METHOD THEREOF

Field of the invention The invention relates to an improved apparatus for production of carbon nanotubes and its method thereof.

Background of the art

Carbon nanotubes (CNTs) are a form of carbon allotropes discovered by Sumio lijima in year 1991. A single nanotube resembles a cylinder of rolled graphene sheet with its body containing hexagonal rings and end cap with a certain number of pentagonal rings. The end cap and body each has a structure similar to that of fullerene and graphite, respectively. Generally, CNTs can be categorized into single-walled CNTs and multi-walled CNTs which can be distinguished by the number of graphitic sheets in the tube wall. The unique arrangement of carbon atom in CNTs give rise to its superior surface property, high tenacity, high electron and thermal conductivity, excellent field emission property, metallic and semiconducting properties. The small dimension, strength yet light and remarkable properties of these nanostructures have resulted in extensive research activities on this carbon chemistry, and result in various potential applications such as cathode catalyst support of fuel cell, microsensor, field emission display, supercapacitor and nanoprobe, etc. However, the applications of CNTs have not been fully commercialized due to two interrelated reasons: the difficulty in mass production of CNTs and thus the high production cost. The imbalance in market demand-supply chain causes the selling price of CNTs remaining high and this slows down the growth of CNT applied products in the market.

Numerous methods have been developed for the synthesis of CNTs, including electric arc-discharge, laser ablation method, chemical vapor deposition (CVD), flame synthesis, solar energy route, etc. Among the synthesis methods as listed above, CVD is the promising method that enables mass production of CNTs at relatively low cost. Besides, CVD allows better control of the CNT structural properties as well as reaction yield through the use of proper catalyst system, corresponding to carbon precursors and thermodynamic reaction conditions. CVD can be conducted in either batch or continuous mode. However, batch process is labor intensive, costly, inefficient, and generally has limited production capacity. Furthermore, the products synthesized by batch process might result in significant batch to batch variation in the quality of CNTs. Continuous process can overcome the shortcomings faced by the batch process and thus, it has been identified as promising method for commercial production of CNTs.

U.S. Publication No. 2004/0151654 (US 7,563,427) describes a method for continuous production of CNTs in a nano-agglomerate fluidization reactor that takes into consideration the agglomeration and agglomerate behaviors of nano-structure during the CVD process. This method involves loading transition metal oxides on a support, followed by activation of catalyst through the flowing a mixture of nitrogen and hydrogen or carbon monoxide into reactor to reduce the nanosized metal oxide particles to nanosized metal particles that are activated for the synthesis of CNTs. The activated catalyst is transported into a fluidized bed reactor and reacted with a gas of lower hydrocarbons with less than 7 carbon atoms to form CNTs. The reactor bed is kept in an agglomerate fluidization state by proper controlling of the reaction rate, operating conditions and also fluidized bed structure in order to realize the continuous production of CNTs.

Another method for continuous production of CNTs is described in U.S. Publication No. 2007/0264187 where any pre-reduced catalyst that effective in forming CNTs is first loaded in a container connected to a modified fluidized bed which is used as an injector. The pre-reduced catalyst is continuously injected as an aerosolized dry powder at constant rate into the top portion of a vertical reaction chamber. The catalyst is allowed to move downward through the vertical reactor and reacted with carbon containing reactant gas for the growth of CNTs. The solid material containing CNTs, unreacted catalyst, and by-product is collected and separated at the bottom of reaction chamber. The unreacted catalyst recovered from the separation can be recycled.

The methods described above require an additional reduction process for the catalyst to reduce the originally oxide state of transition metal prior to the growth of CNTs. Further, the residence time of catalytic particles in the reactor to grow CNTs is not consistent for the methods described above as the reactor designs do not ensure same batch of catalyst fed into reactor leaves at identical time. The methods in the prior art result in the variation of the properties of CNTs produced. Accordingly, there is a need in the art for apparatus and method for production of carbon nanotubes which result in an improved quality of carbon nanotubes, and an improved apparatus for the production of carbon nanotubes. There is further a need in the art to solve the aforementioned residence time problem.

Summary of the invention

The present invention provides an improved apparatus for production of carbon nanotubes. It further provides an improved method for production of carbon nanotubes.

According to a first aspect, the present invention provides an apparatus for production of carbon nanotubes, comprising

a reaction chamber, arranged to receive at least one catalyst and a carbon precursor; and a rotatable helical structure within said reaction chamber arranged to carry the at least one catalyst and arranged to traverse the at least one catalyst along the length of the reaction chamber.

In particular, in the apparatus according to the invention, the helical structure may comprise a series of successive coils, the successive coils may be arranged to contact and traverse said catalyst along the length of the reaction chamber through rotation of the helical structure. The helical movement may progressively push the catalyst along the base of the reaction chamber, such that the rate of movement of the catalyst may be a function of the rate of rotation of the helical structure, or a function of the pitch between the successive coils etc.

According to a particular aspect, the present invention provides an apparatus for production of carbon nanotubes, wherein the helical structure is arranged to traverse said catalyst along the length of the reaction chamber at a predetermined rate of travel.

According to another aspect, the present invention relates to a method for production of carbon nanotubes comprising the steps of:

(a) heating a reaction chamber;

(b) feeding at least one catalyst to a helical structure within said reaction chamber;

(c) rotating the helical structure;

(d) traversing said catalyst along a longitudinal length of the reaction chamber as a result of the rotation of said helical structure;

and so producing the carbon nanotubes.

In particular, the method according to the invention further comprises a step of traversing a carbon precursor along a longitudinal length of the reaction chamber, in a direction opposite to the direction of traverse of said catalyst along a longitudinal length of the reaction chamber as a result of the rotation of said helical structure (as in step (d)).

Brief description of the figures It will be convenient to further describe the present invention with respect to the accompanying drawings that illustrate possible arrangements of the invention. Other arrangements of the invention are possible, and consequently the particularity of the accompanying drawings is not to be understood as superseding the generality of the preceding description of the invention.

FIGURE 1 is a schematic diagram of the structure of the reaction apparatus of the present invention.

FIGURE 2 (a), (b) and (c) are scanning electron microscopy (SEM), transmission electron microscopy (TEM), and high resolution transmission electron (HRTEM) microscopy images respectively of carbon nanotubes produced using the apparatus and the method of the present invention.

Detailed description of the invention

The present invention provides an improved apparatus for production of carbon nanotubes, and its method thereof.

For the purposes of the present invention, the term 'carbon nanotubes' is understood to mean any types, structure and/or categories of carbon nanotubes known in the art. In particular, the carbon nanotubes may comprise single-walled carbon nanotubes, multi-walled carbon nanotubes, carbon nanotorus, carbon nanobud and/or the like. The carbon nanotubes may have varying diameters within a narrow range. The term 'single-walled carbon nanotube' refer to cylindrically shaped thin sheet of carbon atoms having a wall consisting essentially of a single layer of carbon atoms, and arranged in a hexagonal crystalline structure with a graphitic type of bonding. The term 'multi-walled carbon nanotube' as used herein refers to a carbon nanotube comprising more than one concentric tube.

Apparatus for production of carbon nanotubes

According to a first aspect, the present invention relates to an apparatus for production of carbon nanotubes, comprising

a reaction chamber, arranged to receive at least one catalyst and a carbon precursor; and

a rotatable helical structure within said reaction chamber, arranged to carry the at least one catalyst and arranged to traverse the at least one catalyst along the length of the reaction chamber.

In the apparatus according to the present invention, the reaction chamber 6 is located within the reaction section of the apparatus as illustrated in Figure 1. The reaction chamber may comprise any conventional elongated structure with a hollow interior, arranged to receive at least one catalyst and a carbon precursor. For instance, the reaction chamber may be a prismatic, cylindrical or cylindroid elongated structure. Prismatic is defined for the purposes of the present invention as any structure whose sides are parallel and whose two ends are the same in shape and size. The term 'cylindroid' is defined as a cylinder with an elliptical cross section.

In a particular embodiment, the reaction chamber may be a hollow tube with either one or two open ends. The reaction chamber may be fabricated from any inert material which substantially prevents carbon nanotube growth thereon. Any suitable, non-reactive material (eg, metal or ceramic, such as alumina, silicon carbide, nichrome and the like, and any combination thereof) known in the art may be used. The catalyst used herein the present invention may be any catalyst that is capable of allowing the growth of carbon nanotubes. It will be understood by the person skilled in the art that any catalysts known in the art may be suitable for the present invention and the present invention is not limited by the types of catalysts described herein. In particular, the catalyst may include, but is not limited to, a metallic catalyst. The metal or combination of metals selected as the catalyst can be processed to obtain the desired particle size and diameter distribution. In particular, the metal catalyst can be selected from, but not limited to, a Group V metal, such as V or Nb, and mixtures thereof, a Group VI metal including Cr, W, or Mo, and mixtures thereof, VII metal, such as, Mn, or Re, Group VIII metal including Co, Ni, Ru, Rh, Pd, Os, Ir, Pt, and mixtures thereof, or the lanthanides, such as Ce, Eu, Er, or Yb and mixtures thereof, or transition metals such as Cu, Ag, Au, Zn, Cd, Sc, Y, or La and mixtures thereof. Specific examples of mixture of catalysts, such as bimetallic catalysts, which are suitable for the present invention include, but are not limited to, Co- Mo/MgO, Co-Cr, Co-W, Co-Mo, Ni-Cr, Ni-W, Ni-Mo, Ru-Cr, Ru-W, Ru- Mo, Rh--Cr, Rh-W, Rh-Mo, Pd-Cr, Pd-W, Pd-Mo, Ir-Cr, Ir-W, lr--Mo, Pt- Cr, Pt-W, and Pt-Mo. In particular, the metal catalyst may also be iron, cobalt, nickel, molybdenum, or a mixture thereof, such as Fe-Mo, Co-Mo and Ni-Fe-Mo.

In the apparatus according to the present invention, the carbon precursor may be any carbon containing gas. Any carbon containing gas that does not undergo pyrolysis at temperatures up to about 1000°C may be used for the present invention. Pyrolysis is the decomposition or transformation of a compound or substance caused by heat. In particular, the carbon precursor may be solely carbon containing gas, such as lower hydrocarbons or vaporised alcohols, or it can be a mixture of carbon containing gas and diluents gas, selected from nitrogen, neon, xenon, argon or helium and the like. Examples of suitable carbon-containing gases include, but are not limited to, carbon monoxide, aliphatic hydrocarbons, both saturated and unsaturated, such as methane, ethane, propane, butane, pentane, hexane, ethylene, acetylene and propylene; oxygenated hydrocarbons such as acetone, and methanol; aromatic hydrocarbons such as benzene, toluene, and naphthalene; and any mixtures of the above, for example carbon monoxide and methane.

The specific reaction temperature used for the present invention depends on the type of catalyst and the type of carbon precursor. Energy balance equations for the respective chemical reactions can be used to analytically determine the optimum chemical vapor deposition (CVD) reaction temperature to produce (grow) carbon nanotubes. This determines the required reaction temperature ranges. The optimum reaction temperature also depends on the flow rates of the selected carbon precursor and the catalyst used. In particular, the apparatus of the present invention may require CVD reaction temperatures ranging from 500°C to 1100°C. More preferably, reaction temperatures may range from 700°C to 900°C.

In a particular embodiment, the apparatus of the present invention may further comprise any conventional furnace 5 known in the art configured to supply heat energy required for the reaction to produce the carbon nanotubes within the reaction chamber. In particular, the apparatus according to the present invention relates to the production of carbon nanotubes via the chemical vapour deposition method which is known in the art. The furnace may be configured to allow the reaction chamber to be placed inside the furnace. In particular, the whole length of the reaction chamber may be enclosed within the furnace.

The furnace 5, together with the reaction chamber 6, is located within the reaction section of the apparatus according to the present invention as shown in Figure 1. The furnace may be a high temperature furnace, capable of heating the reaction chamber to a desired temperature ranging from 500°C to 1100°C required for the production of the carbon nanotubes. More preferably, the furnace may be capable of heating the reaction chamber to temperatures ranging from 700°C to 900°C.

The term 'helical structure' used herein the present invention is defined as any elongated structure having a series of curvatures and/or elements around the circumference and along the longitudinal axis of the elongated structure, in a constantly changing series of planes. The elements may be coil-like, spiral-like or the like. The series of curvatures and/or elements around the circumference and along the longitudinal axis of the elongated structure may be successive and/or interconnected to each other. In particular, the helical structure may be a device for moving solid or liquid material by means of a rotating helical shaft, wherein the shaft may be hollow or solid. Further, the core of the helical structure and/or the shaft of the helical structure may be solid or hollow. In particular, an example of a helical structure with a hollow core is a coil spring and an example of a helical structure with a solid core may be an Archimedes screw or a corkscrew.

In particular, the helical structure 7 may comprise a shaft having a series of successive screw threads around the circumference of the shaft and along the longitudinal axis of the shaft. Examples of a helical structure include, but are not limited to, a helical coil, a helical coil spring, a screw, an Archimedes screw, a corkscrew, a spiral coil, and the like.

In a particular aspect, in the apparatus accordingly to the present invention, the rotatable helical structure comprises an auger. An auger is defined for the purposes of the present invention as any device for moving solid or liquid by means of a rotating helical shaft, wherein the shaft comprise a series of circumferential helical elements. In a particular aspect of the invention, the helical structure as described herein the present invention may be prismatic. Prismatic is defined for the purposes of the present invention as any structure whose sides are parallel and whose two ends are the same in shape and size.

In another aspect of the invention, the helical structure as described herein may be cylindrical or cylindroid. The term 'cylindroid' used herein the present invention is defined as a cylinder with an elliptical cross section.

In the apparatus according to the present invention, the rotatable helical structure as described herein is capable of rotating along its longitudinal axis.

In the apparatus according to the present invention, the rotatable helical structure is arranged to carry at least one catalyst and arranged to traverse the at least one catalyst along the length of the reaction chamber. In particular, the helical structure is capable of carrying said catalyst by means of the series of curvatures and/or elements around the circumference and along the longitudinal axis of the elongated structure, in a constantly changing series of planes. The said catalyst may be introduced into the pitches of the helical structure. The pitch of a helix is the width of one complete helix turn, measured parallel to the axis of the helix. A helix may be formed through the series of successive curvatures, elements (eg, coil-like, spiral-like elements) around and along the longitudinal axis of the helical structure. Accordingly, the said catalyst is introduced into the plurality of pitches along the length of the helical structure.

The term 'traverse' as used herein the present invention is understood to mean to travel and/or to pass along, over, across or through. In particular, the rotatable helical structure as described herein the present invention is arranged to traverse said catalyst along the length of the reaction chamber. The said catalyst is traversed along the length of the reaction chamber by the rotation of the helical structure for a period of time, said period may be in the range of 5 minutes to 12 hours. In particular, the rotation of the helical structure effects the movement of the pitches of the helical structure along the reaction chamber, wherein said catalyst are introduced into the pitches and said catalyst is in turn being traversed along the reaction chamber. The rotation of the helical structure may be at a pre-determined rotational speed. In particular, the rotational speed ranges from 0.01 rpm to 10rpm, preferably at rotational speed from 0.1 rpm to 0.8rpm.

In a particular aspect, the present invention relates to an apparatus for producing carbon nanotubes, wherein the helical structure comprises a series of successive coils, the successive coils being arranged to contact and traverse said catalyst along the length of the reaction chamber through rotation of the helical structure.

In particular, the present invention relates to an apparatus for production of carbon nanotubes, wherein the helical structure is arranged to traverse the catalyst along the length of the reaction chamber at a pre-determined rate of travel. In particular, the pre-determined rate of travel of the traversed said catalyst is from a range of 0.5mm/min to 150mm/min, preferably from 3mm/min to 20mm/min.

According to another embodiment of the present invention, the apparatus of the present invention further comprises a device which is configured to rotate the helical structure at a pre-determined rotational speed. The device may be any mechanical or electrical actuator. In particular, the device may be a motor. In particular, the device is capable of controlling the rotational speed of the helical structure ranging from 0.01 rpm to 10rpm, preferably at rotational speed from 0.1 rpm to 0.8rpm.

In a particular aspect of the present invention, the apparatus may further comprise a catalyst feeding section connected to one end of the reaction chamber, the catalyst feeding section arranged to feed, at a pre-determined rate, said catalyst to the reaction chamber. The catalyst feeding section as described herein the apparatus according to the present invention further comprises

a vessel for collecting the catalyst, the vessel having an outlet;

a screw conveyor, located at the outlet of the vessel, which is configured to withdraw the catalyst out of the vessel; and

a conduit structure which connects the catalyst feeding section to one end of the reaction chamber. .

As illustrated in Figure 1 , the vessel 1 as described herein the apparatus of the present invention comprises a hollow enclosure which is used as a container for the at least one catalyst. The vessel may be a funnel-shaped container in which said catalyst are stored prior to feeding to the reaction chamber. The vessel may comprise an outlet which is located at the base of the vessel. Further, a screw conveyor 2 is located at the outlet of the vessel, the screw conveyor functions as a means to withdraw the catalyst out of the vessel, and transfer said catalyst through a conduit structure 4, which connects the catalyst feeding section to one end of the reaction chamber. The withdrawal rate of catalyst from the vessel can be varied which is controlled by a motor 3, or by any other mean that is able to control rotation speed of screw conveyor. In particular, the screw conveyor may comprise a screw having screw threads and the said catalyst is introduced within the spaces between the screw threads to transfer said catalyst from vessel through the conduit structure and into the reaction chamber.

In a particular aspect, the apparatus according to the present invention further comprises a gas injector for feeding the carbon precursor into the reaction chamber. The gas injector 9 may be located at the end of the reaction chamber opposite the catalyst feeding section, as illustrated in Figure 1. The gas injector may be configured to allow the carbon precursor to enter the reaction chamber at a constant flow rate. The gas injector is configured to provide a constant flow rate of the carbon precursor into the reaction chamber ranging from 0.5 to 5 liters/minute (Umin). In a particular aspect, the apparatus according to the present invention further comprises a product reservoir 8 connected to an end of the reaction chamber at the opposite side of the catalyst feeding section, as illustrated in Figure 1. The product reservoir may be a vessel which is used to collect the produced carbon nanotubes after the reaction.

In a particular aspect, the apparatus of the present invention is configured to produce carbon nanotubes in a continuous manner. The apparatus according to the present invention is configured to perform like a number of batch-wise reactors in a horizontal series orientation and operated in a continuous manner. The apparatus of the present invention effects in the continuous production of carbon nanotubes through continuous supply of fresh unreacted catalyst into the reaction chamber; and also the continuous withdrawal of carbon nanotubes out of the reaction chamber.

Method for production of carbon nanotubes

According to a particular aspect, the present invention relates to a method for production of carbon nanotubes comprising the steps of:

(a) heating a reaction chamber;

(b) feeding at least one catalyst to a helical structure within said reaction chamber;

(c) rotating the helical structure;

(d) traversing said catalyst along a longitudinal length of the reaction chamber as a result of the rotation of said helical structure; and so producing the carbon nanotubes.

In particular, the reaction chamber is first heated to a desired reaction temperature ranging from 500°C to 1100°C. More preferably, the reaction chamber may be heated to a desired reaction temperature ranging from 700°C to 900°C. Subsequently, at least one catalyst is then fed to the helical structure within said reaction chamber, where catalytic decomposition of a carbon precursor takes place to grow carbon nanotubes. The feeding of the said catalyst to the helical structure within the reaction chamber further comprises the steps of (i) introducing at least one catalyst into a vessel of the catalyst feeding section, (ii) said catalyst is then withdrawn at a pre-determined rate from the vessel by a screw conveyor, located at the base of the vessel; and (iii) said catalyst is then transferred through a conduit structure and onto the helical structure within the reaction chamber. The withdrawal rate of catalyst from the vessel can be varied by controlling the stepper motor, or by any other mean that is able to control rotation speed of screw conveyor. The said catalyst feed is introduced into the pitches of the helical structure, which have the similar function as quartz boat in a horizontal batch-wise reactor to contain a fixed quantity of catalyst for the growth of carbon nanotubes.

The helical structure is then rotated and this effects in traversing said catalyst along a longitudinal length of the reaction chamber as a result of the rotation of said helical structure; and so producing the carbon nanotubes.

-.

In a particular aspect, the method according to the present invention further comprises a step of traversing a carbon precursor along a longitudinal length of the reaction chamber, in a direction opposite to the direction of traverse of said catalyst within the reaction chamber. The effect of traversing the carbon precursor countercurrent to the direction of the catalyst feeding effects in the use of the hydrogen produced from the decomposition of the carbon precursor, to reduce the freshly prepared catalyst which is initially in oxide form. The carbon precursor may be any carbon containing gas. Any carbon containing gas that does not undergo pyrolysis at temperatures up to about 1000°C may be used for the present invention. In particular, the carbon precursor may be solely carbon containing gas, such as lower hydrocarbons or vaporised alcohols, or it can be a mixture of carbon containing gas and diluents gas, selected from nitrogen, neon, xenon, argon or helium and the like. Examples of suitable carbon-containing gases include, but are not limited to, carbon monoxide, aliphatic hydrocarbons, both saturated and unsaturated, such as methane, ethane, propane, butane, pentane, hexane, ethylene, acetylene and propylene; oxygenated hydrocarbons such as acetone, and methanol; aromatic hydrocarbons such as benzene, toluene, and naphthalene; and any mixtures of the above, for example carbon monoxide and methane. In particular, the step of traversing said carbon precursor along the longitudinal length of the reaction chamber is at a constant flow rate. In particular, the constant flow rate ranges from 0.5 to 5 liters/minute (Umin).

According to another aspect, in the method according to the present invention, the catalyst is in contact with said carbon precursor in the reaction chamber for a pre-determined period of time. The pre-determined period of contact time is in the range from 5 minutes to 12 hours.

In a particular aspect, the method according to the present invention comprises the step of traversing said catalyst along the longitudinal length of the reaction chamber at a pre-determined rate of travel. In particular, the predetermined rate of travel is in the range of 0.5mm/min to 150mm/min, preferably from 3mm/min to 20mm/min. In yet another embodiment, the method according to the present invention comprises the step of rotating the helical structure at a pre-determined rotational speed. The rotational speed of the helical structure may be controlled by a motor, or any other mean. The helical structure acts as a means to carry said catalyst through the reaction chamber and allows an effective control of the contact time between said catalyst and carbon precursor. The pre-determined rotational speed may be in the range 0.01 rpm to 10rpm, preferably at rotational speed from 0.1 rpm to 0.8rpm. According to a particular aspect, the method of the present invention relates to the carbon nanotubes being produced in a continuous manner. Bibliographic references mentioned in the present specification are for convenience listed in the form of a list of references and added at the end of the examples. The whole content of such bibliographic references is herein incorporated by reference. Having now generally described the invention, the same will be more readily understood through reference to the following examples which are provided by way of illustration, and are not intended to be limiting of the present invention.

EXAMPLES Example 1

Freshly prepared Co-Mo/MgO catalyst powder was loaded into a vessel, followed by pre-heating of the reaction chamber to the desired reaction temperature of 800°C and purged with the carbon precursor. Once the reactor temperature had reached 800°C, the Co-Mo/MgO catalyst powder was continuously fed into the reaction chamber at a constant rate of 0.5g/min, controlled by the rotation speed of screw conveyor. Gaseous mixture of equal volumetric ratio of methane (carbon precursor) to nitrogen (diluents gas) was introduced at a total flow rate of 1.0L/min in countercurrent to the direction of catalyst feeding. The catalyst powder was conveyed along the reaction section for an hour by controlling the rotation speed of the revolving coil spring to allow the growth of carbon nanotubes. The outlet carbon nanotubes were collected from the product reservoir which is located at the lower end of the apparatus at the end of the reaction chamber opposite to the catalyst feeding section. The unreacted carbon precursor was exhausted through a gas outlet 10. The SEM image of the carbon nanotubes was shown in Fig. 2(a). This image showed that high density filamentous carbon was produced. The TEM image as shown in Fig. 2(b) confirmed the filamentous carbon to be CNTs as clear hollow cores were observed. The as-produced CNTs were sinuous and entangled with the length up to several micrometers and nearly uniform in the diameter. The HRTEM. image in Fig. 2(c) revealed the wall structure of CNT produced from the rotary reactor. As seen from the image, parallel graphene layers were clearly observed which indicated the high degree of graphitization. The production rates of approximately 130g CNTs/hour and about 3kg CNTs/day based on the catalyst feeding rate of 0.5g/min were achieved.