Thin and temperature-resistant TiO2–Sr1−xLaxTiO3 (x = 0.1–0.3) composite ceramics for microwave absorption in the X-band

Recently, high-temperature stability is a challenge in a number of microwave absorption materials. Hence, researchers are still searching for a novel material system preferably with a high-temperature resistance to be applied in the field of microwave absorption. Here, in the current study, toward this aim, lanthanum (La)-doped strontium titanate (SrTiO3) blended with TiO2 were fabricated by hot-press sintering in a vacuum. The as-prepared samples are denoted as TiO2–Sr1−xLaxTiO3 with x varying from 0.1 to 0.3 in steps of 0.1. Scanning electron microscope (SEM), X-ray diffraction (XRD), X-ray photoelectron spectroscope (XPS), and microwave vector network analyzer were carried out to study their morphology, phase composition, structure, and electromagnetic and microwave absorption properties, respectively. It is revealed that the La atom was efficiently doped at the Sr-site in SrTiO3. Benefiting from the tunability of its dielectric and impedance properties, TiO2–Sr1−xLaxTiO3 can be utilized in a highly efficient way to absorb microwave radiations with a decent design. Results illustrated that TiO2–Sr1−xLaxTiO3 (x = 0.2) with a thickness of only 0.42 mm exhibits a high microwave absorption efficiency of −40.89 dB and can achieve a 2.82 GHz bandwidth of reflection loss value below −5 dB. Thus, TiO2–Sr1−xLaxTiO3 composites ceramics can be served as an opening opportunity for the application of high-temperature stability and tunable high-performance effectiveness microwave absorption materials in stealth technology and information security.


Introduction
Along with the fast and fierce evolution of civil network technology and military detection technology, the protection or absorption technology of electromagnetic (EM) wave radiation is becoming of increasing significance in this day and age [1]. In order to address this issue, microwave absorbing materials (MAMs) are increasingly being studied for their isolating and absorbing the EM wave from the surroundings. At present, in addition to the conventional requirements for MAMs: thin thickness, lightweight, wide absorption band, strong absorption capacity, a fifth requirement has been put forward, i.e., high-temperature resistance. It is well known that meeting these five performance requirements simultaneously can be extremely challenging [2]. Currently, one most commonly used MAMs is ferromagnetic material-based absorbing materials [3,4] or carbon-based absorbing materials [5,6], which can absorb or attenuate EM waves using magnetic or electrical losses. However, they are susceptible to high-temperature oxidation, chemical corrosion, or heavyweight, which dramatically limits their applications in the field of microwave absorption.
Perovskite structure oxide, ABO 3 , has been extensively investigated due to properties related to their versatile dielectric, ferroelectric, and spontaneous polarization [7,8]. Among such materials, SrTiO 3 with cubic symmetry (Pm3m) perovskite structure, high chemical, and thermal stability have been applied comprehensively in electronic, energy storage, and ceramic industries. However, due to its nonmagnetic loss (tan l) and low dielectric loss (tan d), it is almost impossible for untreated or modified or pure SrTiO 3 to be employed as one MAM. The nonabsorbent state related to electric and dielectric properties of SrTiO 3 , however, can be altered by chemical substitutions [9]. For instance, after the A-site substitution of Sr 2? by high valence cations (e.g., Gd 3? , La 3? , etc.), SrTiO 3 can be transformed into an n-type electric state. On one hand, it is ascribed to the existence of defects, including oxygen vacancies, polar nanoregions, and defect centers caused by charge imbalance between the substituted ion and the replaced ion [10]. On the other hand, it is attributed to the structural defects created by ionic radius mismatch between host and dopant cations, resulting in elastic lattice strains and leading to the polar states [8]. Therefore, doped SrTiO 3 is of great potential as MAMs with excellent comprehensive performance especially high-temperature resistance for low-cost and sustainable materials resources. However, there are scarce studies on the microwave absorption performance of doped SrTiO 3 . Clearly, there remains a need for systematic research on the impact of substitution ions and amounts on the microwave absorption properties of doping SrTiO 3 . Therefore, in this study, Sr 1-x La x TiO 3 ceramics were synthesized using La 3? donor with a higher valence than the host Sr 2? ion as a dopant for the Sr-site in SrTiO 3 . La was chosen as the dopant mainly for two reasons. One reason is that La is well known for being substituted exclusively at the Sr-site as La 3? [11][12][13][14]. The second reason is that even with a small amount of La doping, the electrical properties of SrTiO 3 change significantly [15][16][17].
Given the foregoing discussion, composite ceramics composed of Sr 1-x La x TiO 3 as absorbers and TiO 2 as matrix were fabricated using a hot-press sintering process in a vacuum atmosphere. The influence of different La doping concentrations on the microstructure, morphology, dielectric, and microwave absorption properties in X-band were systematically studied.

Materials and methods
Specimens were prepared by the conventional mixed oxide method using strontium carbonate (SrCO 3 , 99.7%), lanthanum (III) oxide (La 2 O 3 , 99.99%), and titanium dioxide (TiO 2 , 99.5%) as raw materials, which were purchased from Sinopharm Chemical Reagent Co., Ltd. Firstly, appropriate amounts of SrCO 3 , La 2 O 3 , and TiO 2 were ball milled for 12 h using ZrO 2 balls in alcohol, followed by drying at 100°C in an oven for several hours. Secondly, the mixing powders were sintered at 1250°C for 2 h in the air atmosphere to prepare Sr 1-x La x TiO 3 . Lastly, the calcined Sr 1-x La x TiO 3 and TiO 2 powders were blended in a proportion of 75:25 and pressed into disk samples of u = 35 mm, followed by calcining at 1300°C for 2 h and maintaining a pressure of 15.7 MPa by a hot-press sintering system in a vacuum atmosphere.
The morphologies and microstructures of these asfabricated specimens were investigated by scanning electron microscope (SEM, VEGA3, TESCAN, Czech Republic). The crystal phase compositions of the synthesized specimens were characterized by X-ray diffraction (XRD, X'Pert PRO, PANalytical, the Netherlands) in the 2h range from 10°to 90°with CuKa radiation. X-ray photoelectron spectroscope (XPS, ULVAC-PHI Versa-probe) using a monochromated Al Ka excitation source was conducted to determine surface composition of the TiO 2 -Sr 1-x La x-TiO 3 ceramics. The obtained ceramics were cut into rectangular bars (22.86 9 10.16 9 1.50 mm), and the EM parameters were measured using a two-port microwave vector network analyzer (VNA, N5225B, Keysight Technologies, USA) in the X-band (frequency range: 8.2-12.4 GHz). The microwave reflection loss (RL) value, which reflects the microwave absorption performance, was calculated based on transmission line theory. There are only a small number of pores in these three ceramics, indicating a dense microstructure was formed after the hot-pressing process. Moreover, the grain size increases on the whole with increasing La doping contents, which may be because the substitution of La is beneficial in promoting the grain growth [18,19].

Phase composition analysis
The XRD diffraction experiments with different La doping concentrations were carried out to reveal the influence of La doping on the crystal structure, as displayed in Fig. 2. As shown in Fig. 2a, two main phases were present in the XRD patterns of the TiO 2 -Sr 1-x La x TiO 3 ceramics that could be assigned to the reflections of SrTiO 3 (JCPDS card No. 89-4934) and TiO 2 (JCPDS card No. 21-1276). Furthermore, it indicates that the XRD diffraction pattern of Sr 1-x La x TiO 3 is highly consistent with the standard card of SrTiO 3 to a certain degree. It demonstrates that the crystal structure of SrTiO 3 does not change as La-doped in such concentrations (x = 0.1-0.3). Figure 2b shows one of the diffraction spectra of Sr 1-x La x TiO 3 from 32°to 33°correspond to (110) plane of SrTiO 3 . It can be discovered that the peak position of Sr 1-x La x TiO 3 slightly shifted toward a lower 2h value and then tended to have a higher 2h value with increasing x value. According to the Bragg spacing equation 2dsinh = nk [20], the interplanar spacing d (110) of Sr 1-x La x TiO 3 first increases and subsequently decreases with the raising of x value. Incorporating a certain concentration of La, the lattice parameters of SrTiO 3 can be affected by two mechanisms. One noticeable mechanism is caused by the atom size mismatch among the dopant atom La and the host atom Sr. The other mechanism is electronic effects [21]. The effects of the two mechanisms on the lattice parameters are as follows. On one hand, because of the smaller ion radius La 3? (1.36 Å , coordination number CN = 12) [22] substituting for the larger ion radius Sr 2? (1.44 Å , CN = 12) [23], as to the first mechanism, the interplanar spacing d of Sr 1-x La x TiO 3 should be reduced as La doping contents increases. However, on the other hand, with La 3? as a donor impurity doping in Sr 2? of SrTiO 3 lattice, a considerable proportion of electrons were placed in the conduction band. The system would reduce its energy by lowering the conduction band's position, accompanied by volume deformations, which is described through deformation potential [21]. Consequently, under the influence of deformation potential, the interplanar spacing d of Sr 1-x La x TiO 3 increases with the doping of La. Accordingly, combining size effect and deformation potential effect, the deformation potential effect dominates first, whereas the size effect predominates later.

XPS analysis
Here XPS test was executed to investigate the chemical states of TiO 2 -Sr 1-x La x TiO 3 . The XPS measurement scan was conducted from 0 to 1200 eV binding energy, which covers the core levels of all elements of Sr, La, Ti, and O, as shown in Fig. 3. It confirms the presence of La in all the TiO 2 -Sr 1-x La x TiO 3 ceramics, demonstrating that La successfully substituted Sr-site in SrTiO 3 . Figure 4 presents the high-resolution XPS spectra of TiO 2 -Sr 1-x La x TiO 3 , which belongs to the oxidation states of La, Sr, and Ti. As shown in Fig. 4a, the binding energy positions of La 3d 3/2 and La 3d 5/2 correspond to the oxidation states of La 3? [24]. It can also be observed from Fig. 4 that the binding energy of La first increases slightly and then decreases. It can be attributed to the changes in the relative number of electrons in the outer layer of La due to the twisted structures and changed cell size of Sr 1-x La x TiO 3 . The core-level spectrum of Sr 3d displays the doublet peaks for Sr 3d 5/2 and Sr 3d 3/2 , confirming the Sr 2? state in the sample (Fig. 4b) [25]. The binding energy shift of Sr is opposite to that of La. The XPS spectra of Ti (Fig. 4c)

Dielectric properties
It is well acknowledged that the dielectric permittivity is of the essence to nonmagnetic MAMs. Thus,  Fig. 5a that the real part of all the three samples displays good frequency dispersion relation, i.e., their values decrease rapidly with the increasing frequency, which is beneficial to broaden the absorption band. Furthermore, the e 0 ð Þ value of TiO 2 -Sr 0.8 La 0.2 TiO 3 shows lower on the whole than the other two dopants, indicating that the primary reflection of the microwave is weak, which is conducive to impedance matching [27]. Figure 5b shows the imaginary part of dielectric permittivity of TiO 2 -Sr 1-x La x TiO 3 with three doping contents (x = 0.1-0.3). According to the Debye theory, e 00 ð Þ can be determined as follows [28]: where e ? , e s , and e 0 is the high-frequency limit permittivity, static permittivity, and free space permittivity, respectively. x is the angular frequency, and x = 2pf. s is the relaxation time, and r is the electrical conductivity of the as-prepared ceramics. The first item in Eq. (1) is the loss related to the Maxwell-Wagner-Sillars (MWS) polarization process. This function depends on the electric dipole's charge, and its value is strongly dependent on the microwave energy dissipation originated from the reorientation of electric dipoles. The last item is associated with conductance loss resulting from the movement of charge carriers, such as electronic and ionic. Furthermore, due to the minimal number of electronics in such ceramics we researched the electron conductance loss can be ignored. In contrast, the loss of ionic conductivity, including intrinsic ionic conductivity and impurity ionic conductivity, plays an important role. The intrinsic ionic conductance loss is the energy loss caused by the current formed by the directional movement of defects (including Schottky defects and Frenkel defects) along the electric field's direction after being excited by the external electric/magnetic field and heat. The impurity ionic defect complexes are depicted in Eq. (2), where the Kröger-Vink defect symbol [29] was used: where ''Á,'' '' 0 ,'' and '' 9 '' represent the defect is positively charged, negatively charged, and neutral, respectively. The defect reaction equilibrium equation is built based on the principle of particle positions, charge equilibrium, and mass equilibrium. After La doping in SrTiO 3 , defects, such as La Sr ; La i ; and V 0000 Ti , with the ability to transmit electrons were created that will consume plenty of energy. Figure 5c displays the dielectric loss of TiO 2 -Sr 1-x La x TiO 3 (x = 0.1-0.3) ceramics. It indicates the sample of x = 0.1 possesses the highest dielectric loss compared with the other samples, consistent with the variety imaginary part of dielectric (Fig. 5b).

Microwave absorption properties
To further evaluate the overall EM wave absorbing performance of the TiO 2 -Sr 1-x La x TiO 3 (x = 0.1-0.3) ceramics, the RL value was computed by the following equations based on data from Sect. 3.4 part: where RL is the reflection loss in dB unit, Z in is the input impedance of absorber, e r and l r are, respectively, the complex permittivity and permeability, j ¼ ffiffiffiffiffiffi ffi À1 p is the imaginary unit, f is the frequency of the incident EM wave, d is the thickness of the absorber, and c is the velocity of light. It is worth mentioning that l r = 1 for nonmagnetic material and all the materials prepared in this paper are nonmagnetic. Generally, it was proven that an absorber with excellent EM wave absorbing performance results from both good impedance matching and high energy dissipation capability.
The EM wave absorption performance of the TiO 2 -Sr 1-x La x TiO 3 (x = 0.1-0.3) ceramics with different thicknesses (0.42-0.47 mm) can be gained using Eqs. (3) and (4), as displayed in Fig. 6. On the whole, Fig. 6 illustrates that TiO 2 -Sr 1-x La x TiO 3 (x = 0.1-0.3) ceramics have very different absorption performance at different x values (i.e., different La doping contents) in the same thickness range. It suggests La doping concentration directly reflects the EM wave absorption performance of TiO 2 -Sr 1-x La x TiO 3 ceramics. In purpose to further study the absorption results of TiO 2 -Sr 1-x La x TiO 3 , the effective bandwidth of RL B -5 dB (RL -5 ) and minimum RL (RL min ) value equivalent to the maximum absorption, which are identified in Fig. 6, are indicated in Table 1. It can be seen that among the as-prepared samples, the widest absorption band is 4.2 GHz, which appears in the TiO 2 -Sr 0.8 La 0.2 TiO 3 ceramic. Consequently, when the content of La was 0.2 at.% (x = 0.2), TiO 2 -Sr 1-x La x TiO 3 ceramic possesses better absorption performance in terms of bandwidth within the test frequency band. The minimum absorption peak appeared in TiO 2 -Sr 0.7 La 0.3 TiO 3 ceramic with a thickness of 0.43 mm, and the RL min is -40.89 dB. Therefore, from the point of view of RL min , among the samples, TiO 2 -Sr 0.7 La 0.3 TiO 3 shows the best absorbing performance. Figure 7 exhibits the comparison of RL value of TiO 2 -Sr 1-x La x TiO 3 (x = 0.1-0.3) ceramics with optimum thicknesses (0.43-0.44 mm). As shown in Fig. 7, with the gain of La doping concentration, the RL min value is promoted, and the maximum RL min value reached -40.89 dB when the doping concentration of La is x = 0.3. However, the effective absorption bandwidths have been proved to be a substantial  factor in engineering applications [30]. Therefore, on one hand, the TiO 2 -Sr 0.8 La 0.2 TiO 3 ceramic with a thickness of only 0.44 mm owns an outstanding microwave absorption performance with a maximum absorption peak of -11.7 dB and bandwidth (RL -5 ) of 4.2 GHz. On the other hand, TiO 2 -Sr 0.7 La 0.3 TiO 3 , with a thickness of only 0.43 mm, holds a maximum absorbing peak of -40.89 dB and bandwidth (RL -10 ) of 0.67 GHz, belongs to one of the best candidates of EM wave absorption materials among materials prepared in this work. As demonstrated above, TiO 2 -Sr 1-x La x TiO 3 ceramics can be used as good candidate MAMs at X-band by modifying La doping contents.
The real part Z 0 and imaginary part Z 00 of the complex impedance were calculated with the corresponding thickness for optimum absorption performance. The variations of Z 0 and Z 00 vs. frequency plot of TiO 2 -Sr 1-x La x TiO 3 (x = 0.1-0.3) ceramics are displayed in Fig. 8. It can be seen that the curves are frequency dependent, and both the Z 0 and Z 00 curves show significant differences with different amounts of La doping. The large fluctuation of TiO 2 -Sr 0.7-La 0.3 TiO 3 in Z 0 and Z 00 curves is associated with the complex permittivity fluctuation (Fig. 5). It also can be seen that samples with x = 0.2 and 0.3 achieve good impedance matching, resulting in quite good EM wave transmission [31].

Conclusions
In summary, TiO 2 -Sr 1-x La x TiO 3 ceramics with dense microstructure were fabricated by hot-press sintering process in a vacuum. It has been indicated that, after the introduction of La ion, the lattice parameter and crystallographic structure of SrTiO 3 were affected by both deformation potential and size effect. Benefiting from MWS polarization process and conductance loss, all the as-prepared TiO 2 -Sr 1-x La x TiO 3 ceramics exhibited high dielectric loss. Not only the high dielectric loss but also the good impedance resulted in the excellent wave absorbing properties of TiO 2 -Sr 1-x La x TiO 3 (x = 0.2 and 0.3). The minimum RL of TiO 2 -Sr 0.7 La 0.3 TiO 3 can reach -40.89 dB with a thickness of only 0.43 mm. Therefore, the La-doped