Microwave properties of Ba(Mg1/3Ta2/3)O3, Ba(Mg1/3Nb2/3)O3 and Ba(Co1/3Nb2/3)O3 ceramics revealed by Raman scattering

Five Ba(Co_1/3Nb_2/3)O₃ samples sintered at different temperatures (form 1350 to 1550 °C), one Ba(Mg_1/3Ta_2/3)O₃ and a Ba(Mg_1/3Nb_2/3)O₃ sample were examined by Raman scattering to reveal the correlation of the 1:2 ordered perovskite structure with the microwave properties, such as dielectric constant and Q factors. The Ba(Co_1/3Nb_2/3)O₃ sample sintered at 1400 °C, which possesses the highest microwave Q value and the lowest dielectric constant among five Ba(Co_1/3Nb_2/3)O₃ samples, has the narrowest width and the highest frequency of the stretch mode of oxygen octahedron (i.e. A_1g(O) near 800 cm⁻¹). We found that the dielectric constant is strongly correlated with the Raman shift of A_1g(O) stretch modes, and the width of A_1g(O) stretch mode reflects the quality factor Q × f value in the 1:2 ordered perovskite materials. This concludes that the oxygen octahedron play an important role of the material's microwave performance. Based on the results of Q × f values and the lineshapes of A_1g(O) stretch mode, we found that the propagation of microwave energy in Ba(Mg_1/3Ta_2/3)O₃ and Ba(Mg_1/3Nb_2/3)O₃ shows weak damping behavior, however, Ba(Co_1/3Nb_2/3)O₃ samples sintered at different temperature exhibit heavily damped behavior.     

Temperature and frequency dependence of dielectric loss of Ba(Mg1/3Ta2/3)O3 microwave ceramics

Ba(Mg_1/3Ta_2/3)O₃ ceramic possessing extremely high Q × f value of more than 300 THz at microwave frequency was developed in our previous study. It is of great interest to understand the mechanism of microwave absorption in such a practical material. In the present study we report on the temperature dependence of the dielectric loss in the Ba(Mg_1/3Ta_2/3)O₃. The mechanism of the microwave absorption is discussed using two phonons difference process. The samples were prepared by conventional solid state reaction and sintered at 1893 K in oxygen atmosphere. Dielectric properties in the microwave range were measured by Hakki & Colemann and resonant cavity methods in the temperature range of 20–300 K. Whispering gallery mode technique was used for the measurement of the dielectric properties at the millimeter wave frequency. Dielectric loss of the Ba(Mg_1/3Ta_2/3)O₃ at the microwave frequency increases with temperature between 200 and 300 K in general agreement with the theory of intrinsic dielectric loss derived from the two phonon difference process. However below 200 K, the dielectric loss has shown a distinctive behavior with a loss peak at 40 K. It was inferred that the loss peak of the Ba(Mg_1/3Ta_2/3)O₃ was caused by the local orientation polarization having dispersion at the microwave frequency.     

Microwave dielectric properties of Ba8Ta6(Ni1 − xMx)O24 (M = Zn and Mg) ceramics

The influence of M (M = Zn and Mg) substitution for Ni on the microwave dielectric properties and the crystal structure of Ba₈Ta₆(Ni_1 − xM_x)O₂₄ ceramics was investigated in this study. The Ba₈Ta₆(Ni_1 − xZn_x)O₂₄ (BTNZ) solid solutions showed a single phase in the composition range of 0–1, whereas the limit of Ba₈Ta₆(Ni_1 − xMg_x)O₂₄ (BTNM) solid solutions was approximately x = 0.75; the lattice parameters of both solid solutions increased linearly, depending on the composition x. Although the dielectric constants (ɛ_r) of BTNZ were almost constant over the whole composition range, those of BTNM slightly decreased from 27.8 to 24.3; the decrease in the dielectric constant of BTNM is due to the change in relative density of the sample. The quality factors (Q × f) of both solid solutions were improved by the M substitution for Ni; the maximum Q × f values of BTNZ and BTNM were 91729 and 93127 GHz, respectively. Moreover, the temperature coefficients of resonant frequency (τ_f) of BTNZ and BTNM varied from 33 to 40 ppm/°C and from 33 to 26 ppm/°C, respectively.     

A unique method to fabricate NixMg1 − xO (111) nano-platelet solid solution catalyst for CH4-CO2 dry reforming

A facile solvothermal route is established to generate Ni_xMg_1 − x(OH)(OCH₃) platelet covered by (001) facets. Thermolytic decomposition of Ni_xMg_1 − x(OH)(OCH₃) is topotactic and leads to the formation of Ni_xMg_1 − xO solid solution enclosed by reactive Tasker III type (111) facets. Both Mg and Ni are found to be distributed evenly in the solid solution. The reduced Ni_xMg_1 − xO platelet is a stable catalyst in CH₄-CO₂ dry reforming for a 100 h test.     

Signature of lattice defects in far infrared reflectivity of Ba(Mg1/3Ta2/3)O3

Far infrared reflectivity spectra of Ba(Mg_1/3,Ta_2/3)O₃ prepared at several sintering temperatures were measured, and the eigenfrequencies and damping constants of the TO modes were determined. The reflectivity spectra were fitted with the four-parameter semi-quantum model. The variation in the E_u(O_II) at 222 cm⁻¹ and A_2u(O_II) at 238 cm⁻¹ modes in well ordered ceramics was attributed to the variation in the concentration of the B site defects. It was also found that the change in the oxygen partial pressure of the sintering atmosphere causes a change in the seventh (316 cm⁻¹) and eighth (352 cm⁻¹) modes. We attribute these changes to the oxygen site defect although we cannot evaluate the concentration of this defect at this moment. From the behavior of the damping constants it is suggested that the Ba(Mg_1/3Ta_2/3)O₃ (BMT) attains equilibrium defect density at the heat treatment temperature of more than 1630 °C (120 h).     

Structural,chemical and dielectric properties of ceramic injection moulded Ba(Zn1/3Ta2/3)O3 microwave dielectric ceramics

We report the development of a ceramic injection moulding (CIM) process to produce complex-shaped structures using high-performance microwave ceramic materials. In particular, we describe the synthesis methods and the structural, chemical and dielectric properties of Ba(Zn_1/3Ta_2/3)O₃ (BZT) doped with Ni and Zr ceramics produced using ceramic injection moulding. Sintering the ceramic injection moulded Ba(Zn_1/3Ta_2/3)O₃ to a relative density of ∼94% was possible at a temperature of 1680 °C and a time of 48 h. The best samples to date exhibit a dielectric constant, ɛ_r, of ∼30, a Q value, of ∼31,250 (i.e. tan δ < 3.2 × 10⁻⁵) at 2 GHz, and a temperature coefficient of resonance frequency, τ_f, of 0.1 ppm/°C.     

The effect of cation ordering and domain boundaries on low loss Ba(BI1/3BII2/3)O3 perovskite dielectrics revealed by high-angle annular dark-field scanning transmission electron microscopy (HAADF STEM)

High quality ceramics of Ba((Co_0.7Zn_0.3)_1/3Nb_2/3)O₃ (BCZN), Ba(Mg_1/3Nb_2/3)O₃ (BMN) and Ba(Mg_1/3Ta_2/3)O₃ (BMT) were prepared by the mixed oxide route using sintering temperatures up to 1620 °C. Products with a high degree of cation ordering exhibited dielectric Q × f values from 83,000 GHz (BCZN) to 360,000 GHz (BMT). High Resolution TEM and aberration-corrected scanning transmission electron microscopy (STEM) revealed ordering domains and type I, II and III boundary structures. High-Angle Angular Dark Field (HAADF) STEM images provided direct evidence of 1:2 ordering and stacking sequences, and the presence of disordered regions within domain boundaries. The exceptionally high Q × f values for BMT are associated with a high degree of B-site ordering and the removal of domain boundaries in large, single domain grains. The catastrophic degradation of Q × f values in BMN after prolonged sintering is associated with formation of a lossy ferroelectric secondary phase (Ba₃Nb₂O₈), and changes to composition and stoichiometry of BMN grains.     

Microwave dielectric properties of Bi-substituted La(Mg1/2Ti1/2)O3

The system La(Mg_1/2Ti_1/2)O₃–Bi(Mg_1/2Ti_1/2)O₃ (LMT–BMT) was investigated in respect to formation of perovskite solid solutions based on lanthanum magnesium titanate. Single-phase perovskite (1 − x)LMT–xBMT ceramics (0 ≤ x ≤ 0.3) were prepared and their crystal structure and dielectric properties were studied. It has been found that within the solubility range the crystal structure of Bi-substituted LMT remains monoclinic, P2₁/n. Unit cell volume was evaluated to be almost independent on x, varying within the experimental error. Relative permittivity of the ceramics increases by almost a factor of 3 in the range 0 ≤ x ≤ 0.3 and its value is 40–45 at the compositional region where temperature coefficient of the resonant frequency passes a zero-value. Compositional and temperature variations of the dielectric parameters for LMT–BMT estimated at different frequency ranges are considered in comparison with those observed in other Bi-substituted ceramics based on LMT.     

Broadband dielectric spectroscopy of PbMg1/3Nb2/3O3–PbSc1/2Nb1/2O3 ceramics

Broadband dielectric spectroscopy results of various ordered and disordered (1 ? x)Pb(Mg_1/3Nb_2/3)O₃–(x)Pb(Sc_1/2Nb_1/2)O₃ (PMN–PSN) ceramics are investigated in the temperature range from 80 K to 300 K and frequency range from 20 Hz to 2 THz. Dielectric dispersion is very broad and in the ferroelectrics case (x = 1, 0.95) consists of two parts: low-frequency part caused by ferroelectric domains and higher frequency part caused by soft mode. The relaxational soft mode exhibits pronounced softening close to phase transition temperature, as it is typical for order–disorder phase transitions. By substituting Sc³⁺ by Mg²⁺ in PMN–PSN ceramics relaxation slows down, and for relaxors (x = 0.2) the most probable relaxation frequency decreases on cooling according to Vogel–Fulcher law.     

Structural and electrical properties of bismuth magnesium tantalate pyrochlores

The subsolidus cubic pyrochlore phases in the Bi₂O₃–MgO–Ta₂O₅ (BMT) system were prepared with the proposed formula, Bi_3+(5/2)xMg_2?xTa_3?(3/2)xO_14?x (0.12 ≤ x ≤ 0.22). Replacement of smaller cations, Mg²⁺ and Ta⁵⁺ by larger Bi³⁺ cations with considerable oxygen non-stoichiometry within structure was proposed. The synthesised samples were confirmed phase pure by X-ray powder diffraction and their refined lattice parameters were in the range of 10.5532(4)–10.5672(9) Å. The grain sizes of the samples determined by SEM analysis were in the range of 0.6–10.60 μm and their average relative densities were more than 80%. Five infrared-active modes were also observed in their FTIR spectra due to their metaloxygen bonds. The BMT pyrochlores were highly electrical resistive with high dielectric constants, ?′ in the range of ～70–85; dielectric losses, tan δ in the order of 10^?3 at frequency 1 MHz and a negative temperature coefficient of permittivities, TC?′ of ～?158 to ?328 ppm/°C.     

Structure and dielectric properties of perovskite ceramics in the system Ba(Ni1/3Nb2/3)O3–Ba(Zn1/3Nb2/3)O3

Ceramics in the system Ba(Ni_1/3Nb_2/3)O₃–Ba(Zn_1/3Nb_2/3)O₃ (BNN–BZN) were prepared by the mixed oxide route. Powders were mixed and milled, calcined at 1100–1200 °C then pressed and sintered at temperatures in the range 1400–1500 °C for 4 h. Selected samples were annealed or slowly cooled after sintering. Most products were in excess of 96% theoretical density. X-ray diffraction confirmed that all specimens were ordered to some degree and could be indexed to hexagonal geometry. Microstructural analysis confirmed the presence of phases related to Ba₅Nb₄O₁₅ and Ba₈Zn₁Nb₆O₂₄ at the surfaces of the samples. The end members BNN and BZN exhibited good dielectric properties with quality factor (Qf) values in excess of 25,000 and 50,000 GHz, respectively, after rapid cooling at 240 °C h⁻¹. In contrast, mid-range compositions had poor Qf values, less than 10,000 GHz. However, after sintering at 1450 °C for 4 h and annealing at 1300 °C for 72 h, specimens of 0.35(Ba(Ni_1/3Nb_2/3)O₃)–0.65(Ba(Zn_1/3Nb_2/3)O₃) exhibit good dielectric properties: τ_f of +0.6 ppm °C⁻¹, relative permittivity of 35 and quality factor in excess of 25,000 GHz. The improvement in properties after annealing is primarily due to an increase in homogeneity.     

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.     

Effect of nonstoichiometry on the structure and microwave dielectric properties of Ba(Co1/3Nb2/3)O3 ceramics

The effect of B-site cation deficiency on the structure and microwave dielectric properties of Ba(Co_1/3Nb_2/3)O₃ (BCN) was investigated. Stoichiometric and co-deficient compositions based on Ba(Co_1/3−xNb_2/3)O₃ [x = 0.0, 0.01, 0.02, 0.03 and 0.04] were prepared using the conventional mixed oxide route. Small amounts of V₂O₅ (0.1 wt%) were added to promote densification. The dielectric loss is very sensitive to the composition; it was found that co-deficiency degraded the microwave dielectric properties. The stoichiometric formulation (x = 0) exhibited the best microwave properties. The improvements in the microwave dielectric properties were achieved by increasing the degree of 1:2 cation ordering. The highly ordered, stoichiometric BCN ceramics showed a relative permittivity (ɛ_r) of 32, quality factor (Q × f) of 66,500 GHz and a negative temperature coefficient of resonant frequency (τ_f) of −10 ppm/°C at 4 GHz.     

Microstructure and high-temperature thermoelectric properties of polycrystalline CuAl1−xMgxO2 ceramics

We investigated the effect of Mg-substitution on the microstructure and high-temperature thermoelectric properties of CuAl_1−xMg_xO₂ (0 ≤ x ≤ 0.2) fabricated by the tape casting method. The sintered CuAl_1−xMg_xO₂ bodies crystallized in CuAl_1−xMg_xO₂ solid solutions along with MgAl₂O₄ and Cu₂MgO₃ phases. Mg substitution up to x = 0.12 in the CuAl_1−xMg_xO₂ yielded a higher electrical conductivity and lower Seebeck coefficient mainly because of an enhanced carrier density. The highest value of the power factor (3.47 × 10⁻⁵ W m⁻¹ K⁻²) was attained for CuAl_0.88Mg_0.12O₂ at 800 °C. It is demonstrated that Mg substitution is highly effective for improving high-temperature thermoelectric properties.     

Effect of non-stoichiometry on the densification,phase purity,microstructure, crystal structure,and dielectric loss of Ba(Co1/3Nb2/3)O3 ceramics

The structural, vibrational, densification, and microwave properties of Ba(Co_1/3Nb_2/3)O₃ ceramics with small compositional variations along several tie lines in the ternary BaOCoONb₂O₅ diagram were studied. The results showed that very small deviation from stoichiometric Ba(Co_1/3Nb_2/3)O₃ composition has profound effect on Q × f, degree of ordering, densification, and phase assemblage. The 0.94 Ba(Co_1/3Nb_2/3)O₃–0.06 Ba₅Nb₄O₁₅ ceramic has the highest Q × f value (71 THz) – a value two times larger than that of stoichiometric Ba(Co_1/3Nb_2/3)O₃ (36 THz). Transformation from the (partial) disordered distribution of Co and Nb cations to 1:2 ordered arrangement in the octahedral sites was found to increase the Q factor of the high density and single phase ceramics. It was also observed that formation of very small amount of Ba₉CoNb₁₄O₄₅ second phase degraded Q × f value severely for the dense and highly ordered Nb-rich and Ba-deficient ceramics.     

Composition dependent structure,dielectric and energy storage properties of Pb(Tm1/2Nb1/2)O3-Pb(Mg1/3Nb2/3)O3 antiferroelectric ceramics

In order to stabilize the perovskite structure and improve the storage energy density (U) of Pb(Tm_1/2Nb_1/2)O₃ (PTmN) based materials, Pb(Mg_1/3Nb_2/3)O₃ (PMN) was introduced into PTmN to form binary (1-x)PTmN-xPMN solid solution ceramics. The XRD patterns show that all the compositions belong to orthorhombic phase with space group Pbnm. The Curie temperature (T_C) gradually decreases while the dielectric constant (ε') increases for (1-x)PTmN-xPMN with increasing PMN content. The ε' of each composition above T_C obeys the Curie-Weiss law. The appearance double hysteresis loop confirms the antiferroelectric nature of (1-x)PTmN-xPMN (x = 0.02–0.18) ceramics. With the increase of PMN concentration, the maximum polarization slowly increases from 8.58 μC/cm² to 29.5 μC/cm² while the threshold electric field (E_A-F) gradually declines from 290 kV/cm to 120 kV/cm. The maximum of U (3.12 J/cm³) is obtained in 0.92PTmN-0.08PMN ceramic with moderate E_A-F = 220 kV/cm, which makes (1-x)PTmN-xPMN ceramics safe in practical application.     

Structure and microwave dielectric properties of La(2−x)/3Nax(Mg1/2W1/2)O3

The structure evolution, sintering behavior and microwave dielectric properties of La_(2−x)/3Na_x(Mg_1/2W_1/2)O₃ (x = 0–0.5) were investigated in this paper. The X-ray diffraction (XRD) results show that all samples exhibit single phase, and the structure changed from orthorhombic when 0 ≤ x < 0.3 to monoclinic phase when 0.3 ≤ x ≤ 0.5. The size and ordering degree of A/B-site domains decrease with the increase in x value. The sintering temperature of the Na-doped samples increased compared to the pure La_2/3(Mg_1/2W_1/2)O₃ (LMW) due to the estimated decrease in the concentration of A-site vacancies. The addition of Na⁺ ion does not affect the dielectric permittivity greatly. The Q × f value decreases with the increase in x value, although the estimated concentration of A-site vacancies decreases with increasing x, which may be ascribed to the decrease of A/B-site ordering and domain size with the increase in x. The temperature coefficient of resonant frequency changed from negative values into positive values with the increase in x value.     

Temperature-insensitive large electrostrains and electric field induced intermediate phases in (0.7 − x)Bi(Mg1/2Ti1/2)O3–xPb(Mg1/3Nb2/3)O3–0.3PbTiO3 ceramics

Large electrostrains (∼0.4% at 7 kV/mm) of the new (0.7 − x)Bi(Mg_1/2Ti_1/2)O₃–xPb(Mg_1/3Nb_2/3)O₃–0.3PbTiO₃ ((0.7 − x)BMT–xPMN–0.3PT) ceramics with x = 0.0–0.7 near the morphotropic phase boundary of the BMT–PT–PMN ternary system were found to be associated with the evolution of the compositionally modulated dielectric relaxor behavior. As disclosed by in situ high-resolution synchrotron X-ray diffraction, more intermediate phases of different symmetries induced by external fields largely contribute to smaller strain hysteresis (∼23%) of PMN-rich compositions than BMT-rich ones. Particularly, the x = 0.2 sample exhibits temperature-insensitive electrostrains of ∼0.3% at 5 kV/mm varying less than 10% from room temperature up to 160 °C, which is mainly attributed to the coexistence of two kinds of relaxor states in a broad temperature range owing to special static domain structures typical for BMT-rich compositions. These findings provide a new approach toward an appropriate compositional design for low-hysteresis temperature-insensitive large electrostrains in BMT-based relaxor ferroelectrics.     

Abnormal variation of microwave dielectric properties in A/B site co-substituted (Ca1−0.3xLa0.2x)[(Mg1/3Ta2/3)1−xTix]O3 complex perovskite ceramics

A/B site co-substituted (Ca_1?0.3xLa_0.2x)[(Mg_1/3Ta_2/3)_1?xTi_x]O₃ ceramics (0.1 ≤ x ≤ 0.5) were prepared by solid state reaction and the structures, microstructures and dielectric properties were investigated. B site 1:2 cation ordering and oxygen octahedra tilting lead to monoclinic symmetry with space group P2₁/c for x = 0.1. For x above 0.1, the ordering was destroyed and the crystal structure became orthorhombic with space group Pbnm. The B site 1:2 cation ordering tended to be destroyed to form 1:1 ordering by the A site La³⁺ substitution. The dielectric constant increased linearly with increasing content of Ti⁴⁺ as the increasing second Jahn–Teller distortion enhanced the B site cation rattling. The temperature coefficient of resonant frequency and Qf values showed abnormal variations, which were refined to be caused by the increasing A site cation vacancy and diffused distribution of small size ordering domains respectively. Good combination of microwave dielectric properties was obtained at x = 0.5, where ?_r = 48, Qf = 21,000 GHz and τ_f = 2.2 ppm/°C.     

Microwave dielectric properties of (Zn1/3B2/3)0.5(Ti0.8M0.2)0.5O2 (B = Nb5+, Ta5+, M = Sn4+, Ge4+) ceramics

Dielectric properties of (Zn_1/3Nb_(2?x)/3Ta_x/3)_0.5(Ti_0.8Sn_0.1Ge_0.1)_0.5O₂ (x = 0, 1, 2) and/or (Zn_1/3Nb_1/3Ta_l/3)_0.5(Ti_0.8Sn_0.2(l?y)Ge_0.2y)_0.5O₂ (y = 0, 0.5, 1) were investigated at the microwave frequencies. For the compositions with single phase of rutile structure, the dielectric constant (K) of specimens was not only dependent on the dielectric polarizabilities, but also on the bond length ratio of apical bond (d_apical) to equatorial bond (d_equatorial) of oxygen octahedron in the unit cell. Temperature coefficients of the resonant frequencies (TCF) of the specimens with B = Nb⁵⁺ and/or M = Sn⁴⁺ was larger than those with B = Ta⁵⁺ and/or M = Ge⁴⁺. These results could be attributed to the changes of the degree of oxygen octahedral distortion. Quality factors (Qf) of the specimens with B = Ta⁵⁺ and/or M = Sn⁴⁺ were larger than those with B = Nb⁵⁺ and/or M = Ge⁴⁺.