Tungsten Powder Application in Electrolytic Capacitor

A capacitor which made of tungsten powder has good leakage current (LC) performance

A tungsten powder which has tungsten silicide (such as W5Si3) on the surfaces of particles and has a silicon content of 0.05-7% by mass; a positive electrode body for capacitors; an electrolytic capacitor; a method for producing a tungsten powder; and a method for producing a positive electrode body for capacitors. The tungsten powder has an average primary particle diameter of 0.1-1 μm, and tungsten silicide is localized within 50 nm from the surface of each particle. The tungsten powder contains at least one of tungsten nitride, tungsten carbide and tungsten boride in a part of the surface of each particle. It is preferable for the tungsten powder that the content of phosphorus element is 1-500 ppm by mass, the oxygen content is 0.05-8% by mass, and the content of elements other than tungsten, silicon, nitrogen, carbon, boron, oxygen and phosphorus is 0.1% by mass or less. It is also preferable that the tungsten granulated powder has an average particle diameter of 50-200 μm and a specific surface area of 0.2-20 m2/g. According to the present invention, a tungsten capacitor which has good leakage current (LC) performance can be provided.The present invention relates to: a tungsten powder which has tungsten silicide (such as W5Si3) on the surfaces of particles and has a silicon content of 0.05-7% by mass; a positive electrode body for capacitors; an electrolytic capacitor; a method for producing a tungsten powder; and a method for producing a positive electrode body for capacitors. The tungsten powder has an average primary particle diameter of 0.1-1 μm, and tungsten silicide is localized within 50 nm from the surface of each particle. The tungsten powder contains at least one of tungsten nitride, tungsten carbide and tungsten boride in a part of the surface of each particle. It is preferable for the tungsten powder that the content of phosphorus element is 1-500 ppm by mass, the oxygen content is 0.05-8% by mass, and the content of elements other than tungsten, silicon, nitrogen, carbon, boron, oxygen and phosphorus is 0.1% by mass or less. It is also preferable that the tungsten granulated powder has an average particle diameter of 50-200 μm and a specific surface area of 0.2-20 m2/g. According to the present invention, a tungsten capacitor which has good leakage current (LC) performance can be provided.

The present invention is tungsten powder, the anode body of a capacitor using the same, and to an electrolytic capacitor using the anode.

Size of the shape of the electronic devices such as mobile phones and personal computers, high speed, with the weight reduction, the capacitors used in these electronic devices, lightly smaller, larger capacity, the lower the ESR is sought there.

As such a capacitor, an anode body of a capacitor made of a sintered body of valve metal powder such as that can be anodized tantalum was anodized to form a dielectric layer made of these metal oxides on the surface thereof is the electrolytic capacitor has been proposed.

Tungsten is used as the valve metal, electrolytic capacitors with sintered tungsten powder to the anode body, the volume of the anode body using a tantalum powder of the same particle size, compared to the electrolytic capacitor is obtained in the same formation voltage , it is possible to obtain a large capacitance, leakage current (LC) was not subjected to practical use as a large electrolytic capacitor. To improve this, tungsten and other alloys capacitor is considered that have been but the leakage current with the metal is not sufficient for those improved somewhat.

An object of the present invention, a tungsten powder can be solved the problem of leakage current (LC) in the electrolytic capacitor of the sintered body of tungsten powder and anode as a valve metal, anode body of a capacitor using the same, and an anode The present invention is to provide an electrolytic capacitor using the body as an electrode.

The present inventors have found that the silicon content can be solved the above problems by a sintered body of tungsten powder part was tungsten silicide on the surface so that the specific range is used as the anode body , and the present invention  has been completed .

According to the tungsten powder of the present invention, and conventional tungsten powder, as compared to the tungsten alloy powder in a volume equivalent or more, it is possible that the LC characteristics per volume producing a good electrolytic capacitor.

Tungsten powder used in the present invention (raw tungsten powder) is commercially available. Smaller tungsten powder particle size, for example, tungsten trioxide powder was triturated under a hydrogen atmosphere or a tungstic acid or tungsten halide by using a reducing agent such as hydrogen or sodium, selecting the reducing conditions appropriate It can be obtained by.

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Tungsten Powder and W-Cu Composite Powder II

It is an object of the invention to obviate the disadvantages of the prior art.

It is another object of the invention to produce a W-Cu composite powder which can be used to make W-Cu pseudoalloys having high electrical and thermal conductivities.

It is a further object of the invention to produce a W-Cu composite powder which may be pressed and sintered to near theoretical density without copper bleedout.

It is still a further object of the invention to produce a W-Cu composite powder which may be used to make sintered articles having a high degree of dimensional control.

In accordance with one object the of invention, there is provided a tungsten-copper composite powder comprising individual particles having a tungsten phase and a copper phase wherein the tungsten phase substantially encapsulates the copper phase.

In accordance with another object of the invention, there is provided a W-Cu composite oxide powder comprising individual particles having a copper tungstate phase and tungsten trioxide phase wherein the tungsten trioxide phase exists primarily at the surface of the individual particles.

In accordance with a further object of the invention, there is provided a method for forming a homogeneous W-Cu pseudoalloy comprising pressing a tungsten-coated copper composite powder to form a compact and sintering the compact.

In accordance with a still further object of the invention, there is provided a W-Cu pseudoalloy having a microstructural cross-section having tungsten areas and copper areas, the tungsten areas being less than about 5 µm in size and the copper areas being less than about 10 µm in size.

Several factors influence the solid-state (below 1083°C - the melting point of copper) and liquid-phase (above the melting point of copper) sintering behavior of submicron W-Cu powder systems. Compacted refractory metal powders undergo considerable microstructural changes and shrinkage during solid-state sintering (in the absence of liquid phase). Submicron particle size powders effectively recrystallize and sinter at temperatures (T) which are much lower than the melting temperatures (Tm) of refractory metals (T ≅ 0.3 Tm). The initial sintering temperature for submicron (0.09-0.16 µm) tungsten powder is in the range of 900-1000°C. The spreading of copper and the formation of a monolayer copper coating on tungsten particles occurs in the temperature range of 1000-1083°C. By lowering the activation energy for tungsten diffusion, monolayer copper coatings activate the solid-state sintering of tungsten. Therefore, a number of complementary conditions are met for bonding submicron tungsten particles into a rigid tungsten framework within the composite powder compact during solid-state sintering (950-1080°C). High fineness and homogeneity of the starting composite powders are expected to enhance the sintering of a structurally homogeneous tungsten framework. Such framework should, in turn, aid in making a homogeneous pseudoalloy.

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Tungsten Powder and W-Cu Composite Powder I

Tungsten-copper (W-Cu) pseudoalloys are used in the manufacture of electrical contact materials and electrodes, thermal management devices such as heat sinks and spreaders, and conductive inks and pastes for ceramic metallization. The basic methods for the fabrication of articles composed of W-Cu pseudoalloys include: infiltration of a porous tungsten skeleton with liquid copper, hot pressing of blends of tungsten and copper powders, and various techniques incorporating liquid phase sintering, repressing, explosive pressing, and the like. Complex shapes may be made by injection molding W-Cu composite powders. It is desirable to be able to manufacture articles made from W-Cu pseudoalloys at or near the theoretical density of the pseudoalloy. Besides having improved mechanical properties, the higher density pseudoalloys have higher thermal conductivities which are critical for the application of W-Cu pseudoalloys as heat sink materials for the electronics industry.

The components in the W-Cu system exhibit only a very small intersolubility. Thus, the integral densification of W-Cu pseudoalloys occurs above 1083°C in the presence of liquid copper. The compressive capillary pressure generated by the forming and spreading of liquid copper, the lubrication of tungsten particles by liquid copper and the minute solubility of tungsten in copper above 1200°C combine to cause the relative movement of tungsten particles during sintering and thereby make possible the displacement of tungsten particles. Local densification and rearrangement of the tungsten framework causes an inhomogenous distribution of W and Cu phases in the sintered article and copper bleedout, i.e. the loss of copper from the sintered article. This leads to the degradation of the thermal/mechanical properties of the sintered article.

Prior art methods directed to improving the homogeneity of W-Cu composite powders by coating tungsten particles with copper have not been successful as these copper-coated powders still exhibit a high tendency towards copper bleedout during the consolidation of the composite powder into fabricated shapes.

Thus, it would be advantageous to eliminate copper bleedout from occurring during the liquid-phase sintering of W-Cu pseudoalloys while providing a homogeneous distribution of W and Cu phases in the sintered article.

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Tungsten Powder Classification Application in Coarse Tungsten Carbide Powder Production

In the industrial production, classification treatment on coarse particles tungsten powder, can be an effective solution to the powder clipping coarse or fine powder, manufacturing coarse grain tungsten carbide powder.

Coarse grain WC-Co alloys with high hardness and high toughness, is widely used in mining drilling tools, oil drilling tools, stamping dies, wear resistant parts, high temperature and high pressure resistance, metal pressure processing tools, steel rolling roll ring, hard surface materials and so on. With the rapid development of Chinese economy, the application of tungsten carbide wear-resistant material is widely growing.

Traditional manufacturing processes of coarse tungsten carbide powder are molybdenum wire furnace high temperature reduction and carbon tube furnace high temperature carbonization; in the 90s, mainly the reduction in the middle temperature and the carbonization in high temperature of tungsten oxide doped, this process can produce coarse tungsten carbide powder of about 301xm ~ 401xm; high temperature carbonization adding cobalt and nickel, to get extremely coarse carbide powder; Kennametallne in US developed the aluminum heat production of tungsten carbide and the Russian Research Institute of chemical technology has developed the tungsten concentrate "furnace" thermite reduction method, tungsten carbide can be produced directly from tungsten concentrate, X-ray diffraction analysis of the metal phase containing tungsten carbide obtained by this method shows that the product contains only a tungsten carbide phase, and the grains are coarse; H.C. Stark Co., Ltd. has developed  a process that reduction and carbonization treatment to the tungsten oxide powder by the presence of the alkali metal compounds, producing ultra-coarse single crystal tungsten carbide and thus prepare hard metal . Some coarse tungsten carbide powder produced by the processes above has uneven widespread size, incomplete crystallization, and with much fine particles and abroad particle size distribution; some methods have high requirements of equipment or great impact on environment. A lot of information discusses the micro structure of tungsten carbide powder with the presence of native inheritance among tungsten powder,  the form and structure of tungsten powder directly affect the properties of tungsten carbide. By studying the carbonization process of tungsten powder re-grading, there is an effective solution to the powder clipping coarse or fine powder, manufacturing coarse grain tungsten carbide powder.

Powder properties can be measured not only by the particle size of the powder, but also the powder size, the structure and composition, particle morphology, surface characteristics of the particles and the like must be considered. After classification treatment, A1 powder grain has better morphology consistency. According to the proportion, the particles, the size and shape suffered different gravity and different resistance of medium in the air, being graded with different sedimentation rates, can effectively change the physical properties of the powder. At the same time, despite the use of airflow and iron container as the carrier grades, there is little effect on the oxygen and the iron powder and other trace elements. Within the range of performance indicators, it will not have a negative impact on the process behind the process.

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Tungsten Powder Infiltrated with Copper-Titanium-Bismuth or Copper-Titanium-Tin

Tungsten powder is pure tungsten in powder, apparent black powder. The regular tungsten powder fearture with 2~10um, 99.90% or 99.95%. Tungsten powder used as material for tungsten product usually.

The purity of tungsten powder is of particular importance in PM manufacturing of tungsten metal, since during subsequent sintering further purification through evaporation is only possible to a certain extent. The demand for purity of tungsten powder has increased steadily during the last three decades. Considerable improvements in hydrometallurgy have led to concentrations fairly below 10 µg/g for most of the elements. This trend with time can be demonstrated by comparing today's usual specifications with those given in the last book on "Tungsten" by Yih and Wang, published in 1979.

It was found that using vacuum infiltration techniques, copper-titanium-bismuth or copper-titanium-tin alloys, titanium sandwiched between copper-bismuth or copper tin alloys and a tungsten powder body subsequently heated to form copper-titanium-bismuth or copper titanium-tin alloy, a tungsten powder body coated with titanium by electroplating or vapor phase plating, and the like, wet individual particles of the tungsten powder body so as to allow infiltration of the tungsten particles and the copper-titanium bismuth or the copper-titanium-tin alloy thereby raising the overall electrical conductivity of the copper alloy matrix. The use of vacuum infiltration techniques also decreases the volume of hydrogen present in the resultant tungsten-copper-titanium tin composites by more than an order and decreases the volume of all gaseous components by several orders.

Although complete and substantially instantaneous infiltration of copper into sintered tungsten bodies is conveniently carried out in an atmosphere of hydrogen, a copper melt shows no penetration into tungsten powder bodies in a vacuum atmosphere using comparable time temperature treatments and using standard metallurgical procedures. In carrying out the present invention, it was found that subjecting the tungsten powder body and a contacting copper-titanium-bismuth alloy or a copper titanium-tin alloy to a vacuum infiltration process, the copper-titanium-bismuth alloy and the copper-titanium tin alloy were absorbed into the tungsten body by capillary attraction. It is thought that in each instance the titanium promotes wetting of the tungsten particles by the copper titanium-bismuth alloy and the copper-titanium-tin alloy.

The bismuth and the tin are used in the resultant composite contact materials to sustain an are at low values of current and voltages during the operation of the composite contact material in vacuum environments.

Tungsten is used in electrical contact materials because of its inherent characteristics of hardness and of resistance to arcing which reduce pitting of the tungsten contact material. However, pure tungsten contact material possesses high electrical resistance which lowers the efficiency and reliability of the tungsten contact material.

It has been suggested that a composite of tungsten copper used as an electrical contact material would make advantageous use of the several outstanding characteristics of both metals. In the composite, the copper provides the current carrying capability and thermal conductivity while the tungsten contributes hardness, resistance to are erosion, and superior anti-weld properties. In order to utilize the aforementioned characteristics of the copper and the tungsten, it is necessary to fabricate the metal into a tungsten-copper composite.

Copper and tungsten are mutually insoluble and form no alloys in the metallurgical sense, but mixtures of the "ice two metals are usually referred to as alloys but are, technically speaking, composites. Composites of tungsten-copper may be prepared by pressing the mixed metal powders to the required shape in dies, and subsequently sintering in a hydrogen atmosphere above the melting point temperature of the copper, preferably between l250 and 1350 Centigrade. The hydrogen acts as a flux and the molten copper wets the tungsten particles and cements them together. Another method which provides a harder resultant body consists of first pressing and sintering the tungsten powder so as to form a coherent but porous body, which is then heated at a temperature of about 1200 C. to 1300 C. in a hydrogen atmosphere and in contact with molten copper. The copper is absorbed into the pores of the tungsten powder body by capillary attraction. The copper infiltrated imparts strength and ductility to the tungsten powder body and also provides the resultant body with higher current carrying capability and thermal conductivity. However, using standard metallurgical procedures, a copper melt shows no penetration into the tungsten powder body in a vacuum. It is thought that the lack of penetration of the copper into the tungsten powder body is due to the unfavorable surface energies that are present in the vacuum.

The additions of bismuth and tin to the composites provides a resultant composite contact material that does sustain an are at low magnitudes of current and voltages during the operation of the contact materials in a vacuum atmosphere environment. It is thought that the foregoing occurrence is due to the relatively high vapor pressures of bismuth and of tin.

Therefore, it provides composite materials suitable for use as contact materials in vacuum electrical switching devices.

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Spherical Tungsten Powder Application in Cathode Substrate

Spherical tungsten powder has the advantages of regular shape, good uniformity and adequate liquidity, using spherical tungsten powder to prepare the cathode can meet the current needs of engineering applications, which will play a positive role in cathode studies.

Cathode as the electron emission source of vacuum device, its research direction and development have the large influence on the development level of vacuum devices even weapons systems.

Modern dispenser cathode nowadays is one of the key researches of cathode. The common structures are usually composed of three parts: the porous tungsten matrix, emitting material and heater assembly. Of which the porous tungsten matrix is the framework and launch materials’ support of the entire cathode. As can be seen from the hot cathode development course, the porous tungsten matrix is the results of hot cathode constant pursuit of high emission current density and high reliability. Properties of the porous tungsten matrix, especially the pore structure and the pore distribution will directly affect the emission size, low evaporation rate, emission uniformity and lifetime of the cathode. In the actual production, it requires that the cathode matrix have a uniform distribution of pores of 24-26%. The porous tungsten matrix processes include compression molding process and the sintering process, the development of process is dependent on the progress in powder metallurgy process level, as well as affected by the matrix powder properties. If the matrix porosity and pore distribution can be reasonable controlled by process control, there will be a positive role in the cathode research.

Spherical tungsten powder has the advantages of regular shape, good uniformity and adequate liquidity. These advantages are especially suitable for the automatic press with the automatic filling, and the porous tungsten matrix can be obtained with the suitable size of pore and uniform distribution, compared to conventional tungsten powder, it has quite a lot of advantages. During the process of spherical tungsten powder producing the cathode, relationship between the initial porosity and compaction pressure is in line with Heckel's Law. Cathode pulse emission test results show that in 1050 ℃, cathode substrate prepared by spherical tungsten powder can obtain inflection current density of 20.46A / cm2, which can fully meet the current needs of engineering applications.

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Ammonium Tungsten Bronze Nanoparticles Preparation

Tungsten bronze compounds are a series of important inorganic compounds, tungsten ions exist as mixed valence state of W6 +, W5 + and W4 + in such compounds to make a balance in the overall charge. Rich crystal structure, the tunnel structure and this particular valence state lead to its excellent properties, such as electronic and ionic conductivity, superconductivity, optical properties, which has caught widespread research interest in the aspects of the secondary battery, electrochromism, near-infrared absorption and application of chemical sensors.

Currently, methods to synthesize tungsten bronze compounds mainly are the wet chemical method, heat reduction method and thermal decomposition method. Wet chemical method to synthesize ammonium tungsten bronze is to put the starting material in the reducing solvent refluxing for several days, size of the sample obtained by this method is too large, it’s usually between a few to ten micron, and the preparation process requires a long time and high energy consumption. Thermal reduction method is to uniformly mix tungsten oxide, tungsten powder and metal tungstates in proper proportions, then heated in a vacuum or under an inert atmosphere, the reaction temperature is usually about 1000 ℃, and remove unreacted impurities after the reaction is completed. Since the thermal stability of ammonium tungsten bronze difference is poor and decomposition temperature (300 ℃) is lower than the synthesis temperature, the thermal reduction method can not be used to synthesize ammonium tungsten bronze. The thermal decomposition method to synthesize ammonium tungsten bronze is to heat and decompose ammonium paratungstate in a reducing atmosphere (H2 or a mixed gas of H2, Ar, etc.), the size of the resulting sample is too large, and this method can not obtain completely pure phase ammonium tungsten bronze, ammonium content in sample is too low and easy to excessive decomposed into tungsten oxide.

The pure phase ammonium tungsten bronze nano-powder can not be directly obtained in current study, so usually break the large micron-sized particles obtained into small particles by milling, but these compounds are easily to be oxidized and lost live and decompose in the milling process, also accompanied by crystallization performance degradation. For the above problems, some scholars have proposed a synthesis method to directly synthesize ammonium tungsten bronze powder with controllable particle size.

Preparation of reduced state ammonium tungsten bronze nanoparticles: dissolve 0.01~1g tungsten hexachloride or tungsten tetrachloride in 20~40mL oleic acid solution, and stirred to obtain homogeneous solution, then added 4~30mL oleylamine, and mix evenly, move to supercritical reaction kettle, crystallization reaction at 150~350 ℃ for 0.5~48 hours, the powder samples were centrifuged and washed after reaction, dry under vacuum at 40~250 ℃ for 1~12 hours, and the reduced state ammonium tungsten bronze nanoparticles are obtained, the mole fraction of ammonium group in the composition is between 0.2~0.3. In addition, samples obtained by this method have strong near-infrared absorption ability, the film containing nanoparticles can effectively shield the near infrared rays of 780 ~ 2500nm and maintain high visible light transmittance.

It was proposed a synthesis method to directly synthesize ammonium tungsten bronze powder with controllable particle size since the pure ammonium tungsten bronze nano-powder can not be directly obtained in current study.

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Lithium Tungsten Bronze Photocatalyst Preparation II

Taking into account the practicality of the photocatalyst in the decomposition of harmful substances, sunlight as the light source is indispensable. Wavelength irradiated to the surface of the sun at about 500nm has the maximum intensity of visible light, energy of wavelength at 400nm-750nm in the visible region is about 43% of the total energy of sunlight, so in order to efficiently utilize the solar spectrum, finding the catalyst with light visible effect has aroused people's attention. Kinds of photocatalysts with visible effect reported are still limited currently, so it is necessary to research and develop new photocatalyst with highly efficient visible light effect. Tungsten bronze compounds are a series of important inorganic compounds, tungsten ions exist as mixed valence state of W6 +, W5 + and W4 + in such compounds to make a balance in the overall charge. Rich crystal structure, the tunnel structure and this particular valence state lead to its excellent properties.

A kind of oxide containing lithium tungsten bronze structure as visible effect photocatalyst, the chemical formula of lithium tungsten bronze structure oxide is = Ba5LiFe0.5 (NbxTah) λ A. （0 <x <1） preparation steps of lithium tungsten bronze structure oxide are as follows: mix the 99.9% analytically pure chemicals of BaCO3, Li2CO3, Fe2O3, Nb205 and Ta2O5 as formula Ba5LiFe0.5Nb4.75Ta4. 75 O30, put into the mill pot, add zirconia balls and ethanol, milling for 2-8 hours, mix grinding, remove and dry through 200-mesh sieve; the resulting powder material calcine at 1175-1275 ℃, and incubate for 6-8 hours, cool to room temperature, and then pulverized by ball mill so that the particle size gets smaller to 2 μm, then the lithium tungsten bronze structure oxide powder Ba5LiFe0.5Ta9.5O30 is obtained, 0 <x <10. This method is simple, with low cost, photocatalys obtained has excellent catalytic performance, with effects of decomposing harmful chemical substances, organic biomass and bactericidal under visible light irradiation.

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Lithium Tungsten Bronze Photocatalyst Preparation I

Environment pollution has caught great concern from many countries in the world, it has led to the drinking water source of people's lives and industrial water quality continue to decline, which leads to air pollution grow and results in the continuous destruction of the ecological environment,and poses a serious threat for human survival. In order to solve these problems, a variety of methods to control environmental pollution are carried out.

From the late 1970s, it is proposed the use of photocatalyst to decompose pesticides and malodorous substances and other organic matters in water and the atmosphere, as well as application examples like photocatalyst coated solid surfaces self-cleaning and so on. Currently, the photocatalyst is primarily titanium dioxide, which has been used to decompose pesticides and malodorous substances and other organic matter in water and the atmosphere, however the bandgap of titanium dioxide is 3.2eV, can only show activity in the ratio of 400nm shorter ultraviolet light and can only work indoors or where there is an ultraviolet lamp, hardly with visible light, which greatly limits the use of titanium dioxide photocatalyst.

In order to efficiently utilize the solar spectrum, finding the catalyst with light visible effect has aroused people's attention, so the application of oxide containing lithium tungsten bronze structure as visible effect photocatalyst is found.

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Tungsten Bronze Application in Ceramic Capacitors II

Temperature stability is of great importance for the preparation of multilayer ceramic capacitors in electronic products application, and tantalates with tungsten bronze structure having high dielectric constant (> 100) and low dielectric loss, is expected to be the dielectric materials with temperature stability being used in multilayer ceramic capacitors.

Miniaturized drives and speed of the computer transfer more focus into the high-temperature resistant parts, the capacitor must be able to work at 150℃ or even 200℃ in the future. However, the application of PbTiO3-BaTiO3 composite multilayer ceramic capacitor is not desirable, the first reason is 1¾ toxicity, the second one is that easy to decompose in low oxygen partial pressure, and Ni metal electrode will form into low melting point alloy with it. The same problem also appears in BiTiO3 doped by Bi2O3 base solid solution. The ideal new materials require the phase transition temperature is between -50 ℃ ~250 ℃, without PbO or Bi2O3, containing relatively common and inexpensive raw materials.

Recently, domestic and foreign researchers found that a number of tantalates with tungsten bronze structure having high dielectric constant (> 100) and low dielectric loss, is expected to be the dielectric materials with temperature stability being used in multilayer ceramic capacitors. The ceramic dielectric constant ε r  is between 127~175, dielectric loss tan σ is less than 0.009 at 1MHz, the dielectric constant and temperature coefficient τ ε is between -7¾~-2500ppm / ℃. But the dielectric constant and temperature coefficient of these tantalates with tungsten bronze structure are too large, the dielectric constant is too low, thus the practical applications are limited.

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Tungsten Bronze Application in Ceramic Capacitors I

A ceramic capacitor is a fixed value capacitor in which ceramic material acts as the dielectric. It is constructed of two or more alternating layers of ceramic and a metal layer acting as the electrodes. The composition of the ceramic material defines the electrical behavior and therefore applications. Ceramic capacitors, especially the multilayer style (MLCC), are the most produced and used capacitors in electronic equipment that incorporate approximately one trillion (1012) pieces per year. Ceramic capacitors of special shapes and styles are used as capacitors for RFI/EMI suppression, as feed-through capacitors and in larger dimensions as power capacitors for transmitters. Ceramic capacitors are divided into two application classes: Class 1 ceramic capacitors offer high stability and low losses for resonant circuit applications. Class 2 ceramic capacitors offer high volumetric efficiency for buffer, by-pass, and coupling applications.

Temperature stability is of great importance for the preparation of multilayer ceramic capacitors in electronic products application. Two or more than two of the polyhydric compounds having opposite temperature coefficient and dielectric constant (τ ε) are mixed to produce a solid solution with small temperature coefficient and dielectric constant, this method is frequently used to realize the temperature stability of the materials; or, such as capacitor for BaTiO3, mixed dopant is dispersed in the ceramic body to produce ferroelectric - paraelectric phase transition near room temperature and the relatively stable material has been obtained. Currently, BaTiO3 compound fits X7R standard, its dielectric constant does not exceed ± 15% in the temperature range of -55 ℃ ~125 ℃ compared to the change rate of dielectric constant at room temperature, the dielectric loss tan σ is less than 0.02 at 1MHz. However, if PbTiO3 (Tc = 4950C) is not doped, the maximum operating temperature does not exceed 130 ℃.

Recently, domestic and foreign researchers found that a number of tantalates with tungsten bronze structure having high dielectric constant (> 100) and low dielectric loss, is expected to be the dielectric materials with temperature stability being used in multilayer ceramic capacitors.

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Tungsten Market Short-Term Shocks

Tungsten price rose sharply in short term, which did not only fail to stimulate the enthusiasm of the industry, but made the upstream and downstream market trade sharply compressed, although raw material prices were pulled up rapidly, the price of terminal tungsten carbide products has not been improved.

Two months of tungsten price continuous rising begins to turn around. On May 25, the average price of 65% wolframite is 76,000 yuan / ton, the price of APT is 120,000 yuan / ton, the price of medium grain tungsten powder is 190 yuan / kg, respectively, the average price fell 2,000 yuan / ton, 3,500 yuan / ton, 2 yuan / kg compared with that on last Friday (May 20).

Affected by tungsten enterprises limited production price and purchasing and storage news, tungsten price rose sharply in short term, which did not only fail to stimulate the enthusiasm of the industry, but made the upstream and downstream market trade sharply compressed, the price of tungsten lacking of market demand to support will eventually fall back, so there are reasonable demands for the price returning in the market itself. Price of tungsten products continued to rise, especially tungsten powder, tungsten carbide powder and other powder products, with the rapid rise. Although raw material prices were pulled up rapidly, the price of terminal tungsten carbide products has not been improved. Some alloy enterprises said that the current orders on hand were signed before a month and unable to increase prices. Some small and medium sized carbide producer claimed that they were prepared to discontinued at any time.

Influence by the high costs of raw materials, on the one hand the downstream business are difficult to accept it, on the other hand with the increasing prices of raw materials, there are attempts to raise the offer of their own terminal products. Combined with that the early result of tungsten concentrate prices rising, traders of tungsten concentrate storing a large number of goods leads to backlog of funds, the divergence of interests in the market of all parties, or cause short-term frequent shocks of price tungsten in the market, the base of the sharp decline is still not enough.

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Cesium Tungsten Bronze Hydrothermal Synthesis

Cesium tungsten bronze (CsxWO3) is a kind of non-stoichiometric functional compound, with a specific structure of oxygen octahedral, having a low resistivity and low temperature superconducting properties. In recent years, it's also found that CsxWO3 films have good near-infrared shielding performance, which is expected to replace the existing ITO conductive glass as a window material or that can be used as a good near-infrared heat insulation materials, having very attractive prospects in the automotive and construction sector. Currently the synthesis method of CsxWO3 is mostly confined to high-temperature thermal reduction, such as the vapor chemical transport method by Hussain, the hydrogen reduction method by Takeda, the solid-phase method by Leonova, such methods have shortcomings the high test equipment requirements and experimental process difficult to control.

As a simple and practical synthesis method, hydrothermal synthesis has been widely used in the tungsten bronze synthesis of Li, Na, K, etc., but the hydrothermal synthesis method for the synthesis of CsxWO3 has been reported in recent years. CsOH and WCl6 as raw materials, use solvothermal synthesis at a low temperature (200 ℃) for the first time, CsxWO3 powder is synthesized in ethanol solution. The research shows CsxWO3 films have good near-infrared shielding properties. CsOH and WCl6 are easily hydrolyzed and volatile, which is harmful to humans and the environment, therefore a more moderate material is used to synthesized CsxWO3 powder, which is necessary for achieving CsxWO3 powder scale production and practical application. Thus, cesium carbonate and sodium tungstate were used as raw materials in an experiment, citric acid as the reaction solvent, prepared CsxWO3 powder by hydrothermal synthesis, which studied the effects of the hydrothermal reaction time, the content of citric acid and ethanol on the synthesized CsxWO3 powder's light absorption capacity, and to further explore the effect of UV irradiation on the near-infrared shielding performance of CsxWO3 film, analyzed the mechanism of absorption spectra and transmission spectra.

Cesium Tungsten Bronze Nanopowder in Transparent Insulation Coating

To make the thin, transparent external material like glass, plastic not only insulated and not blocking light, but also energy-saving, the most effective way is to add nanoparticles with the ability to absorb infrared light to resin, such as antimony-doped tin oxide (ATO), indium tin oxide (ITO), lanthanum hexaboride, and cesium tungsten bronze nanoparticles, and made transparent thermal insulation coatings directly onto the glass or shade cloth, or pre-coated on PET (polyester) film, and then stuck the PET film to the glass (such as car film), or made into a sheet of plastic, such as PVB (polyvinyl butyral), EVA (ethylene-vinyl acetate copolymer) plastic, then composite the plastic sheet and tempered glass, which also play a role in blocking the infrared, so as to achieve the effect of transparent insulation.

In the above nanoparticles that are capable of absorbing infrared rays and to achieve transparent insulation , the cesium tungsten bronze nanoparticles(also known as cesium tungstate) has the best near-infrared absorption properties that usually 2 g of addition per square meter coating can reach transmittance of 10% at 950 nm (this data indicates the near infrared absorption), while it can reach transmittance of more than 70% at 550 nm (70% is the majority basic indicators of high levels of transparency film). Although the cesium tungsten bronze nanoparticles have excellent transparent insulation properties, high temperature solid state reaction of raw materials tungsten and cesium is the mainly existing production process. For example, firstly form a tungsten bronze crystal structure at about 600 ℃, and then restore at about 800 ℃ reducing atmosphere, thereby form cesium tungsten bronze nanoparticles with a high carrier concentration (cesium tungsten bronze infrared absorption derived from the carrier).

The process is simple and have stable batch, but the problem is particles being too large, usually in the micron level. To meet the transparent coating requirements, grinding by high-dispersion device for a long time is needed to make the particle size less than 100 nm, which greatly increases the cost, and the presence of large particles increases the coating haze and affects the optical effect of the coating. In addition, using hydrogen reduction at high risk in the production process also increased production costs. Many studies have reported the use of wet chemical liquid phase process, such as the preparation of cesium tungsten bronze nanoparticles by hot water method, hot solvent method and high temperature thermal pyrolysis, but the problems that the high cost of the equipment or severe corrosion, high pressure and low safety factors still exist, and there is still not reports of completely liquid-phase production of small particles cesium tungsten bronze powders.

Tungsten Bronze

Tungsten bronze is a tungsten-containing nonstoichiometric compound, the outside looks like copper and has chemically inert. Tungsten bronzes is typically cubic crystal or tetragonal crystal. Insoluble in water and all acids except hydrofluoric acid, but soluble in an alkaline reagent. It can be used as a catalyst for oxidation of carbon monoxide and fuel cells getter.

A nonstoichiometric compound that empirical formula is MxWO3, and the M is typically an alkali metal, an alkaline earth metal, a rare earth metal ions, ammonium ions and so on. x is between 0 and 1. Tungsten bronzes typically have a metallic sheen and a special color. Species of M and change of the value x make it to have the properties of conductor or semiconductor. Crystal chemistry studies proved that tungsten bronze is essentially a solid solution formed after the alkali metal atoms inserted WO3 lattice. When all the vacancies are filled, the resulting compound is MWO3. The formation of tungsten bronzes is related to the tungsten’s variable atomic value, and if the space is only partially replaced by alkali metal atoms, some tungsten atoms will change from hexavalent to pentavalent.

Rare earth tungsten bronze M0.1WO3 is blue-violet powder, M is a rare earth element with a cubic crystal structure. Yttrium tungsten bronze YxWO3 has two structures of the tetragonal crystal and cubic crystal. Lithium and sodium, lithium and potassium may also form a mixed tungsten bronzes, such as NaxLiyWO3 and KxLiyWO3, where the x can be as small as 0.13, x + y can be up to 0.51. The former is usually a cubic crystal, the latter is hexagonal crystal.

Tungsten Bronzes Near Infrared Cut-Off Characteristics

The various nanoparticles have been investigating the continuous and new methods to reduce solar heat as it ensures a potentially low-cost and high-productivity solution. Not only does it needs high transmittance of ultraviolet radiation but also achieves complete shielding of infrared solar radiation can be used for solar control windows. In the other word, an effective IR absorbent should have high absorbance as well as a broad working wavelength. A well known the kinds of materials to realize the purpose is the nanoparticles of transparent oxide conductors with heat-ray cut-off effect such as tin doped indium oxide (ITO) and antimony doped tin oxide (ATO). They also are well known to provide highly transparent solar filters to absorb heat-ray by the effect of the plasma vibration of the free electrons, as typically observed in gold and silver nanoparticle solution. However, ITO can only shield the IR wavelength ranges longer than 1500 nm as well as indium is an expensive metal resource. In recent years, for practical application, tungsten bronzes actively have been investigating due to their interesting electro-optics, photochromic, electrochromic, and superconducting properties. Tungsten trioxide (WO3) has a wide band gap of 2.6-2.8 eV5 and is transparent in the visible and NIR ranges. A metallic conductivity and a strong NIR wavelength absorption can be induced when free electrons are introduced into crystals by either decreasing the oxygen content or by adding ternary elements. The oxygen deficiency in tungsten oxides leads to a complex-ordered structure known as the Magneli structure, while the ternary addition of the positive ions leads to the tungsten bronze structure. In other words, tungsten bronzes MxWO3 with doping small ions such as H+, Ag+, Li+, Na+, K+ and Cs+ into WO3 have better optical and electrical properties. It has been reported that the tungsten bronzes with the hexagonal phase are of particular interest in the application of electrochromic devices owing to the relatively high diffusion coefficients of hydrogen ions and metal ions compared with those of the orthorhombic phase and pure WO3.

What Are Tungsten Bronzes II

The cubic arrangement described above with an atom in the center of a cube is typical for perovskites, a group of ceramic materials with a variety of interesting electrical properties. The high--temperature superconductors are among these. In the cubic phase, tungsten bronzes are metallic and conduct electricity. However,in the hexagonal phase, they become superconductors. William Moulton, at Florida State University in Tallahassee, has done a lot of work with potassium, rubidium and cesium tungsten bronze superconductors. Dr. Moulton points out that these differences in properties depending on the direction of measurement in the crystal. The temperature at which a material becomes superconducting, of about 6K.

Iowa State University in Ames was another center for tungsten bronze research. There, Douglas Finnemore studied the effects of pressure on the transition temperature of potassium tungsten bronze. The object was to enhance the interaction between electrons and the lattice vibrations, or phonons. However, these tungsten bronzes were still superconductive at only 4K.

Howard Shanks, also at Iowa State, was able to produce sodium tungsten bronze compounds that were superconductors at as high as 10K. Part of his success was due to techniques he developed tp grow large crystals of this material, some as large as 3 inches. Dr. Shanks finds it ironic, in light of today’s superconductor research, that one of the reasons why work on tungsten bronze was dropped was because so many saw no future in oxide superconductors.

Other work at Iowa State has included using sodium tungsten bronze as a coating for one of the electrodes in a fuel cell that used hydrogen and oxygen as fuel to produce electricity. The test cell that was built ran for about a year. Another application that was investigated was using tungsten compounds for hydrogen storage. It was found that for HxW03 with x<0.5 hydrogen could move in and out of the material with ease. Some of this work was also done in Germany.

What Are Tungsten Bronzes I

What are tungsten bronzes?

The tungsten bronzes are a very interesting, but little appreciated, family of materials. They are not related to bronze, an alloy of copper and tin, except coloration. However, the structure of tungsten bronzes are similar to the high-temperature copper oxide superconductors. In fact, the tungsten bronzes were the first oxide superconductors and were the focus of extensive research 10-15 years ago. But by the early 1980s, most of this work had been set aside in favor of other pursuits.

The tungsten bronzes are a group of compounds made up of tungsten trioxide, WO3, and an alkali metal, such as sodium (Na) , potassium (K), rubidium (Rb) , or cesium (Cs). The general chemical form is MxW03, where M=Na, K, Rb, or Cs, and O<x<l. The color of these compounds varies with composition, at x=0.93 the color is a bronzelike golden-yellow, hence the name; at x =0.32 the color is a blue-violet. For this reason tungsten bronzes are use as pigments in dyes allld paints.

The variation in composition also affects the structure of the compound. Imagine a ¢ube with a tungsten atom at each comer, an oxygen atom in the middle of each edge and an atom of an alkali metal in the center of the cube. However, in a tungsten bronze there is not an atom at the center of every cube. When x< 1, only a certain fraction of the cubes will contain an alkali atom. If x is large, close to 1, the structure of the crystal lattice will be cubic. As x decreases, and fewer of the cubes are filled, the structure changes. At about x<0.3, or with less than 30% of the cubes full, the structure becomes hexagonal, with atoms arranged in hexagonal plates.