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.     

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.     

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.     

Stereolithography-based additive manufacturing of polymer-derived SiOC/SiC ceramic composites

In order to overcome challenges typically encountered during additive manufacturing of ceramics via the polymer precursor route, a novel polymer-derived SiOC/SiC composite system suitable for advanced geometric designs achievable by lithography-based ceramic manufacturing was established. The photoreactive resin system filled with 20 wt% SiC exhibits suitable viscosity characteristics, adequate stability against sedimentation, and a fast photocuring behavior. After printing and pyrolytic conversion, SiC particulates were well-dispersed within the polymer-derived SiOC matrix. A direct comparison with the unfilled polysiloxane-based resin system showed that the addition of particulate SiC increases handleability, reduces shrinkage, and significantly increases critical wall thicknesses up to 5 mm. The biaxial Ball-on-Three-Balls testing methodology yielded a characteristic strength of 325 MPa for SiOC/SiC composites. The results highlight the high potential of particle-filled preceramic polymer systems toward the fabrication of high-performance SiC-based materials by lithography-based additive manufacturing.     

The influence of carbon materials on the absorption performance of polymer-derived SiCN ceramics in X-band

Carbon-containing polymer-derived SiCN ceramics (PDCs-SiCN-C) were successfully fabricated with multi-layer graphene (MLG) and multiwalled carbon nanotubes (MWCNTs) as additives at 1100?°C. The effects of MLG and MWCNTs on the microwave absorption properties of PDCs-SiCN-C ceramics were analyzed. The imaginary permittivity and loss tangent of SiCN-MLG and SiCN-MWCNTs were about 3.4, 0.67 at 11.2?GHz and 3.1, 0.57 at 10.6?GHz, respectively. The minimum reflection loss of SiCN-MLG and SiCN-MWCNTs at 3?mm was ??54?dB and ??48?dB with the effective absorption bandwidth (RL ≤ ?10?dB, >90% absorption) about 1.5?GHz and 0.9?GHz in X-band.     

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.     

Structure and microwave dielectric properties of NaSr4V5O17 ceramics for LTCC applications

A new microwave dielectric ceramic, NaSr₄V₅O₁₇ with low firing temperature was fabricated via the traditional mixed oxide method. Rietveld refinement of XRD profiles and Raman spectrum analysis ascertained that the NaSr₄V₅O₁₇ compounds crystallized into Sr₂V₂O₇-like triclinic structure with space group P-1 (2) and Z = 1.6. The variation of Q × f value was explained by the combined effects of mean grain size and cell volume rather than packing fraction and bond valence. The change regulation of ε_r was similar to that of density. The |τ_f| value is mainly related to the cations bond valence. The NaSr₄V₅O₁₇ ceramics sintered at 725 °C showed good compatibility with Ag electrode and superior dielectric properties: ε_r = 8.6, Q × f = 45 900 GHz, τ_f = ?57.0 ppm/K, making it a potential application for LTCC.     

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.     

Macroporous polymer-derived SiO2/SiOC monoliths freeze-cast from polysiloxane and amorphous silica derived from rice husk

A freeze-casting route towards macroporous SiOC/SiO₂ ceramic nanocomposites from preceramic polymers was developed. Amorphous SiOC/SiO₂ monolith with pore channels aligned along the freezing direction were obtained from commercially available methyl-phenyl-vinyl-hydrogen polysiloxane (Silres® H62C) and amorphous silica derived from rice husk ash freeze-cast with water or tert-butyl alcohol, crosslinked and pyrolyzed at 1100 °C in nitrogen. The influence of processing parameters such as solvent (tert-butyl alcohol or water), polymer to silica ratio (2:1, 1:1, 1:2), cooling rate (2, 4, 6 °C/min) and pre-crosslinking of polysiloxane on the porosity and structure of the obtained ceramic nanocomposites were assessed by X-ray tomography, XRD, solid state NMR, scanning electron microscopy and mercury porosimetry. The microstructure of SiOC ceramics derived from the Silres H62C polysiloxane was studied as well.     

Effects of structural transition on microwave dielectric properties of Sr3(Ti1-xSnx)2O7 ceramics

Sr₃(Ti_1-xSn_x)₂O₇ (x = 0–1.0) ceramics were prepared via a standard solid-state reaction method. X-ray diffraction patterns and Rietveld refinement results indicated a composition induced onset of octahedral tilting when x > 0.2, and the crystal structure transformed in sequence: tetragonal (I4/mmm) → coexistence of tetragonal and orthorhombic (I4/mmm + Amam) → orthorhombic (Amam). The τ_f value could be successfully tuned towards zero and the effects of octahedral tilting on the evolution of τ_f value were emphasized. Meanwhile, the role of tolerance factor in tailoring the resultant τ_ε of the present ceramics was revealed and compared with the empirical rule for complex perovskites. Qf value decreased monotonously with increasing x, which could be elucidated by the variations of extrinsic parameters and intrinsic dielectric loss extrapolated from the infrared reflectivity spectra. The optimum microwave dielectric properties were achieved at x = 0.8 (ε_r = 18.6, Qf = 45,250 GHz, τ_f =–14 ppm/^oC).     

The role of carbon in microstructure evolution of SiBCO ceramics

The composition of polymer derived ceramics could be readily tuned through controlling the structure and element content of the polymer precursors, and investigation on the effect of the element on microstructure evolution is important to the design of advanced ceramics. In this article, the effect of carbon content in SiBCO polymer precursors was systematically investigated. The polymer network and thermal stability of polymer precursors and the carbon content of pyrolyzed SiBCO ceramic could be readily tuned by controlling the DVB amount used. Carbon contributed to the formation of graphitic carbon in SiBC_xO ceramics and inhibited the growth of β–SiC and SiO₂ crystals at 1600 °C, but lead to an increase in the graphitic carbon phase at 1800 °C.     

Phase composition,crystal structure,and microwave dielectric properties of Nb-doped and Y-deficient yttrium aluminum garnet ceramics

Nb-doped and Y-deficient yttrium aluminum garnet ceramics were designed and synthesized using the solid-state reaction method according to the chemical equation Y_3?xAl₅Nb_xO_12+x (0 ≤ x ≤ 0.16). The phase composition, sintering behavior, microstructure, and microwave dielectric properties were investigated as functions of the composition and sintering temperature. A single-phase solid solution of yttrium aluminum garnet structure formation was observed in the range of 0 ≤ x ≤ 0.1. Further increments in x prompted the precipitation of the YNbO₄ secondary phase at the grain boundary of Y₃Al₅O₁₂. The complexity of the phase composition degrades the micromorphology and dielectric properties of the ceramics to varying degrees. Transmission electron microscopy results show that the lattice exhibits additional symmetry, which is closely related to the ultrahigh Q×f values of the ceramics. Effectively improving the sintering behaviour and suppressing the secondary phase by simultaneously doping with Nb⁵⁺ and reducing the yttrium stoichiometry. Finally, excellent microwave dielectric properties of ε_r ~ 10.99, Q×f ~ 280,387 GHz (13.5 GHz), and τ_f ~ ? 34.7 ppm/°C can be obtained in x = 0.1 (Y_2.9Al₅Nb_0.1O_12.1) sintered at 1700 °C for 6 h.     

Effect of Li nonstoichiometry and TiO2 addition on the microwave dielectric properties of Li3PO4 ceramics

Li₃PO₄ ceramic is a promising microwave ceramic material with low dielectric constant. The effect of Li nonstoichiometry on phase compositions, microstructures, and microwave dielectric characteristics of Li₃PO₄ ceramics, on the other hand, has been examined infrequently. Therefore, in the first part of this study, the stoichiometry and Li nonstoichiometry compositions based on Li_3+xPO₄(x = 0, 0.03, 0.06, 0.09, 0.12 and 0.15) were prepared by conventional solid-phase method. The results show that a few nonstoichiometric lithium ions enter the lattice of Li_3+xPO₄. Compared with the chemical content of Li₃PO₄, the sintering characteristics, relative dielectric constants and quality factors of Li_3+xPO₄ ceramics can be improved by slightly excessive Li ions, while the properties of Li₃PO₄ ceramics can be deteriorated by excessive Li ions. Li_3.12PO₄ ceramics sintered at 975 °C for 2 h have good dielectric properties (ε_r = 5.89, Q×f = 44,000 GHz, τ_f = ?206 ppm/^°C). In order to improve its large negative temperature coefficient of resonant frequency, in the following study, rutile nano TiO₂ particles were added as τ_f compensator. Adding TiO₂ powders not only effectively improve the temperature stabilities of the multiphase ceramics, but also make the grain growth more uniform. With the increase of TiO₂ content from 0.40 to 0.60, τ_f increases from ?73.5 ppm/^°C to +42.3 ppm/^°C. The best dielectric property of 0.45Li_3.12PO₄-0.55TiO₂ composite ceramic is ε_r = 13.29, Q×f = 40,700 GHz, τ_f = +8.8 ppm/^°C.     

Structural evolution and microwave dielectric properties in Sr2(Ti1-xSnx)O4 ceramics

The structure and microwave dielectric properties of Sr₂(Ti_1-xSn_x)O₄ ceramics were determined in the entire composition range of x?=?0–1.0. X-ray diffraction patterns and Raman spectra indicated a composition-induced onset of octahedral tilting at x?=?0.75, and the crystal structure transformed from tetragonal (I4/mmm) to orthorhombic (Pccn). An obvious change of grain morphology was observed in the phase transformation region as well. The variations of the microwave dielectric properties with composition were systematically investigated and the effect of octahedral tilting on the evolution of τ_f value was emphasized. Moreover, the relationship between τ_ε and tolerance factor of the present ceramics was revealed and compared with the empirical rule in perovskite structure. The role of tolerance factor in designing the materials with required performance was highlighted.     

The effects of dispersants on sinterability and microwave dielectric properties of Zr0.8Sn0.2TiO4 ceramics

This study investigated the effects of dispersants (deionised water and ethanol) on the sinterability, phase compositions and microwave dielectric properties of Zr_0.8Sn_0.2TiO₄ ceramics prepared by a solid-state reaction. Results showed the presence of impurity phases in low-density ceramics with ethanol as dispersant sintered from 1500?°C to 1550?°C. However, pure phase was detected in samples prepared with deionised water as dispersant when sintering temperature ranged from 1512?°C to 1550?°C. The microwave dielectric properties of the samples with deionised water significantly improved compared with those with ethanol. Thus, deionised water was suitable for preparing Zr_0.8Sn_0.2TiO₄ ceramics with a high density of approximately 98%, ε_r of 39.83, Q ×?f of 33,700?GHz and τ_f of +?3.5?ppm/°C.     

Multiscale 2D/3D microshaping and property tuning of polymer-derived SiCN ceramics

Polymer-derived ceramics exhibit excellent properties and are compatible with many shaping techniques due to their liquid precursors. We present a fast and pressureless process for the fabrication of SiCN. Using varied amounts of the filler divinyl benzene, defect-free monolithic disc samples are obtained at high yields. Their electrical conductivity is adjustable across 10 orders of magnitude, flexural strength is improved up to 1.7 GPa, and cytocompatibility is demonstrated. This processing route is applied to a new multiscale microshaping method combining the advantages of two-photon polymerization and casting. The parts’ general shape is defined by KOH-etched silicon molds whereas individual freeform microfeatures like a 3D QR code are implemented through sacrificial 2PP photoresin microstructures added to the mold. The green body is pyrolyzed directly in the mold, whereby the photoresin decomposes and the ceramic part with the submicrometer resolution features imprinted releases itself from the mold undamaged due to ～30% shrinkage.     

Polymer-derived SiOC ceramics: A potential catalyst support controlled by the sintering temperature and carbon content

A series of silicon oxycarbide ceramics with varying carbon content from ca. 10 wt% to ca. 40 wt% were prepared by thermal pyrolysis of four commercially available polysiloxanes and subsequent spark plasma sintering (SPS) at 1200 °C, 1400 °C, and 1600 °C. The results showed that the high carbon content led to a porous microstructure, and for SiOC with ca. 40 wt% carbon content, its porosity and specific surface area at 1600 °C reached 34% and 262 m²/g, respectively. The electrochemical behavior of materials was evaluated. It was shown that SiOC has a certain degree of electrocatalytic activity, and the sample with 10 wt% carbon content obtained at 1200 °C exhibited an overpotential of 450 mV vs. RHE at 10 mA·cm⁻² in acid medium. Finally, it was analyzed that the electrochemical behavior of SiOC is closely related to the phase composition and microstructure of the resulting ceramics.     

Electric conductivity and microstructure evolution of polymer-derived SiAlCO ceramics

The electric conductivity and microstructure of polymer-derived SiAlCO ceramic were studied. The conductivity increased drastically with increasing pyrolysis temperature and exhibited a typical Arrhenius dependence on pyrolysis temperature with the activation energy of ~7.15 eV. The microstructure was analyzed by XRD, Raman spectroscopy and XPS. Unlike previous reported PDCs, the increase in the conductivity of the SiAlCO cannot be explained by the structural changes of the free carbon phase. It is speculated that the increased conductivity is possibly due to the redistribution of C–O bonds within the free carbon phase.     

Anomalous microwave dielectric behaviour induced by the orthorhombic-tetragonal phase transition in CaLaGaO4 ceramics

In this work, orthorhombic (o-) and tetragonal (t-) phase CaLaGaO₄ ceramics were synthesized using a conventional solid-state method. Phase evolution and crystal structure were investigated via XRD and TEM, and o-CaLaGaO₄ phase with Pna21 space group can be obtained at a low temperature (<1260 °C). With the increase in sintering temperature, o-CaLaGaO₄ and t-CaLaGaO₄ phases (I4/mmm) coexisted when sintered in the range of 1280 °C – 1300 °C. The microwave dielectric properties of the o-CaLaGaO₄ ceramic sintered at 1240 °C were ε_r = 11.2, Q × f = 63,900 GHz, and τ_f = ?56.6 ppm/℃. By contrast, the phase transformation in the CaLaGaO₄ ceramic led to a significantly increased ε_r (19.5) in t-CaLaGaO₄ and a near-zero τ_f (+2.3 ppm/℃). The theoretical permittivity (ε_th) was calculated on the basis of the Clausius–Mossotti equation and P-V-L theory. Results indicate that in the low-symmetry-structure o-CaLaGaO₄ ceramics, ε_th can be estimated accurately by the latter.