Calvet differential

Let us consider the application of this technique to a Calvet differential calorimeter. According to the standard procedure an ampoule with the reactant mix is placed in the measuring chamber. If the wall thicknesses of the measuring chamber and the ampoule are sufficiently small, the heat flux q passing through a unit wall area with a stepwise change in temperature is described by the expression ... 

Calvet Differential, ideal flux Main and secondary reactions 0.5-3 g 30 a 300°C 0.1... 

Calvet microcalorimeters are particularly convenient for such studies. Figure 19 show s, for instance, the evolution of the differential heat of adsorption of oxygen, measured at 30°C with a Calvet calorimeter, as a function of the total amount of oxygen adsorbed on the surface of a sample (100 mg) of nickel oxide, NiO(200) (19, 73). The volume of the first... 

Heat-flow calorimetry may be used also to detect the surface modifications which occur very frequently when a freshly prepared catalyst contacts the reaction mixture. Reduction of titanium oxide at 450°C by carbon monoxide for 15 hr, for instance, enhances the catalytic activity of the solid for the oxidation of carbon monoxide at 450°C (84) and creates very active sites with respect to oxygen. The differential heats of adsorption of oxygen at 450°C on the surface of reduced titanium dioxide (anatase) have been measured with a high-temperature Calvet calorimeter (67). The results of two separate experiments on different samples are presented on Fig. 34 in order to show the reproducibility of the determination of differential heats and of the sample preparation. 

The enthalpies of phase transition, such as fusion (Aa,s/f), vaporization (AvapH), sublimation (Asut,//), and solution (As n//), are usually regarded as thermophysical properties, because they referto processes where no intramolecular bonds are cleaved or formed. As such, a detailed discussion of the experimental methods (or the estimation procedures) to determine them is outside the scope of the present book. Nevertheless, some of the techniques addressed in part II can be used for that purpose. For instance, differential scanning calorimetry is often applied to measure A us// and, less frequently, AmpH and AsubH. Many of the reported Asu, // data have been determined with Calvet microcalorimeters (see chapter 9) and from vapor pressure against temperature data obtained with Knudsen cells [35-38]. Reaction-solution calorimetry is the main source of AsinH values. All these auxiliary values are very important because they are frequently required to calculate gas-phase reaction enthalpies and to derive information on the strengths of chemical bonds (see chapter 5)—one of the main goals of molecular energetics. It is thus appropriate to make a brief review of the subject in this introduction. 

A second improvement in Calvet s calorimeter is that a differential set-up was adopted that aimed to suppress temperature drifts and fluctuations of the heat sink. This was achieved by coupling two calorimetric units in opposition to each other, so the measured thermoelectric force was the difference between the thermoelectric forces of the sample cell and the reference cell. The latter may remain at the temperature of the thermostat while the heat output or input related to the event under investigation occurs in the sample cell. 

At the end of the sixties, Godovsky 64 71> developed a fully automatic deformation microcalorimeter based on the Tiang-Calvet principle for simultaneous recording of thermomechanical behaviour of rubbers and solids (films, fibres) at uniaxial deformation. The device consists of two parts a microcalorimeter and a mechanical loading system with dynamometric assembly. The differential microcalorimeter includes the working and the reference cells. The temperature difference between the... 

Methods. The differential heats of adsorption of reagents and the differential heat of their interaction on the nickel oxide surface were measured in a Calvet microcalorimeter with a precision of 2 kcal. per mole. The apparatus has been described (18). For each adsorption of a single gas, small doses of gas are allowed to interact with a fresh nickel oxide sample (100 to 200 mg.) placed in the calorimeter cell maintained at 30°C. At the end of the adsorption of the last dose, the equilibrium pressure is, in all cases, 2 torr. Duplication of any adsorption experiment on a new sample gives the same results within 2 kcal. per mole of heat evolved and 0.02 cc. of gas adsorbed per gram. Electrical conductivities of the nickel oxide sample are measured in an electrical conductivity cell with platinum electrodes (1) by a d.c. bridge. 

In this category of calorimeters, we find the isothermal calorimeters and the dynamic calorimeters where the temperature is scanned using a constant temperature scan rate. The instrument must be designed in such a way that any departure from the set temperature is avoided and the heat of reaction must flow to the heat exchange system where it can be measured. The instrument acts as a heat sink. In this family we find the reaction calorimeters, the Calvet calorimeters [7], and the Differential scanning calorimeter (DSC) [8],... 

The Calvet calorimeters have their roots in the work of Tian [26] and the later modifications by Calvet [7]. Presently this calorimeter type is commercially available from Setaram and the models C80 and BT215 are particularly well adapted for safety studies. It is a differential calorimeter that may be operated isothermally or in the scanning mode as a DSC in the temperature range from room temperature to 300°C for the C80 and -196 to 275°C for the BT215. They show a high... 

Most young stars with optically thick disks appear to be accreting gas, which suggests that gas is continually flowing inwards through the disk and onto the star (Calvet et al. 2004). This inward viscous accretion is probably caused by turbulence in the differentially rotating disk. The existence of turbulence and the strength of the turbulent eddies can have a substantial impact on planetesimal formation, as we will see in Section 10.2.2. 

The effect of surface dehydroxylation of a mesoporous silica on the Ar and N2 energetics of adsorption is illustrated in Figure 10.12. In the work of Rouquerol et al. (1979b) Tian-Calvet microcalorimetry was used to determine the variation of the differential enthalpy of adsorption as a function of surface coverage. Although strong... 

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. 

The adsorption up to 50 bars was carried out by means of a Tian-Calvet type isothermal microcalorimeter built in the former CNRS Centre for Thermodynamics and Microcalorimetry. For these experiments, around 2 g of sample was used which were outgassed by Controlled Rate Thermal Analysis (CRTA) [7]. The experiments were carried out at 30°C (303 K). Approximately 6 hours is required after introduction of the sample cell into the thermopile for the system to be within 1/100 of a degree Celsius. At this point the baseline recording is taken for 20 minutes. After this thermal equilibrium was attained, a point by point adsorptive dosing procedure was used. Equilibrium was considered attained when the thermal flow measured on adsorption by the calorimeter returned to the base line. For each point the thermal flow and the equilibrium pressure (by means of a 0-70 bar MKS pressure transdueer providing a sensitivity of 0.5% of the measured value) were recorded. The area under the peak in the thermal flow, Q eas, is measured to determine the pseudo-differential... 

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... 

To eliminate the effects of external temperature fluctuations in the calorimetric block, the calorimeter has two heat-flow meters, which are connected in opposition. The process under investigation is carried out in one of two identical calorimeter vessels, the other serving as the tare or reference element. This differential arrangement permits the compensation of parasitic phenomena such as external connections and reagent introduction heat, and it provides a good stability of the baseline. (From the development in Section II,A on thermodynamics it follows that for adsorption of gas in a Calvet calorimeter the heat measured corresponds to a differential molar enthalpy of adsorption because all other effects are compensated.)... 

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. 

Differential heats of adsorption of oxygen on NiO(200°) were measured at 30° with a Calvet microcalorimeter (44). The surface coverage corresponding to the break in the adsorption isotherm (Fig. 3) is indicated in Fig. 4 by an arrow. The irreversible adsorption, which is fast, yields high heats (initial value, 60 kcal/mole) (Fig. 4), whereas the heats produced during the partially reversible adsorption are smaller and are evolved slowly. Heats of desorption and readsorption of oxygen have also been measured at 30° (2-4 kcal/mole). These low heats are explained by a molecular adsorption (25). 

On the other hand, for slow reactions, adiabatic and isothermal calorimeters are used and in the case of very small heat effects, heat-flow micro-calorimeters are suitable. Heat effects of thermodynamic processes lower than 1J are advantageously measured by the micro-calorimeter proposed by Tian (1923) or its modifications. For temperature measurement of the calorimetric vessel and the cover, thermoelectric batteries of thermocouples are used. At exothermic processes, the electromotive force of one battery is proportional to the heat flow between the vessel and the cover. The second battery enables us to compensate the heat evolved in the calorimetric vessel using the Peltier s effect. The endothermic heat effect is compensated using Joule heat. Calvet and Prat (1955, 1958) then improved the Tian s calorimeter, introducing the differential method of measurement using two calorimetric cells, which enabled direct determination of the reaction heat. 

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. 

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. 

Other calorimeters described include a microcalorimeter (111) similar to the Calvet instrument (112). high-temperature differential calorimeters (114-117X and others (118-121). 

Figure 9 Calvet-type differential scanning calorimeter...

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... 

Tian A (1933) Researches on Calorimetry. Generalization of the Method of Electrical Compensation. Microcalorimetry. J Chim Phys 30 665-708 and Calvet E (1948) Compensated Differential Microcalorimeter. Compt rend 226 1702-1704. 

---