Physical Chemistry , 1st ed.

One important application of the detailed understanding of solid solution

phases is called zone refining,which is a method for preparing very pure ma-

terials. It is especially useful in the semiconductor industry, where the produc-

tion of ultrapure silicon is the crucial first step in making semiconductors.

Figure 7.25 shows a temperature-composition phase diagram for silicon and

silicon oxides. “Pure” silicon, which would have a composition very near the

zero value for weight percent of oxygen in Figure 7.25, still has enough impu-

rities to cause problems with the electrical properties of silicon, so it must be

purified further.

A solid cylinder of Si, called a boule,is slowly passed through a cylindrical

high-temperature furnace, as shown in Figure 7.26. (Silicon melts at 1410°C.)

When it slowly resolidifies, it does so as very pure silicon, and the impurities

remain in the melted phase. As the boule passes further through the furnace,

this impure layer collects more of the impurities as the ultrapure silicon crys-

tallizes. In the end, as seen in Figure 7.26, the entire boule has passed through

the furnace and the impurities are concentrated in one end, which is cut off.

What remains is a cylinder of ultrapure crystalline silicon that can be cut into

thousands or millions of semiconductors. Other crystals, including synthetic

gemstones, can be fashioned in this way.

192 CHAPTER 7 Equilibria in Multiple-Component Systems

1200

0%

Hypo-eutectoid Hyper-eutectoid

Steel Cast iron

Temperature (

°F)

Temperature (

°C)

0.50%

0.008%

1000

410

3000

2800

2802

2720

2600

2552

2400

2200

2066

2000

1800

1670

1600

1400

1333 723

210

760

910

1130

1400

1492

1539

0.83%1% 2% 3% 4% 5% 6%65%

Austenite solid solution of

carbon in gamma iron

Pearlite and ferrite

Pearlite and cementite

Cementite, pearlite, and

transformed ledeburite

4.3 6.67

Magnetic change of Fe 3 C

Austenite to pearlite

0.025



  

  



  L

  L



  Fe 3 C

  Fe 3 C

L  Fe 3 C

Fe 3 C

Cementite and

ledeburite

Austenite, ledeburite,

and cementite

2066 °F

1333 °F

Austenite

in liquid

Primary austenite

begins to solidify

CM begins

to solidify

  Austenite

  Ferrite

  Delta iron

CM  Cementite

L  Liquid

Figure 7.24 A more complicated solid solution phase diagram, in this case describing the

system Fe/C.

2000

400

0

Weight percent O in Si-O system

0.10

Temperature (

°C)

1800

1600

1400

1200

1000

800

600

0.02 0.04 0.06 0.08

Two liquid phases

Solid cristobalite

Solid tridymite

Solid silicon

T = 1414°C

T = 870°C

T = 1471°C T = 1721°C

Solid quartz

Figure 7.25 The temperature-composition
phase diagram for silicon and silicon oxides. This
phase diagram is very important to the semicon-
ductor industry, where ultrapure silicon is the
first step in making microchips.