METHOD FOR THE SYNTHESIS OF NANOFLUIDS

Abstract

The present invention relates to a method for the synthesis of nanofluids including functionalization of carbon nanostructures through a new method comprising the addition of carbon nanostructures to water; ultrasonication of the solution; addition of persulfate salt and one or several metal hydroxides of the first column of the periodic table to the aqueous solution containing carbon nanostructure; re-exposing the solution to ultrasonic waves; and then, the separation of the functionalized carbon nanostructures from the solution and washing the carbon nanostructures with water to neutralize them and mixing the nanoparticles obtained from the previous step with the fluid. By presenting a new method for the synthesis of the functionalized carbon nanostructures with specific amount of functional groups and their application in the synthesis of nanofluids, an increase in the stability and thermal conductivity of nanofluids takes place.

Claims

1. A method for the synthesis of nanofluids comprising the following steps in ascending order: addition of carbon nanostructures to water; ultrasonication of the solution; addition of one or more persulfate salts and one or more metal hydroxides of the first column of the periodic table to the aqueous solution of carbon nanostructures; reexposing the solution to ultrasonic waves; separation of the functionalized carbon nanostructures from the solution; washing the carbon nanostructures with water to neutralize them to provide nanoparticles; and mixing the nanoparticles obtained from the previous step with a fluid, wherein the functionalization of carbon nanostructures is performed at a temperature of from about 20 to about 30 C.

2. The method of claim 1, wherein the amount of persulfate salt is in an amount of at least one of from about 5 to about 50%, from about 5 to about 20%, and from about 5 to about 10% by weight of water existing in the aqueous solution.

3. The method according to claim 1, wherein the amount of carbon nanostructures is in an amount of at least one of from about 0.01 to about 1%, from about 0.01 to about 0.1%, and from about 0.05 to about 0.1% by weight of water existing in the aqueous solution.

4. The method according to claim 1, wherein the metal hydroxides are added to the aqueous solution in an amount of from about 5 to about 50% by weight of water existing in the aqueous solution.

5. The method according to claim 1, wherein the aqueous solution containing carbon nanostructure is exposed to ultrasound wave for about 5 to about 15 minutes in the first step of ultrasonication.

6. The method according to claim 1, wherein the aqueous solution containing carbon nanostructure is re-exposed to ultrasound waves for about 10 to about 40 minutes.

7. The method according to claim 1, wherein ultrasonication is performed in a frequency range of from about 40- to about 59 kHz in the functionalization step.

8. (canceled)

9. The method according to claim 1, wherein the fluid is one of a hydro-philic fluid, and a hydrophobic fluid.

10. The method according to claim 1, wherein the persulfate salt is at least one of potassium persulfate, sodium persulfate, and ammonium persulfate.

11. The method according to claim 1, wherein the carbon nanostructures include at least one of carbon nanotubes, carbon nanofiber, nanohorns, graphite, graphene, and fullerene.

12. The method according to claim 1, wherein the nanoparticles obtained are mixed in an amount of from about 0.1 to about 1 wt. % of nanofluid with the fluid.

13. The method according to claim 1, wherein the mixing of nanoparticles with the fluid is performed by ultrasonication for about 10- to about 40 minutes within a frequency range of from about 40-to about 59 KHz at 20-30 C.

14. The method according to claim 1, wherein the mixing of nanoparticles with the fluid is performed by ultrasonication for about 10- to about 40 minutes, with predetermined time intervals and then waves created by the ultrasonication are interrupted for about 30 seconds and the suspension is exposed to ultrasound waves in an ultrasonic bath having a frequency in a range of about 40- to about 59 KHz, at a temperature of about 20- to about 30 C.

15. (canceled)

16. The method according to claim 9, wherein the hydrophilic fluid is at least one of water, alkylene glycols, and combinations thereof, and wherein the hydrophobic fluid is at least one of silicone oil and engine oil.

17. The method according to claim 14, wherein the ultrasonificaiton is about 10 minutes, the predetermined time intervals is about every 5 minutes, and the ultrasound wave includes a frequency of about 40 KHz.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0062] FIG. 1 shows a scanning electron microscope (SEM) image of carbon nanotubes.

[0063] FIG. 2 shoes a SEM image of functionalized carbon nanotubes prepared according to example 1.

[0064] FIG. 3 shows a SEM image of functionalized carbon nanotubes prepared according to example 2.

[0065] FIG. 4 shows Raman spectra of carbon nanotubes and functionalized carbon nanotubes prepared according to example 2.

[0066] FIG. 5 shows Raman spectra of carbon nanotubes and functionalized carbon nanotubes prepared according to example 1.

[0067] FIG. 6 shows X-ray diffraction (XRD) patterns of carbon nanotubes and functionalized carbon nanotubes prepared according to example 2.

[0068] FIG. 7 shows zeta-potential curves of functionalized carbon nanotubes prepared according to examples 1(A) and 2(B).

[0069] FIG. 8 shows thermogravimetric (TGA) curves of carbon nanotubes and functionalized carbon nanotubes prepared according to examples 1(A) and 2(B).

EXAMPLES

[0070] The examples below are given for elaborating the subject-matter of the present invention and the invention is not limited to them.

[0071] In all the examples, multi-walled carbon nanotubes of approximate diameter of 1 to 80 nanometers, pore volume of 0.2 to 1.2 cm.sup.3/g, surface area of 100 to 500 m.sup.2/g, and length of 1 to 100 m were used. Also, in all the examples, after mixing the functionalized nanostructures with water as a base fluid, the thermal conductivity of suspensions was measured by KD2 Labcell Ltd UK with temperature kept constant by a circulator at 15 C. To compare the thermal conductivity of the nanofluids containing carbon nanotubes and base fluid, water, 50/50 water/ethylene glycol mixture, and ethylene glycol, the thermal conductivity of distilled water, 50/50 water/ethylene glycol mixture, and ethylene glycol were measured by the KD2 device and the values of 0.58 (W/m.Math.K), 0.43 and 0.28 (W/m.Math.K) were obtained, respectively. Their stability was also measured by Malvern Instrument IncZeta Potential.

Example 1

Functionalization of Carbon Nanotubes Through Oxidation with Sulfuric and Nitric Acids and Thermal Conductivity Test

[0072] One gram of multi-walled carbon nanotube was added to 40 ml of concentrated mixture of nitric and sulfuric acids (volume ratio of 1 to 3) and was refluxed at a temperature higher than 130 C. for one hour. The nanotubes were then filtered and washed with deionized water to reach a pH of about 7. They were then dried in a 150 C. oven for 12 hours. Then, 0.05-0.1 wt. % of nanofluid, the functionalized carbon nanotubes were added to water and the suspension was exposed to ultrasound waves for 10 minutes in an ultrasonic bath with a frequency of 40 KHz at 20-30 C. The results of functionalization are presented in Table 3. Then, the thermal conductivity of nano-suspensions was measured by the KD2 device at 20 C. The results (Table 1) show an amount of 5 to 13% increase in thermal conductivity. The SEM images of the carbon nanotubes and the functionalized carbon nanotubes are depicted in FIGS. 1 and 2, respectively. The destruction of the structure of multi-walled carbon nanotubes is shown in FIG. 2.

TABLE-US-00001 TABLE 1 Measurement results of thermal conductivity of nanofluid containing functionalized multi-walled carbon nanotubes prepared in example 1 Wt. % of the Thermal conductivity functionalized of nanotubes in Base fluid Thermal conductivity of nanofluid (W/m .Math. K) First method (W/m .Math. K) 0.1 0.58 0.66 0.05 0.58 0.61

Example 2

Functionalization of Carbon Nanotubes Through Oxidation with Potassium Persulfate and Thermal Conductivity Test

[0073] To functionalize the carbon nanotubes by the present invention, an aqueous mixture containing 0.01-0.1 wt. % of multi-walled carbon nanotubes was first ultrasonicated for 10 minutes and then, about 20 grams of KPS (potassium persulfate) and 10 grams of KOH (potassium hydroxide) were added to the solution and exposed to ultrasound waves for 10 minutes at ambient temperature. Then, the functionalized carbon nanotubes were separated by a filter and washed with distilled water up to neutral pH. Then, 0.05-0.1 wt. % of nanofluid, functionalized nanotubes were added to water and exposed to ultrasound waves for 10 minutes in an ultrasonic bath with a frequency of 40 KHz at 25 C. The results of functionalization are presented in Table 3. After ultrasonicating the suspensions, the thermal conductivity of the nano-suspensions was measured by KD2 at 20 C. The results (Table 2) show about 43 to 57% increase in thermal conductivity which is a significant increase compared to acid oxidation method (Example 1).

TABLE-US-00002 TABLE 2 Measurement results of thermal conductivity of nanofluids containing functionalized carbon nanotubes prepared in example 2 Wt. % of the CCfunctionalized CCThermal conductivity nanotubes in of Thermal conductivity of 2th nanofluid Base fluid (W/m .Math. K) method (W/m .Math. K) 0.1 0.58 0.91 0.05 0.58 0.83

[0074] FIG. 3 shows the SEM image of functionalized multi-walled carbon nanotubes. This method does not bring about any changes in the tube-shape structure of carbon nanotubes and the tube morphology is preserved like carbon nanotubes prepared through functionalization process.

[0075] As it is observed in Table 3, the amount of the carboxylic functional group in functionalized carbon nanotubes prepared through the present invention method is less than that of the acid oxidation method (Example 1). This amount is about 2.3 mmol/g measured through reverse titration.

TABLE-US-00003 TABLE 3 Percentage of the carboxylic functional groups introduced on the surface of multi-walled carbon nanotubes for the first and the second examples The first Experimental data for method The second method Carboxylic group concentration (example 1) (example 2) Carboxylic group 6.8 mmol/g 2.3 mmol/g concentration

[0076] The Raman spectra of the carbon nanotubes and those functionalized by the method in example 2 are compared in FIG. 4. The presence of oscillations in the cm.sup.1 1500 to cm.sup.1 1700 region shows the generation of different defects and hybridization change in multi-walled nanotubes.

[0077] The intensity ratio of D to G in the functionalized multi-walled carbon nanotubes in this method is 1.03 without destroying the structure of the nanotubes, while this value is 0.92 in the carbon nanotubes. The increase in intensity shows the production of desirable defects in suitable amount (the functional group of carboxylic is one of these defects) on the surface of the nanotubes.

[0078] FIG. 5 shows the value of this proportion in the method of sulfuric and nitric acids (with the ratio of three to one) where carbon structural destruction takes place to be 0.95.

[0079] The comparison of the results of the Raman spectra along with the results obtained through the reverse titration show that in addition to the production of carboxylic functional groups other defects in example 2 which effectuate more stability of the nanoparticles are also produced on the surface of the nanotubes. The spectrum in FIG. 4 shows that sp.sup.2 hybridization of the tube-shape structure of carbon nanotubes in persulfate method is changed into sp.sup.3 hybridization without destroying the nanotubes; this change has a positive effect on stability and thermal conductivity.

[0080] The X-ray diffraction pattern for the multi-walled carbon nanotubes and functionalized multi-walled carbon nanotubes in example 2 are shown in FIG. 6. As it is observed, the intensity of the C (002) peak for the oxidized carbon nanotubes has increased. Also, the peak intensity at 70, related to the catalyst particles remaining from the synthesis of the carbon nanotubes, is greatly reduced, showing an increase in purity after oxidation operations. On the other hand, this pattern is indicative of the non-destruction of the structures of carbon nanotubes after functionalization.

[0081] In general, this method is the first method for the functionalization of carbon nanotubes at ambient temperature causing an increase in theremal conductivity by 57%. Of the main advantages of this method is an increase in thermal conductivity along with more stability of the functionalized carbon nanotubes. The zeta potential curves are presented for the first and the second examples in FIG. 7. The values for the first and the second method is 20.1 and 28.9, respectively. As it is known, the higher absolute value of zeta shows the higher stability rate of the particles, while in this invention in addition to stability, the structure of the carbon nanotubes is not cut denoting the high rate of thermal conductivity.

[0082] The SEM images and curves of zeta potential show that the stability was not singly enough for thermal conductivity and that the amount of the nanostructures destruction was also effective.

[0083] FIG. 8 shows the curve for the thermal properties of nanostructure in which the amount of the nanostructures destruction through acid method results in steeper gradient on the curve, while the matching of the TGA curves of the carbon nanotubes and functionalized carbon nanotubes according to example 2 denotes no destruction of the nanostructures and the existing mild slope in the diagram is related to the functional groups introduced.

Example 3

Functionalization of The Carbon Nanotubes Through Oxidation with Sodium Persulfate and Thermal Conductivity Test

[0084] Based on the present invention, an aqueous mixture 0.01-0.1% of carbon nanotubes was first ultrasonicated for 10 minutes and then about 20 grams of sodium persulfate (Na.sub.2S.sub.2O.sub.8) and 10 grams of potassium hydroxide (KOH) were added to it and ultrasonicated at ambient temperature for 20 minutes. The functionalized carbon nano-tubes were then separated by a filter and washed with distilled water to neutralize their acidity, and then dried in oven at 60 C.

[0085] Thermal Conductivity Test:

[0086] 0.01 grams of the functionalized multi-walled carbon nanotube were added to 10 to 20 milliliters of distilled water. It was then exposed to ultrasound waves for 10 minutes in an ultrasonic bath with a frequency of 40 KHz at 25 C. The value of thermal conductivity was 0.92 (W/m.Math.K) displaying a 58% increase with the zeta value of 28.9 showing its very high stability.

[0087] 0.01 grams of the functionalized multi-walled carbon nanotube were added to 10 to 20 milliliters of distilled water. It was then exposed to ultrasound waves for 10 minutes in an ultrasonic bath with a frequency of 40 KHz at 25 C. The value of thermal conductivity was 0.85 (W/m.Math.K) displaying about a 46% increase.

Example 5

Functionalization of Carbon Nanotubes Through Oxidation with Persulfate Ammonium and Thermal Conductivity Test

[0088] On the basis of the present invention, an aqueous mixture containing 0.01-0.1 wt. % of multi-walled carbon nanotubes were ultrasonicated for 10 minutes and then about 20 grams of APS (Ammonium persulfate) (NH.sub.4) .sub.2S.sub.2O.sub.8 were added and ultrasonicated for 20 minutes at ambient temperature. Then, the functionalized carbon nanotubes were separated through a filter, washed with distilled water until the acidity was neutralized and dried in oven at 60 C.

[0089] Thermal conductivity test:

[0090] 0.01 grams of the functionalized multi-walled carbon nanotube were added to 10 to 20 milliliters of distilled water. It was then exposed to ultrasound waves for 10 minutes in an ultrasonic bath with a frequency of 40 KHz at 25 C. The value of thermal conductivity was 0.91 (W/m.Math.K) displaying about a 57% increase. The zeta value was 28.9 showing its very high stability.

[0091] 0.01 grams of the functionalized multi-walled carbon nanotube were added to 10 to 20 milliliters of ethylene glycol. It was then exposed to ultrasound waves for 10 minutes in an ultrasonic bath with a frequency of 40 KHz at 25 C. The value of thermal conductivity was 0.32 (W/m.Math.K) displaying a 14% increase relative to ethylene glycol thermal conductivity.

[0092] 0.01 grams of the functionalized multi-walled carbon nanotube were added to 10 to 20 milliliters mixture of 50% water and ethylene glycol. It was then exposed to ultrasound waves for 10 minutes in an ultrasonic bath with a frequency of 40 KHz at 25 C. The value of thermal conductivity was 0.52 (W/m.Math.K) displaying a 21% increase relative to mixture of 50% water and ethylene glycol thermal conductivity.

[0093] 0.01 grams of the functionalized multi-walled carbon nanotube were added to 10 to 20 milliliters of distilled water. It was then exposed to ultrasound waves for 10 minutes in an ultrasonic bath with a frequency of 40 KHz at 25 C. . The value of thermal conductivity was 0.84 (W/m.Math.K) displaying a 44% increase.