Heat flow calorimetry sensitivity

In the various sections of this article, it has been attempted to show that heat-flow calorimetry does not present some of the theoretical or practical limitations which restrain the use of other calorimetric techniques in adsorption or heterogeneous catalysis studies. Provided that some relatively simple calibration tests and preliminary experiments, which have been described, are carefully made, the heat evolved during fast or slow adsorptions or surface interactions may be measured with precision in heat-flow calorimeters which are, moreover, particularly suitable for investigating surface phenomena on solids with a poor heat conductivity, as most industrial catalysts indeed are. The excellent stability of the zero reading, the high sensitivity level, and the remarkable fidelity which characterize many heat-flow microcalorimeters, and especially the Calvet microcalorimeters, permit, in most cases, the correct determination of the Q-0 curve—the energy spectrum of the adsorbent surface with respect to... 

DTA Differential Thermal Analysis DSC Differential Scanning Calorimetry TGA Thermo Gravimetric Analysis ARC Accelerating Rate Calorimetry BSC Bench Scale, Heat Flow Calorimetry SEDEX Sensitive Detector of Exothermic Processes Others Oven Tests, Dewar Tests, Hot Plate Tests, etc. 

When discussing the sensitivity of heat flow calorimetry a representative example was chosen of a reaction releasing 27 W/batch on average when performed in a 2 liters glass vessel. A frequently found value for the coolant mass flow rate of a heat balance calorimeter amounts to 70 1/hour. Assuming a specific heat capacity of the coolant of 2600 J/kg K, this reaction power is transferred into a temperature difference between coolant inlet and outlet of 0,54 K. If the heat balance calorimeter and the heat flow calorimeter are to be of equal sensitivity, it follows that a resolution down to 1/100 K is required for the temperature difference. 

Thermal sensitivity is an important criterion with pyrolants as it determines both safety and functionality in terms of rehability of ignition and propagation. Prehmi-nary indicators for the thermal stability are any means of thermal measurements such as Differential Scanning Calorimetry (DSC) (see Chapter 5), Differential Thermal Analysis (DTA) and Thermogravimetry (TG) or Differential Thermogravimetry (DTG). More precisely, the Heat Flow Calorimetry (HFC) measurements at a fixed temperature are able to probe the intrinsic stability of an energetic material [16]. 

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. 

An apparatus with high sensitivity is the heat-flow microcalorimeter originally developed by Calvet and Prat [139] based on the design of Tian [140]. Several Tian-Calvet type microcalorimeters have been designed [141-144]. In the Calvet microcalorimeter, heat flow is measured between the system and the heat block itself. The principles and theory of heat-flow microcalorimetry, the analysis of calorimetric data, as well as the merits and limitations of the various applications of adsorption calorimetry to the study of heterogeneous catalysis have been discussed in several reviews [61,118,134,135,141,145]. The Tian-Calvet type calorimeters are preferred because they have been shown to be reliable, can be used with a wide variety of solids, can follow both slow and fast processes, and can be operated over a reasonably broad temperature range [118,135]. The apparatus is composed by an experimental vessel, where the system is located, which is contained into a calorimetric block (Figure 13.3 [146]). 

The essential requirements for the application of this direct method are a sensitive microcalorimeter (preferably of the heat-flow type, as described in Section 3.2.2) combined with equipment for the determination of the amount adsorbed. Although the assemblage of the apparatus is somewhat demanding, once the effort has been made the advantages of calorimetry are as follows ... 

Immersion calorimetry has much to offer for the characterization of powders and porous solids or for the study of adsorption phenomena. The technique can provide both fundamental and technologically useful information, but for both purposes it is essential to undertake carefully designed experiments. Thus, it is no longer acceptable to make ill-defined heat of immersion measurements. To obtain thermodynamically valid energy, or enthalpy, or immersion data, it is necessary to employ a sensitive microcalorimeter (preferably of the heat-flow isothermal type) and adopt a technique which involves the use of sealed glass sample bulbs and allows ample time (usually one day) for outgassing and the subsequent temperature equilibration. 

H.M. Shirazi, (Quartz Crystal Microbalance/Heat Conduction Calorimetry (QCM/HCC), a new technology capable of isothermal, high sensitivity, mass and heat flow measurements at a solid/gas interface., PhD Thesis, Drexel University, Philadelphia, PA, 2000. 

Thermal relationships between flammability and structure/composition of these polymers was explored. It is found that BPC Il-polyarylate is an extremely fire-resistant thermoplastic that can be used as an efficient flame-retardant agent to be blended with the other polymers. Chakon Il-polyarylate is of interest as a UV/visibk-sensitive polymer with a relatively low HRR and a high char yield. Pyrolysis combustion flow calorimetry (PCFC) results show that the total heat of combustion of the copolymers or blends changes linearly with the composition, but the change of maximum HRR and char yield depends greatly on the chemical structure of the components. 

IMC has been ubiquitously utilized for studying the isothermal crystallization kinetics of amorphous pharmaceuticals owing to the ultimate sensitivity of the technique towards subtle heat flow (Gaisford 2012). Low quantification and detection limit of the technique lead to enhanced crystallization enthalpy and therefore superior S/N ratio as compared to DSC. Several studies report the inert environment in studying crystallization kinetics or at static RH or scanning RH using RH perfusion calorimetry (Yonemochi et al. 1999). Different modified kinetic models are fitted to temporal data... 

An alternative technique is pressure differential scarming calorimetry (PDSC). This technique measures heat flow and temperatures of transitions as a function of temperature, time, and pressure (elevated pressure or vacuum). The ability to vary pressure from 1.3 Pa to 6.8 MPa makes PDSC ideal for the determination of the stability of oxidative polymers and for other pressure-sensitive reactions. 

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