WO2014123532A1

WO2014123532A1 - High strength nanocomposite glass fibers

Info

Publication number: WO2014123532A1
Application number: PCT/US2013/025251
Authority: WO
Inventors: Janet HURST
Original assignee: United States Of America, As Represented By The Administrator Of The National Aeronautics And Space Administration
Priority date: 2013-02-08
Filing date: 2013-02-08
Publication date: 2014-08-14

Abstract

A composite glass fiber with high strength is described along with methods of its manufacture. This glass fiber utilizes the high strength and modulus of nanomaterials such as carbon nanotubes or other compositions of interest such as boron nitride or silicon carbide to synthesize high strength, nanocomposite glass fibers.

Description

High Strength Nanocomposite Glass Fibers

Origin of the Invention

[0001 ] The invention described herein was made by an employee of the United States Government and may be manufactured and used by or for the Government for Government purposes without the payment of any royalties thereon or therefore.

Cross-Reference to Related Applications

[0002] This application claims the benefit of U.S. Provisional Application No. 61 /437,040 filed on January 28, 201 1 .

Background

[0003 ] Glass fibers have been used for many years for a variety of applications. These applications range from the inexpensive glass fibers in the form of fiberglass, the mainstay for insulation in the building industry, to optical glass fibers which are extensively used in communications. While these glass fibers possess useful properties, they have not rivaled the mechanical properties of carbon fibers.

[0004] Carbon fibers are used widely to provide structural reinforcement to many materials. However carbon fibers are difficult to manufacture which renders this useful engineering material a high cost reinforcement. Additionally, carbon fibers also tend to be not oxidatively stable above 500°C, reducing both the potential end-use applications to low temperatures and the potentially available manufacturing methods. Summary

[0005] By developing a method to synthesize glass fibers with enhanced mechanical properties, it is possible to manufacture an alternative to carbon fiber which is also oxidatively stable. These glass fibers may be synthesized by utilizing nanotubes or nanofibers of carbon or boron nitride or other compositions as they become available as a reinforcing phase. Hereafter, nanotubes and nanofibers will collectively be referred to as nanomaterials. "Nanomaterial" as generally used herein is defined to mean a material having nanometric dimensions, I.e. a material at least one of the dimensions of which is at a nanometric scale. For example, a material in at least one of the dimensions of space having a dimension between 1 and 500 nm. The glass matrix provides protection from an oxidative environment to the nanomaterial reinforcing phase. The addition of nanomaterials to several matrix materials have demonstrated enhanced mechanical properties. These enhanced matrices can include a wide variety of materials including polymers, glass, ceramics and metals.

Description of Drawings

[0006] The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate various example systems, methods, and so on that illustrate various example embodiments of aspects of the invention. It will be appreciated that the illustrated element boundaries (e.g., boxes, groups of boxes, or other shapes) in the figures represent one example of the boundaries. One of ordinary skill in the art will appreciate that one element may be designed as multiple elements or that multiple elements may be designed as one element. An element shown as an internal component of another element may be implemented as an external component and vice versa. Furthermore, elements may not be drawn to scale.

[0007] FIGURE 1 illustrates a glass pre-form with non-aligned, non- dispersed nanomaterials.

[0008] FIGURE 2 illustrates a method of forming a glass pre-form with nanomaterials.

[0009] FIGURE 3 illustrates a method of forming glass fiber having substantially aligned and dispersed nanomaterials.

[00 Ί 0] FIGURE 4 illustrates an exemplary glass fiber with improved mechanical properties from aligned and dispersed nanomaterials.

Detailed Description

[00 Ί 1 ] One difficulty in achieving improved properties of reinforced glass has been the difficulty in uniformly dispersing nanomaterials within a matrix. Clumps and/or tangles of nanomaterials actually reduce strength as the agglomerated nanomaterials act as defects within in the matrix. Polymer matrices, with the lower processing temperatures which are required in these systems, have demonstrated the greatest degree of success in obtaining improved strength through uniformly dispersed nanomaterials.

[001 2] Another difficulty in synthesizing glass composite fiber with nanomaterial reinforcement is that nanomaterials are very light and tend to separate within a glass melt due to differences in density. Nanomaterials typically have densities of around 1 gram/cm3 and glass is typically 4 grams/cm3. This results in difficulty mixing nanomaterials into the glass melt as the density difference causes nanomaterials to float near the surface of a glass melt.

[001 3 ] Another difficulty related to the nanomaterials floating near the surface of a glass melt arises as nanomaterials composed of carbon and/or boron nitride are not stable in a glass melt in the presence of oxygen at the surface of a glass melt.

[00 Ί 4] Typical glass processing methods utilized for synthesizing glass fibers have not proven useful for developing reinforced glass fibers with nanomaterials. Nanomaterials tend not to be oxidatively stable at the temperatures necessary in glass melting furnaces. These furnaces operate at temperatures which are typically from 1 200 to 1 400°C or even higher and often utilize additional oxygen in the form of oxygen torches to speed melting. Carbon nanotubes and fibers oxidize violently at only 500°C while the higher temperature capability boron nitride nanotubes are still insufficient with oxidative stability to 1 000°C and are not commercially available.

[001 5] One aspect of the disclosure exploits the unexpected stability of carbon nanotubes (CNT) and potentially nanomaterials in general in a glass melt which is processed under inert atmospheres. Recent work at NASA Glenn Research Center successfully demonstrated nanotube material (Boron Nitride nanotube (BNNT)) as a structural reinforcement for a barium calcium aluminosilicate glass compositions. Both strength and fracture toughness demonstrated marked improvement. Boron Nitride Nanotubes-Reinforced Glass Composites. Narottam P. Bansal, Janet B. Hurst, and Sung R. Choi, NASA Glenn Research Center, Cleveland, Ohio 441 351 . Am. Ceram. Soc, 89 [ 1 ] 388-390 (2006). [001 6] However these composites were manufactured by hot pressing mixtures of well dispersed nanotubes within glass powder at high temperatures and large pressures. This is not a method suitable to producing glass fibers. Additionally, in the publication the nanotube/fiber reinforcing phase was randomly distributed within the matrix. The most desirable situation is for the reinforcing phase to be distributed in the loading direction. Finally, BNNT was chosen for this application as the stability of CNT in a glass was deemed to be poor and BNNT has demonstrated oxidative stability up to 1 000°C in air relative to approximately 500°C for CNT in air.

[001 7] In another aspect of the disclosure, carbon nanotubes were used to reinforce hot pressed Si02 glass. It was expected that CNT would be destroyed or damaged by either the pressure or by oxidation. Surprisingly, carbon nanotubes survived processing intact. Hot pressing was conducted up to temperatures of 1 600° C as pure Si02 is a very high temperature, strong material. It was speculated that the oxidation kinetics involved in reacting Si02 with carbon without the presence of an oxygen containing atmosphere was sufficiently slow so allow the CNT to survive processing. In the presence of an oxidizing atmosphere, for example, in a standard glass melt which is exposed to the atmosphere, CNT will completely oxidize in very short times. In inert conditions, CNT is very stable to extremely high temperatures. The strength of the CNT/Si02 composite though was not enhanced due to existence of agglomerated tangles of CNT throughout the composite which were not infiltrated by glass. This observation is supported by Mechanical and electrical properties of hot-pressed borosilicate glass matrix composites containing multi- wall carbon nanotubes. R. Boccaccini, B. 1 . C. Thomas, G. Brusatin, P. Colombo, J Mater Sci (2007) 42:2030-2036A, describing much the same effect where 1 0 wt% multi-wall carbon nanotubes (MWCNT) were added to a borosilicate glass and condensed via hot pressing.

[00 Ί 8] Similarly, the CNT survived but strength did not benefit due to the existence of agglomerated regions of nanotubes and high porosity. However, the brittleness index value (ratio of hardness to fracture toughness) suggested improved wear resistance and contact damage. Electrical resistivity was also greatly decreased with the addition of CNT to borosilicate glass.

[001 9] In another aspect of the disclosure improved dispersion or alignment of the nanomaterials to overcome agglomeration may be achieved by either pulling fibers from a melt or by extrusion pressing to enhance shear mixing. In this case, so called E-glass fibers containing carbon nanotubes were aligned with the fiber axis. Average strength of these fibers was two times that of fibers pulled without the nanomaterial reinforcement. Again, in this system, similar to the BNNT/barium calcium aluminosilicate glass, CNT appeared to pullout from the glass surface in classical composite behavior. In over 200 individual fiber fracture surfaces which were examined, no regions of CNT agglomeration were detected. Additionally, polished surface sections taken throughout the fiber lengths detected only standard porosity unlike hot pressed samples. This processing method eliminated the porous regions of CNT agglomerations which were not infiltrated by glass which were evident in hot pressed samples and resulted in poorer than hoped for strengths.

[0020] In another aspect of the disclosure, processing methods are shown to provide oxidative protection to the nanomaterials by processing in well controlled, oxygen-free conditions both during incorporation of the nanomaterials into the glass preform and again in the glass fiber drawing step. Additionally, as previously discussed, the density differences between the glass and the nanomaterials are overcome in synthesizing composites with well distributed nanomaterials, a key requirement for optimized mechanical properties. Finally, nanomaterial "float" is reduced to prevent nanomaterial destruction by oxidation.

[0021 ] As expense is a major consideration in most commercial ventures, the economics of carbon nanotube reinforcement must be carefully evaluated. However, strengthening was demonstrated with only 4 wt% BNNT in glass.

Preliminary work suggests 2 wt % of an inexpensive MWCNT product may be sufficient to achieve the same affect. Additionally, while BNNT is more

oxidatively stable, it is difficult to prepare and so still relatively expensive.

[0022] Equipment to manufacture the composite glass/nanomaterial can include a glass melter within an environment chamber or bell jar, or a glass lathe such as typically used for optical glass manufacturing used to create a

feedstock rod of glass with nanotubes. Optimized mixing may not be required as additional melting and nanomaterial alignment may be achieved in a

subsequent step which drew the feedstock into glass fiber. Agglomerated regions of nanomaterial may also be dispersed in this subsequent step.

[0023 ] With regard to Figures 1 and 2, one method to achieve a

nanomaterial/glass composite uses selected processing techniques traditionally reserved for optical glass. A feedstock glass rod 1 00 with somewhat

dispersed, poorly aligned carbon nanotubes/fibers or other nanomaterials may be created by filling a hollow glass rod 1 02 with glass and nanomaterial 1 04. The glass rod may or may not be transparent as illustrated depending largely on the composition of the glass and the particular nanomaterial 1 04 used. Moreover, while the nanomaterial 1 04 appear as single separate strands for ease of illustration and understanding, those skilled will appreciate that, in practice the nanomaterial agglomorates together into clumps of material. The glass may be deposited by a modified chemical vapor deposition process 200 shown generally. The modified chemical vapor deposition process 200 may use various gases 204 to create a wide array of compositions. This is a process more typically associated with the synthesis of optical fiber. The compositions of interest for optical glasses often are mixtures of Si, Ge, and B among others. In this case, simpler compositions such as those similar to so- called e-glass and s-glass may be utilized as the one purpose is mechanical property enhancement rather than improved optical qualities. Nanomaterials 1 04 may be co-deposited with glass material or the feedstock may be periodically removed from the glass lathe 202 and nanomaterials may be added as a separate step. Addition of the nanomaterials may be accomplished by a gas carrier or other methods as well. One variation is to grow nanomaterials in- situ during manufacturing of the preform by incorporating steps utilizing gases such as actelyene and FeCI₃, common reagents for carbon nanotubes. After the glass tube 1 00 has been filled with a glass/nanomaterial mixture, the tube may be collapsed, as is typical in optical glass formation. After removing the glass feedstock from the lathe 202 , a polishing step may be useful to remove surface defects.

[0024] With respect to Figure 3, such a feedstock 1 00 may be inserted into a glass fiber drawing tower 300 and pulled into fibers. The tower 300 includes a preform feed 304 where feedstock 1 00 is supplied to a small furnace 308 that may be configured as a laser. The glass melt zone is very small in this process and the length of time material remains in the molten state is short. Inert gas may be incorporated as the processing atmosphere, protecting the nanotubes/fibers from oxidation damage. This method also reduces the opportunity for nanotubes/fibers to separate from the melt.

Nanotube alignment is not necessary within the feedstock 1 00, only partial dispersion and distribution will suffice. During the pulling and heating of the glass fiber, shear forces urge the nanotubes/fibers toward dispersion and alignment. As the pull continues, a laser micrometer 31 2 or other measuring device ensures desired dimensions are maintained by adjusting the tractor pull rate. The fiber passes through a first coating and curing section 31 6 followed by a second coating and curing section 320. Finished fiber 350 is then wound onto a reel 354.

[0025] Referring now to Figure 4, a glass/nanoparticle composite 400 is shown. The glass/nanoparticle composite 400 defines an axis A. Substantially evenly dispersed within the composite are nanoparticles 41 4 substantially aligned along or parallel to the axis A.

[0026] The exceptional 1 -D mechanical properties of carbon

nanotubes/fibers can then be utilized in axial fiber direction. Also, by altering the nanotube density in the feedstock, one can tailor the nanotube density in the fiber as well. The composite fibers will likely require surface coatings as damage to the glass surface may lead to strength limiting flaws as in the case of any glass fiber.

[0027] In an alternate embodiment, it is to be appreciated that it may be possible to remove many of the chemicals typically used in optical glass compositions, as optical properties are not required in this application. Carbon nanotubes/fibers are black, compromising the optical properties of these fibers. Should enhanced optical properties be desired, BN nanotubes are transparent and may be used to enhance strength without severely damaging optical properties. Further simplifications of the processing methodology are possible, to both reduce the cost and optimize the resultant fiber product.

[0028] Composite theory and practice has both theorized and then demonstrated that improved mechanical properties were possible when matrices were reinforced with high strength fibers. For example, tensile strength tends to be maximized when fibers are oriented in the mechanical loading directions. Additionally, nanotubes are expected to behave similarly to fibers in regards to providing composite toughening mechanisms such as crack deflection, crack bridging and fiber and/or nanotube pullout within a composite. Extrapolation of this phenomena to a fiber suggests that a composite fiber with alignment of dispersed nanotubes would provide substantially improved properties.

Experimental Results

[0029] Improvements of mechanical properties were demonstrated for glass composites which were reinforced with randomly oriented boron nitride nanotubes. In this case tensile strengths were nearly doubled and fracture toughness improvements of 30% were demonstrated. By orienting nanotubes in the loading direction, further improvements in mechanical properties are anticipated. In one example, glass fibers with small additions of carbon nanotubes were made. This method proved to be impractical for several reasons, among them: immediate oxidation of the pyrophoric carbon nanotubes unless they were first encapsulated in a hot presses glass matrix before insertion in the glass melt; density differences between glass and carbon nanotubes resulted in separation of the nanotubes to the surface of the melt where oxidation immediately occurred; difficulty in mixing the melt; oxygen torches commonly utilized to speed melting could not be used due to oxidation of the nanotubes; and loss of nanotubes over time within the melt required rapid processing which was not compatible with commercialization of this approach. However, by methodical examination of thousands of feet of fiber tow, it was possible to find short sections of fiber with nanotubes present.

Likely these short sections were composed of material which passed very quickly through the melter. These sections demonstrated 2-3 times

improvement in tensile strength relative to the majority of the fiber tows, where no nanotubes were observed. Even in these isolated areas, loading density was very low. This result demonstrated that a method of controlling nanotube dispersion and loading density was needed as now provided by the current specification.

[0030] This method enables dispersed nanomaterials in a uniform

concentration throughout long lengths of glass fibers. Nanomaterial

reinforcement has demonstrated enhanced mechanical properties in many composite materials, including those with glass matrices. This method

additionally includes reduced exposure of nanomaterials to high temperature oxidative environments which are detrimental to nanomaterials. Carbon nanotubes are pyrophoric at temperatures of around 500°C while boron nitride nanotubes slowly oxidize at temperatures of around 800-1 000°C.

[0031 ] There are numerous potential commercial applications for high strength glass fibers, including many of the current applications of carbon fibers. Some of these applications include the automotive industry, boating, energy storage applications such as flywheels, aerospace applications such as lightweight airframes, sporting goods.

[0032] While the systems, methods, and so on have been illustrated by describing examples, and while the examples have been described in

considerable detail, it is not the intention of the applicants to restrict or in any way limit the scope of the appended claims to such detail. It is, of course, not possible to describe every conceivable combination of components or methodologies for purposes of describing the systems, methods, and so on provided herein. Additional advantages and modifications will readily appear to those skilled in the art. Therefore, the invention, in its broader aspects, is not limited to the specific details, the representative apparatus, and illustrative examples shown and described. Accordingly, departures may be made from such details without departing from the spirit or scope of the applicants' general inventive concept. Thus, this application is intended to embrace alterations, modifications, and variations that fall within the scope of the appended claims. Furthermore, the preceding description is not meant to limit the scope of the invention. Rather, the scope of the invention is to be determined by the appended claims and their equivalents.

[0033 ] As used herein, "connection" or "connected" means both directly, that is, without other intervening elements or components, and indirectly, that is, with another component or components arranged between the items identified or described as being connected. To the extent that the term

"includes" or "including" is employed in the detailed description or the claims, it is intended to be inclusive in a manner similar to the term "comprising" as that term is interpreted when employed as a transitional word in a claim. Furthermore, to the extent that the term "or" is employed (e.g., A or B) it is intended to mean "A or B or both". When the applicants intend to indicate "only A or B but not both" then the term "only A or B but not both" will be employed. Similarly, when the applicants intend to indicate "one and only one" of A, B, or C, the applicants will employ the phrase "one and only one". Thus, use of the term "or" herein is the inclusive, and not the exclusive use. See, Bryan A. Garner, A Dictionary of Modern Legal Usage 624 (2d. Ed. 1 995).

Claims

I Claim:

1 . A manufacture comprising:

A drawn glass fiber defining a central axis; and

A plurality of nanomaterials substantially evenly dispersed within the glass fiber, where the plurality of nanomaterials are oriented substantially parallel to the central axis.

2. The manufacture as set forth in claim 1 , further comprising a protective surface coating on an exterior of the glass fiber.

3. The manufacture as set forth in claim 1 , where the plurality of

nanomaterials comprise carbon nanotubes.

4. The manufacture as set forth in claim 1 , where the plurality of

nanomaterials comprise boron nitride nanotubes.

5. The manufacture as set forth in claim 1 , where the plurality of

nanomaterials comprise silicon carbide nanotubes.

6. The manufacture as set forth in claim 1 , wherein the manufacture comprises at least twice the tensile strength of a glass fiber without the plurality of nanomaterials substantially evenly dispersed within the glass fiber, and without the plurality of nanomaterials being oriented substantially parallel to the central axis.

7. The manufacture as set forth in claim 1 , further comprising an optical glass fiber surrounded by the drawn glass fiber defining a central axis and plurality of nanomaterials substantially evenly dispersed within the glass fiber, where the plurality of nanomaterials are oriented substantially parallel to the central axis.

8. The manufacture as set forth in claim 1 , where the plurality of

nanomaterials comprise 1 0 percent by weight nanomaterials.

9. The manufacture as set forth in claim 1 , where the plurality of

nanomaterials comprise 4 percent by weight nanomaterials.

1 0. The manufacture as set forth in claim 1 , where the plurality of

nanomaterials comprise 2 percent by weight nanomaterials.

1 1 . The manufacture as set forth in claim 1 , where the plurality of

nanomaterials comprise between 1 0 percent by weight and 2 percent by weight nanomaterials.

1 2. A method of forming glass fiber comprising:

Providing a glass feed rod comprising non-aligned nanomaterials;

Feeding the glass feed rod into a furnace in an inert processing

atmosphere; and

Drawing glass fiber from a melted portion of the glass feed rod; Where the glass fiber comprises nanomaterials substantially parallel with one another and substantially evenly dispersed throughout the glass fiber.

1 3. The method as set forth in claim 1 2, where the providing comprises filling a hollow glass rod with glass and nanomaterials.

1 4. The method as set forth in claim 1 3, further comprising collapsing the filled glass rod into the glass feed rod.

1 5. The method as set forth in claim 1 2, where the furnace comprises a laser.

1 6. The method as set forth in claim 1 2, further comprises coating the drawn glass fiber.

1 7. The method as set forth in claim 1 2, further comprises curing the drawn glass fiber.

1 8. A glass fiber nanomaterial composite material comprising:

A glass fiber defining a central axis, the glass fiber drawn from a melt area of glass feedstock; and

Between 1 0 percent by weight and 2 percent by weight nanomaterials substantially not agglomerated throughout the glass fiber, where the

nanomaterials are predominantly disposed parallel to the axis,

Wherein the glass fiber nanomaterial composite exhibits at least twice the tensile strength of a glass fiber without nanomaterials.

1 9. The glass fiber as set forth in claim 1 8, where the nanomaterials comprise single wall nanotubes, multiwall nanotubes or nanofibers.

20. The glass fiber as set forth in claim 1 8, where the nanomaterials are oxidatively stable at 500°C up to about 1 000°C.