- Email: [email protected]
Synthesis, Optimization and Characterization of Silicon Carbide (SiC) from Rice Husk
Available online at www.sciencedirect.com
ScienceDirect ScienceDirect
Procedia Manufacturing 00 (2019) 000–000 Procedia Manufacturing 00 (2019) 000–000
Available online at www.sciencedirect.com
ScienceDirect
www.elsevier.com/locate/procedia www.elsevier.com/locate/procedia
Procedia Manufacturing 35 (2019) 962–967
2nd International Conference on Sustainable Materials Processing and Manufacturing 2nd International Conference on Sustainable Materials Processing and Manufacturing (SMPM 2019) (SMPM 2019)
Synthesis, Optimization and Characterization of Silicon Carbide Synthesis, Optimization and Characterization of Silicon Carbide (SiC) from Rice Husk (SiC) from Rice Husk
Shatumbu T. Alweendoa*, Oluwagbenga T. Johnsona, Mxolisi B. Shongweb, Frank P. L. a c B. Shongweb, Frank P. L. Shatumbu T. Alweendoa*, Oluwagbenga Johnson , Mxolisi Kavishea and T. Joseph O. Borode Kavishea and Joseph O. Borodec a Department of Mining and Metallurgical Engineering, University of Namibia, Ongwediva, Namibia. a Engineering Research, Department of Chemical, Metallurgical and Materials Engineering, Tshwane University of Institute for Nano Department of Mining and Metallurgical Engineering, University of Namibia, Ongwediva, Namibia. b Technology, Pretoria, South Africa Institute for Nano Engineering Research, Department of Chemical, Metallurgical and Materials Engineering, Tshwane University of c Department of Metallurgical and Materials Engineering, University of Technology, Akure, Nigeria Technology, Pretoria,Federal South Africa c Department of Metallurgical and Materials Engineering, Federal University of Technology, Akure, Nigeria b
Abstract Abstract Although Silicon carbide (SiC) has shown interesting engineering properties, its cost of production continues to limit its applications in the carbide industry.(SiC) Therefore, this paper reports aengineering simple, inexpensive method to synthesize siliconcontinues carbide via Although Silicon has shown interesting properties, its cost of production to pyrolysis limit its of rice huskinwithout the use of catalysts or complex of method rice husk was carried out in a tubeviafurnace at applications the industry. Therefore, this paper reports set-ups. a simple,Pyrolysis inexpensive to synthesize silicon carbide pyrolysis temperatures 1000°C to 1500°C retention time ranging from to 2husk hourswas in an Argonout atmosphere of rice husk ranging without from the use of catalysts or and complex set-ups. Pyrolysis of 1rice carried in a tubeflowing furnaceatata rate of 2l/min.ranging The morphology of the products was analysed by ranging SEM while done using XRD. The yieldatofa temperatures from 1000°C to 1500°C and retention time fromphase 1 to 2analysis hours inwas an Argon atmosphere flowing SiC rice husk was found toofbethe affected significantly by pyrolysis temperature and retention time.using It wasXRD. observed that the rate from of 2l/min. The morphology products was analysed by SEM while phase analysis was done The yield of synthesized SiC powder a mixture of whiskers and particulates. SiC from rice husk was was found to be affected significantly by pyrolysis temperature and retention time. It was observed that the synthesized SiC powder was a mixture of whiskers and particulates. © 2019 The Authors. Published by Elsevier B.V. © 2019 Published B.V. Peer-review under responsibility ofElsevier the organizing © 2019 The The Authors. Authors. Published by by Elsevier B.V. committee of SMPM 2019. Peer-review under responsibility of the organizing committee of SMPM 2019. Peer-review under responsibility of the organizing committee of SMPM 2019. Keywords: Silicon carbide; Rice husk; pyrolysis; carbothermal reduction Keywords: Silicon carbide; Rice husk; pyrolysis; carbothermal reduction
1. Introduction 1. Introduction Silicon carbide (SiC) is one of the non-oxide ceramic materials that finds many engineering applications due to a Silicon carbide (SiC) is mechanical, one of the non-oxide that finds many engineering applications due to a wide range of desirable thermal, ceramic chemicalmaterials [1, 2]. Despite these excellent and electrical properties wide rangeitsof applications desirable mechanical, thermal, chemical electrical Despite these excellent properties, in the industry continue to beand limited by theproperties high cost[1, of 2]. production. Currently, it is properties, its applications in the industry continue to be limited by the high cost of production. Currently, it is 2351-9789 © 2019 The Authors. Published by Elsevier B.V. Peer-review©under the organizing committee 2351-9789 2019responsibility The Authors. of Published by Elsevier B.V. of SMPM 2019. Peer-review under responsibility of the organizing committee of SMPM 2019. *Corresponding Author [email protected] Email: *Corresponding Author Email: [email protected]
2351-9789 © 2019 The Authors. Published by Elsevier B.V. Peer-review under responsibility of the organizing committee of SMPM 2019. 10.1016/j.promfg.2019.06.042
2
Shatumbu T. Alweendo et al. / Procedia Manufacturing 35 (2019) 962–967 Author name / Procedia Manufacturing 00 (2016) 000–000
963
commercially produced via the Acheson process which is based on carbothermal reduction of silica by carbon in an electric arc furnace. However, the Acheson process is based on outdated technology which requires excessive energy inputs, long reaction times, high cost raw materials, as well as products that require further processing before the intended applications [3]. These drawbacks have led to intense research on alternative sources of silicon carbide that are low-cost and renewable, among which rice husk has emerged as one of the promising candidates due to its finely dispersed carbon and silica that are in intimate contact and exhibit high activity with one another [4, 5]. Silicon carbide (SiC) was first synthesized successfully from rice husk by Cutler in the early 1970’s [6, 7]. Consequently, several experimental studies had been carried out on the synthesis of silicon carbide from rice husks via different methods and approaches with a high degree of success. Various techniques such as thermal plasma reactors [8], gas furnaces [9], thermogravimetry-differential scanning calorimetry (TGA-DSC) [10] and hydrothermal carbonization [11] were successfully employed. Despite the success of the synthesis techniques described above, most of them require special equipment and their chamber set-up are complicated, making it difficult to scale up for large scale production of SiC from rice husk. In addition, the variations in rice husk compositions and synthesizing methods led to disparities in the results presented by different researchers, and the commercialization of SiC production from rice husk has been delayed. Therefore in this work, silicon carbide was synthesized by pyrolysis of rice husk using simple inexpensive equipment. 2. Materials and Methods 2.1. Preparation and characterization of starting materials The rice husk used in this study was sourced from Ogongo rice project in Omusati region, northern Namibia. The as-obtained (raw) rice husk was washed with distilled water to remove any dirt such as adhering sand and was then dried in the sunlight for 12 h. A Bruker D2 advance XRD analyzer was used to determine the crystallinity of the rice husk constituents. The D2 advance uses Co-kα radiation produced at 40kV and 40mA, and the measurements were taken at 2θ values ranging from 10°- 90° with step sizes of 0.02°. Furthermore, a proximate analysis was done on the raw rice husk. This is a standardized procedure for determining the proportions of the major components of an organic sample. It is used specifically to determine the amount of moisture, volatile matter, fixed carbon and ash content. The analysis was done according to the ASTM standard D5373-02 (2007), using 5 grams of rice husk samples pulverized using a Fritsch pulverisette 6 ball mill. The proximate analysis of the rice husk is shown in Table 1. The picture and XRD spectrum of the raw rice husk used in this study are presented in Fig. 1(a) and (b), respectively. 2.2. Synthesis of SiC by pyrolysis of rice husk Synthesis of silicon carbide (SiC) was carried out by pyrolysis of rice husk in a tube furnace (THS 15-50-450). Three samples of different features: As received, hand ground and ball milled were prepared by first coking the rice husk in a muffle furnace (635 HT) at 700°C for 10 minutes in order to convert the organic materials, such as cellulose, into amorphous carbon. After coking, the first sample was taken and milled for 3 hours in a Fritsch pulverisette 6 ball mill using 20 mm diameter stainless steel balls at 200 rpm; this sample was then termed as ball milled. The second sample, was ground by hand using a mortar and pestle, which was then termed hand ground, while the last sample was just as received after coking. A mass of 5g of the prepared samples were each pyrolyzed at temperatures of 1000, 1300 and 1500°C at a heating rate of 10°C/min from room temperature to the desired temperature. The pyrolysis time was also varied at 60, 90 and 120 minutes in order to determine which sample feature and at which conditions of temperature and time would yield the most silicon carbide. The process was done under an argon atmosphere at a constant flow rate of 2l/min in order to obtain the formation of silicon carbide from carbothermal reduction of Silica by carbon. After pyrolysis, there was a possibility of having some unreacted carbon left behind, therefore, the pyrolysis products were calcined in a muffle furnace (Huster 635 HT) at 700°C for 2 hours to burn of the residual carbon. After calcination, the pyrolysis products were leached in hydrofluoric acid (HF) to remove any unreacted silica, while leaching in hydrochloric acid (HCl) was done to remove metallic oxide impurities as well as any residual carbon.
964
Shatumbu T. Alweendo et al. / Procedia Manufacturing 35 (2019) 962–967 Author name / Procedia Manufacturing 00 (2016) 000–000
3
2.3. Characterization of pyrolysis products XRD analysis was done on the pyrolysis products before and after leaching to confirm the removal of impurities from the produced SiC materials. Moreover, an FEI Quanta 400 FEG SEM/EDS machine was used for microscopic analysis of the treated pyrolysis products in terms of phase morphology. Table 1. Proximate analysis of rice husk
CONSTITUENT Moisture Volatile matter Ash Fixed carbon
AVERAGE MASS % 5 64 20 11
Fig 1. (a) Picture of raw rice husk and (b) XRD spectrum for the raw rice husk
3. Results and Discussion 3.1 XRD analysis of pyrolysis products Fig. 2 presents the XRD spectra for the three sample features pyrolysed at different temperatures. There was no silicon carbide formed at 1000°C for all sample features, but there was a silica peak observed at 2θ = 26.8° for hand ground and ball milled samples as shown in Fig. 2(b) and 2(c), respectively. This peak means that the amorphous silica had crystallized and transformed to its high temperature state β-cristobalite [12, 13]. On the other hand, the as received rice husk pyrolyzed at 1000°C (Fig. 2(a)) was completely amorphous with no significant crystalline phases. This may have been caused by the coarse nature of the starting materials, where the surface area of the rice husk was not sufficient for chemical reactions to proceed at this temperature. A new silica peak was observed at 2θ = 35.8° for all sample features when the temperature was increased to 1300°C, which implies that more silica had crystallized due to an increase in temperature. It is worth noting that the formation of SiC only started to form at 1300°C for all three sample features but in small quantities as evident from the short XRD peaks. The low yield is believed to be due to the low pyrolysis temperature which does not allow for sufficient chemical reaction between silica and carbon in the rice husk. The peaks for both silica and SiC were intensified as the temperature was increased to 1500°C.
4
Shatumbu T. Alweendo et al. / Procedia Manufacturing 35 (2019) 962–967 Author name / Procedia Manufacturing 00 (2016) 000–000
965
Fig 2. XRD spectra for pyrolysis products of (a) as received, (b) hand ground and (c) ball milled samples treated at different temperatures
Fig. 3 shows the composite XRD plot for leached pyrolysis products. The results revealed that the leaching process has removed the unreacted silica and other oxide impurities. The hydrofluoric acid attacks the silica to form hexafluorosilicic acid vapour that effectively evaporates all silica from the pyrolysis products according to Equation 1. (1) SiO2 (s) + 6HF (l) = H2 SiF6 (g) ↑ + 2H2O (g) ↑
Fig 3. XRD spectra for the three sample features pyrolysed at 1500°C and 120 minutes retention time
3.2 SEM analysis of silicon carbide (SiC) powder SEM images of leached pyrolysis products for the three sample features are presented in Fig. 4. The images show that the silicon carbide powder was a mixture of whiskers and particles. It is believed that the synthesis of SiC by carbothermal reduction of silica is a multi-stage, vapour-solid growth process that involves a series of chemical reactions which produce either particles or whiskers. SiC particles are formed by nucleation through reaction 2 with gas-solid interaction, whereas whiskers grow by reaction 3. SiO (g) + 2C (S) = SiC (s) + CO (g)
(2)
SiO (g) + 3CO (g) = SiC (s) + 2CO2 (g)
(3)
It is also believed that at temperatures above 1450°C, some of the whiskers are converted to particulates via a process called coagulative formation of crystal particulates [6]. It is worth noting that the ball milled powder shown in Fig. 4(a) had more whiskers due to the fine nature of the starting materials which had a greater surface area and sufficient pore volume that allowed the gas-gas reactions to take place at a higher rate. On the other hand, only a few whiskers were observed for hand ground and as received pyrolysis products as evident from Fig. 4(a) and (b), respectively. This is believed to be due to reduction in surface area caused by the larger particle sizes of the hand ground and as received samples.
966
Author name / Procedia Manufacturing 00 (2016) 000–000 Shatumbu T. Alweendo et al. / Procedia Manufacturing 35 (2019) 962–967
5
Fig 4. SEM images of SiC powder obtained from (a) ball milled, (b) hand ground and (c) as received samples pyrolysed at 1500 ᵒC for 120 min
3.3 Effect of temperature and time on the yield of SiC from rice husk The effects of temperature on the yield of silicon carbide from the three rice husk sample features was investigated between 1300°C - 1500°C. The yield of silicon carbide from all three sample features was found to increase with an increase in temperature for all reaction times studied as shown in Fig. 5. The effect of retention time on the yield of SiC from the three sample features was also investigated at different temperatures and the results are presented in Fig. 6. It was observed that the yield of SiC increased with an increase in retention time for all temperatures studied.
Fig 5. Effect of temperature on SiC yield from rice husk at different reaction times for the three sample features
6
Shatumbu T. Alweendo et al. / Procedia Manufacturing 35 (2019) 962–967 Author name / Procedia Manufacturing 00 (2016) 000–000
967
Fig 6. Effect of time on SiC yield from rice husk at different temperatures for the three sample features
4. Conclusions In the present research, silicon carbide (SiC) was synthesized from locally sourced rice husk at different temperatures and retention times without any use of catalysts or complicated equipment set-ups. The following conclusion were drawn: • The synthesized SiC powders were a mixture of particles and whiskers. • Silicon carbide yield was found to increase with increasing temperature and retention time. • The ball milled samples produced the highest yield of SiC (69.2 %) in comparison to the yield obtained from the hand ground (59.8%) and as received samples (56.9%). • The best temperature for production of silicon carbide in this study was found to be 1500ºC, while the best retention time was 120 minutes. References [1] A. A. Adediran, K. K. Alaneme, I. O. Oladele, E. T. Akinlabi, Cog. Eng. 5 (2018) (1) 494-499 [2] R. Pode, Renewable Sust. Ener. Rev. 53 (2016) 1468-1485. [3] J. Li, T. Shirai, M. Fuji, Adv. Powder Tech. 24 (2013) 838-843. [4] Y. Shen, P. Zhao, Q. Shao, Microporous Mesoporous Mater. 188 (2014) 46-76. [5] A. A. Taha, S. A. Abdel Ghani, J. Dispersion Sci. Technol. 37 (2016) (2) 173-182. [6] M. F. Zawrah, M. A. Zayed, M. R. K. Ali, J. Hazard. Mater. 228 (2012) 250-256. [7] M. Ali, A. M. Tindyala, J. As. Ceram. Soc. 3 (2015) (3) 311-316. [8] S. K. Singh, L. Stachowicz, S. L. Girshick, E. Pfender, J. Mater. Sci. Let. 12 (1993) 659-650. [9] V. Martínez, M. F. Valencia, J. Cruz, J. M. Mejía, F. Chejne, Ceram. 32 (2006) 891-897. [10] E. Gorzkwski, S. B. Quadri, B. B. Rath, R. Caldwell, J. Elect. Mater. 42 (2013) (5) 799-804. [11] S. F. Korablev, D.S Korablev, Powd. Metal. Met. Ceram. 54 (2015) (3) 215-219. [12] G. Adylov, S. A Faiziev, M. Paizullakhanov, S. Mukhsimov, E. Nodirmatov, Tech. Phy. Let. (2003) 221-223. [13] S. Chandrasekhar, P.N. Pramada, J. Majeed, J. Mater. Sci. 41 (2006) 7926–7933.
ScienceDirect ScienceDirect
Procedia Manufacturing 00 (2019) 000–000 Procedia Manufacturing 00 (2019) 000–000
Available online at www.sciencedirect.com
ScienceDirect
www.elsevier.com/locate/procedia www.elsevier.com/locate/procedia
Procedia Manufacturing 35 (2019) 962–967
2nd International Conference on Sustainable Materials Processing and Manufacturing 2nd International Conference on Sustainable Materials Processing and Manufacturing (SMPM 2019) (SMPM 2019)
Synthesis, Optimization and Characterization of Silicon Carbide Synthesis, Optimization and Characterization of Silicon Carbide (SiC) from Rice Husk (SiC) from Rice Husk
Shatumbu T. Alweendoa*, Oluwagbenga T. Johnsona, Mxolisi B. Shongweb, Frank P. L. a c B. Shongweb, Frank P. L. Shatumbu T. Alweendoa*, Oluwagbenga Johnson , Mxolisi Kavishea and T. Joseph O. Borode Kavishea and Joseph O. Borodec a Department of Mining and Metallurgical Engineering, University of Namibia, Ongwediva, Namibia. a Engineering Research, Department of Chemical, Metallurgical and Materials Engineering, Tshwane University of Institute for Nano Department of Mining and Metallurgical Engineering, University of Namibia, Ongwediva, Namibia. b Technology, Pretoria, South Africa Institute for Nano Engineering Research, Department of Chemical, Metallurgical and Materials Engineering, Tshwane University of c Department of Metallurgical and Materials Engineering, University of Technology, Akure, Nigeria Technology, Pretoria,Federal South Africa c Department of Metallurgical and Materials Engineering, Federal University of Technology, Akure, Nigeria b
Abstract Abstract Although Silicon carbide (SiC) has shown interesting engineering properties, its cost of production continues to limit its applications in the carbide industry.(SiC) Therefore, this paper reports aengineering simple, inexpensive method to synthesize siliconcontinues carbide via Although Silicon has shown interesting properties, its cost of production to pyrolysis limit its of rice huskinwithout the use of catalysts or complex of method rice husk was carried out in a tubeviafurnace at applications the industry. Therefore, this paper reports set-ups. a simple,Pyrolysis inexpensive to synthesize silicon carbide pyrolysis temperatures 1000°C to 1500°C retention time ranging from to 2husk hourswas in an Argonout atmosphere of rice husk ranging without from the use of catalysts or and complex set-ups. Pyrolysis of 1rice carried in a tubeflowing furnaceatata rate of 2l/min.ranging The morphology of the products was analysed by ranging SEM while done using XRD. The yieldatofa temperatures from 1000°C to 1500°C and retention time fromphase 1 to 2analysis hours inwas an Argon atmosphere flowing SiC rice husk was found toofbethe affected significantly by pyrolysis temperature and retention time.using It wasXRD. observed that the rate from of 2l/min. The morphology products was analysed by SEM while phase analysis was done The yield of synthesized SiC powder a mixture of whiskers and particulates. SiC from rice husk was was found to be affected significantly by pyrolysis temperature and retention time. It was observed that the synthesized SiC powder was a mixture of whiskers and particulates. © 2019 The Authors. Published by Elsevier B.V. © 2019 Published B.V. Peer-review under responsibility ofElsevier the organizing © 2019 The The Authors. Authors. Published by by Elsevier B.V. committee of SMPM 2019. Peer-review under responsibility of the organizing committee of SMPM 2019. Peer-review under responsibility of the organizing committee of SMPM 2019. Keywords: Silicon carbide; Rice husk; pyrolysis; carbothermal reduction Keywords: Silicon carbide; Rice husk; pyrolysis; carbothermal reduction
1. Introduction 1. Introduction Silicon carbide (SiC) is one of the non-oxide ceramic materials that finds many engineering applications due to a Silicon carbide (SiC) is mechanical, one of the non-oxide that finds many engineering applications due to a wide range of desirable thermal, ceramic chemicalmaterials [1, 2]. Despite these excellent and electrical properties wide rangeitsof applications desirable mechanical, thermal, chemical electrical Despite these excellent properties, in the industry continue to beand limited by theproperties high cost[1, of 2]. production. Currently, it is properties, its applications in the industry continue to be limited by the high cost of production. Currently, it is 2351-9789 © 2019 The Authors. Published by Elsevier B.V. Peer-review©under the organizing committee 2351-9789 2019responsibility The Authors. of Published by Elsevier B.V. of SMPM 2019. Peer-review under responsibility of the organizing committee of SMPM 2019. *Corresponding Author [email protected] Email: *Corresponding Author Email: [email protected]
2351-9789 © 2019 The Authors. Published by Elsevier B.V. Peer-review under responsibility of the organizing committee of SMPM 2019. 10.1016/j.promfg.2019.06.042
2
Shatumbu T. Alweendo et al. / Procedia Manufacturing 35 (2019) 962–967 Author name / Procedia Manufacturing 00 (2016) 000–000
963
commercially produced via the Acheson process which is based on carbothermal reduction of silica by carbon in an electric arc furnace. However, the Acheson process is based on outdated technology which requires excessive energy inputs, long reaction times, high cost raw materials, as well as products that require further processing before the intended applications [3]. These drawbacks have led to intense research on alternative sources of silicon carbide that are low-cost and renewable, among which rice husk has emerged as one of the promising candidates due to its finely dispersed carbon and silica that are in intimate contact and exhibit high activity with one another [4, 5]. Silicon carbide (SiC) was first synthesized successfully from rice husk by Cutler in the early 1970’s [6, 7]. Consequently, several experimental studies had been carried out on the synthesis of silicon carbide from rice husks via different methods and approaches with a high degree of success. Various techniques such as thermal plasma reactors [8], gas furnaces [9], thermogravimetry-differential scanning calorimetry (TGA-DSC) [10] and hydrothermal carbonization [11] were successfully employed. Despite the success of the synthesis techniques described above, most of them require special equipment and their chamber set-up are complicated, making it difficult to scale up for large scale production of SiC from rice husk. In addition, the variations in rice husk compositions and synthesizing methods led to disparities in the results presented by different researchers, and the commercialization of SiC production from rice husk has been delayed. Therefore in this work, silicon carbide was synthesized by pyrolysis of rice husk using simple inexpensive equipment. 2. Materials and Methods 2.1. Preparation and characterization of starting materials The rice husk used in this study was sourced from Ogongo rice project in Omusati region, northern Namibia. The as-obtained (raw) rice husk was washed with distilled water to remove any dirt such as adhering sand and was then dried in the sunlight for 12 h. A Bruker D2 advance XRD analyzer was used to determine the crystallinity of the rice husk constituents. The D2 advance uses Co-kα radiation produced at 40kV and 40mA, and the measurements were taken at 2θ values ranging from 10°- 90° with step sizes of 0.02°. Furthermore, a proximate analysis was done on the raw rice husk. This is a standardized procedure for determining the proportions of the major components of an organic sample. It is used specifically to determine the amount of moisture, volatile matter, fixed carbon and ash content. The analysis was done according to the ASTM standard D5373-02 (2007), using 5 grams of rice husk samples pulverized using a Fritsch pulverisette 6 ball mill. The proximate analysis of the rice husk is shown in Table 1. The picture and XRD spectrum of the raw rice husk used in this study are presented in Fig. 1(a) and (b), respectively. 2.2. Synthesis of SiC by pyrolysis of rice husk Synthesis of silicon carbide (SiC) was carried out by pyrolysis of rice husk in a tube furnace (THS 15-50-450). Three samples of different features: As received, hand ground and ball milled were prepared by first coking the rice husk in a muffle furnace (635 HT) at 700°C for 10 minutes in order to convert the organic materials, such as cellulose, into amorphous carbon. After coking, the first sample was taken and milled for 3 hours in a Fritsch pulverisette 6 ball mill using 20 mm diameter stainless steel balls at 200 rpm; this sample was then termed as ball milled. The second sample, was ground by hand using a mortar and pestle, which was then termed hand ground, while the last sample was just as received after coking. A mass of 5g of the prepared samples were each pyrolyzed at temperatures of 1000, 1300 and 1500°C at a heating rate of 10°C/min from room temperature to the desired temperature. The pyrolysis time was also varied at 60, 90 and 120 minutes in order to determine which sample feature and at which conditions of temperature and time would yield the most silicon carbide. The process was done under an argon atmosphere at a constant flow rate of 2l/min in order to obtain the formation of silicon carbide from carbothermal reduction of Silica by carbon. After pyrolysis, there was a possibility of having some unreacted carbon left behind, therefore, the pyrolysis products were calcined in a muffle furnace (Huster 635 HT) at 700°C for 2 hours to burn of the residual carbon. After calcination, the pyrolysis products were leached in hydrofluoric acid (HF) to remove any unreacted silica, while leaching in hydrochloric acid (HCl) was done to remove metallic oxide impurities as well as any residual carbon.
964
Shatumbu T. Alweendo et al. / Procedia Manufacturing 35 (2019) 962–967 Author name / Procedia Manufacturing 00 (2016) 000–000
3
2.3. Characterization of pyrolysis products XRD analysis was done on the pyrolysis products before and after leaching to confirm the removal of impurities from the produced SiC materials. Moreover, an FEI Quanta 400 FEG SEM/EDS machine was used for microscopic analysis of the treated pyrolysis products in terms of phase morphology. Table 1. Proximate analysis of rice husk
CONSTITUENT Moisture Volatile matter Ash Fixed carbon
AVERAGE MASS % 5 64 20 11
Fig 1. (a) Picture of raw rice husk and (b) XRD spectrum for the raw rice husk
3. Results and Discussion 3.1 XRD analysis of pyrolysis products Fig. 2 presents the XRD spectra for the three sample features pyrolysed at different temperatures. There was no silicon carbide formed at 1000°C for all sample features, but there was a silica peak observed at 2θ = 26.8° for hand ground and ball milled samples as shown in Fig. 2(b) and 2(c), respectively. This peak means that the amorphous silica had crystallized and transformed to its high temperature state β-cristobalite [12, 13]. On the other hand, the as received rice husk pyrolyzed at 1000°C (Fig. 2(a)) was completely amorphous with no significant crystalline phases. This may have been caused by the coarse nature of the starting materials, where the surface area of the rice husk was not sufficient for chemical reactions to proceed at this temperature. A new silica peak was observed at 2θ = 35.8° for all sample features when the temperature was increased to 1300°C, which implies that more silica had crystallized due to an increase in temperature. It is worth noting that the formation of SiC only started to form at 1300°C for all three sample features but in small quantities as evident from the short XRD peaks. The low yield is believed to be due to the low pyrolysis temperature which does not allow for sufficient chemical reaction between silica and carbon in the rice husk. The peaks for both silica and SiC were intensified as the temperature was increased to 1500°C.
4
Shatumbu T. Alweendo et al. / Procedia Manufacturing 35 (2019) 962–967 Author name / Procedia Manufacturing 00 (2016) 000–000
965
Fig 2. XRD spectra for pyrolysis products of (a) as received, (b) hand ground and (c) ball milled samples treated at different temperatures
Fig. 3 shows the composite XRD plot for leached pyrolysis products. The results revealed that the leaching process has removed the unreacted silica and other oxide impurities. The hydrofluoric acid attacks the silica to form hexafluorosilicic acid vapour that effectively evaporates all silica from the pyrolysis products according to Equation 1. (1) SiO2 (s) + 6HF (l) = H2 SiF6 (g) ↑ + 2H2O (g) ↑
Fig 3. XRD spectra for the three sample features pyrolysed at 1500°C and 120 minutes retention time
3.2 SEM analysis of silicon carbide (SiC) powder SEM images of leached pyrolysis products for the three sample features are presented in Fig. 4. The images show that the silicon carbide powder was a mixture of whiskers and particles. It is believed that the synthesis of SiC by carbothermal reduction of silica is a multi-stage, vapour-solid growth process that involves a series of chemical reactions which produce either particles or whiskers. SiC particles are formed by nucleation through reaction 2 with gas-solid interaction, whereas whiskers grow by reaction 3. SiO (g) + 2C (S) = SiC (s) + CO (g)
(2)
SiO (g) + 3CO (g) = SiC (s) + 2CO2 (g)
(3)
It is also believed that at temperatures above 1450°C, some of the whiskers are converted to particulates via a process called coagulative formation of crystal particulates [6]. It is worth noting that the ball milled powder shown in Fig. 4(a) had more whiskers due to the fine nature of the starting materials which had a greater surface area and sufficient pore volume that allowed the gas-gas reactions to take place at a higher rate. On the other hand, only a few whiskers were observed for hand ground and as received pyrolysis products as evident from Fig. 4(a) and (b), respectively. This is believed to be due to reduction in surface area caused by the larger particle sizes of the hand ground and as received samples.
966
Author name / Procedia Manufacturing 00 (2016) 000–000 Shatumbu T. Alweendo et al. / Procedia Manufacturing 35 (2019) 962–967
5
Fig 4. SEM images of SiC powder obtained from (a) ball milled, (b) hand ground and (c) as received samples pyrolysed at 1500 ᵒC for 120 min
3.3 Effect of temperature and time on the yield of SiC from rice husk The effects of temperature on the yield of silicon carbide from the three rice husk sample features was investigated between 1300°C - 1500°C. The yield of silicon carbide from all three sample features was found to increase with an increase in temperature for all reaction times studied as shown in Fig. 5. The effect of retention time on the yield of SiC from the three sample features was also investigated at different temperatures and the results are presented in Fig. 6. It was observed that the yield of SiC increased with an increase in retention time for all temperatures studied.
Fig 5. Effect of temperature on SiC yield from rice husk at different reaction times for the three sample features
6
Shatumbu T. Alweendo et al. / Procedia Manufacturing 35 (2019) 962–967 Author name / Procedia Manufacturing 00 (2016) 000–000
967
Fig 6. Effect of time on SiC yield from rice husk at different temperatures for the three sample features
4. Conclusions In the present research, silicon carbide (SiC) was synthesized from locally sourced rice husk at different temperatures and retention times without any use of catalysts or complicated equipment set-ups. The following conclusion were drawn: • The synthesized SiC powders were a mixture of particles and whiskers. • Silicon carbide yield was found to increase with increasing temperature and retention time. • The ball milled samples produced the highest yield of SiC (69.2 %) in comparison to the yield obtained from the hand ground (59.8%) and as received samples (56.9%). • The best temperature for production of silicon carbide in this study was found to be 1500ºC, while the best retention time was 120 minutes. References [1] A. A. Adediran, K. K. Alaneme, I. O. Oladele, E. T. Akinlabi, Cog. Eng. 5 (2018) (1) 494-499 [2] R. Pode, Renewable Sust. Ener. Rev. 53 (2016) 1468-1485. [3] J. Li, T. Shirai, M. Fuji, Adv. Powder Tech. 24 (2013) 838-843. [4] Y. Shen, P. Zhao, Q. Shao, Microporous Mesoporous Mater. 188 (2014) 46-76. [5] A. A. Taha, S. A. Abdel Ghani, J. Dispersion Sci. Technol. 37 (2016) (2) 173-182. [6] M. F. Zawrah, M. A. Zayed, M. R. K. Ali, J. Hazard. Mater. 228 (2012) 250-256. [7] M. Ali, A. M. Tindyala, J. As. Ceram. Soc. 3 (2015) (3) 311-316. [8] S. K. Singh, L. Stachowicz, S. L. Girshick, E. Pfender, J. Mater. Sci. Let. 12 (1993) 659-650. [9] V. Martínez, M. F. Valencia, J. Cruz, J. M. Mejía, F. Chejne, Ceram. 32 (2006) 891-897. [10] E. Gorzkwski, S. B. Quadri, B. B. Rath, R. Caldwell, J. Elect. Mater. 42 (2013) (5) 799-804. [11] S. F. Korablev, D.S Korablev, Powd. Metal. Met. Ceram. 54 (2015) (3) 215-219. [12] G. Adylov, S. A Faiziev, M. Paizullakhanov, S. Mukhsimov, E. Nodirmatov, Tech. Phy. Let. (2003) 221-223. [13] S. Chandrasekhar, P.N. Pramada, J. Majeed, J. Mater. Sci. 41 (2006) 7926–7933.