GLASS

Glass:

Introduction:

          Glass is any substance or mixture of substances that has solidified from the liquid state without crystallisation. The term "Glass" as ordinarily used referred as an amorphous, hard, brittle, transparent or translucent super cooled liquid of infinite viscosity, having no definite melting point obtained by fusing a mixture of number of metallic silicates or borates of sodium, potassium, calcium and lead. Many of its properties corresponding to those of supercooled liquid whose ingredients can't be identified because they have not separated from the solution in in crystallisation form.

Glass is an amorphous solid having a homogeneous texture.

Structure of Glass:

          The glass is a random arrangement of molecules, the great majority of which are oxygen ions bonded together with the network forming ions of silicon, boron or phosphorous. If the network was perfectly regular, each silicon atom would be surrounded by four Oxygen atoms arranged towards the corner of a tetrahedron. Because each oxygen atom in this network is shared by two silicon atoms, the empirical formula of this solid would be SiO2 and the material would have the structure of quartz. A glass made of silica alone has many desirable characteristics but unfortunately the high temperature involved make it expensive, and difficult to prepare. In order to reduce the temperature, required network-modifying ions are added. Sodium, Potassium, Titanium and Calcium are the most common. The network-modifying ions increase the competition for the oxygen ions, thus loosing the Si-O-Si bonds. Certain other ions may substitute for either the network-modifying ions: Aluminium, lead are the few of these intermediate ions.

         

         The amorphous structure of glass makes it brittle. Because glass doesn't contain planes of atoms that can slip past each other, there is no way to relieve stress. Excessive stress therefore forms a crack that starts at a point where there is a surface flaw. Particles on the surface of the crack become separated. The stress that formed the crack is now borne by particles that have fewer neighbours over which the stress can be distributed. As the crack grows, the intensity of the stress at its tip increases. This allows more bonds to break, and the crack widens until the glass breaks. thus, if you want to cut a piece of glass, start by scoring the glass with a file to produce a scratch along which it will break when stressed. 

Uses of Glass

Uses of glass:

          Glass is an unlimited and innovative material that has plenty of applications. It is an essential component of numerous products that we use every day, most often without noticing it.

It is clear that modern life would not be possible without glass!

Glass is used in the following list of products:

• Glass is used to form a riffle barrel which is lighter and stronger than conventional type.
• Thousands of items in the body guided missile are made with glass.
• Glass is used for the roof, and often also for the walls, of green house. The glass in greenhouses functions as a selective transmitter of solar energy. The effect it has is to trap energy within the greenhouse, warming plants, air, soil, water and other things inside the greenhouse. As infrared frequencies are blocked by glass, this thermal energy  generated by plants is also conserved within the greenhouse, adding to the warming effect.
• Packaging (jars for food, bottles for drinks, flacon for cosmetics and pharmaceuticals)
• Tableware (drinking glasses, plate, cups, bowls)
• Housing and buildings (windows, facades, conservatory, insulation, reinforcement structures)
• Interior design and furnitures (mirrors, partitions, balustrades, tables, shelves, lighting), colour changing glass; a window with such a glass will be transparent during a day and it will be source of light at night.
• Appliances and Electronics ( Oven doors, cook top, TV, computer screens, smart-phones)
• Automotive and transport (windscreens, backlights, light weight but reinforced structural components of cars, aircrafts, ships, etc.)
• Optical glass is finding wide application for the development and advancement of sciences of Astronomy, Medical technology, Biotechnology, Life science Engineering.
• Radiation protection from X-Rays (radiology) and Gamma-rays (nuclear)
• The glass lining are applied on equipment likely to be affected by the chemical corrosion such as valves, pipes, pumps etc.,
• Fibre optic cables (phones, TV, computer: to carry information) 
• Renewable energy (solor-energy glass, wind turbines) 

All of this is made possible by the countless properties of the glass substance.

Advanced Ceramics

Advanced ceramics:

1. Glass ceramics:

          Glass-ceramics are polycrystalline materials of fine micro structure that are produced through controlled crystallisation (devitrification) of base glass. Glass ceramic materials share many properties with both glass and ceramics. These are special glass composition that are thermally treated prior to forming operations to devirtify or precipitate a crystalline phase from the material; this phase gives special properties such as zero thermal expansion for applications involving high thermal-shock applications.

         Numerous silicate based wastes, such as coal combustion ash, slag from steel production, fly ash and filter dusts from waste incinerators, mud from metal hydrometallurgy, different types of sludge as well as glass cullet or mixtures of them have been considered for the production of glass-ceramics.

          The composition in which nucleation and crystallisation have been commercially produced are : Li2O-Al2O3-SiO2,  MgO-CaO-SiO2, MgO-CaO-SiO2-P2O5

Charectirestics:

u Very low coefficient of thermal expansion.

u High thermal conductivity.

u High mechanical strength.

u Easily fabricated.

Properties and Uses of Some Glasses and Glass-Ceramics

Composition		Property		Use
		Glasses
Al2O3, MgO, CaO, SiO2		Translucent, chemically resistant		Window glass, bottles
PbO, SiO2		High refractive index		Lead crystal
B2O3, SiO2, Na2O		Acid resistant, low  expansion on heating		Pyrex
		Glass Ceramics
MgO, Al2O3, SiO2		Insulator with high  mechanical strength at  high temperatures		Spark plug insulators
CaSiO3, CaMgSi2O6, CaAl2Si2O8		Wear resistant		Building materials
Li2Si2O5		Resistant to thermal shock		Nose cones on  rockets, cookware

2. Dielectric ceramics:

          These ceramic material are used as both electrical insulators and as functional parts of an electric circuit such as capacitors, memories, sensors and actuators.

          An important property of a dielectric is its ability to support an electrostatic field while dissipating minimal energy in the form of heat. The lower the dielectric loss (The operation of energy lost as heat), the more effective is a dielectric material.

          The electric insulators can be breakdown under high electrical voltages, the insulators are designed with lengthened surface paths to decrease the possibility of surface shorting. Since internal pores and cracks provide opportunity for additional surface failure, the insulator are glazed to make them non-absorbent.

          Non-linear dielectric ceramics are suitable in the miniaturising the electronic parts which have led to the development of increasingly sophisticated electrical circuitry.

Examples : Ferroelectrics such as BaTiO3 and PbTiO3 are used as sensors and actuators.

Lead zirconate titanate (Chemical formula Pb[ZrₓTi₁₋ₓ]O₃ ).

3. Cermets:

Cermets are composites in which ceramic material and metals join together (Ceramic plus metal = Ceramet), typically to give something with the high temperature performance or wear resistance of a ceramic and the toughness, flexibility or electrical conductivity of metal.

• Electrical components are one obvious application, Because they can get extremely hot, they need to behave like ceramics but, since they also need to conduct electricity, it helps if they work like metals. Cermets also make them useful for reducing friction in machine parts.
• These cermets are used in break shoes lining, oxidation resistant parts and inject engines.
• Machine tools are another increasingly common use for cermets, which offer greater toughness and wear resistance than more traditional materials. Cemented carbide such like composites are extensively used as cutting tools for hardened steels.

 

 

 

 

 

WANT TO WATCH VIDEO ABOUT CERMETS CLICK HERE 

Cermets meaning --https://youtu.be/KF6RvzXlXAE