Microscopic Measurement of Particle Size

The use of ultraviolet light in the microscopic measurement of particle size. J. Franklin Inst., 209 777-789. 

In the case of powders, if the particles are of fairly simple form, the surface area (excluding submicroscopic cracks) can be estimated from microscopic measurement of the size of the particles.5 Powders, or porous solids composed of aggregations of small particles, can have the particle size approximately estimated by the width, and imperfection of definition, of the lines in an X-ray diffraction photograph.6 This method has been used by Levi and others7 for finely divided metal such as platinum black the particles were usually extremely small, of the same order of size as the particles in a colloidal sol of the metal. For the platinum group, the size of crystalline particles was estimated as from 20 to 120 A. across only. 

The major advantage of microscopic methods is their direct measurement of particle size. In many of the alternative methods, at least one automated data-interpretation or calculation step is inserted between the instrumental analysis and establishing the estimate of particle size. This reduces the subjectivity of the measurement while increasing the likelihood of interpretive errors. 

Geometrical surface area. This can be determined from particle size, by measurement of particle size, determination of distribution curves, and calculation of surface area values, with the aid of an electron microscope. Although of fundamental importance, this method is too time consuming for production of characteristic data for everyday use. 

For particle size analyses, ultrasonic extinction is measured for a number of different frequencies consecutively. The whole procedure of measurement, including frequency selection, extinction measurement and mathematical evaluation is automated. One measurement of particle size distribution and particle concentration takes about 2 to 5 minutes, depending on the resolution and the reproducibility required. Some cumulative area density distributions measured by ultrasonic spectrometry, compared to the results obtained from a microscopic evaluation, are shown in Fig. 3. Broad and narrow distributions are clearly distinguished, owing to the high resolution and reliability of the method. 

Microscopic observation of aerosol particles permits direct measurement of particle size. This is in contrast to indirect methods such as sedimentation, impaction, mobility analysis, and light scattering, wherein the particle size is estimated from the measurement of a property related to size. Microscopy also provides the opportunity to observe particle shapes, and it requires only an extremely small amount of sample. Linear measurements made with a microscope can be very accurate and, often serve as a primary measurement for the calibration of other aerosol-sizing methods. However, microscopic methods for determining particle size distributions are, in general, tedious and require consistency, skill, and careftd preparation. 

Microscope Methods In microscope methods of size analysis, direct measurements are made on enlarged images of the particles. In the simplest technique, linear measurements of particles are made by using a cahbrated scale on top of the particle image. Alternatively, the projected areas of the particles can be compared to areas of circles. 

Nitrogen adsorption isotherms were measured with a sorbtometer Micromeretics Asap 2010 after water desorption at 130°C. The distribution of pore radius was obtained from the adsorption isotherms by the density functional theory. Electron microscopy study was carried out with a scanning electron microscope (SEM) HitachiS800, to image the texture of the fibers and with a transmission electron microscope (TEM) JEOL 2010 to detect and measure metal particle size. The distribution of particles inside the carbon fibers was determined from TEM views taken through ultramicrotome sections across the carbon fiber. 

Testing. Chemical analyses are done on all manufactured abrasives, as well as physical tests such as sieve analyses, specific gravity, impact strength, and loose poured density (a rough measure of particle shape). Special abrasives such as sintered sol-gel aluminas require more sophisticated tests such as electron microscope measurement ofor-alumina crystal size, and indentation inicroliaidiiess. 

The diameter of latex particles was measured from their transmission electron micrographs which were obtained by use of a Hitachi electron microscope HU-12AF. The uniformity ratio of particle size (U) was calculated from eq. 2 ... 

Microscopic measurements for the determination of particle size of pigments and powders." Ind. Eng. Chem., Anal. Ed., 2 59-62. 

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