Free Carbon Phase Research Articles

Impedance Spectroscopy Study on the Two-phase to Three-phase Transformation in Polymer Derived SiC Pyrolyzed at High Temperature

The electrical behavior and evolution of free carbon phase and ceramic phase in pristine polymer derived (PDC) SiC matrix with high pyrolysis temperature were studied by complex impedance spectroscopy. The results from low pyrolysis temperature (1300-1500°C) showed that the contact resistance at the free carbon and amorphous PDC SiC interphase decreases with increasing pyrolysis temperature. At this temperature range, the dielectric response of the system is not sensitive to the increased ordering of non-ferromagnetic carbon clusters. At higher pyrolysis temperature (1600-1800°C), transformation of free carbon from nanocrystalline carbon to turbostratic phase occurs. This phenomenon leads to the introduction of inductance in the equivalent circuit. The percolative network of turbostratic carbon acts as a conductive helical winding in the PDC system. The study demonstrated that when an alternate current of certain frequency made to flow through the PDC system, it behaves as an inductor. Also, carbon cluster- turbostratic carbon transition observed with microstructural investigation is correlated with the equivalent circuit model.

Effect of SnCl2 addition on the structure and lithium storage performance of SiOC anodes

Sn/SiOC composites with varying Sn content were prepared by the sol–gel method with phenyltriethoxysilane (PhTES) and Tin(II) chloride dihydrate (SnCl2·2H2O) as precursors. The effect of SnCl2 addition on the microstructure, morphology and lithium storage performance of SiOC anodes was investigated. Incorporation of SnCl2 not only reduces carbon-contained units of SiOC ceramics, but also increases the oxygen-containing functional groups on the surface of carbon during the pyrolysis conversion. The formation of Sn involves carbothermal reduction reaction of SnO2 with the free carbon, and the degree of carbothermal reduction increases with the increase of Sn/Si ratio, which results in the reduction of carbon content and destruction of SiOC matrix. The size of Sn nanograins increases from 20 nm to 450 nm with the increasing Sn/Si ratio. The rate capability of Sn/SiOC composites increases first and then decreases with the increase of Sn content. The SiOC and Sn-PhTES-0.15 derived Sn/SiOC composite exhibit better electrochemical performance, which delivers a stable reversible capacity of 485 mAh g−1 and 411 mAh g−1, respectively at 186 mA g−1 after 250 cycles. The uniform dispersed Sn nanograins (below 20 nm), SiOC structure units and the free carbon phase both contribute to better electrochemical properties. However, the free carbon provides a higher reversible capacity than that of Sn and SiOC units, which results in the inferior lithium storage capacity of Sn/SiOC composites.

Rose pollens as sustainable biotemplates for porous SiOC microellipsoids with enhanced lithium storage performance

Exploring of high specific capacity, long cycling stability and high rate capability anode materials with low cost and environmentally benign is closely bound up with the breakthrough of high energy density lithium ion batteries. Inspired by uniform geometries and elaborate morphologies of natural rose pollens, a series of porous SiOC microellipsoids are fabricated by a novel supercritical CO2 fluid assisted carbonization method using rose pollen as biotemplate and carbon source. The high permeability, superior diffusivity and excellent dissolving capability of supercritical CO2 fluid guarantees the efficient mass transfer, leading to the homogeneous elemental distributions in precursor and final product of SiOC. Interestingly, the content of free carbon phase, SiO2C2, SiO3C and SiO4 units in SiOC can be adjusted by varying the calcination temperature. The relationship between mixed SiCxO4-x units and electrochemical performance is systematically investigated. The results demonstrate that SiOC-600 with high content of SiO2C2 and SiO3C units delivers a satisfactory reversible capacity (415.1 mA h g−1 at 0.1 A g−1), a prolonged cycling life (351.3 mA h g−1 after 200 cycles) and enhanced rate capability (175.2 mA h g−1 at 2 A g−1).

Carbon nanofibers-based nanocomposites with silicon oxy-carbide matrix

The subject of the study were nanocomposites made by infiltration of carbon nanofibers in the form of mat (CNF) with a poly(methylphenylsiloxane) resin (PMPS) solution followed by heat-treatment up to 1000 °C. The nanocomposites were studied using scanning electron microscopy (SEM-EDS), X-ray diffraction (XRD), Raman spectroscopy and by means of surface measurement tests. The morphology, microstructure, structure and selected properties of the nanocomposites were analysed. The influence of the CNF oxidative surface treatment on interface properties in CNF/resin and CNF/resin-derived ceramics nanocomposites was investigated. The high surface tension occurring at the nanofiber/resin interface required preliminary chemical treatment of carbon nanofiber surface. The surface chemical state of CNF had a significant influence on their interaction with the resin and formation of SiOC protective layer on nanofibers surface. The SiOC phase formed on the CNF surface at 1000 °C improved thermal stability of the nanocomposites in air. Due to the formation of the free carbon phase during resin heat-treatment the resulting ceramic nanocomposite matrix showed a lower electrical resistance compared to pure carbon mats.

Microstructure of SiC–ZrC composite coatings on TRISO particles via fluidized bed chemical vapor deposition

A SiC and ZrC composite coating layer on a tristructural-isotropic (TRISO) coated particle was deposited by fluidized bed chemical vapor deposition method. Methyltrichlorosilane (MTS) and ZrCl4 powder were used as the source materials of the SiC coating and the ZrC coating, respectively. Two kinds of the SiC and ZrC composite coating layers were fabricated with other source gas fraction. The composite coating layer, with a similar ratio of SiC and ZrC, which was observed in the SEM microstructure, was fabricated, and each phase was confirmed by XRD, Raman, and TEM analysis. Only the SiC and ZrC phases were co-deposited; the ZrSi and free carbon phases were not. The composite coating layer with very small ZrC particles embedded in a SiC matrix was fabricated by decreasing the ZrCl4 source gas ratio. Nano-sized ZrC particles in the coating layer were confirmed by high resolution TEM and EDS analysis.

Effect of heat treatment temperature on microstructure and electromagnetic shielding properties of graphene/SiBCN composites

Three-dimensional (3D) graphene/SiBCN composites (GF/SiBCN) were prepared by depositing SiBCN ceramics in 3D graphene foam via the chemical vapor infiltration technique. The effect of the heat treatment temperature on the microstructure, phase composition, and electromagnetic properties of the GF/SiBCN composite was investigated. The SiBCN ceramics maintained an amorphous structure in the composite below 1400 °C above which the crystallinity of the free carbon phase gradually increased. While the Si3N4 and B4C phases started to crystallize at 1500 °C and their crystallinity increased with temperature, SiC was observed at 1700 °C. The electromagnetic shielding effectiveness of GF/SiBCN increased with the heat treatment temperature.

Low‐frequency dielectric performance of polyimide composites containing amorphous polymer‐derived SiCN ceramics

AbstractHigh‐dielectric‐constant polymer‐matrix composites have potential applications as embedded capacitors. In this paper, an amorphous polymer‐derived SiCN ceramic was incorporated into a polyimide (PI) matrix by in situ polymerization for the first time. The resulting SiCN/PI composite showed good thermal stability below 500°C, with the SiCN filler being homogeneously dispersed in the PI matrix. The dielectric properties of the SiCN/PI composites were investigated over a broad frequency range (0.1‐106 Hz). The dielectric properties of the SiCN/PI composites at low frequencies revealed a weak percolation phenomenon similar to that shown by conductive/polymer composites and the calculated percolation threshold was 22.8 vol%. At SiCN loadings of 25 vol%, the SiCN/PI composites showed a high dielectric constant of 190 at 0.1 Hz, which is 57 times higher than that of pure PI (3.3). The conductivity of the 25 vol% SiCN/PI composite was as low as 8.5 × 10−10 S·cm−1. These results could be explained by the unique microstructure of the polymer‐derived SiCN ceramic, containing an amorphous Si matrix and a free carbon phase. This study revealed SiCN/PI composites as promising dielectric materials with promising applications in energy storage fields.

Optimization of the formation of coatings based on SiAlOC glasses via structural, microstructural and electrochemical studies

The main objective of this work was to determine the tightness and wet corrosion resistance of SiAlOC glasses in the form of coatings on a steel substrate. The coatings were optimized in terms of the formation process, along with their resulting structure and microstructure. The coating material was obtained using sol-gel-synthesized, Al-doped, ladder-like polysilsesquioxanes as glasses' precursors. The layers were deposited by means of the dip-coating technique, for which the influence of both surface preparation (grinding/polishing) and withdrawal speed (5, 15, and 30 cm/min) was investigated. After the deposition, a two-stage thermal treatment (curing −70 °C/air and pyrolysis −800 °C/Ar) was applied. A series of structural (XRD, XPS, Raman) studies confirmed the formation of SiAlOC glass coatings, with a noticeable presence of an additional inner layer, most probably of MnCr2O4 spinel. Furthermore, the direct correlation between the increasing content of aluminum and the decreasing amount of carbon inward the coating can be suggested. Additionally, spectroscopic studies revealed the ongoing amorphization process of the free carbon phase. The result was that more crystalline species were present at the surface of the coating and the amorphous phases were near the inner MnCr spinel layer. Moreover, a set of microstructural (SEM, EDS, Confocal) studies was performed to evaluate the influence of the investigated deposition parameters on the coatings’ compactness, homogeneity, and thickness. The coating with the best characteristics obtained on the ground substrate when using the lowest withdrawal speed was chosen for the final examination of the tightness via electrochemical corrosion studies in a wet environment of chloride ions. The wet corrosion resistance was nearly 60 times higher for the coated specimen in comparison with the bare steel substrate. Additionally, an uncoated specimen after the same thermal treatment was investigated with the use of the same electrochemical studies. It exhibited significantly higher resistance to the corrosion in the wet environment than the rest of the samples due to the pre-formation of a mixed scale containing highly resistant Cr2O3.

Effect of temperature on internal potential barriers in polymer-derived amorphous ceramics

Abstract The behavior of the internal potential barrier in a polymer-derived amorphous ceramic was studied by measuring its complex impedance spectra at various dc bias and temperatures. The results showed that the bias-dependent interfacial capacitance can be described by the double Schottky barrier model. Increasing temperature caused the increase in the charge-carrier concentration and decrease in the barrier height, following Arrhenius dependence. The phenomena are correlated to the electronic structure of the free-carbon phase and the amorphous matrix phase in the material.

An evaluation of the effects of the N2/Ar gas flux ratio on various characteristics of TiC0.3N0.7 nano-structure coatings on the cold work tool steel by pulsed DC-PACVD

As the radius of argon ions is larger than other fragments in the medium of plasma in the plasma assisted chemical vapor deposition (PACVD) technique, it affects the microstructure and mechanical properties of the coatings. Thus, in this paper, the influence of various N2/Ar gas flow ratios on characteristics of TiCN thin coatings on a cold work tool steel have been investigated. These coatings were produced by the method of pulsed DC-PACVD at a temperature of 480 °C. Field emission scanning electron microscopy (FESEM), x-ray diffraction (XRD) and energy dispersive spectroscopy (EDS) were employed for determining the microstructure and chemical composition of various coatings. Various mechanical properties such as micro-hardness, nano-hardness, wear behavior, and toughness were also investigated. The obtained results indicated that the N2/Ar gas flow ratio of 0.6 led to optimal mechanical properties. Such ratio reduced the value of the friction coefficient to 0.15 owing to lower roughness. Besides, the growth rate was found to be highest corresponding to a gas flow ratio of 0.5. It was found that the titanium content in such coatings could enhance the growth rate of TiCN coatings. XRD results also demonstrated that the N2/Ar gas flow ratio value of 0.4 resulted in the formation of the TiN and TiC plus the free carbon phases instead of forming the TiC0.3N0.7 phase. The N2/Ar gas flow ratio value of 0.7 also lowered mechanical properties such as the toughness and the micro-hardness. Their values were found to be 2.3 MPam0.5 and 940 HV0.01, respectively.

SiAlOC glasses derived from sol-gel synthesized ladder-like silsesquioxanes

The main aim of this study was to describe both the formation process of SiOC glasses doped with Al3+ cations and the resulting differences in their structures in comparison with undoped silicon oxycarbide glasses. A novel material was formed by means of the sol-gel method with the application of alkyl-substituted alkoxysilanes and a butoxy derivative of aluminum (Al(OC4H9)3) as a source of Al3+ ions. After the synthesis of a sol, the material was treated with a two-step thermal processing including aging (70 °C) and annealing (800 °C). As a result, yellowish xerogel (aging) and black glasses (annealing) were obtained and subjected to structural investigation, including X-ray diffraction (XRD), middle infrared spectroscopy (MIR), Raman, electron dispersive spectroscopy (EDS), and nuclear magnetic resonance (NMR) methods. The obtained results were compared to corresponding findings for SiOC glasses, obtained with the same experimental procedure. An additional thermogravimetric analysis (TGA) study was carried out for the xerogel in order to determine its thermal behavior in a protective atmosphere up to 1000 °C. The resultant mass loss curve was much smoother for the Al-doped material, indicating the beneficial impact of Al3+ addition on the formation process. XRD and spectroscopic (MIR, Raman, magic angle spinning (MAS) NMR) research revealed the formation of an amorphous silsesquioxane xerogel with a slightly modified ladder-like structure, and silicon oxycarbide glasses successfully doped with Al3+ ions, after aging and annealing, respectively. Moreover, NMR studies confirmed the introduction of aluminum into the glass network, based on the presence of Q4(1Al) and [AlO4] species in the 29Si and 27Al MAS NMR spectra, respectively. Additionally, the results from EDS and Raman spectroscopic studies suggested the partial release of free carbon phase from the glass network because of cationic substitution and ordering influence of Al3+ cations on the glass structure.

A Variable Temperature X- and W-Band EPR Study of Fe-Doped SiCN Ceramics Annealed at 1000, 1100, and 1285 °C: Dangling Bonds, Ferromagnetism and Superparamagnetism

Polymer-derived SiCN ceramics, annealed (also referred to as pyrolyzed) at 1000, 1100, and 1285 °C, and doped with Fe(III) acetylacetonate, are investigated by electron paramagnetic resonance (EPR) from 4 to 120 K at X-band (9.425 GHz). In addition, the SiCN ceramic, annealed at 1100 °C, was studied by EPR at 300 K at W-band (93.96 GHz). There was observed a significant increase in EPR linewidth due to dangling bonds (g = 2.001) below 20 K at X-band. The low-field X-band FMR line (g ≈ 12) indicated the presence of ferromagnetic Fe5Si3 crystallites. There were found two EPR lines due to carbon-related dangling bonds: (1) those present as defects on the surface of the free-carbon phase (as sp2 carbon-related dangling bonds with g = 2.0011) and (2) those present within the bulk of carbon phase (as sp3 carbon-related dangling bonds with g = 2.0033). On the other hand, the intense low-field EPR signal observed at X-band was not observed at W-band. As well, there was observed splitting of the single broad EPR signal observed at g = 2.05 at X-band into two signals at W-band at g = 1.99 and g = 2.06, due to two different Fe-containing superparamagnetic nanocrystallites. Two new EPR signals, not observed at X-band, were observed at W-band, namely at g = 2.28 and g = 3.00, which are also due to g∥ of these superparamagnetic nanocrystallites.

Regenerable, innovative porous silicon-based polymer-derived ceramics for removal of methylene blue and rhodamine B from textile and environmental waters.

The presence of residual color in treated textile wastewater above the regulation limits is still a critical issue in many textile districts. Innovative, polymer-derived ceramics of the Si-C-O system were here synthesized in order to obtain porous nanocomposite materials where a free carbon phase is dispersed into a silicon carbide/silicon oxycarbide network. The sorbents were comprehensively characterized for the removal of two model water-soluble dyes (i.e., the cation methylene blue and the zwitterion rhodamine B). Adsorption is very rapid and controlled by intra-particle and/or film diffusion, depending on dye concentration. Among the nanocomposites studied, the SiOC aerogel (total capacity about 45mg/g, is easily regenerated under mild treatment (250°C, 2h). Adsorption of dyes is not affected by the matrix composition: removals of 150mg/L methylene blue from river water and simulated textile wastewater with high content of metal ions (2-50mg/L) and chemical oxygen demand (800mg/L) were higher than 92% and quantitative for a dye concentration of 1mg/L.

Silicon oxycarbide produced from silicone oil for high-performance anode material in sodium ion batteries

Sodium ion batteries (NIBs) have gained considerable attention for large-scale energy storage applications because of the lower cost and higher abundance of Na than Li. In this study, we demonstrate that silicon oxycarbides (SiOCs), in which free carbon phases are imbedded in amorphous glassy SiOxCy phases, can be promising candidates as anodes for NIBs. The SiOC samples were synthesized by the pyrolysis of low-cost silicone oil in a H2/Ar flow condition. When tested as an anode in NIBs, the SiOC sample calcined at 900 °C exhibited an excellent reversible capacity of 160 mAh g−1 at 25 mA g−1 after 200 cycles and a high rate performance of 62 mAh g−1 at 800 mA g−1. More importantly, the SiOC electrode exhibited an excellent long-term cycling performance of 0.09 mAh g−1 decay per cycle for 650 cycles. The electrochemical properties of SiOC in an NIB were compared with those in an LIB to gain an insight into its ion storage mechanism. The effects of calcination temperature, calcination time, and ball milling time on the physicochemical and electrochemical properties of the SiOC electrode are discussed.

Anomalous piezo-dielectricity of a polymer-derived amorphous silicoaluminum oxycarbide (SiAlCO)

The piezo-dielectricity of a polymer-derived amorphous silicoaluminum oxycarbide (a-SiAlCO) was studied. The material showed a relatively high dielectric constant and anomalous piezo-dielectricity characterized by a positive pressure coefficient as high as 0.10–0.25MPa−1. Further analyses indicated that this anomalous piezo-dielectricity resulted from an increase of polarizability with pressure. This phenomenon was ascribed to the unique self-formed bi-phasic (i.e., low-conductive ceramic and high-conductive free-carbon phases) structure of the SiAlCO material. The huge piezo-dielectric coefficient makes this material a very promising candidate for high-temperature sensing applications.

Poly(methylhydrosiloxane) networks of different structure and content of Si-H groups: Physicochemical properties and transformation into silicon oxycarbide ceramics

In the work, poly(methylhydrosiloxane) was cross-linked by three vinylsiloxanes differing in molecular structure and functionality, i.e. linear difunctional ViMMVi, branched tetrafunctional Q(MVi)4 and cyclic tetrafunctional D4Vi. The cross-linking process was conducted at three molar ratios of Si-H to Si-CH=CH2 groups in order to obtain different amounts of unreacted Si-H groups in the systems. As established by swelling measurements, the applied cross-linker and molar ratio of reactive groups show the influence on the average cross-linking densities of the studied systems. Conducted thermal investigations allowed to conclude that the amount of Si-H groups present in the system affects the number of decomposition steps of the studied networks. The results of thermal studies of the cross-linked samples were complemented by FTIR and XRD analyses of the residues formed after pyrolysis of the cross-linked products conducted at different temperatures in the range between 200 and 1000 °C in Ar atmosphere. The presence of the free carbon phase in the obtained silicon oxycarbide materials was confirmed by Raman spectroscopy.

Polymer‐derived ceramic aerogels as sorbent materials for the removal of organic dyes from aqueous solutions

AbstractPolymer‐derived SiC and SiOC aerogels have been synthesized and characterized both from the microstructural point of view and as sorbent materials for removing organic dyes (Methylene Blue, MB, and Rhodamine B, RB) from water solutions. Their adsorbent behavior has been compared with a polymer‐derived SiC foam and a commercial mesoporous silica. The aerogels can efficiently remove MB and RB from water solution and their capacity is higher compared to the SiC foams due to the higher surface area. The SiOC aerogel remains monolithic after the water treatment (allowing for an easy removal without the need of a filtration step) and its maximum capacity for removing MB is 42.2 mg/g, which is higher compared to the studied mesoporous silica and many C‐based porous adsorbents reported in the literature. The reason for this high adsorption capacity has been related to the unique structure of the polymer‐derived SiOC, which consists of an amorphous silicon oxycarbide network and a free carbon phase.

Complex impedance study on polymer-derived amorphous silicon carbonitride

Abstract Polymer-derived amorphous-SiCN pyrolyzed at 1200 °C was studied by impedance spectroscopy at different frequencies and temperatures. Obtained impedance spectra were analyzed by using equivalent circuit method. Results revealed that two relaxation effects were involved over the entire studied temperature range, which corresponded to free carbon phase at high frequency and the interface at low frequency. The impedance spectroscopy could be well-fitted by two parallel RC circuits in series. Interface behavior dominated the polarization relaxation process, which was controlled by the resistance as well as the capacitance of interface. All processes performed a 1/T1/4 behavior, which follows a three-dimensional (3D) random hopping mechanism. This paper further found that amorphous-SiCN ceramic materials underwent an interfacial charge polarization process over a frequency range from 0.05 Hz to 10 MHz at different temperatures from −50 to 300 °C.

The effect of B‐doping on the electrical conductivity of polymer‐derived Si(B)OC ceramics

AbstractIn this work the room temperature electrical conductivity of Si(B)OC glasses made via polymer pyrolysis at 1200°C and 1400°C (maximum temperature) and having different amount of boron was measured. When B content is increased from zero (pure SiOC glass) up to B/Si=0.5 the electrical conductivity increases in 2 orders of magnitude from 4.09±0.64×10−5 up to 2.93±1.91×10−3 with a corresponding decrease in the activation energy from about 1.08 to 0.51 eV. This results shows for the first time that the electrical conductivity of Si‐based polymer‐derived ceramics can be controlled by the amount of the doping element. The structure of the Si(B)OC glasses has been studied with different techniques including FT‐IR, XRD and Raman spectroscopy. The Raman study indicates that B partially substitutes C into the sp2 C planes of the free carbon phase forming trigonal BC3 units. Accordingly, the evolution of the electrical properties with the B content has been correlated with the corresponding structural evolution and a hypothesis is presented to rationalize the role of boron on the electrical conductivity of SiOBC glasses.

Dielectric property and interfacial polarization of polymer-derived amorphous silicon carbonitride

Abstract The dielectric behavior of polymer-derived amorphous silicon carbonitride was investigated. The material exhibited an interfacial polarization. Although such a polarization exhibited a sharp decrease in the dielectric constant with frequency, the loss peak from this mechanism was not clearly observed because DC conduction dominated the dielectric loss. On the other hand, a sharp increase in the dielectric constant and loss peak were observed at 350 °C at 1 kHz, indicating that the interfacial polarization enhanced at elevated temperatures. This interfacial polarization can be attributed to the migration and accumulation of charge carriers at the interface of matrix and free-carbon phases.