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On the state of W in electrodeposited Ni–W alloys
Electrochimica Acta 54 (2009) 2616–2620
Contents lists available at ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
On the state of W in electrodeposited Ni–W alloys ∗ ˙ ¯ R. Juˇskenas , I. Valsiunas, V. Pakˇstas, R. Giraitis Institute of Chemistry, A. Goˇstauto 9, LT-01108 Vilnius, Lithuania
a r t i c l e
i n f o
Article history: Received 1 July 2008 Received in revised form 16 October 2008 Accepted 29 October 2008 Available online 8 November 2008 Keywords: Electrodeposition Ni–W alloy Phase composition X-ray diffraction
a b s t r a c t We have derived an expression for the theoretical dependence of the Ni–W fcc phase lattice parameter on the atomic fraction of W, in a solid solution. This parameter was used to compare the actual quantity of W, in a solid solution of electrodeposited Ni–W alloys, with the value determined by EDX. It has been shown that not all of the W determined by EDX was present as a solute in the Ni–W fcc phase. The incorporation of tungstates prevailed at lower pH, while the presence of citric complexes dominated at higher pH values. This conclusion was based on the XRD phase analysis of electrodeposited alloys that were annealed in hydrogen or argon atmospheres at 1000 ◦ C. The XRD data correlated well with the morphology results obtained via AFM. The incorporation of tungstates or citric complexes seems to be characteristic of the electrodeposited Ni–W alloys. © 2008 Elsevier Ltd. All rights reserved.
1. Introduction This paper deals with a topical problem for those who study the electrodeposited Ni–W alloys. In what form does tungsten exist within an alloy? Is all of the tungsten present as a solute within a solid solution of Ni–W? As the present work will show not all of the tungsten quantified by EDX is present as a solute within a nickel solution. The majority of the literature that reports on the electrodeposition of Ni–W alloys, presents XRD patterns as evidence suggesting the formation of a nanostructured Ni–W solid solution or a ␥ phase [1–6]. It is generally assumed that the amount of tungsten in the state of solid solution in nickel equals that determined by EDX. However, few researchers report the lattice parameter, aNi–W , or the XRD peak 1 1 1 diffraction angle of the fcc Ni–W phase. With these quantities, one can determine the actual amount of W in the ␥ phase [3,7–10]. If one compares previously reported values for the lattice parameter, a rather appreciable discrepancy can be seen: 20 at.% W, aNi–W = 0.360 nm [3], 19.5 at.% W, aNi–W = 0.3576 nm [7], 20 at.% W, aNi–W = 0.3573 nm [8]. Only Younes et al. [9,10], on the basis of the established lattice parameter (0.3531 nm), noted that only 2 at.% of the W was in the form of a solid solution and that the remaining 7 at.% measured by EDX was likely located in the grain boundaries. Some researchers assumed that a certain proportion of electrodeposited Ni–W can be present as an amorphous state [11] and in the range of tungsten composition, from 20 to 40 at.%, all of the electrodeposited Ni–W is amorphous [2,12].
∗ Corresponding author. Tel.: +370 5 264 8881; fax: +370 5 264 9774. ˙ E-mail address: [email protected] (R. Juˇskenas). 0013-4686/$ – see front matter © 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2008.10.060
The linear relation, between the lattice parameter of the Ni–W fcc phase and the W content within the phase, obeys Vegard’s Law. This relation can either be obtained experimentally or calculated. The dependence of aNi–W on the atomic fraction of W in a Ni–W solid solution can be found in graphical [13] and analytical forms [14]. The goal of the present work is to show that the total amount of W in an electrodeposited Ni–W alloy does not coincide with the tungsten content in a Ni–W solid solution or that in the Ni–W ␥ phase. The work attempts to determine the state in which the remaining W is present within electrodeposited Ni–W alloys; furthermore, a general discussion of the topic is presented. 2. Experimental Ni–W alloys were electrodeposited in an aqueous bath containing 0.1 mol dm−3 NiSO4 , 0.3 mol dm−3 Na2 WO4 , and 0.28 mol dm−3 citric acid (H3 cit). The pH was adjusted by adding sulphuric acid. All of the solutions were prepared from reagent grade (NiSO4 ·7H2 O, Na2 WO4 ·2H2 O) and analytical grade (H3 cit, H2 SO4 ) components, using triple distilled water. The solution temperature was maintained at 60 ◦ C. Steel electrodes, with a 1 cm2 area, served as the working electrode; a platinum plate was used as the anode. Both electrodes were kept in a vertical position and the solution was not agitated. The steel cathodes were boiled for 5 min in an aqueous solution containing 200 g dm−3 NaOH and 20 g dm−3 EDTA, rinsed in distilled water and activated for 10 seconds in an HCl solution (1:1) prior to electrodeposition. The deposition was carried out under galvanostatic conditions at a current density of 4 A dm−2 . The thickness of the Ni–W electroplates was about 12–14 m. After deposition, the coatings were washed in distilled water, and
R. Juˇskenas et al. / Electrochimica Acta 54 (2009) 2616–2620 ˙
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dehumidified. XRD, EDX and AFM measurements were performed approximately 7–10 min after deposition. X-ray diffraction measurements were performed with a Bruker’s D8 Advance diffractometer, equipped with a primary beam monochromator for Cu K␣ radiation. The surface morphology of the Ni–W deposits was examined with AFM in the contact mode (SPM Explorer, ThermoMicroscopes). The chemical composition of the alloys was determined by EDX and WDX (Oxford Instruments). 3. Results and discussion The dependence of the lattice parameter, aNi–W , on the atomic fraction, XW , of W in the solid solution obeys Vegard’s Law. When atoms in a nickel lattice are replaced with tungsten atoms, aNi–W increases. This is due to an enlarged value for the average closest distance between atoms, acd , in the lattice. The increase is directly proportional to the atomic fraction of W, XW , and the difference between the atomic radius of W (rW = 0.13705 nm) and Ni (rNi = 0.1246 nm). While calculating the lattice parameter, one should consider that tungsten has a body centered cubic (bcc) lattice, whereas nickel possesses a face centered (fcc) lattice. In an fcc lattice, the atomic radius of W expands by a factor of 1.03 and equals 0.14116 nm. The relation between the fcc lattice parameter, aNi–W , and the average closest distance between atoms, acd , is expressed by the following equation [15]: √ aNi–W = acd 2 (1) For pure nickel: acd = 2 × rNi
(2)
When nickel atoms are substituted for the atomic fraction, XW , of W: acd = 2 [rNi + (rW − rNi ) · XW ]
(3)
The dependence of aNi–W on the atomic fraction XW was calculated using Eqs. (1) and (3). This relationship is shown in Fig. 1. The trend shown in Fig. 1 is consistent with the results presented by other authors. It should be noted that in [13], the dependence was obtained using different theoretical background — a molecular dynamics simulation. Furthermore, in [14], the value of the lattice parameter of the pure Ni was too low. Fig. 1 presents one more dependence, aNi–W ∼ f(XW ). This was calculated without taking into account the increase in rW , by a factor of 1.03. Using the
Fig. 1. Lattice parameter, aNi–W , of the Ni–W fcc phase as a function of atomic fraction of W in the phase.
Fig. 2. Fragments of XRD patterns for the Ni–W alloys electrodeposited at different pHs.
linear dependence presented in Fig. 1, an expression was derived for the tungsten fraction in the Ni–W solid solution from aNi–W of the ␥ phase: XW = −7.5208 + 2.13429 × aNi–W
(4)
The equation for the fraction of molybdenum in a solid solution with nickel can be constructed by the same procedure: XMo = −7.90906 + 2.24447 × aNi–Mo
(5)
In both expressions, the lattice parameters, aNi–W and aNi–Mo , are given in angstroms. Fig. 2 shows a series of XRD patterns for Ni–W coatings electrodeposited in solutions with different pH. The patterns present ␥ phase peaks of 1 1 1 with different values of 2max and of the full width at the half maximum (FWHM). The patterns indicate that the W content in the electrodeposited ␥ phase and the crystallite’s size (the reciprocal of FWHM) depend on pH. The XRD patterns for coatings obtained at pH 4 and 4.5 present an additional peak at 2 ≈ 41.4◦ . We have shown elsewhere [16] that this peak corresponds to NiWO4 . Hence, in the latter case, a certain proportion of the W determined by EDX is present as a tungstate phase. Using these XRD patterns, aNi–W values were determined; the atomic fraction of W in the ␥ phase was derived using expression (4). These results are shown in Fig. 3. The total amount of W in the alloy was determined by EDX and is also presented in Fig. 3. Not all of the W measured by EDX was in the ␥ phase. To reveal the state in which the rest of the tungsten was present in the alloys the samples were annealed. One group of coatings was annealed for 2 h in argon and another under a hydrogen atmosphere at
Fig. 3. The total quantity of W in the coating and W content in the Ni–W fcc phase, as a function of pH.
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Fig. 4. XRD patterns for the Ni–W alloy, as-deposited at pH 5 and annealed in Ar for 2 h at 1000 ◦ C.
Fig. 5. XRD patterns for the Ni–W alloy, as-deposited at pH 5 and annealed in H2 for 2 h at 1000 ◦ C.
1000 ◦ C. Afterwards, the XRD patterns were evaluated. The atomic fraction of W in the solid solution, for the annealed coatings, is shown in Fig. 3. After annealing, the W content in the solid solution became almost independent of pH; however, this content remained higher than the limiting value of the tungsten solubility in nickel (12.0–13.5 at.%). For the alloys deposited at pH 4–4.5 and 6–7.5 the annealing increased the W content in the solid solution. At pH 5–6, this value decreased slightly. Fig. 4 shows the XRD pattern for a Ni–W coating, electrodeposited at pH 5 and annealed in argon. This pattern is evidence that during annealing some new phases either form or become detectable. The new peaks are attributed to Ni4 W (PDF no. 15755) and Ni6 W6 C. The latter phase was identified using PDF no. 23-1127 for Fe6 W6 C. The corresponding peaks of the pattern were slightly shifted to higher diffraction angles, with respect to peaks in Fe6 W6 C. This is because the atomic radius of nickel in the fcc lattice of Ni6 W6 C is smaller than that of iron. Table 1 presents the measured and calculated values of the interplanar distances, d, for the Ni6 W6 C phase. It is noteworthy that some of the Ni6 W6 C XRD peaks are similar to those of the NiW phase (PDF no. 47-1172), which can lead to the drawing of inappropriate conclusions. According to Fig. 3, the tungsten content in the ␥ phase remained nearly constant after annealing. Thus, some of the W was in the different states (tungstate, Ni4 W) in the as-deposited coating. One of these states could be a nanocrystalline phase of Ni4 W, which became detectable as a result of the recrystallization. However, in our opinion, the formation of Ni4 W during the heat-treatment is more probable. The formation of a carbide, Ni6 W6 C, is evidence that some kind of citrate species was incorporated into the coating. The tungstate, could
be reduced by carbon with the simultaneous formation of carbide at higher temperatures [17]: WO3 + 4C → WC + 3CO
(6)
or 6NiWO4 → 25C = Ni6 W6 C + 24CO
(7)
Fig. 5 shows the XRD pattern for a Ni–W coating, electrodeposited at pH 5 and annealed in the hydrogen. The tungsten XRD peaks only emerged along with those of the ␥ phase after the annealing in the hydrogen. The reduction of nickel tungstate by hydrogen can proceed in two steps [18] resulting in the formation of a pure W phase: NiWO4 + 2H2 = Ni + WO2 + 2H2 O
(8)
and WO2 + 2H2 = W + 2H2 O
(9)
The tungsten XRD peaks are slightly shifted towards higher diffraction angles when than the 2 angles presented in PDF no. 04-0806. This can be indicative of the formation of a nickel solid solution in tungsten. Fig. 6 shows the integral intensity ratios of the Ni6 W6 C 5 1 1 and ␥ NiW 1 1 1 peaks along with the W 1 1 0 and ␥ NiW 1 1 1 peaks.
Table 1 Observed and calculated d values for the Ni6 W6 C phase with lattice parameter a = 10.8941 Å. d (obs. max) (Å)
d (calculated) (Å)
hkl
2.7236 2.4991 2.2239 2.0959 1.9259 – – – – – 1.5254 1.4182
2.7236 2.4994 2.2238 2.0966 1.9259 1.8415 1.8157 1.7226 1.6424 1.5725 1.5255 1.4183
400 331 422 511 440 531 442 620 622 444 551 731
Fig. 6. Integral intensity ratios of XRD peaks W 1 1 0/␥ NiW 1 1 1 and Ni6 W6 C 5 1 1/␥ NiW 1 1 1, as a function of pH.
R. Juˇskenas et al. / Electrochimica Acta 54 (2009) 2616–2620 ˙
These ratios are proportional to the quantity of Ni6 W6 C and W phases in the respective annealed alloys. Due to a wide scatter in the ratio values, the trend is only visible with a polynomial approximation of the data. The annealed coatings deposited at pH 5–6 contain the least amount of carbide and tungsten phases. The quantity of the tungsten phase increased at the lower pH value, while at higher pH, the amount of carbide rose more significantly. The quantity of W and Ni6 W6 C formed could be greater if one considers that a part of the W formed during annealing dissolved in the ␥ phase. This was evident from the increased XW for alloys deposited at low and high pH (Fig. 3). Fig. 7 presents the AFM surface images of the Ni–W coatings, electrodeposited in the pH range from 4 to 7.5. The surface
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morphology of the coatings deposited at pH 4.0–4.5 consists of grains 200–250 nm in diameter. At pH 5–6, the size of grains rises up to 1500–2500 nm. The surface appears to be covered with a fine-granular structure. This was most likely tungstate, since it disappeared after annealing in the hydrogen atmosphere (Fig. 7f). At pH 7.5 (Fig. 7e), the grains were covered with a greater amount of tungstate. This surface consisted of nanosized crystallites. The assumption that the surface was covered with the nanosize structure would be in agreement with a broad XRD peak, Ni–W 1 1 1, on the pattern for pH 7.5 (Fig. 2). A small crystallite size usually points to the fact that the crystallization was inhibited by the adsorption of impurities, or some other species, on the surface of the growing crystallites.
Fig. 7. AFM images of the surface of the Ni–W coatings electrodeposited at different values of pH.
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The results presented prove that not all of the tungsten measured by EDX is present as a solute in a Ni–W solid solution. According to the results of our previous studies [16], and the results of XRD phase analysis of the annealed coatings in the present work, a portion of the tungsten identified by EDX was present as tungstates. The carbide found after annealing in the Ar atmosphere is indicative of incorporation of citric species into the electrodeposited Ni–W alloy. This observation is more significant at higher pHs. According to the increased quantity of the W phase formed during annealing in hydrogen atmosphere, insertion of tungstates should prevail at lower pH. The lowest quantity of the impurities in the electrodeposited alloys studied was observed at pH 5–6. The dependence of the quantity of impurities on the pH at which coatings were deposited correlates with the dependence of the grain and crystallite size on pH. The decrease in grain and crystallite size usually results from the adsorption of various species during crystallization. It may be argued that the reason for the discrepancy between the EDX and XRD data results from an inconsistent composition of the electrolyte. However, in different studies, in which the correct electrolyte composition was used, the difference between the W content determined by EDX and that which should yield XRD data by Eq. (4) is apparent: 20 and 16.3 [3], 19.5 and 11.1 [7], 20 and 10.5 [8], 7 and 1.5 at.% [9]. Furthermore, according to XPS data there was no oxygen present in the deepest layers of the Ni–W coatings, nevertheless, the analysis of the XRD patterns of samples annealed at 500 ◦ C revealed that the XRD peaks of NiWO4 emerged [8]. However, it has been shown in [19] that NiWO4 was formed at annealing temperatures of no less than 1000 ◦ C, even when the heat-treatment of the Ni-3 at.% W alloy was carried out under oxygen flow. It is thus possible that the nickel tungstate was already present in the as-deposited coating in [8]. After annealing at 650 ◦ C, both the XRD peaks of NiWO4 and carbide, Ni6 W6 C, emerged [8]. This carbide phase has also been reported for samples annealed at 600 ◦ C [3] (see Fig. 1 in [3] and Table 1 in the current paper).
4. Conclusions The results we reported suggest that not all of the W measured by EDX, in electrodeposited Ni–W alloys, is present as a tungsten solid solution in nickel, which seems to be a common phenomenon for the electrodeposited Ni–W alloys. Although it is usually assumed that in the bulk of electrodeposited Ni–W alloys, there is no oxygen or carbon, or that the percentage of O and C are negligible, this assumption is not universally true. These are facts to consider when studying the physical and chemical properties of electrodeposited Ni–W alloys. References [1] T. Yamasaki, P. Schloßmacher, K. Ehrlich, Y. Ogino, Nano Struct. Mater. 10 (1998) 375. [2] T. Yamasaki, Mater. Phys. Mech. 1 (2000) 127. [3] P. Schloßmacher, T. Yamasaki, Microchim. Acta 132 (2000) 309. [4] H. Cesiulis, A. Baltutiene, M. Donten, M.L. Donten, Z. Stojek, J. Solid State Electrochem. 6 (2002) 237. [5] H. Wang, S. Yao, S. Matsumura, Surf. Coat. Technol. 157 (2002) 166. [6] M. Donten, Z. Stojek, H. Cesiulis, J. Electrochem. Soc. 150 (2003) C95. [7] H. Somekawa, T.G. Nieh, K. Higashi, Scr. Mater. 50 (2004) 1361. [8] M. Donten, J. Solid State Electrochem. 3 (1999) 87. [9] O. Younes, L. Zhu, Y. Rosenberg, Y. Shacham-Diamand, E. Gileadi, Langmuir 17 (2001) 8270. [10] L. Zhu, O. Younes, N. Ashkenasy, Y. Shacham-Diamand, E. Gileadi, Appl. Surf. Sci. 200 (2002) 1. [11] N.D. Sulitanu, Mater. Sci. Eng. B95 (2002) 230. [12] O. Younes, E. Gileadi, Electrochem. Solid-State Lett. 3 (2000) 543. [13] L.T. Kong, J.B. Liu, W.S. Lal, B.X. Liu, J. Alloy Compd. 337 (2002) 143. [14] A.J. Detor, Ch.A. Schuh, Acta Mater. 55 (2007) 371. [15] B.D. Cullity, S.R. Stock, Elements of X-Ray Diffraction, third edition, AddisonWesley, 2001, p. 664. ˙ ¯ ˙ [16] R. Juˇskenas, I. Valsiunas, V. Pakˇstas, A. Selskis, V. Jasulaitiene, V. Karpaviˇciene, V. Kapoˇcius, Appl. Surf. Sci. 253 (2006) 1435. [17] E. Lassner, W.-D. Schubert, Tungsten, Kluwer Academic/Plenum Publishers, New York, 1999, p. 433. [18] R. Morales, F.J. Tavera, R.E. Aune, S. Seetharaman, Scand. J. Metall. 34 (2005) 108. [19] T.G. Woodcock, Y.L. Cheung, J.R.A. Grenfell, J.S. Abell, Supercond. Sci. Technol. 18 (2005) 721.
Contents lists available at ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
On the state of W in electrodeposited Ni–W alloys ∗ ˙ ¯ R. Juˇskenas , I. Valsiunas, V. Pakˇstas, R. Giraitis Institute of Chemistry, A. Goˇstauto 9, LT-01108 Vilnius, Lithuania
a r t i c l e
i n f o
Article history: Received 1 July 2008 Received in revised form 16 October 2008 Accepted 29 October 2008 Available online 8 November 2008 Keywords: Electrodeposition Ni–W alloy Phase composition X-ray diffraction
a b s t r a c t We have derived an expression for the theoretical dependence of the Ni–W fcc phase lattice parameter on the atomic fraction of W, in a solid solution. This parameter was used to compare the actual quantity of W, in a solid solution of electrodeposited Ni–W alloys, with the value determined by EDX. It has been shown that not all of the W determined by EDX was present as a solute in the Ni–W fcc phase. The incorporation of tungstates prevailed at lower pH, while the presence of citric complexes dominated at higher pH values. This conclusion was based on the XRD phase analysis of electrodeposited alloys that were annealed in hydrogen or argon atmospheres at 1000 ◦ C. The XRD data correlated well with the morphology results obtained via AFM. The incorporation of tungstates or citric complexes seems to be characteristic of the electrodeposited Ni–W alloys. © 2008 Elsevier Ltd. All rights reserved.
1. Introduction This paper deals with a topical problem for those who study the electrodeposited Ni–W alloys. In what form does tungsten exist within an alloy? Is all of the tungsten present as a solute within a solid solution of Ni–W? As the present work will show not all of the tungsten quantified by EDX is present as a solute within a nickel solution. The majority of the literature that reports on the electrodeposition of Ni–W alloys, presents XRD patterns as evidence suggesting the formation of a nanostructured Ni–W solid solution or a ␥ phase [1–6]. It is generally assumed that the amount of tungsten in the state of solid solution in nickel equals that determined by EDX. However, few researchers report the lattice parameter, aNi–W , or the XRD peak 1 1 1 diffraction angle of the fcc Ni–W phase. With these quantities, one can determine the actual amount of W in the ␥ phase [3,7–10]. If one compares previously reported values for the lattice parameter, a rather appreciable discrepancy can be seen: 20 at.% W, aNi–W = 0.360 nm [3], 19.5 at.% W, aNi–W = 0.3576 nm [7], 20 at.% W, aNi–W = 0.3573 nm [8]. Only Younes et al. [9,10], on the basis of the established lattice parameter (0.3531 nm), noted that only 2 at.% of the W was in the form of a solid solution and that the remaining 7 at.% measured by EDX was likely located in the grain boundaries. Some researchers assumed that a certain proportion of electrodeposited Ni–W can be present as an amorphous state [11] and in the range of tungsten composition, from 20 to 40 at.%, all of the electrodeposited Ni–W is amorphous [2,12].
∗ Corresponding author. Tel.: +370 5 264 8881; fax: +370 5 264 9774. ˙ E-mail address: [email protected] (R. Juˇskenas). 0013-4686/$ – see front matter © 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2008.10.060
The linear relation, between the lattice parameter of the Ni–W fcc phase and the W content within the phase, obeys Vegard’s Law. This relation can either be obtained experimentally or calculated. The dependence of aNi–W on the atomic fraction of W in a Ni–W solid solution can be found in graphical [13] and analytical forms [14]. The goal of the present work is to show that the total amount of W in an electrodeposited Ni–W alloy does not coincide with the tungsten content in a Ni–W solid solution or that in the Ni–W ␥ phase. The work attempts to determine the state in which the remaining W is present within electrodeposited Ni–W alloys; furthermore, a general discussion of the topic is presented. 2. Experimental Ni–W alloys were electrodeposited in an aqueous bath containing 0.1 mol dm−3 NiSO4 , 0.3 mol dm−3 Na2 WO4 , and 0.28 mol dm−3 citric acid (H3 cit). The pH was adjusted by adding sulphuric acid. All of the solutions were prepared from reagent grade (NiSO4 ·7H2 O, Na2 WO4 ·2H2 O) and analytical grade (H3 cit, H2 SO4 ) components, using triple distilled water. The solution temperature was maintained at 60 ◦ C. Steel electrodes, with a 1 cm2 area, served as the working electrode; a platinum plate was used as the anode. Both electrodes were kept in a vertical position and the solution was not agitated. The steel cathodes were boiled for 5 min in an aqueous solution containing 200 g dm−3 NaOH and 20 g dm−3 EDTA, rinsed in distilled water and activated for 10 seconds in an HCl solution (1:1) prior to electrodeposition. The deposition was carried out under galvanostatic conditions at a current density of 4 A dm−2 . The thickness of the Ni–W electroplates was about 12–14 m. After deposition, the coatings were washed in distilled water, and
R. Juˇskenas et al. / Electrochimica Acta 54 (2009) 2616–2620 ˙
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dehumidified. XRD, EDX and AFM measurements were performed approximately 7–10 min after deposition. X-ray diffraction measurements were performed with a Bruker’s D8 Advance diffractometer, equipped with a primary beam monochromator for Cu K␣ radiation. The surface morphology of the Ni–W deposits was examined with AFM in the contact mode (SPM Explorer, ThermoMicroscopes). The chemical composition of the alloys was determined by EDX and WDX (Oxford Instruments). 3. Results and discussion The dependence of the lattice parameter, aNi–W , on the atomic fraction, XW , of W in the solid solution obeys Vegard’s Law. When atoms in a nickel lattice are replaced with tungsten atoms, aNi–W increases. This is due to an enlarged value for the average closest distance between atoms, acd , in the lattice. The increase is directly proportional to the atomic fraction of W, XW , and the difference between the atomic radius of W (rW = 0.13705 nm) and Ni (rNi = 0.1246 nm). While calculating the lattice parameter, one should consider that tungsten has a body centered cubic (bcc) lattice, whereas nickel possesses a face centered (fcc) lattice. In an fcc lattice, the atomic radius of W expands by a factor of 1.03 and equals 0.14116 nm. The relation between the fcc lattice parameter, aNi–W , and the average closest distance between atoms, acd , is expressed by the following equation [15]: √ aNi–W = acd 2 (1) For pure nickel: acd = 2 × rNi
(2)
When nickel atoms are substituted for the atomic fraction, XW , of W: acd = 2 [rNi + (rW − rNi ) · XW ]
(3)
The dependence of aNi–W on the atomic fraction XW was calculated using Eqs. (1) and (3). This relationship is shown in Fig. 1. The trend shown in Fig. 1 is consistent with the results presented by other authors. It should be noted that in [13], the dependence was obtained using different theoretical background — a molecular dynamics simulation. Furthermore, in [14], the value of the lattice parameter of the pure Ni was too low. Fig. 1 presents one more dependence, aNi–W ∼ f(XW ). This was calculated without taking into account the increase in rW , by a factor of 1.03. Using the
Fig. 1. Lattice parameter, aNi–W , of the Ni–W fcc phase as a function of atomic fraction of W in the phase.
Fig. 2. Fragments of XRD patterns for the Ni–W alloys electrodeposited at different pHs.
linear dependence presented in Fig. 1, an expression was derived for the tungsten fraction in the Ni–W solid solution from aNi–W of the ␥ phase: XW = −7.5208 + 2.13429 × aNi–W
(4)
The equation for the fraction of molybdenum in a solid solution with nickel can be constructed by the same procedure: XMo = −7.90906 + 2.24447 × aNi–Mo
(5)
In both expressions, the lattice parameters, aNi–W and aNi–Mo , are given in angstroms. Fig. 2 shows a series of XRD patterns for Ni–W coatings electrodeposited in solutions with different pH. The patterns present ␥ phase peaks of 1 1 1 with different values of 2max and of the full width at the half maximum (FWHM). The patterns indicate that the W content in the electrodeposited ␥ phase and the crystallite’s size (the reciprocal of FWHM) depend on pH. The XRD patterns for coatings obtained at pH 4 and 4.5 present an additional peak at 2 ≈ 41.4◦ . We have shown elsewhere [16] that this peak corresponds to NiWO4 . Hence, in the latter case, a certain proportion of the W determined by EDX is present as a tungstate phase. Using these XRD patterns, aNi–W values were determined; the atomic fraction of W in the ␥ phase was derived using expression (4). These results are shown in Fig. 3. The total amount of W in the alloy was determined by EDX and is also presented in Fig. 3. Not all of the W measured by EDX was in the ␥ phase. To reveal the state in which the rest of the tungsten was present in the alloys the samples were annealed. One group of coatings was annealed for 2 h in argon and another under a hydrogen atmosphere at
Fig. 3. The total quantity of W in the coating and W content in the Ni–W fcc phase, as a function of pH.
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Fig. 4. XRD patterns for the Ni–W alloy, as-deposited at pH 5 and annealed in Ar for 2 h at 1000 ◦ C.
Fig. 5. XRD patterns for the Ni–W alloy, as-deposited at pH 5 and annealed in H2 for 2 h at 1000 ◦ C.
1000 ◦ C. Afterwards, the XRD patterns were evaluated. The atomic fraction of W in the solid solution, for the annealed coatings, is shown in Fig. 3. After annealing, the W content in the solid solution became almost independent of pH; however, this content remained higher than the limiting value of the tungsten solubility in nickel (12.0–13.5 at.%). For the alloys deposited at pH 4–4.5 and 6–7.5 the annealing increased the W content in the solid solution. At pH 5–6, this value decreased slightly. Fig. 4 shows the XRD pattern for a Ni–W coating, electrodeposited at pH 5 and annealed in argon. This pattern is evidence that during annealing some new phases either form or become detectable. The new peaks are attributed to Ni4 W (PDF no. 15755) and Ni6 W6 C. The latter phase was identified using PDF no. 23-1127 for Fe6 W6 C. The corresponding peaks of the pattern were slightly shifted to higher diffraction angles, with respect to peaks in Fe6 W6 C. This is because the atomic radius of nickel in the fcc lattice of Ni6 W6 C is smaller than that of iron. Table 1 presents the measured and calculated values of the interplanar distances, d, for the Ni6 W6 C phase. It is noteworthy that some of the Ni6 W6 C XRD peaks are similar to those of the NiW phase (PDF no. 47-1172), which can lead to the drawing of inappropriate conclusions. According to Fig. 3, the tungsten content in the ␥ phase remained nearly constant after annealing. Thus, some of the W was in the different states (tungstate, Ni4 W) in the as-deposited coating. One of these states could be a nanocrystalline phase of Ni4 W, which became detectable as a result of the recrystallization. However, in our opinion, the formation of Ni4 W during the heat-treatment is more probable. The formation of a carbide, Ni6 W6 C, is evidence that some kind of citrate species was incorporated into the coating. The tungstate, could
be reduced by carbon with the simultaneous formation of carbide at higher temperatures [17]: WO3 + 4C → WC + 3CO
(6)
or 6NiWO4 → 25C = Ni6 W6 C + 24CO
(7)
Fig. 5 shows the XRD pattern for a Ni–W coating, electrodeposited at pH 5 and annealed in the hydrogen. The tungsten XRD peaks only emerged along with those of the ␥ phase after the annealing in the hydrogen. The reduction of nickel tungstate by hydrogen can proceed in two steps [18] resulting in the formation of a pure W phase: NiWO4 + 2H2 = Ni + WO2 + 2H2 O
(8)
and WO2 + 2H2 = W + 2H2 O
(9)
The tungsten XRD peaks are slightly shifted towards higher diffraction angles when than the 2 angles presented in PDF no. 04-0806. This can be indicative of the formation of a nickel solid solution in tungsten. Fig. 6 shows the integral intensity ratios of the Ni6 W6 C 5 1 1 and ␥ NiW 1 1 1 peaks along with the W 1 1 0 and ␥ NiW 1 1 1 peaks.
Table 1 Observed and calculated d values for the Ni6 W6 C phase with lattice parameter a = 10.8941 Å. d (obs. max) (Å)
d (calculated) (Å)
hkl
2.7236 2.4991 2.2239 2.0959 1.9259 – – – – – 1.5254 1.4182
2.7236 2.4994 2.2238 2.0966 1.9259 1.8415 1.8157 1.7226 1.6424 1.5725 1.5255 1.4183
400 331 422 511 440 531 442 620 622 444 551 731
Fig. 6. Integral intensity ratios of XRD peaks W 1 1 0/␥ NiW 1 1 1 and Ni6 W6 C 5 1 1/␥ NiW 1 1 1, as a function of pH.
R. Juˇskenas et al. / Electrochimica Acta 54 (2009) 2616–2620 ˙
These ratios are proportional to the quantity of Ni6 W6 C and W phases in the respective annealed alloys. Due to a wide scatter in the ratio values, the trend is only visible with a polynomial approximation of the data. The annealed coatings deposited at pH 5–6 contain the least amount of carbide and tungsten phases. The quantity of the tungsten phase increased at the lower pH value, while at higher pH, the amount of carbide rose more significantly. The quantity of W and Ni6 W6 C formed could be greater if one considers that a part of the W formed during annealing dissolved in the ␥ phase. This was evident from the increased XW for alloys deposited at low and high pH (Fig. 3). Fig. 7 presents the AFM surface images of the Ni–W coatings, electrodeposited in the pH range from 4 to 7.5. The surface
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morphology of the coatings deposited at pH 4.0–4.5 consists of grains 200–250 nm in diameter. At pH 5–6, the size of grains rises up to 1500–2500 nm. The surface appears to be covered with a fine-granular structure. This was most likely tungstate, since it disappeared after annealing in the hydrogen atmosphere (Fig. 7f). At pH 7.5 (Fig. 7e), the grains were covered with a greater amount of tungstate. This surface consisted of nanosized crystallites. The assumption that the surface was covered with the nanosize structure would be in agreement with a broad XRD peak, Ni–W 1 1 1, on the pattern for pH 7.5 (Fig. 2). A small crystallite size usually points to the fact that the crystallization was inhibited by the adsorption of impurities, or some other species, on the surface of the growing crystallites.
Fig. 7. AFM images of the surface of the Ni–W coatings electrodeposited at different values of pH.
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The results presented prove that not all of the tungsten measured by EDX is present as a solute in a Ni–W solid solution. According to the results of our previous studies [16], and the results of XRD phase analysis of the annealed coatings in the present work, a portion of the tungsten identified by EDX was present as tungstates. The carbide found after annealing in the Ar atmosphere is indicative of incorporation of citric species into the electrodeposited Ni–W alloy. This observation is more significant at higher pHs. According to the increased quantity of the W phase formed during annealing in hydrogen atmosphere, insertion of tungstates should prevail at lower pH. The lowest quantity of the impurities in the electrodeposited alloys studied was observed at pH 5–6. The dependence of the quantity of impurities on the pH at which coatings were deposited correlates with the dependence of the grain and crystallite size on pH. The decrease in grain and crystallite size usually results from the adsorption of various species during crystallization. It may be argued that the reason for the discrepancy between the EDX and XRD data results from an inconsistent composition of the electrolyte. However, in different studies, in which the correct electrolyte composition was used, the difference between the W content determined by EDX and that which should yield XRD data by Eq. (4) is apparent: 20 and 16.3 [3], 19.5 and 11.1 [7], 20 and 10.5 [8], 7 and 1.5 at.% [9]. Furthermore, according to XPS data there was no oxygen present in the deepest layers of the Ni–W coatings, nevertheless, the analysis of the XRD patterns of samples annealed at 500 ◦ C revealed that the XRD peaks of NiWO4 emerged [8]. However, it has been shown in [19] that NiWO4 was formed at annealing temperatures of no less than 1000 ◦ C, even when the heat-treatment of the Ni-3 at.% W alloy was carried out under oxygen flow. It is thus possible that the nickel tungstate was already present in the as-deposited coating in [8]. After annealing at 650 ◦ C, both the XRD peaks of NiWO4 and carbide, Ni6 W6 C, emerged [8]. This carbide phase has also been reported for samples annealed at 600 ◦ C [3] (see Fig. 1 in [3] and Table 1 in the current paper).
4. Conclusions The results we reported suggest that not all of the W measured by EDX, in electrodeposited Ni–W alloys, is present as a tungsten solid solution in nickel, which seems to be a common phenomenon for the electrodeposited Ni–W alloys. Although it is usually assumed that in the bulk of electrodeposited Ni–W alloys, there is no oxygen or carbon, or that the percentage of O and C are negligible, this assumption is not universally true. These are facts to consider when studying the physical and chemical properties of electrodeposited Ni–W alloys. References [1] T. Yamasaki, P. Schloßmacher, K. Ehrlich, Y. Ogino, Nano Struct. Mater. 10 (1998) 375. [2] T. Yamasaki, Mater. Phys. Mech. 1 (2000) 127. [3] P. Schloßmacher, T. Yamasaki, Microchim. Acta 132 (2000) 309. [4] H. Cesiulis, A. Baltutiene, M. Donten, M.L. Donten, Z. Stojek, J. Solid State Electrochem. 6 (2002) 237. [5] H. Wang, S. Yao, S. Matsumura, Surf. Coat. Technol. 157 (2002) 166. [6] M. Donten, Z. Stojek, H. Cesiulis, J. Electrochem. Soc. 150 (2003) C95. [7] H. Somekawa, T.G. Nieh, K. Higashi, Scr. Mater. 50 (2004) 1361. [8] M. Donten, J. Solid State Electrochem. 3 (1999) 87. [9] O. Younes, L. Zhu, Y. Rosenberg, Y. Shacham-Diamand, E. Gileadi, Langmuir 17 (2001) 8270. [10] L. Zhu, O. Younes, N. Ashkenasy, Y. Shacham-Diamand, E. Gileadi, Appl. Surf. Sci. 200 (2002) 1. [11] N.D. Sulitanu, Mater. Sci. Eng. B95 (2002) 230. [12] O. Younes, E. Gileadi, Electrochem. Solid-State Lett. 3 (2000) 543. [13] L.T. Kong, J.B. Liu, W.S. Lal, B.X. Liu, J. Alloy Compd. 337 (2002) 143. [14] A.J. Detor, Ch.A. Schuh, Acta Mater. 55 (2007) 371. [15] B.D. Cullity, S.R. Stock, Elements of X-Ray Diffraction, third edition, AddisonWesley, 2001, p. 664. ˙ ¯ ˙ [16] R. Juˇskenas, I. Valsiunas, V. Pakˇstas, A. Selskis, V. Jasulaitiene, V. Karpaviˇciene, V. Kapoˇcius, Appl. Surf. Sci. 253 (2006) 1435. [17] E. Lassner, W.-D. Schubert, Tungsten, Kluwer Academic/Plenum Publishers, New York, 1999, p. 433. [18] R. Morales, F.J. Tavera, R.E. Aune, S. Seetharaman, Scand. J. Metall. 34 (2005) 108. [19] T.G. Woodcock, Y.L. Cheung, J.R.A. Grenfell, J.S. Abell, Supercond. Sci. Technol. 18 (2005) 721.