How to Choose a Thermal Conductivity Test Method

Thermal conductivity has 12 different but contradictory measurement methods. A guarded hot plate requires two identical plates and almost a full day of testing. The flash apparatus needs a 12.7 mm disc and takes less than 2 minutes, but you cannot obtain the conductivity value unless density and specific heat are measured independently.

What Is Thermal Conductivity?

Thermal conductivity, written as λ or k, tells you how fast heat moves through a material under a temperature gradient. The unit is watts per meter-kelvin (W/m · K). One number, measured at one temperature, in one direction.
Materials exhibit an approximate range spanning four orders of magnitude. The silica aerogel blanket material is approximately 0.015 W/m·K. Rigid polyurethane foam is approximately 0.022 W/m · K. Cast acrylic is approximately 0.19 W/m · K. 96% alumina substrate is approximately 25 W/m · K. Aluminum 6061 is approximately 167 W/m · K, and copper is approximately 400 W/m · K. No single apparatus covers this entire range, which is why the standards have fractured.
Two methodologies are available. Steady-state methods maintain a constant temperature difference across the test piece and wait for equilibrium before calculating thermal conductivity using Fourier’s Law applied to the measured heat flux and thickness. The transient methodologies apply pulses of heat energy and observe the temperature transient response. Transient methods measure thermal diffusivity, α, not thermal conductivity. Conductivity is calculated as λ = α × ρ × Cp.

Types of Thermal Conductivity Testing

Specimen form and conductivity range decide the method. Six categories cover most work.
Absolute steady-state plate methods: ASTM C177, the guarded hot plate metho,d involves direct measurement of heat flux without the use of any reference material. The test method is capable of measuring flat specimens with low thermal conductivity in the range of 1 mm to 50 mm.
Comparative steady-state methods. The heat flow meter (ASTM C518) or the guarded comparative longitudinal stack (ASTM E1225) method measures your sample relative to known conductivities of reference materials. ASTM E1225 works for 0.2 to 200 W/mK from 90 K to 1300 K, thus encompassing most metals, ceramics, and composite plastics. It all comes down to reference quality.
Flash diffusivity. ASTM E1461 fires a light pulse at one face of a small disc and records the temperature rise on the other face. Small specimens, fast runs, temperatures past 1000°C. Best suited to conductive, opaque, homogeneous solids.
Transient contact methods. Transient plane source sensor, hot wire, and hot disk sensors are placed adjacent to or within the sample and measure its temperature response to a brief heat input. They can measure pastes, powders, gels, and liquids that cannot be flattened for the plate method.
Interface and stack methods. ASTM D5470 and ASTM E1530 account for thermal resistance across a compressed stack, including resistance through the boundaries of each layer. For gap pads and thermal greases, the sum matters.
Microscale and thin-film methods. Time-domain thermoreflectance resolves layers well under a micron, where every bulk method fails because the film is thinner than the measurement’s own resolution.

Thermal Conductivity Test Methods and Standards

ISO counterparts exist for most of these. ISO 8302 mirrors C177, ISO 8301 mirrors C518, and ISO 22007-2 covers transient plane source. They are close, not identical, and specimen dimensions and reporting requirements differ.
Beware the diffusivity trap. The flash test gives you α in one run, and the lab will multiply by density and specific heat to get λ. However, if Cp was gotten from the literature based on a similar grade instead of measured in your material, there is an error in that conductivity value that no one recorded.

Also read: ASTM E1461: Thermal Diffusivity by the Flash Method

How to Choose the Right Thermal Conductivity Test

Before selecting any test methods, some screening questions and analysis should be reviewed:

1. Where does your material sit on the conductivity scale? For values under 0.5 W/m·K, you have a hot plate or heat flow meter. For values from 0.2 W/m·K to 200 W/m·K, use E1225. Values greater than 20 W/m·K become difficult to handle with the plate method owing to contact resistance, and flash is the solution. The question alone eliminates half of the table.
2. What form and how much material do you have? For C177, the test requires two flat, parallel samples, big enough to cover the metered and guard areas, usually 300 mm by 300 mm. For flash, we need a disc roughly 12.7 mm in diameter and 1 mm to 3 mm thick. In case the only thing available is a 20 mm coupon from a machined component, the guarded hot plate gets eliminated immediately.
3. What temperatures should be covered in the data? Insulation testing is done at an average temperature of 24°C. C201 material is required for the refractory lining, with a maximum temperature of 1260°C. Flash attains even higher temperatures. In most cases, conductivity varies with temperature; thus, the ambient value means little at 400°C operation.
4. Do you need the material property or the assembled thermal resistance? Intrinsic conductivity is a material constant. Thermal impedance under D5470 includes the contact resistance at both interfaces and changes with applied pressure. Electronics thermal designers usually want the second one, reported at the pressure their assembly applies.

Another test whose effects may be more significant than anticipated is direction. Materials such as unidirectional carbon composites, pyrolytic graphite, and rolled metal sheet have highly anisotropic conductivity, sometimes a factor of 100 greater in the longitudinal direction than through thickness. Flash tests heat through thickness by default; specify direction on your test request and machine accordingly.

Thermal Conductivity FAQs