Dissertations / Theses on the topic 'Lanthanum silicate'

A temperatura de operação de células a combustível de óxido sólido (SOFCs) que utilizam zirconia estabilizada com itria (YSZ) como eletrólito é 1000 oC. Essa alta temperatura gera graves problemas relativos a materiais e vida util da célula. Por isso, condutores iônicos que possuem alta condutividade em temperaturas inferiores são pesquisados atualmente. Estudos mostraram que La10Si6O27 tipo apatita possui alta condutividade iônica de oxigenio, que é comparativamente maior que a de YSZ, a 500 oC, sendo, portanto, um potencial candidato como eletrólito para SOFC. O objetivo do presente trabalho é o desenvolvimento de técnicas de síntese de silicato de lantânio tipo apatita. Rotas inéditas de solgel modificada para sintetizar La9,33Si6O26 são propostas. Volumes estequiométricos de soluções de Na2SiO3 e LaCl3 foram misturados para a formação de gel de Si. Em seguida este gel foi calcinado a 900 °C, lavado, filtrado e tratado novamente a 900 °C. Em outra rota, volumes estequiométricos de soluções de Si (Na2SiO3 ou TEOS) e de La (LaCl3) foram utilizados para obtenção de gel de Si. Em seguida, hidróxido de La foi precipitado pela adição de uma base (NaOH ou NH4OH) ao gel. O material resultante foi calcinado a 900 °C, lavado, filtrado e tratado novamente a 900 °C. Pós de aglomerados fracos e alta sinterabilidade foram obtidos. DRX dos pós mostrou a estrutura de apatita monofásica a 900 oC. Morfologia de ceramica densa foi observada em imagens de MEV da superfície das pastilhas sinterizadas a 1200,1300 e 1400 oC por 4 h. Estas temperaturas e tempo de sinterização são significativas, pois no método convencional temperaturas superiores a 1700oC e tempos muito maiores são necessários para obtenção de tais cerâmicas. Densidades relativas superiores a 90% foram obtidas através dos métodos propostos. Uma conclusão importante é que TEOS, o reagente usual de alto custo, pode ser substituído por Na2SiO3, de preço muito mais baixo, para obter La9,33Si6O26 tipo apatita.
Solid oxide fuel cell (SOFCs) operating temperature that uses yttria stabilized zirconia (YSZ) as the electrolyte is 1000ºC. This high temperature causes serious problems concerning cell life and materials. Therefore, the ionic conductors which have high conductivity at lower temperature are currently researched. Studies have shown that the composition of La10Si6O27 apatite type has high oxygen ionic conductivity, which is comparably higher than that of YSZ, at 500 oC, it is therefore a potential candidate as for SOFC electrolyte. The objective of the present work is the development of lanthanum silicate with apatite type synthesis techniques. Novel modified solgel routes to synthesize La9.33Si6O26 are proposed. Stoichiometric volumes of Na2SiO3 and LaCl3 solutions were mixed for the formation of Si gel. This gel was calcined at 900 °C, washed, filtered and again thermally treated at 900 °C. In the other route, stoichiometric volumes of Si (Na2SiO3 or TEOS) and La (LaCl3) solutions were used for obtaining Si gel. Then, La hydroxide was precipitated by adding of a base (NaOH or NH4OH) to gel. Then the material was calcined at 900 °C, washed, filtered and again treated at 900 °C. Highly sinterable weakly agglomerated powders have been obtained. XRD patterns of the powders showed the single-phase apatite structure at 900 oC. Dense ceramic morphology was observed from the SEM images of surface of the pellets sintered at 1200, 1300 and 1400oC for 4h. This low temperature sintering and time of sintering are significant because the conventional method requires superior temperatures of 1700 oC to obtain the same dense ceramics. High relative densities higher than 90% was obtained via proposed methods. An Important conclusion is the TEOS, the usual high cost reagent, may be substituted by a cheap price Na2SiO3, to obtain apatite type La9.33Si6O26.

Cerâmicas de silicato de lantânio tipo apatita têm sido estudadas devido ao grande interesse tecnológico para aplicação como eletrólito em células a combustível de óxido sólido de temperatura intermediária (IT-SOFC: Intermediate Temperature Solid Oxide Fuel Cell). A condutividade iônica dessas cerâmicas em temperaturas intermediárias (600-800°C) é maior do que a da YSZ (Ytria Stabilized Zirconia) utilizada como eletrólito em SOFCs de alta temperatura (800-1000°C). Neste trabalho, silicato de lantânio tipo apatita foi sintetizado pelo método sol-gel seguido de precipitação, a partir de Na2SiO3 como fonte de sílica. No método proposto, estudou-se rotas de síntese em meio ácido e básico para a formação do gel de sílica, seguida de precipitação. A fase cristalina de silicato de lantânio tipo apatita foi obtida pela calcinação de pós sintetizados a 900°C. Esta temperatura é muito inferior às praticadas em outros métodos convencionais de síntese. As análises por difração de raios X (DRX) mostraram silicato de lantânio tipo apatita como fase principal do material sintetizado na rota de síntese em pH ácido. No entanto, uma fase secundária indesejável, La2Si2O7, foi identificada quando o pó cerâmico foi calcinado a 1200°C. Por outro lado, pela rota básica, fase única de silicato de lantânio tipo apatita foi obtida após tratamento térmico dos precursores a 900 e 1200°C. Pastilha cerâmica obtida a partir dos pós obtidos e sinterizados a 1400°C por 4h, apresentaram fase cristalina pura de silicato de lantânio tipo apatita. Microscopia eletrônica de varredura (MEV) foi utilizada para observar a morfologia dos pós e microestrutura das pastilhas sinterizadas. Pós cerâmicos finos com tamanho de partículas submicrométricas e microestrutura típica de apatita foram alcançadas pelo método proposto.
Lanthanum silicate apatite-type ceramics have been studied because of the great technological interest for IT-SOFC applications as electrolyte (Intermediate Temperature Solid Oxide Fuel Cell). Ionic conductivity of those ceramics at intermediate temperatures (600-800°C) is higher than that of YSZ (Ytria Stabilized Zirconia) electrolyte used at high-temperatures (800-1000 °C) SOFCs. In this work, lanthanum silicate apatite-type was synthesized by sol-gel method followed by precipitation from Na2SiO3 as a source of silica. In the proposed method, synthesis routes in acid and basic medium to the formation of silica gel, followed by precipitation were studied. Apatite crystalline phase of lanthanum silicate ceramic was obtained by calcining the powders at 900°C. This temperature is much lower than those other conventional methods of synthesis. Analysis by x-ray diffraction (XRD) showed the lanthanum silicate apatite-type phase as the main phase of the synthesized material at the pH acid synthesis route. However, undesirable secondary phase, La2Si2O7, was recognized when the powder was calcined at 1200°C. On the other hand, by the basic route, single apatite-type phase powder was obtained after thermal treatment of the precursors at 900 and 1200°C. Ceramic pellet obtained from those powders sintered at 1400°C for 4h, presented pure apatite crystalline phase of lanthanum silicate. Scanning electron microscopy (SEM) was used to observe morphology of powders and microstructure of sintered pellets. Sub micrometric size powders and apatite typical microstructure ceramic were reached by the suggested method.

Solid state lithium ion electrolytes are important to the development of next generation safer and high power density lithium ion batteries. Perovskite-structured LLT is a promising solid electrolyte with high lithium ion conductivity. LLT also serves as a good model system to understand lithium ion diffusion behaviors in solids. In this thesis, molecular dynamics and related atomistic computer simulations were used to study the diffusion behavior and diffusion mechanism in bulk crystal and grain boundary in lithium lanthanum titanate (LLT) solid state electrolytes. The effects of defect concentration on the structure and lithium ion diffusion behaviors in LLT were systematically studied and the lithium ion self-diffusion and diffusion energy barrier were investigated by both dynamic simulations and static calculations using the nudged elastic band (NEB) method. The simulation results show that there exist an optimal vacancy concentration at around x=0.067 at which lithium ions have the highest diffusion coefficient and the lowest diffusion energy barrier. The lowest energy barrier from dynamics simulations was found to be around 0.22 eV, which compared favorably with 0.19 eV from static NEB calculations. It was also found that lithium ions diffuse through bottleneck structures made of oxygen ions, which expand in dimension by 8-10% when lithium ions pass through. By designing perovskite structures with large bottleneck sizes can lead to materials with higher lithium ion conductivities. The structure and diffusion behavior of lithium silicate glasses and their interfaces, due to their importance as a grain boundary phase, with LLT crystals were also investigated by using molecular dynamics simulations. The short and medium range structures of the lithium silicate glasses were characterized and the ceramic/glass interface models were obtained using MD simulations. Lithium ion diffusion behaviors in the glass and across the glass/ceramic interfaces were investigated. It was found that there existed a minor segregation of lithium ions at the glass/crystal interface. Lithium ion diffusion energy barrier at the interface was found to be dominated by the glass phase.

Les piles à combustibles à électrolyte solide de type SOFC permettent de transformer directement l’énergie de la réaction chimique de formation de l’eau à partir de l’hydrogène et de l’oxygène, en énergie électrique. De nos jours, les apatites de type silicates de terres rares présentent beaucoup d’intérêt comme électrolyte solide en raison de leurs propriétés de transport élevées avec une forte conductivité ionique et une faible énergie d'activation. Ils peuvent fonctionner de manière stable à une température intermédiaire sur une large plage de pression partielle d'oxygène en maintenant d'excellentes performances. Ils sont ainsi considérés comme de bons candidats pour les électrolytes de piles de type IT-SOFC. Parmi cette série de conducteurs, le type La-Si-O possède une conductivité plus élevée et leur performance serait modifiée par différents éléments dopants.L'objectif de cette thèse est d'étudier les effets des éléments de substitution / dopage ainsi que les méthodes de synthèse sur les propriétés structurales ainsi que sur la conductivité des apatites de type silicates de lanthane. Dans cette étude, nous utilisons une double approche: une approche de simulation et une approche expérimentale pour optimiser la pureté et les performances des matériaux d'électrolyte.Dans l'approche de simulation, le calcul basé sur la DFT (Théorie de la fonctionnelle de la densité) a été réalisée en vue d’étudier l'effet des positions de dopage: dopant Sr à La position de La et dopant Ge à la position de Si. Les résultats obtenus par le calcul concernant la conductivité ionique confirment ceux obtenus par l’expérience.Avec l’approche expérimentale, nous présentons la synthèse et la caractérisation de La10Si6O27 (LSO) dopé par Sr et élaboré par des procédés sol-gel. Les résultats montrent que la conductivité ionique est activé thermiquement et que les valeurs se situent entre 4,5 × 10-2 et 1 x 10-6 S·cm-1 à 873 K et dépend des conditions d’élaboration et de la composition de la poudre
The Solid Oxide Fuel Cell (SOFC) defined by its ceramic and oxide electrolyte, is an electrochemical energy conversion device that produces electricity directly from the chemical reaction of fuel. Nowadays, apatite type rare earths silicates and germaniums attract many interests as the solid electrolyte due to the superior transport properties with high ionic conductivity and low activation energy. They can operate stably at intermediate temperature over a wide oxygen partial pressure range and maintain excellent performances, being considered as a candidate for IT-SOFC electrolytes. Among this series of conductors, the La-Si-O type has a higher conductivity and the performance would be modified by different doping elements.The objective of this thesis is to study the effects of element substitution/doping and synthesis methods on the structural and conductivity properties of apatite type lanthanum silicates. In this study, we use a double approach: a simulation approach and an experimental approach to optimize the electrolyte materials purity and performance.Using simulation approach, a first principle calculation based on DFT (Density Functional Theory) was carried out to investigate the effect on doping positions: Sr dopant at La position and Ge dopant at Si position. The calculation results give a connection to the ionic conductivity obtained by experiments.With experimental approach, we present the synthesis and characterization of Sr-doped La10Si6O27 (LSO) prepared through an optimized water-based sol-gel process. The results show that the ionic conductivity is thermally activated and values lies between 4.5×10-2 and 1×10-6 Scm-1 at 873 K as a function of the composition and powder preparation conditions

Cette thèse a pour objet l’élaboration et la caractérisation d’oxyapatite silicatées de lanthane, matériaux potentiellement utilisables comme électrolyte pour piles à combustible à oxyde solide. Des poudres et céramiques denses d’apatite de stœchiométries en oxygène et de compositions (substitutions) différentes ont été élaborées par réaction à l’état solide à haute température. Il ressort que quelque soit la composition, le paramètre principal qui gouverne les propriétés de conduction est la stœchiométrie en oxygène. La conductivité atteint 1,7 x 10-2 S.cm-1 à 700°C pour la composition La10Si5,5Mg0,5O26,5. La combinaison d’analyses spectroscopiques (Diffusion Raman, Résonance Magnétique Nucléaire) et de diffraction des neutrons a permis de définir un mécanisme de conduction des ions oxydes dans la structure. L’existence d’un site interstitiel localisé dans les canaux de la structure apatite en (-0,01, 0,04, 0,06) est confirmée. Un chemin de diffusion des ions oxyde de type « lacune sur le site 2a – atome interstitiel » est proposé le long de l’axe c. Ce mécanisme semble facilité par le déplacement des tétraèdres situés à la périphérie du canal
This work is devoted to the elaboration and the characterisation of apatite type lanthanum silicates as new electrolyte for SOFC fuel cells. Apatite powders and dense ceramics with different oxygen stoichiometries and compositions (substitutions) were prepared by high temperature solid state reaction. Whatever the composition, we can conclude that the key parameter which governs the conduction properties is the oxygen stoichiometry. The conductivity reaches 1.7 x 10-2 S.cm-1 at 700°C for the La10Si5,5Mg0,5O26,5 composition. By combining Raman spectroscopy, NMR and neutron diffraction, an oxide ion conduction mechanism was deduced. The existence of an interstitial site located at (-0.01, 0.04, 0.06) was confirmed. This allows to proposed a realistic oxygen diffusion pathway through this interstitial site along the c axis. Moreover, this mechanism seems to be facilitated by the displacement of tetrahedral located at the periphery of the oxide channels

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Dissertação (Mestrado)
IPEN/D
Instituto de Pesquisas Energeticas e Nucleares - IPEN-CNEN/SP

Ce travail a permis de mettre en évidence l'influence de différents facteurs tels que le pH, le rapport molaire SiO2/K2O, le sens de mélange des réactifs sur la nature et la composition de silicates et polysilicates de différents éléments polyvalents. Ces éléments ont été choisis, dans le cadre d'un contrat avec le C. E. A. , pour leurs propriétés chimiques proches de celles de certains radionucléides : ce fut le cas pour le lanthane et le thorium ou en raison de leur présence dans la fritte, comme le zirconium et l'aluminium, lors de la fabrication des blocs de verres incluant ces éléments et les oxydes de radionucléides en vue de leur immobilisation comme déchets radioactifs. L'évolution de la composition des corps obtenus en fonction des facteurs cités ci-dessus a été suivie au moyen de différentes méthodes analytiques: analyse chimique quantitative, sonde à dispersion d'énergie, diffraction des rayons X peu opérante du fait de l'état amorphe des composés formes ; en revanche, la spectrométrie de vibration infrarouge a permis de mettre en évidence la formation des ponts siloxanes et les degrés de polymérisation plus ou moins importants des entités structurales silicatées et de les différencier du cas où, par abaissement préalable du pH du réactif, il se forme un gel de silice adsorbant les cations polyvalents sous forme d'hydroxydes. Les résultats obtenus permettent de simuler les composés susceptibles de se produire superficiellement quand le verre nucléaire se trouve au contact de l'eau.

Ausgehend von den binären Systemen La-H und La-Si, wurde das Hydrierungs- und Dehydrierungsverhalten von La5Si3, LaSi und LaSi2 mittels thermogravimetrischer und thermoanalytischer Methoden eingehend untersucht und mit den Aussagen thermodynamischer Gleichgewichtsberechnungen verglichen. Die synthetisierten Silicide und Reaktionsprodukte wurden mittels Röntgenpulverdiffraktometrie charakterisiert. La als starker Hydridbildner dominiert die Eigenschaften seiner Silicide. Ein hoher La-Gehalt führt zu einer starken H2-Affinität des Silicids. Zusätzlich zu den berechneten binären Phasen kommt es zur Bildung von stabilen und metastabilen ternären Hydriden. Während LaSi2 über einen weiten Druck- und Temperaturbereich nicht mit H2 reagiert, nimmt La5Si3 schon bei niedrigen Temperaturen in einer mehrstufigen Reaktion unter Zerfall der Ausgangsverbindung Wasserstoff auf. Die Hydrierung von LaSi verläuft reversibel unter Bildung eines stabilen ternären Hydrides. Die Synthese von La-Siliciden aus LaH3 und Si ist in Abhängigkeit vom La-Gehalt schon bei milden Temperaturen möglich.

Ce travail est consacré à l’élaboration par projection plasma à la pression atmosphérique, et à la caractérisation, de couches d’électrolyte en apatite de type La9Sr1Si6O26,5 pour les piles à combustibles à oxyde solide (SOFC) fonctionnant à température intermédiaire (700 °C). Dans un premier temps, ce travail a porté sur le choix des conditions opératoires permettant l’élaboration de dépôts denses d’apatite, caractéristique essentielle d’un électrolyte. Il est apparu que l’utilisation de mélanges plasmagènes à enthalpie et conductivité thermique élevées ainsi qu’un faible débit massique, soit favorable à l’obtention des dépôts les plus denses. Les deux techniques de frittage essayées (le frittage conventionnel et le frittage par « Spark Plasma Sintering ») ont conduit à la cristallisation et à la densification des dépôts, permettant ainsi d’améliorer leurs propriétés électrochimiques. L’augmentation de conductivité ionique est particulièrement notable pour les dépôts réalisés avec des conditions de projection très énergétiques et frittés de manière classique sous air. La conductivité ionique atteint 1,3.10-3 de Ω-1.cm-1 à 700 °C. Dans un second temps, les nickelates de type (La2, Nd2 et Pr2)NiO4+δ ont été étudiés comme matériau de cathode. Leurs compatibilités avec l’électrolyte en apatite ont été vérifiées et leurs températures de frittage ont été optimisées pour obtenir une microstructure suffisamment poreuse. Les meilleures performances électrochimiques sont ainsi obtenues pour les cathodes Pr2NiO4+δ frittées à 1100 °C dont l’ASR (Area-Specific Resistivity) est de 0,2 Ω.cm2 à 700 °C
This study deals with the elaboration by atmospheric plasma spraying process and the characterization of La9Sr1Si6O26,5 lanthanum silicates electrolyte for SOFC cell working at intermediate temperature (700 °C). In a first step, this work has been devoted to choose the plasma spraying conditions to elaborate dense apatite coatings, fundamental characteristic of an electrolyte. It has been observed that the use of plasma mixture with high enthalpy and thermal conductivity, as well as low gas flow rate, favours dense apatite coatings formation. However, to ensure a sufficient density of the coating while keeping their composition, it is necessary to add a sintering step. The two sintering techniques tested (conventional sintering and Spark Plasma Sintering) have leaded to the coatings crystallization and densification, thus permitted to improve their electrochemical properties. Particularly, a high ionic conductivity increase has been observed for sintered coatings elaborated with high power plasma spraying conditions. The ionic conductivity reaches 1.3.10-3 de Ω-1.cm-1 at 700 °C. In a second step, (La2, Nd2 et Pr2)NiO4+δ type nickelates materials have been studied as cathode. Their compatibility with apatite electrolyte was demonstrated and their sintering temperature was optimized in order to obtain sufficiently porous microstructure. The best electrochemical results were reached with Pr2NiO4+δ cathodes sintered at 1100 °C presenting a 0.2 Ω.cm2 ASR (Area-Specific Resistivity) at 700 °C

La pile à combustible à oxyde solide qui permet de transformer l'énergie chimique en énergie électrique, est l'une des solutions envisagées pour résoudre la crise énergétique grâce à son rendement global élevé et son fonctionnement sans aucun rejet dans l'environnement. Ces dernières années, des apatites dont la formule est La9,33+x(SiO4)6O2+3x /2 ont été développées en tant qu'électrolyte pour la pile de type SOFC fonctionnant à température intermédiaire. Il est possible d'améliorer les propriétés électriques des apatites par le dopage en site de Si4+. Parmi ces matériaux, le matériau La10Si5,8Mg0,2O26,8 présente une bonne propriété électrique. Les objectifs de cette étude sont d'étudier les processus de synthèse du matériau La10Si5,8Mg0,2O26,8 , d'utiliser ce matériau synthétisé pour élaborer par projection plasma l'électrolyte de la pile à combustible et enfin de rechercher les procédés les mieux adaptés pour préparer des cellules complètes de pile combustible de type SOFC comprenant cet électrolyte. Le matériau La10Si5.8Mg0.2O26.8 a été synthétisé à partir des poudres d'oxydes (La2O3, SiO2 et MgO) par voie solide et a été utilisé ensuite en tant que précurseur pour la projection thermique. Les paramètres de frittage ont été optimisés et une gamme de procédures a été établie pour élaborer ce matériau. La réalisation du dépôt par différents procédés de projection plasma (APS, LPPS et VLPPS) a été étudié afin les optimiser pour élaborer cet électrolyte. La recristallisation du dépôt après projection a été étudiée et un processus de traitement thermique est proposé pour obtenir des dépôts cristallisés et denses. Des cellules complètes de structure planaire avec l'électrolyte La10Si5,8Mg0,2O26,8 supporté par le cermet de Ni/Al2O3 ont été finalement élaborées par projection thermique. Les performances électriques des cellules ont été mesurées à température variant de 600°C à 900°C.

博士
大同大學
材料工程學系(所)
95
La3Ga5SiO14 (LGS) has high potential in applications as a new non-ferroelectric, piezoelectric material with high electromechanical coupling factor and almost zero temperature coefficient of delay (TCD). In this study, LGS thin films are prepared by RF sputtering technique on SiO2/Si (111) substrate and MgO buffer layer/Si(100) with stoichiometric LGS polycrystalline targets and sol-gel method with LGS precursors. The as-deposited films are amorphous and crystallization of the films is conducted by annealing. Crystalline LGS thin films on Si substrate with trigonal structure were obtained after annealing at temperature higher than 1200℃ in air. However, the as-deposited films on MgO buffer layers were commenced to crystallization under post-annealing at temperatures higher than 1100�aC for 5hrs in air. These thin films show blue emission from the photoluminescence (PL) investigation. The emission peak of the films was found to be 429 nm under the excited light of �鈃x=381nm. The PL emission of the films is attributed to the formation of oxygen vacancies as examined by the X-ray photoelectron spectroscopy (XPS).

碩士
中華技術學院
電子工程研究所碩士班
96
ABSTRACT This study uses the Czochralski Pulling Method to grow high quality langasite (Langasite, LGS) crystal. The method for growing LGS includes of placing a fixed weight LGS materials into a crucible. After that, insulating ceramics and then long seed crystal are placed inside in sequence into the furnace, before the internal cavity is vacuumed to a pressure of -6.5 × 10-3Pa. 90V heating is used to melt the LGS material uniformly. After that, a LGS crystal seed is placed in contact with the surface of the melted liquid LGS material and it is pulled for crystal growing. The pulling is stopped when the crystal growth reached a saturation stage. Subsequently, a cooling speed of 3v/hr is used. The LGS crystal solidified after around 20hrs. Internal cooling is done for 24 hours before the crystal inner temperature reached room temperature. The crystal can then be removed. The LGS crystal is then exposured under bright light test to study its macro-structure, and to detect any bubbles or inclusions defects. After repeated experiments, a 3.5cm thick, 12.5 cm tall LGS crystal is grown successfully in this study. This study uses a semi-automated process. The control of the crystallization process is done by computer where parameters are setted. On the other hand, observation of weight and voltage change during crystal growth, seeding, cooling and vacuuming are done manually. This study is very dependent on manual operation. Very precise judgment in seeding time is needed to prevent the seed from being melted during seeding. Experience and experiment control are very important to keep the noises from manual operation which influences the experiment to a minimum, and to improve the stability of crystal growing Keywords: Czochralski Pulling Method, Langasite, single crystal

碩士
國立臺北科技大學
材料科學與工程研究所
105
In this experiment, apatite-type lanthanum silicate-based electrolyte (LSMO) films for solid oxide full cells (SOFCs) were prepared by RF magnetron sputtering. In order to effectively enhance the passage of interstitial oxygen ions in the apatite structure, Mg2+ ions were used to substitute parts of Si4+ ions. Input power of 125 W and Ar/O2 ratios of 10/0 and 6/4 were used to deposit the electrolyte films. For Ar/O2 ratio of 10/0, composition of the LSMO film was identified to be La9.8Si5.7Mg0.3O26.4. The open circuit voltage (OCV) and maximum power density (MPD) of the resulting SOFC cell at 850oC were 0.89 V and 0.168 W/cm2, respectively. For the Ar/O2 ratio of 6/4, the composition of the film was the same as that prepared at Ar/O2 ratio of 10/0, while the OCV and MPD of the resulting SOFC cell at 850oC were 0.95 V and 0.982 W/cm2, respectively.

碩士
國立臺北科技大學
材料科學與工程研究所
105
This study was to develop apatite-type lanthanum silicate-based using solid state reaction produced solid electrolytes for Solid Oxide Fuel Cell (SOFC). Using different elements K, Na, B doping at La10Si6O27, replacing La3+ and Si4+ site, because they have different valence and ion radius ,bulk materials increase interstitial oxide concentration. It may increase conductivity that doping different elements. At this research use solid state reaction to develop materials, process mixer slurry, calcined, mixed PVA to the powder and uniaxial press the powder. The samples analysis XRD, SEM, conductivity measure by highest density. On this research the best performance are La9.5K9.5Si6O26.5 and La10Si5.5B0.5O26.75 and conductivities are 9.52×10-4 S/cm, 9.13×10-4 S/cm at 500oC , 2.08×10-2 S/cm, 2.42×10-2 S/cm at 800oC, respectively.

碩士
國立臺灣科技大學
機械工程系
99
Lanthanum silicate structural with formula La10-x(SiO4)6O2+δ (LSO) are potential candidates for IT-SOFC and LT-SOFC because of its high ionic conductivity and low activation energy. Rather than fluorite and perovskite structure with high symmetric structure, lanthanum silicate with apatite-type structure is belong to hexagonal structure and believed to migrate via an interstitial conduction mechanism. Studies have indicated that LSO synthesized via solid state reactions have to be sintered at high temperature (＞1600 ℃) to achieve the desired densities. According to phase equilibrium diagram of La2O3/SiO2, the composition of La9.33(SiO4)6O2 is a intermediate phase , and La2SiO5 or La2Si2O7 are easily formed as secondary phases. In addition, it’s difficult to control the stoichiometry of LSO due to the hygroscopicity of its staring materials La2O3. In this study, apatite-type lanthanum silicates La10(SiO4)6O3 were prepared by solid state reaction using powders of La(OH)3 and SiO2 as starting materials. The calcined samples are characterized by XRD and the results indicate that LSO have been obtained after calcination at 1200 ℃, but the La2Si2O7 impurity is formed after sintered at 1550 and 1600 ℃. TGA of the LSO powders were performed under hydrogen atmosphere, and the results indicates that LSO is a stable SOFC electrolyte materials. The effect of microstructure on the conductivity was also investigated, and the results show that the sintered ceramic with coarse grain size exhibits higher conductivity than fine one at low temperature. In addition, the aging test of LSO indicates that the sintered ceramic with La2Si2O7 impurity maintains a conductive stability.

碩士
國立成功大學
化學工程學系
107
This study reports that the Lewis acid-base properties of peroskite-type LaMnO3 and LaFeO3 can be adjusted by immobilizing them on silica. Bulk LaMnO3 and LaFeO3 were strong base catalyst due to unsaturated-coordinated oxygen on the surface, While, after supported them on silica, the basic properties of bulk materials were diluted and simultaneously acidities improved due to the increased amounts of tetravalent B-site cations. Ethanol reactivity was performed to reflect the different acid-base properties of bulk and silica-supported LaMnO3 and LaFeO3. Under differential analysis conditions, bulk perovskites were active in base-catalyzed reactions such as reverse aldolization and Tishchenko reaction, while silica-supported perovskites were active in aldolization and dehydration. We also notice the higher aldolization activity over LaMnO3/SiO2 than that of LaFeO3/SiO2, the different activity was attributed to existence of excess mobile oxygen on the surface of LaMnO3/SiO2, forming aldolization-active Lewis acid (Mn4+)-base (nonstoichiometric oxygen) pair sites while these sites were absent in LaFeO3/SiO2 which was enriched with oxygen vacancies.

Producing pure amorphous bioactive glasses by sol–gel from a quaternary system containing sodium is extremely difficult as they are prone to undergo readily crystallisation during heat treatment. Moreover, phase segregations and irreversible crystallisation of unwanted phases are likely to occur during the conventional xerogel process. The consequences are reduced homogeneity, excessive time and energy consumptions during the aging and slow drying steps of sol–gel derived bioglasses. A main objective of this thesis is to develop a new and expedite sol–gel synthesis process to prepare high silica quaternary bioactive glasses. Additional concepts are incorporated in the conventional sol–gel synthesis procedure to minimise the aging and drying times, while preserving the typical microstructure and properties and enhancing the degree of homogeneity. The efforts made led to the development of an innovative and rapid sol–gel method for producing bioactive glasses, which skips the ageing process. A comparative study was made between the glasses synthesised by the conventional drying method (CD) and the novel fast drying (FD). The results revealed that both methods produce stabilised bio–glasses with virtually identical behaviour and properties, but FD is 100 times faster in comparison to CD. Moreover, FD glasses exhibited improved bioactivity in simulated body fluid (SBF). The effects of some processing parameters such as calcium concentration, Ca/P ratio, types of metal salts precursors, types and concentrations of catalysts, were investigated to gain an in–depth understanding of their roles in the overall properties. The influence of agitation rate on the extent of bio– mineralisation upon immersing the samples in simulated body fluid (SBF) was also studied. The results revealed that bioactivity was enhanced by using nitrate salt precursors and citric acid as catalyst, and by increasing Ca concentration to some extent. Another aim was doping or co–doping the high silica sol–gel glass (HSSGG) selectively with Cu and La. The individual and combined effects of these doping elements on the glass structure, crystallisation, porosity, specific surface area, particle sizes, density, sintering ability, compressive strength and cytotoxicity were studied. As individual doping elements, Cu and La were found to play opposite roles in some relevant properties. On the other hand, synergistic benefits could be obtained by combining the two dopants. Cytotoxicity was significantly improved for some doping combinations, while the bio– mineralisation activity of doped samples was always very similar to that of the parent HSSGG composition. The significant intrinsic porosity fraction of sol–gel derived glass powders and the consequent uptake of any dispersing liquid medium, which will not be available for dispersion and flow, make them inappropriate for colloidal processing. These features have hindered so far the fabrication of porous scaffolds by additive manufacturing techniques such as robocasting. Overcoming this big challenge is of paramount importance for fully exploiting the specific advantages of the sol–gel derived glasses. Therefore, understanding the effects of each experimental processing variable on the properties of the powders is essential towards achieving optimised processing conditions for the fabrication of scaffolds by robocasting. Accordingly, the effects of heat treatment temperature (HTT), balls to powder ratio (BPR), and solid loading on the flow behaviour and on the viscoelastic properties of the suspensions/pastes were systematically evaluated. Suspensions with solid loadings up to 40 vol.% could be prepared from both HSSGG and Cu–La doped glasses and used to successfully fabricate scaffolds by robocasting. The formation of hydroxyapatite on the scaffold surfaces was observed after 72 h of immersion in SBF for HSSGG. These features make the scaffolds promising candidates for bone regeneration and tissue engineering applications and worthy for further in vivo investigations.
A obtenção de vidros bioactivos amorfos por sol–gel a partir de um sistema quaternário contendo sódio é extremamente difícil, uma vez que são propensos a sofrerem cristalização durante o tratamento térmico. Além disso, é provável que ocorra segregação de fase e cristalização irreversível de fases indesejáveis durante o processo de preparação. As consequências são a redução da homogeneidade, tempo excessivo de processamento e consumo de energia durante as fases de envelhecimento e secagem lenta. Um dos principais objetivos desta tese é desenvolver um novo e rápido processo de síntese por sol–gel de vidros bioativos quaternários com alto teor em sílica. Modificações são incorporadas ao procedimento convencional de síntese por sol–gel para minimizar os tempos de envelhecimento e secagem, preservando a microestrutura e propriedades típicas e aumentando o grau de homogeneidade. Os esforços realizados levaram ao desenvolvimento de um método inovador e rápido para a produção de vidros bioactivos por sol–gel, evitando o processo de envelhecimento. Foi realizado um estudo comparativo entre os vidros sintetizados pelo método de secagem convencional (CD) e a nova secagem rápida (FD). Os resultados revelaram que ambos permitem obter bio–vidros estabilizados com comportamento e propriedades praticamente idênticos; mas o processo designado por FD é 100 vezes mais rápido em comparação com o processo CD. Além disso, os vidros FD exibiram bioatividade melhorada em testes em que se usou fluido corporal simulado (SBF). Os efeitos de alguns parâmetros de processamento, tais como a concentração de cálcio, relação Ca / P, tipos de precursores de sais metálicos, tipos e concentrações de catalisadores, foram investigados para uma compreensão profunda dos seus papéis nas propriedades. A influência da velocidade de agitação na bio–mineralização ao imergir as amostras em fluido corporal simulado (SBF) também foi estudada. Observou–se ainda, que a bioactividade aumenta quando no método de preparação se usaram precursores de sal nitrato e ácido cítrico como catalisador, e quando ocorreu um aumento da concentração de Ca até um dado teor. Outro objetivo foi dopar ou co–dopar o vidro preparado por sol–gel de elevado teor em sílica (HSSGG) seletivamente com Cu e La. Os efeitos individuais e combinados desses elementos de dopagem na estrutura do vidro, cristalização, porosidade, área superficial específica, tamanho de partícula, densidade , capacidade de sinterização, resistência à compressão e citotoxicidade foram estudados. Verificou–se que o Cu e o La isoladamente desempenham papéis opostos em algumas propriedades relevantes. Por outro lado, benefícios sinérgicos poderiam ser obtidos pela combinação dos dois dopantes. A citotoxicidade foi significativamente melhorada para algumas combinações de dopagem, enquanto a biomineralização das amostras dopadas foi sempre muito semelhante à da composição HSSGG. A fracção de porosidade intrínseca dos pós de vidro preparados por sol–gel e a consequente absorção de qualquer meio líquido dispersante, tornam–nos inapropriados para processamento coloidal. Essas características impediram até agora a fabricação de “scaffolds” por técnicas de manufatura aditiva, como o robocasting. A superação desse grande desafio é de suma importância para explorar plenamente as vantagens específicas dos vidros preparados por sol– gel. Assim, a compreensão dos efeitos de cada variável de processamento experimental nas propriedades dos pós é essencial para alcançar condições otimizadas de processamento para a fabricação de “scaffolds” por robocasting. Assim, os efeitos da temperatura de tratamento térmico (HTT), razão bolas / pó (BPR), e carga sólida no comportamento do fluxo e nas propriedades viscoelásticas das suspensões / pastas foram sistematicamente avaliados. Suspensões com cargas sólidas de até 40% em volume podem ser preparadas a partir de vidros HSSGG dopados com Cu–La e usadas para fabricar com sucesso “scaffolds” por robocasting. A formação de hidroxiapatite nas superfícies do “scaffold” foi observada após 72 h de imersão em SBF para o vidro HSSGG. Essas características tornam os “scaffolds” candidatos promissores para aplicações em regeneração óssea e engenharia de tecidos e merecem posterior investigação in vivo.
Programa Doutoral em Ciência e Engenharia de Materiais

Low cost silicates with apatite-structure (general formula of apatite A10-xM6O26±δ, where A = rare earth or alkaline earth and M= Si, Ge, P, V..) have been proposed recently as promising solid electrolyte materials (oxygen ion conductors) for use at intermediate temperature solid oxide fuel cells (SOFCs). These materials exhibit sufficiently high ionic conductivity (e.g. ~ 0.01 S cm-1 at 700 oC), which is dominated by the interstitial site mechanism and can exceed that of yttria-stabilized-zirconia (YSZ), the solid electrolyte used in state-of-the-art SOFCs. The apatite structure is tolerant to extensive aliovalent doping, which has been applied for improving ionic conductivity. In this work are presented results concerning synthesis, conductivity and catalytic characterization of Fe- and/or Al-doped apatite type lanthanum silicates (ATLS) of the general formula La10-zSi6-x-yAlxFeyO26±δ as well as electrochemical characterization of interfaces of ATLS pellets with perovskite and Ni-based electrodes. The aim was to investigate the properties of these ATLS material, in particular as it concerns their potential use as SOFC components or as catalysts in oxidation reactions. The conductivity of pellets prepared from ATLS powders synthesized via four different methods and having different grain size was measured under air and at different temperatures in the range 600 -850 oC, aiming to identification of the effect of composition (doping), method of synthesis, grain size and pellet sintering conditions. For electrolytes of the same composition, those prepared via mechanochemical activation exhibited the highest conductivity, which was improved with increasing Al- and decreasing Fe-content. In state-of-the-art SOFCs perovskite electrodes are used as cathodes and Ni-based electrodes as anodes, thus electrochemical characterization of perovskite and Ni-based/ATLS interfaces was carried out. As it concerns perovskite/ATLS interfaces, the characterization focused on the study of the open circuit AC impedance characteristics of a La0.8Sr0.2Ni0.4Fe0.6O3-δ/La9.83Si5Al0.75Fe0.25O26±δ interface, at temperatures 600 to 800 oC and oxygen partial pressures ranging from 0.1 to 20 kPa. Under the aforementioned conditions, it was observed that the impedance characteristics of the interface were determined by at least two different processes, corresponding to two partially overlapping depressed arcs in the Nyquist plots. The polarization conductance of the interface was found to increase with increasing temperature as well as with increasing oxygen partial pressure, following a power law dependence. The electrochemical characterization of Ni-based electrodes/ATLS interfaces involved study of the electrochemical characteristics of NiO-apatite cermet electrodes as well as a Ni sputtered electrode interfaced with Al- or Fe-doped apatite electrolytes, under hydrogen atmospheres. The impedance characteristics of these electrodes were found to be determined by up to three different processes, their relative contribution depending on the electrode microstructure, Ni content (as it concerns the cermet electrodes), temperature, hydrogen partial pressure and applied overpotential. Aiming to investigation of potential catalytic properties of ATLS materials the catalytic activity for CO combustion of a series of ATLS powders was studied. For this purpose, two series of apatite-type lanthanum silicates La10-xSi6-y-zAlyFezO27-3x/2-(y+z)/2 (ATLS), undoped or doped with Al and/or Fe, were synthesized via sol-gel and modified dry sol-gel methods and tested as catalysts for CO combustion. The experiments revealed that the ATLS powders were catalytically active for CO combustion above approximately 300 oC, with light-off temperatures T50 (50% conversion of CO) ranging from 505 to 629 oC. The study focused on the effect on catalytic activity of the synthesis method and doping with Al and/or Fe. Non-doped ATLS with stoichiometric structure, namely La10Si6O27 prepared via the sol-gel method, exhibited the highest catalytic activity for CO oxidation among all tested compositions, the comparison being based on the measured catalytic rate (expressed per surface area of the catalyst) under practically differential conditions. Compared to La-Sr-Mn-O and La-Sr-Co-Fe-O perovskite powders, the tested ATLS powders exhibited lower catalytic activity for CO oxidation.
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