188 research outputs found
Thermoviscoelasticity in Kelvin--Voigt rheology at large strains
The frame-indifferent thermodynamically-consistent model of thermoviscoelasticity at large strain is formulated in the reference configuration with using the concept of the second-grade nonsimple materials. We focus on physically correct viscous stresses that are frame indifferent under time-dependent rotations. Also elastic stresses are frame indifferent under rotations and respect positivity of the determinant of the deformation gradient. The heat transfer is governed by the Fourier law in the actual deformed configuration, which leads to a nontrivial description when pulled back into the reference configuration. Existence of weak solutions in the quasistatic setting, i.e. inertial forces are ignored, is shown by time discretization
Thermodynamics and analysis of rate-independent adhesive contact at small strains
We address a model for adhesive unilateral frictionless Signorini-type
contact between bodies of heat-conductive viscoelastic material, in the linear
Kelvin-Voigt rheology, undergoing thermal expansion. The flow-rule for
debonding the adhesion is considered rate-independent and unidirectional, and a
thermodynamically consistent model is derived and analysed as far as the
existence of a weak solution is concerned
Thermoviscoelasticity in Kelvin--Voigt rheology at large strains
The frame-indifferent thermodynamically-consistent model of thermoviscoelasticity at large strain is formulated in the reference configuration with using the concept of the second-grade nonsimple materials. We focus on physically correct viscous stresses that are frame indifferent under time-dependent rotations. Also elastic stresses are frame indifferent under rotations and respect positivity of the determinant of the deformation gradient. The heat transfer is governed by the Fourier law in the actual deformed configuration, which leads to a nontrivial description when pulled back into the reference configuration. Existence of weak solutions in the quasistatic setting, i.e. inertial forces are ignored, is shown by time discretization
Finite element approximation of a coupled contact Stefan-like problem arising from the time discretization in deformation theory of thermo-plasticity
AbstractIn the paper we draw on the mathematical formulation of the coupled contact Stefan-like problem in deformation theory of plasticity, which arises from the discretization in time. The problem leads to solving the system of variational inequalities, which is approximated by the FEM. Numerical analysis of the problem is made
A thermodynamically consistent model of magneto-elastic materials under diffusion at large strains and its analysis
The theory of elastic magnets is formulated under possible diffusion and heat
flow governed by Fick's and Fourier's laws in the deformed (Eulerian)
configuration, respectively. The concepts of nonlocal nonsimple materials and
viscous Cahn-Hilliard equations are used. The formulation of the problem uses
Lagrangian (reference) configuration while the transport processes are pulled
back. Except the static problem, the demagnetizing energy is ignored and only
local non-selfpenetration is considered. The analysis as far as existence of
weak solutions of the (thermo)dynamical problem is performed by a careful
regularization and approximation by a Galerkin method, suggesting also a
numerical strategy. Either ignoring or combining particular aspects, the model
has numerous applications as ferro-to-paramagnetic transformation in elastic
ferromagnets, diffusion of solvents in polymers possibly accompanied by
magnetic effects (magnetic gels), or metal-hydride phase transformation in some
intermetalics under diffusion of hydrogen accompanied possibly by magnetic
effects (and in particular ferro-to-antiferromagnetic phase transformation),
all in the full thermodynamical context under large strains
Recycling process of permanent magnets by polymer binder using injection molding technique
Seltene Erden-Elemente (REE) werden aufgrund ihrer technologischen Bedeutung und geopolitischen Versorgungskriterien als kritische Metalle eingestuft. Sie werden in einem breiten Spektrum von Anwendungen eingesetzt, einschließlich der Herstellung von Magneten, Batterieelektroden, Katalysatoren und Polierpulver. Viele dieser Anwendungen sind wichtig für die sog. „grünen“ Technologien. Dauermagneten sind hinsichtlich der Marktgröße die wichtigste Anwendung insbesondere für Neodym-, Praseodym-, Dysprosium- und Terbium-Magnete, die in NdFeB-Magneten verwendet werden. Die Nachfrage nach Seltenerdelementen für die Herstellung von Magneten nimmt zu und es wird erwartet, dass sich dieser Trend in den kommenden Jahren fortsetzt. Um die mit der Nachfrage verbundenen Risiken zu verringern, wurden Maßnahmen zur Entwicklung von Recyclingtechnologien zur Wiederverwendung von NdFeB aus Magneten ergriffen. Während der industrielle NdFeB-Schrott bereits zurückgewonnen wird, ist das Recycling von Magneten aus Altprodukten noch weitergehend auf Labor- und Pilotprojekte beschränkt. Diese Abhandlung stellt die Ergebnisse der Materialanalyse vor, die die Möglichkeit bestätigen, magnetische Materialien durch die Einarbeitung in eine Polymermatrix zu recyceln und mittels Spritzgussprozess vorzubereiten.
Kern der vorliegenden Dissertation ist die Frage, wie der geschlossene Kreislauf und das Recyclingverfahren von Neodynium Magneten aus Elektroschrott gestattet sein soll. Um diese Frage zu beantworten, sind folgende Aspekte relevant:
• Die Wahl der Technologien/Prozesse, die für das Recycling eingesetzt werden.
• Nachweis der Wiederverwendung von Neodym-Magneten, die aus WEEE (Waste of Electrical and Electronic Equipment) gewonnen sind.
• Herstellung und Analyse von Polymer/Magnet- Compound.
• Einfluss der Magnetpartikel, abhängig von ihrer Anzahl und Größe, auf die Viskosität und Fließverhalten des Materials während des Spritzgussprozess.
• Analyse des Einflusses der Restmagnetisierung auf das Fließverhalten und einer gezielten Anordnung von magnetischen Partikeln im Bauteil.
• Technisch-ökonomische Analyse, die entscheidend dazu beitragen wird, ob und in welchem Ausmaß die Einführung des Prozesses erreichbar ist und damit geschlossene Kreisläufe möglich sind.
Auf der Grundlage einer umfangreichen Analyse wurden die optimalen Prozessparameter und die Spritzgussmöglichkeiten des verwendeten Materials vorgestellt. Die Nachfrage nach NdFeB-Magneten in Motoranwendungen wächst und wird in den nächsten Jahren voraussichtlich noch zunehmen. Vor allem die Nachfrage nach E-Bike und E-Autos gewinnt an Bedeutung. Infolgedessen wird die Nachfrage nach schweren Seltenen Erden steigen, was die Entwicklung von Recyclingsystemen für diese Materialien erforderlich macht.Rare earth elements (REE) are classified as critical metals due to their technological importance and geopolitical supply criteria. They are used in a wide range of applications, including the manufacture of magnets, battery electrodes, catalysts, and polishing powders. Many of these applications are important for so-called "green" technologies. Permanent magnets are the most important application in terms of market size, particularly for neodymium, praseodymium, dysprosium, and terbium magnets used in NdFeB magnets. The demand for rare earth elements for the production of magnets is increasing and this trend is expected to continue in the coming years (Langkau S. 2020; Li J. 2020; Goodenough K.M. et al. 2018). To mitigate the risks associated with that demand, have been taken to develop recycling technologies to reuse NdFeB magnets. While industrial scrap is already being recovered, recycling of magnets from end-of-life products is still further limited to laboratory and pilot projects. The following work presents the results of the material analysis, which confirm the possibility to recycle magnetic materials by using a polymer matrix.
The main goal of this dissertation is the question of how the closed-loop and recycling process of neodymium magnets from electronic waste should be designed. To answer this question, the following aspects are relevant:
• The choice of technologies/processes used for recycling and processing.
• Evidence of reuse of neodymium magnets obtained from WEEE (Waste of Electrical and Electronic Equipment).
• Process flow analysis and final product evaluation (polymer/magnet compound).
• The effect of magnetic particles characteristics (size, distribution, and contribution) on the viscosity and flow behavior of the material during the injection molding process.
• Analysis of residual magnetization on the flow behavior and a targeted arrangement of magnetic particles in the component.
• Technical-economic analysis, which decisively contributes to whether and to what extent the introduction of the process is achievable.
Based on an extensive analysis, the optimal process parameters and the maximum injection possibilities of the material used is discussed along the whole processing line. The demand for NdFeB magnets in motor applications is growing and is expected to increase in the coming years. In particular, the demand for e-bikes and e-vehicles is gaining importance (Kampker A. et al. 2021; Pollák F. 2021; Flores P.J 2021). As a result, the demand for heavy rare earths will increase, necessitating the development of recycling systems for these materials, where this thesis is one basic concept to close the loop
Materials and processes for 3D printed electronics
Dissertação de mestrado integrado em Engenharia de MateriaisThe traditional manufacturing of electronic components consists of complex and with
high environmental impact methods. Those materials are potentially dangerous for both
environmental and public health, during the manufacturing process and at the end of the
product lifetime when not correctly handled. Thus, the goal consists of producing in a
simpler/cheaper way and with lower environmental impact, materials to be used into
electronic components. In this work inks based of a natural polymer (carrageenan) and
ultrapure water (a “green” solvent) were used to produce more environmentally friendly
printable electronic components.
To achieve magnetic, conductive and dielectric properties CoFe2O4 (CFO), multiwalled
carbon nanotubes (MWCNTs) and BaTiO3 (BTO) nanoparticles were added, respectively. To
promote a better dispersion and, therefore, to improve the properties of the final product,
Triton X-100 was used as a surfactant. Trition X-100 was selected among other surfactants,
since it has shown better results on initial selection tests. For the printing process, the most
suitable parameters were selected according to the ink viscosity to improve the process as
well as to optimize the method to introduce the ink into the syringe. Morphological, thermal,
and mechanical tests were performed in order to evaluate the effects of fillers addition and
concentration. Dielectric tests were carried out to the samples with BTO. The higher dielectric
constant has been obtained for the sample with 20 wt.% BTO content, reaching 1.3 x 104 at
10 kHz. The electrical conductivity evaluation in the samples with MWCNTs shows that a DC
conductivity of 0.026 S.m-1
is achieved for the sample with 5 wt.% MWCTNs content. Vibrating
sample magnetometer (VSM) test was performed to analyse the magnetic behaviour of the
composite samples with CFO, a saturation magnetization of 11 emu.g-1 being obtained for the
samples with 20 wt.% CFO content. The inks developed on this work highlights the relevance
of the implementation of natural materials as a base for the development of functional and
multifunctional materials. Adding to that, this work can also act as an incentive to the study
of materials and manufacturing procedures with lower environmental risks with the capacity
of still answering society’s needs.A manufatura tradicional de componentes eletrónicos consiste em métodos
complexos, com elevado impacto ambiental, quer por gasto energético quer pelos materiais
usados que sĂŁo potencialmente nocivos para o ambiente e para a saĂşde pĂşblica, durante o
processo de fabrico e no final de vida do produto, quando nĂŁo corretamente processados.
Visto isto, o objetivo deste projeto consiste em produzir de um modo simples, com baixo custo
e com menor impacto ambiental materiais que possam ser usados em componentes
eletrĂłnicos. Assim, neste trabalho foram desenvolvidas tintas Ă base de um polĂmero natural
(carragenina) e água ultrapura (usada como solvente “verde”) para produzir componentes
eletrĂłnicos impressos mais amigos do ambiente.
De modo a fornecer propriedades magnéticas, condutivas e dielétricas foram
adicionadas nanopartĂculas de CoFe2O4 (CFO), Multicamadas de Nanotubos de Carbono
(MWCNTs) e BaTiO3 (BTO), respetivamente. Para promover uma melhor dispersĂŁo foi usado
Triton X-100 como surfactante. No processo de impressão foram estudados os parâmetros
mais adequados de acordo com a viscosidade da tinta para tornar o processo mais rentável
assim como tentar encontrar o melhor método para introduzir a tinta dentro da seringa com
a menor formação de bolhas possĂvel. Os testes morfolĂłgicos, tĂ©rmicos e mecânicos foram
feitos para todas as amostras para comparar as propriedades fornecidas pela adição das
partĂculas, avaliando a sua interferĂŞncia com o aumento da concentração de filler. Os testes
dielétricos foram realizados para as amostras de BTO. A constante dielétrica com valor mais
elevado foi obtido para a amostra com concentração de 20 wt.% BTO, atingindo 1.3 x 104 a 10
kHz. A avaliação dos testes de condutividade elétrica nas amostras de MWCNTs, mostraram
uma condutividade DC de 0.026 S.m-1
foi obtida para a mostra com concentração de 5 wt.%
MWCTNs. O teste de mapeamento de fluxo de valor (VSM) foi realizado para analisar o
comportamento magnĂ©tico do compĂłsito com partĂculas de CFO, a magnetização de
saturação de 11 emu.g-1
foi obtida para a amostra com concentração de 20 wt.% CFO . As
tintas desenvolvidas neste trabalho veem dar relevância à implementação de materiais
naturais como base para o desenvolvimento de materiais funcionais e multifuncionais. Vem
também promover o estudo de materiais e métodos de produção com menos impacto
ambiental e que consigam manter a resposta Ă s necessidades da sociedade
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