Good dielectric performance and broadband dielectric polarization in Ag,Nb co-doped TiO2

Due to the demand of miniaturization and integration for ceramic capacitors in electronic components market, TiO₂-based ceramics with colossal permittivity has become a research hotspot in recent years. In this work, we report that Ag⁺/Nb⁵⁺ co-doped (Ag_1/4Nb_3/4)_xTi_1−xO₂ (ANTOx) ceramics with colossal permittivity over a wide frequency and temperature range were successfully prepared by a traditional solid–state method. Notably, compositions of ANTO0.005 and ANTO0.01 respectively exhibit both low dielectric loss (0.040 and 0.050 at 1 kHz), high dielectric permittivity (9.2 × 10³ and 1.6 × 10⁴ at 1 kHz), and good thermal stability, which satisfy the requirements for the temperature range of application of X9R and X8R ceramic capacitors, respectively. The origin of the dielectric behavior was attributed to five dielectric relaxation phenomena, i.e., localized carriers' hopping, electron–pinned defect–dipoles, interfacial polarization, and oxygen vacancies ionization and diffusion, as suggested by dielectric temperature spectra and valence state analysis via XPS; wherein, electron-pinned defect–dipoles and internal barrier layer capacitance are believed to be the main causes for the giant dielectric permittivity in ANTOx ceramics.     

Influence of Zr dopant on polarization in rutile (In0.5Nb0.5)0.005(Ti1-xZrx)0.995O2 ceramics

(In_0.5Nb_0.5)_0.005(Ti_1-xZr_x)_0.995O₂ (INZT, x = 0-0.10) ceramics were synthesized using a conventional sintering method, and the effects of Zr content on the microstructures, dielectric properties and electron-pinned defect-dipoles (EPDD) polarization of the resultant products were investigated. The solubility limit of INZT was x = 0.075, and a secondary ZrTiO₄ phase appeared at x = 0.10. Ceramics with x = 0-0.10 exhibited excellent dielectric properties, ie, colossal permittivity (CP, εʹ > 10³) and low dielectric loss (tanδ < 0.1), over a wide range of frequencies (10⁰-10⁶ Hz at 300 K) and temperatures (50-350 K at 1 kHz). The dielectric spectra and XPS results confirmed that the CP property of the ceramics could be ascribed to their EPDD polarization. The activation energy (E_a) for EPDD polarization was continuously enhanced by increasing x values. EPDD relaxation parameters at different x values were revealed using Cole-Cole equation fitting. Moreover, α, which characterize the relaxation time τ distribution, increased with x values, thus indicating that Zr was involved in and affected electron localized states. The high E_a, temperature T_p of the peak εʹʹ at 1 kHz, and dielectric relaxation time τ_p at 30 K were related to increases in hopping distance of electrons among defect clusters with Zr addition.     

Broadband impedance spectroscopic characterization of PbTiO3 crystal grown by spontaneous crystallization from molten oxides

Tetragonal lead titanate PbTiO₃ single crystals were grown by spontaneous crystallization from a lead and boron oxide flux and investigated by impedance spectroscopy in the broadband frequency range (10⁻³–5×10⁶ Hz) along the crystallographic a and c axes at room temperature.Three polarization relaxation modes as well as electric conductivity processes were revealed. In the ultralow frequency range (<10 Hz), the relaxation processes were assigned to the electrode and the ferroelectric domains polarizations. In the high frequency range (10⁵ Hz), the relaxation process arises from the crystal lattice polarizations. The electric conduction of the investigated crystals is determined by two fundamental effects – ionic and hopping conductivities. In addition, the anisotropies of relative dielectric constant (300) as well as DC conductivity (1.8×10⁻⁹ S/m) were estimated.     

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.     

Aluminum‐vacancy‐related dielectric relaxations in AlN ceramics

The dielectric properties of AlN ceramics were investigated comprehensively in the temperature range from room‐temperature to 950 K and frequency range of 10² to 5 × 10⁶ Hz. The sample exhibits intrinsic dielectric behavior when T < 500 K, showing a flat dielectric permittivity about 10 and an extremely low dielectric loss factor (tanδ < 2 × 10^?3). In the temperature above 500 K, two thermally activated dielectric relaxations related to bulk and interfacial effects were observed. Both relaxations strongly depend on the concentration of oxygen atoms. Our results indicate that the bulk relaxation, occurring in lower temperature range, is caused by aluminum vacancy hopping motion inside grains. The interfacial relaxation, occurring in higher temperature range, is caused by surface‐layer effect due to aluminum vacancies being blocked by sample‐electrode contact.     

Broadband dielectric properties of multiwalled carbon nanotube/polystyrene composites

This study investigates the dielectric properties of multiwalled carbon nanotube (MWCNT)/polystyrene (PS) composites over the broadband frequency range, i.e., 10^?1 to 10⁶ Hz. The results showed that the real permittivity and imaginary permittivity increased remarkably with increased MWCNT concentration. For instance, at 100 Hz, the real permittivity and imaginary permittivity of the pristine PS was 2.71 and 0.01, respectively, which increased to 5.22 × 10⁴ and 3.28 × 10⁷ at 3.50 wt%, respectively. The increase in the real permittivity was related to the formation of a large number of nanocapacitor structures, i.e., MWCNTs as nanoelectrodes and polymer matrix as dielectric material, i.e., interfacial polarization. The increase in the imaginary permittivity with MWCNT loading was attributed greater number of dissipating charges, enhanced conductive network formation, and boosted polarization loss arising from interfacial polarization. It was also observed that the real and imaginary permittivities were frequency independent in the insulative region, whereas they decreased drastically with frequency in the conductive region. The descending trend of real permittivity with frequency in the conductive region was related to charge polarization relaxation, whereas the reduction in imaginary permittivity with frequency was attributed to lower Ohmic loss and polarization loss. POLYM. ENG. SCI., 55:173–179, 2015. © 2014 Society of Plastics Engineers     

Fabrication and origin of high-k carbon nanotube/epoxy composites with low dielectric loss through layer-by-layer casting technique

The distribution of polarized space charges and their relaxation behavior in high dielectric constant electric conductor/polymer composites are main factors that determine the frequency-dependent dielectric constant and dielectric loss. However, few reports focus on this motif. We present here the dielectric performance and mechanism of a unique kind of composites with multi-layers (coded as [MWCNT/EP]_x, where x refers to the number of layers), fabricated by using layer-by-layer casting technique. Each composite layer with same thickness was composed of multi-walled carbon nanotubes (MWCNTs) and epoxy (EP) resin. When the loading of MWCNTs is 0.5 wt%, the four-layer [MWCNT0.5/EP]₄ material shows the highest dielectric constant (465 at 1 Hz) and low dielectric loss tangent (0.7 at 1 Hz), about 4 and 2.1 × 10⁻² times the values of traditional MWCNT0.5/EP composite, respectively. By investigating the space charge polarization (SCP), Debye polarization and dielectric moduli in [MWCNT/EP]_x materials, the complex relationships and the origin among dielectric constant, dielectric loss, frequency and the content of filler were clearly elucidated. The SCP within each layer is different from that between layers. The greatly improved dielectric properties of [MWCNT/EP]_x materials are believed to be the reinforced SCP and blocked transport of carriers between every two layers.     

Nanoarchitectonics of Niobium-Doped,Lead-Free BLT (Ba0.97La0.02Ti0.98Nb0.016O3) for Electrical Properties with Unusual d.c Bias Voltage Independence

Polycrystalline sample of Ba_0.97La_0.02Ti_0.98Nb_0.016O₃ (distinguished as BLTi_0.98Nb_0.016) has been prepared through Molten-Salt-Flux reaction route. The XRD, surface morphology, absorption spectra, impedance, and dielectric behaviors were employed to typify the prepared polycrystalline ceramic. The XRD analysis reflects that obtained perovskite having the pure-tetragonal structure with space group P4/mmm. As of the absorption spectra, the optical band gap (Eg), Urbach energy (Eu), and refractive index values have designed. The electrical properties of synthesized compound have been inspected via complex impedance spectroscopy vs. frequency (f) (10² Hz–10⁶ Hz) within the d.c-bias voltage range [0.5 V–5 V]. The fitting of the Nyquist plot exposes that both intra- and intra grains contribute to relaxation and the grain limits are more resistive and capacitive than the grains. Modulus analysis confirms that relaxation in our sample is of non-Debye nature and d.c-bias voltages dependent. Depending on the frequency, the change of ε′ can be discussed founded on the principle of interfacial polarization of the Maxwell–Wagner category. BLTi_0.98Nb_0.016 shows notable frequency independent relative studied properties, it is a potential candidate for devices.

     

Complex impedance study on polymer-derived amorphous silicon carbonitride

Polymer-derived amorphous-SiCN pyrolyzed at 1200 °C was studied by impedance spectroscopy at different frequencies and temperatures. Obtained impedance spectra were analyzed by using equivalent circuit method. Results revealed that two relaxation effects were involved over the entire studied temperature range, which corresponded to free carbon phase at high frequency and the interface at low frequency. The impedance spectroscopy could be well-fitted by two parallel RC circuits in series. Interface behavior dominated the polarization relaxation process, which was controlled by the resistance as well as the capacitance of interface. All processes performed a 1/T^1/4 behavior, which follows a three-dimensional (3D) random hopping mechanism. This paper further found that amorphous-SiCN ceramic materials underwent an interfacial charge polarization process over a frequency range from 0.05 Hz to 10 MHz at different temperatures from −50 to 300 °C.     

Electrical properties and light up conversion effects in Bi3.79Er0.03Yb0.18Ti3−xWxO12 ferroelectric ceramics

Modified ceramic compositions of Bi_3.79Er_0.03Yb_0.18Ti_3−xW_xO₁₂ with fixed Er and Yb content, and a varying W content (x=0.0, 0.01, 0.03, 0.06 and 0.10) are prepared. The site selectivity of Er³⁺, Yb³⁺, and W⁶⁺ cations is analyzed, and their influence on the electrical and light up conversion properties is studied. Formation of single phase orthorhombic structure is confirmed with enhanced grain growth up to x=0.03, and for (x≥0.04–0.10) the grain growth is inhibited, and the orthorhombic distortion is relaxed. Raman spectroscopy reveals W⁶⁺ cation substitutes preferentially at the B-site replacing Ti⁴⁺ ions in the Bi₄Ti₃O₁₂ lattice structure. Increasing W⁶⁺ donor concentration reduces the conductivity effects by lowering the oxygen vacancies. Reduced dielectric losses (tan δ=0.003) and dispersion with frequency in the range (10⁻²–10 Hz) are observed, and improvements in the remnant polarization (2P_r=28.86 μC/cm²) are seen up to an optimum content of x=0.03. At higher W content (x>0.03), the properties tend to degrade due to structural relaxation and microstructural changes. Up conversion photoluminescence (UC-PL) under 980 nm excitation shows strong emission in the green and red bands due to enhanced crystal field around the Er³⁺ ions for an optimum W content of x=0.06. A weak blue emission band around (~492 nm) is observed by cooperative emission (CE) due to radiative relaxation of an excited Yb–Yb pair from a virtual level. Variation of UC emission intensity with pump-power confirms a two-photon mechanism for the up conversion process.     

Frequency and Temperature‐Independent Electrical Transport Properties of 2BaO–0.5Na2O–2.5Nb2O5–4.5B2O3 Glass‐Ceramics

The temperature (300–973 K) and frequency (100 Hz–10 MHz) response of the dielectric and impedance characteristics of 2BaO‐0.5Na₂O–2.5Nb₂O₅–4.5B₂O₃ glasses and glass nanocrystal composites were studied. The dielectric constant of the glass was found to be almost independent of frequency (100 Hz–10 MHz) and temperature (300–600 K). The temperature coefficient of dielectric constant was 8 ± 3 ppm/K in the 300–600 K temperature range. The relaxation and conduction phenomena were rationalized using modulus formalism and universal AC conductivity exponential power law, respectively. The observed relaxation behavior was found to be thermally activated. The complex impedance data were fitted using the least square method. Dispersion of Barium Sodium Niobate (BNN) phase at nanoscale in a glass matrix resulted in the formation of space charge around crystal‐glass interface, leading to a high value of effective dielectric constant especially for the samples heat‐treated at higher temperatures. The fabricated glass nanocrystal composites exhibited P versus E hysteresis loops at room temperature and the remnant polarization (P_r) increased with the increase in crystallite size.     

Dielectric and energy storage properties of (La,Li)x[(Bi,Na)BaSrCa]1-xTiO3 high-entropy perovskite ceramics

A series of single-phase (La_0.5Li_0.5)_x[(Bi_0.5Na_0.5)_0.25Ba_0.25Sr_0.25Ca_0.25]_1-xTiO₃ high-entropy perovskite ceramics were designed and successfully synthesized via conventional solid state reaction method. The results of dielectric properties indicate that all samples in different proportions exhibit excellent frequency stability at a wide frequency range (10²–10⁶ Hz) and quintessential relaxation phenomenon. An optimal dielectric constant (ε_r = 920) with low dielectric loss (tanδ = 0.015) was achieved for x = 0.20, which is represented as equimolar high-entropy ceramic. It can be demonstrated that an amazing energy storage efficiency of 95.3% and a discharge density of 1.23 × 10^?2 J/cm³ can be simultaneously achieved in x = 0.24 ceramics. Furthermore, it is confirmed by X-ray photoelectron spectroscopy that the charge compensation mechanism and the oxygen vacancies synergistically cause more Ti⁴⁺ to be reduced, which rationalizes the elevated dielectric properties. We believe that entropy engineering is a credible strategy for tailoring material properties.     

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.     

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.     

Improved energy storage properties of Sr(Zr0.8Nb0.16)O3-doped Bi0.47Na0.47Ba0.06TiO3 ceramics with excellent temperature/frequency stability

Lead-free perovskite dielectric materials for capacitors have received wide concern in recent years, but their energy storage density and efficiency still cannot meet the growing application demand for practical applications. In this work, we prepared a lead-free relaxor ferroelectric ceramic of (1-x)Bi_0.47Na_0.47Ba_0.06TiO₃-xSr(Zr_0.8Nb_0.16)O₃, which was synthesized via a normal solid-state route. The microstructure, dielectric properties and energy storage behavior of the ceramics were explored. The ceramics can be well sintered and situated in the region where rhombohedral and tetragonal phases coexist. The addition of Sr(Zr_0.8Nb_0.16)O₃ (SZN) significantly extends the dielectric-temperature plateau between T_s and T_m and reduces the remnant polarization P_r, but the large saturation polarization P_s is still maintained. Besides, the doping of SZN enhances the relaxation of the material and increases the dielectric breakdown strength (DBS) from 50 kV/cm (x = 0) to 100 kV/cm (x = 0.04 and 0.06). Therefore, the ceramic with x = 0.06 exhibits a high discharging efficiency (η) of 71.1% and energy density (W) of 1.56 J/cm³ at 100 kV/cm and shows the superior thermal stability with the changes in recoverable energy density (W_rec) and η of less than 10% and 30% at the temperature range of 25–180 °C and the excellent frequency stability with the variations of W_rec and η of less than 1.8% and 1% at the frequency range of 10 Hz–100 Hz.     

Analysis of the low-frequency dispersion of the dielectric parameters in AsxSe1-x amorphous layers

This paper reports on the results of the investigation into the dispersion of the permittivity ε’ and the dielectric loss tangent tanδ for amorphous layers in the As_xSe_1-x system (x = 0.4, 0.5) in the frequency range from 10^?3 to 10^?1 Hz. It is found that the permittivity increases with a decrease in the frequency of the polarizing field due to the possible effect exerted by defect surface states on the polarization processes occurring in the layers of this system. The shape of the Cole-Cole plots indicates the existence of several groups of relaxation oscillators that are responsible for the relaxation processes observed in this frequency range.     

Temperature‐stable dielectric and energy storage properties of La(Ti0.5Mg0.5)O3‐doped (Bi0.5Na0.5)TiO3‐(Sr0.7Bi0.2)TiO3 lead‐free ceramics

A new type of (0.7?x)Bi_0.5Na_0.5TiO₃‐0.3Sr_0.7Bi_0.2TiO₃‐xLaTi_0.5Mg_0.5O₃ (LTM1000x, x = 0.0, 0.005, 0.01, 0.03, 0.05 wt%) lead‐free energy storage ceramic material was prepared by a combining ternary perovskite compounds, and the phase transition, dielectric, and energy storage characteristics were analyzed. It was found that the ceramic materials can achieve a stable dielectric property with a large dielectric constant in a wide temperature range with proper doping. The dielectric constant was stable at 2170 ± 15% in the temperature range of 35‐363°C at LTM05. In addition, the storage energy density was greatly improved to 1.32 J/cm³ with a high‐energy storage efficiency of 75% at the composition. More importantly, the energy storage density exhibited good temperature stability in the measurement range, which was maintained within 5% in the temperature range of 30‐110°C. Particularly, LTM05 show excellent fatigue resistance within 10⁶ fatigue cycles. The results show that the ceramic material is a promising material for temperature‐stable energy storage.     

Relaxor behavior in (Ba1−3x/2Bix)(ZryTi1−y)O3 ceramics

New ferroelectric ceramics Bi-BZT of ABO₃ perovskite type were synthesized in the (Ba_1−3x/2Bi_x)(Zr_yTi_1−y)O₃ system by solid state reaction route. The effect of the substitution of barium by bismuth in the A cationic site on structural and dielectric properties was investigated. The dielectric constant was studied in the temperature range from 20 K to 320 K at frequencies ranging from 0.2 to 100 kHz. A clear relaxor behavior was observed for samples with x ≥ 0.075 and y = 0.1. In this case the value of the relaxation parameter γ ≈ 2 estimated from the linear fit of the modified Curie–Weiss law, indicates the relaxor nature of the Bi-BZT ceramics. The dielectric relaxation rate follows Vogel–Fulcher relation with an activation energy of 0.26 eV and 0.27 eV, an attempt frequency f₀ = 3.4 × 10¹² Hz and 5.7 × 10¹² Hz and a static freezing temperature T_VF = 98 K and 94 K respectively for x = 0.075 and x = 0.1.     

Significantly improved dielectric properties of TiO2 ceramics through acceptor-doping and Ar/H2 annealing

The acceptor-doped rutile TiO₂ ceramics, x mol% M₂O₃-(1-x) mol% TiO₂ (M = Al³⁺, Ga³⁺, and In³⁺), were prepared by solid state reaction method. The influence of Ar/H₂ annealing on the structural and dielectric properties of the ceramics were systematically investigated. Our results reveal that the dielectric properties of the ceramics can be significantly improved by the Ar/H₂ annealing. Ga³⁺ is found to be the most suitable dopant with the best doping level of 5 mol%. Excellent dielectric properties of colossal and flat dielectric permittivity (~1.2 × 10⁵ (@1 kHz and 25 °C), low dielectric loss (~0.1), and good frequency stability were achieved over the temperature range of -70–150 °C in the Ar/H₂-annealed 5 mol% Ga₂O₃-95 mol% TiO₂ ceramic. This approach of acceptor-doping and Ar/H₂ annealing leads to two thermally activated relaxations in the sample. The low-temperature relaxation is argued to be a Maxwell-Wagner relaxation caused by frozen electrons, while the high-temperature relaxation is a glass-transition-like relaxation associated with the freezing process of the electrons. This work highlights that engineering low-temperature Maxwell-Wagner relaxation paves a new way other than the frequently used acceptor-donor dual doping to design superior dielectric properties in the TiO₂ system.     

Dielectric properties of Sr0.5Ca0.5TiO3: x Pr3+ ceramics

The frequency dependent dielectric and AC conductivity properties of different concentrations of Pr³⁺ doped Sr_0.5Ca_0.5TiO₃ ceramics were investigated for the frequency range 100 Hz to 2 MHz at different temperatures. The morphology of the prepared samples was analyzed by using Field-Emission Scanning Electron Microscope images. The value of dielectric constant and dielectric loss decreases with increase in the frequency of the applied signal in all the samples. Also, the value of dielectric constant and dielectric loss decreased with doping of different concentrations of Pr³⁺ ions. The conductivity of the samples obeys Jonscher's power-law and shows a decrease with increasing doping concentration of Pr³⁺ ions. The higher value of real and imaginary part of impedance at lower frequency indicates the space charge polarization of the material and its absence at higher frequencies was confirmed from the low value of impedance at higher frequency region. The Cole- Cole parameters of the samples were calculated and the semi-circle observed indicates a single relaxation process and can be modeled by an equivalent parallel RC circuit. At lower frequency region, the value of dielectric constant and dissipation factor increases with increase in the temperature. Also the value of conductivity increases with temperature at high frequency region, due to the enhanced mobility of charge carriers.