- Email: [email protected]
Zinc-nickel alloy whiskers electrodeposited from a sulfate bath
IIGIIMll & ELSEVIER
B
Materials Science and Engineering B38 (1996) 150 155
Zinc-nickel alloy whiskers electrodeposited from a sulfate bath M a s a t o u Ishihara a, Hisami Yumoto", K a z u o A k a s h i b, K a z u t o Kamei c "l)~Tartment q/ ,~taterial,~ S('ienc(, and l'echmdog.v, lyre Sui~'n('c Umrer.~itv o~ I~kyo. \"o~kt, ('hiha 278. Japan hDepartment ~/ Industrial and Engineering ('hemistr.v, ]he Seien('e Unit:er.~ity o/ Tokyo, .Vmh~, Chiha 278. Japan %tdvaneed Tectmolo,~v Research Labs., Sumitomo Metal Ind. Ltd., Amaga,~aki, H.vogo 660, Japan
Received 22 June 1995
Abstract
Zn .Ni alloy whiskers (20-50 itm) were grown from a sulfate bath by electrodeposition. The current density was 20 40 mA cm -~and the pH was above 2. The structure of the whiskers was h.c.p, q-phase or cubic ;.,-phase. The Ni content in the whiskers was 2 7 wt.",~,, which was below that of the bath solution (8.5 wt."/,,), and anomalous co-deposition occurred. ZnO films covered the surface of the whiskers, but Zn(OH) 2 films did not. When the oxide film disappeared, tip growth of the whiskers ceased, and granular particles (10-30 wt.% Ni) composed of q + ~, phases deposited onto the surface of the whiskers; the electrodeposition mechanism changed from anomalous to normal co-deposition. When Ni 2 ' ions were not added to the sulfate solution, neither ZnO films nor whiskers were formed, and only Zn(OH) 2 films grew. In order to grow Z n - N i whiskers. ZnO films were necessary. Keyword¥: Zn Ni: Whisker: Electroplating: Crystal growth: Morphology: ZnO
1. Introduction
Since electrodeposited Z n - N i coatings provide excellent corrosion protection, Zn- Ni coated steel has been used tbr the b o d y panels o f automobiles. In general, the noble metal ion is reduced more easily than the base metal ion in the electodeposition o f alloy. Although Zn is electrochemically less noble than Ni, Zn is electrodeposited more easily than Ni from sulfate solution. Therefore. Z n Ni electrodeposition is classified as a n o m a l o u s co-deposition by Brenner [1]. This a n o m a lous co-deposition is explained by the hydroxide suppression mechanism [2-5] and by the formation o f a mixed intermediate (NiZn)+ls [6]. The hydroxide suppression mechanism is explained here. During electrodeposition, water is also reduced to H2 gas and OH ions at the cathode: 2H20+2e Since the increased becomes Zn(OHL,
~H2T+2OH
(I)
concentration o f OH ions at the interface is by this reaction, the pH near the cathode high. When the value o f pH increases, films are lbrmed on the cathode [7]. Zn is
reduced from these Zn(OH)2 films. However, the reduclions of Ni-'" and H * ions are prohibited by these Zn(OH)z films, so that Zn base metal is electrodeposited onto the cathode preferentially. In general, most whiskers are prepared by chemical vapor deposition (CVD) using expensive reaction gas, so that the cost o f the whiskers is very high. If whiskers are grown by electrodeposition, the cost becomes very low and the time required for production of the whiskers is short. Pure metal whiskers such as A g and Cu are grown by electrodeposition from a simple bath [8]. However, there are few papers discribing the growth o f alloy whiskers by electrodeposition. Recently, it has been reported that Zn Ni alloy whiskers will grow from a sulfate bath under some conditions lable I Standard bath composition and conditions for electrodeposition. ZnSo4.7H20 (mol I i) NiSOa.6H~O(rnol I m) Na2SO 4 (mol I- I1 pH Bath temperature (°C)
()921-51()7.96.$15.(X) ,c. 1996
1.00 0.10 1.00 2.O
50
ElsevierScience S.A. All rights reserved t ") '~ S S D I 0921-5107(95)014-9-_
M. l.*hihara et al. . Materials Science and Engineering B38 (1996) 150.. 155
151
D.C. Power
,
I
÷[1"
'
[ _j
Pt
I
Cu •
&-
J
Stirrerl @
-
Heater ,. -
11
[---
Fig. 1. Schematic view of the electrodeposition apparatus: Pt anode. Cu cathode, and Hull cell heater. [9,10]. However, as the growth mechanism of Z n - N i alloy whiskers is unknown, the purpose of the present study was to investigate the growth mechanism of the whiskers.
2. E x p e r i m e n t a l d e t a i l s
Table l shows the standard bath composition and the conditions for electrodeposition. It was reported that Z n - N i whiskers grew only for an Ni content of 9 wt.'¼,, and did not grow at 0, 5, I I and 13 wt.'¼, Ni [9]. Therefore, we decided on the ratio (8.5) wt.% of Ni 2 ÷ to (Ni 2 " + Zn 2 - ) in the solution of(8.5 wt.%) (9 mol%), giving concentrations of ZnSO4"7H20 and NiSO4-6H20 of 1.00 m o l l ~ and 0.10 mol 1 - t, respectively. Fig. i shows a schematic drawing of the apparatus for electrodeposition. The solution was stirred by a magnetic stirrer (500 rev rain-~), and its temperature was kept at 50°C by a Hull cell heater. Pt anode and Cu cathode plates were covered with insulating tape except for a square area (3 x 3 c m 2) for electrodeposition. Before electrodeposition, the Cu cathode plate was cleaned by electrolytic alkaline degreasing and acid pickling. The morphologies of the electrodeposits were observed by scanning electron microscopy (SEM). In order to study the growth of Z n - N i whiskers using transmission electron microscopy (TEM), a Cu sheet mesh (3 m m in diameter) for T E M was attached to the Cu cathode plate and Z n - N i whiskers were electrodeposited directly onto this sheet mesh. The composition of Zn Ni whiskers was determined by energy dispersive X-ray spectrometry (EDS).
3. R e s u l t s and d i s c u s s i o n
Fig. 2 shows the effect of current density on the morphology of the electrodeposits: (a) dendrite crystals
W
W
Fig. 2. Typical SEM images of electrodeposited crystals: (a) dendrites (5 mA cm- 2), (b) corn-like crystals (10 m A c m 2), (c) Zn Ni whiskers (30 mA cm -2). (d) massive crystals (50 mA cm "-'), and (e) faceted crystals (above 60 mAcm 2).
.~1. I.~htllara ('l al
.llat('rtal.~ .%'~tern'(" a m / l-.),gim.eru l., B 3 8 (1996~ 150
grew at low ct, rrent density (5 mA cm '). (b) corn-like crystals were deposited at 10 mA cm ". (c) Zn Ni whiskers with smooth surfaces grew at 30 m A c m " (d) massive crystals having a rough surface appeared at 50 mA cm 2, and (e) the faceted crystals had ',10/0', and {0001} facets at above 60 mA cm ~. The growth condition for whiskers was a current density o f 20 40 mA cm z. The length and thickness o f the whiskers were 2 0 50 l~m and 2 10 itm respectively. Fig. 3 shows a series o f SEM images o f Zn Ni whiskers grown at 30 m A cm -', suggesting the growth process o f the whiskers. (a) Prismatic hexagonal crystals appeared on the Cu cathode at an electrodeposition time t o f 10 s, and EDS analysis revealed that these crystals were Zn crystals with extremely low Ni content (below 1 wt.% Ni). As the crystal was almost pure Zn. its structure was hexagonal Zn with Ni in solid solution (q phase) and the growth direction was <0001 ~. (b) The hexagonal crystals grew to a whisker at t = 60 s. (c) Whiskers became long and thick at t = 120 s. The shape o f the whiskers was cylindrical, and the concentration of Ni in the whiskers was about 7 wt.'V,,. The concentration o f Ni increased gradually from 1 wl.%, to the value o f the bath solution (8.5 wt.%). (d) After 300 s. granular particles deposited onto the tip o f the whiskers and tip growth ceased. The granular particles were deposited also onto the lateral surfaces o f these whiskers. so the whiskers became thicker and the surface was rough. The a m o u n t o f Ni contained in the granular particles was about 1 0 - 3 0 wt.%. Hence, the electrodeposition mechanism changed to normal co-deposition. and the solubility of Ni in the electrodepositions increased with deposition time. (e) The number o f Zn Ni granular particles increased with increasing deposition time, and the particles covered the whiskers at completely t = 1200 s. The particle size also mcreased. Finally the whiskers were buried under these particles. Fig. 4 shows a T E M image o f a tip o f the Zn Ni whisker electrodeposited onto a Cu sheet mesh at t .... 60 s. In general, the Z n - N i whiskers did not contain dislocations [10]. A thin film (about 100 nm thick) covered the tip o f the whisker and was composed of numerous super-line particles (about 2 0 40 nm in diameter). The electron diffraction image showed thai this film was composed o f ZnO. Fig. 5 showes X-ray diffraction patterns o f the sampie grown at 30 m A c m 2 and pH 2. X-ray diffraction also revealed that Z n O existed at t = 10-300 s. Cu peaks from the substrate were also observed. Open squares, solid squares, open circles and solid circles show the q-phase, 7-phase, e-°Zn(OH) 2 and 7-Zn(OH), respectively. Pourbaix said that ~-Zn(OH)2, 7-Zn(OH)~ and Z n O co-existed for the pH range from about 6 to 12, and that they were formed in the order, ~-Zn(OH)2, Z n O and 7-Zn(OH)2 with increasing pH [7]. In this study, the electrode potential o f the cathode was - 0.73
155
4'
r
mE I.""
W
ml
l.ig. 3. SEM images suggesting the growth process of Zn Ni whiskers (3(I t'n/k cm 2): (a) t = 10 s. (b) 60 s. (el 120 s. (d) 3(R) s. (e) 1200 s. V ~s. N H E (normal hydrogen electrode) and the pH o f the solution was 2. It seemed that the pH near the cathode was increased to about pH 6 by the reaction o f Eq. (I). so that the hydroxides and Z n O could form.
153
M. lshihara et al.. Materials Seient'e and Engineering B38 (1996) 150 155
The 7-phase had a ~,-brass structure (NisZn, 0 [9] and the lattice constant was 0.9 nm, estimated from the diffraction patterns of T E M [10]. The Zn q-phase appeared from t = 30 s, and the 7-phase appeared later at 120 s. It is reported [9] that the transition from q to q + 7 occurs around 4 wt.% Ni, and the transition from q + 7 to 7 occurs near 9.5 wt.% Ni. The 7-phase contains more Ni than the q-phase. However, in the N i Zn phase diagram [11], the solubility of Ni in the q-phase is very low (0.007 wt.%), and the dual phase q + 6 (NiZn~) appears below 9 wt.%, but the 7-phase appears at about 15 wt.°/,, Ni. In general, the solubility of the electrodeposited phase does not agree with the value from the phase diagram [12]. This means that the electrodeposition method has a possibility of producing a new material. Fig. 6 shows the variations in the X-ray peak intensity of (110) Z n O and the length of the whiskers with electrodeposition time. It was found that the whiskers grew longer during the presence of ZnO. When the ZnO disappeared at t = 300 s, the growth of the whiskers ceased. This shows that ZnO is important for growing the whiskers. Fig. 7 shows the variation in the intensities of X-ray of (112) c-Zn(OH),, (631) 7-Zn(OH) 2, and (002) Zn q-phase with time. The amount of e-Zn(OH)2 reached a maximum at t = 120 s, and the amount of 7-Zn(OH)2 reached a maximum at t = 300 s. They disappeared at 1200 s. ZnO appeared only between 10 and 300 s. The period of existence of ZnO was short, and there was less ZnO than zinc hydroxides. The formation of ZnO was more difficult than that of the hydroxides. Figs. 6 and 7 show there was no correlation between the length of the whiskers and the presence of zinc hydroxides, so the zinc hydroxides were not useful for growing the whiskers. The whisker of Fig. 4 at t = 60 s had the q-phase structure, and the Ni content was very low. The growth direction was <0001 >. However, some Z n - N i whiskers grown late had the Z n - N i ),-phase structure, and the growth direction was <11 i > [10]. This suggests there are two types of whisker: q and 7 whiskers.
Fig. 4. TEM image of the tip of a Z n - N i whisker electrodeposited onto Cu mesh for TEM (t = 60 s), and electron diffraction image of zinc oxide film.
a) 10s
'b) 30s
,< (c) 120s
e-a
(d) 3(X)s
(c) 12tl0s 30
40
50
60
70
80
20 ( d e g ) (Cu Kct) Fig 5. X-ray diffraction patterns of the sample grown at pit 2 and ( 3 0 m A c r o 2): (a) t = 1 0 s, (b) 30 s. (c) 120 s, (d1300 s. (e) 1200s. Open squares, solid squares, open circles and solid circles show the q-phase, ;.-phase, ~-Zn(OH) 2 and 7-Zn(OHJ 2 respectively.
Fig. 8 shows the relationship between the electrodeposition time and X-ray peak intensities: (631) 7 and (002) q-phases and (110) ZnO. The time dependences for the q- and ;,-phases were almost the same. The growth rates of the two phases were high during the presence of ZnO (unit t = 300 s), but the growth rates were low at t > 300 s. Therefore, it was confirmed that there were two types of whiskers: r/ and 7 phases. As the intensity of the q-phase was seven times higher than that of the 7-phase, it seemed that most of the whiskers were q-phase
154
II. I~/tihara ct al.
500
.~lat('rtal~ J)'(wnc(" and l','ngme(,ring B38 (1996) 150 155 30(X)
3O
~" ,.-,,
2o0o
20 300
~
~
.o. e,,.,
--
~
'°i
ZnO
10 '-~ frO
0
500 1000 Deposition Time (s)
(a)
1500
~
---"<)--
t-Zn(OH)2
~
,v- Zn(OI 1)2 ZnO
-~
500
a
s
e phase
500 lOOO Deposition Time (s)
15(10
3000
The solubility of Ni in Zn crystals increased to 10%-30'V,, with deposition timc. which was mentioned above, and it was said that the transition from q-phase to q + ;' phase occurcd 4 wt."/,, Ni [9]. therforc, the structure of thc whiskers at the early growth stage was q-phase, and some whiskers grown late were /-phase. so thc q and ;' whiskers grew mixed. Whcn thc ZnO disappeared, the rate of formation of these phases decreased. The efficiency of thc electrodeposition of Ni Zn alloy was decrcased and the reduction of H - ions was increased. The granular particles grew at this time. Therefore. it seems that these particles were made of dual (~1 + 7) phase. Zn N i whiskers were grown also at above pH 3.5, but the amount and length of the whiskers were low. The tormation of ZnO was less in this case. When the pH in the bath solution was decreased to I. neithcr ZnO nor Zn Ni whiskers grew. ¢-Zn(OH): was t\)rmed laster than ;'-Zn(OH)2. Fig. 9 shows X-ray diffraction patterns of the sample electrodeposited from the sulfate solution without NiSO4. There is no ZnO peak. and the whiskers did not grow. ZnO was necessary to grow the whiskers. The
/
h
i 51)0
Fig. 6. Variations of the itltcnsil 3 of l 110) Zn() and the length of the ~hiskers ~ith electrodeposition time.
t-"
p
1000
1500
Deposition Time (s) I"ig. 7. Variations of the X-ray intensities ()1"I112) e-Zn(OII)2. (631) :'-Zn(()H)2 and (002) 8-phase with deposition time.
g 20~10
5(10
f
4(~),~. ~ t~ e",
3(10 :" © 2110~ "~ 1000 edl "E t.o
A
(b)
5110 1000 Deposition Time (s)
ZnO q-phase y-phase
tit0
0 15(~)
Fig. 8. Relationships between the electrodeposition time and the X-ray peak heights: (631) 7-phase, (002) q-phase and (110) ZnO (pH 2 and 30 m A c m 2).
lormation of s-Zn{OH)2 was enhanced, and the maximum peak intensity was at t = 10 s, which was earlier than that of the sample deposited from the solution with NiSO4 ( t = 120 s). The time of the maximum intensity of 7-Zn(OH)2 was shortened from 300 to 30 s with no addition of NiSO4, but the maximum intensity was suppressed to about one-quarter. It seems that ZnO and Zn(OH)2 films are formed by the increasing pH owing to the reaction of Eq. (1), but the amount of Z n ' + ions diffusing for the formation of ZnO or ZntOH)2 is decreased during the electrodeposition by the diffusion limited condition and the formation of these films becomes hard. Zn is reduced from ZnO and Zn(OH)2 films at the cathode, and grows to Zn crystals. Therelbre, these films are exhausted and linally disappear. The reduction rate at the tip of a crystal is much higher than that on the lateral surfaces in the case with ZnO, so that the crystal grows to a whisker. We think that this anisotropy of the growth rate is not so high in
M. Ishihara et al. Material~" Science and Engineering B38 (19961 150- 155
155
the case with zinc hydroxides, so that the whiskers cannot grow from zinc hydroxide films.
4. Summary
(a) IOs
r-~
< (b) 30s
Zn- Ni whiskers (2 7 wt.'¼, Ni) were grown from a sulfate bath (9 moW,, Ni) under the conditions 2 0 - 4 0 mA cm -~, 50°C, pH > 2. The length and thickness of the whiskers were 20 50/tin and 2- 10 l~m respectively. The structure of the whiskers was q or 7 phase, These whiskers were covered by ZnO films which had an important role in the growth of the whiskers. When the ZnO films disappeared, the growth of the whiskers ceased. In order to grow long whiskers, the ZnO films must be retained. The granular particles grew on the whiskers after cessation of the growth of the whiskers. Ni 2 - ions were necessary to form the ZnO films.
Acknowledgement ¢.) a-a
This investigation was partly supported by the Grant in Aid for Fundamental Scientific Research of the Ministry of Education, Science and Culture.
(c) 1212s
(d) 3(~s
30
40
512
612
70
(e) 12012s 80
20 (deg) (Cu Kot) Fig. 9. X-ray diffraction patterns of the sample electrodeposited from the sulfate solution without NiSO a (pH 2 and 30 mA cm-2): (a) t= 10 s. (b) 30 s, (c) 120 s, (d) 300 s, (e)1200 s. Open squares, open circles and solid circles show the r/-phase, ~-Zn(OH), and 7-Zn(OH) 2 respectively.
References [I] A. Brenner. in Electrodepo,sition o/ Alloys. Principles and Practice. Vols. 1 and 2, Academic Press, New York. 1963. [2] H. Dahms and I.M. Croll, J. Electrochem. Sot.. 112 (19651 771. [3] N. Koura, J. Electrochem. Soc. Jpn., 47 (19791 738. [4] D.E. Hall, Plating Surf Finish., 70 (1983) 59. [5] H. Fukushima, T. Akiyama and K. Higashi. Metall. 44 (19901 754. [6] I. Chassaing and R. Wiart, Electrochin~. Acta, 37 (1992) 545. [7] M. Pourbaix, in Atlas o1 electrochemical Equilibria in Aqueou.~ Solutions, Pergamon. London. 1966. p. 4(17. [8] F.R. Nabarro and P.J. Jackson. in R.M. Doremus, B.W. Roberts and D. Turnbull (eds.), Growth and Pe(fection of Crystals. Wiley, New York, 1958, p. 85. [9] M,R. Lambert and R.G. Hart, SAE Technical Paper 9L No. 860266, 1986 (Society of Automotive Engineers). [10] K. Kamei and H. Yumoto, J. Jpn. Inst. Met., 57 (19931 1227. [1 I1 M. Hansen, in M. Hansen (ed.). Constitution of Binao, AIh~ys, McGraw-Hill, New York, 1958, p. 1059. [12] D.E. Hall. Plating Surf Finish., 70 (19831 59.
B
Materials Science and Engineering B38 (1996) 150 155
Zinc-nickel alloy whiskers electrodeposited from a sulfate bath M a s a t o u Ishihara a, Hisami Yumoto", K a z u o A k a s h i b, K a z u t o Kamei c "l)~Tartment q/ ,~taterial,~ S('ienc(, and l'echmdog.v, lyre Sui~'n('c Umrer.~itv o~ I~kyo. \"o~kt, ('hiha 278. Japan hDepartment ~/ Industrial and Engineering ('hemistr.v, ]he Seien('e Unit:er.~ity o/ Tokyo, .Vmh~, Chiha 278. Japan %tdvaneed Tectmolo,~v Research Labs., Sumitomo Metal Ind. Ltd., Amaga,~aki, H.vogo 660, Japan
Received 22 June 1995
Abstract
Zn .Ni alloy whiskers (20-50 itm) were grown from a sulfate bath by electrodeposition. The current density was 20 40 mA cm -~and the pH was above 2. The structure of the whiskers was h.c.p, q-phase or cubic ;.,-phase. The Ni content in the whiskers was 2 7 wt.",~,, which was below that of the bath solution (8.5 wt."/,,), and anomalous co-deposition occurred. ZnO films covered the surface of the whiskers, but Zn(OH) 2 films did not. When the oxide film disappeared, tip growth of the whiskers ceased, and granular particles (10-30 wt.% Ni) composed of q + ~, phases deposited onto the surface of the whiskers; the electrodeposition mechanism changed from anomalous to normal co-deposition. When Ni 2 ' ions were not added to the sulfate solution, neither ZnO films nor whiskers were formed, and only Zn(OH) 2 films grew. In order to grow Z n - N i whiskers. ZnO films were necessary. Keyword¥: Zn Ni: Whisker: Electroplating: Crystal growth: Morphology: ZnO
1. Introduction
Since electrodeposited Z n - N i coatings provide excellent corrosion protection, Zn- Ni coated steel has been used tbr the b o d y panels o f automobiles. In general, the noble metal ion is reduced more easily than the base metal ion in the electodeposition o f alloy. Although Zn is electrochemically less noble than Ni, Zn is electrodeposited more easily than Ni from sulfate solution. Therefore. Z n Ni electrodeposition is classified as a n o m a l o u s co-deposition by Brenner [1]. This a n o m a lous co-deposition is explained by the hydroxide suppression mechanism [2-5] and by the formation o f a mixed intermediate (NiZn)+ls [6]. The hydroxide suppression mechanism is explained here. During electrodeposition, water is also reduced to H2 gas and OH ions at the cathode: 2H20+2e Since the increased becomes Zn(OHL,
~H2T+2OH
(I)
concentration o f OH ions at the interface is by this reaction, the pH near the cathode high. When the value o f pH increases, films are lbrmed on the cathode [7]. Zn is
reduced from these Zn(OH)2 films. However, the reduclions of Ni-'" and H * ions are prohibited by these Zn(OH)z films, so that Zn base metal is electrodeposited onto the cathode preferentially. In general, most whiskers are prepared by chemical vapor deposition (CVD) using expensive reaction gas, so that the cost o f the whiskers is very high. If whiskers are grown by electrodeposition, the cost becomes very low and the time required for production of the whiskers is short. Pure metal whiskers such as A g and Cu are grown by electrodeposition from a simple bath [8]. However, there are few papers discribing the growth o f alloy whiskers by electrodeposition. Recently, it has been reported that Zn Ni alloy whiskers will grow from a sulfate bath under some conditions lable I Standard bath composition and conditions for electrodeposition. ZnSo4.7H20 (mol I i) NiSOa.6H~O(rnol I m) Na2SO 4 (mol I- I1 pH Bath temperature (°C)
()921-51()7.96.$15.(X) ,c. 1996
1.00 0.10 1.00 2.O
50
ElsevierScience S.A. All rights reserved t ") '~ S S D I 0921-5107(95)014-9-_
M. l.*hihara et al. . Materials Science and Engineering B38 (1996) 150.. 155
151
D.C. Power
,
I
÷[1"
'
[ _j
Pt
I
Cu •
&-
J
Stirrerl @
-
Heater ,. -
11
[---
Fig. 1. Schematic view of the electrodeposition apparatus: Pt anode. Cu cathode, and Hull cell heater. [9,10]. However, as the growth mechanism of Z n - N i alloy whiskers is unknown, the purpose of the present study was to investigate the growth mechanism of the whiskers.
2. E x p e r i m e n t a l d e t a i l s
Table l shows the standard bath composition and the conditions for electrodeposition. It was reported that Z n - N i whiskers grew only for an Ni content of 9 wt.'¼,, and did not grow at 0, 5, I I and 13 wt.'¼, Ni [9]. Therefore, we decided on the ratio (8.5) wt.% of Ni 2 ÷ to (Ni 2 " + Zn 2 - ) in the solution of(8.5 wt.%) (9 mol%), giving concentrations of ZnSO4"7H20 and NiSO4-6H20 of 1.00 m o l l ~ and 0.10 mol 1 - t, respectively. Fig. i shows a schematic drawing of the apparatus for electrodeposition. The solution was stirred by a magnetic stirrer (500 rev rain-~), and its temperature was kept at 50°C by a Hull cell heater. Pt anode and Cu cathode plates were covered with insulating tape except for a square area (3 x 3 c m 2) for electrodeposition. Before electrodeposition, the Cu cathode plate was cleaned by electrolytic alkaline degreasing and acid pickling. The morphologies of the electrodeposits were observed by scanning electron microscopy (SEM). In order to study the growth of Z n - N i whiskers using transmission electron microscopy (TEM), a Cu sheet mesh (3 m m in diameter) for T E M was attached to the Cu cathode plate and Z n - N i whiskers were electrodeposited directly onto this sheet mesh. The composition of Zn Ni whiskers was determined by energy dispersive X-ray spectrometry (EDS).
3. R e s u l t s and d i s c u s s i o n
Fig. 2 shows the effect of current density on the morphology of the electrodeposits: (a) dendrite crystals
W
W
Fig. 2. Typical SEM images of electrodeposited crystals: (a) dendrites (5 mA cm- 2), (b) corn-like crystals (10 m A c m 2), (c) Zn Ni whiskers (30 mA cm -2). (d) massive crystals (50 mA cm "-'), and (e) faceted crystals (above 60 mAcm 2).
.~1. I.~htllara ('l al
.llat('rtal.~ .%'~tern'(" a m / l-.),gim.eru l., B 3 8 (1996~ 150
grew at low ct, rrent density (5 mA cm '). (b) corn-like crystals were deposited at 10 mA cm ". (c) Zn Ni whiskers with smooth surfaces grew at 30 m A c m " (d) massive crystals having a rough surface appeared at 50 mA cm 2, and (e) the faceted crystals had ',10/0', and {0001} facets at above 60 mA cm ~. The growth condition for whiskers was a current density o f 20 40 mA cm z. The length and thickness o f the whiskers were 2 0 50 l~m and 2 10 itm respectively. Fig. 3 shows a series o f SEM images o f Zn Ni whiskers grown at 30 m A cm -', suggesting the growth process o f the whiskers. (a) Prismatic hexagonal crystals appeared on the Cu cathode at an electrodeposition time t o f 10 s, and EDS analysis revealed that these crystals were Zn crystals with extremely low Ni content (below 1 wt.% Ni). As the crystal was almost pure Zn. its structure was hexagonal Zn with Ni in solid solution (q phase) and the growth direction was <0001 ~. (b) The hexagonal crystals grew to a whisker at t = 60 s. (c) Whiskers became long and thick at t = 120 s. The shape o f the whiskers was cylindrical, and the concentration of Ni in the whiskers was about 7 wt.'V,,. The concentration o f Ni increased gradually from 1 wl.%, to the value o f the bath solution (8.5 wt.%). (d) After 300 s. granular particles deposited onto the tip o f the whiskers and tip growth ceased. The granular particles were deposited also onto the lateral surfaces o f these whiskers. so the whiskers became thicker and the surface was rough. The a m o u n t o f Ni contained in the granular particles was about 1 0 - 3 0 wt.%. Hence, the electrodeposition mechanism changed to normal co-deposition. and the solubility of Ni in the electrodepositions increased with deposition time. (e) The number o f Zn Ni granular particles increased with increasing deposition time, and the particles covered the whiskers at completely t = 1200 s. The particle size also mcreased. Finally the whiskers were buried under these particles. Fig. 4 shows a T E M image o f a tip o f the Zn Ni whisker electrodeposited onto a Cu sheet mesh at t .... 60 s. In general, the Z n - N i whiskers did not contain dislocations [10]. A thin film (about 100 nm thick) covered the tip o f the whisker and was composed of numerous super-line particles (about 2 0 40 nm in diameter). The electron diffraction image showed thai this film was composed o f ZnO. Fig. 5 showes X-ray diffraction patterns o f the sampie grown at 30 m A c m 2 and pH 2. X-ray diffraction also revealed that Z n O existed at t = 10-300 s. Cu peaks from the substrate were also observed. Open squares, solid squares, open circles and solid circles show the q-phase, 7-phase, e-°Zn(OH) 2 and 7-Zn(OH), respectively. Pourbaix said that ~-Zn(OH)2, 7-Zn(OH)~ and Z n O co-existed for the pH range from about 6 to 12, and that they were formed in the order, ~-Zn(OH)2, Z n O and 7-Zn(OH)2 with increasing pH [7]. In this study, the electrode potential o f the cathode was - 0.73
155
4'
r
mE I.""
W
ml
l.ig. 3. SEM images suggesting the growth process of Zn Ni whiskers (3(I t'n/k cm 2): (a) t = 10 s. (b) 60 s. (el 120 s. (d) 3(R) s. (e) 1200 s. V ~s. N H E (normal hydrogen electrode) and the pH o f the solution was 2. It seemed that the pH near the cathode was increased to about pH 6 by the reaction o f Eq. (I). so that the hydroxides and Z n O could form.
153
M. lshihara et al.. Materials Seient'e and Engineering B38 (1996) 150 155
The 7-phase had a ~,-brass structure (NisZn, 0 [9] and the lattice constant was 0.9 nm, estimated from the diffraction patterns of T E M [10]. The Zn q-phase appeared from t = 30 s, and the 7-phase appeared later at 120 s. It is reported [9] that the transition from q to q + 7 occurs around 4 wt.% Ni, and the transition from q + 7 to 7 occurs near 9.5 wt.% Ni. The 7-phase contains more Ni than the q-phase. However, in the N i Zn phase diagram [11], the solubility of Ni in the q-phase is very low (0.007 wt.%), and the dual phase q + 6 (NiZn~) appears below 9 wt.%, but the 7-phase appears at about 15 wt.°/,, Ni. In general, the solubility of the electrodeposited phase does not agree with the value from the phase diagram [12]. This means that the electrodeposition method has a possibility of producing a new material. Fig. 6 shows the variations in the X-ray peak intensity of (110) Z n O and the length of the whiskers with electrodeposition time. It was found that the whiskers grew longer during the presence of ZnO. When the ZnO disappeared at t = 300 s, the growth of the whiskers ceased. This shows that ZnO is important for growing the whiskers. Fig. 7 shows the variation in the intensities of X-ray of (112) c-Zn(OH),, (631) 7-Zn(OH) 2, and (002) Zn q-phase with time. The amount of e-Zn(OH)2 reached a maximum at t = 120 s, and the amount of 7-Zn(OH)2 reached a maximum at t = 300 s. They disappeared at 1200 s. ZnO appeared only between 10 and 300 s. The period of existence of ZnO was short, and there was less ZnO than zinc hydroxides. The formation of ZnO was more difficult than that of the hydroxides. Figs. 6 and 7 show there was no correlation between the length of the whiskers and the presence of zinc hydroxides, so the zinc hydroxides were not useful for growing the whiskers. The whisker of Fig. 4 at t = 60 s had the q-phase structure, and the Ni content was very low. The growth direction was <0001 >. However, some Z n - N i whiskers grown late had the Z n - N i ),-phase structure, and the growth direction was <11 i > [10]. This suggests there are two types of whisker: q and 7 whiskers.
Fig. 4. TEM image of the tip of a Z n - N i whisker electrodeposited onto Cu mesh for TEM (t = 60 s), and electron diffraction image of zinc oxide film.
a) 10s
'b) 30s
,< (c) 120s
e-a
(d) 3(X)s
(c) 12tl0s 30
40
50
60
70
80
20 ( d e g ) (Cu Kct) Fig 5. X-ray diffraction patterns of the sample grown at pit 2 and ( 3 0 m A c r o 2): (a) t = 1 0 s, (b) 30 s. (c) 120 s, (d1300 s. (e) 1200s. Open squares, solid squares, open circles and solid circles show the q-phase, ;.-phase, ~-Zn(OH) 2 and 7-Zn(OHJ 2 respectively.
Fig. 8 shows the relationship between the electrodeposition time and X-ray peak intensities: (631) 7 and (002) q-phases and (110) ZnO. The time dependences for the q- and ;,-phases were almost the same. The growth rates of the two phases were high during the presence of ZnO (unit t = 300 s), but the growth rates were low at t > 300 s. Therefore, it was confirmed that there were two types of whiskers: r/ and 7 phases. As the intensity of the q-phase was seven times higher than that of the 7-phase, it seemed that most of the whiskers were q-phase
154
II. I~/tihara ct al.
500
.~lat('rtal~ J)'(wnc(" and l','ngme(,ring B38 (1996) 150 155 30(X)
3O
~" ,.-,,
2o0o
20 300
~
~
.o. e,,.,
--
~
'°i
ZnO
10 '-~ frO
0
500 1000 Deposition Time (s)
(a)
1500
~
---"<)--
t-Zn(OH)2
~
,v- Zn(OI 1)2 ZnO
-~
500
a
s
e phase
500 lOOO Deposition Time (s)
15(10
3000
The solubility of Ni in Zn crystals increased to 10%-30'V,, with deposition timc. which was mentioned above, and it was said that the transition from q-phase to q + ;' phase occurcd 4 wt."/,, Ni [9]. therforc, the structure of thc whiskers at the early growth stage was q-phase, and some whiskers grown late were /-phase. so thc q and ;' whiskers grew mixed. Whcn thc ZnO disappeared, the rate of formation of these phases decreased. The efficiency of thc electrodeposition of Ni Zn alloy was decrcased and the reduction of H - ions was increased. The granular particles grew at this time. Therefore. it seems that these particles were made of dual (~1 + 7) phase. Zn N i whiskers were grown also at above pH 3.5, but the amount and length of the whiskers were low. The tormation of ZnO was less in this case. When the pH in the bath solution was decreased to I. neithcr ZnO nor Zn Ni whiskers grew. ¢-Zn(OH): was t\)rmed laster than ;'-Zn(OH)2. Fig. 9 shows X-ray diffraction patterns of the sample electrodeposited from the sulfate solution without NiSO4. There is no ZnO peak. and the whiskers did not grow. ZnO was necessary to grow the whiskers. The
/
h
i 51)0
Fig. 6. Variations of the itltcnsil 3 of l 110) Zn() and the length of the ~hiskers ~ith electrodeposition time.
t-"
p
1000
1500
Deposition Time (s) I"ig. 7. Variations of the X-ray intensities ()1"I112) e-Zn(OII)2. (631) :'-Zn(()H)2 and (002) 8-phase with deposition time.
g 20~10
5(10
f
4(~),~. ~ t~ e",
3(10 :" © 2110~ "~ 1000 edl "E t.o
A
(b)
5110 1000 Deposition Time (s)
ZnO q-phase y-phase
tit0
0 15(~)
Fig. 8. Relationships between the electrodeposition time and the X-ray peak heights: (631) 7-phase, (002) q-phase and (110) ZnO (pH 2 and 30 m A c m 2).
lormation of s-Zn{OH)2 was enhanced, and the maximum peak intensity was at t = 10 s, which was earlier than that of the sample deposited from the solution with NiSO4 ( t = 120 s). The time of the maximum intensity of 7-Zn(OH)2 was shortened from 300 to 30 s with no addition of NiSO4, but the maximum intensity was suppressed to about one-quarter. It seems that ZnO and Zn(OH)2 films are formed by the increasing pH owing to the reaction of Eq. (1), but the amount of Z n ' + ions diffusing for the formation of ZnO or ZntOH)2 is decreased during the electrodeposition by the diffusion limited condition and the formation of these films becomes hard. Zn is reduced from ZnO and Zn(OH)2 films at the cathode, and grows to Zn crystals. Therelbre, these films are exhausted and linally disappear. The reduction rate at the tip of a crystal is much higher than that on the lateral surfaces in the case with ZnO, so that the crystal grows to a whisker. We think that this anisotropy of the growth rate is not so high in
M. Ishihara et al. Material~" Science and Engineering B38 (19961 150- 155
155
the case with zinc hydroxides, so that the whiskers cannot grow from zinc hydroxide films.
4. Summary
(a) IOs
r-~
< (b) 30s
Zn- Ni whiskers (2 7 wt.'¼, Ni) were grown from a sulfate bath (9 moW,, Ni) under the conditions 2 0 - 4 0 mA cm -~, 50°C, pH > 2. The length and thickness of the whiskers were 20 50/tin and 2- 10 l~m respectively. The structure of the whiskers was q or 7 phase, These whiskers were covered by ZnO films which had an important role in the growth of the whiskers. When the ZnO films disappeared, the growth of the whiskers ceased. In order to grow long whiskers, the ZnO films must be retained. The granular particles grew on the whiskers after cessation of the growth of the whiskers. Ni 2 - ions were necessary to form the ZnO films.
Acknowledgement ¢.) a-a
This investigation was partly supported by the Grant in Aid for Fundamental Scientific Research of the Ministry of Education, Science and Culture.
(c) 1212s
(d) 3(~s
30
40
512
612
70
(e) 12012s 80
20 (deg) (Cu Kot) Fig. 9. X-ray diffraction patterns of the sample electrodeposited from the sulfate solution without NiSO a (pH 2 and 30 mA cm-2): (a) t= 10 s. (b) 30 s, (c) 120 s, (d) 300 s, (e)1200 s. Open squares, open circles and solid circles show the r/-phase, ~-Zn(OH), and 7-Zn(OH) 2 respectively.
References [I] A. Brenner. in Electrodepo,sition o/ Alloys. Principles and Practice. Vols. 1 and 2, Academic Press, New York. 1963. [2] H. Dahms and I.M. Croll, J. Electrochem. Sot.. 112 (19651 771. [3] N. Koura, J. Electrochem. Soc. Jpn., 47 (19791 738. [4] D.E. Hall, Plating Surf Finish., 70 (1983) 59. [5] H. Fukushima, T. Akiyama and K. Higashi. Metall. 44 (19901 754. [6] I. Chassaing and R. Wiart, Electrochin~. Acta, 37 (1992) 545. [7] M. Pourbaix, in Atlas o1 electrochemical Equilibria in Aqueou.~ Solutions, Pergamon. London. 1966. p. 4(17. [8] F.R. Nabarro and P.J. Jackson. in R.M. Doremus, B.W. Roberts and D. Turnbull (eds.), Growth and Pe(fection of Crystals. Wiley, New York, 1958, p. 85. [9] M,R. Lambert and R.G. Hart, SAE Technical Paper 9L No. 860266, 1986 (Society of Automotive Engineers). [10] K. Kamei and H. Yumoto, J. Jpn. Inst. Met., 57 (19931 1227. [1 I1 M. Hansen, in M. Hansen (ed.). Constitution of Binao, AIh~ys, McGraw-Hill, New York, 1958, p. 1059. [12] D.E. Hall. Plating Surf Finish., 70 (19831 59.