Programmable magnetic attractive materials: an approach for shape-changing cars' interiors

Programmable

magnetic attractive materials: an

approach for

shape-changing cars’ interiors

João Oliveira da Silva

Mestrado em Engenharia Física

Departamento de Física e Astronomia 2022

Orientador

Prof. Doutor André Pereira, Faculdade de Ciências da Universidade do Porto

Supervisor

Luís Silva, TMG Automotive

Todas as correções determinadas pelo júri, e só essas, foram efetuadas.

O Presidente do Júri,

Porto, / /

U NIVERSIDADE DO P ORTO

M

T

Programmable magnetic attractive materials:

an approach for shape-changing cars’

interiors

Author:

Jo˜ao S^ILVA

Supervisor:

Prof. Doctor Andr´e P^EREIRA

Supervisor:

Lu´ıs S^ILVA

A thesis submitted in fulfilment of the requirements for the degree of MSc. Engineering Physics

at the

Faculdade de Ciˆencias da Universidade do Porto Departamento de F´ısica e Astronomia

December 12, 2022

Sworn Statement

I, Jo˜ao Oliveira da Silva, enrolled in the Master Degree of Engineering Physics at the Faculty of Sciences of the University of Porto hereby declare, in accordance with the pro- visions of paragraph a) of Article 14 of the Code of Ethical Conduct of the University of Porto, that the content of this internship report reflects perspectives, research work and my own interpretations at the time of its submission.

By submitting this internship report, I also declare that it contains the results of my own research work and contributions that have not been previously submitted to this or any other institution.

I further declare that all references to other authors fully comply with the rules of attribution and are referenced in the text by citation and identified in the bibliographic references section. This internship report does not include any content whose reproduc- tion is protected by copyright laws.

I am aware that the practice of plagiarism and self-plagiarism constitute a form of academic offense.

Jo˜ao Silva October 12, 2022

“ When I went to the trial to play for Bar¸ca, my father told me: ’If you come back and they haven’t selected you because they had 20 better players than you, no problem. If they don’t select you because they had 20 guys more determined than you, don’t even come home’. ”

Carles Puyol, ex-player and captain of FC Barcelona

Acknowledgements

First of all, I would like to acknowledge and thank so very much both my supervisors, Professor Andr´e Pereira and Lu´ıs Silva, who made it possible for me to find a Thesis that I loved from the beginning, which combined both the research environment of the University and the application level of a company like TMG automotive. Professor Andr´e pushed me to another level in every aspect of the work, always believing that I would be capable of pulling off every lunatic idea I had and even adding crazy thoughts of their own most of the time. On the other hand, Lu´ıs Silva was also fantastic while dealing with two people that had crazy ideas, being always present to discuss any question and allowing the performance of every procedure related to the company, making the project have a purpose in the interests of TMG. The support from both was something that I immensely appreciated since it allowed me to have the freedom to pursue my plans and learn directly from the mistakes and victories I had. Additionally, I would like to give a word to Ana and Mariana from IFIMUP and Z´e Pedro from TMG, with whom I worked during these months, that helped the development of important parts of the work.

Afterward, I would like to immensely thank my family, with an emphasis on my par- ents and brother, who dealt with me during these five years, in specific this latter one. I received nothing but support and strength from them, even though I left soon and arrived late at home, being a lot of the time tired and not as active as I would like. They are the main responsible for everything. Thank you.

Following my family comes my close friends, Andreia, Miguel, Maria, Jo˜ao, Nuno, Trov˜ao, Afonso, and Francisca from Puats, who were my daily support throughout these five years and were there to help study, lunch at 12:00h, lay on Matematica’ gardens, drink beers, and simply be there whenever needed. In addition, I also need to thank everyone present during my time in Tuna de Ciˆencias do Porto, where I was constantly placed out of my comfort zone. Throughout that, I learned so much stuff that I would not even think about, met so many incredible people that will remain close even after these times, and had memorable nights/stories that I will not forget. Last but not least, my friends from my home town, Andreia, Pereira, Indi, Z´e, and J´essica from Sombrero nº18, with whom I traveled and visited a lot, allowing me to experience and think about completely different worlds from the usual at FCUP and kept me enjoying the life outside Porto.

Finally, I would like to thank the past me, that dealt through five years of engineering physics always with the mindset that it was the correct path and, in the end, it would be the correct one, rewarding all the effort put in.

Congratulations, you were right.

Best regards,

From your almost Master in Science, Jo˜ao Silva

UNIVERSIDADE DO PORTO

Abstract

Faculdade de Ciˆencias da Universidade do Porto Departamento de F´ısica e Astronomia

MSc. Engineering Physics

Programmable magnetic attractive materials: an approach for shape-changing cars’

interiors

byJo˜ao SILVA

For a long time, the Planet’s health was not a concern for most Humans, which changed with the advancement of the problems related to global warming, leading to an immense investment into renewable energy sources and the evolution of components powered by fuel fossils, such as cars. This revolution in the automobile industry, with the fashion change on hybrid and fully electric vehicles, conducted a substantial upgrade in the cars’

interior. The incorporation of more dynamic features with sensors and actuators em- bedded in the seats or dashboards enabled the fashioning of a more comfortable and interactive concept of what once was an overlooked component. Yet, from the multiple multi-functional elements already existent, there are still close to no attempts at changing and varying structural characteristics like the shape of interior panels that remain con- stant throughout the cars’ existence. Hence, a groundbreaking shape-changing material, controlled by magnetic mechanisms, for cars’ interior application was studied, in terms of viability, to redefine the entire interior concept and resulted in the production of three initial prototypes.

The work was divided into two primary components: magnetic field generators, which would cause the programmable deformations, and magnetic samples whose shape would vary. Starting from the ideal choice until the less favorites, planar coils, regular coils, per- manent magnets, and a combination of magnets and regular coils were tested with mixed results in the main characteristics presented, leading to the last option being the more re- liable. Following that, the study entered into the magnetic sample production and charac- terization with an initial focus on theMnFe₂O₄magnetic nanoparticles (M_sat= 63emu/g

and diameter of 10.19nm), which would be embedded into the cars’ interior surface ma- terial’ lacquer or base paste, with a thorough analysis of different fabricated samples af- terward. Results showed a successful embedment of the nanoparticles without property loss. Yet, integrity problems appeared with the increment of nanoparticles in the material, reaching a critical point for samples with 13wt%. Nonetheless, those specimens attained a substantial magnetic behavior with a saturation magnetization of 6.1emu/g.

After a successful proof of concept using an oscillating permanent magnet system, three prototypes were idealized and produced. With the studies performed as a basis, two prototypes using the combo coil+magnet were attempted, powered by a waveform generator and an Arduino. The first concept created an ideal shape deformation yet could not be programmed and would hardly be embedded in a real car. Contrarily, the second prototype allowed programming and presented as an easily embedded solution, but the output power of the Arduino was insufficient for a visible deformation. Due to those mid-results, a mechanical rotating magnet system with a size capable of being embedded in a car was developed and presented promising results, enabling the visualization of continuous shape variation in the material.

UNIVERSIDADE DO PORTO

Resumo

Faculdade de Ciˆencias da Universidade do Porto Departamento de F´ısica e Astronomia

Mestrado em Engenharia F´ısica

Materiais magnetico-atrativos program´aveis: uma idealiza¸c˜ao de revestimentos de interior autom ´ovel de forma vari´avel

porJo˜ao SILVA

Durante bastante tempo, a vida e sa ´ude do Planeta foi ignorada pela grande maio- ria dos Humanos, algo que mudou consideravelmente com o aumento dos problemas relacionados com o aquecimento global, criando um investimento imenso em energias renov´aveis e na inovac¸˜ao de componentes outrora dependentes de combust´ıveis f ´osseis, tal como carros. Esta revoluc¸˜ao na ind ´ustria autom ´ovel, com o aparecimento da mudanc¸a est´etica associada aos autom ´oveis h´ıbridos e el´ectricos, levou a um grande melhoramento nos interiores dos carros. A incorporac¸˜ao de mecanismos dinˆamicos com sensores e ac- tuadores embutidos nos assentos e pain´eis dotablier permite a criac¸˜ao de um conceito mais confort´avel e interactivo do que antes era uma componente do carro desprezada.

Contudo, dentro dos in ´umeros elementos multifuncionais j´a presentes, n˜ao existe prati- camente nenhuma tentativa de variar ou mudar caracter´ısticas estruturais como a forma dos revestimentos interiores, que se mantˆem constantes desde sempre. Assim sendo, um revestimento com forma vari´avel totalmente inovador, controlado por mecanismos magn´eticos, para aplicac¸˜ao em interior de autom ´oveis foi estudado, em termos de viabi- lidade, de modo a permitir uma total redefinic¸˜ao do conceito do interior de um ve´ıculo, resultando em trˆes prot ´otipos iniciais.

O trabalho efetuado foi dividido em duas componentes principais: geradores de campo magn´etico, respons´aveis por causar as deformac¸ ˜oes program´aveis, e amostras magn´eticas que teriam de variar a sua forma. Comec¸ando pela opc¸˜ao ideal at´e `a menos favorita, bo- bines planares, bobines normais, ´ımanes permanentes e uma combinac¸˜ao de ´ıman com

uma bobine regular foram testadas com resultados variados nas caracter´ısticas apresen- tadas, sendo que a ´ultima opc¸˜ao se demonstrou como a mais vi´avel. Seguidamente, o es- tudo entrou na produc¸˜ao das amostras de revestimento magn´eticas e a sua caracterizac¸˜ao, com um foco inicial nas nanopart´ıculas magn´eticas de MnFe2O₄(Msat = 63emu/ge um diˆametro 10.19nm), que seriam depois embutidas na laca ou pasta base dos revestimentos de interior autom ´ovel, e posteriormente uma an´alise profunda das diferentes amostras fa- bricadas. Os resultados demonstraram sucesso na inclus˜ao das nanopart´ıculas sem perda das suas propriedades. Por´em, alguns problemas de integridade do revestimento apare- ceram com o aumento das nanopart´ıculas no material, atingindo um ponto critico para as amostras com uma concentrac¸˜ao de 13wt%. N˜ao obstante, essas amostras apresentaram um comportamento magn´etico substancial, com uma magnetizac¸˜ao de saturac¸˜ao de 6.1 emu/g.

Depois de uma prova de conceito de sucesso usando um sistema de ´ımanes permanen- tes a oscilar, trˆes prot ´otipos foram idealizados e produzidos. Tendo por base os estudos realizados, foram testados dois prot ´otipos usando a combinac¸˜ao bobine+´ıman, controla- dos por um gerador de sinais e um Arduino. O primeiro conceito criava uma deformac¸˜ao ideal mas imposs´ıvel de ser programada, sendo o sistema dif´ıcil de embutir. Contraria- mente, o segundo prot ´otipo permitia que houvesse programac¸˜ao e apresentava-se como uma soluc¸˜ao facilmente inclu´ıda num carro, mas o seu outputera insuficiente para criar uma deformac¸˜ao vis´ıvel. Devido a estes resultados pouco conseguidos, foi desenvolvido um sistema mecˆanico rotativo com tamanho para ser embutido num carro usando ´ımanes permanentes que apresentou resultados promissores, permitindo a visualizac¸˜ao de uma deformac¸˜ao cont´ınua da forma do revestimento.

Contents

Sworn Statement iii

Acknowledgements vii

Abstract ix

Resumo xi

Contents xiii

List of Figures xv

List of Tables xix

Glossary xxi

1 Motivation and outline 1

2 Introduction 5

2.1 Multi-functional materials . . . 5

2.1.1 Multi-functional materials in cars . . . 6

2.1.2 Shape changing material applied in cars. . . 8

2.2 Magnetic materials . . . 9

2.2.1 Magnetic field generators . . . 12

2.2.1.1 Permanent magnets . . . 12

2.2.1.2 Electromagnets. . . 13

2.2.1.3 Planar coils . . . 13

2.2.2 Magnetic nanoparticles . . . 15

3 Fabrication and characterization techniques 17 3.1 Production methods . . . 17

3.1.1 Co-precipitation. . . 17

3.1.2 PVC paste and lacquer production and application . . . 18

3.1.3 Screen printing . . . 19

3.2 Characterization methods . . . 20

3.2.1 X-Ray diffraction . . . 20

xiii

xiv

PROGRAMMABLE MAGNETIC ATTRACTIVE MATERIALS: AN APPROACH FOR SHAPE-CHANGING CARS’INTERIORS

3.2.2 SQUID . . . 22

3.3 TMG quality tests . . . 23

3.3.1 Thermal stabilization. . . 23

3.3.2 Gelification . . . 23

4 Magnetic field generators 25 4.1 Planar coils . . . 25

4.1.1 Substrate choice: PET vs Kapton . . . 27

4.1.2 Design choice and evaluation . . . 29

4.2 Coils . . . 32

4.3 Permanent magnets . . . 33

4.4 Coil + permanent magnet . . . 35

5 Interior cars’ materials embedded with magnetic elements 37 5.1 Magnetic nanoparticles characterization . . . 38

5.1.1 X-Ray diffraction . . . 38

5.1.2 SQUID . . . 40

5.2 Magnetic cars’ interior samples production and analysis . . . 41

5.3 Verification of properties and functionality . . . 44

5.3.1 Basic cars’ interior material properties . . . 44

5.3.1.1 Rigidity and uniformity. . . 44

5.3.1.2 Thermal stabilization . . . 47

5.3.1.3 Gelification . . . 49

5.3.2 X-Ray diffraction analysis . . . 50

5.3.3 SQUID analysis . . . 51

5.3.3.1 Samples with PVC without MNPs. . . 51

5.3.3.2 Samples with PVC with MNPs. . . 55

5.3.4 Magnetic attraction . . . 57

5.4 Proof of concept . . . 58

6 Multi-functional cars’ interior materials prototypes 61 6.1 Waveform generator with coil+magnet prototype . . . 61

6.2 Arduino-based with single coil prototype . . . 63

6.2.1 OP07 OpAmp . . . 64

6.2.2 Low-noise Amplifier . . . 65

6.3 Rotating permanent magnets . . . 67

7 Conclusions and future work 69

A Additional figures, tables and graphs 73

B Arduino scripts for prototypes experimentation 79

Bibliography 81

List of Figures

1.1 Schematic of the shape-changing lining system concept with the magnetic material on top of the magnetic field generators, which are connected to a programmable power source . . . 2 2.1 Two types of vital signs sensors placed in different places of a car to obtain

data from the driver. . . 6 2.2 Continental prototype of embedded speakers into the dashboard (Retrieved

and adapted from [13]) . . . 7 2.3 Two prototypes of Exoskin developed in MIT . . . 8 2.4 Texture wrinkle system based on cephalopods natural behavior. (Retrieved

and adapted from [17]) . . . 9 2.5 Hysteresis loop graphs for various types of magnetic behaviors. (Retrieved

and adapted from [23]) . . . 11 2.6 Different types of planar coils designs: (a) spiral, (b) mesh, (c) meander, (d)

square. (Retrieved and adapted from [40]). . . 14 2.7 Schematic of the nanoparticles energies variation with the size implication

on the magnetic moments flip. (Retrieved and adapted from [51]) . . . 16 3.1 Process diagram ofMnFe₂O₄MNPs . . . 18 3.2 Process diagram of TMG’s magnetic cars’ interior samples . . . 19 3.3 Steps of screen printing fabrication: (a) ink paste is placed on the extrem-

ity of the mesh, (b) squeegee spreads the ink through the entire pattern, (c) pressured spread of the ink with the squeegee, (d) substrate with the pattern printed. (Retrieved and adapted from [6]) . . . 20 3.4 Schematic of an X-Ray diffraction using θ−θ geometry. (Retrieved and

adapted from [58]) . . . 21 3.5 Schematic of the interior of SQUID. (Retrieved and adapted from [60]) . . . 22 4.1 Screen printing mesh (L2) on the left with the respective designs on Auto-

CAD on the right. The mesh possesses sixteen spiral coils, four mesh-type and four squared-spiral, all with different parameters . . . 26 4.2 PET deformation when applied current . . . 28 4.3 Silver ink lines destruction when high temperatures were applied(T =₂₄₆^◦C) 30 4.4 Screw and nut connection for double layer planar coil (front and back) . . . 31 4.5 Images of both coils tested as variable magnetic field generators . . . 33 4.6 Variation of the magnet position with an applied sinusoidal wave function

with a frequency of 10Hzand an amplitude of 15V_pp . . . 36 5.1 Container of 15mLwith approximately 6gofMnFe₂O₄nanoparticles powder 38

xv

xvi

PROGRAMMABLE MAGNETIC ATTRACTIVE MATERIALS: AN APPROACH FOR SHAPE-CHANGING CARS’INTERIORS

5.2 X ray spectrum of MnFe₂O₄nanoparticles . . . 39 5.3 Magnetization dependency on the external magnetic field for theMnFe₂O₄

nanoparticles . . . 40 5.4 Evolution of scratches present in the samples with the increase of the NPs

in the PVC paste. Materials with 2 and 4 wt% have much fewer failures, but in the case of 13wt%, the scratches removed the integrity of the sample 45 5.5 A comparison between the influence of multiple PVC paste application

strokes, with a knife cleaning in between, shows a clear advantage for the multiple strokes approach in terms of sample integrity. . . 46 5.6 Evolution of samples 1, 2, 4, 5, 7, 8, 9, and 10 with the time after being

exposed at a constant temperature of 200^◦C. . . 47 5.7 The procedure of gelification test using 20mLof acetone in samples of 3x2

cmof S1 to S5 (from left to right) . . . 49 5.8 X ray spectrum of the ten samples fabricated from S10 to S1, in downwards

order . . . 50 5.9 Magnetization dependency on the magnetic field of samples without NPs

on PVC paste: with base top coat lacquer (S1) and with top coat lacquer with 1.8 wt% of NPs (S2) . . . 52 5.10 Focused hysteresis loop for the two parts with negative linear behavior. . . 53 5.11 Focused hysteresis loop for the middle part with the diamagnetic compo-

nent extracted . . . 54 5.12 Magnetization dependence on magnetic field of samples with NPs on PVC

paste . . . 55 5.13 Schematic of the oscillating magnet setup. A claw attached to support, with

a permanent magnet placed on top, is moved up and down to create a magnetic interaction with the magnetic material placed and clenched on top of it . . . 58 5.14 Deformation obtained by approximation of a permanent magnet on sample

9 at a distance near 1mm . . . 59 6.1 Magnetic sample deformation obtained in the prototype with a single coil

with a permanent magnet inside . . . 62 6.2 Schematic of the hardware used from the Arduino-based prototype, where

an Arduino, programmed by a computer, emits an analog signal through port 4 that is amplified by an OpAmp by a factor of 1+ ^R_R²

1, powering the coil with the magnet inside . . . 64 6.3 Signals detected by the oscilloscope after the Arduino output (yellow) and

after the OP07 OpAmp application (blue) for without (A) and with (B) the coil present in the circuit . . . 65 6.4 Signals detected by the oscilloscope after the Arduino output (yellow) and

after the low-noise amplifier application (blue) for without (A) and with (B) the coil present in the circuit . . . 66 6.5 Magnetic sample deformation obtained in the Arduino-based prototype . . 66 6.6 Schematic of the experimental setup used on the rotating permanent mag-

nets prototype . . . 67 6.7 Magnetic sample deformation obtained in the rotating permanent magnets

prototype . . . 68 A.1 L1 mesh used for current withstand tests on PET and Kapton substrates . . 74

LIST OFFIGURES xvii

A.2 QR code for a demonstration video of the oscillating movement of the mag- net inside the coil . . . 75 A.3 QR code for the demonstration video of oscillating magnets proof of concept 76 A.4 QR codes for the demonstration video of the two prototypes with the coil . 76 A.5 QR code for a demonstration video of the Rotating magnets prototype . . . 76 A.6 QR code for the oscilloscope videos with and without the coils presence in

the circuit. . . 77

List of Tables

2.1 Magnetic moment alignment on each magnetic behavior without and with an external magnetic field applied . . . 10 4.1 Resistance values of L2 mesh designs using a commercial silver ink . . . 27 4.2 Permanent available and tested with their characteristics and indicative im-

age . . . 34 5.1 List of samples fabricated with its identification number, the weight per-

centage of NPs in PVC paste, presence of lacquer and weight percentage of NPs in it, total thickness (PVC paste + lacquers), and a representative image 42 5.2 Mass susceptibility for PVC without NPs samples (S1 and S2) for regions 1

and 3 . . . 53 5.3 Superparamagnetic hysteresis loop saturation magnetization values for sam-

ples with NPs on PVC (S3-S10) . . . 56 5.4 Magnetic samples capability of being attracted by two different magnets

M3 and M4 at distances of 1, 2, and 3mm . . . 57 A.1 L2 mesh spiral coil design parameters: coil number, internal diameter, ex-

ternal diameter, number of turns, spacing between lines, width of line, in- ternal contact diameter, external contact dimensions . . . 73 A.2 L2 mesh mesh-type coil design parameters: coil number, wave height, num-

ber of waves by column, number of columns, spacing between columns, width of line, contacts dimensions . . . 74 A.3 L2 squared-spiral coil design parameters: coil number, internal square size,

external square size, number of turns, spacing between lines, width of line, internal contact square size, external contact dimensions . . . 74 A.4 Superparamagnetic hysteresis loop characteristic values forMnFe₂O₄nanopar-

ticles powder . . . 75 A.5 Superparamagnetic hysteresis loop characteristic values for samples with

NPs on PVC (S3-S10) . . . 75

xix

Glossary

NPs Nanoparticles

MNPs Magnetic Nanoparticles

MEMS Micro-electro-mechanical systems PVC Polyvinyl chloride

PET Polyethylene terephthalate XRD X-Ray Diffraction

OpAmp Operational Amplifier DAC Digital to Analog converter

xxi

Chapter 1

Motivation and outline

Cars were initially fabricated in 1886, and since then, their development has never stopped, always intending to be a viable, safe, and comfortable way of transportation for those who own them. Nonetheless, in recent years the human acknowledgment of pollution related to fossil fuels raised massively, which led to a revolution that boosted the develop- ment of hybrid and fully electric cars. Consequently, all the technology related to vehicles evolved, appearing various promising ideas for the future in different areas, from design to performance, with some interesting approaches to occupant-car interactions. A fasci- nating proposal is the autonomous car, yet this technological leap is not ready as of now, but the incorporated features within the vehicles will increasingly reduce the drivers’ im- pact. In this regard, it is crucial to think ahead and understand that, without a driver, the car ride resumes itself to a closed space with up to five people seated, thus, a self-driven living room.

In fact, the interior of cars is one of the areas that will suffer a significant transforma- tion, giving rise to futuristic architecture with much cleaner and simple designs. Nonethe- less, the elements used to produce these interiors will be much more complex than ordi- nary materials since they will be mainly multi-functional. This improvement can be ob- tained by embedding electronic components into the textiles and surface linings, giving the final product distinct properties that will enhance the quality and experience of a car occupant. With this change, the cars’ inside becomes much more interactive and enjoy- able, crucial for future vehicles with high degrees of autonomy. Hence, texture or shape- changing materials appear as an exceptional asset for every car interior with multiple applications as system indicators of failures or warnings, as dynamic surfaces controlled by the music or the user, or purely decorative, enabling the creation of different ambients

1

2

PROGRAMMABLE MAGNETIC ATTRACTIVE MATERIALS: AN APPROACH FOR SHAPE-CHANGING CARS’INTERIORS

inside the car. These surfaces could be parts of seats, dashboards, door panels, or even the steering wheel that are initially smooth and then gain a certain roughness, possibly with some patterns, monitored by the user.

In this dissertation, a groundbreaking approach to a shape-changing material for cars’

interior applications is developed, inspired by an animal’s electrical behavior, where mag- netic field generators are used to pull and push sample materials embedded with mag- netic nanoparticles. The schematic presented in figure 1.1 indicates the positioning of both principal features, where the magnetic field generators, represented as orange coils, are placed underneath the cars’ interior material, embedded with magnetic nanoparti- cles, illustrated as a dark gray rectangle oscillating. That way, when the generators are activated by a controlled power source (bottom object in the figure), the material is pulled and pushed with a designated frequency and amplitude, creating a dynamic effect. The magnetic approach allows for a novel concept and contactless surface control, an advan- tage for embedding the new system in an already complex one but gives the project no theoretical background. Additionally, the overall idea of varying the material’s shape for this specific application has little to no reports, originating numerous unanswered ques- tions concerning viability and production, making the project innovative in every aspect possible. Therefore, every step taken during the project was novel and developed without previous work, serving as the base for future developments or related research.

FIGURE1.1: Schematic of the shape-changing lining system concept with the magnetic material on top of the magnetic field generators, which are connected to a programmable

power source

Thus, the dissertation starts with Chapter2, which introduces state-of-art multi-functional materials, ranging from general to smart materials already present in recent cars, with an

1. MOTIVATION AND OUTLINE 3

additional magnetic background focused on the generators and nanoparticles.

Chapter3presents all of the experimental techniques and methods used in nanopar- ticle production and the characterization of magnetic samples and magnetic field genera- tors, whose results will be analyzed in the following chapters.

As for Chapter4, a thorough study of various magnetic field generators is performed with the experimental results and comparisons presented to find the optimum solutions for distinct types of prototypes assembled posteriorly.

For Chapter5, the focus of the analysis turns firstly to the nanoparticles produced and then to the multiple magnetic samples to understand the overall nanoparticle impact on the final structure by studying their crystallographic and magnetic behavior. In the end, the first proof of concept of the initial idea is attempted using an oscillating high-intensity magnet system.

After this inquiry, three different prototypes were made and tested, showing some well-grounded perspectives for application in actual car interior, as presented in Chap- ter6. Finally, Chapter 7 approaches the future work in concept development with the envision of arriving at industrial production, followed by some final remarks.

Chapter 2

Introduction

2.1 Multi-functional materials

In the late 90s, when the technology revolution took place, smart materials appeared and were defined as objects that can sense and respond to environmental changes in a con- figurable manner [1, 2]. This fact created a lot of interest in different fields due to the immense possibility of applications. Specifically, it is possible to divide them into three categories: sensors, actuators, and others.

Sensors are materials capable of detecting when an outsider stimulus is applied, like a variation of strain/stress or temperature, damage, or peak stimulus during a constant one [3]. Fields like biology and medicine were immensely impacted by this innovation, allowing the creation of simpler systems to detect and monitor parameters of the patient’s health. The acquisition can be obtained by inserting, for example, piezoelectric or capac- itive components in the original material, capable of stating the variation felt [2,3]. On the other hand, actuators are systems that respond directly to external interaction, being the shape memory alloys (SMA) [4] one of the most proficient examples. These materials vary in shape depending on the temperature felt, being applied in thermostats [5]. Finally, the other ones are an aggregate of more complex devices that possess properties of both sensors and actuators being used with a specific application like energy harvesting [3].

Nevertheless, integrating these functionalities into existing materials is still a chal- lenge in multiple cases, demanding the creation of novel ways of producing fundamental components such as electric wires. Printing techniques [6] and microelectromechanical systems (MEMS) [7] were crucial for multi-functional materials development, originating in various places dedicated research teams to these subjects.

5

6

PROGRAMMABLE MAGNETIC ATTRACTIVE MATERIALS: AN APPROACH FOR SHAPE-CHANGING CARS’INTERIORS

2.1.1 Multi-functional materials in cars

Cars’ interior future will be drastically different from the present, with a much more so- phisticated environment being created where each passenger can have an almost dedi- cated and personalized space to himself. Consequently, the car’s interior must be more interactive with the occupant (sensing and reacting), have clean and simple lines, possess storage, and be comfortable. Some of these upgrades require electronic devices, making the whole idea difficult unless those gadgets are embedded into elements already present.

That would mean turning fabrics, textiles, and stiff linings into multi-functional objects.

Nowadays, most cars possess their interior covered with artificial materials that replicate finely natural leathers more cheaply and sustainably, allowing more degrees of freedom during functional system production. Usually, the car’s interior is composed of three lay- ers of Polyvinyl chloride (PVC) paste- compact, foam, and adhesive- followed by at least two layers of lacquer- base and top coat. The PVC adhesive paste assures adhesion be- tween the PVC foam and the textile, which is placed underneath everything, making the whole system easier to handle. Afterward comes the foam layer giving the height and cushioning, followed by the compact layer that has the main desired properties of the final material. Finally, the lacquer system is applied on top of the PVC layers, adding chemical and abrasion characteristics to the final material.

At the moment, some cars’ interiors already function as vital signs analyzers (ECG, respiration rate, temperature, etc.) used to prevent not fit drivers from getting on the road. They can be placed in multiple spots, such as the back of the seat (figure 2.1(A)), security belt, or steering wheel (figure2.1(B)) [8–10].

(A) ECG prototype applied on a seat’s back.

(Retrieved and adapted from [8])

(B) Various vital signs sensor on a steering wheel.

(Retrieved and adapted from [8]) FIGURE2.1: Two types of vital signs sensors placed in different places of a car to obtain

data from the driver

2. INTRODUCTION 7

Sensors are connected to analysis systems embedded in the car that process the data and display it to the driver, giving it the chance to warn him in case of unwanted be- havior in his vital signs. Additionally, considering that the interior could also be a living room, light becomes crucial in warm and cozy environment creation. There are fabrics with embedded luminous fibers that can be incorporated into seats, roofs, floors, doors, etc., generating the desired atmosphere [8,9]. Nowadays, some seats also have pressure sensors to indicate passenger location and focus the light in response to that [8], crucial to allow people to read or work while in a passenger’s position. Moreover, it is possible to use those pressure sensors as buttons to open and close windows or, for more complex applications, as interfaces to control music on each seat or steering wheel [11]. This stored data can contain the specific needs and wants of each occupant in that seat, turning the ex- perience more interactive and comfortable for the user. The feeling of warmth is another essential point in comfort, and embedded seat warmers, controllable by the user, are al- ready a reality [8]. In fact, one of the most advanced companies in that topic is LOOMIA, who has created a fully embedded electronic flexible textile capable of heating, sensing touch, and more [12]. This technology shows the viability of having, in a few years, these complex textiles or other materials in cars as operational devices.

Eventually, actuators will become more prominent, appearing in various forms associ- ated with multiple sensors. Those sensors range from humidity and strain to UV radiation or light intensity. As for actuators, cooling systems, diverse alert signals, flexible screens, and displays, security systems actions will certainly emerge [4]. Faurecia and Continental are part of the innovative and development companies in this field, proposing different designs and amenities like embedded sound system on the fabric (figure 2.2) [13, 14], hidden buttons [15], and seat adjustment for different occasions [14].

FIGURE2.2: Continental prototype of embedded speakers into the dashboard (Retrieved and adapted from [13])

8

PROGRAMMABLE MAGNETIC ATTRACTIVE MATERIALS: AN APPROACH FOR SHAPE-CHANGING CARS’INTERIORS

2.1.2 Shape changing material applied in cars

When a passenger is inside a car, everything seems static around him, even though he is moving. In particular, the driver is the occupant with the most access to the dashboard’s dynamic features. Turning the car’s interior into a more vivid space could be attained by making the linings and fabrics change their shape, wrinkle, and extend, granting them the capability of being responsive to external stimuli. This actuator capacity could originate an innovative conceptualization of dynamic interior designs, which may lead to novel ways of displaying visual signs to the occupants, such as door locked/open, ventilation on/off, lights on/off, etc. For instance, a steering wheel with a fabric capable of changing its roughness depending on the velocity of the car or the climate conditions would allow an improved grip in more delicate situations. With a more complex electronic system, it could be possible to create a steering wheel that would indicate, by changing the texture in specific locations, the existence and relative position of the adjacent cars. From a more decorative perspective, fabric linings capable of wrinkling with different amplitudes and frequencies would be a great asset to creating an immersive interior that would deform in response to the music played.

This idea was tested in an MIT masters’ thesis, giving birth to Exoskin [16], where ma- terials, divided into small finite blocks, get pushed or pulled by a pneumatic mechanism like shown in figure2.3(A). The ultimate goal was to apply the system to a steering wheel (figure2.3(B)) or the top of the dashboard, creating a dynamic environment for every car occupant. Even though the results are extremely promising and innovative, the system complexity associated with the multiple pneumatic and space demands will be a substan- tial barrier to cross. Moreover, the moving parts must constantly be in direct contact with the surface, which adds an extra constraint to its embedment in compact space.

(A) Exoskin basic system. (Retrieved and adapted from

[16]) (B) Exoskin applied to a steering wheel. (Retrieved and adapted from [16])

FIGURE2.3: Two prototypes of Exoskin developed in MIT

2. INTRODUCTION 9

Yet, changes in shape/texture are already present in nature, occurring in some ani- mals that use it to blend within objects or to vary their surface area. For example, some cephalopods (molluscans class containing squids and octopus) can control their texture and color by sending electric signals through their nervous system, wrinkling their sur- face. Based on that, Wang et al, 2014 [17] developed work to mimic this phenomenon with elastomer films, where an external power source would be responsible for wrin- kle formation, as shown in figure2.4. Their results showed promising opportunities for the phenomenon application to elastic and deformable surfaces in a programmable way without the need for external pneumatic systems, opening horizons for a simpler concept.

FIGURE2.4: Texture wrinkle system based on cephalopods natural behavior. (Retrieved and adapted from [17])

Modern cars, at the moment, possess artificial formulations of leather and other soft elements used in their interiors that can be additionally molded to become receptive to de- formation, opening doors for the proposed idea implementation. Yet, a concerning point is the feasibility of embedding all the electronic structures since the system could be ap- plied over large areas such as the dashboard or a door panel. This aspect is critical due to the high compaction of cars, yet microelectronics, like MEMS, are increasingly becoming more viable solutions for this type of application. Right now, the leading multi-functional materials are activated or sensed by electrical potentials or mechanical forces, with mag- netic structures barely being used even though they possess some advantages compared to the more common options, like their capability of acting without direct contact.

2.2 Magnetic materials

Magnetic materials were discovered thousands of years ago and have been used since then, in multiple applications, from needles in a compass for guidance gadgets to solid blocks that attach children’s paintings in the fridge. Study around these materials has al- ways been enormous, leading to the discovery of multiple types of objects that can attract

10

PROGRAMMABLE MAGNETIC ATTRACTIVE MATERIALS: AN APPROACH FOR SHAPE-CHANGING CARS’INTERIORS

and repel with various intensities. Those interactions can generally be described through Lorentz law (equation 2.1), assuming the electrical part negligible, where the magnetic force is dependent on the element’s charge,q, and its velocity,v, the magnetic permeabil- ity,µ, the magnetic field applied,H, and magnetization present,M.

⃗_F=qµ(⃗v×(_H⃗ +_M⃗)) (2.1) In fact, depending on their behavior, magnetic elements can be divided into five states:

ferromagnetic, ferrimagnetic, paramagnetic, diamagnetic, and superparamagnetic. These types of magnetic behavior are reigned by the orientation and correlation between the magnetic moments of the material, which depend on atoms’ electrons characteristics. Ta- ble2.1shows the magnetic moments’ relation with an external magnetic field application in every magnetic type enumerated.

TABLE 2.1: Magnetic moment alignment on each magnetic behavior without and with an external magnetic field applied

External magnetic field

Magnetic type H=0 H̸=0

Ferromagnetism

Ferrimagnetism

Diamagnetism

Paramagnetism

Superparamagnetism

Ferro and ferrimagnetism are the states where the materials’ magnetic moments tend to align in a direction during their formation. Nonetheless, their spontaneous alignment can be further enhanced by an external magnetic field application, leading to the creation of high Magnetization (M) values, which is the quantity that defines the density of mag- netic moments [18,19]. Their difference resides in ferromagnetic materials having all the

2. INTRODUCTION 11

moments aligned with equal direction and orientation. In the case of ferrimagnetism ma- terials, such as magnetite,Fe3O₄, it is common to have sublattices, leading to inversion of the orientation of some moments, but with less intensity [20].

Diamagnetism and paramagnetism, in contrast to the previous behaviors, do not dis- play a spontaneous alignment of the magnetic moments, leading to an overall null magne- tization without any field applied. Diamagnetism is present in every material but, due to being weak, is overruled when any other behavior contributes. This phenomenon leads to the non-existence of magnetic moments without an applied field. Yet, when the magnetic induction is present, the diamagnetic material is repelled by it [21]. In paramagnetism, the electrons are not paired, as in diamagnetism, allowing magnetic moment formation. Yet, their interaction is negligible, leading to random orientation. However, with the applica- tion of an external field, all the moments align with it, behaving as ferromagnetism [21].

Lastly, superparamagnetism is a behavior shown in reduced-sized materials, where the magnetic moment cannot maintain orientation for longer than a specific time, Neel relax- ation time [22]. Similar to paramagnetic materials, superparamagnetic elements also align their magnetic moment with the applied field. In the following subsection, nanoparticles and superparamagnetism will be thoroughly studied.

The total magnetization,M, and the applied magnetic field, H, relation is pivotal for a material’s magnetic behavior evaluation. Represented by a hysteresis loop, its correla- tion demonstrates the influence of the applied field in the magnetization of the material, therefore, the magnetic moments’ alignment. As explained, the align dependency varies with the type of behavior, as demonstrated by the various correlations presented in figure 2.5.

FIGURE2.5: Hysteresis loop graphs for various types of magnetic behaviors. (Retrieved and adapted from [23])

12

PROGRAMMABLE MAGNETIC ATTRACTIVE MATERIALS: AN APPROACH FOR SHAPE-CHANGING CARS’INTERIORS

Paramagnetic and diamagnetic possess a linear relation, marking their respective slope as characteristic properties. As shown in equation2.2, that quantity is designated mag- netic susceptibility, χ, which defines the material as attractive or repellent by the field, depending on its sign. For the other cases, the relation between magnetization and field is not linear, giving the susceptibility an extra dependency on the field. Therefore, for those behaviors, the characterization is made by extracting the highlighted points in the graph. The coercive field, H_C, represents the value needed to demagnetize the sample, while the saturation magnetization, Msat, indicates the maximum of the material’s mag- netization. Finally, remanence magnetization, M_R, is the magnetization remaining after the field removal from the object analyzed.

M⃗ =χ_H⃗ _(2.2)

2.2.1 Magnetic field generators

2.2.1.1 Permanent magnets

Certain materials in nature tend to have a spontaneous magnetization and increase it with an external field. After reaching saturation magnetization, some objects can retain those values even with high inverse magnetic fields applied, commonly called hard or permanent magnets. Contrarily, other materials demagnetize more easily, possessing a low coercive field, and are named soft magnets. Nonetheless, the main constituents of both types are ferromagnetic elements that can merge their magnetic domains to create bigger and oriented ones when a sufficiently high field is present. The larger the domains formed, the harder it is to demagnetize the material.

Permanent magnets are used in multiple daily objects like speakers, motors, or sen- sors and in more complex applications like MRI and treatment plants [24]. Their main advantages are the high-intensity magnetic fields produced, which are only dependent on the distance, and their compactness, enabling easy incorporation in static positions. Soft magnets, on their hand, are used for applications that need fast magnetization variation, such as memory disks [25]. Initially, magnets were composed of common ferromagnetic like iron or cobalt, but nowadays, various options are available with multiple complex alloys. This collection goes from ceramic composites, which are cheap but brittle [26,27], to rare-earth elements, which present very high magnetization values [27,28].

2. INTRODUCTION 13

2.2.1.2 Electromagnets

In the early 1800s, Hans Oersted discovered that magnetism and electricity were not sep- arated phenomena, enabling the production of the first electromagnets a few years later.

When a current passes through a conductive medium, such as a copper wire, the charge movement creates small magnetic fields that can be enhanced by winding the wire into a coil shape, forming a solenoid [29]. By doing that, the field becomes more uniform, allow- ing a higher magnetic interaction inside it. Additionally, an iron core placement within the solenoid also increases its intensity since the magnetic moments of the core will align with the produced field. That way, the solenoid and a ferromagnetic combination, two different magnetic generator types, lead to a high-intensity field production capable of having various applications worldwide.

Contrarily to permanent magnets, electromagnets are not restrained to a magnetic intensity, being capable of varying it or even turning it off. That control only depends on the current input chosen by the user. Nonetheless, to obtain enormous magnetic fields (tens of Tesla), like in some permanent magnets, the current flow through the solenoids must be high, commonly leading to heat dissipation. Yet, the needed current can differ depending on the wire characteristics. With a smaller thickness, the number of turns can be higher, which increases the field intensity. However, the wire length also increases, which causes a resistance enhancement. For a thicker wire, the inverse happens, with the number of turns and the resistance decreasing. Therefore, when producing a solenoid, a thorough study of its characteristics is demanded for an optimum choice in the wire properties and power supply to obtain the desired magnetic field intensity.

Due to the immense personalization availability of electromagnets, their uses range from very distinct application fields. Some take advantage of the electromagnet-permanent magnet combo to open and close circuits, like in a relay [30], or to modify the ferri- magnetic material’s orientation, as done in a magnetic storage device [25]. Furthermore, solenoids are also used in electric motors [31], magnetic lenses [32], or spectrometry sys- tems [33].

2.2.1.3 Planar coils

Magnetic MEMS present a novel way of creating electromagnetic interactions using a much smaller and compact system than the previously discussed examples. Moreover,

14

PROGRAMMABLE MAGNETIC ATTRACTIVE MATERIALS: AN APPROACH FOR SHAPE-CHANGING CARS’INTERIORS

when compared with electrostatic or piezoelectric MEMS, they demonstrate multiple ad- vantages, like strength and distance of actuation [7], and are already used in various re- search areas [34,35].

Planar coils can have different patterns and be produced with good resolution, us- ing various techniques, such as photolithography and sputtering [36], molding [7], screen printing [35], 3D printing [37], and PCB printing [34,38]. Nevertheless, the strength of the produced magnetic field depends on multiple factors like shape, thickness, spacing, the number of turns, material, etc., being studied by researchers with simulations, from sim- ple [37] to more complex ones [38,39]. Even though the most famous and use design for planar coils is the spiral (figure2.6(a)), there are other, appearing as alternatives without an internal contact like the mesh (figure2.6(b)) or the meander (figure2.6(c)), and with other shapes, like a square (figure2.6(d)).

FIGURE2.6: Different types of planar coils designs: (a) spiral, (b) mesh, (c) meander, (d) square. (Retrieved and adapted from [40])

It is possible to study planar coils based on three principal parameters: magnetic field (H), inductance (L), and quality factor (Q). The magnetic field is a well-known param- eter, depending on the measurement distance and the applied power. Contrarily, the inductance is an intrinsic characteristic directly proportional to magnetic flux and with a complex dependency on the shape of the coil [41]. Finally, the quality factor indicates the ohmic losses, where largerQmeans fewer losses for a specific current frequency [35]. All of the parameters can be optimized [35,42] for different designs, making them viable for multiple applications, which gives the user a wide range of possibilities depending on the purpose and the fabrication process, as well as its complexity [39,40]. Nonetheless, when producing these optimized types of MEMS, the spacing between the conductor, i.e., the conductive material forming the patterned coil, should always be minimized, contrarily to the cross-sectional area of the conductor that should be maximized [39].

2. INTRODUCTION 15

Nowadays, some applications related to the control of other magnetic elements with planar coils, for example, ferrofluids [43], or ferrous particles in liquids [44], already demonstrate initial viability for the desired system.

2.2.2 Magnetic nanoparticles

Nanotechnology development brought different types of materials, displaying various sizes and dimensions. Nanoparticles (NPs) are materials possessing all three dimensions equal to or below 100 nm [45], making them hold complex and distinct properties ruled by quantic phenomenons. In specific, NPs containing mainly magnetic elements such as iron oxides are widely used, allowing the production of simple compounds likeFe₃O₄or more complex onesMFe₂O₄(M= Fe,Co,Mn) [46,47].

Depending on the materials and the characteristics of the nanoparticles, like shape, size, or functionalization, many techniques appear as viable production solutions. Co- precipitation is one of the most used methods due to its simplicity since it consists of the mixture of two solutions that create a nanoparticle precipitate [47–49]. Equally, thermal decomposition and microemulsion are the other two techniques extensively used to pro- duce magnetic nanoparticles, even though they demand complex systems for fabrication [48,49]. Additionally, it is possible to coat the nanoparticles, maintaining their magnetic properties while adding a functionalization with organic or inorganic molecules on the exterior [48,49], or merely improve their stability with surfactants or polymers [50].

Some magnetic nanoparticles, like certain ones made of iron oxides, possess a specific magnetic behavior called superparamagnetism. This effect happens due to the propor- tional dependency of the magnetic anisotropy energy (U) on the particle volume (V), which can make it smaller than the thermal energy (kT) for small objects such as NPs.

Consequently, the magnetic moment is able to flip in every direction, giving the particle a null spontaneous magnetization like a paramagnet [50,51]. However, when an external field is present, the moment alignment is ruled that same field, giving the nanoparti- cles’ moments an alignment analogous to the ferromagnetic behavior (figure2.7). There- fore, the NPs have the possibility of being ”active” or ”passive” magnetic elements with no hysteresis and low coercivity [50]. In fact, it is possible to obtain superparamagnetic nanoparticles at room temperature, for example 12 nmγ−Fe2O3or 13 nmConanoparti- cles [51].

16

PROGRAMMABLE MAGNETIC ATTRACTIVE MATERIALS: AN APPROACH FOR SHAPE-CHANGING CARS’INTERIORS

FIGURE2.7: Schematic of the nanoparticles energies variation with the size implication on the magnetic moments flip. (Retrieved and adapted from [51])

Superparamagnetic NPs development considerably impacted medicine and biology, opening doors to groundbreaking technologies in the analysis, control, and treatment of humans and animals. With them, it was possible to create drug delivery systems that were more efficient, guiding the particles with external magnetic fields to the desired place where the functional coating of NPs would interact with the receiver, delivering the medication [48,50]. Another significant application is hyperthermia which utilizes high AC fields to vary rapidly the spin direction, which makes the NP irradiate heat to kill the neighbor cells [46,48]. Outside these fields of research, there is less usage of MNPs, especially in the automotive industry, making the conceptualized system an innovative and groundbreaking idea.

Chapter 3

Fabrication and characterization techniques

3.1 Production methods

3.1.1 Co-precipitation

Magnetic nanoparticles (MNPs) can be produced in various ways depending on the mate- rials and characteristics envisioned. In this work, a co-precipitation method, based on the procedure presented atC. Pereira et al[47], was performed at the IFIMUP-IN laboratory to obtainMnFe₂O₄nanoparticles with high saturation magnetization.

Firstly 200cm³of MIPA (alkanolamines isopropanolamine) were heated with smooth stirring up to the boiling temperature (100^◦C). During this, 1,98 g of Manganese salt (MnCl₂·4H₂O) was dissolved in 1 cm³ of a solution of 37% HCl and 4 cm³ of water.

Subsequently, 5,43gof Ferrous salt (FeCl₃·6H₂O) was mixed with 40cm³of water. After obtaining the boiling temperature, both solutions were added to the 200 cm³ of MIPA, creating an immediate dark brown precipitate, which was left for two hours at 100^◦C with vigorous stirring (the first step in figure3.1).

Following the two hours comes a washing process to separate the NPs precipitate from the rest of the solution where the MNPs that do not possess a sufficient high mag- netization, i.e., that are not sufficiently attracted by a hard magnet, are removed. The first step (the second step in figure3.1), repeated four times, begins by filling the NPs’

recipient with 100mLof water, mixing it, and removing the water with a hard magnet on the bottom to maintain the nanoparticles inside. The following step requires two small

17

18

PROGRAMMABLE MAGNETIC ATTRACTIVE MATERIALS: AN APPROACH FOR SHAPE-CHANGING CARS’INTERIORS

containers with 15 mLto retain the nanoparticles (the third step in figure3.1). It starts with the initial recipient receiving some acetone, mixing it inside, and turning the mix- ture over to one of the small containers. The latter is placed over a hard magnet to deposit the nanoparticles, while removing the rest of the solution. This process is replicated un- til there are no NPs left inside the initial recipient. The final step is done three times to assure the complete cleansing of the NPs and consists of filling the small containers with acetone, placing them over the hard magnet, and removing the solution.

Even though most of the liquid is removed during the previous procedures, some can get stuck within the nanoparticles. Therefore, both 15 mL containers are placed in the oven overnight at 40^◦C, assuring that the final result is completely dry MNPs. Since the nanoparticles tend to agglomerate after being in the oven, a mortar and pestle are used to smash them and create a thin powder, which is then stored in a unique 15mLcontainer.

FIGURE3.1: Process diagram ofMnFe₂O₄MNPs

3.1.2 PVC paste and lacquer production and application

Cars’ interior soft materials can differ from car to car in their shape, color, and texture, but, as of now, they will most likely have the same basic structure, which allows the cre- ation of natural material appearances using artificial ones. At TMG Automotive facilities, these artificially structured materials are produced with distinct layers depending on the wanted application. The process of materials fabrication has two ways of adding layers:

film type, using viscous pastes; and coating, using solutions.

The fabrication of a simple example begins with the preparation of the base PVC paste and two lacquers, a base coat and a top coat. Either component production is similar and consists of adding precisely weighted powders and solutions previously chosen, followed by vigorous agitation to assure a homogeneous solution. Afterward, the paste is placed in

3. FABRICATION AND CHARACTERIZATION TECHNIQUES 19

one extremity of a straight paper, which is then pushed uniformly with a knife to the other end, creating a well-defined thickness film with a size of approximately 21x29.7cm(A4 size). Subsequently, the film suffers a heat treatment of one minute at 210^◦C, preceding the application of a base coat lacquer and a top coat lacquer using a rolling method [52].

This method has two rotating cylinders over each other with a gap large enough to barely get the sample through. The bottom one is constantly coated with a bath of the desired lacquer, applying it to the material when it goes between the cylinders. After each lacquer application, an equal heat treatment was applied to the film, finalizing the production of the sample.

Additionally, for this application, different magnetic NPs concentration were also added during the preparation of PVC paste and lacquers, being mixed with the other com- ponents to keep the homogeneity. Therefore, to obtain a considerable analysis, it was produced four PVC pastes, one base (without nanoparticles) and three with different nanoparticle weight percentages. Moreover, it was prepared a single base coat lacquer and two top coat lacquers, one with and the other without NPs. Despite the NP’s addi- tion, the fabrication methods remained the same without any variation.

More complex products can be fabricated similarly by adding different paste layers with distinct functions in the final system. In these cases, the lacquers are only applied in the final film layer to give the object the characteristics wanted on its surface.

FIGURE3.2: Process diagram of TMG’s magnetic cars’ interior samples

3.1.3 Screen printing

Screen printing is one of the most used techniques to fabricate microelectronics devices, among the multiple existences, due to its versatility, low cost, customization of patterns, immense compatibility with flexible substrates, and high capability of being done on large-scale productions [53].

20

PROGRAMMABLE MAGNETIC ATTRACTIVE MATERIALS: AN APPROACH FOR SHAPE-CHANGING CARS’INTERIORS

IFIMUP facilities allowed the fabrication of distinct patterned planar coils films with a screen printing technique, where the two main pillars needed for the production were a mesh with the designs and conductive ink. Through a lithography process, the mesh was exposed to UV light while having a coating of emulsion with a stencil possessing the desired patterns placed on its top. Given that the emulsion is a light-sensitive material, in this case, it hardens when exposed to light, it was possible to, after the exposure, remove the soft emulsion and get a mesh with the pattern wanted [54]. The process can be done in a simple fashion, allowing for an easy way of testing various designs. On the other hand, ink formulation is not a simple operation, resulting in the choice of using a high conductivity generic silver ink from VFP Ink Technologies, already optimized for screen printing.

To produce the printed film, as illustrated in figure3.3, the first step was to place the patterned mesh in the system with ink on one extremity, followed by an equal distribution through the mesh with a squeegee (flood). Afterward, a stronger brush in the opposite direction was made to allow the ink to pass through every mesh hole, printing the desired pattern in the substrate. Additionally, to guarantee the solidification of the ink, the film undergo through heat treatment at 150^◦Cfor 15 minutes.

FIGURE3.3: Steps of screen printing fabrication: (a) ink paste is placed on the extremity of the mesh, (b) squeegee spreads the ink through the entire pattern, (c) pressured spread of the ink with the squeegee, (d) substrate with the pattern printed. (Retrieved and adapted

from [6])

3.2 Characterization methods

3.2.1 X-Ray diffraction

To characterize crystalline materials, i.e., materials possessing an organized, patterned, and recurrent atomic structure, the X-Ray Diffraction (XRD) method is commonly used

3. FABRICATION AND CHARACTERIZATION TECHNIQUES 21

[55]. Discovered by Von Laue and firstly applied by Bragg, this method consists of a beam of X-rays that possess a wavelength similar to the lattice parameter of the sample, which is sent against the specimen, being diffracted by different crystallographic planes. On the other side, a detector will receive the interference of the diffracted beams on the sample, and only certain angles of incidence will result in non-null intensities. This phenomenon is explained by Bragg’s law [56], which relates the incident beam angle or diffraction an- gle,θ_B, beam wavelength,λ, an integer,n, and the crystal lattice constant,d, as presented in equation3.1.

nλ=2dsin(θ_B) (3.1)

Figure3.4 presents a schematic of an X-Ray diffraction measurement using a Bragg- Brentano geometry. In this case, the beam source stays in a fixed position, while the sample is rotated by an angle ofθwhereas the detector is rotated with an angle of 2θ, both regarding the incident angle [57]. When evaluated for multiple incident angles, the result produced will be a graph of intensity peaks as a function of the detector angle, 2θ, repre- senting the diffracted beams in each set of planes of the sample. That way, since it is only dependent on the specimen crystal structure, it is possible to determine its constituents by comparison with other material patterns and calculate the lattice parameter.

FIGURE 3.4: Schematic of an X-Ray diffraction using θ−θ geometry. (Retrieved and adapted from [58])

In this work, every X-Ray diffraction result presented and analyzed was performed using a Rigaku SmartLab high-resolution XRD located at IFIMUP. The system was oper- ated with a Bragg-Brentano (θ−θ) geometry and a power of 9 kW, with beam radiation originating from aCusource, producing aλ=1.540593 ˚A.

22

PROGRAMMABLE MAGNETIC ATTRACTIVE MATERIALS: AN APPROACH FOR SHAPE-CHANGING CARS’INTERIORS

3.2.2 SQUID

Superconducting quantum interference device (SQUID) magnetometers are highly sensi- tive machines capable of measuring the magnetic properties of samples, such as the small magnetic moments or living organisms’ magnetic field [59]. The basis of SQUID is pre- sented in figure3.5. The sample is placed in transparent straw, which is oscillated in the X-direction to create a variable magnetic flux detected by a pick-up coil. That coil trans- mits to the main component of the SQUID, the Josephson junctions (marked as rf-SQUID in figure3.5) that, due to quantum mechanisms, are sensitive to magnetization variation and indicate that sensibility as voltage drop variation,V_{SQU ID}. The value will differ de- pending on the sample, its magnetic behavior, temperature, and the external field applied, which creates the possibility of a good magnetization characterization. The most common analysis is the hysteresis loop which correlates the Magnetization and external magnetic field, a crucial relation to define the magnetic properties of any sample [60,61].

FIGURE3.5: Schematic of the interior of SQUID. (Retrieved and adapted from [60])

The measurements were performed using an MPMS3 SQUID magnetometer from Quantum Design available at the IFIMUP laboratory. To obtain a full-cycle hysteresis loop, a routine was used to vary the external applied magnetic field from 5 T to -5 T and back to 5 T for each sample at 300 K. This method allowed the acquisition of 70 magnetization values at various stable fields, which enabled the characterization of the nanoparticles embedded into cars’ interior samples.

Usually, the procedure starts with a portion of the material, previously weighted, placed in a jelly capsule. Due to the SQUID magnetometer having a magnetization read- ing limit, the amounts of samples used are small. Afterward, that capsule is filled with