Ionic Lattices, Superconductors and Semiconductors

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Ionic Lattices, SuperconductorsIonic Lattices, Superconductors

and Semiconductorsand Semiconductors



Ionic LatticesIonic Lattices

Each sodium ion is surrounded by 6Each sodium ion is surrounded by 6chloride ions and vice versachloride ions and vice versa

Face centred cubic



Ionic LatticesIonic Lattices

Each caesium ion is surrounded by 8Each caesium ion is surrounded by 8chloride ions and vice versachloride ions and vice versa

Body centred cubic



Ionic LatticesIonic LatticesIonic radii:Ionic radii: ± ± Na+: 95pmNa+: 95pm ± ± Cs+: 174pmCs+: 174pm ± ± ClCl--: 181pm: 181pm

The arrangement (face centred cubic or body centredThe arrangement (face centred cubic or body centredcubic) depends on thecubic) depends on the ratio of sizes of the atomsratio of sizes of the atoms ..If theIf the positive ionpositive ion is muchis much smaller smaller than the negativethan the negativeion, it will beion, it will be face centred cubicface centred cubic (like NaCl)(like NaCl)

If theIf the positive ionpositive ion andand negative ionnegative ion areare similar in sizesimilar in size ,,it will beit will be body centred cubicbody centred cubic (like CsCl)(like CsCl)Other structures are possibleOther structures are possible - - especially if the ratio of especially if the ratio of positive:negative ions is not 1:1, e.g. TiO2positive:negative ions is not 1:1, e.g. TiO2





SuperconductorsSuperconductors

All metals conduct electricity but some are better All metals conduct electricity but some are better than others as they have differentthan others as they have different resistanceresistance totothe flow of electricitythe flow of electricity

Some metals show zero resistance when cooledSome metals show zero resistance when cooledclose toclose to - -273oC (zero Kelvin). This means273oC (zero Kelvin). This meanscurrent can be carried without loss of energycurrent can be carried without loss of energyand is calledand is called superconductivity.superconductivity.

Superconductor Superconductor ± ± a material that conductsa material that conductselectricity with zero resistance to the flow of electricity with zero resistance to the flow of electricityelectricity



R esistance vs. temperatureR esistance vs. temperature



Improvements in Superconductor Improvements in Superconductor

technologytechnology



SuperconductorsSuperconductors

Onnes discovered that when mercury, copper,Onnes discovered that when mercury, copper,lead tine and silver were placed in liquid He atlead tine and silver were placed in liquid He at4K, they lost all resistance4K, they lost all resistance

As temperature decreases, the atoms vibrate As temperature decreases, the atoms vibrateless until suddenly there is no resistanceless until suddenly there is no resistance A material loses its resistance at its µcritical A material loses its resistance at its µcriticaltemperature¶, Ttemperature¶, T cc

The TThe T cc of these metals is too low to have anyof these metals is too low to have anypractical usepractical use ± ± cooling He to 4K is far toocooling He to 4K is far tooexpensiveexpensive



µHigh Temperature¶µHigh Temperature¶

SuperconductorsSuperconductorsR ecent research has discovered materials thatR ecent research has discovered materials thatare superconductors at higher temperaturesare superconductors at higher temperatures ± ± YBaYBa 22CuCu 33OO 77 has a Thas a T cc of 92K (known as 1of 92K (known as 1- -22--33

superconductor)superconductor) ± ± Less expensive to run as they can use liquid nitrogenLess expensive to run as they can use liquid nitrogen

(liquefies at 77K)(liquefies at 77K) ± ± Oxides with TOxides with T cc = 125K have now been discovered= 125K have now been discovered

These oxides are known as ceramicThese oxides are known as ceramicsuperconductorssuperconductors ± ± Unfortunately not as malleable as metalsUnfortunately not as malleable as metals



M eissner EffectM eissner Effect

M eissner effect video from Scholar M eissner effect video from Scholar Some superconductors can repel aSome superconductors can repel a

magnetic fieldmagnetic fieldThis means that magnets can levitateThis means that magnets can levitateabove a superconductor above a superconductor



P otential Uses for SuperconductorsP otential Uses for Superconductors

Frictionless transport systemsFrictionless transport systems ± ± A train could levitate above a track and travel with zero friction A train could levitate above a track and travel with zero frictionP ower cablesP ower cables ± ± Currently 3Currently 3- -10% of electrical energy lost as heat caused by10% of electrical energy lost as heat caused by

resistance in the wiresresistance in the wires ± ± superconductors would stop thissuperconductors would stop this ± ± Would allow cables to be stored undergroundWould allow cables to be stored underground ± ± no need for no need for

pylonspylonsMR IMR I ± ± superconductors create high magnetic fieldssuperconductors create high magnetic fieldswhich allow the body to be imagedwhich allow the body to be imaged ± ± removes need for removes need for exploratory surgeryexploratory surgeryElectronicsElectronics ± ± Could make super Could make super- -fast electronic switches and make microfast electronic switches and make micro- -

processors even faster processors even faster



SemiconductorsSemiconductors

Elements such as silicon and germanium areElements such as silicon and germanium areknown asknown as semiconductorssemiconductors ± ± They have a higher conductivity than nonThey have a higher conductivity than non- -metals, butmetals, but

not as a good as metalsnot as a good as metals ± ± They are known asThey are known as metalloidsmetalloids as they are found atas they are found atthe junction of metals and nonthe junction of metals and non- -metals in the P eriodicmetals in the P eriodicTableTable

The conductivity of a semiconductor The conductivity of a semiconductor increasesincreaseswith increasing temperature (unlikewith increasing temperature (unlikesuperconductors and metals whose conductivitysuperconductors and metals whose conductivityincreases withincreases with decreasing decreasing temperature)temperature)



SemiconductorsSemiconductors

Conductivity of a semiconductor alsoConductivity of a semiconductor alsoincreases when exposed to light (knownincreases when exposed to light (knownas theas the photovoltaic effectphotovoltaic effect ))Impurities also affect conductivityImpurities also affect conductivity ± ± They are sometimes deliberately added andThey are sometimes deliberately added and

this is calledthis is called dopingdoping



Conductivity vs. temperatureConductivity vs. temperature





How Semiconductors workHow Semiconductors workSilicon and germanium have tetrahedral networks similar Silicon and germanium have tetrahedral networks similar to diamondto diamondCovalent bonds are weaker in Ge and Si than in CCovalent bonds are weaker in Ge and Si than in CHeat or light can cause the atoms to vibrate more andHeat or light can cause the atoms to vibrate more andcan make electrons move from the filled outer energycan make electrons move from the filled outer energylevel into a higher energy level where they can move andlevel into a higher energy level where they can move andconductconduct

As electrons move up into a higher level, they leave a As electrons move up into a higher level, they leave a

µhole¶ with a positive charge.µhole¶ with a positive charge. As a current is applied, electrons and holes will migrate As a current is applied, electrons and holes will migratethrough the crystal carrying currentthrough the crystal carrying currentGlesga P hysicsGlesga P hysics - - SemiconductorsSemiconductors



How Semiconductors workHow Semiconductors work

Conventionalcurrent

Electronconduction

Holeconduction



PP --type and ntype and n- -type semiconductorstype semiconductors

If we dope Si or Ge with aIf we dope Si or Ge with a G roup 5G roup 5element it becomes aelement it becomes a nn--typetypesemiconductor semiconductor If we dope Si or Ge with aIf we dope Si or Ge with a G roup 3G roup 3element it becomes aelement it becomes a pp--typetypesemiconductor semiconductor



PP --type and ntype and n- -type semiconductorstype semiconductors



D opingD oping



pp--n junctionsn junctions

Silicon and germanium crystals can beSilicon and germanium crystals can bemade with layers of p and n dopingmade with layers of p and n dopingWhere these bands meet is called the pWhere these bands meet is called the p- -nn

junction junctionThe pThe p--n junction acts as an electronn junction acts as an electronbarrier and is the basis of the electronicsbarrier and is the basis of the electronics

industryindustry ± ± D iodes, transistors, circuits etcD iodes, transistors, circuits etcScholar Scholar - - Semiconductor dopingSemiconductor doping



D opingD oping

Only about 1 doping atom for every 1 xOnly about 1 doping atom for every 1 x1010 99 is needed to create the pis needed to create the p- -type or ntype or n- -type superconductor type superconductor This means that the silicon used must beThis means that the silicon used must bevery purevery pureIt is purified by a method calledIt is purified by a method called zonezonerefiningrefining ± ± WikipediaWikipedia - - zone refiningzone refining



P hotovoltaic CellP hotovoltaic Cell

The solar cell converts sunlight into electricityThe solar cell converts sunlight into electricity

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Ionic Lattices, Superconductors and Semiconductors

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