Giant Dielectric Constant Research Articles

Tailoring large magnetodielectric response in core/shell CrO2/Cr2O3 nano-rods

Ferromagnetic/antiferromagnetic CrO2/Cr2O3 core/shell nano-rods were fabricated where the Cr2O3 outer shell was structurally and chemically tailored by controlling the decomposition of the CrO2 core. The dielectric response and magnetodielectric effect (MDE) were studied with respect to the thickness of the dielectric Cr2O3 layer. A giant dielectric constant of ∼104, resulting from space charge polarization, is induced by the accumulation of free charges at the interface of the conducting CrO2 and the insulting Cr2O3 nanostructure. An enhanced MDE (Δε′/ε′ = 3.5%), appearing at the dielectric relaxation frequency, stems from the change of dielectric dispersion under the application of a magnetic field of H = 2400 Oe when the thickness of Cr2O3 shell is ∼6.0 nm. We submit that the MDE is due to the interface electric dipole coupled by ferromagnetic spins at the interface of the ferromagnetic CrO2/antiferromagnetic Cr2O3 heterostructures.

Chemical composition-tailored LixTi0.1Ni1−xO ceramics with enhanced dielectric properties

This paper reports on the synthesis of polycrystalline (Li,Ti)-doped NiO powders (i.e., LixTi0.1Ni1−xO, abbreviated as LTNO) by the solid-state synthesis method. Note that, the doping concentration of Ti is kept constant (x∼0.10) in the stoichiometry, the difference in the material behavior of LTNO samples can only be attributed to the effect of Li. X-ray diffraction patterns confirmed a cubic rock-salt structured NiO-based phase with the presence of minor NiTiO3 phase, were reported elsewhere [Venkata et al., Chem. Phys. Lett., 649 (2016) 115–118.]. Dense microstructures were obtained using ultra high resolution scanning electron microscope. A high dielectric constant (ε∼104) near room temperature at low-frequency was observed in LTNO ceramics. Weak temperature dependence of dielectric constant over the measured compositions (x = 0 to 0.10) was observed in the LTNO ceramics. A giant dielectric constant of 104–105 at high temperatures (120–170 °C) for certain LTNO compositions (x = 0.15 to 0.3) was observed in the sintered ceramics. The origin of the high dielectric constant observed in these LTNO ceramics is attributed to the Maxwell–Wagner polarization mechanism and a thermally activated mechanism.

Colossal dielectric permittivity and relevant mechanism of Bi2/3Cu3Ti4O12 ceramics

Bi2/3Cu3Ti4O12 (BCTO) ceramics with pure perovskite phase were successfully prepared by traditional solid-state reaction technique. Uniformly distributed and dense grains with the grain size of 2–3μm were observed by SEM. A giant low-frequency dielectric permittivity of ~3.3×105 was obtained. The analysis of complex impedance revealed that Bi2/3Cu3Ti4O12 ceramics are electrically heterogeneous. There are three kinds of dielectric response detected in Bi2/3Cu3Ti4O12 ceramics, which existed in the low-frequency range, middle-frequency range, and high-frequency range, respectively. Through the study of dielectric spectrum at different temperatures, the relatively low activation energy of 0.30eV for middle-frequency dielectric response was calculated, which suggested that this Middle-frequency dielectric response can be ascribed to grain boundaries response. In view of the analysis of dielectric spectrum at low temperatures, the activation energy of 0.07eV for high frequency dielectric response was found. This value illustrated that dielectric response at high frequencies was associated with grains polarization effect. The comparison of dielectric spectra of Bi2/3Cu3Ti4O12 ceramics with different types of electrodes revealed that giant low-frequency dielectric constant was attributed to the electrode polarization effect.

Structural and impedance spectroscopic studies of spark plasma sintered CaCu3Ti4O12 dielectric ceramics: an evidence of internal resistive barrier effect

A giant dielectric constant CaCu3Ti4O12 ceramic was prepared by solid-state reaction route. Powders were spark plasma sintered at 1050 °C for 30 min. Rietveld refined XRD pattern reveals the formation of pure phase cubic perovskite CCTO. The dielectric properties were characterized in a broad frequency range of 10 Hz–8 MHz at temperatures 20–120 °C. The grain size of sintered pellets was found in range of 12–15 μm. Impedance spectroscopic analysis shows that the observed semicircle in the Cole–Cole plot was associated with grain boundary relaxation effect. This study reveals that there is an evidence of internal resistive barrier which responsible for a giant dielectric constant in CaCu3Ti4O12 ceramic. An Arrhenius temperature dependent relaxation effect was observed in grain boundary. Using the characteristic frequency of relaxation peak, grain boundary capacitance was calculated to be 0.27 μF. It was found that on increasing the temperature, the resistance of grain boundaries was decreased rapidly as compared to that of the grains. The activation energy of grain boundary was calculated to be 0.165 eV.

Controlled and enhanced dielectric properties of high-titanium containing LixTi0.1Ni1⿿xO via chemical composition-tailoring

Tuning the dielectric properties via composition-tailoring is demonstrated for variable-lithium, constant-titanium co-doped nickel oxide (LixTi0.1Ni1⿿xO; x=0.05⿿0.30; referred to LTNO). The effect of variable Li-content on the structure and dielectric properties of LTNO is investigated. X-ray diffraction studies confirm the formation of a cubic NiO phase in LTNO for x=0.05⿿0.30. However, lattice parameter reduction was evident with increasing Li-content. Composition-driven giant dielectric constant (104⿿105) was observed for a higher lithium content (x=0.15⿿0.30) leading to realization of LTNO ceramics. The origin of the high dielectric constant observed in these low density LTNO ceramics is attributed to the Maxwell⿿Wagner polarization mechanism.

Ion Migration in Organometal Trihalide Perovskite and Its Impact on Photovoltaic Efficiency and Stability.

Organometal trihalide perovskites (OTPs) are emerging as very promising photovoltaic materials because the power conversion efficiency (PCE) of OTP solar cells quickly rises and now rivals with that of single crystal silicon solar cells after only five-years research. Their prospects to replace silicon photovoltaics to reduce the cost of renewable clean energy are boosted by the low-temperature solution processing as well as the very low-cost raw materials and relative insensitivity to defects. The flexibility, semitransparency, and vivid colors of perovskite solar cells are attractive for niche applications such as built-in photovoltaics and portable lightweight chargers. However, the low stability of current hybrid perovskite solar cells remains a serious issue to be solved before their broad application. Among all those factors that affect the stability of perovskite solar cells, ion migration in OTPs may be intrinsic and cannot be taken away by device encapsulation. The presence of ion migration has received broad attention after the report of photocurrent hysteresis in OTP based solar cells. As suggested by much direct and indirect experimental evidence, the ion migration is speculated to be the origin or an important contributing factor for many observed unusual phenomenon in OTP materials and devices, such as current-voltage hysteresis, switchable photovoltaic effect, giant dielectric constant, diminished transistor behavior at room temperature, photoinduced phase separation, photoinduced self-poling effect, and electrical-field driven reversible conversion between lead iodide (PbI2) and methylammonium lead triiodide (MAPbI3). Undoubtedly thorough insight into the ion-migration mechanism is highly desired for the development of OTP based devices to improve intrinsic stability in the dark and under illumination. In this Account, we critically review the recent progress in understanding the fundamental science on ion migration in OTP based solar cells. We look into both theoretical and experiment advances in answering these basic questions: Does ion migration occur and cause the photocurrent hysteresis in perovskite solar cells? What are the migrating ion species? How do ions migrate? How does ion migration impact the device efficiency and stability? How can ion migration be mitigated or eliminated? We also raise some questions that need to be understood and addressed in the future.

Phase morphology control and the selective localization of MWCNT for suppressing dielectric loss and enhancing the dielectric constant of HDPE/PA11/MWCNT composites

A nanocomposite with a giant dielectric constant and suppressed dielectric loss was obtained by tailoring the selective localization of MWCNT in the PA11 phase and improving the interfacial interaction between HDPE and PA11.

Dielectric responses of Na0.65Bi0.45Cu3Ti4O12 ceramics based on the composition design of changing the Na/Bi ratio

Na0.65Bi0.45Cu3Ti4O12 ceramics were successful prepared by the conventional solid-state reaction technique. Compared to Na0.50Bi0.50Cu3Ti4O12 (NBCTO), the composition of Na0.65Bi0.45Cu3Ti4O12 was designed in terms of changing the Na/Bi ratio. Colossal dielectric permittivity of ~1.2 × 104 at 1 kHz was obtained in Na0.65Bi0.45Cu3Ti4O12 ceramics. Interestingly, three frequency dispersions were observed in the frequency dependence of dielectric constant measured at different temperatures. The investigation of electric modulus displayed that the giant low-frequency dielectric constant was attributed to Maxwell–Wagner polarization at the grain boundaries and the frequency dispersion in middle-frequency range was due to the grain polarization. Except grain response and grain boundaries response reflected by two semicircles in the impedance spectroscopy, another electrical response associated with nonzero high frequency intercept was found. The grain resistance Rg and grain boundaries resistance R gb was ~600 Ω and 3.9 × 105 Ω, respectively. The large intrinsic permittivity as high as ~700 was obtained. Furthermore, two dielectric anomalies observed in the temperature dependent of dielectric constant were discussed in detail. The results indicated change in the Na/Bi ratio had a significant effect on the electrical properties of NBCTO ceramics.

Origin of giant dielectric constant and conductivity behavior in Zn1−xMgxO (0 ≤ x ≤ 0.1) ceramics

Zn1−xMgxO (≤x≤0.1) ceramics were fabricated by conventional solid-state reaction of co-precipitated zinc oxide and magnesium hydroxide nanoparticles. Structural and morphological properties of the fabricated ceramics were studied using X-ray diffraction and scanning electron microscopic analysis. The dielectric measurements of the ceramics were carried out as a function of frequency and temperature respectively. Interestingly, Mg doped ZnO (MZO) samples exhibited colossal dielectric response (∼1×104 at 1kHz) with Debye like relaxation. The detailed dielectric studies and thermal analyses showed that the unusual dielectric response of the samples were originated from the defected grain and grain boundary (GB) conductivity relaxations due to the absorbed atmospheric water vapor (moisture). Impedance spectroscopy was employed to determine the defected grain and GB resistances, capacitances and which supported Maxwell–Wagner type relaxation phenomena.

Giant dielectric, low loss tangent and non-ohmics properties of CaCu3Ti4.7O12 ceramics prepared by polymer pyrolysis method

In this work, CaCu3Ti4.7O12 (CCT4.7O) powder was prepared by polymer pyrolysis method. The influence of phase composition and microstructure were characterized using X ray diffraction (XRD) and scanning electron microscopy (SEM). The results show that the TiO2 secondary phase has a remarkable on the microstructure, dielectric loss tangent (tan and dielectric constant () of CCT4.7O ceramics, respectively. Interestingly, low loss tangent of ~0.03 and giant dielectric constant of 31,908 with temperature coefficients less than ±15% in the temperature range of −50–90 oC are observed in a ceramic sintered at 1060 oC for 10h (CCT4.7O-2). Both of ceramics sintered at 1060 oC for 6h (CCT4.7O-1) and 10 h (CCT4.7O-2) display a non linear current voltage with the non-linear coefficient () and breakdown field (Eb) values of 6.4, 5.6 and 1554, 1020, respectively. The dielectric constant, electrical response of grain boundaries, and related nonlinear currentvoltage behavior are found to be associated with the microstructure of CCT4.7O ceramics.

Synthesis of calcium copper titanate (CaCu3Ti4O12) nanowires with insulating SiO2 barrier for low loss high dielectric constant nanocomposites

The discovery of a giant dielectric constant of >105 in calcium copper titanate (CCTO) has generated significant interest for application in electronic devices. Over the past decade, many methods have been developed to synthesize CCTO particles in an effort to capitalize upon the outstanding dielectric properties. Polymer film capacitors are one application where CCTO could greatly increase the typically low dielectric constant of the polymer. Recent work has shown that the aspect ratio of the dispersed filler can greatly affect the dielectric constant of the polymer with high aspect ratio nanowires (NWs) producing greater dielectric constant and energy density than spherical particles. While CCTO NWs hold great potential to improve the properties of nanocomposite capacitors, no method currently exists for their preparation and their integration is challenged by the barrier layer capacitance which leads to high dielectric loss. Here, a novel two-step hydrothermal reaction is developed for the large-scale synthesis of CCTO NWs and an insulating barrier layer is applied to greatly reduce the loss while preserving the high dielectric constant.

Giant dielectric response in Dy2/3Cu3Ti4O12 ceramics

Dy2/3Cu3Ti4O12 ceramics were successfully synthesized by solid-state reaction process, and the crystal structure, microstructure and dielectric properties were systematically investigated. It has been found that Dy2/3Cu3Ti4O12 ceramics exhibit giant dielectric constants (e′ ~ 104 at 1 kHz and room temperature) and have two thermal-activated dielectric relaxations at low and high temperatures, which are quite similar to those reported in CaCu3Ti4O12 ceramics. The low-temperature dielectric relaxation with the activation energy of 0.092 eV is supposed to be the intrinsic response originated from the defect structures of oxygen vacancy and aliovalences of Ti and Cu ions, which might make the grains semiconductive. Moreover, the heterogeneous structures especially the grain boundaries should be responsible for the high-temperature dielectric relaxation with the activation energy of 0.585 eV. The semiconducting grains and insulating grain boundaries thus play the key role as extrinsic origin for the giant dielectric response in Dy2/3Cu3Ti4O12 ceramics according to the internal barrier layer capacitor (IBLC) mechanism.

Influence of sintering conditions on microstructure and electrical properties of CaCu3Ti4O12 (CCTO) ceramics

CaCu3Ti4O12 (CCTO) ceramic was prepared by the solid-state method. The microstructure, dielectric properties, complex impedance and nonlinear I–V characteristic were studied. The results show that increasing the sintering temperature, most grains grow up firstly, and then some diminish, resulting in depravation of the comprehensive properties in varying degrees. While prolonging sintering time promotes grain growth, microstructural densification, and improves the dielectric and nonlinear I–V properties. It should be noted that the CCTO ceramics sintered at 1050 °C for 12 h exhibit giant dielectric constant of 105 and low dielectric loss <0.1 with weak frequency dependence below 1 MHz, as well as a nonlinear I–V coefficient of 5.27.

Dwell time effect on the barrier layer capacitor structure in CaCu3Ti4O12 ceramic

The effects of sintering process on electrical properties of CaCu3Ti4O12 ceramics were investigated. Impedance spectroscopy analysis demonstrates that these materials are electrically heterogeneous. This electrically heterogeneous microstructure plays a major role for the giant dielectric constant in CaCu3Ti4O12 ceramic. We found a very high dielectric constant (~123,000) with low loss (~0.25) at room temperature. Cole–Cole plots provide effective evidence of the high dielectric constant supported by the barrier layer capacitor model. Activation energy was found to be 0.06eV and 0.46eV corresponding to grain and grain boundary, respectively which proves that the grain boundaries are more resistive than grains. Our results indicate that the polarization relaxation has a close relation to the activation energy of CaCu3Ti4O12 ceramics. This study will help to design CaCu3Ti4O12 based materials and lead to more applications in the microelectronics devices.

Characterization of deep level defects and thermally stimulated depolarization phenomena in La-doped TlInS2 layered semiconductor

Lanthanum-doped high quality TlInS2 (TlInS2:La) ferroelectric-semiconductor was characterized by photo-induced current transient spectroscopy (PICTS). Different impurity centers are resolved and identified. Analyses of the experimental data were performed in order to determine the characteristic parameters of the extrinsic and intrinsic defects. The energies and capturing cross section of deep traps were obtained by using the heating rate method. The observed changes in the Thermally Stimulated Depolarization Currents (TSDC) near the phase transition points in TlInS2:La ferroelectric-semiconductor are interpreted as a result of self-polarization of the crystal due to the internal electric field caused by charged defects. The TSDC spectra show the depolarization peaks, which are attributed to defects of dipolar origin. These peaks provide important information on the defect structure and localized energy states in TlInS2:La. Thermal treatments of TlInS2:La under an external electric field, which was applied at different temperatures, allowed us to identify a peak in TSDC which was originated from La-dopant. It was established that deep energy level trap BTE43, which are active at low temperature (T ≤ 156 K) and have activation energy 0.29 eV and the capture cross section 2.2 × 10−14 cm2, corresponds to the La dopant. According to the PICTS results, the deep level trap center B5 is activated in the temperature region of incommensurate (IC) phases of TlInS2:La, having the giant static dielectric constant due to the structural disorders. From the PICTS simulation results for B5, native deep level trap having an activation energy of 0.3 eV and the capture cross section of 1.8 × 10−16 cm2 were established. A substantial amount of residual space charges is trapped by the deep level localized energy states of B5 in IC-phase. While the external electric field is applied, permanent dipoles, which are originated from the charged B5 deep level defects, are aligned in the direction of the applied electric field and the equilibrium polarization can be reached in a relatively short time. When the polarization field is maintained, while cooling the temperature of sample to a sufficiently low degrees, the relaxation times of the aligned dipoles drastically increases. Practically, frozen internal electric field or electrets states remain inside the TlInS2:La when the applied bias field is switched off. The influence of deep level defects on TSDC spectra of TlInS2:La has been revealed for the first time.

Effect of sintering on grain boundary microstructure and electrical properties of CaCu3Ti4O12 ceramics

CaCu3Ti4O12 ceramic with a giant dielectric constant was synthesized by sol-gel method and sintered in three different sintering conditions: 1 035 °C for 48 h, 1 080 °C for 3 h and 48 h. The phase of the ceramics, the element distribution, the valance state of Ti ions at grain boundaries, and the electrical properties were characterized via X-ray diffraction (XRD), energy dispersive X-ray analysis (EDAX), X-ray photoelectron spectroscopy (XPS), electrical conduction and dielectric measurement. The results demonstrate that the grain-boundary microstructure and the electrical properties are influenced by sintering conditions: ➀ By raising sintering temperature, the Cu-rich and Ti-poor grain boundary was formed and grain resistivity was decreased. ➁ By prolonging sintering time, the content of Ti3+ near the grain boundary increased, leading to the decrease of the grain-boundary resistivity and the increase of the activation energy at grain boundary. The ceramic, sintering at 1 080 °C for 48 h, exhibited a small grain resistivity (60.5 Ω · cm), a large grain-boundary activation energy (0.42 eV), and a significantly enhanced dielectric constant (close to 1×105 at a low frequency of 1×103 Hz ). The results of electrical properties accord with the internal boundary layer capacitor model for explaining the giant dielectric constant observed in CaCu3Ti4O12 ceramics.

An Explanation of the Photoinduced Giant Dielectric Constant of Lead Halide Perovskite Solar Cells.

A photoinduced giant dielectric constant of ~10(6) has been found in impedance spectroscopy measurements of lead halide perovskite solar cells. We report similar effects in measurements of a porous lead zirconate titanate (PZT) sample saturated with water. The principal effect of the illumination of the solar cell and of the introduction of water into the pore volume of the PZT sample is a significant increase in conductivity and dielectric loss. This is shown to exhibit low frequency power law dispersion. Application of the Kramers-Kronig relationships show the large measured values of permittivity to be related to the power law changes in conductivity and dielectric loss. The power law dispersions in the electrical responses are consistent with an electrical network model of microstructure. It is concluded that the high apparent values of permittivity are features of the microstructural networks and not fundamental effects in the two perovskite materials.

Fabrication and dielectric properties of Ba(Fe0.5Nb0.5)O3/poly(vinylidene fluoride) composites

To combine good dielectric properties and flexibility in high dielectric constant composite materials, the Ba(Fe0.5Nb0.5)O3 (BFN) particles with giant dielectric constant were dispersed in poly(vinylidene fluoride) (PVDF) polymer matrix. The Ba(Fe0.5Nb0.5)O3 powders were prepared via the conventional solid-state reaction route. The Ba(Fe0.5Nb0.5)O3/poly(vinylidene fluoride) composites were obtained by a physical mixing and a cold press technique. The structure, morphology and dielectric properties have been characterized and discussed in detail. The dielectric constants of the composites increase with the volume fractions of the BFN filler increasing from 20% up to 60% at all frequency (100Hz–1MHz). The BFN/PVDF composite with 60vol% BFN shows the highest dielectric constant (ε′ ~57) at 100Hz at room temperature. The value of dielectric constant of the BFN/PVDF with composite 60vol% BFN is nearly six times as high as that of pure PVDF, which can be attributed to the interfacial polarization between the BFN and PVDF polymer matrix. It is believed that the Ba(Fe0.5Nb0.5)O3/poly(vinylidene fluoride) composites with high dielectric constant would be applied in the embedded capacitor field.

Preparation and properties of ZrO2 and TiO2 films and their nanolaminates by atomic layer deposition

Nanolayers of ZrO2 and TiO2 and their nanolaminates were grown on silicon wafer by an atomic layer deposition process. Precursors of tetrakis (dimethylamido) zirconium and tetrakis (dimethylamido) titanium were used to deposit ZrO2 and TiO2 films at different temperatures, respectively. The XRD results show that ZrO2 films began to crystallize at the deposition temperature of 300°C, and the cubic phase was first formed when the thickness of the film reached 16.5nm, and then monoclinic phase appeared in the film when the thickness further increased. The as-deposited TiO2 films exhibited anatase phase at the deposition temperature range of 50–300°C. AFM morphologies indicate that both ZrO2 and TiO2 films were uniform and smooth. Nanolaminates of ZrO2 and TiO2 were obtained by deposition of ZrO2 and TiO2 layers at 300°C alternatively with precise layer thickness control. The results show that the nanolaminates exhibited a giant dielectric constant of 850 at low frequency which was much higher than the individual dielectric constants of ZrO2 and TiO2. The nanolaminates of ZrO2 and TiO2 with the giant dielectric constant have potential applications for high-k dielectric materials.

Polarization Switching and Light-Enhanced Piezoelectricity in Lead Halide Perovskites.

We investigate the ferroelectric properties of photovoltaic methylammonium lead halide CH3NH3PbI3 perovskite using piezoelectric force microscopy (PFM) and macroscopic polarization methods. The electric polarization is clearly observed by amplitude and phase hysteresis loops. However, the polarization loop decreases as the frequency is lowered, persisting for a short time only, in the one second regime, indicating that CH3NH3PbI3 does not exhibit permanent polarization at room temperature. This result is confirmed by macroscopic polarization measurement based on a standard capacitive method. We have observed a strong increase of piezoelectric response under illumination, consistent with the previously reported giant photoinduced dielectric constant at low frequencies. We speculate that an intrinsic charge transfer photoinduced dipole in the perovskite cage may lie at the origin of this effect.