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N substitution O-site in K0.5Na0.5Nb0.7Ta0.3O3 piezoceramics: Preparation, characterization and piezoelectric properties
Ceramics International 45 (2019) 23458–23466
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N substitution O-site in K0.5Na0.5Nb0.7Ta0.3O3 piezoceramics: Preparation, characterization and piezoelectric properties
T
Xuyu Jianga, Manman Shena, Xiwang Miaoa, Shuoyang Lianga, Min Guoa, Fangqin Chengb, Mei Zhanga,b,∗ a b
School of Metallurgical and Ecological Engineering, University of Science and Technology Beijing, Beijing, 100083, PR China Shanxi Collaborative Innovation Center of High Value-added Utilization of Coal-related Wastes in Shanxi University, Taiyuan, 030006, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: K0.5Na0.5Nb0.7Ta0.3O3-xNx piezoceramics N substitution O-site Piezoelectric properties
In the study, the N-doped K0.5Na0.5Nb0.7Ta0.3O3-xNx (x = 0.035, 0.055, 0.073) lead-free piezoceramics were successfully synthetized by the reaction of KNNT powders with urea. We systematically researched the influences of N3− partially substituted O2− on ceramics phase structure, microstructure, domain structures, piezoelectric properties, and oxygen vacancy defects. The results indicated N3− partially substituted O2− in KNNT ceramics without changing its phase structure, and remained a pure phase perovskite structure. N-doped KNNT ceramics had caused lattice expanded along the c-axis, leading to the increase of the asymmetry of KNNT lattice. In addition, with the increase of N content in KNNT ceramics, TC and TO-T of ceramics gradually decrease. Besides, the doped substitution of N to the O-site in KNNT ceramics would make the ceramics domains more easily reversed. The increase of lattice asymmetry and the easier reversal of electric domains caused by N doping would have positive effects on the d33 of KNNTN ceramics. Nevertheless, as the amount of N in the KNNT ceramics increased, the forbidden band width of the ceramics became smaller and the defects increased; and the oxygen vacancy defect concentrations were 18.9%, 20.2%, 20.9% and 22.5%, which corresponded to KNNTN (x = 0), KNNTN (x = 0.035), KNNTN (x = 0.055) and KNNTN (x = 0.073), respectively. The increase of the oxygen vacancy defect concentrations of KNNT ceramics would have negative effects on its piezoelectric properties. Hence, there was a competitive relationship between the positive and negative effects of N3− doping substituting the O-site in KNNT ceramics. When the doping amount of N was x = 0.035, its positive effects was greater than negative effects, and its relative density also reached the maximum value of 91.12%, making its d33 reached the maximum value of 207 pC/N.
1. Introduction As a kind of high-tech material, piezoelectric ceramic can realize the mutual conversion of mechanical energy and electric energy, and is widely used in machinery, electronics, precision control, national defense, military industry and other fields [1]. Traditional piezoelectric ceramic materials are mainly lead-based ceramics. Although they have good piezoelectric properties, lead has great harm to the environment and human health. Hence, the lead-free piezoelectric ceramics' study has turned into a hot topic. In the last few years, eco-friendly (K0.5Na0.5) NbO3 (KNN) lead-free piezoceramics have attracted extensive attention because of the good piezoelectric properties. In 2014, Zheng et al. [2] prepared 0.96(K0.46Na0.54+x)Nb0.95Sb0.05O3–0.04Bi0.5(Na0.82K0.18) 0.5ZrO3 lead-free piezoceramic based on potassium sodium niobate. Its piezoelectric constant was up to 496 pC/N, equivalent to lead zirconate
*
titanate. The giant piezoelectricity spark enthusiasm for research into lead-free piezoceramics. In recent years, through templated grain growth (TGG) method, Zhang et al. [3,4] have prepared (K, Na)NbO3based lead-free piezoceramics with the piezoelectric constant up to 330pC/N and grain-oriented (Bi1/2Na1/2)TiO3-based lead-free incipient piezoceramics with the large signal piezoelectric coefficient up to 766pm/V, which is of great significance to the study of lead-free piezoceramics. However, compared with lead-based piezoceramics, KNNbased piezoceramics still have the following disadvantages, which cannot meet the requirements of industrial applications. On the one hand, from the KNbO3–NaNbO3 phase diagram, the phase stability is limited to around 1140 °C near n(Na)/n(K) = 1.0 [3], and a large amount of liquid phase is generated when the temperature is slightly increased. Hence, the temperature zone is particularly narrow, and the acquisition of high-density ceramic embryos is very hard. On the other hand, when
Corresponding author. School of Metallurgical and Ecological Engineering, University of Science and Technology Beijing, Beijing, 100083, PR China. E-mail address: [email protected] (M. Zhang).
https://doi.org/10.1016/j.ceramint.2019.08.050 Received 14 June 2019; Received in revised form 31 July 2019; Accepted 5 August 2019 Available online 06 August 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
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the temperature reaches about 1000 °C, K2O and Na2O begin to evaporate [5]. Therefore, KNN-based lead-free piezoceramics behave poor repeatability and performance. For purpose of overcoming the above defects of KNN-based piezoceramics, many experiments have been carried out, mainly including the improvement of the preparation process and composition control. The improvement of the preparation process, including the oriented growth of grain preparation technology [6], special preparation methods of precursor powder (hydrothermal method [7], sol-gel method [8] and solid phase synthesis method, etc.) and special sintering process (hot-pressing method [9], microwave sintering method [10] and discharge plasma sintering [11], etc.), could optimize the performance of the KNN-based ceramics. The composition control was mainly focused on the doping substitution of the A and B sites of KNN-based ceramics, respectively. The research on A-site ion doping of KNN-based ceramics included Li+, Ag+ and alkaline earth metal ions (Mg2+, Ca2+ and Ba2+), which could make the piezoelectric constants d33 of ceramics reach 135–235 pC/N [12,13]. Research on Bsite ion doped Ta5+ and Sb5+ in KNN-based piezoceramics have been reported, and B-doped ceramics’ d33 reached 192–270 pC/N [14,15]. At present, the studies on A-site and B-site ion complex doping (Na0.5K0.5) NbO3 ceramics were mostly reported. If the A-site ion was doped with Li+ or Ag+, the B-site ion was Ta5+ or Sb5+ [16,17]. If the A-site ion was doped with alkaline earth metal ions or Bi3+, the corresponding dopant ion sited Ti4+ or Fe3+ at the B-site [18–21]. For the composite doping within A and B sites of KNN-based ceramics, its d33 could reach 190–271 pC/N. From these reports, it could be seen that for KNN-based piezoceramics, whether it was A-doping, B-doping or A and B sites complexed doping, the researches were relatively mature and have reached the bottleneck, thus KNN-based piezoceramics couldn't achieve a greater breakthrough, so that the group proposed to substitute the Osite (could also be called C-site) of the ceramics. Because the O-site in the KNN-based ceramics is the main skeleton of its perovskite structure, the O-site changes, companying with the great deformation in the ceramic structure and micro-morphology, maybe correspondingly change the ceramic properties. Therefore, it is very necessary to study the substitution of O-sites by other ions in KNN-based lead-free piezoceramics. Because of the similar radius and electronegativity of N3− and O2−, we considered using N3− to partially substitute O2− in KNN-based leadfree piezoceramics. Martha et al. [22] used pure phase KTaO3 to conduct solid phase reaction with urea, and generated N-doped photocatalyst KTaO3-xNx. Since both KNNT and KTaO3 are same oxygen octahedral structures, the O-site by N3− substitution is possible for the KNN-based piezoceramics. So far, there is no report to study the doping modification of O-site in KNN-based piezoceramics. In this work, we synthesized N-doped KNNT powders (KNNTN) by KNNT powders and urea reaction, and then sintered into KNNTN ceramics. The effects of different N doping amounts on the phase structure, morphology, domain structures, defects, density and piezoelectric properties of KNNTN lead-free piezoceramics were systematically investigated. 2. Experimental 2.1. Synthesis of N-doped KNNT ceramics The pure phase K0.5Na0.5Nb0.7Ta0.3O3 (KNNT) was synthesized by two-step microwave hydrothermal method [23]. The starting materials were Ta2O5 (purity: ≥99.99%), Nb2O5 (purity: ≥99.99%), NaOH (AR) and KOH (AR) powders. The precursor synthesized from the starting materials was poured into a hydrothermal kettle made of polytetrafluoroethylene, then heated by microwave at 130 °C (2h)~180 °C (7.5h). The synthesized powders were washed with DI water and ethyl alcohol, and exsiccated at 90 °C for 24h to obtain dried pure phase KNNT powders. Then, 1g of the pure phase KNNT powders and urea (CO(NH2)2) were uniformly mixed according to a certain mass ratio
(designed mKNNT/murea = 1/4, 1/8, 1/12), and heated at 400 °C for 6 h to obtain K0.5Na0.5Nb0.7Ta0.3O3-xNx (KNNTN) powders. These KNNTN powders were denoted as KNNTN (1/4), KNNTN (1/8) and KNNTN (1/ 12). 5 wt% polyvinyl alcohol (PVA) reagent was dropped into KNNTN powders and ground evenly. Then, the KNNTN powders were pressed into disks with Φ13mm*1 mm. The sample was sintered in a closed horizontal tube furnace. In order to prevent the volatilization of Na+ and K+, the sample was buried in KNNT powders. The green bodies were first heated to 100 °C for 30 min, removing the water in the samples. Then the samples were heated to 650 °C for 30 min to evaporate PVA. The heating rate of the drainage and glue removal process was 1 °C/min. Finally, the ceramics were heated to 1100 °C for 2 h with a temperature rise rate of 3 °C/min, and lowered to room temperature with the furnace. For purpose of preventing the oxidation of N in KNNT powders, the whole heating process was carried out in an N2 atmosphere with a flow rate of 160 cm3/min. Finally, KNNTN lead-free piezoelectric ceramics with different nitrogen content doping were obtained. 2.2. Characterization and performance measurements X-ray diffractometer (XRD, MAC-M21XRHF, Japan) was identified the phase composition of the samples by using Cu Kα radiation. The LCR meter (Agilent E4980A; Agilent Technologies Inc, Penang, Malaysia) was operated at 10 kHz to record temperature dependent relative permittivity ε of the ceramics. Field mission scanning electron microscopy (SEM, JSM-6700F, Japan) was determined the microscopic morphology and particle size of the samples. In SEM analysis, the sintered ceramics were firstly cut off, and then the fracture microstructures after carbon coating were observed. The domain structures of the ceramics were observed by transmission electron microscopy (TEM, JEOL, JEM-2200FS, Japan) was used to determine the domain structures of the KNNTN piezoceramics. The binding energy of each element in the sample was determined by X-ray photoelectron spectroscopy (XPS, AXISULTRA-DLD, Japan) using Al Kα radiation. The N element in the sample was quantified by an oxygen-nitrogen hydrogen analyzer (EMGA-830, HORIBA, Germany). UV–Vis spectrophotometer (TU-1901, China) was identified forbidden band width of the samples. The bulk density of ceramics was measured by the Archimedes method. Archimedes method was used to measure the densities of the samples. Quasi-static piezoelectric coefficient meter (YE2730A, China) was used to identified the piezoelectric constant (d33) of the piezoceramics. Before testing the performance, the ceramic samples were needed to be insulated at 800 °C for 30 min for silver plating. Finally, the silverplated ceramic samples were polarized at 130 °C in silicone oil and stored under a 4 kV/mm DC electric field for 10 min. 3. Results and discussion 3.1. Phase composition XRD was used to characterize the crystallinity and crystal phase of KNNTN powders and KNNTN ceramics with different N contents substitution. The XRD patterns of KNNTN powders are showed in Fig. 1. The diffraction patterns of KNNTN (1/4), KNNTN (1/8) and KNNT (1/12) powders are monoclinic perovskite structure (JCPDS:00-077-0038), and no other nitrides compounds appeared. Because urea was completely decomposed into NH3 and carbon oxides at 400 °C, excess NH3 and carbon oxides that do not react with the KNNT powders were escaped. The results showes that partial doping of N3− to replace O2− in KNNT lattice don't lead to significant structural changes. It maybe the fact that the low concentration of doped N3− is insufficient to cause lattice remodeling of KNNT, and there have been reported similar phenomena in previous report [24]. The XRD patterns of KNNTN ceramics with different N contents are
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Table 1 Lattice constant of prepare samples.
Fig. 1. XRD patterns of KNNTN powders with different N contents.
showed in Fig. 2(a). It is obvious observed that all the ceramics are single phase and no second phase is formed. Because the sintering process was carried out under the atmosphere of nitrogen, the N element in KNNT ceramics was well protected from being oxidized at high temperature. However, the main crystalline phase of the sintered ceramic changes from a monoclinic phase of the powders to an orthogonal-tetragonal phase (JCPDS:01-071-2171 and JCPDS:01-071-0945). It is reported that the orthogonal phase and the tetragonal phase with better symmetry are more stable than that of monoclinic phase, and they are easier to retain at high temperatures. Fig. 2(b) shows the magnified views of Fig. 2(a) in the 2θ = 31.0–32.5° range. It is observed that the diffraction peaks of KNNTN piezoceramics gradually move left as the doping amount of element N increases. Since the radii of O2− (0.140 nm) is less than the radii of N3− (0.146 nm), and it is known from the Bragg equation 2dsinθ = nλ, when the doping amount of N increases, the 2θ angle decreases and the diffraction peak shifts to the left. Moreover, the lattice constants of the representative tetragonal phase of KNNTN lattice calculated by Jade software are shown in Table 1, as the doping amount of N increases, the lattice constant of the sample increases accordingly in the c-axis, whereas the variation of lattice constants in a-axis and b-axis are irregular. In the work, these KNN-based piezoceramics undergo the same preparing condition except
sample
a = b/Å
c/Å
KNNT (pure) KNNTN (1/4) KNNTN (1/8) KNNTN (1/12)
7.9725 7.9694 7.9789 7.9626
7.9279 7.9372 7.9630 8.0427
for the different N contents. Besides, in order to avoid consumption of Na+ and K+ in KNN-based piezoceramics, these ceramics were embedded into the same powders for sintering; therefore, the c-axis enlarge caused by the volatilization of the A-site in the ceramics can be ignored. Hence, the lattice constant of the c-axis in KNNT ceramics increases, which is completely caused by the doping substitution of the O-site by N3−. This is because that the A and B sites ions are located on the flat of the oxygen octahedral structure, and the O-site is the skeleton of the oxygen octahedron. N doping the O-site directly enlarges the crystal grains in the c-axis, but has little effect in the a-axis and b-axis. The lattice expansion on the c-axis increases the asymmetry of the KNNTN lattice, which has a positive influence on the piezoelectric properties of ceramics [25]. The relative dielectric constants of KNNTN ceramics related to temperature were tested at 10 kHz, as shown in Fig. 3. As can be seen from the figure, the phase transitions experienced by KNNTN ceramics in the temperature range around room temperature and above are mainly phase transitions between orthogonal phase (O)-tetragonal phase (T)-cubic phase (C), and as the temperature rises, two peaks appear at about 60 °C and 190 °C, corresponding to the orthogonaltetravalent phase transition temperature (TO-T) and Curie temperature (TC), respectively [12]. The existence of TO-T indicates that there exists a two-phase coexisting zone of orthogonal phase and tetragonal phase in KNNTN ceramics near 60 °C. In the two-phase coexisting zone, the asymmetry of grains of ceramics increases, and its piezoelectric properties will reach the maximum value. As can be seen from the figure, with the increase of N content in KNNTN ceramics, its TO-T decreases from about 64 °C to about 51 °C, which is closer to room temperature. The decrease of TO-T temperature of KNNTN ceramics depicts that with the increase of N element, the grains were more likely to transform into the two-phase coexistence zone, and the asymmetry of grains increases, which has a positive impact on the piezoelectric properties of ceramics.
Fig. 2. XRD patterns of KNNTN piezoceramics with different N contents: (a) entire range; (b) magnified diffraction peak in the range of 31–32.5°. 23460
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Table 2 The N content of the KNNTN piezoceramics with different N contents. Designed mKNNT:murea
Real Weight%
Real Atom%
Real n(N):n(O)
1:4 1:8 1:12
0.248 0.391 0.514
0.703 1.108 1.455
0.035:2.965 0.055:2.945 0.073:2.927
N3−, it is widely believed to be the evidence of the presence of Ta-N bond, indicating that the oxygen atom is successfully replaced by a nitrogen atom [28]. And it can be seen from Fig. 4(b) that with the increasing amount of urea reacting with KNNT, the peak of N become more and more obvious, illustrating that the N content in the KNNTN powders increases with the increase of reactant urea amount. Combined with XRD results (Fig. 2) and XPS analysis (Fig. 4), it can be determined that O atoms in KNNT are partial replaced by N atoms successfully, and the K0.5Na0.5Nb0.7Ta0.3O3-xNx (KNNTN) is formed. Fig. 3. Temperature dependences of relative dielectric constants of the KNNTN piezoceramics. Inset: Change trend of TO-T and TC of KNNTN piezoceramics.
3.2. Characterization of N element in KNNTN ceramics From the XRD diffraction pattern (Fig. 2), it can be seen that the ceramics produced were still perovskite structure without the formation of the second phase. However, it is not certain whether N3− doping replaces the O-site in KNNT ceramics. Therefore, the doping of N3− in KNNT ceramics was qualitatively and quantitatively studied by using XPS and oxygen-nitrogen-hydrogen analyzer, respectively. 3.2.1. Qualitative analysis of N3− substitution The chemical states of each element (K, Na, Nb, Ta, and O) in the KNNTN piezoceramics were confirmed by X-ray photoelectron spectroscopy(XPS). Fig. 4(a) shows the XPS spectrum of KNNTN (1/12) ceramic as the representative. The peaks of K 2p1/2 and K 2p3/2 in KNNTN occurred are at 291.7 eV and 294.2 eV, corresponding to K+; and the Na 1s peak at 1071.3 eV is observed, which corresponded to Na+. The O 1s peaks at 529.3 eV is associated with the O of KNNTN lattice. The Ta 4f peaks at 25.11 eV and 26.84 eV mean that Ta mainly exists as Ta5+in the KNNTN lattice [26]. And the Nb 3d peaks at 206.7 eV and 209.5 eV, indicate that the form of Nb atoms in the sample is Nb5+ [27]. Fig. 4(b) shows the N 1s peaks of the KNNTN ceramics prepared with different N doping amounts. The binding energies of N 1s in all the KNNTN ceramics are around 395.3 eV. These peaks are the feature of
3.2.2. Quantitative analysis of N elements The oxygen-nitrogen hydrogen analyzer quantitatively analyzed the N elements in KNNTN ceramics. Table 2 shows the mass percentage, atomic percentage and molar ratio of O atom to N atom of KNNTN ceramics with different N doping proportions. As can be seen from Table 2, when the designed mass ratios of KNNT powders and urea are 1:4, 1:8 and 1:12, the real molar ratios of N and O in the KNNTN ceramics are 0.035:2.965, 0.055:2.945 and 0.073:2.927, respectively. Hence, as the amount of urea reacted with KNNT powders increases, the content of doped N increases accordingly, and this result is also in agreement with the outcomes of XRD (Fig. 2) and XPS (Fig. 4). According to the detecting results, the designed KNNT (pure), KNNTN (1/ 4), KNNTN (1/8) and KNNTN (1/12) can be denoted as KNNTN (x = 0), KNNTN (x = 0.035), KNNTN (x = 0.055) and KNNTN (x = 0.073), respectively. 3.3. Microstructures The SEM images of the KNNTN piezoceramics with different N doping amounts are shown in Fig. 5. It can be seen from the figure that the grain sizes of KNNTN (x = 0), KNNTN (x = 0.035), KNNTN (x = 0.055) and KNNTN (x = 0.073) ceramics are 1.26, 1.35, 1.38 and 1.44 μm, respectively. Since the radius of N3− and O2− are 0.16 nm and 0.14 nm respectively, the radius of N3− is slightly larger than the radius of O2−. When N substitutes O-site doping into the KNNT ceramics’ lattices and occupies the lattice nodes, a certain amount of lattice expansion will be generated, and the expansion will increase with the increase of the amount of N entering the lattice. Hence, the grain size of
Fig. 4. XPS spectrum of KNNTN: (a) survey spectrum; (b) high-resolution XPS spectra of N 1s&Ta 4p in the KNNTN powders with different N contents. 23461
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Fig. 5. SEM images of KNNTN piezoceramics: (a) x = 0; (b) x = 0.035; (c) x = 0.055; (d) x = 0.073. Inset: Grain size distributions of KNNTN piezoceramics.
KNNTN ceramics increases with increasing N3−doping. As can be seen from the figure, the morphology of KNNTN (x = 0) ceramic is more regular than the others. The KNNTN (x = 0.035) ceramic grains has a relatively regular morphology and good grain growth, the most compact. The KNNTN (x = 0.055) ceramic is also dense in grain growth, but has more pores than that of the KNNTN (x = 0.035) ceramic. This should be caused by more N3− substituting in KNNTN (x = 0.055) powders, resulting to more serious lattice distortion. As the amount of N3− doped in KNNTN (x = 0.073) ceramic, the lattice distortion is the most serious, leading to the irregular grain shape the increase of pores, and thus might result the decrease of density. The increase of pores in KNNTN ceramics might negatively affect the piezoelectric properties.
because of the stress of the ceramic grain boundary and the depolarization field greatly limit the 90° domain to reverse, and the reversed 90° domain will revert to its original state under the action of electric field in the grain boundary after strong restrictions gradually returns to the original state [36,37]. By comparing the electric domain structure of KNNTN ceramics with different N contents in Fig. 6, with the increase of N content in KNNTN piezoceramics, the proportion of the domain of 180° gradually increases. Therefore, under the action of electric field, more electric domains has been reversed, thus greatly improving the orientation of KNNTN ceramics’ electric domain, which has a positive impact on their piezoelectric properties. 3.5. Defect analysis
3.4. Electric domain analysis As is well-known that the piezoelectric effect of ferroelectric ceramics consists of two parts, that are the intrinsic contribution and the extrinsic contribution. The former refers to the linear piezoelectric effect associated with lattice displacement, while the latter mainly derives from the motion of the domain wall [29]. The above experimental results proved that the doping substitution of N3− to the O-site in KNNT ceramics causes lattice distortion, and whether it affects the electric domain needed further research. Fig. 6 is the result of observation and analysis of the electric domains of polarized KNNTN (x = 0, 0.035, 0.055 and 0.073) ceramics by transmission electron microscope (TEM). The domain structures of stripe (S), herringbone (H) and watermark (W) can be seen in the figure. The stripe shape corresponded to 90° domain structure, herringbone bone shape is shaped by two alternate 90° domain structure, while watermark shape is corresponding to 180° domain structure [30–32]. The domain structure of 180° is more easily reversed than that of 90°. There have been reports of piezoceramics, only less than 15% of the 90° domain can reverse [33–35], this is
Because the valences of N3− and O2− in KNNTN ceramics are different, oxygen vacancy defects must exist in KNNTN in order to maintain charge balance [38], so that UV–Vis DRS and XPS were used to analyze oxygen vacancy defects. The Tauc plot derived from UV–Vis DRS is applied to confirmed the forbidden band width of materials and analyzes the defect concentrations [39]. Fig. 7 displays the Tauc plot of KNNTN ceramics with different N contents doping. The Tauc equation is expressed as Equa(1):
[F(R) hv]n = A (hv − Eg )
(1)
F(R) = (1-R∞)2/2R∞, R∞ represents the relative reflectivity of the apparatus, h (J·s) represents Planck constant, ν (Hz) represents the vibration frequency. The index n is associated with electron transition (indirect n = 1/2, direct n = 2; here n = 1/2), A is the proportionality constant associated with the sample, Eg (eV) denotes the forbidden band width of the material. The Tauc curve drawn by Equa(1) is shown in Fig. 7 ([F(R)hν]1/2 is the data of vertical axis, Hv is the data of horizontal axis.).
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Fig. 6. TEM images of domain patterns for the polarized KNNTN piezoceramics: (a) x = 0; (b) x = 0.035; (c) x = 0.055; (d) x = 0.073.
Fig. 7. Tauc plots of KNNTN ceramics; inside table is the forbidden width of KNNTN ceramics.
The tangent intercept of Tauc curve represented the forbidden band width of ceramics, and it can be seen when the N doping amount x of KNNTN ceramics increases from 0 to 0.035, 0.055 and 0.073, the forbidden band width of the ceramics decreases correspondingly from 3.638 ± 0.004 to 3.611 ± 0.004, 3.574 ± 0.005 and 3.556 ± 0.003 eV, respectively. These results are consistent with what Rao M P reports [22]. The decrease of the forbidden band width can lead to the decrease of the potential barrier, which may lead to the
increase of the defects. In the Tauc diagram, there is a positive correlation between tail absorption and defect concentration [39]. In Fig. 7, the tail absorption value increases with increasing doping amount of N. That is to say, the increase of the doping amount of N in KNNT ceramics increase the defect concentration of ceramics. Fig. 8(a) is XPS high-resolution view of O 1s, which is divided into peak(Ⅰ) and peak(Ⅱ). The peak(Ⅰ) of binding energy at 529.8 eV is O in the lattice of KNNTN, and the peak(Ⅱ) at 531.3 eV is generally considered as oxygen vacancy defects. Oxygen vacancy concentration can be expressed by the peak area ratio (Ⅰ/(Ⅰ+Ⅱ)) [40,41], and the calculation results are displayed in Fig. 8(b). It is observed that when the N contents of KNNTN ceramics are x = 0, 0.035, 0.055 and 0.073, the oxygen vacancy concentrations are 18.9%, 20.2%, 20.9% and 22.5%, respectively. These results are consistent with the result of uv–vis DRS analysis (Fig. 7), indicating that with the increase of doping N content in KNNTN ceramics, the oxygen vacancy defects of the ceramics increase. Oxygen vacancies are conducive to mass transportation, so the increase of oxygen vacancy is benefit for grain growth [40], which is another reason why the grain size of KNNTN ceramics increases with the increase of N content in ceramics as shown in Fig. 5. However, the increase of oxygen vacancy in KNNTN ceramics will cause the increase of its leakage current, leading to the decline of its insulation, which is easy to be broken down under high electric field, not conducive to the reversal of electric domain [42–44]. The results of TEM figure(Fig. 6) shows that with the increase of N content in KNNTN ceramics, easy reversal of 180° domain proportion increases, benefiting to the raise of ceramic piezoelectric properties; but the increase of oxygen vacancy limits the electrical domain reversal, this may be why there are still some difficult reversal 90° domain in KNNTN ceramics, which is not conductive to the improvement of ceramics piezoelectric properties. In summary, with the increase of N content in KNNTN ceramics, the oxygen vacancy concentration in ceramics increases, which has a
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Fig. 8. (a) High-resolution XPS spectra of O 1s in KNNTN piezoceramics when x = 0, 0.035, 0.055 and 0.073, (b) the oxygen vacancy concentrations of KNNTN piezoceramics.
negative impact on the piezoelectric properties of ceramics.
3.6. Density and piezoelectric properties The results of the relative density and piezoelectric constants of the KNNTN piezoceramics are shown in Fig. 9. When x = 0 is increased to x = 0.035, the relative density of the KNNTN piezoceramics increases correspondingly from 86.24% to 91.12%, and when x is increased to 0.055 and 0.073, the relative density of ceramics correspondingly decreases to 83.09% and 80.67%. Combined with the SEM micrographs of Fig. 5, replacing O-site in KNNT lattice with a very small amount of N3− can improve the relative density of KNNT ceramics, but excessive N3− can cause irregular grain growth and increase the number of pores, thus leading to a decrease in density. As observed from Fig. 9, the piezoelectric constants of ceramics are positively related to the relative density. Combined with the above results of XRD (Fig. 2), SEM (Fig. 5), TEM (Fig. 6) and oxygen vacancy defect analysis(Figs. 7 and 8), the increase of grains asymmetry and more easily reversed domains of KNNTN ceramics are beneficial to the piezoelectric properties, while the increase of oxygen vacancy defects is not conducive to the piezoelectric
Fig. 9. The results of the relative density and piezoelectric constants of the KNNTN ceramics.
properties. Therefore, the piezoelectric properties of KNNTN piezoceramics are the competitive results of these factors. It can be seen from Fig. 9 that when the N doping content in KNNTN ceramics is x = 0.035, the positive effects brought by the asymmetry of ceramic grains, density increase and domain reversal of the ceramics are greater than the negative effects brought by the increase of the oxygen vacancy defects, so that its d33 reaches the maximum value of 207 pC/N. The result is higher than that of KNNTN (x = 0) ceramics (d33 = 186 pC/N). However, when x = 0.055 and 0.073, in contradiction to the above results, resulting in the gradual decrease of d33, which are 180 pC/N and 172 pC/N, respectively. The influences of various factors on d33 of KNNTN ceramics are illustrated in Fig. 10.
4. Conclusions In this study, K0.5Na0.5Nb0.7Ta0.3O3-xNx powders were synthesized from urea and KNNT powders and sintered into K0.5Na0.5Nb0.7Ta0.3O3xNx ceramics (x = 0.035, 0.055 and 0.073). The main crystal phase of KNNT powders after reaction with urea were still pure perovskite phase. And doping N elements in KNNT ceramics caused lattice distortion and leaded to the elongation of crystal cells along the c-axis, which further increased the asymmetry of the crystal lattice of KNNTN ceramics. Besides, with the increase of N content in KNNTN ceramics, TC and TO-T of ceramics decreased from 192 °C and 64 °C to 185 °C and 51 °C, respectively. The decrease of TO-T indicates that ceramic grains are more likely to transform into the two-phase coexistence zone of orthogonal phase and tetragonal phase, which is also conducive to the increase of ceramic grains asymmetry. KNNTN (x = 0.035) ceramic has the densest microstructure and regular grain growth, while KNNTN (x = 0.055) and KNNT (x = 0.073) ceramics showed irregular grain growth and decreased density. Through the observation of the ceramic domain, it could be seen that the doped substitution of N to the O-site in KNNT ceramics would make the ceramics domains more easily reversed. As the amount of N in the KNNTN ceramics increased, the forbidden band width of the ceramic became smaller and the defects increased. When x = 0, 0.035, 0.055 and 0.073, the oxygen vacancy defect concentrations of corresponding ceramics were 18.9%, 20.2%, 20.9% and 22.5%, respectively. Based on the above results and the measured d33 of KNNTN ceramics, when the N content in KNNTN ceramics was x = 0.035, the positive effects brought by the asymmetry of ceramic grains, density increase and domain reversal of the ceramics were greater than the negative effects brought by the increase of the oxygen vacancy defects, so that its d33 reached the maximum value of 207 pC/N. However, when x = 0.055 and 0.073, the positive effects
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Fig. 10. The influences of various factors on d33 of KNNTN ceramics.
brought by the asymmetry of ceramic grains and domain reversal of ceramics were less than the negative effects brought by the decrease of density and the increase of oxygen vacancy defects, resulting in the gradual decrease of d33, which were 180 pC/N and 172 pC/N, respectively. Declaration of interest
[15]
[16] [17] [18]
None. Acknowledgment
[19]
The National Natural Science Foundation of China (Nos. 51672025, 51572020, 51372019), and Major Projects of Science and Technology in Shanxi Province (MC2016-03) are duly acknowledged for their financial support.
[20]
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N substitution O-site in K0.5Na0.5Nb0.7Ta0.3O3 piezoceramics: Preparation, characterization and piezoelectric properties
T
Xuyu Jianga, Manman Shena, Xiwang Miaoa, Shuoyang Lianga, Min Guoa, Fangqin Chengb, Mei Zhanga,b,∗ a b
School of Metallurgical and Ecological Engineering, University of Science and Technology Beijing, Beijing, 100083, PR China Shanxi Collaborative Innovation Center of High Value-added Utilization of Coal-related Wastes in Shanxi University, Taiyuan, 030006, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: K0.5Na0.5Nb0.7Ta0.3O3-xNx piezoceramics N substitution O-site Piezoelectric properties
In the study, the N-doped K0.5Na0.5Nb0.7Ta0.3O3-xNx (x = 0.035, 0.055, 0.073) lead-free piezoceramics were successfully synthetized by the reaction of KNNT powders with urea. We systematically researched the influences of N3− partially substituted O2− on ceramics phase structure, microstructure, domain structures, piezoelectric properties, and oxygen vacancy defects. The results indicated N3− partially substituted O2− in KNNT ceramics without changing its phase structure, and remained a pure phase perovskite structure. N-doped KNNT ceramics had caused lattice expanded along the c-axis, leading to the increase of the asymmetry of KNNT lattice. In addition, with the increase of N content in KNNT ceramics, TC and TO-T of ceramics gradually decrease. Besides, the doped substitution of N to the O-site in KNNT ceramics would make the ceramics domains more easily reversed. The increase of lattice asymmetry and the easier reversal of electric domains caused by N doping would have positive effects on the d33 of KNNTN ceramics. Nevertheless, as the amount of N in the KNNT ceramics increased, the forbidden band width of the ceramics became smaller and the defects increased; and the oxygen vacancy defect concentrations were 18.9%, 20.2%, 20.9% and 22.5%, which corresponded to KNNTN (x = 0), KNNTN (x = 0.035), KNNTN (x = 0.055) and KNNTN (x = 0.073), respectively. The increase of the oxygen vacancy defect concentrations of KNNT ceramics would have negative effects on its piezoelectric properties. Hence, there was a competitive relationship between the positive and negative effects of N3− doping substituting the O-site in KNNT ceramics. When the doping amount of N was x = 0.035, its positive effects was greater than negative effects, and its relative density also reached the maximum value of 91.12%, making its d33 reached the maximum value of 207 pC/N.
1. Introduction As a kind of high-tech material, piezoelectric ceramic can realize the mutual conversion of mechanical energy and electric energy, and is widely used in machinery, electronics, precision control, national defense, military industry and other fields [1]. Traditional piezoelectric ceramic materials are mainly lead-based ceramics. Although they have good piezoelectric properties, lead has great harm to the environment and human health. Hence, the lead-free piezoelectric ceramics' study has turned into a hot topic. In the last few years, eco-friendly (K0.5Na0.5) NbO3 (KNN) lead-free piezoceramics have attracted extensive attention because of the good piezoelectric properties. In 2014, Zheng et al. [2] prepared 0.96(K0.46Na0.54+x)Nb0.95Sb0.05O3–0.04Bi0.5(Na0.82K0.18) 0.5ZrO3 lead-free piezoceramic based on potassium sodium niobate. Its piezoelectric constant was up to 496 pC/N, equivalent to lead zirconate
*
titanate. The giant piezoelectricity spark enthusiasm for research into lead-free piezoceramics. In recent years, through templated grain growth (TGG) method, Zhang et al. [3,4] have prepared (K, Na)NbO3based lead-free piezoceramics with the piezoelectric constant up to 330pC/N and grain-oriented (Bi1/2Na1/2)TiO3-based lead-free incipient piezoceramics with the large signal piezoelectric coefficient up to 766pm/V, which is of great significance to the study of lead-free piezoceramics. However, compared with lead-based piezoceramics, KNNbased piezoceramics still have the following disadvantages, which cannot meet the requirements of industrial applications. On the one hand, from the KNbO3–NaNbO3 phase diagram, the phase stability is limited to around 1140 °C near n(Na)/n(K) = 1.0 [3], and a large amount of liquid phase is generated when the temperature is slightly increased. Hence, the temperature zone is particularly narrow, and the acquisition of high-density ceramic embryos is very hard. On the other hand, when
Corresponding author. School of Metallurgical and Ecological Engineering, University of Science and Technology Beijing, Beijing, 100083, PR China. E-mail address: [email protected] (M. Zhang).
https://doi.org/10.1016/j.ceramint.2019.08.050 Received 14 June 2019; Received in revised form 31 July 2019; Accepted 5 August 2019 Available online 06 August 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
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the temperature reaches about 1000 °C, K2O and Na2O begin to evaporate [5]. Therefore, KNN-based lead-free piezoceramics behave poor repeatability and performance. For purpose of overcoming the above defects of KNN-based piezoceramics, many experiments have been carried out, mainly including the improvement of the preparation process and composition control. The improvement of the preparation process, including the oriented growth of grain preparation technology [6], special preparation methods of precursor powder (hydrothermal method [7], sol-gel method [8] and solid phase synthesis method, etc.) and special sintering process (hot-pressing method [9], microwave sintering method [10] and discharge plasma sintering [11], etc.), could optimize the performance of the KNN-based ceramics. The composition control was mainly focused on the doping substitution of the A and B sites of KNN-based ceramics, respectively. The research on A-site ion doping of KNN-based ceramics included Li+, Ag+ and alkaline earth metal ions (Mg2+, Ca2+ and Ba2+), which could make the piezoelectric constants d33 of ceramics reach 135–235 pC/N [12,13]. Research on Bsite ion doped Ta5+ and Sb5+ in KNN-based piezoceramics have been reported, and B-doped ceramics’ d33 reached 192–270 pC/N [14,15]. At present, the studies on A-site and B-site ion complex doping (Na0.5K0.5) NbO3 ceramics were mostly reported. If the A-site ion was doped with Li+ or Ag+, the B-site ion was Ta5+ or Sb5+ [16,17]. If the A-site ion was doped with alkaline earth metal ions or Bi3+, the corresponding dopant ion sited Ti4+ or Fe3+ at the B-site [18–21]. For the composite doping within A and B sites of KNN-based ceramics, its d33 could reach 190–271 pC/N. From these reports, it could be seen that for KNN-based piezoceramics, whether it was A-doping, B-doping or A and B sites complexed doping, the researches were relatively mature and have reached the bottleneck, thus KNN-based piezoceramics couldn't achieve a greater breakthrough, so that the group proposed to substitute the Osite (could also be called C-site) of the ceramics. Because the O-site in the KNN-based ceramics is the main skeleton of its perovskite structure, the O-site changes, companying with the great deformation in the ceramic structure and micro-morphology, maybe correspondingly change the ceramic properties. Therefore, it is very necessary to study the substitution of O-sites by other ions in KNN-based lead-free piezoceramics. Because of the similar radius and electronegativity of N3− and O2−, we considered using N3− to partially substitute O2− in KNN-based leadfree piezoceramics. Martha et al. [22] used pure phase KTaO3 to conduct solid phase reaction with urea, and generated N-doped photocatalyst KTaO3-xNx. Since both KNNT and KTaO3 are same oxygen octahedral structures, the O-site by N3− substitution is possible for the KNN-based piezoceramics. So far, there is no report to study the doping modification of O-site in KNN-based piezoceramics. In this work, we synthesized N-doped KNNT powders (KNNTN) by KNNT powders and urea reaction, and then sintered into KNNTN ceramics. The effects of different N doping amounts on the phase structure, morphology, domain structures, defects, density and piezoelectric properties of KNNTN lead-free piezoceramics were systematically investigated. 2. Experimental 2.1. Synthesis of N-doped KNNT ceramics The pure phase K0.5Na0.5Nb0.7Ta0.3O3 (KNNT) was synthesized by two-step microwave hydrothermal method [23]. The starting materials were Ta2O5 (purity: ≥99.99%), Nb2O5 (purity: ≥99.99%), NaOH (AR) and KOH (AR) powders. The precursor synthesized from the starting materials was poured into a hydrothermal kettle made of polytetrafluoroethylene, then heated by microwave at 130 °C (2h)~180 °C (7.5h). The synthesized powders were washed with DI water and ethyl alcohol, and exsiccated at 90 °C for 24h to obtain dried pure phase KNNT powders. Then, 1g of the pure phase KNNT powders and urea (CO(NH2)2) were uniformly mixed according to a certain mass ratio
(designed mKNNT/murea = 1/4, 1/8, 1/12), and heated at 400 °C for 6 h to obtain K0.5Na0.5Nb0.7Ta0.3O3-xNx (KNNTN) powders. These KNNTN powders were denoted as KNNTN (1/4), KNNTN (1/8) and KNNTN (1/ 12). 5 wt% polyvinyl alcohol (PVA) reagent was dropped into KNNTN powders and ground evenly. Then, the KNNTN powders were pressed into disks with Φ13mm*1 mm. The sample was sintered in a closed horizontal tube furnace. In order to prevent the volatilization of Na+ and K+, the sample was buried in KNNT powders. The green bodies were first heated to 100 °C for 30 min, removing the water in the samples. Then the samples were heated to 650 °C for 30 min to evaporate PVA. The heating rate of the drainage and glue removal process was 1 °C/min. Finally, the ceramics were heated to 1100 °C for 2 h with a temperature rise rate of 3 °C/min, and lowered to room temperature with the furnace. For purpose of preventing the oxidation of N in KNNT powders, the whole heating process was carried out in an N2 atmosphere with a flow rate of 160 cm3/min. Finally, KNNTN lead-free piezoelectric ceramics with different nitrogen content doping were obtained. 2.2. Characterization and performance measurements X-ray diffractometer (XRD, MAC-M21XRHF, Japan) was identified the phase composition of the samples by using Cu Kα radiation. The LCR meter (Agilent E4980A; Agilent Technologies Inc, Penang, Malaysia) was operated at 10 kHz to record temperature dependent relative permittivity ε of the ceramics. Field mission scanning electron microscopy (SEM, JSM-6700F, Japan) was determined the microscopic morphology and particle size of the samples. In SEM analysis, the sintered ceramics were firstly cut off, and then the fracture microstructures after carbon coating were observed. The domain structures of the ceramics were observed by transmission electron microscopy (TEM, JEOL, JEM-2200FS, Japan) was used to determine the domain structures of the KNNTN piezoceramics. The binding energy of each element in the sample was determined by X-ray photoelectron spectroscopy (XPS, AXISULTRA-DLD, Japan) using Al Kα radiation. The N element in the sample was quantified by an oxygen-nitrogen hydrogen analyzer (EMGA-830, HORIBA, Germany). UV–Vis spectrophotometer (TU-1901, China) was identified forbidden band width of the samples. The bulk density of ceramics was measured by the Archimedes method. Archimedes method was used to measure the densities of the samples. Quasi-static piezoelectric coefficient meter (YE2730A, China) was used to identified the piezoelectric constant (d33) of the piezoceramics. Before testing the performance, the ceramic samples were needed to be insulated at 800 °C for 30 min for silver plating. Finally, the silverplated ceramic samples were polarized at 130 °C in silicone oil and stored under a 4 kV/mm DC electric field for 10 min. 3. Results and discussion 3.1. Phase composition XRD was used to characterize the crystallinity and crystal phase of KNNTN powders and KNNTN ceramics with different N contents substitution. The XRD patterns of KNNTN powders are showed in Fig. 1. The diffraction patterns of KNNTN (1/4), KNNTN (1/8) and KNNT (1/12) powders are monoclinic perovskite structure (JCPDS:00-077-0038), and no other nitrides compounds appeared. Because urea was completely decomposed into NH3 and carbon oxides at 400 °C, excess NH3 and carbon oxides that do not react with the KNNT powders were escaped. The results showes that partial doping of N3− to replace O2− in KNNT lattice don't lead to significant structural changes. It maybe the fact that the low concentration of doped N3− is insufficient to cause lattice remodeling of KNNT, and there have been reported similar phenomena in previous report [24]. The XRD patterns of KNNTN ceramics with different N contents are
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Table 1 Lattice constant of prepare samples.
Fig. 1. XRD patterns of KNNTN powders with different N contents.
showed in Fig. 2(a). It is obvious observed that all the ceramics are single phase and no second phase is formed. Because the sintering process was carried out under the atmosphere of nitrogen, the N element in KNNT ceramics was well protected from being oxidized at high temperature. However, the main crystalline phase of the sintered ceramic changes from a monoclinic phase of the powders to an orthogonal-tetragonal phase (JCPDS:01-071-2171 and JCPDS:01-071-0945). It is reported that the orthogonal phase and the tetragonal phase with better symmetry are more stable than that of monoclinic phase, and they are easier to retain at high temperatures. Fig. 2(b) shows the magnified views of Fig. 2(a) in the 2θ = 31.0–32.5° range. It is observed that the diffraction peaks of KNNTN piezoceramics gradually move left as the doping amount of element N increases. Since the radii of O2− (0.140 nm) is less than the radii of N3− (0.146 nm), and it is known from the Bragg equation 2dsinθ = nλ, when the doping amount of N increases, the 2θ angle decreases and the diffraction peak shifts to the left. Moreover, the lattice constants of the representative tetragonal phase of KNNTN lattice calculated by Jade software are shown in Table 1, as the doping amount of N increases, the lattice constant of the sample increases accordingly in the c-axis, whereas the variation of lattice constants in a-axis and b-axis are irregular. In the work, these KNN-based piezoceramics undergo the same preparing condition except
sample
a = b/Å
c/Å
KNNT (pure) KNNTN (1/4) KNNTN (1/8) KNNTN (1/12)
7.9725 7.9694 7.9789 7.9626
7.9279 7.9372 7.9630 8.0427
for the different N contents. Besides, in order to avoid consumption of Na+ and K+ in KNN-based piezoceramics, these ceramics were embedded into the same powders for sintering; therefore, the c-axis enlarge caused by the volatilization of the A-site in the ceramics can be ignored. Hence, the lattice constant of the c-axis in KNNT ceramics increases, which is completely caused by the doping substitution of the O-site by N3−. This is because that the A and B sites ions are located on the flat of the oxygen octahedral structure, and the O-site is the skeleton of the oxygen octahedron. N doping the O-site directly enlarges the crystal grains in the c-axis, but has little effect in the a-axis and b-axis. The lattice expansion on the c-axis increases the asymmetry of the KNNTN lattice, which has a positive influence on the piezoelectric properties of ceramics [25]. The relative dielectric constants of KNNTN ceramics related to temperature were tested at 10 kHz, as shown in Fig. 3. As can be seen from the figure, the phase transitions experienced by KNNTN ceramics in the temperature range around room temperature and above are mainly phase transitions between orthogonal phase (O)-tetragonal phase (T)-cubic phase (C), and as the temperature rises, two peaks appear at about 60 °C and 190 °C, corresponding to the orthogonaltetravalent phase transition temperature (TO-T) and Curie temperature (TC), respectively [12]. The existence of TO-T indicates that there exists a two-phase coexisting zone of orthogonal phase and tetragonal phase in KNNTN ceramics near 60 °C. In the two-phase coexisting zone, the asymmetry of grains of ceramics increases, and its piezoelectric properties will reach the maximum value. As can be seen from the figure, with the increase of N content in KNNTN ceramics, its TO-T decreases from about 64 °C to about 51 °C, which is closer to room temperature. The decrease of TO-T temperature of KNNTN ceramics depicts that with the increase of N element, the grains were more likely to transform into the two-phase coexistence zone, and the asymmetry of grains increases, which has a positive impact on the piezoelectric properties of ceramics.
Fig. 2. XRD patterns of KNNTN piezoceramics with different N contents: (a) entire range; (b) magnified diffraction peak in the range of 31–32.5°. 23460
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Table 2 The N content of the KNNTN piezoceramics with different N contents. Designed mKNNT:murea
Real Weight%
Real Atom%
Real n(N):n(O)
1:4 1:8 1:12
0.248 0.391 0.514
0.703 1.108 1.455
0.035:2.965 0.055:2.945 0.073:2.927
N3−, it is widely believed to be the evidence of the presence of Ta-N bond, indicating that the oxygen atom is successfully replaced by a nitrogen atom [28]. And it can be seen from Fig. 4(b) that with the increasing amount of urea reacting with KNNT, the peak of N become more and more obvious, illustrating that the N content in the KNNTN powders increases with the increase of reactant urea amount. Combined with XRD results (Fig. 2) and XPS analysis (Fig. 4), it can be determined that O atoms in KNNT are partial replaced by N atoms successfully, and the K0.5Na0.5Nb0.7Ta0.3O3-xNx (KNNTN) is formed. Fig. 3. Temperature dependences of relative dielectric constants of the KNNTN piezoceramics. Inset: Change trend of TO-T and TC of KNNTN piezoceramics.
3.2. Characterization of N element in KNNTN ceramics From the XRD diffraction pattern (Fig. 2), it can be seen that the ceramics produced were still perovskite structure without the formation of the second phase. However, it is not certain whether N3− doping replaces the O-site in KNNT ceramics. Therefore, the doping of N3− in KNNT ceramics was qualitatively and quantitatively studied by using XPS and oxygen-nitrogen-hydrogen analyzer, respectively. 3.2.1. Qualitative analysis of N3− substitution The chemical states of each element (K, Na, Nb, Ta, and O) in the KNNTN piezoceramics were confirmed by X-ray photoelectron spectroscopy(XPS). Fig. 4(a) shows the XPS spectrum of KNNTN (1/12) ceramic as the representative. The peaks of K 2p1/2 and K 2p3/2 in KNNTN occurred are at 291.7 eV and 294.2 eV, corresponding to K+; and the Na 1s peak at 1071.3 eV is observed, which corresponded to Na+. The O 1s peaks at 529.3 eV is associated with the O of KNNTN lattice. The Ta 4f peaks at 25.11 eV and 26.84 eV mean that Ta mainly exists as Ta5+in the KNNTN lattice [26]. And the Nb 3d peaks at 206.7 eV and 209.5 eV, indicate that the form of Nb atoms in the sample is Nb5+ [27]. Fig. 4(b) shows the N 1s peaks of the KNNTN ceramics prepared with different N doping amounts. The binding energies of N 1s in all the KNNTN ceramics are around 395.3 eV. These peaks are the feature of
3.2.2. Quantitative analysis of N elements The oxygen-nitrogen hydrogen analyzer quantitatively analyzed the N elements in KNNTN ceramics. Table 2 shows the mass percentage, atomic percentage and molar ratio of O atom to N atom of KNNTN ceramics with different N doping proportions. As can be seen from Table 2, when the designed mass ratios of KNNT powders and urea are 1:4, 1:8 and 1:12, the real molar ratios of N and O in the KNNTN ceramics are 0.035:2.965, 0.055:2.945 and 0.073:2.927, respectively. Hence, as the amount of urea reacted with KNNT powders increases, the content of doped N increases accordingly, and this result is also in agreement with the outcomes of XRD (Fig. 2) and XPS (Fig. 4). According to the detecting results, the designed KNNT (pure), KNNTN (1/ 4), KNNTN (1/8) and KNNTN (1/12) can be denoted as KNNTN (x = 0), KNNTN (x = 0.035), KNNTN (x = 0.055) and KNNTN (x = 0.073), respectively. 3.3. Microstructures The SEM images of the KNNTN piezoceramics with different N doping amounts are shown in Fig. 5. It can be seen from the figure that the grain sizes of KNNTN (x = 0), KNNTN (x = 0.035), KNNTN (x = 0.055) and KNNTN (x = 0.073) ceramics are 1.26, 1.35, 1.38 and 1.44 μm, respectively. Since the radius of N3− and O2− are 0.16 nm and 0.14 nm respectively, the radius of N3− is slightly larger than the radius of O2−. When N substitutes O-site doping into the KNNT ceramics’ lattices and occupies the lattice nodes, a certain amount of lattice expansion will be generated, and the expansion will increase with the increase of the amount of N entering the lattice. Hence, the grain size of
Fig. 4. XPS spectrum of KNNTN: (a) survey spectrum; (b) high-resolution XPS spectra of N 1s&Ta 4p in the KNNTN powders with different N contents. 23461
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Fig. 5. SEM images of KNNTN piezoceramics: (a) x = 0; (b) x = 0.035; (c) x = 0.055; (d) x = 0.073. Inset: Grain size distributions of KNNTN piezoceramics.
KNNTN ceramics increases with increasing N3−doping. As can be seen from the figure, the morphology of KNNTN (x = 0) ceramic is more regular than the others. The KNNTN (x = 0.035) ceramic grains has a relatively regular morphology and good grain growth, the most compact. The KNNTN (x = 0.055) ceramic is also dense in grain growth, but has more pores than that of the KNNTN (x = 0.035) ceramic. This should be caused by more N3− substituting in KNNTN (x = 0.055) powders, resulting to more serious lattice distortion. As the amount of N3− doped in KNNTN (x = 0.073) ceramic, the lattice distortion is the most serious, leading to the irregular grain shape the increase of pores, and thus might result the decrease of density. The increase of pores in KNNTN ceramics might negatively affect the piezoelectric properties.
because of the stress of the ceramic grain boundary and the depolarization field greatly limit the 90° domain to reverse, and the reversed 90° domain will revert to its original state under the action of electric field in the grain boundary after strong restrictions gradually returns to the original state [36,37]. By comparing the electric domain structure of KNNTN ceramics with different N contents in Fig. 6, with the increase of N content in KNNTN piezoceramics, the proportion of the domain of 180° gradually increases. Therefore, under the action of electric field, more electric domains has been reversed, thus greatly improving the orientation of KNNTN ceramics’ electric domain, which has a positive impact on their piezoelectric properties. 3.5. Defect analysis
3.4. Electric domain analysis As is well-known that the piezoelectric effect of ferroelectric ceramics consists of two parts, that are the intrinsic contribution and the extrinsic contribution. The former refers to the linear piezoelectric effect associated with lattice displacement, while the latter mainly derives from the motion of the domain wall [29]. The above experimental results proved that the doping substitution of N3− to the O-site in KNNT ceramics causes lattice distortion, and whether it affects the electric domain needed further research. Fig. 6 is the result of observation and analysis of the electric domains of polarized KNNTN (x = 0, 0.035, 0.055 and 0.073) ceramics by transmission electron microscope (TEM). The domain structures of stripe (S), herringbone (H) and watermark (W) can be seen in the figure. The stripe shape corresponded to 90° domain structure, herringbone bone shape is shaped by two alternate 90° domain structure, while watermark shape is corresponding to 180° domain structure [30–32]. The domain structure of 180° is more easily reversed than that of 90°. There have been reports of piezoceramics, only less than 15% of the 90° domain can reverse [33–35], this is
Because the valences of N3− and O2− in KNNTN ceramics are different, oxygen vacancy defects must exist in KNNTN in order to maintain charge balance [38], so that UV–Vis DRS and XPS were used to analyze oxygen vacancy defects. The Tauc plot derived from UV–Vis DRS is applied to confirmed the forbidden band width of materials and analyzes the defect concentrations [39]. Fig. 7 displays the Tauc plot of KNNTN ceramics with different N contents doping. The Tauc equation is expressed as Equa(1):
[F(R) hv]n = A (hv − Eg )
(1)
F(R) = (1-R∞)2/2R∞, R∞ represents the relative reflectivity of the apparatus, h (J·s) represents Planck constant, ν (Hz) represents the vibration frequency. The index n is associated with electron transition (indirect n = 1/2, direct n = 2; here n = 1/2), A is the proportionality constant associated with the sample, Eg (eV) denotes the forbidden band width of the material. The Tauc curve drawn by Equa(1) is shown in Fig. 7 ([F(R)hν]1/2 is the data of vertical axis, Hv is the data of horizontal axis.).
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Fig. 6. TEM images of domain patterns for the polarized KNNTN piezoceramics: (a) x = 0; (b) x = 0.035; (c) x = 0.055; (d) x = 0.073.
Fig. 7. Tauc plots of KNNTN ceramics; inside table is the forbidden width of KNNTN ceramics.
The tangent intercept of Tauc curve represented the forbidden band width of ceramics, and it can be seen when the N doping amount x of KNNTN ceramics increases from 0 to 0.035, 0.055 and 0.073, the forbidden band width of the ceramics decreases correspondingly from 3.638 ± 0.004 to 3.611 ± 0.004, 3.574 ± 0.005 and 3.556 ± 0.003 eV, respectively. These results are consistent with what Rao M P reports [22]. The decrease of the forbidden band width can lead to the decrease of the potential barrier, which may lead to the
increase of the defects. In the Tauc diagram, there is a positive correlation between tail absorption and defect concentration [39]. In Fig. 7, the tail absorption value increases with increasing doping amount of N. That is to say, the increase of the doping amount of N in KNNT ceramics increase the defect concentration of ceramics. Fig. 8(a) is XPS high-resolution view of O 1s, which is divided into peak(Ⅰ) and peak(Ⅱ). The peak(Ⅰ) of binding energy at 529.8 eV is O in the lattice of KNNTN, and the peak(Ⅱ) at 531.3 eV is generally considered as oxygen vacancy defects. Oxygen vacancy concentration can be expressed by the peak area ratio (Ⅰ/(Ⅰ+Ⅱ)) [40,41], and the calculation results are displayed in Fig. 8(b). It is observed that when the N contents of KNNTN ceramics are x = 0, 0.035, 0.055 and 0.073, the oxygen vacancy concentrations are 18.9%, 20.2%, 20.9% and 22.5%, respectively. These results are consistent with the result of uv–vis DRS analysis (Fig. 7), indicating that with the increase of doping N content in KNNTN ceramics, the oxygen vacancy defects of the ceramics increase. Oxygen vacancies are conducive to mass transportation, so the increase of oxygen vacancy is benefit for grain growth [40], which is another reason why the grain size of KNNTN ceramics increases with the increase of N content in ceramics as shown in Fig. 5. However, the increase of oxygen vacancy in KNNTN ceramics will cause the increase of its leakage current, leading to the decline of its insulation, which is easy to be broken down under high electric field, not conducive to the reversal of electric domain [42–44]. The results of TEM figure(Fig. 6) shows that with the increase of N content in KNNTN ceramics, easy reversal of 180° domain proportion increases, benefiting to the raise of ceramic piezoelectric properties; but the increase of oxygen vacancy limits the electrical domain reversal, this may be why there are still some difficult reversal 90° domain in KNNTN ceramics, which is not conductive to the improvement of ceramics piezoelectric properties. In summary, with the increase of N content in KNNTN ceramics, the oxygen vacancy concentration in ceramics increases, which has a
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Fig. 8. (a) High-resolution XPS spectra of O 1s in KNNTN piezoceramics when x = 0, 0.035, 0.055 and 0.073, (b) the oxygen vacancy concentrations of KNNTN piezoceramics.
negative impact on the piezoelectric properties of ceramics.
3.6. Density and piezoelectric properties The results of the relative density and piezoelectric constants of the KNNTN piezoceramics are shown in Fig. 9. When x = 0 is increased to x = 0.035, the relative density of the KNNTN piezoceramics increases correspondingly from 86.24% to 91.12%, and when x is increased to 0.055 and 0.073, the relative density of ceramics correspondingly decreases to 83.09% and 80.67%. Combined with the SEM micrographs of Fig. 5, replacing O-site in KNNT lattice with a very small amount of N3− can improve the relative density of KNNT ceramics, but excessive N3− can cause irregular grain growth and increase the number of pores, thus leading to a decrease in density. As observed from Fig. 9, the piezoelectric constants of ceramics are positively related to the relative density. Combined with the above results of XRD (Fig. 2), SEM (Fig. 5), TEM (Fig. 6) and oxygen vacancy defect analysis(Figs. 7 and 8), the increase of grains asymmetry and more easily reversed domains of KNNTN ceramics are beneficial to the piezoelectric properties, while the increase of oxygen vacancy defects is not conducive to the piezoelectric
Fig. 9. The results of the relative density and piezoelectric constants of the KNNTN ceramics.
properties. Therefore, the piezoelectric properties of KNNTN piezoceramics are the competitive results of these factors. It can be seen from Fig. 9 that when the N doping content in KNNTN ceramics is x = 0.035, the positive effects brought by the asymmetry of ceramic grains, density increase and domain reversal of the ceramics are greater than the negative effects brought by the increase of the oxygen vacancy defects, so that its d33 reaches the maximum value of 207 pC/N. The result is higher than that of KNNTN (x = 0) ceramics (d33 = 186 pC/N). However, when x = 0.055 and 0.073, in contradiction to the above results, resulting in the gradual decrease of d33, which are 180 pC/N and 172 pC/N, respectively. The influences of various factors on d33 of KNNTN ceramics are illustrated in Fig. 10.
4. Conclusions In this study, K0.5Na0.5Nb0.7Ta0.3O3-xNx powders were synthesized from urea and KNNT powders and sintered into K0.5Na0.5Nb0.7Ta0.3O3xNx ceramics (x = 0.035, 0.055 and 0.073). The main crystal phase of KNNT powders after reaction with urea were still pure perovskite phase. And doping N elements in KNNT ceramics caused lattice distortion and leaded to the elongation of crystal cells along the c-axis, which further increased the asymmetry of the crystal lattice of KNNTN ceramics. Besides, with the increase of N content in KNNTN ceramics, TC and TO-T of ceramics decreased from 192 °C and 64 °C to 185 °C and 51 °C, respectively. The decrease of TO-T indicates that ceramic grains are more likely to transform into the two-phase coexistence zone of orthogonal phase and tetragonal phase, which is also conducive to the increase of ceramic grains asymmetry. KNNTN (x = 0.035) ceramic has the densest microstructure and regular grain growth, while KNNTN (x = 0.055) and KNNT (x = 0.073) ceramics showed irregular grain growth and decreased density. Through the observation of the ceramic domain, it could be seen that the doped substitution of N to the O-site in KNNT ceramics would make the ceramics domains more easily reversed. As the amount of N in the KNNTN ceramics increased, the forbidden band width of the ceramic became smaller and the defects increased. When x = 0, 0.035, 0.055 and 0.073, the oxygen vacancy defect concentrations of corresponding ceramics were 18.9%, 20.2%, 20.9% and 22.5%, respectively. Based on the above results and the measured d33 of KNNTN ceramics, when the N content in KNNTN ceramics was x = 0.035, the positive effects brought by the asymmetry of ceramic grains, density increase and domain reversal of the ceramics were greater than the negative effects brought by the increase of the oxygen vacancy defects, so that its d33 reached the maximum value of 207 pC/N. However, when x = 0.055 and 0.073, the positive effects
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Fig. 10. The influences of various factors on d33 of KNNTN ceramics.
brought by the asymmetry of ceramic grains and domain reversal of ceramics were less than the negative effects brought by the decrease of density and the increase of oxygen vacancy defects, resulting in the gradual decrease of d33, which were 180 pC/N and 172 pC/N, respectively. Declaration of interest
[15]
[16] [17] [18]
None. Acknowledgment
[19]
The National Natural Science Foundation of China (Nos. 51672025, 51572020, 51372019), and Major Projects of Science and Technology in Shanxi Province (MC2016-03) are duly acknowledged for their financial support.
[20]
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