A convenient method for testing and calibrating surface perfusion sensors has been developed. A phantom tissue model is used to mimic the non-directional blood flow of tissue perfusion. A computational fluid dynamics (CFD) model was constructed in Fluent to design the phantom tissue and validate the experimental results. The phantom perfusion system was used with a perfusion sensor based on the clearance of thermal energy. A heat flux gage measures the heat flux response of tissue when a thermal event (convective cooling) is applied. The blood perfusion and contact resistance are estimated by a parameter estimation code. From the experimental and analytical results, it was concluded that the probe displayed good measurement repeatability and sensitivity. The experimental perfusion measurements in the tissue were in good agreement with those of the CFD models and demonstrated the value of phantom tissue system. This simple, cost effective, and noninvasive convective blood perfusion system was then tested in animal models. The perfusion system was evaluated for repeatability and sensitivity using isolated rat liver and exposed rat kidney tests. Perfusion in the isolated liver tests was varied by controlling the flow of the perfusate into the liver, and the perfusion in the exposed kidney tests was varied by temporarily occluding blood flow through the renal artery and vein. The perfusion estimated by the convective perfusion probe was in good agreement with that of the metered flow of perfusate into the liver model. The liver tests indicated that the probe can be used to detect small changes in perfusion (0.005 ml/ml/s). The probe qualitatively tracked the changes in the perfusion in kidney model due to occlusion of the renal artery and vein. / Ph. D.

In this thesis, MEMS switches actuated using electrostatic actuation is explored. MEMS switches that are lateral switches and clamped-clamped switches are designed, fabricated, and tested in this thesis. This thesis extensively explains the process by which the MEMS Switches were designed and fabricated. In addition, it explains the changes in the switches when issues called for a modification to devices. Contact resistances were extensively studied, in this thesis. There has been a trade-off between the reliability of switches and their contact resistances. Many actions were taken to mitigate this trade-off and to allow both reliable devices with low contact resistances. The efforts to do so ranged from thermal oxidation to reduce the scalloping on the sidewalls, to modifying the dry etching recipe, to modifying the sputtering recipe, to electroplating, and many more. However, reliability of the MEMS Lateral switches was accomplished independent to the contact resistances. In addition, low contact resistances were accomplished independent to reliability. A novel approach to designing clamped-clamped MEMS switches is also showcased in this thesis. These devices experienced unique challenges compared to those faced with lateral switches. Both lateral and clamped-clamped switches are discussed in-depth in this thesis. / Master of Science

Organic electronic devices are receiving growing interest because of their potential to employ lightweight, low-cost materials in a flexible architecture. Typically, indium tin oxide (ITO) is utilized as the transparent positive electrode in these devices due to its combination of high transmission in the visible spectrum and high electrical conductivity. However, ITO may ultimately hinder the full market integration of organic electronics due to its increasing cost, the limited availability of indium, lack of mechanical flexibility, and sustainability with regards to the environment and material utilization. Therefore, alternatives for ITO in organic electronics are currently being pursued. Transparent electrodes comprised of single wall carbon nanotubes (SWNTs) are an appealing choice as a surrogate because of the extraordinary electrical and mechanical properties these 1-D structures posses. As such, the research presented in this dissertation has been conducted to advance the goal of manufacturing SWNT networks with transparent electrode properties that meet or exceed those of ITO. To this end, SWNT films were characterized with regard to the collective and individual optoelectronic properties of the SWNTs that comprise the network. Specifically, corroborative theoretical and experimental observations were employed to expand the understanding of how the optoelectronic properties of polydisperse and monodisperse SWNT networks are enhanced and sustained through chemical treatment and subsequent processing. In addition, the impact of interfacial electrical contact resistance between SWNT electrodes and metallic fingers often used in photovoltaic system applications was elucidated. In summary, the research presented in this dissertation can be leveraged with present state of the art in SWNT films to facilitate future SWNT electrode development.

Contact resistance is the most important and universal characteristic of all types of electrical and thermal contacts. Accurate measurement of contact resistance is important, because it serves as a measure for judging the performance and operational life span of contacts. Rise in contact temperature is one of the major factors that pose a big threat to the stability of electrical contacts. Dissipation of heat by solid conduction through a contact interface is governed by its thermal contact conductance (TCC). This emphasizes the need to study the TCC of an electrical contact along with its electrical contact resistance (ECR). Simultaneous measurement of ECR and TCC is important for understanding the interconnection between these two quantities and the possible influence of one over another. Real time experimental data and analytical correlations can be extremely helpful in developing electrical contacts with improved thermal management capabilities. As a part of the experimental investigation, a test facility has been developed for making simultaneous measurement of ECR and TCC across flat contacts. The facility has the capability of measuring ECR and TCC over a wide range of operating parameters, such as contact pressure, contact temperature, interstitial gaseous media, ambient pressure, etc. It is also capable of determining the electrical resistivity and thermal conductivity of materials as a function of temperature, which is very helpful in analyzing the generated contact resistance data. Using this facility, simultaneous ECR and TCC measurements are made across bare and gold plated contacts of OFHC Cu (oxygen free high conductivity copper) and brass. Simultaneous ECR and TCC measurements are made on nominally flat contacts in the contact pressure range of 0 – 1 MPa and the interface temperature range of 20 – 120 °C. Effect of contact pressure and interface temperature on ECR and TCC is studied on bare and gold coated contacts in vacuum, N2, Ar, and SF6 environments. TCC strongly depends on the thermophysical properties of the interstitial media and shows a significant enhancement in gaseous media, because of the increased interfacial gap conductance compared to vacuum. The gas pressure is varied in the range of 1 – 2.6 bar to study its effect on the gap conductance at different contact pressures and interface temperatures. Minor increase in the ECR observed in gaseous media is found to be independent of the properties of the media. Experimental results indicated that ECR depends on the gas pressure as well as on the applied contact load. Effect of gold coating and its thickness on the ECR and TCC across OFHC Cu and brass contacts is studied. Measurements on electroplated gold specimens having different gold layer thicknesses (0.1, 0.3, and 0.5 µm) indicated that ECR decreases and TCC increases with increasing gold coating thickness. Effect of gold coating on the substrate properties, contact surface tomography, and microhardness is analyzed and correlated to the observed behavior of ECR and thermal gap conductance. An attempt is made to understand and quantify the changes in the contact surface characteristics due to contact loading and heating, by measuring various surface topography parameters before and after the experimentation. Effect of thermal stresses (generated due to temperature variations) on ECR and TCC is studied and inclusion of an experimentally measured temperature dependent load correction factor is suggested in the theoretical models to take into account the effect of thermal stresses in contact assemblies.

Electrical contact resistance (ECR) measurements are needed for judging the performance of electrical appliances. Understanding the behaviour of ECR at low temperature gives a unique opportunity for understanding the contact mechanism itself and controlling the contact resistance for its applications in various areas at these temperatures. In many high-end applications, sophisticated electronic devices are being operated below ambient temperature to improve their performance. The availability of cryogens, improvement in Thermo-Electrical (TE) based Peltier coolers, accelerated the development of these devices. In designing such systems, an accurate measurement of electrical contact resistance below room temperature is important. A detailed experimental investigation has been conducted on electrical contact resistance across bare and coated metal contacts at low temperatures. As a part of the experimental investigation, a test facility capable of varying the contact force, surrounding pressure and temperature, is developed. The design, construction, testing and use of this facility are described. Electrical contact resistance at different contact pressures across copper, OFHC copper and brass with and without gold coatings is measured using 4-wire technique with high accuracy. The test specimen preparation, instrumentation and data acquisition are explained in detail. The setup is standardized by comparing the experimental results obtained across copper-copper contacts in vacuum with the theoretical model. The electrical contact resistance is measured as a function of contact force at different temperatures. The effect of loading and unloading, and the existence of hysteresis are experimentally studied. The electrical properties of conductors improve at low temperature but this is not true for contact resistance. At low temperature the contact resistance increases and it depends on applied contact force, hardness and roughness of the contacting surfaces. Gold-coated contacts exhibited an increase in contact resistance at low temperatures.

Este trabalho trata da aplicação do método Design Construtal para investigar a transferência de calor através de caminhos de alta condutividade térmica com geometrias definidas. O objetivo é obter a configuração que reduz a temperatura máxima em excesso do sistema considerando que as áreas ocupadas pelos materiais de alta e baixa condutividade são tratadas como constantes. Assim, o objeto de estudo é um volume de área finita onde ocorre a geração de calor. O escoamento da energia térmica para fora do volume é feito através de um caminho condutor de alta condutividade térmica. O trabalho considerou a resistência térmica de contato entre o elemento condutivo e o corpo gerador de calor, onde um terceiro material com resistência térmica equivalente à resistência de contato é interposto entre os dois primeiros. Na solução da equação da difusão do calor, foi realizado um tratamento numérico através de um código baseado em elementos finitos e utilizando o toolbox PDETool, Partial Differential Equations Tool, que pertence ao aplicativo comercial MatLab®. O tratamento numérico foi realizado considerando-se caminhos condutivos com geometrias em forma de "I" e em forma de "T", mantendo-se as frações de área constantes e variando-se os comprimentos dos materiais de alta condutividade e os da resistência térmica de contato. A otimização geométrica foi feita considerando-se os graus de liberdade existentes para cada geometria, onde os valores otimizados para a situação ideal, ou de acoplamento térmico perfeito, foram comparados para os resultados envolvendo a resistência térmica de contato (RTC). Os resultados indicam que a RTC pode aumentar a temperatura máxima em excesso, assim como tem efeito significativo sobre as ótimas configurações calculadas quando a resistência de contato é levada em consideração para ambas as configurações "I" e "T" estudadas. / This work applies Constructal Design to investigate the heat transfer through high conductive pathways with defined geometries. The objective is to find the configuration which reduces the maximal excess of temperature considering the areas with high and low thermal conductivity are constants. Thus, the object studied here is a volume with a finite area and heat generation. The outside heat flux is conducted through a high thermal conductive pathway. Here, special attention is given to the thermal contact resistance between the high conductive pathway and the solid body, where a third material with a thermal resistance equivalent to the thermal contact resistance is inserted between them. A numerical treatment was given in order to solve the heat diffusive equation. It was used a numerical code based on finite elements and the toolbox – PDETool, Partial Differential Equations Tool, which is part of the MatLab® applicative. The numerical treatment was achieved considering "I" and "T" geometries for the high conductive pathways keeping the areas fraction constants and varying the lengths of both high conductive and the equivalent thermal contact layer materials. The optimization was performed considering the degrees of freedom of each geometry, where the optimized values for the ideal situation, i.e., perfect thermal contact were compared with the results considering the thermal contact resistance. The results indicate that the thermal contact resistance can increase the excess of temperature, as well as it has a significant effect on the optimal configurations when using perfect thermal contact or taking into account the thermal contact resistance for "I" and "T" shaped geometries.

Este trabalho trata da aplicação do método Design Construtal para investigar a transferência de calor através de caminhos de alta condutividade térmica com geometrias definidas. O objetivo é obter a configuração que reduz a temperatura máxima em excesso do sistema considerando que as áreas ocupadas pelos materiais de alta e baixa condutividade são tratadas como constantes. Assim, o objeto de estudo é um volume de área finita onde ocorre a geração de calor. O escoamento da energia térmica para fora do volume é feito através de um caminho condutor de alta condutividade térmica. O trabalho considerou a resistência térmica de contato entre o elemento condutivo e o corpo gerador de calor, onde um terceiro material com resistência térmica equivalente à resistência de contato é interposto entre os dois primeiros. Na solução da equação da difusão do calor, foi realizado um tratamento numérico através de um código baseado em elementos finitos e utilizando o toolbox PDETool, Partial Differential Equations Tool, que pertence ao aplicativo comercial MatLab®. O tratamento numérico foi realizado considerando-se caminhos condutivos com geometrias em forma de "I" e em forma de "T", mantendo-se as frações de área constantes e variando-se os comprimentos dos materiais de alta condutividade e os da resistência térmica de contato. A otimização geométrica foi feita considerando-se os graus de liberdade existentes para cada geometria, onde os valores otimizados para a situação ideal, ou de acoplamento térmico perfeito, foram comparados para os resultados envolvendo a resistência térmica de contato (RTC). Os resultados indicam que a RTC pode aumentar a temperatura máxima em excesso, assim como tem efeito significativo sobre as ótimas configurações calculadas quando a resistência de contato é levada em consideração para ambas as configurações "I" e "T" estudadas. / This work applies Constructal Design to investigate the heat transfer through high conductive pathways with defined geometries. The objective is to find the configuration which reduces the maximal excess of temperature considering the areas with high and low thermal conductivity are constants. Thus, the object studied here is a volume with a finite area and heat generation. The outside heat flux is conducted through a high thermal conductive pathway. Here, special attention is given to the thermal contact resistance between the high conductive pathway and the solid body, where a third material with a thermal resistance equivalent to the thermal contact resistance is inserted between them. A numerical treatment was given in order to solve the heat diffusive equation. It was used a numerical code based on finite elements and the toolbox – PDETool, Partial Differential Equations Tool, which is part of the MatLab® applicative. The numerical treatment was achieved considering "I" and "T" geometries for the high conductive pathways keeping the areas fraction constants and varying the lengths of both high conductive and the equivalent thermal contact layer materials. The optimization was performed considering the degrees of freedom of each geometry, where the optimized values for the ideal situation, i.e., perfect thermal contact were compared with the results considering the thermal contact resistance. The results indicate that the thermal contact resistance can increase the excess of temperature, as well as it has a significant effect on the optimal configurations when using perfect thermal contact or taking into account the thermal contact resistance for "I" and "T" shaped geometries.

Este trabalho trata da aplicação do método Design Construtal para investigar a transferência de calor através de caminhos de alta condutividade térmica com geometrias definidas. O objetivo é obter a configuração que reduz a temperatura máxima em excesso do sistema considerando que as áreas ocupadas pelos materiais de alta e baixa condutividade são tratadas como constantes. Assim, o objeto de estudo é um volume de área finita onde ocorre a geração de calor. O escoamento da energia térmica para fora do volume é feito através de um caminho condutor de alta condutividade térmica. O trabalho considerou a resistência térmica de contato entre o elemento condutivo e o corpo gerador de calor, onde um terceiro material com resistência térmica equivalente à resistência de contato é interposto entre os dois primeiros. Na solução da equação da difusão do calor, foi realizado um tratamento numérico através de um código baseado em elementos finitos e utilizando o toolbox PDETool, Partial Differential Equations Tool, que pertence ao aplicativo comercial MatLab®. O tratamento numérico foi realizado considerando-se caminhos condutivos com geometrias em forma de "I" e em forma de "T", mantendo-se as frações de área constantes e variando-se os comprimentos dos materiais de alta condutividade e os da resistência térmica de contato. A otimização geométrica foi feita considerando-se os graus de liberdade existentes para cada geometria, onde os valores otimizados para a situação ideal, ou de acoplamento térmico perfeito, foram comparados para os resultados envolvendo a resistência térmica de contato (RTC). Os resultados indicam que a RTC pode aumentar a temperatura máxima em excesso, assim como tem efeito significativo sobre as ótimas configurações calculadas quando a resistência de contato é levada em consideração para ambas as configurações "I" e "T" estudadas. / This work applies Constructal Design to investigate the heat transfer through high conductive pathways with defined geometries. The objective is to find the configuration which reduces the maximal excess of temperature considering the areas with high and low thermal conductivity are constants. Thus, the object studied here is a volume with a finite area and heat generation. The outside heat flux is conducted through a high thermal conductive pathway. Here, special attention is given to the thermal contact resistance between the high conductive pathway and the solid body, where a third material with a thermal resistance equivalent to the thermal contact resistance is inserted between them. A numerical treatment was given in order to solve the heat diffusive equation. It was used a numerical code based on finite elements and the toolbox – PDETool, Partial Differential Equations Tool, which is part of the MatLab® applicative. The numerical treatment was achieved considering "I" and "T" geometries for the high conductive pathways keeping the areas fraction constants and varying the lengths of both high conductive and the equivalent thermal contact layer materials. The optimization was performed considering the degrees of freedom of each geometry, where the optimized values for the ideal situation, i.e., perfect thermal contact were compared with the results considering the thermal contact resistance. The results indicate that the thermal contact resistance can increase the excess of temperature, as well as it has a significant effect on the optimal configurations when using perfect thermal contact or taking into account the thermal contact resistance for "I" and "T" shaped geometries.

As a result of manufacturing processes, real surfaces have roughness and surface curvature. The real contact occurs only over microscopic contacts, which are typically only a few percent of the apparent contact area. Because of the surface curvature of contacting bodies, the macrocontact area is formed, the area where microcontacts are distributed randomly. The heat flow must pass through the macrocontact and then microcontacts to transfer from one body to another. This phenomenon leads to a relatively high temperature drop across the interface. Thermal contact resistance (TCR) is a complex interdisciplinary problem, which includes geometrical, mechanical, and thermal analyses. Each part includes a micro and a macro scale sub-problem. Analytical, experimental, and numerical models have been developed to predict TCR since the 1930's. Through comparison with more than 400 experimental data points, it is shown that the existing models are applicable only to the limiting cases and none of them covers the general non-conforming rough contact. The objective of this study is to develop a compact analytical model for predicting TCR for the entire range of non-conforming contacts, i. e. , from conforming rough to smooth sphere-flat in a vacuum. The contact mechanics of the joint must be known prior to solving the thermal problem. A new mechanical model is developed for spherical rough contacts. The deformation modes of the surface asperities and the bulk material of contacting bodies are assumed to be plastic and elastic, respectively. A closed set of governing relationships is derived. An algorithm and a computer code are developed to solve the relationships numerically. Applying Buckingham Pi theorem, the independent non-dimensional parameters that describe the contact problem are specified. A general pressure distribution is proposed that covers the entire spherical rough contacts, including the Hertzian smooth contact. Simple correlations are proposed for the general pressure distribution and the radius of the macrocontact area, as functions of the non-dimensional parameters. These correlations are compared with experimental data collected by others and good agreement is observed. Also a criterion is proposed to identify the flat surface, where the influence of surface curvature on the contact pressure is negligible. Thermal contact resistance is considered as the superposition of macro and micro thermal components. The flux tube geometry is chosen as the basic element in the thermal analysis of microcontacts. Simple expressions for determining TCR of non-conforming rough joints are derived which cover the entire range of TCR by using the general pressure distribution and the flux tube solution. A complete parametric study is performed; it is seen that there is a value of surface roughness that minimizes TCR. The thermal model is verified with more than 600 data points, collected by many researchers during the last 40 years, and good agreement is observed. A new approach is taken to study the thermal joint resistance. A novel model is developed for predicting the TCR of conforming rough contacts employing scale analysis methods. It is shown that the microcontacts can be modeled as heat sources on a half-space for engineering applications. The scale analysis model is extended to predict TCR over the entire range of non-conforming rough contacts by using the general pressure distribution developed in the mechanical model. It is shown that the surface curvature and contact pressure distribution have no effect on the effective micro thermal resistance. A new non-dimensional parameter is introduced as a criterion to identify the three regions of TCR, i. e. , the conforming rough, the smooth spherical, and the transition regions. An experimental program is designed and data points are collected for spherical rough contacts in a vacuum. The radius of curvature of the tested specimens are relatively large (in the order of m) and can not be seen by the naked eye. However, even at relatively large applied loads the measured joint resistance (the macro thermal component) is still large which shows the importance of surface out-of-flatness/curvature. Collected data are compared with the scale analysis model and excellent agreement is observed. The maximum relative difference between the model and the collected data is 6. 8 percent and the relative RMS difference is approximately 4 percent. Additionally, the proposed scale analysis model is compared/verified with more than 880 TCR data points collected by many researchers. These data cover a wide range of materials, surface characteristics, thermal and mechanical properties, mean joint temperature, directional heat transfer effect, and contact between dissimilar metals. The RMS difference between the model and all data is less than 13. 8 percent.

Today’s electronic society relies on the functionality of electrical contacts. To achieve good contact properties, surface coatings are normally applied. Such coatings should ideally fulfill a combination of different properties, like high electrical conductivity, high corrosion resistance, high wear resistance and low cost. A common coating strategy is to use noble metals since these do not form insulating surface oxides. However, such coatings are expensive, have poor wear resistance and they are often applied by electroplating, which poses environmental and human health hazards. In this thesis, nanocomposite carbide-based coatings were studied and the aim was to evaluate if they could exhibit properties that were suitable for electrical contacts. Coatings in the Cr-C, Cr-C-Ag and Nb-C systems were deposited by magnetron sputtering using research-based equipment as well as industrial-based equipment designed for high-volume production. To achieve the aim, the microstructure and composition of the coatings were characterized, whereas mechanical, tribological, electrical, electrochemical and optical properties were evaluated. A method to optically measure the amount of carbon was developed. In the Cr-C system, a variety of deposition conditions were explored and amorphous carbide/amorphous carbon (a-C) nanocomposite coatings could be obtained at substrate temperatures up to 500 °C. The amount of a-C was highly dependent on the total carbon content. By co-sputtering with Ag, coatings comprising an amorphous carbide/carbon matrix, with embedded Ag nanoclusters, were obtained. Large numbers of Ag nanoparticles were also found on the surfaces. In the Nb-C system, nanocrystalline carbide/a-C coatings could be deposited. It was found that the nanocomposite coatings formed very thin passive films, consisting of both oxide and a-C. The Cr-C coatings exhibited low hardness and low-friction properties. In electrochemical experiments, the Cr-C coatings exhibited high oxidation resistance. For the Cr-C-Ag coatings, the Ag nanoparticles oxidized at much lower potentials than bulk Ag. Overall, electrical contact resistances for optimized samples were close to noble metal references at low contact load. Thus, the studied coatings were found to have properties that make them suitable for electrical contact applications.