3种锆合金的高温氧化行为

对Zr-A和Zr-B锆合金在700-1200℃的O_2中进行了等温氧化实验,观察了合金的氧化动力学行为和基体微观结构演变,并与低Sn的Zr-4合金作了对比.结果表明,700—1200℃氧化时,3种锆合金的动力学曲线基本服从抛物线规律,Zr-B合金在700℃氧化1200 s后速率出现转折;800℃时Zr-A和Zr-B合金的氧化速率出现转折;1000℃时3种合金动力学曲线由抛物线变为近似直线;1100℃以上氧化时,动力学曲线呈抛物线规律,未出现转折.在1100℃以上氧化时,合金成分对3种合金的高温氧化性能影响甚微.依据氧化增重数据得到了3种合金在700—1200℃氧化反应的抛物线速率常数K_p的表达式.     

Phototransformation of water-soluble polymers. Part II: photooxidation of poly(ethylene oxide) in aqueous solution

Poly(ethylene oxide) (PEO) was irradiated in aqueous solution under long wavelengths (λ>300 nm, 20 °C) and in presence of oxygen. The photooxidation of PEO was studied by IR spectrophotometry, viscometry and size exclusion chromatography. The formation of the oxidation photoproducts was studied by infrared analysis of films obtained by evaporation of aliquots of irradiated aqueous solutions. The photoproducts were identified by chemical derivatization treatments coupled with infrared measurements. Viscosimetry and SEC analysis showed that photooxidation was leading to a dramatic decrease of the molecular weights. The influence of the pH of the aqueous solutions was also examined. Unexpected results were obtained for the pH 12 solutions, indicating a strong inhibition of the oxidation.

Comparison with the results obtained in the case of PEO irradiated in the solid state showed that no direct transposition of the knowledge concerning the behavior of the solid polymer could be made.     

Initial Sintering of Submicrometer Titania Anatase Powder

Measurements of surface area reduction of TiO₂ anatase powder (of initially 100 m²· g⁻¹) were carried out for various partial pressures of water and oxygen at 823 K. The kinetic equation obtained for the experimental rates has the formulation r = k[P _H2O]^1/α [ P _O2]^1/β where α and β are equal to 2 and -12, respectively. A mechanistic model involving six consecutive elementary steps was developed, in which hydroxyl species play a dominant role. No geometrical assumption is required. The general expression of the deduced theoretical rates has the same form as previously given. The comparison between the experimental and the predicted rates points out that the rate-limiting step is the surface diffusion of hydroxyl species. This model can easily be used for any other compounds, for particle growth, and for porosity elimination.     

Yeast sequencing reports. CAN1, a gene encoding a permease for basic amino acids in Candida albicans

The first gene coding for an amino-acid permease of Candida albicans was sequenced. The DNA fragment complementing the lysine-permease deficiency was 3385 bp long. An open reading frame of 1713 nucleotides was found encoding a protein of 571 amino acids, with a calculated molecular weight of 63 343. Analysis of the deduced primary structure revealed ten membrane spanning regions and three potential N-glycosylation sites. The protein sequence is strongly homologous to both permeases for basic amino acids (Can1 and Lyp1) of Saccharomyces cerevisiae. C-terminal part of another ORF (105 aa), highly homologous to the gene HAL2 of S. cerevisiae, was found 133 bp downstream, and in tail-to-tail orientation to the permease gene. The sequence data will appear in the EMBL/GenBank/DDBJ Nucleotide Sequence Data Libraries under the accession number X76689.     

Kinetic study of acrylic monomer grafting in the presence of a polyfunctional monomer onto radioperoxided poly(ethylene–tetrafluoroethylene) copolymer

Acrylic acid (AA), methacrylic acid (MA) and diethyleneglycol dimethacrylate (DEGDM) are grafted onto 100 μm radioperoxided poly(ethylene–tetrafluroethylene) copolymer (ETFE). The influences of the homopolymerization inhibitor, the solvent, the grafting temperature, and the monomer content on the grafting kinetics are studied. For the grafting of acrylic monomers, the limitation of the grafting yield is more important for AA grafting whatever the nature of the metallic salt; the difference between the two homopolymerization inhibitors is their respective concentration to obtain a given grafting yield. For the AA + DEGDM cografting, the grafting yield increases with the DEGDM content of the grafting solution. In both cases, the overall activation energy has been calculated, and its variation as a function of the composition of the grafting solution is discussed. © 1994 John Wiley & Sons, Inc.     

Résolution d’un problème d’affectation à l’aide d’une procédure de réarrangement

The problem of allocating a certain number of resources and working out their time schedule to flow a given traffic in a minimum time (under certain constraints) is considered. Unlike other algorithms, which work out the solution from rather long manipulations of the matrix traffic, we use a simple assignement heuristic and solve the very few conflicts through a fast and easy rearranging process. This technique allows a dynamic adjustment to an evolution of the traffic matrix.     

Multiresolution in astronomical image processing: A general framework

Multiresolution transforms, including a wavelet transform, are applied simage visualization, image restoration, filtering and compression, × object detection. Variance stabilization is used, when appropriate, cater for common astronomical image noise models. We discuss idation of such methods in the case of astronomical image processing. A range of examples illustrate the effectiveness of this aproach in handling point source and extended astronomical objects.