A Study on the Microhardness of Organic Molecular Solids

An extensive study of the variation of microhardness with load in a large number of crystals of some organic molecular solids has been carried out. The plots of hardness versus load show peaks at specific loads and these have been attributed to the different slip systems in these crystals. These slip systems are in accordance with the crystal structure of these crystals and are dependent on the shape and way the molecules are geometrically deposed in the unit cell of the crystals.     

An X-band time domain electron paramagnetic resonance spectrometer

A computer-controlled X-band time domain electron paramagnetic resonance (EPR) spectrometer, with a time resolution of the order of 0.5μsec, has been constructed with many of the crucial microwave components designed and fabricated by the Microwave Engineering Group of TIFR. The spectrometer operates either in a microwave power pulsed mode for determination of spin-lattice relaxation times by the saturation recovery technique or in the kinetic mode for determination of the time dependence of EPR signal after laser excitation. It has an automatic frequency control, an automatic phase control and, most importantly, a field-frequency lock which ensures good stability of the EPR line positions enabling signal averaging for extended periods. The constructional details of the spectrometer and its performance in both the modes are described here by reporting results on certain typical systems.     

Hot zones evolution and dynamics in heterogeneous catalytic systems

Stationary and complex moving hot regions formed for temperatures close to the extinction temperature of uniformly ignited states of several catalytic systems, such as thin rings and hollow cylinders, a thin radial flow reactor (RFR) and a shallow packed bed. IR imaging revealed that the hot and cold regions (temperature difference of the order of 100 degrees C) were separated by a sharp (about 3 mm wide) temperature front. The transition from the branch of uniformly ignited to the states with a hot region was usually supercritical. In some experiments a disjoint branch of states with hot regions existed and two qualitatively different states with hot zones existed under the same operating conditions. A very intricate periodic motion of a hot zone was observed in a shallow packed bed reactor. For example, Fig. 16 shows a hot zone which splits and later coalesces several times during the long (14 h) period. Hot pulse motions were observed on a single catalytic pellet. These were caused by global coupling between the surface reaction rate and the ambient reactant concentration and the inherent nonuniformity of the catalytic activity. It is not yet clear what rate processes generate the transversal hot zones in uniform packed bed reactors. (c) 2002 American Institute of Physics.     

Energy dependence of multiplicity in proton-nucleus collisions and models of multiparticle production

This is a continuation of our earlier investigation (Gurtuet al 1974Phys. Lett. 50 B 391) on multiparticle production in proton-nucleus collisions based on an exposure of emulsion stack to 200 GeV/c beam at the NAL. It is found that the ratioR _em = 〈n _s〉/〈n _ch〉, where 〈n _ch〉 is the charged particle multiplicity in pp-collisions, increases slowly from about 1 at 10 GeV/c to 1·6 at 68 GeV/c and attains a constant value of 1·71 ± 0·04 in the region 200 to 8000 GeV/c. Furthermore,R _em = 1·71 implies an effectiveA-dependence ofR _A =A ^0.18,i.e., a very weak dependence. Predictions ofR _em on various models are discussed and compared with the emulsion data. Data seem to favour models of hadron-nucleon collisions in which production of particles takes place through adouble step mechanism,e.g., diffractive excitation, hydrodynamical and energy flux cascade as opposed to models which envisage instantaneous production.     

Mercury(II) ion-selective electrodes based on heterocyclic systems

Mercury ion-selective electrodes (ISEs) were prepared with a polymeric membrane based on heterocyclic systems: 2-methylsulfanyl-4-(4-nitro-phenyl)-l-p-tolyl-1H-imidazole (I) and 2,4-diphenyl-l-p-tolyl-1H-imidazole (II) as the ionophores. Several ISEs were conditioned and tested for the selection of common ions. The electrodes based on these ionophores showed a good potentiometric response for Hg2+ ions over a wide concentration range of 5.0 x 10(5-) - 1.0 x 10(-1)M with near-Nernstian slopes. Stable potentiometric signals were obtained within a short time period of 20 s. The detection limits, the working pH range of the electrodes were 1.0 x 10(-5) M and 1.6-4.4 respectively. The electrodes showed better selectivity for Hg2+ ions over many of the alkali, alkaline-earth and heavy metal ions. Also sharp end points were obtained when these sensors were used as indicator electrodes for the potentiometric titration of Hg2+ ions with iodide ions.     

The band structure and magnetic properties of some transition-metal phosphides. II. V,Cr, and Mn monophosphides

The band structures and density of states for VP, CrP, and MnP are calculated by an LCAO-TB method. The results allow interpretation of the metallic nature of the materials, the trend in magnetic properties observed, and the bonding in such binary phosphides.