Methods of manufacturing energy conversion materials fabricated with boron nitride nanotubes (BNNTs) and BNNT polymer composites

Abstract

Formation of a boron nitride nanotube nanocomposite film by combining a boron nitride nanotube solution with a matrix such as a polymer or a ceramic to form a boron nitride nanotube/polyimide mixture and synthesizing a boron nitride nanotube/polyimide nanocomposite film as an electroactive layer.

Claims

1. A method for forming a boron nitride nanotube nanocomposite comprising: combining a boron nitride nanotube solution with a matrix including at least one of a polymer or a ceramic to form a boron nitride nanotube nanocomposite; synthesizing the boron nitride nanotube nanocomposite film as an electroactive layer from the boron nitride nanotube nanocomposite such that when an electric power source coupled to said electroactive layer is actuated, a mechanical deflection is achieved due to an electroactive characteristic solely in the boron nitride nanotube nanocomposite in the electroactive layer; and forming a plurality of electrodes on the electroactive layer, wherein the boron nitride nanotube nanocomposite film is mechanically deflected when an electrical charge is applied, and electroactivity is achieved when the electrical charge is applied from the electric power source and the plurality of electrodes, and the boron nitride nanotube nanocomposite provides the electroactive characteristic.

2. The method of claim 1, wherein the matrix is synthesized from a diamine, 2,6-bis(3-aminophenoxy)benzonitrile ((-CN),APB) and a dianhydride, pyromellitic dianhydride (PNIDA).

3. The method of claim 1, wherein the polymer is selected from the group consisting of polyvinylidene fluoride, polyvinylidene fluoride copolymer, polycarbonate and epoxy.

4. The method of claim 1, wherein the polymer is selected from the group consisting of polyurethane and polysiloxane.

5. The method of claim 1, wherein the ceramic is selected from the group consisting of silicon dioxides and aluminum oxides.

6. The method of claim 1, wherein a concentration of boron nitride nanotubes in the boron nitride nanotube nanocomposite film is between 0 and 100% by weight.

7. The method of claim 1, wherein the forming of the plurality of electrodes includes coating the boron nitride nanotube nanocomposite film with metal electrodes.

8. The method of claim 7, wherein a metal for the metal electrodes is selected from the group consisting of chrome and gold.

9. The method of claim 7, wherein a metal for the metal electrodes include a mixture of chrome and gold.

10. The method of claim 1, wherein the forming of the plurality of electrodes includes coating the boron nitride nanotube nanocomposite film with compliant electrodes.

11. The method of claim 10, wherein the compliant electrodes are selected from the group consisting of carbon nanotubes, carbon nanotube sheeting, carbon nanotube/polymer composites, gold particles, and silver particles.

12. The method of claim 10, wherein the compliant electrodes are a mixture including one or more of carbon nanotubes, carbon nanotube sheeting, carbon nanotube/polymer composites, gold particles, and silver particles.

13. The method of claim 1, wherein a concentration of boron nitride nanotubes in the boron nitride nanotube nanocomposite film is 2% by weight.

14. A method for forming a boron nitride nanotube nanocomposite film, comprising: combining a boron nitride nanotube solution with a matrix including at least one of a polymer or a ceramic to form a boron nitride nanotube nanocomposite; synthesizing the boron nitride nanotube nanocomposite film as an electroactive layer from the boron nitride nanotube nanocomposite such that when a mechanical deflection is applied to the boron nitride nanotube nanocomposite film, an electrical charge is produced due to an electroactive characteristic solely in the boron nitride nanotube nanocomposite in the electroactive layer; and forming a plurality of electrodes on the electroactive layer, wherein the boron nitride nanotube nanocomposite produces said electrical charge when mechanically deflected, and electroactive properties and the plurality of electrodes enable the electrical charge to be generated when the boron nitride nanotube nanocomposite film is mechanically deflected.

15. The method of claim 14, wherein the matrix is synthesized from a diamine, 2,6-bis(3-aminophenoxy)benzonitrile ((-CN)APB) and a dianhydride, pyromellitic dianhydride (PMDA).

16. The method of claim 14, wherein the polymer is selected from the group consisting of polyvinylidene fluoride, polyvinylidene fluoride copolymer, polycarbonate and epoxy.

17. The method of claim 14, wherein the polymer is selected from the group consisting of polyurethane and polysiloxane.

18. The method of claim 14, wherein the ceramic is selected from the group consisting of silicon dioxides and aluminum oxides.

19. The method of claim 14, wherein a concentration of boron nitride nanotubes in the boron nitride nanotube nanocomposite film is between 0 and 100% by weight.

20. The method of claim 14, wherein the forming of the plurality of electrodes includes coating the boron nitride nanotube nanocomposite film with metal electrodes.

21. The method of claim 20, wherein a metal for the metal electrodes is selected from the group consisting of chrome and gold.

22. The method of claim 20, wherein a metal for the metal electrodes include a mixture of chrome and gold.

23. The method of claim 14, wherein the forming of the plurality of electrodes includes coating the boron nitride nanotube nanocomposite film with compliant electrodes.

24. The method of claim 23, wherein the compliant electrodes are selected from the group consisting of carbon nanotubes, carbon nanotube sheeting, carbon nanotube/polymer composites, gold particles, and silver particles.

25. The method of claim 23, wherein the compliant electrodes is a mixture including one or more of carbon nanotubes, carbon nanotube sheeting, carbon nanotube/polymer composites, gold particles, and silver particles.

26. The method of claim 14, wherein a concentration of boron nitride nanotubes in the boron nitride nanotube nanocomposite film is 2% by weight.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) A more complete description of the subject matter of the present invention and the advantages thereof, can be achieved by reference to the following detailed description by which reference is made to the accompanying drawings in which:

(2) FIG. 1a shows a schematic diagram of a metal electroded BNNT/polymer composite actuator;

(3) FIG. 1b shows a Schematic diagram of a carbon nanotube electroded BNNT actuator;

(4) FIG. 2a shows a graph of thermally stimulated current (TSC) spectra of pristine polyimide and 2 wt % BNNT/polyimide composite;

(5) FIG. 2b shows a graph of remanent polarization (P.sub.r) of pristine polyimide and 2 wt % BNNT/polyimide composite;

(6) FIG. 3 shows a proto-type BNNT actuator fabricated with carbon nanotube electrodes;

(7) FIG. 4 shows a cross-sectional SEM image of a prototype BNNT actuator fabricated with carbon nanotube electrodes;

(8) FIG. 5a shows a graph of the electric field induced strain of the BNNT actuator fabricated with CNT electrodes;

(9) FIG. 5b shows a graph of the piezoelectric response of the BNNT actuator fabricated with CNT electrodes; and

(10) FIG. 5c shows a graph of the electrostrictive response of the BNNT actuator fabricated with CNT electrodes.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

(11) The following detailed description is of the best presently contemplated mode of carrying out the invention. This description is not to be taken in a limiting sense, but is made merely for the purpose of illustrating general principles of embodiments of the invention.

(12) Since the first theoretical prediction of the existence of boron nitride nanotubes (BNNTs) in 1994 and the first experimentally synthesized BNNT report by Zettl's group in 1995, several types of BNNT synthesis methods have been reported. Recently, a new and conceptually simple method of producing extraordinarily long, highly crystalline BNNTs was demonstrated. BNNTs are thought to possess high strength-to-weight ratio, high thermal stability (up to about 800 C. in air), piezoelectricity, and radiation shielding capabilities. Nakhmanson's theoretical analysis predicted that the piezoelectric coefficient of BNNTs can be higher than that of poly(vinylidene fluoride) (PVDF) or poly(vinylidene fluoride-trifluoroethyene) P(VDF-TrFE). However, the piezoelectric properties of BNNTs or BNNT composites have not been reported experimentally as yet. In this invention, we make use of the electroactive characteristics of novel BNNT based materials.

(13) First, a BNNT/polyimide nanocomposite film was synthesized as an electroactive layer by in-situ polymerization under simultaneous shear and sonication. The high temperature piezoelectric polyimide, used as a matrix for this invention, was synthesized from a diamine, 2,6-bis(3-aminophenoxy)benzonitrile ((-CN)APB), and a dianhydride, pyromellitic dianhydride (PMDA). The concentrations of BNNTs in the polyimide were 0 and 2 wt %. In order to characterize electroactive properties of the composites, the samples were coated with metal (chrome/gold) electrodes for both sides (FIG. 1a).

(14) Thermally stimulated current (TSC) spectra of the BNNT nanocomposites were obtained using a Setaram TSC II. Each sample was polarized by a direct current (DC) electric field of 5 MV/m at an elevated temperature (T.sub.p=T.sub.g5 C.) for a selected poling time (t.sub.p=30 min). The glass transition temperatures (T.sub.g) of the pristine polyimide and 2% BNNT/polyimide composite, measured by a differential scanning calorimeter (DSC), are 274.3 and 271.4 C., respectively. After poling, the depolarization current was measured as the sample was heated through its glass transition temperature (T.sub.g) at a heating rate of 7.0 C./min. As shown in FIG. 2a, the pristine polyimide showed negligible depolarization currents until about 225 C., which indicates a good thermal stability of polarization, and then exhibited a rapid depolarization current with a maximum peak of 0.012 mA/m.sup.2 at 255.9 C. On the other hand, the 2 wt % BNNT/polyimide nanocomposite exhibited two depolarization peaks at 119.3 C. and 255.5 C. The magnitude of the depolarization current of the nanocomposite was significantly larger than that of the pristine polyimide as seen in FIG. 2b, and reached a maximum value of about 0.05 mA/m.sup.2, five times greater than that of the pristine polyimide. The remanent polarization (P.sub.r) was calculated by integrating the current with respect to time and is plotted as a function of temperature as shown in FIG. 2b. P.sub.r is given by,

(15) $\begin{array}{cc}{P}_r=\frac{q}{A}=\frac{1}{A}I\left(t\right)\mathrm{dt}& \left(1\right)\end{array}$
where q is the charge, A is the electrode area, I is the current, and t is the time. Details of conventional poling procedures have been described elsewhere [J. H. Kang et al., NANO, 1, 77 (2006)]. The remanent polarization (P.sub.r) of the 2 wt % BNNT/polyimide nanocomposite was 12.20 mC/m.sup.2, almost an order of magnitude higher than that of the pristine polyimide (1.87 mC/m.sup.2). In general, the piezoelectricity of a material is proportional to its remanent polarization. From the TSC result, adding BNNT, even only 2 wt %, was proven to increase the piezoelectricity (remanent polarization) of the polyimide significantly.

(16) An all nanotube film actuator, with a BNNT active layer, was fabricated by a filtering method [J. H. Kang et al., J. Polym. Sci. B: Polym Phys. 46, 2532 (2008)]. Single wall carbon nanotubes (SWCNTs) were used as electrodes for the actuator instead of metal. First, solutions of SWCNTs and BNNTs were prepared in N-methylpyrrolidone (NMP) under sonication. An adequate amount of the SWCNT solution was filtered through the surface of an anodized alumina membrane (pore size: 0.2 m) to form a SWCNT film on the membrane. Then, the BNNT solution and finally the SWCNT solution were sequentially filtered onto the SWCNTs film on the membrane to make a three layered (SWCNT/BNNT/SWCNT) all-nanotube actuator structure shown in FIG. 3. The freestanding all-nanotube actuator film, shown in FIG. 3, was easily delaminated by breaking the brittle membrane. To increase durability, polyurethane resin was infused into the all-nanotube actuator. FIG. 4 shows the cross-sectional scanning electron microscopy (SEM) image of a prototype BNNT actuator fabricated with SWCNT electrodes (Hitachi S-5200 Field Emission Scanning Electron Microscope). The top and bottom layers are SWCNT electrodes and the middle layer is the BNNT actuating layer.

(17) In-plane strain (S.sub.13) was measured using a fiber optic device while the sample was under an alternating current (AC) electric field of 1 Hz. The strain (S.sub.13) of the sample appears as a superposed curve (black solid squares in FIG. 5a) of linear and nonlinear strains as a function of frequency. The superposed curve was de-convoluted to a linear response (red solid circles in FIG. 5a) and a nonlinear response (blue solid triangles in FIG. 5a). The linear response seems to originate from the piezoelectric property of the BNNT active layer. From linear fitting of the data (FIG. 5b), the piezoelectric coefficient, d.sub.13 was calculated to be about 14.80 pm/V. This is comparable to the values of commercially available piezoelectric polymers such as poly(vinylidene fluoride) (PVDF). The nonlinear response showed a quadratic increase with increasing applied electric field, indicating that the mechanism of this strain is mainly an electrostrictive response (FIG. 5c). The electrostrictive coefficient (M.sub.13) of the BNNT active layer, calculated from the slope of a plot of the strain (S.sub.13) to the square of electric field strength (E.sup.2), S.sub.13=M.sub.13 E.sup.2, was 3.2110.sup.16 pm.sup.2/V.sup.2 on average. This value is several orders of magnitude higher than those of electrostrictive polyurethanes (4.610.sup.18 to 7.510.sup.17 m.sup.2/V.sup.2).

(18) Obviously, many modifications may be made without departing from the basic spirit of the present invention. Accordingly, it will be appreciated by those skilled in the art that within the scope of the appended claims, the invention may be practiced other than has been specifically described herein. Many improvements, modifications, and additions will be apparent to the skilled artisan without departing from the spirit and scope of the present invention as described herein and defined in the following claims.