- Email: [email protected]
Kinetics of solid state reaction between zirconium and copper(I) chloride
International Journal of Inorganic Materials 3 (2001) 1083–1089
Kinetics of solid state reaction between zirconium and copper(I) chloride a, b c B. Gillot *, H. Souha , M. Radid a
´ ´ des Solides, UMR CNRS 5613, 9 Avenue Alain Savary, BP 47870 -21078, Dijon Cedex, France Laboratoire de Recherches sur la Reactivite b ` , Morocco Laboratoire de Chimie Physique, Faculte´ des Sciences Dhar El Mehraz, BP 1796, Fes c Universite´ Hassan II, Mohammedia, Faculte´ des Sciences Ben M’ sik, BP 6621, Casablanca, Morocco Received 23 October 2000; received in revised form 13 August 2001
Abstract The kinetics of the reaction between Zr and CuCl powders have been studied under vacuum by means of thermogravimetry in the range from 250 to 3308C. The reactivity of the Zr–CuCl system is significantly affected by the mixing operation, the ratio of Zr and CuCl in the mixture and the thickness of the ZrO 2 film on the Zr particles. It is established that the kinetics of reaction was governed by a nucleation and growth mechanism with an apparent activation energy of 9363 kJ mol 21 and for these low reaction temperatures only Cu and Cu 5 Zr were identified. 2001 Published by Elsevier Science Ltd. Keywords: Zr–CuCl system; Thermogravimetry; Nucleation and growth; Autocatalytic process
1. Introduction This study which originates from a basic research on the reactivity of CuCl with IVb metals (Si, Ge, Sn) and IVa metal (Ti) [1–5] is partly motivated by the aim to examine systematically at low temperature (,3508C) the Cu x Me y alloy formation predicted by the Me–Cu phase diagrams and usually investigated at high temperature from the direct reaction between Cu and Me [6–9]. With CuCl these alloys can be formed through successive reactions where the copper is the final phase generated by the reaction: Me (s) 1 4CuCl (s) → MeCl 4 ( g) 1 4Cu (s)
such as the mixing operation, the proportion of Zr and CuCl in the mixture and the thickness of the ZrO 2 film on the grains. For this system only the kinetics of the reaction at high temperature of intermetallic layer formation by diffusion between Cu–Zr films have been investigated [10,11]. According to the phase diagram [12] (Fig. 1), Cu reacts with Zr to form Cu 5 Zr, Cu 51 Zr 14 , Cu 8 Zr 3 , Cu 2 Zr,
(1)
whereas the alloys are intermediates which react for particular conditions with CuCl. This paper reports the results of a detailed study by means of thermogravimetry and X-ray diffraction of the kinetics of the global reaction Zr 1 4CuCl → ZrCl 4 1 4Cu
(2)
Special attention was paid to the occurrence of intermediate compounds and the effects of a few more factors *Corresponding author. Tel.: 133-3-8039-6142; fax: 133-3-80396167. E-mail address: [email protected] (B. Gillot). 1466-6049 / 01 / $ – see front matter 2001 Published by Elsevier Science Ltd. PII: S1466-6049( 01 )00179-9
Fig. 1. Phase diagram of the Cu–Zr system.
1084
B. Gillot et al. / International Journal of Inorganic Materials 3 (2001) 1083 – 1089
Cu 10 Zr 7 , CuZr, Cu 5 Zr 8 and CuZr 2 containing between 17 and 70 at.% Cu.
2. Experimental details The total mass change due to evolution of ZrCl 4 gas was measured as a function of the temperature and time by means of a MacBain thermobalance with a sensitivity of 0.02 mg for various CuCl and zirconium loadings. Zirconium (80 mesh, 99.98% pure, lot 00012207, Alfa Products) and CuCl (300 mesh, 99.999% pure, lot 400151, Alfa Products) powders were ground manually in an agate mortar or mechanically treated by grinding in a disk oscillating mill for different times in a mole ratio Zr / CuCl$0.25. Since Zr oxidizes readily in oxygen atmosphere and water vapor [13] care was taken in all steps of the sample preparation procedure to avoid contamination by oxygen. Then 20 mg of the mixture was spread as a thin layer on the balance arm; the reactor was subsequently evacuated, outgassed in vacuo (10 22 Pa) for 2 h at room temperature and then for 1 h at 1808C before the sample was heated to the temperature of the experiment. The ZrCl 4( g) liberated during the reaction under vacuum was condensed in a liquid nitrogen trap and the mass loss was calculated per 100 mg CuCl. A few runs were carried out with different dry-grinding times and molar ratios, since in the case of a solid–solid reaction it is necessary to mix the substances at the molecular level [14] in order to enhance their reactivity. To ascertain whether the solid reaction products analysed are the same as in thermogravimetry, a parallel set of these experiments was conducted in a closed system where ZrCl 4( g) was allowed to remain in contact with the sample
Fig. 2. Curves Dm 5 f(T ) obtained in vacuum for the reaction between Zr and CuCl powders. (1) CuCl alone in the scoop, (2) CuCl mixed with an inert oxide (Al 2 O 3 ), (3) mixed sample, (4) grinding sample for 1 min, (5) grinding sample for 5 min.
during the course of the reaction. After grinding in an agate mortar, the mixture was placed in a Pyrex glass capsule, sealed under vacuum (10 23 Pa) and then heated at constant temperatures ranging from 300 to 6008C for different reaction times. X-ray diffraction patterns were recorded on an ‘INEL CPS 120’ linear counter (curved position sensitive) equipped with monochromatized Cu K a radiation and calibrated by a quartz standard. The resolution was 0.028 (2u ) at 28 min 21 . The morphological and energy-dispersive X-ray analyses were carried out with a scanning electron microscope (SEM ‘Cambridge’ 250 MK2).
3. Results and discussion
3.1. Reactivity of the system Zr /CuCl by means of thermogravimetry Fig. 2 shows the thermogravimetric curves Dm 5 f(T ) for the sublimation of CuCl when it is placed alone in the scoop (curve 1) or mixed with an inert oxide such as Al 2 O 3 (curve 2). In the absence of an inert oxide the sublimation temperature of CuCl is lowered by ca. 408C. As a result, the temperature at which CuCl begins to react with zirconium can serve as a criterion for the reactivity of the mixture. For comparison, curves 3, 4 and 5 give the variation of mass loss with temperature for a manually mixed sample for 5 min and a sample ground for 1 and 5 min, respectively, with a Zr / CuCl ratio52.17. The theoretical maximum mass loss of the system is calculated (Dm (cal ) 558.9 mg, dashed line) for the complete reduction of CuCl by Zr according to reaction (2). It can be seen from Fig. 2 that the difference between the experimental Dm (exp) and theoretical maximum loss mass decreases with longer grinding times. This implies that the mass fraction of CuCl which sublimes without reaction with Zr decreases as the grinding time is increased. Because the vaporisation of CuCl starts at about 2708C when it is mixed with an inert oxide, the mass loss from the mixture below this temperature is only due to a solid–solid reaction between CuCl and Zr. Above this temperature, we have also to consider that the solid–gas reaction prevails. So, to study the progress of the reaction some observations are made regarding the effects of some factors in order to impede the CuCl sublimation.
3.1.1. Effect of mixing and grinding operations The mixing operation may be taken into account for an isothermal behavior as represented in Fig. 3. Curves 2–6 show the variation of the total mass loss of the sample ground for 1, 3, 4, 6 and 10 min, respectively, with a molar ratio of 4.10 and a temperature of 2908C. Curve 1 shows the time dependence of the sublimation of CuCl when it is mixed with Al 2 O 3 . In all cases, the experimental maximum mass loss (Dm exp ) was slightly greater than that
B. Gillot et al. / International Journal of Inorganic Materials 3 (2001) 1083 – 1089
Fig. 3. Effect of the grinding duration on the reactivity of the Zr / CuCl system with a molar ratio of 4.10 at 2908C. (1) Sublimation of CuCl; ground sample (2) 1, (3) 3, (4) 4, (5) 6 and (6) 10 min.
calculated (Dm cal ) considering the total consumption of CuCl by Zr. The difference (Dm exp 2Dm cal ) decreases with longer grinding times, the reaction rate being enhanced by grinding the sample. This behavior is believed to be due to the grinding which is effective in removing zirconium dioxide layer (native oxide) from the zirconium particles which results from Me–O bond cleavage and generates clean crystallographic planes [15]. The ZrO 2 film, like a chemical barrier, plays a significant part in the initiation and progress of the reaction. Therefore, we studied the effects of a zirconium oxide layer on the Zr / CuCl reaction where the oxide layer is increased deliberately by thermal oxidation of zirconium or where the native oxide layer is thinned by treatment under vacuum.
3.1.2. Effect of the thickness of the ZrO2 layer The experiments have been performed at 2708C for powdered Zr samples either treated under vacuum (10 24 Pa) at 10008C for 12 h to minimise the native oxide layer thickness [16] (Fig. 4, curve 7) or oxidized at 3008C [17,18] for 3 h at different reaction times to produce the ZrO 2 layer growth (curves 2–5). The thickness of the ZrO 2 layer is calculated from the mass gain after oxidation. It should be stated that the evaluation of thickness of ZrO 2 , although determined from thermogravimetric analysis, is probably not quantitative. In general cases, the effects due to finer powder, mechanochemical change and so on are probably caused at the same time. Experiments were also conducted by substituting Zr by pure ZrO 2 (curve 1) or with a Zr sample which does not undergo any treatment (as-received, curve 6). Moreover, it may be noticed that the sublimation of CuCl slows down by mixing it with only pure ZrO 2 or with Zr covered with the thickest ZrO 2 protective layer. The difference Dm exp 2Dm cal observed for long reaction times was associated with the partial
1085
Fig. 4. Effect of the thickness of the ZrO 2 layer on the reactivity of the Zr / CuCl system at 2708C. (1) CuCl mixed with pure ZrO 2 , (2) 450 nm, (3) 210 nm, (4) 130 nm, (5) 64 nm, (6) native oxide layer, (7) Zr treated under vacuum.
sublimation of CuCl which increased with the thickness of the oxide layer.
3.1.3. Proportion of Zr and CuCl in the mixture The molar ratio of Zr to CuCl was varied from 0.66 to 4.10 for a grinding time of 5 min. Fig. 5 shows that the reactivity of the system at 2908C increases with higher mass percentages of Zr because of the increase in the reactive area of Zr. Moreover, in this case, the mass fraction of sublimated CuCl which does not react with Zr increases with a decrease in the molar ratio. 3.2. Solid products of the reaction In the course of this study it became necessary to
Fig. 5. Effect of the molar ratio (m r ) of Zr to CuCl on the reactivity of the Zr / CuCl system for a grinding time of 5 min at 2908C.
1086
B. Gillot et al. / International Journal of Inorganic Materials 3 (2001) 1083 – 1089
identify the formed phases during the reaction between CuCl and Zr by X-ray diffraction. For this purpose, experiments were first conducted under vacuum in a closed system for some reactant ratios of Zr to CuCl: 0.25, 0.45, 0.525 and 1.25 corresponding to almost selective formation of copper, Cu 5 Zr, Cu 51 Zr 14 and CuZr according to the equations: Zr 1 4CuCl → ZrCl 4 1 4Cu
(3)
9 / 5 Zr 1 4CuCl → ZrCl 4 1 4 / 5 Cu 5 Zr
(4)
21 / 10 Zr 1 4CuCl → ZrCl 4 1 11 / 10 Cu 3.64 Zr
(5)
5Zr 1 4CuCl → ZrCl 1 4CuZr
(6)
After 2 days of reaction at 3008C, the X-ray pattern for a reactant ratio of 0.25 mainly exhibited the presence of copper and CuCl indicating an incomplete reaction. Only at the highest temperatures, e.g. T53408C was there a significant decrease in the intensity of CuCl conducting to a strong peak of Cu located at 2u 543.38. For reactions (4)–(6) other phases were detected and between 20 and 50 at.% Cu there occurs a cascade of reactions which obscure the existence of several intermetallic compounds and there is some ambiguity about the proper identification of one line, only very few lines being unique to a particular crystal structure. Therefore, to determine if Cu 5 Zr was present, the D-spacing 2.4287 corresponding to the (220) plane [19] was used (Fig. 6a); for Cu 51 Zr 14 the D-spacing 2.2066 corresponding to the (213) plane [20] was used (Fig. 6a); for Cu 8 Zr 3 the D-spacing 2.0385 corresponding to the (232) plane [21] was used (Fig. 6b); and for Cu 10 Zr 7 the D-spacing 2.2848 corresponding to the (422) plane [22] was used (Fig. 6c). The diffraction peaks can, thereby, be allocated for reaction (4) to Cu, Cu 5 Zr and Cu 51 Zr 14 (Fig. 6, curve a). For a molar ratio of 0.525, the X-ray pattern mainly shows after 2 days of reaction at 5508C the peaks of Cu 8 Zr 3 (Fig. 6, curve b) although some lines also can be tentatively identified as Cu 51 Zr 14 . When the molar ratio was 1.25 and for a sample heat treated at 6008C for 2 days, the X-ray pattern (Fig. 6, curve c) exhibits almost completely Cu 10 Zr 7 peaks which present a pronounced feature between 2u 537 and 428 [23]. X-ray analysis did not show the presence of the CuZr phase for different heat treatments. Based on these considerations of the X-ray diffraction data, the phases obtained for a reduction carried out in thermogravimetry at 3238C (open system) with zirconium excess (m r 5 4.10) can be confirmed as Cu and Cu 5 Zr (Fig. 7) [24]. For the pronounced feature between 2u 543 and 448, the diffraction peak consists of two distinct components (Fig. 7). The peak at the lower Bragg angle is rather narrow and can be assigned to Cu(111) reflection shifted by 2u 50.058 with respect to pure copper. The second component at higher diffraction angle is attribut-
Fig. 6. X-ray diffraction patterns obtained for the reaction between Zr and CuCl in a closed system for different molar ratios. (a) 0.45, (b) 0.525 and (c) 1.25.
able to the (311) reflection of Cu 5 Zr. The intensity of this diffraction peak decreases with both the reactant ratio and the reaction time (Fig. 7, curves a and b). We can postulate that for longer reaction times, Cu 5 Zr reacts with CuCl to form Cu as final phase according to the reaction: Cu 5 Zr 1 4CuCl → ZrCl 4 1 9Cu
(7)
The other phases obtained in the closed system were not observed until after 2 h of heat treatment.
3.3. Kinetics study The isothermal TG curves Dm 5 f(t) for the reaction
B. Gillot et al. / International Journal of Inorganic Materials 3 (2001) 1083 – 1089
1087
CuCl sublimation is impeded. The curves are the typical S-shape characteristic of the nucleation and growth mechanism of the phases formed at the active sites in the crystal, i.e. the rate increased initially, reached a maximum and then decreased [25]. The d(Dm / dt) curves (Fig. 8b) allow us to determine the maximum rate located at ai 5 Dm i / Dm exp . It is evident from Fig. 8b that ai (ai 5 0.48) can be considered as constant in the range 291–3348C. The isothermal curves a 5 f(t) can be superimposed for 0.10, a ,0.85, where a is the fractional mass change at time t defined by a 5 Dm t /Dm exp . A master run roughly in the middle of the series was chosen and a factor A found for each run such that multiplication of the time scale of the run by A would superimpose it onto the master run curve. The activation energy may then be calculated by plotting log A vs. 1 /T or by plotting the logarithm of the instantaneous rate vi 5 da / dt vs. 1 /T at different constant values of a. In both cases an activation energy of 9363 kJ mol 21 was obtained. The affine character of the curves indicates that the states traversed by the system during the reaction are dependent on intensive variables such as temperature and pressure [26] (in our case, pressure of 10 22 Pa). The rate equation for the process can be written in terms of separated variables as: v 5 da / dt 5 KT Kp e 2E / kT f(a )
(8)
where f(a ) usually represents the kinetic rate expression. At constant temperature and pressure the shape of the curves is determined only by f(a ). We find a very good linear representation of the sigmoid curves if f(a ) is an equation of type:
a f(a ) 5 log a / 1 2 ] 2ai
S S
DD 5 k t 1 C 1
te
(9)
which is the integrated form of the Prout–Tompkins equation [25]. The rate for the process can be thus written as: a da / dt 5 k 1 a 1 2 ] . (10) 2ai
S
Fig. 7. X-ray diffraction patterns obtained at 3238C in thermogravimetry for a molar ratio of 4.10. (a) 15 min of reaction, (b) 50 min of reaction.
between CuCl and Zr (without thermal treatment) at temperatures ranging from 291 to 3348C are shown in Fig. 8a. In order to minimise the mass fraction of CuCl which sublimes without reaction with zirconium and affects the kinetic behavior, we enhanced the reactivity by using a high percentage of zirconium (Zr:CuCl molar ratio of 4.10) and a grinding time of 10 min. For such treatment conditions, the experimental mass loss approaches the calculated value (Dm (exp) ¯Dm (cal) ) suggesting that the
D
From the temperature dependence of k 1 an apparent activation energy of 9263 kJ mol 21 is deduced. This value is very close to that calculated from the affinity ratio. X-ray diffraction results and morphological information show that we must consider an autocatalytic process rather than a branched chain nucleation mechanism, i.e. the Prout–Tompkins model. The plausibility of this is supported by the fact that finely divided copper is one of the reaction products as observed on an SEM micrograph (Fig. 9) of a powder with excess Zr after 20 min of reaction at 3008C. Reactions involving in situ formation of metallic copper are known to be autocatalytic in nature [27]. Thus reduction of CuO and NiO by H 2 as well as by formaldehyde are both autocatalytic reactions [28]. From these considerations the kinetic analysis of TG data can be
1088
B. Gillot et al. / International Journal of Inorganic Materials 3 (2001) 1083 – 1089
Fig. 8. (a) Mass loss vs. time and (b) rate Dm / dt as a function of the temperature.
Fig. 9. SEM micrograph showing the formation of copper nuclei for a reaction time of 30 min at 2908C.
B. Gillot et al. / International Journal of Inorganic Materials 3 (2001) 1083 – 1089
represented by the equation: da / dt 5 k9a (1 2 a )
1089
References (11)
which through integration leads to ai 5 1 / 2. However, this may not always be the case; values of ai other than 0.50 are also possible. Bond [29] in his study on the reduction of CuO (which is autocatalytic) has in fact used a modified form of the Prout–Tompkins equation leading to Eq. (11).
4. Conclusion The present experimental results on the solid state reaction between CuCl and zirconium powders in the temperature range from 290 to 3408C are the primary conclusions of this paper and can be summarized as follows. • From investigations by thermogravimetry under nonisothermal and isothermal conditions the reaction appreciably starts at about 2708C and is significantly affected by the grinding operation and the proportion of Zr and CuCl in the mixture. The reactivity is the greatest for mechanical ground samples with a high molar ratio of Zr to CuCl. • For the reaction of Zr with CuCl the results of the kinetics runs, when plotted as mass loss against time, showed a nucleation–growth mechanism in terms of the Prout–Tompkins equation with an activation energy of 9363 kJ mol 21 . • The presence of both Cu and Cu 5 Zr in an open system was confirmed by X-ray diffraction and the reduction of CuCl by zirconium has been interpreted as an autocatalytic reaction.
[1] Weber G, Viale D, Souha H, Gillot B, Barret P. Solid State Ionics 1989;32–33:250. [2] Gillot B, Radid M. Thermochim Acta 1991;185:63. [3] Radid M, Sbai K, Gillot B. J Alloys Comp 1997;260:172. [4] Gillot B, Radid M, Sbai K, Souha H. J Alloys Comp 1998;274:90. [5] Voorhoeve RJH. Organohalosilanes: precursors to silicones, Amsterdam: Elsevier, 1967. [6] Veer FA, Kolster BH, Burgers WC. Trans Metall Soc AIME 1968;242:669. [7] Becht JGM, Van Loo FJJ, Metselaar R. React Solids 1988;6:61. [8] Tu KN. Acta Metall 1973;21:347. [9] Taguchi O, Iijima Y, Hirano K. J Jpn Inst Metals 1990;54:619. [10] Stelter EC, Lazarus D. Phys Rev B 1987;36:9545. [11] Taguchi O, Iijima Y, Hirano K. J Alloys Comp 1994;215:329. [12] Kneller E, Khan Y, Gorres U. Z Metallkude 1986;77:43. [13] Saur G, Laue HJ, Borchers H. Metallurgia 1961;15:409. [14] Butyagin PY, Pavlichen IK. React Solids 1986;1:361. [15] Watanbe T, Horikoshi Y. Int J Powd Metal Powd Technol 1976;12:209. [16] Jenkins AE. J Inst Metals 1953;82:213. [17] Gubbransen EA, Andrew KF. Trans Metall Soc AIME 1957;209:394. [18] Srivastava LP. Trans Indian Inst Metals 1983;36:335. [19] JCPDS card no. 40-1322 (1997). [20] JCPDS card no. 42-1185 (1997). [21] JCPDS card no. 42-1186 (1997). [22] JCPDS card no. 47-1028 (1997) [23] Rawers JC, Wilson RD. J Mater Sci 1990;25:1392. [24] Forey P, Glimois JL, Feron JL, Devely G, Becle C. CR Acad Sci (Paris) 1980;291:177. [25] Prout EG, Tompkins FC. Trans Faraday Soc 1944;40:488. ´ ´´ ` [26] Barret P. In: Cinetique heterogene, Paris: Gauthier-Villars, 1973, p. 270. [27] Tamhankar SS, Gokarn AN, Doraiswamy LK. Chem Eng Sci 1981;36:1365. [28] Bandrowski J, Bickling CR, Yang KH, Hougen O. Chem Eng Sci 1962;17:379. [29] Bond WD. J Phys Chem 1962;66:1573.
Kinetics of solid state reaction between zirconium and copper(I) chloride a, b c B. Gillot *, H. Souha , M. Radid a
´ ´ des Solides, UMR CNRS 5613, 9 Avenue Alain Savary, BP 47870 -21078, Dijon Cedex, France Laboratoire de Recherches sur la Reactivite b ` , Morocco Laboratoire de Chimie Physique, Faculte´ des Sciences Dhar El Mehraz, BP 1796, Fes c Universite´ Hassan II, Mohammedia, Faculte´ des Sciences Ben M’ sik, BP 6621, Casablanca, Morocco Received 23 October 2000; received in revised form 13 August 2001
Abstract The kinetics of the reaction between Zr and CuCl powders have been studied under vacuum by means of thermogravimetry in the range from 250 to 3308C. The reactivity of the Zr–CuCl system is significantly affected by the mixing operation, the ratio of Zr and CuCl in the mixture and the thickness of the ZrO 2 film on the Zr particles. It is established that the kinetics of reaction was governed by a nucleation and growth mechanism with an apparent activation energy of 9363 kJ mol 21 and for these low reaction temperatures only Cu and Cu 5 Zr were identified. 2001 Published by Elsevier Science Ltd. Keywords: Zr–CuCl system; Thermogravimetry; Nucleation and growth; Autocatalytic process
1. Introduction This study which originates from a basic research on the reactivity of CuCl with IVb metals (Si, Ge, Sn) and IVa metal (Ti) [1–5] is partly motivated by the aim to examine systematically at low temperature (,3508C) the Cu x Me y alloy formation predicted by the Me–Cu phase diagrams and usually investigated at high temperature from the direct reaction between Cu and Me [6–9]. With CuCl these alloys can be formed through successive reactions where the copper is the final phase generated by the reaction: Me (s) 1 4CuCl (s) → MeCl 4 ( g) 1 4Cu (s)
such as the mixing operation, the proportion of Zr and CuCl in the mixture and the thickness of the ZrO 2 film on the grains. For this system only the kinetics of the reaction at high temperature of intermetallic layer formation by diffusion between Cu–Zr films have been investigated [10,11]. According to the phase diagram [12] (Fig. 1), Cu reacts with Zr to form Cu 5 Zr, Cu 51 Zr 14 , Cu 8 Zr 3 , Cu 2 Zr,
(1)
whereas the alloys are intermediates which react for particular conditions with CuCl. This paper reports the results of a detailed study by means of thermogravimetry and X-ray diffraction of the kinetics of the global reaction Zr 1 4CuCl → ZrCl 4 1 4Cu
(2)
Special attention was paid to the occurrence of intermediate compounds and the effects of a few more factors *Corresponding author. Tel.: 133-3-8039-6142; fax: 133-3-80396167. E-mail address: [email protected] (B. Gillot). 1466-6049 / 01 / $ – see front matter 2001 Published by Elsevier Science Ltd. PII: S1466-6049( 01 )00179-9
Fig. 1. Phase diagram of the Cu–Zr system.
1084
B. Gillot et al. / International Journal of Inorganic Materials 3 (2001) 1083 – 1089
Cu 10 Zr 7 , CuZr, Cu 5 Zr 8 and CuZr 2 containing between 17 and 70 at.% Cu.
2. Experimental details The total mass change due to evolution of ZrCl 4 gas was measured as a function of the temperature and time by means of a MacBain thermobalance with a sensitivity of 0.02 mg for various CuCl and zirconium loadings. Zirconium (80 mesh, 99.98% pure, lot 00012207, Alfa Products) and CuCl (300 mesh, 99.999% pure, lot 400151, Alfa Products) powders were ground manually in an agate mortar or mechanically treated by grinding in a disk oscillating mill for different times in a mole ratio Zr / CuCl$0.25. Since Zr oxidizes readily in oxygen atmosphere and water vapor [13] care was taken in all steps of the sample preparation procedure to avoid contamination by oxygen. Then 20 mg of the mixture was spread as a thin layer on the balance arm; the reactor was subsequently evacuated, outgassed in vacuo (10 22 Pa) for 2 h at room temperature and then for 1 h at 1808C before the sample was heated to the temperature of the experiment. The ZrCl 4( g) liberated during the reaction under vacuum was condensed in a liquid nitrogen trap and the mass loss was calculated per 100 mg CuCl. A few runs were carried out with different dry-grinding times and molar ratios, since in the case of a solid–solid reaction it is necessary to mix the substances at the molecular level [14] in order to enhance their reactivity. To ascertain whether the solid reaction products analysed are the same as in thermogravimetry, a parallel set of these experiments was conducted in a closed system where ZrCl 4( g) was allowed to remain in contact with the sample
Fig. 2. Curves Dm 5 f(T ) obtained in vacuum for the reaction between Zr and CuCl powders. (1) CuCl alone in the scoop, (2) CuCl mixed with an inert oxide (Al 2 O 3 ), (3) mixed sample, (4) grinding sample for 1 min, (5) grinding sample for 5 min.
during the course of the reaction. After grinding in an agate mortar, the mixture was placed in a Pyrex glass capsule, sealed under vacuum (10 23 Pa) and then heated at constant temperatures ranging from 300 to 6008C for different reaction times. X-ray diffraction patterns were recorded on an ‘INEL CPS 120’ linear counter (curved position sensitive) equipped with monochromatized Cu K a radiation and calibrated by a quartz standard. The resolution was 0.028 (2u ) at 28 min 21 . The morphological and energy-dispersive X-ray analyses were carried out with a scanning electron microscope (SEM ‘Cambridge’ 250 MK2).
3. Results and discussion
3.1. Reactivity of the system Zr /CuCl by means of thermogravimetry Fig. 2 shows the thermogravimetric curves Dm 5 f(T ) for the sublimation of CuCl when it is placed alone in the scoop (curve 1) or mixed with an inert oxide such as Al 2 O 3 (curve 2). In the absence of an inert oxide the sublimation temperature of CuCl is lowered by ca. 408C. As a result, the temperature at which CuCl begins to react with zirconium can serve as a criterion for the reactivity of the mixture. For comparison, curves 3, 4 and 5 give the variation of mass loss with temperature for a manually mixed sample for 5 min and a sample ground for 1 and 5 min, respectively, with a Zr / CuCl ratio52.17. The theoretical maximum mass loss of the system is calculated (Dm (cal ) 558.9 mg, dashed line) for the complete reduction of CuCl by Zr according to reaction (2). It can be seen from Fig. 2 that the difference between the experimental Dm (exp) and theoretical maximum loss mass decreases with longer grinding times. This implies that the mass fraction of CuCl which sublimes without reaction with Zr decreases as the grinding time is increased. Because the vaporisation of CuCl starts at about 2708C when it is mixed with an inert oxide, the mass loss from the mixture below this temperature is only due to a solid–solid reaction between CuCl and Zr. Above this temperature, we have also to consider that the solid–gas reaction prevails. So, to study the progress of the reaction some observations are made regarding the effects of some factors in order to impede the CuCl sublimation.
3.1.1. Effect of mixing and grinding operations The mixing operation may be taken into account for an isothermal behavior as represented in Fig. 3. Curves 2–6 show the variation of the total mass loss of the sample ground for 1, 3, 4, 6 and 10 min, respectively, with a molar ratio of 4.10 and a temperature of 2908C. Curve 1 shows the time dependence of the sublimation of CuCl when it is mixed with Al 2 O 3 . In all cases, the experimental maximum mass loss (Dm exp ) was slightly greater than that
B. Gillot et al. / International Journal of Inorganic Materials 3 (2001) 1083 – 1089
Fig. 3. Effect of the grinding duration on the reactivity of the Zr / CuCl system with a molar ratio of 4.10 at 2908C. (1) Sublimation of CuCl; ground sample (2) 1, (3) 3, (4) 4, (5) 6 and (6) 10 min.
calculated (Dm cal ) considering the total consumption of CuCl by Zr. The difference (Dm exp 2Dm cal ) decreases with longer grinding times, the reaction rate being enhanced by grinding the sample. This behavior is believed to be due to the grinding which is effective in removing zirconium dioxide layer (native oxide) from the zirconium particles which results from Me–O bond cleavage and generates clean crystallographic planes [15]. The ZrO 2 film, like a chemical barrier, plays a significant part in the initiation and progress of the reaction. Therefore, we studied the effects of a zirconium oxide layer on the Zr / CuCl reaction where the oxide layer is increased deliberately by thermal oxidation of zirconium or where the native oxide layer is thinned by treatment under vacuum.
3.1.2. Effect of the thickness of the ZrO2 layer The experiments have been performed at 2708C for powdered Zr samples either treated under vacuum (10 24 Pa) at 10008C for 12 h to minimise the native oxide layer thickness [16] (Fig. 4, curve 7) or oxidized at 3008C [17,18] for 3 h at different reaction times to produce the ZrO 2 layer growth (curves 2–5). The thickness of the ZrO 2 layer is calculated from the mass gain after oxidation. It should be stated that the evaluation of thickness of ZrO 2 , although determined from thermogravimetric analysis, is probably not quantitative. In general cases, the effects due to finer powder, mechanochemical change and so on are probably caused at the same time. Experiments were also conducted by substituting Zr by pure ZrO 2 (curve 1) or with a Zr sample which does not undergo any treatment (as-received, curve 6). Moreover, it may be noticed that the sublimation of CuCl slows down by mixing it with only pure ZrO 2 or with Zr covered with the thickest ZrO 2 protective layer. The difference Dm exp 2Dm cal observed for long reaction times was associated with the partial
1085
Fig. 4. Effect of the thickness of the ZrO 2 layer on the reactivity of the Zr / CuCl system at 2708C. (1) CuCl mixed with pure ZrO 2 , (2) 450 nm, (3) 210 nm, (4) 130 nm, (5) 64 nm, (6) native oxide layer, (7) Zr treated under vacuum.
sublimation of CuCl which increased with the thickness of the oxide layer.
3.1.3. Proportion of Zr and CuCl in the mixture The molar ratio of Zr to CuCl was varied from 0.66 to 4.10 for a grinding time of 5 min. Fig. 5 shows that the reactivity of the system at 2908C increases with higher mass percentages of Zr because of the increase in the reactive area of Zr. Moreover, in this case, the mass fraction of sublimated CuCl which does not react with Zr increases with a decrease in the molar ratio. 3.2. Solid products of the reaction In the course of this study it became necessary to
Fig. 5. Effect of the molar ratio (m r ) of Zr to CuCl on the reactivity of the Zr / CuCl system for a grinding time of 5 min at 2908C.
1086
B. Gillot et al. / International Journal of Inorganic Materials 3 (2001) 1083 – 1089
identify the formed phases during the reaction between CuCl and Zr by X-ray diffraction. For this purpose, experiments were first conducted under vacuum in a closed system for some reactant ratios of Zr to CuCl: 0.25, 0.45, 0.525 and 1.25 corresponding to almost selective formation of copper, Cu 5 Zr, Cu 51 Zr 14 and CuZr according to the equations: Zr 1 4CuCl → ZrCl 4 1 4Cu
(3)
9 / 5 Zr 1 4CuCl → ZrCl 4 1 4 / 5 Cu 5 Zr
(4)
21 / 10 Zr 1 4CuCl → ZrCl 4 1 11 / 10 Cu 3.64 Zr
(5)
5Zr 1 4CuCl → ZrCl 1 4CuZr
(6)
After 2 days of reaction at 3008C, the X-ray pattern for a reactant ratio of 0.25 mainly exhibited the presence of copper and CuCl indicating an incomplete reaction. Only at the highest temperatures, e.g. T53408C was there a significant decrease in the intensity of CuCl conducting to a strong peak of Cu located at 2u 543.38. For reactions (4)–(6) other phases were detected and between 20 and 50 at.% Cu there occurs a cascade of reactions which obscure the existence of several intermetallic compounds and there is some ambiguity about the proper identification of one line, only very few lines being unique to a particular crystal structure. Therefore, to determine if Cu 5 Zr was present, the D-spacing 2.4287 corresponding to the (220) plane [19] was used (Fig. 6a); for Cu 51 Zr 14 the D-spacing 2.2066 corresponding to the (213) plane [20] was used (Fig. 6a); for Cu 8 Zr 3 the D-spacing 2.0385 corresponding to the (232) plane [21] was used (Fig. 6b); and for Cu 10 Zr 7 the D-spacing 2.2848 corresponding to the (422) plane [22] was used (Fig. 6c). The diffraction peaks can, thereby, be allocated for reaction (4) to Cu, Cu 5 Zr and Cu 51 Zr 14 (Fig. 6, curve a). For a molar ratio of 0.525, the X-ray pattern mainly shows after 2 days of reaction at 5508C the peaks of Cu 8 Zr 3 (Fig. 6, curve b) although some lines also can be tentatively identified as Cu 51 Zr 14 . When the molar ratio was 1.25 and for a sample heat treated at 6008C for 2 days, the X-ray pattern (Fig. 6, curve c) exhibits almost completely Cu 10 Zr 7 peaks which present a pronounced feature between 2u 537 and 428 [23]. X-ray analysis did not show the presence of the CuZr phase for different heat treatments. Based on these considerations of the X-ray diffraction data, the phases obtained for a reduction carried out in thermogravimetry at 3238C (open system) with zirconium excess (m r 5 4.10) can be confirmed as Cu and Cu 5 Zr (Fig. 7) [24]. For the pronounced feature between 2u 543 and 448, the diffraction peak consists of two distinct components (Fig. 7). The peak at the lower Bragg angle is rather narrow and can be assigned to Cu(111) reflection shifted by 2u 50.058 with respect to pure copper. The second component at higher diffraction angle is attribut-
Fig. 6. X-ray diffraction patterns obtained for the reaction between Zr and CuCl in a closed system for different molar ratios. (a) 0.45, (b) 0.525 and (c) 1.25.
able to the (311) reflection of Cu 5 Zr. The intensity of this diffraction peak decreases with both the reactant ratio and the reaction time (Fig. 7, curves a and b). We can postulate that for longer reaction times, Cu 5 Zr reacts with CuCl to form Cu as final phase according to the reaction: Cu 5 Zr 1 4CuCl → ZrCl 4 1 9Cu
(7)
The other phases obtained in the closed system were not observed until after 2 h of heat treatment.
3.3. Kinetics study The isothermal TG curves Dm 5 f(t) for the reaction
B. Gillot et al. / International Journal of Inorganic Materials 3 (2001) 1083 – 1089
1087
CuCl sublimation is impeded. The curves are the typical S-shape characteristic of the nucleation and growth mechanism of the phases formed at the active sites in the crystal, i.e. the rate increased initially, reached a maximum and then decreased [25]. The d(Dm / dt) curves (Fig. 8b) allow us to determine the maximum rate located at ai 5 Dm i / Dm exp . It is evident from Fig. 8b that ai (ai 5 0.48) can be considered as constant in the range 291–3348C. The isothermal curves a 5 f(t) can be superimposed for 0.10, a ,0.85, where a is the fractional mass change at time t defined by a 5 Dm t /Dm exp . A master run roughly in the middle of the series was chosen and a factor A found for each run such that multiplication of the time scale of the run by A would superimpose it onto the master run curve. The activation energy may then be calculated by plotting log A vs. 1 /T or by plotting the logarithm of the instantaneous rate vi 5 da / dt vs. 1 /T at different constant values of a. In both cases an activation energy of 9363 kJ mol 21 was obtained. The affine character of the curves indicates that the states traversed by the system during the reaction are dependent on intensive variables such as temperature and pressure [26] (in our case, pressure of 10 22 Pa). The rate equation for the process can be written in terms of separated variables as: v 5 da / dt 5 KT Kp e 2E / kT f(a )
(8)
where f(a ) usually represents the kinetic rate expression. At constant temperature and pressure the shape of the curves is determined only by f(a ). We find a very good linear representation of the sigmoid curves if f(a ) is an equation of type:
a f(a ) 5 log a / 1 2 ] 2ai
S S
DD 5 k t 1 C 1
te
(9)
which is the integrated form of the Prout–Tompkins equation [25]. The rate for the process can be thus written as: a da / dt 5 k 1 a 1 2 ] . (10) 2ai
S
Fig. 7. X-ray diffraction patterns obtained at 3238C in thermogravimetry for a molar ratio of 4.10. (a) 15 min of reaction, (b) 50 min of reaction.
between CuCl and Zr (without thermal treatment) at temperatures ranging from 291 to 3348C are shown in Fig. 8a. In order to minimise the mass fraction of CuCl which sublimes without reaction with zirconium and affects the kinetic behavior, we enhanced the reactivity by using a high percentage of zirconium (Zr:CuCl molar ratio of 4.10) and a grinding time of 10 min. For such treatment conditions, the experimental mass loss approaches the calculated value (Dm (exp) ¯Dm (cal) ) suggesting that the
D
From the temperature dependence of k 1 an apparent activation energy of 9263 kJ mol 21 is deduced. This value is very close to that calculated from the affinity ratio. X-ray diffraction results and morphological information show that we must consider an autocatalytic process rather than a branched chain nucleation mechanism, i.e. the Prout–Tompkins model. The plausibility of this is supported by the fact that finely divided copper is one of the reaction products as observed on an SEM micrograph (Fig. 9) of a powder with excess Zr after 20 min of reaction at 3008C. Reactions involving in situ formation of metallic copper are known to be autocatalytic in nature [27]. Thus reduction of CuO and NiO by H 2 as well as by formaldehyde are both autocatalytic reactions [28]. From these considerations the kinetic analysis of TG data can be
1088
B. Gillot et al. / International Journal of Inorganic Materials 3 (2001) 1083 – 1089
Fig. 8. (a) Mass loss vs. time and (b) rate Dm / dt as a function of the temperature.
Fig. 9. SEM micrograph showing the formation of copper nuclei for a reaction time of 30 min at 2908C.
B. Gillot et al. / International Journal of Inorganic Materials 3 (2001) 1083 – 1089
represented by the equation: da / dt 5 k9a (1 2 a )
1089
References (11)
which through integration leads to ai 5 1 / 2. However, this may not always be the case; values of ai other than 0.50 are also possible. Bond [29] in his study on the reduction of CuO (which is autocatalytic) has in fact used a modified form of the Prout–Tompkins equation leading to Eq. (11).
4. Conclusion The present experimental results on the solid state reaction between CuCl and zirconium powders in the temperature range from 290 to 3408C are the primary conclusions of this paper and can be summarized as follows. • From investigations by thermogravimetry under nonisothermal and isothermal conditions the reaction appreciably starts at about 2708C and is significantly affected by the grinding operation and the proportion of Zr and CuCl in the mixture. The reactivity is the greatest for mechanical ground samples with a high molar ratio of Zr to CuCl. • For the reaction of Zr with CuCl the results of the kinetics runs, when plotted as mass loss against time, showed a nucleation–growth mechanism in terms of the Prout–Tompkins equation with an activation energy of 9363 kJ mol 21 . • The presence of both Cu and Cu 5 Zr in an open system was confirmed by X-ray diffraction and the reduction of CuCl by zirconium has been interpreted as an autocatalytic reaction.
[1] Weber G, Viale D, Souha H, Gillot B, Barret P. Solid State Ionics 1989;32–33:250. [2] Gillot B, Radid M. Thermochim Acta 1991;185:63. [3] Radid M, Sbai K, Gillot B. J Alloys Comp 1997;260:172. [4] Gillot B, Radid M, Sbai K, Souha H. J Alloys Comp 1998;274:90. [5] Voorhoeve RJH. Organohalosilanes: precursors to silicones, Amsterdam: Elsevier, 1967. [6] Veer FA, Kolster BH, Burgers WC. Trans Metall Soc AIME 1968;242:669. [7] Becht JGM, Van Loo FJJ, Metselaar R. React Solids 1988;6:61. [8] Tu KN. Acta Metall 1973;21:347. [9] Taguchi O, Iijima Y, Hirano K. J Jpn Inst Metals 1990;54:619. [10] Stelter EC, Lazarus D. Phys Rev B 1987;36:9545. [11] Taguchi O, Iijima Y, Hirano K. J Alloys Comp 1994;215:329. [12] Kneller E, Khan Y, Gorres U. Z Metallkude 1986;77:43. [13] Saur G, Laue HJ, Borchers H. Metallurgia 1961;15:409. [14] Butyagin PY, Pavlichen IK. React Solids 1986;1:361. [15] Watanbe T, Horikoshi Y. Int J Powd Metal Powd Technol 1976;12:209. [16] Jenkins AE. J Inst Metals 1953;82:213. [17] Gubbransen EA, Andrew KF. Trans Metall Soc AIME 1957;209:394. [18] Srivastava LP. Trans Indian Inst Metals 1983;36:335. [19] JCPDS card no. 40-1322 (1997). [20] JCPDS card no. 42-1185 (1997). [21] JCPDS card no. 42-1186 (1997). [22] JCPDS card no. 47-1028 (1997) [23] Rawers JC, Wilson RD. J Mater Sci 1990;25:1392. [24] Forey P, Glimois JL, Feron JL, Devely G, Becle C. CR Acad Sci (Paris) 1980;291:177. [25] Prout EG, Tompkins FC. Trans Faraday Soc 1944;40:488. ´ ´´ ` [26] Barret P. In: Cinetique heterogene, Paris: Gauthier-Villars, 1973, p. 270. [27] Tamhankar SS, Gokarn AN, Doraiswamy LK. Chem Eng Sci 1981;36:1365. [28] Bandrowski J, Bickling CR, Yang KH, Hougen O. Chem Eng Sci 1962;17:379. [29] Bond WD. J Phys Chem 1962;66:1573.