Structures of binary metallic nanoparticles produced ...

Powder Technology 288 (2016) 371–378

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Structures of binary metallic nanoparticles produced by electrical explosion of two wires from immiscible elements Marat I. Lerner, Alexander V. Pervikov ⁎, Elena A. Glazkova, Natalya V. Svarovskaya, Aleksandr S. Lozhkomoev, Sergey G. Psakhie Institute of Strength Physics and Materials Science SB RAS, 2/4 Akademicheskii pr., Tomsk 634055, Russia Tomsk Polytechnic University, 30 Lenina pr., Tomsk 634050, Russia

a r t i c l e

i n f o

Article history: Received 24 July 2015 Received in revised form 10 November 2015 Accepted 14 November 2015 Available online 15 November 2015 Keywords: Binary metallic nanoparticles Electrical explosion Twisted wires Immiscible elements Core–shell Janus nanoparticles

a b s t r a c t In this study, Cu/Ag, Сu/Pb and Al/Pb binary metallic nanoparticles were synthesized by the electrical explosion of two twisted wires from immiscible elements (i.e., Сu–Ag, Cu–Pb and Al–Pb). The distribution of metallic components in the nanoparticles depended on the melting temperature, metal density, and degree of initial wire superheating. Different types of nanoparticles, such as nanoparticles with no separation between the metal phases, as well as core-shell and Janus nanoparticles were formed depending on the process parameters and the characteristics of the metals. Certain binary metallic nanoparticles were formed when the metal clusters were combined and formed during the initial stages of dispersing the two wires using a pulse of electrical current. © 2015 Elsevier B.V. All rights reserved.

1. Introduction Most modern studies of the synthesis and properties of metallic particles consider single-component systems. Nanoparticles that contain several metals (two and more) are poorly studied, even though the range of properties governed by structural and phase diversity can be considerably broader. Interest in multi-component nanoparticles is primarily due to their physical and chemical properties, which are determined by both the small particle size and complex phase composition. In addition, their properties can differ significantly from the properties of the initial components [1]. Some of the most interesting nanomaterials are metallic systems that are composed of elements with little mutual solubility in the liquid and solid state, which result in immiscible alloys [2]. These metals are only mutually-soluble under thermodynamically equilibrium conditions in a narrow concentration range. Mixing in these systems is challenging due to the large difference in the densities and melting temperatures of metals [3]. The melting temperature of metals can decrease in nanometric particles, and therefore, the mixing behavior of immiscible metals can be changed in dependence on the shape of the nanoparticles and the ratio of the components [4]. Nanoparticles that are based on these alloys ⁎ Corresponding author. E-mail addresses: [email protected] (M.I. Lerner), [email protected] (A.V. Pervikov).

http://dx.doi.org/10.1016/j.powtec.2015.11.037 0032-5910/© 2015 Elsevier B.V. All rights reserved.

may exhibit unique properties that are not typical of the initial components. These nanoparticles may be used as catalysts and nanomagnets as well as in high-density data storage and biomedical applications. For example, the Cu/Ag binary metallic nanoparticles can be used as components in catalysis [5], and powders based on the Cu/Pb and Al/Pb binary metallic particles have been used to produce antifriction materials [6,7]. The structural states observed as the nanoparticles are formed from immiscible elements range from partial or complete mutual dissolution of elements [3] to the formation of Janus nanoparticles that consist of the initial alloy elements [8]. The binary metallic nanoparticles consisting of Ag/Cu, Pt/Au, Cu/W [9], Ag/Fe [10], Ag/Ni [11], Cu/W [12] and Cu/Ta [13] have been synthesized from immiscible elements using different methods. According to Grigorieva et al. [3] and Barnard [14], the cooling rate is the determining factor for structural stabilization of supersaturated solid solutions based on immiscible elements during quenching from the liquid state. A high-energy method for synthesizing metallic nanoparticles and their chemical compounds involves electric explosion of wires [15,16]. The electrical explosion of wires (EEW) is a process that results in high rate thermodynamic parameter changes in the system, and these changes are achieved due to the electric current flow through a metal wire at a current density of 106–109 A/cm2. A current density of 107– 108 A/cm2 allows for the explosive transition of the metal from the condensed state to a two-phase state, such a liquid metal–gas/plasma [17, 18]. Metal superheating leads to the formation of a dense cylindrical

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core that consists of the products from the electric explosion of wires, which have a foamed structure [17]. The wire explosion products expand to form a gas at rates of several kilometers per second, which provides a subsequent cooling rate up to 1010 K/s [19]. Kim and Nagasawa have demonstrated that the electric explosion of wires can be applied to synthesize a solid solution and intermetallic alloy nanopowders [20,21,22]. In this study, we report the results from studies of the structure and phase composition of nanoparticles formed during the electrical explosion of two wires of elements with little mutual solubility in the solid state (i.e., Cu–Ag, Cu–Pb and Al–Pb).

2. Experimental The synthesis of binary metallic particles was performed using pairs of metal wires (i.e., Cu–Ag, Cu–Pb, Al–Pb). The wires were purchased from the manufacturers. The metal content in the wires declared by the manufacturers was no less than 99.9 wt.%. Prior to use, the wires were cleaned with an organic solvent to remove surface contaminants. A pair of wires was twisted by a special mechanical device. The content of the metals in the twisted couple was regulated by the ratio of the diameters of the wires. Binary metallic particles were synthesized using the apparatus with the circuit diagram shown in Fig. 1a. The twisted wires (Fig. 1b) were wound on spool 1, which was placed in reactor 2, and the air was evacuated to a pressure of 10−2 Pa. Then, the reactor was filled with argon to a pressure of 2·105 Pa. To produce nanoparticles, a high-voltage power supplier (PS) was employed to charge the capacity storage (С) to the required voltage, which was controlled by a kilovoltmeter (kV). The energy storage system (С) consisted of a pulse capacitor bank connected in parallel. After switching on a discharger (P), the energy stored in C was transferred to the interelectrode gap located between high-voltage electrode 3 and grounded electrode 4. High-voltage electrode 1 was connected to the electrical circuit through bushing 5. Supply of wires (EW) in the interelectrode gaps 3–4 was carried out by rotating rollers 6 using a motor that was located on the outside of the reactor (the motor is not shown in Fig. 1a). When the wires were moved, they closed interelectrode gap 3–4. Then, the capacitors discharged, and the wires exploded. The length of the exploding wires was adjusted by the distance between electrodes 3 and 4. To produce nanopowders by EEW in our experiments, the apparatus with electrical energy power consumption equal to 3 kW·h was used. When the frequency of explosions was 0.5 Hz, the productivity of the apparatus was 40–50 g/h. Therefore, the cost of the electrical energy was 60–75 kW·h/kg. The circulation of argon in the apparatus was carried out using pump 7. The gas flow is shown by the arrows in Fig. 1a. The aerosol, which was formed after the electric explosion of wires, entered cyclone 8 where the nanoparticles were deposited. After synthesis of the nanoparticles, the argon pressure in the apparatus was decreased to atmospheric pressure. To prevent rapid oxidation

and spontaneous combustion, the nanoparticle samples were passivated in air for 48 h. The discharge current (I(t)) and voltage (U(t)) were recorded using a current sensor (R3), a voltage sensor (R1–R2) and a TDS2022B storage oscilloscope. R1 and L are the intrinsic resistance and self-inductance, respectively, of the electric circuit of the apparatus. The phase diagrams for the mentioned pairs of metals have been reported elsewhere [23]. They show that the copper-based solid solution in the Cu–Ag system has a maximum silver content of approximately 8.0 wt.%. For the silver-based solid solution, the maximum copper content was approximately 8.8 wt.%. For the Al–Pb and Cu–Pb systems, the lead content in the aluminum and copper is 0.0 wt.%. The wire characteristics and binary metallic nanoparticles synthesis conditions are given in Table 1. Тm is the metal melting temperature, ρ is the specific metal density under normal conditions, R is the radius of the metal atoms, d is the wire diameter, l is the wire length, N is the content of metals in a twist of two wires, U is the voltage on capacitor C, and j is the current density in the wire twist. The key parameter that determines the nanopowder characteristics in wire explosion is the degree of wire metal superheating, which can be calculated for single-metal wires using Eq. (1) as follows [15]: W m ¼ E=ðES  V Þ;

ð1Þ

where Е is the energy supplied to the wire, Еs is the sublimation energy of a particular metal, and V is the wire volume. The energy (E) supplied to the winding EW of two twisted wires was estimated by an oscillogram of the discharge current I(t) and voltage U(t) using the replacement of the integral (2) tZexp

U ðt Þ  Iðt Þdt;

E¼

ð2Þ

t0

by the finite sum. The energy (Еs) was calculated as the sum of the standard sublimation energies of each of the dispersed metals relative to their mass. The quantity Wm varied within the ranges listed in Table 1. The micrographs of the nanoparticles were obtained using a transmission electron microscope (JEM-2100). The elemental composition and distribution in the nanoparticles were determined using an XMax X-ray detector fitted to the microscope. The average nanoparticle size was determined by constructing particle size distribution histograms based on electron microscopy data. The resulting histograms of the particle size distribution were approximated using the normallogarithmic law. Each histogram was constructed based on size measurements for at least 3000 particles. The phase composition of the nanoparticles was determined using a Shimadzu XRD 6000 diffractometer with Co-Kα radiation. The solute element content in the solvent lattice was determined by Vegard's law.

Fig. 1. a–b. Circuit diagram of the apparatus employed for the production of binary metallic nanoparticles (a) and two twisted wires (b).

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Table 1 Wire characteristics and binary metallic particle synthesis conditions. Wire metals

Тm, K

ρ, g/cm3

d, mm

l, mm

N, wt.%

U, kV

С, μF

Wm, a.u.

R, Å

Cu Ag Cu Pb Al Pb

1356 1235 1356 600 933 600

8.9 10.5 8.9 11.3 2.7 11.3

0.20 0.20 0.20 0.28 0.35 0.28

80 80 80 80 100 100

44.0 56.0 29.0 71.0 29.0 71.0

22–32

2.0

1.1–2.8

20–30

1.2

1.3–2.4

25–32

2.0

1.2–2.0

1.28 1.44 1.28 1.75 1.43 1.75

3. Results and discussion 3.1. Cu/Ag nanoparticles Dispersion of the Cu–Ag wire twist at Wm = 1.1–1.6 (Fig. 2а, b) mostly leads to the formation of particles with no apparent separation into individual components. Only a small number of core-shell and Janus nanoparticles were observed in samples. The powders synthesized at Wm = 1.1 consist of particles with either a high copper content with respect to silver or a high silver content with respect to copper. As Wm increased to 2.8, the powder primarily contains Janus and core-shell nanoparticles (Fig. 2c). The micrographs and elemental analysis results for a Cu/Ag Janus nanoparticle are shown in Fig. 3a. There are regions in the particle volume that contain Cu and Ag in approximately equal proportions (Fig. 3a, elemental analysis at points 2 and 3) and those that enriched with copper (Fig. 3a, elemental analysis at point 1). The surface layers of the Cu/Ag core-shell nanoparticles (Fig. 3b) are also rich in copper, which is consistent with the elemental analysis data shown in Fig. 3с. All studied Cu/Ag powders (according to X-ray phase analysis, Fig. 4) contain copper oxides Сu2O, CuO and solid solutions as follows: 1) silver in copper with a lattice parameter of 3.65 ± 0.01 Å (Ag content in the Cu

Fig. 4. X-ray diffraction pattern of the Cu/Ag sample.

of approximately 10.0 wt.%) and 2) copper in silver with a lattice parameter of 4.03 ± 0.01 Å (Cu content in the Ag of approximately 6.0 wt.%.) The Ag content in the Cu lattice and Cu content in the Ag lattice approximately correspond to the previously reported phase diagram data [23]. The splitting of the reflection corresponding to the Ag (111) (2Theta ≈ 45 degree) plane was observed at Wm = 1.1, and at Wm = 1.6, the reflection splitting is less pronounced, which suggests the formation of a more homogeneous silver-based solid solution. At Wm ≈ 2.8, the intensity ratio of the reflections corresponding to the Ag (111) and Сu (111) planes decreased substantially (2Theta ≈ 50°), which indicates a decrease in the content of the copper-based solid solution in the sample. This change may be due to the decomposition of a

Fig. 2. a–b. Micrographs of the Cu/Ag nanoparticles: Wm = 1.1, number-average particle size an = 97 nm (a); Wm = 1.6, an = 87 nm (b); Wm = 2.8; an = 92 nm (с).

Fig. 3. a–c. Micrographs of the Cu/Ag Janus nanoparticle (a), elemental analysis at points: 1 — 76Cu/24Ag wt.%, 2 — 36Cu/64Ag wt.%, 3 — 18Cu/82Ag wt.%; nanoparticle with a core–shell structure (b), and EDS line scans (c) of the particle shown in Fig. 3b. Wm = 2.8 for the particles shown in Fig. 3a and b.

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Fig. 5. a–c. Micrographs of the Cu/Pb particles: Wm = 1.3, number-average particle size an = 95 nm (а); Wm = 1.6, an = 92 nm (b); Wm = 2.4; an = 79 nm (с).

supersaturated copper-based solid solution during crystallization. The metal excess, which is greater than the values allowed by the phase diagram of the Ag/Cu alloy, segregates at the surface to form core–shell and Janus particles (Fig. 3). Silver and copper have similar melting temperatures but the specific density of silver is higher than that of copper (Table 1), which results in the displacement of copper to the particle periphery and formation of a copper-enriched shell during nanoparticle cooling. Janus particles can form if crystallization continues after the displacement of a less dense metal to the near-surface particle layers. In this case, the copperenriched phase on the particle surface tends to assume a spherical

shape under surface tension forces, which leads to the formation of the structure shown in Fig. 3a.

3.2. Cu/Pb nanoparticles The micrographs of the nanoparticles produced by dispersion of the two Cu and Pb wires are shown in Fig. 5. It is difficult to draw conclusions regarding the particle structure based on the micrographs. Unlike Cu/Ag particles, Cu/Pb particles do not exhibit clear separation between the metallic components.

Fig. 6. a–d. Micrographs particles (a), (с), and Cu, Pb and O distribution (b) and (d) along the lines indicated in Fig. (а) and (c), respectively. Wm = 1.3 (a); Wm = 2.4 (с).

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Fig. 7. X-ray diffraction pattern of the Cu/Pb sample.

However, elemental analysis indicates that the metal distribution in the particle volume is inhomogeneous. A majority of the particles have a copper-enriched core (Fig. 6a and b), and the surface layers contain both copper and lead in nearly equal proportions. For particle production from metals with considerably different melting temperatures, such as Cu and Pb (and according to the Cu–Pb phase diagram), the copper-based solid solution is the first to solidify, hindering lead diffusion to the near-surface layers. Therefore, only individual particles with a partial separation of metallic components were observed in the Cu/Pb powder samples (Fig. 6с and d). Similar to the Cu/Ag particles, the amount of these particles and the dispersion products in the samples increased with the wire superheating degree. The elemental analysis data of the Cu/Pb particles indicated that the surface layers of the particles were enriched with lead and contain oxygen. Most likely, lead oxides were formed due to contact between the binary metallic particles and air. Independently of Wm, the XRD data for the Cu/Pb powders indicate the presence of the following crystal structures (Fig. 7): solid solution of lead in copper with a lattice parameter of a = 3.635 ± 0.01 Å, (Pb content in the Cu of approximately 5.0 wt.%); pure lead with a lattice parameter of а = 4.95 Å; α- and β-phases of lead oxide (PbO). 3.3. Al/Pb nanoparticles The micrographs of the nanoparticles produced by dispersion of two Al and Pb wires are shown in Fig. 8. The structure of the particles is approximately the same and independent of Wm. In contrast to the Cu/Ag and Cu/Pb powders, the Al/Pb powders contain a significant number of Janus particles already at Wm = 1.2.

375

Certain Al/Pb particles have lead-enriched surface layers (Fig. 9a, b). Most likely, similar to the Cu/Pb particles, a more refractory metal crystallizes and displaces the metal with a lower melting temperature from the bulk. However, the melting temperature difference between aluminum and lead is not as significant as that between copper and lead. Therefore, the separation of metallic components in most of the particles ends in the formation of Janus nanoparticles (Fig. 9c,d) that are at a relatively low superheating value (Wm = 1.2). The XRD data for the Al/Pb powders indicate the presence of only aluminum and lead crystal structures in all of the samples (Fig. 10). The metal oxides were not revealed by X-ray phase analysis. Based on the size distributions shown in Fig. 11, the sizes of the Cu/Ag, Cu/Pb and Al/Pb particles slightly depend on Wm. All of the samples have positively skewed distributions that are close to the logarithmically normal distribution law. This type of asymmetry might indicate Brownian coagulation of the particles [24]. It is important to that all of the studied powders contain binary metallic Cu/Ag, Cu/Pb and Al/Pb nanoparticles. In addition, according to the TEM data, pronounced disintegration into separate metal phases with a low content of the dissolved element was not observed (Fig. 12, Table 2). The results in Table 2 demonstrate that the central regions of the certain particles are enriched in a more high-melting metal (Cu/Ag particles: elemental analysis at points 2, 4; Cu/Pb: elemental analysis at points 6; Al/Pb: elemental analysis at points 1, 7). The low-melting metal content is higher in the near-surface regions (Cu/Pb particles: elemental analysis at point 7,8; Al/Pb particles: elemental analysis at points 2, 3, 4), which is additional evidence for solidification with the segregation of metallic phases. Elemental analysis at points 5 end 6 in Fig. 12с indicates the elemental composition of the nanoparticles that primarily consist of aluminum and oxygen. According to the Hume–Rothery rules, the factors that determine the probability of the formation of solid solution between two metals are as follows: the difference in the atomic radius of metals, chemical affinity, and valence of interacting metal atoms (metals with a lower valence dissolve a greater amount of the second metal) as well as the type of crystal lattice of the interacting metals (separation predominates in metal pairs with different types of crystal lattices) [25]. The metals pairs including Cu–Ag, Cu–Pb and Al–Pb have the same type of crystal structure (fcc-fcc), and do not form intermetallic compounds based on their phase diagrams. In this context, the probability of these metals to form solid solutions is determined by the difference in the atomic radius and valence of the metals. The difference in the atomic radius is determined by the following expression (3): ΔRMe1Me2 ¼ ððRMe1 −RMe2 Þ=RMe2 Þ  100%

ð3Þ

RMe1 is the radius of the atom of a dissolved element and RMe2 is the atomic radius of the solvent.

Fig. 8. a–c. Micrographs of the Al/Pb particles: Wm = 1.2, number-average particle size an = 113 nm (а); Wm = 1.6, an = 106 nm (b); Wm = 2.0; an = 96 nm (с).

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Fig. 9. a–d. Micrographs of the Al/Pb particles (a) and (с) as well as Al, Pb and O distribution (b) and (d) along the lines indicated in Fig. (a) and (с), respectively. Wm = 2.0.

If the ΔRMe1Me2 N 15%, the formation of the solid solution is difficult, resulting in phase separation. When ΔRMe1Me2 b 15%, solid solutions are formed [25]. The ΔRМе1Ме2 values are ΔRСu–Ag = 12.5%, ΔRСu–Pb = 36% and ΔRAl–Pb = 22% for the selected metal pairs. The formation of solid solutions most likely occurs for Cu–Ag, which is in agreement with the experimental data on ΔRMe1Me2. For pairs of metals consisting of Сu–Pb and Al–Pb, the ΔR Al–Pb values are more than 15%, resulting in separation

into two phases in agreement with the electron microscopy data. In our opinion, the formation of a metastable solid solution consisting of Cu0.985Pb0.015 was caused by the high cooling rate [3] of the dispersed phase during the EEW [19]. The structure of the binary metallic nanoparticles was primarily determined by the formation process [1]. Currently, no generally accepted mechanism is available for particle formation during the electric explosion of wires. The following scenarios are discussed in the literature: 1. The major part particle formation mechanism involves the condensation of metallic gas formed during metal evaporation due to metal superheating above the boiling temperature (see e.g., [26]). 2. The major part particle formation mechanism involves grouping (coagulation or coalescence) of the structural elements formed immediately in the wire dispersion. These elements can be clusters formed in the wire dispersion [16].

Fig. 10. X-ray diffraction pattern of the Al/Pb sample.

Scenario 1 for binary particle formation is unlikely. According to Sarkisov et al. [17] and Baksht et al. [18], the dispersion products in the electrical explosion of single-metal wires are a mixture of a liquid metal and a metallic gas. Pervikov [27] has shown that in wire explosion of a brass alloy with Е N Еs, the liquid phase does not change completely to the gaseous phase. Scenario 2 is more likely. Binary metallic particles as well as monometallic particles may be formed during aggregation of clusters formed during the initial stages of dispersion of each wire. This mechanism is supported by the positive shift in the nanoparticle size distribution functions shown in Fig. 11 and the spherical shape of the nanoparticles.

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Fig. 11. Size distributions of the Cu/Ag (а), Cu/Pb (b) and Al/Pb (с) nanoparticles.

Fig. 12. a–c. Micrographs of the particles: Cu/Ag particles (a), Cu/Pb particles (b) and Al/Pb particles (c) (Wm = 1.9).

Alayan [28] demonstrated that rapid coagulation of clusters yields a spherical particle shape. This suggestion is supported by the nanometric grains structure apparently formed in Cu/Ag particles (Fig. 13), which could form due to coagulation of Cu and Ag clusters. Elemental point analysis indicates that the copper to silver ratios in the particles approximately correspond to the ratio of metals in the two twisted wires. Evidently, complete separation of the metallic phases does not occur during particle structure formation in the rapid cooling of expanding explosion products. The suggested particle formation mechanism might explain the structural features of the nanoparticles shown in Fig. 12. The structure of these particles may represent a mixture of the initial metal clusters. 4. Conclusions During the electric explosion of the two twisted metal wires, the nanoparticles from elements with little mutual solubility in the solid state (i.e., Cu–Ag, Cu–Pb and Al–Pb) were prepared for the first time, and their structural characteristics were investigated. Nanoparticles with different structures, which were controlled by the synthesis

conditions, can be obtained at high cooling rates of the dispersion products. The preferential formation of the nanoparticles with the structure without a clear separation of metallic components as well as core–shell and Janus nanoparticles is determined by the ratio of the melting temperatures and densities of the metals as well as by the degree of wire superheating (Wm). At similar metal melting temperatures, a lower-density metal is displaced to the near-surface layers of the particles. If the melting temperature of one metal is higher, the metal with a lower melting temperature is displaced to the near-surface layers. As the degree of metal superheating increased to Wm = 2.8, the separation of the metallic components results in the formation of Janus nanoparticles. Therefore, the structure of the binary metallic nanoparticles can be adjusted by changing the wire superheating degree. The high cooling rate of the products of destruction wire during the electric explosion of Cu–Pb wires result in the formation of a metastable solid solution Cu0.985Pb0.015. The most likely formation mechanism for the binary metallic nanoparticles involves the coalescence of metal clusters formed during the early dispersion stages of the two electrically exploded wires. The electric explosion of wires is a promising approach for synthesizing and studying new types of binary metallic nanoparticles

Table 2 Elemental distribution in the particles shown in Fig. 12. Elemental analysis points

1 2 3 4 5 6 7 8

Composition of Cu/Ag particles

Composition of Cu/Pb particles

Composition of Al/Pb particles

O, wt.%

Cu, wt.%

Ag, wt.%

O, wt.%

Cu, wt.%

Pb, wt.%

O, wt.%

Al, wt.%

Pb, wt.%

0.0 4.0 3.0 4.0 – – – –

47.0 55.0 49.0 54.0 – – – –

53.0 41.0 48.0 42.0 – – – –

5.0 4.0 6.0 2.0 9.0 2.0 4.0 13.0

43.0 29.0 45.0 45.0 44.0 43.0 34.0 27.0

52.0 67.0 49.0 53.0 47.0 55.0 62.0 60.0

6.0 1.0 5.0 1.0 29.0 47.0 4.0 –

27.0 1.0 3.0 2.0 38.0 46.0 24.0 –

67.0 98.0 92.0 97.0 33.0 7.0 72.0 –

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Fig. 13. Micrographs of a Cu/Ag particle: the elemental composition in the analyzed point is Cu 67.0/Ag 33.0 wt.% (Wm = 2.8).

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