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Simultaneous Recording of Multiple Cooling Curves Ronald A. Bailey, Sudhen B. Desai, Norbert F. Hepfinger, Henry B. Hollinger, Peter S. Locke, and Kenneth J. Miller Department of Chemistry, Rensselaer Polytechnic Institute, Troy, NY 12180 James J. Deacutis and Donald R. VanSteele Department of Materials Science and Engineering, Rensselaer Polytechnic Institute, Troy, NY 12180 Computers have become part of the equipment for many instructional laboratories, particularly for collecting, processing, and graphing data. Computers permit students to collect more data more rapidly than in traditional experiments, reducing drudgery and allowing experimental work to be combined more closely with discussion or lecture activities in a time period that would otherwise be assigned to purely laboratory work. The role of computers is also being expanded to include multimedia presentations, to provide sophisticated model building, to simulate pH meters and other electronic devices, to provide video–audio illustrations, and to allow students to control experimental parameters. The experiment described here is based on an assembly that permits simultaneous recording of six or more heating or cooling curves in 45 minutes or less— providing sufficient data for students to establish a complete phase diagram, with time for discussion. We have combined this with a computer prelab and data-handling modules, which are also described below. The experiment itself is the determination of the phase diagram for the tin–bismuth system. This is a relatively simple binary system with some solid solution, which can be adequately defined by measurements on a small number of concentrations. We felt that 6 were sufficient, but the apparatus could readily be redesigned for a larger number, depending on the number of input channels available on the data acquisition board in the computers. Because the apparatus does not permit stirring, the samples measured must have high thermal conductivity to give good results; consequently, metals are preferred. Traditionally, cooling curves are used to provide the temperature data for establishing the phase diagram. In this case, usable data can be obtained from heating curves as well. In fact, by using heating curves alone, it is possible to define the phase diagram in 20 min, making this experiment practical as part of a combined lecture–laboratory activity in a single period. The complete heating–cooling cycle can be completed in about 45 min. The Laboratory Situation The experiment described here is used in two firstyear courses, General Chemistry for science majors and Chemistry of Materials for engineering majors, which together comprise approximately 800 students. Students work in pairs, and 32 students work on a particular experiment. This experiment, including prelab exercises and data handling, takes less than two hours. The actual experimental work can be completed in 45 minutes. The computers (Gateway 2000 and IBM Valuepoint PCs) are 486 computers with 16 MB RAM networked to a Novell server and a StarLite video server. The software used for the experiment is Asymetrix Multimedia ToolBook, version 3.0, and National Instruments
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Figure 1: Diagram of experimental assembly.
LabVIEW for Windows, version 3.0.1. LabVIEW is used to control a National Instruments data acquisition card AT-MIO-16-L-9 connected to an external interface card (SC-2070) housed in a separate box. The National Instruments card, in turn, receives information from a relay box that collects voltage signals from six T-type thermocouples (Omega JTIN-116G-6) and filters the signals through low-pass filters. The experimental assembly is shown in Figure 1. Mixtures of tin and bismuth with mole percents equal to 0, 10, 30, 57 (eutectic), 80, and 100% bismuth are contained in six stainless steel tubes 0.5 in. in diameter and 3 in. long filled halfway. The tubes were made from stock stainless steel tubing by welding a stainless steel plug on the end. They are capped with 3/8-in. aluminum washers, which serve as covers and keep the thermocouples centered. The thermocouple tips were inserted into the cylinders through the centers of the washers and embedded in the mixtures. Since it is important that the thermocouple junction not contact the steel tube, the thermocouples are fixed to an upper spacer disc with a ring clamp, which is used to maintain both the depth and vertical alignment of the thermocouple tips as shown in Figure 1. The tubes are gathered around a cylindrical heater (Hotwatt HS50-4, 470 W, 120 V) 0.5 in. in diameter and 4 in. long. The sample tubes are held tightly around the heater by a metal hose-clamp. Thin layers of ceramic fiber insulation placed between adjacent cylinders reduce heat transfer between cylinders containing different samples. The heater is connected to a variable transformer adjusted to provide a good heating rate (a setting of 60 proved suitable). A split fiberglass cylinder cut from pipe insulation surrounds the assembly during heating, and is removed during the cooling step. The Laboratory Exercise Before coming to lab, the students will have read the lab manual for the experiment and will have answered some “prelab” questions. The laboratory session
Journal of Chemical Education • Vol. 74 No. 6 June 1997
Information • Textbooks • Media • Resources
Figure 2: LabVIEW front panel from the phase equilibrium experiment.
Figure 3: Sample cooling curve data plotted in Microsoft Excel.
begins with a brief review by a teaching assistant before the students start the ToolBook program and move through some preliminary screens that include safety warnings, a brief experimental outline, and a diagram of the apparatus. The diagram is shown in Figure 1. The students then play a TV-game-show-style game that reviews prelab material by requiring them to answer a certain number of questions related to the prelab before continuing with the experiment. When they achieve a passing score, they view a video clip of the melting metal and then ToolBook runs the LabVIEW program associated with this experiment. The LabVIEW screen shows the LabVIEW “front panel” that allows students to adjust experimental parameters and control data acquisition. Interested students can view a “back panel” that shows how the program is “wired up”. At the front panel, students can adjust scales for temperature data, time intervals, and graphs. Then they manually turn on the heater and click on the “start” button to begin the heating and data monitoring processes. As the samples heat up, the voltages are sent to the computer and translated into tempera-
tures. The evolving heating curves are displayed as colored curves in the graph on the front panel. When all the samples have reached 300 °C, the student is prompted by the computer to turn off the heater and the computer starts recording cooling-curve temperature data. Cooling curves are observed on the screen and the data are automatically transferred to a text data file (one temperature for each sample after each time interval). An image of the front panel with sample data is shown in Figure 2. During this time of heating and cooling the instructor can discuss expected results and the interpretation of the data with students. When the samples have cooled to below 80 °C, LabVIEW stops and tells the students that the experiment is complete, at which time the students close LabVIEW and open the Excel spreadsheet program. They call up the datafile that contains the data for the cooling curves, and simultaneously plot the data for two samples (it is hard to read more than two cooling curves on one). A sample cooling curve is shown in Figure 3. The cooling curves are printed and used to identify melting and freezing points for the samples. Supercooling blips in the curves make it especially easy to identify some of the freezing points. Students could print the graphs directly from the LabVIEW front panel and not use the Excel spreadsheet, but in Excel they can separate the graphs and get graphs with better resolution. Experimental Results Once hard copies of all of the cooling curves have been acquired, the student examines them and determines the freezing and melting points for each set of data. The pure samples of tin and bismuth plus the eutectic mixture all produce only one melting or freezing point. The remaining three mixtures all display both a freezing point and a melting point corresponding to the eutectic. Once these data are collected, they can be plotted to produce the tin–bismuth phase diagram. The experimental data for both heating and cooling curves along with the actual phase diagram (1) are shown in Figure 4. The data are reproducible between runs as well as between experimental setups. Literature Cited
Figure 4: Tin–bismuth phase diagram including cooling curve data, heating curve data, and literature data (1).
1. Binary Alloy Phase Diagrams, 2nd ed.; Massalski, T. B., Ed.; ASM International, 1990; Vol 1.
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