The overall goal of this technique is to measure thermodynamic properties of binary metal systems, such as activity, partial molar entropy's, and partial molar enthalpies. This versatile method can help answer key questions in the metallophilic field about the thermodynamic properties of binary alloys. The main advantage of this technique is that it allows us to accurately probe thermodynamic quantities for binary alloy systems, that are often difficult to measure, such as activity.
Demonstrating the procedure will be Nate Smith, Tim Lichtenstein, and Jarrod Gesualdi, all students from my laboratory. First, prepare in a 1.5 liter bottle a 350 gram mixture of 97 mole percent calcium fluoride and three mole percent barium fluoride. Add to this mixture 1.3 kilograms of a three millimeter yttria-stabilized zirconia milling medium and 25 grams of polyvinyl alcohol.
Suspend the mixture in sufficient isopropyl alcohol to fill 4/5 of the bottle. Then, place the bottle on a ball mill and mill the mixture for 24 hours at 250 rpm. Separate the electrolyte mixture from the milling medium with a 10 mesh sieve.
Use a squeeze bottle of isopropyl alcohol to rinse the remaining salt mixture from the medium. Dry the milled electrolyte mixture in a fume hood for 24 hours. Then, use a mortar and pestle to grind the mixture to a fine powder.
Next, uniformly load 130 grams of the electrolyte powder into a 75 millimeter by 60 millimeter pellet dye. Use a dye press to uniaxially apply 30 mega pascals to the powder for two minutes. Then, invert the pellet dye and place a stainless steel ring on the dye with the pellet centered inside the ring.
Carefully apply one bar to the pellet dye punch to remove the electrolyte pellet from the dye. Next, use a one millimeter drill bit to drill a 0.5 millimeter deep tapping hole in the center of the pellet. Drill six evenly spaced tapping holes around the first, each with a center-to-center distance of 25.4 millimeters to the first hole.
Replace the drill bit with an 11.2 millimeter bit. Guided by the tapping holes. Drill seven, 12 millimeter deep wells in the pellet.
Next place 4.5. grams of finely ground electrolyte powder in a 19 millimeter by 15 millimeter pellet dye. Use the dye press to uniaxially apply 7.5 mega pascals to the powder for one minute to form an electrolyte cap.
Prepare a total of six caps in this way. Use a two millimeter drill bit to drill a hole through the center of each cap. Then lightly cover a 10 centimeter alumina plate with corse alumina powder.
Place each electrolyte piece on the plate ensuring that they are not touching each other. And center the pieces. Place a total of six grams of barium and bismuth in the desired molar ratio at the center of an arc melter stage.
Evacuate and refill the chamber with argon at least three times. Then close the eye protection shield and turn on the arc melter current to melt the pieces together. Turn off the current and arc melter and remove the sample stage.
Flip over the alloy and secure the stage in the arc melter chamber. Repeat the arc melting procedure twice more to form a homogenous alloy. The keys to arc melting properly are to keep the current low enough to melt the metals together without vaporizing them significantly.
And to ensure the metal is homogeneous by re-melting it several times. Cut the re-melted alloy into three to six pieces and melt the pieces together. Store the alloy under an inert argon atmosphere.
Repeat this process for each alloy composition to be tested. Next, cut six 46 centimeter lengths of one millimeter tungsten wire. Sand and clean the wire surface with 100 grit emery paper and acetone.
Insert the wires into six alumina tubes. Position the wires with 12.7 centimeters exposed at the bottom of each tube and 2.5 centimeters exposed at the top. Fix the wires in place with quick cure epoxy.
Secure a 45 centimeter K type thermocouple in another alumina tube. Transfer the electrochemical cell assembly components to an argon filled glove box. Use a mini lathe to shape the alloy's to fit the holes in the electrolyte pellet.
Drill a two millimeter wide hole in the center of each alloy. After this, place the centered electrolyte pellet in the clean alumina crucible of the test chamber. Fill two of the outer wells with a reference barium-bismuth alloy of a known molar ratio.
Fill the remaining outer wells with four barium-bismuth alloys of different molar ratios. Place electrolyte caps over each filled well. Then insert a tungsten wire electrical lead assembly through one outer vacuum port of the chamber cap and the chamber baffle plates.
With the exposed 12.7 centimeter length of wire pointed down. Thread the wire through one electrolyte cap and alloy. Ensure that the tip of the wire is firmly pressed against the electrolyte floor of the well.
Fit electrical leads to the remaining five outer wells in this way. When assembling the electrochemical cell, it is imperative that the electrodes fill the wells in the electrolyte and that the tungsten leads are in contact with the electrodes. Contact issues are the most prevalent causes of failure.
Insert the thermocouple into the center vacuum port and seat it in the center well. Place an o-ring in the groove of the stainless steel vacuum chamber. Carefully load the electrochemical cell assembly into the test chamber.
Tighten the vacuum seals and clamps. Then load the assembled chamber into a crucible furnace. Wrap the exposed surface of the vacuum chamber outside the furnace with two layer of fiberglass insulation.
Attach the coolant lines, the vacuum line and the argon line to the test chamber. Close the gas outlet port and evacuate the test chamber to below 10 millitorr's. Dry the chamber under dynamic vacuum.
Then purge the chamber at least three times. Start flowing argon through the chamber at 50 milliliters per minute at one atmosphere of chamber pressure. Connect the counter and reference electrodes from the cell assembly to a potentiostat.
Connect the working electrodes to a multi-plexing switch box. Connect a ground cable to the stainless steel test chamber and plug the other end into a grounded outlet. Simultaneously start collecting temperature and electromotive force data.
Then heat the furnace from 543 kelvin to 1, 073 kelvin at five kelvin per minute. Program the furnace to cycle from 1, 073 kelvin to 723 kelvin at five kelvin per minute, holding for one to two hours every 25 kelvin. Use the measurements recorded during this cycle to determine the electromotive force values of each alloy as a function of temperature.
A barium-bismuth alloy with a barium mole fraction of 0.05 was used as a reference electrode for electromotive measurements of other barium-bismuth alloys. The potential difference between the identical 0.05 mole fraction alloys was less than two millivolts throughout the measurement, indicating that the alloy was a reliable reference. Symmetric electromotive force profiles were observed for the alloys with barium mole fractions of 0.10 and 0.20, indicating that reproducible electromotive force values were obtained during thermal cycling.
The reference electrode was calibrated with respect to pure barium. The electromotive force values of alloys with barium mole fractions ranging from 0.05 to 0.25 were then evaluated with respect to pure barium. Linear fifths of the data were used to calculate changes in partial molar entropy's and enthalpies.
The activity of barium was also calculated from the electromotive force values. Electromotive force measurements were combined with differential scanning calorimetry, inductively coupled plasma atomic emission spectroscopy and x-ray diffraction data, to determine the phase transition temperatures for each alloy composition. This was used to refine the barium-bismuth alloy phase diagram.
Once mastered, this technique can be done in three days if it is performed properly. While attempting this procedure, remember to be diligent and patient in the setup and execution of all steps. After watching this video, you should have a good understanding of how to perform the electromotive force measurement technique.
To investigate the thermodynamic properties of binary alloy systems.
