Chemical Preparation and Shock Wave Compression of. Carbon Nitride Precursors. Michael R. Wixom. KMS Fusion, Inc., Ann Arbor, Michigan 48106-1567.
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J. Am. Ceram. Soc..73 [7]1973-78 (1990)
Chemical Preparation and Shock Wave Compression of Carbon Nitride Precursors Michael R. Wixom KMS Fusion, Inc., Ann Arbor, Michigan 48106-1567
Two synthetic routes have been developed to produce highmolecular-weight organic precursors containing a high weight fraction of nitrogen. One of the precursors is a pyrolysis residue of melamine-formaldehyde resin. The second precursor is the byproduct of an unusual low-temperature combustion reaction of tetrazole and its sodium salt. These precursors have been shock compressed under typical conditions for diamond and wurtzite boron nitride synthesis in an attempt to recover a new ultrahard carbon nitride. The recovered material has been analyzed by X-ray diffraction, FTIR, and Raman microprobe analysis. Diamond is present in the recovered material. This diamond is extraordinarily well ordered relative to diamond shock synthesized from carbonaceous starting materials. [Key words: shock wave compression, organic compounds, nitrogen, diamonds, synthesis.] I. Introduction
T
two hardest known materials, diamond and boron nitride, exist as metastable phases which have been recovered from shock-compressed graphitic or amorphous starting materials.’f2The possibility of a new ultrahard phase of carbon nitride was first suggested by C ~ h e n He . ~ derived a simple scaling relationship for tetrahedral compounds sharing eight valence electrons: HE
1971 - 220A
B=
d3.5
B is the bulk modulus in gigapascals, d is the interatomic distance in angstroms, and A is a measure of the ionicity of the compound. (A = 0, %, 1 for group IV, 111-IV, and 111-V compounds, respectively.) This expression was shown to be accurate within a few percent for existing group IV and group 111-V compounds. With A = % and d = 1.47 to 1.49 A this model predicts a bulk modulus of 461 to 483 GPa for a hypothetical tetrahedral compound between carbon and nitrogen. Liu and Cohen4 have recently reported the results of an ab initio calculation which yielded a bulk modulus of 427 ? 15 GPa.4 Tetrahedral carbon nitride is thus expected to exceed cubic boron nitride ( B = 369 GPa) and rival diamond (443 GPa) in hardness. We report results from an attempt to recover tetrahedrally bonded carbon nitride from shock-compressed organic precursors. Commercial shock synthesis of diamond and wurtzite boron nitride begins with graphitic-phase starting materials.5z6 By analogy, one might also hope to use a graphitic phase of carbon nitride as a starting material from which to produce the ultrahard tetrahedral material. From stoichiometric con-
R. Rice-contributing
siderations the tetrahedral compound is expected to have a formula of C3N4. One problem is that the existence of crystalline graphitic-phase C3N4 starting material is unlikely from the standpoint of crystal symmetry considerations. The closest approaches to graphitic analogues of carbon nitride have been prepared as derivatives of cyameluric acid? Melon (C6N9H3) and hydromelonic acid (C9N13H3)contain polycyclic rings of alternating carbon and nitrogen atoms. It has been noted that condensation products of melon should .~ to asymptotically approach a (C3N4)nf o r m ~ l a Attempts form such a structure appear to have been limited to condensations of less than ten units as inferred from the average The residual hydrogen content of the products (C3N4.~H1.32).8 high stability of the products was attributed to a large resonance stabilization similar to graphite.’ The observed densities’ (1.40 g/cm3) were well below that of crystalline graphite (2.25 g/cm3). The unavailability of crystalline graphitic phase C3N4does not rule out the possibility of recovering tetrahedral carbon nitride from other starting materials. Numerous reports have described the recovery of diamond from a wide variety of shock-compressed starting materials. These materials include carbon blacks? glassy carbon,” carbonized resins,” and even heptadecane.” It is noteworthy that several of these materials contain significant amounts of hydrogen. Experimental data for several shocked hydrocarbons are consistent with a model assuming dissociation at high pressure into diamond-phase carbon and free molecular hydr~gen.’~ It appears then that extensive crystalline disorder and significant atomic fractions of hydrogen do not rule out materials as precursors for shocksynthesized ultrahard products. This is not to imply that prospective carbon nitride starting materials are free of constraints on the spatial distributions of the carbon and nitrogen atoms. To form a crystalline carbon nitride phase from a noncrystalline starting material, some degree of diffusion must occur to permit crystal growth in the shock-compressed material. Based on the diffusion relationship x = (Dt)’” where x is the diffusion length, D is the diffusion coefficient, and t is time, an estimate of the diffusion length in a shock wave can be made. Estimating a value of D = lo-’ cm’/s as reported for the diffusion of carbon in shocked steel14gives a diffusion length of 0.3 pm in a shock wave of 0.1-ps duration. Consequently, the carbon and nitrogen in the starting material should be well mixed on a submicrometer scale. 11. Experimental Procedure (1) Preparation of C,N,
Two synthetic routes were selected to produce starting materials composed of carbon and nitrogen in a suitable atomic ratio and spatial distribution. One approach was based on an unusual type of combustion reaction in a mixture of tetrazole and its sodium salt.” In the reaction a “liquid spheroidal flame” allows reactants to remain in the combustion zone for a long time. As a result, comparatively slow reactions such as polymerization and polycyclic ring growth can take place.
editor
Manuscript No. 197994. Received November 6,1989; Approved March 19, 1990. Supported by the Department of Ener y through the Small Business Innovative Research program (DE-AC02-8!ER80595).
1973
Journal of the American Ceramic Society - Wixom
1974
The reaction produces a residue with a composition close to the desired 3:4 C:N ratio. High-purity lH-tetrazole was purchased.* The sodium salt was produced by reaction with sodium ethoxide in ethanol. Sample pellets were pressed from a mixture containing 38% sodium tetrazolate. Pellets weighing 1 g were made in a 1.91-cm ram press at 143 MPa. The pellets had a density of 1.59 g/cm3. The combustion reaction was initiated by a hot filament. The reaction was performed in a well-evacuated hood as hydrogen cyanide and nitric acid were among the combustion products. A pellet diameter of at least 1.91 cm is necessary to permit the growth of a stable reaction zone. The reaction ends after burning through the pellet in several seconds. The residue from the reaction weighs 0.10 to 0.12 g independent of the mass of the pellet prior to the combustion reaction. Several grams of residue were collected. The residue was an acrid-smelling, peach- or sand-colored brittle solid which occasionally had a thin gray coating of sodium carbonate. The residue was crushed and sieved to a 100-pm particle size. Gas cell pycnometry gave a bulk density of 1.85 g/cm3. The powder was soluble in water, forming a basic solution of pH 9.7 at 1 g/mL. Elemental analysis for carbon, hydrogen, and nitrogen showed a molar composition of C3N4.5H1.0 with 40% of the mass being other elements, presumably sodium and oxygen. The Fourier transform infrared (FTIR) absorption spectrum is shown in Fig. 1. The second synthetic route was based on the pyrolysis of polymer precursors. Such reactions are used to produce a number of nonoxide ceramics including silicon nitride16-18 (Si3N4)and graphitic boron (gBN). The use of cycloborazenes as a precursor for gBN suggested that the analogous melamine-formaldehyde (MF) copolymer could be used as a precursor for carbon nitride. Another related high-nitrogencontent copolymer of guanidine and formaldehyde (GF) was also selected. A useful feature of these systems is the presence of multiple reaction sites on t h e guanidine and melamine monomers. By varying the amount of formaldehyde used in the copolymerization, the carbon-to-nitrogen ratio in the precursor could be easily adjusted. The GF resins were prepared by adding guanidine carbonate to a stoichiometric volume of 37% formaldehyde solution plus 1 mL of HzO per gram of guanidine carbonate. The reactants were heated slowly to 80°C under a refluxing condenser until the reactants were dissolved. The solution was maintained at 90" to 100°C for 1 h. Upon cooling the solution became a viscous resin which continued to cure slowly and dry at room temperature, eventually becoming a transparent insoluble solid. The MF resins were formed by adding melamine to a stoichiometric volume of 37% formaldehyde solution plus 10 mL
Vol. 73, No. 7
of H z O per gram of melamine. The reactants were heated slowly to 80°C under a reflux condenser to dissolve the solids. The solution was then heated to 90" to 100°C for 1 h. Toward the end of this period the polymer resin began to precipitate out of solution. The resin was very tacky at elevated temperature and could be collected easily on a spatula. Drying and curing were completed in an oven. The polymer resins were analyzed by FTIR spectroscopy, thermogravimetric analysis (TGA), and CHN elemental analysis. The FTIR spectra showed the addition of bridging methylene groups in the polymer chain. Formaldehyde which added without propagating the chain was detectable as CHlOH end groups. The TGA data are shown in Fig. 2. The MF TGA showed a sharp 34% weight loss form 410" to 435"C, followed by a continued gradual loss of an additional 12% up to 550°C. The G F TGA showed a rather continuous weight loss amounting to 70% at 450°C followed by a slower continuous loss of an additional 10% up to 550°C. Polymer precursors were prepared from the reactant ratios shown in Table I. Several of these polymers were pyrolyzed at 600°C under nitrogen. The temperature ramp was not controlled. The samples reached 600°C in 20 min and were held there for 30 min. The recovered samples were brittle, black, foamed bodies with lustrous surfaces. The recovered mass fractions were lower than the fractions remaining at the end of the TGA runs. This was presumably a result of continued weight loss during the 30-min hold time. The recovered materials all had undesirably low nitrogen contents. The pyrolysis mechanism favored a preferential loss of nitrogen. Nitrogen loss is a problem because the upper limit for the nitrogen content of each copolymer is 3:4.5. In an unsuccessful attempt to abate the nitrogen loss, the pyrolysis was repeated under ammonia. The second alternative for limiting nitrogen loss was to use softer pyrolysis conditions. By limiting the samples to 10 min at 5 W C , the nitrogen loss could be held to tolerable levels. Starting with a slightly nitrogen-rich 1:l M F copolymer resulted in a pyrolysis residue with the composition C3N3.6H2.5.The residue amounted to 40% of the mass of the starting material. After pyrolysis 46% of the carbon was retained, but only 38% of the nitrogen and 25% of the hydrogen. The bulk density of the recovered residue as measured by gas cell pycnometry was 1.77 g/cm3. The FTIR spectrum is shown in Fig. 3. The two strong absorption features at 1603 and 1457 cm-' are likely heterocyclic aromatic ring stretching bands related to those that appear at 1552, 1470, and 1440 cm-' in the unpyrolyzed precursor. The spectrum shows a much reduced absorption at 3000 to 3400 cm-' due to residual hydrogen.
*American International Chemical, Inc.
2.01
I
100
Wavenumbers (cm-') Fig. 1. IR absorption spectrum of tetrapole tion residue.
combus-
300 400 Temperature "C
200
500
Fi g . 2. Thermogravimetric a n a l y s i s of 1 : 5 melamine-formaldehyde (MF) and 1 :1.25 guanidine formaldehyde (GF) resins.
July 1990
Table I.
1 2 3 4 5 6 7
1975
Chemical Preparation and Shock Wave Compression of Carbon Nitride Precursors
Monomer ratio*
Precursor C:N (theor)
1:l G : F 1:l G : F 1:l M : F 1:lS M:F 1:4.5 M:F 1:4.5 M : F 1:l M:F
3 :4.5 3 :4.5 3 :4.5 3:4 3 :2.4 3 :2.4 3 :4.5
Polymer Pyrolysis Results Precursor C : N (expt)
Pyrolysis atmospheret
Yield (wt%)
Product C: N (expt)
3 :4.5 3:4 3 :2.2 3 :2.2 3 :4.5
N2 NH3 NH3 N2 N2 NH3 NH3
18 11 8.8 16 16 22 40
3 :2.2 3 :1.9 3 :1.7 3 :2.4 3 :1.6 3:1.5 3 :3.6
*G = guanidine, M = melamine, F = formaldehyde monomers. 'Samples 1 to 6 were pyrolyzed for 30 min at 600°C. Sample 7 was pyrolyzed for 10 min at 5 W C .
(2) Shock Compression The samples were shock compressed by using a mousetraptype plane wave generator and a momentum-trap recovery system.' The shock compression experiment was designed to produce conditions similar to those attained in the commercial shock synthesis of diamond and wBN. A steel flyer-plate was explosively driven at 2.5 km/s. The samples were mixed with copper powder since the addition of copper is known to significantly increase the yields of shock-synthesized wBN and d i a r n ~ n d . ~ , ~ ~ Planar impact loading conditions were calculated using one-dimensional hydrodynamic equations. Carbon equationof-state data were used to approximate carbon nitride. Composite samples were treated by using mass-weighted linear combinations of the state variables.23Porosit was accounted for by using an empirically fitted equation! The results of the calculation are shown in Fig. 4. The one-dimensional planar impact calculations indicate pressures of 29 to 48 GPa. The actual shock pressures must have been higher. More sophisticated numerical simulations of the sample recovery fixture have revealed that twodimensional radial effects dominate the shock pressures in our recovery f i ~ t u r e . ' ~ These simulations show that onedimensional calculations underestimate the shock pressure by a factor ranging from 2 at the outer edge of the sample to 5 at the center. Scaling up the one-dimensional results by this factor gives a pressure range of 60 to 250 GPa, which covers the optimum range for diamond and wBN synthesis. The matrix of shocked samples is summarized in Table 11. Four control samples were included to assure that the targeted pressures were attained. The control samples consisted of graphite and graphitic boron nitride. Each material was mixed with copper powder to form a high-pressure and a low-pressure control sample. The high-pressure control samples (Nos. 9 and 11 in Table 11) experienced the same shock conditions as the highest-pressure carbon nitride
targets (samples 1 and 5). The low-pressure controls (samples 10 and 12) experienced the same shock conditions as the lowest-pressure carbon nitride targets (samples 4 and 8). The remaining carbon nitride targets were shock processed at intermediate pressures. The yields listed in Table I1 report the amount of acidinsoluble material recovered from each sample. Concentrated aqua regia was used to isolate the shocked sample from copper and other metal contaminants. The four samples for which zero yield is reported were high-porosity targets which apparently reached shock temperatures high enough to vaporize and breach the recovery capsules. The nonzero yields reflect losses to chemical attack by aqua regia plus losses in sample handling. Losses to chemical attack were significant for the carbon nitride materials, especially samples 1 and 3.
m
90
a
(3 60
$ v)
$40
n
20 0
0
0.5
1.0 1.5 2.0 2.5 Particle Velocity (kmkec)
3.0
Fig. 4. Upon impact of the stainless steel impactor on the stainless steel sample holder a pressure, Pini,,is produced in the holder. The shock then propagates through the holder into the pressed powder sample. The one-dimensional calculated pressure in each sample is the intersection of the sample Hugoniot with the steel Hugoniot centered at the initial pressure.'* For each sample X is the mass fraction of graphite-like starting material which is mixed with copper powder, and E is the sample density relative to the theoretical maximum density for the composite material.
'The samples were shock processed at the Center for Explosives Technology Research, New Mexico Institute of Mining and Technology, Socorro, NM. For details, see Ref. 21.
Table 11. Shocked Sample Matrix
"
3000
2000
-'
1000
Wavenumbers (cm )
Fig. 3. IR absor tion spectra of melamine formaldehyde resin before fa) and after (b) pyrolysis.
Sample
Starting material
1 2 3 4 5 6 7 8 9 10 11 12
ILSF ILSF ILSF ILSF Pyr 1:l M F Pyr 1:l M F Pyr 1:l MF Pyr 1:l MF Graphite Graphite gBN gBN
Copper Porosity Yield Calculated 1-D (mass fraction) (%) (%) pressure (GPa)
95 95 80 80 95 95 80 80 95 80 95 80
12.8 32.4 15.0 32.4 14.6 33.2 14.6 34.1 11.9 26.6 13.2 27.0
1.7 0 0.7 0 14 10 2.2 0 48 34 60 0
48 34 41 29 48 34 41 29 48 29 48 29
'Yield (%) is the mass of acid-insoluble, shock-processed sample divided by the mass of the starting material.
Journal of the American Ceramic Society - Wixom
1976
While ultrahard carbon nitride was presumed to be inert to aqua regia, this presumption was not true for untransformed carbon nitride starting materials. The listed yields, therefore, are only an indication of chemical inertness rather than of the presence or yield of shock-synthesized ultrahard material. 111. Results and Discussion
The recovered samples were analyzed by powder X-ray diffractometry (XRD) and Raman microprobe analysis (RMA) for the presence of shock-synthesized ultrahard phases. The XRD data from the graphite control samples indicate the recovery of diamond from both the high-pressure and lowpressure samples. The diamond (111) peak at 28 = 43.9" is quite broad, as is typical of shock-synthesized diamond. The substantial diamond yields from both graphite control samples verify that all of the selected target compositions access shock conditions suitable for diamond synthesis and recovery. The results from the boron nitride control sample also confirm the recovery of ultrahard wBN. The near absence of the graphitic phase (002) peak at 28 = 26" is consistent with a wBN fraction of 80% or more. This high yield further affirms the suitability of the chosen shock processing parameters. To test for the presence of crystalline ultrahard carbon nitride phases the XRD data were compared to patterns calculated for hypothetical crystal structures. For example, a diffraction pattern was calculated for an ultrahard phase analogous to the p phase of silicon nitride. By assuming perfectly tetrahedral bonding with a C-N distance of 1.47 A, the atomic parameters were determined for the P6,/m space group: u = 6.533 A, c = 2.400, X N = 0.333, YN = 0.034, X c = 0.184, and YC= -0.224. The density of this structure is 3.45 g/cm3, closely matching that of diamond (3.51) and wBN (3.49). The lattice spacings for the hexagonal lattice planes (hkl) are
The corresponding line positions for CuKa radiation are denoted by symbols along the horizontal axis of Fig. 5. Figure 5 also shows the XRD data for all of the recovered carbon nitride samples. The appearance of several peaks coincident with the calculated positions would indicate a possibility of the presence of carbon nitride in the presumed crystal structure. Unfortunately, none of the data indicated a strong coincidence with the presumed crystal structures. The
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occasional coincident peaks, e.g., 28 = 31.6", were dismissed as contaminants since they also appeared in the XRD data from the control samples. The carbon nitride XRD data revealed the presence of diamond in each recovered sample. This outcome was somewhat surprising considering the near absence of carbon-carbon bonding in the starting materials and given the limited reaction time during the shock pressure pulse (
