Eira Seppälä

Three-dimensional molecular dynamics simulations of void coalescence during dynamic fracture of ductile metals

Void coalescence and interaction in dynamic fracture of ductile metals have been investigated using three-dimensional strain-controlled multi-million atom molecular dynamics simulations of copper. The correlated growth of two voids during the coalescence process leading to fracture is investigated, both in terms of its onset and the ensuing dynamical interactions. Void interactions are quantified through the rate of reduction of the distance between the voids, through the correlated directional growth of the voids, and through correlated shape evolution of the voids. The critical inter-void ligament distance marking the onset of coalescence is shown to be approximately one void radius based on the quantification measurements used, independent of the initial separation distance between the voids and the strain-rate of the expansion of the system. The interaction of the voids is not reflected in the volumetric asymptotic growth rate of the voids, as demonstrated here. Finally, the practice of using a single void and periodic boundary conditions to study coalescence is examined critically and shown to produce results markedly different than the coalescence of a pair of isolated voids.

From Hot-Injection Synthesis to Heating-Up Synthesis of Cobalt Nanoparticles: Observation of Kinetically Controllable Nucleation†

Monodisperse nanoparticles of well-defined size and shape are required in several emerging applications, which take advantage of their size-dependent properties such as the superparamagnetic limit in the case of magnetic nanoparticles. 2] Accurate tuning of the nanoparticle size and shape requires understanding of the mechanisms involved in particle nucleation and growth. In spite of extensive ongoing research, these mechanisms are still not fully understood owing to their complexity and interplay. Moreover, the current small-scale synthesis methods, such as the hotinjection method, can be difficult to scale to industrially relevant levels. Hence, more suitable methods are sought. Herein, we revisit a widely studied hot-injection synthesis of monodisperse cobalt nanoparticles and show that the particle nucleation differs from what is expected for a hotinjection synthesis. Evidence is given that the particles nucleate several tens of seconds or a few minutes after the injection, depending delicately on how the reaction temperature is controlled after the sudden temperature drop caused by the injection. The delayed nucleation is followed by a period during which the cobalt precursor decomposes endothermically, the temperature drops, carbon monoxide evolves, and the nuclei rapidly grow into mature nanoparticles. Particle growth after the endothermic period is negligible, and we show that the final particle size is determined by the rate of temperature increase after the injection-induced temperature drop. A rapid increase results in a higher peak temperature before the endothermic period and more nuclei, hence smaller particles, in comparison to the case of a slower rate of temperature increase. The contribution of the injection to particle nucleation seems minor, and it is shown that injection can be replaced entirely by an accurately controlled heating up of the solution containing all reagents (including the cobalt precursor) from room temperature to the nucleation temperature. This synthetic method, which is often termed either “non-injection synthesis” 16] or “heating-up synthesis”, 11] results in nanoparticles that are nearly identical to those made by the hot-injection method. We synthesized cobalt nanoparticles by injecting dicobalt octacarbonyl, [Co2(CO)8], dissolved in a small amount of ortho-dichlorobenzene (o-DCB, b.p. 181 8C) into a solution of oleic acid and trioctylphosphine oxide (TOPO) in o-DCB at reflux. The injection led to an immediate temperature drop of several tens of degrees, which is characteristic of the hotinjection method in general. It has been shown that [Co2(CO)8] undergoes partial decarbonylation during the injection to form gaseous carbon monoxide and intermediate cobalt carbonyl species (e.g. tetracobalt dodecacarbonyl, [Co4(CO)12], and cobalt tetracarbonyl, [Co(CO)4]) in the solution phase; these species then further decompose more slowly to cobalt atoms. 27] It has been shown that both maintaining the lower temperature and letting the temperature recover to the reflux temperature after the injection can lead to monodisperse nanoparticles. In this study, we concentrated on the latter approach and studied for the first time in detail the kinetics of the temperature recovery to reflux. The recovery rate can be conveniently controlled by tuning the rate of heat transfer from the heat bath to the reaction medium, for example, by using an oil bath at different temperatures or an electric heating mantle with different heating powers. A typical development in the reaction temperature after the injection is shown in Figure 1 for the hot-injection synthesis HI1 (see the Experimental Section for a complete list of syntheses with details). The temperature dropped from 180 to 143 8C during the injection, after which it started to recover, as heat was being transferred from the heat bath to the reaction medium. In contrast to the expected continuous increase until the reflux temperature was reached, one minute after the injection we observed a characteristic endothermic period during which the reaction temperature dropped despite continuous heating. Interestingly, the peak temperature (174 8C) reached just before the endothermic period was very close to the temperature prior to injection (180 8C). Vigorous evolution of carbon monoxide during the endothermic period indicated decomposition of the cobalt carbonyl species and release of cobalt atoms. Further evolution of carbon monoxide after the endothermic period was negligible, even when the reflux temperature was reached. This observation indicated that nearly all cobalt carbonyl species had decomposed during the endothermic period, which was [*] J. V. I. Timonen, Prof. Dr. O. Ikkala, Dr. R. H. A. Ras Department of Applied Physics, Aalto University (formerly Helsinki University of Technology) P.O. Box 15100, FI-02150 Espoo (Finland) E-mail: jaakko.timonen@tkk.fi robin.ras@tkk.fi Homepage: http://tfy.tkk.fi/molmat/

Significance of Nanotechnology for Future Wireless Devices and Communications

This paper reviews the expected wide and profound impact of nanotechnology for future wireless devices and communication technologies.

Graphene and Carbon Nanotube Applications in Mobile Devices

This paper presents potential carbon nanoelectronic applications in battery-powered mobile devices such as mobile phones and laptop computers. Based on the physical behavior of carbon nanotubes (CNTs) and graphene and the specific requirements for portable consumer electronic devices, the main challenges and restrictions for the adoption of carbon-based components by the industry are presented. The main emphasis is on electronic components, particularly transistors and radio-frequency applications. A circuit theory model is used to predict the feasibility of CNT transmission lines, and technical challenges that have to be solved before graphene transistors are suitable for mobile devices are presented. The performance of graphene transistors is compared with the corresponding parameters of silicon. In addition, other potential carbon-based applications in mobile devices aside from transistors such as displays and memory elements are outlined briefly.

Cobalt Nanoparticle Langmuir−Schaefer Films on Ethylene Glycol Subphase

The Langmuir-Schaefer (LS) technique was applied to prepare two-dimensional films of tridodecylamine (TDA)-stabilized Co nanoparticles. Ethylene glycol was used as the subphase because the Co nanoparticles spread better on it than on water. Surface pressure-area isotherms provided very little information on the floating films, and Brewster angle microscopy (BAM) was needed to characterize the film formation in situ. In addition to the subphase, various other experimental factors were tested in the LS film preparation, including solvent and presence of free TDA ligands and poly(styrene-b-ethylene oxide) (PS-b-PEO) in the nanoparticle dispersion. LS films deposited from dispersions from which the excess TDA ligands had been removed by washing the Co nanoparticles with 2-propanol consisted of hexagonally organized particles in rafts that were organized in necklace structures. The addition of PS-b-PEO to the deposition dispersion resulted in small nanoparticle rafts evenly distributed over the substrate surface. The best Co-nanoparticle-PS-b-PEO films were obtained with a mass ratio of 20:1 between Co (9 nm) and block copolymer (38 200 g/mol, PEO content 22 mass %). These films were successfully transferred onto Formvar-coated TEM grids and characterized by transmission electron microscopy (TEM) and a superconducting quantum interference device (SQUID) magnetometer. At room temperature the films showed superparamagnetic behavior with a saturation magnetization M(s) of 100 emu/g (Co). Our work indicates that it is possible to obtain thin superparamagnetic LS films of TDA-stabilized Co nanoparticles. This is an important result as the TDA-stabilized Co nanoparticles show a very good resistance to corrosion.

SIMULATION OF THE TRANSFER FUNCTION FOR A HEAD-AND-TORSO MODEL OVER THE ENTIRE AUDIBLE FREQUENCY RANGE

In this study, a method for simulating the transfer function of a head-and-torso model over the entire audible frequency range is introduced. The simulation method uses the ultra-weak variational formulation (UWVF) which is a finite element type method tailored for wave problems. In particular, the UWVF uses plane wave basis functions which better approximate the oscillatory field than a polynomial basis used in the standard finite element methods (FEM). This leads to reduction in the computational complexity at the high frequencies which, accompanied with parallel computing, extends the feasible frequency range of the UWVF method. The accuracy of the new simulation tool is investigated using a simple spherical geometry after which the method used for preliminary HRTF simulations in the geometry of a widely used head-and-torso mannequin.

Magnetic Nanocomposites at Microwave Frequencies

Most conventional magnetic materials used in the electronic devices are ferrites, which are composed of micrometer-size grains. But ferrites have small saturation magnetization, therefore the performance at GHz frequencies is rather poor. That is why functionalized nanocomposites comprising magnetic nanoparticles (e.g. composed of Fe, Co) with dimensions ranging from a few nm to 100 nm, and embedded in dielectric matrices (e.g. silicon oxide, aluminium oxide) have a significant potential for the electronics industry. When the size of the nanoparticles is smaller than the critical size for multidomain formation, these nanocomposites can be regarded as an ensemble of particles in single-domain states and the losses (due for example to eddy currents) are expected to be relatively small.

Ferromagnetic resonance in ϵ-Co magnetic composites

We investigate the electromagnetic properties of assemblies of nanoscale ϵ-cobalt crystals with size range between 5 to 35 nm, embedded in a polystyrene matrix, at microwave (1-12 GHz) frequencies. We investigate the samples by transmission electron microscopy imaging, demonstrating that the particles aggregate and form chains and clusters. By using a broadband coaxial-line method, we extract the magnetic permeability in the frequency range from 1 to 12 GHz, and we study the shift of the ferromagnetic resonance (FMR) with respect to an externally applied magnetic field. We find that the zero-magnetic field ferromagnetic resonant peak shifts towards higher frequencies at finite magnetic fields, and the magnitude of complex permeability is reduced. At fields larger than 2.5 kOe the resonant frequency changes linearly with the applied magnetic field, demonstrating the transition to a state in which the nanoparticles become dynamically decoupled. In this regime, the particles inside clusters can be treated as non-interacting, and the peak position can be predicted from Kittels FMR theory for non-interacting uniaxial spherical particles combined with the Landau-Lifshitz-Gilbert equation. In contrast, at low magnetic fields this magnetic order breaks down and the resonant frequency in zero magnetic field reaches a saturation value reflecting the interparticle interactions as resulting from aggregation. Our results show that the electromagnetic properties of these composite materials can be tuned by external magnetic fields and by changes in the aggregation structure.

Elastic manifolds in disordered environments: Energy statistics

The energy of an elastic manifold in a random landscape at T = 0 is shown numerically to obey a probability distribution that depends on the size of the box it is put into. If the extent of the spatial fluctuations of the manifold is much less than that of the system, a crossover takes place to the Gumbel distribution of extreme statistics. If they are comparable, the distributions have non-Gaussian, stretched exponential tails. The low-energy and high-energy stretching exponents are roughly independent of the internal dimension and the fluctuation degrees of freedom.

Domain Walls in Random Field Ising Magnets: Wetting

Domain walls in random-field Ising magnets can be investigated in groundstates into which walls are induced by prepared boundary conditions. We outline recent progress, and new results on (domain wall) wetting in random field systems. This is studied in fixed disorder configurations in the presence of an external field, which is varied.