By Arya Shahrostambeik, Laboratory Scientist (M.ScE)
Introduction
Laser Flash Analysis (LFA) was first developed by Parker et al. in 1961. At the time of the invention of this method, it gained popularity due its non-destructive nature and its transience. Compared to steady-state methods, laser flash offers a rapid and direct measurement of thermal diffusivity while not requiring huge sample dimensions. Moreover, samples can be recovered from the test without being damaged and measurements can be done across a wide range of temperatures.
LFA also offers the determination of specific heat capacity by the maximum temperature increase recorded during the measurement. By having the data for specific heat capacity, density, and thermal diffusivity, the value for thermal conductivity can be obtained.
Figure 1: LFA testing being performed by C-Therm’s Thermal Analysis Labs
The Working Principles of the Laser Flash Analysis
A laser flash instrument consists of a few key components. Providing a high-intensity short-duration light pulse is the first step for this analysis. A uniform pulse heating of the specimen is crucial for an accurate measurement, which is provided using an optical fiber. This heat pulse is generated from a laser and applied on the front surface of the sample. This energy source causes the temperature of the sample to increase. This temperature rise is measured on the opposite side of the sample using a thermocouple and recorded by an oscilloscope. The collected data shows a transient temperature increase, and the profile of temperature versus time is then utilized to calculate thermal properties of the specimen. [1]
Since the first development of the method, many efforts have been made to enhance the accuracy and precision of the method as well as reducing the uncertainty, especially at elevated temperatures. For instance, Baba et al. [2] constructed an advanced laser flash apparatus with the following improvements:
- Uniform pulse heating of a specimen, achieved with an improved laser beam using an optical fiber, which in practice reduced the error due to nonuniform heating error.
- A fast infrared radiation thermometer with an absolute temperature scale that decreases the error due to nonlinear temperature detection.
- Developing a new algorithm for advanced data analysis employing a curve-fitting method in which the entire profile of temperature history is fitted by the theoretical solutions.
After successful data collection, the value of thermal diffusivity is calculated through the following formula:
Where a is the thermal diffusivity in cm2/s, d is the thickness of the sample in cm, t1/2 is the time to the half maximum in s and the 0.1388 is a dimensionless coefficient. This formula works for a one-dimensional adiabatic case.
After calculating the thermal diffusivity of the material, using density and specific heat capacity, thermal conductivity can be calculated through a simple formula:
Where k is thermal conductivity, a is thermal diffusivity, Cp is specific heat capacity, and p is density as a function of temperature.
Figure 2: Measuring Principles of Neztsch LFA 447 [3]
Applications
Laser flash analysis allows the operator to obtain a broad range of values for thermal conductivity and diffusivity at extreme temperatures.
High-performance materials across different applications, such as aerospace, can benefit from the capabilities of laser flash analysis. Thanks to the versatility of this method, various materials such as ceramics, composites, and alloys can be tested.
Moreover, LFA can play a key role in designing insulations, optimizing heat sinks in electronics, and understanding the thermal properties in processes such as extrusion molding and metal working.
Another unique advantage of LFA is the capability of measuring thermal conductivity of multilayered composites as well as thermal contact resistance of two- and three-layer structures.
Limitations
Although LFA can be an efficient testing method in various industries for a variety of materials, it has some limitations that should be considered.
The samples for testing with LFA should be homogeneous and when a substantial non-homogeneity or anisotropy is assigned to the sample the results might be substantially in error. However, they can still be used for the means of comparison.
An important assumption of LFA is that all of the energy produced by the laser is absorbed into the sample and the level of emissivity should be better than 95%. It is recommended for samples to be well-coated with graphite before testing.
Although finding the diffusivity is a quick test, it is necessary to have specific heat capacity and density data to calculate thermal conductivity. To obtain these values additional testing such as Differential Scanning Calorimetry (DSC) and Thermomechanical Analysis (TMA) may be required.
Conclusion
LFA has proven to be a valuable method for measuring thermal diffusivity and conductivity across a wide range of materials and temperatures. Its non-destructive nature, rapid measurement capabilities, and versatility make it a popular tool in various industries including aerospace, electronics, and materials science.
Interested in Learning More?
Thermal Analysis Labs (TAL) provides LFA testing services for a wide range of materials, from those with low to high thermal diffusivity. You will be assigned an Account Manager for your testing experience to make sure all of your needs are met. After obtaining data, a C-Therm Subject Matter Expert (SME) will conduct a comprehensive review of the results, and a follow-up meeting can be arranged upon delivery of the final report.
If you are interested in material testing using LFA technology or for any other material testing inquiries, contact TAL at info@thermalanalysislabs.com to schedule a FREE technical consultation today! Or, click on the link below:
https://ctherm.com/thermal-analysis-labs/request-quotation/
Tel: +1 (506) 457-1515
References
[1] G.W.L.J.A. Cape, “Temperature and Finite Pulse-Time Effects in the Flash Method for Measuring Thermal Diffusivity, Journal of Applied Physics, 1963.
[2] A.O. Tetsuya Baba, “Improvement of the Laser Flash Method to Reduce Uncertainty in Thermal Diffusivity Measurement, “Institute of Physics Publishing, Measurement Science and Technology, 2001.
[3] “Neztsch LFA 447 Manual”.
About the Author
Arya Shahrostambeik is a Laboratory Scientist at C-Therm Technologies specializing in the Metravib product line. Having completed his Masters Degree in Chemical Engineering at the University of New Brunswick, Arya has extensive experience in the area of DMA and other materials characterization techniques.