1,721,101 research outputs found
A unicity theorem on the realizations of sem-isimple Lie algebras with quantum canonical variables
No full textGribov problem for gauge theories: a pedagogical introduction
No full textThe functional-integral quantization of non-Abelian gauge theories is affected by the
Gribov problem at non-perturbative level: the requirement of preserving the supplementary
conditions under gauge transformations leads to a nonlinear differential equation,
and the various solutions of such a nonlinear equation represent different gauge configurations
known as Gribov copies. Their occurrence (lack of global cross-sections from the
point of view of differential geometry) is called Gribov ambiguity, and is here presented
within the framework of a global approach to quantum field theory. We first give a simple
(standard) example for the SU(2) group and spherically symmetric potentials, then we
discuss this phenomenon in general relativity, and recent developments, including lattice
calculations
A Boundary Computation Algorithm for the Workspace Evaluation of Continuum Parallel Robots
Full text linkIn this article, a new algorithm for the computation of workspace boundaries of continuum parallel robots (CPRs) is proposed. State-of-the-art techniques are mainly based on time-consuming joint space discretization approaches or task-space discretization algorithms, and only a few approaches are dedicated to the computation of workspace boundaries. The proposed approach for the computation of the workspace boundaries is based on (i) a free-space exploration strategy and (ii) a boundary reconstruction algorithm. The former is exploited to identify an initial workspace boundary location (exterior, interior boundaries, and holes), while the latter is used to reconstruct the complete boundary surface. Moreover, the algorithm is designed to be employed with CPR modeling strategies based on general discretization assumptions, in order to increase its applicability for various scopes. Our method is compared with two state-of-the-art algorithms in four cases studies, to validate the results and to establish its merits and limitations
Singularity Conditions of Concentric Tube Robots
No full textThis paper derives singularity conditions for concentric tube robots (CTRs). To the scope, we build the CTR geometrico-static model using discretization techniques, which are suitable for several currently-used CTRs models. Then, we obtain singularity conditions by performing a linearization of the geometrico-static model. We define Type-1 and Type-2 singularities, which are related to the unsolvability of the inverse and forward kinematostatic problems, respectively. Moreover, we also show the link between Type-2 singularities and CTR equilibrium stability. A case study with a two-tube CTR is proposed to illustrate our results
Design and experimental equilibrium stability assessment of a RFRFR continuum parallel robot
No full textContinuum parallel robots (CPRs) are mainly constituted by flexible links arranged in parallel between a rigid platform and a rigid base, and they promise remarkable performance in human–robot collaboration applications. New CPRs modelling strategies and their experimental validation are continuously investigated due to the nonlinear phenomena complexity and the high computational effort required to solve them. This work focuses on the experimental validation of CPRs equilibrium stability prediction. We demonstrate that models based on planar displacement assumptions may fail in the equilibrium stability prediction, even though the CPR is nominally planar. A new CPR prototype for planar applications is proposed, designed, and tested for the scope. Unstable configurations that limit the robot workspace are theoretically and experimentally investigated. A singularity type, related to out-of-the-plane uncontrolled motions of the planar CPR, is experimentally identified for the first time. Experiments demonstrate that, even though the prototype is theoretically planar, a planar model neglecting out-of-the-plane phenomena is inadequate to assess equilibrium stability limits
Optimizing noble metals exploitation in water oxidation catalysis by their incorporation in layered double hydroxides
No full textWater oxidation (WO) is considered the ideal process for the generation of electrons and protons necessary for the production of sustainable fuels. The unfavorable kinetics of this reaction requires the development of highly efficient catalysts (WOCs). The most promising performance has been obtained with noble metal based systems, which however pose challenges for the careful exploitation of these precious elements. In this Review, one of the most promising strategies for the development of WOCs in a ‘noble metal atom economy’ fashion is described, that is, the incorporation of these metals in layered double hydroxides (LDHs) of earth abundant elements. A rapidly raising number of studies demonstrate that excellent catalytic activity and durability can be obtained by following this strategy. The strong electronic coupling between noble metal nanoparticles or single atoms and the LDHs matrix is typically responsible for such intriguing results. Interestingly, noble metal incorporation as low as 1 wt% allows to boost the catalytic performance of intrinsically active systems like NiFe-LDHs, but also to activate otherwise inert materials like ZnAl-LDHs
– Osservazioni su specie di Fusarium isolate da insetti in un’area della regione Campania.
No full textHemi-metallocene Ti(IV) η3-allyl-type complexes: Structure, dynamics in solution and exploration of reactivity
No full textThe cationic complexes [Cp*(tBu3P=N)Ti(η3-CH2C(C5H11)CH2)]+ (Ti-Alla+) and [Cp*(tBu3P=N)Ti(η3-CH2C(CH3)CH(C4H9))]+ (Ti-Allb+), bearing η3-allyl-type fragments coordinated to a Ti(IV) center with Cp* and phosphinimide ancillary ligands, were easily synthesized from the corresponding cationic Ti-benzyl complex by reaction with 2-methyl-1-heptene. The unusual stability of these allyl species at room temperature allowed detailed NMR spectroscopic investigation of their structure and dynamic behavior in solution. The spectroscopic results provided supporting experimental evidence for η3-coordination mode of the allyls and allowed to determine their preferential configuration. Furthermore, quantitative analysis of the exchange kinetics between syn and anti allylic protons was carried out by 1H EXSY NMR spectroscopy. The activation parameters for the latter process indicate that the faster exchange observed with Ti-Alla+ (ΔH‡ ≈ 16 kcal mol−1; ΔS‡ ≈ 0 cal mol−1 K−1) compared to Ti-Allb+ (ΔH‡ ≈ 11 kcal mol−1; ΔS‡ ≈ −19 cal mol−1 K−1) is primarily of entropic origin. A tentative interpretation for the higher stability of Ti-Alla+ and Ti-Allb+, compared to their bis-Cp titanocene analogues reported in the literature, is proposed based on steric considerations. Finally, an exploratory reaction, using benzaldehyde as representative substrate, was carried out to demonstrate the capability of Ti-Alla+ and Ti-Allb+ to serve as allylating agents for carbonyl compounds
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