Describe the difference between monocrystals and polycrystals? More....?

Question

Describe the difference between monocrystals and polycrystals and the impact this difference has on X-ray and electron scattering.

Answer 1

Monocrystal means one grain. Polycrystal means many grain. A monocrystalline part has the whole thing made of a single lattice so that X-ray diffraction or electron scattering shows a symmetrical large-star-like constellation of orderly spatial atom distribution. An example of a single crystall part is an electronic Si wafer which, depending on the exposed orientation, shows a 120-degree 111 symmetry (because it is two 1/4 diagnally displaced body-centered cubic structures), 180-degree 110 (rectangular), or a 90-degree 100 (square) symmetry. A polycrystalline part has many crystalls of different orientations so that X-ray diffraction or SEM shows a random scatter plot of atom distribution. Annealing can reduce the number of crystalls in a body. Diamond is a good example of a natural annealing process which prolong-heated and cooled very slowly while compressing a polycrystalline carbon structure to make it monocrystalline.