Defect Dipoles Research Articles

Large permittivity, low loss and defect structure in (Nb, Zn) co-doped SnO2 ceramics studied through a combined experimental and DFT calculational method

Dielectric ceramics with high permittivity and low loss are widely used in electronic components and devices. In this study, (Nb, Zn) co-doped NbxZnySn1-x-yO2 with different doping levels and Nb/Zn ratios was designed to tune the defect structure toward the optimal dielectric performance. The lattice parameters firstly increased from x = y = 0.01 to 0.03 and then decreased, while the oxygen vacancy concentration decreased with doping. The co-doped sample with x = y = 0.02 exhibits stable permittivity up to 800 with an ultra-low loss tanδ ∼0.03 at 40 Hz. DFT calculation showed that the oxygen vacancy was formed with single-Zn doping and co-doping at low doping level, while the hole was generated at higher doping level. The achieved large permittivity and low loss of the sample are related to both Electron-Pinned Defect Dipoles (EPDD) and Hole-Pinned Defect Dipoles (HPDD) effects in the lattice, which was determined by the relative positions of donor and acceptor dopants.

Outstanding electromechanical performance in piezoelectric ceramics via synergistic effects of defect engineering and domain miniaturization

Piezoelectric ceramics with high mechanical quality factor Qm and large piezoelectric coefficient d33 are urgently required for advanced piezoelectric applications. However, obtaining both of these properties simultaneously remains a difficult challenge due to their mutually restrictive relationship. Here 0.5Pb(Ni1/3Nb2/3)O3–0.5Pb(Zr0.3Ti0.7)O3 piezoceramic with tetragonal (T)-rich MPB is designed as a matrix to construct the defect engineering by doping low-valent Mn ions. The strong coupling of defect dipole and T-rich phase can effectively hinder the rotation of Ps, restrict domain wall motion and improve Qm. At the same time, the substituted Mn ions will introduce local random field, destroying the long-range ordering of ferroelectric domain and reducing domain size. The miniaturized domain structure can increase poling efficiency and inhibit the reduction of d33. Guided by this strategy, Qm has increased by more than 10 times and d33 has only decreased by about 25%. The optimized electromechanical performance with Qm = 822, d33 = 502 pC/N, kp = 0.55 and tanδ = 0.0069 can be obtained in the present study.

Excellent piezoelectric performance of BiFeO3–BaTiO3– La(Mg2/3Nb1/3)O3 lead-free ceramics via relaxation regulation near the morphotropic phase boundary

Although BiFeO3–BaTiO3-based lead-free ceramics have attracted much attention in recent years as a high temperature piezoelectrics, their d33 value is still difficult to exceed 200 pC/N through conventional methods. In this work, a novel (0.7-x)BiFeO3-0.3BaTiO3-xLa(Mg2/3Nb1/3)O3 (x = 0–0.005) ternary system was reported to own excellent room-temperature piezoelectric constant (d33) ∼216 pC/N and high Curie temperature (480 °C) at x = 0.003 simultaneously. It reveals that the compositions undergo an obvious phase transition from ferroelectric (FE) rhombohedral (R) phase to relaxor pseudo-cubic (PC) phase via an R–PC morphotropic phase boundary (MPB). More importantly, piezoelectric nonlinear Rayleigh analysis and S-E/P-E curves suggest that the significantly enhanced domain dynamics due to the coexistence of macrodomain and nanodomain can be observed in the x = 0.003 sample. The synergy of these two effects induce the improved d33 value observed in the x = 0.003 sample. In addition, the compositions exhibit the inhibition of defect dipoles and the improvement of the high temperature resistivity, leading to the excellent thermal stability with sensitivity coefficients ƞ less than ± 5% in the annealing temperature range of 25–400 °C. The present work provides a guideline for the subsequent design of high-temperature lead-free 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 (ԑr ∼ 62505), low dielectric loss (tanδ ∼ 0.07), as well as excellent temperature stability (−70 °C–180 °C, ΔC/C25°C ≤ ±15%) were obtained in the Sr0.996Ta0.004TiO3 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 Sr1-xTaxTiO3 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.

Piezoelectricity and Thermophysical Properties of Ba0.90Ca0.10Ti0.96Zr0.04O3 Ceramics Modified with Amphoteric Nd3+ and Y3+ Dopants

Lead-free barium calcium titanate zirconate (BCTZ) ceramics doped with a single rare-earth element generally exhibit excellent piezoelectric properties. However, their electrical properties deteriorate at an excessive dopant content, limiting their application. In this study, amphoteric neodymium (Nd3+) and yttrium (Y3+)-codoped BCTZ-NYx ceramics were synthesized via a solid-state reaction at 1240 °C. The influences of the Y3+ content (x) on the structural features, electrical properties, mechanical properties, and thermophysical properties were investigated. At a small x (<0.18 mol%), Y3+ could enhance the fracture strength and electrical properties by eliminating oxygen vacancies, defect dipoles, and/or structural defects. However, the outstanding performance deteriorated with excessive x. Additionally, the mechanism of the defect chemistry at different x was deduced. At an yttrium content of 0.18 mol%, the ceramic exhibited high piezoelectricity and ferroelectricity with low domain-switching activation energy (Ea = 0.401 eV), indicating that it could replace commercial lead-based piezoelectric ceramics.

Dielectric relaxations and low dissipation factor with excellent temperature stability of Ti1-x(Co1/3Ta2/3)xO2 ceramics

Low dissipation factor (tanδ) and excellent temperature stability of high dielectric permittivity are obtained in Ti1-x(Co1/3Ta2/3)xO2 ceramics, which were prepared using a mixed oxide method. The unit cell of the rutile TiO2 structure expanded due to relatively larger ionic radii of the co–dopants. The dopants homogeneously appeared in the microstructure of the TiO2 ceramics. Highly dense sintered–ceramics without second phase exhibit low tanδ of 0.015–0.043 with very high dielectric permittivities of 7.0–9.1 × 104 at 1 kHz. Notably, the temperature coefficient in the range of ±15 % was obtained in the temperature range of −60–200 °C. The electrically heterogeneous microstructure was studied using impedance spectroscopy. The contributions of extrinsic and intrinsic factors were separated by the dielectric relaxations in low and high–frequency ranges, respectively. The dielectric relaxations were well fitted using a modified Cole-Cole model. A low–frequency relaxation was well described based on the Maxwell-Wagner relaxation model. A high permittivity was originated from the intrinsic (i.e., defect dipoles) and extrinsic (internal barrier layer capacitor, IBLC) effects. A low tanδ was primarily caused by the IBLC effect associated with a high resistance of the insulating part and defect dipoles.

Polarization response and thermally stimulated depolarization currents of the modified (Ba,Ca) (Zr,Ti)O3 piezoelectric ceramics

Polarization response and defect mechanism of the (Ba,Ca) (Zr,Ti)O3 piezoelectric ceramics by doping modification were intensively investigated. The Al-doping leads to the reduced piezoelectric polarization responses, but optimizes fatigue resistance and the quality factor, showing the acceptor characteristics. A way to test thermally stimulated depolarization currents (TSDC) is utilized to distinguish the role of defect dipole from oxygen vacancy and uncover the intrinsic reason of improving cyclic fatigue behavior. It has been confirmed that the improvement of fatigue behavior can be attributed to the formation of oxygen vacancies by Al doping. The present study gives a promising method to reveal the origin of piezoelectric characteristics based on defect configuration in advanced dielectrics.

Synthesis and Dielectric Relaxation Studies of KxFeyTi8-yO16 (x = 1.4–1.8 and y = 1.4–1.6) Ceramics with Hollandite Structure

Some solid solutions with the chemical composition KxFeyTi8-yO16 (KFTO) and a hollandite-like structure were successfully synthesized by modified sol–gel method. The obtained powders were characterized using X-ray diffraction (XRD) and scanning electron microscopy (SEM). The ceramic pellets based on KFTO powders were obtained by compressing and sintering at 1080 °C for 4 h. The sinters were characterized by X-ray and impedance spectroscopy. XRD results show that KFTO powders have a mono-phase tetragonal structure at x = 1.4–1.8 and y = 1.4–1.6. However, it was recognized that the hollandite-like phase could be broken during sintering to form TiO2 and Fe2TiO5 crystals distributed throughout the volume of the ceramics. A frequency dependency of dielectric properties for the sinters was studied by impedance spectroscopy. It was found that an increase in the TiO2 (rutile) phase during the sintering contributes to a decrease in dielectric losses. At the same time, the KFTO ceramics with reduced content of potassium had increased permittivity. The contribution of electron-pinned defect dipoles (EPDD) and internal barrier layer capacitance (IBLC) in the permittivity of the obtained ceramics was estimated using the Havriliak–Negami equation. It is shown that the KFTO ceramics have the polydisperse characteristic of dielectric relaxation. The observed grain and grain boundary dipole relaxation times were 1.03 × 10−6 to 5.51 × 10−6 s and 0.197 to 0.687 s, respectively.

Microstructures and physical properties of sol–gel derived Ba2FeMnO6 double perovskite

AbstractWe report microstructures and physical (dielectric, magnetic, and optical) properties of sol–gel derived Ba2FeMnO6 (BFMO) double perovskite powders. The BFMO powders belong to hexagonal crystal structure with P‐6m2 space group. SEM images reveal the powders are nearly homogeneous distribution with spherical morphology and average particle size of 300 nm. Energy‐dispersive spectra gave out the molar ratio of the Ba:Fe:Mn elements equal to 2.16:1.00:1.00. FTIR spectrum verified the [FeO6] and [MnO6] octahedra present in the powders. X‐ray photoelectron spectroscopy spectra identified the chemical valence states of the constituent elements. The BFMO ceramics displayed a strong frequency dispersion dielectric behavior. A relaxor‐like dielectric behavior appeared around ∼475 K due to the contributions of oxygen vacancies of and the defect dipoles. A ferrimagnetic behavior was observed in the powders at 5 K with MS = 0.14 μB/f.u. and HC = 1.51 kOe. The magnetic Curie temperature (TC) was 381 K, whereas the Neel temperature (TN) was 31 K. The ferrimagnetic behavior is governed by the Mn3+–Fe4+ double‐exchange interaction via the mediated oxygen between them. The BFMO powders have a direct optical bandgap of 1.65 eV, which originates from the electron transferring from O 2p to Mn 3d (and/or Fe 3d) levels. The unique combination of high temperature ferrimagnetism and semiconductivity in the BFMO powders makes them particularly appealing for magnetic spintronics and photovoltaics.

Giant Electric Field‐Induced Strain with High Temperature‐Stability in Textured KNN‐Based Piezoceramics for Actuator Applications

AbstractLarge‐strain (K,Na)NbO3 (KNN) based piezoceramics are attractive for next‐generation actuators because of growing environmental concerns. However, inferior performance with poor temperature stability greatly hinders their industrialized procedure. Herein, a feasible strategy is proposed by introducing ‐ defect dipoles and constructing grain orientation to enhance the strain performance and temperature stability of KNN‐based piezoceramics. This textured ceramics with 90.3% texture degree exhibit a giant strain (1.35%) and a large converse piezoelectric coefficient d33* (2700 pm V−1), outperforming most lead‐free piezoceramics and even some single crystals. Meanwhile, the strain deviation at high temperature of 100 °C–200 °C is obviously alleviated from 61% to 35% through texture engineering. From the perspective of practical applications, piezo‐actuators are commonly utilized in the form of multilayer. In order to illustrate the applicability on multilayer actuators, a stack‐type actuator consisted of 5 layers of 0.4 mm thick ceramics is fabricated. It can generate large field‐induced displacement (11.6 µm), and the promising potential in precise positioning and optical modulation are further demonstrated. This work provides a textured KNN‐based piezoceramic with temperature‐stable giant strain properties, and facilitates the lead‐free piezoceramic materials in actuator applications.

Achieving giant electrostrain of above 1% in (Bi,Na)TiO3-based lead-free piezoelectrics via introducing oxygen-defect composition.

Piezoelectric ceramics have been extensively used in actuators, where the magnitude of electrostrain is key indicator for large-stroke actuation applications. Here, we propose an innovative strategy based on defect chemistry to form a defect-engineered morphotropic phase boundary and achieve a giant strain of 1.12% in lead-free Bi0.5Na0.5TiO3 (BNT)-based ceramics. The incorporation of the hypothetical perovskite BaAlO2.5 with nominal oxygen defect into BNT will form strongly polarized directional defect dipoles, leading to a strong pinning effect after aging. The large asymmetrical strain is mainly attributed to two factors: The defect dipoles along crystallographic [001] direction destroy the long-range ordering of the ferroelectric and activate a reversible phase transition while promoting polarization rotation when the dipoles are aligned along the applied electric field. Our results not only demonstrate the potential application of BNT-based materials in low-frequency, large-stroke actuators but also provide a general methodology to achieve large strain.

Sm and Mn co-doped PMN-PT piezoelectric ceramics: Defect engineering strategy to achieve large d33 and high Qm

• A strategy of donor-acceptor (Sm-Mn) co-doping defect engineering is introduced into the PMN-PT to improve the d 33 and Q m simultaneously. • The Optimal performances were obtained in PMN-30PT: 2.5Sm, 1-2Mn ceramics, with d 33 =860-543 pC/N, Q m =495-754, and tan δ =0.0055-0.0086. • This high performance originates from the combined effects of ( Mn Ti ″ − V o • • ) x defect dipoles and the local structural heterogeneity. PbTiO 3 -based piezoelectric ceramics are key materials for developing various electromechanical transducers. For high-power ultrasonic transducers, piezoelectric ceramics are required to possess large piezoelectric coefficient ( d 33 ) and high mechanical quality factor ( Q m ). Although acceptor dopants can improve Q m , they also deteriorate d 33 . If suitable piezoelectricity-beneficial donor dopants can be introduced into acceptor-doped ceramics, it is very possible to obtain large d 33 and high Q m simultaneously in donor and acceptor co-doped ceramics. In this work, a series of x mol% Sm and y mol% Mn co-doped Pb(Mg 1/3 Nb 2/3 )O 3 -30PbTiO 3 (PMN-30PT: x Sm, y Mn) ceramics were prepared by the solid-phase sintered method. The crystal structure, local domain structure and electromechanical properties were systematically analyzed. Optimal performances were obtained in PMN-30PT: 2.5Sm, 1-2Mn ceramics with d 33 =860-543 pC/N, Q m =495-754, and dielectric loss tan δ =0.0055-0.0086. This high performance originates from the combined effects of ( Mn Ti ″ − V o • • ) × defect dipoles and the local structural heterogeneity.

Defect Dipole-Induced HfO2-Coated Ti3C2Tx MXene/Nickel Ferrite Nanocomposites for Enhanced Microwave Absorption

This work reports for the first time the synthesis of a HfO2-coated Ti3C2Tx MXene sandwich-like nanostructure (MXHf) using a simple hydrothermal method and their excellent microwave absorption property in combination with nickel ferrite and epoxy matrix. The nanocomposites of 2, 3, and 4 mm thick samples were tested for microwave absorption in the X-band, and a minimum reflection loss (RLmin) of −20.9 dB with EAB (effective absorption bandwidth) of 2.32 GHz was achieved for the 3 mm thick sample. Its superior performance results from the strong dielectric polarization of defect dipole in the MXene structure, the high number of oxygen vacancies brought about by HfO2 nanoparticles, rich micro-interfaces in the sandwich-like HfO2-Ti3C2Tx-HfO2 structure, and impedance matching by NiFe2O4. A probable mechanism of absorption is proposed. Hence, HfO2/Ti3C2Tx/NiFe2O4 nanocomposites possess essential properties for real-time application in the defense and telecommunication industries.

DFT calculations and giant dielectric responses in (Ni1/3Nb2/3)xTi1-xO2.

The origins of dielectric responses in Ni2+ and Nb5+ co-doped TiO2 were explored considering intrinsic and extrinsic effects. DFT calculations demonstrated that Ni2+ doping induced oxygen vacancies, while Nb5+ doping generated free electrons. Theoretical predictions indicated complex defect dipoles forming in the rutile structure, contributing to overall dielectric responses. Theoretical calculations also showed a possible linear alignment of Ni2+-2Nb5+ without oxygen vacancies, especially in high doping concentrations. Experimentally, (Ni1/3Nb2/3)xTi1-xO2 ceramics (x = 1%, 2.5%, and 10%) were synthesized. The substantial dielectric response at room temperature, attributed to factors like defect dipoles and grain boundary/surface barrier layer capacitor (GBLC/SBLC) effects, increased with higher doping levels. However, in a temperature range where GBLC/SBLC effects were suppressed, the dielectric response decreased with increased doping, likely due to self-charge compensation between Ni2+-2Nb5+. Notably, (Ni1/3Nb2/3)xTi1-xO2 with x = 2.5% exhibited a high dielectric permittivity of 104 and a low loss tangent of 0.029 at 1 kHz. Moreover, the dielectric permittivity changed by less than ±15% (compared to 25 °C) at 150 °C. This work provides an understanding of the origins of dielectric responses in co-doped TiO2 and optimizes the doping concentration to achieve the best dielectric performance.

Defects in motion

Crystalline defects appear when the perfect order of the lattice or ideal arrangements of atoms, molecules, or ionic groups is destroyed, which is inevitable during crystal growth, thereby impacting material functionalities, either in a reinforcing or unwanted way. For functional ferroic materials, natural interfaces called domain walls form, which separate regions of different orientations of a specific order (such as magnetic, ferroelastic, or ferroelectric) in the material. These, including phase boundary, grain boundary, and/or domain boundary, can be reckon as two-dimensional defects. During functioning of ferroic materials, phase transformation and/or dynamic motion of domain walls occur with external stimuli like electric fields or stress. Therefore, domain engineering, phase boundary construction, and grain engineering has long been the most considered effective strategies to enhance the performance of ferroic materials. Meanwhile, lower-dimensional defects, including point defects, defect dipole, and line defects, are another crucial dimension to be considered to tune functionality, since their motion greatly interact with the domain wall dynamics under external stimuli. In all, defect engineering (here we refer to one- and two-dimensional defects) and its coupled motion with order parameters is an interesting topic that has attracted significant attention for functional oxides.

Origins of high–performance giant dielectric properties in TiO2 co-doped with aliovalent ions via broadband dielectric spectroscopy

(Zn1/3Ta2/3)xTi1-xO2 ceramics with x = 0.01 (ZnTTO-1 %) and x = 0.1 (ZnTTO-10 %) were prepared by a solid-state reaction method. Remarkably, the ZnTTO-1 % and ZnTTO-10 % ceramics sintered at 1500 °C for 2 h showed a high dielectric permittivity of approximately 3.90 × 104 and 6.23 × 104 at 1 kHz with low tanδ values of 0.014 and 0.024, respectively. Furthermore, the ZnTTO-1 % ceramic exhibited good temperature stability with a temperature coefficient of <±15 % in the temperature range of 218–473 K (−55–200 °C), which meets the requirement of the X9R capacitor application. The dielectric relaxations in the wide temperature range of 15–483 K was attained using broadband dielectric spectroscopy to clarify the sources of the excellent dielectric properties. The best performance was demonstrated in the ZnTTO-1 % ceramic. The electron pin defect dipole mechanism, electron hopping, and interface effects are attributed to the giant dielectric responses.

Morphology controllable urchin-shaped bimetallic nickel-cobalt oxide/carbon composites with enhanced electromagnetic wave absorption performance

The microscopic morphology of electromagnetic wave absorbers influences the multiple reflections of electromagnetic waves and impedance matching, determining the absorption properties. Herein, the urchin-shaped bimetallic nickel-cobalt oxide/carbon (NiCo2O4/C) composites are prepared via a hydrothermal route, whose absorption properties are investigated by different morphologies regulated by changing calcination temperature. A minimum reflection loss (RLmin) of -75.26 dB is achieved at a matching thickness of 1.5 mm, and the effective absorption bandwidth (EAB) of 8.96 GHz is achieved at 2 mm. Multi-advantages of the synthesized NiCo2O4/C composites contribute to satisfactory absorption properties. First, the interweaving of the needle-like structures increases the opportunities for scattering and multiple reflections of incident electromagnetic waves, and builds up a conductive network to facilitate the enhancement of conductive losses. Second, the carbon component in the NiCo2O4/C composites enhances the interfacial polarization and reduces the density of the absorber. Besides, generous oxygen vacancy defects are introduced into the NiCo2O4/C composites, which induces defect polarization and dipole polarization. In summary, the ternary coordination of components, defects and morphology led to outstanding electromagnetic wave absorption, which lightened the path for improving the electromagnetic wave absorption property and enriching the family of NiCo2O4 absorbers with excellent performance.

Relaxation degree and defect dipoles‐controlled resistivity and leakage current in BF–BT‐based ceramics

AbstractThe low resistivity and high leakage current of bismuth ferrite–barium titanate‐based ceramics greatly limit its development. In this work, the resistivity and leakage current have been regulated by changing the relaxation degree and defect dipoles. With the introduction of Sb, the ceramics endure from weak relaxors with typically ferroelectric behavior to relaxor ferroelectrics and then to relaxors, where nearly 20 times improved resistivity can be obtained in relaxor ferroelectrics with chemical‐homogeneous grain structure and strip‐shaped/submicron‐sized coexisted domains, whereas deteriorated properties appear in relaxors with core–shell structure and tweed‐like ferroelectric domains. The decreased concentration of oxygen vacancies and Ti3+is responsible for the improved insulation in relaxor‐ferroelectrics, whereas the emergence of core–shell structure leads to electrical inhomogeneity and the conductive channels result in decreased resistivity. In addition, the leakage current mechanism is transformed from Ohmic conduction to space‐charge‐limited current as the electric field increases, which is induced by different domain switchings in ceramics with varied relaxation degrees.

Magnetic Porous Carbon Composites for Efficient Electromagnetic Wave Absorption

Biomass carbon composites have the shining points of lightweight, economy, harmlessness, and large specific surface area, casting on them great development prospects in terms of excellent wave absorbing properties. Herein, a facile route to fabricate magnetic porous with a special spongy structure for its great contribution to microwave absorption is reported. It is endowed with a strong absorption capacity and a wide absorption band. The 3D spongy structure and uniform distribution of magnetic Co particles can promote defect polarization, interface polarization, and dipole polarization, which are conducive to scattering and reflection of electromagnetic waves (EMW) in the absorbing body. The magnetic Co particles can enhance the magnetic loss on the one hand and harmonize the impedance matching on the other hand. The minimum reflection loss can reach −54.7 dB. The effective absorption bandwidth at 2.1 mm can reach 7.61 GHz, almost covering the X and Ku bands. This proposed strategy provides more possibilities for the application of biomass microwave absorbers with lightweight and broadband properties.

Contribution of ferroelectric and non-ferroelectric factors to P–E hysteresis loops of CuInP2S6-type single crystals

A defining property of ferroelectricity is the switching between different states by the application of an electric field. Hysteresis loops serve like a fingerprint of ferroelectric materials giving some useful information. Sometimes the interpretation of information extracted from hysteresis measurements can be challenging due to the non-ferroelectric factors such as electrical conductivity, defect dipole presence and/or dielectric properties. In this paper, the ferroelectric and non-ferroelectric factors on P–E hysteresis loops were investigated in a CuInP2S6-type single crystal. The analysis of data obtained for this single crystal allowed extracting the reliable values in the materials with a low polarization and a high conductivity.