Firstly, the analysis of vanadium dioxide (VO₂) XRD and the phase transition temperature. After analysis XRD patterns found VO₂ film crystallinity is not high and smaller particle size. According to the nature of VO₂ which is when the phase changing will cause sudden change of resistivity found vanadium dioxide film will cause phase changing in temperature about 62 ℃.And the resistance change in this process very largely which showed significant calorific value phase change performance, compared to the powder, the phase transition temperature is reduced by about 6 ℃. After many researches the scholars find using microwave plasma method with doping nitrogen to produce tungsten-doped vanadium oxide films has good performance which compare to VO₂ film. On one hand, tungsten-doped vanadium oxide films have low phase transition temperature. On the other hand doping also will reduce the phase transition temperature .so doping nitrogen will good for tungsten-doped vanadium oxide films performance. But analysis the XRD found litter different with VO₂. 

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Mark of tungsten contacts
Each batch of tungsten contacts must be identified and marked:
The type of tungsten contacts and the number of the standard;
Specifications;
Production batch;
Quality;
Production Date;
With manufacturers and related inspection department mark.

Package of tungsten contacts
Firstly packaging tungsten contacts with plastic bags, and then putting them in plastic boxes or metal boxes.
Tungsten contacts with the same batch number, size and type mount on the same box.

Transport of tungsten contacts
Boxes used to hold tungsten contacts need to reinforce and in which put the moisture-proof of paper.
Tungsten contacts in transit should prevent violent impact, corrosion, and must not expose them in acid rain and snow.

Storage of tungsten contacts
Tungsten contacts storage location should be dry and free of acid, alkali and other corrosive gases.

 

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Ammonium tungstate is also called ammonium wolframate, its molecular formula is (NH4)6W7O24·6H2O. It is colorless trapezius crystal which is soluble in water and insoluble in alcohol. Under 100℃ it loses four molecular crystal water. Ammonium tungstate can be used as the raw material for ammonium phosphotungstate and other tungstate compound. It can also be applied in producing metallic tungsten and catalyst.

Ammonium paratungstate is a white crystalline salt of ammonium and tungsten, it is insoluble in water. Its shape is like sheet or needle. Ammonium paratungstate is used as a source for high-purity tungsten oxides, tungsten metal powders, carbides, or as a laboratory reagent. Also used in the manufacture ammonium metatungstate and other tungsten compounds as additives for the petrochemical industry.

Ammonium metatungstate is a tungsten chemical in the form of highly soluble hydrated crystals, solubility is 303.99/loogH:O when 20℃. It is white or slightly yellow crystals which has no harmful components and slightly acidic. Ammonium metatungstate is very versatile and can be used as catalysts, capacitors, nuclear shielding, flame retardants, corrosion inhibitors raw materials, but also excellent raw materials in the preparation of ultra fine tungsten powder, tungsten heavy alloy powder, phosphotungstic acid, arsenic acid, silicon tungstate.



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The structure and morphology of ammonium metatungstate (AMT), (NH4)6[H2W12O40]⋅4H2O, and its thermal decomposition in air and nitrogen atmospheres were investigated by SEM, FTIR, XRD, and TG/DTA-MS.

The thermal decomposition of ammonium metatungstate involved several steps in inert atmosphere: (i) release of crystal water between 25 and 200 °C resulting in dehydrated ammonium metatungstate; (ii) formation of an amorphous phase between 200 and 380 °C, (iii) from which hexagonal WO3 formed between 380 and 500 °C, and (iv) which then transformed into the more stable m-WO3 between 500 and 600 °C. As a difference in air, the as-formed NH3 ignited with an exothermic heat effect, and nitrous oxides formed as combustion products. The thermal behavior of ammonium metatungstate was similar to ammonium paratungstate (APT), (NH4)10[H2W12O42]⋅4H2O, the only main difference being the lack of dry NH3 evolution between 170 and 240 °C in the case of AMT.



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In the step of the leaching of wet ammonium paratungstate, the wet ammonium paratungstate is reacted directly with nitric acid to produce a dilute solution of ammonium metatungstate. In comparison with the prior art, the calcining process is prevented, so it is no longer necessary to use the rotary furnace equipments, thus the fixed investment is reduced, electric power needed is reduced by 120 kWh per ton of product, actual yield can be increased by 3%, and production cost is lowered. 

In the step of the spray-drying of the concentrated solution of ammonium metatungstate, the drying conditions (for example: inlet air temperature, outlet air temperature, and feeding speed) are controlled, such that the concentrated solution of ammonium metatungstate is rapidly dried into a powder with a high speed and a high efficiency, as a result, the ammonium metatungstate product has good flowability and good solubility as well as high purity. The ammonium metatungstate product produced according to the present invention has overcome the shortcoming of low solubility of the ammonium metatungstate produced by the prior art. 



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Below are the further descriptions of preparing steps:

A.The leaching of wet ammonium paratungstate: Pure water is added to a reactor and preheated to boiling; wet ammonium paratungstate [5(NH₄)₂O12WO₃₅H₂O] is added slowly to the pure water at a speed of 150 to 200 Kg/h while stirring; when adding the wet ammonium paratungstate, a 1:3 dilute nitric acid (HNO₃) is added slowly to control the solution at pH 3 to 5; the above process is stopped when the WO₃ content of the solution reaches 150 g/l; After boiling and being stirred for half an hour, the materials are allowed to pass through a suction filter for natural filtration, in order to produce a dilute solution of ammonium metatungstate. 

B.The conversion of dilute solution of ammonium metatungstate to concentrated solution of ammonium metatungstate: the dilute solution of ammonium metatungstate from step A is standing aged for 4 hours, followed by a primarily natural filtration to obtain a filtrate 1; the above filtrate 1 is pumped into a reactor in multiple times for heating to concentrate it, each time the filtrate 1 is pumped, a dilute ammonia water (NH₃H₂O) of 1:5 is used to adjust the solution at pH 3 to 4; when the density of the solution reaches 2, the pumping of the filtrate 1 is stopped, following 1 hour of cooling, the materials are allowed to pass through a suction filter for a secondarily natural filtration to obtain a filtrate 2; the above filtrate 2 is pumped into a settling tank, standing aged for 8 hours, then the materials are allowed to pass through a suction filter for a tertiarily natural filtration to obtain a concentrated solution of ammonium metatungstate. 

C.The spray-drying of the concentrated solution of ammonium metatungstate: The concentrated solution of ammonium metatungstate from step B is dried into a powdery product of ammonium metatungstate by the use of homemade advanced high-speed centrifugal spray-drying equipment or a spray atomizing dryer. The conditions for spray-drying are controlled at: inlet air temperature 220 to 280° C., outlet air temperature 110 to 160° C., and feeding speed 300 to 400 kg/h. 


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Ammonium metatungstate is an important tungsten-containing compound, mainly used as catalyst in petroleum cracking, organic synthesis, nitration, and so on. With the rapid development of petroleum
refining, petrochemical engineering and other industries, the ammonium metatungstate usage is increasing very fast. In the prior art, ammonium paratungstate is used as a raw material, and a crystallization process is utilized for the preparation of ammonium metatungstate; this process is expensive, costly and energy-consuming, and the obtained product is unstable in quality, particularly the solubility is poor.

A process for preparing ammonium metatungstate using ammonium paratungstate as raw material comprising: (A) leaching wet ammonium paratungstate with nitric acid to obtain a dilute solution of ammonium metatungstate; (B) concentrating the dilute solution of ammonium metatungstate to be a concentrated solution of ammonium metatungstate; and (C) spray-drying the concentrated solution of ammonium metatungstate to obtain powder of ammonium metatungstate.


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Spark plasma sintering method (referred SPS) using special pulsed DC current producing discharge plasma between tungsten oxide particle, so that the particle surface activation to give a good performance WC in the materials synthesis and rapid low-temperature sintering has a unique advantage. Besides it has a very significant technology condition advantages. For example: uniform heating, low sintering temperature, sintering time is short, high production efficiency, product organization even small and easy to operate. SPS is currently in thermoelectric materials, magnetic materials, functionally graded materials, composite functional materials and nano-materials and so achieved good results. Tungsten oxide using SPS to produce tungsten carbide the producing process as follows:
1. Producing the carbon content is 12% -17% tungsten oxides and carbon black mixture, while using thermal analyzer the reaction mixture of tungsten oxide and carbon black.
2. ball-milling in drum mill with ethanol for wet grinding 48 hours, drying in 100 ℃, sieving 149um sieve, charged in a graphite mold.
3. Sintered in SPS-3.20-MV type discharge plasma sintering apparatus.
4. Using optical microscopy and scanning electron microscopy analysis samples.

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Spark plasma sintering method (referred SPS), the technology is in between powder particles directly into the heating pulse current sintering. At the beginning the SPS technology has low productivity problem which has not been widely used. In recent years, with further research SPS as new technology for preparing material has attracted attention.
Tungsten carbide (WC) due to its high hardness, high flexural strength and excellent fracture toughness in many areas is widely using in many area. But the key to the producing high-performance WC is the grain size of the raw material powder. Traditional WC production process is: ammonium paratungstate →tungsten oxide → W → WC → ingredients → sintered block. But this process step is complicated and not easy to control the particle size. And using SPS producing WC is by pulsed DC current to produce discharge plasma between the tungsten oxide powder particles, so that the particle surface activation to give a good performance WC in the materials synthesis and rapid low-temperature sintering.

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After analysis using blue tungsten oxide to produce tungsten powder by cyclic oxidation found the tungsten powder particle size is small, and good performance. From the experimental data show that after the first reduction tungsten powder‘s specific surface area is 0.119m2 / g which is 1.95times to blue tungsten oxide’s. From this we can see that tungsten powder particle size is smaller than the blue tungsten oxide and distribution is more concentrated, but there is a certain amount of coarse particles of tungsten powder. In the second reduction which is using obtained tungsten powder in the air restore to tungsten trioxide then using hydrogen to make the second reduction. And the tungsten powder which obtained from the second reduction is finer. Comparing tungsten powder of the first reduction and the second reduction find in the second reduction there has more fine tungsten powder than first reduction, its particle size distribution is more uniform, but there are a small number of coarse particles of tungsten powder. The second reduction of tungsten powder specific surface area is 0.180m2 / g which is the first reduction of tungsten powder of 1.51 times and its diameter mainly in the 3 ~ 10um. It can be found using cyclic oxidation method obtained tungsten powder particles finer than directly by using hydrogen reduction, besides the distribution is more concentrated.

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