Machining long slender workpieces still presents a technical challenge on the shop floor due to their low stiffness and damping. Regenerative chatter is a major hindrance in machining processes, reducing the geometric accuracies and dynamic stability of the cutting system. This study has been motivated by the fact that chatter occurrence is generally in relation to the cutting position in straight turning of slender workpieces, which has seldom been investigated comprehensively in literature. In the present paper, a predictive chatter model of turning a tailstock supported slender workpiece considering the cutting position change during machining is explored. Based on linear stability analysis and stiffness distribution at different cutting positions along the workpiece, the effect of the cutting tool movement along the length of the workpiece on chatter stability is studied. As a result, an entire stability chart for a single cutting pass is constructed. Through this stability chart the critical cutting condition and the chatter onset location along the workpiece in a turning operation can be estimated. The difference between the predicted tool locations and the experimental results was within 9% at high speed cutting. Also, on the basis of the predictive model the dynamic behavior during chatter that when chatter arises at some cutting location it will continue for a period of time until another specified location is arrived at, can be inferred. The experimental observation was in good agreement with the theoretical inference. Moreover, it is shown that vibration spectrum is more sensitive to the chatter evolution than the signal variance. Specifically, when the cutting operation transfers from stable to unstable state, the corresponding vibration frequency features a shift from the spindle rotation frequency or its harmonics to the critical chatter frequency that slightly varies during the chatter-lasting period.

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Coherent twin boundaries in particulate chalcogenide photocatalysts can induce intrinsic homojunctions, which have been demonstrated to be effective toward solar hydrogen production from water. However, limited success up to date has been achieved on pure water splitting because of the self-oxidation of the chalcogenide photocatalyst. Herein, we report the efficient pure water splitting for simultaneous H 2 and H 2 O 2 production under visible light irradiation over a Cd 0.5 Zn 0.5 S (CZS) nanotwin photocatalyst. The success relies on the formation of unique phosphorus bridges at the surface of the photocatalyst that couples bulk twin boundaries and surface red phosphorus (RP) for efficient charge separation and restricted photocorrosion via a two-electron Z-scheme mechanism. The optimal photocatalyst exhibited 5.2 times increased H 2 evolution rate compared to the pristine CZS and good stability as well. This work opens a door toward chalcogenide photocatalyst design for efficient pure water splitting by coupling twin nanostructures with surface P doping and heterojunction construction.

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Slow intramolecular electron transfer (ET) has been observed in the binuclear intermediate of a Ti(III)-Ru(III) redox reaction that involves a dianion derived from a dihydroxyquinone as bridging ligand; this indicates that this ligand provides small but significant cross-bridge Ru(III)-Ti(III) electronic interaction. A Ti(III)-Co(III) reaction with a similar bridging ligand follows a different intimate mechanism (formation of a quinone radical ion), but intramolecular Co(III)-Ti(III) ET through squarate as bridging ligand is so slow that much of the overall redox change is accomplished by a competing outer-sphere reaction of that intermediate with TiOH2+. A two-phase reaction was observed for the Ti(III) reduction (in M LiCl at 25 °C) of Co(en)2SQ+ (en is ethylenediamine; SQ2™ is the dianion of squaric acid). Relatively rapid formation of a binuclear intermediate is followed by competition between outer-sphere attack of TiOH2+ on the deprotonated intermediate and intramolecular ET within that deprotonated species: Ti3+ -TiOH2+ + H+ (ATa); Co(en)2SQ+ + Ti3+ -Co(en)2(SQ)Ti4+ (AT,); Co(en)2(SQ)Ti4+Co(en)2(SQ)TiOH3+ + H+ (Af2); Co(en)2(SQ)TiOH3+producto (*3); Co(en)2(SQ)TiOH3+ + TiOH2+products (Ác4). Here, kjK&Ki = 14s™1 andk3ATiAf2 = 0.8s™1. Reduction of Co(trien)Q+ (trien is triethylenetetramine, and Q2" is the dianion of 2,5-dihydroxy-1,4-benzoquinone) by Ti(III) (in M LiCl at 25 ®C) also involves formation of a dinuclear complex between Ti3+ and the oxidant (XV = 4.5 X 102 M™1) followed by further reaction of that complex by two competing paths. One path is intramolecular ET within the intermediate (*V < 0.5 s_1); the other path is reaction between a second TiOH2+ and the preassembled (not deprotonated) intermediate (k/ = 1.3 X 103 s™1 M™1). Reduction of Ru(trien)CQ+ (CQ2™ is the dianion of 3,6-dichloro-2,5-dihydroxy-1,4-benzoquinone) byTi3+ also involves formation of an intermediate (Xi" = 6 X 10 M™1). Intramolecular ET within that intermediate has a rate constant of 1.1 X 102 s™1, and AH* = -16 kJ/mol and AS* = -150 J/(deg mol). Intramolecular ET in this kinetic intermediate is many orders of magnitude slower than that in analogous pyrazine-bridged mixed-valence diruthenium systems; electronic coupling between donor and acceptor orbitals is smaller in the dihydroxyquinonatebridged species than in the pyrazine-bridged dimers due to orbital symmetry mismatch in the former case.

Chatter vibrations result in poor surface finish, reduced productivity, and shortened tool life in metal cutting. Although chatter stability and monitoring techniques are broadly studied in literature, the deployment of process monitoring system in the shop floor still presents a challenge. In this paper, an online prototype system including two independent modules of chatter stability prediction and monitoring for industrial applications has been constructed cooperatively. Turning and milling processes are considered. Firstly, the stability charts are evaluated by the application of the Laplace transform and the semi-discretization method. The obtained charts can assist the operators in choosing appropriate cutting parameters before operations start. Secondly, the characteristics of chatter are extracted in the time and frequency domains. The norm of chatter arising is formulated in accordance with the variance and spectrum of vibration signals. The corresponding threshold is confirmed relatively in comparison with the characteristics of the stable cutting states recorded in the reference database. Thirdly, the algorithms and modules are implemented with the combination of the advantages of MATLAB and C# programming, in which delay strategy and overlap processing enable to improve the accuracy and rapidity of the monitoring system. Finally, the effectiveness of the proposed system has been verified by experimental results of the industrial trials.

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