Thermal mismatch between different components of a system could be sources of problems like residual stress induced cracking, thermal fatigue or even optical misalignment in certain high technology applications. Use of materials with tailored thermal expansion coefficient is a counter-measure to overcome such problems. With its negative thermal expansion coefficient zirconium tungstate (ZrW2O8) is a candidate component to be used in synthesis of composites with controlled thermal expansion coefficient (CTE). ZrW2O8 is typically produced by solid-state reaction between zirconium oxide and tungsten oxide at 1200oC. However, it has been demonstrated that ZrW2O8 can also be synthesized using wet chemical techniques, which provide a superior chemical homogeneity that often extents down to the atomic scale, and the convenient means of controlling nucleation and growth of the primary crystallites. With the commonly adopted wet chemical approaches, it is possible to crystallize particles with sizes in the submicrometer range at temperatures as low as 600 oC or even lower. In these studies, precursors are aged either below 100 oC (7 days &ndash / 3 weeks), or at 160-180 oC under hydrothermal conditions (1&ndash / 2 days). Besides the obvious disadvantage in the ageing steps, use of tungsten sources with high cost in all approaches, constitutes the other disadvantage. Production of composites with tunable controlled thermal expansion (CTE) has been achieved by blending negatively and positively expanding materials in different proportions. In majority of these studies composites have been produced by conventional sintering methods. Spark Plasma Sintering (SPS) is a recent technique / in which sintering can be achieved at relatively low temperatures in short durations. There is only one study made by Kanamori and coworkers on the use of SPS in sintering of a composite, in which ZrW2O8 is one of the constituents [1]. This study aims the synthesis of ZrW2O8 particles and composites that possess tunable or zero CTE. A novel precursor recipe for ZrW2O8 synthesis was developed. In preparation of the precursor a total of 2 days of ageing and a temperature less than 100 oC was used. It was developed using a cost-effective tungsten source, namely tungstic acid and its final pH was lower than 1. The particles obtained from &lsquo / unwashed&rsquo / procedure had sizes in micrometer range, while those obtained from &lsquo / washed&rsquo / case had sizes in the range of 400-600 nm. These precursors could readily be crystallized at 600 oC, which in turn provided the desired particle sizes for composite applications. Experimental details on the precursor development are hereby presented with a discussion on the effects of solution parameters (i.e. solubility of tungstic acid, adjustment of the stoichiometry, ageing time) on the phase purity of the fired product. Zirconium oxide (ZrO2) has positive vi thermal expansion, therefore ZrW2O8/ZrO2 was selected as the composite system, and for their synthesis both conventional and spark plasma sintering methods were experimented. Composition ranges that provide composites with almost zero CTE&rsquo / s were determined. The composite having a composition of containing 35% ZrW2O8, 65% ZrO2, and 35 w/o Al2O3 and sintered at 1200 oC for 24 hours had an expansion coefficient of 0.20 x 10-6/K for conventional method, while the one having a composition of 55% ZrW2O8, 45% ZrO2 and sintered at 1000 oC for 5 minutes had an expansion coefficient of 0.94 x 10-6/K for spark plasma sintering method. For characterization of the products X-ray diffraction (XRD), scanning electron microscopy (SEM), photon correlation spectroscopy (PCS), and thermal and dilatometer analyses (DTA/TGA/DMA) were used.

The work elucidates the novel physical and thermal properties of thin and ultra-thin films of crosslinked polymer and organized microstructures with a special emphasis on surface and interfacial effects and the structure-property relationships. Two major crosslinked polymer coatings have been thoroughly investigated: polymer microstructures fabricated by multi-laser interference lithography (IL), and plasma polymer coatings. We unveiled intriguing thermal properties of plasma polymer films originating from their physical state and exploiting the same for the design of ultrasensitve chemical sensors. A novel paradigm of surface coatings, single and bi-component periodic, porous crosslinked polymeric structures, has been introduced and thoroughly studied. Surface, interfacial, and mechanical properties of these novel class crosslinked polymer coatings clearly demonstrate the enormous potential of the IL microstructures as organized multicomponent polymer systems. When subjected to external or internal stresses the periodic porous structures can exhibit a sudden and dramatic pattern transformation resulting in remarkable change in the photonic, phononic and mechanical properties of these structures. Furthermore, the confinement of these instabilities to localized regions results in complex hierarchical structures. The two polymer coatings (plasma polymers and IL microstructures) with complementary attributes (such as periodic structure, vertical stratification, residual internal stresses, and high surface and interface tunability) enabled us to understand and design novel multifunctional polymer coatings.

Οι ζεόλιθοι ανήκουν στα νανοπορώδη υλικά ή αλλιώς και μοριακά κόσκινα. Βρίσκουν δε εφαρμογή σε πληθώρα εφαρμογών, καλύπτοντας ένα μεγάλο θερμοκρασιακό φάσμα. Η γνώση της απόκρισής τους κατά τις μεταβολές της θερμοκρασίας είναι καθοριστικής σημασίας για την αποδοτικότητά τους, ακόμα περισσότερο όταν εμφανίζονται ανώμαλα φαινόμενα, όπως αρνητικός συντελεστής θερμικής διαστολής. Η παρούσα εργασία επικεντρώνεται στην ερμηνεία αυτού του φαινομένου με την χρήση προσομοιώσεων και πειραμάτων σκέδασης ακτίνων Χ. Ένα νέο πεδίο δυνάμεων αναπτύχθηκε και χρησιμοποιήθηκε σε προσομοιώσεις δυναμικής πλέγματος, όπου για πρώτη φορά προβλέπεται η συστολή της μοναδιαίας κυψελίδας του σιλικαλίτη, όπως και η διαστολή της μοναδιαίας κυψελίδας του αργιλοπυριτικού φωγιασίτη (NaX) πάνω από την θερμοκρασία δωματίου. Περαιτέρω ανάλυση των αποτελεσμάτων δείχνει πως υπεύθυνες για την συστολή κατά την θέρμανση είναι οι περιστροφές των τετραέδρων SiO4 και AlO4 από τα οποία απαρτίζεται η δομή. Αυτό το αποτέλεσμα είναι σύμφωνο με τον μηχανισμό των περιστροφών άκαμπτων μονάδων που προτείνεται στην βιβλιογραφία (R.U.M.). Στη συνέχεια υπολογίζονται μηχανικές ιδιότητες του υλικού, όπως το μέτρο Young και το μέτρο ελαστικότητας όγκου. Τέλος μελετήθηκε η απόκριση της δομής των ζεολίθων σε ερεθίσματα πέραν της θερμοκρασίας, όπως μεταβολή πίεσης και ρόφηση μορίων. Χαρακτηριστικά καταλήγουμε πως η δομή αποκρίνεται στην άσκηση της πίεσης μέσω του ίδιου μηχανισμού που καθορίζει της θερμοκρασιακή της απόκριση, δηλαδή τις περιστροφές των άκαμπτων ή σχεδόν άκαμπτων τετραέδρων. Επίσης, σε ό, τι αφορά την επίδραση της ρόφησης, η παρουσία των μορίων εξανίου, παρόλο επιφέρει δομικές αλλαγές που βελτιώνουν την αποδοτικότητα των ζεολιθικών μεμβρανών, αυτές οι αλλαγές δεν επηρεάζουν τις ροφητικές ιδιότητες στο εσωτερικό του κρυστάλλου. / Zeolites are nanoporous materials, otherwise called molecular sieves. The knowledge of the response of a zeolite structure to temperature changes or to the presence of host molecules in the pore system is of critical significance for the performance of zeolites as molecular sieves, especially if they are prepared in the form of membranes. Crack formation or grain boundary openings may appear between the substrate and the membrane due to a mismatch between the thermal expansion coefficient of the two materials, affecting the sorption and selectivity properties of the membrane. The problem is enhanced when the zeolite shows negative thermal expansion coefficient. The present work focuses in the mechanism that lies behind contraction upon heating, with the use of simulations and powder XRD experiments. A new force field was developed and was implemented in lattice dynamics simulations. It’s the first time that simulations predict the contraction of silicalite unit cell as well as the expansion of aluminosilicate faujasite (NaX) unit cell above room temperature. Further analysis of the simulation data shows that the Rigid Unit Mode (R.U.M.) mechanism that is proposed in the literature is the dominant mechanism of thermal contraction of the zeolite. Furthermore, mechanical properties such as Young modulus and bulk modulus were computed with the use of the newly derived force field. Finally, the response of framework upon pressure change and sorption of molecules was investigated. The zeolite structure response upon pressure change is similar to this of the temperature response, meaning that the volume and atomic positions change with the help of the rotations of the rigid AlO2/SiO2 tetrahedra. Concerning the response upon sorption, is was found with the help of theoretical calculations that although unit cell size variations that are induced by temperature changes and/or sorption may affect the efficiency of silicalite-1 membrane performance through alteration of the size of the non-zeolitic pores, such changes appear to affect negligibly the bulk sorption capacity of the silicalite-1 crystals.

[pt] O objetivo deste trabalho foi estabelecer uma metodologia de síntese de pós nanométricos a submicrônicos de Y2W3O12, como também relacionar as propriedades óticas, térmicas e mecânicas do Al2W3O12 com métodos distintos de sinterização. As partículas de Y2W3O12 foram obtidas pelo método de co-precipitação reversa modificada mantendo pH constante, com o intuito de manter a supersaturação elevada, garantindo alta taxa de nucleação evitando o crescimento indesejável dos núcleos. As metodologias para cristalização empregadas (tratamento hidrotérmico e calcinação) permitiram obter pós submicrônicos de Y2W3O12 (0.3 ̶ 0.5 Mm) com menor grau de aglomeração em relação ao previamente reportado na literatura. O envelhecimento da dispersão de Y2W3O12 revelou diminuição do tamanho médio de aglomerados em 50 por cento quando comparado à distribuição primária de aglomerados precursores. A dispersão utilizando surfactante CTAB reduziu a aglomeração em 50 por cento. O pó de Al2W3O12 foi sinterizado pelo método de spark plasma sintering (SPS) e comparado com Al2W3O12 sinterizado pelo método de três etapas 1. As propriedades óticas e mecânicas foram estudadas, revelando comportamento distinto entre os materiais consolidados por vias diferentes, evidenciando a importância do método de sinterização sobre a microestrutura final e sobre formação de defeitos pontuais. A espectroscopia de refletância difusa revelou que as amostras obtidas por SPS apresentam absorbância total na faixa da luz visível, sendo coerente com sua coloração preta fosca, assim, a energia de banda proibida dessas amostras situa-se abaixo da energia do espectro visível (EGAP menor que 1,7eV). A amostra sinterizada em três etapas, de coloração branca, possui absorbância parcial no UV-Vis, absorvendo todos os comprimentos de onda até 300nm e conforme o comprimento de onda aumenta e adentra na região visível a absorbância diminui e se torna nula. As amostras de SPS apresentaram ganho expressivo no módulo de Young (80 por cento) e dureza (61-116 por cento) quando comparado com o material obtido por sinterização convencional, previamente descrito na literatura 2, 1. As amostras de SPS quando comparadas com a amostra sinterizada em três etapas 1, apresentou módulo de Young 47 por cento maior e aumento de dureza entre 19-59 por cento, embora, ambas apresentem mesma densidade relativa de 96 por cento, sendo as amostras de SPS com microestrutura de grãos nanométricos proporcionando maior resistência mecânica. O CTE das amostras SPS foi medido, sendo similar ao valor encontrado para a amostra em três etapas 2, constatando que o CTE não foi afetado pela intorução dos defeitos pontuais na Al2W3O12 durante SPS. / [en] The aim of this work is to establish a methodology for the synthesis of submicron nanocomposites of Y2W3O12, as well as to relate the properties, optical and mechanical, of Al2W3O12 with different methods of sintering. The Y2W3O12 particles were obtained by the modified reverse co-precipitation method, maintaining a constant pH along the drip, in order to maintain a high super saturation, ensuring a high nucleation rate, avoiding the undesirable growth of the nuclei. As the methodologies for the crystallization used (hydrothermal treatment and calcination) allowed to obtain post-submicron of Y2W3O12 (0.3 - 0.5 Mm) with lower degree of agglomeration in relation to the report published in the literature. The aging of the dispersion was studied and showed a decrease of 50 percent the average size of agglomerates when compared to the primary distribution of precursor agglomerates. A dispersion using CTAB surfactant reduced 50 percent the agglomeration, as verified by characterization techniques. The Al2W3O12 powder was sintered by the Sintering of Spark Plasma method (SPS) and compared with Al2W3O12 sintered by the three-step method (TSS) 1. The optical and mechanics properties were studied, revealing a differentiated behavior between materials consolidated by different paths, evidencing the importance of the sintering method in a final microstructure. Diffuse reflectance spectroscopy showed that the samples obtained by SPS, it is able to absorb the total wavelengths in the visible light range, being consistent with its black matte color, so the energy gap is below the energy of the spectrum visible (EGAP less than 1.7 eV). The conventional (white) sample absorb all wavelengths up to 300 nm and increasing wavelength decreases absorption and renders it zero. The samples by SPS showed significant gain in the Young s modulus (80 percent) and hardness (61-116 percent) when compared to TSS sample 2, 1. The SPS samples when compared to TSS sample [1], the Young s modulus increased 47 percent and the hardness increased 19-59 percent, although both of them obtained 96 percent relative densities. The reason for the improve of mechanical properties of SPS samples is justified by nanosized grain microstructure providing greater mechanical resistance. The CTE of the SPS samples was measured, being similar to the value found for a three-step sample 2, noting that the CTE was not affected by the introduction of defects in Al2W3O12 after SPS.

No description available.

[pt] O objetivo deste trabalho foi sintetizar pós submicrônicos de Y2W3O12 e Al2W3O12 visando estudar o efeito da incorporação destas cargas no coeficiente de expansão térmica (CET) e nas propriedades mecânicas de compósitos de polietileno de alta densidade (PEAD). As partículas de Y2W3O12 foram obtidas pelos métodos de coprecipitação por mistura de reagentes e coprecipitação reversa modificada, com o intuito de avaliar a influência destas rotas no grau de aglomeração dos pós preparados. Ambos os métodos permitiram obter pós submicrônicos de Y2W3O12 0. 0.7 um) com tamanhos de partículas primárias e graus de aglomeração semelhantes, conforme verificado por diversas técnicas de caracterização. As partículas de Al2W3O12 foram sintetizadas pelos métodos de coprecipitação por mistura de reagentes e coprecipitação normal, utilizando duas fontes de tungstênio e os precursores obtidos foram secos no micro-ondas. Os efeitos do pH da síntese (4.2 e 6.0) no estado de aglomeração dos pós sintetizados foram avaliados. Pós submicrônicos de Al2W3O12 0.22 0.87 um) foram produzidos utilizando estas rotas. A coprecipitação pela mistura de reagentes em pH=4.2 permitiram obter pós com aglomerados de menor tamanho (0.22 um). Os compósitos de PEAD foram fabricados por microextrusão-microinjeção, com partículas de Al2W3O12 e Y2W3O12 (0.70 um) em frações volumétricas na faixa de 0.001 0.011. Os compósitos PEAD/Y2W3O12 exibiram reduções no CET de 20 25 porcento quando comparados ao PEAD na faixa de 25 70 °C, enquanto os compósitos PEAD/Al2W3O12 mostraram reduções no CET de 7 8 porcento. O módulo de Young aumentou 11 32 porcento e 21 porcento para os compósitos reforçados com Y2W3O12 e Al2W3O12, respectivamente, enquanto a estabilidade térmica do PEAD foi preservada para todos os compósitos. / [en] The aim of this work was to synthesize Al2W3O12 and Y2W3O12 submicronic powders and study the effect of incorporation of these fillers on thermal expansion coefficient (CTE) and mechanical properties of high density polyethylene (HDPE) composites. Y2W3O12 was synthesized via co-precipitation by mixing reactants and modified reverse strike co-precipitation, to assess the influence of the method on agglomeration degree of powders. Both methods produced Y2W3O12 submicronic powders (0.60 0.70 um), with similar primary particle sizes and agglomeration degrees, as verified by different characterization techniques. Al2W3O12 was synthesized via co-precipitation by mixing reactants and normal strike method, using two distinct tungsten sources and the assynthesized precursors were microwave dryied. The effects of synthesis pH (4.2 and 6.0) on final agglomeration state of the as-synthesized powders were assessed. Al2W3O12 submicronic powders (0.22 0.87 um) were synthesized by these routes. Co-precipitation by mixing reactants at pH=4.2 led to Al2W3O12 powders with smaller agglomerate sizes (0.22 um). HDPE composites were manufactured by micro-compounding, using Al2W3O12 and Y2W3O12 powders with similar particle sizes (0.70 um), at volume fractions in the range of 0.001 0.011. HDPE/Y2W3O12 composites presented CTE reductions of 20 25 porcento compared to neat HDPE, in the temperature range of 25 70 Celsius degrees, while HDPE/Al2W3O12 composites displayed reductions on CTE of 7 8 porcent. Young s moduli of composites were increased 11 32 porcent nd 21 porcent when filled with Y2W3O12 and Al2W3O12, respectively, whilst thermal stability of HDPE was preserved for all composites.

No description available.

Ca2RuO4 is a structurally-driven Mott insulator with a metal-insulator (MI) transition at TMI = 357K, followed by a well-separated antiferromagnetic order at TN = 110 K. Slightly substituting Ru with a 3d transition metal ion M effectively shifts TMI and induces exotic magnetic behavior below TN. Moreover, M doping for Ru produces negative thermal expansion in Ca2Ru1-xMxO4 (M = Cr, Mn, Fe or Cu); the lattice volume expands on cooling with a total volume expansion ratio reaching as high as 1%. The onset of the negative thermal expansion closely tracks TMI and TN, sharply contrasting classic negative thermal expansion that shows no relevance to electronic properties. In addition, the observed negative thermal expansion occurs near room temperature and extends over a wide temperature interval. These findings underscores new physics driven by a complex interplay between orbital, spin and lattice degrees of freedom. These materials constitute a new class of Negative Thermal Expansion (NTE) materials with novel electronic and magnetic functions.

Synthesis and Characterization of Some Low and Negative Thermal Expansion Materials Tamas Varga 370 pages Directed by Dr. Angus P. Wilkinson The high-pressure behavior of several negative thermal expansion materials was studied by different methods. In-situ high-pressure x-ray and neutron diffraction studies on several compounds of the orthorhombic Sc2W3O12 structure revealed an unusual bulk modulus collapse at the orthorhombic to monoclinic phase transition. In some members of the A2M3O12 family, a second phase transition and/or pressure-induced amorphization were also seen at higher pressure. The mechanism for volume contraction on compression is different from that on heating. A combined in-situ high pressure x-ray diffraction and absorption spectroscopic study has been carried out for the first time. The pressure-induced amorphization in cubic ZrW2O8 and ZrMo2O8 was studied by following the changes in the local coordination environments of the metals. A significant change in the average tungsten coordination was found in ZrW2O8, and a less pronounced change in the molybdenum coordination in ZrMo2O8 on amorphization. A kinetically frustrated phase transition to a high-pressure crystalline phase or a kinetically hindered decomposition, are likely driving forces of the amorphization. A complementary ex-situ study confirmed the greater distortion of the framework tetrahedra in ZrW2O8, and revealed a similar distortion of the octahedra in both compounds. The possibility of stabilizing the low thermal expansion high-temperature structure in AM2O7 compounds to lower temperatures through stuffing of ZrP2O7 was explored. Although the phase transition temperature was suppressed in MIxZr1-xMIIIxP2O7 compositions, the chemical modification employed was not successful in stabilizing the high-temperature structure to around room temperature. An attempt has been made to control the thermal expansion properties in materials of the (MIII0.5MV0.5)P2O7-type through the choice of the metal cations and through manipulating the ordering of the cations by different heat treatment conditions. Although controlled heat treatment resulted in only short-range cation ordering, the choice of the MIII cation had a marked effect on the thermal expansion behavior of the materials. Different grades of fluorinert were examined as pressure-transmitting media for high-pressure diffraction studies. All of the fluorinerts studied became nonhydrostatic at relatively low pressures (~1 GPa).

No description available.