Microwave Dielectric Properties of Ba3.75Nd9.5Ti18O54 Ceramic by the (Co1/3Nb2/3)4+ Substitution

Transcription

1 2017 Asia-Pacific Engineering and Technology Conference (APETC 2017) ISBN: Microwave Dielectric Properties of Ba3.75Nd9.5Ti18O54 Ceramic by the (Co1/3Nb2/3)4+ Substitution Hetuo Chen, Bin Tang and Shuren Zhang ABSTRACT To improve the microwave dielectric properties of the Ba3.75Nd9.5Ti18O54 ceramic, the (Co1/3Nb2/3) 4+ substitution for titanium is performed. When 0 z 1, the system keeps a single phase. The decreasing relative density causes the dielectric constant to decrease. The quality factor (Q f) value increases by 13 %, which is because of the growing grain size. The temperature coefficient of resonant frequency successfully decreases by 70 % and is related to the tolerance factor. For 1.5 z 2, the secondary phase appears and the microwave dielectric properties are degraded rapidly. In addition, the solid solution limit of the Ba3.75Nd9.5Ti18O54 ceramic is found 1 < z < 1.5 when the (Co1/3Nb2/3) 4+ substitution is performed. * INTRODUCTION During the past decades, Ba6-3xNd8+2xTi18O54 based microwave dielectric ceramics have been studied extensively for their high Q f (> 5000 GHz) and high dielectric constant (εr 80) as the rapid growth of the wireless communication industry. [1-3] As for their large temperature coefficient at resonant frequency (+60 ppm/ºc to +150 ppm/ºc), these studies mainly focused on tailoring it to the vicinity of zero. [1-3] Figure 1. The molecular structure of the Ba6-3xNd8+2xTi18O54 ceramics. Hetuo Chen 1, Bin Tang 1,*, and Shuren Zhang 1 1 State Key Laboratory of Electronic Thin Films and Integrated Devices, University of Electronic Science and Technology of China, Chengdu, People s Republic of China * tangbin@uestc.edu.cn 2131

2 The structure of the Ba6-3xNd8+2xTi18O54 ceramics is shown in figure 1. There are three kinds of sites, A1-site (Ba1), A2-site (Nd1) and B-site (Ti1) site which can be substituted by cations of similar ionic radii. The A and B-site equivalent ionic substitution were widely reported. [1-7] For A-site, Ca 2+ et al. substitution always enlarge τf of the system and Sm 3+ replacement requires higher sintering temperature ( 1400 ºC). [2,3] In preceding reports, B-site substitution could always easily obtain near zero temperature coefficient of resonant frequency especially for x = 0.75 (τf ~ +60 ppm/ºc) with Q f value around GHz. [3-8] As a result, in this study we report the result of the B-site substitution on microwave dielectric properties of the Ba3.75Nd9.5Ti18O54 ceramic. In addition, it is reported that minor Nb2O5 addition could help facilitate grain size growth to improve the quality factor. [9,10] And other studies reported that quality factor could be improved by restraining the titanium reduction by adding divalent or trivalent dopants. [3-8] For example, Co2O3 is a very good dopants candidate prohibiting titanium reduction. [11] Considering the charge balance, it is possible that high dielectric constant and Q f value could be simultaneously maintained, if (Co1/3Nb2/3) 4+ substitution is performed in Ba3.75Nd9.5Ti18O54 ceramic. In the present work, we take Ba3.75Nd9.5Ti18O54 as the starting formula to discuss the effect of the (Co1/3Nb2/3) 4+ substitution on the microwave dielectric properties. EXPERIMENTAL PROCEDURE The ceramics are prepared by the conventional solid-state reaction technique according to the formula of Ba3.75Nd9.5Ti18-z(Co1/3Nb2/3)zO54 (0 z 2) with starting materials of BaCO3, Nd2O3, TiO2 and Co2O3, Nb2O5, with purity 99 %. The mixtures were ball-milled in nylon jars with zirconia balls and distilled water for 12 hours. Then these powders were calcined at ºC for 5 hours, from room temperature at the rate of 3 ºC/min. The calcined powders were produced with 5 wt% PVA (polyvinyl alcohol) in the size of 15 mm in diameter and 7 mm in thickness under the pressure of 220 kg/cm 2. At last, the samples were preheated at 600 ºC at the rate of 3 ºC/min for 2 hours to remove the PVA organic binder, and sintered at ºC (3 ºC/min) for 2 hours and cooled naturally down to the room temperature. The densities (ρmeasured) of sintered samples were measured by Archimedes method. The relative density (ρrelative) was calculated measured measuredv relative theory M by, in which ρtheory is obtained from dividing unit cell volume (V) by its molecular mass M (in this study one unit cell volume corresponds to one molecular mass). The phase of the sintered samples was identified by the X-ray diffraction (XRD). The unit cell volume and crystal parameters were calculated by UnitCell, with the XRD data. Microwave dielectric characteristics were examined by the Hakki-Coleman dielectric resonator method in the TE011 mode using a network analyzer at frequency around 3-5 GHZ. The τf values of sintered samples were determined by the equation as following: f f t2 t1 6 f = 10 ft t 1 2 t1 (1) 2132

3 where ft1 and ft2 are the resonate frequencies at the temperature of t1 (ordinary temperature) and t2 = 85 ºC respectively. RESULT AND DISCUSSION Figure 2. The X-ray diffraction patterns of Ba3.75Nd9.5Ti18-z(Co1/3Nb2/3)zO54 ceramics (z = 0-2) sintered at 1350 ºC for 2 hours. First, the phase variation is determined by the X-ray diffraction (XRD) results. Figure 2 shows the XRD patterns of Ba3.75Nd9.5Ti18-z(Co1/3Nb2/3)zO54 (z = 0-2) ceramics sintered at 1350 ºC for 2 hours. For 0 z 1, it showed a single tungstenbronze structure BaNd2Ti4O12 (No ) phase. [7] To clarify this, crystal parameters are calculated (exhibited in Fig. 3) according to XRD data and surface morphology are detected in Figure 4. Parameters of three axes and unit cell volume increase versus z value. Also, the SEM (scanning electron microscopy) results show no evidence of the secondary phase. These phenomena suggested that the solid solution is formed because of similar ionic radius between (Co1/3Nb2/3) 4+ (0.675Å) and Ti 4+ cations (0.605Å). For z 1.5, (Ba,Nd)Ti(Co,Nb)O6 secondary phase appeared and Figure 4 (d) (e) showed obvious appearance of secondary phase. [12] Its appearance indicates the substitution limit for Ti 4+ is around z = 1 for Ba3.75Nd9.5Ti18-z(Co1/3Nb2/3)zO54 ceramics. As reported tolerance factor can be used to describe the structure stableness. [3] Lower tolerance factor value would give rise to more unstable structure and vice versa. Based on previous reports, we derivate tolerance factor as following: [5,7] t 4 x 1 x ( ) R 3 ( ) R 2 R 2 Nd Ba O z z z 2( R 4 R 2 R 5 R 2) Ti Co Nb O where t is the tolerance factor, x = 0.75, RNd 3+, RBa 2+, RO 2-, RTi 4+, RCo 2+ and RNb 5+ represents the ionic radii of Nd 3+, Ba 2+, O 2+, Ti 4+, Co 2+ and Nb 5+, respectively. Taking larger (Co1/3Nb2/3) 4+ (0.675Å) cations to replace Ti 4+ cations (0.605Å) will surly lead to a decreasing tolerance factor. So the decreasing tolerance factor projects the collapse of the structure. Lattice parameters variation trend, SEM results and tolerance factor proved the XRD results. (2) 2133

4 Figure 3. Crystal parameters of Ba3.75Nd9.5Ti18-z(Co1/3Nb2/3)zO54 ceramics (z = 0-1). The typical SEM micrographics of sintered samples as a function of composition (Co1/3Nb2/3) 4+ are illustrated in Fig. 4. From Figs. 4 (a) - (c), most grains, of samples sintered at 1350 o C, are rod-like and the size increases with doping content of (Co1/3Nb2/3) 4+. Then, nubby alternate phases appeared in Figs. 4 (d) - (e) and grain size of main phase decreased rapidly. The EDS examination is performed and results of spots A and B are taken as examples. The data are appended in Table 1. The data approved the main BaNd2(Ti,Co,Nb)4O12 phase and the secondary phase of (Ba,Nd)Ti(Co,Nb)O6. The corresponding quality factor value of these samples was shown in Figure 5. The growing and decreasing grain size corresponds to different quality factor variation trend. [9] Figure 4. SEM photographs of Ba3.75Nd9.5Ti18-z(Co1/3Nb2/3)zO54 ceramics surfaces: (a) z = 0, (b) z = 0.5, (c) z = 1, (d) z = 1.5, (e) z = 2, sintered at 1350ºC; (f) (g), z = 1, 1325 ºC, 1375 ºC. Figure 4 (f), (c) and (g) show the microstructure evolution as the increasing temperature for z = 1. Grain size increases as temperature growing from 1325 ºC (f) 2134

5 to 1350 ºC (c). Then higher temperature again resulted in smaller grain size (g). Also, the relating properties were depicted in Figure 6. Microwave dielectric properties of them vary in accord with the SEM results. [9] Table 1. Element Ratio Data of the Energy Dispersive X-ray Analysis (EDS) of Spots A and B. Spot Atom (%) Ba Nd Ti Co Nb O A B Figure 5. Relative density, dielectric constant and quality factor for z = 0 2 at 1350 o C. Microwave dielectric properties and relative density of Ba3.75Nd9.5Ti18- z(co1/3nb2/3)zo54 (z = 0-2) ceramics sintered at 1350ºC for 2 h are shown in Figure 5. For 0 z 1 sintered at the same temperature, dielectric constant decreases slightly from 87.3 to 85.1 and relative densities of these ceramics show similar trend. It is obviously that dielectric constant of the system is controlled by the relative density. If simply deducting relative density from unity, we can obtain an increasing porosity which would lead to lower dielectric constant because of relative permittivity of air was unity. [13] In Figure 6, the dielectric constant of z = 1 sintered at different temperature is illustrated. The dielectric constant peaked at 1350 o C due to the elimination of pore in samples. The quality factor value increased from z = 0 to 1 which is in consistent with the grain size variation. Usually, many factors, such as pores, secondary phases, impurities, lattice defects or grain size, would influence Q f value and it is always hard to figure out the key reason. [13] For the situation of this paper, in figure 4 (a) (c) at 1350 o C, (c) possesses larger grain size than (a) and (b). Similar trend between grain size and Q f variation indicates that quality factor is controlled by grain size. [9] In [14], authors related the increasing grain size to the decreasing number of grain boundaries in unit volume. The grain boundary could act as a two-dimension defect 2135

6 interrupting the symmetry of the crystal, and then larger grain size, in other words, less defects, will give rise to higher Q f. For 4 (f) at 1325 o C, (c) at 1350 o C and (g) at 1375 o C, grain size drops after an increase, and the corresponding Q f is degraded after the promotion in figure 6. There is probably a limit of grain size. Therefore, we relate Q f to the temperature and grain size. In figure 7, the temperature coefficient at the resonant frequency declines rapidly from z = 0 to 1. According to description on Figure 3, the substitution slightly changes lattice parameters that involve with the degree of tilting of octahedral. [15] The tolerance factor (t) is always also calculated to measure the tilting degree in figure 7. [15] With the decreasing tolerance factor, the degree of the octahedral tilting changes and then τf drops. Similar tendency between τf and tolerance factor confirmed that τf of Ba3.75Nd9.5Ti18-z(Co1/3Nb2/3)zO54 ceramics is controlled by tilting of the Ti-O octahedral structure for 0 z 1. [13,15] Figure 6. Dielectric constant and quality factor of ceramics sintered at 1300 o C 1375 o C. For z 1.5, although of relative low content, secondary phase changed grain size and degraded the microwave dielectric properties of the system obviously, especially for quality factor and temperature coefficient. [12] Dielectric constant is decreased to 83, Q f values of well sintered samples drop rapidly to 3100 GHz and τf value rebounds. Based on above discussion, it is clear that the solid solution is around z = 1. Figure 7. Temperature coefficient at resonant frequency and tolerance factor for z = 0 2 at 1350 o C. 2136

7 CONCLUSIONS The relationship between microstructure, phase composition and microwave dielectric properties of Ba3.75Nd9.5Ti18-z(Co1/3Nb2/3)zO54 (0 z 2) system has been investigated in the study. Microstructure and phase constitution determine microwave dielectric properties of the system. For 0 z 1, relative density, in other words, porosity plays a key role in controlling the dielectric constant. The increasing grain size, less defects in unit volume, is the reason for quality factor promotion. The decreasing tolerance factor predicts the declining τf value and the appearance of the secondary phase. Relative inferior properties of the alternate phase give rise to the decrease of microwave dielectric properties. ACKNOWLEDGEMENTS This work is supported by the National Natural Science Foundation of China (Grant No and ). REFERENCES [1] Y. Li, X.M. Chen, N. Qin, Y.W. Zeng, J. Am. Ceram. Soc. 88, 481 (2005). [2] R. Ubic, I.M. Reaney, W.E. Lee, J. Samuels, E. Evangelinos, Mat. Res. Soc. Symp. Proc. 453, 495 (1997). [3] X. Chen, Y. Li, J. Am. Ceram. Soc. 85, 579 (2002). [4] Y.J. Wu, X.M. Chen, Ferroelectrics, 233, 271, (1999). [5] H.T. Chen, B. Tang, X. Guo, S.X. Duan, Y. Long, Y.X. Li, S.R. Zhang, Int. J. Appl. Ceram. Technol. 12, E170, (2015). [6] H.T. Chen, B. Tang, A.Q. Gao, S.X. Duan, H. Yang, Y.X. Li, H. Li, S.R. Zhang, J Mater Sci: Mater Electron 26, 405, (2015). [7] H.T. Chen, B. Tang, S.X. Duan, H. Yang, Y.X. Li, H. Li, S.R. Zhang, J. Electron. Mater. 44,1081, (2015). [8] X. Guo, B. Tang, J. Liu, H. Chen, S.R. Zhang, J. All. Comp. 646, 512 (2015). [9] N. Ichinose, T. Shimada, J. Euro. Ceram. Soc. 26, 1755 (2006). [10] K.H. Yoon, Y.S. Kim, J. Mater. Res. 10, 2085, (1995). [11] L.X. Li, Y.M. Han, P. Zhang, J. Li, L.F. Cao, Q. W. Liao, Ferroelectrics, 388, 167, (2009). [12] M. Sebastian, S. Solomon, R. Ratheesh, J. George, P. Mohanan, J. America. Ceram. Soc. 84, 1487, (2001). [13] C.L. Huang, W.R. Yang, P.C. Yu, J. Eur. Ceram. Soc. 34, 277, (2014). [14] S.R. Kiran, G. Sreenivasulu, V.R.K. Murthy, V. Subramanian, B.S. Murty, J. Am. Ceram. Soc., 95, 1973, (2012). [15] F. Zhao, Z.X. Yue, Y.C. Zhang, Z.L. Gui, L.T. Li, J. Euro. Ceram. Soc. 25, 3347 (2005). 2137