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3)O3–xBaTi4O9 ceramics
Materials Research Bulletin 42 (2007) 1897–1904 www.elsevier.com/locate/matresbu
Microstructure and microwave dielectric characteristics of (1 x)Ba(Zn1/3Ta2/3)O3–xBaTi4O9 ceramics Wen-Cheng Tzou a,*, Yao-Sheng Yang b, Cheng-Fu Yang c, Ho-Hua Chung d, Chien-Jung Huang e, Chien-Chen Diao b a
Department of Electro-Optical Engineering, Southern Taiwan University of Technology Yung-Kang City, Tainan, Taiwan, ROC b Department of Electronic Engineering, Kao Yuan University, Kaohsiung, Taiwan, ROC c Department of Chemical and Materials Engineering, National University of Kaohsiung, Kaohsiung, Taiwan, ROC d Department of Mechanical and Automation Engineering, Kao Yuan University, Kaohsiung, Taiwan, ROC e Department of Applied Physics, National University of Kaohsiung, Kaohsiung, Taiwan, ROC Received 25 August 2006; received in revised form 3 December 2006; accepted 13 December 2006 Available online 20 December 2006
Abstract (1 x)Ba(Zn1/3Ta2/3)O3–xBaTi4O9 (0.1 x 0.85) composites are prepared by mixing 1150 8C-calcined BaTi4O9 with 1150 8C-calcined Ba(Zn1/3Ta2/3)O3 powders. The crystal structure, microwave dielectric properties and sinterabilites of the (1 x)Ba(Zn1/3Ta2/3)O3–xBaTi4O9 ceramics have been investigated. X-ray diffraction patterns reveal that BaTi4O9, ordered and disordered Ba(Zn1/3Ta2/3)O3 phases exist independently over the whole compositional range. The sintering temperatures of (1 x)Ba(Zn1/3Ta2/3)O3–xBaTi4O9 ceramics are about 1240 – 1320 8C and obviously lower than those of Ba(Zn1/3Ta2/3)O3 ceramics. The dielectric constants (er) and the temperature coefficient of resonant frequency (tf) of (1 x)Ba(Zn1/3Ta2/3)O3– xBaTi4O9 ceramics increase with the increase of BaTi4O9 content. Nevertheless, the bulk densities and the quality values (Q f) of (1 x)Ba(Zn1/3Ta2/3)O3–xBaTi4O9 ceramics increase with the increase of Ba(Zn1/3Ta2/3)O3 content. The results are attributed to the higher density and quality value of Ba(Zn1/3Ta2/3)O3 ceramics, the better grain growth, and the densification of sintered specimens added a small BaTi4O9 content. The (1 x)Ba(Zn1/3Ta2/3)O3–xBaTi4O9 ceramic with x = 0.1 sintered at 1320 8C exhibits a er value of 31.5, a maximum Q f value of 68500 GHz and a minimum tf value of 4.1 ppm/8C. # 2007 Elsevier Ltd. All rights reserved. Keywords: A. Ceramics; C. X-ray diffraction; D. Dielectric properties
1. Introduction The rapidly growing microwave telecommunication devices demand the miniaturization of the size and weight, and the temperature stability [1]. Thus many researchers have devoted to developing the dielectric materials with a high dielectric constant (er), a high quality value (Q) and a small temperature coefficient of resonant frequency (tf). The complex perovskite compounds, such as Ba(Mg1/3Ta2/3)O3 ceramics [2–4] and Ba(Zn1/3Ta2/3)O3 ceramics [5–9], are regarded as the microwave materials of high quality value. Ba(Zn1/3Ta2/3)O3 ceramics possess a suitable er 30, a relatively high quality value (Q 14 000 at 11 GHz), and a stable temperature coefficient of resonant frequency
* Corresponding author. Tel.: +886 7 2518898; fax: +886 945834460. E-mail address: [email protected] (W.-C. Tzou). 0025-5408/$ – see front matter # 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2006.12.009
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(tf 0 5 ppm/8C). Unfortunately, the Ba(Zn1/3Ta2/3)O3 ceramics formed by the solid-state reaction of BaCO3, ZnO, and Ta2O5 are not appropriate, because high sintering temperature (>1500 8C) and long soaking time are required to achieve sufficiently high sintered density for the industrial applications of the Ba(Zn1/3Ta2/3)O3 materials. Moreover, at high sintering temperature zinc departure can be observed in Ba(Zn1/3Ta2/3)O3 ceramics, involving a decrease in dielectric properties [10]. Many efforts had been made to improve the sinterabilities of Ba(Zn1/3Ta2/3)O3 ceramics by the addition of sintering agent. Roulland et al. reported that the sintering temperature of Ba(Zn1/3Ta2/3)O3 ceramics could be reduced to 1400 8C [11]. Nahm et al. presented some investigations into the effects of additives (such as Al2O3, Ga2O3, ZrO2 and TiO2) on the sinterabilities, microstructure and dielectric loss of Ba(Zn1/3Ta2/3)O3 ceramics. In Nahm’s studies, the Q values could be improved effectively due to the increased relative density and grain size [12–15]. Varma et al. studied the effect of different dopants (such as oxides of divalent, trivalent, pentavalent and hexavalent elements) on the microwave dielectric properties of Ba(Zn1/3Ta2/3)O3 ceramics, and concluded that the quality factor increased when the ionic radius of dopant was close to the ionic radius of B site ions (Zn or Ta) [16]. However, high sintering temperatures (1500–1600 8C) were still needed in the researches of Nahm et al. and Varma et al. Numerous investigations exhibited that incorporation of additives, such as Y2O3, Ba5Ta4O15 or other second perovskite type materials, were also found beneficial for the formation of Ba(Mg1/3Ta2/3)O3 ceramics. Lin et al. presented that the sintered density of Ba(Mg1/3Ta2/3)O3 ceramics decreased after adding Ba5Ta4O15, and the dielectric constants (er) were insignificantly changed [17,18]. Cheng reported that the sintering temperature of Ba(Mg1/3Ta2/3)O3 ceramics was decreased to approximately 1320 8C by the addition of BaTi4O9, and the er values increased with the increase of BaTi4O9 content [19]. Previously, we reported that the sinterabilities of Ba(Zn1/3Ta2/3)O3 ceramics could be improved when a small amount of BaTi4O9 was added. The sintering temperature of 0.9Ba(Zn1/3Ta2/3)O3– 0.1BaTi4O9 ceramic was lower than that of Ba(Zn1/3Ta2/3)O3 ceramic. However, the saturated er value of 0.9Ba(Zn1/ 3Ta2/3)O3–0.1BaTi4O9 ceramic was higher than that of Ba(Zn1/3Ta2/3)O3 ceramic [20]. The above-mentioned reasons provided motivation for the systematic study of the influence of the BaTi4O9 addition to Ba(Zn1/3Ta2/3)O3 ceramics. The microwave dielectric properties, microstructure and sintering behavior of (1 x)Ba(Zn1/3Ta2/3)O3–xBaTi4O9 ceramics are thoroughly discussed in this study. 2. Experimental procedures Proportionate amounts of reagent-grade starting materials of BaCO3, ZnO, Ta2O5, and TiO2 were mixed, according to the compositions of Ba(Zn1/3Ta2/3)O3 and BaTi4O9, and ball-milled for 4 h with deionized water. After drying, the reagent was ground with an agate mortar for 1 h. Then the Ba(Zn1/3Ta2/3)O3 and BaTi4O9 powders were calcined for 2 h at 1150 8C. The crystal structure of calcined Ba(Zn1/3Ta2/3)O3 and BaTi4O9 powders was examined by using an Xray powder diffraction. X-ray diffraction (XRD) patterns were taken at 2u = 48 per minute using Cu Ka radiation. Furthermore, the calcined Ba(Zn1/3Ta2/3)O3 and BaTi4O9 powders were used as precursor and mixed in accordance with (1 x)Ba(Zn1/3Ta2/3)O3–xBaTi4O9 system (0.10 x 0.85) by ball milling with deionized water for 2 h. After drying, the powder was pressed into pellets uniaxially in a steel die. Sintering of these specimens was carried out at the temperatures between 1160 and 1360 8C under ambient conditions for a duration of 2 h. The crystal structure of (1 x)Ba(Zn1/3Ta2/3)O3–xBaTi4O9 ceramics was also investigated using X-ray powder diffraction. The bulk densities of sintered specimens, as a function of sintering temperature and composition, were measured by the liquid displacement method using deionized water as the liquid (Archimedes method). To investigate the internal morphology of the samples, the sintered surfaces of specimens were observed, using SEM. Dielectric characteristics at microwave frequency were measured by Hakki and Coleman’s dielectric resonator method [21], which was improved by Courtney [22]. An HP8720ET network analyzer was used for the dielectric characteristic measurements at microwave frequencies between 5.5 and 6.5 GHz. The dielectric constant can be accurately determined by measuring the resonant frequency of the TE011 mode and verified by the TE01d resonant modes. The temperature coefficient of resonant frequency (tf) was defined as follows. tf ¼
f 85 f 25 f 25 60
where f 25 and f 85 were the resonant frequency at 25 and 85 8C, respectively.
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3. Results and discussion The Ba(Zn1/3Ta2/3)O3 composition has hexagonal perovskite-typed structure with Zn and Ta showing 1:2 order in B site. If the Zn and Ta are disorded, the compound has a cubic perovskite-typed structure. For Ba(Zn1/3Ta2/3)O3 ceramics, calcination leads to the disordered phase [19,20]. Typical X-ray diffraction (XRD) patterns collected from different compositions of the (1 x)Ba(Zn1/3Ta2/3)O3–xBaTi4O9 ceramics sintered at 1280 8C are shown in Fig. 1. For x = 0.85, the main crystalline phases were BaTi4O9 phases and the minor ones were disordered Ba(Zn1/3Ta2/3)O3 phases. The intensity of BaTi4O9 phases decreased remarkably with the decrease of BaTi4O9 content. On the contrary, the intensity of ordered and disordered Ba(Zn1/3Ta2/3)O3 phases increased. As the BaTi4O9 content decreased to x = 0.55, the ordered Ba(Zn1/3Ta2/3)O3 phases began to appear more obviously except for the BaTi4O9 phases and the disordered Ba(Zn1/3Ta2/3)O3 phases. The peaks of ordered and disordered Ba(Zn1/3Ta2/3)O3 phases in the (1 x)Ba(Zn1/3Ta2/3)O3–xBaTi4O9 compositions (0.10 x 0.55) became much more intense and sharper. In this study, we also found that the (1 x)Ba(Zn1/3Ta2/3)O3–xBaTi4O9 compositions would melt at 1400 8C for x = 0.1 and 0.25, 1360 8C for x = 0.4 and 0.55, and 1320 8C for x = 0.7 and 0.85, respectively. This result suggests that some types of eutectic phases may exist between BaTi4O9 and Ba(Zn1/3Ta2/3)O3, and the eutectic phases will improve the sinterability of Ba(Zn1/3Ta2/3)O3 ceramics. The SEM micrographs of the (1 x)Ba(Zn1/3Ta2/3)O3–xBaTi4O9 ceramics revealed in Fig. 2 show the changes in the densification and grain growth. Only by using the calcined Ba(Zn1/3Ta2/3)O3 powder as precursor and sintered at 1400 8C could isolated Ba(Zn1/3Ta2/3)O3 particles and pores be easily observed (not shown here). With the addition of BaTi4O9, an obvious densification of Ba(Zn1/3Ta2/3)O3 ceramics is demonstrated in Fig. 2. For 1280 8C-sintered (1 x)Ba(Zn1/3Ta2/3)O3–xBaTi4O9 ceramics, isolated particles and few pores were also observed, as shown in Fig. 2(a-1), (b-1) and (c-1). After increasing the sintering temperature to 1320 8C, homogeneously fine microstructures with almost no pores were observed for all compositions, as shown in Fig. 2(a-2), (b-2) and (c-2). This implies that the pores of all (1 x)Ba(Zn1/3Ta2/3)O3–xBaTi4O9 ceramics decrease and the grain sizes increase with sintering temperatures. As x = 0.1 and 1320 8C was used as sintering temperature, the size of pores could be easily eliminated and the microstructures of (1 x)Ba(Zn1/3Ta2/3)O3–xBaTi4O9 ceramics changed the plate like grains to needle shaped grains (compare Fig. 2(c-1) and (c-2)) [20]. For a given sintering temperature, the specimens with larger BaTi4O9 content had more pores (compare Fig. 2(a-1), (b-1) and (c-1); 2(a-2), (b-2) and (c-2)). These phenomena imply that the
Fig. 1. The X-ray diffraction patterns of (1 x)Ba(Zn1/3Ta2/3)O3–xBaTi4O9 ceramics at different x value and the sintering temperature of 1280 8C. [+: disordered Ba(Zn1/3Ta2/3)O3, : ordered Ba(Zn1/3Ta2/3)O3, *: BaTi4O9].
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Fig. 2. The micrographs of (1 x)Ba(Zn1/3Ta2/3)O3–xBaTi4O9 ceramics. For x = 0.7 and sintered at (a-1) 1280 8C and (a-2) 1320 8C. For x = 0.4 and sintered at (b-1) 1280 8C and (b-2) 1320 8C. For x = 0.1 and sintered at (c-1) 1280 8C and (c-2) 1320 8C.
influences of compositions on the morphologies and microwave dielectric characteristics of (1 x)Ba(Zn1/3Ta2/3)O3– xBaTi4O9 ceramics are evident. Fig. 3 shows the bulk densities of (1 x)Ba(Zn1/3Ta2/3)O3–xBaTi4O9 ceramics under the sintering temperature of 1160–1360 8C. As the sintering temperature increased, the bulk densities of all (1 x)Ba(Zn1/3Ta2/3)O3–xBaTi4O9 ceramics at different composition first increased with the increase of sintering temperatures, and then saturated at a certain temperature. This result was caused by the decrease in pores ratio and the increase in grain growth. Moreover, the temperatures reaching the saturated density values decreased with the increase of BaTi4O9 content. The bulk density values of (1 x)Ba(Zn1/3Ta2/3)O3–xBaTi4O9 ceramics were strongly dependent on the amount of BaTi4O9 content or Ba(Zn1/3Ta2/3)O3 content. The theoretical densities of (1 x)Ba(Zn1/3Ta2/3)O3–xBaTi4O9 ceramics are
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Fig. 3. The bulk densities of (1 x)Ba(Zn1/3Ta2/3)O3–xBaTi4O9 ceramics with different x value as a function of sintering temperature. [+: x = 0.10, ^: x = 0.25, *: x = 0.40, : x = 0.55, ~: x = 0.70, *: x = 0.85].
calculated from 4.525 g/cm3 for BaTi4O9 phase and 7.90 g/cm3 for Ba(Zn1/3Ta2/3)O3 phase [23]. The theoretical densities are calculated by D¼
W1 þ W2 W 1 =D1 þ W 2 =D2
(2)
where W1 and W2 are respectively the weight percent of the BaTi4O9 ceramics and Ba(Zn1/3Ta2/3)O3 ceramics in the mixtures, D1 and D2 the theoretical densities of the BaTi4O9 ceramics and Ba(Zn1/3Ta2/3)O3 ceramics. The bulk densities and the theoretical densities of (1 x)Ba(Zn1/3Ta2/3)O3–xBaTi4O9 ceramics are shown in Fig. 4, both the bulk densities and theoretical densities decrease with the increase of BaTi4O9 content. Fig. 4 also shows the ratios of
Fig. 4. The measured densities and the ratios of measured densities to theoretical densities of (1 x)Ba(Zn1/3Ta2/3)O3–xBaTi4O9 ceramics as a function of BaTi4O9 content. [+: M.D. (measured density), *: T.D. (theoretical density)].
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Fig. 5. The dielectric constants of (1 x)Ba(Zn1/3Ta2/3)O3–xBaTi4O9 ceramics with different x value as a function of sintering temperature. [+: x = 0.10, ^: x = 0.25, *: x = 0.40, : x = 0.55, ~: x = 0.70, *: x = 0.85].
measured densities to theoretical densities. The relative bulk densities of (1 x)Ba(Zn1/3Ta2/3)O3–xBaTi4O9 ceramics exhibited the value between 95.5 and 96.5% at x = 0.85–0.55, because few pores existed. However, those with x = 0.1 sintered at 1320 8C could be higher than 99.5%. These results imply that the changes in composition and grain morphology have a great effect on the bulk densities of (1 x)Ba(Zn1/3Ta2/3)O3–xBaTi4O9 ceramics. The er values of the (1 x)Ba(Zn1/3Ta2/3)O3–xBaTi4O9 ceramics as a function of sintering temperature and BaTi4O9 content are shown in Fig. 5. At first, the er values of all (1 x)Ba(Zn1/3Ta2/3)O3–xBaTi4O9 ceramics increased with the increase of sintering temperatures, and saturated at 1240–1360 8C depending on the BaTi4O9 content. The relationships between er values and sintering temperatures revealed the same trend as those between
Fig. 6. The quality values and temperature coefficients of resonant frequency of (1 x)Ba(Zn1/3Ta2/3)O3–xBaTi4O9 ceramics as a function of BaTi4O9 content.
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densities and sintering temperatures. The increase in the er values with sintering temperatures resulted from the increase of density and grain growth. The maximum er values of (1 x)Ba(Zn1/3Ta2/3)O3–xBaTi4O9 ceramics increased from 31.5 to 37.5 linearly as x was changed from 0.10 to 0.85. In Fig. 1, the BaTi4O9 (er 38.0) and Ba(Zn1/ 3Ta2/3)O3 (er 30) phases coexist in the sintered (1 x)Ba(Zn1/3Ta2/3)O3–xBaTi4O9 ceramics, the increase in theer values of (1 x)Ba(Zn1/3Ta2/3)O3–xBaTi4O9 ceramics is due to the higher dielectric constants of BaTi4O9 ceramics. Fig. 6 shows the Q f values of (1 x)Ba(Zn1/3Ta2/3)O3–xBaTi4O9 ceramics as a function of BaTi4O9 content. As the amount of BaTi4O9 decreased, the Q f values increased linearly and reached a maximum value at x = 0.1. The Q f values increased with the increase of Ba(Zn1/3Ta2/3)O3 content because the Q values of Ba(Zn1/3Ta2/3)O3 ceramics were higher than those of BaTi4O9 ceramics. Moreover, according to the relative density results (see Fig. 4), the relative density of (1 x)Ba(Zn1/3Ta2/3)O3–xBaTi4O9 ceramics increased with the decrease of BaTi4O9 content would result in the improvement of Q f value. The tf values of (1 x)Ba(Zn1/3Ta2/3)O3–xBaTi4O9 ceramics with various x values are also shown in Fig. 6. As the BaTi4O9 content increased, the tf values changed steadily from a small value (+4.1 ppm/8C) to a large value (+14.1 ppm/8C). This result is attributed to that the Ba(Zn1/3Ta2/3)O3 ceramics have a low tf value (tf < 5 ppm/8C) and the BaTi4O9 ceramics have a high tf value (tf 15 ppm/8C) [24]. 4. Conclusions Addition of BaTi4O9 ceramics is found to significantly influence the microwave dielectric properties and densification behavior of the Ba(Zn1/3Ta2/3)O3 ceramics. The sintering temperatures of (1 x)Ba(Zn1/3Ta2/3)O3– xBaTi4O9 ceramics are approximately 1240–1320 8C, close to the sintering temperatures of BaTi4O9 ceramics but far lower than those of Ba(Zn1/3Ta2/3)O3 ceramics. The SEM micrographs reveal that the sinterabilities and densification of Ba(Zn1/3Ta2/3)O3 ceramics can be improved as long as a small amount of BaTi4O9 content is added. The bulk densities of (1 x)Ba(Zn1/3Ta2/3)O3–xBaTi4O9 ceramics can exceed 95% of its theoretical density, because few pores exist in sintered specimens. Furthermore, the bulk densities of (1 x)Ba(Zn1/3Ta2/3)O3–xBaTi4O9 ceramics can be higher than 99.5% at x = 0.1. The er values and the tf values of (1 x)Ba(Zn1/3Ta2/3)O3–xBaTi4O9 ceramics increase with the increase of BaTi4O9 content. The quality values (Q f) of (1 x)Ba(Zn1/3Ta2/3)O3–xBaTi4O9 ceramics are lower than those of Ba(Zn1/3Ta2/3)O3 ceramics, and range from 68,500 to 33,820 GHz as x increased from 0.1 to 0.85. The (1 x)Ba(Zn1/3Ta2/3)O3–xBaTi4O9 ceramic with x = 0.1 sintered at 1320 8C has the microwave dielectric property of er = 31.5, Q f value of 68500 GHz, and tf = 4.1 ppm/8C. Acknowledgments The authors will also acknowledge to the financial support of the National Science Council of the Republic of China (contract NSC-95-2221-E-390-009). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18]
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Microstructure and microwave dielectric characteristics of (1 x)Ba(Zn1/3Ta2/3)O3–xBaTi4O9 ceramics Wen-Cheng Tzou a,*, Yao-Sheng Yang b, Cheng-Fu Yang c, Ho-Hua Chung d, Chien-Jung Huang e, Chien-Chen Diao b a
Department of Electro-Optical Engineering, Southern Taiwan University of Technology Yung-Kang City, Tainan, Taiwan, ROC b Department of Electronic Engineering, Kao Yuan University, Kaohsiung, Taiwan, ROC c Department of Chemical and Materials Engineering, National University of Kaohsiung, Kaohsiung, Taiwan, ROC d Department of Mechanical and Automation Engineering, Kao Yuan University, Kaohsiung, Taiwan, ROC e Department of Applied Physics, National University of Kaohsiung, Kaohsiung, Taiwan, ROC Received 25 August 2006; received in revised form 3 December 2006; accepted 13 December 2006 Available online 20 December 2006
Abstract (1 x)Ba(Zn1/3Ta2/3)O3–xBaTi4O9 (0.1 x 0.85) composites are prepared by mixing 1150 8C-calcined BaTi4O9 with 1150 8C-calcined Ba(Zn1/3Ta2/3)O3 powders. The crystal structure, microwave dielectric properties and sinterabilites of the (1 x)Ba(Zn1/3Ta2/3)O3–xBaTi4O9 ceramics have been investigated. X-ray diffraction patterns reveal that BaTi4O9, ordered and disordered Ba(Zn1/3Ta2/3)O3 phases exist independently over the whole compositional range. The sintering temperatures of (1 x)Ba(Zn1/3Ta2/3)O3–xBaTi4O9 ceramics are about 1240 – 1320 8C and obviously lower than those of Ba(Zn1/3Ta2/3)O3 ceramics. The dielectric constants (er) and the temperature coefficient of resonant frequency (tf) of (1 x)Ba(Zn1/3Ta2/3)O3– xBaTi4O9 ceramics increase with the increase of BaTi4O9 content. Nevertheless, the bulk densities and the quality values (Q f) of (1 x)Ba(Zn1/3Ta2/3)O3–xBaTi4O9 ceramics increase with the increase of Ba(Zn1/3Ta2/3)O3 content. The results are attributed to the higher density and quality value of Ba(Zn1/3Ta2/3)O3 ceramics, the better grain growth, and the densification of sintered specimens added a small BaTi4O9 content. The (1 x)Ba(Zn1/3Ta2/3)O3–xBaTi4O9 ceramic with x = 0.1 sintered at 1320 8C exhibits a er value of 31.5, a maximum Q f value of 68500 GHz and a minimum tf value of 4.1 ppm/8C. # 2007 Elsevier Ltd. All rights reserved. Keywords: A. Ceramics; C. X-ray diffraction; D. Dielectric properties
1. Introduction The rapidly growing microwave telecommunication devices demand the miniaturization of the size and weight, and the temperature stability [1]. Thus many researchers have devoted to developing the dielectric materials with a high dielectric constant (er), a high quality value (Q) and a small temperature coefficient of resonant frequency (tf). The complex perovskite compounds, such as Ba(Mg1/3Ta2/3)O3 ceramics [2–4] and Ba(Zn1/3Ta2/3)O3 ceramics [5–9], are regarded as the microwave materials of high quality value. Ba(Zn1/3Ta2/3)O3 ceramics possess a suitable er 30, a relatively high quality value (Q 14 000 at 11 GHz), and a stable temperature coefficient of resonant frequency
* Corresponding author. Tel.: +886 7 2518898; fax: +886 945834460. E-mail address: [email protected] (W.-C. Tzou). 0025-5408/$ – see front matter # 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2006.12.009
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(tf 0 5 ppm/8C). Unfortunately, the Ba(Zn1/3Ta2/3)O3 ceramics formed by the solid-state reaction of BaCO3, ZnO, and Ta2O5 are not appropriate, because high sintering temperature (>1500 8C) and long soaking time are required to achieve sufficiently high sintered density for the industrial applications of the Ba(Zn1/3Ta2/3)O3 materials. Moreover, at high sintering temperature zinc departure can be observed in Ba(Zn1/3Ta2/3)O3 ceramics, involving a decrease in dielectric properties [10]. Many efforts had been made to improve the sinterabilities of Ba(Zn1/3Ta2/3)O3 ceramics by the addition of sintering agent. Roulland et al. reported that the sintering temperature of Ba(Zn1/3Ta2/3)O3 ceramics could be reduced to 1400 8C [11]. Nahm et al. presented some investigations into the effects of additives (such as Al2O3, Ga2O3, ZrO2 and TiO2) on the sinterabilities, microstructure and dielectric loss of Ba(Zn1/3Ta2/3)O3 ceramics. In Nahm’s studies, the Q values could be improved effectively due to the increased relative density and grain size [12–15]. Varma et al. studied the effect of different dopants (such as oxides of divalent, trivalent, pentavalent and hexavalent elements) on the microwave dielectric properties of Ba(Zn1/3Ta2/3)O3 ceramics, and concluded that the quality factor increased when the ionic radius of dopant was close to the ionic radius of B site ions (Zn or Ta) [16]. However, high sintering temperatures (1500–1600 8C) were still needed in the researches of Nahm et al. and Varma et al. Numerous investigations exhibited that incorporation of additives, such as Y2O3, Ba5Ta4O15 or other second perovskite type materials, were also found beneficial for the formation of Ba(Mg1/3Ta2/3)O3 ceramics. Lin et al. presented that the sintered density of Ba(Mg1/3Ta2/3)O3 ceramics decreased after adding Ba5Ta4O15, and the dielectric constants (er) were insignificantly changed [17,18]. Cheng reported that the sintering temperature of Ba(Mg1/3Ta2/3)O3 ceramics was decreased to approximately 1320 8C by the addition of BaTi4O9, and the er values increased with the increase of BaTi4O9 content [19]. Previously, we reported that the sinterabilities of Ba(Zn1/3Ta2/3)O3 ceramics could be improved when a small amount of BaTi4O9 was added. The sintering temperature of 0.9Ba(Zn1/3Ta2/3)O3– 0.1BaTi4O9 ceramic was lower than that of Ba(Zn1/3Ta2/3)O3 ceramic. However, the saturated er value of 0.9Ba(Zn1/ 3Ta2/3)O3–0.1BaTi4O9 ceramic was higher than that of Ba(Zn1/3Ta2/3)O3 ceramic [20]. The above-mentioned reasons provided motivation for the systematic study of the influence of the BaTi4O9 addition to Ba(Zn1/3Ta2/3)O3 ceramics. The microwave dielectric properties, microstructure and sintering behavior of (1 x)Ba(Zn1/3Ta2/3)O3–xBaTi4O9 ceramics are thoroughly discussed in this study. 2. Experimental procedures Proportionate amounts of reagent-grade starting materials of BaCO3, ZnO, Ta2O5, and TiO2 were mixed, according to the compositions of Ba(Zn1/3Ta2/3)O3 and BaTi4O9, and ball-milled for 4 h with deionized water. After drying, the reagent was ground with an agate mortar for 1 h. Then the Ba(Zn1/3Ta2/3)O3 and BaTi4O9 powders were calcined for 2 h at 1150 8C. The crystal structure of calcined Ba(Zn1/3Ta2/3)O3 and BaTi4O9 powders was examined by using an Xray powder diffraction. X-ray diffraction (XRD) patterns were taken at 2u = 48 per minute using Cu Ka radiation. Furthermore, the calcined Ba(Zn1/3Ta2/3)O3 and BaTi4O9 powders were used as precursor and mixed in accordance with (1 x)Ba(Zn1/3Ta2/3)O3–xBaTi4O9 system (0.10 x 0.85) by ball milling with deionized water for 2 h. After drying, the powder was pressed into pellets uniaxially in a steel die. Sintering of these specimens was carried out at the temperatures between 1160 and 1360 8C under ambient conditions for a duration of 2 h. The crystal structure of (1 x)Ba(Zn1/3Ta2/3)O3–xBaTi4O9 ceramics was also investigated using X-ray powder diffraction. The bulk densities of sintered specimens, as a function of sintering temperature and composition, were measured by the liquid displacement method using deionized water as the liquid (Archimedes method). To investigate the internal morphology of the samples, the sintered surfaces of specimens were observed, using SEM. Dielectric characteristics at microwave frequency were measured by Hakki and Coleman’s dielectric resonator method [21], which was improved by Courtney [22]. An HP8720ET network analyzer was used for the dielectric characteristic measurements at microwave frequencies between 5.5 and 6.5 GHz. The dielectric constant can be accurately determined by measuring the resonant frequency of the TE011 mode and verified by the TE01d resonant modes. The temperature coefficient of resonant frequency (tf) was defined as follows. tf ¼
f 85 f 25 f 25 60
where f 25 and f 85 were the resonant frequency at 25 and 85 8C, respectively.
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3. Results and discussion The Ba(Zn1/3Ta2/3)O3 composition has hexagonal perovskite-typed structure with Zn and Ta showing 1:2 order in B site. If the Zn and Ta are disorded, the compound has a cubic perovskite-typed structure. For Ba(Zn1/3Ta2/3)O3 ceramics, calcination leads to the disordered phase [19,20]. Typical X-ray diffraction (XRD) patterns collected from different compositions of the (1 x)Ba(Zn1/3Ta2/3)O3–xBaTi4O9 ceramics sintered at 1280 8C are shown in Fig. 1. For x = 0.85, the main crystalline phases were BaTi4O9 phases and the minor ones were disordered Ba(Zn1/3Ta2/3)O3 phases. The intensity of BaTi4O9 phases decreased remarkably with the decrease of BaTi4O9 content. On the contrary, the intensity of ordered and disordered Ba(Zn1/3Ta2/3)O3 phases increased. As the BaTi4O9 content decreased to x = 0.55, the ordered Ba(Zn1/3Ta2/3)O3 phases began to appear more obviously except for the BaTi4O9 phases and the disordered Ba(Zn1/3Ta2/3)O3 phases. The peaks of ordered and disordered Ba(Zn1/3Ta2/3)O3 phases in the (1 x)Ba(Zn1/3Ta2/3)O3–xBaTi4O9 compositions (0.10 x 0.55) became much more intense and sharper. In this study, we also found that the (1 x)Ba(Zn1/3Ta2/3)O3–xBaTi4O9 compositions would melt at 1400 8C for x = 0.1 and 0.25, 1360 8C for x = 0.4 and 0.55, and 1320 8C for x = 0.7 and 0.85, respectively. This result suggests that some types of eutectic phases may exist between BaTi4O9 and Ba(Zn1/3Ta2/3)O3, and the eutectic phases will improve the sinterability of Ba(Zn1/3Ta2/3)O3 ceramics. The SEM micrographs of the (1 x)Ba(Zn1/3Ta2/3)O3–xBaTi4O9 ceramics revealed in Fig. 2 show the changes in the densification and grain growth. Only by using the calcined Ba(Zn1/3Ta2/3)O3 powder as precursor and sintered at 1400 8C could isolated Ba(Zn1/3Ta2/3)O3 particles and pores be easily observed (not shown here). With the addition of BaTi4O9, an obvious densification of Ba(Zn1/3Ta2/3)O3 ceramics is demonstrated in Fig. 2. For 1280 8C-sintered (1 x)Ba(Zn1/3Ta2/3)O3–xBaTi4O9 ceramics, isolated particles and few pores were also observed, as shown in Fig. 2(a-1), (b-1) and (c-1). After increasing the sintering temperature to 1320 8C, homogeneously fine microstructures with almost no pores were observed for all compositions, as shown in Fig. 2(a-2), (b-2) and (c-2). This implies that the pores of all (1 x)Ba(Zn1/3Ta2/3)O3–xBaTi4O9 ceramics decrease and the grain sizes increase with sintering temperatures. As x = 0.1 and 1320 8C was used as sintering temperature, the size of pores could be easily eliminated and the microstructures of (1 x)Ba(Zn1/3Ta2/3)O3–xBaTi4O9 ceramics changed the plate like grains to needle shaped grains (compare Fig. 2(c-1) and (c-2)) [20]. For a given sintering temperature, the specimens with larger BaTi4O9 content had more pores (compare Fig. 2(a-1), (b-1) and (c-1); 2(a-2), (b-2) and (c-2)). These phenomena imply that the
Fig. 1. The X-ray diffraction patterns of (1 x)Ba(Zn1/3Ta2/3)O3–xBaTi4O9 ceramics at different x value and the sintering temperature of 1280 8C. [+: disordered Ba(Zn1/3Ta2/3)O3, : ordered Ba(Zn1/3Ta2/3)O3, *: BaTi4O9].
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Fig. 2. The micrographs of (1 x)Ba(Zn1/3Ta2/3)O3–xBaTi4O9 ceramics. For x = 0.7 and sintered at (a-1) 1280 8C and (a-2) 1320 8C. For x = 0.4 and sintered at (b-1) 1280 8C and (b-2) 1320 8C. For x = 0.1 and sintered at (c-1) 1280 8C and (c-2) 1320 8C.
influences of compositions on the morphologies and microwave dielectric characteristics of (1 x)Ba(Zn1/3Ta2/3)O3– xBaTi4O9 ceramics are evident. Fig. 3 shows the bulk densities of (1 x)Ba(Zn1/3Ta2/3)O3–xBaTi4O9 ceramics under the sintering temperature of 1160–1360 8C. As the sintering temperature increased, the bulk densities of all (1 x)Ba(Zn1/3Ta2/3)O3–xBaTi4O9 ceramics at different composition first increased with the increase of sintering temperatures, and then saturated at a certain temperature. This result was caused by the decrease in pores ratio and the increase in grain growth. Moreover, the temperatures reaching the saturated density values decreased with the increase of BaTi4O9 content. The bulk density values of (1 x)Ba(Zn1/3Ta2/3)O3–xBaTi4O9 ceramics were strongly dependent on the amount of BaTi4O9 content or Ba(Zn1/3Ta2/3)O3 content. The theoretical densities of (1 x)Ba(Zn1/3Ta2/3)O3–xBaTi4O9 ceramics are
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Fig. 3. The bulk densities of (1 x)Ba(Zn1/3Ta2/3)O3–xBaTi4O9 ceramics with different x value as a function of sintering temperature. [+: x = 0.10, ^: x = 0.25, *: x = 0.40, : x = 0.55, ~: x = 0.70, *: x = 0.85].
calculated from 4.525 g/cm3 for BaTi4O9 phase and 7.90 g/cm3 for Ba(Zn1/3Ta2/3)O3 phase [23]. The theoretical densities are calculated by D¼
W1 þ W2 W 1 =D1 þ W 2 =D2
(2)
where W1 and W2 are respectively the weight percent of the BaTi4O9 ceramics and Ba(Zn1/3Ta2/3)O3 ceramics in the mixtures, D1 and D2 the theoretical densities of the BaTi4O9 ceramics and Ba(Zn1/3Ta2/3)O3 ceramics. The bulk densities and the theoretical densities of (1 x)Ba(Zn1/3Ta2/3)O3–xBaTi4O9 ceramics are shown in Fig. 4, both the bulk densities and theoretical densities decrease with the increase of BaTi4O9 content. Fig. 4 also shows the ratios of
Fig. 4. The measured densities and the ratios of measured densities to theoretical densities of (1 x)Ba(Zn1/3Ta2/3)O3–xBaTi4O9 ceramics as a function of BaTi4O9 content. [+: M.D. (measured density), *: T.D. (theoretical density)].
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Fig. 5. The dielectric constants of (1 x)Ba(Zn1/3Ta2/3)O3–xBaTi4O9 ceramics with different x value as a function of sintering temperature. [+: x = 0.10, ^: x = 0.25, *: x = 0.40, : x = 0.55, ~: x = 0.70, *: x = 0.85].
measured densities to theoretical densities. The relative bulk densities of (1 x)Ba(Zn1/3Ta2/3)O3–xBaTi4O9 ceramics exhibited the value between 95.5 and 96.5% at x = 0.85–0.55, because few pores existed. However, those with x = 0.1 sintered at 1320 8C could be higher than 99.5%. These results imply that the changes in composition and grain morphology have a great effect on the bulk densities of (1 x)Ba(Zn1/3Ta2/3)O3–xBaTi4O9 ceramics. The er values of the (1 x)Ba(Zn1/3Ta2/3)O3–xBaTi4O9 ceramics as a function of sintering temperature and BaTi4O9 content are shown in Fig. 5. At first, the er values of all (1 x)Ba(Zn1/3Ta2/3)O3–xBaTi4O9 ceramics increased with the increase of sintering temperatures, and saturated at 1240–1360 8C depending on the BaTi4O9 content. The relationships between er values and sintering temperatures revealed the same trend as those between
Fig. 6. The quality values and temperature coefficients of resonant frequency of (1 x)Ba(Zn1/3Ta2/3)O3–xBaTi4O9 ceramics as a function of BaTi4O9 content.
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densities and sintering temperatures. The increase in the er values with sintering temperatures resulted from the increase of density and grain growth. The maximum er values of (1 x)Ba(Zn1/3Ta2/3)O3–xBaTi4O9 ceramics increased from 31.5 to 37.5 linearly as x was changed from 0.10 to 0.85. In Fig. 1, the BaTi4O9 (er 38.0) and Ba(Zn1/ 3Ta2/3)O3 (er 30) phases coexist in the sintered (1 x)Ba(Zn1/3Ta2/3)O3–xBaTi4O9 ceramics, the increase in theer values of (1 x)Ba(Zn1/3Ta2/3)O3–xBaTi4O9 ceramics is due to the higher dielectric constants of BaTi4O9 ceramics. Fig. 6 shows the Q f values of (1 x)Ba(Zn1/3Ta2/3)O3–xBaTi4O9 ceramics as a function of BaTi4O9 content. As the amount of BaTi4O9 decreased, the Q f values increased linearly and reached a maximum value at x = 0.1. The Q f values increased with the increase of Ba(Zn1/3Ta2/3)O3 content because the Q values of Ba(Zn1/3Ta2/3)O3 ceramics were higher than those of BaTi4O9 ceramics. Moreover, according to the relative density results (see Fig. 4), the relative density of (1 x)Ba(Zn1/3Ta2/3)O3–xBaTi4O9 ceramics increased with the decrease of BaTi4O9 content would result in the improvement of Q f value. The tf values of (1 x)Ba(Zn1/3Ta2/3)O3–xBaTi4O9 ceramics with various x values are also shown in Fig. 6. As the BaTi4O9 content increased, the tf values changed steadily from a small value (+4.1 ppm/8C) to a large value (+14.1 ppm/8C). This result is attributed to that the Ba(Zn1/3Ta2/3)O3 ceramics have a low tf value (tf < 5 ppm/8C) and the BaTi4O9 ceramics have a high tf value (tf 15 ppm/8C) [24]. 4. Conclusions Addition of BaTi4O9 ceramics is found to significantly influence the microwave dielectric properties and densification behavior of the Ba(Zn1/3Ta2/3)O3 ceramics. The sintering temperatures of (1 x)Ba(Zn1/3Ta2/3)O3– xBaTi4O9 ceramics are approximately 1240–1320 8C, close to the sintering temperatures of BaTi4O9 ceramics but far lower than those of Ba(Zn1/3Ta2/3)O3 ceramics. The SEM micrographs reveal that the sinterabilities and densification of Ba(Zn1/3Ta2/3)O3 ceramics can be improved as long as a small amount of BaTi4O9 content is added. The bulk densities of (1 x)Ba(Zn1/3Ta2/3)O3–xBaTi4O9 ceramics can exceed 95% of its theoretical density, because few pores exist in sintered specimens. Furthermore, the bulk densities of (1 x)Ba(Zn1/3Ta2/3)O3–xBaTi4O9 ceramics can be higher than 99.5% at x = 0.1. The er values and the tf values of (1 x)Ba(Zn1/3Ta2/3)O3–xBaTi4O9 ceramics increase with the increase of BaTi4O9 content. The quality values (Q f) of (1 x)Ba(Zn1/3Ta2/3)O3–xBaTi4O9 ceramics are lower than those of Ba(Zn1/3Ta2/3)O3 ceramics, and range from 68,500 to 33,820 GHz as x increased from 0.1 to 0.85. The (1 x)Ba(Zn1/3Ta2/3)O3–xBaTi4O9 ceramic with x = 0.1 sintered at 1320 8C has the microwave dielectric property of er = 31.5, Q f value of 68500 GHz, and tf = 4.1 ppm/8C. Acknowledgments The authors will also acknowledge to the financial support of the National Science Council of the Republic of China (contract NSC-95-2221-E-390-009). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18]
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