Accepted Article DR TAO LIU (Orcid ID : 0000-0001-6532-3132) Article type : Article A novel method for preparing dense diffusion barrier limiting current oxygen sensor Tao Liu,1, Hongbin Jin1, Lin Li2, Jingkun Yu1 1 School of Metallurgy, Northeastern University, Shenyang 110819, China 2 School of Science, Shenyang Aerospace University, Shenyang 110136, China Abstract: A novel method for preparation of a dense diffusion barrier limiting current oxygen sensor is presented. The dense thin-layer with the nominal composition Sr-, Mg- and Fe-doped lanthanum gallate (LSGMF) was prepared over La0.8Sr0.2Ga0.8Mg0.2O3–δ (LSGM) electrolyte by a thermal diffusion method. The crystal structure, morphology, elemental distribution, conductivity, and sensing performance of oxygen sensor were investigated. XRD analysis shows a highly crystalline perovskite. SEM-EDS analysis shows that the dense thin-layer with about 40 μm thickness is well attached to the Corresponding author. E-mail address: firstname.lastname@example.org (T. Liu). This article has been accepted for publication and undergone full peer review but has not been through the copyediting, typesetting, pagination and proofreading process, which may lead to differences between this version and the Version of Record. Please cite this article as doi: 10.1111/jace.15302 This article is protected by copyright. All rights reserved. solid electrolyte. Electrical conductivity measurement shows that the dense thin-layer has high Accepted Article conductivity of 8.73 S·cm–1 at 800 °C. The oxygen sensor exhibits good sensing performance. The variations of the limiting current (IL) versus oxygen content (x(O2)) can be obtained as follows: IL (mA) = 96.65 x(O2) (mol%) + 28.62 Keywords: dense diffusion barrier, limiting current oxygen sensor, solid electrolyte, perovskite I. Introduction In order to achieve energy efficiency and environmental protection, it is necessary to regulate the air-fuel ratio of combustion furnaces in the metallurgical industry within a reasonable range. Analytical techniques based on electrochemical solid electrolyte sensors are an effective method for measurement of oxygen content. One of them is the limiting current oxygen sensor (amperometric sensor). By applying a voltage to solid electrolyte, oxygen is transported (pumped) from the cathode to the anode. When the voltage is large enough, oxygen is close to zero at the cathode, and the current related to the measured oxygen content remains unchanged. It is well known that the sensing mechanism is used for the measurement of dissolved oxygen in liquids. Because the diffusion coefficient of oxygen in gas phase (150 mm2·s–1 at 700 °C) is four to five orders of magnitude greater than that of dissolved oxygen in aqueous solutions, it is difficult to observe the limiting current in gas phase . In order to overcome this problem, a diffusion barrier that controls the diffusion rate of oxygen is introduced in front of the cathode. According to the structures, the This article is protected by copyright. All rights reserved. diffusion barrier is classified into aperture, porous and dense mixed oxide-ion/electron conductor Accepted Article (MIEC). Since the plugging and deformation of pore-based diffusion barriers often occur, attention has been paid to dense diffusion barrier limiting current oxygen sensor. Much work has been done on the dense diffusion barrier limiting current oxygen sensor [2–6]. Garzon et al.  reported the preparation of a sensor based on ZrO2 stabilized with Y2O3 (YSZ) as solid electrolyte by magnetron sputtering technique. This sensor had a narrow detection range of oxygen since the dense diffusion barrier was very thin and its oxide-ion conductivity was high. They also reported the preparation of a sensor with YSZ solid electrolyte by screen printing technique . Although the thickness of dense diffusion barrier increased, gas such as H2O and CO2 produced by chemical decomposition of organic matter in the paste caused a large number of pores, leading to a decrease of densification. Peng et al.  reported the preparation of a sensor with YSZ electrolyte by co-pressing and co-sintering technique. Since there was a large thermal expansion coefficients (TEC) mismatch between YSZ and dense diffusion barrier, cracking often occurred, resulting in a relatively narrow detection range. Zou  reported the preparation of a sensor with YSZ electrolyte by spark plasma sintering (SPS) technique. La0.8Sr0.2MnO3 (LSM) underwent a structural phase transition under low oxygen partial pressure. La0.75Sr0.25Cr0.5Mn0.5O3 (LSCM) exhibited high phase stability but TEC mismatch led to cracking. Therefore, the sensor has proven to be unsuccessful. A technique by which electrolyte and dense diffusion barrier pellets are bonded together with Pt avoids the problems associated with the crack formation caused by TEC mismatch and chemical reaction between two materials [5–6]. However, the dense diffusion barrier prepared by conventional solid state sintering method still contains many pores and the sensor exhibits slow response rate. This article is protected by copyright. All rights reserved. Although YSZ exhibits high oxide-ion conductivity at temperatures above 1000 °C, high operating Accepted Article temperature leads to the problems of high-temperature sealing, compatibility between components, and high operating costs. Doped ceria solid electrolytes have high oxide-ion conductivity at low temperature. However, Ce4+ is easily reduced to Ce3+ at reducing conditions, which not only generates high electronic conduction, but also leads to lattice expansion and solid electrolyte fracture. In 1994, Ishihara et al.  and Goodenough et al.  reported LaGaO3 doped with Sr and Mg (LSGM) electrolyte materials that exhibit high oxide-ion conduction over ranges of temperature (600–800 °C) and oxygen pressure (10–15–105 Pa). When transition metal ions replace the Ga ions at the B-sites of LSGM, the formed LSGMN (N=Co, Ni, Cr, Fe, Mn, and V) perovskite oxides exhibit mixed oxide-ion/electron conduction, excellent chemical and physical compatibility with LSGM electrolyte [9–10]. In this work, La0.8Sr0.2Ga0.8Mg0.2O3–δ (LSGM) is used as solid electrolyte. Sr-, Mg- and Fe-doped lanthanum gallate (LSGMF) dense diffusion barrier is prepared on LSGM by a thermal diffusion method, and characterized by XRD, SEM-EDS, Van Der Pauw four-terminal method, EIS, and voltammetry. II. Experimental procedure LSGM was prepared by a conventional solid-state reaction . La2O3 (99.99%, Sinopharm), SrCO3 (99.99%, Sinopharm), MgO (99.99%, Sinopharm), and Ga2O3 (99.99%, Sinopharm) were used as raw materials. La2O3 and MgO were calcined at 1000 °C for 10 h before using. Stoichiometric compositions were mixed in an agate ball-mill for 2 h, pressed into pellets and fired at 1000 °C for 20 h. After cooling down, the pellets were reground, re-pressed, and fired at 1200 °C for 20 h. The This article is protected by copyright. All rights reserved. resulting sample was reground, pressed into circular disks with a diameter of 10 mm and a thickness Accepted Article of about 1 mm, and finally sintered at 1450 °C for 20 h to achieve a pure perovskite structure with space group Pm-3m (No. 221) and cubic symmetry. One side of the sintered LSGM ceramic was evenly covered with Fe2O3 powders and heated at 1400 °C for 10 h. X-ray diffraction (XRD) was measured on a X′Pert Pro MPD X-ray diffractometer to determine the phase composition of the prepared sample, using nickel filtered CuKα radiation produced at 40 kV and 40 mA. The cross-sectional microstructure of prepared sample was observed and analyzed by a Zeiss Ultra Plus scanning electron microscope (SEM) operated at an acceleration voltage of 15 kV. The elemental distribution was determined by SEM-EDS point, and map spectrometry technique. The sample was sputtered with a gold film in order to be conductive before the scanning process. The electrical conductivity measurements were performed from 300 to 850 °C in air using a Van Der Pauw four-terminal method with Agilent 34970A. The four terminals were made of Pt wire. The electrical conductivities (σ) are 1/(4.5×t×R), where t is the thickness of dense thin-layer, and R is the resistance [11, 12]. Electrochemical impedance spectroscopy (EIS) measurements were performed using computer controlled Autolab PGSTAT 204. The impedance spectra were recorded between 105 Hz and 0.1 Hz with the amplitude (rms value) of the ac signal 10 mV. Measurements were carried out over the temperature range of 650 – 850 °C in air. Pt paste was screen-printed on both sides of the resulting sample and fired at 800 °C for 1 h. To prevent oxygen from leaking, a glass was coated around the sample. The sensor I-V characteristics This article is protected by copyright. All rights reserved. were measured with an Autolab PGSTAT 204 Linear Sweep Voltammetry (LSV) potentiostat over the Accepted Article temperature (T) range 750 – 850 °C in 0.1 % – 4.9 % O2/Ar mixtures. The fluxes of O2 and Ar were controlled by a capillary flowmeter and the total flux was approximately 100 ml/min. III. Results and discussion (1) XRD analysis Figure 1 shows the XRD patterns of the top surface of dense thin-layer. It can be seen that the dense thin-layer has a highly crystalline perovskite structure. Traces of additional phases such as Sr4Ga2O7, LaSrGa3O7 and MgFeO4 are observed. This is most probably due to uncompleted solid-state reactions. Minor traces of secondary phases of La-Sr-Ga-O system and/or MgO are common for LaGaO3-based ceramics and it can be assumed that they do not have a significant effect on the transport properties [13,14]. (2) SEM-EDS analysis Figure 2 presents SEM micrograph of the cross-section of prepared sample. The cross sections of the sample show two different zones: a LSGMF dense thin-layer with about 40 μm thickness; and a LSGM electrolyte, which exhibit good densification and are well attached. The elemental distribution was confirmed by SEM-EDS of points in the two different zones. Table 1 shows the depth profile of elemental composition. Spectra in f and m show the EDS microanalysis of the LSGMF dense thin-layer and the LSGM electrolyte, respectively (Fig. 2). This article is protected by copyright. All rights reserved. Figure 3 shows the EDS elemental maps (La, Sr, Ga, Mg, Fe, and O). The element profile once again Accepted Article allows the two zones detected by EDS point analysis. The Fe content is higher in the dense thin-layer, and then reduced steeply at the interface between dense thin-layer and electrolyte. This sharp intensity step is a sign of the reaction/diffusion front observed by SEM (Fig. 2). The depth distribution of La, Sr, Ga, Mg, and O is also accordance with the expected zones. Fig. 3 also shows that the elements are unevenly distributed in the dense thin-layer, indicating that the dense thin-layer is composed of several phases. Further research will be required to elucidate the mechanism resulting in the formation of several phases. (3) Electrical conductivity The introduction of Fe cations in LSGM crystal lattice leads to a transition from pure oxide-ion conductor to mixed oxide-ion/electron conductor. Fig. 4 shows Arrhenius plots of the total conductivity of LSGMF. The dense thin-layer possesses high conductivity of 8.73 S·cm–1 at 800 °C, which is 2 orders of magnitude higher than that of LSGM (0.17 S·cm–1 at 800 °C ). The mechanism of electronic transport is most likely to be hopping of p-type charge carriers (electron hole) between relatively isolated transition metal cations [15, 16]. The slight decrease of conductivity around 600 °C corresponds to the loss of oxygen, which reduces the concentration of p-type electronic charge carrier. This is in good agreement with literature data [5, 17, 18]. 1 2Fe Fe + OO = O2 (g) + 2Fe Fe + VO 2 (1) where VO and OO represent an oxide-ion vacancy and a lattice oxygen, respectively. The reaction equilibrium is shifted to the right side with the temperature rise. This article is protected by copyright. All rights reserved. Accepted Article The activation energy for the conductivity can be calculated as 0 T E exp a kT (2) where σ0 is the pre-exponential factor, k is the Boltzmann constant, and Ea is the activation energy. The activation energies for LSGMF in the high and intermediate temperature ranges are 0.338 eV and 0.183 eV, respectively. As a dense diffusion barrier, mixed conductor needs to have high electronic conductivity, which ensures that the driving force for diffusion of oxygen in dense diffusion barrier is not an electric field, but an oxygen potential gradient. (4) Impedance analysis Figure 5 (a) shows the impedance spectra of prepared sample at 650–850 °C in air. The real axis intercept in high-frequency regions corresponds to the ohmic resistance of LSGM, while the difference between the low-frequency and the high-frequency intercepts on the real axis represents the area specific resistance (ASR) of the electrode/electrolyte interfaces. The last contribution increases significantly with the temperature rise while a relatively slight change in the first one is observed. According to the reaction mechanism of samples proposed by Liu et al. , the semicircles are ascribed to at least two different electrode processes during the O2 reduction. In order to separate these processes, the equivalent circuits are proposed and validated by fitting the impedance spectra data (Fig. 5 (a)), where R1 is ohmic resistance of LSGM; R2 and R3 are the resistance associated with the charge-transfer processes and the diffusion processes, respectively; This article is protected by copyright. All rights reserved. CPE1 and CPE2 are constant phase-angle elements depicting the nonideal capacitance of the surface Accepted Article layer and the double layer, respectively. Fig. 5 (b) shows Arrhenius plots of ASR, electrolyte resistance and total resistance (Rsensor). Rsensor is mainly contributed by the R1, R2, and R3, but not a simply summation of these three individual values. (5) Performance of oxygen sensor Figure 6 shows the I-V characteristic curves of limiting current oxygen sensors with LSGMF dense diffusion barrier at different oxygen content. The I-V characteristic curves include three regions. At low voltage the output current increases in direct proportion to the applied voltage, and the slopes are mainly governed by the ohmic resistance of electrolyte [2,20]. At intermediate voltage we can observe a limiting current plateau that provides very useful information about the oxygen content. In this region, the oxygen diffusion from the dense thin-layer surface to the layer/electrolyte interface is the rate-determining step. The limiting current plateaus increase slightly with increasing applied voltage. This is likely due to the influence of the very reducing applied polarizations on the chemical diffusion coefficient of oxygen in the dense diffusion barrier via an oxide-ion vacancy formation mechanism . The limiting current plateau at each oxygen content is indicated on the plot, and the limiting current is the average of all current in the plateau region. The limiting current plateau increases linearly and shifts to the right side with increasing oxygen content. IL (mA) = 96.65 x(O2) (mol%) + 28.62 (3) At high voltage the limiting current plateau disappears and the output current increases gradually with increasing voltage. According to the thermodynamic calculation, the theoretical decomposition This article is protected by copyright. All rights reserved. potential is (in descending order): La3+/La, Sr2+/Sr, Mg2+/Mg, Ga3+/Ga, Fe2+/Fe, and Fe3+/Fe 2+. Accepted Article Therefore, the current starts to increase at high voltage mainly due to the reduction of Fe ions. The higher the oxygen partial pressure, the higher the decomposition voltage. Figure 7 shows the I-V characteristic curve at different temperatures. The limiting current plateau cannot be observed at temperature below 750 °C due to the lower conductivity of the solid electrolyte, while it appears above 800 °C and increases with the temperature rise due to the increase in conductivity of solid electrolyte and dense diffusion barrier. When the temperature reaches 850 °C, a peak appears at the beginning of the limiting current plateau. Garzon et al. [2,21] postulated that it is due to a partial reduction in oxygen stiochiometry of the metal oxide upon application of reducing potentials. IV Conclusions A state-of-the-art method for the preparation of dense diffusion barrier limiting current oxygen sensor is proposed, and the resulting dense thin-layer is characterized in order to demonstrate the formation of the perovskite structure and the associated conduction performance. The dense thin-layer has a perovskite crystal structure and is well attached to the solid electrolyte. Electrical conductivity measurement shows that the dense thin-layer has high conductivity of 8.73 S·cm–1 at 800 °C. The oxygen sensor exhibits good sensing performance. The variations of the limiting current (IL) versus oxygen content (x(O2)) can be obtained as follows: IL (mA) = 96.65 x(O2) (mol%) + 28.62 This article is protected by copyright. All rights reserved. Accepted Article Acknowledgements This work is financially supported by the National Natural Science Foundation of China (Nos. 51374055 and 61403260). References  Ramamoorthy R, Dutta PK, Akbar SA. Oxygen sensors: materials, methods, designs and applications. J Mater Sci. 2003;38:4271-4282.  Garzon FH, Raistrick ID, Brosha EL, et al. Dense diffusion barrier limiting current oxygen sensors. Sensor Actuat B-Chem. 1998;50:125-130.  Peng ZY, Liu ML, Balko E. A new type of amperometric oxygen sensor based on a mixed-conducting composite membrane. Sensor Actuat B-Chem. 2001;72:35-40.  Zou J. The research of dense diffusion barrier oxygen sensor. Master thesis, Ningbo University, 2006.  Liu T, Gao X, He BG, et al. A limiting current oxygen sensor based on LSGM as a solid electrolyte and LSGMN (N=Fe, Co) as a dense diffusion barrier. J Mater Eng Perform. 2016;25:2943-2950.  Xia H, Li FS, Wu WJ, et al. Limiting current oxygen sensors with LSM as dense diffusion barrier. 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Structure, electrical and thermal properties of (Ba0.5Sr0.5)1−xGdxCo0.8Fe0.2O3−δ perovskite as a solid-oxide fuel cell cathode. Solid State Ionics. 2012;207:38-43.  Liu T, Li L, Yu JK. Cathode materials Sr1−xHoxCoO3−δ (SHC, x ≤ 0.3) for IT-SOFC. Ionics. 2016;22:853-858.  Usui T, Asada A, Nakazawa M, et al. Gas polarographic oxygen sensor using an oxygen/zirconia electrolyte. J Electrochem Soc. 1989;136:534-542.  Brosha EL, Chung BW, Brown DR, et al. Amperometric oxygen sensors based on dense Tb-Y-Zr-O electrodes. Solid State Ionics. 1998;109:73-80. Table and Figure Captions Table 1 Depth profile analysis of elemental composition (atom%) obtained by EDS Fig. 1. XRD analysis of the top surface of dense thin-layer Fig. 2. SEM-EDS analysis of the cross-section of prepared sample Fig. 3. EDS elemental maps of cross-section of prepared sample Fig. 4. Temperature dependence of the total conductivity of LSGMF This article is protected by copyright. All rights reserved. Fig. 5. (a) Complex-plane (Nyquist) impedance plots of prepared sample with the configuration of Accepted Article Pt/LSGM/LSGMF/Pt at different temperatures. The inset is equivalent circuit. (b) Temperature dependence of ASR, electrolyte resistance and total resistance Fig. 6. I-V characteristic curves for different oxygen content Fig. 7. I-V characteristic curves for different temperature Table 1 Elements a b c d e f g h i j k l O 45.16 53.52 57.39 45.08 51.77 52.33 57.60 53.25 52.16 53.10 55.87 54.87 Mg 2.03 1.91 8.53 1.42 0.43 0.56 0.72 2.09 2.79 2.76 3.79 0.72 Fe 25.78 1.82 2.04 0.61 2.89 16.99 8.18 5.78 1.61 1.24 0.18 0.09 Ga 4.68 18.94 19.15 27.25 23.42 6.31 14.04 16.95 17.22 18.39 18.54 25.98 Sr 1.55 1.25 0.84 10.82 7.64 2.12 5.23 2.20 2.40 2.89 3.64 8.05 La 20.80 22.57 12.05 14.83 13.85 21.69 14.22 19.74 23.83 21.62 17.97 10.29 This article is protected by copyright. All rights reserved. m 56.56 3.68 0.15 18.42 3.99 17.20 Accepted Article This article is protected by copyright. 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