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Production of Ba(Zr,Ti)O3 coatings on ternary (Ti,Zr)N thin film electrodes by plasma electrolyte oxidation
Surface & Coatings Technology 385 (2020) 125440
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Production of Ba(Zr,Ti)O3 coatings on ternary (Ti,Zr)N thin film electrodes by plasma electrolyte oxidation Huan-Ping Teng, Fu-Hsing Lu
T
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Department of Materials Science and Engineering, National Chung Hsing University, 145 Xingda Road, Taichung 402, Taiwan
A B S T R A C T
Cubic perovskite barium zirconate titanate [Ba(Zr,Ti)O3; BZT] coatings were produced over ternary (Ti,Zr)N seeding layers by plasma electrolytic oxidation (PEO). (Ti,Zr)N thin films serving as the working electrode were deposited on Si substrates by unbalanced magnetron sputtering. PEO was conducted in the alkaline electrolytes consisting of 0.5 M Ba(OH)2 with a potentiostatic power mode at applied voltages ranging from 30 to 80 V for 30 s. Spark plasma discharges occurred over the thin film electrodes at reaction voltages above 50 V, characterized as a PEO regime. Sintered-like BZT coatings with porous morphology were obtained. The PEO-produced coatings exhibited substantially higher integrated diffraction peak intensities and growth rates, compared to the electrochemically oxidized coatings synthesized at lower applied voltages. X-ray photoelectron spectroscopy revealed chemical compositions of the BZT coatings. This clearly demonstrates that crystalline BZT coatings could be facilely produced over complex (Ti,Zr)N seeding layers by PEO. Possible reaction routes and formation mechanisms of the coatings were also proposed. The PEO-made BZT coatings have great potential for many dielectric and ferroelectric applications.
1. Introduction Barium zirconate titanate [Ba(Zr,Ti)O3; BZT] belonging to a family of perovskite oxides is one of the most promising dielectric and ferroelectric materials [1,2] and can be used in various technological applications, such as tunable microwave devices [3], tunable ceramic capacitors [4], and electrocaloric device [5]. Several methods have been employed to produce BZT coating, as reported in the literature [6–10], including pulse laser deposition [6], multi-target reactive sputtering [7], sol-gel [8], chemical solution deposition [9], and hydrothermal [10] techniques. The above-mentioned methods often require either sophisticated equipment, high temperature post-annealing treatment (> 700 °C), or prolonged processing time to make crystalline materials. Such heat treatment may induce thermal stresses causing cracks in the films. In this work, plasma electrolytic oxidation (PEO) was used to produce BZT coatings. PEO has often been employed to produce oxide coatings over valve metals and alloys [11–18]. PEO possesses several advantages over the other techniques, such as fast growth rate, high crystallinity of oxide coatings, economical set-up, and superior adhesion between obtained coatings and substrates. In our previous work, PEO has been used to produce BaTiO3 (BTO) films on bulk Ti [11], Ti/Si [12], and TiN/Si [13] substrates, BaSrTiO3 (BST) on TiN/Si [14], and ZrO2 coatings on ZrN/Si [15]. In the systematic serial PEO studies, Ba (Zr,Ti)O3 has never been explored before. This is also the first attempt by using the ternary nitride- (Ti,Zr)N thin films as the seeding electrode
⁎
for PEO. Thus, the novelty lies in employing the complex (Ti,Zr)N seeding layers to produce BZT coatings by PEO. Compared to metal seeding electrodes, conductive nitrides exhibiting higher chemical stabilities and melting points may be better candidates for the seeding electrodes in PEO. They can not only act as a supply of metallic components for metallic oxide coatings but enhance the growth rates of the coatings [13–15]. In our previous studies, TiN seeding layers were employed to produce BTO [13] and BST [14], meanwhile ZrN films were used to prepare ZrO2 [15]. (Ti,Zr)N similar to TiN and ZrN but with more complex components could act as seeding electrodes for supplying Ti and Zr sources for BZT. Moreover, (Ti,Zr)N exhibiting superior mechanical properties and corrosion resistance [19,20] could withstand more intense spark discharges during PEO. Thus, the objective of this research is to produce complex BZT coatings by PEO using ternary (Ti,Zr)N seeding electrodes. Crystalline phases, microstructures, and compositions of the resulting oxide coatings are investigated. Possible reaction routes and formation mechanisms of the coatings during PEO are also discussed. 2. Experimental details (Ti,Zr)N seeding layers were deposited on n-type (100) Si substrates by a dc unbalanced magnetron sputtering system. Sintered TiZr compound targets (99.99%) with the composition of Ti:Zr = 50:50 wt % = 66:34 at.% were used. The sputtering power was fixed at 300 W while a bias voltage of −50 V was applied to the substrate. The base
Corresponding author. E-mail address: [email protected] (F.-H. Lu).
https://doi.org/10.1016/j.surfcoat.2020.125440 Received 19 November 2019; Received in revised form 24 January 2020; Accepted 3 February 2020 Available online 04 February 2020 0257-8972/ © 2020 Elsevier B.V. All rights reserved.
Surface & Coatings Technology 385 (2020) 125440
H.-P. Teng and F.-H. Lu
pressure was pumped down to 3.0 × 10−4 Pa before sputtering. Sputtering deposition was performed at room temperature with the working pressure of 0.2 Pa. A TiZr interlayer was deposited on the substrate for 10 min prior to the deposition of (Ti,Zr)N films to enhance the adhesion. The resulting thickness of the TiZr interlayer was 285 ± 10 nm. Subsequently, (Ti,Zr)N films were sputtered onto the interlayer with the Ar/N2 flow ratio of 10 (Ar: 30 sccm; N2: 3 sccm) for 90 min. The coating thickness was about 3.0 ± 0.5 μm and the measured electrical resistivity was 39 ± 4 μΩ-cm, which are compatible with the characteristics of (Ti,Zr)N films reported in the literature [21,22]. Before PEO, each as-deposited (Ti,Zr)N/Si specimen was cleaved into the size of 0.5 × 1.5 cm2 and cleaned ultrasonically with ethanol and deionized water. Two-third of the (Ti,Zr)N/Si specimen was then soaked into a 0.5 M Ba(OH)2 (98%, VETC) electrolyte with an exposed area of 0.5 × 1 cm2. The specimen served as the working electrode while a Pt plate with the size of 1 × 2 cm2 acted as the counter electrode. PEO was performed by applying a 5-kW dc power supply (GP10HH50) under the potentiostatic mode with the voltages varying from 30 to 80 V. The reaction time was fixed at 30 s. The temperature of the electrolytes was kept at 70 °C during PEO. After PEO, the specimens were rinsed in deionized water and air dried. The crystallographic structures of obtained coatings were characterized by X-ray diffraction (XRD) (Mac Science MPX3, Japan) with the copper radiation (λCu,Kα = 0.1542 nm) operated at 40 kV and 30 mA. Both the conventional θ/2θ scanning mode and the grazing incidence X-ray diffraction (GIXRD) mode (2°) were employed. The microstructures of coatings were examined by field-emission scanning electron microscopy (FE-SEM) (JSM 6700F, JEOL, Japan) operated at 3 kV. The chemical compositions were determined by X-ray photoelectron spectroscopy (XPS) (PHI 5000 VersaProbe, ULVAC-PHI, Japan) with the Al-Kα radiation source (1486.6 eV) with a pass energy of 23.5 eV (0.2 eV/step). Ar+ ions with 3 keV were employed to sputter etching the film surface for 5 min. Ti-2p, Zr-3d, N-1s, Ba-3d, and O-1s peaks were deconvoluted by a non-linear least-squares fit with a Gaussian/Lorentzian peak shape (G/L = 0.3). The background was subtracted by the Shirley method prior to each fit [23].
(OH)2 electrolytes at 70 °C by applying fixed voltages ranging from 30 to 80 V for 30 s. Below 40 V, reaction currents were rather small and remained almost unchanged after 2 s, indicating the occurrence of slow electrochemical reactions. As applied voltages were above 50 V, very different current-time behaviors showed up. The currents increased rapidly in an initial short period, reached maxima, and then dropped back. It is noteworthy that evident luminescence over the (Ti,Zr)N surface was observed and accompanied by uniformly spark discharges lasting for about 10–15 s on the specimen surfaces. In this regime, the maximum current changed from 0.4 to 4.6 A with increasing the applied voltage and enhanced spark discharges were spotted. Yerokhin et al. [24] reported that the drastic current drop from the current maximum during PEO was caused by the predominant passivating effect of formed oxide coatings. This means that the (Ti,Zr)N seeding layers were progressively covered by the PEO-produced oxide coatings. Moreover, spark discharges accompanying by the formation and growth of gas bubbles at the anodes were mainly due to dielectric breakdown of the passivated oxide coatings under a strong electrical field [25]. The existence of spark discharges is one of the main characteristics of PEO. 3.2. Crystalline phases As-deposited (Ti,Zr)N films exhibited golden color while after the anodic treatment, the coatings became dark yellow below 40 V and turned gray above 50 V. X-ray diffraction patterns and grazing incidence X-ray diffraction patterns of the as−deposited (Ti,Zr)N films and those after the treatment in 0.5 M Ba(OH)2 electrolytes at various applied voltages for 30 s are shown in Fig. 2 for the (a) θ/2θ and (b) grazing incidence modes. The corresponding relative peak integrated intensities of Ba(Zr,Ti)O3 and (Ti,Zr)N are also plotted against the applied voltages, evaluated from Fig. 2(a), are depicted in Fig. 2(c). TiZr diffraction peaks of the interlayer between the nitride overlayer and the Si substrate were observed for all the specimens in the XRD patterns but not in the GIXRD patterns. These clearly indicate it is the interlayer. On the other hand, the diffraction peaks of (Ti,Zr)N were distinguished in both the XRD and GIXRD patterns. (Ti,Zr)N can be considered as a single-phase solid solution of TiN and ZrN due to the similarity of the (rock-salt) crystal structure. The lattice parameters of the as-deposited (Ti,Zr)N films, evaluated from the diffraction patterns, were about 0.442 ± 0.001 nm, which lies between those of TiN (0.424 nm [26]) and ZrN (0.458 nm [26]). After the anodic treatment, no characteristic diffraction peaks of oxides could be detected below 40 V. As the applied voltages were above 50 V, (100), (110), (111), (200), (211), (220), (310) characteristic diffraction peaks of cubic perovskite BZT were discerned, which were located at the diffraction angles between those of cubic BaTiO3 (ICDD 31-0174 [26]) and BaZrO3 (ICDD 06-0399 [26]). Trace BaCO3 was also found in the patterns, which may attribute to reactions of Ba (OH)2 electrolytes with CO2 containing in air [27]. It is noteworthy that only single-phase BZT was identified without forming either a BaTiO3 or BaZrO3 second phase, indicating the formation of a solid solution for BZT coatings after PEO. Fig. 2(c) shows the relative integrated peak intensities of (Ti,Zr)N and BZT as a function of applied voltages. The relative integrated peak intensity (RI) of either (TiZr)N or BZT is evaluated from the equation:
3. Results and discussion 3.1. Current-time behaviors and spark discharges Fig. 1 displays the reaction currents versus reaction time during the anodic treatment for the (Ti,Zr)N-coated Si specimens in the 0.5 M Ba
RIs [%] =
Is × 100% ∑overall Ioverall
(1)
where I represents the peak integrated intensity and s denotes a specific species, i.e., (Ti,Zr)N or BZT. As sketched in the figure, the relative peak intensity of BZT increased rapidly with applied voltages when the voltages were above 50 V, indicating a fast growth of the oxides. In contrast, the relative peak intensity of (Ti,Zr)N decreased correspondingly since the obtained oxides were resulted from consumption of the
Fig. 1. Current–time curves during PEO conducted in 0.5 M Ba(OH)2 electrolytes for 30 s over (Ti,Zr)N/Si with various applied voltages ranging from 30 to 80 V. 2
Surface & Coatings Technology 385 (2020) 125440
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Fig. 2. (a) X-ray diffraction patterns and (b) Grazing incidence X-ray diffraction patterns of the as-deposited (Ti,Zr)N/Si and obtained coatings after PEO conducted in the Ba(OH)2 electrolytes for 30 s with applied voltages ranging from 30 to 80 V. (c) The relative peak integrated intensities of Ba(Zr,Ti)O3 and (Ti,Zr)N as a function of applied voltages, evaluated from above XRD patterns.
Fig. 3. Lattice parameters of obtained BZT coatings (a) evaluated from the XRD spectra of Fig. 2(b) as a function of the applied voltage.
(Ti,Zr)N seeding layers. The fast growth rate of oxides is one of the main characteristics for PEO. The lattice parameters of obtained BZT were also calculated from the GIXRD patterns [28] and plotted against the applied voltages, as shown in Fig. 3. As depicted in the figure, the obtained lattice parameters of BZT were about 0.410 ± 0.001 nm, which lie between those of BaTiO3 (0.403 nm [26]) and BaZrO3 (0.419 nm [26]). The larger lattice parameter for BaZrO3 is due to the larger ionic radius of Zr4+ (ri = 0.720 Å), compared to Ti4+ (ri = 0.605 Å) [29]. 3.3. Morphologies and microstructures Surface morphologies of the as-deposited (Ti,Zr)N before and after PEO conducted in the 0.5 M Ba(OH)2 electrolytes with varying applied voltages ranging from 30 to 80 V are revealed in Fig. 4. As-deposited (Ti,Zr)N films exhibited nanograins. Tiny rods and nano-network structures were present at applied voltages of 30 and 40 V, respectively. Since no distinct diffraction peaks could be identified, these structures may be related to amorphous hydroxides, similar to those reported in the literature [14]. This regime with slow kinetics and amorphous feature is associated with mainly electrochemical oxidation. Rough surfaces with sintered-like morphologies occurred at applied voltages above 50 V. The dramatic different surface morphologies are mainly due to PEO. It has been reported that sintered-like morphologies with micropores are due to spark discharges occurring over surfaces of the seeding layers [13–15]. Micropores arose from the discharge channels occurring during the PEO process [30–32]. Corresponding cross-sectional micrographs of the as-deposited (Ti,Zr)N thin films before and after PEO conducted at the same voltage ranges are given in Fig. 5(a). It is noteworthy that the TiZr interlayer remained intact, as revealed from the micrographs and thus, BZT was formed from the reactions of (Ti,Zr)N with electrolytes. For comparison, thicknesses of the oxide coatings are plotted as a function of applied voltages, as shown in Fig. 5(b). A thin overlayer < 95 nm could be detected at the applied voltage of 40 V. As mentioned above, this may be resulted from amorphous hydroxides during electrochemical oxidation since no distinct diffraction peaks were found while the film surface revealed nano-network structures. The thickness of the oxide coatings could reach 2.1 μm at the applied voltage of 50 V and about 2.7–2.9 μm at voltages of 60–80 V. As shown in the figure, the thickness 3
Surface & Coatings Technology 385 (2020) 125440
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Fig. 4. Surface morphologies of the as-deposited (Ti,Zr)N/Si and obtained coatings after PEO conducted in the Ba(OH)2 electrolytes with various applied voltages ranging from 30 to 80 V.
similar dc unbalanced magnetron sputtering system [37]. After PEO, high resolution XPS spectra of (a) Ti-2p, (b) Zr-3d, (c) O1s, (d) Ba-3d after sputter etching were acquired, as shown in Fig. 7. As revealed in Fig. 7(a), the Ti-2p peaks were composed of the Ti-2p3/2 peak at lower binding energy (BE: 457.6 eV) and Ti-2p1/2 peak at higher binding energy (BE: 463.0 eV); both of them belong to TieO bonds of BZT [9]. The Ti3+ peak at 456.3 eV might be due to that Ti3+ was generated on the surface region contributing to the overall Ti 2p3/2 spectra [34]. As for Zr-3d3/2 and Zr-3d5/2 peaks in Fig. 7(b), the binding energies were located at 181.2 eV and 183.7 eV, respectively, which could attribute to ZreO bonds of BZT [9]. The O-1 s spectra given in Fig. 7(c) show an asymmetric curve that could be deconvoluted into two peaks with binding energies at 529.0 eV for O-Ti/Zr bonds and 530.8 eV for OeBa bonds [38], respectively. Fig. 7(d) displays the Ba3d spectra consisting of major BaeO bonds for BZT at the binding energies of 779.3 eV (Ba-3d5/2) and 794.7 eV (Ba-3d3/2) [39]. Atomic compositions of Ba, Ti, Zr, and O in the BZT coatings determined from the corresponding XPS spectra were 20.7 at.%, 7.7 at.%, 8.7 at.% and 62.9 at.%, respectively. This corresponds to the constituents of about Ba (Zr0.53Ti0.47)O3 for the BZT coatings. The Ti and Zr constituents in the obtained BZT coatings are rather close to those of the (Ti,Zr)N seeding layer.
increased dramatically above 50 V, i.e., in the PEO regime. The average growth rates of the BZT coatings produced by PEO were in the range of 88 ± 10 nm/s, which are much higher than those synthesized by other techniques, such as 0.06 nm/s by sol gel [8], 0.002 nm/s by hydrothermal synthesis [10], and 0.67 nm/s by chemical solution deposition method [9]. 3.4. X-ray photoelectron spectroscopy analyses Chemical bonding and atomic compositions of the as-deposited (Ti,Zr)N thin films and the PEO-produced BZT coatings were determined by X-ray photoelectron spectroscopy. Fig. 6 depicts high resolution XPS spectra of (a) Ti-2p, (b) Zr-3d, and (c) N-1 s acquired from the as-deposited (Ti,Zr)N films after sputter etching by Ar+ for 5 min to remove surface contaminants. The Ti-2p spectra could be deconvoluted into Three components consisting of TieN (BE: 454.5 eV), Ti3+ (BE: 456.3 eV) and Ti4+ (BE: 458.0 eV), respectively [33,34]. The Zr-3d spectra could then attribute to ZreN (BE: 179.1 eV), Zr3+ (BE: 180.4 eV), and Zr4+ (BE: 183.3 eV) [35,36]. As for the N-1s spectra, the N-(Ti/Zr) (BE: 396.7 eV) and minor NeO (BE: 399.0 eV) components were assigned [33]. Atomic compositions of the as-deposited (Ti,Zr)N films evaluated from the high resolution spectra were Ti:Zr:N = 22.6 at.%:33.6 at.%:43.8 at.%, corresponding to (Ti0.4Zr0.6) N0.8. The obtained N/(Ti + Zr) ratio is compatible with the reported 0.8–1.1 ratios in the literature for (Ti,Zr)xN1−x thin films prepared by a
Fig. 5. (a) Cross-sectional view of the as-deposited (Ti,Zr)N/Si and obtained coatings after PEO conducted in the Ba(OH)2 electrolytes with various reaction voltages ranging from 30 to 80 V and (b) The thickness of Ba(Zr,Ti)O3 coatings as a function of applied voltages. 5
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coatings were facilely produced by PEO using the conductive (Ti,Zr)N seeding layers. Although (Ti,Zr)N is a solid solution of TiN and ZrN, it could act like TiN and ZrN for supplying both Ti and Zr sources while concurrently. That is why a single-phase BZT was produced without forming an either BaTiO3 or BaZrO3 second phase. The possible reactions of the ternary (Ti,Zr)N seeding electrode in the alkaline electrolytes may be similar to those proposed by using binary TiN or ZrN electrode in producing the perovskite oxides as follows [13–15]. (Ti,Zr)N may be firstly oxidized in highly concentrated alkaline solutions to form H(Ti,Zr)O3− ions.
(Ti, Zr)N + 2OH− + 2H2 O → H(Ti, Zr)O−3 + NH 4 OH + e−
(2)
Subsequently, H(Ti,Zr)O3− might react further through two routes: (i) at low voltages (electrochemical oxidation regime) with the OH− ions in the electrolytes to form amorphous hydroxides (Ti,Zr)O2·xH2O.
H(Ti, Zr)O−3 + OH− → (Ti, Zr)O2⋅xH2 O + 1/2O2 + 2e−
(3)
(ii) at high voltages (PEO regime) with Ba2+ contained in the electrolytes to produce crystalline Ba(Zr,Ti) O3 with the aid of spark discharges.
Ba2 + + H(Ti, Zr)O−3 + OH− → Ba(Zr, Ti)O3 + H2 O
(4)
As the voltage increased, the thermal energy provided by spark discharges during PEO would further enhance the growth of BZT. A schematic diagram of the formation mechanisms of BZT coatings on (Ti,Zr)N/Si produced by PEO in the Ba(OH)2 electrolytes at low applied voltages (electrochemical oxidation) and high applied voltages (PEO) is shown in Fig. 8. 4. Conclusions Crystalline Ba(Zr,Ti)O3 coatings have been facilely produced on (Ti,Zr)N-coated substrates by plasma electrolytic oxidation conducted in Ba(OH)2 alkaline solutions for a short period of 30 s above 50 V. Below 40 V, thin amorphous layers were formed, which is due to slow electrochemical oxidation. Above 50 V, thick porous with sintered-like coatings were produced. Visual spark discharges occurring over the specimen surfaces and fast growth rate of the oxides fulfill the characteristics of PEO. Single-phase BZT coatings were obtained without forming an either BaTiO3 or BaZrO3 second phase. The average growth rates of PEO-produced BZT coatings were about 88 ± 10 nm/s, which is much higher than other deposition techniques for BZT. Employing the conductive ternary (Ti,Zr)N seeding electrode can facilitate substantially the growth of the complex BZT coatings by taking advantage of its high conductivity, partially ionic bonding, and multi-metallic constituents. The technique can also be extended to produce more complex oxide coatings with more wide applications. CRediT authorship contribution statement Huan-Ping Teng:Writing - original draft, Conceptualization, Methodology, Investigation, Visualization.Fu-Hsing Lu:Writing review & editing, Conceptualization, Methodology, Resources, Supervision.
Fig. 6. XPS high resolution spectra of (a) Ti-2p, (b) Zr-3d, and (c) N-1s for the as-deposited (Ti,Zr)N seeding electrode.
3.5. Reaction routes and formation mechanisms of Ba(Zr,Ti)O3
Acknowledgement
As mentioned earlier, the fabrication of complex BZT coatings by PEO has not yet been reported in the literature so far. In this work, BZT
This work was partly supported by the Ministry of Science and Technology, Taiwan under the Grant number MOST 106-2221-E-005024-MY3. 6
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Fig. 7. XPS high resolution spectra of (a) Ti-2p, (b) Zr-3d, (c) O-1s and (d) Ba-3d for the Ba(Zr,Ti)O3 coatings produced by PEO over the (Ti,Zr)N seeding electrode in the electrolytes at 60 V.
Fig. 8. A schematic diagram of proposed growth mechanisms of BZT coatings over (Ti,Zr)N/Si in Ba-containing alkaline electrolytes at low applied voltages (electrochemical oxidation) and high voltages (PEO). 7
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Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. References [1] Z. Yu, C. Ang, R. Guo, A.S. Bhalla, Dielectric properties of Ba(Ti1-xZrx)O3 solid solutions, Mater. Lett. 61 (2007) 326–329, https://doi.org/10.1016/j.matlet.2006.04. 098. [2] X.-D. Jian, B. Lu, D.-D. Li, Y.-B. Yao, T. Tao, B. Liang, J.-H. Guo, Y.-J. Zeng, J.L. Chen, S.-G. Lu, Large electrocaloric effect in lead-free Ba(ZrxTi1-x)O3 thick film ceramics, J. Alloy. Compd. 742 (2018) 165–171, https://doi.org/10.1016/j. jallcom.2018.01.143. [3] P.A. Jha, A.K. Jha, Effect of sintering temperature on the grain growth and electrical properties of barium zirconate titanate ferroelectric ceramics, J. Mater. Sci. Mater. Electron. 25 (5) (2014) 2305–2310, https://doi.org/10.1007/s10854-012-0963-7. [4] T. Maiti, R. Guo, A.S. Bhalla, Enhanced electric field tunable dielectric properties of BaZrxTi1-xO3 relaxor ferroelectrics, Appl. Phys. Lett. 90 (2007) 2005–2008, https:// doi.org/10.1063/1.2734922. [5] F. Weyland, T. Eisele, S. Steiner, T. Frömling, G.A. Rossetti Jr., J. Rödel, N. Novak, Long term stability of electrocaloric response in barium zirconate titanate, J. Eur. Ceram. Soc. 38 (2018) 551–556, https://doi.org/10.1016/j.jeurceramsoc.2017.09. 018. [6] J. Ventura, M.C. Polo, C. Ferrater, S. Hernández, J. Sancho-Parramón, L.E. Coy, L. Rodríguez, A. Canillas, L. Fábrega, M. Varela, Heterogeneous distribution of Bsite cations in BaZrxTi1-xO3 epitaxial thin films grown on (100) SrTiO3 by pulsed laser deposition, App. Surf. Sci. 381 (2016) 12–16, https://doi.org/10.1016/j. apsusc.2015.12.224. [7] G. Suchaneck, E. Chernova, A. Kleiner, R. Liebschner, L. Jastrabík, D.C. Meyer, A. Dejneka, G. Gerlach, Vacuum-ultraviolet ellipsometry spectra and optical properties of Ba(Zr,Ti)O3 films, Thin Solid Films 621 (2017) 58–62, https://doi.org/10. 1016/j.tsf.2016.11.023. [8] A. Dixit, S.B. Majumder, A. Savvinov, R.S. Katiyar, R. Guo, A.S. Bhalla, Investigations on the sol–gel-derived barium zirconium titanate thin films, Mater. Lett. 56 (2002) 933–940, https://doi.org/10.1016/S0167-577X(02)00640-7. [9] L.L. Jiang, X.G. Tang, S.J. Kuang, H.F. Xiong, Surface chemical states of barium zirconate titanate thin films prepared by chemical solution deposition, App. Surf. Sci. 255 (2009) 8913–8916, https://doi.org/10.1016/j.apsusc.2009.06.092. [10] Alejandra V. Alvarez, V.M. Fuenzalida, Evidence of transition metal diffusion during hydrothermal ceramic film growth: Ba(Ti,Zr)O3 on layered Ti–Zr alloy, J. Mater. Res. 14 (1999) 4136–4139, https://doi.org/10.1557/JMR.1999.0558. [11] C.-T. Wu, F.-H. Lu, Synthesis of barium titanate films by plasma electrolytic oxidation at room electrolyte temperature, Surf. Coat. Technol. 199 (2005) 225–230, https://doi.org/10.1016/j.surfcoat.2004.10.148. [12] W.-Y. Tsai, C.-J. Yang, J.-L. Zeng, F.-H. Lu, Synthesis and characterization of barium titanate films on Ti-coated Si substrates by plasma electrolytic oxidation, Surf. Coat. Technol. 259 (2014) 297–301, https://doi.org/10.1016/j.surfcoat.2014.01.056. [13] J.-L. Zeng, H.-P. Teng, F.-H. Lu, Electrochemical deposition of barium titanate thin films on TiN/Si substrates, Surf. Coat. Technol. 231 (2013) 297–300, https://doi. org/10.1016/j.surfcoat.2011.12.049. [14] H.-P. Teng, H.-W. Hsu, F.-H. Lu, Formation of BaxSr1-xTiO3 films on TiN-coated substrates by plasma electrolytic oxidation, Ceram. Int. 43 (2017) S584–S590, https://doi.org/10.1016/j.ceramint.2017.05.312. [15] C.-H. Hsiao, H.-P. Teng, F.-H. Lu, Formation of zirconia coatings on ZrN-coated substrates by plasma electrolytic oxidation, Surf. Coat. Technol. 269 (2015) 295–301, https://doi.org/10.1016/j.surfcoat.2015.02.038. [16] L. White, Y. Koo, Y. Yun, J. Sankar, TiO2 deposition on AZ31 magnesium alloy using plasma electrolytic oxidation, J. Nanomater. 2013 (2013) 1–8, https://doi.org/10. 1155/2013/319437. [17] H. Fadaee, M. Javidi, Investigation on the corrosion behaviour and microstructure of 2024-T3 Al alloy treated via plasma electrolytic oxidation, J. Alloy. Compd. 604 (2014) 36–42, https://doi.org/10.1016/j.jallcom.2014.03.127. [18] T.H. Teh, A. Berkani, S. Mato, P. Skeldon, G.E. Thompson, H. Habazaki, K. Shimizy, Initial stages of plasma electrolytic oxidation of titanium, Corros. Sci. 45 (2003) 2757–2768, https://doi.org/10.1016/S0010-938X(03)00101-X.
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Production of Ba(Zr,Ti)O3 coatings on ternary (Ti,Zr)N thin film electrodes by plasma electrolyte oxidation Huan-Ping Teng, Fu-Hsing Lu
T
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Department of Materials Science and Engineering, National Chung Hsing University, 145 Xingda Road, Taichung 402, Taiwan
A B S T R A C T
Cubic perovskite barium zirconate titanate [Ba(Zr,Ti)O3; BZT] coatings were produced over ternary (Ti,Zr)N seeding layers by plasma electrolytic oxidation (PEO). (Ti,Zr)N thin films serving as the working electrode were deposited on Si substrates by unbalanced magnetron sputtering. PEO was conducted in the alkaline electrolytes consisting of 0.5 M Ba(OH)2 with a potentiostatic power mode at applied voltages ranging from 30 to 80 V for 30 s. Spark plasma discharges occurred over the thin film electrodes at reaction voltages above 50 V, characterized as a PEO regime. Sintered-like BZT coatings with porous morphology were obtained. The PEO-produced coatings exhibited substantially higher integrated diffraction peak intensities and growth rates, compared to the electrochemically oxidized coatings synthesized at lower applied voltages. X-ray photoelectron spectroscopy revealed chemical compositions of the BZT coatings. This clearly demonstrates that crystalline BZT coatings could be facilely produced over complex (Ti,Zr)N seeding layers by PEO. Possible reaction routes and formation mechanisms of the coatings were also proposed. The PEO-made BZT coatings have great potential for many dielectric and ferroelectric applications.
1. Introduction Barium zirconate titanate [Ba(Zr,Ti)O3; BZT] belonging to a family of perovskite oxides is one of the most promising dielectric and ferroelectric materials [1,2] and can be used in various technological applications, such as tunable microwave devices [3], tunable ceramic capacitors [4], and electrocaloric device [5]. Several methods have been employed to produce BZT coating, as reported in the literature [6–10], including pulse laser deposition [6], multi-target reactive sputtering [7], sol-gel [8], chemical solution deposition [9], and hydrothermal [10] techniques. The above-mentioned methods often require either sophisticated equipment, high temperature post-annealing treatment (> 700 °C), or prolonged processing time to make crystalline materials. Such heat treatment may induce thermal stresses causing cracks in the films. In this work, plasma electrolytic oxidation (PEO) was used to produce BZT coatings. PEO has often been employed to produce oxide coatings over valve metals and alloys [11–18]. PEO possesses several advantages over the other techniques, such as fast growth rate, high crystallinity of oxide coatings, economical set-up, and superior adhesion between obtained coatings and substrates. In our previous work, PEO has been used to produce BaTiO3 (BTO) films on bulk Ti [11], Ti/Si [12], and TiN/Si [13] substrates, BaSrTiO3 (BST) on TiN/Si [14], and ZrO2 coatings on ZrN/Si [15]. In the systematic serial PEO studies, Ba (Zr,Ti)O3 has never been explored before. This is also the first attempt by using the ternary nitride- (Ti,Zr)N thin films as the seeding electrode
⁎
for PEO. Thus, the novelty lies in employing the complex (Ti,Zr)N seeding layers to produce BZT coatings by PEO. Compared to metal seeding electrodes, conductive nitrides exhibiting higher chemical stabilities and melting points may be better candidates for the seeding electrodes in PEO. They can not only act as a supply of metallic components for metallic oxide coatings but enhance the growth rates of the coatings [13–15]. In our previous studies, TiN seeding layers were employed to produce BTO [13] and BST [14], meanwhile ZrN films were used to prepare ZrO2 [15]. (Ti,Zr)N similar to TiN and ZrN but with more complex components could act as seeding electrodes for supplying Ti and Zr sources for BZT. Moreover, (Ti,Zr)N exhibiting superior mechanical properties and corrosion resistance [19,20] could withstand more intense spark discharges during PEO. Thus, the objective of this research is to produce complex BZT coatings by PEO using ternary (Ti,Zr)N seeding electrodes. Crystalline phases, microstructures, and compositions of the resulting oxide coatings are investigated. Possible reaction routes and formation mechanisms of the coatings during PEO are also discussed. 2. Experimental details (Ti,Zr)N seeding layers were deposited on n-type (100) Si substrates by a dc unbalanced magnetron sputtering system. Sintered TiZr compound targets (99.99%) with the composition of Ti:Zr = 50:50 wt % = 66:34 at.% were used. The sputtering power was fixed at 300 W while a bias voltage of −50 V was applied to the substrate. The base
Corresponding author. E-mail address: [email protected] (F.-H. Lu).
https://doi.org/10.1016/j.surfcoat.2020.125440 Received 19 November 2019; Received in revised form 24 January 2020; Accepted 3 February 2020 Available online 04 February 2020 0257-8972/ © 2020 Elsevier B.V. All rights reserved.
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pressure was pumped down to 3.0 × 10−4 Pa before sputtering. Sputtering deposition was performed at room temperature with the working pressure of 0.2 Pa. A TiZr interlayer was deposited on the substrate for 10 min prior to the deposition of (Ti,Zr)N films to enhance the adhesion. The resulting thickness of the TiZr interlayer was 285 ± 10 nm. Subsequently, (Ti,Zr)N films were sputtered onto the interlayer with the Ar/N2 flow ratio of 10 (Ar: 30 sccm; N2: 3 sccm) for 90 min. The coating thickness was about 3.0 ± 0.5 μm and the measured electrical resistivity was 39 ± 4 μΩ-cm, which are compatible with the characteristics of (Ti,Zr)N films reported in the literature [21,22]. Before PEO, each as-deposited (Ti,Zr)N/Si specimen was cleaved into the size of 0.5 × 1.5 cm2 and cleaned ultrasonically with ethanol and deionized water. Two-third of the (Ti,Zr)N/Si specimen was then soaked into a 0.5 M Ba(OH)2 (98%, VETC) electrolyte with an exposed area of 0.5 × 1 cm2. The specimen served as the working electrode while a Pt plate with the size of 1 × 2 cm2 acted as the counter electrode. PEO was performed by applying a 5-kW dc power supply (GP10HH50) under the potentiostatic mode with the voltages varying from 30 to 80 V. The reaction time was fixed at 30 s. The temperature of the electrolytes was kept at 70 °C during PEO. After PEO, the specimens were rinsed in deionized water and air dried. The crystallographic structures of obtained coatings were characterized by X-ray diffraction (XRD) (Mac Science MPX3, Japan) with the copper radiation (λCu,Kα = 0.1542 nm) operated at 40 kV and 30 mA. Both the conventional θ/2θ scanning mode and the grazing incidence X-ray diffraction (GIXRD) mode (2°) were employed. The microstructures of coatings were examined by field-emission scanning electron microscopy (FE-SEM) (JSM 6700F, JEOL, Japan) operated at 3 kV. The chemical compositions were determined by X-ray photoelectron spectroscopy (XPS) (PHI 5000 VersaProbe, ULVAC-PHI, Japan) with the Al-Kα radiation source (1486.6 eV) with a pass energy of 23.5 eV (0.2 eV/step). Ar+ ions with 3 keV were employed to sputter etching the film surface for 5 min. Ti-2p, Zr-3d, N-1s, Ba-3d, and O-1s peaks were deconvoluted by a non-linear least-squares fit with a Gaussian/Lorentzian peak shape (G/L = 0.3). The background was subtracted by the Shirley method prior to each fit [23].
(OH)2 electrolytes at 70 °C by applying fixed voltages ranging from 30 to 80 V for 30 s. Below 40 V, reaction currents were rather small and remained almost unchanged after 2 s, indicating the occurrence of slow electrochemical reactions. As applied voltages were above 50 V, very different current-time behaviors showed up. The currents increased rapidly in an initial short period, reached maxima, and then dropped back. It is noteworthy that evident luminescence over the (Ti,Zr)N surface was observed and accompanied by uniformly spark discharges lasting for about 10–15 s on the specimen surfaces. In this regime, the maximum current changed from 0.4 to 4.6 A with increasing the applied voltage and enhanced spark discharges were spotted. Yerokhin et al. [24] reported that the drastic current drop from the current maximum during PEO was caused by the predominant passivating effect of formed oxide coatings. This means that the (Ti,Zr)N seeding layers were progressively covered by the PEO-produced oxide coatings. Moreover, spark discharges accompanying by the formation and growth of gas bubbles at the anodes were mainly due to dielectric breakdown of the passivated oxide coatings under a strong electrical field [25]. The existence of spark discharges is one of the main characteristics of PEO. 3.2. Crystalline phases As-deposited (Ti,Zr)N films exhibited golden color while after the anodic treatment, the coatings became dark yellow below 40 V and turned gray above 50 V. X-ray diffraction patterns and grazing incidence X-ray diffraction patterns of the as−deposited (Ti,Zr)N films and those after the treatment in 0.5 M Ba(OH)2 electrolytes at various applied voltages for 30 s are shown in Fig. 2 for the (a) θ/2θ and (b) grazing incidence modes. The corresponding relative peak integrated intensities of Ba(Zr,Ti)O3 and (Ti,Zr)N are also plotted against the applied voltages, evaluated from Fig. 2(a), are depicted in Fig. 2(c). TiZr diffraction peaks of the interlayer between the nitride overlayer and the Si substrate were observed for all the specimens in the XRD patterns but not in the GIXRD patterns. These clearly indicate it is the interlayer. On the other hand, the diffraction peaks of (Ti,Zr)N were distinguished in both the XRD and GIXRD patterns. (Ti,Zr)N can be considered as a single-phase solid solution of TiN and ZrN due to the similarity of the (rock-salt) crystal structure. The lattice parameters of the as-deposited (Ti,Zr)N films, evaluated from the diffraction patterns, were about 0.442 ± 0.001 nm, which lies between those of TiN (0.424 nm [26]) and ZrN (0.458 nm [26]). After the anodic treatment, no characteristic diffraction peaks of oxides could be detected below 40 V. As the applied voltages were above 50 V, (100), (110), (111), (200), (211), (220), (310) characteristic diffraction peaks of cubic perovskite BZT were discerned, which were located at the diffraction angles between those of cubic BaTiO3 (ICDD 31-0174 [26]) and BaZrO3 (ICDD 06-0399 [26]). Trace BaCO3 was also found in the patterns, which may attribute to reactions of Ba (OH)2 electrolytes with CO2 containing in air [27]. It is noteworthy that only single-phase BZT was identified without forming either a BaTiO3 or BaZrO3 second phase, indicating the formation of a solid solution for BZT coatings after PEO. Fig. 2(c) shows the relative integrated peak intensities of (Ti,Zr)N and BZT as a function of applied voltages. The relative integrated peak intensity (RI) of either (TiZr)N or BZT is evaluated from the equation:
3. Results and discussion 3.1. Current-time behaviors and spark discharges Fig. 1 displays the reaction currents versus reaction time during the anodic treatment for the (Ti,Zr)N-coated Si specimens in the 0.5 M Ba
RIs [%] =
Is × 100% ∑overall Ioverall
(1)
where I represents the peak integrated intensity and s denotes a specific species, i.e., (Ti,Zr)N or BZT. As sketched in the figure, the relative peak intensity of BZT increased rapidly with applied voltages when the voltages were above 50 V, indicating a fast growth of the oxides. In contrast, the relative peak intensity of (Ti,Zr)N decreased correspondingly since the obtained oxides were resulted from consumption of the
Fig. 1. Current–time curves during PEO conducted in 0.5 M Ba(OH)2 electrolytes for 30 s over (Ti,Zr)N/Si with various applied voltages ranging from 30 to 80 V. 2
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Fig. 2. (a) X-ray diffraction patterns and (b) Grazing incidence X-ray diffraction patterns of the as-deposited (Ti,Zr)N/Si and obtained coatings after PEO conducted in the Ba(OH)2 electrolytes for 30 s with applied voltages ranging from 30 to 80 V. (c) The relative peak integrated intensities of Ba(Zr,Ti)O3 and (Ti,Zr)N as a function of applied voltages, evaluated from above XRD patterns.
Fig. 3. Lattice parameters of obtained BZT coatings (a) evaluated from the XRD spectra of Fig. 2(b) as a function of the applied voltage.
(Ti,Zr)N seeding layers. The fast growth rate of oxides is one of the main characteristics for PEO. The lattice parameters of obtained BZT were also calculated from the GIXRD patterns [28] and plotted against the applied voltages, as shown in Fig. 3. As depicted in the figure, the obtained lattice parameters of BZT were about 0.410 ± 0.001 nm, which lie between those of BaTiO3 (0.403 nm [26]) and BaZrO3 (0.419 nm [26]). The larger lattice parameter for BaZrO3 is due to the larger ionic radius of Zr4+ (ri = 0.720 Å), compared to Ti4+ (ri = 0.605 Å) [29]. 3.3. Morphologies and microstructures Surface morphologies of the as-deposited (Ti,Zr)N before and after PEO conducted in the 0.5 M Ba(OH)2 electrolytes with varying applied voltages ranging from 30 to 80 V are revealed in Fig. 4. As-deposited (Ti,Zr)N films exhibited nanograins. Tiny rods and nano-network structures were present at applied voltages of 30 and 40 V, respectively. Since no distinct diffraction peaks could be identified, these structures may be related to amorphous hydroxides, similar to those reported in the literature [14]. This regime with slow kinetics and amorphous feature is associated with mainly electrochemical oxidation. Rough surfaces with sintered-like morphologies occurred at applied voltages above 50 V. The dramatic different surface morphologies are mainly due to PEO. It has been reported that sintered-like morphologies with micropores are due to spark discharges occurring over surfaces of the seeding layers [13–15]. Micropores arose from the discharge channels occurring during the PEO process [30–32]. Corresponding cross-sectional micrographs of the as-deposited (Ti,Zr)N thin films before and after PEO conducted at the same voltage ranges are given in Fig. 5(a). It is noteworthy that the TiZr interlayer remained intact, as revealed from the micrographs and thus, BZT was formed from the reactions of (Ti,Zr)N with electrolytes. For comparison, thicknesses of the oxide coatings are plotted as a function of applied voltages, as shown in Fig. 5(b). A thin overlayer < 95 nm could be detected at the applied voltage of 40 V. As mentioned above, this may be resulted from amorphous hydroxides during electrochemical oxidation since no distinct diffraction peaks were found while the film surface revealed nano-network structures. The thickness of the oxide coatings could reach 2.1 μm at the applied voltage of 50 V and about 2.7–2.9 μm at voltages of 60–80 V. As shown in the figure, the thickness 3
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Fig. 4. Surface morphologies of the as-deposited (Ti,Zr)N/Si and obtained coatings after PEO conducted in the Ba(OH)2 electrolytes with various applied voltages ranging from 30 to 80 V.
similar dc unbalanced magnetron sputtering system [37]. After PEO, high resolution XPS spectra of (a) Ti-2p, (b) Zr-3d, (c) O1s, (d) Ba-3d after sputter etching were acquired, as shown in Fig. 7. As revealed in Fig. 7(a), the Ti-2p peaks were composed of the Ti-2p3/2 peak at lower binding energy (BE: 457.6 eV) and Ti-2p1/2 peak at higher binding energy (BE: 463.0 eV); both of them belong to TieO bonds of BZT [9]. The Ti3+ peak at 456.3 eV might be due to that Ti3+ was generated on the surface region contributing to the overall Ti 2p3/2 spectra [34]. As for Zr-3d3/2 and Zr-3d5/2 peaks in Fig. 7(b), the binding energies were located at 181.2 eV and 183.7 eV, respectively, which could attribute to ZreO bonds of BZT [9]. The O-1 s spectra given in Fig. 7(c) show an asymmetric curve that could be deconvoluted into two peaks with binding energies at 529.0 eV for O-Ti/Zr bonds and 530.8 eV for OeBa bonds [38], respectively. Fig. 7(d) displays the Ba3d spectra consisting of major BaeO bonds for BZT at the binding energies of 779.3 eV (Ba-3d5/2) and 794.7 eV (Ba-3d3/2) [39]. Atomic compositions of Ba, Ti, Zr, and O in the BZT coatings determined from the corresponding XPS spectra were 20.7 at.%, 7.7 at.%, 8.7 at.% and 62.9 at.%, respectively. This corresponds to the constituents of about Ba (Zr0.53Ti0.47)O3 for the BZT coatings. The Ti and Zr constituents in the obtained BZT coatings are rather close to those of the (Ti,Zr)N seeding layer.
increased dramatically above 50 V, i.e., in the PEO regime. The average growth rates of the BZT coatings produced by PEO were in the range of 88 ± 10 nm/s, which are much higher than those synthesized by other techniques, such as 0.06 nm/s by sol gel [8], 0.002 nm/s by hydrothermal synthesis [10], and 0.67 nm/s by chemical solution deposition method [9]. 3.4. X-ray photoelectron spectroscopy analyses Chemical bonding and atomic compositions of the as-deposited (Ti,Zr)N thin films and the PEO-produced BZT coatings were determined by X-ray photoelectron spectroscopy. Fig. 6 depicts high resolution XPS spectra of (a) Ti-2p, (b) Zr-3d, and (c) N-1 s acquired from the as-deposited (Ti,Zr)N films after sputter etching by Ar+ for 5 min to remove surface contaminants. The Ti-2p spectra could be deconvoluted into Three components consisting of TieN (BE: 454.5 eV), Ti3+ (BE: 456.3 eV) and Ti4+ (BE: 458.0 eV), respectively [33,34]. The Zr-3d spectra could then attribute to ZreN (BE: 179.1 eV), Zr3+ (BE: 180.4 eV), and Zr4+ (BE: 183.3 eV) [35,36]. As for the N-1s spectra, the N-(Ti/Zr) (BE: 396.7 eV) and minor NeO (BE: 399.0 eV) components were assigned [33]. Atomic compositions of the as-deposited (Ti,Zr)N films evaluated from the high resolution spectra were Ti:Zr:N = 22.6 at.%:33.6 at.%:43.8 at.%, corresponding to (Ti0.4Zr0.6) N0.8. The obtained N/(Ti + Zr) ratio is compatible with the reported 0.8–1.1 ratios in the literature for (Ti,Zr)xN1−x thin films prepared by a
Fig. 5. (a) Cross-sectional view of the as-deposited (Ti,Zr)N/Si and obtained coatings after PEO conducted in the Ba(OH)2 electrolytes with various reaction voltages ranging from 30 to 80 V and (b) The thickness of Ba(Zr,Ti)O3 coatings as a function of applied voltages. 5
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coatings were facilely produced by PEO using the conductive (Ti,Zr)N seeding layers. Although (Ti,Zr)N is a solid solution of TiN and ZrN, it could act like TiN and ZrN for supplying both Ti and Zr sources while concurrently. That is why a single-phase BZT was produced without forming an either BaTiO3 or BaZrO3 second phase. The possible reactions of the ternary (Ti,Zr)N seeding electrode in the alkaline electrolytes may be similar to those proposed by using binary TiN or ZrN electrode in producing the perovskite oxides as follows [13–15]. (Ti,Zr)N may be firstly oxidized in highly concentrated alkaline solutions to form H(Ti,Zr)O3− ions.
(Ti, Zr)N + 2OH− + 2H2 O → H(Ti, Zr)O−3 + NH 4 OH + e−
(2)
Subsequently, H(Ti,Zr)O3− might react further through two routes: (i) at low voltages (electrochemical oxidation regime) with the OH− ions in the electrolytes to form amorphous hydroxides (Ti,Zr)O2·xH2O.
H(Ti, Zr)O−3 + OH− → (Ti, Zr)O2⋅xH2 O + 1/2O2 + 2e−
(3)
(ii) at high voltages (PEO regime) with Ba2+ contained in the electrolytes to produce crystalline Ba(Zr,Ti) O3 with the aid of spark discharges.
Ba2 + + H(Ti, Zr)O−3 + OH− → Ba(Zr, Ti)O3 + H2 O
(4)
As the voltage increased, the thermal energy provided by spark discharges during PEO would further enhance the growth of BZT. A schematic diagram of the formation mechanisms of BZT coatings on (Ti,Zr)N/Si produced by PEO in the Ba(OH)2 electrolytes at low applied voltages (electrochemical oxidation) and high applied voltages (PEO) is shown in Fig. 8. 4. Conclusions Crystalline Ba(Zr,Ti)O3 coatings have been facilely produced on (Ti,Zr)N-coated substrates by plasma electrolytic oxidation conducted in Ba(OH)2 alkaline solutions for a short period of 30 s above 50 V. Below 40 V, thin amorphous layers were formed, which is due to slow electrochemical oxidation. Above 50 V, thick porous with sintered-like coatings were produced. Visual spark discharges occurring over the specimen surfaces and fast growth rate of the oxides fulfill the characteristics of PEO. Single-phase BZT coatings were obtained without forming an either BaTiO3 or BaZrO3 second phase. The average growth rates of PEO-produced BZT coatings were about 88 ± 10 nm/s, which is much higher than other deposition techniques for BZT. Employing the conductive ternary (Ti,Zr)N seeding electrode can facilitate substantially the growth of the complex BZT coatings by taking advantage of its high conductivity, partially ionic bonding, and multi-metallic constituents. The technique can also be extended to produce more complex oxide coatings with more wide applications. CRediT authorship contribution statement Huan-Ping Teng:Writing - original draft, Conceptualization, Methodology, Investigation, Visualization.Fu-Hsing Lu:Writing review & editing, Conceptualization, Methodology, Resources, Supervision.
Fig. 6. XPS high resolution spectra of (a) Ti-2p, (b) Zr-3d, and (c) N-1s for the as-deposited (Ti,Zr)N seeding electrode.
3.5. Reaction routes and formation mechanisms of Ba(Zr,Ti)O3
Acknowledgement
As mentioned earlier, the fabrication of complex BZT coatings by PEO has not yet been reported in the literature so far. In this work, BZT
This work was partly supported by the Ministry of Science and Technology, Taiwan under the Grant number MOST 106-2221-E-005024-MY3. 6
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Fig. 7. XPS high resolution spectra of (a) Ti-2p, (b) Zr-3d, (c) O-1s and (d) Ba-3d for the Ba(Zr,Ti)O3 coatings produced by PEO over the (Ti,Zr)N seeding electrode in the electrolytes at 60 V.
Fig. 8. A schematic diagram of proposed growth mechanisms of BZT coatings over (Ti,Zr)N/Si in Ba-containing alkaline electrolytes at low applied voltages (electrochemical oxidation) and high voltages (PEO). 7
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