An adiabatic calorimetry was used for some investigations of equilibrium and non-equilibrium phase transitions. For one of
the substances studied (4,4′-di-n-heptyloxyazoxybenzene) it was possible to determine temperature dependence of an order parameter and number of clusters of
high temperature phase in a region of a phase transition. For another substance (liquid 3,4 dimethylpiridine) an anomaly on
the specific heat curves was interpreted as being responsible for a decay of molecules’ clusters.
Non-equilibrium phase transitions were investigated for some liquid crystal substances. The process of transformation between
metastable and stable phases was described quantitatively. The conclusions obtained concern the stability of metastable phases.
Monoclinic (I) and orthorhombic (II) polymorphs of paracetamol were studied by DSC and adiabatic calorimetry in the temperature
range 5 - 450 K. At all the stages of the study, the samples (single crystals and powders) were characterized using X-ray
diffraction. A single crystal → polycrystal II→ I transformation was observed on heating polymorph II, after which polymorph
I melted at 442 K. The previously reported fact that the two polymorphs melt at different temperatures could not be confirmed.
The temperature of the II→I transformation varied from crystal to crystal. On cooling the crystals of paracetamol II from
ambient temperature to 5 K, a II→ I transformation was also observed, if the 'cooling-heating' cycles were repeated several
times. Inclusions of solvent (water) into the starting crystals were shown to be important for this transformation. The values
of the low-temperature heat-capacity of the I and II polymorphs of paracetamol were compared, and the thermodynamic functions
calculated for the two polymorphs.
Traditionally, the kinetic treatment of adiabatic calorimetry data has been based on the results of one or more experiments, but always with the assumption of the kinetic model that the reaction follows to calculate the kinetic parameters. In this paper a method for the determination of the activation energy that uses a set of adiabatic calorimetry data is developed. To check the method, the thermal decompositions of two peroxides were studied.
Summary Crystal structures of the room-temperature (RT) and low-temperature (LT) phases of p-methylbenzyl alcohol were reexamined by single-crystal X-ray diffraction method while paying special attention to detect structural disorder in the RT phase involved in successive structural phase transitions at 179 and 210 K. In the RT phase at 250 K, positional disorder of oxygen atoms was detected in contrast to the previous structure report. The structure of the LT phase coincided to the previous one. Heat capacities were measured by adiabatic calorimetry below 350 K, which covers the structural phase transitions and fusion at 331.87 K. The structural phase transitions were of first-order and required long time for completion. The combined magnitude of entropies of transition was ca. 5 J K-1 mol-1, a part of which can be ascribed to the positional disorder observed in the structure analysis. Standard thermodynamic functions are tabulated below 350 K.
Adiabatic calorimetry is a technique that has been introduced as an important approach to hazard evaluation of exothermically reactive systems. In this paper the free radical polymerization of methyl methacrylate (MMA) has been studied. One of the most important aspects of MMA polymerization is its exothermicity and autoaccelerating behaviour, these characteristics can generate the occurrence of a runaway reaction.In a runaway situation the reacting system is close to adiabatic behaviour because it is unable to eliminate the heat that is being generated. An even worse situation can be reproduced in the laboratory with the Phi-Tec pseudo-adiabatic calorimeter. Process design parameters that are usually calculated from thermodynamic data or using semiempirical rules, such as adiabatic temperature rise or maximum attainable pressure, can be directly determined.The existence of the ceiling temperature has been experimentally demonstrated.
Molar heat capacities
of acetaminophen were precisely measured with a small sample precision automated
adiabatic calorimeter over the temperature range from 80 to 330 K. A solid-solid
transition at 149.96 K was found from the Cp,m-T curve. The polynomial functions of Cp,.m(J
K-1 mol-1) vs. T were established
on the heat capacity measurements by means of the least square fitting method.
processes of acetaminophen have been studied by thermogravimetry. And the
thermal decomposition kinetics parameters, such as activation energy E, pre-exponential factor A
and reaction order n, were calculated by
TG-DTG techniques with the Freeman-Carroll method, Kissinger method
and Ozawa method. Accordingly the thermal decomposition kinetics equation
of acetaminophen is expressed as: dα/dt=2.67107e-89630/RT(1-α)0.23.
The process of fusion has been investigated through
DSC. The melting point, molar enthalpy and entropy of fusion are to be (441.890.04)
K, 26.490.44 kJ mol-1 and 59.801.01
J K-1 mol-1,
This paper explains why directly agitated test cells are sometimes required in order to obtain good adiabatic calorimetry data that can be used with confidence to predict large scale plant behaviour. Experiments for methyl methacrylate polymerisation are reported. Simple procedures are presented for calculating genuine thermo-kinetic parameters from data which includes energy dissipation from the stirrer drive system.
The temperature dependence of heat capacity of a natural zinc silicate, hemimorphite Zn4Si2O7(OH)2·H2O, over the temperature range 5–320 K has been investigated by the method of low-temperature adiabatic calorimetry. On the
basis of the experimental data on heat capacity over the whole temperature interval, its thermodynamic functions Cp(T), S(T) and H(T) − H(0) have been calculated. The existence of a phase transition in the area of 90–105 K determined on the basis of vibrational
spectra has been confirmed, and changes of entropy ΔStr. and enthalpy ΔHtr. of the phase transition have been calculated. Hemimorphite heat capacity has also been determined by the calculation methods
according to the valence force field model in LADY program. The values of force constants of valence bonds and angles have
been calculated by semi-empirical method PM5. The calculated IR and Raman spectra concordant with the experimental spectra
have been obtained. The heat capacity values calculated according to the found vibrational states satisfactorily agree with
those experimentally obtained with an accuracy of ±1.7% in the area of 120–200 K, and not more than ±0.8% for the interval
of 200–300 K. This fact testifies that the calculation of thermodynamic characteristics is correct.
A procedure is described for dealing with the error sources inherently present in any real calorimeter: work of powerPs input from stirrer and possibly temperature sensor, and heat exchange at a rate −G(T−Te) whereT andTc are the temperatures of calorimeter and surroundings respectively. The constantsPs andG are calculated from a period of thermal decay, and afterwards are used to correct the entire run. A calorimeter was designed
with high thermal homogeneity and used in a test. The curve of calculated temperature exactly traces the heater energy, even
after 5 h, with a standard deviation of about 1 mK. The relative error inCp is less than 1/1000.