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Synthesis and microwave dielectric properties of Nd2SiO5 ceramics
Journal of Alloys and Compounds 544 (2012) 141–144
Contents lists available at SciVerse ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Synthesis and microwave dielectric properties of Nd2SiO5 ceramics Chan Jiang, Songping Wu ⇑, Qing Ma, Youxian Mei School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510641, China
a r t i c l e
i n f o
Article history: Received 2 July 2012 Received in revised form 14 July 2012 Accepted 16 July 2012 Available online 4 August 2012 Keywords: Synthesis Silicates Nd2SiO5 Dielectric materials/properties Microwaves
a b s t r a c t Nd2SiO5 ceramics were synthesized and their microwave dielectric properties were investigated. The hexagonal Nd4Si3O12 second phase disappeared and the pure monoclinic Nd2SiO5 phase could be obtained when the molar ratio of Nd2O3/SiO2 was 1:1.05 at 1450 °C. The relative density of Nd2SiO5 ceramics increased with increasing temperature. The Nd2SiO5 ceramics sintered at 1500 °C exhibited microwave dielectric properties: a dielectric constant (er) of 7.94, a quality factor (Q f) of 38 800 GHz and a temperature coefficient of resonant frequency (sf) of 53 ppm/°C. High resonant frequency led to a low dielectric constant and low Q f value. Nd2SiO5 ceramics have a wide sintering temperature region. They are promising candidate materials for microwave passive components. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Silicates were important microwave ceramic materials, which have been extensively used to make microwave devices. Silicates, such as magnesium silicate (MgSiO3), forsterite (Mg2SiO4), zinc silicate (Zn2SiO4) and wollastonite (CaSiO3), etc., had low dielectric constant and high quality factor. MgSiO3 ceramics had excellent dielectric properties: er = 6.7, Q f = 121 200 GHz, sf = 17 ppm/ °C [1]; however, they were easy to be powdered due to the inevitable phase transformation. Microwave properties of Mg2SiO4 ceramics containing 0.5 wt.% LMZBS glass were: er = 7.3, Q f = 121 200 GHz [2]. (Mg0.4Zn0.6)2SiO4 ceramics indicated a good combination of microwave dielectric characteristics: er = 6.6, Q f = 95 650 GHz, and sf = 60 ppm/°C [3]. Mg2SiO4 ceramics had a strong processing sensitivity [4]. Zn2SiO4 ceramics with excellent properties were synthesized by a cold isostatic pressing (CIP) at a pressure of 200–300 MPa [5–7]; however, its quality factor was low (Q f = 15 000 GHz) when the conventional solid-state method was employed [7]. CaSiO3 ceramics had a low sintering temperature (1320 °C) and low dielectric constant (6.69). The calcium element was easy to evaporate under high temperature, and the low densification and porous microstructure of CaSiO3 resulted in a low quality factor (Q f = 25398 GHz) [8]. Ternary silicates have also attracted much attention in recent years, for example the Sr2ZnSi2O7 with excellent dielectric properties (er = 8.4, Q f = 105000 GHz and sf = 51.5 ppm/°C) [9] and the low-temperature cofiring of Li2MgSiO4 [10].
⇑ Corresponding author. E-mail address: [email protected] (S. Wu). 0925-8388/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2012.07.076
Study on microwave dielectric properties of Re2O3–SiO2 was lack. Renjini et al. have synthesized Sm2Si2O7 via solid-state method. The dielectric properties of Sm2Si2O7 were: er = 10, Q f = 3 000 GHz and sf = 20 ppm/°C. Dielectric properties of Sm2Si2O7 containing LBS or LMZBS glass were also given: er = 9.89, tan d = 0.024 (LBS, 975 °C) and er = 9.09, tan d = 0.009 (LMZBS, 950 °C), respectively [11]. In our previous study [12], microwave dielectric properties of Sm2SiO5 ceramics were: er = 8.44, Q f = 64 000 GHz, sf = 37 ppm/°C. Even though various studies were conducted on silicates, it is necessary to find new silicate materials for microwave/millimeter-wave application. The monoclinic Nd2SiO5 has the same monoclinic crystal system and space group (P21/c [14]) as the Sm2SiO5. We deduced that Nd2SiO5 might be candidate materials for millimeter-wave devices. In the present work, we reported the preparation, characterization, and microwave dielectric properties of Nd2SiO5 ceramics.
2. Experimental procedure Nd2SiO5 microwave ceramics were synthesized with solid-state synthesis method. Nd2O3 particles were synthesized by thermal decomposition of neodymium oxalate. Nd2O3 and nano-SiO2 (7 nm, Degussa, Auckland New Zealand) were mixed with a suitable molar ratio, and then milled with zirconia balls for 4 h on a planetary milling machine (QM-3SP2, Zhenguang, Nanjing, China). The mixtures were dried, and then calcined at 1100 °C for 2 h. The calcined ceramic powders were firstly re-milled for 4 h, dried, then granulated with PVA binder, and finally pressed into cylindrical disks of 10 mm diameter and 5 mm thickness under 10 MPa pressure isostatically with a hydrostatic press (KSTY70, Haixiang, Changzhou, China). The samples were sintered at 1300–1600 °C for 4 h in air with a high temperature electric furnace (SSJ-1600, Shenjia kiln, Luoyang, China).
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The crystalline phases of the specimen were analyzed by X-ray diffraction (XRD) (D/max-IIIA, Rigaku, Tokyo, Japan) with Cu Ka radiation of 2h from 10° to 60°. The microstructure observations of the ceramic surfaces were performed under a scanning electron microscope (SEM) (LEO 1530 VP, Zeiss, Vertrieb Deutschland, Germany). Element analysis of micro domain was carried out with an energy dispersive spectrometer (EDS) (EPMA1600, Shimadzu, Kyoto, Japan). The bulk density of ceramics was measured by the Archimedes method. Microwave dielectric constants (er) and the quality factor values (Q f) at microwave frequencies were measured by Hakki-Coleman dielectric resonator method using a Network Analyzer (N5230 PNA-L, Agilent, Santa Clara, California, USA). Temperature coefficient of resonant frequency (sf) was also measured by the same method with a changing temperature from 25 to 75 °C, and calculated by the following Eq. (1):
sf ¼
f 75 - f 25 106 f 25 50
ðppm= CÞ
ð1Þ
where f75 and f25 represent the resonant frequency at 75 and 25 °C, respectively.
3. Results and discussion 3.1. Phase identification of Nd2O3–xSiO2 (0.80 6 x 6 1.25) microwave ceramics Fig. 1 showed XRD patterns of Nd2O3–xSiO2 sintered at 1450 °C with different dosage of SiO2. From those curves, one could see the x values had great influences on phase compositions of the samples. According to XRD spectra, monoclinic Nd2SiO5 phase (JCPDS No. 40-0284) was the main crystal phase at x = 0.80–1.0, accompanied with the Nd2O3 (JCPDS No. 41-1089). The Nd2O3 diffraction peak disappeared and uniform phase Nd2SiO5 occurred at x = 1.05. When x = 1.25, monoclinic Nd2SiO5 phase coexists with the hexagonal Nd4Si3O12 (JCPDS No. 42-0171), whose amount increased with an increasing x value. We could obtain a pure hexagonal Nd4Si3O12 phase at x = 1.50. XRD patterns of Nd2O3-1.05SiO2 sintered at different temperatures (from 1300 to 1550 °C) were given in Fig. 2. When sintering temperature was less than 1400 °C, the monoclinic Nd2SiO5 phase was the main crystal phase, accompanied with the hexagonal Nd4Si3O12; however, the pure Nd2SiO5 phase started to form when the specimen was sintered at above 1450 °C. 3.2. SEM and EDS studies The microstructures of Nd2SiO5 ceramics sintered at various temperatures were investigated using SEM, as shown in Fig. 3a– e. For the specimen sintered at 1350 °C, a porous microstructure
Fig. 1. XRD patterns of Nd2O3–xSiO2 with various amounts of SiO2 sintered at 1450 °C for 4 h.
Fig. 2. XRD patterns of Nd2O3–1.05SiO2 sintered at various temperatures.
was developed, a large number of pores existed, and the grain size of the specimen was small, approximately 0.5 lm. The densification and grain size increased as the sintering temperature increased. The dense microstructure was developed, and an average grain size increased to 1.5 lm when the specimen was sintered at 1500 °C, as given in Fig. 3d. The compact interconnected grain microstructures and clear grain boundaries could be observed. The atom ratio (Nd:Si) of grain (marked with a white star in Fig. 3d) was 2:1 by EDS (see Fig. 3f), which clearly displayed the composition of grain was Nd2SiO5. It had also been confirmed by stoichiometric and XRD analysis. As the temperature was above 1550 °C, the abnormal grain growth occurred, and the overfiring might has appeared, as exhibited in Fig. 3e. 3.3. Microwave dielectric properties Fig. 4a–d showed the relative density, dielectric constant (er), quality factor (Q f) and sf values of the Nd2SiO5 ceramics sintered at various temperatures for 4 h, respectively. The relative density of the specimen sintered at 1350 °C was low (78.5%), but increased markedly with increasing sintering temperature to a maximum value of 98.1% of the theoretical density for the specimen sintered at 1500 °C, as exhibited in Fig. 4a. The er value was low (4.87) for the specimen sintered at 1350 °C, probably due to the porous microstructure. It increased with increasing sintering temperature to a maximum value of 7.94 (at 18.35 GHz) for the specimen sintered at 1500 °C (see Fig. 4b). The Q f value of the Nd2SiO5 ceramics sintered at 1350 °C was low (5600 GHz at 17.64 GHz) due to the small grain size, low density and porous microstructure (as exhibited in Fig. 3a). However, it increased considerably with increasing sintering temperature to a value of 38 800 GHz for the specimens sintered at 1500 °C, then the slightly decreased to 34 200 GHz, as given in Fig. 4c. The XRD results revealed the development of inhomogeneous phases consisting of monoclinic Nd2SiO5 and hexagonal Nd4Si3O12 in the specimens sintered at less than 1400 °C. As the temperature increased, the densification of specimen promoted and the pure Nd2SiO5 formed. Grain microstructure was also an important role to affect Q f value. Generally speaking, large grain size, high densification and uniform grain microstructure led to a high Q f value; however, the second phase and abnormal grain growth produced a low Q f value. Therefore, the increased Q f value of specimen sintered at 1400–1500 °C may have related to homogeneous phase and high densification; however, the decreased
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Fig. 3. SEM photographs of Nd2O3–1.05SiO2 sintered at: (a) 1350 °C, (b) 1400 °C, (c) 1450 °C, (d) 1500 °C, (e) 1550 °C, and (f) EDS pattern of micro domain marked in Fig. 3d.
Fig. 4. (a) Relative density and (b) er, (c) Q f, and (d) sf values of the Nd2SiO5 ceramics sintered at various temperatures.
Q f value at 1550 °C could be ascribed to an abnormal grain growth because a boundary defect probably existed among the grains. In this work, microwave dielectric properties of the hexagonal Nd4Si3O12 were: er = 9.94, Q f = 6 300 GHz (at 11.72 GHz) and sf = 12 ppm/°C (at 1450 °C for 4 h). The existence of Nd4Si3O12 should further decrease the quality factor of Nd2SiO5 ceramics according to the well-known mixing rule [5]. The sf values of the
specimens sintered at various temperatures were also shown in Fig. 3d. The sf values decreased with increasing temperatures because the hexagonal Nd4Si3O12 with similar high sf value disappeared. It was clearly that the sf value of monoclinic Nd2SiO5 was 53 ppm/°C. The ceramic powders were pressed into cylindrical disks of 6– 12 mm diameter and 5–6 mm thickness. Those disks were sintered at 1450 °C for 4 h. Influences of resonant frequencies on microwave
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Fig. 5. Influences of resonant frequency on microwave dielectric properties of Nd2SiO5 ceramics.
dielectric properties of samples were exhibited in Fig. 5. The dielectric constant of Nd2SiO5 ceramics decreased as the frequency increased. Because all of samples have a high densification (above 97.5%), we assumed that the main reason for it was that different polarization mechanisms worked in the frequency region of 10– 30 GHz. In general, the difference in dielectric constant of the electrolyte is determined by the different polarization mechanisms. The polarization of dielectric materials mainly contains electronic displacement polarization, ionic displacement polarization and orientation polarization of permanent dipole moment. The latter could be established in microwave band (less than 100 GHz). As the frequency increased to 25 GHz, It might be difficult to establish such a polarization for the ceramic samples completely, leading to a low dielectric constant. The Q f of Nd2SiO5 ceramics decreased as the frequency increased. Within the context of the current theory of the intrinsic dielectric loss, we would expect for Nd2SiO5 a linear frequency dependence of tan d / f in the microwave and millimeter-wave range [13]. In contrast, our data confirm that there exists an additional extrinsic contribution to the dielectric loss which causes a significant departure from the linear tan d / f behavior at low frequencies. This behavior of the Q f value may indicate an effect of the low-frequency extrinsic dielectric loss (possibly Debye-tpye loss) increases at higher frequencies. It is of interest that several other types of dielectric ceramics show similar characteristic extrinsic contributions to the dielectric loss at low-frequency. For example, a sub-linear tan d/f dependence in the 10–40 GHz interval was observed in the Ba(Mg1/3Ta2/3)O3, Ba(Mg1/3Nb2/3)O3 and Ba(Co1/2Nb2/3)O3 ceramics [14]. In our previous study [12], microwave dielectric properties of Sm2SiO5 ceramics were: er = 8.44, Q f = 64 000 GHz (at 11.9 GHz), sf = 37 ppm/°C. The dielectric constant of Nd2SiO5 ceramics (7.94) was lower than that of Sm2SiO5. It could be explained with the following Clausius–Mosotti Eq. (2)
er ¼
3 2 1 bam =V
ð2Þ
where V in cell volume, am is dielectric polarization rate and b = 4p/3. The cell volume of Nd2SiO5 is 438.8 Å3, which is more than that of Sm2SiO5 (425.7 Å3) with the same crystal system and space group. It was generally accepted that the am increases with increasing ionic radius of Re3+ because the larger ionic radius contributed larger electron polarization according to Eq. (2) a = 4pkoR3 (ko is the relative permittivity and R is the atomic or ionic radius). Ionic radius of Nd3+ and Sm3+ are 99.5 and 96.4 pm, respectively, so the electron polarizatnion of Nd3+ was more than that of Sm3+.
Obviously, am and V have a similar trend for RE2SiO5. In the discussion, influence of cell volume on the dielectric constant seems to be dominant, so that the Nd2SiO5 with large cell volume has a low dielectric constant according to Eq. (2). Another possible reason for the difference was the dielectric constants were tested under different frequencies. As reported by Kim [15] and Zheng [16], Q f value of perovskite ceramics decreased with a decreased electronegativity of A-site cation. It was generally accepted that a cation with high electronegativity should produce a strong chemical bond between A-site cation and SiO4 tetrahedron, leading to the decrease in intrinsic loss. Because the electronegativity of neodymium element (1.14) was smaller than that of samarium element (1.17), the low Q f value of Nd2SiO5 (38 800 GHz) may be acceptable. Even though the Q f value of the Nd2SiO5 ceramics was lower than that for MgSiO3 [1], Mg2SiO4 [2] or Zn2SiO4 [5] ceramics and Sm2SiO5 ceramics, it was higher than that reported by Wang et al. for CaSiO3 ceramics [5]. The Q f value of Nd2SiO5 ceramics was not very high, so it may be limited for them to apply in millimeter-wave device with high Q value; however, Nd2SiO5 ceramics may be qualified for some passive components with metallic electrode, for example microwave ceramic capacitor, because the loss of capacitor was dominated by the electrode according to skin effect under high frequency. 4. Summary Nd2SiO5 ceramics were synthesized with a solid- state method. Microwave dielectric properties of Nd2SiO5 ceramics were investigated. Nd2O3 and SiO2 reacted to form pure monoclinic Nd2SiO5 phase at 1450 °C when the molar ratio of Nd2O3/SiO2 was 1:1.05. The relative density, second phase and microstructures have large influences on microwave dielectric properties of Nd2SiO5 ceramics. High resonant frequency resulted in low dielectric constant and low Q f value. The Nd2SiO5 ceramics sintered at 1500 °C exhibited the following microwave dielectric properties: er = 7.94, Q f = 38 800 GHz and sf = 53 ppm/°C. Nd2SiO5 ceramics have a wide sintering temperature region. They could be considered as promising candidate materials for microwave passive components. Acknowledgments This work was supported by the Guangdong-Hong Kong Technology Cooperation Funding Scheme (TCFS) under Grant 2010A090604002 and State Key Laboratory of New Ceramic and Fine Processing Tsinghua University. References [1] M.E. Song, J.S. Kim, M.R. Joung, S. Nahm, J. Am. Ceram. Soc. 91 (8) (2008) 2747– 2750. [2] T.S. Sasikala, C. Pavithran, M.T. Sebastian, J. Mater. Sci. Mater E 21 (2) (2010) 141–144. [3] K.X. Song, X.M. Chen, C.W. Zheng, Ceram. Int. 34 (2008) 917–920. [4] K.X. Song, X.M. Chen, X.C. Fan, J. Am. Ceram. Soc. 90 (6) (2007) 1808–1811. [5] M.Z. Dong, Z.X. Yue, Z. Zhuang, S.Q. Meng, L.T. Li, J. Am. Ceram. Soc. 91 (12) (2008) 3981–3985. [6] Y.P. Guo, H. Ohsate, K.I. Kakimoto, J. Eur. Ceram. Soc. 26 (2006) 375–379. [7] N.H. Nguyen, J.B. Lim, S. Nahm, J. Am. Ceram. Soc. 90 (10) (2007) 3127–3130. [8] H.P. Wang, Q.L. Zhang, H. Yang, H.P. Sun, Ceram. Int. 34 (2008) 1405–1408. [9] T. Joseph, M.T. Sebastian, J. Am. Ceram. Soc. 93 (1) (2010) 147–154. [10] S. George, P.S. Anjana, V.N. Nair Deepu, P. Mohanan, M.T. Sebastian, J. Am. Ceram. Soc. 92 (6) (2009) 1244–1249. [11] S.N. Renjini, A. Thomas, M.T. Sebastian, Int. J. Appl. Ceram. Technol. 6 (2) (2009) 286–294. [12] S.P. Wu, C. Jiang, Y.X. Mei, W.P. Tu, J. Am. Ceram. Soc. 95 (1) (2012) 37–40. [13] J. Petzelt, N. Setter, Ferroelectrics 150 (1993) 89. [14] T. Kolodiazhnyi, G. Annino, T. Shimada, Appl. Phys. Lett. 87 (2005) 212908. [15] W.S. Kim, K.H. Yoon, E.S. Kim, Mater. Res. Bull. 34 (1999) 2309. [16] X.H. Zheng, (Ca, Nd)TiO3/(Li, Nd)TiO3 and BapLn6-PTi8-PM2+PO30 (Ln = Nd, Sm; M = Ta, Nb) Dielectric Ceramics [D], Zhejiang University, 2004.
Contents lists available at SciVerse ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Synthesis and microwave dielectric properties of Nd2SiO5 ceramics Chan Jiang, Songping Wu ⇑, Qing Ma, Youxian Mei School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510641, China
a r t i c l e
i n f o
Article history: Received 2 July 2012 Received in revised form 14 July 2012 Accepted 16 July 2012 Available online 4 August 2012 Keywords: Synthesis Silicates Nd2SiO5 Dielectric materials/properties Microwaves
a b s t r a c t Nd2SiO5 ceramics were synthesized and their microwave dielectric properties were investigated. The hexagonal Nd4Si3O12 second phase disappeared and the pure monoclinic Nd2SiO5 phase could be obtained when the molar ratio of Nd2O3/SiO2 was 1:1.05 at 1450 °C. The relative density of Nd2SiO5 ceramics increased with increasing temperature. The Nd2SiO5 ceramics sintered at 1500 °C exhibited microwave dielectric properties: a dielectric constant (er) of 7.94, a quality factor (Q f) of 38 800 GHz and a temperature coefficient of resonant frequency (sf) of 53 ppm/°C. High resonant frequency led to a low dielectric constant and low Q f value. Nd2SiO5 ceramics have a wide sintering temperature region. They are promising candidate materials for microwave passive components. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Silicates were important microwave ceramic materials, which have been extensively used to make microwave devices. Silicates, such as magnesium silicate (MgSiO3), forsterite (Mg2SiO4), zinc silicate (Zn2SiO4) and wollastonite (CaSiO3), etc., had low dielectric constant and high quality factor. MgSiO3 ceramics had excellent dielectric properties: er = 6.7, Q f = 121 200 GHz, sf = 17 ppm/ °C [1]; however, they were easy to be powdered due to the inevitable phase transformation. Microwave properties of Mg2SiO4 ceramics containing 0.5 wt.% LMZBS glass were: er = 7.3, Q f = 121 200 GHz [2]. (Mg0.4Zn0.6)2SiO4 ceramics indicated a good combination of microwave dielectric characteristics: er = 6.6, Q f = 95 650 GHz, and sf = 60 ppm/°C [3]. Mg2SiO4 ceramics had a strong processing sensitivity [4]. Zn2SiO4 ceramics with excellent properties were synthesized by a cold isostatic pressing (CIP) at a pressure of 200–300 MPa [5–7]; however, its quality factor was low (Q f = 15 000 GHz) when the conventional solid-state method was employed [7]. CaSiO3 ceramics had a low sintering temperature (1320 °C) and low dielectric constant (6.69). The calcium element was easy to evaporate under high temperature, and the low densification and porous microstructure of CaSiO3 resulted in a low quality factor (Q f = 25398 GHz) [8]. Ternary silicates have also attracted much attention in recent years, for example the Sr2ZnSi2O7 with excellent dielectric properties (er = 8.4, Q f = 105000 GHz and sf = 51.5 ppm/°C) [9] and the low-temperature cofiring of Li2MgSiO4 [10].
⇑ Corresponding author. E-mail address: [email protected] (S. Wu). 0925-8388/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2012.07.076
Study on microwave dielectric properties of Re2O3–SiO2 was lack. Renjini et al. have synthesized Sm2Si2O7 via solid-state method. The dielectric properties of Sm2Si2O7 were: er = 10, Q f = 3 000 GHz and sf = 20 ppm/°C. Dielectric properties of Sm2Si2O7 containing LBS or LMZBS glass were also given: er = 9.89, tan d = 0.024 (LBS, 975 °C) and er = 9.09, tan d = 0.009 (LMZBS, 950 °C), respectively [11]. In our previous study [12], microwave dielectric properties of Sm2SiO5 ceramics were: er = 8.44, Q f = 64 000 GHz, sf = 37 ppm/°C. Even though various studies were conducted on silicates, it is necessary to find new silicate materials for microwave/millimeter-wave application. The monoclinic Nd2SiO5 has the same monoclinic crystal system and space group (P21/c [14]) as the Sm2SiO5. We deduced that Nd2SiO5 might be candidate materials for millimeter-wave devices. In the present work, we reported the preparation, characterization, and microwave dielectric properties of Nd2SiO5 ceramics.
2. Experimental procedure Nd2SiO5 microwave ceramics were synthesized with solid-state synthesis method. Nd2O3 particles were synthesized by thermal decomposition of neodymium oxalate. Nd2O3 and nano-SiO2 (7 nm, Degussa, Auckland New Zealand) were mixed with a suitable molar ratio, and then milled with zirconia balls for 4 h on a planetary milling machine (QM-3SP2, Zhenguang, Nanjing, China). The mixtures were dried, and then calcined at 1100 °C for 2 h. The calcined ceramic powders were firstly re-milled for 4 h, dried, then granulated with PVA binder, and finally pressed into cylindrical disks of 10 mm diameter and 5 mm thickness under 10 MPa pressure isostatically with a hydrostatic press (KSTY70, Haixiang, Changzhou, China). The samples were sintered at 1300–1600 °C for 4 h in air with a high temperature electric furnace (SSJ-1600, Shenjia kiln, Luoyang, China).
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The crystalline phases of the specimen were analyzed by X-ray diffraction (XRD) (D/max-IIIA, Rigaku, Tokyo, Japan) with Cu Ka radiation of 2h from 10° to 60°. The microstructure observations of the ceramic surfaces were performed under a scanning electron microscope (SEM) (LEO 1530 VP, Zeiss, Vertrieb Deutschland, Germany). Element analysis of micro domain was carried out with an energy dispersive spectrometer (EDS) (EPMA1600, Shimadzu, Kyoto, Japan). The bulk density of ceramics was measured by the Archimedes method. Microwave dielectric constants (er) and the quality factor values (Q f) at microwave frequencies were measured by Hakki-Coleman dielectric resonator method using a Network Analyzer (N5230 PNA-L, Agilent, Santa Clara, California, USA). Temperature coefficient of resonant frequency (sf) was also measured by the same method with a changing temperature from 25 to 75 °C, and calculated by the following Eq. (1):
sf ¼
f 75 - f 25 106 f 25 50
ðppm= CÞ
ð1Þ
where f75 and f25 represent the resonant frequency at 75 and 25 °C, respectively.
3. Results and discussion 3.1. Phase identification of Nd2O3–xSiO2 (0.80 6 x 6 1.25) microwave ceramics Fig. 1 showed XRD patterns of Nd2O3–xSiO2 sintered at 1450 °C with different dosage of SiO2. From those curves, one could see the x values had great influences on phase compositions of the samples. According to XRD spectra, monoclinic Nd2SiO5 phase (JCPDS No. 40-0284) was the main crystal phase at x = 0.80–1.0, accompanied with the Nd2O3 (JCPDS No. 41-1089). The Nd2O3 diffraction peak disappeared and uniform phase Nd2SiO5 occurred at x = 1.05. When x = 1.25, monoclinic Nd2SiO5 phase coexists with the hexagonal Nd4Si3O12 (JCPDS No. 42-0171), whose amount increased with an increasing x value. We could obtain a pure hexagonal Nd4Si3O12 phase at x = 1.50. XRD patterns of Nd2O3-1.05SiO2 sintered at different temperatures (from 1300 to 1550 °C) were given in Fig. 2. When sintering temperature was less than 1400 °C, the monoclinic Nd2SiO5 phase was the main crystal phase, accompanied with the hexagonal Nd4Si3O12; however, the pure Nd2SiO5 phase started to form when the specimen was sintered at above 1450 °C. 3.2. SEM and EDS studies The microstructures of Nd2SiO5 ceramics sintered at various temperatures were investigated using SEM, as shown in Fig. 3a– e. For the specimen sintered at 1350 °C, a porous microstructure
Fig. 1. XRD patterns of Nd2O3–xSiO2 with various amounts of SiO2 sintered at 1450 °C for 4 h.
Fig. 2. XRD patterns of Nd2O3–1.05SiO2 sintered at various temperatures.
was developed, a large number of pores existed, and the grain size of the specimen was small, approximately 0.5 lm. The densification and grain size increased as the sintering temperature increased. The dense microstructure was developed, and an average grain size increased to 1.5 lm when the specimen was sintered at 1500 °C, as given in Fig. 3d. The compact interconnected grain microstructures and clear grain boundaries could be observed. The atom ratio (Nd:Si) of grain (marked with a white star in Fig. 3d) was 2:1 by EDS (see Fig. 3f), which clearly displayed the composition of grain was Nd2SiO5. It had also been confirmed by stoichiometric and XRD analysis. As the temperature was above 1550 °C, the abnormal grain growth occurred, and the overfiring might has appeared, as exhibited in Fig. 3e. 3.3. Microwave dielectric properties Fig. 4a–d showed the relative density, dielectric constant (er), quality factor (Q f) and sf values of the Nd2SiO5 ceramics sintered at various temperatures for 4 h, respectively. The relative density of the specimen sintered at 1350 °C was low (78.5%), but increased markedly with increasing sintering temperature to a maximum value of 98.1% of the theoretical density for the specimen sintered at 1500 °C, as exhibited in Fig. 4a. The er value was low (4.87) for the specimen sintered at 1350 °C, probably due to the porous microstructure. It increased with increasing sintering temperature to a maximum value of 7.94 (at 18.35 GHz) for the specimen sintered at 1500 °C (see Fig. 4b). The Q f value of the Nd2SiO5 ceramics sintered at 1350 °C was low (5600 GHz at 17.64 GHz) due to the small grain size, low density and porous microstructure (as exhibited in Fig. 3a). However, it increased considerably with increasing sintering temperature to a value of 38 800 GHz for the specimens sintered at 1500 °C, then the slightly decreased to 34 200 GHz, as given in Fig. 4c. The XRD results revealed the development of inhomogeneous phases consisting of monoclinic Nd2SiO5 and hexagonal Nd4Si3O12 in the specimens sintered at less than 1400 °C. As the temperature increased, the densification of specimen promoted and the pure Nd2SiO5 formed. Grain microstructure was also an important role to affect Q f value. Generally speaking, large grain size, high densification and uniform grain microstructure led to a high Q f value; however, the second phase and abnormal grain growth produced a low Q f value. Therefore, the increased Q f value of specimen sintered at 1400–1500 °C may have related to homogeneous phase and high densification; however, the decreased
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Fig. 3. SEM photographs of Nd2O3–1.05SiO2 sintered at: (a) 1350 °C, (b) 1400 °C, (c) 1450 °C, (d) 1500 °C, (e) 1550 °C, and (f) EDS pattern of micro domain marked in Fig. 3d.
Fig. 4. (a) Relative density and (b) er, (c) Q f, and (d) sf values of the Nd2SiO5 ceramics sintered at various temperatures.
Q f value at 1550 °C could be ascribed to an abnormal grain growth because a boundary defect probably existed among the grains. In this work, microwave dielectric properties of the hexagonal Nd4Si3O12 were: er = 9.94, Q f = 6 300 GHz (at 11.72 GHz) and sf = 12 ppm/°C (at 1450 °C for 4 h). The existence of Nd4Si3O12 should further decrease the quality factor of Nd2SiO5 ceramics according to the well-known mixing rule [5]. The sf values of the
specimens sintered at various temperatures were also shown in Fig. 3d. The sf values decreased with increasing temperatures because the hexagonal Nd4Si3O12 with similar high sf value disappeared. It was clearly that the sf value of monoclinic Nd2SiO5 was 53 ppm/°C. The ceramic powders were pressed into cylindrical disks of 6– 12 mm diameter and 5–6 mm thickness. Those disks were sintered at 1450 °C for 4 h. Influences of resonant frequencies on microwave
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Fig. 5. Influences of resonant frequency on microwave dielectric properties of Nd2SiO5 ceramics.
dielectric properties of samples were exhibited in Fig. 5. The dielectric constant of Nd2SiO5 ceramics decreased as the frequency increased. Because all of samples have a high densification (above 97.5%), we assumed that the main reason for it was that different polarization mechanisms worked in the frequency region of 10– 30 GHz. In general, the difference in dielectric constant of the electrolyte is determined by the different polarization mechanisms. The polarization of dielectric materials mainly contains electronic displacement polarization, ionic displacement polarization and orientation polarization of permanent dipole moment. The latter could be established in microwave band (less than 100 GHz). As the frequency increased to 25 GHz, It might be difficult to establish such a polarization for the ceramic samples completely, leading to a low dielectric constant. The Q f of Nd2SiO5 ceramics decreased as the frequency increased. Within the context of the current theory of the intrinsic dielectric loss, we would expect for Nd2SiO5 a linear frequency dependence of tan d / f in the microwave and millimeter-wave range [13]. In contrast, our data confirm that there exists an additional extrinsic contribution to the dielectric loss which causes a significant departure from the linear tan d / f behavior at low frequencies. This behavior of the Q f value may indicate an effect of the low-frequency extrinsic dielectric loss (possibly Debye-tpye loss) increases at higher frequencies. It is of interest that several other types of dielectric ceramics show similar characteristic extrinsic contributions to the dielectric loss at low-frequency. For example, a sub-linear tan d/f dependence in the 10–40 GHz interval was observed in the Ba(Mg1/3Ta2/3)O3, Ba(Mg1/3Nb2/3)O3 and Ba(Co1/2Nb2/3)O3 ceramics [14]. In our previous study [12], microwave dielectric properties of Sm2SiO5 ceramics were: er = 8.44, Q f = 64 000 GHz (at 11.9 GHz), sf = 37 ppm/°C. The dielectric constant of Nd2SiO5 ceramics (7.94) was lower than that of Sm2SiO5. It could be explained with the following Clausius–Mosotti Eq. (2)
er ¼
3 2 1 bam =V
ð2Þ
where V in cell volume, am is dielectric polarization rate and b = 4p/3. The cell volume of Nd2SiO5 is 438.8 Å3, which is more than that of Sm2SiO5 (425.7 Å3) with the same crystal system and space group. It was generally accepted that the am increases with increasing ionic radius of Re3+ because the larger ionic radius contributed larger electron polarization according to Eq. (2) a = 4pkoR3 (ko is the relative permittivity and R is the atomic or ionic radius). Ionic radius of Nd3+ and Sm3+ are 99.5 and 96.4 pm, respectively, so the electron polarizatnion of Nd3+ was more than that of Sm3+.
Obviously, am and V have a similar trend for RE2SiO5. In the discussion, influence of cell volume on the dielectric constant seems to be dominant, so that the Nd2SiO5 with large cell volume has a low dielectric constant according to Eq. (2). Another possible reason for the difference was the dielectric constants were tested under different frequencies. As reported by Kim [15] and Zheng [16], Q f value of perovskite ceramics decreased with a decreased electronegativity of A-site cation. It was generally accepted that a cation with high electronegativity should produce a strong chemical bond between A-site cation and SiO4 tetrahedron, leading to the decrease in intrinsic loss. Because the electronegativity of neodymium element (1.14) was smaller than that of samarium element (1.17), the low Q f value of Nd2SiO5 (38 800 GHz) may be acceptable. Even though the Q f value of the Nd2SiO5 ceramics was lower than that for MgSiO3 [1], Mg2SiO4 [2] or Zn2SiO4 [5] ceramics and Sm2SiO5 ceramics, it was higher than that reported by Wang et al. for CaSiO3 ceramics [5]. The Q f value of Nd2SiO5 ceramics was not very high, so it may be limited for them to apply in millimeter-wave device with high Q value; however, Nd2SiO5 ceramics may be qualified for some passive components with metallic electrode, for example microwave ceramic capacitor, because the loss of capacitor was dominated by the electrode according to skin effect under high frequency. 4. Summary Nd2SiO5 ceramics were synthesized with a solid- state method. Microwave dielectric properties of Nd2SiO5 ceramics were investigated. Nd2O3 and SiO2 reacted to form pure monoclinic Nd2SiO5 phase at 1450 °C when the molar ratio of Nd2O3/SiO2 was 1:1.05. The relative density, second phase and microstructures have large influences on microwave dielectric properties of Nd2SiO5 ceramics. High resonant frequency resulted in low dielectric constant and low Q f value. The Nd2SiO5 ceramics sintered at 1500 °C exhibited the following microwave dielectric properties: er = 7.94, Q f = 38 800 GHz and sf = 53 ppm/°C. Nd2SiO5 ceramics have a wide sintering temperature region. They could be considered as promising candidate materials for microwave passive components. Acknowledgments This work was supported by the Guangdong-Hong Kong Technology Cooperation Funding Scheme (TCFS) under Grant 2010A090604002 and State Key Laboratory of New Ceramic and Fine Processing Tsinghua University. References [1] M.E. Song, J.S. Kim, M.R. Joung, S. Nahm, J. Am. Ceram. Soc. 91 (8) (2008) 2747– 2750. [2] T.S. Sasikala, C. Pavithran, M.T. Sebastian, J. Mater. Sci. Mater E 21 (2) (2010) 141–144. [3] K.X. Song, X.M. Chen, C.W. Zheng, Ceram. Int. 34 (2008) 917–920. [4] K.X. Song, X.M. Chen, X.C. Fan, J. Am. Ceram. Soc. 90 (6) (2007) 1808–1811. [5] M.Z. Dong, Z.X. Yue, Z. Zhuang, S.Q. Meng, L.T. Li, J. Am. Ceram. Soc. 91 (12) (2008) 3981–3985. [6] Y.P. Guo, H. Ohsate, K.I. Kakimoto, J. Eur. Ceram. Soc. 26 (2006) 375–379. [7] N.H. Nguyen, J.B. Lim, S. Nahm, J. Am. Ceram. Soc. 90 (10) (2007) 3127–3130. [8] H.P. Wang, Q.L. Zhang, H. Yang, H.P. Sun, Ceram. Int. 34 (2008) 1405–1408. [9] T. Joseph, M.T. Sebastian, J. Am. Ceram. Soc. 93 (1) (2010) 147–154. [10] S. George, P.S. Anjana, V.N. Nair Deepu, P. Mohanan, M.T. Sebastian, J. Am. Ceram. Soc. 92 (6) (2009) 1244–1249. [11] S.N. Renjini, A. Thomas, M.T. Sebastian, Int. J. Appl. Ceram. Technol. 6 (2) (2009) 286–294. [12] S.P. Wu, C. Jiang, Y.X. Mei, W.P. Tu, J. Am. Ceram. Soc. 95 (1) (2012) 37–40. [13] J. Petzelt, N. Setter, Ferroelectrics 150 (1993) 89. [14] T. Kolodiazhnyi, G. Annino, T. Shimada, Appl. Phys. Lett. 87 (2005) 212908. [15] W.S. Kim, K.H. Yoon, E.S. Kim, Mater. Res. Bull. 34 (1999) 2309. [16] X.H. Zheng, (Ca, Nd)TiO3/(Li, Nd)TiO3 and BapLn6-PTi8-PM2+PO30 (Ln = Nd, Sm; M = Ta, Nb) Dielectric Ceramics [D], Zhejiang University, 2004.