Pyrolysis of Polysilazanes: Relationship between precursor architecture and ceramic microstructure

Composites Science and Technology 37 (1990) 7-19

Pyrolysis of Polysilazanes: Relationship between Precursor Architecture and Ceramic Microstructure F. Sirieix, E Goursat Laboratoire de C6ramiques Nouvelles, Universit6 de Limoges, UA CNRS no. 320, 123 Avenue A. Thomas, 87060 Limoges, France

A. Lecornte & A. Dauger Ecole Nationale Sup~rieure de C~ramiques Industrielles, UA CNRS no. 320, 47 Avenue A. Thomas, 87065 Limoges, France (Received 19 July 1988; revised version received 15 November 1988: accepted 14 June 1989) A BS TRA C T The pyrolysis of polysilazanes to silicon carbo-nitride has been studied on account of their potential as precursors/'or ceramic matrices. First, a polymerisation process using KOCH 3 as a catalyst is investigated to synthesise poO'mers of octamethylc.velotetrasilazane. The poO'silazanes are characterised (e.g.b.v GPC, IR and SA X S ) and heated in nitrogen to 1400°C to determine the influence of structural characteristics on their conrersion into ceramics. The decomposition int~oh'es three steps, hz the first stage oligomer release creates porosity. From 500 to 750°C escape of the organic groups occurs with a density increase. The pyrolysis products are thermally stable up to 1400°C. The final density and the crystallisation kinetics are closely related to the molecular architecture of the precursors.

I INTRODUCTION The preparation of high-performance materials has been the subject of many studies in the field of ceramics, as for example in the manufacture of fibres or synthesis o f ultrafine powders. A number of difficulties encountered during their processing could be solved by using preceramic polymers. Polymer chemistry offers opportunities to tailor molecules in order to obtain 7 Composites &'ience and Technolog.v0266-3538/89/$03"50~ 1989 Elsevier Science Publishers Ltd, England. Printed in Great Britain

8

tE Sirieix, P. Goursat, A. Lecomte, A. Dauger

ceramics with desired properties, such as thermal stability, Young's modulus or high strength. The precursor conversion is governed by such a large number of parameters (thermal cycle, pressure, molecular structure, composition) that it is necessary to use various complementary techniques to follow the chemical reactions and the development of the ceramic microstructure. Silicon carbide and silicon nitride, which are candidates for high temperature mechanical applications, are limited by their brittleness. Recently, ceramic matrix composites have been successfully developed, encouraged by the good performance of carbon-carbon composites. In this context, we decided to study the pyrolysis of polysilazanes as silicon nitride precursors. The aim of this work is to establish relations between the architecture of the polymers and the microstructure of the silicon carbonitrides. Cyclic molecules of organosilicon compounds ~ which are more stable than linear molecules are better suited for pyrolysis. We have thus chosen a cyclic monomer, octamethylcyclotetrasilazane (OMCTS), and a polymerisation catalyst (KOCH3) to promote cyclization. In this paper we present the synthesis conditions and characteristics of the polymers, followed by the thermal decomposition and the structural evolution. 2 SYNTHESIS A N D S T R U C T U R E OF POLYSILAZANES The silazanes consist of a framework of Si-N bonds with alkyl and aryl groups linked.to silicon or nitrogen. In spite of the number of investigations 2 devoted to polymerisation, only a few papers concern ceramic precursors. 3"~ Our goal was to prepare ordered polysitazanes to study structural changes during thermal decomposition. Thus a basic catalyst, KOCH 3, was used to break Si-CH 3 bonds. The polycondensation of OCMTS with KOCH 3 was carried out under reflux conditions in nitrogen (200-350°C) for several hours. The methane and ammonia evolution at temperatures higher than 200°C results from two competing reactions. The methane originates from the breaking of Si-CH3 and N - H bonds, and from the bridging of two rings: CH3

CH3

/Si\ NH NH CH3\ / \ CH3 N/ Si Si / / " F . . . . . . . . . n / Si-C H # NH.. \ NH ~CHa + H N \ ./ .......... Si Si-/ ~ /\ CH 3 CH 3

I - - N \ / CH 3

~-

/ --N I

~, S i - - N + CH~,

Pyrolysis of polysilazanes

9

On the other hand, the evolution of ammonia indicates the occurrence of ring-opening reactions and rearrangement of silazanes. 5 This transamination reaction results in structural heterogeneities. Methane is the main product at higher temperatures, while ammonia evolution is favoured by a high KOCH 3 content. 6 The polymers so obtained are white solids. The influence of the catalyst on the polymerisation kinetics and on the size of macromolecules was studied by determining the molecular weight distribution through gel permeation chromatography (GPC) measurements (Waters Associated equipment) with tetrahydrofurane (THF) as solvent and polystyrene as standard. The chromatograms for different KOCH 3 contents (Fig. 1) show a wide molecular weight distribution (500-40000gmol-~). With increasing catalyst content, the growth of the macromolecules is hindered while the number of heterogeneities is promoted as shown by the evolution of ammonia. Polycondensation occurs on a large number of sites and oligomers are rapidly formed. Owing to the steric hindrance and the increase of viscosity, the reaction progress is slowed down. The infrared spectrum (Nicolet 5DX spectrometer) of the polymers hardly changes with the degree of polymerisation (Fig. 2). The intensity of the Si-CH 3 (as CH3 = 2963 cm- z, s-CH3 = 2904 cm- l, ~5_Si_CH3 = 1265 cm- 1) characteristic absorption bands are almost unaffected by the initial catalyst content. These slight changes agree well with the polymerisation mechanisms. In fact, if no Si-N bonds are broken then only one half of the 16

¼ c o n t e n t (*/,1

content(%1

I % KOCH3

12

5"/,KOCH 3

12

; ,,lllllllJlll;!: ,IUllJllii103

104

~6

2'5% KOCH3

Iz

o

lIE

¼

content (%) t

103

content (%)

10% KOCH3

12

., JtJllllll ,,,.....° 'o "~ Fig. 1.

,.04

Ill/,

• .

w~i~t

"

J E L l , ,, Ill_ ,,

10)

114

Molecular weight distribu(ion for different KOCH 3 contents.

10

1:: Sirieix, P. Goursat, A. Lec'omte, A. Dauger

i

~000

Fig, 2.

3200

2~00

~'~00

~500

;'~00

tl50

650

~.00

IR spectra of(a) OMCTS; polymerisation conditions: 12h, 360:C, with (b) 1%, (c) 2'5%, (d) 5% and (e) 10% KOCH 3.

methyl groups are eliminated. The broadening and the shift of the Si-N-Si absorption band (925 cm- 1) towards higher wave numbers evidence changes in the vicinity of the silicon. Bridged species or different sized cyclosilazanes are formed. We have used small angle X-ray scattering (SAXS) to characterise the

Pyrolysis of polysilazanes

11

molecular architecture of the silazanes. This technique, which is complementary to GPC and IR spectrometry, may also be used in the organic as in the ceramic state. The polymers consist of polycyclic macromolecules surrounded by branched chains. These structural features induce electronic density heterogeneities causing X-ray scattering. SAXS data were recorded with a slit-type small angle camera (Cu K~-quartz monochromator), v The sample-to-detector distance was 300mm and the scattered intensity was counted with a position sensitive proportional counter. The scattering vector, Q=4rc-Zsin0, where 0 is the Bragg angle, ranges from 0"15 to 2nm -1. Experimental results are corrected for parasite scattering and normalised to equivalent sample thickness and incident beam intensity. Powders are pressed to obtain thin plates or dissolved in THF and used in a sample cell with 0-025 mm thick Mylar windows and 0.5-1 mm path length. The main parameter accessible by SAXS is the size of scattering particles, s The electronic radius of gyration, R s, can be deduced from the initial decay of the scattered intensity, I(Q), using the Guinier approximation:

I(Q) =

Nn2(1 - Q2 R~/3)

where N is the number of particles per unit volume and n the number of excess electrons in each particle. The observation of a maximum in the scattered intensity distribution means that the number density of particles is large and that interference effects occur. Then a rough estimation of mean interparticle distance can be derived from a simple hard-sphere model. Finally, out of the small angle scattering Q range, a diffraction effect was observed in the Bragg region. This Bragg ring has been interpreted as evidence of some sort of stacking order and an interplanar distance d was derived. The SAXS spectra for some polymers prepared at 360°C for 12h are shown in Figs 3 and 4; the corresponding measured values are summarised in Table 1. ~'.T

~ Re(. Int. (,~)2~

I

~.

"

-

8

~, I ~ KOCH3 ~- 2.~ ~- KO(H3 5 ~ KOCH3 • [~ ~ KOCH3

~

,, _ 6.98

Fig. 3.

SAXS curves.

1.96

12

F. SirieL~, P. Goursat, A. Lecomte, ,4. Dauger

~el. int.

, ~ ~l~

4

8

1:2

Fig. 4. Bragg ring profile (2-5% KOCH3).

The evolution of the radius of gyration with the catalyst content confirms the GPC results. We observe large non-interacting particles for 1 and 2"5% KOCH 3, and a high number density of small clusters for 5 and 10%. The increase in both the number of active sites and the particle size hinders the reaction. The interplanar distance, d, remains almost unchanged whatever the experimental conditions. Taking into account the high value o f d a n d the polycondensation mechanism which promotes the formation of polycyclic entities, the macromolecules may form flat platelets or folded sheets. The diffraction effect is thus attributed to some order in the stacking of macromolecules. It is then possible to propose a schematic model for the polymer architecture (Fig. 5). In the presence of KOCH3, two mechanisms are competing, The breaking of Si-CH 3 bonds leads to the formation o f a polycyclic bidimensional structure, while the breaking of Si-N bonds gives branched chains. The rearrangement of a part of these chains gives rise to different sized cycles. The increase ofcatalyst content promotes the breaking of Si-N bonds and the number of heterogeneities. This is the origin of interference effects in the SAXS diagrams. The vicinity of areas made of either ordered macromotecules or branched chains creates the electronic density contrast. Particles whose size ranges from 4 to 10 nm, depending on experimental conditions, would be made of stacked polycyclic macroTABLE 1

Catalyst content, KOCH3 (wt%) 1 2"5 5 10

R e (nm)

d (nm)

3"5 5"4 1 1"8

1'17 1'15 1"2 1"19

Pyrolysis t)/polysilazanes stacked ;)tate|ets 7~\

13

folded sheets d : 1.2nm

.../T~

~,rs

Fig. 5.

hettro~ities

Schematic model of the polymer architecture.

molecules or of folded sheets. These organised microdomains are linked to each other by the chains. 3 PYROLYSIS A N D STRUCTURAL EVOLUTION Polysilazanes with various molecular structures were pyrolysed in order to compare the structural evolution. The samples were heated in a thermobalance with a linear temperature increase (750°C h-t) in nitrogen. The volatile species were identified by IR and the pyrolysis products analysed by IR, scanning electron microscopy (JEOL JSM 35) and SAXS. The weight loss (6 W/Wo) curves consist of three stages (Fig. 6). The first

2O

It)

rbt

~0

6O i, o

Fig. 6,

(al

T ('C) 2 0

z,O0

6 0

aoo

1000

1200

Weight loss of different polymers versus temperature (polymerisation conditions: 320"C. 12h), with (a) 1%, (b) 2.5% and (c) 5% KOCH 3

14

,r7 Ser:eix. P. Gol¢rsa r, .4. Lcco*.",'e, .I. De,,~o

,;

ial

(b)

(c)

{d)

(el

If!

Fig. 7. Morphology of pyrolysis products vcrsus catalyst content. Polymerisation conditions: 360-C, 12 h. v,'ith (a) I°:a, (b) 2"5%, it) 5% and {d) 10% KOCH3; (e) 320 C, 12 h, 2% KOCH3; (f) 36OC. 12h. 2"5% KOCH 3.

Pyrolysis of polysila'_anes

15

starts at about 220°C, near the boiling point of the OMCTS, and ends at about 450-500°C. The IR analysis of the evolved gas indicates methane and traces of ammonia which disappear with increasing temperature. The thermal decomposition kinetics and the weight loss decrease when the proportion of high molecular weight species is large. During the second stage, between 500 and 750°C, the yields are very similar in spite of differences in molecular weight distribution. Finally, between 750 and 1250°C, the pyrolysis products are stable. The varying ceramic yields are mainly related to the extent of the first stage, i.e. the amount of oligomers, and not to different molecular structures. To confirm this assumption we have prepared polysilazanes at reflux (360°C) in flowing nitrogen to remove the oligomers. Even though the silazane structures are different (amount of cycles or chains), the Si-N-C frame is preserved and the yields at 1200°C are similar (see Table 2). TABLE2 Catalyst content, KOCH 3 (wt %) Weight loss (%) at 1200~C

1 21

2"5 20

5 22

The micrographs (Fig. 7) illustrate the r61e of the molecular weight distribution on the microstructure of the silicon carbo-nitride. The reactions in the bulk of the precursor induce significant changes in the morphology. For the least polymerised samples the gas evolution creates pores and produces a bloating of the material during the progressive change from the viscous to the solid state (Fig. 7(e)). On the other hand, the highly polymerised silazanes retain their shape. By a combination of a slow temperature increase (10°C h-1) and a slight weight loss to reduce the effect of gas diffusion we obtain fairly compact materials (Fig. 7(f)). Two sets of polymers were prepared using different conditions and heated in nitrogen. The pyrolysis products at different temperatures were analysed by IR spectrometry. The development of the absorption spectra confirms the three stages (Fig. 8). The band intensities are not modified up to 450-500°C. The departure and the decomposition of oligomers do not destroy the structure of the macromolecules and the architecture is preserved. During the second stage (T> 500°C) the decomposition of the precursor is linked to the departure of organic groups. The intensity of Si-CH 3 and N-H bands decreases progressively. Only broad absorption bands due to Si-N-Si and Si-C-Si remain. The ceramic products appear to be amorphous with a distribution of bond lengths. The polymers are ground, pressed into thin slices (0-5 mm thickness) and then heated in nitrogen at a slow rate (5°C h-t) to avoid bloating. Results

E Sirieix, P. Goursat, A. Lecomte,

16

1

i

r

;

Dauger

A.

v

1

]

/

/

/

,

////v,

N

~000

Fig. 8.

3~00

2/..00

1900

I~0

850

1100

650

400

IR spectra of pyrolysis products: (a) 350C, (b] 450~C, (c) 550~C, (d) 800C. Polymerisation conditions: 360~C, 12h, 2-5% KOCH 3. 6.9

I :

Rel. Int. (103) z

.:- 58B" C =

988 ° C

1258" E : :

3.4S cz

8

Fig. 9.

Z'; "n..

I B, 39

B. 78

SAXS profiles for products heated at various temperatures; polymerisation conditions: 360°C, 12 h, 2.5% KOCH 3.

Pyrolysis of polysilazanes

17

TABLE 3 Temperature (~C)

Catalyst c'ot'llenl KOCH 3 (wt%)

- 20

RG

1(0)

(nm) 1

2"5 5

3"5 5"4 1

319 466 106

400

d

Rc

[nm)

(rim)

1"17 I" 15 1"2

7" I 9"1

/(0)

2 416 12 487

500

d

Rc

(nm)

Into)

1"23 1"47

8"4 8"5 9'1

I(0)

1 148 I 289 4 506

700

d

RC

(nm)

(nm)

1"31 1"29 1"47

9"3 8'7

1250

I(0)

RG

/(0)

(rim) 1 089 3 625

9'2 8"4 7'5

561 469 3 942

deduced from SAXS experiments (Fig. 9) are summarised in Table 3. The interference effects disappear at temperatures above 500°C, while the scattered intensities increase significantly. The electronic density contrast between particles and matrix is more pronounced after the departure of oligomers, indicating that interparticular porosity is formed progressively. Moreover, the Bragg ring is shifted with the increase of temperature. At temperatures higher than 500°C the order is lost, an effect starting at lower temperatures for the least polymerised samples. Assuming that the particles consist of stacked platelets or folded macromolecules, then the evolution of oligomers or entangled molecules from the ordered particles would induce a bloating of the structure and an increase of the stacking distance, d. This behaviour agrees with a weakening of the Bragg ring at lower temperatures for compounds with a higher oligomer content. For temperatures above 500°C the breaking of Si-CH3 and N-H bonds of macromolecules is followed by the diffusion of gaseous species (CH4, H2) which distorts the stacking order without destroying the particles. The radius of gyration increases up to 500°C reaching similar values for all samples. The maximum in the scattered intensity distribution is more pronounced when the precursor oligomer content is higher. During mineralisation the particles grow through a polycondensationaggregation mechanism in the presence of a viscous phase. Thus a densification of the material occurs. Above 750-800°C, since the oligomers are dissociated and the methyl groups eliminated, the architecture remains rather stable until 1250°C. Silicon nitride crystals could only be detected after several hours at 1400°C. The first crystallite formation is associated with a slight weight loss (Table 4). For polymers prepared with various catalyst contents the crystallisation kinetics evidence the effect of the molecular weight distribution on the atomic rearrangement process. The annealing time needed for crystallisation is larger when the mean size of the preceramic macromolecules is smaller. The pyrolysis products only exhibit X-ray diffraction when some growth of the microdomains and some reorganisation of the precursor induced network have taken place. As CHz groups and hydrogen are evolving, the Si-N-Si, Si-C-Si or

E Sirieix, P. Goursat, A. Lecomte, A. Dauger

18

TABLE 4

Temperature ('-C)

Annealing time (h)

Weight loss ( % i above 1200:C

Products

1 200 1 300 1 400 1 400 1 400 1 400

1 1 1 7 19 43

--0-1 1-5 1.8

Amorphous Amorphous Amorphous ~-Si3N a traces ~-Si3N ~ ~-Si3N~ and/3-Si3N~ traces

Si-O-Si bonds build up a three-dimensional structure made of randomly distributed tetrahedrons with mixed compositions. Therefore particles generate Si-N rich microdomains separated from each other by C, Si-C and SiO rich areas. Such a network is non-stoichiometric but stable; atomic diffusion is hindered up to 1400:C. After the evolution of volatile species such as H z, SiO or CO, 9 the atomic rearrangement begins when the composition has locally reached the Si3N 4 stoichiometry. This is the reason why precursors having a low average molecular weight and a Si-N threedimensional network with a high heterogeneity content exhibit a slow crystallisation rate.

4 CONCLUSION The aim of this work was to establish relationships between the molecular architecture of the precursors and the microstructure of the pyrolysis products in order to select the proper synthesis route for the polymers. The chosen m o n o m e r was octamethylcyclotetrasilazane. Its catalysed polycondensation arises from two mechanisms, viz. breaking of Si-C bonds with formation of /

Si

Si--N \ Si groups and methane departure or scission of Si-N linkages and ammonia evolution. The polycondensation parameters were studied in order to promote the first reaction. Small angle X-ray scattering and diffraction experiments indicated a structural organisation of the macromolecules in high electronic density microdomains, separated from each other by tow density areas. Moreover, some order was observed inside these 4 - 1 0 n m diameter particles.

Pyrolysis of polysilazanes

19

The behaviour of the preceramic polymers has been characterised by several techniques: TGA, IR, SEM and X-ray analysis. During pyrolysis three typical steps occur with respective magnitudes depending on the molecular weight average of the precursor. The weight loss in the 200-450°C range results from vaporisation and decomposition of oligomers. The second step (450--750°C) is due to the departure of methyl groups. The ordered macromolecular stacking vanishes and the coalescence of the particles promotes an increase of the sample density. Finally, the ct/fl-Si3N,~ crystallisation, occurring at about 1400°C in the pyrolysis products, induces a slight weight loss. We have shown that the final density and crystallisation kinetics of the ceramic bodies are closely related to the molecular architecture of the precursors.

ACKNOWLEDGEMENTS The authors are grateful to Dr I. Baraton (Service Infra-Rouge, University Limoges) and Dr P. Lortholary (Service de Microscopie Electronique, University Limoges) for technical assistance.

REFERENCES 1. 2. 3. 4. 5. 6. 7.

Fink, W., Atomes, 25 (1970) 41-8. Andrianov, K. A., J. OrganometaL Chem., 3 (1965) 129-37. Seyferth, D., Mater. Sci. Res., 17 (1984) 263-9. Seyferth, D., J. Am. Ceram. Soc., 67 (1984) C132-3. Zdhanov, A. A. & Kotrelev, G. V., Polymer Sci. (USSR), 23 (1981) 1290-7. Sirieix, F. & Goursat, P., Rev. Int. Htes Temp, et Refract. (in press). Pouxviel, J. C., Boilot, J. C., Lecomte, A. & Dauger, A., J. Phys. (Paris), 48 (1987) 921-5. 8. Glatter, O. & Kratky, O. (eds), Small Angle X-ray Scattering. Academic Press, New York, 1982. 9. Mah, T., Hecht, N. L., McCullum, D. E., Hoenigham, J. R., Kim, H. M., Katz, A. P. & Lipsitt, M. A., J. Mater. Sci., 19 (1984) 1191-1201.