2021年12月15日星期三

Tungsten Oxide Type

Tungsten oxide is a kind of tungstic anhydride, which is a tungstate product. Tungsten oxide type includes tungsten trioxide and tungsten dioxide. In actual industrial production, it is rare to have tungsten dioxide.

yellow tungsten oxide image

Tungsten dioxide is also known as brown tungsten oxide. The formula WO2 tungsten dioxide is an intermediate transition state of ammonium paratungstate into tungsten trioxide. Its appearance is tan monoclinic powder crystal, and tungsten dioxide is the most commonly known, the role is to manufacture tungsten powder and tungsten trioxide.

Tungsten trioxide, also known as yellow tungsten oxide, chemical formula WO3, is the main tungsten oxide product of our country before the eighties. Yellow powder is insoluble in water, soluble in alkali and slightly soluble in acid. Its density is 7.16 g / cm3; The melting point is 1473 ° C; the boiling point is about 1700 ° C. Mainly used in the system of high melting point alloys and cemented carbide, tungsten wire and fire gas sensitive materials, but also for photocatalyst materials.

blue tungsten oxide image

Blue tungsten oxide is also known as blue tungsten, chemical type W2.90. After the reform and opening up in 80s, China began to produce the blue powder in accordance with the requirements of international orders. Blue tungsten has a certain crystalline form, which is larger than the surface and is easy to be doped. The tungsten carbide powder and tungsten powder are better than those with yellow tungsten. Therefore, after 80s, blue tungsten has gradually replaced the mainstream of the yellow tungsten oxide in the market, and is the main product of the tungsten oxide market.

violet tungsten oxide image

Violet tungsten oxide, also known as purple tungsten, chemical formula 2.72, which in broad terms can be considered a branch of blue tungsten, the appearance is purple or purple red fine crystal powder. Compared with blue tungsten and tungsten, violet tungsten has a special surface and the highest chemical activity. Therefore, ultraviolet tungsten is the most suitable for the production of ultrafine tungsten powder. At the time of increasing the requirements for the performance of cemented carbide, the demand for nanometer tungsten powder is also contending. Therefore, the market of violet tungsten is becoming bigger and bigger.

At present, the tungsten oxide products that are mainly traded on the market are four kinds. Tungsten oxide can only be the final product of tungsten raw material in the whole process of tungsten application, and it is also the main product of China's exports at present.

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The Milling Guide Refinement of Tungsten Trioxide

Ball mill is a fine equipment used in industry and can also be used for mixing.

ball mill tungsten trioxide refinement image

The ball mill is composed of a cylinder and sphere, its working principle is when the cylinder rotates, installed in the cylinder body and the grinding body in grinding by centrifugal force and friction force with the cylinder to rotate to a height, and then automatically fall, the material in the cylinder impact and the grinding and grinding material.

The main factors affecting the efficiency of the ball mill are the speed of the ball mill, the quantity of the ball, the amount of water and electrolyte, the time and the loading. The normal milling time is about 24-72 hours, and the length of the ball milling time directly affects the quality of the ball milling product. Generally speaking, coarse grained material is easier to grind to coffee, and the time is shorter. However, the milling time of gal material to cut below is usually prolonged by several times or longer. However, with the prolongation of ball milling time, the cost of ball consumption and energy consumption will increase correspondingly, and the production capacity of the mill is decreased. Therefore, the appropriate ball milling time must be determined in the actual production.

In the process of ball milling, the ratio of solid to liquid is also a critical process parameter. During the process of mechanical milling, the solid liquid ratio, the shape and size of particles, and the interaction between particles will affect the rheological properties of slurry. The solid-liquid ratio is too low, medium ball received buoyancy, it will reduce the quality of the effective volume, the impact effect is bad, being a medium ball impact the opportunity will be less, so the production efficiency is low, high solid-liquid ratio is opposite, so too high or too low is not conducive to the refinement of tungsten trioxide.

The size of the material ratio directly determines the ball milling fineness, energy consumption, ball consumption and so on. In general, the larger the ratio of the material to the material, the more the relative quantity of the medium, the increase of the impact and the number of shearing of the material, and the enhancement of the effect of refining the crushing.

In summary, mechanical milling can produce high purity tungsten trioxide powder. But the milling conditions have a great influence on the particle size. The ball milling time and the ratio of pellet to particle size tend to be stable. It is better to prolong the time blindly and increase the ball size rather than the thinning effect. On the contrary, it will increase the introduction of impurities.

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Different Crystalline Forms of Tungsten Trioxide Gas Sensitivity Competition

Tungsten trioxide (WO3) is a kind of transition metal oxides, it is not only a kind of N type semiconductor material, is also a kind of "d0" oxide, this dual identity so it has many characteristics.

tungsten trioxide gas sensitive component image

The research shows that there is rod, plate and rod plate three kinds of nano tungsten oxide crystal, gas sensing properties of tungsten oxide with different crystal morphology is slightly different, some scholars use tungsten trioxide for four kinds of common harmful gases were tested and concluded.

(1) The best working temperature of WO3 sensor in ethanol, acetone, ammonia and formaldehyde is 350 ℃, 300 ℃, 325 ℃ and 250 ℃, respectively, and the sensitivity reaches the maximum at gas concentration of 1000 ppm. 21.29, 10.82, 24.32 and 9.68 respectively, and the lowest detection concentration of gas sensors for four gases are 100ppm, 50ppm, 25ppm and 100ppm, respectively.

(2) The best working temperature of nano-plate WO3 gas sensor is 350 ℃, 300 ℃, 325 ℃ and 250 ℃, respectively, and the sensitivity reaches the maximum when the gas concentration is 1000 ppm, which are respectively 25.4 and 18.52 , 30.29 and 18.31, respectively, the sensitivity value of ammonia is up to 30.29; The minimum detectable concentration of nano-plate tungsten trioxide gas sensor for the four gases is 50 ppm, 50 ppm, 25 ppm and 100 ppm, respectively, of which the sensitivity is 2.81 when the ammonia gas concentration is 25 ppm, indicating that the nano- Tungsten trioxide gas sensor is very sensitive to low concentration of ammonia.

(3) Nanoplate rod-shaped tungsten trioxide gas sensor The best working temperature of the four gases was 350 ℃, 300 ℃, 300 ℃ and 250 ℃, respectively, and the sensitivity reached the maximum when the gas concentration was 1000 ppm, the minimum detection concentrations for the four gases at 9.11, 8.73, 14.23 and 6.89 gas sensors were 200 ppm, 50 ppm, 50 ppm and 100 ppm, respectively.

Gas-sensitive test results show that the three kinds of crystal tungsten oxide have their own strengths. However, the overall comparison shows that the plate-shaped nano-tungsten trioxide is slightly better than the other two types of tungsten trioxide in terms of the lowest detection concentration or sensitivity.

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Preparation of Nano Tungsten Trioxide by Hydrothermal Synthesis

Nano tungsten trioxide is a typical N-type semiconductor gas sensing material, which has a large specific surface area, stable chemical properties, a variety of toxic gases have good sensitivity, can effectively improve sensitivity and responsiveness of gas sensors effectively reduce the harmful effects of harmful gases on human health.

tungsten trioxide hydrothermal reactor image

The preparation of nano tungsten trioxide by hydrothermal method can overcome the problems that the nano rods prepared in the prior art have larger diameters, uneven distribution of rods and other forms of particles. 

Using ammonium metatungstate as tungsten source, citric acid as the profile control agent, according to a certain proportion in the mixed solution, adding concentrated hydrochloric acid solution was adjusted by drops in pH magnetic stirring at room temperature is equal to 1.5, half an hour so that the magnetic stirring solution mixing, the mixed solution was transferred to a hydrothermal reaction kettle PTFE capacity of 100 mL in seal, placed in an oven and incubated at 180 ° C for 18 h. After completion of the hydrothermal reaction, the precipitate was collected by centrifugation, washed with deionized water and acetone, and centrifuged three times to remove residual Na2SO4 and organic impurities in the precipitate. The cleaned precipitate was placed in a watch glass, dried in an oven at 80 ° C for 24 h, and finally ground in an agate mortar to obtain a powder.

Through SEM and TEM observation, it is concluded that the best technological conditions for preparing nano structured WO3 are citric acid content of 2 mol, P H value of the system 1.5, hydrothermal reaction 180 degrees, and hydrothermal reaction time of 1D. The prepared nanorod WO3 nanorods have a maximum length of about 2.5 um, a diameter of about 30-60 nm, a maximum aspect ratio of about 1:80 and a growth direction of the nanorods of [002].

The gas sensor made of nano rod tungsten trioxide has the best working temperature of 350 ℃, 300 ℃, 325 ℃ and 250 ℃ in the gas of ethanol, acetone, ammonia and formaldehyde respectively. When the gas concentration is 1000 ppm sensitivity reached the maximum, respectively 21.29,10.82,24.32 and 9.68; Gas sensors for the four gases minimum detection concentrations were 100ppm, 50ppm, 25ppm and 100ppm, gas sensitivity is good, but to be higher than the working temperature of 150 ℃ to play a good gas sensitivity performance, so in the future to find the right of the material on the nano tungsten oxide doping is to improve the performance of gas sensors the main way.

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2021年11月18日星期四

Mixed Tungsten Oxide Pressing Tungsten Plate

Tungsten plate is a tungsten material pressed by special process, it is mainly used in electronics and semiconductor industry parts manufacturing, tungsten sputtering target, high temperature furnace with tungsten boat, sapphire long furnace components etc.

tungsten plate image

The conventional thin tungsten plate pressing process uses yellow tungsten oxide, because the size distribution of yellow oxide powder is narrow and the particle size of powder is uniform, the gap of particles is not easy to be filled in the process of pressing, which eventually leads to lower strength of pressing.

The physical and mechanical properties of the tungsten plate are closely related to the strength of the compact, the strength of compacts is related to the characteristics of tungsten powder morphology and particle size distribution. There are three types of commonly used tungsten trioxide, yellow tungsten oxide, blue tungsten oxide and purple tungsten oxide, its flagship product is divided into three level: ordinary, fine, superfine granularity. The three kinds of tungsten oxide are mixed in proportion, and the coarse and fine powder can complement each other, so as to achieve the ideal pressure intensity of the tungsten plate pressing.

BTO, VTO and YTO are mixed in a biconical mixer according to the ratio of 2 to 5: 10. After 2 hours, the three kinds of tungsten oxide mixture were used as raw materials, and they were laid in the boatpan and reduced in the reduction furnace. The material layer thickness is controlled between 10-12 mm, and the heating rate is 50°C/h. Hydrogen dew point is -70~-75°C. The five zone reduction temperature distribution is in the range of 700~950°C, and the residence time is 180 min in the high temperature zone. The fine powder will pass through the aperture 0.212 mm wire diameter 0.112 mm and the aperture 0.075 mm wire diameter 0.056 mm sieve twice. Finally, the high pressed compact powder was obtained.

Under the action of external loading, the powder with more fine particles is easier to self adjust, by moving, sliding and rotating, the gap between particles is easy to be filled, so that the large porosity in the blank is reduced. Due to the complex shape of the powder particles, the surface is rough, there is a part of the bridge between particles, forming a loose aggregation structure, which makes the powder clench in the pressing between the particles, so as to improve the strength of compacts, it is conducive to the preparation of high performance tungsten products.

Mix the yellow tungsten oxide, blue tungsten oxide and purple tungsten oxide three and proceed a reduction, it is a simple process, low energy consumption and good quality, no need to change the operation methods and processes, but the quality is already the best.

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Violet Tungsten Oxide for Making Ultrafine Tungsten Powder

 What kind of tungsten oxide can produce high quality cemented carbide?

violet tungsten oxide image

At present, there are yellow tungsten oxide, blue tungsten oxide and violet tungsten oxide which are available in the market. Which kind of raw material is most suitable for cemented carbide production? Violet Tungsten oxide is the best raw material for producing superfine tungsten powder and nanometer metal tungsten powder. Ultrafine tungsten carbide was prepared by ultrafine or nano tungsten powder, and then cemented carbide was produced, the properties and requirements of cemented carbide are better than those made of ordinary. tungsten powder. The alloy with violet tungsten as the raw material has better quality and stronger adaptability, and the violet tungsten can make the penetration capability of the military armour piercing bullet increase by 20%, and the abrasion resistance of the carbide cemented carbide tool can be increased by 12%, etc.

The preparation methods of violet tungsten include hydrogen reduction, reverse hydrogen reduction, and liquid ammonia reduction. No matter what kind of reduction, it needs to be carried out in the high temperature reduction furnace. The reduction temperature is between 790 and -900 degrees centigrade. It is generally believed that the optimum reduction temperature of violet tungsten is between 800°C-850°C. In the pusher type reducing furnace environment, the temperature band range from800 - 850°C is generally set. The thickness of the loading layer can not be too large, and the flow of hydrogen should be controlled properly. The reduction reaction of violet tungsten not only on the surface, but also in the internal reduction process.

The crystal structure of violet tungsten is generally needle shape and micro rod-like crystal shape. It is generally believed that the tungsten powder prepared by the violet tungsten oxide of the needle crystal is finer, while the tungsten powder made by the rod shaped crystal violet tungsten oxide is relatively coarse. Of course, if it is the other crystal form or color, the reduction failure can be basically determined.

In summary, violet tungsten oxide has the advantage of manufacturing ultrafine tungsten powder, tungsten nano powder, and preparation of ultrafine tungsten carbide hard alloy for final production, in addition, violet tungsten is a high quality precursor material for gas sensitive element, but again, violet tungsten does not apply doping, doped work should apply to blue tungsten oxide.

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N-doped Tungsten Dioxide Vanadium Thin Films

Scientists discovered that VO2 has a distinctive phase change function long time ago. When the phase transition temperature is 68°C, it can change reversibly from low temperature monoclinic system to high temperature tetragonal system.

N-doped tungsten dioxide vanadium thin films image

The physical properties such as resistivity, optical transmittance, permeability and so on have changed greatly. Doping method is an effective method to change the phase transition temperature of VO2. The preparation methods of doped VO2 films include sol-gel doping method, hydrothermal synthesis doping method, sputtering doping method, metal organic compound vapor deposition doping method, pulse laser deposition process, inorganic sol-gel doping method, and microwave plasma method. We prepare it by microwave plasma method.

Another main content of the preparation of doped VOthin film is the selected doping compounds. Weigh a certain amount of pure VO5 powder and W2O3 into the pot, then stirring, put in the muffle furnace and under the air medium, heating to 900°C to melt. After 15 min of heat preservation, the molten solution is quickly poured into a certain amount of deionized water to quench and stir violently until a stoichiometric V1.96W0.04O5 sol is formed. The prepared V1.96W0.04O5 sol has been plated on glass sheets cleaned with 75% alcohol and deionized water by spin coating method. Dry naturally at the ventilation, then obtain synthesis of stoichiometric V1.96W0.04O5 thin films.

The V1.96 W0.04O5 films have been prepared by microwave plasma enhanced method, control reaction parameters, the V1.96W0.04O5 thin film was reduced to V0.98W0.02O2 and doped with nitrogen. The reaction time was 10~30 min. The V0.98W0.02O2-x N y thin film was obtained.

The phase transition temperature of VOpowders can be reduced by VO2 film, which is about 6°C, with the addition of nitrogen and tungsten ions, the phase transition temperature of V0.98W0.02O2 -x N y thin film can be effectively reduced, when the nitrogen flow rate is changed, the phase transition temperature of vanadium tungsten dioxide thin film can be reduced to 35°C at minimum. This is the closest phase change material known to be the daily temperature.

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Violet Tungsten Oxide Color Identification

 Violet tungsten oxide, abbreviated as VTO, is named after the appearance of purple. It is the best raw material for the preparation of ultrafine or nanometer tungsten powder.

violet tungsten oxide color identification image

It is produced in the process of producing ammonium paratungstate under the condition of precise control of temperature reduction. Violet tungsten oxide is mainly used in the production of tungsten powder and tungsten carbide.

Violet tungsten oxide sample color can be divided into purple, dark purple, dark blue, blue, the color of violet tungsten oxide and its oxygen index, particle degree of tightness, the aspect ratio of the needle.

Some scholars have invented a way to directly control and detect the quality of violet tungsten oxide. The quality of purple tungsten oxide is determined by the unique color of violet tungsten oxide. This method is called "colorimetric method" in the industry.

The color of violet tungsten oxide is related to the oxygen index, the degree of tightness of the particles and the ratio of length to diameter of the needle. A type of violet tungsten oxide is purple, oxygen index is slightly higher and needle is slightly elongated; B violet tungsten oxide is purple red, and the needle is slightly shorter and slightly thicker. A, B class basically maintained the needle-like morphology of violet tungsten oxide, but there are some small particles are not acicular; Category C violet tungsten oxide is black, the needle is severely shrunk and begins to transform into WO2; D and E are produced abnormally, the color of different particles can be seen with a microscope, and the morphology of the sample is also different by scanning electron microscopy.

By colorimetric method, it is found that the quality of violet tungsten oxide with purple and purple color is the best, and the quality of purple black is slightly worse. If it appears purple blue or blue, it has basically declared failure.

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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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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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