Study On The New Silver-based Conductive Ceramic Contact Materials

Electrical contact is a physical and chemical phenomenon that two conductors contact with each other and current and signal transfer through their interface. As the essential components of the electromechanical devices such as switches, relays and connectors, electrical contacts are the key factors which influence the conversion ability and reliability. Owing to the frequent operations of make and break, the contacts have strict requests on the electrical and mechanical properties of their materials. The main requests are: fine conductivity, well anti-welding, high electrical anti-abrasion, low contact resistance and little temperature rise after arcing for a long time. Ag/CdO has been long recognized as the preferred electrical contact material for its outstanding performance in make and break switching. However, due to the toxic nature of cadmium, it has gradually been replaced by some new materials. Although a large part of efforts have been focused on Ag/SnO₂, some of the problems such as high contact resistance and figuration machining are still to be solved. Therefore, to find a predominant contact material becomes a main job in this field.According to the status quo and the unsolved problems, and considering the application request on contact materials used in devices of low-voltage, in this thesis, a new idea of contact material study is provided and the silver-based conductive ceramics contact materials are prepared. With the ceramic preparation technique, several oxides are compounded to get the conductive ceramic which has fine electrical performance and unique micro-structure. The technological operation is simple and is suitable for industrial process. The new contact material compounded with the ceramic and silver has some fine characters such as nonhazardous, low resistivity, fine static and dynamic electrical performance and good mechanical properties. In addition, the ceramic is in low price and can replace the noble metal such as silver to the large extent, which can greatly reduce the manufacturing cost. The experimental results show that the silver-based conductive ceramic composite material has fine electrical and mechanical performance, and it probably can replace the Ag/CdO material. Moreover, it has great competitive power against Ag/SnO₂ because of its fine mechanical working properties.The Main Work and Results of the Thesis:The LaNiO_3-δ ceramic (called LNO in the following) was prepared through the solid reaction process. The LaFeo.25Nio.75O3 ceramic (called LFNO in the following) was prepared by Fe substitution on the B site of LaNiO_3-δ, and the La_0.7Sr_0.3MnO₃ ceramic (called LSMO in the following) by substitution on the A site. These three kinds of ceramics were compounded with silver respectively after a series of powder metallurgy technologies. The law how species and sintering temperature of ceramics and the third addition have effect on the electrical and mechanical properties of the composite contact materials were studied in this thesis.1. The LNO ceramic and Ag/LNO composite contact materialsThe effects of sintering technique on the structure and physical performances of the LNO ceramic were studied. Results show that, for the three LNO ceramics, the two main phases are La₂NiO₄ which is Tetragonal and body-centered as well as NiO which is Hexagonal and rhomb-centered. The ceramic grain grows bigger and the phase ratio of La₂NiO₄ ascends with the increasing of the ceramic sintering temperature. LNO(1400) ceramic was prepared at the sintering temperature of 1400℃, whose grains are regularly and compactly arranged. The density of LNO(1400) ceramic is the highest and its conductivity is the least with the value of 31.8 mΩ·cm. This ceramic, therefore, is the most suitable for the use in the composite contact materials. Ag/LNO(1400) contact material was compounded with the LNO(1400) ceramic and silver. The effects of ceramic sintering condition on the resistivity, tensile strength and working performances of the contacts were studied. The results show that resistivity of Ag/LNO(1400) is 2.10 μΩ·cm and the tensile strength is 171.2 MPa which is the least, indicating the strong anti-welding performance. During the iife test in the condition of 24V DC 15A, Ag/LNO(1400) shows fine anti-erosion and mass transfer properties which qualify it as a high grade electrical contact material.2. The LFNO ceramic and Ag/LFNO composite contact materialsFe is substituted on the B site of LNO and LFNO ceramic is prepared. The effects of sintering technique on the structure and physical performances of LFNO were studied. Results show that all the three LFNO ceramics are complex phases of La(Fe_0.4Ni_0.6)O₃, NiO and La₂NiO_4+δ. In the ceramics, Ni³⁺ coexists with Ni²⁺ and the perovskite structure is concurrent with the spinel structure. The ceramic grain grows bigger with the rise of the ceramic sintering temperature. LFNO(1450) is the fine conductive ceramic with the resistivity value of 6.19 mΩ·cm, which is the least of the three. Ag/LFNO contact materials were compounded with LFNO ceramics and silver, and the effects of ceramic sintering condition on the resistivity, elongation percentage and working performances of the contacts were studied. As to Ag/LFNO(1450) the resistivity is only 2.05 μΩ·cm. Comparing with the more popular contact materials of Ag/CdO and Ag/SnO₂ whose resistivity are about 2.1 μΩ·cm, Ag/LFNO materials have more dominance. Besides, the resistivity of LFNO(1400) is found small too, with the value of 20.6 mΩ·cm and its density is 8.046 g/cm³ which is the highest of the three. Comparing with the results of XRD, LFNO(1400) ceramic shows the stablest performance and is suitable for using in the contact materials. The results indicate that the elongation percentage of Ag/LFNO(1400) is the biggest with the value of 12%, which gives it the best mechanical performance. Moreover, contact materials compounded with this ceramic and silver show fine anti-erosion and material transfer properties.3. Properties of silver-based conductive ceramic composite contact materialsAg/LNO(10) and Ag/LFNO(10) materials were mixed with LNO(1400), LFNO(1400) ceramics respectively with silver in which the accession ratio of ceramic to the whole material was 10%. The two materials were compared with the currently prevailing materials of Ag/CdO(12) and Ag/SnO₂(10) in the market, with respect to the following aspects, such as arc duration, changes of the phase structure after arc erosion, surface morphology and metallographic examinations of the contact surface after arcing erosion. The results show that the resistivity of Ag/LNO(10) and Ag/LFNO(10) are 2.096 μΩ·cm and 2.056μΩ·cm, which leads to their fine anti-erosion and material transfer properties during the contact tests. Better than Ag/CdO(12) and Ag/SnO₂(10), Ag/LNO(10) and Ag/LFNO(10) become the predominant contact materials.The contact samples of Ag/LNO(12), Ag/LNO(10), Ag/LFNO(12) and Ag/LFNO(10) were selected for comparison. The results show that the elongation percentage augments and the tensile strength abates with the increase of ceramic addition, which improves the mechanical and anti-welding properties. However, the resistivity increases simultaneously which can put the materials at a disadvantage. The tensile strength abates and the elongation percentage augments after the annealing processing, which can improve the mechanical property of the contact material and strengthen its anti-welding abilities.4. Effects of the third additions on the composite contact materialsUsing Ni and CuO as the additions, the properties of the composite materials were studied. Results show that the working performance of Ag/LFNO(10) is greatly improved with Ni added. The mass change of the anode and cathode contacts alters from +0.333 mg, -0.401 mg to +0.115 mg, -0.186 mg, which indicates the fine material transfer peroperty. Moreover, Ni decreases the tensile strength of the materials which strengthen their anti-welding properties and play an important role in controlling the mass transfer during the arc erosion. At the current of 24V 3A, the break arc duration is only 2.07 ms and the arc duration increases with the rise of current. The addition of CuO has not taken the expectant effect.Four contact samples Ag/LNO(8)+Ni(1), Ag/LFNO(8)+Ni(1), Ag/LFNO(8)+Ni(2) and Ag/LFNO(10)+Ni(1) were tested for comparison. The elongation percentage of Ag/LFNO(8)+Ni(1) and Ag/LFNO(10)+Ni(1) are both 22.5%, which indicates the fine mechanical performance. The tensile strength of Ag/LFNO(8)+Ni(2) is the least with the value of 152.55 MPa, indicating the fine anti-welding. The life test was taken and the results show the addition of Ni and the accession ratio increase of ceramic to the whole material can ameliorate the mass transfer property under low voltage and weak current. It can also strengthen the ability of resisting high voltage and strong current of the materials.The life test was taken on the Ag/LNO(8)+Ni(1) contacts and the results show that the contacts behave well under the load of 24V DC 10A. The mass transfer and erosion gradually increase with the rise of the current, and the arc duration is generally longer when the mass transfer and erosion is serious. Under the load of 24V DC 15 A, the quantity of mass transfer and erosion takes a trend of increase and then decrease.5. Effects of the third additions on the wettability of the contact materialsWith a small quantity of WO₃, MoO₃ and CuO as the ameliorant, the effects of wettability on the LFNO(1400) ceramic and the Ag/LFNO(1400) contact materials were studied. Results show that the contact angle of ceramic and silver is 80° with the amelioration of WO₃, 54° with MoO₃, and 35° with CuO, which indicates that the effect of CuO is the most prominent. The elongation percentage and the density augment because the wettability amelioration and the mechanical property are greatly improved. Such advantageous aspects for these materials ensure their applicational potential in the correlative fields. Nevertheless, the resistivity and the tensile strength increase as the ameliorant is added, which weakens the anti-welding and mass transfer properties. The results of the life test of the contacts show that the addition, which ameliorated the wettability, does not improve the contact performances.6. The LSMO ceramic and Ag/LSMO composite contact materialsSr is substituted on the A site of LaMnO₃ and therefore the LSMO ceramic is prepared. The effects of sintering technique on the structure and physical performances of the LSMO ceramic were studied. Results show that LSMO(1350) is the complex phase of La_0.65Sr_0.35MnO₃ and LaMnO₃ and the main phase of the latter two ceramics is La_0.7Sr_0.28Mn_0.99O₃. It means that when sintering at 1350℃, Sr has not been completely appended into LaMnO₃. When the sintering temperature is above HOOT, the work of substitution on the A site can be gradually accomplish. The values of resistivity of the LSMO ceramics are much higher and their densities are lower. The crystalline quality of the cramics is improved with the rise of the sintering temperature. The resistivity of LSMO(1450) is the least with the value of 67.6 mΩ·cm, and its density is the highest. Ag/LSMO(1450) contact material was compounded with the LSMO(1450) ceramic and silver. The effects of ceramic sintering condition on the elongation percentage and working performances of the contacts were studied. The results show that the elongation percentage values of these materials are higher and the mechanical properties are better correspondingly. In conditions of direct current, Ag/LSMO materials show worse contact performance than Ag/LNO and Ag/LFNO.