Patent Application: US-201414409128-A

Abstract:
a process for producing boron nitride nanotubes involves providing a one or more sources of boron , nitrogen and hydrogen to a stable induction plasma to form a reaction mixture of boron , nitrogen and hydrogen in the plasma , and cooling the reaction mixture to form bnnts . the process is capable of very efficiently producing small , reasonably pure bnnts continuously in high yield at or around atmospheric pressure without the need to use metals as the catalyst . the process may be further modified by providing one or more sources of carbon to produce bnnts doped with carbon .

Description:
with reference to fig1 , a schematic diagram of an induction plasma reactor suitable for synthesis of boron nitride nanotubes ( bnnts ) in accordance with the present invention is shown . the basic design is adapted from a similar induction plasma reactor for carbon nanotubes ( cnts ) as previously described in the art ( simard 2009 ). in one embodiment , the reactor comprises a 2 - 5 mhz radio frequency ( rf ) inductively coupled plasma torch 100 ( e . g . a tekna pl - 50 from tekna plasma systems , inc .) that can produce high temperature thermal plasma jet 102 in a plasma zone . a stable plasma can be maintained by heating a central inert plasma gas ( e . g . argon ) to a high temperature ( e . g . about 8000 k ). the central plasma gas is provided to the plasma zone through central gas inlet 106 . a sheath gas may also be introduced into the plasma zone through sheath gas inlet 108 , the sheath gas assisting in stabilizing the thermal plasma . the sheath gas may comprise an inert gas ( e . g . argon ) and / or one or more reactant gases that provide a source of nitrogen ( e . g . n 2 ), and hydrogen ( e . g . h 2 ). boron - containing feedstock ( e . g . metal - free h - bn ) may be provided to the thermal plasma through feedstock inlet 110 and the feedstock can be carried by a carrier gas ( e . g . argon ). where the feedstock is a powder , a powder feeder may be used to inject the feedstock into the plasma zone . the boron - containing feedstock and the nitrogen - and hydrogen - containing reactant gases may be continuously injected into the high temperature induction plasma jet 102 to form a reaction mixture of boron and nitrogen species . the boron - containing feedstock evaporates almost immediately (& lt ; 1 ms ) in the plasma releasing abundant boron vapors , and in the case of boron nitride feedstock also releases nitrogen . the nitrogen - containing reactant gas injected into the plasma also generates reactive nitrogen radicals ( e . g ., n , n + , n 2 + ) to improve nitrogen reactivity toward boron for the formation of bnnts . the reaction mixture of reactive boron and nitrogen species is carried from the plasma zone into a reaction zone 112 in a reaction chamber 114 , which is in fluid communication with the plasma zone . the reaction zone contains a refractory liner 116 for maintaining the process temperature and controlling the temperature gradient . in the reaction zone , boron vapors are cooled rapidly through the plasma jet expansion and nano - sized boron droplets are formed as the temperature cools down in the reactor . it is thought that bnnts grow continuously from such boron droplets by adsorbing nitrogen species formed in the plasma . based on the widely accepted “ root growth mechanism ” of bnnts , the effective generation of boron vapors and a controlled cooling of the vapors inside the reactor are of particular importance to abundant nucleation of small diameter boron droplets , which are known to be the practical precursors of small diameter bnnts . vigorous interactions between those boron droplets and the nitrogen species are also important for rapid growth of bnnts from the boron droplets . as the bnnts pass through the reaction chamber 114 their growth slows and is finally terminated . it should be noted that the growth process occurs over the whole of the reaction pathway from when the vapors enter the reaction chamber and begin to nucleate to when the formed bnnts finally leave the reaction chamber . initial cooling of the vapors in the reaction chamber permits nucleation of boron droplets that can then react with nitrogen species to start the formation of bnnts . bnnts continue to grow in their passage through the reaction chamber . as the reaction mixture cools further down in the reaction chamber , the continued growth of the bnnts is ultimately terminated . the reaction chamber is cooled with a water jacket . water flows into the water jacket through water inlet 118 a and out through water outlet 118 b . bnnts formed during the passage through the reaction chamber are collected using a vacuum filtration unit that comprise a filtration chamber 120 in fluid communication with the reaction chamber through a pipe 126 . a vacuum pump connected to vacuum port 124 draws bnnt - laden gases through porous filters 122 in the filtration chamber , whereupon the bnnts are deposited on the filters while the gases are drawn out . boron nitride nanotubes 130 can then be collected off the filters or the pipe . thus , in the present invention , a high enthalpy directional flow ( i . e . plasma jet ) generated from an induction plasma torch is employed for the continuous and effective generation of boron vapors from boron - containing feedstock ( e . g . solid h - bn powder ) while no metallic catalyst is required . although the induction plasma reactor used for a process of the present invention is based on the one described previously in us 2009 - 0214799 ( simard 2009 ), there are a number of important differences between the present process for bnnts and the prior art process for carbon nanotubes ( cnts ). the nucleation mechanism in us 2009 - 0214799 requires metal nanoparticle to play a role as seeds , therefore , the presence of metal catalysts is important . in the present invention , amorphous boron droplets play the role as seeds or growth substrates , so no metal catalyst is needed . further , the growth of cnts in the process of us 2009 - 0214799 involves precipitation of carbon clusters onto the surfaces of metal particles . thus , no chemical reaction occurs and the selection of metal catalyst is important taking into account carbon solubility , radiative heat transfer and other properties of the metal . in the present invention , incorporation of nitrogen into boron droplets ( formed when boron vapors cool ) occurs followed by chemical reaction of boron and nitrogen to form bn , and then precipitation of bn onto boron droplets . this is a fast and vigorous in - flight reaction that permits achieving high yields of bnnts . due to these differences , initial attempts to form bnnts using the process conditions described in us 2009 - 0214799 did not produce bnnts ( fig2 ). it is known in the art that similar induction plasma processes for cnts can produce different materials ( e . g . fullerenes , swcnts , and carbon blacks ) depending on the operating conditions , even when the same feedstock is used . given that the nucleation and growth mechanisms for bnnt production are very different than for cnt production , a new process for efficient bnnt production was invented herein . example 1a : h - bn powder using argon - nitrogen - hydrogen plasma at pressure of 92 kpa ( 0 . 91 atm ) pure hexagonal boron nitride ( h - bn ) powder ( 99 . 5 %, avg . particle size 70 nm , mk - hbn - n70 , m k impex corp .) was chosen as a feedstock . the as - received h - bn powder was sieved ( 300 μm ) with a brush and baked at 100 ° c . overnight . no metallic catalyst was employed . the reaction chamber included a graphite liner ( 80 mm id , 125 mm od and 1000 mm length , sigraform ® hlm , sgl carbon group ) surrounded by thermal insulating carbon felt , in order to extend the high temperature zone desirable for the growth of bnnts . prior to feeding the feedstock , the temperature inside the induction plasma reactor was stabilized using argon - nitrogen - hydrogen plasma for an hour . in this preheating stage , the plasma operating conditions were : a ternary gas mixture of 90 - slpm ar , 3 - slpm h 2 gas and 10 - slpm n 2 in the sheath gas ; 30 - slpm of ar in the central gas ; 3 - slpm of ar in the carrier gas ; 50 kw of plate power ; and , 92 kpa ( 0 . 91 atm ) of reactor pressure . after the stabilization period , the plasma operating conditions were changed for bnnt synthesis as follows : a ternary gas mixture of 45 - slpm ar , 55 - slpm n 2 gas and 20 - slpm h 2 in the sheath gas ; 30 - slpm of ar in the central gas ; 3 - slpm of ar in the carrier gas ; 50 kw of plate power ; and , 92 kpa ( 0 . 91 atm ) of reactor pressure . under these plasma operating conditions , the feedstock was continuously released from a powder feeder ( kt20 twin - screw microfeeder , k - tron , inc .) with a feed rate of about 0 . 5 g / min and delivered to the injection probe located on the top of the torch using 3 - slpm of ar carrier gas . after a 3 - hour operation under these conditions , a total of 20 . 0 g of bnnt material was recovered . this represents a yield rate of about 6 . 7 grams per hour . the product comprised two different materials : a rubbery cloth - like material and an entangled fibril - like material . due to light contamination by amorphous b by - product , the as - grown material was light - beige rather than snow - white . example 1b : h - bn powder using argon - nitrogen - hydrogen plasma at pressure of 92 kpa ( 0 . 91 atm ) another process was conducted following the same procedure as described in example 1a except that the plasma operating conditions were changed . thus , the ternary gas mixture in the sheath gas prior to feeding the feedstock used 110 slpm ar instead of 90 slpm . further , the ternary gas mixture in the sheath gas after the stabilization period was changed to use 25 slpm ar instead of 45 slpm and 30 slpm h 2 instead of 20 slpm . this resulted in a recovery of 60 . 0 g of bnnt instead of only 20 . 0 g , which represents a yield rate of about 20 . 0 grams per hour instead of 6 . 7 grams per hour . example 2a : h - bn — ni mixture using argon - nitrogen - hydrogen plasma at a pressure of 92 kpa ( 0 . 91 atm ) this test was specifically designed and performed to show that metal catalysts can be also used in the present induction thermal plasma process for an effective synthesis of bnnts ( fig3 ). as a typical example , nickel ( ni , 99 . 5 %, & lt ; 1 μm particle size ) was chosen as a metal catalyst . a mixture of h - bn powder ( 99 . 5 %, avg . particle size of 70 nm , mk - hbn - n70 , m k impex corp .) and nickel was chosen as a feedstock . the as - received h - bn powder was well mixed with ni using a rotary mixer at 60 rpm for 4 hours . then the mixture was sieved ( 300 μm ) with a brush and baked at 100 ° c . overnight . the final catalyst concentration of the mixture was 2 . 0 at . %. the reaction chamber included a graphite liner ( 80 mm id , 125 mm od and 1000 mm length , sigraform ® hlm , sgl carbon group ) surrounded by thermal insulating carbon felt , in order to extend the high temperature zone desired for the growth of bnnts . prior to feeding the feedstock , the temperature inside the reactor was stabilized using argon - nitrogen - hydrogen plasma for an hour . in this preheating stage , the plasma operating conditions were : a ternary gas mixture of 90 - slpm ar , 3 - slpm h 2 gas and 10 - slpm n 2 in the sheath gas ; 30 - slpm of ar in the central gas ; 3 - slpm of ar in the carrier gas ; 50 kw of plate power ; and , 92 kpa ( 0 . 91 atm ) of reactor pressure . after the stabilization period , the plasma operating conditions were changed for the bnnts synthesis as follows : a ternary gas mixture of 45 - slpm ar , 55 - slpm n 2 gas and 20 - slpm h 2 in the sheath gas ; 30 - slpm of ar in the central gas ; 3 - slpm of ar in the carrier gas ; 50 kw of plate power ; and , 92 kpa ( 0 . 91 atm ) of reactor pressure . under these plasma operating conditions , the feedstock was continuously released from a powder feeder ( kt20 twin - screw microfeeder , k - tron , inc .) with a feed rate of about 0 . 5 g / min and delivered to the injection probe located on the top of the torch using 3 - slpm of ar carrier gas . after a 3 - hour operation under these conditions , a total of 20 . 0 g of bnnt material was recovered and the product comprises two different materials : a rubbery cloth - like material and an entangled fibril - like material . due to light contamination by partially crystallized b by - product , the as - grown material was dark gray rather than snow - white . example 2b : h - bn — ni mixture using argon - nitrogen - hydrogen plasma at a pressure of 92 kpa ( 0 . 91 atm ) another test was conducted following the same procedure as described in example 2a except that the plasma operating conditions were changed . thus , the ternary gas mixture in the sheath gas prior to feeding the feedstock used 110 slpm ar instead of 90 slpm . further , the ternary gas mixture in the sheath gas after the stabilization period was changed to use 25 slpm ar instead of 45 slpm and 30 slpm h 2 instead of 20 slpm . this resulted in a recovery of 60 . 0 g of bnnt instead of only 20 . 0 g . example 3a : h - bn powder using argon - nitrogen - ammonia plasma at a pressure of 66 kpa ( 0 . 65 atm ) this test was specifically designed and performed to show that any hydrogen - containing gases can be also used in the induction thermal plasma process for an effective synthesis of bnnts . as a typical example , ammonia ( nh 3 , anhydrous , 99 . 99 %) was chosen as a hydrogen - containing gas . pure h - bn powder ( 99 . 5 %, avg . particle size 70 nm , mk - hbn - n70 , m k impex corp .) was chosen as a feedstock . the as - received h - bn powder was sieved ( 300 μm ) with a brush and baked at 100 ° c . overnight . no metallic catalyst was employed . the reaction chamber included a graphite liner ( 80 mm id , 125 mm od and 1000 mm length , sigraform ® hlm , sgl carbon group ) surrounded by thermal insulating carbon felt , in order to extend the high temperature zone desired for the growth of bnnts . prior to feeding the feedstock , the temperature inside reactor was stabilized using argon - nitrogen - hydrogen plasma for an hour . in this preheating stage , the plasma operating conditions were : a ternary gas mixture of 90 - slpm ar , 3 - slpm h 2 gas and 10 - slpm n 2 in the sheath gas ; 30 - slpm of ar in the central gas ; 3 - slpm of ar in the carrier gas ; 50 kw of plate power ; and , 66 kpa ( 0 . 65 atm ) of reactor pressure . after the stabilization period , the plasma operating conditions were changed for the bnnts synthesis as follows : a ternary gas mixture of 55 - slpm ar , 55 - slpm n 2 gas and 10 - slpm nh 3 gas in the sheath gas ; 30 - slpm of ar in the central gas ; 3 - slpm of ar in the carrier gas ; 50 kw of plate power ; and , 66 kpa ( 0 . 65 atm ) of reactor pressure . under these plasma operating conditions , the feedstock was continuously released from a powder feeder ( kt20 twin - screw microfeeder , k - tron , inc .) with a feed rate of about 0 . 5 g / min and delivered to the injection probe located on the top of the torch using 3 - slpm of ar carrier gas . after a 3 - hour operation under these conditions , a total of 20 . 0 g of bnnt material was recovered and the product comprises two different materials : a rubbery cloth - like material and an entangled fibril - like material . due to light contamination by amorphous b by - product , the as - grown material was light - beige rather than snow - white . example 3b : h - bn powder using argon - nitrogen - ammonia plasma at a pressure of 66 kpa ( 0 . 65 atm ) another test was conducted following the same procedure as described in example 3a except that the plasma operating conditions were changed . thus , the ternary gas mixture in the sheath gas prior to feeding the feedstock used 110 slpm ar instead of 90 slpm . this resulted in a recovery of 60 . 0 g of bnnt instead of only 20 . 0 g . most bnnt materials produced with prior art processes are limited to deposits scraped from the reactor walls or cotton - like fibrils . however , real applications or scientific investigation of as - produced materials may require materials in various forms . in contrast , bnnt materials formed in the processes described above show a great diversity in morphology . the present process can produce bnnt materials with several different morphologies in the same run , including i ) laminated flexible cloth - like materials on the surfaces of filters ( fig4 ), ii ) fibril - like materials on the top of filters ( fig5 ), and iii ) thin transparent films on the walls of the pipe between the reactor and the filtration chamber ( fig6 ). the cloth - like material ( 20 cm × 50 cm ) is flexible and mechanically strong which would be ideal for direct uses in manufacturing macroscopic - scale smart materials for civil or mechanical applications . this material is composed of multiple layers where thin membranes can be easily peeled off as shown in fig4 ( a ) . fig4 ( b ) presents a scanning electron microscope ( sem ) image of this material . the purity seems to be reasonably high ( over 50 %), even though non - tubular impurities are present in the samples . the length of bnnts is estimated few μm . fiber or yarn is one of the attractive forms of functional nano - materials . in contrast to the cnt case , macroscopic bnnt yarns have never been tested for their mechanical properties due to the absence of reliable fabrication methods . in the present invention , macroscopic - long fibers can be directly drawn from the fibril - like material simply by pulling them out as shown in fig5 ( b ) . the purity of the fibril - like material seems be much higher than that of the cloth - like material . a large quantity of fibrous material is observed in the sem image of this material with less non - tubular impurities ( fig5 ( d ) ). the purity of the as - produced material is high enough so that spinning fiber directly from the reactor is possible . in the present invention , thin transparent bnnt films can be synthesized in - situ without any substrates in the pipe located between the reactor and the filtration chamber . this as - grown bnnt film which is stretchable , sticky and highly electrostatic uniformly covers the entire surface of the pipe and seems to be formed by diffusion of bnnts towards the cold wall by electrostatic or thermophoretic forces . this thin film peels off readily from the surfaces and is mechanically strong enough to free - stand without polymer supports as shown in fig6 ( a ) . for specific applications , this thin film may be easily transferable to arbitrary surfaces . it is demonstrated that this thin film can be directly transferable to a quartz disk via one single step of spraying methanol on it . in order to investigate its transmission and absorption characteristics in the uv - vis range , two thin transparent bnnt films ( thickness : 160 nm and 198 nm ) were transferred on quartz disks and tested . as shown in fig6 ( b ) , a good transmittance is obtained with a 160 nm - thick film over a wide range of wavelengths , which is very promising for transparent armor applications . the absorption observed around 200 nm ( inset of fig6 ( b ) ) indicates the existence of h - bn materials in the film with a band gap of about 6 . 0 ev . this diversity in the morphology of the product will push the boundaries in the direct uses of as - produced bnnt materials . transmission electron microscope ( tem ) images in fig7 confirm that the fibrous materials seen in the sem images have a tubular structure . the majority of the bnnts are few walled , their diameters being less than 10 nm . large diameter tubes over 20 nm are not observed throughout the samples . the tem images of tubes also reveal that their structural quality is high without any noticeable defects on the surface of the tube . the structural quality the bnnts seems to be improved by the high temperature environment of the process . for the chemical composition analysis , electron energy loss spectroscopy ( eels ) measurements have been conducted . fig7 ( c ) shows eels spectra of the bnnts produced in the present invention . the k - shell ionization edges of b and n can be seen in the spectra which confirm that the tubes are composed of both b and n . the carbon peak between the two peaks is also observable due to sample contamination , probably from carbon grid . the thermal stability of the bnnt materials produced is investigated by the thermogravimetry analysis ( tga ). the thermal oxidation temperatures of h - bn materials are known to be higher than 1000 ° c . fig8 shows the thermogravimetry ( tg ) and derivative thermogravimetry ( dtg ) plots of the fibril - like and cloth - like bnnt materials produced in the present invention . both materials are stable up to 600 ° c . under air oxidation ; however they started to gain weight at 600 ° c . primarily due to the oxidation of amorphous b impurity present in the samples . since the cloth - like material gained more weight compared to the fibril - like bnnt material , it can be concluded that the cloth - like material contains more amorphous b impurity . the use of pure h - bn powder as feedstock in this invention allows for a simple and scalable purification process . various material characterizations have identified three major impurities found in the as - produced bnnt materials : i ) unreacted h - bn powder ; ii ) b - containing polymers ; and , iii ) elemental b . nano - sized h - bn powder and some of the b - containing polymers are readily dispersed in water due to the solvent polarity effect . when the cloth - like material is washed with nh 4 oh or water , the material retains its cloth - like structure as a result of bnnts inherent strength , promoting the physical separation of h - bn and some of the b - containing polymers into solution ( fig9 ( a ) ). the remaining beige material ( amorphous b ) contains elemental b and left - over b - containing polymers that can be easily oxidized to boron oxide , which is water soluble . the amorphous b was easily oxidized using hot ˜ 30 % h 2 o 2 ( fig9 ( b ) ). the oxidation reaction clearly transformed the material from beige to off - white . after several washings with water , the material appears very white in suspension . as a result of chemical characteristics of the as - produced material , the purification process only needs water as the sole solvent and hydrogen peroxide as the sole oxidizer providing a green and accessible purifying method . the use of pure h - bn powder as feedstock in this invention allows for a simple and scalable purification process . various material characterizations have identified three major impurities found in the as - produced bnnt materials : i ) unreacted h - bn powder ; ii ) b - containing polymers and , iii ) elemental b . these impurities can be easily removed by a simple three steps process comprising : 1 ) mulching or fluffing the raw materials using a mulcher ; 2 ) air oxidation at a temperature in a range of 650 ° c . to 850 ° c . ; and , 3 ) removal of boron oxides , unreacted h - bn and derivatives thereof using water or methanol as solvents and filtration . as shown in fig1 the collected solid material is highly pure bnnt material . the contents of the entirety of each of which are incorporated by this reference . arenal d , et al . 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( 2005 ) effective precursor for high yield synthesis of pure bn nanotubes . solid stat . comm . 135 , 67 . the novel features of the present invention will become apparent to those of skill in the art upon examination of the detailed description of the invention . it should be understood , however , that the scope of the claims should not be limited by the preferred embodiments set forth in the examples , but should be given the broadest interpretation consistent with the specification as a whole .