ME EXAM 2 CONCISE

Chapter 5 – Diffusion

Mass transport by atomic motion; atoms move from regions of high concentration to regions of low concentration.

Interdiffusion

Diffusion of atoms of one material into another material across an interface.

Self-diffusion

Migration of host atoms within a pure metal lattice.

Vacancy diffusion

Atoms exchange positions with vacancies; slower process; applies to substitutional atoms.

Interstitial diffusion

Small atoms move between interstitial sites; much faster than vacancy diffusion.

Effect of temperature on diffusion

Higher temperature increases atomic motion and greatly increases diffusion rate.

Effect of concentration gradient

A steeper concentration gradient increases the diffusion flux.

Effect of crystal structure

Open crystal structures (e.g., BCC) allow faster diffusion than close-packed ones (FCC).

Effect of bonding strength

Stronger atomic bonds slow diffusion because they require higher activation energy.

Steady-state diffusion

Diffusion condition where the concentration profile does not change with time and flux is constant.

Non-steady-state diffusion

Diffusion condition where concentration changes with both time and position.

Industrial example – case hardening

Carbon diffuses into steel surface to produce a hard, wear-resistant outer layer.

Industrial example – semiconductor doping

Controlled diffusion of dopant atoms (like P or B) into silicon to tailor electrical behavior.

Why diffusion in solids is slower

Atoms are tightly bonded and have limited mobility compared with liquids or gases.

Stress

The internal force per unit area acting on a material.

Strain

The amount of deformation per original dimension; a measure of shape change.

Elastic deformation

Temporary, reversible deformation that disappears when the load is removed.

Plastic deformation

Permanent deformation that remains after unloading; caused by dislocation motion.

Elastic modulus (stiffness)

Measures a material’s resistance to elastic deformation; depends on bond strength.

Relative stiffness of materials

Polymers < Metals < Ceramics < Composites (stiffest).

Poisson’s ratio

Ratio of lateral contraction strain to longitudinal extension strain; metals ≈ 0.3.

Yield strength

Stress where noticeable plastic deformation begins; defines onset of yielding.

Tensile strength

Maximum stress a material withstands before necking or fracture begins.

Ductility

Ability to plastically deform before fracture; expressed as percent elongation or percent reduction in area.

Toughness

Energy absorbed before fracture; area under the stress–strain curve.

Hardness

Resistance to surface indentation or scratching; correlates with wear resistance and strength.

Design safety concept

A factor of safety divides yield strength to give a safe working stress.

Trend – metals

Moderate stiffness, high toughness, good ductility.

Trend – ceramics

Very stiff and strong but brittle with low toughness.

Trend – polymers

Low stiffness, ductile at room temperature, sensitive to temperature changes.

Trend – composites

Combine materials to achieve high specific strength and tailored behavior.

Concept of Ohm’s Law

Current is proportional to applied electric field; resistance opposes current flow.

Resistivity vs conductivity

Resistivity measures how strongly a material resists current; conductivity is its inverse.

Classes of materials

Conductors (high σ), Semiconductors (moderate σ controlled by T and doping), Insulators (very low σ).

Why metals conduct

Partially filled or overlapping energy bands allow electrons to move freely.

Why semiconductors have moderate conductivity

Small band gap lets some electrons jump to conduction band.

Why insulators resist current

Large band gap prevents electron excitation into conduction band.

Temperature effect – metals

Increasing temperature increases electron scattering, raising resistivity.

Temperature effect – semiconductors

Increasing temperature increases carrier concentration, raising conductivity.

Defects and resistivity

Grain boundaries, dislocations, and impurities scatter electrons and increase resistivity.

Intrinsic semiconductor

Pure material where electrons = holes created thermally in equal numbers.

Extrinsic semiconductor

Doped material where conductivity is dominated by dopant-introduced carriers.

n-type semiconductor

Doped with donor atoms that add extra electrons as charge carriers.

p-type semiconductor

Doped with acceptor atoms that create holes as charge carriers.

Doping purpose

Adjusts carrier concentration to control electrical behavior of semiconductors.

Conduction regions vs temperature

Low T = freeze-out, moderate T = extrinsic dominant, high T = intrinsic dominant.

p–n junction concept

Junction of p- and n-type regions that allows current flow in one direction only.

Forward bias behavior

Holes and electrons recombine across junction; current flows easily.

Reverse bias behavior

Carriers are pulled apart; depletion region widens; current nearly stops.

Ferroelectric materials

Exhibit spontaneous, reversible polarization below a Curie temperature (e.g., BaTiO3).

Piezoelectric materials

Develop an electric voltage when stressed and deform when voltage is applied.

Effect of imperfections on conductivity

More imperfections → more scattering → lower conductivity.

Conductivity and resistance trend

Conductivity increases with carrier mobility and number of charge carriers; resistivity increases with defects and temperature in metals.