Advances in Science and Technology Vol. 45

Abstract: Ethanol-based antimony-tin oxide (ATO) and vanadium-doped ATO (V-ATO) nano-sol solutions were prepared in H2O containing tetramethylammonium hydroxide by peptizing their coagulated precipitates which were synthesized by hydrothermal reaction at 170°C in an autoclave. Also, the ATO and V-ATO films were formed by spin coating to be investigated with dopants (V, Sb) and solid content on their conductivity and transparency.

Abstract: Si-nanoparticles (NP) are attracting steadily growing interest due to their intriguing sizedependent optical properties. The most challenging task in preparing Si-NP is to succeed in controlling the particle size and size distribution and the surface properties. To this respect, the method of laser synthesis of Si-NP from gas phase reactants appears as an ideal tool, being a very versatile and controllable synthesis technique, but the collected Si-NP are strongly agglomerated. Here we report on attempts to decrease the agglomeration of the Si NP by following two complementary strategies: - investigation and control of nanoparticle formation and growth during the synthesis process; - dispersion of the agglomerates and termination of the particle surfaces by controlled chemical etching followed by surface passivation and stabilization treatments. The properties of as-prepared and processed Si NP will be presented and discussed.

Abstract: We have demonstrated the synthesis of lanthanide co-doped nominally SrAl2O4 powders that phosphoresce over long durations. Previous work has reported that the color of phosphorescence depends on the nature of the alkaline earth cation, and that extended phosphorescence in Eu2+ and other lanthanide co-doped MAl2O4 (M= Sr, Ca, Ba) systems occurred with boron incorporation. To gain further insight into the mechanism of extended phosphorescence persistence, we investigated the incorporation of other glass-forming cations, Si and P. The Eu2+ and Dy3+ co-doped SrAl2O4 (SAED) powders were processed by solution polymerization and annealed at 1000oC under a reducing atmosphere. X-ray diffraction spectra revealed that these powders were multiphase. To characterize the optical properties, photoluminescence spectra and phosphorescence persistence curves were measured using a 325-nm wavelength excitation source. We report that the addition of B2O3 alone to SAED powders extended the persistence, while decreasing the luminosity, but more interestingly, the addition of B2O3 along with other glass formers dramatically increased the excitation luminosity.

Abstract: Co-Fe-B/MgO/Co-Fe-B magnetic tunnel junctions were fabricated using UHV magnetron sputtering. Magnetoresistance and spin-transfer switching properties were investigated as a function of Co-Fe-B free layer thickness, between 1.5 nm and 3 nm. The intrinsic switching current and the thermal stability were derived from the pulse duration dependence of the switching current, analyzed based on the thermally activated switching model. Both switching currents, corresponding to parallel (P) to antiparallel (AP) (Ic0 +) and AP to P (Ic0 –) magnetization reversal, were found to be roughly proportional to the free layer thickness. The averaged intrinsic switching current density Jc0 av = (Ic0 +–Ic0 –)/(2A) (where A is the cell area) was in the range of 1–2×107 A/cm2. The experimental values of Jc0 ± agreed with theoretical values, determined taking into account the spintransfer efficiency for the case of magnetic tunnel junction. The thermal stability of the P and AP states was different, but roughly proportional to the free layer thickness in both cases. We attribute this difference to a disparity in the net magnetic field acting on the free layer magnetization in the P and AP states. The average of the thermal stability in the two states varied from 30 to 60 when the free layer thickness was increased. According to our findings, to guarantee the non-volatility of an MRAM device for about 10 years, the Co-Fe-B free layer should be thicker than 2 nm.

Abstract: Fir st prototypes of electrooptic (EO) planar deflector switches (PDS) are fabricated with hybrid integration on Si substrates. Planar optical modules, made in silica-on-silicon technology, consist of input and output waveguide microlenses facing each other and slab waveguides in between. The modules interconnect the input and output fibers with laterally collimated light beams less than 400
m in width at distances up to 100 mm with losses lower than 3 dB. Thin lead lanthanum zirconium titanate (PLZT) films with prism-shaped electrodes grown on SrTiO3 substrates form the deflector elements. The PLZT films are more than 10
m thick with EO coefficients about 40 pm/V. The deflector assembly technology provides chip vertical positioning accuracy better than 1
m. The deflector chips are attached to the optical substrates with thermocompression flip-chip bonding. The optical power losses of the modules with test silica chips can be as low as 3.6 dB. However, the lowest module losses achieved with PLZT are about 10 dB. The channel-to-channel switching operations are demonstrated at about 40 V and switching times less than 300 ns.

Abstract: Epitaxial Mn+1AXn phase (n=1, 2 or 3) thin films from the chemically related Ti-Si-C, Ti-Ge-C, and Ti-Sn-C systems were grown on Al2O3(0001) substrates at temperatures in the region of 700-1000 oC, using d.c. magnetron sputtering from individual sources. In addition to growth of the known phases Ti3SiC2, Ti3GeC2, Ti2GeC, and Ti2SnC the method allows synthesis of the new phases Ti4SiC3, Ti4GeC3, and Ti3SnC2 as well as the intergrown structures Ti5A2C3 and Ti7A2C5 in the Si and Ge systems. Characterization by XRD, TEM and nanoindentation show similarities with respect to phase distribution, mechanical, and electrical properties, particularly pronounced when comparing Si and Ge. The Ti-Sn-C system is, however, the most liable system with respect to surface segregation of the A-element. This causes less favorable growth of MAX phases as seen by a preferential growth of the binary carbide TiC and metallic Sn. Nanoindentation on films from the Ti-Si-C and Ti-Ge-C systems shows large plastic deformation with extensive pile up. The typical thin film hardness is
20 GPa, and the Young’s modulus in the region of 320 GPa. The four-point probe resistivity is low for all systems, but differs depending on materials system and phase, with values of
25 μcm for Ti3SiC2, and
17 μcm for Ti2GeC.

Abstract: Experiments on combustion synthesis for the Ti-2B and Ti-C systems diluted with an inert metal are presented. The paper shows the influence of geometry, composition, density and particle size of diluent on the combustion front velocity. A Ti-2B reactant mixture diluted with Al and Cu and a Ti-C reactant mixture diluted with Al are studied. The metallic diluent and its concentration are varied. Besides, each experiment is based on a stack of cylinders with decreasing diameter in order to vary the heat losses. In some experiments the eventual quenching of the combustion reaction has been observed. Furthermore these experimental results are compared with theoretical calculations based on analytical expressions derived for such systems.

Abstract: An insulated metal substrate (IMS) is a circuit board comprising an insulating layer on a metal base plate. The insulating layer is made from epoxy resin incorporating dense ceramic fillers. The substrates are used in applications where electric parts generate intense heat. It is expected that the insulating layer has higher thermal conductivity as the use of an IMS is expanded. Therefore, the influence of percolation on the equivalent thermal conductivity (ETC) of an insulating layer is considered. The Effect of the volume fraction of ceramic filler on the ETC of insulating layer in IMS is investigated. The ETC as a function of volume fraction of filler is estimated. Based on these experimental and numerical results, an ETC of a filler is evaluated. The ETC of an irregular filler is presumed smaller than that of a spherical filler. It is thought that the control of filler size and shape is important for the formation of high thermal conductivity of an insulating layer. In addition, an improved equation for the ETC of IMS is proposed. The predictive values from the equation for an improved IMS agree with experimental results.