- Thermal Conductivity Definition: Thermal conductivity is a property that measures how well a material can transfer heat without moving itself.
- Heat Transfer in Metals: In metals, heat mainly transfers by conduction, where free electrons move and collide with atoms.
- Factors Affecting Thermal Conductivity: The thermal conductivity of metals is influenced by the number of free electrons, atomic mass and size, crystal structure, and temperature.
- Thermal Conductivity Values: Metals like silver, copper, and gold have high thermal conductivity, making them useful in electronics and electrical applications.
- Importance of Thermal Conductivity: High thermal conductivity in metals is crucial for heat exchangers, thermal management, and thermoelectric devices.
Thermal conductivity k relates conductive heat flux to a temperature gradient through Fourier’s law. Its SI unit is watt per metre-kelvin, written W/(m·K). The conductivity of metals varies with composition, purity, microstructure and temperature, so a useful value must identify the material condition and test temperature.
What is a Metal?
A metal is a material whose electronic structure provides mobile charge carriers and metallic bonding. In a crystalline metal, atoms occupy an ordered lattice and conduction electrons extend through the solid rather than belonging to one bond. These electrons carry electric current and usually carry much of the heat.
Crystallinity is not part of a universal definition: metallic glasses are amorphous, and mercury is liquid near room temperature. Metals also span a wide range of density and transport properties. Their measured behaviour depends on bonding, electronic bands, phase and microstructure.
Metallic materials may offer strength, ductility, formability, lustre or high reflectivity, but no one metal has all of these properties. Many metals are good conductors of electricity and heat because the same mobile electrons transport charge and thermal energy.
How Does Heat Transfer in Metals?
Heat transfer is energy transfer caused by a temperature difference. It occurs by conduction, convection or thermal radiation. A real system may use all three at once.
Conduction transfers energy through microscopic carrier and particle interactions and occurs in solids, liquids and gases. Convection combines conduction with bulk fluid motion. Thermal radiation transfers energy by electromagnetic waves and does not require matter between the surfaces.
In a metal, total thermal conductivity has an electronic contribution and a lattice contribution carried by vibrations called phonons. Mobile electrons usually dominate in pure metals. Conductivity depends on their energy, velocity and mean distance between scattering events, not simply on how many electrons exist. Phonons, impurities, defects, grain boundaries and alloying all scatter carriers.
What Factors Affect the Thermal Conductivity of Metals?
The main influences are:
- Electronic transport: High electrical conductivity often accompanies high thermal conductivity because electrons carry both charge and heat. Silver and high-purity copper have high room-temperature values, but carrier scattering prevents a simple ranking by electron count.
- Composition and bonding: Atomic mass alone does not set thermal conductivity. Electronic band structure, bonding, carrier density and scattering determine the electronic and lattice contributions. Lead conducts heat less effectively than copper, but that result is not a general heavy-atom rule.
- Microstructure and direction: Impurities, vacancies, dislocations, grain boundaries and phase boundaries add thermal resistance by scattering electrons or phonons. Some crystals conduct differently by direction. Cubic symmetry does not guarantee a higher value than hexagonal symmetry.
- Temperature: The number of conduction electrons in an ordinary metal changes little across common engineering temperatures, while electron-phonon scattering increases. Pure-metal conductivity often falls or changes moderately as temperature rises, but alloys can show a different trend. In non-metals, phonon conductivity can rise at low temperature, reach a peak and then fall as phonon-phonon scattering grows. Data must cover the intended range.
What is Wiedemann-Franz Law?
The Wiedemann-Franz-Lorenz law relates the electronic thermal conductivity k_e of a metal to its electrical conductivity σ and absolute temperature T:
k_e / (σT) = L
Where,
- k_e is the electronic part of thermal conductivity in W/(m·K)
- σ is electrical conductivity in S/m
- L is the Lorenz number; the Sommerfeld value is approximately 2.44 × 10^-8 W-ohm/K^2
- T is absolute temperature in kelvins
The relation explains why a pure metal with low electrical resistivity often conducts heat well. It predicts that k_e/σ is proportional to T when L is approximately constant. Total measured conductivity also includes a lattice term, which must be added when it affects the required accuracy.
The Sommerfeld Lorenz value is an approximation based on a simple electron model. Real Lorenz ratios vary with temperature, purity, alloying and scattering mechanism, especially when inelastic scattering or lattice heat transport matters. The relation is therefore used as an estimate for metals and degenerate conductors, not as an exact rule or a model for ordinary insulators.
What are the Thermal Conductivity Values of Some Common Metals?
The table gives illustrative values near 25 °C for high-purity or commonly tabulated material. Published values differ by grade, purity, heat treatment and test method, so design calculations should use a traceable data set for the specified product.
| Metal | Thermal Conductivity (W/(m·K)) |
|---|---|
| Silver | 429 |
| Copper | 398 |
| Gold | 315 |
| Aluminum | 237 |
| Iron | 80 |
| Lead | 35 |
Silver and copper are among the highest-conductivity common metals. Copper is widely used because it combines high conductivity with practical cost and fabrication. Lead’s value is low relative to these metals but far above that of common thermal insulators. Lead shielding relies mainly on density and atomic number, not on thermal insulation.
Alloying often lowers conductivity because solute atoms scatter electrons, although the effect depends on composition and phase. Cold work, porosity, grain size, texture and phase transformations can also change a value. Pressure is usually less important in ordinary service but can matter in high-pressure experiments or through contact resistance.
How to Measure the Thermal Conductivity of Metals?
The method must match the conductivity range, specimen geometry, temperature and required uncertainty. Contact resistance, heat loss, radiation and property variation with temperature must be included in the measurement model.
- Steady-state methods: A known heat flux passes through a specimen while temperature difference and geometry are measured. Comparative cut-bar or divided-bar systems suit conductive solids when contact and lateral losses are controlled. Guarded hot plates and heat-flow meters are more commonly configured for much lower-conductivity insulation.
- Transient methods: A heat pulse or step produces a time-dependent temperature response. Laser flash commonly measures thermal diffusivity; conductivity is then calculated as diffusivity multiplied by density and specific heat. Transient hot-wire and plane-source methods need specimen and sensor arrangements suited to their governing model.
- Optical methods: Modulated or pulsed heating with infrared radiometry can infer thermal properties without a bonded temperature sensor. The result depends on a heat-transfer model and accurate optical absorption, emissivity, geometry and boundary conditions; it is not a direct reading of conductivity.
A reported result should state specimen identity, direction, density, temperature range, method, calibration and measurement uncertainty. Agreement between techniques is meaningful only when they represent the same material state and temperature.
Why is the Thermal Conductivity of Metals Important?
Engineers use conductivity data in several different ways:
- Heat exchangers: Conductive walls move heat between separated fluids. Wall conductivity is one part of the total thermal resistance, alongside convection films, fouling and contact resistance. High conductivity helps most when wall resistance is large enough to limit the required heat flow.
- Thermal management: Heat sinks, spreaders, cold plates and enclosures use metals to carry heat from components to a cooling boundary. Geometry, interfaces, convection and radiation still limit the complete thermal path, so bulk conductivity alone does not predict device temperature.
- Thermoelectric devices use the Seebeck effect for generation and the Peltier effect for heating or cooling. The active material’s figure of merit benefits from low thermal conductivity, high electrical conductivity and a large Seebeck coefficient. High-conductivity metals remain useful for contacts and heat spreaders around that active material.
Conclusion
Metal thermal conductivity combines electronic and lattice heat transport. Purity, alloying, defects, phase, direction and temperature change the result, and the Wiedemann-Franz-Lorenz law provides only an approximate link to electrical conductivity. Select data and measurement methods for the actual grade and temperature, then include interfaces and other heat-transfer modes in the complete design.
