Evaporation of forsterite in H2 gas
Evaporation of forsterite in H2 gas
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镁橄榄石在氢气中的蒸发
DOI：
10.1016/0016-7037(96)00014-2
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发表时间：
1996
影响因子：
5
通讯作者：
K. Ozawa
中科院分区：
文献类型：
--
作者：
H. Nagahara;K. Ozawa
在适合太阳星云的压力范围内，通过高温真空实验研究了镁橄榄石在氢气中的蒸发动力学。总压低于10−6bar时的蒸发速率几乎不变，与真空中的蒸发速率相似，而10−6~10−3bar时的蒸发速率与PTOT有关。对于小于10−4bar的Pot，蒸发率JexpFo可用JexpFo=1.72Pot1.199.87×10−7(g·cm−2·S−1)来拟合。用气体分子动力学的赫兹-克努森方程计算了与表面反应动力学有关的凝结系数α。由实验得到的蒸发速率与由体系化学平衡计算的蒸发速率之比，得出真空下的α值为0.06，当Pott从10−3增加到10−4bar时，该值增加到0.2。镁橄榄石蒸发速率随H_2丰度的增加而明显增加，主要是由于平衡蒸汽压的增加，相应的驱动力增加，部分原因是蒸发动能垒(α)的增加。实验结果被用来理解镁橄榄石尘埃在一个突然加热的模型星云中随时间的行为，该模型星云主要由镁橄榄岩和氢气组成。星云系统可分为完全蒸发和部分蒸发，这是由尘埃浓缩系数定义的。对于完全蒸发状态(低粉尘浓度)，镁橄榄石颗粒完全蒸发的最短时间是总压力、温度和初始颗粒大小的函数。在1700℃下，镁橄榄石颗粒(~lt；10μm)的寿命可小于1h。实验结果进一步探讨了太阳星云中镁橄榄石颗粒同位素分馏的可能性。通过评价镁橄榄岩表面蒸发与元素扩散的竞争关系，表明镁橄榄石的同位素分馏可能只重于镁，而不重于硅和氧。
Kinetics of evaporation of forsterite in hydrogen gas was investigated by high temperature vacuum experiments in the pressure range plausible for the solar nebula. The evaporation rate at total pressure (Ptot) below 10−6bar is nearly constant and is similar to that in vacuum, whereas the rate at 10−6to 10−3bar is dependent on Ptot. The evaporation rate, JexpFo, is fitted by JexpFo= 1.72Ptot1.199.87 × 10−7(g · cm−2· s−1) for Ptotbelow 10−4bar. The condensation coefficient, α, which is a factor related to kinetics of surface reactions, is evaluated by using the Hertz-Knudsen equation for the kinetic theory of gas molecules. The ratio of the experimentally obtained evaporation rate to that calculated from chemical equilibrium in the system Mg2SiO4-H2gives the α value of 0.06 in vacuum, which increases up to 0.2 with increasing Ptotfrom 10−3to 10−4bar. The apparent increase of forsterite evaporation rate with increasing H2abundance is due mainly to increase of the equilibrium vapor pressure, which corresponds to increase in the driving force, and partly to increase in α (reduction of the kinetic barrier) for evaporation. The experimental results were applied to understand behavior of forsterite dusts with time in an abruptly heated model nebula mostly comprising forsterite and H2. The nebular system can be divided into complete and partial evaporation regimes, which is defined by a dust enrichment factor. For the complete evaporation regime (low dust enrichment), the minimum time for forsterite grains to totally evaporate is estimated as a function of total pressure, temperature, and initial grain size. The lifetime of forsterite grains (<10 μm in size) could be less than 1 h at 1700 °C. The experimental results were further applied to examine the possibility of isotopic fractionation for forsterite grains in the solar nebula. By evaluating the competition between evaporation from surface and elemental diffusion in forsterite, it is shown that forsterite grains could have isotopically fractionated to be heavier only for Mg, but not for Si and O.