Dielectric and Magnetic Properties of Epoxy with Dispersed Iron Phosphate Glass Particles by Microwave Measurement

— This paper investigates the dielectric properties and magnetic properties of epoxy composites with different amount of powdered iron phosphate glass (IPG) at 8.2 GHz to 12.4 GHz using microwave technique. IPG composed of 60P 2 O 5 -40Fe 2 O 3 (mol%) was produced by conventional melt and quenching method and crushed into powder. The IPG powder was characterized using energy dispersive X-ray spectroscopy (EDS), particle size analyzer and powder X-ray diffraction (XRD). Different amount of IPG (10-70 wt%) were dispersed in the epoxy. The epoxy-IPG composites were characterized by their morphology, elemental composition and scattering parameters using scanning electron microscope (SEM), energy dispersive X-ray spectroscopy (EDS) and vector network analyzer (VNA), respectively. Reflection coefficient, | S 11 | increases whereas transmission coefficient, | S 21 | decreases with increasing IPG content in epoxy. Dielectric constant, ε r ′ of epoxy-IPG composites were found increased with the IPG content from 3.33 to 4.69.


I. INTRODUCTION
Phosphate glasses are chemical durable compared with most silicate and borosilicate glasses. The addition of iron to phosphate glasses strengthens the chemical bonds in the glass structure and further improves the chemical durability. This property allows iron phosphate glasses to be used in hazardous wastes immobilization [1]- [4]. Iron phosphate glasses exhibit low melting temperature (typically between 950 and 1150 o C). The batch materials of iron phosphate glasses couple well with microwave radiation. Therefore, microwave heating technique is introduced to produce iron phosphate glass [5], [6]. The technique is a fast and energy-saving alternative to the conventional heating technique using furnaces. The microwave radiation interacts with ions and dipoles in the batch materials, and increases their temperature to the melting point [7]. Besides using microwave to heat the batch materials, microwave can also be used to investigate various properties of glasses.
Several microwave measurement techniques such as free-space measurement [8], [9], coaxial probe measurement [10] and waveguide measurement [11], [12] have been employed to investigate Dielectric and Magnetic Properties of Epoxy with Dispersed Iron Phosphate Glass Particles by Microwave Measurement absorption characteristics, shielding effectiveness and dielectric properties of materials. The basic working principle of microwave measurement is based on the interaction between material and incident electromagnetic waves, including reflection, transmission and absorption. However, it is difficult to measure the properties of glasses using microwave measurement techniques because the material under test is normally required to be in specific dimensions and shapes. For instance, material in planar, toroid and rectangular shapes are normally required for free space, coaxial probe, and rectangular waveguide measurement techniques, respectively [13]. Despite this, glasses can be crushed into powders and introduced into polymeric matrices such as epoxy resin, paraffin wax and rubber. The mixture is in viscous liquid form and will solidify after curing in the mould of desired size and shape. Particle filled polymeric composites are often used due to easy preparation and low processing temperature [14]. The composites also provides flexibility in their geometrical design based on the geometry of the mould. In fact, the properties of particle filled polymeric composites were investigated for various applications [15]- [20].
Dielectric properties measurement of phosphate glasses are of considerable research interests because of their applications in various fields of materials science including semiconductors, photonic materials and biomedical materials [21]- [24]. In addition, the dielectric properties of phosphate glasses doped with iron oxides were investigated using microwave measurement technique [12].
Transmission line measurement technique was used to measure the microwave transmission and reflection of samples filled in rectangular waveguide in order to determine the relative complex permittivity and permeability [25]. Furthermore, iron phosphate glasses exhibit interesting electrical and magnetic properties depending on the iron coordination number in which the iron ions can exist in two oxidation states as Fe 2+ and Fe 3+ [26]. However, it is shown that the Fe 2+ ions contribute effectively to the dielectric permittivity due to the interactions of the dipoles with the microwave radiation [27]. The polarization state of the composites can be evaluated by means of measurements of dielectric constant [28]. Although several works have been reported on the dielectric properties of conventional phosphate glasses doped with different transition elements, but there is not much information available on using microwave measurement technique to investigate the powdered iron phosphate glasses dispersed in a polymeric structure. Knowledge of the microwave effects on the glasses may suggest modifications on the composition and processing conditions, which possibly expand their applications.
In this paper, the dielectric properties and magnetic properties of the epoxy composites with dispersed powdered IPG of different composition ratios in X-band microwave frequency range are reported. Epoxy resin is used due to ease of preparation and flexibility in geometrical design. The measured properties of the epoxy-IPG composites can be used to predict the properties of pure IPG.
The powdered IPG are characterized using energy dispersive X-ray spectroscopy (EDS), particle size analyzer and powder X-ray diffraction (XRD). The morphology, elemental composition and scattering parameters of the epoxy-IPG composites are investigated using scanning electron microscope (SEM), homogeneously and placed into a mullite crucible. The powders were heated at a heating rate of 2 °C/min in an electric box furnace (ELITE). The temperature was raised to 1150 °C for 3 hours and the molten glass was poured into a pre-heated stainless steel mould. The obtained glass block were then annealed for 1 hour at 450 °C and cooled to room temperature at a cooling rate of 1 °C/min.
Bulk density of the annealed glass was measured by Archimedes' principle at room temperature using density balance (XS64, Mettler Toledo). The glass was cut into monoliths of 5 x 5 x 20 mm using diamond cutter to be loaded into the dilatometer. Dilatometer (DIL 420C, Netzsch) with 3 °C/min heating rate was used to determine the glass transition temperature (Tg).
The annealed glass was then crushed using a stainless steel percussion mortar and sieved using a test sieve (<75 µm) to obtain powdered glass. Powder density was measured using a helium gas pycnometer (Micromeritics Accupyc II 1340). The chemical composition analysis was done using energy dispersive X-ray spectroscopy (EDS; JSM6400, JEOL). Particle size of the powdered IPG was examined using a laser diffraction particle size analyser (LS 130, Beckman Coulter).
X-ray diffractometer (XRD; D500, Siemens) analysis on the powders was performed to confirm the amorphous property of the glass. The detector operated at 40 kV and 30 mA, utilized the CuKα radiation ( = 1.5406 Å) and scanned over a 2θ range from 10° to 80° at 1°/min intervals with a step size of 0.05°.

B. Epoxy-Iron Phosphate Glass Composite
The epoxy-glass composite samples were prepared by curing the IPG powder in epoxy resin. Liquid epoxy resin (Der 331, Emory) were mixed with hardener (Jointmine 925-3, Emory) at a ratio of 2:1.
The IPG powder were weighted according to its weight percentage in the composite (10-70 wt%) and added to the liquid mixture. The mixture was stirred with a glass rod and poured into an Aluminium Vector Network Analyzer (VNA; E5071C, Keysight) was used to measure the complex reflection 168 coefficient, S11 and transmission coefficient, S21 of the epoxy-IPG composites. X-band rectangular waveguides as shown in Fig. 1 (b) were connected to the vector network analyzer via a pair of coaxial cables. Through-reflect-line (TRL) calibration were carried out before the measurement of the samples. Both S11 and S21 of the samples were measured from 8.2 GHz to 12.4 GHz. The measured S11 and S21 were used to retrieve the properties of the epoxy-IPG composites. Energy absorption coefficient, A of the composites was calculated using Equation (1).
(1) Interfacial reflection coefficient, Γ and interfacial transmission coefficient, Τ were determined using Equation (2) where ko is the free space wavenumber, kc is the cut-off wavenumber and d is the thickness of the composite sample.

A. Iron Phosphate Glass
The chemical compositions and physical properties of prepared IPG are listed in Table 1. The nominal composition of the prepared glass system was 60P2O5-40Fe2O3 (mol%), however 1.07 mol% of Al2O3 was detected by EDS. Contamination by Al was due to the corrosion of the mullite crucibles (b) (a) by the glass melt at high temperature. Bulk density and powder density of the glass are 3.152 ± 0.158 g/cm 3 and 3.1614 ± 0.0009 g/cm 3 respectively. The glass transition temperature, Tg is 457 ± 5 °C. The average particle diameter of the glass powder is 34.8 ± 1.7 µm measured by volume basis and 0.76 ± 0.04 µm measured by number basis. The XRD pattern in Fig. 2 shows diffuse scattering behavior without any crystalline peaks and this confirmed the amorphous character of the glass, which has a random arrangement of atoms.

B. Epoxy-Iron Phosphate Glass Composites
Based on Fig. 3, the bulk density of the epoxy-glass composites increases when the glass content in the composite increases from 0 to 70 wt%. The density of the sample without glass filler is 1.1725 ± 0.0007 g/cm 3 whereas the density of the sample containing 70 wt% of glass content is 1.8966 ± 0.0032 g/cm 3 . The bulk density of the epoxy-IPG composite increases when IPG content increases because the density of IPG is higher than that of epoxy.  was also indirectly proven using the density result (refer Fig. 3).    Based on Fig. 6 (a), the measured |S11| increases when the content of powdered IPG inside the epoxy resin increases. |S11| increases due to the mismatch between the impedance of the composites and the characteristic impedance of waveguide [30]. The impedance mismatch causes the reflection of the electromagnetic signals. Based on Fig. 6 (b), the measured |S21| has an opposite trend compared with |S11|, in which its magnitude decreases when the IPG content increases. There are less transmitted signals (more reflected signals) for composites with higher IPG content. In other words, IPG is a better electromagnetic energy reflector than the epoxy.
Since the measurement used in this work is a closed system technique, absorption coefficient, |A| can be determined using Equation (1). |A| is a parameter to determine the magnitude of incident microwave energy being absorbed and dissipated by the material. Fig. 6 (c) shows the linear magnitude of absorption coefficient, |A|. In general, |A| of the epoxy-IPG composites are all below 0.08. This shows that the absorption capabilities of the composites are insignificant (below ~8 % of the incident energy is absorbed). The incident electromagnetic waves undergo partial reflection and partial transmission with negligible absorption. The composites do not resonate with the electromagnetic wave at X-band frequency range. However, epoxy without IPG fillers exhibits the highest |A| compared with other epoxy-IPG composites. The absorption mechanism of the epoxy is based on its dielectric loss mechanism. Dipole polarization occurs and always lags behind the electrical field inducing it [31]. This lags results in the energy dissipation. Relative complex permittivity, εr and relative complex permeability, µr of composite samples were calculated using Nicolson-Ross-Weir (NRW) conversion technique based on the measured S11 and S21 [29], [32], [33]. The series of curves in Fig. 7 (a) show that the real part of relative permittivity, ɛr′ (c) [35]. The interfacial polarization, which is caused by the accumulation of charges at the interfaces between IPG and epoxy, is enhanced [36]. The presence of intrinsic dipoles formed by charged pairs contributes to the increment of dielectric constant [28], [37].
The imaginary part of relative complex permittivity, ɛr′′ (so-called dielectric loss factor) is in the range of 0.04 to 0.18 as shown in Fig 7 (b). In general, ɛr′ is a measure of how much energy from an external electromagnetic field can be stored in a material whereas ɛr′′ represents how dissipative a material is to the external electromagnetic field. ɛr′′ of the IPG-composites is low (ɛr′′ < 0. 18 (c) which shows that the composite samples exhibit low energy absorption. The absorption coefficient results show that the microwave radiation in the X-band frequency range could not be absorbed by any glassy phase even though there are Fe2O3 in the IPG. Hence, IPG can be considered as a nonmicrowave interactive material at X-band frequency range although its batch materials are a good microwave absorber at certain frequency. This property makes IPG a safer candidate in hazardous wastes immobilization.
Based on Fig. 7 (a), εr′ has considerable consistency of increment trend with each 10 wt% increment of IPG filler content. This proves the applicability of the measurement technique in developing a permittivity sensor for micro-sized particles. εr′ of the particles can be predicted by measuring S11 and S21 of the particles in polymers. Particle-filled polymeric composites can be used to predict the properties of particle fillers which are difficult to be handled in powder form in microwave measurement.