Improved dielectric properties of PDCs-SiCN by in-situ fabricated nano-structured carbons

In order to enhance dielectric properties of polymer derived SiCN ceramics (PDCs-SiCN), nano-structured carbons were in-situ fabricated in PDCs-SiCN by pyrolysis of ferrocene-modified polysilazane. Microstructure evolutions, dielectric and microwave absorption properties of PDCs-SiCN decorated with nano-structured carbons were investigated. Nano-structured PDCs-SiCN ceramics are composed of carbon nanowires as well as interpenetrating graphene-like free carbons, SiC nano-crystals, Si₃N₄ nano-crystals and amorphous SiCN. Relative complex permittivities of nano-structured PDCs-SiCN increase with increasing ferrocene contents and annealing temperatures. Free carbons in PDCs-SiCN play a dominating role on the improved dielectric properties. Polarization loss is the primary dielectric loss. Loss tangent of PDCs-SiCN exceeding 0.7 is obtained when free carbons are only 2.57% in weight. Nano-structured PDCs-SiCN exhibit good microwave absorption property. The reflectivity is smaller than −14 dB in the whole X band when material is composed of both impedance and microwave absorption materials.     

Dielectric,electromagnetic absorption and interference shielding properties of porous yttria-stabilized zirconia/silicon carbide composites

SiC was infiltrated into porous yttria-stabilized zirconia (YSZ) felt by chemical vapor infiltration (CVI), and continuous SiC matrix layer was formed around YSZ fibre. When 86.9 wt.% SiC is introduced into the porous YSZ felt, the mean values of the real part of the permittivity and dielectric loss tangent of porous YSZ felt increase from 1.16 and 0.007 to 8.2 and 1.31, respectively. The electromagnetic interference (EMI) shielding efficiency (SE) increases from 0.069 dB to 16.2 dB over the frequencies ranging from 8.2 GHz to 12.4 GHz. The reflection loss of the composites with a thickness of 5 mm at 8–18 GHz is smaller than ?6.5 dB, and the bandwidth below ?10 dB is 5 GHz at room temperature, which increases to 5.9 GHz at 800 °C. The considerable increases in EMI SE and microwave absorption properties are attributed to the formation of continuous SiC matrix layer composed of SiC nanocrystals in the porous YSZ felt, which is beneficial for the production of induced electric current and the enhancement of dielectric loss.     

Dielectric properties of Si3N4–SiCN composite ceramics in X-band

Si₃N₄–SiCN composite ceramics were successfully fabricated through precursor infiltration pyrolysis (PIP) method using polysilazane as precursor and porous Si₃N₄ as preform. After annealed at temperatures varying from 900 °C to 1400 °C, the phase composition of SiCN ceramics, electrical conductivity and dielectric properties of Si₃N₄–SiCN composite ceramics over the frequency range of 8.2–12.4 GHz (X-band) were investigated. With the increase of annealing temperature, the content of amorphous SiCN decreases and that of N-doped SiC nano-crystals increases, which leads to the increase of electrical conductivity. After annealed at 1400 °C, the average real and imaginary permittivities of Si₃N₄–SiCN composite ceramics are increased from 3.7 and 4.68 × 10^?3 to 8.9 and 1.8, respectively. The permittivities of Si₃N₄–SiCN composite ceramics show a typical ternary polarization relaxation, which are ascribed to the electric dipole and grain boundary relaxation of N-doped SiC nano-crystals, and dielectric polarization relaxation of the in situ formed graphite. The Si₃N₄–SiCN composite ceramics exhibit a promising prospect as microwave absorbing materials.     

Synthesis and microwave absorption properties of SiC nanowires reinforced SiOC ceramic

Silicon carbide nanowires (SiC NWs) reinforced SiOC ceramics were fabricated through in situ growth of SiC NWs in SiOC ceramics by pyrolysis of polysiloxane. SiC NWs were in situ formed by the addition of ferrocene, the content of SiC NWs increased with the increases of annealing temperature and ferrocene content. Due to the formation of SiC NWs in the inter-particle pores of SiOC ceramics, the SiOC particles were bridged by SiC NWs, which led to the increase of electrical conductivity. With the increase of SiC NWs content, the real permittivity and the imaginary permittivity increased from 3.63 and 0.14 to 10.72 and 12.17, respectively, and the minimum reflection coefficient decreased from −1.22 dB to −20.01 dB, demonstrating the SiOC ceramics with SiC NWs had a superior microwave-absorbing ability.     

Precise measurement of the dielectric properties of BaxSr1−xTiO3 thin films by on-wafer through-reflect-line (TRL) calibration method

Polycrystalline Ba_xSr_1−xTiO₃ (x = 0.3, 0.4, 0.5) (BST) thin films with a thickness of 200 nm were deposited on r-cut sapphire substrates by rf sputtering method. The permittivity and loss tangent of the films were successfully observed in the range of 1–3 GHz, by utilizing the on-wafer through-reflect-line (TRL) calibration method although the estimated relative permittivity depended on an applied power to waveguides and the loss tangent had the dispersion around 1 GHz even in the case of 2 μm-thick aluminum. Finally, we concluded that the BST thin film with x = 0.4 is the most suitable for microwave tunable devices because it had the lowest loss tangent and relatively high permittivity.     

The dielectric and microwave absorption properties of polymer-derived SiCN ceramics

The polymer-derived SiCN ceramics were synthesized at different annealing temperature (900 ～ 1400 °C). The XRD, SEM, FT-IR, Raman and XPS were used to analyze the phase composition and microstructure. The result indicated that the crystallization degree and content of free carbon gradually improved with the increase of annealing temperature. The resistivity, dielectric and microwave absorption properties of the samples were studied at 2 ～ 18 GHz. The resistivity decreased gradually as the annealing temperature rose. The dielectric constant of sample decreased with the increase of frequency in 1 ～ 5 MHz. The existence of free carbon could improve the dielectric properties of polymer-derived SiCN ceramics at high frequency. The reflectance of the sample synthesized at 1100 °C was below ?10 dB (> 90% absorption) in a wide frequency range of 6 ～ 16 GHz and the maximum value of dielectric loss angle tangent was about 0.6 at 16 GHz.     

Microstructures,dielectric response and microwave absorption properties of polycarbosilane derived SiC powders

Carbon-rich SiC powders with high dielectric loss were prepared via pyrolysis of polycarbosilane (PCS). The effects of pyrolysis temperature on microstructures, dielectric response and microwave absorption properties in X-band (8.2–12.4 GHz) of PCS-derived SiC powders were investigated. The PCS-derived SiC powders are mainly composed of SiC nanocrystal, turbostratic carbon and amorphous phase (SiC and/or C). The size of SiC nanocrystals and the graphitization degree of carbon both increase with the elevation of pyrolysis temperature. Furthermore, the residual carbon is transformed from amorphous into turbostratic structure with a phenomenon of regional enrichment. Moreover, the relative complex permittivity increases notably with the higher pyrolysis temperature. Meanwhile, the dielectric loss tangent increases from 0.19 to 0.57, while the microwave impedance decreases from 73.20 to 53.58. The optimal reflection loss of ?35 dB for PCS-derived SiC powders is obtained when the pyrolysis temperature is 1500 °C, which exhibits a great application prospect in microwave absorbing materials.     

Microwave performance of thin film ferroelectric varactors in the wide temperature range of −223 °C to +227 °C

Parallel-plate Au(Pt)/Ba_0.25Sr_0.75TiO₃/(Pt)Au thin film varactors are fabricated on high resistive Si substrate and characterized at 1 MHz and microwave frequencies up to 45 GHz in the temperature range of −223 °C to +227 °C. The relative tunability of capacitance decreases with temperature, as capacitance does, from 80 to 90% at −193 °C down to 10% at +227 °C. The temperature coefficient of capacitance in the temperature range −55 to +125 °C is approximately 0.3% at 20 GHz and zero dc field. The temperature dependence of the varactor loss tangent, in general, follows that of the capacitance. The loss tangent at zero dc field and 20 GHz is less than 0.1 at −193 °C and less than 0.02 at +227 °C. The figure of merit of the varactor, taking into account both the tunability and the loss tangent, at 20 GHz is more than 1000 at −193°C and 50 at +227 °C.     

Synthesis and microwave absorption properties of Fe–C nanofibers by electrospinning with disperse Fe nanoparticles parceled by carbon

The Fe–C nanofibers were achieved using electrospinning technique. The microstructure was characterized by field emission scanning electron microscope and high resolution transmission electron microscopy equipped with energy-dispersive X-ray analysis. The results indicated that magnetic Fe nanoparticles uniformly dispersed along nanofibers and were parceled by carbon matrix. For the Fe–C nanofibers/paraffin composite, a minimum reflection loss (RL) value of −44 dB was observed at 4.2 GHz. Moreover, the frequency range with RL peak value below −10 dB was achieved in a wide frequency range from 2.2 to 13.2 GHz. The excellent microwave absorption properties were due to the combination of complex permeability and permittivity resulting from magnetic Fe particles and lightweight carbon.     

Flexible,hydrophobic SiC ceramic nanofibers used as high frequency electromagnetic wave absorbers

In this paper, flexible hydrophobic SiC ceramic nanofibers have been successfully fabricated via electrospinning and subsequent high temperature heat treatment. The synthesized SiC ceramic nanofibers show excellent flexibility without any breakage even under a bending angle of 142.6°, and high hydrophobicity with a water contact angle of 149.05°. The SiC nanofibers exhibit excellent electromagnetic (EM) wave absorption properties with an effective absorption bandwidth (reflection loss (RL) <−10 dB, 90% EM wave absorbed) of 4–18 GHz. The maximum reflection loss of SiC ceramic nanofibers reaches −19.4 dB at 5.84 GHz. In addition, the nanofibers are environmentally stable in 2 mol/L NaOH solution for 2 h and high temperature of 500 °C in air atmosphere. The excellent EM wave absorption performance, flexibility, hydrophobic properties, corrosion resistant properties in alkali environment and high temperature stability make SiC ceramic nanofibers to be a potential candidate for EM wave absorption used in harsh environment.     

Fabrication of microwave absorbing CoFe2O4 coatings with robust superhydrophobicity on natural wood surfaces

A superhydrophobic wood surface with microwave absorption property was prepared based on the formation of CoFe₂O₄ nanoparticles and subsequent hydrophobization using fluorinated alkylsilane (FAS). Meanwhile, sticky epoxy resin was worked as a caking agent by adhering abundant of CoFe₂O₄ nanoparticles to wood surface. The as-prepared superhydrophobic coatings on wood maintain stable superhydrophobicity after suffering a significant abrasion. Moreover, the complex permeability and permittivity of the coated wood composites were measured in the frequency range of 2–18 GHz by vector network analysis. The microwave absorption properties were elucidated by the traditional coaxial line method. The results show that the as-prepared wood composites have excellent microwave absorption properties at the frequency of 16 GHz, and the minimum reflection loss can reach −12.3 dB. The approach presented may provide further routes for designing outdoor wood wave absorbers with a specified absorption frequency.     

Dielectric and microwave-absorption properties of SiC nanoparticle/SiBCN composite ceramics

SiC nanoparticles with different contents (5–20 wt%) were mixed with liquid polyborosilazane. The compound was used to prepare SiC nanoparticle/polymer-derived SiBCN ceramics (SiC/PDCs-SiBCN). Thermal gravity tests (25–1400 °C) in air and helium atmosphere were used to investigate the thermal stability of SiC/PDCs-SiBCN. Dielectric and microwave-absorption properties of SiC/PDCs-SiBCN were determined at frequencies of 8.2–12.4 GHz by waveguide method. Results show that the addition of SiC nanoparticles increased the thermal stability of SiBCN ceramics. The permittivity, dielectric loss and absorption coefficient of ceramics increased as an elevated SiC content, resulting from the increase of carrier concentration. To understand the high-temperature dielectric property of SiC/PDCs-SiBCN, the permittivity of SiBCN ceramics with 15 wt% of SiC was measured at temperatures of 293–773 K. The composite ceramics were found to have a visible increase in the permittivity and dielectric loss, indicating their great potential as the high-temperature microwave absorption materials.     

Microwave dielectric properties of Pb2MoO5 ceramic with ultra-low sintering temperature

Ultra-low firing microwave dielectric ceramic Pb₂MoO₅ with monoclinic structure was prepared via a conventional solid state reaction method. The sintering temperature ranged from 530 °C to 650 °C. The relative densities of the ceramic samples were about 97% when the sintering temperature was greater than 570 °C. The best microwave dielectric properties were obtained in the ceramic sintered at 610 °C for 2 h with a permittivity ∼19.1, a Q × f value about 21,960 GHz (at 7.461 GHz) and a temperature coefficient value of −60 ppm/°C. From the X-ray diffraction, backscattered electron image results of the co-fired samples with 30 wt% silver and aluminum additive, the Pb₂MoO₅ ceramics were found not to react with Ag and Al at 610 °C for 4 h. The microwave dielectric properties and ultra-low sintering temperature of Pb₂MoO₅ ceramic make it a promising candidate for low temperature co-fired ceramic applications.     

Synthesis,characterization, and microwave dielectric properties of ZnTiTa2O8 ceramics with ixiolite structure obtained through the aqueous sol–gel process

Nano-sized ZnTiTa₂O₈ powders with ixiolite structure, with particle sizes ranging from 10 nm to 30 nm, were synthesized by thermal decomposition at 950 °C. The precursors were obtained by aqueous sol–gel and the compacted and sintered ceramics with nearly full density were obtained through subsequent heat treatment. The microstructure and electrical performance were characterized by field emission scanning electron microscopy, x-ray diffraction, and microwave dielectric measurements. All the samples prepared in the range 950–1150 °C exhibit single ixiolite phase and relative density between ~87% and ~94%. The variation of permittivity and Q·ƒ value agreed with that of the relative density. Pure ZnTiTa₂O₈ ceramic sintered at 1050 °C for 4 h exhibited good microwave dielectric properties with a permittivity of 35.7, Q·ƒ value of 57,550 GHz, and the temperature coefficient of resonant frequency of about −24.7 ppm/°C. The relatively low sintering temperature and excellent dielectric properties in the microwave range would make these ceramics promising for applications in electronics.     

Strong effect of atmosphere on the microstructure and microwave absorption properties of porous SiC ceramics

Excellent microwave absorption properties of porous SiC ceramics were successfully synthesized using SiC/camphene slurries with various polycarbosilane (PCS) contents related to the SiC powder. The compositions of the nanowires (NWs) growth in the pore channels of porous SiC ceramics strongly depended on the pyrolysis atmosphere, with N₂-generating Si₃N₄ NWs and Ar SiC NWs. With the increase of PCS content, the minimum reflection coefficient (RC) of porous SiC ceramics decreased from ?7.6 dB to ?67.4 dB in Ar and from ?10.9 dB to ?24.7 dB in N₂, respectively. The effective absorption bandwidth (EAB) of porous SiC ceramics could be up to 8.1 GHz in Ar and 4.5 GHz in N₂. The enhanced microwave absorption properties of porous SiC ceramics could be attributed to the formation of SiC nano-crystalline, nanosized carbon and the NWs, which would increase the amount of boundaries and defects, leading to the electronic dipole polarization and interfacial scattering.     

Influence of synthesis process on the dielectric properties of B-doped SiC powders

Fine powders (～0.7 μm) of SiC doped with 3 mol% and 10 mol% B were successfully produced by mechanical activation assisted self-propagating high-temperature synthesis (MASHS). The experimental results showed that the presence of B caused a reduction in the combustion temperature, shrinkage of the crystal lattice, an increase in the tendency of the grains to be crystallized, and a decrease in the dielectric properties in the frequency range between 8.2 and 12.4 GHz, specifically the real (?′) and the imaginary parts (?″) of complex permittivity and the loss tangent (tan δ). Analysis of the results suggests that B ions should be preferably accommodated in the Si sites of the SiC lattice and cause a reduction in the number of defects (V_Si, V_C, and C_Si), which results in a decrease in the dielectric properties. Comparison of the experimental results of this study with results reported in similar earlier studies reveals that the influence of B on the dielectric properties of the B-SiC powders depends strongly on the synthesis process.     

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.     

Controlled synthesis of Fe3O4@SnO2/RGO nanocomposite for microwave absorption enhancement

A ternary nanocomposite of Fe₃O₄@SnO₂/reduced graphene oxide (RGO) with different contents of SnO₂ nanoparticles was synthesized by a simple and efficient three-step method. The transmission electron microscopy and field emission scanning electron microscopy characterization display that plenty of Fe₃O₄@SnO₂ core–shell structure nanoparticles are well distributed on the surface of RGO sheets. The X-ray diffractograms show that the products consist of highly crystallized cubic Fe₃O₄, tetragonal SnO₂ and disorderedly stacked RGO sheets. The magnetic hysteresis measurement reveals the ferromagnetic behavior of the products at room temperature. The microwave absorption properties of paraffin containing 50 wt% products were investigated at room temperature in the frequency range of 2–18 GHz by a vector network analyzer. The electromagnetic data show that the maximum reflection loss is −45.5 dB and −29.5 dB for Fe₃O₄@SnO₂/RGO-1 and Fe₃O₄@SnO₂/RGO-2 nanocomposite, respectively. Meanwhile, the reflection loss less than −10 dB is up to 14.4 GHz and 13.8 GHz for Fe₃O₄@SnO₂/RGO-1 and Fe₃O₄@SnO₂/RGO-2 nanocomposite, respectively. It is believed that such nanocomposite could be used as promising microwave absorbers.     

Flexible polymer composites with enhanced wave absorption properties based on beta-manganese dioxide nanorods and PVDF

Beta-manganese dioxide (β-MnO₂) nanorods have been fabricated on a large scale by a simple hydrothermal process in a wild condition. Several characterizations such as XRD, SEM, TEM and FESEM have been employed. The wave absorption properties of β-MnO₂/PVDF nanocomposites have been investigated. The results indicated that the β-MnO₂/PVDF nanocomposites exhibit enhanced wave absorption properties. The minimum reflection loss of the β-MnO₂/PVDF nanocomposite reaches − 30.1 dB (> 99.9% attenuation) at 8.16 GHz with a filler loading of 40 wt.%, and the frequency bandwidth less than –10 dB is from 7.12 to 9.20 GHz. The main microwave absorbing mechanism has been also discussed.     

Synthesis of magnetite nanostructures with complex morphologies and effect of these morphologies on magnetic and electromagnetic properties

Magnetite nanostructures with different morphologies (tube, urchin and dendrite) were synthesized via hydrothermal method and consequent reduction by hydrogen gas. Magnetization and electromagnetic (EM) properties were characterized. The results showed that the morphology of the nanostructures has a major role in the microwave magnetic and dielectric properties. The surface area and shape isotropy are the key factors for changes in EM properties of different morphologies. High surface area and anisotropic elongated shape of the dendritic nanostructures lead to their high microwave permittivity, whereas for urchin-like nanostructures their very high surface area resulted to low magnetization and microwave permeability. However, high permeability and low permittivity of tube-like nanostructures caused a good impedance matching and therefore the best EM absorption performance was observed in their case. The tube-like nanostructures produced a reflection loss peak value of about −40 dB at the frequency of 2 GHz with the layer thickness of 2 mm. The results showed that tube-like magnetite nanostructures are promising particles for efficient EM wave absorbing applications.