Active isotropic compliance in redundant manipulators

Multibody System Dynamics - The isotropic compliance property is examined in the Special Euclidean Group SE(3) in the case of redundant manipulators. The redundancy problem is solved by means of...     

上转换纳米棒NaGdF_4:Yb~(3+),Tm~(3+)的合成、形貌调控及其荧光研究

水热法制备了高纯度六方相NaGdF_4:Yb~(3+),Tm~(3+)纳米颗粒,采用以聚乙烯吡咯烷酮(PVP)为表面活性剂,通过X射线衍射(XRD),透射电镜(TEM),元素分析(mapping),荧光光谱学等手段对所制备的NaGdF_4纳米棒的结构与形貌进行表征,发现提高反应时间对粒子尺寸和形貌的影响不明显,表面活性剂的浓度对发光强度有明显影响,并着重研究粒子长径比和发光强弱关系的特点。结果表明:共掺杂NaGdF_4纳米颗粒在980 nm下可发射出475 nm蓝光,以及695 nm红光800 nm的强近红外光。且发现颗粒长径比越大,荧光强度越强,进一步探索了颗粒长径比调控的荧光增强机理。     

嵌段聚合物包埋铕络合物的纳米形貌结构及发光性质研究

一种利用嵌段聚合物作为配体,与稀土离子络合制备一种具备特殊结构的高荧光性能复合材料的方法。嵌段聚合物中的P4VP链段中含有吡啶环,能够为稀土离子提供配位键,邻菲啰啉作为第二配体,并且以DMF作为溶剂。这里用到的稀土离子为铕,它作为交联中心,以稀土离子作为核心与嵌段聚合物的吡啶环配位交联;同时,嵌段聚合物上的亲油部分可以溶于DMF中,这样形成的络合物可以分散于溶剂中,不至于团聚或沉淀。用核磁(~1H NMR)和紫外光谱表征其结构,利用透射电镜(TEM)和荧光光谱仪(FS)等研究其形貌结构和荧光性能,通过调整比率来控制其尺寸大小和荧光强度。     

Nonlinear stress relaxation in palladium(II) complexes with triblock copolymers of styrene and butadiene

Above 200% strain, the mechanical response of triblock copolymers which contain styrene and butadiene is modified significantly by complexation with dichlorobis(acetonitrile)palladium(II). This pseudosquare‐planar transition metal salt forms π‐complexes with, and catalyzes the dimerization of, alkene groups in the main chain and the side group of Kraton's butadiene midblock. Between 10 and 100% strain, the plastic flow regime is similar for undiluted Kraton? and its Pd²⁺ complexes, but the level of engineering stress is approximately twofold larger for the complex that contains 4 mol % palladium(II) [Pd(II)]. Nonlinear stress relaxation measurements in the plastic flow regime (i.e., beyond the yield point but before the large upturn in stress) are analyzed at several different levels of strain. Transient relaxation moduli were modeled by a three‐parameter biexponential decay with two viscoelastic time constants. The longer relaxation time for Kraton? increases at higher strain, and in the presence of 4 mol % palladium chloride. A phenomenological model is proposed to describe the effect of strain on relaxation times. This model is consistent with the fact that greater length scales are required for cooperative segmental reorganization at larger strain. The resistance Ω to conformational reorganization during stress relaxation is estimated via integration of the normalized relaxation modulus versus time data. This resistance increases at higher initial jump strain because conformational rearrangements are influenced strongly by knots and entanglements at larger strain. The effect of strain on Ω is analyzed in terms of time‐strain separability of the relaxation modulus. Linear behavior is observed for Ω versus inverse strain (i.e., 1/ε), and the magnitude of the slope [i.e., ?dΩ/d(1/ε)] is threefold larger in the absence of PdCl₂(CH₃CN)₂. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 93: 1329–1336, 2004     

CONVECTIVE DIFFUSION IN HETEROGENEOUS CATALYTIC DUCT REACTORS WITH NONCIRCULAR CROSS SECTION AND NONUNIFORM CATALYST ACTIVITY

Steady-state simulations of convection, diffusion and first-order irreversible heterogeneous chemical reaction are presented for catalytic channels with rectangular cross section and nonuniform catalyst activity. Finite difference results from the microscopic three-dimensional mass transfer equation also satisfy the cross-section-averaged one-dimensional form of the same equation. Comparisons between viscous flow and plug flow in square cross-section channels suggest how previous inferences of surface-averaged reaction velocity constants from plug flow simulations should be modified when convective diffusion in the mass transfer boundary layer adjacent to the catalytic surface is modeled correctly. Over the following range of Damkohler numbers (i.e., 20 to 10³), viscous flow in rectangular ducts with aspect ratios between two and three can be approximated by the corresponding problem in tubes with the same effective diameter. For Damkohler numbers between 0.5 and 10³, aspect ratios greater than 20 are required to simulate viscous flow between two parallel plates with catalyst coated on both walls. At low Damkohler numbers where reactant diffusion toward the catalytic surface is not the rate-limiting step, nonuniform activity profiles suggest that most of the catalyst should be deposited in regions that are easily accessible to the reactants. However, this strategy for converting reactants to products is not more effective than uniform deposition in the diffusion-limited regime.     

Mixing effects on the mid-kilohertz mobility of polystyrene in glassy polystyrene-diluent blends

Low molecular weight additive effects on the mid-kilohertz mobility of atactic polystyrene (PS) at 25°C via n.m.r. spin-lattice relaxation experiments in the rotating frame (T_1?) are analogous to the results of a previous study of diluents in glassy bisphenol-A polycarbonate. The diluent with a relatively low glass transition temperature (T_g), dioctylphthalate, increases the spectral density of thermal motions of the chain backbone and the pendant group in PS on the order of 38.5 kHz. Of the ¹³C nuclei in the glassy polymer, the relaxation behaviour of which can be differentiated by high-resolution, solid-state n.m.r., the T_1?'s of the aromatic carbons in the side group are affected most by dioctylphthalate. In contrast, the styrene oligomer, which has a higher T_g than that of dioctylphthalate, does not significantly alter the ambient temperature mobility of polystyrene at 38.5 kHz. The PS-styrene oligomer and PS-dioctylphthalate blends are examples of athermal and non-athermal mixtures, respectively. However, the effect of the enthalpy of mixing on the T_1?'s of the polymer is probably obscured by differences in blend mobility due to different blend T_g's.     

Localization of source regions of selected hydrofluorocarbons combining data collected at two European mountain stations

Ground-based in situ measurements of hydrofluorocarbons HFC-125, HFC-134a, and HFC-152a, which are regulated under the Kyoto Protocol, are carried out at four European sites within the SOGE (System of Observation of Halogenated Greenhouse Gases in Europe) program. Concentrations measured at the high mountain stations of Jungfraujoch (Switzerland) and Mte Cimone (Italy) together with back-trajectory statistical analysis are used in order to identify potential source regions on a European scale. Combining concentration data recorded at the two sites allows to reduce one of the problem which is inherent to the back-trajectory approach, i.e. the localisation of "ghost" sources in the wake of real sources. In this way, a more reliable picture of the location of European potential source regions is given.     

On a test bench for studying lubrication in a spherical bearing: simulations and experimental validation

In this paper a test bench dedicated to the analysis of a lubricated spherical pair is presented. The experimental set‐up has been used to validate a numerical code that has been developed by the authors in order to simulate a complex lubricated spherical pair that affects the reliability of a novel continuous variable transmission (CVT). The test bench is modular and can be adapted to other lubricated pairs, either cylindrical or spherical. The stand is equipped with an I/O control card and allows the on‐line acquisition of some important parameters. As for the numerical code, the finite‐difference method has been used to solve the indirect problem in the lubricated spherical pad, under the condition of hydrodynamic regime, with externally pressurized inlets. Two simulators are needed since the physical properties of the test bench and the real system are different. The one that simulates the behavior of the spherical pair in the test stand has been used to ascertain the numeric approach prediction capability. Indeed, the experimental tests gave results which are in very good correspondence with the numerical predictions. Hence, the second simulator, which has been tailored on the particular spherical pair the CVT is equipped with, gained reliability in predicting the behavior of the real system under different load and speed conditions. These predictions have been useful in achieving new insights into the novel CVT, which, under this new light, has been criticized by making the point that its range of application in automotive is, at the actual state of the art, rather limited, in spite of a high intrinsic complexity. Copyright © 2006 John Wiley & Sons, Ltd.     

Chernoff bound of trellis-coded modulation over correlated fadingchannels

We derive a Chernoff upper bound for the pairwise error probability in the presence of an additive white Gaussian noise and a Rayleigh or Rice correlated fading. The bound is useful for situations where perfect interleaving cannot be achieved. We use it to determine some indications in the design of optimum trellis coded modulation for correlated fading channels