Bulk Vacancy Diffusion Research Articles

Annihilating vacancies via dynamic reflection and emission of interstitials in nano-crystal tungsten

Radiation damage not only seriously degrades the mechanical properties of tungsten (W) but also enhances hydrogen retention in the material. Introducing a large amount of defect sinks, e.g. grain boundaries (GBs) is an effective method for improving radiation-resistance of W. However, the mechanism by which the vacancies are dynamically annihilated at long timescale in nano-crystal W is still not clear. The dynamic picture for eliminating vacancies with single interstitials and small interstitial-clusters has been investigated by combining molecular dynamics, molecular statics and object Kinetic Monte Carlo methods. On one hand, the annihilation of bulk vacancies was enhanced due to the reflection of an interstitial-cluster of parallel crowdions by the GB. The interstitial-cluster was observed to be reflected back into the grain interior when approaching a locally dense GB region. Near this region, the energy landscape for the interstitial was featured by a shoulder, different to the decreasing energy landscape of the interstitial near a locally loose region as indicative of the sink role of the GB. The bulk vacancy on the reflection path was annihilated. On the other hand, the dynamic interstitial emission efficiently anneals bulk vacancies. The single interstitial trapped at the GB firstly moved along the GB quickly and clustered to be the di-interstitial therein, reducing its mobility to a value comparable to that that for bulk vacancy diffusion. Then, the bulk vacancy was recombined via the coupled motion of the di-interstitial along the GB, the diffusion of the vacancy towards the GB and the accompanying interstitial emission. These results suggest that GBs play an efficient role in improving radiation-tolerance of nano-crystal W via reflecting highly-mobile interstitials and interstitial-clusters into the bulk and annihilating bulk vacancies, and via complex coupling of in-boundary interstitial diffusion, clustering of the interstitial and vacancy diffusion in the bulk.

Structure and dynamics of C60 molecules on Au(111)

Earlier studies of C 60 adsorption on Au(111) reported many interesting and complex features. We have performed coordinated low-energy electron diffraction, scanning tunneling microscopy (STM), and density functional theory studies to elucidate some of the details of the monolayer commensurate (2Ý3 × 2Ý3)R30° phase. We have identified the adsorption geometries of the two states that image as dim and bright in STM. These consist of a C 60 molecule with a hexagon side down in a vacancy (hex-vac) and a C 60 molecule with a carbon-carbon 6:6 bond down on a top site (6:6-top), respectively. We have studied the detailed geometries of these states and find that there is little distortion of the C 60 molecules, but there is a rearrangement of the substrate near the C 60 molecules. The two types of molecules differ in height, by about 0. A, which accounts for most of the difference in their contrast in the STM images. The monolayer displays dynamical behavior, in which the molecules flip from bright to dim, and vice versa. We interpret this flipping as the result of the diffusion of vacancies in the surface layers of the substrate. Our measurements of the dynamics of this flipping from one state to the other indicate that the activation energy is 0.66 ± 0.03 eV for flips that involve nearest-neighbor C 60 molecules, and 0.93 ± 0.03 for more distant flips. Based on calculated activation energies for vacancies diffusing in Au, we interpret these to be a result of surface vacancy diffusion and bulk vacancy diffusion. These results are compared to the similar system of Ag(111)-(2Ý3 × 2Ý3)R30°-C 60 . In both systems, the formation of the commensurate C 60 monolayer produces a large number of vacancies in the top substrate layer that are highly mobile, effectively melting the interfacial metal layer at temperatures well below their normal melting temperatures.

Ion mass dependence of irradiation-induced local creation of ferromagnetism inFe60Al40alloys

Ion irradiation of ${\text{Fe}}_{60}{\text{Al}}_{40}$ alloys results in the phase transformation from the paramagnetic, chemically ordered B2 phase to the ferromagnetic, chemically disordered A2 phase. The magnetic phase transformation is related to the number of displacements per atom (dpa) during the irradiation. For heavy ions (${\text{Ar}}^{+}$, ${\text{Kr}}^{+}$, and ${\text{Xe}}^{+}$), a universal curve is observed with a steep increase in the fraction of the ferromagnetic phase that reaches saturation, i.e., a complete phase transformation, at about 0.5 dpa. This proves the purely ballistic nature of the disordering process. If light ions are used (${\text{He}}^{+}$ and ${\text{Ne}}^{+}$), a pronounced deviation from the universal curve is observed. This is attributed to bulk vacancy diffusion from the dilute collision cascades, which leads to a partial recovery of the thermodynamically favored B2 phase. Comparing different noble gas ion irradiation experiments allows us to assess the corresponding counteracting contributions. In addition, the potential to create local ferromagnetic areas embedded in a paramagnetic matrix is demonstrated.

Surface mass diffusion and step stiffness on an anisotropic surface; Mo(0 1 1)

Results of step fluctuation experiments for Mo(0 1 1), using low-energy electron microscopy, are re-examined using recently developed procedures that offer accurate coefficients of surface mass diffusion. By these means, surface diffusion D s is documented at T/ T m ∼ 0.5, while the crossover to relaxation driven by bulk vacancy diffusion is inferred for T/ T m ∼ 0.6. Here, T m is the melting temperature T m = 2896 K. We obtain D s = 4 × 10 −4 exp(−1.13 eV/ k B T) cm 2/s for the temperature interval 1080–1680 K. Possible indications of diffusion along step edges appear for T/ T m ∼ 0.4. The same measurements of step fluctuation amplitudes determine also the step stiffness, which by symmetry is anisotropic on Mo(0 1 1). It is shown that three independent procedures yield mutually consistent step stiffness anisotropies. These are (1) step fluctuation amplitudes; (2) step relaxation rate anisotropies; and (3) the observed anisotropies of islands in equilibrium on the Mo(0 1 1) surface. The magnitude of the step stiffness obtained from step edge relaxation is consistent with earlier measurements that determine diffusion from grain boundary grooving.

Surface-diffusion-limited island decay on Rh(0 0 1)

We use low-energy electron microscopy to study the mechanisms of thermal smoothing on Rh(0 0 1) surfaces at high temperature. By examining the change of areas of two-dimensional islands as a function of time and temperature, we find that smoothing from 1210 K to 1450 K is limited by the rate of surface diffusion on terraces and not by bulk vacancy diffusion as observed in other systems in the same temperature range. However, the activation energy we measure for island decay is inconsistent with previous measurements and calculations of the activation energy of surface diffusion and the adatom formation energy. This inconsistency combined with an unexpectedly large activation entropy suggests a surface transport mechanism other than simple hopping of adatoms across the surface.

Crossover from bulk to surface diffusion in the fluctuations of step edges on Pt(111)

Using low-energy electron microscopy we explore the fluctuations of step edges on Pt(111) in the temperature range 1190 K<T<1520 K. Below 1400 K the relaxation times vary with wave vector q as q 3, and above 1400 K as q 2, while the observed activation energy changes from 1.25 to 2.75 eV. A discussion shows that surface diffusion is responsible for the rates at low temperatures, and that bulk vacancy diffusion becomes dominant above 1400 K. We infer that a similar crossover must occur for steps on most metal surfaces. The step stiffness is determined as 175 ± 20 meV nm−1 at 1500 K.

Simulation of the effect of bulk vacancy diffusion on the shape of sputtered films deposited onto trenches and vias

We present a model for the shape evolution of a thin metal film due to the bulk diffusion of vacancies. High-temperature Al sputter deposition and postdeposition annealing are currently becoming significant (very large scale integrated circuit metallization processes. The attractiveness of these processes lies in the ability to fill and planarize high-aspect-ratio vias and contacts with very few process steps. Previous attempts to model these processes have not fully emphasized the role played by bulk diffusion and three-dimensional curvature. We present the differential equation and boundary conditions that describe the flow of vacancies in metal films. An approximate method of solution for this equation system is presented and incorporated into the thin-film deposition simulator SIMBAD (simulation of ballistic deposition). This simulator is then used to present results pertaining to both postdeposition annealing and high-temperature sputter deposition. Particular emphasis is placed upon determining the effects of via and contact geometries, wetting angles and deposition rate during sputtering.

Microwave Radiation Effects on Ionic Current in Ionic Crystalline Solids

ABSTRACTWe describe experiments to test for microwave enhancement of bulk vacancy diffusion in NaCl, bulk interstitial diffusion in AgCl, and grain boundary diffusion in NaCl. Experimental results for bulk vacancy diffusion in NaCl indicate that intrinsic vacancy mobility is not enhanced by microwave field intensities typical of microwave sintering experiments. Nevertheless, a microwave effect is observed with features consistent with a recent theory for microwave-enhanced driving forces.

Dynamic Monte Carlo simulation of spin-lattice relaxation of quadrupolar nuclei in solids. Oxygen-17 in yttria-doped ceria

Although measurement of spin-lattice relation time (T1) can provide valuable information about atomic motion in solids, T1 data for quadrupolar nuclei are often difficult to interpret because the relevant physical interactions cannot be expressed analytically. In order to address this problem, we have developed an extension to the dynamic Monte Carlo method for simulating spin-lattice relaxation of quadrupolar nuclei in solids. In this paper we develop the simulation method generally, and then apply the method to understanding published T1 measurements of oxygen-17 in yttria-doped ceria. We show that even for simple geometries of motion, multiple time scales for electric field gradient fluctuations are possible, resulting in complex relaxation behavior including multiple T1 minima. The method can be used to explain and deconvolute data in which these effects are present. In the case of yttria-doped ceria, we show that two experimentally observed T1 minima result from simultaneous movement of oxygen vacancies and oxygen ions. We discuss the meaning of the kinetic parameters extracted from the data, and show evidence for the relevance of these parameters to bulk vacancy diffusion kinetics.

Long-range ordering kinetics and ordering energy in Ni 3Al-based γ′ alloys

Chemical long-range order kinetics were studied for the Ll 2-ordered γ′-phase domains of the quasi-binary systems Ni 75AL 25− x M x (M = Fe, Mn, Cr, 0 ≤ x ≤ 25), where the γ′-phase stability (ordering energy) rapidly decreases with increasing M-component concentration x. The sample γ′ homogeneity was tested by means of transmission electron microscopy. The ‘order-order’ relaxations measured by means of in-situ resistometry indicated that an increase in x caused a systematic decrease in the activation energy, E A, of homogeneous long-range order kinetics. Discussing a possible influence of various parameters, it was concluded that the observed decrease in E A was predominantly due to an effect of the ordering energy on the bulk vacancy diffusion in these Ll 2-ordered compounds.

Modèle pour les pics de frottement interne observés a haute température sur les monocristaux

Abstract A model for internal friction peaks observed at high temperatures in single crystals A model is presented for treating internal friction peaks observed between 0-3 T t and 0-6 T t (T t being the absolute melting temperature) in high purity copper and aluminium single crystals. The relaxation is related to the migration, by vacancy diffusion, of geometrical jogs along dislocation lines. In the case of peaks observed at about 0-3 T t, the activation energy is close to one half of the self-diffusion energy, and it is supposed that vacancies diffuse along dislocation lines. In the case of higher temperature peaks the activation energy is close to the self-diffusion energy and corresponds to bulk vacancy diffusion. From this model it may also be seen that the peaks probably correspond to isolated dislocation climb and that the high temperature background is due to dislocations belonging to polygonization cells.

Neutron irradiation damage in molybdenum

Abstract An electron microscope study has been made of the time-dependent annealing behaviour of interstitial loops in impure and pure molybdenum neutron-irradiated at 77°K (A77 and C77) respectively and of vacancy loops in pure molybdenum neutron-irradiated at 473°K (C473). The results have been quantitatively analysed and loop growth behaviour has been compared with that predicted theoretically for perfect loops growing by bulk vacancy-limited diffusion mechanism and by a glide and self-climb mechanism. It has been shown that the interstitial loop growth observed in A77 and C77 is accounted for by a glide and self-climb mechanism. On the other hand, vacancy loop growth observed in C473 can be accounted for by a bulk vacancy diffusion mechanism although it is considered that both mechanisms contribute to the observed growth at intermediate stages of the anneal. Finally, two general distinguishing features of loop growth have been exposed. Firstly, under bulk vacancy diffusion-limiting conditions, interst...