Thermal Conductivity Explained: Definition, Units, Material Data & How to Choose the Right Thermally Conductive Material

How well a material dissipates heat depends on its thermal conductivity — and thermal conductivity also affects material price. So before buying a thermally conductive material, you must first establish what thermal conductivity your product actually needs. This article starts with the definition.

1. What Is Thermal Conductivity?

Thermal conductivity (also called the thermal conductivity coefficient) is the key indicator of a material's ability to conduct heat.

Definition: under steady-state heat transfer conditions, the amount of heat conducted in unit time through a 1 m² area of a material 1 m thick when the temperature difference between the two surfaces is 1 K (°C), is that material's thermal conductivity.

Unit: watt per metre-kelvin, W/(m·K), also written W/(m·°C) (K may be replaced by °C). In some engineering data, values are given per hour as W/(m·h·°C). Conversion: 1 W/(m·K) = 3600 W/(m·h·°C).

Symbol: thermal conductivity is normally denoted λ; the heat transfer coefficient is denoted K — the two are different quantities (see Section 5).

Physical meaning: the higher the thermal conductivity, the stronger the material's ability to conduct heat. Materials with high thermal conductivity have excellent heat-transfer performance.

2. What Determines Heat Flux Density?

The heat flux density of conductive heat transfer depends on three factors:

1. the thermal conductivity of the material, λ;
2. the thickness of the material through which the heat flows, δ;
3. the temperature difference between the hot side and the cold side, ΔT.

This relationship is expressed as:

q=λ⋅ΔTδq=δλ⋅ΔT​

Key rules:

• High thermal conductivity, small thickness and a large temperature difference produce a high heat flux density;
• For the same heat flux density and thickness, the temperature difference between the hot-side and cold-side surfaces decreases as thermal conductivity increases;
• Amorphous structures and materials of lower density have lower thermal conductivity;
• At lower moisture content and lower temperature, a material's thermal conductivity is lower.

3. Thermal Conductivity of Common Materials

Note: the source data were given in W/(m·h·°C); the table above has been converted to the standard SI unit W/(m·K) by dividing by 3600.

4. What Thermal Conductivity Means in Practice: Boiler Tube Scaling

When a boiler tube is free of scale, the high thermal conductivity of steel keeps the temperature difference between the inner and outer wall small. The inner wall temperature is also close to the water temperature, so the tube-wall temperature (the average of the inner and outer wall temperatures) stays moderate.

Once scale forms on the inner wall, however, the very low thermal conductivity of the scale causes the temperature difference across the scale layer to rise rapidly as the scale thickens, driving the metal temperature of the tube up quickly. When the scale reaches a substantial thickness (typically 1–3 mm), the tube-wall temperature exceeds the permitted value and the tube fails from overheating.

Engineering takeaway: thermal conductivity determines thermal resistance; thermal resistance determines temperature difference; temperature difference determines service life. This is one of the most easily overlooked points in thermal design.

5. What Is the Heat Transfer Coefficient K — and How It Differs from Thermal Conductivity

The heat transfer coefficient K is the amount of heat transmitted in 1 hour through a 1 m² area of a building envelope when the air temperatures on both sides differ by 1 K (°C). Its unit is W/(m²·K) (K may be replaced by °C).

Thermal conductivity vs. heat transfer coefficient:

6. What Is Thermal Resistance R?

Thermal resistance R is the reciprocal of the heat transfer coefficient K: R = 1/K, in units of (m²·K)/W.

Conclusion: the lower the heat transfer coefficient K of an envelope — or the higher its thermal resistance R — the better its insulation performance.

7. Criteria for Insulating and Thermal Insulation Materials

Thermal insulation materials are part of insulating materials:

• Materials with a thermal conductivity below 0.174 W/(m·K) are generally classed as insulating materials;
• Materials with a thermal conductivity of 0.05 W/(m·K) or lower are generally classed as high-efficiency thermal insulation materials.

The performance of an insulation material is determined by the material itself and by its thermal conductivity: the lower the thermal conductivity, the better the insulation performance. To guarantee the energy-saving performance of a building insulation project, high-efficiency, high-quality thermal insulation materials must be selected.

8. Thermal Conductivity and Material Selection

Measuring thermal conductivity is the first step in assessing heat dissipation, and thermal conductivity also influences price. The correct order of selection is therefore:

1. Define the requirement — the permitted temperature difference, heat flux density and structural thickness;
2. Set the range — derive the required thermal conductivity range from those values;
3. Compare options — with the thermal requirement met, compare price, processing and reliability.

Take LED luminaires as an example: ordinary plastics have a thermal conductivity of only about 0.2 W/(m·K), which places them in the insulating category. By filling the plastic with highly conductive fillers (such as aluminum oxide, aluminum nitride, boron nitride and silicon carbide), thermally conductive plastics reach roughly 1–20 W/(m·K) — 5 to 100 times that of conventional plastics — while remaining insulating, lightweight and injection moldable.

Selection guidelines:

• Heat-dissipation parts (heat sinks, housings, substrates): the higher the thermal conductivity the better, balanced against strength and temperature resistance;
• Thermal insulation parts: the lower the thermal conductivity the better, using 0.174 W/(m·K) and 0.05 W/(m·K) as the grading thresholds;
• Potting compounds and adhesives for precision electronics: also consider electrical insulation, moisture absorption and system viscosity after filling.

9. FAQ

Q1: What is the unit of thermal conductivity? A: The standard international unit is W/(m·K), i.e. watt per metre-kelvin, also written W/(m·°C). Some engineering data use W/(m·h·°C); 1 W/(m·K) = 3600 W/(m·h·°C).

Q2: What is the difference between thermal conductivity and the heat transfer coefficient? A: Thermal conductivity λ describes the heat-conducting ability of the material itself, in W/(m·K); the heat transfer coefficient K describes the overall heat transfer of a component such as a wall or luminaire housing, in W/(m²·K).

Q3: Is a higher thermal conductivity always better? A: It depends on the application. Heat-dissipation parts benefit from the highest possible thermal conductivity; thermal insulation materials need the lowest possible value.

Q4: What thermal conductivity do thermally conductive plastics have? A: Ordinary plastics are around 0.2 W/(m·K). With thermally conductive fillers, thermally conductive plastics reach about 1–20 W/(m·K), which is 5–100 times that of conventional plastics.

Q5: What are the thermal conductivity thresholds for insulation materials? A: Materials below 0.174 W/(m·K) are generally classed as insulating materials, and those at or below 0.05 W/(m·K) as high-efficiency thermal insulation materials.