Investigation of microstructure, ferroelectric and dielectric behavior of CaCu3Ti(4−x)MnxO12 perovskites synthesized through semi-wet route
- 59 Downloads
The Mn-doped CCTO (CaCu3Ti(4−x)MnxO12 system x = 0.25, 0.50 and 1.00) ceramic has been synthesized by using metal nitrates and solid TiO2 powder sintered at 1223 K for 8 h. The crystal phase of CCTMO was confirmed by the XRD at 1223 K for 8 h. The phase-structure, as well as microstructure, was examined by XRD and SEM, respectively. The SEM micrograph revealed the effect of Mn concentration on grain size and grain formation. The bright-field TEM image of the system confirmed the particle size in the range of 22–30 nm. The stoichiometry and purity of synthesized material were observed by EDX. The investigated dielectric permittivity and tangent loss were found to be 150 and 0.6, respectively, at room temperature (10 kHz), which may be used in electronic devices.
KeywordsSemi-wet route Ceramic Microstructure Dielectric properties
An unusual and promising perovskite oxide CaCu3T4O12 (CCTO) was recently found at extraordinarily high dielectric permittivity at room temperature in the range of 104–106, which is frequency independent and possesses good temperature stability over a broad temperature range from 100 to 800 K [1, 2, 3]. The temperature dependency and high dielectric constant of CCTO have attracted much of the attention of scientists. Such special physical behavior and being Ba/Pb free make it a promising application for microelectronics devices, such as an actual application of CCTO perovskites in the modern electronic devices; but the main problems of this type of ceramic material is its high tangent loss and large linkage current. The CCTO ceramic has been combined in a 1:3 ratio of A-site-ordered perovskite A1Cu3Ti4O12 with space group Im3, containing octahedral Ti-site of TiO6 and was also combined with A-site of Cu square-planar [1, 4, 5]. The extrinsic origin relaxation resulting from contact effect, spatial inhomogeneity and the internal barrier layer capacitor mechanism (IBLC) is frequently accepted to be the cause of the colossal dielectric constant (CDC); and intrinsic relaxation for colossal dielectric constant is also active which is responsible for electrical behavior of CaCu3Ti4O12 (CCTO), depending on their synthesis route [6, 7, 8]. Previous researches on CCTO recommended that the local dipoles induced by doping on Ti-site could be responsible for the high dielectric constant . Scientists across the globe carried out an extensive study over the mechanism of dependable high dielectric constant (ɛr) of CCTO but have not reached a definitive conclusion. The internal boundary layer capacitor (IBLC) mechanism explained the high dielectric constant in CCTO ceramics [10, 11, 12]. According to IBLC theory, in CCTO ceramic, the dielectric variation is produced from the semiconducting grains and insulating grains boundary (GB). Therefore, the variation on microstructure can explain up to a good extent by studying the dielectric properties [13, 14]. Recently, researches revealed that the mixed metal-valent structure (e.g., Ti3+/Ti4+, Fe2+/Fe3+, Cu+/Cu2+, Mn3+/Mn4+ and so on) interrelated ordinary behavior of CDC materials [15, 16], which might directly affect the dielectrical behavior of CCTO ceramics. In this way, the different compositions of Mn-doping in CCTO change the grain boundary and also produce a large effect on dielectric behaviors . The microstructure of CCTO ceramic also changes considerably with a small variation in sintering temperature due to irregular grain growth behavior of Mn-doped ceramics [16, 18]. The metal ion doping in CCTO was found to be a good method that strongly controls microstructure, particle size, electrical properties of grains (semiconducting/insulating behaviors) and grain boundaries (dielectric constant, thickness and resistance) . On the basis of structural investigation, the part TiO6 octahedra are sufficient to produce local distortions, which is effectively responsible for the pure ferroelectric properties of CCTO ceramic. In fact, the ferroelectric behavior was observed in CCTO in a broad temperature range. Furthermore, in comparison with other isostructural compounds, only the CCTO has possessed high dielectric constant [20, 21].
CCTO was synthesized by solid-state method from the metal oxide at high temperature. This method needed a long reaction time, high calcination and sintering temperature. In addition, some other secondary phases (CuO, TiO2 and Cu2TiO3) may also come out during synthesis [22, 23]. On the other hand, synthesis by a chemical solution process such as sol–gel using metal alkoxide gives intimate and uniform mixing of the metal ion at the atomic scale. In this route, titanium isopropoxide Ti(OR)4 is very costly. So, we have synthesized CaCu3Ti(4−x)MnxO12 by a semi-wet route and reported their comparative studies of synthesis, microstructure, dielectric and ferroelectric properties. In this method, metal nitrates have been used in powder form instead of using costly titanium isopropoxide.
The CCTMO was calcined as well as sintered at 973 K and 1223 K, respectively, for 8 h. The results confirmed that the Mn-doping can make the dielectric constant decrease with increasing Mn concentration about two orders of magnitude (from 104–102) [21, 24]. These procedures possess the advantage to refine permittivity, dielectric loss and the ferromagnetic response of Mn-doped CCTO ceramics.
2 Experimental details
2.1 Materials and synthesis
CCTMO was synthesized through a semi-wet route. In this method, chemicals calcium nitrate Ca(NO3)2·4H2O (98% Merck, India), copper nitrate Cu(NO3)2·3H2O (99% Merck, India), manganese acetate Mn(CH3COO)2·4H2O (99% Merck, India) and titanium oxide TiO2 (99% Merck, India), were taken in stoichiometric amounts in molar ratio. The solution of Ca(NO3)2·4H2O, Cu (NO3)2·3H2O, and Mn(CH3COO)2·4H2O was prepared in distilled water. All the solutions were mixed together in a beaker, and a stoichiometric amount of solid TiO2 was added in solution. The calculated amount of citric acid (99.5%, Merck India) equivalent to metal ions was dissolved in distilled water and mixed with the solution. The resulting solution was heated on a hot-plate magnetic stirrer at 348–353 K to evaporate water and allows for self-ignition. A fluffy mass of CCTMO powders was obtained after the removal of a lot of gases. Citric acid was used as a complexing agent that acts as fuel in the ignition step. The resulting CCTMO powder was ground with the help of agate and mortar to make a fine powder. The powder was calcined at 1073 K for 6 h. Calcined powder was used to make cylindrical pellets using 2% PVA as a binder on applying 3 tons of pressure using hydraulic pressure for 90 s. Finally, the CCTMO pellets were sintered at 1373 K for 8 h.
The crystalline phase of CCTMO ceramic-sintered sample was identified by X-ray diffractometer (Rigaku miniflex 600, Japan) applying Cu-kα radiation with wavelength 1.5418 Å. The microstructure and elemental composition were confirmed by scanning electron microscope (ZEISS; model EVO18 research, Germany) attached with an energy-dispersive X-ray (EDX) analyzer (Oxford instrument, USA). The particle size was examined by a high-resolution transmission electron microscope (HR-TEM, Technai G2 20 S-Twin). For HR-TEM characterization, the sample was dispersed in acetone and sonicated for 2 h. This suspension was deposited on a carbon-coated copper grid and dried in oven for 4 h. The thickness and surface morphology were analyzed using atomic force microscopy (NTEGRA Prima, Germany). The dielectric data of silver-coated cylindrical pellets were examined by LCR meter (PSM1735, NumetriQN4L, UK). The ferroelectric behaviors of sintered CCTMO ceramics were measured by the ferroelectric tracer (automatic P–E loop tracer, Marin India).
3 Results and discussion
3.1 Microstructure studies
Atomic percentage of elements for CaCu3Ti(4−x)MnxO12 ceramics (x = 0.25, 0.50 and 1.00) sintered at 1223 K for 8 h
Atomic percent of elements
3.2 Dielectric studies
CaCu3T(4−x)MnxO12 (x = 0.25, 0.50 and 1.00) was successfully synthesized by semi-wet route. Solid TiO2 powder and metal nitrates are used in the preparation of CCTMO. The major phase as CCTO along with the minor phase of TiO2 was confirmed by XRD sintered at 1223 K for 8 h. The SEM micrograph shows the clear grain and grain boundaries with an average grain size in the range of 1–2 µm. The particle size was measured by the TEM technique. The dielectric constant decreases with increasing Mn concentration in CCTMO ceramic. The measured tangent loss decreases with increasing frequency. The stoichiometry and purity of CCTMO ceramic were confirmed by the EDX analysis.
The author would like to thank the in-charge of the central instrument facility centre (CIFC), IIT (BHU), Varanasi, for SEM, TEM and AFM facilities. One of the authors Santosh Pandey is grateful to IIT (BHU) for financial support for teaching assistantship.
Compliance with ethical standards
Conflict of interest
There is no conflict of interest among the authors.
- 13.Kim KM, Lee JH, Lee KM, Kim DY, Riu DH, Lee SB (2008) Microstructural evolution and dielectric properties of Cu-deficient and Cu-excess CaCu3Ti4O12 ceramics. Bull Mater Sci 43:284–291Google Scholar
- 16.Sun X, Wang C, Wang G, Lei C, Li T, Liu L (2013) Low-temperature relaxations associated with mixed-valent structure in Sr2TiMnO6. J Am Ceram Soc 96:513–518Google Scholar
- 18.Kawrani S, Boulos M, Cornu D, Bechelany M (2019) From synthesis to applications: copper calcium titanate (CCTO) and its magnetic and photocatalytic properties. Chem Open 8:922–950Google Scholar
- 31.Huang CJ, Li K, Wu SY, Zhu XL, Chen XM (2015) Variation of ferroelectric hysteresis loop with temperature in (SrxBa1−x)Nb2O6 unfilled tungsten bronze ceramics. J Mater 1:146–152Google Scholar