Microwave absorption properties of La1.5Sr0.5NiO4/La0.7Sr0.3MnO3 nanocomposites with and without metal backing

JOURNAL OF SCIENCE OF HNUE DOI: 10.18173/2354-1059.2016-0041 Mathematical and Physical Sci., 2016, Vol. 61, No. 7, pp. 128-137 This paper is available online at MICROWAVE ABSORPTION PROPERTIES OF La1.5Sr0.5NiO4/La0.7Sr0.3MnO3 NANOCOMPOSITES WITH ANDWITHOUTMETAL BACKING Ta Ngoc Bach1, Chu Thi Anh Xuan2, Do Hung Manh1, Ngo Thi Hong Le1, Nguyen Xuan Phuc1 and Dao Nguyen Hoai Nam1 1Institute of Materials Science 2College of Sciences, Thai Nguyen University Abstract. The performance of a microwave absorbing material (MAM) is often characterized by a negative reflection loss (RL) which is due to the material’s strong absorption and weak reflection of electromagnetic radiation. An absolute absorption can theoretically be achieved in a MAM that has equal permittivity and permeability (εr = µr) to satisfy the impedance matching conditionZ = Z0 (Z andZ0 are the intrinsic impedance of the material and the impedance of incident medium, respectively). To establish and study the influence of the balancing of permittivity and permeability on the material’s absorbing property, we have carried out experiments on La1.5Sr0.5NiO4/ La0.7Sr0.3MnO3 nanocomposites and found that with a pertinent La0.7Sr0.3MnO3 concentration, the reflection loss could be significantly reduced. Additionally, the influence of metal-backing on the microwave absorption property is also discussed. With metal backing, we clearly observed the appearance of another resonance related to the phase matching effect in the low frequency region. Keywords: Metal-backing, microwave absorption, nanocomposite, phase matching, impedance matching. 1. Introduction With the rapid increase in the use of microwave radiation in industry and the military, electromagnetic interference (EMI) has become a dominant issue. To prevent interference caused by undesired microwave radiation, one effective solution is the use of electromagnetic wave absorption materials to absorb the radiation. Microwave absorbing materials (MAM) and radar absorbing materials (RAM) are mainly used to absorb microwave energy at a defined frequency or in a narrow frequency band. Additionally, because it minimizes reflected waves from an object’s surface, absorption materials can also be used in many other applications, such as detection avoidance through reducing the radar cross section for military equipment, safe shield in microwave ovens and in the design of microwave circuits. Although research into electromagnetic wave absorbers started in the 1930’s when the first MAM patent was registered in France in 1936 [1], MAM is still being studied today. Studies Received July 3, 2016. Accepted October 25, 2016. Contact Ta Ngoc Bach, e-mail address: tangocbach_b1@yahoo.com 128 Microwave absorption properties of La1.5Sr0.5NiO4/La0.7Sr0.3MnO3 nanocomposites... are usually carried out in three basic directions: (1) reduction of reflection, (2) enhancement of absorption and (3) expansion of the operating frequency band. The materials used for MAM/RAM are quite diverse, consisting of all forms of materials; some of those which have been commonly used in research and applications are carbon black, carbonyl Fe, graphite, carbon nanotubes, magnetic ferrites, ferroelectrics and multiferroics. According to recently published results, the value of reflection loss, which is defined as RL (dB) = 10log10 Pi Pr , where Pi is the power of the incident wave and Pr is the power of the reflected wave, can reach large negative values at the resonance absorption frequency in many magnetic materials, some examples being ZnO-coated Fe particles in paraffin (RL ≈ -60.7 dB) [2], La0.6Sr0.4MnO3/polyaniline composite (RL = -64.6 dB) [3], NiCo2O4 nanoflakes (RL ≈ -38 dB) [4], graphene-coated Fe nanocomposites (RL = -45 dB) [5], BaFe12O19/graphene nanocomposites (RL = -58 dB) [6] and CoFe2O4 nanoparticles/acrylated epoxy (RL ≈ -60 dB) [7]. Magnetic materials are therefore expected to be the best candidate for high performance MAMs and RAMs. Figure 1. X-ray diffraction patterns of the La1.5Sr0.5NiO4 and La0.7Sr0.3MnO3 nanopowders Materials can absorb electromagnetic waves through different mechanisms. Each material shows its best absorption characteristics within a certain frequency range and under certain control parameters. Theoretically, the incident wave is completely absorbed in a material that has equal permittivity (εr) and permeability (µr), satisfying the impedance matching condition Z = Z0 where the impedance of the material equals that of the incident medium. Practically, such an ideal condition is difficult to achieve, but a very large negative RL can still be obtained when Z is close to Z0. To have a balance between εr and µr, MAMs can be fabricated by combining a 129 T. N. Bach, C. T. A. Xuan, D. H. Manh, N. T. H. Le, N. X. Phuc and D. N. H. Nam number of magnetic and dielectric materials in powder form, loaded in various kinds of binders. The performance of MAMs can usually be optimized by controlling the fraction ratios and the thickness of the sample. Figure 2. SEM image of the La1.5Sr0.5NiO4 and La0.7Sr0.3MnO3 nanopowders In our previous studies, we have shown that La1.5Sr0.5NiO4 (LSNO) nanoparticle powder, which was not expected to have good microwave absorbability because of the large difference between permittivity and permeability (εr » µr) and its tiny magnetization, has in fact a rather strong microwave absorption capability in the range of 4 - 18 GHz [8]. La0.7Sr0.3MnO3 (LSMO) is a ceramic ferromagnet that has a considerably large magnetization. According to Zhang and Cao [9], the LSMO nanoparticles showed microwave absorption with the obtained RL value of about -17.47 dB at 15.87 GHz with a sample thickness of 2 mm. In this work, we prepared absorption layers using (100 - x)La1.5Sr0.5NiO4/xLa0.7Sr0.3MnO3 (x is in volume units) nanocomposites mixed in paraffin. By combining LSMO with LSNO, we expected to improve the magnetic loss and to balance the dielectric and magnetic characteristics, consequently enhancing the microwave absorption capability of the nanocomposites. Without an Al backing plate, the RL value reached minimum values for doping content x = 4; with an Al backing plate, the lowest RL is attained for x = 10. Our studies also confirmed that other than the resonance caused by impedance matching (Z-matching), the phase-matching resonance can be only observed in those samples with metal backing. 130 Microwave absorption properties of La1.5Sr0.5NiO4/La0.7Sr0.3MnO3 nanocomposites... Figure 3. Magnetization loops of the La1.5Sr0.5NiO4 and La0.7Sr0.3MnO3 nanopowders at room temperature 2. Content 2.1. Experiment In the present investigation, the La1.5Sr0.5NiO4 and La0.7Sr0.3MnO3 nanoparticle powders were synthesized using a conventional solid-state reaction route combined with high-energy ball milling processes. The quality of the samples was repeatedly checked during preparation via x-ray diffraction (XRD, SIEMENS D5000), scanning electron microscopy images (SEM, Hitachi S-4800) and room temperature magnetization measurement (VSM, noncommercial system) techniques. Absorption layers of 3 mm thickness of (100 - x)La1.5Sr0.5NiO4/xLa0.7Sr0.3MnO3 (x = 0, 2, 4, 6, 8, and 10 in volume units) were prepared by coating the mixture of nanoparticles (60% vol.) and paraffin (40% vol.) on a mica substrate. Measurement of microwave absorption properties of all samples in the frequency range of 4 - 18 GHz was carried out using an Anritsu MS2028B Master Vector Network Analyzer (VNA) and the free-space transmission/reflection measurement method. The angle between the incident radiation and the sample’s plane was found to be 45◦ in both cases. An Al plate was used as the reference sample which produces total reflection and zero transmission. The impedance and the RL can be calculated according to the transmission line theory [10]: Z0 = √ ε0 µ0 (2.1) 131 T. N. Bach, C. T. A. Xuan, D. H. Manh, N. T. H. Le, N. X. Phuc and D. N. H. Nam Z = Z0 √ εR µR tanh [ i ( 2πft c ) (εRµR) 1/2 ] (2.2) RL = 20log ∣∣∣∣Z − Z0Z + Z0 ∣∣∣∣ (2.3) Where Z0 is the impedance of the incident medium, Z is the intrinsic impedance, εr and µr are the relative permittivity and permeability, respectively, f is the frequency of the electromagnetic wave, t is the thickness of the absorption layer, and c is the speed of light. The εr and µr parameters were calculated using an algorism proposed by Nicolson and Ross in [11] and Weir in [12] (hence called the Nicolson-Ross-Weir or NRW method). For the case of the samples with metal backing, the impedance and reflection loss can be written as [13]: Z = Z0 (1 + S11) (1− S11) (2.4) RL = 20 log |S11| (2.5) where S11 is the complex reflection scattering coefficient obtained directly from the reflection measurements. 2.2. Results and discussion The phase formations of La1.5Sr0.5NiO4 and La0.7Sr0.3MnO3 nanoparticle powders were verified by XRD and the typical patterns are shown in Figure 1. All diffraction peaks appearing in the XRD pattern for LSNO and LSMO are very sharp, indicating a high crystallinity of the samples. The Miller index for each diffraction peak was determined by comparison with standard XRD data in JCPDS cards. All the peaks could be indexed to the expected crystal structures of F4K2Ni perovskite-type tetragonal (I4/mmm space group) for LSNO [14] and perovskite-type rhombohedral (R-3cspace group) for LSMO [15]. No trace of impurities or secondary phase was detected. The average grain size (crystallite size) was calculated using a commercial X-Pert-HighScore-Plus program, giving the values of 50 nm and 45 nm for the LSNO and LSMO nanoparticles, respectively. It should be noted that, in the case of the nanoparticles that have a polycrystalline structure or disordered surface, the average particle size calculated from the XRD data is usually less than the actual size of the particles. The SEM images (Figure 2) indicate that particle size for the LSNO powder is in the range of 100 - 300 nm while that of the LSMO is 100 - 200 nm. With both nanopowders, the particles have quite a uniform quasi-spherical shape. The magnetic hysteresis loops, M(H), of LSNO and LSMO nanoparticles at room temperature are presented in Figure 3. With the LSNO powders, the magnetization is very small and no magnetic hysteresis could be observed, though the general shape of the M(H) loop looks similar to that of a superparamagnet or very soft ferromagnet. Because of its miniscule magnetization, the magnetic loss of electromagnetic wave that propagates in LSNO is expected to be negligible. On the other hand, the LSMO nanoparticle powder shows a more typical behavior of a very soft ferromagnet with minimal hysteresis and large magnetization. Figures 4 and 5 present the frequency dependence of the RL and Z/Z0 of (100 - x)LSNO/xLSMO nanocomposites without an Al backing plate. The data for the x = 0 and x = 6 samples (which are not much different from that of x = 0 and x = 4, respectively) are omitted for clarity. It is notable that all of the samples exhibit a resonance notch in the RL(f) curves with a large 132 Microwave absorption properties of La1.5Sr0.5NiO4/La0.7Sr0.3MnO3 nanocomposites... negative value of RL in the high frequency region of 13 GHz to 16 GHz. The minimum values of RL are -14.8 dB, - 28.5 dB, -16.9 dB and -14.5 dB at the resonance frequencies fR1 ≈ 15.8 GHz, 13.6 GHz, 13.2 GHz and 13.1 GHz for absorption layers with x = 2, 4, 8 and 10, respectively. The optimal reflection loss reached -28.5 dB at 13.6 GHz for the x = 4 sample and the RL notch seems to move to a lower frequency with increasing the La0.7Sr0.3MnO3 concentration. The decrease of the resonance frequency with the concentration of LSMO could be due to the effect of the magnetic relaxation loss mechanism. According to this mechanism, the enhancement of magnetic interaction between ferromagnetic LSMO particles results in an increase in their relaxation time and therefore a reduction in resonant frequency. Beside the absorption notch at frequency fR1, we also observed a second absorption notch (at frequency fR2) in the low frequency region near 6 GHz with a smaller negative value of RL. Figure. 4. Unbacked samples: RL of the (100 - x)LSNO/xLSMO absorbers within the frequency range of 4 - 18 GHz Table 1. Summary of the microwave absorption characteristic parameters for paraffin mixed (100 - x)La1.5Sr0.5NiO4/xLa0.7Sr0.3MnO3 nanocomposites x 2 4 8 10 Unbacked samples fp(n = 2) 5.3 5.27 5.26 5.16 fR1 (GHz) 15.8 13.6 13.2 13.1 fR2(GHz) 5.6 5.57 5.8 5.53 RL(fR1)(dB) -14.8 -28.5 -16.9 -14.5 RL(fR2)(dB) -2.6 -2.7 -3.3 -2.9 Al backed samples fR1 (GHz) 17.5 15.9 15.4 16.6 fR2(GHz) 5.8 5.4 6.3 5.5 RL(fR1)(dB) -25.5 -17.8 -8.7 -22.5 RL(fR2)(dB) -10.5 -30.7 -22 -53.8 133 T. N. Bach, C. T. A. Xuan, D. H. Manh, N. T. H. Le, N. X. Phuc and D. N. H. Nam Figure 5. Unbacked samples: The RL(f ) and |Z/Z0|(f) curves for all of the samples in the frequency range of 4 -18 GHz, (a) x = 2, (b) x = 4, (c) x = 8 and (d) x = 10 Materials can absorb electromagnetic waves in several different physical mechanisms. Nevertheless, the resonant phenomena observed at the notch of the RL occur in two general mechanisms, the impedance matching and the phase matching mechanisms. When the impedance matching condition is satisfied, the impedance Z of the material matches the Z0 of the incident medium (Z0 = 377 ω for vacuum or the air approximately). In this case, we will not observe a reflection because the incident wave energy is completely dissipated when it is propagated in the material; a perfect absorption gives a zero reflection coefficient (S11 = 0) when Z = Z0 according to Eq. 3. In the case of phase matching, a resonance occurs if the reflected waves from the two samples’ surfaces have their phases deviated by π. The resonant frequency fp can be determined by fp = (2n + 1) c/(4t √ εRµR), where c is the speed of light in the incident medium and n = 0, 1, 2,. . . All of the characteristic frequencies of the samples, fR1, fR2 (which are determined at the resonant notches in the RL(f) curves) and fp (the calculated phase matching frequency), are summarized in Table 1. In Figures 5a-d, the RL(f) and |Z/Z0|(f) curves for each sample are plotted together for comparison. Obviously, the first resonance at fR1 occurs near |Z/Z0| = 1 for all of the samples. This indicates that the impedance matching is the main mechanism for these resonances. On the other hand, the second resonance at frequency fR2 near 6 GHz does not satisfy the Z–matching condition due to |Z/Z0|» 1. However, the values of fR2 are quite close to the calculated fp according to the phase matching model (as listed in Table 1). We therefore propose that the appearance of the absorption notch near 6 GHz is caused by the phase-matching resonances. It is noticeable 134 Microwave absorption properties of La1.5Sr0.5NiO4/La0.7Sr0.3MnO3 nanocomposites... that the resonance at fR2 is very weak and not as strong as that at fR1. That can be explained by considering that the two reflected waves may have a large difference in intensity thus causing only a small cancellation even though the phase matching condition is reached. Since the reflected wave on the incident surface would be much stronger than that on the back side, to improve the cancellation, we attach to the back of the sample an Al plate which can completely reflect the electromagnetic wave transmitting through the sample. Figures 6a and 6b show the frequency dependence of the reflection coefficient |S11| and the RL of the Al-backed samples. Accordingly, the |S11| signal drops to zero (Figure 6a) corresponding to a very large negative value of RL (Figure 6b) at the resonant notch in the low frequency region near 6 GHz. The minimum RL drops down to -53.8 dB at 5.5 GHz for x = 10. This result convincingly proves the phase-matching nature of the fR2 resonances. The influence of a metal backing plate on reflection measurements was also observed byWang et al. [16]. We propose that using a backing metal in the reflection measurement is an effective method not only to prove the existence of phase-matching, but also to distinguish between the phase-matching and Z-matching resonances. Figure 6. Al-backed samples. (a) Absolute value of the reflection coefficient, |S11|. (b) RL of the (100 - x)LSNO/xLSMO absorbers within the frequency range of 4 - 18 GHz Data of x = 0 are omitted for clarity Additionally, all of the RL notches caused by the Z-matching resonances in the high frequency region seem to move to higher frequencies when the Al plate is attached. Similar phenomena were also observed and discussed in our previous study [17]. In that study, instead 135 T. N. Bach, C. T. A. Xuan, D. H. Manh, N. T. H. Le, N. X. Phuc and D. N. H. Nam of a ferromagnet, as in the present work, spinel ferrite CoFe2O4 (CFO) was used to balance the permeability with the dielectric permittivity in LSNO. Interestingly, despite the large difference between LSMO and CFO, the results obtained were qualitatively quite similar. It’s worth mentioning here that our new study using ferrite NiFe2O4 (NFO) as a partial substitute for LSNO gives similar results. These observations would indicate that the balancing of permittivity and permeability plays the essential role in governing the absorption property of the nanocomposites, irrespective of the magnetic material used. 3. Conclusion By partially substituting La1.5Sr0.5NiO4, which is dielectric with high permittivity, with La0.7Sr0.3MnO3, which is a soft ferromagnet with high permeability, we found that there was a substantial improvement in the microwave absorption capability of the (100 - x)La1.5Sr0.5NiO4/xLa0.7Sr0.3MnO3 nanocomposites. The optimal microwave absorption performance of the nanocomposites was found with a doping content of x = 4 and x = 10 for samples without and with Al backing, respectively. Very low values of RL were observed at the resonant frequencies. We have studied the influence of metal backing and verified that both the Z-matching and phase-matching effects can be detected in metal backed samples while the phase matching effect can only be observed in Al-backed samples. This observation could be used as a method to effectively distinguish these matching effects. Our study also verifies that the balancing of permittivity and permeability plays the major role in governing the absorption property of the LSNO-based nanocomposites, irrespective of the magnetic material that was used. Acknowledgment. This work was sponsored by the Vietnam National Foundation for Science and Technology Development (NAFOSTED) under Grant number 103.02-2012.58. REFERENCES [1] N. V. Machmerieen, 1936. 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