DTC-300 Getting Started Guide
Page 31
Special Sample Testing
Thin Film (Multiple) Sample Testing
Thin samples such as polymeric films, paper products, and elastomeric insulating pads often have a ther-
mal resistance that falls below the minimum value of 0.0005 m
2
K/W that is recommended for accurate
testing with the DTC-300. By stacking several layers of the sample material, the thermal resistance can be
raised above the 0.0005 m
2
K/W limit and the thermal conductivity may then be calculated. A special soft-
ware is offered for this application.
The stack should contain as few layers as possible, preferably less than 7, as each additional layer intro-
duces another physical interface in the stack. The method relies on the fact that the interface resistance
between individual specimen layers is small compared to the thermal resistance of the material itself, and
can therefore be ignored. For thin, conforming materials of relatively low thermal conductivity, this is a
valid assumption.
For testing thin samples of higher thermal conductivity (ceramic, metals, graphite, etc.), refer to the section
“
Testing the Sample
.”
Select the appropriate testing option below:
•
Test one layer of material, using the
Testing the Sample
procedure. If the thermal resistance of this
specimen is within the calibration range of the instrument, there is no need to use the thin films tech-
nique, and the result of the test can be reported. If the thermal resistance of one layer is lower than the
minimum value of the instrument's calibration range, the testing process can continue by testing two,
three, four and five layers of material, stacked. It is recommended to have at least four different num-
bers of layers of material tested.
NOTE
: No thermal compound should be used between the layers of samples or between the samples and
the instrument. Since the thermal resistance of these interfaces will be considered negligible, the number of
samples stacked should be reduced to the minimum necessary. Testing up to five or six layers is usually
sufficient. In cases when the layers cannot be stacked without sliding, and only for non-porous materials, a
very small amount of thermal compound may be placed between the layers.
NOTE
: When using the thin films technique, the thermal conductivity values displayed on the screen at
the completion of each test are not to be considered as the final results of the measurements. The thermal
resistance of each stack tested and its thickness will be used in the program for calculating the material's
thermal conductivity.
NOTE
: When choosing the names for these individual tests, pick related names, so that it will be easy to
identify which files to use when performing the analysis (for example: Film1 - one layer; Film2 - two
layer; Film3 - three layers. etc.).
•
For very thin samples having a tendency to stick to each other under compression, though minimizing
the thermal resistance between them, higher numbers of layers can be tested together (for example:
four, five, six, seven, etc.), forming a new sample that may be treated as a solid piece, with its thermal
resistance being within the instrument's operating range. The total thickness of the test sample should
be considered for the thermal conductivity calculations, paying attention to use the value of the thick-
ness measured under compression, and not the sum of the individual layers' thicknesses.
