Magdalena Lungu

Metallic Porous Parts for Electronics Devices Cooling

The paper presents some results regarding the obtaining of some copper heat pipes with a porous copper internal layer for electronic components cooling. The heat pipes were realized by sintering of spherical copper powders of 90-125 mum size directly on the internal side of a copper pipe of 18 mm in diameter. The obtained pipes were then brazed in order to obtain a heat pipe of 0.5 m in length. After that, the heat pipe was sealed and filled with a small quantity of distilled water as working fluid. To establish the total heat transport coefficient and the thermal flow transferred at the evaporator, some external devices were realized to allow the heating of the evaporator and the cooling of the condenser. Water heat pipes are explored in the intermediate temperature range of 303 up to 500 K. Test data are reported for copper water heat pipe, which was tested under different orientations. The obtained results show that the water heat pipe has a good thermal transfer performance in the temperatures range between 345 and 463 K

Aqueous dispersions of silver nanoparticles in polyelectrolyte solutions

AbstractIn this report, we present the versatile and effective technique, using environmental friendly reductant glucose, to prepare stable silver nanodispersions by reduction of Ag+ ions. Alternant copolymers of maleic anhydride with vinyl acetate and styrene sulphonate sodium acid salt polyelectrolytes were synthesized in aqueous solution and used as stabilizers. The formation of nano silver particles was confirmed by UV-Vis spectrophotometry and TEM measurements. Dynamic Light Scattering (DLS) measurements were needed to study how the reagents and their concentrations influence particle size. SEM images show the nanostructure of the hybrid films and indicate a strong interaction between the polyelectrolyte and the silver NPs. Moreover, the silver NPs could be stored for one year without observation of aggregates or sedimentation. The final solid products obtained after evaporating to dryness can be used to produce stable dispersions upon mixing with water. Few of the final products were found to exhibit a high antibacterial and antifungal activity. Graphical AbstractThe possibility to obtain aqueous dispersions of silver nanoparticles in polyelectrolytes solutions was investigated. In order to synthesize stable silver nanodispersions, glucose, an environmental friendly reductant was used. The polyelectrolytes were alternant copolymers of maleic anhydride with vinyl acetate and styrene sulphonate sodium acid salt in aqueous solutions.

Electrical contact materials obtained by spark plasma sintering technology for vacuum contactors

Contact materials from W-Cu, W-Ag and WC-Ag systems obtained by spark plasma sintering technology to be used as electrical contacts for vacuum contactors were developed. The electrical contact materials have been obtained from homogenized powder composite mixtures of W-Cu (70-30, 60-40), W-Ag (70-30, 60-40), and WC-Ag (70-30, 60-40), by spark plasma sintering technology, in vacuum, under pressure of 50 MPa in the 900-1050 °C sintering temperature range, at a constant dwelling time of 5 minutes and a heating/cooling rate of 100°C/min. The physical, microstructural, electrical, mechanical and functional properties of the obtained electrical contact parts of 20 mm in diameter and 4÷5 mm in height were investigated in respect to chemical composition and sintering temperature. All the analyzed electrical contact materials have shown near fully dense and homogeneous microstructures with enhanced physical, mechanical, electrical and functional properties. Best performances have been obtained on the electrical contacts from WC-Ag with 40 wt. % Ag which had the lowest degree of porosity (4.77 %), the highest value of Vickers hardness (220±3 HV), the highest value of Youngs modulus (180±5 GPa), the lowest value of electrical resistivity (4.65 μΩ·cm) and the lowest value of the medium chopping current (0.59 A).

SHAPE MEMORY NiTi AND NiTiCu ALLOYS OBTAINED BY SPARK PLASMA SINTERING PROCESS

The addition of Cu to near equiatomic NiTi shape memory alloys (SMAs) can provide some modifications of their shape memory properties by affecting their transformation behavior. The same effect was obtained in the case of Ni3Ti2 and Ni4Ti3 precipitates presence in the microstructure of NiTi. Also the substitution of Cu to NiTi alloys increases the hardness of the materials. This paper presents the microstructural and mechanical investigations of NiTi and NiTiCu alloys obtained by spark plasma sintering (SPS) process that represents a great potential for researchers as a new process for the fabrication of intermetallic compounds.

Some Considerations Concerning the Obtaining of Some Ag-SnO2 Sintered Electrical Contacts for Low Voltage Power Engineering Switching Devices

The paper presents some considerations concerning the obtaining and characterisation of some new electrical contact pieces of Ag-SnO2-MeO type, where the MeO is an additive chose from the series of metallic oxides WO3, Bi2O3 and CuO. These have a SnO2 content of 9.5 wt. %, a MeO content of 0.5 wt. %, the balance being Ag. The electrical contact materials were manufactured by powder metallurgy techniques using a new type of powder mixtures obtained from SnO2 and MeO powders which were wet mixed, simultaneously with “in situ” Ag synthesis. These present a very uniform and fine dispersion, which in turn leads to improved functional characteristics of the final electrical contact pieces in comparison to the dry mechanical powders mixtures. The new products were designed for low voltage power engineering switching devices working in air at In = 40, 125 and 200 A.

Some Aspects Regarding the Complex Alloying of the Ni3Al Intermetallic Compound with Substitutional and Interstitial Elements

Research results focussed on the combined influence of iron and boron, in proportions of 0.5 and up to 10 wt.% respectively, in complex alloyed Ni3Al synthesised by Self Propagating High Temperature Synthesis (SHS) in the thermo-explosion mode at two ignition temperatures, 950 and 1150 oC, are presented. By XRD and Mössbauer spectroscopy it was established that for 950 oC ignition temperature, the evolved heat is not high enough for the added Fe to be fully incorporated into the synthesised Ni3Al phase, a temperature of 1150 oC being required. For this temperature, the density of the synthesised alloys, their capacity to be cold deformed by re-pressing, hardness and bending strength variations as a function of B and Fe contents, proved their cumulative effects of the ductility and mechanical properties of complex alloyed Ni3Al enhancing.

Obtaining of Ni Base Intermetallic Alloys by Solidification Control

Directional solidification techniques have been applied to produce Ni based intermetallic alloys with preferentially oriented columnar crystals extended along the complete length and parallel to the solidification direction. Enhanced ductility is expected from such alloys. In this paper we present the research results concerning the application of this technique to some complex Ni3Al- Fe-B alloys obtained from compacted mixtures of elemental powders. The corresponding master alloys have been obtained in a vacuum induction furnace by the known Exo-Melt process [1]. The directional solidification of these alloys was subsequently performed on cylindrical samples, at two solidification rates, 30 and 15 mm/h. The influence of the rate and composition used on the dimensional variations, densities, microstructure, constituent phases and lattice parameters, as well as on the surface chemical composition have been documented and are presented in the paper.