Sensitivity heat flow instruments

Very high sensitivity and the concomitant use of minute samples justify the descriptor microcalorimeter for many heat flow instruments. In general, a calorimeter can be labeled a microcalorimeter when its sensitivity is better than 10 (xW. Note, however, that some authors adopt a tighter definition, indicating 1 (x W as the sensitivity upper limit [160], The cell volume is usually in the range of 0.5-25 cm3. 

The intrinsic sensitivity of a heat-flow calorimeter is defined as the value of the steady emf that is produced by the thermoelectric elements when a unit of thermal power is dissipated continuously in the active cell of the calorimeter 38). In the case of microcalorimeters, it is conveniently expressed in microvolts per milliwatt (juV/mW). This ratio, which is characteristic of the calorimeter itself, is particularly useful for comparison purposes. Typical values for the intrinsic sensitivity of the microcalorimeters that have been described in this section are collected in Table I, together with the temperature ranges in which these instruments may be utilized. The intrinsic sensitivity has, however, very little practical importance, since it yields no indication of the maximum amplification that may be applied to the emf generated by the thermoelements without developing excessive noise in the indicating device. 

Other instruments include the Calvet microcalorimeters [113], some of which can also run in the scanning mode as a DSC. These are available commercially from SETARAM. The calorimeters exist in several configurations. Each consists of sample and reference vessels placed in an isothermally controlled and insulated block. The side walls are in intimate contact with heat-flow sensors. Typical volumes of sample/reference vessels are 0.1 to 100 cm3, The instruments can be operated from below ambient temperatures up to 300°C (some high temperature instruments can operate up to 1000°C). The sensitivity of these instruments is better than 1 pW, which translates to a detection limit of 1 x 10-3 W/kg with a sample mass of 1 g. 

Building a heat flow microcalorimeter is not trivial. Fortunately, a variety of modern commercial instruments are available. Some of these differ significantly from those just described, but the basic principles prevail. The main difference concerns the thermopiles, which are now semiconducting thermocouple plates instead of a series of wire thermocouples. This important modification was introduced by Wadso in 1968 [161], The thermocouple plates have a high thermal conductivity and a low electrical resistance and are sensitive to temperature differences of about 10-6 K. Their high thermal conductivity ensures that the heat transfer occurs fast enough to avoid the need for the Peltier or Joule effects. 

Microcalorimetry has gained importance as one of the most reliable method for the study of gas-solid interactions due to the development of commercial instrumentation able to measure small heat quantities and also the adsorbed amounts. There are basically three types of calorimeters sensitive enough (i.e., microcalorimeters) to measure differential heats of adsorption of simple gas molecules on powdered solids isoperibol calorimeters [131,132], constant temperature calorimeters [133], and heat-flow calorimeters [134,135]. During the early days of adsorption calorimetry, the most widely used calorimeters were of the isoperibol type [136-138] and their use in heterogeneous catalysis has been discussed in [134]. Many of these calorimeters consist of an inner vessel that is imperfectly insulated from its surroundings, the latter usually maintained at a constant temperature. These calorimeters usually do not have high resolution or accuracy. 

Each instrument can deliver the same information, that is, heat flow as a function of temperature (or time). The peak shape, the resolution, and the sensitivity depend on the principle of measurement and the specification of the instrument. 

DSC instruments are sensitive pieces of modem equipment, having the capability to measure heat flows of the order of microwatts. This feature makes the applicability of the technique almost unlimited every physical change or chemical reaction takes place with a change of enthalpy and consequently absorption or release of heat. 

A different approach has been taken by TA Instruments, in using two horizontal balances. The balance arms, each with a thermocouple attached to a pan carrier, hold the sample and reference adjacent to each other in the furnace. Unusually, in this instrument there is no heat flow path between the sample and reference, other than through the surrounding atmosphere, which limits the quality of the DTA data. Mettler-Toledo now offer a technique in which a form of DTA is obtained by comparing the sample temperature with a calculated reference temperature profile. Modern TG-DTA instruments are capable in general of a TG resolution around 1 g, use samples typically from 5 to 100 mg, and can give sensitive and quantitative DTA performance when the head is of... 

Figure 4.67[A] shows a typical isothermal experiment carried out with a DSC. Similar experiments could be carried out with isothermal calorimeters, dilatometry and other teehniques sensitive to crystallinity changes. After attainment of steady state at point 0, the experiment begins. At point 1, the first heat flow rate is observed, and when the heat flow rate reaches 0 again, the transition is complete. The shaded area is the time integral of the heat flow rate, and if there is only a negligible instrument lag, it represents the overall kinetics. In case of an excessive heat flow-rate amplitude, lag calibrations with sharply melting substances of similar thermal conductivity may have to be made (see Figure 4.22). Processes faster than about 1 min...

The best-known calorimeter of this type was developed by Tian and Calvet (Fig. 17). Here the defined heat-conduction path to the thermostated surroundings (a large aluminum block) consists of a large number of differential thermocouples coupled in series (thermopile). This arrangement permits optimum determination of the heat flow rate to the surroundings, and such an instrument can be very sensitive (microcalorimeter). 

It is pertinent to note here again that in these heat flow calorimeters, the sample crucibles and their supports (or vessels) must govern the thermal behavior of the instrument in other words, their heat capacity and thermal resistance must be large compared with that of the sample and reference substance. This naturally affects the sensitivity of the calorimeter this is a factor to be considered in selecting the thermal resistance and thereby the time constant of the instrument. 

For commercial instruments, it is often not possible to judge the sensitivity, it is hidden in internal electronics and computer software, and the given output quantity AX is already transformed into heat or heat flow rate

heat flow rate detectable with that calorimeter. This quantity is closely connected with the noise of the instrument. 

Therefore the T dependence of the relative calorimetric sensitivity is directly governed by the instrument s software. The heat flow to the sample (dHIdt) is given by... 

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