Microwave Synthesis of Ultrafine Silicon Carbide Whiskers

Silicon carbide (SiC) whiskers were formed by microwave and conventional heating in the present study. SiC whiskers with diameters in the nanometer range were synthesized by reducing silica (SiO₂) with various forms of carbon in a microwave furnace. Ultrafine SiO₂ powder, char, phenolformaldehyde resin, and carbon black were used as starting materials. The effects of temperature and type of heating on the production of SiC whiskers, as well as the role of carbon, are discussed here.     

Synthesis of SiC Whiskers from Silicon Nitride in Argon Atmosphere

SiC whiskers were synthesized by carbothermal reduction of silicon nitride. α-Si3N4 and β-Si3N4 powders were used as silicon sources, and graphite, active carbon and black carbon as carbon sources, as well as boron oxide as catalyst. The synthesized SiC whiskers were characterized by XRD and SEM. The results showed that the synthesizing temperature should be above 1 716 K; the decomposition of Si3N4 was the limited step in the synthesis of SiC whiskers; and catalyst not only offered the liquid condition, bu...     

Influences of pre-forming on preparation of SiC by microwave heating

Silicon carbide (SiC) crystals were synthesized by microwave sintering using coal and tetraethoxysilane (TEOS) as raw materials. A sol-gel method was carried out to coat coal mineral particles with silicon dioxide (SiO₂). The mixed raw powders were pre-formed by uniaxial pressing into cylindrical pellets in dimension of ~ 30?×?3?mm². The pre-forming pressure was selected at 0?MPa, 1?MPa, 2?MPa, 3?MPa, 4?MPa and 5?MPa respectively, which led to different apparent density of the green pellets. The influence of apparent density of green pellets on microwave heating behavior was investigated. Different microwave thermal effects were analyzed. Techniques of XRD、SEM were carried out to characterize samples. It was found that pre-forming pressure showed crucial influences on microwave thermal effects and electric field (E-field) intensification. No SiC crystal could be formed without pre-forming pressure. Pre-forming pressure might be the prerequisite for synthesis of SiC by microwave heating. Five consecutive and indispensable heating stages including accumulation of residual air, microwave plasma generation, complex chemical reactions, nucleation and grain growth of SiC crystallites could be distinguished for samples under pre-forming pressure. Different pre-forming pressure leads to changes in heating behavior as well as morphologies of SiC crystals. ~ 4?MPa might be the optimized pre-forming pressure for both microwave plasma effects and E-field intensification.     

Microwave synthesis of worm-like SiC nanowires for thin electromagnetic wave absorbing materials

Thin thickness is always the pursuit of excellent electromagnetic wave absorbing materials. Herein, SiC nanowires with worm-like morphology were synthesized by microwave heating the mixture of expanded graphite and silica. The worm-like SiC nanowires exhibit an excellent microwave absorption ability at a thin thickness. With the filling ratio of SiC nanowires increases in the matrix, the dielectric loss and microwave absorbing ability are significantly enhanced; meanwhile the number of absorption peaks is gradually increased, and the absorption peaks also move toward a thinner thickness. When the nanowires filling ratio was 40?wt%, the minimum reflection loss reached down to ?35.2?dB and the effective absorption (RL?<??10?dB) bandwidth was 1.8?GHz?at a thickness of 1.3?mm. The possible growth mechanism of the worm-like SiC nanowires is that the intermediate reaction gas phases, SiO and CO, were confined in the relatively independent tiny pores of expanded graphite. This resulting in an excessive local gas phase pressure, which causes the nanowire growth direction changes randomly.     

Seeds-induced synthesis of SiC by microwave heating

α-SiC particles were used as heating seeds to prepare SiC from coal minerals with microwave heating method. Coating technique was carried out to prepare composite raw powders in three different methods. Heating temperatures were at 1000?°C, 1050?°C, 1100?°C, 1150?°C, 1200?°C for 10min, respectively. XRD, SEM techniques were carried out to characterize samples. It was found that different distribution between C and α-SiC particles from different mixing method leads to different microwave heating behavior and growth mechanisms. V-V reaction leads to in situ nucleation and grain growth on the surface of α-SiC seeds which contact with C particles. Well-grown β-SiC particulates appear. Hybrid V-V reaction V-L reaction lead to local microwave plasma and diffusion-precipitation with a very thin layer of SiO₂ between raw C particles and α-SiC seeds. Local fine β-SiC whiskers and particulates on the surface of α-SiC seeds co-exist. Primary V-V reaction leads to nucleation and grain growth along reacted C-SiO₂ interface with very thick layer of SiO₂ between raw C and α-SiC seeds. Substantial β-SiC whiskers appear. Transformation from β-SiC to α-SiC on the surface of as-formed whiskers will be enhanced by microwave plasma at high temperature.     

A low-cost lightweight microwave absorber: Silicon carbide synthesized from tissue

Recently, microwave absorbing materials have been widely used with the development of electromagnetic wave technology such as 5G communication. It is urgent to develop low-cost electromagnetic wave absorbing materials to meet the increasing civil market demand. Herein, we report a novel and simple strategy to synthesize lightweight silicon carbide microwave absorber by calcining tissue and glass microspheres at high temperature. The results show that higher synthesis temperature can improve the crystallinity of SiC, generate more whiskers and reduce the impurity content. This in turn increases the dielectric loss of the material. The products synthesized at 1600 °C have excellent microwave absorption properties. At the thickness of 1.3 mm, it achieves a reflection loss of ?32.5 dB and an effective absorption bandwidth of 4.1 GHz. This study broadens the way for the potential reuse of waste paper and glass, and provides a useful reference for the preparation of low-cost microwave absorbing materials.     

Effects of the Susceptor Dielectric Properties on the Microwave Sintering of Alumina

Microwave sintering of alumina has been carried out using SiC and Y‐ZrO₂ based susceptors. The microstructure of the sintered samples has been rigorously compared and correlated to the heating behaviour of the susceptor used. It was found that the nature of the susceptor highly influences the sintering behaviour of alumina. The results show that at high temperatures, the electrical conductivity of the SiC susceptor tends to screen the incident electric field, thus resulting in a surface heating of the alumina sample material. When using ZrO₂ susceptor, the microstructure analysis of the sintered alumina samples reveals a volumetric heating, which is a signature of the microwave dielectric loss mechanism. This could be explained by the lower ZrO₂ electrical conductivity compared to the SiC one. The simulation results confirm this behaviour, particularly showing that in the presence of ZrO₂, the intensity of the electric field within the sample is higher than the one when SiC susceptor is used. Basically, the results of the simulation data are in good agreement with our experimental results. Although the SiC based susceptor is usually used in the microwave heating of materials, the ZrO₂ based susceptor presents numerous advantages over the SiC one, especially in term of direct microwave heating contributions.     

介电材料辅助的微波冷冻干燥的实验研究

设计、加工和装配了一套实验室规模的微波冷冻干燥装置,旨在实验验证介电材料对微波冷冻干燥液体物料的强化作用。介电材料用烧结的碳化硅(SiC),石英玻璃作为介电材料的参照物;甘露醇,一种典型的药物赋形剂被选为待干溶液中的溶质。实验结果表明使用介电材料可以有效地强化微波冷冻干燥过程。与传统冷冻干燥相比干燥速率大大加快,在试验条件下干燥时间可以节省20%。微波加热逐渐生效并且主要体现在干燥过程的后半部分。当溶液中的固含量很低或者固体物质具有很小的介电损耗因子时,如果不用介电材料,微波加热的效果不明显。     

Materials processed by indirect microwave heating in a single-mode cavity

We report in this paper the synthesis, the sintering and the properties of La_0.8Sr_0.2MnO₃ (LSMO) and BaZrO₃ (BZ) compound using microwave heating. The starting raw material was prepared by solid state reaction. Synthesis and sintering were carried out using an indirect microwave heating process. SiC susceptor tube was utilized to produce the indirect heating in a microwave symmetrical single mode cavity TE10p. The LSMO and BZ phases have been successfully sintered in a microwave cavity in a very short time. Scanning electron microscopy (SEM), X-ray diffraction (XRD), magnetic or dielectric and electrical properties were carried out for both processing conditions, i.e., using classical and microwave heating. The results and advantages of this microwave process to synthesize many materials with various physicals properties are discussed.     

Synthesis of SiC whiskers via catalytic reaction method in self-bonded SiC composites

Catalyzed by in-situ formed Fe nanoparticles (NPs), 3C–SiC whiskers were prepared from expanded graphite and Si powders after firing at 1573 K for 3 h in Argon. The density functional theory calculations revealed that Fe catalysts facilitated the formation of SiC nucleus and the epitaxial growth of SiC whiskers via reducing the bonding strength in CC dimer as well as Si–O and C–O bonds. Moreover, using SiC, expanded graphite and silicon powders as starting materials Fe-catalyzed self-bonded SiC composites were fabricated. Lots of SiC whiskers were synthesized in the as-prepared composites, leading to remarkable enhancements in high temperature mechanical behavior, oxidation resistance and cryolite resistance of the self-bonded SiC composites.     

Microstructural analysis of single crystal SiC prepared by novel liquid phase epitaxy

Single crystal SiC has been synthesized by a novel liquid phase epitaxy using mixture of (Sm:Co) as unique solvent. The synthesized high quality crystals have been characterized by field emission gun scanning electron microscopy and field emission gun transmission electron microscopy. The above analysis shows the epitaxial growth of single crystal SiC along [1 1 1] direction parallel to the Si wafer, followed by polycrystalline 3C-SiC and 6H-SiC whiskers. The formation mechanisms of single crystal SiC and SiC whiskers have been proposed.     

Features of vapor-grown cone-shaped graphitic whiskers deposited in the cavities of wood cells

Vapor-grown graphitic whiskers in wood charcoal formed during heat treatment above 2000 °C are discussed through electron microscope observations. The whiskers were composed of conical stacked hexagonal carbon layers, with a cone apex angle of 136°. This angle probably helps the successive helical growth of the whiskers. To grow these whiskers, pyrolyzed gas from the walls of wood cells could be used as the carbon source, and the wood cell cavities functioned as a reactor that can store the pyrolyzed gas and concentrated it to supersaturation levels. Whiskers were produced under various conditions, and their features were compared using electron microscopy to examine the growth mechanism. Samples were prepared by precarbonizing wood along with SiC via a secondary heating process at 2000-2700 °C. The precarbonizing temperature controlled the potential supply of pyrolysate gas. Whisker growth began when the gas concentration reached the supersaturation level, and this was regulated by the secondary heat treatment temperature. The gas concentration can determine the size, features, and yield of the resulting whiskers. Since the carbon source was internal, originating from the surrounding cell walls, the regulation of the gas concentration was due entirely to the heat treatment and the features of the cell tissue.     

稻壳SiC晶须的特性和品质研究

利用现代分析仪器对稻壳法生长的ＳｉＣ晶须（中试条件下生产产品）的结构特性进行分析测试，结果表明：晶须品位较高（＞９７％），晶须外形挺直，表面光洁，长度、直径分布均匀、集中，晶须中缺陷杂质含量和以前产品相比明显降低，各项指标均已达到或接近国外同类产品标准。     

Improved thermal shock stability and oxidation resistance of low-carbon MgO–C refractories with introduction of SiC whiskers

This research focuses on the utilization of SiC whiskers synthesized from rice husk powders in low-carbon magnesia–carbon (MgO–C) refractories, and attempts to reduce the flake graphite content in refractories by adding synthesized SiC whiskers. The effect of the addition amount of SiC whiskers on the microstructure, mechanical properties, thermal shock stability and oxidation resistance of MgO–C refractories with different graphite content was studied. The results indicated that the introduction of SiC whiskers facilitated the generation and growth of ceramic phases in MgO–C refractories. By adding 1 wt% SiC whiskers, the graphite content could be reasonably reduced (from 5 wt% to 4 wt%), and the strength, thermal shock stability and oxidation resistance of refractories were enhanced by the synergistic effect of the introduced SiC whiskers and the generated ceramic phases, the CMOR, CCS, residual CCS, and oxidation resistance were increased by 44, 6, 12 and 27% respectively.     

Preparation of Al2O3-mullite thermal insulation materials with AlF3 and SiC as aids by microwave sintering

Al₂O₃-mullite composites were prepared under the synergy effect of AlF₃ and SiC aids by microwave heating. The phase composition, microstructure, porosity, flexural strength, thermal shock resistance, and thermal conductivity were investigated. The XRD results revealed that the content of mullite phase steadily increased with the increasing of AlF₃ content. The microstructure showed that the lower content (≤1 wt%) of AlF₃ led to the formation of granular mullite and the higher content (≥3 wt%) of AlF₃ led to the formation of mullite whiskers, which could form an interlocking structure. In addition, the SiC hot spots can also promote the generation of mullite whiskers by microwave sintering. The thermal shock resistance was significantly improved by the interlocking structure of mullite whiskers. The residual rate of flexural strength of the composite with 3 wt% AlF₃ was 86%. The composite with 3 wt% AlF₃ additives got its optimized thermal conductivity from 30°C to 950°C, the value was between 0.819 and 1.021 W/(mK), which possess excellent thermal insulation performance.     

SiC-Whisker-Reinforced Al2O3 Composites by Free Sintering of Coated Powders

SiC whiskers were coated with a thick cladding of finegrained Al₂O₃ powder by controlled heterogeneous precipitation in a concentrated suspension of whiskers. After calcination, the coated whiskers were compacted by cold isostatic pressing and sintered at a constant heating rate of 5°C/min in a helium atmosphere. The parameters which control the coating process and the sintering characteristics of the consolidated powders are reported. Starting with an initial matrix density of 40–45% of the theoretical, composites containing up to ≅20 vol% whiskers (aspect ratio ≅15) were sintered freely to nearly theoretical density below 1800°C. By comparison, a similar composite formed by mechanical mixing of the whiskers and the precipitated Al₂O₃ powder reached a density of only 68% of the theoretical after sintering under identical conditions. For a fixed whisker content, the sinterability of the composites formed from the coated whiskers shows a fairly strong dependence on the whisker aspect ratio.     

Effect of SiC nanoparticles on in-situ synthesis of SiC whiskers in corundum–mullite–SiC composites obtained by carbothermal reduction

Corundum–mullite–SiC composites were synthesised using a carbothermal reduction method. The effects of SiC nanoparticles and sintering temperatures on the phase transformation of the composites and the synthesis of SiC whiskers were studied by X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. Results indicated that corundum, mullite, and SiC whiskers were produced as final products at 1600–1650 °C. SiC whiskers were formed through the vapor–solid mechanism. The added SiC nanoparticles worked as nucleating agents to facilitate the carbothermal reduction of aluminosilicates and formation of SiC whiskers. The sample with the added SiC nanoparticles exhibited a high yield of β-SiC of 17.1%. Furthermore, the SiC nanoparticles decreased the formation temperature of SiC whiskers from the original 1600 °C to 1500 °C, and the porosity of the composites was increased from 56.7% to 64.7% by increasing the partial pressure of SiO gas. This study provides an insight into the more efficient synthesis of composites with SiC whiskers through the carbothermal reduction of aluminosilicates.     

Heat and mass transfer model of dielectric-material-assisted microwave freeze-drying of skim milk with hygroscopic effect

A new porous media mathematical model for freeze-drying was developed based on the adsorption-desorption relationship proposed in this paper. A finite difference solution was obtained from a moving boundary problem for the dielectric-material-assisted microwave freeze-drying process. Silicon carbide (SiC) was selected as the dielectric material; and frozen skim milk was used as the aqueous solution to be dried. Simulation results showed that the dielectric material can significantly enhance the microwave freeze-drying process. The drying time was 33.1% shorter than that of ordinary microwave freeze-drying under typical operating conditions. When the solid content of the solution to be freeze-dried was very low, or the solid product had a very small loss factor, microwave heating was less effective without the assistance of dielectric material. The mechanisms of heat and mass transfer during drying were analyzed based on profiles of ice saturation, temperature and vapor concentration. Drying rate-controlling factors were discussed. A comparison was made between the model predictions and the reported experimental data.     

Kinetic Effects on Hydroxyapatite Whiskers Synthesized by the Chelate Decomposition Method

Hydroxyapatite (HA) whiskers have been synthesized using a number of chemical solution methods, including the chelate decomposition method. Numerous previous studies have investigated the effects of the reagents, reagent concentrations, solution pH, and reaction temperature on HA whisker morphology and composition. However, purely kinetic effects, such as the reaction heating and stirring rates, have not been rigorously investigated and are rarely reported in the literature. Therefore, the objective of this study was to investigate kinetic effects on the morphology of HA whiskers synthesized using the chelate decomposition method. In order to study the kinetic effects on the morphology of HA whiskers, three experimental parameters were varied independently: the reaction heating rate (0.36°–3.0°C/min), stirring rate (0–250 rpm), and temperature (80°–200°C). At all heating and stirring rates, precipitated whiskers were confirmed by XRD and FT-IR to comprise phase-pure, calcium-deficient HA (Ca/P=1.57–1.62). The length and aspect ratio of HA whiskers increased with decreased heating rate, decreased stirring rate, and increased reaction temperature. The mean length and aspect ratio of HA whiskers increased approximately twofold with decreased heating rate over the range studied, following a power-law relationship. Therefore, the reaction heating rate is a key variable that can be used to tailor the morphology of HA whiskers and ought to be reported in the literature. The reaction heating rate and temperature had relatively little effect on the width of HA whiskers. However, the precipitate morphology was altered significantly from micro-scale whiskers to nano-scale plates with increased stirring rate. These results offered new insights and provided clarification regarding the reaction mechanism, which is discussed in detail.     

Enhanced microwave‐absorbing property of precursor infiltration and pyrolysis derived SiCf/SiC composites at X band: Role of carbon‐rich interphase

Ceramic matrix composites (CMCs) can be microwave‐absorbent when endowing the composite constituents with proper dielectric properties. In this work, we report a new method to enhance the microwave‐absorbing property of CMCs by in situ fabrication of a carbon‐rich interphase at the fiber/matrix interface. This was achieved in a SiC fiber reinforced SiC matrix (SiCf/SiC) composite fabricated by precursor infiltration and pyrolysis (PIP). We found that as the PIP temperature increased from 800 to 1000°C, the microwave‐absorbing property of the SiCf/SiC composite was significantly enhanced at X band, which also surpassed those of the SiC fiber and monolithic SiC ceramic fabricated at the same temperature. The dominant mechanism was studied by decoupling the effect of individual SiC fibers, SiC matrix, and fiber/matrix interface. The results showed that the SiC fiber and SiC matrix were barely microwave‐absorbent, due to their low dielectric losses. The microwave‐absorbing mechanism was finally ascribed to the fiber/matrix interface, which was carbon‐rich, containing Si and O elements. The interphase showed a conductivity that was superior to that of the fiber and the matrix, and mainly dominated the dielectric property of the overall composite. The results highlight the role of carbon‐rich interphase on the microwave‐absorbing property of CMCs.