Solution-Processed, Surface-Engineered, Polycrystalline CdSe-SnSe Exhibiting Low Thermal Conductivity

Our group focused on developing materials for energy-related applications, emphasizing energy storage and thermoelectricity. We used nano crystals as building blocks or precursors for constructing microscopic materials, and we investigate the transformation the nano crystals undergo into work solids, aiming to enhance performance and by understanding and controlling the properties derived from the nanoscale features. In particular, for thermoelectric materials, we focus on defect control.

Developing thermoelectric materials via solution processing involves numerous challenges. One, mitigating oxidation due to the nanoparticle's high surface volume ratio, reproducibility due to the complexity of the process, and three, dealing with volatile species to ensure stability. Addressing and understanding these challenges is crucial for enhancing thermoelectric materials efficiency for practical applications.

Our research advanced cost-effective solution process thermoelectric materials by fine-tuning nanoparticle properties and their organization. We are uncovering the chemistry involved in the whole process, from the nanoparticle synthesis to the final consolidation, and currently we are focused on how surface species or absorbates, affect materials'microstructure and hence their performance. We enhanced and solenized thermoelectric performance through utilizing solution process surface engineered particles, significantly reducing the thermal conductivity by microstructural tuning and the introduction of defects.

This approach also is advantageous because it uses inexpensive precursors, low temperatures, and also we use water as a solvent. We found that certain molecules absorb at the particle surface and restrict grain growth. Now we are trying to rationalize how different surface species affect microstructure and hence transport properties based on their composition, chemical stability, and bonding nature.

To begin, pass argon gas through a separating funnel placed over a three-neck flask with hot tin solution for five minutes. Remove the rubber septum from a flask with selenium solution and transfer the solution via the separating funnel to the tin solution. Heat the mixture to 101 degrees Celsius and stir for two hours.

Place the flask in a water bath while stirring. Once the mixture is cooled, transfer the flask from the Schlenk line onto a round-bottom flask support. Now pour out 600 milliliters of the supernatant after it is settled for five minutes.

Divide the remaining crude solution into four centrifuge tubes, centrifuge, and then discard the supernatant. Vortex the contents of each tube with 40 milliliters of deionized water`after discarding the supernatant. Following this, sonicate the mixture in a sonicating bath for five minutes.

Then vortex and centrifuge again. Next, wash the precipitate with 40 milliliters of ethanol. After the sixth wash, place the tubes in a desiccator under vacuum for at least 12 hours.

Once the precipitate is dried, transfer the tubes to a nitrogen-filled glove box. With an agate mortar and pestle, grind the tin selenide particles into a fine powder. Transfer four grams of the powder into a 20 milliliter vial.

Add the prepared cadmium-selenium methylformamide mixture to this vial with constant stirring for 48 hours. To purify the cadmium-selenium surface-treated particles, centrifuge the mixture and discard the supernatant. Then add 40 milliliters of anhydrous ethanol.

After discarding the supernatant, dry the cadmium-selenium treated particles in the desiccator under vacuum for 12 hours. Then grind the dried powder in the glove box to obtain a fine powder. To begin open, the in-and-out gas valves of a tubular furnace to allow the forming gas to flow through the quartz tube of the furnace.

Then open one end of the tube and introduce a vial containing the cadmium-selenium surface treated particle into its middle. Set the temperature profile of the furnace to heat to 500 degrees Celsius at a rate of 10 degrees per minute. Hold at this temperature for one hour before cooling to room temperature naturally.

Once annealing is complete, grind the powder in a glove box. Insert a graphite stem halfway into a die. Press two graphite discs flat on the stem.

Place the half-prepared die into the glove box after removing the inserted stem. Now use weighing paper to insert the powder into the die. After removing the inserted stem, place the remaining two graphite discs on top of the powder.

Then place the remaining stem on the discs. Remove the die from the glove box. Use a cold press to compress the powder until the total height of the completed die is about 83 millimeters.

Place the prepared die in the center of the stage of the SPS instrument. Lower the upper electrode to fix the die in place. Then insert the thermocouple.

Set the upper electrodes at Z-axis control to move continuously down and apply vacuum after closing the chamber. Once the manometer has reached minimum pressure, turn on the pirani gauge. After 10 minutes, apply an axial pressure of 47 megapascals at 500 degrees Celsius for five minutes, and set the temperature and pressure controls of the SPS to auto.

Initiate the measurement in the software. Track the pressure and Z-axis. Then press center on to commence the consolidation.

Set the Z-axis to stop step and set the temperature and pressure to manual control. Next, remove the thermocouple from the insert in the vented chamber, In the measurement software, click on Set Up DAQ. Input the sample name followed by dimensions.

Then press Okay. Mount the sample between the electrodes, placing graphite paper between the bar and the electrodes. Place the thermocouples in contact with the samples but separated with graphite paper.

Adjust until the probes are in contact with the bar, and then turn the knob for half a turn. With the software, measure the distance between the probes and input it into the software under Set Up DAQ. Place the in-canal susceptor over the sample and insert the thermocouple.

Close the furnace and apply vacuum for 10 minutes. After conducting a probe test, click on the controller tab followed by the temperature profile, and set the heating cycle to 30 degrees Celsius to 500 degrees Celsius, and the cooling rate between 500 degrees Celsius to 30 degrees Celsius at 20 degrees Celsius per minute. Press start to begin measurement.

To prepare samples for the thermal diffusivity measurement, load a sample holder containing the graphite-coated samples into the magazine of the analyzer. Fill the liquid nitrogen reservoir to cool the detector. After evacuating the analyzer chamber, input the sample name and thickness.

Now load a preset temperature profile from 30 to 500 degrees Celsius at 10 degrees Celsius per minute, measuring every 50 degrees Celsius. After the laser is turned on, conduct a laser shot test. Charge the amplifier to 200 and press start.

Change the mode to automatic mode, then press start to begin measurements. Finally switch off the laser and remove the sample from the vented chamber. Calculate the thermal conductivity with the given equation.

Pure phase tin selenide particles were synthesized. The particles were polydispersed in shape, sized between 50 nanometers to 200 nanometers. SPS increased grain growth, resulting in pellets with a relative density of more than 90%