Synthesis of Graphene Nanofluids with Controllable Flake Size Distributions

The proposed method employs exfoliation process and centrifugation process to control the lower limits and the upper limits of size distributions of resulting graphinate suspension separately. The main advantage of the proposed method is that the final size distribution is controllable by adjusting the process parameters of the exfoliation step and the centrifugation step. In a dry, clean, flat bottom flask, add 20 grams of PVA, and then add 1000 milliliters of distilled water.

Gently swirl the flask until the PVA fully dissolves. Then add 50 grams of graphite powder to the flat bottom flask and gently swirl the flask until the graphite powder fully disperses in the suspension. Transfer 500 milliliters of the resulting suspension to a 500-milliliter beaker.

Place the beaker under a shear mixer, positioning the beaker near the center of the mixing vessel to prevent the formation of a vortex. Lower the mixing head to its lowest position, 30 millimeters from the base plane. Next, prepare a water bath by filling a 5000-milliliter beaker with room temperature water, and position the 500-milliliter beaker in the bath.

Start the mixer, and increase the speed gradually to 4, 500 rpm. Mix at this speed for 120 minutes. Change the water every 30 minutes.

Perform this exfoliation step five more times with different duration. The mixing time determines the lower lateral size limit of the graphene nanoflakes. Collect the 500-milliliter generated suspensions after each exfoliation step.

Label each suspension with the exfolation time, and centrifuge the collected suspension at 140 times g for 45 minutes. To remove the unexfoliated graphite, use a pipette to collect the top 80%of the supernatant from each centrifuge tube for further centrifugation. Centrifuge the supernatant suspension from the last centrifugation step at 8, 951 times g for 45 minutes.

Collect the upper 50%of the supernatant in the centrifuge tube, and label the sample with a number. Then, to recycle the sediment on the bottom of the centrifuge tube, add 50 milliliters of the previously prepared PVA water reagent to the sediment, and shake the tube vigorously by hand until the sediment is well-dispersed in the suspension. Centrifuge the suspension at 8, 951 times g for 45 minutes.

Collect the upper 80%for further measurements. Repeat the centrifugation step for the pellet four more times with four different centrifugation speeds. The centrifugation speed determines the upper lateral size limit of the graphene nanoflakes.

Now, prepare the ultraviolet visible spectroscopy. With the previously prepared PVA water solution in a dry, clean sample cell, calibrate the ultraviolet visible spectrometer, setting the PVA water concentrations to 0%Then, add the PVA water resuspension after centrifugation to a dry, clean sample cell, with a path length of 10 millimeters, and obtain a readout using the manufacturer's software. Click the Obtain button to get the measurement results graph, and save the results.

Next, to determine the graphene weight, vacuum filter the sample suspension using a nylon membrane with a pore size of 0.2 microns. Obtain the membrane film, and wash with approximately 200 milliliters of water into a beaker. Repeat the wash three times, until all the solids are washed away from the membrane.

Determine the washed water mass with a high-precision microbalance to obtain the weight of the solids. Again, vacuum filter the water suspensions using a nylon membrane with a pore size of 0.2 microns. Obtain the membrane, and dry it at room temperature for over 12 hours.

Subsequently, rinse the film with 200 milliliters of deionized water into a beaker. If the desired concentration is less than the production rate at one milligram per milliliter, add the prepared PVA water solution to obtain the desired concentration. If the desired concentration is higher than 1%dry the deionized water under vacuum in the dryer for 24 hours to obtain the graphene nanosheets.

In this protocol, the ultraviolet visible measurement of the various flake size distributions shows the spectra absorbance peak obtained at a wavelength of 270 nanometers, indicating the evidence of the graphene flakes. Suspension with different concentration has different 660-nanometer absorption. The D band and 2D band of the Raman spectroscopy determines the flake thickness of the graphene nanoflakes.

The D band of the Raman spectrum, which is related to graphene sp3 carbon atoms, distinguishes between the initial graphite and the graphene nanoflakes. Low D band intensity indicates the defect-free graphene nanosheets. Distinctive size distribution was observed for the resulting suspension, prepared using different centrifugation speeds.

Both the transmission electron microscopy and the scanning electron microscopy show that graphene was produced, and the exfoliation was successful. However, the centrifugation step only worked on nanoparticles with mean diameters larger than 1000 nanometers. The relationship between the upper limits of size distributions with centrifugation speed should be pre-determined.

Since the size does not influence with the proposed method, the heat transfer efficiency is possible to be manipulated under conditions like some connectivity, convection, and transfer applications at the center. The PVA polymer is harmful to human. Face mask and gloves should be used to protect the operator.