| 1. |
- Kron, J., et al.
(författare)
-
Comparison of numerical simulation models for predicting temperature in solidification analysis with reference to air gap formation
- 2004
-
Ingår i: International Journal of Cast Metals Research. - : Maney Publishing. - 1364-0461 .- 1743-1336. ; 17:5, s. 295-310
-
Tidskriftsartikel (refereegranskat)abstract
- As a result of its influence on heat transfer between cast part and mould, air gap formation is an important problem for many casting processes. The general explanation for gap formation is that, as a result of stresses and distortions that are created from inhomogeneous cooling, shrinkage of the casting and expansion of the mould occur. In this paper, different thermomechanical approaches are applied to a well defined casting process using three commercial and one in-house codes and their predictions are compared with experimental findings. The experimental data were obtained from the solidification and subsequent cooling of cylindrical castings of eutectic Al-13% Si and ternary Al-7% Si-0.3% Mg alloys. Based on these findings, the major differences between the predictions of the models and the actual formation of the air gap are discussed.
|
|
| 2. |
|
|
| 3. |
|
|
| 4. |
|
|
| 5. |
- Sandström, Joakim, 1968, et al.
(författare)
-
Laser photodetachment mass spectrometry
- 2004
-
Ingår i: Nuclear Instruments and Methods in Physics Reseach B. - : Elsevier BV. - 0168-583X. ; 217:3, s. 513-520
-
Tidskriftsartikel (refereegranskat)abstract
- We demonstrate that the technique of laser photodetachment spectroscopy on atomic negative ions can be used as a sensitivity enhancement tool in mass spectrometry, useful for suppressing both isotopic as well as molecular isobaric interferences. In the experiment a beam of negative ions and a laser beam are merged in a collinear geometry and the wavelength of the laser is tuned across the photodetachment threshold region. Due to the large differential Doppler shifts associated with the fast moving ions of different masses, it is possible to selectively detach ions of certain isotopes while leaving others unaffected. By choosing co-propagating laser and ion beams, the heavier isotopes of an element can be selectively detached, whereas counter propagating beams can be used to detach the lighter ones. In this paper we demonstrate the feasibility of the method in an experiment that was designed to selectively detach either 32S− or 34S− ions. The isotopic ratio of 34S/32S was enhanced by a factor of >50 over the natural value. This factor can be improved by using a laser that is better adapted to the measurement and by improving the vacuum conditions in order to further suppress the collisional background. The applicability of this technique to selectivity enhancement in measurements of the abundances of rare or ultra-rare isotopes is discussed.
|
|
| 6. |
- Wendt, Ola, et al.
(författare)
-
Kinetic and Computational Study of Dissociative Substitution and Phosphine Exchange at Tetrahedrally Distorted cis-[Pt(SiMePh2)2(PMe2Ph)2]
- 2000
-
Ingår i: Inorganic Chemistry. - : American Chemical Society (ACS). - 1520-510X .- 0020-1669. ; 39:23, s. 5271-5276
-
Tidskriftsartikel (refereegranskat)abstract
- The substitution kinetics of Me2PhP in cis-Pt(SiMePh2)2(PMe2Ph)2 (1) by the chelating ligand bis(diphenylphosphino)ethane has been followed at 25.0 °C in dichloromethane by stopped-flow spectrophotometry. Addition of the leaving ligand causes mass-law retardation compatible with a dissociative process via a three-coordinate transition state or intermediate. Exchange of Me2PhP in 1 has been studied by variable-temperature magnetization transfer 1H NMR in toluene-d8, giving kex326 = 1.76 ± 0.12 s-1, ΔH⧧ = 117.8 ± 2.1 kJ mol-1, and ΔS⧧ = 120 ± 7 J K-1 mol-1. An exchange rate constant independent of the concentrations of free phosphine, a strongly positive ΔS⧧, and nearly equal exchange and ligand dissociation rate constants also support a dissociative process. Density functional theory (DFT) calculations for a dissociative process give an estimate for the Pt−P bond energy of 98 kJ mol-1 for R = R‘ = Me, which is in reasonable agreement with the experimental activation energy given the differences between the substituents used in the calculation and those employed experimentally. DFT calculations on cis-Pt(PR3)2(SiR‘3)2 (R = H, CH3; R‘ = H, CH3) are consistent with the experimental molecular structure and show that methyl substituents on the Si donors are sufficient to induce the observed tetrahedral twist. The optimized Si−Pt−Si angle in cis-Pt(SiH3)2(PH3)2 is not significantly altered by changing the P−Pt−P angle from its equilibrium value of 104° to 80° or 120°. The origin of the tetrahedral twist is therefore not steric but electronic. The Si−Pt−Si angle is consistently less than 90°, but the Si−Si distance is still too long to support an incipient reductive elimination reaction with its attendant Si−Si bonding interaction. Instead, it appears that four tertiary ligands introduce a steric strain which can be decreased by a twist of two of the ligands out of the plane; this twist is only possible when two strong σ donors are cis to each other, causing a change in the metal's hybridization.
|
|
