Effect of temperature on the ability to synthesize SiC from rice husks

Agricultural production in Vietnam annually generates a substantial volume of by-products and waste, with rice husks constituting the predominant fraction. Due to their meager economic value, rice husks are typically deemed agricultural waste and are commonly disposed of through incineration or discharge into rivers, contributing significantly to environmental pollution. In this investigation, rice husks were employed as the principal raw material for synthesizing silicon carbide. A blend of rice husks and silica gel in a ratio of 1.4/1 was subjected to sintering in a CO2 environment within the temperature range of 800 °C–1300 °C for 30 min. The chemical composition of the resultant product post-pyrolysis was ascertained in accordance with the ISO 21068–2:2008 standard. The capacity for SiC formation was further assessed utilizing Fourier transform infrared spectroscopy and x-ray diffraction. The outcomes revealed that the optimal temperature for SiC synthesis was 1200 °C. The composition of the sample post-pyrolysis was determined as 20.4% SiC, 51.2% SiO2, and 26.4% C (%wt). The primary phase constituents encompass amorphous carbon, cristobalite, α-SiC, and β-SiC. Scanning Electron Microscopy/ Energy Dispersive x-ray imaging of the product at 1200 °C exhibited dispersed SiC crystals on a SiO2-C substrate. The presence of SiC suggests the potential application of the product as a wear-resistant material.


Introduction
Vietnam, ranking third globally in rice exportation as of 2023, recorded a total planted area of 7.24 million hectares with an anticipated rice output of 43.88 million tons.The country exported 6.2 million tons of rice, amounting to 3.2 billion USD, at an average export price of 526 USD per ton.A notable issue within thriving industry is the inadequate treatment methods for by-products, particularly rice husks, which constitute 20% of the grain mass.Vietnam's annual rice husk output is approximately 9 million tons.However, due to the milling of only about 62% of Vietnam's annual rice, the actual rice husk output is about 5.6 million tons [1].Presently, the prevalent practice of rice husk disposal involves combustion, posing risks to human health and the environment.
Recently, there has been a discernible inclination toward harnessing waste by-products from agricultural production, encompassing materials such as bagasse, coconut shells, and coconut fiber.Indeed, rice husk is a type of waste that garners significant attention.In line with the imperative to reduce emissions, Vietnam has initiated the utilization of rice husks for various purposes, such as fuel, fertilizer, and gasification.The multifaceted approach addresses waste management concerns and contributes to sustainable resource utilization and environmental preservation efforts.However, these methods address only approximately 40% of the total rice husk waste, prompting intensified research efforts to extract more excellent value from agricultural by-products [1].
Several studies have explored diverse avenues for utilizing rice husks to produce valuable products.Antonia et al [2] combined rice husks, recycled plastic, and polyurethane adhesives to manufacture composite panels.C. Deiana et al [3] focused on converting rice husks into activated carbon, primarily for environmental treatment applications.The research endeavors by T. Watari and E. O. Onche [4,5] involved fabricating porous materials from rice husks through high-temperature heating in an inert atmosphere.These porous materials exhibit utility as heat-resistant and insulating materials.Within the spectrum of research directions, there is a noteworthy focus on applying rice husks for synthesizing silicon carbide (SiC) materials [6,7].Rice husks, in addition to being predominantly composed of carbon, also contain a substantial quantity of silicon, they represent a suitable raw material for synthesizing SiC.The dual component of rice husks is a promising resource for advancing SiC material synthesis processes.
SiC is a chemical compound formed by covalent bonds between carbon and silicon.SiC possesses excellent mechanical, thermal, and electrical conductivity properties, making it extensively utilized in high-temperature and wear-resistant applications.SiC is typically produced by reacting between silicon and carbon at elevated temperatures.The process is commonly carried out in a vacuum furnace.The primary reaction involved in SiC formation is the reaction between silicon (Si) and carbon (C) to produce SiC (reaction (1)).The kinetics of reaction are exothermic.Temperature plays a crucial role in the SiC synthesis process, requiring it to be sufficiently high for the silicon-carbon reaction, typically ranging between 1500 °C and 2000 °C [8].In certain instances, pressure can indeed influence the reaction process [9].Oxygen-free environments are frequently employed to guarantee product purity.Moreover, the properties of raw materials play a significant role in synthesizing SiC.Among these, biomass materials are a viable solution for producing SiC at lower temperatures [10].
Gorthy et al successfully synthesized silicon carbide (SiC) from rice husks, emphasizing the formation of β-SiC through pyrolysis.The pyrolysis of rice husks demonstrated the production of highly pure β-SiC powder, establishing it as a commercially viable product [11].In another study, K. Janghorban et al researched the impact of catalysts and technological parameters in the production of SiC from rice husks.The research revealed four distinct processes under suitable reduction conditions: (i) silica decomposition, (ii) cellulose decomposition, (iii) granule formation, and (iv) SiC ribbon formation.The rice husks in the study contained 17% silica and underwent pyrolysis at 1400 °C for three hours in an argon atmosphere.Notably, sodium metasilicate (Na 2 SiO 3 ) served as the catalyst in this study.The findings indicated that the Na 2 SiO 3 catalyst played a significant role in enhancing the SiC content.Moreover, soaking rice husks in a sodium silicate solution before pyrolysis facilitated the formation of SiC with a granular ribbon morphology.The efficiency of SiC generation was found to be contingent upon the catalyst concentration and the soaking time [12].
The above studies underscore SiC synthesis as a promising avenue for addressing rice husk waste.The approach offers the potential for reducing rice husk waste in Vietnam.It also contributes to environmental improvement and enhances the economic value of rice.Nevertheless, when utilizing rice husks as the sole raw material, the resulting SiC content tends to be limited [12].The incorporation of a catalyst or additive becomes imperative to enhance SiC content.However, such additives or catalysts must be cost-effective to ensure the feasibility of production.
In the present investigation, SiC was manufactured by sintering rice husks and SiO 2 .nH 2 O in a CO 2 atmosphere.Rice husks were utilized as a primary source of materials, supplying both SiO 2 and carbon, while silica gel, derived from liquid glass and acetic acid, offered an additional source of SiO 2 .Rice husks and silica gel contained Na 2 O, which acted as a catalytic component to facilitate SiC synthesis.Moreover, the investigation explored the effect of temperature on SiC formation.The resulting SiC derived from rice husks demonstrates potential applications as a wear-resistant material.

Experimental procedures
The primary rice husk materials for this study were sourced from the Mekong Delta region in Vietnam.The silica gel utilized in the research was prepared through the synthesis of liquid glass and acetic acid [13].During sintering, rice husks yielded active carbon and SiO 2 , while silica gel contributed active SiO 2 .The interplay between active carbon and active SiO 2 components derived from rice husk and silica gel played a crucial role in promoting the efficient formation of silicon carbide (SiC).
Rice husk (RH) and silica gel (SG) were employed in a finely powdered state for this study.The particle sizes of rice husk and silica gel were determined through laser diffraction using the Horiba LA-920 model.The average particle sizes were 13.12 μm for rice husk and 14.93 μm for silica gel, respectively.Chemical analyses of the raw materials were conducted via x-ray fluorescence (Thermo -ARL ADVANT'X), and the obtained results are summarized in table 1.The mixing ratio of rice husk to silica gel was maintained at 1/1.4 (wt).Subsequently, the mixed blends were compacted into cylindrical pellets with a diameter of 10 mm and a height of 8 mm, employing a pressing pressure of 2 MPa.The shaping machine utilized was the Mognon/s model manufactured by Ceramic Instruments.The resulting pellet samples underwent sintering in a CO 2 atmosphere at various temperatures.The CO 2 flow rate is 120 ml min −1 .The oven heating rate is 10 °C min −1 , with a soaking duration of 30 min.The furnace is a tube furnace with an internal diameter of 170 mm and a hot zone length of 1000 mm.Specifically, the furnace model utilized is the R170/750/13 from Nabertherm.Symbols corresponding to the sintered samples are outlined in table 2. The schematic representation of the prototyping process is shown in figure 1.The temperature profile for heating the sample is depicted in figure 2.
The compositions of SiC, SiO 2 , and C were analyzed using the ISO 21068-2:2008 standards.Different temperatures were applied to free and total carbon to measure the mass loss attributed to carbon loss.Free carbon and SiC contents were calculated using formulas (2)(3)(4).The determination of SiO 2 composition relied on chemical reactions (5) and (6) [14]. ´- Where, %C total is the content of carbon (%); %C free is the contentfree carbon (%); %SiC is the content SiC (%); Δm is the mass of sodium hydroxide solution increased in case of determination of total carbon or free carbon (g); m 1 is the initial mass of the sample when determining total carbon (g); m 2 is the initial mass of the sample when determining free carbon (g); 0.2729 is the carbon dioxide to carbon conversion coefficient; 3.3383 is the stoichiometric coefficient used to convert carbon to silicon carbide.
The analysis of functional groups in the synthesized samples was conducted employing Fourier Transform Infrared Spectroscopy (FTIR).The FTIR model utilized was the Nicolet 6700-Thermo Scientific, covering a scanning range from 450 to 4000 cm −1 with a scanning step of 0.96425.The analyzed samples were in powder form; potassium bromide (KBr) was the adhesive.Phase composition assessments were performed using the x-ray diffraction method (XRD) with the D8 Advance-Bruker model.The sample analysis conditions involved Cu K α radiation (λ = 0.154 nm), powder samples, scanning 2θ degrees ranging from 5 to 70°, and a scanning step of 0.019°.
Microstructure images and element distribution maps were obtained through Scanning Electron Microscopy (SEM)/ Energy Dispersive x-ray (EDX).The equipment employed for these observations was the JSM-IT 200-Jeol.3 present the composition of SiC, SiO 2 , and C in samples sintered at temperatures ranging from 800 to 1300 °C, with 30 min soak in a CO 2 environment.The chemical composition analysis reveals the formation of SiC when sintering a mixture of RH and SG with a mixing ratio of 1.4/1 above 800 °C.

Results and discusstion
The results of the chemical composition analysis indicate that the SiC content rises as the sintering temperature escalates from 800 to 1200 °C.Specifically, the SiC content in the product increases proportionally However, as the sintering temperature increases to 1300 °C, there is a tendency for the SiC content in the sample to decrease.The decrease in SiC content at a sintering temperature of 1300 °C has been observed in previous studies on SiC fabrication.Erdenechimeg K. et al demonstrated that SiC becomes highly vitrified when the sintering temperature reaches 1300 °C.The content and mechanical properties of SiC decrease due to the process [15].When the temperature reaches a sufficiently high level, the oxidation process of SiC begins to surpass the formation process of SiC.Consequently, the SiC content tends to decrease.The oxidation reaction of SiC has been elucidated by Nasiri N A et al denoted by reaction (12) [16].A more pronounced indication of the oxidation process of SiC is the abrupt rise in SiO 2 content observed at the calcination temperature of 1300 °C.According to reactions (9), (10), and (11), an increase in SiC content would typically coincide with a decrease in SiO 2 and C content.However, figure 3 reveals an unexpected rise in SiO 2 content with increasing temperature.The anomaly is   other, as depicted in reactions (9-11), to form SiC. Furthermore, C * is also subject to oxidation through reaction (13).Consequently, the carbon content diminishes with rising temperature, with the rate of carbon reduction escalating as the sintering temperature increases.
The post-reaction product composition was further investigated through the Fourier Transform Infrared Spectroscopy (FTIR) method, and the resulting spectrum is presented in figure 4. The FTIR spectrum of samples sintered at temperatures ranging from 800 to 1300 °C exhibit similar patterns, indicating consistency in their compositions.The characteristic vibration at the wavenumber position of 3400 cm −1 corresponds to the -OH group [17,18].The -OH group represents a chemical water component within the silica gel (SG) structure (SiO 2 .nH 2 O).The dehydration reaction of SG, occurring in the temperature range of 200 °C to over 1000 °C [19], leads to the formation of SiO 2 .SiO 2 is the precursor for SiC synthesis according to reactions (8-10).As the temperature increases, the chemical water gradually separates from the structure, decreasing the intensity of the peak at 3400 cm −1 .
In addition to the -OH peak, the FTIR spectrum in figure 4 exhibits three groups of peaks.The first group is the peak related to the C=C group at 1600 cm −1 [19,20], indicating the presence of pyrolyzed carbon derived from rice husks.The formed carbon is not entirely reacted, demonstrating its existence as free carbon.The second group is characterized by peaks related to the Si-O-Si and Si-O groups at wavenumber positions 1280 and 460 cm −1 [21][22][23].These peaks represent the SiO 2 component in the sample post-participation in the SiC formation reaction.
The wavenumber range of 870 to 1570 cm −1 and 400 to 590 cm −1 exhibits an increase at a temperature of 1000 °C compared to 900 °C.The phenomenon has been demonstrated in numerous prior studies.The reduction in SiC is attributed to the oxidation of SiC, yielding SiO 2 and CO 2 (reaction ( 12)).Reaction (12) initiates at temperatures exceeding 900 °C and intensifies at temperatures surpassing 1200 °C.Consequently, the SiO 2 concentration in the specimen rises within the temperature range above 900 °C [24,25].The third group, observed at Si -C positions at 820 and 660 cm −1 [26], signifies the formation of SiC within the sintered temperature range from 800 to 1300 °C.Notably, the intensity of the SiC peak diminishes at temperatures ranging from 1000 to 1300 °C, corroborating the occurrence of the SiC oxidation reaction at elevated temperatures.
The FTIR findings confirm the synthesis of silicon carbide within the sintered temperature range of 800 °C-1300 °C.In addition to SiC, the resultant products exhibit the existence of carbon and silicon dioxide.To elucidate the crystalline structures, present in the samples, XRD analysis was employed.Figure 5 depicts the XRD patterns of the samples over the 2θ diffraction angle range from 5 to 70°.
The XRD patterns prominently show a distinct peak at the diffraction angle of 21.8°, which is indicative of the characteristic peak associated with the cristobalite allotropy of SiO 2 [27].Interestingly, the 21.8°peak is absent in the sample sintered at 800 °C, only emerging and gradually intensifying at sintered temperatures of 900 and 1000 °C.However, with a further incremental increase in sintered temperature from 1000 to 1300 °C, the intensity of the cristobalite peak tends to diminish.These results imply that SiO 2 formed from reaction (7) initially exhibits an amorphous structure.As the temperature rises (800 °C-1000 °C), the amorphous SiO 2 gradually undergoes a transformation into a crystalline form.Given the presence of Na 2 O in the composition of RH and SG (table 1), the crystal structure of SiO 2 is identified as cristobalite rather than tridymite allotropes [28].The emergence of cristobalite crystals potentially hinders the participatory capacity of SiO 2 in the synthesis reaction, leading to SiC.At temperatures exceeding 1000 °C, the increased internal energy within the cristobalite crystal leads to the reduction of SiO 2 crystals by carbon and carbon monoxide according to reactions (9-10) [29].Consequently, the intensity of the cristobalite peaks at 21.8°and gradually diminishes at temperatures of 1100 and 1300 °C.
Furthermore, the broader diffraction area extending from approximately 20 to 24°at the base of the 21.8°p eak signifies the presence of amorphous carbon formed after pyrolysis [30].The coexistence of amorphous SiO 2 (SiO 2 * ) and carbon (C * ) in the region underscores their crucial role as the foundation for SiC synthesis, particularly at lower temperatures.The results depicted in figure 5 clearly indicate the presence of cristobalite in the product composition.However, the strong intensity of the cristobalite peak may obscure the observation of the formation of other minerals.Figure 6, with a diffraction angle range from 30 to 65°, addressed the problem.In figure 6, in addition to the appearance of cristobalite at positions 31.5 and 36.1°[11], there is also the appearance of SiC.SiC is observed in both α-SiC (34.1, 38.1, and 45.3° [31]) and β-SiC (35.6, 41.6, and 60.2° [32,33]) allotropes.β-SiC is the low-temperature allotropy of SiC, while α-SiC is the high-temperature allotropy.The intensity of the α-SiC peaks gradually increases with rising temperature, indicating an earlier transformation from βto α-SiC.Consequently, as the sintered temperature increases, more α-SiC is formed.These results demonstrate that the SiC was synthesized at the temperature range of 800 to 1300 °C, with the resulting product featuring both α-SiC and β-SiC allotropes.
Previous studies have indicated that SiC can be synthesized from biomass materials at temperatures ranging from 1000 °C [10].However, XRD analysis results reveal that utilizing Na 2 SiO 2 as a source of SiO 2 and Na 2 O catalyst enables the synthesis of SiC at a temperature as low as 800 °C.The synthesized SiC comprises lowtemperature allotropes (β-SiC) and high-temperature allotropes (α-SiC).The α-SiC has favorable mechanical and electrochemical properties, making it suitable for applications in the electronics industry.Conversely, β-SiC exhibits high-temperature resistance and mechanical stability, rendering it ideal for use in wear-resistant and heat-resistant industries.
The formation and distribution of SiC were also observed through the SEM method.Figure 7 shows the SEM analysis results of the sample sintered at 800 °C-1300 °C with the highest SiC content.
Microstructural images of the samples in figure 7 vividly depict the transformations occurring during sintering at temperatures ranging from 800 to 1300 °C.Initially, at 800 to 900 °C sintering temperatures, SEM  images reveal distinct hollow cylindrical structures.These hollow cylinders represent the carbon skeleton remnants from rice husks after pyrolysis.The presence of these structures suggests that the formation reactions of SiC are not significantly active at temperatures of 800 to 900 °C.The observation aligns with the findings of the SiC content determination experiment illustrated in figure 3.However, characteristic structures of both α-SiC and β-SiC persist in SEM images at temperatures of 800 °C and 900 °C.
At sintering temperatures ranging from 1000 to 1100 °C, the rice husk's skeletal structure undergoes gradual destruction due to the SiC creation process.Notably, in the SEM image of the sample at 1100 °C, the β-SiC layer surrounding the rice husk skeleton is visible.Moreover, there is an elevated presence of α-SiC, indicating a stronger allomorphic transition from βto α-SiC at temperatures between 1000 and 1100 °C.The results from determining the chemical composition (figure 3) and mineral composition (figure 6) also demonstrate a sharp increase in α-SiC during the period.The characteristic peaks for α-SiC in figure 6 exhibit heightened intensity within the temperature range of 1100 °C.
At 1200 to 1300 °C sintering temperatures, the rice husk skeletons are scarcely observed, while α-SiC and β-SiC become more prominent.The SEM images at these temperatures closely resemble the SEM images of the produced SiC sample (PS sample), indicating that most of the free carbon participates in the SiC formation reaction and oxidation process.These findings are further supported by elemental composition analysis and elemental distribution maps using the EDX method.
Figures 8-10 depict the results of chemical composition analysis and elemental distribution mapping of samples heated to temperatures ranging from 800 to 1300 °C.The elemental distribution maps reveal areas where carbon (C) and silicon (Si) are simultaneously distributed across all samples, indicating the presence of SiC.These findings indicate that SiC formation occurs within the heating temperature range of 800 to 1300 °C.Furthermore, the distribution of silicon (Si) and oxygen (O) elements suggests that the predominant background phase is primarily SiO 2 .The gradual decrease in the yellow coloration associated with the carbon element with increasing calcination temperature, particularly evident in the samples heated to 1200 and 1300 °C, suggests that free carbon has reacted to produce SiC and subsequently undergone oxidation.
The changes in chemical composition observed in the EDX spectrum further corroborate the observations above.In figure 8, the Si, O, and C content exhibit variation across calcination temperatures ranging from 800 to 1000 °C, indicating a gradual increase in SiC and SiO 2 content with rising calcination temperature.At temperatures of 1100 and 1200 °C (figure 9), the composition of Si, O, and C remains relatively stable, suggesting a consistent generation rate of SiO 2 and SiC during the period.The observation, combined with XRD and SEM analyses, indicates that the primary reaction during the stage is the allomorphic transformation from β-SiC to α-SiC.At a temperature of 1300 °C (figure 10), there is a slight decrease in Si content, a sharp increase in oxygen  content, and a notable decrease in carbon content.These changes indicate a rapid oxidation process of SiC, leading to the formation of SiO 2 and CO 2 .Consequently, the carbon content decreases while the oxygen content increases.
In addition to carbon, silicon, and oxygen, other elements, including sodium, potassium, and sulfur, were detected in the samples.These elements are impurities originating from the raw materials (refer to table 1).The SEM/EDX results depicted in figures 7-10 indicate that the primary constituents of the samples consist of SiC crystals distributed on the SiO 2 -C substrate.Moreover, the samples present impurities such as Na 2 O, K 2 O, and SO 3 at low concentrations.
The findings from the chemical composition analysis (figure 3), mineral composition analysis (figures 5 and 6), and element distribution mapping (figures 8-10) collectively suggest that biomass materials offer a viable route for SiC synthesis at relatively low temperatures.However, the resultant product contains impurities.Previous studies have identified common impurities such as SiO 2 and C [34,35].Moreover, using additives to enhance the SiC synthesis process can introduce unwanted impurities into the product.Therefore, SiC derived from biomass materials may find applications in materials that do not necessitate high purity, such as wear-resistant materials.However, for applications requiring higher purity SiC, these impurities can be mitigated through heat treatment or other specific chemical reactions.

Conclusions
The comprehensive analysis of the chemical composition, functional group composition, phase composition, microstructure, and element distribution map lead to the following conclusions: (1) With a rice husk/silica gel ratio of 1.4/1 and sintering at temperatures ranging from 800 to 1300 °C in a CO 2 atmosphere, silicon carbide (SiC) was successfully formed.Notably, the temperature of 1200 °C yields the highest SiC content, and SiC crystals exhibit both low-temperature (β-SiC) and high-temperature (α-SiC) allotropes.When the sintering temperature is 1300 °C, the SiC oxidation reaction occurs, causing the SiC content to decrease.
(2) As the sintering temperature increases, there is a notable increase in the formation of SiC crystals, accompanied by a conversion from β-SiC to α-SiC.The elevated sintering temperature also initiates the SiC oxidation reaction.Consequently, the formation rate of SiC is rapid in the early stages of the sintering process, slowing down in the later stages.
(3) At a sintering temperature of 800-1300 °C, the primary phase component consists of SiC crystals distributed on the SiO 2 -C substrate.Additionally, the sample contains impurities such as Na 2 O, K 2 O, and SO 3 .These impurities can be mitigated through heat treatment or other specific chemical reactions.

Figure 3
Figure3and table 3 present the composition of SiC, SiO 2 , and C in samples sintered at temperatures ranging from 800 to 1300 °C, with 30 min soak in a CO 2 environment.The chemical composition analysis reveals the formation of SiC when sintering a mixture of RH and SG with a mixing ratio of 1.4/1 above 800 °C.The results of the chemical composition analysis indicate that the SiC content rises as the sintering temperature escalates from 800 to 1200 °C.Specifically, the SiC content in the product increases proportionally

Figure 1 .
Figure 1.A diagram of the prototyping process.

Figure 2 .
Figure 2. The temperature profile for heating the sample.

Figure 3 .
Figure 3.The chemical composition of samples.

0 2. 1 attributed
to the accidental introduction of O 2 gas during the sintering process.Hence, the SiO 2 content increases as the calcination temperature rises, particularly at 1300 °C, likely attributable to the oxidation effect.Additionally, the increase in SiO 2 content and reduction in the SiC formation rate may be influenced by the crystallization process of SiO 2 .A portion of the SiO 2 * produced from reaction (8) reacts with C * to yield SiC, while any unreacted residue may transform into crystalline forms of SiO 2 .These SiO 2 crystals possess low reactivity, hindering the occurrence of reactions (9-11).In addition to the changes in SiC and SiO 2 , the results presented in figure 3 also indicate a decrease in carbon content with increasing temperature.The reduction in carbon content can be elucidated through the SiC generation process.The C * produced from reaction (7) undergoes a reaction with the SiO 2 * produced from reaction (8).Subsequently, C * and SiO 2 * react with each

Figure 4 .
Figure 4.The FTIR spectrum of samples.

Figure 8 .
Figure 8.The EDX spectra and elemental distribution map of T800, T900, and T1000 samples.

Figure 9 .
Figure 9.The EDX spectra and elemental distribution map of T1100 and T1200 samples.

Figure 10 .
Figure 10.The EDX spectra and elemental distribution map of T1300 sample.

Table 1 .
The chemical composition of raw materials (%wt).

Table 2 .
The ingredients and sintering temperature.

Table 3 .
The chemical composition of the samples (%wt).