 sites can be filled by Na+ , K+ , Ca2+ , Sr2+ , Ba2+ , Pb2+ , Bi3+ and some rare earth cations, whereas B1 and B2 sites can be filled by W6+ , Nb5+ , Ta5+ , the smallest interstice C is often empty, and hence a formula is A6 B10 O30 for the filled tungsten–bronze structure. It is well known that the metal cations distribution in the different sites of the tungsten–bronze structure plays a crucial role in tailoring the ferroelectric and other physical properties. Moreover, the properties of the tungsten–bronze structure could be modified in a wide scale, and are complicated and interesting for the complex crystal structure [10,11]. Among these tungsten–bronze niobates, the compound of sodium-modified strontium barium niobate (Ba1−x Srx )2 NaNb5 O15 (BSNN), which can be considered as a binary system of Sr2 NaNb5 O15 (SNN) and Ba2 NaNb5 O15 (BNN), have received increasing interests since their relatively large electro-optic effect and second harmonic generation [12]. On the other hand, the system is one of the most attractive
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Y. Yang et al. / Journal of Alloys and Compounds 453 (2008) 401–406
ones for practical applications, which have the potential morphotropic phase boundary (MPB), since the piezoelectric and dielectric properties peak at the MPB and most of the functional properties will be enhanced near the MPB. The single crystals of BSNN have already been synthesized and investigated by Neurgaonkar et al. [13] and Jiang et al. [14]. In recent years, the polycrystalline ceramics of the BSNN, which have already synthesized by the conventional solid-state reaction [15], have gained immense attention as their possible substitutes in many technological applications because of their low cost, easy fabrication into larger sizes and more complex shapes comparing to the single crystals. Since the physical properties of the ceramics largely depend on the size and morphology of the small particles according to the method of the preparation, the synthesis of ceramics powders with good stoichiometry, homogeneity and sinterability is necessary for the development of the BSNN ceramics. Therefore, the low-temperature combustion synthesis (LCS) developed from self-propagating high-temperature synthesis (SHS) combining with wet chemical techniques for the synthesis of metal oxide based ceramic powders with good stoichiometry and homogeneity. The process combining the advantages of wet chemical routes, which can produce compositionally homogeneous mixtures and uniform sized small crystalline particles, has made both the ignition and calcining temperature lower than those of traditional solid-state reaction [16,17]. In this paper, a series of BSNN ceramics have been synthesized by the combustion synthesis method. In addition, the microstructure, dielectric properties of BSNN ceramics with x = 0.6, 0.65, 0.75 nearer to the MPB and that with x = 0.4 were investigated. It is found that this synthesis method not only obtains the superior homogeneity and sub-micron sized powders but also improves the dielectric constant of the compounds. 2. Experimental Polycrystalline samples of BSNN were prepared by a low-temperature combustion synthesis process (LCS) using pure starting materials: NbCl5 , Ba(NO3 )2 , Sr(NO3 )2 , NaNO3 , NH4 NO3 , citric acid, ammonia solution and H2 O2 solution (30%) in a suitable stoichiometry. The schematic flow chart of the current LCS process is shown in Fig. 1. The hydrated niobium oxide (Nb2 O5 ·nH2 O) was prepared from NbCl5 and H2 O, which was added with citric acid and H2 O2 solution to get the Nb–citric solution used as the source of niobium. H2 O2 solution was added as it facilitated the breaking of polymer chain of the hydrated oxide of Nb2 O5 and the dissolution of Nb2 O5 ·nH2 O in citric acid by complex formation. Ba(NO3 )2 , Sr(NO3 )2 , NH4 NO3 , NaNO3 and citric acid were dissolved in deion water and mixed with the Nb–citric solution made in our laboratory. Citric acid was used as an organic fuel because it could not only form stable watersoluble complexes with metal ions but also act as a rich fuel. Herein Ba(NO3 )2 , Sr(NO3 )2 , NaNO3 and NH4 NO3 containing NO3 − were regarded as oxidizers, as well as the source of barium, strontium, sodium. The required amount of ammonia solution was added to achieve pH < 7 to form transparent mixture solution. This mixture solution was evaporated and then ignited at 300 ◦ C at ambient atmosphere, and then the mixtures were calcined at 800 ◦ C for 1 h. The process utilized the in-built exothermicity of combustion of the reaction system to synthesize the required materials directly, with the advantages of wet chemical routes which could produce a compositionally homogeneous mixture. The calcined powder was compacted into pellets at 200 MPa with 10 mm diameter and around 1 mm thickness, which were sintered at 1250 ◦ C for 12 h.
Fig. 1. Schematic flow chart of low-temperature combustion synthesis process. The formation and quality of the compounds were checked at room temperature using D/max-IIB X-ray Theta-Theta diffractometer with Cu K␣ radiation (λ = 0.15406 nm) in a wide range of Bragg’s angles 2θ (20◦ ≤ θ ≤ 80◦ ) with a scanning rate of 2◦ /min. The microstructure, the particle size and particle distribution studies were performed on the transmission electron microscope (TEM, JEM-1011, JEOL, Japan). The fracture morphology of the sintered pellets was studied at room temperature by a scanning electron microscopy (SEM, XL30, ESEM, FEG, FEI). Ag-electrode was screened on both flat surfaces of the pellets to measure the electrical properties of the compounds. Temperature-dependence dielectric measurement was made using the Agilent 4294A impedance analyzer equipped with a thermostat from room temperature (23 ◦ C) to 500 ◦ C at 1 kHz, 10 kHz, 100 kHz and 1 MHz, respectively.
3. Results and discussion 3.1. XRD analysis of BSNN The XRD analysis of the ceramics powders at room temperature in Fig. 2 show that BSNN are sure of single phase with tungsten–bronze structure and form solid solution. The XRD
Fig. 2. X-ray diffraction pattern of BSNN (x = 0.4–0.75) at room temperature.
Y. Yang et al. / Journal of Alloys and Compounds 453 (2008) 401–406
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Table 1 Crystal structure parameters of (Ba1−x Srx )2 NaNb5 O15 Composition
Ba1.2 Sr0.8 NaNb5 O15
Ba0.8 Sr1.2 NaNb5 O15
Ba0.7 Sr1.3 NaNb5 O15
Ba0.5 Sr1.5 NaNb5 O15
˚ a, b (A) ˚ c (A) ˚ 3) V (A c/a
12.451(5) 3.942(7) 611.2(7) 0.317
12.447(1) 3.944(2) 610.3(6) 0.317
12.438(7) 3.932(0) 608.3(6) 0.316
12.420(0) 3.915(5) 603.9(9) 0.315
analysis reveals that all the BSNN solid solution can be indexed by typical tetragonal structure. However the orthorhombic distortions of ceramics are difficult to confirm. It is obvious that the relative intensities and peak positions of diffraction peaks vary with increasing the content of Sr2 NaNb5 O15 , since it is expected that Sr2+ could substituted for Ba2+ in the system. These substitutions affect not only the structure but also the lattice parameters. The unit cell parameters along with cell volume ˚ 3 ) of the ceramics are shown in Table 1. (V, A The values of lattice parameters obtained in BSNN are summarized in Table 1 and agree well with those reported in the literature [7,15]. The maximum lattice parameter ‘c’ and ‘c/a’ is obtained in the composition in Ba0.8 Sr1.2 NaNb5 O15 , which is probably due to the MPB region. The value of lattice parameter and ‘c/a’ decrease with increasing the content of Sr2 NaNb5 O15 . The decrease in cell volume may be due to the substitution of Sr to Ba as Sr has less ionic radii compared with Ba. 3.2. Microstructure analysis of BSNN
Fig. 3. TEM micrograph of Ba1.2 Sr0.8 NaNb5 O15 powder obtained at 800 ◦ C.
TEM examination of the powders of Ba1.2 Sr0.8 NaNb5 O15 heated for 1 h at 800 ◦ C is shown in Fig. 3, which indicates that the particles have average sizes of about 30–50 nm according
Fig. 4. SEM micrographs of BSNN (x = 0.4–0.75) at different magnification.
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Fig. 5. Frequency dependence of ε and tan δ of BSNN (x = 0.4–0.75).
with the results from the XRD patterns. The other compounds have similar sizes with Ba1.2 Sr0.8 NaNb5 O15 which indicate that the combustion reaction can benefit the formation of small crystalline particles. The BSNN compounds were sintered into dense ceramics without using any additive. To study the surface morphology of the sintered pellets, all the surfaces were made flat and parallel. On the flat and clean surface gold coating was done by a sputtering method to increase the resolution of micrograph. The average grain size was determined by a linear intercept method. The SEM micrographs of the fracture surfaces of the compounds with different magnifications at room temperature are shown in Fig. 4. It is found that the grains distribute homogeneously and uniformly over the entire surface of the samples. The grain size evaluated from the micrograph is found to be in the range of 2–10 m. A similar type of microstructure has been found in many materials of this family [18].
smaller ionic radius than Ba2+ results in the variation in the tungsten–bronze structure. As expected, the substitution of Sr2+ ions induces the significant changes in the dielectric properties. At 100 kHz BSNN exhibit the high dielectric constant and the low dielectric loss from Figs. 6 and 7. The dielectric constants of the solid solution vary slightly with the composition. It is observed that the compounds have dielectric peaks and hence have ferroelectric–paraelectric phase transition of diffuse-type. The phase transition temperatures (Tc ) of the compounds BSNN with x = 0.4–0.75 are 289, 262, 264 and 264 ◦ C, respectively. The trend of Tc coincides well with the literature reported by Oliver [15]. The room temperature dielectric constants of the compounds increase from 195 to 277 with x increasing from 0.4 to 0.75 at 100 kHz. On the other hand, the dielectric constants at Tc decrease from 2693 to 2386 as x increases from 0.6 to 0.75
3.3. Dielectric properties of BSNN Fig. 5 shows the dependence of dielectric constants (ε) and dielectric losses (tan δ) with the composition of the SBNN ceramic samples around room temperature and under different applied frequencies. The value of ε decreases initially with increasing frequency then becomes almost frequency independent in dielectric constant in the range of 10 kHz–1 MHz, which is due to the reduction of active polarization mechanism. However the value of tan δ increases with increasing frequency in the high frequency region. Similar behavior of tan δ with frequency has also been observed in the similar groups [19]. The temperature dependence of ε at 100 kHz for the BSNN ceramics with 0.4 ≤ x ≤ 0.75 is shown in Fig. 6. Sr2+ with
Fig. 6. Temperature dependence of dielectric constant (ε) of the BSNN ceramics at 100 kHz.
Y. Yang et al. / Journal of Alloys and Compounds 453 (2008) 401–406
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Fig. 7. Temperature dependence of dielectric constant (ε) and dielectric loss (tan δ) for the BSNN ceramics at 10 kHz, 100 kHz and 1 MHz with x = 0.4–0.75.
at 100 kHz. The value of dielectric constants for BSNN (x = 0.4) at Tc is 1726. This low value may be due to the composition being farthest from the MPB region. The values of dielectric constant at room temperature and Tc in the present compositions are higher than the reported values [15] which may be due to the compounds synthesized by LCS having the smaller size particles. Temperature-dependence of dielectric constants (ε) and dielectric losses (tan δ) for all the compounds at select frequencies are shown in Fig. 7, respectively. The values of ε for these compounds increase with increasing temperature then decrease after attaining a maximum value. The variation of tan δ with temperature of all the compounds shows similar trend, which is considerably less at low temperature and increases significantly at high temperature. Such variations are strongly dependent on the frequencies. It is observed that the values of tan δ at high frequencies are much lower comparing to those at low frequencies. This kind of dependence of ε and tan δ on frequency is typically associated with losses by conduction [20]. The increase in tan δ at higher temperature may be due to enhanced conductivity in these compounds and transport of ions with higher thermal energy. On increasing the temperature, the electrical conductivity increases due to the increase in thermally activated drift mobility of electric charge carriers according to the hopping conduction mechanism. Therefore, the dielectric polarization increases causing a marked increase in tan δ as the temperature increases at higher temperature. This type of anomaly was also observed in some other type of compounds [21]. It is found that the dielectric constant of BSNN (x = 0.4–0.75) follow the Curie–Weiss law at temperatures much higher than Tc . The degree of disorder or diffusivity (γ) in the BSNN compound has been calculated using a modified empirical relation proposed by Uchino et al, to describe the diffuseness of the ferroelectric
phase transition [22]. 1 1 = ln K + γ ln(T − Tc ) − ln ε εmax
(1)
εmax is the maximum value of ε at Tc and K is an arbitrary constant. A linear relationship is observed for all samples. The value of γ for all the compounds at 100 kHz was obtained from the slope of the curve of ln(1/ε − 1/εmax ) versus ln(T − Tc ) in Fig. 8. The limiting values γ = 1 and 2 reduce the equation to Curie–Weiss law valid for the case of normal ferroelectric and to the quadratic valid for an ideal relaxor ferroelectric, respectively. The values of γ for x = 0.4–0.75 are found to be 1.18, 1.16, 1.17 and 1.34, respectively, which are deviation from Curie–Weiss behavior (γ = 1) as a result of the occurrence of disordering in the system. The γ values of BSNN (x = 0.6, 0.65) are lower
Fig. 8. Variation of ln(1/ε − 1/εmax ) with ln(T − Tc ) at 100 kHz in BSNN (x = 0.4–0.75).
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than those of BSNN (x = 0.4, 0.75) which may be due to the compounds with x = 0.6, 0.65 near to the MPB region. 4. Conclusions The (Ba1−x Srx )2 NaNb5 O15 (0.4 ≤ x ≤ 0.75) solid-solutions were prepared by the low-temperature combustion method, and their dielectric characteristics were investigated. The remarkable advantages of this simple preparation method are the synthesis of uniform sized small crystalline particles. It is concluded that the compounds have typical tetragonal structure at room temperature. This compound shows diffuse type ferroelectric phase transition with transition temperature well above Tc . The compositions in MPB region with x = 0.6 have the lowest Tc (Tc = 262 ◦ C) and the highest dielectric constant εmax (εmax = 2693) at Tc under 100 kHz. Moreover, the dielectric constants of the compounds are higher than those prepared by the solid-state reaction because the superior homogeneity and submicron sized powders were prepared without grinding steps. The improvement of the dielectric properties will further expand the potential application of the material for device fabrication. References [1] X. Wang, H. Chan, Solid State Commun. 125 (2003) 395–399. [2] V. Bobnar, B. Malic, J. Jolc, M. Kosec, J. Appl. Phys. 98 (2005) 0241131–024113-4. [3] H. Nagata, T. Takenaka, J. Eur. Ceram. Soc. 21 (2001) 1299–1302. [4] B. Yang, F. Li, J.P. Han, X.J. Yi, H.L.W. Chan, H.C. Chen, W. Cao, J. Phys. D 37 (2004) 921–924.
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