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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