Microstructure and dielectric properties of ion-doped La_0.7Sr_0.3MnO₃ lossy ceramics at radio frequencies

Abstract

In this paper, the microstructure and dielectric properties of ion-doped La_0.7Sr_0.3MnO₃ are investigated in detail. The polycrystalline ceramics of La_0.7Sr_0.3Mn_1−yM_yO₃ (M = Fe, Ni, Cu; y = 0.3 or 0.5) are prepared by the sol–gel/sintering method. The symmetry of the crystal structure is improved and the dielectric properties obtained in the frequency range from 80 MHz to 1 GHz can be tuned by the doping of Fe, Ni, and Cu ions. With regard to the dielectric properties in lossy ceramics, the decrease of real permittivity ε^′_r for the doping of Fe and Ni will be effective for impedance-matching; the enhanced dielectric losses in the Ni and Cu doping samples will promote stronger absorption. Thus, the improved dielectric properties make ion-doped La_0.7Sr_0.3MnO₃ promising candidates for lossy ceramics in microwave electronics. In addition, the experimental results of permittivity were checked by K–K relations, and the frequency dispersion behaviors of the conductivity within a certain frequency range accord with the Jonscher's power law in the form of σ^′_ac(f) ∝ (2πf)ⁿ, demonstrating that the conductive mechanism is hopping conduction.

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Introduction

Lossy materials have been widely used in microwave electronics, e.g., power amplifiers such as traveling wave tubes and gyroklystrons, in which lossy materials are used to suppress unwanted electromagnetic modes, prevent self-oscillations, broaden device bandwidth, and provide matched electromagnetic terminations.¹ From the viewpoint of applications, the dielectric loss must be relatively large for microwave attenuating materials used in microwave tubes to dissipate unwanted incident power.² Moreover, the application for microwave-absorbing coating on the exterior surfaces of military aircrafts and vehicles requires avoiding detection by radars by the zero-reflection determined by wave impedance-matching at the surface of the absorbing layer, which requires proper dielectric permittivity at a given frequency.³ Hence, it is significant to tune the dielectric constant and dielectric loss to obtain both specific frequency-matching and attenuation characteristics. In some cases, the fabrication of some tubes necessitates smaller sized lossy ceramics. Therefore, the properties of lossy ceramics should be homogenous, besides the attenuation and frequency-matching requirements. However, most of the works focus on composites that comprise insulators with conductors or semiconductor fillers dispersed within, while the single-phase materials are more favorable to homogenous performance. However, few works have reported on the single-phase of distorted ABO₃-type perovskite ceramics with the form of La_1−xSr_xMnO₃ (LSMO) as well as the ion-doped LSMO, even though this structure exhibits a controllable microstructure and various properties via ion doping.

Perovskite La_1−xSr_xMnO₃ (LSMO) ceramics as mixed conducting materials have been extensively investigated as cathode materials for high or intermediate temperature solid oxide fuel cells.^4,5 After 1993, LSMO gained renewed attention for the discovery of “colossal” magneto-resistance (CMR).^6,7 Recently, LSMO that exhibit double exchange,⁸ electron–phonon coupling,⁹ super-exchange function and high chemical stability have attracted considerable attention for their potential applications in magnetic electronics, microwave absorption,¹⁰ catalytic materials¹¹ and so on. The microstructure and grain sizes usually play a significant role in the electrical, magnetic, transport and CMR performances of LSMO systems,^12–15 and a number of investigations on the ion doping of LSMO have been carried out to explore the CMR phenomenon and thermal and electronic transport properties,^16,17 but there are only a few reports on the frequency dispersions of the permittivity and conductivity until now.

It is therefore an objective of this study to clarify the ion-doping effects on the complex dielectric permittivity and dielectric loss (tan δ) of La_0.7Sr_0.3MnO₃. Generally, LSMO-based perovskite ceramic powders can be prepared by a variety of techniques. As compared to co-precipitation synthesis and solid state reactions, the main advantages of the sol–gel process is that the liquid precursor with respect to the solid ones exhibits a high degree of chemical homogeneity and the atomic-level dispersion of the reagents, obtainable even in very complex compositions to improve the reactivity. This results in high perfection powders and is possible only when the solution turns into a solid without any fractional precipitation or intermediate phase segregation.^18,19 Here, we report on the doping effect of Fe, Ni, and Cu on the microstructure and dielectric properties of La_0.7Sr_0.3Mn_1−yM_yO₃ (M = Fe, Ni, Cu; y = 0.3 or 0.5). The improved microstructure and electrical properties are obtained by ion doping, which can be used as a candidate for lossy ceramics in microwave electronics.

Experimental

In this article, lanthanum nitrate La(NO₃)₃ (purity 99.0%) was dissolved in anhydrous ethanol solvent (purity 99.7%), strontium acetate Sr(OAc)₂ (purity 99.0%) and manganese acetate Mn(OAc)₂ (purity 99.0%); further, Fe(NO₃)₃ (purity 98.5%), Cu(NO₃)₂ (purity 99.0%), and Ni(OAc)₂ (purity 98.0%) were fully dissolved in glacial acetic acid (purity 99.5%). Then, both these solutions were mixed and ethanolamine (purity 99.5%) was used as a chelating agent to stabilize the solutions. This mixed solution was stirred to obtain a clear transparent sol. The sol was dried by evaporation in the oven at 80 °C to yield a dry gel; the dry gel was then calcinated at 750 °C to obtain a polycrystalline oxide powder. The powder was pressed into a pellet specimen and sintered at 1200 °C in air for 2 h.

The microstructure of the bulks was investigated by X-ray diffraction (XRD) and field emission scanning electron microscopy (SU-70, FESEM). The measurements in the frequency range from 80 MHz (or 10 MHz) to 1 GHz were carried out at room temperature by using an Agilent E4991A RF Impedance/Material Analyzer (Agilent Technologies). In order to determine the permittivity vs. frequency or various kinds of dielectric parameters, the dielectric test fixture of 16453A was used under an AC voltage of 100 mV, and the samples were processed into square discs (16 mm × 16 mm × 2 mm). During the measurement, the real part (ε^′_r) and imaginative part (ε^′′_r) of permittivity were then determined from the following formula:

where d is the sample thickness, C is the capacitance, R is the resistance, A is the electrode plate area, f is the frequency and ε₀ is the absolute permittivity of free space (8.85 × 10⁻¹² F m⁻¹).

Results and discussion

Fig. 1 shows the XRD patterns of La_0.7Sr_0.3Mn_1−yM_yO₃ (M = Fe, Ni, Cu; y = 0, 0.3, 0.5) sintered at 1200 °C for 2 hours. As shown in Fig. 1a, besides La_0.7Sr_0.3Mn_0.5Ni_0.5O₃ with a little impurity phase (i.e. NiO), all the samples are polycrystalline perovskite structures without secondary phases, and the diffraction peaks in the patterns can be indexed to the (012), (110), (104), (202), (006), (024), (122), (214), (300), (220), (208), and (134) planes in comparison with the theoretical pattern of the system trigonal (R3̄c H (167)). Therefore, ion doping can be considered as displacement doping. Taking the preferential growth (110) surface as an example (as shown in Fig. 1b), the symmetry of the crystal structure is improved by the doping of Fe, Ni, and Cu ions and the diffraction peaks are gradually transformed from the bimodal of (110) and (104) to the unimodal of (110). This is attributed to the role of the average ionic radius. Further, the stability and symmetry of the perovskite (ABO₃) structure can be characterized by the tolerance factor t,²⁰where r_A and r_B are the average ionic radii and r_O is the radius of the oxygen ion. As the radius of the doping ions (r_Fe³⁺ = 0.064 nm, r_Ni²⁺ = 0.069 nm, r_Cu²⁺ = 0.071 nm) is smaller than that of Mn³⁺ (r_Mn³⁺ = 0.072 nm), the value of r_B decreases with the ion doping of Fe, Ni, and Cu; consequently, the tolerance factor t also increases. In general, the perovskite (ABO₃) structure can be stable with a t value between 0.77 and 1.1 for the system of La_1−xSr_xMnO₃ (t < 1); only in the cubic structure with close ion packing, t = 1 (ideal). Therefore, the symmetry of the crystal structure improves because of the ion doping of Fe, Ni, and Cu, and the symmetry of the crystal distortion is diminished. This shows that because of the Jahn–Teller distortion, the crystalline structure of the samples changed from rhombohedral to tetragonal.²¹

Fig. 1 (a) XRD patterns of La_0.7Sr_0.3Mn_1−yM_yO₃ (M = Fe, Ni, Cu; y = 0, 0.3, 0.5) after sintering for 2 h at 1200 °C and (b) is the detailed view of the dominant (110) peak of the bulks.

The SEM images taken from the x-section of La_0.7Sr_0.3Mn_1−yM_yO₃ (M = Fe, Ni, Cu; y = 0, 0.3, 0.5) show that the bulks are densely sintered (Fig. 2). Further, the average grain sizes of La_0.7Sr_0.3MnO₃, La_0.7Sr_0.3Mn_0.7Fe_0.3O₃, La_0.7Sr_0.3Mn_0.5Ni_0.5O₃ and La_0.7Sr_0.3Mn_0.5Cu_0.5O₃ are about 1 μm, 1.8 μm, 0.2 μm and 0.9 μm, respectively. In addition, it can be seen that the doping with different ions significantly affected the micro-shape. The doping of Fe ion increases the crystal grain size (Fig. 2b), while Ni ion doping has the effect of grain refinement (Fig. 2c). The most obvious influence on the micro-shape is Cu ion doping, as shown in Fig. 2d; the grain shape of La_0.7Sr_0.3Mn_0.5Cu_0.5O₃ is regularly cubic.

Fig. 2 SEM images of sintered La_0.7Sr_0.3Mn_1−yM_yO₃ (M = Fe, Ni, Cu; y = 0, 0.3, 0.5). (a) La_0.7Sr_0.3MnO₃; (b) La_0.7Sr_0.3Mn_0.7Fe_0.3O₃; (c) La_0.7Sr_0.3Mn_0.5Ni_0.5O₃; (d) La_0.7Sr_0.3Mn_0.5 Cu_0.5O₃.

The frequency dependence of permittivity and dielectric loss factor for La_0.7Sr_0.3Mn_1−yM_yO₃ (M = Fe, Ni, Cu; y = 0, 0.3, 0.5) is shown in Fig. 3. It can be seen from Fig. 3a that when the doping ions are Fe and Ni, there is an obvious decrease in the real permittivity ε^′_r when compared with La_0.7Sr_0.3MnO₃, which will be conducive to impedance-matching. However, an abnormal frequency dispersion of ε^′_r for La_0.7Sr_0.3Mn_0.5Cu_0.5O₃ was observed, namely, ε^′_r of La_0.7Sr_0.3Mn_0.5Cu_0.5O₃ starts to increase above 300 MHz, reaching a peak value of 306 at 800 MHz. There is an absorption peak above 1 GHz for the imaginary permittivity dispersion, implying that the absorption property becomes stronger. The imaginary permittivity measured by the experiment and calculated by the Kramers–Kronig (K–K) relation (marked as the solid line) is shown in Fig. 3b. It is known that K–K relations express relationships between the real and imaginary parts of the complex frequency response of a linear passive causal system and can be conveyed by the following integral transforms for the permittivity:²²

where P.V. denotes the Cauchy's principal value, and ε₁ and ε₂ represent the real and imaginary parts of the permittivity, respectively. In this work, it is assumed that the real part is known; then, the K–K relation of eqn (4) can be used to rapidly and reliably retrieve the imaginary permittivity. The reliability of the measured real and imaginary permittivity can be validated by comparing the experimental and calculated results, as demonstrated in Fig. 3b. Both these curves meet quite effectively, namely, the measurement results are reliable. Fig. 3c shows that the dielectric loss (tan δ) of Ni-doped La_0.7Sr_0.3MnO₃ is larger than that of undoped La_0.7Sr_0.3MnO₃, and a dramatic increase of tan δ can be seen at about 1 GHz for La_0.7Sr_0.3Mn_0.5Cu_0.5O₃. Therefore, the enhanced dielectric losses in a wide frequency range can be obtained by appropriate ion doping. Combining Fig. 3a and c, La_0.7Sr_0.3Mn_0.5Ni_0.5O₃ with large attenuation and improved impedance-matching (relatively small ε^′_r will be conducive to impedance-matching) will represent perfectly absorbing properties. This improvement in the electrical properties for La_0.7Sr_0.3Mn_0.5Ni_0.5O₃ would lead to a new category of lossy ceramics.

Fig. 3 Frequency dependence of permittivity for La_0.7Sr_0.3Mn_1-yM_yO₃ (M = Fe, Ni, Cu; y = 0, 0.3, 0.5). (a) The real permittivity of the samples, (b) the imaginary permittivity of the experimental results (symbol curve) and calculated by the K–K relation (solid line), (c) the dielectric loss factor (tan δ) of the samples.

The frequency dispersions of the real part of ac conductivity in the frequency range from 10 MHz to 1 GHz are shown in Fig. 4. This was determined by the formula of σ^′_ac = d/RA, where d is the sample thickness, R is the resistance, and A is the electrode plate area. As shown in Fig. 4, the conductivity can be greatly improved by the Cu ion doping. Further, it can be found that the frequency dispersion behavior of the conductivity within a certain range of frequency represents an extension of the universal law, namely, the Jonscher power law²³ in the form of σ^′_ac(f) ∝ (2πf)ⁿ with a different power-law index of n. This shows that the conductive mechanism is hopping conduction. For La_0.7Sr_0.3MnO₃, it exhibits a weak frequency-dependent conductivity corresponding to a small value of the exponent n (n ≈ 0.45), yielding a ‘not-quite-dc’ response when the frequency is below 30 MHz. Above this frequency with a higher value of the exponent n (n ≈ 0.87), several papers have described such a type of conduction behavior in a wide range of materials.²⁴ The values of the exponent n for La_0.7Sr_0.3Mn_0.7Fe_0.3O₃, La_0.7Sr_0.3Mn_0.5Ni_0.5O₃ and La_0.7Sr_0.3Mn_0.5Cu_0.5O₃ are 0.62, 0.57 and 0.75, respectively. The physical significance of dispersion at lower frequencies lies in the rise of C′(f) for the formation of interfacial capacitance—the so-called Maxwell–Wagner effect.²⁵

Fig. 4 Frequency dependence of ac conductivity for La_0.7Sr_0.3Mn_1-yM_yO₃ (M = Fe, Ni, Cu; y = 0, 0.3, 0.5).

Conclusions

In conclusion, the microstructure of La_0.7Sr_0.3MnO₃ can be modified and its dielectric properties and electrical conductivity can be tuned by doping with Fe, Ni, and Cu ions. The XRD results show that ion doping can be considered as displacement doping, and the symmetry of the crystal structure is improved by the doping of Fe, Ni, and Cu ions. According to the SEM images, ion doping significantly influences the size and micro-shape. With regard to Cu ion doping, the grain shape of La_0.7Sr_0.3Mn_0.5Cu_0.5O₃ is regularly cubic; meanwhile, the real permittivity and conductivity can be greatly improved. With regard to the dielectric properties of lossy ceramics, the tunable dielectric permittivity ε^′_r and enhanced dielectric losses for La_0.7Sr_0.3MnO₃ can be obtained by appropriate ion doping. Further, La_0.7Sr_0.3Mn_0.5Ni_0.5O₃ with large attenuation and improved impedance-matching represents perfectly absorbing properties. Therefore, the improved dielectric properties of ion-doped La_0.7Sr_0.3MnO₃ would lead to a new category of lossy ceramics.

In addition, the experimental results of permittivity were checked by the K–K relations, and the frequency dispersion behaviors of the conductivity within a certain range of frequency accord with the Jonscher power law in the form of σ^′_ac(f) ∝ (2πf)ⁿ, confirming that the conductive mechanism is hopping conduction.

Acknowledgements

The authors acknowledge the supports of National Natural Science Foundation of China (51172131).

Notes and references

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