Principles of the Laser Flash Analyzer (LFA)

Thermal-conductivity test methods are divided into steady-state and transient methods. Steady-state methods such as heat-flow, guarded heat-flow, and hot-plate methods are direct measurements based on the Fourier equation, with narrower temperature and conductivity ranges. The flash method, also called the laser flash method, is a transient method for higher-conductivity materials and high-temperature testing. The cited standards are ASTM E1461, DIN EN 821, and DIN 30905.

The flash method directly measures thermal diffusivity. At a controlled temperature T, a laser or xenon-flash source sends a short pulse uniformly to the rear surface of the specimen. Heat travels one-dimensionally toward the front surface, where an infrared detector records the temperature rise over time. Under ideal adiabatic conditions, diffusivity is calculated from the half-rise time t50, also written t1/2:

α = 0.1388 × d² / t50

Here d is specimen thickness. Boundary heat loss, radial radiation, radial heat flow from nonuniform irradiation, partial transmission or subsurface absorption in translucent specimens, and finite pulse width can require mathematical correction.

Thermal conductivity is calculated from λ(T) = α(T) × Cp(T) × ρ(T), using thermal diffusivity, specific heat capacity, and density at temperature T. Density is generally measured at room temperature and may be corrected for temperature using the coefficient of thermal expansion and the temperature-dependent specimen thickness. Specific heat may come from literature, differential scanning calorimetry (DSC), or a comparative method in the laser flash instrument.

The comparative method uses a reference specimen with known Cp and similar cross-section, thickness, thermal properties, and surface finish to the test specimen. Both surfaces are coated for comparable optical absorption and infrared emissivity, then the reference and test specimens are measured in sequence. The specific-heat relation is Cp = Q/(ΔT × m). Under equal pulse-energy and absorption conditions, Cpsam/Cpstd = (ΔUstd × mstd)/(ΔUsam × msam), and therefore Cpsam = Cpstd × (ΔUstd × mstd)/(ΔUsam × msam).

Actual tests include heat loss during the temperature rise, so the measured peak ΔTmeas differs from the adiabatic value ΔTadiabatic. ΔTmeas must be corrected to ΔTcorr before calculating Cp. The Netzsch LFA Proteus software includes this correction in the thermal-diffusivity calculation. If pulse energy or detector gain differs between the reference and test specimens, corresponding scaling factors are applied to Q or ΔU.