2021年9月6日星期一

Decoloration Mechanism of Double Doped Zinc Tungstate

The modification of zinc tungstate by doping rare earth elements is an effective way to improve its luminescent properties.

The single crystal of zinc tungstate is rose red, the color makes the luminous efficiency decrease and the decoloration effect is obvious.

After annealing for a long time, the ZnWo4 crystal will disappear gradually, but the annealing of the bulk crystal still has difficulties which can not be overcome. The main problem is that the defect is incomplete and the defects are introduced into the annealing process. 

double doped zinc tungstate image

In single doped rare earth crystals, only cerium doped Ce3+ can recede the color of the crystal, both doped cerium Ce3+ and lithium li3+ have obvious decoloration effect on zinc tungstate crystal, especially, the former can obtain colorless crystal by proper doping.

The removal mechanism of zinc tungstate may be due to the decomposition of CO2 in Li2CO3 crystals, CO2 molecules will be partially retained in the lattice, CO2 molecules and O2 molecules are linear molecules, and the radius of the two has little difference, can replace each other in the hole. Therefore, when the crystal grows, the CO2 molecules fill in the oxygen vacancies to make the crystal clear.

Through the mechanism of doped zinc tungstate, it can be seen that the segregation coefficient of Ce3+ and Sm3+ in ZnWo4 crystal increases after double doping, it is beneficial to obtain high content of crystals, thereby changing the short life of fluorescence and the quenching temperature of luminescence is too low.

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Toughening Properties of Silver and Nickel Contacts Evaluation by Tungsten and Tungsten Oxide

In life, silver-nickel contact materials are widely used in relays, circuit breakers for low current ratings because of their good ductility and low contact resistance.

When current levels exceed 20 A, conventional AgNi contact materials exhibit weaker resistance to soldering and erosion, limiting their range of application. However, by adding a small amount of additives, the electrical properties can be improved, thus expanding its application area. Some scholars have studied that adding tungsten or tungsten oxide can improve the performance of AgNi contact materials. 

silver nickel contact image

The W or WO3, which accounts for 1.5% of the total, was made into silver-nickel contacts (containing 85% of silver content) by powder mixing, isostatic pressing, sintering, extrusion, subsequent wire drawing and riveting. Each contact material was repeated 5 times using the same resistive load in the test. The current and mechanical structure ensured that all tests had comparable arcing energy with supply voltages and load currents of 220 V and 25 A, respectively. After the 5000 operation, the contact is removed from the equipment and cleaned by ultrasonic in alcohol. After drying, the weight is weighed and the weight loss is calculated. 

The comparison of test results shows that the two different additive silver and nickel contact materials with tungsten powder or tungsten oxide have no obvious difference in the microstructure of the material, but there are big differences in the electrical properties such as material transfer and arc erosion. In the simulation of electrical performance test, the tendency of the addition of WO3 is that the mass loss and the microstructure of arc-affected zone are worse than that of tungsten powder. 

The addition of 1.5% W in AgNi contact materials can reduce the maximum welding power and reduce the arc erosion compared with 1.5% WO3. Therefore, tungsten powder may be better applied at low current level AC contactors.

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Tungsten Phosphide Catalyst Desulfurization

The contradiction between the increase of organic nitrides and sulfides in oil and the heavy crude oil.

The strict environmental protection regulations are becoming more and more serious, so it is necessary to conduct deep hydrofining for oil products.

The research shows that although the traditional molybdenum sulfide based catalyst is effective in hydrotreating process, it is far from meeting the requirements of deep hydrodesulfurization. The current approach is to retrofit existing catalysts and find new catalysts that can be used to meet the need for deep hydrotreating of crude oils.

desulfurization of tungsten phosphide catalyst image

In 1990s, the synthesis and performance of tungsten nitride hydrodesulfurization catalyst have been reported abroad, and the structure and desulfurization performance of P, Ni, W catalyst with phosphorus as auxiliary agent have been studied. However, the research on tungsten phosphide seems to be few. Therefore, the Chinese scholars also try to use tungsten phosphide for hydrodesulfurization test, so as to evaluate the non phosphorus catalytic effect.

The catalyst active component tungsten phosphide was synthesized by the method of temperature programmed reduction of phosphotungstic acid with high purity hydrogen. The PW-r-Al2O3 catalyst was prepared by reduction and mixing with r-Al2O3 as the carrier and tested.

The results show that the specific surface area of the tungsten phosphide is 7.228m2/g, and the specific surface area of tungsten phosphide catalyst Al2O3 is 105.543m2/g, which has better thiophene desulfurization performance. When the pressure is 3.0MPa, the space velocity is 4h-1, the volume ratio of hydrogen to oil is 1000 and the temperature is 340 °C, the thiophene hydrodesulfurization rates of the catalysts are respectively 90.2%. In addition, the stability of the catalyst was investigated. When the mixture was injected (thiophene 0.8%, pyridine 1.2%, cyclohexane 76%, octane 2%, cyclohexene 20%), the quality of the catalyst was all mass fraction, under the same reaction conditions, the denitrification rate and the desulfurization rate of the catalyst were only decreased by about 2%, which indicated that the PW-r-Al2O3 catalyst had good stability after 105h.

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Nickel Tungsten Nitrogen Catalyst for Its Catalytic Strength

With the continuous strengthening of human environmental awareness, the quality of living environment has aroused widespread concern.

All countries have formulated strict standards for the quality indicators of sulfur, nitrogen and other harmful substances in petroleum products. The clean production process and green products have become the mainstream trend in the development of petroleum refining and chemical production. In recent years, the transition metal nitrides hydrotreating catalyst has become a hot topic in the field of new catalysts because of its excellent deep hydrodesulfurization, denitrification activity and selectivity.

Some scholars believe that the current research on oil desulfurization catalyst is too focused on the cobalt molybdenum series earlier, and has excellent mechanical and thermal stability and anti poisoning characteristics of nickel tungsten catalysts study less, but in fact, the nickel tungsten nitrogen catalyst has more potential applications. 

nickel tungsten nitrogen catalyst image

By using temperature programmed reduction technology and r-Al2O3 as carrier, the nickel tungsten hydrofining catalyst NiWN/r-Al2O3 with different metal loading was prepared by impregnation method, and its hydrodesulfurization test was carried out.

 The size of the desulfurization activity level and surface acidity of catalyst was correspondence, namely catalyst surface acidity high desulfurization activity of catalyst, desulfurization activity of small low surface acidity. The surface acidity of catalysts, in addition to direct hydrogenolysis and hydrogenation desulfurization desulfurization, isomerization and cracking degree of strengthening, for deep desulfurization, and high desulfurization activity. The test found in NiWN/r-Al2O3 than the surface of the product is 149.5, surface acidity was 0.205, in the process of cracking gasoline showed good catalytic performance, surface acidity of benzothiophene on two is enough to produce catalytic desulfurization effect, the desulfurization rate is only about 91.2%. In this regard, scholars believe that nickel tungsten nitrogen catalyst can be used for catalytic desulfurization of oil, but there is still room for improvement and improvement.

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2021年7月28日星期三

Chemical Deposition Method Preparing Monolayer Tungsten Disulfide

Monolayer tungsten disulfide is a kind of nano material with layered structure, it has better temperature stability than monolayer disulfide.

It has attracted much attention in recent years because of its better optical and electrical properties, in the semiconductor field such as transistors, tungsten disulfide is an indirect bandgap semiconductor, forbidden band width is  2.0eV, it has more space than the application of graphene in the field of electronic tube. At present, the most important methods to prepare monolayer tungsten disulfide in China are three methods: RF sputtering, reactive magnetron sputtering and chemical deposition.

In recent years, the research on the preparation of two-dimensional TMDCs by CVD(chemical vapor deposition) method is mainly focused on molybdenum disulfide, because MoS2 requires lower temperature and it is easy to control the growth process, tungsten disulfide seems much more complex.

monolayer tungsten disulfide image

There are two main ways to prepare tungsten disulfide film by CVD method in laboratory: one step reaction method and two step method. One step reaction is the method that directly make the reaction of S source with W source to generate WS2 and deposit on the target substrate in CVD furnace. The two step method is to pre deposit a layer of metal W film on the target substrate, the WS2 film is then synthesized by sulfidation in CVD furnace. The above two CVD methods do not perfectly solve the large-area controllable monolayer and minority layer WS2 films.

Overall, growth conditions on the preparation of WS2 is critical. In the process of two-dimensional WS2 prepared by CVD method, it has more stringent requirements on the instrument and growth process, and the cost of the experiment is high. We believe that in the near future, WS2 film development bottleneck will soon be widened to promote its device and industrialization process.

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Application of Monolayer Tungsten Disulfide Film in Photoelectric Field

Two dimensional transition metal chalcogenide (TMDCs) is considered as a potential star material for future applications of electronic devices because of its graphene like structure.

The current research more semiconductor materials are tungsten disulfide WS2, molybdenum disulfide MoS2, MoSe2 molybdenum selenide, WSe2 tungsten selenide and so on. Among them, WS2 has the most promising application in electronic devices because of its bipolar electron transport properties.

Single layer two W is a kind of layered material similar to graphite with a layer spacing of 0.7 nm. WS2 is an important photoelectric material. When WS2 is changed from bulk material to single layer, the material changes from indirect bandgap to direct band gap semiconductor, and the band gap will change from 1.3eV to 2.0eV, which makes the monolayer tungsten disulfide thin films have important potential applications in photoelectric detectors and solar cells.

Monolayers of tungsten disulfide films show excellent performance in photodetection device research. Some scholars have found that based on 10-layer thick WS2 thin-film optical transistor devices, has the ability to detect monochromatic light of different wavelengths. When the wavelength is 514 nm, the photoresponse time of the device is about 5.3 MS, and the photoresponse rate is about 10 times of that of the MoS2 film. 

tungsten disulfide film image

In the field of solar cells, tungsten disulfide has shown great potential due to its excellent light response and absorption characteristics, some scholars have constructed ITO /WS2 /Au Schottky solar cells. The test results show that visible photocurrent can be produced under visible light irradiation. They also studied the effects of single layer, double layer and multilayer graphene electrodes on Al/WS2/rGO, Schottky cells. The results show that the Schottky contact between graphene and WS2 film is easy to form, so that the built-in electric field is formed at the interface between graphene and WS2 film, the photoinduced electron flows from the WS2 film to the Al electrode, and the hole flows from the WS2 film to the graphene, the maximum photoelectric conversion efficiency of the solar cell is up to 3.3%.

At present, the mass production of single-layer tungsten disulfide film is still difficult, the finished product quality is unstable and the cost is also high, this may be that tungsten disulfide is not as famous as graphene main surface. In the future, it is still a difficult problem for scientists to find a cheap and large area continuous preparation method of WS2 film and guarantee the quality and quantity.

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How to Make Rare Earth Terbium Zinc Tungstate More Dazzling

Rare earth ions are rich in energy levels, their luminescence wavelengths can change from ultraviolet to infrared. They are often used as luminescent centers in luminescent materials.

Metal tungstate is a kind of important inorganic functional materials, including tungsten ore stone structure of zinc tungstate (ZnWO4) is a self luminous material type. WO42- itself has self excitation fluorescence, emits blue green light under ultraviolet irradiation, and can effectively transfer energy to rare earth ions. Therefore, it is widely concerned that in the chemical industry, the self luminescent tungstate doped rare earth element is the most effective LED light source acquisition scheme to change its fluorescence color.

rare earth terbium zinc tungstate fluorescence image

At present, the synthesis methods of rare earth doped ZnWO4 are sol gel method, combustion method, hydrothermal method and microemulsion method. Hydrothermal synthesis is a commonly used method for the synthesis of green phosphors ZnWO4:Tb3+. In the process of hydrothermal synthesis of  ZnWO4:Tb3+ phosphor, adding proper surfactant can greatly affect the structure and luminescent properties of  ZnWO4:Tb3+ phosphor.

In the rare earth doped ZnWO4 system, due to the unequal price of trivalent rare earth ions Tb3+ and Zn2+, Na+ is usually introduced into the synthesized ZnWO4:Tb3+as a charge compensation to enhance the luminescence intensity of the rare earth. In addition, some scholars have studied the effect of surfactants on the luminescence of rare earth zinc tungstate, polyvinylpyrrolidone (PVP), cetyltrimethylammonium bromide (CTAB) and polyethyleneglycol 2000 (PEG-2000) were tested for surface active additives, concluded as follow:

Zinc rare earth terbium tungstate added with different surfactants had almost no effect on the position of the excitation spectrum, but had an influence on the emission intensity. The use of PEG-2000 as a surfactant is most effective for enhancing the luminescence intensity of phosphors. The lifetime of  ZnWO4:Tb3+ phosphors synthesized by using PVP as surfactant is longer than that of PEG-2000 and CTAB as surfactants. Combined with luminescence intensity and fluorescence lifetime, PVP is the most suitable surface additive for rare earth terbium zinc tungstate.

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Decoloration Mechanism of Double Doped Zinc Tungstate

The modification of zinc tungstate by doping rare earth elements is an effective way to improve its luminescent properties. The single cryst...