In most cases, the ideal finished piece would be 100% aggregate. A given application's most desirable quality (be it high strength, low cost, high dielectric constant, or low density) is usually most prominent in the aggregate itself. However, the aggregate lacks the ability of a liquid to flow and fill up a volume, and to form attachments between particles.
Aggregate size Experiments and mathematical models show that more of a given volume can be filled with hard spheres if it is first filled with large spheres, then the spaces between (
interstices) are filled with smaller spheres, and the new interstices filled with still smaller spheres as many times as possible. For this reason, control of
particle size distribution can be quite important in the choice of aggregate; appropriate simulations or experiments are necessary to determine the optimal proportions of different-sized particles. The upper limit to particle size depends on the amount of flow required before the composite sets (the gravel in paving concrete can be fairly coarse, but fine sand must be used for
tile mortar), whereas the lower limit is due to the thickness of matrix material at which its properties change (clay is not included in concrete because it would "absorb" the matrix, preventing a strong bond to other aggregate particles). Particle size distribution is also the subject of much study in the fields of
ceramics and
powder metallurgy.
Toughened composites Toughness is a compromise between the (often contradictory) requirements of
strength and
plasticity. In many cases, the aggregate will have one of these properties, and will benefit if the matrix can add what it lacks. Perhaps the most accessible examples of this are composites with an
organic matrix and
ceramic aggregate, such as
asphalt concrete ("tarmac") and
filled plastic (i.e.,
Nylon mixed with powdered
glass), although most
metal matrix composites also benefit from this effect. In this case, the correct balance of hard and soft components is necessary or the material will become either too weak or too brittle.
Nanocomposites Many material 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 nanotubes 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 microspheres. Often called
microballoons, they consist of a hollow shell several tens of
nanometers thick and approximately one
micrometer in diameter. Casting them in a
polymer matrix yields
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
crystalline structure is extremely directional, allowing it to be easily separated into flakes or fibers. The nanotechnology touted by
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
immiscible phases of
polymer (including brittle polystyrene and rubbery
polybutadiene) are mixed together. Special molecules (
graft copolymers) include separate portions which are soluble in each phase, and so are only stable at the
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-ceramics, 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. ==See also==