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.     

Dielectric and EMW absorbing properties of PDCs-SiBCN annealed at different temperatures

Polymer derived SiBCN ceramics (PDCs-SiBCN) were prepared from polyborosilazane and then annealed at 1200–1800 °C in N₂. Effects of annealing temperature on the microstructure, phase composition, dielectric and wave-absorbing properties of ceramics were investigated. Results showed that nano-sized SiC grains were formed in amorphous SiBCN after annealing and the content and crystallization degree of SiC gradually intensified with annealing temperature increasing. The permittivity, dielectric loss and electrical conductivity of PDCs-SiBCN gradually increased as the temperature rose due to the formation of conductivity network of SiC grains and the increase of nano-grain boundary. The increased content of SiC (as the dipole) and interface between SiC nano-grains and amorphous SiBCN phase led to a higher polarization ability and higher dielectric loss. The RC gradually decreased with the annealing temperature increasing, demonstrating the annealed ceramics had the superior wave-absorbing ability and high annealing temperature was conducive to the improvement of wave-absorbing property.     

热解含铁聚硅氧烷原位生长硅氧碳纤维制备硅氧碳复合材料

将铁氯化物混入聚硅氧烷前驱体进行交联成型和热解,利用热解中在聚硅氧烷中形成的孔隙和在孔隙中形成的铁颗粒为催化剂,在硅氧碳陶瓷基体中原位生长出硅氧碳纳米纤维,制备出硅氧碳陶瓷和硅氧碳纤维复合材料。用扫描电子显微镜观察材料断面,结果显示：在硅氧碳陶瓷基体中生长出纳米纤维,部分纤维取向分布,纤维紧贴于硅氧碳陶瓷基体,二者呈良好结合;能谱分析显示纤维中含硅、氧和碳,证实其为硅氧碳。所制得的硅氧碳陶瓷和硅氧碳纤维的复合结构不同于通常热解纯聚硅氧烷形成的单相的硅氧碳结构,在硅氧碳基体中的硅氧碳纤维是在聚硅氧烷前驱体中引入的铁催化剂在热解过程中通过催化聚硅氧烷一维生长形成的,该过程可用于发展一步法原位制备纳米纤维前驱体陶瓷复合材料。     

Microstructure and EMW absorbing properties of SiCnw/SiBCN-Si3N4 ceramics annealed at different temperatures

SiC nanowire/siliconboron carbonitride-Silicon nitride (SiC_nw/SiBCN-Si₃N₄) ceramics were prepared via a low-pressure chemical vapor deposition and infiltration (LPCVD/CVI) technique. The as-prepared ceramics were annealed at varying temperatures (1200–1600 °C) in a N₂ atmosphere, and their crystallization mechanism and absorbing properties were subsequently studied. The absorbing properties of the SiC_nw/SiBCN-Si₃N₄ ceramics improved with the annealing temperature up to a certain value and decreased thereafter. Among the samples tested, the SiC_nw/SiBCN-Si₃N₄ ceramics annealed at 1300 °C showed the highest permittivity (real and imaginary parts) and dielectric loss values in the X-band (ca. 5.34, 2.55, and 0.47 respectively), and this could be attributed to the precipitation of carbon and SiC nanocrystals. The sample treated at 1300 °C decreased its minimum reflection coefficient (RC) from −12.0 to −59.68 dB (compared with the as-received SiC_nw/SiBCN-Si₃N₄ ceramics) and the effective RC (below -10 dB) in the whole X-band could be achieved when the thickness was set to 3–3.5 mm. These results revealed that the absorbing performance was significantly improved after the heat treatment at 1300 °C.     

Electromagnetic wave absorption properties of polymer-derived magnetic carbon-rich SiCN-based composite ceramics

In this paper, the mixture of Fe and Ni nanoparticles (abbreviated as FeNi) was added to liquid polysilazane (PSZ) as a magnetic source, to prepare a series of magnetic carbon-rich SiCN-based composite ceramics by adjusting the mass ratio of FeNi through the polymer derivation method. The phase composition, microstructure, conductivity, electromagnetic wave (EMW) absorption performance and mechanism of composite ceramics prepared were discussed. The analysis shows that the introduction of magnetism has adjusted the impedance matching and improved the magnetic loss performance of composite ceramics on the whole, and the dielectric loss ability of composite ceramics has been strengthened benefiting from the formation of conductive path of CNTs precipitated by FeNi catalysis in the matrix. Therefore, the addition of magnetic particles improves the EMW absorption peak intensity and effective absorption bandwidth (EAB) of composite ceramics. When the addition amount of FeNi was 5 wt%, the sample 5# exhibited the best comprehensive EMW absorption performance: Its minimum reflection loss (RL_min) was ?18 dB and the EAB was 2.5 GHz when the thickness was 1 mm, the EAB covering the C, X and Ku bands can be obtained by adjusting the thickness from 1.0 mm to 4.0 mm. Through calculation, the EAB (EAB_tf) of 5# with a thickness of 1 mm and a filling rate of 1 wt% can reach 50, which is significantly higher than that of a series of SiCN-based composite ceramics previously reported. In addition, the density of 5# was 2.3 g/cm³, and its compressive strength (CS) can reach 337 MPa. The data shows that the composite ceramic 5# prepared in this experiment has the merits of light weight, excellent comprehensive EMW absorption performance and good compression resistance, and is expected to be one of the promising materials in the field of new-generation EMW absorbers.     

UV-curable inorganic precursors enable direct 3D printing of SiOC ceramics

3D ceramic parts are of great interest for various applications including aerospace, defense, electronics, photonics, and biomedical. Yet, additive manufacturing of ceramics is challenging due to their poor machinability. Herein, two approaches based on the chemical modification of silicon resins to obtain UV-curable preceramic precursors of SiOC are described. The dual functionality of the synthesized resins acting both as preceramic precursor and as photopolymerizable entity under UV light is exploited. A set of characterization techniques has allowed the investigation of the mechanisms involved in the synthesis of the inorganic SiOC precursors according to the following approaches: (1) blend of the silicon resin with photoactive monomers and (2) synthesis of a single source UV-curable preceramic silicon resin. A correlation between the nature of the precursor and the properties of the derived SiOC is analyzed. From a technological point of view, the materials can be fabricated as dense or crack-free porous customized objects with low mass loss and optimal surface quality.     

Electrochemical performance of SiOC anodes prepared by a different silicone

SiOC is one of the most promising anodes for lithium-ion batteries, which shows the good structural stability and high capacity comparing to commercial graphite anode. In this paper, different SiOC anodes (SiOC-217, SiOC-H44, and SiOC-MK) were prepared from polymer precursors with different side groups (phenyl, methyl-phenyl, methyl) to investigate the effects of free carbon on the electrochemical performance of SiOC anodes. The results of X-ray photoelectron spectroscopy presented that SiOC was composed by different SiO_xC_4−x units and free carbon phase. The initial discharge capacity of SiOC-217 was 742.67 mA h g⁻¹. After 100 cycles, the reversible capacity of SiOC-217 reached 450.65 mA h g⁻¹ at 0.2 C, indicating a capacity retention rate of 60.68%. After cycling at high current densities, SiOC-217 exhibited a high discharge capacity of 592.88 mA h g⁻¹ at 0.1 C. SiOC-217 exhibited excellent electrochemical performance due to the high content of free carbon phase. Furthermore, the high contents of SiO₂C₂ and SiO₃C units further enhanced the improvement of electrochemical performance.     

Enhanced mechanical properties of amorphous SiOC nanofibrous membrane through in situ embedding nanoparticles

Flexible ceramic nanofibers are highly desired due to their potential applications in free‐standing catalyst supports, fine particulate filters and flexible electronic devices. In this work, robust SiOC fibrous membranes composed of randomly oriented nanofibers with an average diameter of 550 nm were fabricated by a combination of electrospinning and post heat‐treatment process. The mechanical properties of the as‐prepared membranes were enhanced significantly through in situ embedding of palladium nanoparticles into the SiOC fibers. The optimized palladium‐doped SiOC fibrous membrane demonstrated a low flexural modulus of 7.79 kPa and a high tensile strength of 33.2 MPa. Reduced flaw size, initiation of nanocracks and pinning effect were proposed to explain the enhancement mechanism. Furthermore, the flexible SiOC membrane with excellent corrosion resistance exhibits a high filtration efficiency of 99.6% when the membrane weight is 4.8 g m^?2, suggesting efficient filtration applications in harsh environments. This work also provides a feasible strategy for the design and fabrication of the flexible amorphous ceramic fiber membranes for various applications.     

Impact of blending with polystyrene on the microstructural and electrochemical properties of SiOC ceramic

In this work, we present the electrochemical behavior and microstructural analysis of silicon oxycarbide (SiOC) ceramics influenced by an addition of polystyrene (PS). Polymer-derived ceramics were obtained by pyrolysis (1000°C, Ar atmosphere) of different polysiloxanes prepared by sol–gel synthesis. This method is very effective to obtain desired composition of final ceramic. Two alkoxysilanes phenylthriethoxysilane and diphenyldimethoxysilane were used as precursors. Before pyrolysis polysiloxanes were mixed with PS using toluene as a solvent. Blending with PS affects the microstructure and free carbon content in the final ceramic material. Free carbon phase has been confirmed to be a major lithium storage host. Nevertheless, we demonstrate here that capacity does not increase linearly with increasing carbon content. We show that the amount of SiO₄ units in the SiOC microstructure increases the initial capacity but decreases the cycling stability and rate capability of the material. Furthermore, the microstructure of the free carbon influences the electrochemical performance of the ceramic: More ordered graphitic clusters favor better rate capability performance.     

不同温度下退火的二氧化钛纳米薄膜的光催化性能

采用磁控直流溅射的方法,使用TiO2靶材直接制备二氧化钛纳米薄膜,并在不同温度(500、550或600°C)下进行退火处理.利用扫描电子显微镜(SEM)、X射线衍射仪(XRD)和紫外可见光分光光度计(UV-VIS)研究了热处理温度对TiO2纳米薄膜性能的影响.结果表明,热处理温度为550°C时TiO2纳米薄膜的光催化性...     

Dielectric properties and electromagnetic wave absorbing performance of single-source-precursor-derived carbon-rich NbC-SiC-C nanocomposites

For the first time, the present work reports the dielectric properties and electromagnetic wave (EMW) absorbing performance of polymer-derived carbon-rich NbC-SiC-C nanocomposites. In our previous work, NbC-SiC-C nanocomposites with the ultra-high temperature ceramic phase NbC as the main phase were synthesized with the allylhydridopolycarbosilane (AHPCS) and niobium pentachloride (NbCl₅) as starting materials. On this basis, divinyl benzene was chosen as carbon-rich source and introduced into the AHPCS and NbCl₅ to form a single-source-precursor. Finally, carbon-rich NbC-SiC-C nanocomposites were successfully synthesized by polymer-derived ceramic approach. Compared with ceramic samples without Nb and with lower carbon content, the carbon-rich NbC-SiC-C nanocomposites show extremely enhanced EMW absorbing performance with minimum reflection coefficient of −51.1 dB at 6.88 GHz for the thickness of 2.27 mm. As a consequence, the resultant carbon-rich NbC-SiC-C nanocomposite has to be considered as structure&function integrated material with excellent EMW absorption performance, which can be applied in hostile environment.     

Structure and energetics of SiOC and SiOC‐modified carbon‐bonded carbon fiber composites

The incorporation of SiOC polymer‐derived ceramics into porous carbon materials could provide tailored shapeable, mechanical, electrical, and oxidation‐resistant properties for high‐temperature applications. Understanding the thermodynamic and kinetic stability of such materials is crucial for their practical application. We report here the dependence of structures and energetics of SiOC and SiOC‐modified carbon‐bonded carbon fiber composites (CBCFs) on the pyrolysis temperature using spectroscopic methods and high‐temperature oxide melt solution calorimetry. The results indicate that a SiOC ceramic pyrolyzed at 1200°C and 1600°C is energetically stable with respect to an isocompositional mixture of cristobalite, silicon carbide, and graphite by 4.9 and 10.3 kJ/mol, respectively, and more energetically stable than that pyrolyzed at 1450°C. Their thermodynamic stability is related to their structural evolution. SiOC‐modified CBCFs become energetically less stable with increasing preparation temperature and concomitant increase in excess carbon content.     

Synthesize and crystallization behavior of SiOC/SiC nanocomposites derived from organosilane slurry residue

A route preparing SiOC/SiC nanocomposites directly by pyrolysis of organosilane slurry residue was investigated. Organosilane slurry residue's unique composition, containing both silicon and carbon, offers an intriguing platform for developing advanced ceramic materials. The pyrolysis process is examined comprehensively, revealing the chemical reactions and structural changes leading to SiC crystals formation. The phase evolution at various annealing temperatures was revealed. Crystallization behavior in the process were studied. The results reveal that SiOC matrix was generated at annealed temperature 800°C and SiC nanoparticles were formed at 1300°C. In comparison to phase separation of SiOC, carbothermal reduction of SiO₂ was domain in SiC formation. This research advances the understanding of SiOC/SiC nanocomposites, highlighting the value of repurposing industrial byproducts for sustainable and innovative materials development.     

Effect of anionic substitution on the high temperature stability of polymer-derived SiOC glasses

In this study the high temperature stability (crystallization and decomposition) of two silicon oxycarbide glasses with a similar amount of free carbon (8.3 vs 9.6 wt%) but different content of Si-C bonds (SiC_0.22O_1.57 vs SiC_0.07O_1.86) is presented. The two SiOC glasses are obtained from the same precursor (2 µm methyl-silsesquioxane spheres) via pyrolysis at 1100°C in inert (Ar) or reactive (CO₂) atmospheres. Further annealing in Ar flow at temperatures above 1100°C and up to 1500°C is performed and the samples are characterized by Fourier Transformed Infrared Spectroscopy (FT-IR) and X-ray diffraction (XRD). For comparison purposes the same precursor was annealed in air flow to obtain SiO₂ and its high temperature evolution is also studied. Results suggest that the onset for the carbothermal reduction is not dependent on the amount of Si-C bonds. Moreover, contrary to what is usually reported in the scientific literature, silica phase present in the SiOC glasses does not show, in the same experimental conditions, superior crystallization resistance compared to pure silica glass.     

SiOC Ceramic Foams through Melt Foaming of a Methylsilicone Preceramic Polymer

A process for the production of SiOC ceramic foams has been for the first time developed through melt foaming of a siloxane preceramic polymer with the help of a blowing agent, followed by pyrolysis under an inert atmosphere. The raw material consisted of a methylsilicone resin, a catalyst (which accelerated the cross-linking reaction of the silicone resin) and a blowing agent (which generated gas above 210°C). Methylsilicone resin foams were obtained through controlling the melt viscosity around 210°C, temperature where the blowing agent started to decompose, by varying the initial molecular weight of the preceramic polymer and the amount of the catalyst. The obtained SiOC ceramic foams exhibited excellent oxidation stability up to 1000°C, as shown by thermal gravimetric analysis (TGA). As expected, the mechanical properties of the SiOC ceramic foams varied as a function of their bulk density, possessing a flexural strength up to 5.5 MPa and a compression strength up to 4.5 MPa. The main steps in the process, namely foaming and pyrolysis, were analyzed in detail. The viscosity change was analyzed as a function of temperature by the dynamic shear measurement method. The pyrolysis process of foams was analyzed by TGA coupled with infrared spectroscopy (IR).     

Effect of additive structure and size on SiO2 formation in polymer‐derived SiOC ceramics

Silicon oxycarbide (SiOC) ceramics with highly adjustable properties and microstructures have many promising applications in batteries, catalysis, gas separation, and supercapacitors. In this study, additive structures on the nucleation and growth of SiO₂ within SiOC ceramics are investigated by adding cyclic tetramethyl‐tetravinylcyclotetrasiloxane (TMTVS) or caged octavinyl‐polyhedral oligomeric silsesquioxane (POSS) to a base polysiloxane (PSO) precursor. The effects of the 2 additives on the polymer‐to‐ceramic transformation and the phase formation within the SiOC are discussed. POSS encourages SiO₂ nucleation and leads to more SiO₂ formation with significantly increased ceramic yield, which subsequently leads to higher specific surface of 1557 m²/g with a larger pore size of ～1.8 nm for the porous SiOC. High TMTVS content decreases both the specific surface area and pore volume of the resulting porous SiOCs. This study demonstrates a new approach of using Si‐rich additive POSS to increase the SiOC yield while maintaining or even increasing the specific surface area.     

Structural and thermodynamic analysis of metal filler incorporations in SiaOb(M)cCd polymer derived ceramics: Ta,Hf, Nb

This work systematically investigates the thermodynamic stability of Si_aO_b(M)_cC_d structures derived from polymeric precursors incorporating metal fillers: Ta, Nb, and Hf, at 1200 and 1500°C. Structural characterization of the polymer derived ceramics (PDCs) employs X-ray diffraction, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy. Enthalpies of formation relative to crystalline components (metal oxide, silica, silicon carbide, and graphite) are obtained from thermodynamic measurements by high temperature oxide melt solution calorimetry. The enthalpies of formation (∆H°_{f, comp}) of Ta-1200, Hf-1200, Nb-1200, Ta-1500, Hf-1500, and Nb-1500 specimens are −137.82 ± 9.72, −256.31 ± 8.97, −82.80 ± 9.82, −182.80 ± 7.85, −292.54 ± 9.38, −224.98 ± 9.60 kJ/mol, respectively. Overall incorporation of Hf results in most thermodynamically stable structures at all synthesis temperatures. Si_aO_b(M)_cC_d specimens employing Nb fillers undergo the most stable structural evolution in this temperature range. The results indicate strong thermodynamic drive for carbothermal reduction of metal oxide domains. Incorporation of Ta provides the greatest stabilization of SiO₃C mixed bonding environments. Ultimately, the choice of metal filler influences composition, structural evolution, and thermodynamic stability in PDCs.     

Energetics and structure of SiC(N)(O) polymer-derived ceramics

This study presents new experimental data on the thermodynamic stability of SiC(O) and SCN(O) ceramics derived from the pyrolysis of polymeric precursors: SMP-10 (polycarbosilane), PSZ-20 (polysilazane), and Durazane-1800 (polysilazane) at 1200°C. There are close similarities in the structure of the polysilazanes, but they differ in crosslinking temperature. High-resolution X-ray photoelectron spectroscopy shows notable differences in the microstructure of all polymer-derived ceramics (PDCs). The enthalpies of formation (∆H°_{f, elem}) of SiC(O) (from SMP-10), SCN(O) (from PSZ-20), and SCN(O) (from Durazane-1800) are −20 ± 4.63, −78.55 ± 2.32, and −85.09 ± 2.18 kJ/mol, respectively. The PDC derived from Durazane-1800 displays greatest thermodynamic stability. The results point to increased thermodynamic stabilization with addition of nitrogen to the microstructure of PDCs. Thermodynamic analysis suggests increased thermodynamic drive for forming SiCN(O) microstructures with an increase in the relative amount of SiN_xC_4−x mixed bonds and a decrease in silica. Overall, enthalpies of formation suggest superior stabilizing effect of SiN_xC_4−x compared to SiO_xC_4−x mixed bonds. The results indicate systematic stabilization of SiCN(O) structures with decrease in silicon and oxygen content. The destabilization of PDCs resulting from higher silicon content may reach a plateau at higher concentrations.