SubscribeThis 17 page article is a concise and informative overview of particle technology and particle characterisation.
The paper introduces the reader to the theory behind several measurement techniques used to determine particle dimensions.
Fine Particle Technology
Since prehistory man has been aware of the importance of particle size in producing resources and wares with desired properties. Archaeological evidence indicates that paints used for cave wall paintings are mixtures of finely pulverized pigmenting materials, predominantly carbon, ochre and hematite. Man came to realize that adding pulverized materials to clay not only improved its workability, but improved the drying process, reduced shrinkage and changed the characteristics of the resulting vessels.
There also is evidence of using particles of certain sizes to control porosity. For many centuries, finely divided, calcined lime powder or gypsum mixed with sand was used in plasters and binders. Then, about 2000 years ago, the Romans improved upon the formula by adding volcanic (pozzolanic) ash, which produced a superior hydraulic cement that was used in building many structures that still stand.
Today, finely divided particulate materials and objects that incorporate or are produced from these fine particles are everywhere about us. Frequently encountered powders include cement, lime, fertilizer, cosmetic powders, table salt and sugar, detergents, bath and dental powders, coffee creamer, baking soda, and many other household items. Products in which the incorporation of powders is not so obvious include paint, toothpaste, lipstick, mascara, chewing gum, magnetic recording media, many medicinal products, slick magazine covers, floor coverings, and automobile tires.
Such everyday items as fused ceramic bathroom fixtures and many small metal objects produced by powder metallurgy completely obscure their origins as powders. The gold trim of dinnerware, for example, started as a carefully controlled fine powder. Even microwave cooking utilizes particle technology. The desire to brown some foods cooked by microwave was solved by a wrapping of metalised polyethylene terephthalate (PET) film, a material containing fine grained metallic material that absorbs microwaves and produces localized elevated temperatures.
The applications of fine particle technology by no means are limited to commercial products, nor is the need to determine the properties of finely divided materials restricted to one area of technology. It begins in mining with discovering how fine an ore must be ground to release the sought-after mineral.
Detailed physical studies of archaeological items indicate that these processes were known in ancient times. Fine ceramic artifacts indicate a knowledge of processing naturally occurring rocks and minerals to control purity as well as particle size distribution in the clays, glazes, and pigments. Plasters used in decorating the pyramids and mortars used by Roman masons indicate similar attention to particle size.
Today the porosity of limestone and sandstone is characterized by quarry source and related to its expected rate of deterioration in urban environments before it is used in restoration of historic monuments. The mortars and plasters used in ancient and modern times owes its characteristics almost wholly to the selection of the proper sizes of the lime and filler particles.
Earth scientists use particle technology to solve various mysteries of nature. Geologists study the textural characteristics of clastic rocks to extract clues to the methods of transportation, sorting, and deposition of the fine materials incorporated in these rocks. This provides valuable information about the history of natural events and processes such as water flow, winds, glacier movement, and marine currents that occurred at the depositional site prior to lithification.
Petroleum geologists study the physical characteristics of strata deep within the earth in order to determine the capacity of the field and to assess the effort required to remove the petroleum. Oceanographers measure characteristics of marine sediment for clues to its origin as well as to determine its mechanical properties for mooring. Soil scientists examine characteristics of near-surface soils to assess qualities associated with agricultural production.
Many of the physical characteristics of interest to these scientists are dependent upon characteristics of the fine particles from which the materials are composed.
Climatologists are concerned with airborne particles that affect weather, and historical climatologists study particle depositions in ice cores as evidence of weather patterns over thousands of years. Climatologists, palaeontologists and other natural scientists have found evidence linking mass extinction to an excessive number of fine particles in the upper atmosphere that shielded solar energy from the earth's surface, initiating a chain of events that devastated flora and fauna world-wide. Civil engineers study the grain size of subsurface soils to assess load bearing capabilities.
Environmentalists must know the capacity of adsorbents such as carbon granules in order to prevent escape of harmful vapours into the atmosphere. They also must characterize soil to determine the percolation rate, diffusion, and retention characteristics of hazardous substance spills. These bulk characteristics, too, are dependent upon the characteristics of the individual particles that compose the bulk.
There is an optimum particle size, or at least a smallest and largest acceptable size, for most items involving particles. The taste of both peanut butter and chocolate is affected by the size of their respective ingredients. Extremely fine amorphous silica is added to tomato ketchup to control its flow.
Pharmaceutical tablets dissolve in our systems at rates determined in part by particle size and exposed surface area. Pigment size controls the saturation and brilliance of paints. The setting time of concrete, dental fillings, and broken-bone casts proceeds in accordance with particle size and surface area exposure. Some materials, gums in particular, do not dissolve in water but absorb water to form viscous colloidal sols. The particle size of the powder determines the type of dispersion. Larger particles form a discontinuous mucilage and fine powders yield homogeneous dispersions. The former is an effective ingredient in laxatives while the latter finds use in adhesives.
Controlling the surface-to-volume (surface-to-mass) ratio is one reason for manipulating particle size. Another is to control interparticle pore size and pore volume for specialized applications. For example, at the turn of the nineteenth century, filters having sub-micron pore sizes were constructed from diatomaceous earth and used to retain bacterium. However, it was demonstrated that infectious particles far smaller than bacteria could pass through these filters, leading to confirmation of the existence of unfilterable infectious elements called 'viruses'.
Surface area and porosity as a function of particle size or surface area and porosity independent of particle size are other physical characteristics that play an important role in particle technology. The effectiveness of odour removers depends on the active surface area of the adsorbent in them. The tightness of the weave of a cloth raincoat, and therefore its porosity, is adjusted to retard water penetration but permit air and vapor passage for comfort. Adsorbent towels and tissues, on the other hand, are made to have pores that readily wick up liquids. The tips of felt-tip pens have a still different requirement: their pore structure must hold a viscous ink but release it when compressed.
The pore structure of prosthetic devices influences whether or not tissue will attach. There is even a connection between the Sphinx of Egypt's Giza plateau and porosity. The Sphinx may be coerced into revealing its true age thanks to the porosity of the stone from which it is made. A model of the weathering process based on the porosity of the stone has been suggested that may yield a timeline back to the date of its creation.
Many behavioural aspects of fine particle systems come about simply because of the relatively large amount of surface exposed to its surroundings. As matter is subdivided, the free energy of the system increases proportional to the amount of new surface created. The work required to achieve the new surface is equal to or greater than the increase in free energy. However, the laws of thermodynamics dictate that a system spontaneously will seek the lowest free energy state that is possible under the circumstances. The study of the behaviour of the system in seeking this state, and how it can be manipulated and utilized is the domain of fine particle technology.
A thought experiment that exemplifies these principals is as follows. Consider a container of oil and water, the oil floating above the water, the two liquid phases being separated by a surface of minimum area and minimum free energy. Adding work to the system by vigorously shaking the container results in oil droplets being dispersed in the water and the total surface of the oil-water interface being greatly increased.
However, when allowed to again stand at rest, the droplets join to form larger and larger drops of oil, each having less surface than the sum of the surfaces of the individual droplets that formed it, thereby reducing surface free energy. This behaviour continues until the minimum interface is achieved, that is, one mass of oil floating above the mass of water.
The system could be manipulated by adding a surfactant that would be attracted to the surface of the oil droplets, thus lowering the free energy of these surfaces and suppressing or prohibiting their coalescence when the input of agitation energy is ceased. The mechanism employed to achieve minimum energy in the example above is through the mutual attraction of matter. This non-specific attractive force is commonly referred to as van der Waals force. It gives rise to the phenomenon termed physical adsorption (or physisorption) and is also responsible for surface tension and condensation in liquids.
At high temperatures surface energy is likely to be reduced by electron sharing and valence bonding with gas atoms creating the phenomenon known as chemical adsorption (or chemisorption). As has been exemplified, some of the attraction can be reduced by the addition of surfactants, which give rise to what is called double-layer phenomena.. These terms appear again in sections and chapters to follow. Obviously, all the special attributes relating to particle size, surface properties, and pore structure could not have been achieved without precise measuring means.
Particle size probably was measured crudely first in ancient Egypt. Surviving wall paintings show ground foodstuff being sieved -- possibly through a rough cloth of woven reeds -- to remove the large bits for further grinding. While undoubtedly it was recognized long ago that grinding to smaller and smaller sizes exposed progressively more surface area and promoted dissolution, truly assessing the extent of the exposed area and the consequences thereof got its start only in the eighteenth century.
This is when it was discovered that charcoal heated and then cooled without exposure to air would take up several times its own volume of air upon subsequent exposure. That pores in the charcoal accounted for much of the gas uptake by its condensation in them and that all solids exhibited adsorption phenomena to different degrees was learned by the mid-nineteenth century. From that came the realization that gas adsorption measurements could yield much information about the physical surface and pore structure of solids.
Continuing experimentation early in the twentieth century with gases being first adsorbed and then removed by heating revealed that more was involved in some instances than just physical adsorption. Oxygen gas, for example, removed from carbon was found not to be pure oxygen but to contain oxides of carbon. This suggested that two processes were involved in gas uptake on solids: one of purely physical character which, as used above, was given the designation physical adsorption, and one involving a chemical reaction which is termed chemisorption.
Adjacent chemisorbed atoms become susceptible to reaction with one another to form new chemical species when the proper surface structures and conditions are present. This we now know is the action of catalysts. Today, chemists and chemical engineers tailor the pore size and surface properties of catalysts to produce everything from shortening to gasoline.
Providing quantitative measures of the several parameters defining particle size, surface area, pore size and volume, surface activity, object density, and a few other more specialized subjects is the purpose of the instruments and services Micromeritics offers. Following are details of just what is being determined when each measurement is made with Micromeritics' instruments.
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Paul A. Webb and Clyde Orr, Norcross, GA - 12/20/2005