Mikrochemie

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Utersuchung von Mehl und Getreide

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Mikrochemie 1. Elementaranalyse

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Untersuchung von Getreideprodukten und Backwaren

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Formation and properties of some Nb-doped SrTiO3-based solid solutions

It is well known that SrTiO₃ is one of the widely used materials in the electronic ceramic industry. Donor-type dopants are often added in order to improve the electrical properties of SrTiO₃-based ceramics. In the present study, the formation and characteristics of undoped and Nb-doped SrTiO₃ were studied as a function of both the niobium dopant concentration and the Sr/Ti ratio (at the same Nb content). The formation mechanism of the mixed Sr(Ti,Nb)O₃ crystals was investigated by XRD and thermal analysis. The niobium solubility in the SrTiO₃ perovskite lattice was estimated by means of unit cell parameter data. The influence of Sr concentration on the microstructure of the ceramics obtained was investigated by SEM. Ceramic properties pointed out the influence of the dopant concentration and stoichiometry on the sintering behavior of these materials. This revised version was published online in July 2006 with corrections to the Cover Date.     

Nanosized amorphous calcium carbonate stabilized by poly(ethylene oxide)-b-poly(acrylic acid) block copolymers

Particles of amorphous calcium carbonate (ACC), formed in situ from calcium chloride by the slow release of carbon dioxide by alkaline hydrolysis of dimethyl carbonate in water, are stabilized against coalescence in the presence of very small amounts of double hydrophilic block copolymers (DHBCs) composed of poly(ethylene oxide) (PEO) and poly(acrylic acid) (PAA) blocks. Under optimized conditions, spherical particles of ACC with diameters less than 100 nm and narrow size distribution are obtained at a concentration of only 3 ppm of PEO-b-PAA as additive. Equivalent triblock or star DHBCs are compared to diblock copolymers. The results are interpreted assuming an interaction of the PAA blocks with the surface of the liquid droplets of the concentrated CaCO3 phase, formed by phase separation from the initially homogeneous reaction mixture. The adsorption layer of the block copolymer protects the liquid precursor of ACC from coalescence and/or coagulation.