Synthesis of ultra‐low‐molecular‐weight polyethylene wax using a bulky Ti(IV) aryloxide–alkyl aluminum catalytic system

Complexes of titanium(IV) with bulky phenolic ligands such as 2‐tert‐butyl‐4 methylphenol, 2, 4‐di‐tert‐butyl phenol and 3,5‐di‐tert‐butyl phenol were prepared and characterized. These catalyst precursors, formulated as [Ti(OPh*)_n(OPrⁱ)_4?n] (OPh* = substituted phenol), were found to be active in polymerization of ethylene at higher temperatures in combination with ethylaluminum sesquichloride (Et₃Al₂Cl₃) as co‐catalyst. It was observed that the reaction temperature and ethylene pressure had a pronounced effect on polymerization and the molecular weight of polyethylene obtained. In addition, this catalytic system predominantly produced linear, crystalline ultra‐low‐molecular‐weight polyethylenes narrow dispersities. The polyethylene waxes obtained with this catalytic system exhibit unique properties that have potential applications in surface coating and adhesive formulations. Copyright © 2007 John Wiley & Sons, Ltd.     

Low temperature elastic behaviour of As-Sb-Se and Ge-Sb-Se glasses

The ternary glasses of arsenic and germanium with antimony and selenium can be prepared in large sizes for optical purposes. The elastic behaviour of eight compositions of each glass has been studied down to 4.2 K using a 10 MHz ultrasonic pulse echo interferometer. The glasses have a normal elastic behaviour, with the velocities gradually increasing as the temperature is lowered. An anharmonic solid model of Lakkad satisfactorily explains the temperature variations. The elastic moduli of Ge _x Sb₁₀Se_90?x glasses increase linearly as the Ge content is increased up to 25 at. % and beyond this the increase is nonlinear. (AsSb)₄₀Se₆₀ glasses show a linear increase in elastic moduli with increasing Sb content. The elastic moduli of As _x Sb₁₅Se_85?x glasses exhibit a drastic change near the stoichiometric composition As₂₅Sb₁₅Se₆₀. These behaviours have been qualitatively explained on the basis of the structural changes in glasses.     

Surface and catalytic properties of Cu–Ce–O composite oxides prepared by combustion method

Surface and catalytic properties of Cu–Ce–O composite materials prepared by solution combustion method have been investigated. The materials are characterized by X-ray diffraction (XRD), temperature programmed reduction (TPR) and electron paramagnetic resonance spectroscopy (EPR). The results of EPR and TPR show finely dispersed Cu²⁺ species on ceria matrix with low copper content. The Cu²⁺ species exists in the form of dimers and clusters which are not evident in XRD. In addition CuO is also present as small clusters which grow to larger size at higher Cu content. There is no evidence of CuO forming a solid solution with fluorite CeO₂ in combustion method. The Cu²⁺ species mostly appear on surface rather than in the bulk. Hydrogen peroxide decomposition kinetics has been carried out on Cu–Ce–O composite materials to investigate the effect of crystalline and well-dispersed copper oxide phases on CeO₂. From kinetic results, the catalyst materials can be grouped into highly dispersed as well as crystalline CuO phases present on CeO₂ matrix. Two parallel compensating lines for dispersed and crystalline CuO phases on CeO₂ are observed in ln A versus E_a plot indicating the compensation effect in H₂O₂ decomposition. This observation is consistent with XRD and EPR results.     

Conduction studies on electrodeposited indium selenide thin films

Indium Selenide thin films were electrodeposited on Indium Tin Oxide (ITO) coated glass substrates from a mixture of Indium trichloride (InCl₃) and selenium dioxide (SeO₂) in aqueous solution by the potentiostatic electrodeposition technique. The InSe films showed that Mott’s variable-range hopping conductivity (VRH) is dominant under low field (∼2×10⁵ Vcm⁻¹) condition. At high field the Current-Voltage studies (in the temperature range 300 – 380 K) revealed that the conductivity of Indium selenide thin films is of Poole-Frenkel type. The plot drawn between Ln (I/V) and tV showed a straight line and also the experimental data and the theoretical data closely matches. Hence Poole-Frenkel mechanism is confirmed and discussed in detail. Paper presented at the 2nd International Conference on Ionic Devices, Anna University, Chennai, India, Nov. 28–30, 2003.     

Elastic constants of sodium chlorate from 77 K to 350 K

The elastic constantsC ₁₁,C ₁₂ andC ₄₄ of sodium chlorate single crystal have been evaluated using 10 MHz ultrasonic pulse echo superposition technique. The values areC ₁₁=4.90,C ₁₂=1.39,C ₄₄=1.17 (× 10¹⁰ N/m ²) at 298 K and 6.15, 2.16, 1.32 (×10¹⁰ N/m ²) at 77 K. The data agree well with the values measured earlier up to 223 K. Brief mention is also made of the low temperature bonding problems in these soft crystals.     

Electrical transport and high pressure studies on bulk Ge20Te80 glass

The electrical resistivity of bulk Ge₂₀Te₈₀ has been measured as a function of pressure and temperature. At 5 GPa, an amorphous semiconductor-to-crystalline metal transition has been observed. The sample recovered from the high pressure cell, after the application of 7 GPa, has a face-centred cubic structure with a lattice constant of 6·42 A. In crystalline sample, the semiconductor-to-metal transition occurs at 7 GPa. The thermoelectric power has also been measured for glassy samples in the temperature range 300–240 K.     

Stannoxanes and phosphonates: New approaches in organometallic and transition metal assemblies

Phosphonate ligands, [RPO₃]^2-, are extremely versatile in the assembly of multi-tin and multi-copper architectures. We have used organostannoxane cores for supporting multi-ferrocene and multi-porphyrin peripheries. The copper-metalated multi-porphyrin compound is an excellent reagent for facile cleavage of DNA, even in the absence of a co-oxidant. Reaction oft-BuPO₃H₂ with Cu(C10₄)₂. 6H₂O in the presence of 2-pyridylpyrazole (2-Pypz) leads to the synthesis of a decanuclear copper (II) assembly.     

Elastic constants of galena down to liquid helium temperatures

The elastic constants of single crystal galena have been determined from the measured ultrasonic velocities down to liquid helium temperature. A cryostat incorporating an arrangement to inject the liquid bonding material at low temperature is described. At 5 K, the values of elastic constants are C₁₁=14.9₀, C₁₂=3.5₁ and C₄₄=2.9₂×10¹⁰ N/m².