Thao Nguyen1, Dr.Brian L.McIntyre2
1Department of Chemical Engineering, 2The Institute of Optics
Smoke residues from burning coal, wood, scented candle with lead-free wick, and Tostitos white-corn tortilla chips were collected on conductive double-sided tapes mounted on silicon substrates. The substrates were held 5cm above the flame to avoid burning the tapes.
The light microscope Olympus Optical Microscope BXA was first used at low magnification 10x and 50x to confirm the presence of the smoke residues. Since the residues are not conductive, they had to be coated with a gold layer approximately 120Å thick in a sputter coater prior to SEM imaging. SEM Zeiss-Auriga system was employed to record SEM images in both SE2 and BSD modes using using secondary electrons and backscattered electrons, respectively.
X-ray emission was collected using the EDAX spectrometer incorporated in the SEM at various accelerating voltages of 5kV, 10kV and 15kV. The voltage was adjusted so that the x-ray signals were generated from the smoke residues and not from the silicon substrates underneath. On the other hand, the voltages must be large enough to overcome the critical ionization energy of the highest atomic-number element presenting in the specimens. Only one representative EDS graph capturing all of the elements presenting in a given specimen is shown for each sample. The representative graph is not always the one taken at 15kV as long as all the elements are counted.
Two SE2 images of the coal smoke residues were captured when the sample was tilted at 0o and 5o. A 3-D image was created with Anaglyph Maker by placing these two pictures side by side in red and cyan color. The 3-D image can be viewed using red-and-cyan glasses.
The last technique colorization of the coal residues was performed with Photoshop CS6.
1. Coal sample
Figure 2:BSD image of the coal sample at 20kV and 8mm. The bar scale is 1um.
In the BSD mode, brightness constrast represents compositional difference. The smoke particles made of heavy elements appear to be brighter than the ones containing lower atomic-number elements.
Figure 3: EDS analysis of the coal sample taken at 10kV
Coal originates from plant matter and it is a heterogenenous mixture of plant materials and minerals. The EDS test dectected carbon as expected; oxygen presented as the product of the combustion reaction; Mg, Al, K, and Ca were the minerals in the original coal; Si was the substrate material and gold was the coating layer.
Figure 4: Light microscope image of the wood sample. The bar scale is 50um.
Figure 5: BSD image of a wood particle at 15kV and 9mm
Figure 6: EDS analysis of the wood sample taken at 15kV
Wood mainly consists of cellulose which is long chains of glucose molecules. This component released carbon as the wood burnt. Other metals such as Na, Mg, Al, K, Ca, Ti, and Fe detected here are normally found in wood smoke at small amounts.
3. Candle sample
Figure 7: Light microscope image of the candle sample. The bar scale is 30um.
Figure 8: SE2 image of a candle particle at 15kV and 9mm
Figure 9: EDS analysis of the candle particle taken at 5kV
The EDS graph above was taken at 5kV. The EDS graph of the same particle analyzed at 10kV (graph not shown here) demonstrated similar result. Carbon and oxygen presented in the sample as expected. The wick of the candle is said to be lead-free; this advertise is confirmed by the EDS analysis.
4. Tortilla chip sample
Figure 10: Light microscope image of the tortilla chip sample. The bar scale is 30um.
The smoke residues of the tortilla chip agglomerate as drops of varying size rather than individual particles as in other samples. These drops appeared very colorful under the light microscope.
Figure 11: BSD image of the tortilla chip sample taken at 15kV and 9mm
The bright particles in the BSD image is believed to be sillicon chips orginated from the silicon substrate. As have said above, smoke residues of this tortilla chip sample aggregrated and became large drops rather than existing as individual particles. Those drops appear to be the dark spots in the BSD image above. EDS analysis was performed for one of those spots.
Figure 12: EDS analysis of the tortilla chip sample taken at 5kV
The tortilla chip mostlyconsisted of saturated fat, unsaturated fat and carbohydrate which were decomposed to CO2 and water in the combustion reaction. C and O were dectected in the EDS test; H was not detectable because its atomic number is too small. According to the manufacturer, the chips also contained 5% sodium and 2% iron; however, these elements could not be detected in the EDS test. Increasing the accelerating voltage to 10kV and 15kV gave similar result, except that the Si peak was very high attributed to the Si substrate.
Colorization of coal residues using Photoshop CS6
Figure 13: Colorized coal residues using Photoshop CS6
3-D image of coal residues using Anaplyph Maker
Figure 14: 3-D image 3-D image of coal residues using Anaplyph Maker
I would like to thank Dr.Brian L.McIntyre for his advice and instruction, as well as his humor. This project could not be realized without his expertise support.