|Monthly Tech-Tip |
Utlimate particles of ceramic materials are finer than can be measured even on a 325 mesh screen. These particles are the key players in the physical presence of the material.
Processed ceramic materials are typically ground to 200 mesh and feel very fine to the touch. With some you can detect some particle grains between your fingers. The amount of these "physical particles" can be measured by washing or shaking the ceramic powder through a sieve. Using water washing and standard wire mesh sieves it is normally only possible to determine the range of particle sizes of a powder sample down to 325 mesh (about 40 microns). Yet it is common for 90% (even 99%) of a powder to be composed of minus 40 micron ultimate particles. Even the physical particles we can measure on sieves are often agglomerates of hundreds or even thousands of ultimate particles. Almost all ceramic materials are composed mainly of ultimate particles. Ball clays, for example, have particles one tenth of a micron in size, 400 times smaller than 325 mesh. Understanding materials fully means being aware of these particles, their sizes, shapes, densities, etc. An interesting example to illustrate is a water-washed and processed large-particle-size kaolin intended for the casting process. It is likely that 99.9% of such a material will wash right through a 325 mesh screen, making it appear to be a very fine powder. It also feels exceedingly fine to the touch. However, in terms of ultimate particles and in relation to other clays, it has a very large particle size. On the other hand, a plastic kaolin may leave residue on a 200 mesh screen and appear to be coarser, whereas actually its ultimate particles could be 10 times smaller.
To effectively measure ultimate particle sizes advanced testing equipment is needed. These devices use xray or photographic techniques. For example, many devices simply take a micro photograph of an air suspended powder sample and then software analyzes the photo to produce the desired measurement. The rate of sedimentation in water can also reveal information about ultimate particles.
Example of sedimentation test to compare soluble salts water extracts from suspended clay. This simple test also reveals ultimate particle size distribution differences in clays that a sieve analysis cannot do.
These are table salt crystals on a 60 mesh sieve. It has an opening of 250 micro meters (or microns). Half of the crystals passed this sieve, half are retained. Notice on the right, several crystals are in the openings, about to fall through. Imagine that an particle (or crystal) of bentonite or ball clay can be sub-micron in diameter, they can actually be 2500 times smaller on a side than these salt crystals! One-tenth-micron ultimate particles would thus fit 2500x2500 on the flat side of a salt crystal. And, since the clay crystal is much thinner than wide, perhaps ten could stack to the same dimension. That means theoretically 2500x2500x25000 (or 1 with eleven zeros) could pack into a grain of salt!
To measure particle size in a slurry or powder you need sieves. This is the most popular type used in labs. They are made from brass by a company named Tyler. The range of screen sizes for testing particle size is very wide (obvious here: the top screen has an opening of 56 mm, the bottom one 0.1 mm - the wires are almost too small to see). You can buy these on ebay for a lot less than new ones, search for "tyler sieve". The finer sieves (especially 200) are fragile and easily ripped. It is good to have a 50, 100 and 150.
Large particle kaolin (left) and small-particle ball clay (right) DFAC tests (for drying performance) demonstrate the dramatic difference in drying shrinkage and performance between these two extremes (these disks are dried with the center portion covered to set up a water content differential to add stresses that cause cracking). These materials both feel super-smooth, in fact, the white one feels smoother. But the ultimate particles tell the opposite story. The ball clay particles (grey clay) are far smaller (ten times or more). The particles of the kaolin (white) are flatter and lay down as such, that is why it feels smoother.
This 1000 ml 24 hour sedimentation test compares Plainsman A2 ball clay ground to 10 mesh (left) with that same material ball milled for an hour (right). The 10 mesh designation is a little misleading, those are agglomerates. When it is put into water many of those particles break down releasing the ultimates and it does suspend fairly well. But after 24 hours, not only has it settled completely from the upper section but there is a heavy sediment on the bottom. But with the milled material it has only settled slightly and there is no sediment on the bottom. Clearly, using an industrial attrition ball mill this material could be made completely colloidal.
Ceramic clays have a flat particle shape. Various factors determine the extent to which they can bind face-to-face in pugged clay in the presence of particles of other materials.
In ceramics some clays of are of such exceedingly small particle sizes that they can stay in suspension in water indefinitely. But unlike common colloids, clays have a secret weapon.
Particle Size Distribution
Knowing the distribution of particle sizes in a ceramic material is often very important in assessing its function and suitability for an application.
The surface area of a powder can be measured. It is the total surface area of all the particles in a gram of the material, and this number can be alot larger than you might think.
Plasticity (in ceramics) is a property exhibited by soft clay. Force exerted effects a change in shape and the clay exhibits no tendency to return to the old shape. Elasticity is the opposite.
During drying, clay particles draw together and shrinkage occurs. During firing the matrix densifies and shrinkage continues. More vitreous bodies shrink more.
Particle Characterization Instruments at Horiba Scientific
|Tests||Average Particle Size (Microns)|
|Tests||Median Particle Size (Microns)|
|Tests||% < 1 micron|
|Tests||% < 2 microns|
|Tests||% < 10 microns|
|Tests||% < 20 microns|
|Tests||Ultimate Particle Size Distribution|
|Tests||% < 0.5 microns|