Effects of MnO2 addition on the microstructure and dielectric properties of LiTaO3 ceramics

Effects of MnO2 addition on the microstructure and dielectric properties of LiTaO3 ceramics

Vacuum 173 (2020) 109130 Contents lists available at ScienceDirect Vacuum journal homepage: http://www.elsevier.com/locate/vacuum Effects of MnO2 a...

2MB Sizes 0 Downloads 34 Views

Vacuum 173 (2020) 109130

Contents lists available at ScienceDirect

Vacuum journal homepage: http://www.elsevier.com/locate/vacuum

Effects of MnO2 addition on the microstructure and dielectric properties of LiTaO3 ceramics Wanwan Yang, Youfeng Zhang * School of Materials Engineering, Shanghai University of Engineering Science, PR China

A R T I C L E I N F O

A B S T R A C T

Keywords: LiTaO3 ceramics MnO2 Microstructure Dielectric properties

Lithium tantalite (LiTaO3) is an excellent single crystal. At present, only a few research studies have focused on polycrystalline LiTaO3 ceramics because it is beset with difficulties for the fabrication of dense ceramic by conventional sintering. In this study, LiTaO3 composite ceramics with added different MnO2 contents were ob­ tained by hot-pressing sintering at 1300 � C. The sinterability, microstructure, and dielectric properties of LiTaO3 composite ceramics were investigated. The relative densities of the LiTaO3 composite ceramics are remarkably improved by the addition of the MnO2 powder. The LiTaO3 ceramic achieves the highest relative density (98.6%) and exhibits a homogeneous microstructure following the addition of 3 wt% MnO2 sintered in N2. Only the LiTaO3 phase is observed at a MnO2 content addition below 3 wt%, and the secondary phase is found with further increases of added MnO2 (�3 wt%) to the MnO2/LiTaO3 (MLT) composite ceramics. The dielectric properties of the MnO2/LiTaO3 composite ceramics are significantly influenced by the MnO2 contents, and the relatively optimal values are obtained when the addition of 3 wt% MnO2 sintered in N2.

1. Introduction Lithium tantalite (LiTaO3) is well known for its unique optical and ferroelectric properties, which have been extensively studied for appli­ cations in optical waveguides, modulators and surface acoustic devices [1–4]. Because of its excellent optical properties, it not only has unique piezoelectric, acousto-optic, electro-optical, and nonlinear optical properties but also exhibits good mechanical and chemical stability. Improvements in the performance of LiTaO3 single crystals, powders, films, and ceramics have become an important research field. In recent years, extensive researches have focused on the performance of LiTaO3 single crystals. Although few have examined dense LiTaO3-based ce­ ramics, many studies have achieved some success using various sintering aids [5–10]. For example, Huanosta et al. have researched the Curie temperature and electrical properties of the Mg-doped LiTaO3 ceramics. Bamba et al. have examined the influence of adding CaTiO3 on electrical properties and microstructure of LiTaO3 ceramics, and Tahiri et al. have studied the effect of Cu doping on the corresponding substitution mechanism and microstructure of LiTaO3 ceramics. Similarly, Zhang et al. have researched the effect of adding Al2O3 on the microstructure and electrical properties of LiTaO3 ceramics and other systems, such as

LiF and MgF2 co-doped LiTaO3 system, or by widely replacing the A-site, B-site, or both [11–17]. It is difficult to densify LiTaO3 ceramics due to the great difference between the cation activity and mobility during sintering. In addition, the thermal expansion coefficients of LiTaO3 ce­ ramics exhibit conspicuous crystal anisotropy because of their trigonal crystal structure, which causes great pressure when cooling the consolidated body [18]. Mn is able to improve the performance of many functional ceramics, and researchers have shown that doping Mn can improve the dielectric properties of ceramics [19–21]. However, few research studies have examined the addition of MnO2 into LiTaO3 ce­ ramics. In general, high-density composite ceramics can be fabricated using the hot-pressing technique. In this study, a series of MnO2/LiTaO3 composite ceramics with different MnO2 mass fractions were fabricated by hot-pressing sintering. The effects of the MnO2 contents on the microstructure and dielectric properties of the MnO2/LiTaO3 composite ceramics are studied in detail. 2. Experimental details Commercially available MnO2 powder (Shanghai Meixing Chemical Limited Company, Shanghai, China) and LiTaO3 powder (Fangxiang

* Corresponding author. Shanghai University of Engineering Science, School of Materials Engineering, 1619 Room, Xing Zheng Building, 333 Long Teng Road, Shanghai, 201620, PR China. E-mail address: [email protected] (Y. Zhang). https://doi.org/10.1016/j.vacuum.2019.109130 Received 26 September 2019; Received in revised form 5 December 2019; Accepted 7 December 2019 Available online 10 December 2019 0042-207X/© 2019 Elsevier Ltd. All rights reserved.

W. Yang and Y. Zhang

Vacuum 173 (2020) 109130

Industry Co. Ltd., Shanghai, China) were used as starting materials. The powders were weighed according to a given ratio (mass fractions of 1%, 3%, 5%, and 7%) and then ball-milled for 24 h with agate balls using ethanol as the medium. The slurry was stirred and dried to generate MnO2/LiTaO3 (MLT) composite powders. A vacuum hot pressing furnace was utilized to prepare dense ceramics. The mixed powders of 3 wt% of MnO2 were placed in a graphite die and then sintered at 1300 � C for 0.5 h under nitrogen (N2) or vacuum, respectively. Then, the mixed powders doped with different MnO2 contents were sintered in the N2. The heating rate was set as 25 � C/min from room temperature to 1300 � C, after which the samples were removed and cooled to room temper­ ature, After sintering, the surfaces of the samples were then individually polished and coated with Pt electrodes. X-ray diffractometry (XRD) with Cu Ka radiation (X’Pert PRO PANalytical) was used to characterize the sample crystal structures. The XRD data was collected within the range of 10� –90� at 2θ under room temperature with a counting time of 1s and a step size of 0.02� . The MIP method was used to determine the relative density of the samples. The microstructures of the composite ceramics were characterized using scanning electron microscopy (SEM, Hitachi S–3400 N). Chemical composition studies were performed following the standard method, which employed an energy dispersive spectrometer (EDS). The SEM and EDS studies were carried out at an accelerating voltage ranging from 15 to 20 kV with Pt sputtering on the surface of the samples. X-ray photoelectron spectroscopy (XPS) studies were per­ formed on an ESCALAB 250Xi spectrometer with monochromatic Al Ka X-Ray source with excitation energy of 1486.6 eV. The frequency dependent dielectric constant and dielectric loss were carried out at room temperature from 100 Hz to 1 MHz using an impedance analyzer (Agilent E4294A). The variations in the dielectric constant and loss tangent at a temperature of 0.1, 1, 10, 100 kHz, and 1 MHz from room temperature (RT) to 750 � C were determined using a broadband dielectric spectrometer (LCR-HP4284A).

samples are sintered under N2 in order to decrease the vaporization of lithium in the research of Huang et al. [29]. Therefore, the N2 is used for sintering MLT ceramics doped with different MnO2 contents in this paper. 3.2. Relative density The relative densitiy of the MLT ceramics with different MnO2 con­ tents sintered in N2 measured by MIP method is presented in Fig. 2 [30], wherein all the samples exhibit a comparatively high relative density (above 95%) as compared to the theoretical density. It can be seen from the previous research that the relative density of pure LiTaO3 ceramics is only 91.5% [23], so the addition of MnO2 can significantly increase the density of the sample. The relative density of the ceramics increases following the addition of <3 wt% MnO2 as compared to the addition of 3 wt% MnO2, which exhibits a decrease with the further increase of MnO2 content. A maximum relative density of 98.6% is observed, indicating that the appropriate addition of MnO2 is conducive to the densification of LiTaO3 ceramics during sintering. However, an exces­ sive addition of MnO2 reduces the relative density of the ceramics possibly due to the uniformity of the grain growth during the sintering process. From these results, a reasonable addition of about 3 wt% MnO2 is added to the LiTaO3 ceramics. 3.3. Microstructural characterization The crystal structures of all the prepared samples with different MnO2 contents sintered in N2 were examined using XRD, as shown in Fig. 3, wherein the diffraction patterns of all the samples are wellindexed by the trigonal system and the corresponding indices of the diffraction peaks are marked. No additional diffraction peaks associated with secondary or structural phase transitions are detected in all of the samples, which may be due to the diffusion of the Mn ions into the LiTaO3 lattice to form a solid solution without changing the structure of the MLT ceramics. However, an enlarged section of Fig. 3(b) exhibits gentle shifting of the corresponding diffraction peaks toward the low angle following an increase in the MnO2 contents, such that the inter­ planar pacing distance increases with the increases of MnO2 addition. The Mn ions exhibits different valence states at various temperatures, which follows the previously reported mechanism [31]:

3. Results and discussion 3.1. Sintering atmospheres The optimum relative density and performance of ceramic samples sintered with manganese dioxide are obtained at 1300 � C by the research of Shimada et al. [22]. In our previous research, the LiTaO3 composite ceramics added with Al2O3 are obtained and the better sin­ tering temperature is 1300 � C [23]. The sinterability of ceramics is affected by the sintering atmosphere [24–28]. Firstly, LiTaO3 composite ceramics with added MnO2 of 3 wt% are sintered by hot-pressing sin­ tering at 1300 � C in vacuum and N2, respectively. Fig. 1 shows the SEM micrographs of the 3MLT ceramics sintered at vacuum and N2. Fig. 1(a) displays the uniform microstructure of the sample sintered in N2, Fig. 1(b) reveals the cracks occurring in the sample sintered in vacuum, and the number of pores also significantly increases. And, it can be seen from Fig. 1 that sintering ceramic of 3MLT in N2 is found the suitable with high density and unifrom microstructure, compared to sintered in vacuum, which may be due to the inhibit lithium evaporation sintered in N2 at high temperature. In addition, the

þ4

535℃

þ3

1080℃

þ2;þ3

1650℃

þ2

MnO2 �! Mn2 O3 ��! Mn3 O4 ��! MnO Mn

Fig. 1. SEM micrographs of the 3MLT ceramics with different atmospheres: (a) N2, (b) vacuum.



and Mn



(1)

are present in the LiTaO3 composite ceramics with

Fig. 2. Relative density of the MLT ceramics with different MnO2 contents sintered in N2. 2

W. Yang and Y. Zhang

Vacuum 173 (2020) 109130

Fig. 3. XRD patterns of MLT ceramics with different MnO2 contents sintered in N2.

added MnO2 contents since the sintering temperature is 1300 � C. Similar to the Mn-modified perovskite piezoelectric ceramics, the above phe­ nomenon may be a result of the partial replacement of the Ta5þ(0.64 Å) ions by the Mn ions given that the radii of Mn2þ (0.83 Å) and Mn3þ (0.645 Å) are larger than that of Ta5þ [14,19]. Therefore, small amounts of Mn2þ and Mn3þ substitute Ta5þ in the LiTaO3 lattice due to radius matching, thus resulting in the expansion of the LiTaO3 phase unit cells. XPS studies were conducted to supplement the composition, elec­ tronic configuration, and surface state of the MLT ceramics. The 3MLT ceramic sintered in N2 was chosen as a representative specimen for Mn (2p), O (1s), and Ta (4f) spectra as well as to provide a more detailed description. Many articles have pointed out that it is not appropriate to use the contaminated C1s peak as a reference, because there are obvious

changes in the C1s peak on different substrates or the amount of carbon contamination. In addition, the Fermi energy level position is not easy to determine, making the binding energy of the sample uncertain [32,33]. Therefore, the Au element is used to calibrate the peak position in XPS measurement and not the sample itself. Fig. 4(a) presents the wide-range XPS spectrum of the 3MLT ceramic, which indicates the presence of Mn, O, and Ta. Fig. 4(b) shows that the Mn 2p spectrum is consisted of two wide peaks of Mn 2p3/2 and Mn 2p1/2, which are mainly ascribed to the Mn–O bonds. In addition, the Mn 2p3/2 and 2p1/2 exhibit photoelectron emission energy of 642.4 eV and 654.1 eV, respectively. The different valence states of the doped Mn ions are investigated by fitting the overlapping Mn 2p3/2 and 2p1/2 peaks. Three 2p3/2 peaks of the 3MLT ceramic are observed at ~640.5 eV, ~642.6 eV, and ~646.3 eV. The

Fig. 4. XPS spectra of the 3MLT ceramic sintered in N2: (a) full spectra, (b) Mn (2p),(c) O(1s), and (d) Ta(4f). 3

W. Yang and Y. Zhang

Vacuum 173 (2020) 109130

Mn2þ ion peak is observed at a lower binding energy, while the peaks of the Mn3þ and Mn4þ ions are observed at higher binding energies, revealing the co-existence of the Mn2þ, Mn3þ, and Mn4þ ions. In addi­ tion, it can be seen from formula (1) that the oxide of Mn is present in the form of Mn3O4 at a temperature of 1300 � C, and the appearance of a small quantity of Mn4þ is related to the fact that MnO2 does not fully participate in the reaction, and there are three valence states of Mn appearing in the MnO2-doped K0.5Na0.5NbO3 ceramics studied by Wang et al. [34]. The O (1s) spectrum (Fig. 4(c)) exhibits a binding energy of 530.5 eV, which defines the lattice oxygen (O2 ), and a peak at 532.1 eV, which represents the oxygen vacancies, chemisorbed oxygen, and coordinated lattice oxygen. Fig. 4(d) indicates the presence of Ta 4f7/2 and Ta4f5/2 electron configurations at 26.0 eV and 27.9 eV, respectively, which is a representative characteristic of the Ta5þ ions. These results reveal the existence of an abundance of surface valence states and oxygen va­ cancies, which is beneficial to grain growth of the LiTaO3 composite ceramics. The SEM micrographs of the hot-pressed MLT ceramics with different MnO2 contents sintered in N2, as shown in Fig. 5. Fig. 5(a) indicates the presence of distinct pores at the grain boundaries of the 1MLT ceramic. Comparatively, Fig. 5(b) and (c) display more uniform microstructure in the 3MLT and 5MLT ceramics. In addition, there are small amount of pores at the grain boundaries in the 5MLT ceramic. The 1MLT ceramic exhibits inhomogeneous grain size distribution and small grains are surrounded by larger grains. The grain sizes in ceramics increase with the amount of MnO2 adding, such that the SEM micrograph of the 7MLT ceramic exhibits abnormal grain growth (Fig. 5(d)). The secondary phases are observed following further increasing of MnO2 (�3 wt%). As mentioned above, the B-site ions (Ta5þ) in the LiTaO3 lattice are replaced by the Mn ions, which have lower valences (Mn 2þ or Mn 3þ), thus creating oxygen vacancies and the reaction can be described using Kroger–Vink notation as follows: Ta5þ

2Mn2þ → 2Mn’’’Ta þ 3V€ O Ta5þ

Mn3þ → Mn00Ta þ V€ O

sintered in N2 to determine the chemical composition of the identified intermediate phase. The distribution of the Mn, Ta, and O elements can be characterized by the data presented in Fig. 6, which indicates Mn as the main component of spot A and Ta as the main component of spot B. These results reveal that the Mn concentration in the grain boundaries exceeds that of the LiTaO3 grains (Fig. 6(b) and (c)). The EDS analysis in Fig. 6 reveals manganese oxide as the second phase. Therefore, the gray particles are LiTaO3 phase and the black prismatic particles that are observed at the grain boundaries are manganese oxide. According to formula (1), Mn ions have different valence states at different temper­ atures. Therefore, it is speculated that the Mn ions in the MLT ceramics coexists with the Mn3þ and Mn2þ valence states as the temperature increaseing up to 1300 � C during sintering, of which the resulting black prismatic particles belong to Mn3O4. However, the absence of the Mn3O4 peak in the XRD patterns may be ascribed to the relatively low diffraction peak intensity of manganese oxide in comparison with LiTaO3 phase. A similar situation has occurred in the study of Mn-doped PbZrO3 ceramics by Liu [35]. Therefore, there is reason to believe that the second phase is in low relative intensities of peaks and has the small amount in the ceramics, resulting in the peaks of Mn3O4 are obscured by the LiTaO3 peaks. 3.4. Dielectric properties The frequency-dependent dielectric constant (εr) and dielectric loss (tanδ) of the MLT ceramics sintered in N2 were measured at room temperature as a function of frequency (Fig. 7). The εr and tanδ of all samples rapidly decrease with increasing frequency, after which εr and tanδ stabilize at a certain frequency. This low-frequency behavior can be explained by the Maxwell-Wagner effect or interface polarization [36]. Fig. 7(a) indicates that the dielectric constant of the MLT ceramics first increases and then decreases following an increase in the MnO2 con­ tents. In addition, the maximum value of εr is 75 for the 3MLT ceramic due to the uniform microstructure and highest density of the 3MLT ceramic. Dielectric properties are influenced by many factors, such as the relative density, dipole polarization, and space charge [37,38]. Firstly, the relative density affected the dielectric constant of the ce­ ramics. As shown in Fig. 7(a), the dependence of εr is dependent on the amount of doped MnO2, which exhibits an increasing-then-decreasing trend and reaches its maximum value at the 3MLT ceramic that corre­ sponded to the densification process observed in Fig. 2. In addition, when the solid solubility of MnO2 is limited in lithium tantalite, the excessive MnO2 addition induces second phase of manganese oxide accumulate at the grain boundaries, thus resulting in domain wall clamping. The clamp suppresses the macro-micro domain switching to some extent, which results in the relative dielectric constant decline [39]. The manganese oxide is a non-ferroelectricphase, its increasing causes decreasing of dielectric constant as well. The inset of Fig. 7(b) exhibits an increase in the dielectric loss with the addition of MnO2 at high frequencies. The dielectric loss affected by the phase composition, densification or porosity, and leakage current [40,41], given that the dielectric loss tends to increase with decreasing densification. Meanwhile, the presence of some second phase, especially of the conductive material, may still significantly increase the dielectric loss of ceramics [42]. According to the above analysis, the second phase belongs to Mn3O4 and has electrical conductivity [43]. Therefore, the second phase increases following increases in the MnO2 contents (Fig. 5), which results in an increase in the dielectric loss of the ceramics. The values of dielectric loss increase with the addition of MnO2, as indicated by the appearance of the second phase in the 3, 5, and 7MLT ceramics. Thus, the 1MLT sample exhibits the least tanδ. Furthermore, the dielectric loss of 3MLT ceramic is approximated with relation to 5MLT ceramic, which is simultaneously affected by the relative density and composition. Increases in the dielectric loss may be associated with various factors at MnO2 contents above 3 wt%. Thus, the dielectric properties of the LiTaO3 ceramics can be improved by the appropriate

(2) (3)

The generation of oxygen vacancies due to valence ion imbalances results in an increase in the mass and energy transfer between the different particles, thus improving the density and increasing the grain size of the ceramics. However, excessive addition can drastically worsen the sintering behavior of the ceramics due to the inhomogeneity of grain growth. EDS analysis was performed on the micro-areas of the 5MLT ceramic

Fig. 5. SEM micrographs of the MLT ceramics with different MnO2 contents sintered in N2: (a) 1MLT, (b) 3MLT, (c) 5MLT, and (d) 7MLT. 4

W. Yang and Y. Zhang

Vacuum 173 (2020) 109130

Fig. 6. (a) SEM micrograph; EDS results of spot (b) A and (c) B of the 5MLT ceramic sintered in N2 and EDS mapping of Mn, Ta, and O.

Fig. 7. (a) Frequency-dependent dielectric constant and (b) dielectric loss of the MLT ceramics sintered in N2.

addition of MnO2. The temperature-dependent dielectric constant (εr) and dielectric loss (tanδ) of the MLT ceramics sintered in N2 as a function of temper­ ature were measured at different frequencies, the results of which are

shown in Fig. 8(a)-(f). Fig. 8(a)–(e) indicate that only the 3MLT ceramic exhibits one dominant dielectric peak (referred to as Tc at which LiTaO3based ceramics make a phase transition from the ferroelectric to the paraelectric phase) at different frequencies due to the uniform 5

W. Yang and Y. Zhang

Vacuum 173 (2020) 109130

Fig. 8. Temperature-dependent dielectric constant and loss of the MLT ceramics sintered in N2 at different frequencies: (a) 0.1 kHz, (b) 1 kHz, (c) 10 kHz, (d) 100 kHz, (e) 1 MHz, and (f) the 3 MLT ceramic at different frequencies.

6

W. Yang and Y. Zhang

Vacuum 173 (2020) 109130

microstructure and the increase of the order of B-site in the sample. The values of εr are almost constant at low temperatures (<300 � C) due to the very small thermal motion energy, which almost “froze” the polar­ ized particles. Therefore, the dielectric constant is relatively stable at low temperature. In addition, the continuous increase of the dielectric constant is observed above 300 � C. The toggling of valence state be­ tween Mn3þ and Mn2þ increases with the rise in temperature. Under the applied electric field, the dipoles Mn3þ ↔ Mn2þ become oriented along the applied field, thus contributing to the dielectric constant and resulting in the continuous increase of the dielectric constant with the temperature. However, the dielectric constant decreases as the tem­ perature further rising at a higher frequency (Fig. 8(c)–(e)) due to sample transformation into a paraelectric phase above the Curie tem­ perature, which exhibits polarization disorder at high frequencies. The values of εr and tanδ increase with the increase of the MnO2 contents at 0.1 kHz and 1 kHz (Fig. 8(a) and (b)). The observed εr of the dielectric materials is a combined effect of different polarizations such as ionic polarization (Pi), dipolar polarization (Pd), space charge (Ps) and so on [44]. Generally, all polarizations are responsible for the increase of εr at low frequencies. Therefore, the observed increases in the dielectric constant can be ascribed to the fact that the sample contained more ions, space charges, and defects (oxygen vacancy) with the increase of the MnO2 contents. In addition, the charges on the defects follow the applied electric field, which in turn increase εr and tanδ. Furthermore, the grain boundaries are generally known to be more effective at lower fre­ quencies. Therefore, the grain growth may also contribute to improve­ ments in εr based on the observed SEM results. The εr of the 3MLT ceramic is larger than that of the 5MLT ceramic and smaller than that of the 7MLT ceramic at 10 kHz (Fig. 8(c)). The 3MLT ceramic exhibits the largest relative densities (Fig. 2), and the grain size of the 3MLT ceramic is less than that of the 7MLT ceramic (Fig. 5), which is affected by the influence of the grain boundary and relative density. In addition, the εr reaches a maximum of the 3MLT ceramic at 100 kHz and 1 MHz (Fig. 8 (d) and (e)) because the inability of the hopping electrons to follow the high-frequency AC field. Overall, the relative density plays a dominant role in the εr of the samples. Fig. 8(a)–(e) display the increase in the dielectric loss with increasing MnO2 contents. In this system, the variation in dielectric loss is the result of the combination of porosity and the presence of the second phase. The porosity is the main reason for the decrease in dielectric loss because of the absence of a second phase in the 1MLT ceramic, thus, the dielectric loss of the 1MLT ceramic is the lowest. With increasing of added MnO2, the excessive MnO2 results in Mn ions oversaturation in the lattice of the MLT ceramics, and redundant Mn ions will be accumulated at the grain boundaries as a second phase, thereby resulting in an increase in the electrical loss. However, only the loss of 1MLT ceramic suddenly in­ creases when the temperature exceeds phase-transition temperature, which is ascribed to the increase of the leakage current in the sample at high temperatures both under 0.1 kHz and 1 kHz. The Curie temperature (Tc) at which εr occurs shifts towards the low temperatures as the addition of the MnO2 contents while slightly in­ creases at 5 wt% at 100 kHz and 1 MHz (Fig. 8(d) and (e)). Yang et al. have reported that the phase transition temperature is affected by the valence state of the Mn ions, oxygen vacancies, and defect dipoles [45]. Thus, we believe that the valence state of the Mn ions causes the nonlinear change of Tc. Moreover, it can be also observed that all MLT ceramics have clear Tc peaks at high frequencies. The Tc for the 3MLT ceramic is relatively sharp and then broaden following the further in­ crease of MnO2. The widening of the dielectric peak near Tc resultes in diffusion phase transition over a wide range of temperature. This phe­ nomenon may be due to the heterogeneous composition and the increase degree of B-site disorder in the LiTaO3 crystal structure [46]. The 3MLT ceramic sintered in N2 was chosen as an example to further examine the temperature dependence of the dielectric constant and loss of the sample at different frequencies. Fig. 8(f) presents the εr and tanδ of the LiTaO3 sample with 3 wt% MnO2 as a function of

temperature at frequencies 0.1, 1, 10, 100 kHz, and 1 MHz. The dielectric constants are almost constant at lower temperatures and in­ crease at higher temperatures for all frequencies. The increase of εr with temperature can be attributed to the change in ordering of the electric dipoles. However, further increaseing in temperature resultes in dielectric constant decreases due to the random vibrational motion of the electrons and ions. The dielectric loss has a temperature relationship similar to that of the dielectric constant. An increase in the dielectric loss with the temperature can be related to the thermal activation relaxation mechanism [47]. As a whole, the improvement of dielectric constant can be realized by added sintering aid of MnO2 for LiTaO3 ceramics, but the dielectric loss of samples also accordingly increases. Hence, the added amount of sintering aid MnO2 needs to consider both sinterability and electric properties. The optimal content relatively is 3 wt% of MnO2 in the LiTaO3 matrix ceramic fabricated by hot-pressing sintering in N2 and the εr of 1647 at 1 MHz is obtained. 4. Conclusion This study examines the microstructure and dielectric properties of LiTaO3 matrix ceramics following the addition of different MnO2 con­ tents. The relative density of the LiTaO3 matrix ceramics obviously improves, specifically a maximum relative density of 98.6%, and a ho­ mogeneous composite ceramic microstructure is observed at an MnO2 amount of 3 wt% sintered in N2. Excess MnO2 is observed from the micrographs in the form of Mn3O4. The grain sizes of the LiTaO3 com­ posite ceramics continuously increases with the increase of the MnO2 mass fraction, which effectively promotes grain growth. The dielectric constant of the LiTaO3 composite ceramics at room temperature first increases within the frequency range of 100 Hz to 1 MHz, and then decreases following an increase in the MnO2 content at the same frequency. A maximum value of 75 is observed at a MnO2 content of 3 wt%. The dielectric constant and dielectric loss dependent temperatures increases with the increase of MnO2 contents. The Curie temperature decreases with the increase of the additive amount of MnO2, while has a small increase in the 5MLT ceramic. Based on both sinterability and electric properties considering, the highest relative density, well-grained microstructure, and optimal dielectric properties are relatively obtained following the addition of 3 wt% MnO2/LiTaO3 composite ceramic sintered in N2. Declaration of competing interest The authors declare no conflict of interest. Acknowledgments This work was supported by the National Natural Science Foundation of China (Nos. 11604204 and 51603120). References [1] C. Fu, A.J. Quan, J.T. Luo, et al., Vertical jetting induced by shear horizontal leaky surface acoustic wave on 36� Y-X LiTaO3, Appl. Phys. Lett. 110 (17) (2017), 173501. [2] M. Gruber, A. Leitner, D. Kiener, et al., Incipient plasticity and surface damage in LiTaO3 and LiNbO3 single crystals, Mater. Des. 153 (2018) 221–231. [3] A. Tarafder, K. Annapurna, R.S. Chaliha, et al., Processing and properties of Eu3þ: LiTaO3 transparent glass–ceramic nanocomposites, J. Am. Ceram. Soc. 92 (9) (2009) 1934–1939. [4] N.E. Yu, S. Kurimura, Y. Nomura, et al., Efficient optical parametric oscillation based on periodically poled 1.0mol% MgO-doped stoichiometric LiTaO3, Appl. Phys. Lett. 85 (22) (2004) 5134–5136. [5] M. Gruber, I. Kraleva, P. Supancic, et al., Strength distribution and fracture analyses of LiNbO3 and LiTaO3 crystals under biaxial loading, J. Eur. Ceram. Soc. 37 (14) (2017) 4397–4406. [6] S. Sanna, S. Neufeld, M. Rüsing, et al., Raman scattering efficiency in LiTaO3 and LiNbO3 crystals, Phys. Rev. B. 91 (22) (2015), 224302.

7

W. Yang and Y. Zhang

Vacuum 173 (2020) 109130

[7] Y.S. Cho, Electrical conduction in nanodomains in congruent lithium tantalate single crystal, Appl. Phys. Lett. 104 (4) (2014), 042905. [8] X.L. Kang, L.Y. Liang, W. Song, et al., Formation mechanism and elimination methods for anti-site defects in LiNbO3/LiTaO3 crystals, CrystEngComm 18 (42) (2016) 8136–8146. [9] X.Y. Fang, G. Yang, D.Z. Wei, et al., Coupled orbital angular momentum conversions in a quasi-periodically poled LiTaO3 crystal, Opt. Lett. 41 (6) (2016) 1169–1172. [10] D.Z. Wang, B.X. Yan, Y. Bi, et al., Three-wavelength green laser using intracavity frequency conversion of Nd:Mg:LiTaO3 with a MgO: PPLN crystal, Appl. Phys. B 117 (4) (2014) 1117–1121. [11] A. Huanosta, E. Alvarez, M.E. Villafuerte-Castrej� on, et al., Electrical properties of Mg-doped LiTaO3 ceramics, Mater. Res. Bull. 39 (14–15) (2004) 2229–2240. [12] N. Bamba, T. Yokouchi, J. Takaoka, et al., Effects of CaTiO3 on electrical properties in LiTaO3 ceramics, Ferroelectrics 304 (1) (2004) 135–138. [13] M. Tahiri, N. Masaif, A. Jennane, et al., Experimental and theoretical study of the density of Cu-doped LiTaO3, Opt. Quant. Electron. 48 (5) (2016) 278. [14] Y.F. Zhang, D.C. Jia, Y. Zhou, et al., Sintering mechanism and dielectric properties LiTaO3 matrix composites with the addition of Al2O3 particles, Ceram. Int. 35 (8) (2009) 3475–3478. [15] T. Yang, Y.G. Liu, L. Zhang, et al., Powder synthesis and properties of LiTaO3 ceramics, Adv. Powder Technol. 25 (3) (2014) 933–936. [16] Z.G. Ye, R.V.D. Mühll, J. Ravez, New oxyfluorides and highly densified ceramics related to LiNbO3, J. Phys. Chem. Solids 50 (8) (1989) 809–812. [17] Y.J. Song, Q.H. Zhang, X. Shen, et al., Room-temperature magnetism realized by doping Fe into ferroelectric LiTaO3, Chin. Phys. Lett. 31 (1) (2014), 017501. [18] C.F. Chen, G.L. Brennecka, G. King, et al., Processing of crack-free high density polycrystalline LiTaO3 ceramics, J. Mater. Sci. Mater. Electron. 28 (2017) 3725–3732. [19] M.Q. Awan, J. Ahmad, A. Berlie, et al., The upper Manganese doping limit and its effects on physical properties of lead-free Bi0.5Na0.5TiO3 ceramics, Ceram. Int. 44 (2018) 12767–12773. [20] H.M. Qiao, C. He, Z.J. Wang, et al., Improved thermal stability of ferro/piezoelectric properties of Mn-doped Pb(In1/2Nb1/2)O3- PbTiO3 ceramics, J. Eur. Ceram. Soc. 38 (2018) 3162–3169. [21] K. Sultan, M. Ikrama, K. Asokan, Structural, optical and dielectric study of Mn doped PrFeO3 ceramics, Vacuum 99 (2014) 251–258. [22] S. Shimada, K. Kodaira, T. Matsushita, Sintering LiTaO3 and KTaO3 with the aid of manganese oxide, J. Mater. Sci. 19 (1984) 1385–1390. [23] Y.F. Zhang, Y. Zhou, D.C. Jia, et al., Microstructure and mechanical properties of Al2O3 particle reinforced LiTaO3 piezoelectric ceramic matrix composites, Mater. Sci. Eng. A 448 (2007) 330–334. [24] X.M. Chen, W.Y. Pan, H.H. Tian, et al., Microstructure, dielectric and ferroelectric properties of 0.94Bi0.5Na0.5TiO3–0.06BaTiO3(NBTB) and 0.05BiFeO3–0.95 NBTB ceramics: effect of sintering atmosphere, J. Alloy. Comp. 509 (2011) 1824–1829. [25] S.W. Zhang, H.L. Zhang, B.P. Zhang, et al., Dielectric and piezoelectric properties of (Ba0.95Ca0.05)(Ti0.88Zr0.12)O3 ceramics sintered in a protective atmosphere, J. Eur. Ceram. Soc. 29 (2009) 3235–3242. [26] Y.J. Zhao, Y. Zheng, W.M. Ding, et al., Effect of TiO2 addition on the densification behavior, microstructure and mechanical properties of Al2O3-Cr cermets prepared via vacuum sintering, Vacuum 146 (2017) 274–277. [27] J.X. Xue, H.T. Liu, Y. Tang, et al., Effect of sintering atmosphere on the densification behavior of hot pressed TiN ceramics, Ceram. Int. 39 (7) (2013) 8531–8535.

[28] M.S. Nikovaa, V.A. Tarala, D.S. Vakalov, et al., Temperature-related changes in the structure of YSAG:Yb garnet solid solutions with high Sc3þ concentration, J. Eur. Ceram. Soc. 39 (15) (2019) 4946–4956. [29] J.T. Huang, Y.G. Liu, M.H. Fang, et al., Synthesis and characterization of singlecrystalline phase Li-α-Sialon, Ceram. Int. 38 (2012) 3391–3395. [30] W. Li, Z.H. Chen, Characterization of partially densified 3D-Cf/SiC composites by using mercury intrusion porosimetry and nitrogen sorption, Ceram. Int. 34 (2008) 531–535. [31] Y.S. Ng, S.M. Alexander, Structural studies of manganese stabilised lead- zirconatetitanate, Ferroelectrics 51 (1) (1983) 81–86. [32] G. Greczynski, L. Hultman, X-ray photoelectron spectroscopy: towards reliable binding energy referencing, Prog. Mater. Sci. 107 (2020), 100591. [33] W.P. Dianis, J.E. Lester, External standards in X-ray photoelectron spectroscopy. Comparison of gold, carbon, and molybdenum trioxide, Anal. Chem. 45 (8) (1973) 1416–1420. [34] T. Wang, D.M. Wang, Y. Liao, et al., Defect structure, ferroelectricity and piezoelectricity in Fe/Mn/Cu-doped K0.5Na0.5NbO3 lead-free piezoelectric ceramics, J. Eur. Ceram. Soc. 38 (15) (2018) 4915–4921. [35] H.B. Liu, Universal dielectric relaxation induced giant dielectric permittivity in Mn-doped PbZrO3 ceramics, Ceram. Int. 45 (2019) 10380–10384. [36] T. Wang, J.C. Hu, H.B. Yang, et al., Dielectric relaxation and Maxwell-Wagner interface polarization in Nb2O5 doped 0.65BiFeO3-0.35BaTiO3 ceramics, J. Appl. Phys. 121 (8) (2017), 084103. [37] M. Xiao, Y. Wei, P. Zhang, The effect of sintering temperature on the crystalstructure and microwave dielectric properties of CaCoSi2O6 ceramic, Mater. Chem. Phys. 225 (2019) 99–104. [38] G.L. Song, J. Su, H.G. Yang, et al., Modified crystal structure, dielectric properties, and magnetic phase transition temperature of Ca doped BiFeO3 ceramic, J. Sol. Gel Sci. Technol. 85 (2) (2018) 421–430. [39] M.J. Iqbal, A. Ullah, I.U. Rehman, et al., Dielectric, ferroelectric and electromechanical properties of (1 x)(Bi0.5Na0.5TiO3)–xBa(Ti0.8Zr0.2)O3 ceramics, J. Mater. Sci. Mater. Electron. 30 (11) (2019), 106386-110693. [40] T. Luo, X.X. Shan, J.W. Zhao, et al., Improvement of quality factor of SrTiO3 dielectric ceramics with high dielectric constant using Sm2O3, J. Am. Ceram. Soc. 102 (7) (2019) 3849–3853. [41] Z.P. Xu, L.X. Li, S.H. Yu, et al., Magnesium fluoride doped MgTiO3 ceramics with ultra-high Q value at microwave frequencies, J. Alloy. Comp. 802 (2019) 1–5. [42] J.H. Park, B.K. Kim, J.G. Park, et al., Effect of microstructure on the microwave properties in dielectric ceramics, J. Eur. Ceram. Soc. 21 (2001) 2669–2672. [43] P. Rosaiah, J.H. Zhu, O.M. Hussain, et al., Synthesis of flower-like reduced graphene oxide–Mn3O4 nanocomposite electrodes for supercapacitors, Appl. Phys. A Mater. 124 (9) (2018) 597. [44] B.N. Parida, D.K. Pattanayak, R.K. Parida, et al., Ferroelectric and optical modulations of double perovskite Ba2BiVO6, J. Mol. Struct. 1189 (2019) 288–298. [45] L.H. Yang, Z.R. Li, T.T. Wang, et al., Effect of Mn-doping on the structure and electrical properties of (Pb0.325Sr0.675 )TiO3 ceramics, Ceram. Int. 44 (2018) 16654–16659. [46] H.W. Du, Y.Q. Huang, H.L. Li, et al., Effects of NaSbO3 on phase structure and electrical properties of K0.5Na0.5NbO3–LiTaO3–NaSbO3 piezoelectric ceramics, J. Mater. Sci. Mater. Electron. 24 (2013) 855–860. [47] V. Kuldeep, O. Subohi, R. Kurchania, Investigations on dielectric, electrical and ferroelectric properties of xBaTiO3–(1 x)NiFe2O4 multiferroic composite ceramics, Appl. Phys. A Mater. 125 (1–12) (2019) 631.

8