DM308 Production Techniques 2 Lecture 5 – Properties and Processing of Glasses (PDF)

DM308 Production Techniques 2 Lecture 5 – Properties and processing of glasses Dr. Vassili Vorontsov Department of Design, Manufacture and Engineering management, Faculty of Engineering, University of Strathclyde [email protected] Historic use of glass Glass occurs in nature within a nu...

DM308 Production Techniques 2 Lecture 5 – Properties and processing of glasses Dr. Vassili Vorontsov Department of Design, Manufacture and Engineering management, Faculty of Engineering, University of Strathclyde [email protected] Historic use of glass Glass occurs in nature within a number of minerals, including obsidian (volcanic), moldavite (meteor impact) and fused sand (lightning strike). Some of these can be knapped like flint and have been used by since prehistoric times by people to make tools and artefacts. First man made glass was produced as early as 3000BCE probably as a by product of metal-working (slags). It remained a luxury material until the its more widespread adoption in the Roman Empire. The middle ages saw more widespread the use of glass as a building material for stained glass widows in religious buildings and palaces. Full glass windows only became commonplace with the advancement of modern plate glass technologies. Silicon dioxide The building block of glass is silicon dioxide SiO2, also known as silica. Silicon can bond with 4 other atoms (tetravalent) and oxygen can bond with 2 other atoms (bivalent). As a stand-alone molecule SiO2 would have double covalent bonds between the silicon and oxygen atoms. These bonds are quite weak and molecular silica rarely exists in nature. Silicon dioxide tends to form larger structures. Silicon-oxygen tetrahedra Instead, silicon-oxygen tetrahedra are formed. Each silicon atom is bonded with four oxygen atoms and each oxygen atom is bonded with two silicon atoms. Thus there are no double Si=O bonds, only single Si-O bonds. There are many ways in which the tetrahedra can connect with one another. Quartz Silica can have a range of possible crystal structures. Their stability depends on the pressure and temperature. The most common crystalline form of silica is Quartz. It has a diamond-like structure. All the oxygen atoms act as bridges between silicon atoms. Thus, these oxygen atoms are termed “bridging oxygens”. Quartz There are two forms / polymorphs of quartz, which differ in the way that the silicon-oxygen tetrahedra are connected. ` a-quartz b-quartz (trigonal) (hexagonal) Quartz uses Quartz occurs naturally in many minerals including sand and rock crystal. Historically rock crystal has been carved to produce decorative art objects. Today quartz is value more for its piezoelectric properties. (straingenerated voltage and voltageinduced strain). It is used to make quartz resonators for timing electronic signals (inc. wathches, computers). It can also be used to make very sensitive balances – quartz crystal microbalances. Silica phase diagram Consider the phase diagram below for phase transitions in silica. Atmospheric pressure is 1 bar. (Close to 0 kbar) Before melting, a quartz undergoes several phase transitions. This limits the applicability of pure quartz as a hightemperature material. Alloying is usually required to stabilise a particular polymorph. (e.g. with Al2O3) Structure of Glass Glassy silica does not have the long-range order of quartz. There is no definable crystal structure of lattice+basis that obeys long range translational symmetry (tessellation). Glassy silica without additives is called Fused Silica. Glass Quartz Fused silica - Near zero thermal expansion - Exceptional resistance to thermal shock - Chemically inert - Good UV transparency - Low dielectric constant and dielectric loss Uses: High-temperature lamp envelopes Lenses and mirrors in variable temperature environments Raw material for optical communication fibres Refractories Glass transition Unlike crystalline silica, amorphous glass has no defined melting point. Instead, it gradually transitions from a stiff brittle glassy state to a soft rubbery state whereby it flows under applied stress. Specific volume Liquid Supercooled liquid Shrinkage due to freezing Glass Crystalline solid Tg Temperature Tm Modifying glass properties Fused silica has a high glass transition temperature (1600ºC) and high viscosity that make it difficult to work with. Additives are thus frequently introduced to alter the properties of glasses to facilitate their processing and to alter their properties. Network formers – are additives that help form the molecular network in the glass. Network modifiers – break up the molecular network of glass by creating more “non-bridging oxygen” atoms. Network formers and modifiers Silica-based glasses readily incorporate oxides of other elements as either network formers or network modifiers. - network former - network modifier - intermediate behaviour Soda-lime glass Approximately 90% of all manufactured glass is soda-lime glass. Consists of 70-75 wt% silica. Soda, Na2O, is added as a network modifier to lower the glass transition temperature. Addition of Na2O makes the glass soluble in water. Lime, CaO, is added to improve the chemical stability and makes the glass insoluble in water. The glass transition is ~570°C, allowing for convenient forming. Uses: construction, containers Borosilicate glass Soda-lime glass has poor thermal shock resistance and cracks if the temperature differential exceeds 37°C. Borosilicate glass can withstand thermal stresses of 165°C. Boric oxide, B2O3, is added as a network former. A smaller amount of Na2O is also added. The glass transition is ~820°C Low thermal expansion Max. use temperature ~500°C Uses: lab-ware, cookware, telescope mirrors, low-refraction optics, medical equipment, 3D printing platforms, lamp envelopes, nuclear waste immobilisation, solar heat collectors. Lead glass Lead glass is also know as “lead crystal” and is made with the addition of lead oxide PbO. PbO lowers the melt viscosity Easier to form when hot High refractive index and light dispersal Absorbs radiation Evidence of toxicity Poor thermal shock resistance Uses: glassware, jewellery, decorative arts, optics, radiation shielding (e.g. hospital x-ray booths, hot cells, electron microscopes) Bioglass Bioglass is a glass composition designed for medical applications that require repair of damaged bone tissue – i.e. manufacture of bioactive scaffolds. The formulation of bioglass is such that it is water soluble and chemically similar to hydroxyapatite – the main mineral in bone. Bioglass- 45% SiO2, 24.5% CaO, 24.5% Na2O, 6.0% P2O5 by weight. (Hydroxyapatite is Ca10(PO4)6(OH)2) Bioglasses dissolve in the body and their minerals are used in growth of new bone tissue. Glass ceramics Materials containing both crystalline and amorphous phases can be produced using controlled crystallisation. The crystalline phase has a negative thermal expansion coefficient. Controlling the phase proportions allows for materials with net zero thermal expansion. The also materials have very low thermal conductivity and can be made transparent to infra-red radiation. Glass ceramic systems: LAS – Li2O-Al2O3-nSiO2 MAS – MgO-Al2O3-nSiO2 ZAS – ZnO-Al2O3-nSiO2 Uses: cooktops, fire doors, fireplaces, cookware, high temperature applications Uranium glass and vitrification Glass made with uranium oxide was produced by glass-blower before the onset of the nuclear age and the resulting tight control of uranium. It was yellow-green in appearance and was mostly used in artistic glass-blowing. It also fluoresces under UV light. Today glass is used to immobilise highly radioactive nuclear waste. (Glass incorporates oxides and most nuclear fuel is in uranium oxide). This process is known as vitrification. A highly-stable water-insoluble formulation of glass is used and sealed in stainless steel containers. The waste is still highly-radioactive and very hazardous to humans for centuries. There are no long-term studies to confirm the safety. (Fingers crossed it works.) Float glass process The Pilkington float glass process is used to produce uniformly flat glass sheet. It relies on floating the molten glass on top of a bath of molten tin, which has higher density than the glass. Float glass process stages 1. Raw materials are weighed and loaded automatically into the furnace. Water is added to improve mixing and minimise airborne dust hazards. 2. (I) Raw materials are melted at 1600°C (II) Molten glass is homogenised and bubbles are removed (III) Glass melt is cooled to 1100°C to give correct viscosity for float bath. 3. Glass flows out over a smooth bath of molten tin leaving the bath at ~500°C. The tin bath is in a protective atmosphere of nitrogen and hydrogen. 4. Glass is annealed within a long cooling tunnel (Lehr) where it is gradually cooled to room temperature. This avoids residual stresses in the glass. 5. The glass ribbon is electronically inspected and cut into sheets. 6. The glass sheets are packed and loaded for offline coating and shipment. Fusion forming process Fusion forming is also known as “overflow and down-draw method”. Melting Powder Refining Flow control It is another process for producing flat glass sheet. It is another process for producing flat glass sheet. Forming Melt glass The molten glass overflows from a wedge shaped tank, flowing down its sides and fusing when the two sides meet. Since there is no contact with molten tin, the end product is flatter and its thickness is more uniform. Very thin glass sheet can be produced. Uses: electronic display panels Annealing Cutting Blow moulding Blow moulding is used to manufacture containers such as bottles and jars, as well as light-bulb envelopes. The glass is inflated using compressed gas inside a forming mould. Often, a two-step “blow and blow” process is used for production. The first step is used to make a blank from a molten glass gob, and the second step forms the blank into the final product. Glass fibre production Glass can be made into fibres that may be woven into yarns and fabrics. These find practical uses either as thermal insulation or in glass fibre reinforced polymer composites (GFRP). Nozzle drawing is a process that is widely used to manufacture glass fibres. Liquid glass is fed into a tank with a perforated bottom plate. The glass flows out of the perforations as filaments, which further thinned by gravity. Water is sprayed to cool the fibres and they are spooled up. A separate nozzle may spray the fibres with binders or other surface treatments. Glass wool production Glass wool production uses a similar principle to produce fibrous glass. The molten glass is fed into a fiberiser machine that rotates at high speed. The centrifugal forces cause the glass to scape radially through numerous holes in the spinner wall. A jet of hot air blows the escaping fibres downwards, where they are cooled with sprayed water. The fibres are also sprayed with a binder on their way down. The fibres land on the conveyor, which feeds them into a furnace to cure the binder. Slicing and packing follows. Optical communication fibre production 1 preform An optical communications fibre must have a core with a low refractive index that is clad by material with a high refractive index. A special glass tube is fixed in a lathe and rotated. Special reagent gases are fed through the rotating tube wile a burner traverses its entire length. The hot gases react to form a glassy soot on the inside of the tube. Once the inner wall of the tube is completely coated it is pulled apart to make a preform with a pointed end. Optical communication fibre production 2 Rod drawing is the process used to manufacture high quality optical communication fibres. A specially made preform is lowered into a furnace at 2000°C. A droplet called a “gob” is formed at the end of the preform. The gob drops away under gravity and thus drawing out a single fibre from the preform. The outer preform material forms the optical fibre cladding, and the silicon dioxide soot inside forms the core. Fibre dimensions are checked electronically and a protective coating is applied. Up to 300km of continuous fibre can be produced this way. preform Precision glass forming Precision glass forming allows the shaping of glass preforms into components without the need for grinding and polishing. It is used to produce optical lenses with complex profiles for digital cameras and other imaging instruments. It also has other applications for shaping custom glass parts for the transportation industry. A glass gob or sheet is placed into the dies/moulds and heated using infrared radiation. The travel of the moulding dies is then carefully controlled to produce glass products with a high-quality surface finish. Quality of die materials and their surface smoothness are also very important. Tolerances must be very high. Thermally toughened glass Glass can be toughened thermally by heating and rapid quenching. This creates a compressive stress at the surface and tensile stress in the centre. The surface compression acts to close any microcracks and lowers tensile stresses (hopefully to zero or below) which would otherwise propagate cracks. Thermally toughened glass The process can be used to create either thermally strengthened glass or tempered glass. Tempered glass has much higher surface compression and hence higher internal tension. When tempered glass breaks it shatters into many small pieces rathaer than large dangerous shards. Chemically toughened glass Glass can be toughened chemically by placing it in a molten salt bath. Exchange of ions takes place between the glass and the salt. For soda-lime glass, potassium ions can be used which are larger than the sodium ions. Potassium ions are larger than sodium ions. This creates compressive stresses at the surface, closing any microcracks and strengthens the glass. Summary Glasses are a versatile materials family that has a variety of uses: construction, optics, mass communications, containers and biomaterials. Glass is an amorphous form of silicon dioxide SiO2 (silica), which is built up of interconnected silicon-oxygen tetrahedra. Glasses do not have a melting point, but instead have a softening temperature known as the glass transition temperature (Tg). Below Tg they behave as brittle solids and immediately above Tg glasses behave like supercooled liquids and will flow under applied stress. The properties of glasses can be adjusted by changing the molecular structure using network formers and network modifiers. These additives are oxides of other chemical elements. Network modifiers break up the glass structure, increasing the number of non-bridging oxygens, resulting in a lower T g. Soda-lime glasses are the most widely used formulation and have a low Tg that facilitates processing. Glasses can be processed using a variety of methods: float glass process, fusion forming, blow-moulding, extrusion, rod-drawing, precision forming. Glasses can be toughened using surface treatments that create compressive stresses in their outer surfaces.