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

• f 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

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

such as M1-xO2 ( Metal Vacnacy) or M1+yO2 (Metal Interstitial)

• Oxides with Anion defects

such as MO2+x (oxygen Interstitial) and MO2-y ( oxygen vacancy)

In practice when many of oxides are formed they are not formed in exact molecular

• proportion. There is

either less or more

• f one of the atoms.

These are defined 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
• f 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