EMA 3010 EXAM #3

Intermediate and terminal solid solutions

A terminal phase or terminal solution is one that exists in the extremes of concentration (0 and 100%) of the phase diagram. One that exists in the middle, separated from the extremes, is called an intermediate phase or solid solution.

Thermocouples

A thermocouple is an electrical device consisting of two dissimilar electrical conductors forming electrical junctions at differing temperatures. A thermocouple produces a temperature-dependent voltage as a result of the thermoelectric effect, and this voltage can be interpreted to measure temperature.

Pieozelectricity

An electric field is generated by mechanical stresses on this type of material (ceramic) Microphones, speakers, etc.

Fiber reinforcement, influence of length orientation, concentration, composition

Fiber-reinforced composites have high strength, stiffness on a weight basis. load transmittance from matrix to fiber, affected by magnitude of interfacial bond between fiber and matrix phase As the length orientation increases, the fiber reinforcement becomes more effective. parallel alignment of the longitudinal axis of fibers in a single direction and a totally random alignment. Continuous fibers are normally aligned whereas discontinued fibers can be aligned. Composition affects the modulus of elasticity

Solubility limits, melting points, phase identification, % composition

Practice problems

Development of microstructure

Defined by: number of phases present proportions manner in which they are arranged Equilibrium conditions: slow cooling causing motion down vertical line at set weight percentage, use tie line to find various solid/liquid compositions, microstructure is not realized until we hit the liquidus line moving downward Microstructures develop as a result of cooling and the solid has homogenous compositions SEE PAGE 371

Nonequilibrium cooling

Development of microstructures through quenching (fast cooling rate) Diffusion is very fast for liquids comparatively to solids, and decreases with decreasing temperature (smaller temperature gradient) This causes the microstructures to have layers of different alloy compositions, largest composition of the alloy specified in the center and decreasing as with distance from center. Tie lines to determine, (dotted line from tie line to tie line on solidus) SEE PAGE 371

Iron-carbon phase diagram

PAGE 399

Types of Composites (particle filled, fiber filled)

Partical filled: large-particle and dispersion-strengthened composites. The term large is used to indicate that particle-matrix interactions cannot be treated on an atomic or molecular level; rather continuum mechanics is used. The particulate phase tends to be harder and stiffer than the matrix. Load bearing. Think concrete, concrete is matrix, sand and gravel is particulate. Dispersion-strengthened composites, particles, are normally much smaller. Particle-matrix interactions that lead to strengthening occur on the atomic or molecular level. Fiber-Reinforced Compsites: Goal to have high strength and or stiffness on weight basis Critical length

Ohm's law, resistivity, conductivity

Practice problems

Tie line and lever rule

Practice problems

Rule of mixtures (upper and lower limits)

Rule of mixtures equation predict that the elastic modulus should fall between an upper bound or lower bound.

Electronic bands and band gaps, Electron mobility

An electronic band is a series of electron states that are closely spaced with respect to energy, and one such band may exist for each electron subshell found in the isolated atom. Band gap - The electronic bands are separated by gaps, where electrons cannot exist. PAGE 487 Electron mobility - Free electrons being acted on by an electric field are scattered by imperfections in the crystal lattice. The magnitude of electron mobility is indicative of the frequency of these scattering events. In many materials, the electrical conductivity is proportional to the product of electron concentration and the mobility. PAGE 491

Heat capacity, atomic description of heat content

Heat capacity - indicates a material's ability to absorb heat from external surroundings; it represents the amount of energy required to produce a unit temperature rise. Waves amongst several atoms, causing waves to propagate about them. The vibrational thermal energy for a material consists of a series of these elastic waves which have a range of distributions and frequencies, which propagate through a material at the speed of sound. A single quantity of vibrational energy is a phonon.

Hypoeutectic and proeutectoid compositions

Hypoeutectic composition - The microstructures of alloys having carbon contents less than the eutectoid, see microstructure PAGE 403 Proeutectoid composition - Proeutectoid signifies is a phase that forms (on cooling) before the eutectoid austenite decomposes. It has a parallel with primary solids in that it is the first phase to solidify out of the austenite phase. Thus, if the steel is hypoeutectoid it will produce proeutectoid ferrite and if it is hypereutectoid it will produce proeutectoid cementite.

Factors influencing conductivity (temperature, impurities, dopants)

In metals, higher temperatures, impurities, and deformation causes its resistivity to go up. Refer to equations to see relationship. In semiconductors, dopants cause electron and hole mobility to stay at max mobility until a max concentration is met and causes the mobility to decline (think logarithmic but backwards). The mobility of electrons is always higher than hole mobility. Page 504 For dopant concentrations 10E24 and below, both electron and hole mobilities decrease in magnitude with rising temperature. This is due to enhanced thermal scattering of the carriers. As the dopant level increases there is less and less temperature dependence on electron and hole mobility

Intrinsic vs. extrinsic semiconductors

Intrinsic semiconductors are those in which the electrical behavior is based on the electronic structure inherent in the pure material. Empty conduction band, thin forbidden band gap, and a filled valence band. In intrinsic semiconductors, for every electron excited into the conduction band there is left behind a missing electron in one of the covalent bonds, or in the band scheme, a vacant electron state in the valence band. The way conduction works is that when an electric field is applied, valence electrons repeatedly fill in the incomplete bond. Incompleteness is referred to as a hole and has the same magnitude but opposite charge as an electron. Two types of charge carries, holes and free electrons. Refer conduction eq for intrinsic conductors. When electrical characteristics are dictated by impurity atoms , the semiconductor is said to be extrinsic. Impurity atoms may only bond four out of five available electrons leaving a nonbonding electron loosely bound to the region around the impurity, can become a free or conducting electron. There is a single energy state in the forbidden band gap directly below the band gap for this electron. See page 499 and 500 Because this electron comes from the impurity as a donor electron, there is no valence spot for it. In n-type extrinsic semiconductor the electrons are majority carries by virtue of density and their concentration. p-type extrinsic semiconduction Opposite of n-type, there is an acceptor hole from impurity, leaving a shortage of valence electrons or a traveling hole. Page 501

Factors influencing the properties of particle composites (concentration, size, shape, distribution, orientation)

Large particle composites should be small and evenly distributed in the matrix, these particles should be approximately the same dimension (shape), the more particulate content, the more enhanced the mechanical properties. Uniform dispersion-strenghthened composites in metals can harden metals with fine particles of a very hard inert material. The dispersed phase can be metallic or nonmetallic.

Critical fiber length

Length orientation is important, the fibers should be at least the critical length to exhibit desired properties of stiffening and strengthening.

Eutectics (composition, temperatures, and microstructure)

Liquid forming two different solids L --> Alpha + Beta Eutectic point where liquid touches combined solid solution of A + B This point gives us the eutectic composition (of liquid) and a tie line for A and B compositions the isotherm or eutectic temperature in which the eutectic occurs The microstructure will likely be laminar, which is a layered structure with composition proportional to composition at eutectic point Eutectic is the lowest melting temperature At the Eutectic isotherm, we will have a partial eutectic structure

Solidus, liquidus, and solvus lines

Liquidus - The liquidus line separates the liquid phase from solid or solid + liquid phases. That is, the solution is liquid above the liquidus line. Solidus - The solidus line is that below which the solution is completely solid (does not contain a liquid phase.) Solvus - The solvus is represented by a line on a phase diagram that separates a solid phase from a solid1 + solid2 phase, where solid1 and solid2 are different microstructures.

Conductors, insulators, semiconductors

Metals are good conductors, typically having conductivities to the order of 10E7 Insulators have very low conductivities, ranging from 10E-10 to 10E-20 Semiconductors are materials with intermediate conductivities, generally from 10E-4 to 10E4

Nanocomposites

Nanocomposite is a multiphase solid material where one of the phases has one, two or three dimensions of less than 100 nanometers, or structures having nano-scale repeat distances between the different phases that make up the material. Nanocomposites are materials that incorporate nanosized particles into a matrix of standard material. The result of the addition of nanoparticles is a drastic improvement in properties that can include mechanical strength, toughness and electrical or thermal conductivity.

Thermal conductivity

The ability for a material to transfer heat. AKA heat flux (transport bb)

Mechanisms of conduction in different materials

The difference in metals, insulators, and semiconductors is in the number of free electrons and hole charge carriers. For an electron to become free, it must be excited or promoted into one of the empty and available energy states above Ef. METALS Page 489 There are vacant energy states adjacent to the highest filled state Ef. Thus very little energy is required to promote electrons into the low-lying empty states. Can usually be promoted from Efield alone. INSULATORS AND SEMICONDUCTORS Page 490 For insulators and semiconductors, empty states adjacent to the top of the filled valence band state are not available. To become free, therefore, electrons must be promoted across the energy band gap and into empty states in the bottom of the conduction band. Required energy difference between electron in valence band to conduction band roughly equal to the band gap energy. The larger the band gap the lower the conductivity. Increasing temperature gives rise to more conduction for these types.

Euctectoid and peritectic reactions

The eutectoid (eutectic-like) reaction is similar to the eutectic reaction but occurs from one solid phase to two new solid phases. It also shows as V on top of a horizontal line in the phase diagram. Solid Phase 1 --> Solid Phase 2 + Solid Phase 3 The peritectic reaction also involves three solids in equilibrium, the transition is from a solid + liquid phase to a different solid phase when cooling. The inverse reaction occurs when heating. Solid Phase 1 + liquid --> Solid Phase 2 Practice problems

Specific modulus

The modulus of elasticity to specific gravity. High moduli in fiber-reinforced composites produced that use low-density fiber and matrix materials.

Ionic conductivity

Typically insulating materials Have to excite electrons with thermal energy Ionic conductivity is caused by movement of ions and electrons. It is quantitatively described as the sum of electronic and ionic contributions to conduction.

Thermal expansion

extent at which something expands look at EQ