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Nanocrystalline bulk materials are desirable for many applications as they combine mechanical strength and specific electronic transport properties. Our bottom up approach starts with tailored nanoparticles. Compaction and thermal treatment are crucial, but usually the final stage sintering is accompanied by rapid grain growth which spoils nanocristallinity. For electrically conducting nanoparticles, field activated sintering techniques overcome this problem. Small grain sizes have been maintained in spite of consolidation. Nevertheless, the underlying principles, which are of high practical importance, have not been fully elucidated yet.In this combined experimental and theoretical work we show, how the developing microstructure during sintering correlates to the percolation paths of the current through the powder using highly doped silicon nanoparticles as a model system. It is possible to achieve a nanocrystalline bulk material and a homogeneous microstructure. For this, not only the generation of current paths due to compaction, but also the disintegration due to Joule heating is required. The observed density fluctuations on the micrometer scale are attributed to the heat profile of the simulated powder networks. 1 arXiv:1011.6225v2 [cond-mat.mes-hall]

Two dimensional simulations of non-cohesive granular matter in a biaxial shear tester are discussed. The effect of particle elasticity on the mechanical behavior is investigated using two complementary distinct element methods (DEM): Soft particle molecular dynamics simulations (Particle Flow Code, PFC) for elastic particles and contact dynamics simulations (CD) for the limit of perfectly rigid particles. As soon as the system dilates to form shear bands, it relaxes the elastic strains so that one finds the same stresses for rigid respectively elastic particles in steady state flow. The principal stresses in steady state flow are determined. They are proportional to each other, giving rise to an effective macroscopic friction coefficient which is about 10 % smaller than the microscopic friction coefficient between the grains. Correspondence to: kadau@comphys.uni-duisburg.de der to calculate dense granular flows. These models contain parameters whose connection to the properties of the grains is not yet understood. It is the aim of distinct element simulation methods (DEM) to establish the connection between the grain scale and the macroscopic behavior directly [2,3].The stress-strain behavior of a dense granular assembly consists of two parts: the rearrangement of the particles on the one hand, and their individual elastic or plastic deformation on the other. In this paper we address the question, how strongly the flow properties are influenced by the grain deformations compared to particle rearrangements. Therefore, two different distinct element methods are used: soft particle molecular dynamics modelling elastic particles (used here: PFC) and contact dynamics (CD) to simulate perfectly rigid particles. By comparing the results of the two methods the influence of particle elasticity can be separated from the effect that particle rearrangements have on the macroscopic stress-strain behavior found in e.g. the biaxial shear tester considered here.We simulate dense granular flow in a biaxial tester, which allows for larger displacements than the Jenike shear cell. The biaxial shear tester is a rectangular box [4,5] in which the material is sheared under constant strain rate in one direction and constant stress in a perpendicular direction while the load plates in the third direction are fixed. In this setup one will reach steady state flow (constant volume and stress tensor), and the yield locus can be determined.2 Models Both models we use simulate the trajectory of each individual particle by integrating Newton's equations. They mainly differ in the way, how the contact forces between grains are determined. In soft particle molecular dynamics (PFC) microscopic elastic deformations of each particle have to be taken into account: They determine the forces. By contrast, in contact dynamics (CD) particles are considered as perfectly rigid, and forces are calculated from the volume exclusion constraint.In order to make the comparison between the two methods as stringent as possible, we simulate a very simple...

A nanoparticular p-n junction was realized by a field-assisted sintering process, using p-type and n-type doped silicon nanoparticles. A spatially resolved Seebeck microscan showed a broad transition from the positively doped to the negatively doped range. Overshoots on both sides are characteristic for the transition. Despite the tip size being much larger than the mean particle size, information about the dopant distribution between the particles is deduced from modeling the measured data under different assumptions, including the limited spatial resolution of the tip. The best match between measured and modeled data is achieved by the idea of doping compensation, due to the sintering process. Due to a short time at high temperature during the field-assisted sintering process, solid state diffusion is too slow to be solely responsible for the observed compensation of donors and acceptors over a wide range. Therefore, these measurements support a densification mechanism based on (partial) melting and recrystallization.