EXPERIMENTAL STUDY ON MINERAL CONDENSATION FROM GAS

为了探索不同恒星环境中凝结过程中出现的相的时间尺度和控制因素，采用实验室实验的方法获取尘埃。凝结包括成核和晶体生长两个基本过程，在地球上由于重力的作用，凝结是难以控制的。宇宙中构成固体物质的最丰富的元素是Si、Mg和Fe。在年轻、演化恒星和星际空间周围的化学平衡计算和天体物理观测表明，长辉石、顽辉石和金属铁是主要相。因此，我们计划在低压条件下，在无成核的基底上进行铁灰、铁长石和金属铁粉的凝结实验。首先，尝试研制实验设备，用不锈钢制作真空室，加热钨网，制备W和Mo热屏蔽，在冷凝点处获得大量的热气体。然而，结果被发现不适合硅酸盐粉尘的凝结，因为钨阻止氧原子或分子凝结成硅酸盐。然后，利用氧化铁（Fe2O3）作为蒸发源，金属Mo和forsterite作为衬底，实现金属铁的冷凝。在1350℃下加热1 ~ 120 h，用扫描电镜（SEM）和电子背散射衍射（EBSD）技术对残渣和凝析物进行了观察。在Mo和forsterite上均观察到缩聚物，其大小和数量随时间增加而增加，确定为金属铁。结果表明，由于实际温度的差异，恒星辐射场中物质的温度对尘埃的生长起着关键作用。

In order to explore the time scale and controlling factors of phases appear during condensation in various stellar environments, experimental approach to get dusts in laboratory was employed. Condensation includes two elementary processes, nucleation and crystal growth, in which condensation is hard to control on the Earth due to the gravitational force. The most abundant elements that form solid materials in the universe are Si, Mg and Fe. Calculation of chemical equilibrium and astrophysical observation around young and evolved stars and interstellar space reveal that forserite, enstatite and metallic iron are the dominant phases. Therefore, experiments of condensation of forsterite, ensatite, and metallic iron dust on substrates without nucleation are planned at low pressure conditions.At first, development of an experimental equipment was attempted, and an vacuum chamber made of stainless steel with tungsten mesh heated and W and Mo thermal shields were prepared to obtain large amount of hot gas at the condensation spot. The results, however, were found to be unsuitable for condensation of silicate dusts, because the tungsten prevents condensation of oxygen atoms or molecules into silicates. Then, condensation of metallic iron was intended by using iron oxide (Fe2O3) as an evaporation source, and metallic Mo and forsterite for substrates. The source was heated at 1350℃ for 1 to 120 hours, and the reside and condensates were observed with a scanning electron microscopy (SEM) and analyzed with electron back scattered diffraction (EBSD) technique. Condensates were observed on both Mo and forsterite, of which size and amount increase with time, and are determined to be metallic iron. They show cubic shape and the size is larger on forsterite than Mo. The results show that the temperature of substance at a radiation field from a star plays key role on dust growth due to difference in actual temperature.