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The physicochemical properties of Si-B alloys
407
Journal of the Less-Common Metals, 67 (1979) 407 - 413 0 Elsevier Sequoia S.A., Lausanne - Printed in the Netherlands
THE PHYSICOCHEMICAL
PROPERTIES
OF Si-B ALLOYS*
S. V. LUKIN, V. I. ZHUCHKOV, N. A. VATOLIN and Yu. S. KOZLOV Institute of Metallurgy, Ural Scientific U.S.S.R., Suerdlovsk (USSR)
Research
Centre, Academy
of Sciences of the
Summary The surface tension and the density of Si-B alloys with a boron content of 22.4 at.% were studied in a helium atmosphere in the temperature range 1410 - 1700 “C. We calculated the activity coefficients of the component in the melt and the composition of the surface layer. We investigated the enthalpies of the mixing of liquid Si-22.4 at.% B alloys at 1813 K. The oxidation kinetics of liquid Si-B alloys with boron contents of 1.28, 12.02 and 22.4 at.% was investigated at temperatures of 1450, 1500 and 1550 “C. The effect of temperature and of boron concentration on the oxidation rate of the liquid phase was studied.
1. Introduction The physicochemical properties of the interaction process between Si-B alloys and steel which is vitally important to metallurgy have been studied very little. The oxidation of alloys occurs from the surface. However, the surface properties have been used to investigate the mechanism of the oxidation rate of liquid metals only to a limited extent. Thus it is the objective of the work reported in this paper to study the surface tension, the density, the oxidation kinetics and the thermal effects of the mixing of liquid Si-B alloys.
2. Experimental The surface tension and density of the system were studied with boron contents up to 22.4 at.% (10 wt.%) over the temperature range 1410 - 1700 “C using the droplet method. The experiments were carried out in a grade “HF” purified helium atmosphere. To purify the helium, gas was slowly
*Paper presented at the 6th International Symposium on Boron and Borides, Varna, Bulgaria, October 9 - 12, 1978.
408
passed through a cell of ZrIs heated to 1000 “C and then trough an activated-coal trap submersed in a liquid nitrogen vessel. Purified gas was collected in an intermediate container from which it was passed into the working chamber of the furnace. The temperature was recorded by a W- Rh thermocouple. To obtain alloys, we used crystalline silicon (of purity 99.999~) and crystalline boron (of purity 99.0%). The surface tension (I and the density p were calculated from the dimensions of a drop on a negative by a modified Dorsay method [l] using an EVM-222 computer. The errors in the density and surface tension me~uremen~ with respect to the heats did not exceed f 1 and + 2 respectively .
The heats of mixing of liquid Si-B alloys were measured in a high temperature isothermal shell calorimeter at a temperature of 1823 K. The partial molar heats were calculated using an electronic computer according to the technique described by Jesin and Geld [2]. The calculation was based on the equation
where AH$sssx is the enthalpy of the pure component added to the bath, Ann the number of gram atoms of admixture, AT the increase in temperature and W the heat equivalent of the bath.
3. Results and discussion The surface temsion and density data we obtained were treated using the least-squares method. The temperature dependences of p and CTwere approximated by the equations P = P1410
-K,@-
u = 0141(-)-K,(t-
1410)
(2)
1410)
(31
The values of the temperature coefficients are given in Table 1. Equations (1) and (2) were employed to construct the isotherms of free surface energy and of molar volumes. The experimental isotherm of free surface energy at a temperature of 1410 “C is a smooth curve and almost coincides with the isotherm calculated from Pavlov and Popel’s [3] equation, even without taking into account the differences in the components with respect to the interparticle binding energies (Fig. 1). The fusibility diagram of the Si-B system over the concentration range under consideration exhibits a eutectic point at a boron content of 7.4 at.% according to ref. 4 or at 18 at.% according to ref. 5 but the surface tension isotherm is not affected. This confirms the earlier conclusions that the majority of systems have eutectic points.
409
TABLE 1 Values of temperature coefficients Composition
in the linear equations of surface tension and density
of alloy
Si (at.%); Si (wt.%)
B (at.%); B (wt.%)
100 99.75; 99.9 98.72; 99.5 92.57; 97.0 87.98; 95.0 77.6; 90
-
700
0
0.25; 0.10 1.28; 0.5 7.43; 3.0 12.02; 5.0 22.4; 10.0
I 5
I 10
I 15
I 20
KP x 10-l (kg mP3 K-l)
G (MJ m -2
1.2910 1.301 1.681 4.332 1.8340 1.1030
0.1044 0.1076 0.1321 0.2298 0.1998 0.2187
J
t?,at.%
Fig. 1. The free surface energy isotherms for Si-B alloys at melting point (1410 “C): curve 1, experimental isotherm; curve 2, calculated isotherm [ 41.
Fig. 2. The molar volume isotherms for Si-B alloys: curve 1, additive straight line; curve 2, experimental isotherm.
K-1)
410
The isotherm of the molar volumes has a negative deviation from the additive straight line (Fig. 2). It is a feature peculiar to the Si-B system that the volume compression observed when small amounts of boron are added reaches its maximum (4.5%) for an alloy with 7.43 at.% B (3 wt.% B). The results obtained agree with the data for solid Si-B alloys. It is shown in ref. 5 that admixtures of boron and silicon lead to lattice compression, with a solution which depends on the type of substitution. Over the boron concentration range 1.28 - 22.4 at.% the experimental data for the volume variation A V fit a straight line satisfactorily on a A V uersus NSiV, plot where Nti and NB are the molar proportions of the components in the bulk (Fig. 3). This indicates that the thermodynamic properties of the system involved are described to a zero approximation by equations of regular solutions [ 61. To calculate the boron activity we used the values of the molar volumes AV and of the calorimetric data AH. The activity values are given in Table 2. The results obtained agree satisfactorily with each other, but the activities calculated according to the enthalpies of mixing are apparently somewhat higher.
Fig. 3. The dependence
TABLE Boron
of AV on NsiN,.
2 aB and silicon
activity
‘BAV
‘BAH
0
0
0
1
1
1
0.013 0.074 0.120 0.224
0.008 0.061 0.075 0.099
0.073 0.071 0.108 0.201
0.987 0.926 0.878 0.776
0.974 0.853 0.764 0.588
0.987 0.925 0.860 0.768
NB
activity
asi in the bulk of the melt Nsi
“SiAV
“si
AH
411
From the surface tension isotherms and using the activity values, we calculated the surface boron concentrations. The estimation was performed according to Rusanov’s [ 71 equation : x,
=
N,g,,
+ aoz(doI~,)
(a01 -
aod(da/~d
(4)
+g,l
where X, is the boron concentration at the surface, sol and uo2 are the molar surface areas of silicon and boron, g,, is the Gibbs molar thermodynamic potential which can be determined from
g,, = RT
1
NB(l
-NB)
+
dln(udusi1 WB
I
The data obtained showed that there was an appreciable difference between the surface boron concentration and the bulk boron concentration. The oxidation kinetics of liquid Si-B alloys with boron contents of 1.28,12.02 and 22.4 at.% was studied using the technique of continuous weighing on an apparatus described by Kiselev et al. [ 81. Oxidation was effected in a pure O2 atmosphere at temperatures of 1450, 1500 and 1550 “C. For these temperatures, an increase in the rate of liquid phase oxidation with increasing boron concentration was observed (Fig. 4). The increase in temperature also appreciably increased the rate of oxidation. At a boron content of 1.28 at.% we found a parabolic time dependence of melt oxidation and an exponential temperature dependence of the quantity of oxides produced (the gain in grams for 1 cm2 s-l). The quantity of oxides produced is given by q = Slfiexp
- ( 23:6)
For a boron content of 12.02 at.% and with the temperature increased from 1450 to 1550 OC, the mode of oxidation changed from parabolic to linear. An increase in the boron concentration to 22.4 at,% under the same temperature conditions led to a reversal of the oxidation mode from linear to parabolic. These changes in the oxidation modes are due to the occurrence of films which are different in texture. Dense films slow down the rate at which the elements reach the place of reaction; the rate of the process is determined by the diffusion in the oxide layer, and this leads to a parabolic oxidation law. The formation of friable porous films is described by a linear oxidation law because in this case the rate of the process is determined by the crystallochemical event. For the Si-B system over the concentration range studied, a selective oxidation was observed which disagrees with the thermodynamic data. An analysis of the composition of films obtained when subject to oxidation using an LMA-1 microanalyser showed that the content
412
0
405
010
0.15
020
MB
Fig. 4. The variation of the boron content at the surface with the boron in the bulk: curve 1, line of equal concentrations; curve 2, boron content at the surface of the metallic melt; curve 3, boron content in the oxide film; curve 4, dependence of the alloy oxidation on the boron concentration.
of boron in the oxidation products was lower compared with the content in the bulk (Fig. 4), whereas the affinity of boron for oxygen was higher than that of silicon for oxygen. Figure 4 also shows the curves of the surface concentration of boron in the metal (Fig. 4, curve 2) and the dependence of the rate of alloy oxidation on the boron content in the bulk (Fig. 4, curve 4). There are some similarities in the behaviour of Fig. 4, curves 2,3 and 4. This possibly arises because the oxidation process is determined by the boron content at the surface of the metal.
4. Conclusions (1) Over the Si-B system concentration and temperature ranges studied, the surface tension increased with increasing boron content. (2) The system is characterized by negative deviations from Raoult’s law with the maximum of compression (4.5%) at 7.43 at.% B. (3) The composition of the alloy surface layer was calculated. It was shown that it differs appreciably from the composition of the alloy in the bulk. (4) The oxidation kinetics of the Si-B system was studied. An increase in boron content and an increase in the temperature led to an increase in the rate of oxidation.
413
References 1 Yu. N. Ivashenko, B. B. Bogatyrenko and V. I. Eremenko, K voprosu o rastchetje povjerkhnostnogo natjazhenija zhidkosti po razmjeram ljezhashej kapli, in Poujerkhnostnyje Javtjenija v Rasplavakh i Processakh Poroshkovoj Mjetallurgii, Akademii Nauk Ukrainskoi S.S.R., Kiev, 1963, pp. 391 - 417. 2 Yu. 0. Jesin and P. V. Geld, Tjeplofizika vysshikh tjemperatur, !I’?. Nouosib. Elektrotekh. Inst., 4 (1974). 3 S. I. Pope1 and V. V. Pavlov, Tjermodinamitsheskii rastchjet povjerkhnostnogo natjazhenija rastvorov, in Povjerkhnostnyje JavZjenija v Rasplavakh i Voznikqjushikh u Nikh Tuerdych Fasakh, KB, Nalchik, 1965, pp. 46 - 60. 4 C. Brosset and B. Magnusson, The silicon-boron system, Nature (London), 187 (1960) 54 - 55. 5 V. M. Sleptsov and G. V. Samsonov, K voprosu o rastvorimosti bora v krjemnii, DokJ. Akad. Nauk SSSR, (9) (1959) 982 - 984. 6 V. F. Ukhov, 0. A. Jesin, N. A. Vatolin and E. L. Dubinin, Rastchjet aktivnostjej binarnykh splavov po ikh moljarnym ob’jemam, Fizitsheskaja khimija mjetallurgitsheskikh rasplavov, Tr. Inst. Metall. Akad. Nauk SSSR, Ural. Fil., 13 (1969) 87 - 103. 7 A. I. Rusanov, Fasovyje Ravnovjesija i Povjerkhnostnyje JavZjenija, Khimiya, Leningrad, 1967. 8 V. I. Kiselev, B. M. Lepinskikh and V. I. Zhutchkov, Kinetika okisljenija zhidkhikh mjetallov kislorodom iz gazovoj fazy, Strojenije i svoistva mjetallurgitsheskikh rasplavov, Tr. Inst. Metall., Akad. Nauk SSSR, Ural. Nauchn. Tsentr., 28 (1974) 117 120.
Journal of the Less-Common Metals, 67 (1979) 407 - 413 0 Elsevier Sequoia S.A., Lausanne - Printed in the Netherlands
THE PHYSICOCHEMICAL
PROPERTIES
OF Si-B ALLOYS*
S. V. LUKIN, V. I. ZHUCHKOV, N. A. VATOLIN and Yu. S. KOZLOV Institute of Metallurgy, Ural Scientific U.S.S.R., Suerdlovsk (USSR)
Research
Centre, Academy
of Sciences of the
Summary The surface tension and the density of Si-B alloys with a boron content of 22.4 at.% were studied in a helium atmosphere in the temperature range 1410 - 1700 “C. We calculated the activity coefficients of the component in the melt and the composition of the surface layer. We investigated the enthalpies of the mixing of liquid Si-22.4 at.% B alloys at 1813 K. The oxidation kinetics of liquid Si-B alloys with boron contents of 1.28, 12.02 and 22.4 at.% was investigated at temperatures of 1450, 1500 and 1550 “C. The effect of temperature and of boron concentration on the oxidation rate of the liquid phase was studied.
1. Introduction The physicochemical properties of the interaction process between Si-B alloys and steel which is vitally important to metallurgy have been studied very little. The oxidation of alloys occurs from the surface. However, the surface properties have been used to investigate the mechanism of the oxidation rate of liquid metals only to a limited extent. Thus it is the objective of the work reported in this paper to study the surface tension, the density, the oxidation kinetics and the thermal effects of the mixing of liquid Si-B alloys.
2. Experimental The surface tension and density of the system were studied with boron contents up to 22.4 at.% (10 wt.%) over the temperature range 1410 - 1700 “C using the droplet method. The experiments were carried out in a grade “HF” purified helium atmosphere. To purify the helium, gas was slowly
*Paper presented at the 6th International Symposium on Boron and Borides, Varna, Bulgaria, October 9 - 12, 1978.
408
passed through a cell of ZrIs heated to 1000 “C and then trough an activated-coal trap submersed in a liquid nitrogen vessel. Purified gas was collected in an intermediate container from which it was passed into the working chamber of the furnace. The temperature was recorded by a W- Rh thermocouple. To obtain alloys, we used crystalline silicon (of purity 99.999~) and crystalline boron (of purity 99.0%). The surface tension (I and the density p were calculated from the dimensions of a drop on a negative by a modified Dorsay method [l] using an EVM-222 computer. The errors in the density and surface tension me~uremen~ with respect to the heats did not exceed f 1 and + 2 respectively .
The heats of mixing of liquid Si-B alloys were measured in a high temperature isothermal shell calorimeter at a temperature of 1823 K. The partial molar heats were calculated using an electronic computer according to the technique described by Jesin and Geld [2]. The calculation was based on the equation
where AH$sssx is the enthalpy of the pure component added to the bath, Ann the number of gram atoms of admixture, AT the increase in temperature and W the heat equivalent of the bath.
3. Results and discussion The surface temsion and density data we obtained were treated using the least-squares method. The temperature dependences of p and CTwere approximated by the equations P = P1410
-K,@-
u = 0141(-)-K,(t-
1410)
(2)
1410)
(31
The values of the temperature coefficients are given in Table 1. Equations (1) and (2) were employed to construct the isotherms of free surface energy and of molar volumes. The experimental isotherm of free surface energy at a temperature of 1410 “C is a smooth curve and almost coincides with the isotherm calculated from Pavlov and Popel’s [3] equation, even without taking into account the differences in the components with respect to the interparticle binding energies (Fig. 1). The fusibility diagram of the Si-B system over the concentration range under consideration exhibits a eutectic point at a boron content of 7.4 at.% according to ref. 4 or at 18 at.% according to ref. 5 but the surface tension isotherm is not affected. This confirms the earlier conclusions that the majority of systems have eutectic points.
409
TABLE 1 Values of temperature coefficients Composition
in the linear equations of surface tension and density
of alloy
Si (at.%); Si (wt.%)
B (at.%); B (wt.%)
100 99.75; 99.9 98.72; 99.5 92.57; 97.0 87.98; 95.0 77.6; 90
-
700
0
0.25; 0.10 1.28; 0.5 7.43; 3.0 12.02; 5.0 22.4; 10.0
I 5
I 10
I 15
I 20
KP x 10-l (kg mP3 K-l)
G (MJ m -2
1.2910 1.301 1.681 4.332 1.8340 1.1030
0.1044 0.1076 0.1321 0.2298 0.1998 0.2187
J
t?,at.%
Fig. 1. The free surface energy isotherms for Si-B alloys at melting point (1410 “C): curve 1, experimental isotherm; curve 2, calculated isotherm [ 41.
Fig. 2. The molar volume isotherms for Si-B alloys: curve 1, additive straight line; curve 2, experimental isotherm.
K-1)
410
The isotherm of the molar volumes has a negative deviation from the additive straight line (Fig. 2). It is a feature peculiar to the Si-B system that the volume compression observed when small amounts of boron are added reaches its maximum (4.5%) for an alloy with 7.43 at.% B (3 wt.% B). The results obtained agree with the data for solid Si-B alloys. It is shown in ref. 5 that admixtures of boron and silicon lead to lattice compression, with a solution which depends on the type of substitution. Over the boron concentration range 1.28 - 22.4 at.% the experimental data for the volume variation A V fit a straight line satisfactorily on a A V uersus NSiV, plot where Nti and NB are the molar proportions of the components in the bulk (Fig. 3). This indicates that the thermodynamic properties of the system involved are described to a zero approximation by equations of regular solutions [ 61. To calculate the boron activity we used the values of the molar volumes AV and of the calorimetric data AH. The activity values are given in Table 2. The results obtained agree satisfactorily with each other, but the activities calculated according to the enthalpies of mixing are apparently somewhat higher.
Fig. 3. The dependence
TABLE Boron
of AV on NsiN,.
2 aB and silicon
activity
‘BAV
‘BAH
0
0
0
1
1
1
0.013 0.074 0.120 0.224
0.008 0.061 0.075 0.099
0.073 0.071 0.108 0.201
0.987 0.926 0.878 0.776
0.974 0.853 0.764 0.588
0.987 0.925 0.860 0.768
NB
activity
asi in the bulk of the melt Nsi
“SiAV
“si
AH
411
From the surface tension isotherms and using the activity values, we calculated the surface boron concentrations. The estimation was performed according to Rusanov’s [ 71 equation : x,
=
N,g,,
+ aoz(doI~,)
(a01 -
aod(da/~d
(4)
+g,l
where X, is the boron concentration at the surface, sol and uo2 are the molar surface areas of silicon and boron, g,, is the Gibbs molar thermodynamic potential which can be determined from
g,, = RT
1
NB(l
-NB)
+
dln(udusi1 WB
I
The data obtained showed that there was an appreciable difference between the surface boron concentration and the bulk boron concentration. The oxidation kinetics of liquid Si-B alloys with boron contents of 1.28,12.02 and 22.4 at.% was studied using the technique of continuous weighing on an apparatus described by Kiselev et al. [ 81. Oxidation was effected in a pure O2 atmosphere at temperatures of 1450, 1500 and 1550 “C. For these temperatures, an increase in the rate of liquid phase oxidation with increasing boron concentration was observed (Fig. 4). The increase in temperature also appreciably increased the rate of oxidation. At a boron content of 1.28 at.% we found a parabolic time dependence of melt oxidation and an exponential temperature dependence of the quantity of oxides produced (the gain in grams for 1 cm2 s-l). The quantity of oxides produced is given by q = Slfiexp
- ( 23:6)
For a boron content of 12.02 at.% and with the temperature increased from 1450 to 1550 OC, the mode of oxidation changed from parabolic to linear. An increase in the boron concentration to 22.4 at,% under the same temperature conditions led to a reversal of the oxidation mode from linear to parabolic. These changes in the oxidation modes are due to the occurrence of films which are different in texture. Dense films slow down the rate at which the elements reach the place of reaction; the rate of the process is determined by the diffusion in the oxide layer, and this leads to a parabolic oxidation law. The formation of friable porous films is described by a linear oxidation law because in this case the rate of the process is determined by the crystallochemical event. For the Si-B system over the concentration range studied, a selective oxidation was observed which disagrees with the thermodynamic data. An analysis of the composition of films obtained when subject to oxidation using an LMA-1 microanalyser showed that the content
412
0
405
010
0.15
020
MB
Fig. 4. The variation of the boron content at the surface with the boron in the bulk: curve 1, line of equal concentrations; curve 2, boron content at the surface of the metallic melt; curve 3, boron content in the oxide film; curve 4, dependence of the alloy oxidation on the boron concentration.
of boron in the oxidation products was lower compared with the content in the bulk (Fig. 4), whereas the affinity of boron for oxygen was higher than that of silicon for oxygen. Figure 4 also shows the curves of the surface concentration of boron in the metal (Fig. 4, curve 2) and the dependence of the rate of alloy oxidation on the boron content in the bulk (Fig. 4, curve 4). There are some similarities in the behaviour of Fig. 4, curves 2,3 and 4. This possibly arises because the oxidation process is determined by the boron content at the surface of the metal.
4. Conclusions (1) Over the Si-B system concentration and temperature ranges studied, the surface tension increased with increasing boron content. (2) The system is characterized by negative deviations from Raoult’s law with the maximum of compression (4.5%) at 7.43 at.% B. (3) The composition of the alloy surface layer was calculated. It was shown that it differs appreciably from the composition of the alloy in the bulk. (4) The oxidation kinetics of the Si-B system was studied. An increase in boron content and an increase in the temperature led to an increase in the rate of oxidation.
413
References 1 Yu. N. Ivashenko, B. B. Bogatyrenko and V. I. Eremenko, K voprosu o rastchetje povjerkhnostnogo natjazhenija zhidkosti po razmjeram ljezhashej kapli, in Poujerkhnostnyje Javtjenija v Rasplavakh i Processakh Poroshkovoj Mjetallurgii, Akademii Nauk Ukrainskoi S.S.R., Kiev, 1963, pp. 391 - 417. 2 Yu. 0. Jesin and P. V. Geld, Tjeplofizika vysshikh tjemperatur, !I’?. Nouosib. Elektrotekh. Inst., 4 (1974). 3 S. I. Pope1 and V. V. Pavlov, Tjermodinamitsheskii rastchjet povjerkhnostnogo natjazhenija rastvorov, in Povjerkhnostnyje JavZjenija v Rasplavakh i Voznikqjushikh u Nikh Tuerdych Fasakh, KB, Nalchik, 1965, pp. 46 - 60. 4 C. Brosset and B. Magnusson, The silicon-boron system, Nature (London), 187 (1960) 54 - 55. 5 V. M. Sleptsov and G. V. Samsonov, K voprosu o rastvorimosti bora v krjemnii, DokJ. Akad. Nauk SSSR, (9) (1959) 982 - 984. 6 V. F. Ukhov, 0. A. Jesin, N. A. Vatolin and E. L. Dubinin, Rastchjet aktivnostjej binarnykh splavov po ikh moljarnym ob’jemam, Fizitsheskaja khimija mjetallurgitsheskikh rasplavov, Tr. Inst. Metall. Akad. Nauk SSSR, Ural. Fil., 13 (1969) 87 - 103. 7 A. I. Rusanov, Fasovyje Ravnovjesija i Povjerkhnostnyje JavZjenija, Khimiya, Leningrad, 1967. 8 V. I. Kiselev, B. M. Lepinskikh and V. I. Zhutchkov, Kinetika okisljenija zhidkhikh mjetallov kislorodom iz gazovoj fazy, Strojenije i svoistva mjetallurgitsheskikh rasplavov, Tr. Inst. Metall., Akad. Nauk SSSR, Ural. Nauchn. Tsentr., 28 (1974) 117 120.