20January2017

Nano-Micro Letters

A Short Review on Copper Calcium Titanate (CCTO) Electroceramic: Synthesis, Dielectric Properties, Film Deposition, and Sensing Application

Mohsen Ahmadipour1, Mohd Fadzil Ain2, Zainal Arifin Ahmad1,*

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Nano-Micro Letters,Volume 8, Issue 4pp 291–311

First Online:  30 March 2016 (Review)

DOI: 10.1007/s40820-016-0089-1 

*Corresponding author.

E-mail: srzainal@usm.my Tel: +60-04-599 6128

 

Abstract

 


Flow chart for the synthesis of materials by solid-state route.

Electroceramic, calcium copper titanate (CaCu3Ti4O12, CCTO) with high dielectric permittivity (ε), approximately 105 and 104 for respective single crystal and bulk materials, that are produced for a number of well-established and emerging applications such as a resonator, capacitor, and sensor. These applications take advantage of the unique properties achieved through the structure and properties of CCTO. This review comprehensively looks into the primary processing routes, effect of impurity, dielectric permittivity, and deposition technique used for the electroceramics processing with the chemical composition and micro- and nano-structures. Emphasis is given to versatile and basic approaches that allow one to control the micro-structural features that ultimately determine the properties of the CCTO ceramic. Despite the intensive research in this area, there is no literature available that provides all the possible relevant information about CCTO fabrication, structure, the factorsinfluencing its dielectric properties, CCTO immobilisation and sensing applications. 


 

Keywords

CCTO;Chemical synthesis;Dielectric permittivity; Loss factor;Deposition;Sensitivity

 

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1. Introduction

 

Calcium copper titanate (CCTO) has the chemical formula of CaCu3Ti4O12,a novel electroceramic material with high dielectric permittivity (ε), approximately 100,000 for single crystal and 10,000 for bulk material at room temperature. In addition, CCTO shows moderate dielectric loss (tanδ~0.15) at a broad frequency region (upto 106 Hz), high positive temperature coefficient of resonant frequency (τf ~ +9.13 ppmK-1), and phase transition stability against temperatures (100-400 K).Subramanian et al. [1] discovered in 2000 thatCCTO belongs to family of ACu3Ti4O12 (A = Ca, Sr, Ba, Bi2/3, Y2/3, La2/3) type oxide of pseudo-cubic perovskite-related structure (space group: Im3) with a lattice parameter of 7.391 Å. The huge value of εkeeping constant from 100 to 400 K, leading to widepotential applications [2-4].

The advancement of technologyrequires a material with giant ɛ to reduce the size of electronic components, whereas the effective performance of these electronic components requires substantially low tanδ. For this reason, a great number of theoretical and experimental researches have been done to reveal the nature and origin of the giant ɛ of CCTO ceramics. Various processing routes (chemical and physical methods) of CCTO were adopted such as solid-state reaction, wet-chemistry route, sol-gel, solution combustion synthesis, sonochemical assisted route, and co-precipitation [5-10]. The solid-state reaction is common forsynthesisof CCTO from CaCO3, CuO, and TiO2 at high temperature accompanyingheterogeneitydrawbacks of precursor materials and long reaction time [11-14]. These drawbacks are someother secondaryphases which appears during synthesis because of limited atomic diffusion throughmicrometer sized grains. It is well known that the electrical properties can be remarkably enhanced when ceramic has a homogeneous microstructure. Othermethods improve CCTOcharacteristics, but not without some drawbacks.

Study of CCTO has been interesting in terms of its molecular structural and wideapplications.For instance, ithas been applied in capacitor, antenna, microwave devices and sensors. CCTO for sensor applications shows benefit from its polycrystalline porous nanostructures point of view. One of the most significant properties of CCTO sensor is to detect and monitor gases and toxic species without decomposition or change in their structural arrangements. Researches have been done on CCTO to enhance its sensitivity and selectivity on gas sensing, chemical- and bio-sensors. However, few work has been published on some aspects of CCTO sensing applications and this still remains asa potential research area. The CCTO gas sensor is considered non-ohmic device because electrical properties are greatly dominated by grain-boundary interface states[15]. This non-ohmic ceramic devices are also known as “metal-oxide” varistors (variable resistors) whose applications are technologically important because of their electrical characteristics that enable them to be used as solid state switches with large-energy-handling capabilities. The varistors are also know as voltage-dependent resistors because they show a highly nonlinear current–voltage (I-V) characteristic.The first voltage-dependent resistors polycrystalline ceramics were developed around early the 1930s by the Bell System and consisted of partially sintered compacts of SiC. A voltage-dependent resistors system with very superior performance based on CCTO composition was announced in 2005 by Il-Doo Kim[16] although parallel developments were conducted in Brazil in the early of 2008 [17]. This research  revealed a mechanismresponsible for remarkable electron transportation (n- type or p-type conductivity) which depends on the synthesis methods and experimental conditions [18, 19].

This study reviewes the principal methods for CCTO fabrication, structure, factor influence on dielectric properties, immobilisation of  CCTO, and sensing applications. Following the limited studies by researchers on CCTO electroceramic as a gas sensor, there is no comprehensive reviews on its sensing applications while a few available onlyfocus on the dielectric property of bulk ceramics and dense films.

 

2. Synthesis Methods of CCTO

 

Different synthesis methods have been adopted by various reseachers to obtain CCTO electroceramic in order to tailor its dielectric properties.

 

2.1 Solid-state Reaction Method

 

CCTO is generally synthesized by a conventional solid-state method or dry route[20, 21].  The method normally uses stoichiometric amount of the common precursors i.e. CaCO3, CuO, and TiO2 with suitable liquid (acetone or ethanol), mixed with ball milling. To remove volatile impurities, the fine powder is calcined at 930 °C for a marked period of time (12 h). An appropriate amount (1ccmL-1) of a suitable binder (PVA) is added to powder and mixed uniformly before beingpressed into a pellet shape. The resulting product is first heated slowly (heating rate 5 °Cmin-1) to a particular temperature (1,040 °C) to burn off the binder. The mixture is then maintained at this temperature for 10h for annealing. The sample is later cooled under controlled rate of a cooling (cooling rate 5°Cmin-1). A systematic flow chart of a solid-state reaction method is shown in Fig. 1.

Flow chart for the synthesis of pure CCTO by solid-state route.

Fig.1 Flow chart for the synthesis of pure CCTO by solid-state route.

CCTO chemical reaction during conventional solid-state reaction method is shown in Eq. 1:

                                                                                               eq1                                                                                                    (1)

Shao et al. [22] prepared CaCu3Ti4O12 ceramics by the conventional solid-state reaction method under various sintering temperatures from 1,000 to 1,120 °C at an interval of 10 °C. It was reported that the morphologies changed significantly with the sintering temperature. Ceramic specimens prepared by this method had a good polycrystalline structure in spite of the different microstructures. The ɛ value was found to increase with the sintering temperatureand have a close relation with the polycrystalline microstructure and particularly the grain size.

However, solid-state route method suffers the disadvantages of inhomogeneity,and requires repetitive grinding,requirement of firing at high temperatures and a prolonged reaction time.

 

2.2 Wet-Chemistry Method

 

Wet-chemistry method is a type of combustion synthesis technique that is based on redox reaction between a fuel and oxidant in a precursor solution. Generally, citric acid, urea, and ethylene glycoletc. are used as a fuel while nitrates of different metals are used as oxidants. The chelating agents like EDTA and acetic acid etc. can form complex with metal ions in the precursor solution and act as a fuel. This complex in the dehydration process produces a viscous gel which can further self-ignites with the evolution of huge amount of gases. In the combustion method, the calcium/copper/titanium/citrate aqueous solution becomesa viscous, transparent blue gel after evaporation of water. After heat treatment, all organic materials burned out and a voluminous porous black sample is left behind. This leads to the remnant of fine phase pure powder. The systematic flow chart of the wet-chemistry method is depicted in Fig. 2.

 Flow chart for the synthesis of materials by a wet-chemistry method.

Fig.2 Flow chart for the synthesis of pure CCTO by a wet-chemistry method.

Liu et al. [23] reported synthesis of fine CCTO powder using the wet-chemistry method at relatively low temperatures and a shorter reaction time. The pure-phase sample was obtained at 800 °C for 5 h and the grain size of a pellet sample sintered at 1,030 °C for 4 h. The sampleshad a homogeneous distribution in the range of 0.4-1.5 μm. This method started with a homogeneous liquid solution of cation ingredients mixed in stoichiometric ratio at the atomic scale. Therefore, pure samples at nanoscale could theoretically be obtained at lower temperatureand a shorter reaction time than that obtainedby solid-state reactions.

 

2.3 Sol-gel Method

 

This method is also known as the Pechini, or liquid mix, process and widely used in the fields of material science and ceramic engineering.The sol-gel process is also known as chemical solution deposition. In 1967, Maggio P. Pechini developed a sol-gel method for lead and alkaline earth titanates and niobates, materials that do not have favorable hydrolysis equilibria. Such methods are used primarily for the fabrication of materials (typically a metal oxide) starting from a chemical solution (or sol) that acts as the precursor for an integrated network (or gel) of either discrete particles or network polymers. Typical precursors are metal alkoxides and metal chloriddes, which undergo various forms of hydrolysis and poly condensation reactions. Factors thatneed to be considered in a sol-gel process are solvents, temperature, precursors, catalysts, pH, additives, and mechanical agitation. These factors can influence the kinetics, growth reactions, and hydrolysis and condensation reactions. The solvent influences the kinetics and conformation of the precursors while pH affects the hydrolysis and condensation reactions. Acidic conditions favor hydrolysis, which means that fully or nearly fully hydrolyzed species are formed before condensation begins. Under acidic conditions there is a low cross-link density, which yields a denser final product when the gel collapses. These reactions are carried out at room temperature and further heat treatments need to be done to get to the final crystalline state. Sol-gel routes can be used to prepare pure, stoichiometric, dense, and particles of CCTO [24-26]. The flow chart of synthesis of CCTO is shown in Fig. 3.

 Flow chart for the synthesis of materials by a sol-gel method.

Fig.3 Flow chart for the synthesis of materials by a sol-gel method.

Liu et al. [27] have synthesized CCTO with the giant ɛ material by a sol-gel method using nitrate and alkoxide precursors. The ɛ of CCTO was found to be three times smaller than that synthesized by another low-temperature methodsincludingsolid-state reaction method. Theirresults were explained by internal barrier layer capacitor (IBLC) model of Schottky barriers at grain boundaries between semiconducting grains. A sol-gel process showed considerable advantages, including excellent chemical stoichiometry, compositional homogeneity and lower crystallization temperature due to the mixing of liquid precursors on the molecular level [28, 29] compared with other techniques. Ion diffusing displacement is shortened in a sol-gel process. The pure phases of the powders were obtained on calcinations at 900 °C for 1 h. The ɛof CCTO ceramics was found to be 35,000 at 1 kHz in sintered samples at 1,060 °C for 48 h.

 

2.4 Combustion Synthesis Technique

 

Combustion synthesis in solid-state chemistry is also known as metathesis reactions or self-propagating high-temperature synthesis (SHS). From this method, homogeneous nanopowder, and multi-component single phase material can be produced. The SHS is one of the effective and economic methods [30] and has emerged as an important technique for the synthesis and processing of advanced ceramics, catalysts, composites, alloys, intermetallic, and nanomaterial [31]. The method exploits self-sustaining solid flame combustion reaction for the internal development for a limited short period [32]. The combustion synthesiswas first studied by Alexander Merzhavov in 1970s [33] and was reviewed by Gillan and Kaner [34]. A typical reaction occurred between a metal halide and an alkali or alkaline earth main group compounds.

The emphasis in the ceramic synthesis is the need for high reaction temperatures in order to achieve usable diffusion coefficients for the atoms in the reacting solids. Mechanical alloying offers an alternative to high reaction temperatures (i.e., high-pressure pulses), which themselves can be a source for local rapid heating. In the combustion synthesis, high temperatures again take center stage. Consequently, the high temperature is achieved not by placing the sample in a furnace, but using the heat generated by exothermic chemical reaction that sustained the high temperature for around 30 to 45 seconds to form desired product at the end of the reaction. Thus, the heating is spontaneous, and createsa large amount of gases during the combustion process with the consequent formation ofa nano size porous and foamy product. The mechanism of the combustion reaction is quite complex. The parameters that influence the reaction kinetics and mechanism include the type of fuel, the fuel to oxidizer ratio (Ψ), the use of excess oxidizer, the ignition temperature, and water content of the precursor mixture. It is well recognized that the fuel is an important component for the preparation of oxides by combustion synthesis.If the fuel to oxidizer ratio is unity, the energy released by the combustion is maximized.Fuel to oxidizer ratio and the calculation is expressed as Eq. 2:

 eq2                                                                                                              (2)

The ratio is crucial and effective in this method [35, 36].It can change the properties of the nanomaterial as the reaction temperature depending on fuel to oxidizer ratio. Generally, citric acid, urea, ethylene glycol is used as fuel, and nitrates of different metals are used as an oxidant.

Patra et al. [37] have completely synthesized the nanopowder CCTO by citrate-nitrate gel combustion technique followed by calcination in the temperature range of 700-800 °C. The average crystalline size of synthesized powders was 66 nm and the particle size range was in between 189-300 nm. Sintering of the powders was conducted in air at 1000°C for 4 to 6 h. The X-rany diffraction (XRD) study revealed that CCTO powder calcined in 700 °C had some extra peaks that were unable to be identified at 800 °C, which XRD showed pure CCTO without any impurity. Scanning electronic microscopy (SEM)images of the sintered CCTO ceramics showed submicron grain size distribution of the sample in 1,000 °C (4 h)-1 and an illustrated matrix consisting of large grains, wherein the small grains were embedded between the large grains at 1,000 °C (6 h)-1. The samples exhibit a giant dielectric permittivityof 6800 and 23200, and dielectric loss 0.21 and 0.61for 1,000 °C (4 h)-1 and 1,000 °C (6 h)-1at 1 kHz, respectively. This method was found useful due to its relative effective and economy for obtaining a homogeneous and fine powder precursor. Figure 4 shows the combustion reaction flow chart.

Flow chart of CCTO powder synthesis by a solution combustion method [37].

Fig.4 Flow chart of CCTO powder synthesis by a solution combustion method [37].

 

2.5 Sonochemical-assisted Process

 

Sonochemical originates from the extreme transient conditions induced by ultrasound, which produces unique hot spots that can withstand temperatures and pressure above 5,000 K and 1,000 atmosphereswith the  heating and cooling rates in excess of 1,010 K s-1. The speed of sound in a typical liquid is 1,000 to 1,500 m s-1, and ultrasonic wavelengths will vary from roughly 10 cm down to 100 µm over a diminishing frequency range of 20-15 MHz,which is much larger than the molecular size scale. The chemical and physical effects of ultrasound therefore, arise not from a direct interaction between chemical species and sound waves, rather from the physical phenomenon of acoustic cavitation [38] that involves: a) formation, b) developing, and c) the implosive collapse of the microcavities. The acoustic waves crossing the liquids are generating a cavity phenomenon. When sound waves with sufficient amplitude propagate through a liquid, the liquid is under dynamic tensile stress and the density changes with alternating expansive and compressive waves. Bubbles aregenerated from pre-existing impurities (e.g., gas-filled crevices in dust motes) and oscillate with the applied sound field. Bubbles can grow through a slow pumping of gas from the bulk liquid into the oscillating bubble (rectified diffusion). Bubbles at a critical size (usually tens of micrometres) can couple strongly to the extremetransient conditions produced during acoustic cavitation. Thisallows the formation of unique materials and also permit syntheses on the benchtop in a room-temperature liquid which would otherwise require high temperatures, high pressures, or long reaction times. When a liquid is irradiated by high intensity ultrasound, high-energy chemical reactions occur. Sonochemistry was employed in the synthesis of materials from volatile or nonvolatile precursors, but generally through different mechanisms. Sonochemical activation of a starting material led to a precipitation reaction that improves the homogeneity of the final product.

Wongpisutpaisan et al. [39] synthesized the giant-dielectric-constant material CCTO by aonochemical-assisted process containing stoichiometric amounts of the metal nitrate, at a shorter reaction time than that of a conventional solid-state reaction.Synthesis from a solution affords intimate and homogeneous mixing of the metal ions at the atomic scale. Thus, this leads to the reduction of the diffusion path length. However a short diffusion length reduces reaction time. The diameter of the multiphase CCTO powders was 500 nm obtained by SEM and the average crystalline size calculated using the Scherrer’s equation from the full width at half-maximum (FWHM) of the strongest diffraction peak (220) of∼75nm. Wongpisutpaisan et al. [39] observed that as the frequency increased over 1 kHz, the ɛ rapidly decreased. In addition, the higher ɛvalues at lower frequency could be related to the possibility of presence of interfacial polarization. The systematic flow chart is illustrated in Fig. 5.

Flow chart of CCTO powder synthesis by sonochemical assisted process [39].

Fig.5 Flow chart of CCTO powder synthesis by sonochemical assisted process [39].

 

2.6 Co-precipitation Method

 

Co-precipitation reactions are among the oldest techniques for the synthesis of nanomaterials.The reaction precipitate substances are normally soluble under the reaction conditions. There are three main mechanisms of co-precipitation: inclusion, occlusion, and adsorption mechanisms [40]. An inclusion occurs when the impurity occupies a lattice site in the crystal structure of the carrier. The inclusion of impurities results in a crystallographic defect that occurs when the ionic radius and charges of the impurity are similar to those of the carrier. An adsorbate is an impurity that is weakly bound (adsorbed) to the surface of the precipitate. An occlusion occurs when an adsorbed impurity gets physically trapped inside the crystal as it grows.

Barbier et al. [10] synthesized CCTO powders by a soft chemistry method (co-precipitation method). The sintered pellets showed a high room temperature ɛr(∼1.4×105) and relatively small tanδ (∼0.16) at 1 kHz. The study suggested that the high dielectric permittivity observed in the material were not related to an interface but, related to mechanism due to the type of the capacitor (an internal barrier layer capacitor (IBLC)) used. The samples prepared from the powder was found to exhibit a bimodal grain size distribution, with small grains of about 20 μm and large grains of size ranging from 50  to 200 μm. Further, they found that the nature of the electrode contact had no influence on the ɛrand the losses values of CCTO pellets. The ɛrstrongly depends on the sample diameter but, the tanδ remains constant regardless of diameter. The systematic flow chart of preparation of CCTO powder by a co-precipitation method is showed in Figs. 6 and 7. Table 1 shows the comparison of CCTO powder preparation methods in terms of particle size. Similarly, the advantages and disadvantages of the various production methods are explained.

Various route for CCTO synthesis.

Table 1 Various route for CCTO synthesis.

A flow chart of CCTO powder synthesis procedure by a co-precipitation method.

Fig.6 A flow chart of CCTO powder synthesis procedure by a co-precipitation method.

Preparation of CCTO by a co-precipitation method.

Fig.7 Preparation of CCTO by a co-precipitation method.

 

3. Strcuture and Dielectric properties

 

The crystal structure of CCTO [41] shown in Fig. 8 can be obtained from the ideal cubic perovskite structure by superimposing a body centered ordering of Ca2+ and Cu2+ ions share in A-site [42]. The size difference between Ca2+ and Cu2+ causes the TiO6 octahedra to undergo remarkable tilting, leading to a body centred cubic supercell of space group Im3. Consequently, the Ti4+ ions engrossed centrosymmetric position in the octahedral sites. The tilted angle is so appropriate such that the Cu2+ ions occupy mostly a square-planer environment [43].Tilting also significantly changes the coordination environments of the A-site cations which  lead to a 4-coordinate square-planar environment for Cu and a 12-coordinate icosahedral environment for Ca. It is the mismatch in size and bonding preferences of these two ions and the titanium that drives the large octahedral tilting distortion. The Ti4+cations could be displaced off center along their one-threefold axis. However, this cannot be a pure ferroelectric transition, because the displacements occur along four different directions. Thus, CCTO has a perovskite-type structure where ɛr is increased by tension on the Ti-O bonds. Figure 9 illustrates the unit cell of body-centered cubic CCTO with an Im3 space group, which consists of two formula units. The Ti atoms sit at the center of the canted TiO6 octahedra (the tilt angle is nominally 141°), with bridging Cu atoms bonded to the oxygen atoms, and large Ca atoms at the corners and center of the unit cell.

A carystal structure of CaCu3Ti4O12. Large white blue atoms are Ca, medium-size dark blue atoms are Cu, red atoms are O, and atoms in the octahedra centers are Ti [43].

Fig.8 A carystal structure of CaCu3Ti4O12. Large white blue atoms are Ca, medium-size dark blue atoms are Cu, red atoms are O, and atoms in the octahedra centers are Ti [43]

Two main features are needed for any dielectric material in practical applications: high ɛrand low tanδ.

 

3.1 Dielectric Permittivity

 

Theɛris the ratio of the capacitance of a capacitor filled with the given material to thecapacitance of an identical capacitor in a vacuum without dielectric material. The insertion of a dielectric between the plates of a parallel-plate capacitor always increases its capacitance, or ability to store opposite charges on each plate, compared with the ability when the plates are separated by a vacuum. If Cʹ is the value of the capacitance of a capacitor filled with a given dielectric and C is the capacitance of an identical capacitor in a vacuum which is showed in Fig. 9. The ɛr is dimensionless, and simply expressed as ɛrCʹ/C. It denotes a large-scale property of dielectrics without specifying the electrical behavior on the atomic scale.

Dielectric permittivity concept.

Fig.9 Dielectric permittivity concept.

The ɛrof CCTO has been reported to range from 100 to 300,000 in pellets, films, single crystals and poly crystalline [44- 46].The highest value of εremains essentially unchanged from 100 to 400 K. A completely satisfactory explanation is not yet available for the unique and remarkable behavior. However, it is established that the behavior is not related to the ferroelectric transition [47], and the consensus is that the behavior is not intrinsic [48]. According to a well-established barrier layer mechanism for high ɛr, the high ε of CCTO has been interpreted by the conducting grains with insulating grain boundaries and its possible source for high ε values [49]. However, the exceptionally high εin CCTO crystals dictates that the insulating barriers must be inside the crystals rather than between them. Very high concentrations of twin boundaries seem always to be present in CCTO crystals. It has been suggested that these somehow become the insulating barriers. An understanding of the dielectric properties of CCTO requires an explanation of how this titanate develops, its conducting regions, and a description of the insulating boundaries within the conducting regions. Although, the high ɛrof CCTO can be rationalized based on its atomic structure, there is a good reason to suspect that the ɛrof this phase is enhanced by its microstructure. The dielectric behavior is scientifically interesting and technologically intriguing. Therefore, the origin of large dielectric permittivity of CCTO has attracted much attentions. So far, several modelsthough controversial have been proposed to explain the dielectric behavior. The mechanism for the giant dielectric constant of CCTO is still questionable, and investigators are studying it in an effort to understand whether the giant dielectric constant is intrinsic to a perfect crystal or extrinsic [50].

 

3.2 Dielectric Loss

 

Tangentδ (tanδ ) or energy loss in a dielectric is due to an alternating electric field, which is a material property rather than  geometry property of a capacitor. Usually the tanδ, expressed as the dissipation factor (Df) or loss tangent (tanδ) can be mathematically defined as the Eq. 3:

                                                                                                                          eq3                                                                                                                                              (3)

Where the angle δis supplementary angle of the phase difference between the applied electric field and the induced current. ʹʹand ʹ is the real and imaginary part of the ɛr. The value of tanδ has been found to be about 0.115 at 1 MHz at room temperature [51]. Therefore, CCTO which presents a large ɛr, fails to meet the second requirement. To reduce the dissipation factor (Df) by keeping the ɛrconstant, different methods were employed. These include the techniques such as applying different sintering methods or doping additional oxides.In general, dielectric loss of a dielectric material results from distortional, dipolar, interfacial, and conduction loss. The distortional loss is related to electronic and ionic polarization mechanisms. The interfacial loss originates from the excessive polarized interface induced by the fillers, and specifically the movement or rotation of the atoms or molecules in an alternating electric field. The conduction loss is attributed to the dc electrical conductivity of the materials which represents the flow of actual charge through the dielectric materials.

 

3.3 Factors Effecting on the Property and Morphology of CCTO

 

Like the influences of doping onCCTO, it seems that different processing conditions can significantly affect the dielectric properties as well. ForCCTO, Bender and Pan [52] examined the effects of the processing conditions in detail by using various conditions including the powder mixing, firing temperature (both calcination and sintering), and annealing. They revealed that the ɛrincreased when the CCTO powder was mixed via a milling method accompanied with a higher sintering temperature and a longer sintering time. They also suggested that improvement ofɛrcan be attributed to higher concentration of defects in the grain core.

 

3.3.1 Effect of Sintering Temperature and Time

 

Sintering of CCTO electroceramic is the method that involves heating the CCTO powder green compact part to a high temperature below the melting point when the material of the  separate particles diffuse to the neighboring powder particles. The driving force of sintering process is reduction of surface energy of the particles caused by decrease in their vapor-solid interfaces. The reduction in energy is accomplished by an atomic diffusion process. The process leads to densification of the body by transporting matter from inside the grains into the pores microstructure of the matter between different parts of the pore surfaces leading to a decrease in the pore volume. Sintering process may be conducted in different atmospheres: inert, and air atmosphere.

Typically, Sinclair et al. [53] reported that  the impedance measured at 1 kHz showed great enhancement of ɛrfrom ~ 7,700 of 900°C to ~60,000 of 1,050 °C at 3 h, as illustrated in Table 2.  Consequently, the temperature shows a compelling effect on the CCTO grain size, as in Fig. 10a, b. It is also obvious that with increasing grain size, there is corresponding decrease in the total volume faction of boundary layer of the CCTO. Further, the overall permittivity decreases because of the total number of capacitors that constructed by the micro layers increases. Whereas, the total conductivity decreases as parts of the insulating boundary layer vanishes. Furthermore, time has a characteristic effect on the grain size of CCTO with consequent effect on the ɛr. The grain size reported for the CCTO electroceramic increases from less than 30µm to around 300µm as in Fig. 10c, d.The ɛrare ~ 9,000 for 3 h to ~280,000 for 24 h for CCTO ceramics sintered at1,100°C, as revealed in Table 3.The ɛrobviously increases with the sintering temperature and time as shown in Tables 2 and 3. It is also accordingly closely related to the polycrystalline microstructure, particularly to the grain sizes.

Effect of sintering temperatures on dielectric permittivity of CCTO.

Table 2 Effect of sintering temperatures on dielectric permittivity of CCTO.

Effect of sintering times on dielectric permittivity of CCTO.

Table 3 Effect of sintering times on dielectric permittivity of CCTO.

SEM images of ceramic microstructure for CCTO ceramics sintered for 3h for a) 900 °C,and b) 1050 °C. CCTO ceramics sintered at1100 °C for 3 h (c) and 24 h (d).

Fig.10 SEM images of ceramic microstructure for CCTO ceramics sintered for 3h for a) 900 °C, and b) 1050 °C. CCTO ceramics sintered at 1100 °C for 3 h (c) and 24 h (d).

 

3.3.2 Effect of Doping

 

A doping method opens an effective way to alter the electric performance, both high ɛand low tanδare essential for different applications. The dielectric material that has a low dielectric loss and low ɛris useful in insulators. However, high ɛr and low tanδare desirable for a capacitor component. Co2+/Co3+ doping was studied by Chiodelli et al. [61]. The adopted doping dramatically increased the ɛrof CCTO as shown in Table 4. Especially in the case of 5% Co doping, the ɛrwas raised to 147,000, which is 15 times larger than that of CCTO.

Effects of doping in CCTO.

Table 4 Effects of doping in CCTO.

The reasons for the observed wide variation in dielectric properties for CCTO are unknown: it is because the nature of its giant permittivity is still open to scientific debate.

Hong et al. [64] studied the effects of isovalent dopant Nb on the dielectric properties of CCTO. The CCTO was doped with different concentration Nb5+(x= 0, 0.02, 0.1, 0.2, 0.4). They found that the dopant concentration has profound effect on the ɛrof CCTO. The resistivity of boundary layers and ɛrof the CCTO increase with increasing dopant concentrations.

XRD was utilized in their work to observe changes in lattice parameters and phase evolution. The XRD data showed a modest linear increase in lattice parameters with the addition of dopant up to x = 0.4. Therefore, they concluded that the added Nb5+ was most likely present in the microstructure either in the grain boundaries or as a secondary phase. The XRD data also showed that there was no secondary phase as the grain size reduced to ~7 µm with x = 0.1 Nb5+ substitution.

Vangchangyia et al. [69] synthesized Ca1−xSrxCu3Ti4O12 (x= 0.05, 0.1, 0.15, 0.2) ceramics by using a simple thermal decomposition methodand studied their dielectric properties. It was found that by increasing the Sr2+ concentrations from 5 to 15 at%, there was slightlydecreased in tanδ (004). However, εr was reduced with the value is still higher than104.

Jin et al. [83] examined the effect of La3+ on the compositions of Ca1-xLa2x/3Cu3Ti4O12. It was found that these ceramics show enhancing in εr (1,700 to 1,1000 at 103Hz) and sufficiently low tanδ (02). Over the frequency range from 102 to 105 Hz, tanδ values at x = 0.05–0.20 were small and almost not varied with frequency. La3+ doping plays important roles in the observed excellent dielectric properties in Ca1-xLa2x/3Cu3Ti4O12 ceramics.

 

4. Deposition Techniques of CCTO Thin Films

 

Many researchers focused on the preparation of CCTO thin films [91-93] due to their unusual dielectric properties and potential applications for microelectronic devices. The relationship between the dielectric properties and microstructures of the deposited thin films was studied in terms of the different substrates via pulsed laser deposition (PLD) [92, 94- 96] or sputtering [97] to metal organic chemical vapour deposition (MOCVD) [98, 99] or sol-gel [100-102].

 

4.1 Pulsed Laser Deposition (PLD)

 

The PLD technique is very effective and well suited for developing epitaxial films, and allows fabrication of multilayer’s, hetero-structures, and super lattices. The technique was first used by Smith and Turner in 1965 for the preparation of semiconductors and dielectric thin films [103]. The first research on CCTO thin films was published in IEEE proceedings by Cho et al. [92]. Adoption of PLD method caused ceramic thin films to grow on a substrate like LaAlO3 (LAO) (100) and SrRuO3. A study about the PLD parameters to obtain optimized properties in epitaxial CCTO films and comparison between these films and polycrystalline thin films was also done. They concluded that the dielectric responses and mechanisms were similar in both cases; i.e. for both cases, the dielectric behavior can be modelled by series combination of two parallel R-C circuits. The gigantic relaxation process in oriented epitaxial thin-film CCTO was proposed to be caused by the mutual interaction of the domain volume resistance and the domain boundary capacitance. Domain boundaries are likely to be at twin boundaries within the oriented epitaxial thin CCTO film.

Fang et al. [94] reported to obtain results comparable to epitaxial thin films with thin films produced by PLD on Pt/Ti/SiO2/Si substrate. The influence of PLD parameters was studied and the best films were obtained with oxygen pressure of 26.6 Pa and 720°C. The values obtained were 2,000 for ɛrat 10 kHz and below 0.5 for the tanδ. Fang et al. [104] also studied the influence of multilayers CaTiO3/CCTO and CaTiO3/CCTO/CaTiO3. Samples were produced by PLD with thicknesses of 8-24 nm for CaTiO3 (CTO) and 500 nm for CCTO. Simple CCTO was made as comparison and the results showed that loss is largely reduced by the introduction of CTO buffer layers while ɛrvalue also increased. At 10 kHz the ɛrof CCTO was about 1,005. The ɛrvalue for CTO/CCTO/CTO was about 1507 (w/ CTO 16 nm). Besides, the tanδof CCTO was 0.17 while the tanδfor the double-buffered was 0.106. The authors also presented a study about the introduction of a SiO2 layer with different thicknesses between two layers of CCTO [105]. Films were obtained also by PLD technique. They reported the lower value of tanδand the leakage current density of the multilayer thin films with decreased value of ɛr. Two reasons were pointed out to explain this behavior: one was the improvement in the crystallinity and the other was the reduction of the free carriers in the multi-layered films. Multilayer films with 20 nm SiO2 layer showed a tanδof 0.065 at 100 kHz and a ɛrof approximately 150. The main drawbacks of PLD are an inhomogeneous energy distribution in the laser beam profile which consequently gives rise to an inhomogeneous energy profile and angular energy distribution in the laser plume.  Due to the involvement of the high laser energies, macroscopic and microscopic particles from the target can be ejected. This effect isdetrimental to the desired properties of films and multilayers [94].

Deng et al. [96] investigated experimentally the relationship between the electrical properties and the oxidation states of Cu and Ti in CCTO thin films deposited by pulsed laser deposition (PLD) on top of 150 nm thick SrRuO3 conductive layer on top of a LaAlO3 single crystal.Their results demonstrated that the as-deposited CCTO film was made of insulating grain boundaries with semiconducting grains, indicating that the high dielectric-constant can be attributed to barrier layer capacitor (BLC) effects. The capacitance of the grains and grain boundaries can be tuned by changing the annealing atmosphere and temperature. Under an oxygen-absent annealing atmosphere, the electric resistances of the grain boundaries changed greatly but the resistance of the grains showed almost no change. While under an oxygen annealing atmosphere, the reverse process occurred. On the basis of this result, it is demonstrated that the origin of the dielecteric of the grains in CCTO films arises from their oxygen-loss, while the grain boundaries are close to oxygen-stoichiometry. The high apparent ɛrvalue in the CCTO thin film can be suppressed by annealing the sample in air.

 

4.2 RF Sputtering

 

Prakash et al. [97] reported high quality films with preferential (220) orientationobtained at a substrate temperature of 650°C under total pressureof 4.86 Pa with 1% O2.The ɛrwas reported to be ~5,000 at 1 kHz and 400 K. Also, the frequency of the dielectric relaxation in thin films was found to be much lower than that observed in bulk ceramics and the dielectric relaxation was much higher.

 

4.3 Metal Organic Vapour Deposition (MOCVD)

 

Nigro et al. [98] analyzed a microstructural of thin films of CCTO prepared by MOCVD on LaAlO3 substrates. Unlike other referenced works in this review, they did not report the dielectric response of the CCTO. However, they asserted that the deposited MOCVD films consisted of CuO grains embedded in a quite amorphous matrix of Ca–Ti oxides. After the in-situ annealing step at 900 °C, CCTO phase was formed and the XRD patterns showed the formation of (100) oriented CCTO films. In the case of LaAlO3 (100) CCTO films, it was found that 1,100 °C treatment for 24 h resulted in a very rough and porous surface. By contrast, the same annealing treatment carried out on amorphous CCTO samples, revealed the formation of large rounded grains (600–1,000 nm) that were homogenously distributed on the surface. However, after rapid thermal annealing processes at 1,100 °C,  substantial and flat grains (about 5 μm) were observed, which were similar to those of CCTO ceramics with very high ɛrvalues. Evidences of the formation of CCTO and CaTiO3 phases were also reported for the films. The group reported in another paper [100] the influence of low ɛr(low-k) layers of SiO2 and Si3N4 on CCTO morphological properties without conducting dielectric characterization. SiO2 showed better crystallinity than Si3N4 due to oxide nature of the buffer layer.

Fiorenza et al. [106] also presented an overview from the process of growing thin films of CCTO to the assessment of the permittivity. Hot-wall MOCVD technique was used to obtain thin films on LaAlO3 (001) substrates. The microscopy property was investigated with atomic force microscope(AFM) equipped with the scanning capacitance microscopy module used in scanning impedance configuration. The investigation demonstrated the presence of a surface depleted layer at the electrode/CCTO film interface, and simultaneously a huge ɛrof 8,000 was measured as an extrinsic local behavior. The absence of the barrier at the macroscopic scale for the films thermal treated at 900 °C was explained by the presence of conducting leaking regions on the films. However, the presence of Schottky barrier was observed for the larger grains of the sample heat-treated at 1,000 °C with the rising of a local colossal permittivity.

 

4.4 Sol-Gel

 

A sol-gel is another method which yields thin film layers. Compared to other methods, sol-gels offer several advantages such as: (i) good physical and mechanical strength, (ii) low swelling capacity of the material in aqueous or organic solvents, (iii) high chemical inertness, (iv)lower temperatures for processing, and (v) high thermal and photochemical stability. Furthermore, sol-gel based films require low immobilization temperatures which result in preservation of the properties of the CCTO thin film. Although the sol- gel process is known for over 150 years, interest in the process began only in the 1980s after some studies showed the possibility of incorporating organic molecules and active proteins in porous ceramic matrices.

Jiménez et al. [107] achieved appreciable electrical properties in CCTO films obtained by sol-gel method using a non-methoxyethanol route. Solution was produced using a titanium diol-based precursor, obtained by refluxing of Ti (IV) with 1,3-propanediol in the ratio of 1:1 with two solutions of copper acetate and calcium acetate in 2-ethyl-hexanoic acid in the ratio of 1:10. The group reported tanδvalues at maximum in the range of 0.2-0.5 and ɛrof 200-400 at room temperature,that depended on the frequency for spin-coated films heat treated at 650°C.Moreover, Shen et al. [108] studied the switching resistance characteristics dependence on the annealing parameters of thin CCTO films obtained by a sol-gel on silicon based substrates. Solution without methoxyethanol was composed of calcium and cupric acetates, acetic acid, titanium isopropoxide, ethylenglycol and formamide. The CCTO films showed resistance switching phenomena when annealed at 700 °C and above. With increasing annealing temperature the crystallinity of the films improved. No report on the dielectric properties of these films was published.

Consequently, reliable results were reported for CCTO films prepared by physical deposition methods as sputtering and PLD. For these techniques,ɛrwas measured to be 6,000 for epitaxial films, 2,000 for polycrystalline ones with PLD, and 5,000 for polycrystalline sputtered thin films. The tanδreported with these techniques were 0.5-0.2 for PLD and no values were found for sputtered films. Compared to chemical solution deposition methods, these results are very good but those techniques used are very expensive and complex with long time procedures. In addition, it is difficult to control the stoichiometry of the films. Within the chemical solution deposition methods, the sol-gel [109, 110] and MOCVD have been the most preferred methods used to prepare CCTO thin films.

 

5. Appliaction of CCTO Films as Sensors

 

Semiconducting metal oxide (SMO) sensors are one of the most widely studied groups of chemiresistive gas sensors. These sensors are designed to react with one class of gases whereby the SMO undergoes reduction and oxidation. This process causes the SMO sensors to exchange electrons with the target gas at a certain characteristic rate, thereby affecting the sensor’s resistance and yielding a certain signal. The reaction of SMO materials with gases and the result of the conductometric changes were introduced in the early 1950's by Brattein et al. [111]. The direct applications of the SMO sensors as catalysts and electric conductive detectors towards various gases were then introduced by Seiyama et al. [112].

During the past few decades, SMO gas sensors have become a prime technology in many industrial gas sensing systems. Gas sensor technologies are still evolving and yet to reach their full potential in capabilities and usage. Gas sensors based on metal oxides thin films such as MgO, Cr2O3, NiO, SrO, In2O3, GeO2, Nb2O5, Ta2O5, La2O3, [113] are commonly used in the monitoring of humidity. Gases are based on electrochemical behavior, catalytic combustion, or resistance modulation of SMO [114-120] and can provide the necessary sensitivity, and stability required by such system. However, it has difficulties in measurement of electrical conductivity because of the fact that those materials have large band gaps thereby causing easy formation of electrons and holes. Due to this reason,CCTO pioneered by Subramanian et al. [1] is being usedto improve the sensor properties.

A CCTO based sensor - that only few researchers’ have worked on - can be defined as an analytical device, which consists of an immobilized layer of CCTO on a solid support. If the structure and properties of the CCTO are preserved after immobilization, it will recognize the analyte. The chemical reaction between the immobilized CCTO and analyte will then be transformed into an electrical signal, which will be amplified and converted by signal processing equipment into a display (Fig. 11). Chemical sensors should transform chemical quantities into electrical signals. Chemical sensor should respond exclusively to one analytic, or at least be selective to a group of analyses.

Schematic

Fig.11 Schematic representation on how an appropriate CCTO based electrochemical sensor works.

 

5.1 Sensing Mechanism

 

Considering the influential factors on gas sensing properties of metal oxides, it is necessary to know the sensing mechanism of a metal oxide gas sensor. The exact fundamental mechanisms that cause a gas response are still adaptable, but the main responsible reason for conductivity is trapping of electrons at adsorbed molecules by charged molecules.

Figure 12 illustrates the surface chemical reaction mechanism of CCTO gas sensor upon exposure to reference gas with gas analytes and the microstructure of the sensor on the electrical response to the gas [121]. The surface chemical reaction can be referred to as the receptor function of the sensor, because it converts a chemical reaction with the gas analyte into a charge transfer event. This motivates an electrical signal, which is amplified by the charge transport mechanism through the sensor or the so-called transducer function of the sensor [122-124].

(a)

Fig.12 (a) Chemisorption and reaction between reducing gases and oxygen adions (O-) at the surface give rise to the receptor function. (b) electronic charge transport through the grains and across grain boundaries gives rise to the transducer function. The latter depends on the microstructure of the sensing layer, e.g. on the grain size and pore size. (c) shows the sensor element comprises of the sensing layer, electrodes for electrical measurements, substrate and integrated microheater [125].

The important factor that influences the performance of CCTO as a gas sensor is the environmental humidity. However, mechanism of sensing water vapor and other pollution gases is different. For CCTO humidity gas sensors, ionic-type humidity sensors are the most common patterns. The conduction mechanism depends on H+ or H3O+, from dissociation of adsorption water [126]. The reaction between the surface oxygen and the water molecules conduces to a decrease in baseline resistance of the gas sensor, and results in a decrease of the sensitivity [127]. Secondly, the adsorption of water molecules leads to less chemisorption of oxygen species on the sensing layer due to the decrease of the surface area that is responsible for the sensor response. Thus, the sensitivity decreases and the response and recovery times increase. 

 

5.2 Sensor Properties 

 

Two important factors in sensor properties are sensitivity and selectivity [128-132]. In general, sensitivity depends on the sensor size, activity of the deposited CCTO towards the specific analyte,success in CCTO immobilization by keeping as much activity as possible [16] and also dopant [116]. Figure 13 gathers examples of these characteristics for a Al2O3 substrate coated with CCTO. The selectivity deals with the study of interferences thatresults mostly from the presence of electroactive species in the test medium. A CCTO based sensor will be selective if the response to the substrate is high and the response to interferences is low. However, the selectivity of sensors for a given gas in a mixture of two or more reactive gases is debatable.

Main

Fig.13 Main characteristics of CCTO based sensor constructed and deposited on Al2O3 substrates with interdigitated Pt electrode and Amperometric response showing the sensitivity and selectivity of the sensor to successive injections of hydrogen [16].

On the other hand, the nonlinear response of a sensor should result from the slow rate of oxidation or reduction of the CCTO in the presence of the analyte. Highly nonlinear I-V characteristics with strong thermal activation were observed, as shown in Fig. 14. Similar nonlinear behavior has been reported recently for polycrystalline CCTO ceramics by Chung et al.[133].

I-V

Fig.14 I-V characteristics for CCTO based sensor at temperature 20 and 200 °C [16].

Li et al. [127] investigated the effect of addition of Mg to CCTO ceramic humidity sensors, which resulted in improved sensitivity and longtime stability as well as decreased hysteresis. However, sensitivity of doping enhanced CCTO can be seen in Fig. 15.

The

Fig.15 The humidity sensing characteristic of CaCu3−xMgxTi4O12 (x = 0, 0.1, 0.2, 0.3, 0.4, and 0.5) [127].

Gas sensing properties have also been reported by few researchers for CCTO thin films; the mechanism of a sensor signal is based on conductivity of the CCTO thin film but the type of conductivity in thin films are not well defined in the literature. Kim et al. [16], reported the preparation of macroporous CCTO thin films by PLD onto PMMA (polymethyl methacrylate) microsphere templated substrates. These films have n-type conductivity and higher H2 sensitivity compared to CO and CH4. N-type conductivity was also reported by Parra et al. [134], in mesoporous thin films prepared by the sol-gel method. However, Joanni et al. [17] showed that films prepared by rf-sputtering display p-type conductivity. Dependingon the synthesized method and experimental conditions, n or p type conductivity can be attained. Some of the representative developments in this area are summarized in Table 5.

Compilation of sensor-based on CCTO applied for gas monitoring.

Table 5 Compilation of sensor-based on CCTO applied for gas monitoring.

Also researchers reported that CCTO modified electrodes can catalyze and recognize specific analyte species. In order to do so, CCTO modified electrodes have to exhibit certain characteristics of sensors including response time, sensitivity, and concentration which is shown in Table 5. An overview of the literature in this field revealed that most modified electrodes satisfy the criteria of molecular recognition between the immobilized CCTO and the specific analyte species in terms of sensitivity and linear range, in some cases.

Although all described sensors comply with the concept of molecular recognition between the CCTO and the specific analyte. Unfortunately not all of them can be considered as appropriate sensors because most of them lack high selectivity and low response time. These are the most important characteristics that a good sensor should display. Given the great potential of CCTO in sensing, further research and development of these processes will allow the understanding of mechanisms of the sensing pathways.

 

5.3 Sensor Testing Setup

 

Figure 16 shows the sensor array which mainly consists of a target gas, a multi-component gas mixer, a mass flow controller unit, a testing chamber, a power supplier and heaters, and an electrometer for resistance measurement [121,135-138]. LabVIEW based software is mainly used to control all testing parameters and measurements during the experiment. The testing chamber consists of CCTO sensor platforms with the ability to control and measure each sensor’s temperature and resistance. The CCTO films are deposited on the sensing element as thin or thick film substrates. Thin film deposits are made via RF sputtering, LPD, and sol gel techniques. The sensor platform is bonded into a standard header and then placed in a test chamber and temperature adjusted before gas exposure into the chamber.

A

Fig.16 A general schematic for CCTO gas sensor devices.

 

6. Conculsions

 

The urgent demand for high ɛrceramic has been a key issue leading to the development of many application technologies. In view of this, scientists and technologists have shown the potential CCTO with high ɛrand low tanδvalues for such applications. The critical success of new CCTO materials lies in their methods of preparation which controls the morphology, particle size and its dielectric properties. In order to obtain these features, small particle size, narrow size distribution, uniform morphology, optimum crystallinity degree, high specific surface area, minimum defects, agglomeration, and homogeneous metal doping are required for the practical applications of CCTO powders. The only limitation which restricts the broad applications of this compound is the tanδ. Methods described in this overview were designed to obtain CCTO nano and micro-crystalline powders with desired structures and high ɛr. Since the number of surface atoms or ions becomes a significant fraction of their total number, and the surface energy plays an important role in the crystal structure. The materials in the nanometer size range exhibit some remarkable properties which can be exploited to get CCTO with ahigh ɛrand low tanδ. Besides, the gas sensing process is strongly related to the surface reactions. So, high surface areas can provide large reaction contact area between gas sensing materials and target gases. Porous structure with high surface areas seems to be the standard structure of CCTO gas sensor layers. Fabrication of microstructure based CCTO for novel generation of future advanced CCTO based sensors with appropriate properties in terms of sensitivity, detection limit; response time and stability are required. Adaptability should also be considered because the final goal of all these sensors is the discovery of analytes in real analytical, environmental, and biomedical samples. Other causes, such as temperature and humidity, also play important roles in the testing of sensitivity. Humidity will decrease the sensitivity and may be harmful to repeatability.The most important characteristic of these sensors are: very high sensitivity, simple signal (resistance), small size, simple constitution, possibility of manufacturing in very high volumes (these sensors can be manufactured using the microelectronic and MEMS technologies), and easy integration in electronic boards. All these advantages make it a promising technology for the future. This sensors can be envisaged to detect polluting gases as CO, SO2, NO2, O3 fuel gases (hydrocarbon and hydrogen), toxic gases (H2S, NH3) and volatile organic compounds.

 

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Citation Information

Mohsen Ahmadipour, Mohd Fadzil Ain, Zainal Arifin Ahmad. A Short Review on Copper Calcium Titanate (CCTO) Electroceramic: Synthesis, Dielectric Properties, Film Deposition, and Sensing Application. Nano-Micro Lett. 8(4), 291-311 (2016). http://dx.doi.org/10.1007/s40820-016-0089-1

History

Received: 14 December 2015 / Accepted: 7 March 2016

 

 

 

 


Additional Info

  • Type of Publishing: JOUR - Journal
  • Title:

    A Short Review on Copper Calcium Titanate (CCTO) Electroceramic: Synthesis, Dielectric Properties, Film Deposition and Sensing Application

  • Author: Mohsen Ahmadipour, Mohd Fadzil Ain, Zainal Arifin Ahmad
  • Year: 2016
  • Journal Name: Nano-Micro Letters
  • Publisher: OPEN ACCESS HOUSE SCIENCE & TECHNOLOGY
  • ISSN: 2150-5551
  • URL: http://dx.doi.org/10.1007/s40820-016-0089-1
  • Abstract:

    Electroceramic, calcium copper titanate (CaCu3Ti4O12, CCTO) with high dielectric permittivity (ε), approximately 105 and 104 for respective single crystal and bulk materials, that are produced for a number of well-established and emerging applications such as a resonator, capacitor, and sensor. These applications take advantage of the unique properties achieved through the structure and properties of CCTO. This review comprehensively looks into the primary processing routes, effect of impurity, dielectric permittivity, and deposition technique used for the electroceramics processing with the chemical composition and micro- and nano-structures. Emphasis is given to versatile and basic approaches that allow one to control the micro-structural features that ultimately determine the properties of the CCTO ceramic. Despite the intensive research in this area, there is no literature available that provides all the possible relevant information about CCTO fabrication, structure, the factorsinfluencing its dielectric properties, CCTO immobilisation and sensing applications. 

  • DOI: 10.1007/s40820-016-0089-1