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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
21

THE INFLUENCE OF WATER ON THE DEGRADATION AND WEAR OF AL2O3 SURFACES

Pickett, Ammon T. 16 January 2010 (has links)
As alumina plays ever more important roles in advanced technologies, such as substrates for in vivo biological sensors, catalysts for water purification and components of novel fuel devices, it is exposed to various environments. These environments lead to wear and degradation due to chemical and mechanical forces. Macroscale tribological tests including the scratch test and indentation are unable to analyze nanoscale properties due to their limited sensitivity. Today, nanotribologists incorporate three main tools to analyze nanoscale tribological properties: atomic force microscopy (AFM), the nanoindentor and surface force apparatus. Of these, AFM is the instrument of choice due to the sensitivity to and diversity of surface properties analyzed in a single setting. Through adhesion measurements and scratch testing under known loads the AFM was used to analyze the effect of OH- on the degradation of the alumina surface. Fourier transform infrared spectroscopy (FTIR) and AFM were also used to analyze the stability of self-assembled monolayers (SAMs) on the alumina surface. Through scanning a region of the surface with a defined force in an aqueous environment, the tribochemical surface properties were found. The pressure required to induce degradation of the first O-Al layer (~2 �) ranged from 3.10 GPa at a pH 3 to 1.58 GPa at pH 10. Further analysis of adhesion across the surface and within a defect region exposed significant changes in the forces of adhesion as the alumina surface experienced wear. At pH 3, 5, 7, and 10 the adhesion was approximately double that of the defect free surface. Two well known molecules were examined for their ability to protect the surface from bond rupture due to the catalytic effect of OH-. SAMs of octadyclephosphonic acid (OPA) and Octadycletrichlorosilane (OTS) were assembled on the alumina surface. FTIR spectra indicated well ordered monolayers formed from each molecule. The exposure of OPA to aqueous environments of pH >7 lead to the degradation of the SAM. OTS, on the other hand, demonstrated significantly more resilience to degradation as indicated through FTIR and AFM analysis.
22

Microstructure of Al2O3/5 vol% SiC with Transmission Electron Microscope

Yeh, Jhy-Shyong 11 July 2002 (has links)
Hot-pressing sintering Al2O3/5 vol% SiC gets detail research. Sub-boundaries and dislocations observed in grains. Most Burgers vectors are ¡Õ1120¡Ö.
23

Effect of growth conditions on the formation of the alumina/NiAl(110)

Chang, Yun-dian 26 July 2008 (has links)
In this work, STM measurements are employed to study the growth behaviors of alumina formed on the NiAl(110) surface under low coverage of oxygen dose (3 Langmuir and 120 Langmuir). Utilizing the capability of STM, the surface structures and the corresponding electronic properties of these alumina can be probed, respectively. The alumina nanoclusters are observed at sample bias 4V after the NiAl(110) surface is oxidized. However, when the bias is lower to 2V, the electronic states of the oxide are not accessible, so that the electrons can only tunnel between the tip and the metal support. In this case, the oxide just modulates the potential barrier as a function of position. Compared with the topographic image of the alumina formed by 3L and 120L oxygen dose, the growing behavior of the alumina/NiAl(110) at the initial stage could be measured.
24

The effects of nickel on an amorphous silica-alumina cracking catalyst

Brinen, Jeffrey Lawrence 05 1900 (has links)
No description available.
25

Inclusion behaviour in steel flows through convergent nozzles

Duncombe, John Trevor January 1989 (has links)
It is well established that nozzle blockage in Al-killed continuously cast steels is caused by the deposition of preexisting Al 0 inclusions at the narrowest part of the nozzle. Thisa adhere strongly at the wall by interfacial forces. The theory examined in this thesis is that during the curved flow path through the nozzle, inclusions move towards the nozzle wall by the action of centripetal force. A mathematical model has been developed which takes account of centripetal, gravitational, and drag forces. A computer programme has been written to predict particle behaviour in nozzle flows. To study the behaviour of second phase particles, a water model using a perspex tundish and an interchangeable glass nozzle has been built. Turbulence has been almost completely removed from the experimental tundish. Hydrogen bubbles electrolytically generated in water have been used to simulate the inclusions in steel and their flow paths through the nozzle have been recorded by cine photography. With frame by frame analysis of the films, the effects of bubble size, (50 pm to 1000 um), and flow velocity have been studied. Two experimental nozzles have been tested which have demonstrated the importance of limiting the centripetal force. It has been shown that there is a good relationship between hydrogen bubble behaviour and model predictions. This has been particularly true of small bubbles near the nozzle wall. By comparing the results with other experimental work it has been shown that centripetal forces can have a significant effect on the rate of blocking. Practical suggestions for reducing the turbulent behaviour in steel tundishes and nozzles are made. The merits of an improved nozzle design which takes advantage of the results of the model work are discussed.
26

The structure of alumina coatings on titanium dioxide

Hare, A. S. January 1979 (has links)
No description available.
27

Estudo da forma e da distribuição de tamanho de partículas de alumina em dispersões coloidais relacionado a sedimentação gravitacional /

Varanda, Laudemir Carlos. January 1999 (has links)
Resumo: Amostras comerciais de alumina foram caracterizadas quanto à estrutura, características de superfície e morfologia. Sais de polifosfato com diferentes tamanhos de cadeia foram isolados por cromatografia por troca iônica, tendo como precursor polifosfato de sódio. Os sais isolados na forma de tetraetilamoniomagnésio foram caracterizados e uma das amostras foi ensaiada como agente dispersante para alumina. As técnicas de caracterização utilizadas foram: análise termogravimétrica e térmica diferencial, difratometria de raios X, espectroscopia vibracional infravermelho, microscopia eletrônica de varredura e de transmissão, medidas de mobilidade eletroforética, área de superfície, ressonância magnética nuclear de 31P no estado sólido e sedimentometria. Sedimentometria de partículas de alumina foi utilizada para verificar a influência da natureza e da concentração de sais de polifosfato como agente dispersante na estabilidade coloidal da dispersão. Também foi investigada a influência da forma da partícula na velocidade terminal de sedimentação de partículas micrométricas, através da relação entre o coeficiente de arraste e o número de Reynolds. Modelos estatísticos, a saber: normal, lognormal e Rosin-Rammler foram ajustados aos perfis de sedimentação facilitando o tratamento dos resultados experimentais e permitindo caracterizar as dispersões através de parâmetros dos modelos teóricos, tais como o desvio padrão, que fornece informações sobre a distribuição do tamanho de partículas com relação à media. Metodologia que permitem a linearização dos dados experimentais através dos modelos estatísticos foram empregados e parece contribuir para definir rapidamente faixas de concentração, cada vez mais restritas, até a concentração ideal, bem como a natureza do agente dispersante que minimize a formação de agregados. Os resultados...(Resumo completo, clicar acesso eletrônico abaixo) / Abstract: The influence of the nature and concentration of the polyphosphate salts as dispersant agents on the colloidal stability of alumina dispersions by particle gravitational sedimentation was investigated. Shape of colloidal particle influence on the terminal velocity of sedimentation has been correlated using a simple relationship involving the usual drag coefficient and Reynolds number. In order to treat the experimental results, statistic models: normal, lognormal and Rosin-Rammler were adjusted to the sedimentation curves. Theoretical parameters of these models, such as standard-deviation, gives information about the dispersion of the particle size distribution in relationship to average particle size, and are not usually obtained directly from the experimental data set.A linear data set starting from the experimental data by calculation of the statistical models probabilities was obtained. This procedure quickly define dispersion agent concentration range, more and more restricted, up to the ideal concentration, as well as the nature of the dispersant agent that minimizes the aggregates formation. Structure, surface properties and morphology of the commercial alumina samples were characterized.Ion exchange chromatography for separating polyphosphate salts with different chain size was used.The isolated salts as tetraetylammonium magnesium were characterized and tested as dispersion agent for alumina. However, the salt behaved as flocculant agent due to the group of tetraetylammnium.The samples were characterized by thermogravimetry and diferential thermal analysis, X ray diffratometry, infrared spectroscopy, scanning and transmission electron microscopy, electrophoretic mobility and surface area measurements, 31P MAS NMR and sedimentometry.The results showed that the particle shape influences the settling velocity and they allowed to insert a proportionality term in the Stokes' equation.This term includes the projected area of the particle. / Orientador: Miguel Jafelicci Junior / Coorientador: Romeu Magnani / Banca: José Renato Coury / Banca: Nilso Barelli / Mestre
28

Effect of TiO₂ on the electrical conductivity of Al₂O₃

Uppal, Rajeev January 2000 (has links)
No description available.
29

Produção por dip coating e caracterização de revestimentos compósitos de poliuretano/alumina sobre fibras de poliamida 6

Sánchez, Felipe Antonio Lucca January 2009 (has links)
Este trabalho teve como objetivo produzir, pela técnica de dip coating contínuo, revestimentos compósitos poliuretano/alumina sobre fibras de poliamida 6. Para tanto, foram produzidas suspensões aquosas de poliuretano com partículas de alumina (tamanho médio de partícula de 2,1 µm) e empregadas fibras sintéticas de poliamida 6 com 0,1, 0,2 e 0,4 mm de diâmetro como substrato. Diferentes parâmetros para produção das suspensões foram avaliados visando a otimização desta etapa processual. A viscosidade das suspensões foi controlada pela adição de carboximetilcelulose e avaliada por reometria rotacional. A distribuição granulométrica das suspensões também foi determinada. Os parâmetros operacionais do equipamento para dip coating contínuo (i.e. velocidade de bobinamento e temperatura dos fornos) foram avaliados na busca das condições operacionais ótimas para obter revestimentos uniformes ao longo da fibra. A caracterização morfológica dos revestimentos foi conduzida com auxílio de microscopias óptica, eletrônica de varredura e de força atômica. Os revestimentos foram caracterizados ainda por termogravimetria e sua densidade medida por picnometria de gás hélio. Testes de fricção, sobre as fibras revestidas foram usados para qualificar o revestimento quanto à resistência ao desgaste. Os resultados evidenciaram que o processo adotado possibilita a deposição de um revestimento compósito uniforme. A espessura dos revestimentos variou de acordo com a velocidade de bobinamento, com a viscosidade das suspensões utilizadas e com a espessura da fibra original. Os revestimentos formados mostraram melhorar a resistência ao desgaste das fibras de poliamida 6 e essa propriedade está diretamente ligada à espessura do revestimento obtido. A tecibilidade das fibras revestidas foi avaliada pela produção de tecido com uma máquina de tecelagem convencional. As fibras de poliamida 6 (Ø = 0,1 mm) revestidas mantiveram o revestimento aderido mesmo após o processo de tecelagem. Assim, a técnica de dip coating mostrou-se promissora para o revestimento de fibras, combinando características de flexibilidade e elasticidade típica de polímeros com a dureza típica de materiais cerâmicos na forma de um revestimento protetor. / The aim of this work was to produce, using a continuous dip coating technique, a polyurethane/alumina composite coating over polyamide 6 fibers. For that, slurries based on aqueous polyurethane with dispersed alumina particles (mean size of 2.1 µm) were obtained and polyamide 6 synthetic fibers (diameter: 0.1, 0.2 and 0.4 mm) were used as substrate. Different slurry characteristics were evaluated seeking the optimization of the procedure, including particle size distribution and slurry viscosity which was varied using carboxymethylcellulose as filler and evaluated by rotational rheometry. The operational conditions of the dip coating machine (i.e. roll-up speed and temperature of the ovens) were also evaluated seeking to optimize the conditions to obtain a uniform coating over the fibers. Morphological characterization of the coatings was carried out using optical, scanning electron, and atomic force microscopy techniques. The coatings were also characterized by thermogravimetric analysis, density measurement and wear testing on coated fibers. The textile characteristics of the coated fibers were assessed by producing a knitted fabric in a conventional weave machine. The results showed that the adopted process enables the deposition of a uniform composite coating. Thickness of the coatings was a function of the roll-up speed, the slurry viscosity and also the diameter of the original fiber. The composite coating improved the wear resistance of the polyamide 6 fibers and this property was connected to its thickness. Polyamide 6 (Ø = 0.1 mm) coated fibers were able to maintain coating integrity even after the knitting process. In all, the dip coating technique showed to be a promising alternative for the production of a composite coating for the polyamide fibers, combining the flexibility and elasticity characteristics of typical polymers with the hardness typical of ceramics.
30

Estudo da forma e da distribuição de tamanho de partículas de alumina em dispersões coloidais relacionado a sedimentação gravitacional

Varanda, Laudemir Carlos [UNESP] 17 May 1999 (has links) (PDF)
Made available in DSpace on 2014-06-11T19:25:35Z (GMT). No. of bitstreams: 0 Previous issue date: 1999-05-17Bitstream added on 2014-06-13T19:12:30Z : No. of bitstreams: 1 varanda_lc_me_araiq.pdf: 4063066 bytes, checksum: 6543acba4b206125bc43665f5db92ef2 (MD5) / Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) / Amostras comerciais de alumina foram caracterizadas quanto à estrutura, características de superfície e morfologia. Sais de polifosfato com diferentes tamanhos de cadeia foram isolados por cromatografia por troca iônica, tendo como precursor polifosfato de sódio. Os sais isolados na forma de tetraetilamoniomagnésio foram caracterizados e uma das amostras foi ensaiada como agente dispersante para alumina. As técnicas de caracterização utilizadas foram: análise termogravimétrica e térmica diferencial, difratometria de raios X, espectroscopia vibracional infravermelho, microscopia eletrônica de varredura e de transmissão, medidas de mobilidade eletroforética, área de superfície, ressonância magnética nuclear de 31P no estado sólido e sedimentometria. Sedimentometria de partículas de alumina foi utilizada para verificar a influência da natureza e da concentração de sais de polifosfato como agente dispersante na estabilidade coloidal da dispersão. Também foi investigada a influência da forma da partícula na velocidade terminal de sedimentação de partículas micrométricas, através da relação entre o coeficiente de arraste e o número de Reynolds. Modelos estatísticos, a saber: normal, lognormal e Rosin-Rammler foram ajustados aos perfis de sedimentação facilitando o tratamento dos resultados experimentais e permitindo caracterizar as dispersões através de parâmetros dos modelos teóricos, tais como o desvio padrão, que fornece informações sobre a distribuição do tamanho de partículas com relação à media. Metodologia que permitem a linearização dos dados experimentais através dos modelos estatísticos foram empregados e parece contribuir para definir rapidamente faixas de concentração, cada vez mais restritas, até a concentração ideal, bem como a natureza do agente dispersante que minimize a formação de agregados. Os resultados... / The influence of the nature and concentration of the polyphosphate salts as dispersant agents on the colloidal stability of alumina dispersions by particle gravitational sedimentation was investigated. Shape of colloidal particle influence on the terminal velocity of sedimentation has been correlated using a simple relationship involving the usual drag coefficient and Reynolds number. In order to treat the experimental results, statistic models: normal, lognormal and Rosin-Rammler were adjusted to the sedimentation curves. Theoretical parameters of these models, such as standard-deviation, gives information about the dispersion of the particle size distribution in relationship to average particle size, and are not usually obtained directly from the experimental data set.A linear data set starting from the experimental data by calculation of the statistical models probabilities was obtained. This procedure quickly define dispersion agent concentration range, more and more restricted, up to the ideal concentration, as well as the nature of the dispersant agent that minimizes the aggregates formation. Structure, surface properties and morphology of the commercial alumina samples were characterized.Ion exchange chromatography for separating polyphosphate salts with different chain size was used.The isolated salts as tetraetylammonium magnesium were characterized and tested as dispersion agent for alumina. However, the salt behaved as flocculant agent due to the group of tetraetylammnium.The samples were characterized by thermogravimetry and diferential thermal analysis, X ray diffratometry, infrared spectroscopy, scanning and transmission electron microscopy, electrophoretic mobility and surface area measurements, 31P MAS NMR and sedimentometry.The results showed that the particle shape influences the settling velocity and they allowed to insert a proportionality term in the Stokes’ equation.This term includes the projected area of the particle.

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