JP2012516827A - Boron nitride nanotube fibrils and yarns - Google Patents

Abstract

Boron nitride nanotube fibrils made of long, high aspect ratio boron nitride nanotubes and single or small number of walled boron nitride nanotubes in bundles of 20 μm or more at a speed in excess of about 1 meter per second Method and apparatus for manufacturing. A nanotube yarn comprising such a twisted bundle of nanotube fibrils is also described.
[Selection] Figure 1

Description

The present invention relates to nanostructure fabrication, and more particularly to a method for high speed fabrication of long strand boron nitride nanotube fibers, boron nitride nanotube fibrils and boron nitride nanotube yarns.

Since the announcement in 1995 that the synthesis of boron nitride nanotubes with a relatively high aspect ratio and few walls (FW-BNNT) was successful, the scale of the synthesis has little progress. One proof is that despite the theoretical ability of FW-BNNT to provide high strength / weight, high temperature resistance, piezoelectric operation and radiation shielding (depending on boron content), the aerospace industry is using structural Still rely on micron-sized graphite or boron fibers. FW-BNNT is widely used because such materials currently available are very small, even in the aerospace and other manufacturing industries that are generally very willing to pay a premium for high performance. It has not been.

To date, relatively high aspect ratio FW-BNNTs have been produced in small quantities (from individual tubes to milligrams) by arc discharge or laser heating methods. Another type of boron nitride nanotubes has also been produced by chemical vapor deposition on a ball milled precursor of a nitrogen compound (eg, ammonia), but these tubes are larger in diameter and are of most theoretical interest. It does not show the continuous crystal sp2-type bond structure.

Boron nitride nanotubes (BNNTs) are required because of their exceptional mechanical, electronic, thermal, structural, textural, optical and quantum properties. Most of these applications require long BNNTs to give these desirable properties to distances of the visible scale in real world objects and devices. Although progress has been made in synthesizing long strands and yarns of carbon nanotubes, no similar progress has been reported for BNNTs.

International application PCT / US2008 / 006227 filed on May 2, 2008 and published as International Publication WO2009 / 17526 (Patent Document 1) describes a method for producing boron nitride nanotubes having a length of at least a centimeter. Has been. The entire disclosure of that application is incorporated herein by reference.

In US patent application Ser. No. 12 / 322,591, filed Feb. 4, 2009, an apparatus for large-scale production of boron nitride nanotubes in which a boron-containing target is supplied continuously; a thermal energy source, preferably A pressure chamber apparatus containing a focused laser beam; a cooled condenser; a pressurized nitrogen gas source; and a mechanism for extracting from the pressure chamber the boron nitride nanotubes condensed on or in the cooled condenser. Including the above apparatus is described. The entire disclosure of that application is also incorporated herein by reference.

International Publication WO2009 / 17526

Although each of the above applications is a highly useful and efficient method and apparatus for producing BNNTs, the production of boron nitride fibrils that can be bundled and spun into yarn has not been demonstrated. Thus, there is a need for a method of producing BNNTs that exhibit very large aspect ratios (ie, very long) that can be processed into useful macroscale yarns.

Accordingly, an object of the present invention is to provide a method for producing boron nitride nanotube fibrils that can be relatively easily spun into a boron nitride nanotube yarn or processed by other methods.

Another object of the present invention is to provide a method for producing such large aspect ratio BNNT fibrils at speeds exceeding 1 meter per second.

In accordance with the present invention, fibrils made of very high aspect ratio boron nitride nanotubes and single or multi-walled nanotubes aligned in tube bundles of length 20 μm or more can exceed about 1 meter per second. A method of manufacturing is provided. Also described are nanotube yarns made of twisted bundles of such nanotube fibrils.

FIG. 1 is a conceptual side view of an apparatus useful for successful implementation of the present invention.

FIG. 2 shows an agglomeration of boron nitride nanotube fibrils harvested according to the present invention.

FIG. 3 shows a yarn formed by simply spinning the boron nitride mass shown in FIG.

FIG. 4 is a conceptual representation showing transmission electron microscope images of single-walled and three-walled nanotubes according to the present invention.

The benefits of the present invention are achieved by a method of producing boron nitride nanotubes and nanostructures, which comprises:
(a) preparing a boron-containing target in a chamber under nitrogen that is at a higher pressure than the atmosphere;
(b) thermally exciting the boron-containing target;
Including that.

Particularly advantageous results are obtained when the boron-containing target is excited by a laser, for example a free electron laser or a carbon dioxide laser.

Beneficial results are obtained when the boron-containing target is a compressed boron powder or a compressed boron nitride powder.

It is advantageous if the target rotates in a cylindrical shape and receives irradiation from the radial direction, or rotates in a cylindrical shape and receives irradiation on one side. However, the target may be fixed. Further preferred target orientation and irradiation will be described in detail below.

A highly desirable and very advantageous result is that the method
(a) creating a boron vapor source;
(b) Boron vapor is mixed with nitrogen gas so that a mixture of boron vapor and nitrogen gas is present at the nucleation site. At this time, the nitrogen gas is supplied at a temperature higher than about 2 atm and less than about 250 atm. Done;
(c) harvesting boron nitride nanotubes formed at the aggregation sites, advantageously in the absence of a catalyst,
Obtained when including

The boron vapor source is advantageously provided by supplying energy to a solid boron-containing target, where such energy is sufficient to break bonds in the solid boron-containing target, resulting in boron Steam can enter the steam state.

This energy is preferably focused thermal energy. This energy is conveniently and advantageously in the state of a laser beam directed at a solid boron containing target. Useful examples of lasers used to provide such a laser beam include free electron lasers and carbon dioxide lasers, among other lasers known to those skilled in the art. Other thermal techniques that can produce a boron vapor plume with an appropriate shape under high pressure ambient nitrogen pressure may also be useful.

Excellent results have been obtained when the solid boron-containing target is a piece or lump of compressed boron powder or compressed boron nitride powder. In addition, the laser beam is applied to a solid boron-containing target, but as the laser beam is applied to the target, the target is depressed, so that the interior of the recess is laser heated to produce a boron vapor stream. It turned out to be advantageous and convenient. As the boron vapor flows upward from the bottom of the recess through the recess, the vapor then contacts nitrogen gas. Nitrogen gas is maintained in a pressurized state in a synthesis chamber containing a solid boron-containing target and containing pressurized nitrogen gas.

Nitrogen gas can be advantageously used above about 2 atmospheres and below about 250 atmospheres, but very good results have been achieved when nitrogen gas is supplied at pressures above about 2 atmospheres and up to about 12 atmospheres. The

The boron nitride nanotubes described herein are produced according to the present invention at the aggregation sites, preferably in the absence of a catalyst. The agglomeration site is advantageously a surface, in particular an uneven surface. It has been found to be very beneficial when the agglomeration site is in the upper edge of an indentation in a solid boron containing target and there is asperity there. Boron nitride nanotubes are produced at this agglomeration site and propagate from there in the direction of the flow of the boron vapor flow, which is generated by heating the inside of the recess.

After production, the boron nitride nanotubes are advantageously harvested continuously by standard means known to those skilled in the art. As an example of such continuous harvesting, a growth rate of about 10 cm / sec was achieved with the method of the present invention for boron nanotubes. A harvesting method and apparatus that is suitable, but not exclusively useful, will now be described along with a description of the apparatus depicted in FIG.

The method of the present invention produces crystalline boron nitride nanotubes with sp2 bonded walls that are continuous, parallel, and substantially free of defects. Such nanotubes are single, double, few and / or many-walled nanotubes. Highly preferred nanotubes made according to the present invention have an elongated tubular wall with an inner diameter of about 1.5 to about 2.5 nanometers and a thickness of at least one molecule of boron nitride. A representative such nanotube is shown in the conceptual diagram of the transmission electron micrograph shown in FIG. 4 of a single wall and multiple wall nanotube produced in accordance with the present invention.

Under high ambient pressure (for example, about 12 bar (1.2 MPa)), with appropriate raw materials, few wall boron nitride nanotubes (FW-BNNT) fibrils are linear in the appearance of any irregularities due to surface aggregation Grows continuously at high speed (at least 10 / sec). These fibers or fibrils are referred to as “streamers” because they follow the steam stream line in the synthesis chamber and sway like the movement of the tail of the heel. Because it looks like.

As described in International Publication WO2009 / 17526, a laser beam including a 1.6 micron wavelength, 8 mm diameter, unfocused, 1 kW beam emitted from a FEL (free electron laser) is preferred. The heat source propagates vertically downward into the target. According to this example, the target, a 2.5 cm piece of compressed boron metal powder, rotates on the turntable at a speed of 20 seconds / revolution. As a result of the target center of rotation being offset from the center of the beam by about half the beam diameter, the laser drills a depression about twice the diameter of the laser as the target spins. High pressure nitrogen gas at ambient temperature is continuously fed into the synthesis chamber.

At the edge of this laser drilling recess, a streamer is created and is lengthened by the upward flow of boron vapor. The flapping motion occurs as the fibril / fiber follows the turbulent streamline of boron vapor. This boron vapor is created at the bottom of the recess by laser heating. Streamers are quickly generated and reach a length of more than 1 cm within about 1 / 30th of a second. The streamer portion is snapped up and rolled up above the target, and then carried out of the chamber by an apparatus described later. The high nitrogen pressure in the chamber is important for streamer production. When the nitrogen pressure drops, for example from 12 bar to a little over 1 bar (close to atmospheric pressure), no streamer is seen, instead a spark shower is emitted from the laser illumination zone. In post-run analysis, the spark is a drop of boron metal that solidifies after emission from the laser impact / illumination zone and appears to deposit at the bottom of the chamber. When the synthesis chamber was opened, there was an odor of boron vapor indicating no reaction with nitrogen.

Streamers are collected from both the target surface and downstream on the collector surface (described below). When the ends are fixed, the streamer feels like a spider silk piece, looks similar to it, and has a dull, medium gray color. Like a guitar string, it can be pulled 2 or 3 times its length and then return to its original shape.

This behavior is explained in more detail and more fully by the numerous photomicrographs presented above in International Publication WO2009 / 17526, which is incorporated herein by reference.

At this point, an important upward flow of boron vapor is established. According to the post-implementation analysis of the target, the streamer appears to be generated by the aerodynamic mechanism described in detail below.

As explained in International Publication WO2009 / 17526, at the base of each long streamer, a number of shorter individual BNNT feeder roots are found. It is concluded that after these short fine roots have grown to a few millimeters, they are intertwined with each other because of the turbulent force of boron vapor flow induced by rising heating. The main streamers are fed by high-speed mutual growth of their fine roots and grow to multi-centimeter lengths. Inspection with light and SEM microscopes showed that individual fine roots were bonded to various irregularities on the surface. That is, grain boundaries in solidified boron metal, micron-sized drops on the surface, and the surface of apparent white particles of boron nitride crystals.

Since the multi-centimeter long fiber becomes entangled after the laser is shut down, a fully grown length streamer cannot be photographed in its extended state. However, some streamers in the early stage (fine roots) of BNNT growth were seen along the edge of the target and were photographed with an optical microscope as shown in FIG. 5 of the above International Publication WO2009 / 17526.

Based on these measurements, it was concluded that unlike the formation of carbon nanotubes, boron nitride nanotubes do not require a chemocatalytic surface for aggregation. These are irregularities in any suitable surface, for example a zone where hot boron vapor and cold nitrogen gas are in a precise stoichiometric mixture, and are produced simply and continuously by fine root growth. Under the high pressure employed, the growth rate is on the order of centimeters per second (preferably at least 10) in localized fibers / fibrils.

Based on these measurements, the production of BNNTs is fundamentally less complicated than the production of carbon nanotubes (CNTs) that must create a gas transport cloud or coated surface of catalyst particles and remain active during the growth process.

It is important to note that the laser is only one means of heating the powdered boron metal to create boron vapor. The heating zone and the BNNT production zone are physically separated. Although the laser drilling mechanism that forms the recess in this implementation may be unique to the FEL beam characteristics, this technique can be applied with other lasers and other heat sources given the appropriate geometric configuration. is there. Of course, the boiling point of boron is high, for example 12 bar (3250 ° C.), so there are substantial engineering obstacles. This temperature can be easily accommodated by laser and arc heating. Other heating methods that can achieve this appropriate temperature and can generate a boron vapor stream from the recess should be equally effective.

That is, the method of the present invention requires that the boron be evaporated under high pressure and the resulting buoyant gaseous boron (plume) impinge on the condenser to initiate the generation of BNNTs. This technique is fast and scalable and apparently depends only on the rate at which boron can evaporate in the chamber. A survey of previous work shows the limited success of BNNT synthesis technologies available to date, namely those using arc discharge, laser evaporation, chemical vapor deposition, chemical reaction, and atomic deposition. The technology is most closely related to “laser evaporation” but introduces the use of forced condensation (a new growth mechanism) and operates at pressures that have not been studied.

Our technique uses laser heating to create boron vapor, but this method is not the “laser” method itself. The type of heating source, in this case a laser, is not critical. The growth zone is isolated from the heating zone and is near or above the surface of the separated capacitor. This technology works equally well with the two heat sources currently available to us, a near infrared free electron laser and a far infrared commercial metal-cutting laser.

To produce the fibrils of the present invention, a high power laser is used to boil boron in a high pressure nitrogen environment, as described more fully below. The high ambient pressure creates a large density ratio between the boron vapor plume and nitrogen and, as a result, creates a strong levitation force that accelerates the boron vapor vertically toward the ceiling of the chamber. Fibril growth begins when the boron vapor plume crosses the condenser and rises rapidly toward the ceiling of the chamber. Visualizing the flow shows that a 10 cm growth often occurs in less than 33 milliseconds within a single video frame, showing a growth rate of more than 3 meters per second. Fibrils on the order of about 1 mm in diameter were observed under these production conditions. Such fibrils constitute bundles or collections of individual vertical strands of BNNTs oriented in parallel in one direction, extending the full height of the image, and the material is in the direction of growth. It can be seen that they have parallel alignment axes. Along the growth axis, the material can stretch elastically like a cobweb. In the direction transverse to the growth axis, the material can be easily separated into individual strands with a finger. On the micron and submount micron scale, the structure is a network of long-branched nanotubes and tube bundles, often connected at nodes. SEM showed that these nodes were mainly boron nanodroplets coated with a boron nitride layer.

Yarns can be spun from raw BNNT fibril masses grown as described herein. Such yarns are made from fibril conglomerates as shown / drawn in FIG. About 60 mg of this mass with a soft and slender cotton ball appearance and feel is a crude product of a 30-minute production run. A collection of fibrils weighing about 10 mg (indicating a synthesis time of 5 minutes) is separated from the mass, pulled in the growth direction (ie, lengthwise), and twisted with a finger to make a simple, A yarn (see FIG. 3) having a twist angle of about 45 degrees was formed. Prior to twisting, the fibrils had a delicate feel and almost no mechanical strength, but the spun yarn was able to support the load, demonstrating the improvement in strength obtained by spinning. The yarn of FIG. 3 had a relatively large diameter of about 1 mm, was loosely clogged, and was twisted dry from unwashed raw materials—none of which was favorable for mechanical strength. This suggests that the basic staple fibers (BNNT bundles / fibrils) are relatively long to alleviate these disadvantages, and that significant improvements in length can be expected with more sophisticated spinning methods.

Turning now to FIG. 1 of the accompanying drawings, an apparatus 10 useful for producing BNNT fibrils according to the present invention has a pressure chamber 12 which is continuously fed or rotated with a boron nitride target 14. A thermal energy source, preferably a focused laser beam 16; a rotating cooled condenser ring 18; a pressurized nitrogen gas source 20; and a cooled condenser ring 18 in which the boron nitride evaporated by the thermal energy source rotates. A mechanism 22 is provided for extracting boron nitride nanotubes from the pressure chamber 12 after producing a boron nitride plume 24 that condenses on or near the substrate.

As shown in FIG. 1 of the accompanying drawings, the thermal energy source 16 is preferably introduced into the pressure chamber 12 via a convex lens 26 that allows the laser beam / thermal energy source 16 to be focused inside the pressure chamber 12. Laser beam. As also shown in FIG. 1, the cooled condenser ring 18 is attached to the rotating shaft 28 so that its new surface is brought into the vicinity of the mechanism 22, in the embodiment shown in FIG. Continuously rotated.

In the preferred embodiment depicted in FIG. 1, the boron nitride target 14 is a commercially available hot-pressed hexagonal boron nitride rod 14 that is driven by a motor driven plunger rod, rotator, or similar device 30. The laser beam 16 is continuously supplied to the field. According to one preferred embodiment, the boron nitride target 14 has an approximately 0.050 inch square cross-section on one end and a constriction that focuses the laser beam 16 on the laser beam 16. Rather than a (focal waist), rather, the laser beam 16 is introduced at a location that is approximately the same size as the boron nitride target rod 14. According to this preferred embodiment, the target rod 14 is advanced at a rate of about 1 mm / second into a beam 16 of a ₂ kW CO ₂ laser having a wavelength of about 10.6 microns and a diameter of about 12 mm.

A small flow of nitrogen gas, approximately 40 SCFH, is maintained in the pressure chamber 12 via the nitrogen supply path 20, for example, regulated by the needle valve 32 in the chamber exhaust path 34.

According to the preferred embodiment depicted in FIG. 1, the laser beam 16 terminates in a copper mass 38 cooled with water supplied to the copper mass 38 by inlets and outlets 38 and 40. The copper mass 38 is designed to absorb the continuous power of the laser beam 16 without being completely damaged.

Similarly, the rotating condenser 18 is well water cooled and maintained at about 20 ° C. by conventional means within the skill of the artisan. According to the preferred embodiment shown in FIG. 1, the rotating condenser 18 is an approximately 0.025 inch hoop attached to a water-cooled rotating copper shaft 28, although other materials such as stainless steel, tungsten, Niobium, hot pressed boron nitride, boron powder, boron castings, etc. are useful as well.

In operation, as the target rod 14 is continuously converted from solid to vapor by the action of the laser beam 16, the buoyant plume 24 rises vertically from the laser interaction zone. The plume 24 is trapped by the condenser ring 18. Where the plume 24 is trapped by the condenser ring 18, boron nitride nanotubes are generated at high speed. A web-like covering of boron nitride nanotube fibrils 4 inches or more in length is produced with a vertical structure bonded to the lower end of the condenser ring 18. These boron nitride nanotube structures are produced in a fraction of a second, as recorded in video image visualization. As the condenser ring 18 rotates, new boron nitride nanotubes grow on each newly exposed surface portion of the advancing capacitor.

Boron nitride nanotube fibrils are removed from the rotating condenser 18 by a collection tube 22 that leads to the exterior of the pressure chamber 12. When a ball valve (not shown) in the collection tube 22 is opened, boron nitride nanotube fibrils are “suctioned down” from the rotating condenser ring 18 with nitrogen gas exhausted to 1 atmosphere. Boron nitride nanotube fibrils can be collected, for example, in a wire mesh placed in series with a collection tube 22 (not shown), or simply spun into a yarn upon exiting the collection tube 22.

While the above describes specific preferred devices for obtaining the materials of the present invention, it is clear that other variations of these specific parameters can be utilized. For example, hot-pressed boron nitride was used as the preferred target, but other targets may be suitable if interaction with the laser is acceptable. For example, hot-pressed boron powder is manufactured in the literature, but a good target could be made if adequate laser characteristics are obtained. Any density boron or boron nitride plays a role as long as the laser interacts with its target to produce a continuous vapor stream. The wavelength or other laser characteristics are not important. Any kW class laser should reproduce these results. Nd: YAG or free electron lasers are two examples; the shape of the condenser 18 can be varied insofar as it provides a free flow of the plume. It worked and Nb wire, W wire, Nb plate and Cu plate all proved useful; both mechanical and suction were found to be useful for collecting boron nitride nanotubes. The material tends to adhere to itself or to any surface if present, so it can be wrapped around a number of geometric structures, attached to the structure, or sucked into the structure. Also, since boron nitride nanotubes respond well to static electricity, they can also be collected by their mechanism. Further, although the cooled condenser has been depicted as a cooled ring, the condenser can also be provided with a similarly cooled vibrating structure, and the boron nitride nanotube collection mechanism can include one or more collection tubes in the cooled ring. It can also be in the vicinity of the extreme amplitude of the oscillating capacitor.

Thus, highly crystalline and long BNNTs, BNNT fibrils, and bundles of BNNT fibrils—long enough to be spun directly into the yarn—was grown for the first time. Since these are high aspect ratio nanotubes with straight, parallel walls and few defects, they are expected to fully embody a number of theoretically desirable material properties. These grow at a high linear rate (several meters per second) without a catalyst or additive, suggesting a promising and environmentally favorable production means.

While the invention has been described, it will be apparent to those skilled in the art that various modifications can be made without departing from the spirit and scope thereof. Any and all such modifications are intended to be included within the scope of the appended claims.

10 Apparatus used for BNNT fibril production 12 Pressure chamber 14 Target rod 16 Thermal energy or laser beam 18 Condenser ring 20 Supply path 22 Condenser tube, collection tube 24 Plume 26 Convex lens 28 Rotating shaft 30 Motor driven plunger rod 32 Needle valve 34 Chamber exhaust passage 36 Inlet 38 Copper lump 40 Outlet

Claims (17)

  Boron nitride nanotube fibrils composed of bundles of boron nitride nanotubes aligned in the longitudinal direction and having a length of 20 μm or more.   The boron nitride nanotube fibril of claim 1, wherein the longitudinally aligned bundle of boron nitride nanotubes is at least 10 cm long.   The boron nitride nanotube fibril of claim 1, wherein the boron nitride nanotube is a single wall, a few walls, or a number of walls.   A boron nitride yarn comprising spun boron nitride nanotube fibrils composed of longitudinally aligned bundles of boron nitride nanotubes having a length of 20 μm or more.   5. The boron nitride yarn of claim 4, wherein the longitudinally aligned bundle of boron nitride nanotubes is at least 10 cm long.   5. The boron nitride yarn of claim 4, wherein the boron nitride nanotube comprises a single wall, a few walls, or a number of walls. An apparatus for large-scale production of boron nitride nanotubes, comprising a pressure chamber,
Said pressure chamber is a continuously supplied boron-containing target; a focused thermal energy source for evaporating said continuously supplied boron-containing target; a cooled condenser; a pressurized nitrogen gas source; A mechanism for extracting boron nitride nanotubes that are condensed on or in a cooled condenser from the pressure chamber;
Including the above devices.   8. The apparatus of claim 7, wherein the cooled condenser includes a cooled condenser ring.   9. The apparatus of claim 8, wherein the continuously supplied boron-containing target comprises a rod of boron nitride or hot pressed boron powder.   8. The apparatus of claim 7, wherein the focused thermal energy source comprises a focused laser beam.   11. The apparatus of claim 10, wherein the continuously supplied boron-containing target is in a laser beam at a position where the laser beam diameter is approximately equal to the cross-sectional area of the continuously supplied boron-containing target. A device as described above, which is continuously advanced.   9. The apparatus of claim 8, wherein the cooled condenser ring is maintained at a temperature of about 20 degrees Celsius.   12. The apparatus of claim 11, wherein the cooled condenser ring is maintained at a temperature of about 20 degrees Celsius. The apparatus of claim 10, wherein the laser beam is CO _2, Nd: a YAG or free electron lasers, having a diameter of about 12 mm, the apparatus.
  8. The apparatus of claim 7, wherein the mechanism for extracting boron nitride nanotubes comprises a vacuum or suction collection tube.   11. The apparatus of claim 10, further comprising a cooled mass that terminates the laser beam by completely absorbing the continuous power of the laser beam. An apparatus for large-scale production of boron nitride nanotubes, comprising a pressure chamber,
The pressure chamber is a continuously supplied boron nitride or hot pressed boron powder target; a focused laser beam for evaporating the continuously supplied boron nitride or hot pressed boron powder target; A condenser; a pressurized nitrogen gas source; and a collector tube that draws, by vacuum or suction, boron nitride nanotubes that are condensed on or in a cooled and rotating condenser from a pressure chamber;
Including the above device.

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