Generally the heat transfer in solid conduction has two components: Lattice thermal conduction Electronic thermal conduction Both types of heat conduction occur in solids but one is dominant over the other depending upon the type of material. Figure 2: Valence band vs Conduction band In case of insulating materials, lattice conduction contributes to heat conduction. The metals that present the FCC structure have the highest thermal conductivity, since they present the highest compaction Figure 4: Crystalline structure Figure 5: Atomic packing factor Thermal conduction in ceramics and polymers Theory In these materials, electrons are not free, so their contribution to electrical conductivity, and also to thermal conductivity, is practically nil except at very high temperatures.
Scattering of phonons by point defects see figures such as impurities, isotopes; crystal atoms with the same number of protons but different numbers of neutrons, etc. Scattering of phonons along the boundaries of the specimen or crystallites. Phonon scattering by dislocations.
Eq 2 Thermal conductivity is strongly influenced by the different phonon scattering mechanisms that can manifest themselves in the transfer of thermal energy in solids. Figure Vibration model Atoms vibrate around their equilibrium position, given by the average distance of separation from their neighbors, with a specific frequency and amplitude.
The characteristic vibration frequency of the harmonic oscillator obeys the expression: Eq 3 where k is a constant related to the elastic deformations around the equilibrium point in the bond potential energy-interatomic distance graph, different in each material. The maximum amplitude of vibration A0 is a function of the temperature T: Eq 4 Increases in temperature cause vibrations of greater amplitude and energy and also the excitation of some electrons towards the conduction band is facilitated, which causes the conductivity to increase with temperature.
The above equation justifies several experimentally proven facts: Materials with more compact structures and higher modulus of elasticity have higher conductivities.
Crystalline solids have higher conductivity than if they are amorphous. Bonding Forces and Energies Considering the interaction between two isolated atoms as they are brought into close proximity from an infinite separation. Figure Bonding Forces vs distance Figure Atomic mechanism of elastic deformation Figure Types of Bonds Examples Metal vs Metal oxides Slack carried out a systematic comparison of calculated and measured thermal conductivities for face-centered cubic lattices.
That is due to the following factors: Electronegativity difference between atoms ionic character Crystal structure of crystallization packing density Defects in the crystal structure Mass difference between atoms Figure Comparison of Ionic and Covalent Bonding The thermal conductivity of non-metallic crystals, including the majority of oxides, exhibits four distinct regimes when plotted against temperature. At low temperatures, in the A or I regime , there is very little thermal energy to excite phonons.
Therefore, the phonon mean-free-path is limited by the physical dimensions of the material, the grain size, and the spacing between dislocations. The T 3 temperature dependence of thermal conductivity must then be explained by the heat capacity having a T 3 dependence at very low temperatures. The B or II regime, characterized by a maximum in thermal conductivity occurs when the phonon mean-free-path of phonon-phonon collisions is equal to phonon defect collisions.
Although this temperature will certainly vary with defect type and concentration, this value to be approximately 0. This is because anharmonic phonon scattering, due to phonon-phonon interactions dominate. If more phonons are present then the likelihood of scattering should increase. Finally, at very high temperatures the D or IV regime tends towards having no temperature dependence.
The physical explanation for this is that phonons are waves of displaced atoms and the mean-free-path cannot be decreased smaller than the distance between two neighboring atoms. Figure Thermal conductivity versus temperature for non metallic crystal. The four characteristics regimes correspond to different scattering processes Diamond vs graphite Diamond is one of the best known thermal conductors, in fact, diamond is a better thermal conductor than many metals.
Figure 19 : Crystal structure of diamond and graphite The reason why diamond, in particular, is an especially good thermal conductor, even compared to other well-ordered crystals, comes down to two factors: the mass of the carbon atoms and the strength of the bonds that connect them.
Figure 20 : Allotropes of carbon The same happens to crystallized boron nitride in the hexagonal system similar to graphite , along the sheets there is a very good conduction but between layers the conductivity is low.
This result suggests four criteria for choosing materials with high thermal conductivity: low atomic mass, strong interatomic bond, simple crystal structure and low anharmonicity. Dispersion of phonons in different directions of the crystal. Orderly structures of crystals Impurities Types of atoms types of bonds: covalent, ionic … etc Difference in masses between the different atoms.
Single-element structures or elements of similar atomic weight no distorted lattice. Download our ebook. Related posts. And when one is looking to "ground" a charge i. This is common with electrical circuits where exposed metal is a factor, ensuring that people who accidentally come into contact are not electrocuted.
Insulating materials, such as rubber on the soles of shoes, is worn to ensure that people working with sensitive materials or around electrical sources are protected from electrical charges. Other insulating materials like glass, polymers, or porcelain are commonly used on power lines and high-voltage power transmitters to keep power flowing to the circuits and nothing else! In short, conduction comes down to the transfer of heat or the transfer of an electrical charge. Both happen as a result of a substance's ability to allow molecules to transfer energy across them.
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By using our site, you acknowledge that you have read and understand our Privacy Policy and Terms of Use. Share Twit Share Email. Home Physics General Physics. Diagram showing the transfer of thermal energy via conduction. Credit: Boundless. Heat conduction occurs through any material, represented here by a rectangular bar.
The rate at which it is transfers depends in part on the thickness of the material rep. Conduction, as demonstrated by heating a metal rod with a flame. Credit: Thomson Higher Education. This view of the nose section of space shuttle Discovery, build of heat-resistance carbon-composites. Electrons are freed from a molecule when heat is applied, which forces the electrons to travel toward the colder end of the metal.
At the colder end, the electrons collide into many molecules, and therefore, pass energy to the molecules at the other side. However, in metals, due to the presence of free electrons, thermal energy is spread through electron diffusion.
Electrons gain kinetic energy and move rapidly and collide with the atoms in the cooler parts of the metal to pass on their energy. The process of conduction in metals is important for engineers to understand when preparing to pass the PE Mechanical exam. This website uses cookies to ensure the best experience possible.
Process of conduction has both the negative as well as positive consequences in our lives. Zhu Yugang answered.
Because it can help us transfer electricity energy. Answer Question. Connect Connect Connect.
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