Conduction transfers energy through two primary mechanisms: the vibration of atoms in a material’s structure and the movement of free electrons. In metals, both processes work simultaneously, while in non-metals, atomic vibrations do most of the work. Understanding these mechanisms explains why some materials conduct heat and electricity far better than others.
Atomic Vibrations: How Energy Spreads Through a Structure
Every solid material is made of atoms arranged in a lattice, a repeating three-dimensional grid. When one end of that lattice gets heated, those atoms start vibrating more intensely. They bump into their neighbors, which bump into their neighbors, passing kinetic energy along like a chain reaction. Physicists describe these vibrations as “phonons,” which are simply packets of vibrational energy moving through the lattice.
This phonon-based transfer is the dominant mechanism in insulators and semiconductors, materials like glass, ceramics, rubber, and silicon. Because these materials have very few free electrons available to carry energy, nearly all heat moves through these lattice vibrations. The efficiency depends on how orderly the atomic structure is. A perfect crystal transfers vibrations smoothly, while defects, grain boundaries, and interfaces between different materials scatter phonons and slow the transfer down. Research on semiconductor interfaces has shown that when phonon energies don’t match across a boundary, the vibrations can’t simply pass through. Instead, they must scatter and exchange energy indirectly, which creates localized hot spots at the nanoscale.
Free Electron Movement: Why Metals Conduct So Well
Metals have a pool of electrons that aren’t bound to any single atom. These free electrons roam through the material, and when one region gets hotter, the electrons there pick up kinetic energy and carry it rapidly to cooler regions. This is far faster than waiting for atoms to vibrate against each other, which is why metals like copper and aluminum are such excellent thermal conductors.
The same free electrons responsible for thermal conduction also carry electrical current. This connection is so consistent that it’s captured in a principle called the Wiedemann-Franz Law: in a normal metal, thermal conductivity and electrical conductivity are directly proportional to each other. A metal that conducts electricity well will also conduct heat well, and vice versa. The relationship holds because the same electrons are doing both jobs, scattering off lattice defects and impurities in the same way regardless of whether they’re carrying heat or charge.
How Conduction Works in Liquids and Gases
Conduction doesn’t only happen in solids. In liquids and gases, molecules transfer energy through direct collisions. A fast-moving (hot) molecule slams into a slower (cooler) one, transferring some of its kinetic energy in the process. These collisions happen on extraordinarily short timescales, roughly a trillionth of a second each. When no internal changes happen to the molecules during the collision, it’s called elastic. When the collision changes something inside the molecule, like its rotational or vibrational state, it’s called inelastic.
The reason liquids and gases are poor thermal conductors compared to solids is spacing. Their molecules are farther apart, so collisions happen less frequently and energy transfer is slower. Water and air both conduct heat poorly through this mechanism alone, which is why still air trapped in insulation or a double-paned window is such an effective thermal barrier.
Thermal vs. Electrical Conduction
The word “conduction” covers two related but distinct types of energy transfer. Thermal conduction moves heat energy from hot regions to cold ones. Electrical conduction moves charge from one point to another under the influence of a voltage. Both rely on many of the same physical carriers, particularly free electrons in metals, but they can also diverge. Diamond, for instance, is one of the best thermal conductors in existence thanks to its rigid, highly ordered crystal lattice that transmits phonons efficiently, yet it conducts almost no electricity because it has virtually no free electrons.
In semiconductors, both electrons and “holes” carry charge. A hole is simply the empty space left behind when an electron breaks free from its atom. It behaves like a positive charge moving through the material. Depending on how the semiconductor is engineered, either electrons or holes can be the dominant carrier. In n-type semiconductors, free electrons do most of the work. In p-type semiconductors, holes are the majority carriers. Both types contribute to thermal conduction as well, though phonons still carry most of the heat in these materials.
What Governs the Rate of Transfer
The rate of heat conduction through any material follows a straightforward relationship described by Fourier’s Law. The heat flowing through a material equals its thermal conductivity multiplied by the surface area and the temperature difference, divided by the thickness. In practical terms, this means three things control how fast conduction happens: how conductive the material is, how large the contact area is, and how steep the temperature difference is across the material.
Thermal conductivity (the “k” value in Fourier’s Law) is the material property that captures all the microscopic activity described above. Copper has a high k because free electrons shuttle energy quickly. Foam insulation has a low k because it traps pockets of air with few molecular collisions. Heat always flows from hot to cold regions, moving against the temperature gradient until equilibrium is reached.
Why Materials Change Conductivity During Phase Transitions
When a material melts, freezes, or otherwise changes phase, its thermal conductivity can shift dramatically. The orderly lattice that efficiently transmits phonons in a solid becomes disordered in a liquid, scattering vibrations and reducing thermal transport. Research on materials like copper selenide and silver sulfide has shown that during a phase transition, thermal diffusivity (how quickly heat spreads) drops abnormally below its true value. This happens because the material is absorbing extra energy to fuel the phase change itself rather than simply passing it along. Engineers working with phase-change materials in electronics cooling take advantage of this: the material absorbs a large amount of heat as it melts, acting as a thermal buffer that keeps components cool during power spikes.