An Inexpensive Route to Synthesize High-Purity CrO_𝟐 for EMI Shielding in X-Band Frequencies

Abstract

Rod-shaped high-purity samples of CrO₂ have been synthesized by an inexpensive and simplified procedure. Here, we have prepared pure CrO₂ without applying any external pressure or control it during synthesis. The sample prepared exhibited an improvement in saturation magnetization values, 68 emu/g at 300 K, 136 emu/g at 80 K, and uniform grained microstructure. The complex permittivity, permeability, and microwave absorption properties of high-purity CrO₂ sample were investigated in the 8.2–12.2 GHz (X-band) microwave frequency range. Microwave measurements have shown the high shielding effectiveness due to absorption (SE_A) of 20.3 dB. The high value of SE_A suggests that CrO₂ can be used as a promising electromagnetic shielding, EMI, material in 8.2–12.2 GHz (X-band) microwave frequency range.

1. Introduction

The ferromagnetic chromium oxide (CrO₂) is considered to be one of the most promising candidates for new generation of spintronics devices [1–3] and is a well-established magnetoresistive material. CrO₂ is a peculiar compound, which behaves as a half-metallic ferromagnet, with a Curie temperature T_c ~ 114°C [4, 5]. It shows giant magnetoresistance (GMR) and optical properties for spintronics and optoelectronic devices [6]. There is an increased interest in shielding against electromagnetic radiation [7] in commercial, military equipments, scientific electronic devices, and communication instruments that are widely being used. The ferromagnetic CrO₂ with high permeability has been utilized in many microwave applications [8–12]. This study is inspired by the recent advances in the development of materials with magnetically controlled attenuation.

2. Experimental

The most widely accepted synthesis route for CrO₂ is initiated by using a highly hygroscopic compound CrO₃, which is treated at elevated pressures (270 GPa) in raw form or by mixing with other oxides of Cr and some catalysts that are often needed to bring down the working pressure [13–16]. In this paper, we report microwave absorption properties of high-purity sample of CrO₂ synthesized by a simplified procedure, where the pressure is not a control parameter, thus drastically reducing the cost of production. This synthesis technique involves a simple two-step process. The first step involves the preparation of intermediate precursors oxide, which can be prepared under ambient pressure by heating CrO₃ for 3 hours at a temperature of 250°C in air. In a second step, the sample is sealed in an evacuated quartz tube at ambient pressure followed by palletization and subsequently treated in furnace at a temperature of 400°C for 3 hours.

3. Results and Discussion

The XRD pattern of the calcined powder of CrO₂ determined by Rigaku Miniflex II, step size = 0.02, with Cu Kα radiation of wave length λ = 1.5406 Å, is shown in Figure 1(a). It is observed that the sample consists of the pure CrO₂ phase confirmed by PCPDFWIN card no. 76-1232. The lattice parameters corresponding to the tetragonal structure of CrO₂ (P4₂/mnm) were calculated, a = 4.421 Å and c = 2.913 Å. Figure 1(b) shows the SEM micrograph of pure CrO₂ sample which depicts uniform grain size distribution.

(a)

(b)

Figure 1 

(a) XRD diffraction pattern of pure CrO2 and shielded sample in quartz tube. (b) Scanning electron micrograph depicting long rod-like particles of CrO2 (~4 μm).

The magnetic properties of a representative CrO₂ sample have been determined by the M-H hysteresis loop at 300 K and 80 K as shown in Figure 2. The saturation magnetization (Ms) value of the sample has been measured as 68 emu/g and 136 emu/g at 300 K and 80 K, respectively, by vibrating sample magnetometer (Lakeshore, 7304). The saturation magnetization value obtained in our samples is higher as reported by A. Bajpai and A. K. Nigam [15]. The coercive force is found nearly the same at 300 K and 80 K as shown in the inset of Figure 2. Curie temperature T_c = 391 K is depicted from M-T curve at 3 k Oe for CrO₂ sample as shown in the inset of Figure 2. Comparisons of relevant physical parameters as exhibited by our sample to those reported in literatures are given in Table 1.

Table 1 

Comparisons of relevant physical parameters as exhibited by our sample to that are reported in literatures.

Figure 2 

M-H curves at 300 K and 80 K and M-T curve (inset) of CrO2 sample.

The complex permittivity, permeability, and 𝑆₁₁ (𝑆₂₂), 𝑆₂₁ (𝑆₁₂) measurements were carried out on an Agilent E8362B vector network analyzer in the microwave frequency range of 8.2–12.2 GHz (X-band). The rectangular pellet of 2 mm thickness with a dimension to fit the waveguide dimensions has been prepared for microwave measurements. From 𝑆₁₁ and 𝑆₂₁ measurements, the reflectivity (𝑅), transmissivity (𝑇), and absorptivity (𝐴) were calculated. The real and imaginary parts of complex permittivity (ε′ and ε′′) and permeability (μ′ and μ′′) versus frequency are shown in Figures 3(a) and 3(b). Compared with conventional ferrite materials, the CrO₂ exhibits larger saturation magnetization and complex permeability value in X-band frequency range. In CrO₂ sample, permittivity and permeability are found to be decreasing with an increase in frequency because the dipole present in the system cannot reorient themselves along with the applied electric field. The CrO₂ sample shows highest real permittivity (ε′ = 33), imaginary part (ε′′ = 1.4), and permeability (μ′ = 6) which ultimately enhances the shielding effect. Magnetic materials with high permeability have been utilized in many microwave applications. The EMI shielding effectiveness (SE) of a material is defined as the ratio of transmitted power to incident power and is given by SE(dB)=−10log(𝑃_𝑡/𝑃₀), where 𝑃_𝑡 and 𝑃₀ are the transmitted and incident electromagnetic powers, respectively. For a shielding material, total SE=SE_𝑅+SE_𝐴+SE_𝑀, where SE_𝑅 is due to reflection, SE_𝐴 is due to absorption, and SE_𝑀 is due to multiple reflections. In two-port network, 𝑆 parameters 𝑆₁₁ (𝑆₂₂), 𝑆₂₁ (𝑆₁₂) represent the reflection and the transmission coefficients𝑇=|𝐸_𝑇/𝐸_𝐼|²=|𝑆₂₁|²=|𝑆₁₂|², 𝑅=|𝐸_𝑅/𝐸_𝐼|²|𝑆₁₁|²=|𝑆₂₂|² and absorption coefficient (𝐴)=1−𝑅−𝑇. Here, 𝐴 is given with respect to the power of the incident EM wave. If the effect of multiple reflections between both interfaces of the material is negligible, the relative intensity of the effectively incident EM wave inside the material after reflection is treated equally to 1−𝑅. Hence, the effective absorbance [17] (𝐴_eff) can be described as 𝐴_eff=(1−𝑅−𝑇)/(1−𝑅) with respect to the power of the effectively incident EM wave inside the shielding material. It is convenient to express the reflectance and effective absorbance in the form of −10log(1−𝑅) and −10log(1−𝐴_eff) in decibel (dB), respectively, which results in SE_𝑅 and SE_𝐴 as SE_𝑅=−10log(1−𝑅) and SE_𝐴=−10log(1−𝐴_eff) =−10log[𝑇/(1−𝑅)].

(a)

(b)

Figure 3 

(a) Response of real and imaginary (a) permittivity, (b) permeability with frequency of pure CrO2.

Figure 4 shows the variation of EMI shielding effectiveness of SE_𝐴 and SE_𝑅 versus frequency for CrO₂ sample. It explicitly confirms that the pure CrO₂ possesses much more effective electromagnetic absorbing effect compared to γ-iron and ferrites [18–20]. CrO₂ has shielding effectiveness mainly due to absorption, which is found in maximum ~20 dB. The shielding effectiveness due to reflection was nominal and of value ~10 dB.

Figure 4 

The variation of EMI shielding effectiveness of SE𝐴 and SE𝑅 versus frequency for CrO2 samples.

4. Conclusions

In conclusion, the pure phase CrO₂ can be prepared by an inexpensive and simplified procedure adopted. Structural and magnetic properties are compatible as reported earlier [6, 15]. These CrO₂ have a relatively uniform dimension and well-defined rod-shaped structure. The microwave absorption properties strongly depend on the intrinsic properties of CrO₂. The value of EMI shielding effectiveness due to absorption SE_𝐴 (~20 dB) is quite interesting for strategic technology applications.