STRUCT. & PROP. I
STRUCT. & PROP. I MATRL 100A
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This 4 page Class Notes was uploaded by Miss Alberto Prohaska on Thursday October 22, 2015. The Class Notes belongs to MATRL 100A at University of California Santa Barbara taught by Staff in Fall. Since its upload, it has received 46 views. For similar materials see /class/226930/matrl-100a-university-of-california-santa-barbara in Material Science and Engineering at University of California Santa Barbara.
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Date Created: 10/22/15
Materials 100A Class 14 Optical Properties Ram Seshadri MRL 2031 x6129 seshadri mrlucsbedu httpwwwmrlucsbeduNseshadriteachhtml Light Some basics Electromagnetic radiation has electric and magnetic eld components that are perpendicular to each other and to the direction of propagation The propagation velocity is 57 and this is constant for all electromagnetic radiation in vacuum 5 N 3 X 10 m s c is related to the vacuum permittivity 60 and the vacuum permeability M0 in the following manner 1 c m c is equal to the product of the frequency and the wavelength 5 VA The energy of any light photon is proportional to the frequency of the photon so E hl where 1 is in Hz s71 and h is the Planck constant it 663 X 10 34 Js It is also possible then to write he 7 Electromagnetic radiation with different frequencies have correspondingly different energies and wavelengths Visible electromagnetic radiation light has wavelengths ranging from 400 to 700 nm Light and metals reflection Since metals have free electrons7 all energies are absorbed and reemitted Hence the re ectivity Yellowred metals such as Cu and Au are so colored because they absorb a little in the blue Refraction The refractive index n of a material is the ratio of the velocity of light in vacuum always 0 to the velocity of light in that material 1 c n 7 v If the material has a dielectric constant e and permeability V then Materials 100A Class 5 Crystal structures ll Ram Seshadri MRL 2031 x6129 seshadrimrlucsbedu httpwwwmrlucsbeduwseshadriteachhtml This class closely follows the third chapter of Callister Bond Distances We have seen that in the P cubic structure the cell parameter a 2r d where r is the radius of an atom If two atoms touch the distance between their centers 7 the bond distance or length 7 corresponds to the sums of their radii Therefore 0 In the primitive cubic structure d 2r a o In the I cubic structure 3a 4r so that the bond distance is d 2r 32a o In the F cubic structure x2a 4r so that the bond distance is d 2r ax2 We can compare bond lengths in fcc Ni 1 3524 A d 2492 A and bcc Fe 1 28665A d 24825 We nd that they are very similar as we might expect for two elements that are nearly neighbors Densitites If the cubic unit cell has the side 1 its volume is V 13 If we know the cell parameter and the name of the element within the unit cell we can calculate the density of the material precisely As an example conmsider Ni in fcc Ni where a 3524 A We have V a3 3524A3 3524 gtlt 10 8cm3 4376 gtlt 10 23cm3 The atomic mass of Ni is 58693 so one Ni atom weighs 586936023gtlt1023 g 97448gtlt10 23 g There are four Ni atoms per unit cell so that the density p is given by mass 7 4 gtlt 9745 gtlt 10 23g 7 8907 1 p volume 4376 gtlt 10 23cm3 g cm We can perform other similar calculations where the unknown is different Number of neighbors In the P cubic structure each atom has 6 nearest neighbors in I cubic the number is 8 and in F cubic the number is 12 Materials 100A Class 5 Crystal structures II More c0mplex elemental structures More complex structrures than the four we have mentioned P cubic7 I cubic7 F cubic and hexagonal close packedl We will start with the diamond structure that is adopted by C diamond7 Si etc Diamond This structure can be built up from the fcc structure by inserting some extra layers 134 3212 E Z Note that the layer at z 1 is identical to z 0 and is not shown Also note that z 0 and z 12 are identical to fcc The cell parameter of Si is a 54309 A For carbon diamond it is a 356780 A The packing fractionef ciency is only 034 1We Will return to a discussion of this structure presently 2 Materials 100A Class 5 Crystal structures ll Graphite And here is an image of the top surface using a technique called Scanning Tunnelling Microscopy STM From httpWWWphysicslouisVilleeduwwwpublicfacultystm The structures of simple c0mpounds As we go to compounds structures can become more complex One of the simplest structures is that of common salt NaCl Adopted by a number of halides oxides chalcogenides Note that each cation has 6 neighbors and each anion has 6 neighbors etc The cell parameter of NaCl itself has a 563 A Other examples AgCl BaS CaO CeSe DyAs GdN KBr Lap LiCl LiF MgO NaBr NaF NiO PrBi PuC RbF ScN SrO TbTe UC YN YbO ZrO