A study on the effects of silica particle size and milling time on synthesis of silicon carbide nanoparticles by carbothermic reduction

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Abstract

Silicon carbide nanoparticles were produced by a carbothermic reduction of nano and micro size silica with graphite at 1450 °C for 1 h. The SiC nanoparticles were characterized by XRD, SEM and TEM. The results showed that in the case of nano silica, milling up to 20 h could develop SiC particles of 5–25 nm with some residual SiO2 particles. By extending milling time to 40 h, more energy was provided and produced Fe contamination, which could act as catalyst and increased SiC yield as well as Fe2Si phase formation after heat treatment. Coarser particles of micro silica caused higher Fe erosion, more SiC formation with 20–70 nm size and presence of Fe2Si phase at shorter milling times after heat treatment. Leaching treatment could purify SiC nanoparticles. Increase of milling from 20 to 40 h changed the morphology from polygonal shape to spherical with some reduction in the particle size.

Graphical abstract

Silicon carbide (SiC) nanoparticles were produced by a carbothermic reduction of nano and micro size silica (SiO2) with graphite at relatively low temperature of 1450 °C for 1 h. The particle size of silica and milling time were important parameters determining the rate of carbothermic reduction and size and morphology of SiC particles produced. The SiC nanoparticles were between 5 and 25 nm by using nano silica and were in the range of 20–70 nm by using micro silica.

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Research highlights

► SiC nanoparticles were synthesized by silica with graphite at 1450 °C for 1 h. ► Fe contamination during milling act as catalyst and increased SiC yield. ► Leaching treatment led to leaving behind single phase SiC nanoparticles. ► Size of SiO2 and milling time affected on the size and morphology of SiC. ► SiC from micro silica and nano silica were 20–70 and 5–25 nm, respectively.

Introduction

Silicon carbide (SiC) is a very interesting ceramic due to its promising properties like high hardness, high strength, low bulk density, good wear and corrosion resistance, high oxidation resistance which makes it useful for a wide range of industrial applications (abrasives, cutting tools, heating elements, thermal barriers for aeronautic or aerospace applications) [1], [2], [3]. Four methods are principally used for preparing silicon carbide, namely: (1) direct carbonization [4], [5], (2) chemical vapor deposition [6], (3) sol–gel method [7] and (4) carbothermic reduction [8], [9]. Carbothermic reduction of silica is known to be a simple and economical process for the synthesis of SiC nanostructure [9]. The Acheson process, which is one type of the carbothermic reduction method, is the most popular process for production of silicon carbide. The raw materials for this process are quartz sand and petroleum coke and the reaction is carried out in an electrical resistance furnace. This process requires temperatures over 2400 °C and produces α-phase SiC. If the reaction is carried out between 1400 °C and 2000 °C, β-phase SiC can be synthesized [10].

The overall reaction for the formation of SiC powder through carbothermic reduction of silica can be represented as:SiO2(s) + 3C(s) = SiC(s) + 2CO(g)

This reaction is strongly endothermic with ΔH˚298 = 618.5 kJ [11]. Agarwal et al.[12] investigated the thermodynamic analysis of this reaction. They determined the equilibrium phases by calculation of Gibbs free energies of the individual phases. They suggested that the SiC formation began at a temperature of about 1500 °C. Also, above 1600 °C, the stable phases were SiC and CO(g)[12].

Reaction 1 is believed to proceed via a two-step process, [13] first with the formation of SiO gas:SiO2(s) + C(s) = SiO(g) + CO(g)

In the presence of carbon, the chemical potential of O2 gas is reduced by forming a thermodynamically stable gas CO:2C (s) + O2(g) = 2CO(g)

SiO gas subsequently reacts with carbon and CO, as follows:SiO(g) + 2C(s) = SiC(s) + CO(g)andSiO(g) + 3CO(g) = SiC(s) + 2CO2(g)

Any CO2 formed is expected to be consumed immediately by the surrounding carbon particles to yield CO gas:CO2(g) + C(s) = 2CO(g)

At lower temperatures (1500 °C or lower), the reaction is attributed to the SiO(g)–C(s) gas–solid reaction (4), favoring powder formation. Because of low temperatures, the yield of SiC is usually low. At higher temperatures (1600 °C or higher), the reaction is believed to be a SiO(g)–CO(g) gas–gas reaction, favoring whisker formation [14], [15].

In this study, the formation of very high yield nano SiC particles by carbothermic reduction at lower temperatures compared to conventional process is reported. The effect of mechanical activation by high energy ball mill and the size of silica powder on the process have been investigated.

Section snippets

Materials

Mixtures of amorphous pure silica with two different sizes, nano SiO2 (Degussa, purity > 99.5%, particle size  20 nm) and micro SiO2 (Aldrich, purity > 99%, particle size  5 μm) and crystalline graphite powders (Fluka, 99.9% purity, particle size  100 μm) in the stoichiometric ratio according to the reaction (1) were used as the starting materials. Fig. 1 shows the morphology of the initial powders. Both SiO2 powders were spherical and strongly agglomerated, as shown in Fig. 1(a–b). The graphite

Phase evolution and reaction mechanism

During milling at ambient temperature, reaction (1) can not thermodynamically occur due to positive ΔH˚, indicating that the SiO2 + C reaction is endothermic. Fig. 2 shows the XRD patterns of nano SiO2 and graphite powder mixtures at different milling times. This patterns show the amorphous halo and no trace of SiC peaks. It was indicated that ball milling up to 40 h had no effect on initiation of the reaction except destroying the crystalline structure of graphite and increasing the defect

Conclusions

In this study, experiments were conducted to examine the role of silica particle size and milling time of staring powder mixtures on synthesis of silicon carbide nanoparticles by carbothermic reduction at a relatively low temperature of 1450 °C for 1 h. From these investigations, the following conclusions can be drawn.

  • 1–

    In the case of nano silica by increasing the milling time to 20 h, the synthesis was improved and SiC yield increased. At longer milling times, more energy is provided and the

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