Local Frustration Determines Molecular and Macroscopic Helix Structures

The effect of displaying cytochromes from an amyloid fibre is modelled as perturbation of -strands in a bilayer of helical -sheets, thereby explaining the spiral morphology of decorated amyloid and the dynamic response of morphology to cytochrome conformation. The morphology of the modelled fibre, which consists of minimal energy assemblies of rigid building blocks containing two anisotropic interacting units, depends primarily on the rigid constraints between units rather than the soft interactions between them. The framework is a discrete version of the bilayered frustration principle that drives morphology in Bauhinia seedpods. We show that self-assembly of frustrated long range structures can occur if the building blocks themselves are internally frustrated, e.g. amyloid morphology is governed by the conformation of the misfolded protein nucleating the fibre. Our model supports the idea that any peptide sequence can form amyloid if bilayers can form first, albeit stabilised by additional material such as chaperones or cytochromes. Analysis of experimentally derived amyloid structures supports our conclusions and suggests a range of frustration effects, which natural amyloid fibres may exploit. From this viewpoint, amyloid appears as a molecular example of a more general universal bilayered frustration principle, which may have profound implications for materials design using fibrous systems. Our model provides quantitative guidance for such applications. The relevance to longer length scales was proved by designing the morphology of a series of macroscopic magnetic stacks. Finally, this work leads to the idea of mixing controlled morphologically defined species to generate higher-order assembly and complex functional behaviour. The systematic kinking of decorated fibres and the nested frustration of the Bauhinia seed pod are two outstanding examples.

Neutronic optimization in high conversion Th-233U fuel assembly with simulated annealing

This paper reports on fuel design optimization of a PWR operating in a self sustainable Th-233U fuel cycle. Monte Carlo simulated annealing method was used in order to identify the fuel assembly configuration with the most attractive breeding performance. In previous studies, it was shown that breeding may be achieved by employing heterogeneous Seed-Blanket fuel geometry. The arrangement of seed and blanket pins within the assemblies may be determined by varying the designed parameters based on basic reactor physics phenomena which affect breeding. However, the amount of free parameters may still prove to be prohibitively large in order to systematically explore the design space for optimal solution. Therefore, the Monte Carlo annealing algorithm for neutronic optimization is applied in order to identify the most favorable design. The objective of simulated annealing optimization is to find a set of design parameters, which maximizes some given performance function (such as relative period of net breeding) under specified constraints (such as fuel cycle length). The first objective of the study was to demonstrate that the simulated annealing optimization algorithm will lead to the same fuel pins arrangement as was obtained in the previous studies which used only basic physics phenomena as guidance for optimization. In the second part of this work, the simulated annealing method was used to optimize fuel pins arrangement in much larger fuel assembly, where the basic physics intuition does not yield clearly optimal configuration. The simulated annealing method was found to be very efficient in selecting the optimal design in both cases. In the future, this method will be used for optimization of fuel assembly design with larger number of free parameters in order to determine the most favorable trade-off between the breeding performance and core average power density.

Detailed design of242mAm breeding in pressurized water reactors

There is growing interest in the use of 242mAm as a nuclear fuel. Because of its very high thermal fission cross section and its large number of neutrons released per fission, it can be used for various unique applications, such as space propulsion, medical applications, and compact energy sources. Since the thermal absorption cross section of 242mAm is very high, the best way to obtain 242mAm is by the capture of fast or epithermal neutrons in 241Am. However, fast spectrum reactors are not readily available. In this paper, we explore the possibility of producing 242mAm in existing pressurized water reactors (PWRs) with minimal interference in reactor performance. As suggested in previous studies on the subject, the 242mAm breeding targets are shielded with strong thermal absorbers in order to suppress the thermal neutron flux that causes 242mAm destruction. Since 242mAm enrichment within the Am target mainly depends on the neutron energy distribution, which in turn depends on the Am target thickness and on the neutron filter cutoff energy (thermal absorber type), this unique Am target design was developed. In our study, Cd, Sm, and Gd were considered as thermal neutron filters, as suggested by Cesana et al. The most favorable results were obtained by irradiating Am targets covered either with Gd or Cd. In these cases, up to 8.65% enrichment of 242mAm is obtained after 4.5 yr (three successive PWR fuel cycles) of irradiation. It was also found that significant quantities [up to 1.3 kg/GW (electric)-yr] of 242mAm can be obtained in PWR reactors without notable interference with reactor performance. However, in order to maintain the original fuel cycle length, the enrichment of the driver (UO2) fuel must be increased by ∼1%, raised from the conventional 4.5 to 5.5%, depending on the thermal neutron filter used. The most important reactivity feedback coefficients for fuel assemblies containing the 242mAm breeding targets were evaluated and found to be close to those of a standard PWR. Another product of neutron capture in the 241Am reaction is 238Pu. It was found that in a typical 1000 MW (electric) PWR core with one-third of the fuel assemblies containing 241Am targets, up to 15.1 kg of 238Pu enriched to 80% can be produced per year.

Consideration of reactivity initiated accidents for TRU recycling in LWRs

Response of a PWR core loaded with Combined Non-Fertile and UO2 (CONFU) fuel assemblies to control rod ejection accident was evaluated. A number of core arrangements and TRU fuel compositions were considered and the results were compared with the performance of a reference all-UO2 core. The comparison was based on the results of a simple point kinetics model with thermal reactivity feedbacks and temperature dependant materials properties. The reactivity coefficients and core average kinetics parameters were obtained from the full core 3D neutronic simulations. The results show that application of the CONFU assembly concept causes only minor deviation of fuel performance characteristics in reactivity initiated accidents. This is a consequence of relatively small loadings of TRU in the CONFU assembly and therefore dominating role of conventional UO2 fuel in the neutronic performance of the core.

Fibers with integrated mechanochemical switches: minimalistic design principles derived from fibronectin.

Inspired by molecular mechanisms that cells exploit to sense mechanical forces and convert them into biochemical signals, chemists dream of designing mechanochemical switches integrated into materials. Using the adhesion protein fibronectin, whose multiple repeats essentially display distinct molecular recognition motifs, we derived a computational model to explain how minimalistic designs of repeats translate into the mechanical characteristics of their fibrillar assemblies. The hierarchy of repeat-unfolding within fibrils is controlled not only by their relative mechanical stabilities, as found for single molecules, but also by the strength of cryptic interactions between adjacent molecules that become activated by stretching. The force-induced exposure of cryptic sites furthermore regulates the nonlinearity of stress-strain curves, the strain at which such fibers break, and the refolding kinetics and fraction of misfolded repeats. Gaining such computational insights at the mesoscale is important because translating protein-based concepts into novel polymer designs has proven difficult.

Use of axially graded burnable boron for hot-spot temperature reduction in a pressurized water reactor core

Shortly after the loading of a pressurized water reactor (PWR) core, the axial power distribution in fresh fuel has already attained the characteristic, almost flat shape, typical of burned fuel. At beginning of cycle (BOC), however, the axial distribution is centrally peaked. In assemblies hosting uniform burnable boron rods, this BOC peaking is even more pronounced. A reduction in the axial peaking is today often achieved by shortening the burnable boron rods by some 30 cm at each edge. It is shown that a two-zone grading of the boron rod leads, in a representative PWR cycle, to a reduction of the hot-spot temperature of approximately 70 °C, compared with the nongraded case. However, with a proper three-zone grading of the boron rod, an additional 20 °C may be cut off the hot-spot temperature. Further, with a slightly skewed application of this three-zone grading, an additional 50 °C may be cut off. The representative PWR cycle studied was cycle 11 of the Indian Point 2 station, with a simplification in the number of fuel types and in the burnup distribution. The analysis was based on a complete three-dimensional burnup calculation. The code system was ELCOS, with BOXER as an assembly code for the generation of burnup-dependent cross sections and SILWER as a three-dimensional core code with thermal-hydraulic feedback.

Effects of cooling time on a closed LWR fuel cycle

In this study, the effects of cooling time prior to reprocessing spent LWR fuel has on the reactor physics characteristics of a PWR fully loaded with homogeneously mixed U-Pu or U-TRU oxide (MOX) fuel is examined. A reactor physics analysis was completed using the CASM04e code. A void reactivity feedback coefficient analysis was also completed for an infinite lattice of fresh fuel assemblies. Some useful conclusions can be made regarding the effect that cooling time prior to reprocessing spent LWR fuel has on a closed homogeneous MOX fuel cycle. The computational analysis shows that it is more neutronically efficient to reprocess cooled spent fuel into homogeneous MOX fuel rods earlier rather than later as the fissile fuel content decreases with time. Also, the number of spent fuel rods needed to fabricate one MOX fuel rod increases as cooling time increases. In the case of TRU MOX fuel, with time, there is an economic tradeoff between fuel handling difficulty and higher throughput of fuel to be reprocessed. The void coefficient analysis shows that the void coefficient becomes progressively more restrictive on fuel Pu content with increasing spent fuel cooling time before reprocessing.

Fabrication and integration of horizontally aligned carbon nanotube C60 films for UV sensors

Carbon nanotubes (CNTs) are promising for microsystems applications, yet few techniques effectively enable integration of CNTs with precise control of placement and alignment of the CNTs at sufficiently high densities necessary for compelling mechanical or electrical performance. This paper explores new methods for scalable integration of dense, horizontally aligned (HA) CNTs with patterned electrodes. Our technique involves the synthesis of vertically aligned (VA) CNTs directly on a conductive underlayer and subsequent mechanical transformation into HA-CNTs, thus making electrical contact between two electrodes. We compare elasto-capillary folding and mechanical rolling as methods for transforming VA-CNTs, which lead to distinctly different HA-CNT morphologies and potentially impact material and device properties. As an example application of this novel CNT morphology, we investigate fabrication of electrically addressable CNT-C60 hybrid thin films that we previously demonstrated as photodetectors. We synthesize these assemblies by crystallizing C60 from dispersion on HA-CNT thin-film scaffoldings. HA-CNTs fabricated by rolling result in relatively low packing density, so C 60 crystals embed inside the HA-CNT matrix during synthesis. On the other hand, C60 crystallization is restricted to near the surface of HA-CNT films made by the elasto-capillary process. © 2013 IEEE.

Biofunctionalization of PET/SiO2 surfaces for single molecule experiments and medical applications

PET/SiO2 layers were chemically modified to maintain immobilization of functional single molecules. GFP molecules provide an ideal system due to their stability and intrinsic fluorescence. GFP in vivo biotinylated within its NH2-terminal region and attached on the substrate via the biotinstreptavidin bond was further investigated with confocal microscopy, atomic force microscopy (AFM) and spectroscopic ellipsometry (SE). AFM revealed monolayered donut-like structures representing assemblies of biotinstreptavidinbiotinGFP immobilized onto PET/SiO2 surfaces via mPEG. In particular, regions with an approximate height of 12 nm, which approaches the molecular dimensions of the above complex given by molecular modeling, could be detected. The dimensions of the donut-like structures suggest a close-to-each-other positioning of the GFP molecules - which, however, retain their functionality, as evidenced by confocal microscopy. © 2011 World Scientific Publishing Company.

Al2O3 : Cr3+ nanotubes synthesized via homogenization precipitation followed by heat treatment

Cr3+-doped NH4Al(OH)(2)CO3 nanotubes, templated by surfactant assemblies, were successfully synthesized via the homogenization precipitation method, and various crystallographic phase Al2O3:Cr3+ nanotubes were also obtained by postannealing at different temperatures. The characteristic R-1, R-2 doublet line transitions of ruby can be observed in the high crystalline alpha-Al2O3 nanotubes calcined at temperatures higher than 1200 degrees C. The results also indicate that the formation mechanism of the tubular nanostructures should result from the self-rolling action of layered compound NH4Al(OH)(2)CO3 under the assistance of the surfactant soft-template. The convenient synthetic procedure, excellent reproducibility, clean reactions, high yield, and fine quality of products in this work make the present route attractive and significant. Aluminum oxide nanotubes with high specific surface area could be used as fabricating nanosized optical devices doped with different elements and stable catalyst supports of metal clusters.

Optical properties and electrical bistability of CdS nanoparticles synthesized in dodecanethiol

We reported the synthesis of CdS semiconductor nanoparticles using a simple one-pot reaction by thermolysis of cadmium acetylacetonate in dodecanethiol. Optical measurements of the as-obtained CdS nanoparticles revealed that their optical properties were closely related to surface effects. Based upon the cocktail of poly (N-vinylcarbazole) (PVK) and CdS nanoparticles, a bistable device was fabricated by a simple solution processing technique. Such a device exhibited a remarkable electrical bistability, which was attributed to the electric field-assisted charge transfer between PVK and the CdS nanoparticles capped by dodecaethiol. The conduction mechanism changed from an injection-controlled current to a bulk-controlled one during switching from OFF-state to ON-state.

可生物降解的两亲性嵌段共聚物的自聚集行为研究

可生物降解的两亲性嵌段共聚物PLA-PEG飞所制备的胶束或纳术粒子，作为潜在的药物控制释放体系弓！起人们广泛的兴趣。，方们授有寿比山于PEG链的空间位阴．效应可以避免单核噬菌体的吞噬，、并且可以通过控制可降解部分的降解行为实现药物的持续释放，使在微载体内所包载的药物分子持续释放出来。尽管高聚物的胶束和纳米粒子作为药物的胶体载体已作厂泛研究，但是对其本身物理化学性质与应用之间的联系研究甚少。因此本文对一系列PLLA和PEG两嵌段和三嵌段共聚物的自聚集行为进行了细致研究，得到了以卜结论：1．以花为"模型药物"，通过荧光探针技术对一系列两亲性共聚物在水呀招夜和NaCI溶液种的胶束化行为进行了研究。这些共聚物是由一种新型氨钙催化利，以人分J，的聚乙二醇（PEG）为引发剂，引发丙交酷开环聚合得到，，其中囚定长度阴 PEG段分剐为44，104和113环氧乙烷早兀，PLLA的长没在15-280乳酸中元之间。由于氨钙准活性的特点，这些共聚物的分散度较低，均在1.1-1.3之间。其临界胶束浓度cmc发现随PLLA的含量增加I荆氏。具有同一PEG长度的两嵌段和三嵌段共聚物cmc值的截然差别为它们胶束的构型不同提供了证据。同时也发现了NaCI的加入对丫EG段和争LLA段较短洪聚物的cmc的降低有明掀笋作用，而对具有较长PEG段或较长PLLA段的共聚物的cmc基本上没有什么影响。2．通过荧光探余十技术测定花在这一系列共聚物胶束溶液锄勺配分系数在0.2*10~5至1.9*10~5之间，对于同-PEG段的共聚物，花在其胶柬相中的配分系数随PLLA的含量的增加而增加。另外发现NaCl的加入能够促进花在胶束相中的配分。3．通过透射电子显微镜研究了两嵌段共聚物水溶液胶束的形貌，发现胶束的粒径和分散度均随PLLA段的增加而增加：通过原子力显微镜研究"这些纳米粒子退火前后的形貌变化，发现退火后纳米粒子重新自聚集为类似于神经网络红脚乏的"纳米条带"结构，其中心为类似"神经元"的团簇结构，而周困为支化的车由突"分支结构，这与文献上提到的只有三嵌段共聚物能够形成支化的"纳米条带"结构截然不同，其自聚集机理在进，步研究之中。4．以亲水性的荧光素为荧光探针研究了两嵌段共聚物在甲苯中的胶束化行为，发现其clnc值随PLLA段的含量增加而降低，相对于PEG段，PLLA段在其胶宋化过程中起主要作用。通过1HNMR证明两嵌段共聚物在甲苯中的胶束具伯以PLLA段为"核"、PEG段为"壳"的"核-壳"结构，这种胶柬化行为通过溶解度参数的差异进行了解释。5.通过原子力显微镜发现，当这些胶束滴加在云母表面上经过热处理后，这些胶束重新自聚集成为规则的具有平缓隆起的纳米结构，这与由水中得到的胶柬热处理后的形貌截然不同，并对此进行了进一步解释。由XPS分析认为主要是PEG段覆盖在PLLA段表面。

固体表面的分子组装体和纳米结构

本文评叙了分子组装体和纳米结构在固体表面的自组装及其应用，并对控制其自组装过程中的分子间作用力和表面力进行了介绍．采用扫描探针显微学结合电化学的方法对自组装膜（SAMs）、仿生膜（biomimetic mebane），金属和聚合物纳米颗粒（nanoparticles）等分子组装体系进行了研究。主要结果如下：(l)我们研究了2-巯基，-3-烷基唾吩（MOT）在金电极表面的自组装行为。测量了MOT膜的电容，结果表面MOT膜能够选择性的透过溶液中的探针物种。交流阻抗和欠电位沉积研究表明湘T膜的覆盖度达99.9%以上。在硫酸溶液中评价了MOT膜的稳定性，MOT膜的电位窗阳极极限为1.2V。次外，我们还研究了一系列有机小分子在MOT膜上的通透行为，实验结果显示于MOT膜的选择性透过能力最密切相关的是探针分子的溶剂化程度，溶剂化程度约高通过能力越差。我们进一步研究呢MOT膜与表面活性剂的相互作用，发现表面活性剂分子能够在MOT膜表面强烈的吸附，并使MOT膜对电极的封闭作用大大地增强。（2）我们在基于MOT的单组分和电活性多组分的自组膜上进一步构筑了杂化:仿生双层膜，并且研究了双层膜的形成对其中电活性物质的氧化还原反应热力学的影响。跟踪了该双层的形成动力学并研究了其界面属性。得到交叉构象的双层膜．研究了重组其中的HRP的电活性以及磷脂酶A2对该膜的水解动力学。证明我们得到的双层膜比基于烷基硫醇的双层膜有更好的稳定性，而且在仿生膜基础研究和生物传感器应用的有很好的应用前景。（3）我们在MOT和C10SH自组膜上利用表面力，憎水力和电泳力协同作用组装了电活性金纳米粒子。用伏安法跟踪了纳米粒子在界面成膜的过程。用扫描探针显微术对表面得到的纳米结构经行了研究。结果表面这些弱相互作用力协同作用可形成二维纳米结构的表面。(4)我们在单晶Au（111）表面制备了分子导线和长链烷基硫醇的混合自组装膜。以该自组装膜为模板，我们在其表面选择性的生长了聚苯胺纳米结构，这些纳米结构可以作为为阵列电极，而且具有开关特性；同时这些结构为研究分子导线的导电性质提供了一个理想平台。

两亲性三嵌段共聚物在选择性稀溶液中的自组装及应用

本论文主要研究了ABA和ABC型两亲性三嵌段共聚物在选择性稀溶液中的自组装行为，得到了多种形态新颖、结构复杂多样的胶束，研究了这些复杂胶束的形成过程，探讨了影响胶束形态的各种因素并通过适当的方法对胶束形态进行调控。研究了聚乙烯基毗陡（P4VP）／聚苯乙烯（PS）三嵌段共聚物P4VP-b-PS-b-P4VP在二氧六环／水中的自组装行为，成功得到了嵌段共聚物环状胶束，并通过实验研究了环状胶束的形成过程。结果表明，胶束形态依赖于退火时间的长短。随着退火时间的延长，胶束结构从棒状过渡到环形结构。以实验结果为基础提出了环状胶束形成的新的机理，即囊泡塌陷形成环。同时，通过改变实验条件还得到了一些新型的环状复合结构，如环套环形、鸟状、哑铃形、戒指形、网络状等结构，并得到了与计算机模拟一致的结果。通过不同的方法对ABA型三嵌段共聚物（P4VP-b-PS-b-P4VP）在选择性溶液中的自组装胶束形态进行调控：首先，详细研究了P4VP-b-PS-b-P4VP在不同的共溶剂中的自组装行为。结果表明通过单纯改变共溶剂的性质可以很方便地调节胶束的形态，得到了球、棒、囊泡等结构。并用混合溶剂的方法得到了长度和直径可控的纳米线胶束。同时，通过加入第二种选择性溶剂（核层嵌段PS的选择性溶剂甲苯）的方法使形成胶束的核层嵌段在胶核中的伸展程度增加，从而使胶束形态发生转变。其次，研究了加入表面活性剂十五烷基苯酚（PDP）以构建分子间氢键来调节P4VP-b-PS-b-P4VP的胶束形态。结果表明，通过调节PDP的加入量可以使胶束形态发生从球到棒，到网络状、再到囊泡结构的转变。通过实验对比系统地研究了PDP的加入对胶束形态转变的影响，提出了相应的形态转变机理。再次，研究了不同分子量的嵌段共聚物之间共混及共聚物与均聚物共混对胶束形态的影响。结果表明加入亲油嵌段的均聚物对共聚物胶束形态影响非常明显，胶束形态与加入的均聚物的分子量及加入量直接相关。同时得到了一些新形态的胶束，如海绵状、笼子状等。共聚物共混的研究结果表明：通过两种不同分子量的共聚物共混可以得到这两种共聚物胶束的过渡态结构。用共聚物混合的方法还可以得到一些具有生物模拟性的胶束结构，如乌贼状、章鱼状等。这加深了人们对囊泡的形成机理及各种胶束形态之间形态转变的认识。P4VP-b-PS-b-P4VP通过在二氧六环／水中的自组装形成了囊泡，结果表明囊泡的尺寸依赖于初始状态下共聚物在共溶剂中的浓度及退火时间。除得到常规的球形囊泡外，还得到一些非球形囊泡，如长条形、三角形、项链形等囊泡结构。结合计算机模拟的方法研究了囊泡的形成机理，发现这些不同结构的囊泡的形成是由于初始状态下密度涨落所引起的。研究了实验中经常出现的各种胶束形态共存现象的原因，发现体系中亚稳态的存在是多形态共存的重要原因之一。通过聚苯乙烯一左聚乙烯基毗睫一左聚氧乙烯（PS一b一PZVP一b一PEO）在THF／水中的二次自组装首次得到了一种具有生物模拟性的巨大的节状蠕虫胶束（SWM）。研究发现，SWM是由重复单元盘状结构和丝状结构相连组成的。最令人吃惊的是这种SWM与自然界中的一些生命体如蛆叫、蛹、昆虫类的幼虫结构非常相似。通过对SWM形成过程中的中间态胶束结构的深入研究发现SWM是由球形胶束通过二次自组装形成的。SWM的形成过程可以分为三个阶段：ABC三嵌段共聚物先组装形成球形结构；这些球型结构粘连在一起形成梭形的中间结构；这些梭状中间结构中的球经过重组和重新调整各嵌段的排布最终形成SWM。用所得到的嵌段共聚物胶束为模板，采用无电沉积的方法成功制备了各种形态的金属一有机高分子纳米复合材料。用简单的方法还得到了导电金属金一银的双金属纳米结构材料。这些纳米结构材料在微电子器件等领域有潜在应用价值。以上研究结果丰富了人们对嵌段共聚物在选择性介质中自组装行为的理解，为人们提供了对生物材料自组装本质的理解的依据。这在两亲性分子在溶液中自组装的基础研究方面以及基于这些自组装形态而构建结构及功能更复杂的纳米结构材料等方面都有一定的意义。

Fabrication of silicon-on-reflector for Si-based resonant-cavity-enhanced photodetectors

A novel silicon-on-reflector substrate for Si-based resonant-cavity-enhanced photodetectors has been fabricated by using Si-based sol-gel and smart-cut techniques. The Si/SiO2 Bragg reflector is controlled in situ by electron beam evaporation and the thickness can be adjusted to get high reflectivity. The reflectance spectra of the silicon-on-reflector substrate with five pairs of Si/SiO2 reflector have been measured and simulated by transfer matrix model. The reflectivity at operating wavelength is close to 100%. Based on the silicon-on-reflector substrate, SiGe/Si multiple quantum wells resonant-cavity-enhanced photodetectors for 1.3 mu m wavelength have been designed and simulated. Ten-fold enhancement of the quantum efficiency of resonant-cavity-enhanced photodetectors compared with conventional photodetectors is predicted.