【作者】 高丽娟；

【导师】 赵良举；

【作者基本信息】 重庆大学 ， 热能工程， 2006， 硕士

【摘要】 超音速冷喷涂是一种新型的表面涂层制备方法,具有诸多优点,得到了国内外学者的广泛关注。然而冷喷涂作为一个新兴技术,其理论尚不完善,喷涂质量和效率尚待提高,尽管其特点适合于纳米粒子喷涂,但尚未在工程中加以实现。为揭示冷喷涂中的气固两相超音速流动激波机理及表面沉积机理,本文作了以下几方面研究:气固两相介质的音速与激波;纳米金Au的熔点分子动力学模拟;纳米粒子表面沉积过程分子动力学模拟。在两相平衡条件下,从定熵音速出发,推导了气固两相介质的音速,分析计算了不同气体与固体粉末两相介质的音速。结果表明对音速影响最大的参数是压力和体积含气率,与普朗特的理论分析吻合,现有的实验结果也验证了本文分析的正确性。建立了基于两相音速的气固两相流激波模型,得到了较基于单相音速模型更为合理的气固两相流激波结果;分析了相间滑移、相间热交换、不同固相粉末和波前参数对激波的影响。结果表明,相间滑移和波前压力对激波有一定影响,对气固激波影响最大的因素是波前马赫数、波前速度、波前音速和体积含气率。采用两相音速模型分析两相流激波,比用单相音速模型所得的结果更合理,适用性更广。采用多体势函数,用分子动力学模拟方法计算模拟了不同原子数( Au 256～Au 32000)的Au纳米团簇,通过计算团簇的径向分布函数分析其熔点随团簇大小的变化情况。结果表明,随着纳米团簇尺寸的减小,其熔点降低。当纳米团簇的原子数小于5000时,本研究得到的熔点与前人研究结果吻合,利用径向分布函数可以较准确得到纳米团簇的熔点;当纳米团簇的原子数大于5000时,本研究得到的熔点高于前人研究结果,当团簇原子数大于9000时,纳米团簇的熔点与大块金的熔点相同;从微观结构看,当团簇的熔化过程发生时,团簇表面的原子排列首先开始紊乱。通过对Au团簇在Au基体上的沉积过程的分子动力学模拟,观察了团簇撞击基体并在基体上沉积的过程以及团簇和基体的形貌变化;观察了基体的温度变化过程;通过计算团簇原子进入基体表面层的数量,初步探讨了影响团簇沉积过程的主要因素。通过计算基体和团簇的径向分布函数,发现团簇在基体上沉积后形成了与基体层类似结构的沉积层。团簇撞击基体的过程中,基体的受撞击影响区域有熔化现象产生,但基体的其余部位并没有发生熔化。入射速度越大或是团簇更多还原

【Abstract】 Supersonic cold gas spray is a new coating process which has many good attributes and it has attracted attentions from scholars in and abroad. But as a newly-developed technology, it is short of perfect theory and the quality and the efficiency of spray should be improved. And what’s more, it has not been carried out in practical project although it can be applied to nanometer-sized particle spray. In order to illustrate the mechanism of supersonic gas-solid two-phase flow and the deposition behavior on metal surface in cold gas spray, this paper has done the following researches: gas-solid two-phase sound velocity and shock wave,Molecular Dynamics Simulation of the melting point of nanometer-sized Au,Molecular Dynamics Simulation of the deposition behavior of nanometer-sized Au clusters on a Au surface.First, considering the balance of the two-phase mixture, we derive the sound velocity of gas-solid two-phase flow from Laplace’s Equation of isentropical sound velocity and analyze the sound velocity of different gas-solid two-phase mixture. The experimental results show that pressure and volume ratio of gas has much influence on sound velocity. This conclusion is consistent with Prandtl’s analysis and agrees well with the experimental results. Second, we present a two-phase shock wave model based on two-phase sound velocity which is more suitable for the two-phase shock wave than single- phase sound velocity model and analyze the effects to shock wave by phase slip, heat transfer between phases, different materials and parameters upstream shock. The results show phase slip, pressure upstream shock have a little effects on shock wave, but Mach number, velocity, sound velocity and gas volume ratio upstream shock are the main working factors.Using many-body potential function, we simulate nanometer-sized Au clusters which have atoms ranging from 256 to 32000 by Molecular Dynamics Simulation and then analyze the melting point change with the change of size of clusters by calculating the radial distribution function. We can see that the melting point decrease along with the decreasing of the size of clusters. when the atoms is below 5000, the melting point we calculate is same as the ones calculated before and the melting point of the nanometer-sized clusters can be calculated by the radial distribution function with precision. When the number of atoms is above 5000, the melting point we calculate is higher than ones calculated before. When the number of atoms is above 9000, the更多还原