5: Looking Inside Materials Flashcards
Name a natural and a man-made polymer
Rubber - natural
Polythene - man-made
What was the aim of Rayleigh’s oil drop experiment?
To estimate the size of an atom
What was an assumption Rayleigh made?
The oil would spread as much as it could, so this thickness of the oil patch h would be the size of one molecule of oil
POTENTIAL SKIP Describe X-ray crystallography
Involves firing x-rays at a sample and using their diffraction patterns to investigate atomic spacing and structure
POTENTIAL SKIP Explain how a scanning tunnelling microscope (STM) works (5)
1) Has a very fine tip which a voltage is applied to
2) Electrons from the sample surface tunnel from the surface to the tip and cause a current to flow
3) The tip is moved across the surface of the sample, and the height of the tip is adjusted to keep the current constant…
4) ….meaning any small bumps/dips in the surface can be detected
5) STMs have such a fine resolution that individual atoms can be resolved and their size and spacing measured
What do modern techniques say the diameter of an atom is?
0.1-0.5 nm
What do Scanning Electron Microscopes (SEM) and Atomic Force Microscopes (AFM) do?
Measure atomic sizes
They build up an atom-by-atom image of the surface on a computer screen. By knowing the magnification of the image on the computer screen and the size of the ‘blobs’ representing each atom, the sizes of atoms can be calculated
[3] LiF chips need to be handled with care. Use the internal structure of such ionic crystals to explain how this structure gives these properties
Information given in insert:
Its an ionic lattice.This makes the structure hard but brittle, and easily cleaved
LiF brittle – so snapping risk/cracks will propagate (1)
due to LiF being an ionic lattice structure/ has
directional bonds (1)
so there is a lack of (mobile) dislocations (1)
how did rayleigh carry out his experiment
measure the diameter d of the oil drop to calculate the radius r
place the oil drop in still water and let it spread
measure diameter of patch D after it’s spread to calculate radius R
use equation to calculate thickness of oil patch which matches size of atom
big uncertainty in calculating size of atom
the largest percentage uncertainty comes from trying to measure diameter of drop of 2 mm to more precise than +-0.5mm, which is a considerable factor,
radius is cubed so this is a considerable uncertainty
what do stm and afm images show
show individual atoms
what do sem images show
larger scale structures than stm and afm
ductility of pure metals vs metal alloys
why
metal alloys tend to be less ductile than pure metals
this is because metal alloys are formed by the addition of other metals with different sized atoms which can pin dislocations in the structure making slippages more difficult
