Chemistry - First Year Engineering, Lecture notes of Chemistry

Download Chemistry - First Year Engineering and more Lecture notes Chemistry in PDF only on Docsity! 1 Notes: FE Engg. Chemistry Unit-III: Engineering Materials A] Specialty polymers B] Nanomaterials A] Specialty polymers: In this unit we shall cover following speciality polymers as per syllabus. 1. Engineering Thermoplastic : Polycarbonate 2. Bio-degradable polymer: Poly(hydroxybutyrate-hydroxyvalerate) 3. Conducting polymer : Polyacetylene 4. Electroluminescent polymer : Polyphenylenevinylene 5. Polymer Composites : Fiber reinforced plastics – Glass fiber and carbon fiber reinforced polymer composites Engineering Thermoplastic: Engineering Thermoplastics are the group of materials obtained from high polymer resins, which provide one or more outstanding properties when compared with commodity thermoplastics like polystyrene, polyethylene, polypropylene, etc. Advantages of Engineering Thermoplastic: 1. High thermal stability, 2. Better chemical resistance 3. High tensile strength 4. High flexibility 5. High mechanical strength 6. Light weight 7. Easily mouldable Due to these properties, they can be used like metals, alloys and ceramics. Many of such polymers can be used as substitutes for metals, ceramics. They are useful in various sectors like automobiles, electrical and electronic, telecommunication, textiles, computer components, etc. 1) Engineering Thermoplastic – Polycarbonate Preparation: Polycarbonates are thermoplastic polyesters. They are commercially known as Lexan or Merlon. It is obtained by interacting Bisphenol - A with diphenyl carbonate. 2 Properties: 1. It has very high impact strength over wide range of temperature. 2. It is a highly transparent plastic, refractive index = 1.58, means higher light transmittance. 3. It is resistant to water and many organic compounds but is soluble in number of organic solvents and alkalis. 4. It has a good heat resistance upto 140oC. 5. It has low combustibility. It melts around 265oC. 6. Polycarbonates have limited chemical and scratch resistance and become yellow with long term UV exposure. 7. It has a good thermal and oxidative stability in melt. 8. Its specific gravity = 1.2 gm/cc, Tm = 230 -250 ̊C, Tg = 145 ̊C Applications: 1. Electrical and electronic components: It is useful in making electrical and electronic components like industrial plugs, sockets, switches, cell phone covers, laptops, pagers. 2. Data storage: It is useful in making data storage devises like CD’s, DVD’s by injection moulding. 3. Optical applications: Because of high optical clarity it is used in making sunglasses, swimming and scuba goggles, head lamp lenses, wind screens for small planes and helicopters. It is also used in making electronic display screens for mobile phones and some portable LCDs. They are also useful in thin UV eyeglass lenses. 4. Construction material: as they have high impact strength useful as substitute for window glass, dome lights, sound walls, etc. 5. Security components: It can be laminated to make bullet proof glass. 6. General applications: hair drier bodies, camera bodies, transparent food containers, cooking utensil covers, glasses, water bottles, toys, etc. 2) Biodegradable polymers: Definition: It is the process of converting polymer material into harmless simple gaseous products, by the action of enzymes of micro-organisms and water. Bacteria derive energy (food) from such fragments, survive and finally the fragments are converted to gases like N2, CO2, CH4 etc. However the overall process of biodegradation is slow. Need for Biodegradable Polymers: i) To reduce the non biodegradable synthetic plastic solid waste, this affects environment badly. ii) Degradation of synthetic polymers produces harmful products, which causes pollution. iii) To reduce the dependence of man on synthetic polymers, which are made from the petroleum fractions whose reserves are limited. 5 Types of conducting polymers: 1) Intrinsically conducting polymer (ICP): An intrinsically conducting polymer material is the one which can conduct electricity by its own e.g. trans -polyacetylene, polypyrrole, etc Conductivity range of insulators, semiconductors and conductors in siemens /cm, is given below- 2) Extrinsically conducting polymer (ECP): An extrinsically conducting polymer material is the one which is filled with metal powder, metal filaments or graphite powder to make it conducting. Types of Extrinsically conducting polymers / Doping of Polymers: Conductivity of polymer chains with conjugation, can be increased to the extent of metals, on doping. In the doping process, the polymer is either oxidized (removal of electrons) or reduced (addition of electrons), so that the polymer chain carries the resonating charge. The important doping reactions are given below, for polyacetylene, a) Oxidative or P-doping: The oxidizing reagents for P-doping are like iodine vapours, I2 dissolved in CCl4, HBF4, HClO4, Br2, etc. The positive charge resonates throughout the polymer chain & can be transferred to neighbouring chains. b) Reductive or n-doping: The reducing agents for n-doping are like Na metal, FeCl2, lithium metal, sodium naphthalide, etc. The negative charge resonates throughout the chain and transferred to neighbouring chains, through Na+ during the conduction. c) Protonic doping: A polymer like polyaniline, can be H+- doped, with acid solution. Applications: 1) Rechargeable batteries: The doped conducting polymers have high charge carrier concentrations, they are used as promising charge storage material. These batteries of small size, long lasting can be produced and they can produce high current density upto 50 mA/cm2. They have high charge to weight ratio. 2) Optical filters: Radiations from computer screens, other electrical devices can be absorbed by conductive polymers by coating on casing. 6 3) As antistatic material: To avoid static electricity in plastic carpets in offices, theatres, doped polyacetylene can be used as antistatic material. Coating an insulator with conductive polymer is also important for explosives industry, computer industry as antistatic materials. 4) Sensors: The conductive polymers have chemical properties suitable to use them as sensors for pH, O2, NO2, SO2, NH3, glucose, reducing and oxidizing chemicals, for the study of their even very low concentrations. 5) In electronics: They are used for photodiodes, light emitting wall-papers, light emitting diodes (LED)and data storage. 6) Photovoltaic cells. 7) In telecommunication systems. 4) Electroluminescent Polymer -: Definition: The property in which a material produces bright light of different colours when stimulated electronically is known as electroluminescence. The material which shows electroluminescence, is called as Electroluminescent polymers Ex. Polyparaphenylene Vinylene (PPV) Preparation: Properties: 1. PPV is diamagnetic material. 2. It has a very low intrinsic electrical conductivity. 3. Gives bright-yellow electroluminescence. 4. It is insoluble in water. 5. Its conductivity increases on doping. Applications of PPV: 1. In the form of thin films for information displays. 2. Photovoltaic cells 3. Automotive instrument panel backlighting. 4. Backlight for liquid crystal displays. 5. Electroluminescent night lamps. 6. Long life, full colour displays. 7. Flat panel displays. 8. Theatre, assembly hall decoration. 5) Fibre Reinforced Plastic (FRP) / Polymer Composite: A polymer and a reinforcing material as a two phase mixture, with interfaces between them, is The polymer composite has properties of both the materials in combination. The polymer phase is called as matrix phase and the composite used for mixing is known as dispersed phase. The boundaries between the matrix and dispersed phase are known as interface. 7 Functions of Matrix Constituent in Polymer Composite: i) To bind the reinforcing particles/fibres strongly. ii) It acts as medium for distribution of applied load to the dispersed phase. iii) It keeps the reinforcing fibres in proper orientation for the high strength development. iv) It prevents propagation of cracks due to its plasticity. Some Important Dispersed Phase Constituents are given below: a) Glass Fibres: The glass fibres are produced by passing a glass melt through small orifices and cooled. It offers very high tensile strength, higher thermal stability, high toughness and impact strength, to the polymer matrix. b) Carbon fibres: They are prepared from carbon by oxidation under tension at low temperature and then carbonization at 1000 ̊C. these are stiff, strong, even at high temperature. c) Aramid fibres: Aramid is the aromatic polyamide e. g. Nomex, Kevlar. It has liquid – crystal polymer property. The fibres have very high tensile strength, impact resistance, high thermal stability. Properties: 1) Low coefficients of expansion. 2) Low cost of production. 3) High dimensional stability. 4) High tensile strength. 5) High heat stability and therefore usable at higher working temperature. 6) Better abrasion/wear resistance. 7) Better toughness and impact strength. Applications: 1) Automobile bodies, chassis parts, racing vehicle components. 2) Boats body, propeller shafts. 3) Parts of aircrafts. 4) Sport goods, musical instruments, toys etc. 5) High speed machinery parts, PCB, equipment parts, bodies of refrigerator, coolers, cabins for offices, windows, doors. 3 3. Optical properties: The optical properties of nanomaterials depend on number of factors like size, shape, surface functionalization, doping, and interactions with other materials, etc. The size-dependent optical property of them is due to change in the optical energy band gap. The optical band gap increases with the decrease in particle size, especially for the semiconductor nanomaterials. Thus the colloidal gold or silver nanoparticles produce different colors for different sizes, especially in the range of 1–10 nm. E.g. simple change in size of CdSe semiconductor to nano-size particle alters the optical properties of the CdSe nanoparticles. 4. Electrical properties : Nanomaterials exhibit electrical properties in between semiconductors and metals depending on the chirality of the molecules and diameter of the molecules. Electrical properties of nanomaterials are different than their bulk materials. In nanomaterials especially nanotubes / nanowires with decreasing diameter, the number of electron wave modes contributing to the electrical conductivity become very smaller, which increases electrical conductivity. For example, in electrically conducting carbon nanotubes only one electron wave mode is observed which transport the electrical current.  Structure, Properties and Applications of 1) Graphene, 2) CNT, 3) Quantum dot I) Graphene Structure: 1) It is single layer of graphite. It is an allotrope of Carbon, discovered in 2003. 2) 2D Carbon nanomaterial (atomic thickness: 0.345 nm), all Carbon atoms are SP2 hybridised. 3) Each carbon has 3 hybrid orbitals and 1 unhybridized orbital. 4) Single atom-thick layer of tightly bonded Carbon atoms, it has Hexagonal arrangement with planer structure 5) Each carbon atom is joined to 3 other carbon atoms by sigma bonds. 6) The unhybridized orbital of adjacent carbon overlaps laterally to form Pi bond double bond. 7) Due to resonance phenomenon, the electrons are delocalized over the graphene structure, hence graphene is good conductor of electricity. 8) The c-c bond length is 1.42 AO (142pm). 4 General Properties: It is not only one of the thinnest but also strongest materials. It conducts heat better than all other materials; it is a great conductor of electricity; It is optically transparent, yet so dense that it is impermeable to gases – not even helium, can pass through it. These amazing properties, and its multifunctionality, make graphene suitable for a wide spectrum of applications ranging from electronics to optics, sensors, and biodevices. Chemical Properties 1) An inert material and does not readily react with other atoms (Even though all of graphene’s atoms are exposed to the environment) 2) However, “absorb” different atoms and molecules. 3) It can be functionalized by various chemical groups, which can result in different materials such as graphene oxide (functionalized with oxygen and helium) or fluorinated graphene (functionalized with fluorine). 4) This can lead to changes in the electronic properties, and may also be used to make sensors or other applications. Physical / Mechanical Properties: 1) Graphene is one of the thinnest (only one carbon atom thick ~0.345 nm) but strongest material. 2) It is flexible & most stretchable crystal (stretch upto 20% initial size w/o breaking it) 3) It is toughest 2D material, much harder than either steel (200 times stronger than steel) or diamond of the same dimensions. 4) High tensile strength (of over 1 Tpa), Lightweight; it weighs just 0.77 mg/m2 ) 5) Highest surface area, since single 2D sheet 6) Highly impermeable (even helium gas cannot go through it) 7) Available in variety of forms: it can be wrapped into balls, rolled into tubes, or stacked to make graphite once again. 8) The impressive intrinsic mechanical properties of graphene are - its stiffness, strength and toughness. Thus graphene stand out both as an individual material and as a reinforcing agent in composites. Electrical Properties 1) High electrical current density (million times that of copper) and intrinsic mobility (100 times that of silicon) 2) Lower resistivity than any other known material at room temperature, including silver. 3) There are also some methods to turn it into a superconductor (it can carry electricity with 100% efficiency) 4) Although graphene, the fastest and most efficient conductor, it cannot be readily used to make transistors as it does not have a bandgap. There are several methods to open a bandgap that are in existence and some that are under development 5) It is Perfect thermal conductor – Higher than that of CNT, graphite and diamond (over 5,000 W/m/K). 6) Conducts heat in all directions - it is an isotropic conductor 5 Optical Properties 1) Extremely thin, but still a visible material. It absorbs about 2.3% of white light (which is quite a lot for a 2D material) i.e. 98% of visible light passes through graphene, making it transparent 2) Due to graphene’s amazing electronic properties, it can theoretically be useful to make very efficient solar cells 3) As graphene absorbs 2.3% of visible light, it is very much transparent to the human eye, which may have various uses eg. Used to make transparent conductors  Applications of Graphene: a) Energy storage and solar cells: 1) Graphene improves both energy capacity and charge rate in rechargeable batteries; 2) Activated graphene makes superior super capacitors for energy storage; 3) Graphene electrodes are useful for making solar cells that are of low cost, lightweight, flexible and multifunctional. 4) Graphene is useful for solar cells, super capacitors, graphene batteries, and catalysis for fuel cells 5) Defect-free graphene might solve lithium-metal batteries dendrite problem i.e. electrical short circuit and overheating of electrodes. b) Photovoltaic devices: Due to their excellent electron-transport properties and extremely high carrier mobility, graphene and other direct bandgap monolayer materials [such as Transition-Metal Dichalcogenides (TMD CS) and black phosphorus] show great potential to be used for low-cost, flexible, and highly efficient photovoltaic devices. c) Sensor applications: It is found that Graphene Oxide (GO), due to its super permeability to water molecules, leads to sensing devices with an high speed. d) Composites: It is used in many composite materials. Graphene-infused carbon fiber helmet being thin, strong, conductive, flexible and light characteristics is amazing, which absorbs and dissipates heat more efficiently, so it's cooler. e) Water Filtration Systems: It is useful in purification of water as it allows water to pass, but not other liquids and gases. f) Medical Sensors & Drug delivery: It is useful in several biomedical applications like drug delivery, cancer therapy and as a sensor. But, its toxicity profile must be checked before clinical use. g) In Touch Screens in Devices: It is also useful in displays and touch screens in many devices, due to transperancy and conductivity. But it is more expensive to produce. (than currently used indium tin oxide) 8 Chemical Applications: e) Air pollution filters: These are useful in air filters, due to high adsorption capacity and large surface area. f) Water filters: These are useful in water filtration as they are so thin & can block small & big sized ions. g) Chemical Nanowires: They are useful in nanowire manufacturing using materials like gold, ZnO, etc which are very sensitive and selective for H2S detection. h) Sensors: CNT based sensors are useful for detection of temperature, pressure, chemical gases, etc. 3) Structural applications: a) Textiles: CNTs can produce water proof and tear-resistant clothing. b) Concrete: CNTs in concrete increases its tensile strength and stop crack propagation. CNTs may be able to replace steel in suspension and bridges, making flywheels. c) Sports equipment: They are useful in making sports equipments as golf balls, golf clubs, bicycle parts, stronger and lighter tennis rackets. d) Fire protection: Thin layers of bucky paper along with compact layer of CNT can protect the object from fire. 4) Wind Mill Blades: Nano tubes are also used in the wind mill blades because of their low weight to better wind and produce more electricity at faster rate. 5) Air Craft Stress Reduction: Nanotubes are also used in the space and air craft’s to reduce the weight and stress of the various components working tighter. 6) Catalyst in some chemical reactions. 7) CNT as Nanocylinders: A gas like H2, can be safely stored inside the CNT for battery or vehicles and problem of H2 storage-hazzards can be solved. 8) As CNTs can absorb Infrared light and may have applications in the I/R Optics Industry III) Quantum dots: Definition: These are tiny man-made semiconductor nanoparticles that glow a particular colour after being illuminated by light, whose sizes are normally 2-10nm. When UV light hits these semiconducting nanoparticles, they can emit light of various colours. Types of Quantum dots- There are three main types of Quantum dots 1) III-V Semiconductor Quantum dots: They are made up of elements from Gr. III of the periodic table (B, Al, Ga, In) and from Gr. V (N, P, As, Sb, Bi) 9 Best known example of this type is gallium arsenide (GaAs). It is used as a light source in optical data processing and used as an amplification medium in lasers. 2) II-VI Semiconductor Quantum dot: They are made up of elements from Gr. II i.e. transition metals (Zn, Cd) and from group VI (O, S, Se, Te). The important examples of this type are cadmium selenide (CdSe), cadmium telluride (CdTe) and Zinc oxide (ZnO). They show outstanding fluorescence properties and widely used in electronics, photonics, photovoltaics and biomedicine. 3) Silicon (Si) Quantum dots: They are made up of element silicon, which is the standard material of semiconductor and chip industry. Silicon quantum dots have great potential to be used as a component of optical chip, optical sensors etc. Properties of quantum dots (semiconductor nanoparticles): 1) Their properties are intermediate between bulk semiconductors and discrete atoms / molecules. 2) The properties of a quantum dot are determined by size, shape, composition and structure. Their optoelectronic properties change as a function of both size and shape. 3) Many semiconductor substances can be used as quantum dots. Nanoparticles of any semiconductor substance have the properties of a quantum dot. 4) The gap between valence band and conduction band, which is present for all semiconductor materials, causes quantum dots to show fluorescence. 5) They show colour glow when illuminated by UV light. 6) When illuminated by UV light, some of the electrons receive enough energy to get free from the atoms. Thus they create a conductance band in which electrons move free in a material and conduct electricity. When these electrons drop back into the outer orbit around the atom (the valence band), they emit light. 7) The colour of emitted light depends on the energy difference between the conductance band and the valence band. 8) A majority of QDs have the ability to emit light of specific wavelengths if excited by light or electricity. Typically, smaller QDs (e.g., radius of 2~3 nm) emit shorter wavelengths generating colours such as violet, blue or green. While bigger QDs (e.g., radius of 5~6 nm) emit longer wavelengths generating colors like yellow, orange or red. 10 Absorbance and luminescence spectrums are blue shifted with deceasing particle size as in diagram below. Photoluminescence of alloyed CdSxSe1-x/ZnS quantum dots of 6nm diameter. The material emits different colour of light at different particle sizes. Potential Uses / Applications of Quantum Dots 1) Optical / Light Emitting Diodes / Display screens: a) The most commonly known use of quantum dots nowadays may be LEDs and TV screens. (Samsung and LG launched their QLED TVs in 2015, and a few other companies also followed) b) Quantum dots, will be useful in next-generation displays because they are both photo-active (photoluminescent) and electro-active (electroluminescent) and have unique physical properties, c) Compared to organic luminescent materials, QD-based materials have purer colors, longer lifetime, lower manufacturing cost, and lower power consumption. d) Another key advantage is that, because QDs can be deposited on virtually any substrate, you can expect printable and flexible – even rollable – quantum dot displays of all sizes. 2) Photovoltaics / Solar Cells a) Quantum dots are used for making solar cells. b) They can be manufactured in an energy-saving room-temperature process; c) They can be made from abundant, low cost materials that do not require extensive purification, as silicon does; d) They can be applied to a variety of inexpensive and even flexible substrate materials, such as lightweight plastics. 3) Biological Applications / in medicine a) With the help of Quantum dots it is possible to study cell processes at the level of a single molecule and found improvement in the diagnosis and treatment of cancers like diseases b) They are widely used to study intracellular processes, tumor targeting, diagnostics and cellular imaging at high resolutions. c) They can also be used as biosensors. d) Quantum dots could transform medicines, but most of them are toxic.