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Thermal Diffusivity
1. The thermal diffusion is expected to play an important role in determining the crystal growth morphology and the nature of crystalline defects, which may be generated by thermal stress during growth.
The measurements for the thermal diffusivity a will investigate the anisotropy and explore the temperature dependence for
crystal.
2. Laboratory assembled system for the measurement includes instruments in Figure and lists at below.
an Ar+ laser (Coherent model Sabre, 5 W),
a photographic shutter,
a type T thermocouple (signal),
a Si photodiode (trigger)and
a data acquisition system (National Instruments, model TC-2190) working under LabVIEW environment was used.
3. The method of the measurement:
Measurement for thermal diffusivity was performed on single crystal samples using the flash pulse method.
The thermal diffusivity can provide for an approximate measure of thermal response time, t = d2/a where d is the distance from the heat input pulse.
In processing or during applications, t might be viewed as the characteristic time of cool down (heating) without imposing significant transient temperature gradients, see the Figure.
Laser flash technique is set up for producing the heat pulse by briefly exposing the whole uncoated (front) face of the wafer to the expanded beam from a Coherent INNOVA 90 Argon-ion laser
operating with all lines, at a total power of 5 W.
The timed-shutter produces rectangular pulses of durations from 0.125 to 1.0 s as the range of t produced the heat pulse
The amplified thermocouple signal, indicating the temperature-time response at the back faceof the sample was recorded.
4. From the value of thermal diffusivity a, which is much important for the
computing of thermal conductivity k = aCpρ and the vectorial equation of the
energy flux: ju = -kÑT, where ÑT is the temperature gradient.
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