Adiabatic scanning calorimetry (ASC) is a time-proven calorimetric technique, developed in the 1970s by a group at the Catholic University of Leuven to investigate phase transitions of liquid mixtures and liquid crystals. It has been in academic use ever since. It provides the user with detailed curves for the temperature dependence of both heat capacity and enthalpy.

An adiabatic scanning calorimeter is conceptually a very simple instrument. A sample cell is heated with a constant power and its temperature is measured continuously. The ASC is constructed such that all the power is used to heat the sample and the sample assembly, due to the presence of adiabatic conditions. From the measured power $P(t)$ and temperature $T(t)$ as a function of time $t$, the specific heat capacity $c_P(T)$ and enthalpy $h(T)$ of the sample can simply be evaluated:

$$m c_P + C_{bg} = P/{{dT}/{dt}} = P/T↖{.}$$

$$m h + H_{bg} = P t$$

The background heat capacity $C_{bg}$ and the background enthalpy $H_{bg}$ (of the sample cell and assembly) are calibrated in a separate experiment. $m$ is the sample mass.

The challenge for an adiabatic scanning calorimeter is to maintain the adiabatic conditions. Although an ASC is conceptually simple, it has remained until now largely a research instrument to be operated by skilled and trained personnel, not in the least because complicated sample cell mounting and frequently delicate thermistor calibrations for monitoring the temperature difference between the sample cell and the adiabatic shield. Moreover, quite large samples, typically a couple of hundred milligram or more, had to be used.

The novel pASC instrument

These problems have recently been eliminated by the ASC-TC team in its innovative design of a Peltier-element-based Adiabatic Scanning Calorimeter (pASC) (patent pending), allowing measurements on mg size samples. The pASC is constructed to achieve absolute values of the heat capacity and enthalpy as a function of temperature. To this end the electrically applied power to the sample cell assembly and the temperature of the sample, are continuously measured and heat leaks eliminated or controlled. Besides the sample mass, the only parameters needed to get the specific heat capacity and enthalpy of the sample are the heat capacity of the calorimeter addenda and of the measuring cell. But these are calibrated in the construction phase and are of no concern to the user. This calibration of the calorimeter allows to determine the absolute values of the heat capacity and the enthalpy from a single run.

The basic accuracy of this approach is about 1%, and can be enhanced by running the adiabatic scanning calorimeter in step mode. The combination of the scanning measurements and the short step runs allows an accuracy much better than 1%, rivalling the performance of adiabatic step calorimeters while using samples that are 10-1000 times smaller.

This new pASC instrument provides a high resolution on mg size samples while ease of use is preserved. DSC pans as well as custom designed measurement cells can be used for an easy workflow and optimal results.

Comparison with DSC

The core idea of differential scanning calorimetry (DSC) is to compare the heat capacity of an unknown sample with the known heat capacity of a reference material. To achieve this, there are two common designs, the heat-flux DSC (by far the most common) and the power compensated DSC.

ASC is an absolute technique that does not require a reference.

In heat-flux DSC, the shield surrounding both sample and reference is heated, and heat is allowed to flow from the shield into the sample and the reference. The measured quantity is then the differential heat flow between the sample and the reference, which is a measure for the difference in heat capacity. In order to evaluate the absolute value of the heat capacity, a calibration of the heat flow with respect to known standards has to be made. This calibration itself depends on the heating rate.

In power compensated DSC, the reference sample is heated by a constant power through a heater on the reference sample holder, while a variable power is applied to the sample. The difference/ratio between these two powers is proportional to the heat capacity difference/ratio of sample and reference.

In ASC, no calibration is necessary, because all calibrations are done in the construction phase.

The differential approach in DSC is used to compensate for the influence of the environment on the sample: it is assumed that sample and reference undergo the same distortions, and hence these are compensated in the final result.

In ASC, the environment is completely controlled, and by definition does not influence the measurement.

In particular for the heat-flux design, the sensitivity of the instrument is directly proportional to the sensitivity of the heat-flux sensor. As a consequence, DSCs are often rated and compared by the sensitivity of this sensor, usually expressed in V/W.

The operation of an ASC depends on a zero-detector, and the size of the signal is not critical to the instrument's performance.

But more importantly, the larger the heat-flux signal in a DSC, the better the result. And when the heating rate is larger, the heat flux is larger (because only a maximum power can flow through the heat-flux detector, the sample lags further behind, increasing the flux until it balances). This has led to a tendency to favour higher heating rates, because this gives apparently better curves. However, higher rates lead to substantial thermal gradients over the sample. And also the thermodynamic equilibrium of the sample is obviously compromised in such circumstances (even outside the phase transition regions discussed below).

The performance of an ASC is essentially rate independent, because the sample controls the rate; performance in principle increases with decreasing rate, and remains excellent with increasing rate, as long as thermal and thermodynamic equilibrium of the sample is assured.

The mean difference between ASC and DSC becomes clear when a phase transition needs to be crossed. Existing DSCs operate in a constant rate mode. However, when a phase transition is reached, this means that the transition heat must be supplied in a very narrow time interval (in the ideal case: immediately), thus requiring a high power (ideally: infinite power). In heat-flux DSC, the power is limited by the thermal resistance of the connections between shields and cells through which the heat has to flow. In power-compensated DSC, this is limited by the maximum power that the cell heater systems can deliver. But in either case, insufficient heat can be delivered in time, and the sample lags behind its equilibrium behaviour, waiting until sufficient heat is delivered for the transition to complete. Thus, phase transitions in DSC are always broadened, and the data do not reflect thermodynamic equilibrium.

The temperature rate of an ASC decreases on approaching the phase transition (due to the increasing heat capacity), leading to, in fact, a better performance. Especially, when a latent heat is present, the sample stays at the same temperature until all the latent heat is delivered to the sample.