A novel strategy to enhance dielectric performance and non-Ohmic properties in Ca2Cu2−xMgxTi4O12
Introduction
Over the past few years, CaCu3Ti4O12 (CCTO) has been extensively studied in the field of high-permittivity dielectric materials. This is due to the fascinating physics underlying the origin of an ultra-high dielectric permittivity (ɛ′) in CCTO without any detectable phase transition over a wide temperature range. It holds promise for a new generation of multilayer ceramic capacitors.1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16 The origin of the giant dielectric response in CCTO is still unclear today. According to several elegant works, CCTO is clearly electrically heterogeneous, consisting of n-type semiconducting grains and relatively low conductivity at grain boundaries (GBs).2, 4 Therefore, it is believed that the overall dielectric performance can be tuned by engineering the internal interfaces at the GBs. Generally, tan δ of CCTO ceramics is still too high (>0.05).1, 5, 8, 9, 10 This is one of the most serious problems preventing the use of CCTO ceramics in practical applications. Another difficulty is the poor temperature stability of ɛ′, , where and are ɛ′ (at 103 Hz) at room temperature and at any selected temperature, respectively. Usually, ɛ′ at 103 Hz of CCTO-based compounds is strongly dependent on temperature when temperatures are higher than 100 °C.17, 18, 19, 20, 21 Such an increase in the ɛ′ value is also accompanied by an increase in the low-frequency tan δ. These undesirable behaviors are closely associated with high dc conductivity ().22
Generally, lowering to reduce low-frequency tan δ can be done by enhancing the resistances of internal interfaces in CCTO-based compounds. These could be the interface between grains (GB) or the interface between a CCTO grain and a second phase particle. Enhancement may be accomplished by (1) doping CCTO with suitable metal ions to intrinsically improve electrical properties of GBs,5, 18, 20, 23 (2) altering Ca2+ and Cu2+ molar ratios to produce CCTO/CaTiO3 (CTO) composites,6, 7, 24, 25, 26, 27, 28, 29 (3) filling oxygen vacancies at GBs,30 among others. Unfortunately, improved dielectric properties of CCTO produced using these strategies rarely result in materials that fulfill all of the requirements of electronic applications, i.e., high ɛ′, low tan δ, and dielectric response with good temperature stability. It was also found that most of the metal ion substitutions or other strategies, which have been successfully used to improve a particular dielectric property, simultaneously worsen other important dielectric properties of CCTO. For example, a large decrease in tan δ (∼0.02 at 1 kHz) observed in a binary compound system of Ca2Cu2Ti4O12 (consisting of 33.3 mol% of CCTO and 66.7 mol% of CTO) caused a large decrease in ɛ′ (∼2 × 103).6, 26 Given a very low-tan δ value of CCTO/CTO, with its low-ɛ′ value are still acceptable values for use in capacitor applications compared to commercial BaTiO3 and Pb(Sc1/2Ta1/2)O3 ceramics.6 Unfortunately, the condition of within CCTO/CTO exists only in a narrow temperature range of −60 to 90 °C.26, 29 Hence, these properties are consistent with the EIA temperature standard for application in X5R capacitor only, but not for X7R or X8R capacitors.31 It is very important to note the advantages of CCTO/CTO composite ceramics. First, this composite system can be synthesized using a one-step process from a nominal composition of Ca2Cu2Ti4O12.6, 25, 26, 27, 28, 29 Second, this composite ceramic showed no piezoelectricity, which is advantageous to reduce mechanical damage in ac operation.1, 6 Third, its sintering temperature is lower than that of BaTiO3.6, 26, 29 Thus, it is better if all of the dielectric properties of Ca2Cu2Ti4O12 ceramics can simultaneously be improved. This will support progress in communications technology. Enhanced ɛ′, reduced tan δ, and increased are valuable for enhancing volumetric efficiency, reducing dissipation of stored energy into heat, and extending the temperature use range of capacitors, respectively.
It was reported that Mg2+ substitution in CCTO ceramics can enhance their dielectric response.8, 9 Interestingly, low-frequency tan δ values of Mg-doped CCTO ceramics were slightly reduced to ≈0.05 compared to un-doped CCTO.8 Although this value is still slightly higher than the standard value, these behaviors are rarely found in CCTO ceramics. Normally, the variation of both ɛ′ and tan δ are directly proportional.10 It is likely that the temperature stability of Mg-doped CCTO was slightly higher than the un-doped material.8 We hypothesize that substitution of Mg2+ into Ca2Cu2Ti4O12 ceramics may enhance the overall dielectric properties if the majority of Mg2+ doping ions are substituted into Cu2+ sites of the CCTO phase. Therefore, the aim of this work was to provide a novel strategy to improve the overall dielectric properties of CCTO-based ceramics by substitution of Mg2+ ions to a binary compound system of Ca2Cu2Ti4O12 to form CaCu3−xMgxTi4O12/CaTiO3 composite ceramics.
Section snippets
Experimental procedure
An Mg-doped binary compound Ca2Cu2Ti4O12 system with a nominal chemical composition of Ca2Cu2−xMgxTi4O12 (x = 0, 0.05, 0.10, 0.20, and 0.30) ceramics was prepared using a solid state reaction method. These materials are referred to as CCTO/CTO, Mg05, Mg10, Mg20, and Mg30 ceramics, respectively. CaCO3 (Cerac, 99.95% purity), CuO (Cerac, 99.9% purity), TiO2 (Sigma–Aldrich, 99.9% purity), and MgO (Sigma–Aldrich, 99.99% purity) were used as starting raw materials. A stoichiometric mixture of the
Results and discussion
Fig. 1 shows the XRD patterns of Ca2Cu2−xMgxTi4O12 (x = 0, 0.05, 0.10, 0.20, and 0.30) ceramics sintered for 24 h. All of the 24 h samples (as well as the 6 h samples) consisted of two phases of CCTO (JCPDS 75-2188) and CTO (JCPDS 82-0231). This result is similar to that reported in the literature.24, 25, 26, 27, 28, 29, 32, 33 According to the nominal formula of Ca2Cu2Ti4O12, two phases of ∼33.3 mol% of CCTO and ∼66.7 mol% of CTO should be formed due to the imbalance between Ca2+ and Cu2+ ions. This
Conclusions
The dielectric properties of CCTO/CTO composites were improved by doping with Mg2+ ions. It was found that Mg2+ was incorporated into only the CCTO phase. An enhanced ɛ′ of 3550 with good temperature stability (Δɛ′ ≤ ±15% in the temperature range of −60 to 160 °C) and a reduced tan δ value to ∼0.014 (at 30 °C and 1 kHz) were achieved in a CaCu2.7Mg0.3Ti4O12/CTO ceramic sintered at 1100 °C for 6 h. Furthermore, the nonlinear coefficient of this ceramic was greatly enhanced to 27.2. Using impedance
Acknowledgments
This work was financially supported by the Thailand Research Fund (TRF) and Khon Kaen University, Thailand (Grant No. TRG5680047). J.J. would like to thank the Thailand Graduate Institute of Science and Technology (TGIST) for his Master of Science Degree scholarship.
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