Reduced pressure and temperature epitaxial silicon CVD kinetics and applications

In the rapidly advancing VLSI technology the scaling of devices for higher density and better performance imposes new requirements on semiconductor materials and processes. The ability to grow thin Si epitaxial layers with sharp transition widths and of high crystallographic quality is essential for high speed technology. Using a Rapid Thermal Processing system we have studied the kinetic aspects of the most useful reacting gases like: silane, dichlorosilane and disilane. We have obtained significant results specific to the working temperatures (650 to 1100° C) and pressures (around 2 Torr). A comparative study of these systems will be presented in terms of growth rate, selectivity and defect production. We also show the feasibility of some devices and techniques where the thermal budget is of primary importance: (i) epitaxial silicon deposition on porous silicon; (ii) epitaxial silicon growth over the gate structure of a permeable base transistor and (iii) the selective deposition of TiSi₂ with no substrate consumption.     

Gadolinium‐Doped Persistent Nanophosphors as Versatile Tool for Multimodal In Vivo Imaging

Recent breakthroughs in the rational development of multifunctional nanocarriers have highlightened the advantage of combining the complementary forces of several imaging modalities into one single nanotool fully dedicated to the biomedical field and diagnosis applications. A novel multimodal optical‐magnetic resonance imaging nanoprobe is introduced. Designed on the basis of a spinel zinc gallate structure doped with trivalent chromium and gadolinium, this nanocrystal bears the ability to serve as both a highly sensitive persistent luminescence nanoprobe for optical imaging, and a negative contrast agent for highly resolved magnetic resonance imaging (MRI). Additional proof is given that surface coverage can be modified in order to obtain stealth nanoparticles highly suitable for real‐time in vivo application in mice, showing delayed reticulo‐endothelial uptake and longer circulation time after systemic injection.     

Notion of metabolic adaptation to exercise

During muscular exercise, adenosine triphosphate (ATP) must be resynthesized in order to allow subsequent muscle contraction. ATP can be immediately resynthesized from adenosine diphosphate (ADP) and from creatine phosphate (CP). However, such resynthesis from phosphorylated compounds is not possible in very short-duration exercise. For further exercise an energy source, mainly carbohydrates and lipids, is needed. While anaerobic (glycolysis and glycogenolysis) and aerobic (mitochondrial oxidation) pathways are used for carbohydrates, lipids can only undergo oxidation. Carbohydrates are used preferentially with rising exercise intensity because of increased muscle uptake, progressive recruitment of type II fibers and catecholamine release. Lipid oxidation rises with the duration of exercise and falls with increasing intensity. The observation that training plays an important role in the use of these energy substrates has led to the development of the crossover theory by which the use of carbohydrates and lipids in all individuals depends at all times on the combined effect of training and exercise intensity. Training facilitates lipid oxidation but, by allowing higher intensity, also implies inevitable use of carbohydrates. Thus there is an exercise level, or a crossover point, at which carbohydrate energy predominates over lipid energy. Beyond this point, increasing energy requirements are satisfied by preferential use of carbohydrates. Finally, although it was incorrectly thought that proteins play a very minor role, actually amino acids coming from the voluminous tissue mass provide an important source of energy during prolonged exercise.     

Nucleation in polymer crystallization: A physical or a chemical mechanism?

Primary nucleation is in general heterogeneous in polymer crystallization. In unseeded polymers, the crystallization originates from a very small concentration (<0.1 percent) of unknown submicroscopic heterogeneous nuclei. Only in three cases have the nucleation mechanisms been identified with precision. The first example is self-nucleation, which relates to the nucleation of a polymer melt by fragments of its own crystals previously present in the melt. Another example of heterogeneous nucleation is represented by epitaxial nucleation of polymers on inorganic and organic substrates recently established by Lotz and Wittmann. More recently, it was discovered that finely divided organic salts added to reactive polymers do not behave as inert heterogeneous substrates but rather dissolve and cut molten macromolecules producing ionic chain ends which precipitate into the melt and form organized aggregates which are the true nucleating species. It is clearly shown that mechanisms of physical and chemical origin are involved in heterogeneous primary nucleation of polymer crystallization.     

Stability study of cermet-supported solid oxide fuel cells with bi-layered electrolyte

Performance and stability of five cermet-supported button-type solid oxide fuel cells featuring a bi-layered electrolyte (SSZ/SDC), an SSC cathode, and a Ni-SSZ anode, were analyzed using polarization curves, impedance spectroscopy, and post-mortem SEM observation. The cell performance degradation at 650 °C in H₂/air both with and without DC bias conditions was manifested primarily as an increase in polarization resistance, approximately at a rate of 2.3 mΩ cm² h⁻¹ at OCV, suggesting a decrease in electrochemical kinetics as the main phenomenon responsible for the performance decay. In addition, the initial series resistance was about ten times higher than the calculated resistance corresponding to the electrolyte, reflecting a possible inter-reaction between the electrolyte layers that occurred during the sintering stage. In situ and ex situ sintered cathodes showed no obvious difference in cell performance or decay rate. The stability of the cells with and without electrical load was also investigated and no significant influence of DC bias was recorded. Based on the experimental results presented, we preliminarily attribute the performance degradation to electrochemical and microstructural degradation of the cathode.     

Mechanical properties of impregnated asbestos-cement

This study is devoted to the mechanical properties of impregnated asbestos-cements. We studied the influence of the degree of impregnation and of the nature of the resin on the modulus of elasticity and on the strength of the materials. The impregnation of asbestos-cement by polymeric resins produces composites of outstanding mechanical properties. The relative increase in flexural and tensile strength is much higher for mortar and concrete but the improvement produced by the polymer on asbestos-cement is especially significant if one takes into consideration the already high absolute strengths of the materials.     

Ethynyl terminated bisquinazolone

Summary The reaction of -2(-4 ethynylphenyl)-4H-1,3 benzoxazin-4 one with bis-2,6(3-aminophenoxy pyridine leads to a soluble ethynylated bisquinazolone. The latter starts to polymerize at 150°C to yield a network which exhibits a Tg at 290°C. This network is stable in air up to 405°C.