Materials Handbook 15th ed - G. Brady_ H. Clauser_ J. Vaccari (McGraw-Hill_ 2002) WW Part 13 doc

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Silica flour, made by grinding sand, is used in paints, as a facing for
sand molds, and for making flooring blocks. Silver bond silica is
water-floated silica flour of 98.5% SiO
2
, ground to 325 mesh. In zinc
and lead paints it gives a hard surface. Pulverized silica, made
from crushed quartz, is used to replace tripoli as an abrasive.
Ultrafine silica, a white powder having spherical particles of 157 to
984 ␮in (4 to 25 ␮m), is made by burning silicon tetrachloride. It is
used in rubber compounding, as a grease thickener, and as a flatting
agent in paints. Aerosil, of Cabot Corp., is this material. Silica pow-
der, of Praxair Inc., is a white, amorphous powder with maximum
particle size of 1,969 nin (50 nm). Other natural amorphous silicas
come in an average particle size of 59 ␮in (1.5 ␮m) with no particles
larger than 394 ␮in (10 ␮m). Quso, of Philadelphia Quartz Co., is a
soft, white powder with small particles, 394 to 787 nin (10 to 20 nm).
It is used in cosmetics and paper coatings and as an anticaking agent
in pharmaceuticals. As a filler in plastics, it gives a plasticizing action
that aids extrusion. These fine silicas are also marketed as dust-free
agglomerate particles which disperse easily in solution to the discrete
hydrophyllic particle. Arc silica, of PPG Industries, used as a flat-
ting agent in clear lacquers, is produced directly from silica sand in
an arc furnace at 5432°F (3000°C). It has crystals of 0.59 ␮in (0.015
␮m) agglomerated into translucent grains, 79 to 118 ␮in (2 to 3 ␮m).
Valron, of Du Pont, originally called Estersil, is ester-coated silica
powder of 0.3- to 0.4-in (8- to 10-mm) particle size, for use as a filler
in silicone rubbers, printing inks, and plastics. Ludex, of the same
company, is another colloidal silica with the fine particles nega-
tively charged by the incorporation of a small amount of alkali. It
forms a sol, or high-concentration solution, without gelling. Min-U-
Sil, of Pennsylvania Glass Sand Corp., for making molded ceramics,

has tiny crystalline particles. Syton, of Monsanto, is a water disper-
sion of colloidal silica for treating textiles. Translucent silica particles
deposited on the fibers increase the coefficient of friction, giving uni-
formly high-strength yarns.
A polymer-impregnated silica, Polysil, produced by
Westinghouse, has twice the dielectric strength of porcelain as well as
better strength. It is also cheaper to make, and its composition can be
tailored to meet specific environmental and operating conditions.
Silica aerogel is a fine, white, semitransparent silica powder, the
grains of which have a honeycomb structure, giving extreme lightness.
It has a density of 2.5 lb/ft
3
(40 kg/m
3
) and is used as an insulating
material in the walls of refrigerators, as a filler in molding plastics, as
a flatting agent in paints, as a bodying agent in printing inks, and as a
reinforcement for rubber. It is produced by treating sand with caustic
soda to form sodium silicate and then treating with sulfuric acid to
form a jellylike material called silica gel, which is washed and ground
840 SILICA
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Materials, Their Properties and Uses
to a fine, dry powder. It is also called synthetic silica. Syloid is this
material. It is a fluffy, white powder with a pH of 7.2. Silica hydrogel
is a colorless, translucent, semisolid hydrated silica of composition
SiO
2

и xH
2
O, bulking about 44 lb/ft
3
(705 kg/m
3
). It contains 28% solids
and 72 water. It becomes fluid by mixing with water and regels on
standing. It is used for paper and textile coatings, ointments, and
water suspensions of silica. Hi-Sil, of PPG Industries, and Santocel,
of Monsanto, are silica gels. Mertone WB-2, of the same company, is
silica gel used as a coating material for blueprint papers to deepen the
blue and increase legibility. When silica gel is used as a pigment, the
vehicle surrounds the irregular particle formation, producing greater
rigidity and hardness of paint surface than when a smooth pigment is
used. For insulation use, the thermal conductivity of silica gel powder
is given as 0.1 Btu/(h и ft
2
и °F [0.57 W/(m
2
и K)] at Ϫ115°F (Ϫ81°C).
Silicon monoxide, SiO, does not occur naturally but is made by
reducing silica with carbon in the electric furnace and condensing the
vapor out of contact with air. It is lighter than silica, having a specific
gravity of 2.24, and is less soluble in acid. It is brown powder valued
as a pigment for oil painting, as it takes up a higher percentage of oil
than ochres or red lead. It combines chemically with the oil. Monox is
a trade name for silicon monoxide. Fumed silica is a fine, translucent
powder of the simple amorphous silica formula made by calcining
ethyl silicate. It is used instead of carbon black in rubber compounding

to make light-colored products, and to coagulate oil slicks on water so
that they can be burned off. It is often called white carbon, but the
“white carbon black” of Cabot Corp. called Cab-O-Sil, used for rubber,
is a silica powder made from silicon tetrachloride. Cab-O-Sil EH5, a
fumed colloidal form, is used as a thickener in resin coatings. The
thermal expansion of amorphous fused silica is only about one-eighth
that of alumina. Refractory ceramic parts made from it can be heated
to 2000°F (1093°C) and cooled rapidly to subzero temperatures with-
out fracture. QLF silicon oxide, of Airco Coating Technology, is a
vapor-deposited barrier coating for resistance to oxygen and moisture
in paperboard/polyethylene laminate products.
SILICON. A metallic element, symbol Si, used chiefly in its combined
forms. Pure silicon metal is used in transistors, rectifiers, and elec-
tronic devices. It is a semiconductor and is superior to germanium for
transistors, as it withstands temperatures to 300°F (149°C) and will
carry more power. Rectifiers made with silicon instead of selenium
can be smaller and will withstand higher temperatures. Its melting
point when pure is about 2615°F (1434°C), but it readily dissolves in
molten metals. It is never found free in nature, but combined with
oxygen, it forms silica, SiO
2
, one of the most common substances in
SILIICON 841
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Materials, Their Properties and Uses
the earth. Silicon can be obtained in three modifications. Amorphous
silicon is a brown powder with a specific gravity of 2.35. It is fusible
and dissolves in molten metals. When heated in air, it burns to form

silica. Graphitoidal silicon consists of black, glistening spangles
and is not easily oxidized and is not attacked by the common acids,
but is soluble in alkalies. Crystalline silicon is obtained in dark,
steel-gray globules or crystals or six-sided pyramids of specific gravity
2.4. It is less reactive than the amorphous form, but is attacked by
boiling water. All these forms are obtainable by chemical reduction.
High-quality crystalline silicon is the most efficient material for pho-
tovoltaic cells used to generate electricity from sunlight. Amorphous
silicon films are also used and are less costly, but the cells are less effi-
cient because the atoms are random. Silicon is an important con-
stituent of commercial metals. Molding sands are largely silica, and
silicon carbides are used as abrasives. Commercial silicon is sold in
the graphitoidal flake form, or as ferrosilicon, and silicon-copper. The
latter forms are employed for adding silicon to iron and steels.
Commercial refined silicon contains 97% pure silicon and less than 1
iron. It is used for adding silicon to aluminum alloys and for fluxing
copper alloys. High-purity silicon metal, 99.95% pure, made in an arc
furnace, is too expensive for common uses, but is employed for elec-
tronic devices and in making silicones. For electronic use, silicon must
have extremely high purity, and the pure metal is a nonconductor
with a resistivity of 118,000 ⍀иin (300,000 ⍀иcm). For semiconductor
use it is “doped” with other atoms, yielding electron activity for con-
ducting current. Epitaxial silicon is higher purified silicon doped
with exact amounts of impurities added to the crystal to give desired
electronic properties. Thus, silicon doped with boron has resistivities
in grades from 394 to 3,940 ⍀иin (1,000 to 10,000 ⍀иcm). Silicon
ribbon of Westinghouse, for semiconductors, consists of dendritic sili-
con crystals grown into thin continuous sheets 0.5 in (1.3 cm) wide,
thus eliminating the need to saw slices from ingots. Pure single-crys-
tal silicon ribbon of Dow Chemical is as thin as 49 ␮in (1.25 ␮m)

and is made as a membrane formed by surface tension between two
growing dendritic crystals. Float-zoned single-crystal silicon is 100
times purer than semiconductor-grade silicon. It is used in wafer form
for laser and infrared detectors in guided bombs and missiles and for
high-power switching devices, such as thyristors.
Silicon does not possess a metallic-type lattice structure and, like
antimony, is a semimetal and lacks plasticity, but is more akin to the
diamond in structure. Because of its feeble electronegative nature, it
has a greater tendency to form compounds with nonmetals than with
metals. Silicon forms silicon hybrids of general formula Si
x
H
2x+2
,
842 SILIICON
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Materials, Their Properties and Uses
similar to the paraffin hydrocarbons, but they are very unstable and
ignite in air. But a mixture of ferrosilicon and sodium hydroxide,
called hydrogenite, which yields hydrogen gas when water is added,
is used for filling balloons. Silicon, like carbon, has a valence of 4 and
links readily to carbon in SiC chain formations. The SiC bond acts as
the CᎏC bond of organic chemistry, but silicon does not enter into
animal or plant structures.
SILICON BRONZE. A family of wrought copper-base alloys (C64700 to
C66100) and one cast copper alloy (C87200), the wrought alloys con-
taining from 0.4 to 0.8% silicon (C64700) to 2.8 to 4.0 silicon (C65600),
and the cast alloy 1.0 to 5.0, along with other elements, usually lead,

iron, and zinc. Other alloying elements may include manganese, alu-
minum, tin, nickel, chromium, and phosphorus. The most well-known
alloys are probably silicon bronze C65100, or low-silicon bronze B,
and silicon bronze C65500, or high-silicon bronze A, as they were
formerly called. As these names imply, they differ mainly in silicon
content: 0.8 to 2.0% and 2.8 to 3.8, respectively, although the latter
alloy also may contain as much as 0.6 nickel. C87200 contains at least
89% copper, 1.5 silicon, and as much as 5 zinc, 2.5 iron, 1.5 aluminum,
1.5 manganese, 1 tin, and 0.5 lead. Regardless of alloying ingredients,
copper content is typically 90% or greater.
Both of the common wrought alloys are quite ductile in the
annealed condition, C65500 being somewhat more ductile than
C65100, and both can be appreciably strengthened by cold working.
Annealed, tensile yield strengths are on the order of 15,000 to
25,000 lb/in
2
(103 to 172 MPa) depending on mill form, with ulti-
mate tensile strengths to about 60,000 lb/in
2
(414 MPa) and elonga-
tions of 50 to 60%. Cold working can increase yield strength to as
much as 70,000 lb/in
2
(483 MPa). Electrical conductivity is 12% for
C65100 and 7 for C65500 relative to copper, and thermal conductiv-
ity is 33 Btu/(ft и h и °F) [57 W/(m и K)] and 21 Btu/(ft и h и °F) [(36
W/m и K)], respectively. The alloys are used for hydraulic-fluid lines
in aircraft, heat-exchanger tubing, marine hardware, bearing plates,
and various fasteners.
Silicon bronze C87200, which is suitable for centrifugal, invest-

ment, and sand-, plaster-, and permanent-mold casting, also has
been known by the trade names Everdur, Herculoy, and Navy
Tombasil. Typical as-sand-cast tensile properties are 55,000 lb/in
2
(379 MPa) ultimate strength, 25,000 lb/in
2
(172 MPa) yield strength,
and 30% elongation. Brinell hardness is 85, electrical conductivity
6%, and, relative to free-cutting brass, machinability is 40%. Uses
include pump and valve parts, marine fittings, and bearings.
SILICON BRONZE 843
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Materials, Their Properties and Uses
SILICON CARBIDE. A bluish-black, crystalline, artificial mineral of
composition SiC having a Knoop hardness of 2,500. It is used as an
abrasive as loose powder, coated abrasive cloth and paper, wheels,
and hones. It withstands temperatures to its decomposing point of
4175°F (2301°C) and is valued as a refractory. It retains its strength
at high temperatures and has low thermal expansion, and its heat
conductivity is 10 times that of fireclay. It is used for butterfly valves
that control the flow of hot blasts through the tuyeres of blast fur-
naces, and for high-pressure, high-temperature mechanical seals in
polymer-processing reactors. Silicon-carbide particulates are used as
reinforcements in aluminum-alloy composites, and silicon-carbide
fibers and whiskers serve as reinforcements in emerging
metal-matrix and ceramic-matrix composites. The material is also a
potential matrix material for composites. Fibers are used to
strengthen and toughen glass ceramics. Thermal-insulation blankets

of spacecraft, which can withstand repeated exposure to tempera-
tures as high as 3632°F (2000°C), comprise layers of silicon-carbide
and aluminoborosilicate fabrics, and silicon-carbide thread is used to
stitch the fabrics. The material also holds promise for integrated cir-
cuits able to withstand higher temperatures than silicon-based ICs
and for mirrors of superior mechanical, thermal, and optical proper-
ties in space systems, solar collectors, and astronomical telescopes.
Silicon carbide is made by fusing sand and coke at a temperature
above 4000°F (2204°C). It can also be made from polymer precursors
and by vapor-phase reactions. One such precursor, developed at
Rensselaer Polytechnic Institute, is hydridopolycarbosilane. When
it is heated to 1832°F (1000°C), 90% of the polymer converts to the
carbide. Silicon carbide can also be made from wood or sawdust. The
Glenn Research Center of the National Aeronautics and Space
Administration reports that parts formed to net shape are pyrolyzed
at 1800°F (982°C) and infiltrated with molten silicon or silicon alloys.
Unlike aluminum oxide, the crystals of silicon carbide are large,
and they are crushed to make the small grains used as abrasives.
They are harder than aluminum oxide, and as they fracture less eas-
ily, they are more suited for grinding hard cast irons and ceramics.
The standard grain sizes are usually from 100 to 1,000 mesh. The
crystalline powder in grain sizes from 60 to 240 mesh is also used in
lightning arrestors. Carborundum, of Standard Oil Engineered
Materials Co., Crystolon, of Norton Co., and Carbolon, of Exolon
Co. of Canada Ltd., are trade names for silicon carbide. Many other
trade names are used, such as Carborite, Carbolox, Carbolite,
Carbobrant, Storalon, Sterbon, and Natalon. Ferrocarbo is a
silicon carbide of Carborundum Co. in briquettes for adding to the
iron cupola charge. It breaks down in the cupola above 2000°F
844 SILICON CARBIDE

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Materials, Their Properties and Uses
(1093°C) to form nascent carbon and silicon for adding to the iron and
also for deoxidizing. It produces more-uniform iron castings. Alsimag
539 is a fine-grained silicon carbide in the form of molded parts for
brazing fixtures and furniture for kilns for high-temperature sinter-
ing. The siliconized graphites produced by Pure Carbon Co.,
named Purebide, are graphite materials with surfaces chemically
converted to silicon carbide. They have the wear resistance of silicon
carbide, but retain some of the lubricity of graphite. Cost savings are
achieved by machining graphite into intricate shapes before conver-
sion, and subsequently impregnating parts to control leakage or mod-
ify strength and/or wear properties.
When used as a refractory in the form of blocks or shapes, silicon
carbide may be ceramic-bonded or self-bonded by recrystallization. A
standard silicon carbide brick has about 90% SiC, with up to 8 silica.
The specific gravity is about 3.2. It has very high resistance to
spalling. The thermal conductivity is about the same as that of mul-
lite, and the coefficient of expansion is about 2.6 ϫ 10
Ϫ6
/°F (4.7 ϫ
10
Ϫ6
/K). Carbex is a silicon carbide firebrick of General
Refractories Co. Refrax silicon carbide of Carborundum Co. is
bonded with silicon nitride. It is used for hot-spray nozzles, for heat-
resistant parts, and for lining electrolytic cells for smelting alu-
minum. Silicon carbide KT, of the same company, is molded without

a binder. It has 96.5% SiC with about 2.5 silica. The specific gravity is
about 3.1, and it is impermeable to gases. Parts made by pressing or
extruding and then sintering have a flexural strength of 24,000 lb/in
2
(165 MPa) and compressive strength of 150,000 lb/in
2
(1,034 MPa).
The Knoop hardness is 2,740. It is made in rods, tubes, and molded
shapes, and the rough crystal surface can be diamond-ground to a
smooth, close tolerance. The operating temperature in inert atmo-
spheres is to 4000°F (2204°C) and in oxidizing atmospheres to 3000°F
(1649°C). For reactor parts, it has a low neutron-capture cross section
and high radiation stability. The thermal conductivity is 2.5 times
that of stainless steel. Crystolon R of Norton Co. is a stabilized sili-
con carbide bonded by recrystallization. It has a specific gravity of
2.5, a tensile strength of 5,500 lb/in
2
(38 MPa), compressive strength
of 25,000 lb/in
2
(172 MPa), and Knoop hardness of 2,500. The porosity
is 21%. It is for parts subject to temperatures to 4200°F (2316°C), and
it withstands high thermal shock. Crystolon C is a self-bonding sili-
con carbide for coating molded graphite parts to give high wear and
erosion resistance. The coatings, 0.003 to 0.020 in (0.008 to 0.051 cm)
thick, produced by high-temperature chemical reaction, form an inte-
gral part of the graphite surface. Vitropore filter candles, of Pall
Corp., are made from rigid silicon carbide and are used to recover
particulates from hot gas streams. They are especially effective in
SILICON CARBIDE 845

MPa) and compressive strength of 750 lb/in
2
(5 MPa). Its porosity is
80%. It is inert to hot chemicals and can be machined.
Silicon carbide crystals are used for semiconductors at tempera-
tures above 650°F (343°C). As the cathode of electronic tubes instead
of a hot-wire cathode, the crystals take less power and need no
warm-up. In the silicon carbide crystal, both the silicon and the crys-
talline carbon have the covalent bond in which each atom has four
near neighbors and is bonded to each of these with two electrons sym-
metrically placed between the atoms; but since there is an electroneg-
ative difference between silicon and carbon, there is some ionic
bonding which results in a lesser mobility for lattice scattering. The
silicon carbide semiconductor crystals of Westinghouse have less than
1 part of impurities to 10 million, and the junction is made by diffus-
ing aluminum atoms into the crystal at a temperature of 3900°F
(2149°C), making a p-type junction.
Silicon carbide fiber is one of the most important fibers for high-
temperature use. It has high strength and modulus and withstands
temperatures even under oxidizing conditions up to 3272°F (1800°C),
though the fibers show some deterioration in tensile strength and
modulus properties at temperatures above 2192°F (1200°C). It has
advantages over carbon fibers for some uses, having greater resis-
tance to oxidation at high temperatures, superior compressive
strength, and greater electrical resistance. SCS silicon-carbide
fibers, of Textron Specialty Materials, maintain strength at tempera-
tures over 2500°F (1371°C) and are useful as reinforcements for
ceramic-matrix composites.
There are two commercial processes for making continuous silicon
carbide fibers: (1) by coating silicon carbide on either a tungsten or a

carbon filament by vapor deposition to produce a large filament [3,937
to 5,906 ␮in (100 to 150 ␮m) in diameter], or (2) by melt-spinning an
846 SILICON CARBIDE
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Materials, Their Properties and Uses
organic polymer containing silicon atoms as a precursor fiber followed
by heating at an elevated temperature to produce a small filament
[394 to 1,181 ␮in (10 to 30 ␮m) in diameter]. Fibers from the two
processes differ considerably, but both are used commercially.
Silicon carbide whiskers as small as 276 ␮in (7 ␮m) in diameter
can be made by a number of different processes. Although these
whiskers have the disadvantage in some applications of not being in
continuous-filament form, they can be made with higher tensile
strength and modulus values than continuous silicon carbide filament.
SILICON CAST IRON. An acid-resistant cast iron containing a high
percentage of silicon. When the amount of silicon in cast iron is above
10%, there is a notable increase in corrosion and acid resistance. The
acid resistance is obtained from the compound Fe
3
Si, which contains
14.5% silicon. The usual amount of silicon in acid-resistant castings is
from 12 to 15%. The alloy casts well but is hard and cannot be
machined. These castings usually contain 0.75 to 0.85% carbon.
Lesser amounts decrease acid resistance. Too much carbon also sepa-
rates out as graphite in silicon irons, causing faulty castings.
Increasing the content of silicon in iron reduces the melting point pro-
gressively from 2786°F (1530°C) for pure iron to 2282°F (1250°C) for
iron containing 23% silicon. A 14 to 14.5% silicon iron has a silvery-

white structure, a compressive strength of about 70,000 lb/in
2
(483
MPa), and Brinell hardness 299 to 350, and it is resistant to hot sul-
furic acid, nitric acid, and organic acids. Silicon irons are very
wear-resistant and are valued for pump parts and for parts for chemi-
cal machinery. They are marketed under many trade names.
Duriron, of Duriron Co., contains 14.5% silicon and 1 carbon and
manganese. The tensile strength is 16,000 lb/in
2
(110 MPa) and den-
sity 0.253 lb/in
3
(7,003 kg/m
3
).
SILICON-COPPER. An alloy of silicon and copper used for adding sili-
con to copper, brass, or bronze, also employed as a deoxidizer of cop-
per and for making hard copper. Silicon alloys in almost any
proportion with copper, and is the best commercial hardener of cop-
per. A 50–50 alloy of silicon and copper is hard, extremely brittle, and
black. A 10% silicon, 90 copper alloy is as brittle as glass; in this pro-
portion silicon copper is used for making the addition to molten cop-
per to produce hard, sound copper-alloy castings of high strength. The
resulting alloy is easy to cast in the foundry and does not dross.
Silicon-copper grades in 5, 10, 15, and 20% silicon are also marketed,
being usually sold in slabs notched for breaking into small sections
for adding to the melt. A 10% silicon-copper melts at 1500°F (816°C);
a 20% alloy melts at 1152°F (623°C).
SILICON-COPPER 847

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Materials, Their Properties and Uses
SILICON-MANGANESE. An alloy employed for adding manganese to
steel and as a deoxidizer and scavenger of steel. It usually contains 65
to 70% manganese and 12 to 25 silicon. It is graded according to the
amount of carbon, generally 1, 2, and 2.5%. For making steels low in
carbon and high in manganese, silicomanganese is more suitable
than ferromanganese. A reverse alloy, called manganese-silicon,
contains 73 to 78% silicon and 20 to 25 manganese, with 1.5 maxi-
mum iron and 0.25 maximum carbon. It is used for adding man-
ganese and silicon to metals without the addition of iron. Still
another alloy is called ferromanganese-silicon, containing 20 to
25% manganese, about 50 silicon, and 25 to 30 iron, with only about
0.50 or less carbon. This alloy has a low melting point, giving ready
solubility in the metal.
Silicon-spiegel is an alloy of silicon and manganese with iron
employed for making additions of silicon and manganese to open-
hearth steels, and also for adding manganese to cast iron in the
cupola. A typical analysis gives 25 to 30% manganese, 7 to 8 silicon,
and 2 to 3 carbon. Both the silicon and manganese act as strong deox-
idizers, forming a thin, fusible slag, making clean steel.
SILICON NITRIDE. Si
3
N
4
is a hard, lightweight, heat- and creep-resis-
tant polycrystalline ceramic having low coefficients of friction and
thermal expansion and good resistance to corrosion and thermal

shock. Powder, the starting stock for parts production, is commonly
made by the nitridation of metallic silicon. Other methods include
gas-phase ammonolysis of silicon tetrachloride, carbothermic reduc-
tion of silicon dioxide, and thermal decomposition of silicon diimide.
In Japan, the Isuzu Ceramic Research Institute begins with silicon
powder containing by weight as much as 2% iron and up to 5 alu-
mina, tantalum oxide, and yttria. The mixture is evenly dispersed,
put into a mold and heated in a nitrogen atmosphere at 9-bar pres-
sure and stepped temperatures of 2552 to 3362°F (1400 to 1850°C) for
3 days, forming Si
3
N
4
.
Parts are usually made by reaction bonding without sintering or
hot-pressing and liquid-phase sintering. Reaction bonding involves
reacting a consolidated and shaped mass of pure silicon powder with
nitrogen at high temperature. Resulting parts, commonly designated
reaction-bonded silicon nitride (RBSN), are 15 to 20% porous,
thus only moderate in strength, but essentially shrink-free, thus
quite accurate as formed. Hot-pressing powder, using powder with
sintering additives, followed by sintering results in parts commonly
designated hot-pressed silicon nitride (HPSN). These are nearly
full-density parts of more-robust mechanical performance. Density
ranges from 0.111 to 0.122 lb/in
3
(3,072 to 3,377 kg/m
3
), and the coeffi-
848 SILICON MANGANESE

2
(600 MPa).
Noralide, of Norton Co., is an HPSN used for ball and roller bear-
ings. Such bearings, used in machine-tool spindles and instruments,
are noted for their light weight, low friction, and good wear and
fatigue resistance. Other silicon nitride applications include valves,
seals, and cutting tools. Ceralloy 147, of Ceradyne, Inc., is a cast sili-
con nitride material for check-valve balls and mechanical seals. Its
key features are resistance to abrasion, oxidation, corrosion, and ther-
mal shock. Ceralloy 147-3 Needlelok is toughened by interlocking,
needlelike grains. It has a Vickers hardness of 1,600, a tensile
strength of 113,000 lb/in
2
(780 MPa), and a fracture toughness of
5,600 (lb/in
2
) и ͙in
ෆ
(6.2 MPa и ͙m
ෆ
), and it is used for oil-drilling
applications. Roydazide, of Materials Research Corp., is for coatings
as well as parts production.
Silicon nitride auto-engine valves made by cold isostatic pressing at
Hoechst CeramTec in Germany have demonstrated 2 to 6% fuel sav-
ings over metal valves, while reducing nitrous oxide and carbon monox-
ide emissions. At Japan’s Agency of Industrial Science and Technology,
a 17,076-Btu/min (300-kW) ceramic turbine using Si
3
N

4
in the
high-temperature sections attained 29% thermal efficiency in a 39-h
test compared with 15 to 20% for conventional turbines by allowing gas
inlet temperatures of 2192°F (1200°C) rather than 1652°F (900°C) or
less for the conventional. Crystalline silicon nitride applied by chemical
vapor deposition can protect carbon-carbon composites from oxidation
at temperatures as high as 3200°F (1760°C) for up to 5 h.
Silicon nitride fibers have been made by reacting silicon oxide
and nitrogen in the presence of a reducing agent in an electrical-
resistance furnace at 2552°F (1400°C). Discontinuous fibers are
used as reinforcements in composites for specialty aircraft and elec-
trical parts, and in radomes (microwave windows). Whiskers have
been made by the reaction of nitrogen and a mixture of silicon and
silica. For the emerging ceramic-matrix composites, silicon nitride
is a potential matrix and reinforcement material.
As of 2000, the silicon nitride in use was beta silicon nitride. The
long, thin rods of bonded beta-silicon nitride crystals account for its
high strength and toughness. At that time a new form—alpha silicon
nitride—was discovered by researchers at the University of
Pennsylvania in work sponsored by Air Force Office of Scientific
Research. Having a similar but slightly more complicated structure
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Materials, Their Properties and Uses
than the beta type, it combines strength, toughness, and hardness
superior to all other engineering ceramics on the market at the time,
but it is less stable. However, it can slowly convert to the beta type at

high temperature, with the newly grown crystals consuming the
unstable matrix and forming long rods much like the crystals in geo-
logical formations. On the other hand, starting with beta silicon
nitride mixed with additives, the new crystals also form long rods and
create the same toughening effect. This new form of the ceramic is
40% harder than the beta type and equivalent to silicon carbide, the
hardest commercial abrasive currently in use. Potential uses include
aircraft bearings, cutting tools, engine valves, and other applications.
SILICON STEEL. All grades of steel contain some silicon, and most
contain from 0.10 to 0.35% as a residual of the silicon used as a deoxi-
dizer. But from 3 to 5% silicon is sometimes added to increase the
magnetic permeability, and larger amounts are added to obtain
wear-resisting or acid-resisting properties. Silicon deoxidizes steel,
and up to 1.75% increases the elastic limit and impact resistance
without loss of ductility. Silicon steels within this range are used for
structural purposes and for springs, giving a tensile strength of about
75,000 lb/in
2
(517 MPa) and 25% elongation. A common low-silicon
structural steel contains up to 0.35% silicon and 0.20 to 0.40 carbon,
but the structural silicon steels are ordinarily silicon-manganese
steel, with the manganese above 0.50%. Low-carbon steels used as
structural steels are made by careful control of carbon, manganese,
and silicon and with special mill heat treatment. LT-75, of Lukens
Steel, contains 0.2% carbon, up to 1.35 manganese, and 0.3 silicon.
The tensile strength is 90,000 lb/in
2
(621 MPa), with elongation of
24%. European silicon structural steels contain 0.80% or more silicon,
with manganese above 0.50, and very low carbon. The silicon alone is

a graphitizer and, to be most effective, needs the assistance of man-
ganese or other carbide-forming elements. It is useful in
high-strength, low-alloy steels and has a wide range of utility when
used in alloy steels. Considerable addition of silicon above 1.75%
increases the hardness and the corrosion resistance, but reduces the
ductility and makes the steel brittle. The lower grades can be rolled,
however, and silicon-steel sheet is used for electric transformer lami-
nations. Silicon forms a chemical combination with the metal, form-
ing an iron silicide.
The value of silicon steel as a transformer steel was discovered by
Hadfield in 1883. Silicon increases the electrical resistivity and
decreases the hysteresis loss, making silicon steel valuable for mag-
netic circuits where alternating current is used. Electrical steel, or
electric sheet, is sheet steel for armatures and transformers, in var-
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ious grades from 1 to 4.5% silicon. Hipersil is a high-permeability sil-
icon steel, and Cubex is a silicon steel containing 3% silicon which
has been processed so that each cubic crystal of the steel structure is
oriented with the faces symmetric, giving alignment in four directions
instead of the normal two. The steel is easily magnetized across as
well as along the sheet. In transformers it lowers energy losses, and
also gives greater flexibility in designing shapes. One silicon iron is
double-oriented, with the cubic crystals of the iron in exact alignment
in all directions with the sides of the cubes parallel to the sides and
ends of the sheets. It gives high permeability with low induction loss.
Relay steel, used for relays and magnets, contains 0.5 to 2.75% sili-

con. Orthosil is silicon steel sheet, 0.004 in (0.010 cm) thick, for elec-
trical laminations. NK Super E-core, of NKK Corp. of Japan, is a
6.5% silicon electrical steel in which about half of the silicon is dif-
fused into the sheet after cold rolling.
Several cold-work steels and shock-resisting tool steels con-
tain 1 to 2.25% silicon. Of the cold-work air-hardening type, A10 con-
tains 1.25% silicon. Of the cold-work oil-hardening type, O6 contains
1% silicon. And of the shock-resisting type, S2 contains 1%; S4 and
S5, 2%; and S6, 2.25%.
SILICONES. A group of resinlike materials in which silicon takes the
place of carbon of the organic synthetic resins. Silicon is quadrivalent,
like carbon. But while carbon also has a valence of 2, silicon has only
one valence of 4, and the angles of molecular formation are different.
The two elements also differ in electronegativity, and silicon is an
amphoteric element, having both acid and basic properties. The
molecular formation of the silicones varies from that of the common
plastics, and they are designated as inorganic plastics as distinct
from the organic plastics made with carbon.
In the long-chain organic synthetic resins, the carbon atoms repeat
themselves, attaching on two sides to other carbon atoms, while in the
silicones the silicon atom alternates with an oxygen atom so that the
silicon atoms are not tied to each other. The simple silane formed by
silicon and hydrogen corresponding to methane, CH
4
, is also a gas, as
is methane, and has the formula SiH
4
. But, in general, the silicones do
not have the SiH radicals, but contain CH radicals as in the organic
plastics. Basically, silicon is treated with methyl chloride and a cata-

lyst to produce a gas mixture of silanes, (CH
3
)
x
(SiCl)
4Ϫx
. After condens-
ing, three silanes are fractioned, methyl chlorosilane, dimethyl
dichlorosilane, and trimethyl trichlorosilane. These are the common
building blocks of the siloxane chains, and by hydrolyzing them cyclic
linear polymers can be produced with acid or alkali catalysts to give
fluids, resins, and rubbers. Silicone resins have, in general, more
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heat resistance than organic resins, have higher dielectric strength,
and are highly water-resistant. Like organic plastics, they can be com-
pounded with plasticizers, fillers, and pigments. They are usually
cured by heat. Because of the quartzlike structure, molded parts have
exceptional thermal stability. Their maximum continuous-use service
temperature is about 500°F (260°C). Special grades exceed this and go
as high as 700 to 900°F (371 to 482°C). Their heat-deflection tempera-
ture for 265 lb/in
2
(1.8 MPa) is 900°F (482°C). Their moisture absorp-
tion is low, and resistance to petroleum products and acids is good.
Nonreinforced silicones have only moderate tensile and impact
strength, but fillers and reinforcements provide substantial improve-

ment. Because silicones are high in cost, they are premium plastics
and are generally limited to critical or high-performance products such
as high-temperature components in the aircraft, aerospace, and elec-
tronic fields.
A great variety of molecular combinations are available in the sili-
cone polymers, giving resins of varying characteristics, and those hav-
ing CH radicals with silicon bonds are termed organosilicon
polymers. Silicon tetramethyl, Si(CH
3
)
4
, is a liquid boiling at 79°F
(26°C). Trichlorosilane, HSiCl
3
, is also called silicochloroform,
and it corresponds in formation to chloroform. By replacing the hydro-
gen atom of this compound with an alkyl group, the alkylchlorosi-
lanes are made which have high adhesion to metals and are used in
enamels. Methyl chlorosilane, (CH
3
)
2
SiCl
2
, is a liquid used for
waterproofing ceramic electrical insulators. The material reacts with
the moisture in the ceramic, forming a water-repellent coating of
methyl silicone resin and leaving a residue of hydrochloric acid which
is washed off.
Silicone insulating varnishes will withstand continuous operating

temperatures at 350°F (177°C) or higher. Silicone enamels and
paints are more resistant to chemicals than most organic plastics,
and when pigmented with mineral pigments, they withstand temper-
atures up to 1000°F (538°C). For lubricants the liquid silicones are
compounded with graphite or metallic soaps and operate between
Ϫ50 and 500°F (Ϫ46 and 260°C). The silicone liquids are stable at
their boiling points, between 750 and 800°F (399 and 427°C), and
have low vapor pressures, so that they are also used for hydraulic flu-
ids and heat-transfer media. Silicone oils, used for lubrication and
as insulating and hydraulic fluids, are methyl silicone polymers. They
retain a stable viscosity at both high and low temperatures. As
hydraulic fluids, they permit smaller systems to operate at higher
temperatures. In general, silicone oils are poor lubricants compared
with petroleum oils, but they are used at high temperatures, 302 to
392°F (150 to 200°C), low speeds, and low loads.
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Materials, Their Properties and Uses
Silicone resins are blended with alkyd resins for use in outside
paints, usually modified with a drying oil. Silicone-alkyd resins are
also used for baked finishes, combining the adhesiveness and flexibil-
ity of the alkyd with the heat resistance of the silicone. A phenyl
ethyl silicone is used for impregnating glass-fiber cloth for electrical
insulation, and it has about double the insulating value of ordinary
varnished cloth.
Silicone rubber is usually a long-chain dimethyl silicone which
will flow under heat and pressure, but can be vulcanized by
cross-linking the linear chains. Basically, it consists of alternate sili-

con and oxygen atoms with two methyl groups attached to each sili-
con atom. The tensile strength is 300 lb/in
2
(2 MPa), but with fillers it
is raised to 600 lb/in
2
(4 MPa). It is usually compounded with silica
and pigments. It is odorless and tasteless, is resistant to most chemi-
cals but not to strong acids and alkalies, resists heat to 500°F
(260°C), and remains flexible to Ϫ70°F (Ϫ57°C). The dielectric
strength is 500 V/mil (20 ϫ 10
6
V/m). Silicone adhesive sealants have
similar advantages and bond well to various metals and nonmetallics.
Ordinary silicone rubber has the molecular group H и CH
2
и Si и
CH
2
и H in a repeating chain connected with oxygen linkages, but in
the nitrile-silicone rubber one of the end hydrogens of every fourth
group in the repeating chain is replaced by a C:N radical. These polar
nitrile groups give a low affinity for oils, and the rubber does not
swell with oils and solvents. It retains strength and flexibility at tem-
peratures from Ϫ100°F (Ϫ73°C) to above 500°F (260°C) and is used
for such products as gaskets and chemical hose. As lubricants, sili-
cones retain a nearly constant viscosity at varying temperatures.
Fluorosilicones have fluoroalkyd groups substituted for some of the
methyl groups attached to the siloxane polymer of dimethyl silicone.
They are fluids, greases, and rubbers, incompatible with petroleum

oils and insoluble in most solvents. The greases are the fluids thick-
ened with lithium soap, or with a mineral filler.
SILK. The fibrous material in which the silkworm, or larva of the moth
Bombyx mori, envelops itself before passing into the chrysalis state. Silk
is closely allied to cellulose and resembles wool in structure, but unlike
wool, it contains no sulfur. The natural silk is covered with a wax or silk
glue which is removed by scouring in manufacture, leaving the glossy
fibroin, or raw-silk fiber. The fibroin consists largely of the amino acid
alanine, CH
3
CH(NH
2
)CO
2
H, which can be synthesized from pyruvic
acid. Silk fabrics are used mostly for fine garments, but are also val-
ued for military powder bags because they burn without a sooty residue.
The fiber is unwound from the cocoon and spun into threads. Each
cocoon has from 2,000 to 3,000 yd (1,829 to 2,743 m) of thread. The chief
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Materials, Their Properties and Uses
silk-producing countries are China, Japan, India, Italy, and France.
Floss silk is a soft silk yarn practically without twist, or is the loose
waste silk produced by the worm when beginning to spin its cocoon.
Hard silk is thrown silk from which the gum has not been discharged.
Soft silk is thrown silk yarn, degummed, dyed or undyed. Souple silk
is dyed skein silk from which little gum has been discharged. It is

firmer but is less lustrous. Organizine silk is from the best grade of
cocoons. Marabout silk, used for making imitation feathers, is a white
silk, twisted and dyed without discharging the gum. Silk waste is silk
other than that reeled from the cocoon. It includes cocoons not fit for
reeling, partly unwound cocoons, broken filaments, mill waste, and dis-
carded noils. It is used in the spun-silk yarn industry. Noils consist of
the short, staple knotty combings.
In China the cultivation of the silkworm is claimed to date back to
2640 B.C. Silk was first woven in Rome about 50 B.C. The eggs of the
silkworm were smuggled into Europe in the year 552. Sericulture,
or silkworm culture, is a highly developed industry. The larvae, which
have voracious appetites, are fed on mulberry leaves for 24 days, after
which they complete their cocoons in 3 to 4 days. In 7 to 70 days these
are heated to kill the chrysalis to prevent bursting of the shell. The
reeling is done by hand and by machine. Wild silk is from a night
peacock moth which does not feed on the mulberry. It is coarser and
stronger, but darker in color and less lustrous. Tussah silk is a vari-
ety of wild silk from South China and India. Charka silk is raw silk
produced in Bengal on native hand-reeling machines. Byssus silk is
a long fiber from a mussel of Sardinia and Corsica which spins the
thread to attach itself to rocks. The fiber is golden brown, soft, lus-
trous, and elastic, and not dissolved by acids or alkalies. It was for-
merly used for fine garments but is no longer obtained commercially.
Canton silk is soft and fluffy, but is greenish and lacks firmness. It
is from B. textor and is used for weft yarns and in crepes. The silk
grown in India and known as Indian silk is the finest of all silks
with fibers 0.0004 in (0.0016 cm) compared with 0.001 in (0.003 cm)
for Japanese silk. Before World War II Japan produced most of the
silk of the world from a cultivated moth of the tussah variety,
Antheria yama mai. Shantung silk is from a tussah moth, A. pernyi,

which feeds on oak leaves.
The fabric called shantung is a rough-textured, plain-woven silk
with irregular fillings. It is heavier and more bumpy than pongee.
Grosgrain is a heavy, close-woven, corded fabric of silk. It is used for
tapestry and in narrow widths for ribbons. China silk, or habutai, is
an unweighted, all-silk fabric of close, firm, but uneven texture woven
of low-quality, unthrown raw silk in the gum, but it is also imitated
with textiles with a silk warp and a rayon filling. The lightweight
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Materials, Their Properties and Uses
grades of 3, 3.5, and 4 momme, or 371, 317, and 273 ft
2
/lb (76, 65, and
56 m
2
/kg), are classified as sheer fabrics and are used for impregnated
fabrics for umbrellas, raincoats, and hospital sheetings. Unimpregnated
habutai is used for curtains, lampshades, handkerchiefs, and caps.
Heavyweight habutai of 12 momme [53 ft
2
/lb (18 m
2
/kg)] is used for
parachutes. Pongee is a rough-textured, plain-woven, silk fabric with
irregular filling yarns. It is made in natural color or dyed and, like
China silk, has a gummy feel. Bolting cloth, for screening flour, is a
fine, strong, silk fabric. The yarn is a fine-thread, hard-twist tram

thrown in the gum from high-quality raw silk. The fabric has a lino
weave with two warp threads swiveled around the weft. It comes in
various meshes, the finest having 166 to 200 threads/linear inch (65 to
79 threads/linear cm). It is produced on handlooms in Switzerland and
France. Cartridge cloth is a thin, strong fabric for powder bags for
large-caliber guns. It is made of silk waste and noils. The silk is con-
sumed in the explosion without leaving residues that would cause pre-
mature explosion of the subsequent charge. It also does not deteriorate
in storage in contact with the powder.
The kente cloth of Ghana is a silk fabric of fine weave in delicate col-
ors, hand-woven in long, narrow strips which are sewn together to make
a pattern. Satin is a heavy silk fabric with a close twill weave in which
the fine warp threads appear on the surface and the weft threads are
covered up by the peculiar twill. Common satin is of eight-leaf twill, the
weft intersecting and binding down the warp at every eighth pick, but
16 to 20 twills are also made. In the best satins a fine quality of silk is
used. It was originally called zayton, derived from the Arab name of
the Chinese trading post where the fabric was produced. Varieties of
imitation satin are made with a cotton weft. Satins are dyed to many
colors and much used for linings and trimmings.
Qiana, of Du Pont, originally called Fiber Y, produces synthetic
resin fabrics with the feel and drape of silk. They are resilient and
take dyes readily. The fiber is a polyamide based on an alicyclic
diamine. A-Tell is a Japanese textile fiber of great silkiness. It is a
polyethylene oxybenzoate, the molecule having both ester and ether
linkages. Another Japanese fiber is 50% polyvinyl chloride and 50
polyvinyl alcohol. Called Cordelan, it produces fabrics with the feel
of wool.
SILVER. A white metal, symbol Ag, very malleable and ductile, and
classified with the precious metals. It occurs in the native state, and

also combined with sulfur and chlorine. Copper, lead, and zinc ores
frequently contain silver; about 70% of the production of silver is a
by-product of the refining of these metals. Mexico and the United
States produce more than half of the silver of the world. Canada,
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Materials, Their Properties and Uses
Peru, and Bolivia are also important producers. Although nearly 90%
of the silver produced in Arizona comes from copper ores, most of that
produced in California is a by-product of gold quartz mining. Silver is
the whitest of all the metals and takes a high polish, but easily tar-
nishes in air because of the formation of a silver sulfide. It has the
highest electrical and heat conductivity: 108% IACS relative to 100%
for the copper standard and about 244 Btu/(ft и h и °F) [422 W/(m и K)],
respectively. Cold work reduces conductivity slightly. The specific
gravity is 10.7, and the melting point is 1764°F (962°C). When heated
above the boiling point (3925°F, 2163°C), it passes off as a green
vapor. It is soluble in nitric acid and in hot sulfuric acid. The tensile
strength of cast silver is 41,000 lb/in
2
(283 MPa), with a Brinell hard-
ness of 59. The metal is marketed on a troy-ounce value.
Since silver is a very soft metal, it is not normally used industrially
in its pure state, but is alloyed with a hardener, usually copper.
Sterling silver is the name given to a standard high-grade alloy con-
taining a minimum of 925 parts in 1,000 of silver. It is used for the
best tableware, jewelry, and electrical contacts. This alloy of 7.5% cop-
per work-hardens and requires annealing between roll passes. Silver

can also be hardened by alloying with other elements. The old alloy
silanca contained small amounts of zinc and antimony, but the name
sterling silver is applied only to the specific silver-copper alloy.
The standard types of commercial silver are fine silver, sterling sil-
ver, and coin silver. Fine silver is at least 99.9% pure and is used for
plating, making chemicals, and for parts produced by powder metal-
lurgy. Coin silver is usually an alloy of 90% silver and 10 copper, but
when actually used for coins, the composition and weight of the coin
are designated by law. Silver and gold are the only two metals which
fulfill all the requirements for coinage. The so-called coins made
from other metals are really official tokens, corresponding to paper
money, and are not true coins. Coin silver has a Vickers hardness of
148 compared with a hardness of 76 for hard-rolled pure silver. It is
also used for silverware, ornaments, plating; for alloying with gold;
and for electric contacts. When about 2.5% of the copper in coin silver
is replaced by aluminum, the alloys can be age-hardened to Vickers
190. Silver is not an industrial metal in the ordinary sense. It derives
its coinage value from its intrinsic aesthetic value for jewelry and
plate, and in all civilized countries silver is a controlled metal.
Silver powder, 99.9% purity, for use in coatings, integrated cir-
cuits, and other electrical and electronic applications, is produced in
several forms. Amorphous powder is made by chemical reduction and
comes in particle sizes of 35 to 591 ␮in (0.9 to 15 ␮m). Powder made
electrolytically is in dendritic crystals with particle sizes from 394 to
7,874 ␮in (10 to 200 ␮m). Atomized powder has spherical particles
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Materials, Their Properties and Uses

and may be as fine as 400 mesh. Silver-clad powder for electric con-
tacts is a copper powder coated with silver to economize on silver.
Silver flake is in the form of laminar platelets and is particularly
useful for conductive and reflective coatings and circuitry. The tiny,
flat plates are deposited in overlapping layers permitting a metal
weight saving of as much as 30% without reduction in electrical prop-
erties. Nickel-coated silver powder, for contacts and other parts
made by powder metallurgy, comes in grades with 0.25, 0.5, 1, and 2%
nickel by weight.
The porous silver of the Pall Corp. comes in sheets in standard
porosity grades from 79 to 2,165 ␮in (2 to 55 ␮m). It is used for chemi-
cal filtering. Doré metal used for jewelry is silver containing some
gold, but the material known as doré metal, obtained as a by-product
in the production of selenium from copper slimes, is a mixture of silver,
gold, and platinum. Silver plating is sometimes done with a silver-tin
alloy containing 20 to 40 parts silver and the remainder tin. It gives a
plate having the appearance of silver but with better wear resistance.
Silver plates have good reflectivity at high wavelengths, but reflectiv-
ity falls off at about 13,780 nin (350 nm), and is zero at 118,110 nin
(3,000 nm), so that it is not used for heat reflectors. Silvar, of Texas
Instruments, is a silver and nickel-iron composite made by infiltrating
silver into a porous preform of the alloy. It is intended for heat-sink
and thermal management applications in electronics.
Silver-clad sheet, made of a cheaper nonferrous sheet with a coat-
ing of silver rolled on, is used for food processing equipment. It is
resistant to organic acids but not to products containing sulfur.
Silver-clad steel, used for machinery bearings, shims, and reflec-
tors, is made with pure silver bonded to the billet of steel and then
rolled. For bearings, the silver is 0.010 to 0.35 in (0.025 to 0.889 cm)
thick, but for reflectors the silver is only 0.001 to 0.003 in (0.003 to

0.008 cm) thick. The silver-clad stainless steel of American
Cladmetals Co. is stainless-steel sheet with a thin layer of silver
rolled on one side for electrical conductivity.
Silver iodide is a pale-yellow powder of composition AgI, best
known for its use as a nucleating agent and for seeding rain clouds.
Silver nitrate, formerly known as lunar caustic, is a colorless,
crystalline, poisonous, and corrosive material of composition AgNO
3
.
It is used for silvering mirrors, for silver plating, in indelible inks, in
medicine, and for making other silver chemicals. The high-purity
material is made by dissolving silver in nitric acid, evaporating the
solution and crystallizing the nitrate, then redissolving the crystals in
distilled water and recrystallizing. It is an active oxidizing agent.
Silver chloride, AgCl, is a white, granular powder used in silver-
plating solutions. This salt of silver and other halogen compounds of
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silver, especially silver bromide, AgBr, are used for photographic
plates and films. The image cast on the plate by the lens breaks down
the atomic structure of the compound in proportion to the intensity of
light waves received and time of exposure. Electrons gather on the
positive lower side of the bromide grains, causing the formation of
black threads of silver when the film is placed in a developing solu-
tion of ferrous oxalate, FeC
2
O

4
, or other reducing chemical. The
comparative values, or tones, in the picture come from the different
color wavelengths in the white light and the different intensities of
incoming waves. Measured in seconds, the action of violet light, the
shortest wavelength, on the compound is more than 40 times greater
than the action of the long wavelength of red light. To prevent further
action by light, the film is transferred to a fixing bath of sodium thio-
sulfate which dissolves out the unreduced silver bromide.
VerdeFilm, developed by Xerox Corp. for commercial printing,
requires only electrostatic sensitizing prior to use and no silver halide
or chemical developers. Its use avoids wastewater disposal problems
associated with developers, fixers, and solubilized silver salts.
Silver chloride crystals in sizes up to 10 lb (4.5 kg) are grown
synthetically. The crystals are cubic and can be heated and pressed
into sheets. The specific gravity is 5.56, index of refraction 2.071, and
melting point 851°F (455°C). They are slightly soluble in water and
soluble in alkalies. The crystals transmit more than 80% of the wave-
lengths from 1,969 to 7,874 ␮in (50 to 200 ␮m). Cerargyrite, some-
times called horn silver, an ore of silver, found in the upper zone of
silver veins in Nevada, Colorado, Idaho, Peru, Chile, and Mexico, is a
silver chloride containing theoretically 75.3% silver, with sometimes
some mercury. The Mohs hardness is 2.3 and specific gravity 5.8. It is
massive, resembling wax, with a pearl-gray color.
Silver sulfide, Ag
2
S, is a gray-black, heavy powder used for inlay-
ing in metal work. It changes its crystal structure at about 355°F
(179°C), with a drop in electrical resistivity, and is also used for
self-resetting circuit breakers. Silver potassium cyanide,

KAg(CN)
2
, is a white, crystalline, poisonous solid used for silver-plat-
ing solutions. Silver tungstate, Ag
2
WO
4
, silver manganate,
AgMnO
4
, and other silver compounds are produced in high-purity
grades for electronic and chemical uses.
SILVER SOLDER. High-melting-point solder employed for soldering
joints where more than ordinary strength and, sometimes, electrical
conductivity are required. Most silver solders are copper-zinc braz-
ing alloys with the addition of silver. They may contain from 9 to
80% silver, and the color varies from brass yellow to silver white.
Cadmium may also be added to lower the melting point. Silver sol-
858 SILVER SOLDER
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Materials, Their Properties and Uses
ders do not necessarily contain zinc, and may be alloys of silver and
copper in proportions arranged to obtain the desired melting point
and strength. A silver solder with a relatively low melting point con-
tains 65% silver, 20 copper, and 15 zinc. It melts at 1280°F (693°C),
has a tensile strength of 64,800 lb/in
2
(447 MPa), and elongation 34%.

The electrical conductivity is 21% that of pure copper. A solder melt-
ing at 1400°F (760°C) contains 20% silver, 45 copper, and 35 zinc.
ASTM silver solder No. 3 is this solder with 5% cadmium replacing
an equal amount of the zinc. It is a general-purpose solder. ASTM sil-
ver solder No. 5 contains 50% silver, 34 copper, and 16 zinc. It melts
at 1280°F (693°C) and is used for soldering electrical work and refrig-
eration equipment.
Any tin present in silver solders makes them brittle; lead and iron
make the solders difficult to work. Silver solders are malleable and
ductile and have high strength. They are also corrosion-resistant and
are especially valuable for use in food machinery and apparatus
where lead is objectionable. Small additions of lithium to silver sol-
ders increase fluidity and wetting properties, especially for brazing
stainless steels or titanium. Sil-Fos, of Handy & Harmon, is a phos-
phor-silver brazing solder with a melting point of 1300°F (704°C). It
contains 15% silver, 80 copper, and 5 phosphorus. Lap joints brazed
with Sil-Fos have a tensile strength of 30,000 lb/in
2
(207 MPa). The
phosphorus in the alloy acts as a deoxidizer, and the solder requires
little or no flux. It is used for brazing brass, bronze, and nickel alloys.
The grade made by this company under the name of Easy solder
contains 65% silver, melts at 1325°F (718°C), and is a color match for
sterling silver. TL silver solder of the same company has only 9%
silver and melts at 1600°F (871°C). It is brass yellow in color and is
used for brazing nonferrous metals. Sterling silver solder, for braz-
ing sterling silver, contains 92.8% silver, 7 copper, and 0.2 lithium.
Flow temperature is 1650°F (899°C).
A lead-silver solder recommended by Indium Corp. of America to
replace tin solder contains 96% lead, 3 silver, and 1 indium. It melts

at 590°F (310°C), spreads better than ordinary lead-silver solders,
and gives a joint strength of 4,970 lb/in
2
(34 MPa). Silver-palladium
alloys for high-temperature brazing contain from 5 to 30% palla-
dium. With 30%, the melting point is about 2250°F (1232°C). These
alloys have exceptional melting and flow qualities and are used in
electronic and spacecraft applications.
SISAL. The hard, strong, light-yellow to reddish fibers from the large
leaves of the sisal plant, Agave sisalana, and the henequen plant, A.
fourcroydes, employed for making rope, cordage, and sacking. About
80% of all binder twine is normally made from sisal, but sisal ropes
SISAL 859
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Materials, Their Properties and Uses
have only 75% of the strength of Manila rope and are not as resistant
to moisture. Sisal is a tropical plant, and grows best in semiarid
regions. The agave plant is native to Mexico, but most of the sisal
comes from Haiti, east Africa, and Indonesia. The retting, separation,
and washing of the fiber are done by machine, and less than 5% of the
weight of the leaf results in good fiber. Mexican sisal is classified in
seven grades from the Superior white fiber 41 in (105 cm) in length to
Grade C-1, short-spotted fiber 24 in (60 cm) in length. Yucatan sisal,
or henequen, is from the henequen plant and is reddish, stiffer, and
coarser, and is used for binder twine. The Indian word henequen
means knife, from the knifelike leaves. The plant is more
drought-resistant than sisal. Henequen also comes from Indonesia as
the spotted or reddish grades of sisal. Maguey, or cantala, is from

the leaves of A. cantala of India, the Philippines, and Indonesia. It is
used principally for binder twine. The fibers are white, brilliant, stiff,
and lightweight. The fibers are not as strong as sisal, but have a bet-
ter appearance and greater suppleness. Zapupe fiber, of Mexico, is
from A. zapupe. The fiber is similar to sisal, finer and softer than
henequen. Salvador sisal, of El Savador, is from A. letonae. The
leaves are more slender than those of Mexican sisal, and the fiber is
softer and finer. It is used for cordage and fabrics.
The fibers of sisal are not as long or as strong as those of Manila
hemp, and they swell when wet, but they are soft and are preferred
for binder twine either alone or mixed with Manila hemp. Sisal fiber
is also used instead of hair in cement plasters for walls and in lami-
nated plastics. Corolite is a molded plastic made with a mat of sisal
fibers so as to give equal strength in all directions. Agave fibers
from other varieties of the plant are used for various purposes,
notably tampico, from A. rigida, which yields a stiff, hard, but pliant
fiber employed for circular power brushes, and istle, a similar stiff
brush fiber from several plants. Tampico is valued for polishing
wheels, as the fibers hold the grease buffing compositions, and it is
not brittle but abrades with flexibility. Jaumave istle is from A.
funkiana of Mexico. It yields long, uniform fibers finer than tampico.
Lechuguilla is a type of istle from A. lechuguilla.
There are at least 50 species of agave in Mexico and the southwest-
ern United States which yield valuable by-products in addition to
fiber. From some varieties saponin is obtained as a by-product. From
a number of thick-leaved species the buds are cut off, leaving a cavity
from which juice exudes. This juice is fermented to produce pulque, a
liquor with a ciderlike taste containing about 7% alcohol. The juice
contains a sugar, agavose, C
12

H
22
O
11
, which is used in medicine as a
laxative and diuretic. Agava, of Agava Products, Inc., is a dark-
brown, viscous liquid extracted from the leaves of agave plants, used
860 SISAL
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Materials, Their Properties and Uses
as a water conditioner for boiler-water treatment. It is a complex mix-
ture of sapogenines, enzymes, chlorophyllin, and polysaccharides.
A fine strong fiber is obtained from the long leaves of the pineap-
ple, Ananus comosus, native to tropical America. The plant is grown
chiefly for its fruit, known in South America under its Carib name
ananá and marketed widely as canned fruit and juice, preserves, and
confections. Pineapple concentrate is also sold as a flavor
enhancer, as much as 10% being added to apricot, cherry, or other
fruit juices without altering the original flavor. For fiber production
the plants are spaced widely for leaf development and are harvested
before the leaves are fully mature. The retted fibers are long, white,
and of fine texture and may be woven into water-resistant fabrics.
The very delicate and expensive piña cloth of the Philippines is
made from pineapple fiber. The fabrics of Taiwan are usually
coarser and harder.
SLAG. The molten material that is drawn from the surface of iron in
the blast furnace. Slag is formed from the earthy materials in the ore
and from the flux. Slags are produced in the melting of other metals,

but iron blast-furnace slag is usually meant by the term. Slag is used
in cements and concrete, for roofing, and as a ballast for roads and
railways. Finely crushed slag is used in agriculture for neutralizing
acid soils. Blast-furnace slag is one of the lightest concrete aggre-
gates available. It has a porous structure and, when crushed, is angu-
lar. It is also crushed and used for making pozzuolana and other
cements. Slag contains about 32% silica, 14 alumina, 47 lime, 2 mag-
nesia, and small amounts of other elements. It is crushed, screened,
and graded for marketing. Crushed slag weighs 1,900 to 2,100 lb/yd
3
(1,127 to 1,245 kg/m
3
), or is about 30% lighter than gravel.
Honey-comb slag weighs only about 30 lb/ft
3
(481 kg/m
3
). The finest
grade of commercial slag is from 0.1875 in (0.48 cm) to dust; the
run-of-crusher slag is from 4 in (10 cm) to dust. Basic phosphate
slag, a by-product in the manufacture of steel from phosphatic ores,
is finely ground and sold for fertilizer. It contains not less than 12%
phosphoric oxide, P
2
O
5
, and is known in Europe as Thomas slag.
Foamed slag is a name used in England for honeycomb slag used for
making lightweight, heat-insulating blocks. A superphosphate cement
is made in Belgium from a mixture of basic slag, slaked lime, and

gypsum.
SLATE. A shale having a straight cleavage. Most shales are of sedi-
mentary origin, and their cleavage was the result of heavy or
long-continued pressure. In some cases slates have been formed by the
consolidation of volcanic ashes. The slaty cleavage does not usually
SLATE 861
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Materials, Their Properties and Uses
coincide with the original stratification. Slate is of various colors:
black, gray, green, and reddish. It is used for electric panels, chalk-
boards, slate pencils, tabletops, roofing shingles, floor tiles, and treads.
The terms flagstone and cleftstone are given to large, flat sections of
slate used for paving, but the names are also applied to blue sand-
stones cut for this purpose. Slate is quarried in large blocks, and then
slabbed and split. The chief slate-producing states are Pennsylvania,
Vermont, Virginia, New York, and Maine. Roofing slates vary in size
from 12 by 6 in (30 by 15 cm) to 24 by 14 in (61 by 36 cm), and from
0.125 to 0.75 in (0.32 to 1.91 cm) in thickness, and are usually of the
harder varieties. The roofing slate from coal beds is black,
fine-grained, and breaks into brittle thin sheets. It does not have the
hardness or weather resistance of true slate. As late as 1915 more
than 85% of all slate mined was used for roofing, but the tonnage now
used for this purpose is small. Ribbon slate, with streaks of hard
material, is inferior for all purposes. Lime impurities can be detected
by the application of dilute hydrochloric acid to the edges and noting if
rapid effervescence occurs. Iron is a detriment to slates for electric
purposes. The average compressive strength of slate is 15,000 lb/in
2

(103 MPa) and the density 175 lb/ft
3
(2,804 kg/m
3
). Slate granules
are small, graded chips used for surfacing prepared roofing. Slate
flour is ground slate, largely a by-product of granule production. It is
used in linoleum, caulking compounds, and asphalt surfacing mix-
tures. Slate lime is an intimate mixture of finely divided, calcined
slate and lime, about 60% by weight lime to 40 slate. It is employed for
making porous concrete for insulating partition walls. The process
consists in adding a mixture of slate lime and powdered aluminum,
zinc, or magnesium to the cement. The gas generated on the addition
of water makes the cement porous.
SMOKE AGENTS. Chemicals used in warfare to produce an obscuring
cloud of fog to hide movements. Smokes may be harmless and are
then called screening smokes, or smoke screens, or they may be
toxic and called blanketing clouds. There are two types of
smokes: those forming solid or liquid particles and those forming
fogs or mists by chemical reaction. White smokes, which do not
have light-absorbing particles, such as carbon, are formed by chem-
ical reaction and have the best opacity or screening action. The first
naval smoke screens were made by limiting the admission of air to
the fuel in the boilers, and the first Army smoke pots contained
mixtures of pitch, tallow, saltpeter, and gunpowder. The British
smoke candles contained 40% potassium nitrate, 29 pitch, 14 sul-
fur, 8 borax, and 9 coal dust. They gave a brown smoke, but one
that lifted too easily.
862 SMOKE AGENTS
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Materials, Their Properties and Uses
Fog or military screening may be made by spraying an oil mixture
into the air at high velocity. The microscopic droplets produce an
impenetrable fog which remains for a long period. White phospho-
rus gives a dense, white smoke, called WP smoke, by burning to the
pentoxide and changing to phosphoric acid in the moisture of the air.
Its vapor is toxic. Smoke from red phosphorus is known as RP
smoke. Sulfuric trioxide, SO
3
, is an effective smoke producer in
humid air. It is a mobile, colorless liquid vaporizing at 113°F (45°C) to
form dense, white clouds with an irritating effect. The French
opacite is tin tetrachloride, or stannic chloride, SnCl
4
, a liquid
that fumes in air. When hydrated, it becomes the crystalline pentahy-
drate, SnCl
4
и 5H
2
O. The smoke is not dense, but it is corrosive and it
penetrates gas masks. Sulfuryl chloride, SO
2
Cl
2
, is a liquid that
decomposes on contact with the air into sulfuric and hydrochloric
acids. FS smoke is made with a mixture of chlorosulfonic acid and

sulfur trioxide. Silicon tetrachloride, SiCl
4
, is a colorless liquid
that boils at 140°F (60°C), and fumes in the air, forming a dense
cloud. Mixed with ammonia vapor, it resembles a natural fog. The
heavy mineral known as amang, separated from Malayan tin ore,
containing ilmenite and zircon, is used in smoke screens. Titanium
tetrachloride, TiCl
4
, is a colorless to reddish liquid boiling at 277°F
(136°C). It is used for smoke screens and for skywriting from air-
planes. In most air it forms dense, white fumes of titanic acid,
H
2
TiO
3
, and hydrogen chloride. The commercial liquid contains about
25% titanium by weight.
A common smoke for airplanes is oleum. It is a mixture of sulfur
trioxide in sulfuric acid, which forms fuming sulfuric acid, or pyro-
sulfuric acid, H
2
S
2
O
7
. The dense liquid is squirted in the exhaust
manifold. Zinc smoke is made with mixtures of zinc dust or zinc
oxide with various chemicals to form clouds. HC smoke is zinc chlo-
ride with an oxidizing agent to burn up residual carbon so that the

smoke will be gray and not black. Signal smoke is colored smoke
used for ship distress signals and for aviation marking signals. They
are mixtures of a fuel, an oxidizing agent, a dye, and sometimes a
cooling agent to regulate the rate of burning and to prevent decompo-
sition of the dye. Unmistakable colors are used so that the signals
may be distinguished from fires, and the dyes are mainly
anthraquinone derivates, together with mixtures of azo, azine, and
diphenyl-methane compounds.
SNAKESKINS. The snakeskins employed for fancy leathers are in gen-
eral the skins of large, tropical snakes which are notable for the
beauty or oddity of their markings. Snakeskins for shoe-upper
leathers, belts, and handbags are glazed like kid and calfskin after
SNAKESKINS 863
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Materials, Their Properties and Uses
tanning. Small cuttings are used for inlaying on novelties. The leather
is very thin, but is remarkably durable and is vegetable-tanned and
finished in natural colors, or is dyed. Python skins are used for
ladies’ shoes. Regal python skins from Borneo, the Philippines, and
the Malay Peninsula sometimes measure 30 ft (9 m) in length and
have characteristic checked markings. Diamond-backed rattlesnakes
are raised on snake farms in the United States. The meat is canned as
food, and the skins are tanned into leather. Only the back is used for
leather, as the belly is colorless.
SOAP. A cleansing compound produced by saponifying oils, fats, or
grease with an alkali. When caustic soda is added to fat, glycerin sep-
arates out, leaving sodium oleate, Na(C
17

H
33
O
2
), which is soap. But
since oils and fats are mixtures of various acid glycerides, the soaps
made directly from vegetable and animal oils may be mixtures of
oleates, palmitates, linoleates, and laurates. Soap oils in general,
however, are those oils which have greater proportions of nearly satu-
rated fatty acids, since the unsaturated fractions tend to oxidize to
form aldehydes, ketones, or other acids, and turn rancid. If an excess
of alkali is used, the soap will contain free alkali; and the greater the
proportion of the free alkali, the coarser is the action of the soap.
ASTM standards for milled toilet soap permit only 0.17% free alkali.
Soap makers now employ refined and bleached oils, which are then
hydrolyzed into fatty acids and glycerol prior to saponification with
caustic. This allows the fatty acids to be distilled, resulting in a more
stable product. Sodium soaps are always harder than potassium
soaps with the same fat or oil. Hard sodium soaps are used for chips,
powders, and toilet soaps. Soft, caustic potash soaps are the liquid,
soft, and semisoft pastes. Mixtures of the two are also used. Soaps are
made by either the boiled process or the cold process. Chip soap is
made by pouring the hot soap onto a cooled revolving cylinder from
which the soap is scraped in the form of chips or ribbons which are
then dried to reduce the moisture content from 30 to 10%. Soap
flakes are made by passing chips through milling rollers to make
thin, polished, easily soluble flakes.
Powdered soap is made from chips by further reducing the mois-
ture and grinding. Milled soaps are made from chips by adding color
and perfumes to the dried chips and then passing through milling

rollers and finally pressing in molds. Toilet soaps are made in this
way. Soap is used widely in industrial processing, and much of the
production has consisted of chips, flakes, powdered, granulated, and
scouring powders. Soaps have definite limitations of use. They are
unstable in acid solutions and may form insoluble salts. In hard
waters they may form insoluble soaps of calcium or magnesium
864 SOAP
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Materials, Their Properties and Uses

Materials Handbook 15th ed - G. Brady_ H. Clauser_ J. Vaccari (McGraw-Hill_ 2002) WW Part 13 doc

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