Microwave Dielectric Properties and Thermally Stimulated Depolarization Currents Study of (1−x)Ba0.6Sr0.4La4Ti4O15–xTiO2 Ceramics

Microwave dielectric properties and thermally stimulated depolarization currents (TSDC) of (1?x)Ba_0.6Sr_0.4La₄Ti₄O₁₅–xTiO₂ (x = 0, 0.01, 0.02, 0.05, and 0.1) ceramics were studied. X‐ray diffraction analysis indicates that the specimens show a hexagonal perovskite structure; however, with an increase of x to 0.1, TiO_2?δ as a secondary phase can be detected in the ceramics. The variation of TiO₂ content has a significant effect on the dielectric properties of (1?x)Ba_0.6Sr_0.4La₄Ti₄O₁₅–xTiO₂ at microwave frequency. The dielectric permittivity of ceramics increases from 44 to 49 with the increase of TiO₂ content. The Qf value is in the range of 39 300–53 400 GHz. However, the temperature coefficient of resonant frequency (τ_f) changes from ?7.5 to–9.4 ppm/°C, and then turns to +3.9 ppm/°C. A near zero τ_f value can be obtained by tuning the content of TiO₂ addition. TSDC was also employed to analysis the extrinsic loss mechanism. Utilizing a fixed polarization condition, the TSDC relaxation peaks are present, which are generated by oxygen vacancies. And the concentration of oxygen vacancies increases with the increase of TiO₂ content. It can be concluded that the extrinsic dielectric loss is dominated by microstructure and oxygen vacancy defects.     

Microwave Dielectric Properties and Thermally Stimulated Depolarization Currents of (1−x)Ba(Mg1/3Nb2/3)O3–xBaSnO3 Solid Solutions

Microwave dielectric ceramics of (1?x)Ba(Mg_1/3Nb_2/3)O₃‐xBaSnO₃ [(1?x)BMN‐xBS] with high quality factors was synthesized by the solid‐state reaction method. The effects of BaSnO₃ additions (x = 0–0.2) on the sinterability, crystal structures, microwave dielectric properties, and microwave dielectric loss mechanisms of BMN were investigated systematically. The degree of 1:2 cation ordering was decreased with increasing Sn content and eventually faded away as x ≥ 0.1, where the low‐temperature relaxations disappeared coincidently through the thermally stimulated depolarization current technique. It was supposed to be the short‐range misplacements of the B‐site cations within the long‐range ordered structure. Meanwhile, the high‐temperature relaxations associated with the in‐grain oxygen vacancies were found in all the title compounds. Though the concentrations of oxygen vacancies of 0.8BMN‐0.2BS were higher than BMN, high Q × f values could also be obtained even in the absence of 1:2 cation ordering. Specifically, the excellent characteristics like ε_r = 29.02, Q × f = 90 000 GHz and τ_f = 6.3 ppm/°C were achieved in the specimens of x = 0.2 sintered at 1450°C.     

Microwave Dielectric Properties and Thermally Stimulated Depolarization Currents of (1 − x)MgTiO3–xCa0.8Sr0.2TiO3 Ceramics

(1 ? x)MgTiO₃–xCa_0.8Sr_0.2TiO₃ (0.04 ≤ x ≤ 0.2, MT‐CST) composite ceramics were prepared by the conventional solid‐state reaction process. The phase composition, microwave dielectric properties, and microwave dielectric loss mechanisms were studied. Ca_0.8Sr_0.2TiO₃ was employed as a τ_f compensator for MgTiO₃, and they coexisted well without forming any secondary phases. Interestingly, significant dielectric relaxations associated with oxygen vacancy defects were observed in all the MT‐CST ceramics through the dielectric‐temperature spectra. Thermally simulated depolarization current was therefore conducted to obtain the defects associated with extrinsic dielectric loss mechanisms. The concentrations of both defect dipole and in‐grain oxygen vacancies increased with the increasing x, which could induce microwave dielectric loss consequently. It demonstrated that the behaviors of Q × f were basically influenced by phase composition and defects here. Temperature‐stable ceramics can be achieved at x = 0.06, where the microwave dielectric properties were ε_r = 21.19, Q × f = 110 900 GHz (f = 9.295 GHz), and τ_f = ?0.9 ppm/°C, respectively.     

Microwave Dielectric Properties and Thermally Stimulated Depolarization Currents of MgF2‐Doped Diopside Ceramics

A new low‐fired dielectric material derived from CaMg_0.9Zn_0.1Si₂O₆ (CMZS) ceramics with high quality factor was synthesized by solid‐state reaction method. The effects of MgF₂ addition on the sinterability, phase composition, crystal defects, and microwave dielectric properties of CMZS were investigated. MgF₂ was proved not only to lower the sintering temperature to ~1000°C but also to remarkably modify the microwave dielectric properties of CMZS. In addition to the main diopside phase, forsterite was identified as the secondary phase in all MgF₂‐doped samples. Dielectric temperature spectra showed that MgF₂ induced significant dielectric relaxations associated with oxygen vacancy defects to CMZS. Thermally stimulated depolarization current was, therefore, considered to obtain the defects associated with extrinsic microwave dielectric loss mechanisms. Compared with undoped CMZS, although the concentration of oxygen vacancies showed a notable increase in the 5 wt% MgF₂‐doped CMZS, the Q×f values were still improved. Here, with proper MgF₂‐doping, it demonstrated that the microwave dielectric loss was basically influenced by phase composition. The excellent characteristics of ε_r = 7.78, Q×f = 151 800 GHz, and τ_f = ?26.40 ppm/°C were achieved from the 5 wt% MgF₂‐doped specimens sintered at 1000°C.     

Effects of Postdensification Annealing upon Microstructures and Microwave Dielectric Characteristics in Ba((Co0.6−x/2Zn0.4−x/2Mgx)1/3Nb2/3)O3 Ceramics

Effects of postdensification annealing upon microstructures and microwave dielectric characteristics in Ba((Co_0.6?x/2Zn_0.4?x/2Mg_x)_1/3Nb_2/3)O₃ (x = 0, 0.1, 0.2, and 0.3) complex perovskite ceramics have been investigated. Long‐time annealing at temperatures below the order–disorder transition temperature enhances the cation ordering degree and promotes the ordering domain growth. The most significant improvement of Qf value is obtained together with the suppressed temperature coefficient of resonant frequency in the samples annealed at 1400°C for 12 h, while the dielectric constant decreases slightly. The Qf value of ceramics annealed at 1400°C mainly attributes to the enhanced cation ordering degree, because their low‐energy domain boundaries are not detrimental to the Qf value. As the annealing temperature increases close to the transition temperature, coarse ordering domains with high‐energy boundaries are formed, and then the Qf value steadily decreases because of the inferior domain structure, even the cation ordering degree increases. The microwave dielectric characteristics of Ba((Co_0.6?x/2Zn_0.4?x/2Mg_x)_1/3Nb_2/3)O₃ ceramics are affected by the common function of ordering degree and domain structure. The best combination of microwave dielectric characteristics is obtained in the composition of x = 0.3 after annealing at 1400°C for 12 h: ε_r = 33.2, Qf = 117 200 GHz, and τ_f = 8.6 ppm/°C.     

钨青铜陶瓷Sr0.6Ba0.4Nb2O6的合成、结构与介电性能

通过固相反应法合成了Sr0．6Ba0．4Nb2O6陶瓷，并对其进行了结构、介电性能的表征。结果表明Sr0．6Ba0．4Nb2O6陶瓷为四方钨青铜结构单相，其在60℃附近存在一个明显的弥散介电峰，峰值温度随频率向高温偏移，为典型的弛豫铁电相变。室温时，10kHz频率下，其介电常数约为1404，介电损耗为0．03。     

Low Dielectric Loss and Good Dielectric Thermal Stability of xNd(Zn1/2Ti1/2)O3–(1−x)Ba0.6Sr0.4TiO3 Thin Films Fabricated by Sol–Gel Method

xNd(Zn_1/2Ti_1/2)O₃–(1?x)Ba_0.6Sr_0.4TiO₃ (xNZT–BST) thin films were fabricated on Pt/Ti/SiO₂/Si substrates by sol–gel method with x = 0, 3%, 6%, and 10%. The structures, surface morphology, dielectric and ferroelectric properties, and thermal stability of xNZT–BST thin films were investigated as a function of NZT content. It was observed that the introduction of NZT into BST decreased grain size, dielectric constant, ferroelectricity, tunability, and significantly improved dielectric loss and dielectric thermal stability. The corresponding reasons were discussed. The 10%NZT–BST thin film exhibited the least dielectric loss of 0.005 and the lowest temperature coefficient of permittivity (TCP) of 3.2 × 10^?3/°C. In addition, the figure of merit (FOM) of xNZT–BST (x = 3%, 6%, and 10%) films was higher than that of pure BST film. Our results showed that the introduction of appropriate NZT into BST could modify the dielectric quality of BST thin films with good thermal stability. Especially for the 3%NZT–BST thin film, it showed the highest FOM of 33.58 for its appropriate tunability of 32.87% and low dielectric loss of 0.0098.     

凝胶预碳化处理对Ca0.4Sr0.6Bi4Ti4O15纳米晶粉体的影响

用凝胶预碳化处理工艺的溶胶-凝胶法制备了晶粒粒径小且分散性能较好的钙锶铋钛(Ca0.4Sr0.6Bi4Ti4O15)纳米晶粉体.借助差热-热重分析仪、X射线衍射仪和扫描电镜等分别确定凝胶的预碳化处理温度,研究了预碳化处理工艺对粉体的物相结构、粉体的微观形貌以及分散性能的影响,并分析讨论了预碳化机理.结果表明:在300℃对前驱体凝胶进行预碳化处理增强了粉体的分散性,降低粉体的粒度,提高粉体的均匀性.凝胶预碳化处理工艺并未对粉体的物相结构造成影响.经过预碳化处理制备的粉体的颗粒尺寸集中在100nm左右;未经预碳化处理的粉体的颗粒尺寸为100nm～1 μm.凝胶预碳化处理后,高吸附活性的有机碳包覆在前驱体的表面是有效减少粉体团聚的原因.     

Evaluation of Using LaNi0.6Fe0.4O3–δ Contact Layer Between La0.6Sr0.4FeO3 Cathode and Crofer22APU Interconnect for Solid Oxide Fuel Cells

Interconnect‐cathode interfacial adhesion is important for the durability of solid oxide fuel cell (SOFC). Thus, the use of a conductive contact layer between interconnect and cathode could reduce the cell area specific resistance (ASR). The use of La_0.6Sr_0.4FeO₃ (LSF) cathode, LaNi_0.6Fe_0.4O_3–δ (LNF) contact layer and Crofer22APU interconnect was proposed as an alternative cathode side. LNF‐LSF powder mixtures were heated at 800 °C for 1,000 h and at 1,050 °C for 2 h and analyzed by X‐Ray power diffraction (XRD). The results indicated a low reactivity between the materials. The degradation occurring between the components of the half‐cell (LSF/LNF/Crofer22APU) was studied. XRD results indicated the formation of secondary phases, mainly: SrCrO₄, A(B, Cr)O₃ (A = La, Sr; B = Ni, Fe) and SrFe₁₂O₁₉. Scanning electron microscopy with energy dispersive X‐Ray spectroscopy (SEM‐EDX) and the X‐Ray photoelectron spectroscopy (XPS) analyzes confirmed the interaction between LSF/LNF and the metallic interconnect due to the Cr vaporization/migration. An increment of the resistance of ∼0.007 Ω cm² in 1,000 h is observed for (LSF/LNF/Crofer22APU) sample. However, the ASR values of the cell without contact coating, (LSF/Crofer22APU), were higher (0.31(1) Ω cm²) than those of the system with LNF coated interconnect (0.054(7) Ω cm²), which makes the proposed materials combination interesting for SOFC.     

Characterization of La0.6Sr0.4Co0.2Fe0.8O3–δ + La2NiO4+δ Composite Cathode Materials for Solid Oxide Fuel Cells

The structure, electrical conduction, thermal expansion and electrochemical properties of the La_0.6Sr_0.4Co_0.2Fe_0.8O_3–δ + La₂NiO_4+δ (LSCF‐LNO) composite cathodes were investigated with regard to the volume fraction of the LNO composition. No chemical reaction product between the two constituent phases was found for the composite cathodes sintered at 1,400 °C for 10 h within the sensitivity of the XRD. Compared to the performance of the LSCF cathode, the LNO composition in the composite cathode plays a role in deteriorating both electrical conductivity and electrochemical properties, however, improving the thermal expansion properties. The trade‐off between electrical conducting and thermal expansion classifies the composite cathode containing 30 volume percent (vol.%) LNO as the optimum composition. For characterizing cathode performance in a single cell, a slurry spin coating technique was employed to prepare a porous cathode layer as well as a YSZ/Ce_0.8Sm_0.2O_3–δ (SDC) electrolyte. The optimum conditions for fabricating the YSZ/SDC electrolyte were investigated. The resulting single cell with 70 vol.% LSCF‐30 vol.%LNO (LSCF‐LNO30) cathode shows a power density of 497 mW cm^–2 at 800 °C, which is lower than that of the cell with a LSCF cathode, but still within the limits acceptable for practical applications.     

Flame Spray Synthesis of Nanoscale La0.6Sr0.4Co0.2Fe0.8O3–δ and Ba0.5Sr0.5Co0.8Fe0.2O3–δ as Cathode Materials for Intermediate Temperature Solid Oxide Fuel Cells

Electrochemical performance and degradation was analysed by conductivity measurements as well as thermogravimetric analysis (TGA) under different atmospheres. CO₂ was identified as a critical parameter in terms of carbonate formation from Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3–δ and causes a strong increase in the material resistivity, whereas La_0.6Sr_0.4Co_0.2Fe_0.8O_3–δ is unaffected. The oxygen exchange kinetic of both compositions is affected by CO₂ containing atmospheres.     

A Novel Magnetodielectric Solid Solution Ceramic 0.4LiFe5O8–0.6Li2MgTi3O8 with Excellent Microwave Dielectric Properties

In this study, a novel spinel solid solution ceramic of 0.4LiFe₅O₈–0.6Li₂MgTi₃O₈ (0.4LFO–0.6LMT) has been developed and investigated. It is found that the 40 mol% LiFe₅O₈ and 60 mol% Li₂MgTi₃O₈ are fully soluble in each other and a disordered spinel phase is formed. The ceramic sample sintered at 1050°C/2 h exhibits both good magnetic and dielectric properties in the frequency range 1–10 MHz, with a permeability between 29.9~14.1 and magnetic loss tangent between 0.12~0.67, permittivity between 16.92~16.94 and dielectric loss tangent between 5.9 × 10^?3–2.3 × 10^?2. The sample also has good microwave dielectric properties with a relative permittivity of 16.1, a high quality factor (Q × f) ~28 500 GHz (at 7.8 GHz). Furthermore, 3 wt% H₃BO₃–CuO (BCu) addition can effectively lower the sintering temperature to 925°C and does not degrade the magnetodielectric properties. The chemical compatibility with silver electrode indicates that this kind of ceramics is a good candidate for the low‐temperature cofired ceramic (LTCC) application.     

Structure Stability and Microwave Dielectric Properties of (1 − x) Ba(Mg1/2W1/2) O3 + xBa(Y2/3W1/3)O3 Ceramics

The structure stabilities of double perovskite ceramics‐ (1 ? x) Ba(Mg_1/2W_1/2)O₃ + xBa(Y_2/3W_1/3)O₃ (0.01 ≤ x ≤ 0.4) have been studied by X‐ray powder diffraction (XRD), scanning electron microscopy (SEM), and Raman spectrometry in this study. The microwave dielectric properties of the ceramics were studied with a network analyzer at the frequency of about 8–11 GHz. The results showed that all the compounds exhibited face‐centered cubic perovskite structure. Part of Y³⁺ and W⁶⁺ cations occupied 4a‐site and the remaining Y³⁺ and Mg²⁺ distributed over 4b‐site, respectively, and kept the B‐site ratio 1:1 ordered. Local ordering of Y³⁺/Mg²⁺ on 4b‐site and Y³⁺/W⁶⁺ cations on 4a‐site within the short‐range scale could be observed with increasing Y‐doping content. The decomposition of the double perovskite compound at high temperature was successfully suppressed by doping with Y on B‐site. However, Ba₂Y_0.667WO₆ impurity phase appeared when x > 0.1. The optimized dielectric permittivity increased with the increase in Y doping. The optimized Q × f value was remarkably improved with small amount of Y doping (x ≤ 0.02) and reached a maximum value of about 160 000 GHz at x = 0.02 composition. Further increasing in Y doping led to the decrease in Q × f value. All compositions exhibited negative τ_f values. The absolute value of τ_f decreased with increasing Y‐doping content. Excellent combined microwave dielectric properties with ε_r = 20, Q × f = 160 000 GHz, and τ_f = ?21 ppm/°C could be obtained for x = 0.02 composition.     

Dielectric and Piezoelectric Properties of (1−x)K0.5Bi0.5TiO3–xBa(Ti0.8Zr0.2)O3 Ceramics

The properties of relaxor ceramics in the compositional series (1?x)K_0.5Bi_0.5TiO₃–xBa(Ti_0.8Zr_0.2)O₃ have been investigated. Values of T_m, the temperature of maximum relative permittivity, decreased from 380°C at x = 0.0 to below room temperature for x > 0.7. Compositions x = 0.1 and 0.2 were piezoelectric and ferroelectric. The maximum value of d₃₃ piezoelectric charge coefficient, 130 pC/N, and strain, 0.14%, occurred at x = 0.1. Piezoelectric properties of x = 0.1 were retained after thermal cycling from room temperature to 220°C, consistent with results from high‐temperature X‐ray diffraction indicating a transition to single‐phase cubic at ~300°C.     

Performance of Cobalt‐free La0.6Sr0.4Fe0.9Ni0.1O3–δ Cathode Material for Intermediate Temperature Solid Oxide Fuel Cells

A series of cobalt‐free perovskite‐type cathode materials La_0.6Sr_0.4Fe_1–xNi_xO_3–δ (0 ≤ x ≤ 0.15) for intermediate temperature solid oxide fuel cells (IT‐SOFCs) are prepared by a citric‐nitrate process. The conductivities of the cathode materials are measured as functions of temperature (300–800 °C) in air by AC impedance method, and the La_0.6Sr_0.4Fe_0.9Ni_0.1O_3–δ (LSFN10) has the highest conductivity to be 160 S cm^–1 at 400 °C. A single IT‐SOFC based on LSFN10 cathode, BaZr_0.1Ce_0.7Y_0.2O_3–δ electrolyte membrane and Ni–BaZr_0.1Ce_0.7Y_0.2O_3–δ anode substrate was fabricated by a simple spin‐coating process, and the performances of the cell using hydrogen as fuel and air as the oxidant were researched by electrochemical methods at 600–700 °C. The maximum power densities of the cell are 405 mW cm^–2 at 700 °C, 238 mW cm^–2 at 650 °C, and 140 mW cm^–2 at 600 °C, respectively. The results indicate that the LSFN10 is a promising cathode material for proton conducting IT‐SOFCs.     

Synthesis and conductivity behaviour of proton conducting (1−x)Ba0.6Sr0.4Ce0.75Zr0.10Y0.15O3−δ‐xGDC (x=0, 0.2, 0.5) composite electrolytes

An attempt has been made here to synthesize (1?x)Ba_0.6Sr_0.4CZY‐xGDC (x=0, 0.2, 0.5) composite electrolytes and investigated their phase(s), X‐ray photo spectra (XPS) and conduction properties. All compositions possess dual phases (perovskite‐type as well as cubic fluorite structure) and show proton conduction in various atmospheres. Homogeneous formation and compatibility between phases have been confirmed from X‐ray diffraction analysis. Detailed X‐ray photoelectron spectroscopy (XPS) studies on the oxidation states of barium, strontium, gadolinium, cerium, zirconium, yttrium, and oxygen was performed. With increasing “x”, oxygen vacancy concentration increases as cerium ions in 4+ oxidation state decreases. The conduction behavior of composites depicts the protonic in nature and total activation energy lying in the range of 0.16‐0.24 eV. This study indicates that the conductivity increases with GDC content in composite electrolytes and highest conductivity is found for composite with x=0.5. These characteristics are useful to make (1?x)Ba_0.6Sr_0.4CZY‐xGDC composite electrolytes as promising candidate of central membrane for advanced fuel cell technology.     

Optimisation and Evaluation of La0.6Sr0.4CoO3 – δ Cathode for Intermediate Temperature Solid Oxide Fuel Cells

In this work, La_0.6Sr_0.4CoO_{3 – δ}/Ce_{1 – x}Gd_xO_{2 – δ} (LSC/GDC) composite cathodes are investigated for SOFC application at intermediate temperatures, especially below 700 °C. The symmetrical cells are prepared by spraying LSC/GDC composite cathodes on a GDC tape, and the lowest polarisation resistance (R_p) of 0.11 Ω cm² at 700 °C is obtained for the cathode containing 30 wt.‐% GDC. For the application on YSZ electrolyte, symmetrical LSC cathodes are fabricated on a YSZ tape coated on a GDC interlayer. The impact of the sintering temperature on the microstructure and electrochemical properties is investigated. The optimum temperature is determined to be 950 °C; the corresponding R_p of 0.24 Ω cm² at 600 °C and 0.06 Ω cm² at 700 °C are achieved, respectively. An YSZ‐based anode‐supported solid oxide fuel cell is fabricated by employing LSC/GDC composite cathode sintered at 950 °C. The cell with an active electrode area of 4 × 4 cm² exhibits the maximum power density of 0.42 W cm^–2 at 650 °C and 0.54 W cm^–2 at 700 °C. More than 300 h operating at 650 °C is carried out for an estimate of performance and degradation of a single cell. Despite a decline at the beginning, the stable performance during the later term suggests a potential application.     

Stability of High‐Temperature Dielectric Properties for (1 − x)Ba0.8Ca0.2TiO3–xBi(Mg0.5Ti0.5)O3 Ceramics

Ceramics in the solid solution system, (1 ? x)Ba_0.8Ca_0.2TiO₃–xBi(Mg_0.5Ti_0.5)O₃, were prepared by a conventional mixed oxide route. Single‐phase perovskite‐type X‐ray diffraction patterns were observed for compositions x < 0.6. A change from tetragonal to single‐phase cubic X‐ray patterns occurred at x ≥ 0.1. Dielectric measurements indicated relaxor behavior for x ≥ 0.1. Increasing the Bi(Mg_0.5Ti_0.5)O₃ content improved the temperature sensitivity of relative permittivity ?_r at high temperatures. At x = 0.5, a near‐plateau relative permittivity, 835 ± 40, extended across the temperature range, 65°C–550°C; the permittivity increased at x = 0.6 to 2170 ± 100 for temperatures 160°C–400°C (1 kHz). The corresponding loss tangent, tanδ, was ≤0.025 for temperatures between 100°C and 430°C for composition x = 0.5; at x = 0.6, losses increased sharply at >300°C. Comparisons of dielectric properties with other materials proposed for high‐temperature capacitor applications suggest that (1 ? x)Ba_0.8Ca_0.2TiO₃–xBi(Mg_0.5Ti_0.5)O₃ ceramics are a promising base material for further development.     

Low‐Temperature Sinterable (1−x)Ba3(VO4)2–xLiMg0.9Zn0.1PO4 Microwave Dielectric Ceramics

The novel low‐temperature sinterable (1 ? x)Ba₃(VO₄)₂–xLiMg_0.9Zn_0.1PO₄ microwave dielectric ceramics were prepared by cofiring the mixtures of pure‐phase Ba₃(VO₄)₂ and LiMg_0.9Zn_0.1PO₄. The phase structure and grain morphology of the ceramics were evaluated using X‐ray diffraction, Raman spectra, and scanning electron microscopy. The results indicated that Ba₃(VO₄)₂ and LiMg_0.9Zn_0.1PO₄ phases can well coexist in the sintered body. Nevertheless, a small amount of LiZnPO₄ and some vanadate phases with low melting points were observed, which not only can influence the microwave dielectric properties of the ceramic but also can obviously improve the densification behavior at a relatively low sintering temperature. The near‐zero temperature coefficients of the resonant frequency (τ_f) could be achieved by adjusting the relative content of the two phases owing to their opposite τ_f values and simultaneously a desirable quality factor Q × f value can be maintained. No chemical reaction between the matrix ceramic phase and Ag took place after sintering at 800°C for 4 h. The ceramics with 45 vol% LiMg_0.9Zn_0.1PO₄ can be well sintered at only 800°C and exhibit excellent microwave dielectric properties of ε_r ~ 10, Q × f ~ 64 500 GHz, and τ_f ~ ?2.1 ppm/°C, thus showing a great potential as a low‐permittivity low‐temperature cofired microwave dielectric material.