The absorption properties of submicron SiC particles between 2.5 and 40 μm

0020-0891/83 s3.00 + 0.00

Infrared Phm

Vol. 23. No. 6, pp. 32lL328, 1983 Printed in Great Britain. All rights reserved

THE

ABSORPTION PARTICLES A. BORGHESI,’ ‘Cruppo

“Gruppo

Nazionale

PROPERTIES BETWEEN

E. BUSSOLETTI,’

Astrofisico, Struttura

Copyright

Physics

L.

Department,

della Materia, University

OF SUBMICRON 2.5 AND 40pm

COLANGELI’ University

SIC

and C. DE BLASI’ of Lecce, Lecce, Italy

Consiglio Nazionale delle Ricerche, of Lecce, Lecce. Italy

(Received

(* 1983 Pergamon Press Ltd

Physics

Department.

26 July 1983)

Abstract-Submicron a-Sic particles have been studied in the near and middle i.r. at room temperature and their properties measured after different treatment processes: namely, grinding, ultrasonic washing and sedimentation. TEM analysis allows determination of grain morphology and size distributions. The extinction curves reveal the presence of a central main peak at I I .4 pm and of two shoulders, respectively at 10.6 and 12.7 pm, due to phonon resonances. Our data show good agreement with those previously reported by the University of Jena Group. On the other hand, B-Sic spectra differ substantially, indicating their crucial dependence on the crystallographic characteristics of the samples. Experimental astronomical observations seem to indicate that a-Sic is the best candidate to simulate the actual Sic present in cosmic sources.

INTRODUCTION Silicon and silicon compounds represent some of the main constituents of interstellar, circumstellar and interplanetary dust according to the large amount of experimental evidence available on the subject. Grains originate predominantly in high-density conditions that occur in the atmosphere or the surroundings of various classes of stars. Thermodynamic calculations clearly show that two “families” of products may arise from the cooling of a stellar gas according to the oxygen-to-carbon ratio, corresponding therefore, to carbon stars (O/C < 1) and to oxygen stars (O/C > 1). In general magnesium silicates dominate in oxygen stars, while carbon and silicon carbide dominate in carbon stars.” 4, Sic, one of the most refractory compounds, therefore plays an important role in the condensation processes. It is present in several celestial sources, as has been revealed by a series of i.r. observations of circumstellar shells and planetary nebulae which have evidenced a band of the band was obtained by applying Mie centered around 11 pm. (5 12)Initially the identification theory to refractive indices measured on SIC single crystals of exaedric or cubic structure in special orientation. This feature is suggested to be due to a fundamental solid-state transition in the grain lattice. Recently, a large program has been developed, by the Jena University Group, to improve the knowledge of spectroscopic properties of particulates with cosmic importance, including Sic.” 16) Mass absorption coefficients in the near and middle i.r. have been determined accurately, in order to simulate the experimental astronomical observations. In parallel, similar work using different particle-production methods was performed by others in U.S.A.“7.‘X’ Following our systematic analysis of materials’ analogues to cosmic dust”‘) (hereinafter identified as Paper I), we present here the results concerning SIC in the 2.5-40 pm range. Particular attention has been paid to the criticism of particle preparation and to a comparison with the laboratory results mentioned above. Actually, in spite of being a relatively new field, i.r. spectroscopy represents one of the most promising research fields for the near future-as is evidenced by the large number of papers which appear in the literature reporting astronomical observations or related laboratory results (see Refs (20) and (21) for very good recent reviews). The aim of this work is to offer a further contribution to the general call for accurate measurements of optical and physical properties of materials’ analogues to cosmic dust. EXPERIMENTAL

PROCEDURE

It is well known that different types of SIC do exist according composition. Several samples with different composition (both chemical 321

to their crystallographic and crystallographic) are

A. BORGHESI rt cd.

322

presently under analysis in our laboratory-a comparative study is in progress and will be described in a forthcoming paper. Here, we present the preliminary results obtained for a sample of a commercial mixture of polytypes of cl-Sic (Silcar N) produced by Elektroschmeltz Werk GmbH, Kempten, F.R.G. Table 1 lists the chemical composition and other characteristics of the raw material. The choice of this particular sample, with low Sic purity, was deliberate in order to verify if some spectral differences exist with respect to samples of higher purity studied by other authors. Table

and I. Chemical composition ameters of Silca1.N

Black r-SK Chemical composition (“J

physical

mixture Crystallographvz compo\ltlon

Exaedric 89.80 SIC Cubic (tracea) 0.90 Free Si 0.42 Metal. Fe 4 25 Free C 4.65 Free SiO, Mass density of grains = 3.21 g cm ’ Mean dia = 4.0 /~cm

Sample preparation was in accordance with the well-established procedure defined by the Jena Group (I3 16)(hereinafter identified as F.G.S.D.). The material was initially ground for different time periods (0.5, 1.5 and 3 hr). The tendency of grains to clump together suggested ultrasonic treatment (-20 min) in order to produce acceptable dispersion. Suspension in acetone and sedimentation (1 and 2 hr) allowed selection of appropriate particle-dimension ranges. The above procedure enabled us to study the influence of the different preparation steps on the physical and spectroscopic properties of the dust. Transmission electron micrographs (TEMs) with various magnifications (Fig. 1) were taken to identify grain dimensions and their distribution. Mass absorption coefficients were measured in the 2.5-40~m range by a double-beam i.r. Perkin-Elmer 683 spectrophotometer.

Fig. I. TEM micrograph

of a G-3.0

sample;

single particles irregular.

are distinguishable.

The shapes

arc clearly

Absorption properties of Sic particles

323

RESULTS (a) Electron

microscope

analyses

TEM photographs show clearly that SIC grains present irregular shapes. We take, therefore, the maximum elongation of the particles as a characteristic dimension, d. The experimental evidence indicates that grinding times longer than 1.5 hr do not affect appreciably the diameter distribution (according to the results shown by F.G.S.D. this time is even shorter: only 1 hr). The size distributions of the grains have been determined by analyzing several TEM images, containing more than 1500 single grains, for different samples. Actually, a range of magnifications (from 1000 x to 10,000 x ) has been used for any one single dust sample in order to prevent possible selection effects for very large grains and to reduce those for very small ones.

(a)

20

(b)

1

d (pm)

Fig. 2. Typical size distributions obtained by measuring the maximum dimensions, single grains on several TEM images with different magnifications (1000 x -10,000 (b) GUS-2.0 results.

d, of more than 1500 x ). (a) G-3.0 results;

Figures 2a,b show typical results of our statistical analysis. Figure 2a applies to a 3-hr-ground (G-3.0) sample: the mean diameter of the grains is (d) = 0.9 pm. Figure 2b represents the extreme case of 3-hr grinding plus 20 min ultrasonic treatment and 2 hr sedimentation (GUS-2.0). A mean diameter (d) = 0.7 pm is found in this case, as a consequence of the selection effect due to the sedimentation. A common feature in all our results is the cutoff appearing in the size distributions below 0.5 pm, which is due to selection effects of the TEM images. As already noted by F.G.S.D. the histograms in Figs 2a,b can be fitted by a power law of the form n (d)ocdF.

(1)

Least-squares fitting of this function allows us to determine the exponent ~1, and therefore the corrected values of the mean diameters of the grains according to the extrapolation towards sizes smaller than 0.5 pm. We find (a) = 0.6 pm and (d) = 0.4 pm, respectively, for the distributions reported in Figs 2a,b, using the appropriate CI valuesPz = 1.7 and c( = 2.0 (see Table 2).

324

A.

bKGHES1

Table 2. Morphological

et ul.

properties

of the grains

Sample

Cd) (Pm)

II

(2) (Pm)

G-3.0 GUS-2.0

0.9 0.7

1.7 2.0

0.6 0.4

It is worthwhile to note here that there is good agreement in the general trend of distributions and mean sizes of particles between our results and those of F.G.S.D., within the experimental errors. (6) Spectral measurements The SIC particles have been embedded in KBr pellets, and the i.r. transmission a double-beam spectrophotometer, in order to determine the mass absorption

measured coefficient:

using

where
6

250

500

750

loo0

125d

3500

3750

4(

li(cm-‘I

Fig. 3. Sic mass absorption coefficient measured according to equation (2) for G-3.0 samples. This is the mean curve obtained from several spectra. The characteristic absorption band at about c4 - 845 cm-’ is clear with the two phonon resonances at S, - 940 and & - 790 cm ‘,

Fig. 4. SiC mass absorption coefficient normalized to the “baseline” in the band region measured according to equation (3). Mean curves obtained from several spectra in each case are given here. ~~, G-3.0 sample; ----. GUS-I .O sample; --@P. GUS2.0 sample.

325

Absorption properties of Sic particles

continuum in the band region (dashed line in Fig. 3) according to the new mass absorption coefficient: (3) Here, T,(C) represents the G-3.0 and GUS-I.0

the “baseline” transmission curve. No significant differences appear between samples, within the experimental errors; GUS-2.0 presents a higher mass absorption coefficient at the peak position, Z?(?*). Table 3 presents the results of this study which, we reiterate, are averages obtained by measuring different samples at each step of the production procedure. Column 1 indicates the type of sample; column 2, the wavenumber position (respective of the longitudinal phonon resonance PL, the absorption peak fA and the transverse phonon resonance VT); and columns 3 and 4 report the absorption peak intensities, K(.r;A) and R(c,& according to equations (2) and (3). Table 3. Spectroscopic Peak wavenumber Sample cs G-0.5 G-I.5 G-3.0 GUS-l.0 GUS-2.0

% 940 940 940 940 940 940

results

(cm- ‘)

“A

%

K(~AA)*

B(<,)C

830 830 845 845 845 845

790 790 790 790 790 790

9100 I600 9900 f 800 10,400 + 500 10,400 i 900 10,300 2 300 11,300_+600

8100 i 500 9000 i 700 9400 15 500 9600&900 9700 i: 300 10,700 i: 600

*Units of cm’gg’. CS, commercial Silcar N; G-0.5, material ground for 0.5 hr; G-l .5, material ground for 1.5 hr: G-3.0, material ground for 3.0 hr; GUS-1.0, material ground for 3.0 hr, ultrasonically treated for 20min and sedimented for l.Ohr; GUS-2.0, material ground for 3.0 hr. uitrason~cally treated for 20 min aad sedimented for 2.0 hr.

DISCUSSION

As previously mentioned, our TEM analyses reveal that, apparently, grinding times longer than 1.5 hr do not seem to have a significant effect on the dust size distribution. Actually, the mean grain diameter that we found for sample G-l.5 is about 0.9 pm-note that it is in good agreement with the value obtained by F.G.S.D. after only 1 hr of grinding. On the other hand, these authors do not mention the typical dimensions of their starting material; therefore, if these dimensions were comparable to ours (- 4 pm), we may conclude that 1 hr grinding time is sufficient to reach the observed (d). If not, the only reasonable interpretation is that F.G.S.D.‘s starting material had a smaller mean diameter. Let us now discuss sedimentation effects, remembering that t=

9%Z VP - &Jr*g

(4)

is the time necessary to precipitate particles with radii larger than a fixed value r, suspended at height Z; pi0and p0 are, respectively, the solvent viscosity and mass density (acetone in our case), while p is the mass density of particles. Our results show that in 1 hr no appreciable sedimentation of particles is evident, leaving size distributions similar to those presented in Fig. 2a. On the other hand, a sedimentation time of 2 hr or longer produces a consistent change both in size distribution and in (d), as is evident from the histogram of Fig. 2b. (b) Spectral evidence

The first indication from our data (Table 3) is the occurrence - 830 and -v 845 cm-‘, respectively, for the starting raw material 1 hr or more. The simplest explanation which may be given is that dimensions: in the first sample the particles have diameters (d) -

of the main peak absorption at and for material processed for this is due to the effects of grain 4 ,um (with a significant fraction

326

A. &HZGHESl

et al.

of larger grains). On the other hand, we have observed that grinding times longer than l-l.5 hr reduce drastically the mean dust diameters to values of the order of (u’) _ 0.9 pm. Apparently, the interaction of i.r. radiation with grains characterized by dimensions comparable with the wavelength 3., tends to shorten the wavenumber position of the SIC main absorption peak. In addition we note, as already found by F.G.S.D., that a significant dependence of the mass absorption coefficient on the size distribution of the particles is revealed (see Tables 2 and 3). The smaller (d), the larger K(PA) is. This result is confirmed further by the constancy in K(va) found for our samples G-l .5, G-3.0 and GUS-l .O, which present similar grain dimensions, (d) _ 0.9 pm. Our averaged data derived for samples G-3.0 and GUS-2.0, which appear to be the most useful for astrophysical applications, according to their size distributions, are tabulated in the Appendix. It is worthwhile to note that, according to the computations performed by Dorschner et u/.,~‘~) F.G.S.D. and Schmidt,“41 some corrections to the experimental data may eliminate the influence of the KBr matrix on the spectra and, hence, data relevant to a vacuum may be obtained. These corrections are as follows: (a) a wavenumber shift D;= +30cm-’ (Dj, = -0.39 pm) must be applied within the range 630 cm-’ < v’ 6 1100 cm-‘; (b) a correction factor h = 0.7 must be applied to the mass absorption coefficient K(G) over the entire band profile. Finally, we wish to discuss the results of a detailed comparison that we performed between our data and those presented by F.G.S.D. As already mentioned, the two kinds of samples analyzed (ours and F.G.S.D.‘s) appear to have very similar size distributions and mean grain diameters, if we consider material which has undergone grinding only. When the material is sedimented also, the mean grain diameters obtained by F.G.S.D. reach values of about (d) - 0.7 pm in 0.5 hr. We obtained the same value of (d) only after 2 hr. By comparing the mass absorption coefficients, we note that the absolute values of K (S) are quite different in the two cases for any sample at every stage of the processed material. K (3) for simple ground samples appears higher, by -2O”/i, for our G-3.0. A reversal of the absolute values, of times. Actually, about 25”j,, occurs and remains stable for any sample after different sedimentation if we consider, as in Figs 5a,b, the spectra normalized to the peak value of K (v”), i.e. K (CA), we observe good agreement between our samples and those derived from F.G.S.D., within the experimental errors. We are not able at present to interpret the previously-cited discrepancies between absolute values of K (v”); we tentatively suggest, that these may be attributed to possible differences in the raw material (industrial production method, slight difference in chemical composition etc.). CONCLUSIONS Here, we have reported the results of morphological and spectral analyses of submicron Sic particles in the range 2.5-40~~. The grains have been processed through grinding, ultrasonic washing and sedimentation in order to select different ranges of dimensions. TEM analysis has allowed the measurement of particle size distributions: as already found by other authors this distribution follows a power law, n (d)cx&‘, with !Xvalues in the range 1.7 < CYd 2.0. In this case the mean typical dimensions are within the range 0.4 pm < (d) < 0.6 pm, which is in good agreement with the results of F.G.S.D. The observed spectra are reproducible, so we have considered average properties. A typical spectrum shows the characteristic peak absorption band occurring at I 1.4 pm in a vacuum, as well as the phonon resonances falling, respectively, at 10.6 and 12.7 pm. These data agree with values usually attributed to Sic particles supposed to be present in space in circumstellar shells of carbon stars and planetary nebulae. Similar results due to Cc-Sic were obtained by the Jena Group with higher-purity samples, indicating that the residue material still present in our samples does not affect appreciably the spectral properties. On the other hand, we note that a large disagreement in spectral behavior exists between our using laser pulses, for B-Sic (cubic) [see results and those reported by Stephens,(‘@ produced

Absorption

properties

of Sic particles

321

IO

R

c

0:

I

/

600

800

I 600

IO00

1000

800

Fig. 5. A comparison of the normalized mass absorption coefficient, R, between our data (---) and those of F.G.S.D. (---). Good agreement between the two sets of data, within the experimental errors, is evident. (a) Data for grinding-only samples (our G-3.0 and F.G.S.D.‘s BO); (b) data for ground, washed and sedimented samples (our GUS-2.0 and F.G.S.D.‘s B2).

Fig. 3b in Stephens(“)]. In this case the SIC band is split into two peaks falling, respectively, at about 11.0 and 12.4 pm. If these peaks are interpreted as due to phonon resonances, as by Stephens,“” we note the absence of the central main absorption peak that we and F.G.S.D. located at 11.4 pm. We are inclined to interpret this difference as due to the different crystallographic composition of the samples (polytypes for cr-Sic, cubic for P-Sic). It is worthwhile to note, however, that cr-Sic appears to be the best candidate to simulate the astrophysical observations, as indicated in the recent measurements by Goebel et ~1.““’ for the carbon star Y Canum Venaticorum. Acknowledgements-This 82-00872-02 and PSN

work WS supported 82-012, by the Consiglio

by the Minister0 Pubblica Nazionale delle Ricerche.

Istruzione

and,

under

Contracts

CNR

REFERENCES 1 2. 3. 4. 5. 6. 7. 8. 9. 10. Il. 12. 13. 14. 15. 16. 17. 18.

Friedemann C., Physica 41, 139 (1969). Gilman R. C., Astrophys. J. 155, L185 (1969). Hackwell J. A., Ph.D. Thesis, University College, London (1971). Hackwell J. A., Astr. Astrophys. 21, 239 (1972). Treffers R. and Cohen M., Astrophys. J. 188, 545 (1974). Forrest W. J., Gillett F. C. and Stein W. A., Astrophys. J. 195, 423 (1975). Merrill K. M. and Stein W. A., Pubis astr. Sot. Pacif. 88, 294 (1976). Willner S. P., Jones B.. Puetter R. C., Russell R. W. and Soifer B. T., Astrophys. J. 234, 496 (1979). Aitken D. K., Roche P. F., Spencer P. M. and Jones B., Astrophys. J. 233, 925 (1979). Goebel J. M., Bregman J. D., Goorvitch D., Strecker D. W., Puetter R. C., Russell R. W., Soifer B. T., Willner S. P., Forrest W. J., Houck J. R. and McCarthy J. F., Astrophys. J. 235, 104 (1980). Mitchell R. M. and Robinson G., Mon. Not. R. mfr. Sot. 190, 661 (1980). Aitken D. K. and Roche P. F., Mon. Not. R. asfr. Sot. 200, 2 I7 (1982). Dorschner J., Friedemann C. and Giirtler J., Astrophys. Space Sri. 48, 305 (1977). Dorschner J., Friedemann C. and Giirtler J., Astr. Nuchr. 298, 279 (1977). Dorschner J., Friedemann C. and Giirtler J., A.m. Nachr. 299, 269 (1978). Friedemann C., Giirtler J., Schmidt R. and Dorschner J., Astr0ph.v.t Space Sci. 79, 405 (1981). Stephens J. R. and Kothari B. K., Moon Planers 19, 139 (1978). Stephens J. R., Astrophys. J. 237, 450 (1980).

328

A.

kMGHES1

et rd.

19. Borghesi

A.. Bussoletti E.. Colangeli L.. Minafrd A. and Rubini F., S. P., Gulactic and E,~trapluctic Iqficrred Spectroscopy-XVIth Phillips J. P. and Guyenne T. D.). Reidel. Dordrecht (1983). Allamandola L. J., ihid. Gilra D. P.. Nature 229, 237 (1971). Huffman D. R., Adc. Ph~x. 26, 129 (1977). Schmidt R.. Diploma Thesis (1980).

20. Willner

21. 22.

23. 24.

Infrared Phys. ESLAB

23, 85 (1983). .‘$vnposium (edited by Kessler

M. F..

APPENDIX Table A.I. Measured

mass

absorption coeficients for G-3.0 and GUS-2.0

samples

K(S)

K(i)

(cm?g-1)

(cm~&?‘I

a (cm

‘)

4000

G-3.0 2 500

5210

GUS-?.0

G-3.0

5230

980

10.204

1460

GUS-2 1230

3900

2.564

5130

5080

970

IO.309

IX70

1690

3800

2.632

5010

4900

960

10.417

2860

2730

3700

2.703

4910

4760

950

10.526

3930

3x90

3600

2.778

4X20

4590

940

IO.638

4350

4290

3500

2.857

4710

4430

930

10.753

4440

4390

3400

2.941

4600

4260

920

10.870

4720

4690

3300

3.030

4450

4050

910

10.989

5340

5420

3200

3.125

4330

3890

900

ll.IlI

6160

6360

3100

3.226

4190

3700

890

II.236

7020

7290

3000

3.333

4040

3510

880

Il.364

7840

8310

2900

3.448

3910

3310

870

Il.494

X780

9470

2800

3.571

3770

3130

860

Il.628

9570

10.440

2700

3.704

3610

2940

x50

Il.765

10.280

II.170

2600

3 846

3460

2750

845

11.834

10.390

II.280

2500

4.000

3310

2570

840

Il.905

10,190

II.150

2400

4.167

3140

2380

830

12.048

9460

10,040

2300

4.348

2970

2200

820

12.195

7870

82X0

2200

4.545

2800

2020

810

12.346

63Y0

6440

2100

4762

2640

1840

800

12.500

5310

5020

2000

5.000

2460

1680

795

12.579

50x0

4760

1950

5.128

23x0

1580

790

12.658

4960

4520

1900

5.263

2280

1510

7x5

12.739

4800

4220

1850

5.405

2190

1420

780

12.820

4580

3810

1800

5.556

2090

1350

770

12.987

4120

3040

1750

5.714

1990

1290

760

13.158

3690

2460

1700

5.X82

1890

1240

750

13.333

3320

2030

1650

6.061

1810

II90

740

13.514

3030

1710

1600

6.250

1710

II30

730

13.699

2780

1470

1550

6452

1640

IOU0

720

I3 X89

2580

I290

1500

6.667

1550

1030

710

14.084

2410

1160

1450

6.X97

1480

980

700

14.286

2270

1030

1400

7.143

1390

930

650

15.385

1760

760

1350

7.407

1310

890

600

16.667

1450

600

1300

7.692

1240

880

550

18.182

1240

520

I250

8.000

1150

830

500

20000

1050

450 400

I?00

8.333

1100

850

450

22.222

890

II50

8.696

1050

890

400

25000

720

300

II00

9.091

1000

940

350

28.571

580

230

1050

9.524

060

980

300

33.333

460

I60

10.000

II60

1040

250

40.000

350

I30

10.101

1270

Ill0

IO00 990

0