Specific Heat Capacity Measurement by The Flash Diffusivity Method

Table of Contents

Introduction
Rise in Temperature
Laser Pulse Magnitude
Reflection by Other Media
Sample-Related Problems
     Emissivity
     Heat Loss
Conclusion

Introduction

Specific heat capacity is related to thermal diffusivity and thermal conductivity as follows:

          thermal diffusivity = thermal conductivity/(density x specific heat capacity)

As per the above equation, the Flash method for measurement of thermal diffusivity can be extended for indirect measurement of thermal conductivity of a sample by measuring its thermal diffusivity and specific heat capacity. To decide whether the same method can be used to measure the specific heat capacity of a sample, the following factors that affect measurement during thermal diffusivity tests need to be analyzed:

  • Rise in temperature
  • Laser pulse magnitude
  • Reflection by other media
  • Sample-related problems

Rise in Temperature

During the measurement of diffusivity, the front surface of the sample is heated by a laser pulse and the rise in the absolute temperature in the rear surface of the sample is measured. Measuring thermal diffusivity requires only the determination of the time dependent functionality of the temperature rise at the other surface and not its absolute magnitude. On the other hand, any error encountered in the determination of absolute magnitude of the temperature rise would directly affect the specific heat capacity values determined. Quasi-differential sensors with fast response are deployed in such tests. Maintaining the calibration of sensors is usually done by training a second pyrometer on the sample. Even still, determination of temperature rises in the range 1-3° is possible only with high measurement uncertainties.

Laser Pulse Magnitude

Another parameter that directly affects specific heat capacity measurement is the absolute magnitude of the laser pulse. Errors in determination of laser pulse magnitude will be reflected in specific heat measurement also. The magnitude of a laser pulse depends on the following factors:

  • the stored energy in capacitor banks for discharge,
  • the power of the laser and
  • the efficiency of the flash tube.

Reflection by Other Media

Usually, samples are isolated from the normal environment by windows. These materials can partly absorb and reflect the laser beam. The loss due to absorption and reflection may be minimized by coating windows. Nevertheless, nearly 10 to 15% of the energy may be lost per surface. One solution would be to determine the power input using a sample of known material for which the specific heat is known and then applying a correction factor. This approach will work if side-by-side comparison of samples are done.

Sample-Related Problems

Sample-related problems include:

  • Emissivity
  • Heat loss

which are covered in the following sections.

Emissivity

Many factors affect the fraction of the laser pulse that the sample actually absorbs. The surface of the sample is flat and exposed, and the ability of the sample to absorb light will largely depend on the emissivity of the surface. An accurate determination of surface emissivity is difficult because of varying surface roughness and formation of oxide films. Moreover, emissivity could also change with time and environment. It is therefore difficult to prove that the emissivities of known and unknown samples are identical. A 10% error in emissivity measurement will result in a 10% error in the interpretation of heat input and translates to the same percentage of error in the final specific heat capacity measured. Using Anter Corporation’s FL-5000 system in multi sample configuration, the sample’s emissivity can be determined first and quantitative corrections made later.

Heat Loss

Thermal diffusivity measurement is very fast, yet heat losses from the sample even during such short times can be too large to be ignored. Therefore corrections need to be applied for heat loss during diffusivity testing. Temperature dependence of losses and secondary effects due to radiative losses also need to be considered. Applying the Boltzman law to this heat loss problem, the higher the sample emissivity the more it will radiate as its temperature will be higher. Then one will be forced to conclude that the greater emissivity sample reaches a lesser final temperature and thus its specific heat is higher, which is entirely wrong as the steps discussed above are related to emissivity and not specific heat. Side-by-side testing of multiple samples will help reduce errors induced by heat loss in measurements.

Conclusion

From the above discussion, we can conclude that the measurement of specific heat capacity of a single sample using a flash diffusivity system is prone to errors. To reduce the probability of errors, parallel testing of multiple samples is recommended. With its multi-sample configuration, Anter Corporation’s FL-5000 system allows users to determine specific heat capacity with reasonable accuracy.

Anter Thermal Analysis Equipment

Anter Corporation manufactures thermal properties analyzers to measure:

This information has been sourced, reviewed and adapted from materials provided by Anter Corporation.

For more information on this source, please visit Anter Corporation

 

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