ONLINE EXCLUSIVE: Measuring Particles With Laser Diffraction
The particle size and size distribution of a ceramic powder are key factors in defining the performance of the final ceramic product or component. A direct relationship exists between the particle size and the pore size observed within the green body prior to sintering. Large particles or agglomerates tend to pack inefficiently, resulting in the formation of large pores that persist during sintering, and these pores increase the probability of component failure.
Pore formation can be controlled by using powders with small particle sizes or polydispersed size distributions, in which the fine particles fill the voids between the large particles present in the powder. Particle size can also help define the time and temperature required to achieve full density during sintering, with finer particles requiring shorter sintering times. Thus, reliable measurement of particle size is an important requirement of product development and quality control.
Characterizing Particle SizeOne effective technique for measuring particle size is laser diffraction. Also known as low-angle laser light scattering (LALLS), laser diffraction relies on the fact that particles passing through a laser beam scatter light at an angle that is inversely proportional to their size. Measuring the scattering pattern associated with a sample as a function of angle therefore enables rapid and reliable particle size measurement, with the dynamic range and resolution being defined by the positioning of the detectors and the range of angles that are sampled. (See ISO13320-1, the international standard for laser diffraction measurements, available online at www.iso.org.)
A schematic of a basic laser diffraction instrument is given in Figure 1. The system consists of:
- A light source. Typically a red laser (HeNe, l = 0.63 µm) is used as a source of coherent intense light of fixed wavelength.
- Some means of passing the sample through the laser beam. Laser diffraction is a flexible technique in this regard, since dry powders, emulsions, suspensions and sprays can all be easily presented to the measurement zone.
- A lens system. This is used to collect the light scattered by the particles and focus it onto the detector system. The properties of the lens system ensure that light scattered at the same angle is focused to the same point on the detector, regardless of the particle position within the measurement zone.
- A suitable detector. Usually this is a silicon diode detector with a number of discrete detectors placed at a range of angles.
One drawback with early diffraction systems was the need to change lenses to measure different particle size ranges due to the limited angular range sampled by the detector. Modern systems measure particles over a wider angular range, allowing much broader distributions to be measured without making changes to the instrument configuration. In addition, a patented duel-wavelength measurement system available on some diffraction instruments* allows red-light measurements to be combined with those at smaller wavelengths to increase the sensitivity of the system within the sub-micron range. As a result, consistent resolution can now be achieved across an extremely wide dynamic range (for example 0.02-2000 microns), ensuring that both well-dispersed and agglomerated materials can be detected simultaneously.
Calculating particle size distributions requires fitting an appropriate model to the light scattering data. Older diffraction systems rely on the Fraunhofer Approximation to calculate size distributions. This approximation assumes that the particles being measured are opaque disks that scatter light at small angles. As such, it is only applicable to large particles and will give an incorrect assessment of the fine particle fraction. Modern diffraction systems instead rely on the Mie Scattering model. This model accurately predicts the scattering intensities observed from particles across the entire dynamic range accessible using laser diffraction, providing a correct assessment of both the fine and coarse particle fraction present in modern ceramics. The use of Mie theory is particularly important when particles below 50 microns in size are present, as defined in ISO 13320-1.
*The Mastersizer 2000, developed and supplied by Malvern Instruments Ltd., Worcestershire, UK
Testing the Method's SensitivityISO 13320-1 suggests that the resolution or sensitivity of a laser diffraction system can be tested by determining how the system responds to the addition of small amounts of one material in the presence of another. To test the accuracy of the laser diffraction technique in ceramic applications, a laser diffraction system was used to measure the particle size for a relatively coarse ceramic powder (see Figure 2). The particle size of this material was determined with a measurement variability of less than 1%, which shows the robustness of the measurement technique and defines the experimental limit of detection of any "problem" particles. The ability of the system to detect over-sized material was determined by adding small amounts of larger particles to the powder.
Modeling AgglomerationTo model the agglomeration of the powder, known weights of a coarse particle fraction (particle size >90 micron by sieving) were added to the ceramic powder. Figure 3 shows the particle size distributions reported by the laser diffraction instrument during the addition of the coarse material, along with the percentage of coarse material in each of the samples.
Modeling Oversized MaterialThe final sample measured in the first experiment was doped with small amounts of 225-micron glass beads to model the presence of oversized, unmilled material. The particle size distributions reported by the laser diffraction instrument are shown in Figure 5. Again, the instrument detected the presence of the oversized material at the lowest concentration used. These results also demonstrate that the instrument was accurately able to track the concentration of the doped material.
Obtaining Accurate ResultsAccurate measurement of ceramic powder particle size distributions is important in the production of modern ceramic components. Laser diffraction has proven to be very sensitive to the presence of oversized particles in the ceramic powders tested, even at extremely low concentrations. These experiments have shown that the technique can be a valuable tool in determining the particle size and agglomeration of ceramic powders. With this information, ceramic manufacturers can reduce defect formation and ensure more consistent production of their products and components.
For more information about laser diffraction particle size analysis, contact Malvern Instruments Ltd., Enigma Business Park, Grovewood Road, Malvern, Worcestershire WR14 1XZ, UK; (44) 1684-892456; fax (44) 1684-892789; e-mail email@example.com; or visit www.malvern.co.uk.