Differential scanning calorimeter output

Figure 16 represents the differential scanning calorimeter output of hydrated CA at different scanning rates (at 28 days hydration). The dehydration peaks of AH3 and the C3 AH<5 phases are well separated, the heat energy being much higher for AH3. 

In a differential scanning calorimeter, a sample and reference material are heated in separate, but identical, metal heat sinks. The temperatures of the sample and reference material are kept the same by varying the power supplied to the two heaters. The output is the difference in power as a function of heat added. 

The output of a differential scanning calorimeter is a measure of the power (the rate of energy supply) supplied to the sample cell. The thermogram in the third illustration shows a peak that signals a phase change. The thermogram does not look much like a heating curve, but it contains all the necessary information and is easily transformed into the familiar shape. 

Although the output traces of a differential scanning calorimeter (DSC) are visually similar to a DTA, the operating principle of this device is entirely different. Figure 3.3 shows... 

Figure 2. Modification of differential scanning calorimeter for photocure kinetics analysis, (a) Dual light guides inserted through instrument cover and (b) light guide output into sample compartment...

The differential scanning calorimeter evolved from an older instrument known as a differential thermal analyzer, or DTA. The DTA, which is based on the work of Le Chatelier in 1887, was developed in 1899 for identification of specific types of clays, which are difficult to differentiate by more traditional methods. The concept of the DTA is quite simple. A differential thermocouple, which consists of two otherwise identical thermocouples connected in opposing polarities, is placed in a furnace in a position which allows the bead of one thermocouple to be inserted into an inert reference material, while the bead of the other thermocouple is inserted into the sample. The difference in temperature between the reference and sample materials is obtained directly as a function of temperature as the entire assembly is heated at a controlled, usually linear, rate. In the absence of any thermal difference between the sample and reference material, the output of the differential thermocouple will be zero. When a thermal event occurs, c.g., heat released during crystallization, the change in specific heat at the glass... 

This equipment can be used for research work to measure the heat output of new compositions, to compare the laboratory result with the theoretical heat output value. In addition, calorimetry can be used for quality-control work in manufacturing. As an example each batch of energetic material that is produced, or perhaps purchased, can be tested in the calorimeter for heat output. The new material should yield the same heat value as the previous sample of the material. If it doesn t, the reason needs to be determined before the new material is put into production. Also, a calibrated differential scanning calorimeter (DSC) can be used to determine the heats of various processes that occur in energetic materials as the sample tempera-tnre is raised. 

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