Saxs and BET studies of aging and densification of silica aerogels

Journal of Non-Crystalline Solids 95 & 96 (I 987) North-Holland. Amslerdnn~

SAXS

AND BET STUDIES

Thierry

LOURS

Laboratory University France Aldo

and

of of

OF AGING

Jerzy

Science Montpellier

I I SI - I I XI

AND DENSIFICATIDN

OF SILICA

AEROGELS

ZARZYCKI of

Vitreous

Materials

II

CRAIEVICH

Laboratorio and Instituto Dayse

I.

National de Luz de Fisica/USP, DOS SANTOS and

Instituto Sao Carlos,

de

Sincrotron/CNPq, Sgo Paulo,

Michel

Campinas,

SP

Brazil.

A. AEGERTER

Fisica e Quimica SP, Brazil.

de

Sao

Carlos/USP,

Aerogels prepared by hypercritical drying of gels obtained by hydrolysis of TMDS/methanol 50% solution in acid and basic conditions were studied combining SAXS and BET methods. Diffraction results do not seem to reveal a fractal character as the PORDD's limit I(q) q4 + Cte was obtained in all cases without well characterised portions in log I(q) vs log q graphs. Aging produces a small increase in the density of the matrix. Densification studies indicate that for moderate temperatures 530"-810°C the poresshow a shrinkage due to a diffusion controlled mechanism with an activation energy AEsg Kcal/mole. For higher temperatures expansion due to bloating was observed. The density of the matrix undergoes only a slight increase.

1.

INTRODUCTION The

structure

tion

and

ther

evolution

te

stages The

and

aging

of

BET surface

this

gel-into-glass

angle

scattering

of

ments

introducing

the

study

2.

METHOD

The texture

that

of

cles" by

the

of the

the

the

GUINIER's

in

can

intensity

to

prove explore

be described the I(q)

can law

way In

might

between

(which

limiting

direct

order

aerogels

The v

are (SAXS)

these

influence

be either

electronic scattered material

:

OD22-3093/87/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

The

however,

more

characterising

interesting these

textural

recent

in

this

developrespect.

The

possibilities.

by a "two density by a diluted particles

understood.

conventional the

more

fur-

intermedia-

poorly

with

particular,

prepara-

their

detailed

still

together of

on the

dependent

that

treatments.

structures.

concepts

difference

pores.

volume

strongly

known

X-rays

a most gel

undertaken

of

Ape is

which

are

well

conversion of

provide

was

is

densification

intricate fractal

present

aerogels

it

thermal

analysis

features

of and

during

small

SAXS

texture

conditions

density" of

model the

matrix

system or

voids)

in and

of

N "partiis

given

1152 I(q) where

q

is

scattering

modulus

angle,

ration

of R

the

the

can

= NApe ' of

h the

the

(-

This

$ RE q21

scattering

vector,

X-ray wavelength

particle.

be obtained

v2 exp

and

approximation

from

SAXS

q = 4n

R6 the is

valid

measurements

(sinBl/x,

with

(electronic) for

28

radius

q R6 <<

on a relative

of

the gy-

1.

scale

from

log

I(q)

vs q 5 plots. For

a polydispersed

tion

a The

of

system

heavily

intensity

a power

this

weighted

procedure

towards

scattered

at

law

using

log

I(q)

the

to

has

been

shown

"2

that

particles

higher

angles

(q

vs

q plots

log

s

an

RD > 1)

average

can

be

radius

of

analysed

gyra-

in

terms

:

q-a

a can

be related

fracta2

to

characteristics

of

:

* for

"mass

* for

"surface

For

particles

the

classic

In this the

to

particles.

I(q) It

leads

larger

fractals" fractals" having

of

for

the

dimension

1<0<3,u=D

of

2 < DS < 3,a=

dimension

a smooth

PDRDD's

case,

area

of

law

:

lim

q,

1 I(q)

a system

interface

surface,

of

per

Q,

is

the

phases

s V

integrated

and

-t cte

a =4 which

corresponds

= p

with

volume

S/V

volume

-.-= 1 &(I-$) where

q41

two

unit

DS = 2

6-DS

is

fractions

given

Q

by the

and

l-$

relation

:

P

9,

intensity m

9, =

I(q)

q2 dq.

f 0 which

serves From

to

obtained,

where

Combining obtained,

density of

zero

da

this thence

This a)

nonalise

model the

the

BET measurements

leaves

p = d,/$,the density.

is

value the

scattering

the the S' two

apparent with

due

remaining

I(q) of

SAXS

the

fraction

per

of

the

the

1-g

arbitrary mass

units.

S'=

S/Vd,

which corresponding

is

sample. K = @(l-a)

can

phases. for

porticks

in unit

parameter

coexisting

possibilities

soZid

expressed

interface

results of

two to

the density

fractions

us with is

intensities value

interpreting form

Ape the

fraction

to air-filled

: $ of pores

be

b)

the

scattering

remaining

"particles"

fraction

l-

P ' = d,/(l-$1 For K < 0.25 is

usually

values

3.

the

possible of

concept to

p and

OF THE SAMPLES were

prepared

in

(TMOS), bidistilled

water

constant

stirring.

Two series

NH40H(pH=9)

as

catalyser.

a few

minutes,

the

closed.

placed

in

tical

After

an

The red

the

The

samples

phase

has

and

$ , the

a density

b)

a meaning from

the

and

it

calculated

in

proportion

of

of

were

samples

were

bar

by

and

by hydrolysis

4 mole

(1

the

:l

into

were

added

under

HN03(pH=2)

tubes

and

herme-

opened

evacuation4.

reached

te-

To this

immediately

solvent

3OO'C

either

Pyrex

were

hypercritical

of

volume).

TMOS was

using

tubes

solutions in

H20/mole

prepared

transferred

tc=

of

methanol

by the

and

The cri-

addition

of

. 5'

their

of

the

apparent

destined

No additional

for

oxydation

residual

dry

aerogels

density

da

sintering

The

densification samples

faces.

with

da

or

organic

The both

obtained by

studies

in

this

way

was

measu-

Hg volumetry.

were

dried

without

hypercritically

was destined

Their ; it

of

and

treatments

water

followed for

studies

order

of

at

for 0.5

were

used

densification

were

adjusted

the

before

isothermally

SAXS

thickness,

was

chlorination

groups

cut

530°C. into

optimal

in

this

study

to

treatments. 66O"C, thin

X-ray

810°C

slices,

and

912°C.

polished

transmission,

on

varied

mm.

EXPERIMENTAL The

SAXS

(Orsay, priate

experiments

France). collimation

of

systems

The

using used

white

monochromator X =1.55

i.

type

mathematical

of data

"infinite

for

of

of

Si

Two sets

the the

a very

intense of

slits

define

synchrotron storage

slits

and fractal

ring

is

8 KeV. the

radiation ring)

monochromatic

for

possible

for

using positron

collimation

desmearing

storage

tuned of

out (DC-1

geometry

height"

testing

beam

carried source

provides This

cessity

results

were

The X-ray

cross-section.

in

fraction of

aging.

remove

4.

major a)

way

drying

200

surface

N2 BET and

previous

and

the

matrix

hypotheses

hasoccurred

autoclave.

specific

by

minor

occupy solid

dissolved

solutions

for p,=

to

in

gelation

autoclave

conditions

methanol

the

a classic

(Fluka)

solution

tically

the

between

Silica

After

which

to

of

choose

PREPARATION gels

pores

p13.

tramethoxysilane

or

are

$ corresponding

beam avoids

the which

may affect

beam

LURE

an

with

appro-

point-like

troublesome

effects the

ne-

inevitable precision

of

behaviour.

monochromatised The

the

slit-height

at

with

resulting cross-section

by a double X-ray

wavelength and

reduce

crystal was the

para-

sitic

scattering. A one-dimensional

red

X-ray

a standard

5.

This

study I gives

dbring

SAXS

curves

performed

the

variations

It

aging.

was used

detector

sample.

The

were

a slight

Fig.1

shows

I(q)

dent

for

vs

parasitic

determined

the

limiting

on

is

record

the

scattering

was

subtracted

a relative

scale.

to

present

in the

while,

are

scattein

the

basic

q

and

only

minor

as

4

a result The

interval to

samples

which

a constant

. This

indicates

Aging time

series

density S'

aging

that

plotted can for

time

approach

law

without

shown

on

coincivalues

a larger

progressively any

with

indeed, the

q

lol<

with aerogel

the

intermediate 4 could

values

be

loI

presents

characteristics

seem

> 4 a sharp

to

be

studied. the

same

da(gcme31

;

as

be made

lower

are

to

slope

occurs

no fractal

limiting

slope

TABLE

pH

with

curves

These

seen

PDROD's

slope

that

ncid

aging.

PORDD's

and

the

differences

are to

to the

apparent series

increases

different

of

curves

corresponding

seen effect 0 s 4 A scale

series

for

of

loI=

the

curves the

Fig.1.

and

acid-catalysed

series.

that

apparent

series. 5'

actually

seen

of

often

for

5'

both

catalvsed surface

be

q-

at

that

intensity

insert

an

be seen

series

base-

It can

The approach

interface

specific

SAXS

well-characterised

which

and

the

for

values

slope

ascribed.

acid-

of

increase

q.

i-')

in

can

the

log

higher

(q < 0.04

on

base-catalysed

shows

scale

was

for

log

For

the

AND DISCUSSION

decreases, da

All

from

Aging

Table da

way.

RESULTS 5-1

position-sensitive

intensity

S'(m'g-')

Ial

= 4 is

observed,

the

curves

I
p(gcms3)

p'(gcme3)

o

(days) 2

cl

0.39

465

61

5.0

0.47

0.18

2

7

0.445

440

48

4.5

0.56

0.20

2

14

0.45

413

46.5

5.3

0.55

0.18

2

21

0.47

353

45

5.6

0.57

0.18

9

0

0.17

380

129

3.6

0.18

0.05

9

8

0.18

423

102

2.5

0.19

0.07

9

16

0.19

621

99

2.0

0.21

0.09

show

however

2.37

respectively

fractal

for

particles

The

of

seen

the

calculated

absolute

values

glass

phase,

responding

or

are

not

the

scattering

for

acid

correspond

phase

or

to

pores

during

values

of

P

being

an

[al could

around

100

either characterised

o'

= 2.18,

2.43

correspond i

and

to

formed

plots

are

which

should

increase

of

shown

a

elementary

in

Table

correspond

by

slightly

phase

BET.

a

1. to

of

bl

St The aging

matrix

P'

for

cross-linking the

Table

due

1.

value

for

was

contains

The meso-

and

during the

in

limiting

hypothesis

light

or

by

contraction

included 2.2,

series, This

SAXS

of

also than

basic p'.

by

are

greater

the

increases

isotropic

GUINIER decreases

P and

density

units

the

the

much

detected

to

This

adopted

closed and

cor-

nanopores

macro-pores

values both

The solid

are

series.

while

still

to

a syneresis

in

are higher This

the

liquid

effect.

bJ,oq -17

4.8 4.0

-16

-15

-

3.2 r 22.4 0" A

-

1.6

-

0.8 -

o.oI -2.0

-1.7

-1.4

-1.1 Log,o

Scattered intensity shifted to make the I(q) variations for

curves high-q lower

It the

aging. 9,

for of

; p'

values

aging.

dimensions

from

systematically

decreasing

series

could

of

with

of

s 15 i.

obtained

a matrix

portion

16 days


of

to

8 and

radius

z

that of

The

linear

0,

aggregates of

values

be

filling-in

which

intermediate for

regime

smooth

can

an

-0.8

-0.5

9

FIGURE 1 for different aging portions co'incide. q values.

stages ; they are vertically The insert shows the detail

of

T. L.ours CI al. / Aging

1156

LOOO-

-A d ”

aerogels

3000-

I 0

-30-c 1 50

2 of

o/silica

-

2000 1

Variation

and dmsificarion



during

100

150 200 Slnlermg lime (mn)

FIGURE isothermal

2 sintering

250

for

300

temperatures

indicated

.?

0.60

t

. x

660-C BlO’C

.

912’C

oLk----150

Smlermg

Evolution

of

matrix

densities

p'

200 tme lmn)

FIGURE for

3 various

250

densification

temperatures

The

pore

fraction

contraction with

of

solid

a transient

phase

and

increase

then

consecutive

decreases

as

the

to

the

syneresis

initial

progresses

time. 5-2

SINTERING

The ned

log

for

I(q)

This

indicates for

during

this

Fig.2

thermal

the

is

at

an

to

observed

those

of

that

the

for and

of

tering

mechanism

about

matrix

indicate

Fig.1

previously

obtain-

and

time

is

process

be

smoothing

c

shrinkage

Kcal/mole

of

in

such

different

as

samples

be

seen

a linear

may be

gases

RG is

isthat

decrease

interpreted

as

- the

behaviour

closed

pores.

expansion

is

at It

is

noted

observed5.

constant

obtained.

for

can

pores

no

velocity

was

shows

the

of

of

the

It

This

oxydized

q. time

may differ

observed.

by expansion

of

of of

reasons.

810°C

pteviously

This

an

would

average

activation

confirm

an

initial

sin-

by OH migration6. shows

previous

surprisingly

small

processes reesterification

investigations - Work

of

shrinkage

densification

by

des

as would

further

values

technical

660

analysis

Further

Ministere

for

with

controlled

ACKNOWLEDGEMENT

sintering

undergo

as a function values

used

continuous

E%9

the

hindered

during

only

intermediate

t = 530,

samples

density

that

treatment.

for

initial

were

increase

Arrhenian

energy

obeyed

might

a2 plotted The

a bloating

1080°C

From

is

which

a diffusion-controlled

due

be noted

behaviour

of

batches

912°C of

912°C

is values

temperatures

indicative

are

to

treatment.

several

for

The

similar

interfaces

temperatures.

lower

even

are

POROD's

behaviour

shows

from

while

q plots

defined

densification

for

log

that

well

No fractal

sued

vs.

aging.

normal

to

$ shows

the

are

supported

Relations

in

the

of

surface

and

(Fig.31

temperature

which

range

layers

in

the

studied

might here

autoclave

progress.

by FAPESP,

Exterieures

variations

in

FINEP

CNRS

and

CNPq

(Brasill.

,s

(1984)

1383

the

(France).

REFERENCES 1) D.W.

Schaefer,

2)

K.D. C.J.

Keefer, Brinker,

K.D.

3)

A.F. Craievich, J. of Non Cryst.

Keefer,

Phys.

in Mat. Rev. Sot. O.E.Clark, D.R.

Zarzycki,

M.A. Sol.

4)

J.

5)

A.F. Craievich, D.I. presented at the 7" Materiais. Florianopolis,

6)

M. Prassas, in "Science

Aegerter, 96 (1986)

M. Prassas,

J. of

J.

Rev.

Symp. Ulrich D.I. 394

DOS Santos,

Phalippou,

DOS Santos, M.A. Congress0 Brasileiro SC (Dec.1986).

Phalippou, Ceramic

Letters

Proc.32 (1984) 1 eds. I%-th Holland.

J. Zarzycki, Processing",

J.

T.

Mater.Sci.

Woignier, 17

J.Zarzycki, (1982)

Aegerter, T. LoFs, de Engenharia e

p.156

Sintering (L.Hench,

of

monolithic D.Ulrich

3371

J. Zarzycki, Ciencia dos silica gels Eds.) Wiley 1986.