0016-2361(94)00119-0

Fuel Vol. 74 No. 11, pp. 1722-1725, 1995 Elsevier Science Ltd

Printed in Great Britain 0016-2361/95/$9.50 + 0.00

Surface analyses of pyrolysed rice husk using scanning force microscopy

Rajan K. Vempati, Sirish C. Musthyala, M. Yousuf A. Mollah and David L. Cocke P.O, 10022 Chemistry (Gill Chair), Lamar University, Beaumont, TX 77710, USA (Received 10 March 1994; revised 1 August 1994)

Scanning force microscopy (SFM), also referred to as atomic force microscopy (AFM), has been used for the surface characterization of untreated and pyrolysed rice husk samples. This technique, unlike scanning electron microscopy, provides both qualitative and quantitative measurements. The principles of SFM and sample preparation are discussed. Quantitative data in the form of surface roughness, void and fractal dimension are presented. The rice husks were pyrolysed in air at 573, 673 and 773 K for 30 min in a muffle furnace. The surfaces of the rice husk pyrolysed at 773 K showed voids due to the decomposition of the organic matter. Quantitative analysis of these samples indicated that the surface roughness parameters decreased with increasing temperature owing to the increased decomposition of the organic matter. In addition, the number of voids and their z dimensions increased with increasing temperature. The fractal dimension increased from 2.12 to 2.31 with increasing temperature, indicating that the voids were becoming rougher. Furthermore, the decomposition of the materials followed a fractal geometry.

(Keywords: scanning force microscopy; combustion; surface roughness)

Rice husk is an agricultural waste material generated in rice-producing countries, especially in Asia. The annual world rice production is approximately 500 million tons, of which 10-20% is rice husk I . Dry rice husk contains 70-85% of organic matter (lignin, cellulose, sugars, etc.) and the remainder consists of silica, which is present in the cellular mem- brane. In the USA, rice husk is mostly used as a bedding material for animals and the industrial applications of this material are limited. Therefore the rest is landfilled. However, landfilling is becom- ing expensive because of the increasing demand of land for urban and agricul- tural usage. Therefore, there is a tremen- dous need for making useful and/or value-added products from rice husk. In the developing countries, it is being used as a source for: (1) fuel in the rice milling industries 2-4, (2) pozzolanic material to enhance the lime treatment of degraded

5,6 soil , (3) increasing compressive strength 7 8 10 in cement and cementitious materials - ,

(4) particle board technology 11 and (5) activated carbon 2'12. Since the purity of silicon in rice husk ash is about 99.9%, some advanced countries extract Si to be utilized in the silicon industry. Some industrial applications for the utilization of Si from rice husk are extraction of silicon 13, fillers in polypropylene 14, manufacture of SiC for the electronic industries 15'16, fabrication of solar cells 17, synthesis of porous or dense vitreous silica 18, materials having higher density and lower sintering temperature for ceramics applications 19 and the manu- facture of monosilanes and disilanes for electronic applications 2°-22.

Gasification is one of the viable routes for the effective use of rice husk for thermal and mechanical/electrical appli- cations 23. The morphological changes

are important features that have to be characterized for the efficient utilization of rice husk as a source of fuel and activated charcoal 2 and for grain devel- opment 24. This is due to the fact that in the gasification process it is important to monitor the changing geometrical shapes when the solid materials are reacted. In addition, to maximize and/or model adsorption processes in porous material one should understand the pore struc- tures 2'25. It has become increasingly clear that the rice husk pores are not of simple Euclidean geometric shapes but of fractal geometry 2.

Previous work on understanding the pore geometry of the pyrolysed rice husk has been reported 2,26, and conclusions were derived from a scanning electron microscopy (SEM) study. However, scanning force microscopy (SFM) is a more powerful tool for understanding these processes because it provides both qualitative and quantitative data, as we have shown for soils and environmental materials 27. There are several advantages of SFM over other microscopic tech- niques: (1) it provides structural and surface topographical information and three-dimensional resolution down to atomic level, (2) it needs minimum sample preparation, (3) samples can be imaged in air, liquid or vacuum and (4) data can be manipulated quantitatively. To the best of our knowledge, we are the first to report applications of SFM to understand the surface roughness and pore geometry of rice husk pyrolysed at various temperatures.

SFM background Binning et al. 28 first introduced SFM

in 1983, and since then considerable improvements have been made. SFM is used primarily to image non-conducting

samples in addition to semiconducting and conducting samples. SFM consists of a soft cantilever spring, sensing detector, piezoelectric ceramics, feed- back electonic circuit and a computer for generating and presenting images (Figure 1). A laser light is focused on to the tip of the cantilever, which moves up and down depending on the topography of the sample. The feedback causes the tip and the sample to maintain a given force set point. The distance that the tip is pulled is a measure of the height of the sample. These movements are captured on a four-section photodiode detector. The amount of motion of the cantilever is calculated from the changes in light intensities. The photodetector can sense sub-angstrom cantilever deflection, and hence atomic resolution is achieved.

The strength of SFM lies in image processing and data manipulation. The collected data can be enhanced either in qualitative and/or quantitative forms. Some of the qualitative manipulations are levelling, shading, smoothing, colour- ing, fast Fourier transformation and three-dimensional display. The quantita- tive manipulations are line profile, sur- face roughness, fractal analysis, grain boundary analysis, etc.

Surface quantitative analysis SFM data can be manipulated to des-

cribe the surface roughness parameters and the fractal geometry of materials. Some of the surface roughness param- eters used in this paper are discussed in the Appendix. The surface roughness measurements are obtainable at the macroscopic, microscopic and atomic scales. These parameters provide con- siderable information about the surface texture which results in an approximate classification of surfaces as level, wavy,

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smooth or rough. Fractal geometry is defined as a shape made of parts similar to the whole in some way 25,29. The fractal dimension of a surface is important because (1) surfaces can be characterized by only one non-integer number, (2) a surface can be directly correlated with observable phenomena such as cleanability, corrosion, adsorp- tion, catalysis and degassing and (3) it helps to distinguish between surfaces that other engineering parameters would classify in the same category.

EXPERIMENTAL

Raw material Two grams of the rice husk (obtained

from Douget Rice Milling, Beaumont, TX, USA) were taken in a crucible and placed in a muffle furnace, which was preset for 30min at the required tem- perature. Three different samples were pyrolysed in air at 573,673 and 773 K for 30 min. Subsequently, the samples were cooled to room temperature and stored in glass vials for SFM analysis.

Scanning force microscopy For analysing samples using SFM, the

following sample preparation technique was used. A 5 mm square of Si [100] or an A1 wafer was placed on a hot-plate which

had been preset to 333 K. Tempfix epoxy (manufactured by Structure Probe, West Chester, PA, USA) was used to embed the rice husk samples. This is necessary because any sample movement or vibra- tion would result in a poor image. The epoxy was melted at 333 K; then, with a flat spatula, it was spread evenly on the wafer surface. The sintered rice husk particle was then placed carefully into the warm epoxy using forceps. Care was taken that no epoxy stuck to the rice husk surface. The Si or A1 wafer was then removed from the hot-plate and cooled to room temperature. This solidified the epoxy and held the embedded material intact. All this was done in such a way that there was no damage to the surface of the particle. This is important because SFM is a surface technique.

The outer surfaces of the untreated and the pyrolysed rice husk particle were scanned with a 10 #m scanner using a TopoMetrix TMX 2000 SFM. Duplicate runs were performed for the 673 and 773 K samples. The roughness parameter and fractal analysis were performed on 8.7 × 8.7#m size scans with 200 × 200 pixel resolution. The data were then analysed using the TopoMetrix 2.3A software.

RESULTS AND DISCUSSION

The treated pyrolysed rice husk at 573

LASER

~ g ~ e l

'ect°r !"

i C al

Sample ~ ~

Cantilever

Schematic diagram of SFM

Table 1 Surface parameters and fractal dimensions of the untreated and treated rice husk

Sample z-Level treatment (Ra) Rrms (nm) Fd

Untreated 32 38.5 - - 573 K 17 23.0 46 2.12 673 K 18 23.0 59 2.26 773 K 3 6.0 208 2.31

SD ~ 11.8 13.2 90 0.09

a Standard deviation

and 673 K was black but the sample at 773 K was grey, indicating the oxidation of organic-C at higher temperature. X-ray diffraction of the 573 K sample indicated the presence of amorphous SiOa (not shown). The untreated rice husk sample consists of organic material and cellulose on the outer epidermis of the rice husk 3°. As the samples were pyrolysed at 573, 673 and 773 K the organic matter on the surface of the rice husk decomposed to create a porous structure or voids. The SFM images of untreated and 773 K- treated rice husks are shown in Figure 2a and b, respectively. The untreated samples contained globular materials on the sur- faces, whereas the 773 K sample showed voids on the surface.

Table 1 summarizes the surface param- eters. The average surface roughness (Ra) and root mean square of average rough- ness (Rrms) values for the untreated rice husk sample were higher, indicating a rough surface. This is because of the presence of globular organic matter. With increase in the pyrolysis tempera- ture, the surface roughness values decreased, indicating the decomposition of the organic matter (compare Figure 2a and b). The Ra and R~ms values for the untreated and 773K-treated samples were statistically different, as observed from their standard deviation values. The surface void dimension, as indicated by the z-level, increased with increasing pyrolysis temperature (Table 1). The fractal dimension calculated from the slope of log L versus log A (Figure 3), where L is the length of the perimeter of the void at the sample surface and A is the surface area of the void, showed that these values increased with increase in temperature, indicating that the voids were irregularly shaped. The number of lakes (Ln) increased with increase in temperature, indicating an increase in porosity (data not shown). The fractal dimensions increased with increasing pyrolysis temperature, indicating the creation of rougher voids. Furthermore, the Fd values of -->2.12 for the pyrolysed samples indicated that the void geometry was fractal in nature.

CONCLUSIONS

We have shown that scanning force microscopy can be successfully used for qualitative and quantitative analyses of untreated and treated rice husk surfaces. The advantages of SFM are that data can be manipulated to provide surface rough- ness parameters and fractal dimensions. These parameters might be easily corre- lated with the adsorption, mechanical/ thermal properties and reaction kinetics of rice husk conversion to charcoal. Also, the above techniques might be used to understand controlled growth studies of nanophase, non-crystalline or crystalline Si phases synthesized from rice husk.

Fuel 1995 Volume 74 Number 11 1723

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(a)

Figure 2 SFM images of (a) untreated rice husk and (b) 773 K-pyrolysed rice husk

(b)

Figure 3 SFM surface analyses of 777 K-pyrolysed rice husk showing fractal dimension

A C K N O W L E D G E M E N T S

We thank the Texas Advanced Tech- nology Program and the Texas State Coordinating Board for funding to support this work. Partial support for work relating to amorphous materials came from Welch Foundations. We thank N A S A - J o h n s o n Space Center, Houston, TX for use of their X-ray diffractom- eter. Mr Steven J. Pytel is thanked for technical assistance.

R E F E R E N C E S

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Figure AI

Ra(1) = Ra(2)

Fd(1)<Fd(2) Internal features of (1) smooth and (2) rough voids

International Soil Science Proceedings Alcapulco, Mexico, 1994', in press

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APPENDIX

Average surface roughness ( Ra), and root mean square surface roughness ( Rrms)

An arithmetic mean (Z) is defined as the sum of all height values (Zi) divided by the number of data points (N) in the profile. In the TopoMetrix algorithms for

surface profile measurements of this type, N is 20, which takes into account the 20 highest features in the chosen area.

1 N Z = ~ _ Z i (A1)

Surface roughness ( Ra). The most fre- quently used roughness parameter is R a. This value is the arithmetic mean of the deviations in height from the profile mean value. R a is expressed as

1 N Ra = ~"~.(IZi~ - Z[) (A2)

Root mean square determination ( Rrms). Rrms is defined as the root mean square of the deviations in height from the profile

mean, defined as

Rr~s= -~.= Zi - Z 2 (A3)

Surface fractal dimension The lake filling method has been used

successfully to characterize electrodepos- ited gold surfaces and crystal growth properties and to quantify differences in paper quality 31. This method involves choosing a z-height, such that approxi- mately half the structures of the surface are above the chosen z-value. The other half of structures are below the z-value. Lake patterns are then generated by simulating a water level at that z-height. This method generates lakes for a chosen depth (z). The perimeter (L) and area (A) of the lake are calculated. The fractal dimension of the surface is obtained by plotting log L versus log A. Lake filling is expressed as

L cx A F'd/2 (A4)

or rewritten as

F ¢ L = a A (A5) d/2

where a is the proportionality constant. The three-dimensional surface fractal

dimension (Fd) is related to F~ by

F~ = Fd -- 1 (A6)

The fractal dimension is the slope of a plot of log L versus log A. For smooth pores F~ = 1; hence an Fd value of 2 means the surface is smooth and a value of 3 indicates an extremely rough surface.

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