What is thermal conductivity, and how is it measured?

Short Answer:

Thermal conductivity is the ability of a material to transfer heat through it. If a material has high thermal conductivity, it can pass heat quickly (like metals). If it has low thermal conductivity, it blocks heat (like wood or rubber). It tells how fast heat moves from one part of a material to another.

Thermal conductivity is measured by applying heat to one side of a sample and observing how much heat reaches the other side. The measurement is usually given in units of watts per meter per kelvin (W/m·K). Different methods are used like steady-state and transient techniques depending on the material and accuracy needed.

Detailed Explanation:

Thermal Conductivity and Its Measurement

Thermal conductivity is one of the most important thermal properties in engineering and materials science. It helps engineers understand how materials behave when exposed to heat, and how quickly they can conduct that heat. It is especially important in fields like heat exchangers, insulation, electronics cooling, and aerospace.

What Is Thermal Conductivity

  1. Definition
    • Thermal conductivity (k) is the measure of how well a material conducts heat.
    • It shows how easily heat energy moves through a material when there is a temperature difference.
    • Materials with high thermal conductivity (like copper and silver) are good heat conductors.
    • Materials with low thermal conductivity (like foam, plastic, or air) are good insulators.
  2. Physical Meaning
    • If you heat one side of a metal rod, the other side becomes hot after some time.
    • The rate at which this heat spreads from hot to cold side depends on thermal conductivity.
  3. Units
    • The SI unit of thermal conductivity is Watt per meter per kelvin (W/m·K).
    • Example:
      • Copper: ~400 W/m·K (excellent conductor)
      • Glass: ~1 W/m·K
      • Wood: ~0.1 W/m·K
      • Air: ~0.025 W/m·K (very poor conductor)

How Thermal Conductivity Is Measured

There are two main categories of measurement:

  1. Steady-State Methods

In steady-state, heat flows through the material at a constant rate, and temperatures stay stable at each point.

  • Guarded Hot Plate Method
    • Heat is applied to one face of a flat sample.
    • The opposite face is cooled or kept at a lower temperature.
    • The difference in temperature and the heat flow is used to calculate conductivity.
  • Heat Flow Meter Method
    • Used in insulation testing.
    • Two plates with known heat flux sensors sandwich the test sample.
    • Sensors measure how much heat flows through.

Formula Used:

k=Q⋅LA⋅ΔTk = \frac{Q \cdot L}{A \cdot \Delta T}k=A⋅ΔTQ⋅L​

Where:

  • kkk = thermal conductivity
  • QQQ = heat flow per second (Watts)
  • LLL = thickness of the material (m)
  • AAA = cross-sectional area (m²)
  • ΔT\Delta TΔT = temperature difference (K)
  1. Transient Methods

These methods are faster and used when steady state is hard to achieve.

  • Laser Flash Method
    • A laser pulse heats the front surface of a small sample.
    • The back surface’s temperature rise is recorded.
    • This gives thermal diffusivity, from which conductivity is calculated.
  • Hot Wire Method
    • A wire is inserted into the sample and heated.
    • The temperature rise is recorded over time.
    • Used for liquids and insulation materials.
  • Transient Plane Source (TPS)
    • A disk sensor placed between two parts of a sample heats and records temperature.
    • Gives thermal conductivity and heat capacity.

Importance of Measuring Thermal Conductivity

  • Electronics: Helps in choosing materials for heat sinks and chip cooling.
  • Building materials: Determines insulation quality.
  • Automotive and Aerospace: Selects materials that can resist or conduct heat efficiently.
  • Medical devices: Ensures safe temperature control in instruments.
Conclusion

Thermal conductivity is the measure of a material’s ability to transfer heat, and it plays a key role in many engineering designs. It is measured using various methods, depending on the material type and application. Knowing thermal conductivity helps engineers select the right materials for insulation, heat dissipation, or thermal protection, improving safety, performance, and energy efficiency in systems.