Patent Application
20120042806
The invention relates to a method for concentrating nano-reinforcements, such as multi-walled and single-walled carbon nanotubes, graphene oxide sheets, and other nano-reinforcements in suspension for use in making cementitious or other composites as well as to the concentrated suspensions and composite cementitious or other products.
The utilization of highly dispersed multi-walled carbon nanotubes (MWCNTs) in cementitious materials has shown to substantially improve the mechanical and other properties of the cementitious matrix. For example, copending patent application U.S. Ser. No. 12/322,842 filed Feb. 6, 2009, discloses that the utilization of highly dispersed carbon nanotubes (CNTs) in cementitious materials substantially improves the performance of the cementitious matrix. In particular, by adding a very low amount of MWCNTs or carbon nanofibers (CNFs), at concentrations of 0.025 wt. % to 0.08 wt. % of cement, the strength and stiffness of cement beams increases significantly [U.S. Ser. No. 12/322,842 and references 1-6]. The application of low concentration of MWCNTs and CNFs enables the control of matrix cracks at the nanoscale level [reference 7]. Also, the cost of CNTs at such low concentrations is comparable or lower than that of conventional reinforcement which makes the introduction of CNTs in concrete economically feasible. In addition to the benefits of reinforcement, autogenous shrinkage tests have demonstrated that MWCNTs can also have beneficial effects on the early age strain capacity of cementitious materials, which leads to improved durability of the cement matrix [reference 1].
The current preparation method of MWCNT suspensions for use in cementitious materials includes a one step technique involving the application of ultrasonic energy and the use of a commercially available surfactant to disperse the MWCNTs in the mixing water prior to their addition to cement [U.S. Ser. No. 12/322,842 and references 2, 8]. However, in order to have widespread use of MWCNT-cement nanocomposites, there is a need to produce MWCNT suspensions in large scale production for full-scale application in concrete to decrease the transportation and storage cost of the large volume suspensions for this application.
A number of solution-phase processes exist where carbon nanomaterials, such as CNTs and graphene flakes, are concentrated by the removal of their solvent. This can be achieved by precipitation via addition of organic solvent and vacuum filteration [9], solvent exchange utilizing polymer-organic solvent [10] and sedimentation and decantation by ultracentrifugation [11]. Among these processes, the ultracentrifugation method is ideal due to its simplicity and also for applications where the introduction of organic solvents will become a hindrance. Ultracentrifugation process has been proven as a facile method to increase the concentration of CNTs in aqueous solutions prior to being used in a technique called density gradient ultracentrifugation (DGU). DGU is a solution phase purifying technique that is widely used to separate various forms of carbon nanomaterials by their physical and electronic structures, which depend on the subtle buoyant densities of different species [12-16]. In this technique, the materials of interest are suspended in an aqueous solution and layered within a density gradient, thus their high initial concentration is essential for the optimal yield of separation after ultracentrifugation. To address this issue, a preparative ultracentrifugation process called pelleting, which is generally used to sediment solidified organic compounds out of solutions, has been adapted from biology [17-18]. During this technique, nanomaterials in aqueous suspensions are presented under a centrifugal force inside a tube and travel towards the bottom at certain sedimentation rate, forming a highly concentrated region which can be recovered after decantation.
In an embodiment of the present invention, a method is provided for providing concentrated MWCNT and other nano-reinforcement suspensions for applications as admixtures in cement based materials. An illustrative embodiment of the invention involves dispersing nano-reinforcements in an aqueous solution containing a surfactant to form a suspension and subjecting the suspension to centrifugation to form a concentrated suspension of nano-reinforcements followed by removal of excess solution from the container, leaving a centrifuged and concentrated mass of nano-reinforcements which can be subsequently used as admixtures with cement base materials to form composite material.
Another embodiment of the present invention provides a method of making such a cement based composite using the concentrated suspension of MWCNT or other nano-reinforcements wherein the dispersibility of the MWCNT suspension and its contribution in reinforcing the cementitious matrix is maintained and can be used effectively for large-scale production of cementitious admixtures. An illustrative method involves diluting the centrifuged and concentrated mass of nano-reinforcements to form an aqueous suspension and mixing the suspension with a cementitious material to form a composite material. Practice of embodiments of the invention by mixing suspensions of MWCNT reinforcements with cement based material yield similar strength and even marginally higher modulus of elasticity as compared to materials prepared using the non-concentrated suspensions.
The present invention provides a centrifuged and concentrated mass of nano-reinforcements (e.g. concentrated carbon nanotube suspension) that can find wide applications for highway structures, bridges, pavements and in general in all applications of conventional and high strength concrete, as well as in manufactured precast elements for residential and commercial buildings. The centrifuged and concentrated mass can be shipped dry or wet or re-suspended in water to users. Potential users of the masses or suspensions are concrete plants across the world, individual users, national and international contractors, developers and construction companies. The method for preparing highly dispersed and concentrated MWCNTs will be advantageous and useful to cement manufacturers and companies that develop admixtures for concrete or supplementary cementitious materials.
Other advantages of the present invention will become more apparent from the following detailed description taken with the following drawings.
a, 2b, 2c depict MWCNTs suspensions ultracentrifuged for 30 min, 45 min and 60 min, respectively.
a and 7b show effect of admixtures prepared using different ultracentrifugation methods on flexural strength and Young's modulus of cement paste (w/c=0.3), respectively.
The present invention involves a method for the preparation of highly concentrated MWCNT suspensions. The invention can be practiced to prepare highly concentrated suspensions of other nano-reinforcments including, but not limited to, single-walled carbon nanotubes, graphene sheets, graphene oxide sheets, and carbon nanofibers. For purposes of illustrating and not limiting the invention, the method is descried below and employed to reduce the amount of water in a MWCNT/water/surfactant suspension, increasing the MWCNT concentration in a final suspension.
Two ultracentrifuge rotors, the swing bucket and the fixed angle rotor, were used to highly concentrate the suspensions. Ultracentrifugation as used herein means a process of spinning these rotors at a significantly high speed of greater than about 10,000 rpm, producing an amplified effective gravitational force field for the contained samples. Optical absorbance spectroscopy was used to evaluate the concentration of the suspensions after ultracentrifugation. Cement paste samples were prepared using the highly concentrated MWCNT suspensions after they were diluted in water. Three point bending tests were performed in order to assess the effect of the concentration method on the mechanical properties of the produced nanocomposites. The results of the nanocomposites reinforced with the highly concentrated/diluted MWCNT suspensions were compared with the respective nanocomposites produced with the initial, non-concentrated MWCNT suspensions.
Purified multiwalled carbon nanotubes (MWCNTs), produced by the chemical vapor deposition method (CVD), with a diameter of about 20-40 nm, length of about 10-30 μm and purity >95% were used as received. The suspensions were prepared using MWCNTs at a concentration of 0.26 wt % which were dispersed in an aqueous solution containing a surfactant to MWCNTs weight ratio of 4.0. The mixture was then ultrasonicated using a 500 W cup-horn high intensity ultrasonic processor with a 13 mm diameter tip, operating at 50% of its maximum amplitude delivering 1900-2100 J/min. Energy was applied in cycles of 20 sec to prevent the suspensions from overheating.
Ultracentrifuges are typically available with a wide variety of rotors suitable for a great range of experiments. The most widely used configurations of rotors are the swing bucket and the fixed angle. The swing bucket rotors allow the tubes to hang on hinges so that they spin perfectly horizontally [reference 19]. During ultracentrifugation the material travels down the entire length of the centrifuge through the media within the tube [reference 20]. Fixed angle rotors contain cavities that hold the tubes at a predetermined angle [reference 19]. The materials are forced against the side of the centrifuge tube, and then slide down the wall of the tube [reference 20]. In this experiment, the effectiveness of the swing bucket and fixed angle rotors for concentrating MWCNT suspensions was investigated.
Following dispersion as described above, the suspensions were concentrated by ultracentrifugation. Initially, sedimentation of the MWCNTs was explored using a swing bucket SW41 rotor (from BeckmanCoulter) with ambient temperature at 22° C. and at 41,000 r.p.m. using centrifuge tubes that can hold 12 ml of suspension. In the aforementioned DGU process, OptiPrep containing 60% (w/v) of iodixanol was used as the density medium for the concentration process to increase the viscosity of concentrated dispersion for the subsequent density gradient separations. However, since the MWCNTs used have a density of 2.1 g/ml which is higher than the density of OptiPrep (1.32 g/ml) and furthermore there is no need for their viscosity adjustment, the suspensions in water with uniform density were simply added to the centrifugation tubes. The sedimentation of the MWCNTs was monitored at 30, 45 and 60 min.
After centrifugation, the supernatant solution was decanted down to approximately 2 cm from the bottom of the tube, enabling retrieval of 2.5 ml of concentrated suspension, which corresponds to about 20% of the total volume of the suspension. The concentration of MWNTs in the solution before and after centrifugation was quantified using optical absorbance spectroscopy (OAS). It was observed that the absorption of the suspension after centrifugation was lower than the sample before centrifugation. Several researchers have reported that the presence of MWCNTs agglomerates causes a decrease in the absorption spectrum because the MWCNTs bundles do not optically absorb in the wavelength region between 200 and 1200 nm [reference 21]. After a close observation of the samples, it was seen that a solid pellet of MWCNTs formed at the bottom of the tube within the residual water, causing the agglomeration of the MWCNTs and thus the reduction of the absorption.
In an optimized experiment, the MWCNT suspension was ultracentrifuged at 20,500 r.p.m. and it was observed that the MWCNTs had fully concentrated at the bottom of the tube after four hours. After ultracentrifugation and decantation, the hardened pellet of concentrated MWCNTs and surfactant-encapsulated MWNTs were re-suspended by ultrasonication inside the centrifuge tube submerged in an ice bath using the Sonic Dismembrator 500 from Fisher Scientific with a ⅛″ microtip attachment at 20% power for 5 min.
The aforementioned technique can load approximately 12 ml of MWCNT suspension per ultracentrifuge tube, which limits the yield of concentrated materials and precludes even the production of laboratory scale specimens. To be able to apply the method to cementitious samples, higher scale ultracentrifugation preferably should be employed to increase the loading and yield.
A preliminary investigation of scaling up the process with two larger capacity rotors was performed. Firstly, a swing bucket Ti 32 rotor was used, which can hold tubes of 38 ml capacity. The samples were centrifuged for 11 hours at 28,000 r.p.m to make sure of complete sedimentation of the MWCNTs at the bottom of the centrifuge tubes. After centrifugation, the supernatant solution was decanted from the tube, leaving 7.6 mL, or 20% of initial solution, of concentrated suspension. The remaining material was re-suspended by ultrasonication at 25% power for 40 min.
The second rotor studied was the JLA-16.250 fixed-angle style rotor, which can hold about 200 ml per tube. The samples were centrifuged for 11 hours applying 14,000 rpm which is close to the maximum speed of the rotor. Generally, substances in a fixed rotational environment precipitate faster with fixed angle rotors [reference 20]. The speed and the time of ultracentrifugation were estimated by calculating the cut-off threshold for sedimentation rate of MWCNTs, using the geometry of the rotors used and their average gravitational force applied so as to simulate the laboratory scale procedure. Though, complete sedimentation was not achieved. The material primarily sedimented at the side of the tube. Thus, during decantation, some MWCNTs were removed from the mix. After decantation, the remaining 50 ml of concentrated suspension was ultrasonicated at 40% power for 90 min to re-disperse any MWCNTs agglomerates created by the process.
After concentration, the MWCNT suspensions were diluted back to their initial concentration by adding the same amount of water that was removed during decantation. The diluted suspensions were then used to prepare cementitious samples with Type I ordinary Portland cement (OPC), at a water to cement ratio (w/c) of 0.3. The materials were mixed using a standard Hobart mixer following the procedure outlined by the ASTM 305. After mixing, the paste was placed in 20×20×80 mm molds. After demolding, the samples were cured in water saturated with lime until testing.
Initial evaluation of the concentration of MWCNTs in the aqueous surfactant solution was performed by optical absorbance spectroscopy (OAS). The test was conducted at a wavelength range of 260 nm to 400 nm using a Cary 5000 UV-Vis-NIR spectrophotometer from Varian Instruments. Three-point bending tests of beams with a 6 mm notch cut at the midspan were performed to investigate the effect of the different concentration techniques on the mechanical properties of cement based nanocomposites reinforced with MWCNTs. Following the testing procedure described in [reference 2], the beams were tested at the age of 3, 7 and 28 days. Based on ASTM C 348, three specimens were tested for each curing age. The tests were performed with a closed-loop MTS servo-hydraulic testing machine with a 20 kip (about 89 kN) capacity. A crack mouth opening displacement extensometer was used to control the test with a constant opening velocity of 0.009 mm/min. Load versus CMOD graphs were created from the test results. Young's modulus was then calculated from these graphs using the two-parameter fracture model by Jenq and Shah [reference 22]. Flexural strength was calculated using the net specimen depth.
The results of the absorbance spectra of the suspensions before and after centrifugation using the laboratory method (swing bucket SW41 rotor) are presented in
The produced admixtures were then diluted with water and mixed with the cement following the ASTM 305 standard. The absorbance spectra of the samples after dilution compared to the reference suspension are shown in
The effectiveness of the two different concentration methods was evaluated by three-point bending tests.
From the above examples, it is apparent that the present invention provides a method for the production of highly concentrated MWCNT suspensions that can be used as admixture in fabricating cementitious nanocomposites has been developed. It has been demonstrated, by optical absorbance spectroscopy, that the swing bucket rotor centrifugation can be used to effectively concentrate the MWCNTs suspensions, increasing the concentration of the MWCNTs by approximately five times. The produced suspensions were then diluted and used as admixture in the cementitious matrix. Specimens cast using suspensions prepared by the swing bucket rotor have been shown to outperform specimens prepared using the fixed angle rotor. Also, they exhibited similar flexural strength and marginally higher stiffness at 7 and 28 days, when compared to samples prepared using the non-concentrated suspensions.
The following detailed EXAMPLES set forth below provide more information on the method for concentrating the MWCNT suspensions and also a method for concentrating graphene oxide sheet nano-reinforcements in aqueous solution.
Graphene oxide was produced by a modified Hummer's Method described in Hummers W S, Offeman R E. Preparation of Graphitic Oxide. J. Am. Chem. Soc. 1958; 80: 1339, as follows:
The invention provides a simple method for preparing highly concentrated multi-walled carbon nanotube (MWCNT) and other nano-reinforcement suspensions, for applications as admixtures in cement based materials. Ultracentrifugation process was employed to increase the concentration of the MWCNTs dispersed with surfactant in an aqueous solution by removal of solvent, which will lead to reduction in cost and increase in efficiency of transportation and storage of such suspensions. The centrifuged and concentrated mass can be shipped dry or wet or resuspended in water to users. Two different ultracentrifuge rotors, the swing bucket and the fixed angle rotor, were used. The concentration of the produced suspensions was quantified using optical absorbance spectroscopy. It was found that the ultracentrifugation process enhanced the concentration of the MWCNT suspension by five times. To evaluate the effectiveness of the concentration process, three point bending tests were conducted on nanocomposites produced using suspensions before and after ultracentrifugation. Specimens cast using the swing bucket concentrated suspensions exhibited similar flexural strength and marginally higher stiffness compared to the samples prepared using the non-concentrated suspensions. These results demonstrate that the ultracentrifugation concentration method preserves the solubility of the MWCNT suspension and its contribution in reinforcing the cementitious matrix and thus it is an effective preparation step for large-scale production of such admixtures.
Although the invention has been described in connection with certain embodiments thereof, those skilled in the art will appreciate that changes and modifications can be made therein within the scope of the invention as set forth in the appended claims.
References which are incorporated herein by reference:
This application is a continuation-in-part of U.S. Ser. No. 12/322,842 filed Feb. 6, 2009, which claims benefits and priority of U.S. provisional application Ser. No. 61/027,160 filed Feb. 8, 2008, the disclosures of which are incorporated herein by reference.
The invention was made with government support under grant numbers DMR-0706067 and DMR-1006391 awarded by the National Science Foundation. The Government has certain rights in the invention.
| Number | Date | Country | |
|---|---|---|---|
| 61027160 | Feb 2008 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 12322842 | Feb 2009 | US |
| Child | 13135004 | US |
