Mechanochemical synthesis of NaNbO3, KNbO3 and K0.5Na0.5NbO3

Mechanochemical synthesis of the K0.5Na0.5NbO3 solid solution (KNN) is studied. In order to explore the mechanochemical interactions between the constituents in the Na2CO3 - K2CO3 - Nb2O5 system, NaNbO3 and KNbO3 as the boundary compositions of the KNN solid solution are also studied. It has been shown that NaNbO3 can be prepared by a single-step mechanochemical synthesis, while in the case of K2CO3 and Nb2O5, and Na2CO3, K2CO3 and Nb2O5 mixtures, only amorphisation occurs even after prolonged milling

Barium titanate thick films prepared by screen printing technique

The barium titanate (BaTiO3) thick films were prepared by screen printing technique using powders obtained by soft chemical route, modified Pechini process. Three different barium titanate powders were prepared: i) pure, ii) doped with lanthanum and iii) doped with antimony. Pastes for screen printing were prepared using previously obtained powders. The thick films were deposited onto Al2O3 substrates and fired at 850°C together with electrode material (silver/palladium) in the moving belt furnace in the air atmosphere. Measurements of thickness and roughness of barium titanate thick films were performed. The electrical properties of thick films such as dielectric constant, dielectric losses, Curie temperature, hysteresis loop were reported. The influence of different factors on electrical properties values was analyzed

Synthesis of nanocrystalline Co3O4 nanopowders via self-propagation combustion reaction

Over the past two decades, a considerable effort has been devoted to research of the p-type semiconducting materials, which found diverse applications in industry. As the p-type material with a spinel AB2O4 structure and direct optical band gap of 2.19 eV, cobalt oxide, Co3O4 has drawn special interests in gas-sensing and solar energy absorption research community. At the same time, it has found the usage as an electrode material in lithium ion batteries, catalyst, ceramic pigment, field-emission and magnetic material. A powerful strategy to improve the Co3O4 performances in some of the mentioned applications is the utilization of a nanocrystalline powder with a high surface to volume ratio, which can be successfully produced by self-propagation combustion reaction. In this work, cobalt nitrate (oxidizer) and glycine (fuel) of required amounts were dissolved in the minimum amount of deionized water to obtain a clear solution. According to the rules of propellant chemistry, the glycine/metal ion ratio was adjusted to provide stoichiometric or fuel-lean conditions of the redox reaction. The sols were dried at 80 oC to obtain the gels, and then subjected to the rapid heating, where the gels auto-ignited at approximately 250 oC (depending on the amount of the fuel) and spontaneously underwent a smouldering combustion with evolution of large amounts of gases, subsequently forming a voluminous nanosized Co-oxide powder. It is worth to mention that the dual role of the glycine (chelating agent and fuel) in this synthesis was of great importance in preventing the precipitation of the solution during rapid heating, and in liberation of the required energy for the synthesis of the Co3O4 powder with desired Co+2/Co+3 oxidation state, necessary for its further use in production of gas sensors and catalysts. The processes involved in formation of the pure Co3O4 powder and especially the exothermicity of the reaction, were followed by simultaneous thermal analysis (thermogravimetric and differential thermal analysis coupled with evolved gas analysis). According to the X-ray diffraction analysis the phase-pure Co3O4 was obtained only when the precursor powder was prepared from the fuel-lean redox reaction. The field emission scanning electron micrographs revealed the spongy aspect of the calcined powder, where small primary particles formed the agglomerates. The transmission electron microscopy was used to get deeper insight in the particle size and purity of the as prepared nanosized powders. The formation of the single Co3O4 powder from the solution to the combusted powder was also analysed by Fourier transformed infra-red spectroscopy

Put prema otvorenom repozitoriju doktorskih disertacija: kako prevladati neusklađenost propisa

Sveučilišna knjižnica Rijeka izgrađuje digitalnu zbirku doktorskih disertacija obranjenih na Sveučilištu u Rijeci. Disertacije obranjene od 1974. do 2015. godine pohranjene su u institucijskom repozitoriju Sveučilišne knjižnice dok se disertacije obranjene nakon 2015. godine pohranjuju u institucijski repozitorij sastavnice Sveučilišta na kojoj je rad obranjen. Cjelovita zbirka vidljiva je u repozitoriju Sveučilišta u Rijeci. Iako je prema Zakonu o znanstvenoj djelatnosti i visokom obrazovanju rad potrebno „...objaviti na javnoj internetskoj bazi doktorskih disertacija...“, zbog neusklađenosti sa Zakonom o autorskom pravu i srodnim pravima, za objavu rada u otvorenom pristupu potrebno je pribaviti suglasnost autora rada. Iz tog razloga krenulo se u sustavno slanje upita autorima te prikupljanje suglasnosti za objavu u otvorenom pristupu. U tu svrhu pripremljen je obrazac koji je usklađen s Općom uredbom o zaštiti podataka 2016/679 ili GDPR (General Data Protection Regulation), odnosno hrvatskim Zakonom o provedbi Opće uredbe o zaštiti podataka. U izlaganju je prikazan proces prikupljanja suglasnosti u kojem se nailazi na brojne izazove – od samog pronalaženja kontakta autora do arhiviranja pribavljenih Izjava. Pokazalo se da ovakav pristup daje izvrsne rezultate, jer većina autora dozvoli objavu rada u otvorenom pristupu što zahvaljujući interoperabilnosti repozitorija u sustavu Dabar pridonosi većoj vidljivosti autora, rada i samog Sveučilišta

Microstructural analysis of Bulk Molding Compounds and correlation with the flexural strength

In this study, the influence of the glass fiber (GF) content on the microstructure and flexural strength of bulk molding compounds (BMCs) is investigated. Three sets of BMCs with different weight fractions of GF (5/10/12.5 wt%) were commercially prepared and compression molded into test specimens. The microstructure of the composites was analysed by scanning electron microscopy and further quantitatively characterized by Voronoi analysis in order to define the degree of the fiber distribution homogeneity. The experimental results were compared to the modelled microstructures. The results revealed that the fiber distribution in the composite with 5 wt% of GF is considered as the most homogeneous. Through the obtained microstructural descriptors, the fiber weight content and their distribution were correlated to the flexural strength of BMCs. The flexural strength was the highest for the composite with 10 wt% of GF

Preparation and dielectric properties of CuAlO2 ceramics

Within this work, the focus was on preparation of the delafossite CuAlO2 single phase powder and ceramic with a high density by the solid state synthesis, and on dielectric properties of the as-synthesized ceramic. The reaction between the nanoboehmite γ-AlOOH, with a high specific surface area, and the Cu2O, with the particles below 1 μm, was enhanced by comminution in a high energy mill, which resulted in reduction of the particle size and consequently shorter diffusion paths between constituent powders. The phase pure CuAlO2 powder was synthesized upon heating the reagent powder mixture two times for 10 h at 1100oC in the inert argon atmosphere as confirmed by the X-ray analysis. The ceramic with 86% of theoretical density was obtained after sintering the CuAlO2 powder compact at 1100oC for 2 h in air. According to the X-ray analysis the ceramic sample was single-phase. The bulk of the sample revealed a dense microstructure with a uniform distribution of porosity within the delafossite matrix. However, traces of Cu-rich impurities have been identified at the surface of the pellets by the EDXS analysis. The semiconducting nature of the ceramic sample was confirmed by the temperature dependent dielectric parameters measurements (ε’ and tgδ) in the 10 kHZ-1MHz frequency range between 297 and 473 K