Effect of ambient air pressure on synthesis of copper and copper oxide nanoparticles by wire explosion process

Nanoparticles of copper and copper oxide were synthesized by wire explosion technique in the environment of different ambient air pressure. Copper wire with a diameter of 125 μm and 6.1 cm in length has been exploded in air at 1 bar, 500 mbar, 100 mbar and 50 mbar. Current density in the order of 10⁶ A cm⁻² has been applied to disintegrate the wire by discharging a 1.85 μF capacitor at 10 kV. It is found that the particle size distribution for nano-sized particles follows the log-normal distribution and the median diameter is found to decrease from 31.3 nm to 23.6 nm as the pressure is decreased from 1 bar to 50 mbar.     

Influence of ambient gas on formation process of Si nanoparticles by laser ablation

We described the influence of a type of gas and its pressure upon the size distribution of Si nanoparticles fabricated by laser ablation in an ambient gas and the plume dynamics during the synthesis. The plume dynamics was investigated by laser-induced fluorescence and ultraviolet Rayleigh scattering. Based on the results, the importance of the gas flow within the ablation plume in the formation of the nanoparticles is understood.     

介质环境对铜丝电爆炸制备纳米粉体的影响

为了探究介质环境对电爆炸制备纳米粉体的影响, 搭建了相应的电爆炸实验平台, 以铜丝为例分别在水和不同空气压力下开展了电爆炸制备纳米粉体实验.通过Rogoswki线圈和高压探头分别测试了电爆炸过程中的电流和电压波形图.通过电压、电流及能量沉积特征分析了电爆炸的基本过程以及介质环境在电爆炸过程中的作用.运用透射电子显微镜对爆炸产物进行了粒度分析.研究发现, 介质环境对于电爆炸过程的影响主要表现在汽化化阶段以后, 包括介质对蒸汽膨胀的抑制作用, 介质的电离对于铜丝表面击穿的影响以及其对高温金属蒸汽及等离子体的冷却作用.水中铜丝电爆炸能够制备局部均匀的小尺寸纳米粉体, 粒度多数集中在10–20 nm之间, 但粉体易积聚, 且整体粒度跨越较大.空气中制备的粉体分散良好, 符合对数正态分布, 基本上分布于20–100 nm之间, 平均粒度约为40 nm.     

Understanding the mechanism of nano-aluminum particle formation by wire explosion process using optical emission technique

Nano-aluminum particles are produced by a wire explosion process in different inert gas ambience. It is observed that generated particles have different sizes and it follows log–normal probability distribution. The particle size produced by the wire explosion process varies depending on the thermal conductivity of the medium and the operating pressure of the gas. To understand the mechanism of nano-particle formation, the optical-emission spectroscopic technique is used for measuring characteristics of plasma generated during the wire explosion process. Strong emission lines were observed from the species formed during the wire explosion process. Plasma temperatures are estimated based on local thermal equilibrium principle and using Al emission lines. Plasma temperature of more than 8000 K is observed in an Ar ambient. The optical emission study clearly indicates that the intensity of plasma increases with an increase in the ambient pressure. Further, it is observed that an increase in the pressure of the gas, the plasma temperature also increases. The study shows that the plasma temperature in the He gas is lesser than in the Ar gas. The plasma temperature due to the discharge plays a significant role on nano-particle formation. In addition, it is observed that irrespective of polarity, emission characteristics are almost the same.     

Effect of gas flows on fullerene formation process

The effect of helium flow on the fullerene formation process is studied. It has been shown that an annular flow of helium directed into the fullerene formation zone along the discharge axis reduces the content of fullerenes in soot. If the velocity of the additional flow of helium pumped through a hole in the cathode becomes close to that of the gas-plasma jet, practically no fullerenes are formed.     

丝电爆过程的电流导入机理

丝电爆制备纳米粉时, 电流从电极导入金属丝的过程直接影响电极烧损和粉末中微米级大颗粒产生. 分别通过接触和气体放电两种方式导入电流进行电爆试验. 结果表明, 光测量装置检测到的丝端部光电流几乎与回路放电电流同时产生, 而中间位置的光电流则要滞后一段时间; 由探针收集的产物确定, 金属丝端部主要形成熔融粒子, 中间部分主要形成气相粒子. 分析可知, 接触方式导入电流时, 丝端部也存在气体放电现象, 大电流主要通过气体放电形成的等离子体导入. 等离子体对电流的旁路作用会阻碍能量向金属丝沉积, 这是产生微米级大颗粒和"积瘤"主要原因. 通过气体放电方式导入电流时, 电极烧损明显减轻, 并可以避免"积瘤"产生.     

铝丝电爆炸过程的光学诊断

采用100 m和40 m两种直径的铝丝,在不同放电电压下,通过分幅成像技术和光谱诊断方法,对铝丝电爆炸过程放电特性及放电等离子参数进行了诊断。实验研究表明:铝丝电爆炸过程中金属蒸气的二次击穿分为内部击穿和沿面击穿两种类型,较细的铝丝更容易发生内部击穿,发生内部击穿时产生的等离子体具有更好的空间均匀性和对称性,其放电过程具有更高的稳定性和可重复性。通过光谱诊断可知铝丝电爆炸等离子体电子温度在104 K量级,电子密度在1018 cm-3量级。     

铝丝电爆炸过程的光学诊断

采用100 m和40 m两种直径的铝丝,在不同放电电压下,通过分幅成像技术和光谱诊断方法,对铝丝电爆炸过程放电特性及放电等离子参数进行了诊断。实验研究表明:铝丝电爆炸过程中金属蒸气的二次击穿分为内部击穿和沿面击穿两种类型,较细的铝丝更容易发生内部击穿,发生内部击穿时产生的等离子体具有更好的空间均匀性和对称性,其放电过程具有更高的稳定性和可重复性。通过光谱诊断可知铝丝电爆炸等离子体电子温度在104 K量级,电子密度在1018 cm-3量级。     

Effect of gas pressure on ionization of ambient gas

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高压击穿铜丝物相研究

采用高压放电的方式对材料进行击穿,可以方便地制造纳米颗粒．搭建了高压击穿实验装置,对铜丝进行高压击穿实验;分别采用透射电子显微镜（TEM）、扫描电子显微镜（SEM）和元素能谱（EDS）、X射线衍射（XRD）测试,对铜丝击穿丝状物进行了形貌和成份分析．研究了铜丝高压击穿后的物相特性．研究结果发现,在高压作用下铜丝被充分电离,产生丝状分布,其构成为纳米颗粒的凝结;纳米颗粒的直径分布主要集中在30—60nm之间;颗粒产物由铜元素和氧元素组成;它们以单晶Cu,Cu2O和CuO组成混合物;粒径大小、产物成分与铜丝长度、直径及电压等因素相关．     

Synthesis of Cu nanoparticles by condensation from the gas phase

A series of computer experiments was conducted by the raw material evaporation and subsequent condensation to determine the most efficient regimes of copper nanoparticles synthesis. By variation of the cooling rate of the metal vapor the formation of Cu clusters were studied. The computer experiments showed the influence of different final temperatures on the shape of the resultant particles. This allowed to determine the conditions for a predominant formation of worm-like clusters of a spherical shape in the gas phase synthesis.     

氩气中铝丝电爆炸沉积能量对制备铝纳米粉体特性的影响

研制了基于脉冲电容器放电回路的亚微秒金属丝电爆炸纳米粉体制备实验平台,包括电爆炸过程电流和电压测量系统。利用透射电子显微镜(TEM)观察纳米粉体的形态与结构,并通过电镜统计观察法分析TEM图像得到纳米粉体的粒度大小及其分布。在氩气中电爆炸铝丝制备铝纳米粉体,通过改变电容器充电电压,即初始储能,实验研究沉积能量对铝纳米粉体特性的影响规律。结果表明：铝纳米粉体颗粒形态与结构主要由氩气气压的高低决定,与沉积能量基本无关。增大丝爆过程的沉积能量可显著缩小铝纳米粉体粒度分布范围,减小颗粒平均粒径,并有效地抑制纳米粉体中亚微米颗粒的形成。随着沉积能量E与氩气气压p比值(Ep-1)增大,铝纳米粉体颗粒平均粒径、最大粒径和粒径大于100 nm颗粒所占比例均呈指数函数单调减小。     

Homogeneous electrical explosion of tungsten wire in vacuum

Experimental results on Joule energy deposition upon initiation of a fast electrical explosion of 16-μm tungsten wire in vacuum at current densities of more than 10⁸ A/cm² are reported. We have found that explosion with a fast current rise time (～170 A/ns into a short) results in homogeneous and enhanced deposition of electrical energy into the tungsten before surface flashover. The maximum tungsten wire resistivity reaches a value of up to ～185 μΩ cm before surface flashover that significantly exceeds the melting boundary and corresponds to a temperature of ～1 eV. The highest values for light radiation and expansion velocity of wire ～1 km/s were observed for the fast explosion. For the explosion mode with a slower current rise time (～22 A/ns into a short), we observed the existence of an “energy deposition barrier” for tungsten wire. In the slow explosion mode, the current is reconnected to the surface shunting discharge before melting. The maximum tungsten wire resistivity in this case reaches the value of ～120 μΩ cm, which is less than indicative of melting. Also, the energy deposition along the wire is strongly inhomogeneous, and wire is disintegrated into parts. We attribute the early reconnection of the current to the surface discharge for the slow explosion to high electron emission from the wire surface, which starts before melting.     

真空中铝单丝电爆炸的实验研究

开展了铝单丝在负极性电流脉冲作用下电爆炸特性的研究.利用皮秒激光探针,搭建了阴影、纹影和干涉的光学诊断平台,得到了不镀膜铝丝典型的能量沉积过程,在电压崩溃时刻其沉积能量为2.4 eV/atom.为了增加金属丝内的沉积能量,开展了相同电参数及金属丝尺寸下的镀膜铝丝电爆炸实验,其沉积能量可达到5 eV/atom,实现了在电压崩溃之前铝丝完全气化(完全气化所需能量为4 eV/atom).阴影图像展示了高密度丝核区域的膨胀过程,不镀膜铝丝平均膨胀速度为2.2 km/s,而镀膜铝丝因为沉积能量大,其膨胀速度约为不镀膜铝丝的2.3倍,高密度区域膨胀速度为5 km/s.由于阴影不能反映低密度等离子体的膨胀,开展了平行双丝实验,通过测量自发光辐射,估算了低密度等离子体的膨胀速度.利用条纹相机拍摄了不镀膜铝丝电爆炸过程中自发光区域的图像.纹影图像清晰地展示了不镀膜铝丝在电爆炸过程中形成的核冕结构,而镀膜铝丝电爆炸过程中核冕结构得到了一定程度的抑制.从干涉图像计算了相移,在轴对称假设下对相移进行阿贝尔逆变换,重构了三维的铝原子数密度分布.     

Effects of target-substrate geometry and ambient gas pressure on FePt nanoparticles synthesized by pulsed laser deposition

FePt nanoparticles, in the forms of nanoparticle agglomerates and floccules-like nanoparticle networks, can be synthesized by pulsed laser deposition (PLD) at different ambient gas pressures. Backward plume deposition (BPD), as special target-substrate geometry, can achieve higher uniformity in terms of agglomerate size and size distribution, compared to conventional PLD. Both as-deposited FePt nanoparticles exhibit low K_u fcc phase and post-annealing at 600 °C is required for the phase transition to high K_u fct phase. FePt nanoparticle agglomerates deposited by BPD were found to have better fct phase crystallinity after annealing, which may be caused by the higher kinetic energy of backward moving ablated species due to shorter travel distance.     

Corona-free electrical explosion of polyimide-coated tungsten wire in vacuum

We present experimental evidence of corona-free electrical explosion of dielectric-coated W wire in vacuum. A fast current rise of approximately 150 A/ns and a coating of 2 microm polyimide are both needed to achieve the corona-free regime of explosion. Breakdown is absent in corona-free explosion; the wire remains resistive, and this allows anomalously high energy deposition (approximately 20 times atomization enthalpy). MHD simulations reproduce the main differences between corona and corona-free explosions. A corona-free explosion of a wire can be useful for the generation of a hot plasma column by direct energy deposition.     

Effect of titanium dioxide nanoparticles on polymer network formation

This work presents a study of the effect of nanoparticles on polymer composites to develop a powerful polymer dispersed liquid crystal materials. Tri propylene glycol diarcrylate/titanium dioxide nanocomposites were produced at various titanium dioxide fractions ranging between 0 and 1?wt%, through ultraviolet curing technique during 30?min. Different technics such as polarized optical microscopy, Fourier transform infrared spectroscopy, differential scanning calorimetry, and thermal gravimetric analysis were used to characterize the samples. A dynamic swelling of polymer network was also investigated. The evolution of the acrylic double-bond conversion shows a decrease in the absorption intensity at 1637?cm^?1. It is noted that the conversion rate decreases from 0.1 to 1?wt% of titanium dioxide nanoparticles. An increase in glass transition temperature is observed by differential scanning calorimetry. The thermogravimetric analysis results reveal a highly improved thermal stability upon the addition of the reinforcing phase. The follow of kinetics swelling of polymer network shows a decrease of the swell ratio with the inclusion of titanium dioxide nanoparticles.     

并联金属丝提高电爆炸丝沉积能量的数值模拟

对于低气压或真空环境中的电爆炸丝,因丝沿面击穿会过早终止能量沉积过程,使丝中沉积能量(Ed)大大低于金属丝完全汽化时所需能量(Es).本文提出并联金属丝法延缓沿面击穿时刻以提高电爆炸丝沉积能量.对电流上升时间为几十纳秒、幅值约为1 kA级作用下的金属丝电爆炸过程进行了数值模拟.结果表明,在电爆炸丝两端并联一定尺寸的金属丝可降低爆炸丝端电压上升率,从而推迟电压上升过程中沿面击穿时刻,显著提高丝中沉积能量和过热系数.