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Crystal structure and microwave dielectric properties of CaTiO3–La[Ga(1−δ)Alδ]O3 ceramics system
Materials Research Bulletin 57 (2014) 140–145
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Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu
Crystal structure and microwave dielectric properties of CaTiO3–La [Ga(1 d)Ald]O3 ceramics system Fei Liang *, Meng Ni, Wenzhong Lu, Shaoshuai Feng School of optical and electronic information, Huazhong University of Science and Technology, Wuhan 430074, PR China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 18 November 2013 Received in revised form 24 February 2014 Accepted 24 May 2014 Available online 28 May 2014
The crystal structure and microwave dielectric properties of 0.64CaTiO3–0.36La[Ga(1 d)Ald]O3 specimens have been investigated in this work. When d 0.5, the crystal structure of 0.64CaTiO3– 0.36La[Ga(1 d)Ald]O3 was orthorhombic, while it transformed into the tetragonal structure when d > 0.5. When d = 0.5, specimens exhibited good microwave dielectric properties with er = 45.54, Q f = GHz) and t f = 1.7 ppm/ C. The 50 at% substitution of Ga3+ by Al3+ in 0.64CaTiO3–0.36LaGaO3 benefited the Q f value, the results showed that all samples of xCaTiO3–(1 x)La[Ga0.5Al0.5]O3 (x = 0.62, 0.63, 0.64, 0.65, 0.66) exhibited rr 96.8%, Q f 39,000 GHz, 44 er 48 and 8 ppm/ C t f +6 ppm/ C. ã 2014 Elsevier Ltd. All rights reserved.
Keywords: B. Phase transitions B. Microstructure D. Crystal structure
1. Introduction Microwave dielectric materials with a high Q f value, a high dielectric constant and a good stability of temperature coefficient of resonant frequency have been widely investigated all over the world because of their extensive applications to microwave devices, such as satellite communications, mobile technologies and microwave devices [1–5]. Among various candidates, microwave dielectric ceramics based on CaTiO3 system such as CaTiO3–LaGaO3 system and CaTiO3–LaAlO3 system are attracting great interests around the world due to their intermediate dielectric permittivity (er 45), high Q f value (Q f 40,000 GHz), and the low dielectric permittivity temperature coefficient (t f 0) [6,7]. CaTiO3 is an orthorhombic distorted perovskite at room temperature with high relative permittivity (er = 170), modest quality factor (Q f = 3500 GHz) and high positive temperature coefficient of resonant frequency (t f + 800 ppm/ C). On the contrary, LaGaO3 is a orthorhombic perovskite at room temperature but with low relative permittivity (er = 27), high quality factor (Q f = 97,000 GHz) and negative temperature coefficient of resonant frequency (t f = 80 ppm/ C). The high positive t f of CaTiO3 can be suppressed to low value or zero value t f with the
* Corresponding author. Tel.: +86 27 87542594; fax: +86 27 87543134. E-mail address: [email protected] (F. Liang). http://dx.doi.org/10.1016/j.materresbull.2014.05.042 0025-5408/ ã 2014 Elsevier Ltd. All rights reserved.
addition of LaGaO3, which make it a potential candidate for the microwave application [8,9]. According to previous investigations [10–12] and our work, 0.64CaTiO3–0.36LaGaO3 system showed quite good microwave dielectric properties (er 48, Q f 42,000 GHz, t f 1.5 ppm/ C). However, the price of Ga2O3 is recently rather high, which greatly limits the application of 0.64CaTiO3–0.36LaGaO3 microwave ceramic. Moon et al. [13] suggested that the improvement of the microwave dielectric properties could be achieved by the substitution of B-sites and the increase in ordering degree of the Bsite 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 electrical properties and ionic radius. The electrical properties of the Al3+ are similar to that of Ga3+ at B site and the ionic radius of Al3+ (0.535 Å) is also close to the ionic radius of Ga3+(0.62 Å), but the ionic radius of La3+(1.36 Å) at A-site is much larger than that of Al3+ [14]. So Ga3+ at B site instead of La3+ at A-site is more likely substituted by Al3+ when Al3+ ions are doped in the 0.64CaTiO3–0.36LaGaO3 compounds. When Ga3+ in 0.64CaTiO3– 0.36LaGaO3 is substituted partly by Al3+, it would be possible to develop a novel microwave dielectric material with a good microwave dielectric properties and a relatively low price. The aim of this work was to investigate the crystal structure of xCaTiO3–(1 x)La[Ga(1 d)Ald]O3 and improve the microwave dielectric properties of 0.64CaTiO3–0.36LaGaO3 by partial substitution of B-site ions and optimizing the composition and preparation process. The effects of the part substitution of B-site
F. Liang et al. / Materials Research Bulletin 57 (2014) 140–145
ions and the optimization of composition on the crystal structure and the microwave dielectric properties of ceramics were also discussed in details. 2. Experimental procedure The powders were prepared via the conventional solid-phase synthesis technique from the raw materials of CaCO3 (99.3%), TiO2 (99.6%), La2O3 (99.99%), Ga2O3 (99.99%), Nd2O3 (99.9%) and Al2O3 (99.6%). They were weighed according to the composition of 0.64CaTiO3–0.36La[Ga(1 d)Ald]O3 (d = 0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9) and xCaTiO3–(1 x)La[Ga0.5Al0.5]O3 (x = 0.62, 0.63, 0.64, 0.65, 0.66). The weighed chemicals were mixed by ZrO2 balls with the deionized water for 8 h and then calcined in the range of 1250–1400 C for 4 h in the air after being dried. The calcined mixture was milled in the deionized water with ZrO2 balls again for 8 h. Then the samples were prepared by pressing the powders into pellets in a steel die of 12 mm diameter at 10 MPa. After draining the glue, the pellets were sintered at 1550 C for 6 h and the cooling rate was 0.5 C/min. The density of sintered samples was tested by Archimedes method. After the determination of bulk density results, the relative density of the samples was obtained by dividing the bulk density to the theoretical density. The theoretical density of the samples was calculated by using the lattice parameters obtained from the XRD analysis, Avogadro constant and molar weight of the 0.64CaTiO3–0.36La[Ga(1 d)Ald]O3 (CTLGA). The phase identification was carried out by X-ray diffraction (XRD-7000, Shimadzu Corporation, Kyoto, Japan) working with conventional Cu Ka radiation and scanning at 2 /min in the range of 10–90 . The microstructure of the sintered specimens was measured by the scanning electron microscopy (SEM, VEGA3, TESCAN, Brno, Czech Republic) equipped with energy dispersive spectroscopy (EDS). The lattice parameters were measured by indexing and leastsquares powder diffraction program. The dielectric constant (er) and the unloaded Q f (@4.4–4.6 GHz) value were measured by the Hakki and Coleman method using an Agilent E8362B (Agilent Technologies, Inc., Santa Clara, USA) network analyzer and two parallel silver plates. The temperature coefficient of resonant frequency (t f) was calculated in the temperature range of 25–80 C. 3. Results and discussion
141
Table 1 The Q f values and the relative density of 0.64CaTiO3–0.36La[Ga0.5Al0.5]O3 ceramics with different claining temperatures. Calcination temperature ( C) Q f value (GHz) Relative density r (%)
1270 33,034 95.65
1290 35,775 96.14
1310 38,490 96.48
1330 41,225 96.79
1350 43,897 97.42
Fig. 1, the intensity of the peak increased with the increase of the calcining temperature, suggesting high calcining temperature makes contributions to the formation of 0.64CaTiO3–0.36La [Ga0.5Al0.5]O3. The peaks marked with arrows represented intermediate phases. Fig. 1 shows a single phase of the perovskite structure with the orthorhombic crystal system, which is obtained only at 1350 C. Table 1 shows that the calcining temperature is very important for the Q f value and the relative density of 0.64CaTiO3–0.36La[Ga0.5Al0.5]O3 ceramics. For example, the Q f value of 0.64CaTiO3–0.36La[Ga0.5Al0.5]O3 ceramics calcined at 1330 C was markedly lower than that at 1350 C under the same sintering condition, even if there was only a small amount of intermediate phase at 1330 C calcining temperature. However, higher calcining temperature was not discussed in our study because it could result in the difficulty with decreasing particle size when powders were ball-milled again. Therefore, 1350 C was chosen as the best calcining temperature for the CTLGA ceramics in our work [15,16]. The sintering temperature of 1550 C was chosen finally according to our previous work, and the Q f value of 0.64CaTiO3–0.36 La[Ga0.5Al0.5]O3 ceramics with the cooling rates of 2 C/min, 0.5 C/min and 0.1 C/min were 40,694 GHz, 43,897 GHz and 45,168 GHz, respectively. It was evident that the Q f value escalated markedly with the slowing cooling rate. In the study of Wang et al. [17], no conclusive reason was provided concerning the cooling rate slowing to explain the dependence of Q f value on the cooling rate. Jancar et al. [18] suggested that the Q f value of various complex perovskite systems could be greatly improved by decreasing the cooling rate because the formation of the high ordering degree of B-site cations was generally a relatively slow process. However, the lower the slowing cooling rate was, the more time the preparation process needed. With the Q f value and the preparation time taken into comprehensive consideration, the cooling rate of 0.5 C/min was selected.
3.1. Calcination and sintering properties Fig. 1 shows the X-ray diffraction patterns of 0.64CaTiO3–0.36La [Ga0.5Al0.5]O3 ceramics calcined over 1270–1350 C. According to
Fig. 1. XRD patterns of 0.64CaTiO3–0.36La[Ga0.5Al0.5]O3 samples calcined at different temperatures (a) 1270 C, (b) 1290 C, (c) 1310 C, (d) 1330 C, (e) 1350 C.
Fig. 2. XRD patterns of CTLGA samples with different LaAlO3 contents (a) d = 0, (b) d = 0.1, (c) d = 0.3, (d) d = 0.5, (e) d = 0.7, (f) d = 0.9.
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F. Liang et al. / Materials Research Bulletin 57 (2014) 140–145 Table 4 Performance of xCaTiO3–(1 contents.
Fig. 7. XRD patterns of xCaTiO3–(1 x)La[Ga0.5Al0.5]O3 samples with different CaTiO3 contents (a) x = 0.62, (b) x = 0.63, (c) x = 0.64, (d) x = 0.65, (e) x = 0.66.
Table 3 Lattice parameters of xCaTiO3–(1
x)La[Ga0.5Al0.5]O3 ceramics.
Composition Lattice parameters (Å)
x = 0.62 x = 0.63 x = 0.64 x = 0.65 x = 0.66
a
b
c
a=b=g
5.4342 5.4291 5.4257 5.4224 5.4182
5.4435 5.4270 5.4296 5.4273 5.4236
7.6857 7.6779 7.6778 7.6757 7.6716
90 90 90 90 90
Structure
Unit cell volume (Å)
Orthorhombic Orthorhombic Orthorhombic Orthorhombic Orthorhombic
227.35 226.22 226.18 225.89 225.44
Fig. 8. SEM patterns of xCaTiO3–(1
x)La[Ga0.5Al0.5]O3 samples with different CaTiO3
Performance of the samples
x = 0.62 x = 0.63 x = 0.64 x = 0.65 x = 0.66
Relative density rr (%) Relative permittivity er Q f value (GHz) Temperature coefficient (t f) (ppm/ C)
97.25 44.52 43,000 7.83
96.84 45.09 42,088 5.66
97.42 45.54 43,897 1.7
97.12 45.95 39,037 2.53
96.97 47.36 41,498 5.66
calculated from the X-ray diffraction patterns and the results were listed in Table 3. As CaTiO3 had smaller unit cell volume, the unit cell volume of xCaTiO3–(1 x)La[Ga0.5Al0.5]O3 samples decreased with the increased contents of CaTiO3. Table 4 shows the variation of the relative density, relative permittivity, the Q f value, the temperature coefficient of xCaTiO3–(1 x)La[Ga0.5Al0.5]O3 samples with the increase amount of CaTiO3. As CaTiO3 had a high relative permittivity (er = 170) and a positive temperature coefficient (t f = +800 ppm/ C), both the relative permittivity and the temperature coefficient of xCaTiO3– (1 x)La[Ga0.5Al0.5]O3 increased linearly with the increased amount of CaTiO3. All the relative density values of xCaTiO3– (1 x)La[Ga0.5Al0.5]O3 were above 96.8% and all the Q f values of xCaTiO3–(1 x)La[Ga0.5Al0.5]O3 were above 39,000 GHz. Fig. 8 shows the SEM patterns of xCaTiO3–(1 x)La[Ga0.5Al0.5]O3 samples among x = 0.62, x = 0.64, x = 0.66. According to Fig. 8, all the samples of the ceramics exhibit homogeneous and compact structure.
x)La[Ga0.5Al0.5]O3 samples sintered at 1550 C (g) x = 0.62, (h) x = 0.64, (i) x = 0.66.
F. Liang et al. / Materials Research Bulletin 57 (2014) 140–145
4. Conclusions The crystal structure and microwave dielectric properties of 0.64CaTiO3–0.36La[Ga(1 d)Ald]O3 (CTLGA) were investigated in this paper. As d increased in 0.64CaTiO3–0.36La[Ga(1 d)Ald]O3 up to d = 0.5, the solid solution with the orthorhombic perovskite structure was observed. The solid solution with the tetragonal perovskite structure was formed when d > 0.5. The CTLGA ceramics showed good microwave dielectric performance with er = 45.54, Q f = 43,897 GHz, t f = 1.7 ppm/ C near the concentration region of the transition boundary, which showed good prospects for the application in the microwave technology. Acknowledgements This work was supported by the National Natural Science Foundation of China through Grant No. 61172004. The provided supports are gratefully acknowledged. References [1] S.F. Wang, J.H. Chen, Y.F. Hsu, Y.T. Wang, Effects of CaTiO3 addition on the densification and microwave dielectric properties of BiSbO4 ceramics, Ceram. Int. 39 (2013) 2857–2861. [2] E.R. Kipkoech, F. Azough, R. Freer, C. Leach, S.P. Thompson, C.C. Tang, Structural study of Ca0.7Nd0.3Ti0.7Al0.3O3 dielectric ceramics using synchrotron X-ray diffraction, J. Eur. Ceram. Soc. 23 (2003) 2677–2682. [3] B. Jancar, M. Valant, D. Suvorov, Solid-state reactions occuring during the synthesis of CaTiO3-NdAlO3 perovskite solid solutions, Chem. Mater. 16 (2004) 1075–1082. [4] D. Suvorov, M. Valant, B. Jancra, S.D. Skapin, CaTiO3-based ceramics: microstructural development and dielectric properties, Acta Chim. Slov. 48 (2001) 87–99.
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[5] C.L. Huang, C.S. Hsu, Improved high Q value of 0.5LaAlO3–0.5SrTiO3 microwave dielectric ceramics at low sintering temperature, Mater. Res. Bull. 36 (2001) 2677–2687. [6] H.X. Yuan, X.M. Chen, M.M. Mao, Structure and microwave dielectric characteristics of Ca1+xNd1 xAl1 xTixO4 ceramics, J. Am. Ceram. Soc. 92 (10) (2009) 2286–2290. [7] D.D. Khalyavin, A.N. Salak, A.M.R. Senos, P.Q. Mantas, V.M. Ferreira, Structure sequence in the CaTiO3–LaAlO3 microwave ceramics-revised, J. Am. Ceram. Soc. 89 (5) (2006) 1721–1723. [8] G.A. Ravi, F. Azough, R. Freer, Effect of Al2O3 on the structure and microwave dielectric properties of Ca0.7Ti0.7La0.3Al0.3O3, J. Eur. Ceram. Soc. 27 (2007) 2855–2859. [9] Y.X. Hu, Y.M. Zhang, H. Liu, D.X. Zhou, Microwave dielectric properties of PTFE/CaTiO3 polymer ceramic composites, Ceram. Int. 37 (2011) 1609–1613. [10] C.S. Hsu, C.L. Huang, J.F. Tseng, C.C. You, Low temperature sintering and microwave dielectric properties of 0.5LaAlO3–0.5SrTiO3 ceramics using copper oxide additions, Ceram. Int. 30 (2004) 2067–2073. [11] H. Zheng, I.M. Reaney, Microstructure and microwave properties of CaTiO3– LaGaO3 solid solutions, J. Mater. Sci. 40 (2005) 5207–5214. [12] E.A. Nenasheva, L.P. Mudroliubova, N.F. Kartenko, Microwave dielectric properties of ceramics based on CaTiO3–LnMO3 system (Ln–La, Nd; M–Al, Ga), J. Eur. Ceram. Soc. 23 (2003) 2443–2448. [13] J.H. Moon, H.M. Jang, H.S. Park, J.Y. Shin, H.S. Kim, Sintering behavior and microwave dielectric properties of (Ca, La) (Ti, Al)O3 ceramic, Jpn. J. Appl. Phys. 38 (1999) 6821–6826. [14] T.Y. Chen, K.Z. Fung, A and B-site substitution of the solid electrolyte LaGaO3 and LaAlO3 with the alkaline-earth oxides Mgo and SrO, J. Alloys Compd. 368 (2004) 106–115. [15] M.H. Kim, C.S. Woo, S. Nahm, C.H. Choi, H.J. Lee, H.M. Park, Crystal structure and microwave dielectric properties of (1 x)NdAlO3–xCaTiO3 ceramics, Mater. Res. Bull. 37 (2002) 605–615. [16] F. Liang, M. Ni, W.Z. Lu, G.F. Fan, Microwave dielectric properties and crystal structures of 0.7CaTiO3–0.3[LaxNd(1 x)]AlO3 ceramics, J. Alloys Compd. 568 (2013) 11–15. [17] S.F. Wang, Y.R. Wang, J.C. Hsu, Densification, microstructural evolution, and dielectric properties of CaTiO3–LaGaO3, J. Phys. Chem. Solids 72 (2011) 1011–1014. [18] B. Jancar, D. Suvorov, M. Valant, G. Drazic, Characterization of CaTiO3–NdAlO3 dielectric ceramics, J. Eur. Ceram. Soc. 23 (2003) 1391–1400.
Contents lists available at ScienceDirect
Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu
Crystal structure and microwave dielectric properties of CaTiO3–La [Ga(1 d)Ald]O3 ceramics system Fei Liang *, Meng Ni, Wenzhong Lu, Shaoshuai Feng School of optical and electronic information, Huazhong University of Science and Technology, Wuhan 430074, PR China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 18 November 2013 Received in revised form 24 February 2014 Accepted 24 May 2014 Available online 28 May 2014
The crystal structure and microwave dielectric properties of 0.64CaTiO3–0.36La[Ga(1 d)Ald]O3 specimens have been investigated in this work. When d 0.5, the crystal structure of 0.64CaTiO3– 0.36La[Ga(1 d)Ald]O3 was orthorhombic, while it transformed into the tetragonal structure when d > 0.5. When d = 0.5, specimens exhibited good microwave dielectric properties with er = 45.54, Q f = GHz) and t f = 1.7 ppm/ C. The 50 at% substitution of Ga3+ by Al3+ in 0.64CaTiO3–0.36LaGaO3 benefited the Q f value, the results showed that all samples of xCaTiO3–(1 x)La[Ga0.5Al0.5]O3 (x = 0.62, 0.63, 0.64, 0.65, 0.66) exhibited rr 96.8%, Q f 39,000 GHz, 44 er 48 and 8 ppm/ C t f +6 ppm/ C. ã 2014 Elsevier Ltd. All rights reserved.
Keywords: B. Phase transitions B. Microstructure D. Crystal structure
1. Introduction Microwave dielectric materials with a high Q f value, a high dielectric constant and a good stability of temperature coefficient of resonant frequency have been widely investigated all over the world because of their extensive applications to microwave devices, such as satellite communications, mobile technologies and microwave devices [1–5]. Among various candidates, microwave dielectric ceramics based on CaTiO3 system such as CaTiO3–LaGaO3 system and CaTiO3–LaAlO3 system are attracting great interests around the world due to their intermediate dielectric permittivity (er 45), high Q f value (Q f 40,000 GHz), and the low dielectric permittivity temperature coefficient (t f 0) [6,7]. CaTiO3 is an orthorhombic distorted perovskite at room temperature with high relative permittivity (er = 170), modest quality factor (Q f = 3500 GHz) and high positive temperature coefficient of resonant frequency (t f + 800 ppm/ C). On the contrary, LaGaO3 is a orthorhombic perovskite at room temperature but with low relative permittivity (er = 27), high quality factor (Q f = 97,000 GHz) and negative temperature coefficient of resonant frequency (t f = 80 ppm/ C). The high positive t f of CaTiO3 can be suppressed to low value or zero value t f with the
* Corresponding author. Tel.: +86 27 87542594; fax: +86 27 87543134. E-mail address: [email protected] (F. Liang). http://dx.doi.org/10.1016/j.materresbull.2014.05.042 0025-5408/ ã 2014 Elsevier Ltd. All rights reserved.
addition of LaGaO3, which make it a potential candidate for the microwave application [8,9]. According to previous investigations [10–12] and our work, 0.64CaTiO3–0.36LaGaO3 system showed quite good microwave dielectric properties (er 48, Q f 42,000 GHz, t f 1.5 ppm/ C). However, the price of Ga2O3 is recently rather high, which greatly limits the application of 0.64CaTiO3–0.36LaGaO3 microwave ceramic. Moon et al. [13] suggested that the improvement of the microwave dielectric properties could be achieved by the substitution of B-sites and the increase in ordering degree of the Bsite 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 electrical properties and ionic radius. The electrical properties of the Al3+ are similar to that of Ga3+ at B site and the ionic radius of Al3+ (0.535 Å) is also close to the ionic radius of Ga3+(0.62 Å), but the ionic radius of La3+(1.36 Å) at A-site is much larger than that of Al3+ [14]. So Ga3+ at B site instead of La3+ at A-site is more likely substituted by Al3+ when Al3+ ions are doped in the 0.64CaTiO3–0.36LaGaO3 compounds. When Ga3+ in 0.64CaTiO3– 0.36LaGaO3 is substituted partly by Al3+, it would be possible to develop a novel microwave dielectric material with a good microwave dielectric properties and a relatively low price. The aim of this work was to investigate the crystal structure of xCaTiO3–(1 x)La[Ga(1 d)Ald]O3 and improve the microwave dielectric properties of 0.64CaTiO3–0.36LaGaO3 by partial substitution of B-site ions and optimizing the composition and preparation process. The effects of the part substitution of B-site
F. Liang et al. / Materials Research Bulletin 57 (2014) 140–145
ions and the optimization of composition on the crystal structure and the microwave dielectric properties of ceramics were also discussed in details. 2. Experimental procedure The powders were prepared via the conventional solid-phase synthesis technique from the raw materials of CaCO3 (99.3%), TiO2 (99.6%), La2O3 (99.99%), Ga2O3 (99.99%), Nd2O3 (99.9%) and Al2O3 (99.6%). They were weighed according to the composition of 0.64CaTiO3–0.36La[Ga(1 d)Ald]O3 (d = 0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9) and xCaTiO3–(1 x)La[Ga0.5Al0.5]O3 (x = 0.62, 0.63, 0.64, 0.65, 0.66). The weighed chemicals were mixed by ZrO2 balls with the deionized water for 8 h and then calcined in the range of 1250–1400 C for 4 h in the air after being dried. The calcined mixture was milled in the deionized water with ZrO2 balls again for 8 h. Then the samples were prepared by pressing the powders into pellets in a steel die of 12 mm diameter at 10 MPa. After draining the glue, the pellets were sintered at 1550 C for 6 h and the cooling rate was 0.5 C/min. The density of sintered samples was tested by Archimedes method. After the determination of bulk density results, the relative density of the samples was obtained by dividing the bulk density to the theoretical density. The theoretical density of the samples was calculated by using the lattice parameters obtained from the XRD analysis, Avogadro constant and molar weight of the 0.64CaTiO3–0.36La[Ga(1 d)Ald]O3 (CTLGA). The phase identification was carried out by X-ray diffraction (XRD-7000, Shimadzu Corporation, Kyoto, Japan) working with conventional Cu Ka radiation and scanning at 2 /min in the range of 10–90 . The microstructure of the sintered specimens was measured by the scanning electron microscopy (SEM, VEGA3, TESCAN, Brno, Czech Republic) equipped with energy dispersive spectroscopy (EDS). The lattice parameters were measured by indexing and leastsquares powder diffraction program. The dielectric constant (er) and the unloaded Q f (@4.4–4.6 GHz) value were measured by the Hakki and Coleman method using an Agilent E8362B (Agilent Technologies, Inc., Santa Clara, USA) network analyzer and two parallel silver plates. The temperature coefficient of resonant frequency (t f) was calculated in the temperature range of 25–80 C. 3. Results and discussion
141
Table 1 The Q f values and the relative density of 0.64CaTiO3–0.36La[Ga0.5Al0.5]O3 ceramics with different claining temperatures. Calcination temperature ( C) Q f value (GHz) Relative density r (%)
1270 33,034 95.65
1290 35,775 96.14
1310 38,490 96.48
1330 41,225 96.79
1350 43,897 97.42
Fig. 1, the intensity of the peak increased with the increase of the calcining temperature, suggesting high calcining temperature makes contributions to the formation of 0.64CaTiO3–0.36La [Ga0.5Al0.5]O3. The peaks marked with arrows represented intermediate phases. Fig. 1 shows a single phase of the perovskite structure with the orthorhombic crystal system, which is obtained only at 1350 C. Table 1 shows that the calcining temperature is very important for the Q f value and the relative density of 0.64CaTiO3–0.36La[Ga0.5Al0.5]O3 ceramics. For example, the Q f value of 0.64CaTiO3–0.36La[Ga0.5Al0.5]O3 ceramics calcined at 1330 C was markedly lower than that at 1350 C under the same sintering condition, even if there was only a small amount of intermediate phase at 1330 C calcining temperature. However, higher calcining temperature was not discussed in our study because it could result in the difficulty with decreasing particle size when powders were ball-milled again. Therefore, 1350 C was chosen as the best calcining temperature for the CTLGA ceramics in our work [15,16]. The sintering temperature of 1550 C was chosen finally according to our previous work, and the Q f value of 0.64CaTiO3–0.36 La[Ga0.5Al0.5]O3 ceramics with the cooling rates of 2 C/min, 0.5 C/min and 0.1 C/min were 40,694 GHz, 43,897 GHz and 45,168 GHz, respectively. It was evident that the Q f value escalated markedly with the slowing cooling rate. In the study of Wang et al. [17], no conclusive reason was provided concerning the cooling rate slowing to explain the dependence of Q f value on the cooling rate. Jancar et al. [18] suggested that the Q f value of various complex perovskite systems could be greatly improved by decreasing the cooling rate because the formation of the high ordering degree of B-site cations was generally a relatively slow process. However, the lower the slowing cooling rate was, the more time the preparation process needed. With the Q f value and the preparation time taken into comprehensive consideration, the cooling rate of 0.5 C/min was selected.
3.1. Calcination and sintering properties Fig. 1 shows the X-ray diffraction patterns of 0.64CaTiO3–0.36La [Ga0.5Al0.5]O3 ceramics calcined over 1270–1350 C. According to
Fig. 1. XRD patterns of 0.64CaTiO3–0.36La[Ga0.5Al0.5]O3 samples calcined at different temperatures (a) 1270 C, (b) 1290 C, (c) 1310 C, (d) 1330 C, (e) 1350 C.
Fig. 2. XRD patterns of CTLGA samples with different LaAlO3 contents (a) d = 0, (b) d = 0.1, (c) d = 0.3, (d) d = 0.5, (e) d = 0.7, (f) d = 0.9.
144
F. Liang et al. / Materials Research Bulletin 57 (2014) 140–145 Table 4 Performance of xCaTiO3–(1 contents.
Fig. 7. XRD patterns of xCaTiO3–(1 x)La[Ga0.5Al0.5]O3 samples with different CaTiO3 contents (a) x = 0.62, (b) x = 0.63, (c) x = 0.64, (d) x = 0.65, (e) x = 0.66.
Table 3 Lattice parameters of xCaTiO3–(1
x)La[Ga0.5Al0.5]O3 ceramics.
Composition Lattice parameters (Å)
x = 0.62 x = 0.63 x = 0.64 x = 0.65 x = 0.66
a
b
c
a=b=g
5.4342 5.4291 5.4257 5.4224 5.4182
5.4435 5.4270 5.4296 5.4273 5.4236
7.6857 7.6779 7.6778 7.6757 7.6716
90 90 90 90 90
Structure
Unit cell volume (Å)
Orthorhombic Orthorhombic Orthorhombic Orthorhombic Orthorhombic
227.35 226.22 226.18 225.89 225.44
Fig. 8. SEM patterns of xCaTiO3–(1
x)La[Ga0.5Al0.5]O3 samples with different CaTiO3
Performance of the samples
x = 0.62 x = 0.63 x = 0.64 x = 0.65 x = 0.66
Relative density rr (%) Relative permittivity er Q f value (GHz) Temperature coefficient (t f) (ppm/ C)
97.25 44.52 43,000 7.83
96.84 45.09 42,088 5.66
97.42 45.54 43,897 1.7
97.12 45.95 39,037 2.53
96.97 47.36 41,498 5.66
calculated from the X-ray diffraction patterns and the results were listed in Table 3. As CaTiO3 had smaller unit cell volume, the unit cell volume of xCaTiO3–(1 x)La[Ga0.5Al0.5]O3 samples decreased with the increased contents of CaTiO3. Table 4 shows the variation of the relative density, relative permittivity, the Q f value, the temperature coefficient of xCaTiO3–(1 x)La[Ga0.5Al0.5]O3 samples with the increase amount of CaTiO3. As CaTiO3 had a high relative permittivity (er = 170) and a positive temperature coefficient (t f = +800 ppm/ C), both the relative permittivity and the temperature coefficient of xCaTiO3– (1 x)La[Ga0.5Al0.5]O3 increased linearly with the increased amount of CaTiO3. All the relative density values of xCaTiO3– (1 x)La[Ga0.5Al0.5]O3 were above 96.8% and all the Q f values of xCaTiO3–(1 x)La[Ga0.5Al0.5]O3 were above 39,000 GHz. Fig. 8 shows the SEM patterns of xCaTiO3–(1 x)La[Ga0.5Al0.5]O3 samples among x = 0.62, x = 0.64, x = 0.66. According to Fig. 8, all the samples of the ceramics exhibit homogeneous and compact structure.
x)La[Ga0.5Al0.5]O3 samples sintered at 1550 C (g) x = 0.62, (h) x = 0.64, (i) x = 0.66.
F. Liang et al. / Materials Research Bulletin 57 (2014) 140–145
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