High-power pulsed lasers provide an ingenious method for launching metal foils to generate high-speed flyers for high-pressure loading in material science or aerospace engineering.At high-temperature and high-pressure...High-power pulsed lasers provide an ingenious method for launching metal foils to generate high-speed flyers for high-pressure loading in material science or aerospace engineering.At high-temperature and high-pressure laser-induced conditions,the dynamic response of the metals and the mechanism of flyer formation remain unclear.In this study,the overall process of the laser-driven aluminum flyer,including laser ablation,rupture of metal foil,and the generation of the flyer was investigated by molecular dynamics combined with the two-temperature model.It was found that under high laser fluence(over 1.3 J/cm;with 200-fs laser pulse duration),the laser induced a shock wave with a peak pressure higher than25 GPa,which led to shear bands expanding from the edge of the laser ablation zone in the foil.Compared with the cases of low laser fluence less than 0.5 J/cm^(-1),the shear band induced by high laser fluence promotes the rupture of the foil and results in a high-speed flyer(>1 km/s)with better flatness and integrity.In addition,the shock wavefront was found to be accompanied by aluminum crystal phase transformation from face-centered cubic(FCC)to body-centered cubic structure.The crystal structure reverts with the decrease of pressure,therefore the internal structure of the generated flyer is pure of FCC.The results of this study provide a better understanding of the laser-induced shock effect on the foil rupture and flyer quality and forward the development of the laser-driven flyer.展开更多
The thermal expansion coefficient(TEC)and thermal conductivity(k)of thermal fillers are key factors for designing thermal management and thermal protection composite materials.Due to its unique advantages,hexagonal bo...The thermal expansion coefficient(TEC)and thermal conductivity(k)of thermal fillers are key factors for designing thermal management and thermal protection composite materials.Due to its unique advantages,hexagonal boron nitride(h-BN)is one of the most commonly used thermal fillers.However,its TEC and k values are still unclear due to the inconsistency of characterization techniques and sample preparations.In this work,these disputes were addressed using the quasi-harmonic approximation(QHA)method and phonon Boltzmann transport equation(BTE)theory based on the density functional theory(DFT),respectively.The accuracy of our calculated TEC and k values was confirmed by previously reported experimental results,and the underlying physical principles were analyzed from the phonon behaviors.Our TEC results show that the h-BN has small in-plane negative value and large cross-plane positive value,which are-2.4×10^(-6) and 36.4×10^(-6) K^(-1) at 300 K,respectively.And the anisotropic TEC is mainly determined by the anisotropic isothermal bulk modulus and the low-frequency out-of-plane longitudinal phonon modes.We found that the convergence of cutoff radius and q-grid size have significant effect on the accuracy of k of h-BN.Our results show that the in-plane k is much higher than the cross-plane k,and the values at 300 K are 286.6 and 2.7 W m^(-1) K^(-1),respectively.The anisotropic phonon group velocity arising from the vibration behaviors of acoustic phonon modes should be primarily responsible for the anisotropic k.Our calculated TEC and k values will provide important references for the design of h-BN composite materials.展开更多
基金supported by Singapore Ministry of Education Ac RF Tier 2 (MOE2019-T2-2-105), Ac RF Tier 1 RG4/17 and RG161/19。
文摘二维(2D)过渡金属氧化物(TMO)的地球丰度高,并且具有独特的物理化学性质和较好的催化性能,是新能源工业领域中非常有应用前景的电催化剂.然而,由于合成高质量和可控厚度的2D TMO具有一定的难度,目前有关2D TMO的微观电化学研究的报道较少.本文采用化学气相沉积法直接合成了2D钴铁氧体(CoFeO),所制得的2D CoFeO呈现结晶性良好的超薄尖晶石结构,其最薄厚度可达到6.8 nm.采用超微电极测试平台考察了碱性条件下2DCoFeO催化析氧反应(OER)的性能.结果表明,2D CoFeO(111)面在10 m Acm^(-2)的电流密度下表现出330 m V的低过电位,在570 m V的过电位下表现出142 m Acm^(-2)的高电流密度.密度泛函理论计算发现2DCoFeO表面上的双金属位点降低了反应能垒.此外,2DCoFeO的超薄厚度使体电阻率降低,同时增加了活性位点的利用率,进而提高了对OER的催化活性,这与在超微电极平台上测得的2D CoFeO厚度-OER活性依赖关系的结果一致.本研究还合成了大面积的2D CoFeO薄膜,其标准三电极体系研究表明2D CoFeO样品仍然表现出较高的催化OER活性和较好的寿命,说明所制备的2D CoFeO具有较好的实际应用潜力.综上,本文采用气相化学沉积法直接合成了超薄2DCoFeO纳米片,其最薄厚度可达6.8 nm,2DCoFeO表现出良好的OER性能,为2DTMOs电催化剂的可控合成开辟了新途径.此外,本文还分析了2DCoFeO电催化OER反应的机理,为二维电催化剂设计提供了新思路.
基金financial supports from Singapore Ministry of Education Academic Research Fund Tier 2(Grant No.MOE-T2EP10220-0005)Academic Research Fund Tier 1(Grant No.RG104/18)。
基金supported by the National Natural Science Foundation of China(Grant No.11832006)the Opening Fund of State Key Laboratory of Explosion Science and Technology in China(Grant No.KFJJ20-04M)。
文摘High-power pulsed lasers provide an ingenious method for launching metal foils to generate high-speed flyers for high-pressure loading in material science or aerospace engineering.At high-temperature and high-pressure laser-induced conditions,the dynamic response of the metals and the mechanism of flyer formation remain unclear.In this study,the overall process of the laser-driven aluminum flyer,including laser ablation,rupture of metal foil,and the generation of the flyer was investigated by molecular dynamics combined with the two-temperature model.It was found that under high laser fluence(over 1.3 J/cm;with 200-fs laser pulse duration),the laser induced a shock wave with a peak pressure higher than25 GPa,which led to shear bands expanding from the edge of the laser ablation zone in the foil.Compared with the cases of low laser fluence less than 0.5 J/cm^(-1),the shear band induced by high laser fluence promotes the rupture of the foil and results in a high-speed flyer(>1 km/s)with better flatness and integrity.In addition,the shock wavefront was found to be accompanied by aluminum crystal phase transformation from face-centered cubic(FCC)to body-centered cubic structure.The crystal structure reverts with the decrease of pressure,therefore the internal structure of the generated flyer is pure of FCC.The results of this study provide a better understanding of the laser-induced shock effect on the foil rupture and flyer quality and forward the development of the laser-driven flyer.
基金the National Natural Science Foundation of China(51621091,51225203,and 51672060)the National Key Research and Development Program of China(2017YFB0310400)。
文摘The thermal expansion coefficient(TEC)and thermal conductivity(k)of thermal fillers are key factors for designing thermal management and thermal protection composite materials.Due to its unique advantages,hexagonal boron nitride(h-BN)is one of the most commonly used thermal fillers.However,its TEC and k values are still unclear due to the inconsistency of characterization techniques and sample preparations.In this work,these disputes were addressed using the quasi-harmonic approximation(QHA)method and phonon Boltzmann transport equation(BTE)theory based on the density functional theory(DFT),respectively.The accuracy of our calculated TEC and k values was confirmed by previously reported experimental results,and the underlying physical principles were analyzed from the phonon behaviors.Our TEC results show that the h-BN has small in-plane negative value and large cross-plane positive value,which are-2.4×10^(-6) and 36.4×10^(-6) K^(-1) at 300 K,respectively.And the anisotropic TEC is mainly determined by the anisotropic isothermal bulk modulus and the low-frequency out-of-plane longitudinal phonon modes.We found that the convergence of cutoff radius and q-grid size have significant effect on the accuracy of k of h-BN.Our results show that the in-plane k is much higher than the cross-plane k,and the values at 300 K are 286.6 and 2.7 W m^(-1) K^(-1),respectively.The anisotropic phonon group velocity arising from the vibration behaviors of acoustic phonon modes should be primarily responsible for the anisotropic k.Our calculated TEC and k values will provide important references for the design of h-BN composite materials.
