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MechDatum

Thermal conductivity of materials

The thermal conductivity of materials is the property that determines their ability to transfer heat by conduction. In the steady state, pure metals such as copper reach values of 401 W/(m·K) / 231.8 BTU/(h·ft·°F) at 0 °C / 32 °F, while unfilled polymers rarely exceed 0.2 W/(m·K) / 0.116 BTU/(h·ft·°F). This difference of up to three orders of magnitude has driven the development of polymer-matrix composites with high-conductivity additives, capable of achieving between 1 and 20 W/(m·K) / 0.58 and 11.6 BTU/(h·ft·°F).

Chemical composition

Section titled “Chemical composition”

The composition of thermally conductive materials varies drastically by family. Conductive metals are usually pure elements or alloys with a crystalline structure that favors electron mobility. Conductive polymer composites consist of a thermoplastic or thermoset matrix and a dispersed phase of highly conductive particles.

Material Major component Additive / alloying element Typical additive content
Electrolytic tough pitch copper (ETP) Cu ≥ 99.9% Oxygen ≤ 0.05% <0.05% by mass
Aluminum 3003 Al 96.8–99% Mn 1.0–1.5%, Cu 0.05–0.2% 1–2% by mass
Stainless steel 304 Fe 66–74% (balance) Cr 18–20%, Ni 8–10.5% 26–30% by mass
Polymer with HC particles (Al₂O₃) Polyethylene (PE) / Nylon Alumina (Al₂O₃) 30–60% vol. / 30–60% vol.
Polymer with HC particles (Graphite) Polypropylene (PP) Graphite 40–70% vol. / 40–70% vol.
Polymer with carbon nanotubes Epoxy CNTs 1–10% vol. / 1–10% vol.

Mechanical properties by condition and treatment

Section titled “Mechanical properties by condition and treatment”

Mechanical properties depend on the metallurgical condition or, in the case of polymers, on the curing state. The following table lists representative values for selected materials in the most common supply conditions.

Material Condition / Treatment Yield strength (MPa / ksi) Tensile strength (MPa / ksi) Young’s modulus (GPa / Msi) Elongation (%)
Copper ETP Annealed 69 / 10.0 220 / 31.9 115 / 16.7 45
Copper ETP Cold rolled (H04) 310 / 45.0 345 / 50.0 117 / 17.0 12
Aluminum 3003 Annealed (O) 41 / 5.95 110 / 15.9 69 / 10.0 30
Aluminum 3003 Rolled (H14) 145 / 21.0 150 / 21.8 69 / 10.0 16
Stainless steel 304 Annealed 215 / 31.2 505 / 73.2 193 / 28.0 70
PP + 40% graphite composite Injection molded 35 / 5.08 45 / 6.53 8.5 / 1.23 3
Epoxy + 5% CNT composite Cured – 65 / 9.43 12 / 1.74 1.5

Physical properties

Section titled “Physical properties”

Density and coefficient of thermal expansion (CTE) are critical parameters in applications that require heat dissipation and low weight simultaneously.

Material Density at 20 °C (kg/m³ / lb/ft³) CTE 20-100 °C (10⁻⁶/°C / 10⁻⁶/°F) Specific heat (J/(kg·K) / BTU/(lb·°F))
Copper ETP 8960 / 559.3 17.0 / 9.44 385 / 0.0920
Aluminum 3003 2730 / 170.4 23.6 / 13.1 893 / 0.213
Stainless steel 304 8000 / 499.4 17.3 / 9.61 500 / 0.119
Unfilled polypropylene 905 / 56.5 100 / 55.6 1925 / 0.460
PP + 40% graphite composite 1350 / 84.3 35 / 19.4 1250 / 0.299
Epoxy + 5% CNT composite 1180 / 73.7 25 / 13.9 1050 / 0.251

Thermal properties

Section titled “Thermal properties”

Thermal conductivity (k) is defined through Fourier’s law for one-dimensional steady-state conduction:

q = –k · (dT/dx)

Where q is the heat flux per unit area (W/m²), k is the thermal conductivity (W/(m·K)), and dT/dx is the temperature gradient (K/m). Values of k for selected materials are given in the following table.

Material Temperature (°C / °F) Thermal conductivity k (W/(m·K) / BTU/(h·ft·°F))
Copper (ETP) 0–25 / 32–77 391 / 225.9
Aluminum (pure) 0–25 / 32–77 236 / 136.3
Aluminum 3003 0–25 / 32–77 190 / 109.8
Wrought iron 0–25 / 32–77 59 / 34.1
Stainless steel 304 20 / 68 14.4 / 8.32
Carbon steel 0.5% C 20 / 68 54 / 31.2
Polyethylene (PE) unfilled 20 / 68 0.35 / 0.202
Polyamide (Nylon 6) unfilled 20 / 68 0.25 / 0.144
PE + 50% Al₂O₃ composite 20 / 68 2.1 / 1.21
PP + 60% graphite composite 20 / 68 12 / 6.93
Epoxy + 5% CNT composite 20 / 68 8 / 4.62

Applications by industry

Section titled “Applications by industry”

Materials with high thermal conductivity are selected based on the balance between thermal performance, weight, and cost.

Industry Typical components Preferred materials
Electronics and semiconductors Heat sinks, chip encapsulants, IGBTs, LEDs Copper, aluminum, polymer composites with graphite
Automotive Battery trays, motor housings, sensors Aluminum, polyamide composites with ceramic filler
Aerospace Satellite radiators, solar panel structures Aluminum-lithium alloys, epoxy-CNT composites
Appliances Heat exchangers, injection molds Copper, carbon steels
Energy Boiler tubes, solar collectors Stainless steel, coated aluminum
Protective equipment Helmets, portable heat shields Polypropylene composites with graphite

Comparison with similar materials

Section titled “Comparison with similar materials”

Compared to classic metals, conductive polymer composites offer lower density, better chemical resistance, and greater design flexibility, although with still lower thermal conductivity. The following table summarizes the key attributes.

Material Typical conductivity (W/(m·K) / BTU/(h·ft·°F)) Density (kg/m³ / lb/ft³) Corrosion resistance Relative cost
Copper 391 / 225.9 8960 / 559.3 Medium (oxidizes) High
Aluminum 236 / 136.3 2730 / 170.4 Good (alumina layer) Medium
Stainless steel 304 14.4 / 8.32 8000 / 499.4 Excellent Medium
Unfilled polymer 0.2–0.4 / 0.12–0.23 900–1200 / 56–75 Excellent Low
Polymer composite with conductive filler 2–20 / 1.2–11.6 1100–1500 / 69–94 Good Medium-Low

Sources Consulted

Section titled “Sources Consulted”