_Article_  
**Fabrication of ZrO2(MgO)/CaAl2O4+CaAl4O7**  
**Bilayer Structure Used for Sulfur Sensor by Laser Cladding**  
**[PERSON]**  
**[PERSON]**  
**[PERSON]**  
[PERSON]**  
to form a bilayer structure, the calcium sulfide would be formed on the side of the calcium aluminate coating when inserted into the liquid iron, and furthermore the calcium sulfide could work as the same function as the traditional auxiliary electrode. The aforementioned electrolyte could be kept for a long time and could be prepared in sufficient quantities for further processes, which could make the production process more complete and reduce unnecessary consumption.  
The thermodynamics of the sulfur determination using the bilayer system in the liquid iron can be written as follows.  
\[\text{CaO}(\text{CaAl}_{2}\text{O}_{4}+\text{CaAl}_{4}\text{O}_{7})+[\text{S }]=\text{ CaS}+[\text{O}] \tag{1}\]  
\[\ln a_{[\text{S}]}=\ln a_{[\text{O}]}+\ln\frac{a_{\text{CaS}}}{a_{\text{CaO}}}- \ln K \tag{2}\]  
where \(K\) is the equilibrium constant, \(a_{[\text{S}]}\) and \(a_{[\text{O}]}\) are the activities of [S] and [O] in the liquid iron, respectively, \(\ln\frac{a_{\text{CaS}}}{a_{\text{CaO}}}\) does not change with the sulfur content and it can be considered to be a constant value. [PERSON] et al. [15] have analyzed the relationship between the CaS and CaO incorporated in \(\text{CaAl}_{2}\text{O}_{4}\) and \(\text{CaAl}_{4}\text{O}_{7}\) in liquid iron, and found that \(a_{[\text{O}]}\) was very low in the carbon-saturated liquid iron. It should be noted that the effect of other elements in molten iron on reaction (1) should be eliminated for ensure its occurrence, such as the element P. Therefore, the purity of liquid iron for testing should be high, and the trace elements such as P and Si elements containing in the liquid iron should be less and would not have any effect on the reaction (1) in this stage. The effect of other elements on the reaction would be examined in the next study. Under these conditions, when the sulfur content was significantly higher than the oxygen content (for example 0.02-0.1 wt% S), the equilibrium of reaction (1) would proceed to the right to form CaS used as the auxiliary electrode and eventually the [S] and [O] would come to equilibrium at the phase boundary of the auxiliary electrode and the liquid iron. Then the electromotive force (EMF) would be obtained and the sulfur content could be calculated from the following equation.  
\[\ln a_{[\text{S}]}=f(E,T)+\ln\frac{a_{\text{CaS}}}{a_{\text{CaO}}}-\ln K \tag{3}\]  
where \(E\) is the EMF of the sulfur sensor (V), \(T\) is the temperature of the molten iron (K).  
Based on the above principle, a bilayer structure of non-sulfide solid electrolyte with a coating of calcium aluminate has been proposed to use in the sulfur sensor, as this unique structure would solved the disadvantages of the traditional sulfide coating with poor stability and easy oxidation. Meanwhile, this structure would also help more effectively to increase the conductivity in the sensor and avoid high electronic conductivity. The traditional methods of producing this bilayer structure is by coating the desired layer of material either by plasma spraying, flame spraying, or screen printing [6; 8; 9]. However, the disadvantages of these methods are that the coating may contain pinholes, and can fall off easily due to erosion of the liquid iron.  
In this paper, an attempt has been made to fabricate the ZrO\({}_{2}\)(MgO)/CaAl\({}_{2}\)O\({}_{4}\)+CaAl\({}_{4}\)O\({}_{7}\) bilayer system used for sulfur sensor by using the laser powder cladding (LPC) method. The properties of this structure in terms of the microstructure, composition and the ionic conductivity were investigated. The application of the bilayer structure in sulfur determination was also studied.