Kinetic studies of the formation of nanocrystalline chromium silicide-silicon carbide from homogeneous amorphous precursor powders

MATERIALS SCIENCE & ENGINEERING ELSEVIER

Materials Science and Engineering B31 (1995) 243-248

B

Kinetic studies of the formation of nanocrystalline chromium silicide-silicon carbide from homogeneous amorphous precursor powders P i n g L u o a,b,

Peter R. Strutt b, Owen F. Devereux c, Hemant K. Gupta b

aL-370, P.O. BOX 808, Lawrence Livermore National Laboratory, Livermore, CA 94550, USA bAdvanced Manufacturing Center for Precision Manufacturing, University of Connecticut, Longley Building, Storrs, CT06269, USA Clnstitute ofMaterials Science, U-136, University of Connecticut, Storrs, CT06269, USA Received 30 March 1994; in revised form 30 September 1994

Abstract The kinetics of formation of nanocrystalline chromium silicide and silicon carbide composites from homogeneous amorphous precursor powders has been investigated. This study was conducted in a dynamic argon atmosphere at 1 atm in the temperature range 900-1300 °C for time periods ranging from 3 min to 62 h. Two precursors with different concentration ratios of Cr, Si and C were examined. The phase percentages of the final composites, ( 1 ) hexagonal Cr~Si3C x and cubic fl-SiC and (2) tetragonal CrsSi 3 and cubic fl-SiC, vs. temperature and time were determined using X-ray diffraction. The relationship between the phase percentages of the crystalline material, the processing temperature and time follow the Avrami-Erofe'ev equation (J.D. Verhoeven, Fundamentals of Physical Metallurgy, Wiley, 1975). The activation energies have also been determined, and the formation of nanocrystalline chromium silicide and silicon carbide has been discussed.

Keywords: Self-catalytic formation; Intermetallic silicides; Nanocrystalline formation; Silicon carbide

1. Introduction 2. Experimental procedure The technological importance of nanostructured materials is becoming well recognized because of their enhanced mechanical properties, such as high strength, ultrahigh hardness, high fracture toughness and wear resistance [1,2]. Nanostructured intermetallics and ceramics (chromium silicide and silicon carbide composites) have been successfully synthesized using chemical synthesis routes which involve converting homogeneous precursors to nanocrystalline forms at high temperatures [3]. In order to control the particle size and the crystallinity of the final product, it is necessary to study the kinetics of formation of nanocrystalline material. As far as we know, no investigation on the kinetics of formation of nanocrystalline composites has been experimentally conducted using a homogeneous precursor.

A solution chemical synthesis route was selected for the synthesis of chromium silicide and silicon carbide nanocomposites. The approach involved the spray drying of water- soluble inorganic compounds to form an intermediate amorphous precursor, and the thermal chemical conversion of the intermediate precursor to form the final product material at an elevated temperature in a dynamic gas environment. Chromium chloride (CrC13"6H20), sodium methylsiliconate (CH3Si(ONa)3) with 30 wt.% water and dextrose (D-glucose, C6HI 206) were dissolved in distilled water to produce an aqueous solution; the solution was spray dried using compressed air and molar ratios of 1:4:2 and 1:2:2 were obtained. The resulting precursor powder was cyclone separated from the flowing gas stream. The system was

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Materials Science and Engineering B31 (1995) 243-248

cyclically evacuated to 10 3 Torr, flushed with argon gas several times and then back filled with argon gas to near ambient pressure. During this process, the system was maintained under an argon flow rate of 70 cm 3 rain-1 The kinetic study of the formation of chromium silicide and silicon carbide nanocomposites was conducted by investigating the phase percentages (concentration) of nanocrystalline chromium silicide and silicon carbide as a function of the reaction temperature and time. X-ray diffractometry was applied to analyze the phase percentages using an image analyzer to measure the integrated intensities of the peaks at the maximum intensities of both phases.

JL

k

'

/•

A i 20

m 30

~

.*

i 40

I

20

I

• , v "

V

V

I

I

I

60

CrsSi3 0 CrSi2 I • SiC * N a C l *

I

1:4:2

[I v

v

V

"

a

V I

~ V

i 50

1:2:2 0

125o c

1250% 5h~l

•

V

E 3. R e s u l t s and d i s c u s s i o n

I 20

I

I

I

30

3.1. Chromium silicide and silicon carbide nanocomposite powders The overall reaction for the formation of chromium silicide and silicon carbide was determined experimentally to be [4]

I

I

40

I

I

50

2(~

I

60

I ~ Ci~S&Cx+y I

Fig. I. X - r a y analysis of the precursor powder and the final composites.

CrC13 + CH3Si(ONa)3 + HCI + 02 + C6H1206

100 I 90 80 70 60

dry --* amorphous precursor (Cr, Si, C complex)

spray

+ NaC1 (crystalline, see Fig. 1 )

50 40

1250°C

30 20 10 0 200

--,- Cr~Si3Cx+ SiC + COt + NaClt + H20t The final products, chromium silicide and silicon carbide, were identified by X-ray analysis. No other crystalline phases, such as sodium chloride, were detected (Fig. 1 ). This is because sodium chloride has a melting point of 801 °C [5]. Above this temperature, sodium chloride melts and is carried out of the system under a continuous flow of argon gas. Fig. 2 shows the chemical analytical results of sodium vs. the processing temperature. The amount of sodium in the final composite is 2 wt.%. The investigation of the concentration distribution of chromium and silicon elements in the precursor powders (1:4: 2) was conducted by energy dispersive X-ray analysis (EDAX). The window, line and spot functions from scanning electron microscopy (SEM) were selected to analyze the precursor powders. The relative intensity ratios of Cr/Si vs. the examined areas of the powders are plotted in Fig. 3. A value of around 1.5 is maintained. This result shows that the precursor powder (1:4:2) has a relatively homogeneous concentration distribution of chromium and silicon elements. In a previous study [3], it was observed that spray dried

4;0

660 800 1000 Temperature (°C)

1200

1400

Fig. 2. Chemical analysis showing the d e c r e a s e in sodium c o n c e n t r a t i o n vs. p r o c e s s i n g t e m p e r a t u r e .

1.8

o 1.6

mmm

'- 1.4 m

>,

=

e-

•-

"mm''"

•

If

1.2

0.8

> 0.6 111

•"6 0.4 0.2 0

20 --) x4.13/~m z

40

60

Areas of agglomerated

80

100

particle

Fig. 3. Cr/Si intensity ratios vs. e x a m i n e d areas o f the p r e c u r s o r p o w d e r s by E D A X .

P. Luo et al. /

MaterialsScience and Engineering B31 (1995) 243-248

material (1:2:2) had a homogeneous concentration distribution compared with chemically precipitated material.

245

3 min

3.2. Calculation of the crystalline phase percentage ratio from X-ray diffraction analysis The relative crystalline phase percentages can be calculated by measuring the X-ray peak intensities, i.e. the area of the most intense peak for each phase. The most intense peak intensities were assumed to be proportional to the concentration of the respective crystalline phase [6] Isic = A Csic Icr,si,cx =

•

o

.

,

•

(1)

BCcrsSi,Cx

(2)

where A and B are constants for the composite, Is~c and IosSi3C x are the integrated intensities and Csic and CcrsSi3C x are the concentrations of the crystalline phases in the composite. The relative intensity ratio of silicon carbide, fsio is written as =

/sic

Is~c + Ic~,Si,Cx

(3)

Combining Eqs. ( 1 ), (2) and (3), we obtain fs,c-

1

1 + (BCc~,s~cx)/(ACs~c)

(4)

Eq. (4) shows the relationship between the silicon carbide relative intensity ratio and the crystalline percentage ratio of the composite phases. Fig. 4 shows the X-ray diffraction spectra vs. time collected on assynthesized samples (1:4:2) at 1250°C. Table 1 lists the measured areas under the most intense X-ray diffraction peaks (i.e. integrated intensities) and the ratios of the corresponding crystalline phases.

3720

30

rain

.

35

40

45

50

20

3.3. Determination of kinetic parameters using nonlinear regression method The powders are apparently strain free if no external forces are applied. Nanocrystalline powders are made up of single crystal grains with dimensions of several hundred angstroms. It can be assumed that strain-free nanocrystalline silicon carbide and chromium silicide grains form as a result of solid state reactions. The rate at which the volume will be transformed into nanocrystalline grains is a function of both the nucleation rate and the growth rate of these grains. When a reaction occurs by nucleation and growth of the product in the solid state, the Avrami-Erofe'ev (AE)

Fig. 4. X-ray spectra of chromium silicide and silicon carbide nanocomposite (1:4:2) crystallization vs. time at 1250°C, • , Cr5Si3Cx; o, SiC.

equation is frequently obeyed [7] Cs~c = Ms~c[ 1 - exp( - ksic t"s'c)]

(5)

CCrsSi3Cx = m c r s s i 3 C x [ 1 - exp( - kc~si,cxt" ......~)]

(6)

where Msi c and Mc~,s~,cx are the limiting concentrations of crystalline silicon carbide and chromium silicide as the time t goes to infinity. Both values are determined by the Cr/Si/C concentrations in the precursor

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Materials Science and Engineering B31 (1995) 243-248

Table 1 Calculation of the relative intensity ratio of silicon carbide and chromium silicide nanocrystalline phases from X-ray analysis t (min) 3 8 30 60 150 300 480 720 1080 3720

Area of the SiC peak

Area of the

2376 3105 1854 2082 5419 2979 4573 3839 7864 1917

2216 2247 1329 1549 3158 1982 5203 3293 8067 2042

fsic

1.00 fl, l --

I + P I-exp ( - k v t n" )

0.70

I - e x p (-k u t nu)

fCr,Si,(:x

O

Cr 5Si3C x peak

0.66

,._

0.52 0.58 0.58 0.57 0.63 0.6 0.47 0.54 0.49 0.48

O,,-, 0.33

0.48 0.42 0.42 0.43 0.37 0.4 0.53 0.46 0.51 0.52

0.10

I

I

413

926

I

1430

I

1960

I

2463

I

2976

I

3488

t (min) k,, =0.01986 k v =0.01992 n u =0.8668 n v =0.7060 P =1.0802

Accuracy: SumSqr = 0.009758

Fig. 5. Curve fitting using non-linear regression method to determine the kinetic parameters.

powders. T h e sum of Msi c and Mc,r,S~c~ is equal to unity. T h e parameters ks~c and kcr,s~c~ are the reaction constants in units of m i n - " for silicon carbide formation and chromium silicide formation; ns~c and ncr,s~cx are exponents and depend on the geometry and the growth process. Substitution of Eqs. (5) and (6) into Eq. (4) gives fsic

1 + (BMc,~si~c,)/(AMs~c)/{[1 - e x p ( -

kcrssi3cxtcrssi3Cx)]/[1

If P=BMcr,s~cx/AMsic and the SiC phase CrsSi3Cx phase are represented by u and v then

l+P{[1-exp(

_

kvt ,,v) ] / [ 1 - e x p ( - k u t n0)]}

-

(7)

e x p ( - ksict"~")]}

and

1

L =

T h e values of the exponents ns~c and ncrsSi3Cx a r e between 0.5 and 1. According to Schults [8], if the exponent is between 0.5 and 1, an instantaneous nucleation is involved in the crystallization, the growth is either of rods or disks and the growth is controlled by diffusion or interracial reaction. Transmission electron microscopy was applied to confirm the

(8)

A non-linear regression method was applied to define the parameters P, ku, kv, nu and nv of Eq. (8). Ten groups of data (fsiot) listed in Table 1 were used to define these five parameters. "Ps-Plot" software was used to perform the non-linear regression (curve fitting) by selecting the "user defined equation" function. T h e calculation was continued until the sum of absolute accuracy ( fsic - f u ) reached a minimum. Fig. 5 shows a graph of the raw data points (fsiot) and fitted curve (f~,t). T h e five parameters in Eq. (8) are P=1.0802, ku=0.01986, kv=0.01992, nu=0.8668 and nv = 0.7060. Eq. (8) can be written as

geometry of nanocrystalline silicon carbide and chromium silicide. Fig. 6 shows that silicon carbide and chromium silicide are in the shapes of hexagonal and square platelets (disks). Some of the particulates are also in the form of whiskers and short fibers (rods) in agglomerated powders (Fig. 7).

3. 4. Studies of the activation energy of formation o[ nanocrystalline silicon carbide and chromium silicide T h e Arrhenius equation for the temperature dependence of solid state processes is written as [7] k= k ° exp(- E/RT)

(10)

where k and k ° are the reaction rate constants and E is the activation energy. Combining Eqs. (8), (9) and (10)

f~(t) - 1 + 1.0802{[1 - exp( - 0.01992t° 7"6°)]/[1 - exp( - 0.01986t°s~6s)]}

(9)

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247

Materials Science and Engineering B31 (1995) 243-248

Q

Q

IlL

O 0

|

O 0

!

•

fll

0 Q

I

Ii

Q

.

lit

Q I

o

tlb V

i

t,~u ,,m

Fig. 6. Transmission electron microscopy bright field image showing the size and shape of particulates (1:4:2, 1250°C, 12h).

~

-

3•tl ilIIi

Fig. 7. Transmission electron microscopy bright field image showing the agglomeration of particulates ( 1 : 4 : 2, 12 5 0 °C, 1 h).

gives

L=

1 + 1.0802{[1 - e x p ( - k ° exp( - Ev/RT)t°7°6°)]/[1 - e x p ( - k~ exp( - Eu/RT)t°8668)]}

(11)

At T = 1523 K, from Eq. (9) we have 0 . 0 1 9 9 2 = k~ exp( - Ev/8.314 x 1523) 0.01986 = k~ exp( - Eu/8.314 x 1523)

(12)

Table 2 lists the phase percentages vs. temperature and time. These data are input into Eq. ( 11 ) and combined with Eq. (12); if we assume that k ° and E are constant in the temperature range 1373-1573 °C, the activation energies of silicon carbide and chromium silicide are obtained as E u = 1 0 3 . 3 8 kJ mo1-1 and E v = 55.67 kJ mol- l respectively. These activation energies for silicon carbide and chromium silicide formation are much lower than those reported in the literature [9-11]. For example, Ono and Kurachi [9] have reported that the activation energy for single phase silicon carbide formation from homogeneous precursors is 391 kJ mol-1; the activation energies of chromium silicides have been reported as 326 kJ mol ] for CrsSi 3 and 164.645 kJ mo1-1 for CrSi 2 [ 10]. These former studies focused on the formation of a single phase instead of a composite. Therefore, we may conclude that the formation of the two phases in the composite is accelerated, since the diffusivity of the nanocrystalline material is much larger than that of the conventional material. Suguro et al. [12] found that chromium disilicide formation reduced the formation temperature of tungsten silicide in a thin layered tungsten/barrier metal/silicon W - C r - S i system.

In order to confirm the activation energies, another experiment was conducted on a precursor with a molar concentration ratio of 1:2:2. By carefully preparing the same amount of as-synthesized sample powder for X-ray analysis each time and maintaining the procedural parameters constant, a series of X-ray spectra of silicon carbide peak intensities vs. temperature is obtained and shown in Fig. 8. By calculating the peak intensities vs. temperature, a linear Arrhenius relationship (Fig. 9) is obtained Cu = Mu exp( - Eu/RT )

(13)

l n A C u = _ Eu/RT+ l n A M u

(14)

l n I u = _ Eu/RT+ I°u

(15)

where M u is the equilibrium concentration in the fully crystallized composite and A is a constant. By calculating the slope from Fig. 9 and Eq. (15), the activation energy of silicon carbide crystalline phase formation is obtained as E u = 9 2 . 2 4 kJ tool -1 with Ev=27.71 kJ tool-1. These values agree with the results obtained from the precursor ( 1 : 4: 2). One of the data points at 1250 °C in the graph of chromium silicide formation (Fig. 9) is far below the line. It is difficult to explain this observation. However, if this point is included, the activation energy will reach negative values. More experiments are necessary to provide an accurate interpretation.

248

P. Luo et al.

/

Materials Science and Engineering B31 (1995) 243-248 10.0

Table 2 Calculation of the activation energy (for the precomposite concentration ratio 1: 4 : 2) f,

T

(K)

8.7 7.3

t (min) O

0.68 0.6 0.56

1373 1523 1573

300 300 240

6.0

O < =

InACsic=l 1455.78(-1/'1)+15.93 4.7 3.3 2.0 -0.0009

•

I

,

I

,

I

-0i ~0S

900%

,

I

,

I

,

-0.0006

-0.0007 lfr(K)

14 /

•

•/

13.5

1100°C

•

•

13

0

12.5 r~

¢ ' ~

1200%

.--"

L_t_

12 rj <

________.__--.----~-= ___I-----------lnACcr5Si3=3333.33(- IFF):I 0.26

11.5 11 lO.5 10 -0.0009

j

•

•0

I I

1300°C •

•

I 40

20

I

0

°°•°m

I

-0.00085

-0.0008

-0.00075

-0.0007

-0.00085

-0.0006

lIT(K)

Fig. 9. Study of the Arrhenius relationship for the formation of nanocrystalline silicon carbide and chromium silicide.

I

50

60 •

sic

0 CrSi2

• Cr~i3 Fig. 8. X-ray diffraction analysis for the kinetic study of chromium silicide and silicon carbide crystalline phase formation vs. temperature ( 1 : 2: 2).

formation of nanocrystalline materials. Further research on nucleation and grain growth mechanisms is currently being continued.

References 4. Conclusions T h e kinetics of formation of nanocrystalline c h r o m i u m silicide and silicon carbide composites was studied by measuring the relative X-ray peak intensities of the corresponding phases. T h e kinetic behavior was related to the A v r a m i - E r o f e ' e v equation describing the instantaneous nucleation and growth of hexagonal and square platelets (disks). Activation energies for the formation of nanocrystalline c h r o m i u m silicide and silicon carbide are very low c o m p a r e d with single phase formation using commercial synthesis methods. This has been explained by a self-catalytic behavior of n a n o c o m p o s i t e formation, i.e. the formation of the silicon carbide phase accelerates the formation of the c h r o m i u m silicide phase and vice versa. This may be due to the large mobility of silicon through nanograins. This p a p e r represents an initial study of the kinetics of

[1] R.W. Siegel, S. Ramasamy, H. Hahn, Z. Li, T. Lu and R. Gronsky, J. Mater. Res., 3(1988) 1376. [2] R. Roy, Mater. Res. Soc. Symp. Proc., 286 (1993) 241. I3] P. Luo, P.R. Strutt and T.D. Xiao, Mater. Sci. Eng., BI7 (1993) 126. [4] E Luo and P.R. Strutt, J. Mater. Sci., submitted for publication. f5] R.C. Weast, Handbook of Chemistry and Physics, 56th edn., 1975-1976. [6] B.D. Cullity, Elements of X-ray Diffraction, AddisonWesley, 1978. [7] J.D. Verhoeven, Fundamentals of Physical Metallurgy, Wiley, 1975. [8] J. Schults, Polymer Materials Science, Prentice-Hall, 1974. I9] K. Ono and Y. Kurachi, J. Muter. Sci., 26 ( 1991 ) 388, I10] G,V. Samsonov and I.M. Vinitskii, Handbook of Re(ractoO, Compounds, IFI/Plenum, New York, 1980. [111 T.L. Francis and R.L. Colbe, J. Am. Ceram. Soc., 51 (1968) 115. [12] K. Suguro, Y. Nakasaki, T. Inoue, S. Shima and M. Kashiwagi, Thin Solid Films, 166 (1988) 1.