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Low Temperature Measuring Service
Measurement of Thermal Properties at Low Temperatures
Many physical properties of materials are temperature-dependent. Heat conduction, heat capacity and linear expansion in particular are temperature-dependent variables.
For the temperature range above room temperature, numerous values are available in the literature for almost all materials. Values are available for the temperature range below room temperature up to -197°C / -323°F (liquid nitrogen level), but not as detailed as above room temperature. There are hardly any precise values for temperatures below -197°C / -323°F.
Test set up for determining the thermal conductivity
In order to be able to make reliable statements about heat conduction in selected materials in the cryogenic temperature range, a test rig for investigating thermal conductivity was set up at the ILK Dresden. The thermal conductivity \( \lambda \) of a material is defined as the heat flow \( \dot{Q} \) over a defined cross-section \( A \) and a length \( l \) at an externally imposed temperature difference \( \Delta T \). \begin{equation} \lambda = \frac{\dot{Q} \cdot l}{A \cdot \Delta T}\left[ \mathrm{\frac{W}{m\:K}} \right] \end{equation} Thermal conductivity can also be understood as thermal resistance. With the measuring principle used, a heat flow or heating power is specified and the temperature difference across the sample to be tested is measured. It must be ensured that the applied heat flow can only flow over the sample cross-section and not over the surrounding structure or atmosphere. To minimize these lateral influences, a symmetrical measurement setup can be selected, see Figure 1, left. The sample heater is located in the middle between two identical samples. Both samples are surrounded by a thermally conductive structure, the measuring cell. A defined heat output is applied with the heater. The resulting heat flow, yellow arrows, is transferred to the measuring cell via the two samples. The measurements are carried out in a stationary state and in a vacuum. With the aid of the temperature control system, these measurements can be carried out very precisely and fully automatically at different temperatures. For larger sample dimensions, simpler arrangements are also possible, see Figure 1, right.
Several test setups are available for examining the samples. Figure 2 shows a set-up specifically for testing samples in shape of pipe sections. The height of the measuring cell used can be adjusted to different sample lengths and adaptations to other sample shapes are also possible. In other setups, rod-shaped samples or flat samples can also be examined.
Sample Paremeter | Limits |
---|---|
Materials | plastics, metals |
Dimension: pipe section Da x L Dimension: flate sample L x B x H | 30 x 20 x 1 mm, 1.2" x ¾" 80 x 10 x 2 mm |
Temperature range | 4 ... 333 K / -269 ... +60°C / -452 ... 140°F |
Test set up for heat capacity measurement
Like thermal conductivity, the heat capacity is a temperature-dependent physical quantity. The specific heat capacity of a substance measures the ability to store thermal energy and indicates how much energy is required to heat one kilogram of a certain substance by 1 K, for example. The specific heat capacity c can be determined using the heating pulse method. This involves heating a sample equipped with a heater and a temperature sensor for a defined period of time. The integral of the electrical heating power supplied over this period of time gives the amount of energy Q required to heat the sample with a mass m by the measured temperature difference \( \Delta T \). \begin{equation} c = \frac{Q}{m \cdot \Delta T} \left[ \mathrm{\frac{J}{kg\:K}} \right] \end{equation}.
Figure 3 shows the test set-up, with the temperature control system with cryocooler on the left and the sample carrier with a sample and sample heater mounted on it on the right.
[Translate to EN:]
[Translate to EN:]
Der Probenträger wurde speziell gestaltet um einen minimalen Wärmeeintrag in die Probe zu gewährleisten. Die Messungen erfolgen in thermisch stationärem Zustand und in einem Isolationsvakuum kleiner 1 x 10-4 mbar in Kombination mit Superisolationsfolie um Wärmeströme von der Umgebung in die Probe auszuschließen. Die Heizung und der Temperatursensor verfügen ebenfalls über eigene Wärmekapazitäten. Diese werden in einem separaten Versuch ohne die Probe ermittelt. Die Messwerte der Probe werden wiederum mit diesen Anteilen korrigiert.
Parameter | Grenzwerte |
---|---|
Materialien | Kunststoffe, Metalle |
Abmessungen, L x B x H | 30 x 30 x 10 mm |
Temperaturbereich | 20 ... 333 K / -253 ... +60°C < 20 K auf Anfrage |
[Translate to EN:]
[Translate to EN:] Ausgehend von einer Referenztemperatur \( T_0 \), und einer Länge \( L_0 \) ändern Materialien ihre geometrischen Abmessungen bei einer Temperaturänderung. Beim Erwärmen dehnen sich die meisten Werkstoffe aus und beim Abkühlen schrumpfen diese. Der physikalische Kennwert für dieses Verhalten ist der temperaturabhängige und werkstoffspezifische Ausdehnungskoeffizient \( \alpha(T) \) \begin{equation} \alpha(T) = \frac{dL(T)}{dT} \cdot \frac{1}{L_0} \left[ \mathrm{K^{-1}} \right]
[Translate to EN:]
[Translate to EN:]
Die Proben werden einseitig auf einen Probenhalter, siehe Abbildung 5, gespannt und in den Versuchsaufbau montiert. Der Probenhalter ist mit dem Kryokühler thermisch und schwingungsentkoppelt verbunden. Die Proben sind von einer thermischen Abschirmung umgeben um ein Erwärmen durch Wärmestrahlung und damit Temperaturgradienten in den Proben zu reduzieren. Bei schlecht wärmeleitenden Materialien wie z.B. bei Kunststoffen ist dieser Gradient deutlich stärker ausgeprägt als bei metallischen Proben. Um diesen Gradienten zu vermindern wird zusätzlich ein wärmeleitendes Austauschgas im Probenraum verwendet. Die freien Enden der Proben werden mit einem Lasermesssystem, einem Triangulationsmesssystem durch das optische Fenster abgetastet. Die Messwerte Probenlänge, Temperaturen und Messzeit werden vom PC aufgezeichnet und anschließend ausgewertet. Abbildung 5 zeigt nochmals den Versuchsstand und das Lasermesssystem vor dem optischen Fenster.
Parameter | Grenzwerte |
Materialien | Kunsstoffe, Metalle |
Abmessungen, L x B x H | 100 x 10 x 2 mm |
Temperaturbereich | 20 ... 333 K / -253 ... +60°C |
Anzahl Proben | 2 Proben, 1 Referenz |
Thermal Tests (Cycling)
Indicator | Minimal | Maximal |
---|---|---|
Cool down rate | 25 K/h | 200 K/min |
Cool down cycles | 1 | Customer preference |
Measuring accuracy | 1 % | 5 % |
Temperature range | 80 K (5 K) | 450 K |
Sample preparation | Customer material | |
Sample dimension | up to | Ø 400 mm x 550 mm |
other | dimenions on request |
Heat Capacity
Indicator | Minimal | Maximal |
---|---|---|
Measuring range | 10 J/(kg K) | 10 J/(kg K) |
Measuring accuracy | 1 % | 5 % |
Temperature range | 10 K | 350 K |
Sample preparation | Customer material | |
Sample dimension | up to | 150 x 150 x 150 mm3 |
other | dimenions on request |
Coefficient of Thermal Expansion
Indicator | Minimal | Maximal |
---|---|---|
Measuring range | 10-6 K-1 | 10-4 K-1 |
Measuring accuracy | 0.1 % | 5 % |
Temperature range | 10 K | 350 K |
Sample preparation | Customer material | |
Sample dimension | up to | 200 x 200 x 200 mm3 |
other | dimenions on request |
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