We all are aware of the convection process and how it takes place in fluids in natural or in forced form. However, interesting as it might sound, there can be thought of an analogous concept of convection in solids. Before I go further, for those who are not able to appreciate the remarkable thinking behind it, let me describe the convection in a bit more detail.

    According to Wikipedia, Convection is defined as the movement of molecules within fluids (i.e. liquids, gases) and rheids. It is one of the major modes of heat transfer and mass transfer. Convective heat and mass transfer take place through both diffusion – the random Brownian motion of individual particles in the fluid – and by advection, in which matter or heat is transported by the larger-scale motion of currents in the fluid. In the context of heat and mass transfer, the term “convection” is used to refer to the sum of advective and diffusive transfer.

Hence looking from the conventional definition of convection, we can safely assume, as we have been doing so, that convection is not feasible in solids since bulk current flows and significant diffusion cannot take place in solids.

However, a team of researchers at Massachusetts Institute of Technology (MIT) led by Prof. Gang Chen, the Soderberg Professor of Power Engineering have carried out simulations which show phonons travelling through a crystal lattice. A Phonon here is a quasiparticle analogous to a photon but does the job of a particle which transfers heat from one region of space to another just like photon which instead carries packets of electromagnetic energy. In a way, phonon is just a fancy word for a particle of heat.

The important thing here is the application of this idea in engineering problems one of which is heat dissipation in electronic gadgets. For the engineers who design cell phones, solar panels and computer chips, it’s increasingly important to be able to control the way heat moves through the crystalline materials — such as silicon — that these devices are based on. In computer and cell-phone chips, for example, one of the key limitations to increasing speed and memory is the need to dissipate the heat generated by the chips.

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.

To be able to get the most out of it, this will require a thorough understanding of how heat spreads through a material, considering that heat as well as sound is actually the motion or vibration of atoms and molecules. Just as photons of a given frequency can only exist at certain specific energy levels, so too can phonons.

In a crystal, the atoms are neatly arranged in a uniform, repeating structure which when heated, the atoms can oscillate at specific frequencies. The bonds between the individual atoms in a crystal behave essentially like springs, Chen says. When one of the atoms gets pushed or pulled, it sets off a wave (or phonon) travelling through the crystal, having a specific frequency of vibration.

In the quest for better ways to dissipate heat from computer chips, a key requirement in future as chips get faster and pack in more components, finding ways to manipulate the behavior of the phonons in those chips, so the heat can be removed easily, is the essential requirement of the future. Conversely, in designing thermoelectric devices to generate electricity from temperature differences, it’s important to develop materials that can conduct electricity easily, but block the motion of phonons (heat).

AMRITPAL SINGH MANN