Editing Aggregate (composite) (section)

===Nanocomposites===
Many materials properties change radically at small length scales (see [[nanotechnology]]). In the case where this change is desirable, a certain range of aggregate size is necessary to ensure good performance. This naturally sets a lower limit to the amount of matrix material used.

Unless some practical method is implemented to orient the particles in micro- or nano-composites, their small size and (usually) high strength relative to the particle-matrix bond allows any [[macroscopic]] object made from them to be treated as an aggregate composite in many respects.

While bulk synthesis of such nanoparticles as [[carbon nanotube]]s is currently too expensive for widespread use, some less extreme nanostructured materials can be synthesized by traditional methods, including [[electrospinning]] and spray [[pyrolysis]]. One important aggregate made by spray pyrolysis is [[glass microsphere]]s. Often called ''microballoons'', they consist of a hollow shell several tens of [[nanometer]]s thick and approximately one [[micrometre|micrometer]] in diameter. Casting them in a [[polymer]] matrix yields [[glass microsphere|syntactic foam]], with extremely high compressive strength for its low density.

Many traditional nanocomposites escape the problem of aggregate synthesis in one of two ways:

'''Natural aggregates''': By far the most widely used aggregates for nano-composites are naturally occurring. Usually these are ceramic materials whose [[crystal]]line structure is extremely directional, allowing it to be easily separated into flakes or fibers. The nanotechnology touted by [[General Motors Corporation|General Motors]] for automotive use is in the former category: a fine-grained [[clay]] with a laminar structure suspended in a [[thermoplastic]] [[olefin]] (a class which includes many common plastics like [[polyethylene]] and [[polypropylene]]). The latter category includes fibrous [[asbestos]] composites (popular in the mid-20th century), often with matrix materials such as [[linoleum]] and [[Portland cement]].

'''In-situ aggregate formation''': Many micro-composites form their aggregate particles by a process of self-assembly. For example, in high impact [[polystyrene]], two [[solubility|immiscible]] phases of [[polymer]] (including brittle polystyrene and rubbery [[polybutadiene]]) are mixed together. Special molecules ([[graft copolymer]]s) include separate portions which are soluble in each phase, and so are only stable at the [[Interface (chemistry)|interface]] between them, in the manner of a [[detergent]]. Since the number of this type of molecule determines the interfacial area, and since spheres naturally form to minimize [[surface tension]], synthetic chemists can control the size of polybutadiene droplets in the molten mix, which harden to form rubbery aggregates in a hard matrix. [[Dispersion strengthening]] is a similar example from the field of [[metallurgy]]. In [[glass-ceramic]]s, the aggregate is often chosen to have a negative [[coefficient of thermal expansion]], and the proportion of aggregate to matrix adjusted so that the overall expansion is very near zero. Aggregate size can be reduced so that the material is transparent to [[infrared]] light.