Thermal Conductivity of Aluminium
Aluminum is A Good Conductor of Heat
Aluminum is the second most widely used metal after steel, and it is found throughout both daily life and industry. Its popularity is due in part to its excellent physical properties, including good thermal conductivity. In this article, we will provide a clear and thorough overview of the thermal conductivity of aluminum, exploring what it is, what affects it, and why it matters in practical applications.
Thermal Conductivity of Pure Aluminum
Thermal conductivity (λ) is a measure of a material’s ability to conduct heat, defined as the amount of heat energy transferred through a unit thickness of the material per unit area and per unit temperature difference.
For pure aluminum at room temperature (20°C), the thermal conductivity is approximately 237 W/(m·K). As temperature increases, the thermal conductivity of pure aluminum decreases slightly; for example, at 100°C, it is about 230 W/(m·K).
Aluminum Alloys Thermal Conductivity
The high thermal conductivity of pure aluminum is primarily due to the efficient movement of free electrons. When alloying elements are added, several mechanisms reduce thermal conductivity:
Formation of Solid Solutions: Solute atoms such as Cu, Mg, and Zn distort the aluminum lattice structure, increasing electron scattering and thus reducing the rate of heat conduction.
Precipitation of Second Phases: The formation of intermetallic particles such as the θ phase (Al₂Cu) and Mg₂Si further impede the flow of electrons, resulting in an additional decline in conductivity.
Influence of Alloying Elements: Among common alloying elements, their impact on reducing thermal conductivity generally follows this order of effectiveness: Cu > Zn > Mg > Si > Mn ≈ Fe.
Below is a simplified table showing representative alloys, their tempers, and their typical thermal conductivity ranges:
| Grade | Temper | Thermal Conductivity (W/(m·K)) |
|---|---|---|
| 1050 | O | 220–237 |
| 1060 | H12 | 220–235 |
| 1100 | O | 218–222 |
| 2014 | T6 | 150–160 |
| 2024 | T3/T4 | 140–150 |
| 3003 | H14 | 190–210 |
| 3004 | H34 | 160–180 |
| 5052 | H32 | 138–160 |
| 5083 | H112 | 125–145 |
| 5754 | O | 130–150 |
| 6061 | T6 | 150–170 |
| 6063 | T5 | 180–200 |
| 6082 | T6 | 140–160 |
| 7075 | T6 | 130–150 |
| 7050 | T74 | 140–155 |
Aluminium Oxide Thermal Conductivity
When exposed to air, aluminum naturally forms a thin oxide film on its surface, and many aluminum products are further treated by anodization to intentionally thicken this oxide layer. The primary component of this film is aluminum oxide (Al₂O₃), which has a much lower thermal conductivity—approximately 30 W/(m·K)—compared to pure aluminum.
The presence of an oxide layer reduces the overall thermal conductivity of aluminum surfaces. For naturally oxidized layers, the thickness is typically only 2–5 μm, resulting in a negligible impact on heat dissipation performance. However, anodizing produces a standard oxide layer around 15 μm thick, which begins to noticeably diminish the thermal conductivity. Hard anodized films can exceed 50 μm in thickness, dramatically decreasing heat transfer capabilities.
For this reason, aluminum components designed primarily for thermal dissipation—such as heat sinks—are generally not subjected to anodizing treatments, so as to maintain optimal heat transfer efficiency.
Aluminum Thermal Capacity
Specific heat capacity is a physical quantity that measures the amount of heat required to raise the temperature of a unit mass of a substance by one degree Celsius (or one Kelvin). This parameter plays a crucial role in a material’s thermal management behavior, influencing how quickly it heats up or cools down during operation. Pure aluminum is notable for its relatively high specific heat capacity, which is approximately 0.900 kJ/(kg·K) at room temperature.
The table below presents the specific heat capacities of common aluminum alloy grades along with those of pure copper and carbon steel for direct comparison:
| Material / Grade | Specific Heat Capacity (kJ/(kg·K)) |
|---|---|
| Pure Aluminum (1050) | 0.900 |
| Aluminum Alloy 1060 | 0.895 |
| Aluminum Alloy 1100 | 0.895 |
| Aluminum Alloy 2024 | 0.875 |
| Aluminum Alloy 3003 | 0.897 |
| Aluminum Alloy 5052 | 0.883 |
| Aluminum Alloy 6061 | 0.896 |
| Aluminum Alloy 7075 | 0.870 |
| Pure Copper (C11000) | 0.385 |
| Carbon Steel (AISI 1010) | 0.490 |
Aluminum Coefficient of Thermal Expansion
The coefficient of linear thermal expansion (CTE) for pure aluminum is typically in the range of 23 to 24 × 10⁻⁶ /°C (or /K). This means that for each 1°C increase in temperature, a one-meter aluminum bar will expand by approximately 23 to 24 micrometers. While aluminum alloys exhibit slight variations in thermal expansion coefficients, these differences are generally minimal and do not significantly alter their expansion behavior.
In contrast, the oxide layer (primarily alumina, Al₂O₃) formed on aluminum surfaces has a thermal expansion coefficient that is only about one-third that of metallic aluminum. This mismatch can lead to the oxide layer cracking and losing its protective function under high thermal cycling, as the underlying aluminum expands more than the oxide layer can accommodate.
Aluminum’s thermal expansion coefficient is significantly higher than that of steel (about 12 × 10⁻⁶ /°C) and copper (approximately 16–17 × 10⁻⁶ /°C), making this a critical factor to consider in applications where dimensional stability and joint compatibility with other materials are important.
Thermal Conductivity of Copper VS Aluminium
Both aluminum and copper are widely used in radiators, air conditioning tubes, and similar heat dissipation applications. Although copper offers higher thermal conductivity, aluminum is more prevalent due to its lower cost, lower density, and easier processing.
| Property | Pure Aluminum (1050) | Pure Copper (C11000) |
|---|---|---|
| Thermal Conductivity (20°C) | 237 W/(m·K) | 398 W/(m·K) |
| Density | 2.70 g/cm³ | 8.96 g/cm³ |
| Thermal Conductivity per Density | 87.8 W·cm³/(g·K) | 44.4 W·cm³/(g·K) |
Material Selection for Thermal Aluminum Products
The choice of aluminum alloy for thermally conductive applications depends on the specific performance requirements of each industry and product type. Below is a summary of recommended alloy grades for common applications, including their primary attributes and additional notes:
| Application Area | Key Performance Requirements | Recommended Alloys & Notes |
|---|---|---|
| Heat Sinks | High thermal conductivity, formability | 1050, 1060, 1070, 6063 — Pure aluminum is often chosen for excellent conductivity and workability; 6063 offers a good balance of cost, strength, and formability for extrusion. |
| Aluminum Foil | High conductivity, ductility, thin rolling | 1100, 1235, 8011 — 1100 and 1235 are widely used as pure aluminum foil grades. 8011 provides higher strength and processability with slightly lower conductivity. |
| Cookware | Conductivity, corrosion resistance, strength, formability | 3003, 3004, 1100, 1050 — 3003/3004 are typical for pot and pan bodies; 1050/1100 are used in conductive layers. All alloys must ensure food safety and corrosion resistance. |
| AC & Heat Exchange Tubes | Conductivity, corrosion resistance, weldability | 1050, 1070, 3103, 3003 — 1050/1070 (high-purity, easy to bend) are common in AC tubing; heat exchanger fins use 3003/3103 for improved strength and corrosion resistance. |
| Enclosures for Electrical and Electronic Equipment | Thermal management, shielding, mechanical protection, corrosion resistance | 5052, 6061, 6063 — 5052 provides good workability and corrosion resistance, widely used for enclosures; 6061 and 6063 offer higher mechanical strength and can be easily extruded or machined for custom designs. |
Thermally Insulating Aluminum Materials
Although aluminum itself has a high thermal conductivity, various engineering techniques are employed to significantly reduce heat transfer in applications where insulation is required. Below are some commonly used methods, along with typical thermal conductivity values for each structure or material:
Thermal Break Aluminum Profile:
Widely used in windows, doors, and curtain walls. By incorporating an insulating material, such as polyamide (thermal conductivity ≈ 0.3 W/(m·K)), between two aluminum profiles, the overall thermal conductivity of the system can be reduced to approximately 2–4 W/(m·K).
Aluminum Honeycomb Structures:
Composed of thin aluminum skins surrounding a honeycomb core, these panels utilize trapped air to reduce heat transfer. The effective thermal conductivity generally ranges from 2–10 W/(m·K), much lower than solid aluminum.
Foamed Aluminum :
This material features a porous structure filled with closed air cells, resulting in thermal conductivity typically between 1–10 W/(m·K), depending on density and pore structure.
Hybrid Composites with Insulating Materials:
Aluminum is often laminated or bonded with other materials—such as plastics, rubbers, ceramics, or mineral wool—to form composite panels with highly enhanced insulation properties. Depending on the materials used, the overall thermal conductivity of these composites can range from less than 1 W/(m·K) (when combined with high-performance thermal insulators) to several W/(m·K).
Thermal Reflectivity of Aluminum
While aluminum is well known for its high thermal conductivity, it also serves as an effective insulation material due to its exceptional ability to reflect radiant heat, particularly infrared radiation. The infrared reflectivity of aluminum can reach 90–95%, making it highly efficient at reflecting thermal energy rather than absorbing it.
In practical applications, thermal insulation is typically achieved by combining multiple materials with different insulating properties. Within such composite systems, aluminum foil is commonly used as a radiant barrier. It reflects a significant portion of incident thermal radiation away from the insulated surface, helping to minimize heat transfer and enhance the overall insulation performance. This makes aluminum foil an essential component in building envelopes, HVAC ducts, and packaging for temperature-sensitive goods.
Relationship of Thermal and Electrical Conductivity
Both thermal and electrical conductivity in aluminum are primarily governed by the movement of free electrons. This shared mechanism underpins the strong correlation observed between these two properties in metallic materials.
- Thermal Conductivity: Free electrons carry thermal energy, diffusing from regions of higher temperature to lower temperature.
- Electrical Conductivity: When subjected to an electric field, free electrons move directionally, generating electric current.
The effectiveness of both heat and electrical transport is enhanced as the average free path of electrons increases. Thus, materials with high electrical conductivity, such as aluminum, also tend to exhibit high thermal conductivity.
This relationship is quantitatively described by the Wiedemann-Franz Law, which states that the ratio of thermal conductivity (λ) to electrical conductivity (σ) is proportional to the absolute temperature (T):
Where:
- λ = Thermal conductivity (W/(m·K))
- σ = Electrical conductivity (S/m)
- L = Lorenz constant (approximately 2.44×10−8 W·Ω/K², for free electron metals)
- T = Absolute temperature (K)
