Spelling suggestions: "subject:"egative athermal expansion"" "subject:"egative athermal dexpansion""
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Structural and high pressure studies of some low and negative thermal expansion materialsÇetinkol, Mehmet 17 November 2008 (has links)
The research presented in this thesis focuses on the structural studies and the high pressure behavior of oxide negative thermal expansion (NTE) materials that can be classified as framework materials. First two chapters were devoted to TaO2F which adopts the ReO3-type cubic structure. Our studies under pressure revealed a rather complicated high pressure behavior for this deceivingly simple compound. The diffraction measurements at variable temperature and high pressure indicated that pressure had a significant effect on the linear coefficient of thermal expansion of TaO2F. In the remainder of the thesis, compounds that belong to the Sc2W3O12 family were examined. High-pressure in-situ powder diffraction studies were conducted on Zr2WO4(PO4)2, Zr2MoO4(PO4)2, Hf2WO4(PO4)2, and Sc2W3O12 in order to investigate the effects of pressure on the coefficients of thermal expansion, existence of phase transitions, phase transition pressures and structural changes occurring upon phase transitions.
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First principles approach to understanding stability and phase transitions of metal A(II)B(IV)hexafluoridesPueschel, Charles A. 24 November 2015 (has links)
No description available.
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Programmable Mechanical Metamaterials with Negative Poisson's Ratio and Negative Thermal ExpansionHeo, Hyeonu 12 1900 (has links)
Programmable matter is a material whose properties can be programmed to achieve particular shapes or mechanical properties upon command. This is an essential technique that could one day lead to morphing aircraft and ground vehicles. Metamaterials are the rationally designed artificial materials whose properties are not observed in nature. Their properties are typically controlled by geometry rather than chemical compositions. Combining metamaterials with a programmable function will create a new area in the intelligent material design. The objective of this study is to design and demonstrate a tunable metamaterial and to investigate its thermo-mechanical behavior. An integrated approach to the metamaterial design was used with analytical modeling, numerical simulation, and experimental demonstration. The dynamic thermo-mechanical analysis was used to measure base materials' modulus and thermal expansion coefficient as a function of temperature. CPS, the unit cell of the metamaterial, is composed of circular holes and slits. By decomposing kinematic rotation of the arm and elastic deformation of a bi-material hinge, thermo-mechanical constitutive models of CPS were developed and it was extended to 3D polyhedral structures for securing isotropic properties. Finite element based numerical simulations of CPS and polyhedral models were conducted for comparison with the analytical model. 3D printing of multi-materials was used for sample fabrication followed by tests with uniaxial compressive mechanical tests and thermal tests at 50℃. From the analytical model of the metamaterial, the contour plots were obtained for the effective properties – Poisson's ratio, the effective coefficient of thermal expansion of the metamaterial as a function of geometry and materials. A controllable range of temperature and strain was identified associated with maximized thermal expansion mismatch and contact on the slit surface of CPS, respectively. This work will pave the road toward the design of programmable metamaterials with both mechanically- and thermally- tunable capability and provide unique thermo-mechanical properties with a programmable function.
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Synthesis, Phase Development, and the Mechanism for Negative Thermal Expansion in Aluminum TungstateRose, Kyle 05 1900 (has links)
An in-depth study of Al2W3O12 negative thermal expansion (NTE) ceramic was performed, focused on synthesis, phase mappings, and the underlying mechanisms shown to be responsible for NTE. Review of the literature has shown inconsistencies in reported values of the dilatometry measured coefficients of thermal expansion, and the temperature for the known monoclinic to orthorhombic phase transition. Two synthesis techniques are introduced: an ionic-liquid non-hydrolytic sol-gel synthesis route; and a low temperature solid state reaction synthesis for Al2W3O12. X-ray diffraction, Raman spectroscopy, and attenuated total reflectance Fourier-transform infrared spectroscopy (ATR-FTIR) were used to verify the techniques. Two differential scanning calorimetry (DSC) experiments (high and low temperature) were performed on the material showing the transition between -5 and -20 °C and no other phase changes until a reported degradation above 1100 °C. Extensive dilatometry on the material led to the discovery of elastic transitions occurring in the polycrystalline sample capable of explaining the inconsistencies in reported dilatometry results. This is further developed into a proposed model defining the regions between these transitions. Each region has a different thermal expansion as well as a direct effect on the reaction of the material upon cooling. This proposed model may allow more consistent reporting of dilatometry results for NTE materials. Raman spectroscopy was performed from 25-725 °C on the material showing both a joining in the tungsten-oxygen bending modes as well as a broadening in the tungsten-oxygen stretching modes. This is consistent with Al-O-W angle changes along the same temperature range reported in literature as well as the transverse vibrational modes responsible for NTE.
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Autohydration of Nanosized Cubic Zirconium TungstateBanek, Nathan A. 19 September 2011 (has links)
No description available.
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Cation Influence on Negative Thermal Expansion in the A<sub>2</sub>M<sub>3</sub>O<sub>12</sub> FamilyGates, Stacy D. 30 September 2008 (has links)
No description available.
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Amorphisation sous pression dans des aluminophosphates à coefficient de dilatation thermique négatif / Pressure Induced Amorphisation in Negative Thermal Expansion AluminophosphatesAlabarse, Frederico 23 November 2012 (has links)
L'objectif de ce travail concerne l'étude de l'amorphisation sous pression (PIA) dans des aluminophosphates, qui présentent des coefficients de expansion thermiques négatif (NTE). La synthèse de ces aluminophosphates, leur stabilité en température (dilatation) ainsi que leur comportement sous pression (amorphisationant) été étudiés. Les aluminophosphates choisis pour réaliser cette étude sont AlPO4-54, premier tamis moléculaire uniforme ayant des pores (unidimensionnelles parallèles à l'axe c) de diamètre supérieur à 1 nm, et AlPO4-17, qui présente une porosité tridimensionnelle et le plus fort coefficient de dilatation thermique négatif connu dans les matériaux oxydes. Chaque matériau déshydrataté a été étudié par diffraction des rayons-X sous pression en cellule à enclume de diamants (CED). AlPO4-54 subit une transformation de phase vers AlPO4-8 sous pression avant l'amorphisation. Au contraire, AlPO4-17 qui s'amorphise à basses pressions, est extrêmement compressible et présente une instabilité élastique, avec valeur négative pour B0'. Des valeurs anormales (négatives) pour B0' sont rares et ont déjà été observées pour des cyanures et des MOF (metal organic framework). Cette instabilité semble être caractéristique des matériaux présentant une forte NTE, montrant le lien entre la NTE et la compressibilité anormale. L'influence de la basse température sur l'eau confinée dans les pores de la structure d'AlPO4-54•xH2O a été étudié par diffraction des rayons X et par spectroscopie Raman sur monocristal. Les résultats ont été comparés à des simulations Monte Carlo sur le matériau. Les résultats ont montré que les molécules d'eau en contact avec la surface de la structure du pore unidimensionnel sont plus ordonnées vers 173 K, alors que l'eau du centre du pore est restée dans un état de type verre (liquide) à toutes les autres températures étudiées. L'amorphisation d'AlPO4-54•xH2O a été suivie par diffraction des rayons X et par spectroscopie Raman in-situ sous pression dans une CED en utilisant huile de silicone et l'eau comme milieux transmetteurs de pression. Des analyses ex-situ des échantillons de la forme amorphe d'AlPO4-54•xH2O ont été réalisées par résonance magnétique nucléaire et spectroscopie d'absorption de rayons x au synchrotron Soleil (ligne LUCIA). L'analyse de la structure locale a révélé qu'un changement de coordinence est responsable pour la déstabilisation de la structure : deux molécules d'eau ont été contraintes à entrer dans la sphère de coordination de l'Al tétraédrique, devenant ainsi un site octaédrique. / The aim of this study is to investigate pressure-induced amorphisation (PIA) in negative thermal expansion (NTE) aluminophosphates. The aluminophosphates AlPO4-17 and AlPO4-54 are of particular importance, the first exhibits the highest known degree of negative thermal expansion for an oxide and the second exhibits the largest pores known for zeolites with a diameter of 12 Å. These materials exhibit unusual behavior upon compression due to the softening of a large number of low frequency modes leading to pressure-induced amorphisation. The pressure-induced amorphisation in the exceptional NTE material AlPO4-17 was studied by in situ X-ray powder diffraction. AlPO4-17 shows anomalous behavior under pressure with elastic instability and negative value of B0'. Anomalous compressibility behavior may be observed in other materials that exhibit strong NTE, indicating a link between these two physical properties due by low-energy lattice vibrations. The pressure-induced phase transition of the AlPO4-54 to AlPO4-8 and the consequent amorphisation of the final structure, was studied by X-ray diffraction in situ at high pressures. The freezing of nanoconfined water in the AlPO4-54•xH2O was studied by Single Crystal X-ray Diffraction and Raman spectroscopy experiments and were compared to Monte Carlo and Molecular Dynamics simulations. Results shows that, at the pore surface, the adsorbed layer of water molecules had a crystal-like orientational order, in contrast, a cylindrical core of glassy water in the pore center is present due to the frustration arising from the curvature. The effect of H2O on the (PIA) of AlPO4-54•xH2O was studied by in situ X-ray powder diffraction and Raman spectroscopy under high pressures. Ex situ analysis were used to investigate the local structure of pressure-amorphized microporous AlPO4-54•xH2O by nuclear resonance magnetic and by X-ray absorption spectroscopy at the synchrotron Soleil (beamline LUCIA) which shows that, upon increasing the pressure, two water molecules enters in the coordination sphere of IVAl, changing the coordination from 4- to 6-fold, which destabilizes the structure.
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[en] PRINCIPAL COMPONENT ANALISYS AND NEURAL NETWORK APPROACHES TO THERMAL EXPANSION OF CERAMICS OXIDES / [pt] ANÁLISE DA EXPANSÃO TÉRMICA DE ÓXIDOS CERÂMICOS UTILIZANDO AS ABORDAGENS DE ANÁLISE DE COMPONENTES PRINCIPAIS E REDES NEURAISADALVARO GARCIA ROSA 04 November 2013 (has links)
[pt] Alguns óxidos cerâmicos apresentam Coeficiente de Expansão Térmica
Negativa (ETN). Estes materiais usados como compósitos podem controlar a
expansão térmica em valores determinados, inclusive zero. Este trabalho tem
como objetivo propor novos óxidos da família A2M3O12 com coeficiente de
expansão próximo a zero. Foi usada a Análise de Componentes Principais (PCA)
para se verificar estatisticamente a causa de ETN em 45 óxidos conhecidos
desta família e a partir daí foi usada uma Rede Neural para simulação de novos
óxidos. / [en] Some ceramic oxides exhibit negative coefficient of thermal expansion.
These material generally used as compounds can control the thermal expansion
in determined values, including zero. The aim of this work is to propose new
oxides of the family type A2M3O12 with coefficient of thermal expansion close to
zero. The Principal Component Analysis (PCA) is used to statistically verify the
reason for negative thermal expansion in 45 known oxides of this family and then,
the neural network approach is used for simulation of new oxides.
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[en] SYNTHESIS AND CHARACTERIZATION OF LOW THERMAL EXPANSION OXIDES / [pt] SÍNTESE E CARACTERIZAÇÃO DE ÓXIDOS COM BAIXA EXPANSÃO TÉRMICAMONICA ARI SANO 19 February 2008 (has links)
[pt] Materiais que apresentam expansão térmica baixa ou negativa
possuem
grande potencial de emprego em diversas aplicações que
requerem resistência
ao choque térmico, assim como para aplicações
odontológicas, em placas de
circuitos eletrônicos, em componentes ópticos e para
produzir compósitos em
que compensam a expansão térmica positiva de outros
materiais. Por este
motivo, o estudo e a produção de materiais com expansão
térmica controlada
têm crescido nos últimos anos. No presente trabalho, foram
estudadas algumas
famílias de óxidos com estruturas que apresentam este tipo
de comportamento.
Foi avaliada a expansão térmica em três sistemas da família
A2M3O12 com o
intuito de produzir materiais com expansão térmica
controlada pela substituição
química do cátion A pelos cátions Al, Cr e Fe. Os sistemas
produzidos foram:
Cr2xFe2-2xMo3O12 (molibdato de cromo - ferro), Al2xCr2-
2xMo3O12 (molibdato de
alumínio - cromo) e Al2xFe2-2xMo3O12 (molibdato de
alumínio - ferro). Além
destes, o composto HfMgMo3O12 foi também sintetizado para
testar a viabilidade
de substituição dos cátions A trivalentes por um cátion
divalente e um
tetravalente. Foi possível obter soluções sólidas
monofásicas e seus parâmetros
de rede variam linearmente com o aumento no conteúdo do
cátion de maior
tamanho, conforme a lei de Vegard. Análise térmica
realizada por DSC permitiu
determinar a temperatura de transição de fase da estrutura
monoclínica (P21a) à
ortorrômbica (Pbcn). Os valores encontrados foram: para o
Al2Mo3O12, 200oC,
para o Cr2Mo3O12 , 403oC e para o Fe2Mo3O12, 512oC.
Coeficientes de expansão
térmica intrínsecos foram determinados por difração de
raios-X utilizando luz
síncrotron, encontrando-se valores bem reduzidos para todos
os sistemas
estudados, a saber: para o Al2Mo3O12, (alfa)l = 2,32 x 10-
6/oC, para o Cr2Mo3O12 , (alfa)l =
0,65 x 10-6/oC e para o Fe2Mo3O12, (alfa)l = 1,72 x 10-6/oC. / [en] Negative and low thermal expansion materials have important
potential
applications as resistance to thermal shock, as well as,
for odontological
applications, printed circuit boards, optical components
and to produce
composites to compensate the positive thermal expansion of
materials. For this
reason, the study and production of materials with
controlled thermal expansion
have increased in the recent years. In the present work,
some oxide families with
structures that present this type of behavior were
investigated. The thermal
expansion in three systems of A2M3O12 family was evaluated
in order to produce
materials with controlled thermal expansion through the
chemical substitution of
cation A with Al, Cr and Fe. The produced systems were:
Cr2xFe2-2xMo3O12
(chromium - iron molybdate), Al2xCr2-2xMo3O12 (chromium -
aluminum molybdate)
and Al2xFe2-2xMo3O12 (aluminum - iron molybdate). Besides
that, HfMgMo3O12 was
synthesized to test the viability of substitution of
trivalent cations for one divalent
plus one tetravalent. It was possible to obtain single-
phase solid solutions and
their lattice parameters increased linearly with the
increase of the largest cation
content, following the Vegard`s law. Thermal analysis
carried out by DSC allowed
the determination of the phase transition temperature from
monoclinic (P21a) to
orthorhombic (Pbcn) structure. The values were 200oC for
Al2Mo3O12, , 403oC for
Cr2Mo3O12 , and 512oC for Fe2Mo3O12. Intrinsic thermal
expansion coefficients
were determined by X-rays diffraction using syncrotron
radiation, and low values
were found for all studied systems: (alfa)1 = 2,32 x 10-
6/oC for Al2Mo3O12, (alfa)1 = 0,65 x
10-6/oC for Cr2Mo3O12 , and (alfa)1 = 1,72 x 10-6/oC for
Fe2Mo3O12.
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Low Temperature Synthesis and Characterization of Some Low Positive and Negative Thermal Expansion MaterialsWhite, Kathleen Madara 10 July 2006 (has links)
LOW TEMPERATURE SYNTHESIS AND CHARACTERIZATION OF SOME LOW POSITIVE AND NEGATIVE THERMAL EXPANSION MATERIALS
Kathleen Madara White
151 pages
Directed by Dr. Angus P. Wilkinson
Low temperature non-hydrolytic sol-gel synthesis was used to explore the possibility of lowering the crystallization temperatures of some known AIVMV2O7 compounds. Crystallization temperatures for ZrP2O7 and ZrP2O7 were unaffected by the use of non-hydrolytic sol-gel methods; however, successful synthesis of these compounds broadens the range of materials that can be produced using this method and suggests the possibility of synthesizing solid solutions (or composites) including ZrP2O7 or ZrV2O7.
This research presents for the first time the direct synthesis of ZrP2O7 from separate zirconium and phosphorus starting materials using mild autoclave methods.
Characterization of some AIVMV2O7 compounds, using lab and high resolution synchrotron powder XRD, led to the assignment of a new symmetry for CeP2O7 and to the suggestion that the reported structure for PbP2O7 was inadequate. Studies using in situ high temperature lab and synchrotron powder XRD for PbP2O7 and CeP2O7 provided the opportunity to report their thermal properties for the first time, and to compare their behavior to that of some other AIVMV2O7. High pressure diffraction measurements on CeP2O7 provided data for the estimation of bulk moduli and suggested two possible pressure-induced phase transitions.
A broad range of MIIIMVP4O14 compounds were prepared using low temperature hydrolytic sol-gel synthesis. Thermal studies revealed nearly linear trends in CTEs and lattice constants with respect to the sizes of MIIIMV cations. Some lower ionic radii compounds had CTEs comparable to that of ZrP2O7 at low temperature, suggesting a similar superstructure. Three compounds were found to exhibit temperature-induced phase transitions.
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