Strongly enhanced elastic modulus of solid nitrogen in nanopores

In this paper we show that solid nitrogen has considerably enhanced elastic properties in the nanopores of a porous glass. Particularly, the first adsorbed molecular layer near the pore wall possesses a shear modulus more than twice as high as the bulk modulus. The interaction between pore surface and pore filling provides a mean for the enhancement of elastic properties. Our discovery shows that nanoconfinement in porous samples can be used to influence the mechanical properties of substances.

Figure 1

Schematic sketch of nanoporous Vycor glass for different fillings. (a) Structure of the empty sample. (b) At low fillings with nitrogen the adsorbate forms a surface layer (red) with enhanced elastic properties on the pore walls. (c) The completely filled sample. The influence of the pore wall on the filling decreases gradually with increasing distance from the wall. Our analysis shows that in the pore center bulk properties (marked in blue) are reached.

Figure 2

Effective shear modulus of the sample scaled with the modulus of the empty sample at two different fillings: During cooling the effective modulus starts to increase below a temperature of about 55 K. The increase is considerably higher for complete filling ($f=1$) than for one surface layer ($f\approx 0.12$). At the lowest temperature, however, the increase for the surface layer corresponds to about 20% of the increase for the completely filled sample.

Figure 3

Shear modulus of one surface layer, of the adsorbate for completely filled pores, and of bulk nitrogen as a function of temperature. (a) Below $\approx 40$ K the modulus of the surface layer deviates considerably from the modulus of the complete pore filling, reaching a value of $\approx 1.7$ GPa at low temperatures. In comparison, the complete pore filling has a modulus of only $\approx 1.0$ GPa. (b) At low temperatures the shear modulus of bulk nitrogen [calculated from ultrasonic measurements (Ref. [28])] corresponds to less than $50\%$ of the value for one surface layer.