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Investigation on microwave dielectric properties and microstructures of (1−x) LaAlO3-xCa0.2Sr0.8TiO3 ceramics
Journal of Alloys and Compounds 649 (2015) 254e260
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Investigation on microwave dielectric properties and microstructures of (1x) LaAlO3-xCa0.2Sr0.8TiO3 ceramics Liangzhu Zhang a, b, *, Huixing Lin a, XiangYu Zhao a, Xiaogang Yao a, Shaohu Jiang a, Fei He a, Botao Li a, Lan Luo a a Key Laboratory of Inorganic Functional Materials and Devices, Shanghai Institute of Ceramics, Chinese Academy of Sciences, 215 Chengbei Road, Jiading, Shanghai 201800, China b University of Chinese Academy of Sciences, Beijing 100049, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 20 April 2015 Received in revised form 14 July 2015 Accepted 16 July 2015 Available online 18 July 2015
The structural, microstructural and microwave dielectric properties of (1x) LaAlO3-xCa0.2Sr0.8TiO3 ceramics prepared by the conventional solid state ceramic route have been investigated. The formation of solid solutions (1x) LaAlO3-xCa0.2Sr0.8TiO3 was confirmed by the X-ray diffraction patterns and the EDS analysis. The lattice parameter, average grain size and dielectric constant (εr) increase with increasing amount of Ca0.2Sr0.8TiO3 whereas the quality factor (Q f) decreases. The increasing temperature coefficient of resonant frequency (tf) caused by the decreasing tolerance factor with x ranging from 0.3 to 0.7. The tf can be tuned near zero at x ¼ 0.5. Specimen with the composition of 0.5LaAlO3 e0.5Ca0.2Sr0.8TiO3 possesses an excellent combination of microwave dielectric properties: εr ~ 32.7, Q f ~ 33400 GHz, tf ~ 2.5 ppm/ C. © 2015 Elsevier B.V. All rights reserved.
Keywords: Crystal structure Microwave dielectric properties
1. Introduction Miniaturization of dielectric resonators in modern microwave telecommunication such as 3G, global positioning systems and wireless local area networks is a primary requirement. These dielectric resonators must satisfy three main criteria: a high dielectric constant, a high quality factor, and a near-zero temperature coefficient of resonant frequency for the small size, low loss and high temperature stability, respectively [1,2]. However, highdielectric-constant materials exhibit high dielectric loss (low Q f value), whereas low-loss ceramics are usually accompanied by a low εr value. Achieving all three requirements in one material is a formidable problem and therefore a number of materials have been proposed. LaAlO3 has been widely used as substrate for superconducting microwave devices because it provides a high-quality factor, excellent lattice matching for thermal expansion [3e5]. LaAlO3
* Corresponding author. Key Laboratory of Inorganic Functional Materials and Devices, Shanghai Institute of Ceramics, Chinese Academy of Sciences, 215 Chengbei Road, Jiading, Shanghai 201800, China. Fax: þ86 2169906330. E-mail address: [email protected] (L. Zhang). http://dx.doi.org/10.1016/j.jallcom.2015.07.127 0925-8388/© 2015 Elsevier B.V. All rights reserved.
possess high microwave dielectric properties (εr ~ 23, Qf ~ 65000, tf ~ 44 ppm/ C) [6,7]. To tune tf near zero, another material with positive tf should be introduced to form a solid solution or composite. To avoid deteriorating the Q f value, the solid solution rather than the composite is preferred, like MgTiO3eCaTiO3, LaAlO3eSrTiO3, NdAlO3eCaTiO3 [8e14] etc. Therefore, Ca0.2Sr0.8TiO3 (εr ~ 255, Q f ~ 3960) with a large positive temperature coefficient of resonant frequency (tf ¼ þ1534 ppm/ C) can be selected as to form a solid solution with LaAlO3 [15,16]. It is well known that the tolerance factor (t) determine the stability of the perovskite phase for a given set of anions and cations.
t ¼ R A þ RB
.pffiffiffi 2ðRB þ RO Þ
(1)
Compounds with t > 0.985 are untiled and 0.965 < t < 0.985 are antiphase tilted whereas t < 0.965 have undergone in-phase and antiphase tilted. Reaney et al. [17e19], in their investigations of the dielectric properties of complex perovskites, demonstrated the relationship of t with temperature coefficient of dielectric constant, tf, (tf ¼ 12 tε aL Þ where aL is linear thermal-expansion
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255
coefficient. Decreasing t from 1.01 to 0.98 in complex perovskites will induce octahedral tilt transitions and force tf towards zero. In this work, it was reported a systematic study of (1x) LaAlO3xCa0.2Sr0.8TiO3 ceramics with different Ca0.2Sr0.8TiO3, prepared by solid-state method, in which the structural and microwave properties were studied. 2. Experimental Ceramics with the compositions of (1x) LaAlO3-xCa0.2Sr0.8TiO3 (x ¼ 0.3, 0.4, 0.5, 0.6, 0.7) were prepared. High purity La2O3 (99.8%), Al2O3 (99%), CaCO3 (99.9%), SrCO3 (99.9%) and TiO2 (99%) were used as raw materials. The stoichiometrically weighed raw materials were mixed and ball milled with zirconia media in water for 24 h. Then the slurry was dried and caclined at 1450 C in air for 4 h. The calcined powders were re-milled for 24 h in water. After drying, the powders were palletized with a 3wt% solution of PVA (polyvinyl alcohol), and pressed into the cylindrical compact disks of 15 mm in diameter and 8 mm in height. Then these disks were sintered at temperatures 1450 Ce1650 C for 4 h in air. The crystalline phases of the sintered sample after crushing and grinding was determined by powder X-ray diffraction (XRD) using CuKa radiation (RIGAKU D/max 2550V/PC, Rigaku CO., Tokyo, Japan). The microstructural observations and analysis of the ceramics were performed by a scanning electron microscopy (S-4800, Hitachi, Tokyo, Japan) and an energy-dispersive X-ray spectrometer (EDS, Philips). The bulk densities of the sintered pellets were measured by the Archimedes methods. The dielectric constant (εr) and the quality factor (Q f) at 4.2e5.5 GHz were measured by Hakki-Coleman dielectric method [20,21], using a network analyzer (Agilent E8363A, PNA series network analyzer). The temperature coefficient of frequency (tf) was measured in the temperature range of 20e80 C. The tf value (ppm/ C) was calculated by noting the change in resonant frequency
f2 f1 tf ¼ f1 ðT2 T1 Þ
(2)
Wheref1 and f2 represent the resonant frequencies at T1 and T2, respectively. 3. Results and discussion 3.1. Structural analysis by X-ray diffraction Fig. 1a shows the XRD patterns of (1x) LaAlO3xCa0.2Sr0.8TiO3 ceramics sintered 1550 C for 4 h. All of the XRD patterns are indexed with rhombohedral symmetry of space group R3 c. Fig. 1b shows the split of superlattice reflections of (222) and (400), which can be used to identify the crystal structure. The observation of all the samples imply that it corresponds to R3 c group, and the similar results also observed by Victor et al. [22]. The XRD results indicate that (1x) LaAlO3-xCa0.2Sr0.8TiO3 is a solid solution when x ranges from 0.3 to 0.7. The XRD patterns were refined by using Rietveld simulations under the Jana2006, developed by Vaclav Petricek [23], by taking into consideration the R3 c space group for all the studied compositions. Fig. 2 presents the results of these
Fig. 1. a. X-ray diffraction patterns of a Microwave dielectric properties of (1x) LaAlO3-xCa0.2Sr0.8TiO3 ceramics with different x values sintered at 1550 C for 4 h. 1b. XRD patterns at the regions of (222) and (400) superlattice reflections of (1x) LaAlO3xCa0.2Sr0.8TiO3 ceramics sintered 1550 C for 4 h.
refinements which show a good fit. The cell parameters and the unit-cell volumes (abbr. V unit) of the (1x) LaAlO3xCa0.2Sr0.8TiO3 samples can be obtained from Rietveld refinement, and are listed in Table 1. Fig 3 shows that the lattice parameter increase almost linearly with the Ca0.2Sr0.8TiO3. The enlarged lattice parameter can be explained that the Sr2þ crystal radius (1.58 Å) is larger than La3þ (1.50 Å) and the Ti4þ crystal radius (0.765 Å) is also larger than Al3þ (0.675 Å). The data for each ionic crystal radius are taken from Shannon [24].
3.2. Microstructural characterization Fig. 4 shows the bulk density, theoretical density and the relative density of the (1x) LaAlO3-xCa0.2Sr0.8TiO3 ceramics sintered at 1550 C for 4 h as a function of x. The calculated theoretical density decrease linearly from 5.98 g/cm3 to 5.34 g/cm3 with increasing x because of the smaller molar weight of Ca0.2Sr0.8TiO3 than that of LaAlO3. That is, the relative density increases with increasing amount of Ca0.2Sr0.8TiO3, suggesting that Ca0.2Sr0.8TiO3 is
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Fig. 2. X-ray patterns obtained by using Rietveld refinement for the (1x) LaAlO3-xCa0.2Sr0.8TiO3 (x ¼ 0.3 0.4 0.5 0.6, 0.7).
Table 1 The refinement results of the (1x) LaAlO3-xCa0.2Sr0.8TiO3 (x ¼ 0.3, 0.4, 0.5, 0.6, 0.7). Compounds
0.7LaAlO3e0.3Ca0.2Sr0.8TiO3 0.6LaAlO3e0.4Ca0.2Sr0.8TiO3 0.5LaAlO3e0.5Ca0.2Sr0.8TiO3 0.4LaAlO3e0.6Ca0.2Sr0.8TiO3 0.3LaAlO3e0.7Ca0.2Sr0.8TiO3
Lattice parameters(Å) a
b
c
5.408978 5.42977 5.438417 5.457993 5.46963
5.408978 5.42977 5.438417 5.45799 5.46963
13.26513 13.27377 13.33817 13.34459 13.37574
Vunit (Å3)
Rp (%)
Rwp (%)
c2
336.1 339.1 341.6 344.4 346.6
4.55 3.60 3.85 3.44 3.26
6.07 4.75 4.98 4.54 4.25
1.38 1.14 1.25 1.22 1.18
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by the high sintered temperature. Typical energy-dispersive X-ray (EDX with a resolution of 183 eV) analysis of the surface of 0.5LaAlO3e0.5Ca0.2Sr0.8TiO3 sintered at 1550 C for 4 h is shown in Fig. 7 and the corresponding EDX data of the 0.5LaAlO3e0.5Ca0.2Sr0.8TiO3 also presented in Table 2. All the examined grains showing La:Ca þ Sr ¼ 1:1, further confirming the formation of a solid solution.
3.3. Microwave dielectric properties
Fig. 3. The evolution of the lattice parameter of the (1x) LaAlO3-xCa0.2Sr0.8TiO3 (x ¼ 0.3 0.4 0.5 0.6, 0.7) solid solution system.
Fig. 4. The evolution of theoretical, bulk and relative density of the (1x) LaAlO3xCa0.2Sr0.8TiO3 (x ¼ 0.3 0.4 0.5 0.6, 0.7) ceramics.
more sinterable than LaAlO3 Fig. 5 Shows the scanning electron microscopy (SEM) micrographs of the (1x)LaAlO3-xCa0.2Sr0.8TiO3 ceramics sintered at 1550 C for 4 h. Porous microstructure can be seen for specimen x ¼ 0.3. Well-developed microstructure can be achieved at x ¼ 0.4, 0.5, 0.6 and 0.7. All of the grains shows irregular (spherulite-like) grains and the average grain sizes increase from 2.1 mm to 5.2 mm. The boundary of each sample are very clear which imply no liquid phase occurred during the sintering process. Fig. 6 shows the scanning electron microscopy (SEM) micrographs of 0.5LaAlO3e0.5Ca0.2Sr0.8TiO3 ceramics sintered at various temperatures for 4 h. Porous microstructure can be seen for specimen sintered at 1450 C and the grain size increases as the sintering temperature increases due to a grain growth. Welldeveloped microstructure can be achieved at temperature 1550 C-1650 C. However, degradation in grain uniformity and abnormal grain growth appeared at 1650 C, which might cause
Fig. 8 shows the dielectric constant of the (1x) LaAlO3xCa0.2Sr0.8TiO3 ceramics as a function of x. With increasing x from 0.3 to 0.7, the dielectric constant εr increased nonlinearly from 26.7 to 50.9. Its mixture-like behavior is similar to that observed for (1x) LaAlO3-xSrTiO3 [6]. The results can be explained by the empirical rule: lnεr ¼ V1lnε1 þ V2 lnε2 [25], where Vi and εi are the volume fraction and dielectric constant of each phase, respectively. The dielectric constant of LaAlO3 and Ca0.2Sr0.8TiO3 is 23 and 255, respectively. When x ranges from 0.3 to 0.7, the volume fraction of Ca0.2Sr0.8TiO3 increase, which leads to the increasing of εr. Fig. 9 shows the Q f and the tolerance factor of the (1x) LaAlO3-xCa0.2Sr0.8TiO3 as a function of x. Q f decreased from 43,500e18300 GHz as the x value increased from 0.3 to 0.7. The great deterioration of Q f value of the system is caused by the increase of Ca0.2Sr0.8TiO3 composition. Because the Ca0.2Sr0.8TiO3 possesses bad Q f 3960. When its content goes up, we can suggest that the compounds quality factor will be worsen. Fig. 10 shows the tf and the tolerance factor of (1x) LaAlO3xCa0.2Sr0.8TiO3 ceramic system as a function of x sintered at 1550 Cfor 4 h. The tf value also had a nonlinear variation analogous to εr for the samples, varying from 35.5 ppm/ C to 90.08 ppm/ C as the x value increased from 0.3 to 0.7. This may cause by the deceasing of t. According to the equation tf ¼ 12 tε aL , where aL is between 10 and 20 ppm/ C. tε will ranges from negative to positive as the t decrease from 1.012 to 0.935 [2] and when t ¼ 1.012 the tε is near zero. One can suggest that the tf changes from negative to positive with decreasing t in this system. At x ¼ 0.5, a near zero tf value of 2.5 ppm/ C was obtained. For practical applications, a new microwave dielectric material, 0.5LaAlO3e0.5Ca0.2Sr0.8TiO3, has been developed, exhibiting a dielectric constant εr of 32.7, a Q f value of ~33400.
4. Conclusions The microwave dielectric properties of (1x) LaAlO3xCa0.2Sr0.8TiO3 ceramics were investigated. LaAlO3 and Ca0.2Sr0.8TiO3 combine to form a solid solution. The lattice parameter decease with increasing amount of Ca0.2Sr0.8TiO3. The average grain sizes increase from 2.1 mm to 5.2 mm with increasing x at 1550 C. As the x increase from 0.3 to 0.7, εr increased from 26.7 to 50.9, the Q f decrease from 43,500 to 18,30 GHz and tf varied in a wide range from 35.5 ppm/ C to 90.08 ppm/ C. A new microwave dielectric material, (1x) LaAlO3-xCa0.2Sr0.8TiO3 ceramics with x ¼ 0.5, having a dielectric constant εr of ~32.7, a Qf value of ~33400 GHz, and a tf value of ~ 2.5 ppm/ C, is suggested as a candidate material for low loss microwave applications.
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L. Zhang et al. / Journal of Alloys and Compounds 649 (2015) 254e260
Fig. 5. Scanning electron microscopy photographs of (1x) LaAlO3-xCa0.2Sr0.8TiO3 ceramics with (a) x ¼ 0.3, (b) x ¼ 0.4, (c) x ¼ 0.5, (d) x ¼ 0.6, (e) x ¼ 0.7, sintered at 1550 C for 4 h.
Fig. 6. Scanning electron microscopy photographs of 0.5LaAlO3e0.5Ca0.2Sr0.8TiO3 ceramics: (a) sintered at 1450 Cin air for 4 h, (b) sintered at 1550 C in air for 4 h, (c) sintered at 1600 C in air for 4 h, and (d) sintered at 1650 C in air for 4 h.
L. Zhang et al. / Journal of Alloys and Compounds 649 (2015) 254e260
Fig. 7. Typical energy-dispersive X-ray analysis lO3e0.5Ca0.2Sr0.8TiO3 sintered at 1550 C for 4 h.
of
the
surface
of
0.5LaA-
Table 2 The EDX Data of the 0.5LaAlO3e0.5Ca0.2Sr0.8TiO3 Ceramics for spots A, B and C. Spots
A B C
259
Atom (%)
Fig. 10. tf and the tolerance factor of (1x) LaAlO3-xCa0.2Sr0.8TiO3 ceramic system as a function of x sintered at 1550 C for 4 h
Acknowledgments
OK
Al K
Ca K
Ti K
Sr L
La L
60.75 61.41 59.46
9.03 8.71 9.31
2.03 1.83 2.14
9.34 9.22 9.83
8.44 8.77 8.34
10.41 10.06 10.91
The authors would like to express thanks to Ruan YinJie, Zeng Yi and Zhai MingLong for their technical support. Zhang LiangZhu expresses his gratitude to Yu ZeXiao for useful discussions related to this work. References
Fig. 8. Dielectric constants of the (1x) LaAlO3-xCa0.2Sr0.8TiO3 ceramic system as a function of x sintered at 1550 C for 4 h.
Fig. 9. Q f values of the (1x) LaAlO3-xCa0.2Sr0.8TiO3 system as a function of x sintered at 1550 C for 4 h
[1] T. Sebastian, Dielectric Materials for Wireless Communications, Elseiver Publishers, Oxford, U.K, 2008, pp. 1e11. [2] I.M. Reaney, D. Iddles, Microwave dielectric ceramics for resonators and filters in mobile phone networks, J. Am. Ceram. Soc. 89 (2006) 2063e2072. [3] A. Ohtomo, H.Y. Hwang, A high-mobility electron gas at the LaAlO3/SrTiO3 heterointerface, Nature 427 (2004) 423e426. [4] M. Huijben, A. Brinkma, G. Koster, G. Rijnders, H. Hilgenkamp, H.A.D. Blank, StructureeProperty Relation of SrTiO3/LaAlO3 Interfaces, Adv. Mater 21 (2009) 1665e1677. [5] T. Konaka, M. Sato, H. Asano, S. Kubo, Relative permittivity and dielectric loss tangent of substrate materials for high-T superconducting film, J. Supercond. 4 (1991) 1665e1667. [6] Y.C.H.O. S-, H.S. Hong, K.Y. Ko, Mixture-like behavior in the microwave dielectric properties of the (1x) LaAlO3exSrTiO3system, Mater. Res. Bul. 34 (1999) 511e516. [7] P-h Sun, T. NAakamura, Y.J. Shan, Y. Inaguma, M. Itoh, T. Kitamura, Dielectric behavior of (1x)LaAlO3-xSrTiO3 solid solution system at microwave frequencies, Jpn. J. Appl. Phys. 37 (1998) 5625e5629. [8] V.M. Ferreira, F. Azough, R. Freer, J.L. Baptista, The effect of Cr and La on MgTiO3 and MgTiO3 exCaTiO3 microwave dielectric ceramics, J. Mater. Res. 12 (1998) 3293e3299. [9] C.-L. Huang, C.-S. Hsu, Improved high Q value of 0.5LaAlO3e 0.5SrTiO3 microwave dielectric ceramics at low sintering temperature, Mater. Res. Bul. 36 (2001) 2677e2687. [10] C.-L. Huang, K.-H. Chiang, Dielectric properties of B2O3-doped (1x)LaAlO3xSrTiO3 ceramic system at microwave frequency, Mater. Res. Bul. 37 (2002) 1941e1948. [11] C.-S. Hsu, C.-L. Huang, Effect of CuO additive on sintering and microwave dielectric behavior of LaAlO3 ceramics, Mater. Res. Bul. 36 (2001) 1939e1947. [12] B. Jancar, D. Suvorov, M. Valant, Microwave dielectric properties of CaTiO3eNdAlO3 ceramics, J. Mater. Sci. Lett. 20 (2001) 71e72. [13] C.-L. Huang, J.-Y. Chen, C.-C. Liang, Dielectric properties and mixture behavior of Mg4Nb2O9eSrTiO3 ceramic system at microwave frequency, J. Alloy. Compd. 478 (2009) 554e558. [14] J.-Y. Chen, C.-Y. Jiang, C.-L. Huang, Low-loss microwave dielectrics in the Mg2(Ti0.95Sn0.05)O4e(Ca0.8Sr0.2)TiO3 ceramic system, J. Alloy. Compd. 502 (2010) 324e328. [15] P.L. Wise, I.M. Reaney, W.E. Lee, T.J. Price, D.M. Iddles, D.S. Cannell, Structuremicrowave property relations of Ca and Sr titanates, J. Eur. Ceram. Soc. 21 (2001) 2629e2632. [16] J.-Y. Chen, C.-L. Huang, A new low-loss microwave dielectric using (Ca0.8Sr0.2) TiO3-doped MgTiO3 ceramics, Mat. Lett. 64 (2010) 2585e2588. [17] P.L. Wise, I.M. Reaney, W.E. Lee, Tunability of tf in perovskites and related compounds, J. Mater. Res. 17 (2002) 2033e2040. [18] I.M. Reaney, P. Wise, R. Ubic, On the temperature coefficient of resonant frequency in microwave dielectrics, Philos. Mag. 81 (2001) 501e510. [19] I.M. Reaney, E.L. Colla, N. Setter, Dielectric and structural characteristics of Ba-
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and Sr-based complex perovskites as a function of tolerance factor, Jpn. J. Appl. Phys. 33 (1994) 3984. [20] B.W. Hakki, P.D. Coleman, IEEE Trans. Microw. Theory Tech. 8 (1960) 402e410. [21] W.E. Courtney, IEEE Trans. Microw. Theory Tech. 18 (1970) 47e85. [22] D.D. Khalyavin, A.N. Salak, A.M.R. Senos, Structure sequence in the CaTiO3eLaAlO3 microwave ceramicsdrevised, J. Am. Ceram. Soc. (2006) 1721e1723.
[23] V. Petricek, M. Dusek, L. Palatinus, Crystallographic computing system JANA2006: general features, Z. für Kristallogr. Mater. 229 (2014) 345e352. [24] R.D. Shannon, Revised effective ionic radii and systematic studies of interatomic distances in halides and chalcogenides, Acta Crystallogr. Sect. A Cryst. Phys. Diffr. Theor. General Crystallogr. (1976) 751e767. [25] K. Wakino, T. Okada, N. Yoshida, A new equation for predicting the dielectric constant of a mixture,, J. Am. Ceram. Soc. 76 (1993).
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Investigation on microwave dielectric properties and microstructures of (1x) LaAlO3-xCa0.2Sr0.8TiO3 ceramics Liangzhu Zhang a, b, *, Huixing Lin a, XiangYu Zhao a, Xiaogang Yao a, Shaohu Jiang a, Fei He a, Botao Li a, Lan Luo a a Key Laboratory of Inorganic Functional Materials and Devices, Shanghai Institute of Ceramics, Chinese Academy of Sciences, 215 Chengbei Road, Jiading, Shanghai 201800, China b University of Chinese Academy of Sciences, Beijing 100049, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 20 April 2015 Received in revised form 14 July 2015 Accepted 16 July 2015 Available online 18 July 2015
The structural, microstructural and microwave dielectric properties of (1x) LaAlO3-xCa0.2Sr0.8TiO3 ceramics prepared by the conventional solid state ceramic route have been investigated. The formation of solid solutions (1x) LaAlO3-xCa0.2Sr0.8TiO3 was confirmed by the X-ray diffraction patterns and the EDS analysis. The lattice parameter, average grain size and dielectric constant (εr) increase with increasing amount of Ca0.2Sr0.8TiO3 whereas the quality factor (Q f) decreases. The increasing temperature coefficient of resonant frequency (tf) caused by the decreasing tolerance factor with x ranging from 0.3 to 0.7. The tf can be tuned near zero at x ¼ 0.5. Specimen with the composition of 0.5LaAlO3 e0.5Ca0.2Sr0.8TiO3 possesses an excellent combination of microwave dielectric properties: εr ~ 32.7, Q f ~ 33400 GHz, tf ~ 2.5 ppm/ C. © 2015 Elsevier B.V. All rights reserved.
Keywords: Crystal structure Microwave dielectric properties
1. Introduction Miniaturization of dielectric resonators in modern microwave telecommunication such as 3G, global positioning systems and wireless local area networks is a primary requirement. These dielectric resonators must satisfy three main criteria: a high dielectric constant, a high quality factor, and a near-zero temperature coefficient of resonant frequency for the small size, low loss and high temperature stability, respectively [1,2]. However, highdielectric-constant materials exhibit high dielectric loss (low Q f value), whereas low-loss ceramics are usually accompanied by a low εr value. Achieving all three requirements in one material is a formidable problem and therefore a number of materials have been proposed. LaAlO3 has been widely used as substrate for superconducting microwave devices because it provides a high-quality factor, excellent lattice matching for thermal expansion [3e5]. LaAlO3
* Corresponding author. Key Laboratory of Inorganic Functional Materials and Devices, Shanghai Institute of Ceramics, Chinese Academy of Sciences, 215 Chengbei Road, Jiading, Shanghai 201800, China. Fax: þ86 2169906330. E-mail address: [email protected] (L. Zhang). http://dx.doi.org/10.1016/j.jallcom.2015.07.127 0925-8388/© 2015 Elsevier B.V. All rights reserved.
possess high microwave dielectric properties (εr ~ 23, Qf ~ 65000, tf ~ 44 ppm/ C) [6,7]. To tune tf near zero, another material with positive tf should be introduced to form a solid solution or composite. To avoid deteriorating the Q f value, the solid solution rather than the composite is preferred, like MgTiO3eCaTiO3, LaAlO3eSrTiO3, NdAlO3eCaTiO3 [8e14] etc. Therefore, Ca0.2Sr0.8TiO3 (εr ~ 255, Q f ~ 3960) with a large positive temperature coefficient of resonant frequency (tf ¼ þ1534 ppm/ C) can be selected as to form a solid solution with LaAlO3 [15,16]. It is well known that the tolerance factor (t) determine the stability of the perovskite phase for a given set of anions and cations.
t ¼ R A þ RB
.pffiffiffi 2ðRB þ RO Þ
(1)
Compounds with t > 0.985 are untiled and 0.965 < t < 0.985 are antiphase tilted whereas t < 0.965 have undergone in-phase and antiphase tilted. Reaney et al. [17e19], in their investigations of the dielectric properties of complex perovskites, demonstrated the relationship of t with temperature coefficient of dielectric constant, tf, (tf ¼ 12 tε aL Þ where aL is linear thermal-expansion
L. Zhang et al. / Journal of Alloys and Compounds 649 (2015) 254e260
255
coefficient. Decreasing t from 1.01 to 0.98 in complex perovskites will induce octahedral tilt transitions and force tf towards zero. In this work, it was reported a systematic study of (1x) LaAlO3xCa0.2Sr0.8TiO3 ceramics with different Ca0.2Sr0.8TiO3, prepared by solid-state method, in which the structural and microwave properties were studied. 2. Experimental Ceramics with the compositions of (1x) LaAlO3-xCa0.2Sr0.8TiO3 (x ¼ 0.3, 0.4, 0.5, 0.6, 0.7) were prepared. High purity La2O3 (99.8%), Al2O3 (99%), CaCO3 (99.9%), SrCO3 (99.9%) and TiO2 (99%) were used as raw materials. The stoichiometrically weighed raw materials were mixed and ball milled with zirconia media in water for 24 h. Then the slurry was dried and caclined at 1450 C in air for 4 h. The calcined powders were re-milled for 24 h in water. After drying, the powders were palletized with a 3wt% solution of PVA (polyvinyl alcohol), and pressed into the cylindrical compact disks of 15 mm in diameter and 8 mm in height. Then these disks were sintered at temperatures 1450 Ce1650 C for 4 h in air. The crystalline phases of the sintered sample after crushing and grinding was determined by powder X-ray diffraction (XRD) using CuKa radiation (RIGAKU D/max 2550V/PC, Rigaku CO., Tokyo, Japan). The microstructural observations and analysis of the ceramics were performed by a scanning electron microscopy (S-4800, Hitachi, Tokyo, Japan) and an energy-dispersive X-ray spectrometer (EDS, Philips). The bulk densities of the sintered pellets were measured by the Archimedes methods. The dielectric constant (εr) and the quality factor (Q f) at 4.2e5.5 GHz were measured by Hakki-Coleman dielectric method [20,21], using a network analyzer (Agilent E8363A, PNA series network analyzer). The temperature coefficient of frequency (tf) was measured in the temperature range of 20e80 C. The tf value (ppm/ C) was calculated by noting the change in resonant frequency
f2 f1 tf ¼ f1 ðT2 T1 Þ
(2)
Wheref1 and f2 represent the resonant frequencies at T1 and T2, respectively. 3. Results and discussion 3.1. Structural analysis by X-ray diffraction Fig. 1a shows the XRD patterns of (1x) LaAlO3xCa0.2Sr0.8TiO3 ceramics sintered 1550 C for 4 h. All of the XRD patterns are indexed with rhombohedral symmetry of space group R3 c. Fig. 1b shows the split of superlattice reflections of (222) and (400), which can be used to identify the crystal structure. The observation of all the samples imply that it corresponds to R3 c group, and the similar results also observed by Victor et al. [22]. The XRD results indicate that (1x) LaAlO3-xCa0.2Sr0.8TiO3 is a solid solution when x ranges from 0.3 to 0.7. The XRD patterns were refined by using Rietveld simulations under the Jana2006, developed by Vaclav Petricek [23], by taking into consideration the R3 c space group for all the studied compositions. Fig. 2 presents the results of these
Fig. 1. a. X-ray diffraction patterns of a Microwave dielectric properties of (1x) LaAlO3-xCa0.2Sr0.8TiO3 ceramics with different x values sintered at 1550 C for 4 h. 1b. XRD patterns at the regions of (222) and (400) superlattice reflections of (1x) LaAlO3xCa0.2Sr0.8TiO3 ceramics sintered 1550 C for 4 h.
refinements which show a good fit. The cell parameters and the unit-cell volumes (abbr. V unit) of the (1x) LaAlO3xCa0.2Sr0.8TiO3 samples can be obtained from Rietveld refinement, and are listed in Table 1. Fig 3 shows that the lattice parameter increase almost linearly with the Ca0.2Sr0.8TiO3. The enlarged lattice parameter can be explained that the Sr2þ crystal radius (1.58 Å) is larger than La3þ (1.50 Å) and the Ti4þ crystal radius (0.765 Å) is also larger than Al3þ (0.675 Å). The data for each ionic crystal radius are taken from Shannon [24].
3.2. Microstructural characterization Fig. 4 shows the bulk density, theoretical density and the relative density of the (1x) LaAlO3-xCa0.2Sr0.8TiO3 ceramics sintered at 1550 C for 4 h as a function of x. The calculated theoretical density decrease linearly from 5.98 g/cm3 to 5.34 g/cm3 with increasing x because of the smaller molar weight of Ca0.2Sr0.8TiO3 than that of LaAlO3. That is, the relative density increases with increasing amount of Ca0.2Sr0.8TiO3, suggesting that Ca0.2Sr0.8TiO3 is
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Fig. 2. X-ray patterns obtained by using Rietveld refinement for the (1x) LaAlO3-xCa0.2Sr0.8TiO3 (x ¼ 0.3 0.4 0.5 0.6, 0.7).
Table 1 The refinement results of the (1x) LaAlO3-xCa0.2Sr0.8TiO3 (x ¼ 0.3, 0.4, 0.5, 0.6, 0.7). Compounds
0.7LaAlO3e0.3Ca0.2Sr0.8TiO3 0.6LaAlO3e0.4Ca0.2Sr0.8TiO3 0.5LaAlO3e0.5Ca0.2Sr0.8TiO3 0.4LaAlO3e0.6Ca0.2Sr0.8TiO3 0.3LaAlO3e0.7Ca0.2Sr0.8TiO3
Lattice parameters(Å) a
b
c
5.408978 5.42977 5.438417 5.457993 5.46963
5.408978 5.42977 5.438417 5.45799 5.46963
13.26513 13.27377 13.33817 13.34459 13.37574
Vunit (Å3)
Rp (%)
Rwp (%)
c2
336.1 339.1 341.6 344.4 346.6
4.55 3.60 3.85 3.44 3.26
6.07 4.75 4.98 4.54 4.25
1.38 1.14 1.25 1.22 1.18
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257
by the high sintered temperature. Typical energy-dispersive X-ray (EDX with a resolution of 183 eV) analysis of the surface of 0.5LaAlO3e0.5Ca0.2Sr0.8TiO3 sintered at 1550 C for 4 h is shown in Fig. 7 and the corresponding EDX data of the 0.5LaAlO3e0.5Ca0.2Sr0.8TiO3 also presented in Table 2. All the examined grains showing La:Ca þ Sr ¼ 1:1, further confirming the formation of a solid solution.
3.3. Microwave dielectric properties
Fig. 3. The evolution of the lattice parameter of the (1x) LaAlO3-xCa0.2Sr0.8TiO3 (x ¼ 0.3 0.4 0.5 0.6, 0.7) solid solution system.
Fig. 4. The evolution of theoretical, bulk and relative density of the (1x) LaAlO3xCa0.2Sr0.8TiO3 (x ¼ 0.3 0.4 0.5 0.6, 0.7) ceramics.
more sinterable than LaAlO3 Fig. 5 Shows the scanning electron microscopy (SEM) micrographs of the (1x)LaAlO3-xCa0.2Sr0.8TiO3 ceramics sintered at 1550 C for 4 h. Porous microstructure can be seen for specimen x ¼ 0.3. Well-developed microstructure can be achieved at x ¼ 0.4, 0.5, 0.6 and 0.7. All of the grains shows irregular (spherulite-like) grains and the average grain sizes increase from 2.1 mm to 5.2 mm. The boundary of each sample are very clear which imply no liquid phase occurred during the sintering process. Fig. 6 shows the scanning electron microscopy (SEM) micrographs of 0.5LaAlO3e0.5Ca0.2Sr0.8TiO3 ceramics sintered at various temperatures for 4 h. Porous microstructure can be seen for specimen sintered at 1450 C and the grain size increases as the sintering temperature increases due to a grain growth. Welldeveloped microstructure can be achieved at temperature 1550 C-1650 C. However, degradation in grain uniformity and abnormal grain growth appeared at 1650 C, which might cause
Fig. 8 shows the dielectric constant of the (1x) LaAlO3xCa0.2Sr0.8TiO3 ceramics as a function of x. With increasing x from 0.3 to 0.7, the dielectric constant εr increased nonlinearly from 26.7 to 50.9. Its mixture-like behavior is similar to that observed for (1x) LaAlO3-xSrTiO3 [6]. The results can be explained by the empirical rule: lnεr ¼ V1lnε1 þ V2 lnε2 [25], where Vi and εi are the volume fraction and dielectric constant of each phase, respectively. The dielectric constant of LaAlO3 and Ca0.2Sr0.8TiO3 is 23 and 255, respectively. When x ranges from 0.3 to 0.7, the volume fraction of Ca0.2Sr0.8TiO3 increase, which leads to the increasing of εr. Fig. 9 shows the Q f and the tolerance factor of the (1x) LaAlO3-xCa0.2Sr0.8TiO3 as a function of x. Q f decreased from 43,500e18300 GHz as the x value increased from 0.3 to 0.7. The great deterioration of Q f value of the system is caused by the increase of Ca0.2Sr0.8TiO3 composition. Because the Ca0.2Sr0.8TiO3 possesses bad Q f 3960. When its content goes up, we can suggest that the compounds quality factor will be worsen. Fig. 10 shows the tf and the tolerance factor of (1x) LaAlO3xCa0.2Sr0.8TiO3 ceramic system as a function of x sintered at 1550 Cfor 4 h. The tf value also had a nonlinear variation analogous to εr for the samples, varying from 35.5 ppm/ C to 90.08 ppm/ C as the x value increased from 0.3 to 0.7. This may cause by the deceasing of t. According to the equation tf ¼ 12 tε aL , where aL is between 10 and 20 ppm/ C. tε will ranges from negative to positive as the t decrease from 1.012 to 0.935 [2] and when t ¼ 1.012 the tε is near zero. One can suggest that the tf changes from negative to positive with decreasing t in this system. At x ¼ 0.5, a near zero tf value of 2.5 ppm/ C was obtained. For practical applications, a new microwave dielectric material, 0.5LaAlO3e0.5Ca0.2Sr0.8TiO3, has been developed, exhibiting a dielectric constant εr of 32.7, a Q f value of ~33400.
4. Conclusions The microwave dielectric properties of (1x) LaAlO3xCa0.2Sr0.8TiO3 ceramics were investigated. LaAlO3 and Ca0.2Sr0.8TiO3 combine to form a solid solution. The lattice parameter decease with increasing amount of Ca0.2Sr0.8TiO3. The average grain sizes increase from 2.1 mm to 5.2 mm with increasing x at 1550 C. As the x increase from 0.3 to 0.7, εr increased from 26.7 to 50.9, the Q f decrease from 43,500 to 18,30 GHz and tf varied in a wide range from 35.5 ppm/ C to 90.08 ppm/ C. A new microwave dielectric material, (1x) LaAlO3-xCa0.2Sr0.8TiO3 ceramics with x ¼ 0.5, having a dielectric constant εr of ~32.7, a Qf value of ~33400 GHz, and a tf value of ~ 2.5 ppm/ C, is suggested as a candidate material for low loss microwave applications.
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Fig. 5. Scanning electron microscopy photographs of (1x) LaAlO3-xCa0.2Sr0.8TiO3 ceramics with (a) x ¼ 0.3, (b) x ¼ 0.4, (c) x ¼ 0.5, (d) x ¼ 0.6, (e) x ¼ 0.7, sintered at 1550 C for 4 h.
Fig. 6. Scanning electron microscopy photographs of 0.5LaAlO3e0.5Ca0.2Sr0.8TiO3 ceramics: (a) sintered at 1450 Cin air for 4 h, (b) sintered at 1550 C in air for 4 h, (c) sintered at 1600 C in air for 4 h, and (d) sintered at 1650 C in air for 4 h.
L. Zhang et al. / Journal of Alloys and Compounds 649 (2015) 254e260
Fig. 7. Typical energy-dispersive X-ray analysis lO3e0.5Ca0.2Sr0.8TiO3 sintered at 1550 C for 4 h.
of
the
surface
of
0.5LaA-
Table 2 The EDX Data of the 0.5LaAlO3e0.5Ca0.2Sr0.8TiO3 Ceramics for spots A, B and C. Spots
A B C
259
Atom (%)
Fig. 10. tf and the tolerance factor of (1x) LaAlO3-xCa0.2Sr0.8TiO3 ceramic system as a function of x sintered at 1550 C for 4 h
Acknowledgments
OK
Al K
Ca K
Ti K
Sr L
La L
60.75 61.41 59.46
9.03 8.71 9.31
2.03 1.83 2.14
9.34 9.22 9.83
8.44 8.77 8.34
10.41 10.06 10.91
The authors would like to express thanks to Ruan YinJie, Zeng Yi and Zhai MingLong for their technical support. Zhang LiangZhu expresses his gratitude to Yu ZeXiao for useful discussions related to this work. References
Fig. 8. Dielectric constants of the (1x) LaAlO3-xCa0.2Sr0.8TiO3 ceramic system as a function of x sintered at 1550 C for 4 h.
Fig. 9. Q f values of the (1x) LaAlO3-xCa0.2Sr0.8TiO3 system as a function of x sintered at 1550 C for 4 h
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