Modern industry requires more and more materials resisting to high and very high temperatures. A good characterization of these materials is becoming necessary in order to precisely measure their properties, to know their field of applications and also their limits of use. Among all the properties needed, heat capacity data are required at high temperature to be combined with thermal conductivity and thermal diffusivity.For material scientists there is a key issue to accurately measure the heat capacity of samples, especially at high temperatures. Many calorimetric devices and methods have been used to meet the requirements but the DSC technique remains the most often used for such a determination. The major difficulties in this case, besides the matters of simply reaching high temperatures or building measurement systems with materials than can stand high temperatures, is to overcome the drop of sensitivity of commonly employed thermocouples when approaching their upper limit of use or to deal with perturbations linked with the radiation effects of the measured samples. Another difficulty is related to the plate-type DSC technique that limits the amount of material to be investigated.The solution is to use the technology developed around the Calvet principle (3D) sensors that has been successfully used in the accurate determination of specific heat with various types of thermal analyzers. Based on this expertise, a 3D detector was designed to be used on the Labsys DSC for the Cp determination at high temperature.
CP DETERMINATION WITH THE LABSYS 3D CP DETECTOR
Modern industry is requiring more and more materials to be resistant to very high temperatures. Good characterization of these materials is necessary in order to measure their precise properties to know the limits of their use and which applications they can be best applied to. Properties that are needed include, heat capacity data at high temperatures as well as thermal conductivity and thermal diffusivity data.
Material scientists must therefore accurately measure the heat capacity of samples, especially at high temperatures. Many calorimetric devices and methods have been used for these measurements, but the DSC technique remains the most common. The biggest difficulty, besides simply reaching these high temperatures or building measurement systems with materials than can withstand high temperatures, is to overcome the drop in sensitivity of commonly employed thermocouples when approaching their upper limit of use and to deal with perturbations associated with the radiation effects of the samples being measured. Another difficulty with plate-type DSC technique is the limited amount of material that can be tested.
The solution to these problems is to use the Calvet principle. Calvet (3D) sensors have been successfully used in the accurate determination of specific heat. Based on expertise, a 3D detector was designed for the LABSYS evo DSC for CP determination at high temperatures.
THE LABSYS 3D CP DETECTOR
The CP measurement system has benefited from the development of the LABSYS evo, whose furnace has been improved in terms of the maximum temperature rate and temperature homogeneity. This new furnace can scan temperatures at rates up to 100K.min-1 along the entire temperature range, i.e. 20-1600°C. The scanning rate can therefore be increased, thus increasing the magnitude of the heat signal.
Moreover, the wider homogeneous temperature zone allows the design of a larger platinum sample holder with capacity up to 380μL and an optional platinum cover (see figure 1), increasing the mass parameter. The 380μL sample holder is positioned on the alumina structure of the rod.
Figure 1: Cross section of the Labsys thermoanalyzer
and the CP 3D DSC detector
The sensitivity coefficient of the CP measurement system is increased by building a thermopile made of a series of 18 thermocouples, following the Calvet principle. Platinum and platinum-rhodium threads are successively soldered on the whole surface of both alumina holders. Then, heat exchanged by the sample is fully captured and measured.
DSC TECHNIQUES FOR MEASURING THE HEAT CAPACITY
The ideal technique for measuring the heat capacity of a material is the calorimetric technique. The DSC signal for a given sample at a temperature T is equal to:
– dq/dt is the DSC signal
– dT/dt is the temperature scanning rate
The first DSC technique for measuring the heat
capacity is called the continuous heating (or cooling) mode (Figure 2). A linear heating rate is applied between T1 and T2.
The As curve corresponding to the DSC signal of the sample is corrected from the Ab curve corresponding to the DSC signal of the blank experiment (obtained with two empty crucibles).
Figure 2: CP determination using the continuous heating mode
The second DSC technique, called the step heating (or cooling) mode is based on the application of an incremental temperature ramp (Figure 3). In this case the Cp determination is related to the integration of the DSC signal on a given temperature ramp, according to the following relationship
The Qs area corresponding to the sample curve is corrected from the Qb area which corresponds to the blank curve (with two empty crucibles). In this case a mean Cp value is measured for the given temperature range.
Figure 3: Cp determination in the step heating mode
With the Labsys Cp detector it is possible to measure the Cp of solids from room temperature up to 1600°C. Some examples are given below to illustrate the capacity and the accuracy of the 3D DSC detector.
1- CP OF ALUMINA NANOPOWDER (CONTINUOUS HEATING)
361 mg of alumina nanopowder is used for the Cp determination from room temperature to 1400°C with the continuous method at 20°C.min-1 . The sapphire calibration is used for this determination. Two types of confidence intervals are drawn on Figure 4 at ±1% and ±5% of the sapphire Cp value. Above 300°C, the Cp of alumina nanopowder stays within ±1% of the sapphire Cp interval up to 1400°C.
Figure 4: Cp determination of alumina nanopowder using the continuous heating mode
2- CP OF TUNGSTEN (STEP HEATING)
4213 mg of tungsten is used for the Cp determination from room temperature to 1500°C using the step heating mode under argon. The selected scanning rate for the ramp is 8°C.min-1 and an isothermal step of 1200 seconds is applied after each ramp. A calibration using sapphire was applied for this determination. The obtained Cp values of tungsten are compared with the JANAF values. A confidence interval of ±5% is drawn on figure 5. It shows that the accuracy of the Cp determination for tungsten using the 3 D Cp rod is better than ±5%.
Figure 5: Cp determination of tungsten in the step heating mode
3- CP OF GLASS AND GLASS TRANSITION DETERMINATION (CONTINUOUS HEATING)
508 mg of white glass is used for the Cp determination from room temperature to 700°C at 10°C.min-1 under argon with the continuous heating mode. Figure 6 shows the Cp variation and clearly shows a jump in the Cp curve after 500°C corresponding to the glass transition of the material. This Cp jump (see table) characterizes the amorphous content of the material.
Figure 6: Cp determination of à glass in the continuous mode
With the 3D DSC technology, it is possible to accurately measure the heat capacity of materials over a broad range of temperatures up to 1600°C. The main interest in the technology is the ability to work with a large amount of sample with detectors that completely surround the sample, providing a very good integration of the thermal exchanges. Tests can be run using both continuous and stepwise methods.
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