Synthesis of SiC from rice husk in a plasma reactor

A new route for production of SiC from rice husk is reported by employing thermal plasma technique. The formation of β-SiC is observed in a short time of 5 min. The samples are characterized by XRD and SEM.     

Thermal plasma synthesis of SiC

Synthesis of silicon carbide has been carried out using thermal plasma processing technique using SiO₂ as the solid feed and CH4 as the gaseous reducing agent. Thermochemical calculations have been performed varying the molar ratio of silicon dioxide and methane to determine the feasibility of the reaction. Experiments using a molar ratio of SiO2:CH4 equal to 1:2 produced maximum yield of SiC of about 65 mol % at a solid feed rate of 5 g/min.
Mostly spherical morphology with some nanorods has been observed. The presence of Si had been observed and was quantified using XRD, HRTEM, Raman spectroscopy and X-ray photoelectron microscopy (XPS). Si acts as a nucleating agent for SiC nanorods to grow.     

Thermal plasma synthesis of SiC nano-powders/nano-fibers

Thermal plasma synthesis of nano-powders/nano-fibers is a relatively new technology with great potential for future application in industries. The paper presents the effects of molar ratio on synthesis of SiC by thermal plasma technology. The experimental results show that SiC can be synthesized by thermal plasma technology. The average size of SiC powders is less than 100 nm and SiC fiber-like microstructure were observed.     

Effect of cobalt catalyst on the formation of SiC from rice husk silica-carbon black mixture

Cobalt has been found to act as a strong catalyst in increasing the formation of total SiC by decreasing the crystallization of carbon black. The formation of SiC whiskers in the presence of cobalt catalyst is found to be low. No -SiC has formed in the presence of Co catalyst. With Co catalyst needle-like whiskers are formed. Without any catalyst (a) slow heating has decreased the reactivities of carbon black and rice husk silica by increasing their degree of crystallization and (b) rapid heating has increased the formation of SiC particulates. With Co catalyst rapid heating has been found to be beneficial in increasing the whisker content.     

稻壳SiO_2/环氧树脂纳米复合材料的热性能研究

将稻壳用10%的盐酸处理后在600℃焚烧得到纯度为99.3%,比表面积为212g/cm2的SiO2,经硅烷偶联剂γ-氨丙基三乙氧基硅烷(KH550)改性后的SiO2为无定形态,尺寸在30~50nm。将改性后的稻壳SiO2与EP复合,对SiO2/EP纳米复合材料的热膨胀过程进行了详细的研究。结果表明:稻壳SiO2的加入,能有效降低SiO2/EP纳米复合材料的热分解性能,其降低量随稻壳SiO2含量增加而增加。纳米复合材料在玻璃态(30℃≤T≤140℃)以及在大于180℃的橡胶态,热膨胀系数随温度变化不大,而在玻璃态与橡胶态的过渡区,材料随温度升高出现收缩。进一步研究发现材料的热循环次数对纳米复合材料的热收缩现象影响较大,热循环次数的增加能消除降低材料在升温过程的中的收缩现象,并有效降低材料的热膨胀系数。     

Short-period synthesis of high specific surface area silica from rice husk char

Porous silica with high specific surface areas is prepared from rice husk char within a short period by avoiding the commonly used hydrothermal treatment step and using polyethylene glycol (PEG, molecular weight = 20,000) as a template. The preparation process mainly involves sodium silicate production, precipitation with ortho-phosphoric acid, and calcination. The amount of used PEG significantly affects the silica textural properties like the specific surface area, total pore volume and mesopore volume. The reason should be attributed to the amount of PEG removed from the PEG-silica composites by calcination. By varying the PEG dosage from 100 to 176 mg, porous silica with specific surface areas ranging from 709 to 936 m²/g could be successfully prepared within 10 h.     

Sorption of Cd(II) and Se(IV) from aqueous solution using modified rice husk

A carbonaceous sorbent was prepared from rice husk via sulfuric acid treatment. Removal of Cd(II) and Se(IV) from aqueous solution was studied varying time, pH, metal concentration, temperature and sorbent status (wet and dry). Cd(II) sorption was found fast reaching equilibrium within approximately 2 h while Se(IV) sorption was slow reaching equilibrium within approximately 200 h with better performance for the wet sorbent than for the dry. Kinetics data for both metals were found to follow pseudo-second order model. Cd(II) sorption was low at low pH values and increased with pH increase, however, Se(IV) sorption was high at low pH values, and decreased with the rise in initial pH until pH 7. A fall in the final pH was noticed with Cd(II) sorption due to the release of protons indicating an ion exchange mechanism. However, for Se(IV) sorption, a rise in the final pH was observed due to protons consumption in the process. For both metals, sorption fit well the Langmuir equation with higher uptake by rising the temperature. Analysis by scanning electron microscope and X-ray powder diffraction for the sorbent after the reaction with acidified Se(IV) confirmed the availability of elemental selenium, Se(0), as particles on the sorbent surface. The reduction process of acidified Se(IV) to Se(0) is accompanied by surface oxidation. Physicochemical tests showed an increase in sorbent acidity, cation exchange capacity and surface functionality after the reaction with acidified Se(IV) indicating that oxidation processes took place on the sorbent surface. On the other hand, no changes in physicochemical tests were found after Cd(II) sorption indicating the absence of redox processes between Cd(II) and the sorbent.     

Silicon carbide materials obtained from rice husk

Thermal processing of a rice husk sol in air at T≥750°C leads to the phase transformation of amorphous silicon dioxide (SiO₂) into crystobalite (type B samples). Thermal treatment of the same sol at T≈ 1400 °C in a closed graphite crucible leads to the formation of a mixture comprising hexagonal SiC phases and graphite (type A samples). Unannealed type A samples showed high refractory properties, being stable up to 1650°C. Using kaolin binder in silicon carbide articles leads to a decrease in the material refractoriness.     

The sorption of lead(II) ions on rice husk ash

Present study deals with the adsorption of Pb(II) from aqueous solution on rice husk ash. Rice husk is a by-product generally obtained from rice mill. Rice husk ash is a solid obtained after burning of rice husk. Batch studies were performed to evaluate the influences of various experimental parameters like pH, initial concentration, adsorbent dosage, contact time and the effect of temperature. Optimum conditions for Pb(II) removal were found to be pH 5, adsorbent dosage 5 g/L of solution and equilibrium time 1h. Adsorption of Pb(II) followed pseudo-second-order kinetics. The effective diffusion coefficient is of the order of 10(-10)m(2)/s. The equilibrium adsorption isotherm was better described by Freuindlich adsorption isotherm model. The adsorption capacity (q(max)) of rice husk ash for Pb(II) ions in terms of monolayer adsorption was 91.74 mg/g. The change of entropy (DeltaS(0)) and enthalpy (Delta H(0)) were estimated at 0.132 kJ/(mol K) and 28.923 kJ/mol respectively. The negative value of Gibbs free energy (Delta G(0)) indicates feasible and spontaneous adsorption of Pb(II) on rice husk ash. The value of the adsorption energy (E), calculated using Dubinin-Radushkevich isotherm, was 9.901 kJ/mol and it indicated that the adsorption process was chemical in nature. Application study was also carried out to find the suitability of the process in waste water treatment operation.     

稻壳基白炭黑的制备研究

以炭化稻壳为原料,Na2CO3为活化剂,采用沉淀法对稻壳基白炭黑的制备工艺进行了研究.结果表明沉淀法制备白炭黑的最佳工艺条件为:反应温度100℃,Na2CO3溶液浓度为13％(w/w),稻壳灰与Na2CO3溶液质量配比为1:40,反应时间4h,提取率可以达到93％以上.整个工艺流程中,Na2CO3溶液循环回用,有利于工业化放大,产品的相关质量检测均符合国家标准.     

Nano filter from sintered rice husk silica membrane

A nano filter showing the Knudsen flow was demonstrated by a modification of a membrane constructed from rice husk silica. The membrane was prepared by pressing and sintering micron sized rice husk silica with 4 nm pores. The membrane showed a permeability of 5.2 x 10(-8) mol m(-1) sec(-1) Pa(-1) for H2 and ratios of gas permeability 2.1 and 3.2 for k(H2)/k(CH4) and k(H2)/k(CO2), respectively. When the membrane was treated by filtration of approximately 100 nm sized rice husk silica particles, the permeability decreased to 4.9 x 10(-8) mol m(-1) sec(-1) Pa(-1) and the ratios increased to 2.2 and 3.4. In the case of the membrane after treatments with the dispersion and chemical deposition of tetraethylorthosilicate (TEOS), the corresponding permeability and ratios of the membrane were 1.8 x 10(-8) mol m(-1) sec(-1) Pa(-1), and 2.9 and 4.5, respectively. From the change of the ratio of gas permeability for the membrane with modifications, it is suggested that approximately 100 nm sized rice husk silica particles pack the large pores among the micron sized rice husk silica particles while the chemical deposition of tetraethylorthosilicate (TEOS) reveals the gas flow through 4 nm pores in the rice husk silica by blocking large pores.     

Silica nanoparticles and frameworks from rice husk biomass

Biogenic silica nanoparticles (25-30 nm in diameter) were synthesized from rice husks. The characterizations revealed that the silica nanoparticles were composed of smaller primary particles (ca. 4.2 nm in diameter), and their clustering led to a porous structure with a surface area of 164 m(2)/g. Under the controlled melting catalyzed by K(+), such silica nanoparticle clusters can gradually fuse to form semicrystalline porous silica frameworks with tunable pore size and structural integrity.     

Fixed bed column study for Cd(II) removal from wastewater using treated rice husk

A fixed bed of sodium carbonate treated rice husk was used for the removal of Cd(II) from water environment. The material as adopted was found to be an efficient media for the removal of Cd(II) in continuous mode using fixed bed column. The column having a diameter of 2 cm, with different bed depths such as 10, 20 and 30 cm could treat 2.96, 5.70 and 8.55 l of Cd(II) bearing wastewater with Cd(II) concentration 10 mg/l and flow rate 9.5 ml/min. Different column design parameters like depth of exchange zone, adsorption rate, adsorption capacity, etc. was calculated. Effect of flow rate and initial concentration was studied. Theoretical breakthrough curve was drawn from the batch isotherm data and it was compared with experimental breakthrough curve. An amount of 0.01 mol/l HCl solution was used for desorption of adsorption column. Column regeneration and reuse studies were conducted for two cycles of adsorption-desorption.     

混合活化制备稻壳基活性炭研究

以脱硅稻壳灰为原料,采用混合活化法制取活性炭,通过4种单一活化剂物料比的实验,确定了最佳物料比为1∶3;设计了5种混合活化的配比方案,实验结果表明在NaOH&Na2CO3和KOH&K2CO3配比为2.5∶0.5时碘吸附值和亚甲基蓝吸附值分别达到最优,说明辅助活化剂的加入可有效提高稻壳基活性炭的吸附性能。在总物料比和活化剂混合配比确定的条件下,进行了浸渍液质量分数、活化温度、活化时间3个单因素实验,结果显示,浸渍液质量分数为30%、活化温度为500℃、活化时间为40min时活化效果最佳,其中碘吸附值最高可达1528.76mg/g,可知混合活化对制备稻壳基活性炭有显著作用。     

Induction plasma synthesis of ultrafine SiC powders from silicon and CH4

Ultrafine SiC powders have been synthesized from elemental silicon and methane using induction plasma technology. The powder products were characterized by X-ray diffraction, thermogravimetric analysis, scanning and transmission electron microscopy, electron probe microanalysis, infrared spectroscopy, and surface area measurement. The powders collected from various sections of the reactor system showed different features reflecting different compositions and powder morphologies. The purest SiC powder was collected in the metallic filter. It was composed of both -and -phase of SiC with small levels of free silicon and carbon. The reaction route used is based on the evaporation of the injected pure silicon starting powder, followed by carburization of the silicon vapour using methane. The silicon evaporation rate was found to depend strongly on the particle size of the silicon powder. Using silicon powder with a mean particle diameter of 100 m, at a plasma power level of P=43.2 kW, the conversion of silicon to SiC and the overall SiC content in the product powder was 44.2% and 50.8 wt%, respectively. The injection probe position was Z=9.3 cm, the silicon feed rate was 4 g min^–1, and the C/Si molar ratio was 0.7. Using silicon particles with a mean diameter of 45 m, the conversion and overall content of SiC increased to 70.4% and 73.9 wt%, respectively, under the same plasma operating conditions and powder feed rates. By appropriate selection of experimental conditions, ultrafine SiC powder of high quality was achieved.     

Growth of SiC nanowires and nanocones using mixture of oil palm fibres and rice husk ash

3C-SiC nanowires and nanocones were grown by pyrolysing mixture of acid-treated oil palm empty fruit bunch fibres and rice husk ash (RHA), which acted as carbon and silicon source, respectively. The effects of different RHA amounts and pyrolysis temperature were studied. When the amount of RHA was increased to 80 % of the mixture, there was a change in the morphology from nanowires to nanocones. Overall, it was found that 40 % of RHA in the mixture was the ideal amount in growing the nanowires with the maximum yield and with the least amount of impurities. When the pyrolysis temperature was raised, there was an increase in the amount, diameter and length of the nanowires. The proposed main growth mechanism for the SiC nanowires were combination of solid-state reaction and vapour–solid mechanisms, with some nanowires grown under vapour–liquid–solid (VLS) mechanism induced by trace metals as well. The growth of the nanocones could also be related to VLS mechanism.     

Rapid carbothermal synthesis of nanostructured silicon carbide particles and whiskers from rice husk by microwave heating method

This paper reported a simple and rapid route to large-scale synthesis of nanostructured SiC powders using rice husk as the precursor. Rapid carbothermal reduction reactions were achieved in a 2.45 GHz microwave field in an argon atmosphere. The XRD patterns revealed that complete carbothermal reduction of silica was achieved at 1300 °C for 60 min or at 1500 °C for only 15 min by microwave heating, resulting in β-SiC formation. The FE-SEM images showed that the β-SiC powders were mixtures of particles and whiskers. The β-SiC particles had diameters of 60–130 nm and the β-SiC whiskers, which were several to tens of micrometers in length, had diameters of 110–170 nm. The β-SiC powder synthesized at 1500 °C for 15 min showed the highest BET surface area of 12.2 m²/g. Compared to the conventional heating method, the microwave heating method proved to be an efficient approach for synthesis of SiC in terms of energy and time saving, as well as for fabrication of nanostructured SiC.