This paper present the investigation carried out to study the formation of silicon nitride and silicon carbide compound from the reaction of silica sand powders with carbon in nitrogen with 5% hydrogen atmosphere at temperatures between 1350oC to 1550oC. The effect of mechanical milled silica sand and different temperature during carbothermal reduction process was determined. The morphology of the synthesis products was characterized using scanning electron microscopy and its composition was determined by elemental and X-ray phase analysis. The formation of silicon nitride compound was facilitated by using silica sand with distorted structure by mechanical milling method. Further increased on temperature will lead to the formation of silicon carbide compound.
Silicon nitride is one of the ceramic materials that exhibit very good behavior at high temperature because of their excellent chemical stability, mechanical and thermal properties. An important characteristic of silicon nitride ceramics is the in-situ mechanism produced by their microstructures. The high melting point and the low self-diffusivity of silicon atoms make the material processing difficult. Si3N4 powders have been reported to be produced by self-propagating high temperature synthesis , reaction silicon compound with ammonia  and carbothermal nitridation of silica [3-4]. In the present study, the carbothermal nitradition method is used produce Si3N4 powder due to lower cost compare to the other. The indigenous source of raw materials will be used and treated by mechanical milling method. During the process, microstructural modification of silica sand will take place because of the high energy milling . The source of carbon is treated carbon black from Petronas refinery. With the different temperature chosen for carbothermal reduction process, the formation of Si3N4 powder was reported.
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The precursor material of silica source, which is silica sand, was obtained from the beach near Mersing, Johor. The silica sand with the purity of 99.5% was screened to less than 0.1 mm. The mechanical treatment of the silica sand was performed in a planetary ball mill of four stations using a stainless steel jar with a ball to powder ratio (BPR) 20:1 by weight and grinding speed of 200 rpm. Carbon powder was used in as-received condition and was obtained from Petronas refinery in Melaka. The carbon and mechanical milled silica sand mixture in a molar ratio 6/1 were prepared by mixing them in rotary mixer.
The carbothermal reduction process was carried out in tube furnace of working volumes of 250 cm3. The 1-2 mm thick layer of mixture was introduced in the furnace in a ceramic boat. The furnace was purged with a mixture of 95% N2 and 5% H2 gaseous for 15 minutes before the carbotermal nitridation process. The furnace was heated up to the required temperature at a chosen rate and at temperature of 1350o, 1450o, 1550 o and 1650oC for the required time. After nitridation process, the access carbon was removed by exposing the product in air at 1000oC and access silica by hydrofluoric acid treatment. The morphology of the sample surface was observed using LEO 1510 Scanning Electron Microscopy (SEM) and chemical composition was analyzed by Energy Dispersive Spectroscopy (EDS) and X-ray Fluorescence Spectroscopy (XRF). The phase composition was determined by X-ray Diffraction method (XRD).
RESULTS AND DISCUSSION
Figure 1 shows the XRD patterns of silica sand before mechanical milling. The diffraction line of silica can be clearly seen before milling. In crystalline forms, the silica structures are characterized by tetrahedral configuration of atoms within the crystals. It shows discrete reflection in X-ray diffraction from the internal planes formed by the orderly patterns of atoms.
FIGURE . XRD diagram before mechanical milling of silica sand.
After 100 hours of milling, the diffraction intensity of silica sand decreased (Fig. 2). These changes indicate that the crystalline structure of silica sand abruptly altered after mechanical milling. For intrinsic brittle powders, such as silicon powder or ionic compound (e.g. carbide and oxide) powders, the reduction of the grain size is a natural outcome of the transgranular fracturing and cold welding, and the minimum grain size is achieved when there is no more propagation of cracks within grains. Instead, the reduction of the grain size is due to localization of plastic deformation in the form of shear bands containing a high density of dislocations, formation of subgrains or cells by annihilation of dislocations and the conversion of subgrains into grains through mechanically driven grain rotation and subgrain boundary sliding . Dislocation and fracturing of the grains also accumulated microstrain within it.
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The mixture of carbothermal reduction process powder was done with the excess of carbon powder over the stoichiometry of reduction. This will reduce the unconverted silica during the reduction process. The carbothermal synthesis of Si3N4 takes place according to the overall reaction written as,
The actual mechanism of the formation of Si3N4 is more complicated than reaction (1). After carbothermal reduction process using nitrogen gas with 5% of hydrogen, the formation of Si3N4 can be detected by XRD (Fig. 2) at the temperature of 1450oC and 1550oC. Meanwhile at the temperature of 1650oC, SiC peak was clearly observed. It has been reported that Si3N4 can be synthesized at 1540oC under the flow of nitrogen gas . The formation of SiC also can occur at that particular temperature but tend to be unstable to Si3N4 in nitrogen atmosphere and holding time during heat treatment. At the temperature of 1650oC, the formation of SiC increased with the high intensity of XRD peak.
FIGURE 2. XRD diagrams of silica sand after mechanical milling and final products after carbothermal reduction process.
Figure 3 shows product of C/SiO2 mixture after heat treatment at 1450oC and 1650oC of mechanical milled of silica sand. The presence of whisker and spiky-like with the diameters between 1 to 3 Âµm (Fig. 3(a)) of Si3N4 in large amount, was observed in heat-treated mixture at 1450oC. The morphology for the product treated at 1650oC is also whisker like although some consists of non-filamentary and fibrous structures. From the EDX analysis, it is confirmed that the morphology are mainly SiC. The diameter of the product varies from sub-micron to 1 Âµm. The formation and growth of most whiskers can be explained by Vapor-Liquid-Solid (VLS) mechanism  which is generally accepted. This mechanism is characterized by the existence of a liquid phase (impurity or catalyst) that acts as a preferred site for the deposition of gases intermediate, leading to the supersaturation of the liquid in the elements forming the crystals. The crystal grows by precipitation from the liquid at the liquid-solid interface, originating a filament that carries the particle from which it was formed on top of it .
FIGURE 3. SEM micrograph of materials produced at the temperature of (a) 1450oC
and (b) 1650oC.
It is known that the introduction of 5% hydrogen in the nitrogen atmosphere during reaction process tend to reduces the limiting temperature for SiC production and yield of reacted product . In this experiment, the atmosphere and reaction time was fixed for all reaction temperature. After removing of the excess carbon and silica, the yield of product is shown in Fig. 4. The formation of SiC can be detected at the temperature of 1450oC. The composition of Si3N4 and SiC compound is relatively higher at the temperature of 1450oC and 1650oC respectively. About 80% yield of Si3N4 formed at 1450oC. It has been reported that the formation of Si3N4 in contact with the solid carbon is thermodynamically possible up to about 1550oC . At higher temperature, Si3N4 is not stable in the SiO2(s)-C(s)-N2(g)-Si3N4(s) system and SiC preferably formed if overpressure of nitrogen is not used, with the reaction of:
It was suggested that the boundary temperature between Si3N4 and SiC formation is about 1450oC . Thus the formation of Si3N4 decreased for about 40% and SiC increased to 20% from overall yield.
The treatment of the silica sand by high energy mechanical milling process also contributed to the high yield of product as well as reported by . The higher surface area of silica can be achieved by mechanical milling process. The intimate contact among the mixed powder will occur with the higher surface area. Decreasing in size of grain boundaries and microstrain within it has also increased the tendencies of reaction.
FIGURE 4. The influence of temperature on the product composition.
In summary, the treatment to the precursor material which is silica sand has been done by mechanical milling process. During the carbothermal reduction process, with the flow of nitrogen gas with 5% hydrogen and heat treated between the temperature of 1350o to 1650oC, the formation of Si3N4 and SiC has been observed. The formation of silicon nitride compound was facilitated by using silica sand with distorted structure by mechanical milling method. The mechanical milling also contributed in increasing the product yield. Further increased on temperature will lead to the formation of silicon carbide compound.
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