Properties of yttria doped barium cerium oxide ceramics as an electrolyte for solid oxide fuel cells

Predmet ove doktorske disertacije je ispitivanje uticaja različitih količina Nb5+ i Ta5+ na hemijsku stabilnost, mikrostrukturna i električna svojstva BaCe0,9Y0,1O3–δ perovskitne strukture, kao elektrolita za gorivne ćelije na bazi oksida u čvrstom stanju. Pored toga, izvršeno je poređenje dve različite metode sinteze keramičkih prahova kod uzoraka dopiranih sa Nb5+. Prahovi BaCe0,9–xMxY0,1O3–δ, pri čemu je M = {Nb; Ta}, a x = {0,01; 0,03; 0,05}, sintetisani su klasičnim postupkom reakcije u čvrstom stanju. Na osnovu rezultata diferencijalne skenirajuće kalorimetrije i termogravimetrijske analize utvrđeno je da se reakcija u čvrstom stanju odvija već na 1000 °C. To je potvrđeno rendgenskom difrakcijom na prahu uzoraka koji su dobijeni žarenjem na 1000 °C u toku 5 h, kada je uočeno samo prisustvo perovskitne faze. Uzorci BaCe0,9–xNbxY0,1O3–δ, sa istim sadržajem Nb kao u prethodnom slučaju, sintetisani su i postupkom reakcije samosagorevanja. Na osnovu izračunatih vrednosti kristalografskih parametara uočeno je da se zapremina jedinične kristalne ćelije smanjuje sa porastom koncentracije dopanata, kao i da je veličina kristalita veća kod uzoraka sintetisanih metodom reakcije u čvrstom stanju. Veći stepen aglomeracije prahova dobijenih metodom reakcije u čvrstom stanju je potvrđen i skenirajućom elektronskom mikroskopijom visoke rezolucije. Takođe, BET analizom se pokazalo da mnogo veću specifičnu površinu postižu prahovi sintetisani metodom reakcije samosagorevanja. Neporozni elektroliti su dobijeni sinterovanjem uniaksijalno presovanih keramičkih prahova na 1550 °C u toku 5 h. Mikrostrukture elektrolita su ispitane skenirajućom elektronskom mikroskopijom, i uočeno je da veličina zrna blago opada sa povećanjem koncentracije dopanata. Osim toga, elektroliti sintetisani metodom reakcije samosagorevanja pokazuju nešto veću srednju veličinu zrna u odnosu na uzorke istog sastava sintetisanih metodom reakcije u čvrstom stanju. Električna karakterizacija sinterovanih uzoraka je izvršena elektrohemijskom impedansnom analizom u temperaturnom intervalu od 550–750 °C u atmosferama vlažnog...The subject of this doctoral dissertation was to investigate the influence of various amounts of Nb5+ and Ta5+ on chemical stability, microstructure and electrical properties of perovskite-structured BaCe0,9Y0,1O3–δ as an electrolyte for solid oxide fuel cells. Besides, two different synthesis methods of ceramic powders were compared for samples doped with Nb5+. BaCe0,9–xMxY0,1O3–δ powders, where M = {Nb; Ta}, and x = {0,01;0,03;0,05}, were synthesized by the classical solid state reaction method. Based on the results of differential scanning calorimetry and thermogravimetric analysis it was found that the solid state reaction occurred already at 1000 °C. It was confirmed by X-ray powder diffraction analysis of the samples obtained by firing at 1000 °C for 5 h, when only the presence of the perovskite phase was observed. BaCe0,9–xNbxY0,1O3–δ samples, with the same Nb content as in the previous case, were also synthesized by the method of autocombustion reaction. Based on the calculated crystallographic parameters it was found that unit cell volume decreased with an increase in dopant concentration, as well as the crystallite size was larger for the samples synthesized by the solid state reaction method. A higher degree of agglomeration for powders obtained by the solid state reaction method was also confirmed by field emission scanning electron microscopy. In addition, BET analysis revealed that the samples synthesized by the method of autocombustion reaction acquired much higher specific surface area. Non-porous electrolytes were obtained by sintering of uniaxially pressed ceramic powders at 1550 °C for 5 h. Microstructures of the electrolytes were investigated by scanning electron microscopy, and it was observed that the grain size slightly decreased with an increase in dopant concentration. Besides, the electrolytes synthesized by the autocombustion reaction method showed somewhat larger average grain size compared to the samples of the same composition synthesized by the method of solid state reaction..