Particle Characterization by Advanced Electron Microscopy - Essay Example

?Particle Characterization by Advanced Electron Microscopy Characterization of particle has been performed using advanced electron microscopy and microanalytical instruments like Scanning Electron Microscope (SEM), Scanning Electron Microscope with Energy Dispersive Analysis of X-rays (SEM-EDAX) and Transmission Electron Microscope (TEM). Different attributes of particles like particle shape, size and size distribution and chemical composition of different particles were determined. SEM analysis shows that there is multimodal distribution of particle size with modes at 25 ?m, 60 ?m and 115 ?m. SEM-EDAX analysis shows that the powder particles are those of ytterbium oxide and TEM analysis shows morphology of fine TiO2 and Fe2O3. Results and analysis of these experiments are presented in this report. Introduction Electron microscopy has been a leap jump over optical microscopy in the field of micro imaging. While resolution in case of optical microscopy is limited by the wavelength of light, the same gets tremendously improved because accelerated electrons have much smaller wavelengths and hence much better resolution in case of electron microscopy. Besides, micro imaging there are other benefits associated with using electron beam as probe. This is because electron – matter interaction produces a wide variety of signals like secondary electrons, back scattered electrons, fluorescence and characteristic X-rays, transmission electrons, auger electrons etc. to name a few [1]. These signals do not contain information about only topography but also about chemistry of the region of interaction and hence micro-analytical information can also be extracted. This is the reason why a large number of micro-imaging and microanalytical instruments have been developed using electron beam as probe and these are being widely used in the industry as well as in the advanced research. A brief introduction of some of these instruments like Scanning Electron Microscope (SEM), Scanning Electron Microscope with Energy Dispersive Analysis of X-rays (SEM-EDAX) and Transmission Electron Microscope (TEM) is presented in the following sections. Scanning Electron Microscope (SEM) [3]: This microscope scans the probe electron beam over a raster and response signals like secondary and backscattered electrons are collected and used for image formation [2]. It consists of a column always under vacuum consisting of different subcomponents. It consists of an electron gun or electron source. This is followed by condenser lenses to focus the beam. There are apertures in the path to allow only the useful (central) portion of the beam to the subsequent stages. The focused beam is double scanned and made to pass through an aperture to fall onto the specimen. The beam is scanned over the specimen in a raster and the generated signals – secondary or back scattered electrons are collected, amplified and again scanned in a raster onto a CRT screen in synchronized manner. Thus image is formed pixel by pixel. The magnification is ratio of the CRT screen size to the area of the raster onto the specimen. Because, the CRT size is fixed, therefore, magnification can be increased by scanning lower area onto the specimen and vice – versa. While secondary electron provides topographic contrast, the backscattered electrons provide atomic number or Z-contrast. SEM with Energy Dispersive Analysis of X-rays (SEM-EDAX): EDAX is essentially a detector or an attachment to an instrument. This is capable of detecting energy of the X-rays falling onto it. This uses a semiconductor – SiLi or GeLi and as X-rays fall onto this semiconductor electrons are produced in the proportion of the energy of the X-rays and this produces a current which is used to determine energy of the X-ray. This attachment can be attached to different instruments like SEM, EPMA or Electron Probe Microanalyser or even with a TEM or Transmission Electron Microscope. When probe electron beam falls onto a sample characteristic X-rays are produced, which are collected and analyzed using ESAX attachment. Transmission Electron Microscope or TEM In transmission electron microscope a focused electron beam of high energy usually more than 160 keV in old microscope to more than 300 keV or even higher in high resolution TEMs is used as probe beam. The sample is kept thin enough (usually less than 1000 Ao) so that a significant proportion of the primary beam gets transmitted through the sample and is used for image formation. In electron microscope electromagnetic lenses are used to focus the beam and for image formation. The image is formed on a fluorescent phosphor screen. Besides high resolution imaging, TEM is used for crystallographic studies using diffraction and for microanalytical studies using different attachments like EDAX, EELs etc. Therefore, it is only natural that TEM is the most sought after instrument for advanced studies in materials science. Essential Instruments: The FEI Quanta 250 ESEM with EDAX was used for imaging and chemical identification of the powders while Philips 410 TEM for transmission electron imaging. A Cressington sputter system was used to apply conducting carbon (graphite) coating for SEM and SEM-EDAX studies. CARNOY software package was used to measure particle size and size distribution. Silica powder was used for SEM studies and TiO2 and Fe2O3 powders were used for TEM studies. Experimental Procedure: The experiment consisted of preparing samples for SEM and TEM and then examination of these samples in SEM and TEM. Sample preparation for SEM The powders used in these studies are non-conducting powders and these cannot be used in SEM as such. This is because electrons will accumulate on a non-conducting sample and will not allow examination of the samples. Therefore, these particles were made conducting by applying a conducting graphite coating. For this purpose, a stub was used to hold the powder sample and a small amount of silica powder was placed on the top of the stub. The sample was placed in a small chamber inside a sputter coater and coated with conducting carbon coating. Examining the Sample in FEI Quanta 250 ESEM with EDAX The coated sample was placed in the SEM with EDAX and the SEM was made ready to work. This required evacuating the sample chamber to high vacuum level ~10-7 torr and then beam was put on. BSE images of the powder samples were recorded and used for particle size analysis. Analysis morphology of silica particles was recorded in a FEI SEM at 4kv, 10kv and 20kv. The same procedure was used to prepare the sample for SEM-EDAX. Then the sample was analyzed for its micro-structure and elemental compositions under EDX-SEM operating at 15KV. For TEM observation, the powders were mixed with distilled water and TEM compatible glue ultrasonically. A drop of this sample was placed onto a copper grid coated with a layer of amorphous carbon. A Philips 410 TEM was used to study the morphology. RESULTS and ANALYSIS OF EXPERIMENT 1: Particle size distribution as obtained from SEM imaging and using CARNOY software program is presented in Table 1. The same is presented as a histogram is Fig. 1. It can be seen that the particle size distribution is multimodal in nature. This essentially means that there are many peaks in the histogram. Three peaks can be clearly seen one around 25 ?m, the other around 60 ?m and the next one around 115 ?m. The mean particle size is approximately 72 ?m. Table 1: Particle size distribution of silica powder Diameter range (µm) of silica particles Number of Particles 0-10 0 10-20 2 20-30 265 30-40 12 40-50 64 50-60 198 60-70 142 70-80 3 80-90 7 90-100 25 100-110 87 110-120 291 120-130 26 130-140 5 140-150 10 150-160 4 160-170 0 A pictorial representation of the particle size distribution can be seen in Fig. 1. This is a histogram that shows number of particles in different size range. It can be seen that the particles size is centered on three main sizes. This means this powder sample is a mixture of powders from different sources. This is because powder from one source should have a Gaussian distribution and only if three such distributions are mixed then one can get a multimodal distribution, which is the present case. Fig. 1: Histogram showing particle size distribution in silica sample RESULTS and ANALYSIS OF EXPERIMENT 2: Figure 2 shows BSE image of a powder sample. It can be seen that some powders are bright while some are dull. This is because; BSE image contains information about atomic number or Z of the sample. Accordingly, the bright particles belong to high atomic number and vice versa. Morphology (round or angular) and size (small or big) of the particles is clearly seen in this image. Fig. 2: BSE Image of the powder sample Figure 3 shows, characteristic X-ray peaks belonging to the elements in the bright particles as seen in Fig. 2 as recorded by SEM-EDAX. This gives chemical information of the bright particles. It can be seen that there are multiple peaks of yttrium and some peaks of oxygen. Therefore, it can be concluded that the bright particles are yttrium oxide particles. As can be seen from Fig. 2 that area fraction of the bright particles is much smaller as compared to that of the dull particles. This implies presence of yttrium oxides as impurity in the silica powder. Fig. 3: Characteristic X-ray peaks of bright particles as recorded by SEM-EDAX RESULTS and ANALYSIS OF EXPERIMENT 3: TEM images of TiO2 and Fe2O3 particles are presented in Fig. 4 and Fig. 5 respectively. From Fig. 4 it can be seen that TiO2 particles are of two shapes – spherical and cylindrical; Fig. 5 on the other hand shows that Fe2O3 particles are faceted and equiaxed in morphology. Fig. 4: TEM image of TiO2 particles Fig. 5: TEM image of Fe2O3 particles Conclusions: From these experiments it can be concluded that SEM is a very good instrument to perform particles size distribution characterization by direct viewing of the particles. Besides, SEM-EDAX is a very good instrument to augment the size distribution analysis by complementing it with chemical analysis. In the present study while SEM has helped in characterizing particle size distribution of silica powder, SEM-EDAX has helped in identifying the yttrium oxide impurity in the sample. When particle size is extremely fine then TEM has helped in characterizing the powder in terms of shape, size and distribution of the same by direct viewing of the particles images formed by transmitted electrons. References: [1] Hesseler-Wayser A., “Intensive SEM/TEM Training: Electron Matter Interaction”, 2009, Retrieved on January 15, 2011 from http://cime.epfl.ch/files/content/sites/cime2/files/shared/Files/Teaching/Doctoral%20School%202009/Chapter%201%20-%20Interaction%20of%20electrons%20with%20matter.pdf [2] Hesseler-Wayser A., “Intensive SEM/TEM Training: SEM”, 2009, Retrieved on January 15, 2011 from http://cime.epfl.ch/files/content/sites/cime2/files/shared/Files/Teaching/Doctoral%20School%202009/Chapter%201%20-%20Interaction%20of%20electrons%20with%20matter.pdf [3] www.zeiss.com [4] www.edax.com [5] www.fei.com Read More