Low-toxic gelcasting of giant dielectric-constant CaCu3Ti4O12 ceramics from the molten salt powder

https://doi.org/10.1016/j.jeurceramsoc.2015.05.034Get rights and content

Highlights

  • CCTO nano powder was synthesized by a molten salt method in NaCl flux.

  • Aqueous CCTO slurry with high solid loading and low viscosity was obtained.

  • A low-toxicity monomer, DMAA was used as the gelling agent.

  • CCTO green body with homogeneous microstructure and high strength was obtained.

  • CCTO ceramics prepared by gelcasting have giant dielectric constant and low loss.

Abstract

CaCu3Ti4O12 (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 acid-2-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.

Introduction

Recently, the perovskite-type ternary oxide compound CaCu3Ti4O12 (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], [2], [3], [4], [5], [6], [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], [11], [12], [13], [14], [15]. In this work, CCTO powder was synthesized by a molten salt synthesis (MSS) method using NaCl as the flux. As a low-temperature synthesis technique, MSS has been applied to synthesize many different kinds of powders, such as LaAlO3, MgAl2O4, SrBi4Ti4O15, Na2Ti6O13, CCTO, and so on [16], [17], [18], [19]. CCTO powder has been synthesized in the molten salt systems of KCl, NaCl–KCl or Na2SO4–K2SO4 [20], [21], [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, hot-press 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 Al2O3, SiC, AlN, Si3N4, ZrO2, SiO2, ZrB2–SiC and so on [27], [28], [29], [30], [31], [32], [33], [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 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], [36], [37].

Section snippets

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

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],

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

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.

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