Aggregate is the component of a
composite materialComposite materials, often shortened to composites or called composition materials, are engineered or naturally occurring materials made from two or more constituent materials with significantly different physical or chemical properties which remain separate and distinct at the macroscopic or...
that resists compressive stress and provides bulk to the composite material. For efficient filling, aggregate should be much smaller than the finished item, but have a wide variety of sizes. For example, the particles of stone used to make
concreteConcrete is a composite construction material, composed of cement and other cementitious materials such as fly ash and slag cement, aggregate , water and chemical admixtures.The word concrete comes from the Latin word...
typically include both
sandSand is a naturally occurring granular material composed of finely divided rock and mineral particles.The composition of sand is highly variable, depending on the local rock sources and conditions, but the most common constituent of sand in inland continental settings and non-tropical coastal...
and
gravelGravel is composed of unconsolidated rock fragments that have a general particle size range and include size classes from granule- to boulder-sized fragments. Gravel can be sub-categorized into granule and cobble...
.
Comparison to fiber composites
Aggregate composites tend to be much easier to fabricate, and much more predictable in their finished properties, than
fiber compositesA fiber-reinforced composite consists of three components: the fibers as the discontinuous or dispersed phase, the matrix as the continuous phase, and the fine interphase region, also known as the interface. This is a type of advanced composite group, which makes use of rice husk, rice hull,...
. This is because fiber orientation and continuity can have an overwhelming effect, but can be difficult to control and assess. Fabrication aside, aggregate materials themselves also tend to be less expensive; the most common aggregates (mentioned above) are found in nature and can often be used with only minimal processing.
Not all composite materials include aggregate in their design. This is because aggregate particles tend to have about the same dimensions in every direction (that is, an aspect ratio of about one), so that aggregate composites do not display the level of synergy that fiber composites often do. A strong aggregate held together by a weak
matrixThe matrix or groundmass of rock is the finer grained mass of material in which larger grains, crystals or clasts are embedded.The matrix of an igneous rock consists of finer grained, often microscopic, crystals in which larger crystals are embedded. This porphyritic texture is indicative of...
will be weak in
tensionIn physics, tension is the magnitude of the pulling force exerted by a string, cable, chain, or similar object on another object. It is the opposite of compression. As tension is the magnitude of a force, it is measured in newtons and is always measured parallel to the string on which it applies...
, whereas fibers can be less sensitive to matrix properties, especially if they are properly oriented and run the entire length of the part (i.e., a
continuous filament).
Most composites are filled with particles whose aspect ratio lies somewhere between oriented filaments and spherical aggregates. A good compromise is
chopped fiber, where the performance of filament or cloth is traded off in favor of more aggregate-like processing techniques.
Ellipsoid and plate-shaped aggregates are also used.
Aggregate properties
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; all the aggregate lacks is the ability to flow on a small scale, and form attachments between particles. The matrix is specifically chosen to serve this role, but its abilities should not be abused.
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
tileA tile is a manufactured piece of hard-wearing material such as ceramic, stone, metal, or even glass. Tiles are generally used for covering roofs, floors, walls, showers, or other objects such as tabletops...
mortarMortar is a workable paste used to bind construction blocks together and fill the gaps between them. The blocks may be stone, brick, cinder blocks, etc. Mortar becomes hard when it sets, resulting in a rigid aggregate structure. Modern mortars are typically made from a mixture of sand, a binder...
), 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
ceramicA ceramic is an inorganic, nonmetallic solid prepared by the action of heat and subsequent cooling. Ceramic materials may have a crystalline or partly crystalline structure, or may be amorphous...
s and
powder metallurgyPowder metallurgy is the process of blending fine powdered materials, pressing them into a desired shape , and then heating the compressed material in a controlled atmosphere to bond the material . The powder metallurgy process generally consists of four basic steps: powder manufacture, powder...
.
Some exceptions to this rule include:
Toughened composites
ToughnessIn materials science and metallurgy, toughness is the ability of a material to absorb energy and plastically deform without fracturing; Material toughness is defined as the amount of energy per volume that a material can absorb before rupturing...
is a compromise between the (often contradictory) requirements of
strengthIn materials science, the strength of a material is its ability to withstand an applied stress without failure. The applied stress may be tensile, compressive, or shear. Strength of materials is a subject which deals with loads, deformations and the forces acting on a material. A load applied to a...
and
plasticityIn physics and materials science, plasticity describes the deformation of a material undergoing non-reversible changes of shape in response to applied forces. For example, a solid piece of metal being bent or pounded into a new shape displays plasticity as permanent changes occur within the...
. 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
organicAn organic compound is any member of a large class of gaseous, liquid, or solid chemical compounds whose molecules contain carbon. For historical reasons discussed below, a few types of carbon-containing compounds such as carbides, carbonates, simple oxides of carbon, and cyanides, as well as the...
matrix and
ceramicA ceramic is an inorganic, nonmetallic solid prepared by the action of heat and subsequent cooling. Ceramic materials may have a crystalline or partly crystalline structure, or may be amorphous...
aggregate, such as
asphalt concreteAsphalt concrete is a composite material commonly used in construction projects such as road surfaces, airports and parking lots. It consists of asphalt and mineral aggregate mixed together, then laid down in layers and compacted...
("tarmac") and filled plastic (i.e.,
NylonNylon is a generic designation for a family of synthetic polymers known generically as polyamides, first produced on February 28, 1935, by Wallace Carothers at DuPont's research facility at the DuPont Experimental Station...
mixed with powdered
glassGlass is an amorphous solid material. Glasses are typically brittle and optically transparent.The most familiar type of glass, used for centuries in windows and drinking vessels, is soda-lime glass, composed of about 75% silica plus Na2O, CaO, and several minor additives...
), 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 materials properties change radically at small length scales (see
nanotechnologyNanotechnology is the study of manipulating matter on an atomic and molecular scale. Generally, nanotechnology deals with developing materials, devices, or other structures possessing at least one dimension sized from 1 to 100 nanometres...
). 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
macroscopicThe macroscopic scale is the length scale on which objects or processes are of a size which is measurable and observable by the naked eye.When applied to phenomena and abstract objects, the macroscopic scale describes existence in the world as we perceive it, often in contrast to experiences or...
object made from them to be treated as an aggregate composite in many respects.
While bulk synthesis of such nanoparticles as
carbon nanotubeCarbon nanotubes are allotropes of carbon with a cylindrical nanostructure. Nanotubes have been constructed with length-to-diameter ratio of up to 132,000,000:1, significantly larger than for any other material...
s is currently too expensive for widespread use, some less extreme nanostructured materials can be synthesized by traditional methods, including
electrospinningElectrospinning uses an electrical charge to draw very fine fibres from a liquid. Electrospinning shares characteristics of both electrospraying and conventional solution dry spinning of fibers. The process does not require the use of coagulation chemistry or high temperatures to produce solid...
and spray
pyrolysisPyrolysis is a thermochemical decomposition of organic material at elevated temperatures without the participation of oxygen. It involves the simultaneous change of chemical composition and physical phase, and is irreversible...
. One important aggregate made by spray pyrolysis is
glass microsphereGlass microspheres are microscopic spheres of glass manufactured for a wide variety of uses in research, medicine, consumer goods and various industries. Glass microspheres are usually between 1 to 1000 micrometers in diameter, although the sizes can range from 100 nanometers to 5 millimeters in...
s. Often called
microballoons, they consist of a hollow shell several tens of nanometers thick and approximately one
micrometerA micrometer , is by definition 1×10-6 of a meter .In plain English, it means one-millionth of a meter . Its unit symbol in the International System of Units is μm...
in diameter. Casting them in a
polymerA polymer is a large molecule composed of repeating structural units. These subunits are typically connected by covalent chemical bonds...
matrix yields
syntactic foamGlass microspheres are microscopic spheres of glass manufactured for a wide variety of uses in research, medicine, consumer goods and various industries. Glass microspheres are usually between 1 to 1000 micrometers in diameter, although the sizes can range from 100 nanometers to 5 millimeters in...
, 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
crystalA crystal or crystalline solid is a solid material whose constituent atoms, molecules, or ions are arranged in an orderly repeating pattern extending in all three spatial dimensions. The scientific study of crystals and crystal formation is known as crystallography...
line 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
clayClay is a general term including many combinations of one or more clay minerals with traces of metal oxides and organic matter. Geologic clay deposits are mostly composed of phyllosilicate minerals containing variable amounts of water trapped in the mineral structure.- Formation :Clay minerals...
with a laminar structure suspended in a
thermoplasticThermoplastic, also known as a thermosoftening plastic, is a polymer that turns to a liquid when heated and freezes to a very glassy state when cooled sufficiently...
olefin (a class which includes many common plastics like
polyethylenePolyethylene or polythene is the most widely used plastic, with an annual production of approximately 80 million metric tons...
and
polypropylenePolypropylene , also known as polypropene, is a thermoplastic polymer used in a wide variety of applications including packaging, textiles , stationery, plastic parts and reusable containers of various types, laboratory equipment, loudspeakers, automotive components, and polymer banknotes...
). The latter category includes fibrous
asbestosAsbestos is a set of six naturally occurring silicate minerals used commercially for their desirable physical properties. They all have in common their eponymous, asbestiform habit: long, thin fibrous crystals...
composites (popular in the mid-20th century), often with matrix materials such as
linoleumLinoleum is a floor covering made from renewable materials such as solidified linseed oil , pine rosin, ground cork dust, wood flour, and mineral fillers such as calcium carbonate, most commonly on a burlap or canvas backing; pigments are often added to the materials.The finest linoleum floors,...
and
Portland cementPortland cement is the most common type of cement in general use around the world because it is a basic ingredient of concrete, mortar, stucco and most non-specialty grout...
.
In-situ aggregate formation: Many micro-composites form their aggregate particles by a process of self-assembly. For example, in high impact
polystyrenePolystyrene ) also known as Thermocole, abbreviated following ISO Standard PS, is an aromatic polymer made from the monomer styrene, a liquid hydrocarbon that is manufactured from petroleum by the chemical industry...
, two
immiscibleSolubility is the property of a solid, liquid, or gaseous chemical substance called solute to dissolve in a solid, liquid, or gaseous solvent to form a homogeneous solution of the solute in the solvent. The solubility of a substance fundamentally depends on the used solvent as well as on...
phases of
polymerA polymer is a large molecule composed of repeating structural units. These subunits are typically connected by covalent chemical bonds...
(including brittle polystyrene and rubbery
polybutadienePolybutadiene is a synthetic rubber that is a polymer formed from the polymerization process of the monomer 1,3-butadiene.It has a high resistance to wear and is used especially in the manufacture of tires, which consumes about 70% of the production...
) are mixed together. Special molecules (graft copolymers) include separate portions which are soluble in each phase, and so are only stable at the
interfaceAn interface is a surface forming a common boundary among two different phases, such as an insoluble solid and a liquid, two immiscible liquids or a liquid and an insoluble gas. The importance of the interface depends on which type of system is being treated: the bigger the quotient area/volume,...
between them, in the manner of a
detergentA detergent is a surfactant or a mixture of surfactants with "cleaning properties in dilute solutions." In common usage, "detergent" refers to alkylbenzenesulfonates, a family of compounds that are similar to soap but are less affected by hard water...
. Since the number of this type of molecule determines the interfacial area, and since spheres naturally form to minimize
surface tensionSurface tension is a property of the surface of a liquid that allows it to resist an external force. It is revealed, for example, in floating of some objects on the surface of water, even though they are denser than water, and in the ability of some insects to run on the water surface...
, synthetic chemists can control the size of polybutediene 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
metallurgyMetallurgy is a domain of materials science that studies the physical and chemical behavior of metallic elements, their intermetallic compounds, and their mixtures, which are called alloys. It is also the technology of metals: the way in which science is applied to their practical use...
. In
glass-ceramicGlass-ceramics are polycrystalline material produced through controlled crystallization of base glass. Glass-ceramic materials share many properties with both glasses and ceramics...
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
infraredInfrared light is electromagnetic radiation with a wavelength longer than that of visible light, measured from the nominal edge of visible red light at 0.74 micrometres , and extending conventionally to 300 µm...
light.
See also
- Construction aggregate
Construction aggregate, or simply "aggregate", is a broad category of coarse particulate material used in construction, including sand, gravel, crushed stone, slag, recycled concrete and geosynthetic aggregates. Aggregates are the most mined material in the world...
- Saturated-surface-dry
Saturated surface dry is defined as the condition of an aggregate in which the surfaces of the particles are "saturated" , but the inter-particle voids are otherwise dry...