In order to achieve higher quality, advanced functionality and improved sophistication for processes involving fine particulate material, recent advances in particle optimization have focused on particle shape as well as the more traditional particle size distribution.
Coating films and porous material obtained from fine particles (size of a few microns) are used in electromagnetic materials, drugs and chemical tablets as well as ceramics. In particular, achieving the optimal adjustment of particle characteristics for coating film for battery electrodes has become increasingly important. For example, research has been carried out into improving the recharging-discharging characteristics of batteries by better osmosis and permeation of the electrolyte relative to the void structure of the coating film though adjustment of the shape of the fine particles. This application note will demonstrate the effect of particle shape adjustment on the film coating and will introduce appropriate instrumentation for measuring the particle shape of the graphite material used.
A variety of dry particle processing equipment is available. For this research a batch-type particle shape adjustment mechanism that operates on rotational impact and shear-compression force, combined with a high-speed dispersion and mixer were employed.
The Sysmex Flow Particle Image Analyzer FPIA-3000 was used to analyze the shape of the particles as well as the size. The sample is dispersed in a liquid and is introduced to the instrument which uses a sheath flow mechanism to force the sample into a planar flow. The sample is transversely illuminated with visible light and images of the particles are captured by a camera perpendicular to the flow. From the contour coordinates of each particle a number of size and shape indexes can be calculated.
|
Figure 1 shows an example result with distributions of size (in this case Circular Equivalent Diameter (CED)) and shape (in this case Circularity) and a correlation between the two in terms of a scattergram.
The shape of graphite particles that had been processed with and without high-speed dispersion where analyzed and the resulting scattergrams in terms of CED vs Circularity are shown in Figure 2. The proportion of particles present in each sample that were larger than 3 µm is shown in Table 1 along with the mean CED and mean circularity after exclusion of particles smaller than 3µm from the final result. Example images of the large particles are shown in Figure 3.
|
|
% particles >3µm | Mean volume based CED (all particles <3 µm excluded) | Mean Circularity (all particles <3 µm excluded) | |
|---|---|---|---|
Without high speed dispersion | 61.97 % | 26.48 µm | 0.878 |
After high speed dispersion | 40.22 % | 11.49 µm | 0.912 |
|
Differences were found between the two samples: The sample that was not treated with the high-speed dispersion machine contained a higher percentage of larger particles (>3 µm) than the treated sample. Moreover the former sample also presented a lower overall circularity. The results indicate that treatment of the material with a high- speed dispersion machine reduces the amount of non-circular agglomerated particles present. Therefore analysis of particle shape along with particle size better enables the assessment of the particles with respect to their effectiveness in a coating film.
Raw carbon material is usually in the form of flaky particles as shown by the SEM image in Figure 4a. Treatment of the raw carbon material with the particle processing machine leads to more spherical (conglobulated) particles as time elapses and the revolving speed of the machine increases. An SEM of such a conglobulated particle is shown in Figure 4b. Therefore, analyzing the shape of the material as well as the size distribution with an instrument such as the FPIA-3000 allows a better understanding of the packing structure of the resulting coating film and therefore the permeability and recharging-discharging characteristics.
|
|
The particle specimen in the form of a paste is spread out to prepare a thin green coating film, using a scalpel, it is subsequently allowed to dry and is pressed with rollers to a uniform thickness. The coating film is immersed in resin liquid allowing it to be impregnated with the resin and to be solidified. Then, the solidified coating film is sectioned across the top face exposing newly cut-faces which are photographed with an electron microscope. The SEM images are processed to allow the contour coordinates of individual particles to be detected, and imaginary circles of various diameters are superimposed on these faces: consequently, void distribution information is obtained which indicates the probability of voids that successfully form in spaces between particles.
|
A number of coating films were prepared using different particle shapes. An example of a cross-section of the packing structure of one of them is shown in Fig. 5. A coating film comprising of a greater percentage of spherical particles has wider pore size distribution and greater mean pore size as illustrated in Figure 6.
|
A coating film is placed on a filtering material. Using a fixed displacement pump, water is allowed to vertically permeate through this test piece: during the course, the resultant differential pressure is measured with a micro pressure difference manometer. When the differential pressure is similarly measured with varied permeating flow rate, a value can be determined which defines the relation between pressure loss and flow velocity and is equivalent to the permeability coefficient of the Kozeny-Carman formula for fluid dynamics. It was found that at a given void ratio of a particle material, permeation pressure with the material decreases with increasing pore diameter, consequently, the more spherical material permits better permeation of fluid.
A single cell is constructed using a coating film as a negative electrode, and its recharging-discharging characteristics are determined with a constant current, constant voltage system. Various coating films were used, for which each coating film type featured different shaped particles.
It was found that when a greater portion of particles in the coating film was spherical, the pore size distribution was greater and the electrolyte permeated through the coating film more readily. The plot presented in Figure 7, shows that the use of spherical conglobulated particles in a negative electrode material results in improved performance and helps improve high-rate discharging performance of a secondary battery, particularly as the electrode thickness increases.
|
Material with appropriately designed particle shape as well as size will help increase the potential of control for a number of fine particle applications. The example of coating film for battery electrodes presented here demonstrates that the recharging-discharging characteristics of a secondary battery can be improved by adjusting the particle shape thereby altering the packing structure of the coating film. This promotes the permeation of electrolyte through the film. Therefore the packing structure can be controlled by using the appropriately shaped particles.
The Sysmex FPIA-3000 is an easy-to-use automated instrument that is well suited to analyzing fine particles by capturing images of hundreds of thousands of particles as they pass in front of the camera. Results in terms of individual particle size and shape and associated distributions from the sample are produced. Additionally the correlation between the size and shape can be assessed from the scattergram of the size vs the shape distribution. The FPIA-3000 is suitable not only for research and development applications to develop higher particle quality and functionality, but also for routine measurement for quality management in commercial particle treatment processes.
Thanks to Kunio Shirohara of Hokkaido University for providing the data presented.
