A geometrically nonlinear finite element shell theory

Civil EngineeringCivil Engineering and Geoscience

Moisture absorption analysis of high performance polyimide adhesive

The high temperature resistant polymers and metal composites are used widely in aviation, space, automotive and electronics industry. The high temperature resistant polymers and metals are joined together using high temperature adhesives. Polyimide and epoxy adhesives that can withstand high temperature (200 °C-300 °C) are commonly used for joining high temperature metals and polymers. The performance of adhesively bonded metals and polymers depends upon physical properties of these high temperature adhesives. The physical properties like modulus, Tg. coefficient of thermal expansion (CTE) etc., are affected by external factors such as force, temperature, humidity etc. The external factors play a vital role in the adhesive bond strength and the durability of bond between metal and polymer. In this investigation moisture absorption analysis of polyimide adhesive is performed using Q5000 moisture absorption analyzer. The moisture absorption data of polyimide at different temperatures and humidity level is obtained. Further, the moisture absorption data is fitted to well known Fickian-fit model to determine the diffusion coefficient (D) and saturated moisture gain Msat. Diffusion coefficient (D) and Msat of polyimide and epoxy adhesive are calculated at different temperature and different humidity level. It is observed that diffusion coefficient changes with the change in temperature and humidity level. The diffusion coefficient (D) and M sat data are used in Fick’s second law of diffusion to estimate the time needed for preconditioning of the adhesively bonded titanium samples in humidity chamber at elevated temperature and higher moisture level. After preconditioning of adhesively bonded Titanium samples in moisture chamber for estimated time, samples will be subjected to lap shear tensile test to study the effect of these elevated conditions on adhesive bond strength.Precision and Microsystems EngineeringMechanical, Maritime and Materials Engineerin

Surface Modification of Titanium by Atmospheric Pressure Plasma Treatment for Adhesive Bonding and Its Application to Aviation and Space

Titanium is one of the most effective materials for structural application of space craft and aviation. Titanium alloys are widely used in solid rocket booster cases, guidance control pressure vessel and other different applications demanding light weight and reliability. Aerospace industry is also a larger market for titanium products and adhesive bonding is advantageous in terms of its fabrication. However, surface treatment of titanium alloy is critical in improving the adhesive bond strength and long term durability of the adhesive joint. In this investigation surface treatment of titanium is carried out by plasma ion implantation in order to increase adhesive bond strength and durability. Optical microscopic and SEM analysis of untreated and atmospheric plasma treated specimens is carried out to examine the surface characteristics. A substantial improvement in the surface energy of Titanium is observed after the atmospheric plasma treatment. The Treated surface was basically characterized by contact angle analyzer for the activation property on the surface. The surface energy of titanium surface increases with increasing exposure time of atmospheric pressure plasma. The optimized time of plasma treatment suggested in this investigation results maximum adhesive bond strength with polyimide adhesive and consequently, this technology is highly acceptable for aviation and space applications.Precision and Microsystems EngineeringMechanical, Maritime and Materials Engineerin

Durability of polyimide to titanium bonds

Titanium and its alloys are usually bonded together using a high temperature resistant polyimide or epoxy adhesives. Such adhesives can withstand temperatures from 200°C to300°C. Earlier research work indicates that Surface modification of titanium with mechanical treatment and atmospheric pressure plasma treatment techniques leads to improved adhesive bond strength of polyimide with titanium at room temperature. In this investigation, durability of these improved adhesive bond strength titanium samples is studied at elevated temperature and moisture level. Samples were exposed at 80°C and 60% Relative humidity conditions in moisture oven. Another set of samples was placed in heating oven at 80°C for dry aging of samples. After conditioning these samples were subjected to single lap shear tensile test. Combined effect of relatively high temperature and moisture on bond strength of titanium samples and only high temperature dry aged samples are studied. Single lap shear tensile test results indicate that conditioning at high temperature and moisture resulted in significant decrease of adhesive bond strength, where as dry aging at same temperature has no effect on adhesive bond strength. A comparison of the bond strength of these elevated condition samples and normal room condition samples will be presented in this paper.Precision and Microsystems EngineeringMechanical, Maritime and Materials Engineerin

Surface modification of polyimide by atmospheric pressure plasma for adhesive bonding with titanium and its application to aviation and space

It is noted that in search of long term and efficient service performance in the context of future generation of aerospace materials, there is increasing need of metal-high performance polymer composite. Based on these considerations, high temperature resistant polymeric sheet such as Polyimide Meldin7001 sheet, is joined with Titanium sheet by employing ultra high temperature resistant Polyimide adhesive. In order to increase surface energy of Polyimide surface, atmospheric pressure plasma treatment is used to modify the Polyimide surface. Atmospheric pressure plasma treatment creates physical and chemical changes such as cross linking, formation of free radicals and oxygen functionalization in the form of polar groups on polymer surface resulting in improvement of wetting and adhesion characteristics. Surface of Polyimide (PI) sheet is treated with atmospheric pressure plasma for different exposure periods. Surface energy of PI sheet increases with increase in exposure time. However, after a certain exposure time of plasma, deterioration of surface layer of PI substrate results in degradation and embitterment of PI which is not suitable for adhesive bonding. Optical microscopic, SEM (EDS), analysis of treated and untreated specimen is carried out to examine the surface characteristics. Treated samples and untreated samples of Polyimide are bonded together with overlap joints. Lap shear bond strength of treated and untreated samples was measured by tensile test to study the effect of treatment on adhesive bond strength. The optimized time of plasma treatment suggested in this investigation results in maximum adhesive bond strength and consequently, this technology is highly acceptable for aviation and space applications.Precision and Microsystems EngineeringMechanical, Maritime and Materials Engineerin

Ultra-Thin Deformable Silicon Substrates with Lateral Segmentation and Flexible Metal Interconnect

Our progress in developing technology modules for deformable single-crystalline-silicon electronics is presented in this contribution. Additional deformability/reliability is accomplished by modifications of the previously reported ultra-thin and flexible CIRCONFLEX technology (1). The flexibility is added in the last steps of the process flow using a combination of lateral segmentation and flexible metal interconnects. The post-processing nature of the added mechanical flexibility is thought to allow sensors and electronics to be built with proven technologies before they are rendered flexible. The additional deformability/reliability is achieved by lateral segmentation of silicon/dielectric layers and connecting these using flexible electrical interconnect. In the current study, segment thickness (silicon/SiO2) of ~1 ?m, segment size between 150 and 450 ?m, spacing of 20-200 ?m and serpentine-shaped aluminum interconnect transferred onto 8- 10 ?m thick polyimide film are characterized by tensile stretching to find out the reliability limits. Compared to our previous reports (2, 3), next to the processing issues also new electrical integrity results obtained from passive electrical test structures implemented on fully segmented polymer-embedded silicon islands are presented.Electrical Engineering, Mathematics and Computer Scienc

Micromechanical modeling of stress evolution induced during cure in a particle-filled electronic packaging polymer

Mechanical, Maritime and Materials Engineerin

Fononit ja raekohina

Sähköinen raekohina riippuu elektronien kokemista vuorovaikutuksista. Erityisesti elektronien ja fononien välinen vuorovaikutus pyrkii vähentämään raekohinaa. Tyypillisesti virran raekohina onkin voimakkaasti vaimentunut makroskooppisissa johteissa, mutta mesoskooppisissa järjestelmissä sen voi havaita. Miten muutos parametrialueiden välillä tapahtuu, riippuu elektroni-fononivuorovaikutuksen muodosta ja voimakkuudesta. Tämä diplomityö käsittelee elektroni-fononivuorovaikutuksen raekohinaan aiheuttamaa muutosta. Ilmiötä mallinnetaan semiklassisella Boltzmann-Langevin-kuljetusteorialla, joka sopii hyvin elektronien vuorovaikutusten mallintamiseen, mutta jättää huomiotta esimerkiksi kvanttimekaaniset interferenssi-ilmiöt. Työssä käydään läpi ja laajennetaan aiempia tuloksia diffuusin ja yksiulotteisen kuljetuksen tapauksissa, ja kytkentää erityyppisiin fononeihin tutkitaan yksinkertaistettujen mallien avulla. Työ sisältää myös teorian kvalitatiivisen vertailun hiilinanoputkien kohinan mittauksiin. Tulokset yhtenevät kirjallisuudessa esitettyjen kanssa ja osoittavat kytkennän elektronifononivuorovaikutuksen tarkan muodon ja raekohinan vaimenemisen välillä. Hiilinanoputkien kohinaan vertailussa ilmenee, ettei yksinkertainen teoria kuvaa hyvin pienten jännitteiden aluetta, mutta suurilla jännitteillä kvalitatiiviset tulokset voi periaatteessa ymmärtää. Syinä eroavuuteen pienillä jännitteillä voivat olla malliin kuulumattomat varautumis- ja interferenssi-ilmiöt

Effect of Atmospheric Pressure Plasma Modification on Polyimide and Adhesive Joining with Titanium

This investigation highlights the effect of surface modification on polyimide by atmospheric pressure plasma treatment with different exposure time. Surface modification of polymer by plasma treatment essentially creates physical and chemical changes such as cross-linking and formation of free radicals. It also forms oxygen functionalization in the form of polar groups on polymer surface, hence improving the wetting and adhesion properties. It is observed that surface energy of the polymer increases with increasing exposure time of atmospheric pressure plasma. However, prolonged exposure time of plasma results in deterioration of the surface layer of polyimide resulting in degradation and embrittlement. Scanning electron microscopy and atomic force microscopy analysis reveal that there is a considerable morphological change on the polymer surface due to atmospheric pressure plasma treatment. X-ray photo electron spectroscopy analysis reveals that the oxygen functionalities of polymer surface increases significantly when polyimide is exposed to atmospheric pressure plasma. Untreated and atmospheric pressure plasma-treated polyimide sheet are adhesive bonded by employing polyimide adhesive as well as with titanium substrate. Due to surface modification of polyimide, it is observed that there is a significant increase in lap shear tensile strength, and therefore, this technology is highly acceptable for aviation and space applications.Mechanical, Maritime and Materials Engineerin