Difference between revisions of "Powder"
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Revision as of 15:18, 4 June 2016
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A powder (sometimes referred to as powdered material) is a dry bulk solid, composed of a large number of extremely fine particles, that may flow freely or become airborne when shaken or tilted. To be classified as powder, the material must be dry, as in free of any liquid. If the powder contains liquid, then it's either a gel, paste or solid emulsion.
Contents
Powder vs other granular material
The main difference between powders and other granular materials is the size of their particles. The constituent particles that compose granular materials should be large enough such that they are not subject to thermal motion fluctuations. Thus, the lower size limit for grains in granular material is about 1 µm. In powders, the particles are generally small enough that they can become airborne if the powder is agitated or scattered in air, while granular materials cannot become airborne.
Types of powder
Metal
Metal in their powdered form are generally more reactive than in their bulk form. Extremely finely powdered iron is pyrophoric, while bulk iron is less reactive. Osmium and iridium are also a good example, in that they're very inert in their bulk form, but very reactive as powder. However, in case of some metals, such as gold or copper, there is little difference between the reactivity of their powdered and bulk forms.
Soft metals are difficult to turn into powders, as their granules will tend to coalesce over time.
Metallic powders are commonly used in fireworks and flash powders.
Nonmetal
Powdered nonmetallic materials, such as powdered elements (boron, carbon, phosphorus, sulfur, silicon), nonmetallic compounds (oxides, nitrides, carbides) are somewhat more reactive than in their bulk form, due their higher surface-to-volume ratio.
Organic
Airborne powdered organic materials can pose a serious fire or explosion hazard, such as finely powdered starch (grain flour) sugar suspended in air in confined areas, phenomenon known as dust explosion.
Other
Powdered metallic oxides are useful as catalysts or as building block in most construction materials (cement, clay, ceramic materials).
Insoluble organometallic compounds can only exist as powder, as most will decompose when heated, rather than melt. This is also true for most insoluble coordination complexes.
Availability
Metallic powders are sold by various suppliers.
Starch and sugar powder can be purchased from any food store.
Preparation
Metallic powders can be prepared by finely grinding metals of small size using ball mills.
Projects
- Flash powder
- Powder metallurgy
- Sintering
Handling
Safety
Flammable powders can pose a serious fire and explosion risk.
Storage
In closed containers.
Disposal
Disposal depends on the type of powder.
