The silver tungsten points’ preparation methods: infiltration method and re-pressuring method.

1. Infiltration method. Infiltration method is a method of producing refractory metal and. low melting point metal prosthesis alloy Firs of all , to form a porous skeleton, you should press and pre-sinter the metal powder which possess high melting point, secondly, you should control the temperature until its initial temperature is higher than the metal that owns low melting point, then place the metal which has low melting point under/above the porous skeleton, and make the metal melt and immerge porous skeleton’s pores, to get the final product- silver tungsten points which own high density and other good properties.

2. Mechanical alloying. Mechanical alloying, as a new material preparation process, is gradually stepping into people's vision in recent years. And its concept was first put forward in the 1960's. Mechanical alloying has gradually been developed and is widely used nowadays. Mechanical alloying refers to: adopting high-energy ball milling, mixing two or more kinds of elements powder for a long time. Mixing powder will bear attrition force, impact, shear force, compression force and other forces and absorb mechanical energy in this process. In other words, under the role of milling media, after repeatedly pressing, welding, and crushing and fracture process and through solid state reaction or diffusion, the mixing powder will combined with each other to get the result-the product- silver tungsten points.



 

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Silver tungsten points to produce electric locomotives regulating switch:

Firstly, batching and mixing tungsten powder, silver powder and a small amount of nickel powder;
Secondly, to form porous skeleton, you should press the mixing powder;
Thirdly, you should stack the porous skeleton and the infiltration silver pieces, and place them in tungsten boat;
Finally, to make the molten silver infiltrated into porous skeleton pores completely, you should carry out infiltration sintering; and you can get the product-silver tungsten points after cooling. Sintering conditions: under the protection atmosphere, you should putt them in the furnace of molybdenum wire, and the infiltration sintering temperature is higher than the melting point of silver.

Using silver tungsten as raw material to produce electric locomotives regulating switch by infiltration processes:

Batching (tungsten powder (70%), silver powder, nickel powder) → Mixing powder(silver powder, tungsten powder(70%),nickelpowder) → Pressing → Semi-inspection,infiltration+hydrogen → sintering → Checking → Cleaning → Packing → Silver tungsten points



 

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The steps of silver tungsten points made by infiltration method:

First of all, pressing porous the skeleton with partial-induced silver or the silver blank and tungsten skeleton without silver; then, at the condition of high temperature, pre-sintering the porous skeleton; putting the two parts in a special melt sintering furnace after stacking up them together, and the next step is liquid sintering, the temperature should be controlled above the melting point of silver of 100℃. Under the molten state, silver will fill the pores of the porous skeleton through capillary action, ultimately silver tungsten points with high density is formed. The main steps of silver tungsten points as described bellow:

A small amount of silver, tungsten powder and adding other elements---mixing---preprocessing---skeleton molding + pure silver---pre-sintering (the temperature is over the melting point of silver) --- infiltrating---cooling 



 

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Different tungsten points material possess different properties, different ratio of the same kind of tungsten points material also have different properties, their density, hardness, specific resistance have a different scope.
Main performance index of silver tungsten contacts (with different ratio) as shown below:

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A coprecipitation method using calcium tungstate was developed to remove ⁸⁷Sr isobaric interference with ⁸⁷Rb prior to measurement of Rb by ID-ICP-MS. Precipitation of calcium tungstate was obtained by adding Ca(NO₃)₂ solution and (NH₄)₂WO₄ solution to the sample, where (NH₄)₂WO₄ was added more than the stoichiometric proportion to precipitate Ca(NO₃)₂ completely and remove Sr effectively. Furthermore, in order to reduce matrix burden to the ICP-MS instrument, the residual (NH₄)₂WO₄ was removed by adding conc. HNO₃. Prior to the application, thorough purification of coprecipitant reagent was carried out to reduce the blank. The effectiveness of the present method was verified by analyzing two brown rice flour certified reference materials (CRMs), NIES CRM 10a and NIES CRM 10b. Finally, the present method was applied to the measurement of Rb in a white rice flour RM sample being developed by National Institute of  Metrology of Japan.

 

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The main methods of silver tungsten contacts’ preparation are re-pressuring method and infiltration method.

1. Infiltration method. Infiltration method is a common method of producing low melting point metal prosthesis alloy and refractory metal. Firstly, pressing and pre-sintering the metal powder with high melting point to form a porous skeleton, and then you should control the temperature higher than the metal with low melting point, and put the metal with low melting point below or above the porous skeleton, to make the metal melt and fill the pores of the porous skeleton, to obtain silver tungsten contacts with high density.

2. Mechanical alloying. In recent years, mechanical alloying as a new material preparation process is gradually stepping into people's vision, the concept was first proposed in the 1960's, and now, mechanical alloying has gradually been developed and is widely used. Mechanical alloying refers to mixing two kinds or more kinds of elements powder by high-energy ball milling for a long time., in this process, powder will withstand friction, impact, shear and compression and other forces and absorb mechanical energy, that is to say, under the role of milling media, the mixing powder will be combined with each other after repeatedly pressing, welding, crushing and fracture process and through solid state reaction or diffusion, in order to achieve alloying.



 

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Silver tungsten contacts to produce electric locomotives regulating switch by infiltration process:

Firstly, batching silver powder, tungsten powder and a small amount of nickel powder, and then mixing;
Then pressing the mixing powder to form porous skeleton;
And then stacking the infiltration silver pieces and the porous skeleton, placing them in the burned tungsten boat;
Finally, conducting infiltration sintering, to make the molten silver completely soaked porous skeleton pores, you can get silver tungsten contacts after letting it cool down. Sintering conditions are: putting them in the furnace of molybdenum wire, under the protection of the atmosphere, the temperature range is above the melting point of silver over 100 ℃.

Infiltration processes to produce electric locomotives regulating switch, using silver tungsten as raw material:

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The process of silver tungsten contacts produced by infiltration method:

Firstly, pressing the silver blank and tungsten skeleton without silver or porous skeleton with partial-induced silver; then, pre-sintering the porous skeleton at high temperature; after stacking up the two parts together, putting them in a special melt infiltration sintering furnace, and then liquid sintering, the sintering temperature is controlled above the melting point of silver. Silver under the molten state fills the pores of the porous skeleton by capillary action, forming silver tungsten contacts with high density eventually. The main process of silver tungsten contacts are as follows:

Tungsten powder + a small amount of silver, adding additional elements → mixing powder → pretreatment →skeleton molding + pure silver → pre-sintering (temperature > melting point of silver + 100 ℃) → infiltration → cold treatment



 

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Different tungsten contacts material have different characteristics, different proportions of the same tungsten contacts material also own different characteristics, their density, hardness, specific resistance have a different scope.

Main performance index of silver tungsten contacts (with different proportions) can be seen as follows:

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The geff=30/7 isotropic signal in the EPR of high-spin Fe3+ compounds is discussed for both tetragonal rhombic crystal fields by reference to the resonance in calcium tungstate. The Fe3+/CaWO4 EPR signals are shown to arise from Fe3+ in two slightly different interstitial sites in the crystal; the crystal field in both sites is predominantly rhombic. Highly anisotropic signals arising from transitions within the other two Kramers' doublets were positively identified leading to an unambiguous calculation of the spin Hamiltonian parameters. The results are likely to be of great value in analysing similar Fe3+ spectra in crystals of biological importance.

 

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