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BaSixO1+2x (1.61 ≤ x ≤ 1.90) and LiF-doped BaSi1.63O4.26 ceramics were prepared by using a traditional solid-state method at the optimal sintering temperatures. The evolution of phase compositions of BaSixO1+2x (1.61 ≤ x ≤ 1.9) ceramics was revealed. The coexistence of Ba5Si8O21 and Ba3Si5O13 phases was obtained in BaSixO1+2x (1.61 ≤ x ≤ 1.67) ceramics. The BaSi2O5 phase appeared inBaSixO1+2x (1.68 ≤ x ≤ 1.90) ceramics. At 1.68 ≤ x ≤ 1.69, only BaSi2O5 and Ba3Si5O13 phases existed. With the further increase in x, the Ba5Si8O21 phase appeared, and BaSi2O5, Ba5Si8O21 and Ba3Si5O13 phases coexisted in BaSixO1+2x (1.70 ≤ x ≤ 1.90) ceramics. The phase compositions of BaSixO1+2x (1.61 ≤ x ≤ 1.90) ceramics were controlled by the ratio of Ba:Si. The BaSixO1+2x (x = 1.68) ceramics with 98.15 wt% Ba3Si5O13 and 1.85 wt% BaSi2O5 phases exhibited a negative τf value (−37.53 ppm/°C), and the good microwave dielectric properties of εr = 7.51, Q × f = 13,038 GHz and τf = +3.95 ppm/°C were obtained for BaSi1.63O4.26 ceramics with 70.05 wt% Ba5Si8O21 and 29.95 wt% Ba3Si5O13 phases. The addition of LiF sintering aids were able to reduce the sintering temperatures of BaSi1.63O4.26 ceramics to 800 °C. The phase composition of BaSi1.63O4.26 ceramics was affected by the sintering temperature, and the coexistence of Ba5Si8O21, Ba2Si3O8, BaSi2O5 and SiO2 phases was achieved in BaSi1.63O4.26-3 wt% LiF ceramics. The BaSi1.63O4.26-3 wt% LiF ceramics sintered at 800 °C exhibited dense microstructures and excellent microwave dielectric properties (εr = 7.10, Q × f = 12,463 GHz and τf = +5.75 ppm/°C), and no chemical reaction occurred between BaSi1.63O4.26-3 wt% LiF ceramics and the Ag electrodes, which indicates their potential for low-temperature co-fired ceramic (LTCC) applications. [ABSTRACT FROM AUTHOR]

The traditional solid‐state reaction method was used to prepare Ca2Sn2−xMxAl2O9 (M = Ti, Zr, and Hf) ceramics. Then, the impact of an M4+ substitution of Sn4+ on the phase transition, crystal structural parameter, and microwave dielectric properties of Ca2Sn2−xMxAl2O9 (0 ≤ x ≤ 0.4) ceramics were investigated. Ti4+ could not replace the Sn4+ of Ca2Sn2Al2O9 due to its small ionic radius, and the Al‐based second phases of Ca2Sn2−xTixAl2O9 ceramics were confirmed by the X‐ray diffractometer and EDS map scanning results. With the Zr4+ and Hf4+ substitutions of Sn4+, the SnO2 and CaSnO3 second phases of Ca2Sn2Al2O9 ceramic were inhibited, and the Ca2Sn2−xMxAl2O9 (M = Zr and Hf) (0.05 ≤ x ≤ 0.2) single‐phase ceramics with orthorhombic structure (Pbcn space group) were obtained. New MO2 (M = Zr and Hf) and CaAl2O4 second phases appeared in the Ca2Sn2−xMxAl2O9 (M = Zr and Hf) (0.3 ≤ x ≤ 0.4) ceramics, and their contents increased gradually with the increase in x. The Ca2Sn2−xMxAl2O9 (M = Zr and Hf) (0.05 ≤ x ≤ 0.2) ceramics exhibited high Q × f because of their pure phase compositions, and the Q × f of Ca2Sn2Al2O9 ceramic was improved to 77 800 GHz (12.6 GHz) in the Ca2Sn1.9Zr0.1Al2O9 ceramic. The Q × f values of Ca2Sn2−xMxAl2O9 single‐phase ceramics were mainly controlled by rc (Sn/M–O) and rc (Al–O). The τf values of single‐phase Ca2Sn2−xMxAl2O9 ceramics were related to octahedral distortions. The Zr4+ and Hf4+ substitution of Sn4+ optimized the phase compositions and microwave dielectric properties of the Ca2Sn2−xMxAl2O9 ceramics, and the Ca2Sn1.9Zr0.1Al2O9 ceramic sintered at optimal temperature exhibited excellent microwave dielectric properties (εr = 8.67, Q × f = 77 800 GHz at 12.6 GHz and τf = −69.8 ppm/°C). [ABSTRACT FROM AUTHOR]

Hyperbola detection is an important application field of ground-penetrating radar (GPR) systems as underground threats and targets detected by these systems are presented in the form of a hyperbola. However, the low-amplitude hyperbola detection of deeply-buried targets and targets with low metal content has remained a major challenge due to the increase in the attenuation of radar echo with an increase in the detection depth and the features with low resolution extracted by existing methods. In this study, first, a high-level visual feature, namely, phase symmetry, is proposed to effectively improve the feature resolution and the robustness of amplitude change in GPR images. Then, we propose a handcrafted feature descriptor based on phase symmetry, namely, histogram of oriented vector phase symmetry (HOVPS) to improve the detection of hyperbola in GPR images. In constructing HOVPS, we first cite our previous work to enhance the descriptive ability of hyperbola in GPR images. Subsequently, we extend a phase symmetry model to develop a vector feature model [namely, vector phase symmetry (VPS)]. Finally, HOVPS is developed based on the VPS to extract structural information from GPR images. The proposed HOVPS describes the shape features in GPR images using a symmetrical structure, and it is used as the feature input of the classifier for hyperbola detection. The qualitative analysis of the proposed method is performed by comparing the performance of the proposed method for the extraction of features on different GPR data with those of other methods. In addition, we also provide quantitative analysis on different signal-to-noise ratio (SNR) test sets, and the results reveal that the proposed method outperforms various state-of-the-art methods (e.g., GPR histogram of oriented gradient (gprHOG), edge histogram descriptor (EHD), and log-Gabor (LG) feature). [ABSTRACT FROM AUTHOR]