Microwave Dielectric Properties of Low-Loss (1-x) Ca0.6Sm0.267TiO3–xSmAlO3 Ceramics
Xu Wang1, 2, *, Renli Fu1, Yue Xu1
1College of Material Science and Technology, Nanjing University of Aeronautics and Astronautics, Nanjing, China
2School of Materials Engineering, Yancheng Institute of Technology, Yancheng, China
To cite this article:
Xu Wang, Renli Fu, Yue Xu. Microwave Dielectric Properties of Low-Loss (1-x) Ca0.6Sm0.267TiO3–xSmAlO3 Ceramics. International Journal of Materials Science and Applications. Vol. 4, No. 5, 2015, pp. 368-370. doi: 10.11648/j.ijmsa.20150405.24
Abstract: A novel compositions in the (1-x) Ca0.6Sm0.267TiO3–xSmAlO3 system were prepared via conventional mixed oxide route in order to modify the postive temperarure coefficient of resonnant frequency(ôf) of Ca0.6Sm0.267TiO3.The microwave dielectric properties and phase composition of this system ceramics were investigated. X-ray powder diffraction results showed that Ca0.6Sm0.267TiO3 and SmAlO3 formed a solid solution when 0.05≤x≤0.20. Microstructures of the sintered ceramics were observed by SEM; dielectric properties of the samples were also measured. The ôf values showed a near linear decrease with increasing additions of SmAlO3. A dielectric constant of 53.1, Qf value of 24,085GHz and ôf value of +13.3ppm/°C were obtained for 0.80Ca0.6Sm0.267TiO3–0.20 SmAlO3 ceramics sintered at 1450°C for 4h.
Keywords: Dielectric Ceramic, High Dielectric Constant, Zero Temperature Coefficient, Low Loss
In resent years, increased attentions has been paid to develop dielectric materials with high quality factor (Q × f), high dielectric constant (εr) and zero temperature coefficient of resonant frequency (τf) as dielectric resonators and microwave device substrates. High dielectric constant material can effectively reduce the size of resonators. The inverse of the dielectric loss (Q = 1/tan δ) is required to be high for achieving prominent frequency selectivity and stability in microwave transmitter components. Small temperature coefficient of the resonant frequency is required to ensure the stability of the microwave components at different working temperatures. Numerous perovskite-type compounds and their solid solutions have been investgated and applied in microwave frequency devices [2–6]. However, in MW frequency range the number of reported dielectrics with high εr, τf ~0, and low loss is relatively poor.
Research focused on discovering new compositions is still active as there is a great demand for various permittivities. Combining two compounds with negative and positive temerature coefficients is the most means of obtaining a near-zero temperature coefficient of resonant frequency. Since Ca0.6Sm0.267TiO3 has a high dielectric constant of 101, Q × f of 14,000GHz and a large positive temperature coefficient of resonant frequency of +220 ppm/°C ; rare-earth aluminates, SmAlO3 possesses microwave dielectric properties (εr ~ 20.4, Q × f ~ 65,000GHz, τf~−74 ppm/°C) . So in the present work, to compensate the τf value and improved the Q × f, SmAlO3 has been introduced to form the solid solution (1-x) Ca0.6Sm0.267TiO3–xSmAlO3.
2. Experimental Procedure
2.1. Sample Preparation
The samples were prepared by the conventional solid-state reaction technique. For the synthesis of (1-x) Ca0.6Sm0.267TiO3–xSmAlO3 (x = 0.05–0.20)powders, High purity CaCO3 (99.5%), Sm2O3 (99.9%) and TiO2 (99.9%), Al2O3 (99.5%) powders were used as raw materials. The starting materials were mixed according to the stoiciometries of Ca0.6Sm0.267TiO3 and SmAlO3 separately. Mixtures were ball-milled in a nylon jar with zirconia balls and ethanol for 12 h, and after drying these powders were calcined at 1050–1200°C for 2 h. Different amounts of SmAlO3 were added to the calcined Ca0.6Sm0.267TiO3 and the desired compositions prepared from these mixtures were grounded again in ethanol for 12h. Then, the dried powders added with 5 wt% polyvinyl alcohol solution as binder were passed through a mesh and pressed into cylindrical samples with 15 mm in diameter and 7 mm in thickness under a pressure of 250 kg/cm2. At last, these compacts muffled with powder of the same composition were sintered at 1450°C for 4h.
The phase composition was identified by X-ray diffraction (XRD) using CuKa radiation(Bruker D8 advanced X). The microstructure of the sintered sample was characterized by scanning electron microscopy (SEM) (Model Hitachi -SU8010,Hitachi Ltd., Japan). All samples were polished and thermally etched at a temperature which was about 100°C -150°C lower than its sintering temperature. The dielectric characteristics at microwave frequencies were measured by the Hakki–Coleman dielectric resonator method in the TE011 mode  using a network analyzer (Advantest R3767C, Tokyo, Japan) and parallel conductor boards.The τf value was determined from the difference between the resonant frequencies obtained at -35°C and 85°C.
3. Results and Discussion
Fig. 1 denoted the powder XRD patterns of (1-x) Ca0.6Sm0.267TiO3–xSmAlO3 (x = 0.05–0.20) ceramics sintered at 1450℃ for 4 h. All the four XRD peaks can be assigned to the orthorhombic perovskite type compounds (JCPDS-PDF#42-0423) with increasing addition of SmAlO3, extrapeaks were not observed even at the highest concentration when x=0.20.The X-ray diffraction study showed that Ca0.6Sm0.267TiO3 and SmAlO3 can form a solid solution across the range (0.05≤x≤0.20) because of the similar ionic radii between Ca (1.34A) and Sm (1.24A) and between Ti (0.605A) and Al (0.535A ) [10,11]. It can be deducted that the most probable mechanism for solid solution formation is the substitution of Sm on the A (Ca)site and B on the (Ti)site in the perovskite structure.
Fig. 2 showed typical hexagonal grain morphology usually can be observed in perovskite stucture ceramics. With increasing content of SmAlO3 additions the grain sizes tended to become smaller as SmAlO3 additions suppressed the grain growth and enhanced the densification process, which was analogy with the report by the researchers[12,13]. No significant amount of porosity is observed even at higher concentrations of SmAlO3. No evidence of second phases were observed in the SEM micrographs too.
Fig.3 demonstrated the dielectric constant of Ca0.6Sm0.267TiO3 ceramics with increasing additions of SmAlO3 sintered at 1450°C for 4h. In all the compositions, additions of rare earth aluminates deteriorate the dielectric constant obviously. The observed change in permittivities is attributed to the low permittivities of the dopants SmAlO3. In a similar to the variation in permittivity in Fig.3, as Ca0.6Sm0.267TiO3 and SmAlO3 exhibit positive and negative temperature dependencies of permitivity respectively, a linear decrease with increasing content of the additives of the τf values was shown in Fig.4. SmAlO3 decreased the τf of (1-x)Ca0.6Sm0.267TiO3-xSmAlO3 from +95ppm/°C to +13.3ppm/°C for samples with 5-20% additions. It is known that the temperature coefficient of composite ceramic was derived from the rule: τf=v1τf1 +v2τf2 , where the τf1 and τf2 are the τf values of the Ca0.6Sm0.267TiO3 and SmAlO3 phase, respectively. In all the compositions it is apparent that the τf values of perovskite structured Ca0.6Sm0.267TiO3 ceramics can be tuned to near zero with suitable additions of SmAlO3. Fig.5 showed a notable variation in Q×f values of Ca0.6Sm0.267TiO3 with various additions of SmAlO3 ceramics. The entire range of compositions investigated exhibited a significant increase in the Q×f values with x=0.20 additions of SmAlO3. The high Q×f values of SmAlO3 attributes to the increase in the Q×f values of Ca0.6Sm0.267TiO3 under the condition of good densification. Q×f values as high as 24,085GHz were achieved with addition of x=0.20 to (1-x) Ca0.6Sm0.267TiO3–xSmAlO3.
(1-x)Ca0.6Sm0.267TiO3–xSmAlO3 (x= 0.05–0.20) microwave ceramics were prepared by solid-state reaction route at 1450°C for 4h for the purpose of investigating effect of SmAlO3 ratio on the sintering behavior and microwave dielectric properties. As the content of SmAlO3
increased from 0.05 to 0.20, no secondary phase appeared.
All samples of the (1-x) Ca0.6Sm0.267TiO3–xSmAlO3 ceramics exhibited formed perovskite phase system only. And furthermore,SmAlO3 functioned as sintering aids, which promoted the densification and suppressed the grain growth and thus, influenced the microwave dielectric properties. When the ratio of SmAlO3 was increased, the dielectric constant decreased monotonically, τf values decreased continuously and Q×f values increased. Ceramics with composition 0.80Ca0.6Sm0.267TiO3–0.20 SmAlO3 sintered at 1450°C for 4h had compact and possessed excellent microwave dielectric properties: relative permittivities constant εr= 53.1, Q×f = 24,085GHz and the temperature coefficient of resonant frequency τf = 13.3 ppm/°C.
This research is financial supported by the National Natural Science Foundation of China (No. 51402251) and Science and technology projects of Guangdong Province (2011A091103002).