Thermal characterization

Thermal analysis focuses on the study of physical or chemical changes of a sample which occur as the temperature is increased or decreased. Several methods are commonly used to study the thermal behaviour of alloys. These are distinguished from one to another by the property which is measured. However, all of them have common features. The sample, positioned in an appropriate crucible (depending on the nature of the alloy that will be analyzed), is placed in a furnace and subjected to the desired temperature programme. During this procedure, one or more properties of the sample are monitored by means of suitable transducers for converting the properties to electrical quantities, such as voltages or currents.

2.2.1.1. Differential thermal analysis (DTA)

DTA is the simplest and most widely used thermal analysis technique. The difference in temperature between the sample and a reference material is recorded while both are subjected to the same heating programme.

The principle and a scheme of a DTA device are illustrated Figure 2.2. As the specimen is heated or cooled in a controlled manner, its temperature (Ts) will depart from the normal rate as it undergoes a reaction or transformation. When, for example, an endothermic event occurs in the sample, the temperature of the sample will lag behind the temperature of the reference material, which follows the programmed heating. For exothermic processes (Figure 2.2b), the response will be a faster heating. Furthermore, second order transitions characterized by for example a change in thermal conductivity or heat capacity of the sample, would give rise to a change in slope during thermal analysis and an offset of the baseline in differential thermal analysis.

The typical DTA device uses a pair of matched temperature sensors, generally thermocouples, one of them is in contact with the sample or its container and the other one is in contact with the reference material, as shown in Figure 2.2a. The output of the differential thermocouple is amplified and fed to the recorder or data acquisition system.

The use of a reference sample, besides the specimen of interest, is essential in order to eliminate disturbances in the recorded signal resulting from unintended fluctuations in

the rate of heating or cooling. This reference sample should be inert during the temperature interval of interest and should have a similar heat capacity. Since both the sample and reference material will react similarly to possible fluctuations, the final effect should be cancelled, leaving the baseline undisturbed.

A typical DTA-curve is the temperature difference between sample and reference monitored against the reference temperature, the actual sample temperature or time.

Ideally, the area under the DTA peak should be proportional to the enthalpy of the process to give rise to the peak. However, changes in thermal transport properties of the system, detector sensitivity, etc. with temperature will generally decrease the response of the DTA device with increasing temperature and are not compensated for in a traditional DTA curve. To compensate those effects Differential Scanning Calorimetry (DSC) was developed.

Figure 2.2: a) scheme of a DTA device b) general principle for an exothermic event where Ts is the sample temperature, Tr is the reference temperature, Ti is the initial temperature and Tf is the final

temperature

DTA in this work was used to study the thermal behaviour at high temperatures of the Fe36Co36B19.2Si4.8Nb4 alloy. DTA experiments were performed with a Perkin Elmer DTA at the Tohoku university at a heating rate of 40 K/min.

2.2.1.2. Differential Scanning Calorimetry (DSC)

This technique aims to maintain the sample and the reference material at the same temperature at any time during the experiment, by compensating independently the supplies of power to the sample and reference.

A schematic representation of a power compensating is shown in Figure 2.3. The inventive concept keeps the value of ∆T equal to zero by placing the temperature sensor, Pt resistance thermometers, into a bridge. Any imbalance drives a heater to compensate in the appropriate (sample or temperature) part of the cell. The power needed to the bridge circuit in balance is proportional to the change in heat capacity or enthalpy. The integral of power over the time of the total event gives the energy difference between sample and reference, which corresponds to the enthalpy difference of the process.

Therefore, if power is supplied to the sample, the process is endothermic, if it is supplied to the reference side it is exothermic.

The International Union of Pure and Applied Chemistry, IUPAC, has given precise definitions of the terminology used for the interpretation of DSC curves, as illustrated in Figure 2.4:

Baseline: part or parts of the DSC in which dQ/dt is nearly zero (lines AB and DE).

Peak: part of the curve which initially goes off and later goes back on the baseline (line BCD).

Peak width: interval of time or temperatures in which the DSC curve goes off the baseline (line B’D’).

Peak height distance, perpendicular to the time or temperature axis, between the interpolation of the baseline and the peak vertex (line CF).

Peak area: closed area by the peak and the interpolation of the baseline (area enclosed by the lines BC, CD and BD)

Initial temperature of transformation: temperature corresponding to the intersection point between the tangent at the maximum slope point at the beginning of the peak and the interpolation of the baseline point (point G)

Final temperature of transformation: temperature corresponding to the intersection point between the tangent at the maximum slope point at the end of the peak and the interpolation of the baseline point (point H).

Figure 2.3: Schematic representation of a power compensating DSC instrument and its operation

DSC in this work was mainly used to study the thermal stability of the amorphous alloys. Furthermore, in some cases, it was also used to study the change in mechanical and structural properties induced by heating a metallic glass below the crystallization temperature (to induce structural relaxation) or between the various crystallization steps.

D'

Temperature or time

A B G H D E

C F

B'

0 dQ/dt

Figure 2.4: DSC curve and its interpretation following the IUPAC rules.

DSC experiments were performed with a Perkin Elmer DSC-7 at the Universitat Autònoma de Barcelona at a heating rate of 40 K/min.

All our experiments were carried out up to high temperatures. As a consequence, graphite crucibles were used as a reference holder as well as a sample holder. The reference material in all the DSC experiments was simply an empty graphite holder.

For each experiment, two consecutive scans were made: the first one to let the sample crystallize and measure its thermal properties; the second one (in the same sample) was made for baseline correction.