Superior Plasticity of Silver-Based Composites with Reinforcing Pyrochlore

: Silver (Ag) has difﬁcult forming strong bonding with oxides due to its deep d band beneath the Fermi level and completely ﬁlled 4d orbital. Thus, it is difﬁcult to fabricate silver-based composites with superior plasticity and processability because of the easy debonding at their interface. Herein, La 2 Sn 2 O 7 pyrochlore was used as a reinforcing phase for a silver matrix. The enhanced interfacial bonding strength of Ag-La 2 Sn 2 O 7 was conﬁrmed both theoretically and experimentally, indicating that Ag could form more localized ionic bonding with La 2 Sn 2 O 7 than with SnO 2 . The superior plasticity was further conﬁrmed for the Ag-La 2 Sn 2 O 7 composite, as the uniform elongation (UE) of the Ag-La 2 Sn 2 O 7 composite was ~19%, i.e., ~14% higher than and 2.8 times that of the conventional Ag-SnO 2 composite. The plasticity enhancement mechanism was also unraveled by calculating the interfacial mobility. This work veriﬁed the usefulness of pyrochlore to fabricate silver-based composites with superior plasticity and also provides a new strategy for the construction of advanced silver-based composites for application in the electrical contact ﬁeld.


Introduction
Silver (Ag) is widely used in many scientific and industrial fields, such as in heat sink materials, conducting components, electrodes, aircraft circuit breakers, motor controllers, pressure controllers and electric switches due to its excellent thermal and electrical conductivity [1,2].However, its inferior strength, low hardness and thermal stability do not allow meeting the design requirements for practical applications, especially under severe conditions.
Silver-ceramic particle composites, due to their integrated properties of high hardness and strength, a low thermal expansion coefficient, good arc resistance arising from the ceramic particles and high electrical and thermal conductivity arising from Ag, offer excellent thermal, electrical and mechanical performance, allowing a wide range of applications [3,4].Great efforts have been made to fabricate the desired silver-ceramic particle composites.As reported previously, mono-oxides such as tin oxide (SnO 2 ) [5], zinc oxide (ZnO) [6] and nickel oxide (NiO) [7] are promising candidates for high-strength silver-based composites obtained by dispersion strengthening [8].However, the plasticity and deformability of these composites rapidly degrade with increasing amounts of SnO 2 , ZnO and NiO [9].
Since metal-based composites consist of metal and ceramic particles, the characteristics of the metal-ceramic interface are a vital factor that affects the monolithic mechanical properties of the composites [10,11].A decreased bonding strength in metal-based composites may lead to the formation of cracks during the deformation process and thus to a decrease in plasticity.Generally, the charge transfer between the metal matrix and the metal oxides dispersed phase, which increases the oxidation of the interfacial metal, can enhance the bonding strength by electrostatic attraction.Silver, due to its completely filled d orbital, has difficulty forming a strong interface with metal oxides [12,13].Typically, the introduction of heteroelements or defects into the metal matrix was considered an effective way to improve the interface properties of metal-based composites.The introduced elements or defects would affect the electronic structure and bond properties of the interfaces, thus improving or weakening the bonding strength.The introduction of chromium (Cr) improved the bonding strength between SiO 2 and a silver matrix [14], that of both titanium (Ti) and Cr improved the bonding strength between SiO 2 and a copper matrix [15], and that of Ni improved the bonding strength between a copper matrix and Tungsten (W) [16].Meanwhile, introduced defects such as oxygen vacancies can also improve the metal-ceramic bonding strength by regulating the interfacial electrons distribution and enhancing the interaction between metal cations and oxygen anions [17][18][19].However, the heteroatoms or defects introduced in a metal matrix can restrain dislocation motions and act as scattering centers, hindering the motion of electrons and thus decreasing plasticity and conductivity.Thus, changing the components of dispersed metal oxides is a more desirable way to enhance the interfacial bonding strength.In addition, it is of great importance to reveal how different incorporated ceramic particles affect the interfacial properties, thus affecting the plasticity of metal-based composites, and their potential positive effects on conductivity, and thereby guide to the discovery of advanced silver-based composites with superior plasticity and deformability.
In addition, for particle-reinforced composites, the thermal mismatch (difference of the coefficient of thermal expansion between the matrix and the reinforcement particles) would induce residual strain and produce a high density of defects that will restrain the dislocations and decrease the plasticity [20,21].A high thermal mismatch can induce dislocation accumulation and stress concentration around the two-phase interface, which will also decrease the plasticity of the composites and facilitate microcracks nucleation and growth.The coefficient of thermal expansion (CTE) of silver is about 19.0 × 10 −6 K −1 , while the CTE of SnO 2 , a typical reinforcing phase in silver-based composites, is about 3.8 × 10 −6 K −1 .Thus, Ag-SnO 2 composites usually show poor plasticity and deformability.Bearing the negative effect of thermal mismatch on plasticity in mind, pyrochlore, a typical ceramic with a high CTE of 7-9 × 10 −6 K −1 [22], can be a competitive candidate as the reinforcing phase.So far, although Ag-SnO 2 composites fabricated by internal oxidation and powder metallurgy have been widely used as electrical contact materials, their poor plasticity (the elongation is up to 13% for Ag-SnO 2 composites fabricated by internal oxidation, and to 3~7% for those fabricated by powder metallurgy) impedes the process of replacing the toxic Ag-CdO composites.
Herein, La 2 Sn 2 O 7 pyrochlore was chosen as the reinforcing phase for a silver matrix.We performed calculations based on the density functional theory (DFT) for the Ag-La 2 Sn 2 O 7 and Ag-SnO 2 composites to establish the theoretical properties of the silverceramic interface for these two composites (e.g., the bonding properties based on the electron localization function, diffusion coefficient, theoretical displacement under shear stress and binding energy) and predict their mechanical performance, especially plasticity.Accordingly, Ag-La 2 Sn 2 O 7 composites were prepared by the powder metallurgy process combined with the extrusion method and demonstrated superior plasticity.The as-prepared Ag-La 2 Sn 2 O 7 composites showed enhanced plasticity, which was much higher than that of the Ag-SnO 2 composite and other composites.The plasticity enhancement mechanism was also revealed by theoretical calculations and by comparing the experimental values of conductivity with the theoretical values for the Ag-La 2 Sn 2 O 7 and Ag-SnO 2 composites, respectively.The good conformity between the experimental and the theoretical conductivity of the Ag-La 2 Sn 2 O 7 composites indicated interfaces with less defects, which can scatter moving electrons, and good interfacial bonding strength, which hinders the formation of cracks.

Materials
All chemicals were of analytical grade and were used without further purification.Oxalic acid, stannic chloride, an ammonia aqueous solution and polyethylene glycol (PEG2000) were purchased from Sinopharm Chemical Reagent Co., Ltd.(Shanghai, China).Lanthanum nitrate hexahydrate was purchased from Aladdin.The silver powder (purity > 99.9% mass%) was purchased from Wenzhou Hongfeng Electrical Alloy Co., Ltd.(Wenzhou, China).

Material Synthesis 2.2.1. Synthesis of La 2 Sn 2 O 7 and SnO 2 Powders
In a typical synthesis of La 2 Sn 2 O 7 powders, 0.025 mol La(NO 3 ) 3 •6H 2 O and 0.025 mmol SnCl 4 •5H 2 O were first dissolved in 500 mL of deionized water, and then 0.98 g of PEG2000 was added as a dispersant.Ammonia was added to the above solution drop by drop until reaching pH 7~8, and then the solution was subjected to vigorous stirring for 3 h.The precursor powders were obtained by centrifugation, washed with deionized water for 3 times and dried at 80 • C for 12 h.La 2 Sn 2 O 7 pyrochlore was obtained by calcinating the precursor powder at 1200 • C for 2 h (XRD and SEM images are shown in Figure S1 in Supplementary Information).The SnO 2 powder was synthesized by the same method.

Composite Fabrication
The mixed Ag-La 2 Sn 2 O 7 powders were first subjected to ball milling t a rotation speed of 300 rpm for 4 h using a planetary ball milling machine and the mass fraction of La 2 Sn 2 O 7 of 12%.Then, the milled Ag-La 2 Sn 2 O 7 powders were added to a die (Φ 40 mm) and uniaxially pressed at a hydraulic pressure of 7 MPa for 10 s.The obtained disks, 33.10 mm in height and 38.11 mm in diameter, were sintered at 900 • C for 6 h in a muffle furnace.Then, hot extrusion was carried out at 900 • C at an extrusion pressure of 10 MPa to help densification.Subsequently, further heat treatment was carried out at 400 • C for 6 h.The Ag-SnO 2 composites were synthesized through the same process.The ceramic volume fraction was 17.2% in Ag-SnO 2 and 17.5% in Ag-La 2 Sn 2 O 7 .The relative densities of Ag-SnO 2 before and after extrusion were 93.96 ± 0.62% and 95.88 ± 0.27%, respectively, while the corresponding values were 95.37 ± 0.76% and 97.86 ± 0.14% for Ag-La 2 Sn 2 O 7 .

Material Characterization
Transmission electron microscopy (TEM) was carried out on a JEM-1200 TEM (JEOL, Akishima, Japan) at an acceleration voltage of 120 kV and on a JEM-2100 (JEOL, Akishima, Japan) at an acceleration voltage of 200 kV.Scanning electron microscopy (SEM) was carried out on a HITACHI S-8010 scanning electronic microscope (HITACHI, Tokyo, Japan).

Tensile Test
The Ag-SnO 2 and Ag-La 2 Sn 2 O 7 composites were used for the tensile test along the hot extrusion direction.The length of the scale distance of the tensile samples was 100 mm.The tensile test was carried out on a unidirectional tensile testing machine (CMT5504) (Sansi Yongheng Technology (Zhejiang) Co., Ltd., Ningbo, China) at a loading rate of 0.5 mm/min.Each sample was measured at least three times.

Computational Method
The compound Ag-SnO 2 was modelled by building the (1 1 1) plane of Ag and the (1 1 0) plane of SnO 2 .Similarly, the model of Ag-La 2 Sn 2 O 7 was built by the (2 2 0) plane of Ag and the (4 0 0) plane of La 2 Sn 2 O 7 .Structure optimization was performed by the Vienna Ab initio Simulation Package (VASP) [23,24] with a force convergence smaller than 0.05 eV/Å.Monkhorst-Pack k-points of 1 × 1 × 1 were used for all structures.Generalized gradient approximation of the Perdew-Burke-Ernzerhof (PBE) functional [25] was used to describe the exchange-correlation effects of the electrons.The projected augmented wave (PAW) potentials [26,27] were chosen to describe the ionic cores, taking valence electrons into account using a plane wave basis set with a kinetic energy cutoff of 500 eV.In order to calculate the diffusion coefficients, ab initio molecular dynamics (AIMD) studies were carried out.The optimized structures were firstly equilibrated by NVT ensemble at 300 K, then the production run was carried out with NVT ensemble, holding the temperature at 300 K for 10 ps with a timestep 0.2 fs.The shear strength was calculated according to a previous work [28] on the (1 1 1) plane of Ag in Ag-SnO 2 and on the (2 2 0) plane of Ag in Ag-La 2 Sn 2 O 7 .In order to investigate the microscopic origin of the mechanical properties, the electron localization function (ELF) and static electronic potential were calculated based on the optimized structure.

Theoretical Evaluation of Interfacial Bonding Strength
A theoretical calculation of the electron localization function (ELF) and the evaluation of the bonding energy in the interface of the composites were carried out to confirm the enhanced interfacial bonding strength of the Ag-La 2 Sn 2 O 7 composites.The models of Ag-SnO 2 and Ag-La 2 Sn 2 O 7 composites on which our calculations were based are shown in Figure 1a,b, respectively.As shown in Figure 1c, the enhanced localization and ionicity induced by charge transfer between metal and metal oxide could be confirmed at the interface of Ag-La 2 Sn 2 O 7 , while less charge transfer was generated at the interface of Ag-SnO 2 , indicating a weaker interaction between Ag and SnO 2 .Moreover, the bonding energy calculated was −0.8944 eV for Ag-La  Based on the theoretical prediction of the interfacial bonding strength, the Ag-La 2 Sn 2 O 7 composites were prepared by the powder metallurgy process combined with extrusion methods.Figure 2 presents the TEM morphologies of the as-prepared Ag-SnO 2 composite and Ag-La 2 Sn 2 O 7 composite.As shown in Figure 2a, a large number of dislocation walls and dislocation cells were observed near the SnO 2 particle.Meanwhile, the debonding of the SnO 2 particles from the Ag matrix was also detected, as shown in Figure 2a.In contrast, as shown in Figure 2b, dislocation slip bands and movable dislocations were observed near the La 2 Sn 2 O 7 particles, where no dislocation walls were detected.It is often the case that the mismatch of the CTE between the reinforcements and the Ag matrix will generate geometrically necessary dislocations (GNDs) [29].The larger density of dislocations and dislocation accumulation observed for Ag-SnO 2 can be attributed to the larger CTE mismatch between SnO 2 and the Ag matrix, while the lower CTE mismatch between La 2 Sn 2 O 7 and the Ag matrix produced a lower dislocation accumulation and less hardening [30].Figure 2c,d shows the HRTEM images of the Ag-SnO 2 and Ag-La 2 Sn 2 O 7 interfaces, respectively.A clear and tight Ag-SnO 2 interface was observed for the Ag-SnO 2 composite, while a blurred interface indicating an interfacial transition layer was observed for Ag-La 2 Sn 2 O 7 .The presence of an interfacial transition layer indicated enhanced interfacial bonding strength in the Ag-La 2 Sn 2 O 7 composite, which was consistent with the calculated bonding energy of the Ag-La 2 Sn 2 O 7 interface.The enhanced interfacial bonding strength confirmed by both theoretical calculations and TEM observations will decrease the formation and growth of microcracks and microcavities, which is beneficial to increasing fracture elongation and plasticity.

Tensile Test
Furthermore, a tensile test was conducted on both the Ag-La 2 Sn 2 O 7 composite and the conventional Ag-SnO 2 composite to confirm the superior plasticity of the Ag-La 2 Sn 2 O 7 composite experimentally.The engineering stress-strain curves of the Ag-La 2 Sn 2 O 7 composite and Ag-SnO 2 composite are shown in Figure 3a.The tensile strength (TS) and uniform elongation (UE) are reported in Table 1.It was found that the Ag-La 2 Sn 2 O 7 com-posite exhibited superior UE of 19%, which was ~14% higher than that of the Ag-SnO 2 composite.Meanwhile, the TS of the Ag-La 2 Sn 2 O 7 composite was lower than that of the Ag-SnO 2 composite, indicating good dislocation mobility and a lower density of defects that restrain the motion of dislocation (the typical jagged stress-strain curve induced by twinning was absent).Figure 3b-d shows the fracture morphologies of pure silver, Ag-SnO 2 and Ag-La 2 Sn 2 O 7 , respectively.The fracture surface of pure silver consisted of quite coarse dimples, while the fracture surface of the Ag-La 2 Sn 2 O 7 and Ag-SnO 2 composites consisted of relatively fine dimples, apparently indicating typical plastic fracture features in all samples [31,32].For the Ag-SnO 2 composite, the dimples became much denser and shallower than those of pure silver and Ag-La 2 Sn 2 O 7 , indicating a much higher density of microcracks or microcavities in the Ag-SnO 2 composite, primarily formed in the early stage of fracture [31], which is consistent with the lower relative density of the Ag-SnO 2 composite.

Experimental Demonstration of Enhanced Interface Bonding Strength
In order to further confirm the enhanced interfacial bonding strength, the theoretical conductivity of the Ag-La 2 Sn 2 O 7 and Ag-SnO 2 composites was also calculated and related to its experimental values.A theoretic formula for silver-based composites was defined by theoretic derivation based on the Effective Medium Theory (EMT) [33].
The assumption of this theory is that the second phase disperses well in the matrix phase, and the structure of second phase is spherical.It was assumed that the finely dispersed second-phase particles were well distributed in the Ag matrix and were nearly spherical.Based on the Bruggeman model [34], the function of the electrical conductivity of the Ag-La 2 Sn 2 O 7 or Ag-SnO 2 composite is: where φ is the volume fraction of Ag in the composites, k e is the electrical conductivity of Ag-La 2 Sn 2 O 7 or Ag-SnO 2 , k 1 is the theorical electrical conductivity of Ag (k 1 = 0.6305 µΩ −1 •cm −1 ), k 2 is the electrical conductivity of La 2 Sn 2 O 7 or SnO 2 , which is six orders smaller than k 1 ; therefore, k 2 could be neglected.Considering k 2 as zero in Equation ( 1), the linear dependence of k e on φ was obtained: By connecting φ with the mass fraction of Ag, denoted as w, Equation (3) was obtained: where ρ is the measured density of the Ag-La 2 Sn 2 O 7 or Ag-SnO 2 composites, and ρ 1 is the theorical density of Ag (ρ 1 = 10.49g/cm 3 ).Then, by combining Equation (3) with Equation ( 2), the final expressions of the electrical conductivity (Equation ( 4)) and electrical resistivity (Equation ( 5)) of the composites were: where

Mechanism of Enhanced Interface Bonding Strength
Silver, due to its deep d band beneath the Fermi level and a completely filled 4d orbital, has difficult forming strong bonding with oxides.Enhancing the Fermi level difference between Ag and the reinforcing phases can produce a strong interfacial bonding, facilitating the charge transfer at the interface [35,36].Figure 5a

Geometric Phase Analysis (GPA) around the Interfaces
The higher dislocation density of the Ag-SnO 2 composite around the interface compared to that of the Ag-La 2 Sn 2 O 7 composite was confirmed with the TEM images shown in Figure 2. The effect of different reinforcing phases on the interfacial microstructures was further characterized by HRSTEM and corresponding strain mapping.As shown in Figure 6, the ε xx difference between the Ag phase and La 2 Sn 2 O 7 was small, and only a few high strain zones were observed, indicating a small and homogenous strain in the x direction.In contrast, large, high-strain zones were observed for ε xx of the Ag phase in the Ag-SnO 2 composite.The comparison of the ε yy values showed that the strains in the y direction were similar for the Ag-SnO 2 and Ag-La 2 Sn 2 O 7 interfaces.Therefore, it can be concluded that the introduced SnO 2 particles produced enhanced strain at the interface, which could induce more defects, thus decreasing the plasticity by restraining dislocation motions and dislocation accumulations.

Enhanced Interfacial Mobility and Plasticity
Other than the interfacial bonding strength and CTE mismatch, the interfacial mobility of the Ag-based composites is also another very important factor that affects the plasticity of metal-based composites because of the high proportion of metal-ceramic interface in the composites [40,41].As shown in Figure 7, the shear stress needed for a certain displacement at the Ag-La 2 Sn 2 O 7 interface was lower than that needed for the Ag-SnO 2 interface, signifying enhanced mobility of the Ag-La 2 Sn 2 O 7 interface.An enhanced mobility of an interface can be beneficial for releasing the stress concentration at the interface to inhibit the formation and growth of microcracks and the debonding phenomenon, which will further improve the plasticity of the composite [40].Generally, increased interfacial mobility combined with enhanced interfacial bonding strength and decreased CTE mismatch could finally lead to Ag-La 2 Sn 2 O 7 composites with good plasticity, boosted deformability mechanics and good processability.

Conclusions
In summary, a successful strategy to obtain Ag-based composites based on theoretical bonding energy and bonding type was demonstrated, and a relevant Ag-La 2 Sn 2 O 7 pyrochlore composite was successfully fabricated.Generally, the enhanced interfacial bonding strength, decreased mismatch of the CTE and enhanced interfacial mobility can help to improve the plasticity and processability of Ag-La 2 Sn 2 O 7 composites.

Figure 4 .
1 is the theorical electrical resistivity of Ag (R 1 =1.586 µΩ•cm), and R e is the electrical resistance of the Ag-La 2 Sn 2 O 7 or Ag-SnO 2 composite.It was showed that the electrical conductivity and/or the electrical resistance of the Ag-La 2 Sn 2 O 7 or Ag-SnO 2 composite were related to its density and the mass fraction of Ag.In contrast to the ideal conditions assumed in the derivation, microcavities, microcracks and severe dislocations accumulating at the Ag-ceramic interface would induce great electron scattering and cause a deviation between the experimental and the theoretical conductivity.As shown in Figure4, the curve of the experimental conductivity of Ag-La 2 Sn 2 O 7 fits well with the theoretical curve, while that of the experimental conductivity of Ag-SnO 2 shows an enhanced deviation from the theoretical curve for increasing SnO 2 amounts and decreasing Ag amounts, indicating enhanced electron scattering, increased interfacial defects and weakened interfacial bonding strength in the Ag-SnO 2 composites.ρw-k e curve of (a) Ag-SnO 2 composites and (b) Ag-La 2 Sn 2 O 7 composites.

Figure 5 .
Figure 5. Calculations of the Fermi level difference between Ag and the reinforcing phases: (a) SnO 2 , (b) La 2 Sn 2 O 7 ; projected density of the state plots (pDOS) of the O 2p band and Ag 4d band for the Ag-SnO 2 interface (c) and the Ag-La 2 Sn 2 O 7 interface (d).

Figure 6 .
Figure 6.HRSTEM images of the Ag-SnO 2 interface (a) and Ag-La 2 Sn 2 O 7 interface (d); strain mapping at the Ag-SnO 2 interface in the x direction (ε xx , (b)) and y direction (ε yy , (c)) and corresponding strain-distance curves ((b1,c1); strain mapping at the Ag-La 2 Sn 2 O 7 interface in the x direction (ε xx , (e)) and y direction (ε yy , (f)) and corresponding strain-distance curves ((e1,f1).(red and blue in strain mapping indicate large strain in different directions, while the green indicate a area free from strain).

( 1 )
The as-prepared Ag-La 2 Sn 2 O 7 composite showed enhanced plasticity, approximately 2.8 times that of the traditional and widely used Ag-SnO 2 composite.(2) The good conformity between the experimental conductivity and the theoretical conductivity of the Ag-La 2 Sn 2 O 7 composites indicated interfaces with less defects, which can scatter moving electrons, and good interfacial bonding strength, which hinders the formation of cracks.(3) The enhanced mobility of the Ag-La 2 Sn 2 O 7 interface helped to release the stress concentration at the interface, inhibiting the formation and growth of microcracks and the debonding phenomenon.

Table 1 .
Mechanical properties of the Ag-SnO 2 composite and Ag-La 2 Sn 2 O 7 composite.