Heat Calvet type

Calvet and Guillaud (S3) noted in 1965 that in order to increase the sensitivity of a heat-flow microcalorimeter, thermoelectric elements with a high factor of merit must be used. (The factor of merit / is defined by the relation / = e2/pc, where e is the thermoelectric power of the element, p its electrical resistivity, and c its thermal conductivity.) They remarked that the factor of merit of thermoelements constructed with semiconductors (doped bismuth tellurides usually) is approximately 19 times greater than the factor of merit of chromel-to-constantan thermocouples. They described a Calvet-type microcalorimeter in which 195 semiconducting thermoelements were used instead of the usual thermoelectric pile. 

The efficiency of this method has been demonstrated for several types of heat-flow calorimeters. The rather long time constant of a Calvet-type calorimeter (200 sec), for instance, is decreased to 10 sec, when exact Peltier cooling is used (61). Similarly, the time constant of calorimeters... 

Classical DTA has been developed into heat-flux DSC by the application of multiple sensors (e.g., a Calvet-type arrangement) or with a controlled heat... 

A heat-flow calorimeter of Tian-Calvet type from Setaram maintained at a desired temperature, from room temperature up to 400°C, was used in connection with a volumetric apparatus equipped with a Me Leod gauge. Sample weights were typically 100 mg and ammonia doses 0.1 cm NTP. 

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]). 

A batch microcalorimetric experiment, very similar to the one just described, is possible with a diathermal heat flowmeter type of microcalorimeter, which is less versatile than the Tian-Calvet microcalorimeter (especially in its temperature range and ultimate sensitivity), but of a simpler design. In the Montcal microcalorimeter (Partyka et al., 1989), the thermopile with up to 1000 thermocouples is replaced by a few thermistors. 

This work is a continuation of our earlier study [1] of the hydrogen interaction with intermetallic compound (IMC) AB2-type Tio.9Zro.1Mn . 3V0.5. The measurements were carried out in twin-cell differential heat-conducting Tian-Calvet type calorimeter connected with the apparatus for gas dose feeding, that permitted us to measure the dependencies of differential molar enthalpy of desorption (AHdes.) and equilibrium hydrogen pressure (P) on hydrogen concentration x (x=[H]/[AB2]) at different temperatures simultaneously. The measurements were carried out at 150°C, 170°C and 190°C and hydrogen pressure up to 60 atm. 

In a joint work with A. A. Isirikyan with the participation of G. U. Rakhmat-Kariyev, we carried out direct measurements of differential heats of adsorption of water vapors on crystalline and molded zeolite NaA at 22 °C using a Tian-Calvet-type calorimeter. The calorimetric installation enabled us to measure thermal effects for each point of the adsorption isotherm for a period of 300 hours and more (Figure 1). The squares and circles in the upper part of the graph denote our data for... 

We built a conduction calorimeter of the Tian-Calvet type to measure the heat of adsorption of gases on zeolites. Figure 1 shows the construction of this calorimeter. The metal block reaches a temperature constancy of 0.01°-0.03°C. At about 300°C, we obtain the same values with an automatic adjustment. The calorimeter attains a time constancy... 

The heat-flow calorimeter of the Tian-Calvet type used for the determination of the adsorption heats of ammonia and the applied experimental technique were recently reported [9]. Ammonia adsorption was carried out at 80 C. All samples were pretreated under vacuum at 200 and 450 C, respectively, prior to any calorimetric measurement. 

The heats of adsorption of the probe molecules were measured in a heat-flow microcalorimeter of the Tian-Calvet type from Setaram, linked to a glass volumetric line to permit the introduction of successive small doses of gases [6]. The equilibrium pressure relative to each adsorbed amount was measured by means of a differential pressure gauge (Datametrics). Successive doses were sent onto the sample until a final equilibrium pressure of 133 Pa was obtained. The adsorption temperature was maintained at 353 K in order to limit physisorption interactions between the probe molecules and the zeolites. All the samples were pretreated at 773 K under vacuum overnight prior to any calorimetric measurement. 

The acid-base properties of the samples were investigated using adsorption of appropriate probe molecules, namely ammonia and sulfur dioxide, monitored by microcalorimetry. The microcalorimetric studies were performed at 353 K for sulfur dioxide adsorption and at 423 K for ammonia adsorption in a heat flow calorimeter of Tian-Calvet type (Setaram C80), linked to a conventional volumetric apparatus. Before each experiment the samples were outgassed overnight at 673 K. 

Microqalorimetry. Following a pretreatment at 623 K under vacuum (<10 torr) overnight, the differential heat of adsorption of ammonia was measured at 423 K using a SETARAM - Tian-Calvet type microcalorimeter associated with a volumetric equipment allowing the simultaneous determination of the adsorption isotherm. 

Microcalorimetric studies of the adsorption of ammonia (Critical diameter 0.3 nm) have been carried out using a Tian-Calvet type heat-flow microcalorimetric (C-80 model, Setaram, France). The microcalorimeter has been connected to a volumetric vacuum adsorption unit for catalyst treatment and probe molecule delivery. A validyne low pressure transducer (USA) attached with vacuum system has been used for precision pressure mecisurement. 

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. 

Care must be taken for polymer solntions with respect to the lesolntion of the instrument, which has to be higher than for common solntions with larger enthalpic effects. Usnally employed calorimeters for snch purposes are the Calvet-type calorimeters based on heat-flnx principle. Details can be fonnd in 1984HEM and 1994MAR. 

Calorimetry is a highly accurate method to measure the heat of adsorption, be it physisorption or chemisorption. It is typically performed using microcalorimeters of the Tian-Calvet type, in which known volumes of the adsorbate are sequentially dosed onto the solid from the gas phase at the required temperature and the liberated heat is determined from the temperature rise. In this way very accurate plots of heats of adsorption against uptake can be obtained directly for both weakly and strongly bound sorbates. 

A Calvet type calorimeter was used to estimate the interaction heat of the samples with tetrachloroethane or 5N MaOH. Device for thensooptical method was that used and described elsewhere. ... 

Assuming that the heat of adsorption is directly depending on the acidic or basic strength one obtains information about the amount, strength and distribution in strength of such sites but not about their nature. In such a technique a vacuum line with gas manifolds is attached to a cell placed into a Tian-Calvet type calorimeter maintained at the desired temperature. Successive increments of the probe molecules are introduced over the sample placed in the calorimetric cell and the heat of adsorption is then measured. One may then plot the differential heats of adsorption versus coverage or even its derivative, i.e. the number of... 

Figures 5 and 6, respectively, show the isosteric heats of adsorption of pure CO2 and C2H6 as functions of adsorbate loadings (surface excess) on three different microporous adsorbents (silicalite crystals, NaX zeolite crystals, and granular BPL activated carbon). Figure 7 shows similar plots for adsorption of three different pure gases (CH4, SFf, and CO2) on NaX zeolite crystals and pellets. The data for adsorption of CO2 and C2H6 on BPL carbon and those for adsorption of CH4 on NaX zeolite pellets were calculated with Eq. (11) using the pure gas adsorption isotherms at different temperatures [22]. The CH4 isotherms on the zeolite pellets were measured in our laboratory. All other data were measured by using a Tian-Calvet type of calorimeter [19,20].

Busey et al. (1984) constructed a flow mixing device from a cylindrical coil of stainless steel tube which fitted into a high-temperature Tian-Calvet type, heat-flux calorimeter. Fluids were pumped at the same flow rate through both sample and reference cells. Platimun-rhodimn capillary... 

Research of polymerization s kinetics was made with application of the calorimetric method on Calvet type differential automatic micro calorimeter DAK-1-1 in the mode of immediate record of heat emission rate in isothermal conditions at 60 °C. Kinetic parameters of polymerization were calculated basing on the calorimetric data as in the Ref. [5]. The value of polymerization enthalpy Aff = -73.8 kJ xmol [5] was applied in processing of the data in the calculations. 

Various commercial calorimeters are now available for routine heat of immersion measurements. For research it is preferable to use a calorimetric technique which is consistent with thermodynamic requirements. We recommend the employment of a Tian-Calvet type of microcalorimeter, which by means of two thermopiles composed of a large number of thermocouple junctions allows the heat flux to be measured accurately at practically constant temperature (AT < 10 K). Whichever technique is used, the experiments must be devised in a manner which will allow the evaluation of a number of corrective terms due to partial evaporation of the liquid, bulb breaking, stirring and effect of atmospheric pressure. In practice, this does not present difficulties because the detailed procedures and calculations are described in the literature. ... 

In a gas-solid open system, as the one described above (i.e., a Tian-Calvet type microcalorimeter connected to a high-vacuum gas-volumetric apparams) the system will exchange with the environment not only heat but also work and matter. Work is due to the reversible, isothermal transfer of matter to both gas (V dp) and adsorbed (RT dn ) phase. So, the isothermal heat measured in the calorimeter at constant T is defined by the Eq. 1.17 ... 

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