Measurement of Thermal Conductivity & Thermal Diffusivity
Thermal conductivity measurement overview:
Depending on sample properties and measurement range, there are many different direct or indirect ways to measure the thermal conductivity and LINSEIS is producing devices for almost every method. There is never “the right” method because it is always depending on the sample and factors like its shape, surface, diameter, composition and so on. The following pages give a little overview what method can be used for which application and how the corresponding devices are working.
The different Methods LINSEIS offers are:
The scheme below shows the possible range of temperature and thermal conductivity of each method in comparison.
The THB Method can be used for a variety of applications and is not as dependent from sample geometries and sample properties like other methods. By its means, thermal conductivity, thermal diffusivity and specific heat capacity can be determined within only a few minutes.
The method itself is considerable simple: The sensor is clamped between two parts of the same sample with at least one flat surface at each part. This sensor consists of an isolating capton layer that contains heating wires and some resistances. During the measurement, a defined heat flow is brought into the sample by the heaters and the measurement results are detected by the change of the bridge voltage that is caused by the transfer of the heat flow through the sample.
By choosing the right sensor that fits to the sample properties it is possible to measure insulating materials as well as the investigation of materials that show a very high thermal conductivity like metals. There are also sensors with a tightened frame available that can measure powders, liquids and gels, as well as sensors that are made of ceramic materials to go up to temperatures of 800°C. The needed heat is then generated by an external furnace that heats up the sample and the sensor together.
LINSEIS is producing different models of the THB device, beginning with the THB-1 that possesses only a standard sensor for thermal conductivities up to 1 W/mK. This goes up to the THB-500 that can cover every temperature range between -150°C and 800°C and thermal conductivity range between 0.001 W/mK and 500 W/mK.
The Heat Flow Meter is a special device for the investigation of insulation materials. It consists of a closed sample holder that has a heating plate (resistance heater foil) and a cooling plate (peltier element) at its top and at its bottom. Using various heat flow conductors inside, a constant heat gradient is generated that includes the sample itself. A measurement then lasts up to 1 or 2 hours, until a homogenous temperature field is established inside what determines directly the thermal conductivity and thermal resistance of the sample.
The whole proceeding is according to ASTM C518, JIS A1412, ISO 8301 and DIN 12667. The only disadvantage is that the measurement requires sample geometries of at least 30 x 30 cm in square and minimum 10 cm in height to give reliable results. However, if such samples are available it is the most accurate method for determination of thermal conductivity in a range of 0.001 up to 2.0 W/mK.
LINSEIS is offering a broad variety of models here, that are designed for different measurement conditions like different temperature ranges (from -30°C to 100°C) or sample geometries (from 30 x 30cm up to 60 x 60cm).
The Xenon- or Laser Flash method is a type of measurement to directly determine the thermal diffusivity. Xenon or Laser means the source of radiation the stimulation energy is coming from that heats up the sample for a short moment. To run a measurement, first of all a sample has to be placed into a sample holder and covered by a heat radiation adsorbing graphite layer. Then the sample holder is placed in the system where it is heated up by a furnace to the desired temperature for the measurement. If the temperature is reached, a short stimulation laser pulse brings a certain amount of heat into the sample. On the opposite site the thermal reflection of the sample is detected by a detection laser. This usually shows an increase of temperature during the heating pulse followed by a decrease that is shorter or longer, depending on the sample. This data is evaluated by a mathematical model and this gives the thermal diffusivity.
If the thermal conductivity is needed as well, the density and specific heat capacity of the sample have to be known or determined by another experiment using another method. By variation of the sample holders, measurements are possible in-plane as well as through-plane. It is also possible to measure pastes, gels, liquids and even powders. The thickness of the sample can be reduced down to some hundred micrometers; however it is also possible to measure bigger samples of some millimeters thickness. Because of the graphite layer, a reproducible heat entry is ensured for almost every sample and the type of material is almost not having any influence on the method itself.
The measurement conditions are covering a range from -125°C up to 1600°C and can give thermal diffusivities from 0.01 mm2/s up to 1000 mm2/s. Vacuum and inert atmosphere are also possible to be used.
This method is a measuring method for the determination of thermal conductivity and thermal diffusivity of thin layers. It is pretty similar to the LFA technique. For this method, a thin film sample is put onto a substrate and is covered with a top coating of gold. The sample is then inserted into a sample holder and placed in a furnace, similar to the LFA measurement. It is then heated by a short laser pulse.
The difference to the classical LFA measurement is, that there is used a detection laser from the same side that measures the sample temperature. Usually there is a strong temperature increase during the pulse that is followed by a temperature decrease. From this decrease the thermal conductivity and diffusivity can be determined using a 2- or 3 layer evaluation model.
The properties of thin films are completely different from their corresponding macroscopic samples. If there are films of different thicknesses used it is also possible to see, that a small change in the layer thickness of only a few nanometers already can have a massive influence on the sample properties.
The thin film laser flash method is suitable for samples with a thickness of 80 nanometers up to 20 micrometers and can be used under almost every kind of atmosphere within a temperature range from -100°C up to 500°C.
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