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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Tungsten copper electrode can be classified into 3 types.

1. Tungsten copper (Class 10) is used for flash and butt welding electrodes in applications in which high heat resistance, higher electrical and thermal conductivity, high malleability, and low thermal expansion are required. This metal is created by combining 45% copper and 55% tungsten, resulting in a dense and hard metal with superior wear resistance and strength.

2. Tungsten copper (Class 11) contains 25% copper and 75% tungsten. Like Class 10, it has superior wear resistance and strength and good thermal and electrical conductivity. However, it is harder than Class 10 and used in applications that require moderate pressure.

Common uses include projection welding electrodes, flash and butt welding electrodes, light upsetting and seam welding bushings, and spot welding low conductivity steels. It is also widely used in chip carriers, substrates, flanges, and frames for power semiconductor devices.

3. Tungsten copper (Class 12) contains 20% copper and 80% tungsten, being stronger than Class 10 and 11 tungsten copper, it is commonly used in heavy duty projection welding electrodes, and cross wire welding. It is also used in the liners of some specialty shaped explosive charges for the defense and oil and gas industries.

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The electron spin resonance spectrum of the S-state ion Gd3+ in a calcium tungstate host lattice has been examined at 37.5 GHz over the temperature range 290 K to 4.2 K. Low concentration Czochralski-grown single crystals having gadolinium concentrations of about 100 ppm were used to ensure substitution by Gd3+ in calcium sites; the crystals were vacancy compensated. Although the general features of the 290 K spectrum agreed well with that reported previously by Hempstead and Bowers, extra lines were observed very close to the main ΔM=1 transitions. Examination in theφ-plane showed that the extra lines were most pronounced nearφ=55° and that their intensites relative to those of the main transitions increased as the temperature was reduced. The extra lines are explained in terms of the ordering of oxygen vacancies.

 

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Gamma irradiation of calcium tungstate at 77°K produces two paramagnetic species in high yield. Measurements of yields and of rates of disappearance upon warming indicate the two species are formed and disappear upon warming in one to one correspondence. The principal axis directions and gtensors have been measured and indicate that one species contains a surplus electron while the other is electron‐deficient (hole). Hyperfine effects of W¹⁸³ have been observed. The electron‐deficient species contains two tungsten atoms with small isotropic hyperfineinteractions. Its unpaired electron must be highly localized in orbitals of atoms other than tungsten. The electron‐surplus species contains one tungsten atom with an anisotropichyperfineinteraction. This center could be WO₄ ^3-, but from the lack of symmetry of the measured g values it must be formed near a lattice defect. Experiments with heat‐treated crystals indicate lattice defects to be important in the radiation effect.


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The energy levels of the ⁵I ground term of Ho³⁺ in single crystals of CaWO₄ were established by absorption and fluorescence spectra measurements. The measurements were made using crystals at temperatures near 2, 20, and 85°K. Most of the observed transitions can be accounted for by the electric dipole selection rules. Magnetic dipole selection rules cannot be ruled out for some of the lower energy transitions, however. Calculations were made to determine the effects of the crystalline host material on the Ho³⁺ energy levels. An effective Hamiltonian of the form suggested by Karayianis was diagonalized in a basis of Russell–Saunders wavefunctions to obtain the calculated energy levels and wavefunctions for Ho³⁺. Such a calculation takes into account the complete J mixing of the states within the ground term and is equivalent to determining the effects of the spin–orbit interaction to better than second order. In the calculation, an rms deviation of 10 cm−1 was found as the best agreement between the theoretical and experimental energy levels using the Hamiltonian, H = λ1(L⋅S) + λ2(L⋅S)² + λ3(L⋅S)³ + ΣlmB+lmClm. The empirically determined parameters yielding this rms deviation are (in cm⁻¹): λ₁= − 487.9, λ₂ = − 6.601, λ₃ = − 0.1801, B₂₀ = 436, B₄₀ = − 664, B₄₄ = 779, B₆₀ = − 33,ReB₆₄ = 558, and ImB₆₄ = 196. The g_‖ factor for the ground state of Ho³⁺ in CaWO₄ was calculated to be 13.675. This is in agreement to within 0.12% of the experimental value of 13.691 ± 0.006 reported by Kirton.

 

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