Laser flash analysis

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An established laser flash system ( LFA 427 from NETZSCH ) with which the thermal diffusivity of many materials can be determined over a wide temperature range (−125 to 2800 ° C).

The laser flash analysis (LFA) or laser flash method is used to determine the thermal diffusivity of a large number of different materials. An energy pulse heats a plane-parallel sample from below. The temperature of the top of the sample then rises. The higher the thermal diffusivity of the sample, the faster this temperature rise occurs. This increase is measured and evaluated with an infrared detector. A commercial LFA is shown in the picture on the right.

In the one-dimensional, adiabatic case , the thermal diffusivity is described as follows: ${\displaystyle a}$

${\displaystyle a=0{,}1388\cdot {\frac {d^{2}}{t_{1/2}}}}$

With

• ${\displaystyle a}$ is the thermal diffusivity
• ${\displaystyle d}$ is the thickness of the sample
• ${\displaystyle t_{1/2}}$ is half time

Measuring principle

LFA measurement principle: A laser / energy pulse (red) heats the sample (yellow) on the underside and a detector records the temperature rise as a function of time on the top of the sample (green).

Laser flash analysis has been reported by Parker et al. Developed in 1961.

In a vertical setup, the energy pulse from a flash lamp or laser heats the underside of a sample. An infrared detector is arranged above, which records the temperature rise on the top of the sample. The thermal diffusivity is calculated from this signal using the above formula, for example. Since the conductivity is very dependent on the temperature, the sample is heated with an oven. The measurement itself then takes place isothermally.

Excellent experimental conditions are

• homogeneous material,
• a uniform energy input over the entire sample surface
• a short pulse of energy; preferably in the form of a delta function .

Many improvements have been made on the first model. In 1963, Cowan took radiation losses and convection on the surface into account. In the same year Cape and Lehman described the transient heat transport, effects of finite energy impulses and heat loss. Blumm and Opfermann improved the Cape Lehman model with regard to radial and surface heat loss and an innovative, patented pulse correction.

Further articles

• Thermal diffusivity
• Specific heat capacity
• Thermal conductivity
• Thermal analysis

Individual evidence

1. ^ WJ Parker, RJ Jenkins, CP Butler, GL Abbott: Method of Determining Thermal Diffusivity, Heat Capacity and Thermal Conductivity . In: Journal of Applied Physics . 32, No. 9, 1961, p. 1679. bibcode : 1961JAP .... 32.1679P . doi : 10.1063 / 1.1728417 .
2. ^ RD Cowan: Pulse Method of Measuring Thermal Diffusivity at High Temperatures . In: Journal of Applied Physics . 34, No. 4, 1963, p. 926. bibcode : 1963JAP .... 34..926C . doi : 10.1063 / 1.1729564 .
3. ↑ JA Cape, GW Lehman: Temperature and Finite-Pulse-Time Effects in the Flash Method for Measuring Thermal Diffusivity . In: Journal of Applied Physics . 34, No. 7, 1963, p. 1909. bibcode : 1963JAP .... 34.1909C . doi : 10.1063 / 1.1729711 .
4. ↑ J. Blumm, J. Sacrificial man: Improvement of the mathematical modeling of flash measurements . In: High Temperatures - High Pressures . 34, 2002, p. 515. doi : 10.1068 / htjr061 .