Patent Application: US-38900203-A

Abstract:
method and apparatus for producing filamentary structures . the structures include single - walled nanotubes . the method includes combusting hydrocarbon fuel and oxygen to establish a non - sooting flame and providing an unsupported catalyst to synthesize the filamentary structure in a post - flame region of the flame . residence time is selected to favor filamentary structure growth .

Description:
the term ‘ catalyst ’ refers to particles introduced to the flame gases to initiate filamentary structure growth and control the nature of the formed structures . the term ‘ unsupported catalyst ’ refers to catalyst particles ( or precursor reagents that decompose to form components that coalesce and consequently form catalyst particles ) that are introduced to the flame environment independent of any physical support affixed to a point or surface outside of the post - flame domain . the term ‘ filamentary structures ’ refers to materials where there exists a dominant linear dimension , giving the structure of the material a filament - like or filamentary appearance . see also the definition of aspect ratio below . the term ‘ filamentary nanostructures ’ refers to filamentary structures that have one or more dimensions on the scale of nanometers . filamentary nanostructures include nanotubes , nanowires , nanocones , peapods , and nanofibers . the term ‘ fullerenic ’ refers most specifically to an allotropic form of carbon that exhibits a three - dimensional curved structure comprising one or more layers or shells each including five - and sometimes seven - membered rings within a network of otherwise six - membered rings . the term ‘ nanotube ’ implies a tubular structure of nanoscale dimensions . nanotubes may be fullerenic in nature , implying they have end caps that close the surface of the structure , or may be tubular yet with end regions affixed to features ( such as metal particles ) other than curved end - caps , or the nanotubes may be open at one or multiple ends of the structure . more formally , there are essentially four categories that define the structure of nanotubes . 1 . single or multiwalled : nanotubes can be considered as a graphitic plane rolled to form a cylinder . there are two main classes of carbon nanotubes . a single - walled nanotube ( swnt ) is a single graphitic layer in the form of a tube . multi - walled nanotubes ( mwnt ) consist of multiple layers arranged concentrically about a common axis . double - walled nanotubes ( dwnt )( 63 ) are occasionally described as a distinct class , however they can be considered as the smallest category of mwnt . 2 . diameter : single wall nanotubes have diameters of order 1 nm . typical range of diameters spans from 0 . 7 nm ( the diameter of c 60 ) through to 10 nm . the smallest observed nanotube diameter is 0 . 4 nm ( 4 å )( 64 , 65 ). the diameter of multi - walled nanotubes varies between around 1 nm up to 100 nm . 3 . aspect ratio : one of the most striking properties of nanotubes is the disparity in their dimensions . the length of nanotubes can extend to order of microns and more , giving an aspect ratio ( length to diameter ) of 1000 to 1 . the longest nanotubes reported to date ( 66 ) are 20 cm , giving an aspect ratio of 200 , 000 , 000 to 1 ! as used herein , filamentary structures have an aspect ratio of at least 10 to 1 . 4 . chirality : the chirality of a nanotube refers to the ‘ twist ’ in the graphitic layer that makes up the tube wall . certain chiralities can give metallic conduction while others are semiconductive . the chirality of a nanotube can be described uniquely by two indices ( m , n ). by folding a graphene sheet into a cylinder so that the beginning and end of a ( m , n ) lattice vector in the graphene plane join together , one obtains an ( m , n ) nanotube ( 4 ). ( m , m ) nanotubes are said to be ‘ arm - chair ’, ( m , 0 ) and ( 0 , m ) nanotubes are ‘ zig - zag ’, and ( m , n ) nanotubes are chiral . all arm - chair nanotubes are metallic but only one third of possible zig - zag and chiral nanotubes are metallic , the other two thirds being semiconducting ( 67 ). the term ‘ nanowire ’ implies a linearly contiguous and non - hollow length of metal based material with diameter on a scale of nanometers . nanowires can be formed by filling the internal cavity of carbon nanotubes with metals and other elements . the term ‘ nanocone ’ refers to a class of materials that have a dominant linear dimension with a non - constant diameter increasing or decreasing relative to the position along the length of the structure . the term ‘ peapod ’ refers to carbon nanotubes that have one or more carbon fullerenes occupying the internal cavity of the nanotube . the term ‘ nanofiber ’ refers to filamental structures that are similar in structure to multi - walled nanotubes as they possess multiple structural layers in the wall area . nanofibers are much more disordered and irregular relative to nanotubes and the walls are non - graphitic . carbon nanofibers may alternately be described as carbon fibrils , vapor grown carbon fibers ( vgcf ), filamental carbon , filamental coke or simply filaments . the term ‘ graphitic ’ refers most specifically to an allotropic form of carbon that exhibits a flat , two - dimensional , planar structure . the term graphitic in the context of this document refers to the flat geometric structure and the high degree of order associated with a planar structure , and does not necessarily imply an elemental composition of carbon . a graphitic plane rolled into a cylinder can therefore describe the structure of a single - walled nanotube . the term ‘ post - flame region or zone ’ is the part of the flame located downstream of , or farther from a burner than , the oxidation region or zone of the flame . the beginning of the post - flame region is marked by the approximate completion of the consumption of molecular oxygen and the conversion of the original fuel to intermediates and products including carbon monoxide , carbon dioxide , acetylene , other carbon containing species , hydrogen and water . the post - flame region includes the tail of the flame , extends to the transition between the flame and the exhaust , and consists of hot but usually cooling gases which are approximately well mixed within a given cross section of the flow perpendicular to the direction of flow at a given distance from the burner . the well - mixed condition is achieved by mixing the fuel and oxygen together before feeding them to the burner ( premixed combustion ) or by feeding the fuel and oxygen as separate streams which rapidly mix within the combustor over a downstream distance from the burner that is much smaller than the diameter or the equivalent diameter of the post flame region ( non - premixed combustion ). the residence time in the post - flame region is much larger than the residence time in the oxidation region of premixed flames or the mixing and oxidation region of non - premixed flames . the term ‘ sooting flame ’ refers to a flame system including a fuel and oxygen undergoing combustion in such a way that carbon soot is generated in visibly significant quantities . almost all non - premixed flames of hydrocarbon fuels exhibit soot formation . the sooting limit for premixed flames is defined as the lowest equivalence ratio ( or carbon to oxygen ratio ) at which soot is observed in the flame gases . a sooting flame has a distinctive , visibly luminous glow caused by emission from the soot particles . a non - sooting flame is established by a fuel equivalence ratio ( or carbon to oxygen ratio ) lower than the sooting limit . addition of metal bearing compounds to the flame may induce visible luminosity yet the flame is not sooting as in this instance the sooting limit is defined for the base - flame ( fuel , oxygen only ). for non - sooting flames the radiance is caused by emission from the metal particles rather than soot particles . in flames containing nanotube formation catalysts , the critical equivalence ratio for soot formation depends not only on equivalence ratio , but also on the type and concentration of catalysts present . metal catalysts may augment soot formation such that a non - sooting condition may become sooting upon catalysts addition if the flame were at an equivalence ratio near the sooting limit and the type and concentration of catalyst added were sufficient . a premixed acetylene / oxygen / argon flame formed the basis of the experiments disclosed in this patent application . an argon dilution of 15 molar percent , cold gas feed velocity of 30 cm / s , and burner pressure of 50 torr were used throughout the experiments . a variety of fuel equivalence ratios ranging from 1 . 4 through 2 . 2 were considered . iron pentacarbonyl ( fe ( co ) 5 ) was used as the source of metallic catalyst necessary for nanotube synthesis . with reference to fig2 a controlled flow of iron pentacarbonyl vapor was supplied through a temperature - controlled ( 4 ° c .) single - stage bubble saturator 20 unit using argon as the carrying gas . the argon gas flow could be accurately proportioned between the saturator 20 and a bypass line , allowing control of the catalyst feed rate . typical iron pentacarbonyl feed concentrations were 6000 ppm ( molar ). a burner 22 consisted of a 100 mm diameter copper plate 24 with 1500 uniformly spaced 1 mm diameter holes drilled through the surface . only the inner 70 mm diameter burner section was utilized for this study with the outer annular section used during flame startup . the burner plate 24 is attached to a burner cavity filled with stainless steel wool to facilitate uniform flow distribution of premixed gases 25 entering from the base of the cavity . suitable premixed gases include acetylene , oxygen and argon . it is also contemplated that modifying agents for altering the structure or morphology of the condensed material may be co - injected . in addition , a secondary oxidant may be injected in the post - flame region to oxidize carbon contamination . it is also contemplated to quench the filamentary structures by injecting an inert fluid that will quench by sensible energy , latent energy or chemical reaction . a flow of cooling water passes through copper tubing 26 coiled around the outside of the burner body . burner plate temperatures were typically 70 - 80 ° c . the burner was mounted on a vertical translation stage 28 , which allows measurements to be taken at various heights - above - burner ( hab ). the burner 22 and translation stage 28 are contained in a stainless - steel pressure chamber 30 . an upper chamber plate is water - cooled and exhaust gases are withdrawn through two ports 32 in the upper flange . a variety of ports in the sidewall of the chamber provide access to sampling and diagnostic instruments . a large ( 15 cm ) window 34 is provided for visual observation of the flame ( 68 ). an electronic proportioning valve 36 and pid controller coupled to the exhaust extraction system allows accurate control of the chamber pressure . table 1 shows operational settings and parameters to obtain good quality nanotubes . with reference still to fig2 a preferred embodiment includes an electric field represented by the arrow 27 aligned with flame gas flow and having a selected field strength . those skilled in the art will recognize that the electric field 27 could also be a magnetic field or the combination of an electric and magnetic field to alter the characteristics of the filamentary structures produced . for example , an electric and / or magnetic field may be used to alter residence time profiles and / or particle trajectories to alter the structure or morphology of the produced structures . the electric field 27 aligned with the flame gas flow will induce preferential growth of the structures with either metallic or semiconductor chirality . a thermophoretic sampling technique ( 69 ) was used to collect condensed material in the flame gases at various hab and the samples were then analyzed using transmission electron microscopy ( tem ). a thermophoretic sampling system 38 included a pneumatic piston coupled with a timing mechanism to give precise control over immersion time within the flame . an insertion time of 250 ms was used throughout the experiments . tem grids 40 ( ladd research industries , 3 mm lacy film ) were affixed to a thin metal stage attached via a 6 mm diameter rod and pressure seal feedthrough to a pneumatic plunger . after insertion into the flame gases , each tem grid was removed and subsequently taken to the microscope for analysis . a joel 200cx was used for the bulk of the microscopy work to allow rapid screening and turnaround of samples to be examined . more detailed microscopy was performed on a 2010 and 2000fx for high resolution images . the elemental composition of any condensed material is of particular interest in terms of the nanotube formation processes occurring in the flame . stem combined with electron dispersive x - ray spectroscopy ( edxs ) allows a high resolution transmission electron microscopy image to be correlated with an elemental map that gives insight into the distribution of specific elements ( such as c , fe , o ) relative to the material structures imaged using tem . a vg hb603 system was used for stem analysis performed in this study . raman spectroscopy can be used to obtain information relating to the diameter and also the chirality of single - walled carbon nanotubes ( 70 , 71 ). when single - wall nanotubes are irradiated with 514 . 5 nm argon - ion laser light , at least two distinct resonant modes are observed in the resulting raman spectrum . modes around the 100 to 300 cm − 1 frequency range correspond to the ‘ radial breathing mode ’ ( rbm ) of nanotubes where the cylindrical nanotube vibrates in a concentric expansion and contraction . the frequency of the rbm is inversely proportional to tube diameter and so the spectrum can be used to obtain tube diameter information . the second major feature in the spectrum is the ‘ g - band ’ at around 1590 cm − 1 which corresponds to transverse vibrations along the plane of the nanotube wall . shifts in the shape of the g - band peak can indicate the nature of the nanotube chirality ( semiconducting or metallic ). raman spectroscopy on condensed samples collected from the burner chamber wall was performed using a kaiser hololab 5000r raman spectrometer with raman microprobe attachment . the spectrometer was operated at 514 . 5 nm at 0 . 85 mw power in stokes configuration . thermophoretic samples were taken at regular height intervals above the burner 22 and images obtained using transmission electron microscopy . each sampling height corresponds to a residence time away from the burner and so this technique enables characterization of the dynamics of the nanotube growth processes occurring in the flame . flame characterization sampling was performed on flames with equivalence ratios ( φ ) between 1 . 4 and 2 . 2 . for each flame , samples were obtained along the axis - line in the post - flame region between 10 and 75 mm above the burner . a typical progression of nanotube morphologies observed in a flame with equivalence ratio of 1 . 6 is shown in fig3 . the initial post - flame region ( up to 40 mm ) as shown in fig3 is dominated by the presence of discrete particles . particle formation and growth leads to larger particle sizes as height above burner increases . iron pentacarbonyl decomposes rapidly upon exposure to the flame and the particles size growth most likely occurs through coagulation of the iron resulting from this decomposition ( 62 ). the composition of the particles is most likely metallic iron as observed in flames of higher equivalence ratio ( 72 ). nanotube growth is generally accepted to occur through a decomposition - diffusion - precipitation mechanism whereby carbon bearing species ( primarily co ) catalytically decompose on the surface of a metal particle , followed by elemental carbon dissolving into the metal lattice and diffusing to the adjacent side of the particle , where the carbon precipitates in a curved tubular graphitic structure ( 73 - 75 ). based on this mechanism it is likely that catalytic decomposition and ‘ loading ’ of carbon into the particles is also occurring concurrently with particle growth in this initial post - flame region . carbon nanotubes are observed after an inception time of approximately 30 milliseconds . a small number of discrete nanotube segments with length of the order of 100 nm are observed as early as 25 ms and longer tube lengths up to a micron in length are observed to form in the following 10 ms . it appears that the metallic particle population has reached a critical level after 25 ms and nanotube growth proceeds rapidly after this point for the next 10 to 20 ms . the critical condition may be sufficiently large particle size , carbon content , surface properties , internal lattice structure transition ( 41 ), or point of relative concentrations for co and h 2 within the flame gases ( 42 ). for times after 40 ms the dominant mechanism appears to be coalescence of the condensed material in the flame gases . disordered networks of nanotube bundles form tangled webs decorated with metallic and soot - like particles . the complexity and size of the webs increases significantly in the upper region of the system , between 45 and 70 ms . from the structures observed in the post - flame gases it is clear that , once initiated , nanotube growth occurs quite rapidly . an order of magnitude estimate for the nanotube growth rate is 100 μm / s based on the images and observed increase in length of 1 00 nm to 1 micron over a period of 10 ms ( between 25 to 35 ms ). the effect of different equivalence ratios upon nanotube formation was also investigated . samples were extracted from 70 mm above burner ( approx . 67 ms ) for equivalence ratios between 1 . 4 and 2 . 0 . representative tem images over the range of equivalence ratios are shown in fig4 . nanotubes are observed to form between equivalence ratios of 1 . 5 and 1 . 9 . this range of equivalence ratios can be considered as a ‘ formation window ’ where conditions within the flame are suitable for nanotube synthesis . a particularly preferred equivalence ratio range is 1 . 5 ≦ φ & lt ; 1 . 7 . for low equivalence ratios ( 1 . 4 and 1 . 5 ) the condensed material in the flame is dominated by discrete particles , although nanotubes may form at higher hab than those described in the present system ( see fig1 ). the range of equivalence ratios that could support nanotube growth can therefore potentially extend from 1 . 7 to 1 . 0 . equivalence ratios of 1 . 9 and higher are dominated by soot - like structures displaying clustered networks of primary particles ( of either metallic or carbon encapsulated metal centers ) with the occasional nanotube within this matrix . it is interesting to note that within the formation window range , relatively ‘ clean ’ nanotubes are formed at the lower equivalence ratios while an increasing level of encrusting with disordered carbon is observed on the nanotubes as the equivalence ratio increases . a continuum of morphologies is apparent ranging between clean nanotubes at low equivalence ratios through to an increasing proportion of soot - like material as the equivalence ratio increases . a competition between carbon precipitation pathways is likely , with one pathway leading to filamentary or tube structures and the other to disordered carbon clusters . this observation is consistent with the nanotube formation mechanism and how this would relate to a flame environment . as fuel equivalence ratio increases from unity , the level of excess carbon available in the flame gases increases , so one would also expect an increasing potential to form carbon nanotubes . this trend is tempered as the sooting equivalence ratio limit is reached and the availability of carbon exceeds the capacity of the nanotube formation pathway and disordered carbon is formed . therefore the lower formation limit corresponds to insufficient availability of carbon , while the upper limit is due to dominance of soot formation pathways close to the sooting limit . the observed change in morphology as equivalence ratio is changed is described quantitatively in fig1 . a metric of nanotube quality , defined in this instance as the product of filament length and filaments counted in a tem image divided by the image area covered by condensed material . high quality material by this metric would have many filaments of significant length within a matrix of minimal non - structured condensed material . a plot of this metric against equivalence ratio indicates quite clearly that nanotube quality improves dramatically as equivalence ratio moves from high ( 2 . 0 +) to lower equivalence ratios . this trend reinforces the importance of using non - sooting flames to enhance the growth of filamental structures in the flame . furthermore , the tem insets and schematic plot give context to this phenomena relative to other flame parameters . higher magnification tem analysis shows that the condensed filamental material is predominantly bundles of single - wall nanotubes ( fig5 ). the structures shown in fig5 resulted from an equivalence ratio of 1 . 6 with a hab of 70 mm . the inset shows detail of a nanotube bundle with an outer wall shown in dark contrast . the flame synthesis process preferentially forms single - wall as opposed to multi - wall nanotubes . this observation is in agreement with other flame studies ( 43 ) and indicates a high degree of selectivity in the material synthesis despite the ensemble of competing processes occurring in the flame system . a tem image for material collected from the water - cooled chamber wall ( used for raman measurements also ) is shown in fig6 . note the dominant features of nanotube bundles encrusted with agglomerates of carbon with internal metallic particles . the raman spectroscopy technique yielded a number of observations about the flame generated nanotube material . a typical raman spectrum for flame generated material is shown in fig7 . spectra for flame generated material ( bold line ) are compared to materials obtained from plasma arc processes ( light grey lines ) in fig8 . in fig7 features centered around 200 cm − 1 are radial breathing modes corresponding to a range of single - walled nanotube diameters ( approximately 0 . 9 to 1 . 3 nm ). the shape of the large peak at 1590 cm − 1 relates to chirality effects . the raman spectrum for the flame generated material shows a wide distribution of peaks corresponding to radial breathing modes ( rbm ). the corresponding range of tube diameters that generate this spectrum are between approximately 0 . 9 and 1 . 3 nm . when compared to the rbm modes obtained from the spectra of material generated with a plasma - arc technique , a difference in diameter distribution is clear . the flame generated material has a broader distribution of diameters and the diameters extend to smaller sizes . other differences are apparent based on the shape of the g - band . the flame generated material has a significant ‘ hump ’ profile on the side of the g - band , with a peak at about 1330 cm − 1 and an apparent peak seen as a shoulder on the g - band , which is indicative of nanotube chirality ( semiconducting or metallic ). compared to the plasma - arc generated material , the flame material appears to be more metallic in nature . scanning transmission electron microscopy ( stem ) was performed on material sampled directly from the flame as per the previously described tem measurements . the composition of the particles associated with the carbon nanotubes is of particular interest and electron dispersive x - ray spectroscopy ( edxs ) was used in scanning mode to obtain spatial maps of elemental intensity which could be compared to the stem image in order to correlate composition with position . images for this measurement on flame generated material are shown in fig9 . the stem bright field image shows a bundle of single - wall carbon nanotubes with a dark particle agglomerate overlaying the bundle ( apparently sitting next to rather than a part of the bundle ). the elemental map for iron clearly shows a close correlation between the iron and the particle position , indicating the particle is composed largely of iron . the oxygen map also shows a correlation although at much lower intensity . the particle is most likely predominately iron but may have a small oxide content . the carbon map shows rather poor contrast due to the bundle sitting on a carbon substrate yet an increased carbon intensity is observed in correlation with the nanotube bundle and around the particle . the particle associated with the nanotube bundle is likely composed of iron surrounded by non - structured carbon , as can also be observed in tem images shown in fig5 and 6 . the yield of nanotube material from the flame was estimated by a probe sampling technique and gravimetric analysis . a quartz tube ( od 11 mm , id 9 mm ), surrounded by a water cooled jacket , was inserted into the post - flame region with the mouth opening of the probe positioned 70 mm above the burner surface . the quartz tube was attached directly to a sintered metal filter assembly ( swagelok ) that had been modified by placing a custom made disc of filter fabric ( balston , grade cq ) in - line before the metal filter disc . a vacuum pump was coupled to the filter to allow extraction of flame gases and flame - born condensed material through the probe and filter unit . sampled gases were vented from the sample pump exhaust directly to a water column ( gas collection bell ) to allow determination of the volumetric concentration in the flame . after sampling the flame for a measured period of time , the filter disc was removed from the filter unit and weighed to determine the mass of material collected . the amount of condensed material collected on the filter , scaled to the cross - section area of the burner face , over the sampling time ( 90 sec ), gave the following estimates for condensed material yield per component of burner feed ( per c fed : 1 . 1 %; per fe fed 24 . 8 %; per fe ( co ) 5 fed 9 . 8 %). based on inspection of representative tem micrographs for the flame sampled material ( fig6 ), it is estimated that roughly 50 % of the image area covered by condensed material is associated with nanotubes ( typically in the form of bundles ) and this would equate roughly to a mass percentage of 10 % or so . the yield of nanotubes relative to components fed to the burner can therefore be estimated as ( per c fed 0 . 1 %; per fe fed 2 . 5 %; per fe ( co ) 5 fed 1 . 0 %). these estimated yields indicate that there are significant quantities of nanotubes generated in the flames described in this study , and would certainly amount to more than 1 % of the condensed material . the effect of using non - sooting flames is illustrated in fig1 . nanotube quality and yield as a proportion of condensed material clearly improves as equivalence ratios shift away from the sooting limit . note that yield of the filamentary material peaks at an equivalence ratio of approximately 1 . 6 . however , it is likely that higher yields may be obtained at lower equivalence ratios and higher hab ( or longer residence times ) as is indicated in fig1 . single - walled nanotubes have thus been observed in a premixed acetylene / oxygen / argon flame operated at 50 torr with iron pentacarbonyl vapor used as a source of metallic catalyst necessary for nanotube growth . a thermophoretic sampling method and transmission electron microscopy were used to characterize the solid material present at various heights above burner ( hab ), giving resolution of formation dynamics within the flame system . catalyst particle formation and growth is observed in the immediate post - flame region , 10 to 40 mm hab , with coagulation leading to typical particle sizes on the order of 5 to 10 nm . nanotubes were observed to be present after 40 mm (˜ 34 ms ) with nanotube inception occurring as early as 30 mm hab (˜ 25 ms ). between 40 and 70 mm hab ( period of approx . 30 ms ), nanotubes are observed to form and coalesce into clusters . based on the rapid appearance of nanotubes in this region , it appears that once initiated , nanotube growth occurs quite rapidly , on the order of 100 μm per second . a nanotube formation ‘ window ’ is evident with formation limited to fuel equivalence ratios between a lower limit of 1 . 5 and an upper limit of 1 . 9 , although this range may extend to lower equivalence ratios in samples withdrawn from higher ( or after more time ) in the post - flame region . a continuum of morphologies ranging from relatively clean clusters of nanotubes to disordered material is observed between the lower and upper limits . the yield of nanotubes in the condensed material increases at compositions lower than the sooting limit . it is recognized that modifications and variations of the invention disclosed herein will occur to those skilled in the art and it is intended that all of such modifications and variations be included within the scope of the appended claims . 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