Journal of Inorganic Materials
无 机 材 料 学 报
Vol. 26No. 2 Feb. , 2011
Article ID: 1000-324X(2011)02-0219-06 DOI: 10.3724/SP.J.1077.2011.10387
Studies on the (1−x)(Mg0.7Zn0.3)TiO3-xCa0.61La0.26TiO3 Microwave Dielectric
Ceramics System
ZHAO Li, SHEN Chun-Ying, QIU Tai
(College of Materials Science and Engineering, Nanjing University of Technology, Nanjing 210009, China)
Abstract: The microstructures and the microwave dielectric properties of the (1−x)(Mg0.7Zn0.3)TiO3-xCa0.61La0.26TiO3
(MZT-CLT) ceramic system were investigated. The objective of the present work is to compensate for the negative temperature coefficient of resonant frequency of (Mg0.7Zn0.3)TiO3 by the addition of (Ca0.61La0.26)TiO3. MZT-CLT ce-ramics exhibited mixed phases of (Mg0.7Zn0.3)TiO3 as the main phase associated with minor phases of Ca0.61La0.26TiO3 and (Mg0.7Zn0.3)Ti2O5. The microwave dielectric properties are strongly correlated with the sintering temperature and the composition. Very dense MZT-CLT ceramics were synthesized by sintering at 1275℃. When the samples were sintered at temperatures above 1300℃, the evaporation of Zn resulted the decrease in density and degradation in di-electric properties. With the increasing of x, the relative dielectric constant (εr) of the ceramics increases and the prod-uct of quality factor and resonant frequency (Q·f) decreases. The εr, Q·f and temperature coefficient of resonant fre-quency (τf) of 0.87(Mg0.7Zn0.3)TiO3-0.13Ca0.61La0.26TiO3 ceramic sintered at 1275℃ are 26, 86000 GHz and −6×10−6/℃, respectively.
Key words: (1−x)(Mg0.7Zn0.3)TiO3-x(Ca0.61La0.26)TiO3 ceramics; dielectric constant; quality factor; temperature stable
resonant frequency
In the past decades, microwave dielectric resonators and antennas have been developed for applications in communication systems such as cellular phone, direct broadcasting satellite and global positioning systems. These microwave dielectric devices require the combined dielectric properties of high dielectric constant (relative dielectric constant, εr), high quality factor (product of quality factor and resonant frequency, Q·f) and near-zero temperature coefficient of resonant frequency (τf). These three parameters are correlated to the size, frequency se-lectivity and temperature stability of the system, respec-tively. To meet the demands of microwave circuit designs, each dielectric property should be precisely controlled[1-2]. MgTiO3-CaTiO3 (MCT) is well known as a material for temperature-compensating capacitors, dielectric resonators and patch antennas. The material is made of a mixture of modified α-Al2O3 structured magnesium titanate (MgTiO3: εr=17, Q·f=110000GHz and τf=−55×10−6℃) and perovskite structured calcium titanate (CaTiO3: εr=170, Q·f=3600 GHz and τf=800×10−6/℃). With a ratio of
n(Mg):n(Ca)=95:5, 0.95MgTiO3–0.05CaTiO3 ceramics gives εr=20, Q·f= 56000GHz and a zero τf value. However, it required sintering temperatures as high as 1400− 1450℃[3].
(Mg0.7Zn0.3)TiO3 ceramic has been studied as promising candidate since it possesses a εr of 18.6, a Q·f of 90000 GHz and a τf value of −51×10−6/℃[4], while there were still limits of the Q·f value for the compositions having near-zero τf. In order to achieve a temperature-stable ma-terial, combining a positive one with a negative tempera-ture coefficient material was applied in this study. Ca0.61La0.26TiO3 (εr=109, Q·f= 17600GHz and τf= 212×10−6/℃)[5], having a large positive τf value, was added to (Mg0.7Zn0.3)TiO3 to compensate its τf. The subsequent property changes of the system were primarily investi-gated to find ultimately an optimal composition. The re-sultant microwave dielectric properties were analyzed based upon the densification, the X-ray diffraction (XRD) patterns and the microstructures of the ceramics. The cor-relation between the microstructure and the Q·f value was
Received date: 2010-06-24; Modified date: 2010-08-03; Published online: 2010-11-10 Biography: ZHAO Li (1986−), female, candidate of master. E-mail: zlgrace2009@163.com Corresponding author: SHEN Chun-Ying, professor. E-mail: shency@njut.edu.cn
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also investigated.
1 Experimental
Compositions based on the system of (1−x)(Mg0.7Zn0.3) TiO3-xCa0.61La0.26TiO3, where x=0.11–0.19, were prepared using the conventional solid state reactions. High-purity oxide powders (>99.9%) (MgCO3)4·Mg(OH)2·5H2O, ZnO, TiO2, CaCO3 and La2O3 were used as raw materials. They were weighed and then milled in distilled water for 6h. The mixture slurry was dried and calcined at 1100℃ for 3h. The calcined powders were re-milled and then pressed into pellets with a diameter of 11mm and a thickness of 6mm. These pellets were sintered at 1225–1325℃ for 4h in ambient atmosphere.
The bulk densities of the sintered pellets were measured by the Archimedes method. The dielectric properties of the samples at microwave frequency were measured by the modified Hakki and Coleman’s method in the TE011 mode using a network analyzer[6]. The τf value was calcu-lated using the equation τf =(f80−f25)/(f25·55℃), where f80 and f25 are the resonant frequency of the samples at 80 and 25℃, respectively. The microstructures of the sintered samples were observed by scanning electron microscope (SEM) and energy dispersive spectroscope (EDS). X-ray power diffraction (XRD) data were collected for phase identification using CuKα radiation.
Fig. 1 XRD patterns of 87MZT-13CLT ceramics sintered at different temperatures for 4h
2 Results and discussion
The XRD patterns of the 0.87(Mg0.7Zn0.3)TiO3- 0.13Ca0.61La0.26TiO3 (87MZT-13CLT) ceramic system did not significantly change with the sintering temperature in the range of 1225−1325℃, as shown in Fig. 1. 87MZT-13CLT ceramics showed mixed phases of MZT as the main phase associated with the minor phases of CLT, (Mg0.7Zn0.3)Ti2O5(εr=17.4, Q·f=47000GHz and τf= −66×10−6/℃)[7]. The XRD patterns of the 87MZT- 13CLT ceramic system have not change significantly with sintering temperatures in the range of 1225–1325℃ except that rational intensity of second phase (Mg0.7Zn0.3)Ti2O5 was enhanced at higher temperatures. The crystal structure of MgTiO3 is known as hexagonal structure with lattice parameter a=b=0.50787nm, c=1.3898nm. ZnTiO3 is also hexagonal structure. MgTiO3 and ZnTiO3 form the solid solutions of the zinc magnesium titanates easily because of their same ilmenite structure and similar ionic radius (r(Mg2+)=0.066nm, r(Zn2+)=0.074nm)[8–10]. The formation of the solid solutions helps to decrease the sintering tem-perature needed to densify the ceramics.
The XRD patterns of MZT-CLT ceramics system has
not significantly changed with x ranging from 0.11 to 0.19 sintered at 1275℃ for 4h, as shown in Fig. 2. With in-creasing content of CLT in the system, the intensity of MZT reflection lines decreases gradually. CLT is perovskite structure. The dielectric constant of CLT has a high εr of around 109, a Q·f value higher than 17600 GHz and a large τf value of 212×10−6/℃. According to the well-known mixing rules, addition CLT to the MZT sys-tem can enhance the dielectric constant and adjust the temperature coefficient of dielectric constant to near zero. When CLT was added to MZT to form a MZT-CLT ce-ramic system, the lattice parameters of MZT hasn’t change with CLT content. Furthermore, the formation of mixed phases in the MZT-CLT ceramics system was due to structural differences and because the average ionic radii of Ca2+ (0.099nm) and La3+ (0.115nm) were larger than those of Mg2+ (0.066nm) and Zn2+ (0.074nm).
The SEM images of MZT−CLT system sintered at 1275℃ for 4h are illustrated in Fig. 3. All specimens
Fig. 2 XRD patterns of MZT-CLT system sintered at 1275℃ for 4h
第2期 ZHAO Li, et al: Studies on the (1−x)(Mg0.7Zn0.3) TiO3-xCa0.61La0.26TiO3 Microwave Dielectric Ceramics System
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Fig. 3 SEM images of MZT-CLT system sintered at 1275℃ for 4h with x = (a) 0.11; (b) 0.13; (c) 0.15; (d) 0.17 and (e) 0.19
showed a similar surface morphology. The average grain size of the MZT phase reduced remarkablely with an in-crease in CLT content. This phenomenon reveals that the existing of CLT phase may inhibit abnormal grain growth of the main phases, which is cooperative to gain excellent dielectric properties. However, excess amount of CLT increased the loss value of the system because of its low Q·f value of 17600 GHz.
The SEM images of 87MZT-13CLT ceramics sintered at different temperatures for 4h are illustrated in Fig. 4. The ceramics are already dense at 1275℃. The grain
growth is uniform and increased with increasing sintering temperature. However, inhomogeneous grain growth is observed at temperatures higher than 1300℃, indicating 1275℃ should be an appropriate firing temperature for the specimens. Porosity and abnormal grain growth may di-rectly affect the microwave dielectric properties of the ceramic samples. The EDS patterns of 87MZT-13CLT ceramics sintered at 1300℃ are shown in Fig. 5. EDS analysis of Fig. 4 (d) exhibited several types of grains: large grains (Spot A) are identified as MZT, small grains (Spot B) are CLT, and rectangle-like grains (Spot C) are
Fig. 4 SEM images of 87MZT-13CLT ceramics sintered at different temperatures
(a) 1225℃; (b) 1250℃; (c) 1275℃; (d) 1300℃; (e) 1325℃
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Fig. 5 EDS patterns of 87MZT-13CLT ceramics in Fig.4 (d)
(a) A Point; (b) B Point; (c) C Point
analyzed as (Mg0.7Zn0.3)Ti2O5.
The bulk densities of MZT-CLT ceramics sintered at different temperatures for 4h are shown in Fig. 6. With the increase of sintering temperature, the bulk density in-creased to a maximum at 1275℃ and thereafter slightly decreased. The decrease of density at 1300℃ and 1325℃ as compared to 1275℃ may result from the decomposition of CLT as observed in Fig. 1 and the evaporation of Zn at high temperatures (>1300℃)[11]. Bulk densities of these materials are also influenced by the composition and in-creased with the increase of CLT content, which possesses a higher density than MZT[12].
The dielectric constants of MZT-CLT ceramics system sintered at different sintering temperatures for 4h are in-dicated in Fig. 7. The variation of εr value was consistent with that of densities. Initially, the dielectric constant of MZT-CLT also increased with increasing sintering tem-perature. After reaching maximum at 1275℃, it decreased. As can be expected from mixing rule, the dielectric con-stant increased from 25.5 to 29 as the content of CLT in-creased from 0.11 to 0.19, owing to a much higher εr value of CLT than MZT. It also suggests an εr-tunable system through the control of the x value. The quality factor value MZT-CLT ceramics sintered at different temperatures are shown in Fig. 8. With increasing sintering temperature, the Q·f value increased to a maximum value at 1275℃. It is
Fig. 7 εr value of MZT-CLT ceramics system sintered at dif-ferent temperatures
Fig. 8 Q·f value of MZT-CLT ceramics system sintered at dif-ferent temperatures
consistent with the variation of density, which plays an important role in controlling the dielectric loss[13]. As a low Q·f value of CLT (17600GHz), ceramics show a gradual decrease in Q·f value with increasing x and the maximum Q·f value of 86000GHz was obtained for 87MZT–13CLT ceramics sintered at 1275℃. The tem-perature coefficient of the resonant frequency (τf) of MZT–CLT ceramics sintered at 1275℃ is illustrated in Fig. 9. In general, τf is related to the phase composition of
Fig. 6 Bulk density of MZT-CLT ceramics system sintered at different temperatures
第2期 ZHAO Li, et al: Studies on the (1−x)(Mg0.7Zn0.3) TiO3-xCa0.61La0.26TiO3 Microwave Dielectric Ceramics System
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obtained for ceramics sintered at 1275℃.
References:
[1] Ferreira V M, Baptista J L. Preparation and microwave dielectric
properties of pure and doped magnesium titanate ceramics. Mater. Res. Bull., 1994, 29(10): 1017−1023.
[2] Huang C L, Hou J L, Pan C L, et al. Effect of ZnO additive on
sintering behavior and microwave dielectric properties of 0.95MgTiO3–0.05CaTiO3 ceramics. J. Alloys Compd., 2008, 450(1/2): 359−363.
[3] Ferreira V M, Azough F, Freer R, et al. The effect of Cr and La on
Fig. 9 τf value of MZT-CLT ceramics system sintered at 1275℃ for 4h
MgTiO3 and MgTiO3-CaTiO3 microwave dielectric ceramics. Journal of Materials Research, 1997, 12(12): 3293−3299. [4] Li L, Shen C Y, Qiu T. Study on microwave dielectric properties of
(Mg1–xZnx)TiO3 system ceramics. Electronic Components and Ma-
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the ceramics and insensitive to the sintering temperature.
[5] Huang C L, Tasi J T, Chen Y B. Dielectric properties of
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tive value, which increased from −18×10−6/℃ to
23×10−6/℃ as the addition of CLT increased from 0.11 to 0.19. It gives a cross-zero line which implies that zero τf can be achieved through appropriate adjustment of the x value in the ceramics.
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[6] Kobayshi Y, Katoh M. Microwave measurement of dielectric
properties of low-loss materials by the dielectric rod Resonator method. IEEE Trans. Microwave Theory Technol., 1985, 33(7): 586−592.
[7] Liou Y C, Yang S L. Calcium-doped MgTiO3-MgTi2O5 ceramics
prepared using a reaction-sintering process. Materials Science and Engineering, 2007, 142(2/3): 116−120.
[8] Kim H T, Byun J D, Kim Y. Microstructure and microwave dielec-tric properties of modified zinc titanates. Mater. Res. Bull., 1998, 33(6): 963−973.
[9] Hsieh M L, Chen LS, Wang S M, et al. Low-temperature sintering
of microwave dielectrics (Zn,Mg)TiO3. Jpn. J. Appl. Phys., 2005, 44(7A): 5045−5048.
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3 Conclusions
The dielectric characteristics of MZT-CLT ceramics were investigated. Very dense MZT-CLT ceramics have been synthesized by sintering at 1275℃. When the sam-ples were sintered at temperatures above 1300℃, the ab-normal grain growth and the evaporation of Zn lead to the decrease in density and degradation in dielectric properties. MZT-CLT ceramics exhibit mixed phases of MZT as the main phase associated with minor phases of CLT and (Mg0.7Zn0.3)Ti2O5.
With the molar fraction of CLT in MZT-CLT ceramics increasing from 0.11 to 0.19, both εr and τf value increased, whereas Q·f value decreased gradually. With x = 0.13, near-zero τf value can be obtained for MZT-CLT ceramics. A relatively ideal dielectric properties with a dielectric constant of 26, Q·f value of 86000 GHz, and temperature coefficient of resonance frequency of −6×10−6/℃ were
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(1−x)(Mg0.7Zn0.3)TiO3-xCa0.61La0.26TiO3
系陶瓷的微波介电性能研究
赵 莉, 沈春英, 丘 泰
(南京工业大学 材料科学与工程学院, 南京 210009)
摘 要: 本实验研究了(1−x)(Mg0.7Zn0.3)TiO3-x(Ca0.61La0.26)TiO3(MZT-CLT)系陶瓷的微观结构和微波介电性能, 通过(Ca0.61La0.26)TiO3来协调(Mg0.7Zn0.3)TiO3陶瓷的谐振频率温度系数. MZT-CLT陶瓷的主晶相为(Mg0.7Zn0.3)TiO3, 第二相为Ca0.61La0.26TiO3和(Mg0.7Zn0.3)Ti2O5. 烧结温度和陶瓷组成对微波介电性能影响显著, 当烧结温度为1275℃时, 可以获得良好的致密度, 当烧结温度超过1300℃时, Zn的蒸发导致陶瓷致密度和介电性能下降. 随着(Ca0.61La0.26)TiO3含量的增大, 材料的介电常数增大, 品质因数减小. 当x=0.13, 烧结温度为1275℃保温4h, (MZT-CLT)陶瓷具有优良微波介电性能, εr =26, Q·f=86000 GHz, τf = −6×10−6/℃.
关 键 词: (1-x)(Mg0.7Zn0.3)TiO3–x(Ca0.61 La 0.26)TiO3陶瓷; 介电常数; 品质因数; 谐振频率温度系数 中图分类号: TQ174 文献标识码: A
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