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Calorimetric Investigation of the Heats of Formation of La1-xSrxCoO3-y (0.00 < X < 0.40)
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Energetics of Substitution Effects on the Stability of Metal-Organic Frameworks
Full text linkMetal-organic frameworks are hybrid materials constructed by inorganic metal nodes and organic linkers. The presence of both organic and inorganic moieties in the structure allows MOFs to have a significant flexibility and tunability in compositions, structures, and functional properties. This in turns enables MOFs to have very desirable and interesting properties, such as very high porosity, very high surface area, multiferroicity, electrical conductivity, proton conductivity and others. Consequently, MOFs have potentials in a wide array of applications, including catalysis, gas storage and separation, energy storage and drug delivery.Despite the significant amount of research performed on the development and synthesis of MOFs, fundamental study of their intrinsic stability is still fragmentary. The thermodynamic properties of materials are crucial in understanding their properties and evaluating their intrinsic stability. To accelerate the discovery of new MOFs, it is necessary to study the energetics of MOFs systematically to further understand how the compositions and structures affect stability. By relating these composition – structure – energetics relationships to other measurable, calculable, or available chemical properties, it should be possible to predict the stability of new MOFs. Therefore, this dissertation work aims to fulfill this goal of understanding and predicting MOF stability.
Chapter 1 is a general introduction. Chapter 2 discusses the synthesis, characterization and thermodynamic study of mineral paceite CaCu(OAc)4‧6H2O as well as its cadmium-based analogue CaCd(OAc)4‧6H2O. The enthalpy of formation revealed the destabilization introduced by the Cd to the structure. Additionally, it also explained the most possible synthesis route of paceite, which is through the formation of a calcium acetate – acetic acid solvate intermediate.
Chapter 3 presents a systematic study of the energetics of A-site cation substitution on the thermodynamic stability of protonated-amine manganese formate perovskite. This system was chosen representative of a dense MOF system. The measured enthalpy of formation revealed that the stability of hybrid perovskites is not strongly influenced by the structural factor, unlike its inorganic perovskite counterparts. Instead, the stability is governed more by the interaction of the A-site cation with the framework, which further can be explained by the dissolution enthalpy of the cation salts.
Chapter 4 reports the interplay between theoretical calculations and experimental measurements to determine the thermodynamic stability of porous metal organic frameworks, in this case fluorinated zeolitic imidazolate frameworks (ZIFs). The enthalpy of formations revealed the strong influence of ligand functionalization and topology on the stability.
Chapter 5 presents a systematic study of the energetics of ligand substitution on the stability of ZIFs with sodalite structure. Based on the results obtained in Chapter 4, this study was designed to see only the contribution of ligand substitution to stability, while keeping the topology the same. It was found that the stability trend of sodalite ZIF samples with different ligand functionalization can be explained by a tabulated chemical property from physical organic chemistry, namely Hammett -para(+) constant.
Lastly, in Chapter 6, all the results are related with summarizing points and suggestions for future research will be presented
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Calorimetric studies on the thermodynamic properties of M2Mn3O8 materials
Full text linkManganese oxides are abundant in the environment and are essential to various technologies. The four most common binary manganese oxides are MnO2 (pyrolusite), Mn2O3 (bixbyite), Mn3O4 (hausmannite) and MnO (manganosite). In 1965, Oswald et al. reported a new binary manganese oxide with the nominal stoichiometry Mn5O8 or Mn(II)2Mn(IV)3O8. Mn5O8 exhibits a unique monoclinic layered crystal structure. Subsequent studies revealed that the divalent manganese ions in Mn5O8 can be replaced by other divalent metal cations, including Ca2+, Cu2+, and Cd2+ ions, which results in ternary oxides that bear the same crystal structure. In this dissertation, samples of Mn5O8, Ca2Mn3O8, Cd2Mn3O8, Cu2Mn3O8 and a 50-50 solid solution of Ca2Mn3O8-Cd2Mn3O8 have been synthesized to study their thermodynamic properties. Their structures have been confirmed by X-ray diffraction and compositions by iodometric titration and/or electron microprobe analyses. High temperature oxide melt solution calorimetry has been carried out on all samples. Their formation enthalpies from binary oxide end members have been calculated from measured and literature data. The results are discussed in terms of stability, structure, bonding, and significance.
Mn5O8 exhibits a slightly endothermic (< 6 kJ/mol) enthalpy of formation from an isochemical mixture of bixbyite and pyrolusite, suggesting that it is energetically metastable. Given that a solid-state reaction involving no gas species usually has very small change in entropy due to the small differences in heat capacities, the free energy of formation of Mn5O8 from an isochemical mixture of bixbyite and pyrolusite is likely to be slightly endothermic or close to zero. Therefore, Mn5O8 is probably a metastable phase not found on the equilibrium Mn - O phase diagram.
The energetic stabilities of the ternary M2Mn3O8 samples depend on the M2+ cation (M = Ca, Cd, Cu) in the structure. The enthalpies of formation of M2Mn3O8 from MO and MnO2 are -133.25 ± 4.87 kJ/mol, -50.86 ± 3.52 kJ/mol and +35.42 ± 3.70 kJ/mol for M = Ca, Cd and Cu respectively, suggesting energetic stability decreases in the order Ca, Cd, Mn, and Cu. The same trend is observed in their thermal decomposition behavior, where Ca2Mn3O8 is the most refractory and does not decompose until over 900 °C while Cu2Mn3O8 decomposes well below 500 °C. The 50-50 solid solution of Ca2Mn3O8-Cd2Mn3O8 or CaCdMn3O8 has an enthalpy of formation from CaO, CdO and MnO2 of -117.6 ± 4.75 kJ/mol. It is noticeably energetically more stable than a 1:1 mechanical mixture of Ca2Mn3O8 and Cd2Mn3O8, whose enthalpy of formation from CaO, CdO and MnO2 would be -96.09 ± 3.01 kJ/mol. The enthalpy of mixing for this solid solution is hence -22.14 ± 3.78 kJ/mol, indicating energetically favorable mixing, possibly related to some sort of short range ordering of cations.
The results of this dissertation work can aid in future computational and experimental studies on M2Mn3O8 materials. The quantitative study of thermodynamic properties can aid in assessing their stabilities in the environment which will be of interest to heavy/toxic metal immobilization. Also, the choice of materials for applications in areas such as electrochemical energy conversion and storage can directly benefit from these well documented thermodynamic data
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Energetics of Rare Earth Solids Containing Hydroxide and Halide Ions
Full text linkRare earth (RE) materials have widespread applications and constitute an indispensable part of modern lives. Tailored electrical, optical, and magnetic properties can be achieved by various RE elements in isomorphic structures, providing materials with the desired properties for specific applications. However, the processing of RE materials encounters difficulties in separation and recycling of RE elements due to the similar physical and chemical properties. For better utilization of REs, including mining, recovery, and incorporation into devices, it is essential to have comprehensive knowledge about thermodynamic properties of RE compounds.The present work comprises not only new experimental thermodynamic studies on selected RE compounds but also a fundamental understanding of the energetics of RE compounds. By means of high temperature oxide melt solution calorimetry, formation enthalpies of RE oxycompounds (REOOH, REOF, and REOCl) and ternary sodium fluorides (NaREF4) were measured to provide reliable data for energetic assessments of REs in diverse structures. NaF–NdF3 nanocrystals and Ca1−xRExF2+x solid solutions were also investigated to shed light on the energetics of RE fluoride nanocrystals and solid solutions containing defect clusters. Combining other published thermochemical data, lattice energies of numerous RE compounds were evaluated to reveal the correlations between the energetics and structures of RE materials.
RE oxycompounds are important components of RE precipitates and play a key role in the leaching of RE elements. By different synthesis methods, high-purity single-phase REOOH, REOF, and REOCl with various REs were prepared and characterized by powder X-ray diffraction (PXRD). The formation enthalpies from binary components, determined by high temperature oxide melt solution calorimetry, are all negative but become less exothermic from light to heavy RE elements. They confirm the thermodynamic stability of these RE compounds relative to binary oxides and/or halides, The thermal decomposition of REOOH and a structural phase transition in YbOF were studied by differential scanning calorimetry (DSC).
Mixed cation (alkali or alkaline earth) RE fluorides have drawn extensive research interest due to their unique luminescent properties and potential applications in phosphors and laser hosts, but limited thermodynamic studies hinder the rational design and synthesis of these materials. The formation enthalpies of NaREF4 follow an opposite trend compared to most ternary RE oxide compounds, becoming increasingly negative for heavier REs. Energetic studies of NaF–NdF3 nanocrystals coupled with chemical analysis and structural characterizations indicate that the α → β phase conversion in aqueous solutions is associated with compositional variations and distinct from the β → α transition driven by temperature for stoichiometric NaNdF4. Formation enthalpies of Ca1−xRExF2+x (RE = La, Pr, and Nd) solid solutions were measured and the relationships between the energetics and structures were discussed.
Based on experimental thermodynamic studies, an energetic evaluation of RE crystalline compounds was conducted to identify correlations between structures and thermodynamics. Lattice energies were calculated by Born–Haber cycles, which highly depend on ionic strength and increase in magnitude (becoming more exothermic) from light to heavy RE elements. The slopes of lines relating lattice energies to RE ionic radii are related to RE coordination numbers, contributing to the opposite trends in formation enthalpies for RE oxides and fluorides. A correlation between the magnitude of lattice energies and the structures occurring in RE compounds across the RE series was identified
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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