Effects of Ta5+ doping on microstructure evolution,dielectric properties and electrical response in CaCu3Ti4O12 ceramics

The effects of Ta⁵⁺ substitution on the microstructure, electrical response of grain boundary, and dielectric properties of CaCu₃Ti₄O₁₂ ceramics were investigated. The mean grain size decreased with increasing Ta⁵⁺ concentration, which was ascribed to the ability of Ta⁵⁺ doping to inhibit grain boundary mobility. This can decrease dielectric constant values. Grain boundary resistance and potential barrier height of CaCu₃Ti₄O₁₂ ceramics were reduced by doping with Ta⁵⁺. This results in enhancement of dc conductivity and the related loss tangent. Influence of charge compensations on microstructure and intrinsic electrical properties of grain boundaries resulting from the effects of replacing Ti⁴⁺ with Ta⁵⁺ are discussed. The experimental data and variation caused by the substitution of Ta⁵⁺ can be described well by the internal barrier layer capacitor model based on space charge polarization at the grain boundaries.     

Crystal structure,phase transition,vibrational spectra,and microwave dielectric characteristics of Ta5+ ionic substituted Co0.5Ti0.5NbO4 ceramics

In the present research, structural representation, phase transition, and dielectric characteristics of Ta⁵⁺ ionic substituted Co_0.5Ti_0.5(Nb_1-xTa_x)O₄ (x = 0–1) ceramics were thoroughly investigated. Combined with the changes of diffraction peaks in the X-ray diffraction spectra, such as (002) and (101) crystalline planes near 20°, and the evolutionary trend of the strongest peak in the Raman spectra, a phase transition from rutile to trirutile structure at x = 0.6 was confirmed, suggesting the emergence of structural evolution. Thus, the structural evolution showed a significant impact on the dielectric characteristics, as evidenced by the weakening of the dielectric polarizability with the increasing amount of Ta⁵⁺ ion, which led to a reduction in the dielectric constant; the dielectric loss is also closely related to the packing fraction (PF) arising from the structural changes. Additionally, infrared and THz time-domain spectroscopy revealed that the dielectric losses in the microwave band were originated from the oscillatory absorption of structural phonons and supported that the effects of structural evolution on the intrinsic dielectric properties due to the introduction of Ta⁵⁺ ion.     

Experimental study and DFT calculations of improved giant dielectric properties of Ni2+/Ta5+ co-doped TiO2 by engineering defects and internal interfaces

High-performance ceramics with chemical formula (Ni_1/3Ta_2/3)_xTi_1?xO₂ with excellent dielectric properties are demonstrated. The dopants of Ni²⁺ and Ta⁵⁺ in TiO₂ caused the formation of oxygen vacancies and free electrons. The (Ni_1/3Ta_2/3)_xTi_1?xO₂ exhibited low loss tangent of 0.046 and a high dielectric permittivity of 3.5–4.5 × 10⁴ with a very weak dependence on temperature (?60 to 200 °C). Broadband dielectric spectroscopy shows at least four dominant sources in the dielectric relaxation response in the temperature range of ? 253–210 °C. DFT calculations indicate the formation of defect clusters, which are the largest contributors to the dielectric response, and these are found to be dominant even at temperatures down to ? 253 °C. Both grain boundary and surface layer mechanisms in the ceramics contribute to the dielectric response at the relatively high temperatures. The sample–electrode contact effect associated with oxygen vacancy diffusion is dominant at high temperatures above 150 °C.     

Influence of Sb-doping on dielectric properties of NaCu3Ti3TaO12 ceramics and relevant mechanism(s)

A series of NaCu₃Ti₃Ta_1−xSb_xO₁₂ ceramics were prepared by the conventional solid-state reaction technique, and their dielectric properties, crystalline structures, microstructures and complex impedance were investigated systematically. All the ceramics show the main phases of perovskite-related crystallographic structure, and their dielectric properties change significantly with the increasing Sb-doping. All these ceramics exhibit giant dielectric-permittivity properties, and impedance spectroscopy analysis reveals that NaCu₃Ti₃Ta_1−xSb_xO₁₂ ceramics are electrically heterogeneous and composed of insulating grain boundaries and semiconducting grains. Moreover, CuO secondary phase and Cu²⁺/Cu¹⁺, Ti⁴⁺/Ti³⁺, Sb⁵⁺/Sb³⁺ and Ta⁵⁺/Ta³⁺ aliovalences are found to exist in NaCu₃Ti₃Ta_1−xSb_xO₁₂ ceramics through XRD, EDS and XPS analysis. Therefore, CuO segregation and aliovalences of metal ions were suggested to contribute greatly to the internal barrier layer capacitance effect formation in NaCu₃Ti₃Ta_1−xSb_xO₁₂ ceramics. Furthermore, Sb-doping could decrease the tanδ of NaCu₃Ti₃Ta_1−xSb_xO₁₂ ceramics at low frequencies, and the reason was discussed.     

Effects of Sr2+ substitution on the crystal structure,Raman spectra,bond valence and microwave dielectric properties of Ba3-xSrx(VO4)2 solid solutions

The effects of Sr²⁺ substitution for Ba²⁺ on microwave dielectric properties and crystal structure of Ba_3-xSr_x(VO₄)₂ (0 ≤ x ≤ 3, BSVO) solid solution were investigated. Such Sr²⁺ substitution contributes to significant reduction in sintering temperature from 1400 °C to 1150 °C. Both permittivity (∑_r) and quality factor (Q × f) values decreased with increasing x value, which was determined to be related with the descending values of average polarizability and packing fraction, whereas the increase in τ_f value was explained by the decreased average VO bond length, A-site bond valence. BSVO ceramics possessed encouraging dielectric performances with ∑_r = 12.2–15.6 ± 0.1, Q × f = 44,340 - 62,000 ± 800 GHz, and τ_f = 24.5–64.5 ± 0.2 ppm/°C. Low-temperature sintering was manipulated by adding B₂O₃ as sintering additive for the representative Sr₃V₂O₈ (SVO) ceramic and only 1 wt.% B₂O₃ addition successfully contributed to a 21.7% decrease in sintering temperature to 900 °C, showing good chemical compatibility with silver electrodes, which render BSVO series and SVO ceramics potential candidates in multilayer electronic devices fabrication.     

Correlation between crystal structure and dielectric characteristics of Ti4+ substituted CaSnSiO5 ceramics

The crystal structure and dielectric properties of Ti⁴⁺-substituted CaSnSiO₅ ceramic were investigated. Ti⁴⁺ entirely substituted Sn⁴⁺ of CaSnSiO₅, and the solid solutions were formed at Ca(Sn_1-xTi_x)SiO₅ (0 ≤ x ≤ 1.0) ceramics. The evolutions of crystal structure were analysed through Rietveld refinement and transmission electron microscopy, and the phase transition from A2/a to P2₁/a space groups at Ca(Sn_1-xTi_x)SiO₅ (0.9 ≤ x ≤ 1.0) ceramics was clarified. The change in dielectric properties was related to the structural evolution of Ca(Sn_1-xTi_x)SiO₅ (0 ≤ x ≤ 1.0). The τ_f values of Ca(Sn_1-xTi_x)SiO₅ (0 ≤ x ≤ 0.4) ceramics initially decreased to +49.8 ppm/°C and then increased to +98.3 ppm/°C because of Sn/TiO₆ octahedral distortion. The temperature coefficient of capacitance and ε_r anomaly peak were controlled by Ti⁴⁺ substitution for Sn⁴⁺ at Ca(Sn_1-xTi_x)SiO₅ (0.4 ≤ x ≤ 1.0) ceramics. The results provided a way to control the τ_f value of microwave dielectric ceramics.     

Thermal stability improvement of dielectric properties and non-ohmic characteristic of CaCu3+xTi4O12 ceramics via a Cu-nonstoichiometric approach

In this work, the effects of Cu composition on the thermal stability of the dielectric and nonlinear properties of CaCu_3+xTi₄O₁₂ (?0.2 ≤ x ≤ 0.2) ceramics obtained via a polymer-pyrolysis chemical process were studied. The mean grain sizes of Cu-stoichiometric (x = 0), Cu-deficient (x < 0) and Cu-excess (x > 0) CaCu_3+xTi₄O₁₂ ceramics were found to be ~3.2, ~3.4 and ~3.7 μm, respectively. Interestingly, very good dielectric properties (0.020 ≤ tanδ ≤ 0.038 and 4000 ≤ ε′ ≤ 7065) were attained in CaCu_3+xTi₄O₁₂ (?0.2 ≤ x ≤ 0.1, excluding x = 0.2) ceramics. Moreover, the variation of dielectric constant (ε′) within a limit of ±15% (Δε′_{± 15%}) over a wide temperature range (T_R) of ?70 – 220 °C with low tanδ < 0.05 (tanδ_<0.05) over a T_R of ?70 to 80 °C were achieved in a CaCu_2.8Ti₄O₁₂ ceramic. These results suggest that this ceramic could be applicable for X9R capacitors and energy storage devices that require high thermal stability. Additionally, the nonlinear properties of Cu-nonstoichiometric ceramics could be improved when compared with those of the Cu-stoichiometric material. The incremental changes of dielectric and nonlinear properties of CaCu_3+xTi₄O₁₂ (?0.2 ≤ x ≤ 0.2) ceramics revealed the significant role of Cu composition on grain boundary resistance (R_gb), which was confirmed by impedance spectroscopy analysis. In addition, XANES results revealed the proper ratios of Cu⁺:Cu²⁺ and Ti³⁺:Ti⁴⁺ found in these ceramics, indicating the semiconducting behavior of these grains.     

Enhancing giant dielectric properties of Ta5+-doped Na1/2Y1/2Cu3Ti4O12 ceramics by engineering grain and grain boundary

Various strategies to improve the dielectric properties of ACu₃Ti₄O₁₂ (A = Sr, Ca, Ba, Cd, and Na_1/2Bi_1/2) ceramics have widely been investigated. However, the reduction in the loss tangent (tanδ) is usually accompanied by the decreased dielectric permittivity (ε′), or vice versa. Herein, we report a route to considerably increase ε′ with a simultaneous reduction in tanδ in Ta⁵⁺–doped Na_1/2Y_1/2Cu₃Ti₄O₁₂ (NYCTO) ceramics. Dense microstructures with segregation of Cu– and Ta–rich phases along the grain boundaries (GBs) and slightly increased mean grain size were observed. The samples prepared via solid-state reaction displayed an increase in ε′ by more than a factor of 3, whereas tanδ was significantly reduced by an order of magnitude. The GB–conduction activation energy and resistance raised due to the segregation of Cu/Ta–rich phases along the GBs, resulting in a decreased tanδ. Concurrently, the grain–conduction activation energy and grain resistance of the NYCTO ceramics were reduced by Ta⁵⁺ doping ions owing to the increased Cu⁺/Cu²⁺, Cu³⁺/Cu²⁺, and Ti³⁺/Ti⁴⁺ ratios, resulting in enhanced interfacial polarization and ε′. The effects of Ta⁵⁺ dopant on the giant dielectric response and electrical properties of the grain and GBs were described based on the Maxwell–Wagner polarization at the insulating GB interface, following the internal barrier layer capacitor model.     

Influence of Zn0.5Ti0.5NbO4 on the structure and microwave dielectric properties of ZrTiO4 ceramics

In this work, xZn_0.5Ti_0.5NbO₄-(1-x) ZrTiO₄ (0.1 ≤ x ≤ 0.5) ceramics were prepared through the traditional solid-state route. The microstructure, phase evolution and crystallographic variations were investigated in detail. An orthorhombic structure solid solution was formed with the composition of Zr_1-xZn_0.5xTi_1-0.5xNb_xO₄ (0.1 ≤ x ≤ 0.5). Variations of lattice parameters were responsible for the opposite movement of different XRD peaks as increasing x value. Chemical bond theories include that bond ionicity, bond susceptibility, lattice energy, bond energy and linear thermal expansion coefficient are correlated with dielectric properties. Temperature stable ceramics were obtained when x = 0.2 with excellent dielectric performances: Q × f = 26741 GHz, ε_r = 41.93 and τ_f = −2.3 ppm/^oC.     

Impedance spectroscopy,B-site cation ordering and structure–property relations of (1 − x) La[Al0.9(Mg0.5Ti0.5)0.1]O3–x CaTiO3 ceramics for 5G dielectric waveguide filters

Dense (1 ? x) La[Al_0.9(Mg_0.5Ti_0.5)_0.1]O₃–x CaTiO₃ ceramics were synthesized via solid-state reaction. The crystal structure and microwave dielectric properties of the ceramics were systematically investigated. Rietveld refinement revealed that when x ≤ 0.2, the ceramics had a rhombohedral structure with an R-3c space group. When x ≥ 0.5, the ceramics had an orthorhombic structure with a Pbnm space group. Selected area electron diffraction and Raman spectroscopy analyses proved that the microwave dielectric ceramics had a B-site order, which accounted for the great improvement in microwave dielectric properties. The content of oxygen vacancies was identified through X-ray photoelectron spectroscopy, and the change rule of Q × f was closely related to oxygen vacancy content. The perturbation of A-site cations had an important influence on dielectric constant. Specifically, with the increase in Ti⁴⁺ content, the perturbation effect of the A-site cations was enhanced and dielectric constant increased. When x = 0.65, the temperature coefficient of resonant frequency of the (1 ? x) La[Al_0.9(Mg_0.5Ti_0.5)_0.1]O₃–x CaTiO₃ microwave dielectric ceramics was near zero. The optimal microwave dielectric properties of 0.35LaAl_0.9(Mg_0.5Ti_0.5)_0.1O₃–0.65CaTiO₃ were ε_r = 44.6, Q × f = 32,057 GHz, and τ_f = +2 ppm/°C.     

Large enhancement of piezoelectric properties and resistivity in Cu/Ta co-doped Bi4Ti3O12 high-temperature piezoceramics

In this study, the electrical properties of Bi₄Ti₃O₁₂-based Aurivillius-type ceramics were tailored by a B-site co-doping strategy combining high valence Ta⁵⁺ and low valence Cu²⁺. A series of Bi₄Ti_3−x(Cu_1/3Ta_2/3)_xO₁₂ (BTCT) (x = 0, 0.005, 0.01, 0.015, 0.02, 0.025, and 0.03) ceramics were prepared by the conventional solid-state reaction method. The effect of Cu/Ta co-doping on the crystal structure, microstructure, dielectric properties, piezoelectric properties, ferroelectric properties, and electrical conductivity of these ceramics was systematically investigated. Co-doping significantly enhanced the piezoelectric properties and DC electrical resistivity of the resulting composites. The optimized comprehensive performances were obtained at x = 0.015 with a large piezoelectric coefficient (34 pC/N) and a relatively high resistivity of 9.02 × 10⁶ Ω cm at 500°C. Furthermore, the ceramic also exhibited stable thermal annealing behaviors and excellent fatigue resistance. The results of this study demonstrated great potential of the Cu/Ta co-doped Bi₄Ti₃O₁₂ ceramics for high-temperature piezoelectric device applications.     

(Cu1/3Nb2/3)4+ ionic substituting effects,low-temperature sintering behaviors,and improved temperature stability of Ba3Nb4Ti4O21 microwave dielectric ceramics

In this study, (Cu_1/3Nb_2/3)⁴⁺ complex cation and BaO–ZnO–B₂O₃ glass frit were adopted to solve the high sintering temperature and poor temperature stability of Ba₃Nb₄Ti₄O₂₁ ceramics. It is shown that pure Ba₃Nb₄Ti₄O₂₁ phase was formed when Ti site was partially replaced by (Cu_1/3Nb_2/3)⁴⁺ cation. The increasing number of dopants decreases the dielectric polarizability, correspondingly, the dielectric constant and temperature coefficient of the resonance frequency values are reduced consistently. The variation of the Q × f value is determined by internal ionic packing fraction and external sintering densification. The (Cu_1/3Nb_2/3)⁴⁺ cation effectively decreases the suitable sintering temperature from 1200 to 1050 °C while greatly improving the temperature stability. BaO–ZnO–B₂O₃ glass was used to further improve the low-temperature sintering characteristics of Ba₃Nb₄Ti₄O₂₁ ceramics. It is proven that the addition of glass frits effectively decreases the temperature to 925 °C with combinational excellent microwave dielectric properties: ε_r ～55.6, Q × f ～5700 GHz, τ_f ～3 ppm/^°C, making the Ba₃Nb₄Ti₄O₂₁ ceramics promising in the applications of low-temperature cofired ceramic technology.     

Modified Ba6−3xNd8+2xTi18O54 microwave dielectric ceramics

In order to improve the microwave dielectric properties of Ba_6−3xNd_8+2xTi₁₈O₅₄ solid solution ceramics, the effects of Bi₂O₃ and Bi₄Ti₃O₁₂ additives were determined. The results of SEM and EDS analyses suggested that the present ceramics with Bi₄Ti₃O₁₂ additives consisted of Ba_6−3xNd_8+2xTi₁₈O₅₄ solid solution matrix phase, and secondary phase of BaTi₄O₉, but this was not the situation of Bi₂O₃ added ceramics. XRD analysis also revealed that the unit cell volume of the matrix phase increased with increasing the amount of Bi₄Ti₃O₁₂ additive. With addition of Bi₄Ti₃O₁₂ into the present ceramics, the dielectric constant increased and the temperature coefficient of resonator frequency decreased, while the Qf value slightly decreased. The excellent microwave dielectric characteristics (ε=94·9, Qf=5620 GHz, τ_f=21·4 ppm/°C could be achieved in the present ceramics through the microstructure control.     

The effect of co-substitution on the microwave dielectric properties of Ba6-3xNd8+2xTi18O54(x=2/3) ceramics

In this work, the microwave dielectric properties of Ba₄(Nd_1-yBi_y)_28/3Ti_18-x(Al_1/2Ta_1/2)_xO₅₄(0≤x≤2, 0.05≤y≤0.2) ceramics co-substituted by A/B-site were studied. Firstly, (Al_1/2Ta_1/2)⁴⁺ was used for substitution at B-site. At 0≤x≤1.5, the above mentioned ceramic was found to exist in single-phase tungsten bronze structure, but at x = 2.0, the secondary phase appeared. Although the dielectric constant decreased by doping the (Al_1/2Ta_1/2)⁴⁺, but the quality factor was observed to improve by 40% and the temperature coefficient of resonant frequency decreased by 75%. Based on the above results, Bi³⁺ was introduced to Ba₄Nd_28/3Ti₁₇(Al_1/2Ta_1/2)O₅₄. The introduction of Bi³⁺ reduced the sintering temperature, greatly improved the dielectric constant, and ultimately decreased the temperature coefficient of resonant frequency, but it led to deterioration of quality factor. At last, with appropriate site-substitution content control (x = 1.0,y = 0.15), excellent comprehensive properties (ε_r = 89.0, Q × f = 5844 GHz @ 5.89 GHz，TCF = +8.7 ppm/°C) were obtained for the samples sintered at 1325 °C for 4 h.     

Effect of Ta5+ doping on the thermal physical properties of defective fluorite Y3NbO7 ceramics

The effects of substituting the B cation in A₃BO₇ ceramics on their thermal physical properties were investigated by applying a large mass difference. Y₃(Nb_1-xTa_x)O₇ (x = 0, 0.1, 0.2, 0.3, 0.4, and 0.5) ceramics were synthesized, and their structural characteristics were determined. All the fabricated Y₃(Nb_1-xTa_x)O₇ ceramics showed defective fluorite structures and glass-like low thermal conductivity (1.18−2.04 W/m∙K at 25°C) because of the highly distorted crystal structure and significant mass difference. Substitution with Ta⁵⁺ enhanced the sintering resistance, leading to superior thermal-insulating performance via grain boundary scattering. Furthermore, the ceramics exhibited excellent coefficients of thermal expansion, implying the promising applicability of Y₃(Nb_1-xTa_x)O₇ as new thermal barrier materials. The effect of mass difference on the thermomechanical properties of the ceramics was examined, suggesting a simple strategy for engineering the chemical composition of new thermal barrier materials.     

Thermally-stimulated defect relaxations and microwave/terahertz dielectric response of La,Al co-doped (Ba,Sr)La4Ti4O15 ceramics

The thermally-stimulated relaxation of defects in La,Al co-substituted (Ba,Sr)La₄Ti₄O₁₅ dielectric ceramics with hexagonal perovskite structure were investigated systematically with the thermally stimulated depolarization current (TSDC) measurements. The ion substitution decreased the lattice parameters and the bond lengths of the cations and O²⁻. TSDC spectra indicated the main extrinsic defects in the ceramics were oxygen vacancies and the concentration of oxygen vacancies decreased with the increasing ion substitutions, which was derived from the decrease in the lattice parameters. The terahertz time-domain spectra indicated the damping factor of lattice vibration was reduced with the decline of the concentration of oxygen vacancies, which decreased the microwave/terahertz dielectric loss. High-performance (Ba_0.2Sr_0.8)_0.75La_4.25Ti_3.75Al_0.25O₁₅ (ε_r = 43.9, Q × f = 84300 GHz, τ_f = -18.0 ppm/°C) ceramics were obtained. The correlations between crystal structures, defect behaviors and dielectric properties discussed in this work could provide guidance for the modification of microwave dielectric ceramics.     

Microwave dielectric properties of BaO–Ta2O5–TiO2 system

Microwave dielectric properties of the BaO–Ta₂O₅–TiO₂ system were investigated by the solid-state reaction method. It was recognized that the Ba₁₀Ta_7.04(Ti_1.2 − xSn_x)O₃₀ solid solutions have the higher Q · f value in comparison with the Ba₈(Ta_4 − xNb_x)Ti₃O₂₄ solid solutions. The limit of the Ba₁₀Ta_7.04(Ti_1.2 − xSn_x)O₃₀ solid solutions was approximately x = 0.75; the lattice parameter c of the solid solutions, which is related to the change in the B(1)O₆ octahedron, was significantly increased in the composition range from 0 to 0.75. The Q · f values of the Ba₁₀Ta_7.04(Ti_1.2 − xSn_x)O₃₀ solid solutions are remarkably improved by the Sn substitution for Ti; the highest Q · f value of 59,100 GHz is obtained at x = 0.75. Moreover, the ɛ_r and τ_f values of the Ba₁₀Ta_7.04(Ti_1.2 − xSn_x)O₃₀ solid solutions at x = 0.75 were 25.6 and 30.3 ppm/°C, respectively.     

Colossal dielectric response and relaxation behavior in novel system of Zr4+ and Nb5+ co-substituted CaCu3Ti4O12 ceramics

In this work, we developed a novel system of isovalent Zr⁴⁺ and donor Nb⁵⁺ co-doped CaCu₃Ti₄O₁₂ (CCTO) ceramics to enhance dielectric response. The influences of Zr⁴⁺ and Nb⁵⁺ co-substituting on the colossal dielectric response and relaxation behavior of the CCTO ceramics fabricated by a conventional solid-phase synthesis method were investigated methodically. Co-doping of Zr⁴⁺ and Nb⁵⁺ ions leads to a significant reduction in grain size for the CCTO ceramics sintered at 1060 °C for 10 h. XRD and Raman results of the CaCu₃Ti_3.8-xZr_xNb_0.2O₁₂ (CCTZNO) ceramics show a cubic perovskite structure with space group Im-3. The first principle calculation result exhibits a better thermodynamic stability of the CCTO structure co-doped with Zr⁴⁺ and Nb⁵⁺ ions than that of single-doped with Zr⁴⁺ or Nb⁵⁺ ion. Interestingly, the CCTZNO ceramics exhibit greatly improved dielectric constant (~10⁵) at a frequency range of 10²–10⁵ Hz and at a temperature range of 20–210 °C, indicating a giant dielectric response within broader frequency and temperature ranges. The dielectric properties of CCTZNO ceramics were analyzed from the viewpoints of defect-dipole effect and internal barrier layer capacitance (IBLC) model. Accordingly, the immensely enhanced dielectric response is primarily ascribed to the complex defect dipoles associated with oxygen vacancies by co-doping Zr⁴⁺ and Nb⁵⁺ ions into CCTO structure. In addition, the obvious dielectric relaxation behavior has been found in CCTZNO ceramics, and the relaxation process in middle frequency regions is attributed to the grain boundary response confirmed by complex impedance spectroscopy and electric modulus.     

Investigation of phase composition and microwave dielectric properties of MgO‐Ta2O5 ceramics with ultrahigh Qf value

Four MgO‐Ta₂O₅ ceramics with the MgO/Ta₂O₅ mole ratio x = 1, 2, 3, and 4 were prepared by traditional solid‐state reaction method, and the influence of x on the phase composition, microstructure, and dielectric properties (the dielectric constant ε_r, the temperature coefficient of resonant frequency τ_f and the quality factor Qf) of the materials was investigated using XRD, SEM, etc. The results indicated that the ceramics were composed of two crystalline phases MgTa₂O₆ and Mg₄Ta₂O₉ in the composition range studied, and that the dielectric properties ln ε, 1/Qf, and τ_f changed proportionally to the fraction of main crystal phases, which meet perfectly with the mixing model proposed in this study. It is obvious that the proportion of the two crystal phases could be precisely controlled by x, and thereby, the dielectric properties can be conveniently and precisely tailored. Our research provided a new microwave dielectric ceramic with the composition of 2MgO‐Ta₂O₅, which has an ultrahigh Qf value (211 000 GHz), low dielectric constant ε_r (19.9), and near zero temperature coefficient of resonant frequency τ_f (8 ppm/°C).     

Effects of (Cr0.5Ta0.5)4+ on structure and microwave dielectric properties of Ca0.61Nd0.26TiO3 ceramics

The Ca_0.61Nd_0.26Ti_1-x(Cr_0.5Ta_0.5)_xO₃ (CNT-CTx) ceramics with orthorhombic perovskite structure were prepared using the conventional solid-state method. The X-ray diffraction (XRD), Raman spectra and X-ray photoelectron spectra (XPS) were employed to investigate the correlations between crystal structure and microwave dielectric properties of CNT-CTx ceramics. The XRD results showed that all CNT-CTx samples were crystallized into the orthorhombic perovskite structure. The SEM micrographs indicated that the average grain size of samples depended on the sintering temperature. As (Cr_0.5Ta_0.5)⁴⁺ concentration increased, there was a significant decrease in the average grain size of samples. The short range order (SRO) structure and structural distortion of oxygen octahedra proved to exist in CNT-CTx crystals according to the analysis of Raman spectra results. The microwave dielectric properties highly depended on the full width at half maximum (FWHM) of Raman spectra, oxygen octahedra distortion, reduction of Ti⁴⁺ to Ti³⁺ and bond valence. At last, the CNT-CT0.05 ceramic sintered at 1420?°C for 4?h exhibited the good and stable comprehensive microwave dielectric properties: relative permittivity of 96.5, quality factor of 14,360?GHz, and temperature coefficient of resonant frequency of +153.3?ppm/°C.