Progress and Application of Contemporary Laser Particle Analysis Technology
Either in Beijing
( Institute of Particle Testing, Shandong Institute of Building Materials , Jinan 250022, China )
Abstract: This paper introduces the latest progress of major contemporary laser particle analysis technology. The content involves the development of test principles, the improvement of instrument structure, the breakthrough of data processing technology, the treatment of multiple scattering, the diversification of sample dispersion systems, the influence of particle shape on testing, particle scattering model, industrial online application and so on. Application problem .
Keywords : laser , powder , particle , scattering , test
1 Introduction
The famous physicist Feynman once said : If due to some kind of catastrophe , all scientific knowledge is lost , only one sentence
Passed to the next generation , then how can we use the least vocabulary to express the most information ? I believe this sentence is an atomic assumption , all objects are made up of atoms [1] . " What is the significance of visible material composition in human civilization .
20th century, people in the physical world macro and micro has been considerable in-depth understanding, but for a large number of physical phenomena between microscopic particles to macroscopic objects are poorly understood. Particles are the intermediary between the two . For example, large particles mainly exhibit solid characteristics . As the particles become smaller , the fluidity is significantly enhanced , much like a liquid ; the particles become smaller , and it will fly like a gas ; the particle size is smaller , its surface area increases rapidly , and the surface molecules are in a large state. The particles are completely different and the properties of the particles will mutate , showing some shocking quantum properties ! Now , many excellent in the world
Scientists are working hard in this mesoscopic field , and a large number of materials with special properties will be born in this field . Lead to particles
The first feature of such a change in properties is its size [2] . Particle size is also very important in people's lives and production . If the cement particles are finely ground , the early strength of the cement will be significantly improved ; the finer the particle size of the drug , the better the absorption of the body ; the finer the magnetic recording material , the higher the storage density . There are countless examples of this . Therefore , the ultrafine refinement of particles has become an improvement
An important means of material performance . It is not surprising that particle size measurements are valued by people . In order to determine the particle size ,
Almost all the methods that can be thought of [3, 4] . Due to space limitations , this article only introduces the overview of laser particle analysis technology .
2 laser how to measure particle size
There are many methods for laser particle size measurement, including light scattering, light diffraction, Doppler effect, photon correlation spectrum, light transmission method, extinction method, photo counter, holography, etc. By detecting the scattering spectrum distribution of the particle group, the laser scattering ( diffraction ) particle analysis technique of its size and distribution is analyzed. It is well known that a beam of parallel laser light is incident on the particles, and the famous Fraunhofer diffraction will occur. The diffraction light will be collected using a Fourier transform lens, and the diffraction spectrum of the particle can be obtained at the back focal plane of the lens. If the particle is a sphere, the diffraction spectrum is a well-known Airy pattern, and the center of the Airy spot diameter is inversely proportional to the particle diameter [3] . If a concentric ring-shaped photodetector is placed on the back focal plane for the detection of the diffraction spectrum, and then combined with a signal processing system , it constitutes a basic laser diffraction particle analysis system [5] ( see Figure 1) .
When no particles are present in the beam, the light converges at the center of the detector ; when the small particles enter the beam , the intensity distribution of the detector is wider; when the large particles enter the beam, the intensity distribution of the detector is narrower. If the particle detection area enters the particle group with a certain particle size distribution , the output of the detector is a linear superposition of the diffraction spectrum of all the particles, and the inversion technique can be used to reverse the particle size distribution of the measured particle group according to the diffraction spectrum [6] . The laser diffraction particle analysis system is suitable for particles with a particle size larger than the laser wavelength. The measurement range is above 6Lm . The upper limit of measurement depends on the focal length of the lens. It is known to be up to 2000Lm. The advantages of the laser particle analysis system are very prominent, including (1) The measuring speed is fast, other methods can't be compared; (2) The measuring process is highly automated, free from human factors, accurate and reliable; (3) The diffraction spectrum is only related to the particle size, and has nothing to do with the physical and chemical properties of the particles. It has a wide range of applications.
3 from diffraction to scattering
The main disadvantage of the laser particle analysis system using the diffraction principle is that the measurement error is large in the small particle range, and in particular, the size of the submicron particles cannot be measured. With the advancement of particle technology, the particle size is rapidly developing to ultra-fine, and Fraunhofer diffraction can not meet the test requirements, and a more accurate Mie theory must be adopted .
The Mie scattering theory is a rigorous analytical solution of the scattering field distribution of spherical particles to monochromatic light. Fraunhofer diffraction is an approximation of the Mie scattering theory under specific conditions. Mie scattering theory states that when the particle diameter is much smaller than the incident light wavelength, the forward scattering and backscatter field distribution of the particle are symmetric; when the particle diameter is similar to the incident light wavelength, the forward scattering is stronger than the backscattering, and the scattering The field is periodically distributed with respect to the incident optical axis; when the particle diameter is much larger than the incident light wavelength, the particle will have only a forward scattering field, which is consistent with the Fraunhofer diffraction theory ( see Figure 2) . It can be seen that the Mie scattering theory is more widely and more accurate than the Fraunhofer diffraction theory [7, 8] .
In order to adapt to the measurement of small particle scattering spectrum, the optical path has also undergone major changes, and the original parallel light route is replaced by a concentrated optical path. After the particle sample is changed from the front of the lens to the lens, the acceptable scattering angle reaches 70b . The improved particle analysis new optical path measurement range from 0.1u m to hundreds of u m , the measurement range can be easily adjusted by changing the sample position without having to change the lens [9] . At this point, Mie scattering theory officially served as the protagonist of particle analysis.
4 multiple scattering
Laser scattering particle analysis requires, in principle, that the particles to be measured are randomly dispersed in the same plane perpendicular to the optical path. However, this requirement is actually difficult to achieve. For example, the dry powder is usually three-dimensionally distributed from the nozzle. The front particles scatter the parallel laser, and the scattered light encounters the subsequent particles to scatter again. This process is repeated several times, and the scattering spectrum is repeated. The distribution is greatly broadened and this phenomenon is called multiple scattering. It can be shown that the N-th complex amplitude of light scattering is a complex amplitude of light scattering from single N fold convolution. The thicker the particles are distributed, the more severe the scattering spectrum broadens, and the particle analysis results will be severely shifted to small particles. In order to suppress multiple scattering, various methods have been adopted.
Chinese scholars analyzed the relationship between multiple scattering and particle concentration, and found that there is still an optimal diffraction concentration when the particle is three-dimensionally distributed. At this concentration, multiple scattering can be effectively suppressed [10] . The thicker the particle distribution, the smaller the optimum diffraction concentration. Under the guidance of this theory, the dry powder laser particle analyzer developed in China can be compared with the wet laser particle analyzer.
5 inversion - the pursuit of real efforts
Our measurement objects rarely have a single particle size collection, often a particle size distribution. The spectral distribution we measured is the integral of all particle sizes by the particle scattering spectrum distribution weighted by the particle distribution function. The inversion operation in the particle analysis is to reverse the particle size distribution through the measured spectral distribution ( the scattering spectrum distribution of the particles is known as a theory ) . The correctness of the inversion is directly related to the success or failure of this technology. This article does not want to comprehensively discuss the inversion technique, only briefly introduces the two inversion ideas. One popular method is to assume that the measured particle size obeys a certain distribution function ( such as normal distribution, lognormal distribution, R - R distribution, etc., and then iteratively obtains the distribution parameters. If the presumption is wrong) Then the inversion result must be wrong.
How can we obtain real and reliable results ? Chinese researchers have developed an unconstrained free-fitting inversion technique that does not impose any constraints on the particle size distribution function, so that each weighting factor independently approaches the optimal value. This technology has been applied to the instrument and achieved good results, improving particle size resolution and ensuring the true reliability of the inversion results. This technology also has application value in other occasions.
6 Is the size related to the shape ?
It is generally believed that the size of an object and the shape of the object are two concepts that are not related to each other. Recent studies on granules have shown that particle size characterization is not only related to particle shape, but also to particle testing methods, which is probably unexpected. Take the sedimentation method as an example to illustrate. In the gravitational field, the final settling velocity of a non-spherical particle A is the same as the final sedimentation velocity of another homogenous spherical body B. The particle size of the particle A is defined as the spherical diameter of the particle B , which is called the sedimentation particle size. The actual volume of the two is not the same. In contrast, two particles of the same volume have different sedimentation particle sizes if they are different in shape, one is a sphere and the other is aspheric. From this it appears that the particle size is related to the shape. Similar to the sedimentation method, the particle size measured by the laser scattering method is also related to the shape. For two particles with the same cross-sectional area, the diffraction spectrum of the non-sphere is wider than the spectrum of the sphere. If the non-sphere is measured by the spherical diffraction spectrum, the test result is too small. In order to solve this contradiction, Chinese scholars introduce an elliptical particle diffraction model, that is, the smallest outer diameter of the non-spherical particles is the long axis, and the largest inner diameter is the short axis. The ellipse is the elliptical model of the particle. The development of the spherical model of the particle to the elliptical model is an advancement in granulation. The essence of the elliptical model is to recognize that the particle size is related to the shape of the particle and to introduce the shape factor into the category of size. The introduction of the elliptical model lays a theoretical foundation for the analysis of laser particles for non-spherical particles and effectively improves the measurement accuracy [12, 13] .
7 from the laboratory to the first line of industrial production
In fact, the particle testing line already needs a particle online inspection device. For example, the main function of the grinding equipment is to grind the raw materials, so the particle size becomes the primary detection index of the grinding process, but whether it is the sedimentation method or the Coulter method, whether it is the image method or the ultrasonic method, it is difficult to carry out this task. . At present, people can only judge the working state by detecting the sound of the mill load and the sound of the monitoring mill. As for the product granularity, it takes several hours to sample at intervals, go to the laboratory for analysis, and then return to the site to adjust the mill. As a result, the product is too rough or overgrown, and the waste caused cannot be calculated. Now, the emergence and maturity of laser particle analysis technology has made it possible for online testing of particles. In addition to the many advantages mentioned above, laser particle analysis technology has not yet attracted people's attention: (1) it can be used for real-time particle analysis of moving particle groups; (2) it is not only suitable for particles in liquids, but also Suitable for particles in gases. All of these advantages are doomed to be the protagonist of online particle size testing in modern particle production lines. The application of this technology in the grinding system will definitely change the control mode of the mill, the mill will exert greater potential and the energy consumption will be maximized. China's particle size online measurement and control research work on airflow pulverizers has achieved gratifying results. It is expected that many processes, such as powder selection, granulation, spraying, drying, and crystallization, will be the responsibility of online analysis by laser particle analyzers. By then, the potential of such technology can be fully utilized [11, 14] .
8 Conclusion
The research of laser particle analysis technology started in the 1970s , but it is only 20 years ago. It has far surpassed traditional analysis methods in measurement accuracy, measurement speed, resolution, dynamic detection capability, etc. in many laboratories around the world. Production enterprise applications show unparalleled superiority, and more and more products are choosing laser particle analysis technology as product inspection standard. This technology has not stopped, and there are compelling developments and innovations every year [15 -1 7] . No wonder some people exclaimed that a laser whirlwind is being blown up in the field of material testing. It can be expected that this advanced particle analysis technology will play an increasingly important role in China's material science research and powder industry modernization.
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