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The obtaining of Sn-Ag powder alloy by contact displacement in aqueous solutions

Authors:
  • Research Institute of Research Institute for Physical Chemical Problems of the Belarusian State University

Abstract and Figures

Sn-Ag powder alloy of eutectic composition is demanded in the production of powders for soldering pastes used in electronics. Non-eutectic alloy has found its application in catalysis for CO2 reduction, in 3D printing, as the promising material for lithium ion batteries. In this work the way of synthesis of Sn-Ag nanostructured powder alloy with near-eutectic composition based of cementation reaction in the system Sn"/Ag. in iqueous, solutions was proposed. The peculiarities of alloy powder synthesis in acid and slightly acid solutions were studied. Factors infiuencing on powder micristructure, phase and elemental composition were identified. Electrochemical behavior of tin in aqueous solutions for silver deposition was studied by potentiometic method. KEYWORDS: CEMENTATION, POWDER, Sn-Ag EUTECTIC ALLOY, PHASE COMPOSITION, MELTING POINT.
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THE OBTAINING OF SnAg POWDER ALLOY BY CONTACT DISPLACEMENT IN
AQUEOUS SOLUTIONS
MSc. Shikun M.1, Ass. Prof. PhD Vrublevskaya O.2, Rabenok A.1
Belarusian State University1,
Research Institute for Physical Chemical Problems of the Belarusian State University2, Belarus
E-mail: mariawerner21@gmail.com, vrublevskaya.olga@gmail.com
Abstract: Sn-Ag powder alloy of eutectic composition is demanded in the production of powders for soldering pastes used in electronics.
Non-eutectic alloy has found its application in catalysis for CO2 reduction, in 3D printing, as the promising material for lithium ion
batteries. In this work the way of synthesis of SnAg nanostructured powder alloy with near-eutectic composition based of cementation
reaction in the system Sn0/Ag+ in aqueous solutions was proposed. The peculiarities of alloy powder synthesis in acid and slightly acid
solutions were studied. Factors influencing on powder microstructure, phase and elemental composition were identified. Electrochemical
behavior of tin in aqueous solutions for silver deposition was studied by potentiometric method.
KEYWORDS: CEMENTATION, POWDER, SnAg EUTECTIC ALLOY, PHASE COMPOSITION, MELTING POINT.
1. Introduction/Введение
Ultra- and nano-dispersed powders from SnAg alloy with
different metal content are used in catalysis, electrocatalysis, in
printed electronics, as the promising material for lithium ion
batteries [14]. Powder alloys with eutectic or close to eutectic
compositions (Sn96.5Ag3.5 (wt.%); phases -Sn, Ag3Sn) are in
demand in the production of solder pastes, due to the low melting
point of 221C [5] and the correspondence to a number of
physicochemical requirements for solders [6]. Powders from the
SnAg alloy are obtained by thermal and plasma chemical spraying
[7, 8], by mechanochemical treatment of high-purity tin and silver
powders [9], in the way of Sn(II) chemical reduction in aqueous and
non-aqueous solutions with subsequent Ag(I) reduction on the
surface of tin in the result of contact displacement (cementation)
process [1, 3, 10]:
(1) 2Ag+ + Sn0 = 2Ag0 + Sn2+
Contact displacement is proceeded in case of the reduction of
metal ions in solution with more negative metal of the substrate
(E(Ag+/Ag0) = +0.08 V, E(Sn2+/Sn0) =0.76 V).
In the work [10] the possibility of synthesis an ultrafine powder
alloy SnAg, close in composition to the eutectic, Ag(I)
cementation with tin powder (99.9 wt.%) is shown. Two strongly
acidic solutions containing thiourea or citrate- and iodide- ions
simultaneously as ligands for inhibition of oxidation of Sn(II) to
Sn(IV) with oxygen dissolved in water and hydrolysis of Sn(II, IV)
were used for alloy synthesis.
The purpose of this work was to evaluate the effect of pH, the
duration of tin powder treatment in cementation solution, the role of
side processes accompanied cementation on the elemental, phase
compositions and microstructure of the alloy in the solution with
thiourea as ligand.
2. Experimental
For SnAg alloy synthesis the cementation solution with next
composition was used (moldm3): silver nitrate(I), thiourea,
sulphuric acid (pH 0.5, 4.0). Deposition of silver coating was
proceeded on tin powder (99.9 wt.%) with molar ratio Sn : Ag(I)
equal to 56 : 1 (or on the surface of tin foil). Duration of tin powder
or tin foil treatment in the solution was 116 min. The experiment
with tin foil was needed to assess the role of side processes
accompanying Ag(I) reduction.
Electrochemical measurements were conducted in standard
electrochemical cell with tin working, platinum counter and
Ag/AgCl reference electrodes with PG Autolab, controlled by Nova
2.1. Concentrations of Ag(I) and Sn(II, IV) ions in the solutions
after the tin foil treatment were determined by X-ray fluorescence
spectroscopy (XFS) (PANalitical): error of concentration
determination was 3% for Ag(I) and 17% for Sn(II, IV). The
obtained powder alloys washed repeatedly with distilled water and
air dried were analyzed by energy-dispersive X-ray spectroscopy
(EDX), scanning electron microscopy (SEM) (LEO 1420),
differential scanning calorimetry (DSC) (NETZXH SRA 449 F 3),
X-ray phase analysis (DRON-3.0, CuK).
3. Results and discussion
The elemental analysis results of Sn-Ag powders show that the
quota of silver in the alloys does not depend on the duration of tin
powder treatment in cementation solution, but depend on the
solution pH, Table 1. So, silver content in the obtained alloys after
two minutes of tin treatment in the solutions with pH 0.5 and 4.0
reaches 11.4 and 6.0 wt.%, accordingly, and changes little with the
increase of treatment time.
Table 1. The dependence of the silver content in SnAg alloys
obtained in the solutions with pH 0.5 and 4.0 on the duration of tin
powder treatment.
Time, min
Silver content, wt.%
pH = 0.5
pH = 4.0
2
11.4 1.1
6.00.5
5
14.21.3
6.60.4
10
13.41.1
6.80.4
15
14.31.2
6.30.5
X-ray phase analysis showed that powders with silver content
11.414.3 wt.% consist of two crystalline phases which are -Sn
and Ag3Sn. Low intensity peaks at 36.0, 38.11 and 39.59 2
correspond to Ag3Sn phase (Fig. 1, a). The powders with silver
content 6.06.8 wt.% formed in cementation solution with pH 4.0
consist of -Sn phase only (Fig. 1, b).
DSC analysis in nitrogen atmosphere (Fig. 2, curve 1) shows
that the alloy obtained from the solution with pH 0.5 with silver
content 11.4 wt.% starts to melt at 220.4 °C, which corresponds to
the melting point of the eutectic SnAg alloy. It is necessary to note
that the DSC curves for all alloys with a silver content 11.4
14.3 wt.% were the same. Powders containing silver in quota 6.0
6.8 wt.% obtained from the solution with pH 4.0 wt.% (Fig. 2, curve
2) melt at 230.9 °С, at melting point of tin.
a
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b
Figure 1. XRD-patterns of SnAg-powders with Ag content 11.4
wt.% (а) and 6.3 wt.% (b) obtained from the cementation solutions
with pH 0.5 and 4.0, accordingly. Duration of tin powder treatment
was 2 min (curves 1, 3), 15 min (curves 2, 4)
Figure 2. DSC-curves of SnAg-powders with Ag content 11.4 wt.%
(curve 1) and 6.3 wt.% (curve 2) obtained from the cementation
solutions with pH 0.5 and 4.0
The results of DSC and phase analysis of SnAg alloy powders
with silver content 6.06.8 wt.%, indicate that there is no Ag3Sn
phase in the alloys obtained from solutions with pH 4.0 and they
melt at tin melting point in spite of the fact that powder elemental
composition is close to eutectic one. However, the alloy obtained
from the solution with pH of 0.5, with nearly two-fold high silver
content melts as eutectic one and consists of two crystalline phases
distinguishing the eutectic. The presented results indicate the non-
equilibrium compositions of the alloys synthesized in the result of
cementation.
The authors of the work [11] showed that one of the reasons
of non-equilibrium state of the alloy is related to particle size it
forming. So, in case of eutectic SnAg alloy formed with nanometer
particles its melting point is not higher than 209 °С. SEM analysis
was conducted for the comparison of the surface morphology of the
particles making up the alloys obtained at different conditions in
cementation solutions, Fig. 3. Sizes of the initial tin particles do not
change (520 μm) during cementation regardless the solution pH,
but the agglomerates of small grains with sizes 0.20.5 μm appear
on the surface of each particle of the alloy powder, which are
associated with Ag(I) reduction (Fig. 3, a and c). At the same time,
the process of tin dissolution occurs with the formation of pores and
loosening of the surface of initial particles. Quantity of the
agglomerates on the initial particles surface slightly increases with
the duration of cementation. A smaller number of grains, their
agglomerates and pores on the surface of alloys obtained in a
solution with pH 4.0, in comparison with a solution with pH 0.5,
indicate a lower rate of cementation.
b
d
Figure 3. SEM photos of SnAg-powders obtained from the
cementation solutions with pH 0.5 (a, c) and 4.0 (b, d) for the
samples treated for 2 min (a, b) and 15 min (c, d)
It was important to find out the reasons for the slowdown of the
cementation process in solutions with pH 4.0. The reasons for the
low rate of cementation can be the following: the formation on
insoluble film of Sn(II, IV) oxides on tin particles surface; the
adsorption on the surface of initial particles of oxy-hydroxy
compounds of tin(II, IV), formed in the result of tin(II, IV) ions
hydrolysis. Tin(II, IV) ions passed into the solution during
cementation, at tin(II) oxidation by oxygen dissolved in water and
also as a result of the reaction 2:
(2) 2Ag+ + Sn2+ = 2Ag0 + Sn4+.
The hydrolysis of tin (II, IV) ions proceeded slightly in the
solutions with pH 0.5, but tin dissolution according to the reaction
(3) is possible:
(3) 2Sn0 + 2H+ = 2Sn2+ + H2
Two special experiments were conducted in order to elucidate
the role of side processes accompanying cementation and to explain
the established fact of lower cementation rate in the solution with
pH 4.0, the reasons for the higher melting temperature of powders
with lower silver content. In the first experiment the transients of
open circuit potential (OCP) of tin electrode from the duration of
treatment in cementation solutions and in the solutions without
Ag(I) ions (background solutions) were analyzed (Fig. 4). In the
second experiment concentrations of Ag(I) and Sn (II, IV) ions
were analyzed with XFS method after tin foil treatment in
cementation solutions and in the background solutions (Fig. 5).
Figure 4. Transients of tin electrode OCP in the solutions with pH
0.5 (curves 1, 2) и 4.0 (curve 3, 4): background solutions curves
1, 3, cementation solutions curves 2, 4
Tin electrode OCP in the background solution with рН 0.5
increases by 0.003 V during the first 1.7 min and then does not
change (Fig. 4, curve 1). The increase in potential is associated with
the dissolution of tin. Passivation of tin surface due to the oxides
formation leads to the potential stabilizing. Tin electrode OCP in
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the background solution with рН 4.0 decreases during first 0.7 min
by 0.010 V and then changes a little (Fig. 4, curve 3), but
dependence does not reach a plateau, as in case of the solution with
pH 0.5 and the potential varies up to 0.008 V. Probably the reason
of potential decrease is the formation of thiourea complexes of
tin(II) compounds such as SnTu22+; SnTu52+ [12]:
(4) Sn + 2Tu + 2H3O+ SnTu22+ + H2 + 2H2O
Thiourea complexes of tin(II) are not stable and Sn(II) ions
hydrolysis with the formation of colloidal particles of oxy-hydroxy
compounds is proceeded. Oxy-hydroxy compounds adsorbed and
desorbed on the surface of tin changing its electrode potential
within the specified limits.
In cementation solution with pH 0.5 tin electrode OCP
decreases by 0.007 V in the first 2 min and then changes a little
(Fig. 4, curve 2). It can be explained in the next way: in the first 2
min dissolution of tin is passed less effective than cementation
process.
In case of cementation solution with pH 4.0 OCP decreases by
0.002 in the first 0.3 min, then increases by 0.08 V to 10 min, after
that there is a slight drop in potential (Fig.4, curve 4). It is obvious
that cementation efficiency is different at the indicated three stages
in the solution with pH 4.0. At the first stage (up to 0.3 min), tin
dissolves, probably due to the formation of thiourea complexes
Sn(II) [12] and cementation process proceeded slowly. At the
second stage (0.310 min), cementation process occurs mainly. At
the third stage, the tin surface is passivated by hydrolysis products
that impede the reaction 1.
In the result of cementation reaction (1) the molar ratio of
reduced silver and oxidizes tin in the solutions should be 2 : 1. The
comparison of the experimental results shows (Fig. 5, a, b, curves 1,
4) that the concentration of Ag(I) ions left the solution with pH 0.5
is greater than for the solution with pH 4.0 in 1.1 1.4 time and the
increase in cementation duration leads to the growth of the
difference in Ag(I) concentrations. Concentrations of tin ions
experimentally determined and calculated (according to the reaction
1 and concentrations Ag(I)) in cementation solution with pH 0.5 are
coincide up to the tin treatment time equal to 8 min. After this
period of time, the experimentally determined concentration of
Sn(II, IV) becomes 1.5 times lower than calculated one (for 16
min). The established fact indicates that the reduction of Ag(I)
occurs according to reactions 1 and 2.
a
b
Figure 5. The dependences of metal ions concentrations in
cementation solutions with pH 0.5 (a) and pH 4.0 (b) on treatment
time: 1, 4 cemented Ag(I); 2, 5 Sn(II, IV) passed onto the
solution; 3, 6 Sn(II) calculated according to the reaction 1 and
concentrations Ag(I). Tin foil with square 25 cm2 was treated in 20
ml of the solutions
For the solution with pH 4.0, the experimental and calculated
concentrations of tin ions at cementation duration from 2 to 16 min
differ in ~4.5 times. The reason for such low concentration may be
due to the fact that tin passing into the solution is easily hydrolyzed
at pH 4.0 with formation of oxy-hydroxy compounds adsorbed on
the surface of the tin and glass beaker in which the cementation
process takes place, all these lead to underestimated results of tin
concentration determinination. So, it follows, that almost all Sn(II)
ions passed into the solution through the reaction 1 participate in the
reduction of Ag(I) by reaction 2 in the solution with the pH 4.0. To
determine the quota of the reactions 3 and 4 in the total process of
tin dissolution, tin plates were kept in background solutions with pH
0.5 and 4.0 from 2 to 16 min. It was found that in the solution with
pH 4.0, the concentration of Sn (II, IV) is below the level
determined by XFS method. The absence of Sn(II, IV) in the
cementation solution with pH 4.0 probably connected with the
formation of insoluble oxy-hydroxy Sn(II, IV) compounds on tin
plate surface. In case of the solution with pH 0.5 Sn(II, IV)
concentration in the background solution is not more than 3% of the
quantity determined after cementation. The slowdown of Ag(I)
reduction by tin in cementation solution with pH 4.0 is caused by
the formation of a SnO2 film on the particles surface, which
prevents diffusion of silver atoms into the tin crystal lattice as it was
shown in [13], which explains the absence of the Ag3Sn
intermetallic compound in the obtained powder alloy. The absence
of characteristic peaks of tin oxides on the X-ray powder diffraction
patterns is probably due to the small thickness of oxide film. It is
necessary to pick out that characteristic peaks of SnO2 with low
intensity are presented on the X-ray diffraction patterns obtained for
tin plates after cementation.
4. Conclusion
The way of synthesis of SnAg powder alloy with the eutectic
phase composition (-Sn, Ag3Sn) and melting point (220.4 °C)
based on the cementation reaction in the system Ag+/Sn0 in aqueous
strongly acid (pH 0.5) solution contained thiourea was proposed.
It was found that cementation process in the system Ag+/Sn0 at
pH 4.0 allow to receive alloy powder with elemental composition
close to eutectic (6.0-6.3 wt.%) but included only one crystalline
phase (-Sn) and melted at tin melting point.
It was shown, that cementation in the solution with pH 4.0 is
accompanied with tin surface oxidation, with Sn(II, IV) hydrolysis
and formation of oxy- hydroxy- compounds adsorbed on the surface
of tin. These side processes depressed Ag(I) reduction and
formation of intermetallic Ag3Sn.
5. Acknowledgements
The study was carried out in the frames of the project 4.1.22
(2019-2020) of the State Scientific Research Institute “Mechanics,
Metallurgy, Diagnostics in Mechanical Engineering”, subprogram 4
“Electroplating”.
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