- Email: [email protected]
Low-toxic gelcasting of giant dielectric-constant CaCu3Ti4O12 ceramics from the molten salt powder
G Model
ARTICLE IN PRESS
JECS-10149; No. of Pages 6
Journal of the European Ceramic Society xxx (2015) xxx–xxx
Contents lists available at www.sciencedirect.com
Journal of the European Ceramic Society journal homepage: www.elsevier.com/locate/jeurceramsoc
Low-toxic gelcasting of giant dielectric-constant CaCu3 Ti4 O12 ceramics from the molten salt powder Wei Wan a,b , Changkun Liu a , Hongyuan Sun a , Zhongkuan Luo a , Wen-Xiang Yuan a,∗ , Huisi Wu c,∗ , Tai Qiu b a Shenzhen Key Laboratory of New Lithium-ion Battery and Mesoporous Materials, College of Chemistry and Chemical Engineering, Shenzhen University, Shenzhen 518060, PR China b College of Materials Science and Engineering, Nanjing Tech University, Nanjing 210009, PR China c College of Computer Science and Software Engineering, Shenzhen University, Shenzhen 518060, PR China
a r t i c l e
i n f o
Article history: Received 11 February 2015 Received in revised form 25 May 2015 Accepted 29 May 2015 Available online xxx Keywords: CaCu3 Ti4 O12 Molten salt method NaCl Low-toxic gelcasting Dielectric properties
a b s t r a c t CaCu3 Ti4 O12 (CCTO) nano powder was synthesized using a molten salt synthesis method in NaCl flux. Synthesis temperature and holding time were investigated. The suitable synthesis condition is 800 ◦ C for 2 h. Aqueous CCTO slurry with high solid loading and low viscosity was prepared by using acrylic acid2-acrylamido-2-methypropane sulfonic acid copolymer (AA/AMPS) as the dispersant. AA/AMPS dosage and pH condition have been optimized as AA/AMPS dosage of 3 wt% and pH about 9.08. A low-toxicity and water-soluble monomer, N,N-dimethylacrylamide (DMAA) was used as the gelling agent. CCTO green body fabricated by the gelcasting method has the homogeneous microstructure and relatively high mechanical strength of 9.27 MPa. CCTO ceramics obtained by the gelcasting method have higher dielectric constant than those prepared by the cold isostatic pressing method and show relatively low dielectric loss of below 0.2 in the wide frequency range of 102 –105 Hz. © 2015 Elsevier Ltd. All rights reserved.
1. Introduction Recently, the perovskite-type ternary oxide compound CaCu3 Ti4 O12 (CCTO) has drawn much interest for its potential microelectronic applications including capacitors and memory devices due to its giant dielectric constant of ∼105 over the wide frequency and temperature ranges [1–7]. CCTO powder is usually prepared by the traditional solid-state reaction from metal oxides with a few intermediate grinding/heating cycles [8,9]. However, this method is tedious with relatively long reaction time and high temperature (typically at 1000 ◦ C for 10–20 h), and might still generate unwanted phases owing to the limited atomic diffusion. Other synthesis techniques such as mechanochemical synthesis, sol–gel, wet chemical method, microwave heating, mechanical alloying and polymer pyrolysis have also been reported recently, which are found to have considerable influence on dielectric properties of CCTO ceramics [10–15]. In this work, CCTO powder was synthesized by a molten salt synthesis (MSS) method using NaCl as the flux. As a low-temperature
∗ Corresponding authors. Tel.: +86 755 26536627; fax: +86 755 26536141. E-mail addresses: [email protected] (W.-X. Yuan), [email protected] (H. Wu).
synthesis technique, MSS has been applied to synthesize many different kinds of powders, such as LaAlO3 , MgAl2 O4 , SrBi4 Ti4 O15 , Na2 Ti6 O13 , CCTO, and so on [16–19]. CCTO powder has been synthesized in the molten salt systems of KCl, NaCl–KCl or Na2 SO4 –K2 SO4 [20–22]. However, single NaCl salt system has not been tried yet. So we will try to synthesize CCTO powders in NaCl flux in this experiment. CCTO ceramics are conventionally prepared by the cold isostatic pressing method followed by pressureless sintering. Recently, hotpress sintering has also been used in preparing CCTO ceramics [23,24]. Gelcasting, an advanced ceramic process, is developed by Janney et al. at Oak Ridge National Laboratory (ORNL, USA) to make complex-shaped parts. Gelcasting is a colloidal processing method, and has established itself for its simplicity and its ability to produce a high degree of homogeneity as well as green body strength, resulting in good machinability [25,26]. It has been successfully used to prepare many kinds of ceramics, such as Al2 O3 , SiC, AlN, Si3 N4 , ZrO2 , SiO2 , ZrB2 –SiC and so on [27–34]. However, to our knowledge, gelcasing has never been used to prepare CCTO ceramics. So it is meaningful to try using gelcasting to prepare CCTO ceramics. Acrylamide (AM) is the first and most widely used gel monomer in gelcasting. Unfortunately, industries have been reluctant to use gelcasting in this way, because AM is a neurotoxin. In this study, we will try using a low-toxicity and water-soluble
http://dx.doi.org/10.1016/j.jeurceramsoc.2015.05.034 0955-2219/© 2015 Elsevier Ltd. All rights reserved.
Please cite this article in press as: W. Wan, et al., Low-toxic gelcasting of giant dielectric-constant CaCu3 Ti4 O12 ceramics from the molten salt powder, J Eur Ceram Soc (2015), http://dx.doi.org/10.1016/j.jeurceramsoc.2015.05.034
G Model JECS-10149; No. of Pages 6
ARTICLE IN PRESS W. Wan et al. / Journal of the European Ceramic Society xxx (2015) xxx–xxx
2
Fig. 1. XRD patterns of CCTO powders synthesized at (a) different temperatures for 2 h and (b) 800 ◦ C for different holding times.
monomer, N,N-dimethylacrylamide (DMAA), as the binder to gelcast CCTO ceramics. In the previous works of our lab, it has been proved that the DMAA gel system shows excellent performance same as even superior to the AM system [35–37]. 2. Experimental 2.1. CCTO powder synthesis First, a stoichiometric mixture of CaCO3 (≥99.0%), CuO (≥99.0%) and TiO2 (≥99.0%) was ball-milled in ethanol for 2 h. Then NaCl (≥99.5%) was added into the mixture with a 3:1 salt/oxides mass ratio and continued further ball-milling for 2 h. The final obtained mixture was divided into several parts. Some were heated in an alumina crucible for 2 h in the temperature range of 750–850 ◦ C, the others at 800 ◦ C for 2–6 h. The heating rate for all samples was 5 ◦ C/min. Next, after furnace cooling to room temperature, the reacted powers were washed in 80 ◦ C distilled water to remove NaCl salt. The washing process was repeated about 15 times until the filtrate gave no reaction with silver nitrate solution. At last, the resultant powders were oven-dried at 100 ◦ C. 2.2. CCTO slurry, green body and ceramics preparation N,N-dimethylacrylamide (DMAA, Kowa, Japan), N,Nmethylenebisacrylamide (MBAM, Tianjing Chemical Reagent Institute, China), acrylic acid-2-acrylamido-2Research methypropane sulfonic acid copolymer (AA/AMPS, Taihe Water Treatment Co., Ltd., China) and ammonium persulfate (APS) were used as the gel monomer, crosslinker, dispersant and initiator for the gel-casting process, respectively. CCTO powder used below was synthesized at 800 ◦ C for 2 h. First, a pre-mix solution was prepared by dissolving DMAA (10 wt%) and MBAM (1 wt%) in distilled water. Next, the pre-mix solution, CCTO powder, dispersant (0.1–3.5 wt% of CCTO powder), pH regulator (ammonia water) and zirconia mill balls were added to a nylon jar, and then ball-milled for 1 h in a planetary mill at a speed of 200 rpm to obtain CCTO slurry. The studied solid volume loading was the percentage of CCTO powders volume to the total slurry volume. After adding a small amount of APS (dosage: 1 wt% of DMAA) aqueous solution (5 wt%), the slurry was degassed in a vacuum deaeration mix and then cast into stainless steel molds (with lids) and soaked in a water bath at 75 ◦ C for an hour. APS decomposes to create free radicals ((NH4 )2 S2 O8 2NH4 SO4 −• ) at this temperature, which initiates the polymerization reaction of the DMAA monomer and
create the macromolecular gel network to solidify the ceramic suspension in situ. Then the solidified wet green bodies were carefully de-molded and dried in an oven at constant humidity and temperature of 100% and 40 ◦ C. For comparison, the other CCTO powders were mixed with little PVA solution, and pressed into pellets. These pellets were further densified using a cold isostatic pressing method (300 MPa). Sintering of the gelcasting green bodies and these pellets was carried out in an ordinary electric furnace at 1080 ◦ C for 4 h, assisted by heating at a rate of 1 ◦ C/min, from 400 to 600 ◦ C to decompose the organics. 2.3. Characterization and measurements R/S Rheometer (R/S CC25, Brookfield Corporation, American) was used to characterize rheological behaviors of slurries. The measuring shear rate was from 0 to 400 s−1 and the values of viscosity and shear stress under each shear rate were automatically recorded by the computer. Zeta potential was determined by the Zeta Potential Analyzer Ver. 3.54 (Brookhaven Instruments Corp, PALS). The phases of the specimens were identified by an X-ray diffractometer (RIGAKU, CuK␣, Japan). Microstructures of the as-synthesized powder and sintered ceramics were observed under a field emission scanning electron microscope (FE-SEM, HITACHI, SU8010, Japan). Mechanical strength of the green body was measured using an universal testing machine (WT-6002, Shenzhen Reger instrument Co., Ltd., China), by the three-point flexural method with a sample dimension of 3 mm × 4 mm × 40 mm and a crosshead speed of 0.5 mm/min. For dielectric measurements, both sides of ceramic pellets (thickness: 1 mm, diameter: 9.5 mm) were polished, coated with silver conductive paste, and then heated at 650 ◦ C for 30 min. Dielectric properties were performed using an impedance analyzer (Agilent 4294A, America) over the frequency range of 100 Hz–10 MHz with the applied voltage of 500 mV at room temperature. 3. Results and discussion Fig. 1 shows XRD patterns of the resultant powders synthesized at different temperature and holding time. All the synthesized powders contain CCTO phase. Fig. 1(a) indicates XRD patterns of powders synthesized at 750, 800, and 850 ◦ C for 2 h, which show CCTO has been successfully synthesized at the temperature as low as 750 ◦ C by using NaCl salt as the flux. There are little secondary phases of CuO and CaTiO3 in all powders, which is the same as other MSS systems for preparing CCTO powders [17–19]. The CuO amount
Please cite this article in press as: W. Wan, et al., Low-toxic gelcasting of giant dielectric-constant CaCu3 Ti4 O12 ceramics from the molten salt powder, J Eur Ceram Soc (2015), http://dx.doi.org/10.1016/j.jeurceramsoc.2015.05.034
G Model JECS-10149; No. of Pages 6
ARTICLE IN PRESS W. Wan et al. / Journal of the European Ceramic Society xxx (2015) xxx–xxx
Fig. 2. The FE-SEM image of the as-synthesized CCTO powder.
3
Fig. 4. Effect of AA/AMPS dosage on rheological behavior of CCTO slurry (solid loading: 25 vol%).
Fig. 3. Zeta potential of CCTO slurry with and without the dispersant AA/AMPS.
decreases slightly when the synthesis temperature increases from 750 to 800 ◦ C. Relatively higher temperature may enhance diffusion coefficients of the molten chloride liquid phase and result in more adequate reaction of reagents. But further increase of the temperature to 850 ◦ C leads to the slight increase of CuO amount. It has been reported that the NaCl–KCl melts start to vaporize and small weight losses are evident and thus difficult to control the reaction at high synthesis temperatures [22]. Single NaCl melt may have the same trouble. In general, increasing the reaction temperature has limited effect on the formation and development of the CCTO phase. And 800 ◦ C is selected as the suitable temperature in this experiment. In Fig. 1(b), however, it can be seen that increasing synthesis time has little effect on purifying of resultant powders. So CCTO powder obtained at 800 ◦ C for 2 h was used in the following research, and its micromorphology is shown in Fig. 2. Most CCTO powders are smaller than 1 m, and seriously agglomerate together. For gelcasting, perhaps the preparation of ceramic suspension with high solid loading and low viscosity is the most important step. Figs. 3–6 show our effort on preparing CCTO slurry with solid loading as high as possible. As we know, stabilizing mechanisms of particles in solution mainly include electrostatic stabilization, steric stabilization, and electrical steric stabilization. According to DLVO theory, it is known that the greater the absolute value of zeta potential, the greater the electrostatic repulsion between particles, and thus the more stable the slurry. Fig. 3 shows zeta potential
Fig. 5. Effect of pH on rheological behavior of CCTO slurry (solid loading: 30 vol%, AA/AMPS dosage: 3 wt%).
of CCTO slurry with and without dispersant. It can be seen that the isoelectric point (IEP) of CCTO powders without dispersant is about 1.85 and the absolute value of zeta potential increases with pH. The maximum value of zeta potential is obtained in pH range of 10–12. Variation of zeta potential of CCTO powders with 3 wt% AA/AMPS dispersant will be discussed later. Fig. 4 indicates the effect of AA/AMPS dispersant on the viscosity of CCTO slurry with solid loading of 25 vol%. When AA/AMPS was added, the viscosity decreases sharply. From the inset, it is seen that the optimum dosage of AA/AMPS is about 3 wt%. AA/AMPS ( ) is a polyelectrolyte (molecular weight: 3000–6000), and has sulfonic groups, whose dissociation is not affected by pH value [38]. Thus, it can produce steric stabilization and electrostatic stabilization mechanisms at the same time and shows good dispersion effect. With the increase of AA/AMPS dosage, its absorption amount on CCTO surface increases, and the dispersion effect enhances. When the dosage increases to 3 wt%, the absorption of AAAMPS on CCTO surface becomes saturated, which results in the best dispersion effect and the lowest viscosity of CCTO slurry. Further increase of AA/AMPS increases the amount of dissociative dispersant, and causes the
Please cite this article in press as: W. Wan, et al., Low-toxic gelcasting of giant dielectric-constant CaCu3 Ti4 O12 ceramics from the molten salt powder, J Eur Ceram Soc (2015), http://dx.doi.org/10.1016/j.jeurceramsoc.2015.05.034
G Model
ARTICLE IN PRESS
JECS-10149; No. of Pages 6
W. Wan et al. / Journal of the European Ceramic Society xxx (2015) xxx–xxx
4
Fig. 6. Effect of solid loading on rheological behavior of CCTO slurry (AA/AMPS dosage: 3 wt%, pH: 9.08).
Table 1 Flexural strength and density of CCTO green body. Flexural strength (MPa)
Apparent porosity (%)
Bulk density (g/cm3 )
Relative density (%)
9.27 ± 0.52
52.4 ± 0.2
2.19 ± 0.01
44.73 ± 0.02
increase of viscosity. In Fig. 3, the absolute value of zeta potential of CCTO powders with 3 wt% AA/AMPS is higher than that of CCTO powders without dispersant when pH is below 9, which is caused by the dissociation of AA/AMPS. Higher zeta potential value leads to better dispersion effect. In fact, when no dispersant is added, CCTO slurry can not flow even at the solid loading as low as 25 vol%. Another important factor that influences the rheological behaviors of ceramic slurry is pH. Fig. 5 illustrates the effect of pH on viscosity of CCTO slurry with 3 wt% AA/AMPS dosage and 30 vol% solid loading. The viscosity decreases rapidly with the increase of pH. As shown in Figs. 3 and 5, increase of pH leads to the increase of absolute value of zeta potential and results in stronger electrostatic repulsion, which brings about better dispersion effect. In addition, the carboxyl of AA/AMPS dissociates in alkaline solution and further increase the zeta potential of CCTO powder. What is more, owing to the dissociation of carboxyl, the molecular chains of AA/AMPS becomes straight in alkaline solution, which contributes to the dispersion of CCTO powder. The optimum pH value is 9.08. Excessive alkaline pH increases the ionic concentration in solution and compresses the electronic double-layer of CCTO particle surface, which results in lower zeta potential (Fig. 3) and larger CCTO slurry viscosity. Fig. 6 shows the effect of solid loading on the rheological behaviors of CCTO slurry with the optimum dispersant dosage and pH conditions. The maximum solid loading can be up to 47 vol%, but the viscosity of this slurry is too high to be suitable for casting. CCTO ceramics were prepared using the 45 vol% solid loading slurry in this work. The viscosity of slurries with high solid loading can be characterized by Quemada Model [39]:
r = (1 +
ϕ −2 ) ϕm
(1)
Fig. 7. The FE-SEM image of CCTO green body prepared by gelcasting.
where the r is the relative viscosity, is the solid loading of slurry and m is the maximum solid loading. As can be seen from the Quemada model, the relative viscosity increases when the solid loading increases. Table 1 shows the properties of CCTO green body obtained by the present method. Flexural strength of CCTO green body reaches 9.27 MPa, which enables them to be machined without breakage. Also, it can be seen that CCTO green body has homogeneous microstructure and no obvious defects are observed in Fig. 7. Table 2 lists the density values of CCTO ceramics obtained by gelcasting and cold isostatic pressing. CCTO ceramics prepared by the two methods have the similar density, whose microstructures are shown in Fig. 8. The similar density of the two ceramics sug-
Table 2 Density of CCTO ceramics prepared by glecasting and isostatic pressing, respectively. Methods
Apparent porosity (%)
Bulk density (g/cm3 )
Relative density (%)
Gelcasting Isostatic pressing
0.76 ± 0.07 0.86 ± 0.05
4.67 ± 0.005 4.65 ± 0.006
95.30 ± 0.10 94.90 ± 0.13
Please cite this article in press as: W. Wan, et al., Low-toxic gelcasting of giant dielectric-constant CaCu3 Ti4 O12 ceramics from the molten salt powder, J Eur Ceram Soc (2015), http://dx.doi.org/10.1016/j.jeurceramsoc.2015.05.034
G Model JECS-10149; No. of Pages 6
ARTICLE IN PRESS W. Wan et al. / Journal of the European Ceramic Society xxx (2015) xxx–xxx
5
Fig. 8. FE-SEM images of CCTO ceramics prepared by (a) the gelcasting and (b) cold isostatic pressing methods.
Fig. 9. (a) and (b) tanı of CCTO ceramics prepared by the gelcasting and cold isostatic pressing methods.
gests that the sintering time is enough for the densification of both ceramics. It can be deduced that gelcasting does improve the densification of CCTO ceramics due to the fact that CCTO powders in the gelcasting green bodies have closer packing. The frequency dependences of their dielectric constant ( ) and loss (tanı) are shown in Fig. 9(a) and (b), respectively. of the gelcasting ceramics is twice as that of the ceramics obtained by isostatic pressing (Fig. 9(a)). From the microstructures in Fig. 8, CCTO ceramics prepared by gelcasting method have larger grains than those prepared by isostatic pressing, which is the main reason for the higher permittivity because it has been proven by many researchers that larger CCTO grains lead to higher . Both kinds of ceramics have relatively low tanı, which is below 0.2 in the wide frequency range of 102 –105 Hz.
4. Conclusions CCTO powder was successfully synthesized at a low temperature 800 ◦ C using the molten salt method in NaCl flux. High solid loading CCTO aqueous slurry with low viscosity was obtained by using AA/AMPS as the dispersant. Dispersant dosage and pH conditions were optimized in this work. The optimum dispersant dosage is 3 wt% and pH is about 9.08. CCTO green body with relatively high mechanical strength and homogeneous microstructure was obtained. CCTO ceramics obtained by the gelcasting method have higher dielectric constant than those by the isostatic pressing method and show relatively low dielectric loss below 0.2 in the wide frequency range of 102 –105 Hz.
Acknowledgements This work was financially supported by the Science and Technology Innovation Commission of Shenzhen (No. JCYJ20120817163755065 and JCYJ20140418091413506), Seedling Project of Guangdong Provincial Department of Education (No. 2013LYM 0079), Shenzhen Peacock Plan (No. KQCX20130625164044956 and KQCX20130621101205783), and Startup Project of Shenzhen High-end Talent Scientific Research.
References [1] B.A. Timothy, C.S. Derek, R.W. Anthony, Giant barrier layer capacitance effects in CaCu3 Ti4 O12 ceramics, Adv. Mater. 14 (2002) 1321–1323. [2] C.C. Homes, T. Vogt, S.M.S. Shapiro Wakimoto, A.P. Ramirez, Optical response of high-dielectric-constant perovskite-related oxide, Science 293 (2001) 673–676. [3] F. Tsang-Tse, S. Hsu-kai, Mechanism for developing the boundary barrier layers of CaCu3 Ti4 O12 , J. Am. Ceram. Soc. 87 (2004) 2072–2079. [4] H. Wanbiao, L. Yun, L.W. Ray, J.F. Terry, N. Lasse, S. Amanda, K. Melanie, S. Paul, G. Bill, C. Hua, S. Jason, B. Frank, W.L. Jennifer, Electron-pinned defect-dipoles for high-performance colossal permittivity materials, Nat. Mater. 12 (2013) 821–826. [5] D. Zhi-Min, Z. Tao, Y. Sheng-Hong, Y. Jin-Kai, Z. Jun-Wei, S. Hong-Tao, L. Jian-Ying, C. Qiang, Y. Wan-Tai, B. Jinbo, Advanced calcium copper titanate/polyimide functional hybrid films with high dielectric permittivity, Adv. Mater. 21 (2009) 2077–2082. [6] C.C. Wang, L.W. Zhang, Surface-layer effect in CaCu3 Ti4 O12 , Appl. Phys. Lett. 88 (2006) 42906. [7] F. Amarala, E. Clementea, M.A. Valentea, L.C. Costaa, F.M. Costa, Effects of Mn doping on the electrical and dielectric properties of CaCu3 Ti4 O12 fibres, Ceram. Int. 40 (2014) 16503–16511. [8] C.S. Derek, B.A. Timothy, D.M. Finlay, R.W. Anthony, CaCu3 Ti4 O12: one-step internal barrier layer capacitor, Appl. Phys. Lett. 80 (2002) 2153–2155.
Please cite this article in press as: W. Wan, et al., Low-toxic gelcasting of giant dielectric-constant CaCu3 Ti4 O12 ceramics from the molten salt powder, J Eur Ceram Soc (2015), http://dx.doi.org/10.1016/j.jeurceramsoc.2015.05.034
G Model JECS-10149; No. of Pages 6 6
ARTICLE IN PRESS W. Wan et al. / Journal of the European Ceramic Society xxx (2015) xxx–xxx
[9] M.A. Mohamad, A.L. Eman, A.J. Abdullah, G. Syed, Y. Koji, Mechanochemical synthesis and giant dielectric properties of CaCu3 Ti4 O12 , Appl. Phys. A: Mater. Sci. Process 116 (2014) 1299–1306. [10] H. Awater, G. Monique, C.C. Jérôme, T.P. Vinh, G. Franc¸ois, Synthesis of Ca0.25 Cu0.75 TiO3 and infrared characterization of role played by copper, Mater. Sci. Eng. B 87 (2001) 164–168. [11] J. Shuhua, X. Haiping, Z. Yuepin, G. Juping, X. Jun, Synthesis of CaCut3 T4 O12 ceramic via a sol–gel method, Mater. Lett. 61 (2007) 1404–1407. [12] P. Thomas, K. Dwarakanath, K.B.R. Varma, T.R.N. Kutty, Synthesis of nanoparticles of the giant dielectric material, CaCu3 Ti4 O12 from a precursor route, J. Therm. Anal. Calorim. 95 (2009) 267–272. [13] A.F.L. Almeida, P.B.A. Fechine, M.P.F. Grac¸a, M.A. Valente, A.S.B. Sombra, Structural and electrical study of CaCu3 Ti4 O12 (CCTO) obtained in a new ceramic procedure, J. Mater. Sci.: Mater. Electron. 20 (2009) 163–170. [14] A.F.L. Almeida, R.S.D. Oliveira, J.C. Góes, J.M. Sasaki, A.G. Souza Filho, J. Mendes Filho, A.S.B. Sombra, Structural properties of CaCu3 Ti4 O12 obtained by mechanical alloying, Mater. Sci. Eng. B 96 (2002) 275–283. [15] L. Jianjun, S. Yucheng, D. Chun-gang, M. Wai-Ning, W.S. Robert, R.H. John, CaCu3 Ti4 O12 : low-temperature synthesis by pyrolysis of an organic solution, Chem. Mater. 18 (2006) 3878–3882. [16] L. Zushu, Z. Shaowei, E.L. William, Molten salt synthesis of LaAlO3 powder at low temperature, J. Eur. Ceram. Soc. 27 (2007) 3201–3205. [17] Z. Shaowei, D.J. Daniel, B. Goutam, E.L. William, Molten salt synthesis of magnesium aluminate (MgAl2 O4 ) spinel powder, J. Am. Ceram. Soc. 89 (2006) 1724–1726. [18] H. Hua, L. Han-xing, L. Yang, C. Ming-he, O. Shi-xi, Lead-free SrBi4 Ti4 O15 and Bi4 Ti3 O12 material fabrication using the microwave-assisted molten salt synthesis method, J. Am. Ceram. Soc. 90 (2007) 1659–1662. [19] L. Zhen, C.Y. Xu, W.S. Wang, C.S. Lao, Q. Kuang, Electrical and photocatalytic properties of Na2 Ti6 O13 nanobelts prepared by molten salt synthesis, Appl. Surf. Sci. 255 (2009) 4149–4152. [20] B.S. Prakash, K.B.R. Varma, Molten salt synthesis of nanocrystalline phase of high dielectric constant material CaCu3 Ti4 O12 , J. Nanosci. Nanotechol. 8 (2008) 5762–5769. [21] H. Yanmin, L. Laijun, S. Danping, Z. Shaoying, W. Shuangshuang, F. Liang, H. Changzheng, B. Elouadi, Giant dielectric permittivity and non-linear electrical behavior in CaCu3 Ti4 O12 varistors from the molten-salt synthesis powder, Ceram. Int. 39 (2013) 6063–6068. [22] C. Ke-pi, Z. Xiao-wen, Synthesis of calcium copper titanate ceramics via the molten salts method, Ceram. Int. 36 (2010) 1523–1527. [23] K.K. Byeong, S.L. Hyung, W.L. Jung, E.L. Seung, S.C. Yong, Dielectric and grain-boundary characteristics of hot pressed CaCu3 Ti4 O12 , J. Am. Ceram. Soc. 93 (2010) 2419–2422.
[24] H. Wentao, Z. Jialiang, Microstructure and dielectric property of hot-pressed high density CaCu3 Ti4 O12 ceramics, J. Alloys Compounds 559 (2013) 16–19. [25] A.J. Mark. Method of Molding Ceramic Powders. U. S. Patent No.4894194 1990: January 16. [26] C.Y. Albert, O.O. Ogbemi, A.J. Mark, A.M. Paul, Gelcasting of alumina, J. Am. Ceram. Soc. 74 (1991) 612–618. [27] A.J. Millàn, M.I. Nieto, R. Moreno, Near-net shaping of aqueous alumina slurries using carrageenan, J. Eur. Ceram. Soc. 22 (2002) 297–303. [28] V. Sujith, N. Rajaram, K. Prabhakaran, Gelcasting of alumina using the hexamethylenediamine-paraformaldehyde monomer system, Ceram. Int. 40 (2014) 3185–3191. [29] Z. Tao, Z. Zhaoquan, Z. Jingxian, J. Dongliang, L. Qingling, Preparation of SiC ceramics by aqueous gelcasting and pressureless sintering, Mater. Sci. Eng. A 443 (2007) 257–261. [30] X. Jianfeng, D. Manjiang, L. Jun, Z. Guohong, W. Shiwei, Gelcasting of aluminum nitride ceramics, J. Am. Ceram. Soc. 93 (2010) 928–930. [31] Y. Liu-yan, Z. Xin-gui, Y. Jin-shan, W. Hong-lei, Z. Shuang, L. Zheng, Y. Bei, Preparation of Si3N4 ceramic foams by simultaneously using egg white protein and fish collagen, Ceram. Int. 39 (2013) 445–448. [32] Z. Huoping, Y. Chunsheng, F. Zitian, A simple and effective method for gel casting of zirconia green bodies using phenolic resin as a binder, J. Eur. Ceram. Soc. 34 (2014) 1457–1463. [33] W. Wei, Y. Jian, Z. Jinzhen, Q. Tai, Gelcasting of fused silica glass using a low-toxicity monomer DMAA, J. Non-Cryst. Solids 379 (2013) 229–234. [34] H. Rujie, Z. Rubing, Z. Xiaolei, W. Kai, Q. Zhaoliang, P. Yongmao, F. Daining, Improved green strength and green machinability of ZrB2–SiC through gelcasting based on a double gel network, J. Am. Ceram. Soc. 97 (2014) 2401–2404. [35] W. Wei, Y. Jian, Z. Jinzhen, Y. Lichun, Q. Tai, Aqueous gelcasting of silica ceramics using DMAA, Ceram. Int. 40 (2014) 1257–1262. [36] X. Weiliang, Y. Jian, J. Yulong, Q. Tai, Aqueous gelcasting of yttrium iron garnet, J. Eur. Ceram. Soc. 33 (2013) 1023–1028. [37] Z. Chao, Q. Tai, Y. Jian, G. Jian, The effect of solid volume fraction on properties of ZTA composites by gelcasting using DMAA system, Mater. Sci. Eng. A 539 (2012) 243–249. [38] S. Durmaz, O. Okay, Acrylamide/2-acrylamido-2-methylpropane sulfonic acid sodium salt-based hydrogels: synthesis and characterization, Polymer 41 (2000) 3693–3704. [39] R.S. Jones, An introduction to rheology, Endeavour 15 (1991) 35.
Please cite this article in press as: W. Wan, et al., Low-toxic gelcasting of giant dielectric-constant CaCu3 Ti4 O12 ceramics from the molten salt powder, J Eur Ceram Soc (2015), http://dx.doi.org/10.1016/j.jeurceramsoc.2015.05.034
ARTICLE IN PRESS
JECS-10149; No. of Pages 6
Journal of the European Ceramic Society xxx (2015) xxx–xxx
Contents lists available at www.sciencedirect.com
Journal of the European Ceramic Society journal homepage: www.elsevier.com/locate/jeurceramsoc
Low-toxic gelcasting of giant dielectric-constant CaCu3 Ti4 O12 ceramics from the molten salt powder Wei Wan a,b , Changkun Liu a , Hongyuan Sun a , Zhongkuan Luo a , Wen-Xiang Yuan a,∗ , Huisi Wu c,∗ , Tai Qiu b a Shenzhen Key Laboratory of New Lithium-ion Battery and Mesoporous Materials, College of Chemistry and Chemical Engineering, Shenzhen University, Shenzhen 518060, PR China b College of Materials Science and Engineering, Nanjing Tech University, Nanjing 210009, PR China c College of Computer Science and Software Engineering, Shenzhen University, Shenzhen 518060, PR China
a r t i c l e
i n f o
Article history: Received 11 February 2015 Received in revised form 25 May 2015 Accepted 29 May 2015 Available online xxx Keywords: CaCu3 Ti4 O12 Molten salt method NaCl Low-toxic gelcasting Dielectric properties
a b s t r a c t CaCu3 Ti4 O12 (CCTO) nano powder was synthesized using a molten salt synthesis method in NaCl flux. Synthesis temperature and holding time were investigated. The suitable synthesis condition is 800 ◦ C for 2 h. Aqueous CCTO slurry with high solid loading and low viscosity was prepared by using acrylic acid2-acrylamido-2-methypropane sulfonic acid copolymer (AA/AMPS) as the dispersant. AA/AMPS dosage and pH condition have been optimized as AA/AMPS dosage of 3 wt% and pH about 9.08. A low-toxicity and water-soluble monomer, N,N-dimethylacrylamide (DMAA) was used as the gelling agent. CCTO green body fabricated by the gelcasting method has the homogeneous microstructure and relatively high mechanical strength of 9.27 MPa. CCTO ceramics obtained by the gelcasting method have higher dielectric constant than those prepared by the cold isostatic pressing method and show relatively low dielectric loss of below 0.2 in the wide frequency range of 102 –105 Hz. © 2015 Elsevier Ltd. All rights reserved.
1. Introduction Recently, the perovskite-type ternary oxide compound CaCu3 Ti4 O12 (CCTO) has drawn much interest for its potential microelectronic applications including capacitors and memory devices due to its giant dielectric constant of ∼105 over the wide frequency and temperature ranges [1–7]. CCTO powder is usually prepared by the traditional solid-state reaction from metal oxides with a few intermediate grinding/heating cycles [8,9]. However, this method is tedious with relatively long reaction time and high temperature (typically at 1000 ◦ C for 10–20 h), and might still generate unwanted phases owing to the limited atomic diffusion. Other synthesis techniques such as mechanochemical synthesis, sol–gel, wet chemical method, microwave heating, mechanical alloying and polymer pyrolysis have also been reported recently, which are found to have considerable influence on dielectric properties of CCTO ceramics [10–15]. In this work, CCTO powder was synthesized by a molten salt synthesis (MSS) method using NaCl as the flux. As a low-temperature
∗ Corresponding authors. Tel.: +86 755 26536627; fax: +86 755 26536141. E-mail addresses: [email protected] (W.-X. Yuan), [email protected] (H. Wu).
synthesis technique, MSS has been applied to synthesize many different kinds of powders, such as LaAlO3 , MgAl2 O4 , SrBi4 Ti4 O15 , Na2 Ti6 O13 , CCTO, and so on [16–19]. CCTO powder has been synthesized in the molten salt systems of KCl, NaCl–KCl or Na2 SO4 –K2 SO4 [20–22]. However, single NaCl salt system has not been tried yet. So we will try to synthesize CCTO powders in NaCl flux in this experiment. CCTO ceramics are conventionally prepared by the cold isostatic pressing method followed by pressureless sintering. Recently, hotpress sintering has also been used in preparing CCTO ceramics [23,24]. Gelcasting, an advanced ceramic process, is developed by Janney et al. at Oak Ridge National Laboratory (ORNL, USA) to make complex-shaped parts. Gelcasting is a colloidal processing method, and has established itself for its simplicity and its ability to produce a high degree of homogeneity as well as green body strength, resulting in good machinability [25,26]. It has been successfully used to prepare many kinds of ceramics, such as Al2 O3 , SiC, AlN, Si3 N4 , ZrO2 , SiO2 , ZrB2 –SiC and so on [27–34]. However, to our knowledge, gelcasing has never been used to prepare CCTO ceramics. So it is meaningful to try using gelcasting to prepare CCTO ceramics. Acrylamide (AM) is the first and most widely used gel monomer in gelcasting. Unfortunately, industries have been reluctant to use gelcasting in this way, because AM is a neurotoxin. In this study, we will try using a low-toxicity and water-soluble
http://dx.doi.org/10.1016/j.jeurceramsoc.2015.05.034 0955-2219/© 2015 Elsevier Ltd. All rights reserved.
Please cite this article in press as: W. Wan, et al., Low-toxic gelcasting of giant dielectric-constant CaCu3 Ti4 O12 ceramics from the molten salt powder, J Eur Ceram Soc (2015), http://dx.doi.org/10.1016/j.jeurceramsoc.2015.05.034
G Model JECS-10149; No. of Pages 6
ARTICLE IN PRESS W. Wan et al. / Journal of the European Ceramic Society xxx (2015) xxx–xxx
2
Fig. 1. XRD patterns of CCTO powders synthesized at (a) different temperatures for 2 h and (b) 800 ◦ C for different holding times.
monomer, N,N-dimethylacrylamide (DMAA), as the binder to gelcast CCTO ceramics. In the previous works of our lab, it has been proved that the DMAA gel system shows excellent performance same as even superior to the AM system [35–37]. 2. Experimental 2.1. CCTO powder synthesis First, a stoichiometric mixture of CaCO3 (≥99.0%), CuO (≥99.0%) and TiO2 (≥99.0%) was ball-milled in ethanol for 2 h. Then NaCl (≥99.5%) was added into the mixture with a 3:1 salt/oxides mass ratio and continued further ball-milling for 2 h. The final obtained mixture was divided into several parts. Some were heated in an alumina crucible for 2 h in the temperature range of 750–850 ◦ C, the others at 800 ◦ C for 2–6 h. The heating rate for all samples was 5 ◦ C/min. Next, after furnace cooling to room temperature, the reacted powers were washed in 80 ◦ C distilled water to remove NaCl salt. The washing process was repeated about 15 times until the filtrate gave no reaction with silver nitrate solution. At last, the resultant powders were oven-dried at 100 ◦ C. 2.2. CCTO slurry, green body and ceramics preparation N,N-dimethylacrylamide (DMAA, Kowa, Japan), N,Nmethylenebisacrylamide (MBAM, Tianjing Chemical Reagent Institute, China), acrylic acid-2-acrylamido-2Research methypropane sulfonic acid copolymer (AA/AMPS, Taihe Water Treatment Co., Ltd., China) and ammonium persulfate (APS) were used as the gel monomer, crosslinker, dispersant and initiator for the gel-casting process, respectively. CCTO powder used below was synthesized at 800 ◦ C for 2 h. First, a pre-mix solution was prepared by dissolving DMAA (10 wt%) and MBAM (1 wt%) in distilled water. Next, the pre-mix solution, CCTO powder, dispersant (0.1–3.5 wt% of CCTO powder), pH regulator (ammonia water) and zirconia mill balls were added to a nylon jar, and then ball-milled for 1 h in a planetary mill at a speed of 200 rpm to obtain CCTO slurry. The studied solid volume loading was the percentage of CCTO powders volume to the total slurry volume. After adding a small amount of APS (dosage: 1 wt% of DMAA) aqueous solution (5 wt%), the slurry was degassed in a vacuum deaeration mix and then cast into stainless steel molds (with lids) and soaked in a water bath at 75 ◦ C for an hour. APS decomposes to create free radicals ((NH4 )2 S2 O8 2NH4 SO4 −• ) at this temperature, which initiates the polymerization reaction of the DMAA monomer and
create the macromolecular gel network to solidify the ceramic suspension in situ. Then the solidified wet green bodies were carefully de-molded and dried in an oven at constant humidity and temperature of 100% and 40 ◦ C. For comparison, the other CCTO powders were mixed with little PVA solution, and pressed into pellets. These pellets were further densified using a cold isostatic pressing method (300 MPa). Sintering of the gelcasting green bodies and these pellets was carried out in an ordinary electric furnace at 1080 ◦ C for 4 h, assisted by heating at a rate of 1 ◦ C/min, from 400 to 600 ◦ C to decompose the organics. 2.3. Characterization and measurements R/S Rheometer (R/S CC25, Brookfield Corporation, American) was used to characterize rheological behaviors of slurries. The measuring shear rate was from 0 to 400 s−1 and the values of viscosity and shear stress under each shear rate were automatically recorded by the computer. Zeta potential was determined by the Zeta Potential Analyzer Ver. 3.54 (Brookhaven Instruments Corp, PALS). The phases of the specimens were identified by an X-ray diffractometer (RIGAKU, CuK␣, Japan). Microstructures of the as-synthesized powder and sintered ceramics were observed under a field emission scanning electron microscope (FE-SEM, HITACHI, SU8010, Japan). Mechanical strength of the green body was measured using an universal testing machine (WT-6002, Shenzhen Reger instrument Co., Ltd., China), by the three-point flexural method with a sample dimension of 3 mm × 4 mm × 40 mm and a crosshead speed of 0.5 mm/min. For dielectric measurements, both sides of ceramic pellets (thickness: 1 mm, diameter: 9.5 mm) were polished, coated with silver conductive paste, and then heated at 650 ◦ C for 30 min. Dielectric properties were performed using an impedance analyzer (Agilent 4294A, America) over the frequency range of 100 Hz–10 MHz with the applied voltage of 500 mV at room temperature. 3. Results and discussion Fig. 1 shows XRD patterns of the resultant powders synthesized at different temperature and holding time. All the synthesized powders contain CCTO phase. Fig. 1(a) indicates XRD patterns of powders synthesized at 750, 800, and 850 ◦ C for 2 h, which show CCTO has been successfully synthesized at the temperature as low as 750 ◦ C by using NaCl salt as the flux. There are little secondary phases of CuO and CaTiO3 in all powders, which is the same as other MSS systems for preparing CCTO powders [17–19]. The CuO amount
Please cite this article in press as: W. Wan, et al., Low-toxic gelcasting of giant dielectric-constant CaCu3 Ti4 O12 ceramics from the molten salt powder, J Eur Ceram Soc (2015), http://dx.doi.org/10.1016/j.jeurceramsoc.2015.05.034
G Model JECS-10149; No. of Pages 6
ARTICLE IN PRESS W. Wan et al. / Journal of the European Ceramic Society xxx (2015) xxx–xxx
Fig. 2. The FE-SEM image of the as-synthesized CCTO powder.
3
Fig. 4. Effect of AA/AMPS dosage on rheological behavior of CCTO slurry (solid loading: 25 vol%).
Fig. 3. Zeta potential of CCTO slurry with and without the dispersant AA/AMPS.
decreases slightly when the synthesis temperature increases from 750 to 800 ◦ C. Relatively higher temperature may enhance diffusion coefficients of the molten chloride liquid phase and result in more adequate reaction of reagents. But further increase of the temperature to 850 ◦ C leads to the slight increase of CuO amount. It has been reported that the NaCl–KCl melts start to vaporize and small weight losses are evident and thus difficult to control the reaction at high synthesis temperatures [22]. Single NaCl melt may have the same trouble. In general, increasing the reaction temperature has limited effect on the formation and development of the CCTO phase. And 800 ◦ C is selected as the suitable temperature in this experiment. In Fig. 1(b), however, it can be seen that increasing synthesis time has little effect on purifying of resultant powders. So CCTO powder obtained at 800 ◦ C for 2 h was used in the following research, and its micromorphology is shown in Fig. 2. Most CCTO powders are smaller than 1 m, and seriously agglomerate together. For gelcasting, perhaps the preparation of ceramic suspension with high solid loading and low viscosity is the most important step. Figs. 3–6 show our effort on preparing CCTO slurry with solid loading as high as possible. As we know, stabilizing mechanisms of particles in solution mainly include electrostatic stabilization, steric stabilization, and electrical steric stabilization. According to DLVO theory, it is known that the greater the absolute value of zeta potential, the greater the electrostatic repulsion between particles, and thus the more stable the slurry. Fig. 3 shows zeta potential
Fig. 5. Effect of pH on rheological behavior of CCTO slurry (solid loading: 30 vol%, AA/AMPS dosage: 3 wt%).
of CCTO slurry with and without dispersant. It can be seen that the isoelectric point (IEP) of CCTO powders without dispersant is about 1.85 and the absolute value of zeta potential increases with pH. The maximum value of zeta potential is obtained in pH range of 10–12. Variation of zeta potential of CCTO powders with 3 wt% AA/AMPS dispersant will be discussed later. Fig. 4 indicates the effect of AA/AMPS dispersant on the viscosity of CCTO slurry with solid loading of 25 vol%. When AA/AMPS was added, the viscosity decreases sharply. From the inset, it is seen that the optimum dosage of AA/AMPS is about 3 wt%. AA/AMPS ( ) is a polyelectrolyte (molecular weight: 3000–6000), and has sulfonic groups, whose dissociation is not affected by pH value [38]. Thus, it can produce steric stabilization and electrostatic stabilization mechanisms at the same time and shows good dispersion effect. With the increase of AA/AMPS dosage, its absorption amount on CCTO surface increases, and the dispersion effect enhances. When the dosage increases to 3 wt%, the absorption of AAAMPS on CCTO surface becomes saturated, which results in the best dispersion effect and the lowest viscosity of CCTO slurry. Further increase of AA/AMPS increases the amount of dissociative dispersant, and causes the
Please cite this article in press as: W. Wan, et al., Low-toxic gelcasting of giant dielectric-constant CaCu3 Ti4 O12 ceramics from the molten salt powder, J Eur Ceram Soc (2015), http://dx.doi.org/10.1016/j.jeurceramsoc.2015.05.034
G Model
ARTICLE IN PRESS
JECS-10149; No. of Pages 6
W. Wan et al. / Journal of the European Ceramic Society xxx (2015) xxx–xxx
4
Fig. 6. Effect of solid loading on rheological behavior of CCTO slurry (AA/AMPS dosage: 3 wt%, pH: 9.08).
Table 1 Flexural strength and density of CCTO green body. Flexural strength (MPa)
Apparent porosity (%)
Bulk density (g/cm3 )
Relative density (%)
9.27 ± 0.52
52.4 ± 0.2
2.19 ± 0.01
44.73 ± 0.02
increase of viscosity. In Fig. 3, the absolute value of zeta potential of CCTO powders with 3 wt% AA/AMPS is higher than that of CCTO powders without dispersant when pH is below 9, which is caused by the dissociation of AA/AMPS. Higher zeta potential value leads to better dispersion effect. In fact, when no dispersant is added, CCTO slurry can not flow even at the solid loading as low as 25 vol%. Another important factor that influences the rheological behaviors of ceramic slurry is pH. Fig. 5 illustrates the effect of pH on viscosity of CCTO slurry with 3 wt% AA/AMPS dosage and 30 vol% solid loading. The viscosity decreases rapidly with the increase of pH. As shown in Figs. 3 and 5, increase of pH leads to the increase of absolute value of zeta potential and results in stronger electrostatic repulsion, which brings about better dispersion effect. In addition, the carboxyl of AA/AMPS dissociates in alkaline solution and further increase the zeta potential of CCTO powder. What is more, owing to the dissociation of carboxyl, the molecular chains of AA/AMPS becomes straight in alkaline solution, which contributes to the dispersion of CCTO powder. The optimum pH value is 9.08. Excessive alkaline pH increases the ionic concentration in solution and compresses the electronic double-layer of CCTO particle surface, which results in lower zeta potential (Fig. 3) and larger CCTO slurry viscosity. Fig. 6 shows the effect of solid loading on the rheological behaviors of CCTO slurry with the optimum dispersant dosage and pH conditions. The maximum solid loading can be up to 47 vol%, but the viscosity of this slurry is too high to be suitable for casting. CCTO ceramics were prepared using the 45 vol% solid loading slurry in this work. The viscosity of slurries with high solid loading can be characterized by Quemada Model [39]:
r = (1 +
ϕ −2 ) ϕm
(1)
Fig. 7. The FE-SEM image of CCTO green body prepared by gelcasting.
where the r is the relative viscosity, is the solid loading of slurry and m is the maximum solid loading. As can be seen from the Quemada model, the relative viscosity increases when the solid loading increases. Table 1 shows the properties of CCTO green body obtained by the present method. Flexural strength of CCTO green body reaches 9.27 MPa, which enables them to be machined without breakage. Also, it can be seen that CCTO green body has homogeneous microstructure and no obvious defects are observed in Fig. 7. Table 2 lists the density values of CCTO ceramics obtained by gelcasting and cold isostatic pressing. CCTO ceramics prepared by the two methods have the similar density, whose microstructures are shown in Fig. 8. The similar density of the two ceramics sug-
Table 2 Density of CCTO ceramics prepared by glecasting and isostatic pressing, respectively. Methods
Apparent porosity (%)
Bulk density (g/cm3 )
Relative density (%)
Gelcasting Isostatic pressing
0.76 ± 0.07 0.86 ± 0.05
4.67 ± 0.005 4.65 ± 0.006
95.30 ± 0.10 94.90 ± 0.13
Please cite this article in press as: W. Wan, et al., Low-toxic gelcasting of giant dielectric-constant CaCu3 Ti4 O12 ceramics from the molten salt powder, J Eur Ceram Soc (2015), http://dx.doi.org/10.1016/j.jeurceramsoc.2015.05.034
G Model JECS-10149; No. of Pages 6
ARTICLE IN PRESS W. Wan et al. / Journal of the European Ceramic Society xxx (2015) xxx–xxx
5
Fig. 8. FE-SEM images of CCTO ceramics prepared by (a) the gelcasting and (b) cold isostatic pressing methods.
Fig. 9. (a) and (b) tanı of CCTO ceramics prepared by the gelcasting and cold isostatic pressing methods.
gests that the sintering time is enough for the densification of both ceramics. It can be deduced that gelcasting does improve the densification of CCTO ceramics due to the fact that CCTO powders in the gelcasting green bodies have closer packing. The frequency dependences of their dielectric constant ( ) and loss (tanı) are shown in Fig. 9(a) and (b), respectively. of the gelcasting ceramics is twice as that of the ceramics obtained by isostatic pressing (Fig. 9(a)). From the microstructures in Fig. 8, CCTO ceramics prepared by gelcasting method have larger grains than those prepared by isostatic pressing, which is the main reason for the higher permittivity because it has been proven by many researchers that larger CCTO grains lead to higher . Both kinds of ceramics have relatively low tanı, which is below 0.2 in the wide frequency range of 102 –105 Hz.
4. Conclusions CCTO powder was successfully synthesized at a low temperature 800 ◦ C using the molten salt method in NaCl flux. High solid loading CCTO aqueous slurry with low viscosity was obtained by using AA/AMPS as the dispersant. Dispersant dosage and pH conditions were optimized in this work. The optimum dispersant dosage is 3 wt% and pH is about 9.08. CCTO green body with relatively high mechanical strength and homogeneous microstructure was obtained. CCTO ceramics obtained by the gelcasting method have higher dielectric constant than those by the isostatic pressing method and show relatively low dielectric loss below 0.2 in the wide frequency range of 102 –105 Hz.
Acknowledgements This work was financially supported by the Science and Technology Innovation Commission of Shenzhen (No. JCYJ20120817163755065 and JCYJ20140418091413506), Seedling Project of Guangdong Provincial Department of Education (No. 2013LYM 0079), Shenzhen Peacock Plan (No. KQCX20130625164044956 and KQCX20130621101205783), and Startup Project of Shenzhen High-end Talent Scientific Research.
References [1] B.A. Timothy, C.S. Derek, R.W. Anthony, Giant barrier layer capacitance effects in CaCu3 Ti4 O12 ceramics, Adv. Mater. 14 (2002) 1321–1323. [2] C.C. Homes, T. Vogt, S.M.S. Shapiro Wakimoto, A.P. Ramirez, Optical response of high-dielectric-constant perovskite-related oxide, Science 293 (2001) 673–676. [3] F. Tsang-Tse, S. Hsu-kai, Mechanism for developing the boundary barrier layers of CaCu3 Ti4 O12 , J. Am. Ceram. Soc. 87 (2004) 2072–2079. [4] H. Wanbiao, L. Yun, L.W. Ray, J.F. Terry, N. Lasse, S. Amanda, K. Melanie, S. Paul, G. Bill, C. Hua, S. Jason, B. Frank, W.L. Jennifer, Electron-pinned defect-dipoles for high-performance colossal permittivity materials, Nat. Mater. 12 (2013) 821–826. [5] D. Zhi-Min, Z. Tao, Y. Sheng-Hong, Y. Jin-Kai, Z. Jun-Wei, S. Hong-Tao, L. Jian-Ying, C. Qiang, Y. Wan-Tai, B. Jinbo, Advanced calcium copper titanate/polyimide functional hybrid films with high dielectric permittivity, Adv. Mater. 21 (2009) 2077–2082. [6] C.C. Wang, L.W. Zhang, Surface-layer effect in CaCu3 Ti4 O12 , Appl. Phys. Lett. 88 (2006) 42906. [7] F. Amarala, E. Clementea, M.A. Valentea, L.C. Costaa, F.M. Costa, Effects of Mn doping on the electrical and dielectric properties of CaCu3 Ti4 O12 fibres, Ceram. Int. 40 (2014) 16503–16511. [8] C.S. Derek, B.A. Timothy, D.M. Finlay, R.W. Anthony, CaCu3 Ti4 O12: one-step internal barrier layer capacitor, Appl. Phys. Lett. 80 (2002) 2153–2155.
Please cite this article in press as: W. Wan, et al., Low-toxic gelcasting of giant dielectric-constant CaCu3 Ti4 O12 ceramics from the molten salt powder, J Eur Ceram Soc (2015), http://dx.doi.org/10.1016/j.jeurceramsoc.2015.05.034
G Model JECS-10149; No. of Pages 6 6
ARTICLE IN PRESS W. Wan et al. / Journal of the European Ceramic Society xxx (2015) xxx–xxx
[9] M.A. Mohamad, A.L. Eman, A.J. Abdullah, G. Syed, Y. Koji, Mechanochemical synthesis and giant dielectric properties of CaCu3 Ti4 O12 , Appl. Phys. A: Mater. Sci. Process 116 (2014) 1299–1306. [10] H. Awater, G. Monique, C.C. Jérôme, T.P. Vinh, G. Franc¸ois, Synthesis of Ca0.25 Cu0.75 TiO3 and infrared characterization of role played by copper, Mater. Sci. Eng. B 87 (2001) 164–168. [11] J. Shuhua, X. Haiping, Z. Yuepin, G. Juping, X. Jun, Synthesis of CaCut3 T4 O12 ceramic via a sol–gel method, Mater. Lett. 61 (2007) 1404–1407. [12] P. Thomas, K. Dwarakanath, K.B.R. Varma, T.R.N. Kutty, Synthesis of nanoparticles of the giant dielectric material, CaCu3 Ti4 O12 from a precursor route, J. Therm. Anal. Calorim. 95 (2009) 267–272. [13] A.F.L. Almeida, P.B.A. Fechine, M.P.F. Grac¸a, M.A. Valente, A.S.B. Sombra, Structural and electrical study of CaCu3 Ti4 O12 (CCTO) obtained in a new ceramic procedure, J. Mater. Sci.: Mater. Electron. 20 (2009) 163–170. [14] A.F.L. Almeida, R.S.D. Oliveira, J.C. Góes, J.M. Sasaki, A.G. Souza Filho, J. Mendes Filho, A.S.B. Sombra, Structural properties of CaCu3 Ti4 O12 obtained by mechanical alloying, Mater. Sci. Eng. B 96 (2002) 275–283. [15] L. Jianjun, S. Yucheng, D. Chun-gang, M. Wai-Ning, W.S. Robert, R.H. John, CaCu3 Ti4 O12 : low-temperature synthesis by pyrolysis of an organic solution, Chem. Mater. 18 (2006) 3878–3882. [16] L. Zushu, Z. Shaowei, E.L. William, Molten salt synthesis of LaAlO3 powder at low temperature, J. Eur. Ceram. Soc. 27 (2007) 3201–3205. [17] Z. Shaowei, D.J. Daniel, B. Goutam, E.L. William, Molten salt synthesis of magnesium aluminate (MgAl2 O4 ) spinel powder, J. Am. Ceram. Soc. 89 (2006) 1724–1726. [18] H. Hua, L. Han-xing, L. Yang, C. Ming-he, O. Shi-xi, Lead-free SrBi4 Ti4 O15 and Bi4 Ti3 O12 material fabrication using the microwave-assisted molten salt synthesis method, J. Am. Ceram. Soc. 90 (2007) 1659–1662. [19] L. Zhen, C.Y. Xu, W.S. Wang, C.S. Lao, Q. Kuang, Electrical and photocatalytic properties of Na2 Ti6 O13 nanobelts prepared by molten salt synthesis, Appl. Surf. Sci. 255 (2009) 4149–4152. [20] B.S. Prakash, K.B.R. Varma, Molten salt synthesis of nanocrystalline phase of high dielectric constant material CaCu3 Ti4 O12 , J. Nanosci. Nanotechol. 8 (2008) 5762–5769. [21] H. Yanmin, L. Laijun, S. Danping, Z. Shaoying, W. Shuangshuang, F. Liang, H. Changzheng, B. Elouadi, Giant dielectric permittivity and non-linear electrical behavior in CaCu3 Ti4 O12 varistors from the molten-salt synthesis powder, Ceram. Int. 39 (2013) 6063–6068. [22] C. Ke-pi, Z. Xiao-wen, Synthesis of calcium copper titanate ceramics via the molten salts method, Ceram. Int. 36 (2010) 1523–1527. [23] K.K. Byeong, S.L. Hyung, W.L. Jung, E.L. Seung, S.C. Yong, Dielectric and grain-boundary characteristics of hot pressed CaCu3 Ti4 O12 , J. Am. Ceram. Soc. 93 (2010) 2419–2422.
[24] H. Wentao, Z. Jialiang, Microstructure and dielectric property of hot-pressed high density CaCu3 Ti4 O12 ceramics, J. Alloys Compounds 559 (2013) 16–19. [25] A.J. Mark. Method of Molding Ceramic Powders. U. S. Patent No.4894194 1990: January 16. [26] C.Y. Albert, O.O. Ogbemi, A.J. Mark, A.M. Paul, Gelcasting of alumina, J. Am. Ceram. Soc. 74 (1991) 612–618. [27] A.J. Millàn, M.I. Nieto, R. Moreno, Near-net shaping of aqueous alumina slurries using carrageenan, J. Eur. Ceram. Soc. 22 (2002) 297–303. [28] V. Sujith, N. Rajaram, K. Prabhakaran, Gelcasting of alumina using the hexamethylenediamine-paraformaldehyde monomer system, Ceram. Int. 40 (2014) 3185–3191. [29] Z. Tao, Z. Zhaoquan, Z. Jingxian, J. Dongliang, L. Qingling, Preparation of SiC ceramics by aqueous gelcasting and pressureless sintering, Mater. Sci. Eng. A 443 (2007) 257–261. [30] X. Jianfeng, D. Manjiang, L. Jun, Z. Guohong, W. Shiwei, Gelcasting of aluminum nitride ceramics, J. Am. Ceram. Soc. 93 (2010) 928–930. [31] Y. Liu-yan, Z. Xin-gui, Y. Jin-shan, W. Hong-lei, Z. Shuang, L. Zheng, Y. Bei, Preparation of Si3N4 ceramic foams by simultaneously using egg white protein and fish collagen, Ceram. Int. 39 (2013) 445–448. [32] Z. Huoping, Y. Chunsheng, F. Zitian, A simple and effective method for gel casting of zirconia green bodies using phenolic resin as a binder, J. Eur. Ceram. Soc. 34 (2014) 1457–1463. [33] W. Wei, Y. Jian, Z. Jinzhen, Q. Tai, Gelcasting of fused silica glass using a low-toxicity monomer DMAA, J. Non-Cryst. Solids 379 (2013) 229–234. [34] H. Rujie, Z. Rubing, Z. Xiaolei, W. Kai, Q. Zhaoliang, P. Yongmao, F. Daining, Improved green strength and green machinability of ZrB2–SiC through gelcasting based on a double gel network, J. Am. Ceram. Soc. 97 (2014) 2401–2404. [35] W. Wei, Y. Jian, Z. Jinzhen, Y. Lichun, Q. Tai, Aqueous gelcasting of silica ceramics using DMAA, Ceram. Int. 40 (2014) 1257–1262. [36] X. Weiliang, Y. Jian, J. Yulong, Q. Tai, Aqueous gelcasting of yttrium iron garnet, J. Eur. Ceram. Soc. 33 (2013) 1023–1028. [37] Z. Chao, Q. Tai, Y. Jian, G. Jian, The effect of solid volume fraction on properties of ZTA composites by gelcasting using DMAA system, Mater. Sci. Eng. A 539 (2012) 243–249. [38] S. Durmaz, O. Okay, Acrylamide/2-acrylamido-2-methylpropane sulfonic acid sodium salt-based hydrogels: synthesis and characterization, Polymer 41 (2000) 3693–3704. [39] R.S. Jones, An introduction to rheology, Endeavour 15 (1991) 35.
Please cite this article in press as: W. Wan, et al., Low-toxic gelcasting of giant dielectric-constant CaCu3 Ti4 O12 ceramics from the molten salt powder, J Eur Ceram Soc (2015), http://dx.doi.org/10.1016/j.jeurceramsoc.2015.05.034