Dielectric properties of CaCu3Ti4O12 improved by chromium/lanthanum co-doping

The dielectric properties of Cr + La co-doped CaCu₃Ti₄O₁₂ ceramics prepared by a solid-state reaction method were evaluated and compared to Cr-doped, La-doped, and parent CaCu₃Ti₄O₁₂ (CCTO). Their structure and grain size were evaluated by X-ray diffraction and scanning electron microscopy, respectively. No secondary phase was detected based on the XRD analysis. The results show that, the room temperature dielectric loss of the co-doped samples is reduced to 43% compared to CCTO and their dielectric permittivity is higher than the un-doped, Cr-doped, and La-doped samples at frequencies over 325 kHz, 30 kHz, and 12 Hz, respectively. Furthermore, the temperature stability of the co-doped sample is significantly more convenient than that of CCTO, and its dielectric loss is three times lower. The results also indicated that the co-doping method is effective in reducing the dielectric loss, still maintaining the high dielectric permittivity.     

CaCu3Ti4O12 ceramics from co-precipitation method: Dielectric properties of pellets and thick films

Dielectric properties of CaCu₃Ti₄O₁₂ (CCTO)-based ceramics and thick films (e ～50 μm) prepared from powders synthesized by a soft chemistry method (co-precipitation) are presented and discussed. The characteristics of pellets and thick films are compared.The pellets exhibit high values of the dielectric permittivity (?_r ～1.4 × 10⁵) and relatively small dielectric losses (tan δ ～0.16) at 1 kHz and room temperature. These properties are independent of the nature of the metallization of the electrodes. In addition, the dielectric permittivity decreases when the diameter of the electrodes of the pellets increases, while the losses remain constant. This result, which is strongly related to the nature of the dielectric material in between the electrodes, constitutes a strong indication that the high dielectric permittivity values observed in this material are not related to an interfacial (electrode material) related mechanism but is an internal barrier layer capacitor (IBLC) type.Very high values of the dielectric permittivity of CCTO thick films are measured (?_r ～5 × 10⁴). The differences in dielectric permittivity between thick films and dense pellets may be attributed to the difference in grain size due to different CuO contents, and to the different reactivity of the materials.     

Effects of LiF addition on microstructure and dielectric properties of CaCu3Ti4O12 ceramics

The effects of small amounts of lithium fluoride sintering aid on the microstructure and dielectric properties of CaCu₃Ti₄O₁₂ (CCTO) ceramics were investigated. CCTO polycrystalline ceramics with 0.5 and 1.0 mol% LiF, and without additive were prepared by solid state synthesis. Good densification (>90% of the theoretical density) was obtained for all prepared materials. Specimens without the sintering aid and sintered at 1090 °C exhibit secondary phases as an outcome of the decomposition reaction. The mean grain size is controlled by the amount of LiF in specimens containing the additive. Impedance spectroscopy measurements on CaCu₃Ti₄O₁₂ ceramics evidence the electrically heterogeneous nature of this material consisting of semiconductor grains along with insulating grain boundaries. The activation energy for grain boundary conduction is lower for specimens prepared with the additive, and the electric permittivity reached 53,000 for 0.5 mol% LiF containing CCTO.     

Evolution of the intergranular phase during sintering of CaCu3Ti4O12 ceramics

CaCu₃Ti₄O₁₂ (CCTO) ceramics have been processed by solid state reaction and sintered at 1100 °C for different times. A clear increase of the dielectric constant of the material up to values of 6 × 10⁴ has been observed with the sintering time. This increase is accompanied by a limited grain growth and intergranular Cu-oxide phase thickness reduction. The disappearance of the Cu-oxide phase is caused by the incorporation of Cu cations into the grains, contributing to the increase of the dielectric constant. Raman spectroscopy shows the decrease of TiO₆ octahedral rotational modes with the sintering time due to the incorporation of Cu cations into the CCTO grains. XANES measurements show that the Cu main oxidation state is Cu²⁺ and does not change with the sintering time. The fitting of the experimental dielectric constant to the Internal Barrier Layer Capacitance (IBLC) model reveals the change of the intergranular phase dielectric constant, caused by a compositional change due to the incorporation of Cu into the CCTO grains.     

Origin of giant dielectric response in LiCuNb3O9 distorted perovskite ceramics

LiCuNb₃O₉ ceramics with the distorted cubic perovskite structure were prepared by a solid-state reaction method. The ceramic exhibited a very large value of permittivity (∼4.4 × 10⁴ at 100 kHz) at room temperature (∼300 K) and a low-temperature dielectric relaxation behaviour following the Arrhenius law. The origin of the giant dielectric response of the LiCuNb₃O₉ ceramics was correlated with the structure of the ceramics. The barrier layers in the grain boundaries and the mixed-valent structure of Cu⁺/Cu²⁺ were found to contribute to the giant permittivity of the ceramics and confirmed by X-ray spectroscopy and complex impedance spectroscopy analyses.     

Microstructures and dielectric relaxation behaviors of pure and tellurium doped CaCu3Ti4O12 ceramics prepared via vibro-milling method

In this work, we have reported microstructures and the dielectric properties of CaCu₃Ti₄O₁₂ (CCTO) ceramics doped with different proportions of TeO₂ dopant (mol%, x=0, 0.5%, 1.0%, 2.0%). The pure and tellurium doping CCTO ceramics were prepared by a conventional solid-state reaction method and the effects of TeO₂ doping on the electrical properties and microstructures of these ceramics were investigated. XRD analysis confirmed the formation of single-phase material in samples. Scanning electron microscopy (SEM) is used in the micro structural studies of the specimens, which showed that TeO₂ doping can reduce the mean grain size and increasing size of an abnormal grain growth. Lattice parameter increases slightly with tellurium doping in CCTO, the dielectric constant reached a value as high as 18,000 (at 1 kHz) at a tellurium-doping concentration of 2.0 mol% and showed temperature stability at high frequency. The loss tangent of Te-doped CCTO ceramics was less than 0.05 at 1 kHz region below 105 °C. The loss tangent properties could be interpreted by the internal barrier layer capacitor model and the impedance measurement data.     

Effects of sintering temperature on the internal barrier layer capacitor (IBLC) structure in CaCu3Ti4O12 (CCTO) ceramics

The formation of the internal barrier layer capacitor (IBLC) structure in CaCu₃Ti₄O₁₂ (CCTO) ceramics was found to be facilitated by the ceramic heat treatment. Electrically insulating grain boundary (GB) and semi-conducting grain interior areas were characterized by impedance spectroscopy to monitor the evolution of the IBLC structure with increasing sintering temperature T_S (975–1100 °C). The intrinsic bulk and GB permittivity increased by factors of ≈2 and 300, respectively and the bulk resistivity decreased by a factor of ≈10³. These trends were accompanied by increased Cu segregation from the CCTO ceramics as detected by scanning electron microscopy and quantitative energy dispersive analysis of X-rays. The chemical changes due to possible Cu-loss in CCTO ceramics with increasing T_S are small and beyond the detection limits of X-ray absorption spectroscopy near Cu and Ti K-edges and Raman Spectroscopy.     

Decrease in the dielectric loss of CaCu3Ti4O12 at high frequency by Ru doping

CaCu_3−xRu_xTi₄O₁₂ (x=0, 0.03, 0.05 and 0.07) electronic ceramics were fabricated using a conventional solid-state reaction method. The microstructure, grain sizes and dielectric properties as well as the impedance behaviours of the ceramics were carefully investigated. Scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) results indicate that ruthenium (Ru) dopant inhibits the growth of grains during the sintering process by promoting the formation of high melting point oxides of Ca and Ti. The study on the frequency dependence of dielectric properties suggests that Ru doping shifts the dielectric loss peak of CCTO to a much lower frequency, thereby reducing the dielectric loss of CCTO at high frequency (f>1.0 MHz) accordingly. When doped with proper amount of Ru, the high frequency dielectric loss of CCTO is reduced to a very low value (tanδ<0.05). Our study conclusively suggests that Ru-doped CCTO, with sufficiently low dielectric loss and decent permittivity, presents potential applications at high frequency.     

Fabrication of lanthanum doped BaTiO3 fine-grained ceramics with a high dielectric constant and temperature-stable dielectric properties using hydro-phase method at atmospheric pressure

Lanthanum doped BaTiO₃ powders were synthesized by the hydro-phase method at atmospheric pressure, which controls the uniformity and particle size of the ceramic powders. The effects of La³⁺ ions concentration on the microstructure and dielectric properties of BaTiO₃ ceramics were studied. The results suggested that both the average grain size and dielectric constants (ε_r and ε_max) of the ceramics decreased as the concentration of La³⁺ increased. The ceramic met the X8R specifications: La³⁺ ions concentration of 3 mol%, a permittivity of 2322, low dielectric loss of less than 0.6% at room temperature, and average grain size of about 260 nm.     

Effect of SiO2 additive on dielectric response and energy storage performance of Ba0.4Sr0.6TiO3 ceramics

SiO₂-added barium strontium titanate ceramics Ba_0.4Sr_0.6TiO₃-xwt%SiO₂ (x=0, 0.5, 1, 3, BSTSx) were prepared via a traditional solid state reaction method. The effect of SiO₂ additive on the microstructure, dielectric response and energy storage properties was investigated. The results confirmed that with the increase of SiO₂ additive, diffuse phase transition arises and the dielectric constant decreases. An equivalent circuit model and Arrhenius law were used to calculate the activation energy of grain and grain boundary, which indicated that the dielectric relaxation at high temperature was caused by oxygen vacancy. While appropriate SiO₂ additive led to improve the breakdown strength, further increase of SiO₂ deteriorated the energy storage because of the low densification. Finally, optimized energy storage performance was obtained for BSTS0.5 ceramics: dielectric constant of 1002, dielectric loss of 0.45%, energy density of 0.86 J/cm³ and energy storage efficiency of 79% at 134 kV/cm.     

“Dark hole” cure in Ba4.2Nd9.2Ti18O54 microwave dielectric ceramics

Large size Ba_4.2Nd_9.2Ti₁₈O₅₄ (BNT) ceramics doped with MnCO₃, CuO and CoO were prepared by the conventional solid-state method. Only a single BaNd₂Ti₄O₁₂ phase was formed in all samples. No second phase was found in the XRD patterns. The bulk density increases slightly because of the dopants. The SEM results showed that the grain size of Mn²⁺and Cu²⁺-doped BNT ceramics became larger with the increasing amount of dopants. The permittivity of all samples stays the same. However, the Q×f value of BNT ceramics increases by doping, especially with Mn²⁺ ions. The conductivity of BNT ceramic doped with Mn²⁺(0.5 mol‰) under high temperature is lower than that without doping. There are fewer defects in Mn²⁺-doped BNT ceramics. The XPS results indicated that Ti reduction was suppressed in BNT ceramics doped with 0.5 mol‰ Mn²⁺. BNT ceramics doped with 0.5 mol‰ Mn²⁺ ions sintered at 1320 °C for 2 h exhibited good microwave dielectric properties, with ε_r=88.67, Q×f=7408 GHz and τf = 82.98 ppm/°C.     

Influence of Li2O–B2O3–SiO2 glass on the sintering behavior and microwave dielectric properties of BaO–0.15ZnO–4TiO2 ceramics

This paper reports the investigation of the performance of Li₂O–B₂O₃–SiO₂ (LBS) glass as a sintering aid to lower the sintering temperature of BaO–0.15ZnO–4TiO₂ (BZT) ceramics, as well as the detailed study on the sintering behavior, phase evolution, microstructure and microwave dielectric properties of the resulting BZT ceramics. The addition of LBS glass significantly lowers the sintering temperature of the BZT ceramics from 1150 °C to 875–925 °C. Small amount of LBS glass promotes the densification of BZT ceramic and improves the dielectric properties. However, excessive LBS addition leads to the precipitation of glass phase and growth of abnormal grain, deteriorating the dielectric properties of the BZT ceramic. The BZT ceramic with 5 wt% LBS addition sintered at 900 °C shows excellent microwave dielectric properties: ε_r=27.88, Q×f=14,795 GHz.     

Structural,thermal and mechanical properties of aluminum nitride ceramics with CeO2 as a sintering aid

AlN ceramics have been prepared with CeO₂ as a sintering aid at a sintering temperature of 1900 °C. The effect of CeO₂ contents on the microstructure, density, thermal conductivity and hardness was investigated. Addition of CeO₂ exerted a significant effect on the densification of AlN ceramics and hence on the microstructure. Thermal conductivity of AlN ceramics increased with CeO₂ content and was greater than that of Y₂O₃-doped AlN ceramics at a similar sintering temperature. The resulting AlN ceramics with 1.50 wt% of CeO₂ had the highest relative density of 99.94%, thermal conductivity of 156 W m⁻¹ K⁻¹ and hardness of 72.46 kg/mm².     

Grain size effect on electromechanical properties and non-linear response of dense nano and microstructured PIN–PT ceramics

Nanopowders of 0.63Pb(In_1/2Nb_1/2)O₃–0.37PbTiO₃ were synthesized by solid state reaction using the continuous attrition milling followed by high-energy ball milling techniques in air at room temperature. After milling for 8 h nanopowders of 20–30 nm grain size are obtained. Sintering by hot pressing of PIN–37PT green pellets leads to dense ceramics with average grain size varying from 100 nm to 1 μm. The dielectric and piezoelectric properties of PIN–37PT nanostructured ceramics with grain size bigger than about 160 nm remain roughly unchanged and comparable to those of microstructured ceramics. In addition, the stability of the permittivity and dielectric losses under high ac electric field grows when the grain size decreases. The material becomes less non-linear with decreasing grain size. This result is attractive for acoustic transducer applications.     

Dielectric behaviour of Hf-doped CaCu3Ti4O12 ceramics obtained by conventional synthesis and reactive sintering

CaCu₃(Ti_4?xHf_x)O₁₂ ceramics (x = 0.04, 0.1 and 0.2) were prepared by conventional synthesis (CS) and through reactive sintering (RS), in which synthesis and sintering of the material take place in one single step. The microstructure and the dielectric properties of Hf-doped CCTO (CCTOHf) have been studied by XRD, FE-SEM, AFM, Raman and impedance spectroscopy (IS) in order to correlate the structure, microstructure and the electrical properties. Samples prepared by reactive sintering show slightly higher dielectric constant than those prepared by conventional synthesis in the same way than the pure CCTO. Dielectric constant and dielectric losses decrease slightly increasing Hf content. For CCTOHf ceramics with x > 0.04 for CS and x > 0.1 for RS, a secondary phase HfTiO₄ appears. As expected, the reactive sintering processing method allows a higher incorporation of Hf in the CCTO lattice than the conventional synthesis one.     

Synthesis of fine-grain Ba0.96La0.04TiO3 dielectric ceramics by different routes for multilayer ceramic capacitors

Spherical Ba_0.96La_0.04TiO₃ powders were synthesized by a wet chemical method-based on a precipitation process, able of controlling uniformity and particle size. After sintering, fine-grain Ba_0.96La_0.04TiO₃ ceramics with good dielectric properties were obtained. Different synthesis routes of Ba_0.96La_0.04TiO₃ ceramics were used, and their effects on microstructure and dielectric properties were examined. Results showed that the ceramics samples prepared by La³⁺ and Ba²⁺ added together (abbreviated as, BLT-T) resulted in a mixture of large and small grains, sharp permittivity peaks, and high dielectrics loss. By comparison, another ceramics samples prepared by La³⁺ and Ba²⁺ added separately (BLT-S) exhibited uniform grain size and broadened/flattened permittivity peaks. The high value of ε_r (3255) and low tan δ (0.01) at room temperature and negligible temperature coefficient of capacitance from − 35 to 85 °C suggested the suitability of the above materials for multilayer capacitor applications.     

Dielectric properties of manganese and cobalt doped lead iron tantalate ceramics

This paper reports the results of synthesis and sintering studies as well as dielectric properties of Pb(Fe_1/2Ta_1/2)O₃ (PFT) relaxor ferroelectric ceramics. Influence of doping with MnO₂ and Co₃O₄ (0.1–1 mol%) on resistivity and dielectric characteristics were investigated. The dielectric permittivity and dissipation factor of the ceramics were determined as a function of temperature in the range from −55 to 500 °C at frequencies 10 Hz to 1 MHz. DC resistivities of the samples were measured in the temperature range 20–500 °C. Two maxima in dielectric permittivity versus temperature curves were observed, dependent on frequency and the content of dopants. The investigated PFT ceramics were characterized by high dielectric permittivity of 3500–6700 at the transition temperature and 900–17,000 at the second maxima.     

(Na,K)NbO3–CaCu3Ti4O12 perovskite composites for supercapacitor based piezoelectric devices

The supercapacitor based piezoelectric material composite (Na,K)NbO₃–CaCu₃Ti₄O₁₂ (NKN–CCTO) is investigated for possible application in piezoelectric devices. (1−x)NKN–xCCTO (0.015≤x≤0.06) with different sintering conditions is researched for supercapacitor based piezoelectric applications. The 0.94NKN–0.06CCTO composite sintered at 975 °C shows the highest dielectric permittivity of 796. Clear SEM images of (1−x)NKN–xCCTO reveal that these compositions have high density well-crystallized structures. The composition and sintering temperature dependence of dielectric permittivities and piezoelectric coefficients, plotted in three dimensions, show that the 0.985NKN–0.015CCTO composite sintered at 1025 °C has a moderate dielectric permittivity of 405 and a piezoelectric constant of 98 pC/N.     

Low temperature sintering and microwave dielectric properties of TiO2 ceramics

The TiO₂ ceramics were prepared by a solid-state reaction in the temperature range of 920–1100 °C for 2 h and 5 h using TiO₂ nano-particles (Degussa-P25 TiO₂) as the starting materials. The sinterability and microwave properties of the TiO₂ ceramics as a function of the sintering temperature were studied. It was demonstrated that the rutile phase TiO₂ ceramics with good compactness could be readily synthesized from the Degussa-P25 TiO₂ powder in the temperature range of 920–1100 °C without the addition of any glasses. Moreover, the TiO₂ ceramics sintered at 1100 °C/2 h and 920 °C/5 h demonstrated excellent microwave dielectric properties, such as permittivity (Ɛ_r) value >100, Q × f  > 23,000 GHz and τ_f ≅ 200 ppm/°C.     

Energy storage density and tunable dielectric properties of BaTi0.85Sn0.15O3/MgO composite ceramics prepared by SPS

(100-x) wt.% BaTi_0.85Sn_0.15O₃–x wt.% MgO (BTS/MgO) composite ceramics were prepared by spark plasma sintering (SPS) technology. Phase constitution, microstructure, dielectric and electrical energy storage properties of BTS/MgO composite ceramics were investigated. The samples prepared by SPS had smaller grain size and presented layer-plate substructure. Dielectric permittivity and dielectric loss of BTS/MgO composite ceramics decreased significantly with the content of MgO increasing, and dielectric tunability maintained a relatively high value (>45%). Meanwhile, the dielectric breakdown strength was improved when addition of MgO in BTS matrix, which resulted in a significant improvement of energy storage density. The high dielectric breakdown strength of 190 kV/cm, energy storage density of 0.5107 J/cm³ and energy storage efficiency of 92.11% were obtained in 90 wt.% BaTi_0.85Sn_0.15O₃–10 wt.% MgO composite ceramics. Therefore, BTS/MgO composites with good tunable dielectric properties and electrical energy storage properties could be exploited for energy storage and phase shifter device applications.