| With large surface energy, silver nanoparticles(Ag NPs) can generally surface melt or sinter at temperatures far below the melting points of the bulk silver. In addition, the resultant structures after sintering present excellent thermal conductivities and electrical conductivities. Therefore, as the thermal interface materials to be applied in high-power devices and an inkjet-printed conductive material for flexible printed circuits, Ag NP has attracted extensively attentions. However, according to the existing reports, the sintered structures of single-sized Ag NPs are faced with numerous challenges. For small-sized Ag NPs, the sintering temperature is low while its porosity is high, and grain size is small with more defects. For large-sized sintered Ag NPs, the grain size is large with fewer defects, but the sintering temperature and porosity are high. In this study, a composite Ag NP with different sizes was synthesized, in which the small Ag NPs serve as the “filler” to increase the initial stacking density and weld the large Ag NPs together, while the large Ag NPs play as the framework to decrease the initial crystallographic defects and stabilize the sintered structures. To improve the thermal conductivities, service reliabilities and electrical conductivities of the sintered structure of Ag NPs, composite Ag NPs were synthesized for preparing thermal interface materials and conductive ink. In this study, the sintering mechanism of composite Ag NPs at low temperature was focally investigated, and the influencing factors on the thermal conductivities, service reliabilities and electrical conductivities for the sintered structure of composite Ag NPs as well as the influencing rules were analyzed.Through the chemical reduction method, Ag NPs with average particle sizes of 11.9 nm and 52.8 nm were prepared respectively and covered with the organic coating layer of citrates. Subsequently, a composite Ag NP was synthesized via ultrasonic mixing of two types of Ag NPs(with mass ratio of 2:1). A composite Ag NP paste was prepared via mixing of composite Ag NP participles and deionized water at a certain proportion with uniform dispersion. Compared with the Ag NP pastes of a single size, the porosity of sintered composite Ag NP paste always kept the lowest(the porosity at the sintering temperature range of room temperature to 250 °C basically maintained around 25.5%). When the sintering temperature was- IV-higher than 130℃, the participle size of the sintered composite Ag NP paste was always larger than other Ag NP pastes of a single size(the average grain size at the sintering temperature of 250 °C was 36.8nm). The thermal conductivity of the sintered composite Ag NP paste was always higher than other Ag NP pastes of a single size. After sintering at 250 °C for 30 min, the average thermal conductivity of this paste could reach 278.5 Wm-1K-1, close to 65% of the theoretical value of the bulk Ag. The major reasons include: on the one hand, the sintered structure of the composite Ag NP paste was featured with low and stable porosity as well as large grain size, which means the sintering defects and crystal defects were reduced greatly. On the other hand, a large number of coherent twins were formed inside the sintered structure of the composite Ag NP paste according to the observation of TEM image, which was beneficial to improve the heat-conducting property of the sinter. Moreover, the low temperature sintering mechanism of the composite Ag NP paste was discussed in details. The distribution intervals and forming reasons of exothermic peaks during the sintering process of the composite Ag NP paste were analyzed by characterizing differential scanning calorimetry(DSC) heat flow and thermogravimetric analysis(TG) curves. By comparing the Raman spectral characteristics of the sintered structure for the composite Ag NP pastes formed at different temperatures, this study proposed a structural model connecting citrate and Ag NP surface and discussed the interaction mechanism of Ag NPs and citrates during the sintering process at the low temperature in details. In addition, the spontaneous sintering behavior of the Ag NPs in two sizes of Ag NPs paste and the formation mechanism of accompanying twins were revealed through in-situ observations.Dimensional instability mechanism of sintered structure for Ag NP pastes was discussed in this study. Thermal expansion behavior test results of sintered structure for Ag NP pastes at different temperatures indicated that, almost all sintered Ag NP pastes of a single size would shrink violently when the service temperature was higher than its initial sintering temperature. However, when the sintering temperature was higher than 200 °C, the structure stability at high temperature of the composite Ag NP paste sinter was obviously superior to the Ag NP pastes of a single size. Interestingly, the structure of the composite Ag NP paste after sintering at 250 °C for 30 min has a stable coefficient of thermal expansion during 30-600 °C, which may be owing to the stable state of the sintered structure and the effective blockage of the dislocation motion by the many coherent twins inside. Based on this, Cu/Ag NPs/Cu sandwiched bonding joints was prepared by using the composite Ag NP paste as the interlayer and their service reliability was investigated. The results indicated that the service reliability of composite Ag NP paste bonding joints was obviously better than the Ag NP pastes in a single size. The average shear strength of the composite Ag NP paste bonding joints sintered at 250°C before and after thermal cycling at 50–200°C for 1000 cycles could reach 41.80 and 28.75 MPa respectively, which is far higher than the strength of Sn-based solders in similar conditions. Moreover, the failure mechanism of composite Ag NP paste bonding joints was analyzed in details. By observing the fracture morphologies of the composite Ag NP paste bonding joints, the change rules of the fracture models before and after thermal cycling test were analyzed.A composite Ag NP ink was prepared with composite Ag NP participles, ethylene glycol and deionized water at a certain proportion in this paper. The experiment indicated that the resistivity of composite Ag NP ink printing patterns was obviously lower than the Ag NP ink of a single size. The resistivity of composite Ag NP ink printing patterns will decrease with the increase of sintering temperature and printing cycles and the decrease rate of resistivity will decrease with the increase of printing cycles. The composite Ag NP ink is featured with excellent conductivity and sintering ability at room temperature. When the sintering temperature increases to 180 °C with thinkness of 1.7 um, the resistivity can fall to 3.54 μΩcm while the minimum electrical resistivity of printed patterns at room temperature can be approximately 5.64 μΩcm. Moreover, the room-temperature sintering mechanisms of the composite Ag NP ink were discussed in details. Results showed that, oxhydryls could effectively facilitate the quick sintering of Ag NPs at room temperature. The reason may be that oxhydryls can replace the citrates on the surface of Ag NPs, causing the potential unbalance and the disappearance of space steric hinerance effect among the adjacent Ag NPs which enable their coalescence and sintering of Ag NPs. |
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