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1 Introduction

Because AgCdO electrical contact material contains toxic metal Cd, it has been banned in the use of household appliances and cars. It is very necessary to develop a new type of non-polluting and non-toxic electrical contact material as a substitution. AgSnO2, which has excellent resistance to arc erosion, wear resistance and good welding resistance, is used to replace the AgCdO. But the usage of AgSnO2material is limited by its shortcomings, such as the great contact resistance, the high temperature rise and so on. The main reason is that SnO2is a nearly insulated semiconductor with wide band gap, which makes the material resistance increase and the conductivity becomes poor[1-3]. Therefore, improving the conductivity of SnO2is a major problem which needs to be solved urgently[4-5].

The recent study shows that the problem can be solved by doping element. Yu et al. studied the band structure by the first principle calculation[6]. Xie et al. studied the electronic structure of Ru doped SnO2semiconductor and the results proved that the conductivity of Ru doped SnO2can be greatly improved[7]. Shan et al. calculated the electronic structure and optical properties of Ce doped SnO2, and discovered that the doped SnO2had half-metallic property[8]. Wang et al. studied the density of states and band structure of n-layers transition metal Cr doped SnO2superlattice by using full potential linearized augmented plane wave method, and the results showed that doped SnO2were half-metallic[9]. Long et al. studied the electronic structure and optical properties of Sb-doped SnO2, which showed that Sb doped SnO2was still a direct band gap semiconductor, the conduction band moved to the lower energy side and the band gap became smaller[10]. Jia et al. calculated the electronic properties of different ratio of Ti doped SnO2, which pointed out that the band gap reduced gradually along with the increasing of doping ratio[11]. Yu et al. calculated the density of states and optical properties of V、Cr、Mn doped SnO2, and discovered that the V, Cr doped SnO2had the properties of half-metallic, but Mn doped SnO2didn’t[12]. However, the first principle calculations for La doped SnO2semiconductors have not been reported. La is a rare earth element, there is an electron on the 5d orbit but no electron on 4f orbit, which can be known by the Hund Rule. And the resistivity of La is higher than that of Al and Cu. La doped SnO2semiconductor is proved to be a good conductive material only in experiments[13]. Therefore, theoretical analysis of La doped AgSnO2contact material has a theoretical guidance for the optimization of performance.

In this paper, the electronic property and structure of La doped SnO2based on density functional theory (DFT) are calculated. The lattice constant, band structure and density of states are contained in this calculation. Finally, the contact resistance and the arc energy with different doping ratios are measured by experiments. By theoretical and experimental analysis, the optimal doping ratio of La is obtained, which provides a theoretical basis for the research of contact materials.

2 Lattice model and calculation method of La doped SnO2

2.1 Model of the cell of SnO2

The structure of SnO2is rutile, which belongs to 136 P4/MNM space group. In the calculation, the lattice constants are based on the experimental results (a=b=0.4737nm, c=0.3816nm, α=β=γ=90°). As shown in Fig.1, each unit cell contains two Sn atoms and four O atoms, the Sn atoms are located at the apex and the body core respectively[14].

In this paper, atomic alternative method is used in the calculation process. Firstly, different model of supper cells are established, and then one La atom is used to replace one Sn atom. The correspondence relationship between doping ratio and super cell is shown in Table 1.

Table 1 Correspondence relationship between doping ratio and super cell

2.2 Method for calculation

The CASTEP software module of Material Studio software package developed by Accelrys company was used. The calculations were based on the plane wave ultra soft pseudo potential method, which included two steps[15]: First, the structure of the super cells were optimized to find the most stable state of the semiconductor structure; Second, the energy band structure, density of states and other properties of the optimized super cell were calcu-lated. The energy cut-off of plane wave chose 340eV, while Monkhorst-Pack mesh of Brillouin-Zone sampling took 4×4×6, the convergence criterion for the interaction between atoms was 0.3eV/nm, and the self-consistent convergence of the total energy took 2×10-5eV/atom for both super cells. The parameters after optimized were all achieved as convergence criteria. The atomic configuration for O, Sn and La were 2s22p4, 5s25p2and 5p65d1respectively. All energy calculations were carried out in the reciprocal space[16].

3 Calculation results and analysis of La doped SnO2

3.1 crystal structure

Firstly, the crystal structures of different doping ratios are optimized, and the parameters after optimized are shown in Table 2.

Table 2 Crystal parameters of geometry optimization and experimentation

The optimized data shows that the parameters of unit cells are consistent with the available experimental data, so the unit cell can be calculated in the next step. At low doping ratio, the volume expansion is small. With the increasing of doping ratio,the volume expansion becomes larger and larger. But in short, the volume is larger than it’s original. According to quantum chemistry theory, the radius of La3+ion (0.106nm) is larger than that of Sn4+ion (0.083nm[17]), La3+ion with larger ionic radius replaces the Sn4+ion leads to the volume of cell decreasing. And the bond length of the La-O (0.2417nm) is larger than that of the Sn-O (0.2081nm) also leads to the volume of cell decreasing. The calculated results are in accordance with the theoretical foundation.

3.2 Band structure and density of state

The energy band structures of the pure SnO2and La doped SnO2are shown in Fig.2.

As shown in Fig.2, the highest point of the valence band and the lowest point of the conduction band are both in the Brillouin Zone G, which shows that the SnO2belongs to a direct band gap semiconductor material. The Fermi level is chosen to be zero of the energy scale. The calculated band gap value of SnO2is 1.580eV, and the result is similar to 1.4eV[18]and 1.258eV[19], which were calculated by Liu et al. and Jiang et al. respectively. The calculated band gaps are all lower than available experimental data (3.6eV[20]) mentioned in paper, and it is because GGA is ground state theory, but the energy-gap belongs to property of excited state. This does not affect the theoretical analysis of the electronic structure of SnO2. As for the La doped SnO2, there are more energy levels in both conduction band and valence band than pure SnO2, which is consistent with the rich energy levels of rare earth impurity. La doping enhances the interaction between atoms, and the band gap becomes smaller, which makes the metallic properties of the material enhance.

Fig.2 Band structure of SnO2 (a) pure SnO2; (b) 16.67% La doped SnO2

As shown in Fig.3 is the total density of state (TDOS) and partial density of states (PDOS) of pure SnO2and La doped SnO2.According to the Fig.3(a), The valence band, which is located at -15～-20eV regions is mainly dominated by O 2s orbitals and a little Sn 5sand Sn 5p orbitals for pure SnO2. Because of its far from Fermi level, the influence of it can be ignored. Near the band edge the valence band can be divided into three parts, the top valence band in a range of about 2.5eV is O 2p orbitals contribution, the bottom valence band in a range of about 3.3eV is O 2p and Sn 5s orbitals contribution, and the rest of the contribution is to O 2p and Sn 5p orbitals. The conduction band is composed by Sn 5s, 5p and O 2p orbitals, and the electron has a transition from Sn 5s to O 2p, which causes the density of states moves to the lower energy level. It is indicated that SnO2is an ionic bond crystal with some covalent properties[21-22].

Fig.3 Density of states of SnO2 (a) pure SnO2; (b) 16.67% La doped SnO2

According to the Fig.3(b), a new peak which is dominated by La 5d orbitals appears in the range of -14.53～-13.30eV. -8.3～0 eV region is dominated by Sn 5s, Sn 5p, and O 2p orbitals. At Fermi level, the valence band is mainly composed of O 2p orbitals, while the conduction band is mainly composed of Sn 5p orbitals. La 5p orbit enter into the part of the conduction band resulting in a narrower band gap, a little change in the top of the valence band and a movement to the lower energy side.

The energy gap can be obtained from the band structure as shown in table 3. In general, doping makes the band gap smaller. With the increasing of doping ratio, the band gap decreases firstly and increases afterward. When the doping ratio is 16.67%, the band gap is the smallest. The band gap means that the carriers needs how much energy when they are excited from the valence band to the conduction band. So we can come to the conclusion that the conductivity is best at the doping ratio which has the minimum band gap.

Table 3 Band gap parameters

The density of states can be obtained after calculating as shown in Fig.4.With the doping ratio increasing, the conduction band of the density of states moves to the lower energy level firstly and then broadens, while the number of electrons near the Fermi level increases firstly and then decreases. From the figure we can see the conduction band moves to the lowest energy level at the doping ratio of 16.67%, meanwhile, the band gap is the smallest and the conductivity is the best.

Fig.4 Comparison of the total density of States

3.3 The experimental verification of La doped AgSnO2 contact materials

3.3.1The measurement experiment of contact resistance The contact material with different doping ratio was prepared by powder metallurgy method. The composition and doping ratio of contact materials are shown in table 4. Contact processing process included mixing powder, initial pressing, initial burning, re-pressing, re-burning and polishing. Finally, the contact material was made and cut into a simple one which was 4.5mm in diameter and 3.5mm in thickness.

Table 4 Doping ratio and the corresponding composition of the contact material

The electrical contact performance was tested by using JF04C contact test system. The current of test system of 13A and the voltage of 24V were set. Before conducting a new round of performance measurement, the instrument must be adjusted firstly. The protection voltage of the system needed to set to ±40V, and the voltage was needed to correct to zero in the parameter setting system, which was used to ensure that the relative pressure of contact point was zero before the test. After the the experiment started, the contact pressure was regulated to 86cN by adjusting the closed drive voltage and the open drive voltage in the parameters setting system. All of the electric contact experiments were conducted at room temperature. Measured a contact resistance after each 100 times electric contact. The test data was arranged and analyzed at the end of the experiment.

The results of the contact resistance test are shown in Fig 5.

Fig.5 Contact resistance (a) Ag∶SnO2∶La2O3=88∶11∶1； (b) Ag∶SnO2∶La2O3=88∶10.5∶1.5； (c) Ag∶SnO2∶La2O3=88∶10∶2； (d) Ag∶SnO2∶La2O3=88∶9∶3； (e) Ag∶SnO2∶La2O3=88∶6∶6

The testing data of the contact resistance of La doped AgSnO2is listed in table 5. When the ratios of material are Ag∶SnO2∶La2O3=88∶11∶1 and Ag∶SnO2∶La2O3=88∶6∶6, the variation of contact resistance is mainly between 0.3mΩ and 2mΩ, and the average contact resistance of the later reaches 1.096 mΩ. When the ratios of material are Ag∶SnO2∶La2O3=88∶10.5∶1.5, Ag∶SnO2∶La2O3=88∶10∶2, Ag∶SnO2∶La2O3=88∶9∶3, the variation of contact resistance is mainly between 0.2mΩ and 1.5mΩ, and the average of contact resistance is around 0.55mΩ.

Table 5 Experimental data of contact resistance of La doped contact material

3.3.2The measurement experiment of arc energy JF04C contacts test system can record the arc energy. The collected data was too big to analyze, so we averaged every 100 equal intervals from the 25000 collected data and then got a new set of data which can be chosen for the further analysis at the end of the experiment. The results of the arc energy of La doped contact material are shown in Fig 6.

Fig.6 Arc energy (a) Ag∶SnO2∶La2O3=88∶11∶1； (b) Ag∶SnO2∶La2O3=88∶10.5∶1.5； (c) Ag∶SnO2∶La2O3=88∶10∶2； (d) Ag∶SnO2∶La2O3=88∶9∶3； (e) Ag∶SnO2∶La2O3=88∶6∶6

The experimental data are shown in table 6. The table illustrates that when the ratios of contact material are Ag∶SnO2∶La2O3=88∶10∶2, Ag∶SnO2∶La2O3=88∶9∶3, Ag∶SnO2∶La2O3=88∶6∶6, the average arc energy is about 185mj. When the ratios of contact material are Ag∶SnO2∶La2O3=88∶11∶1 and Ag∶SnO2∶La2O3=88∶10.5∶1.5, the average arc energy is about 190mj.

Table 6 Experimental data of the arc energy of La doped contact material

As is well known, contact resistance and arc energy are important parameters for the electrical properties of contact material. The contact materials with excellent performance should have smaller contact resistance and arc energy. The results show that when the ratios of contact material are Ag∶SnO2∶La2O3=88∶10.5∶1.5, Ag∶SnO2∶La2O3=88∶10∶2, or Ag∶SnO2∶La2O3=88∶9∶3, the contact resistance is smaller, and when the ratios of contact material are Ag∶SnO2∶La2O3=88∶10∶2, Ag∶SnO2∶La2O3=88∶9∶3, the arc energy is minimum. After comprehensive analysis, we can conclude that the electrical properties of the material are the best when the ratio of contact material is Ag∶SnO2∶La2O3=88∶10∶2.

4 Conclusion

In conclusion, the electronic structures and properties of La doped SnO2by DFT are studied. La doping can induce the lattice distortion, but the band gap becomes narrow, making the material half metallic. The density of states are analyzed, which shows the energy needed for the electronic transition from valence band to conduction band. The analysis of lattice constant, energy band structures and density of states show that when the doping ratio of La doped SnO2is 16.67%, the conductivity of the material is the best. Finally, the best doping ratio is verified by the experiment of contact resistance and arc energy. It provides a theoretical method of the design of La doped AgSnO2contact material, which is simple and low cost.

Reference

[1] Liu ZY. Calculation and Study of Nano-doping AgSnO2Electrical Contact Materials[D]. Tianjin University, Zheng J, Tianjin, Tianjin University, 2007.06.

[2] Li X, Deng R, Li YF, et al. Effect of Mg Doping on Optical and Electrical Properties of SnO2Thin Films: an Experiment and First-Principles Study[J]. Material And Design, 2015, 42(4): 5299～5303.

[4] Lu Y, Wang JP, Zhang CW, et al. First-Principle Study on the Electronic and Optical Properties of Mn-Doped SnO2[J]. Physica B, 2011, 406(17):3137～3141.

[5] Lai KG, Sun Y, Chen HM, et al. Effect of Oxygen Vacancy and Al-doping on the Electronic and Optical Properties In SnO2[J]. Physica B, 2013, 428(10):48～52.

[6] Yu F, Wang PJ, Zhang CW. First-Principle Study of Optical and Electronic Properties of SnO2[J]. Journal of University of Ji Nan, 2009, 23(4): 414～417.

[7] Xie XJ, Zhong LP, Liang ZH, et al. Electronic Structure of Ru-doped SnO2Semiconductor Solid Solutions[J]. Chinese Journal of Inorganic Chemistry, 2013, 29(12): 2514～2520.

[8] Shan LT, Ba DC, Lin YH, Li JC. Structure and Optical Properties of Ce Doped SnO2[J]. Vacuum, 2014, 51(1):25～28.

[9] Wang J, Feng XY, Wang PJ. Study on Magnetics and Optical Properties of Transitionmetal Doped SnO2Superlattice[J]. Journal of Functional Materials, 2014, 45(3): 3070～3074.

[10] Long M, Yan AY. Research on the Photoelectric Property of SnO2Doped Sb[J]. Journal of Southwest University for Nationalities, 2014, 40(5): 768～771.

[11] Jia JQ, Xie XJ, Liang ZH, et al. First-principles Study of Ti-doped SnO2Semiconductor Solid Solutions[J]. Chemical Journal of Chinese Universities, 2012, 33(5): 1050～1056.

[12] Yu L, Zheng G, He KH, et al. Electronic Structure and Magnetism of Transition Metal Doped SnO2[J]. Acta Physico-Chimica Sinica, 2010, 26(3): 763～768.

[13] Zhu YC, Wang JQ, An LQ, Wang HT. Influence of Rare Earth Oxide on Anti-Welding Performance of Ag/SnO2Electrical Contact Materials[J]. Journal of Hebei University of Thechnology, 2014, 43(3): 16～20.

[14] Godinho KG, Walsh A, Watson GW. Energetic and Electronic Structure Analysis of Intrinsic Defects in SnO2[J]. Journal of Physical Chemistry C, 2008, 113(1): 439～448.

[15] Li YJ, Tian HM, Liu JF. First-Principle Study of Anatase TiO2by Al-Doping[J].Journal of Materials Science & Engineering, 2013, 31(2): 305～309.

[16] Zhu ZG, Ramesh C D, Arunabhiram C, et al. Enhanced Gas-sensing Behaviour of Ru-doped SnO2Surface: Aperiodic Density Functional Approach[J]. Journal of Physics and Chemistry of Solids, 2009, 70(9):1248～1255.

[17] Murakami Y, Ito M, Kaji H, Takasu Y. Surface Characterization of Ruthenium-Tin Oxide Electrodes[J]. Applied Surface Science, 1997, 121(6): 314～318.

[18] Jiang L, Wang PJ, Zhang CW,et al. Electronic Structure and Optical Properties of Cr Doped SnO2Superlattice[J]. Acta Physica Sinica, 2011, 60(9): 227～232.

[19] Liu YM, Yao SW, Liu YB. First-principles Study of the Electronic and Optical Properties of SnO2[J]. Journal of Henan Institute of Science and Technology, 2011, 39(1): 78～82.

[20] Dolbec R, Khakani MAE, Serventia A M, et al. Microstructure and Physical Properties of Nanostructured Tin Oxide Thin Films Grown by Means of Pulsed Laser Deposition[J]. Thin Solid Films, 2002, 419(1～2): 230～236.

[21] Barbarat Ph, Matar S F. First-principle Investigations of the Electronic, Optical and Chemical Bonding Properties of SnO2[J]. Journal of Materials Chemistry, 1997, 7(12): 2547～2550.

[22] Sensato F R, Filho O T, Longo E, et al. Theoretical Analysis of the Energy Levels Induced by Oxygen Vacancies and the Doping Process (Co, Cu and Zn) on SnO2(110) Surface Models [J]. Journal of Molecular Structure Theochem, 2001, 541(1): 69～79.