Characterization of Magneto-Dielectric Material for Microwave devices[01-20]

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<p>UPTEC Q 20010</p><p>Examensarbete 30 hp</p><p>September 2020</p><p>Characterization of Magneto-Dielectric</p><p>Materials for Microwave Devices</p><p>Joseph Lazraq Byström</p><p>Teknisk- naturvetenskaplig fakultet</p><p>UTH-enheten</p><p>Besöksadress:</p><p>Ångströmlaboratoriet</p><p>Lägerhyddsvägen 1</p><p>Hus 4, Plan 0</p><p>Postadress:</p><p>Box 536</p><p>751 21 Uppsala</p><p>Telefon:</p><p>018 – 471 30 03</p><p>Telefax:</p><p>018 – 471 30 00</p><p>Hemsida:</p><p>http://www.teknat.uu.se/student</p><p>Abstract</p><p>Characterization of Magneto-Dielectric Materials for</p><p>Microwave Devices</p><p>Joseph Lazraq Byström</p><p>There is an increasing interest in using new composite materials in microwave</p><p>devices, to reduce size and weight while maintaining similar performances. A new</p><p>promising material group is named magneto-dielectric materials, which have the</p><p>permittivity and permeability values both larger than one. Compared to the</p><p>commercially used dielectric materials, magneto-dielectric materials can achieve a</p><p>larger miniaturization factor with the equivalent properties as dielectric materials.</p><p>There is a very limited availability of commercial magneto-dielectric materials. A</p><p>recent addition was from Rogers Corporation with MAGTREX 555, [1], that is</p><p>available as a printed circuit board laminate. The material is limited to 500 MHz</p><p>operational frequency due to its increased magnetic and dielectric losses.</p><p>In this thesis the purpose is to understand the loss mechanisms, characterize and</p><p>understand the state-of-the-art magneto-dielectric materials at microwaves, and to</p><p>produce a magneto-dielectric material in the lab to understand the material better. A</p><p>new material was developed with magneto-dielectric properties. The material was</p><p>based on a polymer base of polystyrene that serves as a dielectric material and doped</p><p>with nickel nanoparticles that produce the magnetic properties. The contents of the</p><p>nanoparticles in the mix is a design variable. Nickel-polystyrene samples with different</p><p>nickel contents of 0%, 2.3% and 4.5%, were produced in the lab and measured</p><p>in-house to understand the loss mechanism and RF performance.</p><p>ISSN: 1401-5773, UPTEC Q 20010</p><p>Examinator: Åsa Kassman Rudolphi</p><p>Ämnesgranskare: Dragos Dancila</p><p>Handledare: Darwin Blanco och Christos Kolitsidas</p><p>Karakterisering av magneto-dielektriska material för</p><p>mikrovågsapplikationer</p><p>Efterfrågan på nya kompositmaterial för att minska storleken och vikten på kompo-</p><p>nenter för mikrovågsapplikationer ökar. Ett lovande material med en permittivitet och</p><p>permeabilitet större än ett är magneto-dielektriska material. Komponenter i magneto-</p><p>dielektriska material kan göras mindre än komponenter i kommersiella dielektriska mate-</p><p>rial, med bibehållna eller liknande egenskaper. Det finns väldigt få magneto-dielektriska</p><p>material på marknaden. Ett exempel är MAGTREX 555 från Rogers Corporation, [1],</p><p>som nyligen släpptes på marknaden och säljs som ett kretskortslaminat. Materialet är</p><p>begränsat till att användas upp till 500 MHz på grund av sina ökande förluster.</p><p>Syftet med projektet är att karakterisera och försöka förstå magneto-dieletriska mate-</p><p>rial genom att identifiera de nyaste rapporterade materialen, samt att minska eller kon-</p><p>trollera de dielektriska och magnetiska förlusterna i materialet. Under projektet valdes</p><p>två Magneto-dielektriska materail, MAGTREX 555 från Rogers Corporation och nickel-</p><p>polystyren där valet inspirerades från Kyu Hwan Han’s doktorsavhandling.</p><p>MAGTRX 555 tillhandahölls av Rogers Corporation medan nickel-polystyrenprover med</p><p>olika nickelhalt producerades på Ångströmlaboratoriet i Uppsala. Permittiviteten och</p><p>dielektriska förluster mättes med en dielektrisk testfixtur från Keysight kopplad till en</p><p>Impedance Analyzer. Permeabiliteten och magnetiska förluster mättes med ett egengjort</p><p>mätinstrument byggt för att likna en magnetisk testfixtur från Keysight, också kopplad</p><p>till en Impedance Analyzer. Röntgendiffraktion (XRD) användes för att mäta nickel-</p><p>halten och faserna i nickelpulvret. Vibrating-Sample Magnetometer (VSM) användes</p><p>för att mäta de magnetiska egenskaperna av nickelpulvret och de producerade nickel-</p><p>polystyrenproverna.</p><p>Data från Rogers Corporation och egna mätningar av MAGTREX 555 valdes som kon-</p><p>troll för vår metod och de uppmätta värdena av de producerade proverna. Resultaten för</p><p>de producerade nickel-polystyrenproverna visar en stabil permittivitet och permeabilitet</p><p>med låga förluster upp till 1 GHz. Valet av material och kompositstruktur för de pro-</p><p>ducerade proverna, som gav upphov till de låga förlusterna, visar lovande resultat och</p><p>är en bra kandidat för uppföljningsstudier. En rekommendation för uppföljningsstudier</p><p>skulle vara att testa andra mätmetoder för att säkerställa de uppmätta värdena vid högre</p><p>frekvenser.</p><p>Examensarbete 30 hp på civilingenjörsprogrammet</p><p>Teknisk fysik med materialvetenskap</p><p>Uppsala universitet, September 2020</p><p>Contents</p><p>1 Introduction 4</p><p>2 Theory 4</p><p>2.1 Permittivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4</p><p>2.2 Permeability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5</p><p>2.3 Losses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5</p><p>2.3.1 Dielectric Losses . . . . . . . . . . . . . . . . . . . . . . . . . . . 6</p><p>2.3.2 Magnetic Losses . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7</p><p>2.4 Dielectric Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8</p><p>2.5 Magnetic Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9</p><p>2.6 Magneto-Dielectric Materials . . . . . . . . . . . . . . . . . . . . . . . . . 10</p><p>3 Methods 11</p><p>3.1 Evaluated Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11</p><p>3.1.1 Sample preparation . . . . . . . . . . . . . . . . . . . . . . . . . . 12</p><p>3.2 Measurement Equipment . . . . . . . . . . . . . . . . . . . . . . . . . . . 17</p><p>4 Results 23</p><p>4.1 Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23</p><p>4.2 Measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27</p><p>5 Discussion 32</p><p>5.1 Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32</p><p>5.2 Measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32</p><p>5.3 Dielectric and Magnetic Losses . . . . . . . . . . . . . . . . . . . . . . . . 33</p><p>6 Conclusion 34</p><p>7 Future Works and Applications 35</p><p>1 Introduction</p><p>As modern radio frequency (RF) antennas and filters are becoming more advanced with</p><p>the aim of efficiency, the microwave devices used are optimized to the limit. Therefore,</p><p>it is of importance to characterize new materials and understand their properties to fur-</p><p>ther improve them. Until today, dielectric materials have been used in many microwave</p><p>devices. Instead, this thesis will focus on understanding the loss mechanism and how to</p><p>characterize magneto-dielectric materials, [2].</p><p>There are a few past reports on magneto-dielectric materials which show good results</p><p>at lower frequencies, [3], [4], [5]. However, both the magnetic and dielectric losses are</p><p>higher from dielectric materials and especially once the frequency rises. Besides the high</p><p>losses, magneto-dielectric materials are interesting because of the property of having a</p><p>permeability and permittivity larger than one. This property can be interesting for some</p><p>particular applications where the losses are not a critical parameter, [6]. With this taken</p><p>into account, magneto-dielectric materials are interesting to further characterize with</p><p>more modern production methods and better understanding in material properties.</p><p>The purpose of this master thesis is to characterize magneto-dielectric materials, un-</p><p>derstanding the methods to fabricate the composite, and to measure the permittivity,</p><p>permeability, and the losses. The goal is to understand why the losses of the material are</p><p>so high and if it is possible to design or fabricate a composite with lower losses compared</p><p>to the commercial materials.</p><p>2 Theory</p><p>This section is intended to describe the basic knowledge to understand the magneto-</p><p>dielectric materials better by introducing</p><p>both the magnetic and dielectric properties</p><p>and materials.</p><p>2.1 Permittivity</p><p>The permittivity or the dielectric constant comes from the electric susceptibility χe and</p><p>shows the dielectric properties of a specific material. The permittivity was discovered in</p><p>the 18th century by Charles-Augustin de Coulomb.</p><p>When a material is polarized from an electric field, the atomic and molecular dipoles</p><p>lines up and gives the electric susceptibility which is stated in Equation 1. The permit-</p><p>tivity can then be expressed as in Equation 2. The permittivity in free space is called</p><p>the vacuum permittivity. When an electric field is applied to matter, the susceptibility</p><p>obtains a value and relative permittivity occur as stated in Equation 2. The polarization</p><p>of a material P in Equation 1, can be described as:</p><p>P “ ε0χeE (1)</p><p>4</p><p>Where P is the polarization, E is the electric field, and ε0 is the vacuum permittivity,</p><p>[7]. The relative permittivity is stated in Equation 2 as:</p><p>εr “ 1` χe “</p><p>ε</p><p>ε0</p><p>(2)</p><p>Where ε is the permittivity (ε0εr), [7]. The permittivity has both a real part (ε1) and an</p><p>imaginary part (jε2) as stated in Equation 3:</p><p>ε “ ε1 ´ jε2 (3)</p><p>This is important when measuring the permittivity and the dielectric loss as will be seen</p><p>in the following section.</p><p>2.2 Permeability</p><p>The permeability was discovered in the 19th century by Jean-Baptiste Biot and Félix</p><p>Savart. Like the permittivity, the permeability comes from the magnetic susceptibility</p><p>χm (Equation 4) and is a measurement on how magnetic a material is:</p><p>Mi “ χm,ijHj (4)</p><p>Equation 4 describes the relation between the magnetic field, the magnetic susceptibility,</p><p>and magnetization of a 3-dimensional material with M: Magnetization, H: Magnetic field,</p><p>and χm,ij: Magnetic susceptibility as a tensor, [7]. When there is matter to magnetize,</p><p>the susceptibility obtains a value and permeability occurs as stated in Equation 5:</p><p>µ “ µ0p1` χmq (5)</p><p>With µ0 as the permeability in free space, [7]. As the permittivity, the permeability also</p><p>has both a real part (µ1) and an imaginary part (jµ2) as stated in Equation 6:</p><p>µ “ µ1 ´ jµ2 (6)</p><p>This is important when measuring the permeability and the magnetic loss as will be seen</p><p>in the following section.</p><p>2.3 Losses</p><p>The loss tangent (tan(δ)) is the measure of signal loss due to the inherent dissipation of</p><p>electromagnetic energy in the substrate. In our case, this means that all of the energy</p><p>from the radio frequency is not transmitted. Some part of the transmitted energy in the</p><p>material in terms of time-varying external E-field and H-field, is irrecoverably transformed</p><p>into heat, [8]. Both magnetic losses and dielectric losses are occurring in a material.</p><p>Figure 1 shows schematically the loss tangent with, the real and imaginary permeability</p><p>in this case which creates the complex relative permeability vector, [9].</p><p>5</p><p>Figure 1: The loss tangent displayed schematically with the real and imaginary perme-</p><p>ability, and µ˚r as the complex conjugate of the relative permeability, [9].</p><p>2.3.1 Dielectric Losses</p><p>The dielectric losses are associated with the substrate (insulating material) used in guided</p><p>wave applications. When an electric field is applied, polarization occurs and charges are</p><p>displaced relative to the electric field. Dielectric losses cause a radiation of the electric</p><p>field. As the alternating electric field switches the polarization in the material with a</p><p>certain frequency, losses are created in the form of friction and heat energy. As mentioned</p><p>in Section 2.1, the dielectric loss tangent depends on both the real part and complex part</p><p>of the permittivity from Equation 3. The relation can be seen in Equation 7:</p><p>tanpδeq “</p><p>ε2</p><p>ε1</p><p>(7)</p><p>As for the permeability, the real and imaginary part of the permittivity contributes to</p><p>the losses. However, they are more stable at lower frequencies. The real part stay high</p><p>and the imaginary part stay low which leads to lower losses from Equation 7. This is</p><p>displayed in Figure 2, [10].</p><p>6</p><p>Figure 2: Real permittivity (ε1) and imaginary permittivity (ε2) as a function of the</p><p>frequency, [10].</p><p>2.3.2 Magnetic Losses</p><p>According to Han [6], in his thesis on how to develop a material characterization method</p><p>for magneto-dielectric materials, the losses increases when the particle size exceeds a</p><p>critical dimension and domain walls are formed. The losses further increase when the</p><p>frequency reaches Ferromagnetic Resonance (FMR, explained in the Magnetic Materials</p><p>section), combined with the resonance of the domain walls. However, losses from Excess</p><p>Eddy currents and the domain walls can be decreased by reducing the particle size to</p><p>nanoscale. Eddy current is created when a magnetic field is applied on a material which</p><p>creates current loops in it, [11].</p><p>In Peng et al. [12], FMR was used to evaluate the energy loss in the material. The</p><p>loss from magnetic friction is translated into heat by the applied external magnetic field</p><p>(Hex). However, if Hex rotates in the opposite direction of the magnetic moment (M),</p><p>the magnetic material exhibits low loss. This means that the magnetic loss can be altered</p><p>depending on the direction of rotation.</p><p>According to Snoek’s law, a high initial permeability decreases the FMR, which leads to</p><p>higher magnetic losses, [6]. Snoek’s law for a bulk polycrystalline material is stated in</p><p>Equation 8:</p><p>pµ0 ´ 1qF0 “</p><p>4</p><p>3</p><p>γMs (8)</p><p>With F0 as the Resonance frequency, γ as the Gyromagnetic factor, and Ms as the</p><p>Magnetic saturation, [13].</p><p>As mentioned in Section 2.2, the magnetic loss tangent depends on both the real part and</p><p>complex part of the permeability from Equation 6. The relation can be seen in Equation</p><p>9:</p><p>tanpδmq “</p><p>µ2</p><p>µ1</p><p>(9)</p><p>7</p><p>Equation 9 is important to take into account when increasing the frequency. At a mod-</p><p>erate frequency (« 500MHz), the real part of the permeability is high and the imaginary</p><p>part is low. As the frequency increases (>1GHz), the real part starts to decrease and</p><p>the imaginary part increases. From Equation 9 one can see that a decreased real part</p><p>and increased imaginary part of the permeability will increase the magnetic losses. The</p><p>increased loss with increased frequency is displayed in Figure 3, [10].</p><p>Figure 3: Real permeability (µ1) and imaginary permeability (µ2) as a function of the</p><p>frequency, [10].</p><p>2.4 Dielectric Materials</p><p>High dielectric, and low loss materials are usually used for radio frequency (RF) compo-</p><p>nents. Dielectric materials are electrical insulators with a permittivity larger than one</p><p>and a permeability equal to one. They are generally cheap, easy to produce and can be</p><p>used in many different applications. Dielectric materials can be both ceramics and poly-</p><p>mers. An example of a well known dielectric material used in many different applications</p><p>is PTFE (Teflon). PTFE has a relative permittivity of εr « 2.1 and a dielectric loss of</p><p>tanpδdq “ 15 ¨ 10´4 at 3 GHz, [14]. The fabrication of dielectrics is simple depending on</p><p>the material class. Ceramics can be made by oxidizing metals and polymers by letting</p><p>monomers react and create the wanted polymer chains which are then pressed to the</p><p>requested shape.</p><p>Dielectric materials have the ability to be polarized and hold an electric field with a</p><p>minimum transformation of heat energy. The transformation to heat energy is referred</p><p>to as losses and therefore, the less heat transformation, the lower losses, and the more</p><p>effective dielectric properties. The dielectric constant of a material and the losses can be</p><p>used to specify the applications. For example, a material such as aluminum oxide with</p><p>a high dielectric constant is light weighted and works well where the weight aspect are</p><p>important, such as in antennas. However, the drawback is that high dielectric materials</p><p>8</p><p>lack the ability to resist higher electrostatic fields and will reach dielectric breakdown</p><p>and start to conduct current. The dielectric breakdown can then damage fragile cir-</p><p>cuits in electronic devices. Instead, low dielectric materials, can be used as they resist</p><p>electrostatic fields better, [15].</p><p>2.5 Magnetic</p><p>Materials</p><p>A magnetic material is a material with a permeability greater than one. There are five</p><p>different classes of magnetic materials which can be seen in Figure 4 and Table 1.</p><p>Figure 4: The definitions of the magnetic materials are classed in Table 1, [16].</p><p>Table 1: Classification of Magnetic Materials, [12].</p><p>Type of Magnetic Material Magnetic Susceptibility</p><p>Diamagnetic Negative and small</p><p>Paramagnetic Positive and small</p><p>Ferromagnetic Positive and large</p><p>Antiferromagnetic Positive and small</p><p>Ferrimagnetic Positive and large</p><p>In ferromagnetic materials, there are magnetic domains which can be affected and changed</p><p>by an external magnetic field. The direction of the domains is random until a magnetic</p><p>field H starts to magnetize the material and direct the domains in the same direction</p><p>as the magnetic field. Since the magnetization follows a hysteresis loop (see Figure</p><p>5), the material will still be magnetized when the magnetic field is removed. However,</p><p>the material reaches a limit of magnetization as the material is saturated when a high</p><p>enough magnetic field is applied. Equation 10 describes the relation between the B-field,</p><p>the magnetic field (H), and the magnetization (M):</p><p>B “ µ0pM`Hq (10)</p><p>9</p><p>Figure 5: Hysteresis loop of a ferromagnetic material, [7].</p><p>Ferromagnetic Resonance (FMR) is a magnetic property usually used to evaluate the</p><p>performance of a magnetic material. The ferromagnetic material is put in a magnetic</p><p>field alterning at high frequencies. At a certain frequency the magnetic material reaches</p><p>resonance (FMR). The resonance leads to heat effects which is caused by heavily oscil-</p><p>lating of the materials electron spins, [12].</p><p>Equation 11 describes the ferromagnetic resonance stated as follows:</p><p>FMR “</p><p>γ</p><p>2π</p><p>Heff (11)</p><p>With γ as the gyromagnetic ratio and Heff as the effective field anisotropy.</p><p>2.6 Magneto-Dielectric Materials</p><p>Magneto-dielectric materials are ceramic or composite materials with a relative permit-</p><p>tivity and permeability both larger than one, hence, they posses both the properties</p><p>of magnetic and dielectric materials. Magneto-dielectric materials can not be found in</p><p>nature and therefore have to be synthesised in a lab. It is important to obtain a high</p><p>permeability and permittivity, as well as low losses. This can be achieved with the right</p><p>synthesis technique. A magneto-dielectric material can be understood as a former dielec-</p><p>tric material which is then doped with another magnetic material to add the magnetic</p><p>behavior. The added magnetic properties often comes with an increase of the total ma-</p><p>terial losses.</p><p>When it comes to the synthesis of magneto-dielectric materials a ferromagnetic or fer-</p><p>rimagnetic filler is put into a polymer matrix, or a ferrite substrate. Three examples</p><p>follows:</p><p>• Composites with ferromagnetic metals, which gives higher permeability, higher mag-</p><p>netic loss, and εr " µr</p><p>• Composites with ferrite ceramic powders, which gives lower permeability, lower</p><p>magnetic and dielectric loss, and εr ą µr</p><p>10</p><p>• Ferrite substrates, which gives higher permeability, higher magnetic and dielectric</p><p>loss, εr « µr, and a size limited by effective size of sintered ceramic.</p><p>The synthesis technique chosen affects the losses and efficiency, [2]. According to Horn</p><p>et al. [3], who used polymer-ferrite composites, there are three different arrangements</p><p>which can be used and affect the properties differently. The three arrangements are</p><p>parallel, serial, and dispersed. Su et al. [10] did a review on the three different magneto-</p><p>dielectric groups and found that the composite magneto-dielectric materials had low</p><p>losses at lower microwave frequencies. Depending on the choice of arrangement, the</p><p>anisotropy of both the permittivity and permeability can be affected in some grade. The</p><p>chosen arrangement is of importance when the application for the produced material</p><p>are taken into account where the material properties can be crucial and affect both</p><p>the permittivity/permeability, and losses in the material. Another way to influence the</p><p>material properties is to change the metal loading in the polymer-ferrite composition, [3].</p><p>As it can be seen in Table 1, the suitable magnetic materials wanted for magneto-dielectric</p><p>materials are ferromagnetic and ferrimagnetic materials since they have a positive and</p><p>large susceptibility and therefore a high permeability (see Equation 5).</p><p>3 Methods</p><p>3.1 Evaluated Materials</p><p>The aim of this master thesis is to evaluate state-of-the-art magneto-dielectric materials</p><p>with low magnetic and dielectric losses at low microwave frequencies (up to 1 GHz).</p><p>There are a few magneto-dielectric materials reported in the literature. However, most of</p><p>them are either patented or having high losses in the frequency range examined. In Mos-</p><p>allaei et al. [4], a promising magneto-dielectric material with the composition BaFe12O19</p><p>with some additional CoO and BaCO3 was reported with relatively low losses up to 1</p><p>GHz. However, since there were no commercial supply and the production process was</p><p>complicated without any synthesis information in the article, the material was not chosen</p><p>for this study. In another paper from Ikonen et al. [5], an interesting hexagonal ferrite</p><p>material was reported with low losses and a stable complex permeability up to 5 GHz.</p><p>Unfortunately, the material composition was not included due to patent. After some</p><p>further research, cobalt-fluoropolymer and nickel-fluoropolymer with relatively low losses</p><p>up to 4 GHz and a manageable production process to produce samples in the lab were</p><p>found, [6]. A third material, from Rogers Corporation called MAGTREX 555, was found</p><p>to have some interesting properties and low losses. The material was reported to have</p><p>good properties up to 500 MHz and was available commercially from Rogers Corporation,</p><p>[2]. Both the properties and the synthesis technique reported in Horn et al. [3], were</p><p>really helpful to understand and characterize the magneto-dielectric materials.</p><p>Two materials were chosen from the-state-of-the-art based in their losses at low microwave</p><p>frequencies. They are Nickel-Polystyrene produced in the lab by me [6], and MAGTREX</p><p>555 from Rogers Corporation [1].</p><p>Han [6] used the fluoropolymer ALX-507 from AGC (former Asahi Glace Chemicals).</p><p>11</p><p>However, since the Ph.D. thesis was written in 2015, ALX-507 was not available in the</p><p>market any more from AGC. Instead, polystyrene (product# 331651) with the similar</p><p>properties as ALX-507 was chosen and bought from Sigma-Aldrich. It was also hard to</p><p>find cobalt nanoparticles in the same size as mentioned in Han [6] with a particle size of</p><p>20-30 nm. Therefore, 100 nm nickel particles with 99% purity (product# 577995) were</p><p>bought from Sigma-Aldrich. Both MAGTREX 555 and cobalt-fluoropolymer with their</p><p>properties are listed in Table 2. The dielectric constant for polystyrene is expected to be</p><p>εr “ 2.53 and a dielectric loss tangent of tanpδeq « 10´4 at 1GHz, [17]. The permeability</p><p>of the nickel powder is expected to be µr “ 100´ 600, [18].</p><p>Table 2: Evaluated Magneto-Dielectric Materials and their properties.</p><p>Material Permittivity Permeability tanpδdq tanpδmq Frequency</p><p>Cobalt-fluoropolymer r6s 9.5 2.18 0.004 0.018 1 GHz</p><p>MAGTREX 555 r1s 6.5 6 0.005 0.05 400 MHz</p><p>3.1.1 Sample preparation</p><p>MAGTREX 555 with two different thicknesses 0.5 mm and 1.52 mm were provided by</p><p>Rogers Corporation with the dimension 130 ˆ 180 mm without copper cladding. The</p><p>dimension was chosen to fit in a Split Post Dielectric Resonator (SPDR) measurement</p><p>equipment.</p><p>The nickel-polystyrene samples were produced to evaluate and understand the synthesis</p><p>process of magneto-dielectric materials. Nickel-polystyrene was produced by me with</p><p>the help of Dr Roland Mathieu and Ph.D. Student Pierfrancesco Maltoni in their lab at</p><p>Uppsala University as there were no commercial samples available. As mentioned in the</p><p>previous section, 50g of 100 nm nickel particles with 99% purity and 500g of polystyrene</p><p>with Mw « 35000 were bought from Sigma-Aldrich. In Figure 6, one can see a flow chart</p><p>of the</p><p>lab process. The first step was to solve the polystyrene to add the nickel nanopar-</p><p>ticles. This was done by mixing polystyrene and acetone with a magnetic stirrer in 40˝C</p><p>covered with aluminum foil until all the polystyrene were solved, [19]. After some trails to</p><p>demonstrate that the process worked, the wanted amount of polystyrene were measured</p><p>and mixed with the acetone. When all the polystyrene was solved, the magnetic stirring</p><p>was removed and the nickel particles were added to the solvent and put in a sonicator</p><p>bath for 1 hour to disperse the particles, also covered with aluminum foil. The sonicator</p><p>bath uses the sound energy from ultrasound to mix and disperse solvents. After 1 hour,</p><p>the aluminum foil was removed and the polystyrene-nickel-acetone solvent was put on a</p><p>heat plate at 90 ˝C until most of the acetone was evaporated (Figure 7). Then the glass</p><p>beaker was put in a furnace at 100 ˝C for 40 minutes until the polystyrene started to</p><p>react. As the polystyrene reacted, the nanoparticles were mixed with the polymer, and</p><p>a more homogeneous mix was achieved (see Figure 8).</p><p>12</p><p>Figure 6: Flow chart displaying the lab process of the nickel-polystyrene samples.</p><p>Figure 7: Acetone evaporating at 90 ˝C in the polystyrene-nickel solvent</p><p>13</p><p>Figure 8: Hardened polystyrene-nickel mix.</p><p>Immediately after the polystyrene-nickel mix had reacted, the material was put in a mold</p><p>to press the sample. The material was pressed in the mold with a pressing machine at a</p><p>pressure of 10 kN (Figure 9). In Figure 10, one can see the first sample made in a test</p><p>mold.</p><p>Figure 9: Manual pressing machine used to press the samples.</p><p>14</p><p>Figure 10: The first sample made in the mold.</p><p>Since the samples material properties needed to be measured, a specific shape of the</p><p>samples was required. The shape wanted was a toroidal shape. In Figure 11 and 12, one</p><p>can see the chosen method which includes a round mold with the outer diameter 1.8 cm</p><p>and the inner diameter 5 mm.</p><p>Figure 11: The chosen mold to make the sample with a diameter of 1.8cm.</p><p>15</p><p>Figure 12: The chosen mold with a sample inside to be pressed in the manual pressing</p><p>machine.</p><p>Three different sample compositions were made for the measurements as listed in Table 3.</p><p>Two of them had both polystyrene and nickel particles in them and the third contained</p><p>only polystyrene as a control sample.</p><p>Table 3: Composition of the three Nickel-Polystyrene samples made in the lab.</p><p>Sample Polystyrene Acetone Nickel Metal Loading</p><p>1 3300 mg 100 ml 0 mg 0 %</p><p>2 3225 mg 100 ml 75 mg 2.3 %</p><p>3 3150 mg 100 ml 150 mg 4.5%</p><p>To characterize the nickel particles, the powder was evaluated in a XRD to see the phase,</p><p>oxidation amount over time, and the crystal planes.</p><p>To characterize the magnetic properties of the nickel particles and the nickel-polystyrene</p><p>samples, a Vibrating Sample Magnetometer (VSM) was used. The VSM measures the</p><p>magnetic properties of the nanoparticles and creates a hysteresis loop with the magneti-</p><p>zation (M) as a function of the magnetic field (H). The method was also used to see if</p><p>the magnetic properties changed for the Nickel particles due to oxidation over time.</p><p>According to the literature, to further enhance the magnetic properties of the nickel,</p><p>phosphorus can be used. According to Knyazev et al. [20], the right amount of phospho-</p><p>rus leads to changes in the magnetic properties depending on how the alloy is treated.</p><p>For an annealed amorphous sample at 500 ˝C with 7% phosphorus, the magnetic satu-</p><p>ration, residual magnetization, and coercive field are increased. For a non-treated amor-</p><p>phous sample with 11% phosphorus, the material showed magnetic properties like an</p><p>unchanged coercive force, a decreased magnetization, a more shallow hysteresis loop, and</p><p>an increased Curie temperature. As a result, the heat treated samples with phosphorus</p><p>16</p><p>had a higher coercive force compared to a sample with only nickel, [20]. However, due</p><p>to time constraints and the safety restrictions of phosphorus, it was not evaluated in this</p><p>master thesis.</p><p>3.2 Measurement Equipment</p><p>In Han [6], the two measurement methods Cavity Perturbation Technique (CPT) and</p><p>Substrate Integrated Wave-guide (SIW), which was exploited in their study to make</p><p>cavities, were used to measure the permittivity and permeability. CPT works well for</p><p>magneto-dielectric materials with moderate losses. The material is put into a cavity and</p><p>the material properties can be obtained when a resonator is used which creates changes in</p><p>the resonant frequency. In order to measure the permeability of a material, the materials</p><p>permittivity must be known. The sample is either introduced in the E-field or H-field</p><p>maximum position which changes the resonant frequency and quality factor of the SIW</p><p>cavity depending on the samples complex permittivity or permeability. If the quality</p><p>factor and resonant frequency are slightly changed, the permittivity and permeability</p><p>can be found by using Equation 12:</p><p>f2 ´ f1</p><p>f1</p><p>“ ´</p><p>ş</p><p>Vs</p><p>p∆εE2E</p><p>˚</p><p>1 `∆µH2H</p><p>˚</p><p>1 qdV</p><p>ş</p><p>Vc</p><p>pε1E2E˚1 ` µ1H2H˚</p><p>1 qdV</p><p>(12)</p><p>Where f1 and f2 are the resonant frequencies before and after the introduction of the</p><p>sample, ε1 and ε2 are the complex permittivities of the original medium in the cavity, and</p><p>sample, and µ1 and µ2 are the complex permeability of the original medium in the cavity</p><p>and the sample. E1 and E2 are the electric fields in the cavity before and after perturba-</p><p>tion, and H1 and H2 are the magnetic fields in the cavity before and after perturbation.</p><p>Vc and Vs are the volumes of the cavity and the sample. By putting in the complex per-</p><p>mittivity and permeability in Equation 12, one can obtain the real and imaginary part of</p><p>the permittivity and the permeability. As mentioned, the sample is introduced where the</p><p>E-field is maximum for permittivity measurement and for permeability measurements,</p><p>the sample is introduced where the H-field is maximum.</p><p>The Substrate Integrated Wave-guide is used to fabricate the cavity for CPT measure-</p><p>ment. SIW can give good measurements of high quality factors and radiation effects. A</p><p>sample inserted in the cavity covered with copper tape can be seen in Figure 13, [6].</p><p>17</p><p>Figure 13: A sample inserted in the cavity covered with copper tape, [6].</p><p>Instead, the measurement methods as follows were used to measure the permittivity, per-</p><p>meability, and losses.</p><p>The measurement equipment used to measure the permittivity and dielectric losses for</p><p>the chosen materials is a Split Post Dielectric Resonator (SPDR) bought from QWED.</p><p>The setup can be seen in Figure 14.</p><p>Figure 14: SPDR setup (left) and its internal construction (right), [21].</p><p>The measurements can be done in two different ways:</p><p>• SPDR combined with additional Microwave Frequency Q-Meter from QWED</p><p>• SPDR combined with a Vector Network Analyser</p><p>The first technique is easier as the additional Microwave Frequency Q-Meter from QWED</p><p>can be connected directly to a computer and holds the information needed for different</p><p>measurements. In order to obtain the data, the VNA is connected to a computer and</p><p>uploaded to the QWED software included in the order. The VNA used was a miniVNA</p><p>Tiny from mini Radio Solutions with a wide frequency range from 1 MHz to 3 GHz.</p><p>Before performing the measurements, the User Guide provided by QWED was carefully</p><p>followed step by step for the equipment setup, [22].</p><p>18</p><p>Figure 15: Connection between SPDR and VNA, [22].</p><p>An alternative way used to measure the permittivity was an Impedance Analyzer of</p><p>model 4291B Agilent HP together with an additional Dielectric Test Fixture (model no:</p><p>Permittivity 16453). After setup calibration, the frequency was set to 1GHz and the</p><p>sample thickness was inserted in the equipment. Both the complex permittivity and the</p><p>dielectric loss tangent were measured. The Dielectric Test Fixture is displayed in Figure</p><p>16.</p><p>Figure 16: Dielectric Test Fixture connected to the Impedance Analyzer setup.</p><p>The permeability measurements were executed by using the the same Impedance Ana-</p><p>lyzer as for the permittivity measurements</p><p>with another additional measurement setup,</p><p>a Magnetic Test Fixture (16454A) from Keysight (see Figure 17). The sample shape used</p><p>for this setup was a toroidal shape. The sample was inserted in the test fixture, the height,</p><p>and inner and outer radius data were inserted in the Impedance Analyzer software and</p><p>then the measurements were made. The complex permeability and the magnetic loss tan-</p><p>gent were obtained from the commercial software in the Impedance Analyzer. Equation</p><p>19</p>