Liquid crystal 'blue phases' with a wide temperature range

Liquid crystal 'blue phases' are highly fluid self-assembled three-dimensional cubic defect structures that exist over narrow temperature ranges in highly chiral liquid crystals. The characteristic period of these defects is of the order of the wavelength of visible light, and they give rise to vivid specular reflections that are controllable with external fields. Blue phases may be considered as examples of tuneable photonic crystals with many potential applications. The disadvantage of these materials, as predicted theoretically and proved experimentally, is that they have limited thermal stability: they exist over a small temperature range (0.5-2 degrees C) between isotropic and chiral nematic (N*) thermotropic phases, which limits their practical applicability. Here we report a generic family of liquid crystals that demonstrate an unusually broad body-centred cubic phase (BP I*) from 60 degrees C down to 16 degrees C. We prove this with optical texture analysis, selective reflection spectroscopy, K?ssel diagrams and differential scanning calorimetry, and show, using a simple polarizer-free electro-optic cell, that the reflected colour is switched reversibly in applied electric fields over a wide colour range in typically 10 ms. We propose that the unusual behaviour of these blue phase materials is due to their dimeric molecular structure and their very high flexoelectric coefficients. This in turn sets out new theoretical challenges and potentially opens up new photonic applications.     

Novel Schmitt trigger with wide temperature range

A novel Schmitt trigger circuit, implemented by a reset-set (RS) trigger and two simple distinct inverters, is proposed. Its trigger levels are determined by two Complementary Metal-Oxide-Semiconductor (CMOS) inverters. Contrast with traditional six transistors Schmitt trigger, its temperature and supply voltage characteristics have been analyzed. These two triggers are applied into relaxation oscillator and the result shows that the proposed one in this paper has better performance over wide temperature range, whose temperature coefficient from 0 to 85 °C is 1/5 of the traditional one. Biography: ZOU Zhige (1975–), male, Ph.D. candidate, research direction: analog and mix-signal IC design.     

大量程位移传感器的几何非线性分析

为测得结构的较大位移,利用柔性悬臂梁的大变形研制了一种量程为±500mm的位移传感器.利用数值积分建立了柔性悬臂梁受法线方向力时的几何非线性本构关系,借助Matlab中Lobatto法对本构方程进行求解并获得了相应的挠度与集中力和固端弯矩曲线,分析了挠度与柔性悬臂梁各参数的关系及产生大变形的主要原因,通过试验和误差分析得出了本构关系的正确性和有效性.研究结果表明:柔性悬臂梁的几何非线性本构关系与试验结果符合,固端弯矩理论值在y方向上分量是产生大变形的主要原因,各项静态参数指标良好,此柔性悬臂梁大变形的本构关系可应用于构件或结构的大位移测量.     

中国大范围雾霾期间大气污染特征分析

为分析我国2013年1月份大范围雾霾成因及特点,在收集相关污染物与气象数据的基础上,运用主成分及相关性分析,对雾霾期间我国8个重点城市大气细颗粒物(PM2.5)浓度、粒径分布,时空变化规律,雾霾与气象因素的关系以及雾霾期间各城市大气污染指标的主成分及相关性进行了分析.结果显示雾霾期间8个城市PM2.5平均超标2.34倍,11~14号超标最为严重,PM2.5/PM10浓度比值平均为0.72,高湿、逆温、低压、静风等气象条件有利于雾霾的形成,PM2.5与SO2,NO2等表现出较好的相关性,主成分分析表明多数城市表现出明显的复合污染特征.此次雾霾是以特殊气象条件为主导的机动车尾气及煤烟型复合污染引起的大范围污染现象.     

An abundant population of small irregular satellites around Jupiter

Irregular satellites have eccentric orbits that can be highly inclined or even retrograde relative to the equatorial planes of their planets. These objects cannot have formed by circumplanetary accretion, unlike the regular satellites that follow uninclined, nearly circular and prograde orbits. Rather, they are probably products of early capture from heliocentric orbits. Although the capture mechanism remains uncertain, the study of irregular satellites provides a window on processes operating in the young Solar System. Families of irregular satellites recently have been discovered around Saturn (thirteen members, refs 6, 7), Uranus (six, ref. 8) and Neptune (three, ref. 9). Because Jupiter is closer than the other giant planets, searches for smaller and fainter irregular satellites can be made. Here we report the discovery of 23 new irregular satellites of Jupiter, so increasing the total known population to 32. There are five distinct satellite groups, each dominated by one relatively large body. The groups were most probably produced by collisional shattering of precursor objects after capture by Jupiter.     

Detection of Earth-like planets around nearby stars using a petal-shaped occulter

Direct observation of Earth-like planets is extremely challenging, because their parent stars are about 10(10) times brighter but lie just a fraction of an arcsecond away. In space, the twinkle of the atmosphere that would smear out the light is gone, but the problems of light scatter and diffraction in telescopes remain. The two proposed solutions--a coronagraph internal to a telescope and nulling interferometry from formation-flying telescopes--both require exceedingly clean wavefront control in the optics. An attractive variation to the coronagraph is to place an occulting shield outside the telescope, blocking the starlight before it even enters the optical path. Diffraction and scatter around or through the occulter, however, have limited effective suppression in practically sized missions. Here I report an occulter design that would achieve the required suppression and can be built with existing technology. The compact mission architecture of a coronagraph is traded for the inconvenience of two spacecraft, but the daunting optics challenges are replaced with a simple deployable sheet 30 to 50 m in diameter. When such an occulter is flown in formation with a telescope of at least one metre aperture, terrestrial planets could be seen and studied around stars to a distance of ten parsecs.     

一种高速宽动态陀螺信号采集电路的设计

根据捷联导航系统中陀螺信号采集的要求,设计了一种新型自动程控增益的A/D数据采集电路,系统根据预采样值判断放大倍数,有效解决了A/D动态范围宽的问题.给出了电路的硬件结构、器件选择与应用等,软件给出数据采集流程图.最后给出的测试结果表明:该系统具有实时性、高精度、大动态范围的优点,具有较高的实用价值.     

内埋式永磁同步电机宽调速范围运行控制策略

为了改善内埋式永磁同步电机驱动系统的性能、效率和可靠性,提出了一套适用于其在宽调速范围运行下的控制策略。该文研究的调速系统采用转子磁链定向的矢量控制作为基本的控制策略,利用模型参考自适应系统对转子位置和转速进行估算。在低速域,采用单位电流最大转矩控制来降低电机定子电流幅值。而在高速域,利用过调制控制策略来增加脉宽调制(pulse width modulation,PWM)逆变器的电压传输比和永磁同步电机的转矩输出能力,同时采用弱磁控制策略实现永磁同步电机驱动系统高速稳定运行。研究了在过调制下的相电流重构技术对永磁同步电机高速域运行性能的影响,并讨论了从低速域的单位电流最大转矩控制到高速域的弱磁控制之间的无缝切换问题。仿真和实验结果证明了上述组合控制策略的可行性与有效性,实现了内埋式永磁同步电机驱动系统在宽调速范围的高性能控制。     

高效率宽输入范围电荷泵2倍压电路的设计

提出一种无阈值损耗的电荷泵倍压电路(2倍压电路),该电路采用衬底可变单元代替二极管连接的MOS管,消除了MOS器件的体效应及阈值损耗的影响,与传统电荷泵相比效率提高了20%,获得低至1.0V的宽输入范围.基于该结构采用TSMC 0.25μm BCD工艺设计了一款2.0AUSB功率开关芯片,使用spec-tre对整体电路进行了仿真验证,结果表明:该电荷泵电路的工作状态良好,同比输出电压提高了1.0V,效率最高可达90%,基于该工艺实现的电荷泵电路的版图面积仅为0.04mm2.     

Detection of carbonates in dust shells around evolved stars.

Carbonates on large Solar System bodies like Earth and Mars (the latter represented by the meteorite ALH84001) form through the weathering of silicates in a watery (CO3)2- solution. The presence of carbonates in interplanetary dust particles and asteroids (again, represented by meteorites) is not completely understood, but has been attributed to aqueous alteration on a large parent body, which was subsequently shattered into smaller pieces. Despite efforts, the presence of carbonates outside the Solar System has hitherto not been established. Here we report the discovery of the carbonates calcite and dolomite in the dust shells of evolved stars, where the conditions are too primitive for the formation of large parent bodies with liquid water. These carbonates, therefore, are not formed by aqueous alteration, but perhaps through processes on the surfaces of dust or ice grains or gas phase condensation. The presence of carbonates which did not form by aqueous alteration suggests that some of the carbonates found in Solar System bodies no longer provide direct evidence that liquid water was present on large parent bodies early in the history of the Solar System.     

Efficient delivery of meteorites to the Earth from a wide range of asteroid parent bodies

Almost all meteorites come from asteroids, but identifying their specific parent bodies, and modelling their transport to the Earth, has proved to be difficult. The usual model of delivery through orbital resonances with the major planets has recently been shown to deplete the supply of meteorites much too rapidly to explain either the observed flux at the Earth, or the length of time the meteorites have spent in space (as measured by cosmic-ray exposure ages). Independently, it has been found that a force arising from anisotropically emitted thermal radiation from asteroidal fragments (the 'Yarkovsky effect') influences the fragments' orbits in important ways. Here we report the results of a detailed model for the transport of meteorites to the Earth, which includes the Yarkovsky effect and collisional evolution of the asteroidal fragments. We find that the Yarkovsky effect significantly increases the efficiency of the delivery of meteorites to the Earth, while at the same time allowing a much wider range of asteroids to contribute to the flux of meteorites. Our model also reproduces the observed distribution of cosmic-ray exposure ages of stony meteorites.     

The 'solar model problem' solved by the abundance of neon in nearby stars

The interior structure of the Sun can be studied with great accuracy using observations of its oscillations, similar to seismology of the Earth. Precise agreement between helioseismological measurements and predictions of theoretical solar models has been a triumph of modern astrophysics. A recent downward revision by 25-35 per cent of the solar abundances of light elements such as C, N, O and Ne (ref. 2) has, however, broken this accordance: models adopting the new abundances incorrectly predict the depth of the convection zone, the depth profiles of sound speed and density, and the helium abundance. The discrepancies are far beyond the uncertainties in either the data or the model predictions. Here we report neon-to-oxygen ratios measured in a sample of nearby solar-like stars, using their X-ray spectra. The abundance ratios are all very similar and substantially larger than the recently revised solar value. The neon abundance in the Sun is quite poorly determined. If the Ne/O abundance in these stars is adopted for the Sun, the models are brought back into agreement with helioseismology measurements.     

A compact system of small planets around a former red-giant star

Planets that orbit their parent star at less than about one astronomical unit (1?AU is the Earth-Sun distance) are expected to be engulfed when the star becomes a red giant. Previous observations have revealed the existence of post-red-giant host stars with giant planets orbiting as close as 0.116?AU or with brown dwarf companions in tight orbits, showing that these bodies can survive engulfment. What has remained unclear is whether planets can be dragged deeper into the red-giant envelope without being disrupted and whether the evolution of the parent star itself could be affected. Here we report the presence of two nearly Earth-sized bodies orbiting the post-red-giant, hot B subdwarf star KIC 05807616 at distances of 0.0060 and 0.0076?AU, with orbital periods of 5.7625 and 8.2293 hours, respectively. These bodies probably survived deep immersion in the former red-giant envelope. They may be the dense cores of evaporated giant planets that were transported closer to the star during the engulfment and triggered the mass loss necessary for the formation of the hot B subdwarf, which might also explain how some stars of this type did not form in binary systems.     

Substantial reservoirs of molecular hydrogen in the debris disks around young stars

Circumstellar accretion disks transfer matter from molecular clouds to young stars and to the sites of planet formation. The disks observed around pre-main-sequence stars have properties consistent with those expected for the pre-solar nebula from which our own Solar System formed 4.5 Gyr ago. But the 'debris' disks that encircle more than 15% of nearby main-sequence stars appear to have very small amounts of gas, based on observations of the tracer molecule carbon monoxide: these observations have yielded gas/dust ratios much less than 0.1, whereas the interstellar value is about 100 (ref. 9). Here we report observations of the lowest rotational transitions of molecular hydrogen (H2) that reveal large quantities of gas in the debris disks around the stars beta Pictoris, 49 Ceti and HD135344. The gas masses calculated from the data are several hundreds to a thousand times greater than those estimated from the CO observations, and yield gas/dust ratios of the same order as the interstellar value.     

A close halo of large transparent grains around extreme red giant stars

An intermediate-mass star ends its life by ejecting the bulk of its envelope in a slow, dense wind. Stellar pulsations are thought to elevate gas to an altitude cool enough for the condensation of dust, which is then accelerated by radiation pressure, entraining the gas and driving the wind. Explaining the amount of mass loss, however, has been a problem because of the difficulty of observing tenuous gas and dust only tens of milliarcseconds from the star. For this reason, there is no consensus on the way sufficient momentum is transferred from the light from the star to the outflow. Here we report spatially resolved, multiwavelength observations of circumstellar dust shells of three stars on the asymptotic giant branch of the Hertzsprung-Russell diagram. When imaged in scattered light, dust shells were found at remarkably small radii (less than about two stellar radii) and with unexpectedly large grains (about 300 nanometres in radius). This proximity to the photosphere argues for dust species that are transparent to the light from the star and, therefore, resistant to sublimation by the intense radiation field. Although transparency usually implies insufficient radiative pressure to drive a wind, the radiation field can accelerate these large grains through photon scattering rather than absorption--a plausible mass loss mechanism for lower-amplitude pulsating stars.     

Deep and stable interferometric nulling of broadband light with implications for observing planets around nearby stars

The number of indirectly detected planetary systems around nearby stars has grown tremendously since their initial discovery five years ago. But the direct observation of light reflected from these systems remains a formidable task, because of the high contrast ratios between them and their parent stars, and because of the tiny angular separations. Theoretically, these difficulties can be overcome by using a dual-aperture stellar interferometer in which the starlight is cancelled, or 'nulled' by broadband destructive interference, leaving the planet's light visible. Although the basic requirement of equal and oppositely directed electric fields is easy to state, an experimental demonstration of deep broadband nulling has been lacking, owing to difficulties engendered by the needs for extreme symmetry and stability, and low dispersion in the optical system. Here we report the deep (10(-4)) and stable nulling of broadband (18% bandwidth) thermal light. These results validate the physical principles underlying future planet-searching interferometers, and our laboratory instrument will serve as a prototype for the nulling instrument to be implemented on the Keck interferometer in 2001.     

Iontronic pressure sensor with high sensitivity and linear response over a wide pressure range based on soft micropillared electrodes

Electronic skins and flexible pressure sensors are important devices for advanced healthcare and intelli-gent robotics.Sensitivity is a key parameter of flexibl...     

一种大量程疲劳试验机实现小试样预制裂纹的方法

根据简单的力学原理，提出了一种使用大量程程控高频疲劳试验机，按照国标2038-91标准预制三点弯曲小试样裂纹的方法——双试样法，并与单试样法进行了对比，给出了实验数据，说明了双试样法的优点，详细给出了三点弯曲试样裂纹预制的相关公式及确定各个载荷具体方法。     

纳入蠕变变形机理的宽范围应力区下P91钢本构模型

以P91钢为对象,构建宽范围应力区基于蠕变变形机理(CDMWRS)的本构模型,采用ABAQUS软件,结合构建的CDMWRS本构模型,模拟单轴蠕变拉伸试验,并与文献数据进行对比.结果表明:宽范围应力区的不同蠕变变形机理和长期热老化都会对P91钢的蠕变应变率产生较大影响,CDMWRS模型能够较好地描述宽范围应力下P91钢的蠕变第一阶段和第二阶段变形行为,蠕变第三阶段相对偏差较大,主要原因是因为构建的CDMWRS模型未纳入损伤的影响.