2794
Anal. Chem. 1987, 59, 2794-2797
absolute number densities of neutral atom and ion a t the ground states by using the absorption or fluorescence technique (10, 51, 52). Registry No. Ar, 7440-37-1; Mg, 7439-95-4.
LITERATURE CITED (1) Demers, D. R.; Allemand, C. 0. Anal. Chem. 1981, 53, 1915. (2) Fassel. V. A.; Knlseley, R. N. Anal. Chem. 1974, 46, 1llOA . (3) Houk, R. S.; Fassel, V. A.; Flesch, G. D.; Svec, H. J.; Gray, A. L.; Taylor, C. E. Anal. Chem. 1980, 52,2283. (4) Haraguchl, H. I n Inducnvely Coupled Plasma Atomic Emlsslon Spectrometry-Fundamentals and Appllcatlons ; Kdansha Sclentlflc Publishers: Tokyo, 1986. (5) Imlucthely CoupM Plasmas in Anaiyfcal Atom& Spectrometry; Montaser, A., Golightly, D. W., Eds.; VCH Publishers: New York, 1987. (6) Inductively Coupled Plasma Emlsslon Spectrometry, Part I 8 Part I I ; Boumans, P. W. J. M., Ed.; Why: New York, 1987. (7) Boumans. P. W. J. M.; de Boer, F. J. Spectrochlm.Acta, Part B 1977, 328,365. (8) Mermet, J. M. C . R . Acad. Scl., Ser. B 1975, 2878,273. (9) Robin, J. P. Pfog. Anal. At. Spectrosc. 1982, 5 , 79. (IO) Nojlri, Y.; Tanabe, K.; Uchida, H.; Haraguchl, H.; Fuwa, K.; Wlnefordner, J, D. Spectrochlm. Acta, Part B 1983, 388,61. (11) Hart, L. P.; Smlth, B. W.; Omenetto, N. Spectrochim. Acta. Part B 1986, 418, 1367. (12) Boumans, P. W. J. M. Spectrochlm. Acta, Part 8 1982, 378, 75. (13) Schram, D. C.; Raaymakers, I. J. M. M.; van der Sijde, B.; SchenkelBars, H. J. W.; Boumans, P. W. J. M. Spectrochlm. Acta, Part B 1983, 388, 1545. (14) Aeschbach, F. Spectrochlm. Acta, Part B 1982, 378,987. (15) Blades, M. W.; Hietje, G. M. Spectrochlm. Acta, Part B 1972, 378, 191. (16) Blades, M. W. Spectrochlm. Acta, Part B 1982. 378,869. (17) de Galan, L. Spectrochlm. Acta, Part B 1984, 398. 537. (18) Rayson, 0. D.; Hleftje, G. M. Spectrochim. Acta, Part B 1988, 418,
883. (19) Goldwasser, A.; Mermet, J. M. Spectrochlm. Acta, Part B 1986, 418, 725. (20) Hasegawa, T.; Fuwa, K.; Haraguchi, H. Chem. Lett. 1984, 2027. (21) Hasegawa. T.: Haraguchl, H. Spectrochlm. Acta, Part B 1985, 408, 1067. (22) Hasegawa, T.; Haraguchi, H. Spectrochim. Acta Part B 1985, 408, 1505. (23) Moore, C. E. I n Atomic Energy Levels; NSRDS-NBS 35; National Bureau of Standards: Washington, DC. 1971. I
(24) Leep, D.; Qallagher, A. Phys. Rev. 1978, A13, 148. (25) Wllllams. W.; Trajmar. S.J. Phys. B 1978, 1 1 , 2021. (26) Kennedy, J. V.; Myerecough, V. P.; McDowell, M. R. C. J . Phys. 8 1978, 17, 1303. (27) Mensoza, C. J. Phys. B 1981, 14, 2465. (28) Larche, K. Z . Phys. 1931, 67,440. (29) Savchenko, V. N. Opt. Spectrosc. 1971, 30, 6. (30) Wellenstein. H. F.; Robertson, W. W. J. Chem. Phys. 1972, 56,1072. (31) Martin, S. 0.; Peart, F.; Dolder, K. T. J. Phys. B 1968, 1 , 537. (32) Karstensen, F.; Schneider, M. J. Phys. B 1978, 1 1 , 167. (33) Penkin, N. P.; Endko, T. P. Opt. Spectrosc. 1987, 2 3 , 353. (34) Seaton, M. J. Phys. Rev. 1959, 113, 814. (35) Wiese, W. L.; SmRh. M. W.; Miles, B. M. I n Atomic Transition Probabilities ; NSRDS-NBS 22; National Bureau of Standards: Washington, DC, 1969; Vol. I. (36) Schaefer, A. R. J. Quant. Spectrosc. Radiat. Transfer 1971, 7 1 , 197. (37) Andersen, T.; Sorensen, G. J. Quant. Spectrosc. Radiat. Transfer 1973, 13, 369. (38) Fischer, C. F. Can. J. Phys. 1975, 53, 184. (39) Fischer, C. F. Can. J . Phys. 1975, 5 3 , 338. (40) Llndgard, A.; Nlelsen, S. E. At. Data Nucl. Data Tables 1977, 19, 570. (41) Zllltis, V. A. Opt. Spectrosc. 1971, 37. 86. (42) Blalas-Zabawa, A.; Kucharski, M.; Skulska, E.; Urvaczka, J.; Walach, 2. Acta Phys. Pol. 1986, 30, 897. (43) Bates, D. R.; Damgaad, A. Phllos. Trans. R . SOC.London, A 1949, 242, 101. (44) Burgess, A.; Seaton, M. J. Rev. M o d . Phys. 1958, 3 0 , 992. (45) Katsuura, K. J. Chem. Phys. 1965, 42,3771. (46) Ferguson, E. E. Phys. Rev. 1982, 128, 210. (47) Gryzinski, M. Phys. Rev. 1965, 138, A336. (48) Uchida, H.; Kosinski, M. A.; Omenetto, N.; Winefordner, J. D. Spectrochim. Acta, Part B 1984, 398,63. (49) Kornblum, G. R.; Smeyers-Vereke, J. Spectrochlm. Acta, Part B 1982, 378, 83. (50) Lovett. R. J. Spectrochim. Acta, Part B 1982, 378,969. (51) Gillson, G.; Horlick, G. Spectrochim. Acta, Part B 1986, 418, 431. (52) Gillson, G.; Horlick, G. Spectrochim. Acta, Part 8 1986, 4 1 8 , 1323.
RECEIVED for review April 20,1987. Accepted August 17,1987. The present research has been supported by Grant-in-Aids for Special Research Project (No: 62117005) and for Environmental Science (No*61030026) from the Ministry Of Education, Science, and Culture, Japan.
Determination of Phase Composition of Silicon Nitride Powders by Silicon-29 Magic Angle Spinning Nuclear Magnetic Resonance Spectroscopy K. R. Carduner,* R. 0. Carter 111, M. E. Milberg, and G . M. Crosbie Research Staff, Ford Motor Company, Dearborn, Michigan 48121
The use of aolkhtate %I NMR spectroscopy to analyze q N 4 powders b demonstrated for a series of commercial and research samples. I n particular, two dlfferent commercial powders contaln 20% and 30% amorphous Si3N4content, as identtfied by the applicatlon of methods described herein. These same materlals had been analyzed by X-ray dmtaction as only crystalline SiaNp NYR spectroscopy is capable of dlstlngulohlng among dlfferent amorphous silicon specks normally found In preparatlons of S13N4 powders. These Include amorphous S13N4,sHlcon oxynltrldes, sllicates, and elemental slllcon. The measurement of concentration of these species Is made for correlatlon wHh properties of powder sinterability.
Sinterable silicon nitride (Si3N4) has been intensely investigated since 1974 because of potential applications as a tough, refractory ceramic material (1). The production of reliable and cost-effectivestructural Si3N, ceramic by sintering
of powders must begin with powders that have, among other qualities, an a-phase content in excess of 85% (2). Excessive @-Si3N4 in the powder interfers with microstructure changes that accompany sintering. On the other hand, some a-Si3N4 (amorphous) can aid densification of the final material (3). Oxygen and elemental silicon can also aid sintering, although concentrations in excess of 2% can deteriorate mechanical properties at elevated temperatures. Oxygen normally appears as amorphous silicon oxynitrides and silicates. In view of what is known about the effects of the powder’s phase and purity on sinterability and fiial ceramic mechanical properties, it is necessary to have rapid, reliable methods for the determination of the crystallinity and purity of batches of Si3N4powder before carrying out final product formation and sintering. Traditionally, X-ray powder diffraction has been used to determine the presence of @-Si3N4, silicon oxynitrides, silicates, and silicon in the a-Si,N4 powders. The powder diffraction technique fails, however, in the identification of amorphous species. All amorphous species contribute to the background diffraction signal whose intensity is difficult
0003-2700/87/0359-2794$01.50/0 @ 1987 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 59, NO.
to quantify. a//3 ratios have also been measured by infrared spectroscopic methods (4), but difficulties arise as a result of spectral overlap by bands from oxides and from amorphous species. Elemental analyses for oxygen and nitrogen, while accurate, are without regard for molecular speciation. Windowless SEM techniques are available for determination of particle size, shape, and oxygen and nitrogen content but do not probe the crystallinity of submicron particles. We describe the use of solid-state 29SiNMR spectroscopy for the chemical and physical analysis of Si3N4for siliconcontaining species. Silicon-29 NMR spectroscopy is readily applied to the analysis of Si3N4. Since 29Siis spin there are no quadrupole couplings to broaden the NMR lines (5). Low 93i natural abundance (4.7%) eliminates homonuclear dipolar couplings (6). A substantial amount of data exists on solid-state 2gSichemical shifts, (7-11).Silicon, silicon oxynitrides, and silicates as well as Si3N4are distinguished by their unique chemical shifts. The method is easily implemented and provides for the assay of the these species in both crystalline and amorphous phases.
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DECEMBER 1, 1987
2795
1 n
EXPERIMENTAL SECTION Solid-state =Si NMR spectra were acquired under conditions of magic angle spinning (MAS)at ambient temperature by using a Bruker MSL 300 spectrometer (=Si resonance frequency of 59.5 M H z ) and a Bruker MAS probe. Filling of the alumina MAS rotor required 'Iz g of powder. MAS speeds were typically 4 kHz. Two pulse sequences were employed. The first used a single read pulse (tip angle 759 followed by acquisition and relaxation-recycle delay. The second was a multiple-pulse saturation method for measurement of the spin-lattice relaxation time, Tl,a sequence appropriate for measurement of long T,'s (12). All free induction decays were subjected to standard Fourier transformation and phasing. The chemical shifts are referenced to tetramethylsilane (TMS) whose resonance was periodically checked by inserting a MAS rotor full of liquid. Lack of discernible magnet drift over days suggests that the shifts are correct to at least hO.1 ppm. Negative shifts represent more shielded environments. The determination of the relative concentration of different silicon species must take into account differences in TI.A rather wide dispersion of values was observed for the relevant species of Si: 43 h 2 min for Si3N4,3 f 0.5 min for silicon, and 40 h 3 min for SiOz (sample of glass wool). Normally, a wait of five times the longest Tl is required between repetitions of the experiment. In the present case, this would require 200 min. Instead, use of a 75O read pulse requires only a 1-h wait since this tip angle only destroys 74% of the initial magnetization, the amount relaxed in 1h (assuming exponential relaxation). The price paid for the opportunity to repeat the experiment more than three times faster is a reduction of the NMR signal to 97% of its potential value after a 90' pulse on a fully relaxed system-a loss of only 3%. SiBN4and other powders were acquired from both commercial and private sources: GTE Sylvania SN502, lot no. 20429 (a-Si3N4); Aldrich Chemical 24,862-24,lot no. 00822PM (P-Si3N4);Cookson 1002, lot no. 71070 (a-Si3N4);UBE E-10, lot no. A-22 (a-Si3N4); Alfa Products 89985, lot no. 012985 (a-Si3N4);Vertex Chemical, lot no. 39-R-1074-R (powdered silicon crystals); Du Pont Si3N4, lot no. 3-23 (phase unspecified), and three research Si3N4-containing materials prepared at Ford Motor Co. With the exception of some annealing experiments described below, samples were studied as received.
RESULTS AND DISCUSSION Reference Spectra. MAS spectra of the three known phases of Si3N4are illustrated in Figure 1. X-ray characterization had shown each of the crystalline samples to be at least 99 f 1 vol. % of the major component with no diffuse (amorphous) background. No crystalline features were recorded in the X-ray diffraction pattern of the a-Si3N,. The spectra of the a-and P-Si3N4powders agree with those recently presented (13). The spectrum for the amorphous phase is presented for the first time here. Two peaks appear in the a-phase spectrum since this phase contains two unique silicon
PPM
Figure 1. Silicon-29 MAS spectra of Si3N4: (A) a-Si3N4;(B) /3-Si3N,, SSB is a spinning skleband; (C) a-Si,N4 (lamour frequency = 59.4 MHz; rotor speed = 4 kHz; 12-20 acquisitions: 1-h recycle time).
Table I. Spectral Parameters for SiJNl Analysisa phase ab
bd ae oxynitridef
peak maxima, ppm -46.8 -48.9 -48.7 -46.4 -57 -60
Si?N208 sihconh SiOz (glass wool)
-63 -81.1
fwhh, ppm 1.@
1.8 2.5 27 3 4
5 0.9
15 nError on values is f 2 of the last significant figure quoted. GTE SN502. 'Doublet. dAldrich Chemical, annealed under H2 at 1500 O C for 160 min. eDu Pont. fNonstoichiometric phases. #From Dupree et al. (13). hVertex Chemical. -110
positions. The 6-phase, containing an additional center of inversion, has only one unique site and thus only one peak in its MAS spectrum (13).Although the shape of the a-Si3N4 resonance peak does not correspond to either the Gaussian or Lorenzian line shape (14),it may be approximately characterized by a full width a t half-height (fwhh) of 27 ppm. Fwhh's for the crystalline phases along with the peak positions from all three spectra are reported in Table I. The greater line width of the amorphous species is related to heterogeneity of local Si electronic environments (distribution of bond lengths and angles), a typical property of amorphous phases. As shown in Table I, the single line of the P-phase has a geater line width than either of the individual peaks of CY. The reason for this is possibly related to the nonequivalence of the four Si-N bonds in both the a-and P-phases. This inequivaience is greater in the P-phase, with a gap of 0.007 nm between the longest and shortest bonds (1.9, compared with 0.0045 and 0.0023 nm for the two CY tetrahedra (16). The average Si-N bond lengths in the two phases are similar (0.1733 nm for P and 0.1747 and 0.1740 nm for CY)so it is not surprising that the resonance frequencies of the two phases differ only slightly (17). a-Si3N4Powders. Figure 2 presents MAS spectra for seven predominantly a-Si3N4powders. Results of analysis of each by X-ray diffraction are given in the first column of Table
2796
ANALYTICAL CHEMISTRY, VOL. 59, NO. 23, DECEMBER 1, 1987 Table 11. Characterization of NMR spectrum
(I-
component
2Ab
and &SiJN, Powderso X-ray, %
NMR, %
97 3 neg 32 97 3 neg 32 95e 5 unspec unspec 19 nag na na na na 13 na na na na na 7 na na na na na 3 5 86 10
67 3 30 22 76 4 20 19 82 3 8 7 27 39 2 33 14 12 20 24 2 18 23 32 12 22 12 0 28 37 2 na 84 16
a
P a
4 P
2B'
a
P a
fflP
2Cd
a
P
a oxyni
alp
2w
a
P
a oxyni silicate
b L I
0
'
~
.lP
2Eh
a
P
a oxyni silicate ,
-50
~
I
-100 PPM
~
~
~
~
I
~
-
'
-150
'
I
~
RIP
'
2F'
a
P
Figure 2. Silicon-29 MAS spectra of a-Si3N4powders: (A) Cookson; (B) UBE; (C) Alfa Products; (0)research sample 1; (E) research sample 2; (F) research sample 3: (0)GTE SN502 (from Figure 1A). Same condkions as for Figure 1.
II. Employing the spectra of the pure Si3N4samples of Figure 1in a spectral analysis method that uses weighted sums of the pure phase spectra to match the spectra of the powders, these powders were reanalyzed for phase distribution and silicon-containing impurities. Two assumptions underlie the procedure. First, the NMR line shapes of the individual Si3N4 phases were assumed the same in the various powders. This assumption is justified, as will be shown, since this technique generally left no unaccounted for intensity in the spectra. Second, the weights used for the pure phase spectra were assumed to be directly proportional to the relative concentration of the phase. This assumption was safely made since care was taken to ensure complete TIrelaxation. A consistent procedure for the spectral manipulations was maintained throughout. After integration of the spectrum, the amorphous component was removed by subtraction of a scaled version of Figure 1C from the spectrum of the powder. The result of this process for the Cookson a-Si3N4is illustrated in Figure 3B. The processed spectrum was then reintegrated, the difference representing the amorphous fraction, in this case 30%. The a-phase content was then determined by subtraction of the pure a-Si3N4spectrum of Figure 1A. This was done by software scaling of the pure phase spectrum to match up the intensity of the low-field peak a t -46.8 ppm. Any residual spectral intensity, shown for the Cookson in Figure 3C, was taken to represent the P-phase content. As a further example, in the case of the UBE a-Si3N4powder the X-ray diffraction pattern had essentially no background. However, the present method revealed 20% amorphous content (see Table 11). Other silicon species may also be readily identified by their chemical shifts. The three research samples, whose spectra are illustrated in Figure 2D-F, show the two types of silicon-containing impurities that are significant in powder preparations-SiOp at -1 10 ppm and silicon oxynitride between -50 and -65 ppm. Dupree et al. (13)report the resonance of SizNzOat -63 ppm. This resonance was not observed in the present study. The two silicon oxynitride bands ob-
a oxyni silicate
.lP
4Aj
a
P
silicon
nError on values is f 2 of the last significant figure quoted. *Cookson. 'UBE E-10. dAlfa Products. e 1986/1987 Alfa Products catalog. !Research sample 1. 8Not analyzed. hResearch sample 2. 'Research sample 3. jAldrich Chemical (see footnote c of Table I).
I 1
il
1
0
1
1
1
1
1
1
1
1
,
1
4
1
1
1
1
1
,
-20 -40 -60 -80 -100 -120 -140 -160 PPM
Figure 3. Example of analysis of an a-Si,N, spectrum: (A) Cookson, same spectrum as Figure 2A; (B) resultant after subtraction of amorphous component; (C) residual P-Si,N, spectrum after subtractbn of a-component. Numerical results are listed in Table 11.
served here at -57 and -60 ppm are new, presumably nonstoichiometric silicon oxynitride phases. An interesting result
ANALYTICAL CHEMISTRY, VOL. 59, NO.
23, DECEMBER 1,
1987
2797
This spectrum required 2000 s to acquire. To just detect the presence of less than 1%silicon would require approximately 2.2 h.
CONCLUSION
/I
L
l
0
'
l
~
l
-20 -40 -60
r
l
-so
'
l
~
l
~
l
~
l
l
l
-100 -120 -140 -160
PPM
Flgure 4. Silicon-29 MAS spectra of (A) @-SI,N, annealed at 1500 OC for 160 mln under strongly reducing conditions, (B) crystalline silicon, (C) subtraction of (B) from (A), and (D) 0-Si,N, using a 100-s recycle time and 20 acquisitions.
of this effort was the analysis of 7% silicon oxynitride in the commercial sample of a-Si3N4purchased from Alfa Products (Figure 2C). B-Si3N4Powders. Prior to acquisition of the MAS spectrum of @-Si3N4illustrated in Figure lB, the powder was annealed at 1500 "C under reducing conditions (an H2atmosphere in the furnace) to ensure the complete conversion to 0 of any residual a. Sometimes, an annealed sample would show production of a small quantity of silicon, as illustrated by the spectrum of Figure 4A. Except for the presence of the asymmetric silicon peak a t -81 ppm, the spectrum is identical with Figure 1B. Figure 4B presents the spectrum of powdered crystalline silicon. The obvious difference in line shapes between the peak in parts A and B of Figure 4 is behind the discrepancy between the X-ray diffraction analysis and NMR spectroscopy analysis of silicon concentration in Figure 4A (see Table 11, last entry). The difference is due to the sensitivity of NMR to amorphous as well as crystalline silicon, thus accounting for the greater line width of the silicon peak in Figure 4A as compared to that of Figure 4B. Figure 4C, resulting from subtraction of Figure 4B from Figure 4A, illustrates this difference in line width. Since the Tl of silicon is much shorter than that for the other silicon species, it is possible to acquire the spectrum of silicon a t a much faster rate than one pulse per hour. With a rapid pulse, a siliconenhanced spectrum is generated, as illustrated in Figure 4D.
l
With solid-state 29Si NMR spectroscopy one is able to identify a substantial amorphous fraction in certain Si3N4 powders of low oxygen content. In at least these two cases, conventional X-ray diffraction displays a negligible diffuse background and no indication of any amorphous material. In oxygen-containing powders, NMR spectroscopy can distinguish among different chemical species often found in these powders, including silicon oxynitrides and silicates. The major drawbacks to the %i NMR spectroscopy technique are the long time period required for acquisition of spectra with sufficient signal-to-noise ratios. Other samples might show values for T1different from those reported here, and so the acquisition time may vary. Unfortunately, Si3N4 powders are unprotonated so that cross-polarization techniques (18) to enhance the %Sisignal and reduce the recycle delay may not be employed. Use of larger sample volumes are presently limited by the size requirements of the MAS rotors (1/2 g or less). The solid-state NMR spectrometer can be justified as the first choice instrument for the analysis of Si3N4powders when the high sensitivity to amorphous phases and the unique capability to distinguish among different amorphous species are critical. Registry No. Si3N4, 12033-89-5; Si, 7440-21-3; Si2N20, 12033-76-0;SO2,7631-86-9;silicon oxynitride, 11105-01-4;silicate, 12627-13-3.
LITERATURE CITED Terwilliger, 0.R. J. Am. Chem. SOC. 1974, 5 7 , 48. Engei, W. Powder Metall. Int. 1978, 10, 124. Kanzaki, S.; Abe, 0.; Tabata, H. Yogyo Kyokaishi 1986, 9 4 , 189. Luongo, J. P. J. EEectrochem. SOC. 1983, 130. 1560. Cohen, M. H.; Relf, F. SolM State Phys. 1957, 5 , 321. Fyte, C. A. SolU State NMR for Chemlsts; Guelph; Ontario, 1963; p 18. (7) Oldfieid, E.; Kirkpatrick, R. J. Nature (London) 1984, 308, 523. (8) Magi, M.; Lippmaa, E.; Samoson, A.; Englehardt, G.; Grimmer, A. R. J. Phys. Chem. 1984. 8 8 , 1518. (9) Smith, J. V.; Blackwell. C. S. Nature (London) 1983, 303, 223. (IO) Smith, J. V.; Blackwell, C. S.; Harris, 0.L. Nature (London) 1984, 300, 140. (11) Flnlay, G. R.; Hartman, J. S.; Richardson, M. F.; Williams, B. L. J. Chem. SOC.,Chem. Commun. 1985, 159. (12) Fukushima, E.; Roeder, S. 8. W. Experimental Pulse NMR; AddlsonWesley: London, 1981. (13) Dupree, R.; Lewis, M. H.; Leng-Ward, G.; Williams, D. S. J. Mater. Scl. Len. 1985, 4, 393. (14) Dupree, E.; Pettifer. R. F. Nature (London) 1984, 308, 523. (15) Grun, I?. Acta Crystallogr., Sect. B : Struct. Crystallog. crysr. Chem. 1979. 8 3 5 . 800. (16) Kato, K.; Inque, 2.; Kijlma. K.; Kawada, I.; Tanaka, H.; Yamane, T. J. Am. Ceram. SOC. 1975, 5 8 , 90. (17) Smith. K. A.; Kirkpatrick, R. J.; Oldfield, E.; Henderson, D. M. Am. Mlneral. W63, 68. 1206. (18) Pines, A.; Glbby. M. G.;Waugh, J. S. J. Chem. Phys. 1973, 5 9 , 569. (1) (2) (3) (4) (5) (6)
RECEIVED for review April 17,1987. Accepted August 7,1987. A portion of the support for this work was provided by the US. Dept. of Energy, Office of Transportation Systems, Advanced Materials Program through the Ceramic Technology for Advanced Heat Engines Program at Oak Ridge National Laboratory, Oak Ridge, TN, under Contract DE-AC05840R21400 with Martin Marietta Energy Systems, Inc.
