812 research outputs found
Experimental analysis and numerical simulation of sintered micro-fluidic
This paper investigates the use of numerical simulations to describe solid state diffusion of a sintering stage during a Powder Hot Embossing (PHE) process for micro-fluidic components. Finite element analysis based on a thermo-elasto-viscoplastic model was established to describe the densification process of a PHE stainless steel porous component during sintering. The corresponding parameters such as the bulk viscosity, shearing viscosity and sintering stress are identified from dilatometer experimental data. The numerical analyses, which were performed on a 3D micro-structured component, allowed comparison between the numerical predictions and experimental results of during a sintering stage. This comparison demonstrates that the FE simulation results are in better agreement with the experimental results at high temperatures
Physical modelling of amorphous thermoplastic polymer and numerical simulation of micro hot embossing process
Micro hot embossing process is considered as one of the most promising micro replication processes for manufacturing of polymeric components, especially for the high aspect ratio components and large surface structural components. A large number of hot embossing experimental results have been published, the material modelling and processes simulation to improve the quality of micro replication by hot embossing process are still lacking. This paper consists to 3D modelling of micro hot embossing process with amorphous thermoplastic polymers, including the mechanical characterisation of polymers properties, identification of the viscoelastic behaviour law of the polymers, numerical simulation and experimental investigation of micro hot embossing process. Static compression creep tests have been carried out to investigate the selected polymersâ viscoelastic properties. The Generalized Maxwell model has been proposed to describe the relaxation modulus of the polymers and good agreement has been observed. The numerical simulation of the hot embossing process in 3D has been achieved by taking into account the viscoelastic behaviour of the polymers. The microfluidic devices with the thickness of 2 mm have been elaborated by hot embossing process. The hot embossing process has been carried out using horizontal injection/compression moulding equipment, especially developed for this study. A complete compression mould tool, equipped with the heating system, the cooling system, the ejection system and the vacuum system, has been designed and elaborated in our research. Polymer-based microfluidic devices have been successfully replicated by the hot embossing process using the compression system developed. Proper agreement between the numerical simulation and the experimental elaboration has been observed. It shows strong possibility for the development of the 3D numerical model to optimize the micro hot embossing process in the future
An Exploration of Self-Identity Experiences Within the Lives of Afro-Caribbean Women Undergraduate College Students: A Feminist Phenomenological Study
The purpose of this feminist phenomenological dissertation was to explore the self-identity experiences of Afro-Caribbean women undergraduate college students. In doing so, self-identity experiences, ethnic marginalization, and counseling experiences were explored for six participants. Data was collected and analyzed using Simone De Beauvoir\u27s feminist framework of self-discovery/ rediscovery where two semi-structured interviews were conducted for each participant. This study resulted in six individual profiles illuminating the voices of each participant as well as collective themes. Findings from this study show that Afro-Caribbean women undergraduate college students filter their self-identity experiences through their ethnicity; meaning that participants understand other pieces of their self-identity (i.e., gender and race) and self-identity as a whole through their primary social location of ethnicity. Also, the findings represent that ethnic marginalization fueled participants\u27 push back against stereotypical and colonial narratives about the Afro-Caribbean community and having an inadequate counseling experiences affects college social and academic success. Based on the findings of this data, implications center on multicultural and intersectional training of college counselors and counselor educators; as well as implications for Afro-Caribbean women undergraduate college students. Recommendations for future research also showcases the need for larger research sample sizes, including qualitative and quantitative studies about self-identity and related subjects (i.e., self-concept, self-esteem, and ethnic identity development)
Exact quantum master equation for a molecular aggregate coupled to a harmonic bath
We consider a molecular aggregate consisting of identical monomers. Each
monomer comprises two electronic levels and a single harmonic mode. The
monomers interact with each other via dipole-dipole forces. The monomer
vibrational modes are bilinearly coupled to a bath of harmonic oscillators.
This is a prototypical model for the description of coherent exciton transport,
from quantum dots to photosynthetic antennae. We derive an exact quantum master
equation for such systems. Computationally, the master equation may be useful
for the testing of various approximations employed in theories of quantum
transport. Physically, it offers a plausible explanation of the origins of
long-lived coherent optical responses of molecular aggregates in dissipative
environments
First-passage time theory of activated rate chemical processes in electronic molecular junctions
Confined nanoscale spaces, electric fields and tunneling currents make the
molecular electronic junction an experimental device for the discovery of new,
out-of-equilibrium chemical reactions. Reaction-rate theory for
current-activated chemical reactions is developed by combining a Keldysh
nonequilibrium Green's functions treatment of electrons, Fokker-Planck
description of the reaction coordinate, and Kramers' first-passage time
calculations. The NEGF provide an adiabatic potential as well as a diffusion
coefficient and temperature with local dependence on the reaction coordinate.
Van Kampen's Fokker-Planck equation, which describes a Brownian particle moving
in an external potential in an inhomogeneous medium with a position-dependent
friction and diffusion coefficient, is used to obtain an analytic expression
for the first-passage time. The theory is applied to several transport
scenarios: a molecular junction with a single, reaction coordinate dependent
molecular orbital, and a model diatomic molecular junction. We demonstrate the
natural emergence of Landauer's blowtorch effect as a result of the interplay
between the configuration dependent viscosity and diffusion coefficients. The
resultant localized heating in conjunction with the bond-deformation due to
current-induced forces are shown to be the determining factors when considering
chemical reaction rates; each of which result from highly tunable parameters
within the system
Manifestation of nonequilibrium initial conditions in molecular rotation: the generalized J-diffusion model
In order to adequately describe molecular rotation far from equilibrium, we
have generalized the J-diffusion model by allowing the rotational relaxation
rate to be angular momentum dependent. The calculated nonequilibrium rotational
correlation functions (CFs) are shown to decay much slower than their
equilibrium counterparts, and orientational CFs of hot molecules exhibit
coherent behavior, which persists for several rotational periods. As distinct
from the results of standard theories, rotational and orientational CFs are
found to dependent strongly on the nonequilibrium preparation of the molecular
ensemble. We predict the Arrhenius energy dependence of rotational relaxation
times and violation of the Hubbard relations for orientational relaxation
times. The standard and generalized J-diffusion models are shown to be almost
indistinguishable under equilibrium conditions. Far from equilibrium, their
predictions may differ dramatically
Water soluble Invar 36 feedstock development for ”PIM
A water soluble binder system based on cellulose acetate butyrate (CAB) and polyethylene glycol (PEG) is proposed and investigated to carry out a micro powder injection moulding (muPIM) process with an Invar 36 alloy powders. The overall process was optimised with an emphasis on the determination of the optimal solid loading. Several methodologies were evaluated and compared to determine this parameter. A full muPIM was performed with different powder content feedstocks. Dog bone-type micro test parts were fabricated thereof and their mechanical properties were evaluated. Solid loadings up to 65 vol.% resulted to have the most equilibrated properties to successfully fabricate Invar 36 micro parts with the selected powder and binder system.The authors wish to thank GUZMĂN GLOBAL S.L. and MIMTECH ALFA for their collaboration on the ECOPIM project (Ref. IPT-2011-0931-20000) that was funded by the Spanish Ministry of the Economy and Competitiveness. Furthermore, the authors would like to acknowledge the strong support from the ESTRUMAT projects (Ref. S2009/MAT-1585), which were funded by the CAM ConsejerĂa EducaciĂłn Dir. Gral. Universidades e InvestigaciĂłn, and from the COMETAS project (Ref. MAT2009/14448-C02-02), which was funded by the Spanish Ministry of the Economy and Competitiveness.Publicad
Dynamics of a one-dimensional Holstein polaron: The multiconfigurational Ehrenfest method
We have extended the multiconfigurational Ehrenfest (MCE) approach to investigate the dynamics of a one-dimensional Holstein molecular crystal model. It has been shown that the extended MCE approach yields results in perfect agreement with benchmark calculations by the hierarchy equations of motion method. The accuracies of the MCE approach in describing the dynamical properties of the Holstein polaron over a wide range of exciton transfer integrals and exciton-phonon couplings are carefully examined by a detailed comparison with the fully variational multiple Davydov D2 ansatz. It is found that while the MCE approach and the multi-D2 ansatz produce almost exactly the same results for a small transfer integral, the results obtained by the multi-D2 ansatz start to deviate from those by the MCE approach at longer times for a large transfer integral. A large number of coherent state basis functions are required to characterize the delocalized features of the phonon wavefunction in the case of large transfer integral, which becomes computationally too demanding for the multi-D2 ansatz. The MCE approach, on the other hand, uses hundreds to thousands of trajectory guided basis functions and converges very well, thus providing an effective tool for accurate and efficient simulations of polaron dynamics
Effect of the particle size and solids volume fraction on the thermal degradation behaviour of Invar 36 feedstocks
Degradation kinetics and the thermal stability of Invar 36 powder injection moulding feedstocks (PIM) based on cellulose acetate butyrate (CAB) and polyethylene glycol (PEG) binders were investigated using simultaneous thermogravimetric analysis (STA) and differential scanning calorimetry (DSC). The initial decomposition temperature (IDT) and the integral procedure decomposition temperature (IPDT) were used to analyse the thermal stability of the binder system as a function of the solid loading content and powder particle size. The degradation kinetics was studied, and the process apparent activation energies were assessed using isoconversional methods. All the methodologies revealed changes in the thermal degradation behaviours of the feedstocks for solid loadings that were previously determined to correspond to optimal solid loadings using other experimental procedures. The studies also contrast previous similar findings with a ceramic powder. Therefore these results strengthen the proposal of thermodynamic degradation studies of feedstocks as an alternative or complementary technique to determine optimal solid loading contents in metal injection moulding (MIM).The authors a wish to thank GUZMĂN GLOBAL S.L. and MIM
TECH ALFA for their collaboration on the ECOPIM project (ref. IPT
2011 0931 20000) that was funded by the Spanish Ministry of
the Economy and Competitiveness. Furthermore, the authors
would like to acknowledge the strong support from the ESTRUMAT
projects (ref. S2009/MAT 1585), which were funded by the CAM
ConsejerĂa EducaciĂłn Dir. Gral. Universidades e InvestigaciĂłn, and
from the COMETAS project (ref. MAT2009/14448 C02 02), which
was funded by the Spanish Ministry of the Economy and Compet
itiveness. J.P.F. B. acknowledges support from âMarie Curieâ
Amarout Europe Program
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