AAAS. The American Association for the Advancement of Science was
founded in 1848 and incorporated in 1874. Its objectives are to further the
work of scientists, to facilitate cooperation among them, to foster scientific
freedom and responsibility, to improve the effectiveness of science in
promoting human welfare, to advance education in science, and to increase
public understanding and appreciation for the importance and promise of
the methods of science in human progress. The AAAS head quarters is
in Washington, DC. Additional information on the AAAS can be found at
http://www.aaas.org/ and http://www.sciencemag.org/.
ABACA. The sclerenchyma bundles from the sheathing leaf bases of
Musa textilis, a plant closely resembling the edible banana plant. These
bundles are stripped by hand, after which they are cleaned by drawing over
a rough knife. The fiber bundles are now whitish and lustrous, and from
six to twelve feet (1.8-3.6 meters) long. Being coarse, extremely strong
and capable of resisting tension, they are much used in the manufacture of
ropes and cables. Since the fibers swell only slightly when wet, they are
particularly suited for rope that will be used in water. Waste manila fibers
from rope manufacture and other sources are used in the making of a very
tough grade of paper, known as manilla paper. The fibers may be obtained
from both wild and cultivated plants, the latter yielding a product of better
grade. The cultivated plants, propagated by seeds, by cuttings of the thick
rhizomes or by suckers, are ready for harvest at the end of three years,
after which a crop may be expected approximately every three years.
ABHERENT. Any substance that prevents adhesion of a material to
itself or to another material. It may be in the form of a dry powder (a
silicate such as talc, mica, or diatomaceous earth); a suspension (bentonitewater);
a solution (soap-water); or a soft solid (stearic acid, tallow waxes).
Abherents are used as dusting agents and mold washes in the adhesives,
rubber, and plastics industries. Fats and oils are used as abherents in the
baking industry. Fluorocarbon resin coatings on metals are widely used on
ABLATING MATERIAL. A material, especially a coating material,
designed to provide thermal protection to a body in a fluid stream through
loss of mass. Ablating materials are used on the surfaces of some reentry
vehicles to absorb heat by removal of mass, thus blocking the transfer of
heat to the rest of the vehicle and maintaining temperatures within design
limits. Ablating materials absorb heat by increasing in temperature and
changing in chemical or physical state. The heat is carried away from the
surface by a loss of mass (liquid or vapor). The departing mass also blocks
part of the convective heat transfer to the remaining material in the same
manner as transpiration cooling.
(1) Fibers made from white silica, fused in an oven, cut into blocks, and
coated with borosilicate glass; these are extremely efficient at temperatures
up to 2300F. (2) An all-carbon composite (called reinforced carbon-carbon)
make by laminating and curing layers of graphite fiber previously coated
with a resin, which is pyrolized to carbon. The resulting tile is then treated
with a mixture of alumina, silicon, and silicon carbide. Such composites
are used for maximum-temperature (nose cone) exposure up to 3000F.
Both types are undamaged by the heat and are reusable. The tiles are
adhered to the body of the spacecraft with a silicone adhesive. Ablative
materials used on early spaceship trials were fluorocarbon polymers and
glass-reinforced plastics, but these were wholly or partially decomposed
ABLATION. The removal of surface material from a body by vaporization,
melting, chipping, or other erosive process; specifically, the intentional
removal of material from a nose cone or spacecraft during high-speed
movement through a planetary atmosphere to provide thermal protection
to the underlying structure. See also Ablating Material.
ABRASION. All metallic and nonmetallic surfaces, no matter how
smooth, consist of minute serrations and ridges that induce a cutting or
tearing action when two surfaces in contact move with respect to each
other. This wearing of the surfaces is termed abrasion. Undesirable abrasion
may occur in bearings and other machine elements, but abrasion is also
adapted to surface finishing and machining, where the material is too hard
to be cut by other means, or where precision is a primary requisite.
Temperature is a significant factor: friction may raise the temperature of
the surface layers to the point where they become subject to chemical
attack. Abrasion causes deterioration of many materials, especially of
rubber (tire treads), where it can be offset by a high percentage of carbon
black. Other materials subjected to abrasion in their service life are textiles
(laundering), leather and plastics (shoe soles, belting), and house paints
and automobile lacquers (airborne dust, grit, etc.).
See also Abrasives.
ABRASION pH. A term originated by Stevens and Carron in 1948 "to
designate the pH values obtained by grinding minerals in water." Abrasion
pH measurements are useful in the field identification of minerals. The
pH values range from 1 for ferric sulfate minerals, such as coquimbite,
konelite, and rhomboclase, to 12 for calcium-sodium carbonates, such
as gaylussite, pirssonite, and shortite. The recommended technique for
determining abrasion pH is to grind, in a nonreactive mortar, a small
amount of the mineral in a few drops of water for about one minute.
Usually, a pH test paper is used. Values obtained in this manner are given
in the middle column of Table 1. Another method, proposed by Keller
et al. in 1963, involves the grinding of 10 grams of crushed mineral in 100
milliliters of water and noting the pH of the resulting slurry electronically.
Values obtained in this manner are given in the right-hand column.
Keller, W.D., W.D. Balgord, and A.L. Reesman: "Dissolved Products of Artificially
Pulverized Silicate Minerals and Rocks," Jrnl. Sediment. Petrol., 33(1), 191-204
ABRASIVES. An abrasive is a substance used to abrade, smooth, or
polish an object. If the object is soft, such as wood, then relatively soft
abrasive materials may be used. Usually, however, abrasive connotes very
hard substances ranging from naturally occurring sands to the hardest
material known, diamond.
There are three basic forms of abrasives: grit (loose, granular, or
powdered particles); bonded materials (particles are bonded into wheels,
segments, or stick shapes); and coated materials (particles are bonded to
paper, plastic, cloth, or metal).
Properties of Abrasive Materials
Hardness. Table 1 lists the various scales of hardness used for
Toughness. An abrasive's toughness is often measured and expressed
as the degree of friability, the ability of an abrasive grit to withstand impact
without cracking, spalling, or shattering.
Refractoriness (Melting Temperature). Instantaneous grinding temperatures
may exceed 3500C at the interface between an abrasive and the
workpiece being ground. Hence melting temperature is an important property.
Chemical Reactivity. Any chemical interaction between abrasive grains
and the material being abraded affects the abrasion process.
Thermal Conductivity. Abrasive materials may transfer heat from the
cutting tip of the grain to the bond posts, retaining the heat in a bonded
wheel or coated belt. The cooler the cutting point, the harder it is.
Fracture. Fracture characteristics of abrasive materials are important, as
well as the resulting grain shapes. Equiaxed grains are generally preferred
for bonded abrasive products and sharp, acicular grains are preferred for
coated ones. How the grains fracture in the grinding process determines
the wear resistance and self-sharpening characteristics of the wheel or belt.
Microstructure. Crystal size, porosity, and impurity phases play a major
role in fixing the fracture characteristics and toughness of an abrasive grain.
Naturally occurring abrasives are still an important item of commerce,
although synthetic abrasives now fill many of their former uses. They
include diamonds, corundum, emery, garnet, silica, sandstone, tripoli,
pumice, and pumicite.
Manufactured abrasives include silicon carbide, fused aluminum oxide,
sintered aluminum oxide, sol-gel sintered aluminum oxide, fused zirconia-alumina,
synthetic diamond, cubic boron nitride, boron carbide, slags,
steel shot, and grit.
Sizing, Shaping, and Testing of Abrasive Grains
Sizing. Manufactured abrasives are produced in a variety of sizes that
range from a pea-sized grit of 4 (5.2 mm) to submicrometer diameters.
Shaping. Desired shapes are obtained by controlling the method of
crushing and by impacting or mulling. In general, cubical particles are
preferred for grinding wheels, whereas high aspect-ratio acicular particles
are preferred for coated abrasive belts and disks.
Testing. Chemical analyses are done on all manufactured abrasives,
as well as physical tests such as sieve analyses, specific gravity,
impact strength, and loose poured density (a rough measure of particle
shape). Special abrasives such as sintered sol-gel aluminas require more
sophisticated tests such as electron microscope measurement of [alpha]-alumina
crystal size, and indentation microhardness.
Coated abrasives consist of a flexible backing on which films of adhesive
hold a coating of abrasive grains. The backing may be paper, cloth, openmesh
cloth, vulcanized fiber (a specially treated cotton rag base paper), or
any combination of these materials. The abrasives most generally used are
fused aluminum oxide, sol-gel alumina, alumina-zirconia, silicon carbide,
garnet, emery, and flint.
A new form of coated abrasive has been developed that consists of tiny
aggregates of abrasive material in the form of hollow spheres. As these
spheres break down in use, fresh cutting grains are exposed; this maintains
cut-rate and keeps power low.
Grinding wheels are by far the most important bonded abrasive product
both in production volume and utility. They are produced in grit sizes
ranging from 4, for steel mill snagging wheels, to 1200, for polishing the
surface of rotogravure rolls.
Marking System. Grinding wheels and other bonded abrasive products
are specified by a standard marking system which is used throughout most
of the world. This system allows the user to recognize the type of abrasive,
the size and shaping of the abrasive grit, and the relative amount and type
of bonding material.
Bond Type. Most bonded abrasive products are produced with either a
vitreous (glass or ceramic) or a resinoid (usually phenolic resin) bond.
Special Forms of Bonded Abrasives. Special forms of bonded abrasives
include honing and superfinishing stones, pulpstone wheels, crush-form
grinding wheels, and creep feed wheels.
Superabrasive wheels include diamond wheels and cubic boron nitride
Grinding fluids or coolants are fluids employed in grinding to cool the
work being ground, to act as a lubricant, and to act as a grinding aid.
Soluble oil coolants in which petroleum oils are emulsified in water
have been developed to impart some lubricity along with rust-preventive
In addition to their use in bonded and coated products, both natural and
manufactured abrasive grains are used loose in such operations as polishing,
buffing, lapping, pressure blasting, and barrel finishing.
High pressure jet cutting with abrasive grit can be used on metals to produce
burn-free cuts with no thermal or mechanical distortion.
Health and Safety
Except for silica and natural abrasives containing free silica, the abrasive
materials used today are classified by NIOSH as nuisance dust materials
and have relatively high permissible dust levels.
Charles V. Rue
Arpe, H.-J.: Ullmann's Encyclopedia of Industrial Chemistry, Abrasives to Aluminum
Oxide, Vol. 1, 5th Edition, John Wiley & Sons, Inc., New York, NY, 1997.
Coes, L. Jr., Abrasives, Springer-Verlag, New York, NY, Vienna, 1971.
Ishikawa, T. 1986 Proceedings of the 24th Abrasive Engineering Society Conference,
Abrasive Engineering Society, Pittsburgh, PA, 1986, pp. 32-51.
Shaw, M.C.: Principles of Abrasive Processing, Oxford University Press, New York,
Sluhan, C.A. Lub. Eng., 352-374 (Oct. 1970).
ABSOLUTE 1. Pertaining to a measurement relative to a universal
constant or natural datum, as absolute coordinate system, absolute altitude,
absolute temperature. See also Absolute Temperature. 2. Complete, as in
ABSOLUTE TEMPERATURE. The fundamental temperature scale
used in theoretical physics and chemistry, and in certain engineering
calculations such as the change in volume of a gas with temperature.
Absolute temperatures are expressed either in degrees Kelvin or in degrees
Rankine, corresponding respectively to the centigrade and Fahrenheit
scales. Temperatures in Kelvins are obtained by adding 273 to the
centigrade temperature (if above C) or subtracting the centigrade
temperature from 273 (if below C). Degrees Rankine are obtained by
subtracting 460 from the Fahrenheit temperature.
ABSOLUTE ZERO. Conceptually that temperature where there is
no molecular motion, no heat. On the Celsius scale, absolute zero is
-273.15C, on the Fahrenheit scale, -459.67F; and zero Kelvin (0 K).
The concept of absolute zero stems from thermodynamic postulations.
Heat and temperature were poorly understood prior to Carnot's analysis
of heat engines in 1824. The Carnot cycle became the conceptual
foundation for the definition of temperature. This led to the somewhat
later work of Lord Kelvin, who proposed the Kelvin scale based upon
a consideration of the second law of thermodynamics. This leads to a
temperature at which all the thermal motion of the atoms stops. By using
this as the zero point or absolute zero and another reference point to
determine the size of the degrees, a scale can be defined. The Comit'e
Consultative of the International Committee of Weights and Measures
selected 273.16 K as the value for the triple point for water. This set the
ice-point at 273.15 K.
From the standpoint of thermodynamics, the thermal efficiency E of an
engine is equal to the work W derived from the engine divided by the heat
supplied to the engine, Q2. If Q1 is the heat exhausted from the engine,
E = (W/Q2) = (Q2 - Q1)/Q2 = 1 - (Q1/Q2)
where W, Q1, and Q2 are all in the same units. A Carnot engine is a
theoretical one in which all the heat is supplied at a single high temperature
and the heat output is rejected at a single temperature. The cycle consists
of two adiabatics and two isothermals. Here the ratio Q1/Q2 must depend
only on the two temperatures and on nothing else. The Kelvin temperatures
are then defined by the relation where Q1/Q2 is the ratio of the heats
rejected and absorbed, and T1/T2 is the ratio of the Kelvin temperatures
of the reservoir and the source. If one starts with a given size for the
degree, then the equation completely defines a thermodynamic temperature
Q1/Q2 = T1/T2
Excerpted from Van Nostrand's Encyclopedia of Chemistry
by Glenn D. Considine Excerpted by permission.
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