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3O3 ceramics
Materials Research Bulletin 70 (2015) 678–683
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Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu
Effects of structural characteristics on microwave dielectric properties of (Sr0.2Ca0.488Nd0.208)Ti1xGa4x/3O3 ceramics Fei Liua , Changlai Yuanb,* , Xinyu Liua,b , Jingjing Quc, Guohua Chenb , Changrong Zhoub a b c
College of Material Science and Engineering, Central South University, Changsha 410083, PR China College of Material Science and Engineering, Guilin University of Electronic Technology, Guilin 541004, PR China Department of Information Engineering, Guilin University of Aerospace Technology, Guilin 541004, PR China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 24 December 2014 Received in revised form 26 May 2015 Accepted 29 May 2015 Available online 1 June 2015
Structural features and microwave dielectric properties of (Sr0.2Ca0.488Nd0.208)Ti1xGa4x/3O3 (SCNTGx, 0.2 x 0.6) samples were studied. The X-ray patterns revealed that a single phase with orthorhombic perovskite-like structure was obtained in the range of x = 0.2–0.4. A certain amount of Ga2O3 phase was detected at x = 0.5 from the XRD data. However, some unknown phase was formed as a secondary phase when x increased up to 0.6. Moreover, as Ga3+ concentration increased, the er decreased due to the smaller ionic polarization of Ga3+ than that of Ti4+, the Q f value increased firstly and then decreased because of the existence of the unknown secondary phase. And the t f values changed from a positive to negative value, arising from a reduction in tolerance factor (t) for tilted region (t < 1) in perovskite structure. Furthermore, an extra Raman band centered at about 240/236 cm1 was detected in SCNTGx ceramics at x = 0.5, 0.6, which was caused by the appearance of the Ga2O3 phase. The SCNTGx (x = 0.5) ceramic sintered at 1350 C for 4 h showed good microwave dielectric properties with er = 43.7, Q f = 60,100 GHz (at 4.71 GHz) and t f = 8.3 ppm/ C. Obviously, the introduction of Ga2O3 could efficiently improve Q f value of the SCNTGx specimens. ã2015 Elsevier Ltd. All rights reserved.
Keywords: Ceramics Raman Spectroscopy Ga2O3 Dielectric properties
1. Introduction Materials with the perovskite structure, such as CaTiO3 and SrTiO3, are widely employed as dielectric resonators because of their higher dielectric constant (er) [1]. However, their dielectric loss is rather high, compared with the complex perovskites. Therefore, in order to ensure temperature stability and reduce the dielectric loss, much attention has been paid to improve their microwave dielectric properties through La3+, Nd3+, and Sm3+ substitution for Ca2+ at A-site ions [2–5]. Even though such substitution is done, the temperature coefficient of resonant frequency (t f) is still too large. Generally, the high positive t f of CaTiO3-based materials can be suppressed to low value or zero value by addition of perovskite compounds with negative t f values, which make it a potential candidate for the microwave application [6–8]. On the other side, SrTiO3, as a well-known incipient ferroelectric material, has a higher er (300) than that of CaTiO3 (168) [9–11]. However, little consideration has been given to the partial substitution of the Sr2+ ion for A-site ions in ABO3 perovskite materials.
In our previous work, an optimized microwave dielectric properties with er130.4, Q f 9500 GHz and t f +332.5 ppm/ C can be achieved for specimens using (Sr0.2Ca0.488Nd0.208)TiO3 ceramics after sintering at 1400 C for 4 h [12]. The replacing of slight Sr2+ for Ca2+ and Nd3+ on the A-site develops a novel microwave dielectric material with a good microwave dielectric properties. But the t f value is still too large to apply in the microwave communication and passive component applications. According to previous investigations [13,14], Moon et al. and Liang et al. suggested that the t f value could be tuned effectively by the substitution of B-site ions. As we know, when some kind of ions in the crystal lattice are substituted by another kind of ions, these two kinds of ions should have similar ionic radius. Similarly, the ionic radius of Ga3+ (0.62 Å) is close to that of Ti4+ (0.605 Å) [15]. Consequently, the aim of this work is to investigate the crystal structure, microstructures and sintering behavior on SCNTGx (0.2 x 0.6) and improve the microwave dielectric properties of (Sr0.2Ca0.488Nd0.208)TiO3 ceramics by partial substitution of B-site ions and optimizing the composition. 2. Experimental procedure
* Corresponding author. Tel.: +86 773 229 1434; fax: +86 773 229 5903. E-mail addresses: [email protected], [email protected] (C. Yuan). http://dx.doi.org/10.1016/j.materresbull.2015.05.043 0025-5408/ ã 2015 Elsevier Ltd. All rights reserved.
The polycrystalline ceramic samples of (Sr0.2Ca0.488Nd0.208) Ti1xGa4x/3O3 (SCNTGx) with x = 0.2, 0.3, 0.4, 0.5 and 0.6 were
F. Liu et al. / Materials Research Bulletin 70 (2015) 678–683
prepared by solid-state synthesis using the following chemical reaction: 0.2SrCO3 + 0.488CaCO3 + 0.104Nd2O3 + (1x)TiO2 + 2x/3Ga2O3 ! (Sr0.2Ca0.488Nd0.208)Ti1xGa4x/3&2x/3O3 + CO2(") (1) Where “&” represents the B-site vacancy created. It was assumed that there were vacancies in crystallographic sites formed to keep the change balance. The starting materials were highly-pure (99.9%) powers of CaCO3, SrCO3, Nd2O3, TiO2, Ga2O3. All powders were dried at 600 C for 8 h to remove moisture before the use. After weighing, these powders were mixed for 24 h in a ball mill with zirconia balls. The mixture was dried at 100 C and thoroughly milled before it was calcined at 1150 C for 4 h. The sieved powders with 5 wt.% PVA as a binder were pressed into pellets 10 mm in diameter and 4.5–5 mm in thickness under a pressure of 150 MPa and sintered at 1280–1380 C for 4 h in air with a heating rate of 240 C/h and cooling rate of 180 C/h. Phase structural characteristics were examined by X-ray diffraction employing Cu-Ka radiation and Raman spectrum analysis including peak-fitting, data smoothing, quantitative analysis and peak picking. The relative density of the sintered samples was determined from geometry and weight measurements. And their microstructures were examined by the scanning electron microscopy (SEM) equipped with energy dispersive spectroscopy (EDS). The dielectric constant (er) and the unloaded Q f value were measured by the parallel plate method using a network analyzer [16]. The temperature coefficient of resonant frequency (t f) at microwave frequencies was calculated in the temperature range of 25–75 C [17]. Moreover, the tolerance factor (t) was calculated using the following equations: R þ RO t ¼ pffiffiffi A 2ðRB þ RO Þ
(2)
Where RA and RB are the average ionic radii of the ions in A site and also in B site, and RO is oxygen ionic radii [18,19]. 3. Results and discussion Fig. 1 illustrates the room-temperature XRD patterns of the SCNTGx (0.2 x 0.6) ceramics sintered at 1350 C for 4 h. It is clear that a single phase with orthorhombic perovskite, belonging to the space group Pnma (62), is obtained in the range of 0.2 x 0.4. This result is similar to our previous reports on (Sr0.2Ca0.488Nd0.208)TiO3 ceramics without Ga doped [12]. It can be seen from Fig. 1 that the orthorhombic perovskite phase with a small amount of Ga2O3 phase can be indexed in the composition of x = 0.5. Furthermore, the new unknown phase is
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detected at about x = 0.6. The formation of this unknown phase may be due to a different chemical behavior between Ti and Ga at B-site in the SCNTGx specimens. For the sample with x = 0.5, the most intensive (111) peak for the beta-Ga2O3 phase is at about 35.2 , coinciding with the (11 5) peak in perovskite-like phase, it become more intensive at higher Ga contents (x = 0.6). Additionally, the other weaker diffraction peaks at 30.5, 31.7 37.4, 38.4, 45.8, 57.6 and 59.2 degree are shown at x = 0.5, respectively. Furthermore, the variation of lattice parameters and unit cell volume with respect to different x from the XRD patterns are listed in Table 1. According to Shannon ionic radii table [15], the ionic radii of Ti4+ (0.605 Å, CN = 6) is smaller than that of Ga3+ (0.620 Å, CN = 6). Therefore, the substitution of Ga3+ for Ti4+ leads to the increase in unit cell volume, which accords with the Vegard’s law. Fig. 2 shows the BEIs (backscattered electron images) of the polished and thermally etched surface of the SCNTGx (0.2 x 0.6) ceramics sintered at 1350 C for 4 h. Fig. 2(a)–(c) illustrate that the color of all the grains is basically the same, which suggests that a single phase exists in the SCNTGx (0.2 x 0.4) ceramics. However, some white flecks grains are observed in the specimens with x = 0.5 [see Fig. 2(d)]. For the ceramics with x = 0.6, the amount of white flecks grains decrease, while another grains with black sticky shape are discovered, as shown in Fig. 2(e). These results also show that the SCNTGx solid solutions occur as x increases from 0.2 to 0.4, which is in agreement with XRD patterns (in Fig. 1). Moreover, it can also be observed that the ceramics of all compositions show inhomogeneous grain sizes, with large grains of 8 mm and small grains of 2–4 mm. The difference in grain sizes may be due to structural disorder, and strain developed in the lattice because of different ionic radii between Ti and Ga at B-site. S. Parida had also reported similar results in Ca(ZrxTi1x)O3 systems [20]. On the other hand, an increasing porous microstructures are developed in specimens with 0.4 x 0.5, but the ratio still remains below 5%. In order to identify the compositional difference among these three types of grains, the composition analysis is conducted by using EDS on the grains marked ‘A’,‘B’ and ‘C’, as shown in Fig. 2(d) and (e). The EDS results and the atom ratio (%) are shown in Fig. 3(a)–(c). By the EDS data of grain ‘A’, there is only a small difference in stoichiometry between the observed value and the theoretical value for SCNTGx (x = 0.5) sample. While the atom ratio of Ga to O is almost 2:3, and the white flecks grains ‘B’ are therefore confirmed to be Ga2O3, in which a small amount of Ca and Ti atoms dissolves [see Fig. 3(b)]. Moreover, the EDS spectra reveals that less Ga element exists in the grain ‘C’ region, the SCNTGx (x = 0.6) ceramic is offstoichiometric from the beginning of powder processing. It illustrates that the isolated phase is mainly composed of Ca, Ti, Nd, Sr and O, as shown in Fig. 3(c). Fig. 4(a) shows the room-temperature Raman spectra of the SCNTGx (0.2 x 0.6) ceramics in the frequency range of 100–1000 cm1. In the Raman spectrum of the SCNTGx (x = 0.2) sample, eight scattering bands centered at 188, 283, 318, 410, 475,
Table 1 Lattice parameters and tolerance factor (t) of SCNTGx (0.2 x 0.6) specimens sintered at 1350 C for 4 h. x
Fig. 1. X-ray diffraction patterns of SCNTGx (0.2 x 0.6) specimens sintered at 1350 C for 4 h.
0.2 0.3 0.4 0.5 0.6
Lattice parameters a (Å)
b (Å)
c (Å)
5.4494 5.4569 5.4445 5.4475 5.4547
7.7032 7.6899 7.6838 7.7038 7.6906
5.3592 5.3673 5.4486 5.4459 5.4547
Unit cell volume (Å3)
Tolerance factor (t)
224.97 225.24 227.94 228.55 228.83
0.787 0.783 0.780 0.776 0.773
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Fig. 2. BEIs images of the polished and thermally etched surface of SCNTGx ceramics sintered at 1350 C for 4 h: (a) x = 0.2; (b) x = 0.3; (c) x = 0.4; (d) x = 0.5; (e) x = 0.6.
639, 779, and 912 cm1 are observed. Since no relevant work on SCNTGx ceramics is reported, a comparison is not possible here. However, it is similar to the Raman spectrum of CaTiO3 [21], where eight Raman bands are also observed at 183, 227, 247, 288, 339, 470, 494 and 641 cm1. Moreover, the band observed at 641 cm1 can be assigned to the TiO symmetric stretching vibration. Due to the different ionic sizes and force constants of Ga3+ and Ti4+, two adjacent corner sharing oxygen octahedra may result in the rotation of oxygen octahedral cage associated with the orthorhombic distortion [22]. Therefore, the band at 639 cm1 in SCNTGx (x = 0.2) ceramic is attributed to the stretching in B O site, a similar results has been reported by Balachandran et al. [23]. Furthermore, an extra weak Raman band centered at about 236 cm1 is observed in the sample with x = 0.5, then the band becomes stronger with increasing x, which implies that the B-site ions occupied by Ti4+ and Ga3+ exceed the limit of solubility as x > 0.4. Moreover, the relative intensity of the Raman peaks located at 410/427 cm1 is increasingly stronger and the widths gradually decrease, as shown in Fig. 4(b). It has been suggested that the variations of these bands are characteristic of the increase in B-site ordering for complex perovskites [22]. Fig. 5 shows the variation of the relative density with x for SCNTGx ceramics sintered at various temperatures for 4 h. All the samples display a rather high relative density (>96%). It can be observed that with the increasing sintering temperature, the
relative densities firstly increase and then reach a saturation value at about 1320–1350 C. The relative density obtains a maximum 98.8% in the SCNTGx ceramics with x = 0.3. Fig. 6 is a plot of er and Q f values with x for SCNTGx ceramics sintered at various temperatures for 4 h. Fig. 6(a) shows that the er of SCNTGx ceramics linearly decreases with the increasing amount of Ga3+ content. Because the ionic polarizability of Ga3+ (1.50 Å3) is lower than that of Ti4+ (2.93 Å3) on the B-site [24], leading to the decrease of the dielectric polarizability in SCNTGx ceramics with increasing x. The dotted line in the figures of er indicates that it is insensitive to the sintering temperature. Therefore, dielectric polarizability plays an important factor in affecting the er for these ceramic systems. In addition, there may be a lower er for the new unknown second phase, which can further reduce the er in SCNTGx (x = 0.6) ceramics. As can be seen in Fig. 6(b), the Q f value increases with increasing x from 0.2 to 0.5. For the SCNTGx (x = 0.5) ceramics sintered at 1350 C for 4 h, the Q f value reaches the maximum 60100 GHz (at 4.91 GHz). It is generally believed that the Q f value of the specimens is dependent on secondary phase, grain size and porosity [25,26]. An impact on the higher Q f value for SCNTGx (x = 0.5) ceramics mainly depends on an obvious secondary phase of Ga2O3. In fact, the introduction of Ga2O3 can efficiently improve Q f value, which accords with several reports in different microwave ceramic systems including Ba(Zn1/3Ta2/3)
F. Liu et al. / Materials Research Bulletin 70 (2015) 678–683
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Fig. 3. EDS spectra of SCNTGx (x = 0.5, 0.6) ceramics: (a) grain ‘A’; (b) grain ‘B’; and (c) grain ‘C’.
Fig. 4. (a) Raman spectra of SCNTGx (0.2 x 0.6) ceramics sintered at 1350 C; (b) the enlarged figure of the 410/427 cm1 band.
O3, Yb3Al5O12 and BaTiO3 ceramics [27–29]. On the other hand, the Q f value decreases sharply at x = 0.6. It may be caused by the appearance of new unknown second phase although there can still be a small amount of Ga2O3. Besides this, the relationship between Q f values and sintering temperature shows the same trend as that between relative densities and sintering temperature because higher density means lower porosity (see Fig. 5). Therefore, the lower Q f values can occur at a lower or a higher sintering temperature for the same x value [30,31]. The orthorhombic perovskite lattice with tilted oxygen octahedra surrounding the Ga/Ti atoms leads to the variation of stability and symmetry of the perovskite phase for SCNTGx
(0.2 x 0.6) specimens. The tolerance factor (t) is calculated from Eq. (2) and the results are listed in Table 1. In general, the t f value is strongly dependent on tolerance factor in tilted region (t < 1) if the perovskite compound has the tilted oxygen octahedral [32]. In this work, the t f values are measured at 4.3–6.1 GHz from temperatures 25 C to 75 C. The dependence of t f value on tolerance factor (t) of SCNTGx (0.2 x 0.6) ceramics sintered at 1350 C for 4 h is shown in Fig. 7. The decreasing t means that the symmetry in perovskite phase gradually reduces with an increasing of x, which results in the decrease of the rattling effect. Therefore, the t f value linearly decreases from 132.2 ppm/ C at x = 0.2 to 9.7 ppm/ C at x = 0.6.
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F. Liu et al. / Materials Research Bulletin 70 (2015) 678–683
Fig. 5. The variation of the relative density with x for SCNTGx ceramics sintered at various temperatures for 4 h.
of the Raman peaks located at 410/427 cm1 were increasingly stronger and the widths decreased with increasing x, implying the degree of order was enhanced. The er had a quasi-linear decrease due to the fact that the ionic polarizability of Ga3+ was lower than that of Ti4+ on the B-site. The introduction of the Ga2O3 was found to be beneficial in improving Q f values of the SCNTGx (x = 0.5) specimens. Moreover, the t f values decreased with the increase of x, arising from a reduction in tolerance factor (t) for tilted region (t < 1) in perovskite structure. The SCNTGx (x = 0.5) ceramic sintered at 1350 C for 4 h shows good microwave dielectric properties with er = 43.7, Q f = 60,100 GHz (at 4.91 GHz) and t f = 8.3 ppm/ C. Acknowledgements
Fig. 6. (a) The er and (b) Q f values of SCNTGx (0.2 x 0.6) ceramics as functions of various sintering temperatures for 4 h.
Financial supports of the National Natural Science Foundation of China (Grant no. 11464006), the Natural Science Foundation of Guangxi (Grant no. 2014GXNSFBA118254), the research fund of Guangxi Key Laboratory of Information Materials through 131018-Z, 131004-Z and Guangxi Experiment Center of Information Science through 20130115 are gratefully acknowledged by the authors. References
Fig. 7. The dependence of t f values on tolerance factor (t) of SCNTGx (0.2 x 0.6) ceramics sintered at 1350 C for 4 h.
4. Conclusion The crystal structures and microwave dielectric properties of SCNTGx (0.2 x 0.6) ceramics were investigated in this paper. The X-ray patterns revealed that the orthorhombic perovskite-like structure could be obtained in the range of x = 0.2–0.6. However, a certain amount of Ga2O3 phase and unknown second phase were also detected at x = 0.5 and 0.6, respectively. The relative intensities
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Contents lists available at ScienceDirect
Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu
Effects of structural characteristics on microwave dielectric properties of (Sr0.2Ca0.488Nd0.208)Ti1xGa4x/3O3 ceramics Fei Liua , Changlai Yuanb,* , Xinyu Liua,b , Jingjing Quc, Guohua Chenb , Changrong Zhoub a b c
College of Material Science and Engineering, Central South University, Changsha 410083, PR China College of Material Science and Engineering, Guilin University of Electronic Technology, Guilin 541004, PR China Department of Information Engineering, Guilin University of Aerospace Technology, Guilin 541004, PR China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 24 December 2014 Received in revised form 26 May 2015 Accepted 29 May 2015 Available online 1 June 2015
Structural features and microwave dielectric properties of (Sr0.2Ca0.488Nd0.208)Ti1xGa4x/3O3 (SCNTGx, 0.2 x 0.6) samples were studied. The X-ray patterns revealed that a single phase with orthorhombic perovskite-like structure was obtained in the range of x = 0.2–0.4. A certain amount of Ga2O3 phase was detected at x = 0.5 from the XRD data. However, some unknown phase was formed as a secondary phase when x increased up to 0.6. Moreover, as Ga3+ concentration increased, the er decreased due to the smaller ionic polarization of Ga3+ than that of Ti4+, the Q f value increased firstly and then decreased because of the existence of the unknown secondary phase. And the t f values changed from a positive to negative value, arising from a reduction in tolerance factor (t) for tilted region (t < 1) in perovskite structure. Furthermore, an extra Raman band centered at about 240/236 cm1 was detected in SCNTGx ceramics at x = 0.5, 0.6, which was caused by the appearance of the Ga2O3 phase. The SCNTGx (x = 0.5) ceramic sintered at 1350 C for 4 h showed good microwave dielectric properties with er = 43.7, Q f = 60,100 GHz (at 4.71 GHz) and t f = 8.3 ppm/ C. Obviously, the introduction of Ga2O3 could efficiently improve Q f value of the SCNTGx specimens. ã2015 Elsevier Ltd. All rights reserved.
Keywords: Ceramics Raman Spectroscopy Ga2O3 Dielectric properties
1. Introduction Materials with the perovskite structure, such as CaTiO3 and SrTiO3, are widely employed as dielectric resonators because of their higher dielectric constant (er) [1]. However, their dielectric loss is rather high, compared with the complex perovskites. Therefore, in order to ensure temperature stability and reduce the dielectric loss, much attention has been paid to improve their microwave dielectric properties through La3+, Nd3+, and Sm3+ substitution for Ca2+ at A-site ions [2–5]. Even though such substitution is done, the temperature coefficient of resonant frequency (t f) is still too large. Generally, the high positive t f of CaTiO3-based materials can be suppressed to low value or zero value by addition of perovskite compounds with negative t f values, which make it a potential candidate for the microwave application [6–8]. On the other side, SrTiO3, as a well-known incipient ferroelectric material, has a higher er (300) than that of CaTiO3 (168) [9–11]. However, little consideration has been given to the partial substitution of the Sr2+ ion for A-site ions in ABO3 perovskite materials.
In our previous work, an optimized microwave dielectric properties with er130.4, Q f 9500 GHz and t f +332.5 ppm/ C can be achieved for specimens using (Sr0.2Ca0.488Nd0.208)TiO3 ceramics after sintering at 1400 C for 4 h [12]. The replacing of slight Sr2+ for Ca2+ and Nd3+ on the A-site develops a novel microwave dielectric material with a good microwave dielectric properties. But the t f value is still too large to apply in the microwave communication and passive component applications. According to previous investigations [13,14], Moon et al. and Liang et al. suggested that the t f value could be tuned effectively by the substitution of B-site ions. As we know, when some kind of ions in the crystal lattice are substituted by another kind of ions, these two kinds of ions should have similar ionic radius. Similarly, the ionic radius of Ga3+ (0.62 Å) is close to that of Ti4+ (0.605 Å) [15]. Consequently, the aim of this work is to investigate the crystal structure, microstructures and sintering behavior on SCNTGx (0.2 x 0.6) and improve the microwave dielectric properties of (Sr0.2Ca0.488Nd0.208)TiO3 ceramics by partial substitution of B-site ions and optimizing the composition. 2. Experimental procedure
* Corresponding author. Tel.: +86 773 229 1434; fax: +86 773 229 5903. E-mail addresses: [email protected], [email protected] (C. Yuan). http://dx.doi.org/10.1016/j.materresbull.2015.05.043 0025-5408/ ã 2015 Elsevier Ltd. All rights reserved.
The polycrystalline ceramic samples of (Sr0.2Ca0.488Nd0.208) Ti1xGa4x/3O3 (SCNTGx) with x = 0.2, 0.3, 0.4, 0.5 and 0.6 were
F. Liu et al. / Materials Research Bulletin 70 (2015) 678–683
prepared by solid-state synthesis using the following chemical reaction: 0.2SrCO3 + 0.488CaCO3 + 0.104Nd2O3 + (1x)TiO2 + 2x/3Ga2O3 ! (Sr0.2Ca0.488Nd0.208)Ti1xGa4x/3&2x/3O3 + CO2(") (1) Where “&” represents the B-site vacancy created. It was assumed that there were vacancies in crystallographic sites formed to keep the change balance. The starting materials were highly-pure (99.9%) powers of CaCO3, SrCO3, Nd2O3, TiO2, Ga2O3. All powders were dried at 600 C for 8 h to remove moisture before the use. After weighing, these powders were mixed for 24 h in a ball mill with zirconia balls. The mixture was dried at 100 C and thoroughly milled before it was calcined at 1150 C for 4 h. The sieved powders with 5 wt.% PVA as a binder were pressed into pellets 10 mm in diameter and 4.5–5 mm in thickness under a pressure of 150 MPa and sintered at 1280–1380 C for 4 h in air with a heating rate of 240 C/h and cooling rate of 180 C/h. Phase structural characteristics were examined by X-ray diffraction employing Cu-Ka radiation and Raman spectrum analysis including peak-fitting, data smoothing, quantitative analysis and peak picking. The relative density of the sintered samples was determined from geometry and weight measurements. And their microstructures were examined by the scanning electron microscopy (SEM) equipped with energy dispersive spectroscopy (EDS). The dielectric constant (er) and the unloaded Q f value were measured by the parallel plate method using a network analyzer [16]. The temperature coefficient of resonant frequency (t f) at microwave frequencies was calculated in the temperature range of 25–75 C [17]. Moreover, the tolerance factor (t) was calculated using the following equations: R þ RO t ¼ pffiffiffi A 2ðRB þ RO Þ
(2)
Where RA and RB are the average ionic radii of the ions in A site and also in B site, and RO is oxygen ionic radii [18,19]. 3. Results and discussion Fig. 1 illustrates the room-temperature XRD patterns of the SCNTGx (0.2 x 0.6) ceramics sintered at 1350 C for 4 h. It is clear that a single phase with orthorhombic perovskite, belonging to the space group Pnma (62), is obtained in the range of 0.2 x 0.4. This result is similar to our previous reports on (Sr0.2Ca0.488Nd0.208)TiO3 ceramics without Ga doped [12]. It can be seen from Fig. 1 that the orthorhombic perovskite phase with a small amount of Ga2O3 phase can be indexed in the composition of x = 0.5. Furthermore, the new unknown phase is
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detected at about x = 0.6. The formation of this unknown phase may be due to a different chemical behavior between Ti and Ga at B-site in the SCNTGx specimens. For the sample with x = 0.5, the most intensive (111) peak for the beta-Ga2O3 phase is at about 35.2 , coinciding with the (11 5) peak in perovskite-like phase, it become more intensive at higher Ga contents (x = 0.6). Additionally, the other weaker diffraction peaks at 30.5, 31.7 37.4, 38.4, 45.8, 57.6 and 59.2 degree are shown at x = 0.5, respectively. Furthermore, the variation of lattice parameters and unit cell volume with respect to different x from the XRD patterns are listed in Table 1. According to Shannon ionic radii table [15], the ionic radii of Ti4+ (0.605 Å, CN = 6) is smaller than that of Ga3+ (0.620 Å, CN = 6). Therefore, the substitution of Ga3+ for Ti4+ leads to the increase in unit cell volume, which accords with the Vegard’s law. Fig. 2 shows the BEIs (backscattered electron images) of the polished and thermally etched surface of the SCNTGx (0.2 x 0.6) ceramics sintered at 1350 C for 4 h. Fig. 2(a)–(c) illustrate that the color of all the grains is basically the same, which suggests that a single phase exists in the SCNTGx (0.2 x 0.4) ceramics. However, some white flecks grains are observed in the specimens with x = 0.5 [see Fig. 2(d)]. For the ceramics with x = 0.6, the amount of white flecks grains decrease, while another grains with black sticky shape are discovered, as shown in Fig. 2(e). These results also show that the SCNTGx solid solutions occur as x increases from 0.2 to 0.4, which is in agreement with XRD patterns (in Fig. 1). Moreover, it can also be observed that the ceramics of all compositions show inhomogeneous grain sizes, with large grains of 8 mm and small grains of 2–4 mm. The difference in grain sizes may be due to structural disorder, and strain developed in the lattice because of different ionic radii between Ti and Ga at B-site. S. Parida had also reported similar results in Ca(ZrxTi1x)O3 systems [20]. On the other hand, an increasing porous microstructures are developed in specimens with 0.4 x 0.5, but the ratio still remains below 5%. In order to identify the compositional difference among these three types of grains, the composition analysis is conducted by using EDS on the grains marked ‘A’,‘B’ and ‘C’, as shown in Fig. 2(d) and (e). The EDS results and the atom ratio (%) are shown in Fig. 3(a)–(c). By the EDS data of grain ‘A’, there is only a small difference in stoichiometry between the observed value and the theoretical value for SCNTGx (x = 0.5) sample. While the atom ratio of Ga to O is almost 2:3, and the white flecks grains ‘B’ are therefore confirmed to be Ga2O3, in which a small amount of Ca and Ti atoms dissolves [see Fig. 3(b)]. Moreover, the EDS spectra reveals that less Ga element exists in the grain ‘C’ region, the SCNTGx (x = 0.6) ceramic is offstoichiometric from the beginning of powder processing. It illustrates that the isolated phase is mainly composed of Ca, Ti, Nd, Sr and O, as shown in Fig. 3(c). Fig. 4(a) shows the room-temperature Raman spectra of the SCNTGx (0.2 x 0.6) ceramics in the frequency range of 100–1000 cm1. In the Raman spectrum of the SCNTGx (x = 0.2) sample, eight scattering bands centered at 188, 283, 318, 410, 475,
Table 1 Lattice parameters and tolerance factor (t) of SCNTGx (0.2 x 0.6) specimens sintered at 1350 C for 4 h. x
Fig. 1. X-ray diffraction patterns of SCNTGx (0.2 x 0.6) specimens sintered at 1350 C for 4 h.
0.2 0.3 0.4 0.5 0.6
Lattice parameters a (Å)
b (Å)
c (Å)
5.4494 5.4569 5.4445 5.4475 5.4547
7.7032 7.6899 7.6838 7.7038 7.6906
5.3592 5.3673 5.4486 5.4459 5.4547
Unit cell volume (Å3)
Tolerance factor (t)
224.97 225.24 227.94 228.55 228.83
0.787 0.783 0.780 0.776 0.773
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Fig. 2. BEIs images of the polished and thermally etched surface of SCNTGx ceramics sintered at 1350 C for 4 h: (a) x = 0.2; (b) x = 0.3; (c) x = 0.4; (d) x = 0.5; (e) x = 0.6.
639, 779, and 912 cm1 are observed. Since no relevant work on SCNTGx ceramics is reported, a comparison is not possible here. However, it is similar to the Raman spectrum of CaTiO3 [21], where eight Raman bands are also observed at 183, 227, 247, 288, 339, 470, 494 and 641 cm1. Moreover, the band observed at 641 cm1 can be assigned to the TiO symmetric stretching vibration. Due to the different ionic sizes and force constants of Ga3+ and Ti4+, two adjacent corner sharing oxygen octahedra may result in the rotation of oxygen octahedral cage associated with the orthorhombic distortion [22]. Therefore, the band at 639 cm1 in SCNTGx (x = 0.2) ceramic is attributed to the stretching in B O site, a similar results has been reported by Balachandran et al. [23]. Furthermore, an extra weak Raman band centered at about 236 cm1 is observed in the sample with x = 0.5, then the band becomes stronger with increasing x, which implies that the B-site ions occupied by Ti4+ and Ga3+ exceed the limit of solubility as x > 0.4. Moreover, the relative intensity of the Raman peaks located at 410/427 cm1 is increasingly stronger and the widths gradually decrease, as shown in Fig. 4(b). It has been suggested that the variations of these bands are characteristic of the increase in B-site ordering for complex perovskites [22]. Fig. 5 shows the variation of the relative density with x for SCNTGx ceramics sintered at various temperatures for 4 h. All the samples display a rather high relative density (>96%). It can be observed that with the increasing sintering temperature, the
relative densities firstly increase and then reach a saturation value at about 1320–1350 C. The relative density obtains a maximum 98.8% in the SCNTGx ceramics with x = 0.3. Fig. 6 is a plot of er and Q f values with x for SCNTGx ceramics sintered at various temperatures for 4 h. Fig. 6(a) shows that the er of SCNTGx ceramics linearly decreases with the increasing amount of Ga3+ content. Because the ionic polarizability of Ga3+ (1.50 Å3) is lower than that of Ti4+ (2.93 Å3) on the B-site [24], leading to the decrease of the dielectric polarizability in SCNTGx ceramics with increasing x. The dotted line in the figures of er indicates that it is insensitive to the sintering temperature. Therefore, dielectric polarizability plays an important factor in affecting the er for these ceramic systems. In addition, there may be a lower er for the new unknown second phase, which can further reduce the er in SCNTGx (x = 0.6) ceramics. As can be seen in Fig. 6(b), the Q f value increases with increasing x from 0.2 to 0.5. For the SCNTGx (x = 0.5) ceramics sintered at 1350 C for 4 h, the Q f value reaches the maximum 60100 GHz (at 4.91 GHz). It is generally believed that the Q f value of the specimens is dependent on secondary phase, grain size and porosity [25,26]. An impact on the higher Q f value for SCNTGx (x = 0.5) ceramics mainly depends on an obvious secondary phase of Ga2O3. In fact, the introduction of Ga2O3 can efficiently improve Q f value, which accords with several reports in different microwave ceramic systems including Ba(Zn1/3Ta2/3)
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Fig. 3. EDS spectra of SCNTGx (x = 0.5, 0.6) ceramics: (a) grain ‘A’; (b) grain ‘B’; and (c) grain ‘C’.
Fig. 4. (a) Raman spectra of SCNTGx (0.2 x 0.6) ceramics sintered at 1350 C; (b) the enlarged figure of the 410/427 cm1 band.
O3, Yb3Al5O12 and BaTiO3 ceramics [27–29]. On the other hand, the Q f value decreases sharply at x = 0.6. It may be caused by the appearance of new unknown second phase although there can still be a small amount of Ga2O3. Besides this, the relationship between Q f values and sintering temperature shows the same trend as that between relative densities and sintering temperature because higher density means lower porosity (see Fig. 5). Therefore, the lower Q f values can occur at a lower or a higher sintering temperature for the same x value [30,31]. The orthorhombic perovskite lattice with tilted oxygen octahedra surrounding the Ga/Ti atoms leads to the variation of stability and symmetry of the perovskite phase for SCNTGx
(0.2 x 0.6) specimens. The tolerance factor (t) is calculated from Eq. (2) and the results are listed in Table 1. In general, the t f value is strongly dependent on tolerance factor in tilted region (t < 1) if the perovskite compound has the tilted oxygen octahedral [32]. In this work, the t f values are measured at 4.3–6.1 GHz from temperatures 25 C to 75 C. The dependence of t f value on tolerance factor (t) of SCNTGx (0.2 x 0.6) ceramics sintered at 1350 C for 4 h is shown in Fig. 7. The decreasing t means that the symmetry in perovskite phase gradually reduces with an increasing of x, which results in the decrease of the rattling effect. Therefore, the t f value linearly decreases from 132.2 ppm/ C at x = 0.2 to 9.7 ppm/ C at x = 0.6.
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Fig. 5. The variation of the relative density with x for SCNTGx ceramics sintered at various temperatures for 4 h.
of the Raman peaks located at 410/427 cm1 were increasingly stronger and the widths decreased with increasing x, implying the degree of order was enhanced. The er had a quasi-linear decrease due to the fact that the ionic polarizability of Ga3+ was lower than that of Ti4+ on the B-site. The introduction of the Ga2O3 was found to be beneficial in improving Q f values of the SCNTGx (x = 0.5) specimens. Moreover, the t f values decreased with the increase of x, arising from a reduction in tolerance factor (t) for tilted region (t < 1) in perovskite structure. The SCNTGx (x = 0.5) ceramic sintered at 1350 C for 4 h shows good microwave dielectric properties with er = 43.7, Q f = 60,100 GHz (at 4.91 GHz) and t f = 8.3 ppm/ C. Acknowledgements
Fig. 6. (a) The er and (b) Q f values of SCNTGx (0.2 x 0.6) ceramics as functions of various sintering temperatures for 4 h.
Financial supports of the National Natural Science Foundation of China (Grant no. 11464006), the Natural Science Foundation of Guangxi (Grant no. 2014GXNSFBA118254), the research fund of Guangxi Key Laboratory of Information Materials through 131018-Z, 131004-Z and Guangxi Experiment Center of Information Science through 20130115 are gratefully acknowledged by the authors. References
Fig. 7. The dependence of t f values on tolerance factor (t) of SCNTGx (0.2 x 0.6) ceramics sintered at 1350 C for 4 h.
4. Conclusion The crystal structures and microwave dielectric properties of SCNTGx (0.2 x 0.6) ceramics were investigated in this paper. The X-ray patterns revealed that the orthorhombic perovskite-like structure could be obtained in the range of x = 0.2–0.6. However, a certain amount of Ga2O3 phase and unknown second phase were also detected at x = 0.5 and 0.6, respectively. The relative intensities
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