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j m a t e r r e s t e c h n o l . 2 0 1 6;5(4):345–352

www.jmrt .com.br

Available online at www.sciencedirect.com

riginal Article

etal–metal bonding process using cuprous oxideanoparticles

oshio Kobayashia,∗, Takafumi Maedaa, Yusuke Yasudab, Toshiaki Moritab

Department of Biomolecular Functional Engineering, College of Engineering, Ibaraki University, Hitachi, Ibaraki, JapanHitachi Research Laboratory, Hitachi Ltd., Hitachi, Ibaraki, Japan

r t i c l e i n f o

rticle history:

eceived 12 May 2015

ccepted 21 May 2016

eywords:

uprous oxide

anoparticle

etal–metal bonding

a b s t r a c t

This work performs metal–metal bonding by using cuprous oxide (Cu2O) nanoparticles

prepared by reduction in aqueous solution. A colloid solution of Cu2O nanoparticles was

prepared by mixing Cu(NO3)2 aqueous solution and NaBH4 aqueous solution. Cu2O nanopar-

ticles with a size of 111 ± 34 nm, cubic crystal phase, and crystal size of 21.2 nm were

produced at initial concentrations of 0.010 M Cu(NO3)2 and 0.010 M NaBH4 at a tempera-

ture of 40 ◦C. The Cu2O particles contained not only Cu O bonds but also Cu0 Cu0 bonds,

which indicated formation of fine cluster-like domains composed of Cu0 Cu0 bonds. The

shear strength required for separating the metallic Cu discs bonded by using the particles

as a filler at 400 ◦C in H2 gas was as high as 27.9 MPa, which was comparable to the shear

strengths of metallic Cu particles and CuO particles reported in our previous works. Metallic

Cu single crystallites were produced during the bonding process. The presence of the fine

cluster-like domains promoted epitaxial particle growth of the metallic Cu and formation

of the micron-sized domains composed of nano-sized and submicron-sized single crystals,

which provided the strong bonding.

© 2016 Brazilian Metallurgical, Materials and Mining Association. Published by Elsevier

Editora Ltda. This is an open access article under the CC BY-NC-ND license (http://

creativecommons.org/licenses/by-nc-nd/4.0/).

both types of alloys have a problem due to their low melt-

. Introduction

etal–metal bonding is an important process in fields suchs electronics, metalworking industry, structural materials,nd materials science [1–4]. A process using either solder orller is an example of the various metal–metal bonding pro-

esses that are mainly used in the field of electronics. Fillersften used in conventional solder-based bonding are metalliclloys composed mainly of lead and tin [5–7], because they

∗ Corresponding author.E-mail: ykoba@mx.ibaraki.ac.jp (Y. Kobayashi).

ttp://dx.doi.org/10.1016/j.jmrt.2016.05.007238-7854/© 2016 Brazilian Metallurgical, Materials and Mining Assocrticle under the CC BY-NC-ND license (http://creativecommons.org/lic

have low melting points so that the bonding can be madewith little energy with no thermal damage to the materialsto be bonded or the joints. Since lead is harmful to living crea-tures [8–10], its use tends to be limited. Accordingly, lead-freealloys are desirable as fillers. Among lead-free alloys, tin-basedalloys have been developed as new fillers [11–15]. However,

ing points. If the joints are kept at temperatures higher thantheir melting points, the alloys remelt, which may release thejoints.

iation. Published by Elsevier Editora Ltda. This is an open accessenses/by-nc-nd/4.0/).

n o l

346 j m a t e r r e s t e c h

The field of electronics requires fillers to be electrically andthermally conductive. Metals such as Au, Ag, and Cu are candi-dates for fillers because of their high electric conductivity andthermal conductivity. However, metal–metal bonding usingsuch metallic fillers has to be performed at high temperaturebecause their melting points are higher than those of the con-ventional lead-based and tin-based fillers. The metallic fillerspresent in the joints are exposed to high temperature duringthe bonding, which damages the joints.

Nanoparticles of metallic materials have melting pointslower than those of bulk metals [16–18]. The low meltingpoints due to the decrease in size of materials make it possi-ble to perform the bonding at low temperature. Bonding usingmetallic nanoparticles has another advantage: the nanoparti-cles become metallic bulk after the bonding. The metallic bulkhas a melting point higher than that of the nanoparticles. Asa result, the bonded materials are not released for use belowtheir melting points.

Among electrically and thermally conductive metals,metallic Cu is promising from the viewpoints of cost andelectrical migration. Our group has studied preparation ofan aqueous colloid solution of metallic Cu nanoparticles byreducing the Cu2+ ions and CuO nanoparticles with hydrazineand metal–metal bonding processes using the metallic Cunanoparticles [19–24]. The metallic Cu nanoparticles are oxi-dized easily with the oxygen in the air and the aqueoussolution, which has been pointed out by several researchers[25–29]. This spoils their bonding properties, which causestheir unreliability in bonding.

Cu oxide can be reduced to metallic Cu with a reducingagent or reducing atmosphere. This means that nanoparti-cles of Cu oxide may form metallic Cu nanoparticles duringthe reduction. If metal–metal bonding is performed using Cuoxide nanoparticles as the filler in a reducing atmosphere,the transformation from Cu oxide nanoparticles to metallicCu nanoparticles and the metal–metal bonding probably takeplace simultaneously. The use of Cu oxide has another advan-tage: since Cu oxide is chemically stable in air, unlike metallicCu, the metal–metal bonding properties of Cu oxide nanopar-ticles is reliable for a long period after their fabrication. Fromthis viewpoint, nanoparticles of cupric oxide (CuO) have beenexamined as the filler for metal–metal bonding in our previousworks [30–33]. Powder of the CuO nanoparticles was simplyfabricated by mixing a copper salt aqueous solution and asodium hydroxide aqueous solution in air at 20–80 ◦C, andmetallic Cu discs were successfully bonded using the powder.

Cuprous oxide (Cu2O) is also promising as a filler sinceCu2O is more easily reduced thermodynamically to metallicCu than CuO. Cu2O can be produced by electrochemical reac-tion [34], sonication assistance [35], microwave assistance [36],and hydrothermal reaction [37]. Although these methods workwell, they need processes other than chemical reactions thatcomplicate the production processes. Cu2O can also be pro-duced with Fehling’s reagent [38]. The final solution containssulfate, potassium sodium tartrate, and a reductant such asglucose as well as Cu ions, which may function as impurities

that deteriorate the bonding properties.

In a preliminary experiment, Cu2O was produced by opti-mizing the concentrations of raw chemicals in a reactionbetween Cu(NO3)2 and NaBH4 in aqueous solution. Because

. 2 0 1 6;5(4):345–352

this method consisted of only mixing Cu(NO3)2 aqueous solu-tion and NaBH4 aqueous solution, it was simple like the CuOproduction. The aim of the present work is to find a methodfor producing Cu2O nanoparticles. The metal–metal bondingproperties of the Cu2O particles were also studied.

2. Experimental work

2.1. Chemicals

Copper (II) nitrate trihydrate (Cu(NO3)2·3H2O) (Kanto ChemicalCo., Inc., 77.0–80.0% (as Cu(NO3)2)) and sodium borohydride(NaBH4) (Kanto Chemical Co., Inc., >92%) were used as Cu2Oprecursors. All chemicals were used as received. Water thatwas ion-exchanged and distilled with Yamato WG-250 wasused in all the preparations.

2.2. Preparation

Colloid solutions of Cu2O nanoparticles were synthesizedby using a reaction between copper ions and reductant. Anaqueous solution of NaBH4 was added to a Cu(NO3)2 aque-ous solution under vigorous stirring at 20–80 ◦C. Our researchgroup has studied on development of copper-related nanopar-ticles for metal–metal bonding for several years, in whichthe nanoparticles were produced from an aqueous solutionof copper salt. According to Wu and Chen’s work producingmetallic Cu nanoparticles in aqueous solution [39], concentra-tions of copper salt were adjusted to 0.01 M in those studies,which was within their concentrations. Thus, the initial con-centration of Cu(NO3)2 was also adjusted to 0.01 M in the finalsolution in the present work. An expected chemical equationfor the reduction is ‘4Cu2+ + BH4

− + 8OH− → B(OH)4− + 4H2O’[40]. The metallic Cu is formed at a stoichiometric ratio of4:1 of Cu2+:NaBH4 in the reduction. The NaBH4 concentra-tions were adjusted to 0.01–0.03 M in the final solution inthe present work, which were higher than the stoichiomet-ric ratio. A preliminary experiment confirmed that color ofthe solution turned dark after the addition of NaBH4, whichimplied a progress of reaction. The color turning almost fin-ished over 5 min after the NaBH4 addition, so that the reactionwas regarded as having been almost completed in 5 min. Thereaction time was adjusted to 3 h in the present work to com-plete the reaction.

2.3. Characterization

The particles were characterized by X-ray diffractometry(XRD), transmission electron microscopy (TEM), X-ray photo-electron spectroscopy (XPS), and thermal analysis (TG-DTA).The XRD measurements were performed with a Rigaku UltimaIV X-ray diffractometer at 40 kV and 30 mA with CuK�1

radiation. For preparing a powder sample for the XRD mea-surement, supernatant of the particle colloid was removedby decantation, and then the residue of the colloid was dried

at room temperature for 24 h in vacuum. TEM photographswere taken with a JEOL JEM-2100 microscope operating at200 kV. Samples for the TEM were prepared by dropping andevaporating the particle colloid on a collodion-coated copper

o l . 2 0 1 6;5(4):345–352 347

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10 20

a

b

c

d

e

30 40

2θ (degrees), CuK α

Inte

nsity

(ar

b. u

nits

)

50 60 70 80

Fig. 1 – XRD patterns of particles fabricated at NaBH4

concentrations of (a) 0.010, (b) 0.015, (c) 0.020, (d) 0.025, and(e) 0.030 M. Reaction temperature was 40 ◦C. �: cubic Cu2O,

j m a t e r r e s t e c h n

rid. Dozens of particle diameters in the TEM images wereeasured to determine the number-averaged particle size

nd standard deviation of particle size distribution. The XPSpectra were obtained using a JEOL JPS-9010 equipped with

monochromatic Mg K� radiation source (200 W, 10 kV, and253.6 eV). The XPS samples were powders that were obtainedy drying the particles at room temperature in vacuum afterhe final removal of the supernatant during the washing pro-ess. To study the composition below the surface, the particlesere etched using 500 kV Ar+ sputtering. To clean the par-

icle surface, etching was performed once prior to the maineasurements. Thermal analysis was performed in 3% (v/v)

2/N2 gas at a heating rate of 10 ◦C/min with a Mettler-ToledoGA/SDTA851 thermal analyzer. Samples for thermal analy-is were obtained in the same manner as that for the XRDamples.

The metal–metal bonding properties were investigatedith the same set-up as used in our previous works [41–44].

owder samples were sandwiched between copper discsstage (diameter: 10 mm, thickness: 5 mm) and plate (diam-ter: 5 mm, thickness: 2.5 mm)), and pressed at 1.2 MPa undernnealing in H2 at 400 ◦C for 5 min with a Shinko Seiki vac-um reflow system. To examine the bonding strength, sheartrengths that were required to separate the bonded plate andhe stage were measured with a Seishin SS-100KP bond tester.o investigate the microstructure of the particle layer-to-stageoint after the measurement of shear strength, the joint wasmbedded with resin and then cross-sectioned, metallograph-cally polished, and observed with a Hitachi S-4800 scanninglectron microscope (SEM) operating at 30 kV. The copper plateas observed with a JEOL JSM-5600LV microscope after theeasurements of shear strengths. The microstructure of the

rystal orientation of the particles after the measurementsf shear strengths was characterized by electron backscat-er diffractometry (EBSD). The EBSD was performed with aitachi SU-70 field emission SEM (FE-SEM) operating at 5 kV,hich was equipped with an EDAX/TSL orientation imagingicroscopy (OIM) system operating with an acceleration volt-

ge of 20 kV, measured area of 3 �m × 6 �m, and measurementtep of 20 nm. The sample for the EBSD was fabricated bynishing the cross-sectional sample with polishing oil andilling the finished sample with an Ar-ion milling instrument

perating at an accelerating voltage of 15 kV for 180 s.

. Results and discussion

.1. Crystal structure

.1.1. Effect of NaBH4 concentrationig. 1 shows the XRD patterns of particles fabricated at variousaBH4 concentrations and constant temperature of 40 ◦C. ForaBH4 concentrations of 0.010 and 0.015 M, diffraction peaksere detected at 29.6, 36.4, 42.3, 52.5, 61.3, and 73.5◦. Theseeaks were assigned to the cubic phase of Cu2O in accor-ance with the works of Labidi et al. [45] and Wang et al.

46] as well as JCPDS card #05-0667. This result indicated thatu2+ was reduced to Cu+ to form Cu2O. Since the amountsf NaBH4 were small, the Cu2+ was not reduced to Cu0. The

ntensities of peaks due to Cu2O decreased with increasing

©: face-centered cubic Cu.

NaBH4 concentration to 0.020 M. With NaBH4 increase, newpeaks appeared at 43.3 and 50.4◦. They were assigned to theface-centered cubic phase of metallic Cu in accordance withthe works of Harne et al. [47] and Dong et al. [48] as well asJCPDS card #04-0836. The increase in NaBH4 concentrationpromoted reduction of Cu2+ to Cu0 to form metallic Cu, thoughall the Cu2+ was not reduced to Cu0 because of the shortage ofNaBH4. Over 0.020 M, the intensities of Cu2O and metallic Cudecreased further and increased, respectively, with increas-ing NaBH4 concentration. The increase further promoted thereduction to metallic Cu, though the Cu2O was still producedeven at high NaBH4 concentrations. The average crystal sizesof Cu2O were estimated from the XRD line broadening of the36.4o peak in accordance with the Scherrer equation. Thecrystal sizes were 21.2, 20.3, 13.9, 13.8, and 12.5 nm for theNaBH4 concentrations of 0.010, 0.015, 0.020, 0.025, and 0.030 M;the size decreased with increasing NaBH4 concentration. Theincrease in NaBH4 concentration increased the rate of reduc-tion. The increase in the reduction rate increased the numberof nuclei generated in the early reaction stage, which pro-duced a lot of fine particles. As a result, small crystallites wereproduced at higher NaBH4 concentrations.

3.1.2. Effect of reaction temperatureFig. 2 shows the XRD patterns of particles fabricated at var-ious reaction temperatures. The NaBH4 concentrations wereadjusted to 0.010 M. At this concentration only the peaks dueto cubic Cu2O were detected, as shown in Fig. 1. For all thetemperatures examined, the peaks attributed to cubic Cu2Owere obtained at 36.4, 42.3, and 61.3◦. There was no large dif-ference in peak intensities. The average crystal sizes of Cu2Owere estimated in accordance with the Scherrer equation. Thecrystal sizes were ca. 25 nm; the exact sizes were 29.0, 22.3,

21.2, 27.3, 21.4, 28.0, and 29.2 nm for 20, 30, 40, 50, 60, 70, and80 ◦C, respectively. The crystal size was not dominantly corre-lated with the reaction temperature. Accordingly, the reaction

348 j m a t e r r e s t e c h n o l . 2 0 1 6;5(4):345–352

10 20 30 40 50 60 70 80

a

b

c

d

e

f

g

2θ (degrees), CuK α

Inte

nsity

(ar

b. u

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Fig. 2 – XRD patterns of particles fabricated at temperaturesof (a) 20, (b) 30, (c) 40, (d) 50, (e) 60, (f) 70, and (g) 80 ◦C. Initial

a

b

c

d

e

Inte

nsity

(ar

b. u

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)

NaBH4 concentration was 0.010 M. �: cubic Cu2O.

temperature was found not to be a dominant factor, whichgoverns both the crystal structure and crystal size of Cu2O par-ticles. For 20 ◦C, the particle size of 111 nm was larger than thecrystal size of 21.2 nm, which indicated that the Cu2O particlesfabricated at 0.010 M NaBH4 and 40 ◦C were polycrystalline.

3.2. Specifications of particles

3.2.1. NanostructureFig. 3 shows the TEM image of the Cu2O particles fabricatedwith 0.010 M NaBH4 at 40 ◦C. Several particles appeared to forman aggregate. The particle colloid solution was concentratedby drying a colloid solution dispersant in the preparation of

the TEM sample, which flocculated the particles to form theaggregate. The particles were angular and had an average sizeof 111 ± 34 nm. The particle size was larger than the crystal

Fig. 3 – TEM image of Cu2O particles. Sample was same asin Fig. 1(a).

928 930 932

Binding energy (eV)934 936

Fig. 4 – Cu 2p3/2 XPS spectra of Cu2O particles. Sample wassame as in Fig. 1(a). CPS sample was etched by Ar+

sputtering; number of etchings were (a) 1, (b) 2, (c) 3, (d) 4,and (e) 5. Spectra were fitted with Gaussian peaks due to

( ) Cu Cu, ( ) Cu+ O and ( ) Cu2+ O bonds.

size of 21.2 nm. Accordingly, the Cu2O particles were polycrys-talline.

Fig. 4 shows the Cu 2p3/2 XPS spectra of Cu2O particlesafter etching. A peak was clearly detected for each sample.

The peak position increased from 932.50 to 932.59 eV withincreasing the number of etchings from 1 to 5. According tothe works of Ghodselahi et al. [49] and Luo et al. [50], the peakpositions of (Cu+ O/Cu0 Cu0) and Cu2+ O are 932.3–933.8

j m a t e r r e s t e c h n o l . 2 0 1 6;5(4):345–352 349

60

50

40

10

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Number of etchings (–)

Rat

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d to

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bond

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Fig. 5 – Atomic ratios of various bonds for surfaces of Cu2Oparticles as function of number of Ar etching steps. ( )Cu Cu, ( ) Cu+ O, and ( ) Cu2+ O. Samples were same asi

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100

95

90

100 200 300 400Temperature (ºC)

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Fig. 6 – TG-DTA curves for Cu2O particles. Sample was

indicated that the Cu2O particles could function as the filler

n Fig. 4.

nd 934.5–935.2 eV. It is difficult to distinguish the peak pos-tions between Cu+ O and Cu0 Cu0 because the differencen binding energy between these two bonds is only 0.1–0.2 eVn XPS analysis [49–51]. It has been confirmed in the worksf Luo et al. [50] and Park et al. [52] performing Auger spec-roscopy that the binding energy of Cu0 Cu0 is higher thanhat of Cu+ O. Accordingly, the increase in peak position wasonsidered to take place with an increase in the amount ofu0 Cu0. Taking into account the possible assignments foreak positions of these bonds, the XPS spectra were fittedith a Gaussian peak using peak position, peak intensity, and

WHM (full width at half maximum) value as fitting param-ters. The curve-fitting gave binding energies of 932.5, 932.7,nd 933.8 eV. According to the works of Ghodselahi et al. [49],uo et al. [50] and Mansikkamäki et al. [51], these peaks weressigned to bonds of Cu+ O, Cu0 Cu0, and Cu2+ O, respec-ively. The XRD measurement revealed that only the peakue to Cu2O was detected, as shown in Fig. 1(a). Accordingly,he Cu2O particles were speculated to have fine cluster-likeomains composed of Cu0 Cu0 bonds, which were too smallo be detected by the XRD. Fig. 5 shows the atomic ratios of theonds estimated from the XPS peak area intensity. The ratiosf Cu+ O, Cu0 Cu0, and Cu2+ O bonds decreased, increased,nd stayed almost constant, respectively, as the etching timencreased. These results indicated that the Cu2O particles con-ained the Cu0 Cu0 bond, of which there were many at theirore, and the Cu2+ O bond was distributed uniformly in them.hese assignments suggested two possible routes for the pro-uction of Cu2O. One was some Cu2+ ions were reduced to Cu0,nd the rest of the Cu2+ ions were reduced to Cu0 to form Cu2O.he other was all the Cu2+ ions were reduced to Cu0, and thenome Cu0 species were oxidized with the oxygen in the air tou+ to form Cu2O.

.2.2. Effect of annealing in reducing atmospherehe TG-DTA curves for the Cu2O particles are shown in Fig. 6.

n the range of 220–370 ◦C, an exothermic peak and a weight

same as in Fig. 1(a).

loss were detected, and the weights did not change above330 ◦C. The Cu2O was reduced in the reducing gas to formmetallic Cu in the temperature range. The reduction resultedin the removal of oxygen from the Cu2O, which caused theweight loss. In a preliminary experiment on thermal anal-ysis of commercially available Cu2O, such weight loss andthe exothermic peak accompanied with the weight loss tookplace at 570 ◦C. This result indicated that the Cu2O particlesproduced in the present work were reduced at a tempera-ture lower than that of the commercially available Cu2O. Inaccordance with the XPS measurements, the Cu2O particlescontained the Cu0 Cu0 bond derived from metallic Cu withlow crystallinity. The metallic Cu may have functioned as apromoter for the reduction of Cu2O, though the detailed mech-anism of the promoted reduction is still unclear.

3.3. Metal–metal bonding properties

3.3.1. Shear strengthA preliminary experiment revealed that the Cu discs were notbonded with the commercially available Cu2O powder, mean-ing that its shear strength was 0 MPa. The Cu2O powder wasnot reduced in H2 gas at 400 ◦C in accordance with the thermalanalysis. The non-reduction resulted in the lack of bonding. Incontrast, the Cu discs were strongly bonded with the Cu2O par-ticles fabricated in the present work, and the shear strengthof the Cu2O particles was 27.9 MPa. It is possible that the Cu2Owas reduced to metallic Cu, the metallic Cu grew and formedmetallic Cu nanoparticles, and then the nanoparticles bondedwith the Cu discs. The fine cluster-like domains composed ofCu0 Cu0 bonds probably promoted the growth of metallic Cuepitaxially. Consequently, strong bonding was attained withthe Cu2O particles in the present work. The shear strengthvalue of 27.9 MPa was comparable to the shear strength val-ues of metallic Cu particles and CuO particles, which wererecorded in our previous works [19,31]. Accordingly, this result

for metal–metal bonding. Fig. 7(a) shows a photograph of theCu stage after the measurement of shear strength. The reddishbrown product that was obviously metallic Cu was widespread

350 j m a t e r r e s t e c h n o l . 2 0 1 6;5(4):345–352

Fig. 7 – Photograph (a) and SEM image (b) of copper plateafter measurement of shear strength. Powder used was

Fig. 9 – Results of EBSD analysis for particle layer afterbonding and measurement of shear strength. Image (a)shows band contrast map. Image (b) shows mean angulardeviation map. Images (c), (d), and (e) showEBSD-determined inverse pole figure maps in directions ofx, y, and z, respectively. Powder used was Cu2O particles (a)

Cu2O particles (a) shown in Fig. 1.

on the stage. This indicated that the as-prepared particleswere reduced to metallic Cu annealing at 400 ◦C in H2 gas,which had been implied by the thermal analysis (Fig. 6). Con-sequently, the metallic Cu was bonded to the Cu discs. Fig. 7(b)shows an SEM image of the surface of the Cu stage afterthe measurement of shear strength. Sintering appeared totake place among the particles, which caused strong bond-ing. Dimples accompanied with sharp tips, which often formin strongly bonded regions when metallic materials are sepa-rated with shear stress [53], were not observed on the Cu disksurface. The discs appeared to be brittly fractured by the shearstress. Brittle fracture often takes place in ceramics. All the

Cu2O was not reduced, which left a small amount of Cu oxidein the bonded region. The existence of Cu oxide brought aboutby the metal–metal bonding process taking on an aspect of

Fig. 8 – SEM images of plate-to-stage joint made usingCu2O particles after measurement of shear strength.Images (b), (c), and (d) are high magnification images forareas surrounded with rectangles in images (a), (b), and (c),respectively. Powder used was Cu2O particles (a) shown inFig. 1.

shown in Fig. 1.

ceramic bonding, resulted in the brittle fracture in the presentwork.

3.3.2. Microstructure of particle layerFig. 8 shows the microstructure of the particle layer-to-Custage joint. Sintering of particles took place, and consequentlymicron-sized domains were formed. The domains and theCu disk also sintered. This observation confirmed the strongbonding. Voids were also observed among the domains. Thevolume of Cu2O particles decreased through their reductionto metallic Cu, which is denser than the Cu2O, leading to theformation of voids.

3.3.3. Crystal orientation of particles in particle layerFig. 9 shows the results of the EBSD analysis for the parti-cle layer after bonding and measurement of shear strength.Image (a) shows a band contrast map of the particle layer.Particles with light contrast were observed, and their sizeswere in the range of ca. 50–300 nm that are orders of nano-meter and submicron-meter. Image (b) shows a mean angulardeviation map of the same place as shown in image (a). Con-trast between grayish sites and blackish sites was clearlyobserved, which meant that the EBSD analysis was success-fully performed for the particle layer. Images (c)–(e) showEBSD-determined inverse pole figure maps in the directions ofthe x, y, and z axes, respectively. They were obtained by analyz-ing the back scattering intensity in the directions in image (b).

Each particle had almost a single color, which indicated thatthe Cu2O particles became nano-sized or submicron-sizedmetallic Cu single crystals, or fine metallic Cu single crystals.

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he process of annealing in H2 gas under pressure was con-idered to provide not only the reduction of Cu2O to metallicu, but also the formation of micron-sized domains com-osed of fine single crystals. The XPS measurements and theeasurement of bonding strength gave rise to speculation

hat the epitaxial growth of metallic Cu promoted by the fineluster-like domains composed of Cu0 Cu0 bonds might haveccelerated the formation of single crystals. Solid materialsomposed of particles with small grain size are mechanicallytrong compared to with large grain size due to the Hall–Petchffect [54,55]. Thus, the metallic Cu particles produced withhe reduction form mechanically strong domains composed ofhe fine single crystals, which resulted in the strong bondingf Cu discs in the present work.

. Conclusions

he Cu2O nanoparticles were used as the filler for metal–metalonding. The Cu2O nanoparticle colloid solutions were pre-ared by reacting 0.01 M Cu(NO3)2 with 0.01–0.03 M NaBH4

n aqueous solution at 20–80 ◦C. A NaBH4 concentration of.01 M and reaction temperature of 40 ◦C resulted in the pro-uction of cubic CuO nanoparticles that had an average sizef 111 ± 34 nm and crystal size of 21.2 nm. The XPS mea-urements supported the hypothesis that the Cu2O particlesontained fine metallic cluster-like domains composed ofu0 Cu0 bonds. The bonding examinations in H2 gas at 400 ◦Cevealed that the Cu2O particle powder had a shear strengths high as 27.9 MPa. The metallic Cu single crystalline parti-les, production of which during bonding was confirmed byBSD analysis, made the metallic Cu grow epitaxially, whichroduced the micron-sized domains composed of fine singlerystals, and consequently the metallic Cu discs were stronglyonded. Our finding that the Cu2O nanoparticles could per-orm metal–metal bonding will increase interest and activeesearch in the field of metal–metal bonding processes usinganoparticles. However, some discussions are still based onpeculation. Further studies on metal–metal bonding mecha-isms are awaited.

onflicts of interest

he authors declare no conflicts of interest.

cknowledgements

his work was partially supported by Hitachi, Ltd. We expressur thanks to Prof. T. Noguchi in the College of Science ofbaraki University, Japan (current affiliation: Faculty of Artsnd Science of Kyushu University, Japan) for his help with TEMbservation.

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