Thermal conductors are materials that transfer heat efficiently from one area to another. Metals like silver, copper, and aluminum are the most familiar examples, but some non-metals, particularly diamond, actually outperform every metal. The defining measurement is thermal conductivity, expressed in watts per meter-kelvin (W/m·K), which tells you how much heat a material can move through a given thickness for each degree of temperature difference.
How Heat Moves Through a Material
Heat travels through solids by two main mechanisms, and which one dominates depends on whether the material is a metal or not.
In metals, free electrons do most of the work. Metal atoms share their outermost electrons in a “sea” that flows through the material. When one end of a copper bar is heated, those free electrons pick up kinetic energy and carry it rapidly toward the cooler end. This is also why good thermal conductors tend to be good electrical conductors: the same mobile electrons carry both heat and electric current.
In non-metals and insulators, heat moves through lattice vibrations called phonons. Picture atoms in a crystal lattice connected by springs. When one atom vibrates faster (gets hotter), it pushes on its neighbors, passing that energy along in a wave. Diamond conducts heat almost entirely through phonons, yet it reaches a staggering 2,200 W/m·K in natural single-crystal form. That works because diamond’s carbon atoms are light, tightly bonded, and arranged in an extremely rigid lattice, allowing vibrations to travel with very little energy loss.
Best Thermal Conductors and Their Values
Among common metals at room temperature, the ranking is consistent:
- Silver: 406 W/m·K
- Copper: 385 W/m·K
- Gold: 314 W/m·K
- Aluminum: 205 W/m·K
Silver tops the list, but copper is the practical winner for most engineering uses because it’s far cheaper while being nearly as conductive. Aluminum, at roughly half copper’s conductivity, is favored where weight matters, like aircraft components and lightweight heat sinks.
Natural single-crystal diamond (1,000 to 2,200 W/m·K) surpasses all of them. Other materials with diamond-like crystal lattices also perform well above 100 W/m·K, including silicon carbide, boron nitride, and aluminum nitride. These are electrical insulators, which makes them especially useful when you need to pull heat away from electronics without creating a short circuit.
Solids, Liquids, and Gases Compared
Thermal conductivity drops dramatically as you move from solids to liquids to gases. Water conducts heat at about 0.6 W/m·K, roughly 640 times less than copper. Air sits at just 0.026 W/m·K. Even helium, the best-conducting common gas, only reaches 0.151 W/m·K.
This gap exists because molecules in liquids and gases are farther apart and less rigidly connected, so vibrations and energy transfer happen much less efficiently. It’s also why trapped air pockets make such good insulation: air is a terrible thermal conductor, and if you prevent it from circulating (which would transfer heat by convection), very little heat gets through.
What Affects a Material’s Conductivity
A material’s thermal conductivity isn’t fixed. Several factors raise or lower it.
Impurities have one of the biggest effects. Foreign atoms dissolved in a metal’s crystal structure act as scattering centers, disrupting the flow of electrons and reducing conductivity significantly. This is why pure copper conducts heat better than brass (a copper-zinc alloy), and why high-purity metals are specified for demanding thermal applications.
Crystal structure matters too. In aluminum alloys, changing the shape and distribution of internal particles through heat treatment can improve conductivity. Finer, rounder, and more uniformly distributed particles offer less resistance to electron flow. Faster cooling rates during manufacturing also promote these favorable structures.
Temperature itself plays a role. Most metals become slightly less conductive as they heat up, because increased atomic vibration scatters electrons more frequently. For non-metals, the relationship can go either direction depending on the material and temperature range.
Conductivity vs. Diffusivity
Two terms often get confused. Thermal conductivity measures how much heat a material can transfer. Thermal diffusivity measures how quickly temperature changes spread through it. A material can be highly conductive but slow to change temperature if it’s dense and has a high heat capacity, because there’s more thermal energy to move per unit volume.
The relationship is straightforward: thermal diffusivity equals thermal conductivity divided by the product of density and specific heat capacity. In practical terms, diffusivity tells you how fast a material responds to temperature changes, while conductivity tells you how much heat it can shuttle in steady state. A copper pan has high values for both, which is why it heats up quickly and distributes heat evenly.
Everyday and Engineering Applications
Thermal conductors show up everywhere heat needs to be moved. Copper and aluminum form the basis of radiators, heat exchangers, and cookware. Your computer’s CPU sits on a copper or aluminum heat sink specifically because those metals pull heat away from the processor and spread it across fins where air can carry it off.
In electronics, managing heat from concentrated sources is a critical bottleneck. Microprocessors, smartphones, and telecommunication systems all generate intense heat in tiny areas. Heat spreaders made from high-conductivity materials conduct that heat outward to a larger surface where it can dissipate. Graphite films produced through chemical vapor deposition are increasingly used in flexible electronics, where they can outperform metal films.
Bridging the microscopic gaps between components requires thermal interface materials. These include thermal greases (the oldest and simplest option), adhesive-backed thermal tapes, gels that cure into a stable structure, and dielectric pads that insulate electrically while conducting heat. Phase change materials offer another approach: they’re solid at room temperature but soften into a viscous liquid around 55 to 65°C, conforming tightly to surface imperfections and improving contact.
Diamond and diamond-like materials are used in specialized high-power electronics where extreme heat densities would overwhelm copper. Synthetic diamond heat spreaders sit beneath laser diodes and high-frequency transistors, pulling heat away fast enough to prevent damage.