Melting point | EPFL Graph Search

The melting point (or, rarely, liquefaction point) of a substance is the temperature at which it changes state from solid to liquid. At the melting point the solid and liquid phase exist in equilibrium. The melting point of a substance depends on pressure and is usually specified at a standard pressure such as 1 atmosphere or 100 kPa. When considered as the temperature of the reverse change from liquid to solid, it is referred to as the freezing point or crystallization point. Because of the ability of substances to supercool, the freezing point can easily appear to be below its actual value. When the "characteristic freezing point" of a substance is determined, in fact, the actual methodology is almost always "the principle of observing the disappearance rather than the formation of ice, that is, the melting point." Examples For most substances, melting and freezing points are approximately equal. For example, the melting and freezing points of mercury is . However, cer

Grain Boundary Relaxation in 18-carat Gold Alloys

Ann-Kathrin Audren

Grain boundaries (GBs) play an important role for the mechanical properties of metals. In addition to dislocation motion inside the grains, some metal alloys deform along the GBs at high temperatures. GBs can also be the cause of a brittle behaviour of some metallic alloys, where crack propagation along the GBs is observed. This thesis aims to understand the role of GBs on the thermo-mechanical behaviour of metals and especially to define the microscopic mechanism, which leads to a deformation at the GBs. In this thesis, polycrystals, single crystals and bi-crystals of a yellow gold alloy are studied in detail, primarily by mechanical spectroscopy. The analysis and interpretation of the experimental data identifies different anelastic relaxations (internal energy dissipation processes), that produce peaks in the mechanical loss spectrum. The mechanical loss spectrum of polycrystals shows a relaxation peak P2 at about 780K, which is absent in single crystals made from the same alloy. Stepwise deformation of a single crystal causes an increase of the high temperature mechanical loss background and the appearance of a high temperature peak (P3). Above a critical deformation, P3 disappears and the peak P2, which is normally observed in polycrystals, appears. The increase of the exponential background is interpreted as due to the introduction of new dislocations whereas the peak P3 is attributed to a dislocation relaxation mechanism in the sub-grain boundaries. The peak P2 located at intermediate temperatures depends on the grain size: with grain growth, the peak position shifts to higher temperatures. It is shown that the relaxation time is proportional to the grain size d. Such a grains size dependence is in agreement with the Zener model based on geometrical considerations of an assembly of elastic grains, which can slide against each other. The investigations on bi-crystals containing a single GB with a specific misorientation between crystal lattices and a specific boundary plane orientation show that the relaxation peak P2 is closely related to a mechanism taking place at the boundary. The peak height is proportional to the GB density in a bi-crystal, whereas a single crystalline part cut from a bi-crystal does not show a relaxation peak. Molecular dynamics simulations are performed in order to illustrate the potential microscopic mechanisms responsible for the stress relaxation peak P2 in polycrystals. A Sigma5 grain boundary is submitted to a shear deformation parallel to the boundary plane. The grain boundary shows a migration perpendicular to the boundary plane coupled to shear for temperatures below 700K. Above 1000K, only grain boundary sliding is observed. Two models are developed that provide expressions for the relaxation strength and the relaxation time that are compared to experimental measurements performed on polycrystals. The observed grain size dependence favours the sliding model over the migration model. Measurements as a function of stress allow a refinement of the sliding model. The stress amplitude dependence of the peak P2 indicates a depinning mechanism, which is interpreted as due to GB dislocations between zones where sliding of different amount occurred. Obstacles to the sliding movement like steps or extended coincidence sites in the GB plane act as pinning points. The pinning points can be overcome at high temperatures close to the melting point, which results in the onset of local microscopic creep.

EPFL2014

Titanium carbide-carbon porous nanocomposite materials for radioactive ion beam production

João Pedro Fernandes Pinto Ramos

The Isotope Separator OnLine (ISOL) technique is used at the ISOLDE - Isotope Separator OnLine DEvice facility at CERN, to produce radioactive ion beams for physics research. At CERN protons are accelerated to 1.4 GeV and made to collide with one of two targets located at ISOLDE facility. When the protons collide with the target material, nuclear reactions produce isotopes which are thermalized in the bulk of the target material grains. During irradiation the target is kept at high temperatures (up to 2300 °C) to promote diffusion and effusion of the produced isotopes into an ion source, to produce a radioactive ion beam. Ti-foils targets are currently used at ISOLDE to deliver beams of K, Ca and Sc, however they are operated at temperatures close to their melting point which brings target degradation, through sintering and/or melting which reduces the beam intensities over time. For the past 10 years, nanostructured target materials have been developed and have shown improved release rates of the produced isotopes, due to the short diffusion distances and high porosities. In here a new Ti-based refractory material is developed to replace the currently used Ti-foils. Since nanometric TiC can't be maintained at high temperatures (T>1200 °C) due to sintering, a processing route was developed to produce TiC-C nanocomposites where the carbon allotropes used were either graphite, carbon black or multi wall carbon nanotubes (MWCNT). The developed nanocomposites sinterability was tested up to 1800 °C and they were characterized according to dimensional changes, relative density, mass losses, surface area, TiC particle size and microstructure morphology. All carbon allotropes had a significant effect on the stabilization of the nanometric TiC where the best result was obtained for a 1:1 volume ratio of TiC:carbon black at 1800 °C where TiC crystallite sizes were of 76 nm (from 51 nm in green) and density of 55 %, followed by TiC:MWCNT with TiC of 138 nm (58 % dense). The processing introduced a ZrO2 contamination from the milling media, forming ZrC that solubilizes in the TiC phase, increasing its lattice parameter. TiC sintering kinetics were studied through the master sintering curve and the activation energy determined for sintering, 390 kJ/mol, were close to the ones obtained in the literature. Using the same method, the calculated activation energy for TiC-carbon black was 555 kJ/mol resulting from the carbon which reduces the TiC sintering, reducing its coordination number. The nanocomposites referred (and the TiC) were irradiated and studied in terms of isotope (Be, Na, Mg, K, Sc and Ca) release, where the nanocomposite with the highest isotope released fraction, TiC-carbon black was selected for the final target material. To produce a full target the processing was scaled up and a target prototype was build and tested at ISOLDE. Li, Na and K isotope intensities and release time-structure were measured from the target prototype, where in comparison with Ti-based materials, Na and Li intensities were higher, K were slightly lower and Ca were lower. The target presents an apparently longer release time structure when comparing with standard materials, as seen in other nanomaterial targets, which is likely related with effusion of the isotopes in the material porosity. Furthermore, contrarily to the Ti-foil targets, the obtained intensities were stable over the full operation time. At the end of this thesis suggestions for a future work which include a second iteration of the TiC-C nanocomposite (already developed) and further modeling.

EPFL2017

Enhanced Electrical Conductivities of Complex Hydrides Li-2(BH4)(NH2) and Li-4(BH4)(NH2)(3) by Melting

Yu Zhou

The electrical conductivities of complex hydrides Li2(BH 4)(NH2) and Li4(BH4)(NH 2)3 consisting of (BH4)- and (NH2)- anions are investigated focusing on their low melting temperatures (365 K for Li2(BH4)(NH2) and 490K for Li4(BH4)(NH2)3). The two hydrides show lithium fast-ion conductivities of about 1 × 10 -4 S·cm-1 at room temperature. After melting, the total ion conductivities OfLi2(BH4)(NH2) and Li4 (BH 4)(NH2)3 reach 6 × 10-2 S·cm-1 (378 K) and 2 x 10-1 S·cm -1 (513 K), respectively. The crystal structure and local atomistic structures closely correlated with the lithium ion conduction before and after melting are also discussed. ©2011 The Japan Institute of Metals.

2011