# Shared Flashcard Set

## Details

Thermal Behavior
Thermal Behavior
26
Mechanical Engineering
05/13/2014

Term
 Define thermal behaviors of materials
Definition
 -Heat capacity-specific heat
Term
 When a material is heated, what happens
Definition
 -thermal expansion-heat flow
Term
 Thermal conductivity
Definition
 indicates the heat flow rate of certain material
Term
 Heat capacity
Definition
 Defines the amount of heat required to raise temperature by 1k (=1 Celsius)
Term
 Specific heat
Definition
 heat capacity per unit mass
Term
 Thermal expansion
Definition
 Increase the thermal vibration of atomsLeads to increase in distance between atoms
Term
 Thermal conductivity
Definition
 Thermal gradient between different heat zone causes heat flowFree electrons transfer heat rapidly (metal)Randomized electron spins in ferromagnetic materials
Term
 Bonding Force
Definition
 Steeper slope at F=0  higher elastic modulus
Term
 Bonding Energy
Definition
 Deeper well  higher bonding energy
Term
 Equilibrium point
Definition
 Zero forceMinimum energy
Term
 Highest to lowest for thermal expansion and why
Definition
 polymer>metals>ceramics and glassesb/c ceramics and glasses have deeper wells than other materials (higher bonding energy)higher melting point-less change in anatomic distance with increasing temp-higher elastic modulus
Term
 Weakly bonded solids
Definition
 -low melting point-low elastic modulus-high thermal expansion coefficient
Term
 Strongly bonded solids
Definition
 -high melting point-high elastic modulus-low thermal expansion coefficient
Term
 thermal expansion coefficient
Definition
 -function of temperature-higher coefficient at higher temperature in general-some glass-ceramics has a negative coefficient, good for thermal shock
Term
 Thermal conductivity
Definition
 K: thermal conductivitydQ/dt: the rate of heat transferA: cross sectional areadT/dx: teperature gradientDifferent material has their own thermal conductivitythermal conductivity is a function of temperature
Term
 Two primary mechanisms of conduction
Definition
 Atomic vibrationsConduction of free electrons
Term
 What material is a good conductor and why?
Definition
 Metal: better conduction by kinetic energy of conduction electrons
Term
 What material is a poor conductor and why?
Definition
 electronsPoor conductor (ceramic, polymer): only by atomic vibration
Term
 Structural disorder by chemical impurities results
Definition
 in a similar decrease in thermal conductivity
Term
 Structural disorder interferes
Definition
 with the movement of the waveform
Term
 True of false: Metal alloys tend to have lower thermal conductivities than pure metals
Definition
 true
Term
 The thermal conductivity of ceramics and polymers can be further reduced by
Definition
 the presence of porosity. (air pocket has very low conductivity)
Term
 Thermal shock
Definition
 -Fracture of the material as a result of a temperature change (usually a sudden cooling from high temperature)-common for brittle materials (ceramics, glasses
Term
 The mechanism of thermal shock
Definition
 1. Thermal expansion2. Residual stress
Term
 Thermal Shock by Thermal expansion
Definition
 A failure stress can be built up by constraint of thermal expansion
Term
 Thermal Shock by residual stress
Definition
 Rapid temperature changes produce temporary temperature gradients resulting in surface tensile stressesSurface tensile stress compress the interior and the interior pull the surfaceThis causes internal residual stressResidual stress – temperature gradients by finite thermal conductivity(thick shape)
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