Development of magnetically switchable high permittivity microwave dielectrics using La(Al1-xFex)O3

The introduction of transition metal doping, particularly Fe³⁺, into high-performance microwave dielectrics can make “smart” materials that switch between a high-Q, low loss state and a low-Q, high loss state using a small external magnetic field. In this study, the dielectric and magnetic properties of the high permittivity host material LaAlO₃ (ε_r = 22.5), when doped with Fe³⁺, are reported. Spin losses dominate the loss tangent at cryogenic temperatures and survive up to room temperature. Peaks in the loss tangent versus temperature relation are observed near 40, 75, and 215 K. Additional measurements of samples exposed to annealing in varying environments, combined with Debye analysis and the results of native defect energy predictions from density functional calculations[Phys Rev B. 2009;80:104115], allows us to associate the 40, 75, and 215 K peaks to the following reactions, , , and , respectively.     

Ultralow dielectric loss in Y-doped SrTiO3 colossal permittivity ceramics via designing defect chemistry

Effects of doping of Y and sintering atmosphere on the dielectric properties of Sr_1-1.5xY_xTiO₃ ceramics (SYT, x = 0-0.014) were systematically investigated. The SYT14 (x = 0.014) ceramic sintered in N₂ attains a colossal permittivity (CP, Ɛ_r = 28 084@ 1kHz, 27 685@ 2MHz) and an ultralow dielectric loss (tanδ = 0.007@ 1kHz, 0.003@ 2MHz) at room temperature. Because of using of the A-site deficient, there are in SYT ceramics. Through the comprehensive analysis of dielectric responses, X-ray photoelectron spectroscopy (XPS), electron paramagnetic resonance (EPR), and complex impedance data, it is proved that doping of Y promotes the formation of (Y³⁺ are located at Sr²⁺ site), (Y³⁺ are located at Ti⁴⁺ site), and Ti³⁺, and sintering in reducing atmosphere of N₂ results in more (oxygen vacancy) and (strontium vacancy) generating in SYT ceramics. The defect dipoles, , , , , , and formed by introduced defects make charge carriers localized in SYT ceramics. The combined action of the massive defect dipoles is responsible for the ultralow tanδ and CP in SYT14 ceramics sintered in N₂.     

Role of oxygen-vacancy in piezoelectric properties and fatigue behavior of (Bi0.5Na0.5)0.93Ba0.07Ti1+xO3 ceramics

In this reported study, (Bi_0.5Na_0.5)_0.93Ba_0.07Ti_1+xO₃ (abbreviated as BNBT_1+x) ceramics, containing Ti-nonstoichiometry that ranged from a 2% deficiency to a 1% excess, were designed and systematically characterized. The results of the X-ray diffraction Rietveld refinement and X-ray photoelectron spectroscopy analysis of these materials revealed that the amount of Ti in the BNBT_1+x ceramics significantly affected the degree of coexistent rhombohedral/tetragonal phases and also affected the content of singly/doubly charged oxygen-vacancy (/) in the ceramics. After poling and 10⁵ fatigue cycles, the variation in Raman resonance line-width of the Ti–O bond in the BNBT_1+x ceramics was found to be strongly dependent on the amount of Ti in the ceramic. The → transformation and clustering of the defects under electrical loading were considered to be a critical factor in electric field-induced structural transition and fatigue properties of the material.     

Thermoelectric properties and phase transition of doped single crystals and polycrystals of Bi2Te3

The temperature dependences of the electrical conductivity , Seebeck coefficient , and heat capacity C_p(T) of polycrystalline samples of Bi₂Te₃, Bi₂Te₃+1%CuI, and Bi₂Te₃+1%(CuI+1/2Pb) are investigated in the temperature range below room temperature. Based on the temperature dependences of all investigated physical properties, it is discovered that phase transition occurs at 120–200 K. Investigation of single crystals shows that anomalies in the electrical resistivity occur only across the crystal growth axis (across the well-conducting Bi–Te plane). Investigation of the low-temperature dependence of electrical conductivity shows that all polycrystalline samples exhibit quasi-two-dimensional electron transport. Additionally, quasi-two-dimensional transport is detected in single crystals based on anisotropy analysis (where is the resistivity along the crystal growth axis, and is resistivity across the crystal growth axis) and temperature dependence below 50 K. The Fermi energy is estimated using the temperature dependence of . It is discovered that an increase in at T > 200 K is associated with the phase transition. For single-crystal samples, the maximum thermoelectric figure of merit ZT, as observed along the crystal growth axis, increases with doping. A maximum ZT value of ∼1.1 is observed for the Bi₂Te₃+1%(CuI+1/2Pb) sample at room temperature ().     

Crystal structure and magneto-dielectric properties of Co-Zr co-substituted Co2Z hexaferrites

A series of Ba_1.5Sr_1.5Co_2+xZr_xFe_24-2xO₄₁ hexaferrites (x = 0.00, 0.01, 0.03, 0.05, 0.07, and 0.09) were successfully prepared by the conventional solid-state reaction method. It was found that as the Co²⁺ and Zr⁴⁺ ions entered the hexaferrite structure, the lattice parameters increased, whereas the relative density increased when x = 0.00-0.03 and then decreased. A suitable amount of substitution increased the DC resistivity, reduced the magnetic and dielectric losses, and made the ${\mu }^{\prime }$ and ${\epsilon }^{\prime }$ closer to each other. At x = 0.03, the relative density and DC resistivity of the samples reached their maxim. Besides, both the magnetic and dielectric losses were lowest within the frequency range of 10 MHz-1 GHz. Meanwhile, the hexaferrite was impedance matched to free space, and the miniaturization factor was about 15. Therefore, this low-loss ferrite with almost equal permeability and permittivity could be meaningful for antenna miniaturization and high-frequency applications.     

Synchrotron x-ray spectroscopy investigation of the Ca1−xLnxZrTi2−x(Al,Fe)xO7 zirconolite ceramics (Ln = La,Nd, Gd,Ho, Yb)

When incorporating actinides into zirconolite for high-level radioactive waste immobilization, Al³⁺ and Fe³⁺ ions generally act as charge compensators. In this study, we rationally designed a series of (Ln = La, Nd, Gd, Ho, Yb) to unravel the dopant solubility and evolutions of the crystalline phase and local environment of cations through synchrotron X-ray methods. It was found that single zirconolite phase is difficult to obtain and the fraction of perovskite have an increase with x from 0.1 to 0.9 in . Formation of both zirconolite-2M and zirconolite-3O phases was observed in and . Phase transformation from zirconolite-2M to 3O occurs at x = 0.7 for while x = 0.9 for . The solubility of and to form single zirconolite-2M can reach to 0.9 f.u. and 0.7 f.u., respectively. The evolution of lattice parameters of zirconolite in is greatly related to the ionic radii of cations and substitution mechanism among the cations. X-ray absorption near edge spectroscopy revealed that Fe³⁺ ions replace both five- and six-coordinated Ti sites and the ratio of TiO₅ to TiO₆ decreases when increasing dopant concentration in the . For the local environment of Zr⁴⁺, the major form is ZrO₇ with a trace of ZrO₈.     

Diffused and successive phase transitions of (K,Na)NbO3-based ceramics with high strain and temperature insensitivity

Phase boundaries realize enhanced piezoelectricity in lead-free (K, Na)NbO₃-based ceramics but suffer from the weakness of undesirable temperature sensitivity. Here, an effective method is designed to develop temperature-insensitive piezoelectricity (small signal piezo-coefficient [d₃₃] and large signal piezo-coefficient []) in KNN-based piezoceramics by constructing the diffused and successive phase transitions, which results in a broadness of the optimal temperature range of the electrical properties. The room-temperature value in KNN-based ceramics modified with BaZrO₃ and (Bi_0.5Na_0.5)HfO₃ reaches as high as 540 (±10) pm V⁻¹, which is higher than PZT-5H and most reported KNN-based systems. Notably, superior temperature insensitivity of the and P_r values is also observed among the diffused and successive phase transitions region (20-100°C), with <5% fluctuation. In addition, the in situ temperature-dependent d₃₃ measurement shows a high-temperature reliability and less fluctuation (<15%) in a wide temperature range (20-120°C). These results open a new window for further development of highly temperature-insensitive lead-free piezoceramics.     

Effect of Al2O3 on the formation of color centers and CdSe/Cd1−xZnxSe quantum dots in SiO2–Na2O–ZnO glasses

color centers and CdSe/Cd_1−xZn_xSe quantum dots (QDs) were formed in silicate glasses, respectively, by carefully adjusting the content of Al₂O₃. The absorption coefficient and broad visible emission at 400-700 nm of color centers increased firstly and then decreased upon increasing concentration of Al₂O₃ from 0 to 5 mol. %. With 2 mol. % Al₂O₃ adding, the band edge emission centered at 497 nm from CdSe QDs in glasses was observed after heat treatment. Raman spectra and TEM characterization confirmed the formation of CdSe/Cd_1−xZn_xSe QDs. The small amount addition of Al₂O₃ had a significant effect on the formation of color centers and precipitation of CdSe/Cd_1−xZn_xSe QDs in glasses due to its complicated effect on the network structure and optical basicity of the glasses.     

Formation criterion for binary metal diboride solid solutions established through combinatorial methods

Establishing the formation criterion is urgent for accelerating the discovery and design of solid-solution materials with desirable properties. The previously reported formation criterion mainly focused on solid-solution alloys, while the formation criterion was rarely established in solid-solution ceramics. To solve this problem, herein, we take a class of solid-solution ceramics, namely binary metal diboride ((M_xN_1-x)B₂) solid solutions, as a prototype. Through combinatorial methods including high-throughput molten salt syntheses and high-throughput first-principles calculations combined with the machine learning approach, the correlation between influential factors, including atomic size difference (δ), mixing enthalpy at 0 K and 0 Pa (), doping condition (φ), and valence electron concentration (VEC), and the formation ability of (M_xN_1-x)B₂ solid solutions was first studied systematically, and then their formation criterion was well established. The results showed that the influential degree of the aforementioned four factors on the formation ability of (M_xN_1-x)B₂ solid solutions could be described as follows: δ > > φ > VEC. In addition, a newly proposed parameter, β, could well reflect the formation ability of (M_xN_1-x)B₂ solid solutions: when β > 0, the single-phase (M_xN_1-x)B₂ solid solutions could be successfully synthesized in our work and vice versa. This study may provide a theoretical guidance in the discovery and design of various solid-solution ceramics, such as the metal borides, carbides, nitrides, etc, with desirable properties.     

Phase transition,microstructure and electrical properties of K1-xNaxNbO3-based ceramic sintered in reducing atmosphere

Lead-free 0.955K_1-xNa_xNbO₃-0.045Bi_0.5Na_0.5ZrO₃+0.4% mol MnO ceramics (Abbreviated as K_1-xN_xN-0.045BNZ+0.4Mn) were prepared by a conventional solid-state sintering method in a reducing atmosphere (oxygen partial pressure : 1 × 10⁻¹¹ MPa). All K_1-xN_xN-0.045BNZ+0.4Mn samples show a pure perovskite structure with a polymorphic phase boundary (PPB) composed of rhombohedral (R) and tetragonal (T) phases. A high Na/K ratio and a low Na/K ratio can both induce an increase in the rhombohedral phase. The reverse piezoelectric coefficient and its temperature stability in K_1-xN_xN-0.045BNZ+0.4Mn ceramics can be improved by controlling the Na/K ratio. The increase in the Na/K ratio from x = 0.46 to x = 0.56 can decrease the A-site cation vacancies. The activation energy of the grain is higher than that of the grain boundary due to the accumulation of oxygen vacancies at the grain boundary. K_1-xN_xN-0.045BNZ+0.4Mn ceramics with excellent piezoelectric properties (quasi-static piezoelectric coefficient d₃₃ = 326 pC/N, and  = 472 pm/V at E_max = 25 kV/cm) were obtained at x = 0.52.     

Enhanced spontaneous polarization in double perovskite Bi2FeCrO6 films

We report the pulsed-laser deposition of epitaxial double-perovskite Bi₂FeCrO₆ (BFCO) films on the (001)-, (110), and (111)-oriented single-crystal SrTiO₃ substrates. All of the BFCO films with various orientations show the and superlattice-diffraction peaks. The intensity ratios between the -superlattice and the main 111-diffraction peak can be tailored by simply adjusting the laser repetition rate and substrate temperature, reaching up to 4.4%. However, both optical absorption spectra and magnetic measurements evidence that the strong superlattice peaks are not correlated with the B-site Fe³⁺/Cr³⁺ cation ordering. Instead, the epitaxial (111)-oriented Bi₂FeCrO₆ films show an enhanced remanent polarization of 92 μC/cm² at 10 K, much larger than the predicted values by density-functional theory calculations. Positive-up-negative-down (PUND) measurements with a time interval of 10 μs further support these observations. Therefore, our experimental results reveal that the strong superlattice peaks may come from A- or B-site cation shifts along the pseudo-cubic [111] direction, which further enhance the ferroelectric polarization of the BFCO thin films.     

Crystal structure,phonon modes,and bond characteristics of AgPb2B2V3O12 (B = Mg,Zn) microwave ceramics

AgPb₂B₂V₃O₁₂ (B = Mg, Zn) ceramics with low sintering temperature were synthesized via the conventional solid-state reaction route. Rietveld refinements of the X-ray diffraction patterns confirm cubic symmetry with space group . The number of observed vibrational modes and those predicted by group theoretical calculations also confirm the space group. At the optimum sintering temperature of 750°C/4 hours, AgPb₂Mg₂V₃O₁₂ has a relative permittivity of 23.3 ± 0.2, unloaded quality factor () of 26 900 ± 500 GHz (), and temperature coefficient of resonant frequency of 19.3 ± 1 ppm/°C, while AgPb₂Zn₂V₃O₁₂ has the corresponding values of 26.4 ± 0.2, 28 400 ± 500 GHz () and –18.4 ± 1 ppm/°C at 590°C/4 hours. Microwave dielectric properties of a few reported garnets and Pb₂AgB₂V₃O₁₂ (B = Mg, Zn) ceramics were correlated with their intrinsic characteristics such as the Raman shifts as well as width of A_1g Raman bands. Higher quality factor was obtained for lower full width at half-maxima (FWHMs) values of A_1g modes. The increase in B-site bond valence contributes to high and low |τ_f| with the substitution of Zn²⁺ by Mg²⁺. Furthermore, the high ionic polarizability and unit cell volume with Zn²⁺substitution contribute to increased relative permittivity.     

Ionic conductivity of Li2O–P2O5 glasses from thermodynamic modeling of their chemical structure

Modeling the transport properties of glasses, and ionic conductivity in particular, has been a recurrent issue motivated for their high interest in the rapid developing field of energy conversion and storage devices for a wide range of applications such as in lithium rechargeable batteries. Despite the absolute conductivity of phosphate-based glasses is not among the highest, their ease of preparation and ability to host transition metals and secondary anions has contributed to foster their research from which good examples are the LiPON electrolytes or the LATP-based glass ceramics. In this work, the structure and ionic conductivity of lithium phosphate glasses with nominal composition Li₂O⋅()P₂O₅, and x between 46 and 58 mol%, have been studied through the thermodynamic approach of the associated solutions model that was introduced by Shakhmatkin and Vedishcheva in glasses. The weak electrolyte model and the Anderson and Stuart model have also been employed herein in order to calculate the activation energy of the conduction of lithium ions within the glass matrix. Experimental ionic conductivity and activation energy data have been determined and were compared with the ones calculated using the models mentioned above. The structural units of the network have also been obtained by means of the thermodynamic approach and compared with those determined by ³¹P nuclear magnetic resonance (NMR) spectra. All of them showed that the glass network structure can be reliably represented by a solution of chemical species and whose properties can then be derived from the respective weights in the system.     

Room-temperature mechanochemical synthesis of RE molybdates: Impact of structural similarity and basicity of oxides

The feasibility of room-temperature synthesis of R₁₀Mo₂O₂₁ (R = La, Y, Er) molybdates via mechanochemical processing of the 5R₂O₃+2MoO₃ oxide mixtures has been studied using X-ray powder diffraction (XRD) with Rietveld refinement and electron spin resonance spectroscopy (ESR). In all systems, the initial stage of mechanochemical synthesis is associated with MoO₃ dissolution in R₂O₃ despite the difference in the crystal structure of initial oxides: La₂O₃ (, no. 164), Er₂O₃ and Y₂O₃ (, no. 206). Only for the 5Er₂O₃ + 2MoO₃ system containing magnetic cations, ESR detects A-type Mo⁵⁺ centers, thus enabling to follow the decrease in MoO₃ content during room-temperature mechanochemical synthesis. In the Er- and Y-based systems, the starting oxides and synthesized R₁₀Mo₂O₂₁ (R = Y, Er) molybdates have the same bixbyite structure and mechanochemical MoO₃ dissolution in these oxides leads to R₁₀Mo₂O₂₁ synthesis at room temperature within 80 min. In the La-based system, MoO₃ dissolution leads mainly to mechanochemical synthesis of La₁₀Mo₂O₂₁ with the same structure as the starting La₂O₃ (, no. 164), but the amorphization of La₂O₃ and formation of basic lanthanum hydroxide during milling also took place. The use of nanosized MoO₃ facilitates the slightly formation of La₁₀Mo₂O₂₁ (, no. 164), but makes no impact on the synthesis of La₁₀Mo₂O₂₁ ( (RI), no. 166).     

Enhanced piezoelectricity and temperature stability in LaFeO3-modified KNN-based lead-free ceramics

(0.96-x)K_0.48Na_0.52NbO₃-0.04Bi_0.5Na_0.5ZrO₃-xLaFeO₃ ceramics (abbreviated as KNN-BNZ-LF1000x) with enhanced piezoelectric performance and temperature stability were prepared by the conventional solid-state sintering method. It was found that the incorporation of LaFeO₃ gradually shifted the O-T phase boundary toward room temperature, while maintaining the Curie temperature above 300°C. The optimal piezoelectricity was found at x = 0.006, with relatively high piezoelectric constant d₃₃ of 345 pC/N as well as a high level of unipolar strain (0.126% at 3 kV/mm). Benefiting from the diffused phase transition induced by appropriate amount of LaFeO₃ content, the KNN-BNZ-LF6 sample possessed greatly enhanced the temperature stability of , which varied less than 8% in the temperature range of 20°C-100°C.     

Structure and Microwave Dielectric Behavior of A‐Site‐Doped Sr(1−1.5x)CexTiO3 Ceramics System

The ion valence state, phase composition, microstructure, and microwave dielectric properties of Sr_(1?1.5x)Ce_xTiO₃ (x = 0.1–0.67, SCT) ceramics were systematically investigated. Sr_(1?1.5x)Ce_xTiO₃ ceramics were produced with gradual structural evolution from a cubic to a tetragonal and turned to an orthorhombic structure in the range of 0.1 ≤ x ≤ 0.67. Above a critical Ce proportion (x = 0.4), microstructural changes and normal grain growth initially occurred. On the basis of chemical analysis results, the reduction of Ti⁴⁺ ions was hastened by tetravalent ions (Ce⁴⁺). By contrast, this reduction was inhibited by trivalent ions (Ce³⁺). The observed dielectric behavior was strongly influenced by phase composition, oxygen vacancies (), and defect dipoles, namely, () and (). Temperature stable ceramics sintered at 1350°C for 3 h in air yielded an intermediate value of dielectric constant (ε_r = 40), with the smallest reported value of temperature coefficient of resonant frequency (τ_f = +0.9 ppm/°C), and quality factor (Q × f = 5699 GHz) at x = 0.6.     

Phase Equilibria in the ZrO2–MgO–MnOx System

Phase equilibria were experimentally investigated in the MgO–MnO_x and the ZrO₂–MgO–MnO_x systems for different oxygen partial pressures by powder X‐ray diffractometry, scanning electron microscopy, and differential thermal analysis. The formation of two compositionally and structurally different β‐spinel solid solutions was observed in the MgO–MnO_x system in air in the temperature interval 1473–1713 K. Isothermal sections of the ZrO₂–MgO–MnO_x phase diagram were constructed for air conditions ( = 0.21 bar) at 1913, 1813, 1713, 1613, and 1523 K. In addition, isothermal sections at 1913 and 1523 K were constructed for = 10^?4 bar. The β‐spinel and halite phases of the MgO–MnO_x system were found to dissolve up to 2 and 5 mol% ZrO₂. A continuous c‐ZrO₂ solid solution forms between the boundary ZrO₂–MnO_x and ZrO₂–MgO systems. It stabilizes in the ZrO₂–MgO–MnO_x system down to at least 1613 K in air and down to 1506 K at = 10^?4 bar.     

Microwave dielectric properties and thermally stimulated depolarization of Al-doped Ba4(Sm,Nd)9.33Ti18O54 ceramics

Ba₄(Sm_0.15Nd_0.85)_9.33Ti_18-zAl_3z/4O₅₄ (BSNT-zAl, 0.0 ≤ z ≤ 2.5) ceramics were prepared via a solid-state reaction, and the effects of Al doping on the microwave dielectric properties and defect behavior of the title compound were studied. X-ray diffraction (XRD) analysis and scanning electron microscopy (SEM) photographs suggested that Al ions successfully entered the lattice to form tungsten-bronze-like solid solutions. With a small amount of Al substitution, the relative dielectric constant (ε_r), and the temperature coefficient of resonant frequency (τ_f) values decreased, whereas the quality factor (Q × f) substantially increased by approximately 50%. The defect-related extrinsic dielectric loss was clarified via the thermally stimulated depolarization current (TSDC) technique. With Al doping, the TSDC relaxation of across-grain-boundary oxygen vacancies () vanished, whereas that of defect dipoles () appeared at relatively low temperatures. Therefore, in the BSNT-zAl ceramics, oxygen vacancies were more inclined to interconnect with to form defect dipoles. This could reduce the activity of and account for the notable improvement in the Q × f values. In particular, the excellent characteristics of ε_r = 67.33, Q × f = 16 530 GHz, and τ_f = +0.87 ppm/°C were achieved in the specimens with z = 1.5 sintered at 1350°C for 4 hours.     

Fabrication and characterization of tetragonal yttria-stabilized zirconia single-crystalline thin film

Tetragonal yttria-stabilized zirconia thin film was successfully fabricated by a pulsed laser deposition method. The thin film grew heteroepitaxially with the orientation relationship of ZrO₂‖Al₂O₃. Energy dispersive X-ray spectroscopy mapping revealed that Y³⁺ ions were distributed homogeneously without local segregations. X-ray and electron-diffraction analysis confirmed a single crystalline structural feature of the film. On the other hand, high-resolution scanning transmission electron microscopy observations show that this film contains small-angle tilt grain boundaries, which is composed of the periodic array of dislocations with the Burgers vector .