Patent Application: US-201313904610-A

Abstract:
a process for the production of pyrolytic carbon comprising the steps of : depositing pyrolytic carbon on a substrate , and controlling the structure of the deposited pyrolytic carbon through use of a volmer - weber island growth model .

Description:
pyrolytic carbon is preferably deposited onto a suitable substrate by the thermal decomposition of a gaseous hydrocarbon at high temperature , using a process called chemical vapor deposition ( cvd ). cvd is a very versatile process used in the production of coatings , powders , fibers and monolithic parts . with cvd , it is possible to produce almost any metallic or non - metallic element , including carbon and silicon , as well as compounds such as carbides , nitrides , borides , oxides , and many others . a key advantage of the cvd process lies in the fact that the reactants used are gases , thereby taking advantage of the many characteristics of gases . one result is that cvd is not a line - of - sight process as are most other plating / coating processes . graphite has properties that are particularly well suited for pyrolytic carbon coating , most notably its thermal expansion coefficient that avoids weakening the coated substrate . in order to appear visible on x rays , graphite is soaked in tungsten . this permeation does not change the mechanical properties of the substrate . for example , to make pyrolytic carbon coated orthopedic implants , a graphite substrate is introduced into a chamber that is heated to between 1 , 200 ° and 1 , 500 ° celsius . a hydrocarbon gas , typically propane , is introduced into the chamber . these high temperatures facilitate the decomposition of the hydrocarbon precursor via a complex radical cascade , producing a variety of carbon containing species which can participate in pyrolytic carbon deposition onto the graphite substrate . over a period of time the substrate is completely coated with between 300 and 600 microns of pyrolytic carbon . reaction byproducts are then exhausted out of the system . the bed consists of small ceramic particles and parts to be coated a levitating gas creates required random motion of parts within the bed heating elements raise furnace temperature to 1000 °- 1500 ° c . an introduced hydrocarbon gas undergoes decomposition at these temperatures creating species that deposit on the surface of the substrate . further details relating to the process of applying a pyrolytic coating to a substrate may be readily found in the prior art including u . s . pat . no . 6 , 274 , 191 . a gas flow control and mixing panel was supplied with methane ( linde , 99 . 95 %), oxygen ( linde , 99 . 9 %) helium ( boc , 99 . 99 %) and argon ( boc , 99 . 99 %). the gas flow rates were controlled by mass flow controllers ( brooks 5850e ) operating within the range of 10 - 90 % of their total flow . the mass flow controllers were calibrated using a bios definer 220 flow meter and calibration curves were checked for linearity . feed gases were blended prior to being introduced into the reactor . the pressure was monitored at the inlet and outlet of the reactor by pressure transducers . the feed and product streams were analysed by gas chromatography using an on - line shimadzu gc17a chromatograph set up with two flow lines . a gas sample was simultaneously injected into the two flow lines . one flow line flow line , setup with a varian cp - molsieve 5a column ( 25 m , 0 . 53 mm , 50 μm ) and a thermal conductivity detector , provided analyses of helium , hydrogen and oxygen . the second flow line , equipped with a varian cp - poraplot q column ( 27 . 5 m , 0 . 53 mm , 20 μm ), methaniser and flame ionisation detector , was used for the analysis of carbon dioxide , carbon monoxide and c 1 - c 3 hydrocarbons . analyses were conducted under isothermal conditions at 30 ° c . a reactor tube comprised of zirconia with 10 . 5 % yttria was stacked with wafers of zirconia with 6 % scandia ( ceramatec ) and heated to 1673k under a flow of argon 12 . once the temperature had stabilized methane ( 1 %) and dioxygen ( ch 4 : o 2 = 1 . 1 ) were introduced into the feed ( total flow = 5000 sccm ) and this flow was maintained for a period of 240 minutes . during this time the product gas composition was analyzed periodically by gas chromatography ( gc ). after 240 minutes the reactive components were removed from the feed and the argon flow rate was reduced to 150 sccm . the reactor was cooled to ambient temperature and the wafers were removed for analysis . the surfaces of the zirconia wafers were analyzed by scanning electron microscopy ( sem ; fe - sem philips xl30 and fei helios nanolab 600 fib - sem ) combined with energy disperse x - ray ( edx ) analysis and micro - raman spectroscopy ( reinshaw invia , λ 0 = 514 . 5 nm ). a longitudinal temperature profile within the reactor was measured using an s - type thermocouple , under identical conditions to the experiment , but without wafers inside the reactor . the surface area to free volume ratio ( nv ) in the section of the reactor stacked with wafers was 1 . 13 mm − 1 we have defined residence times , t , as the time that the gas phase was exposed to temperatures above 1300 k , as little reactivity was observed below this temperature . using this definition the total residence time was 0 . 015 s . sem examination of the surface of the zirconia wafers showed the presence of carbon deposits that resembled johnson - mehl tessellations , with polygonal elements with triple - point grain boundaries ( fig2 a ). some areas of the zro 2 surface were not covered by a continuous film , but rather by non - continuous carbon deposits , as shown in fig2 b and c . these deposits were either isolated carbon hemispheres or non - continuous johnson - mehl tessellations . a number of isolated hemispherical deposits were sliced by fib milling and the resulting cross - sections examined by sem . one such cross - section is shown as an inset in fig2 c . the appearance of these cross - sections supported the assessment that the deposits were hemispheres , and edx analysis showed that they were comprised of solid carbon . because of the likelihood that the fib milling technique had changed the internal structure of the deposits , the inside of a deposit that had been mechanically damaged was examined and appeared to be comprised of concentric layers of carbon ( fig3 b ). the concentric arrangement of the carbon layers was also evident when the underside of a deposit was analyzed . a zirconia surface bearing carbon deposits was scraped with a sharp blade and on examination by sem an upturned deposit was observed ( fig3 c ). it appeared that on breaking from the substrate the deposit had fractured . our interpretation of this image is that on breaking from the surface the central area of some grains remained on the surface , leaving hemispherical - shaped cavities in the underside of the center these grains . this interpretation was difficult to verify as the central areas that are proposed to remain on the surface would be expected to be similar in appearance to small hemispherical deposits . grain boundaries were clearly evident between adjacent grains . in some cases there appeared to be a void along the grain boundary , however we could not determine whether this was a feature of the structure or had formed as a consequence of breaking the deposit from the zirconia surface . the nature of the carbon structure was examined using micro - raman spectroscopy . from the raman spectra the i d / i g ratio was found to be ca . 1 . 6 , consistent with graphite with a low degree of order . these observations are consistent with growth by the v - w mechanism . growth via this mechanism has been well described with comprehensive mathematical models 13 , however , for our purposes a simple model was developed to investigate the consequences of this mechanism on the structure of pyrolytic carbon materials . the simplified model defined a v - w mechanism which seeds nuclei randomly on a flat surface at a constant rate per unit area and once a nucleus is generated it grows at a constant radial growth velocity in free ( non - occupied ) directions . within this embodiment , seed points accumulate only on the substrate . a time sequence generated using this model is presented in fig4 . this model generates all characteristic morphological features of the deposits observed in this study . the pyrolytic carbon growth was interrupted prior to coalescence of the hemispherical islands due to a relatively low nucleation density and a slow rate of growth . we postulated that growth by the v - w mechanisms may be common , but the initial stages are seldom seen due to higher densities of nucleation sites resulting in rapid film formation . the model was used to investigate this by increasing the nucleation site density by ca . 1000 . under these conditions rapid coverage of the substrate surface was shown to occur ( fig5 ). in addition , with the higher nucleation density the effect of the growth mechanism on the structure of the material also became apparent . from the cross - section in fig5 c , it is clear that the structure is columnar . this is consistent with numerous experimental reports of the structure of anisotropic pyrolytic carbon 4 , 7 , 8 , 14 - 17 . it can also be seen that the number of elements ( i . e . the top of columns ) in the tessellated surface decreases with film growth , as described previously 5 . this is a consequence of the constant rate of nucleation . for any two neighboring columns , the column that originated for the earliest seeded nuclei impinges on the column seeded later . consequently , columns based on the earliest forming nuclei progressively dominate the structure over time by terminating the growth of columns seeded later . this is most obvious in the early stages of growth where the cross - sectional structure of the deposit looks somewhat disordered due to the rapid termination of columns originating from the latest formed nuclei ( see the cross - section in fig5 b ). this behavior accounts for the observation of “ granular ” regions at pyrolytic carbon / substrate interfaces 6 , 8 . the formation of grain boundaries between columns is also consistent with v - w growth . for each column the graphitic layers are oriented concentrically around its seed point , thus the layers in adjacent columns are misaligned . this can be seen in the results for the simulation shown in 6 , in which red lines , that indicate the surface of the deposit at periodic growth intervals , show the arrangement of carbon layers in the structure . another structural feature of anisotropic pyrolytic carbon , often described in the literature , is the presence of growth cones that result from surface asperity . the simple model of v - w growth was applied to surfaces with surface texture or roughness ( fig7 ). four situations were compared : a flat surface ( a ) ( i . e . the base surface of the substrate ); surfaces with single cylindrical - shaped protrusions of different heights ( b and c ); a surface with single gaussian hump - shaped protrusion ( d ). the hyperboloid structures above the protrusions in b , c and d demonstrate that v - w growth over surface asperity can form growth cones such as those reported ( see fig1 ). the columns formed from these nuclei have a similar advantage to those that are formed earlier ( vide supra ). during the growth of the film they impinge on the neighboring columns of nuclei formed lower . comparison of b and c shows that the higher above the substrate nuclei are formed the more privileged , and hence larger , is the resulting growth cone . a simple check of the literature confirms this , whenever the base of a growth cone is visible ( note : this is not always the case in cross - sections ), the largest cone grows from the highest point on the substrate , for example see fig1 [ 4 , 5 , 18 ]. a consequence of the growth cone impinging on surrounding columns is that the column or columns in the growth cone have a larger cross - sectional area than the columns from nuclei seeded lower . thus a pyrolytic carbon deposit with growth cones will have fewer grain boundaries in the mature film than one without . in the example shown in fig7 ( d ) the protrusion on the surface is a gaussian hump with the same height as the cylinder in ( c ). comparison of the two cases shows that the shape of the protrusion may affect the shape of the growth cones , in each case the shape of the cross section is a hyperbola . in particular , the elevated surface or the gaussian hump ( d ) has a narrower apex which results in fewer nucleation sites ( and hence fewer seeded nuclei ) from which pyrolytic carbon growth ( i . e . columns or cones ) is able to impinge on the growth of columns emanating from the base of the substrate surface ( a ) in addition to pyrolytic carbon growth from beneath the apex of the elevated portion of the protrusion . as a result , an elevated substrate with a smaller number of preferential nucleation sites result in larger grained pyrolytic carbon growth . in this example ( d ) the growth cone consists of one column at the surface of the elevated substrate . the other main factor influencing the shape of the growth cone is the height of the protrusion above the plane of the substrate . an important consequence of the v - w mechanism is that surface texture or roughness will have a significant effect on the structure of the growth columns formed . fig8 shows the result of a simulation of growth over a flat surface ( a ), a surface with two protrusions with different heights ( b ), and a surface with periodic protrusions of the same height ( c ). the example of two protrusions demonstrates that growth cones seeded higher dominate and impinge on growth cones seeded lower . because growth cones and columns are one and the same , this is in essence the behavior as described previously for columns impinging on columns and growth cones impinging on columns . the relationship between columns and growth cones appears to have been a point of confusion in the literature . although we are continuing to use the term “ growth cone ” to describe the structures produced above protrusions , it is important to recognize that these are simply dominant columns , or collections of columns . in the case of periodic protrusions the columns formed from higher nuclei are less privileged than the example of a single protrusion . this results in fewer growth columns at the surface than the case of no substrate roughness ( a ), and columns with a smaller cross - sectional area than that of the large growth cone formed in the case of a single protrusion ( b - d ). the results of this computer simulation model highlight the benefits of using the computer simulation model designing a patterned series of protrusions sufficiently separated to result in the desired grain size , which in turn produce the desired mix of functional properties of the pyrolytic carbon structure . fig9 to 11 , 13 and 14 further illustrate the effect of the protrusion shape ( height and elevated surface area ) on the structure of the pyrolytic carbon . fig1 illustrates how increasing nucleation density ( figures a1 to c1 and figures d1 to f1 ) influence the resultant pyrolytic carbon structure . for a flat substrate surface ( figures a1 to c1 ) a larger structural variation is observed relative to a substrate comprising a plurality of protrusions ( figures d1 to f1 ). this demonstrates that through manipulation of the substrate surface , variations in process conditions which may lead to increased nucleation density ( e . g . changes in temperature and / or pressure ) may be significantly negated , in terms of their impact on the resultant pyrolytic carbon structure . by considering the action of v - w growth in anisotropic pyrolytic carbon formation it is possible to identify the factors that will control the columnar structure . three factors will control the shape and cross - sectional area of the columns within an anisotropic pyrolytic carbon deposit formed on a surface : the rate of nucleation ; the rate of growth ; and the texture of the surface ( nucleation site height distribution over the surface ). the first two of these factors will be determined by the nature of the substrate and the growth conditions , and the third by the substrate surface . although controlling each of these factors is not trivial , with an understanding of the mechanism of pyrolytic carbon growth it is possible that rational strategies can be devised to produce pyrolytic carbon materials with desired structures and properties . clearly the growth cones and columnar structures are a consequence of the same mechanism . this appears to have been a point of confusion in the prior art . 2 delhaes , p . chemical vapor deposition and infiltration processes of carbon materials . carbon 40 , 641 - 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