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Microstructure and microwave dielectric properties of Ba6−3xSm8+2xTi18O54 ceramics with various BaxSr1−xTiO3 additions
JOURNAL OF RARE EARTHS, Vol. 30, No. 2, Feb. 2012, P. 142
Microstructure and microwave dielectric properties of Ba6–3xSm8+2xTi18O54 ceramics with various BaxSr1–xTiO3 additions ZHOU Lingling (਼⦆⦆), ZHOU Hongqing (਼⋾ᑚ), SHAO Hui (䚉 䕝), ZHU Haikui (ᴅ⍋༢) (College of Materials Science and Engineering, Nanjing University of Technology, Nanjing 210009, China) Received 14 July 2011; revised 22 August 2011
Abstract: The Ba6–3xSm8+2xTi18O54 (x=2/3) microwave dielectric ceramics were prepared by traditional solid sate reaction technique. The experiment was based on the Ba6–3xSm8+2xTi18O54 (BST) microwave dielectric ceramics doped with a certain amount of Bi2O3, then the effects of BaxSr1–xTiO3 additives on the structure and microwave dielectric properties of Ba6–3xSm8+2xTi18O54 ceramics were investigated using X-ray diffraction and scanning electron microscopy. In this study, the small amount substitution of Sr for Ba was effective for the microwave dielectric properties of BST, especially the f could be tuned to near zero. The result showed that the BST possessed excellent dielectric properties when the addition of Bi2O3 and BaxSr1–xTiO3 was 1 wt.% respectively: r=79.6, Q·f=10789 GHz, f=–1.5 ppm/ºC. Keywords: dielectric properties; microstructure; BaxSr1–xTiO3; rare earths
The combination of a high dielectric constant (r), low dielectric loss and near-zero temperature coefficient of resonant frequency makes BaO-Ln2O3-kTiO2 (where Ln=Sm, Nd, Pr, La are lanthanide elements with k=3–5) very important for the applications as microwave dielectric resonators and filters. Since the first report on the excellent microwave dielectric properties in this system, many studies concerning these have been investigated[1,2]. Ohsato[3] has studied the structural and dielectric properties of Ba6–3xSm8+2xTi18O54. The crystal structure of these ceramics belongs to the tungsten bronze structural family and consists of a three-dimensional frame-work of corner-sharing perovskite-like TiO6 and the special point with x=2/3 at which the Q·f value becomes the highest[3]. However, the high densification temperature around 1300–1460 ºC and the high-negative temperature coefficient of resonant frequency (f=–15 ppm/ºC)[4] are the disadvantages of this system, thus a lot of work have been done to reduce the high temperature as well as to tune the f to near zero. In previous studies, the addition of a small amount of Bi2O3 was found to reduce the sintering temperature[5], Li et al.[6] have studied the BaO-Sm2O3-TiO2 microwave dielectric ceramics doped with bismuth and zinc, they also found doping with Bi2O3 and ZnO can decrease sintering temperature of BST and the performances of the microwave dielectric ceramics are most excellent. Substitution of Sr2+ with Ba2+ [7,8] on the system have been found to be very helpful to tune the f. Zhu et al.[9] reported that near zero TCf values (–1.5 ppm/ºC) could be achieved with the addition of SrTiO3 to BST ceramics. In the past decades, (Ba, Sr)TiO3 has attracted much atten-
tion in the development of tunable microwave devices as it exhibits high dielectric permittivity and low dielectric loss with an applied DC electric field[10]. In the present study, BaxSr1–xTiO3 have been used as additives, effects of composition and sintering temperature on the characteristics of BST with microwave dielectric ceramics were investigated.
1 Experimental The BaxSr1–xTiO3 and Ba6–3xSm8+2xTi18O54 (x=2/3) samples were prepared by the general solid-state reaction method. Reagent grade BaTiO3, SrTiO3, BaCO3, Sm2O3, TiO2 and Bi2O3 were used as the raw materials. After weighing the raw powders as the stoichiometries of BaxSr1–xTiO3 (0.4x 0.6) and Ba6–3xSm8+2xTi18O54 (x=2/3), the mixtures were put into two polyethylene bottles respectively, milled with agate balls in distilled water for 24 h. After drying, calcination of the two kinds mixtures were conducted at 1100 ºC for 3 h in air, the phases of BaxSr1–xTiO3 and Ba6–3xSm8+2xTi18O54 (x=2/3) were obtained. 1 wt.% Bi2O3 and different contents of the BaxSr1–xTiO3 were added to the calcined BST. The mixed powders were then remilled for 12 h, mixed with 5 wt.% polyvinyl alcohol, pressed into pellets of 13 mm in diameter and 5–8 mm in thickness under a pressure of 100 MPa. The samples were sintered at 1250–1340 ºC for 3 h in air. The phases of these samples were determined by X-ray diffraction with Cu K radiation (XRD ARL XTRA Switzerland). The bulk densities were measured by Archimedes’ method. Polished and thermally etched surfaces of the samples were executed by scanning electron microscopy (SEM JSM-5900, Japan). The quality factor (Q·f) and the tempera-
Foundation item: Project supported by the Priority Academic Program Development of Jiangsu Higher Education Institutions Corresponding author: ZHOU Hongqing (E-mail: [email protected]; Tel.: +86-25-86639976) DOI: 10.1016/S1002-0721(12)60011-8
ZHOU Lingling et al., Microstructure and microwave dielectric properties of Ba6–3xSm8+2xTi18O54 ceramics with various …
ture coefficient of resonant frequency (f) were measured by Agilent 8722ET network analyzer.
2 Results and discussion The BST ceramics with Bi and BaxSr1–xTiO3 are identified as single phase solid solutions. Fig. 1 shows the X-ray diffraction patterns of Ba6–3xSm8+2xTi18O54 ceramics with various amounts of BaxSr1–xTiO3 at 1310 ºC. It is shown that the major phase is an orthorhombic tungsten-bronze structure of Ba6–3xSm8+2xTi18O54 (x=2/3) solid solution formula, assigned as BaSm2Ti4O12, and meanwhile another phase is BaTi4O9. Just as Zhu et al.[9] reportedˈno significant changes were found in the XRD patterns when 0–8 wt.% SrTiO3 were added to Ba4.2Sm9.2Ti18O54 ceramics. Fig. 2 is the SEM images at different sintering temperatures. Fig. 2(a) shows the small amount main crystals with lots of porosity around. As the sintering temperatures are above densified temperatures, long bar-shaped grains come to form, the size of the grain increases with the sintering temperatures increasing as shown in (b), (c) and (d). However, when the sintering temperature is above 1310 ºC, the long bar-shaped grain comes into contact, continual growth squeezes them away from each other[11], which results in the formation of new and larger vacant positions where the short
Fig. 1 XRD patterns of BST ceramics with various amounts of BaxSr1–xTiO3 at 1310 ºC for 3 h
143
grains are originally located as shown in (c) and (d). The bulk densities of the samples as a function of the different additions of BaxSr1–xTiO3 and sintered at temperatures from 1250 to 1340 ºC for 3 h are shown in Fig. 3. It can be seen that the densities increase with the increase addition of BaxSr1–xTiO3, the densities of the samples increase greatly with the increase of sintering temperature in the beginning. However, it decreased slightly as the temperature range from 1310 to 1340 ºC. The ceramics sintered at 1310 ºC have the highest densities. The formation of new and larger vacant causes expansion of the space between bar-shaped grains and results in the increase porosity and hence decreases the bulk density at 1340 ºC, which can also be seen in micrographs. Fig. 4 shows the variations in the r value of the BST+ 1 wt.% Bi2O3+(y)wt.% BaxSr1–xTiO3 ceramics with 0y7 sintered at various temperatures. It can be seen that the variation in the r value is the same as that of the density. Density is an important factor which influences the r, so the ceramics sintered at 1310 ºC have the highest densities and therefore exhibit the highest permittivity. Ohsato[3] reported that dielectric constant (r) of Ba63xR8+2xTi18O54 solid solutions was affected by TiO6 octahedral volume. The radius of Sr2+ are bigger than that of Sm3+, which results in getting crystal cell volume and the octahedron become bigger, the moving space of Ti4+ broadens, so the permittivity increase and with the increase of the BaxSr1–xTiO3, the effects on permittivity are increasing. Fig. 5 shows the variations in the Q·f values of the BST ceramics with different BaxSr1–xTiO3 contents sintered at various temperatures. The BST (x=2/3) only with oxide addition (1 wt.% Bi2O3) have virtually no effect on the Q·f values at different temperatures, a small amount of addition of BaxSr1–xTiO3 exhibits an increase in the Q·f values, The fundamental structural formula of Ba6–3xSm8+2xTi18O54 is [Ba23xSm8+2xVx]A1[Ba4]A2Ti18O54 (V:vacancy). Okawa et al.[12] reported the substitute of Bi for Ba in A2 sites, Suzuki et al.[13] investigated about the (Ba1Sr)63xSm8+2xTi18O54 solid solutions. They found that Sr ions occupied only the A1 sites. In this study, at first Ba ions in A1 sites are substituted by Sr ions. After all Ba ions in A1 sites are substituted, vacancies in A1 sites are filled by Sr ions. When all vacancies in A1 sites are filled, decreasing of inner strain and increasing of lattice stability by substitution make quality factor (Q·f ) increase significantly. Further addition of BaxSr1–xTiO3 leads
Fig. 2 SEM images of BST ceramics with 1 wt.% Bi2O3 and 1 wt.% BaxSr1–xTiO3 at different sintering temperatures (a) 1250 ºC; (b) 1280 ºC; (c) 1310 ºC; (d) 1340 ºC
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Fig. 3 Densities of BST ceramics with various amounts of BaxSr1–xTiO3 at different temperatures
JOURNAL OF RARE EARTHS, Vol. 30, No. 2, Feb. 2012
Fig. 6 f values of BST ceramics as a function of sintering temperatures and various amounts of BaxSr1–xTiO3
tuned to near zero when 5 wt.% BaxSr1–xTiO3 was added. Further addition of BaxSr1–xTiO3 makes the f value decrease greatly. The reason might be considered to be the formation of nanoscale secondary phases[5] with a large negative f created by excess Ba and Ti ions due to the substitution of Sr for Ba.
3 Conclusions
Fig. 4 Permittivity of BST ceramics with various amounts of BaxSr1–xTiO3 at different temperatures
The effects of a certain amount of Bi2O3 and small addition of BaxSr1–xTiO3 on the microstructures and microwave dielectric properties of BST ceramics (x=2/3) were investigated. The additions were shown beneficial to the BST system. First, doping with Bi2O3 and BaxSr1–xTiO3 could decrease sintering temperature of BST; second, the addition of 1 wt.% BaxSr1–xTiO3 had obviously excellent effect on the Q·f values, at the same time, near zero f values could be achieved with the additions of Bi2O3 and BaxSr1–xTiO3. However, further addition of BaxSr1–xTiO3 led to a rapid decrease of the Q·f values and f. When the addition of Bi2O3 and BaxSr1–xTiO3 was 1 wt.% respectively, the sintering temperature was 1310 ºC, the ceramics exhibited excellent dielectric properties: r= 79.6, Q·f=10789 GHz, f=–1.5 ppm/ºC.
References: Fig. 5 Q·f values of BST ceramics with various amounts of BaxSr1–xTiO3 at different temperatures
to a rapid decrease. It is probable that, as the concentration of BaxSr1–xTiO3 increases, the lower solubility limit (x) of Ba6–3xSm8+2xTi18O54 (x=2/3) is reached and a second metatitanate phase is thought to form. However, the XRD shows no secondary phase. It is possible that the reflections form the secondary phase overlap by the tungsten bronze type reflections[9]. The variation in the f values of the specimens with various amounts of BaxSr1–xTiO3 at different sintering temperatures are shown in Fig. 6. It shows that the f values increase with the content of BaxSr1–xTiO3 increasing, the f can be
[1] Kolar D, Gaberscek S, Volavsek B, Parker H S, Roth R S. Synthesis and crystal chemistry of BaNd2Ti3O10, BaNd2Ti5O14, and Nd4Ti9O24. Journal of Solid State Chemistry, 1981, 38(2): 158. [2] Suvorov D, Valant M, Kolar D. The role of dopants in tailoring the microwave properties of Ba6–3xR8+2xTi18O54 R=(La-Gd) ceramics. Journal of Materials Science, 1997, 32: 6483. [3] Ohsato H. Science of tungsten bronze-type like Ba6–3xR8+2xTi18O54 (R=rare earth) microwave dielectric solid solutions. Journal of the European Ceramic Society, 2001, 21: 2703. [4] Ohsato H, Nishigaki S, Okudat T. Superlattice and dielectric properties of BaO-R2O3-TiO2 (R=La, Nd and Sm) microwave dielectric compounds. Japanese Journal of Applied Physics, 1992, 31: 3136. [5] Okawa T, Imaeda M, Ohsato H, Harada A. Site occupancy of Bi ions and microwave dielectric properties in Bi-doped
ZHOU Lingling et al., Microstructure and microwave dielectric properties of Ba6–3xSm8+2xTi18O54 ceramics with various … Ba63xR8+2xTi18O54 (R=rare earth, x=2/3) solid solutions. Materials Chemistry and Physics, 2003, 79: 199. [6] Lin B, Zhang Q T. Study on BaO-Sm2O3-TiO2 microwave dielectric ceramics doped with bismuth and zinc. Journal of Rare Earths, 2006, 24: 231. [7] Imaeda M, Mizuta M, Ohsato H. Microwave dielectric properties of Ba6–3xSm8+2xTi18O54 solid solutions with Sr substituted for Ba. Japanese Journal of Applied Physics, 1997, 36(9B): 6012. [8] Kagomiya I, Suzuki M, Kakimoto K, Ohsato H. Microwave dielectric properties of tungsten bronze type like (Ba1Sr)63xR8+2xTi18O54 (R=Sm, Nd) solid solutions. Journal of the European Ceramic Society, 2007, 27: 3059. [9] Zhu J H, Lu W Z, Lei W, Wan S. Effects of SrTiO3 additives on the structure and microwave dielectric properties of Ba4.2Sm9.2Ti18O54 ceramics. Ceramics International, 2009, 35:
145
855. [10] Zhang D, Button T W, Sherman V O, Tagantsev A K, Price T, Iddles D. Effects of glass additions on the microstructure and dielectric properties of barium strontium titanate (BST) ceramics. Journal of the European Ceramic Society, 2010, 30: 407. [11] Huang C L, Chen Y C. Microwave dielectric properties and microstructure of Ba2xSm4+2x/3Ti8+yO24+2y ceramics. Materials Science and Engineering: A, 2003, 345: 106. [12] Okawa T, Imaeda M, Ohsato H. Site occupancy of Bi ions and microwave dielectric properties in Ba63xNd8+2xTi18O54 solid solutions. Materials Science and Engineering, 2002, B88: 58. [13] Suzuki M, Ohsato H, Kakimoto K, Nagatomo T, Otagiri T. Crystal structure and microwave dielectric properties of (Ba1Sr)63xR8+2xTi18O54 solid solutions. Journal of the European Ceramic Society, 2006, 26: 2035.
Microstructure and microwave dielectric properties of Ba6–3xSm8+2xTi18O54 ceramics with various BaxSr1–xTiO3 additions ZHOU Lingling (਼⦆⦆), ZHOU Hongqing (਼⋾ᑚ), SHAO Hui (䚉 䕝), ZHU Haikui (ᴅ⍋༢) (College of Materials Science and Engineering, Nanjing University of Technology, Nanjing 210009, China) Received 14 July 2011; revised 22 August 2011
Abstract: The Ba6–3xSm8+2xTi18O54 (x=2/3) microwave dielectric ceramics were prepared by traditional solid sate reaction technique. The experiment was based on the Ba6–3xSm8+2xTi18O54 (BST) microwave dielectric ceramics doped with a certain amount of Bi2O3, then the effects of BaxSr1–xTiO3 additives on the structure and microwave dielectric properties of Ba6–3xSm8+2xTi18O54 ceramics were investigated using X-ray diffraction and scanning electron microscopy. In this study, the small amount substitution of Sr for Ba was effective for the microwave dielectric properties of BST, especially the f could be tuned to near zero. The result showed that the BST possessed excellent dielectric properties when the addition of Bi2O3 and BaxSr1–xTiO3 was 1 wt.% respectively: r=79.6, Q·f=10789 GHz, f=–1.5 ppm/ºC. Keywords: dielectric properties; microstructure; BaxSr1–xTiO3; rare earths
The combination of a high dielectric constant (r), low dielectric loss and near-zero temperature coefficient of resonant frequency makes BaO-Ln2O3-kTiO2 (where Ln=Sm, Nd, Pr, La are lanthanide elements with k=3–5) very important for the applications as microwave dielectric resonators and filters. Since the first report on the excellent microwave dielectric properties in this system, many studies concerning these have been investigated[1,2]. Ohsato[3] has studied the structural and dielectric properties of Ba6–3xSm8+2xTi18O54. The crystal structure of these ceramics belongs to the tungsten bronze structural family and consists of a three-dimensional frame-work of corner-sharing perovskite-like TiO6 and the special point with x=2/3 at which the Q·f value becomes the highest[3]. However, the high densification temperature around 1300–1460 ºC and the high-negative temperature coefficient of resonant frequency (f=–15 ppm/ºC)[4] are the disadvantages of this system, thus a lot of work have been done to reduce the high temperature as well as to tune the f to near zero. In previous studies, the addition of a small amount of Bi2O3 was found to reduce the sintering temperature[5], Li et al.[6] have studied the BaO-Sm2O3-TiO2 microwave dielectric ceramics doped with bismuth and zinc, they also found doping with Bi2O3 and ZnO can decrease sintering temperature of BST and the performances of the microwave dielectric ceramics are most excellent. Substitution of Sr2+ with Ba2+ [7,8] on the system have been found to be very helpful to tune the f. Zhu et al.[9] reported that near zero TCf values (–1.5 ppm/ºC) could be achieved with the addition of SrTiO3 to BST ceramics. In the past decades, (Ba, Sr)TiO3 has attracted much atten-
tion in the development of tunable microwave devices as it exhibits high dielectric permittivity and low dielectric loss with an applied DC electric field[10]. In the present study, BaxSr1–xTiO3 have been used as additives, effects of composition and sintering temperature on the characteristics of BST with microwave dielectric ceramics were investigated.
1 Experimental The BaxSr1–xTiO3 and Ba6–3xSm8+2xTi18O54 (x=2/3) samples were prepared by the general solid-state reaction method. Reagent grade BaTiO3, SrTiO3, BaCO3, Sm2O3, TiO2 and Bi2O3 were used as the raw materials. After weighing the raw powders as the stoichiometries of BaxSr1–xTiO3 (0.4x 0.6) and Ba6–3xSm8+2xTi18O54 (x=2/3), the mixtures were put into two polyethylene bottles respectively, milled with agate balls in distilled water for 24 h. After drying, calcination of the two kinds mixtures were conducted at 1100 ºC for 3 h in air, the phases of BaxSr1–xTiO3 and Ba6–3xSm8+2xTi18O54 (x=2/3) were obtained. 1 wt.% Bi2O3 and different contents of the BaxSr1–xTiO3 were added to the calcined BST. The mixed powders were then remilled for 12 h, mixed with 5 wt.% polyvinyl alcohol, pressed into pellets of 13 mm in diameter and 5–8 mm in thickness under a pressure of 100 MPa. The samples were sintered at 1250–1340 ºC for 3 h in air. The phases of these samples were determined by X-ray diffraction with Cu K radiation (XRD ARL XTRA Switzerland). The bulk densities were measured by Archimedes’ method. Polished and thermally etched surfaces of the samples were executed by scanning electron microscopy (SEM JSM-5900, Japan). The quality factor (Q·f) and the tempera-
Foundation item: Project supported by the Priority Academic Program Development of Jiangsu Higher Education Institutions Corresponding author: ZHOU Hongqing (E-mail: [email protected]; Tel.: +86-25-86639976) DOI: 10.1016/S1002-0721(12)60011-8
ZHOU Lingling et al., Microstructure and microwave dielectric properties of Ba6–3xSm8+2xTi18O54 ceramics with various …
ture coefficient of resonant frequency (f) were measured by Agilent 8722ET network analyzer.
2 Results and discussion The BST ceramics with Bi and BaxSr1–xTiO3 are identified as single phase solid solutions. Fig. 1 shows the X-ray diffraction patterns of Ba6–3xSm8+2xTi18O54 ceramics with various amounts of BaxSr1–xTiO3 at 1310 ºC. It is shown that the major phase is an orthorhombic tungsten-bronze structure of Ba6–3xSm8+2xTi18O54 (x=2/3) solid solution formula, assigned as BaSm2Ti4O12, and meanwhile another phase is BaTi4O9. Just as Zhu et al.[9] reportedˈno significant changes were found in the XRD patterns when 0–8 wt.% SrTiO3 were added to Ba4.2Sm9.2Ti18O54 ceramics. Fig. 2 is the SEM images at different sintering temperatures. Fig. 2(a) shows the small amount main crystals with lots of porosity around. As the sintering temperatures are above densified temperatures, long bar-shaped grains come to form, the size of the grain increases with the sintering temperatures increasing as shown in (b), (c) and (d). However, when the sintering temperature is above 1310 ºC, the long bar-shaped grain comes into contact, continual growth squeezes them away from each other[11], which results in the formation of new and larger vacant positions where the short
Fig. 1 XRD patterns of BST ceramics with various amounts of BaxSr1–xTiO3 at 1310 ºC for 3 h
143
grains are originally located as shown in (c) and (d). The bulk densities of the samples as a function of the different additions of BaxSr1–xTiO3 and sintered at temperatures from 1250 to 1340 ºC for 3 h are shown in Fig. 3. It can be seen that the densities increase with the increase addition of BaxSr1–xTiO3, the densities of the samples increase greatly with the increase of sintering temperature in the beginning. However, it decreased slightly as the temperature range from 1310 to 1340 ºC. The ceramics sintered at 1310 ºC have the highest densities. The formation of new and larger vacant causes expansion of the space between bar-shaped grains and results in the increase porosity and hence decreases the bulk density at 1340 ºC, which can also be seen in micrographs. Fig. 4 shows the variations in the r value of the BST+ 1 wt.% Bi2O3+(y)wt.% BaxSr1–xTiO3 ceramics with 0y7 sintered at various temperatures. It can be seen that the variation in the r value is the same as that of the density. Density is an important factor which influences the r, so the ceramics sintered at 1310 ºC have the highest densities and therefore exhibit the highest permittivity. Ohsato[3] reported that dielectric constant (r) of Ba63xR8+2xTi18O54 solid solutions was affected by TiO6 octahedral volume. The radius of Sr2+ are bigger than that of Sm3+, which results in getting crystal cell volume and the octahedron become bigger, the moving space of Ti4+ broadens, so the permittivity increase and with the increase of the BaxSr1–xTiO3, the effects on permittivity are increasing. Fig. 5 shows the variations in the Q·f values of the BST ceramics with different BaxSr1–xTiO3 contents sintered at various temperatures. The BST (x=2/3) only with oxide addition (1 wt.% Bi2O3) have virtually no effect on the Q·f values at different temperatures, a small amount of addition of BaxSr1–xTiO3 exhibits an increase in the Q·f values, The fundamental structural formula of Ba6–3xSm8+2xTi18O54 is [Ba23xSm8+2xVx]A1[Ba4]A2Ti18O54 (V:vacancy). Okawa et al.[12] reported the substitute of Bi for Ba in A2 sites, Suzuki et al.[13] investigated about the (Ba1Sr)63xSm8+2xTi18O54 solid solutions. They found that Sr ions occupied only the A1 sites. In this study, at first Ba ions in A1 sites are substituted by Sr ions. After all Ba ions in A1 sites are substituted, vacancies in A1 sites are filled by Sr ions. When all vacancies in A1 sites are filled, decreasing of inner strain and increasing of lattice stability by substitution make quality factor (Q·f ) increase significantly. Further addition of BaxSr1–xTiO3 leads
Fig. 2 SEM images of BST ceramics with 1 wt.% Bi2O3 and 1 wt.% BaxSr1–xTiO3 at different sintering temperatures (a) 1250 ºC; (b) 1280 ºC; (c) 1310 ºC; (d) 1340 ºC
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Fig. 3 Densities of BST ceramics with various amounts of BaxSr1–xTiO3 at different temperatures
JOURNAL OF RARE EARTHS, Vol. 30, No. 2, Feb. 2012
Fig. 6 f values of BST ceramics as a function of sintering temperatures and various amounts of BaxSr1–xTiO3
tuned to near zero when 5 wt.% BaxSr1–xTiO3 was added. Further addition of BaxSr1–xTiO3 makes the f value decrease greatly. The reason might be considered to be the formation of nanoscale secondary phases[5] with a large negative f created by excess Ba and Ti ions due to the substitution of Sr for Ba.
3 Conclusions
Fig. 4 Permittivity of BST ceramics with various amounts of BaxSr1–xTiO3 at different temperatures
The effects of a certain amount of Bi2O3 and small addition of BaxSr1–xTiO3 on the microstructures and microwave dielectric properties of BST ceramics (x=2/3) were investigated. The additions were shown beneficial to the BST system. First, doping with Bi2O3 and BaxSr1–xTiO3 could decrease sintering temperature of BST; second, the addition of 1 wt.% BaxSr1–xTiO3 had obviously excellent effect on the Q·f values, at the same time, near zero f values could be achieved with the additions of Bi2O3 and BaxSr1–xTiO3. However, further addition of BaxSr1–xTiO3 led to a rapid decrease of the Q·f values and f. When the addition of Bi2O3 and BaxSr1–xTiO3 was 1 wt.% respectively, the sintering temperature was 1310 ºC, the ceramics exhibited excellent dielectric properties: r= 79.6, Q·f=10789 GHz, f=–1.5 ppm/ºC.
References: Fig. 5 Q·f values of BST ceramics with various amounts of BaxSr1–xTiO3 at different temperatures
to a rapid decrease. It is probable that, as the concentration of BaxSr1–xTiO3 increases, the lower solubility limit (x) of Ba6–3xSm8+2xTi18O54 (x=2/3) is reached and a second metatitanate phase is thought to form. However, the XRD shows no secondary phase. It is possible that the reflections form the secondary phase overlap by the tungsten bronze type reflections[9]. The variation in the f values of the specimens with various amounts of BaxSr1–xTiO3 at different sintering temperatures are shown in Fig. 6. It shows that the f values increase with the content of BaxSr1–xTiO3 increasing, the f can be
[1] Kolar D, Gaberscek S, Volavsek B, Parker H S, Roth R S. Synthesis and crystal chemistry of BaNd2Ti3O10, BaNd2Ti5O14, and Nd4Ti9O24. Journal of Solid State Chemistry, 1981, 38(2): 158. [2] Suvorov D, Valant M, Kolar D. The role of dopants in tailoring the microwave properties of Ba6–3xR8+2xTi18O54 R=(La-Gd) ceramics. Journal of Materials Science, 1997, 32: 6483. [3] Ohsato H. Science of tungsten bronze-type like Ba6–3xR8+2xTi18O54 (R=rare earth) microwave dielectric solid solutions. Journal of the European Ceramic Society, 2001, 21: 2703. [4] Ohsato H, Nishigaki S, Okudat T. Superlattice and dielectric properties of BaO-R2O3-TiO2 (R=La, Nd and Sm) microwave dielectric compounds. Japanese Journal of Applied Physics, 1992, 31: 3136. [5] Okawa T, Imaeda M, Ohsato H, Harada A. Site occupancy of Bi ions and microwave dielectric properties in Bi-doped
ZHOU Lingling et al., Microstructure and microwave dielectric properties of Ba6–3xSm8+2xTi18O54 ceramics with various … Ba63xR8+2xTi18O54 (R=rare earth, x=2/3) solid solutions. Materials Chemistry and Physics, 2003, 79: 199. [6] Lin B, Zhang Q T. Study on BaO-Sm2O3-TiO2 microwave dielectric ceramics doped with bismuth and zinc. Journal of Rare Earths, 2006, 24: 231. [7] Imaeda M, Mizuta M, Ohsato H. Microwave dielectric properties of Ba6–3xSm8+2xTi18O54 solid solutions with Sr substituted for Ba. Japanese Journal of Applied Physics, 1997, 36(9B): 6012. [8] Kagomiya I, Suzuki M, Kakimoto K, Ohsato H. Microwave dielectric properties of tungsten bronze type like (Ba1Sr)63xR8+2xTi18O54 (R=Sm, Nd) solid solutions. Journal of the European Ceramic Society, 2007, 27: 3059. [9] Zhu J H, Lu W Z, Lei W, Wan S. Effects of SrTiO3 additives on the structure and microwave dielectric properties of Ba4.2Sm9.2Ti18O54 ceramics. Ceramics International, 2009, 35:
145
855. [10] Zhang D, Button T W, Sherman V O, Tagantsev A K, Price T, Iddles D. Effects of glass additions on the microstructure and dielectric properties of barium strontium titanate (BST) ceramics. Journal of the European Ceramic Society, 2010, 30: 407. [11] Huang C L, Chen Y C. Microwave dielectric properties and microstructure of Ba2xSm4+2x/3Ti8+yO24+2y ceramics. Materials Science and Engineering: A, 2003, 345: 106. [12] Okawa T, Imaeda M, Ohsato H. Site occupancy of Bi ions and microwave dielectric properties in Ba63xNd8+2xTi18O54 solid solutions. Materials Science and Engineering, 2002, B88: 58. [13] Suzuki M, Ohsato H, Kakimoto K, Nagatomo T, Otagiri T. Crystal structure and microwave dielectric properties of (Ba1Sr)63xR8+2xTi18O54 solid solutions. Journal of the European Ceramic Society, 2006, 26: 2035.