<p>U ovom radu su sintetisani prahovi na bazi lantan-galata, citratnom sol-gel<br />metodom i hemijskom sintezom u parnoj fazi (CVS), a njihovim presovanjem<br />i sinterovanjem su dobijeni keramički uzorci kontrolisane mikrostrukture. Cilj<br />disertacije je bio dobijanje guste keramike koja bi mogla da se primeni kao<br />jonski provodni elektrolit u gorivnim ćelijama sa čvrstim elektrolitom (SOFC),<br />a koje bi radile na srednjim temperaturama od 500–700 °C (IT-SOFC). Po prvi<br />put su sintetisani nanoprahovi lantan-galata u gasnoj fazi i utvrđeno je da ovako<br />dobijeni polazni prahovi, zbog svojih superiornih karakteristika omogućavaju<br />snižavanje temperature sinterovanja za 150 °C, što je veoma povoljno kako sa<br />stanovišta dizajniranja mikrostrukture, tako i zbog uštede energije u procesu<br />proizvodnje gorivnih ćelija. Ipak, zbog nedovoljne kontrole hemijskog sastava<br />i stehiometrije sintetisanih prahova, što je i najveći nedostatak CVS metode<br />dobijanja perovskitnog LaGaO<sub>3</sub>, nije bilo moguće dobiti keramiku koja bi ispunjavala zahteve za primenu u SOFC. S druge strane, citratna sol-gel metoda<br />pruža veliku kontrolu pomenutih parametara zbog čega je bila moguća sinteza<br />čitavog niza čvrstih rastvora lantan-galata. Ovom metodom su tako sintetisani<br />čist lantan-galat (LG) i dopirani prahovi kod kojih je deo lantana supstituisan<br />stroncijumom, a deo galijuma magnezijumom: La<sub>0,85</sub>Sr<sub>0,15</sub>Ga<sub>0,85</sub>Mg<sub>0,15</sub>O<sub>3-</sub><br />La<sub>1-x</sub>Sr<sub>x</sub>Ga<sub>0,8</sub>Mg<sub>0,2</sub>O<sub>3-</sub>, gde je x = 0,10, 0,15 ili 0,20 (LSGM). Svi ovako sintetisani</p><p>prahovi su zahtevali naknadnu kalcinaciju na 900 °C, ali su potrebne<br />gustine (>95% teorijske gustine) postignute sinterovanjem na temperaturi<br />od 1450 °C već nakon 2 h, što je izuzetno kratko imajući u vidu literaturne<br />podatke. Takođe, sinterovani uzorci su po faznom sastavu bili čista LSGM<br />keramika, a impedansna merenja su pokazala da je najveću provodljivost imao<br />uzorak La<sub>0,85</sub>Sr<sub>0,15</sub>Ga<sub>0,8</sub>Mg<sub>0,2</sub>O<sub>3-</sub>. Ovaj sastav je dalje korišćen u cilju provere</p><p>mogućnosti za dodatno povećanje provodljivosti te su pripremljeni i uzorci kod kojih je izvršena parcijalna supstitucija magnezijuma sa niklom ili kobaltom:<br />La<sub>0,85</sub>Sr<sub>0,15</sub>Ga<sub>0,8</sub>Mg<sub>0,2-y</sub>MyO<sub>3-</sub>, gde je M = Ni ili Co, a y = 0,03 ili 0,05 (LSGMN i</p><p>LSGMC). Pokazano je da dodatak male količine prelaznih metala značajno utiče<br />na mehanizam provođenja, ali da je na višim temperaturama jonska provodljivost<br />i dalje dominantna. Konstatovano je da se dodavanjem male količine Ni ili Co u<br />LSGM mogu dobiti materijali koji bi služili kao elektroliti u IT-SOFC, pri čemu je<br />potencijal nikla kao dopanta nešto veći nego kobalta.</p> / <p>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: La<sub>0,85</sub>Sr<sub>0,15</sub>Ga<sub>0,8</sub>5Mg<sub>0,15</sub>O<sub>3- </sub>and La<sub>1-x</sub>Sr<sub>x</sub>Ga<sub>0,8</sub>Mg<sub>0,2</sub>O<sub>3-</sub>, 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 La<sub>0,85</sub>Sr<sub>0,15</sub>Ga<sub>0,8</sub>Mg<sub>0,2</sub>O<sub>3-</sub>. 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: La<sub>0,8</sub>5Sr<sub>0,1</sub>5Ga<sub>0,8</sub>Mg<sub>0,2-y</sub>MyO<sub>3-</sub>, 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.</p>
| Identifer | oai:union.ndltd.org:uns.ac.rs/oai:CRISUNS:(BISIS)85671 |
| Date | 28 December 2012 |
| Creators | Stijepović Ivan |
| Contributors | Srdić Vladimir, Nikolić Ljubica, Stojanović Goran |
| Publisher | Univerzitet u Novom Sadu, Tehnološki fakultet Novi Sad, University of Novi Sad, Faculty of Technology at Novi Sad |
| Source Sets | University of Novi Sad |
| Language | Serbian |
| Detected Language | English |
| Type | PhD thesis |
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