Modern Engineering Materials (U-II) SCET R-23 PDF

Unit-2 Modern Engineering Materials Syllabus: Semiconductors-introduction, basic concept, applications; semiconductor preparation by distillation method and purification by zone refining; Super conductors: introduction, basic concept, applications; Nano materials:...

Unit-2 Modern Engineering Materials Syllabus: Semiconductors-introduction, basic concept, applications; semiconductor preparation by distillation method and purification by zone refining; Super conductors: introduction, basic concept, applications; Nano materials: Introduction, classification, properties and applications of Fullerenes, carbon nano tubes and Graphines nanoparticles. 1. Explain different types of semiconductors and applications. Semiconductor:  It is a solid material whose conductivity is in between conductor and insulator.  Types of semiconductors: Semiconductors Elemental Non-elemental Chalcogen photo Semiconductors Semiconductors Semiconductors Intrinsic Extrinsic Stoichiometric Non-Stoichiometric Controlled valency Semiconductors Semiconductors Semiconductors Semiconductors Semiconductors Ex: Si, Ge etc. P type n type  Intrinsic semiconductors: o In these, Ef is small, so that the electrons from valence band are excited to conduction band. o In this, electrons in conduction band = holes left in valence band. o As the temperature increases, the conduction of electrons from valence band to conduction band also increases. o For example: Si, Ge, GaAs, etc.  Extrinsic semiconductors: o In these, Ef is decreased by introducing impurity into the semiconducting material by doping. Unit-2 (Modern Engineering materials) SCET R-23 Pa ge1 of 11 Srinivasa Rao Kadali, Asst. prof.  o The impurity forms an impurity band between Valence band and Conduction band to increase conductivity. o n- type semiconductor:  In this, Si or Ge is doped with penta valent impurity atom like P, As etc.  P forms four covalent bonds with Si or Ge atoms and the extra 5 th electron conduct between valence band and conduction band.  Since the conductivity is due to negative electrons, it is called n-type semiconductor. o P-type semiconductor:  In this, Si or Ge is doped with trivalent atoms like Al, B and Ga.  B forms three covalent bonds with Si or Ge and the hole left conducts between valence band and conduction band.  Since the conductivity is due to hole (+ve charge), it is called p-type semiconductor.  Stoichiometric semiconductor: o It is a compound in which the elements are in integer ratio. For example: GaP, GaAs, GaSb, InAs etc.  Non Stoichiometric (or defect) semiconductor: o It is compound in which elements are in decimal (or non-integer) ratio For example: Zn1.1O, Fe0.6-0.94O  Controlled valency semiconductor: o It is a compound in which concentration of elements are controlled. For example: Li+ 2+ 3+ 0.05 Ni0.9 Ni0.05 O  Chalcogen photo semiconductors: o O, S, Se (Selenium), Te (Tellurium) are collectively called as Chalcogens or ore forming elements. o The semiconductors are formed by these compounds are called chalcogen photo semiconductors. Applications of Semiconductors:  Semiconductors are used in  Temperature sensors, 3D printing machines, microchips and self-driving cars  calculators, solar plates, computers and other electronic devices., microchips, LEDs, solar cells.  Transistors, diodes, photosensors, microcontrollers, integrated chips Unit-2 (Modern Engineering materials) SCET R-23 Pa ge2 of 11 Srinivasa Rao Kadali, Asst. prof.   Microprocessor (used for controlling the operation of space vehicles, trains, robots, etc.,) is manufactured by semiconductor materials. Semiconductors: 2.Explain 2. Write how Geon a note (orSuperconductors Si) is prepared by distillation with process. to types, application and properties. special reference Preparation of Germanium by distillation:  Ge concentrate is recovered from sphalerite (ore of Zn), Flue dust of Ag, Pb and Cu ores and fly ash (of certain coals) of thermal power plants. Note: Flue dust: Finely divided metal or metallic compounds escaping with the flue gases of a smelter or metallurgical furnace.  Ge concentrate is reduced with C (smelting) to form GeO2 concentrate.  It is then chlorinated with dry HCl gas to form GeCl4 (Germanium tetra chloride) liquid. GeO2 + 4HCl → GeCl4 + 2H2 O (Boiling point: 83.10 C)  So formed GeCl4 is distilled in fractions based on boiling points to remove all solid impurities and to get crude GeCl4. Note: Boiling points of different metal halides are given below: Metal halide b p (0C) Metal halide b p (0C) Metal halide b p (0C) Metal halide b p (0C) AlCl3 180 CuCl2 993 MoCl4 322 AsCl3 130 BCl3 12.6 FeCl3 315 NiCl2 Solid SbCl3 220 CrCl3 1300 MnCl2 1225 PCl3 76.1 ZnCl2 732 Chlorination Smelting Dry HCl gas Ge concentrate GeO2 concentrate (0.5 – 6%) Fractional Distillation (6 – 60%) Hydrolysis Saturated Extra pure H2O Cl2/HCl Soln. Pure GeCl4 Crude GeCl4 GeO2 ( 99.9999 %) ( 98 %) Reduction, H2 Czochralsky Ge Zone refining Ge Crystal pulling ( 99.9999 %) 6N Pure ( 99.99999999999 %) Many times 13N Pure or Ultra-pure Chips Single crystal of Ge  The crude GeCl4 is passed through saturated solution of Cl2 & HCl to remove some impurities like As, P, etc. and then distilled to get pure GeCl4. For example: arsenic acid & phosphoric acids are more soluble in water Cl2 Cl2 AsCl3 → H3 AsO4 PCl3 → H3 PO4  Now the pure GeCl4 is reacted with extra pure water to get pure GeO2 powder. GeCl4 + 2H2 O → GeO2  GeO2 is reacted with H2 to get pure Ge metal (6N pure). GeO2 + 2H2 → Ge + 2H2 O  6N pure Ge is further purified with zone refining many times to get ultra-pure Ge (13N).  So formed 13N pure Ge is poly crystalline and hence Czochralsky crystal pulling is used to get single Unit-2 (Modern Engineering materials) SCET R-23 Pa ge3 of 11 Srinivasa Rao Kadali, Asst. prof.  crystal of Ge.  The single crystals are cut into chips (wafers) using laser beam and then semiconductor device is made. Preparation of Si by distillation:  MGS (Metallurgical grade Silicon) – 98-99%, EGS (Electronic grade Silicon) – 99.9999% (6N pure) and Ulra pure Si - 99.99999999999% (13N pure).  Ore: Quarzite gravel  It is done in two methods: 1. Trichloro silane (or Siemens) process: 2. Silane (SiH4) Process: Trichloro silane (or Siemens) process:  SiHCl3( tri chloro silane b.p. 31.50C) is prepared by heating powdered MGS with dry HCl gas at 3000C in fluidized bed reactor. 3000 C Si + 3 HCl ⇌ SiHCl3 + H2 + Heat Note: Compound b.p. (0C) Compound b.p. (0C) SiH4 -112.3 SiHCl3 31.5 SiH3Cl -30.4 SiCl4 57.6 SiH2Cl2 8.3  It is an exothermic reaction, heat must be removed to get maximum yield.  This reaction gives mixture of SiH3Cl, SiH2Cl2, SiHCl3, SiCl4 and chlorides of other impurities like Fe,Al & B.  So formed chlorides are separated by fractional distillation easily.  A mixture of SiHCl3 and pure H2 is decomposed in Fluidized bed reactor at 3000C to get granular Si. 3000 𝐶 SiHCl3 + H2 → Si (EGS) + HCl  The yield of this process is only 30% due to the following side reaction. 3000 𝐶 SiHCl3 + HCl → SiCl4 + H2 SiCl4 Low boiling impurities Carbon, 18000C HCl Mixture of SiH3 Cl, Quarzite gravel MGS Fractional distillation SiH2Cl2, SiHCl3 & SiCl4 H2 EGS Chemical Vapour Deposition 30% yield only SiHCl3 in Fluidized Bed reactor SiCl4 (byproduct) Silane (SiH4) Process: Method 1: Silane can be prepared by the reaction of MGS powder with Mg at 5000C in H2 atmosphere to form Mg2Si, and then reacted with NH4Cl in liquid ammonia. Unit-2 (Modern Engineering materials) SCET R-23 Pa ge4 of 11 Srinivasa Rao Kadali, Asst. prof.  5000 𝐶 MGS + 2 Mg → Mg 2 Si Mg 2 Si + 4 NH4 Cl → SiH4 + 2 MgCl2 + 5 NH3 In this process, Boron impurities are removed in the form of BH3 – NH3 complex. Method 2: In this, SiCl4 is reduced with LiAlH4 or LiH. SiCl4 + LiAlH4 → SiH4 + LiCl + AlCl3 Production of granular Silicon in a Fluidized Bed (FB) Reactor:  Tiny EGS particles are fluidized in SiH4, H2 and inert gas.  EGS particles act as seed crystal onto which poly crystalline Si deposits to form free flowing spherical particles of EGS (diameter 0.1 – 1.5 mm).  Large EGS particles are fired at each other by high speed inert gas and the collision breaks them down in a seed reactor. 3. Explain how Zone refining is used to purify 6N pure semiconductor material into 13N pure? Zone Refining:  It was invented by pfann.  In this, a metal (like Si or Ge) is liquefied (or melted) and recrystallized to purify it.  It is based on “impurities are more soluble in liquid metal (or melt) than in the solid metal”.  Process: o A (Electronic grade) metal rod (like Ge [m.p.- 9380C] or Si [m.p.-14140C]) is clamped vertically. o It is heated (by RF coil) to about its melting point in a reducing atmosphere (like H2). Unit-2 (Modern Engineering materials) SCET R-23 Pa ge5 of 11 Srinivasa Rao Kadali, Asst. prof.  o As the heater is very slowly moved from top to bottom, the impurities (soluble in liquid metal) move along the metal rod from the top to bottom. o After several passes, the impurities are concentrated in the bottom part of the metal which is cut down to get pure metal rod. o In this process, the impurities can be reduced to 1 atom per 10 12 (trillion) atoms of metal. Super Conductors 4. Write a note on Superconductors with special reference to types, application and properties. Super Conductor:  It exhibits both zero electrical resistance and expulsion of magnetic field.  Super conductivity is exhibited by many metals at very low temperatures (TC).  Super conducting Transition Temperature or Critical temperature (Tc): o It is the temperature at which the normal metal passes into super conducting state. o For ex. Tc of solid Hg is 4.2 K (-268.80C).  Resistivity - Temperature behaviors for Super conductors:  Types of Super conductors: o These are two types based on their magnetic response. 1. Type-I Super conductors (soft): 2. Type-II Super conductors (hard): o Type-I Super conductors:  These are completely diamagnetic in super conducting state.  These exhibit Meissner effect (ie. Expulsion of magnetic flux from the super conducting material).  Examples: Al, Pb ,Hg and TaSi2. o Type II Super conductors:  These exhibit superconductivity up to lower critical magnetic field, after that super conductivity decreases slowly and becomes zero at upper critical magnetic field.  Examples: Niobium-Tiatanium, Niobium-Tin and Cuprate-perovskite.  Properties of Super conductors: o These are brittle. o At room temperature, the resistivity of Super conductor material is higher than other elements. o These are diamagnetic which leads to levitation (means cause to raise in air) effect. o Thermoelectric effect (conversion of heat into electricity) disappears in super conducting state. Unit-2 (Modern Engineering materials) SCET R-23 Pa ge6 of 11 Srinivasa Rao Kadali, Asst. prof.  o Super conductivity is lost, if strong magnetic field is applied below Tc. o When current is passed through super conductor, the heating loss is zero.  Applications: They can be used as They can be used in 1. Super chips in computers 6. MRI & NMR 2. Super cables 7. Nuclear fusion reactions 3. Strong electromagnets (up to 50 Tesla) 8. Frictionless, high speed, levitating 4. Gas sensors (Ex. La2-x SrxCuO4 is good sensor trains for alcohol vapours) 9. Electric filters 5. Catalysts (Ex. YBa2Cu3O7-x) 10. Electrical generators and transformers. Nanomaterials 5. Write a short note on Nano materials. What are carbon nano tubes? Explain their types and engineering aplications. Nanochemistry:  Nanochemistry is a branch of science.  Nanochemistry uses the scientific knowledge and system of methods to make chemical compounds and their analysis in Nano liter scale.  Nano means 10-9 parts of a unit. For example: Nano liter means 10 -9 liter. Nano particles:  The size of Nano particles is in between 1-50 nm. Nano materials:  The size of all particles of Nano materials is less than 100 nm atleast in one dimension.  Nano materials are three types o Nano films and surface coatings (1- D nanomaterials) o Nano tubes and Nano wires (2- D nanomaterials) o Quantum dots, precipitates and colloids (3- D nanomaterials). Carbon nano tubes (CNTs):  CNTs are made up of carbon and look like a long, thin cylinders.  In CNT, each carbon atom is covalently bonded to three other nearby carbon atoms.  CNTs are very big molecules having very small size, shape and rare physical properties.  CNT is one of the physical forms (allotropes) of carbon and others are diamond, graphite fullerenes etc.  Length to diameter ratio of a CNT is greater than 100,000.  CNT is formed, when hexagonal graphene sheet is turned around into a cylinder and its edges joined  So the CNTs are longer tubes of graphite sheet.  Types of CNTs: o There are two types of CNTs depends on their arrangement.  Single walled nano tubes (SWNTs)  Multi walled nano tubes (MWNTs)  Single walled nano tubes (SWNTs): Unit-2 (Modern Engineering materials) SCET R-23 Pa ge7 of 11 Srinivasa Rao Kadali, Asst. prof.  o SWNT is formed, when single one atom thick graphene sheet is turned around into a cylinder and its edges joined. o SWNTs have a diameter of 1 nm. o The length of SWNT is many millions times of its diameter. o SWNT show electrical conductivity. o It is used to make field affect transistors (FETs). o There are three types of SWNTs based on the arrangement of single one atom thick graphene sheet.  Armchair  Zigzag  Chiral o Graphene sheet is represented by a pair of indices (n,m) called Chiral vector. Where n and m are number of unit vectors along two directions in the hexagonal crystal lattice of graphene.  If n = m, the nano tubes are called “armchair”.  If m = 0, the nano tubes are called “zigzag”.  Otherwise, they are called “Chiral”. It has a bend shape around the nano tube.  Multi walled nano tubes (MWNTs): o MWNT is formed, when more than two one atom thick graphene sheets are turned around into a cylinder and its edges joined. o These are concentric tubes of graphene. o The distance between graphene layers of MWNT is ≈ 3.3 A0. o It the Russian doll model, sheets of graphene are arranged in concentric cylinders, for example: a large (0, 10) SWNT in a small (0, 8) SWNT. o In the parchment (means animal skin) model, a single sheet of graphene is turned around itself, like a roll of newspaper. Properties of CNTs (or Nano materials):  Mechanical properties: o Strength:  The strength of SP2 C-C bonds gives CNTs a special type of properties.  CNTs are strongest and stiffest materials than diamond in terms of tensile (extension) strength and elastic modulus.  This strength is due to the SP2 bonds formed between the carbon atoms.  It has low density than diamond. o Hardness:  CNTs have highest elastic modulus and harder than diamond.  CNTs can withstand damage from physical forces.  Young’s modulus of MWNTs is 1200 G Pa where as for diamond it is 600 G Pa.  Electrical properties: o CNTs are metallic or semiconducting depending on the structure of graphene sheet. o For a given (n,m) nano tube,  If n=m, the nano tube is metallic.  If n-m is multiple of 3, then the nano tube is semiconducting.  Otherwise the nano tube is moderate semiconductor.  Vibration properties: Unit-2 (Modern Engineering materials) SCET R-23 Pa ge8 of 11 Srinivasa Rao Kadali, Asst. prof.  o Atoms in CNTs are continuously vibrating outwards and forwards and they have two modes of vibrations.  A1g mode: It has in and out movements.  E2g mode: It has movement between Sphere and ellipse. Engineering applications of CNTs:  Uses in fuel cells: o The SWNTs are used to store hydrogen. o The SWNTs are used to make fuel cell electrodes. o CNTs are used as catalysts in fuel cell. o Pt/CNT electrodes produce more electricity than Pt/CB (carbon black) electrodes.  Uses in catalysis: o When catalyst having CNTs used, the reaction takes place with a medium rate and gives the required product. o CNTs are used as both catalyst and catalyst supportive materials. o CNTs containing nitrogen are used as cathodes in highly alkaline solution. o Oxidizable CNTs containing phosphorous are used in oxidative dehydrogenation of butane and butadiene. o Some chemical reactions are also done inside the nano tubes. For example:  Reduction of Nickel oxide (NiO) to Ni.  Reduction of AlCl3 to Al.  Uses in medicine: o Drug molecules joined to the CNTs are used to leave the drug molecule at the required part of the body; for testing and control of diseases. o This is very useful in cancer treatment. o CNTs are used to check the presence of DNA. 6. Discuss the preparation, properties and applications of fullerenes. Fullerenes:  Fullerene is the third newly discovered physical form (allotrope) of carbon.  The structure of C-60 fullerene looks like a circular building (dome) constructed by Buckminster Fuller and hence called as Buckminster fullerene or Bucky ball (by considering his hard work).  The structure of C-60 fullerene:  Types of fullerenes: o There are three types  Spherical fullerenes: They look like a soccer ball and many times called as Bucky balls.  Cylindrical fullerenes: These are called CNTs of Bucky tubes.  Planar fullerenes: Graphene is an example of planar fullerene sheet.  Preparation: o Fullerenes are made by vaporizing a graphite rod in a helium atmosphere. o Mixture of fullerenes like C-60, C-70 etc. is formed. o These are separated by solvent extraction. o C-60 is gotten from this mixture by column chromatography using alumina and hexane.  Structure: o Fullerene has closed cage structure having SP2 hybridization (like the structure of C-60) and it has chemical formula C n. Unit-2 (Modern Engineering materials) SCET R-23 Pa ge9 of 11 Srinivasa Rao Kadali, Asst. prof.  o (Every fullerene contains 12 pentagons and changeable number of hexagons). o Its crystals are spherical in shape. o The C-60 molecule has bent icosohedron structure. o An icosohedron is a polygon with 60 vertices and 32 faces, 12 of which are pentagonal and 20 hexagonal.  Properties: o C-60 is brown to black color solid and soluble in aromatic hydrocarbons. o It dissolves in benzene to form purplish red solution. o It is stable up to 6000C and it becomes liquid under vacuum at 6000C. o It has highest tensile (extension) strength and packing density. o It can be pressed to lose 30% of its volume without spoiling its cage like structure.  Engineering applications: o Fullerenes are used to 1. Carry charge in batteries 5. Stop cell damages in human body 2. Make photovoltaic cells 6. Make catalyst (used in hydrogenation and 3. Make lubricants dealkylations). 4. Make protective covering in 7. Make alkali metal fullerides that are superconductors wars 8. Make soft ferromagnets. 7. Write a note on graphene nanoparticles. Graphene Nanoparticles:  Graphene nano particles are in the size of 1 to 100 nm.  A monolayer of graphite is called graphene.  Graphene is a single-layered 2D material with a large surface area and a sp2 hybridized honeycomb- shaped structure. Properties of graphene: The thermal conductivity of graphene is excellent (up to 500 W/mK).  Few layered Graphenes shows room temperature ferromagnetism.  It has good intrinsic electron mobility of up to 200,000 cm2/Vs  Young's modulus is up to 1.0 TPa  The optical transmittance is up to 97.7%  It is two hundred times stronger as compared to steel. Types of graphene nanomaterials:  monolayer graphene  few-layer graphene  graphene nanosheets  graphene nanoribbons  graphene oxide (GO)  reduced graphene oxide (rGO)  thermally reduced graphene oxide (TRG) Applications: Unit-2 (Modern Engineering materials) SCET R-23 Pa ge10 of 11 Srinivasa Rao Kadali, Asst. prof. Uses in Bio-sensors:  It is an advanced diagnostic device that converts a biological or physical event into a detectable signal.  Graphene nano materials are used in the make of biosensors to detect rotavirus, Glucose, L- cystein, food toxins, hepatitis-B, progesterone, cancer genes, cancer protein, urea, levodopa in sweat etc,. Uses in Drug delivery:  Graphene nano particles are used to deliver the required drug molecule ( doxorubicin) at the tumour site for reducing growth. Uses in Batteries:  Graphene nano materialcomposites are used as anodes in batteries. Uses in Agriculture:  Graphene nano materialcomposites are used as antifungal-agent, anti-bacterial agent, coating materials on plants, fertilizer, aloe vera plant development, improvement of tomato plant Used as lubricant additives and greases Unit-2 (Modern Engineering materials) SCET R-23 Pa ge11 of 11 Srinivasa Rao Kadali, Asst. prof.