[Reproduced] Ke Lu also posted Science: unexpectedly found that the smaller the nanocrystal size, the more stable it is!
 
Highlights:
    Breaking the convention, it is found that when the size of the nanocrystalline grains on the metal surface is smaller than the critical size, the smaller the grains, the higher the thermal stability of the nanocrystals.
   The nanometerization of metal surfaces was initiated by Ke Lu, a famous Chinese nanoscientist. The surface grain nanometer greatly improves the mechanical strength and hardness of the metal, but at the same time it also increases the surface grain boundary density. Due to the high surface energy of nanocrystals, the high-density nanocrystal boundaries lead to the thermodynamic instability of nanocrystals, which are prone to coarsening at high temperatures or even room temperature, thereby losing the nano effect and reducing the mechanical properties.
   With the miniaturization and high integration of microelectronic devices, the thickness and line width of metal connecting wires have entered the nanometer scale, and the use of electronic devices will inevitably bring about a rise in temperature. The melting temperature is generally lower than the equilibrium melting point of the corresponding bulk material, and decreases significantly as the particle diameter or film thickness decreases.
Figure 1. The first realization of metal surface nanometer 
   Classical theory believes that the smaller the nanocrystal size, the more unstable the thermodynamics and the lower the melting point. In order to enhance the thermal stability of the nano-metal under the premise of ensuring the mechanics of the nano-metal, the usual method is to use alloying to reduce the grain boundary energy of the nano-crystal. Although alloying has a certain degree of gain, it is still difficult to avoid the reduction of metal mechanical properties.
    Therefore, how to improve the thermal stability of the nanocrystals on the pure metal surface has become a key issue that needs to be solved urgently in the practical application of nanometal materials.
    In view of this, the team of researcher Lu Ke and researcher Li Xiuyan of the Shenyang Institute of Metal Research of the Chinese Academy of Sciences discovered a phenomenon that broke through the conventional thinking: when the size of the nanocrystalline grains on the metal surface is smaller than the critical size, the smaller the grains, the higher the thermal stability of the nanocrystals.
Figure 2. The relationship between grain size and thermodynamic instability temperature 
    The researchers used pure Cu rods with a purity of 99.97%, coarse crystals on the surface and no oxygen as the original sample material, and mechanically polished the surface at the temperature of liquid nitrogen. After plastic deformation, a gradient nanostructured surface was obtained. After the treatment, the average size of randomly oriented nanocrystals in the cross-section of the outermost layer of the Cu rod surface is about 40 nm and the aspect ratio is about 1.7. From the outermost layer down, the grain size is getting bigger and bigger. At 20 μm, the average cross-sectional grain size is about 70 nm; at 150 μm, the average cross-sectional grain size is about 200 nm.
   The study found that at a temperature of 373 K, the sub-surface layer with a grain size of about 70-110 nm at 20-50 μm begins to coarsen first. The higher the temperature, the more obvious the coarsening, and the coarsening behavior migrates from top to bottom. Performance is also reduced from top to bottom. The strange thing is that within the temperature of 453K, the outermost layer with a grain size of about 40 nm has not been coarsened and morphologically changed, and the mechanical properties have not changed. 
Figure 3. Annealing leads to changes in the gradient nanostructure of pure Cu surface 
Figure 4. Annealing leads to changes in the gradient nanostructure of pure Ni surface 
   As the temperature rises further, the nanocrystals on the outermost layer begin to coarsen and recrystallize. The morphology of the recrystallized coarse crystals and the coarse crystals within the surface layer are completely different, and they are located between the nanocrystals and the coarse crystals in the inner surface layer, forming a sandwich structure. . The thermodynamic instability temperature of this recrystallized coarse crystal is higher than that of the coarse crystal of the same size in the inner surface layer.
  That is to say, when the size of the nanocrystalline grains is small to a certain critical value, the instability temperature of the nanocrystalline grains increases as the size of the nanocrystalline grains becomes smaller. 
Figure 5. Annealing causes coarsening of submicron crystals on the surface of pure Cu 
    Further mechanism studies have shown that the abnormal thermodynamic stability of nanocrystals is due to: the activation of some dislocations during plastic deformation causes the formation of low-angle grain boundaries between nanocrystal grains, causing the nanocrystal grain boundaries to automatically evolve from a high-energy state to a low-energy state. State, thereby enhancing thermal stability.
  In short, this discovery reveals the thermal stability mechanism of the nanocrystalline grain boundary on the metal surface, and plays a major role in enhancing the thermal stability of nanometals and making them suitable for practical high-temperature applications! 
Lu Ke (left) and Li Xiuyan (right) 

X.Zhou, X. Y. Li, K. Lu. Enhanced thermal stability of nanograined metals below acritical grain size. Enhanced thermal stability of nanograined metals below acritical grain size. Science 2018, 360, 526-530.