100 GPa以上冲击压力区蓝宝石的近红外发光

采用三层夹心靶结构,利用铜箔与抛光蓝宝石之间良好的接触条件,使用辐射式高温计观测到c取向蓝宝石在130～172 GPa冲击高压下的红外辐射。实验信号显示,位于近红外波段的蓝宝石冲击辐射随着压力的增加而变强;强度对比显示,蓝宝石的近红外发射强于界面发射。基于冲击作用下滑移带的温度高于冲击平衡温度的概念,将蓝宝石体内滑移带温度作为蓝宝石发射温度代入线性吸收公式,从而将蓝宝石发光强度的增加和界面冷却引起的发光强度下降相结合;将得到的界面一蓝宝石发射强度数值模拟结果与实验结果相比较,发现二者能够重合。由实验信号的上升趋势得到三个压力下0.85μm近红外波长处蓝宝石的冲击吸收系数数值。

Shock-induced Infrared Radiation from Sapphire over 100 GPa Pressure

Alumina is very important ceramic material;it has a unique combination of mechanical and optical properties.Alumina single crystal (sapphire) is commonly used as transparency window in shock wave mea-surements.When doped with Cr³⁺,alumina is used as a pressure calibrant in static high pressure experiments.In addition it is believed to be one of the major constituent of the Earth’s mantle.Understanding of its optical property under high pressure is very important for the use of it in high pressure experiments and other material regions.However,sapphire or other transparent windows are indispensable in shock temperature measurements of opaque material.Recently,a few of sapphire’s emission at lower than 50 GPa pressure range had been reported;whether this phenomenon detected at higher pressures is yet an open question.In this paper,infrared radiation of shock loaded sapphire has been observed to 172 GPa,using 1 ns time-resolved photomuhiplier.As an un-polished 20-micro-thickness copper foil being sandwiched by two sapphires for a shock target,the intensity received by photomnltiplier demonstrates a durative increase characteristic.It is the fact that thermal diffusion occurred at contact interface can’t cause this intensity-amplified current,the observed radiance continuously increases with time must origin from sapphire window.Under 130～172 GPa,the near infrared radiance of sapphire increases with shock stress.The fitting curve shows the absorptive parameter varies from 0.8 to 2.1 cm^-1 at 850 nm in the experimental pressure range.Up to now,there is no quantitative calculation about the color temperature of shocked sapphire for explaining this emission.In the previous works,inhomogeneous hot spots distributed in shocked sapphire are regarded as the radiance source of sapphire,these hot spots,named"adiabatic shear band",are yielded by high speed local deformation.To our interpretation,adiabatic shear band usually exists in high-speed shock compressed material;its survival time depends upon the thermal conduction.Because thermal conduction of sapphire is obviously smaller than that of metals,as such shear band’s temperature can keep a high value in the period of experiment;and this temperature may also track the melting line which is suggested by D.E.Hare(2002).Therefore,obvious emission can be observed from residual heated spots behind shockwave.Based on the mechanism of local deformation,we made a preliminary quantitative model to explain the inhomogeneous emission of sapphire.This mathematic model is derived from an established slipping band theory by D.E.Grady (1982);the calculation shows the model can predict the unique temperature or apparent emission of shocked sapphire.In addition,sapphire’s slipping band temperature in calculation is roughly consistent with the emission temperature values estimated by D.P.Sebbana (2005) and D.E.Hare at lower pressures.In other words,the shear band’s temperature tacitly assumed that the emission is generated by the disjunet heated regions in sapphire window,not by sapphire’s total bulk.The difference of sapphire’s emissivity between the lower pressure and the more than 100 GPa pressure reveals that the density of emission eenters increases with pressure.Therefore,over 100 GPa pressure,sapphire’s radiation may disturb the intedaeial temperature measurement under shock compression.