A kinetic model for radio frequency plasma-activated chemical vapour deposition

A kinetic model for r.f. plasma-activated chemical vapour deposition is proposed. With this model we are able to explain the deposition profiles of silicon nitride films obtained from a gaseous mixture of SiH₄ and N₂. The plasma reactor is modelled in three zones: a central zone with electron impact dissociation, diffusion and convection and two lateral zones with only diffusion and convection. The mass transfer equation is solved for the case of convective diffusion (Péclet numbers from 0 to 2). The axial concentration of the activated species in the gas phase is correlated with the deposition profile. The model explains why the deposition profile is influenced by the r.f. power and the gas flow rate. The agreement between calculation and the Si₃N₄ mass profile is good for the deposition range investigated.     

Variable energy positron beam study of Xe-implanted uranium oxide

Doppler broadening of annihilation gamma-line combined with a slow positron beam was used to measure the momentum density distribution of annihilating pair in a set of sintered UO₂ samples. The influence of surface polishing, of implantation with 800-keV ¹³⁶Xe²⁺ at fluences of 1 × 10¹⁵ and 1 × 10¹⁶ Xe cm^?2, and of annealing were studied by following the changes of the momentum distribution shape by means of S and W parameters. The program used for this purpose was VEPFIT. At the two fluences in the stoichiometric as-implanted UO₂, formation of Xe bubbles was not detected. The post-implantation annealing and over-stoichiometry in the as-implanted sample caused Xe precipitation and formation of Xe bubbles.     

Conformational study of the sequential (Tyr-Glu)n copolymer in aqueous solution

Conformational properties of the sequential poly(Tyr-Glu) copolymer in aqueous solution were investigated as a function of pH. No transition could be detected over the range pH 13-10.5. Below pH 10.5 an aggregation process takes place, causing a drastic change in all optical properties of the polymer (optical rotation, c.d. and u.v. spectra). By means of i.r. spectroscopy in D₂O, aggregates were found to be in antiparallel β conformation. The c.d. spectrum of β-aggregates is very similar to that of α-helical poly(l-tyrosine) in organic solvents, except in the 280 nm region. In addition the aggregation process is accompanied by a strong hyperchromic effect in the 277 nm absorption band. Using space-filling models, different arrangements of the chains in the β-aggregates were shown to be realizable. All result in a close stacking of tyrosyl phenyl groups, which can explain the drastic changes in all optical properties.     

Synthesis and characterization of high molecular weight poly(tert.butyl acrylate)

High molecular weight poly(tert. butyl acrylate) s have been synthesized anionically using a technique derived from that of Teyssié et al. It was established that under proper conditions the sites remained living, allowing proper control of the molecular weight and yielding samples with narrow molecular weight distributions.     

Direct Ni Electroless Metallization of Poly(etherimide) without Using Palladium as a Catalyst

ELECTROLESS METALLIZATION of insulatingmaterials is currently used in industry since severaldecades.In its most common applications it consists ina redox reaction between Ni(+2)or Cu(+2)ions and astrong reducer(respectively hypophosphite,H2PO2,forNi(+2)and formaldehyde,HCHO,for Cu(+2))bothcontained in the same solution.The reaction initiationis generally catalyzed by palladium,then the redoxreaction goes on thanks to the remarkable property ofNi and Cu in their metallic state to …     

Impact of agglomeration on the relaxometric properties of paramagnetic ultra-small gadolinium oxide nanoparticles

Ultra-small gadolinium oxide nanoparticles (US-Gd(2)O(3)) are used to provide 'positive' contrast effects in magnetic resonance imaging (MRI), and are being considered for molecular and cellular imaging applications. However, these nanoparticles can aggregate over time in aqueous medium, as well as when internalized into cells. This study is aimed at measuring in vitro, in aqueous medium, the impact of aggregation on the relaxometric properties of paramagnetic US-Gd(2)O(3) particles. First, the nanoparticle core size as well as aggregation behaviour was assessed by HRTEM. DLS (hydrodynamic diameter) was used to measure the hydrodynamic diameter of nanoparticles and nanoaggregates. The relaxometric properties were measured by NMRD profiling, as well as with (1)H NMR relaxometers. Then, the positive contrast enhancement effect was assessed by using magnetic resonance scanners (at 1.5 and 7 T). At every magnetic field, the longitudinal relaxivity (r(1)) decreased upon agglomeration, while remaining high enough to provide positive contrast. On the other hand, the transverse relaxivity (r(2)) slightly decreased at 0.47 and 1.41 T, but it was enhanced at higher fields (7 and 11.7 T) upon agglomeration. All NMRD profiles revealed a characteristic relaxivity peak in the range 60-100 MHz, suggesting the possibility to use US-Gd(2)O(3) as an efficient 'positive-T(1)' contrast agent at clinical magnetic fields (1-3 T), in spite of aggregation.