Differential Thermal Analysis and Differential Scanning Calorimetry
First, the basic principle
The basic principle of classical DTA is to place the sample and the reference substance (a thermally inert substance, such as a-Al2O3) in an environment of the same temperature state heated or cooled at a certain rate, and record the temperature difference between the sample and the reference substance Delta T in relation to time or temperature. The two thermocouples of the DTA are connected in reverse series, and the thermoelectromotive force measured by the two output terminals of the thermocouple corresponds to Delta T. The thermocouple of the traditional DTA is directly inserted into the sample or reference object, and both the thermocouple and the sample will be contaminated. Modern DTA generally heats the atmosphere around the sample and the reference object in a large thermal furnace, so it is easy to obtain a more linear instrument baseline, but it takes a long time to stabilize the instrument. In short, the advantage of DTA is that it can be used for high temperature measurement (the highest temperature can reach 1500 ° C, and some instruments even reach 2400 ° C), but the measurement sensitivity is poor, suitable for the analysis of inorganic materials such as minerals and metals, generally used for qualitative analysis, quantitative accuracy is poor.
DSC instruments are divided into two types, one is the heat flow type, and the other is the power compensation type. The principle of the former is similar to that of DTA, and the quantification is also converted by Delta T, but the thermocouple is closely attached to the bottom of the sample or reference material support, and some instruments have separate heaters for the sample and the reference material. Because this design reduces the influence of thermal resistance changes caused by the sample body, and the application of computer technology, its quantitative accuracy is better than that of traditional DTA, so it is also called quantitative DTA. The principle of power compensation DSC is special. During the process of temperature control, the temperature of the sample and the reference material is always kept the same. For this purpose, a separate heater and temperature detector are used for the sample and the reference material. When the endothermic effect occurs in the sample, the heat is increased by the compensating heater to maintain the same temperature between the sample and the reference; and vice versa. Then the power of this compensation is directly recorded, which is exactly equal to the endothermic and exothermic heat. Using a cyclic signal source on the instrument, the positive half-cycle controls the linear temperature rise (drop), and the negative half-cycle controls the temperature difference between the sample and the reference to zero, so that two different temperature control loops are cleverly operated almost simultaneously.
DSC resolution, repeatability and accuracy are good, more suitable for the analysis of organic and polymer materials, measuring the temperature range of 170-700 ℃ (some instruments can also reach high temperature). Not only for qualitative analysis, but also for quantitative analysis.
Typical DTA and DSC curves are depicted in the figure, respectively, and the transitions measured by the two curves were quantitatively analyzed.
Typical DTA and DSC curves are shown in the figure, respectively, and the transitions and thermal effects measured by the two curves are similar. Since the endothermic/exothermic directions set by each instrument are different, the directions of endothermic (endo) and/or exothermic (exo) must be indicated on the curve. The transition temperature value is sometimes based on the peak maximum value; but sometimes the peak onset temperature (onset) shall prevail, that is, the intersection point of the tangent line between the baseline and the peak front. The stew corresponds to the area surrounded by the curve and the baseline, as shown in the shaded part of the figure. The glass transition temperature (claw) is generally taken as the starting temperature or midpoint (midpoint).
It should be noted that the DSC records the relationship between the heat flow rate (dH/dt or dQ/dt) and temperature. The unit of heat flow rate can be W (i.e. J · s-1) or W · g-1, which is independent of the sample volume, also known as heat flow. The abscissa sometimes uses time instead of temperature, especially when doing kinetic studies or constant temperature measurements.
The main information provided by the DTA or DSC is:
(1) Temperatures at the beginning, peak, and end of the thermal event (provided by the abscissa of the curve).
(2) The magnitude and sign of the thermal effect (provided by the area and direction of the peak, respectively).
(3) The type and amount of substances involved in the thermal event (provided by the transition temperature value and peak area, respectively).
Since the peak area A is proportional to the thermal effect, i.e.
△H=k·A/m
Where m is the mass of the sample and k is the instrumental constant. The k-value is calculated using the melting peak area of a known mass of high purity (99.999%) and the heat of fusion (28.59J/g), and then the k-value is used to calculate the thermal effect of the unknown substance. High purity indium (melting point 156.634 ° C) is also used for temperature correction of the instrument. In turn, the mass m of the substance participating in the thermal event can be determined with the known delta H.
In addition, the longitudinal heat flow Y sample/Y standard = m sample/m standard rate dH/dt (abbreviated as Y) of the DSC is proportional to the instantaneous specific pressure heat capacity Cp of the sample, that is
dH/dt=mCpdT/dt (1一1)
Where m is the mass of the sample and dT/dt is the heating rate. Therefore, DSC can be used to determine the specific pressure heat capacity of the sample. Under the same conditions, the DSC curves of the sample and the standard (usually synthetic sapphire, that is, high purity a-Al2O3) are measured. At a certain temperature, the change rate of the ordinate of the DSC curve Y sample and Y standard (compared with the blank baseline) are obtained. The specific pressure heat capacity of the unknown sample is obtained by the following formula:
Y sample/Y standard = m sample Cp sample/m standard Cp standard
There is a sudden change in specific heat capacity during glass transition, and a step appears on the curve as a baseline shift, which can be used to determine the glass transition temperature.
It can also be seen from the formula (1-1) that the ordinate is proportional to the mass of the sample or the heating rate. Therefore, it is important to control the appropriate sample quality and heating rate. Generally, the sample volume is 5-10mg, and the standard heating rate is 10 ° C · min-1. When the sample is large, the sensitivity is higher, but the resolution is reduced, and the former has a greater impact. When the heating rate is slow, the resolution is higher, but the sensitivity decreases, and both have a greater impact. Therefore, a slower heating rate is generally selected to maintain good resolution, and an appropriate increase in the sample volume is used to improve the sensitivity.
Second, apply
In general, the applications of DTA/DSC can be divided into two categories: physical transformation and chemical reaction. Physical transformation includes crystallization/melting, solid-solid transformation (such as polycrystalline transformation), liquid-liquid transformation, liquid crystal phase transformation, sublimation, vaporization, adsorption, desorption, glass transformation, etc. Chemical reactions include oxidation/reduction, isomerization, dissociation, dehydration, polymerization, crosslinking, decomposition, etc. Therefore, DTA/DSC can determine the temperature of each transition and transition baking, reaction heat, specific heat capacity and glass transition temperature, crystallinity, crystallization kinetics, reaction kinetics, purity, phase diagram, thermal stability, etc.
Melting, vaporization, sublimation, precipitation, dehydration, desorption, reduction, etc. are endothermic processes, while crystallization, adsorption, oxidation, chemisorption, polymerization, cross-linking, etc. are exothermic processes. Decomposition or other chemical reactions are endothermic and exothermic. Crystal transition or liquid crystal phase transition depends on the direction of change in the degree of order, from a higher degree of order to a lower degree of order to an endothermic process, and vice versa.
Examples of inorganic, organic and polymer substances are given below for illustration.
1. Various phase transitions of sulfur
The endothermic peak at temperature 113 ° C is caused by the transformation of orthogonal crystal form into monoclinic crystal form; the endothermic peak at temperature 124 ° C is the melting peak; the peak at temperature 179 ° C belongs to the further transformation of liquid sulfur (i.e. liquid-liquid transition); the final vaporization peak temperature is 446 ° C.
2. Crystal transformation of quartz
The a —β’，β’～β crystal transformation of quartz is the endothermic peak when it warms up and the exothermic peak when it cools down. The position of the peak during cooling occurs at a lower temperature, which is called supercooling. Supercooling often occurs in the crystallization/melting transition. The crystallization peak at cooling is much lower than the melting peak at warming, and the supercooling degree is above 20-30 ° C. However, for glass transition, as long as the rising and cooling rates are consistent, the transition temperature can be reproduced.
3. Quality identification of chocolate
Genuine chocolate mainly contains cocoa butter V-shaped crystals, which show a single molten endothermic peak at 29.5 ° C and have a good taste. Defective chocolate has two endothermic peaks at 28.3 ° C and 32.9 ° C, which belong to the molten endothermic peaks of cocoa butter V-shaped and VI-shaped crystals, and the taste is poor.
4. Curing of epoxy resin
Commercially available AB glue is a two-component adhesive between epoxy resin and curing agent. It is the DSC curve of epoxy resin mixed with curing agent. When heated for the first time, the glass transition was observed to 64 ° C (superimposed stress relaxation endothermic peak), and the curing exothermic peak appeared in a wide temperature range, with a peak value of about 150 ° C. No curing peak was observed when heated again, and the glass transition temperature moved to a high temperature of 100 ° C (crosslinking increased the glass transition temperature), indicating that the curing has been completed after the first heating.
