Nanoparticle fragmentation at solid state under single picosecond laser pulse stimulation

Understanding the laser-nanomaterials interaction including nanomaterial fragmentation has important implications in nanoparticle manufacturing, energy, and biomedical sciences. So far, three mechanisms of laser-induced fragmentation have been recognized including non-thermal processes and thermomechanical force under femtosecond pulses, and the phase transitions under nanosecond pulses. Here we show that single picosecond (ps) laser pulse stimulation leads to anomalous fragmentation of gold nanoparticles that deviates from these three mechanisms. The ps laser fragmentation was weakly dependent on particle size, and it resulted in a bimodal size distribution. Importantly, ps laser stimulation fragmented particles below the melting point and below the threshold for non-thermal mechanism. This study reveals a previously unknown regime of nanoparticle fragmentation.

Analysis of the thermal mechanism and temporal and spatial evolution of the thermal field of deep sandstone under microwaves

In the practice of the deep engineering, it is expected to improve engineering efficiency by introducing the microwave energy. Therefore, based on 1050 m deep sandstone, the heating characteristics of sandstone and its constituent minerals in the microwave field are comprehensively explored through experiments and nu?merical simulations. In the paper, the asynchronism of the temperature rise in different areas of the sandstone depends on the local characteristics of dielectric loss and maximum heat storage capacity. With increase of the temperature, the evaporation of the water leads to the decrease of the dielectric properties, the increase in the constant-pressure heat capacity and the increase in the heat dissipation coefficient, which suppresses the temperature growth trend. The temperature rise of the amplitude of the material is lower than that expected from the microwave power. The maximum temperature of dolomite, feldspar and quartz under the power of 2000 W is 1.86, 1.71, and 1.63 times that of the power of 1000 W, respectively. It is necessary to select the reasonable microwave power to maximize the engineering efficiency. The results are expected to provide the theoretical and technical supports for the electromagnetic heat generation in deep engineering.

Model of Thermal Mechanism of Subclass A1 Fire Extinguishing with General Purpose Fire Extinguishing Powder in Non-Stationary Heat Exchange Conditions

The aim of the paper was to develop a model of thermal extinguishing mechanism using dry chemical powder taking into account the inertia of heat transfer to powder particles during unsteady heat exchange to identify the optimal conditions for extinguishing of A1 class fires with powders.The method of experimental and mathematical modelling of fire extinguishing process using dry chemical powder with short-term effect on the fire was used to achieve the goal. The experimental dependences of the extinguishing time and unit consumption of the extinguishing powder on the intensity of the powder supply to the combustion zone in extinguishing of subclass A1 fire in same area and in a limited volume were obtained. The mathematical model of a thermal extinguishing mechanism using dry chemical powder has been developed, taking into account the inertia of heat transfer to powder particles during unsteady heat exchange.Analysis of the regularities of extinguishing the subclass A1 fire using powder with a short feeding it into the fire indicates the presence of optimum values of unity consumption of powder on the fire from the intensity of feeding it into the fire. The presence of this optimum is due to the inertia of extinguishing the subclass A1 fire using powder due to the finiteness of the heat transfer time to the particles of the extinguishing powder and the limited time of interaction of particles with the combustible material.The theoretical analysis of the fire extinguishing process over the area taking into account the inertia of heat transfer to the powder particles at non-stationary heat exchange are carried out. The results of the analysis are in qualitative agreement with the results of the experimental study of the regularities of extinguishing of model fire foci of subclass A1 with General-purpose fire extinguishing powder.