Dissertations / Theses on the topic 'Lanthanum chromite'

Orientador: Prof. Dr. Daniel Zanetti de Florio
Dissertação (mestrado) - Universidade Federal do ABC, Programa de Pós-Graduação em Nanociências e Materiais Avançados, 2017.
A obtencao de nanoparticulas por exsolucao e uma tecnica que tem sido extensivamente pesquisada e aprimorada recentemente. As particulas exsolvidas apresentam propriedades superiores quando comparadas as depositadas. O metodo de exsolucao consiste na reducao de ions previamente solubilizados em uma matriz, dando origem a particulas fortemente ligadas ao substrato. Materiais com composicoes (La1-xSrx)z(Cr1-yNiy)O3-¿Â (x e y = 0%, 5%, 10%, 15% e 20%; z = 100% e 80%) foram sintetizados, utilizando o metodo de polimerizacao de complexos. A resina produzida no processo de sintese foi analisada utilizando tecnicas de analise termica, tal que a temperatura de calcinacao foi determinada sendo igual a 900 ¿C. Foi utilizada a tecnica de difracao de raios X para caracterizacao estruturas e identificacao das fases presentes. Todas as amostras foram reduzidas com objetivo de criar, atraves de exsolucao, nanoparticulas de niquel metalico. Analises magneticas foram conduzidas para determinar o comportamento magnetico dos materiais produzidos. Materiais nao estequiometricos foram sintetizados para verificar a influencia de diferentes estequiometrias no processo de exsolucao. O sitio A nao estequiometrico prejudicou a solubilizacao do Ni e Sr na estrutura da cromita de lantanio. Todas as amostras com dopantes apresentaram uma diminuicao da temperatura de Neel da cromita de lantanio. Entretanto, o tratamento de reducao provocou um aumento da temperatura de Neel, se comparado com as amostras oxidadas. Esse e um indicativo da diminuicao de concentracao de niquel em solucao solida, um resultado condizente com o mecanismo de exsolucao. Utilizando as medidas de magnetismo foi possivel verificar o mecanismo de exsolucao e fracao massica de niquel metalico para cada amostra. Utilizando tecnicas de microscopia eletronica foi possivel detectar a presenca de nanoparticulas apos o tratamento de reducao com diametro de aproximadamente 20 nm. Ao realizar uma analise quimica local utilizando um microscopio eletronico de transmissao, foi possivel verificar que as particulas em questao sao de niquel metalico.
The production of nanoparticles by exsolution is a technique that has been extensively researched and improved recently. Exsolved particles have superior properties when compared to deposited particles. The method of exsolution consists in the reduction of previously solubilized ions in a matrix, giving rise to particles strongly attached to the substrate. Materials with compositions (La1-xSrx)z(Cr1-yNiy)O3-ä (x and y = 0%, 5%, 10%, 15% and 20%, z = 100% and 80%) were synthesized using the polymerization of complexes technique. The resin produced in the synthesis process was analysed using thermal analysis and the calcination temperature was determined to be 900 ° C. The X-ray diffraction technique was used to characterize structures and identify the present phases. All samples were reduced in order to create nanoparticles of metallic nickel by exsolution. Magnetic analysis was conducted to determine the magnetic behaviour of the produced materials. Non-stoichiometric materials were synthesized to verify the influence of different stoichiometry in the exsolution process. Non-stoichiometric A site impaired the solubilisation of Ni and Sr in the lanthanum chromite structure. All samples with dopants showed a decrease in the temperature of Neel of the lanthanum chromite. However, the reduction treatment caused an increase in the Neel temperature when compared to the oxidized samples. This is indicative of the decrease in nickel concentration in solid solution, a result consistent with the exsolution mechanism. Using the measurements of magnetism it was possible to verify the mechanism of exsolution and mass fraction of metallic nickel for each sample. Using electron microscopy techniques it was possible to detect the presence of nanoparticles after the reduction treatment with a diameter of approximately 20 nm. When performing a local chemical analysis using a transmission electron microscope, it was possible to verify that the particles in question are of metallic nickel.

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - CAPES
In the present work, we performed a study of the structural, electrical and optical properties of the compound LaCr1-xAlxO3 (x = 0.0, 0.05, 0.5, 0.95 and 1.0) produced by the combustion method. The structural analyzes were performed by X-ray diffraction (XRD) and Rietveld refinement, the electrical measurements using impedance spectroscopy and IV measurements, and optical measurements using UV-Vis spectroscopy. The results of XRD allied to the Rietveld method of refinement indicate that the samples with x = 0.0 and 0.05, have an orthorhombic structure belonging to the space group Pnma (62), and for x = 0.5, 0.95 and 1.0, a rhombohedral structure belonging to the space group R-3c (167), thus taking place a structural transition in the material. The impedance spectroscopy measurements associated to the IV curves show that the samples with higher concentration of Al present higher resistance, and the samples with lower concentration of Al present less resistance. We observe that for high frequencies a decrease of dielectric constant with frequency occurs. In addition, the appearance of only a semicircle establishes the presence of non-Debye type relaxation, where an equivalent circuit composed of two resistors (R1 and R2) and a constant phase element (CPE) were used. By means of the absorption measurements in the ultraviolet to visible region (UV-Vis), we estimated the optical gap of the samples, through the tauc equation, which vary between 3.27 - 3.43 eV. Still, we observed that the increase in Al concentration favors the increase in transmittance in the material.
No presente trabalho, realizamos um estudo das propriedades estruturais, elétricas e ópticas do composto LaCr1-xAlxO3 (x=0,0; 0,05; 0,5; 0,95 e 1,0) produzido pelo método da combustão. As análises estruturais foram realizadas por meio de difração de raios-X (DRX) e refinamento Rietveld, as elétricas por meio de espectroscopia de impedância e medidas IV, e ópticas por meio de espectroscopia de UV-Vis. Os resultados de DRX aliados ao método de refinamento Rietveld indicam que as amostras com x = 0,0 e 0,05, apresentam uma estrutura ortorrômbica pertencente ao grupo espacial Pnma (62), e para x = 0,5; 0,95 e 1,0, uma estrutura romboédrica pertencente ao grupo espacial R-3c (167), ocorrendo assim uma transição estrutural no material. As medidas de espectroscopia de impedância associadas às curvas IV mostram que as amostras com maior concentração de Al apresentam maior resistência, e as amostras com menor concentração de Al apresentam menor resistência. Observamos que para altas frequências ocorre uma diminuição da constante dielétrica com a frequência. Além disso, o aparecimento de apenas um semicírculo estabelece a presença do relaxamento do tipo não-Debye, onde foi utilizado um circuito equivalente composto por duas resistências (R1 e R2), e um elemento de fase constante (CPE). Através das medidas de absorção na região do ultravioleta ao visível (UV-Vis) estimamos o gap óptico das amostras, através da equação de tauc, os quais variam entre 3,27 - 3,43 eV. Além disso, observamos que o aumento da concentração de Al favorece o aumento da transmitância no material.
São Cristóvão, SE

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Conselho Nacional de Desenvolvimento Cient?fico e Tecnol?gico
Doped lanthanum chromite ( LaCrO3 ) has been the most common material used as interconnect in solid oxide fuel cells for high temperature ( SOFC-HT ) that enabling the stack of SOFCs. The reduction of the operating temperature, to around 800 ? C, of solid oxide fuel cells enabled the use of metallic interconnects as an alternative to ceramic LaCrO3, From the practical point of view, to be a strong candidate for interconnect the material must have good physical and mechanical properties such as resistance to oxidizing and reducing environments, easy manufacture and appropriate thermo-mechanical properties. Thus, a study on the physic-mechanical interconnects La0,8Sr0,2Cr0,92Co0,08O3 ceramics for SOFC -AT obtained by the method of combustion , as well as thermo-mechanical properties of metallic interconnects (AISI 444) covered with La0,8Ca0,2CrO3 by deposition technique by spray-pyrolysis fuel cells for intermediate temperature (IT-SOFCs). The La0,8Sr0,2Cr0,92Co0,08O3 was characterized by X -ray diffraction(XRD) , density and porosity , Vickers hardness (HV) , the flexural strength at room temperature and 900 ?C and scanning electron microscopy (SEM). The X -ray diffraction confirmed the phase formation and LaCrO3 and CoCr2O4, in order 6 GPa hardness and mechanical strength at room temperature was 62 MPa ceramic Interconnector. The coated metal interconnects La0,8Ca0,2CrO3 passed the identification by XRD after deposition of the film after the oxidation test. The oxidative behavior showed increased resistance to oxidation of the metal substrate covered by La0,8Ca0,2CrO3 In flexural strength of the coated metal substrate, it was noticed only in the increased room temperature. The a SEM analysis proved the formation of Cr2O3 and (Cr,Mn)3O4 layers on metal substrate and confirmed the stability of the ceramic La0,8 Ca0,2CrO3 film after oxidative test
A cromita de lant?nio (LaCrO3) dopada tem sido o material mais utilizado como interconector nas pilhas a combust?vel de ?xido s?lido de alta temperatura(PaCOS-AT), possibilitando o empilhamento(stack) da PaCOS. A redu??o da temperatura de opera??o, em torno de 800 ?C, das pilhas a combust?vel de ?xido s?lido, tornou poss?vel o uso de interconectores met?licos como alternativa aos LaCrO3 cer?micos. Do ponto de vista pr?tico para o material ser forte candidato a interconector deve ter boas propriedades f?sicas e mec?nicas, como resist?ncia a ambientes oxidantes e redutores, f?cil fabrica??o e propriedades termo-mec?nicas adequadas. Por este motivo realizou-se um estudo sobre as propriedades fisico-mec?nicas de interconectores de La0,8Sr0,2Cr0,92Co0,08O3 cer?mico para PaCOS-AT obtido pelo m?todo da combust?o, como tamb?m sobre as propriedades termo-mec?nicas de interconectores met?licos (AISI 444) recobertos com La0,8Ca0,2CrO3 pela t?cnica de deposi??o por spray-pir?lise para Pilhas a combust?vel de temperatura intermediaria (PaCOS-TI). A La0,8Sr0,2Cr0,92Co0,08O3 foi caracterizada por meio de difra??o de raios X (DRX), densidade e porosidade, dureza Vickers (HV), resist?ncia mec?nica a flex?o na temperatura ambiente e a 900?C e microscopia eletr?nica de varredura (MEV). A difra??o de raios X comprovou a forma??o das fases LaCrO3 e CoCr2O4, dureza na ordem de 6 GPa e resist?ncia mec?nica a temperatura ambiente de 62 MPa do interconector cer?mico. A avalia??o das fases formadas no interconector met?lico recoberto com La0,8Ca0,2CrO3 tanto na deposi??o quanto ap?s o ensaio oxidativo foi realizado por DRX. O comportamento oxidativo evidenciou o aumento da resist?ncia a oxida??o do substrato met?lico recoberto por La0,8Ca0,2CrO3. Na resist?ncia mec?nica a flex?o do substrato met?lico recoberto, notou-se o aumento apenas na temperatura ambiente. Com aux?lio do MEV comprovou-se a forma??o das camadas de Cr2O3 e (Cr,Mn)3O4 sobre o substrato met?lico e confirmou-se a estabilidade do filme cer?mico de La0,8Ca0,2CrO3 ap?s o ensaio oxidativo

Des poudres de chromite de lanthane ont ete preparees par procede sol-gel en milieu aqueux a partir de deux precurseurs: nitrates ou acetates. Les sols colloidaux ont ete obtenus par precipitation et peptisation en ajoutant de l'ammoniaque. Les zones de ph de precipitation et de peptisation ont ete determinees par des dosages acidobasiques. Apres gelification a l'air, la decomposition des gels a ete etudiee par analyse thermique (atg - atd) et par spectroscopie ir ; la cristallisation du chromite de lanthane, obtenu par traitement thermique des gels a ete determinee par diffractometrie des rx sur poudre. Des elements chauffants tubulaires ont ete realises par la methode d'extrusion avec les poudres ainsi obtenues. La composition de pates ceramiques permettant cette mise en forme a ete determinee. Une analyse thermomecanique par dilatometrie a permis de suivre le comportement au frittage des preformes, et de determiner la temperature de frittage convenable pour differentes compositions. Les poudres et les ceramiques ont ete caracterisees par meb, bet, porosimetrie a mercure, et par la mesure de la conductivite electrique

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Dissertacao (Mestrado)
IPEN/D
Intituto de Pesquisas Energeticas e Nucleares, IPEN/CNEN-SP

Hydrogen is a vital component in several different chemical reactions as well as a potential fuel source for the future. The water gas shift (WGS) reaction converts CO and water to hydrogen and CO2. The objective of this work is to first, characterize the potential benefits of the addition of lanthanum oxide (lanthana) to the iron-chromium-copper (Fe-Cr-Cu) oxide catalysts industrially used in high temperature water gas shift processes, and second, analyze these catalysts using in-situ UV-Visible spectroscopy. The benefits of each component in the catalyst are discussed as well as potential benefits from the addition of lanthana. Lanthana is a rare earth oxide that dramatically increases the surface area of the iron based WGS catalysts, and small concentrations of other rare earth oxides (i.e. cerium) have been shown to increase the rate of desorption of CO2 from iron surfaces (Hu Yanping 2002). Lanthana has similar chemical properties to other rare earth oxides tested and has not been previously tested as an additive to the WGS catalyst. Therefore catalysts with 0, 1, 2, 5, 10, and 20 wt% lanthana were made via a co-precipitation method in order to measure changes in activity, physical stability, and thermal stability. Catalyst characterization techniques utilized include electron dispersive X-ray spectroscopy (EDX), temperature programmed reduction (TPR) with hydrogen, and nitrogen physisorption (BET). The kinetic analysis was performed utilizing both mass spectroscopy (MS) and gas chromatography (GC). The addition of 1 wt% lanthana to the Fe-Cr-Cu catalysts increases WGS reaction rates of the catalyst at 425°C and 350°C, however the 0% La catalyst has the highest rates at 375°C and 400°C. The 0% La catalyst shows significant drop off in rate at 425°C, suggesting that the lanthana provides a small thermal stabilizing, i.e. the addition of lanthana prevents catalyst sintering at higher temperatures. Traditionally, chromia acts as the sole thermal stabilizer in these catalysts. The addition of large amounts of lanthana inhibits the chromia stabilizing effect, however small additions of lanthana appear to have an additional catalyst promotional effect without interfering with the chromia thermal stabilization. The increased WGS reaction rates at higher temperatures could allow for greater throughput of reactants in industrial settings. Higher concentrations of lanthana decrease the activity due to what is believed to be disruption of the chromia stabilizing effect as well as reduced amount of the active phase of catalyst. In-situ UV-Visible analysis shows that the oxidation state of the iron in the catalyst has a direct correlation to the UV-Visible light absorbance of the surface of the iron catalyst. Extent of reduction is traditionally measured with a synchrotron which is significantly more expensive than UV-Vis spectroscopy. This study uses the more economical UV-Vis spectrometer to determine similar information. The lanthana doped catalysts show an over-reduction of iron during WGS conditions (i.e. rapid reduction of Fe2O3 to Fe3O4 and FeO).

The interactions between different components of solid oxide cells (SOCs) are critical issues for achieving the tens of thousands of hourâ s goal for long-term performance stability and lifetime. The interactions between the ceramic electrolyte, porous ceramic air electrode, and metallic interconnect materials â including solid state interfacial reactions and vaporization/deposition of some volatile elements â have been investigated in the simulated SOC operating environment. The interactions demonstrate the material degradation mechanisms of the cell components and the effects of different factors such as chemical composition and microstructure of the materials, as well as atmosphere and current load on the air electrode side. In the aspect of materials, this work contributes to the degradation mechanism on the air electrode side and provides practical material design criteria for long-term SOC operation. In this research, an yttria-stabilized zirconia electrolyte (YSZ)/strontium-doped lanthanum manganite electrode (LSM)/AISI 441 stainless steel interconnect tri-layer structure has been fabricated in order to simulate the air electrode working environment of a real cell. The tri-layer samples have been treated in dry/moist air atmospheres at 800°C for up to 500 h. The LSM air electrode shows slight grain growth, but the growth is less in moist atmospheres. The amount of Cr deposition on the LSM surface is slightly more for the samples thermally treated in the moist atmospheres. At the YSZ/LSM interface, La enrichment is significant while Mn depletion occurs. The Cr deposition at the YSZ/LSM interface is observed. The stoichiometry of the air electrode is an important factor for the interactions. The air electrode composition has been varied by changing the x value in (La0.8Sr0.2)xMnO3 from 0.95 to 1.05 (LSM95, LSM100, and LSM105). The enrichment of La at the YSZ/LSM interface inhibits the Cr deposition. The mechanisms of Cr poisoning and LSM elemental surface segregation are discussed. A 200 mA·cm-2 current load have been applied on the simulated cells. Mn is a key element for Cr deposition under polarization. Excessive Mn in the LSM lessens the formation of La-containing phases at the YSZ/LSM interface and accelerates Cr deposition. Deficient Mn in LSM leads to extensive interfacial reaction with YSZ forming more La-containing phase and inhibiting Cr deposition.
Ph. D.

L'etude des monocristaux d'hexaaluminates mixtes de lanthane et magnesium, de formule lamgal11019 (lma), dopes aux ions cr3+ et ti3+, pour la realisation de nouveaux materiaux laser accordables dans le visible et proche infra-rouge (600-1200 nm), s'est deroulees en trois etapes: cristallogenese et amelioration de la qualite cristalline, etude des proprietes spectroscopiques d'absorption et d'emission et essais laser en cavite. Aucun des deux materiaux n'a donne lieu a une emission laser: le lma:cr de bonne qualite cristalline possede des proprietes d'emission mediocres, principalement dues a l'existence de plusieurs sites emetteurs de sections efficaces d'emission stimulee faible et de transferts d'energie entre ces sites. Le lma:ti presente de nombreux defauts cristallins, lies a une forte segregation du titane. De plus il presente des bandes d'absorption parasites dans la zone d'absorption et d'emission de l'ion ti#3#+, liees sans doute a la presence d'ions ti#4#+, ti#2#+ et/ou de lacunes cationiques. Des recherches pour diminuer ces bandes parasites ont ete entreprises (codopages, recuits. . . )

Etudes de nouveaux materiaux laser, dans le domaine du proche ir (compose lamgal::(11)o::(19):cr ou dans le domaine visible pour lamgal::(11)o::(19):pr) et d'autre part a la sensibilisation de la fluorescence du neodyme dans la matrice par les ions cr**(3+) et ce**(3+). Le materiau cr**(3+) pourrait etre un bon candidat dans la gamme des lasers accordables dans le rouge. Dans les composes lamgal::(11)o::(19):ce,nd l'analyse des proprietes optiques revele que le cerium sensibilise la fluorescence du neodyme dans le lna. Dans le compose lamgal::(11)o::(19):ce,cr,nd, nous observons non seulement un transfert ce -> nd mais egalement un transfert ce -> cr

Conception d'un pot catalytique et etude de l'efficacite de la catalyse. Mise au point d'un support metallique. Preparation du support par passivation de l'alliage fer-chrome-aluminium. Stabilisation de l'alumine par ajout de terres rares. Utilisation de platine et de palladium comme materiaux catalytiques

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Tese (Doutoramento)
IPEN/T
Instituto de Pesquisas Energeticas e Nucleares - IPEN-CNEN/SP

Cette étude apporte des données sur l'intérêt des chromites de lanthane strontium dopés au ruthénium La1-xSrxCr1-yRuyO3δ (LSCrRu) comme anode de pile à combustible ITSOFC. La synthèse sous champ micro-onde d'oxydes mixtes a permis la mise en forme de demi-cellules et cellules complètes modèles par pulvérisation cathodique magnétron et par sérigraphie. Les propriétés de reformage de CH4 ont pu être étudiées dans un dispositif mono-chambre couplé à un spectromètre de masse en collaboration avec l'Institut de REcherche Hydro-Québec. La réactivité à l'interface anode/électrolyte a été analysée par SIMS et XRD sur demi-cellules modèles vieillies sous H2 à 800°C. Elle a permis de préciser la chimie du film après traitement. Les cellules complètes, testées en condition de fonctionnement, ont mis en évidence, après dissolution sélective de LSCrRu, une modification de la microstructure à l'interface anode/électrolyte. Des analyses XPS et SEM-EDX n'ont pas montré de réactivité chimique.

Cette étude apporte des données sur l'intérêt des chromites de lanthane strontium dopés au ruthénium La1-xSrxCr1-yRuyO3δ (LSCrRu) comme anode de pile à combustible ITSOFC. La synthèse sous champ micro-onde d'oxydes mixtes a permis la mise en forme de demi-cellules et cellules complètes modèles par pulvérisation cathodique magnétron et par sérigraphie. Les propriétés de reformage de CH4 ont pu être étudiées dans un dispositif mono-chambre couplé à un spectromètre de masse en collaboration avec l'Institut de REcherche Hydro-Québec. La réactivité à l'interface anode/électrolyte a été analysée par SIMS et XRD sur demi-cellules modèles vieillies sous H2 à 800°C. Elle a permis de préciser la chimie du film après traitement. Les cellules complètes, testées en condition de fonctionnement, ont mis en évidence, après dissolution sélective de LSCrRu, une modification de la microstructure à l'interface anode/électrolyte. Des analyses XPS et SEM-EDX n'ont pas montré de réactivité chimique
New data on the interest of ruthenium doped lanthanum strontium chromite La1-xSrxCr1-yRuyO3δ (LSCrRu) as anode in ITSOFC are presented. Microwave synthesis of mixed oxides allowed shaping of half-cells and complete cells using sputtering and screen printing methods. LSCrRu methane reforming property had been studied in a mono-chamber system coupled with a mass spectrometer in collaboration with Institut de REcherche Hydro-Québec. Anode/electrolyte interfacial reactivity had been analysed using SIMS and XRD on half cells models after ageing under H2 at 800°C. This experiment gave the chemical composition of the film after treatment. Complete cells, tested under working conditions, highlighted, after selective dissolution of LSCrRu film, a microstructure modification at anode/electrolyte interface. XPS and SEM-EDX techniques did not show chemical reactivity

Dans les systèmes d’Electrolyse Haute Température (EHT), le matériau choisi comme interconnecteur doit avoir une bonne résistance à la corrosion sous air et sous mélange H2/H2O à 800 °C, et maintenir une bonne conductivité sur de longues durées. Dans ce cadre, l’objectif de ce travail était, d’une part, d’évaluer un alliage ferritique commercial (l’alliage K41X) comme matériau d’interconnecteur pour l’application EHT. Dans ce but, ont été mis en place des essais d’oxydation en four et en thermoblance pour accéder aux cinétiques d’oxydation, et des mesures de résistivité pour évaluer le paramètre ASR (Area Specific Resistance) à 800°C. D’autre part, l’étude a permis d’apporter des éléments de compréhension plus fondamentaux sur les mécanismes d’oxydation des alliages chromino-formeurs, en particulier sous mélange H2-H2O, par le biais d’essais et de caractérisations spécifiques (Photoélectrochimie, traçage isotopique, essais de longues durées). Cette double stratégie est également appliquée pour l’étude d’une solution de revêtement (obtenu à l’aide de la MOCVD) basée sur l’oxyde pérovskite LaCrO3 qui présente des propriétés de conductivité élevée particulièrement intéressante en vue de l’application EHT. Ainsi, cette étude amène également des éléments de compréhension sur le rôle du lanthane comme élément réactif dont l’effet est souvent discuté dans la littérature. Pour les deux milieux, à 800°C, la couche d’oxyde formée est une couche duplexe Cr2O3/(Mn,Cr)3O4 , recouverte dans le cas du mélange H2-H2O par une fine couche d’oxyde spinelle Mn2TiO4 . Sous air, le mécanisme de croissance déterminé ici est cationique, en accord avec la littérature. La présence d’un revêtement LaCrO3 ne modifie pas ce mécanisme mais ralentit la cinétique de croissance de la couche sur les premières centaines d’heure. De plus, le revêtement améliore l’adhérence et la conductivité de la couche d’oxyde. Sous mélange H2-H2O, le mécanisme de croissance se révèle anionique. La présence de revêtement ralentit la cinétique d’oxydation. Bien que .d’épaisseurs similaires, les couches d’oxyde présentent sous air une résistivité d’un ordre de grandeur inférieure à celle mesurée sous H2-H2O. Il est mis en évidence que la forte résistivité de l’alliage en milieu H2-H2O est liée à la présence de protons issus de la vapeur d’eau présents dans la couche d’oxyde. Le revêtement ne permet néanmoins pas d améliorer la conductivité sous H2-H2O
In High Temperature Vapor Electrolysis (HTVE) system, the materials chosen for the interconnectors should have a good corrosion behaviour in air and in H2-H2O mixtures at 800°C, and keep a high electronic conductivity over long durations as well. In this context, the first goal of this study was to evaluate a commercial ferritic alloy (the K41X alloy) as interconnect for HTVE application. Oxidation tests in furnace and in microbalance have therefore been carried out in order to determine oxidation kinetics. Meanwhile, the Area Specific Resistance (ASR) was evaluated by Contact Resistance measurements performed at 800°C. The second objective was to improve our comprehension of chromia-forming alloys oxidation mechanism, in particular in H2/H2O mixtures. For that purpose, some specific tests have been conducted: tracer experiments, coupled with the characterization of the oxide scale by PEC (PhotoElectroChemistry). This approach has also been applied to the study of a LaCrO3 perovskite oxide coating on the K41X alloy. This phase is indeed of high interest for HTVE applications due to its high conductivity properties. This latter study leads to further understanding on the role of lanthanum as reactive element, which effect is still under discussion in literature.In both media at 800°C, the scale is composed of a Cr2O3/(Mn,Cr)3O4 duplex scale, covered in the case of H2-H2O mixture by a thin scale made of Mn2TiO4 spinel. In air, the growth mechanism is found to be cationic, in agreement with literature. The LaCrO3 coating does not modify the direction of scale growth but lowers the growth kinetics during the first hundreds hours. Moreover, with the coating, the scale adherence is favored and the conductivity appears to be slightly higher. In the H2-H2O mixture, the growth mechanism is found to be anionic. The LaCrO3 coating diminishes the oxidation kinetics. Although the scale thickness is about the same in both media, the ASR parameter is one order of magnitude higher in H2/H2O than in air. Specific contact resistance tests show that the higher resistivity in the H2/H2O mixture is closely linked to the presence of protons in the scale. Moreover, tracer experiments show that these protons come from the water molecule dissociation, and not from the H2 molecule. In H2/H2O, the LaCrO3 coating does not increase the conductivity

Lanthanum chromite (LaCrO3), Lanthanum manganite (LaMnO3) and 40% strontium doped lanthanum cobalt iron oxide, La0.6Sr0.4Co0.2Fe0.8O3 (LSCF-6428) are perovskite oxides which are widely used as interconnect or cathode materials in solid oxide fuel cells(SOFCs) due to their high electrical conductivity, good oxygen reduction kinetics, and good chemical stability. The solid state reaction route is the most commonly used method for synthesis of these materials. However, the solid state reaction method usually involves long-time mixing and high synthesis temperature (typically, >1200 ºC), which makes it time-consuming and costly. Molten salt synthesis, which occurs at much lower temperatures (350 ºC – 550 ºC) can offer better particle size and compositional control and reduced energy usage during materials synthesis. In this study, LaCrO3, LaMnO3, and LSCF were synthesized in a molten salt eutectic of LiCl-KCl. A range of reaction temperature from 370 ºC to 600 ºC was investigated. It was found that a pure LaMnO3 perovskite phase can be formed at as 400 ºC using the molten salt method and that LSCF powders were successfully synthesized at 500 ºC. When forming LaCrO3 using the molten salt method, LaOCl was also formed at or above 400 ºC. The X-ray diffraction (XRD) results show this is an attractive alternative route of synthesis to decrease the reaction temperature. Both Scanning Electron Microscopy (SEM) images and XRD patterns for LaCrO3 showed that only cubic structures were formed at low temperature (400 ºC and 450 ºC) and then hexagonal structures started to appear at temperatures above 500 ºC. The molten salt synthesis method was then used to prepare core-shell structures with LaCrO3 or LSCF particles as the core and LaMnO3 as the shell. Core-shell structures were characterized by Scanning Electron Microscopy (SEM), Scanning Transmission Electron Microscope (STEM) and Energy Dispersive X-ray Spectroscopy (EDS). It was found that the expected core-shell structures were successfully formed with the overall cubic structures. Therefore, the molten salt synthesis method is a feasible method to decrease the operation temperature and form the core-shell structure.

碩士
國立成功大學
化學工程研究所
83
The precursors with OH- ligands were prepared by coprecipitated method from various ratio of La(NO3)3.6H2O，Cr(NO3)3.9H2O and ZrOCl2.8H2O agueous solution with Sodium hydroxide solution. After freeze drying, the precursors were calcined at 1073K for 8 hrs to obtain corresponding compounds. It indicated from X- ray diffraction patterns that the major compounds were LaCrO3 and a new phase La1-xZrxCrO3.In this study , we investigated not only the properties of the ternary system, but a X-ray diffraction oatterns show that La1-xZrxCrO3 was a major compound when the mole fraction of zirconium was smaller than 0.06 and that ZrO2 was discovered as the mole fraction of zirconium was bigger than 0.07. From the variations of susceptibility with temperature analyzed by SQUID, we found that all of compounds were antiferromagnetism. The compounds are characteristics of a polaron mechanism of conductivity from the variations in resistivity with temperature. This semiconductor is a characteristic of resistence changing with electricwave frequency. Of the four sintered atmospheres,the sintered body in atmosphere of chromium oxide powder has the least resistivity coefficient in electrical property.And the sintered body in N2 atmosphere has the best hardness in Vickers Hardness Testing.

碩士
國立成功大學
材料科學及工程學系碩博士班
101
BiFeO3(BFO),which is one of the most potential multiferroics ,has high Curie temperature TC(~1123 K) and high Néel temperature TN(~643 K) resulting to the coexistence of ferroelectricity and ferromagnetism at room temperature.This study mainly reported the influence and change by the co-doping of lanthanum and different content of chromium.Bi3+ ions have close size as La3+ ions, and Fe3+ ions have close size as Cr3+ ions, so we hope to improve the ferroelectric properties with the substitution of La3+ ions for Bi3+ ions and Cr3+ ions for Fe3+ ions in the perovskite BiFeO3 structure. Bulks of BiFeO¬3 and BiFeO3 co-doping with La3+ and Cr3+ were prepared by the usual solid state reaction method. X-ray diffraction(XRD), Scanning electron microscopy(SEM), ferroelectric hysteresis measurements, leakage current measurements and dielectric measurement are used for understanding the effect of codoping. We found that second phases appeared when the doping amount of Cr were more than 5%, indicating that there was a limit for Cr doping in BFO.With the XPS analysis,La doping and the increased doping amount of Cr make more Fe ions change from Fe3+ to Fe2+,which resulting to higher conductivity by transfer of electrons between the neighboring Fe2+ and Fe3+ ions. Because of more Fe2+ ions, the conductive mechanism changes from ohmic conduction to space charge limit conduction by fitting the IV curve. The dielectric measurement can find out that the space charge is the main polarization mechanism.