Papers by Keyword: Precipitation

Abstract: Recent progress is presented in the understanding of grown-in defects in Czochralskigrown germanium crystals with special emphasis on intrinsic point defects, on vacancy clustering and on interstitial oxygen. Whenever useful the results are compared with those obtained for silicon.

Abstract: Changes in the concentration of interstitial iron in multicrystalline silicon wafers after high temperature annealing (900°C) have been monitored by carrier lifetime measurements. Two cooling rates were investigated. The first was considered ‘fast’, meaning the interstitial Fe had no time to diffuse to precipitation sites, and should therefore be frozen-in, despite being far above the solubility limit at lower temperatures. A second ‘slow’ cool down to 650°C allowed ample time for the Fe to reach the surfaces or other internal precipitation sites. Surprisingly, in both cases the Fe remained in a supersaturated state. This indicates the precipitation process is not diffusion-limited, and that another energetic barrier to precipitate formation must be present. Since the slow cooling used here is similar to the cooling rate experienced by multicrystalline ingots after crystallisation, this precipitate-impeding mechanism is probably responsible for the surprisingly high interstitial Fe concentrations often found in as-grown multicrystalline silicon wafers.

Abstract: Influence of annealing in molecular hydrogen as well as of treatment in hydrogen plasma (hydrogenation) on the electrical properties of NiSi2 precipitates in n- and p-type silicon has been studied by means of deep level transient spectroscopy (DLTS). Both annealing and hydrogenation gave rise to noticeable changes of the shape of the DLTS-peak and of the character of its dependence on the refilling pulse duration that according to [1] allows one to classify the electronic states of extended defects as “band-like” or “localized”. In both n- and p-type samples DLTS-peak in the initial as quenched samples showed bandlike behaviour. Annealing or hydrogenation of n-type samples converted the band-like states to the localised ones but differently shifted the DLTS-peak to higher temperatures. In p-type samples, the initial “band-like” behaviour of DLTS peak remained qualitatively unchanged after annealing or hydrogenation. A decrease of the DLTS-peak due to precipitates and the appearance of the peaks due to substitutional nickel and its complexes were found in hydrogenated p-type sample after removal of a surface layer of 10-20µm.

Abstract: We have investigated the effect of infrared laser irradiation on the oxygen precipitation in silicon crystals during the heat-treatments. The laser light of the wave numbers of 1085 cm-1 was adopted, which is close to the wave number of the infrared absorption by oxygen in silicon (Si-O-Si) at high temperature. We have found that for the high-temperature anneals around 1200 °C for 1-2 h, the infrared laser irradiation during the anneals significantly (almost completely) suppresses the oxygen precipitation.

Abstract: The paper presents the results of experimental in-situ observations of cobalt disilicide nucleation in Co+ implanted silicon and ab initio simulations of energies of small cobalt and cobaltvacancy clusters. Based on these results, microscopic nucleation mechanisms of different types of CoSi2 precipitates are discussed.

Abstract: From a “macroscopic” point of view, steel composition is assumed to vary smoothly along its microstructure. A closer look reveals that, on the atomic level the material composition does not change so smoothly. Single atoms jump randomly along the crystal lattice due to their thermal energy. These random jumps create sporadic zones of the crystal with higher concentration of certain species, and they are responsible for many phenomena, such as precipitation, Ostwald ripening, some phase transformations… This paper proposes a model to simulate the evolution of C-N-V precipitates in microalloyed steels heat treated in the range of warm temperatures (800-900 °C); when the matrix is austenite (fcc), thus taking into account for the local composition fluctuations. The model works by dividing the space into very small cells, containing a single atomic cell each. If during the random movement of atoms a cell that touches a precipitate reaches some critical composition, it is very easy to stick it to the precipitate by changing its “phase”. But it is also possible that some atoms escape from the precipitate by jumping to the austenitic matrix. Both processes happening simultaneously, and which one is leading depends on the atoms energy, i.e. system temperature.

Abstract: Niobium is a strong carbide forming element which is often used in microalloyed steels to control the grain size during thermomechanical treatments and to provide strengthening through precipitation processes. A detailed microscopic investigation is one of the keys for understanding the first stages of the precipitation sequence, thus Transmission Electron Microscopy (TEM) is required. The main difficulty of TEM studies is due to the nanometre scale dimensions of the particles, which makes their detection, structural and chemical characterization delicate. Model Fe- (Nb0.06%,C0.05%) and Fe-(Nb0.05%,C0.03%,N0.03%) ferritic alloys subjected to isothermal annealing treatments have been investigated. High Resolution TEM (HRTEM) and conventional TEM (CTEM) were used to characterise the morphology, nature and location of precipitates. Volume fraction measurements and a statistical approach to the determination of precipitate size histograms have been investigated using Energy Filtered TEM (EFTEM) and High Angle Annular Dark Field (HAADF) imaging. Chemical compositions were quantified by Electron Energy Loss Spectroscopy (EELS). The evolution of precipitate composition with time and temperature is compared with previous simulations obtained from new thermodynamic models based on equilibrium boundary conditions.

Abstract: The solubility limit of copper in iron at temperature lower than 700°C is not precisely known because copper diffusion is too slow to reach an equilibrium with classical experimental techniques involving long range diffusion. However, fine precipitation of copper can lead to an equilibrium in a reasonable ageing time. Hence, coupling ThermoElectric Power and Small Angle X-ray Scattering techniques leads to a precise estimation of this solubility limit in the temperature range 500°C-700°C. Values obtained are confirmed by Tomographic Atom Probe and give results much higher than what is usually extrapolated from high temperature experiments.

Abstract: The development of microstructure of Nb,Ti-bearing microalloyed steel during the CSP process was studied. Three samples were taken from the as-cast slab prior to tunnel furnace, intermediate bar after stand F2 and the hot band, respectively. In the as-cast slab, the average austenite grain size is 654 µm with a large size range from 150 to 2000 µm. In the intermediate bar after stand F2, the austenite grains are remarkably refined, but are heterogenous due to the incomplete recrystallization, which are in the size range of 23 to 116 µm. In the hot band is mainly non-polygonal ferrite. Microstructural heterogeneity exists in the hot band. It is attributed to the heterogeneous austenite grain size in the intermediate bar and the less rolling reduction after stand F2. With regards to precipitation, cubic TiN and fine precipitates less than 20nm are commonly observed in the as-cast slab and the intermediate bar. Some complex (Ti,Nb)(C,N) precipitates with a slightly larger size also exist. In the hot band, most particles are complex (Ti,Nb)(C,N) precipitates, in a shape of irregular or cruciform. The fine precipitates which can strengthen the ferrite matrix are seldom seen. These results are in good agreement with the size distribution of the precipitates determined using small angle X-ray scattering method. The chemical phase analysis reveals that 45%Nb of the total and 43%Ti of the total are still in solution in ferrite of the hot band.