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The effect of ZnO on phase composition, microstructure, thermal and dielectric properties of AlN ceramics was studied. The X-ray diffraction indicated that ZnAl2O4 and ZnO phases were detected in samples. With 0.5 wt% ZnO added, the thermal conductivity of AlN sample increased to 193.7 W/m·K. Whereas, the thermal conductivity dropped sharply due to excess ZnO. Moreover, the addition of ZnO increased the permittivity and altered the relaxation mechanism of AlN ceramics. The fitting data and activation energy from impedance spectroscopy revealed the variation of defect mechanisms in samples. The addition of 0.5 wt% ZnO reduced the concentration of aluminum vacancy in AlN grains to improve thermal conductivity. Nevertheless, excess ZnO dissolved into the AlN lattice and created additional defects, causing the decrease in thermal conductivity and electrical resistivity for AlN ceramics. Overall, suitable addition of ZnO can effectively increase the thermal conductivity and alter the dielectric properties of AlN ceramics. [ABSTRACT FROM AUTHOR]

High‐gain, low‐weight, wide bandwidth, and miniaturized lens antennas with stable properties against temperature are intensely required in extreme environments such as aerospace field. However, high‐performance microwave dielectric ceramics with near‐zero thermal expansion and frequency shift with temperature are very rare; moreover, the requirement of 3D printing processes also restricts their developments. Ba0.4Sr0.6Zn2Si2O7 with negative coefficient of thermal expansion (CTE), which originates from the stretched [ZnO4] tetrahedral chain to the twisted one along b‐axis with temperature, and CaTiO3 with positive temperature coefficient of resonant frequency (τf) can effectively adjust both CTE and τf of Zn1.8SiO3.8 to near‐zero value in 0.95(0.9Zn1.8SiO3.8–0.1Ba0.4Sr0.6Zn2Si2O7)–0.05CaTiO3 ceramic. Then, a Ku‐band Luneburg lens integrative antenna fabricated by stereolithography 3D printing technology exhibits an average gain of 8.06 dBi at 10.45–11.39 GHz and 10.3 dBi at 12.27–13.45 GHz, which has potential applications in temperature‐stable satellite communication. [ABSTRACT FROM AUTHOR]

The effect of oxidation in air on the phase composition, microstructure, flexural strength and thermal property of AlN ceramics was investigated. Oxidation was found to produce a continuous oxide layer on the surface of AlN samples. The flexural strength and thermal conductivity of AlN ceramics significantly improved after oxidation at 1000 °C and 1100 °C. Residual stress in the AlN ceramic matrix was enhanced by oxidation. The enhanced residual compressive stresses inhibited crack propagation and reduced interfacial thermal resistance, thereby improving the flexural strength and thermal conductivity of AlN ceramics. Electrochemical impedance spectroscopy was further used to analyze the defects in AlN ceramics. The increase in fitting grain resistance revealed a decrease in aluminum vacancy concentration in oxidized AlN sample, which resulted in high thermal conductivity. Therefore, oxidation at a certain temperature is pretty effective to obtain excellent performance for AlN ceramics. [ABSTRACT FROM AUTHOR]

Ba5LaMgNb9O30 ceramics with the addition of Li3NbO4 sintered under nitrogen atmosphere were synthesized and characterized. The phase structure, microstructure, dielectric properties and impedance spectroscopy of the prepared ceramics were investigated. All samples showed a diffuse phase transition and the diffuse exponent γ was calculated. With the addition of Li3NbO4, Li+ could enter the C-sites of BLMN ceramics, which modified the composition of the tungsten bronze structure and improved the performance. Results show that addition of Li3NbO4 decreased the sintering temperature of BLMN ceramics to 1050 °C and improved dielectric properties of nitrogen-sintered BLMN ceramics. Impedance spectroscopy analysis indicated that the performance for nitrogen-sintered ceramics was comparable to air-sintered samples at the temperature below 400 °C. The dielectric properties of BLMN ceramics were influenced by the amount of Li3NbO4, and the 15 wt% Li3NbO4-added BLMN ceramic sintered at 1050 °C in nitrogen for 3 h showed good dielectric properties at 1 MHz of εr = 122.19, tanδ = 0.00686 and τε = −631 ppm/°C. The Li3NbO4-added BLMN ceramics have application potential for the multilayer ceramic capacitors co-fired with base metal electrode. [ABSTRACT FROM AUTHOR]

(Ba 0.6 Sr 0.4 ) 0.85 Bi 0.1 TiO 3 (abbreviated as BSBT) ceramics doped with x wt% Zn 2 P 2 O 7 ( x  = 0, 1, 5, 10, 15) were synthesized. The influence of Zn 2 P 2 O 7 on microstructures and electrical properties of BSBT ceramics were studied. Microstructural observation indicated that Zn 2 P 2 O 7 doping could inhibit the growth of grain and improve the microstructure homogeneity of BSBT ceramics. Dielectric measurements exhibited a relaxor-like characteristic for all samples. Breakdown strength measurements indicated that undoped BSBT ceramics owned a breakdown strength of 122 kV/cm. However, with increasing Zn 2 P 2 O 7 content, the breakdown strength enhanced monotonously, and the maximum breakdown strength reached 326 kV/cm at x  = 15. Meanwhile, the energy storage efficiency was improved greatly by Zn 2 P 2 O 7 addition. The results suggest that BSBT ceramics doped with Zn 2 P 2 O 7 could be taken for a candidate for energy storage applications. In addition, the correlation between energy storage efficiency and interface polarization was qualitatively discussed by complex impedance spectra. [ABSTRACT FROM AUTHOR]

Ba1+1/ M[Si2O5+1/ M] low-permittivity microwave dielectric ceramics are prepared using the conventional solid-state method. Pure phases are obtained in barium silicates with M = 1, 3, 4, 5, and ∞, except for M = 7, in which two phases, Ba5Si8O21 and BaSi2O5, are observed. As the complexity of the crystal structure described by the Shannon information per reduced unit cell increases, the τf value tends to change from a negative to a positive value, except for M = 5, which has the highest complexity. A single Ba5Si8O21 phase with εr anomaly peak at −180°C exhibits a rare positive τf value (+25 ppm/°C), which is a novel temperature compensator. [ABSTRACT FROM AUTHOR]

Sintered in a reducing atmosphere, the dielectric properties of Ba(NdSm)TiO (BNST) ceramics are deteriorated because of the ionized oxygen vacancies and conduction electrons. The feasibility of reduction resistance in BNST ceramics sintered in a reducing atmosphere was investigated in this work. Prompted by the example of BaTiO ceramics used in base-metal electrode multilayer ceramics capacitor, the effects of acceptor dopant MnCO on the microstructure, insulation resistivity (IR) and dielectric properties of BNST ceramics doped with 1 wt% BaCuBO (BCB) sintered under a reducing atmosphere of moist N-1%H were investigated. Results showed that MnCO effectively rendered BNST-BCB ceramics resistant to reduction. Mn ions completely substituted into the Ti site of BNST at <1.5 wt% MnCO doping content, whereas secondary phases were formed with increased MnCO addition. Resistivity as high as 10 Ω cm was effectively achieved in ceramics with the addition of >1.5 wt% MnCO, and the dielectric constant ( ε ) of BNST-BCB was not obviously modified by the presence of additive. Moreover, 2 wt% MnCO-doped BNST-1wt% BCB ceramics sintered in a reducing atmosphere exhibited attractive properties with ε = 75.1, tanδ = 0.00115, TCC = −42.3 ppm/°C and IR = 4.56×10 Ω cm. [ABSTRACT FROM AUTHOR]

The effects of eutectic addition of 0.8LiF–0.2CaF 2 on microwave dielectric properties and structure of MgO ceramics were investigated. Addition of LiF–CaF 2 decreased the sintering temperature of MgO ceramics to 900 °C but did not affect the dielectric properties of the ceramics. The dielectric constant ( ε r ) and temperature coefficient of the resonant frequency ( τ f ) of MgO ceramics were not influenced by the LiF–CaF 2 eutectic content within the range of 0.02–0.08. However, the quality factor ( Q×f ) was dependent on the amount of LiF–CaF 2 added. Moreover, open or closed sintering condition influenced the microstructure and microwave dielectric properties of LiF–CaF 2 -doped MgO ceramics. The X-ray diffraction and impedance spectroscopy analyses indicated that Li and Ca substitution for Mg and F substitution for O occurred in MgO ceramics, and a high amount of F was incorporated into the MgO lattice under closed sintering condition. Overall, 0.96MgO–0.04(0.8LiF–0.2CaF 2 ) ceramics sintered at 950 °C in an open crucible exhibited excellent microwave dielectric properties of ε r =9.18, Q×f =130198 GHz and τ f =−65 ppm/°C. This study shows that eutectic addition of LiF–CaF 2 effectively decreases the sintering temperature of MgO without deteriorating the ceramic performance. [ABSTRACT FROM AUTHOR]

The effect of yttrium fluoride (YF 3) on the densification behavior, microstructure, phase composition and thermal conductivity of aluminium nitride (AlN) ceramics with yttrium oxide (Y 2 O 3) and YF 3 additives were studied. Since YF 3 provided liquid phases and promoted densification at a lower temperature, the sintering temperature required to reach the full density of AlN samples decreased with the increase in YF 3 content. Appropriate addition of YF 3 could improve the thermal conductivity of AlN ceramics, but the values of thermal conductivity decreased as YF 3 increased further. It is attributed to the ability of YF 3 to react with oxygen impurity was worse than that of Y 2 O 3. Moreover, the reducing atmosphere significantly affected the phase composition, and the oxygen content in grain boundary phases decreased at 1750 °C and 1800 °C. Therefore, the proper proportion of Y 2 O 3 –YF 3 additives could simultaneously improve densification and the thermal conductivity of AlN samples at a low sintering temperature. [ABSTRACT FROM AUTHOR]

The phase composition, microstructure, dielectric properties and breakdown strength of (Mg 0.9 Zn 0.1 ) 2 (Ti 1− x Mn x )O 4 ceramics synthesized using a conventional solid-state reaction were investigated. The substitution of Ti 4+ by Mn 2+ can form a single spinel phase and result in a homogenous microstructure, which improves breakdown strength and reduces dielectric loss. Impedance spectroscopy results show that grain and grain boundary exhibit similar impedance properties and that the Schottky barrier in the grain boundary cannot be observed. When the x value is equal to 0.01, (Mg 0.9 Zn 0.1 ) 2 (Ti 1− x Mn x )O 4 ceramics with a high breakdown strength of 102 kV/mm (at 0.2 mm) and a low dielectric loss of 5.3×10 −5 (at 6 GHz) can be obtained, which are beneficial in the development of high voltage solid-state pulse-forming lines. [ABSTRACT FROM AUTHOR]

Dense (1 − x) La[Al 0.9 (Mg 0.5 Ti 0.5) 0.1 ]O 3 – x CaTiO 3 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 0.5 Ti 0.5) 0.1 ]O 3 – x CaTiO 3 microwave dielectric ceramics was near zero. The optimal microwave dielectric properties of 0.35LaAl 0.9 (Mg 0.5 Ti 0.5) 0.1 O 3 –0.65CaTiO 3 were ε r = 44.6, Q × f = 32,057 GHz, and τ f = +2 ppm/°C. [ABSTRACT FROM AUTHOR]

The phase compositions and low-permittivity microwave dielectric properties of Ca 2 Sn (1− x) O (4−2 x) (0 ≤ x ≤ 1.0) ceramics were investigated through a solid-state reaction at 1350 °C-1450 °C for 5 h. A small Sn-deficiency is beneficial to inhibiting a CaSnO 3 second phase and forming a Ca 2 SnO 4 single phase with excellent microwave dielectric properties (ε r = 9.9, Q × f = 120,100 GHz, and τ f = −45.6 ppm/°C) in the Ca 2 Sn (1− x) O (4−2 x) (x = 0.02) ceramics. With an increase in x from 0.04 to 1.0, the amount of CaO phase increased, and the Ca 2 SnO 4 phase decreased gradually. Finally, a CaO pure phase (ε r = 10.6, Q × f = 229,600 GHz, and τ f = −37.5 ppm/°C) was obtained at x = 1.0. • The CaSnO 3 phase was difficult to be eliminated in preparing Ca 2 SnO 4 ceramics. • The excessive CaO can inhibit the CaSnO 3 second phase in Ca 2 SnO 4 ceramics. • The Ca 2 SnO 4 single phase ceramics were synthesised through Sn-deficiency. • Microwave dielectric properties of Ca 2 Sn (1− x) O (4−2 x) ceramics were investigated. [ABSTRACT FROM AUTHOR]

Abstract Low-fired fluoride microwave dielectric ceramics (LiF, CaF 2 , SrF 2 and BaF 2) were prepared through a simple one-step sintering process. Fluoride ceramics, especially LiF, which had the lowest sintering temperature of 800 °C, could be well sintered below 1050 °C. Rietveld refinement results showed that LiF, CaF 2 , SrF 2 and BaF 2 ceramics crystallized into a cubic structure with space group Fm -3 m. The relative permittivity (ε r), quality factor (Q × f) and temperature coefficient of the resonant frequency (τ f) of the fluoride ceramics were closely related to relative density, the ionic polarizability of the primitive unit cell, the packing fraction and the bond valence. In this series of low-permittivity fluoride ceramics, LiF, CaF 2 and BaF 2 could be co-fired with Ag powders, and LiF ceramic exhibited the highest Q × f value of 73880 GHz, which is comparable to those of traditional oxide microwave dielectric ceramics. [ABSTRACT FROM AUTHOR]

BaAl 2−2 x (ZnSi) x Si 2 O 8 ( x = 0.2–1.0) ceramics were prepared using the conventional solid-state reaction method. The sintering behaviour, phase composition and microwave dielectric properties of the prepared compositions were then investigated. All compositions showed a single phase except for x = 0.8. By substituting (Zn 0.5 Si 0.5 ) 3+ for Al 3+ ions, the optimal sintering temperatures of the compositions decreased from 1475 °C ( x = 0) to 1000 °C ( x = 0.8), which then slightly increased to 1100 °C ( x = 1.0). Moreover, the phase stability of BaAl 2 Si 2 O 8 was improved. A novel BaZnSi 3 O 8 microwave dielectric ceramic was obtained at the sintering temperature of 1100 °C. This ceramic possesses good microwave dielectric properties with ε r = 6.60, Q × f = 52401 GHz (at 15.4 GHz) and τ f = −24.5 ppm/°C. [ABSTRACT FROM AUTHOR]

Abstract: The microstructures, phase compositions and microwave dielectric properties of ZnAl2O4–TiO2 spinel-based composites have been investigated. It is found that ZnAl2O4 cannot form a solid solution with TiO2. As TiO2 content increases, the ε r and τ f values increase gradually, while the Q · f values degrade by degrees. Under the same amount of TiO2 content, the ε r and Q · f values increase initially and then decrease slightly with increasing sintering temperature, while the τ f values increase slowly. The optimal microwave dielectric properties are achieved in (1− x)ZnAl2O4–xTiO2 (x =0.21) sintered at 1500 °C for 3 h with ε r value of 11.4, Q · f value of 71,810 GHz (at about 6.5 GHz), and τ f value of −0.5 ppm/°C. [Copyright &y& Elsevier]