2021年10月14日星期四

Solid Phase Chemical Method Composing Nano Tungsten Oxide

Nano tungsten oxide is considered to be one of the most promising new oxides gas sensing materials that detects nitrogen oxides, hydrogen sulfide, and acetone and other harmful gases.

The preparation methods of nano tungsten oxide mainly include hydrothermal method, sputtering method, sol gel method and gas phase deposition method. Compared with other synthetic methods, as a new method for composing nano-materials, low heating solid phase chemical reaction has the characteristics of simple operation, friendly environment and mild conditions. 

Mix sodium tungstate and sodium sulfate for 30min, the sample was placed in a cone bottle and placed for 24h to obtain the precursor. Then put the precursor calcined at 600°C in the muffle furnace 2h, nano tungsten carbide was obtained. 

nano tungsten oxide image

Based on the product of nano tungsten carbide test, the material in the lower working temperature of low concentration acetone gas showed the performance of fast response, high selectivity and high sensitivity. This material can be used for the detection of early diabetes, in addition, under the UV light and simulated sunlight conditions it has better performance for the degradation of methylene blue solution, the degradation efficiency of not less than 70%, these excellent properties concern with nano tungsten carbide smaller particle size and surface activity. 

Solid phase chemical synthesis of nano tungsten carbide can be obtained with high purity, good crystallinity and particle size of tungsten oxide nanoparticles, this method overcomes some traditional experimental hard conditions, the tedious experiment process and long reaction time, it is a simple and efficient, new green composing method.

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Tungsten Titanium Thin Films Electrochromic Properties

Electrochromic is the phenomenon that the optical properties of materials change stably and reversibly under the action of applied electric field.

It can cause light penetration through the electric field, changing color with the environment, and no longer need to curtain.

WO3 thin film is one of the most promising electrochromic materials. WO3 thin films have been widely used in photochromic, photochromic, filter, dye sensitized solar cells and so on. 

With the implantation and extraction of ions, the dissolution and ion residues of WO3 thin films will occur, and the electrochromic properties will decline obviously, which limits the wide use of electrochromic devices. TiO2 has excellent stability, and has excellent transmittance in wide waveband. The doping of electrochromic films with TiO2 composite WO3 can improve the stability and lifetime of devices. 

titanium tungsten film image

Pure Ti/WO3 composite films were prepared by radio frequency sputtering method, tungsten titanium alloy with tungsten and titanium content of 5% is used as target material. Indium tin oxide (ITO) conductive glass was selected for thin film substrate. The chamber is pre evacuated to 1 x 10–5 Pa before entering the gas. High purity argon (Ar) was used as working gas, high purity oxygen (O2) was used as reaction gas, base pressure was 0.5 Pa, O2/Ar was 0.15, target base distance was 8 cm, sputtering power was 300 W, sputtering time was 40 min. Finally, Ti/WO3 films were prepared by annealing for 2 hours under N2 atmosphere.

Electrochemical testing results show that Ti doping can improve the reversibility of ion implantation / extraction and improve the cycle stability of the thin films. At the same time, the response speed and optical modulation performance of the films were also improved. The response time of the doped and bleached thin films was reduced from 9.8 and 3.5 s to 8.4 and 2.7 s, respectively. Therefore, the Ti doped WO3 thin films had better electrochromic properties.

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Powdery White Tungstic Acid and Its Application

Tungstic acid can be roughly divided into yellow tungstic acid and white tungstic acid.

More and more practice has shown that tungstic acid not only has an important use in tungsten chemistry, but also has great value in tungsten metallurgy.

Powdery white tungstic acid can compose tungsten niobium and tungsten tantalum Heteropoly salt. Tungsten phosphoric acid and tungstic acid can also be composed. The heteropoly acid prepared from powdered white tungstic acid has better catalytic performance than that prepared with tungstic acid. 

white powdery tungstic acid image

The preparation of white tungstic acid is to drop Na2WO4 solution into dilute acid. In the middle of the process, there is no acid anion, so the precipitation rate is high. In the Na2WOsolution tungsten exists as monomeric WO42-form. The presence of dilute acid immediately results in protonation and formation of tungstic acid precipitation. Generally, the prepared white tungstic acid is a mixture of mono - tungstic acid and poly - tungstic acid. When the mono tungstic acid was converted into poly acid, the tungstic acid was converted into colloid, and its activity was similar to that of yellow tungstic acid.

Pour out method for impurity removal of white tungstic acid. The 0.5-0.8m / L Na2WOsolution was dripped into H2SO4 or H N solution until the pH value <=1, the powdery white tungstic acid can be obtained with a precipitation rate of 100%. When the P, A, s or Si in Na2WOexist in less than 30 m g / L, the resulting white tungstic acid does not contain these three impurities, and these impurities are dissolved in water in the form of twelve tungsten heteropoly acid.

In the preparation of ammonium tungstate, ammonium tungstate, tungsten phosphorus heteropoly acid, white tungstic acid is much better than that of yellow tungstic acid, and the yield is almost 100%. Considering the low acidity and high yield of white tungstic acid, the advantage of white tungstic acid in tungsten metallurgy is more obvious.

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Super Wave Absorbing Material—Nanometer Tungsten Disulfide/Fullerene

Microwave absorbing material is an important stealth form in stealth technology.

High temperature absorbing composites with new wave absorbing materials should be a kind of structure and function integrated material, which can replace metal parts and realize dual functions of bearing and stealth.

Fullerene is characterized by cage-like structure, hollow in the middle, and the most common fullerene is C60. Because of the particularity of inorganic fullerene hollow cage-like structure, it is found that the inorganic fullerene’s structure has excellent resistance to compression and wave absorption. Its maximum compressive resistance is up to 25Gpa, which is the most hard molecular cage structure found recently, and its compressive resistance performance is better than other carbon cage like structures. Even when it under lower pressure, the collapse or transformation to other diamond structures will occur. Therefore, when inorganic fullerene structure is used as wave absorbing material, it can withstand more hard load conditions, such as high temperature oxidation. 

F35 absorbing materials image

The reason why nano tungsten disulfide / fullerene has excellent absorbing electromagnetic wave property lies in that: (1) nano tungsten disulfide / fullerene particles have small size and large specific surface area, the quantum size effect causes the splitting of the electronic energy levels, and the splitting energy levels are in the energy range corresponding to the microwave, which creates a new absorbing channel for the materials. (2) The atomic and electronic properties of tungsten disulfide / fullerene increase the efficiency of electromagnetic energy conversion to thermal energy, thus increasing the absorption properties of electromagnetic wave. (3) nano tungsten disulfide / fullerene has higher coercive force, it can cause large hysteresis loss, and it is conducive to the absorption of radar waves and other heat energy conversion and consumption.

In summary, inorganic fullerene/ nano tungsten disulfide is an excellent microwave absorbing material. It is not enough to use stealth materials only against single frequency bands. Only a wide spectrum stealth material that can resist a variety of detection instruments can meet the needs of military equipment stealth. Therefore, the stealth material in the military field is a more complex material system engineering.

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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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Solid Phase Chemical Method Composing Nano Tungsten Oxide

Nano tungsten oxide is considered to be one of the most promising new oxides gas sensing materials that detects nitrogen oxides, hydrogen s...