Theory
Similarly that in electricity, one defines the resistance and the
conductivity, one can define the thermal resistance and the thermal
conductivity.
The resistance depends on dimensional parameters while the conductivity
is an intrinsic characteristic to the material.
The electrical resistance is determined by the measure of the current
and the electrical tension:
Knowing the value of the resistance Re and dimensional parameters,
one can calculate the electrical conductivity σ:
σ = L/ (Re * S)
The thermal flow that crosses a wall of thermal conductivity λ
shown in permanent regime a difference of temperature between the
two faces of the wall.
The thermal resistance is determined by the measure of the thermal
flow Φ
(in Watt) and the difference of temperature Δ
T.
Knowing the value of the thermal resistance
RT
and dimensions of the sample, one can calculate the value of the
thermal conductivity.
λ = e / (RT * S ) (Watt/ m.K)
Conductivimeter
The conductivimeter we have is composed of a thermal flow sensor,
a surface heated element and a differential thermocouple.
The element heated allows to create a uniform thermal power through
the sample.
The sensor allows to measure the power in the sample.
Thermocouples measure the temperature of the sensor and the temperature
of the thermal well.
Protocol of measure
With a measure of the power in the sample and the difference of
temperature, one can calculate a thermal resistance. However the
obtained value will be erroneous because it will be equal to the
sum of thermal resistances: the sample and the thermal sensor.
To eliminate this disadvantage, the trick consists in using several
thickness samples. For each sample one measures the global thermal
resistance. Then one traces the curve of the thermal resistance
according to the thickness:
Original value Rc is the sum of resistances of contact. The thermal
resistance of the sample of thickness e j is equal to Rj - Rc.
Then one can calculate the conductivity λ that is independent
to the thickness.