【作者】 解文军；

【导师】 魏炳波；

【作者基本信息】 西北工业大学 ， 材料物理与化学， 2002， 博士

【摘要】 声悬浮是一种重要的无容器处理实验技术，可用于材料的无容器凝固、液体物理性质的非接触测量以及自由液滴动力学研究。其优点是不受材料电磁学性质的限制，并且可以处理低熔点金属材料。本文针对当前单轴式声悬浮能力较弱，而且主要应用于低密度材料无容器处理的研究现状，以提高声悬浮能力、实现低熔点高密度材料的声悬浮无容器凝固为主线，采用理论和实验相结合的方法，对单轴式声悬浮过程及其主要影响因素进行了较为系统、深入的研究，主要取得以下几方面的研究结果。 建立了单轴式声悬浮的优化设计理论模型，采用边界元方法求解入射声场，在表征声悬浮性能的时间平均势U、声辐射力F_i和回复力常数κ_i的表达式中分别引入了一个涉及悬浮器几何参数的因子。该模型揭示了声悬浮性能与声悬浮器几何参数以及声源强度之间的相关规律。它可以成功地预测声场的谐振模式，并解释了靠近反射端和发射端的样品偏离对称轴的实验现象。 针对球面、旋转抛物面和旋转双曲面三种形状的凹面反射端，系统研究了悬浮力与反射端截面半径R_b和曲率半径R(或深度D)之间的本征关系。发现通过选择最优化的R或D值，可以显著提高声悬浮力，并且谐振模式数越小，反射端截面半径越大，提高的程度就越大。在这三类反射端中，采用球面反射端通过上述方式提高悬浮力，可以获得最好的效果。靠近反射端的样品在R_b较小、D较大以及模式较高的情况下更容易偏离对称轴。对计算所得到的悬浮力和R、R_b以及模式数之间的关系进行了实验验证，计算结果和实验结果吻合得相当好。 对自行研制的单轴式声悬浮器进行了优化设计，极大地提高了悬浮力和悬浮稳定性，首次悬浮起自然界中密度最大的固体铱(I-r，密度22.6g／cm~3)和液体汞(Hg，密度13.6g／cm~3)。这再次证实了本文所提出的单轴式声悬浮参数优化理论，同时说明，声悬浮原则上可以在地面条件下悬浮起任何固体和液体。 对材料声悬浮无容器处理过程中的主要影响因素如气体媒质、重力水平、声场调谐以及温度变化等进行了研究。发现不同媒质中产生的最大悬浮力不仅取决于媒质密度与声速之比，还取决于该媒质的波长与悬浮器几何参数的相对大小。重力水平的提高会使悬浮势阱变浅甚至消失。声场调谐过程中，声压分布、 摘要悬浮力和样品位置等在谐振状态两侧基本呈对称性变化。悬浮样品所允许的反射端一发射端间距调节范围只有0.005兄量级。在准静态加热和冷却过程中，谐振条件、悬浮力以及声压闲值p。(最小俘获声压)和pM(保持液滴不雾化的最大声压)随温度变化发生很大的变化。第一谐振间距Hl的温度依赖性主要来自于波长与温度的关系，即正比于T’/2。由于对几何参数与波长比值的敏感性，最大悬浮力瓜随温度上升而迅速下降。pm和pM随温度的升高分别增大和减小，从而缩小了在高温区稳定安全地悬浮液态样品的声压范围。 研究了液滴在声悬浮状态下的运动规律。提出了实现较大液滴声悬浮的声场条件，即将反射端一发射端间距调节在略大于谐振间距的状态。采用数值模拟方法研究了水滴在声悬浮状态下的变形和振荡，发现随着声压级的提高，水滴被压扁，并且上下表面的中心部位由凸面变成凹面，与实验观察一致。由于发生了较大变形，水滴的表面振荡主频率略高于Rayleigh频率。重力场对振荡频率的影响不明显。在正常重力条件下，水滴在竖直方向的平移振动频率的模拟值与解析值定量吻合。 发展了一种保持较高声源强度，适当弱化谐振条件，通过调节反射端一发射端间距来补偿温度改变引起的声场失谐和维持样品的稳定悬浮，用于低熔点高密度材料的声悬浮无容器熔化与凝固的实验方法。该方法在水和Pb一Sn共晶合金的声悬浮无容器处理实验中证明是非常成功的。 采用单轴式声悬浮方法在氢气气氛下完成了Pb一sn共品合金(密度8.59/cm，)的无容器熔化和凝固，获得了38K(O.o83几)的过冷度。随着过冷度的增大，发生了“层片共晶。碎断层片。两相枝晶”的组织转变。这一组织演化规律可以用共晶共生区和超声场的作用来解释。发现规则共晶层片的碎断和(Pb)、(Sn)枝晶宏观重力偏析的抑制主要是由悬浮液滴内部的复杂流动引起的。液相流动起源于悬浮液滴的形态振荡、总体振动以及旋转运动。同时发现，超声场可以激发液滴表面的受迫振动，进而在液滴表面激发毛细波并诱发表面形核。更多还原

【Abstract】 Acoustic levitation (AL) is an important technique for containerless processing, which can be applied to containerless solidification of materials, contactless measurement of physical properties of liquids, and experimental investigation of free drops. The advantage of AL is its applicability to non-conducting substances and also to low-melting-point metallic materials. Due to the fact that the levitation capability of single-axis acoustic levitation (SAAL) is comparatively weak and its applications are mainly limited to low-density substances, this paper aims at enhancing the levitation capability of SAAL and realizing the containerless solidification of those materials with low melting temperature and high density.A two-cylinder model for SAAL incorporating BEM simulations is proposed, which introduces a factor concerning the geometric parameters of the levitator into the expressions for the time-averaged potential U, acoustic radiation force F, and restoring force constant Ki, respectively, and builds up the relationship between the levitation capabilities and the geometric parameters of a single-axis acoustic levitator. This model proves to be successful in predicting resonant modes of acoustic field and explaining the axially symmetric deviation of the levitated samples near the reflector and emitter.Concave reflecting surfaces of spherical cap, paraboloid and hyperboloid of revolution are investigated with regard to the dependence of the levitation force on the section radius Rb and curvature radius R (or depth D) of the reflector. It is found that the levitation force can be remarkably enhanced by choosing optimum value of R or D, and the possibility of this enhancement for spherically curved reflectors is the largest. The degree of levitation force enhancement by this means can also be facilitated by enlarging Rb and employing lower resonant mode. The deviation of the sample near the reflector is found likely to occur in circumstance of smaller Rb, larger D and higher resonant mode. The calculated dependence of levitation force on R, Rb and resonant mode is also verified by experiment and finally demonstrated to be in good agreement with experimental results.Single-axis acoustic levitation of the heaviest solid (iridium, p = 22.6g/cm3) and liquid (mercury, p = 13.6g/cm3) on the earth is achieved by greatly enhancing both the levitation force and stability through optimizing the geometric parameters of the self-developed single-axis acoustic levitator. This indicates that all the solids and liquids can be acoustically levitated on the earth in principle.The main influencing factors for containerless processing, such as the gas medium, gravitational levels, resonance adjustment, and temperature variations are studied. For a gas medium, the levitation force is determined not only by the ratio of its density to sound speed, but also by the ratios of its wavelength to the geometric parameters of the levitator. The increase of the gravitational level makes the levitation potential wells shallower or even disappearing. During the resonance adjustment, the sound pressuredistribution, levitation force and sample positions vary symmetrically with respect to the resonant state, and the allowed adjusting range of the reflector-emitter spacing is only the order of 0.005/1. In a quasi-static heating and cooling process, the resonant conditions, levitation force and threshold pressures pm (the minimum entrapping pressure) and pM (the maximum pressure to keep the integration of a liquid drop) are changed significantly. The temperature dependence of the first resonant spacing H\ comes mainly from the variation of wavelength, which is proportional to T1/2 The maximum levitation force FM has a drastic decreasing tendency with temperature rise due to its sensitivity to the geometric parameters/wavelength ratios. pm increases whereas pM decreases with the rise of temperature, which narrows the allowed pressure range for the safe and stable levitation of the processed drops at higher temperatures.The cond更多还原