Investigations under quasi-isothermal and quasi-isobaric conditions by means of the Derivatograph

This paper examines the influence of residual pressures in the range from 10-~to 5 torr on the course of thermal analysis. With the help of examples concerning in particular the thermolysis of gibbsite, AI(OH)3 , it is shown that a) the control of residual pressure is of virtually no use unless the rate of decomposition is also controlled (otherwise, the TG curves represent a composite phenomenon, which is practically unintelligible); b) the influence of residual pressure may be unexpectedly high both on the shape of the TG curves (and therefore on the apparent kinetic parameters) and on the nature (porosity, structure) of the products.The influence of the gas atmosphere on a thermal decomposition of the type solid A ~ solid B + gas is widely accepted. In 1951, Rowland and Lewis [1] in this way explained the usual DTA trace for calcite thermolysis, where the onset of decomposition may be observed as soon as 650 ~ (low partial pressure of CO~) and where "the reaction progressively becomes more vigourous until a peak is reached at about 925 ~ the temperature at which the reaction would begin if the furnace had been filled with COz at one atmosphere pressure. In 1960, Garn suggested simplification of the situation by operating under self-generated atmospheres [2], and several devices were proposed and used in order to control the overall pressure, e.g. the "cold sink", especially convenient in the case of water evolution [3], or the discontinuous and automatic gas extractor [4]. Nevertheless, "vacuum" was often considered as one pressure at which it may be worthwhile to carry out one experiment in order to complement a set of data obtained under higher pressures. Indeed, various authors, such as Eyraud et al. at the time of the first vacuum TG experiments, pointed out the "simplifying" part played by vacuum-operation, which was supposed to eliminate disturbing phenomena such as adsorption or diffusion processes [5] or secondary reactions [6] and to give rise to the simplest mechanism with higher decomposition rate [7]. Now pressures lower than I torr, which are usually referred to as "vacuum", actually cover a wide experimental range, down to 10 -~ torr, whereas the range of medium and *

Section: Methodssupporting

confidence: 62%

Thermolysis under vacuum: Essential influence of the residual pressure on thermoanalytical curves and the reaction products

Rouquérol¹,

Ganteaume²

1977

“…The thermal decomposition processes were studied on a Q-derivatograph under quasi-equilibrium conditions [2] using different sample holders and with linear heating. The temperatures of decomposition under quasi-equilibrium conditions were used for qualitative estimation of the thermodynamic stabilities of the compounds investigated.…”

Section: Methodsmentioning

confidence: 99%

Study of the thermal decomposition processes of clathrate compounds

Gavrilova

Kislykh

Логвиненко

1988

The decompositions of the clathrate compounds [M(NCS)2(4-MePy)4 ]"nG (where M = Mn, Co, Ni or Cd; G = 4-methylpyfidine (4-MePy), benzene or xylenes) were studied on a Q-derivatograph under quasi-equilitlrium conditions and with linear heating. These clathrates can be divided into two groups, in which the loss of guest is either (I) accompanied by destruction of the host complex, or (II) occurs before decomposition of the host complex. Kinetic parameters were obtained.Due to their structures clathrate compounds are interesting subjects for the research of heterogeneous thermal reactions of the type As -* Bs + CgasIn this work we have studied the features of the decompositions of the clathrate compounds M(NCS)2(4-MePy)4nG, where M = Mn(II), Co(II), Ni(II) or Cd(II), and G ---= 4-methylpyridine (4-MePy), benzene or xylenes. ExperimentalSynthesis of the host coordination complexes was carried out by a known method [1]. The clathrate compounds were sysnthesized through the reaction between the host coordination complex (crystal compound) and the guest in the vapour state at room temperature.The thermal decomposition processes were studied on a Q-derivatograph under quasi-equilibrium conditions [2] using different sample holders and with linear heating. The temperatures of decomposition under quasi-equilibrium conditions were used for qualitative estimation of the thermodynamic stabilities of the compounds investigated.The kinetic curves of gas evolution were obtained by means of a continuous reactor [3] under non-isothermal conditions. The samples had a mass of 5-10 rag; the heating rate was 4-6 deg min -1 ; the helium flow rate was 60 cm a min -1 ; a conductometric detector was used. Experimental data were processed on a computer with the program TA IB [4]. For the compounds studied, the equation of a contracting sphere was the equation best describing the process. The values of E and A were calculated from 5 experiments; experimental errors were found with a confidence level of 95%.

“…We performed our experiments with a O.-derivatograph (Hungarian Optical Works, Budapest) using either a dynamic heating programme [10] (5 deg min-1 heating rate) or the quasi-isothermal-quasi isobaric technique [10][11][12] (0.6 mg min -1 rate of weight change). The measurements were made with different types of sample holders, in the presence of air, using about 600 mg of sample.…”

Section: Methodsmentioning

confidence: 99%

Thermogravimetric examination of the dehydration of calcium nitrate tetrahydrate under quasiisothermal and quasi-isobaric conditions

Paulik

Arnold

1983

Self Cite

The progressive nature of the process of dehydration of Ca(NO3) 2 -4 H20 has been shown not to be due to consecutive decomposition reactions, as believed so far, but is the result of various physical processes, such as the incongruent melting of the material, the boiling and evaporation of the unsaturated or saturated solution formed, as well as the crust formation and drying of the surface of the solidifying residue. The quasi-isothermal--quasiisobaric thermogravimetric technique and the phase diagram of the Ca(NO3)2--H20 system were used to evaluate the results.There are many contradictory data in the literature regarding the process of dehydration of calcium nitrate tetrahydrate [1][2][3][4][5][6][7][8][9]. The cause of this is that it is rather difficult to interpret the conventional thermoanalytical curves.The simultaneous TG, DTG and DTA curves in Fig. 1 indicate that the thermal decomposition of the hydrated salt under the given conditions takes place in four steps. Although the existence of Ca(NO3) 2 -3 H20 and Ca(NO3) 2 9 2 H20 is known, it is impossible that in the present case the formation of these intermediates causes the stepwise transformation. These intermediates decompose even above 52 ~ (Fig. 7), while the TG and DTG curves reveal that the four subsequent processes in question took place with maximum rate at 120 ~ 155 ~ 160 ~ and 210 ~ respectively. What then was the cause of the gradual water loss?We also observed that, even under identical experimental conditions, the courses of the curves could not be reproduced. Further, when the-exl~erimental conditions were varied in a set manner, the courses of the curves changed not in the expected way but irregularly and apparently randomly. Curves 2 and 3 in Fig. 2 show examples of this. These curves were obtained in covered and uncovered crucibles, respectively, but under otherwise identical conditions. The courses of these curves relative to each other are not regular. Despite the fact that the liberated water vapour can depart more easily from the uncovered crucible than from the covered one [10], the water loss ended at a higher temperature in the former crucible than in the latter one. How can this contradiction be explained?Only the results of our experiments carried out by using the quasi-isothermalquasi-isobaric technique could give answers to these questions.