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.

If you have any interest in our products, please feel free to contact us by email: sales@chinatungsten.comsales@xiamentungsten.com or by telephone:+86 592 5129696.

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.

If you have any interest in our products, please feel free to contact us by email: sales@chinatungsten.comsales@xiamentungsten.com or by telephone:+86 592 5129696.

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.

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.