# A Brief Review on the Recent Experimental Advances in Thermal Rectification at the Nanoscale
## 2 Experimental Study
### Thermal Rectification at the Binaterial Nanocontact Interface  
[PERSON] et al. studied the interface-induced thermal rectification in nanowires through experiments and molecular dynamics simulations [25]. They built an organic/inorganic system consisting of silicon (Si) nanowires and polyamide (PA) nanofibers. The thermal conductivity was measured by a high heat flux resolution test platform [26]. Figure 1 shows the change of the thermal rectification coefficient with respect to the heat flux at room temperature. Except for the first point in the figure, the thermal rectification coefficient equates to approximately 3%. The experimental results show that there is no significant correlation between the heat flux and the thermal rectification coefficient, which they believe is due to the low heat flux used in the experiment. However, molecular dynamics simulations show that high heat flux significantly affects the thermal rectification coefficient, but high heat flux could not be used in the experiment in order to avoid damage to the test samples. Molecular dynamics simulations were used to analyze the physical mechanisms of the thermal rectification effects, performed by the LAMMPS package [27]. The simulation results show that the heat flux of Si-to-PA is always larger than that of PA-to-Si, which is consistent with the experimental observations. The result indicates that a strong edge effect exists in the nanowires, which reduces the amplitude of the phonon spectrum and moves the peak to a lower frequency region. It is a sign that the phonon localization occurs at the interface between PA and Si nanowires, resulting in a decrease of thermal conductivity. The thermal rectification effect at the PA/Si nanowire interface could be understood as the different degrees of phonon localization in different heat flux directions. In the process of using phonon spatial distribution to describe the degree of phonon localization, it was found that phonon localization only occurs at the interface. Moreover, in the heat flux direction from Si to PA, the phonon localization at the interface is smaller than that from PA to Si. The internal phonons have no obvious difference in two opposite directions.  
In order to further confirm the origin of this interface-induced thermal rectification effect, another kind of sample composed of PA and Si nanowires was experimentally tested. The contact area of the second kind of sample was much larger than that of the first kind, which means that the contact heat resistance could be smaller. However, the experimental results show that the thermal rectification coefficient was almost unchanged for these two samples, so there is no obvious correlation between the thermal rectification and contact area (Figure 2).  
Figure 1: Thermal rectification coefficient as a function of heat flux in the Si/PA nanowire [25].  
It needs to be mentioned that the mechanism of thermal rectification at the nanocontact interface is different from the conventional model of bulk materials. In bulks, one important reason for the thermal rectification effect is the asymmetric temperature dependence of thermal conductivity. In the case of the Si-to-PA system, the thermal conductivity of polymers increases with increasing temperature, while the temperature dependence of thermal conductivity is opposite for Si. This indicates that the PA-to-Si heat flux direction has larger thermal conductivity, where PA is in the high temperature region and Si is in the low temperature region. On the other hand, the thermal rectification in the PA/Si nanowire structure is caused by the asymmetric temperature dependence of thermal resistance at the nanocontact interface.