Application and development of laser particle size analyzer1

1. Introduction of laser particle size analyzer The laser particle size analyzer analyzes the particle distribution by measuring the diffraction spectrum of the particle group and processing it by computer. It can be used to measure the particle size distribution of various solid particles, mist droplets, bubbles and any two-phase suspended particulate matter, and to measure the particle size distribution of moving particle groups. It is not limited by the physicochemical properties of the particles. This type of instrument has a wide measurement range (the measurement range can reach 0.02~2000 microns, some even wider) because of its ultrasonic, stirring and circulating sample dispersion system; high degree of automation; convenient operation; fast test speed; measurement results Accurate, reliable and repeatable. It can be widely used in the determination of particle size of petrochemical, ceramics, dyes, cement, coal powder, abrasive materials, metal powder, sediment, ores, fog droplets, emulsions, etc. Such as domestic JL, WJL series laser particle size analyzers, British-made Mastersizer series laser particle size analyzers, and PCS nano particle size analyzers that can be used to measure nanoscale. 2. The basic principle of measurement The laser particle size analyzer measures the particle size distribution according to the physical phenomenon that particles can scatter laser light. Because the laser has good monochromaticity and strong directionality, a parallel laser beam will irradiate to an infinite distance in an unobstructed infinite space, and there will be little divergence during the propagation process. When the beam encounters a particle block, a portion of the light will scatter. The propagation direction of the scattered light will form an angle with the propagation direction of the main gloryθ. Scattering theory and results show that the scattering angleθThe size of the particle is related to the size of the particle, the larger the particle, the greater the amount of scattered light produced.θThe smaller the angle; the smaller the particle, the moreθthe bigger the angle. The classic optical route of the laser particle size analyzer consists of three parts: emission, reception and measurement windows. The emission part is composed of a light source and a beam processing device, mainly providing monochromatic parallel light as illumination light for the instrument. The receiver is the key to the optical structure of the instrument. The measurement window is mainly to let the sample to be tested pass through the measurement area in a completely dispersed suspension state, so that the instrument can obtain the particle size information of the sample. The receiver consists of Fourier mirror selection and photodetector array. The so-called Fourier lens selection is a lens selection that eliminates aberrations when the object side is at infinity and the image side is at the back focal plane. The optical structure of the laser particle size analyzer is an optical Fourier transform system, that is, the observation plane of the system is the back focal plane of the system. Since the light intensity distribution on the focal plane is equal to the square of the modulus of the mathematical Fourier transform of the light amplitude distribution function of the object (no matter where it is placed in front of the lens), the spectrum of the object light amplitude distribution. The laser particle size analyzer places the detector on the back focal plane of the lens, so parallel light in the same direction of propagation will focus on the same point on the detector. The detector consists of multiple concentric rings centered on the optical axis, and each ring is an independent detection unit. Such detectors are also called annular photodetector arrays, or photodetector arrays for short. After focusing, low-pass filtering and collimation, the laser beam emitted by the laser becomes parallel light with a diameter of 8~25 mm. Scattering occurs when the parallel beam hits the particles within the measurement window. After the scattered light passes through the Fourier lens, the light of the same scattered angle is focused on the same radius of the detector. The photoelectric signal output by a detection unit represents the scattered light energy in an angular range (the size is determined by the difference between the inner and outer radii of the detector and the focal length of the lens), and the signal output by each unit constitutes the distribution of scattered light energy. Although the intensity distribution of scattered light is always large in the center and small at the edge, because the area of ​​the detection unit is always small inside and outside, the peak of the measured light energy distribution is generally on a certain unit between the center and the edge. When the particle diameter becomes smaller, the distribution range of scattered light becomes larger, and the peak value of light energy distribution also moves outward. Therefore, particles of different sizes correspond to different light energy distributions, and conversely, the particle size distribution of the sample can be inferred from the measured light energy distribution. The lower measurement limit is an important technical index of the laser particle size analyzer. The improvement of the optical structure of the laser particle size analyzer is basically to expand its lower measurement limit or the resolution of the small particle segment. The basic idea is to increase the measurement range of scattered light, measurement accuracy or reduce the wavelength of illumination light. 3. The structure of the laser particle size analyzer The composition of the instrument system mainly includes three parts, 1) the host (optical element), marked as MasterSizer 2000; the host is used to collect the raw data for measuring the particle size in the sample. 2) Accessory (sampler), identified as Hydro 2000G (normal wet method); the only purpose of the accessory is to fully disperse and mix the sample and transmit it to the host for measurement. 3) Computer and Malvern measurement software; Malvern software can define and control the entire measurement process, and simultaneously process the measured particle size distribution data, display the results and print reports. 4. Application of laser particle size analyzer Laser particle size analyzer based on light scattering theory has been widely used in powder metallurgy, thin film, diaphragm material, catalyst, insulating material, lubricating oil, superconductor, radio technology and other industries, involving chemical, pharmaceutical, food and other industries. , building materials and other industrial fields and play an increasingly important role.

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