Influence of the electric field on flash-sintered (Zr + Ta) co-doped TiO2 colossal permittivity ceramics

In the preparation process for advanced ceramics, how to reduce the sintering temperature, shorten the processing time and refine grains is the key to obtaining high-performance ceramic materials. The flash sintering (FS) provides an effective method to solve this issue. Here, (Zr + Ta) co-doped TiO₂ colossal permittivity ceramics were successfully fabricated by conventional sintering (CS) and flash sintering under electric fields from 500 V/cm to 800 V/cm. The flash behavior, sintered crystal structure and microstructure, dielectric properties, and varistor characteristics were systematically investigated. The effects of the applied electric fields on the above behaviors were discussed. The results show that flash sintering can reduce the sintering temperature by 200 °C, decrease the processing time by 10 times and reduce grain sizes in TiO₂ ceramics. All sintered samples were single rutile structures. Flash sintering led to similar electrical properties to conventional sintering. In the flash-sintered samples, with increasing the electric field, the permittivity of co-doped TiO₂ ceramics increased at a frequency of 10³–10⁴ Hz. The flash-sintered sample under an electric field of 800 V/cm possessed the best comprehensive properties, a dielectric permittivity of >10⁵, a dielectric loss of ～0.77 at 10³ Hz, and a nonlinear coefficient of 5.2.     

Large-size-mismatch co-dopants for colossal permittivity rutile TiO2 ceramics with temperature stability

Colossal permittivity (CP) in donor-accepter co-doped rutile TiO₂ has attracted significant interest. Here, the CP behavior of (Ta?+?La) co-doped rutile TiO₂ ceramics were studied, where the ionic radii of Ta⁵⁺ and La³⁺ are much larger than that of Ti⁴⁺. The ceramics with an extremely low doping exhibit colossal dielectric permittivity (～2.6?×?10⁴) with an acceptable low dielectric loss (<0.07) in the frequency range from 40 to 10⁶?Hz. The CP properties obtained in (Ta?+?La) co-doped TiO₂ ceramics show excellent temperature stability over a wide temperature range of 20–400?°C. The X-ray diffraction analysis and the density functional theory calculation illustrates that the La₂³⁺${\text{V}}_{\text{o}}^{??}$Ti₂³⁺ and Ta₂⁵⁺Ti³⁺Ti⁴⁺ defect complexes with the lowest energy are responsible for the enhanced dielectric properties. Moreover, the defect complex formed by large-size trivalent substitutions and oxygen vacancy is very stable, and assists in improving temperature stability of the dielectric properties of co-doped rutile TiO₂ ceramics.     

Controllable low-temperature flash sintering and giant dielectric performance of (Zn,Ta) co-doped TiO2 ceramics

In this study, the challenge of high-temperature and long-time sintering of (Zn, Ta) co-doped TiO₂ ceramics is solved successfully using flash sintering technology. Joule heating and a high heating rate make the sample compact rapidly at low temperatures (1050 °C in 24 min). When the electric field was equal to 200 V/cm, high permittivity (ε' ～ 1.32 × 10⁴), low dielectric loss tangent (tan δ ～ 0.27), and nonlinear coefficient ($\alpha$ ～ 5.8) values were obtained. Flash sintered samples have more free electrons, resulting in a high dielectric constant. Further, the higher the electric field, the smaller the grain resistance of the sample; this condition is conducive to reducing dielectric loss. giant dielectric performance is explained by the combined action of the electron-pinned defect dipole theory and the internal barrier layer capacitance effect. Therefore, this study provides a promising prospect for the green preparation of co-doped TiO₂ giant dielectric ceramics.     

Microstructure,colossal permittivity,and humidity sensitivity of (Na,Nb) co-doped rutile TiO2 ceramics

(Na_0.25Nb_0.75)_xTi_1−xO₂ (NNTO) ceramics (x = 0, 0.005, 0.01, 0.02, and 0.05) were prepared by the conventional solid-state reaction. The microstructure, dielectric, and humidity sensitivity of the ceramics were systematically investigated. Results showed that all ceramics exhibit pure rutile TiO₂ phase with dense microstructures. Co-doping of (Na, Nb) can effectively improve the microstructure homogeneity of the ceramics. When the doping level x ≥ 0.01, the co-doped samples show colossal permittivity higher than 10⁴ and dielectric loss tangent lower than 0.38. This dielectric behavior features the merit of both frequency and temperature stability in the range of 10²-10⁶ Hz and 100-300 K, respectively. The co-doped ceramics were found to be sensitive to the environment moisture. The humidity sensitivity incurs a Maxwell-Wagner relaxation near room temperature, which further enhances the dielectric permittivity. Excellent humidity sensitive properties of sensitivity to be 102.6 pF/%RH, response/recovery time to be 115/20 seconds, as well as good repeatability, were achieved in the sample with the doping level x = 0.05. This work underscores that the room temperature dielectric properties of doubly doped TiO₂ system depends strongly on the environmental condition and suggests that the (Na + Nb) co-doped TiO₂ ceramics might be promising humidity sensing materials.     

The dielectric properties for (Nb,In,B) co-doped rutile TiO2 ceramics

Recently, colossal permittivities (~10⁵) and low loss factors (<0.1) were reported in (Nb+In) co-doped rutile TiO₂ ceramics, which have attracted considerable attention. In this work, (Nb,In,B) co-doped rutile TiO₂ ceramics were investigated for achieving temperature- and frequency- stable dielectric properties in TiO₂ based colossal dielectric ceramics. The (Nb,In,B) co-doped rutile TiO₂ ceramics were prepared by conventional solid-state reaction method. The microstructures, dielectric properties and complex impedance of 1 mol.% (Nb+In) co-doped rutile TiO₂ (TINO) and xwt% B₂O₃ (x=0.5, 1, 2 and 4) doped TINO were systematically investigated and compared. It was found that by doping B₂O₃ the sintering temperature of TINO ceramics can be reduced by 100 °C. Meanwhile, the dielectric loss of TINO ceramics was decreased by doping B₂O₃. In the ₂wt% B₂O₃ doped TINO ceramics, the dielectric permittivity kept a high value of >2.0×10⁵ and the dielectric loss was lower than 0.1 in a frequency range of 10²−10⁵ Hz and a temperature range of 25–200 °C.     

Low-temperature Maxwell-Wagner relaxation in (Na + Nb) co-doped rutile TiO2 colossal permittivity ceramics

The exact mechanism of the stunning colossal permittivity behavior found in (donor-acceptor) co-doped TiO₂ system still remains enigmatic. This behavior results from a thermally activated dielectric relaxation occurring below 50 K. Herein, thermally stimulated depolarization current analysis combined with dielectric investigation was used to disentangle this relaxation in (Na + Nb) co-doped TiO₂ ceramics. We find that this relaxation is related to frozen electrons and features the Vogel-Fulcher behavior and negative dielectric tunability. Our results reveal that this low-temperature relaxation is a new kind of Maxwell-Wagner relaxation. Differences between the low-temperature Maxwell-Wagner relaxation and its high-temperature counterpart are discussed. This study provides new insights into the physics of the eye-catching dielectric properties in co-doped TiO₂ system.     

Colossal dielectric permittivity in (Al+Nb) co-doped Ba0.4Sr0.6TiO3 ceramics

Stimulated by the outstanding colossal permittivity behavior achieved in trivalent and pentavalent cations co-doped rutile TiO₂ ceramics, the co-doping effects on the dielectric behavior of Ba_0.4Sr_0.6TiO₃ ceramics were further explored. In this work, (Al + Nb) co-doped Ba_0.4Sr_0.6TiO₃ ceramics were synthesized via a standard solid state ceramic route. The structural evolution was analyzed using X-ray diffraction patterns and Raman spectra. Dense microstructures with no apparent change of grain morphology were observed from the scanning electron microscopy. A huge enhancement of dielectric permittivity was obtained with 1 mol% (Al + Nb) doping and excellent dielectric performances (ε_r ∼ 20,000, tanδ ∼ 0.06 at 1 kHz) were achieved after further heat treatment. The formation of electron pinned defect dipoles localized in grains may account for the optimization of dielectric behaviors and the corresponding chemical valence states were confirmed from the XPS results.     

Colossal permittivity in (In1/2Nb1/2)xTi1−xO2 ceramics prepared by a glycine nitrate process

(In + Nb) co-doped TiO₂ nanoparticles with very low dopant concentrations were prepared using a glycine nitrate process. A pure rutile—TiO₂ phase with a dense microstructure and homogeneous dispersion of dopants was achieved. By doping TiO₂ with 1.5% (In + Nb) ions, a very high dielectric permittivity of ε′ = 42,376 and low loss tangents of tanδ = 0.06 (at room temperature) were achieved. The large conduction activation energy at the grain boundary decreased with decreasing dopant concentration. The colossal permittivity was primarily attributed to the internal barrier layer capacitor (IBLC) effect. The dominant effect of interfacial polarization at the non–Ohmic sample–electrode contact was observed when the dopant concentration was ≤1.0 mol%. Interestingly, the sample–electrode contact and resistive–outer surface layer effects, i.e., surface barrier layer capacitor (SBLC) effect, has also an effect on the colossal dielectric response in (In + Nb) co-doped TiO₂ ceramics.     

Correlation between the radius of acceptor ion and the dielectric properties of co-doped TiO2 ceramics

There is an urgent need to seek colossal dielectric materials with better dielectric properties because of the rapid development of the electronics industry. In this work, (M_0.5Nb_0.5)_0.01Ti_0.99O₂ (M = Al, In, Eu) ceramics are synthesized by traditional solid-state reaction, and recorded as ANTO, INTO and ENTO ceramics, respectively. The effects of different radius acceptor ions on the phase structure, microstructure and dielectric properties of TiO₂ based ceramics are investigated. It is found that all samples are rutile phases without the generation of the second phase, but the difference of ion radius resulted in lattice distortion of different degrees, which affects the defect structure in the material. The dielectric properties of the materials are related to the defect structure in the material, and the ability to localize electrons in different defect structures is different. The dielectric properties of ANTO ceramics are similar to those of un-doped TiO₂; INTO ceramics have the lowest dielectric loss (0.039), the best temperature and frequency stability, and there are no dielectric loss peak near the room temperature; The ENTO ceramics has the maximum permittivity (2.01 × 10⁵).     

Effect of ZnO doping on the microstructure and microwave dielectric properties of 0.2CaTiO3-0.8(Li0.5Sm0.5)TiO3 ceramics

0.2CaTiO₃-0.8(Li_0.5Sm_0.5)TiO₃-xZnO(x = 0, 0.3, 0.6, 0.9, 1.2 wt%, 0.2CT-0.8LST-xZnO) with orthogonal perovskite structure were fabricated by the solid state method. The effects of ZnO additives on the microwave dielectric properties of 0.2CT-0.8LST ceramics were systematically investigated. With increasing the dopant (x) concentration, the dielectric constant (ε_r) and the temperature co-efficient of resonance frequency (τ_f) decreased, however, the Q × f values increased. The relationship between vibration mode and microwave dielectric properties was studied using Raman spectroscopy. The Q × f value of ceramics was related to the half-height width of Raman scattering. Narrower Raman scattering peaks corresponded to longer microwave energy propagation decay times and higher Q × f value. Based on X-ray photoelectron spectroscopy (XPS), the addition of Zn²⁺ ions limited the reduction of Ti⁴⁺ cations. The excellent dielectric properties were obtained when x = 1.2 wt% with ε_r = 100.25, Q × f = 6525 GHz, and τ_f = ?12.12 ppm/°C.     

Structure and dielectric properties of Ln3NbO7 (Ln = Nd,Gd, Dy,Er, Yb and Y)

The structure and dielectric properties of Ln₃NbO₇ (Ln = Nd, Gd, Dy, Er, Yb and Y) ceramics are investigated. With decreasing ionic radius of Ln³⁺, the stable crystal structure of the compounds shifts from orthorhombic weberite to defect fluorite. It is experimentally observed that, with the exception of Gd₃NbO₇, the room temperature real part of the relative permittivity of Ln₃NbO₇ ceramics decreases from approximately 40 to 30 (at 1 MHz) with increasing ionic radius of Ln³⁺. The observed imaginary part of the relative permittivity is in order of 10⁻² to 10⁻¹ (room temperature and 1 MHz) and it is relatively stable up to 80 °C, where it increases with a rise in temperature. Interesting exceptions of these trends are Nd₃NbO₇ that crystallizes with a weberite-type structure and shows large positive temperature variation of the dielectric properties, and Gd₃NbO₇ that crystallized in a weberite related structure and displays frequency and temperature dependent dielectric relaxation behavior.     

Effect of MnO2 on the dielectric properties of Nb-doped BaTiO3-(Bi0.5Na0.5)TiO3 ceramics for X9R MLCC applications

MnO₂ and Nb₂O₅ co-doped 0.9BaTiO₃-0.1(Bi_0.5Na_0.5)TiO₃ powders with excellent dielectric properties were fabricated using a conventional solid-state reaction method and sand milling. The doping effects of various amounts of MnO₂ on the dielectric properties were investigated. The results revealed that the dielectric properties greatly depended on the concentration of MnO₂. All the ceramics met the X9R specification. The dielectric loss decreased with an increasing concentration of MnO₂. The specimen with an appropriate amount of 0.2 mol% MnO₂ exhibited the most enhanced properties: high insulation resistance (2.49 × 10¹³ Ω/cm) and improved degradation properties. Multilayer ceramic capacitor (MLCC) chips were prepared by tape casting using a 0.2 mol% Mn-doped 9010BTBNT-based ceramic powder. The capacitance of the MLCC chip was approximately 100 nF, and the dielectric loss was approximately 1.75% at room temperature. The high-temperature accelerated lifetime was over 1000 hours under 250 V (five times the working voltage) and at 230°C, indicating that the MLCC chips possess superior reliability.     

Structural,dielectric and mechanical behaviors of (La,Nb) Co-doped TiO2/Silicone rubber composites

Ceramic/polymer composites have great potential to achieve the concomitant enhancement of both dielectric constant and breakdown field while maintaining other superior properties of the polymer matrix, ideal for elastomer sensors, actuators, capacitive energy storage, and many other applications. However, material incompatibility between the ceramic filler and the polymer matrix often leads to void formation, particle aggregation and phase separation, with significantly degraded performance. Herein, through surface modification, co-doped TiO₂ particles were uniformly dispersed and bridged onto the silicone rubber matrix via a silane coupling agent for fabricating composites via mechanical mixing and hot-pressing. The synthesized composites exhibit enhanced dielectric constant, increased from 2.78 to 5.06 when 50 wt% co-doped TiO₂ particles are incorporated. Their dielectric loss is less than 0.001 in a broad frequency range. Theoretical modelling and experimental results reveal that the morphology and dispersion state of co-doped TiO₂ particles were crucial to the dielectric properties of the silicone rubber-based composites. Besides, the composites are thermally stable up to 400 °C. Significantly increased tensile strength (612 kPa) and elongation at break (330%) were obtained for the composite incorporated with 30 wt% co-doped TiO₂ particles, accompanied by a moderate increased elastic module (540 kPa). Such composites have the potential for different applications.     

Crystal structure,spectra analysis and dielectric characteristics of Ba4M28/3Ti18O54 (M= La,Pr, Nd,and Sm) microwave ceramics

High dielectric constant Ba₄M_28/3Ti₁₈O₅₄ (M = La, Pr, Nd, and Sm) ceramics were synthesized via standard solid-state reaction route. All the samples possessed single-phase orthorhombic tungsten-bronze structure, and the unit cell volumes decreased monotonously when rare-earth ion changed from La to Sm. With the decreasing radius of rare-earth ions, the dielectric constant (ε_r) gradually decreased, which was correlated with the decrease of total ionic polarizability and blue shift of Raman vibration modes. The increase of the quality factor (Q × f) was associated with the increase of packing fraction and the decrease of FWHM of Raman vibration modes (A_g and B_1g). The temperature coefficient of resonant frequency (TCF) shifted in the negative direction, which was highly related to the decrease of the TiO₆ octahedral tilt angle. Infrared reflection (IR) spectroscopy analysis indicated that the infrared vibration modes between TiO₆ octahedron and A-site cation (M³⁺ and Ba²⁺) at low frequency dominated microwave dielectric polarization in the Ba₄M_28/3Ti₁₈O₅₄ ceramics.     

Effect of Ta2O5 in (Ca,Si, Ta)-doped TiO2 ceramic varistors

TiO₂ varistors doped with 0.2 mol% Ca, 0.4 mol% Si and different concentrations of Ta were obtained by ceramic sintering processing at 1350 °C. The effect of Ta on the microstructures, nonlinear electrical behavior and dielectric properties of the (Ca, Si, Ta)-doped TiO₂ ceramics were investigated. The ceramics have nonlinear coefficients of α = 3.0–5.0 and ultrahigh relative dielectric constants which is up to 10⁴. Experimental evidence shows that small quantities of Ta₂O₅ improve the nonlinear properties of the samples significantly. It was found that an optimal doping composition of 0.8 mol% Ta₂O₅ leads to a low breakdown voltage of 14.7 V/mm, a high nonlinear constant of 4.8 and an ultrahigh electrical permittivity of 5.0 × 10⁴ and tg δ = 0.66 (measured at 1 kHz), which is consistent with the highest and narrowest grain boundary barriers of the ceramics. In view of these electrical characteristics, the TiO₂–0.8 mol% Ta₂O₅ ceramic is a viable candidate for capacitor–varistor functional devices. The characteristics of the ceramics can be explained by the effect and the maximum of the substitution of Ta⁵⁺ for Ti⁴⁺.     

Colossal permittivity of (Gd + Nb) co-doped TiO2 ceramics induced by interface effects and defect cluster

Material with high dielectric constant plays an important role in energy storage elements. (Gd + Nb) co-doped TiO₂ (GNTO) ceramics with giant dielectric permittivity (>10⁴), low dielectric loss, good temperature and frequency stability in broad range of 30–150 °C and 10²–10⁶ Hz have been systematically characterized. Especially, a low dielectric loss of 0.027 and a giant dielectric permittivity of 5.63 × 10⁴ at 1 kHz are attained for the composition with x = 0.01. Results of complex impedance spectroscopy, I–V curve and frequency dependent dielectric constant under DC bias indicate that internal barrier layer capacitance (IBLC) effect, electrode effect and electron-pinned defect-dipole (EPDD) effect contribute to the colossal permittivity (CP) property simultaneously.     

Tailoring the microwave dielectric properties of Ba(Mg1/3Ta2/3)O3 ceramics

Ba(Mg_1/3Ta_2/3)O₃ (BMT) is a very important microwave dielectric resonator material with a high dielectric constant of about 25, high quality factor and small temperature variation of resonant frequency. The preparation temperature of BMT ceramics is relatively high, about 1650 °C. The microwave quality factor of BMT depends on the ordering of Mg–Ta–Ta ions on the perovskite B-site. In the present paper we report how one can tailor the properties of BMT ceramics by glass addition, slight non-stoichiometry and by dopant addition. (a) It is found that addition of small amount of glasses such as B₂O₃, ZnO–B₂O₃, ZnO–2B₂O₃, ZnO–B₂O₃–SiO₂ reduces the sintering temperature, increases density, order parameter and quality factor. (b) Slight Mg or Ba deficiency improves densification, order parameter and quality factor whereas excess Mg or Ba deteriorated the properties (c) Small amounts of dopants such as Sb₂O₅, MnCO₃, ZrO₂, WO₃, SnO₂ and ZnO improve the microwave dielectric properties. It is found that the quality factor is maximum when the ionic radii of the dopant ions are close to the weighted average ionic radii of B site ions (i.e. Mg_1/3Ta_2/3), which is 0.653 Å.     

Dielectric temperature stability of Nb-modified Bi0.5(Na0.78K0.22)0.5TiO3 lead-free ceramics

In this study, the phase structure, microstructure and dielectric properties of Bi_0.5(Na_0.78K_0.22)_0.5(Ti_1-xNb_x)O₃ lead-free ceramics prepared by traditional solid phase sintering method were studied. The second phase pyrochlore bismuth titanate (Bi₂Ti₂O₇) was produced in the system after introduction of Nb⁵⁺. The dielectric constant of the sample (x = 0.03) sintered at 1130 °C at room temperature reached a maximum of 1841, and the dielectric loss was 0.045 minimum. It had been found that the K⁺ and Nb⁵⁺ co-doped Bi_0.5Na_0.5TiO₃ (BNT) lead-free ceramics exhibited outstanding dielectric-temperature stability within 100–400 °C with T_cc ≤±15%. Result of this research provides a valuable reference for application of BNT based capacitors in high temperature field.     

Effect of Mn-doping on the morphological and electrical properties of (Ba0·7Sr0.3) (MnxTi1−x)O3 materials for energy storage application

Lead-free (Ba_0·7Sr_0.3) (Mn_xTi_1?x)O₃ (x = 0.0, 1.0, 2.0, 4.0, 6.0, and 8.0%) ceramics were effectively synthesized by the sol-gel process. XRD and Raman spectroscopy confirm the single-phase perovskite structure with tetragonal symmetry, for all compositions. An in-depth analysis of the chemical composition and thermochemistry of the ceramics was carried out via FT-IR and TG-DTG. Morphological analyses of the samples revealed that doping Mn at higher concentrations suppresses the grain size and grain growth rate. The dielectric properties increased first and then decreased with increasing Mn content. The ferroelectric properties represented similar trend in polarization and energy storage efficiency as showed in dielectric properties. 2% Mn-doped sample exhibits the best dielectric and energy storage performance which is attributed to increased densification and grain size effects. Dielectric anomaly caused by defect dipole polarization was observed in temperature-dependent dielectric constant curves, for 6% and 8% Mn-doped samples. This study is helpful to establish the relationship between the structure, morphology, dielectric and ferroelectric properties of Mn substituted Ba_0·7Sr_0·3TiO₃ ceramics.     

Colossal permittivity in Ta-doped SrTiO3 ceramics induced by interface effects and defect structure: An experimental and theoretical study

The lack of systematic research on the phase structure, defect structure, and polarization mechanism hinders the full comprehension of the colossal permittivity (CP) behavior for SrTiO₃-based ceramics. For this purpose, Ta-doped SrTiO₃-based ceramics were synthesized in an N₂ atmosphere with a traditional method. When the appropriate amount of Ta was doped, colossal permittivity (ԑ_r ∼ 62505), low dielectric loss (tanδ ∼ 0.07), as well as excellent temperature stability (−70 °C–180 °C, ΔC/C_25°C ≤ ±15%) were obtained in the Sr_0.996Ta_0.004TiO₃ ceramic. The relationship between Ta doping, polarization mechanism, and dielectric performance was systematically researched according to experimental analysis and theoretical calculations. The first-principle calculations indicate that the Ta⁵⁺ ion prefers to replace the Sr-site. The defect dipoles and oxygen vacancies formed by heterogeneous-ion doping play an active role in regulating the dielectric performance of ceramics. In addition, the interface barrier layer capacitance (IBLC) effect associated with semi-conductive grains and insulating grain boundaries is the primary origin of colossal permittivity for Sr_1-xTa_xTiO₃ ceramics. The polarization mechanism and defect structure proposed in the study can be extended to the research of SrTiO₃ CP ceramics. The results have a good development prospect in colossal permittivity (CP) materials.