INFLUENCE OF EXPERIMENTAL CONDITIONS ON DATA OBTAINED BY THERMAL ANALYSIS METHODS Monika ŽALUDOVÁ, Bedřich SMETANA, Simona ZLÁ, Jana DOBROVSKÁ, Silvie ROSYPALOVÁ, Daniel PETLÁK, Ivo SZURMAN, Alice ŠTVRTŇOVÁ Faculty of Metallurgy and Materials Engineering, VŠB-Technical University of Ostrava, 17. listopadu 15, CZ 708 33, Ostrava - Poruba, Czech Republic, monika.zaludova@vsb.cz Abstract Differential thermal analysis (DTA) and Differential Scanning Calorimetry (DSC) are two of the methods that are suitable for obtaining thermo-physical and thermo-dynamical data. This paper deals with the influence of the experimental conditions settings of the used DTA and DSC methods in relation to the obtained data. The paper is focused on the study of heating/cooling rate and mass influence of the sample on shifting of the phase transformations temperatures and size of the thermal effects of alpha-gamma, gamma-delta phase transformations and melting of pure Fe. Sample mass and heating/cooling rate belong to the frequently and easily changing parameters and their influence on the obtained data is substantial. The study was aimed both at low temperature and high temperature regions. DTA measurements were realised using the Setaram Setsys 18 TM laboratory system, and DSC measurements using the Setaram MHTC laboratory system. The samples of pure Fe (with different mass) were analysed at several controlled heating/cooling rates. On the basis of adjusted experimental conditions (different heating/cooling rates and different masses of samples), the experimental data set was obtained and corresponding dependences were derived. Using the corresponding dependences the corrections can be applied to minimise the influence of experimental conditions on studied data for investigated alloys (samples). Application of derived dependences can substantially contribute to lowering (minimising) of the experimental conditions influence and enables an extrapolation to the so called zero mass and zero heating rate. Keywords: DTA, DSC, experimental conditions, heating/cooling rate, sample mass 1. INTRODUCTION Thermo-physical and thermo-dynamical properties of Fe based metallic systems are still a subject of extensive research. In spite of that the number of experimental data about these systems is still insufficient. Important data are for example temperatures [1] and latent heats of phase transformations [2], specific heat [3, 4], surface tensions [5] and others. Although it is possible to find in available literature the values of some of the above physical quantities, differences exist even between the available data. These differences may be caused also instead of others, by use of different experimental conditions of measurements at one method. The paper deals with possibilities of elimination of some influences at experiments. Differential Thermal Analysis (DTA) and Differential Scanning Calorimetry (DSC) are methods suitable for obtaining of thermo-physical data about metallic systems. These methods make it possible to obtain some thermo-dynamical and thermo-physical data, such as temperatures and latent heats of phase transformations. Using the DSC method it is possible to measure heat capacities and their variation with temperature as well. Experimental conditions may have a substantial influence on thermo-dynamical and thermo-physical data obtained by these methods. It is possible to include among experimental conditions for example temperature mode (heating x cooling), rate of heating/cooling, mass of the analysed sample, type and purity of atmosphere surrounding the sample during analysis, reference sample, kind of crucible.
Mass of the sample and heating rate are the most frequently changing experimental conditions. The heating rate and mass of the sample should be chosen with regard to the specific behaviour of the sample during the thermal analysis. It is necessary to take into account namely the possible change of the chemical composition during the analysis [3, 6] and the shape of the DTA curves (size and position of the peaks) [7]. This paper is focused on investigation of influence of the mass of analysed samples and of the heating rate on shifting on the phase transformation temperatures and thermal effects size of phase transformation of iron sample, obtained by the DTA and DSC methods. 2. THERMAL ANALYSIS Differential thermal analysis and Differential Scanning Calorimetry are dynamic thermal analytical methods used for investigation of temperature effects of investigated sample connected with its physical, chemical or physical-chemical changes during its continuous linear heating or cooling [8]. DTA method is used for measurement of temperature differences between the investigated and reference sample, DSC method is used for measurement of heat flow differences between the investigated and reference sample. Temperature of the reference sample follows the selected temperature program, temperature of the investigated sample is subject to changes, which reflect physical and chemical transformations occurring in the sample. These methods makes it possible to express all physical, chemical or physical-chemical changes that are accompanied by sufficiently big change of enthalpy [9]. 3. EXPERIMENT 3.1 Material Samples of iron were made from powder iron with purity of 98 wt. % Fe. Tablets were pressed from powder iron. The tablets were then plasma re-melted. Iron was during plasma melting also repeatedly purified. Chemical composition of the analysed iron is given in Table 1. Samples for the analysis were prepared in the form of cylinders with diameter of 3.5 mm (DTA) and with diameter 5mm (DSC). Mass of cylinders was approximately 82, 153, 219, 318, 459 mg (DTA) and 379, 612, 1071, 1695, 2353 mg (DSC). The samples were prior to the thermal analysis ground off and cleaned in acetone with use of ultrasound. Table 1. Chemical composition of the analysed iron in wt. % Sample (S) C Si Mn P S Cr Ni Mo Co V W O Fe Fe (DTA) 0.015 0.005 0.042 0.002 0.006 0.004 0.015 0.010 0.011 0.022 0.011 0.110 99.90 Fe (DSC) 0.006 0.017 0.081 0.005 0.008 0.004 0.020 <0.002 0.013 0.041 <0.010 0.015 99.80 3.2 Experimental equipments Laboratory system for thermal analysis Setaram Setsys 18 TM, measuring rod DTA and thermocouple of the type S (Pt/PtRh 10%) were used for obtaining of temperatures and thermal effects of phase transformations. The thermocouple was ended by three thermocouple connections, which are located below the bottom of the measuring crucible. The samples were analysed in corundum crucibles with volume of 100 µl. An empty corundum crucible served as a reference sample. Dynamic atmosphere of Ar (purity>6n) was maintained in the furnace during the analysis in order to protect the sample against oxidation. Each sample was analysed at the controlled rates of heating: 1, 2, 5, 10 and 20 K/min. Laboratory system for thermal analysis Setaram MHTC 96, measuring rod 3D DSC and thermocouple of the type B (PtRh 6%/ PtRh 30%) were used for obtaining of temperatures and thermal effects of phase transformations. The thermocouple consists of 20 thermocouple connections, which surround the crucible walls (3D DSC sensor). The samples were analysed in corundum sleeves inserted into platinum crucibles with volume of 400 µl. An empty platinum crucible with corundum sleeve served as a reference sample. Dynamic atmosphere of He (purity 6N) was maintained in the furnace during analysis in order to protect the
sample against oxidation. Each sample was analysed at the controlled rates of heating: 2, 3, 5, 7 and 10 K/min. Altogether 25 measurements were thus performed on each device. 4. RESULTS AND DISCUSSION Temperatures and thermal effects of alpha-gamma and gamma-delta phase transformations and melting were obtained on the basis of evaluation of DTA and DSC curves. It is possible to determine from the DTA and DSC curves of Fe plasma (heating) the Curie point temperatures (T C ), the start of the transformation α γ (T α γ, s ), the end of the transformation α γ (T α γ, e ), the start of the transformation γ δ (T γ δ, s ), the end of the transformation γ δ (T γ δ, e ), solidus (T S ) and liquidus (T L ), as well as thermal effects of individual phase transformations. Figure 1 shows a DSC curve with marked temperatures and thermal effects of phase transformations. Fig.1. DSC curve For each obtained temperature and thermal effect a table was compiled containing the resulting values from all 25 measurements (for each method separately). Due to the large amount of data it is impossible to present all the obtained temperatures and thermal effects (contained in 20 tables). Table 2 presents for clarity the liquidus temperatures obtained by the DSC method. The obtained data will be described in the text. Average values of the determined temperatures are given in Table 3, those of thermal effects in Table 4. Table 2. Experimentally obtained liquidus temperatures (3D DSC) in C Heating rate Sample mass 379.75 mg 612.58 mg 1071.61 mg 1695.21 mg 2353.65 mg 2 K/min 1542.5 1543.9 1544.5 1545.4 1546.7 3 K/min 1545.1 1546.0 1547.2 1547.6 1549.1 5 K/min 1548.4 1549.8 1551.3 1551.9 1553.0 7 K/min 1551.0 1552.6 1554.3 1555.5 1556.6 10 K/min 1553.9 1556.4 1558.7 1560.5 1561.3 Table 3. Average value of phase transformation temperatures in C DTA 3D DSC T C 770.4 772.6 T α γ,s 917.5 917.9 T α γ,e 926.0 922.4 T γ δ,s 1391.6 1393.0 T γ δ,e 1396.1 1398.0 T S 1525.4 1533.6 T L 1542.2 1550.9 Table 4. Average values of thermal effects in μv.s DTA 3D DSC α γ 1246.951 4832.3 γ δ 700.524 4562.8 Melting 10592.19 69090.1
4.1 Influence of the mass of the sample on shifting of temperatures The mass of the sample affects the temperature of the Curie point and the liquidus temperature both in the DTA and DSC methods. With the increasing mass of the sample the Curie point temperature and liquidus temperature shift to the higher values. The mass has greater influence on shift of the liquidus temperature than on the temperature of the Curie point. The Curie point temperature shifts with the increasing mass of the sample at maximum by 2.3 C (DSC 3D), or by 2.4 C (DTA), while the liquidus temperature shifts in dependence on the increasing mass even by 7.4 C (DSC 3D), or by 7.9 C (DTA). A slight shift of temperatures with the increasing mass was observed also at the temperatures of the end of γ δ transformation. The trend of the temperature shift was, however, irregular and it was observed at the temperature determined by the DSC method only at higher heating rates (> 5 C/min). The shift to higher temperatures with the increasing mass can be explained by the fact that the larger sample requires more heat for the phase transformation, and that the phase transformation takes place over a longer period of time (i.e. that phase transformation of the larger sample will be terminated at higher temperature). The mass of the sample does not have substantial influence on shifting of temperatures of the starts of transformations (corresponding to the temperatures of starts of the peaks). No trend in the temperature shift was observed in dependence on the increasing mass of the sample. The differences between the values may be caused by the difficult identification of the temperatures of starts of peaks on the DTA and DSC curves. Temperatures of the starts of peaks can be determined with certainty only in certain temperature range (of the order of centigrades). 4.2 Influence of heating rate on shifting of temperatures Heating rate of the samples affects all temperatures of phase transformations obtained by the DSC method. A shift to the higher temperatures occurs with the increasing heating rate. Influence of the heating rate is more significant at the temperatures corresponding to the tops of the peaks (T α γ, e, T γ δ, e, T L ) than at temperatures the corresponding to the temperature of starts of the peaks (T α γ, s, T γ δ, s, T S ). The greater the thermal effect of phase transformation, the greater the effect of the heating rate on the temperature shift. It follows from the above that the heating rate has the greatest influence on shift of the liquidus temperature. In the DTA method, the influence of the heating rate shows at the temperatures corresponding to the tops of the peaks. The temperatures are shifted with the increasing heating rate to the higher values, similarly as in the case of the DSC method. The shift of temperatures to higher values with the increasing rate can be explained in both methods by dynamics of the process and by detection capabilities of instruments. In the temperatures corresponding to the temperatures of starts of the peaks no trend was observed in the shift of temperatures in dependence on the heating rate. This may again be caused by the difficult identification of the temperatures of starts of the peaks. Table 5 summarises, which of the experimental conditions influences the given temperature (SM-sample mass, HR-heating rate, N-not influenced). On the basis of the observed influences graphs of dependences were plotted for individual temperatures on the basis of the mass of the sample and the heating rate. The dependence of shift of the obtained temperatures on the mass of the sample is in all cases linear. The dependence of the obtained Curie point temperature on the heating rate is also linear. The dependence on the heating rate for the temperatures of the end of γ δ transformation and for the liquidus temperatures is expressed as a polynomial of the 2 nd order. It is not easy to determine clearly for other temperatures whether the dependence is linear or polynomial. For this reason, the values were smoothed both by linear and polynomial dependence. Both dependences showed very close reliability value R 2, and the resulting temperatures obtained by extrapolation to the "zero" heating rate by individual dependences are also very close (the maximum difference is 0.6 C).
Table 5. Summary of influence of experimental conditions on the obtained temperatures DTA 3D DSC T C SM, HR SM, HR T α γ,s N HR T α γ,e HR HR T γ δ,s N HR T γ δ,e SM, HR HR T S N HR T L SM, HR SM, HR Table 6. Average value of phase transformation temperatures after correction in C DTA 3D DSC T C 768.3 767.7 T α γ,s 917.5 914.2 T α γ,e 919.9 916.4 T γ δ,s 1391.6 1390.3 T γ δ,e 1391.6 1392.6 T S 1525.4 1531.5 T L 1533.0 1537.0 On the basis of the obtained dependences of the shift of temperatures on the heating rate and on the mass of the sample, it is possible to extrapolate the experimentally obtained temperatures to the so-called "zero heating rate" or the so-called "zero mass" of the sample [10]. Thus corrected temperatures should be therefore very close to the equilibrium temperatures [11]. The temperatures obtained after extrapolation to the "zero" conditions are listed in Table 6. It is evident form comparison of Tables 3 and 6 that the extrapolation reduced the difference between the temperatures obtained by the DTA and DSC methods. The temperature interval of individual transformations was also shortened in both methods. 4.3 Influence of the mass of the samples on magnitude of thermal effects In thermal effects obtained by both methods the magnitude of the thermal effect increases with the increasing mass of the sample. The bigger the sample, the more heat must be added for realisation of phase transformation. Influence of the mass of the sample on the magnitude of the thermal effect is considerable. In the DSC method the mass of the biggest sample is approximately six times bigger than the mass of the smallest sample. The magnitude of the thermal effect also increases proportionally to that. Thermal effect of the biggest sample is for in phase transformations approximately six times bigger than thermal effect of the smallest sample. In the DTA method the trend of increasing thermal effect is not so regular as in the DSC method. The biggest sample is also approximately six times bigger than the smallest sample, but the thermal effect is only 2.5 to 3.5 times bigger than the thermal effect of the smallest sample. Influence of the mass of the sample can be partially eliminated by relating the thermal effect to the mass of the sample (μv.s/mg). 4.4 Influence of the heating rate on magnitude of thermal effects No trend in changes of the magnitude of thermal effect was observed in dependence on the increasing heating rate in the DTA method. In the DSC method a slight decrease of the magnitude of thermal effect was observed for transformations α γ transformation and for melting in dependence on the increasing heating rate. In the transformation γ δ no trend in changes of magnitude of thermal effects was observed. Influence of heating rate on the magnitude of the obtained thermal effect can be partially eliminated by an enthalpy calibration. The calibration is performed with use of standard metals at the same heating/cooling rate as the analysis of studied sample. An extrapolation of the sizes of thermal effects to the "zero" heating rate is not required at performing of the enthalpy calibration. 5. CONCLUSIONS The influence of experimental conditions, of the mass of the sample and of the heating rate on the temperatures of phase transformations and on magnitude of thermal effects of phase transformations was investigated on the iron samples with use of the experimental equipment Setaram SETSYS 18 TM and method DTA, and the device Setaram MHTC 96 and DSC method.
It follows from the obtained results that the mass of the sample and the heating rate affect majority of the obtained temperatures of phase transformations. A summary of the monitored parameters is presented in Table 5. At the given experimental arrangement the heating rate has greater influence on the shift of the phase transformation temperatures than the mass of the sample for both devices. On the basis of dependences that express the influence of the mass of the sample and of the heating rate on the shift of the phase transformation temperatures, the temperatures were extrapolated to the so-called "zero" mass and "zero" heating rate. As a result of this extrapolation the difference between the temperatures determined by the DTA method and the temperatures determined by the DSC was reduced. The extrapolation also shortened the temperature interval of phase transformations. It is also evident from the obtained results that the degree of the influence of experimental conditions (the mass of the sample and the heating rate) on the obtained results is different at each device (method), i.e. that different dependences of the influence of the mass of the sample and of the heating rate on the data are obtained. These differences are mainly due to different design of devices (experimental arrangement). It follows that the determination of the influence of experimental conditions on the obtained data should be performed on each device before the actual measurement. This may thus help to set the optimum experimental conditions at measurement for obtaining the phase transformation temperatures with minimal influence of experimental conditions. On the basis of the obtained dependences that express the influence of the mass of the sample and of the heating rate it is possible to correct the data obtained for other investigated Fe-based metallic systems, such as steels, to the "zero" mass and "zero" heating rate. We can thus obtain for the investigated systems the temperatures close to equilibrium temperatures, without time-consuming experimental measurements. It follows from the obtained thermal effects that the mass of the sample has a significant influence on the magnitude of thermal effects. The influence of the mass of the sample can be partially eliminated by relating the thermal effect to the mass of the sample. The heating rate does not have a significant effect on magnitude of the thermal effect. Influence of the heating rate can be reduced by an enthalpy calibration. ACKNOWLEDGEMENTS The work was carried out within the scope of the project of the Czech Science Foundation (P 107/11/1566) and the project No. CZ.1.05/2.1.00/01.0040 "Regional Materials Science and Technology Centre" within the frame of the operation programme "Research and Development for Innovations" financed by the Structural Funds and from the state budget of the Czech Republic and of the project of the Technology Agency of the Czech Republic (TA 03011277). LITERATURE [1] SMETANA, B., et al. Application of High Temperature DTA to Micro-Alloyed Steels. Metalurgija, 2012, vol. 51, no. 1, p. 121-124. [2] SMETANA, B., et al. Latent heats of phase transitions of FE-C based metallic systems in high temperature region. In METAL 2011: Proceedings of 20th International Metallurgical and Materials Conference, 2011 Brno. Czech Republic. Ostrava: TANGER, spol. s r.o., 2011. p. 486-491. [3] SMETANA, B., et al. Experimental verification of hematite ingot mould heat capacity and its direct utilisation in simulation of casting process. Journal of thermal analysis and calorimetry. 2013, DOI 10.1007/s10973-013-2964-z [4] RAJU, S., et al. Drop calorimetry studies on 9Cr-1W-0.23V-0.06Ta-0.09C reduced activation steel. International Journal of Thermophysics, 2010, vol. 31, no. 2, p. 399-415 [5] ROSYPALOVÁ, S., DUDEK, R., DOBROVSKÁ, J. Influence of SiO 2 on interfacial tension between oxide system and steel. In METAL 2012: Proceedings of 21th International Metallurgical and Materials Conference, 2012 Brno. Czech Republic. Ostrava: TANGER, spol. s r.o., 2012. s. 109-114.
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