Thermal Conductivity
Also: Heat conduction, Thermal conductance, λ (lambda)
Thermal conductivity describes how well a material transports heat energy through itself and is expressed in watts per metre and kelvin (W/(m·K)).
Thermal conductivity (symbol λ, unit W/(m·K)) is a material property that indicates how quickly a solid, a liquid or a gas transports heat energy from a warmer to a cooler zone. The higher the λ value, the more efficiently the material conducts heat. Precious metals are among the best thermal conductors of all – a property in as much demand in industry as it is relevant in the melt value calculator for alloys.
Physical Background
Heat transport in metals occurs predominantly through free electrons, not through lattice vibrations (phonons) alone. This is why good electrical conductors are usually also good thermal conductors – a relationship described by the Wiedemann-Franz law:
λ / σ = L · T
Here σ is the electrical conductivity, T the absolute temperature (in kelvin) and L the Lorenz number (≈ 2.44 × 10⁻⁸ W·Ω/K²). The law holds fairly well for pure metals at room temperature.
Precious Metals in Comparison
Silver tops the ranking of the purest metals with 429 W/(m·K) and is thus the best metallic thermal conductor of all. Copper follows close behind at 401 W/(m·K) – which is why copper dominates as the cheaper alternative in heat sinks and heat exchangers. Gold reaches 318 W/(m·K) and is used where corrosion resistance takes priority, for instance in aerospace or microelectronics. Platinum and palladium lie considerably lower at around 72 W/(m·K) but score through their high-temperature resistance.
| Metal | λ in W/(m·K) | Main application (thermal) |
|---|---|---|
| Silver | 429 | Contacts, paste, soldering technology |
| Copper | 401 | Heat sinks, heat exchangers |
| Gold | 318 | Microchips, aerospace |
| Platinum | 72 | Thermocouples, high temp. |
| Palladium | 72 | Catalysts, electrodes |
Practical Significance
In electronics, thermal conductivity determines how effectively processors and power semiconductors are cooled. Silver-bearing thermal pastes and sinter layers are regarded as the state of the art for high-performance chips. In jewellery and the alloy field, every alloying partner changes the λ value: sterling silver (925) already conducts noticeably worse than pure silver, because lattice defects disturb electron movement.
Thermal conductivity also influences the so-called ice melt test for silver: pure silver conducts heat so well that an ice cube on a silver bar melts noticeably faster than on most other metals – a simple plausibility test for home use.
In Brief
Silver is the best metallic thermal conductor of the known elements; copper and gold follow at some distance. This property makes precious metals indispensable materials in high-performance electronics, aerospace and heating technology – far beyond their function as an investment precious metal.