Self-diffusion in liquid copper as seen by quasielastic neutron scattering

Abstract

Quasielastic neutron scattering has been used to study atomic dynamics in liquid Cu. At small wave numbers q the intermediate scattering function is dominated by incoherent scattering contributions. From the decay of the quasielastic signal, self-diffusion coefficients D are obtained on an absolute scale. In a temperature range from 1370 to 1620 K, D values exhibit an Arrhenius-type temperature dependence and are significantly smaller than those from previous tracer experiments that are hampered by convective flow.Self-diffusion is a fundamental property for an understanding of liquid dynamics, nucleation, crystal growth, and vitrification. Diffusion data serve as a vital input to the modeling of microstructure evolution and are an essential control to molecular dynamics ͑MD͒ simulation results. A common way to measure self-diffusion coefficients in liquid metals are capillary techniques using isotopes as tracers. However, in most cases the influences of convective flow on the evolving diffusion profile during annealing are not known. A comparison to long capillary experiments under microgravity conditions in space, where gravity driven convective flow is suppressed, shows that convective contributions indeed influence the measurement, the more so the larger the temperatures involved. 1,2 As a consequence, self diffusion coefficients are usually overestimated in the range of a few 10% to 100% and even their temperature dependence may exhibit systematic deviations from the actual one without convection. Therefore, accurate experimental self diffusion data in liquid metals are rare.Recently, the field of liquid diffusion experiments advanced through the use of quasielastic neutron scattering ͑QNS͒ for accurate measurements of self-diffusion coefficients in metallic liquids. QNS probes the dynamics of a liquid on atomic length scales and on a picosecond time scale; short enough to be undisturbed by the presence of convective flow. In the case of an incoherent scattering contribution, e.g., from a liquid containing Ni, Ti, or Cu the quasielastic signal at small q is dominated by the incoherent contributions. From the resulting incoherent intermediate scattering function the self diffusion coefficient can be obtained on an absolute scale. 3 This was also experimentally checked for a viscous Pd-Ni-Cu-P alloy via a comparison of QNS data to results of a long-capillary ͑LC͒ diffusion experiment under microgravity conditions. 4 In combination with containerless processing via electromagnetic levitation, QNS gives even access to the measurement of self diffusion coefficients in chemically reactive metallic liquids at high temperatures. Through the absence of a container wall that enhances heterogeneous nucleation, dynamics can even be measured in the undercooled state, several 100 K below the melting point. [5][6][7] Liquid copper has evolved as a simple model system in computer simulations to study atomic transport properties, 8-11 nucleation, melting, and crystal growth, 12,13 as well as relations of melt structure and s...

Atomistics of self-diffusion in liquid metals

“…In [36] (on page 3652) S m was taken as 2.8k, S f is taken from [6] and d derived from the density of liquids reported in [6]; the obtained D 0 is compared with those reported in [6,14,16,19] in Figure 2. It is seen that the calculated and the reported D 0 are comparable in order of magnitude.…”

“…Q SD~Em . Iida&Guthrie (compiled) [6] Meyer (QENS) [14,16] Demmel et al (QENS) [19] Chathoth et al (QENS) [38] Itami et al (microgravity) [9,39] Li Q SD =E m /2 Fig. 4.…”

“…Besides measurements under microgravity, quasielastic neutron scattering (QENS) [14][15][16][17][18][19] was argued to probe "the dynamics on atomic length scales and on a picosecond time scale, short enough to be undisturbed by the presence of convective flow." [18] "In order to derive the self-diffusion coefficient from QENS one has to assume that concepts developed in the framework of hydrodyanmics [20], i.e.…”

Entropy and transport properties of liquid metals along the melting curve

“…As revealed in Table I, our calculated self-diffusion coefficients and viscosity of liquids at melting points are in agreement with available experimental data and other calculations. 4,[33][34][35][36][37] The self-diffusion coefficients and viscosity of liquids along the melting curve are presented in Figure 2, parts a and b, as a function of the pressure. Contrary to a large enhancement in the viscosity by 5∼12 orders of magnitude from ambient to Earth's pressure, 2 the self-diffusion coefficients and viscosity of liquids increase slightly with increasing pressure along the melting curves, which is consistent with the predication of the Andrade's model.…”

Temperature dependence of Self-diffusion coefficient (SDC) of liquid aluminum using Embedded Atom Method-Finnis-Sinclair (EAMFS) potential

“…Experimentally, the results for diffusion coefficients in liquid metals usually show deviations up to 100% (Banish and Lyle, 1999). According to Meyer (2010), SDCs are normally overestimated in the range of 10% -100% and also their temperature dependence deviates from the actual value without convection. It is also well known in the literature that the use of different potential functions gives slightly varying results for the self-diffusion coefficient, and the data for liquid metals are scarce and contains large errors (Merher, 2007;Ju et al, 2013).…”