Biosynthesis and chemical modification of poly(γ-glutamic acid)

Summary The water soluble poly(-glutamic acid) (-PGA) was synthetized by bacteria. A series of new, less water soluble -esters were prepared by repeated esterification.The characterization of the resulted polymers was performed by 200 MHz ¹H and 75.4 MHz ¹³C NMR spectroscopy.     

Obituaries

RILEM Bulletin

Obituaries     

Low temperature dielectric studies of zinc oxide (ZnO) nanoparticles prepared by precipitation method

Using zinc nitrate as a precursor and NaOH starch as a stabilizing agent, hexagonal zinc oxide (ZnO) nanoparticles has been synthesized by precipitation method. The transmission electron microscopy (TEM) images show particles of nearly uniform spherical size of around 40 nm. The infrared spectroscopy (FT-IR) measurement reveals the peak at 500 cm^?1, corresponding to the Zn–O bond. Dielectric studies of ZnO nanoparticles show frequency dependence dielectric anomaly at low temperature (85–300 K). Results reveal that the capacitance and loss tangent decrease with the frequency while these parameters improve with the increasing of temperature. The increase of a.c. conductivity with the temperature indicates that the mobility of charge carriers is responsible for hopping and electronic polarization in ZnO nanoparticles.     

Microstructure evolution in densification of SiC ceramics by aluminium vapour infiltration and investigation of mechanical properties

The densification and phase formation of 6?wt% Y₂O₃ containing SiC compacts infiltrated by aluminium vapour were investigated. The densification occurred through infiltration of aluminium vapour that formed through a reaction between alumina and carbon powders at 2000?°C. Infiltrated specimens were evaluated concerning the density, phase, microstructure and mechanical properties including hardness and fracture toughness. X-ray diffraction studies showed the presence of yttrium aluminium garnet (YAG) and Al₂O₃ as the secondary phases along with other minor phases. Sectioning of the infiltrated specimen showed two regions: a dense layer starting from the surface of about 1?mm thickness followed by a relatively porous structure at the core. The effect of infiltration depth on densification and evolution of microstructure are studied. Also, the changes in Vickers’ hardness and fracture toughness with the increase in specimen depth are discussed.