简介概要

Experiment and finite element method analysis mass erosion and transfer of Ag/La₂NiO₄-based electrical contacts during operation

来源期刊：Rare Metals2013年第1期

论文作者：Song Chen Wei-Ming Guan Kun-Hua Zhang Zhi-Long Tan Ming Xie

文章页码：93 - 99

摘要：A uniform transient temperature field model of electrical contacts operation was found by analyzing the process of closing arc → constriction resistance Joule heat → breaking arc. Essential parameters of Ag/La2NiO4 electrical contact material for transient temperature field calculation were obtained through tests of electrical contact experimental instrument under 18 V DC in different currents, other correlation experiments, and calculation analysis. The finite element method was applied to solve the transient temperature field, and the features and distribution of the transient temperature field were obtained. The condition of material erosion and mass transfer can be forecasted by those calculation results. It is beneficial to research about the lifetime of Ag/La2 NiO4 electrical material.

Rare Metals 2013,32(01),93-99+2-8

Experiment and finite element method analysis mass erosion and transfer of Ag/La₂NiO₄-based electrical contacts during operation

Song Chen Wei-Ming Guan Kun-Hua Zhang Zhi-Long Tan Ming Xie

Kunming Institute of Precious Metals

State Key Laboratory of Advanced Technologies for Comprehensive Utilization of Platinum Metals

摘要：

A uniform transient temperature field model of electrical contacts operation was found by analyzing the process of closing arc → constriction resistance Joule heat → breaking arc. Essential parameters of Ag/La2NiO4 electrical contact material for transient temperature field calculation were obtained through tests of electrical contact experimental instrument under 18 V DC in different currents, other correlation experiments, and calculation analysis. The finite element method was applied to solve the transient temperature field, and the features and distribution of the transient temperature field were obtained. The condition of material erosion and mass transfer can be forecasted by those calculation results. It is beneficial to research about the lifetime of Ag/La2 NiO4 electrical material.

作者简介：Wei-Ming Guan e-mail:cs@ipm.com.cn;

收稿日期：22 February 2012

基金：financially supported by the National Science Foundation of China-Yunnan United Foundation(No.U0837601);the National Natural Science Foundation of China(No.51267007);the Natural Science Foundation of Yunnan Province(No.2010CD126,No.2012FB195);

Experiment and finite element method analysis mass erosion and transfer of Ag/La₂NiO₄-based electrical contacts during operation

Abstract：

A uniform transient temperature field model of electrical contacts operation was found by analyzing the process of closing arc→constriction resistance Joule heat → breaking arc. Essential parameters of Ag/La2NiO4 electrical contact material for transient temperature field calculation were obtained through tests of electrical contact experimental instrument under 18 V DC in different currents, other correlation experiments, and calculation analysis. The finite element method was applied to solve the transient temperature field, and the features and distribution of the transient temperature field were obtained. The condition of material erosion and mass transfer can be forecasted by those calculation results. It is beneficial to research about the lifetime of Ag/La2NiO4 electrical material.

Keyword：

Electrical contact; Finite element; Mass transfer; Erosion; Temperature field model;

Received： 22 February 2012

1 Introduction

Silver-based electrical contact materials are widely used in low voltage apparatus to control the break or close of electrical circuit.Ag/Me O,including Ag/Cd O,Ag/Sn O2,Ag/Zn O,and Ag/Yb₂O₃[1],have been applied most widely in many fields.Because different metal oxides can control the feature of electrical arc,researches about different metal oxides’influence on Ag-based electrical contact materials have been studied for many years[2–4].La₂Ni O₄is a new typical conducting ceramic,and its conductivity,anti-arc erosion,and melting point were all higher than Sn O₂[5].Ag/La₂Ni O₄was prepared by our research group[5,6],and difference parameters[6–9]were measured during simulation application process by electrical contact experimental instrument.

During operation of electrical contact,the electrical contact material experience has the following process:closing arc?constriction resistance Joule heat?breaking arc?natural cooling.The electrical contact materials were eroded in this process by coupling of different facts and many physical fields.The erosion of contact materials is a very fundamental problem of contact material research;moreover,the lifetime of contact has a relationship with this problem,and the study about this problem has always been a heat point.The investigation on this problem usually adopts the method of experiment connecting with theoretic calculation.Robertson[10]and Nied and Schlansker[11]were pioneers in this research filed.In recent years,Borkowski and Walczuk[12]and Swingler and Mc Bride[13]not only built a phenomenological model to describe arc effect but also calculated and analyzed this model.Shun[14]and Wang Qiping and Rong[15]investigated the temperature field of constriction resistance and constructed a simple temperature model of arc action with materials,and then,the model was calculated by finite different method.The calculation result agreed with experimental examination.Li and coworkers[16]not only built up a complex arc action model but also calculated and analyzed this model by finite element method.However,in the actual operation of electrical contact including closing arc?constriction resistance Joule heat?breaking arc?natural cooling,the transient state temperature field model was not set up.

In our researches[8,9],transient temperature field model was constructed based on theoretic analysis of the process of contact operation.Then,the transient temperature field of Ag/La₂Ni O₄electrical contact in operation was given by finite element method.Based on the transient temperature field of Ag/La₂Ni O₄,information about material melting and evaporation,features of material erosion,and direction of mass transfer were forecasted.

2 Experimental

2.1 Preparation of samples

The mixed powder of La₂Ni O₄and Ag(99.95%)in mass ratio of 1–9,and then,Ag/La₂Ni O₄wire(Φ1.4 mm)was made by pressing,sintering,extruding,and drawing.At last,electrical contacts(Φ3.0 mm 9 1.0 mm+Φ1.5 mm 92mm)were made using this wire.

2.2 Experimental arrangement

When contact open voltage was 18 V and close current was5,10,15,20,25,and 30 A,frequency of operation was1 Hz and 5,000 times operation.Those parameters including voltage,current,and contact resistant were measured by electrical contact experimental instrument.The surface of anode and cathode was investigated by SEM after 5,000 times operation.The mass change of anode and cathode was measured by optical balance.

3 Results and analysis

3.1 Mass transfer

The arc was observed both in close and break operation.Figure 1 shows the mass change and total loss in anode and cathode after 5,000 times operation.It is found that mass transfer direction from cathode to anode,when current<22 A,and if current is larger than 22 A,will reverse.At last,the anode mass loss is larger than cathode mass addition.Total mass loss increased as current increased.

3.2 Morphology of anode and cathode surface after 5,000 times operation(I<22 A)

Figure 2 shows the morphology of anode and cathode surface on 20 A after 5,000 times operation by PHLIPS-XL30E scanning electronic microscope.It is found in Fig.2 that hillock is present in anode,and pits,droplets,and solidified texture are present in anode.When current<22 A,the morphology of anode and cathode surface is the same.It was generally believed that hillock,droplet,and solidified texture in anode have a close relationship with cathodic arc.

Fig.1 Curves of current with mass change of anode and cathode

3.3 Morphology of anode and cathode surface after 5,000 times operation(I>22 A)

Figure 3 shows the morphology of anode and cathode surface on 30 A after 5,000 times operation by scanning electronic microscope.Figure 3 shows that pits,large droplets,and solidified texture are present in anode,and hillock is present in cathode.The morphology of anode and cathode surface is the same when current is larger than22 A.It is generally believed that hillock,droplet,and solidified texture in cathode have close relationship with anodic arc.

3.4 Results and discussion

The operation of electrical contact material includes closing arc→constriction resistance Joule heat→breaking arc→natural cooling.It is believed that mass transfer and mass loss appear in the stage of close arc and break arc.Many research results indicated that close arc was represented by anodic arc which made mass transfer from anode to cathode,and if break arc was represented by cathodic arc,it will make mass transfer from cathode to anode[2,3].The anodic arc was made of electron,but cathodic arc was made of ion of gas.It was noticed in the two arcs that directions of mass transfer were opposite[2,3].The two arcs contribute to erosion and mass transfer.The arc which had dominant influence could control the direction of mass transfer.Based on many results and researches[5–7],a conclusion was drawn as follows:(1)When current<22 A,break electrical arc which was an anodic arc,controlled the mass transfer.(2)When current larger than 22 A,close electrical arc which was a cathodic arc,controlled the mass transfer.Arc erosion of anode was larger than cathode,and material erosion and loss were carried out by sputtering and evaporation of material.

Fig.2 SEM images of anode and cathode surface after 5,000 times operation under 20 A:a anode,and b cathode

Fig.3 SEM images of anode and cathode surface after 5,000 times operation under 30 A:a anode,and b cathode

4 Investigation of finite element method

The sketch of transient temperature field model is shown in Fig.4.The model was built as following:the heat flow P(t)was produced by constriction resistance joule heat or electrical arc.The heat flow P(t)×η g acted on a round area on Ag/La₂Ni O₄electrical contact anode or cathode surface the radius of which was r(t).The heat flux J=k(t)=P(t)×η/2πr(t)²(0<η≤1,where g is power input parameter[3,17]),and heat flux k(t)was homogeneously distributed on that round area.It was noticed that the g was different for anode or cathode when arc action(η=0.5 or η=1 were chosen in calculation),and g was 0.5 under constriction resistance joule heat.The other contact surface kept on T=25°C or thermal insulation,and the electrical contact initial temperature was 25°C.The whole electrical contact was made of Ag/La₂Ni O₄.

It is noticed that in this model,the P(t),r(t),and k(t)are changed with time.This feature not only can represent the process of close arc,constriction resistance Joule heat,break arc,and natural cooling but also exceeds that of other models[14–16].This model applies actual boundary condition that could overcome the shortcoming of previous models[14–16],wherein the boundary of calculation area and boundary condition artificially gave a result.Moreover,the enthalpy method is used to deal with material melt and evaporation in finite element method calculation.The fundamental equations are listed as follows:

Fig.4 Sketch of transient temperature field model

4.1 Calculation of model

At first,thermal conductivity and enthalpy of Ag/La₂Ni O₄material are given in Figs.5 and 6 through differential thermal analysis and calculation.Then,the parameters of P(t)and I(t)were determined through tests of our electrical contact experimental instrument.During arc action,the r(t)(lm)can be obtained by the relationship between r(t)and I(t)[2–4]:πr(t)²=1.67×10³I(t).η=0.5 or η=1 were inpidually chosen in k(t)calculation.r(t)approximately20 lm was obtained through microhardness[15]during constriction resistance joule heat action;η=0.5 was chosen in k(t)calculation.The powers of arc or constriction resistance joule heat were directly measured from electrical contact experimental instrument.

When contact operation condition was 18 V DC and30 A,those parameters were measured by electrical contact experimental instrument.The relationship of P(t)–t with r(t)–t is shown in Figs.7 and 8 during electrical contact operation:closing arc(0–19.7 ms)→constriction resistanceJouleheat(19.7–485.7 ms)→breakingarc(485.7–501.7 ms)→natural cooling(t>501.7 ms).It is found that the numerical values of P(t)and r(t)are changed too sharply in the stage of arc action,but P(t)and r(t)are constant under constriction resistance Joule heat action.

Fig.5 Enthalpy of Ag/La2Ni O4

Fig.6 Thermal conductivity of Ag/La2Ni O4

The finite element method was applied in this problem.At the beginning of calculation,the model must be pided into a lot of elements,especially the density of element in heat flux acting area was more intensive(size of element was0.5 lm)than the element in stalk of contact(size of element was 50 lm).The size of element becomes larger and larger from top to stalk.The calculation time ranges from 0 to 0.6 s and time step of calculation:△t=0.20–20 ms.The computer programs of FEM calculation were compiled by Fortran[8,9,18].The calculations were inpidually carried for η=0.5 and η=1.0.

Fig.7 Heat flux P(t)on contact during contact operation

Fig.8 Heat flux radius r(t)of on contact during contact operation

4.2 Results and analysis

The results of transient temperature field of Ag/La₂Ni O₄contact at different current were calculated for η=0.5 or η=1.0.It is found that the position of maximum temperature T_maxon contact is located on the center of contact surface of top.It is shown in Fig.9 that the temperature field map in contact is at 1.59 ms when temperature rises to a maximum T_max.The curves of that position temperature and time were shown in Fig.10 under 18 V/30 A when η=1.0.Based on temperature field results,the same conclusions are gained from these figures:(1)The position of maximum temperature on contact located on the center of contact surface of top.(2)The temperature of contact was not only high but also increased sharply in the period of electrical arc action.The maximum temperature of contact appeared in the range of 1.0–10 ms after arc beginning action.(3)During constriction resistance action,the temperature was constant,and temperature field was steady.(4)At the junction time of electrical arc action period and constriction resistance period,because the value of heat flux and size of radius changed discontinuously,the temperature changed sharply.(5)It was noted for g=1.0or η=1.0 that the electrical contact temperature sharply decreased to room temperature 25°C about 10 ms after breaking arc extinction.It was concluded that the electrical contact was kept at 25°C before the next operation because the time distance was generally larger than 10 ms.

The relationship of current and maximum temperature T_maxare shown in Figs.11 and 12 for η=1.0 or η=1.0.It is found in Figs.11 and 12 that T_maxis very close in process of close arc and break arc,when current is smaller than 20 A.The T_maxof close arc is far greater than the T_maxthat the break arc produced after the electrical current is[20 A.It is obvious that the influence of both kinds of electrical arcs on material has changed at 20 A.

Fig.9 The Temperature field map of Ag/La2Ni O4base contact top under 18 V/30 A(η=1.0)at 1.59 ms

Fig.10 Temperature on the center of Ag/La2Ni O4base contact top under 18 V/30 A(η=1.0)

Fig.11 Curves of maximum temperature and current(η=0.5)

Fig.12 Curves of maximum temperature and current(η=1.0)

Fig.13 Curves of current and maximum volume(η=1)

The relationship of current with molten pool volume at the time of the maximum temperature is shown in Fig.13.When current is<20 A,molten pool volume produced by close arc or break arc is very close.However,molten pool volume of close arc is ten times larger than the molten pool volume of break arc after current larger than 20 A.It is concluded that the higher the temperature,the larger the molten pool volume.

The relationship of current with lifetime of molten pool is showed in Fig.14.The lifetime of molten pool produced by close arc is larger than break arc in all ranges of current.The curve of molten pool lifetime produced by break arc is smooth.When current is larger than 20 A,the material begins to evaporate.Therefore,the curve of molten pool lifetime,produced by close arc,changes sharply.

The two curves of current with product of maximum molten pool volume and lifetime of molten pool for break arc and close arc are given in Fig.15.The two curves cross between 20 and 25 A.Those curves have relationship with the mass transfer curves in Fig.1.Because the mass transfer is directly affected by molten pool volume and molten pool lifetime,the direction of mass transfer can be forecasted by the curves in Fig.15.Mass loss curve begins to increase after the current was larger than 20 A in Fig.1.This result coincided with the calculation that the mass began to form vapor during close arc operation after 20 A.The close arc is anodic arc,so the mass transfer direction is from anode to cathode.

Fig.14 Curves of current and melting pool lifetime(η=1)

Fig.15 Relationship of current and maximum volume multiple lifetime for break arc and close arc

5 Conclusion

Based on the experiment and calculation results,the conclusion could be drawn as follows:

This model is rational,and the finite element method calculation results are correct and reasonable.

These calculation results are important for erosion and forecast direction of electrical contact material transfer and have reference to the development of new electrical contact material.