Improved Thermoelectric Performance of Monolayer HfS2 by Strain Engineering.

Strain engineering can effectively improve the energy band degeneracy of two-dimensional transition metal dichalcogenides so that they exhibit good thermoelectric properties under strain. In this work, we have studied the phonon, electronic, thermal, and thermoelectric properties of 1T-phase monolayer HfS2 with biaxial strain based on first-principles calculations combined with Boltzmann equations. At 0% strain, the results show that the lattice thermal conductivity of monolayer HfS2 is 5.01 W m-1 K-1 and the electronic thermal conductivities of n-type and p-type doped monolayer HfS2 are 2.94 and 0.39 W m-1 K-1, respectively, when the doping concentration is around 5 × 1012 cm-2. The power factors of the n-type and p-type doped monolayer HfS2 are different, 29.4 and 1.6 mW mK-2, respectively. Finally, the maximum ZT value of the n-type monolayer HfS2 is 1.09, which is higher than 0.09 of the p-type monolayer HfS2. Under biaxial strain, for n-type HfS2, the lattice thermal conductivity, the electronic thermal conductivity, and the power factor are 1.55 W m-1 K-1, 1.44 W m-1 K-1, and 22.9 mW mK-2 at 6% strain, respectively. Based on the above factor, the ZT value reaches its maximum of 2.29 at 6% strain. For p-type HfS2, the lattice thermal conductivity and the electronic thermal conductivity are 1.12 and 1.53 W m-1 K-1 at 7% strain, respectively. Moreover, the power factor is greatly improved to 29.5 mW mK-2. Finally, the maximum ZT value of the p-type monolayer HfS2 is 3.35 at 7% strain. It is obvious that strain can greatly improve the thermoelectric performance of monolayer HfS2, especially for p-type HfS2. We hope that the research results can provide data references for future experimental exploration.

Theoretical study for anisotropic responses of the condensed-phase RDX under shock loadings.

We have performed quantum-based molecular dynamics (MD) simulations in conjunction with multiscale shock technique (MSST) to investigate the initial chemical processes and the anisotropy of shock sensitivity of the RDX under shock loading applied along the different directions. The results show that there is a difference between x (or y)-direction and z-direction in the response to a shock wave velocity of 12 km/s. It was shown that detonation temperature and pressure in the z-direction lags behind that of x-direction (or y-direction). Moreover, from the time evolution of the population of various key fragments, we also observe that along with z-direction significantly later than that of x (or y)-direction, which the reaction rate is also slower. Thus, we draw a conclusion that sensitive for shock propagation along x or y-direction, but less sensitive for shock propagation along z-direction.

Shock response of condensed-phase RDX: molecular dynamics simulations in conjunction with the MSST method.

We have performed molecular dynamics simulations in conjunction with the multiscale shock technique (MSST) to study the initial chemical processes of condensed-phase RDX under various shock velocities (8 km s-1, 10 km s-1 and 11 km s-1). A self-consistent charge density functional tight-binding (SCC-DFTB) method was used. We find that the N-NO2 bond dissociation is the primary pathway for RDX with the NO2 groups facing (group 1) the shock, whereas the C-N bond scission is the dominant primary channel for RDX with the NO2 groups facing away from (group 2) the shock. In addition, our results present that the NO2 groups facing away from the shock are rather inert to shock loading. Moreover, the reaction pathways of a single RDX molecule under the 11 km s-1 shock velocity have been mapped out in detail, NO2, NO, N2O, CO and N2 were the main products.

Effect of Mn doping on electroforming and threshold voltages of bipolar resistive switching in Al/Mn : NiO/ITO.

We investigated the bipolar resistive switching (BRS) properties of Mn-doped NiO thin films by sol-gel spin-coating. As the Mn doping concentration increased, lattice constant, grain size and band gap were found to decrease simultaneously. Moreover, the electroforming voltages and threshold voltages were gradually reduced. It can be ascribed to the increase in the density of grain boundaries, and the defects caused by doping Mn and lower formation energy of Mn-O. They would be helpful for the formation of oxygen vacancies and conductive filaments. It is worth mentioning that excellent BRS behaviors can be obtained at a low Mn-doped concentration including enlarged ON/OFF ratio, good uniformity and stability. Compared with other samples, the 1% Mn-doped NiO showed the highest ON/OFF ratio (>106), stable endurance of >100 cycles and a retention time of >104 s. The mechanism should be determined by bulk properties rather than the dual-oxygen reservoir structure. These results indicate that appropriate Mn doping can be applied to improve the BRS characteristics of NiO thin films, and provide stable, low-power-consumption memory devices.

Lattice Dynamics Study of Phonon Instability and Thermal Properties of Type-I Clathrate K8Si46 under High Pressure.

For a further understanding of the phase transitions mechanism in type-I silicon clathrates K8Si46, ab initio self-consistent electronic calculations combined with linear-response method have been performed to investigate the vibrational properties of alkali metal K atoms encapsulated type-I silicon-clathrate under pressure within the framework of density functional perturbation theory. Our lattice dynamics simulation results showed that the pressure induced phase transition of K8Si46 was believed to be driven by the phonon instability of the calthrate lattice. Analysis of the evolution of the partial phonon density of state with pressure, a legible dynamic picture for both guest K atoms and host lattice, was given. In addition, based on phonon calculations and combined with quasi-harmonic approximation, the specific heat of K8Si46 was derived, which agreed very well with experimental results. Also, other important thermal properties including the thermal expansion coefficients and Grüneisen parameters of K8Si46 under different temperature and pressure were also predicted.

Pressure-induced metallization of condensed phase ß-HMX under shock loadings via molecular dynamics simulations in conjunction with multi-scale shock technique.

The electronic structure and initial decomposition in high explosive HMX under conditions of shock loading are examined. The simulation is performed using quantum molecular dynamics in conjunction with multi-scale shock technique (MSST). A self-consistent charge density-functional tight-binding (SCC-DFTB) method is adapted. The results show that the N-N-C angle has a drastic change under shock wave compression along lattice vector b at shock velocity 11 km/s, which is the main reason that leads to an insulator-to-metal transition for the HMX system. The metallization pressure (about 130 GPa) of condensed-phase HMX is predicted firstly. We also detect the formation of several key products of condensed-phase HMX decomposition, such as NO2, NO, N2, N2O, H2O, CO, and CO2, and all of them have been observed in previous experimental studies. Moreover, the initial decomposition products include H2 due to the C-H bond breaking as a primary reaction pathway at extreme condition, which presents a new insight into the initial decomposition mechanism of HMX under shock loading at the atomistic level.

Anisotropic responses and initial decomposition of condensed-phase ß-HMX under shock loadings via molecular dynamics simulations in conjunction with multiscale shock technique.

Molecular dynamics simulations in conjunction with multiscale shock technique (MSST) are performed to study the initial chemical processes and the anisotropy of shock sensitivity of the condensed-phase HMX under shock loadings applied along the a, b, and c lattice vectors. A self-consistent charge density-functional tight-binding (SCC-DFTB) method was employed. Our results show that there is a difference between lattice vector a (or c) and lattice vector b in the response to a shock wave velocity of 11 km/s, which is investigated through reaction temperature and relative sliding rate between adjacent slipping planes. The response along lattice vectors a and c are similar to each other, whose reaction temperature is up to 7000 K, but quite different along lattice vector b, whose reaction temperature is only up to 4000 K. When compared with shock wave propagation along the lattice vectors a (18 Å/ps) and c (21 Å/ps), the relative sliding rate between adjacent slipping planes along lattice vector b is only 0.2 Å/ps. Thus, the small relative sliding rate between adjacent slipping planes results in the temperature and energy under shock loading increasing at a slower rate, which is the main reason leading to less sensitivity under shock wave compression along lattice vector b. In addition, the C-H bond dissociation is the primary pathway for HMX decomposition in early stages under high shock loading from various directions. Compared with the observation for shock velocities V(imp) = 10 and 11 km/s, the homolytic cleavage of N-NO2 bond was obviously suppressed with increasing pressure.

Initial decomposition of the condensed-phase ß-HMX under shock waves: molecular dynamics simulations.

We have performed quantum-based multiscale simulations to study the initial chemical processes of condensed-phase octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) under shock wave loading. A self-consistent charge density-functional tight-binding (SCC-DFTB) method was employed. The results show that the initial decomposition of shocked HMX is triggered by the N-NO(2) bond breaking under the low velocity impact (8 km/s). As the shock velocity increases (11 km/s), the homolytic cleavage of the N-NO(2) bond is suppressed under high pressure, the C-H bond dissociation becomes the primary pathway for HMX decomposition in its early stages. It is accompanied by a five-membered ring formation and hydrogen transfer from the CH(2) group to the -NO(2) group. Our simulations suggest that the initial chemical processes of shocked HMX are dependent on the impact velocity, which gain new insights into the initial decomposition mechanism of HMX upon shock loading at the atomistic level, and have important implications for understanding and development of energetic materials.