Patent Application: US-62617300-A

Abstract:
carbon nitride powder prepared by solid - state reaction between cyanuric chloride or its fluoro analogue and lithium nitride . the determined , by elemental analysis , atomic n / c ratio in the synthesized material is consistent with c 3 n 4 stoichiometry . combined material characterization data , obtained by ftir , raman , uv - vis , mas nmr , xps , tga / dta and pyrolysis - eims methods , provide substantial evidence for graphite - like sp 2 - bonded structure composed of building blocks of s - triazine rings bridged by the three - fold coordinated nitrogen atoms in the bulk carbon nitride .

Description:
the present process uses dry powdered reagents and is carried out at temperatures that are easily within the capabilities of conventional manufacturing furnaces . according to one embodiment , bulk quantities of sp 2 - bonded a - c 3 n 4 can be manufactured in bulk quantities using a halogenated triazine and an alkaline metal nitride as reagents . more specifically , the present process comprises mixing dry powdered c 3 n 3 x 3 , where x is a halogen , with m 3 n where m is an alkaline metal . hence , x can be fluorine , chlorine , bromine , or iodine and m can be lithium , sodium , potassium , rubidium or cesium . it has been found that combining the two reagents in dry powdered form , heating them to a temperature above the boiling point of the triazine ( c 3 n 3 x 3 ) and holding them at an elevated temperature for a predetermined period of time produces bulk quantities of a compound having a c : n ratio of 3 : 4 . the use of cyanuric chloride c 3 n 3 cl 3 or its fluoro - analogue , c 3 n 3 f 3 , as sp 2 c ═ n precursors and the use of lithium nitride li 3 n as a nitridation and cross - linking agent have been discovered to be effective for the syntheses of nitrogen - rich carbo - nitride powders . these reactions are most likely to result in a three - fold n - bridged s - triazine ring structure , as shown in fig1 and to proceed according to the following general scheme : where x ═ f , cl . it is expected that other halogens will likewise be useful in combination with the traizine reagent , and that other alkaline metals will be useful in the nitridation and cross - linking agent . cyanuric chloride ( 99 %, sigma / aldrich ) was sublimated twice in vacuum before the use . cyanuric fluoride ( 99 %, alfa ) and lithium nitride ( 99 . 5 %, strem ) were used as received . the small - scale reactions were carried out in a sealed thick - wall pyrex glass ampules . the larger scale syntheses were performed in a closed stainless steel or monel tube reactor . in a typical example of these experiments , 2214 mg ( 12 mmol ) of c 3 n 3 cl 3 and 630 mg ( 18 mmol ) of li 3 n fine powders were mixed together in a n 2 - filled dry box , loaded into a stainless steel reactor , which afterwards was sealed with a leak - tight con - flat cap . the reactor was taken out of the dry box and suspended in a vertical furnace by a sample holder of an air - driven vibrator , which was kept in operation until the end of reaction . the furnace temperature was gradually and slowly raised to about 380 ° c ., held for 2 - 4 hours , and then brought back to room temperature . it has been found that a certain minimum temperature must be obtained before the reaction will begin to take place . for the reagents described above , that temperature is about 320 ° c . a preferred reaction temperature is 380 ° c . while the reactor was not heated above this temperature , it is believed that temperatures inside the sample may be significantly higher during the early phases of the reaction , as a result of the heat released by the exothermic reaction . in the present case , the small sample sizes and the thermal inertia of the equipment surrounding the sample reduced the effect of the heat generated by the reaction . in some instances , such as where the sample is much larger , it may be necessary to control the sample temperature more actively to avoid overheating , ask discussed below . when the reaction was complete , the resulting powder was washed with water on a 0 . 2 μm cole - parmer ptfe filter membrane to remove the reaction byproduct licl . the remaining dark - brown product was dried overnight at about 100 ° c . under vacuum . the powder obtained in this manner weighed 1092 mg ( 98 . 5 % of the theoretically expected for c 3 n 4 ). a microprobe analysis indicated that the powders prepared by the described method are nitrogen - rich with the following varying compositions : c 0 . 37 - 0 . 42 n 0 . 55 - 0 . 58 o 0 . 02 - 0 . 05 cl 0 . 002 - 0 . 005 . the same reaction proceeded faster ( 0 . 5 hr to completion ) and at lower temperature ( 300 ° c .) in a monel reactor ; however , the obtained orange - colored powder contained up to 2 - 5 at . % cl . a similar product was prepared when the synthesis was carried out in a sealed glass tube , but had a significantly lower yield and much longer reaction time ( 24 hrs ). this strongly suggests that transition metals ( fe , ni , cu ) present in the metal reaction containers had a catalytic effect on the reaction . at the same time , the presence of extraneous elements ( fe , ni , cu ) in the reaction product is significantly increased , thus reducing its purity . the ftir spectra of the carbon nitride powders pressed into a kbr pellet for purposes of optical ( ir ) analysis were collected on a perkin elmer paragon 1000 ftir spectrometer with 1 cm − 1 resolution . raman spectroscopy measurements for the powders placed on the top of a standard microscope slide were carried out on a reninshaw system 1000 micro - raman spectrometer with an 1800 line / mm grating using 514 . 5 nm ar ion laser . uv - vis spectra were recorded on gbc uv / vis 918 spectrophotometer . quantitative elemental analyses were performed with the cameca sx - 50 electron microprobe analyzer equipped with pgt energy dispersive spectrometer ( eds ) using the following parameters : acceleration voltage of 15 kv , beam current of 15 na , beam diameter of 20 μm , and peak and background counting time of 20 s . xps data were collected with the help of physical electronics phi 7500 x - ray photoelectron spectrometer using al kα radiation source ( 1486 . 6 ev ) with a power setting of 350 w and an analyzer pass energy of 23 . 5 ev . 13 c mas nmr spectra of powders packed into a 7 mm rotor were recorded using bruker instruments 200 mhz spectrometer with a 90 ° pulse width of 4 μs and relaxation delay of 10 s and referenced to 13 c chemical shift of glycine carbonyl at 176 . 2 ppm . thermal gravimetric analyses were done using a ta instruments tga - dta 2960 analyzer . pyrolysis - ms data were obtained with the finnigan - mat 95 mass spectrometer . scanning electron microscopy ( sem ) was carried out using a jeol model jsm - 6320f field emission microscope at 30 kev beam energy . x - ray diffraction data were collected with a siemens powder diffractometer using a cu kα radiation source . the powder produced as described above was placed into a hardened steel die and densified at 5000 psi for 5 min to form pellets . disk - shaped specimens of carbon nitride were fabricated by cold isothermal pressing at 55000 psi of the pellets sealed into a latex bag . electric conductivity measurements were performed on disk - shaped ceramic samples using a four - point method . the orange or dark - brown colored powders , isolated after complete removal of the reaction byproducts ( lici or lif ), did not melt at temperatures up to as high as 400 ° c . and also did not dissolve in common organic solvents , hinting at a high - molecular nature of synthesized materials . solvation at some degree ( about 10 - 20 mg / l ) was achieved for finely ground powder by sonication in water or alcohol , which produced a clear light - yellow solution , stable to precipitation for hours . according to elemental analyses ( galbraith laboratories , inc . ), the overall n / c at . % ratio in the prepared powders was approximately 1 . 33 , which is in agreement with the c 3 n 4 stoichiometry . besides carbon and nitrogen , the analyses have yielded about 0 . 7 wt . % hydrogen content in the samples . the electron microprobe studies of powders also revealed that they contain 2 - 4 at . % oxygen and trace amounts of halogens ( cl or f ). these data thus indicate the likely presence of some oh , nh and nh 2 functional groups in the polymer structure of powders studied . the ftir spectra of these newly synthesized carbon nitride bulk materials have shown some features very similar to those of nitrogen - rich carbo - nitride thin films , observed earlier 29 , 16 , 30 , 32 . as shown in fig2 a , the spectrum of orange powder shows broad bands of the stretching and deformation modes of nh 2 groups at 3424 and 1627 cm − 1 ( as well as an overlapping band of oh stretching ), weak band at 2162 cm − 1 due to the cyano group stretch , and a group of multiple bands characteristic for s - triazine ring vibrations , at 1561 cm − 1 ( quadrant stretch ), 1490 and 1421 cm − 1 ( double semicircle stretchings ) and 805 cm − 1 ( out - of - plane ring bending by sextants ). 32 the strong band observed in this spectrum at 1314 cm − 1 characterizes the c — n stretch in the three - fold n - bridge linking the triazine rings . the nh bridge - coupled triazine rings are also likely to be present , as well as terminal nh 2 and nhcn groups . they are likely to induce the loss of three - fold axis of symmetry in the polymer structure shown in fig1 and result in appearance of double bands in the spectrum , e . g ., an already mentioned band of semicircle ring stretch at 1490 cm − 1 and a band of double ring quadrant stretch at about 1580 - 1600 cm − 1 , which is overlapped by a strong nh 2 deformation band , as shown in fig2 . annealing of the powders to 650 ° c . in vacuum caused an increase of absorption of the c ═ n group at 2162 cm − 1 and reduction of intensities of bands of nh 2 and n — c vibrations at 3424 , 1627 and 1314 cm − 1 , respectively , in the ftir spectra of fig2 b due to a loss of ammonia probably accompanied by a partial rearrangement and destruction of s - triazine rings under high temperatures , as observed earlier for melamine polymerization products . 33 - 36 since somewhat similar secondary processes are likely to take place during the carbon nitride preparation in stainless steel reactor , proceeding at a higher reaction temperature than one done in monel reactor , the spectrum of the obtained powder shown in fig2 c shows as strong absorption of the cyano group at 2162 cm − 1 as the spectrum of annealed material shown in fig2 b . referring now to fig3 a typical raman spectrum of carbon nitride powder shows two broad peaks with maxima at about 1340 and 1545 cm − 1 , corresponding to d and g raman bands of amorphous graphitic carbon , respectively . the locations of these peaks resemble ones typically observed for carbon nitride thin films of an average c 2 n composition 37 - 39 ; however , unlike them , in the raman spectra of a - c 3 n 4 powder due to a much higher nitrogen content the d band appears to be significantly stronger than the g band . the presence of triazine rings in the a - c 3 n 4 polymer structure is also evidenced by uv - vis studies of the light - yellow solution produced by sonication of carbon nitride powder in water . as can be seen in fig4 the observed strong absorption at 250 nm lies in the range characteristic for π - π * electronic transition in the aromatic 1 , 3 , 5 - triazine compounds . 40 additional absorption , very broad and weak , was observed in visible region at about 410 nm . presumably , it belongs to the n - π * transitions involving lone pairs of nitrogen atoms in the polymer . the solid - state ( 13 c ) mas nmr spectrum shown in fig5 exhibits two broad peaks : the larger at 168 . 9 ppm ( with two shoulder peaks at 165 . 3 and 155 . 7 ppm ) corresponding to the sp 2 - hybridized carbon atoms from the s - triazine rings and the smaller at 123 . 4 ppm for the sp carbon of the cyano group . 40 , 41 according to this spectrum and the ftir spectral data for non - annealed carbon nitride powder , there are practically no hydrogen atoms bonded to carbon atoms present in the polymer structure . the s - triazine ring carbon peak positions in the nmr spectrum are found to lie close to those reported for b — c — n powder synthesized from melamine and bcl 3 and being similarly composed from layers of triazine rings bridged by nh groups , nitrogen and boron atoms . 35 at the same time , these and our nmr data do not agree with the ( 13 c ) nmr spectra of free - standing films of c 3 n 4 stoichiometry observed earlier by kouvetakis et al . 16 , which exhibited two resonance peaks in much a higher field , at 104 and 115 ppm . these peaks were also assigned to sp 2 hybridized carbons in the proposed nitrogen - bridged s - triazine ring structure , identical to the structure we suggest for a - c 3 n 4 powder in present work ( fig1 ). based on large discrepancy of these data , it is not easy to agree with the structure proposed earlier for the purported c 3 n 4 film . 16 the xps data support the results obtained by ftir , uv - vis , and nmr spectroscopy . in the xps survey spectrum , given in fig6 three observed peaks indicate that the synthesized material is composed primarily from carbon and nitrogen ; however , some oxygen impurity ( about 5 %) is also present . higher resolution xps data , referenced to a peak at 284 . 6 ev of graphite powder taken as a standard , show that the c and n lines can be split and deconvolved . the c1s peak deconvolves as shown in fig7 into a four components at binding energies of 284 . 5 , 285 . 5 , 286 . 6 and 288 . 1 ( major component ) ev , which are attributed to the c — c , c — o , c ═ n and c ═ n bonds , respectively . the c — c peak originates from graphitic carbon , presumably formed during minor decomposition of carbon nitride sample under x - ray irradiation . the deconvolved n1s peak shown in fig8 shows a major component at 398 . 5 ev due to nitrogen , sp 2 - bonded to carbon , and a shoulder peak at a higher binding energy , 400 . 1 ev , assigned to sp - bonded nitrogen in the terminal c ═ n groups . the xps data thus strongly suggest that the graphite - like sp 2 - bonded structure of fig1 is most likely for the carbo - nitride powders . these observations also agree with the tga / dta and vacuum pyrolysis - ms analyses . the tga / dta data plots for carbon nitride powder are shown in fig9 and 10 . they show that this material is significantly less thermally stable in air ( fig9 ) than in an inert atmosphere ( fig1 ), undergoing stepwise decomposition in both cases . major weight loss in argon is observed at temperatures above 550 ° c ., in comparison with the 400 ° c . on air . according to mass spectral study of evolution products , in which only ions with mass numbers ( m / z ) higher than 35 were detected ( fig1 ), the mass loss above 450 ° c . is in part due to cyanic acid hocn ( mn / z = 43 ) and most likely ammonia , as evidenced by ftir studies of annealed material at similar temperatures ( fig2 b ). at a higher temperatures ( up to 900 ° c .) the evolution of other species , detected by the mass fragments at m / z = 52 ( c 2 n 2 + ), 64 ( c 3 n 2 + ), 78 ( c 3 n 3 + ), and 92 ( c 3 n 4 + ), were observed as well . these data provide clear evidence for formation of cyanogen ( cn ) 2 and heavier c 3 n x species as a result of fragmentation of triazine rings being a major unit in the suggested polymer structure of the c 3 n 4 material . all synthesized a - c 3 n 4 powders are amorphous , therefore , only a very broad reflection centered at about 3 . 0 å could be detected in xrd measurements . these data do not look like the xrd patterns of the carbon nitride synthesized , according to kawaguchi et al . 43 , from the similar reagents ( c 3 n 3 cl 3 and li 3 n ) but under reaction conditions that differ from those described herein . based on diffuse reflectance uv - vis spectra of the powder , optical bandgap for a - c 3 n 4 material was estimated to be approximately 3 . 1 ev . we were able to perform densification of micronized c 3 n 4 powders by cold isothermal pressing at pressure of 55000 psi . the disk - shaped ceramics formed had densities of about 1 . 34 - 1 . 38 g / cm 3 , which is much lighter than graphite ( d = 2 . 25 g / cm 3 ). the linear electric resistivities of these ceramics , measured at about 0 . 67 megaohm . cm , lie close to those of blue diamond semiconductors . because of wide bandgap semiconducting properties , the undoped and doped carbon nitride ceramics suggest testing for possible application as secondary electron emitters . in addition , the ceramic disks of a - c 3 n 4 can be studied for application as targets for laser ablation in the deposition processes of potentially ultrahard c — n films . finally , based on availability of stoichiometric sp 2 - bonded carbon nitride ( a - c 3 n 4 ) as a result of present work , we have carried out a series of high pressure experiments in pursuit of large crystals of superhard β - c 3 n 4 and other predicted polymorphs while using the amorphous a - c 3 n 4 powder as a precursor . according to sem ( fig1 ), edax , micro - raman and powder x - ray studies , under pressures of 8 to 12 gpa the structure of carbon nitride changes from amorphous ( fig1 a ) to a more ordered layered graphite - like structure with interplane d - spacing of about 3 . 0 - 3 . 1 å , retaining the c 3 n 4 stoichiometry at temperatures of up to 400 ° c . ( fig1 c ). the tem studies , however , did not reveal the presence of any ( nano - or micro ) crystalline phases of carbon nitride , which means that the material still remains amorphous although well densified under pressure . this material loses nitrogen at higher temperatures ( up to 1200 ° c .) and converts into a pure carbon phase of graphite with d - spacing of 3 . 34 å ( fig1 d ). these preliminary experiments allowed us to determine so far the temperature range where the phase transformation of carbon nitride starts to occur without change of the c 3 n 4 composition . the formation of crystalline phases , predicted to be superhard , probably will require application of higher pressures than tested so far , or catalysts . the difficulties in the synthesis of hard carbon nitrides , such as β - c 3 n 4 , are very likely related to their low thermodynamic stability with respect to the elements ( c and n 2 ), indicated by a positive values of enthalpies of formation 23 . in other words , the atoms readily revert to their unbonded carbon and nitrogen states . lda calculations predict that sp 3 - bonded β - c 3 n 4 phase is only about 20 kj / mol less stable than sp 2 - bonded a - c 3 n 4 . this implies that , similar to the graphite / diamond and h - bn / c - bn transformations that occur under high pressure / high temperature conditions , the crystalline sp 3 - bonded carbon nitride can be synthesized from the sp 2 - bonded graphite - like carbon nitride . the syntheses of large crystals of β - c 3 n 4 as well as other polymorphs appear to be very promising using the bulk quantities of stoichiometric sp 2 - bonded carbon nitride as precursor . as mentioned above , the techniques of the present invention are not limited to the synthesis of a - c 3 n 4 . other binary compositions , as well as ternary and quaternary systems can be made using the present techniques . examples of such compositions include , but are not limited to : b 3 c 3 n 7 ( made from nabf 4 / c 3 n 3 x 3 and li 3 n ). the powder synthesis of gram quantities of amorphous carbon nitride with the stoichiometry very close to c 3 n 4 has been achieved in the present work . the demonstrated approach , which is based on fast solid state reactions , is particularly attractive since : ( i ) it uses the relatively cheap reagents and does not require synthesis of single - source precursors , as in the previously reported preparation of carbo - nitride 16 , 44 , 45 ; ( ii ) it produces powders with a higher nitrogen content than , for example , the carbon nitride powders of approximately c 4 n 5 stoichiometry described in a german patent 45 , ( iii ) the reaction routes , leading to production of covalently bonded not only binary , but also ternary and quaternary carbo - nitride materials with controlled stoichiometry , morphology , mechanical and electric properties can probably be designed . we have recently verified the flexibility of this approach by successful synthesis of amorphous b — c — n powders of approximate b 3 c 3 n 7 stoichiometry , and extended our current work to preparation of other carbo - nitride materials . 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