Melting Point Depression and Fast Diffusion in Nanostructured Brazing Fillers Confined Between Barrier Nanolayers

Abstract

Successful brazing using Cu-based nanostructured brazing fillers at temperatures much below the bulk melting temperature of Cu was recently demonstrated (Lehmert et al. in, Mater Trans 56:1015-1018, 2015. The Cu-based nano-fillers are composed of alternating nanolayers of Cu and a permeable, non-wetted AlN barrier. In this study, a thermodynamic model is derived to estimate the melting point depression (MPD) in such Cu/AlN nano-multilayers (NMLs) as function of the Cu nanolayer thickness. Depending on the melting route, the model predicts a MPD range of 238-609 K for Cu 10nm /AlN 10nm NMLs, which suggests a heterogeneous pre-melting temperature range of 750-1147 K (476-874°C), which is consistent with experimental observations. As suggested by basic kinetic considerations, the observed Cu outflow to the NML surface at the temperatures of 723-1023 K (450-750°C) can also be partially rationalized by fast solid-state diffusion of Cu along internal interfaces, especially for the higher temperatures.

Interdiffusion at Room Temperature in Cu-Ni(Fe) Nanolaminates

“…The present room-temperature Cu-Ni(Fe) result is the data point marked by the largest (solid red) circle in Figure 6. The diffusion coefficient computed at 23 • C rests just below the guideline for grain-boundary diffusion mechanism, and may well represents an extension of the grain-boundary diffusion data [38][39][40][41][42][43] for both Ni and Fe in Cu as well as Ni self-diffusion. The lack of grain growth, i.e., recrystallization, implies that the role of grain-boundary motion induced diffusion [48] is not a significant factor for the nanolaminates of this study whereas the use of grain boundaries and dislocation pipes provide paths for accelerated atomic transport between layers.…”

“…High diffusivities are found for semiconductor interfaces at lower temperatures as well. A high atomic mobility of Si and Ge at (Si,Ge)-Al interfaces is reported [37] at only 80 K. Similarly, the diffusion of Hf at 950 • C is reported [38] along linear defects in HfN-ScN nanolaminate; for Cu diffusion at 450 • C [39] along internal interfaces in Cu-AlN nanolaminates; and for Si-Al [40] as well. In these high-temperature studies, high-resolution electron microscopy (HREM) and atom probe methods are used for measuring the effect of solute concentration due to atomic diffusion along dislocations and grain grain boundaries.…”

On the Configurational Entropy of Nanoscale Solutions for More Accurate Surface and Bulk Nano-Thermodynamic Calculations

“…The majority of papers written so far on nanoscale phase equilibria discuss one-component systems [2][3][4][5][6][7][8][9][10][11][12][13][14][15] (see also references in these selected papers). A relatively small number of papers have been devoted to nanoscale phase equilibria of two-component systems [16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35].…”

Nano-Structured Cu/W Brazing Fillers for Advanced Joining Applications

“…The traditional approach to reduce the process temperature is based on the survey of low melting point alloys such as deep eutectic systems. Joining using nano-multilayer (NML) based brazing fillers has been shown to be a promising route for low-temperature joining applications [1,3,4]. The basic concept of this approach is to exploit nano-scale effects such as (i) the size dependency of the melting point and (ii) the high density of internal interfaces (e.g.…”

“…The basic concept of this approach is to exploit nano-scale effects such as (i) the size dependency of the melting point and (ii) the high density of internal interfaces (e.g. grain boundaries, inter phase boundaries) of nano-structured materials to tailor both of the thermodynamics (temperature) [5][6][7] and the kinetics (time) [4,8] of the joining process. The NML brazing fillers are composed of alternating nanolayers (individual thickness ≤ 10 nm) of a metal (or alloy) and a chemically-inert barrier (e.g.…”