TL;DR High molecular structural symmetry and lower wave scattering properties are why metals are good conductors.

Your life is a lie

Movement of delocalized electrons has little to do with conduction of heat How do we know that? Think about it. Resistance in wires causes the wire to heat up. By simple logic then, if the wire was heated, resistance should increase, not conductance. By that logic, the more you heat a metal, the less heat it will conduct because of high resistance. (Which is in fact true) Which means there must be something other than delocalized electrons that transfer heat in a metal, because they can’t promote and condemn conduction at the same time.



In a solid, molecules don’t randomly start bumping into each other and transferring kinetic energy, until or unless the solid is melted. (They do, but very, very slowly)

Phonons – Heat waves in solids

There’s a certain dichotomy in physics between waves and particles where they can be treated similarly and differently depending on context. A Phonon is a name given to the effect of creating a wave within an atomic structure, and that wave propagates through the lattice and does things that simulate the effects of if it was a particle. It isn’t a real particle (it a ‘quasi-particle’), but the effects of what it does to things around it make it look like one. (Source – r/explainlikeimfive)

The number of phonons changes when a lattice, such as a solid, absorbs energy. But overall their position is not localized and their amount cannot be determined as due to their quasiparticle or emergent nature they are incapable of existing independently. Which is why they can rather be seen as a secondary property or a characteristic of a lattice arrangement.

A computer simulation shows phonons, depicted as color variations, traveling through a crystal lattice. The lattice, in this case, is broken up by round rods whose spacing has been chosen to block the passage of phonons of certain wavelengths.

In crystalline solids, lattice vibrations are manifested as ‘heat’ and is often a dominant mode of energy transfer (heat conduction) at room temperature. In this case, we often talk of phonons as a quantum of these lattice vibrations. So, a phonon can be rightly thought of as a quantum of ‘heat energy’ in crystalline solids. (Source)

But unlike photons (the particles that carry light or other electromagnetic radiation), which generally don’t interact at all if they have different wavelengths, phonons of different wavelengths can interact and mix when they bump into each other, producing a different wavelength. This makes their behavior much more chaotic and thus difficult to predict and control. (Source)

So why are good conductors good? Answer: They scatter less phonons.

We categorize materials as good, poor conductors or insulators. One way to determine the thermal efficiency of a conductor is to look for the rate of phonon scattering in its lattice. The more phonon scattering it has, the less thermal conductivity.

Phonon scattering is basically the diversion of phonons from their path due to any obstacle. It is classified into four types-

Umklapp Phonon – Phonon scattering Phonon – Impurity scattering Phonon – Electron scattering Phonon – Boundary scattering ~ (Reference)

Also, follow this link for more info on phonon scattering.

Diamond – High Symmetry

Another way of determining the thermal conductivity of a material is by analyzing its internal structure. This is one such way of clearly specifying a good conductor and a poor conductor. If we start by the exceptional non – metal which is the best thermal conductor – Diamond. Then, it can provide a great insight into the ideal structure of a good conductor of heat.

Diamond has a very strong atomic bonding due to its strong covalent bonds. Its compact structure provides a smooth path for heat to flow in the form of phonons, which is the major form of Thermal Conduction in Diamond due to the absence of electrons.

If we look into the whole non-metal tribe, we find that most of them are either liquids or gases or even those which are solids do not stand to be good conductors or even conductors of heat due to their weak inter-molecular forces.

Cubic Boron Arsenide – Low Scattering

Diamond has recently found a rival in the form of a compound of Boron and Arsenic. The chemical compound, named Cubic Boron Arsenide, has been successful in beating the Thermal Conductivity of Diamond. It is known to have a Thermal Conductivity of more than 2000W/m.K at room temperature and even exceeding that of Diamond at high temperatures. (Reference)

The peculiarity in its structure

Generally, in a solid lattice, there are disturbances within the conduction process for vibrational waves, like Phonons, but having the opposite effects. These disturbances can also be thought of as Wave – Scattering. They result in creating resistance in the conduction process eventually leading to less conductibility. These disturbances are pretty common in most solids under certain frequencies. But, the case of Cubic Boron Arsenide is different. Under the same magnitude of frequencies, the number of such disturbances has been found significantly low in Cubic Boron Arsenide. This results in less resistance to heat flow, hence, more heat conduction.

The reason for this unique vibrational property of Cubic Boron Arsenide has surprised scientists and is still being researched. This is due to the fact that the methods which have been used to compute the thermal conductivity of earlier well-known materials made scientists expect the Thermal Conductivity of Cubic Boron Arsenide to be ten times less than that of Diamond. But the result turned out to the absolute contrary. (Source)

Alloys

Alloys are not insulators but also not very good conductors of heat and electricity, which is why they are often known and used as resistors. Their low thermal conductivity is because of their non – uniform composition of atoms. Since they have different types of atoms which often vary in size, an obstructive path is created for the thermal vibrations to travel, eventually leading to their diversion and reduction.

Another reason for lower thermal conductivity among alloys is that they often contain atoms of both heavy and light elements. So when light atoms pass on the vibrations to the heavier atoms from the same heat source, then the heavier atoms vibrate comparatively less due to their higher inertia.

Metals

Metals have a compact structure just like Diamond. They have a uniform composition of atoms having the same atomic radius and mass throughout. The metallic structure facilitates a smooth and clear flow of thermal vibrations, which is the main reason for its high thermal conductivity.

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