High thermal conductivity describes a material’s ability to transfer heat quickly and efficiently. It’s measured in watts per meter-kelvin (W/m·K), and materials above roughly 100 W/m·K are generally considered highly conductive. At the top of the scale, copper conducts heat at 385 W/m·K, silver at 406 W/m·K, and single-crystal diamond reaches an extraordinary 2,200 W/m·K.
How Heat Moves Through Materials
Heat travels through solids by two main carriers: free electrons and phonons. Phonons are vibrations that ripple through a material’s atomic lattice, like a wave passing through a crowd. Free electrons, the same particles that carry electrical current in metals, also pick up thermal energy and shuttle it through the material.
In metals like copper and silver, free electrons do the heavy lifting. Because metals have enormous numbers of loosely bound electrons, heat energy spreads rapidly. This is also why good thermal conductors tend to be good electrical conductors. In non-metals like diamond or ceramic, there are almost no free electrons available. Instead, heat travels entirely through phonon vibrations. Diamond conducts heat so well because its carbon atoms are light, tightly bonded, and arranged in an extremely stiff, orderly crystal. That stiffness lets phonons travel long distances before losing energy, producing thermal conductivity five times higher than copper, all without any electrical conduction at all.
Where Common Materials Fall on the Scale
To appreciate what “high” means, it helps to see the full range. Air conducts heat at about 0.025 W/m·K. Water sits around 0.6 W/m·K. Typical plastics land between 0.1 and 0.5 W/m·K. These are all thermal insulators.
Stainless steel, often thought of as a metal that gets hot, is actually a modest conductor at roughly 15 W/m·K. Aluminum comes in around 250 W/m·K, which is why it’s the standard material for cookware and lightweight heat sinks. The precious metals rank higher: gold at 314 W/m·K, copper at 385, and silver at 406. Among bulk materials found in nature, single-crystal diamond tops the list at up to 2,200 W/m·K.
Carbon-based materials push even further in laboratory settings. Measurements of suspended single-layer graphene, a sheet of carbon just one atom thick, have yielded values between 4,840 and 5,300 W/m·K near room temperature. Those numbers outpace even carbon nanotubes, though they apply to isolated samples rather than bulk materials you can hold in your hand.
What Makes One Material Better Than Another
Several factors determine where a material lands on the conductivity scale. Crystal structure matters enormously. A perfect, defect-free crystal lets phonons travel uninterrupted, which is why single-crystal diamond vastly outperforms polycrystalline or industrial diamond. Any impurity, grain boundary, or void in the lattice scatters phonons and slows heat transfer.
Temperature also plays a role, and it works differently depending on the material. In pure metals near room temperature, increasing temperature causes more phonon vibrations that scatter electrons, which raises electrical resistivity and lowers thermal conductivity. In crystalline insulators like diamond, rising temperature increases a process called umklapp scattering, where phonon vibrations collide and redirect each other, also reducing conductivity. The practical takeaway: most high-conductivity materials perform best near or below room temperature, and their advantage narrows as things heat up.
Density and atomic weight factor in too. Lighter atoms vibrate faster and transmit phonon energy more efficiently, which is one reason carbon-based materials dominate the top of the conductivity chart while heavy metals like lead (35 W/m·K) lag far behind.
Why It Matters in Electronics and Engineering
The most visible everyday application of high thermal conductivity is the heat sink inside your computer. CPUs and GPUs generate intense, concentrated heat. If that heat isn’t pulled away quickly, the chip throttles its performance or fails. Heat sinks are typically made from aluminum or copper, chosen specifically for their ability to conduct heat away from the chip and spread it across a larger surface where fans can dissipate it. Copper’s thermal conductivity is roughly double aluminum’s, so it appears in higher-performance cooling systems, though it’s heavier and more expensive.
Beyond consumer electronics, high-conductivity materials are critical in power plants, industrial equipment, and electric vehicle battery packs, where managing heat directly affects safety and lifespan. Copper tubing is standard in solar thermal water systems, HVAC units, and geothermal heating loops, all situations where heat needs to move efficiently from one fluid to another through a solid wall.
In semiconductor manufacturing, the mismatch between silicon chips and metal heat sinks creates a design challenge. Silicon and copper expand at very different rates when heated, which can crack solder joints over time. Diamond’s combination of extreme thermal conductivity and low thermal expansion makes it an appealing heat-sink material for high-power chips, though cost limits its use to specialized applications.
Engineering High Conductivity Into Low-Conductivity Materials
Sometimes the best material for a job is a poor thermal conductor. Plastics are lightweight, moldable, and electrically insulating, but their thermal conductivity hovers below 0.5 W/m·K. Engineers bridge this gap by mixing thermally conductive fillers into polymer matrices.
Boron nitride flakes and aluminum oxide particles are two of the most common fillers. They boost thermal performance while preserving the electrical insulation that makes plastics useful around electronics. Results vary widely depending on filler type, concentration, and how well the filler bonds to the surrounding plastic. In published studies, adding boron nitride and carbon nanotubes to a flexible polyurethane-based plastic increased conductivity by 210%, reaching 4.52 W/m·K. A polyurethane composite loaded with boron nitride alone hit 39 W/m·K, an 85-fold improvement. These filled polymers won’t match solid copper, but they don’t need to. They fill the gap between bare plastic and bulk metal in applications where weight, flexibility, or electrical insulation matters more than peak conductivity.
Silicon carbide and aluminum nitride ceramics occupy a middle ground for applications that need fairly high thermal conductivity paired with low electrical conductivity. These materials are already widely used as substrates in power electronics, sitting between the heat-generating chip and the metal heat sink below.
How Thermal Conductivity Is Measured
The standard laboratory technique for solid materials is the flash method, codified as ASTM E1461. A short energy pulse, typically from a laser, hits one face of a thin sample. Sensors on the opposite face record how quickly the temperature rises. That speed is the material’s thermal diffusivity. Multiply it by the material’s density and specific heat capacity, and you get thermal conductivity. The method covers a wide range, from about 75 K to 2,800 K, and handles diffusivity values from 0.1 to 1,000 mm²/s, making it suitable for everything from ceramics to metals to composites.