Phenomenological model of grain boundary behaviour under a bias field in Nb-doped CaCu3Ti4O12 ceramics

In this work, the dielectric behaviour and capacitance-voltage (C-V) curves under an applied DC bias field of 1 wt% Nb-doped CaCu₃Ti₄O₁₂ ceramics have been studied. The dielectric properties reveal the existence of grain boundaries of different electrical nature. A new model is proposed to simultaneously explain the presence of insulating and conducting grain boundaries. At low frequency, the capacity curve of the material exhibits a double metal oxide semiconductor (MOS) capacitor-like behaviour and as the frequency is increased, the curve suffers an inversion showing a ferroelectric-like response. This behaviour does not correspond to ferroelectric domain movement phenomena but seems associated to charge accumulation on grain boundary regions.     

Dielectric relaxation behaviors of pure and Pr6O11-doped CaCu3Ti4O12 ceramics in high temperature range

Pure and Pr₆O₁₁-doped CaCu₃Ti₄O₁₂ (CCTO) ceramics were prepared by conventional solid-state reaction method. The compositions and structures were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The influences of Pr-ion concentration on dielectric properties of CCTO were measured in the ranges of 60 Hz-3 MHz and 290-490 K. The third phase of Ca₂CuO₃ was observed from the XRD of CCTO ceramics. From SEM, the grain size was decreased obviously with high valence Pr-ion (mixing valence of Pr³⁺ and Pr⁴⁺) substituting Ca²⁺. The room temperature dielectric constant of Pr-doped CCTO ceramics, sintered at 1323 K, was an order of magnitude lower than the pure CCTO ceramics due to the grain size decreasing and Schottky potential increasing. The dielectric spectra of Pr-doped CCTO were flatter than that of pure CCTO. The loss tangent of Pr-doped CCTO ceramics was less than 0.20 in 2 × 10²-10⁵ Hz region below 440 K. The complex impedance spectra of pure and Pr-doped CCTOs were fitted by ZView. From low to high frequency, three semicircles were observed corresponding to three different conducting regions: electrode interface, grain boundary and grain. By fitting the resistors R and capacitors C, the activation energies of grain boundary and electrode contact were calculated. All doped CCTOs showed higher activation energies of grain boundary and electrode than those of pure CCTO ceramics, which were concordant with the decreasing of dielectric constant after Pr₆O₁₁ doping.     

Dielectric properties of iron doped calcium copper titanate, CaCu2.9Fe0.1Ti4O12

CaCu_2.9Fe_0.1Ti₄O₁₂ (CCFTO) has been prepared by a novel semi-wet route and its dielectric properties have been studied in the temperature range 300-500 K. It is found that dielectric constant (?) decreases drastically in the frequency range 100 Hz to 1 MHz. Complex plane impedance and modulus analysis was done to understand this drastic decrease in ?. Oxidation state of various ions was studied using X-ray photoelectron spectroscopy (XPS). The decrease in the permittivity of CCFTO can be attributed to two factors: the suppression of the Ca/Cu disorder in CCFTO which is observed in CaCu₃Ti₄O₁₂ (CCTO) and the absence of the grain boundary internal barrier layer capacitance mechanism.     

Preparation and characterization of Mg-doped CaCu3Ti4O12 thin films

Mg-doped CaCu_3?xMg_xTi₄O₁₂ (x=0, 0.05, 0.1, 0.15, 0.2, at.%) thin films were prepared by a modified sol?gel method. A comparative study on the microstructure and electrical properties of Mg-doped CaCu₃Ti₄O₁₂ (CCTO) thin films was carried out. The grain sizes of the Mg-doped CCTO thin films were smaller in comparison to the undoped CCTO films. Furthermore, compared to undoped CCTO films, Mg-doped CCTO thin films obtained higher dielectric constant as well as excellent frequency stability. Meanwhile, Mg doping could reduce the dielectric loss of CCTO thin films in the frequency range of 10⁴?10⁶ Hz. The results showed that the Mg-doped CCTO thin films had the better electrical characteristics compared with the undoped CCTO films. The nonlinear coefficient of Mg-doped CCTO thin films at x=0.15 and x=0.1 was improved to 7.4 and 6.0, respectively.     

Charge compensation, electrical and dielectric behavior of lanthanum doped CaCu3Ti4O12

Mechanism of charge compensation on lanthanum, (La³⁺) substitution on Ca site in calcium copper titanate (CaCu₃Ti₄O₁₂), and its effect on resulting electrical and dielectric properties has been studied in the present investigation. For this purpose samples were prepared according to two stoichiometries viz. La_xCa_(1−3x/2)Cu₃Ti₄O₁₂ (x ≤ 0.09) and La_xCa_(1−x)Cu₃Ti₄O₁₂ (x = 0.03) by solid state ceramic route. The former represents ionic compensation while the later is in accordance with electronic compensation. Nature of charge carriers is identified by measuring Seebeck coefficient which is found to be negative in the entire range of measurement. In order to understand the mechanism of conduction, ac conductivity is measured as a function of temperature and frequency. Space charge polarization is the dominant polarization mechanism phenomenon at low frequency and high temperature while orientation polarization dominates at low temperature and high frequency. Impedance analysis confirms the formation of internal barrier layers which is responsible for high dielectric constant in these samples.     

Microstructures and electrical responses of pure and chromium-doped CaCu3Ti4O12 ceramics

Pure and chromium-doped CCTO (CaCu₃Ti₄O₁₂) ceramics were prepared by a conventional solid-state reaction method, and the effects of chromium doping on the microstructures and electrical properties of these ceramics were investigated. Efficient crystalline phase formation accompanied by dopant-induced lattice constant expansion was confirmed through X-ray diffraction studies. Scanning electron microscopy (SEM) results show that doping effectively enhanced grain growth or densification, which should increase the complex permittivity. The dielectric constant reached a value as high as 20,000 (at 1 kHz) at a chromium-doping concentration of 3%. The electrical relaxation and dc conductivity of the pure and chromium-doped CCTO ceramics were measured in the 300-500 K temperature range, and the electrical data were analyzed in the framework of the dielectric as well as the electric modulus formalisms. The obtained activation energy associated with the electrical relaxation, determined from the electric modulus spectra, was 0.50-0.60 eV, which was very close to the value of the activation energy for dc conductivity (0.50 ± 0.05 eV). These results suggest that the movement of oxygen vacancies at the grain boundaries is responsible for both the conduction and relaxation processes. The short-range hopping of oxygen vacancies as “polarons” is similar to the reorientation of the dipole and leads to dielectric relaxation. The proposed explanation of the electric properties of pure and chromium-doped CCTO ceramics is supported by the data from the impedance spectrum.     

CaCu3Ti4O12 ceramics from basic co-precipitation (BCP) method: Fabrication and properties

High dielectric CaCu₃Ti₄O₁₂ (CCTO) ceramics have been successfully prepared by a novel basic co-precipitation (BCP) method. Compared with the conventional solid-state and/or soft chemistry methods, the BCP method has many advantages such as relatively lower sintering temperature, shorter sintering time and lower costs. The XRD patterns confirm the formation of CCTO crystal phase in the as-prepared samples. Influences of initial ingredients and sintering condition on phase composition, microstructure and dielectric property have been investigated through series of trials. The correlation between the process of the grain growth and dielectric properties of final products has been explored. The final products exhibit the dielectric constants higher than 10,000 and the dielectric losses lower than 0.15 at 1 KHz.     

Electric and dielectric behavior of CaCu3Ti4O12-based thin films obtained by soft chemical method

CaCu₃Ti₄O₁₂ (stoichiometric) and Ca_1.1Cu_2.9Ti₄O₁₂ (non-stoichiometric) thin films have been prepared by the soft chemical method on Pt/Ti/SiO₂/Si substrates, and their electrical and dielectric properties have been compared as a function of the annealing temperature. The crystalline structure and the surface morphology of the films were markedly affected by the annealing temperature and excess calcium. The films show frequency-independent dielectric properties at room temperature which is similar to those properties obtained in single-crystal or epitaxial thin films. The room temperature dielectric constant of the 570-nm-thick CCTO thin films annealed at 600 °C at 10 kHz was found to be 124. The best non-ohmic behavior (α = 12.6) presented by the film with excess calcium annealed at 500 °C. Resistive hysteresis on the I-V curves was observed which indicates these films can be used in resistance random access memory (ReRAM).     

Dielectric properties of soft chemical method derived CaCu3Ti4O12 thin films onto Pt/TiO2/Si(1 0 0) substrates

Calcium copper titanate, CaCu₃Ti₄O₁₂ (CCTO), thin film has been deposited by the soft chemical method on Pt/Ti/SiO₂/Si (1 0 0) substrates at 700 °C for 2 h. The peaks were indexed as cubic phase belonging to the Im−3 space group. The film exhibited a duplex microstructure consisting of large grains of 130 nm in length and regions of fine grains (less than 80 nm). The CCTO film capacitor showed a dielectric loss of 0.031 and a dielectric permittivity of 1020 at 1 MHz. The J-V behavior is completely symmetrical, regardless of whether the conduction is limited by interfacial barriers or by bulk-like mechanisms. Based on impedance analyses, the equivalent circuit of CCTO film consisting of a resistor connected in series with two resistor-capacitor (RC) elements.     

Improved dielectric and non-ohmic properties of Ca2Cu2Ti4O12 ceramics prepared by a polymer pyrolysis method

Non-ohmic and dielectric properties of Ca₂Cu₂Ti₄O₁₂ (CaCu₃Ti₄O₁₂/CaTiO₃ composite) ceramics prepared by a polymer pyrolysis method (PP-ceramic samples) are investigated. The PP-ceramics show a highly dense structure and improved non-ohmic and dielectric properties compared to the ceramics obtained by a solid state reaction method (SSR-ceramic samples). ?′ (tan δ) of the PP-ceramic samples is found to be higher (lower) than that of the SSR-ceramic samples. Interestingly, the PP-ceramic sintered at 1050 °C for 10 h exhibits the high ?′ of 2530 with weak frequency dependence below 1 MHz, the low tan δ less than 0.05 in the frequency range of 160 Hz-177 kHz, and the little temperature coefficient, i.e., |Δ?′| ≤ 15 % in the temperature range from −55 to 85 °C. These results indicate that the CaCu₃Ti₄O₁₂/CaTiO₃ composite system prepared by PP method is a promising high-?′ material for practical capacitor application.     

High permittivity and low dielectric loss of the Ca1−xSrxCu3Ti4O12 ceramics

The Ca_1−xSr_xCu₃Ti₄O₁₂ (CSCTO) giant dielectric ceramics were prepared by conventional solid-state method. X-ray diffraction patterns revealed that a small amount of Sr²⁺ (x < 0.2) had no obvious effect on the phase structure of the CSCTO ceramics, while with increasing the Sr²⁺ content, a second phase of SrTiO₃ appeared. Electrical properties of CSCTO ceramics greatly depended on the Sr²⁺ content. The Ca_0.9Sr_0.1Cu₃Ti₄O₁₂ ceramics exhibited a higher permittivity (71,153) and lower dielectric loss (0.022) when measured at 1 kHz at room temperature. The ceramics also performed good temperature stability in the temperature range from −50 °C to 100 °C at 1 kHz. By impedance spectroscopy analysis, all compounds were found to be electrically heterogeneous, showing semiconducting grains and insulating grain boundaries. The grain resistance was 1.28 Ω and the grain boundary resistance was 1.31 × 10⁵ Ω. All the results indicated that the Ca_0.9Sr_0.1Cu₃Ti₄O₁₂ ceramics were very promising materials with higher permittivity for practical applications.     

An aqueous-solution based low-temperature pathway to synthesize giant dielectric CaCu3Ti4O12—Highly porous ceramic matrix and submicron sized powder

A simple, cost-effective and environment-friendly pathway for preparing highly porous matrix of giant dielectric material CaCu₃Ti₄O₁₂ (CCTO) through combustion of a completely aqueous precursor solution is presented. The pathway yields phase-pure and impurity-less CCTO ceramic at an ultra-low temperature (700 °C) and is better than traditional solid-state reaction schemes which fail to produce pure phase at as high temperature as 1000 °C (Li, Schwartz, Phys. Rev. B 75, 012104). The porous ceramic matrix on grinding produced CCTO powder having particle size in submicron order with an average size 300 nm. On sintering at 1050 °C for 5 h the powder shows high dielectric constants (>10⁴ at all frequencies from 100 Hz to 100 kHz) and low loss (with 0.05 as the lowest value) which is suitable for device applications. The reaction pathway is expected to be extended to prepare other multifunctional complex perovskite materials.     

Microstructure and electrical properties of sol–gel derived Ni-doped CaCu3Ti4O12 ceramics

Dielectric properties and varistor performance of sol–gel prepared Ni-doped calcium copper titanate ceramics (CaCu₃Ni_xTi₄O_12+x, x=0, 0.1, 0.2, 0.3) were investigated. SEM and XRD were used in the microstructural studies of the specimens and the electrical properties were investigated for varistors. XRD patterns show that the CCTO ceramics were single phase with no Cu-rich phase. SEM results indicated that the samples had smaller grain sizes than those synthesized by traditional solid-state reaction methods. The experimental results show that the highest dielectric constant and lower dielectric loss occur when x=0.2. When x=0.3, the lowest leakage current is obtained and the maximum value reaches 0.295; meanwhile, the lowest threshold voltage and nonlinear coefficient are found, the minimum values of them are 1326 V/mm and 3.1, respectively.     

Complex impedance analysis of (Y2O3 + CeO2)-YCr0.5Mn0.5O3 composite NTC ceramics

Ceramic compositions based on (aY₂O₃ + bCeO₂)-0.4YCr_0.5Mn_0.5O₃ (a + b = 0.6) were prepared by conventional solid state reaction at 1200 °C, and sintered under air atmosphere at 1600 °C. For 0 ≤ a < 0.6, XRD patterns have shown that the major phases presented in the calcined powders are Y₂O₃, CeO₂ and orthorhombic perovskite YCr_0.5Mn_0.5O₃ phase, respectively. SEM and EDAX observations confirm the YCr_0.5Mn_0.5O₃ phases mostly exist at the grain, whereas the Y₂O₃ and CeO₂ phases mainly exist at the grain boundaries. Complex impedance analysis shows that, for 0 < a ≤ 0.6, single semicircular arc whose shape does not show any change with temperature. Nevertheless, for a = 0, two overlapping semicircular arcs are observed at and above 300 °C. The grain boundary properties exhibit thermistor parameters with a negative temperature coefficient characteristic. The relaxation behavior and conduction for the grain boundary could be due to a space-charge relaxation mechanism and oxygen vacancies, respectively.     

Effect of Nb doping on the phase transition and optical properties of sol-gel TiO2 thin films

Various content Nb-doped TiO₂ thin films were prepared by sol-gel process. XRD analysis shows that the existence of crystalline TiO₂ in anatase and rutile form depends on the Nb content in the examined samples. It is observed that Nb promotes the anatase to rutile phase transition but has a depression effect on the anatase grain growth. It is found that incorporation of about 4 at.% of Nb completely transforms anatase TiO₂ to the rutile form at a calcination temperature as high as 900 °C. The mechanism is proposed. Optical analyses show that the films have an average of 60% transmission in visible region. The energy gap values using Tauc's formula have also been estimated. The band gap of rutile Ti_1−xNb_xO₂ solid solutions increases with increasing x.     

Dielectric properties and crystal structure of La(Mg1/2Ti1/2)O3 ceramics with Mg substituted by Co

Solid solutions of (1 − x)La(Co_1/2Ti_1/2)O₃-xLa(Mg_1/2Ti_1/2)O₃ were used to prepare La(Mg_1−xCo_x)_1/2Ti_1/2O₃ using solid-state synthesis. X-ray diffraction patterns of the sintered samples revealed single phase formation. A maximum density of 6.01 g/cm³ was obtained for La(Mg_1−xCo_x)_1/2Ti_1/2O₃ (x = 1) ceramics sintered at 1375 °C for 4 h. The maximum values of the dielectric constant (?_r = 29.13) and the quality factor (Q × f = 80,000 GHz) were obtained for La(Mg_1−xCo_x)_1/2Ti_1/2O₃ with 1 wt% ZnO additive sintered at 1375 °C for 4 h. The temperature coefficient of resonant frequency τ_f was −59 ppm/°C for x = 0.3.     

The effect of calcining temperatures on the phase purity and electric properties of CaCu3Ti4O12 ceramics

CaCu₃Ti₄O₁₂ (CCTO) ceramics are prepared by the traditional solid-state reaction method under the same sintering conditions. The effect of calcining temperatures for the powders before sintering on the microstructure and electric properties of CCTO ceramics has been investigated. The XRD patterns for the powder calcined at 950 °C show that some measure of second phases (CaTiO₃, TiO₂ and CuO) can be found except a considerable amount of CCTO phase in them and the content of second phases decrease markedly as the calcining temperature is raised to 1000 °C. The XRD patterns for the powder calcined at 1050 °C indicate that the powder has been basically formed into a single CCTO phase except a small quantity of CaTiO₃ phase, which is attributed to CuO volatilizing in the calcining process. Furthermore, the XRD patterns for the CCTO pellets sintered at 1080 °C/10 h manifest that all the second phases have disappeared after the sintering process except that a very weak peak of CaTiO₃ can still be seen in the XRD pattern for the pellets made of the powder calcined at 1050 °C. The electric properties measurement demonstrates that the lower calcining temperature for the raw powder is helpful to increase the values of permittivity and the higher calcining temperature is helpful to improve the non-ohmic properties. The non-ohmic characteristic has a behavior reverse to that of the permittivity, which can be ascribed to the change in the height of Schottky barriers.     

Impedance and electric modulus analysis of Sm-modified Pb(Zr0.55Ti0.45)1−x/4O3 ceramics

The polycrystalline ceramic samples of Pb_1−xSm_x(Zr_0.55Ti_0.45)_1−x/4O₃ (x = 0.00, 0.03, 0.06 and 0.09) were prepared by solid-state reaction technique at high temperature. Electric impedance (Z) and modulus (M) properties of the materials have been investigated within a wide range of temperature and frequency using complex impedance spectroscopy (CIS) technique. The complex impedance analysis has suggested the presence of mostly bulk resistive (grain) contributions in the materials. This bulk resistance is found to decrease with the increase in temperature. It indicates that the PSZT compounds exhibit a typical negative temperature coefficient of resistance (NTCR) behavior. The bulk contribution also exhibits an increasing trend with the increase in Sm³⁺ substitution to PZT. The complex modulus plots have confirmed the presence of grain (bulk) as well as grain boundary contributions in the materials. Both the complex impedance and modulus studies have suggested the presence of non-Debye type of relaxation in the materials.     

Phase evolution, crystal structure and dielectric behavior of (1 − x)Nd(Zn0.5Ti0.5)O3 + xBi(Zn0.5Ti0.5)O3 compound ceramics

The phase evolution, crystal structure and dielectric properties of (1 − x)Nd(Zn_0.5Ti_0.5)O₃ + xBi(Zn_0.5Ti_0.5)O₃ compound ceramics (0 ≤ x ≤ 1.0, abbreviated as (1 − x)NZT-xBZT hereafter) were investigated. A pure perovskite phase was formed in the composition range of 0 ≤ x ≤ 0.05. The B-site Zn²⁺/Ti⁴⁺ 1:1 long range ordering (LRO) structure was detected by both XRD and Raman spectra in x ≤ 0.05 samples. However, this LRO structure became gradually degraded with an increase in x. The dielectric behaviors of the compound ceramic at various frequencies were investigated and correlated to its chemical composition and crystal structure. A gradually compensated τ_f value was obtained in (1 − x)NZT-xBZT microwave dielectrics at x = 0.03, which was mainly due to the dilution of dielectric constant in terms of Claussius-Mossotti differential equation.     

Crystal structure and dielectric properties of La(Mg1−xZnx)1/2Ti1/2O3 ceramics at microwave frequencies

Ceramics in the system La(Mg_1−xZn_x)_1/2Ti_1/2O₃ with B₂O₃ additions (1 wt.%) have been investigated by the conventional solid-state route. The XRD patterns of the sintered samples (0.3 ≤ x ≤ 1.0) revealed single phase formation with a structure. The unit cell volume slightly increased with increasing Zn content (x). La(Mg_1−xZn_x)_1/2Ti_1/2O₃ were found to form perovskite solid solutions in the whole compositional range. The maximum values of the dielectric constant and the quality factor multiples resonant frequency (Q × f) can be obtained when the La(Mg_0.7Zn_0.3)_1/2Ti_1/2O₃ with 0.5 wt.% B₂O₃ additive were sintered at 1475 °C for 4 h. The temperature coefficient of resonant frequency τ_f (−63 ppm/°C) was measured for x = 0.7.