Diffusion couple experiment

Figure 3-18 (a) Fit of an H20t profile (from low to high H20t) from a diffusion-couple experiment. The misfit at x —120 imi is attributed to the convolution effect (limited spatial resolution of the measurements), (b) The dependence of DhjO, on H20t from the fit in (a). From Zhang and Behrens (2000). 

Figure 3-28 H2O diffusion profile for a diffusion-couple experiment. Points are data, and the solid curve is fit of data by (a) error function (i.e., constant D) with 167 /irn ls, which does not fit the data well and (b) assuming D = Do(C/Cmax) with Do = 409 /im ls, which fits the data well, meaning that D ranges from 1 /rm /s at minimum H2O content (0.015 wt%) to 409 firn ls at maximum H2O content (6.2 wt%). Interface position has been adjusted to optimize the fit. Data are adapted from Behrens et al. (2004), sample DacDC3.

For the calculation of convective dissolution rate of a falling crystal in a silicate melt, the diffusion is multicomponent but is treated as effective binary diffusion of the major component. The diffusivity of the major component obtained from diffusive dissolution experiments of the same mineral in the same silicate melt is preferred. Diffusivities obtained from diffusion-couple experiments or other types of experiments may not be applicable because of both compositional effect... 

Figure 3-18 H2O profile from a diffusion-couple experiment 243...

ZOU] SEM, diffusion couple experiment 930-1000°C, Cu transport in austenite... 

When two solid reactants can form several compounds, some of them are not found in a diffusion-couple experiment. Give three possible reasons for their absence and suggest experiments to determine the correct one. 

Nis] Diffusion couple experiments, chemical analyzes 0-1.04 mass% Cr and 20-180 mass ppm C, 700-1100°C... 

Diffusion couple experiments (25 to 35kbar, 1260 to 1400C) were performed, in a piston-cylinder apparatus, on natural pyrope- and almandine-rich garnets. The results were combined with a re-analysis of previous profiles to obtain the Arrhenius expression ... 

In a significant contribution, [1995Pal] determined two complete isofiiermal sections at 800 and 1000°C. They prepared 59 ternary alloys in a cracible-free levitation furnace and cast into a copper mold. They used elements of following purity 99.99% Al, 99.97% Fe and 99.77% Ti. The alloys were heat treated at 1000 and 800°C for 100 and 500 h, respectively, followed by quenching in brine solution. In addition, they also performed six diffusion couple experiments at 1000°C. 

A general treatment of a diffusion-controlled growth of a stoichiometric intermetallic in reaction between two two-phase alloys has been introduced by Paul et al. (2006). A reaction couple in which a layer of Co2Si is formed during inter-diffusion from its adjacent saturated phases was used as a model system. In the discussion it has been emphasized that the diffusion couple is undoubtedly one of the most efficient and versatile techniques in solid-state science it is therefore desirable to have alternative theories that enable us to deduce the highest possible amount of information from the data that are relatively easily attainable in this type of experiments. 

One of the first studies of how these secondary phases form was performed by van Roosmalen and Cordfunke. These authors used SEM/EDS and XRD to study postannealed diffusion couples of LSM and YSZ as well as pressed and fired powder mixtures of LSM and YSZ. These experiments showed that reaction products in sufficient quantity to detect by XRD (1—3%) form at temperatures as low as 1170 °C. The two principle reaction products observed were La2Zr207 (LZ) and SrZrOs (SZ), with the relative amount of LZ and SZ depending on the La/Sr ratio in the LSM. Calcia- and baria-doped LaMnOs were found to be similarly reactive with YSZ, and reactivity of LSM with YSZ having 3% or 8% yttria was found to be similar. In the case of the diffusion couples, the layer of reaction products formed at the interface was found (using SEM) to be on the order of 1 /xm after 600 h at 1280 °C and 10—15 fim after 600 h at 1480 °C. By employing Pt diffusion markers... 

The purpose of most experimental studies of diffusion is to obtain accurate diffusion coefficients as a function of temperature, pressure, and composition of the phase. For this purpose, the best approach is to design the experiments so that the diffusion problem has a simple anal3hical solution. After the experiments, the experimental results are compared with (or fit by) the anal3hical solution to obtain the diffusivity. The method of choice depends on the problems. The often used methods include diffusion-couple method, thin-source method, desorption or sorption method, and crystal dissolution method. 

Equation 3.23 gives the velocity of the local C-frame with respect to the V-frame (i.e., the velocity of local mass flow measured by the velocity of an embedded inert marker relative to the ends of a diffusion couple such as in Figs. 3.3 and 3.4). The measurement of and D at the same concentration in a diffusion experiment thus produces two relationships involving Di and D2 and allows their determination. In the V-frame, the diffusional flux of each component is given by a simple Fick s-law expression where the factor that multiplies the concentration gradient is the interdiffusivity D. In this frame, the interdiffusion is specified completely by one diffusivity. 

A diffusion measurement at the temperature To is made by annealing a diffusion couple comprised of two semi-infinite bars. However, there is a complication after the completion of the isothermal anneal, carried out at To for the time to, the specimen must be cooled to room temperature at a finite rate. During this cooling period, a small amount of additional nonisothermal diffusion occurs. If an expression can be found for the amount of time, At, required to produce this same additional increment of diffusion at the constant temperature To, the specimen could be analyzed very simply at the end of the experiment by assuming that it was annealed at To for the time to + At. Assume that D = D0 exp —E/(kT)] and that the temperature during the cooling period is... 

For quenching experiments, which were necessary to freeze-in the equilibrium states between the phase bands, the samples were reheated in a vacuum furnace under 500 mbar N2 to the original annealing temperature and dropped onto a copper plate kept below 470 K (200°C). With this treatment the Group 5 transition metal nitrides underwent a slight change in surface composition because it was impossible to adjust the appropriate nitrogen pressures. This, however, did not influence the compositions at the interface boundaries inside the diffusion couples. 

Diffusion couple in the Kirkendall experiments. Because zinc atoms diffused faster than copper atoms, the molybdenum wires appeared to move toward the center of the specimen. 

All attempts to carry out marker experiments with Ni-Zn and Co-Zn diffusion couples in the same way as with Ni-Bi ones (see Section 1.8 of Chapter 1) were unsuccessful because of crack formation, not allowing an unambiguous determination of the diffusing species in the growing intermetallic layers. These proved, however, extremely useful for understanding the mechanism of multiple-layer formation. Namely, it became quite clear that this phenomenon is most frequently a result of secondary reactions occurring in cracked couples. 

With IR light sources like this one, a technology is available which, in terms of day-to-day reliability and long-term and short-term stability, is entirely comparable with Ti sapphire regenerative amplifiers. As shown in this article, it was possible to perform femtosecond experiments on all kinds of condensed phase phenomena involving vibrational transitions (such as energy relaxation, dephasing, spectral diffusion, coupled systems) with essentially the same facility and accuracy as can be achieved in visible and near-infrared experiments. 

Table 2.4. Binary metal/metal systems observed to exhibit solid-state amorphiz-ation by interdiffusion or other type of driven mixing (e.g. mechanical codeformation). The table lists the type of experiment, the typical reaction temperature, TR (for the case of interdiffusion reaction) and references. (B thin-film bilayer diffusion couples, M thin-film multilayer diffusion couple, S interdiffusion of polycrystalline layer of one component with single crystal of another component, MA mechanical alloying of the metals, MAT thermal reaction of a mechanically deformed composite...

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