Free Carbon Phase Research Articles

Construction of ZrC@SiC shell-core structure in SiBCN aerogel for enhanced thermal stability and thermal insulation

SiBCN aerogels have drawn increased attention as thermal insulation material, but they still suffer from precipitation and phase transition causing the microstructural collapse and performance deterioration under high-temperature environments. Here, ZrC/SiBCN aerogels were prepared using zirconium (IV) butoxide modified polyborosilazane as precursor through solvothermal reaction, frozen casting followed by frozen drying and pyrolysis. Benefiting from its unique hierarchical cellular structure, ZrC/SiBCN aerogel exhibits super-low density (0.042 g/cm3), high specific surface area (287.9 m2/g) as well as low thermal conductivity (24 mW/(m·K)). Shell-core ZrC@SiC nanocrystals are in-situ formed in amorphous SiBCN matrix through step-by-step carbothermal reduction of oxygen-containing phases and free carbon phase during the pyrolysis process, which effectively inhibits the crystallization and growth of SiC crystals, further enhancing the thermal stability under high-temperature environments. It could maintain a high specific surface area (163 m2/g) and low thermal conductivity (29 mW/(m·K)) after thermal treatment at 1800 °C for 2 h. The excellent structural and performance stability of the SiBCN-based aerogels will expand their applications under high-temperature extreme environments.

Structure–property relations of silicon oxycarbides studied using a machine learning interatomic potential

AbstractSilicon oxycarbides show outstanding versatility due to their highly tunable composition and microstructure. Consequently, a key challenge is a thorough knowledge of structure–property relations in the system. In this work, we fit an atomic cluster expansion potential to a set of actively learned density‐functional theory training data spanning a wide configurational space. We demonstrate the ability of the potential to produce realistic amorphous structures and rationalize the formation of different morphologies of the turbostratic free carbon phase. Finally, we relate the materials stiffness to its composition and microstructure, finding a delicate dependence on Si‐C bonds that contradicts commonly assumed relations to the free carbon phase.

Temperature and frequency dependent conductive behavior study on polymer-derived SiBCN ceramics

Conductive behavior research is vital for in-depth understanding the conductive capabilities of materials as well as promoting the development of their applications in fields such as electronics, energy and sensing. In the article, the AC conductive behavior was systematically evaluated for polymer-derived SiBCN ceramics by complex impedance spectroscopy and electrical modulus methods. The results suggest that SiBCN ceramics have a frequency-independent conductivity at low frequencies, whereas it exhibits a relaxation process with increasing frequency at high frequencies. In addition, the SiBCN ceramics follow the electron hopping conductive mechanism in both low and high-frequency bands over the testing temperature range. By considering the relationship between the prefactor σ0 and T0, the conductive mechanism is revealed to be the band-tailed state hopping instead of the variable range hopping. It is worth noting that although the material has the same conductive mechanism at various testing temperatures, hopping is mainly controlled by the matrix phase at low frequencies; while at high frequencies, hopping is dominated by free carbon phase. Our results provide the underlying insights needed to guide the design of amorphous SiBCN ceramics for applications in electrically relevant fields.

Effect of electric field on the structure of SiCN pyrolyzed at 1600 °C

Application of electric field is known to influence the phase transformation and sintering conditions of ceramics. In this work, the effect of electric field on the phase structure evolution of SiCN pyrolyzed at 1600 °C was studied in comparison with conventional pressureless pyrolyzed SiCN. The structural information was characterized via X-ray diffraction, X-ray photoelectron spectroscopy, transmission electron microscopy, and Raman spectroscopy. It was found that SiCN ceramic prepared via conventional pressureless pyrolysis was composed of SiC, Si3N4 and free carbon phases, while that produced via electric field-assisted pyrolysis consisted of Si3N4 and free carbon phases. No crystalline SiC was detected in SiCN ceramic pyrolyzed via the field-assisted method, indicating that the electric field affected the carbothermal reaction between Si3N4 and C phases and changed the formation mechanism of SiC. Moreover, the electric field promoted structural ordering rearrangement of the free carbon phase and increased the defect content therein.

Preparation of polymer-derived SiOC-Cu ceramic composites and their tribological performance

Cu-containing silicon oxycarbide (SiOC) ceramic composites have been prepared by thermal pyrolysis of polymeric precursors and subsequent spark plasma sintering at 1600 °C. The resulting composites consist of SiO2, SiC, free carbon, and Cu-Si alloys. The microstructure of the composites has been identified via optical microscope and SEM. The mechanical properties, including the Vickers hardness, nano-indentation hardness, and Young’s modulus, of the SiOC composites have been assessed via indentation methods. The effects of Cu content on the tribological properties of the composites have additionally been investigated under different sliding frequencies against Al2O3 spheres at room temperature. It is shown that the introduction of the Cu-Si alloys is effective in reducing the friction coefficient of the material system, which decreases linearly from 0.27 of monolithic SiOC to 0.15 of SiOC with 7 wt% Cu content at 1 Hz. Raman spectroscopy has highlighted the friction-driven evolution of the free carbon phase in the SiOC ceramics. Finally, a correlation between the sliding frequency, Cu-Si content, and tribological properties of SiOC is established.

The effect of graphitization degree of carbon and Si─O─C network on the electrochemical performance of SiOC anodes

AbstractSilicon oxycarbide (SiOC) is a promising anode material for lithium‐ion batteries, owing to the high capacity and superior structural stability. However, the application of SiOC anode is limited by its low conductivity and poor rate performance. Here, SiOC anodes were prepared by the pyrolysis of silicone resin (RSN‐217) at different temperatures (700, 850, 1000, and 1150° C), and the effect of graphitization degree and Si─O─C network on the electrochemical performance of SiOC anodes was investigated. The elemental and structural characteristics of the pyrolyzed SiOC ceramics at different temperatures were inspected using a broad spectrum of characterization techniques, and electrochemical performance testing methods, such as cyclic voltammetry and galvanostatic charge/discharge, were conducted on SiOC anode. SiOC‐1000 showed excellent electrochemical performance due to the highest graphitization degree of free carbon phase and the highest content of SiO2C2 and SiO3C units. Considerable research effort laid a foundation for exploring the lithium storage mechanism of SiOC anode.

Temperature-dependent electrical and dielectric behavior of polymer-derived SiAlCN ceramics

Polymer-derived SiAlCN ceramics as sensing materials are promising materials for wireless temperature sensors to achieve application in high-temperature harsh environments. However, limited understanding for the electric and dielectric properties of SiAlCN ceramics restricts the development of wireless sensors. Herein, the electrical and dielectric properties of polymer-derived SiAlCN ceramics pyrolyzed at different temperatures are investigated systemically. The results reveal that the electric resistance of the free-carbon phase and amorphous SiAlCN ceramic phase are comparable, however, the dielectric constant of the ceramic phase is much higher than that of the free-carbon phase. The maximum dielectric constant can reach up to 2.4E+5. Furthermore, the detailed mechanism analysis reveals that the SiAlCN ceramics with different pyrolysis temperatures all obey the same relaxation process, which is interfacial polarization and non-Debye relaxation. The deep understanding of the dielectric property of polymer-derived SiAlCN ceramics is very practical to design wireless sensors for potential applications in high-temperature and harsh environments.

Low‐temperature synthesis of HfC/HfO2 nanocomposites from a commercial single‐source precursor

AbstractA liquid‐phase polymer‐to‐ceramic approach is reported for the synthesis of hafnium carbide (HfC)/hafnium oxide (HfO2) composite particles from a commercial precursor. Typically, HfC ceramics have been obtained by sintering of fine powders, which usually results in large particle size and high porosity during densification. In this study a single‐source liquid precursor was first cured at low temperature and then pyrolyzed at varying conditions to achieve HfC ceramics. The chemical structure of the liquid and cured precursors, and the resulting HfC ceramics was studied using various analytical techniques. The nuclear magnetic resonance and Fourier transform infrared spectroscopy indicated the presence of partially hydrated hafnium oxychloride (Hf–O–Cl·nH2O) in the precursor. Scanning electron microscopy of the resulting HfC crystals showed a size distribution in the range of approx. 600–700 nm. The X‐ray diffraction of the pyrolyzed samples confirmed the formation of crystalline HfC along with monoclinic‐HfO2 and free carbon phase. The formation of HfO2 in the ceramics was significantly reduced by controlling the low‐temperature curing temperature. Pyrolysis at various temperatures showed that HfC formation occurred even at 1000°C. These results show that the reported precursor could be promising for the direct synthesis of ultrahigh temperature HfC ceramics and for precursor infiltration pyrolysis of reinforced ceramic matrix composites.

Structural Evolution of Polyaluminocarbosilane during the Polymer-Ceramic Conversion Process.

Polyaluminocarbosilane (PACS) is an important precursor for silicon carbide (SiC) fibers and ceramics. The structure of PACS and the oxidative curing, thermal pyrolysis, and sintering effect of Al have already been substantially studied. However, the structural evolution of polyaluminocarbosilane itself during the polymer-ceramic conversion process, especially the changes in the structure forms of Al, are still pending questions. In this study, PACS with a higher Al content is synthesized and the above questions are elaborately investigated by FTIR, NMR, Raman, XPS, XRD, and TEM analyses. It is found that up to 800-900 °C the amorphous SiOxCy, AlOxSiy, and free carbon phases are initially formed. With increasing temperature, the SiOxCy phase partially separates into SiO2 then reacts with free carbon. The AlOxSiy phase changes into Al3C4 and Al2O3 by reaction with free carbon at around 1100 °C. The complicated reactions between Al3C4, Al2O3, and free carbon occur, leading to the formation of the Al4O4C and Al2OC phases at around 1600 °C, which then react with the SiC and free carbon, resulting in the formation of the Al4SiC4 phase at 1800 °C. The amorphous carbon phase grows with the increasing temperature, which then turns into a crystalline graphitic structure at around 1600 °C. The growth of β-SiC is inhibited by the existence of the Al4O4C, Al2OC, and Al4SiC4 phases, which also favor the formation of α-SiC at 1600-1800 °C.

A distinct structure of TiC for electromagnetic interference shielding and thermal stability of SiTiOC ceramic nanocomposites

Materials capable of thermal stability electromagnetic interference (EMI) shielding have practical significance in the fields of aerospace. In this paper, a brand-new polymer-derived SiTiOC ceramic nanocomposites with tubular carbon was designed. The titanium atoms were uniformly introduced into the polysiloxane backbone by the transesterification between tetrabutyl titanate and silanol prepolymer, leading to excellent thermal stability properties. By controlling the pyrolysis temperature, a distinct structure of TiC was formed between the amorphous SiTiOC matrix and free carbon phase for enhanced electromagnetic interference shielding performance. More conductive paths were established, and the TiC structure initiated interfacial polarization and space charge polarization, contributing to the improvement of the dielectric loss and relaxation phenomena. Simultaneously, a total electromagnetic interference shielding efficiency of 27.85 dB at the entire X-band was reported for SiTiOC ceramic nanocomposites. A maximum SET of 38.59 dB was examined at around 12 GHz, suggesting that over 99.99% of the electromagnetic waves were shielded. The SiTiOC ceramic nanocomposites exhibited outstanding prospects in thermal stability electromagnetic interference shielding application in a severe environment.

Spectroscopic studies on phosphate-modified silicon oxycarbide-based amorphous materials

Vibrational spectroscopy is the most effective, efficient and informative method of structural analysis of amorphous materials with silica matrix and, therefore, an indispensable tool for examining silicon oxycarbide-based amorphous materials (SiOC). The subject of this work is a description of the modification process of SiOC glasses with phosphate ions based on the structural examination including mainly Infrared and Raman Spectroscopy. They were obtained as polymer-derived ceramics based on ladder-like silsesquioxanes synthesised via the sol-gel method. With the high phosphate’s volatility, it was decided to introduce the co-doping ions to create [AlPO4] and [BPO4] stable structural units. As a result, several samples from the SiPOC, SiPAlOC and SiPBOC systems were obtained with various quantities of the modifiers. All samples underwent a detailed structural evaluation of both polymer precursors and ceramics after high-temperature treatment with Fourier-transformed infrared spectroscopy (FTIR), Raman spectroscopy, X-ray diffraction (XRD) and magic angle spinning nuclear magnetic resonance (MAS-NMR). Obtained results proved the efficient preparation of desired materials that exhibit structural parameters similar to the unmodified one. They were X-ray-amorphous with no phase separation and crystallisation. Spectroscopic measurements confirmed the presence of the crucial Si-C bond and how modifying ions are incorporated into the SiOC network. It was also possible to characterise the turbostratic free carbon phase. The modification was aimed to improve the bioperformance of the materials in the context of their future application as bioactive coatings on metallic implants.

Bridged polysilsesquioxane‐derived SiOCN ceramic aerogels for microwave absorption

AbstractThe development of high‐performance microwave absorption materials in harsh environment is highly desirable but challenging. Herein, lightweight silicon oxycarbonitride (SiOCN) ceramic aerogels were fabricated by the pyrolysis of bridged polysilsesquioxane aerogels, which was presynthesized by a sol–gel method followed by vacuum drying. The structural order and content of free carbon phase determined by pyrolysis temperature play a critical role in modulating the impedance matching and attenuation capacity. The as‐prepared SiOCN aerogel pyrolyzed at 1000°C achieves a minimal reflection loss of −64.2 dB at 8.96 GHz and a broad bandwidth of 5.4 GHz at a low thickness of 2.15 mm. The hierarchical pore structure, abundant heterogeneous amorphous SiOCN/free carbon interfaces, and conductive free carbon phase benefit the superior absorption performance by forming good impedance matching, multiple scattering, interface polarization, and conduction losses. This work provides an insight for the rational design of polymer‐derived ceramic aerogel‐based microwave absorption materials for potential application under extreme conditions such as high‐temperature and oxidation environments.

Preparation of fully dense boron carbide ceramics by Fused Filament Fabrication (FFF)

Boron carbide is the third hardest material known, with a high melting point (2450 °C) and poor sintering ability. Therefore, boron carbide is a challenging material for shaping by conventional processing routes and can still be considered as unsuitable for commercial production of ceramics parts by additive manufacturing technologies. This work reports the first successful preparation of boron carbide ceramics fabricated by fused filament fabrication from a newly developed composite filament containing 65 wt% of micron-sized boron carbide powder dispersed in a thermoplastic binder. A commercial FFF desktop printer with a 0.40 mm nozzle was used for manufacturing of complex-shaped green bodies. Almost fully dense boron carbide ceramics with printed parts sized up to 4 centimeters and relative density higher than 96% after sintering were prepared. The DTA/TG analysis of composite filament and heat microscopy technique were used to set the debinding temperature program with critical temperature at 140 °C, due to the thermal decomposition of the binder. Microstructure SEM images after sintering showed excellent material homogeneity, while micro-CT images showed very well retained experimental shapes of collimator-like printed grids. The x-ray diffraction proved the presence of boron carbide phase with the free carbon phase at the level of about 1 wt% without significant influence on the measured hardness value of 29.88 ± 1.27 GPa.

Characteristics of (Mo-Ta-W)-C and (Nb-Ta-W)-C refractory multi-principal element carbide thin films by non-reactive direct current magnetron co-sputtering

The (Mo-Ta-W)-C and (Nb-Ta-W)-C carbide thin films were prepared by non-reactive magnetron co-sputtering. The (Mo-Ta-W)-C thin films exhibited the simple BCC structure with the strip-like surface morphologies and a double-layer structure for cross sections with a columnar upper layer and an featureless lower layer. In the (Mo-Ta-W)-C thin films, the metal-metal and metal-oxygen bonds were observed with no metal-carbon bonds. The carbon-carbon bonds were observed, indicating free carbon phases existed in the (Mo-Ta-W)-C thin films. The surface roughness of (Mo-Ta-W)-C thin films was small with Ra value of 3.11–3.74 nm. The hardness of (Mo-Ta-W)-C thin films was 32.6–33.5 GPa. The (Nb-Ta-W)-C thin films formed the amorphous structure. Different from (Mo-Ta-W)-C thin films, the metal-carbon bonds existed in the (Nb-Ta-W)-C thin films, except the metal-metal and metal-oxygen bonds. The C-C bonds were also observed, indicating free carbon phases existed in the (Nb-Ta-W)-C thin films. The surfaces of (Nb-Ta-W)-C thin films displayed a granular structure with Ra of 0.86–1.00 nm, and the cross sections exhibited a fine fibrous structure. The hardness values of (Nb-Ta-W)-C thin films ranged from 34.5 to 36.1 GPa with elastic modulus of 319.6–349.5 GPa. The corrosion current density and polarization resistance of (Nb-Ta-W)-C thin films were better than 304 stainless steel, indicating good anti-corrosive performance of (Nb-Ta-W)-C thin films. The (Nb23Ta21W23)C33 thin film had low reflectivity in the visible light band (300–800 nm), and high reflectivity in the near-infrared band (800–2500 nm), with the solar absorption ratio αs of 0.4577 and the infrared emissivity εH of 0.178.

Polymer-derived porous ceramics from novel silicon-based preceramic/sucrose systems

Porous Si/C/O/(N) ceramic bodies were developed by the direct consolidation of novel liquid silicon-based preceramic polymer/porogen (methacryloxypropyl silsesquioxane/sucrose) systems, burnout, and N2 pyrolysis (1300–1500 °C), and they were characterized via open porosity, volumetric shrinkage, and mass loss measurements. The evolution of phases as temperature increased was analyzed using XRD, TGA-mass spectrometry tests, and 29Si NMR. The free carbon phase was characterized via Raman spectroscopy, and its content was determined using a carbon analyzer. Porous microstructures were analyzed by SEM/EDS and Hg-porosimetry, and by measuring the N2 adsorption/desorption and specific surface area. The final ceramics exhibited a hierarchical porosity composed of large irregular pores that grew with temperature, together with a lower volume of meso- and micropores. β-SiC whiskers and faceted hexagonal crystals of α-Si3N4 were observed inside of cavities. The presence of meso- and micropores explained the high specific surface area achieved in the material pyrolyzed at 1500 °C.

Defective structure of doped carbons obtained from preceramic polymers through Cl2‐ and NH3‐assisted thermolysis

AbstractThe processing parameters such as the heat treatment temperature, type of preceramic precursor, and post‐synthesis treatments are key factors for the development the different microstructures in polymer‐derived ceramics. Moreover, doping with different heteroatoms has increased the ability of the polymer‐derived ceramics to produce tunable nanostructures with a controlled pore size and distributions. A preceramic precursor containing P has been prepared from a commercial polysiloxane polymer and a phosphate alkoxide. It has been subjected to thermal treatments in N2, NH3, and Cl2 atmospheres in different order sequences to create differentiated microstructures either in the ceramic matrix and the carbon phase. The structural, textural, and spectroscopic characterization revealed that the P atoms play a key role in the evolution of the microstructure during the thermal treatments. If the chlorination is carried out before the treatment in NH3, a silicophosphate matrix is formed and prevents from nitrogen incorporation into the free carbon phase. On contrary, if the NH3 treatment is carried out before the chlorination, the carbon phase is predominantly modified by the incorporation of P atoms within the free carbon network.

Enhanced piezoresistivity of polymer-derived SiCN ceramics by regulating divinylbenzene-induced free carbon

In-situ formed free carbon nanodomain is an unique feature of polymer-derived ceramics (PDCs), which plays a significant role to affect the properties of PDCs, especially for the electoral property and piezoresistivity. In this study we report an enhanced piezoresistive properties of amorphous silicon carbonitrides (SiCN) ceramics derived by divinylbenzene (DVB)-modified polysilazane. Microstructure analysis indicated that the free carbon phase in SiCN ceramics is gradually increasing in order degree and concentration with increasing content of DVB. The average spacing between the conducting particles decreases with increasing concentration of free carbon, resulting in an enhanced internal electric field. The effects of DVB concentration on the piezoresistive properties of SiCN ceramics are also discussed, finding that the DVB has a significant effect on the piezoresistive properties.

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

AbstractIn 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.

Electrochemical performance of SiOC anodes prepared by a different silicone

AbstractSiOC 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 SiOxC4−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.

Revealing Nanodomain Structures of Bottom-Up-Fabricated Graphene-Embedded Silicon Oxycarbide Ceramics.

Dispersing graphene nanosheets in polymer-derived ceramics (PDCs) has become a promising route to produce exceptional mechanical and functional properties. To reveal the complex nanodomain structures of graphene–PDC composites, a novel reduced graphene oxide aerogel embedded silicon oxycarbide (RGOA-SiOC) nanocomposite was fabricated bottom-up using a 3D reduced graphene oxide aerogel as a skeleton followed by infiltration of a ceramic precursor and high-temperature pyrolysis. The reduced graphene oxide played a critical role in not only the form of the free carbon phase but also the distribution of SiOxC4−x structural units in SiOC. Long-ordered and continuous graphene layers were then embedded into the amorphous SiOC phase. The oxygen-rich SiOxC4−x units were more prone to forming than carbon-rich SiOxC4−x units in SiOC after the introduction of reduced graphene oxide, which we attributed to the bonding of Si atoms in SiOC with O atoms in reduced graphene oxide during the pyrolysis process.