An electrothermographic method was used to study the regularities of heat release ... heat-release stage due to the crystallization of the disilicide phase from a ...
Combustion, Explosion, and Shock Waves, Vol. 36, No. 3, 2000
Regularities
o f H e a t R e l e a s e in T u n g s t e n
in a G a s l e s s C o m b u s t i o n
Siliconizing
Wave
S. L. Kharatyan 1 and A. A. Chatilyan 1
UDC 536.46+546.281
Translated from Fizika Goreniya i Vzryva, Vol. 36, No. 3, pp. 65-71, May-June, 2000. Original article subnfitted January 18, 1999; revision submitted July 27, 1999. An electrothermographic method was used to study the regularities of heat release a n d f o r m a t i o n o f t h e m i c r o s t r u c t u r e o f t u n g s t e n d i s i l i c i d e w i t h w i d e v a r i a t i o n in t h e temperature regime of sample heating, including conditions modeling combustionw a v e p r o p a g a t i o n in m i x t u r e s o f t u n g s t e n a n d s i l i c o n p o w d e r s . E m p h a s i s is o n t h e heat-release stage due to the crystallization of the disilicide phase from a supersaturated melt of WSi2-Si. At this stage of the process, self-acceleration of the heat r e l e a s e is o b s e r v e d e v e n w h e n t h e s a m p l e t e m p e r a t u r e d e c r e a s e s . G r o w t h o f t u n g s t e n d i s i l i c i d e c r y s t a l s o c c u r s o n l y a t t h e h e a t - r e l e a s e s t a g e . W h e n t h e s y s t e m is k e p t a t t h e m a x i m u m t e m p e r a t u r e o f t h e t h e r m o g r a m , f u r t h e r c r y s t a l g r o w t h is n o t o b s e r v e d over a period of time exceeding the duration of heat release by a factor of 10-20.
INTRODUCTION
EXPERIMENTAL
A previous paper [1] describes an electrothermographic study of the kinetics and mechanism of tungsten siticonizing in a gasless combustion wave. Typical thermograms for combustion of metals with silicon show t h a t during heating of tungsten filaments coated with a silicon layer, heat release proceeds in two successive stages. At the first (quasistationary) stage, interaction of liquid silicon with the metal surface and passage (dissolution) of the WSi2 product into the silicon melt are observed. The second stage of the process goes with acceleration. It is suggested that the second stage is due mainly to crystallization of the WSi2 phase from the supersaturated eutectic melt of WSi2-Si. It is of interest that the accelerating nature of heat release is preserved under isothermal conditions. The nature of interaction of components in the W - S i system corresponds to the mechanism established by Aleksandrov et al. [2, 3] for some systems of carbides, borides, and intermetallides by "in situ" transmission electron microscopy on "particlefilm" model samples.
1Nalbandyan Institute of Chemical Physics, National Academy of Armeniya, Erevan 375044, Republic of Armenia. 342
0010-5082/00/3603-0342
TECHNIQUE
The studies described here were performed by the electrothermographic method of [1] on an IBM PC-controlled facility. This allowed us not only to control the experiment but also to conduct automatic data. processing. After recrystallization by annealing, tungsten filaments with diameter of 100 pm and working length of 8.5 cm were coated with a silicon layer of various thicknesses (SSl = 1-10 #m) and heated in an argon atmosphere (p = 10 torr) by various standard thermograms for combustion of a Me Si powder mixture. Deposition of silicon on the tungsten surface was carried out in rarefied monosilane at T = 700 ~ and P S i H 4 = 2 tort. Metaltographic cross sections and the morphology of the surface of the starting and reacted samples were studied on an optical (Jenavert) microscope and a scanning electron (BS-300) microscope at various magnifications. The frequency of variation in physical quantities (voltage, current, and temperature) was 1 kHz. The rate of heat release due to the exothermal chemical reaction was determined using the nonstationary equation of heat balance for an electrically heated metal filament from the measured electric powers released on the filament during the first; (reaction) and second (inert) heating of the same sample by identical thermograms. The measurement accuracies for the heatrelease rate and the amount of heat released were
$25.00 @ 2000 Kluwer Academic/Plenum Publishers
R e g u l a r i t i e s of H e a t R e l e a s e i n T u n g s t e n S i l i c o n i z i n g in a G a s l e s s C o m b u s t i o n W a v e
a
T,~
dq/dt, cal/(cm2.sec) T,~
1600
c
dq/dt, cal/(cm2.sec)
1600
12
10 0
1400
9
1400
6
-10 1200
1200
- 20 1000
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9
0
i
II
0.1
l
I
0.2
0.3
b
T, ~
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3
1000
0.4 L sec 0.5
dq/dt, cal/(cm 2.sec)
T~ ~
12
1600
9
1400
1600
0
0.1
0.2
0.4 t, sec 0.5
0.3
d
dq/dt, cal/(cm2 .sec) 12
I
9
6 1200
6 1200
3
1000 0.25
.
0.5
0.75
.
.
.
.
.
.
.
0
1.0 t, sec 1.25
3
1000 , 0
0.1
0.2
0.3
0.4 t, sec 0.5
Fig. 1. Effect of the heating rate on the heat-release function (T,..... = 1600~ and 5Si = 3 pro): (dT/dt> = 8 (a and c), 1.6 (b), and 32~ (d).
0.2 cal/(cm2.sec) and 10 2 cal/cm 2, respectively. In the present study of the interaction of components of the W Si system, we focus on the effects of the thermogram parameters (rate of heating to inaximum temperature, maximum temperature, duration of exposure at maximum tempera. ture, and cooling rate) on the heat-release function type. In the experiments of [1], the effect of these parameters was insignificant since at average rates of heating from the initial to the maximum temperature 1000~ d T / d t = 0.5-3.5~ the heatrelease process was completed practically before attainment of the maximmn temperature (T,..... ). In the present work, we studied interaction regimes in which the average heating rate at T < Tm~x was varied in the range {dr/dt> = 7-35~ (in the region r > 1000~ = 2-20~ which is more characteristic of combustion-wave propagation in the metal-silicon system [4]. On the other hand, we took into account that the duration of full consumption of silicon does not exceed 0.5 sec (at T I . . . . ---- 1600~ and dSi ~< 3 #m). Under these conditions, to trace the heat-release rate during cooling too, we sharply reduced the length of the isothermal segment at the maximum temperature (from
t ...... = 1 2 sec to t ...... = 0 0.5 sec). The sample was then cooled at various rates (0.1 4.5~ Implementation of the indicated interaction regimes is required, in addition, to determine the role of WSi2 crystallization from the melt in the general mechanism of siliconizing. We also studied the effects of the temperature regime of heating (thermogram parameters, particularly T,.... ) and the silicon-layer thickness on the WSi2 grain size alter completion of the siliconizing reaction.
EXPERIMENTAL
RESULTS
Figure l a shows the heat-release function type (dq/dt) obtained from the combustion thermogram for heating of a tungsten filament coated with a silicon layer. It is evident that this function has nonzero values in the interval between the beginning of silicon melting and the time of its full consumption. The rather narrow region of negative values of the heat-release function corresponds to silicon melting (endothermic process). The region of positive values of the function consists of two parts: a segment with quasiconstant velocity and a segment with acceler-
344
Kharatyan and Chatilyan TABLE 1 Heat-Release Rate versus Heating Rate and Maximum Temperature of Sample (dsi = 3 tim)
dT/dt, ~ mean value
7.0 8.0 8.5 14 16 17 28 32 34 Note.
T>
for 1000~ 2.2 3.3 3.8 5.0 7.2 8.5 10 20 22
T.,~x, ~C
(dq/dt)l, c~l/(cm 2. se~)
(dq/ dt )2 ....... e a l / ( c m 2. sec)
A-t~ sec
2.5 2.3 3.0 2.5 3.2 3,0 2.5 4.3 5.3
5.3 10 14 5.2 I0.3 14 4.5 13 14
0.23 0.15 0.11 0.23 0.17 0.10 0.26 0.12 0.08
1400 1600 1700 1400 1600 1700 1400 1600 1700
At is t h e t o t a l d u r a t i o n of h e a t release.
ated heat release. For simplicity, the figures below show only the positive part of the heat-release function. Effect of the Heating Rate. Measurements of the heat-release function with variation in the heating rate (for T, ..... /> 1600~ and dsi ~< 3 pm) showed that with increase in the latter, the heat-release function is shifted to the maximum temperature (Fig. l b d) and at (dT/dt} > 30~ it is completely concentrated in the region T = T~..... (see Fig. ld). Furthermore, transition of the process from the region of temperature increase (see Fig. lb) to the region of T,,~x (see Fig. ld) leads to a more distinct separation of the two heat-release stages, which are conditionally called dissolution and crystallization. At the same time, it is established that an increase in the heating rate leads to an increase in the heat-release rate at the first and second stages and to a corresponding decrease in their duration. Table 1 gives values of the heat-release rate with variation in the heating rate and maximum temperature of the sample. As can be seen from Table 1, these values are in the ranges of 2.3-5.3 and 4.5-14 cal/(cm 2- see), respectively. Effects of the D u r a t i o n of E x p o s u r e at T = Tm~x and the Cooling R a t e . As was noted above, the duration of the process at T = Tm~x up to full consumption of silicon is ~0.1 0.4 see (at T , , ~ ~> 1600~ and dSi ~< 3 pro). An analysis of the results of experiments with a reduced time of exposure of the system at the maximum temperature (to 0.05 see, and in some cases, to zero) shows (Fig. 2) that heat release with maximum rate can also take place during cooling of the sample. Such regimes model the behavior of large particles in combustion waves of mixtures of tungsten and silicon powders because precisely an increase in particle size (in the
present experiments, in silicon-layer thickness) can be responsible for tile fact, that the intense interaction of the components will not be completed at the maximum temperature but will continue at decreased temperature - - in the afterburning zone. Taking into account that at a temperature below the eutectic temperature (Teut), interaction of tungsten with silicon practically does not occur, it is concluded that the residual reaction zone extends from T, ..... to the melting point (Tmett) of the WSi2-Si eutectic (according to [5], Tmelt ~ 1400~ 9 Thus, in the case of high heating rates and short duration of the process at TI~..... the rate of cooling of the system assumes significance. It is established that at rather low cooling rates (up to 0.5~ and small thicknesses of the silicon layer, a decrease in the time of exposure at T, ..... (to t ...... = 0) does not play an important role because of the small decrease in temperature before complete consmnption of silicon (see Fig. 2a and b). It should be noted again that the presence of self-accelerating heat. release, which was observed at both constant temperature [1] and decreased temperature (see Fig. 2), is fairly strong evidence that the reaction proceeding in this case is not an ordinary chemical reaction (whose rate, according to the Arrhenius law, should drop with decrease in temperature) but crystallization of tungsten disilicide from the saturated melt. The observed slight increase in maxinmm heat-release rate at low cooling rates is apparently caused by additional activation of the crystallization process due to supersaturation of the melt during cooling. It is known that the nucleation rate depends strongly on both the supersatura. tion (supercooling) of the melt, and temperature. As the temperature decreases, the nucleation rate first increases due to supersaturation and then drops be-
R e g u l a r i t i e s of H e a t R e l e a s e i n T u n g s t e n S i l i c o n i z i n g in a G a s l e s s C o m b u s t i o n W a v e
T, ~
a
dq/dt, cal/(cm2. sec)
1600
~12 9
1 400
T, ~
c
345
dq/dt, cal/(cm2. sec)
1600
12 g
1400
6
6
1200
1200 3 100C
g
0
I
0.1
T, ~
#
I
kl
0.2
0.3
b
. . --,
-
3 1000
0,4 t, sec 0.5
dq/dt, cal/(cm 2 . sec)
0
0.1
0.2
d
T, ~
0.3
dq/dt, cal/(cm 2, sec)
1600
1600
0 t, sec
12 T
9
1400
9
1400
6
6
1200
1200 3 100s
0.05
3 0
1000
0.15
0.25
0.35
0.45 t, sec
0,05
0.1
0.15
0.2
0.25
0.3
0.35 t, sec
Fig. 2. Effect of the sample cooling stage on the heat-release function type (T,.... = 1600~ 5Si = 3 pro) for t ..... = 0 (a-c) and =0.05 sec (d) and cooling rate equal to 0.1 (a), 0.5 (b), and 2~ (d).
cause of the reduction in the mobility of the liquidphase particles. Nucleation is severely hampered by low mobility, even at large supersaturation [61. From the aforesaid, the most significant changes in the heat-release function occur at, high (higher than 1.5~ rates of cooling of the smnple (see Fig. 2c and d). The high cooling rates and small length of the region of T = T,..... lead to a considerable decrease in heat-release rate (including the maximum value) and an increase in the duration of the process (see Fig. 2c and d). Thus, at the first stage of the process, the heat-release rate practically does not change because the relative decrease in temperature is small and, in addition, this stage only partly falls in the cooling zone. From the results of [1], under conditions of temperature decrease, the main source of heal release can be crystallization of the WSi2 phase from the melt, which, on cooling, becomes supersaturated tungsten disilicide. On the other hand, passage of the unreacted metal into the melt is practically not observed under these conditions, and the lowest silicide phase (WsSi3) is not formed as long as the liquid phase is present. This suggests that in combustion of a stoiehiometric W + 2Si powder mixture, large tungsten particles are not completely transformed into disili-
cide and the synthesis products contain unreacted tungsten and silicon (without marked traces of the WsSia phase). Effect of t h e M a x i m u m T e m p e r a t u r e . The effect of the value of Tmax on the regularities of heat release is more or less pronounced when the heating rates of the sample are high and the heat release proceeds mainly at this (maximum) temperature. In this case, an increase in Tn,~• leads to an increase in heat-release rate and a decrease in the duration of heat release. The heat release completely proceeds at the maxinnnn temperature. At T,..... ~< Teut (here Teut ~ 1400~ practically corresponds to the minimum combustion temperature for the present system), interaction is ahnost absent, and at 7].... > Teut and (dT/dt) > 7~ heat release begins and ends (dSi ~< 3 pm and tm.~• ~> 0.5 see) practically at the maximum temperature and, hence, the heating rate is of no significance. As regards the absolute value of the heat-release rate (see Table 1), at the first stage, it depends weakly on the maximum temperature, and as the latter increases from 1400 to 1700~ it increases by approximately 30%. In contrast to the first, stage, the heat-release rate at. the second stage increases considerably, by more than a factor of two.
346
Kharatyan
and Chatilyan
a
I 5.am
I
Fig. 3. Microstructures of the zone of interaction of tungsten with silicon (surface of filaments) and ~Si = 3 pro) for t. = 0.18 at various stages of process (T,,a• = 1600~ ( d T / d t } = 8~ (a), 0.23 (b), 0.27 (c), 0.35 (d), and 15 sec (e) (the magnification in the photographs is tile same everywhere).
R e g u l a r i t i e s o f H e a t R e l e a s e in T u n g s t e n S i l i c o n i z i n g in a G a s l e s s C o m b u s t i o n W a v e
a
347
b
I
5~lm
I
Fig. 5. Microstructures of WSi~ formed at various temperatures (dSi = 6 pm): T = 1400 (a), 1500 (b), 1600 (c), and 1700 (d) (the magnification in the photographs is the same everywhere). Effect o f t h e T h e r m o g r a m P a r a m e t e r s o n t h e F i n a l S i z e o f W S i 2 C r y s t a l s . The final size of the crystals (the size of WSi2 crystals upon full consumption of silicon) depends primarily on the siliconlaver thickness and increases with increase in the latter. At the first stage of heat release, the melt is an unsaturated solution of tungsten disilicide in silicon, irrespective of the silicon-layer thickness. Formation of WSi2 crystals, as noted in [1], occurs only during cooling of the sample, because of the shift of equilibrium. The dynamics of crystal growth during interaction under heating determined from a combustion thermogrmn is given in [1]. In the study described herein, the final size of the crystals was measured with variation in the thermogram parameters. It is established that the final size of the WSi2 crystals depends mainly on the silicon-layer thickness and the maximum temperature of the smnple. Thus, an increase in values of these parameters leads to forma-
tion of larger disilicide particles. It should be noted that after full consumption of silicon, even when the sample was kept at high temperatures for a long time (tens of seconds), crystal growth (secondary crystallization) was not. observed. Figure 3 gives electron micrographs of the interaction zone at the various stages of development of the process. It is evident that WSi2 crystals grown for 0.3 sec practically do not change in size on being kept at T = T,,,~x for more than 10-15 sec, i.e., we can state with assurance that, in this case, crystal growth is observed only at the heat-release stage (as long as there is the liquid WSi2-Si phase). Figure 4 gives curves of the grain size of WSi~ formed at the stage of heat release versus the maximum temperature at various thicknesses of the silicon layer. It is evident that an increase in both parameters has a significant effect on the value of /~wsi2. This is also clearly seen from Fig. 5, which
348
Kharatyan
~WSi2,,urn
REFERENCES
3
2
1
t
1400
and Chatilyan
I
1500
I
I
1600
h
T, ~
I
1700
Fig. 4. Grain size of WSi2 versus the maximum temperature of the thermogram for various silicon-layer thicknesses: 5Si = 6.5 (1), 4.5 (2), 3 (3), and 1.5 (4).
shows WSi2 microstructures formed at various temperatures. In summary, we note t h a t a n u m b e r of d a t a obtained in [1] and in the present study (particularly, those on the heat-release function type) are ignored and c a n n o t be explained within the framework of the available combustion models. Therefore, the theoretical concepts of the mechanism of transformations in gas-fl'ee combustion waves require further development.
1. S. L. Kharatyan and H. A. Chatilyan, "Nonisothermal kinetics and mechanism of tungsten siliconizing in gasless combustion wave," Int. J. Self-Propag. HighTemp. Synth., 8, No. 1, 31-42 (1999). 2. V. V. Aleksandrov, M. A. Korchagin, and V. V. Boldyrev, "Mechanism and macrokinetics of interaction of components in powder mixtures," Dokl. Akad. Nauk SSSR, 292, No. 4, 879 881 (1987). 3. V. V. Aleksandrov and hi. A. Korchagin, "On the mechanism and macrokinetics of reactions in SHSsystem combustion," Fiz. Goreniya Vzryva, 23, No. 5, 55-63 (1987). 4. A. R. Sarkisyan, "Self-propagating high-temperature synthesis of transient metal silicides," Candidate's Dissertation in Tech. Sci., Kiev (1980). 5. G. V. Samsonov, L. A. Dvorina, and V. M. Rud', Silicides [in Russian], Metallurgiya, Moscow (1979). 6. B. K. Vainshtein (ed.), Modern Crystallography, Part 3: Formation of Cr~ystals [in Russian], Nauka, Moscow (1980).
