A wide variety of engineered materials have been developed that exhibit advanced magneto-dielectric properties. Such materials can significantly extend the range of microwave characteristics found in common substrates, thus improving the performance of microwave components. In particular, ferrites, ferroelectrics and multiferroics have been widely studied as functional materials with enhanced microwave properties while enabling tunable microwave devices. In addition, polymer nanocomposites with unique absorption properties have been identified as an effective functional material for microwave electromagnetic interference (EMI) shielding. Emerging types of metamaterials show promising magneto-dielectric properties. These materials often involve the inclusion of multiple layers and/or periodic resonant arrays in order to produce tailored microwave properties, although typically within a relatively small bandwidth. Magneto-dielectrics have been shown to enable considerable improvements in the bandwidth and/or size reduction of microwave antennas. However, the works do not simultaneously satisfy many crucial requirements for microwave device applications such as low dielectric and magnetic losses, low power consumption, low biasing electric or magnetic fields, structural simplicity and ease of integration with existing fabrication processes.
One of the promising ways to develop materials showing magneto-dielectric properties is to exploit polymer composites reinforced with magnetic nanoparticles. However, the dispersion of nanoparticles into a polymer matrix has been a challenging task for nanocomposite fabrication. Since the polymer matrix and inorganic nanoparticles often possess different polarities, a simple blending of particles and polymer will result in aggregation of particles. In fabrication of magneto-dielectric materials, the key challenge is the formation of morphologically controlled and highly ordered arrays of nanoparticles over an extended area.
Due to the challenges in the development of these magneto-dielectric materials, there has been very little progress in exploring their potential for tunable microwave applications. There is still a need for improvement with respect to keeping the enhanced magneto-dielectric properties in a wider frequency range, lowering fabrication complexity and reducing size and cost. As a result, extensive utilization of polymer nanocomposites has yet to occur.
Accordingly, there is a need in the art for a low-loss microwave material that exhibits wide tunability of its effective dielectric and magnetic properties.
The invention is a new superparamagnetic Magneto-Dielectric Polymer Nanocomposite (MDPNC), with uniformly dispersed magnetic nanoparticles that has resulted in a new class of low-loss microwave substrates with very large tunability of the dielectric and magnetic properties. The substrate losses were retained at a negligible level due to the mono-dispersion of the sub-10 nm magnetic nanoparticles with zero coercivity and zero remanence, thus leading to low hysteresis losses. These properties provide the potential for advanced performance in many microwave and high-speed devices, including the capability to vary the operating characteristics of the devices (e.g. the operational frequency) in real-time.
In a particular embodiment, a superparamagnetic low-loss polymer nanocomposite is provided comprising magnetic nanoparticles coated with a surfactant and substantially uniformly dispersed in a low-loss polymer. In a particular embodiment, the magnetic nanoparticles are Fe3O4 nanoparticles having an average diameter of 8 nm. In an additional embodiment, the magnetic nanoparticles are CoFe2O4 nanoparticles having an average size of 10 nm. Other magnetic nanoparticles are within the scope of the present invention. The surfactants used in the formation of the superparamagnetic low-loss polymer nanocomposite may include oleylamine and oleic acid. Other surfactants are within the scope of the present invention. In a particular embodiment, the low-loss polymer is about 25% butadiene resin and copolymer dissolved in xylene. Other low-loss polymers are within the scope of the present invention.
The present invention provides a method for preparing a polymer nanocomposite, useful as a superparamagnetic low-loss material. The method includes, coating magnetic nanoparticles with surfactants, dissolving a low-loss polymer and the coated magnetic nanoparticles in hexane and magnetically stirring the dissolved low-loss polymer and coated magnetic nanoparticles to obtain a polymer nanocomposite having substantially uniform dispersion. The magnetic nanoparticles may be Fe3O4 nanoparticles or CoFe2O4 nanoparticles. In addition, other magnetic nanoparticles are within the scope of the present invention. The surfactants used in the formation of the superparamagnetic low-loss polymer nanocomposite may include oleylamine and oleic acid. Other surfactants are within the scope of the present invention. In a particular embodiment, the low-loss polymer is about 25% butadiene resin and copolymer dissolved in xylene. Other low-loss polymers are within the scope of the present invention.
In accordance with the present invention, novel superparamagnetic magneto-dielectric polymer nanocomposites are synthesized using a novel process. The tunability of the dielectric/magnetic properties demonstrated by this novel polymer nanocomposite, when an external DC magnetic field is applied, exceeds what has been previously reported for magneto-dielectric polymer nanocomposite materials.
In a particular embodiment, Fe3O4 (magnetite) nanoparticles with mean size of 8 nm and coated with surfactants (oleylamine and oleic acid) were synthesized following a standard chemical co-precipitation procedure. The low-loss polymer was dissolved in hexane along with different amounts of surfactant-coated nanoparticles to obtain the polymer nanocomposites with uniform dispersion of nanoparticles at the desired concentration.
The surfactant plays a dual role in the synthesis of polymer nanocomposites. The surfactant completely encapsulates and isolates individual particles and thus weakens the magnetic exchange interactions between them. Moreover, the choice of surfactant is also important to enhance the binding between the macromolecular chains of the polymer and the individual nanoparticles; this binding prevents the particle diffusion during the formation of the polymer nanocomposite, thus effectively suppressing the tendency of agglomeration. This chemical process is important because the goal is to retain the superparamagnetic properties of both individual magnetic nanoparticles and entire macroscopic magneto-dielectric polymer nanocomposite (MDPNC) material, even at relatively high packing densities. Superparamagnetism or lack of coercivity, which implies no hysteresis losses, is desirable for low-loss microwave magnetic materials. Thus the polymer nanocomposites in accordance with the present invention are well-suited for implementation of numerous tunable and low-loss RF and microwave devices.
In order to verify that the superparamagnetic response retained in the MDPNC, due to the homogenously dispersed ferrimagnetic nanoparticles in the polymer matrix, magnetization measurements were done by Physical Property Measurement System (PPMS).
To evaluate the tunability of the MDPNC in accordance with the present invention, a multi-layer microstrip linear resonator (MLR) filled with polymer nanocomposites was designed to study the variation of the MDPNC's microwave properties. The resonance frequency of the microstrip resonator depends on the effective material properties of the substrate given by:
where lr is the length of the center conductor in the MLR and n represents the nth resonant frequency of the MLR. The characteristic impedance of the microstrip feed lines was designed to be 50Ω.
The MLR test fixture is formed by bonding two printed circuit board (PCB) laminates together. The RF PCB laminate chosen was a thermoset low loss polymer composite (∈r=10.2, tan δ=0.0023) with a thickness of 635 μm, which offers a high dielectric constant and thus good contrast with the MDPNC material. The MDPNC is deposited in the 435 μm cavity (bottom laminate), and heated in a vacuum oven at 90° C. for 4 hours to harden (cure) the composite materials.
Two-port S-parameters measurements were performed and concurrently, an external magnetic field in the range from 0 to 4 kOe is applied to the MLR. The orientation of the magnetic field lies perpendicular to the direction of signal propagation. The base material was composed of ferromagnetic nanowires under magnetic fields up to 9 kOe to modulate the response of such device. In the case of the MDPNC in accordance with the present invention, a maximum magnetic field of 4 kOe was needed to obtain the peak performance due to the superparamagnetic nature of the material. This field can be easily obtained using small and low-cost commercial Neodymium magnets.
The influence of the DC magnetic field on the resonance frequency of the device is shown in
A non-resonant multi-layer microstrip transmission line was employed to extract the microwave properties of the MDPNC (e.g., ∈r, μr and tan δ). The structure of this device is similar to the multi-layer MRL. The key difference is that the through transmission line between the two ports is uninterrupted.
Microwave properties of the material were extracted using an improved technique derived from the Nicolson-Ross-Weir method, and a conformal mapping method was used to extract analytical relations for the filling factor of the multi-layer structure.
∈r, μr and tan δ were extracted from 0.65 to 6 GHz at room temperature conditions (300 K) and plotted vs. the applied magnetic biasing field.
As shown in
The accuracy of the parameters extraction procedure has been validated by comparison of the calculated attenuation obtained from the extracted material properties, and the attenuation calculated from the S-parameter measurements. From the extracted parameters:
The calculated attenuation from the extracted parameters is expressed as:
On the other hand, the measured attenuation is calculated from the S-parameters:
The present invention illustrates that magneto-dielectric polymer nanocomposites with 8 nm Fe3O4 nanoparticles have great potential to be implemented in the fabrication of low-loss and tunable microwave substrates and devices. An important novelty of such material resides in its superparamagnetic properties that guarantee low loss at microwave frequencies. Implementing a MLR as the carrier of the MDPNC, measured frequency tunability of 57 MHz, and marked enhancement of the quality factor from 13 to 67 (5.1× improvement) were achieved with an externally applied DC magnetic field of less than 4 kOe. The observed variations in the resonance frequency, insertion loss and quality factor of the fabricated device clearly indicate the large sensitivity of the device to magnetic bias fields. Undoubtedly, this nanocomposite material shows fascinating properties that has never been reported and will be applicable in improved microwave device applications.
In an additional embodiment, CoFe2O4(CFO), which is a well-known hard magnetic material in its bulk form with large coercivity, exchange bias and high saturation magnetization is used in the fabrication of magneto-dielectric polymer nanocomposite in accordance with present invention.
In a particular embodiment of the present invention the high temperature synthesis for CoFe2O4NPs includes taking 2 mmol of a mixture of cobalt (II) acetylacetonate and iron (III) acetylacetonate in 1:2 ratio by weight. Then the mixture was added to 10 mmol 1,2, hexadecanediol, 6 mmol oleic acid, 6 mmol oleylamine, and 20 ml benzyl ether. The mixture was heated to 200° C. for 2 h with constant stirring and then reflexed at 300° C. for 1 hour in the presence of Ar gas. The reaction mixture was allowed to cool to room temperature and ethanol was added to the cooled mixture. The black precipitate was separated by centrifugation. The final product CoFe2O4NPs was dispersed in hexane. The resulting NPs were 10±1 nm in size on average and had no obvious indication of agglomeration over several regions of the samples observed, as verified by transmission electro microscope (TEM) images.
The polymer nanoparticle composites (PNCs), consisting of a thermoset low loss polymer composite polymer and CFO, were prepared by adding a calculated amount of CFO to the polymer by weight to get the desired compositions. Nanocomposites with 30, 50 and 80% wt of CFO in the thermoset low loss polymer composite were prepared. Both the polymer and the NPs were dissolved in hexane and magnetically stirred overnight to obtain uniform dispersion.
To test the CoFe2O4 polymer nanocomposite, the microstrip test fixture previously was utilized. The x-ray diffraction (XRD) patter of CFO NPs is shown with reference to
To examine the dispersion of CFO NPs in the polymer matrix, TEM images of the PNCs were taken. TEM images of 30, 50 and 80% wt PNCs are depicted in
The magnetization (M) measurements were done in the temperature (7) range from 330 degrees K down to 10 degrees K and magnetic fields (H) up to 50 kOe using a commercial physical properties measurement (PPMS). The DC magnetic characterizations were done using field cooled-zero field cooled (FC-ZFC) mode M-T and M-H hysteresis loop measurements in ZFC mode. For this purpose the samples were loaded in a standard gelatin capsule.
where K is the magnetocrystalline anisotropy, V is the volume of the nanoparticle and kB is the Boltzmann constant. It can be observed from
In the present invention, TB remains nearly constant for all PNCs, with the value being exactly same as CFO NPs, which indicates that the interparticle interactions are less prominent here because of a homogeneous dispersion of particles with average size of 10±1 nm in the polymer matrix, as shown in TEM. It also indicates that the surfactant coating of the particles is robust and preserved during the PNC formation. This observation is very important for tunable microwave applications as problems with particle dispersion are known to affect the response and often yield results that are not reproducible from sample to sample.
In order to investigate the superparamagnetic nature and magnetization profile of the PNCs, M-H data have been measured at 300 degrees K and 10 degrees K.
The saturation magnetization (Ms) increases with increasing particle loading in the composites, as shown in the table of
To test the microwave response of these PNCs, a two-port microstrip linear resonator was designed using the multilayer structure shown schematically in
in which μr and ∈r are the effective permeability and permittivity, respectively, for the multilayer system.
The influences of the DC magnetic field on the resonance frequency and quality factor of the microstrip linear resonators with 80 wt %, 50 wt % and 30 wt % loadings of PNC are shown in
However, as compared with the 80 wt % sample of PNC, the other samples with reduced loading of 50 and 30 wt % only demonstrate subtle changes in their measured frequency responses under the influence of the externally applied DC magnetic field, as shown in
The present invention illustrates the successfully synthesis of three different thermoset low loss polymer nanocomposites embedded with CoFe2O4 nanoparticles and a uniform particle dispersion has been achieved throughout the polymer matrix, as shown in the TEM images. Magnetic measurement data revealed superparamagnetic behavior at room temperature for all the PNCs. The important magnetic parameters, namely blocking temperature, coercivity and reduced remnant magnetization, do not vary with changing loading percentage of the NPs. A strategically designed multilayer microstrip linear resonator embedded with different loadings of PNC was chosen as a test fixture to evaluate the susceptibility of the microwave properties of the PNC under the influence of an externally applied magnetic field. For the device with 80 wt % loading, a measured frequency tunability of 518 MHz, and marked enhancement of the quality factor from 2 to 11.46 (5.6 fold improvement) were achieved with an externally applied DC magnetic field of less than 4.5 kOe. The observed variations in the resonance frequency, insertion loss and quality factor of the fabricated device clearly indicate the high sensitivity of the device to magnetic bias fields. Significant microwave responses are observed for the highest CFO loading nanocomposite. On the contrary, devices with reduced loading of magnetic nanoparticles demonstrated much less severe changes in their measured responses, such as the resonance frequency and quality factor, under the influence of the externally applied DC magnetic field. Clearly, loading of PNC beyond a certain threshold value might be preferred to enable great tunability of the nanocomposite material. However, the incorporation of higher levels of magnetic nanoparticles also slightly compromises the performance of the device by introducing additional losses. A design strategy taking into account all of the performance metrics would provide a guideline to achieve the best tradeoff between tunability and losses for this new class of nanocomposite materials.
This application is a continuation of and claims priority to International Patent Application No. PCT/US2011/38366 which was filed on May 27, 2011 which claims priority to U.S. Provisional Patent Application No. 61/349,053, filed on May 27, 2010 which is hereby incorporated by reference into this disclosure.
This invention was made with Government support under Grant No. CMMI #0728073 awarded by the National Science Foundation. The Government has certain rights in the invention.
Number | Date | Country | |
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61349053 | May 2010 | US |
Number | Date | Country | |
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Parent | PCT/US2011/038366 | May 2011 | US |
Child | 13686447 | US |