Results 1 - 10 of 134875
Results 1 - 10 of 134875. Search took: 0.071 seconds
|Sort by: date | relevance|
[en] Ferritic-martensitic steels are excellent potential candidates for a use as structural materials in future fusion reactors. For this application, they will have to withstand high fluxes of 14 MeV neutrons that will create atomic displacement cascades and transmutation reactions which will produce large quantities of helium. In order to understand the basic mechanisms under irradiation with helium, we have studied the effects of helium and those of chromium. Pure iron and a Fe-5.4 wt.% Cr model alloy were irradiated at the JANNuS platform in dual-beam mode with Fe+ and He+ ions and in single-beam mode with Fe+ ions at 500 C. This platform enabled us to follow the evolution of damage up to low doses (1 dpa) and to characterize the microstructure at high doses (100 dpa). It also allowed us to observe in situ irradiation/implantation kinetic effects in a TEM coupled to two accelerators. The nature and distribution of irradiation defects was determined: they are essentially dislocation loops with a<100> Burgers vectors and cavities/bubbles. We have demonstrated that the co-implantation of helium and the addition of chromium both reduce the mobility of dislocation loops. The addition of chromium reduces swelling for all the irradiation conditions studied, with or without helium. In pure iron irradiated with helium, an original phenomenon was discovered for the first time: cavities/bubbles nucleate heterogeneously on the planes of the dislocation loops. We have also interpreted our experimental results in pure iron irradiated with helium using the cluster dynamics code CRESCENDO. (author)
[fr]Les aciers ferritiques/martensitiques au chrome sont d'excellents materiaux candidats pour les structures des futurs reacteurs de fusion. En conditions de service, ils devront resister a des flux intenses de neutrons de 14 MeV qui creeront des cascades de deplacements atomiques et des produits de transmutation tels que l'helium. Afin de mieux comprendre le comportement de ces materiaux sous irradiation en presence d'helium, nous avons etudie les effets de l'helium et ceux du chrome dans le cadre de ce travail de these. Du fer pur et un alliage modele Fe-5,4%pds Cr ont ainsi ete irradies dans la plateforme JANNuS a 500 C en bi-faisceau avec des ions Fe et He et en mono-faisceau avec des ions Fe+. L'utilisation de cette plateforme a permis de suivre l'evolution du dommage jusqu'a des doses faibles (1 dpa) et de caracteriser la microstructure apres irradiation a forte dose (100 dpa) Elle a egalement permis l'observation in situ dans un MET couple a deux accelerateurs des effets cinetiques d'implantation/irradiation. La nature et la repartition des defauts d'irradiation ont ete determines: ce sont essentiellement des boucles de dislocations de vecteur de Burgers de type a<100> et des cavites/bulles. Nous avons montre que la co-implantation d'helium et l'addition de chrome reduisaient la mobilite des boucles. Par ailleurs, avec ou sans helium, l'addition de chrome reduit le gonflement dans toutes les conditions etudiees. De plus, dans le fer pur irradie avec helium, un phenomene original de germination heterogene de cavites dans les plans des boucles a ete mis en evidence. Enfin, nous avons egalement utilise le code de dynamique d'amas CRESCENDO pour interpreter les resultats experimentaux dans le fer pur irradie avec helium
[en] A real-time observation of the microstructure evolution of irregularly shaped silicon carbide powders during solid state sintering is realized by using synchrotron radiation computerized topography (SR-CT) technique. The process of sintering neck growth and material migration during sintering are clearly distinguished from 2D and 3D reconstructed images. The sintering neck size of the sample is presented for quantitative analysis of the sintering kinetics during solid state sintering. The neck size-time curve is obtained. Compared with traditional sintering theories, the neck growth exponent (7.87) obtained by SR-CT experiment is larger than that of the two-sphere model. Such condition is discussed and shown in terms of sintering neck growth, in which the sintering process slows down when the particle shape is irregular rather than spherical.
[en] Hydrogels (gellan or agarose) reinforced with nanocrystalline carbonated hydroxyapatite (nCHA) were prepared by the GELPOR3D technique. This simple method is characterized by compositional flexibility; it does not require expensive equipment, thermal treatment, or aggressive or toxic solvents, and yields a three-dimensional (3D) network of interconnected pores 300-900 μm in size. In addition, an interconnected porosity is generated, yielding a hierarchical porous architecture from the macro to the molecular scale. This porosity depends on both the drying/preservation technology (freeze drying or oven drying at 37 deg. C) and on the content and microstructure of the reinforcing ceramic. For freeze-dried samples, the porosities were approximately 30, 66 and below 3% for pore sizes of 600-900 μm, 100-200 μm and 50-100 nm, respectively. The pore structure depends much on the ceramic content, so that higher contents lead to the disappearance of the characteristic honeycomb structure observed in low-ceramic scaffolds and to a lower fraction of the 100-200-μm-sized pores. The nature of the hydrogel did not affect the pore size distribution but was crucial for the behavior of the scaffolds in a hydrated medium: gellan-containing scaffolds showed a higher swelling degree owing to the presence of more hydrophilic groups.
[en] We have prepared silica glass by the sol-gel method and studied its ability to disperse the Ca-α-SiAlON:Eu2+ phosphor for application in white light emitting diodes (LEDs). The emission color generated by irradiating doped glass with a blue LED at 450 nm depended on the concentration of SiAlON and the glass thickness, resulting in nearly white light. The luminescence efficiency of 1-mm-thick glass depended on the SiAlON concentration, and was highest at 4 wt% SiAlON.
[en] In this article, we review the applications of a novel theory (Ohshima 2009 Sci. Technol. Adv. Mater. 10 063001) to the analysis of electrokinetic data for various soft particles, that is, particles covered with an ion-permeable surface layer of polyelectrolytes. Soft particles discussed in this review include various biological cells and hydrogel-coated particles as a model of biological cells. Cellular transformations increase the concentration of sialic acid of glycoproteins and are associated with blocked biosynthesis of glycolipids and aberrant expression of the developmentally programmed biosynthetic pathway. The change in shape or biological function of cells may affect their surface properties and can be detected by electrokinetic measurements. The experimental results were analyzed with Ohshima's electrokinetic formula for soft particles and soft surfaces. As a model system, hydrogel surfaces that mimic biological surfaces were also prepared and their surface properties were studied. (topical review)
[en] We report the fabrication of shortened electrospun polymer fibers with a well-defined concentrated polymer brush. We first prepared electrospun nanofibers from a random copolymer of styrene and 4-vinylbenzyl 2-bromopropionate, with number-average molecular weight Mn=105 200 and weight-average molecular weight Mw=296 700 (Mw/Mn=2.82). The fibers had a diameter of 593±74 nm and contained initiating sites for surface-initiated atom transfer radical polymerization (SI-ATRP). Then, SI-ATRP of hydrophilic styrene sodium sulfonate (SSNa) was carried out in the presence of a free initiator and the hydrophobic fibers. Gel permeation chromatography confirmed that Mn and Mw/Mn values were almost the same for free polymers and graft polymers. Mn agreed well with the theoretical prediction, and Mw/Mn was relatively low (<1.3) in all the examined cases, indicating that this polymerization proceeded in a living manner. Using the values of the graft amount measured by Fourier transform infrared spectroscopy, the surface area, and Mn, we calculated the graft density σ as 0.22 chains nm-2. This value was nearly equal to the density obtained on silicon wafers (σ=0.24 chains nm-2), which is categorized into the concentrated brush regime. Finally, we mechanically cut the fibers with a concentrated poly(SSNa) brush by a homogenizer. With increasing cutting time, the fiber length became shorter and more homogenous (11±17 μm after 3 h). The shortened fibers exhibited excellent water dispersibility owing to the hydrophilic poly(SSNa) brush layer.
[en] Superconducting materials have contributed significantly to the development of modern materials science and engineering. Specific technological solutions for their synthesis and processing helped in understanding the principles and approaches to the design, fabrication and application of many other materials. In this review, we explore the bidirectional relationship between the general and particular synthesis concepts. The analysis is mostly based on our studies where some unconventional technologies were applied to different superconductors and some other materials. These technologies include spray-frozen freeze-drying, fast pyrolysis, field-assisted sintering (or spark plasma sintering), nanoblasting, processing in high magnetic fields, methods of control of supersaturation and migration during film growth, and mechanical treatments of composite wires. The analysis provides future research directions and some key elements to define the concept of 'beautiful' technology in materials science. It also reconfirms the key position and importance of superconductors in the development of new materials and unconventional synthesis approaches. (topical review)
[en] The lateral super junction (SJ) power devices suffer the substrate-assisted depletion (SAD) effect, which breaks the charge balance of SJ resulting in the low breakdown voltage (BV). A solution based on enhancing the electric field of the dielectric buried layer is investigated for improving the BV of super junction LDMOSFET (SJ-LDMOS). High density interface charges enhance the electric field in the buried oxide (BOX) layer to increase the block voltage of BOX, which suppresses the SAD effect to achieve the charge balance of SJ. In order to obtain the linear enhancement of electric field, SOI SJ-LDMOS with trenched BOX is presented. Because the trenched BOX self-adaptively collects holes according to the variable electric field strength, the approximate linear charge distribution is formed on the surface of the BOX to enhance the electric field according to the need. As a result, the charge balance between N and P pillars of SJ is achieved, which improves the BV of SJ-LDMOS to close that of the idea SJ structure. (semiconductor devices)
[en] We present an improved large-signal device model of GaAs/GaN HEMTs, amenable for use in commercial nonlinear simulators. The proposed model includes a new exponential function to independently control the transconductance compression/tail-off behaviors. The main advantage of this model is to provide a simple and coherent description of the bias-dependent drain current (I-V) that is valid in all regions of operation. All aspects of the model are validated for 0.25-μm gate-length GaAs and GaN HEMT processes. The simulation results of DC/pulsed I-V, RF large-signal power and intermodulation distortion products show excellent agreement with the measured data. (semiconductor devices)
[en] Appendix E-9, 'Elastic-Plastic Fracture Mechanics Evaluation,' in the JSME Rules on Fitness-for-Service for Nuclear Power Plants (Codes for Nuclear Power Generation Facilities) uses the load multiplier Z-factor that is applied to elastic-plastic fracture mechanics evaluation for a pipe circumferential flaw of austenitic stainless steel piping and ferritic steel piping. The Z-factor is defined as the ratio of the limit load to the load at tearing instability. Here, the Z-factor of the current JSME Rules on Fitness-for-Service for Nuclear Power Plants was developed for a through-wall pipe circumferential flaw with the angle of 60deg. In Code Case JSME S NA-CC-002, 'Alternative Rule of Criteria on Allowable Flaw Angle for Circumferential Crack' published in 2005, the Z-factor equations are applicable to a circumferential surface flaw including the angle of greater than 60deg, but the Z-factor equations of the Code Case were conservatively formulated as upper-bounded those Z-factors for through-wall pipe circumferential cracks including the angle of greater than 60deg. However, the original purpose of the Code Case was to eliminate the limitation of flaw angle and evaluate non-through-wall flaws. Accordingly, Z-factors for non-through-wall flaws should be developed to have the consistency. Therefore, Z-factor equations of austenitic stainless steel piping and ferritic steel piping have been developed for non-through-wall flaws. (author)