Self-diffusion studies of pore fluids in unconsolidated sediments by PFG NMR

The self-diffusion of water and hexadecane in medium and coarse sands from glacial sand deposits in central Germany were investigated by pulsed field gradient nuclear magnetic resonance (PFG NMR). Due to the restriction of the diffusion path at the pore/grain interface, the measured apparent self-diffusion coefficients (D(Δ)) in the pore space depend on the observation time (Δ) in the PFG NMR experiment. Although the bulk self-diffusion coefficients of water and hexadecane differ by about one order of magnitude, the apparent self-diffusion coefficients in the pore space obey the same characteristic time-behaviour, which depends only on geometrical properties of the pore system. Using the “short-time diffusion” model, surface-to-volume (S/V) ratios and inherent self-diffusion coefficients (D₀) of the pore fluids were extracted from these diffusion measurements. The S/V ratios obtained are independent of the pore fluid used and agree with known geometrical properties of the sand grains. Moreover, the D₀ values are consistent with the corresponding bulk self-diffusion coefficients measured separately. In contrast to these results of PFG NMR, simultaneous investigations of longitudinal (T₁) nuclear magnetic relaxation reveal that the relaxation time of the pore fluid is a less suitable parameter for a quantitative estimation of geometrical properties of the pore/grain interface in these unconsolidated sediments since it depends on chemical properties of the fluid/grain interface.     

钙长石成分熔体粘滞度和自扩散系数压力效应的分子动力学研究

用分子动力学方法,研究了1999 K下,压力由23 MPa上升到15183 MPa的过程中,CaAl_2Si_2O_8成分熔体的微观结构、剪切粘滞度和粒子自扩散系数的压力效应。在此基础上,探讨压力对剪切粘滞度与粒子自扩散系数之间关系的影响,并将它同微观结构的变化联系起来。结果表明,粒子自扩散系数的压力效应与熔体结构有很强的相关性;压力的挤压效应阻碍了粒子的扩散,而Si-O和Al-O 5次配位体的形成又加速了扩散过程,两种相反的作用相互抵消,造成的结果是在0～5 GPa范围内,Si~(4 ),O~(2-)和Al~(3 )等网架形成粒子的自扩散系数随压力变化不明显;当压力继续增大时,挤压效应占了主导,导致自扩散系数值快速减小。Ca~(2 )作为网架修饰粒子,自扩散系数随压力升高单调下降。压力小于5 GPa时,粒子自扩散系数的大小关系是:D_(Ca)>D_(Al)>D_O>D_(Si)。系统粘滞度随压力的变化与熔体中BO的含量密切相关:BO含量小于域值时,一定范围内BO含量的变化不会对粘滞度产生很大的影响,超过域值,BO含量的微小增加会导致粘滞度值迅速增大。有效应用Eyring方程的关键是方程中粒子跳跃距离的确定,本研究发现,Si~(4 )和O~(2-)的跳跃距离可以通过系统中非桥氧的百分含量来获得。这一发现使得我们能够利用系统中NBO的含量,结合Eyring方程有效进行不同压     

NaAlSi3O8熔体粒子扩散行为压力效应的分子动力学研究

为了研究压力对钠长石成分熔体粒子微观扩散行为的影响，本文用分子动力学方法，探讨了2001．5K温度下，压力由50MPa上升到19862MPa的过程中，熔体的微观结构、粒子自扩散系数随压力的变化规律。研究表明，在升压过程中，低次配位体Al和Si向5次和6次配位体转变，Al的含量在15GPa达到极大值，而Si含量的极大值在20GPa仍未出现。Na的自扩散系数随压力升高单调下降，Al、Si和O的自扩散系数随压力升高先增后减，在8～10GPa左右达到极大值。网架形成粒子自扩散系数的极大值与Al和Si含量的极大值对应的压力点不一致。所有粒子的自扩散系数与它们与O之间键的寿命呈线性负相关。     

Self-diffusion in liquid Fe at high pressure

The self-diffusivity of liquid iron has been measured between 2 and 20 GPa and 1883 and 2393 K. There is a large pressure dependence across the pressure range studied and simple Arrhenius extrapolation to outer core conditions yields unrealistically low diffusivities. The large pressure and temperature range covered, however, allows different a priori diffusion models to be tested using the experimental results. The free volume model [J. Chem. Phys. 31 (1959) 1164] is found to be in excellent agreement with the experimental data and provides a basis for extrapolation of the results to outer core conditions. An extrapolation to core–mantle boundary (CMB) pressure at 4300 K yields a self-diffusivity for liquid iron of 2.5×10⁻⁵ cm² s⁻¹, which corresponds to a viscosity of 11 mPa s, in reasonable agreement with previous, theoretical, estimates.