含AgI人工冰核粒子的电镜分析

Particle Sizing Analysis on AgI Pyrotechnics with the Electron Microscope

为深入认识当前人工影响天气作业中广泛使用的AgI焰剂的成冰特性,利用电子显微镜对含AgI焰剂产生的人工冰核粒子尺度特征进行分析研究。利用环境场扫描电镜对焰剂颗粒的尺度分布和形态学特征进行研究,利用场发射高分辨透射电镜纳米区域的X射线成分分析(EDS)对实验样品的颗粒结构特点和主要组成成分进行研究。实验结果表明:不同配方焰剂燃烧产生的颗粒谱分布特征有明显差异,所取5种焰剂产生的颗粒平均谱分布,其直径在0.02~0.50μm之间的粒子数占98.96%,即产生粒子绝大部分都可直接参加云内的成冰核化过程,但其谱宽、峰值直径,分布特征都不相同。透射电镜结果表明:焰剂颗粒的主要组成是KCl,其表面附着AgI小颗粒,该结构特征可能更有利于焰剂颗粒的成冰核化。利用中国气象科学研究院1 m3等温冷云室对AgI焰剂阈温对比实验表明:5种焰剂的成冰阈温在-3.5^-4.4℃范围内,不同焰剂配方的阈温不同,最大相差1℃。焰剂成冰核化速率主要由颗粒的大小(均立方根直径)决定,同时受到谱宽、主峰位置等多种分布特征量影响,改进配方时应综合考虑。同时,由于高于-4.4℃时,焰剂产生颗粒接触过冷水滴缺少活性,即含AgI焰剂不适于云中较暖区的催化。

Pyrotechnics flares, containing silver iodide（AgI） as active ice-nuclei （IN）, are widely used for seeding in hail suppression and precipitation enhancement operations over China nowadays. The idea and procedure for developing newly high efficient AgI pyrotechnics widely used in weather modification in China are given. The different nucleation mechanisms are strongly depended on the size of the nuclei aerosols. Furthermore, it must be known how long it is possible to work with one and the same aerosol before coagulation has seriously altered the spectrum. Five formulations of pyrotechnics with high nucleating effectiveness, containing 1.5%-5% by weight of AgI, including the flare is made according to the given formulation of silver spare （USSR）, are examined their particle size with the environmental scanning electron microscope FEI Quanta 200 （ESEM） and transmission electron microscope（TEM）. At the same time, their nucleating effectiveness and temperature thresholds are tested. The characteristics of five kinds of AgI-type pyrotechnics are investigated and compared with other widely used product-silverspare, or A2 USSR. From the electron microscopic analyses, the size distributions of the particles produced from all five types of flares are found to be similar. For each of pyrotechnics, more than 98 % of AgI effluent concentration is contained in particles less than 0.5μm in diameter, and more than 90 % is contained in particles less than 0.3μm. Because of the particle sizes, a diffusive contact and condensation-freezing mechanism is probably playing an important role in the nucleation of water droplets. Different kinds of AgI-type pyrotechnic has differ size distribution. Based on TEM micrographs or five kinds of AgI-type pyrotechnics particles, several less AgI particles adhered on a KCl particle and the structure characteristics is maybe the cause that resulted in a high nucleating effectiveness. The AgI contents and temperature thresholds of five kinds of pyrotechnic nucleate and Ice crystal formation kinetics for AgI aerosols in the isothermal cloud chamber at given temperature are studied in the research. For five kinds of AgI agents, their thresholds temperature change in the range of - 4.4- - 3.5 ℃. Their differences are less, in other words, the temperature thresholds of AgI pyrotechnics are mainly decided by AgI itself. The interaction of cloud, CCN and AgI artificial ice nuclei （AAIN） is important to the formation of cloud, precipitation and weather modification. The size distribution of AgI effluent from the five flares appears to be similar based on the carbon membrane samples that are collected static subsidence and analyzed with an ESEM. The deactivation of AgI smoke in contact with liquid water above - 4.4℃ is considered, it therefore should not be introduced for cloud-base seeding at warm temperatures. The chemical composition of the pyrotechnic is another important factor that directly affects the effectives and nucleating mechanism of the AgI pyrotechnics. In the future research, the experimental study on the chemical composition of flares at the main mode is helpful to interpret the nucleating effectiveness differences in the flares. The interrelation of them is complex and further study is needed.