使用激光粒度仪5200对多孔粉末粒径分布的研究

Application Note138 Page 2 of 6Application Note 138 Particle Size Distribution Analysis of Porous PowdersUsing the Saturn DigiSizer 5200 Porous powders find application in many industries these days. These range from catalysts to pharmaceu-ticals； from environmental cleanup to liquid chromatography. Not only is it important to know the poresize distribution of these powders， but it is also crucial to have a reliable particle size distribution analysisof these materials. In general， these analyses can be performed in much the same way as analyses ofnon-porous particles. When using laser-scattering particle size analysis， however， a few additional precautionsare necessary. Laser light scattering has been used for particle size analysis for more than 30 years. In 2000，Micromeritics introduced the Saturn DigiSizer 5200 High-Definition Digital Laser Particle SizeAnalyzer， the first such instrument to utilize a CCD detector for high-resolution particle size distributionanalysis. Because of the high level of resolution and sensitivity of this analyzer， particle size results areinfluenced by all types of light scatter phenomenon， including some not related to size but rather to themorphology of the particle. Not only does light scatter at the surface of the interface between the particle and the suspending medium(often a liquid)， it will also scatter as it passes through the pores of a sample. Since the pores are filledwith the suspending medium， each time the light passes through a pore， it encounters two additional phaseboundaries， and scatters again. The effect directs some light back into the particle， or at very wide anglesfrom the incident light， often away from the light-scattering detector， and definitely in a direction notpredicted by spherical particle scattering models regardless of the size of the particles. This phenomenonis similar but not equal to what would be expected for non-transparent particles. In other words， theabsence of light at wide scattering angles is similar to the effect of absorption of light within the particle.But even a higher degree of absorption used in the scattering model cannot account for all of the missinglight. Due to the high level of sensitivity in the particle size analysis capabilities of the Saturn DigiSizer 5200，the particle size distribution produced for porous materials can be misleading unless that portion of thescattering pattern due to morphology， and not size， is omitted from the particle size calculations. Such canbe accomplished by using a feature of the DigiSizer calculation software. The scattering data can betruncated at a specified scattering angle before the particle size distribution is calculated using non-negative least squares deconvolution methods.This， combined with allowance for some apparentabsorption of light by the particles through the use of an appropriate imaginary refractive index， results ina reliable particle size distribution analysis. Simply using a higher imaginary refractive index is not sufficient to account for the missing light， asillustrated in Figure 1. In this case， a porous catalyst (ZSM5) was suspended in water and analyzed fourtimes using the DigiSizer. A scattering model typical for use with silicas was used to model the expectedlight scattered by spherical particles to produce a particle size distribution for the powder. Figure 1. Overlay of four analyses ofa sample ofZSM5 catalyst powder using the Saturn DigiSizer5200. The sample was dispersed in water containing a small amount of sodium metaphosphate. Thescattering model used was calculated using a particle real refractive index of 1.45， a particle imaginaryrefractive index of 0.100i， and a medium real refractive index of 1.331. Notice that there are a number of modes present at the fine end of the analysis， between 1 and 4micrometers in diameter. Also note that the fit between this particle size distribution and the measuredscattering pattern is not that good at wide angles， as can be seen in the goodness of fit plot for thisanalysis shown in Figure 2. An imaginary refractive index of 0.100i was used in these calculations， whichis much higher than that normally used with transparent materials such as silicas. Since the modelintensity is still above the measured intensity in the goodness of fit plot， there is still less light presentthan predicted by this absorptive model. Figure2. Goodness of fit plot between measured scattering pattern and that predicted from thecalculated particle size analysis ofZSM5 powder using a scattering model of1.45， 0.100i in water. Simply recalculating the results using only a portion of the scattering pattern results in a distributionwithout most of the additional modes at small diameters， extends to a smaller diameter， and fits thescattering data better. This is because less light is scattered at wide angles than predicted for sphericalparticles according to the remainder ofthe scattering pattern. Not using the scattered pattern in the areawhere light is missing results in a wider and smoother particle size distribution. Figure 3 shows the resultof using scattering data only out to 26.2 degrees. Figure 4 shows the goodness of fit for this calculation，and shows that the weighted residual has improved from 24.58% (Figure 2) to 2.05%. Figure 3. Particle size distributions calculated after truncating the scattering pattern at 26.2 degrees. Figure 4. Goodness of fit calculated after truncating the scattering pattern at 26.2 degrees. While this calculation is better， it has not yet been optimized. Further improvement is possible bytruncating the scattering pattern at 15.2 degrees， as shown in Figures 5 and 6. Note that the weightedresidual has improved to 0.15%. Figure 5. Particle size distribution calculated after truncating the scattering pattern at 15.2 degrees. Figure 6. Goodness offit calculated after truncating the scattering pattern at 15.2 degrees. By truncating the scattering pattern in this manner， the morphology portion of the scattering pattern is notused and only the particle size information remains. Further truncation of the scattering pattern results inloss of information from the fine particles in the distribution. When doing this， the distribution becomesnarrower， reversing what had happened up to this point. An overlay of particle size distributions calcu-lated with different ending diameters is shown in Figure 7. Note that the distribution is widest， with thesmallest particles detected， when truncating the scattering pattern at 15.2 degrees. Figure 7. Overlay of particle size distribution calculated at different maximum scattering angles. The angles utilized in this case correspond to the maximum angle at which data are collected for differentmaximum beam angles. Since the DigiSizer moves the scattering beam at increments of 5 degrees， thosescattering angles corresponding to these incident light positions make natural values for truncating thescattering pattern. Note that the incident angle and the scattering angle are significantly different due tothe refractive index of the suspending medium， water in this case. This is because the scattering takesplace inside the sample cell (filled with water)； however， the detector is outside the cell， so that the lightrefracts when moving from water to air. Table 1 shows the nominal scattering angle that corresponds tothe 10-beam angle positions used by the DigiSizer for a number of typical dispersing media. Table 1. Nominal scattering angle equivalent to incident scattering beam angles for the Saturn DigiSizer IncidentBeamAngle Water RI= 1.331 Ethanol RI=1.359 IsopropanolRI=1.376 40 % Sucrosein WaterRI= 1.400 Odorless Mineral SpiritsRI=1.420 MineralOil RI=1.467 0 4.0 3.9 3.8 3.8 3.7 3.6 5 7.7 7.6 7.5 7.3 7.2 7.0 10 11.5 11.2 11.1 10.9 10.7 10.4 15 15.2 14.9 14.7 14.4 14.2 13.8 20 18.9 18.5 18.3 18.0 17.7 17.1 25 22.6 22.1 21.8 21.4 21.1 20.4 30 26.2 25.6 25.3 24.8 24.5 23.7 35 29.7 29.0 28.7 28.1 27.7 26.8 40 33.1 32.4 31.9 31.4 30.9 29.9 45 36.0 35.6 35.1 34.5 33.9 32.8 To use this feature： Open the sample information file in the Advanced mode. Select the MaterialsProperties tab， then click Options to access the Scattering Model Advanced Options dialog. Select theTruncate data at scattering angle option and enter the desired maximum scattering angle. Click Applychanges to recalculate the particle size distribution using the truncated scattering pattern. After theanalyses are finished， a message dialog indicating so is displayed.Click OK to close the dialog and thengenerate a new report with the recalculated results. mi micromeriticsOne Micromeritics Drive， Norcross， Georgia . ( www.micromeritics.com