Calorimeter high-temperature

The papers that consider determination of the heat effects that accompany physical and chemical processes present a wide spectmm of types of calorimeters. These devices have been given various names by the authors, who made their choices on the basis of different criteria. Names such as low-temperature calorimeters, high-temperature calorimeters and high-pressure calorimeters come from the conditions of temperature and pressure under which the measurements are performed. In some cases, the type of process investigated is decisive calorimeters for heat of mixing, heat of evaporation, specific heat measurements, and others. The names of calorimeters often have to contain information about their constmction features, e.g. labyrinth flow calorimeter, calorimetric bomb, drop calorimeter, or stopped-flow calorimeter. The name of the device sometimes stems from the name of its creator. Examples here include the calorimeters of Lavoisier, Laplace, Bunsen, Calvet, Swietoslawski, Junkers, and others. This diversity of the names of calorimeters justifies an attempt to find features that classify the devices unambiguously. 

Figure Bl.27.2. Schematic vertical section of a high-temperature adiabatic calorimeter and associated thennostat (Reprinted with penuission from 1968 Experimental Thermodynamics vol I (Butterworth).)...

Reviews of batch calorimeters for a variety of applications are published in the volume on Solution Calorimetry [8] cryogenic conditions by Zollweg [22], high temperature molten metals and alloys by Colinet andPasturel [19], enthalpies of reaction of inorganic substances by Cordfunke and Ouweltjes [16], electrolyte... 

Figure Bl.27.9. High-temperature heat-leak calorimeter. (Reproduced by pemiission from Cliristensen J J and Izatt R M 1984 An isothemial flow calorimeter designed for high-temperature, high-pressure operation...

Magee J W, Blanco J C and Deal R J 1998 High-temperature adiabatic calorimeter for constant-volume heat capacity of compressed gases and liquids J. Res. Natl Inst. Stand. Technol. 103 63... 

A high-temperature and high-pressure reaction calorimeter. 

In earlier times, ethyl ether was commonly used as an anesthetic. It is, however, highly flammable. When five milliliters of ethyl ether, C HuQC/), (d - 0.714 g/mL) is burned in a bomb calorimeter, die temperature rises from 23-5°C to 39.7°Q The calorimeter heat capacity is 10.34 k)/°C. 

A survey of the literature shows that although very different calorimeters or microcalorimeters have been used for measuring heats of adsorption, most of them were of the adiabatic type, only a few were isothermal, and until recently (14, 15), none were typical heat-flow calorimeters. This results probably from the fact that heat-flow calorimetry was developed more recently than isothermal or adiabatic calorimetry (16, 17). We believe, however, from our experience, that heat-flow calorimeters present, for the measurement of heats of adsorption, qualities and advantages which are not met by other calorimeters. Without entering, at this point, upon a discussion of the respective merits of different adsorption calorimeters, let us indicate briefly that heat-flow calorimeters are particularly adapted to the investigation (1) of slow adsorption or reaction processes, (2) at moderate or high temperatures, and (3) on solids which present a poor thermal diffusivity. Heat-flow calorimetry appears thus to allow the study of adsorption or reaction processes which cannot be studied conveniently with the usual adiabatic or pseudoadiabatic, adsorption calorimeters. In this respect, heat-flow calorimetry should be considered, actually, as a new tool in adsorption and heterogeneous catalysis research. 

Fig. 4. Vertical cross section of a high-temperature Calvet calorimeter (16) cell guides (A) thermal insulation (B) top (C) and bottom (N) electrical heaters thermostat consisting of several metal canisters (D, G, and H) switch (E) electrical heater (F) thermometers (I, J, and K) microcalorimetric element (L) and heat sink (M).

Several features of the early model (Fig. 6) have been modified in the present-day, high-temperature version of this calorimeter (Fig. 7) (37). Depending upon the temperature range envisaged, the block is made of refractory steel, alumina, or beryllium oxide and is machined to house the calorimeter itself. The thermoelectric pile (about 50 platinum to platinum-rhodium thermocouples) is affixed in the grooves of an alumina plate (A), which is permanently cemented to two cylindrical tubes of alumina (B). Cylindrical containers of platinum (C) ensure the uniformity of the temperature distribution within the calorimeter cells. 

Figure 15 gives a diagrammatic representation of a volumetric line which is used in connection with a high-temperature Calvet microcalorimeter 67). Other volumetric lines which have been described present the same general features (15, 68). In the case of corrosive gases or vapors, metallic systems may be used 69). In all cases, a sampling system (A in Fig. 15) permits the introduction of a small quantity of gas (or vapor) in a calibrated part of the volumetric line (between stopcocks Ri and Ro in Fig. 15) where its pressure Pi is measured (by means of the McLeod gage B in Fig. 15). The gas is then allowed to contact the adsorbent placed in the calorimeter cell C (by opening stopcock Ro in Fig. 15). The heat evolution is recorded and when it has come to completion, the final equi-...

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. 

Figure 10.4 Adiabatic high-temperature calorimeter [15], 1 Calorimeter proper 2 Silver guard 3 Silver shield 4 Shield heater 5 Thermocouple sleeve 6 Silica glass container 7 Sample 8 Calorimeter heater 9 Pt resistance thermometer 10 Silica ring spacer 11 Type S thermocouple 12 Guardheater 13 Removable bottom. Reproduced by permission of F. Grpnvold.

Heat capacities at high temperatures, T > 1000 K, are most accurately determined by drop calorimetry [23, 24], Here a sample is heated to a known temperature and is then dropped into a receiving calorimeter, which is usually operated around room temperature. The calorimeter measures the heat evolved in cooling the sample to the calorimeter temperature. The main sources of error relate to temperature measurement and the attainment of equilibrium in the furnace, to evaluation of heat losses during drop, to the measurements of the heat release in the calorimeter, and to the reproducibility of the initial and final states of the sample. This type of calorimeter is in principle unsurpassed for enthalpy increment determinations of substances with negligible intrinsic or extrinsic defect concentrations... 

The solution experiments may be made in aqueous media at around ambient temperatures, or in metallic or inorganic melts at high temperatures. Two main types of ambient temperature solution calorimeter are used adiabatic and isoperibol. While the adiabatic ones tend to be more accurate, they are quite complex instruments. Thus most solution calorimeters are of the isoperibol type [33]. The choice of solvent is obviously crucial and aqueous hydrofluoric acid or mixtures of HF and HC1 are often-used solvents in materials applications. Very precise enthalpies of solution, with uncertainties approaching 0.1% are obtained. The effect of dilution and of changes in solvent composition must be considered. Whereas low temperature solution calorimetry is well suited for hydrous phases, its ability to handle refractory oxides like A1203 and MgO is limited. 

High-temperature solution calorimeters [34-36] are in general of the twin heat flux type. They are applicable from around 900 K to around 1500 K and a... 

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. 

The first attempt in our laboratory to apply flow techniques to high temperature operation was the construction by Dr. E.E. Messikomer of a flow, heat-of-mixing calorimeter(lO). Unfortunately, because the thermopiles used in this instrument did not work above 100°C the instrument was limited to this temperature. However, the results were encouraging because they showed that very rapid and accurate thermodynamic data could be obtained and that the operation of the calorimeter was as easy at 100°C as it was at room temperature. 

Heat contents can be measured accurately by a number of techniques based on a drop method. This involves heating the sample to a high temperature and dropping it directly into a calorimeter held at a lower temperature. The calorimeter then measures the heat evolved while the sample cools to the temperature of the calorimeter. The temperature at which the sample is initially heated is varied and a plot of Ht — 298.15 vs temperature is drawn (Fig. 4.1). Heat capacities can then be calculated using Eq. (3.9). A popular calorimeter for this is the diphenyl ether calorimeter (Hultgren et al. 1958, Davies and Pritchard 1972) but its temperature range is limited below about 1050 K. 

The problems associated with direct reaction calorimetry are mainly associated with (1) the temperature at which reaction can occur (2) reaction of the sample with its surroundings and (3) the rate of reaction which usually takes place in an uncontrolled matmer. For low melting elements such as Zn, Pb, etc., reaction may take place quite readily below S00°C. Therefore, the materials used to construct the calorimeter are not subjected to particularly high temperatures and it is easy to select a suitably non-reactive metal to encase the sample. However, for materials such as carbides, borides and many intermetallic compounds these temperatures are insufficient to instigate reaction between the components of the compound and the materials of construction must be able to withstand high temperatures. It seems simple to construct the calorimeter from some refractory material. However, problems may arise if its thermal conductivity is very low. It is then difficult to control the heat flow within the calorimeter if some form of adiabatic or isothermal condition needs to be maintained, which is further exacerbated if the reaction rates are fast. 

B) R.L. McKisson L.A. Bromley, "A New High-Temperature Calorimeter , USAEC (US Atomic Energy Commission) Report UCRL 688 (1950) (A calorimeter for use in the temp interval 600 to 1500° K contg a thermostat of molten tin surrounding a cavity in which the sample is placed. The calorimeter is resistance-heated, and control of the thermostat temp to lQ.5°C is effected by means of a modified "single-speed floating control . 

R.L. McKisson L.A. Bromley, "A New High-Temperature Calorimeter , USAEC Rept UCRL-688(1950) 32) F.D. Rofcsini,... 

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