5: Looking Inside Materials_checked Flashcards
<p>Name a natural and a man-made polymer</p>
<p>Rubber - natural
<br></br>Polythene - man-made</p>
<p>What was the aim of Rayleigh's oil drop experiment?</p>
<p>To estimate the size of an atom</p>
<p>What was an assumption Rayleigh made?</p>
<p>Rayleigh assumed that 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</p>
<p>Describe X-ray crystallography</p>
<p>X-ray Crystallography Involves firing X-rays at a sample and using their diffraction patterns to investigate atomic spacing and structure</p>
<p>Explain how a scanning tunnelling microscope (STM) works. (5)</p>
<ol><li><strong>Scanning Tunneling Microscopes</strong> have a <strong>very fine tip</strong> which a voltage is applied to.</li><li><strong>Electrons</strong> from the sample surface <strong>tunnel</strong> from the <strong>surface</strong> to the <strong>tip</strong> and cause a <strong>current</strong> to flow</li><li>The tip is moved across the surface of the sample, and the <strong>height</strong> of the tip is adjusted to keep the <strong>current</strong> constant...<br></br>4)....meaning any<strong> small bumps/dips</strong> in the <strong>surface</strong> can be <strong>detected</strong><br></br>5) STMs have such a fine resolution that individual atoms can be resolved and their size and spacing measured</li></ol>
<p>What do modern techniques say the diameter of an atom is?</p>
<p>0.1 - 0.5 nm depending on the size of the atom</p>
<p>What do Scanning Electron Microscopes (SEM) and Atomic Force Microscopes (AFM) do?</p>
<p>Measure <strong>atomic sizes</strong><br></br>They don't let you see the surface of the material directly. They build up an <strong>atom-by-atom</strong> image of the surface on a computer screen. By knowing the <strong>magnification</strong> of the <strong>image</strong> on the computer screen and the size of the 'blobs' representing each atom, the sizes of atoms can be calculated</p>
<p>LiF chips need to be handled with care. Use the internal structure of such ionic crystals to explain how this structure gives these properties<br></br>Information given in insert:<br></br>Its an ionic lattice.This makes the structure hard but brittle, and easily cleaved</p>
<p>[3]</p>
<p>LiF brittle – so snapping risk/cracks will propagate (1)
<br></br>due to LiF being an ionic lattice structure/ has
<br></br>directional bonds (1)
<br></br>so there is a lack of (mobile) dislocations (1)</p>
<p>How did rayleigh carry out his experiment</p>
<ol><li>Measure the diameter, d, of the oil drop to calculate the radius, r </li><li>Place the oil drop in still water and let it spread fully.</li><li>Measure diameter of patch, D, after it's spread to calculate radius, R</li><li>The volume of the oil drop (a sphere) is equal to the volume of the patch of oil (approximate a cylinder).</li><li>Use equations for Volume and rearrange for h, the thickness of the oil patch which matches size of atom</li></ol>
<p>What is the biggest uncertainty in calculating the size of the atom?</p>
<ol><li>The largest percentage uncertainty comes from trying to measure diameter of the oil drop of 2 mm to more precise than +-0.5mm, which is a considerable factor</li><li>In Volume, radius is cubed so this the % uncertainty in radius is a considerable uncertainty</li></ol>
<p>What do STM and AFM images show?</p>
<p>STM and AFM images show individual atoms</p>
<p>What do SEM images show?</p>
<p>SEM images show larger scale structures than STM and AFM</p>
<p>Which has the greater ductility? Pure metals or metal alloys<br></br>Why?</p>
<ol><li><strong>Metal alloys</strong> tend to be <strong>less ductile </strong>than pure <strong>metals</strong></li><li>This is because metal alloys are formed by the addition of other metals with different sized atoms which can <strong>pin dislocations</strong> in the structure making slippages more difficult</li></ol>
