Materials Science and Engineering B 186 (2014) 101–105 Contents lists available at ScienceDirect Materials Science and Engineering B journal homepage: www.elsevier.com/locate/mseb Microwave absorption properties of dielectric La1.5 Sr0.5 NiO4 ultrafine particles P.T Tho a,b , C.T.A Xuan a,b , D.M Quang c , T.N Bach a , T.D Thanh a , N.T.H Le a , D.H Manh a , N.X Phuc a , D.N.H Nam a,∗ a Institute of Materials Science, Vietnam Academy of Science and Technology, 18 Hoang-Quoc-Viet, Hanoi, Viet Nam College of Sciences, Thai-nguyen University, Thai-nguyen, Viet Nam c Department of Physics, Hanoi National University, Hanoi, Viet Nam b a r t i c l e i n f o Article history: Received 30 August 2013 Received in revised form 18 March 2014 Accepted 20 March 2014 Available online April 2014 Keywords: Dielectric Ultrafine particle Microwave absorption Impedance matching Phase matching Reflection loss a b s t r a c t La2−x Srx NiO4 compounds are well known dielectric materials that have colossal permittivities (εR > 107 ) In the present work, the powder of La1.5 Sr0.5 NiO4 ultrafine particles was prepared by a combinatorial method of solid-state reaction and high-energy ball milling Magnetic measurements, M(H), show a very small magnetization and paramagnetic characteristics at room temperature Flat layers of La2−x Srx NiO4 /paraffin mixture of different thicknesses (t) exhibits strong microwave absorption resonances in the 4–18 GHz range The reflection loss (RL) decreases with t and reaches down to −36.7 dB for t = 3.0 mm The impedance matching (|Z| = Z0 = 377 ), rather than the phase matching mechanism, is found responsible for the resonance for 1.5 mm ≤ t ≤ 3.0 mm Further increase in the thickness leads to |Z| > Z0 at all frequencies and a reduced absorption The influence of non-metal backing is also discussed The obtained low RL suggests that La1.5 Sr0.5 NiO4 particles could be a potential filler for high performance radar absorbing material © 2014 Elsevier B.V All rights reserved Introduction The rapid development and utilization of microwave applications today make electromagnetic interference (EMI) [1,2] a serious problem that needs to be solved Although high conductivity metals are very effective for high frequency electromagnetic wave shielding, in many cases they are not suitable when weak or zero reflection is required (such as for radar stealth technology) While metals shield the object by reflecting the incident radiation away, microwave absorbing materials (MAM) are designed to absorb the radiation and therefore effectively reduce the reflection Strong absorption and weak reflection will lead to a large negative value of reflection loss (RL) and are therefore identified as two strict requirements for high loss MAMs Minimum RL values as low as down to less than −60 dB have been reported for some materials, most of them are ferrite/ferromagnet-based particles or composites, e.g., carbonyl iron/BaTiO3 composite (RL = −64 dB, at 3.5 GHz) [3], ZnO/carbonyl-iron composite (RL = −60.7 dB, at 11.7 GHz) [4], La0.6 Sr0.4 MnO3 /polyaniline composite (RL = −64.6 dB, at 16.4 GHz) ∗ Corresponding author Tel.: +84 38364403 E-mail address: daonhnam@yahoo.com (D.N.H Nam) http://dx.doi.org/10.1016/j.mseb.2014.03.015 0921-5107/© 2014 Elsevier B.V All rights reserved [5], indicating the dominant role of magnetic losses over the others such as dielectric and conduction losses For ferroelectric and multiferroic fillers, such as BiFeO3 and its based compounds, where the magnetic components are rather weak, typical RL values of about −25 dB (or less) have been obtained at room temperature in several studies [6–8] Interestingly, by partially substituting La for Bi in (Bi,La)FeO3 , the saturation magnetization Ms is greatly enhanced and RL reaches down to about −30 dB (at ≈11.5 GHz) for Bi0.8 La0.2 FeO3 [8] However, it is also worth noting that, for the purpose of practical use, the performance of microwave absorbers is characterized by not only the maximum absorption intensity, but also the absorption frequency range and bandwidth [9–11] Dielectrics usually have small permeability and, visa versa, most magnetic materials have small permittivity To maximize the absorption capability by combining dielectric and magnetic losses, and since zero reflection can be achieved in a MAM that has equal permittivity and permeability (εR = R ) to satisfy the impedance matching condition Z = Z0 (Z0 is the impedance of the free space), much attention has been paid to multiferroic and magneto-dielectric materials La1.5 Sr0.5 NiO4 (LSNO) is known as a dielectric compound that has a colossal dielectric constant of up to more than 107 (at about f = 20 Hz) at room temperature [12,13] Although some authors have argued that the colossal P.T Tho et al / Materials Science and Engineering B 186 (2014) 101–105 Experiments The La1.5 Sr0.5 NiO4 ultrafine particle powder was synthesized using a solid state reaction route combined with high-energy ball milling processes The polycrystalline La1.5 Sr0.5 NiO4 bulk sample was first synthesized at 1100 ◦ C for 10 hrs in air using pure (>99.9%) La2 O3 , SrCO3 and NiO as starting raw materials A high energy ball mill (Spex 8000D) was then used to grind the samples into ultrafine powder A pertinent post-milling heat treatment was performed to reduce possible surface and structural damages (such as lattice distortion and chemical disorder) caused by the highenergy milling The structure, phase quality, and particle size of the powder were examined by X-ray diffraction (XRD) measurements using a SIEMENS-D5000 diffractometer XRD patterns indicated an improvement in the crystallization and phase quality of the powder by the post-milling annealing Scanning electron microscopy (SEM) images were taken by a FESEM Hitachi S-4800 microscope Magnetization measurements, M(H) (H: magnetic field), were carried out by a Quantum Design PPMS-6000 To prepare the samples for microwave measurements, the powder was mixed with paraffin in 40/60 vol percentage, respectively, and finally coated (on an area of 10 × 10 cm2 and with different coating thicknesses t = 1.0, 1.5, 2.0, 3.0, and 3.5 mm) on thin plates of mica that are almost transparent to microwave radiation (the plate also could later be removed for more accurate measurements) The free-space microwave measurement method in the frequency range of 4–18 GHz was utilized using a vector network analyzer (Anritsu MS2028B VNA Master) An aluminum plate was used as the reference material with 0% of attenuation or 100% of reflection The permittivity and permeability are calculated according to the algorism proposed by Nicolson and Ross [23], and Weir [24] (hence called the NRW method) The impedance and the reflection loss are then calculated according to the transmission line theory [25]: Z = Z0 R εR RL = 20 log 1/2 i ft c Z − Z0 Z + Z0 ( R εR ) 1/2 (103) La1.5Sr0.5NiO4 30 40 50 2θ (deg) 60 (224) (303/208) (310) (301) (206) (118) (220) (008/213) (211) (116) (204/107) (114) (105) (112) 20 (200) (110) (004) (101) permittivity would stem from extrinsic effects such as, e.g., the dielectric–metal contact interfaces or the grain boundaries [14,15], some others claimed that the effect should be intrinsic [12,16,17] For the magnetic properties, while La2 NiO4 is an antiferromagnet, the substitution of Sr for La introduces hole carriers into the system and suppresses the antiferromagnetic order [18–21] Experimental magnetic data show that La1.5 Sr0.5 NiO4 is a paramagnet at room temperature [20–22], suggesting that the magnetic loss may be negligibly small With a large imbalance between permittivity and permeability, εR R , and insignificant magnetic loss, the material has been therefore not expected to have a low RL; this would be the reason why microwave absorption experiments have not been carried out for this material In this paper, we show that La1.5 Sr0.5 NiO4 indeed exhibits a strong microwave absorption capability at the resonant frequencies; for a layer of 3.0 mm, the minimum RL reaches down to −36.7 dB at ≈9.7 GHz The resonance mechanism is found to be impedance matching with |Z| ≈ Z0 = 377 The RL value of −36.7 dB is not as low as that of magnetic particles and composites, but remarkably small for a single-dielectric-compound filler Intensity (arb units) 102 70 Fig X-ray diffraction pattern of the La1.5 Sr0.5 NiO4 powder at 300 K The peaks in the XRD patterns are marked by the Miller indices size of ≈50 nm was calculated using the Scherrer’s equation, d = K · /(ˇ · cos Â) (where K is the shape factor, is the X-ray wavelength, ˇ is the line broadening at half the maximum intensity, and  is the Bragg angle) It’s worth noting that this value of d may not reflect the actual sizes of the particles but of the crystalized domains contained inside each particle The particle size obtained by the Scherrer’s equation via XRD data would be therefore accurate for single crystal particles, but under-calculated for polycrystal particles As illustrated in Fig 2, the SEM images indicate that the particle size is significantly larger than that obtained from the XRD technique, ranging from 100 nm to 300 nm The magnetization loop, M( H), in Fig shows very small magnetic moments with no hysteresis (a small opening near the center of the loop is determined to be due to the fluctuation of the measurement system), verifying the paramagnetic characteristic of the material at room temperature The initial relative permeability, R = ( H + M)/ H, calculated from the magnetization curve is of ≈1.005, which is only slightly higher than that of the air (1.00000037) [27] All of the high-frequency characteristic parameters of the samples are summarized in Table The calculated permeability and permittivity, | R | and |εR |, respectively, for the samples with 1.5, 2.0, 3.0 and 3.5 mm are presented in Fig and the corresponding |Z|(f) and RL(f) curves are plotted in Fig (1) (2) Results and discussion X-ray diffraction data (Fig 1) indicate that the La1.5 Sr0.5 NiO4 particles are single phase of a tetragonal structure (F4 K2 Niperovskite-type, I4/mmm space group) [22,26]; no impurity or secondary phase could be distinguished A mean particle 80 Fig Scanning electron microscopy image of the La1.5 Sr0.5 NiO4 powder P.T Tho et al / Materials Science and Engineering B 186 (2014) 101–105 T = 300 K 1.5 |Z| -10 -15 -20 fz2 -25 -1 12 fz1 13 14 0.5 377 Ω 15 16 17 18 0.9 RL |Z| -5 -2 -0.5 0.5 μ0H (T) Fig Magnetization loop, M( H), b) of the La1.5 Sr0.5 NiO4 powder at 300 K RL (dB) -1 t = mm -10 0.8 0.7 fz1 -15 0.6 -20 Table Summary of the microwave absorption characteristics for the paraffin-mixed La1.5 Sr0.5 NiO4 ultrafine particle layers with different thicknesses Here, t is in mm; fr , fz1 , fz2 , fp are in GHz; and |Z | is in See text for details 1.5 14.7 14.3 13.2 13.9 209.5 317.2 −24.5 2.0 12.18 12.22 – 12.7 34.6 – −28.2 3.0 9.7 9.7 9.2 10.9 18.5 242 −36.7 3.5 8.2 – – 10.4 – – −9.9 For t = 1.0 mm (not shown), no significant absorption or distinguishable resonance could be observed The RL value is large (>−5 dB) and has a tendency to decrease when approaching GHz (from above) and 18 GHz (from under) It is possible that a resonance peak for this sample would occur at a frequency very close to (but higher than) 18 GHz, considering the variation of the resonance frequency fr on the thickness, as presented below The RL(f) curve for t = 1.5 mm in Fig 5a exhibits a deep minimum of RL = −24.5 dB at fr = 14.7 GHz, which is very close to the frequency fz1 (≈14.0–14.3 GHz) where |Z| ≈ Z0 = 377 The close value of fr to fz1 suggests that the strong microwave absorption would be attributed to a resonance caused by impedance matching However, the resonance could also be caused by a phase matching 0.4 377 Ω -30 0.3 12 12.5 13 13.5 fz2 fz1 t = mm -10 RL (dB) 1.0 – – – 4, 18 – – – -25 14 1.2 c) 0.8 -20 RL |Z| 377 Ω -30 0.6 0.4 0.2 -40 8.5 9.5 10 10.5 11 11.5 12 t = 3.5 mm d) -2 RL (dB) t fr fz1 fz2 fp |Z |(fz1 ) |Z |(fz2 ) RL(fr ) 0.5 |Z| (kΩ) -1 |Z| (kΩ) RL (dB) M (10 emu/g) RL a) |Z| (kΩ) -5 t = 1.5 mm -4 -6 RL |Z| -8 |Z| (kΩ) 103 377 Ω -10 25 10 11 12 f (GHz) 20 Fig RL(f) (squares) and |Z|(f) (circles) curves of the paraffin-mixed La1.5 Sr0.5 NiO4 particle layers with different thicknesses: (a) t = 1.5 mm, (b) t = 2.0 mm, (c) t = 3.0 mm, and (d) t = 3.5 mm fz1 and fz2 are the upper and lower frequencies, respectively, where |Z| = Z0 = 377 t (mm) R R |μ |, |ε | 15 1.5 2.0 3.0 3.5 1.5 2.0 3.0 3.5 { { |μ | R 10 |ε | R 0 12 16 f (GHz) Fig Permeability, | R |(f), and permittivity, |εR |(f), of the paraffin-mixed La1.5 Sr0.5 NiO4 particle layers with different thicknesses if the phases of the reflected waves from the two sample’s surfaces differ by In this case, the resonance frequency and its harmonics are given by fp = (2n + 1)c/ 4t |εR |.| R | , where c is the speed of light in the incident medium and n = 0, 1, 2, The phase matching frequency is determined when the calculated resonance frequency matches that of the incident wave, i.e., f = fp Nevertheless, since the closest fp value (13.9 GHz, obtained for n = 1) is also quite close to fr , it is difficult to determine conclusively which mechanism is responsible for the deep negative RL at fr for this t = 1.5 mm sample The phase-matching calculation for the t = mm sample predicts 104 P.T Tho et al / Materials Science and Engineering B 186 (2014) 101–105 fp ≈ GHz for n = and ≈18 GHz for n = 1; both are the lower and upper frequency limits of our measurement system Fig 5b and c displays the |Z|(f) and RL(f) curves for the t = 2.0 mm and 3.0 mm samples With increasing thickness from 1.5 mm to 3.0 mm, the resonance shifts to lower frequencies while the notch in RL becomes deeper For t = 2.0 mm, the minimum of RL appears almost at the same frequency as that of Z-matching while the phase matching frequency is a little higher, i.e., fr ≈ fz1 = 12.2 GHz and fp = 12.7 GHz for n = Similar scenario is also obtained for t = 3.0 mm: fr fz1 = 9.7 GHz whereas fp = 10.9 GHz It is then quite clear that, although the shift of the resonance to lower frequencies is qualitatively in agreement with the phase matching model, there is still a considerable difference between the calculated values of fp and the measured fr that seems to even develop with increasing the sample’s thickness Hence, both of the increasing deviation of fp from fr and the coincidence of fr and fz1 indicate that the resonance observed in these samples belongs to the Z-matching mechanism As mentioned above, the obtained La1.5 Sr0.5 NiO4 particles may have a polycrystal structure However, the influence of this type of structure on microwave absorption property of material is not very clear and may specifically depend on each individual case Commonly, dielectrics absorb microwave’s energy and convert it to heat (so-called dielectric heating effect) via the rotation of polar molecules at high frequencies and the ion-drag at low frequencies Since La1.5 Sr0.5 NiO4 is a good insulator [12] and in paramagnetic state at room temperature, losses caused by eddy currents and ferromagnetic resonance [28] could be ruled out The sample’s thickness dependence of the resonance frequency (as can be seen in Fig 5) indicates that the effect is not caused by dielectric resonance, which occurs at a frequency characterized for the material intrinsic nature rather than the sample’s size The dielectric relaxation loss is thus expected to dominate the absorption property of the La1.5 Sr0.5 NiO4 powder However, the frequency range (4–18 GHz) used for the measurements is considered too low for electronic and ion displacement polarizations; the loss could perhaps mainly come from the thermal ion and dipole rotation polarizations, which have the relaxation time of about 10−8 –10−2 s [29] The Z-matching resonance itself does not necessarily cause any energy dissipation of the electromagnetic wave, but it favors the wave’s propagation into the sample and hence promotes the absorption Vice versa, a perfect microwave absorption of the material will result in the Z-matching condition The |Z|(f) curves in Fig 5a–c shows that there are at least two frequencies (fz1 and fz2 ) where the |Z| = Z0 condition is satisfied Nevertheless, a strong absorption is obtained only at fz1 while there is no observable anomaly (except for a shoulder for the t = 1.5 mm sample) in the RL(f) curve at fz2 This implies that, although Z-matching occurs at both fz1 and fz2 , the energy dissipation is not promoted at fz2 According to Eq (2), perfect energy absorption, RL =− ∞, occurs if Z = Z0 = 377 , i.e., |Z| = 377 and the imaginary part Z = A deviation of Z from zero will reduce RL to a finite value; the larger the relative value of |Z | is, the larger the minimum RL will be at the resonance The obtained data for the t = 1.5 mm sample show |Z | = 209.5 and 317.2 at fz1 and fz2 , while those for t = 3.0 mm are |Z | = 18.5 and 242 , respectively For both the samples, the larger values of |Z | may explain the absence of resonant absorption at the fz2 frequencies This seems to be similar to the analysis for magnetic loss absorbers reported by Pang et al [30,31] where the authors introduced an entity of imaginary thickness component, dA , that becomes zero at the absorption resonance In addition, the variation of |Z | at fz1 (see Table 1) is also in agreement with the decrease of the minimum RL values (from −24.5 dB to −36.7 dB) as t increases from 1.5 mm to 3.0 mm |Z | is therefore could be considered as the mismatch at the Z-matching condition It is also noticeable that the RL(f) curves not show any deep minimum at the phase matching frequencies The reason may lie into the use of the transparent backing plates for the samples The electromagnetic wave reflects at all the boundaries between two different impedance media However, without a metal backing plate, the internal reflection at the back side of the sample would be much weaker than the reflection at the front side Moreover, the internal reflection wave is also absorbed for the second time by the sample So even the phase matching resonance does occur, no significant cancelation of the reflected signals would be detected This observation seems to be in agreement with the results reported earlier by Wang et al [32], where the influence of metal backing was examined in details and, importantly, a phase matching resonance is observed only when the samples were shorted by metal backing plates The shoulder appearing in the |RL|(f) curves in Fig 5a may belong to the phase matching effect because of the sample’s small thickness (t = 1.5 mm) A Z-matching mechanism for this shoulder is ruled out due to the large value of |Z | at fz2 Considering the proximity of fz and fp in the La1.5 Sr0.5 NiO4 absorbers, we expect that using metal backing plates would either further deepen the minimum of RL or widen the absorption band by combining phase-matching and Z-matching phenomena; a similar case has been discussed in Ref [33] The minimum RL of −36.7 dB obtained in this work is not very large comparing to many high magnetic loss fillers, but is very significant for a single compound filler that is not a ferrite or ferromagnet The absorption performance could be improved by adjusting other parameters such as the particle size and concentration, or by doping with magnetic fillers to increase the magnetic loss and to balance out the permeability and permittivity Some principle designs for matching conditions of single-layer radar absorbing coatings can be referenced in Ref [34] A recent trend of improving microwave absorption by controlling the particle microstructure to induce multiple scattering and absorption in hierarchical structural materials has also been of high interest [35,36] When the thickness is increased to 3.5 mm, as displayed in Fig 5c, the microwave absorption is strongly suppressed No Zmatching condition could be observed because the whole |Z|(f) curve lies well above Z0 Though, the RL(f) curve still exhibits a notch at fr = 8.2 GHz A calculation according to the phase matching model gives fp = 10.4 GHz (with n = 1), which is far above fr Apparently, none of the mentioned matching phenomena would be the mechanism for the absorption peak at fr = 8.2 GHz However, since |Z| reaches its minimum of 718 at 8.4 GHz that is closely equal to fr , this minimum in |Z| could be responsible for the deep of RL at fr Conclusions In summary, we have observed very low microwave RL values for the first time for the powders of La1.5 Sr0.5 NiO4 ultrafine particles despite the large imbalance between permittivity and permeability and possibly a very small magnetic loss contribution For t ≤ mm, the resonance takes place according to the Z-matching mechanism, where |Z | could be considered as a mismatch parameter The smallest minimum RL is observed for the absorber with the matching thickness of t = 3.0 mm and in the radar X-band The Z-matching condition is not attained in a thick sample (t = 3.5 mm) that has |Z| > Z0 at all frequencies but the peak absorption occurs where the impedance reaches its minimum The results also suggest that using a metal backing plate to combine the Z- and phase-matching resonances would further improve the material’s microwave absorption performance The minimum RL of −36.7 dB is remarkably small for a single-compound filler and could be smaller if magnetic fillers are added to induce magnetic loss and to balance out the permittivity and permeability This observation of low RL would reveal La1.5 Sr0.5 NiO4 as a promising material for high performance microwave absorption In fact, after the present P.T Tho et al / Materials Science and Engineering B 186 (2014) 101–105 work on pure La1.5 Sr0.5 NiO4 , by adding CoFe2 O4 particles to the La1.5 Sr0.5 NiO4 /paraffin mixture and using a Al backing plate, we have obtained very strong microwave absorption with RL close to −55 dB [37] 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Scherrer’s equation via XRD data would be therefore accurate for single crystal particles, but under-calculated for polycrystal particles As illustrated in Fig 2, the SEM images indicate that the particle... indicates that the effect is not caused by dielectric resonance, which occurs at a frequency characterized for the material intrinsic nature rather than the sample’s size The dielectric relaxation