Lezione 12: Materials and heat Flashcards
How does heat affect a material?
Heat is related to atoms’ motion and vibration. It affects mechanical and physical properties, as strength, stiffness, creep. Thermal design keeps into account (or exploits) thermal properties of materials.
Which temperatures influence materials?
Two temperatures are directly related to the strength of a material:
-Tm, melting temperature: crystalline materials have a defined temperature at which they melt
-Tg, glass temperature: non-crystalline solids are characterized by a temperature at which they transition from a true solid to a very viscous liquid.
Materials also have operating temperatures:
-Tmax: highest T at which the material can be used continuously without oxidation, chemical change or excessive distortion
-Tmin: T below which the material becomes brittle or otherwise unsafe to use
What is heat capacity Cp?
It’s the energy needed to heat 1kg of a material by 1K at atmospheric pressure. It’s measured in J/kgK, symbol Cp
It’s measured by a calorimeter that records T and heat (energy pumped to a known material mass)
What is thermal expansion coefficient α?
It’s the thermal strain per degree of temperature change 1dL/LdT. The unit is K^-1 or better microstrain/K. To characterize anisotropic material more thermal coefficients are necessary.
What is thermal conductivity λ?
It’s the rate at which heat is conducted through a solid at steady-state (when T does not change with time). The unit is W/mK. It’s measured recording q, the heat flux, measured in W/m^2 and knowing ∆T/∆x.
What is thermal diffusivity a?
If T changes with time, heat flow changes according to thermal diffusivity. It can be measured directly by measuring the time it takes for a temperature pulse to travers a specimen of known thickness when a heat source is applied briefly to one side.
Which graphs can help us in material selection for thermal properties?
- Thermal expansion - conductivity: contours show thermal distortion parameter
- Thermal conductivity - diffusivity: contours show the specific heat per unit volute, pCp. As a general rule this value is almost constant for all solids. Porous materials have low conductivity but high diffusivity: they don’t transmit much heat, but they do change T quickly.
- Thermal conductivity - yield strength: many applications require a combination of high strength and high conductivity
How does thermal expansion affect materials?
A solid expands when heated because its atoms are moving farther apart. The bond is stiffer when atoms are pushed together and less stiff when pulled apart.
Materials with a high modulus have a low expansion coefficient
All crystalline solids expand approximately 2% from absolute zero to their melting point, independently from how high is their melting point.
How does thermal conductivity affect materials?
Heat transmission in solids takes place through:
-thermal vibration
-movement of free electrons
-irradiation (materials transparent to IR)
About thermal vibration, when a solid is heated the heat enters as elastic wave packets or phonons. The phonons travel at the speed of sound, but only move at a short distance before they are scattered. The distance a phonon can travel before bouncing off something else is called the mean free path - typically less than 0.01µm
Heat in metals is more efficiently conducted by free electrons.
Is it possible to manipulate thermal properties?
- Thermal expansion (coefficient) cannot be modified, because ti depends on stiffness and strength of the atomic bond.
- Thermal conductivity can only be manipulated by modifying the free path of phonons or electrons, for example by introducing impurities. The glassy state of some materials introduces disorder and scatter centers, making conductivity very low. Also increasing strength in metal usually works. It can be decreased by changing density.
Strengthening mechanisms can increase the yield strength of metals. On the other hand, these processes introduce scattering sites which can reduce thermal conductivity bu shortening the mean free length that phonons can travel. Work hardening doesn’t reduce thermal conductivity much.
What can cause damage from thermal stresses and how can we prevent it?
When two materials have two different thermal expansion coefficients and are constrained to each other, the difference in their α values can bring to thermal stresses (one materials expands more than the other). The easiest way to prevent this is to choose material with similar α values.
If this is not possible, it can be solved by having graded joints between the two materials that acts like a buffer to minimize the expansion difference.
What is thermal sensing and actuation?
Designs also can utilize materials with different thermal expansions to sense or actuate.
What are thermal gradients?
Thermal gradients are developed when a material is rapidly cooled or heated. When the material is subjected to differential temperature, the surface tries to expand/contract while the rest of the body opposes this motion. This phenomenon can result in cracking, especially in brittle materials. If the deformation stays in the elastic field, cracks are still possible but when the T is uniform all stresses are released. If material yields, stress are not released and residual stresses may cause distortion and failure.
The resistance of a material to this behavior is measured by its thermal shock resistance, ∆Ts. Thermal distortion due to thermal gradients is a problem for precision equipment. Solution is to use materials with lowest α possible and with high conductivity λ. So, high values of λ/α
What’s the job of heat exchangers?
They must be able to transfer heat and withstand internal pressure from the fluids. Best materials for this job are the ones with highest values of λσy
How do we ensure insulation?
Best material to minimize power loss are those with low thermal conductivities. However, many of the materials with very low conductivity also have low Tmax values. This must be considered since Tmax is generally a constraint.
