Nonlinear Size-Dependent Melting of Silica-Encapsulated Ag-Cu Alloy

Subscriber access provided by Kaohsiung Medical University

C: Physical Processes in Nanomaterials and Nanostructures

Nonlinear Size-Dependent Melting of SilicaEncapsulated Ag-Cu Alloy Nanoparticles Ting Su, Haolin Xiao, Wei Shen, Chaohao Hu, and Chengying Tang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b09156 • Publication Date (Web): 06 Nov 2018 Downloaded from http://pubs.acs.org on November 10, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 17 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

Nonlinear Size-Dependent Melting of Silica-Encapsulated AgCu Alloy Nanoparticles Ting Su, Haolin Xiao, Wei Shen, Chaohao Hu, *Chengying Tang School of Materials Science and Engineering and Guangxi Key Lab for Informational Materials, Guilin University of Electronic Technology, 1 Jinji Road, Guilin, Guangxi, 541004, P. R. China

ABSTRACT: We report on the size dependence of the Ag-Cu nano-alloy melting temperatures via differential scanning calorimetry (DSC) measurements of silica-coated Ag-Cu nanoparticles (NPs) and the thermodynamic modeling. Four Ag-Cu alloy NPs samples with compositions ranging from Ag8Cu2 to Ag5Cu5 were prepared via NaBH4 co-reduction and silica-coating. The experimental size dependence of melting temperature was obtained via UV-visible absorption spectra, transmission electron microscopy, and differential scanning calorimetry. Based on the measured melting temperatures of the Ag-Cu nano-alloys in this work as well as the thermodynamic properties of the pure Ag and Cu NPs and Ag-Cu systems in the literature, the theoretically calculated size dependence of Ag-Cu NPs melting temperatures was then obtained via thermodynamic modeling of the Ag-Cu nano-system. Thermodynamically calculated size dependence of the melting temperatures for Ag-Cu alloy NPs show a nonlinear function with respect to the inverse of the particle size and satisfactory agreement with experimental observations. This work establishes a methodology for determining the melting temperature as a function of size for different nano-alloy systems.

■ INTRODUCTION

nanoparticle diameter,24,25 while a nonlinear function with respect to the inverse of the particle size for Ag nanoparticles is found by differential scanning calorimetry and thermodynamic modeling and verified by high-resolution in situ TEM.18,25 Ag-Cu alloy NPs are attractive because of their electronic,26 antibacterial,27 electrochemical activity28 and their good selfcatalytic stability29 which can be used as catalysts in photocatalysis. The phase stability of Ag-Cu NPs is of great importance when they are applied to catalysis and microjoining at high temperatures. To date, there is only a few experimental investigations on the melting behavior of Ag-Cu alloy NPs using differential scanning calorimetry (DSC),30-33 or differential thermal analysis (DTA),34 as well as computer simulations on the size dependent melting of Ag-Cu alloy combining thermodynamic models35 and first principles (FP) calculation36 have been reported, respectively. The first experimental data on the melting temperature depression of the Ag-Cu eutectic nanoalloy encapsulated in carbon shells was reported by Huang et al.37 A decrease of 75℃ melting depression with respect to the temperature of the bulk samples was observed for eutectic NPs with size of 10nm. Recently, Delsante et al.31 synthesized the Ag, Cu, and Ag-Cu NPs and investigated the thermodynamics of Ag-Cu NPs. However, since agglomeration takes place for the samples shown in TEM images, the melting temperature depression about 14℃ of the eutectic Ag-Cu nanoalloy was observed, which has a difference up to 60℃ from the data reported by Huang et al. In

Nanoparticles are of considerable interest due to their sizedependent properties, which are different from those of bulk materials.1-5 It is well known that the melting point of nanoparticles is lower than that of their bulk counterparts due to a significant size effects.6 size-dependent melting of NPs is studied by theoretical prediction7-11 as well as computer simulations in varying models12 and experimental works.13-15 Experimental verification of Pawlow’s16 predication was first given by Takagi in 1954 via transmission electron microscopy(TEM) observation. The melting temperature was determined as the temperature at which the diffraction patterns change from individual rings to halos.17 These experimental observations yielded the first evidence of size-dependent melting point depression for Pb, Sn and Bi NPs that can lower the lead-free solder reflow temperature and lower the thermal stress. The melting transition process of Ag18 and Pb-Bi,19 BiSn,20 and Pb-Sn.21Nanoparticles was directly observed by developing a direct heating type-sample holder for use with high-resolution in situ TEM. Buffet and Borel later used scanning electron diffraction to demonstrate that the melting temperature of gold can be varied by as much as ~500oC.22 Recently, calorimetry measurements are used to determine both melting temperature and melting enthalpy of In, Sn, and Bi, nanocrystals dispersed in a polymer matrix23 or Bi nanoparticles in Ag matrix for phase-change materials.24 Their data and thermodynamic model indicate that the decrease in melting temperature of nanoparticles depends inversely on

1 ACS Paragon Plus Environment

The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 17

ensure complete complexation of the amine groups with the silver-copper alloy surface. A solution of active silica was prepared by lowering the pH of a 0.54 wt% sodium silicate solution to 10–11 by progressive addition of cation-exchange resin. Activated sodium silicate solution was added to the NP solution 20 min later with vigorous stirring. The resulting solution was allowed to stand for 4 days so that the active silica was polymerized onto the primed metal particles surface. The silica shell thickness was about 2 nm thick. After encapsulation, the AgCu@SiO2 alloy NPs were collected by centrifugation at 5000 to 8000 rpm for 0.5 h and further washed twice by ethanol. The solid was dried in vacuo. Characterization of Ag-Cu NPs. The UV–Visible absorption spectra of Ag-Cu alloy suspensions with proper concentration were recorded on a YUANXI UV–6000PC spectrophotometer in a 1 cm quartz cuvette with the range of 300 nm–700 nm. Deionized water was used as the reference sample to take the “blank” spectrum for all measurements. The size and morphology of the particles was analyzed by TEM instrument (FEI TECNAI G2 F20) with a point-to-point resolution of 0.24 nm and an accelerating voltage of 200 kV. TEM samples were prepared by placing a drop of the colloidal dispersion on a copper grid (Ted Pella, coated with amorphous carbon). Melting Temperature Measurements of Ag-Cu NPs. The melting temperatures of the particles were defined as the onset temperature of melting. This was determined using Netzsch STA449 F3 Jupiter® DSC under pure argon. Each sample was heated from room temperature to 1233.15 K. The DSC equipment was first calibrated with three high purity standard samples (Sn 99.9%, Al 99.999%, and Ag 99.99%) at a heating rate of 5 K/min, 10 K/min and 15 K/min, respectively. One baseline with no sample was then measured with the same temperature range. The model of sample plus baseline was used to monitor the invariant reaction (eutectic) temperature of Ag-Cu alloy nanoparticles during heating. Calculation of the Melting Temperature of Ag-Cu NPs. Thermodynamic modeling is shown to be an effective technique to predict equilibria and the melting temperature for pure Au and Ag NPs with any size larger than 5 nm. Here it is used to obtain the thermodynamic properties and predict the phase transformation temperature for Ag-Cu alloy NPs. For Ag-Cu nano system, the total Gibbs energy of one alloy nanoparticle is written as Eq(1). GTotal = GBulk + GSurface (1),

the work performed by Sopousek et al,38 the depression of the eutectic Ag-Cu melting point was calculated but not observed, since the eutectic Ag-Cu microparticles are formed before melting. Consequently, there is no consistent experimental and theoretical investigation on the size dependence of the melting of Ag-Cu alloy NPs yet reported. We present here a consistent approach to determine the size dependence of the melting temperature and the essentially thermodynamic properties of Ag-Cu alloy NPs encapsulated in silica via experimental measurements and thermodynamic modeling. Wet colloidal chemistry techniques were used to prepare well-defined Ag-Cu alloy particles with a relatively narrow size distribution. These were encapsulated in silica suggesting that it does not influence the temperature measurements. After the alloy NPs were characterized, DSC measured the invariant reaction temperatures of these alloy NPs. The computer coupling of phase diagrams and thermochemistry for nano-systems method obtained thermodynamic properties and predicted the melting temperatures of Ag-Cu alloy NPs with sizes larger than about 5 nm.

■ METHODS Preparation of Colloidal Ag-Cu Alloy NPs. We used silver nitrate (AgNO3) (99.9999%), copper sulfate (CuSO4) (anhydrous, powder), 3–aminopropyltrimethoxysilane (APS) (H2N(CH2)3Si(OCH3)3) (97%), and Dowex 50wx4–50 ion exchange resin ((C10H12·C10H10·C8H8)x). We also used sodium silicate solution (Na2O(SiO2)3–5, 27wt.% SiO2); tri–sodium citrate dehydrate (Na3C6H5O7·2H2O); and sodium borohydride (NaBH4). All were used as received. Pure grade ethanol and double-distilled de-ionized water were used in all the preparations. All glassware was cleaned with aqua regia. In this work, four Ag-Cu nano-alloy samples with compositions of Ag8Cu2, Ag7Cu3, Ag6Cu4, and Ag5Cu5 were first prepared by NaBH4 co-reduction of AgNO3 and CuSO4 with desired molar ratio in the presence of sodium citrate. A constant total metal molar concentration of 2×10-4M was used. The molar ratio of Ag: Cu was adjusted through the addition of the corresponding concentration of AgNO3 and CuSO4. A mixed solution of metal salts and 4 ml of sodium citrate solution (1wt.%; used as capping reagents) was added into a 100 ml conical flask with deionized water. The resulting solution was then heated to boiling and cooled to room temperature quickly by ice water under inert Ar flow and vigorous magnetic stirring to prevent possible oxidation of Ag-Cu NPs. A fresh excess aqueous solution of 1 ml of NaBH4 was added rapidly to the mixed solution. The stirring was maintained for a few minutes and then allowed to stand for some minutes resulting in the Ag-Cu alloy NPs. Silica Coating and Collection of Ag-Cu NPs. Silica coating can eliminate surface contamination and aggregation of nanoparticles. After preparation of these Ag-Cu suspensions, the pH value was first adjusted to 5 by addition of cation exchange resin. The sample was then added to a freshly prepared solution of 1ml bi-functional linker molecule APS under vigorous stirring as a primer. We only used enough APS form one monolayer on the particles. A surface coverage of 40 Å2 per APS molecule was assumed. The mixture of APS and Ag-Cu alloy dispersion was allowed to stand for 20 min to

where GBulk and GSurface are the Gibbs energy of the bulk and the surface of the Ag-Cu alloy nanoparticle, respectively. The molar Gibbs energy of the bulk phase is expressed as GBulk = XAgG0Ag + XCuG0Cu + RT(XAglnXAg + XCulnXCu) + GEx, Bulk XAg, XCu, G0Ag,

(2), G0Cu

where are the molar fractions and standard Gibbs energy of pure Ag and Cu, respectively, R is the gas constant, and T is the absolute temperature; GEx,Bulk is the excess Gibbs energy of the bulk phase and is expressed by the Redlich-Kister Polynomial as35 GEx,Bulk = XAgXCu∑Lv(XCu ― XAg)v (v = 0,1,2,…) (3), Lv = a + bT + cTln(T) +…


(4).

Page 3 of 17

The free energy of surface for spherical particles is expressed as39 2CσV

(5).

r

Here, σ is the surface tension, V is the molar volume, and r is the radius of the particle, C is a correction factor considering the effects from the shape, the surface strain due to the nonuniformity, and the uncertainty of the surface tension, respectively. The correction factor is obtained by fitting one measured eutectic temperature of one nanoparticle in this work. For Ag-Cu binary alloys, the total molar volume is the sum of the fraction of each constituent. V = XAgVAg + XCuVCu (6)

The C is obtained by fitting and optimizing the experimental data on the eutectic melting temperatures of four Ag-Cu alloys. The thermodynamically calculated size dependence of the melting temperature with any size larger than 5 nm calculated from the chemical potential equilibrium was then obtained based on the assessed thermodynamic parameters for Ag-Cu nano-system.

■ RESULTS AND DISCUSSION

The surface tension of Ag-Cu liquid binary alloy can be obtained via Butler’s equation.40 XSurface RT Ag ln σAgCu = σAg + AAg XBulk Ag

Formation Process and Determination of Ag-Cu@SiO2 NPs. The Ag-Cu bimetallic alloy NPs with compositions of Ag8Cu2, Ag7Cu3, Ag6Cu4, and Ag5Cu5 were successfully obtained via the simultaneous reduction of silver (Ag+) and copper (Cu2+) ions using NaBH4 reducing agent in the presence of sodium citrate. The fundamental reaction can be involved simplified as follows: Ag + (aq) + Cu2 + (aq) + 3BH4― (aq)

( )

+

1 AAg

= σCu +

Bulk( (T, XSurface ) - GEx, [GEx,Surface T,XBulk Ag Ag Ag Ag )]

RT

ln

ACu

( ) XSurface Cu XBulk Cu

+

1

(T,XBulk - GEx,Bulk Cu )] Cu

1.8 1.5

(

a′

) (

(

c′

)

b′ γ

440

0.0 0.2 0.4 0.6 0.8 1.0

Cu at% Ag Ag8Cu2

0.9

Ag7Cu3 Ag6Cu4

0.6

Ag5Cu5 Cu

400

500

600

700

Wavelength(nm) Figure 1. Normalized UV-vis absorption spectra of Ag, Cu and Ag-Cu alloys with compositions Ag8Cu2, Ag7Cu3, Ag6Cu4 and Ag5Cu5. The inset shows the change of absorbance peak with the atomic fraction of Cu.

The formation of nanosized Ag-Cu alloy can be characterized by their absorption in the UV-visible region with optical spectroscopy due to the localized surface Plasmon resonance (SPR). This is attributed to the coherent collective oscillation of conduction electrons across the particle surface. Figure 1 shows the UV-visible spectra of Ag8Cu2, Ag7Cu3, Ag6Cu4, and Ag5Cu5 together with monometallic Ag and Cu NPs. The silver and copper NPs had a SPR maxima at 385 and 563 nm. Only one SPR band was detected for each of alloys and the absorption maximum was always located between the SPR of monometallic Ag (385 nm) and monometallic Cu (563 nm). The results indicate that the NPs were an alloy rather than core-shell NPs or a mixture of monometallic NPs. This is because a physical mixture of monometallic Ag and Cu NPs has two absorption peaks. Core-shell NPs would exhibit the

Then the excess Gibbs energy of constituents of Ag or Cu in the nanoparticles, G0,Nano is expressed by the Redlich-Kister i polynomials as GEx,Nano = XAgXCu∑Lv(XCu ― XAg)v (v = 0,1,2,…) (11), Where Lv,Nano = LBulk + LSurface = a + γ + b +

1.2

0.0 300

(10).

γ

480

0.3

where βMix is a parameter corresponding to the ratio of the coordination number in the surface to that in the bulk. This is estimated to be 0.85 for liquids and 0.84 for solids, respectively. For the thermodynamic assessment of the nanoparticles, the standard Gibbs energy should be redefined as a function of the particle size. Here, homogeneous particles of pure metal of the same size were chosen as the standard state. Consequently, the chemical potential of pure Ag or Cu of radius r is given by 2CσiVi

520

400

(T,XSurface ), GEx,Surface (T,XSurface ), GEx,Bulk (T, GEx,Surface Ag Ag Cu Cu Ag Bulk Ex,Bulk Bulk (T,XCu ) are the partial excess Gibbs XAg ) and GCu energy of surface and bulk phase of Ag and Cu as a function of T and XSurface , respectively. According to the Yeum’s Cu model,41 the surface excess energy is related to that of their bulk phase as (T,XSurface ) = βMixGEx,Bulk (T,XBulk GEx,Surface (9), Ag Ag Ag Ag )

+ c + γ Tln(T) +…

560

(7),

where AAg and ACu is the molar surface area of pure Ag and Cu, calculated from the Avogadro’s number, N0, and the molar volume data VAg and VCu, respectively. This assumes a closedpacked monolayer. 2/3 Ai = 1.091N1/3 (8), 0 V0

= G0i + G0,Nano i

(14).

= AgCu(s) + 1.5H2 +1.5B2H6

) [GEx,Surface(T,XSurface Cu ACu Cu

max

GSurface =

The parameters a, b, and c in Eq. (12) are the same as those for the bulk phase in Eq. (4) and a’, b’ and c’... is expressed by a′ + b′T + c′Tln(T) +… = 2C(σalloyvalloy ― XAgσAgVAg ― XCu σCuVCu) (13).

Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


)T (12).



same two absorption peaks, where one increases as a function of the concentration of that component. When the shell is very thick, only the SPR from the shell is detectable. Figure 1 shows that the absorption peak red-shifted continuously from 385 nm to 430 nm. The inset shows the change in absorbance with atomic fraction of Cu. These findings further confirm that the NPs are core/shell products or a mixture of Cu and Ag. This agrees well with the experimental and theoretical results for Ag-Au and Ag-Cu alloys based on Mie theory. 42,43 Figure 2 shows the TEM image of silica-coated Ag-Cu bimetallic alloy NPs with compositions Ag8Cu2, Ag7Cu3, Ag6Cu4, and Ag5Cu5. The inset of Figure 2a that alloy NPs are nicely coated with a ~2 nm silica shell. Most alloy particles were nearly spherical and the average diameters of them are shown in Table 1. There is a systemic decrease in the Ag-Cu alloy NP diameter with increasing Cu content. This is expected because the size of Cu (r=0.135 nm) atoms is smaller than that of Ag (0.160 nm) despite having identical fcc crystal structures 44. This finding is similar to the preparation of AgAu alloy NPs via a replacement reaction between Ag NPs and HAuCl4.45

Page 4 of 17

HRTEM studies of the nanoparticles reveal that a fraction of the alloy nanoparticles have icosahedral morphology with multiple-twinned structures except for a fraction of the truncated octahedral (TO) single crystalline nanoparticles. This multi-twinned icosahedral structure with their surfaces bounded by the lowest-energy {111} facets is often observed in fcc metallic NPs at the nanometer size.46-49 Lu and coworker found that there are approximate 50% of stepped phase interfaces with the step surface parallel to (111) and an orientation relationship between phases Cu and Ag symmetrized to the stepped interface in the eutectic structure.50 The radii of the electron diffraction pattern of particles sampled from Ag5Cu5 shown in Figure 3c have ratios of 3: 4: 8: 11, corresponding to the 111, 200, 220 and 311 reflection of the fcc structure. This confirms that alloy NPs are isostructural with the starting Ag or Cu NPs. The electron diffraction patterns of other alloy NPs are similar to this one.

Figure 3. High-resolution images of a multi-twinned nanometersized particle of Ag7Cu3 (a) and Ag6Cu4 (b) and electron diffraction (ED) pattern of nanoparticles from Ag5Cu5 (c) alloy. The four rings (from inner to outer) correspond to the 111, 200, 220, and 311 reflections

Experimental Size Dependence of Melting. Figure 4 shows the DSC curves of four Ag-Cu alloy NPs samples encapsulated in silica. Similar melting behaviors have been observed in these four samples. No obvious event can be detected until 882.9 K, 849.9 K, 837.9 K and 827.5 K for Ag8Cu2, Ag7Cu3, Ag6Cu4, and Ag5Cu5, respectively, where an endotherm can be detected. This event is attributed to the melting of these four Ag-Cu NPs, respectively. Another endothermic effect is observed at 1053.1 K, 1053.1 K, 1060.6 K and 1048.5 K for Ag8Cu2, Ag7Cu3, Ag6Cu4, and Ag5Cu5, respectively, which correspond to the well-established invariant reaction temperature of bulk Ag-Cu alloy—Ag-Cu, is a simple eutectic system with a eutectic temperature of 1052.5K. Nanoparticles are often coated or embedded in a solid matrix to eliminate their surface contamination and preserve the identity. The coating material is chosen with the following conditions for studying the melting behavior of small particles: 1) The coating material has a higher melting point than particles; 2) The coating and the particles form an immiscible system, with a negligible inter-diffusion at least up to the melting point of particles. Silica coating for nanoparticles is a very efficient approach to preserve the identity of the nanoparticles even at high temperatures, due to the stability of silica at high temperatures. This acts as a nano-crucible for the melting of gold with little effect on the melting temperature itself.51 The melting endotherm at about 1053 K was observed

Figure 2. TEM images of the Ag-Cu NPs. Samples have molar ratios of Ag8Cu2, Ag7Cu3 ,Ag6Cu4, and Ag5Cu5 from (a) to (d).

Figure 3 shows the representative HRTEM images for the Ag7Cu3 and Ag6Cu4 NPs and electron diffraction (ED) pattern for Ag5Cu5 NPs. The nanoparticles in Ag8Cu2 and Ag5Cu5 had a similar appearance except for the size of the particles. Figure 3 shows an excellent atomic ordering within each particles. There is a 12.2% mismatch in the {111} facets as expected from the various lattice constants of 0.4086 nm for Ag and 0.3615 nm for Cu, respectively. Direct lattice imaging of Ag and Cu in one particle is seen. The inter-planar distance measured from the adjacent lattice fringes in the HRTEM image (Figure 3a and 3b) are 0.240 nm and 0.220 nm apart, respectively. This agrees well the (111) planes of fcc Ag and Cu, respectively. The diffraction contrast throughout the particle volume confirmed good atomic level alloying.


Page 5 of 17

Heat flow(mW/mg)

in all alloy samples regardless of particle size and composition. This is attributed to the porosity and the less thickness of the coated silica that allows flow of the melting Ag-Cu alloy to form large interconnected particles. This also demonstrates that the vapor pressure with the silica is maintained at atmospheric pressure—the pressure effect on the sizedependent melting of silica coating can be neglected.48,49 The melting behavior of alloy nanoparticles is not affected by the matrix. The measured bulk melting temperatures generally agree well with each other. These findings are in good agreement with the melting of silica-encapsulated Au51 and Ag NPs21, 25 and disagreement with the Bi NPs dispersed in polyimide (PI) resin matrix 23 and metal matrix. 24 (a)

(b)

1053.1K

EXO

9

3

882.9K

6

EXO

Measured TM(K)

Calculated TC(K)

Measured TB(K)

Ag8Cu2 Ag7Cu3 Ag6Cu4 Ag5Cu5

10.5±1.65 9.2±2.43 8.9±1.70 8.5±2.05

882.9 849.9 837.9 827.5

884.8±3 850.9±3 840.9±3 826.9±3

1053.1 1053.1 1060.6 1048.5

2.28 ∗ 10 ―8

( ) + x ∗ ( ―9.51 + x x ∗ {(3.67 + ) + (x ― x ) ∗ ( ―4 + ) ― (x ― x ) a = xAg ∗ 8.89 +

-3

1060.6K

EXO

12

(d)

Ag Cu

EXO

837.9K

0.75

1048.5K

0.00

(

4

-0.75

Ag

D

Ag

D

827.5K

b = xAg ∗ ―

Cu

2

Cu

1.04 ∗ 10 ―12

)+x ∗(―

1.53 ∗ 10 ―12

Cu

D

)+

D

―9

5.43 ∗ 10 ―9

8

7.3 ∗ 10 ―10

Cu

D

6.06 ∗ 10

(c)

400

Size (nm)

The parameters a, b, c, e are functions of particle size (D) and mass fraction (xAg 、 xCu), while d is a function of mass fraction(xAg 、 xCu), respectively. These parameters are expressed as

849.9K

0

3

Alloy

Consequently, the size-dependent melting temperature T of the Ag-Cu NPs can by de determined by thermodynamic modeling and written as aT + bT2 +cT3 +dT7 +e = 0 (15).

1053.1K

1.50

Heat flow(mW/mg)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


D

)

0 600

800

1000

Temperaure(K)

1200 400

600

800

1000

Temperaure(K)

1200

c = xCu ∗

Figure 4. DSC profiles of Ag8Cu2 with size of 10.5 nm (a), Ag7Cu3 with size of 9.2 nm (b), Ag6Cu4 with size of 8.9 nm (c) and Ag5Cu5 with 8.5nm (d) alloy NPs, respectively.

(

7.53 ∗ 10 ―7

)

D

d = xAg ∗ ( ―1.03 ∗ 10 ―20) + xCu ∗ ( ―5.84 ∗ 10 ―21) 4.57 ∗ 10 ―5

(

Size Dependence of Melting by thermodynamic Modeling. In this work, the thermodynamic parameters for the Ag-Cu bulk system were taken from the recent updated thermodynamic description by Witusiewicz et al.52 Recently, Tang et al.25 measured the melting points of Ag nanoparticles, and reported that the correction factor C for solid Ag was 1.265. By fitting the experimental work on the melting of Cu nanoparticles by Luo,53 we concluded that the C value for solid Cu was 1.006. Similarly, after the surface tension of solid Ag-Cu alloys is carried out with the reported surface tension and Butler’s equation, the C value for solid Ag-Cu alloy was obtained to be 1.305 by fitting the melting temperature of 882.9 K with size of 10.5 nm for Ag8Cu2 nanoparticles and optimizing the results of 9.2 nm, 8.9nm, 8.5nm for Ag7Cu3, Ag6Cu4, Ag5Cu5 NPs, respectively. The thermodynamic properties for Ag-Cu nano system were then obtained. Finally, the calculated size dependence of the melting temperature for the Ag-Cu NPs was obtained from the chemical potential equilibrium. The measured and calculated melting temperatures for Ag-Cu alloy nanoparticles with various sizes and bulk alloy with various compositions are summarized in Table 1. Table 1. Summary of the sizes, measured melting temperatures (TM for alloy NPs and TB for bulk alloy), and calculated melting temperatures (TC) for Ag-Cu alloy nanoparticles and bulk alloy with various compositions.

e = xAg ∗ 11025.08 ―

(12964.74 ― ( ―16284.5 + (7853.2 ―

)+x

D

)+x x ) + (x

2.99 ∗ 10

Ag Cu

D

2.874 ∗ 10 ―6

Ag

D

1.157 ∗ 10 ―6 D

Cu

∗

―5

) + (x

Ag

∗{ ― xCu) ∗

(

― xCu)2 ∗ 492.7 ―

4.289 ∗ 10 ―6 D

)}

Figure 5 (a) shows the size dependence of the melting temperatures of Ag-Cu NPs in this work. The theoretical results agree well with the experimental results. It is indicated that a nonlinear size-dependent melting temperature function with respect to the inverse of the particle size can be determined for Ag-Cu alloy NPs, which is in good agreement with previous work on the melting of Ag NPs,25 and disagreement with the literature works .22-23,54 Implications of the Lowered Melting Temperature of Ag-Cu alloy NPs. From the viewpoint of classical thermodynamic point, the melting point, the latent heat of fusion of pure bulk metal should be a constant value, respectively. The consequence of this classical melting depression effect for nanoparticle is expected to be a decrease in the enthalpy of fusion. Studies by Lai et al.55 and by Liu and Wang24 have shown that a decrease in enthalpy of fusion accompanies melting point depression by specialized microfabricated nanocalorimeters or standard DSC, respectively.



-12

Fig. 5(b) shows the calculated heat contents, 𝐻𝑇 ― 𝐻298.15, of the bulk Ag5Cu5 alloy and Ag8Cu2, Ag6Cu4 NPs with the size of 10.5nm, 8.9nm, respectively. It is shown that the heat contents decrease with the decreased size of Ag-Cu NPs accompanying melting temperature depression. This finding is in good agreement with the experimental results in this work. Another consequence of this classical effect is expected to be an increase in the self-diffusion of the metal atoms. Dick and coworkers 51 calculated the self-diffusion coefficient for the 2 nm gold NPs to be about 10-24 cm2·s-1 at room temperature—this is orders of magnitude higher than the value of 10-32

1100

(a)

TM(K)

10

-13

-2

Diff.Coeff(cm /s)

800

0.12

1/D(nm-1)

(b)

60 40 20

Bulk

200

400

600

H(T)-H(298K),K J/mol

80

10.5nm

-17

10

0

10

20

Size(nm)

30

cm2·s-1 for bulk gold, which is in agreement with the melting point depression of Au NPs. Fig. 5(c) show an estimate of the diffusion coefficient, D, as a function of size, of Ag, Cu and Ag8Cu2 NPs. It is found that the of diffusion coefficients of Ag, Cu, and Ag8Cu2 alloy NPs increase with the decrease of size. The diffusion coefficient of Ag8Cu2 alloy NPs is orders of magnitude higher than that for Ag and Cu NPs. Boltzmann-Arrhenius describes the dependence of the selfdiffusion coefficient D on temperature: D = D0exp ( ― ∆Hd/kT) (16).

0.16

100

8.9nm

-16

10

Figure 5. (a) The calculated size-dependent melting temperatures of Ag-Cu NPs (solid curve) with the measured melting temperature of Ag-Cu alloy NPs with the sizes of 8.5 nm, 8.9 nm, 9.2 nm and 10.5 nm, respectively. (b) The heat contents, HTH298.15, for the bulk Ag5Cu5 alloy and Ag8Cu2, Ag6Cu4 NPs with the diameter of 10.5nm, 8.9nm, respectively. (c) The diffusion coefficient of silica-encapsulated Ag, Cu and Ag8Cu2 NPs.

900

0.08

-15

10

-18

TM experimental

0.04

-14

10

10

TM calculated

700

Ag Cu Ag8Cu2

(c)

10

TM Bulk (1052.5K)

1000

) (nm Size

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 17

Here, the activation energy of diffusion is ∆Hd = 2.1504eV for silver and 2.1791eV for Cu, and the pre-exponential factor is D0= 1.2×10-3 m2·s-1 for Ag and 1.2×10-4 m2·s-1 for Cu56,57 respectively, the bulk diffusion coefficient at room temperature is 1.2×10-17 m2·s-1 and 7.0×10-17 m2·s-1, respectively. This is slow enough to prevent any spontaneous diffusion from the surface to the bulk. At the melting point of 1234 K, the diffusion coefficient in the bulk Ag is calculated to be 1.2×10-7 m2·s-1. Assuming Dm is same for alloy Ag particles, independent the melting temperature, and therefore independent of the size, the size dependence of diffusion coefficient D can be calculated at ambient temperature on the size by using the following equation, D(r) = Dmexp [( ― ∆Hd(r)(T ―1 ― Tm(r) ―1) (17). As suggested in Eq. 17, the enthalpy of activation of diffusion is expected to be size dependent. Assuming ∆Hd(r) is proportional to the average number of bonds that needs to be broken in the melting process, it is corrected slightly and amounts to 75% of ∆Hd when all atoms that compose the particle on the surface of a fcc lattice (assumed to be a 111 face). The diffusion coefficient for an 8 nm particle of silver

1200 800 1000

T(K)


Page 7 of 17 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(5)

Alloyeau, D.; Ricolleau, C.; Mottet, C.; Oikawa, T.; Langlois, C.; Le Bouar, Y.; Braidy, N.; Loiseau, A. Size and shape effects on the order-disorder phase transition in CoPt nanoparticles. Nat. Mater. 2009, 8(12), 940-946. (6) Xiong, S. Y.; Qi, W. H.; Huang, B. Y.; Wang, M. P.; Li, Z.; Liang, S. Q. Size-Temperature Phase Diagram of Titanium Nanosolids. J. Phys. Chem. C 2011, 116(1), 237-241. (7) Fernández, J. L; Walsh, D. A; Bard, A. J. Thermodynamic Guidelines for the Design of Bimetallic Catalysts for Oxygen Electroreduction and Rapid Screening by Scanning Electrochemical Microscopy. M-Co (M: Pd, Ag, Au). J. Am. Chem. Soc. 2005, 127(1), 357-365. (8) Monji, F.; Jabbareh, M. A. Thermodynamic Model for Prediction of Binary Alloy Nanoparticle Phase Diagram Including Size Dependent Surface Tension Effect. CALPHAD 2017, 58, 1-5. (9) Park, J.; Lee, J. Phase Diagram Reassessment of Ag-Au System Including Size Effect. CALPHAD 2008, 32(1), 135-141. (10) Kaptay, G. Nano-Calphad: Extension of the Calphad Method to Systems with Nano-Phases and Complexions. J. Mater. Sci. 2012, 47(24), 8320-8335. (11) Qi, W. H. Nanoscopic Thermodynamics. Acc. Chem. Res. 2016, 49(9), 1587-1595. (12) Zhao, Z.; Fisher, A.; Cheng, D. J. Phase Diagram and Segregation of Ag-Co Nanoalloys: Insights from Theory and Simulation. Nanotechnology 2016, 27(11), 115702. (13) Guisbiers, G.; Mendozacruz, R.; Bazán-Díaz, L.; VelázquezSalazar, J. J.; Mendoza-Perez, R.; Robledo-Torres, J. A.; Rodriguez-Lopez, J.-L.; Montejano-Carrizales, J. M.; Whetten, R. L.; José-Yacamán, M. Electrum, the Gold−Silver Alloy, from the Bulk Scale to the Nanoscale: Synthesis, Properties, and Segregation Rules. ACS Nano 2015, 10(1), 10618-10619. (14) Wu, Zhikun; MacDonald, Mark, A.; Chen, Jenny. Kinetic Control and Thermodynamic Selection in the Synthesis of Atomically Precise Gold Nanoclusters. J. Am. Chem. Soc. 2011, 133(25), 9670-9673. (15) Yin, F.; Wang, Z. W.; Palmer, R. E. Controlled Formation of Mass-Selected Cu-Au Core-Shell Cluster Beams. J. Am. Chem. Soc. 2011, 133(27), 10325-10327. (16) Pawlow, P. The Dependency of The Melting Point on the Surface Energy of a Solid Body. (Supplement). Z. Phys. Chem. 1909, 65, 545–548. (17) Takagi, M. Electron-Diffraction Study of Liquid-Solid Transition of Thin Metal Film. J. Phys. Soc. Jpn. 1954, 9, 359-363. (18) Chen, C. L.; Lee, J. G.; Arakawa, K.; Mori, H. In situ Observations of Crystalline-to-Liquid and Crystalline-to-Gas Transitions of Substrate-Supported Ag Nanoparticles. Appl. Phys. Lett. 2010, 96(25), 253104. (19) Jesser, W. A.; Shneck, R. Z.; Gile, W. W. Solid-Liquid Equilibria in Nanoparticles of Pb-Bi Alloys. Phys. Rev. B 2004, 69(14), 1124-1133. (20) Allen, G. L.; Jesser, W. A. J. The Structure and Melting Character of Sub-micron In-Sn and Bi-Sn particles. J. Cryst. Growth 1984, 70(1), 546-551. (21) Matsuki, H.; Ibuka, H.; Saka H. TEM Observation of Solder Joints Using Focused Ion Beam Process. J. Electron. Micro. 1999, 34, 196. (22) Buffat, P.; Borel, J. P. Size Effect on Melting Temperature of Gold Particles. Phys. Rev. A 1976, 13(16), 2287–2298. (23) Liu, M.; Wang, R. Y. Size-Dependent Melting Behavior of Colloidal In, Sn, and Bi Nanocrystals, Scientific Reports, 2015, 5, 16353. (24) Liu, M.; Ma, Y.; Wu, H.; Wang, R. Y. Metal Matrix-Metal Nanoparticle Composites with Tunable Melting Temperature and High Thermal Conductivity for Phase-Change Thermal Storage, ACS Nano, 2015, 9(2), 1341-1351. (25) Tang, C.; Sung, Y. M.; Lee, J. Nonlinear Size-Dependent

nanoparticle is estimated to be 7.65×10-16 m2·s-1 based on experiment.25 This is orders larger than that at room temperature. The increase in the diffusion coefficient may lead to spontaneous alloying of bimetallic particles. In addition, rapid interdiffusion of metal atoms on the nanoscale has been experimentally confirmed by the rapid alloying of two metals when one of them was deposited on the nanoparticles of the other even at ambient temperature. Schumacher and coworkers measured the interdiffusion of the silver in nanocrystalline copper (average crystal size 8 nm). They found that it ranged from 0.3×10-18 m2·s-1 at 303 K to 1.2×10-17 m2·s-1 at 373 K. The activation enthalpies were 0.63 eV (T > 353𝐾) and 0.39 eV (T < 343𝐾).58 There are a large number of vacancies defects at the interface between Ag and Cu metals and these contribute to the interfusion rate; these were created during alloy formation.

■ CONCLUSIONS The nonlinear size dependence of the melting temperature of Ag-Cu alloy NPs has been successfully investigated via a combination of experimental measurements and thermodynamic modeling. A co-reduction method was used to prepare the Ag-Cu alloy NPs. Silica coating is been proved to be a very effective approach to prevent the aggregation and preserve the identity of the Ag-Cu alloy NPs—the product is stable at high temperatures. The experimental size dependence of the melting temperature was obtained using DSC measurements. Thermodynamic modeling was used to predict the melting temperatures for sizes larger than 5 nm of Ag-Cu NPs. A consistent experimental and calculated melting temperature depression of Ag-Cu alloy NPs was obtained. The reliability of the experimental and theoretical approaches was verified by the good agreement between the experiments and calculations for size dependence melting of pure Ag and Cu NPs and Ag-Cu bimetallic NPs.

AUTHOR INFORMATION Corresponding Author *[email protected]

ACKNOWLEDGMENTS The financial support from the National Natural Science Foundation of China (51761005, 11464008) and the Natural Science Foundation of Guangxi (2016GXNSFGA380001), the Talents Project of Guilin University of Electronic Technology, are greatly acknowledged.

REFERENCES (1)

Radnóczi, G.; Bokányi, E.; Erdélyi, Z.; Misják, F. Size Dependent Spinodal Decomposition in Cu-Ag Nanoparticles. Acta. Mater. 2017, 123, 82-89. (2) Masitas, R. A.; Allen, S. L.; Zamborini, F. P. Size-Dependent Electrophoretic Deposition of Catalytic Gold Nanoparticles. J. Am. Chem. Soc. 2016, 138(47), 15295-15298. (3) Zhang, S. D.; Qi, W. H.; Huang, B. Y. Size Effect on Order Disorder Transition Kinetics of FePt Nanoparticles. J. Chem. Phys. 2014, 140, 044328. (4) Kauffman, D. R.; Alfonso, D; Matranga, C. Experimental and Computational Investigation of Au-25 Clusters and CO2: A Unique Interaction and Enhanced Electrocatalytic Activity. J. Am. Chem. Soc. 2012, 134(24), 10237-10243.



Melting of the Silica-Encapsulated Silver Nanoparticles. Appl. Phys. Lett. 2012, 100(20), 201903. (26) Gamerith, S.; Klug, A.; Scheiber, H.; Scherf, U.; Moderegger, E.; List, E.J.W. Direct Ink-Jet Printing of Ag-Cu Nanoparticle

Page 8 of 17

(42) Link, S.; Wang, Z. L.; El-Sayed, M.A. Alloy Formation of GoldSilver Nanoparticles and the Dependence of the Plasmon Absorption on Their Composition. J. Phys. Chem. B 1999, 103(18), 3529-3533. (43) Hövel, H.; Fritz, S.; Hilger, A.; Kreibig, U.; Vollmer, M. Width of Cluster Plasmon Resonances: Bulk Dielectric Functions and Chemical Interface Damping. Phys.Rev. B 1993, 48(24),18178. (44) WebElements, https://www.webelements.com (last accessed 3rd November 2018). (45) Zhang, Q.; Lee, J. Y.; Yang, J.; Boothroyd, C; Zhang, J. Size and Composition Tunable Ag-Au Alloy Nanoparticles by Replacement Reactions. Nanotechnology 2007, 18(24), 245605. (46) Wang, C.; Yin, H.; Chan, R.; Peng, S.; Dai, S.; Sun, S. One-Pot Synthesis of Oleylamine Coated AuAg Alloys and Their Catalysis for CO Oxidation. Chem. Mater. 2009, 21(3), 433-435. (47) Xiong, Y.; McLellan, J. M.; Yin, Y.; Xia, Y. Synthesis of Palladium Icosahedra with Twinned Structure by Blocking Oxidative Etching with Citric Acid or Citrate Ions. Angew. Chem. Int. Ed. 2010, 46(5), 642. (48) Lee, J. G.; Mori, H. Direct Evidence for Reversible Diffusion Phase Change in Nanometer-Sized Alloy Particles. Phys. Rev. Lett. 2004, 93(23), 235501. (49) Sun, Y.; Xia, Y. Shape-Controlled Synthesis of Gold and Silver Nanoparticles. Science 2002, 298, 2176-2179. (50) Papavassiliou, G. C. Surface Plasmons in Small Au-Ag Alloy Particles. J. Phys. F: Met. Phy. 2008, 6(4), L103-L106. (51) Dick, K.; Dhanasekaran, T.; Zhang, Z.; Meisel, D. SizeDependent Melting of Silica-Encapsulated Gold Nanoparticles. J. Am. Chem. Soc. 2002, 124 (10), 2312-2317. (52) Witusiewicz, V.T.; Hecht, U.; Fries, S. G.; Rex, S. The Ag-AlCu System: Part I: Reassessment of the Constituent Binaries on the Basis of New Experimental Data. J. Alloys Compd. 2004, 385, 133-143. (53) Luo, W.; Deng, L.; Su, K.; Li, K.; Liao, G.; Xiao, S. Gibbs Free Energy Approach to Calculate the Thermodynamic Properties of Copper Nanocrystals. Physica B 2011, 406, 859-863. (54) Allen. G. L.; Bayles, R. A.; Gile, W. W.; Jesser, W.A. Small Particle Melting of Pure Metals. The solid Films 1986, 144(2), 297-308. (55) Lai, S. L.; Guo, J. Y.; Petrova, V.; Ramanath G.; Allen, L. H.; Size-Dependent Melting Properties of Small Tin Particles: Nanocalorimetric Measurements, Phys. Rev. Lett. 2010, 104(18), 18960. (56) Butrymowicz, D. B.; Manning, J. R.; Read, M. E. Diffusion in Copper and Copper Alloys, Part II. Copper-Silver and CopperGold Systems. J. Phys. Chem. Ref. Data. 1974, 3(2), 527-602. (57) Wang, C. P.; Yan, L. N.; Han, J. J.; Liu, X.J. Diffusion Mobilities in the fcc Ag-Cu and Ag-Pd alloys. CALPHAD 2012, 37, 57-64. (58) Schumacher, S.; Birringer, R.; Strauβ, R.; Gleiter, H. Diffusion of Silver in Nanocrystalline Copper Between 303 and 373 K. Acta Metall. 1989, 37(9), 2485-2488.

and Ag-Precursor Based Electrodes for OFET Applications. Adv. Funct. Mater. 2007, 17(6), 3111-3118. (27) Valodkar, M.; Modi, S.; Pal, A. Thakore, S. Synthesis and AntiBacterial Activity of Cu, Ag, and Cu-Ag Alloy Nanoparticles: A Green Approach. Mater. Res. Bull. 2011, 46(3), 384-389. (28) Liu, J.; Chen, F. Plasmon Enhanced Photoelectrochemical Activity of Ag-Cu Nanoparticles on TiO2/Ti Substrates. Int. J. Electrochem. Sci. 2012, 7(10), 9560-9572. (29) He, L.; Liu, C.; Zhou, Y.; Yang, H. Self-Catalytic Stabilized AgCu Nanoparticles with Tailored SERS Response for Plasmonic Photocatalysis. Appl. Surf. Sci. 2018, 434, 265-272. (30) Pandey, O. P.; Mishra, N. S.; Ramachandra, C.; Lele, S.; Ojha, S. N. Nonequilibrium Solidification of Undercooled Melt of Ag-Cu alloy Entrained in the Primary Phase. Metall. Mater. Trans. A. 1994, 25(11), 2518-2523. (31) Delsante, S.; Borzone, G.; Novakovic, R.; Plazza, D.; Pigozzi, G.; Janczak-Rusch, J.; Pilloni, M.; Ennas, G. Synthesis and Thermodynamics of Ag-Cu Nanoparticles. Phys. Chem. Chem. Phys. 2015, 17(42), 28387-28393. (32) Tan, K. S.; Cheong, K. Y. Advances of Ag-Cu Alloy Nanoparticles Synthesized via Chemical Reduction Route. J. Nanopart. Res. 2013, 15(4), 1-29. (33) Arfat, Y. A.; Ahmed, J.; Jacob, H. Preparation and Characterization of Agar-Based Nanocomposite Films Reinforced with Bimetallic (Ag-Cu) Alloy Nanoparticles. Carbohydr. Polym. 2016, 155, 382-390. (34) Bhagathsingh, W.; Nesaraj, A. S. Low Temperature Synthesis and Thermal Properties of Ag-Cu Alloy Nanoparticles. Trans. Nonferrous Met. Soc. China 2013, 23, 128-133. (35) Lee, J.; Tanaka, T.; Mori, H. Effect of Substrates on the Melting temperature of Gold Nanoparticles. CALPHAD 2007, 31(1), 105111. (36) Shin, K.; Kim, D. H.; Yeo, S. C.; Lee H. M. Structural Stability of AgCu Bimetallic Nanoparticles and Their Application as A Catalyst: A DFT Study. Catal. Today 2012, 185(1), 94-98. (37) Huang, C. H.; Wang, H. P.; Chang, J. E.; Eyring, E. M. Synthesis of Nanosize-Controllable Copper and Its Alloys in Carbon Shells. Chem. Commun. 2009, 31(31), 4663-4665. (38) Sopousek, J.; Pinkas, J.; Broz, P. Ag-Cu Colloid Synthesis: Bimetallic Nanoparticle Characterisation and Thermal Treatment. J. Nanomater. 2014, 2014(36), 1. (39) Lee, J.; Sim, K. J. General Equations of CALPHAD-Type Thermodynamic Description for Metallic Nanoparticle Systems. CALPHAD 2014, 44(1), 129-132. (40) Butler, J. A. V. Proc. R. The Thermodynamics of the Surfaces of Solutions. Proc. Roy. Soc. Lond. A 1932, 135(827), 348-375. (41) Yeum, K. S.; Speiser, R.; Poirier, D. R. Estimation of the Surface tensions of Binary Liquid Alloys. Metall. Trans. B 1989, 20(5), 693-703.


Page 9 of 17 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


TOC Graphic



Figure 1 297x229mm (300 x 300 DPI)

ACS Paragon Plus Environment

Page 10 of 17

Page 11 of 17 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 2 83x83mm (300 x 300 DPI)



Figure 3 41x20mm (300 x 300 DPI)


Page 12 of 17

Page 13 of 17 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 4 297x215mm (300 x 300 DPI)



Figure 5a 297x232mm (300 x 300 DPI)


Page 14 of 17

Page 15 of 17 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 5b 297x224mm (300 x 300 DPI)



Figure 5c. 297x237mm (300 x 300 DPI)


Page 16 of 17

Page 17 of 17 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


85x47mm (300 x 300 DPI)


Nonlinear Size-Dependent Melting of Silica-Encapsulated Ag-Cu Alloy

Recommend Documents