|Monthly Tech-Tip |
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.
Surface area (or SSA - Specific Surface Area) is a physical property you will see listed on the data sheets and certificates of analysis of many materials. Some materials (e.g. sedimentary clays) can contain particles that have a wide range of sizes, shapes, densities, surface texture, reactivities and unique chemistries and mineralogies. Other materials (e.g. kaolins) contain mostly one particle shape with the only significant difference being particle size. More than any other material, the physical properties of plastic clays and clay-containing slurries are directly tied to surface area. In clays, the total surface area of all particles in a sample help explain many properties, (e.g. plasticity, drying shrinkage, dry strength, melting behaviour).
In clays, more surface means a greater ability of the clay to exhibit plasticity. Water acts as a glue, holding all the particles together (because the surface chemistry of clay particles has an electrolytic affinity for water). The total forces by which they attract to it increase exponentially as surface area does.
Kaolins have comparatively large ultimate particles and thus have less surface area than ball clays (one popular kaolin has an SSA of 20-30 square meters per gram). Ball clays, having particle sizes up to ten times smaller, should have dramatically more surface area. But, their SSAs can sometimes be in the same range as kaolins, this is counterintuitive since they are much more plastic. Bentonites have particle sizes up to ten times smaller than ball clays, only a gram can have hundreds of square meters of surface area! Notwithstanding this, some manufacturers quote SSA values in the range of ball clays (or even kaolins)! It can thus be a little confusing, perhaps we can take from this that SSA is not much of an absolute indicator of the magnitude of any property (e.g. plasticity). Remember, surface area is just one part of understanding the physical properties of a material, especially clays (the reactivity and topography of that surface will contribute to properties also).
Surface area values are thus best suited as quality control indicators for a specific material. When the SSA value changes it is a heads-up to possible changes in process or materials that have occurred.
When companies ship materials they often include these with the shipment. The information reported is often very basic and properties important to ceramics are often not found.
Water in Ceramics
Water is the most important ceramic material, it is present every body, glaze or engobe and either the enabler or a participant in almost every ceramic process and phenomena.
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.
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.
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.