Measurements of Heat Capacity and Enthalpy of Phase Change Materials by Adiabatic Scanning Calorimetry
Summary (3 min read)
1 Introduction
- Phase change materials (PCMs) are substances exhibiting phase transitions with large latent heats and can be used as thermal storage materials with a large energy storage capacity in a relatively narrow temperature range [1] [2] [3] .
- In nowadays applications mainly two types of PCMs are in use.
- A first category includes organic materials, mainly paraffins and to some extent fatty acids.
- In particular, the temperature dependence of the enthalpy around the phase transition has to be known with good accuracy.
- Usually, the phase transitions of PCMs are investigated with differential scanning calorimetry (DSC).
2 Experimental Method
- The so-called ASC technique was introduced around 1980 [5] and extensively used for the study of many different types of phase transitions, in particular in liquid mixtures and liquid crystals.
- Here the authors will only give a description of the two principal modes of operation of an ASC and some key features of the data analysis.
2.1 Principal Modes of Operation of an ASC
- Since the beginning of the twentieth century, several different calorimetric techniques with varying degrees of accuracy and precision have been developed.
- Traditionally, heat-capacity measurements are carried out by means of the adiabatic heat pulse method, where a known amount of heat, Q, is (usually electrically) applied to the sample and the corresponding temperature rise, T , is measured.
- Rewriting Eq. 1 in the following way: EQUATION (with t time and P power), shows the possibility of operating in dynamic modes.
- These modes require different settings for the thermal environment (thermal shields) of the sample.
- Implementing a cooling run with constant power is less obvious and has to be realized by imposing a constant leaking power between the sample (cell) and its isothermal environment.
2.2 Implementation of the ASC Concept
- Figure 1 gives a schematic diagram of a four-stage ASC that can operate between room temperature and about 470 K.
- Each of the stages (1 to 4) has its own thermometer (Th i ) and its own electrical (e.g., constantan) heating wires.
- To minimize further thermal transfer between stages, all electric connecting wires are, on passing from one stage to another, several thermal diffusion lengths long (for temperature variations at relevant time scales), and neatly coiled not to touch the wall of either stage.
- Different sizes of sample cells can be suspended in the calorimeter.
- For liquid samples it is also possible to stir the samples inside the cell.
2.3 Analysis of the Direct Experimental Data
- These results are graphically displayed in the two central boxes of Fig. 2 for a weakly first-order transition.
- Depending on the temperature range to be covered, a typical run can take several days or weeks (for very slow scans).
- Proper calibration of the (only weakly T dependent) heat capacity of the empty cell and knowing the total amount of sample allows one to calculate the specific heat capacity of the sample.
- This is, however, an idealized situation for a perfectly pure one-component sample.
- For single components as well as for eutectic mixtures, the two-phase region can be very small depending on the unknown (and unavoidable) small amounts of impurities.
3 Materials
- Adiabatic scanning calorimetry (ASC) measurements were carried out for three n-paraffinand waxes-based PCM materials.
- These materials were purchased from Rubitherm GmbH, Germany.
- The first material has a quoted melting range of 25 C).
- On the outside of both cells, an electric heating wire was distributed and glued over the entire length.
- Adiabatic scanning calorimetry (ASC) measurements results are also reported for the pure salt hydrate calcium chloride hexahydrate (CaCl 2 6H 2 O).
4 Results and Discussion
- The characteristics of the ASC runs as well as relevant results for the different materials are included in Table 1 .
- For heating runs this is the temperature at which the whole sample has melted, and for cooling runs, the temperature where the sample starts to solidify.
- In the lower part of that figure the temperature dependence of the enthalpy for a heating run is given by the thick curve and by the thin one for a cooling run.
- It should be noted that in this these curves are not fit curves through the data points but collections of very large numbers of closely spaced direct data points.
- These effects are ascribed to premelting of hydrocarbon tails in the crystalline phase with increasing temperature, and gradual loss of the orientational order around the chain axes [11] .
4.2 Results for Calcium Chloride Hexahydrate
- The unusual shape of the curve for the cooling run at the hightemperature side is the result of substantial supercooling and then a sudden release of solidification heat in the thermally well insulated cell, resulting in an abrupt increase of the temperature to the real melting temperature of the substance.
- At that temperature the sample suddenly solidified and the temperature raised to near the melting temperature.
- Further enthalpy release (to the controlled thermal environment of the cell) occurred in the continuation of the cooling run.
- In the heating run and also in the cooling run, pretransitional enthalpy changes are clearly present, similar to what is observed for the paraffin-based PCMs.
- The argument of increased chain mobility does not apply here.
5 Summary and Conclusions
- ASC is introduced as a suitable tool for simultaneous measurements of the temperature dependence of the enthalpy and of the heat capacity of PCMs near their solid-liquid phase transition.
- Moreover, because of the very slow rates in heating the samples, the ASC measurements result in the equilibrium temperature dependence of the enthalpy.
- Results are reported for three paraffin-and wax-based PCMs obtained from an industrial supplier (Rubitherm GmbH, Germany).
- Results are also represented in several figures.
- For the heating run in the salt hydrate the shape of the enthalpy curve is analogous to that of the paraffin-based materials.
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Frequently Asked Questions (10)
Q2. Why do the ASC measurements result in the equilibrium temperature dependence of the enthalpy?
because of the very slow rates in heating the samples, the ASC measurements result in the equilibrium temperature dependence of the enthalpy.
Q3. How many modes of operation are used in the ASC?
By keeping P or Ṫ constant, while increasing or decreasing the temperature of the sample (P and Ṫ positive or negative), four practical modes of operation are obtained.
Q4. How slow is the heating and cooling of the PCMs?
Measurements have been performed at very slow heating and cooling rates, typically three orders of magnitude slower than the ones usually applied in differential scanning calorimetry (DSC).
Q5. What is the main purpose of the paper?
In this paper the authors present adiabatic scanning calorimetry (ASC) as an interesting complementary tool to measure simultaneously the temperature dependence of the enthalpy as well as of the heat capacity near the phase transitions in PCMs.
Q6. How can the authors avoid problems with superheating or supercooling?
With ASC the problems with superheating or supercooling can in many cases be avoided and true equilibrium data can be obtained by using very slow rates as slow as 2 to 3 orders of magnitude slower than in DSC.
Q7. What is the temperature of the cell at the end of the first order?
After a long temperature stabilization time of stage 2 (shield around the sample cell) with zero power to the cell (stage 1), the cell attains the same temperature (within a few tenths of a mK).
Q8. What is the temperature of the transition between heating and cooling?
For PX 42 their transition temperatures (44.5 ◦C for heating and 43.7 ◦C for cooling) are at the upper edge of the 38 ◦C to 43 ◦C range of the manufacturer.
Q9. What is the purpose of the paper?
In this paper, ASC is introduced as a suitable tool for simultaneous measurements of the temperature dependence of the enthalpy and of the (effective) heat capacity of PCMs near their solid–liquid phase transition.
Q10. How do the authors overcome the problems of latent heat measurements?
Efforts to (partly) overcome these problems for latent heat measurements of PCMs have resulted in running a DSC in an isothermal step mode and/or by applying a T -history method [4].