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Phases, microstructure and microwave dielectric properties of hexagonal perovskite Ca(La1−xNdx)4Ti4O15 ceramics
Journal of Alloys and Compounds 395 (2005) 126–131
Phases, microstructure and microwave dielectric properties of hexagonal perovskite Ca(La1−xNdx)4Ti4O15 ceramics Zhenxing Yue∗ , Fei Zhao, Zhilun Gui, Longtu Li State Key Laboratory of New Ceramics and Fine Processing, Department of Materials Science and Engineering, Tsinghua University, Beijing 100084, PR China Received 24 August 2004; accepted 16 September 2004 Available online 8 January 2005
Abstract Hexagonal perovskite microwave ceramics of Ca(La1−x Ndx )4 Ti4 O15 with x changing from 0 to 0.5 were prepared by solid-state reaction method. The phase relationship, microstructure and microwave dielectric properties were investigated. The substitution of Nd for La has marked effect on the phases. For x = 0 and x = 0.125, the hexagonal perovskite phase remained in sintered ceramics. With x increasing further, a secondary phase with orthorhombic perovskite, Ca(La1−x Ndx )4 Ti5 O17 , was formed. The lattice parameters of hexagonal perovskite cell decrease linearly with increasing Nd content. The dielectric constant of sintered ceramics decreases from 43.61 to 42.30, the Q × f value from 33856 to 15207 GHz with x from 0 to 0.50. The τ f value decreases from −17.1 ppm/◦ C for x = 0 to −5.77 ppm/◦ C for x = 0.50. The mechanisms responsible for the change of dielectric properties with the substitution of Nd for La in perovskite and with the sintering temperature were also investigated. © 2004 Elsevier B.V. All rights reserved. Keywords: Microwave dielectric ceramics; Hexagonal perovskite; Ca(La1−x Ndx )4 Ti4 O15 compound; Dielectric properties
1. Introduction Microwave dielectric ceramics have received much attention in recent years due to the great progress in mobile and satellite telecommunications. Commercial microwave ceramics must have excellent dielectric properties combining a high dielectric constant with a low dielectric loss or high Q and a near zero temperature coefficient of resonant frequency (τ f ) [1]. Several types of microwave dielectric materials have been investigated to meet these requirements for the microwave applications. The large majority of microwave dielectric ceramics have the perovskite or a related structure [2]. The cubic perovskite-type ceramics, such as Ba(Mg1/3 Ta2/3 )O3 (BMT) [3] and Ba(Zn1/3 Ta2/3 )O3 (BZT) [4], have been extensively investigated and gotten to the commercial applications due to their particularly high qual∗
Corresponding author. Tel.: +86 10 62784579; fax: +86 10 62771160. E-mail address: [email protected] (Z. Yue).
0925-8388/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2004.09.076
ity factors. In contrast, there are very few reports on the phases and dielectric properties of perovskites containing mixed cubic/hexagonal stacking sequences [5–7]. Recently, Vineis et al. [8] reported that some hexagonal perovskite ceramics with general formula La4 Ban Ti3+n O12+3n with n = l and 2 possess good dielectric properties combining a relatively high permittivity (39 < εr < 46) with a low dielectric loss (11583 < Q × f < 31839) and small temperature coefficient of resonant frequency (−36 < τ f < 79). Jawahar et al. [9] also reported the excellent dielectric properties of several cation-deficient hexagonal perovskites, making them suitable for dielectric resonators. In the present study, we incorporated a small amount of Nd into the hexagonal perovskite CaLa4 Ti4 O15 in order to adjust the τ f value. In this paper, we report on the effect of the substitution of Nd for La on the phases, microstructure and dielectric properties of hexagonal perovskite ceramics. The influence of sintering temperature on the dielectric properties was also investigated.
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2. Experimental procedures Microwave dielectric ceramics with compositions of Ca(La1−x Ndx )4 Ti4 O15 (noted as CLNT) with x = 0–0.5 were prepared by the conventional solid-state reaction method. High purity CaCO3 , La2 O3 , Nd2 O3 , and TiO2 were used as raw materials. The oxides and carbonates were weighted in the appropriate molar ratio and ball milled using zirconia balls in alcohol medium. The mixtures were dried and calcined at 1300 ◦ C for 4 h. The calcined powders were mixed with an appropriate amount of 5 wt.% solution of polyvinyl alcohol (PVA) as a binder, granulated, and uniaxially pressed into cylindrical disks of diameter 10 mm and height about 5 mm under a pressure of 200 MPa. The green pellets were preheated at 600 ◦ C for 1 h to expel the hinder and then sintered at temperatures from 1450 to 1600 ◦ C for 4 h. The bulk densities of the samples were measured by the Archimedes method. The phase purity of the sintered samples was examined by X-ray diffraction (XRD) techniques using Rigaku D/MAX IIIB X-ray diffractometer with Cu K␣ radiation. Microstructural characterization was conducted by scanning electron microscopy (SEM). The dielectric properties, such as dielectric constant and unloaded quality factor at microwave frequencies were measured using the Hakki–Coleman dielectric resonator method [10]. A HP8720ES network analyzer was employed in the measurement. An identified technique was applied in measuring the temperature coefficient of resonant frequency (τ f ). The measurement was carried out in the temperature range from +10 to +100 ◦ C. The if value can be calculated by the following relationship: τf =
f100 − fRT × 106 (ppm/◦ C) (T100 − TRT )fRT
(1)
where fRT and f100 represent the resonant frequencies at room temperature and 100 ◦ C, respectively.
Fig. 1. XRD patterns of synthesized CLNT powders at various temperatures: (a) 1150 ◦ C, (b) 1200 ◦ C, (c) 1250 ◦ C, (d) 1300 ◦ C, (e) 1350 ◦ C, and (f) 1400 ◦ C.
1400 ◦ C. Thus, the calcination temperature of 1200 ◦ C was selected for all compositions in the preparation of CLNT ceramics. Fig. 2 shows the XRD patterns of sintered Ca(La1−x Ndx )4 Ti4 O15 ceramics with different x. The ceramic samples were sintered at 1550 ◦ C. It can be seen that the substitution of Nd for La has marked effect on the crystal phases of sintered ceramics. For x = 0 and x = 0.125, the pure hexagonal perovskites remained after sintering. For x = 0.25 and 0.50, however, a secondary phase Ca(La1−x Ndx )4 Ti5 O17 , with orthorhombic perovskite structure, was formed, and its amount increases with increasing Nd content. This phenomenon can be interpreted as a variation of the tolerance factor with Nd substitution for La in Ca(La)1−x Ndx )4 Ti4 O15 . It is well known that the stable cubic perovskite structure can be formed in the range of tolerance factor (t) from 0.77 to 1.1, and the stability of perovskite structure increases with increasing t in this range. The tolerance factor (t) of an ABO3
3. Results and discussion 3.1. Crystal phases It was reported that pure phase hexagonal perovskites were difficult to synthesize. Thus, two-step process is generally adopted [9]. In order to get the pure phase perovskite and to determine the optimizing calcination temperature, the mixed powders were calcined at different temperatures and then the phase characterization was performed on them. Fig. 1 shows the XRD patterns of the synthesized powders at various temperatures from 1150 to 1400 ◦ C. It can be seen that all diffraction peaks for powders calcined above 1200 ◦ C can be indexed to be hexagonal perovskite unit cell (JCPDF Card No. 36–1278). This indicates that pure phase hexagonal perovskite was obtained at calcination temperature of 1200 ◦ C for 4 h. This experiment also indicates that the hexagonal perovskite is stable in the temperature range from 1200 to
Fig. 2. XRD patterns of sintered Ca(La1−x Ndx )4 Ti4 O15 ceramics with different x, (a) x = 0, (b) x = 0.125, (c) x = 0.25, and (d) x = 0.50.
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compound can be calculated from the following equation: t=√
rA + rO
(2)
2(rB + rO )
where t is the tolerance factor, rA and rB are the ionic radii of ions on A- and B-sites in ABO3 , respectively, and rO the ionic radius of O2− . In Ca(La1−x Ndx )4 Ti4 O15 , we can suppose the Ca2+ , La3+ and Nd3+ ions are located on the A-sites due to their larger ionic radius relative to Ti4+ . Therefore, Eq. (2) can be rewritten as follows: t=
[(1/5)rCa + (4/5)(1 − x)rLa + (4x/5)rNd ] + rO √ 2((4/5)rTi + rO )
(3) Fig. 3. Variation of lattice parameters with Nd content.
where rCa , rLa , rNd , and rTi are the ionic radii of Ca2+ , La3+ , Nd3+ and Ti4+ , respectively. From Eq. (3), the tolerance factors (t) were calculated for Ca(La1−x Ndx )4 Ti4 O15 , and listed in Table 1. It can be seen that the tolerance factor decreases with Nd substitution for La in the present ceramics. This is the reason that the hexagonal perovskite structure became unstable and the alternative phases were formed for x > 0.25. In view of the layered hexagonal perovskite with general formula An Tin−1 O3n (A = Ca, Ba, Sr, La, etc.) in which the perovskite structure is based on an (hhccc) repeat of the close packed AO3 layers [8], we can conclude that the substitution of NdO3 layer for LaO3 layer decreases the stability of hexagonal perovskite structure, resulting in the formation of orthorhombic structure. In order to investigate the effect of Nd substitution for La on the unit cell of hexagonal perovskite, the lattice parameters of samples were measured and calculated from the diffraction peaks of (126) and (133). The calculated lattice parameters were also listed in Table 1 and their variations with Nd content were shown in Fig. 3. It is clearly seen that the lattice parameters and unit cell volume decrease almost linearly with increasing Nd content before x = 0.25. The smaller diameter of Nd3+ than La3+ can be considered to be responsible for the changes of lattice parameters and unit cell volume. The linear decrease of lattice parameters is also the evidence that the Nd3+ is entered into the unit cell of hexagonal perovskite in place of La3+ . The departure from the linearity for x = 0.50 may be caused by the large calculation errors due to the formation of the secondary phase which cause the relatively low diffraction peaks of hexagonal perovskite phase.
3.2. Bulk density and microstructure The bulk densities of the samples were measured by Archimedes method. Fig. 4 shows the bulk density of samples as a function of sintering temperature. With increasing sintering temperature, the densities increase first, and then decrease, with a maximum at 1550 ◦ C for every composition. From this experiment, the optimizing dielectric properties can be expected at sintering temperature of 1550 ◦ C. The microphotographs of sintered samples were taken by the SEM technique. The typical SEM photographs for different compositions, which were sintered at 1550 ◦ C, were shown in Fig. 5. One can see from the figures that the dense and well-developed microstructures were formed for all compositions. For x = 0 and 0.125, the grains are platelike which are the feature of the hexagonal perovskite ceramics. For x = 0.25 and 0.50, however, the number of platelike grains decreases and some rodlike grains increase. The rodlike grains may be the orthorhombic perovskite phase, Ca(La1−x Ndx )4 Ti5 O17 .
Table 1 Tolerance factors (t) and lattice parameters of Ca(La1−x Ndx )4 Ti4 O15 ceramics X
t
˚ a (A)
˚ c (A)
˚ 3) V (A
0.0 0.125 0.25 0.50
1.034 1.031 1.027 1.021
5.393 5.384 5.376 5.344
11.328 11.327 11.325 11.327
285.32 284.34 283.40 280.13
Fig. 4. Density of ceramic samples as a function of sintering temperature.
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Fig. 5. SEM photographs of sintered CLZT ceramics with (a) x = 0.0, (b) x = 0.125, (c) x = 0.25, and (d) x = 0.50.
3.3. Dielectric properties The dielectric properties of sintered samples at various temperatures were measured in the microwave frequency range. Using these data, the effects of sintering temperature on the dielectric properties were investigated. Fig. 6 shows the variations of dielectric constant with sintering temperature. It
Fig. 6. Variation of dielectric constant with sintering temperature for Ca(La1−x Ndx )4 Ti4 O15 ceramics.
can be seen that the dielectric constant increase rapidly with increasing sintering temperature from 1500 to 1550 ◦ C, and the increase then become slowly above 1550 ◦ C. This is the similar trend with the variations of bulk density with sintering temperature, as shown in Fig. 2. Therefore, the increase in dielectric constants is caused by increasing densification. As the sintering temperature is increased to 1600 ◦ C, the dielectric constants still increase although the bulk densities start to decrease. This may be attributed to the grain growth at high temperature. Fig. 7 shows the variations of Q × f value with sintering temperature. It is interesting to note that the Q × f value increases first with increasing sintering temperature and then decreases after a temperature at which a Q × f value maximum occurs for every composition. In view of the variation of density with sintering temperature, as shown in Fig. 2, the lower Q × f values at low and high temperatures, respectively, may be attributed to the low bulk density. We can conclude from Fig. 7 that the high Q × f values can be obtained at an optimum sintering temperature of about 1550 ◦ C for x = 0.0, 0.125 and 0.25. The measured dielectric properties for all samples sintered at optimizing temperatures were listed in Table 2, and their variations with Nd content were shown in Fig. 8. It can be seen that the dielectric constant decreases slightly
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Fig. 7. Variations of Q × f value with sintering temperature for Ca(La1−x Ndx )4 Ti4 O15 ceramics.
Table 2 Microwave dielectric properties of sintered Ca(La1−x Ndx )4 Ti4 O15 ceramics X
Sintering temperature (◦ C)
f0 (GHz)
εr
Q×f (GHz)
τ f (ppm/◦ C)
0.0 0.125 0.25 0.50
1550 1550 1550 1525
7.757 7.519 7.853 8.283
43.61 43.36 43.10 42.30
33856 32921 29819 15207
−17.1 −13.0 −9.06 −5.77
from 43.61 to 42.30 with increasing Nd content from 0 to 0.50. The Q × f value, however, decreases significantly at relatively high Nd content from 32921 GHz for x = 0.125 to 15207 GHz for x = 0.50. The temperature coefficient of resonant frequency (τ f ) shifts toward zero with increasing Nd content. The τ f value decreases from −17.1 ppm/◦ C for x = 0 to −5.77 ppm/◦ C for x = 0.50. This result indicates the substitution of Nd for La in hexagonal perovskite ceramics can improve the temperature stability of sintered ceramics. In view of the crystal phases and microstructures, the variations of the dielectric properties with Nd content may be at-
tributed to the change of phase relations and microstructures caused by the substitution of Nd for La in perovskite. The volume shrinkage of perovskite unit cell caused by the substitution of Nd for La and the formation of secondary phase may be responsible for the decrease of dielectric constant. The former may decrease the polarizability of Ti4+ ions in perovskite cell due to the decrease of the volume of [TiO6 ] octahedral. The dielectric properties of orthorhombic perovskite compound, Ca(La1−x Ndx )4 Ti5 O17 , have not been reported before. We can deduce from the present study that the dielectric properties, such as dielectric constant and Q × f value, of Ca(La1−x Ndx )4 Ti5 O17 may be lower than hexagonal perovskite. Therefore, the formation of orthorhombic perovskite phase decreases the dielectric constant and Q × f value of the sintered ceramics. Moreover, the dielectric loss of the perfect dielectric crystals at microwave frequency is considered to originate from the anharmonic lattice forces that mediate the interaction between the crystal’s phonons [1]. For the present ceramics, as discussed above, the substitution of Nd for La resulted in the unstable hexagonal perovskite structure. Thus, the anharmonic effect of the interaction between the crystal’s phonons will increase. As a result, the dielectric loss at microwave frequency will be increased. In other words, the existence of the secondary phase has positive effect in improving the temperature stability, as observed from Fig. 8. Although we have not known the exact dielectric properties of the secondary phase, we can refer that the orthorhombic perovskite compound, Ca(La1−x Ndx )4 Ti5 O17 , may have positive τ f value. The detailed mechanisms responsible for the effect of the secondary phases should be performed further.
4. Conclusions Hexagonal perovskite microwave ceramics of Ca(La1−x Ndx )4 Ti4 O15 with x changing from 0 to 0.5 were prepared by conventional solid-state oxides method. The substitution of Nd for La has marked effect on the phases. For x = 0 and x = 0.125, the hexagonal perovskite phase was remained in sintered ceramics. With x increasing further, a secondary phase with orthorhombic perovskite, Ca(La1−x Ndx )4 Ti5 O17 , was formed. The lattice parameters of hexagonal perovskite cell decrease linearly with increasing Nd content. The dielectric constant of sintered ceramics decreases from 43.61 to 42.30, and the Q × f value from 33856 to 15207 GHz with x from 0 to 0.50. The τ f value decreases from −17.1 ppm/◦ C for x = 0 to −5.77 ppm/◦ C for x = 0.50. The structural change and the secondary phase may be responsible for the variations of dielectric properties with the substitution of Nd for La.
Acknowledgements Fig. 8. Variation of microwave dielectric properties with Nd content for CLNT ceramics.
This work has been financially supported by the National High-Tech Development Project of China (Grant No.
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2001AA325100) and the National Major Fundamental Research Project (Grant No. 2002CB613307), and SRFDP (No. 20040003003). References [1] W. Wersing, Curr. Opin. Solid State Mater. Sci. 1 (1996) 715. [2] M. Reaney, R. Ubic, Int. Ceram. 1 (2000) 48. [3] D.J. Barber, K.M. Moulding, J. Zhou, M.Q. Li, J. Mater. Sci. 32 (1997) 1531. [4] S. Kawasima, M. Nishida, I. Ueda, H. Ouchi, J. Am. Ceram. Soc 66 (1983) 421–434.
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[5] H. Sreemoolanadhan, M.T. Sebastian, P. Mohanan, Mater. Res. Bull. 30 (1995) 653. [6] S. Kim, W.H. Jung, Y. Inaguma, T. Nakamura, M. Itoh, Mater. Res. Bull. 30 (1995) 307. [7] S. Kamba, J. Petzelt, E. Buixaderas, D. Haubrich, P. Vanek, P. Kuzel, L.N. Jawahar, M.T. Sebastian, P. Mohanan, J. Appl. Phys. 89 (2001) 3900. [8] C. Vineis, P.K. Davies, T. Negas, S. Bell, Mater. Res. Bull. 31 (1996) 431. [9] N. Jawahar, N.I. Santha, M.T. Sebastian, P. Mohanan, J. Mater. Res. 17 (2002) 3084. [10] B.W. Hakki, P.D. Coleman, IRE Trans. Microwave Theory Tech. 8 (1960) 402.
Phases, microstructure and microwave dielectric properties of hexagonal perovskite Ca(La1−xNdx)4Ti4O15 ceramics Zhenxing Yue∗ , Fei Zhao, Zhilun Gui, Longtu Li State Key Laboratory of New Ceramics and Fine Processing, Department of Materials Science and Engineering, Tsinghua University, Beijing 100084, PR China Received 24 August 2004; accepted 16 September 2004 Available online 8 January 2005
Abstract Hexagonal perovskite microwave ceramics of Ca(La1−x Ndx )4 Ti4 O15 with x changing from 0 to 0.5 were prepared by solid-state reaction method. The phase relationship, microstructure and microwave dielectric properties were investigated. The substitution of Nd for La has marked effect on the phases. For x = 0 and x = 0.125, the hexagonal perovskite phase remained in sintered ceramics. With x increasing further, a secondary phase with orthorhombic perovskite, Ca(La1−x Ndx )4 Ti5 O17 , was formed. The lattice parameters of hexagonal perovskite cell decrease linearly with increasing Nd content. The dielectric constant of sintered ceramics decreases from 43.61 to 42.30, the Q × f value from 33856 to 15207 GHz with x from 0 to 0.50. The τ f value decreases from −17.1 ppm/◦ C for x = 0 to −5.77 ppm/◦ C for x = 0.50. The mechanisms responsible for the change of dielectric properties with the substitution of Nd for La in perovskite and with the sintering temperature were also investigated. © 2004 Elsevier B.V. All rights reserved. Keywords: Microwave dielectric ceramics; Hexagonal perovskite; Ca(La1−x Ndx )4 Ti4 O15 compound; Dielectric properties
1. Introduction Microwave dielectric ceramics have received much attention in recent years due to the great progress in mobile and satellite telecommunications. Commercial microwave ceramics must have excellent dielectric properties combining a high dielectric constant with a low dielectric loss or high Q and a near zero temperature coefficient of resonant frequency (τ f ) [1]. Several types of microwave dielectric materials have been investigated to meet these requirements for the microwave applications. The large majority of microwave dielectric ceramics have the perovskite or a related structure [2]. The cubic perovskite-type ceramics, such as Ba(Mg1/3 Ta2/3 )O3 (BMT) [3] and Ba(Zn1/3 Ta2/3 )O3 (BZT) [4], have been extensively investigated and gotten to the commercial applications due to their particularly high qual∗
Corresponding author. Tel.: +86 10 62784579; fax: +86 10 62771160. E-mail address: [email protected] (Z. Yue).
0925-8388/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2004.09.076
ity factors. In contrast, there are very few reports on the phases and dielectric properties of perovskites containing mixed cubic/hexagonal stacking sequences [5–7]. Recently, Vineis et al. [8] reported that some hexagonal perovskite ceramics with general formula La4 Ban Ti3+n O12+3n with n = l and 2 possess good dielectric properties combining a relatively high permittivity (39 < εr < 46) with a low dielectric loss (11583 < Q × f < 31839) and small temperature coefficient of resonant frequency (−36 < τ f < 79). Jawahar et al. [9] also reported the excellent dielectric properties of several cation-deficient hexagonal perovskites, making them suitable for dielectric resonators. In the present study, we incorporated a small amount of Nd into the hexagonal perovskite CaLa4 Ti4 O15 in order to adjust the τ f value. In this paper, we report on the effect of the substitution of Nd for La on the phases, microstructure and dielectric properties of hexagonal perovskite ceramics. The influence of sintering temperature on the dielectric properties was also investigated.
Z. Yue et al. / Journal of Alloys and Compounds 395 (2005) 126–131
127
2. Experimental procedures Microwave dielectric ceramics with compositions of Ca(La1−x Ndx )4 Ti4 O15 (noted as CLNT) with x = 0–0.5 were prepared by the conventional solid-state reaction method. High purity CaCO3 , La2 O3 , Nd2 O3 , and TiO2 were used as raw materials. The oxides and carbonates were weighted in the appropriate molar ratio and ball milled using zirconia balls in alcohol medium. The mixtures were dried and calcined at 1300 ◦ C for 4 h. The calcined powders were mixed with an appropriate amount of 5 wt.% solution of polyvinyl alcohol (PVA) as a binder, granulated, and uniaxially pressed into cylindrical disks of diameter 10 mm and height about 5 mm under a pressure of 200 MPa. The green pellets were preheated at 600 ◦ C for 1 h to expel the hinder and then sintered at temperatures from 1450 to 1600 ◦ C for 4 h. The bulk densities of the samples were measured by the Archimedes method. The phase purity of the sintered samples was examined by X-ray diffraction (XRD) techniques using Rigaku D/MAX IIIB X-ray diffractometer with Cu K␣ radiation. Microstructural characterization was conducted by scanning electron microscopy (SEM). The dielectric properties, such as dielectric constant and unloaded quality factor at microwave frequencies were measured using the Hakki–Coleman dielectric resonator method [10]. A HP8720ES network analyzer was employed in the measurement. An identified technique was applied in measuring the temperature coefficient of resonant frequency (τ f ). The measurement was carried out in the temperature range from +10 to +100 ◦ C. The if value can be calculated by the following relationship: τf =
f100 − fRT × 106 (ppm/◦ C) (T100 − TRT )fRT
(1)
where fRT and f100 represent the resonant frequencies at room temperature and 100 ◦ C, respectively.
Fig. 1. XRD patterns of synthesized CLNT powders at various temperatures: (a) 1150 ◦ C, (b) 1200 ◦ C, (c) 1250 ◦ C, (d) 1300 ◦ C, (e) 1350 ◦ C, and (f) 1400 ◦ C.
1400 ◦ C. Thus, the calcination temperature of 1200 ◦ C was selected for all compositions in the preparation of CLNT ceramics. Fig. 2 shows the XRD patterns of sintered Ca(La1−x Ndx )4 Ti4 O15 ceramics with different x. The ceramic samples were sintered at 1550 ◦ C. It can be seen that the substitution of Nd for La has marked effect on the crystal phases of sintered ceramics. For x = 0 and x = 0.125, the pure hexagonal perovskites remained after sintering. For x = 0.25 and 0.50, however, a secondary phase Ca(La1−x Ndx )4 Ti5 O17 , with orthorhombic perovskite structure, was formed, and its amount increases with increasing Nd content. This phenomenon can be interpreted as a variation of the tolerance factor with Nd substitution for La in Ca(La)1−x Ndx )4 Ti4 O15 . It is well known that the stable cubic perovskite structure can be formed in the range of tolerance factor (t) from 0.77 to 1.1, and the stability of perovskite structure increases with increasing t in this range. The tolerance factor (t) of an ABO3
3. Results and discussion 3.1. Crystal phases It was reported that pure phase hexagonal perovskites were difficult to synthesize. Thus, two-step process is generally adopted [9]. In order to get the pure phase perovskite and to determine the optimizing calcination temperature, the mixed powders were calcined at different temperatures and then the phase characterization was performed on them. Fig. 1 shows the XRD patterns of the synthesized powders at various temperatures from 1150 to 1400 ◦ C. It can be seen that all diffraction peaks for powders calcined above 1200 ◦ C can be indexed to be hexagonal perovskite unit cell (JCPDF Card No. 36–1278). This indicates that pure phase hexagonal perovskite was obtained at calcination temperature of 1200 ◦ C for 4 h. This experiment also indicates that the hexagonal perovskite is stable in the temperature range from 1200 to
Fig. 2. XRD patterns of sintered Ca(La1−x Ndx )4 Ti4 O15 ceramics with different x, (a) x = 0, (b) x = 0.125, (c) x = 0.25, and (d) x = 0.50.
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compound can be calculated from the following equation: t=√
rA + rO
(2)
2(rB + rO )
where t is the tolerance factor, rA and rB are the ionic radii of ions on A- and B-sites in ABO3 , respectively, and rO the ionic radius of O2− . In Ca(La1−x Ndx )4 Ti4 O15 , we can suppose the Ca2+ , La3+ and Nd3+ ions are located on the A-sites due to their larger ionic radius relative to Ti4+ . Therefore, Eq. (2) can be rewritten as follows: t=
[(1/5)rCa + (4/5)(1 − x)rLa + (4x/5)rNd ] + rO √ 2((4/5)rTi + rO )
(3) Fig. 3. Variation of lattice parameters with Nd content.
where rCa , rLa , rNd , and rTi are the ionic radii of Ca2+ , La3+ , Nd3+ and Ti4+ , respectively. From Eq. (3), the tolerance factors (t) were calculated for Ca(La1−x Ndx )4 Ti4 O15 , and listed in Table 1. It can be seen that the tolerance factor decreases with Nd substitution for La in the present ceramics. This is the reason that the hexagonal perovskite structure became unstable and the alternative phases were formed for x > 0.25. In view of the layered hexagonal perovskite with general formula An Tin−1 O3n (A = Ca, Ba, Sr, La, etc.) in which the perovskite structure is based on an (hhccc) repeat of the close packed AO3 layers [8], we can conclude that the substitution of NdO3 layer for LaO3 layer decreases the stability of hexagonal perovskite structure, resulting in the formation of orthorhombic structure. In order to investigate the effect of Nd substitution for La on the unit cell of hexagonal perovskite, the lattice parameters of samples were measured and calculated from the diffraction peaks of (126) and (133). The calculated lattice parameters were also listed in Table 1 and their variations with Nd content were shown in Fig. 3. It is clearly seen that the lattice parameters and unit cell volume decrease almost linearly with increasing Nd content before x = 0.25. The smaller diameter of Nd3+ than La3+ can be considered to be responsible for the changes of lattice parameters and unit cell volume. The linear decrease of lattice parameters is also the evidence that the Nd3+ is entered into the unit cell of hexagonal perovskite in place of La3+ . The departure from the linearity for x = 0.50 may be caused by the large calculation errors due to the formation of the secondary phase which cause the relatively low diffraction peaks of hexagonal perovskite phase.
3.2. Bulk density and microstructure The bulk densities of the samples were measured by Archimedes method. Fig. 4 shows the bulk density of samples as a function of sintering temperature. With increasing sintering temperature, the densities increase first, and then decrease, with a maximum at 1550 ◦ C for every composition. From this experiment, the optimizing dielectric properties can be expected at sintering temperature of 1550 ◦ C. The microphotographs of sintered samples were taken by the SEM technique. The typical SEM photographs for different compositions, which were sintered at 1550 ◦ C, were shown in Fig. 5. One can see from the figures that the dense and well-developed microstructures were formed for all compositions. For x = 0 and 0.125, the grains are platelike which are the feature of the hexagonal perovskite ceramics. For x = 0.25 and 0.50, however, the number of platelike grains decreases and some rodlike grains increase. The rodlike grains may be the orthorhombic perovskite phase, Ca(La1−x Ndx )4 Ti5 O17 .
Table 1 Tolerance factors (t) and lattice parameters of Ca(La1−x Ndx )4 Ti4 O15 ceramics X
t
˚ a (A)
˚ c (A)
˚ 3) V (A
0.0 0.125 0.25 0.50
1.034 1.031 1.027 1.021
5.393 5.384 5.376 5.344
11.328 11.327 11.325 11.327
285.32 284.34 283.40 280.13
Fig. 4. Density of ceramic samples as a function of sintering temperature.
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Fig. 5. SEM photographs of sintered CLZT ceramics with (a) x = 0.0, (b) x = 0.125, (c) x = 0.25, and (d) x = 0.50.
3.3. Dielectric properties The dielectric properties of sintered samples at various temperatures were measured in the microwave frequency range. Using these data, the effects of sintering temperature on the dielectric properties were investigated. Fig. 6 shows the variations of dielectric constant with sintering temperature. It
Fig. 6. Variation of dielectric constant with sintering temperature for Ca(La1−x Ndx )4 Ti4 O15 ceramics.
can be seen that the dielectric constant increase rapidly with increasing sintering temperature from 1500 to 1550 ◦ C, and the increase then become slowly above 1550 ◦ C. This is the similar trend with the variations of bulk density with sintering temperature, as shown in Fig. 2. Therefore, the increase in dielectric constants is caused by increasing densification. As the sintering temperature is increased to 1600 ◦ C, the dielectric constants still increase although the bulk densities start to decrease. This may be attributed to the grain growth at high temperature. Fig. 7 shows the variations of Q × f value with sintering temperature. It is interesting to note that the Q × f value increases first with increasing sintering temperature and then decreases after a temperature at which a Q × f value maximum occurs for every composition. In view of the variation of density with sintering temperature, as shown in Fig. 2, the lower Q × f values at low and high temperatures, respectively, may be attributed to the low bulk density. We can conclude from Fig. 7 that the high Q × f values can be obtained at an optimum sintering temperature of about 1550 ◦ C for x = 0.0, 0.125 and 0.25. The measured dielectric properties for all samples sintered at optimizing temperatures were listed in Table 2, and their variations with Nd content were shown in Fig. 8. It can be seen that the dielectric constant decreases slightly
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Fig. 7. Variations of Q × f value with sintering temperature for Ca(La1−x Ndx )4 Ti4 O15 ceramics.
Table 2 Microwave dielectric properties of sintered Ca(La1−x Ndx )4 Ti4 O15 ceramics X
Sintering temperature (◦ C)
f0 (GHz)
εr
Q×f (GHz)
τ f (ppm/◦ C)
0.0 0.125 0.25 0.50
1550 1550 1550 1525
7.757 7.519 7.853 8.283
43.61 43.36 43.10 42.30
33856 32921 29819 15207
−17.1 −13.0 −9.06 −5.77
from 43.61 to 42.30 with increasing Nd content from 0 to 0.50. The Q × f value, however, decreases significantly at relatively high Nd content from 32921 GHz for x = 0.125 to 15207 GHz for x = 0.50. The temperature coefficient of resonant frequency (τ f ) shifts toward zero with increasing Nd content. The τ f value decreases from −17.1 ppm/◦ C for x = 0 to −5.77 ppm/◦ C for x = 0.50. This result indicates the substitution of Nd for La in hexagonal perovskite ceramics can improve the temperature stability of sintered ceramics. In view of the crystal phases and microstructures, the variations of the dielectric properties with Nd content may be at-
tributed to the change of phase relations and microstructures caused by the substitution of Nd for La in perovskite. The volume shrinkage of perovskite unit cell caused by the substitution of Nd for La and the formation of secondary phase may be responsible for the decrease of dielectric constant. The former may decrease the polarizability of Ti4+ ions in perovskite cell due to the decrease of the volume of [TiO6 ] octahedral. The dielectric properties of orthorhombic perovskite compound, Ca(La1−x Ndx )4 Ti5 O17 , have not been reported before. We can deduce from the present study that the dielectric properties, such as dielectric constant and Q × f value, of Ca(La1−x Ndx )4 Ti5 O17 may be lower than hexagonal perovskite. Therefore, the formation of orthorhombic perovskite phase decreases the dielectric constant and Q × f value of the sintered ceramics. Moreover, the dielectric loss of the perfect dielectric crystals at microwave frequency is considered to originate from the anharmonic lattice forces that mediate the interaction between the crystal’s phonons [1]. For the present ceramics, as discussed above, the substitution of Nd for La resulted in the unstable hexagonal perovskite structure. Thus, the anharmonic effect of the interaction between the crystal’s phonons will increase. As a result, the dielectric loss at microwave frequency will be increased. In other words, the existence of the secondary phase has positive effect in improving the temperature stability, as observed from Fig. 8. Although we have not known the exact dielectric properties of the secondary phase, we can refer that the orthorhombic perovskite compound, Ca(La1−x Ndx )4 Ti5 O17 , may have positive τ f value. The detailed mechanisms responsible for the effect of the secondary phases should be performed further.
4. Conclusions Hexagonal perovskite microwave ceramics of Ca(La1−x Ndx )4 Ti4 O15 with x changing from 0 to 0.5 were prepared by conventional solid-state oxides method. The substitution of Nd for La has marked effect on the phases. For x = 0 and x = 0.125, the hexagonal perovskite phase was remained in sintered ceramics. With x increasing further, a secondary phase with orthorhombic perovskite, Ca(La1−x Ndx )4 Ti5 O17 , was formed. The lattice parameters of hexagonal perovskite cell decrease linearly with increasing Nd content. The dielectric constant of sintered ceramics decreases from 43.61 to 42.30, and the Q × f value from 33856 to 15207 GHz with x from 0 to 0.50. The τ f value decreases from −17.1 ppm/◦ C for x = 0 to −5.77 ppm/◦ C for x = 0.50. The structural change and the secondary phase may be responsible for the variations of dielectric properties with the substitution of Nd for La.
Acknowledgements Fig. 8. Variation of microwave dielectric properties with Nd content for CLNT ceramics.
This work has been financially supported by the National High-Tech Development Project of China (Grant No.
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