Investigation of the Microwave Absorption Properties of Bi_1.7Pb_0.3Sr₂Ca₂Cu₃O₁₀-Based Ceramic Composites

Abstract

This study investigates the microwave absorption properties of the cuprate ceramic material Bi_1.7Pb_0.3Sr₂Ca₂Cu₃O₁₀ (BSCCO) and its composites with bismuth oxide (Bi₂O₃) in the 4–25 GHz frequency range. Composites with varying BSCCO contents were fabricated and characterized using the Nicolson–Ross–Weir method and Agilent Materials Measurement Software 85071E to determine complex permeability and permittivity. The 4 wt.% BSCCO composite exhibited a peak reflection loss of −32.6 dB at 12.5 GHz, while the 40 wt.% BSCCO composite reached a 52% microwave absorption ratio at 23 GHz. These results demonstrate that microwave absorption is strongly influenced by dielectric properties and the ratio of BSCCO and Bi₂O₃ composites. This work highlights the potential of BSCCO-Bi₂O₃ ceramics for microwave absorption applications, particularly in environments experiencing significant temperature gradients due to their thermal stability and electromagnetic performance.

1. Introduction

The attenuation through the absorption and shielding of electromagnetic (EM) waves have become a subject of research due to the potential applications in the military, medical, and other areas [1,2,3,4]. Militaries have a particular interest in EM wave attenuation so as to make vessels resistant to electromagnetic interference (EMI). With the increasing use of digital communication devices, radio frequency (RF) wave energy can produce deleterious health effects on the human body, which is an issue that can be alleviated by microwave absorption materials (MAMs). To ascertain if a material is a proper candidate for EM attenuation and applicable, the EM wave absorption and EMI shielding properties, as well as the complex permittivity (ε′—jε′) and permeability (µ′—jµ″) of the material must be studied. The quantities ε′ and µ′ are associated with energy storage, and ε″ and µ″ are associated with energy dissipation. From the analysis of permittivity and permeability, an ideal MAM for EMI shielding should also have good EM wave absorption capabilities and a minimal reflection coefficient. Metals are commonly used for EMI shielding because they attenuate electromagnetic waves very well. However, metals tend to reflect EM waves rather than absorb them, meaning that they can still contribute to electromagnetic pollution in the surrounding environment. For some applications, materials with high EM absorption and low reflection are preferred, but developing such materials is still a challenge.

For the MAMs to be applicable, the MAMs should also have good mechanical properties and a wide absorption bandwidth [5]. A common method for EM attenuation is conducted with ferrite-based polymer composites [6,7]. Another group of MAMs studied incorporated carbon nanotubes (CNTs) embedded within polymer matrices, yielding excellent results [8]. However, polymers used to embed CNTs or ferrites face challenges in maintaining mechanical properties under large temperature gradients due to inherent limitations such as low melting points and thermal degradation [9,10]. Polymers have a specific temperature range known as the glass transition temperature T_g. Below T_g, polymers are rigid and brittle, while above T_g, they become soft and rubbery. Large temperature gradients can cause parts of the polymer material to cross T_g, leading to inconsistent mechanical properties within the matrix.

In environments where large temperature gradients are encountered, such as the vacuum of space around Earth’s orbit, ceramics should be considered because of their heat resistance and resistance to ultraviolet (UV) degradation [11,12,13]. Ceramics for high-temperature environments for microwave absorption is an area of active research, with the focus of the dielectric loss properties of ceramics [14,15]. Ayeni et al. studied the microwave absorption properties of Mullite-Al₂O₃-SiC ceramic composites with activated carbon using wide sintering temperatures and demonstrated that the introduction of activated carbon can improve dielectric properties and reflection loss [16]. Xiang et al. investigated porous heterogeneous SiC/SiO₂ microspheres and found that porous structures in the material exhibited good reflection loss properties [17]. Ceramics also have an advantage over metals, in that they are more chemically stable to chemical reactions such as corrosion. Ceramics have also proven to have high tensile strengths. In order to have ceramics be a suitable material for EM absorption, the synthesis techniques must be optimized so that the porosity, particle size, magnetic properties, and dielectric properties can contribute to absorption. Silica-based ceramics have been studied as a candidate for EM absorption, but one of the challenges is high heat treatment temperatures [18]. A class of ceramics that does not require such high heat treatment is copper-oxide-based ceramics, commonly called cuprates. Cuprates can also be high-T_c superconductors, allowing electrical current conduction with zero measurable resistance. This work does not focus on the superconducting properties of cuprates but rather explores their EM wave absorption and EMI shielding properties, which are not well studied in this field.

The cuprate ceramics possess promising microwave absorption properties, particularly when particle sizes are sufficiently small to exploit skin depth effects [19]. Ceramics made with cuprates could be used, for example, as tiles for vessels to shield critical components from electromagnetic interference, or conceal them from radar detection in the low-Earth orbit environment. Although ceramics are typically electrical insulators with poor intrinsic microwave absorption, their absorption capabilities can be enhanced by doping with conductive particles [20]. Cuprate ceramics such as BSCCO stand out as conductive ceramics with superior electrical conductivities compared to conventional ceramics, making them suitable candidates for microwave absorption applications [21].

In this work, Bi_1.7Pb_0.3Sr₂Ca₂Cu₃O₁₀ (BSCCO)-based ceramic composites with Bi₂O₃ (bismuth oxide) were investigated to understand their microwave absorption properties. From a thermodynamic and bonding perspective, oxide–oxide interfaces (BSCCO-Bi₂O₃) can exhibit a higher intrinsic work of adhesion than a typical metal–ceramic contact because of oxygen-mediated ionic/covalent bonding [22]. Moreover, Bi₂O₃ can act as an effective sintering aid that promotes wetting and liquid-phase necking at moderate temperatures, increasing real contact area and producing graded interfacial layers that increase interfacial fracture energy and retard de-cohesion [23]. Previous studies [16,17] primarily reported the data of the reflection loss of their samples. In contrast, this work provides a comprehensive characterization of the microwave absorption properties of these composites, including reflection loss, transmission loss, and absorption ratio.

2. Experimental Preparation

2.1. Material Synthesis

For this study, the synthesis of BSCCO was achieved by combining bismuth oxide (Bi₂O₃), lead oxide (PbO), strontium carbonate (SrCO₃), calcium carbonate (CaCO₃), and copper oxide (CuO). The mentioned compounds were added in ratios to achieve Bi_1.7Pb_0.3Sr₂Ca₂Cu₃O₁₀ [24]. The samples were then mixed with a mortar and pestle and then transferred to a stainless-steel die. The die was transferred to a press and cold-pressed in a tonnage machine at between 4 and 5 tons for about five (5) minutes. When removed from the cold-press, the BSCCO was sintered five (5) times in a box furnace for twenty (20) hours at temperatures of 790–830 °C using ten (10) degree increasing increments for each sintering period. Between the increments, the sample was crushed, re-pressed, and re-sintered.

After the final sintering of the BSSCO at a temperature of 830 °C, Bi₂O₃ powder was then used as the matrix to make the BSCCO-based composites. The BSCCO was crushed into a powder and then mixed with Bi₂O₃. BSCCO’s percentages in the composites were controlled at 2%, 4%, 10%, 20%, and 40% when combined with Bi₂O₃. For each percentage, the BSCCO powder was mixed with Bi₂O₃ powder and sintered four (4) times in the box furnace at a temperature of 710 °C for 20 h. The sintering as a combined powder of BSCCO and Bi₂O₃ was repeated three (3) times at a temperature of 710 °C for 20 h, and the combined powder was agitated before each subsequent sintering period. The combined powder was then placed in a cold-pressed die at a tonnage of one (1) ton. The final dimension of the pellet was that of a hollow cylinder with an inner diameter of 1.5 mm and an outer diameter of 3.5 mm with a length of about 4.5 mm. After being cold-pressed into a pellet, the sample was re-sintered for the fourth and final time at a temperature of 710 °C for twenty (20) hours. Figure 1 shows the steps of the synthesis.

2.2. Superconducting and SEM Properties

In this experiment, BSCCO was tested using the Quantum Design Physical Property Measurement System (PPMS). A critical temperature (T_c) of 77 K for zero-field cooling and a T_c of 72 K for field cooling were achieved, which is indicative of a superconductor. For field cooling, a magnetic field of 15 Oe was applied to the sample (Figure S1). The Meissner effect was observed in the BSSCO along with its diamagnetic properties under both zero-field and field cooling, which was also more evidence that a superconductor was successfully made. Magnetic flux trapping was also observed when the sample was under field cooling. The resistivity of the BSCCO sample was measured to be 0.01047 Ω·m at a temperature of 300 K under a 0.1225 Oe magnetic field (Figure S2).

The scanning electron microscopy (SEM) morphologies of the powder samples were analyzed at the micrometer level, which was achieved with a Phenomworld system (Model: MVE0265761429) manufactured by nanoScience Instruments. The Bi₂O₃ powder was measured to have an average particle size of 2.96 µm (Figures S3 and S4), and the BSCCO powder was measured to have an average particle size of around 1.91 µm (Figures S5 and S6). The SEM images of the composite powder sample with 40 wt.% BSCCO powder indicated an average particle size of around 1.66 µm (Figures S7 and S8). The SEM images of the solid composite samples were obtained with a ThermoFisher Scientific Quanta 3D DualBeam FEG FIB-SEM (Waltham, MA, USA), as shown in Figure 2, which reveal that the composite surfaces are rough and jagged, indicating some roughness and porosity at the micrometer level, with the BSCCO particles being randomly distributed within the Bi₂O₃ particles. Porosity may also have a role in microwave absorption through multiple scattering and dissipation of the EM waves inside the sample [25]. Surface mapping was carried out with electron dispersive spectroscopy (EDS) on the surface of the 40 wt.% BSCCO sample using an Ametek EDAX Pegasus EDS manufactured by Gatan, Inc. (Warrendale, PA, USA) Figure S9, which also shows a random distribution of the BSCCO elements within the sample (Figure S9).

2.3. Crystalline Structures

The crystalline structures of the materials were analyzed using a Rigaku MiniFlex 600 X-ray diffractometer (XRD) equipped with Cu Kα radiation (λ = 1.5405 Å), with the operational conditions of 36 kV and 20 mA. Figure 3 shows the results of the samples with 4%, 10%, 20%, and 40% weight loadings of BSCCO, along with pure BSCCO and Bi₂O_3. The pure Bi₂O₃ has the phase δ-Bi₂O₃ with discernable peaks at 2θ ≈ 28°, 32°, 46°, and 55°, which is indicative of a cubic structure. The pure BSCCO is in an orthorhombic structure in Bi-2223 phase [26,27], with discernable peaks at 2θ ≈ 18°, 22°, 30°, 31°, 34°, 39°, and 48°. The peaks of δ-Bi₂O₃ are visible throughout the composite samples with various wt.% of BSCCO, and Bi-2223 peaks become more visible and discernable as its wt.% BSCCO is increased.

3. Microwave Absorption

Microwave EMI shielding can be achieved in a material by either increasing the absorption of the incident EM wave or decreasing the reflection over a large frequency bandwidth. The EM wave absorption of the material can depend on multiple factors such as impedance matching, dielectric losses, and magnetic losses [28]. When the EM wave is incident on the material, absorption, reflection, and transmission will occur, and these traits will be studied. In this work, the analysis and measurements of these traits were performed by ascertaining the S-parameters in the measurement using an Agilent Network Analyzer N5230C PNA-L and a coaxial transmission line method in the frequency range from 4 to 25 GHz. The reflectance (R), transmittance (T), and absorbance (A) can be calculated using the measured S-parameters, as follows:

R = |S₁₁|² = |S₂₂|²

(1)

T = |S₁₂|² = |S₂₁|²,

(2)

and

A = 1 − R − T,

(3)

where S₁₁, S₂₂, S₁₂, and S₂₁ are the measurements of the S-parameters.

3.1. Reflection, Transmission, and Absorption Properties

Figure 4 shows the microwave reflection loss (RL) properties of the composites with various weight percentages of BSCCO over the frequencies 4–25 GHz. In Figure 4, the negative in dB in reflection means the reflection loss. The composite samples show some high reflection losses (or reflection dips) at various frequencies. An RL value at −10 dB corresponds to 90% of EM wave reflection reduction and is regarded as effective for some applications. A reflection loss of −20 dB corresponds to 99% of EM wave reflection reduction. The composite samples with 4, 10, 20, and 40 wt.% BSCCO show some high reflection losses in the X-band frequency region in the electromagnetic wave spectrum.

Table 1. RL peak values, positions, and effective bandwidth for the composite samples with different BSCCO weight percentages.

Sample	1st RL Peak			2nd RL Peak
	Value [dB]	Peak Position [GHz]	Effective Bandwidth [GHz]	Value [dB]	Peak Position [GHz]	Effective Bandwidth [GHz]
0 wt.%	−14.0	10.3	1.45	−28.7	18.6	1.19
4 wt.%	−32.6	12.5	2.51	−20.8	20.2	2.64
10 wt.%	−23.3	11.3	1.98	−14.9	19.2	1.06
20 wt.%	−30.7	10.0	1.46	−19.4	18.6	1.98
40 wt.%	−11.0	9.08	0.523	−12.1	23.7	1.45

shows the RL peak (or reflection dip) values and effective bandwidth (frequency bandwidth where RL is less than −10 dB) for the samples with different BSCCO loadings.

The transmission properties of the samples with various BSCCO weight percentages are shown in Figure 5. The negative dB in transmission means the transmission loss. The 40 wt.% BSCCO sample had a transmission loss of −7 dB at 4.9 GHz with the widest band of absorption. Figure 6 shows the absorption ratio of the composite samples with varying weight percentages of BSCCO in the samples. The composite sample with 40 wt.% BSCCO had an absorption of 52% around 23 GHz, which had the widest band of absorption. As the weight percentage increases, the absorption bands widen and allow for a greater frequency range of absorption. Because the samples were formed from the cold-presses of powders and heat treatment procedures as discussed in the Material Synthesis Section, the composite samples possess a large amount of porosity and interfaces, which can cause multiple scattering and absorption of electromagnetic waves. The porous structures inside the composite samples may also play some kind of role in resonant absorption, which may lead to “resonant-like” features in reflection, transmission, and absorption, as shown in Figure 4, Figure 5 and Figure 6. Further understanding may require deeper theoretical studies beyond the scope of this experimental report for the phenomena.

3.2. Dielectric and Magnetic Properties

The effective complex permeability µ and permittivity ε were also studied for microwave absorption. The real parts of the permeability µ′ and permittivity ε′ are related to the magnetic and electrical energy a material can store, respectively. The imaginary part of the permeability µ″ and permittivity ε″ are related to the magnetic and electrical energy that a material can dissipate through magnetic and dielectric loss, respectively [29]. For efficient microwave absorption, materials should ideally have both dielectric loss and magnetic loss to fully interact with EM waves [30]. In this work, the complex µ and ε were determined for the samples using the Nicolson–Ross–Weir method, and the measured S-parameters were determined using the Agilent Materials Measurement Software 85071E in the calculations. As discussed in the above sections, the porous structures and interfaces in the composite samples can also lead to a complex feature in the effective complex permittivity ε and permeability µ, as shown in the following sections.

Figure 7 shows the ε′ for the composite samples of various weight percentages of BSCCO. It was observed that as the BSCCO weight percentage increases in the samples, the ε′ also exhibits an observable increase. The Bi₂O₃ is a wide band gap insulator. The sintering of BSCCO—Bi₂O₃ bulk samples can lead to oxygen vacancies, which can cause a “hopping” between neighboring ion sites. Interfacial polarizations, along with the introduction of BSCCO, can contribute to mobile charge carriers, which cause dielectric dispersion. Figure 8 gives the ε″ for the samples. The 40 wt.% BSCCO sample shows a significantly high and wide ε″ across a wide frequency spectrum as compared to other samples due to the porous structures and interfaces, which can cause multiple scattering and even resonant absorption peaks of microwaves, as shown in Figure 8. The 40 wt.% BSCCO sample shows a high and wide ε″ across a wide frequency spectrum as compared to other samples. The µ′ and µ″ are shown in Figure 9 and Figure 10, respectively. The behavior is indicative of a non-ferromagnetic material, where relaxation and eddy current effects occur as a result from the microwave signal. As BSCCO is introduced, the measurements of µ″ of the samples exhibit weak magnetic loss. The complex permeability results are due to the fact that BSCCO is a cuprate superconductor; however, its diamagnetic properties can only be observed at relatively low temperatures below the transition temperature T_c [31]. In the normal state at around room temperature, BSCCO may still possess some weak magnetic properties [32]. In this case, dielectric loss is much greater than the magnetic loss; therefore, the absorption depends largely on the dielectric properties.

4. Conclusions

In this study, BSCCO-based ceramic samples with different weight percentages blended with Bi₂O₃ were synthesized using the cold-press and sintering techniques. XRD results show that δ-Bi₂O₃ and Bi-2223 of BSCCO were synthesized. The microwave absorption properties of the composite samples with various BSCCO weight percentages were studied for their microwave reflection, transmission, dielectric loss, and magnetic loss in a frequency range of 4–25 GHz. The studies showed that increasing the BSCCO weight percentage in the sample resulted in an increasing microwave absorption percentage and wider frequency band, as demonstrated by the 40 wt.% BSCCO sample. The measurements show that the dielectric losses contribute more to the microwave absorption than the magnetic losses. The increase in the BSCCO weight percentage also led to higher dielectric loss, which was indicated by the complex permittivity in the 40 wt.% BSCCO sample. The results show that BSCCO-based ceramic composites at various weight percentages could have some potential applications where polymers would be inapplicable.