Enhanced microwave absorption properties of Fe-doped SiOC ceramics by the magnetic-dielectric loss properties

The combination of multiple loss characteristics is an effective approach to achieve broadband microwave wave absorption performance. The Fe-doped SiOC ceramics were synthesized by polymer derived ceramics (PDCs) method at 1500 °C, and their dielectric and magnetic properties were investigated at 2–18 GHz. The results showed that adding Fe content effectively controlled the composition and content of multiphase products (such as Fe₃Si, SiC, SiO₂ and turbostratic carbon). Meanwhile, the Fe promoted the change of the grain size. The Fe₃Si enhanced the magnetic loss, and the SiC and turbostratic carbon generated by PDCs process significantly increased the polarization and conductance loss. Besides, the magnetic particles Fe₃Si and dielectric particles SiO₂ improved the impedance matching, which was beneficial to EM wave absorption properties. Impressively, the Fe-doped SiOC ceramics (with Fe addition of 3 wt %) presented the minimum reflection coefficient (RC_min) of ?20.5 dB at 10.8 GHz with 2.8 mm. The effective absorption bandwidth (EAB, RC < ?10 dB) covered a wide frequency range from 5 GHz to 18 GHz (covered the C, X and Ku-band) when the absorbent thickness increased from 2 mm to 5 mm. Therefore, this research opens up another strategy for exploring novel SiOC ceramics to design the good EM wave-absorbing materials with broad absorption bandwidth and thin thickness.     

Dielectric permittivity and microwave absorption properties of transition metal Ni and Mn doped SiC nanowires

In order to enhance the microwave absorption properties of SiC nanowires, two transition metals Ni and Mn were selected as doping elements to improve their electromagnetic parameters. The experimental results indicate that Ni and Mn as catalysts reduce the stacking defect density of SiC nanowires, which will weaken the interfacial polarization loss induced by stacking defects. However, they can increase the electrical conductivity of SiC nanowires and generate new impurity defects, thereby effectively improving the conductance loss and dipole polarization loss. Therefore, the dielectric loss of SiC nanowires is significantly enhanced, but they still do not have considerable magnetic loss capability. In addition, Ni and Mn doping also improves the impedance matching characteristics of SiC nanowires. Therefore, the microwave absorption ability of SiC nanowires is effectively enhanced. As the nanowire filling ratio is 20 wt%, the minimum reflection loss of the Ni_0.01Si_0.99C nanowire is −11.1 dB and the effective absorption bandwidth is 1.1 GHz (9.3–12.4 GHz) at a thickness of 2.8 mm; Mn_0.01Si_0.99C nanowires have a minimum reflection loss of −16.8 dB and an effective absorption bandwidth of 3.1 GHz (9.3–12.4 GHz) at a thickness of 2.8 mm.     

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.     

Simultaneously improving mechanical and microwave absorption properties of a novel SiCf/SiOC & mullite hybrid ceramic matrix composite

For enhanced mechanical and microwave absorption properties at the same time, the SiC_f/hybrid matrix composites were fabricated by precursor infiltration and pyrolysis (PIP) method with polysiloxane (PSO) ethanol solution, alumina sols and silica sols. As the first layer of the hybrid matrix, the SiOC ceramic was pyrolyzed from PSO solution. The remained hybrid matrix was mullite, which sintered from alumina sols and silica sols. The effects of different content of PSO solution on the morphologies, flexure strength and reflection loss values of composites were studied. Additionally, the XRD patterns, Fourier Transform Infrared (FTIR) and Raman spectrum of hybrid matrix were also investigated. With the increasingly content of PSO solution from 0% to 10 % and 20%–60% in the first infiltration-pyrolysis process, the flexure strength of composites was increased from 175.18 MPa to 301.94 MPa and decreased from 263.33 MPa to 221.30 MPa, respectively. The complex permittivity was increased with the increasing content of PSO solution from 0%–40% due to the free carbon conductive network from excessive SiOC. Moreover, the complex permittivity of SiC_f/hybrid matrix composites with 50 % and 60 % content of PSO solution was reduced due to more open porosity and broken free carbon conductive network. Additionally, the maximum reflection loss values of SiC_f/hybrid matrix composite with 50 % PSO solution were over -60 dB and the effective absorption bandwidth (EAB) of this composite reaches 3.89 GHz in the X band.     

In situ growth of one-dimensional carbon-rich SiC nanowires in porous Sc2Si2O7 ceramics with excellent microwave absorption properties

For enhancing the absorption ability of dielectric and electromagnetic wave (EMW), C-rich SiC NWs /Sc₂Si₂O₇ ceramics are successfully fabricated through in-situ growth of SiC nanowires (NWs) into porous Sc₂Si₂O₇ ceramics by precursor infiltration and pyrolysis (PIP) at 1400?°C in Ar. SiC NWs are in-situ formed in the pore channels via a vapor-liquid-solid (VLS) mechanism, the relative complex permittivity increases notably with the content of absorber (C-rich SiC NWs), which tune the microstructure and dielectric property of C-rich SiC NWs/Sc₂Si₂O₇ ceramics. Meanwhile, the minimum reflection coefficient (RC) of C-rich SiC NWs/Sc₂Si₂O₇ ceramic decreases from ?9.5?dB to ??35.5?dB at 11?GHz with a thickness of 2.75?mm, and the effective absorption bandwidth (EAB) covers the whole X band (8.2–12.4?GHz) when the content of absorber is 24.5?wt%. The results indicate that Sc₂Si₂O₇ ceramics decorated with SiC NWs and nanosized carbon have a superior microwave-absorbing ability, which can be contributed to the Debye relaxation, interfacial polarization and conductivity loss enhanced by in-situ formed SiC NWs and nanosized carbon phases. The C-rich SiC NWs /Sc₂Si₂O₇ ceramics can be a promising microwave absorbing materials within a broad bandwidth.     

Simultaneously enhancing mechanical and microwave absorption properties of Cf/SiC composites via SiC nanowires additions

C_f/SiC composite is an excellent structural and functional material, silicon carbide nanowires (SiCnws) are not only a toughening material but also a great application in the field of microwave absorption. In this study, SiCnws are grown on the surface of carbon fiber (C_f) by polymer impregnation and pyrolysis, and the SiC matrix was prepared by chemical vapor osmosis method. The SiCnws are introduced to enhance the mechanical and microwave absorption properties simultaneously. After 3 impregnations, the flexural strength of the composite was 107.35 ± 10 MPa. When the thickness is 1.86 mm, the minimum reflection loss value is ?41.08 dB, and the effective absorption bandwidth (RL ≤ ?10 dB) is 3.86 GHz. Furthermore, the microwave absorption mechanism of the material is discussed. This work provides a new method to prepare lightweight, stable and high-performance microwave absorption materials, and these materials are expected to be used in high temperature environments.     

Multifunction properties of SiOC reinforced with carbon fiber and in-situ SiC nanowires

Herein, the SiC nanowires were successfully fabricated via chemical vapor infiltration (CVI) into carbon fiber felts (CFs) and then the SiOC/SiCnws/CFs composites were synthesized by precursor infiltration and pyrolysis (PIP) processes. Results indicated that the lightweight composites possessed enhanced mechanical performance, low thermal conductivity, and excellent electromagnetic wave absorption properties. Detailedly, the compressive strength reached to 22.0 MPa and 9.6 MPa after two PIP processes cycles in z and x/y directions, respectively. Meanwhile, the composites exhibited tailored electromagnetic wave absorption performance with the effective absorption bandwidth of 3.06 GHz, and the minimum reflection loss (RL_min) was -48.2 dB with a thickness of 3.6 mm. The present work has a guidance to prepare and design multifunction properties for application in harsh environment.     

Enhanced electromagnetic microwave absorption of SiC nanowire-reinforced PDC-SiC ceramics catalysed by rare earth

Rare earth elements can modulate the dielectric constant of materials and significantly improve their dielectric properties. Herein, SiCnws/SiC ceramics were prepared through polymer derived ceramics (PDCs) technology with rare earth Sc particles as the catalyst. The Sc particles promote the precipitation of SiC and C from the matrix. Furthermore, the SiCnws, grown via the vapour-liquid-solid (VLS) mechanism, construct the three dimensional (3D) network structure to improve impedance matching and loss characteristics. Remarkably, the SiCnws/SiC ceramics minimum reflection coefficient (RC_min) achieved a value of ?33.2 dB at 9.4 GHz with a thickness of 2.75 mm, and the effective absorption bandwidth (EAB) was 4.2 GHz covering the whole X band. When microwaves permeated into the SiCnws/SiC ceramics, those trapped in the 3D network structure underwent a variety of microwave energy dissipation processes, including multiple reflections, scattering, and interface and dipole polarisation. Consequently, SiCnws-reinforced PDC-SiC ceramics catalysed by rare earth emerge as a promising new approach to enhance electromagnetic (EM) wave absorption performance.     

Preparation,dielectric property and microwave absorption property of Cu doped SiC nanopowder by combustion synthesis

Abstract

Cu doped SiC nanopowders have been prepared via combustion synthesis, using silicon powder and carbon black as the raw materials, copper powder as the doping source and polytetrafluoroethylene as the chemical activator respectively. The microstructure of prepared nanopowders has been characterised by X-ray diffraction and scanning electronic microscope. The electric permittivities of prepared SiC nanopowders in the frequency range of 8·2–12·4 GHz have been determined. Results show that prepared β-SiC nanopowders have fine spherical particles and narrow particle size distribution, and a quantity of SiC whisker increases with increasing Cu doping content. The Cu₃Si impurity has been generated when Cu content is up to 10%. The β-SiC doped with 10% Cu has the highest real part ?′ and dielectric loss tanδ values. The 5% Cu doped SiC nanopowder with matching thickness of 2 or 2·5 mm exhibits the best microwave absorption properties in the frequency range of 8·2–12·4 GHz.     

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.     

Enhanced high-temperature dielectric properties and microwave absorption of SiC nanofibers modified Si3N4 ceramics within the gigahertz range

Si₃N₄ ceramics modified with SiC nanofibers were prepared by gel casting aiming to enhance the dielectric and microwave absorption properties at temperatures ranging from 25?°C to 800?°C within X-band (8.2–12.4?GHz). The results indicate that the complex permittivity and dielectric loss are significantly increased with increased weight fraction of SiC nanofibers in the Si₃N₄ ceramics. Meanwhile, both complex permittivity and dielectric loss of SiC nanofibers modified Si₃N₄ ceramics are obviously temperature-dependent, and increase with the higher test temperatures. Increased charges mobility along conducting paths made of self-interconnected SiC nanofibers together with multi-scale net-shaped structure composed of SiC nanofibers, Si₃N₄ grains and micro-pores are the main reason for these enhancements in dielectric properties. Moreover, the calculated microwave absorption demonstrates that much enhanced microwave attenuation abilities can be achieved in the SiC nanofibers modified Si₃N₄ ceramics, and temperature has positive effects on the microwave absorption performance. The SiC nanofibers modified Si₃N₄ ceramics will be promising candidates as microwave absorbing materials for high-temperature applications.     

Enhanced microwave-absorption properties of polymer-derived SiC/SiOC composite ceramics modified by carbon nanowires

Novel microwave-absorbing SiOC composite ceramics with dual nanowires (carbon nanowires (CNWs) and SiC nanowires) with high performances were fabricated by using the polymer-derivation method and heat treatment in Ar atmosphere. The introduction of CNWs in the amorphous SiOC ceramics promotes the ceramic crystallization into SiC nanoparticles and SiC nanowires at lower annealing temperatures, which leads to multi-phases and multiple nano heterogeneous interfaces. The distinctive architectures largely increase the interfacial and dipole polarizations of the composite ceramics. The CNWs/SiC/SiOC composite ceramics exhibit excellent microwave-absorption properties in the Ku band (12.4–18 GHz). The minimum reflection coefficient (RC) is -24.5 dB at a thickness of 1.8 mm, while the maximum effective absorption bandwidth (EAB, the corresponding frequency band in which RC is smaller than -10 dB) is 4.8 GHz at a thickness of 1.9 mm, which make the CNWs/SiC/SiOC composite ceramics promising electromagnetic-wave-absorbing materials.     

Dielectric and microwave absorption properties of polymer-derived TiC/SiC/SiOC multiphase ceramics in X band

Polymer-derived TiC/SiC/SiOC ceramics were prepared using tetrabutyl titanate (TBT)-modified polysiloxane (PSO) as precursor. The effects of heat treatment temperature and TBT content in precursor on the microstructure, phase composition, and microwave absorbing properties of TiC/SiC/SiOC ceramics were investigated. The crystallinity of the ceramics increases with the increase of heat treatment temperature. With the increase of TBT content, the TiC content of the ceramics increases and the SiC content decreases. When the TBT content ranges from 1 to 5 wt.%, the increase of TBT content has little effect on the real part of the dielectric constant of TiC/SiC/SiOC ceramics. When the TBT content is 7 wt.%, the imaginary part of the dielectric constant of the ceramics changes. For TiC/SiC/SiOC ceramic obtained from the pyrolysis of PSO-TBT precursor with 7 wt.% TBT, the dielectric constant is within the target electromagnetic parameters. Therefore, it has an effective absorption bandwidth of 4.2 GHz, covering the entire X band, showing an excellent microwave absorbing performance.     

Controllable dielectric properties and strong electromagnetic wave absorption properties of SiC/spherical graphite-AlN microwave-attenuating composite ceramics

Fully dense SiC/spherical graphite-AlN microwave-attenuating composite ceramics were manufactured via hot-pressing sintering, in which, apart from the primary SG (spherical graphite) attenuating agent, 5–30 wt% semiconductive α-SiC was employed as the second attenuating agent. The incorporation of SiC contributed to a slightly decreasing electrical conductivity and enhanced polarization relaxation. Controllable complex permittivities were obtained, namely, both the real and imaginary permittivities exhibit first a decrease and then an increase with the SiC addition, and which delivers an optimized impedance matching of the composites. RL_min values below ?10 dB (more than 90% absorption) were achieved by all the composites containing 5–20 wt% SiC with the sample thickness of 1–1.4 mm, and the absorption performance characteristics were significantly tunable by controlling the of SiC content at 8.2–12.4 GHz. Impressively, a superior reflection loss of ?46 dB (1.1 mm) and wide effective absorption bandwidth of 2.1 GHz were achieved at a 5 wt% SiC content, respectively, rendering SiC/SG–AlN composites a potential ultra-thin and highly efficient microwave-attenuating ceramic candidate.     

Si3N4-BN-SiCN ceramics with unique hetero-interfaces for enhancing microwave absorption properties

SiCN-based ceramics with broadband and strong microwave absorption properties are desired for the structural and functional integration of ceramic matrix composites. The elemental composition and thermal expansion coefficients of the ceramics matrix crucially affect its microstructure and electromagnetic wave (EMW) absorption properties. BN layer with lower electrical conductivity and higher specific area, exhibits both the impedance matching characteristic and EMW attenuation in the process of multiple reflections, electrical conductivity loss, dipole polarization and interfacial polarization. Therefore, Si₃N₄-BN-SiCN ceramics, which were synthesized using chemical vapor infiltration (CVI) method, construct unique hetero-interface of Si₃N₄-BN, Si₃N₄–SiCN and BN-SiCN. Therefore, the Si₃N₄-BN-SiCN ceramics have outstanding EMW absorption performance and realize an effective absorption bandwidth (EAB) that covers the whole X band and the minimum reflection coefficient (RC) reaches -18.43 dB at a thickness of 3.37 mm.     

Enhancement of the microwave absorption properties of PyC-SiCf/SiC composites by electrophoretic deposition of SiC nanowires on SiC fibers

The employment of coating technique on the silicon carbide fibers plays a pivotal role in preparing SiC fiber-reinforced SiC composites (SiC_f/SiC) toward electromagnetic wave absorption applications. In this work, SiC nanowires (SiCNWs) are successfully deposited onto the pyrolytic carbon (PyC) coated SiC fibers by an electrophoretic deposition method, and subsequently densified by chemical vapor infiltration to obtain SiCNWs/PyC-SiC_f/SiC composites. The results reveal that the introduction of SiCNWs could markedly enhance the microwave absorption properties of PyC-SiC_f/SiC composites. Owing to the increasing of SiCNWs loading, the minimum reflection loss of composites raises up to −58.5 dB in the SiCNWs/PyC-SiC_f/SiC composites with an effective absorption bandwidth (reflection loss ≤ −10 dB) of 6.13 GHz. The remarkable enhancement of electromagnetic wave absorption performances is mainly attributed to the improved dielectric loss ability, impedance matching and multiple reflections. This work provides a novel strategy in preparing SiC_f/SiC composites with excellent electromagnetic wave absorption properties.     

Effect of SiC nanowires on the high-temperature microwave absorption properties of SiCf/SiC composites

SiC-nanowire-reinforced SiC_f/SiC composites were successfully fabricated through an in situ growth of SiC nanowires on SiC fibres via chemical vapour infiltration. The dielectric and microwave absorption properties of the composites were investigated within the frequency range of 8.2–12.4 GHz at 25–600 °C. The electric conductivity and complex permittivity of the composites displayed evident temperature-dependent behaviour and were enhanced with increasing temperature. The composites exhibited superior microwave absorption abilities with a minimum reflection loss value of ?47.5 dB at 11.4 GHz and an effective bandwidth of 2.8 GHz at 600 °C. Apart from the contribution of the interconnected SiC nanowire network and multiple reflections, the excellent microwave absorption performance was attributed to dielectric loss that originated from SiC nanowires with abundant stacking faults and heterostructure interfaces. Results suggested that the composites are promising candidates for high-temperature microwave absorbing materials.     

Effects of Ni2+, Zr4+, or Ti4+ doping on the electromagnetic and microwave absorption properties of CaMnO3 particles

Although CaMnO₃ has been widely studied and used for its thermoelectric properties and giant magnetoresistance effect, little information exists about its application for microwave absorption. In this study, we synthesized CaMnO₃, CaNi_0.05Mn_0.95O₃ CaTi_0.05Mn_0.95O₃ and CaZr_0.05Mn_0.95O₃ with an orthorhombic system using a simple high-temperature solid-phase method. The minimum reflection loss value and effective absorption bandwidth could be efficiently improved due to the enhanced match complex permittivity produced after the Ni, Ti or Zr ions were substituted for Mn ions in CaMnO₃. The minimum reflection loss value increases to ?39.7 dB from ?14.1 dB and the effective absorption bandwidth increases to 4.9 GHz from 2.7 GHz. The magnetic loss results only in a negligible influence on the microwave absorption. The enhancement of microwave absorption properties was primarily due to the stronger polarization effect. When Ni²⁺, Ti⁴⁺, or Zr⁴⁺ is introduced in the CaMnO₃ lattice, the charge balance is broken, and the crystal lattice distortion increases because of the substitutive ions, interstitial ions, oxygen vacancy and exchange effect of Mn³⁺~Mn⁴⁺. The results indicate that CaMnO₃ with reasonable doping at the Mn-site could achieve excellent microwave properties of wide bandwidth, high-efficiency absorption, and adjustable response frequency.     

Microstructural evolution and microwave transmission/absorption transition in polymer-derived SiOC ceramics

Herein, we present the structural evolution of polymer-derived SiOC ceramics with the pyrolysis temperature and the corresponding change in their microwave dielectric properties. The structure of the SiOC ceramics pyrolyzed at a temperature lower than 1200 °C is amorphous, and the corresponding microwave complex permittivity is pretty low; thus, the ceramics exhibit wave transmission properties. The Structural arrangement of free carbon in the SiOC ceramics mainly happens in the temperature range of 1200 °C-1300 °C due to the separation from the Si–O–C network and graphitization, while the structural arrangement of the Si-based matrix mainly occurs in the range of 1300 °C-1400 °C owing to the separation of SiC₄ from the Si–O–C network to form nanocrystalline SiC. In pyrolysis temperature range of 1200 °C-1400 °C, the microwave permittivity of SiOC shows negligible change. At a pyrolysis temperature exceeding 1400 °C, the carbothermal reaction of free carbon and the Si–O backbone becomes significant, leading to the formation of crystalline SiC. The as-formed SiC and residual defective carbon improve the polarization loss of SiOC ceramics. In this case, the SiOC ceramics show significantly increased complex permittivity, exhibiting electromagnetic absorption characteristics. These characteristics promote the application of polymer-derived SiOC ceramics to high-temperature electromagnetic absorption materials.     

Multiscale SiCnw and carbon fiber reinforced SiOC ceramic with enhanced mechanical and microwave absorption properties

The hierarchical SiOC/SiCnws/CF composites (A/B/C structure) were designed via precursor infiltration and pyrolysis process. The SiOC ceramics served as the wave-transparent characteristic materials to adjust the impedance matching. Herein, the in situ growth of SiCnws not only was used for the absorber but served as the reinforcement of SiOC matrix composites. In detail, the SiOC/SiCnws/CF-1200°C-2 exhibits excellent electromagnetic absorption performance with minimum reflection loss of −46 dB at 14.4 GHz with the thickness of 1.6 mm, and its effective absorption band reaches 4.3 GHz. Its compressive strength reaches 8.69 and 16.41 MPa in z and x/y directions, separately. This contribution has a guidance for the application of carbon fiber/ceramics matrix composites in harsh environment.