Colossal permittivity of (Li,Nb) co-doped TiO2 ceramics

In this work, (Li, Nb) co-doped TiO₂ ceramics (LNTO_x, x?<?0.1), were synthesized through a conventional solid state reaction method. As revealed by X-ray diffraction (XRD) spectra, all LNTO ceramics exhibited pure tetragonal rutile structure. The LNTO_0.01 ceramic showed a colossal permittivity over 7000 and a low dielectric loss (tgδ?<?0.06) in a wide frequency range of 10²?Hz–10⁷?Hz. The dielectric spectra under DC biases were tested at different temperatures. The experimental data could fit the modified Debye equation well. It was found that there are multiple dielectric polarization mechanisms in LNTO ceramics including space charge polarization, relaxor-type relaxation, polaron hopping and dipole polarization related with localized electrons.     

Enhanced relative permittivity in niobium and europium co-doped TiO2 ceramics

The appearance of colossal permittivity materials broadened the choice of materials for energy-storage applications. In this work, colossal permittivity in ceramics of TiO₂ co-doped with niobium and europium ions ((Eu_0.5Nb_0.5)_xTi_1-xO₂ ceramics) was reported. A large permittivity (ε_r ～ 2.01?×?10⁵) and a low dielectric loss (tanδ ～ 0.095) were observed for (Eu_0.5Nb_0.5)_xTi_1-xO₂ (x?=?1%) ceramics at 1?kHz. Moreover, two significant relaxations were observed in the temperature dependence of dielectric properties for (Eu, Nb) co-doped TiO₂ ceramics, which originated from defect dipoles and electron hopping, respectively. The low dielectric loss and high relative permittivity were ascribed to the electron-pinned defect-dipoles and electrons hopping. The (Eu_0.5Nb_0.5)_xTi_1-xO₂ ceramic with great colossal permittivity is one of the most promising candidates for high-energy density storage applications.     

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 BaTiO3-0.5wt%Na0.5Ba0.5TiO3 ceramics with high insulation resistivity induced by reducing atmosphere

The donor-acceptor co-doping grain-fine BaTiO₃-0.5wt%Na_0.5Ba_0.5TiO₃ (BT-0.5wt%NBT) ceramics are obtained by conventional solid-state reaction method and sintered in different atmospheres. The dielectric properties are sensitively influenced by the sintering atmosphere, which the colossal permittivity can be activated and increased by enhanced atmospheric reducibility with the increase of H₂ content. However, the excessive H₂ content can make a significant deterioration in dielectric loss and insulation resistivity. The impedance spectra, XPS and EPR measurements indicate that sintering atmosphere can effectively regulate the concentration and distribution of charge carriers (delocalized electrons, oxygen vacancies and defects), which induce the interfacial polarization and hopping polarization. When sintered in 0.1% H₂/N₂, the sample possesses a relatively good comprehensive performance with colossal permittivity (ε_γ>4×10⁴), ultra-low dielectric loss (tanδ=0.0218) and high insulation resistivity (ρ_v>10¹¹Ω·cm), which related to the moderate values of resistance and conduction activation energy for the grain boundaries.     

Effect of ionic radius on colossal permittivity properties of (A,Ta) co-doped TiO2 (A= alkaline-earth ions) ceramics

(A, B) co-doped TiO₂ ceramics attract great interests due to the excellent dielectric properties. In this work, the (A, Ta) co-doped TiO₂ ceramics were prepared by a solid state reaction process. The effect of the acceptors ionic radius on the structure and properties of TiO₂ ceramics was investigated. According to XRD analysis, the main phase is rutile TiO₂ for all samples. Due to the larger ionic radius, it is hard to replace Ti site in TiO₆ octahedron. As a result, the content of the secondary phase increased with increasing ionic radius. The dielectric properties were significantly enhanced by co-doping of alkaline-earth ions and tantalum ions, and the best dielectric constant obtained at 3% (Sr, Ta) co-doped compositions, where ε’ = 2.1 × 10⁵, tanδ = 0.21. Meanwhile, the XPS analysis suggested that the concentration of the defect dipoles exhibit a maximum in Sr-doped TiO₂ ceramics. The larger ionic radius of the acceptors leads to the more stability of the defect structure. However, for Ba ions, the replacement concentration decreased due to the excessive ionic radius, which in turn reduces the defect concentration. This work is meaningful for the further investigations on TiO₂-based colossal permittivity materials.     

Disappearance and recovery of colossal permittivity in (Nb+Mn) co-doped TiO2

We investigate the effects of doping and annealing on the dielectric properties of metal ions doped TiO₂ ceramics. Colossal permittivity (CP) above 10⁴ was observed in single Nb ion doped TiO₂, which was dominated by electron transport related interfacial polarization. Moreover, the CP can be dropped to 120 when simultaneously introducing Mn ion into the sample. The disappearance of CP behaviors maybe due to the multivalence of Mn which would inhibit the reduction of Ti⁴⁺ to Ti³⁺, and thus reduce delocalized electrons. Interestingly, the CP was recovered for the (Nb+Mn) co-doped TiO₂ after post-sintering heat treatment in N₂ atmosphere. The recovery of CP in the sample after annealing can be ascribed to the semiconducting grain and the insulating grain boundary, according to impedance spectroscopy. We therefore believe that this work can help us understand the mechanism of CP from a new perspective.     

An investigation into the enhanced permittivity properties of Zr co-doped (Ga0.5Nb0.5)0.03Ti0.97O2 ceramics

Dielectric materials with high permittivity and low dielectric loss have a range of promising applications within electronic devices. Here, we report on Zr co-doped (Ga_0.5Nb_0.5)_0.03Ti_0.97O₂ ceramics, fabricated using a solid-state reaction. The colossal permittivity (CP) of (Ga_0.5Nb_0.5)_0.03-(Zr_xTi_1-x)_0.97O₂ ceramics was investigated (x = 0%, 1%, 4%, 6%, 10%, 20%). When the doping value of Zr was 4%, the dielectric loss was reduced to 0.098 and, at room temperature and at a frequency of 1000 Hz, the dielectric permittivity was recorded as 2420. In addition, the material's dielectric permittivity exhibited good stability at temperatures ranging from −50 °C to 200 °C. Using X-ray photoelectron spectroscopy (XPS) and Scanning electron microscopy (SEM), we have observed that Zr doping reduces grain size and increases grain boundary regions. According to our XPS and impedance analysis, Zr doping also reduces the concentration of oxygen vacancies, which are considered to be the main cause of dielectric loss. We believe that the Zr doping is an effective method for reducing the dielectric loss of CP materials.     

Niobium and divalent‐modified titanium dioxide ceramics: Colossal permittivity and composition design

Colossal permittivity (CP) (ε_r=10⁴~10⁵) is attained in (A_1/3Nb_2/3)_xTi_1‐xO₂ (A=Ba²⁺, Ca²⁺, Zn²⁺, Mg²⁺) ceramics. Here, (Ca_1/3Nb_2/3)_xTi_1‐xO₂ material was studied as a typical example, and effects of Ca and Nb on their microstructure, dielectric properties and stability were studied. Both backscattering and elements mapping strongly confirmed the formation of secondary phases due to the addition of Ca and/or Nb. Secondary phases‐induced by Ca cannot affect dielectric properties of the ceramics when low Ca and Nb contents were doped, while secondary phases formed by Ca and Nb strongly affected their dielectric properties in a high doping level. In particular, their dielectric properties can be well modified by the optimization of sintering temperatures. In addition, the (Ca_1/3Nb_2/3)_xTi_1‐xO₂ ceramics with x=0.01 exhibited the optimum dielectric properties (ε_r=130500 and tan δ=0.19). Electron‐pinned defect‐dipoles may be suitable to explain CP phenomenon of this work. We believed that this profound investigation can benefit the development of new TiO₂ ceramics as a CP material.     

Colossal permittivity and ultralow dielectric loss in (Nd0.5Ta0.5)xTi1-xO2 ceramics

Giant permittivity ceramic is one of the most significant classes of material to realize the miniaturization and integration of a high-performance capacitor. In this paper, to realize good giant dielectric properties, the (Nd_0.5Ta_0.5)_xTi_1-xO₂ ceramics (NTTO x = 0.005, 0.01, 0.03, 0.05) were synthesized by a standard conventional solid-state reaction. Comparing with the previous co-doped TiO₂ ceramics giant permittivity material system, NTTO ceramics perform extremely colossal permittivity and ultralow dielectric loss (1%NTTO: ε = 82052, tanδ = 0.008 at 1 kHz; 5%NTTO: ε = 170131, tanδ = 0.090 at 1 kHz). The broad distinction of the dielectric behavior between the (Nd_0.5Ta_0.5)_xTi_1-xO₂ ceramics can be explained by the impedance analysis and the calculated polarization activation energies. The main electron-pinned defect-dipole (denoted as EPDD) polarization corresponds to the ultralow loss, embodying in the maximum value of E_gb (the activation energy of the grain boundary), E_a2 (the EPDD polarization activation energies) and the minimum value of E_a1 (the total polarization activation energies). Though the interfacial polarization can cause the permittivity increase, it can also give rise to poor frequency stability and higher loss.     

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.     

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.     

Grain-boundary engineering inducing thermal stability,low dielectric loss and high energy storage in Ta+Ho co-doped TiO2 ceramics

How to achieve a giant dielectric constant and high energy storage density at the same time has been the problem to be solved for donor-acceptor co-doped TiO₂ ceramics. In this work, (Ho_0.5Ta_0.5)_0.01Ti_0·99O₂ - x SiO₂, where x = 0, 1, 3, 5 and 7 wt% (HTTO - x wt% SiO₂), nanocomposites were prepared via a conventional mixed oxide technique. Significantly, the HTTO - 5 wt% SiO₂ composite ceramic exhibits a low dielectric loss (tanδ ～ 0.012) and an ultrahigh permittivity (ε_r ～ 1.29 × 10⁴) at 1 kHz. Also, excellent energy storage property with a high breakdown field strength (E_b ～1.86 kV/cm) and energy storage density (η ～ 1.97 mJ/cm³) was obtained in HTTO - 5 wt% SiO₂ ceramic. Besides, the enhancement of E_b is attributed to the finer grains and the presence of SiO₂ blocking layers in the grain boundaries, which hinder the long-range motion of electrons. It can be concluded that the CP and high energy storage properties arise from the combined contribution of enhanced grain boundary effects and electron-pinning type of defect dipole (EPDD) effects. This study not only proposes an effective method improving E_b, but also offers a new routine for how to simultaneously achieve CP and high η in TiO₂ dielectric materials.     

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.     

Giant permittivity up to 100 MHz in La and Nb co-doped rutile TiO2 ceramics

Dielectric spectroscopy was carried out for reduced and stoichiometric La_0.0025Nb_0.0025Ti_0.995O₂ ceramics synthesized by sintering in different atmospheres. A giant permittivity (~1 × 10⁴) was obtained at a frequency of 100 MHz and temperature range from 170 to 350 K. Three dielectric relaxation mechanisms were observed within the temperature range of 10-300 K via dielectric spectroscopy. A low temperature dipole relaxation peak (in the temperature range of 10-30 K) in the spectra was identified to be associated with the giant permittivity specifically measured at 100 MHz. The origin of such giant permittivity was attributed to dipole orientation polarization. Hopping polaron and interfacial effect contributed to giant permittivity. After annealing treatment, all the relaxation contributions were weakened. Low dielectric loss was attributed to high resistance of grain and grain boundaries. Annealing in ambient conditions led to decreased relaxation times which gives the signature of decreased concentration of oxygen vacancies and Ti³⁺. Dipoles which were related to oxygen vacancies and Ti³⁺, resulted in giant permittivity up to 100 MHz.     

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.     

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.     

Double pentavalent (Sb5+, Nb5+) and trivalent (Sm3+, Y3+) co-doped Ti0.9Zr0.1O2 colossal dielectric permittivity multilayer ceramics for the miniaturization of the next-generation electronics

Materials with colossal dielectric permittivity (CP) are in the focus of interest for the development of miniaturization and integration of electronic components. Despite the extensive study of these new classes of co-doped TiO₂ CP materials, the preparation of multilayer ceramics using this kind of CP materials is still challenging work. Here, we synthesize a series of (Sb⁵⁺, Nb⁵⁺) and (Sm³⁺, Y³⁺) co-doped Ti_0.9Zr_0.1O₂ ceramics (SNSYTZO) through the conventional solid-state reaction method. XRD spectrum identifies that ceramics under x = 0.04 show a perfect rutile phase with the tetragonal crystal structure; however, minor brookite orthorhombic crystal structure appears when x > 0.04. FESEM images show the prepared ceramics have excellent densification and low porosity. Dielectric, modulus, and impedance spectrum are systematically explored the underlying CP mechanism and compared with each other to find the optimal materials composition to prepare further multilayer ceramics, which is fabricated by the industrial tape casting method. FESEM, together with surface element mapping, indicates that all doping elements are homogeneously distributed. Also, we investigate the dielectric response without/with DC bias. This work sheds light on a promising feasible route to prepare the miniaturization of the next-generation electronics via a large scale industrial tape casting method.