2021年9月22日星期三

RF Sputtering Method Preparing Nanometer Tungsten Disulfide Membrane

Tungsten disulfide is an excellent solid lubricant material, it has high application value in the field of super solid lubrication.

Tungsten disulfide membrane have layered structure, it has lower hardness and better stability under high temperature, it is suitable to be used as a solid lubricant for friction parts in special environment. There are three main methods for preparing tungsten disulfide membrane which include RF sputtering method, reactive magnetron sputtering method, chemical deposition method.

The method of RF sputtering crystal growth is the method of using RF sputtering to make the components of the crystal material gasified and then recrystallized to grow the crystal. RF sputtering is a sputtering deposition method suitable for all kinds of metal and nonmetal materials, the frequency range is 5~30MHz, the frequency of 13.56MHz is commonly used internationally. It is mainly used to prepare thin membranes and also to prepare small size crystals.

RF sputtering method image

WS2 powder with 99.99% purity was prepared by cold pressing to form WS2 target, the surface of the stainless steel substrate is mechanically polished, and the surface is cleaned by ultrasonic wave. The background vacuum is 3 x 10-3Pa, fill in argon to the working pressure 1Pa, sputtering power 30W, deposition time for 1hour, depositing WS2 on a substrate to form a thin membrane.

The surface of tungsten sulfide membrane prepared by RF sputtering method is smooth, no voids and gaps were observed under SEM, it has high density and good quality, the surface morphology of the samples with different working pressure, different sputtering power and deposition time is similar, and no obvious changes.

Compared with other sputtering methods, RF sputtering method has a wide range of target materials, better advantages of high deposition rate, the RF power can be effectively input by adjusting the discharge impedance and the power impedance matching.

The membrane obtained by this method are generally amorphous. But by strictly controlling the sputtering pressure, power and other technological parameters, the sputtering method is still an important method for the preparation of the laboratory or the production of small workpieces.

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Review on the Effectiveness of Tungsten Disulfide and Molybdenum Disulfide Lubricant

 Solid lubrication is a method of coating or plating on the surface of the friction pairs to form a thin film and reducing friction.

tungsten disulfide and molybdenum disulfide image

The technique of using solid lubricants to lubricate is called solid lubrication. Tungsten disulfide and molybdenum disulfide are excellent solid lubricating materials, in the field of ultra-solid lubrication has a very high value, this technology is applied to the hard disk drive bearing of computer, such as rotation and sliding in ultra high vacuum system.

Molybdenum disulfide has a graphite-like structure, molybdenum and sulfur through the covalent bond to form a hexagonal structure, each molybdenum is surrounded by six sulfur atoms, the molecular layer surface only sulfur atoms, each layer molecular layer thickness of 0.626nm. The thermal stability of MoS2 is better in the air environment lower than 400 degrees. When the temperature is higher than 400 degrees, MoS2 will be oxidized to MoS3 in a short time, MoS3 is a granular material with higher hardness, attached to the friction surface of abrasive formation. But in a vacuum and inert gas environment, the structure of MoS2 at 1100 DEG C can keep steady.

Tungsten disulfide WS2 also has a hexagonal lamellar structure similar to molybdenum disulfide. Tungsten atoms and sulfur atoms covalently bonded, the molecular layer surface only sulfur atoms, between the sulfur atoms between the weak molecular bonds. WS2 is insoluble in almost all media, including acids, alkalis, oils and water, but is sensitive to free gaseous fluoride, hydrofluoric acid and hot sulphuric acid. Compared with MoS2, the thermal stability of WS2 is better than that of MoS2, and the decomposition temperature of WS2 is 510 °C in the air. In 539 degrees will be rapid oxidation, decomposition temperature in vacuum and inert gas is 1150 DEG C. Compared to the molybdenum disulfide, more widely applicable temperature range WS2.

Several domestic and foreign lubricant manufacturers have tungsten disulfide, molybdenum disulfide and graphene three lubrication performance comparison, through the test found that tungsten disulfide is not only stronger than molybdenum disulfide, stronger than graphene. In addition, in the high radiation operating environment, tungsten disulfide has a higher anti-radiation properties, can adapt to more complex operating environment, therefore, the major manufacturers are tungsten disulfide lubricant showed a more intense interest in production.

In the United States, aerospace industry, mainly used as MoS2 solid lubricating material, this is because the United States is the production of molybdenum power, convenience with local resources. In China, we have not only molybdenum more tungsten, relative to molybdenum, tungsten, whether it is heat resistance, physical and chemical stability, or radiation resistance are better than molybdenum, so tungsten disulfide lubricant will be better choice.

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Electrical Properties of Tungsten Disulfide

The structure of tungsten disulfide is a hexagonal layered structure with excellent electrical properties.

In tungsten disulfide, the interaction between the W atoms in the layer and the surrounding S atoms is strong, and the interaction between the tungsten disulfide layer and the layer is weak.

monolayer of tungsten dioxide image

The thermal stability of two tungsten sulfide is stronger than that of molybdenum disulfide. The temperature range is wide, and it is not easy to decompose. Oxidation occurs at 539 degrees centigrade. Therefore, tungsten disulfide is an excellent semiconductor material for indirect bandgap semiconductors. When the thickness of tungsten disulfide is reduced to a single layer, it changes into a direct band gap, which means that tungsten disulfide can effectively pass through the bandgap transition absorb or emit photons. The band gap of monolayer tungsten disulfide is about 2eV. This electrical performance characteristic is the main advantage of tungsten disulfide in semiconductor tubes, transistor applications that can go beyond graphene.

In fact, graphene has a very important defect in the field of semiconductors, which is the lack of band gap between electrons. In theory, graphene has very high mobility electrons that allow it to process data at very high rates, although it's very fast, there's no electron gap, and it's hard to turn it off once it starts to transmit data, this seriously hinders the logical operation, because all the problems in logic operations are open and close. While the two tungsten sulfide with planar high current switch, a higher than the mobility and the effective conductivity modulation performance, there is no logic problem, which can completely replace graphene in transistors, lighting diodes and optical sensors and other devices. The future, even now produce more than 100 times the calculation function of CPU, the use of the material is probably two tungsten sulfide.

In addition, tungsten disulfide also has a very strong photoluminescence properties, can replace the traditional transparent electrode materials and graphene, as a liquid crystal display, touch screen, solar cells and other equipment, transparent electrodes, with the addition of tungsten disulfide, the future of such equipment or will have a folding function.

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Tungsten Carbide / Cobalt Sprayed Coating

Thermal sprayed tungsten carbide / cobalt cermet is widely used in aerospace, metallurgy, machinery and other fields due to its good hardness and toughness.

tungsten carbide / cobalt sprayed coating image

Thermal spraying (spray welding), surfacing and other processes are gradually becoming the commonly used alloy, steel material anti-wear and surface corrosion technology measures. Tungsten carbide / cobalt thermal spraying technology has a simple process compared with other thin film and coating preparation technology, the coating and substrate have a wide range of selection, a large range of coating thickness, high deposition efficiency and easy to form composite coating. 

The wear resistance of alloy is a systematic engineering problem. The material is only an important aspect to measure the grindability. In addition, it is also related to the stress state, surface structure, lubrication conditions, environmental temperature and other factors. The characteristics of thermal spraying powder vary with the milling process. The usual preparation methods include sintering crushing method, agglomeration sintering method, mixing method, coating method, melting method, etc..

The powders prepared by sintering method are generally rough and angular shape, and the particles are compact inside. A large number of WC particles are distributed in the Co matrix, and a single WC particle is combined with the Co matrix. Agglomeration sintering is a kind of solid aggregate particles containing organic binder in the liquid medium. The WC powder and Co powder are mixed evenly and spray drying is formed. Finally, the heat treatment is carried out. 

The mixed powder prepared by WC and Co phase, the WC particles were loose co polymerization together; coating on the surface of WC particles is coated with cobalt form; melting method is made by crushing and melting casting method, powder particle size is generally big. 

With the progress of nanotechnology, a new spraying process cold spraying has been developed in recent years. The nanostructured WC/Co coating prepared by cold spraying has no negative effect on decarburization, there is no porosity, and there is no gap and crack between the matrix and it has good bonding property. Cold spraying is a 100% solid state process without particle melting, which can solve the problems of decarburization, phase transformation and re solidification of nano coating melt particles during the conventional process.

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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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