An Undergraduate Experiment Using Differential Scanning

Dec 22, 2011 - A homogeneous alloy with a unique composition having the lowest possible melting point is called a eutectic. A binary eutectic alloy is...
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Laboratory Experiment pubs.acs.org/jchemeduc

An Undergraduate Experiment Using Differential Scanning Calorimetry: A Study of the Thermal Properties of a Binary Eutectic Alloy of Tin and Lead Ronald P. D’Amelia,* Daniel Clark, and William Nirode Department of Chemistry, Hofstra University, Hempstead, New York 11549, United States S Supporting Information *

ABSTRACT: An alloy is an intimate association of two or more metals, with or without a definite composition, which has metallic properties. Heterogeneous alloys, such as tin−lead (Sn/Pb) solders, consist of a mixture of crystalline phases with different compositions. A homogeneous alloy with a unique composition having the lowest possible melting point is called a eutectic. A binary eutectic alloy is a mixture of two metals with a well-defined composition that melts at the lowest temperature of any composition of the mixture. The purpose of this project was to study the thermal properties of various compositions of Sn/Pb alloys. The conclusions from this work were as follows: (i) the eutectic composition was found to be 62 wt % Sn with a melting temperature of 183 °C; (ii) as the Sn and Pb compositions decreased from 100%, the alloy melting temperature also decreased; and (iii) by measuring the differential scanning calorimetry (DSC) peak melting point of the Sn/Pb alloy, the alloy composition can be obtained from the constructed binary phase diagram. Overall, the DSC provided a method of analysis for the determination of a binary alloy composition of Sn/Pb and the construction of a simple binary phase diagram. KEYWORDS: Second-Year Undergraduate, Upper-Division Undergraduate, Laboratory Instruction, Physical Chemistry, Hands-On Learning/Manipulatives, Instrumental Methods, Materials Science, Metals, Phases/Phase Transitions/Diagrams, Thermal Analysis

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have become increasingly important to incorporate into the undergraduate chemistry curriculum. Differential scanning calorimetry (DSC) is commonly used to determine many different thermal properties and used in a wide variety of applications ranging from polymer analysis to food analysis.1−3 DSC has been used in general chemistry and physical chemistry experiments.4−10 DSC is a rugged, easy-to-use instrumental method for thermal property determination of binary eutectic alloys. The DSC provides for a hands-on laboratory experiment that can be used in the undergraduate chemistry curriculum. In this experiment, students determine the thermal properties of a binary eutectic mixture of Sn and Pb using the DSC in an undergraduate physical chemistry laboratory. The goals of this experiment are • To supplement the classroom lecture on binary eutectics with a hands-on experiment; • To study the thermal properties of various compositions of Sn and Pb alloys ranging from 0.0% to 100% Pb; • To determine the Sn/Pb eutectic composition, melting temperature, and the enthalpy of fusion; • To establish the liquidus and solidus lines of the binary phase diagram.

ndergraduate chemistry students are first introduced to phase diagrams in general chemistry. These phase diagrams are often a single substance such as water or carbon dioxide and are typically a plot of pressure versus temperature showing the equilibrium boundaries between the three phases of matter. In physical and organic chemistry classes, binary phase diagrams of azeotropic and eutectic mixtures of two components are often introduced. Phase diagrams of these binary systems are plots of temperature versus the composition of the binary system under a constant pressure condition. Frequently, a eutectic alloy is used to construct a classic binary phase diagram. A eutectic composition is a homogeneous mixture of two or more elements or compounds that has a melting temperature that is lower than those of the pure components and the lowest of any composition. A binary eutectic alloy is a mixture of two metals with a well-defined composition that melts at the lowest temperature of any composition of the mixture. One example of a binary eutectic alloy is tin (Sn) and lead (Pb), which has been used for centuries as solders and is important in today’s modern technology of electronics and plumbing. A eutectic alloy composition will melt sharply at the eutectic temperature to form a liquid of the same composition. Classical calorimetry methods of analysis are used to determine the thermal properties of eutectic alloys and facilitate the plotting of a binary phase diagram. Instrumental techniques that can routinely and easily be used for thermal property determination © 2011 American Chemical Society and Division of Chemical Education, Inc.

Published: December 22, 2011 548

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Laboratory Experiment

The experiment was performed with approximately 20 students in a physical chemistry laboratory with the students working in pairs over a two-week period (approximately 8−10 h). The students performed the experiment with little difficulty and had a favorable response to doing an instrumental experiment and applying it to a real-world sample.

Table 1. DSC Thermal Data for Sn/Pb Binary Mixtures Upon Heating from Solid to Molten Liquid State First Peak: Eutectica

Second Peak: Componenta

Wt % of Sn

Onset/ °C

Peak/ °C

ΔH/ (J/g)

Onset/ °C

Peak/ °C

ΔH/ (J/g)

90 80 70 62b 60 50 40 30 20 10

183.0 182.9 182.7 182.4 182.7 182.8 183.2 182.2 182.8 184.7

185.0 187.5 187.9 183.3 188.4 186.2 187.7 185.8 185.6 185.9

10.9 24.3 34.6 43.8 43.0 32.7 23.5 14.7 6.9 1.3

207.7 195.3 190.7 NAc d 192.3 208.2 230.9 254.1 285.2

217.8 204.2 192.5 NA d 210.0 231.2 252.2 273.6 297.2

27.5 10.5 0.4 NA d 2.4 5.1 7.5 12.3 17.8

EXPERIMENTAL DETAILS

Materials

The high purity metals were used for the preparations of the alloys. Sn (Baker Analyzed, 99.999%) was purchased from JT Baker. Pb was obtained from Aesar in wire form, 1 mm in diameter, having a purity of 99.95%. The NaOH was reagent grade, purchased from JT Baker. a

Equipment and Calibration

Second heating cycle from solid to molten liquid state was from 100 to 350 °C at 10 °C/min. bThe data in boldface are for the eutectic alloy. cNA is not applicable. dThe dashed line for the 60% tin is due to the fact that no second peak was observed in the DSC thermogram.

The study of the thermal properties of the binary eutectic alloys of Sn and Pb was performed using a Perkin-Elmer powercompensated DSC model Pyris 1. The DSC was used in its high-temperature mode. All experiments were run with samples ranging from 20 to 30 mg under dry nitrogen flowing at 20 mL/min. The flowing nitrogen was used to prevent moisture pick-up or oxidative degradation. All samples were placed in aluminum sample pans having an internal volume of 20 μL and sealed using the crimping tool made for these types of pans. A thermal baseline was established by running empty cells for both the sample and reference compartments. Calibration of the temperature and energy outputs of the DSC were performed using high purity indium, tin, and lead standards. The three standards were subjected to three thermal ramps: two heating and one cooling using a rate of 10.0 °C/min. The Perkin-Elmer thermal analysis software Pyris for Windows was used to determine the endothermic and exothermic transition temperatures and enthalpies of fusion and crystallization. The melting points of the standards were identified as the onset of the endothermic change from the thermal baseline obtained from the second heating thermogram. The onset melting point temperatures and the enthalpies of fusion of the indium, tin, and lead standards are shown in the Supporting Information.

30 s before starting the next ramp. All scanning rates were 10.0 °C/min. The Perkin-Elmer thermal analyzer software, Pyris for Windows, was used to determine the onset and peak melting and freezing points and enthalpies of fusion and crystallization or solidification. The onset melting and freezing points were taken as the point of first inflection from the baseline. The peak temperature is the temperature at the maximum height of the peak.

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HAZARDS Tin is nontoxic, but care should be taken to avoid contact with the mouth. Lead is toxic and can be harmful if inhaled or ingested and should be handled with gloves. Care should be taken to avoid contact with the skin and mouth. Small quantities (approximately 20−30 mg) of samples help minimize any risks. All sample preparation should be performed in a fume hood. Students were only required to prepare the lowestmelting sample (i.e., the eutectic composition alloy), thus minimizing the risk of exposures. Alternatively, the instructor can prepare all samples in the fume hood and can reuse already prepared sealed DSC sample pans, thus eliminating any exposure of lead to the students. Sodium hydroxide is caustic; it causes burns to any area of contact. Any waste metal and their alloys are collected in an appropriate heavy metal hazardous waste container in the lab and are disposed of according to local requirements and procedures and OSHA regulations. Indium metal may be considered a less toxic alternative to lead in this experiment. Details of the experimental procedure and thermal property data for both lead/tin and indium/tin binary systems are included in the Supporting Information.

Alloy Preparation

The appropriate mass of each metal was weighed in a ceramic glazed CoorsTek crucible to provide the weight percent required to obtain 5.00 g of alloy. Approximately 0.05 g of solid NaOH (equivalent to about a half of a pellet) was added to the crucible as a flux, and the entire mixture was heated with an open microburner in a fume hood until both metals were molten. The molten mixture was then stirred with a capillary melting point tube until uniform. The molten samples were subsequently left to stand in the fume hood to cool until solidified and formed a 5.00 g ingot. The student prepares only the eutectic mixture, and the instructor prepares all other mixtures. A sliver of alloy was cut using a single-edge blade, and a 20−30 mg sample was weighed in an aluminum sample pan. The weight percents of each metal used in the composite alloy are shown in Table 1.

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RESULTS AND DISCUSSION A summary of the thermal data obtained from the DSC second heating ramp is listed in Table 1. A thermogram for the 62 wt % Sn mixture on an expanded temperature axis is shown in the Supporting Information. The 62 wt % Sn mixture is the eutectic mixture of the Sn/Pb alloy and has an onset melting temperature of 182.4 °C and a ΔfusionH of 43.8 J/g. Heating thermograms, taken from the second heating ramp for all samples, are shown in Figures 1 and 2. Two heating cycles are used to remove any thermal history in the samples, thus

Procedure To Obtain the DSC Thermograms

Each alloy sample was subjected to three consecutive thermal ramps: (i) first heating from 100.0 to 350.0 °C; (ii) cooling from 350 to 100 °C; and (iii) second heating from 100 to 350 °C. The sample was equilibrated at each final ramp temperature for 549

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(i.e., that of pure component tin) is observed at increasingly higher temperatures with increasing tin content in the alloy. This is seen in the 70 to 90 wt % Sn alloys with the 90 wt % having the highest melt temperature and the 70 wt % the lowest. The binary phase diagram for the Sn/Pb alloys used in this experiment is shown in Figure 3. It is a plot of the peak

Figure 1. The five thermograms for the alloy compositions ranging from 10 to 50 wt % Sn are shown. Each alloy has a first peak onset melt temperature of approximately 182 °C, which corresponds to the melting point of the binary eutectic mixture of Sn/Pb.

Figure 3. Phase diagram for the binary system of tin and lead. The binary phase diagram shows both the liquidus and solidus lines. The point of intersection of the Sn liquidus line and the Pb liquidus line corresponds to the eutectic composition of 62 wt % Sn.

temperature (°C) versus wt % Sn. The binary phase diagram shows both the liquidus and solidus lines for the Sn/Pb alloys studied. The point of intersection of the Sn liquidus line and the Pb liquidus line corresponds to the eutectic composition of 62 wt % Sn. The binary phase diagram and eutectic composition corresponds well with data previously obtained by others.11−15 A plot of ΔfusionH for the eutectic component (first peak) versus temperature (°C) is shown in Figure 4. As the wt % Sn increases,

Figure 2. The second heating thermograms for the 50 to 90 wt % Sn alloys. The eutectic onset peak melting temperature of approximately 182 °C is shown in each of the alloys.

providing the samples with the same thermal history for improved precision and accuracy. The five thermograms for the alloy composition ranging from 10 wt % Sn to 50 wt % Sn are shown in Figure 1. Each alloy has an onset melt temperature of approximately 182 °C, which corresponds to the melting point of the binary eutectic mixture of Sn/Pb. For those binary systems richer in lead than the eutectic mixture, the second melting peak (i.e., that of pure component lead) is observed at increasingly higher temperatures with increasing percent lead content in the alloy. Similarly shown in Figure 2 are the second heating thermograms for the 50 to 90 wt % Sn alloys. Once again, the eutectic onset melting temperature of approximately 182 °C is found for each of the alloys. For those binary systems poorer in lead than the eutectic mixture the second melting peak

Figure 4. A plot of the eutectic peak ΔfusionH against alloy composition. The maximum corresponds to the eutectic composition (62 wt % Sn) and the ΔfusionH (43.8 J/g).

the ΔfusionH first increases until it reaches the eutectic composition of 62 wt % Sn at its highest (ΔfusionH = to 43.8 J/g), and then it decreases. The point of intersection can be used as an additional method of determining the binary eutectic composition for the Sn and Pb alloys.

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CONCLUSIONS The eutectic composition was found to be 62 wt % Sn and 38 wt % Pb having a eutectic melt temperature of 183 °C and a 550

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ΔfusionH of 43.8 J/g. As the Sn content in the alloy decreased from 100% to that of the eutectic composition, the alloy melting temperature also decreased. As the Pb content in the alloy decreased from 100% to that of the eutectic composition, the alloy melting temperature also decreased. Overall, the DSC is an excellent tool of thermal analysis for undergraduate chemistry students to investigate the thermal properties of the Sn/Pb alloy and its eutectic behavior.

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ASSOCIATED CONTENT

S Supporting Information *

Student handouts; instructor notes; fact sheet on thermal analysis; and safety precautions using the DSC. This material is available via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: Ronald.P.D’[email protected].

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ACKNOWLEDGMENTS We acknowledge the support from a Hofstra HCLAS Faculty Research & Development Grant. REFERENCES

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