1 Introduction
There are many methods for powder particle size measurement. The traditional methods mainly include sieving method, microscope method, Coulter induction method, gravity sedimentation method, centrifugal sedimentation method, etc. These test methods generally have cumbersome and long test time. , can not be measured on-line and other shortcomings, has become less and less suitable for the requirements of modern industrial production and scientific research. Laser particle size analyzer is a kind of widely used at present. It has a wide range of particle size measurement, wide application range, good reproducibility, fast measurement speed, convenient operation and online measurement. A variety of particle size data can be obtained by one measurement. , such as volume average particle size, particle size curve, interval particle size distribution and cumulative particle size distribution, etc.
2 Principle and structure of laser particle size analyzer
The laser particle size analyzer is mainly manufactured according to two optical theories of Fraunhofer diffraction and Mie scattering . Its theory points out that when the light passes through the inhomogeneous medium, the scattering phenomenon . The scattered light includes particle size, shape, structure and Information such as composition Therefore, the size distribution of particles can be measured using light scattering techniques . Figure 1 shows the classic optical structure of a laser particle size analyzer, which consists of three parts: emission, acceptance and measurement window. The light emitting part mainly provides monochromatic parallel illumination light for the instrument. The receiving part is the key to the optical . The measurement window is mainly to let the measured sample pass through the measurement area in a completely dispersed state, so that the instrument can obtain the particle size .
3 Major laser particle size analyzers at home and abroad
The performance of the laser particle size analyzer is mainly reflected in the particle size measurement range, the type of laser source, the shape and size of the Detector, the scanning speed , accuracy and stability. The following introduces several typical laser particle size analyzers produced by major instrument manufacturers at home and abroad .
3.1 Main foreign laser particle size analyzers
(1) Mastersizer 2000 developed by British Malvern Company applies Mie theory throughout. Mastersizer 2000 uses an inverse Fourier transform optical system, the Fourier lens is located between the measurement window and the laser , the He-Ne gas laser with a wavelength of 632.8nm is used as the main light source, and the solid blue light with a wavelength of 466nm is used as an auxiliary light source, and the built-in non-uniform Cross-arranged three-dimensional fan-shaped detection system, which is equivalent to 175 circular or cross star-shaped and 93 semi-circularly arranged, can make the direct detection angle reach 135°, the scanning speed can reach 1000 times/s, the resolution is high, and the measurement The lower limit reaches 0.02 μm. The circulation system adopts a centrifugal circulation pump with adjustable speed , and the circulation pipe is an ordinary plastic pipe, which is easy to replace. It can be configured with two sampling devices, dry method and wet method, with a measurement range of 0.02 μm to 2000 μm and a repeatability of ±0.5%.
(2) Germany's Sympatec is famous for its dry particle size measurement. Its HELOS/RODOS is the world 's first patented dry laser particle size analyzer, which can effectively measure dry powder as fine as 0.1μm. Dispersion, less sample consumption, good repeatability, measuring range 0.1μm ~ 3500μm. HELOS/RODOS adopts an optical structure similar to that in Figure 2, uses a 5mW, 632.8nm He-Ne gas laser as the light source, and adopts 8 groups of Fourier lenses with different focal lengths, and automatically changes the lens according to the particle size distribution range of the sample. It has 31 built-in sensors with a scanning speed of 2000 times/s. Because it adopts the Fraunhofer theory, the measurement results can be obtained without knowing the refractive index of the sample, which is convenient for users. Due to the use of dry sampling, HELOS/RODOS combined with automatic sampling manipulator can realize online detection, which has been applied in the cement industry.
(3) The LS13320 produced by Mie theory and Fraunhofer theory throughout the process, with a measuring range of 0.04 μm to 2000 μm. It uses 5mW, 750nm solid-state semiconductor laser as the main light source, 12W, 450nm, 600nm, 900nm as auxiliary light source, with a service life of up to 70,000 hours , no need for preheating, and no loss when not working. It is equipped with 132 Detectors, which are fan-shaped and cross-arranged within the range of 0~135°. Each Detector is equipped with an independent integral noise reduction amplifier, which can accurately capture scattered light at large angles, and the reproducibility reaches ±1. %. Figure 2 is the optical structure diagram of LS13320 . It adopts double Fourier lens and polarized light intensity difference (PIDS) technology to ensure the measurement accuracy of 0.04μm~0.4μm particles. Information is available in a single analysis. Its circulation system adopts a speed-adjustable high-speed centrifugal circulation pump, and the circulation tube is a silicone tube. After the measurement, it automatically completes the intelligent cleaning of the sample measurement window, sample tank and pipeline.
(4) The LA-950V2 full-range has a measuring range of 0.01 μm to 3000 μm and a reproducibility of ±0.1%. LA-950V2 adopts dual solid-state light sources (5mW, 650nm semiconductor laser as the main light source, 3mW, 405nm LED as the auxiliary light source) configuration, 87 effective Detectors are arranged in a logarithmic cross, and equipped with side Detectors and large angle (155°) backward Detector to realize seamless detection of scattering space. LA-950V2 adopts a unique overlapping inverse Fourier optical system, which increases the scattering optical path by 4 times, and the scanning speed reaches 5000 times/s. LA-950V2 adopts a fully automatic built-in circulation, dispersion and sampling system, and a high-speed centrifugal circulation pump with adjustable speed, and the circulation tube is a silicone tube. LA-950V2 can be equipped with dry and wet sample injectors. The switch between dry and wet methods is very convenient and fast, without any tools, just push and pull with one hand to complete the switch between , and the measurement software can also automatically identify it.
(5) CILAS-1190 produced by CILAS company in France , the measurement range of dry dispersion is 0.1-2500 μm, and the measurement range of wet dispersion is 0.04-2500 μm. CILAS-1190 uses semiconductor laser diodes and optical fibers as the laser source. In order to fully ensure the accurate positioning of the laser, it uses a short optical bench as the optical long-term correction system. All optical components are fixed on the cast iron base. The spatial geometric design of the iron base ensures that the laser, measurement window, Fourier lens and Detector are firmly matched together, ensuring that the Detector collects all laser scattering signals. There is no need to recalibrate the optical system or replace the lens during the test , and the reproducibility of the measurement results is better than ±1%. Its circulation system uses a built-in peristaltic pump, which does not contaminate the sample, but the pump tube needs to be replaced frequently. CILAS-1190 can be connected with a CCD video camera to observe the dispersion and aggregation of particles, and can observe the shape of particles with a particle size larger than 20 μm.
3.2 Main domestic laser particle size analyzers
(1) The LS900 produced by Zhuhai Euro-American Technology Co., Ltd. has a measuring range of 0.05-500 μm. It mainly adopts wet method for sampling, and the scanning speed is 1000 times/s. The reproducibility of the measurement results is less than ±3%. LS900 also adopts inverse Fourier transform optical system, adopts 2.0mW integrated He-Ne gas laser as light source, 53 independent detection units are arranged in a circle, and applies the spherical receiving technology of large-angle scattered light in Figure 3 to detect large-angle The Detectors are arranged on a suitable spherical surface to achieve precise focusing of scattered light at large angles . It uses the backscattered light receiving technology to extend the lower limit of measurement to 0.1 μm, and the bidirectional polarization compensation technology can eliminate the fluctuation of scattered light intensity caused by the competition of the polarization mode of the laser, and overcome the problem of the decrease in the measurement accuracy of small particles caused by the competition of the polarization mode , to ensure that the lower limit of instrument measurement extends to 0.05 μm. Its circulation system uses a 200W booster pump with a maximum speed of 2800r/ min, which can effectively realize the circulation of large particles.
(2) The Bettersize2000 produced by Dandong Better Instrument Co., Ltd. applies the Mie scattering theory and adopts a wet method for sample injection. The measurement range is 0.02 μm to 2000 μm, and the reproducibility of the measurement results is less than ±1%. It adopts the single-beam double-lens optical structure shown in Figure 4, uses a solid-state semiconductor laser as the light source, has an array of 90 sector-shaped photoDetectors , and adopts a layout of 80 forward and 10 backward Detectors, ensuring Effective reception of particle scattering signals of various particle sizes. It has a built-in automatic circulating dispersion system and automatic centering system, including an ultrasonic disperser that can run with an empty cup, a centrifugal circulating pump with continuously adjustable flow rate and a water level measuring device.
3 ）济南微纳仪器有限公司生产的Winner2000ZD，采用湿法进样，测量范围分为3档（0.1～40μm，0.6～120μm，1～300μm）。它采用图7所示的反傅里叶变换光学结构，以2mW的He-Ne激光器作为光源，该结构的优点是最大接收角不受傅里叶透镜孔径的限制，将焦距缩至最短，有效提高仪器的分辨能力，同时采用精密四项混合式步进电机，自动进行光路调整，消除了手动对中光路时带来的麻烦和困难，使重现性小于±1%。它可以采用多种粒度的国家标准物质，对仪器进行全量程校准标定，保证了测量结果的准确性。
（4）成都精新粉体测试设备有限公司生产的JL-6000全量程Mie散射理论，采用干法和湿法进样，测量范围0.02～2000μm，测量结果重现性小于±1%。它应用经典傅里叶变换光学系统，以波长632.8nm的He-Ne激光器作为主光源、紫光源为辅助光源，80个检测器呈前、后、侧三维排列，最大检测角度165°。另外，它的离心泵循环系统、超声波分散、样品池都设置于主机内部，全密封抗干扰能力强，有效的防止粉尘污染。
4 激光粒度仪的发展方向
激光粒度仪的应用极大地提高了粒度测试效率，保证了粉体产品的质量，促进了粉体技术的研究、生产和应用。随着粉体技术研究的进一步发展，对激光粒度仪的要求也越来越高。未来激光粒度仪的发展主要有几个方向：
(1) Further improve the light scattering theory of non-spherical particles , further optimize the optical path system, and design a laser particle size analyzer with a wider range and higher resolution;
(2) The progress of processing technology for various difficult samples including extremely heavy, extremely light, difficult to disperse, extremely low surface activity, and mixed substances with complex ;
(3) On-line in-situ analysis of samples and development of on-line dry and wet analysis systems to improve the measurement accuracy of on-line analysis instruments;
(4) A reliable and accurate algorithm , especially for the inversion of multi-peak and narrow-distribution particle samples, to improve the measurement accuracy.
references
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2 Zhang Fugen. Optical structure of laser particle size analyzer[C]. Proceedings of the 2006 Annual Meeting of Chinese Particle Society and Cross-Strait Particle Technology Symposium, Beijing: 2006.
3 Yang Zhenghong. How to judge and choose a laser particle size analyzer[J]. Modern Scientific Instruments, 2000, (1): 58~59.
4 Yang Yuying, Xie Qinghong, Zhao Hong, etc. LS230 laser particle size analyzer and its application [J]. Modern Scientific Instruments, 2002, (3): 41~43.
5 Li Xiangzhao, Xie Kang, Huang Zhifan, etc. Technology development and prospect of laser particle size analyzer [J]. Modern Scientific Instruments, 2009, (4): 146~148.
About the author: Liu Shu, Ph.D candidate, engineer, mainly engaged in the testing and characterization of inorganic materials.
