Question

1.Fused silica glass is used for expensive space vehicle windows , whereas sodalime glass is used for low cost glass windows for homes. Why is the cost of making these two glasses so much different. Answer this question by;

a)comparing the specific processing conditions required for each

b)comparing the specific properties which require these processing conditions

c)comparing the specific microstructural features which result in these properties

Answer 1

FUSED SILICA GLASS CHEMICAL COMPOSITION

Fused Silica (also called fused quartz). This glass is composed exclusively of high-purity silica (SiO2) in amorphous (non-crystalline) form. In contrast, other common glasses (Pyrex, borosilicate, soda-lime, etc.) are mixed composition glasses – in addition to SiO2, they include such ingredients as Na2O, CaO, and B2O3. These additional ingredients modify the optical and mechanical characteristics of the glass and lower the melt temperature of the mix. Fused Silica, on the other hand, is pure SiO2. It has a much higher working and melting temperature than other common optical glasses, it also has exceptional optical, thermal, and mechanical properties.

FUSED SILICA glass CHEMICAL PURITY

Contaminants, despite their presence at very low levels, have subtle yet significant effects. Purity is mostly determined by the raw material, the manufacturing method and subsequent handling procedures. Special precautions must be taken at all stages of the manufacturing process to maintain high purity.

The most common impurities are metals (Al, Na and Fe among others), water (present as OH groups), and chlorine. These contaminants affect the viscosity, optical absorption and electrical properties of the glass.

CHEMICAL COMPATIBILITY

Due to its pure composition, Fused Silica is an extremely inert glass. It is compatible with the vast majority of chemicals used in the industry and in the laboratory. There are only a few uncommon materials that will attack it, including HF (hydrofluoric acid) and hot KOH (Potassium hydroxide)

THERMAL PROPERTIES

COEFFICIENT OF THERMAL EXPANSION (CTE)

Fused Silica has a remarkably low coefficient of thermal expansion, which varies only slightly with temperature.

Fused Silica Mean Coefficient of Thermal Expansion (Source: Präzisions Glas & Optik GmbH, Menden, Germany):

- 0.52 x 10-6/K over the 5°C to 35°C temperature range
- 0.57 x 10-6/K over the 0°C to 200°C temperature range
- 0.48 x 10-6/K over the -100°C to 35°C temperature range
This material characteristic imparts fused silica a high resistance to thermal shock, and makes it an excellent material for applications that require the utmost in dimensional stability over a large temperature range. For comparison, INVAR (a nickel-iron alloy), which has the lowest thermal expansion among all metals and alloys near room temperature, has a mean coefficient of thermal expansion that is more than twice that of Fused Silica (1.3 x 10-6°/K over the 20°C to 100°C temperature range).

THERMAL CONDUCTIVITY

Fused Silica is an excellent thermal insulator. Its thermal conductivity is only 1.38W/mºK (measured at 25ºC), which is one of the lowest for any solid. For comparison, this value is 100-fold lower than that of silicon (148W/mºK @ 25 C); and one eighth of that of INVAR (10.5 W/mºK @ 25 C).

OPTICAL PROPERTIES

Fused Silica offers a set of optical characteristics that compare favorably with other optical materials. It is transparent from the deep ultraviolet (UV) to the mid-infrared.

OPTICAL ABSORPTION

Fused Silica is transparent in the ultraviolet, visible, and near infrared.

TEMPERATURE DEPENDENCE OF REFRACTIVE INDEX

The refractive indices of the glasses are not only dependent on wavelength, but also upon temperature. The dn/dT for Fused Silica is 1.1x10-5/ºC at 1064nm, near room temperature.

ELECTRICAL PROPERTIES

Fused Silica is an excellent electrical insulator well into the microwave, with a dielectric loss factor of under 0.0004 at 1 MHz.

MECHANICAL PROPERTIES

Less known is the fact that Fused Silica has excellent elastic properties, making it a superior material for manufacturing micro-mechanical pieces that must flex to provide some type of motion. This is counter-intuitive: To the casual observer, glass is the personification of a breakable material, or what material scientists call a brittle material; and thus, one naturally assumes it can’t be used to manufacture dynamic MEMS. When using standard fabrication processes, such as ultrasonic drilling, lapping, or sand-blasting, this is indeed true. These processes create microscopic cracks, which lower the tensile strength of the glass and make it break easily. However, when silica glass is machined with our ultra-low stress femtoEtch process, microcrack formation is prevented, and the resulting part exhibits an exceptional elastic behavior. With this low-stress approach, one can manufacture glass MEMS (GMEMS).  

Soda-lime glass
Soda-lime glass, also called soda-lime-silica glass, is the most prevalent type of glass, used for windowpanes and glass containers (bottles and jars) for beverages, food, and some commodity items. Glass bakeware is often made of tempered soda-lime glass. Soda-lime glass accounts for about 90% of manufactured glass.

Soda-lime glass is relatively inexpensive, chemically stable, reasonably hard, and extremely workable. Because it can be resoftened and remelted numerous times, it is ideal for glass recycling.

Soda-lime glass is prepared by melting the raw materials, such as sodium carbonate (soda), lime, dolomite, silicon dioxide (silica), aluminium oxide (alumina), and small quantities of fining agents (e.g., sodium sulfate, sodium chloride) in a glass furnace at temperatures locally up to 1675 °C. The temperature is only limited by the quality of the furnace superstructure material and by the glass composition. Relatively inexpensive minerals such as trona, sand, and feldspar are usually used instead of pure chemicals. Green and brown bottles are obtained from raw materials containing iron oxide. The mix of raw materials is termed batch.
Soda lime is not as chemical resistant as borosilicate glass. Its lower melting point and higher coefficient or expansion and contraction make it ideal for certain glass to metal operations as well as inexpensive glassware such as pipettes or plate glass. However those same coefficients make it unusable where high heat or great temperature fluctuations are necessary. Because its characteristics it is not recommended that soda lime "apparatus" be purchased in many cases. It is much more difficult (or sometimes impossible) to repair if damaged.

Chemical Composition

Composition

(percent approx.)

SiO2-73%
Na2O-14%
CaO-7%
MgO-4%
Al2O3-2%

Physical Properties

Coeff. of Exp.-89 x 10-7cm/cm/°C
Strain Point-511°C
Anneal Point-545°C
Soften Point-724°C
Density-2.40 g/cm3
Youngs Mod.-Not available