Heat-flux differential scanning calorimeter

Figure 12.1 Scheme of a disk-type heat flux differential scanning calorimeter. A cell B furnace C temperature sensors S sample R reference. 

Figure 2 (a) Power-compensation differential scanning calorimeter, (b) Heat flux differential scanning calorimeter... 

Pak J, Wunderlich B (2001) Heat Capacity by Sawtooth-modulated, Standard Heat-flux Differential Scanning Calorimeter with Close Control of the Heater Temperature. Thermochim Acta 367/368 229-238. 

Pak, (. and Wimderlich, B. (2001) Heat capacity by sawtooth-modulated, standard heat-flux differential scanning calorimeter with dose control of the heater temperature. Thermochim. Acta, 367/368, 229-238. 

Adiabatic calorimeters are complex home-made instruments, and the measurements are time-consuming. Less accurate but easy to use commercial differential scanning calorimeters (DSCs) [18, 19] are a frequently used alternative. The method involves measurement of the temperature of both a sample and a reference sample and the differential emphasizes the difference between the sample and the reference. The two main types of DSC are heat flux and power-compensated instruments. In a heat flux DSC, as in the older differential thermal analyzers (DTA), the... 

The heat flux and energy calibrations are usually performed using electrically generated heat or reference substances with well-established heat capacities (in the case of k ) or enthalpies of phase transition (in the case of kg). Because kd, and kg are complex and generally unknown functions of various parameters, such as the heating rate, the calibration experiment should be as similar as possible to the main experiment. Very detailed recommendations for a correct calibration of differential scanning calorimeters in terms of heat flow and energy have been published in the literature [254,258-260,269]. 

There are two types of differential scanning calorimeters (a) heat flux (AT) and (b) power compensation (AT). Subsequent sections of this experiment will not distinguish between the two types. In either type of calorimeter, the measurement is compared to that for a reference material having a known specific heat [16,17], As AT and AT have opposite signs there is some potential for confusion [3], e.g., at the melting point, Tm, Ts < Tr, and AT < 0, whereas Ts > Tr and AT > 0 because latent heat must be supplied (subscripts s and r refer to the sample and the reference material, respectively) [3]. 

In the CSM laboratory, Rueff et al. (1988) used a Perkin-Elmer differential scanning calorimeter (DSC-2), with sample containers modified for high pressure, to obtain methane hydrate heat capacity (245-259 K) and heat of dissociation (285 K), which were accurate to within 20%. Rueff (1985) was able to analyze his data to account for the portion of the sample that was ice, in an extension of work done earlier (Rueff and Sloan, 1985) to measure the thermal properties of hydrates in sediments. At Rice University, Lievois (1987) developed a twin-cell heat flux calorimeter and made AH measurements at 278.15 and 283.15 K to within 2.6%. More recently, at CSM a method was developed using the Setaram high pressure (heat-flux) micro-DSC VII (Gupta, 2007) to determine the heat capacity and heats of dissociation of methane hydrate at 277-283 K and at pressures of 5-20 MPa to within 2%. See Section 6.3.2 for gas hydrate heat capacity and heats of dissociation data. Figure 6.6 shows a schematic of the heat flux DSC system. In heat flux DSC, the heat flow necessary to achieve a zero temperature difference between the reference and sample cells is measured through the thermocouples linked to each of the cells. For more details on the principles of calorimetry the reader is referred to Hohne et al. (2003) and Brown (1998). 

Thermal conductivity was measured by the hot wire method. The principle involved in the measurement has been well explained by Carislaw and Jaeger (1959). Specific heat was measured by DSC (Differential Scanning Calorimeter) System TA 2910 (DuPont, U.S.A.) with heat flux type. The temperature differences between the reference material and the target specimen were measured during heating. Measurements were conducted twice at a specified temperature and temperature is varied from room temperature to 100 °C. Relationship between specific heat and temperature was linear and the result is summarized in table 1 and figure 2. 

Most differential scanning calorimeters fall into one of two categories depending on their operating principle power compensation or heat flux. 

Differential scanning calorimetry (DSC) is a thermoanalytical technique used to study the thermal properties of the polymer using a differential scanning calorimeter. In this process, the difference in the amount of heat required to increase the temperature of a sample and reference is measured as a function of temperature. Sample and reference will be maintained at same temperature throughout the experiment. DSC curves were plotted based on heat flux versus temperature or time. Thermal transitions of polymer can be determined by this technique. DSC is widely used for the decomposition behavior determination of the polymer. Figure 17.8 shows the DSC curves of PHB. 

Fig. 13.7. Heat flux during heating of poly(ethylene terephthalate) (PET) at a constant rate in a differential scanning calorimeter (DSC), showing changes in specific heat at Tg, crystallization exotherm at Tc, and melting endotherm at I m.

Temperature difference >> > Differential Thermal Analysis (DTA). Heat flux variation >>> Differential Scanning Calorimeter (DSC). Heat variation >>> Calorimetry. 

Differential Scanning Calorimeter, operating in either power compensation or heat flux mode, capable of heating at 10 rc/min from 15°C to 150°C. Controlled cooling capability is preferred but not essential. The calorimeter must be able to record automatically the differential signal (WE or WT) versus temperature with a temperature repeatability of 0.5°C. If the differential record is versus time, the calorimeter must have the capability to make a simultaneous record of temperature versus time. 

The term differential scanning calorimetry has become a source of confusion in thermal analysis. This confusion is understandable because at the present time there are several entirely different types of instruments that use the same name. These instruments are based on different designs, which are illustrated schematically in Figure 5.36 (157). In DTA. the temperature difference between the sample and reference materials is detected, Ts — Tx [a, 6, and c). In power-compensated DSC (/), the sample and reference materials are maintained isothermally by use of individual heaters. The parameter recorded is the difference in power inputs to the heaters, d /SQ /dt or dH/dt. If the sample is surrounded by a thermopile such as in the Tian-Calvet calorimeter, heat flux can be measured directly (e). The thermopiles surrounding the sample and reference material are connected in opposition (Calvet calorimeter). A simpler system, also the heat-flux type, is to measure the heat flux between the sample and reference materials (d). Hence, dqjdi is measured by having all the hot junctions in contact with the sample and all the cold junctions in contact with the reference material. Thus, there are at least three possible DSC systems, (d), (c), and (/), and three derived from DTA (a), [b), and (c), the last one also being found in DSC. Mackenzie (157) has stated that the Boersma system of DTA (c) should perhaps also be called a DSC system. 

In calorimetry techniques, enthalpy changes accompanying physical or chemical events, whether they are exothermic or endothermic, are measured and monitored either as a function of temperature or time. Thus, a calorimeter is able to collect a heat flux exchanged between the sample and the sensible part of the apparatus, generally made of thermocouples, and to register it. The result is a profile of the rate of enthalpy change, either as a function of temperature as the sample is heated at a known linear rate in differential scanning calorimetry (DSC), or as a function of time when the calorimeter is held at constant temperatnre in isothermal differential calorimetry (DC). 

In practice, the enthalpy of gasification is rarely calculated because detailed and reliable thermodynamic data for the polymer and its decomposition products are generally unavailable. Direct laboratory measurement of Lg using differential thermal analysis and differential scanning calorimetry has been reported, but Lg is usually measured in a constant heat flux gasification device or fire calorimeter (see under Steady Burning). In these experiments a plot of mass loss rate per unit surface area (mass flux) versus external heat flux has slope 1/Lg where... 

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