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Section 8. Physics
DOI: http://dx.doi.org/10.20534/AJT-17-1.2-114-117
Nadareishvili Malkhaz, Tbilisi State University, Institute of Physics, senior researcher E-mail: malkhaz.nadareishvili@tsu.ge Kiziria Evgeni, Institute of Physics, Leading researcher E-mail: evgenikiziria@hotmail.com Sokhadze Viktor, Institute of Physics, senior researcher E-mail: vsokhadze@yahoo.com Tvauri Genadi, Institute of Physics, Engineer E-mail: gena_tvauri@yahoo.com Tsakadze Severian, Institute of Physics, senior researcher E-mail: ztsakadze@gmail.com
Differential calorimeter of a new type
The work was supported of Sh. Rustaveli Foundation grant № FR/500/6-130/13
Abstract: Discussed new type differential calorimeter, which uses impulse heating of cells during the scanning across the temperature, instead of continuous, which is commonly used in differential calorimetry. Pulse heating allows combine high speed of warming with measurements in equilibrium conditions, which strongly enhances the sensitivity and accuracy of the measurement of this installation. Keywords: calorimeter, a differential calorimeter, continuous heating, pulse heating.
Introduction the increase in temperature AT is measured. The heat
Calorimetric studies play an important role for so- capacity of the sample is calculated by the formula:
lution many problems existing in modern science and C = AQ/AT. (1)
technology. These devices of different kinds are widely The differential calorimeter consists of two cells
used in physics, chemistry, biology, materials science, equipped with heaters and a thermometer and connect-
medicine etc. [1-3], including for diagnosing cancer [4]. ed via thermocouples. In the cells, two samples, reference
Modern calorimeters are divided into two basic groups: and analytical, are placed [3].
classical calorimeters, measuring the absolute heat ca- Equal power is applied to both samples, but they are
pacity of bodies in the impulsive regime and differential heated differently because of different heat capacities of
scanning calorimeters (DSC) measuring the heat capac- the samples. Leveling of heating rates happens with the
ity difference between the research sample and the stan- help of the heat that transfers from one cell to another via
dard in the continuous heating regime. the thermocouples and temperature difference ST be-
The classical calorimeter consists of a single cell with tween them. The difference between heat capacities is
a heater and a thermometer, where a sample is placed. A calculated by the formula:
certain amount of heat AQ is supplied to the sample, and AC = 2 AP/(AT/ At), (2)
where AP is the power transferring from the one cell to another and AT/At is the heating rate of the samples. In the experiment, the heating rate AT/At is calculated by measuring the temperature and time with the help of a thermometer and a timer, respectively. From the formula (2), it is evident that, the higher is the heating rate, the more power AP transfers from one cell into another. This means that, the smaller is the difference between heat capacities, the higher heating rate must be used to make the measurement of AP possible. At the same time, temperature gradients emerge in the sample, and the measurement becomes non-equilibrium, which causes mismatching of the measured and real values. Incompatibility of the high heating rate with the providing the measurement in equilibrium conditions, which does not allow studying many "fine" effects, is main important disadvantage of modern differential calorimeters [5].
It should be noted that DSC are characterized by higher sensitivity to the heat effects and by comparatively shorter time of measurements (that can be regulated by scanning rate) than the classical calorimeters, and they are used more widely [1]. That is why it is very important to eliminate all drawbacks of these calorimeters.
To eliminate the above-mentioned drawback of differential calorimetry, it was elaborated a new method the novelty of which consists in the use of pulse heating mode [6] instead of continuous heating, which is usually used in modern differential calorimeters. The high heating rate is achieved within a short, but powerful, pulse. At the same time the measurement proceeds under equilibrium conditions, because, at the beginning and at the end of the pulse, the samples are under equilibrium conditions.
Results and discussion
Based on the above-described pulse method, the differential calorimeter of a new type, a differential pulsed calorimeter (DPC), was designed. The main part of this calorimeter is differential container (fig.1). It consists of two identical cells (1), in which a sample and a standard (5) are placed. The cells are connected via thermocouples (3) that measure the temperature difference between the cells and, at the same time, provide the required thermal link between them. Before applying a heat pulse of At duration and after a certain time of relaxation t>> At, the sample under study and the standard are in thermal equilibrium and their temperatures are similar. During the entire process of measurement, the differential container is thermally isolated by radiation shields (6), and the process proceeds under adiabatic conditions.
Fig. 1. Container of the differential calorimeter: 1 - Cells; 2 - Heaters; 3 - Thermocouples;
4 - Thermometer; 5 - Samples; 6 - Radiation shields
Measuring the difference between heat capacities, similar heat pulses Q = IUAt are applied to both cells by means of electric heater (2), where I is the current in heaters, U is the voltage at the heater ends, and At is the pulse duration, are applied to both cells by means o fan electric heater (2). Due to the difference between the heat capacities of the samples, they are heated differently, the difference between the temperatures of cells emerges, and a certain amount of heat AQ transfers from one cell to another. Thus, after switching off of the heaters, after a certain time t, which is called the relaxation time, the cells come into equilibrium again. During the whole process, the difference between the cell temperatures is registered by thermocouples located between the cells. A typical time dependence of emf of the thermocouples has the shape shown in fig. 2. As was shown in work [6], the difference between heat capacities of samples is calculated by the equation:
AC = 2 AQ/AT, (3)
where AT is the increase in the temperature, and AQ is the amount of heat transferred from one call into another, being proportional to the area under the curve in fig. 2 and calculated by integration of this curve over time.
t, t2 t3 t
Fig. 2. The dependence of the emf of thermocouples versus time during a warm-Impulse
The cryostat of the calorimeter allows measuring the Heat capacity from 1.5 K. For this purpose, it is equipped by a special chamber where liquid helium is pumped off. Liquid helium is supplied into the chamber through a special throttle. Parts of the container are made with high precise, the difference between the weights of the cells is less than a milligram.
The design of the cryostat, in combination with the electronics, provides high level of adiabatic conditions of differential container. This aspect is very important because the difference ofheat flows to the cells should be less than the sensitivity of calorimeter (otherwise it will not be possible to use the maximum sensitivity). To solve this problem, some special technological and methodological measures was taken: to decrease the heat flows in the conducting wires, sapphire contacts will be used, which, at present, are considered as the best removals of heat flows from wires, to decrease the transfer of heat by conductors to the cells, the thinnest (0.03 mm. in diameter) golden wires with tungsten core was used, that are characterised by a good breaking strength at small diameters, by a rather poor heat conductivity and rather good electric conductivity. A special scheme of connection of wires with cells was used providing maximum similar values ofheat flows, etc. To protect the cells against heat radiation, the gold-plating ofcells and basic parts will be made. The design of cryostat provides the cooling of samples to the working temperature under the high-vacuum condition, this provides by using the special distribution of turn-off systems of thermal links. For the differential container the mine problem is the providing of the maximum precision of temperature difference measurements between cells and very small thermal conductivity of the thermocouples defining the sensitivity and precision of the calorimeter. For this aim our original technology of production of thermocouples from a great number (~400 pcs) of thermoelectric couple for differential container was used. One should emphasize that the thermocouples are the most important element of the device and such a great number of thermoelectric couple in it provides a high sensitivity of the calorimeter. Most the work associated with the cryostat is performed under the microscope.
As for the electronics, first of all, the proportionalintegral-differential amplifiers was created, which was assembled on the modern microcircuits and provide such
regulations ofheat screen temperatures that heat flows in the cells are very small. In the measuring part of electronics the elements of high accuracy was used to provide a small error in measurements. The program of automatic control of calorimeter controls the experiments and the measuring units, creates the database and displays the obtained graphical results on the monitor.
The sensitivity of the pulsed differential calorimeter at low temperatures is 10-8 W/div. (1 div=0.01 ^V) and the precision SC/ C is 10-4, where SC is the error of measurement of AC, and C is the absolute heat capacity of sample.
The device is multifunctional and wide-range, it could be used for precision studies of solid state, liquids, and powders in the wide range oftemperatures (1.5 k - 400 k).
The calorimeter is able to operate in two modes of heating: in the pulse and continuous, similar to the usual DSC. Such combination of modes makes it possible when it is necessary and it is not required a high precision or at carrying out the search experiments to carry out fast measurements in the continuous mode, to establish curves of heat capacity dependence on temperature, which frequently differs from real curves, according to our experience, but in difference with usual DSC, in our case they could be corrected using exact values of heat capacities obtained at several temperatures in pulsed mode.
Simultaneously with AC, the absolute heat capacities of studied and reference samples C1 and C2, respectively, can be measured with the same accuracy as in the case of calorimeter measures absolute heat capacity of sample. The new calorimeter created by us is very effective also in case of measurement of heat capacity of a small sample (several milligrams of mass) in the equilibrium conditions. In this case, the investigated sample should be placed in one cell, and other cell remains empty. The difference of heat capacities between cells, measured in pulsed mode, would present the absolute heat capacity of sample.
Conclusion
Based on the pulse method developed by us for differential calorimetry, the differential calorimeter of a new type was designed. The calorimeter is highly sensitive and accurate over a wide temperature range, and shows high possibilities of pulse heating in differential calorimery.
Acknowledgments
The authors express their gratitude to Prof. J. Monas-elidze for very useful discussions.
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