Powder Characterization by SEM, SEM-EDAX, and TEM - Essay Example

?Powder Characterization by SEM, SEM-EDAX and TEM Determination of important characteristics like particle shape, size, size distribution chemistry etc. of different oxide powder samples was carried out using Scanning Electron Microscope (SEM), SEM with Energy Dispersive Spectroscopy (SEM-EDS) and Transmission Electron Microscope (TEM). SEM images were used to carry out size distribution analysis of the powder particles using a software package CARNOY. BSE image of the powder particles was taken and EDS was performed to get chemical information about bright particles in the powder sample. Analysis shows that even though the particle size range is from 10 ?m to 160 ?m; however most of the particles are confined in three size ranges: 10 - 20 ?m, 50 – 70 ?m and 110 – 120 ?m. SEM-EDS analysis shows that the bright particle in the silica sample are yttrium oxide. TEM images show two kinds of shape – spherical and cylindrical for TiO2 and faceted equiaxed morphology for Fe2O3 particles. The detailed results and analysis is presented in this report. Introduction Powders play very important role in materials science and industry so much so that one stream of metallurgy is known as powder metallurgy. Besides, ceramics engineering revolved around powders. Consolidation of many materials becomes possible only through powder metallurgy route, which involves filling, compaction and sintering of powders. All these processes depend heavily on powder characteristics like shape, size, size distribution etc. to name a few. Experimental determination of these attributes of powder is very important. Some of these attributes like size and size distribution can be determined by indirect methods like laser particle size analysis. However, only a direct method like microscopy gives the confidence in the result. Beside, many attributes like shape and chemistry can be determined only by advance electron microscopy. Electron microscopy involves obtaining high magnification images of the samples using focused beam of accelerated electrons as probe and then forming images by collecting the different signals like backscattered electrons, secondary electrons, transmitted electrons etc [1]. As wavelength of accelerated electrons is much smaller as compared to light; therefore, it can be focused to much finer spots and much higher resolution and magnification is possible in case of electron microscopes as compared to the same in case of optical microscopes. Besides, many signals generated by electron – matter interaction like auger electron, characteristic X-rays etc. contain information about chemistry of the matter and these signals can be used to determine chemistry of the sample using different detectors like Wavelength Dispersive Spectrometer (WDS), Energy Dispersive Spectrometer etc. Accordingly there are different instruments like Scanning Electron Microscope (SEM), SEM-EDS, Electron Probe Microanalyser (EPMA), Transmission Electron Microscope (TEM), High Resolution Transmission Electron Microscope (HRTEM) etc. A basic description of SEM, SEM-EDS and TEM which were used in these experiments is provided in the subsequent sections. Scanning Electron Microscope (SEM) [2]: As suggested by the name, in this microscope a focused beam of electron is scanned over the sample in a raster using scanning coils. This leads to generation of signals like secondary electrons and backscattered electrons; which are used for image formation on a CRT screen. The image is formed in a pixel by pixel manner and therefore, the raster size corresponds to the CRT screen size magnification is arrived by dividing the CRT length by the raster length. The magnification can thus be increased gradually by reducing the rater size on the sample as the CRT size is fixed. An SEM consists of an electron source or an electron gun, apertures to block unwanted beam, electromagnetic lenses to focus the beam, different detectors like secondary electron detector and backscattered electron detector for image formation and a computer for operation of the instrument and different digital processing of the image. This entire system is kept under very high vacuum of the order of 10-7torr or better, otherwise electrons will collide with the air molecule and their energy will get attenuated and also the beam will get defocussed. Secondary electrons and backscattered electrons are two modes of imaging in an SEM and provide topographic and atomic number contrast respectively. This makes an SEM a versatile instrument to perform microstructural studies and fractography. SEM with Energy Dispersive Spectrometer (SEM-EDS) [3]: When focused electron beam strikes a sample it produces characteristic X-rays. These X-rays are characteristic of the elements present in the sample and energy of the X-rays provides information about the chemistry of the sample. Spectral analysis of these X-rays is performed by EDS detector. This detector is thus not standalone equipment but an attachment which is attached with instruments like SEM, EPMA, TEM etc. There is uses a semiconductor – SiLi or GeLi, which is heart of this detector. When the X-rays are collected onto this detector electrons are produced in the proportion of the energy of the X-rays. These electrons produce a current in the circuit which is used to determine energy of the X-ray and thus the spectrum of the X-rays is produced. Transmission Electron Microscope (TEM) [4]: A focused beam of very high energy (200 – 1000 keV) electrons are made to strike a thin sample (usually 50 to 100 nm). The electrons get transmitted through the sample and are used to form image onto a phosphor screen. The basic construction of a Tem is similar to an SEM except that there are electromagnetic lenses even below the sample in case of a TEM, which is not there in case of an SEM. Also, scanning coils are normally not there in a TEM. As energy of the electrons is much higher (> 200 keV) in TEM as compared to the same in an SEM (~ 20 keV); therefore, wavelength of the electron beam is much smaller and one can get much higher resolution in case of a TEM. Modern high resolution TEMs are able to image even a single atom and they are able to image even the double dumbbell shape of a silicon atom. Thus TEM is a sophisticated high end electron microscope and is highly sought after by professionals engaged in state of the art materials science research. Materials and Instruments: The SEM with EDS used in these experiments was FEI Quanta 250 ESEM. This was used for imaging and chemical identification of the powders. The TEM used in these experiments was 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: Samples were made for imaging and chemical analysis using SEM, SEM-EDS and TEM and subsequently, these samples were analyzed using these high end electron microscopes. Sample preparation for SEM Silica is a non-conducting material and therefore, it cannot be used as such in an SEM. Therefore, these powders were coated with a conducting coating in a sputter deposition chamber. 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; the sample is now ready for examination by an SEM. Examining the Sample in FEI Quanta 250 ESEM with EDAX The coated sample was placed in the SEMsample holder and the chamber was closed. The chamber was then evacuated to very high vacuum level and as the vacuum level was reached, the beam current was put on. BSE images of the powder sample were taken at different accelerating voltages and these images were recorded in the PC for further analysis of size distribution using CARNOY package. Similarly, the sample was prepared for SEM-EDS examination. BSE images were taken at 15 keV accelerating voltage and EDS spectrum of the bright particle was recorded. For making TEM sample, 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: Table 1 presents particle size distribution as obtained from SEM imaging and using CARNOY software program is presented in Table 1. It can be seen that the particle size range is from 10 ?m to 160 ?m. However, most of the particles are in three size ranges 10 ?m - 20 ?m, 50 ?m – 70 ?m and 110 ?m – 120 ?m. This means there are three modes in particle size distribution. The mean particle size is 71.75 ?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 The particle size distribution is presented as a histogram in Fig. 1. In this Fig. one can see three peaks corresponding to three modes in particle size distribution. Such a distribution is termed as multimodal distribution. Such a distribution generally arises from a powder made by mixing powders from different sources. 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. Particles of different sizes and brightness are seen in this figure. In BSE image the contrast is due to differences in the atomic number. A region with higher average atomic number yields more backscattered electrons and appears bright and opposite is true for regions with low average atomic number. Therefore, it can be said from that bright particles in Fig. 2 have higher average atomic number than those appearing relatively dull. In order to identify elements present in the bright particles EDS analysis of the same was done. Fig. 2: BSE Image of the powder sample EDS spectrum of characteristic X-rays coming from the bright particles is shown in Figure 3. In this spectrum one can notice multiple peaks of Yb and peak of oxygen is also seen. Therefore, one can conclude that the bright particles are made of Yb and oxygen and from the knowledge of chemistry it can be further concluded that these particles must be Yb2O3. Fig. 3: Characteristic X-ray Spectrum of Bright Particles RESULTS and ANALYSIS OF EXPERIMENT 3: High magnification images of TiO2 and Fe2O3 particles as recorded by using TEM are presented in Fig. 4 and Fig. 5 respectively. Figure 4 shows spherical and elongated particles of TiO2. On the other hand the particle of Fe2O3 are faceted and equiaxed in morphology. Fig. 4: TEM image of TiO2 particles Fig. 5: TEM image of Fe2O3 particles Conclusions: It can be concluded that SEM, SEM-EDS and TEM are very useful instruments for complete characterization of powders having very fine particle size. The silica powder sample analyzed in these experiments have particles concentrated in three size ranges - 10 ?m - 20 ?m, 50 ?m – 70 ?m and 110 ?m – 120 ?m and the average particle size was 71.75 ?m. The bright particles were analyzed to be Yb2O3 by SEM-EDS. TEM images have shown that particles of titanium oxide are spherical and elongated in morphology while those of ferric oxide are faceted and equiaxed. References: [1] Goodhew P. J. and Humphreys F. J. “Electron Microscopy and Analysis”, Taylor & Francis, London, New York, Philadelphia, 1988 p. 34. [2] www.zeiss.com [3] www.edax.com [4] www.jeol.com Read More