Search

A CoSe2/N-doped carbon heterojunction with a three-dimensional porous structure is synthesized via a topological transformation process, which delivers superior sodium storage performance.

Novel ZrB2-matrix composites were designed and prepared by in-situ introducing SiC and Zr2[Al(Si)]4C5 simultaneously for the first time. The obtained composites were dense and showed good mechanical properties, especially the strength and toughness, 706 MPa and 7.33 MPa·m1/2, respectively, coupled with high hardness of 21.3 GPa, and stiffness of 452 GPa. SiC and Zr2[Al(Si)]4C5 constituted a reinforcing system with synergistic effects including grain refinement, grain pull-out as well as crack branching, bridging, and deflection. Besides, the oxidation results of the composites showed that the oxidation kinetics followed the parabolic law at 1600 °C, and the oxidation rate constants increased with the increase of Zr2[Al(Si)]4C5 content. The formation and evolution model of the oxidation structure was also investigated, and the oxide scale of the composite exhibited a three-layer structure.

To provide a basis for the high-temperature oxidation of ultra-high temperature ceramics (UHTCs), the oxidation behavior of Zr3[Al(Si)]4C6 and a novel Zr3[Al(Si)]4C6-ZrB2-SiC composite at 1500 °C were investigated for the first time. From the calculation results, the oxidation kinetics of the two specimens follow the oxidation dynamic parabolic law. Zr3[Al(Si)]4C6 exhibited a thinner oxide scale and lower oxidation rate than those of the composite under the same conditions. The oxide scale of Zr3[Al(Si)]4C6 exhibited a two-layer structure, while that of the composite exhibited a three-layer structure. Owing to the volatilization of B2O3 and the active oxidation of SiC, a porous oxide layer formed in the oxide scale of the composite, resulting in the degradation of its oxidation performance. Furthermore, the cracks and defects in the oxide scale of the composite indicate that the reliability of the oxide scale was poor. The results support the service temperature of the obtained ceramics.

Titanium carbide (TiC) fibers are attractive functional materials that find application in the areas of energy, environment, and biology. Herein, a simple precursor route is designed to prepare TiC fibers by means of electrospinning. The precursor is prepared by adding polyvinylpyrrolidone (PVP) to as-synthesized polytitanoxane (PTO), and the synthesis process, structure, and thermal behavior of the resulting TiC fibers are analyzed. Smooth and continuous precursor fibers with a uniform diameter of ~1.22 μm are obtained after the electrospinning of the precursor. With increase in the preparation temperature, the phase composition of the fibers evolves into TiC from TiCxOy via pyrolysis and carbothermal reduction. Chemical-state analysis results show that the surface of as-prepared TiC fibers exhibits very weak oxidation, thus leading to TiO2 and TiCxOy formation. In addition, all the prepared fibers show uniform morphologies without adhesion. With the proposed approach, continuous TiC fibers with a diameter of ~0.55 μm can be obtained.

In this letter, a dual-polarized frequency-selective absorptive structure is presented with two wideband transmission windows. The transmission windows are achieved within a wide –10-dB reflection band. At the frequencies of the passbands, the out-of-phase currents are excited without flowing through the lossy elements. Therefore, the passbands can be obtained with high transmittance. Moreover, only four resistors are loaded on each unit cell to consume the out-of-band reflective wave. By combining lossy flower-shaped and lossless arrays together, the desired dual-band and low reflection performances can be realized. Within the operating band, the reflection is below –10 dB with the fractional bandwidth of 127%. Meanwhile, around 6.5 and 12 GHz, the passband widths are 39% and 40% and the corresponding insertion losses are 0.07 and 0.1 dB, respectively. Finally, the fabricated prototype is measured to demonstrate the feasibility of the presented method.

Herein, in-situ Zr3[Al(Si)]4C6-based composites with 10–40 vol% ZrB2–SiC (2-to-1 molar ratio) were prepared by hot-pressing sintering at 1850 °C. The simultaneously incorporated ZrB2–SiC constitute multicomponent reinforcements and has a synergistic effect on the matrix, which improves the sinterability, mechanical properties, and oxidation resistance of materials. It is found that both of the toughness and strength increase first and then decrease with the increasing content of ZrB2–SiC, while the hardness increases near linearly. Zr3[Al(Si)]4C6–ZrB2–SiC shows high strength (623 MPa), toughness (7.59 MPa m1/2), and hardness (18.6 GPa), which can be ascribed to the synergistic mechanisms of the binary ZrB2–SiC including fine-grained strengthening, particle reinforcement, intragranular microstructure, grain's pull-out and crack bridging, etc. In addition, the oxidation kinetics of as-prepared materials follow the parabolic law, and the composite shows a low oxidation rate of 0.87 × 10−5 kg2 m−4 s−1 when oxidized at 1400 °C.

In this letter, a novel absorptive structure with a high-transmittance bandpass behavior is proposed based on the electromagnetically induced transparency-like effect. The desired response is achieved by employing cascaded lossy π-shaped and lossless dual-band reflective arrays. The lossy array consists of radiative and subradiant resonators. A resistor is loaded on the radiative resonator to consume the excited current within the stopband. The bright-dark mode coupling contributes to a transparency window with an insertion loss of 0.07 dB. By combining with the reflective array, two absorption bands are obtained at both sides of the passband, and the maximum absorptivity is above 98%. Meanwhile, the 10 dB reflection reduction behavior is realized within the operating band, and a good stealth performance is provided. The fabricated prototype is measured, and the simulation and experimental results are in a good agreement.

SiO2 nanoparticles (SiO2 NPs) modified 0.02B2O3–0.98SiO2 ceramics were prepared by solid state sintering. The microstructure and microwave dielectric properties were investigated. All the modified samples exhibited low-cristobalite phase with small amount of quartz. The quartz phase inhibited the cracking caused by crystallization process of low-cristobalite. The densification of 0.02B2O3–0.98SiO2 ceramics improved with SiO2 NPs addition. The introduction of 1.5 wt% SiO2 NPs brings excellent microwave dielectric properties with er = 4.4, Q × f = 42000 GHz, τf = − 5 ppm/°C for the samples sintered at 1250 °C, which can be a low-temperature sintering system as microwave substrate materials.

In this letter, a new frequency-selective absorber is presented that has wideband transmission/absorption/reflection reduction responses. The presented absorber is completed by lossy cross-dog-bone and lossless second-order frequency-selective surfaces. The modal interaction pole generated by the lossy array is employed to achieve a wideband transmission. Three absorption bands are combined to realize a continuous stopband with high absorptivity. Furthermore, the improved absorber structure is investigated to reduce the strong reflection between the absorption and transmission bands. The 10 dB reflection reduction is obtained with a fractional bandwidth of 126.4%. Meanwhile, the –3 dB fractional passband width is 41.3% with minimum insertion loss of 0.37 dB. The absorption bandwidth of 91.5% is provided with at least 90% absorptivity. The distance between the lossy and lossless layers is only 0.08 $\lambda _{L}$ at the lowest operating frequency. Finally, the proposed design is validated by a good agreement between the numerical simulations and experimental measurements.

(1-x)ZnGa2O4-xTiO2 (x = 0.05–0.20) composite ceramics were fabricated via traditional solid state method. The experimental and theoretical value of ɛr, τf and Q × f shows similar trend. Tunable microwave dielectric performance was achieved due to the effect of TiO2, especially for temperature coefficient and quality factor. The large negative temperature coefficient at resonant frequency (τf) of ZnGa2O4 was continuously tailored from negative (−70 ppm/oC) to positive (+13 ppm/oC) 2 For practical application, 15 mol% (about 6.9 V%) TiO2 can adjust τf value to near zero. The tailoring effect on quality factor (Q × f) was significant. Due to the smaller Q × f values of TiO2, the Q × f value decreases with the increasing content of TiO2. The inhomogeneous microstructure results in the increasing difference between calculated and experimented Q × f value. In addition, compositing TiO2 with ZnGa2O4 ceramic not only promote the grain growth but also reduce its sintering temperature from 1400 °C to 1300 °C. The dielectric constant (ɛr) was not significant affected by TiO2, only varied from 10.8 to 12.7. The ɛr value of 12.3, Q × f value of 73,000 GHz (at 11.8 GHz) and τf value of +3 ppm/°C were achieved for 0.85ZnGa2O4-0.15TiO2, sintered at 1300 °C for 2 h.

In this paper, a novel linear-to-circular polarization converter based on meander-line metasurfaces is presented. The proposed metasurfaces is a two-dimensional periodic structure which consists of two metallic layers separated by dielectric substrate. The structure of each unit is meander-line which is printed on the top and bottom metallic layers. By controlling different phase for vertical and horizontal electric field components of incident wave, the phase difference is 90° between two components, after passing through the metasurfaces. The metasurfaces converts the linear polarization wave to circular polarization wave with the bandwidth of 10.5~11.5 GHz. The simulation results show the metasurfaces is valuable for polarization conversion application.

A novel switch-type ultra-wideband (UWB) active frequency selective surfaces (AFSSs) for radar radome applications is proposed in this paper. The switch-type AFSSs includes two dielectric substrates layers and three metal layers. PIN diodes are arranged on the upper and lower metallic layers of the AFSSs, which can reconstruct the performance of the AFSSs. In the OFF-state, the transmission bandwidth of AFSSs with the insertion loss is less than 1 dB, is 7.95-11.82 GHz, and the fractional bandwidth is 39.2%. And in the OFF-state, the maximum transmission coefficient is -10 dB in the frequency range of 2–18 GHz. In addition, the AFSSs has good polarization insensitivity and oblique insensitivity characteristics. The numerical simulation results show that the AFSSs structure is a potential multifunctional radomes application scheme.

A compact circular-polarization discriminator is presented in this paper. The proposed structure is composed of three simple layers, which are connected by air spacers. The first and third layers act as circular polarisers (CPs), and the second one is a linear polarizer (LP). By combing the three layers together, the right-handed circularly-polarized wave (RHCPW) can be reflected, while the left-handed circularly-polarized wave (LHCPW) can be transmitted through the proposed structure. Within the operating band, the insertion loss is lower than 2.5 dB. Finally, the proposed design is validated by both simulated and measured results.

This paper presents a new compact circular-polarized Huygens source antenna with low profile. A circular-ring that can generate electric and magnetic resonances is employed to complete this design. The operating principle of the Huygens source is introduced. The input power with 90° phased differences is provided by a coupled feeding system. A low profile with a thickness of $\mathbf{0.046}\ \lambda_{\mathbf{0}}(\lambda_{\mathbf{0}}$ is the wavelength in free space at the center work-frequency) is achieved. The -10 dB operating bandwidth is 3.7%, which covers 1.57 GHz to 1.63 GHz. The fractional 3 dB axial ratio bandwidth is 2.75%. In addition, the maximum gain and the half-power beamwidth are 2.85 dBi and 128°, respectively. Finally, good simulated performances validate this concept.

A reconfigurable active frequency selective surface (AFSS) with ultra-wideband (UWB) characteristics is presented in this article. The proposed AFSS is a periodic array structure which consists of three metal layers and two dielectric layers. PIN diodes are arranged on the top and bottom metal layers which can rebuild the function of AFSS. The transmission bandwidth of the OFF-state AFSS is 8-12.5 GHz with a fractional bandwidth of 44%, and the transmission coefficient of the ON-state AFSS is lower than -10 dB from 2 to 18 GHz. Additionally, the AFSS has dual polarization characteristics, and it is angular stable and polarization-insensitive for two polarizations. The simulation results show the proposed AFSS is a valuable candidate for radome applications.

In this study, a circularly polarised (CP) patch array antenna based on reactive impedance surface (RIS) is presented. The proposed 4 × 4 patch array antenna possesses a RIS layer which is utilised to replace the conventional ground plane in order to enhance the bandwidth. Meanwhile, patches are cut with two diagonal corners and a rectangular slot for the fulfilment of wideband CP radiation and fed by a one-to-sixteen network simultaneously. Furthermore, in order to explain the effect of RIS in the axial ratio (AR), the CP antenna elements with (or without) RIS are designed. There is the same trend in simulated and measured results. Measured results show the proposed antenna exhibits good performances, such as wideband impedance matching, low AR, high gain. It is testified that pure CP radiation and symmetry broadside patterns are achieved in a wide frequency band, indicating the proposed antenna can exhibit significant application in global satellite communication systems and 5G Wi-Fi application.

(Ba 1- x Ln 2 x /3 )(Co 0.7 Mg 0.3 ) 1/3 Nb 2/3 O 3 (0.01 ≤ x ≤ 0.04, Ln=La, Nd) ceramics were synthesized by solid-state reaction process and their sintering behavior, microstructure and microwave dielectric properties were investigated in this paper. The X-ray diffraction results show that all the samples exhibit a single peroskite phase except for the Nd-substituted compositions with x > 0.02. The sinterability and grains distribution are improved by the La-substitution. Though the long range ordering degrees on B-site are similar in the Ln-substituted compositions, the size of ordered domains observed by HRTEM images are quite different. Compared with the Nd-subsituted compositions, the higher Q × f values in the La-substituted compositions are mainly attributed to the better densification and ordered domain structure with lower energy barriers. Both e r and τ f values increase slightly with the increase of Ln-substitution. A good combination of microwave dielectric properties was obtained in the La-substituted composition with x = 0.01: e r = 32.5, Q × f = 79812 GHz, τ f = 4 ppm/°C.

This paper presents a frequency-selective rasorber whose transmission window locates at the higher frequency of absorption band. The accomplished rasorber is composed of dipole-like and slot arrays, and has realized the transmissive/absorptive performance. In every unit cell, each pair of dipole-like elements connected by vias is printed on the two sides of the substrate, and the coupling between long and short dipoles is suppressed by this structure. A guiding circuit is studied based on the analysis of the current path, and the insertion loss of transmission window is significantly reduced by the surface current at the pass-band that is hindered to pass through lossy elements. The presented rasorber acts as an absorber at the low frequencies, while providing a high transmittance window at 5.6 GHz. This design is elaborately optimized to achieve low reflection and angle-insensitive performance. Finally, the presented structure is validated by numerical simulations and experimental measurements. This rasorber could be used for secrecy communications among stealth facilities while providing stable broad-band absorptive properties.

The 0.90Al2O3–0.10TiO2 ceramics have been prepared by the conventional solid-state method. MnCO3 was added into the ceramic system for the purpose of eliminating Al2TiO5 and improving the microwave dielectric performance. For the samples of MnCO3 addition, once the sintering temperature was over 1200°C, the Al2TiO5 phase was suppressed without annealing treatment. In combination with scanning electron microscope (SEM) micrographs, the optimized sintering temperature for the ceramics system was 1250°C. The grain size increased with the increasing of MnCO3 addition, accompanied with the abnormal grain growth. Moreover, the energy dispersive spectrometer (EDS) data illustrate that the Mn element exists at the grain boundary. The compact uniform microstructure and excellent microwave dielectric performance were obtained in the samples of 0.90Al2O3–0.10TiO2 with 0.3 mol.% MnCO3 sintered at 1250°C for 4 h: e r = 12.6, Q × f = 125500 GHz, τ f = −1.4 ppm/°C.

The sintering behavior, microstructure and microwave dielectric properties of (Ba1 − x Sm2x/3)((Co0.7Mg0.3)1/3Nb2/3)O3 (0.01 ≤ x ≤ 0.04) ceramics has been investigated. A small amount of Sm-substitution facilitates the densification of Ba (Co0.7Mg0.3)1/3Nb2/3O3 ceramics. All ceramic samples are well-sintered and the relative densities are beyond 97%. Small homogeneous grains and low porosity have been observed by scanning electron microscopy photograghs. Single-phase solid solutions are achieved in all compositions and no secondary phase was detected by X-ray diffraction patterns. The appropriate amount of Sm-substitution on A-site leads to the improved degree of 1:2 B-site cation ordering, while the ordered degree decreases gradually as the Sm content increases, which is related to the A-site deficiency caused by the inequitable substitution. The degree of 1:2 ordering structure is responsible for the microwave dielectric properties of the Sm-substituted samples, especially Q × f values. A good combination of microwave dielectric properties have been obtained for (Ba0.98Sm0.04/3)((Co0.7Mg0.3)1/3Nb2/3)O3 sintered at 1420 °C: e r = 32.9, Q × f = 50309 GHz and τ f = 6 ppm/oC.

In this paper, a novel periodic 2D all-dielectric metamaterial based on dielectric trimer is proposed. The electromagnetic responses are explained by the corrected equations of motion using coupled mode theory (CMT). An abnormal vanishment mode phenomenon is also discovered and explained using the zero-sum effect of magnetic dipole, by which the relative bandwidth of the metamaterial has been improved significantly compared with other structures. The presented design is easy for fabrication and can be applied in microwave region by scaling the dimensions of the cubes.

A reconfigurable active frequency selective surface (AFSS) with ultra-wideband (UWB) characteristics is presented in this article. The proposed AFSS is a periodic array structure which consists of three metal layers and two dielectric layers. PIN diodes are arranged on the top and bottom metal layers which can rebuild the function of AFSS. The transmission bandwidth of the OFF-state AFSS is 8-12.5 GHz with a fractional bandwidth of 44%, and the transmission coefficient of the ON-state AFSS is lower than -10 dB from 2 to 18 GHz. Additionally, the AFSS has dual polarization characteristics, and it is angular stable and polarization-insensitive for two polarizations. The simulation results show the proposed AFSS is a valuable candidate for radome applications.

In this paper, a tunable ferroelectric unit cell for wideband beam-steering reflectarray antennas is proposed. The proposed element is composed of three parts, i.e., a shorted parallel plate waveguide, a bifurcated ferroelectric phase shifter and two symmetric four-section stepped impedance transformers. Employing the DC bias voltage to vary the permittivity of ferroelectric material, the reflection phase of proposed unit cell can be continuously tuned. Full-wave simulated results show that the reflectarray element can achieve more than 360o linear phase tunability from 5.0 GHz to 6.5 GHz. The impact of ferroelectric loss on the performance of unit cell is also investigated. The proposed ferroelectric unit cell can be used to implement wideband beam-steering reflectarray antennas, providing an alternative of complicated phased array antennas.

Highly confined waves of surface plasmon polaritons (SPPs) in monolayer graphene are efficiently excited using an etched diffractive grating on silicon. In this paper, an inhomogeneous graphene-covered grating is proposed and the excitation of which is analyzed as an analogy with the two-level system in quantum mechanics. Based on the equivalent circuit theory, the excitation of plasmonic waves in an etched diffractive grating is equivalent to the resonance of an oscillator. Thus, according to the governing equation, the electric currents for the inhomogeneous graphene-covered grating is correspond to the ket of energy states in the two-level system. In addition, the excitation of SPPs in an inhomogeneous graphene-covered grating is numerically analyzed to validate the theoretical model. Our work offers a new inroad of the excitation of SPPs in an inhomogeneous graphene-covered grating in an analogy with the two-level system.

Ba 4.2 Nd 9.2 Ti 18 O 54 (BNT) ceramics have been prepared by solid-state ceramic method. Dark holes were observed in the center of the ceramics. All the samples showed typical columnar grain morphology in SEM images. TEM and XRD analysis indicated that there was only a BaNd 2 Ti 4 O 12 (JCPDS Card No. 44-0061) phase in the ceramics. In HRTEM images, the grain boundary was clear and no impurities and defects were observed. The XPS results showed the difference in valence of Ti. Ti 3 + ions appeared in all the samples. The Q × f value significantly dropped when the amount of Ti 3 + ions was considerable. BNT ceramics sintered at high temperature (1300 °C to 1400 °C) usually contain dark holes in the center (caused by Ti 3 + ions) which could lead to a significant drop in Q × f value.

(Zr0.8Sn0.2)TiO4 (ZST) ceramics were prepared by conventional solid-state ceramic method and investigated by XRD, SEM and dielectric measurements. 1 wt% ZnO and different concentrations of Er2O3 ranging from 0.25 to 1 wt% were introduced to study their effect on the densification, microstructure and microwave dielectric properties of the ZST ceramics. It was found that the addition of ZnO and Er2O3 can significantly improved the density at lower temperature. Meanwhile, the microwave dielectric properties of ZST ceramics were also heavily influenced by the sintering aids. The maximum dielectric constant of 39.2 and Q × f value of 28,000 were found to be for the samples added with 0.25 wt% of Er2O3 sintered at 1240 °C for 4 h.

0.90Al2O3–0.10TiO2 doped with x mol% ZnO (x = 0.2, 0.3, 0.4, 0.5) microwave dielectric ceramics, have been prepared by the conventional solid-state method with various sintering temperatures. A relationship between phase composition and microwave dielectric properties was systematically discussed for millimeter-wave applications. The formation of Al2TiO5 impurity phase was inhibited by ZnO addition without annealing treatment. The existence of Al2TiO5 impurity phase deteriorated seriously the microwave dielectric properties, especially quality factor (Q × f) and temperature coefficient (τ f ). Due to the liquid sintering effect, the addition of ZnO could reduce the sintering temperature to 1300 °C. To a large extend, ZnO additive improved the density of 0.90 Al2O3–0.10 TiO2 ceramics, and ultimately enhanced the microwave dielectric properties. The ZnAl2O4 impurity phase could also enhance the Q × f value and permittivity (e r ) of 0.90Al2O3–0.10TiO2 ceramics. Excellent microwave dielectric properties was achieved for 0.4 mol% ZnO doped 0.90Al2O3–0.10TiO2 ceramics sintered at 1300 °C for 4 h: e r = 12.4, Q × f = 159,000 GHz, τ f = −0.2 ppm/°C.

(Zr0.8Sn0.2)TiO4 (ZST) ceramics were prepared by conventional solid-state ceramic method. The effects of ZnO + CeO2 on the phase composition, microstructure, sintering characteristics and microwave dielectric properties of ZST ceramics were investigated. Only a single phase (Zr0.8Sn0.2)TiO4 was identified for XRD patterns. The sintering temperature decreased from 1550 to 1220 °C owning to the composite addition of ZnO and CeO2. The excellent microwave dielectric properties have been obtained in the samples of ZnO (1 wt%) + CeO2 (0.5 wt%) doped ZST ceramics sintered at 1220 °C for 4 h: e r = 37, Q × f = 33,000 GHz, τ f = −0.76 ppm/°C.

SrTiO3/PTFE composites with high dielectric constant and low loss properties were prepared via powder processing technique. The structure, morphology, dielectric and thermal properties of the composites were characterized by X-ray diffraction, scanning electron microscope, coefficient of thermal expansion, and network analyzer. SrTiO3/PTFE composites with 60 wt% SrTiO3 are of high er and high Q: er = 12.19, Q × f = 1930 GHz. Additionally, theoretical models like Logarithmic mixture rule, Maxwell–Garnet, Effective medium theory and Smith’s model were also used to predict the dielectric constant of these composites. The values obtained by the EMT model are in well agreement with the experimental values.

(Ba1−xSrx)(Co1/3Nb2/3)O3 (0 ≤ x ≤ 0.5) microwave dielectric ceramics had been prepared using the solid-state reaction method. The dense microstructures with small grains were obtained. All Sr2+ substituted samples exhibited the perovskite phase. The structure transition from cubic to hexagonal occurred with the increase of x, which led to higher degree of 1:2 ordered structure in the B site. The improved Q × f value was correlated monotonously with the ordering parameters S. The dielectric constant (e r ) and the temperature coefficient of resonant frequency (τ f ) could be effectively tuned by tailoring x. A good combination of microwave dielectric properties was obtained for (Ba0.95Sr0.05)(Co1/3Nb2/3)O3 sintered at 1380 °C: e r = 33.3, Q × f = 87,120 GHz, τ f = 4 ppm/°C.

Ba4.2Nd9.2Ti18−xSnxO54(x = 0, 0.25, 0.5, 1, 1.5, 2) microwave dielectric ceramics with high Q × f value were prepared by conventional solid state route. The microstructure and microwave dielectric properties of Ba4.2Nd9.2Ti18−xSnxO54(x = 0, 0.25, 0.5, 1, 1.5, 2)(BNTS) ceramics were systematically investigated. XRD patterns showed that there was only a single BaNd2Ti5O14 phase identified in all samples and no second phase was found. SnO2 substitution increases the lattice parameters of BNT ceramic. Moreover, the bulk density of BNT ceramics increases as the x increases. Both the permittivity and τ f value decrease when the Sn concentration increases. The SnO2 substitution has a big influence on Q × f value. When x = 0.5, the BNTS ceramic gains the highest Q × f value. Afterwards, the Q × f value decreases sharply when x value increases. Excellent microwave dielectric properties were achieved in Ba4.2Nd9.2Ti18−xSnxO54(x = 0.5) ceramics sintered at 1340 °C for 2 h: e r = 80.6, Q × f = 9177 GHz, τ f = 61 ppm/°C.

Microwave dielectric ceramics with the composition of Ba(Co1/3Nb2/3)O3–MnO2 (BCN–MnO2) were prepared by the solid-state reaction method. The effects of MnO2 on phase composition, microstructure, sintering characteristics and microwave dielectric properties of BCN ceramics were investigated. The additive MnO2 slightly decreased the sintering temperature and improved the densification of BCN as sintering aids. The multivalent Mn was beneficial for maintaining the compositional stability of BCN, which broadened the region of the sintering temperature. The Q × f value and temperature coefficient of resonant frequency (τ f ) were improved by 0.3 wt% MnO2. BCN–0.3 wt% MnO2 ceramic sintered at 1,340 °C for 12 h in air exhibited good microwave dielectric properties with e r = 32.9, Q × f = 90,431 GHz, τ f = −12 ppm/°C.

Ba(Co1/3Nb2/3)O3 (BCN) ceramics were prepared using a conventional solid-state route. Effects of sintering on microstructure and microwave dielectric properties of Ba(Co1/3Nb2/3)O3 ceramics were systematically investigated. Appropriate sintering temperature could restrain the evaporation of CoO and the formation of Nb-rich phase. The dielectric constants and temperature coefficients of resonant frequency changed slightly with the variation in sintering temperatures. The high Q × f value with prolonged sintering time was related to the improvement of 1: 2 ordered arrangement in the B site, rather than the compactness of ceramics. Excellent microwave dielectric properties (e r =32.6, Q × f =86857 GHz, τ f =−14 ppm/°C) were obtained in the Ba(Co1/3Nb2/3)O3 ceramics sintered at 1380°C for 12 h.

In this paper, a frequency-selective rasorber (FSR) with a high efficiency transmission window is proposed for stealth and security applications. By utilizing a new equivalent circuit model to generate the transmission window, there is no lumped and distributed inductor/capacitor (LC) element is used in this structure. Moreover, communication security and stealth performance are promised on the account of the low reflection at operation band. The reflection power is lower than 10 percent from 2.8 GHz to 5.7 GHz, while the transmission window locates at 5.6 GHz with 0.17 dB insertion loss. Finally, simulated results are consistent with circuit computed results, and the rasorber is endurable with the oblique incidence.

Parity-time (PT) symmetric structures present the unidirectional invisibility at the spontaneous PT-symmetry breaking point. In this paper, we propose a PT-symmetric circuit consisting of a resistor and a microwave tunnel diode (TD) which represent the attenuation and amplification, respectively. Based on the scattering matrix method, the circuit can exhibit an ideal unidirectional performance at the spontaneous PT-symmetry breaking point by tuning the transmission lines between the lumped elements. Additionally, the resistance of the reactance component can alter the bandwidth of the unidirectional invisibility flexibly. Furthermore, the electromagnetic simulation for the proposed circuit validates the unidirectional invisibility and the synchronization with the input energy well. Our work not only provides an unidirectional invisible circuit based on PT-symmetry, but also proposes a potential solution for the extremely selective filter or cloaking applications.

MnO2 doped Ba4.2Nd9.2Ti18O54–NdAlO3(13 wt%) (BNT–NA) microwave dielectric ceramics with the near zero τ f and the wide range of sintering temperature were prepared by conventional solid state method. The effects of Mn4+ doping on the microstructures and microwave dielectric properties of BNT–NA ceramics were investigated. XRD patterns showed only a single BaNd2Ti5O14 phase was identified in all samples and there was no second phase. The sintering temperature decreased from 1,380 to 1,320 °C as MnO2 content increased from 0.1 to 0.9 wt%. The MnO2 doped BNT–NA ceramics could be densified at a lower sintering temperature. The MnO2 additive had a positive effect on lowing sintering temperature of BNT–NA ceramics. The τ f varied from negative to positive with the increase of MnO2. Excellent microwave dielectric properties were achieved in Ba4.2Nd9.2Ti18O54–NdAlO3 ceramics doped with 0.3 wt% MnO2 and sintered at 1,380 °C for 2 h: e r = 66.5, Q × f = 13,948 GHz, τ f = 0.4 ppm/°C.

Microwave dielectric ceramics of Ba(Co 0.7 Zn 0.3 ) 1/3 Nb 2/3 O 3 (BCZN) with MnO 2 addition have been prepared by the solid-state reaction method. The low-temperature annealed samples were added to make comparison. The results revealed that the improvement of Q×f value was attributed to 1 mol% MnO 2 addition in the unannealed BCZN, but the annealed BCZN had a sharp decrease on Q×f value. The microstructures and crystal structures were characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD) analysis and electrical conductivity results to reveal the mechanisms.