Browsing by Subject "Polyethylene"
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Item Four-probe thermal transport measurements of mesoscale structures(2017-08-11) Zhou, Yuanyuan, active 21st century; Shi, Li, Ph. D.; Wang, YaguoA number of thermal transport measurement methods have been reported in the past few decades for bulk materials and nano-structures. However, challenges have remained for measuring the thermal conductivity of mesoscale structures with the dimension in the range between 1 [mu]m and 1 mm. A recent reported four-probe thermal transport measurement method for nanostructures can separately obtain the intrinsic thermal resistance and the contact thermal resistance. This thesis reports an investigation of the applicability of this method for mesoscale samples. Mesoscale four-probe measurement devices have been designed, fabricated and calibrated with a SiNx beam sample. Multiple error sources have been evaluated. The measured thermal conductivity of SiN{subscript x], 3.7- 3.8 W m⁻¹ K⁻¹ at room temperature, is in agreement with the literature values. In addition, consistent thermal conductivity values of the SiN[subscript x] layer have also been obtained based on the measured thermal and electrical conductances of the Pd/Cr/SiN[subscript x] beam of the measurement device and the Wiedemann-Franz law. The mesoscale four probe devices have also been used to perform thermal transport measurements of three polyethylene ribbon samples with different dimensions. The measured thermal conductivity of polyethylene ranges between 4 W m⁻¹ K⁻¹ and 6 W m⁻¹ K⁻¹ at the temperature range between 150 K and 350 K, with the peak value appearing at 275K. The decreasing thermal conductivity with increasing temperature reveals the effect of lattice anharmonicity at temperatures higher than 275 K. These results demonstrate that the mesoscale four-probe thermal transport measurement device can obtain both the contact thermal resistance and intrinsic thermal resistance of mesoscale structures with acceptable errors, which are mainly caused by increasing radiation heat loss from the thermometer lines and the sample with increasing lengths. Improvement in the measurement devices can be made by reducing the length and width of the thermometer lines to reduce the radiation loss without reducing the conduction thermal resistance. Hence, this mesoscale four-probe thermal transport measurement method can fill the gap between thermal transport measurements of bulk materials and nano-structures and provide experimental data of mesoscale structures.Item Modeling and simulation of linear thermoplastic thermal degradation(2012-05) Bruns, Morgan Chase; Ezekoye, Ofodike A.; Ganesan, Venkat; Howell, John R.; Koo, Joseph H.; Nyden, Marc R.; Ruoff, Rodney S.Thermal degradation of linear thermoplastics is modeled at several scales. High-density polyethylene (HDPE) is chosen as an example material. The relevant experimental data is surveyed. At the molecular scale, pyrolysis chemistry is studied with reactive molecular dynamics. Optimization is used to calibrate several pyrolysis mechanisms with thermogravimetric analysis (TGA) data. It is shown that molecular scale physics may be coupled to continuum scale transport equations through a population balance equation (PBE). A PBE solution method is presented and tested. This method has the advantage of preserving detailed information for the small species in the molecular weight distribution with minimal computational expense. The mass transport of these small species is modeled at the continuum scale with a bubble loss mechanism. This mechanism includes bubble nucleation, growth, and migration to the surface of the condensed phase. The bubble loss mechanism is combined with a random scission model of pyrolysis to predict TGA data for HDPE. The modeling techniques developed at these three scales are used to model two applications of engineering interest with a combined pyrolysis and devolatilization PBE. The model assumes a chemically consistent form of the random scission pyrolysis mechanism and an average, parameterized form of the bubble loss mechanism. This model is used to predict the piloted ignition of HDPE. Predictions of the ignition times are reasonable but the model over predicts the ignition temperature. This discrepancy between model and data is attributed to surface oxidation reactions. The second application is the prediction of differential scanning calorimetry (DSC) data for HDPE. The model provides detailed information on the energy absorption of the thermally degrading sample, but the literature data is too variable to validate the model.Item Stretch-induced wrinkling of thin sheets(2013-08) Nayyar, Vishal; Huang, Rui, doctor of civil and environmental engineering; Ravi-Chandar, K.Thin sheets and membrane structures are widely used in space applications such as solar sails, sunshields and membrane optics. Surface flatness over a large area is one of the key requirements for many applications using the flexible thin structures. However, wrinkles are commonly observed in thin sheets. It is thus important to understand the mechanics of thin sheets for practical applications that require reliable control of surface wrinkles. In this study, a model problem of stretch-induced wrinkling of thin sheets is considered. First, a two-dimensional (2-D) finite element model was developed to determine stretch-induced stress distribution patterns in hyperelastic thin sheets, assuming no wrinkles. As a prerequisite for wrinkling, development of compressive stresses in the transverse direction was found to depend on both the length-to-width aspect ratio of the sheet and the applied tensile strain. Next, an eigenvalue analysis was performed to predict the critical conditions for buckling of the elastic sheet under the prescribed boundary conditions, followed by a nonlinear post-buckling analysis to simulate evolution of stretch-induced wrinkles. Experiments were conducted to measure stretch-induced wrinkling of polyethylene thin sheets, using the three-dimensional digital image correlation (3D-DIC) technique. It was observed that the wrinkle amplitude first increased and then decreased with increasing nominal strain, in agreement with finite element simulations for a hyperelastic thin sheet. However, unlike the hyperelastic model, the stretch-induced wrinkles in the polyethylene sheet were not fully flattened at high strains (> 30%), with the residual wrinkle amplitude depending on the loading rate. The hyper-viscoelastic and the parallel network nonlinear viscoelastic material models were adopted for finite element simulations to improve the agreement with the experiments, including the wrinkle amplitude, residual wrinkles and rate dependence. Finally it is noted that wrinkling is sensitive to defects and material inhomogeneity in thin sheets. By varying the elastic stiffness in a narrow region, numerical simulations show drastically different wrinkling behavior, including the critical strain and evolution of wrinkle amplitude and wavelength. In conclusion, a comprehensive understanding of stretch-induced wrinkling is established, where geometry, material, and boundary conditions all play important roles.