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Microwave synthesis of high dielectric constant CaCu3Ti4O12
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 8 ( 2 0 0 8 ) 145–148
journal homepage: www.elsevier.com/locate/jmatprotec
Microwave synthesis of high dielectric constant CaCu3 Ti4 O12 Hongtao Yu, Hanxing Liu ∗ , Dabing Luo, Minghe Cao State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Luoshi road 122#, Wuhan 430070, PR China
a r t i c l e
i n f o
a b s t r a c t
Article history:
In this work, single-phase cubic perovskite CaCu3 Ti4 O12 powders have been synthesized
Received 8 June 2007
successfully by microwave heating with a relatively low energy consumption and short
Received in revised form
time, compared with conventional synthesis. The reaction evolution of forming CCTO has
16 October 2007
been suggested according to the XRD data. The ceramics from microwave-synthesized pow-
Accepted 24 December 2007
der was found to have higher dielectric constant than that from conventional synthesized powder under the same sintering conditions. © 2008 Elsevier B.V. All rights reserved.
Keywords: CaCu3 Ti4 O12 Dielectrics Perovskite Microwave synthesis
1.
Introduction
The drive for ultra-miniaturization of electronic devices in automobiles and aircrafts requires the development of high dielectric materials that are stable over a wide range of temperatures. Recently, a ABO3 -type perovskite material, CaCu3 Ti4 O12 (CCTO) has attracted extensive attentions because it exhibits an extraordinarily high dielectric constant at room temperature of about 104 to 105 and good temperature stability in a wide temperature range from 100 to 600 K (Subramanian et al., 2000; Ramirez et al., 2000). In general, the conventional ceramic (solid-state) processing is used as synthesis of this material. However, this synthesis technique usually needs long reaction time at elevated temperature (Subramanian et al., 2000; Choudhary and Bhunia, 2002; Bender and Pan, 2005). There are some cases reported after heating for several days at temperatures up to 1000 ◦ C with some intermittent regrinding stages for obtaining singlephase CCTO powder (Choudhary and Bhunia, 2002). To date, researchers have studied other methods to synthesis CCTO
∗
Corresponding author. E-mail address: [email protected] (H. Liu). 0924-0136/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2007.12.104
powder, such as mechanical alloying, polymeric citrate precursor route and so on (Almedia et al., 2002; Jha et al., 2003), but no report on microwave synthesis of CCTO. Microwave heating has been suggested as a means of reducing the time of preparation and the energy consumption (Baghurst et al., 1988) as its effect originates from the natural ability of certain substances to efficiently absorb, and then to transform electromagnetic (radiation) energy into heat. If sufficient heat can be generated at a local level, chemical reactions may be initiated at a very rapid rate. This novel method has been found to result in better reaction yields and superior structural uniformity of products as compared with conventional ceramic methods. Furthermore, the microwave method is very clean and non-polluting and results in better reaction yields. In past 10 years, some important functional ceramics, such as LiCoO2 , YBa2 Cu3 O7 , Ba(Mg1/3 Ta2/3 )O3 , and La2−x Srx CuO4 had been synthesized using microwave dielectric heating with a lower consumption of time and energy (Subramanian et al., 2001; Agostino et al., 2004; Vaidhyanathan et al., 2000; Gibbons et al., 2000). In present work, we have reported on the reaction
146
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 8 ( 2 0 0 8 ) 145–148
Fig. 1 – Steps of synthesis of CCTO by microwave dielectric heating.
evolution of the Ca, Cu, and Ti oxides mixture to synthesis the giant dielectric constant CCTO by microwave heating, and investigated the dielectric properties of the sintered ceramics.
2.
Experimental details
The starting materials for synthesis of CCTO powder were highly pure CaCO3 , CuO and anatase-TiO2 (99%). The stoichiometric amount of precursor powders was mixed and grinded in a ball mill for 24 h with distilled water. The dried and mixed precursor powders were pressed into pellets (Ø12 mm × 2 mm). Some of these pellets were calcined by microwave and others were calcined at 1000 ◦ C in a conventional electric furnace for comparison. For microwave heating, the pellets were placed into an alumina crucible surrounded by SiC powder, which readily absorbs microwave energy and then causes a rapid increase of the sample temperature. This system was then introduced into a commercial multi-mode, 1600 W microwave oven (NJZ4-3, China) operating at 2.45 GHz. This oven is equipped with a radiation thermometer and an automatic time controller. The samples were heated for periods of 30 min. After each heating step, the samples were allowed to cool for about 15 min and an aliquot was taken for X-ray analysis (PANalytical Xpert-PRO) till the single phase formed. The steps for caclining CCTO by microwave dielectric heating were shown in Fig. 1. After the desired single phase was confirmed by X-ray analysis, CCTO powder synthesized by microwave synthesis (MS) and conventional synthesis (CS) was ground and milled with 2.5% polyvinyl alcohol (PVA) solution as binder for 24 h, respectively. These two kinds of powder were pressed into circular pellets of 12-mm diameter and about 1 mm thickness under a pressure of 160 MPa. After de-binded, these pellets were sintered at 1100 ◦ C for 3 h in an electric furnace in air. The crystalline phases of sintered ceramics were investigated by powder X-ray diffraction and the microstructure was observed by a scanning electron microscope (SEM, Akashi Seisakusho Jsm-5610LV). The relative densities of sintered pellets were measured by the Archimedes method. The room temperature dielectric properties were collected by using a parallel plate capacitor arrangement with a Tonghui 2818 Auto Component Analyzer.
Fig. 2 – XRD patterns of CCTO powder calcined at different temperature for 30 min by microwave heating: (a) 750 ◦ C; (b) first, 800 ◦ C; (c) second, 800 ◦ C; (d) third, 800 ◦ C.
3.
Results and Discussion
Fig. 2 shows the XRD patterns of CCTO powder calcined by microwave treatment according to the experimental steps in Fig. 1. After heated at the temperature of 750 ◦ C for 30 min, the XRD data illustrates that there are four products, CaO, CaTiO3 (CT), rutile-TiO2 and CCTO beside two starting materials, CuO and anatase-TiO2 in the powders, as shown in Fig. 2a. So the reactions occurring during this first step of microwave heating can be summarized as: CaCO3 + 3TiO2 + 3CuO → CaCu3 Ti4 O12 + CO2 ↑
(1)
CaCO3 → CaO + CO2 ↑
(2)
CaCO3 + TiO2 → CaTiO3 + CO2 ↑
(3)
and a phase transition from anatase-TiO2 to rutile-TiO2 .
Fig. 3 – XRD patterns of CCTO calcined (a) first at 1000 ◦ C for 24 h, (b) second at 1000 ◦ C for 24 h, and (c) third at 1000 ◦ C for 24 h by conventional solid-state reaction synthesis.
147
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 8 ( 2 0 0 8 ) 145–148
Table 1 – Phase composition of CCTO oxide powders during every step of microwave heating Heating conditions
Phase composition
750 ◦ C, 30 min 800 ◦ C, 30 min (first) 800 ◦ C, 30 min (second) 800 ◦ C, 30 min (third)
CCTO + CT + CaO + CuO + TiO2 b CCTO + CT + CaO + CuO + rutile CCTO + CT + CuO + rutile CCTO
a b
Intensity ratioa (CCTO/CT) 10.8 58.8 108.7 –
Intensity ratioa (CCTO/CaO)
Intensity ratioa (CCTO/CuO)
2.2 47.4 – –
1.1 27.9 208.3 –
Estimated from the relative ratio of the maximal peak intensity of the two phases. TiO2 means the sum of anatase and rutile.
The samples were then cooled for 15 min followed by treatment at 800 ◦ C for 30 min, the contents of samples are still CuO, rutile-TiO2 , CaO, CT and CCTO, respectively. But the relative amount of CCTO increases, which can be confirmed by the increased relative intensity of X-ray peaks of CCTO in Fig. 2b. After the second calcination at 800 ◦ C for 30 min, the main phase was CCTO in the oxide powders. Only a slight amount of other phases such as CT, CuO and rutile-TiO2 can be observed, as shown in Fig. 2c. Uncorrected X-ray integrated peak intensities show that the relative amount sum of these
phases is about 2 wt%. For obtaining single phase, above powders were calcined at 800 ◦ C for 30 min again. Fig. 2d shows the X-ray pattern of CCTO oxides powders after the third treatment at 800 ◦ C for 30 min by microwave. It illustrates that a single phase of CCTO is formed whose diffraction peaks are quite sharp indicating that the samples are well crystallized. In contrast, conventional solid reaction synthesis takes higher temperature and longer time to form single-phase CCTO. Fig. 3 shows the XRD patterns of CCTO synthesized by conventional synthesis in this work. The starting mate-
Fig. 4 – Dielectric properties (A) and SEM micrograph (B) of sintered pellets from MS and CS powder.
148
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 8 ( 2 0 0 8 ) 145–148
rials were calcined at 1000 ◦ C for 24 h, there were still other phases, CuO, rutile-TiO2 and CaTiO3 , except for CCTO in the samples as shown in Fig. 3a. Fig. 3b is the X-ray pattern of the second calcinations indicating that slight CaTiO3 and CuO still presence. The single phase of CCTO could be obtained, after the third processes of milling and calination as shown in Fig. 3c. As discussed above, the total microwave heating time is about 2 h, which is much shorter than that used in the conventional synthesis with a relatively low temperature (Choudhary and Bhunia, 2002; Bender and Pan, 2005). Table 1 shows the details of phase composition and the estimated intensity ratio of CCTO to other phases during the each heating step. The obtained single-phase powders exhibit centro-body perovskite structure with Im3 space group as determined from Rietiveld refine method. The estimated parameter is ˚ in agreement well with the data reported by lita = 7.3937(1) A, eratures Subramanian et al. (2000) and Choudhary and Bhunia (2002). The dielectric properties of ceramics from two kinds of powders were investigated too. After sintering at 1100 ◦ C for 3 h, the relative dielectric constant from MS powder (∼21400, at 1 KHz) is higher than that from CS powder (∼10240, at 1 KHz) at room temperature as shown in Fig. 4A. This can be attributed to that the ceramics from MS powder have larger grains than that from CS powder in Fig. 4B, based on the internal barrier layer capacitor model (Prakash and Varma, 2006; Fang and Shiau, 2004). And the relatively high dielectric loss in the samples from MS powders can be attributed to the porosity that not be observed in the samples from CS powders, as shown in Fig. 4B. It can be suggested that this difference in the microstructures arises from the different sintering abilities of two kinds of precursor powders, which should be confirmed by more detailed works.
4.
Conclusions
In this work, the use of microwave resulted in the formation of single-phase CCTO in just 2 h and at a lower temperature than required for conventional synthesis. The reactions during the formation of CCTO are suggested from the X-ray analysis. The sintered pellets from microwave synthesis powders possess a higher dielectric constant than that from conventional ceramic processing. Further studies should be considered for commercial applications of CCTO.
Acknowledgements This work is supported by 973-Project under 2002 CB613303, NSFC under 50472016 and Foundation for innovation of research team of Hubei province (grant number 2005ABC004).
references
Agostino, A., Benzi, P., Castiglioni, M., Rizzi, N., Volpe, P., 2004. YBa2 Cu3 O7 synthesis using microwave heating. Supercond. Sci. Technol. 17, 685–688. Almedia, A.F.L., De Oliveira, R.S., Goes, J.C., Sasaki, J.M., Souza Filho, A.G., Mendes Filho, J., Sombra, A.S.B., 2002. Structural properties of CaCu3 Ti4 O12 obtained by mechanical alloying. Mater. Sci. Eng. B 96, 275–283. Baghurst, D.R., Chippindale, A.R., Mingos, D.M.P., 1988. Microwave synthesis for superconducting ceramics. Nature 332, 311–314. Bender, B.A., Pan, M.J., 2005. The effect of processing on the giant dielectric properties of CaCu3 Ti4 O12 . Mater. Sci. Eng. B 117, 339–347. Choudhary, R.N.P., Bhunia, U., 2002. Structural, dielectric and electrical properties of ACu3 Ti4 O12 (A = Ca, Sr and Ba). J. Mater. Sci. 37, 5177–5182. Fang, T.T., Shiau, H.K., 2004. Mechanism for developing the boundary barrier layers of CaCu3 Ti4 O12 . J. Am. Ceram. Soc. 87 (11), 2072–2079. Gibbons, K.E., Jones, M.O., Blundell, S.J., Mihut, A.I., Gameson, I., Edwards, P.P., Miyazaki, Y., Hyatt, N.C., Porch, A., 2000. Rapid synthesis of colossal magnetoresistance manganites by microwave dielectric heating. Chem. Commun., 159–160. Jha, P., Arora, P., Ganguli, A.K., 2003. Polymeric citrate precursor route to the synthesis of the high dielectric constant oxide, CaCu3 Ti4 O12 . Mater. Lett. 57, 2443–2446. Prakash, B.S., Varma, K.B.R., 2006. Effect of sintering conditions on the dielectric properties of CaCu3 Ti4 O12 and La2/3 Cu3 Ti4 O12 ceramics: a comparative study. Physics B 382, 312–319. Ramirez, A.P., Subramanian, M.A., Garbel, M., Blumberg, G., Li, D., Vogt, T., Shapiro, S.M., 2000. Giant dielectric constant response in copper-titanate. Solid State Commun. 115, 217–222. Subramanian, M.A., Dong, L., Resiner, B.A., Sleight, A.W., 2000. High dielectric constant in ACu3 Ti4 O12 phases. J. Solid State Chem. 151, 323–325. Subramanian, V., Chen, C.L., Chou, H.S., Fey, G.T.K., 2001. Microwave-assisted solid-state synthesis of LiCoO2 and its electrochemical properties as a cathode material for lithium batteries. J. Mater. Chem. 11, 3348–3353. Vaidhyanathan, B., Agrawal, D.K., Shrout, T.R., Fang, Yi, 2000. Microwave synthesis and sintering of Ba(Mg1/3 Ta2/3 )O3 . Mater. Lett. 42, 207–211.
journal homepage: www.elsevier.com/locate/jmatprotec
Microwave synthesis of high dielectric constant CaCu3 Ti4 O12 Hongtao Yu, Hanxing Liu ∗ , Dabing Luo, Minghe Cao State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Luoshi road 122#, Wuhan 430070, PR China
a r t i c l e
i n f o
a b s t r a c t
Article history:
In this work, single-phase cubic perovskite CaCu3 Ti4 O12 powders have been synthesized
Received 8 June 2007
successfully by microwave heating with a relatively low energy consumption and short
Received in revised form
time, compared with conventional synthesis. The reaction evolution of forming CCTO has
16 October 2007
been suggested according to the XRD data. The ceramics from microwave-synthesized pow-
Accepted 24 December 2007
der was found to have higher dielectric constant than that from conventional synthesized powder under the same sintering conditions. © 2008 Elsevier B.V. All rights reserved.
Keywords: CaCu3 Ti4 O12 Dielectrics Perovskite Microwave synthesis
1.
Introduction
The drive for ultra-miniaturization of electronic devices in automobiles and aircrafts requires the development of high dielectric materials that are stable over a wide range of temperatures. Recently, a ABO3 -type perovskite material, CaCu3 Ti4 O12 (CCTO) has attracted extensive attentions because it exhibits an extraordinarily high dielectric constant at room temperature of about 104 to 105 and good temperature stability in a wide temperature range from 100 to 600 K (Subramanian et al., 2000; Ramirez et al., 2000). In general, the conventional ceramic (solid-state) processing is used as synthesis of this material. However, this synthesis technique usually needs long reaction time at elevated temperature (Subramanian et al., 2000; Choudhary and Bhunia, 2002; Bender and Pan, 2005). There are some cases reported after heating for several days at temperatures up to 1000 ◦ C with some intermittent regrinding stages for obtaining singlephase CCTO powder (Choudhary and Bhunia, 2002). To date, researchers have studied other methods to synthesis CCTO
∗
Corresponding author. E-mail address: [email protected] (H. Liu). 0924-0136/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2007.12.104
powder, such as mechanical alloying, polymeric citrate precursor route and so on (Almedia et al., 2002; Jha et al., 2003), but no report on microwave synthesis of CCTO. Microwave heating has been suggested as a means of reducing the time of preparation and the energy consumption (Baghurst et al., 1988) as its effect originates from the natural ability of certain substances to efficiently absorb, and then to transform electromagnetic (radiation) energy into heat. If sufficient heat can be generated at a local level, chemical reactions may be initiated at a very rapid rate. This novel method has been found to result in better reaction yields and superior structural uniformity of products as compared with conventional ceramic methods. Furthermore, the microwave method is very clean and non-polluting and results in better reaction yields. In past 10 years, some important functional ceramics, such as LiCoO2 , YBa2 Cu3 O7 , Ba(Mg1/3 Ta2/3 )O3 , and La2−x Srx CuO4 had been synthesized using microwave dielectric heating with a lower consumption of time and energy (Subramanian et al., 2001; Agostino et al., 2004; Vaidhyanathan et al., 2000; Gibbons et al., 2000). In present work, we have reported on the reaction
146
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 8 ( 2 0 0 8 ) 145–148
Fig. 1 – Steps of synthesis of CCTO by microwave dielectric heating.
evolution of the Ca, Cu, and Ti oxides mixture to synthesis the giant dielectric constant CCTO by microwave heating, and investigated the dielectric properties of the sintered ceramics.
2.
Experimental details
The starting materials for synthesis of CCTO powder were highly pure CaCO3 , CuO and anatase-TiO2 (99%). The stoichiometric amount of precursor powders was mixed and grinded in a ball mill for 24 h with distilled water. The dried and mixed precursor powders were pressed into pellets (Ø12 mm × 2 mm). Some of these pellets were calcined by microwave and others were calcined at 1000 ◦ C in a conventional electric furnace for comparison. For microwave heating, the pellets were placed into an alumina crucible surrounded by SiC powder, which readily absorbs microwave energy and then causes a rapid increase of the sample temperature. This system was then introduced into a commercial multi-mode, 1600 W microwave oven (NJZ4-3, China) operating at 2.45 GHz. This oven is equipped with a radiation thermometer and an automatic time controller. The samples were heated for periods of 30 min. After each heating step, the samples were allowed to cool for about 15 min and an aliquot was taken for X-ray analysis (PANalytical Xpert-PRO) till the single phase formed. The steps for caclining CCTO by microwave dielectric heating were shown in Fig. 1. After the desired single phase was confirmed by X-ray analysis, CCTO powder synthesized by microwave synthesis (MS) and conventional synthesis (CS) was ground and milled with 2.5% polyvinyl alcohol (PVA) solution as binder for 24 h, respectively. These two kinds of powder were pressed into circular pellets of 12-mm diameter and about 1 mm thickness under a pressure of 160 MPa. After de-binded, these pellets were sintered at 1100 ◦ C for 3 h in an electric furnace in air. The crystalline phases of sintered ceramics were investigated by powder X-ray diffraction and the microstructure was observed by a scanning electron microscope (SEM, Akashi Seisakusho Jsm-5610LV). The relative densities of sintered pellets were measured by the Archimedes method. The room temperature dielectric properties were collected by using a parallel plate capacitor arrangement with a Tonghui 2818 Auto Component Analyzer.
Fig. 2 – XRD patterns of CCTO powder calcined at different temperature for 30 min by microwave heating: (a) 750 ◦ C; (b) first, 800 ◦ C; (c) second, 800 ◦ C; (d) third, 800 ◦ C.
3.
Results and Discussion
Fig. 2 shows the XRD patterns of CCTO powder calcined by microwave treatment according to the experimental steps in Fig. 1. After heated at the temperature of 750 ◦ C for 30 min, the XRD data illustrates that there are four products, CaO, CaTiO3 (CT), rutile-TiO2 and CCTO beside two starting materials, CuO and anatase-TiO2 in the powders, as shown in Fig. 2a. So the reactions occurring during this first step of microwave heating can be summarized as: CaCO3 + 3TiO2 + 3CuO → CaCu3 Ti4 O12 + CO2 ↑
(1)
CaCO3 → CaO + CO2 ↑
(2)
CaCO3 + TiO2 → CaTiO3 + CO2 ↑
(3)
and a phase transition from anatase-TiO2 to rutile-TiO2 .
Fig. 3 – XRD patterns of CCTO calcined (a) first at 1000 ◦ C for 24 h, (b) second at 1000 ◦ C for 24 h, and (c) third at 1000 ◦ C for 24 h by conventional solid-state reaction synthesis.
147
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 8 ( 2 0 0 8 ) 145–148
Table 1 – Phase composition of CCTO oxide powders during every step of microwave heating Heating conditions
Phase composition
750 ◦ C, 30 min 800 ◦ C, 30 min (first) 800 ◦ C, 30 min (second) 800 ◦ C, 30 min (third)
CCTO + CT + CaO + CuO + TiO2 b CCTO + CT + CaO + CuO + rutile CCTO + CT + CuO + rutile CCTO
a b
Intensity ratioa (CCTO/CT) 10.8 58.8 108.7 –
Intensity ratioa (CCTO/CaO)
Intensity ratioa (CCTO/CuO)
2.2 47.4 – –
1.1 27.9 208.3 –
Estimated from the relative ratio of the maximal peak intensity of the two phases. TiO2 means the sum of anatase and rutile.
The samples were then cooled for 15 min followed by treatment at 800 ◦ C for 30 min, the contents of samples are still CuO, rutile-TiO2 , CaO, CT and CCTO, respectively. But the relative amount of CCTO increases, which can be confirmed by the increased relative intensity of X-ray peaks of CCTO in Fig. 2b. After the second calcination at 800 ◦ C for 30 min, the main phase was CCTO in the oxide powders. Only a slight amount of other phases such as CT, CuO and rutile-TiO2 can be observed, as shown in Fig. 2c. Uncorrected X-ray integrated peak intensities show that the relative amount sum of these
phases is about 2 wt%. For obtaining single phase, above powders were calcined at 800 ◦ C for 30 min again. Fig. 2d shows the X-ray pattern of CCTO oxides powders after the third treatment at 800 ◦ C for 30 min by microwave. It illustrates that a single phase of CCTO is formed whose diffraction peaks are quite sharp indicating that the samples are well crystallized. In contrast, conventional solid reaction synthesis takes higher temperature and longer time to form single-phase CCTO. Fig. 3 shows the XRD patterns of CCTO synthesized by conventional synthesis in this work. The starting mate-
Fig. 4 – Dielectric properties (A) and SEM micrograph (B) of sintered pellets from MS and CS powder.
148
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 8 ( 2 0 0 8 ) 145–148
rials were calcined at 1000 ◦ C for 24 h, there were still other phases, CuO, rutile-TiO2 and CaTiO3 , except for CCTO in the samples as shown in Fig. 3a. Fig. 3b is the X-ray pattern of the second calcinations indicating that slight CaTiO3 and CuO still presence. The single phase of CCTO could be obtained, after the third processes of milling and calination as shown in Fig. 3c. As discussed above, the total microwave heating time is about 2 h, which is much shorter than that used in the conventional synthesis with a relatively low temperature (Choudhary and Bhunia, 2002; Bender and Pan, 2005). Table 1 shows the details of phase composition and the estimated intensity ratio of CCTO to other phases during the each heating step. The obtained single-phase powders exhibit centro-body perovskite structure with Im3 space group as determined from Rietiveld refine method. The estimated parameter is ˚ in agreement well with the data reported by lita = 7.3937(1) A, eratures Subramanian et al. (2000) and Choudhary and Bhunia (2002). The dielectric properties of ceramics from two kinds of powders were investigated too. After sintering at 1100 ◦ C for 3 h, the relative dielectric constant from MS powder (∼21400, at 1 KHz) is higher than that from CS powder (∼10240, at 1 KHz) at room temperature as shown in Fig. 4A. This can be attributed to that the ceramics from MS powder have larger grains than that from CS powder in Fig. 4B, based on the internal barrier layer capacitor model (Prakash and Varma, 2006; Fang and Shiau, 2004). And the relatively high dielectric loss in the samples from MS powders can be attributed to the porosity that not be observed in the samples from CS powders, as shown in Fig. 4B. It can be suggested that this difference in the microstructures arises from the different sintering abilities of two kinds of precursor powders, which should be confirmed by more detailed works.
4.
Conclusions
In this work, the use of microwave resulted in the formation of single-phase CCTO in just 2 h and at a lower temperature than required for conventional synthesis. The reactions during the formation of CCTO are suggested from the X-ray analysis. The sintered pellets from microwave synthesis powders possess a higher dielectric constant than that from conventional ceramic processing. Further studies should be considered for commercial applications of CCTO.
Acknowledgements This work is supported by 973-Project under 2002 CB613303, NSFC under 50472016 and Foundation for innovation of research team of Hubei province (grant number 2005ABC004).
references
Agostino, A., Benzi, P., Castiglioni, M., Rizzi, N., Volpe, P., 2004. YBa2 Cu3 O7 synthesis using microwave heating. Supercond. Sci. Technol. 17, 685–688. Almedia, A.F.L., De Oliveira, R.S., Goes, J.C., Sasaki, J.M., Souza Filho, A.G., Mendes Filho, J., Sombra, A.S.B., 2002. Structural properties of CaCu3 Ti4 O12 obtained by mechanical alloying. Mater. Sci. Eng. B 96, 275–283. Baghurst, D.R., Chippindale, A.R., Mingos, D.M.P., 1988. Microwave synthesis for superconducting ceramics. Nature 332, 311–314. Bender, B.A., Pan, M.J., 2005. The effect of processing on the giant dielectric properties of CaCu3 Ti4 O12 . Mater. Sci. Eng. B 117, 339–347. Choudhary, R.N.P., Bhunia, U., 2002. Structural, dielectric and electrical properties of ACu3 Ti4 O12 (A = Ca, Sr and Ba). J. Mater. Sci. 37, 5177–5182. Fang, T.T., Shiau, H.K., 2004. Mechanism for developing the boundary barrier layers of CaCu3 Ti4 O12 . J. Am. Ceram. Soc. 87 (11), 2072–2079. Gibbons, K.E., Jones, M.O., Blundell, S.J., Mihut, A.I., Gameson, I., Edwards, P.P., Miyazaki, Y., Hyatt, N.C., Porch, A., 2000. Rapid synthesis of colossal magnetoresistance manganites by microwave dielectric heating. Chem. Commun., 159–160. Jha, P., Arora, P., Ganguli, A.K., 2003. Polymeric citrate precursor route to the synthesis of the high dielectric constant oxide, CaCu3 Ti4 O12 . Mater. Lett. 57, 2443–2446. Prakash, B.S., Varma, K.B.R., 2006. Effect of sintering conditions on the dielectric properties of CaCu3 Ti4 O12 and La2/3 Cu3 Ti4 O12 ceramics: a comparative study. Physics B 382, 312–319. Ramirez, A.P., Subramanian, M.A., Garbel, M., Blumberg, G., Li, D., Vogt, T., Shapiro, S.M., 2000. Giant dielectric constant response in copper-titanate. Solid State Commun. 115, 217–222. Subramanian, M.A., Dong, L., Resiner, B.A., Sleight, A.W., 2000. High dielectric constant in ACu3 Ti4 O12 phases. J. Solid State Chem. 151, 323–325. Subramanian, V., Chen, C.L., Chou, H.S., Fey, G.T.K., 2001. Microwave-assisted solid-state synthesis of LiCoO2 and its electrochemical properties as a cathode material for lithium batteries. J. Mater. Chem. 11, 3348–3353. Vaidhyanathan, B., Agrawal, D.K., Shrout, T.R., Fang, Yi, 2000. Microwave synthesis and sintering of Ba(Mg1/3 Ta2/3 )O3 . Mater. Lett. 42, 207–211.