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Constructing a High-Sensitivity, Computer-Interfaced, W Differential Thermal Analysis Device for Teaching and Research L. M. Martínez,* M. Videa, F. Mederos, and J. Mesquita Department of Chemistry, Tecnológico de Monterrey, Campus Monterrey, 64849 Monterrey, NL México; *
[email protected]
The increasing demands to characterize ever more materials, such as alloys, polymers, glasses, pharmaceuticals, and liquid crystals, has made clear the importance of including thermoanalytical techniques in the analytical chemistry curriculum. One particular case is differential thermal analysis (DTA), which provides significant information regarding structural changes, phase transitions, and chemical reactions through the detection of thermal effects produced by heat evolution or absorption. A review of the literature of the last 40 years shows several efforts to assemble homemade differential thermal analysis devices (1–5). Wiederholt (6, 7) described the simplest DTA design for basic teaching purposes. His model included an electrical soldering pot used as the oven, an aluminum cylinder that served as the cell holder, and glass tubes that functioned as the sample and reference cells. A chart recording apparatus with two channels was used to display the signals, recording the thermovoltage corresponding to the temperature of the sample and the thermovoltage difference between the sample and a reference; those signals were obtained using a simplified connection of thermocouples. Although this model provided the essential functions of a DTA instrument, it had some limitations; for example, it just detected enthalpic changes; it was not possible to determine the exact temperature corresponding to the endothermic and exothermic peaks (7). To overcome the design limitations of previous instrument assemblies reported in the literature it is important to meet the following requirements to obtain ideal functioning of a DTA instrument: 1. Temperature measurement precision must be within 0.1 °C. This can be achieved with an appropriate design of the cell holder and heating source, allowing an adequate balance between heating and heat dissipation. 2. Good thermal mass balance must be maintained between sample and reference. Sample amounts have to be carefully adjusted to minimize thermal inertia effects that may broaden the peaks or alter the baseline. 3. Allow for optimal thermal contact and thermal conductivity between the thermocouple tip and sample. 4. Since the thermovoltage and thermovoltage differences are small (∼0.001 mV), high-resolution data acquisition is necessary. Also, the signal-to-noise ratio has to be adjusted so that small signals can be detected; for example, the magnitude of the thermal signal produced during a glass transition process, a pseudo-second order transition, is usually small compared to that of a melting process (see the section on research applications), and a simple recorder might not have sufficient resolution. Fortunately, data acquisition boards with sufficient digital resolution are available at accessible prices.
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5. A simple and efficient way to handle and display the signal must be included. An excellent way to achieve this is through computer interfacing. The commercial software LabVIEW from National Instruments is suitable for this kind of programming. This software does not require previous knowledge of computer programming and the time to develop an application for acquisition, processing, and recording the data is very short compared to other computer languages like Visual C++ or Visual Basic (8–10).
All of these requirements were considered for the construction of the DTA instrument presented here. Constructing the Differential Thermal Analysis Device The principle of differential thermal analysis is the measurement of temperature and difference of temperature of a sample and a thermally inert reference during simultaneous heating or cooling under identical conditions. The design for the DTA setup presented here includes the construction of a computer-based instrument panel. This panel consists of a program written with LabVIEW, which, besides handling the data acquisition, also displays the measurements in real time. Users can customize the functionality of this panel as required by the measurement. Figure 1 shows the complete assembly of the instrument. A detailed description of the construction of this device can be found in the Supplemental Material.W The program controls measurement of temperature and difference of temperature between sample and reference, both displayed in real time on the computer screen. The heating rate during the measurement is also monitored to keep control of the experimental conditions, since the heating rate determines the peak definition. At the end of the experiments, a plot of difference in thermovoltage versus temperature is generated on the computer screen (Figure 2). The data are saved in ASCII code and can be analyzed in detail using graphing software such as SigmaPlot, GNUPlot, Origin, or Excel. This device is more affordable compared to commercial instruments, which makes DTA measurements accessible to undergraduate laboratories. This DTA setup also has sufficient sensitivity to be used for academic and research purposes. Experimental Results Construction and use of the instrument described in the previous section has been included in the physical chemistry undergraduate laboratory at our institution. Students are introduced to the LabVIEW software at the first lab session so they can become familiar with the programming and the software for the DTA. During the same session the theoretical basis of thermal analysis and the construction of the device is presented to them so they can understand the principles of the technique. In the second lab session, students build a
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Figure 1. DTA instrument components: (a) Computer with PCI-4351 board; (b) TBX-68T-NI connector; (c) thermocouple arrangement; (d) 300 W heating cartridges; (e) sample and reference cells; (f) aluminum heating block; (g) plug connector box; (h) variable voltage regulator (Variac).
Figure 2. Thermogram of cyclohexane showing solid–solid and solid– liquid transformations obtained with the instrument in Figure 1.
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binary phase diagram using the data they obtained through DTA runs of mixtures of different compositions. A document in the Supplemental MaterialW describes the procedure followed in this experiment and shows the experimental results corresponding to the phase diagram of naphthalene and pdichlorobenzene obtained by the students. The DTA instrument described here has also been used in various research projects to characterize different types of samples, including materials showing phase transitions below room temperature (see Figure 2) and corrosive substances that could damage expensive elements of commercial DTA instruments. Furthermore, given the instrument’s high sensitivity, it has also been used in the measurement of thermal processes as small in magnitude as a glass transition temperature belonging to sugar glasses used for the preservation of biomolecules relevant to the pharmaceutical industry. As an example Figure 3 shows experimental results for the determination of the glass transition temperature, crystallization temperature, and the melting temperature of a sucrose sample. Conclusion With the construction of the DTA instrument the students were introduced simultaneously to the concepts of phase transitions, binary phase diagrams, thermal analysis, graphic programming (LabVIEW) and interfacing. This is also a relevant example of using new computer technologies to teach experimental chemistry. The simplicity of the DTA assembly offers an excellent opportunity to introduce students to the construction of measurement devices, keeping in mind that regardless of the field of work (industry, research, or academia), chemists need to have the ability to install, implement, transform, and even construct devices to measure physical or chemical properties. It is therefore very important to offer undergraduate students the means to learn the essential components of digital data acquisition that are the working principles of modern devices. Furthermore, using LabVIEW to construct instrument panels that allow the display of experimental results in real time gives students more time to analyze and discuss results, compared to lengthy manual data acquisition in traditional physical chemistry experiments. WSupplemental
Material A detailed DTA device construction description and phase diagram notes are available in this issue of JCE Online. Literature Cited 1. 2. 3. 4. 5. 6. 7. Figure 3. Thermogram of the experimental determination of glass transition temperature (Tg), crystallization temperature (Tc) and melting temperature (Tm) for a sucrose sample. The difference in magnitude between the Tg and Tc or Tm signals proves the instrument’s high sensitivity. Insert is the magnified signal for glass transition process.
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8. 9. 10.
Borchardt, H. J. J. Chem. Educ. 1956, 33, 103–107. Scism, A. J. J. Chem. Educ. 1970, 47, 772–774. Malawer, E.; Allen, E. J. Chem. Educ. 1977, 54, 582–583. Wiederholt, E.; Plempel, M. Thermochim. Acta 1984, 77, 375– 382. Lötz, A. J. Chem. Educ. 1996, 73, 195–196. Wiederholt, E. J. Chem. Educ. 1983, 60, 431–434. Wiederholt, E.; Heinemann, St. Thermochim. Acta 1985, 94, 215–218. Drew, S. M. J. Chem. Educ. 1996, 73, 1107–1111. Gostowski, R. J. Chem. Educ. 1996, 73, 1103–1107. Bailey, R. A.; Desai, S. B.; Hepfinger, N. F.; Hollinger, H. B.; Locke, P. S.; Miller, K. J.; Deacutis, J. J.; VanSteele, D. R. J. Chem. Educ. 1997, 74, 732–733.
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