## 3 Results and Discussion  
Figure 2 shows the XRD patterns recorded on the surface of the laser cladding layer and the electrolyte substrate. From Figure 2, it could be clearly seen that the laser cladding layer was composed of three phases, namely Ca\({}_{0.15}\)Zr\({}_{0.85}\)O\({}_{1.85}\), CaAl\({}_{2}\)O\({}_{4}\) and CaAl\({}_{4}\)O\({}_{7}\). Meanwhile, the intensities of 30.15\({}^{\circ}\), 34.95\({}^{\circ}\), 50.23\({}^{\circ}\) and 59.68\({}^{\circ}\) peaks increased after laser cladding and the overlap diffraction of monoclinic, tetragonal and cubic phase in the substrate was all transited to the cubic phase in the laser cladding layer because of the formation of c-ZrO\({}_{2}\)(Ca\({}_{0.15}\)Zr\({}_{0.85}\)O\({}_{1.85}\)). It was a change like this that increased the content of cubic phase in the layer and made this structure possess a high ionic conductivity. Figure 3 present the SEM micrographs and line scan of the cross-section of the fabricated sample by LCP method. Figure 3a,b show the SEM micrographs of the cross-section of the substrate with the calcium aluminate layer. Compared with those of the electrolyte substrate, the grain sizes of laser cladding layer are much larger, presenting a strip attached to the electrolyte substrate. Meanwhile, in the part of the laser cladding layer near the interface between layer and substrate, the grain sizes are larger than the substrate and smaller than the laser cladding layer apart from the electrolyte substrate. Most of the nuclei might not have enough time to grow before solidification in this area because of rapid cooling of the substrate. As such, this structure makes the laser cladding layer ad  
Figure 2: X-ray diffraction (XRD) patterns of the samples (**a**) before and (**b**) after laser cladding. electrolyte firmly. In this experiment, the mean thickness of the laser cladding layer was measured to be about 100 \(\upmu\)m.  
The line scan of the cross-section of the substrate with the calcium aluminate layer is illustrated in Figure 3c. It could be understood that the Al, Ca and O contents remained at a relatively high level in the laser cladding layer, where the variation tendency of Al and Ca contents was identical and the Al content was higher than Ca content. The above analysis indicated that the laser cladding layer was rich in Ca, Al and O. The Al, Ca and O contents then reduced steeply at the interface between the laser cladding layer and electrolyte. This sharp content step was a sign of the reaction/diffusion front observed with a SEM (Figure 3a,b). Meanwhile, there existed Zr element in the laser cladding layer due to the synthesis of Ca\({}_{0.15}\)Zr\({}_{0.85}\)O\({}_{1.85}\) in the laser cladding process. The distributions of O, Mg, and Zr agree in the expected zones in the substrate. Figure 4 shows the Arrhenius plots for the ionic conductivity of the fabricated sample. It could be obvious that its ionic conductivity increased linearly with the increase of temperature and no sudden change in conductivity was observed in the Arrhenius plots. The ionic conductivity of the fabricated bilayer structure was up to 2.06 \(\times\) 10\({}^{-3}\) S\(\cdot\)cm\({}^{-1}\) at 850 \({}^{\circ}\)C and the value would be larger under the  
Figure 3: (a,b) Scanning electron microscope (SEM) micrographs and (c) line scan of the cross-section of the fabricated sample. temperature used for sulfur determination. As for the solid electrolyte used for the potential reversible cell, the ionic conductivity was required to be greater than 10\({}^{-6}\) S-cm\({}^{-1}\)[16]. The ionic conductivity of the bilayer structure in this study was hundreds of times, even thousands of times, the required conductivity, which could meet the required ionic conductivity of ionic conductor for solid electrolyte sulfur sensor. For this bilayer structure, the CaS would be formed on the side of the laser cladding layer when inserting the liquid iron according to the reaction (1), which has been proved experimentally in previous studies [15].