MAT FINAL EXAM DEFINITIONS

Electric Current

Flor of charged particles

Electrical conductors

• Allow electricity to flow through

• Conductivness (10^7 (Ω*m)-1

Electrical insulators

• Difficult for electricity to flow through it

• Conductiveness 10^-10 - 10^-20 (Ω*m)-1

Semi Conductors

• Between conductors and semiconductors

• Conductivess (10^-6 to 10^4) (Ω*m)-1

Electrical conduction

Flow of charge carriers

Ionic conduction

• Diffusion of charged ions

• Net motion of charged ions

Energy Band Gap

• Difference between valence and conduction bands

• Electrical conductivity requires charge carriers to be in conduction band

Valence band

• Outermost shell of atoms

Fermi Energy

• Energy possesed by electrons in highest orbital

• Electrons with energy greater than Ef can accelerate in an electric feild

Conductivity of a metal decreases with:

• Increasing temperature

• Increasing impurities

• Increasing number of dislocations

Intrinsic Semiconductors

• Electrical behaviour depends on the properties of the pure material

  • More ionic bonding= more likley material is insulative

  • Holes are left behind by excited electrons

Extrinsic semiconductors

• Electrical behaviour depends on presents of dopants (impurities)

• N-tytpe (electrons ) and P-type(holes)

PN junctions

• Combining N-type and P-type semiconductors

Forward Bias in PN junctions

• Increases electric conductions

• Shrinks depletion zone

Reverse Bias in PN junctions

• Conducts little current

• Widens depletion zone

Diodes

• PN junction at room temperature

• Diodes flow currents in 1 direction

Insulators

• Bad conductors

• Band Gap energy (Eg> 3eV)

• Ceramics, polymers

Dielectrics

• Bad conductors

• All Dielectrics are insulators, but not all insulators are dielectrics

• Polarized by Electric feild:

  • Algin dipoles to crease layers of charge particles in each surface

  • More dipole moments= more polarization= higher dielectric constant

Capacitors

• Store electrical energy in an electric feild

Piezoelectricity

• “Pressure electricity”

• Mechanical strain polarizes materials, resulting in voltage

Heat properties

• Heat capacity

• Thermal expansion coefficient

• Thermal conductivity

• Latent heat

Temperature

Average Ek associated with atomic motion

Thermal energy

Sum of all particles in material

Heat

Transfer of thermal energy

Heat Capacitance

Ability of material to absorb heat from environment

Phonon

• Vibrational energy

• Lattice wave energy allows transfer of thermal energy

• Thermal energy can move via free electrons

Dubye Temperature

• Temperature after the heat capacitance becomes independent of temperature

• Solids: Debye temperature< room temperature

  • Except titanium and diamond

Thermal conduction

• Heat transport from high to low temperature regions

• Combination of phonons and free electron movement

Thermal Expansion

• Result of solid materials xpanding upon heating and contracting when cooled

Thermal Expansion coefficient trend:

• Polymers, metals, ceramics

Thermal Stress

• Results from changes in temperature (gradients)

  • Leads to fractures or undesired plastic deformations (thermal shock)

Seebeck Thermoelectric effect

• Voltage generated from temperatures as electrons move from high to low temperatures

Fouriers Law

• Direction of heat transfer is perpendicular to lines of constant temperature (isotherms)

• Heat transfer is done through conduction, convection, and radiation

  • Cond’s need change in temperature

  • Conduction(dominant in solids)

  • Convection(only in fluids)

Phase meaning

• Chemically uniform (phase has same composition)

• Physically distinct

• Mechanically seperable

• 1phase, 1 component: pure water

• 1 phase, 2 component: Brass

• 2 phases, 1 component: Ice + Water

Phase diagrams

• Tell us what phases are most stable and at what conditions

• isomorphous: complete solubility of two components in solid state

• Eutectic: limited solubility of the 2 components in solid state

Deterioration of Materials

• Metals corrode

• Ceramics are resistant

  • Immune to temp change

• Polymers degrade (physiochemical

  • Swelling and dissolution: liquid diffuses into and is absorbed within polymer

  • Heat energy

  • Radiation (bond rupture)

Corrosion pervention

• Material selectrion

• Change the enviornment (temp, concentration of corrosion)

• Physical barriers

Cathodic Protection: Galvanizing

• Make metal interested in cathode

• Apply layer of zinc to surface of steel

Passivation

When metals lose their chemical resistivity and become intert

Purpose of heat exchangers

To transfer heat between 2 fluids

Difference between counterflow and parallel heat exchangers

• Counterflow: opposite directions

• Parallel: same direction

What happens to the cold fluid in a heat exchanger

• Temperature increases

Which of the following improves heat transfer in a heat

exchanger?

A) Decreasing the surface area.

B) Reducing fluid flow rate.

C) Using materials with low thermal conductivity.

D) Increasing the temperature difference between fluids

D

A solid solution occurs when

two or more elements are completely soluble in one another in the solid state,