GLASS

• Glass- not just a functional material to let light into an area

• Used to add decorative effect. • Important to choose the right kind of glass

for the right place- to be effective, attractive and safe.

• The wrong type of glass used in the wrong position can be unsatisfactory and present a serious hazard to personnel safety.

Different types of glass1. 'Ordinary' sheet glass 2. Float glass (plate) 3. Energy efficient glass 4. Patterned (obscured glass) 5. Toughened (Safety glass) 6. Laminated glass 7. Wired glass8. Mirrors 9. Picture frame glass 10. Soda-lime glass (or lime glass) 11. Lead-alkali glass (also called lead glass)12. Borosilicate glass13. Alumino-silicate glass14. Ninety-six percent silica glass15. Fused silica glass16. Fused Quartz17. Colouring Glass

1. 'Ordinary' sheet glass

• This glass is made by passing the molten glass through rollers; this process gives an almost flat finish but the effects of the rollers upon the molten glass makes some distortion inevitable. The glass can be used in domestic windows etc. but the relatively low cost of float glass (with its lack of distortion) has tended to restrict ordinary sheet glass to glazing greenhouses and garden sheds where the visual distortions do not matter.

• Sheet glass can be cut a glass cutter and no special equipment is necessary. The glass is often available in standard sizes to suit 'standard' glasshouses, these sizes tend to be comparatively cheaper than glass cut to size.

2. Float glass (plate)

• Float glass gets its name from the method of production used to manufacture it. The molten glass is 'floated' onto a bed of molten tin - this produces a glass which is flat and distortion free.

• Float glass can be cut using a glass cutter and no special equipment is necessary. Float glass is suitable for fixed and opening windows above waist height.

3. Energy efficient glass

• Some manufacturers produce float glass with a special thin coating on one side which, allows the suns energy to pass through in one direction while reducing the thermal transfer the other way. The principle behind this is the difference in thermal wavelength of energy transmitted from the sun and that transmitted from the heat within a room.

• The special coating often gives a very slight brown or grey tint to the glass. The coating is not very robust and would not last very long if subjected to normal cleaning or external weather conditions - for these reasons, this type of glass is normally only used in sealed double (or triple) glazed units with the special coating on the inside

4. Patterned (obscured glass)

• Made from flat glass, this type has a design rolled onto one side during manufacture. It can be used for decorative effect and/or to provide privacy. Patterned glass is available in a range of coloured tints as well as plain.

• A variety of pattern designs are available, each pattern normally has an quoted distortion number, from 1 to 5, 1 being very little distortion, 5 being a high level of diffusion.

• On external glazing, the patterned side is usually on the inside so that atmospheric dirt can easily be removed from the relatively flat external face.

5. Toughened (Safety glass)

• Toughened glass is produced by applying a special treatment to ordinary float glass after it has been cut to size and finished. The treatment involves heating the glass so that it begins to soften (about 620 degrees C) and then rapidly cooling it. This produces a glass which, if broken, breaks into small pieces without sharp edges. The treatment does increase the surface tension of the glass which can cause it to 'explode' if broken; this is more a dramatic effect than hazardous.

• It is important to note that the treatment must be applied only after all cutting and processing has been completed, as once 'toughened', any attempt to cut the glass will cause it to shatter.

• Toughened glass is ideal for glazed doors, low level windows (for safety) and for tabletops (where it can withstand high temperature associated with cooking pots etc.

6. Laminated glass

• As the name suggests, laminated glass is made up of a sandwich of two or more sheets of glass (or plastic), bonded together by a flexible, normally transparent material.

• If the glass is cracked or broken, the flexible material is designed to hold the glass fragments in place.

• The glass used can be any of the other basic types (float, toughened, wired etc.) and they retain their original breaking properties.

• Some laminated glass is laminated for other reasons than just keeping any broken glass in place, some provide decorative internal finishes to the glass while others act as fire breaks.

7. Wired glass

• Wired glass incorporates a wire mesh (usually about 10mm spacing) in the middle of the glass. Should be glass crack or break, the wire tends to hold the glass together. It is ideal for roofing in such areas as a garage or conservatory where its 'industrial' look is not too unattractive.

• Wired glass is generally not considered a Safety glass as the glass still breaks with sharp edges.

• Wired glass is available as clear or obscured.

8. Mirrors

• • Mirrors are usually made from float glass 4-6mm

thick, and silvered on one side. Mirrors are available for use without a surrounding frame, these usually are made from a type of safety glass. Old mirrors, and modern mirrors supplied within a frame, should not be used unframed as any damage to them might cause the glass to shatter dangerously.

9. Picture frame glass

• Glass (and plastics) are available specifically for picture framing, these tend to be referred to as 'diffused reflection' glass or plastic. They have high transparency but low reflective properties to reduce reflections when the picture or photograph is viewed.

• Most of these materials can easily be cut by the average diy person providing suitable tools and safety precautions are taken.

10. Soda-lime glass ( lime glass)

• With only minor compositional differences designed to optimize the glass for the forming process and application.

• Soda lime glass is used as the outer shell for both incandescent and fluorescent lighting applications.

• Also used in non-lighting applications such as Christmas Glass

most common glass. It is made of oxides of silicon (SiO2), calcium

(CaO) and sodium(Na2O).

cheap to make and can be made into a

wide variety of shapes; medium resistance to high temperatures and

sudden changes of temperature, fair

resistance to corrosive chemicals.

used to make bottles and windows

11. Lead-alkali glass (also called lead glass)

• lead oxide (PbO) is used in place of calcium oxide. more expensive than soda-lime glass; excellent electrical insulating properties; poor resistance to high temperatures and sudden changes of temperature. used for electrical applications.

• . Lead Glass• Lead glass is melted and drawn into small-

diameter tubing at the Versailles, KY plant. This glass has a much higher electrical resistivity and remains workable over a wider temperature range than soda lime glass. Its primary use is for flare and exhaust tubing in incandescent and fluorescent lighting products. It also is used extensively for neon signs.

12. Borosilicate glass

• appreciable resistance to high temperature or sudden changes in temperature; medium resistance to chemical attack. Moderate cost to make. used for light bulbs, photochromic glasses, sealed-beam headlights, laboratory ware, and some bake ware products.

Borosilicate Glass Borosilicate glass is melted and formed

into blown bulbs and small diameter tubing at our Central Falls, RI, facility.

This glass has high durability, high thermal shock resistance, and high

electrical resistivity. Its optical transmission is controlled to cut off

harmful ultraviolet radiation. Borosilicate glass is designed for HID (high-intensity discharge) lighting applications, in which

hot lamps are exposed to outdoor conditions for many years.

The tubing drawn from this glass seals well to both the tungsten lead wires and the blown bulbs, and therefore is used

for flare and exhaust tubing in HID applications.

13. Alumino-silicate glass

• alumina (Al2O3) is added to the glass batch to improve the properties of the glass.

• good resistance to high temperature or sudden changes in temperature; difficult to make.

• used in electronics

14. Ninety-six percent silica glass

• special type of glass made by a proprietary method, at temperatures up to 900°C.

• used to furnace sight glasses, for outer windows on space vehicles.

15. Fused silica glass

• only made of silicon dioxide (SiO2) in the noncrystalline state.

• expensive and difficult to make; maximum resistance to high temperature (900°C for extended periods, 1200°C for short periods).

• used in special applications such as optical waveguides, crucibles

16. Fused Quartz

• Fused Quartz are ultra pure, single component glasses (SiO2) with a unique combination of thermal, optical and mechanical properties, which make them the preferred materials for use in a variety of processes and applications where other materials are not suitable.

• The very high purity (over 99.9% ) ensures minimum contamination in process applications.

• These materials can routinely withstand temperatures of over 1250ºC, and due to their very low coefficient of thermal expansion can be rapidly heated and cooled with virtually no risk of breakage due to thermal shock.

• Fused Quartz are inert to most substances, including virtually all acids, allowing their use in arduous and hostile environments.

• The dielectric properties and very high electrical receptivity of these materials over a wide range of temperatures, together with their low thermal conductivity allow their use as an electrical and thermal insulating material in a range of environments.

• Fused Quartz is manufactured using powdered quartz crystal as a feedstock and is normally transparent; the fusion process is carried out at high temperature (over 2000ºC).

Fused Quartz Fused quartz products are melted and formed at our Exeter facility. Major applications include both the lighting and semiconductor industry.In lighting products, fused quartz is widely used in high-temperature arc and filament

lamps requiring high purity to minimize devitrification and provide optimum sag

resistance. These attributes contribute to the long life of these lamps at high operating

temperatures. Major semiconductor manufacturers

worldwide use OSRAM SYLVANIA's fused quartz for its high chemical purity, high-

temperature resistance, and precise dimensional tolerances. Common applications include furnace tubes for oxidation, CVD and

diffusion processes, end caps, transfer carriers, thermocouple tubes, wafer carriers, end plates, baffles and bell-jars for epitaxial

reactors.

Fused quartz and fused silica • Vitreous silica is the generic term used to describe all

types of silica glass, with producers referring to the material as either fused quartz or as fused silica.

• originally used to distinguish between transparent and opaque grades of the material.

• Fused quartz products - those produced from quartz crystal into transparent ware, and fused silica - manufactured from sand into opaque ware.

• Advances in raw material bonification permit transparent fusions from sand as well as from crystal.

• Consequently, if naturally occurring crystalline silica (sand or rock) is melted, the material is simply called fused quartz.

• If the silicon dioxide is synthetically derived the material is referred to as synthetic fused silica.

• Controlled Process: The performance of most fused quartz products is closely related to the purity of the material. The proprietary raw material bonification and fusion processes are closely monitored and controlled to yield typically less than 50 ppm total elemental impurities by weight. Clear fused quartz varieties have a nominal purity of 99.995 W % SiO2. Structural hydroxyl (OH-) impurities are also shown. The strong IR absorption of OH- species in fused quartz provides a quantitative method for analysis. Beta Factor: The term Beta Factor is often used to characterize the hydroxyl (OH-) content of fused quartz tubing. This term is defined by the formula shown below.

Property Typical Values• Density 2.2x103 kg/mm3• Hardness 5.5 - 6.5 Mohs' Scale 570 KHN 100• Design Tensile Strength 4.8x107 Pa (N/mm2) (7000 psi)• Design Compressive Strength Greater than 1.1 x l09 Pa (160,000 psi)• Bulk Modulus 3.7x1010 Pa (5.3x106 psi)• Rigidity Modulus 3.1x1010 Pa (4.5x106 psi)• Young's Modulus 7.2x10-10 Pa (10.5x106 psi)• Poisson's Ratio .17• Coefficient of Thermal Expansion 5.5x10 -7 cm/cm . oC (20øC-320oC)• Thermal Conductivity 1.4 W/m . oC • Specific Heat 670 J/kg . oC• Softening Point 1683oC• Annealing Point 1215oC• Strain Point 1120 oC• Electrical Receptivity 7x107 ohm cm (350oC)• Dielectric Properties (20oC and 1 MHz) • Constant 3.75

• Strength 5x107 V/m • Loss Factor Less than 4x10 -4 • Dissipation Factor Less than 1x10 -4• Index of Refraction 1.4585• Contingence (Nu) 67.56• Velocity of Sound-Shear Wave 3.75x103 m/s• Velocity of Sound/Compression Wave 5.90x103 m/s• Sonic Attenuation Less than 11 db/m MHz• Permeability Constants (cm3 mm/cm2 sec cm of Hg) (700oC)

• Helium 210x10 -10 • Hydrogen 21x10 -10 • Deuterium 17x10 -10• Neon 9.5x10 -17

• Electrical Properties Electrical conductivity in fused quartz is ionic in

nature. Alkali ions exist only as trace constituents. Fused quartz is preferred for electrical insulation

and low loss dielectric properties. The electrical insulating properties of clear fused

quartz are superior to those of the opaque or translucent types.

Both electrical insulation and microwave transmission properties are retained at very high temperatures and over a wide range of frequencies.

• Effects Of Temperature

Fused quartz is a solid material at room temperature, but at high temperatures, it behaves like all glasses.

It does not experience a distinct melting point as crystalline materials do, but softens over a fairly broad temperature range.

This transition from a solid to a plastic-like behavior, called the transformation range, is distinguished by a continuous change in viscosity with temperature.

• Viscosity • Viscosity- the measure of the resistance to flow. • viscosity scale is generally expressed

logarithmically. • Common glass terms for expressing viscosity

include: strain point, annealing point, and softening point, which are defined as: Strain Point: The temperature at which the internal stress is substantially relieved in four hours. This corresponds to a viscosity of 10 14.5 poise, where poise = dynes/cm2 sec.

• Annealing Point: The temperature at which the internal stress is substantially relieved in 15 minutes, a viscosity of 10 13.2 poise.

• Softening Point: The temperature at which glass will deform under its own weight, a viscosity of approximately 10 7.6 poise. The softening point of fused quartz has been variously reported from 1500 ºC to 1670ºC, the range resulting from differing conditions of measurement.

• Cristobalite Growth The growth rate of cristobalite from the nucleation site depends on

certain environmental factors and material characteristics. Temperature and quartz viscosity are the most significant factors,

but oxygen and water vapor partial pressures also impact the crystal growth rate.

Consequently, the rate of devitrification of fused quartz increases with increasing hydroxyl (OH-) content, decreasing viscosity and increasing temperature.

High viscosity, low hydroxyl fused quartz materials produced , therefore, provide an advantage in devitrification resistance.

The phase transformation to Beta-cristobalite generally does not occur below 1000ºC. This transformation can be detrimental to the structural integrity of fused quartz if it is thermally cycled through the crystallographic inversion temperature range (250 ºC). This inversion is accompanied by a large change in density and can result in spalling and possible mechanical failure.

• Mechanical Properties • Mechanical properties of fused quartz are much

the same as those of other glasses. Material is extremely strong in compression, with design compressive strength of better than 1.1 x 10 9 Pa (160,000 psi).

• Surface flaws can drastically reduce the inherent strength of any glass, so tensile properties are greatly influenced by these defects. The design tensile strength for fused quartz with good surface quality is in excess of 4.8 x 10 7 Pa (7,000 psi). In practice, a design stress of .68 x 10 7 Pa (1,000 psi) is generally recommended.

• Thermal Properties • One of the most important properties of fused quartz is

its extremely low coefficient of expansion: 5.5 x 10 -7 mm ºC (20-320ºC). Its coefficient is 1/34 that of copper and only 1/7 of borosilicate glass. This makes the material particularly useful for optical flats, mirrors, furnace windows and critical optical applications which require minimum sensitivity to thermal changes. A related property is its unusually high thermal shock resistance.

• For example, thin sections can be heated rapidly to above 1500 ºC and then plunged into water without cracking. The residual stress or design, depending on the application, may be in the range of 1.7 x 10 7 to 20.4 x 10 7 Pa (25 to 300 psi). As a general rule, it is possible to cool up to 100ºC /hour for sections less than 25 mm thick.

• Optical Properties • Optical transmission properties provide a means for

distinguishing among various types of vitreous silica as the degree of transparency reflects material purity and the method of manufacture.

• Specific indicators are the UV cutoff and the presence or absence of bands at 245 nm and 2.73 um. The UV cutoff ranges from ~155 to 175 nm for a 10 mm thick specimen and for pure fused quartz is a reflection of material purity.

• The presence of transition metallic impurities will shift the cutoff toward longer wavelengths. When desired, intentional doping, e.g., with Ti in the case of Type 219, may be employed to increase absorption in the UV.

• The absorption band at 245 nm characterizes a reduced glass and typifies material made by electric fusion. If a vitreous silica is formed by a "wet" process, either flame fusion or synthetic material, for example, the fundamental vibrational band of incorporated structural hydroxyl ions will absorb strongly at 2.73 um.

17. Colouring Glass

• Unless the raw materials are very pure, glass is normally green.

• In order to change the colour of glass, one can decolourise the glass by adding colorants which produce the complementary colour to green.

• The colour depends on the state of oxidation of the colorant, the composition of the glass and the thermal treatment.

COLORANT GLASS COLOUR/S

iron green and aqua

iron and sulfur amber

copper light blue

cobalt dark blue

manganese Purple

tin and calcium opaque white

lead plus antimony opaque yellow

selenium red

neodymium purple

praseodymium green

cerium yellow

carbon and sulphur amber, brown

cadmium sulphide yellow

antimony sulphide red

gold red

Strength of Glass

• Glass is a strong material. Like most materials, glass can be bent until a certain limit. Imagine bending a long rod of glass. If you release the tension before this limit, the rod returns to its original shape. The deformation is elastic. If you pass the limit, the glass breaks

Why does glass shatter?

•The strength of glass is only slightly affected by composition but is highly dependent on the surface condition. If stress is applied on the damaged surface, the stress at the damaged points will be increased and the glass will shatter.

• Glass does not age quickly: glass windows remain clear and undamaged after many years of exposure to the elements.

Properties of Glass

• The thermal, optical, electrical and chemical properties of glass vary with its composition.

• Glass is a good thermal conductor. • Glass is an electrically insulating material: it does not conduct

electricity. • When light falls on glass, part of the light is reflected at the surface,

part of the light is absorbed in the glass, and part of the light is transmitted.

• If most of the light is transmitted, the glass is transparent. By colouring the glass or changing its composition, it is possible to transmit selectively some wavelengths of the spectrum.

• Common glass does not transmit ultra-violet radiation (short wavelengths): you will not get a tan behind a window! However it does transmit infrared radiation

Some Chemical Properties of Glass

• The chemical properties of glass vary with its composition.

• Glass is highly resistant to most chemicals: this is why it is in use in all chemistry labs. Its big enemy is hydrofluoric acid which dissolves glass quickly.

• Water can corrode glass at very high temperatures, especially if the water is alkaline (Ph > 7). At low temperatures however, water corrosion is very

Light and Glass

• Scientists and engineers have experimented with light and ways to guide light for many centuries: glass was the prime choice material.

• It was not until the 1950's that the first optical fibres were made. Although these optical fibres could transmit light, they did not carry information very far: most of the signal was lost due to a high absorption.

• In 1966, Dr. C.K. Kao and George A. Hockham published a paper in which they discussed and proved the possibility of long distance communications over optical fibres provided that the optical fibres had low absorption.

• In 1970, three Corning scientists Dr Robert Maurer, Dr Donald Keck and Dr Peter Schultz developed the first low absorption optical fibre.

• Just a dream more than 3 decades ago, long distance communication through optical fibres has now become a reality.

Glass at the Atomic Scale

• Glass holds a special place in physics: it is not a crystalline solid and it is not a liquid.

• A crystal is made of atoms which are arranged in a unit cell. This unit cell is repeated in all three directions. This order is retained over long atomic distances: it is referred to as long range order.

• A liquid on the other hand lacks this order: the atoms are not rigidly bound to each other and they "flow" in the material. A liquid has no order.

• Glass is an amorphous or non-crystalline solid: it is in a state between the crystalline state and the liquid state. It does not have the long range order of crystals but it is not a liquid either. The atoms in the glass are bound to each other but they lack the long range order. However locally they can be ordered and possess a short range order.

OPTICAL FIBRE

• Made of extremely pure silica. • Thinner than a human hair and stronger than

a steel fibre of similar thickness. It can carry thousands of times more information than a copper wire!

• Optical fibre cables have the advantage of being lighter and taking less space than copper wire cables for the same information capacity.

Fabrication of Optical Fibres• The best cakes are made of the best ingredients. • To make a good optical fibre, we need to start with good quality materials, that is

highly purified materials.

• The presence of impurities alter the optical properties of the fibre.

• There are several ways to manufacture optical fibres:

• Directly drawing the fibre from what is called a preform.

• The fibre is then drawn from the preform

i) Direct Techniques

Two methods can be used to draw a fibre directly: 1. Double Crucible method 2. Rod in Tube method

1. Double Crucible

The molten core glass is placed in the inner crucible.

The molten cladding glass is placed in the outer crucible.

The two glasses come together at the base of the outer cucible and a fibre is

drawn. Long fibres can be produced (providing you don't let the content of the crucibles

run dry!). Step-index fibres and

graded-index fibres can be drawn with this method.

2. Rod in Tube A rod of core glass is placed inside a tube of cladding glass. The end of this assembly is heated; both are softened and a fibre is drawn.

Rod and tube are usually 1 m long. The core rod has typically a 30 mm diameter. The core glass and the cladding glass must have similar softening temperatures.

This method is relatively easy: just need to purchase the rod and the tube.

However, must be very careful not to introduce impurities between the core and the cladding.

ii) Deposition Techniques

• Most optical fibres are made from preforms. The preforms are made by deposition of silica and various dopants from mixing certain chemicals; the fibre is then drawn from the preform.

• Many techniques are used to make preforms. Among them:

• · Modified Chemical Vapour Deposition or MCVD • · Plasma-Enhanced Modified Chemical Vapour

Deposition or PMCVD • · Outside Vapour Deposition or OVD • · Axial Vapour Deposition or AVD

The Chemicals

• Oxygen (O2) and silicon tetrachloride (SiCl4) react to make silica (SiO2).

• Pure silica is doped with other chemicals such as boron oxide (B2O3), germanium dioxide (GeO2) and phosphorus pentoxide (P2O5) are used to change the refractive index of the glass.

Modified Chemical Vapour Deposition (MCVD)

• The chemicals are mixed inside a glass tube that is rotating on a lathe.

• • They react and extremely fine particles

of germano or phosphoro silicate glass are deposited on the inside of the tube.

• A travelling burner moving along the tube:

causes a reaction to take place and then fuses the deposited material.

• The preform is deposited layer by layer starting first with the cladding layers and followed by the core layers.

• Varying the mixture of chemicals changes the refractive index of the glass.

• When the deposition is complete, the tube is collapsed at 2000 C into a preform of the purest silica with a core of different composition.

• The preform is then put into a furnace for drawing.

Plasma-Enhanced Modified Chemical Vapour Deposition (PMCVD)

Plasma-Enhanced Modified Chemical Vapour Deposition is similar in principle to MCVD. The

difference lies in the use of a plasma instead of a torch. The plasma is a region of

electrically heated ionised gases. It provides sufficient heat to

increase the chemical reaction rates inside the tube and the

deposition rate. This technique can be used to

manufacture very long fibres (50 km).

It is used for both step index and graded index fibres.

Outside Vapour Deposition (OVD)

The chemical vapours are oxidised in a flame in

a process called hydrolysis.

The deposition is done on the outside of a silica rod as the torch moves

laterally.When the deposition is

complete, the rod is removed and the

resulting tube is thermally collapsed

Axial Vapour Deposition (AVD)

The deposition occurs on the end of a rotating

silica boule as chemical vapors react to form silica. Core preforms and very long fibres can be made

with this technique. Step-index fibres and

graded-index fibres can be manufactured this w

From Preform to Fibre• All these deposition

techniques produce preforms. These are typically 1 m long and have a 2 cm diameter but these dimensions vary with the manufacturer.

• The preform is one step away from the thin optical fibre. This step involves a process called drawing.

Fibre Drawing and Spooling • During this last step

of the fabrication process, many things will happen to the fibre:

• · the fibre is drawn from the preform.

• · it is quality checked

• · it is coated for protection

• · it is stored on a spool (just like a photographic film).

• The tip of the preform is heated to about 2000°C in a furnace. As the glass softens, a thin strand of softened glass falls by gravity and cools down.

• As the fibre is drawn its diameter is constantly monitored

• A plastic coating is then applied to the fibre, before it touches any components. The coating protects the fibre from dust and moisture.

• The fibre is then wrapped around a spool.

Fabrication of an Optical Fibre

Heating the preform Drawing the fibre