Composites: Part B 42 (2011) 1994–1998

Contents lists available at ScienceDirect

Composites: Part B

journal homepage: www.elsevier .com/locate /composi tesb

Effect of temperature on nano-crystalline silica and carbon compositesobtained from rice-husk ash

M. Sarangi a,⇑, P. Nayak a, T.N. Tiwari b

a Department of Physics, Sambalpur University, Jyoti Vihar, Sambalpur 768 019, Indiab Unique Research Centre, Koel Nagar, Rourkela 769 014, India

a r t i c l e i n f o

Article history:Received 24 October 2010Received in revised form 29 April 2011Accepted 16 May 2011Available online 6 June 2011

Keywords:A. Nano-structuresSilica–carbon compositesD. Electron microscopyD. Thermal analysisE. Heat treatment

1359-8368/$ - see front matter � 2011 Elsevier Ltd. Adoi:10.1016/j.compositesb.2011.05.026

⇑ Corresponding author.E-mail address: madhumita_sarangi@rediffmail.co

a b s t r a c t

Rice husk (RH), an agricultural waste, was used to produce nano-crystalline silica and carbon compositepowder. The rice-husk ash enriched with carbon was obtained in N2 atmosphere after pyrolysing at700 �C/2 h. This pyrolysed product enriched with carbon, hence named as black ash (BA), was heat-trea-ted at different temperatures (400–1200 �C)/2 h in a temperature-controlled muffle furnace, in the pres-ence of static air. Different phases of silica obtained from charred BA were observed with respect tochanges in temperature. Changes in phase transitions of silica and carbon were studied by using X-raydiffractometry (XRD). Crystallite sizes of silica and carbon obtained from charred BA at each temperaturewere worked out using Scherer’s relation. Nano-crystalline silica of various crystallite sizes was obtainedfrom charred black ash at different temperatures (400–1200 �C). A comparative study between raw ricehusk (RH) as received, BA (prepared by pyrolysing RH at 700 �C/2 h in N2 atm.) and BA charred in air wasdone by using scanning electron microscopy (SEM), X-ray diffractometry (XRD) and Fourier transforminfra-red (FTIR) spectroscopy. TG/DSC of RH was also done to study the decomposition process.

� 2011 Elsevier Ltd. All rights reserved.

1. Introduction process, about 20% mass of rice husk remains as ash, this contains

Ultrafine silica powder finds potential applications in manyindustrially important products. Silica is an important ingredientof high surface area catalyst, sorption media, glass and cementmanufacturing and dehydration systems [1]. This silica can be syn-thesized using complicated chemical route [2,3]. The rice husk,coming out of rice mills as a cheap by-product, offers great oppor-tunity for silica production more conveniently and at reduced ex-penses. At present, nano-silica materials are prepared usingseveral methods, including vapor phase reaction, sol–gel and ther-mal decomposition technique. The powder production of silica is ahighly energy intensive process. Unlike this, economically viableand high-grade, nano-size amorphous silica from rice husk canbe produced simply by burning under appropriate conditions [4].

The major constituents of rice husk are hydrated silicon and or-ganic materials consisting of cellulose (55–60 wt% including cellu-lose and hemicelluloses) and lignin (22 wt%) [5]. The mainadvantage of producing silica in this manner is highly reactive nat-ure of silica particle, requiring minimum grinding. The energy forburning is mostly supplied from the carbonaceous part of husk it-self. Therefore, attempts by many workers have been made to pro-duce silica from rice husk. Silica having large surface area and highporosity by burning rice husk has been produced [6]. During the

ll rights reserved.

m (M. Sarangi).

95 wt% of silica. It was found that leaching of rice husk with HCl at75 �C for 1 h prior to combustion produces amorphous silica ofcomplete white color [7]. Nano-structured silica powders withhigh specific surface area were obtained from non-isothermaldecomposition of rice husk in an air atmosphere [8]. Effect of tem-perature on phase transitions of nano-crystalline silica obtainedfrom rice husk was already reported [9]. Furthermore, charring ofrice husk produces nanosize silica carbon intermixed compositewhich may find newer application because such product is notavailable synthetically. This may directly be converted to siliconcarbide and silicon nitride at high temperature [10,11].

The present paper deals with the preparation of nano-crystal-line silica and carbon composite powder obtained from pyrolysisof carbon-enriched rice husk ash (RHA) and study of differentphases silica so formed is done by X-ray diffractometer (XRD) anal-ysis. The phase transformations of silica with respect to tempera-ture and carbon were also studied from XRD plots. The crystallitesizes were determined and a quantitative estimation was donefrom XRD plot. FT-IR was carried out to confirm the presence of sil-ica and carbon. Microstructural analysis was carried out to showthe presence of ultrafine particles.

2. Experimental details

The rice husk was collected from a local rice mill with the huskdimensions of 7–10 mm long, 1.5–2.0 mm wide and 0.10–0.15 mm

15 20 25 30 35 401000

2000

3000

4000

5000

6000

7000

Inte

nsity

2θ

Δ

As available from market

After proper washing

Fig. 2. XRD plot of market available and properly washed raw rice husk (RRH).

M. Sarangi et al. / Composites: Part B 42 (2011) 1994–1998 1995

thick. As collected from local rice mill, raw rice husks were washedthoroughly with running tap water followed by distilled water toremove adhering soil and other contaminants present in themand then dried in the sunlight for 24 h. These were designatedraw rice husk (RRH). Chemical analysis of these washed RRH wascarried out and already reported [9]. After proper cleaning, the soiland dirts were removed from the husk. RRH was heat treated at700 �C for 2 h in N2 atmosphere (0.5 kg/cm2) to remove volatilematter. N2 atmosphere was used here to preserve the fixed carbonin the system. The physical appearance of the residual material isblack in color and hence it is named black ash (BA). The BA con-tains silica, fixed carbon and metallic oxide impurities.

About 5 g samples of BA were taken in cylindrical alumina cru-cibles and introduced into a muffle furnace for pyrolysis at differ-ent temperatures varying from 400 �C to 1200 �C at an interval of200 �C for 2 h soaking time in static air. The pyrolysed productswere analyzed by XRD. A Philips XRD (Model PW 1830) with CuKa radiation through Ni filter was used. Scanning electron micros-copy (SEM) (JSM-6480 LV, JEOL) was used to study the surfacemorphology of the charred products. Thermo-gravimetric/differen-tial scanning calorimeter (TG/DSC) studies of RRH were carried outfrom room temperature to 1000 �C (Fig. 1) with a heating rate of10 �C/min in the presence of air using a thermal analyzer (NET-ZSCH-STA 409C) to investigate the decomposition process. FT-IR(Perkin Elmer, model Spectrum RX1) analysis of the pellet (usingKBr press) samples were carried out to confirm the different bondsof Si/O system in RRH and BA.

3. Results and discussion

A detailed and systematic heat treatment of RRH has been car-ried out by thermal analysis (TG/DSC) from room temperature to1000 �C (Fig. 1). The TG curve clearly indicates that the thermaldecomposition of RRH takes place in three different stages. In theinitial stage, weight loss (�5 wt%) is observed up to 100 �C whichis mainly due to removal of physically bound water (i.e., moisture).Maximum weight loss (�60 wt%) takes place in the second stage(250–350 �C). DSC plot also shows two close exothermic peaksduring this stage, due to two components of organic matters (cel-lulose and lignin) participating in the second stage of decomposi-tion reaction, well-known as pyrolysis [8]. This stage ofdecomposition is the loss of volatile compounds, followed by thepyrolysis of cellulose and lignin to the intermediates which maybe the organic materials of smaller molecular weight and/or car-bon. In the third stage, gradual weight loss in the TG is observed

100 200 300 400 500 600 700 800-2

0

2

4

6

Temperature (oC)

20

30

40

50

60

70

80

90

100

110

exo

TG/%DSC/mW/mg

Fig. 1. Thermal analysis (TG/DSC) of RRH.

with no sharp DSC peak. Hence, the thermal decomposition mech-anism of the rice husk in this stage is attributed to the further oxi-dation of carbon in the residual intermediate to form other volatilespecies, tar and char. The final product mainly contains silica [8].Cleaned RRH was analyzed through XRD (Fig. 2). The raw rice huskas received from market has a sharp peak at 2h � 26� which may bedue to dust or sand (silica) present in the husk. After proper wash-ing or cleaning, the dust or sand particles were removed. So, nopeaks can be observed. A hump at 2h � 22� can be observed dueto the presence of amorphous silica in rice husk. The amorphoussilica changed to crystalline silica at 700 �C (Fig. 3). The presenceof fixed carbon can also be detected at 2h � 26�. This is also con-firmed from the SEM and FTIR results discussed later. The peaksat 2h � 22� and 26� (Fig. 3) confirm the presence of silica. Fig. 4shows the composite XRD plot of pyrolysed BA at different temper-atures (400–1200 �C). In this plot mainly different phases of silicaand graphite peaks can be observed. a-Quartz can be observed at2h � 21� till 800 �C. A phase transformation of silica can be ob-served from 1000 �C to 1200 �C. Another silica peak at 2h � 26�shows the existence of both carbon and silica. The peak at2h � 26� mainly shows carbon at 400 �C and carbon and silica com-posite at 600 �C. From temperature 800 �C onwards, carbon cannotbe detected at 2h � 26�, instead a peak of silica can be observed.Usually carbon gets oxidized at 700 �C in the presence of air [9].With increase in temperature (i.e., at 1000 �C) this peak becomessharper, which may be due to the fact that with increase in tem-perature, the crystallite size increases. At 1200 �C, two major peaksof silica can be observed. A distinct change of silica phase can be

20 30 40 50 60 70

Inte

nsity

2θ

- silica/carbon- quartzx

oo

x

Fig. 3. XRD plot of black ash (BA).

20 30 40 50 60 70

4000C

Inte

nsity

(a.u

.)

2θ (degree)

xx

x

x

xx

- quartz

x

x

- cristobalite**

- graphite- silica

x

xx x

xο

ο

12000C

10000C8000C6000C

Fig. 4. XRD plot of BA pyrolysed in air at different temperature (400–1200 �C).

Table 1Crystallite size of SiO2 (h k l) and carbon (h k l) in nm of black ash charred at differenttemperature (400–1200 �C).

Temperature (�C) Crystallite size (nm) (h k l) Identified phases

400 24.39 (0 0 2) C600 33.88 (1 1 1), (0 0 2) SiO2, C800 44.68 (1 1 1) SiO2

1000 67.27 (1 1 1) SiO2

1200 41.47 (1 1 1) SiO2

Fig. 6. High resolution micrograph of charred BA at 600 �C.

1996 M. Sarangi et al. / Composites: Part B 42 (2011) 1994–1998

detected. a-Quartz gets transformed into b-cristobalite, and thepeak is shifted from 2h � 21� to 2h � 22� (Fig. 4).

Crystallite size was calculated from the XRD graph by usingScherer’s equation [12]

t ¼ 0:9kB cos h

ð1Þ

Fig. 5. Micrograph of (a) RRH, (b) B

where t is the crystallite size; k is the wavelength of Cu Ka line; B isangular width (in radians) at half the maximum intensity and h isBragg’s angle.

Again,

B ¼ ðB2meas � B2

equipÞ12 ð2Þ

where Bmeas is the measured FWHM (full-width at half maximum)and Bequip is FWHM due to instrumental broadening.

The crystallite sizes were found in nano-range (24.39–67.27 nm) and are listed in Table 1. The crystallite sizes increasegradually with increase in temperature from 400 �C to 1000 �C.This is because of the heat-induced expansion of crystallite sizeby reduction of crystallite thickness and agglomeration of smallcrystallites. The crystallite size observed here is quite large as com-pared to our earlier report (Ref. [9]). This is may be due to extraheat absorbed by the sample during (a) charring for preparation

A, and (c) charred BA at 600 �C.

(a)

(b)

(c)

Fig. 7. FT-IR spectra of (a) RRH, (b) BA, and (c) charred BA at 600 �C.

Table 2Absorption bands in the FTIR spectra and their assignment.

Region (cm�1) Assigned to

3600–3100 OAH stretching3000–2850 CAH stretching1600 Aromatic C@C vibrations activated

by neighboring oxygen groups1500–1415 CAH deformation1500–1200 CH2, CH3, Si(CH3), C(CH3)1500–1000 Aromatic C@C skeletal vibration1360–1310 CAC and CAO skeletal vibration1100–1000 SiAO stretching460, 796, 1093 Characteristic of SiAO bonds

M. Sarangi et al. / Composites: Part B 42 (2011) 1994–1998 1997

of BA (which was absent in earlier case) and (b) high heating rate(200 �C for 2 h soaking time, which is almost twice of the earlierheating rate) of the sample. At 1200 �C the phase change takesplace and two major peaks of silica at 2h � 22� and 2h � 26� canbe detected. The crystallite sizes of these two peaks also get re-duced to 41.47 nm (at 2h � 26�) and 51.39 nm (at 2h � 22�).

From the XRD plot, it can be clearly observed that both the car-bon and silica phases are present at 600 �C. So, scanning electron

micrographs of raw rice husk (RRH), black ash (BA) and calcinedBA at 600 �C are done and shown in Fig. 5a–c to study the surfacemorphology. Fig. 5a shows the outer epidermis of rice husk, whichis well organized and has a corrugated structure. The silica ismainly localized in the tough interlayer (epidermis) of the ricehusk; mainly concentrated in protuberances and hair trichomeson the outer epidermis and also adjacent to rice kernel [10]. TheBA sample in Fig. 5b shows many residual pores distributed withinthe ash indicating the presence of highly porous silica with a largeinternal surface area [8]. The black color indicates that sufficientpresence of carbon. The physical appearance of the BA charred at600 �C in air as shown in Fig. 5c is comparatively white in color,which may be due to the oxidation of carbon. From the morphol-ogy, we also find that it is difficult to distinguish between silicaand carbon. This may be due to the fact that silica is in intimatecontact with carbon. This is also confirmed from the XRD results.A higher magnification SEM of the BA sample calcined at 600 �Cin air was also done to observe the particle size (Fig. 6). The submi-cron particles were mainly cubical in nature. Some agglomeratedparticles along with very few whiskers can also be observed. Theparticle size ranges from 0.1 lm to 1 lm.

Results obtained from the FT-IR spectroscopy of RRH, BA and BAcharred at 600 �C are shown in Fig. 7. As can be seen, RRH containsseveral absorption bands, which have been assigned to individualstructural units as listed in Table 2 [13]. The indication of dehydra-tion is confirmed by the negligible intensity of the OAH stretchingband (3600–3100 cm�1) in BA and BA charred in air when com-pared with that in RRH. Organic decomposition can be observedfrom the absence of a 2900 cm�1 band representing CAH absorp-tion, and peaks of CH2 and CH3 at 1500–1200 cm�1 dramaticallydiminished. In addition, absorbance bands 1600 cm�1 (C@C) and1700 cm�1 (C@O) implies that there are some aromatic structurespresent in the BA. In BA charred in air, the band from 1600 cm�1

implies due to aromatic C@C vibrations by neighboring oxygengroups [13] and 1500 cm�1 may be due to CAH deformation.Improvement in aliphatic CAH deformation band indicates theenhancement of carbon contents of the composite powder becauseof charring of BA at 600 �C. The band from 1050 cm�1 to 1100 cm�1

corresponds to the SiAO bond vibration mode which appearedsharper as the organic matter was pyrolysed.

4. Conclusions

Nano-crystalline silica and carbon composite powder was ob-tained by the pyrolysis of RH under non-isothermal heating inpresence of N2 atmosphere. Silica and carbon composite can co-ex-ist till 600 �C as detected from XRD and confirmed by FTIR. Carboncannot be detected from 800 �C onwards. Silica produced afterpyrolysis (mainly below 800 �C) was a-quartz as determined fromthe XRD plots. Transformation of a-quartz to cristobalite phasesoccurred at higher calcined temperature (above 1000 �C). Fromthe XRD plot, the crystallite sizes were determined using Scherer’s

1998 M. Sarangi et al. / Composites: Part B 42 (2011) 1994–1998

formula. Crystallite sizes of silica varied from 24.39 nm to67.27 nm with increase in temperature till 1000 �C, mainly dueto agglomeration. The crystallite sizes increases gradually with in-crease in temperature from 400 �C to 1000 �C. At 1200 �C the phasechange takes place and two major peaks of silica at 2h � 22� and2h � 26� can be detected. The crystallite size of these two peaksalso gets reduced to 41.47 nm (at 2h � 26�) and 51.39 nm (at2h � 22�). These changes in crystallite sizes may occur due to thephase transformation.

Results of TG revealed the three stages of thermal decomposi-tions due to the removal of physically adsorbed water, pyrolysisof volatile organic matters, and oxidation of carbons and residualvolatile species. Morphology of the RRH, BA and BA charred at600 �C were studied by SEM. Higher magnification of the BA sam-ple charred at 600 �C shows some particles within the range 0.1–1 lm. FT-IR study confirms the presence of SiAO, C@C, C@O,OAH and CAH bands in case of RRH, BA and BA charred at600 �C samples.

Acknowledgments

Authors are grateful to Department of Science and Technology,Government of India, for providing grants under the Project (No.DST/TSG/WM/2005/3) to support the present work. XRD, SEMand FT-IR analyses were performed at the National Institute ofTechnology, Rourkela, India.

References

[1] Wu G, Wang J, Shen J, Yang T, Zhang Q, Zhou B, et al. Properties of sol–gelderived scratch-resistant nano-porous silica films by a mixed atmospheretreatment. J Non-Cryst Solids 2000;275:169–74.

[2] Tomozawa M, Kim DL, Lou V. Preparation of high purity, low water contentfused silica glass. J Non-Cryst Solids 2001;296:102–6.

[3] Tanner PA, Yan B, Zhang H. Preparation and luminescence properties of sol–gelhybrid materials incorporated with europium complexes. J Mater Sci2000;35:4325–8.

[4] Kapur PC. Production of reactive bio-silica from the combustion of rice husk ina Tube-in-Basket (TiB) Burner. Powder Technol 1985;44:63–7.

[5] Patel M, Karare A, Prasanna P. Effect of thermal and chemical treatments oncarbon and silica contents in rice husk. J Mater Sci 1987;22:2457–64.

[6] Hayasi H, Nakashima S. Synthesis of tri octahedral smectite from rice husk ashas agro-waste. Clay Sci 1992;8:181–93.

[7] Chakravarty A, Kaleemullah S. Conversion of rice husk in to amorphous silicaand combustible gas. Energy Convers Manage 1991;32:565–70.

[8] Liou TH. Preparation and characterization of nano-structured silica from ricehusk. Mater Sci Engg A 2004;364:313–23.

[9] Sarangi M, Bhattacharyya S, Behera RC. Effect of temperature on morphologyand phase transformations of nano-crystalline silica obtained from rice husk.Phase Trans 2009;82:377–86.

[10] Krishnarao RV, Godkhindi MM. Distribution of silica in rice husks and its effecton the formation of SiC. Ceram Inter 1992;18:243–9.

[11] Ray AK, Mahanty G, Ghose A. Effects of catalysts and temperature on siliconcarbide whiskers formation from rice husk. J Mater Sci Lett 1991;10:227–9.

[12] Cullity BD. Elements of X-ray diffraction, 2nd ed. Massachusetts: Addition-Wesley Publishing Company; 1978. p. 102.

[13] Sujirote K, Leangsuwan P. Silicon carbide formation from pretreated ricehusks. J Mater Sci 2003;38:4739–44.