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General Refractories Co. Refrax silicon carbideof 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 aluminum. 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 (165 MPa) and compressive strength of

150,000 lb/in (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

atmospheres is to 4000F (2204C) and in oxidizing atmospheres to 3000F

(1649C). 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 Rof Norton Co. is a stabilized silicon carbide bonded by

recrystallization. It has a specific gravity of 2.5, a tensile strength of 5,500

lb/in (38 MPa), compressive strength of 25,000 lb/in (172 MPa), and Knoop

hardness of 2,500. The porosity is 21%. It is for parts subject to temperatures

to 4200F (2316C), and it withstands high thermal shock. Crystolon Cis a

self-bonding silicon 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 integral

part of the graphite surface.Vitroporefilter 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 recovering catalysts from fluid-

catalytic-cracking processes. Diasil,of Crestel Pty of Australia, comprises

low-value, crushed diamond dust in silicon carbide and is used for cutting,

drilling, and grinding ceramics.

For humidity protection of integrated circuits, Dow Corning and National

Semiconductor developed a dual thin-film coating for application prior to

plastic encasement. A silicon dioxidelayer smooths circuit topology, then a

silicon carbide layer serves as a barrier against moisture and activated ion

impurities.

Silicon carbide foamis a lightweight material made of self-bonded silicon

carbide foamed into shapes. The low-density foam has a density of 17 lb/ft

(272 kg/m ), a porosity of 90%, and tensile and compressive strengths of 30

lb/in (0.2 MPa). The high-density foam of 33 lb/ft (529 kg/m ) has a tensile

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they can be made with higher tensile strength and modulus values than

continuous silicon carbide filament.

4.150. SILICON CAST IRON.

An acid-resistant cast ironcontaining 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 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 separates

out as graphite in silicon irons, causing faulty castings. Increasing thecontent of silicon in iron reduces the melting point progressively from 2786F

(1530C) for pure iron to 2282F (1250C) for iron containing 23% silicon. A 14

to 14.5% silicon ironhas a silvery-white structure, a compressive strength

of about 70,000 lb/in (483 MPa), and Brinell hardness 299 to 350, and it is

resistant to hot sulfuric acid, nitric acid, and organic acids. Silicon irons are

very wear-resistant and are valued for pump parts and for parts for chemical

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 (110 MPa) and density 0.253 lb/in (7,003 kg/m ).

4.151. SILICON-COPPER.

An alloy of silicon and copper used for adding silicon to copper, brass, or

bronze, also employed as a deoxidizer of copper and for making hard copper.

Silicon alloys in almost any proportion with copper, and is the best

commercial hardener of copper. A 5050 alloy of silicon and copper is hard,

extremely brittle, and black. A 10% silicon, 90 copper alloy is as brittle as

glass; in this proportion silicon copper is used for making the addition to

molten copper 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 1500F (816C); a 20% alloy

melts at 1152F (623C).

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