This paper presents a novel approach for the characterization of microwave properties of carbon-based nanopowders, decorated or not with magnetic nanoparticles. Their microwave parameters, dielectric constant, electrical conductivity, and complex magnetic permeability are extracted from measurements performed using a single transmission line acting as a test cell. Two geometries of transmission line were tested, and successful results were obtained with each one of them. The measurement results are assessed by a phenomenological model enabling to fit the measurement of the dielectric constant and conductivity, providing an insight on the compacity quality of the powder sample. Also, the extraction of the permeability is validated by the detection of a ferromagnetic resonance showing a linear dependence on external DC magnetic field.
Microwave characterization of materials is of prime interest for a number of applications. The knowledge of the electromagnetic constitutive parameters (complex permittivity and the associated electrical conductivity and magnetic permeability) is mandatory to design a number of microwave devices including electromagnetic absorbers, attenuators, and resistors. Over the past years, various methods were proposed using different topologies. The quantities most often measured are the reflection and/or transmission coefficient of the microwave signal in free space in the presence of the material acting as a screen or a reflector, the resonant frequency and loaded
A lot of work has been done in previous years by our team on the characterization of (nano)materials, but embedded in an alumina [
In order to achieve a broadband characterization, we exploit here a property of the line-return-line (LRL) method [
The main originality of our work is that it allows us to extract the parameters of the sole nanopowder. In the literature, nanostructures are usually dispersed in an insulating polymer [
Another originality of our method is to make the retrieval of
Section
This section describes the preparation of the nanocomposite powders under scope.
Three types of graphenic nanocarbon supports (NcS) were employed: graphene oxide (GO), reduced graphene oxide (rGO) (both from Nanoinnova Technologies SL, Spain), and graphene nanoplatelets (GNPs; TIMCAL, Switzerland). These NcS were microwave characterized as received.
Metallic zerovalent iron (ZVI) and nickel (Ni) nanoparticles (NPs) were deposited onto graphene oxide (GO). The synthetic methodologies employed will be briefly described below. In both cases, the engaged quantities aimed a 50% metal/GO weight loading rate (LR). Corresponding TEM pictures are given in Figures
TEM images of graphene oxide decorated with zerovalent iron nanoparticles (ZVI@GO) at different magnifications.
TEM images of graphene oxide decorated with nickel nanoparticles (Ni@GO) at different magnifications.
The preparation of ZVI nanoparticles supported on GO was performed using a Pechini-type sol-gel method. In a typical synthesis, 50 mg of graphene oxide (GO), 180.8 mg of iron (III) nitrate nonahydrate ((FeNO3•9H2O) 99+%, Across), 516 mg of citric acid (CA; analytical grade, Merck), and a magnetic stir bar were introduced in a small round-bottom flask. 3.3 ml of ethanol (TechniSolv, VWR) was added, the flask was closed with a septum, and the mixture was stirred until complete dissolution of the reactants. The suspension was then sonicated at 80 W for 1 hour (VWR ultrasound cleaner USC 1200THD) in order to disperse GO. 112
GO was decorated with nickel nanoparticles using a one-pot solvothermal method. In a typical synthesis, 50 mg of GO was dispersed in 40 ml of ethylene glycol (EG) and sonicated for 120 minutes in a VWR ultrasound cleaner USC 1200-THD at 80 W sonication power. 110 mg of nickel (II) acetylacetonate (Ni(acac)2, 95%, Sigma-Aldrich) was then added to the above suspension under vigorous stirring. Subsequently, an aqueous 1 M NaOH (Normapur, VWR) solution was added dropwise, under vigorous stirring, until a pH value of 10.5 was reached. After stirring for 45 minutes, the mixture was transferred into a 300 ml stainless steel autoclave vessel with a Teflon lining and heated at 190°C for 24 hours. The autoclave temperature was automatically regulated by a Parr 4836 temperature controller (Parr Instrument Company, USA) having an average heating rate of 4°C/min. The autoclave inner atmosphere was never purged or modified in any manner during the synthesis, while the system pressure was left to build up during heating without any type of external control. Reaction solutions were continuously stirred at 300 rpm while heating. When reaction time was completed, the autoclave was immediately immersed in an ice bath and cooled down to ambient temperature. Reaction products were separated from the reaction media by magnetic decantation and then recovered by filtration over a PVDF filter (Millipore GVWP02500, 0.22
Samples were dispersed in hexane by sonication. Three drops of the supernatant were then deposited onto a holey carbon film supported on a copper grid (C-flat, Protochips, USA), and left to dry, overnight, at room temperature under vacuum. TEM images were obtained on a LEO 922 OMEGA Energy Filter Transmission Electron Microscope operating at 120 kV.
Two geometries of transmission lines (TLs) were tested: a coplanar waveguide (CPW) and a microstrip. The CPW device consisted of a 0.762 mm thick Teflon substrate of type Ro4350B onto which a 17
Dimensions of the coplanar waveguide (CPW) line for measurements (not in scale). (a) top view and (b) side view. The Teflon substrate is represented in light grey whereas the top copper layer is colored in dark pink and the nanopowder is black.
Geometry of the microstrip line for measurements (not in scale). (a) top view and (b) side view. The Teflon substrates are represented in light grey and grey white whereas the strip and ground copper layers are colored in dark pink, the cellotape in light blue, and the nanopowder in black.
In both cases, the same method was employed to fill the TL. The nanopowders were mixed with ethanol in order to form a slurry that could be introduced into the TLs’ rectangular cavities. A Teflon spatula was then used to compact the samples. The procedure was repeated thrice in order to compactly fill the cavities. Afterwards, the as-filled TLs were dried overnight under vacuum, at ambient temperature, to eliminate the solvent. Finally, nanopowder leftovers were removed with a thin cotton swab impregnated with ethanol in order to clean the TL surface. After filling, a piece of cellotape was placed on top of the microstrip substrate in order to maintain the low-density nanopowders in the cavities. In the case of the CPW substrate, cellotape was placed before and after filling on the bottom and top sides, respectively. Tests were performed to verify that the cellotape had no effect on the TL line electromagnetic characteristics. It has to be emphasized that the same microstrip (or CPW) line was used for all nanopowders characterized in this paper. Cavities were simply carefully cleaned using ethanol before moving from one powder sample to another.
The microwave response of the empty and sample-filled microstrip and coplanar waveguide lines (CPW) was measured under DC magnetic field applied perpendicularly to the substrate using a NTM 10400M-260 electromagnet supplying magnetic field values ranging from 0 to 9 kOersteds (kOe). The samples were placed in the gap of the electromagnet and connected to one end of a set of long coaxial cables using a pair of Anritsu 36801K right angle launchers. The other coaxial cables ends were connected to the ports of a 12-term calibrated Agilent N5245A PNA-X 70GHz vector network analyzer (VNA), placed away from the electromagnet’s intense DC magnetic field. Data acquisition of the transmission
In this section, we will present the extraction procedure for the characterization of nanopowders. It is important to stress here that the extraction of permittivity and permeability is based on the propagation constant that takes into account the whole cross-section of the TL used, including the surrounding air visible in Figures
Figure
Schematic of the measurement configuration.
As
Permeability calculated according to (
For the microstrip topology of Figure
Ratio of relative by effective permeability
For the CPW line topology of Figure
Finally, we are able to retrieve the contribution of the sole nanopowder by using a simple volumetric factor taking into account the volume occupied by the nanopowder in the cavities as compared to the total volume present around the central strip conductor. If the width of each cavity is noted at
Factor
To summarize, the full procedure for the retrieval of the parameters The effective parameters The parameters The parameters
In order to assess the quality of the extraction method, we have simulated the
Nominal (bold) and retrieved (italics) values of
Case | Topology | |||
---|---|---|---|---|
1 | Microstrip | |||
2 | Microstrip | |||
3 | CPW | |||
4 | CPW |
In order to assess the efficiency of the method, extracted permittivity
Schematic representation of model. (a) Topology of a nanocomposite, (b) equivalent circuit, and (c) two-layered equivalent transmission line.
The inputs for the phenomenological model (
In this section, we present the extraction of parameters for nonmagnetic and magnetic conductive nanopowders characterized with the techniques described in Sections
Dielectric constant and conductivity extracted from (
Dielectric constant and conductivity extracted from (
Dielectric constant and conductivity extracted from (
Dielectric constant and conductivity extracted from (
Figure
It is worth mentioning here that oscillation ripple observed in the experimental curves is due to the residual mismatch between the 50
Also, we have checked the reproducibility of our procedure by making 5 measurements spread over 8 months on a same sample. Observed variation on the extracted permittivity is less than 3%. This illustrates the good repeatability of the extraction procedure and the excellent stability of our samples over time.
As stated in the Introduction, there is no other method published in the scientific literature that allows to directly characterize the EM properties of nanopowders without prior sample preparation. The closest-matching methodology loads the nanopowder into a paraffin matrix, and a toroidal-shape test sample is produced from this mixture [
Figures
Figures
Permeability extracted from (
Permeability extracted from (
FMR dispersion relation for Ni@GO nanopowder, extracted from Figure
FMR dispersion relation for ZVI@GO nanopowder, extracted from Figure
As another assessment, the permeability of our ferromagnetic material can be modelled by a FMR formalism derived from [
In these equations,
In this paper, we have demonstrated the efficiency of a method based on a single transmission line reusable for all samples to extract both complex permittivity and magnetic permeability of low density nanocomposite powders. We present a novel method for the extraction of dielectric constant and conductivity, which is successfully validated by implementing a phenomenological model. This model includes, as input parameters, the dielectric constant and conductivity of nanoparticles and a filling factor modelling the distance between particles. In consequence, it also enables to assess the quality of the filling of the cavities with nanopowders. The magnetic permeability is also extracted and modelled successfully and further assessed by an accurate retrieval of the FMR frequency that closely follows the linear theoretical law. This highlights that the procedure is efficient for this kind of low density samples. Although the proposed extraction method used here is for nanopowders, it can also be applied to other topologies of lines and other materials such as ferrites, liquids, or ferrofluids, provided they can easily be inserted in the cavities.
This is of prime interest for the design of numerous microwave devices such as absorbers, filters, and metamaterials.
The data used to support the findings of this study are included within the article.
The authors declare no conflicts of interest regarding this publication.
The authors thank the participants to the Nano4waves project for fruitful discussions, Mr. Rajkumar Jaiswar for the design of the microstrip line, and Mr. Tommy Haynes for performing the TEM imagery. The authors are grateful to the National Fund for Scientific Research (Fonds De La Recherche Scientifique - FNRS, Belgium) for supporting this research. This work is also supported by the Walloon region and by the “Communaute Française de Belgique”, through the project “Nano4waves” funded by its research program “Actions de Recherche Concertees.”