Free Carbon Content Research Articles

Preparation and structure of SiOCN fibres derived from cyclic silazane/poly-acrylic acid hybrid precursor.

Ceramic matrix composite (CMC) materials have been considered a desired solution for lightweight and high-temperature applications. Simultaneously, among all different CMC reinforcements, polymer-derived ceramic (PDC) fibres have gained attention for the intrinsic thermal stability and mechanical strength with simple and cost-effective synthesis techniques. Here, carbon-rich SiOCN fibres were synthesized via hand-drawing and polymer pyrolysis of a hybrid precursor of 1,3,5,7-tetramethyl-1,3,5,7-tetravinylcyclotetrasilazane (TTCSZ) and poly-acrylic acid (PAA). The type of silazane reported in this work is considered as a major precursor for SiCN; however, it is unspinnable, due to its unfavourable physical properties (low viscosity) and chemical structure (cyclic rather than linear structure). The introduction of PAA to TTCSZ to create a hybrid precursor remarkably improved the spinnability of the silazane and should be widely applicable to other unspinnable PDC pre-ceramic polymers. Investigations on the structural and compositional development of the fibres were mainly conducted via Raman spectroscopy, Fourier-transform infrared spectroscopy, scanning electron microscopy, X-ray photoelectron spectroscopy, nuclear magnetic resonance and thermo-gravimetric analysis to determine spinnability, free carbon content, cross-linking and pyrolysis behaviour of the fibres, respectively.

Elastic properties and fracture toughness of SiOC‐based glass‐ceramic nanocomposites

AbstractFour different SiOC glass ceramics were synthesized and their fracture toughness (KIc) and fracture surface energy (γ) were assessed by means of the single‐edge precracked beam (SEPB) method. In addition, the elastic moduli were measured and the Vickers indentation behavior (hardness and microcracking) was characterized. In particular, the dependence of KIc on the free carbon content and on the fraction of crystallized nanoparticles (SiC, ZrO2, HfO2) was investigated. An increase in KIc, from about 0.73 to 0.99 MPa √m is observed as the free carbon content is increased from less than 1 to 12 vol%. The addition of Hf and Zr (resulting in 4.5 to 7.8 vol% HfO2 and ZrO2 nanoparticles) was found to increase KIc to an extent similar to the free carbon content. Moreover, predicted KIc values, assuming that the crack travels through all phases accounting for their respective volume fractions, disrupting the weakest links within the structural units, are in agreement with the experimental values.

Electromagnetic interference shielding properties of polymer derived SiC-Si3N4 composite ceramics

SiC-Si3N4 composite ceramics are successfully fabricated by pyrolysis of ferrocene-modified polycarbosilane (PCS) mixed with inert filler Si3N4 powders, followed by thermal treatment from 1100 °C to 1400 °C in Ar atmosphere. The porosity of SiC-Si3N4 ceramics decreases to 6.4% due to the addition of inert filler Si3N4. And the content and crystallization degree of free carbon and SiC derived from PCS are improved simultaneously with the increase of thermal treatment temperature. Finally, the free carbon and SiC interconnect, forming the conductive network. As a result, the electromagnetic interference (EMI) shielding performance of the as-prepared ceramic annealed at 1400 °C reaches up to 36 dB, meaning more than 99.9% of EM energy is shielded. The low porosity and high EMI shielding performance enable SiC-Si3N4 composite ceramics to be a promising electromagnetic shielding and structural material.

Synthesis of Reinforced Ceramic Matrix Composite Based on SiC and Nanocarbon Mesh

Most modern matrix composite materials employ a variety of carbon nanofillers to improve their mechanical, electrical, and functional properties. Nanofillers are separately implanted into the initial ceramic matrix, which complicates the composite manufacturing technology and increases the final cost. In this work, the synthesis of nanocarbon fillers was carried out using high-temperature (1200°C) pyrolysis of phenolic resin directly inside the silicon carbide matrix. This results in the formation of a continuous 3D nanocarbon mesh uniformly binding and reinforcing the final product throughout the volume. The nanocarbon filler synthesized in the SiC matrix contains two allotropic carbon forms: nanographite and nanofibers. The study of the features of the carbon structure and morphology showed that during the pyrolysis the multilayered nanographite structures had been formed on the surface of SiC grains of average crystallite size equal to 20–35 nm. In the matrix pores, carbon nanofibers a few micrometers in length and 20–40 nanometers in diameter are synthesized. The reiteration of the phenolic resin impregnation–pyrolysis cycle increased the free carbon content from 4 to 7 wt.% for once and twice impregnated and pyrolyzed samples, respectively, and the Young's modulus, from 50.7 to 94.3 GPa. The obtained carbon content and structure are appropriate to produce C/SiC composite for application of ceramics and electrodes.

Carbothermic synthesis of boron carbide with low free carbon using catalytic amount of magnesium chloride

Microcrystalline boron carbide was synthesized from boric acid and carbon black in the presence of a small amount of magnesium chloride by carbothermal reduction method at 1600 °C. The presence of magnesium chloride in the mixture has a determining effect in increasing the yield of the reaction and lowering the residual free-carbon content of the resulting boron carbide. We have also shown that, compared to several reported carbothermal reduction methods, the present catalyst also considerably lowers the excess amount of boron compound precursor required to afford boron carbide with negligible free carbon. X-ray powder diffraction (XRD) pattern indicated that the product was rhombohedral B4C. No peak, corresponding to free carbon, was observed in the XRD diffraction pattern of the B4C product formed from H3BO3:C:MgCl2 in the molar ratio of 1.0:1.45:0.01. Scanning electron microscopy image also showed that the product is fully crystalline and consisted of uniformly sized particles.

C/SiOC Composites by a Modified PIP Using Solid Polysiloxane: Fabrication, Microstructure and Mechanical Properties

A modified polymer infiltration and pyrolysis method (PIP) was developed to enhance the densification of C/SiOC composites, using molten MK resin (polymethylsilsesquioxane) as precursor. Organic sulfonic acid was added as cross-linking agent to lower the curing temperature. The cross-linking mechanism and ceramization behavior of MK resin was studied. A high ceramic yield of about 85wt% and low free carbon content below 3wt% are achieved, with excellent high-temperature stability of the derived SiOC. The modified PIP approach presents a high densification efficiency. After only 8 PIP cycles, the final C/SiOC composites possess a density of about 1.81 g/cm(3). Compared with those composites fabricated by conventional PIP process, the C/SiOC composites prepared by modified PIP process show a much denser microstructure with much more improved densification efficiency. Bending strength of the as-fabricated C/SiOC composites is of (312 +/- 25) MPa, showing obvious non-brittle fracture behavior.

Electrical conductivity change induced by porosity within polymer-derived SiCN ceramics

Polymer-derived ceramics (PDCs) which is synthesized by pyrolysis of organosilicon polymers have recently attracted an increasing attention on the electrical property with possible application in Li-ion batteries, micro electro mechanical systems (MEMS) and harsh-environmental sensors. In this paper, the chemical composition, nanostructure evolution and the electrical conductivity variation which are induced by porosity are explored systematically in polymer-derived SiCN ceramics. The SiCN ceramics with 0, 4.82%, 8.94%, 16.81% and 21.32% porosities were prepared by mixing the polymer precursors with different crosslinking degree. There are significant changes in the content of N element, free carbon, crystallization SiC and the morphology of free carbon with the increasing porosity. As the porosity increases, the electrical conductivity of the SiCN ceramics decrease gradually from 0.015 to 0.003 S cm−1. By comparing the relationship between the conductivity and the porosity from experiment and model prediction (only intrinsic porosity considered), it can be concluded that the chemical composition, nanostructure evolution dramatically decrease the conductivity of SiCN ceramics with 56.41% maximum decline, and the decrease of the free carbon content is the most significant aspect influencing the electrical conductivity of SiCN ceramics. Our findings can realize the regulation of the nanostructure and electrical conductivity in polymer-derived SiCN ceramics by the control of porosity with a new insight on the design of PDCs functional materials.

Influence of free carbon on the Young's modulus and hardness of polymer‐derived silicon oxycarbide glasses

AbstractSilicon oxycarbide (SiOC) glasses in the form of thin, dense, and crack‐free samples were fabricated according to the polymer pyrolysis route starting from cross‐linked polysiloxane. The amount of free carbon in the final SiOC materials was varied in the range 18‐60 vol%. The mechanical properties of the SiOC glasses were measured by nanoindentaion technique and revealed that both the Young's modulus and the hardness decrease with increase in the free carbon content and follow a simple rule of mixtures model.

Tuning the Properties of Refractory Carbide Nanopowders

Two methods for transition metal carbide nanopowder production such as plasma chemical synthesis and high-energy ball milling are considered. The control of the chemical, phase, and dispersed composition of carbides produced from oxide and halogenide raw materials is studied by an electric arc plasma plant with a power of 20 kW. A study of the grinding time effect of micron-sized carbide powders on a Retsch PM-400 mill tool in hard-alloy containers is carried out to obtain nanosized powders, as well as to determine fractional composition and particle shapes. It is shown that powders obtained by plasma chemical synthesis have a grain size of 20–80 nm and are subjected to a spheroidized or equiaxed shape. The process allows one to tune the phase composition and the content of common and free carbon. Polydispersed carbide powders with a specific surface area of 3–25 m2/g and predominately bimodal particle size distribution in a range from less than 0.1 μm to dozens of microns are thus obtained by high-energy ball milling.

Effect of free carbon on micro-mechanical properties of a chemically vapor deposited SiC coating

SiC coatings with different content of free carbon were deposited on polycrystalline silicon substrate by chemical vapor deposition (CVD) by using CH3SiCl3-H2-Ar reactive system. The composition, microstructure and micro-mechanical properties of coatings were investigated. The results show that, when H2/MTS is 10, there is no free carbon in coatings. However, at the ratio of H2/MTS of 8.89, 7.97 and 7.32, the content of free carbon is falling in the range of 0.025–0.065, 0.100–0.800 and 0.005–0.035 respectively. The supersaturation of [H] is a key factor to control the generation of free C and the nucleation and growth of SiC coating. As to mechanical properties of SiC coatings, there may be a critical value of the content of free carbon in the coating. When the content of carbon is less than the critical value, the free carbon can enhance the mechanical properties of the coating; otherwise, the free carbon weakens the mechanical properties.

Silicon Oxycarbide Beads from Continuously Produced Polysilsesquioxane as Stable Anode Material for Lithium-Ion Batteries

Silicon oxycarbides are promising anode materials for lithium-ion batteries. In this study, we used the continuous MicroJet reactor technique to produce organically modified silica (ORMOSIL) spheres which were pyrolyzed to obtain silicon oxycarbides. The continuous technique allows the production of large quantities with a constant quality. Different alkoxysilanes were used to produce the silicon oxycarbides with different compositions. Thereby, the amounts of silicon–carbon bonds, as well as the free carbon content, were modified. Electrochemical testing was carried out in 1 M LiPF6 in ethylene carbonate/dimethyl carbonate. A mixture of vinyl- and phenyltrimethoxysilane was identified as the best anode material with a stable performance due to the increased carbon content. The first-cycle delithiation capacity of the most stable material was 922 mA h/g, and the capacity retention after 100 cycles was 83% (767 mA h/g).

Influence of reaction temperature and time on nanoboron carbide morphology and its thermal stability, optical and electronic properties

Boron carbide powder was prepared via the carbothermic reduction reaction (CRR) using lactose (C12H22O11) and boric acid (H3BO3) as carbon and boron sources. Starting materials were mixed with an excess amount of H3BO3 stoichiometric ratio to reduce the presence of free carbon content in the produced boron carbide samples caused by the great loss of boron during the reaction. The mixed solutions of starting materials were heated in a stainless steel pot at 280 °C for 24 h. The resulted powder was carbothermally reacted under the flowing of an argon gas at different reaction temperatures and times ranging from 1300 to 1500 °C for 3 h and at 1500 °C for 1–3 h, respectively. The synthesized samples at each procedure conditions were characterized using X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FT-IR) techniques. The results showed that the produced samples were in the nanocrystalline scale with no carbon traces. The structural morphology of the synthesized powder studied using scanning electron microscope (SEM) and transmission electron microscope (TEM). The results indicated the possibility of producing B4C with minimal carbon residue via controlling both reaction temperature and time. A significant morphology change of the produced samples was well detected by changing the reaction time. Thermochemical properties of the as-synthesized samples at 1500 °C for 1 and 3 h also studied in order to be exploited in the proper applications. In addition, the optical properties of boron carbide was investigated by UV/visible and photoluminescence measurements along with its electronic properties.

Role of W in W/Ni Bilayer Ohmic Contact to n-Type 4H-SiC From the Perspective of Device Applications

Ohmic contacts to n-type 4H-SiC using Ni layer and W/Ni bilayer were investigated and compared. The phase composition, electronic states, and carbon structural evolution of the contact layer were analyzed by X-ray diffraction, X-ray photoelectron spectroscopy, and Raman spectroscopy. The surface roughness and cross-sectional morphology were characterized by atomic force microscopy and high-resolution transmission electron microscopy. The Keithley 4200-SCS semiconductor parameter analyzer was used to measure the current–voltage curves of the contacts. The specific contact resistance $\rho _{c}$ was calculated based on the circular transmission line model. $\rho _{c}$ is $\textsf {3.2}\times \textsf {10}^{-\textsf {5}}\,\,\Omega \cdot $ cm2 for W/Ni/SiC and $\textsf {4.2} \times \textsf {10}^{-\textsf {4}} \,\,\Omega \cdot $ cm2 for Ni/SiC. Ni2Si with minor W substitution and tungsten carbide with minor Ni substitution were identified as the dominant phases for W/Ni/SiC after annealed. The contact surface morphology is improved, and the content of free carbon and the quantity of voids at the interface are reduced when W is introduced into Ni/SiC. The wire bonding is easy to carry out and the thermal stability of ohmic contact is greatly enhanced. From the perspective of device applications, W/Ni bilayer ohmic contact to n-type 4H-SiC is very competitive.

Synthesis of nanocrystalline boron carbide by sucrose precursor method-optimization of process conditions

Nanocrystalline boron carbide powder was synthesized by a precursor method using B2O3 as the source of boron and sucrose as the source of carbon. Precursor was prepared at different temperatures ranging from 300 to 800°C. The optimum temperature for the precursor preparation was found to be 600°C. All the precursors were heat treated at different temperatures from 1000 to 1600°C for different duration of heating, ranging from 5 to 240min under vacuum. The products thus obtained after heat treatment were characterized using X-ray diffraction. The boron carbide obtained was nanocrystalline and the average X-ray crystallite size was found to be ~ 33nm. Boron, total carbon and free carbon contents also were determined. The free carbon content was found to be less than 3% for samples heated at 1600°C for 10min. Effect of heat treatment temperature on the morphology of the synthesized product was studied using scanning electron microscope.

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∼18GHz. The resistivity decreased gradually as the annealing temperature rose. The dielectric constant of sample decreased with the increase of frequency in 1∼5MHz. 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 −10dB (>90% absorption) in a wide frequency range of 6∼16GHz and the maximum value of dielectric loss angle tangent was about 0.6 at 16GHz.

Cross-linking behavior and dielectric properties of SiCN precursor

The SiCN precursor was prepared by cross-linking process using polysilazane (PSZ) as raw materials. The cross-linking behavior and dielectric properties of SiCN precursor were analyzed. The results indicated that the main reactions during cross-linking process were double bond addition reaction, hydrosilylation reaction, dehydrogenation coupling reaction and transamination reaction. The SiCN precursor was made up of Si-C-N network structure and the main chemical bonds were Si-C, Si-N and C-C. The appropriate cross-linking temperature of SiCN precursor was 600°C. Furthermore, the increasing content and crystallization degree of free carbon could improve the dielectric properties of SiCN precursor.

Removal of free carbon and crystal structure change of amorphous silica fume by calcination

The calcination process was used to remove free carbon and to change the structure of amorphous microsilica powder. The results show that the carbon content in amorphous silica powder strongly depends on the calcination temperature. When the calcination temperature is higher than 900[Formula: see text]C, the free carbon content, SiO2 content and weight loss of microsilica powder tend to be stable, with values of 0.05%, 83.8% and 4.8%, respectively. The crystalline structure, particle size and morphology start changing at a calcination temperature of 850[Formula: see text]C. When the calcination temperature is higher than 900[Formula: see text]C, the amorphous microsilica powder is fully crystallized. The morphology of the silica fume kept its original spherical shape under 850[Formula: see text]C, while above 850[Formula: see text]C the microsilica powder recrystallizes. These results provide a basis for chemical removal of metal impurities in the preparation of high-purity silica powder.

Thermodynamic Analysis and Reduction of MnO2 by Methane–Hydrogen Gas Mixture

Methane and hydrogen reduction has emerged as promising technology for clean and efficient metallurgy. To understand the reduction process of MnO2 by CH4-H2 gas mixture, the effects of various CH4/(CH4 + H2) molar ratios (0–1) and temperatures (1100°C–1250°C) on the reduction of pure MnO2 were researched. Characterization of the solid carbon derived from the decomposition of CH4 gas was also determined. Thermodynamic analysis indicated that the MnO2 was first reduced to MnO by CH4-H2 gas mixture, and then MnO was reduced to Mn7C3 according to this reaction, 7MnO(s) + 10CH4(g) = Mn7C3(s) + 7CO(g) + 20H2(g). Experimental results showed that under the CH4/(CH4 + H2) molar ratio of 0.2, the reduction extent of MnO2 increased with the rising of temperatures. Nevertheless, the deep reduction of MnO to Mn7C3 was retarded even by increasing the temperature and prolonging the time, which was a result of the formation of deposited solid carbon. The free carbon contents in the reduced samples increased with the increasing of the CH4/(CH4 + H2) molar ratio and reduction time. The reduction of MnO2 and formation of carbon can be regulated by adjusting the CH4/(CH4 + H2) molar ratio in a gas mixture. For example, when MnO2 was reduced by 50 vol.%CH4-50%N2, large amounts of MnO and Mn7C3 particles were wrapped by the platelike and rodlike carbon with nanoscale. Yet, the wrapping phenomenon cannot be observed under 10 vol.%CH4-40 vol.%H2-50 vol.%N2.

High-temperature evolution behavior of polymer-derived SiAlOC ceramics under inert atmosphere

SiAlOC ceramics were fabricated by pyrolysis of Al-doped polymethyl(phenyl)siloxane at 1000 °C. Then SiAlOC ceramics were annealed under inert atmosphere at 1200 °C, 1400 °C, 1500 °C and 1600 °C, respectively, to investigate their high-temperature evolution behaviors. The transformations of microstructural units resulting from bonds redistribution are observed from 1000 °C to 1400 °C. Creation of mullite at 1400 °C and its crystallization at higher temperatures are confirmed. Carbothermal reaction is effectively suppressed due to low content of free carbon and formation of mullite in SiAlOC ceramics. Accordingly, enhanced thermal stability of SiOC ceramics is realized by incorporation of Al element.

Effect of KOH etching on the structure and electrochemical performance of SiOC anodes for lithium-ion batteries

The SiOC ceramic was prepared with vinyltriethoxysilane as the sol−gel precursor, and followed by etching with KOH. The effect of KOH etching on the morphology, structure and electrochemical properties of SiOC anodes was investigated. The amorphous SiOC ceramics with different free carbon content is obtained by KOH etching. The reaction of oxygen-enriched units with KOH and the activation of free carbon resulted in the formation of micro and mesopores. The galvanostatic charging/discharging and rate capability suggest that the electrochemical performance of SiOC ceramics is improved by KOH etching. As compared with SiOC sample, KOH-SiOC-5 (KOH/SiOC weight ratio of 5:1) with a high BET surface area of 249.2m2g−1 deliveres a discharge capacity of 607mAhg−1 at the current density of 36mAg−1. Nano-sized pores, mixed SiOxC4-x structures and the free carbon phase both work together for the excellent electrochemical performance. The contribution of carbon to reversible capacity is higher than that of SiOxC4-x phases and surface area. However, the specific surface area, pore volume and free carbon content are decreased by excessive etching with KOH, which resulted in the decrease of electrochemical properties.