Stick-slip instability analysis

A theoretical investigation of friction-induced self-excited oscillations for systems with one degree of freedom is proposed. The friction force is assumed as an odd function of the relative sliding velocity with a jump discontinuity at a value of zero for the relative sliding velocity. The friction characteristic is approximated with a piecewise linear function, i.e. straight line segments with a suitable slope. For the generic system belonging to the class in question, the stick-slip instability region is located on a suitable dimensionless map.

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Sommario Viene proposta un'indagine teorica sulle oscillazioni autoeccitate indotte dall'attrito per sistemi ad un grado di libertà. La forza d'attrito viene assunta come funzione dispari della velocità relativa tra le superfici accoppiate, con una discontinuità di prima specie in corrispondenza del valore nullo della velocità. La caratteristica d'attrito viene approssimata mediante una funzione lineare a tratti con segmenti di opportuna pendenza. Per il generico sistema appartenente alla classe in esame, si perviene all'individuazione, su opportuna mappa adimensionale, della regione di instabilità da stick-slip.

     

Spectres infrarouges du furanne et du thiophéne aux basses températures

Recent X-ray determination of the structure of solid furan allowed us to reexamine its fundamental vibrations. Analysis of the low temperature phase spectrum has been facilitated by comparisons with the spectra of the liquid, the high temperature solid phase and in an inert gas matrix. An analogous study has been performed for thiophene in the liquid phase and in the four solid phases. We suggest a quadratic structure to account for the crystalline cell of the solid (IV) phase.     

The Decomposition of Methane,Ethane, Ethylene and n-Butane in Electrical Discharges of Moderate Pressures

The decomposition of CH₄, C₂H₆, C₂H₄, and n-C₄H₁₀ has been investigated in radiofrequency plasmas operated at power levels of 1–8 cal cm^?3 sec^?1, 10–40 torr and flow rates of the hydrocarbons between 2 and 20 1 (STP) min^?1. It is shown that values of the rate constants are very close for all the hydrocarbons investigated and several orders of magnitude greater than those observed in their pyrolisis at comparable gas temperatures. Energy yields up to 30% can be observed for all hydrocarbons. It is suggested that the rate determining step for the decomposition process of all hydrocarbons investigated involves the rupture of a C-H bond in vibrotationally excited molecules.     

Electro-oxidation of CO on Pd black in phosphotungstic acid

Electro-oxidation of CO by Pd black in phosphotungstic acid (PWA) has been studied on porous gas-diffusion electrodes. Galvanostatic steady-state polarization data show that the reaction proceeds under strong activation control at low current densities. The current density at 0.5 V_NHE is found to be three times higher than that in a H₂SO₄ reference electrolyte. X-ray photoelectron spectroscopic data on various electrochemically tested electrodes show lower oxidized Pd species in PWA than in H₂SO₄. It is conjectured that an increase in the electron density around Pd sites is ameliorated by PWA in relation to H₂SO_4, with a consequent promoting effect of the heteropolyacid towards CO electro-oxidation. Received: 29 July 1998 / Accepted: 15 September 1998