Many efforts have been dedicated to improve the solar steam generation by using a bi-layer structure. In this paper, a two-dimensional mathematical model describing the water evaporation in a bi-layer structure is firstly established and then the finite element method is used to simulate the effects of different influence factors on the evaporation rate. Results turn out that: besides the high solar energy absorptivity of the first-layer, an optimum porosity of the second-layer porous material should be applied and the optimum porosity is about 0.45 in this work. This optimum porosity is determined by the balance between the positive effect of the lowering effective thermal conductivity of the second layer and the negative effect of the reduced vapor diffusivity in the second layer when the porosity is decreased. The influence of the thermal conductivity of the second-layer porous material is negligible because the effective thermal conductivity of the second layer is determined by the porosity while a larger porosity means more water in the second layer. The ambient air velocity could greatly enhance the evaporation rate, and the evaporation rate will decrease linearly with the increase of the air relative humidity. This study is expected to supply some information for developing a more effective bi-layer solar steam generation system.

The hot-wire method and the four-probe resistivity method are applied to probe the thermal conductivity (k) and the electric conductivity (σ) of Cu and Ni nanoparticle packed beds (NPBs). A fitting method based on classical physical theory is devised to separate ke (electronic thermal conductivity) and kp (phonon thermal conductivity) from k at room temperature. Results turn out that kp only accounts for a small proportion of k (4-20%); the proportion decreases with increasing porosity or temperature. Most importantly, this fitting method provides a simple way to separate ke and kp from k at room temperature. The Wiedemann-Franz law is checked and is found to be unsuitable for NPBs. The Lorenz number (L) is calculated from measurements of ke, k, and σ. Results turn out that L is found to be 50-60 times that of the bulk. With a Seebeck coefficient (S) measured, the thermoelectric property of NPBs is also calculated. We find that the NPB possess an advantage in thermoelectric property than bulk, the thermoelectric figure of merit (ZT) of Ni (Cu) NPBs can be 20.17 (1.87) times that of bulk Ni (Cu). The effect of porosity on ZT is also discussed, and results show that a NPB with a small porosity is more preferable as a thermoelectric material. With a small porosity, ZT can be even 1.73 times that of a large porosity. Although metals are not good thermoelectric material, the method in this paper supplies a way to improve the thermoelectric property of other thermoelectric materials.

No abstract

Two-dimensional materials (2D materials) show great advantages in high-performance lithium ion battery materials due to the inherent ion channels and rich ion sites. Unfortunately, rare 2D materials own all desired attributes to meet complex scenarios. Further enriching the 2D materials database for lithium ion battery use is of high interest. In this work, we extend the list of candidates for lithium ion batteries based on a 2D material identification theory. More importantly, a usability identification framework leveraging the competitive mechanism between the adsorbability and reversibility of ions on a 2D material is proposed to assist the deeper screening of practicable 2D materials. As a result, 215 2D materials including 158 anodes, 21 cathodes, and 36 solid electrolytes are predicted to be practicable for lithium ion battery use. The comparison between the identified 2D materials with the known ones verifies the reliability of our strategy. This work significantly enriches the choices of 2D materials to satisfy the various battery demands and provides a general methodology to assess the usability of unexploited 2D materials for lithium ion batteries.

No abstract