Characterization Of Secondary Phases Research Articles

Characterisation of secondary phases in Ni-base superalloy Rene 65

ABSTRACTThe Ni-base superalloy Rene 65 is a newly introduced cast and wrought alloy, used for turbine disc applications in aero-engines. The fine-grained and γ′-strengthened alloy was developed to increase service temperatures up to over 700°C and therefore, enhance the efficiency of aerospace turbines.In this work, the phases occurring in the as-received material as well as after continuous cooling experiments are characterised by transmission electron microscopy (TEM). In the as-received material, a tri-modal γ′ size distribution is found, ranging from a few nanometres up to approx. 5 µm. In addition, borides are found preferentially at grain boundaries with a size of approx. 1 µm. For different cooling rates, a change in morphology and size distribution of γ′ precipitates is found.This is part of a thematic issue on Nanoscale Materials Characterisation and Modeling by Advances Microscopy Methods - EUROMAT.

Characterisation of Secondary Phases in Cu Poor CuInSe2: Surface and in-depth resolved Raman scattering analysis of polycrystalline layers

AbstractThis work reports the surface and in-depth resolved Raman scattering analysis of polycrystalline CuInSe2 layers grown with different chemical composition, as function of the Cu to In content ratio (0.48 ≤ × ≤ 1.14). Measurements performed at the surface of the Cu-poor layers corroborate the formation of both Cu-poor Ordered Vacancy Compound (OVC) and CuAu ordered CuInSe2 phases for compositions corresponding to x ≤ 0.82 (OCV) and x ≤ 0.66 (CuAu), respectively. In-depth resolved measurements performed on the layers with lower Cu content have allowed observing a strong decrease with the depth of the intensity of the Raman peak from the chalcopyrite ordered CuInSe2 phase. This suggests a preferential formation of both OVC and CuAu ordered phases at the region close to the interface with the back Mo layer. On the other hand, the comparison between the spectra directly measured on the front and back surfaces of the layers -after removal of the layers from the Mo coated glass substrates- suggests also a worsening of the crystalline quality at the back region of the layers in the whole range of compositions.

Microstructural Characterization of U Coprecipitated Phases Formed in Bentonic-Granitic Groundwater and under Anoxic Conditions

AbstractFor improving the accuracy of the performance assessment studies related to the spent fuel safety under storage conditions it is necessary to develop a new matrix alteration model. These models must be based on laboratory experiences and they should be capable to extrapolate to storing environmental conditions. Most of recently models developed included the oxidation and dissolution process of the spent fuel matrix, but the influence of a possible process of secondary phase formation over the spent fuel surface is not taken into account yet. This is a key process that could produce a reduction of the matrix dissolution rate, radiation shielding behaviour; however, the surface precipitation of the secondary phase could induce a localized corrosion process, which in this case dissolution rate of the spent fuel would be increased. This paper is focussed on microstructural characterization of secondary phases formed in coprecipitation experiments performed under anoxic conditions in granitic-bentonitic simulated groundwater. In order to simulate the influence of the container material, the coprecipitation experiments were performed in absence and presence of iron powder. The solid phases formed were characterized using the following techniques: XRD; SEM-EDX and TEM-EDX. The XRD diffraction pattern showed that under anoxic conditions a mixture of phases were obtained (sodium and potassium uranate and schoepite), whereas uranate phases were detected when only iron was present. The characterization study indicates that the U secondary phase formed (under reducing conditions and presence of iron powder) growth from iron surface. The crystal size of the secondary phase is independent of the presence of iron powder (and it is always less than 3 μm). Furthermore, the microstructural study showed the growing of U phases over iron powder.

The Role of Oxygen Vacancies on the Microstructure Development and on the Electrical Properties of SnO2-Based Varistors

Variations on the microstructure development and on the electrical properties of SnO2-based varistors are discussed on the basis of the oxygen vacancies created or annihilated by the presence of different additives. Electron paramagnetic resonance (EPR) analysis of sintered samples evidenced a substantial increase in the paramagnetic oxygen vacancies concentration when Nb2O5 is added to the SnO2 ⋅ Co3O4 system. On the other hand, the observed diminution in the concentration of such species after the addition of Fe2O3 indicates solid solution formation. The quantification of paramagnetic oxygen vacancies allowed to confirm the proposed substitutions taking place in the lattice during sintering. These findings are supported by scanning electron microscopy, by density measurements and by current density versus electric field curves. The characterization of secondary phases through EDS assisted SEM and TEM is also reported in this work.

Identification of the mechanism limiting the alteration of clad spent fuel segments in aerated carbonated groundwater

Leaching experiments were performed with five spent fuel samples (20 mm segments of clad fuel rods) from French power reactors (four UO 2 fuel samples with burnup ratings of 22, 37, 47 and 60 GW d t HM −1 and a MOX fuel sample irradiated to 47 GW d t HM −1) to determine the release kinetics of the matrix containing most of the radionuclides. The experiments were carried out with carbonated groundwater on previously leached sections of clad fuel rods in static mode, in an aerated medium at room temperature (25 °C) in a hot cell. Until 313 days of leaching and below uranium saturation, the Sr/U congruence ratios for all the UO 2 fuel samples ranged from 1 to 2; allowing for the experimental uncertainty, strontium can thus be considered as a satisfactory matrix alteration tracer. No significant burnup effect was observed on the alteration of the UO 2 fuel matrix. The daily strontium release factor was approximately 2.7 × 10 −8 d −1 for UO 2 fuel after 706 days of leaching, and seven to eight times higher for MOX fuel. Several alteration mechanisms (radiolysis, solubility, precipitation/clogging) are examined to account for the experimental findings. All the available experimental data (characterization of secondary phases and leaching data) indicate that the mechanism limiting the spent fuel alteration kinetics, for the conditions studied, is likely based on the transport and accessibility of oxidizing species and/or water within the segment.

Characterization of Secondary Phases in Lead Zirconate Titanate Film Surface Deposited with Excess Lead Content

The characterization of secondary phases in lead zirconate titanate (PZT) film surface deposited with excess lead content has been carried out using high-voltage transmission electron microscopy. The high-resolution transmission electron microscopy images indicate that the PZT surface was largely covered with the Pb-rich amorphous phase and a small amount of pyrochlore phase among the PZT grains. The particle size of pyrochlore phase is about 2.5–3 nm. The Pb-rich PZT surface in Auger electron spectroscopy depth profile seems to be attributed to the existence of the Pb-rich amorphous phase, which is likely to PbO.