Carbon microtubes, the bigger brother of carbon nanotubes, have been synthesized by Los Alamos National Laboratory scientists (LANL) (Los Alamos, NM). The materials, which can be made from seaweed, could compete with carbon nanotubes, cotton and Kevlar.
Carbon microtubes have a diameter of 10 to about 150 microns. Carbon nanotubes have a diameter of 1 to 1000 nanometers. A carbon microtube is a hollow, substantially tubular structure with a porous wall and a diameter of from about 10 microns to about 150 microns, and a density of less than 20 mg/cm3. The wall has a structure characterized by rectangular voids extending parallel to the central axis of the tube.
LANL inventors Huisheng Peng, Yuntian Theodore Zhu, Dean E. Peterson and Quanxi Jia found that this unique structure results in a number of desirable properties which are superior to other forms of carbon, including low density (comparable to that of nanofoams), high strength, excellent ductility, and high conductivity. These properties indicate a potential for a variety of advanced applications. For example, the diameter and the length of carbon microtubes are comparable to cotton fibers, and yet carbon microtubes may be over 200 times stronger than cotton fibers.
Conventional textile technologies may be used to make carbon microtube fabrics suitable for applications such as body armor and other applications requiring lightweight, high strength composite structures, according to U.S. Patent Application 20100035019, the rights to which are held by Los Alamos National Security, LLC and the U.S. Government.
Carbon microtubes were synthesized using a chemical vapor deposition (CVD) process. A mixture of ethylene and paraffin oil (with kinematic viscosity of 33.5 centistokes or less at 40.degree. C.) was used as the precursor. Ar with 6% H.sub.2 was used to carry the precursor to a 1-inch quartz tube furnace where the growth took place in the temperature range of 750-850.degree. C. The carbon microtube growth was carried out with 80 sccm ethylene and 120 sccm carrier gas in a process similar to that used to synthesize carbon nanotubes, with a difference being that both ethylene and paraffin oil were used as precursors. No catalyst was used. FIG. 1 depicts scanning electron microscopy (SEM) images of a typical carbon microtube having a diameter of about 50 microns.
FIG. 1 shows images of carbon microtubes of the present invention, obtained by scanning electron microscopy (SEM). FIG. 1a shows carbon microtubes grown for 30 min. and FIG. 1b shows carbon microtubes grown for about 3 hours.
FIG. 1 shows images of carbon microtubes of the present invention, obtained by scanning electron microscopy (SEM). FIG. 1a shows carbon microtubes grown for 30 min. and FIG. 1b shows carbon microtubes grown for about 3 hours.
Also described in U.S. Patent Application 20100035019, is a carbon microtube, with a diameter of at least 10 microns and comprising a hollow, substantially tubular structure with a porous wall, wherein the porous wall contains a plurality of voids, the voids substantially parallel to the length of the microtube, and defined by an inner surface, an outer surface, and a shared surface separating two adjacent voids.
FIG. 2 is a schematic depiction of a carbon microtube of the present invention, showing the rectangular columnar pores, or voids, extending the length of the tube wall. FIG. 2a depicts an angular view of the entire carbon microtube. FIG. 2b depicts a cutaway view showing approximately one-half of the carbon microtube
Surprisingly, the porous walls of the carbon microtubes exhibit a highly ordered lamellar structure, and the wall exhibits a layered graphite crystal structure.
As depicted in FIG. 2 and FIG. 3, carbon microtube (200) comprises a hollow, tubular inner space (210) surrounded by porous wall (220). The porous wall comprises a plurality of voids (230), said voids substantially parallel to the central axis of the microtube, and defined by an inner surface (240), an outer surface (250), and a shared surface (260), which separates two adjacent voids. The thickness of the shared surface may be from about 10 nm to about 150 nm.
The thickness of the porous wall may be from about 0.5 microns to about 2 microns, and alternatively from about 1.0 microns to about 1.5 microns. The porous wall may have a density of from about 50 mg/cm3 to about 150 mg/cm3. The size of the voids may be substantially similar, or may vary. For example, the width (as measured at two oppositionally-positioned points on adjacent shared surfaces) may vary from about 500 nm to about 2 microns.
FIG. 3 is a schematic depiction of the carbon microtube of FIG. 1, as viewed along the z-axis (lengthwise).
FIG. 3 is a schematic depiction of the carbon microtube of FIG. 1, as viewed along the z-axis (lengthwise).
The carbon microtubes also exhibit surprising and useful properties. The microtubes have a density of less than 20 mg/cm3, which is comparable to carbon nanofoams.
The microtubes possess excellent mechanical, semiconductor and electrical properties. The LANL carbon microtubes have a tensile strength of at least 1 GPa, and alternatively a tensile strength of from about 6 to about 10 GPa, per individual carbon microtube. In comparison, carbon nanotubes, one of the strongest materials known, have been measured to have a tensile strength of approximately up to 65 GPa. However, to exploit this superior property for practical applications, individual carbon nanotubes generally are assembled into macroscopic fibers, which exhibit a tensile strength of less than 3.3 GPa.
The carbon microtubes have a specific strength of at least 1 to times 108 cm, which results from the combination of high strength and low density. The specific strength may be calculated by dividing the tensile strength in GPa by the density in g/cm3, to yield a specific strength in cm.
The carbon microtubes have a tenacity of at least 1 times to 103 g/tex, which is about 30 times that of Kevlar and about 224 times that of individual cotton fibers. Alternatively, the microtubes may have a tenacity of from about 2 to 103g/tex. The tenacity may be calculated from the tensile strength measurements by using the conversion factor of 1 g/tex=105 cm.
The carbon microtubes have an electrical conductivity of at least 102 S/cm at room temperature (298K), as measured on an individual carbon microtube. Alternatively, the carbon microtubes may have an electrical conductivity of from about 100 S/cm to about 2000 S/cm at room temperature, and alternatively from about 1000 S/cm to about 2000 S/cm at room temperature.
It was found that several biopolymers which are contained in seaweeds, especially in red or brown seaweeds or agarophytes, are very suitable precursors for carbon microtubes. Examples of such biopolymers are alginic acid, alginate salts, agar and carrageenan (iota and kappa). The polymers might be extracted from the seaweeds before carbonization.
Alternatively, the raw seaweeds containing such polymers can be used directly as precursor without previous extraction of the biopolymer. Thus, by carbonization of carrageenan-rich red seaweeds like Hypnea Musciforme or of alginate rich brown seaweeds like Lessonia Nigrescens or of agar-rich seaweeds, carbonaceous material suitable as electrode material for electrochemical capacitors can be produced. This route is very favorable for economic reasons, since the extraction process is omitted. Another suitable precursor biopolymer is chitin.
The carbon microtubes have a tenacity of at least 1 times to 103 g/tex, which is about 30 times that of Kevlar and about 224 times that of individual cotton fibers. Alternatively, the microtubes may have a tenacity of from about 2 to 103g/tex. The tenacity may be calculated from the tensile strength measurements by using the conversion factor of 1 g/tex=105 cm.
The carbon microtubes have an electrical conductivity of at least 102 S/cm at room temperature (298K), as measured on an individual carbon microtube. Alternatively, the carbon microtubes may have an electrical conductivity of from about 100 S/cm to about 2000 S/cm at room temperature, and alternatively from about 1000 S/cm to about 2000 S/cm at room temperature.
It was found that several biopolymers which are contained in seaweeds, especially in red or brown seaweeds or agarophytes, are very suitable precursors for carbon microtubes. Examples of such biopolymers are alginic acid, alginate salts, agar and carrageenan (iota and kappa). The polymers might be extracted from the seaweeds before carbonization.
Alternatively, the raw seaweeds containing such polymers can be used directly as precursor without previous extraction of the biopolymer. Thus, by carbonization of carrageenan-rich red seaweeds like Hypnea Musciforme or of alginate rich brown seaweeds like Lessonia Nigrescens or of agar-rich seaweeds, carbonaceous material suitable as electrode material for electrochemical capacitors can be produced. This route is very favorable for economic reasons, since the extraction process is omitted. Another suitable precursor biopolymer is chitin.
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