Structural Evolution of SiC From Polycarbosilane

7/26/2019 Structural Evolution of SiC From Polycarbosilane

1/8

J O U R N A L O F M A T E R I A L S S C I E N C E 2 5 I I 9 9 O ) 3 8 8 6 - 3 8 9 3

Structural

evolutions

rom

polycarbosilane

o

SiGceramic

G . D .

S O R A R U - ,

L O R E N C EA B O N N E A U l ,. D . M A C K E N Z I E

Departmentof MaterialsScienceand Engineering,Universityof California,Los Angeles,California 90024. USA

The

pyrolysis rocess

f a

polycarbosilane

nto

a

microcrystalline

iliconcarbideceramic

has

been ol lowedup to 1700"C mainlyby means f sol idstate

sSi

and

13C

nuclear

magnetic

resonance,ransmission lectronmicroscopy nd X-ray diffractionanalysis. structuralmodel

has

been

proposed

or the amorphous iliconcarbide

phase

hat is formedduring he

pyrolysis

process.

he ceramic btained t

h igh

emperature

s formed

by a

mixture

f

p-SiC

and a-SiC;

however,

ome difficulties

n

the

identification

f the crystalline

hases

have

been

pointed

out.

1. lnt roduct ion

Recently, t has beenshown hat non-oxideceramics

suchascarbides nd nitridescanbe obtainedby

firing

suitablemetal-organic

olymer

precursors

n con-

trolled atmospheres

].

A common feature of the

polymer

oute o carbides r nitrides s the formation

of intermediates hich are amorphoussolids.These

are formed after the removal of the organic com-

ponents

and before crystallization

[2].

We have

already

pointed

out the mportance

f regarding hese

amorphouscovalent

ceramics

ACC)

as an entirely

new family of disordered olidswhosestructureand

propertiesareworthy of independent tudy

3,

4].

Among the

various

systemsalready synthesized,

SiC obtained

from

polycarbosilane

s certainly the

most widely

studied

[5-7].

Commercially available

Nicalon

SiC

ibres

are obtained

rom

polycarbosilane

following the

process

irst developed y

Yajrma

et al.

[8].

The feasibilityof this method

n

producing

ibres

or coatings

s not restricted o this material. It is

mainly due to the

polymeric

nature of the ceramic

precursors,

and it accounts or the many research

efforts hat arecurrentlybeingconducted.

Despite he

increasing umberofstudieson SiC

ibres

obtained

by

the Yajimaprocess,he structural onversionrom the

starting

polycarbosilane

o the resulting ceramic

matedal

is not well

understood.

n this

paper

the

pyrolysis

mechanism f

polycarbosilane,

he structure

ofthe

intermediate CC

phase

nd

ts conversionnto

the microcrystalline ilicon carbideceramics,

will

be

discussedmainly basedon

2eSi

and

r3C

magic-angle

spinning uclearmagnetic esonance

MAS-NMR),

electron spin

resonance

ESR),

X-ray diffraction

(XRD)

and ransmission lectron

microscopy/selected

areaelectrondiffraction

(TEM/SAED)

experiments.

2. Exper imental

procedure

Commercially

available

polycarbosilane

PC,

Dow

CorningX9-6348)

with

a

molecular eightof 1400,

was used n this study.All the firing

treatmentswere

performed

n flowing argon with a heating rate of

2'C min

I

up

to the complete

emoval

of the organic

components

t around8400C.

The resulting norganic

solid, that is amorphous according to XRD and

TEM/SAED experiments,can be considered as

the ACC

phase precursor

or

SiC

microcrystalline

ceramics.n order o study he subsequentensification

and crystallization

process,

he ACC

phase

was

fired at different temperatures

up to

1700'C

al

l0'Cmin

I.

The

amountsof silicon,carbon and

hydrogenwere

analysed

or

selected amples.Oxygen

contentwasnot analysed. hermogravimetricnalysis

was

performed

n flowing argonusinga PerkinElmer

equipment.

esi,rC

and

H

liquid NMR

spectra

were

recorded n a AM 360Brukerspectrometerl. '71.5,

90.5 and 360

MHz, respectively.

he

polymer

wa s

dissolvedn CDC!. For

"Si

NMR experiments,

pulsewidth

of l0

p

secwas applied with a relaxation

delayof 6 sec. H and

rrC

NMR spectrawere ecorded

wi th

pulsewidths

f24sec orrH and

5gsec

or

|C.

and delays etween

ulses

f

I

sec

or

I

H

and

2

sec

or

'rC.

Solid-state

esi

and

r3C

NMR

spectra were

obtainedon a

MSL

300 Bruker spectrometer t 59.6

and75.5MHz. A pulsewidth f 2.5p secand a delay

between

ulses

of 60sec

were

used for the

zosi

MAS-NMR spectra. contact ime of 2mse c was

appliedor thecross-polarization

xperimenrs.

'C

CP

MAS-NMR

spectra

were

ecordedwith a contact ime

of 3 m sec.Tetramethylsilane

TMS)

was used as a

reference or all the NMR data. ESR expedments

were carried out on a Varian E09

spectrometer.

Bruker

gaussmeter

asused o measure

he

magnetic

field with

diphenylpicrylhydrazyl

DPPH)

as a stan-

dard. The numberof spinswas estimated y com-

parison

with copper sulphate

as

reference.XRD

pattems

were recorded

on a Philips diffractometer

usinga Cu,(c radiationwith a

nickel ilter. For TEM

observations he sampleswere

ground

to very fine

*

Permanent ddress. ipartimentodi Ingegneria, nivrsitdi Trento, 38050Mesiano,Trento, talia.

I

Permanent ddress:Chimiede la Matire

Condense.

nivelsilParis6. Tour 54. 4

olace

Jussieu. 5005Paris.

France.

3886

0022-2461/q001.00

.12

O

1990

Chapman

d Hall Ltd.


2/8

50

0

-50

Chemical

shift

{g.o.m.}

Frgrre 1

(a)

?'Si

MAS-NMR and

O),,Si

Cp MAS-NMR

sr,ctra f

polycarbosilane.

powders

which

were then dispersed

on to a TEM

copper grid using an eye dropper. TEM/SAED

investigations

were

performed

using a Jeol

STEM

100

CX equipment. Density measurements

were

performed

on fine

powders

by immersion n

CClo

following

the Archimedesmethod.

BET measure-

ments ryere

one with a Flow

Sorb

II 2300

Micro-

meritics

equipment.

3. Results

3.1 Characterization

f the

polymer

orecursor

The chernical

analysisof

PC, reported n

Table I,

shows Si :C:H rat ioof l :2.2:5.bs i MAS-NMR

and

2esi

CP MAS-NMR

spectrahave

been ecorded

on this starting material

(Fig.

l).

The two spectra

reveal

he

same

eatures,

with an

enhancement f

the

resolution for

the

cross-polarizedspectrum. The

spectra show two different

silicon

units, already

reported n

the iterature

7].

The

peak

at

-

0.8

p.p.m.

is

due to silicon

atoms bonded o four

carbonatoms

(SiCa)

as type I units and

the second

peak

at

-

17.6p.p.m.s

due to type II

units n which the

silicon atoms are

surroundedby three

carbon atoms

and one hydrogen

atom

(SiCrH).

No

distinct

peaks

appeararound

-

35

p.p.m:

Si-Si bonds, f

they exist,

are thus not abundant n this starting PC

[7].

These

two typesof units

are shown

below.

CH '

I

CH"_Si_CH,

CH"

l

CH,

(r)

TABLE

I Atomic ratio

Si:C:H in

the

preculsor

and in some

fired

samples

Sample

PC

PC8,l0

PCl200

PCt500

cH.,

I

-si-cH,

I

H

OD

ffi

ffi

Chemicalshift

(

p.p.m.l

F8rle

2"Si,rH andr3C

NMR spectra f

polycarbosilane

issolved

in

CDClr.

PC

wasdissolved

n CDClr.

2esi,

H and

,C

NM R

spectra ere ecorded

n

solution

Fig.2).

The,,Si

NMR

spectrums

quite

similar o the

CP MAS-NMR

spectrum

on th

powder.

The ratio

between

he two

kindsof units (I and II) appearso be I :0.8.The

'H

NMR

spectrum

of PC shows

two regions for

the

resonance,rom

4 to

5p.p.m. due o

Si-H bonds

and

around0

p.p.m.

due o C-H

bonds.The ntegration

of

the

peaksgives

a value

of

11

or the

C-H/Si-H ratio.

In

the Si-H region,

everal

eaks

re

present

t 4.1,4.3

and 4.6p.p.m.

due to

differentSi-H

sites.The C-H

region

hows main

peak

at 0.17p.p.m.,

ue o CHj

groups,

and two

shoulderswith lower

chemical

hift

values,

at 0 and

0.5p.p.m. assigned,

espectively,

o

CH, and CH

groups.

The

'3C-{rH}

spectrum

of the

solution

shows a broad

peak

centred

on 3

p.p.m.

corresponding

o the aliphatic

carbon

atoms

present

in PC in CH3,CH, and CH units.Somesharppeaks

are superimposed,

ertainly

due o some

quite

mobile

units

nside he

polymer.

All the NMR

spectrandeed

have

broad

peaks,

and

0. 0.0.0

2. 2

1 .6

t.44

1.43

5

0.65

0.10

0.07

3887


3/8

no

resolution as

obtained speciallyor

rH

and

r3C

NMR experiments.

hisseemso ndicate

hata arge

distribution

f units s

present

n

this

polycarbosilane.

The

structure f

polycarbosilane

hat emergesrom

these esults

s morecomplicated

hana simple inear

chainof

type and I unitswith

a

I

:

I ratio

suchas

CH. CH

t t

-cH,

si cH, si-cH,-

l l

CH.. H

In thiscase he

chemical nalysis hould

be Si C : H

:

I :2.5:7. The low

carbon and hydrogen

contents n

the

studiedPC and the

presence

f CH units n the

'

H

NMR

spectrum suggest

hat some cross-linkngha s

already

occurred between he

chains. This has

already

been suggested y

Okamura et a/.

[9].

3.2.

Character izal ion

f he

pyrolysis

rocess

3.2.1 From the prccursorpolymer to the

ACC

phase

The

low molecular

weightcomponents

f the

poly-

carbosilane

ave been

previously

emoved

by melt-

ing

the

polymer

n flowing

nitrogen

gas.

Thermo-

gravmetric

nalysis

TGA)

performed

n this material

(PCD)

showed hat

the

weight

osses nd at

around

800'C.At this emperature

he

precursorolymer

as

been onvertednto

an inorganic olid

hat appea$

to

be amorphous y XRD

and TEM/SAED nvesti-

gations Fig.

3) .

According

to TGA

experiments,he

pyrolysis

processeading o the formation of the amorphous

slcon carbide

phase,

consists

of two stages: rom

300

o 500'C with a weight oss

of l37o

and

from

500 to 800'C with

a

further

weight oss

of

1270.

The PCD was heated

at 2'Cmin

'

in argon up

to

500"c

(PC500),

700'c

(PC700)

and

840.C

(PC840).

he

chemical nalysis

f PC840appeared

o

b e

S i : C : H :

l : 1 . 6 : 0 . 6 5

T a b l e

) .

E x c e s sa r b o n

TABLE II

"Si

MAS-NMR data for the

precursor

nd rhe

fircd samples

Sample Chemical hift

(p.p.m.)

Linewidth

p.p.m.)

Fgurc

3 TEM bright-field

micrograph with

SAED

pattems

of

polycarbosilane

yrolysed

t 8,10" .

was

hus

present

n

this

amorphous ilicon

carbide

phase.

The

"Si

MAS-NMR and

r3C

CP MAS-NMR

spectra ecorded

n these amples re shown

n Fig. 4

aswell as he specra

fthe

precursor

s eference.

he

"Si

MAS-NMR

spectrum f PC

shows he wo

peaks

assignedo SiCo nd

SiCrHunits. n thesample

red

at 500'C, the peakat l6p.p.m. assignedo the

SiC,H

units has disappeared.

he major

peak

at

0

p.p.m.

due o SiCa nits

appears lmostunchanged

with

onlya slight hift

ofthe

position

frhe

maximum

and a small ncrease

f the linewidth

(Table

II). By

increasing

he firing

temperature ver

500oC and up

to

840'C his rend s maintained:

he

peak

elated

o

the

SiCa nits s continuousl y

oving oward

ower

values

of chemical

shift typical of the

crystalline

silicon

carbide

phase

while ts Iine',vidth

s increasing

up to 700'C. The

evolution

i

rhe

esi

MAS-NMR

spectra as

alreadybeen explained

n detail in

a

previous tudy 3]. n the firststage f the pyrolysis

process!

up to 500"C, a

possible

reaction is

the

consumption f

Si-H

groups

and the formation

of

bridging Si-C

bonds between he

polycarbosilane

chains.Above

this temperature

he shift of the

peak

due o

SiCo nits eflects n ncrease

fthe connectivity

29si

laes-rulrR

13c

cp

uls-t'tl,,ta

*ito---*-t*

-Zo-:n-*-

Chemical hift

(p.p,m

FErre 4 Evolutionwith the firing

temperature f

DSi

MAS-NMR

and

''C

CP MAS-NMR spectra

f

polycarbosilane.

PC

PC500

PCTOO

PC84O

0 1 6

-

4. 5

-6 .3

-8 .0

l0

12.5

3888


4/8

Fur ESRspectrum

ecordedt room emperature

n PC840-

of the network by the formation

of Si C-Si bonds.

At the same time

the evolution of t he linewidth is

correlated

to an increase of the disorder

of the local

environment

around the silicon atoms. During

th e

pyrolysis

process,

condensation reactions lead

to

the

consumption of CH., and

CH,

groups

and to the

formation of CH or C u nits with the evolution of H,

or CH"

[7].

These reactons

obviously increase he

number

of different SiC, units that can exist in

th e

material

and

may account for the observed ncreaseof

the

peak

linewidth already mentioned.

"Si

MAS-NMR

study of the fired samples shows no other

peaks

than

thoseassignedo SiCa nits. No

Si-Si

nor

Si O bonds

seem o be ormed

during the

pyrolysisprocess.

Information

about the evolution of the local

environment

of the carbon sites during the

pyrolysis

process

can be obtained from the

rrC

CP

MAS-NMR

spectra n Fig. 4.

The spectrumof PC shows

one

peak

at 4.2p.p.m. and a small absorption near 55p.p.m.

that has been dentified

as a spinning side band. By

increasing he firing

temperature he main

peak

shifts

toward

higher values of chemical shift with a cor-

responding increase

of its line\,/idth. The resonance

present

at 4.2p.p.m. n

the spectrumof PC is due to

the cont r ibu t ion f a l l the

carbon

groupspresenr

n

the starting

polymer,

namely

CH,, CH, and CH. It is

wel known

that the chemical shifts of

CH,, units

increase

when the r value s decreased. hen,

accord-

ing

to thesedata, the shift

of the

peak

with the f ir ing

temperature reflects

a co nsumption of the more

hydrogenated

species due to the occurrence of

the

condensation eactionsduring the pyrolysisprocess.

The

ncreasen the i newidthhas o

be

related

with the

same evolution in the

resi

MAS-NMR

spectra. It

reveals

n

increasen

the disorderofthe Ioca

environ-

ment ofthe aliphatic

carbon atoms during the

pyrolysis

process.

Moreover,

an

interesting

eatureappearsat

above 700'C: new

peaks

are

present

n

the

100

to

200p.p.m.

range.These

peaks

become

more ntense

t

840'C. The broad

peak

centred around 135p.p.m.,

can

be assigned o the

presence

f aromatic

carbon

atoms.and

could be

related

o th e formation of

C:C

bonds. t has alreadybeen

suggestedhat suchbonds

could be present n the ntermediate morphousphase

but

no

experimental videncewas

given.

They

should

be

precursor

bonds

or

graphitic

carbon hat is formed

around

1200'C

according o Raman

data

[9].

ESR was

perormed

on the

sample fired at 840" C

r '- '.1

'.

u- l.,.tn\,

," /

r..,2" ',.- lr

i ,

, L .

S t

I

,').,-'./"'t.-i.

I

.r'"

-c '

|

|

I

s l - " \

,

- l

- s r - c \

/ s i -

)sr..

/

r,-cr

I

-cH-t.

i' i,-

cfl7.r

./

_s f

si-uc/

\

/ \

Fr,g/fe Proposedtructure

ftheamorphousilcon arbide

hase

obtainedrom

polycarbosilane-

(Fig.

5).

The

spectrum xhibitsa single sotropic

signal

centredat

g

:

2.0030,with

a

linewidth

of 3.9G. This

valuecorresponds

o carbon danglingbonds

0].

Th e

integration

of the signal

gave

the number

of defect:

to

be

2.5 x

lOrecm

I,

considering

a densit,v o

2.2gcm-l for

this sample.This value s in

agreemenL

with the number

of defect s ound in amorphous

SiC,

samples

prepared

rom chemical vapour

deposition

l l0 l .

3.2.2.

Characterization

f the ACC

phase

The

proposed

structure

of the amorphous

silicon

carbide

phase

obtained

at 840'C is il lustrated

in

Fig.

6.

It is

basedmainly on rhe

2esi

MAS-NMR and

rC

CP

MAS-NMR

resultsalthough

he broadness

f

the NMR

peaks

n these

samplesmade it diflculr

to

obtain

precise

structural data. These

amorphous

phases

how

a

wide

distribution of

siliconand carbon

atoms types. However, some relevant

features

can be

pointed

out.

(a)

The silicon

carbide

phase

is not

stoichiometric,but an excessof carbon is present

(C/Si

:

1.6).

(b)

All

the sili con atoms

seem to be

bonded to four carbon

atoms and Si Si or Si-H

bonds, f they

are

present

hould

be

minimal.

(c)

Some

C:C bonds

are

present

sclearly

shown by the

rrC

CP

MAS-NMR

experiments.

d)

The residual

hydrogen

content

H/Si

:

0.65)should

be mainly

present

n

th e

structure

as CH

groups

as suggested

by the chemical

shift of the main

peak

in

the

rrc

Cp MAS-NMR

spectra.

(e)

The

condensation reactions

occurdng

during the

pyrolysisprocess

an ead

o the formation

of

six-member ings like

those

present

n

crystal l ine

SiC,but

the

ormation

ofdistorted

ive-or seven-aton

rings cannot be ruled out and should lead to the

presence

of C{

bonds.

(f)

The

presence

of

para-

magnetic defectshas

been shown

by

ESR

experiments

and

these defects were

assigned to carbon

dangling

bonds.

A

density of 2.21gcm

r

has

been measured

on a

sample of fine

powders

of this

phase.

This

value is

lower

than the

density of

p-SiC

(3.21gcm

r).

A

theoretical density

(2.70gcm

r)

can be estimated

from the

chemical nalysis

fPC840

(Table

)

by using

the rule

of

mixtures

and assuming

hat al l

the silicon

atoms are engaged n forming

amorphous

SiC

(density

3.0gcm

r)

[tl]while the remainingcarbon s present

as

graphite density

2.2gcm

3;.

The low

value

of the

density

of the amorphous

phase

annot be

ascribed o

the

presence

f

porosity

n

the material.

SEM investi-

gations (Cambridge

Sterescan)have

shown only few

3889


5/8

10000c

9500c

9000c

o 2 4

6 I 1 0 1 2

Time

(h )

Figwe 7 Etoltion of the

density,

p,

of the ACC

phase

with the

lring

time.

isolated

pores

for

the

ACC

phase

while TEM studies

(Fig.

3)

have

shown

no

porosity

at a sub-microscopic

level- This

evidence

s

also supported by BET analysis

that

gave

a

value

of surfacearea

ower

than I m' g-

|

lor

the samesample.

Therefore,

a

quite

open structure,

with a l arge amount of free volume must be invoked

to account for the low value of density

of

the

amor-

phous

ceramics. With the aim of following

the

evolution

of

the

density of

PC840

as a function of

the firing time, isohermal treatments in

an argon

atmosphere

at 900, 950 and

1000'C

were

performed

and the results epo rted n Fig .

7. An activa tionenergy

of 82 kcal mol

I

was obtained rom the nitial slopeof

the densilcatio n urves

Fig.

8).

Assuming

a negligible

value ol

porosity

in

PC840, he observed

ncrease

f

densityduring the firing treatment

must

be

primarily

ascribed o a reduct ion of its free volume. Two main

processesmay account for this effect. (i) The progress

of the condensation

eactions

between esidual CH

groups

n the structurewith the elimination

ofH, and

CHo and the formation of new

Si-C Si bridges with a

consequentncrease

fthe crosslinkingofthe network.

Actually, chemical

analysis f the

ACC

phase

ired at

950 and 1000"C for

the

longest

t imes, showed a

decrease

f the

hydrogen

down to H/Si

:

0.2 com-

pared

to the initial value of H/Si

:

0.65;

ii)

a

rear-

rangement

of the open amorphous covalent structure

toward more

compact configurations with no

change

in

chemical composition. For both

of these mech-

anisrns, key stepshould be the

cleavage f chemical

bonds.either C-H

(99

kcal mol

r)

in

the

first

caseor

S i -C

(T6kca lmol

' )

and C C

(82kca lmol

r )

[2 ]

i n

the

atter one.The obtained alueofactivation energy

suggestshat, at

least n

the

nitial

stages,

he densifi-

cation

process

occurs via the cleavage f chemical

bonds

present

n the material.On the

ground

of

' esi

MAS-NMR experiments,t hasalreadybeen eported

in a

previous

study

[3]

that, during the isothermal

treatments, he rarrangement f the structure eads

to an orderingof the local environment f the

silicon toms.Moreover, t the highest emperature

(1000'C),

XRD

and

TEM/SAED investigations

showeda concomitant

eorganization

f the

network

also

n

the

medium rangewith the forrnation of

SiC

microcrystals

3] .

3.2.3. From

the

ACC

Dhase to

microcrystalline SiC

The amorphous

iliconcarbide

hase

an

beconverted

into

a

microcrystalline

eramic by firing it

at

high

temperatures.n

order o follow

such ransformation,

thePC840washeated !t10'Cmin-r

in argon low at

1000'c

PCl000),

200"

(PCl200),

500.c(pcls00)

and 1700'C (PCl700).XRD and "Si MAS-NMR

spectra recorded on these

samplesare shown in

Figs

9a and b, respectively,

ogetherwith the spectra

obtainedon a commerical

B-SiC

as reference.

In

the XRD

patterns Fig.9a),

broad

peaks,

or -

responding

o crystallineSiC

phase,

tart to appearat

1000'C

and sharpen

y

increasing

he firing tempera-

ture. The correspondng

rystallite sizes,evaluated

from the diffraction

resultsby usinga

peak

broaden-

ing

procedure,

re reported n Table III. The

micro-

structure

f thesample eated t 15000

, as

revealed

by

TEM investigations,

s

shown

n

Fig. 10.Although

a detailed nalysis f the crystalsizes rom TEM

micrographs asnot

performed,

he meancrystal

size

seems

o be slightly igher ompared

o XRD results.

This could be due o the fact that the crystallites ave

a range

of sizedistribution n the

sample nd the

largest

particles

are more readily

observed. Fine

porosity,

as revealed

y the white spots n the micro-

graph,

seems

o be

present

in

this sample. This

observation s in

agreementwith a recent

study

[

3]

that

showed he formation

of

porosity

n Nicalon

fibresafterannealing

t

14000

in argonatmospheres.

BET analysis esultedn

a valueoi surface

rea

ower

than m' g

I

indicatinghatclosed

orosity

spresent

in these amples.

The

position

of the diffraction lines in the fired

samplesndicateshat the microcrystall ine

hase

s

mainly

B-SiC.

n

the XRD

pattern

of the sample

heated

up to

1700"C

a small

shoulder

s

seenat

around20

:

34". t hasbeen

ssignedo a-SiC, ug-

gesting

that, at the highest

temperatures he

crystalline

hase

onsists

f a

mixture

of cubic

p-SiC

with traces

of the

hexagonal

orm

TABLE III

Sizes

f the

SiC microcrystalsn thc fired

poly-

carbosilane alculatedrom

the broadness f the main

peak

n the

diffractionpattern

Firing

temperature

"C)

^

2.4

-

2.3

f= 82

kco lmo t - r

4 . 1

1 0 4 1 R r

Fre 8

Arrhenius

plot

for the inital densificalion

ate.

3890

-1

4 . "

4 .2

. Q

1700

Crystalsize

nm)

1000 t 500

16 .0


6/8

Chemical

shift

(p.p.m.)

In the X-ray patterns

of the fired

samples a small

peak

around 20

:

26'is

clearly visible. ts intensi ty

reaches the maximum

value in the

sample heated at

1200'C

and

decreases ith further heating. t

could be

assigned

ither o the

(l

0 l) ref lect ion

f(-quartz or to

the

(002)

l ine of carbon. In the literature, he dis-

appearance

ofthis

peak

after a treatment with HF has

been reported

[7].

This

result has

been assumed o be

proof

for the existence

of crystalline

silica in the fired

ceramics. However,

in the

present

study, the

samples

were heatcd

in an inert

atmosphere

to avoid major

oxygen

contamination; moreover

SiOo

units should

give

rise ro a

peak

in

the

"Si

MAS-NMR

spectra

around

-

I l0 p.p.m.

Such

a

peak

s completely

bsent

in

the spectra. Therefore

it seems hat

the X-ray dif-

fraction

peak

af 20

:

26"

should be assigned

o the

presence

of small clusters

of

graphite

rather

than

crystal l i tes

f d-quartz.

The evolutionwith the firing temperature fthe' eSi

MAS-NMR

spectra s

shown in Fig.

9b.

The

main

peak

corresponding

o the SiC4

nits s shiftingdown-

field

approaching the value

corresponding

to the

crystalline form

(Table

IV). At the

same time, irs

linewidth is

decreasing suggesting

an ordering

of the

ocal environment

of the

sil icon atoms in

the SiCr

units. At 1500"C

some

structures tart to

appear n

this

peak

hat becomemore

evidentat 1700"

C. At this

temperature,

he MAS-NMR

spectrum eveals

hree

TABLE

IV

2'Si

MAS-NMR

dataof the ired

oolvcarbosilane

PC840

PC 000 PCr200 PCl500

PCt700

Figule

9 Evolution of

(a)

XRD

patterns

and

(b)

"Si

MAS-NMR

spectra of ACC

phase

during

the firing

processP-SiC

ampl

rovided

by

Superior raphite).

dist inct

eaks

t

-16.2, -20and -25 p.p.m.Sucha

spectrum as alreadybeen

published

n

the literature

[4,

l5]. In a first approximation,

he

main

peak

at

16.2

.p.m.

can

be assignedo

B-SiC

and the two

minor

peaks

ould be due o some

a

phase.

However,

this assignmentwill be discussedater in the dis-

cussion.

4. Discussion

and conclus ion

In the

pyrolysis

of

PC,

the removal of organiccom-

ponents

occur

via

condensation eactions beween

CH.,and CH,

groups

of the strting

polymer.

When

this

process

s completehe

polymer

hasbeen

onverted

into an amorphous

covalentceramic

ACC)

phase.

The

temperature t which the condensationeactions

end and the ACC is formed can be

obtained

rom

a

TGA experiment:t

canbe defined s he emperature

at which

he

weight osses

re complete nd the

curve

approachesconstant eight alue.However,his s

FBrre 10 TEM bright-fildmicrograph f PC

frredat 1500'C.

\ ,[ Pcl7oo

\--./q*JL_L

PC1500

\"*-/"..***--".-

\.-l\.*....-...,-

\-_-.r,r.*.,*.,.*"-

40

20

(des

PC1200

PC1000

Pc84o

i

"t*"*,/

200

10 0

Chemical

8

shift

(p.p.m.)

-

t6.2

-

16.2

20

-20

l 6

3891


7/8

not an absolute hreshold alue: t is reasonableo

think that

the emperaturef formationof the ACC

phase

s

dependentupon the heating rate and the

heating

atmosphere. or example,by

processing

he

PC in vacuum

or by using a lower heating rate it

shouldbe

possible

o

complete he

pyrolysis rocess

t

lower emperature

nd hus

t

shouldbe

possible

o

get

an amorphous norganic solid at lower temperature.

Moreover, he structure tself

and the compositionof

the orming norganicdisordered hase an be affected

by the same

processingarameters

.e. emperature,

heating ate and heating

atmosphere.

In the

present

study,

TGA

experiments btained

usinga

heating

ate of 2"Cmin

I

in flowingargon,

showed hat the weight osses nd at around

800oC.

Thus

according to these results the PC has been

converted

nto

the ACC

phase

or the subsequent

structuraland crystallization tudies y heating t with

the same

processing

parameters

at temperatures

slightly

higher

hen800'C, namely 40'C.

A

wide tange of different ypesof defectseemso

exist n thisamorphousil icon arbide hase sshown

in Fig. 6. NMR and ESR experiments

howed he

presence

f C:C bonds

and carbon dangling

bonds

respectively.

he

presence

f distorted

ive- or seven-

atom rings can

be reasonably

assumed

3];

excess

carbonand

residual ydrogen

C/Si

:

1.6;

H/Si

:

0.65)

havebeen vinced y

chemical nalysis.

hemical

and structural modificationsoccur n this

phase

by

increasing he temperature

ver 840'C.

Residual

hydrogen ontentand carbonexcess

re considerably

reduced

Table

)

at

1200'C

H/Si

:

0.1;C/Si

:

1.44)

and are sti l l decreasing t l500oc

(H/Si:0.07;

C/Si

:

1.43) ue mainly

o the completion f con-

densationreactions.The other major modification

occurring uring he iring

process

f theACC

phase

is its structural rearrangement

hat

leads

to the

formation

of a

microcrystalline

eramic.This trans-

formaton

tarts, ccordingo XRD andTEM/SAED

experiments,

t around

1000'C.

t is known

16]

hat

the crystallizationmechanism f disordered ovalent

four-coordinatedmaterials ike

amorphous iliconor

germanium

nvolves he ruptureofthe

Si-Si or Ge-Ge

bonds, espectively.n these

ases he experimentally

observed ctivationenergies re close o the covalent

bond energies or both silicon

!7]

and

germanium

[8]. In thepresent ase,he crystallizationmechanism

ofthe amorphous ilicon arbide

hase

houldbe more

complicated because

t

occurs together with the

mentioned hemicalmodificationof the system.The

crystallization f the ACC

phase

esults n an ncrease

of

its

density.

Kinetic

studies of the densification

process

n the early stagesof crystallization,below

1000'C,

gave

an activation nergy lose o the energies

of the

Si-C

and

C-C bonds.

However,t is not

possible

to

regard

this value as the activation energy lor

crystallizationbecause he

observeddensification s

due not only

to crystallizationbut also to the con-

comitant ompletion f the condensationrocess.

At 1200'C

crystallites f SiC with dimensio n f

2.5nm are

present

n

the

material

Table

II); brighr

fieldTEM

observations erestill featurelessike those

obtained on PC840. However, he

diffraction rings

3892

Chemical

hi f t

(p.p.m.)

Fgrle / 1

Coparison betweenhe

zesi

MAS-NMR

spectra f the

sampleired

at 1700'Canda commerical

-SiC

(Suprior

raphite).

were

quite

narrow,

confirming the microcrystalline

nature of this sample.As suggested y X-ray dif-

fraction

pattems,

lusters f

graphite

re

present

n

the material.As

already

eported

9]

they should

be

present

t the edgeof

SiC

microcrystals

nd could

play

an

important

role

preventing

r slowing he rate

of crystal

rowth.

At 1500 nd 1700"Checr ls ta l

ize

ncrease

p Lo

8 and 16nm, espectively

Table

ll). As reported

n

the

iterature

7]

his

process

houldbeconnected ith

an evolution of CO from the system. n the

present

case, owever,fthis reactionakes

lace,

t, should

e

minimal concerningonly the oxygen

present

n

the

system s mpurities:ndeed,n the firing

process

f

PC care has

been aken to avoid

major

oxygencon-

taminaion.

The

density fthe sample

ired

at 1500o approaches

Lhe

value

of

2.7gcm-' .

The thcoret ical ensi t l .

calculated s

previously

described rom the chemical

analysis nd using

n

this case he density f

B-SiC

(3.2gcm

).

is2.9gcm

.

Thedi f ferenceetweenhe

two values an be ascribedo a

possible

lose

porosity

present

n the sample s evealed

y

TEM

observations.

A value

ol 77o of

porosity

shouldaccount or the

observed ifference

n

the densityvalues.

According o XRD analysis

-SiC

eemso be the

principalcrystallinephasen the samplesired at the

highest

emperaturesogetherwith small amountsof

the hexagonalorm. The

presence

fa-SiC in Nicalon

fibresheated t temperaures igher

han

1400'C has

already een eportedn the iterature

13].

However,

due to the broadnessof the diffraction

peaks,

a

definitiveassignment eems ificult. Many

different

polytypes

fthe

hexagonal

hase

re

known,

differing

from each

other only

in

the stackingsequence f the

sil icon nd carbon ayers

20].

"Si

MAS-NMR has beensuccesslullypplied o

distinguish between the different SiC

polytypes

4. 21.1.n Fig. l l a comparisonerweenhe

-"Si

MAS-NMR spectrum f the sampleiredat 1700'C

and that recordedon commerical

P-SiC

s reported.

The NMR spectrum of SiC from

polycarbosilane

shows hree distinct

peaks

at

-16.2, -20

and

80

40

0

-40 -B0


8/8

TABLE V

Chemical hifts of siliconcarbide

polytypes

Sample

Chemical hift

(p.p.m.,

silicon

siteswith slight

differencesn

the Si-C bond-

lengths.

his s

only an assumption,

nd he dentif i-

cation of

the crystalline

phase

formed

during the

pyrolysis rocess

fPC

seems orthy

offurther studies.

Ackn ow ledg

eme nts

Mike

Jeckle

and Richard Lysse

are thanked or

their

respective

ontribution n

the MAS-NMR

and TEM/

SAED nvestigations.SF is alsoacknowledgedor

financial

upport fthis study,

Contract o.

DMR 87

063'79.

References

LL K. J. WYNNE and R. W. P.ICE,

Ann. Re\,.Mater.

Sci.

14

(1984\

291

2 .

B . A . BENDER, R. W. R ICE and

J .

R.

SPANN, " / .

Amet. Ceram.Soc.70

(1987)

C58.

3 .

G .

D.

SORARU, F . BABONNEAU and

J , D .

MACKENZIE.

J. Non-Cr))st.olrZr 06

1988)

56 .

4 .

G . D . SORARU, F . BABONNEAU

ANd J . D . MAC.

KENZIE,

"Procedings

of the 7th

International

Symposium n Ceramics

,

14

o

16

December 988,Bologna,

Italy, in

prcss.

5. S. YAJIMA et al.,J. Matet.

Scl.

13

1978)

569.

6 . Y . HASEGAWA,M. I IMURA

andS. YAJ IMA, i d 15

Reference

,-sic,

3c

a-SiC,6H

d-SiC, 5R

-

18 .3

-

18 .9

I2r]

Present

[2 t

12u

-

25

p.p.m.

whereas he commerical

one displays

just

one

peak

at l9

p.p.m.

A detailedNMR

study

of silicon

carbide

polytypes

has

been

published

by Hartman ef al.

l2ll.

Among

the large number

of known

polytypes,

t seemshat

only four types

of

silicon nvironment

xist,designated

, B, C and D

by these uthors.The

cubicsiliconcarbide

B phase

r

3C

polytype)

has only type A silicon

sites,while the

H or 15Rpolytypesexhibit three silicon sitesA, B

and C in relative ntensities

: I : I and I

:

2

:

2, respect-

ively. The

chemical shifts for these

polytypes

are

reported

n Table V. The

fype D site s more unusual:

it is the only

site

present

n the 2H

polytype,

but

no

NMR data seem

available.The chemical shift

has

been

predicted

o be

-31p.p.m.

[21].

The two

resonances

t

-20

and

-25p.p.m.

in thePC

sample

pyrolysed

at 1700'C,

can be assignedo the

presence

of type B

and C silicon units

of somea

phases.

he

third

componentdue to type A

units could lie under

the major

peak

at

-

16.2

.p.m.

The assignment

f

this

peak

s more complicated, ecausehe chemical

shift

does

not

correspondo the

usually eported alue

for

-SiC.

Thisvalue,

round l6

p.p.m.

as

already

been eported or

powdered

samples hat were sup-

posed

o be

P-SiC

4,

l5],

and also or

p-SiC

single

crystals

22.

nkrott

e/ a/.

[

5] found such

peak

n a

plasma

synthesized

iC sample.After annealing

his

material

above 1600'C under inert

atmosphere,he

expected

-SiC

peak

at l8.3p.p.rn. ppeared.

-SiC

has only type A

silicon units. However,

he chemical

shift s different rom

that oftype A units n

6H or l5R

polytypes

-13.9

and

-

14.9p.p.m.,

espectively).

Hatman et

al.

[21]

assignedhis

differenceo different

Si-C bondlengths:n p-SiC, he siliconsite has a full

tetrahedra

ymmetry,while

one

ong

and three

short

Si-C

distances re

present

n

the 6H

polytype.

The

anomalous

eak

around

l6

p.p.m.,

ancertainly e

assignedo typeA

units.The

shift compared o

the 3C

polytype

could

be due to a lower

symmetry

of the

and K. OKAMURA,

rr id l8

(1983)

HAYASHandM.

OMORt , Chcm.,? ! ! .

-

13 .9 )

- " - |

, r4.e

-

20.81

24.4)

(1980)

20.

7 - Y . HASEGAWA

3633.

8 . S , YAJ IMA, J ,

(1975)

31.

9. K.

OKAMURA et d1.,

"Ultrastructure

Processing f

Advaced

Ceramics", edited by J. D. Mackenzie

and

D. R.

Ulrich

(Wiley,

New York, 1988)

.

501.

i0. S. LIEDIKE

et al., J. Non-Crts.Solrzs 7198

1987)

1083.

l l .

J. LIPOWITZet al.,Adv.

Ceram.Matet.2

(1987)

2l .

12. T.

L. COTTRELL,

"The

Strength f ChemicalBonds"

(Butterworths,

ondon,

1958).

13 . R . CHAIM, A . HEUER and R. T . CHEN,

Ceran.

Soc.7l

(1988)

960.

14. c. L. MARSHALL

et al., Proc.

Symp. Sc.

(t987)

341

K. E . INKROTT,

S .

M. WHARRY

and

D. J .

O

DONNEL.

Mater.Res. oc.

Symp. roc.73

1986)

55 .

D. TURNBULL,

J. Non-Cr)st. o/rzs. 5

(1985)

9? .

L.

CZEPREGI et al., J. Appl. Phrs.

49

(1978)

3906.

L-

CZEPRECI et al.,

SolidStateCommun.21

19'7'l)

1019.

J. AYACHE

et al.,J. Mater.Srt.a?r. 7

(1988)

85.

v. HAASE

er a/.,

"Gmeln

Handbook

of Inorganic

Chemistry",

Springer,

Berlin,

Heidelberg,New York,

1984)

Supplement olume82,

Si-Silicon,

Propertes

of Crystalline

SiliconCarbide".

J.

S.

HARTMAN

et al., J. Amer.

Chen. Soc. 109

(198'7)

6059.

22.

J. R. GUTH and W. T.

PETRUSKEY. . Ph},s,

Chem.

9t

(1987)

361.

Received

4

May

and accepted

29 September

1989

Cerum. l4

1 5 .

6 .

1 7 .

8 .

19 .

20.

21.

3893

Structural Evolution of SiC From Polycarbosilane

Documents

Transcript of Structural Evolution of SiC From Polycarbosilane