利用皮秒超声技术测量高压下Fe和Fe-Si合金的声速

Iron is the main element of the Earth’s outer core, and Silicon is considered as one of the leading candidates of the light elements. So far, the high pressure performances of Fe-Si systems are still ambiguous. Sound velocity measurement is one of the most efficient way to explore the intrinsic changes of the material under high pressure. Based on a new method by combining an ultrashort time-resolved pump-probe technique - Picosecond Ultrasonics and the Diamond Anvil Cell, sound velocity measurement of pure iron and Fe-Si alloys at high pressure can be achieved. For better stimulating acoustic pulses in the sample, a thin layer of aluminum will be deposited on the sample surface as a reassuring opto-acoustic transducer. And in order to detect the acoustic signal with more accuracy, the Michelson interferometer will be applied. The sound velocity can be revealed from time domain analysis, while the density can be extracted as well from the amplitude and profile analysis of the echo signals. The results will be compared with the seismological PREM model for trying to constrain the amount of Si present in the Earth’s outer core. This new experimental method has not only potential for applications ranging from material fundamental physics to Earth sciences, but also for any kinds of solid state materials, both opaque and transparent.

铁（Fe）是外地核的主要成分，而硅（Si）被认为是主要候选轻元素之一。迄今，Fe-Si合金体系在高压下的性质表现还存在很大争议。对高压下物质的声速进行研究是探索其内在性质变化的最有效方法之一。本项目将结合超短时间分辨泵浦探测技术-皮秒超声技术和金刚石压砧技术来测量纯铁Fe及Fe-Si合金在超高压下的声速。本项目将采取在待测样品上镀铝膜作为光声换能介质的方法，和在测量端采用迈克尔逊干涉仪的方法，来最大限度地提高对超声波的激发效率和测量精度。通过对回声信号的时域分析，可以精确地测量声速；通过对信号幅值以及波形的分析，待测材料的密度同样可以被获得。本项目拟将所得实验数据与地震学提出的PREM模型进行比较，以期修定外地核中的Si含量。本项目所提出的高压光声测量方法，不仅有潜力应用于物理及地球科学的各个领域，而且其研究对象可以是任何透光或不透光的固体材料。