Table of ContentsIntroductionRise in
TemperatureLaser Pulse MagnitudeReflection by Other MediaSample-Related
Heat Loss Conclusion
Specific heat capacity is related to thermal diffusivity and thermal
conductivity as follows:
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 include:
which are covered in the following sections.
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.
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.
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
Corporation manufactures thermal properties analyzers to measure:
This information has been sourced, reviewed and adapted from
materials provided by Anter Corporation.
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