Study Of Containerless Material Experiment Technology By Electrostatic Levitaion

The containerless electrostatic levitation technique did the material experiment under the state of levitation. Since there is no external container introduced heterogeneous nucleation caused by impurities, the material can obtain a larger degree of supercooling, to develop new materials or to do research about solidification. Materials in a state of levitation without covering are more easily measured with thermal physical properties of higher temperature. The containerless electrostatic levitation technique is an advanced method on researching of material deep-cooling solidification and thermal physical properties. NASA and JAXA have carried out the experiments about containerless electrostatic levitation materials on the ground or space, and made a series of achievements. China still lacks of experimental equipments in this regard.The purpose of the project is to break through the containerless electrostatic levitation technique, to establish experimental facility and carry out deep-cooling solidification and thermal physical properties measurement. It includes the following aspects:(1) Building the electrostatic levitation experiment platform. The platform has the high vacuum and clean chamber environment. The platform also has the function of vibration isolation to protect the measurement devices. The structure of electrodes is designed by comparing with different structures of electrodes. The sample changing mechanism is designed to do experiments without opening the vacuum chamber to increase efficiency.(2) Breaking through the key techniques of electrostatic levitation cntrol and heating and temperature meausrement. The sample 3-dimensional positions are detected by two groups of Parallel light sources and 2-dimensional PSD sensors. By calculating with PID control algorithm and driving with high-voltage amplifiers, the position is controlled stably. Because of the coulomb force and mirror force, the control model of levitation stably is different with that of leaving electrode. The PID with integral separation is used to decrease the overshoot of levitating and oscillation in stable levitation. The sample is heated by three powerful semi-conductor lasers and measured by dual-wavelength pyrometers. The design of hardware solves circuit difficulties of PSD capture, main control, AD/DA, signal isolation and communicating interface. The design of software solves the architectures, communicating protocol and real-time data saving problems.(3) The phenomenon of charges lose is analyzed by measuring film thickness. The main reason is found that the the falling of oxide firm whiling heating. By calculating the sample charges whiling levitating, the three methods are analyzed. The capacitive charging is related with the electrode shape and smple size. The photoelectric effect is related with the wavelength of the UV light source and the thermoelectric effect is obvious after a period of temperature. The pre-heating method is promoted to accelerate the heating speed.(4) Deep-cooling solidification experiment. The material Zr is selected as the test material, because the melting temperature of Zr is high and the vapor pressure of Zr is low. the Zr material is melt by vacuum arc furnace firstly. Then the spheral sample is levitated and heated. The sample is heated above melting temperature to ensure complete melting. Then the heating is stopped and the sample is cooling. The sample is still liquid below melting temperature about 300 ℃ and solidified with recalescence. By scanning with the electron microscope, the deep-cooling sample is different with the sample melted by the vacuum arc furnace. The surface has rapid solidification dendrite structure.(5) The thermophysical properties are measured, including the density, thermal expansion coefficient, surface tension, viscosity, and deep-cooling specific heat. During the heating process, the levitated sample images are recorded by CCD camera. The image edge is extracted by direction gradient algorithm, and the volume can be calculated by a spherical harmonic function. Then density and thermal expansion coefficients at different temperatures can be calculated. After the levitating sample was heated by semiconductor lasers and melt, the output position controlling signal was added by a period of sinusoidal oscillatory signal which frequency and amplitude were adjusted. The decay signal after stable oscillation was captured by one-dimensional PSD and analyzed by spectrum transform to get the resonant frequency and fitted by exponential function to get decay time. Then the surface tension and viscosity can be calculated. By combining the cooling curve and emissivity of material, the deep-cooling specific heat can be calculated.