As estruturas cristalinas e algumas propriedades elásticas das fases a e g do tungstato de zircônio, ZrW2O8, foram calculadas de acordo com a Teoria do Funcional da Densidade Eletrônica usando o funcional B3LYP (DFT/B3LYP). Para a fase a-ZrW2O8, a estrutura foi otimizada em diferentes pressões e suas constantes elásticas foram estimadas. As interações interatômicas, ordenadas em termos das compressibilidades das ligações, diminuem de acordo com a seqüência W-O > Zr· · ·W > Zr-O. O tetraedro em torno dos átomos do tungstênio é muito mais rígido do que os octaedros de ZrO6. Estes últimos são, de fato, mais compressíveis que a cela unitária da fase a-ZrW2O8. As constantes elásticas calculadas no limite atérmico estão em excelente acordo com os recentes resultados experimentais obtidos próximo de 0 K. O mecanismo de compressão em torno dos átomos W1 e W2 é completamente diferente. Enquanto o primeiro é descrito essencialmente em termos de uma rotação correlacionada dos poliedros, o último envolve a rotação correlacionada dos poliedros de primeira coordenação e a translação das unidades WO4 para baixo, ao longo dos eixos < 111>. Na medida em que estes modos de deformação são semelhantes aos modos de baixa energia responsáveis pela expansão térmica negativa do tungstato de zircônio, este resultado pode auxiliar na elaboração do mecanismo microscópico responsável por este fenômeno. Para a fase g-ZrW2O8 foram otimizadas as estruturas a pressão ambiente e para V/V0 = 0,97 (limiar da transição de fase g ® amorfa) com o objetivo de estudar a evolução da estrutura desta fase com a pressão e obter indícios sobre o mecanismo de amorfização induzida por altas pressões. Com a redução de 3% no volume da cela unitária ocorre uma variação maior que 1,5% nas distâncias interatômicas entre o oxigênio terminal de um determinado poliedro com o átomo de tungstênio do poliedro vizinho. Portanto, é esperado que a redução de 15% no volume molar com a transição g ® amorfa promova a formação de novas ligações W-O, responsáveis pela retenção metaestável da fase amorfa após alívio da pressão. / Fundação de Amparo à Pesquisa do Estado do Rio Grande do Sul. / The crystalline structure and some elastic properties of the a and g phases of zirconium tungstate, ZrW2O8, were calculated according to Density Functional Theory using the B3LYP functional (DFT/B3LYP). The structure of a-ZrW2O8, was optimized at different pressures and its elastic constants were estimates. The interatomic iterations, ranked in terms of bond compressibilities, decrease according to the sequence W-O > Zr· · ·W > Zr-O. The tetrahedra around tungsten atoms are found to be much stiffer than the ZrO6 octahedra. These latter are, in fact, more compressible than the a-ZrW2O8 unit cell. The elastic constants calculated in the athermal limit are in excellent agreement with recent experimental results obtained near 0 K. The compression mechanism around W1 and W2 atoms is quite different. While the former can be described essentially in terms of a correlated polyhedral rotation, the latter involves correlated rotation of the first coordination polyhedra and translation of WO4 units downward along the < 111> axis. As far as these modes of deformation should bear some resemblance to the low-energy modes responsible for the negative thermal expansion in zirconium tungstate, this result can shed some light on the microscopic mechanism behind this phenomenon. The structure of g-ZrW2O8 was optimized at ambient pressure and for V/V0 = 0.97 (threshold of the g ® amorphous phase transition) aiming to study the evolution of the structure of this phase with the pressure and the pressure induced amorphization mechanism. A reduction of 3 % in the volume of the unit cell leads to a variation greater than 1,5 % in the interatomic distances between the terminal atoms and the tungsten of the nearest tetrahedron. Therefore, it is expected that the reduction of 15 % in the molar volume with the g ® amorphous transition promotes the formation of new W-O bonds, which would be responsible for the metaestable retention of the amorphous phase upon pressure release.

O carbeto de boro é um material sintético com ligações químicas essencialmente covalentes, tem um alto ponto de fusão só é sinterizável em elevada temperatura. Possui excepcional dureza, baixa densidade, resistência a abrasão, elevada velocidade sônica e boas propriedades mecânicas, características ideais para as aplicações balísticas. Tem como principal característica a alta seção de choque para nêutrons térmicos para aplicações nucleares. O presente trabalho teve por objetivo avaliar as propriedades mecânicas do carbeto de boro, pela introdução de diferentes teores de diboreto de titânio, pela reação in situ com pós de carbeto de titânio, e adição do carbono durante a sinterização, em forno resistivo sem pressão e prensagem isostática a quente dos componentes cerâmicos. Em menores temperaturas valores obtidos da densidade teórica para o carbeto de boro puro, foram alcançados com o emprego do aditivo. Os resultados obtidos na sinterização mostram a eficiência da introdução do carbeto de titânio para o aumento da densificação do material. Com percentuais de 20% de carbeto de titânio, obteve-se os máximos valores para microdureza (HV) de 35 GPa e tenacidade a fratura (KlC) de 3,16 MPa.m1/2. Comprovadamente a dificuldade de sinterização em elevadas temperaturas, para maior densificação, de componentes cerâmicos de carbeto de boro pode ser minimizada com a introdução de percentuais de carbeto de titânio. / Boron Carbide is a synthetic material with essentially covalent chemical bonds with high fusion point. The main characteristics are: it is sintered at high temperature, high hardness, low density, abrasion resistant, high sonic velocity, good mechanical properties and high neutron absorption cross section (10BxC, x>4). Those features are ideal for ballistics applications. The aim of this study was to evaluate the changes in mechanical properties of Boron Carbide with different concentration of Titanium Diboride by reaction in situ with TiC powders. The addition of carbon during sintering without pressure and hot isostatic pressing of ceramic components was studied. At low temperatures, the nearly values for the theoretical density for pure Boron Carbide were obtained only with the use of additives. In sintering, the use of TiC increased Boron Carbide density. At concentrations up to 20% of TiC, the maximum values for hardness (HV) and fracture toughness (KlC) were obtained. With the introduction of different levels of additive, the difficulty of sintering at high temperatures was minimized and the density of ceramic components was maximized.

Abstract In gravure offset printing, ink is transferred with the help of an offset material from a patterned gravure plate to a substrate. This thesis is concerned with the study and further development of this printing process for electronics; on alumina, glass and polymers. The work has been divided into five parts. In the first section, the printing process is described. The second section describes the composition of the inks for gravure offset printing and the resulting ink properties. It also presents the ink transfer mechanism; the model that explains how the ink is transferred between an offset material and a substrate. The third chapter details the printing process explained by a solvent absorption mechanism. The forth chapter describes the firing/curing of printed samples and their properties. The last chapter describes applications of the method. The inks used to produce conductors on ceramics (ceramic inks) and conductors on polymers (polymer inks) contain silver particles, and were under development for gravure offset printing. The major achieved properties were the high ink pickup to the offset blanket and high transfer percentage to the substrate. 100% ink transfer from blanket to substrate for ceramic inks and almost 100% ink transfer for polymer inks was obtained. The printing of ceramic inks was able to produce 8 μm of relatively thick, 300 μm wide lines with &lt; 10 mΩ/sq. resistance. The minimum line width for conducting lines was 35 μm, with one printing. Multi printing was applied producing as many as 10 times wet-on-wet multiprinted lines with 100 % ink transfer from blanket to substrate resulting in a square resistance of 1mΩ/sq. Polymer inks were able produce a square resistance of 20 mΩ/sq. for 300 μm wide lines after curing at 140 °C for about 15 min, and the minimum width was down to 70 μm. In the optimised manufacturing process, the delay time on the blanket was reduced to 3 s. In addition to ultra-fine-line manufacturing of conductors, the method enables the manufacture of special structures e.g. laser-solder contact pads with 28/28 μm lines/spaces resolution. With industrial printing equipment it is possible to produce 100 m2/h with the demonstrated printing properties.

Abstract The thesis describes the development of a novel LTCC material system for RF and microwave telecommunication purposes. The work has been divided into three parts. In the first section, the compositional and firing properties of this novel LTCC dielectric have been studied as well as its thermomechanical and dielectric properties. The second section describes the multilayer component preparation procedure for the ceramic material including tape casting and lamination parameters and the selection of the conductor paste. In the last section, the novel LTCC material system has been used to demonstrate its properties in RF multilayer resonators and a bandpass filter. The dielectric material for the novel LTCC system was prepared using magnesium calcium titanate ceramic, the firing temperature of which was decreased to 900°C by the addition of a mixture of zinc oxide, silicon oxide and boron oxide. The powder was made without any prior glass preparation, which is an important process advantage of this composition. The fired microstructure was totally crystalline with high density (3.7 Mg m-3) and low porosity (0.5 %). The mechanical properties were virtually identical to the values of the commercial LTCCs, but the higher thermal expansivity makes it most compatible with alumina substrates. The dielectric values were also good. The permittivity was 8.5 and the dissipation factor (0.9·10-3 at 8 GHz) less than that of the commercial LTCCs. Furthermore, the temperature coefficient of the resonance frequency was demonstrated to be adjustable between the range of +8.8 ... -62 ppm/K with a simple compositional variation of titanium oxide. The slurry for the tape casting was prepared using poly(vinyl butyral) -base organic additives and the 110 μm thick tapes had a smooth surface (RA &lt; 0.5 μm). The multilayer components were prepared using 20 MPa lamination pressure, 90°C temperature and 1 h dwell time. The most suitable conductor paste for this composition was found to be commercial silver paste (duPont 6160), which produced satisfactory inner and outer conductor patterns for multilayer components. Finally, resonators with a resonant frequency range of 1.7 ... 3.7 GHz were prepared together with a bandpass filter suitable for the next generation of telecommunication devices. This demonstration showed the potential of the developed novel LTCC material system at high RF frequencies.

Uma das grandes aplicações dos sensores magnetoelásticos está relacionada aos sistemas antifurto. Devido ao seu excelente desempenho e natureza passiva sem fio, também têm sido amplamente utilizados no sensoriamento ambiental em resposta às mudanças de temperatura, pressão atmosférica, velocidade de fluxo do fluido através da superfície do sensor, densidade e viscosidade de líquidos e variação de massa. O presente trabalho investiga a utilização de fitas magnetoelásticas, tais como Metglas®2826MB3, para estudar o comportamento de secagem de materiais cerâmicos. Duas argilas extraídas da região do Vale do Caí, principal produtor de materias cerâmicos para a construção civil, foram analisadas. Suspensões de argila foram preparadas e sua caracterização foi realizada por meio de técnicas tradicionais que considerou a cinética de secagem, a retração e o comportamento das trincas de acordo com a massa úmida do material depositado em um molde de alumínio. Posteriormente as suspensões foram depositadas sobre a superfície da fita sensora com auxílio de um dispositivo doctor blade. Aplicando um campo magnético variável, foi possível determinar a frequência de ressonância experimental do sensor. Essa frequência é inversamente proporcional ao comprimento da fita e dependente das propriedades físicas dos materiais. Um modelo matemático foi utilizado para relacionar a frequência de ressonância com as propriedades elásticas da cobertura da fita. Além das propriedades físicas dos materiais também foi incluído ao modelo, a dependência do módulo de elasticidade da fita em campo magnético, conhecido como efeito E. A tensão desenvolvida durante o processo de secagem pode gerar trincas e deformações no material quando não dissipadas. Essa tensão contribui para a anisotropia magnética e é introduzida no problema através do efeito E. A técnica do cantilever foi utilizada para verificar a presença de tensões durante a secagem. Medindo a deflexão do substrato é possível determinar a tensão gerada pela suspensão. As curvas da frequência de ressonância em função do tempo mostram várias regiões distintas. Na região que distingue os dois principais estágios de secagem da suspensão, caracterizada pela taxa de secagem que diminui aceleradamente, a medida da frequência se mostrou bastante sensível à presença de tensões. Portanto, as estruturas observadas nas curvas experimentais de frequência em função do tempo puderam ser reproduzidas pelo modelo, utilizando a distribuição de tensões experimentais e as propriedades elásticas do sensor e da suspensão de argila. Esse resultado promissor sugere que as tensões desenvolvidas durante a secagem de materiais cerâmicos poderiam ser determinadas por medidas da frequência de ressonância dos sensores. / An important application of magnetoelastic sensors is in antitheft systems. Because of their excellent performance and their passive wireless nature, they have also been extensively used in environmental sensing of changes in temperature, atmospheric pressure, flow speed of fluid over the sensor surface, density and viscosity of liquids and mass change. The present work investigates the use of magnetoelastic sensors based on Metglas®2826MB3 to study the drying behavior of ceramic materials. Two clays from the Vale do Caí region, the main producer of ceramic materials for the construction industry, were analyzed. Aqueous clay suspensions were prepared and their characterization was performed using traditional techniques that considered the drying kinetics, shrinkage and cracking behavior as a function of the wet weight of the material deposited into an aluminum mold. Subsequently, the suspensions were deposited on the sensor tape surface with a doctor blade device. Applying a variable magnetic field, it was possible to determine the experimental resonance frequency of the sensor. This frequency is inversely proportional to the ribbon length and dependent upon the physical properties of the materials. A mathematical model was used to correlate the resonance frequency with the elastic properties of the coating. Besides the physical properties of the materials, also included in model was the dependence of the tape's elastic modulus on magnetic field, known as the E effect. The stress developed during the drying process may cause cracks and deformations in the material when it is not dissipated. This stress contributes to the magnetic anisotropy which is introduced into the problem through the E effect. The cantilever technique was used to determine the presence of tensions during drying. Measuring the deflection of the substrate, it is possible to determine the stress generated by the suspension. The curves of resonance frequency versus time show several distinct regions. For the region that separates the two main rying stages of the suspension, the frequency measurement proved to be very sensitive to the presence of tension. Therefore, the structures observed in the experimental curves of frequency versus time could be modeled using the experimental distribution of stress and the elastic properties of the sensor and clay suspension. This promising result suggests that the tensions developed during the drying of ceramic materials could be determined by measurements of the resonance frequency of the sensors.

Silicon nitride thin films are of interest in the biomedical engineering field due to their biocompatibility and favorable tribological properties. Evaluation and understanding of the properties of these films under diverse loading and failure conditions is a necessary prerequisite to their use in biomedical devices. Three wafers of silicon nitride-coated silicon were obtained from Lawrence Livermore National Laboratory and used to create 96 samples. Samples were subjected to nanoindentation testing to evaluate the mechanical properties of the film. Samples were subjected to nanoimpact testing to compare the damage resistance of the film to separate nanoimpact types. Samples were subjected to nanoscratch testing to evaluate the consistency of the critical load of the film. Results showed that there were no significant differences in the mechanical properties of the film across the tested groups. There was a significant difference observed in the rate of damage to the film between pendulum oscillation nanoimpact testing and sample oscillation nanoimpact testing, with the former causing more damage with all experiment variables controlled for. Results showed that the critical load measure for the film was significantly different between different nanoscratch test parameters. The conclusions from this study will support future work for in vitro and in vivo testing of ceramic thin films for biomedical applications.

Polymer-derived Ceramics (PDCs) represent a unique class of high-temperature stable materials synthesized directly by the thermal decomposition of polymers. This research first focuses on the fabrication of high temperature stable siliconcarbonitride (SiCN) fibers by electrospinning for ceramic matrix composite (CMC) applications. Ceraset™ VL20, a commercially available liquid cyclosilazane, was functionalized with aluminum sec-butoxide in order to be electrospinnable. The surface morphology of the electrospun fibers was investigated using the fibers produced from solvents. The electrospun fibers produced from the chloroform/N,N-dimethylformamide solutions had hierarchical structures that led to superhydrophobic surfaces. A “dry skin” model was proposed to explain the formation of micro/- and nanostructures. The second objective of the research is to align the multiwalled carbon nanotubes (MWCNTs) in PDC fibers. For this purpose, a non-invasive approach to disperse carbon nanotubes in polyaluminasilazane chloroform solutions was developed using a conjugated block copolymer synthesized by ATRP. The effect of the polymer and CNT concentration on the fiber structure and morphology was also examined. Detailed characterization using SEM and TEM was performed to demonstrate the orientation of CNTs inside the ceramic fibers. Additionally, the electrical properties of the ceramic fibers were investigated. Finally, the structural evolution of polymer-derived amorphous siliconborocarbonitride (SiBCN) ceramics with pyrolysis temperatures was studied by solid-state NMR, Raman and EPR spectroscopy. Results suggested the presence of three major components: (i) hexagonal boron nitride (h-BN), (ii) turbostratic boron nitride (t-BN), and (iii) BN2C groups in the final ceramic. iv The pyrolysis at higher temperature generated boron nitride (BN3) with a simultaneous decomposition of BN2C groups. A thermodynamic model was proposed to quantitatively explain the conversion of BN2C groups into BN3 and “free” carbon. Such structure evolution is believed to be the reason that the crystallization of Si4.0B1.0 ceramics starts at 1500 ° C, whereas Si2.0B1.0 ceramics is stable upto 1600 ° C.

Processing of ceramics has always been difficult due to how hard and brittle the material is. Fused Deposition of Ceramics (FDC) is a method of additive manufacturing which allows ceramic parts to be built layer by layer, abetting more complex geometries and avoiding the potential to fracture seen with processes such as grinding and milling. In the process of FDC, a polymeric binder system is mixed with ceramic powder for the printing of the part and then burned out to leave a fully ceramic part. This experiment investigates a new combination of materials, zirconia and acrylic binder, optimizing the process of making the material into a filament conducive to the printer system and then performing trials with the filament in the printer to assess its feasibility. Statistical analysis was used to determine optimal parameter levels using response surface methodology to pinpoint the material composition and temperature yielding the highest quality filament. It was discovered that although the mixture had adequate melting characteristics to be liquefied and printed into a part, the binder system did not provide the stiffness required to act as a piston to be fed through the printer head. Further studies should be completed continuing the investigation of zirconia and acrylic binder, but with added components to increase strength and rigidity of the filament.

No description available.

A novel class of inorganic/ organic hybrid materials referred to as CERAMERs have been successfully prepared by a sol-gel process using a polymeric acid catalyst, poly(styrene sulfonic acid), to form a network from tetraethylorthosilicate (TEOS) and triethoxysilane end capped oligomers of poly( tetramethylene oxide) (PTMO). The structure-property relationships were determined for these hybrid networks from their dynamic mechanical behavior, mechanical properties, and SAXS behavior. Thermal gravimetric data coupled with FfIR analysis were used to obtain a qualitative measure of the extent of conversion of the metal alkoxides as a function of acid catalyst. Another new class of CERAMERs was successfully developed by the sol-gel processing of three different metal alkoxides in conjuction with the triethoxy silane end capped PTMO oligomers. The first novel hybrid is based upon complexes of aluminum tri-n-butoxide with ethyl acetoacetonate or titanium tetra-iso-propoxide with ethyl acetoacetate reacted with the triethoxysilane end capped PTMO. The hybrid networks were optically clear and could be easily formed as relatively large monoliths. A novel method of preparation of stable sols without the use of ethyl acetoacetate, developed by Dr. Bing Wang in this laboratory, led to more stable CERAMERS of titanium tetra-iso-propoxide (TiOPr) reacted with the functionalized P'TMO as well as a fourth inorganic/organic hybrid material based upon zirconium tetra-n-propoxide and PTMO. Indeed, the mechanical properties of the TiOPr/P'TMO CERAMERs and ZrOPr/P'TMO CERAMERs were significantly enhanced over the TEOS/PTMO CERAMERs in terms of both Young's modulus and stress at break. The structure-property relationships of these materials were adequately described by the morphological model originally developed for the CERAMERs based upon TEOS/PTMO. / Ph. D.