j m a t e r r e s t e c h n o l .2 0 1 4;3(2):114–121 Available online at www.sciencedirect.com www.jmrt.com.br Original Article A metal–metal bonding process using metallic copper nanoparticles produced by reduction of copper oxide nanoparticles Yoshio Kobayashi a,∗, Yuki Abe a, Takafumi Maeda a, Yusuke Yasuda b , Toshiaki Morita b a b Department of Biomolecular Functional Engineering, College of Engineering, Ibaraki University, Hitachi, Ibaraki, Japan Hitachi Research Laboratory, Hitachi Ltd., Hitachi, Ibaraki, Japan a r t i c l e i n f o a b s t r a c t Article history: Metal–metal bonding was performed using metallic Cu nanoparticles fabricated from CuO Received 20 September 2013 nanoparticles. Colloid solutions of CuO nanoparticles were prepared by the reaction of Accepted 24 December 2013 Cu(NO3 )2 and NaOH in aqueous solution at reaction temperatures (TCuO s) of 20, 40, 60 and Available online 6 February 2014 80 ◦ C. CuO single crystallites with a size of ca. 10 nm were produced, and they formed leaflike aggregates. The longitudinal and lateral sizes of the aggregates decreased from 522 to Keywords: 84 nm and from 406 to 23 nm, respectively, with an increase in TCuO . Colloid solutions of Metallic Cu metallic Cu nanoparticles were prepared by reducing the CuO nanoparticles with hydrazine Copper oxide in the presence of cetyltrimethylammonium bromide. The size of the metallic Cu nanopar- Nanoparticle ticles decreased from 92 to 73 nm with increasing TCuO . Metallic copper discs were bonded Metal–metal bonding with the metallic Cu nanoparticles by annealing at 400 ◦ C and 1.2 MPa for 5 min in H2 gas. The shear strength required to separate the bonded discs increased with increasing TCuO . A maximum shear strength of 39.2 MPa was recorded for a TCuO of 80 ◦ C. © 2014 Brazilian Metallurgical, Materials and Mining Association. Published by Elsevier Editora Ltda. All rights reserved. 1. Introduction In metal–metal bonding processes, which are important in many fields, solders or fillers have conventionally been used for efficient bonding [1–5]. These solders are melted at high temperatures and spread between metallic surfaces, thus bonding the surfaces together. A decrease in temperature solidifies the metallic materials and completes the metal–metal bonding. Metallic alloys composed mainly of lead and tin have been used as solders [1–4]. These metallic alloys ∗ melt at temperatures as low as 184 ◦ C, lower than the melting points of many other metallic alloys. The lead- and tin-based alloys diffuse into the materials to be bonded, and then, they can be bonded at low temperatures. It is well known that lead is harmful to living bodies, which limits its use. Various lead-free alloys have been developed as new solders [6–11]. Although low-temperature metal–metal bonding can be performed using the lead-free solders, there is a serious problem: the bonded materials may break apart when exposed to temperatures higher than their melting points due to remelting of the solders. Corresponding author. E-mail:
[email protected] (Y. Kobayashi). 2238-7854/$ – see front matter © 2014 Brazilian Metallurgical, Materials and Mining Association. Published by Elsevier Editora Ltda. All rights reserved. http://dx.doi.org/10.1016/j.jmrt.2013.12.003 j m a t e r r e s t e c h n o l .375 × 10−2 M. After 30 min.5 mm was placed on top of the powder sample. Ag and Cu are ca. the copper stage . Samples for XRD were prepared by removing the supernatant of the nanoparticle colloid solution with decantation and drying the residue at room temperature for 24 h in a vacuum.25]. Copper oxide can also be used as a precursor.2. 2.0% (as Cu(NO3 )2 )) and sodium hydroxide solution (NaOH) (5 M). High-temperature annealing is required during the bonding process to successfully bond metallic materials. the addition of water. their melting points are ca. and shaking of the mixture with a vortex mixer to disperse the nanoparticles. hydrazine was added. The copper stage and plate were pressed at 1.000 rpm for 30 min.2 MPa while annealing in H2 at 400 ◦ C for 5 min with a Shinko Seiki vacuum reflow system.and tin-based solders. and it was deaerated by bubbling with N2 gas for 30 min prior to preparation of the aqueous solutions of Cu(NO3 )2 . The melting points of metallic materials such as Au. and these high temperatures thermally damage the material near the bonding site. 1000 ◦ C.9 in the final solution. This decrease in the melting point decreases the temperature needed for the metal–metal bonding process.3. The initial concentrations of Cu(NO3 )2 and NaOH were 1. and the reaction time was 3 h. Several researchers have studied metal–metal bonding using metallic Cu nanoparticles [20. Chemicals The reactants for the production of copper oxide were copper (II) nitrate trihydrate (Cu(NO3 )2 ·3H2 O) (77. respectively. Experimental work 2. respectively.1. Dozens of particle diameters were measured in TEM images to determine an average particle size and a standard deviation. 40. 0.0–80.0 × 10−2 . Both the additions were performed in air with vigorous stirring at a constant solution temperature of 30 ◦ C. Hydrazine monohydrate (>98. Colloid solutions of metallic Cu nanoparticles were synthesized by reducing the copper oxide nanoparticles. 2. 1000 ◦ C in the bulk state but decrease as the material size is decreased to several nanometers [12–16]. This process was performed three times. Characterization The particles were characterized by transmission electron microscopy (TEM) and X-ray diffractometry (XRD). The metal–metal bonding properties are related to the morphology of the metallic Cu nanoparticles.17–23] because metallic Ag is chemically stable.33–35]. Various researchers have studied metal–metal bonding using metallic Ag nanoparticles as the filler [15. Metallic Cu is promising as a filler for bonding because it is inexpensive and because electric migration does not take place as often as it does with metallic Ag. which were prepared using the same process as that used for the XRD samples. The reaction times were 3 h at TCuO s of 60 and 80 ◦ C and 24 h at TCuO s of 20 and 40 ◦ C. All chemicals were purchased 115 from Kanto Chemical Co.24.8 and 5. and were used as received. they have some disadvantages: metallic Ag is relatively expensive and prone to migration under an applied voltage.0 × 10−3 M..0%) and cetyltrimethylammonium bromide (CTAB) (99%) were used as a reducing reagent for copper oxide and as a stabilizer for the resulting metallic Cu nanoparticles. TEM observation was performed with a JEOL JEM-2100 microscope operating at 200 kV.25 × 10−2 and 2.3(2):114–121 Metallic materials such as Au. Samples for TEM were prepared by dropping and evaporating the nanoparticle colloid solution on a collodion-coated copper grid. as described in our previous work [31]. they remain bonded even at temperatures higher than the melting points of the metallic nanoparticles because the nanoparticles convert to a bulk state during bonding. Preparation Colloid solutions of copper oxide nanoparticles were synthesized using a reaction between copper ions and a base. However. We also varied the morphologies of CuO nanoparticles and CuO-nanoparticle aggregates with several parameters such as the reaction temperature [32]. metallic Cu nanoparticles are fabricated from CuO nanoparticles with various preparation parameters and are examined with respect to metal–metal bonding. The morphology of metallic Cu particles depended on the morphologies of the CuO nanoparticles and CuO-nanoparticle aggregates as well as on the concentrations of chemicals. An aqueous solution of CTAB was added to the colloid solution of copper oxide nanoparticles. The powdered nanoparticles were obtained by drying the nanoparticles at room temperature in a vacuum after the final removal of the supernatant. 2 0 1 4. Our research group has developed a method for producing metallic Cu nanoparticles for use as the filler in metal–metal bonding [26–30]. hydrazine and CTAB in the final colloid solution were 1. The resulting nanoparticles were washed first with centrifugation at 10. our research group proposed a method for fabricating metallic Cu nanoparticles using CuO nanoparticles as the precursor and examined the metallic Cu nanoparticles as a filler for bonding [31]. was spread on a copper stage with a diameter of 10 mm and a thickness of 5 mm. Although Ag nanoparticles work well as a filler for metal–metal bonding. 60 and 80 ◦ C. XRD patterns were obtained with a Rigaku Ultima IV X-ray diffractometer at 40 kV and 30 mA with Cu K␣1 radiation. 2. Water that was ion-exchanged and distilled with a Yamato WG-250 system was used in all preparations. A copper plate with a diameter of 5 mm and a thickness of 2. In the present work. higher than those of the conventional lead. Ag and Cu can be used as the filler because they have excellent electrical and thermal conductivities. The powdered particles. Previously. The metal–metal bonding properties of metallic Cu nanoparticles were investigated by a method described in our previous work [21. respectively. which may damage an electric circuit. Initial concentrations of Cu. Metallic Cu nanoparticles are usually prepared using methods based on the direct reduction of copper ions with reducing reagents in the liquid phase. After bonding. followed by the removal of the supernatant. An aqueous solution of NaOH was added to an aqueous solution of Cu(NO3 )2 with vigorous stirring at reaction temperatures (TCuO s) of 20. which resulted in an Na/Cu molar ratio of 1. Once the metallic materials are bonded with the metallic nanoparticles. Inc. The aggregate size decreased with increasing TCuO .1. 3. and small aggregates were produced at high TCuO s. Fig. These peaks were assigned to Cu2 (OH)3 NO3 (JCPDS card No. This increase in pH decreased the surface charge of the nanoparticles. 2 shows the XRD patterns of the CuO nanoparticles. 208 ± 22/151 ± 9 at 40 ◦ C. At a TCuO of 20 ◦ C. 38. and 80 ◦ C. At TCuO s over 20 ◦ C.9◦ . 15-0014). 40. and only the peaks due to CuO were detected. the Cu2 (OH)3 NO3 peaks disappeared. Morphology of copper oxide nanoparticles The blue aqueous solution of Cu(NO3 )2 became a black colloid solution after the addition of the NaOH solution. A high magnification image (inset of Fig. This trend corresponded with our previous work [32]. which is only an estimate. the pH did not closely approach the IEP. In our previous work [31]. The surface of each copper plate was observed by scanning electron microscopy (SEM) with a JEOL JSM-5600LV microscope after the measurement of shear strength. Application of the Scherrer equation to the XRD line broadening of the 35. as the outlines of the nanoparticles were not well defined in the TEM images. 5-0661) were recorded at 35.6◦ .6◦ peak provided average CuO crystal sizes of 12. and 48. The appearance of Cu2 (OH)3 NO3 peaks indicated that the TCuO of 20 ◦ C was too low for complete reaction of Cu(NO3 )2 to CuO. Therefore.8◦ and 25. 1(a)) revealed that the aggregates appeared to be composed of nanoparticles with a size of ca. leaf-like aggregates with an average longitudinal size of 522 nm and an average lateral size of 406 nm were produced. which examined the fabrication of CuO particles at initial concentrations of 1. several peaks attributed to monoclinic CuO (JCPDS card No. The bonding and the measurement of shear strength were performed two or three times for each sample and averaged to determine the bonding strength of the nanoparticles. The addition of NaOH made the pH of the solution approach its isoelectric point (IEP) without exceeding it because the Na/Cu ratio of 1. the IEP of CuO particles prepared at 80 ◦ C was higher than that of CuO nanoparticles prepared at 20 ◦ C. 1 – TEM images of various nanoparticles.0 × 10−2 M Cu(NO3 )2 and 1. 60 and 80 ◦ C. Fig. 40.8◦ .9 was lower than the stoichiometric ratio of 2 required for complete reaction of NaOH and Cu2+ . There Fig.8◦ .116 j m a t e r r e s t e c h n o l . of which colloid solutions were prepared at various TCuO s. at a TCuO of 80 ◦ C. the longitudinal/lateral aggregate sizes (in nm) were 522 ± 43/406 ± 39 at 20 ◦ C.4 and 10.3(2):114–121 and plate were separated by applying a shear strength. Consequently. 14. 2 0 1 4. Results and discussion 3.6. 60. which was measured with a Seishin SS-100KP bond tester. Accordingly. which resulted in their aggregation. it was found that the purity of CuO nanoparticles increased with increasing TCuO . In addition. For the particles prepared at 20 ◦ C.9 × 10−2 M NaOH. 13. respectively.3. electrostatic repulsions between the nanoparticles limited aggregation. 75 ± 10/48 ± 9 at 60 ◦ C and 84 ± 15/23 ± 5 at 80 ◦ C. 10 nm. this observation implied the production of CuO. the dominant peaks were at 12. Colloid solutions of samples (a)–(d) were prepared with the salt–base reaction at 20. 1 shows the TEM images of the prepared nanoparticles.7 nm for 20. . Because CuO is black. Colloid solutions of samples (a)–(d) were prepared by reducing the samples (a)–(d) in Fig. 1. the reddening implied the production of metallic Cu nanoparticles.j m a t e r r e s t e c h n o l . Fig. This peak was assigned to cubic metallic Cu2 O (JCPDS card No. 101 ± 37 and 73 ± 23 nm for TCuO s of 20. . The average crystal sizes of the metallic Cu nanoparticles. The particles had rough surfaces. 4 shows the XRD patterns of the metallic Cu nanoparticles. The patterns showed peaks at 43.3◦ .2◦ . 93 ± 33. The detection of Cu2 O indicated that the nanoparticles were partially oxidized. Accordingly.3(2):114–121 was no large difference in crystal size at the TCuO s examined. As shown in Fig. This similarity indicated that single crystals of CuO formed the aggregates. 40.3◦ and 74. b a 10 20 117 30 40 50 60 70 80 2θ (degree). 5-0667). 2 – XRD patterns of various nanoparticles. Fig. units) c 3. aggregate size was considered a reflection of particle size. the aggregate size decreased with increasing TCuO . respectively. respectively. 2 0 1 4.6◦ in each pattern. 3 shows TEM images of the metallic Cu particles. 1. the particle size was approximately constant in the TCuO range of 20–60 ◦ C but decreased when the TCuO was increased to 80 ◦ C. 60 and 80 ◦ C. These crystal sizes of ca. d Intensity (arb. almost all of the nanoparticles examined had sizes in the range of 50–150 nm. 10 nm were similar to the sizes of the nanoparticles composing the aggregates. Since surface plasmon resonance absorption of metallic Cu nanoparticles results in a red color. Morphology of metallic copper nanoparticles The addition of hydrazine reddened the colloid solution of CuO nanoparticles. These peaks were attributed to cubic metallic Cu (JCPDS card No. a faint peak was detected at 36. 3 – TEM images of various nanoparticles. Although some particles larger than 1 m were observed. CuKα Fig. Fig. In addition to the metallic Cu peaks. 50. Samples (a)–(d) were the same as in Fig.2. The average particle sizes were 92 ± 33. 4-0836). Symbols () and () stand for CuO and Cu2 (OH)3 NO3 . respectively. 1. 3. All of the shear strengths measured were over 20 MPa. There are other possible causes as well.6 nm at TCuO s of 20. Our previous work showed Fig. The shear strength increased with increasing TCuO and reached 39. For all the particles examined.1. 6 shows shear strength as a function of TCuO . Bonding properties of metallic copper nanoparticles Intensity (arb. were 33. This large contact area for the small particles probably caused the strong metal–metal bonding. As shown in Fig. 28. 3. The particle size observed with TEM was larger than the crystal Fig.2 MPa at a TCuO of 80 ◦ C. which indicated that the nanoparticles were polycrystalline. which were estimated from the XRD line broadening of the 43. . Fig.3◦ peak using the Scherrer equation. This observation implies that the metallic Cu powders strongly bonded the copper discs together. One possible cause involves impurities in the metallic Cu nanoparticles. 5 shows the photographs of the copper plates after the measurement of shear strength. Therefore. 3. than for the large particles. respectively. The powders used were the metallic Cu particles (a)–(d) shown in Fig. Samples (a)–(d) were the same as in Fig. Small particles have a larger surface area than large particles if the amount of material is the same. 3. as a result of the larger surface area. 5 – Photographs of the copper plates after the measurement of shear strength. 60 and 80 ◦ C.1. which was the largest in the present work. 4 – XRD patterns of various nanoparticles. units) d c b a 10 20 30 40 50 60 70 80 2θ (degree). 5. Symbols (䊉) and () stand for metallic Cu and Cu2 O. 31. the contact area between the particles and the Cu discs was larger for the small particles. 2 0 1 4.3.3(2):114–121 size. CuKα Fig. 40. respectively.118 j m a t e r r e s t e c h n o l . the pressed powders on the plates appeared to be faintly reddish due to the presence of metallic Cu: the powders were still metallic after the bonding process. the particle size decreased with increasing TCuO .2 and 32. which indicated that the copper discs were strongly bonded for all the particles examined and also corresponded to the strong bonding implied in Fig. thus decreasing the shear strength. TCuO . at which the smaller nanoparticles were produced. Fig. as indicated by the XRD measurements (Fig. dimples often form in the bonded Fig. 7 shows the SEM images of the surfaces of the copper discs separated with shear stress. This intensive sintering might have contributed to the strong metal–metal bonding with high TCuO . the CuO nanoparticles fabricated at the 119 low TCuO contained Cu2 (OH)3 NO3 .3(2):114–121 50 Shear strength (MPa) 40 30 20 10 0 20 40 60 80 TCuO (ºC) Fig. and elimination of NO3 and H2 O did not occur during bonding in the metallic Cu nanoparticles at high TCuO . no decrease in bonding ability as a result of impurities was expected for the high TCuO sample. Some dimples accompanied by sharp tips were observed on the surfaces for the samples at TCuO s of 20 and 80 ◦ C. When the metallic materials are separated with shear stress. Sintering of particles takes place intensively for small particles because of the size effect related to the depression of the melting point. the metallic Cu nanoparticles derived from the CuO nanoparticles produced at the high TCuO probably did not contain much Cu2 (OH)3 NO3 . Thus. The powders used were the metallic Cu particles (a)–(d) shown in Fig. Therefore.j m a t e r r e s t e c h n o l . In the present work. The other possible cause of strong metal–metal bonding in the high TCuO sample involves a size effect. the elimination of NO3 and H2 O from Cu2 (OH)3 NO3 produced pores that prevented the particles from sintering. 3. The powders used were the metallic Cu particles (a)–(d) shown in Fig. 7 – SEM images of the copper plates after the measurement of shear strength. 2 0 1 4. These three factors are the possible mechanisms for the strong bonding achieved by the nanoparticles fabricated at the TCuO of 80 ◦ C. that the presence of Cu2 (OH)3 NO3 decreased the bonding ability [31]: during bonding. while the purity of CuO nanoparticles increased with increasing TCuO . The purity of the metallic Cu nanoparticles might increase with increasing TCuO . 6 – Shear strength vs. while Cu2 (OH)3 NO3 might remain in the metallic Cu nanoparticles derived from the CuO nanoparticles produced at the low TCuO . 2). 3. . Because the CuO nanoparticles were the precursor of the metallic Cu particles. the purity of the metallic Cu nanoparticles should depend on that of the CuO nanoparticles. the observation of dimples on the copper disks suggests that they were strongly bonded using the metallic Cu nanoparticles. . Effect of iron and indium on IMC formation and mechanical properties of lead-free solder.8 M hydrazine in the presence of 5. Hu A. Hu X. An innovative method for joining materials at low temperature using silver (nano) particles derived from [AgO2 C(CH2 OCH2 )3 H].41:1886–92. Xu SH. Li H. Maleki M. Japan for assistance with the TEM observations. [18] Acknowledgements [19] This work was partially supported by Hitachi. We express our thanks to Prof. Interfacial microstructure and shear strength of Ag nano particle doped Sn–9Zn solder in ball grid array packages. Guo Z.498:323–7. 40. Maes M. Fukusumi M.53:2385–93.2 MPa at a TCuO of 80 ◦ C. Effects of joining conditions on joint strength of Cu/Cu joint using Cu nanoparticle paste. Models for mean bonding length.34:1–58. but the bonded materials appeared to be sharply cut off. Mangelinck-Noël N. Terada N. Ferrara S. Jpn J Appl Phys 2009. Low-temperature low-pressure die attach with hybrid silver particle paste.7Cu0. Evolution of thermal property and creep resistance of Ni and Zn-doped Sn–2. Ide E. Microelectron Reliab 2009. Surf Coat Technol 2005. Yasuda Y. Bobzin K. Islam RA. Silver nanoparticle-assisted diffusion brazing of 3003 Al alloy for microchannel applications. Microelectron Reliab 2012. Omar MS.29:589–93. Arezodar AF. Zhou YN. 4. Mater Sci Eng B 2004.3:60–4.51:789–96. Osório WR. Li XF. Suganuma K. A low-temperature bonding process using mixed Cu–Ag nanoparticles. Polymer-protected Cu–Ag mixed NPs for low-temperature bonding application. For the TCuO s of 40 and 60 ◦ C. 60 and 80 ◦ C. Calata JN. Mater Design 2012. Nagaoka T. Peixoto LC. Zhang LD. Appl Surf Sci 2013. Mater Chem Phys 2013. Size-dependent melting point of nanoparticles based on bond number calculation. Mater Sci Eng A 2012.375:1746–50. were reduced with 0. Wilden J.106:120–5. Wong NB. Mikula A. Low-temperature sintered nanoscale silver as a novel semiconductor device-metallized substrate interconnect material. Lerch M. Yamamoto M. Low-temperature bonding using silver nanoparticles stabilized by short-chain alkylamines. Morisada Y. Chen Z. as reported by Morisada et al. J Electron Mater 2012. Botsis J. Yan J. J Alloys Compd 2013. Morita T. Such dimples were produced in materials that underwent a strong bonding process using Ag nanoparticles. Metal–metal bonding process using Ag metallo-organic nanoparticles.9 and at TCuO s of 20. Kim KS. Zou G. Angata S.265:239–44. Microstructure-based modeling of the ageing effect on the deformation behavior of the eutectic micro-constituent in SnAgCu lead-free solder.48:125004. Ltd. The aggregate size decreased from 522 to 84 nm for longitudinal size and from 406 to 23 nm for lateral size with an increase in TCuO from 20 to 80 ◦ C.3(2):114–121 region. [5] Lugscheider E. El-Taher AM. Bai JG. [20] references [21] [22] [1] Tu KN. Mater Design 2013. Eluri R. Chan YC.23:325–35. Garcia LR. Further studies on the practical use of metallic Cu nanoparticles and the precise bonding mechanism are in progress. Oestreicher A. Wu A. Shibuta Y. Han PD. Chem Phys Lett 2010. Sn–Ag and Sn–Zn lead-free solder alloys. Hirano T.0 × 10−3 M CTAB in water at 30 ◦ C to produce the metallic Cu nanoparticles. The CuO nanoparticles. Li M. Zeng K. Nishikawa H. Jakob A. Fallahi H. Sharif A. Size-dependent melting behavior of indium nanowires.0Ag–0.2 MPa for 5 min in H2 gas. El-Daly AA. Nogi M. Microstructure evolution and mechanical properties of Sn0. The shear strength required to separate the bonded discs increased with increasing TCuO . Tin–lead (SnPb) solder reaction in flip chip technology. Solid state interfacial reaction of Sn–37Pb and Sn–3. Noguchi at the College of Science of Ibaraki University. Garcia A. 10 nm were prepared by reacting 1. The size of the metallic Cu nanoparticles decreased from 92 to 73 nm with an increase in TCuO from 20 to 80 ◦ C. Kagami N. T.52:2047–56. Abdullah J. Paul BK. clear dimples were not observed. Melting and solidification point of fcc-metal nanoparticles with respect to particle size: a molecular dynamics study. Zhang XB.25 × 10−2 M in the colloid solution. J Alloys Compd 2013. 2 0 1 4. Lang H.566:239–45. Effect of volume in interfacial reaction between eutectic Sn–Pb solder and Cu metallization in microelectronic packaging. Cugnoni J. J Chem Thermodyn 2012. Mixed mode I/II fracture and fatigue crack growth along 63Sn–37Pb solder/brass interface. [2] Nayeb-Hashemi H. Hindler M.5Cu lead-free solders. Zhang Y. Microstructure and mechanical properties of Sn–Bi. Acta Mater 2013. Open Surf Sci J 2011.52:375–80. [3] Sharif A. Nurulakmal MS.47:3518–22.200:444–7.572:97–106. Hirose A. J Electron Mater 2010. Kobayashi KF. Mater Res Bull 2012. Röhrich T. [4] He M. Min Z. Takemoto T. Advancement in low melting solder deposition by pulsed [23] [24] [25] Magnetron Sputter-PVD process for microsystem technology. The bonding strength of the particles was examined by putting the metallic Cu nanoparticles between two metallic copper discs and then annealing the discs at 400 ◦ C at a pressure of 1. Sakamoto S. Phys Lett A 2011.55:102–9. [20] Therefore. Li K. Ren J.7Bi lead-free solders produced by directional solidification.61:103–14.5Ag solders with Ni–P under bump metallization. Yang P. Suzuki T. Chan YC. Kashiwagi Y. IEEE Trans Compon Pack Technol 2006. Jin Z. Wakuda D.553:22–31. Nakamoto M. melting point and lattice thermal expansion of nanoparticle materials.25 × 10−2 M Cu(NO3 )2 with NaOH in aqueous solution at an Na/Cu ratio of 1. Gain AK. Lu GQ. reaching a maximum of 39. Fei GT. This sharp cut-off also indicated strong bonding.137:1007–11. Acta Mater 2004.39:1283–8.49:746–53. Qi G.120 j m a t e r r e s t e c h n o l . Lead-free solder alloys: thermodynamic properties of the (Au + Sb + Sn) and the (Au + Sb) system. Zhang ZZ. Yung WKC. Erdle A.36:13–23. Acta Mater 2005. Int J Fatigue 2001. with a Cu concentration of 1. [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] Conflicts of interest The authors declare no conflicts of interest. Ide E. Mater Sci Technol 2001. Conclusions Colloid solutions of leaf-like aggregates composed of CuO single crystallites with a size of ca. Yasuda Y.3(2):114–121 [26] Kobayashi Y. [32] Kobayashi Y. J Nanoparticle Res 2011. World J Eng 2010. Preparation of metallic copper nanoparticles in aqueous solution and their bonding properties. [27] Kobayashi Y. Study of bonding technology using silver nanoparticles. Morita T.13:5365–72. Microstructure of metallic copper nanoparticles/metallic disc interface in metal–metal bonding using them. [29] Kobayashi Y. Hirose A. Yasuda Y. Hirose A.7:583–4. [34] Morita T. Watanabe K. Shirochi T. Yasuda Y.33:50–5. Morita T. Ihara K.49:2875–80. Maeda T. Morita T. Morita T. [30] Kobayashi Y. Direct bonding to aluminum with silver-oxide microparticles. World J Eng 2013. [35] Morita T. Metal–metal bonding process using copper nanoparticles. Morita T. Surf Int Anal 2013. [28] Kobayashi Y. Hirose A. Yasuda Y. Yasuda Y. Solid State Sci 2011. Sci Technol Weld Join 2012. Yasuda Y. Bonding technique using micro-scaled silver-oxide particles for in-situ formation of silver nanoparticles.j m a t e r r e s t e c h n o l . Akada Y. 2 0 1 4.50:226–8. Morita T. Kobayashi Y. . Yasuda Y. Ishida S. Kobayashi K. Ihara K. [33] Morita T.47:6615–22. Mater Trans 2009. Synthesis of metallic copper nanoparticles using copper oxide nanoparticles as precursor and their metal–metal bonding properties. Shirochi T. Maeda T. Jpn J Appl Phys 2008. Yasuda Y. Yasuda Y. Ide E. Yasuda Y.45:1424–8. Metal–metal bonding process using metallic copper nanoparticles prepared in aqueous solution. Mater Trans 2008. Preparation of CuO nanoparticles by metal salt–base reaction in aqueous solution and their metallic bonding property. 121 [31] Maeda T. Int J Adhes Adhes 2012. Shirochi T.10: 113–8.13:553–8. Ihara K. Ide E. Ishida S. Morita T. Preparation of copper nanoparticles and metal–metal bonding process using them. Abe Y.17:489–94. Ide E.