Principle of SIMS Atomic Force Microscope Can resolve two atoms - PowerPoint PPT Presentation

Principle of SIMS

Atomic Force Microscope Can resolve two atoms Resolution 0.1-1 nm

• It uses a cantilever made from silicon or silicon nitride having a very low spring constant to image a sample. • At one end of the cantilever, a very sharp tip (around 100 – 200µm long and 20 – 60 nm radius of curvature) is fabricated using semiconductor processing techniques. • The cantilever scans above the surface of the sample by progressively moving backward and forward across the surface. • A piezo-electric crystal raises or lowers the cantilever to maintain a constant bending of the cantilever. • The force exerted on the tip varies with the difference in the surface height and thus leads to the bending of the cantilever. • A laser beam gets constantly reflected from the top of the cantilever towards a position-sensitive photodetector consisting of four side by-side photodiodes

PRESENTATION OF DATA • Determination of “n” – Double Log Plots • Determination of Activation Energy • Carry out test at various temperatures • Determine K at each temperature • Plot a curve between Log K and 1/T • Slope gives the activation energy

Application of High Temperature in Industry • Power Plants • Aerospace • Chemical Process and Petrochemicals • Refineries

Defect Structure of Oxides

Perfect Crystal Exist at absolute Zero Temperatures All atoms are on their lattice positions There are no missing atoms from lattice

Notation of Point Defects

Schottky Defect A pair of Metal Ions and Metal Vacancy Frenkal Defect A pair of metal and Oxygen Vacancy

Non Stoichiometry A Stoichimetric Compound has elements with given proportion such as FeO, which means for one atom of Fe there is one atom of oxygen • Oxides with cation defects In practice when many of oxides are such as M 1-x O 2 ( Metal formed they are not Vacnacy) or M 1+y O 2 (Metal formed in exact Interstitial) molecular • Oxides with Anion defects proportion. There is either less or more such as MO 2+x (oxygen of one of the atoms. Interstitial) and MO 2-y ( oxygen These are defined vacancy) as:

Formation of Defects Ratio of No of Anion to Cation Sites Remain Constant Charge balance Mass Constant – the number of atoms remains constant before and after defect formation

Formation of Oxygen Vacancy • At sufficient low pressure, the oxygen atom can slip out of oxide lattice resulting in creating a oxygen vacancy :

Formation of Metal Vacancy Defect • At very high pressures of oxygen there can be • Adsorption and dissociation of oxygen molecules on the surface of solid • Formation of oxygen ions by taking electrons from adjacent metal ions which are thus oxidised • Formation of metal vacancy which becomes necessary because of creation of O – ion.

Defect Equilibria of Metal Deficient Oxide A plot of defect concentration with partial pressure will give a line whose slope will be 1/6