Obtaining of lanthanum-gallate for fuel cells application

U ovom radu su sintetisani prahovi na bazi lantan-galata, citratnom sol-gel metodom i hemijskom sintezom u parnoj fazi (CVS), a njihovim presovanjem i sinterovanjem su dobijeni keramički uzorci kontrolisane mikrostrukture. Cilj disertacije je bio dobijanje guste keramike koja bi mogla da se primeni kao jonski provodni elektrolit u gorivnim ćelijama sa čvrstim elektrolitom (SOFC), a koje bi radile na srednjim temperaturama od 500–700 °C (IT-SOFC). Po prvi put su sintetisani nanoprahovi lantan-galata u gasnoj fazi i utvrđeno je da ovako dobijeni polazni prahovi, zbog svojih superiornih karakteristika omogućavaju snižavanje temperature sinterovanja za 150 °C, što je veoma povoljno kako sa stanovišta dizajniranja mikrostrukture, tako i zbog uštede energije u procesu proizvodnje gorivnih ćelija. Ipak, zbog nedovoljne kontrole hemijskog sastava i stehiometrije sintetisanih prahova, što je i najveći nedostatak CVS metode dobijanja perovskitnog LaGaO3, nije bilo moguće dobiti keramiku koja bi ispunjavala zahteve za primenu u SOFC. S druge strane, citratna sol-gel metoda pruža veliku kontrolu pomenutih parametara zbog čega je bila moguća sinteza čitavog niza čvrstih rastvora lantan-galata. Ovom metodom su tako sintetisani čist lantan-galat (LG) i dopirani prahovi kod kojih je deo lantana supstituisan stroncijumom, a deo galijuma magnezijumom: La0,85Sr0,15Ga0,85Mg0,15O3- La1-xSrxGa0,8Mg0,2O3-, gde je x = 0,10, 0,15 ili 0,20 (LSGM). Svi ovako sintetisani prahovi su zahtevali naknadnu kalcinaciju na 900 °C, ali su potrebne gustine (>95% teorijske gustine) postignute sinterovanjem na temperaturi od 1450 °C već nakon 2 h, što je izuzetno kratko imajući u vidu literaturne podatke. Takođe, sinterovani uzorci su po faznom sastavu bili čista LSGM keramika, a impedansna merenja su pokazala da je najveću provodljivost imao uzorak La0,85Sr0,15Ga0,8Mg0,2O3-. Ovaj sastav je dalje korišćen u cilju provere mogućnosti za dodatno povećanje provodljivosti te su pripremljeni i uzorci kod kojih je izvršena parcijalna supstitucija magnezijuma sa niklom ili kobaltom: La0,85Sr0,15Ga0,8Mg0,2-yMyO3-, gde je M = Ni ili Co, a y = 0,03 ili 0,05 (LSGMN i LSGMC). Pokazano je da dodatak male količine prelaznih metala značajno utiče na mehanizam provođenja, ali da je na višim temperaturama jonska provodljivost i dalje dominantna. Konstatovano je da se dodavanjem male količine Ni ili Co u LSGM mogu dobiti materijali koji bi služili kao elektroliti u IT-SOFC, pri čemu je potencijal nikla kao dopanta nešto veći nego kobalta.Powders based on lanthanum-gallate have been synthesised in this work by using citrate sol-gel method in the liquid phase and by chemical vapour synthesis (CVS). As-synthesised powders were calcined, pressed and finally sintered in order to produce ceramic samples with controlled microstructure. The main goal of this dissertation has been obtaining of dense ceramics for application in ion conducting electrolyte for solid oxide fuel cells working at 500–700 °C (IT-SOFC). Lanthanum-gallate nanopowders have been synthesised in the gas phase for the first time and it has been determined that these starting powders posses superior properties which could lower down the sintering temperature for about 150 °C. This is very significant considering microstructure design, but also high energy consumption during the manufacturing process of fuel cells. However, it has not been possible to obtain ceramics with exact properties needed for SOFC application using CVS due to the lack of control of chemical composition and stoichiometry of the as-synthesised powders, which are the main drawbacks of this method. On the other hand, citrate sol-gel method offered a possibility to precisely control aforementioned parameters which enabled synthesis of a whole range of lanthanum-gallate sollid solutions. So, pure perovskite lanthanum-gallate (LG) has been synthesised by using this liquid phase method, but also doped powders where part of lanthanum and gallium was supstituted with strontium and magnesium, respectively: La0,85Sr0,15Ga0,85Mg0,15O3- and La1-xSrxGa0,8Mg0,2O3-, where x = 0.10, 0.15 or 0.20 (LSGM). Calcination at 900 °C was necessary step during the processing of these sol-gel powders but the sintering time at 1450 °C was only 2 h which is quite short in comparison with available literature data. Additionaly, the sintered samples were phase pure LSGM ceramics and impedance measurement showed that the highest conductivity had sample La0,85Sr0,15Ga0,8Mg0,2O3-. Accordingly, this composition has been used to check the possibility of further improvement of conductivity. A set of new samples has been prepared where one part of magnesium has been substituted with nickel or cobalt: La0,85Sr0,15Ga0,8Mg0,2-yMyO3-, where M = Ni or Co and y = 0.03 or 0.05 (LSGMN i LSGMC). It has been shown that addition of small amount of transition metals significantly influences conduction mechanism, but at higher temperatures the ionic conductivity is still dominant. It has been found that electrolyte materials for IT-SOFCs could be obtained by incorporation of small quantities of Ni or Co into LSGM and that nickel is more promissing for this purpose than cobalt

Obtaining of lanthanum-gallate for fuel cells application

U ovom radu su sintetisani prahovi na bazi lantan-galata, citratnom sol-gel metodom i hemijskom sintezom u parnoj fazi (CVS), a njihovim presovanjem i sinterovanjem su dobijeni keramički uzorci kontrolisane mikrostrukture. Cilj disertacije je bio dobijanje guste keramike koja bi mogla da se primeni kao jonski provodni elektrolit u gorivnim ćelijama sa čvrstim elektrolitom (SOFC), a koje bi radile na srednjim temperaturama od 500–700 °C (IT-SOFC). Po prvi put su sintetisani nanoprahovi lantan-galata u gasnoj fazi i utvrđeno je da ovako dobijeni polazni prahovi, zbog svojih superiornih karakteristika omogućavaju snižavanje temperature sinterovanja za 150 °C, što je veoma povoljno kako sa stanovišta dizajniranja mikrostrukture, tako i zbog uštede energije u procesu proizvodnje gorivnih ćelija. Ipak, zbog nedovoljne kontrole hemijskog sastava i stehiometrije sintetisanih prahova, što je i najveći nedostatak CVS metode dobijanja perovskitnog LaGaO3, nije bilo moguće dobiti keramiku koja bi ispunjavala zahteve za primenu u SOFC. S druge strane, citratna sol-gel metoda pruža veliku kontrolu pomenutih parametara zbog čega je bila moguća sinteza čitavog niza čvrstih rastvora lantan-galata. Ovom metodom su tako sintetisani čist lantan-galat (LG) i dopirani prahovi kod kojih je deo lantana supstituisan stroncijumom, a deo galijuma magnezijumom: La0,85Sr0,15Ga0,85Mg0,15O3- La1-xSrxGa0,8Mg0,2O3-, gde je x = 0,10, 0,15 ili 0,20 (LSGM). Svi ovako sintetisani prahovi su zahtevali naknadnu kalcinaciju na 900 °C, ali su potrebne gustine (>95% teorijske gustine) postignute sinterovanjem na temperaturi od 1450 °C već nakon 2 h, što je izuzetno kratko imajući u vidu literaturne podatke. Takođe, sinterovani uzorci su po faznom sastavu bili čista LSGM keramika, a impedansna merenja su pokazala da je najveću provodljivost imao uzorak La0,85Sr0,15Ga0,8Mg0,2O3-. Ovaj sastav je dalje korišćen u cilju provere mogućnosti za dodatno povećanje provodljivosti te su pripremljeni i uzorci kod kojih je izvršena parcijalna supstitucija magnezijuma sa niklom ili kobaltom: La0,85Sr0,15Ga0,8Mg0,2-yMyO3-, gde je M = Ni ili Co, a y = 0,03 ili 0,05 (LSGMN i LSGMC). Pokazano je da dodatak male količine prelaznih metala značajno utiče na mehanizam provođenja, ali da je na višim temperaturama jonska provodljivost i dalje dominantna. Konstatovano je da se dodavanjem male količine Ni ili Co u LSGM mogu dobiti materijali koji bi služili kao elektroliti u IT-SOFC, pri čemu je potencijal nikla kao dopanta nešto veći nego kobalta.Powders based on lanthanum-gallate have been synthesised in this work by using citrate sol-gel method in the liquid phase and by chemical vapour synthesis (CVS). As-synthesised powders were calcined, pressed and finally sintered in order to produce ceramic samples with controlled microstructure. The main goal of this dissertation has been obtaining of dense ceramics for application in ion conducting electrolyte for solid oxide fuel cells working at 500–700 °C (IT-SOFC). Lanthanum-gallate nanopowders have been synthesised in the gas phase for the first time and it has been determined that these starting powders posses superior properties which could lower down the sintering temperature for about 150 °C. This is very significant considering microstructure design, but also high energy consumption during the manufacturing process of fuel cells. However, it has not been possible to obtain ceramics with exact properties needed for SOFC application using CVS due to the lack of control of chemical composition and stoichiometry of the as-synthesised powders, which are the main drawbacks of this method. On the other hand, citrate sol-gel method offered a possibility to precisely control aforementioned parameters which enabled synthesis of a whole range of lanthanum-gallate sollid solutions. So, pure perovskite lanthanum-gallate (LG) has been synthesised by using this liquid phase method, but also doped powders where part of lanthanum and gallium was supstituted with strontium and magnesium, respectively: La0,85Sr0,15Ga0,85Mg0,15O3- and La1-xSrxGa0,8Mg0,2O3-, where x = 0.10, 0.15 or 0.20 (LSGM). Calcination at 900 °C was necessary step during the processing of these sol-gel powders but the sintering time at 1450 °C was only 2 h which is quite short in comparison with available literature data. Additionaly, the sintered samples were phase pure LSGM ceramics and impedance measurement showed that the highest conductivity had sample La0,85Sr0,15Ga0,8Mg0,2O3-. Accordingly, this composition has been used to check the possibility of further improvement of conductivity. A set of new samples has been prepared where one part of magnesium has been substituted with nickel or cobalt: La0,85Sr0,15Ga0,8Mg0,2-yMyO3-, where M = Ni or Co and y = 0.03 or 0.05 (LSGMN i LSGMC). It has been shown that addition of small amount of transition metals significantly influences conduction mechanism, but at higher temperatures the ionic conductivity is still dominant. It has been found that electrolyte materials for IT-SOFCs could be obtained by incorporation of small quantities of Ni or Co into LSGM and that nickel is more promissing for this purpose than cobalt

Preparation and photocatalytic activity of the layered titanates

Titanate structures were synthesized in highly alkaline solution using hydrothermal procedure. As-preparedpowders were characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR)and transmission electron microscopy (TEM). A specific surface area of the powders was measured by BETmethod. Results confirmed formation of layered trititanates, already after one hour of hydrothermal synthesis.To examine the photocatalytic activity of the as-prepared layered titanates, methylene blue (MB) was employedas a target compound in response to visible light at ambient temperature. It was observed that the specificsurface area, size distribution and crystallinity are important factors to get high photocatalytic activity for thedecomposition of MB

Direct synthesis of nanocrystalline oxide powders by wet-chemical techniques

In a recent period there is a great need for increasing the knowledge of tailoring the innovative procedures for the synthesis of electroceramic nanopowders and materials with improved quality for specific application. In order to produce electroceramics with desirable microstructure and properties, synthesis of stoichiometric, ultra-fine and agglomerate free powders with narrow size distributions is one of the most important steps. Within this scope, in the present paper we summarize our recent results on direct synthesis of some important perovskites and ferrites nanopowders by wet-chemical techniques

Cubic structured core/shell NiFe2O4@ZnFe2O4 nanocomposite: Structural, morphological and magnetic properties

Double ferrite nanocomposites with NiF2O4 as the core and ZnFe2O4 as the shell, were synthesised through combination of co-precipitation and hydrothermal methods. This two-step process resulted in formation of a cubic structured core of around 100 nm and a shell of zinc ferrite nanoparticles which are below 10 nm in size. The core/shell structures were characterised using various techniques such as XRD, Raman, TEM/EDS and VSM. In addition, Ni leaching out of these structures was tested in the ultra-pure water and saline solution. These analyses demonstrated that the shell of zinc ferrites effectively prevents Ni leaching from the core keeping the concentration of nickel well below the level usually found in drinking water. These core/shell nanocomposites exhibit a ferrimagnetic behaviour

Development of a Kinetic Model for Catalytic Reforming of Naphtha and Parameter Estimation Using Industrial Plant Data

In the present paper, a semi-empirical kinetic model for catalytic reforming has been developed. In the developed model, the component "lumping" strategy is based on a paraffins, olefins, naphthalenes, and aromatics (PONA) analysis. "Activation energy lumps" are introduced to take into account different values of activation energies within specific reaction classes. The parameters of the model have been estimated by bench marking with industrial data. Simulation results have been found to be in very close agreement with plant data. One of the advantages of the present kinetic model is that it predicts the concentration of hydrogen and light gases very well. Because it is formulated from basic principles, this kinetic model with some modification can be applied to any catalytic reformer

Oil absorption in mesoporous silica particles

Mesoporous silica particles were prepared from highly basic sodium silicate solutions, having different silica modulus and SiO2 concentrations, by adding sulphuric acid at different temperatures. Pore structure of prepared silica particles (aggregates) is strongly influenced by processing conditions and easy controllable in broad range of the specific surface area, pore size, pore volume and size distribution. It is shown that there is a clear correlation between volume of absorbed oil and processing parameters used in preparation of silica aggregates. Thus, oil absorption is higher in the samples prepared from sodium silicate solution with higher SiO2 concentration and at higher synthesis temperature

Functionalization of zinc ferrite nanoparticles: influence of modification procedure on colloidal stability

The present work describes the synthesis of zinc ferrite nanoparticles by a chemical coprecipitation method and its surface modification with poly(diallyldimethylammonium chloride) (PDDA), tetramethylammonium hydroxide (TMAOH) and poly(ethylene glycol) (PEG). The effect of coating on the colloidal stability of obtained systems as well as the effect of modification procedure on the final characteristics of particles were studied. The unmodified and modified nanoparticles were characterized by X-ray diffractometry (XRD), scanning electron microscopy (SEM) and Raman spectroscopy. The particle size distribution and stability of these systems were investigated by dynamic light scattering (DLS) and zeta potential measurements. The results have shown the profound effect of the used modification procedure on colloidal stability of the investigated particles

Effect of process parameters on the morphology and adsorption properties of nanocrystalline boehmite

In the last decade, exploration of transition alumina phases with good adsorption properties has attracted a great research interest from both a fundamental and a practical point of view. The transition phases of alumina are metastable polymorphs of aluminum oxide formed through the thermal dehydration of aluminum trihydroxide and aluminum oxyhydroxide. Powder X-ray diffraction (XRD), Fourier-transformed infrared spectroscopy (FT-IR), scanning electron microscopy (SEM) and low-temperature nitrogen absorption studies were employed to trace the formation of the transition phases of alumina. In this work transition alumina powders were synthesized starting from sodium aluminate solution prepared from Bayer liquor. The neutralization of sodium aluminate solution was performed with the use of sulphuric acid, while glucose was added in the starting solution. In this way, the single phase nanocrystalline boehmite was obtained. As-synthesized boehmite powders have high surface area (above 360 m2/g) and the average crystallite size less than 5 nm. The results showed that the properties of the powders (structure, morphology) are strongly influenced by the initial pH value of sodium aluminate solution, as well as by the duration of neutralization step. [Projekat Ministarstva nauke Republike Srbije, br. III 45021

Properties and Potential Application of Lead-Free (BaZr_0.2Ti_0.8O₃) and Lead-Based (PbZr_0.52Ti_0.48O₃) Flexible Thick Films

For the last several decades, energy harvesters based on piezoelectricity from mechanical vibration have emerged as very promising devices that are being explored extensively for their functionality in energy technologies. In this paper, a series of flexible lead-free BaZr0.2Ti0.8O3 (BZT)/PVDF and lead-based PbZr0.52Ti0.48O3 (PZT)/PVDF piezocomposites with variable filler content up to 50 vol% were prepared by a hot pressing method. The structure and morphology of the BZT and PZT powders, as well as the distribution of the piezo-active filler in the obtained flexible films were characterized by XRD and SEM analysis. In addition, the remnant polarization (Pr) and leakage current were also investigated to evaluate the breakdown strength in both types of flexible films. The calculations of storage energies and output voltage obtained for the investigated materials revealed an increasing trend with an increasing amount of BZT and PZT active phases. The maximum storage energy of 0.42 J/cm3 (and energy efficiency of 40.7 %) was obtained for the PZT–PVDF (40–60) films, while the maximum output voltage of about 10 V (~10 μA) was obtained for the PZT–PVDF (50–50) flexible film. In addition, a comparison between the properties of the lead-based and lead-free flexible films, as well as the potential use of these films as energy storage and energy harvesting systems were analyzed