What are the common problems of particle size testing and the difficulties in the development of particle size analyzers?

The laser particle size analyzer refers to a particle size analysis instrument that uses a laser as a detection light source. The development of laser particle size analyzers in our country began in the 1980s. Tianjin University, Jinan University, University of Shanghai for Science and Technology, Dandong Institute and other units have done a lot of work and have made significant breakthroughs in the past decade.

The laser particle size analyzer uses a laser light scattering method. This method has a wide measuring range (usually 0.1 to 3500 μm), fast particle size analysis, good reproducibility, and online measurement. It is also suitable for scientific research and production process. Control plays an important role. In addition, the laser particle size analyzer can obtain a variety of particle size data, such as volume, average particle size, specific surface area, interval particle size distribution, and cumulative particle size distribution.

First, the four principles of laser particle size analyzer

1, based on the principle of static light scattering, that is, according to different sizes of particles scattered on the light have different angular distribution principle of measuring particle size of the instrument;

2, based on dynamic light scattering (photon correlation spectroscopy) principle, that is, according to the Brownian motion of tiny particles (usually less than 1μm) in the liquid, causing the frequency of scattered light to move (or phase change), scattering light interference with each other, making a certain observation point The principle of the change of the scattered light intensity with time, the instrument for measuring the particle size;

3, Photoresist particle counter;

4, light pulsation particle size analyzer.

Second, the common problems in the process of particle testing

1, the particles are too small

The finer the particles, the smaller the angle of scattered light. When the particle geometry reaches the sub-micron level, scattered light scattering angle greater than 90 degrees will be significantly enhanced. Scattered light from tiny particles is even distributed within 360 degrees, as shown in Figure 1.

2, too large particles

The scattering angle of large particles is very small and it is not easy to distinguish and measure, as shown in Figure 2. When the particle geometry is more than 500 microns, the scattered light scattering angle is extremely small, and the scattering angle of the scattered light generated by the difference between the particle size and the time scale is small, and the prior art is not easy to accurately measure.

To effectively resolve the light intensity distribution of large particles, it is possible to simply lengthen the focal length of the focusing lens (for example, 500 or even 1000 mm or more), but a large focal length will cause the size of the laser particle size analyzer to increase significantly and it is very inconvenient for large particles to be large angles. Scattered light detection. At the same time, the processing precision of the lens will be higher. This technical difficulty makes it difficult for a conventionally designed laser particle sizer to actually exceed the upper limit of 1000 microns.

3.There is a blind spot for scattered light detection. Regardless of the design of the laser particle size analyzer, there is a measurement window in which the sample is fully dispersed in the window and is irradiated with laser light to generate scattered light. As shown in FIG. 3 , due to the total reflection of the mechanical structure and the optical glass in the conventional measurement window, there is always a scattered light detection dead zone. This blind spot is roughly distributed in the area of ​​75-105 degrees and 255-285 degrees.

4, scattered light intensity is too small

The smaller the particles, the smaller the scattered light intensity distribution in the 360-degree spatial range. When the particles are small to a certain limit, the light intensity difference will be almost too small to be distinguished. At this point, the measurement limit of the laser particle size analyzer was reached. As can be seen from Figure 4, when the particles are small to a certain extent, the vector of light intensity is infinitely close to a circle (the particle is infinitely close to the center of the circle), and the light intensity difference at this time is difficult to distinguish. The obstacles in the optical design and the nature of the scattered light itself determine that the lower limit of measurement of conventional laser particle size analyzers is generally around 0.2 microns.

Third, the prospect of China-made laser particle size analyzer

As a mainstream instrument for sub-micron particle measurement laboratories (as opposed to on-line), the Chinese product brand of laser particle size analyzer has occupied a major market share in China, but in terms of technical level, domestic equipment is in the sub-micron range and widely distributed samples. In terms of measurement, there is still a certain gap compared to the advanced level in the world.

Although our country’s granularity testing technology has made great progress, we should work harder on the original work, especially from the whole machine or the entire method introduced by the Chinese people.

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