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.

Tungsten Oxide Ceramic Nonlinear I-V Characteristics

A varistor is an electronic component with an electrical resistance that varies with the applied voltage. Also known as a voltage-dependent resistor (VDR), it has a nonlinear, non-ohmic current–voltage characteristic that is similar to that of a diode. In contrast to a diode however, it has the same characteristic for both directions of traversing current. At low voltage it has a high electrical resistance which decreases as the voltage is raised.

Varistors are used as control or compensation elements in circuits either to provide optimal operating conditions or to protect against excessive transient voltages. When used as protection devices, they shunt the current created by the excessive voltage away from sensitive components when triggered.

In recent years, studies on low-voltage varistor materials have gotten widespread attention, such as TiO2, Sr TiO3 and WO3. In 1994, WO3 ceramics nonlinear behavior was first reported. The study shows that WO3 ceramic material has low breakdown voltage and good dielectric properties, which makes it ideal for low voltage varistor material.

With the further study of nonlinearity WO3 ceramics, we found that non-linear characteristics and mechanism of WO3 are not significantly different from conventional ceramic varistor materials of ZnO and SnO2. WO3 ceramic normally sintered exhibits significant nonlinear characteristics. The high temperature quenching sample has no nonlinear behavior. quenched ceramic samples restored after heat treatment under oxygen-rich conditions. Impedance spectroscopy analysis showed that the ceramic samples with non-linear characteristic have high resistivity layer at grain boundaries,  while the sample without non-linear behavior not. It’s believed that a high-impedance grain boundary layer is a non-equilibrium grain defects occurred inside and outside and migration of ceramic during cooling, forming high-resistivity layer on the grain surface under the action of oxygen adsorbed. Because of internal and external features of this huge grain of resistance differences, the electronic barrier informs in the grain boundary. This is the origin of the nonlinear characteristics of WO3 ceramics.

Tungsten Oxide Ceramic Target

Tungsten trioxide film is an important functional material, it’s been used in the system because of its excellent electrochromic properties. The main evaporation materials used in Electron beam evaporation preparing electrochromic WO3 film are WO3 powder and WO3 target. More current research is on the WO3 powder as evaporation material, but the form of powder is generally inappropriate, so the evaporation material is chosen. So choosing WO3 target as a evaporation material is an optimization program. However, there are few reports of WO3 target preparation at home or abroad. WO3 target prepared in domestic laboratories has disadvantages of low density or impure, so the preparation of high purity and high densityWO3 target is significant.

To prepare high purity ultra-compact WO3 target, it can be researched in powder, forming and sintering, the optimal combination of which is the necessary condition to achieve the above goals. 

(1) Preparation of WO3 powders of high purity, high sintering activity and suppression;
Use domestic high-purity tungsten oxide (greater than 99.99%) as raw materials and low-energy rolling ball milling process to prepare the experimental powder, investigate the crushing mechanism and the effects of milling technology, media, etc. on the size of the structure and purity.

(2) WO3 ceramics pressing process optimization;
Put WO3 granulated powders under different pressures in the mold, and analyze the relationship between the pressure and the green density and discrete explore optimal pressing process.

(3) WO3 ceramics sintering process optimization.
In order to suppress the sublimation of WO3 sintering process, the sintering process should be in oxygen throughout the experiment. Investigate the effects of temperature and density of sintered compacts caused and find the optimum.

Rare Earth Oxide Doped Tungsten Oxide Ceramic

The microstructure, the phase structure, nonlinear electrical properties and dielectric properties of rare earth doped WO3 have been studied and following are the main conclusions.

(1) Rare earth doped affects the growth of WO3 grains. A small amount of Gd and Ce doped has restrictions on grain growth, while heavily doped contributes to grain growth. Dy and La doped contributes to WO3 grain growth. Yb doped WO3 restrict the growth of crystal grains. The grain size of the sample is substantially between 10 ~ 20μm. EDS analysis showed that the main adulterants doped out at the grain boundary.
(2) Rare earth doped can significantly inhibit the generation of triclinic phase WO3 and make WO3 single-phase, thereby improving the ceramic electrical stability of WO3 at high electric field. Rare-earth doped can reduce ion transport in the depletion layer, so that the sample also has stable electrical properties at low electric field, indicating that WO3 has good prospects in the field of low voltage.
(3) Rare earth doped WO3 ceramics have a low varistor voltage and the barrier voltage, therefore WO3 is particularly suitable for low pressure varistors.
(4) Rare earth doped can not improve nonlinear coefficient of WO3 ceramics. Nonlinear coefficient is substantially in the range of 2 to 5.
(5) Rare earth doped can increase dielectric constant of WO3 in varying degrees, on the whole, which can be improved about one magnitude. Increasing the dielectric makes WO3 more suitable for Capacitive - Varistor Double Functional Materials.
(6) Samples doped with Dy and La have a special grain boundary phase. Rod material occurs at grain boundaries of La-doped sample that make the barrier between the grains disappear, so the sample shows a linear voltage characteristics. Grain boundary resistance of La-doped sample is less different from the resistance of the grain, but still has a large dielectric constant, indicating that the normal grain boundary barrier layer capacitor model (GBBLC) cannot well explained the phenomenon that La doped samples have a high dielectric constant.
(7) Rare earth doped WO3 ceramics Schottky barrier model is proposed. The results show grain boundary barrier of WO3-based ceramic has the similar properties of ZnO grain boundary barrier.
(8) The high-temperature electrical behavior of Tb doped WO3 ceramics has been studied. Sample still has some non-linear electrical characteristics at high temperature of 300 ~ 500 ℃. Two phases coexisting at a high temperature is considered to be the non-linear sources.
(9) Tb doped WO3 ceramics have some hot electric current output at high temperatures when no external field, this electric current is neither induced by the thermoelectric effect, nor a simple pyroelectric phenomenon. The act of which is like a direct thermoelectric conversion cells. Therefore we believe that this extraordinary thermoelectric effect may become new ways of heat energy -electrical energy conversion.

Tungsten Oxide Ceramic Varistor Property

A varistor is an electronic component with an electrical resistance that varies with the applied voltage. Also known as a voltage-dependent resistor (VDR), it has a nonlinear, non-ohmic current–voltage characteristic that is similar to that of a diode. In contrast to a diode however, it has the same characteristic for both directions of traversing current. At low voltage it has a high electrical resistance which decreases as the voltage is raised.

Varistors are used as control or compensation elements in circuits either to provide optimal operating conditions or to protect against excessive transient voltages. When used as protection devices, they shunt the current created by the excessive voltage away from sensitive components when triggered.

The nonlinear WO3 ceramics (pressure-sensitive behavior) was first reported by Makarov in 1994, and it’s pointed out that it can be used as a low pressure-sensitive materials in the field of microelectronics since the WO3 ceramics has low varistor voltage. But there are not further studies and reports in earlier studies about behavior origin and mechanism of WO3 ceramics issues.

Research shows that the varistor characteristics of WO3 ceramic is different from traditional ZnO and SnO2 Varistor material, the conventional Schottky barrier model can not explain the behavior of WO3 ceramic varistor yet, so the conventional grain boundary Schottky barrier model needs to be modified. In view of which, WO3 ceramics was prepared, and the mechanism of the pressure-sensitive characteristics were studied, and amendments WO3 ceramic grain boundary Schottky barrier model was proposed.

Undoped WO3 ceramics sintered has obvious nonlinear behavior, AES spectra shows the ceramic crystals surface has excess of oxygen. Results of the quenching and the atmosphere treatment of ceramic samples showed that the excess of oxygen on the grains surface is the result of the oxygen adsorption of the ceramic during cooling. With the effect of oxygen of the grain surface adsorption and W ions, and electrons combine to provide grain interior surface of the grains in the form of O- and O2- interface states at grain boundaries further Schottky barrier is WO3 ceramic varistor origin of behavior. Thus according to the conventional pressure-sensitive ceramic grain boundary Schottky barrier model, a revised WO3 ceramic grain boundary barrier model.

Tungsten Oxide Ceramic Preparation

Produce nano WO3 ceramic powder preparation by ball mill method. 

A ball mill, a type of grinder, is a cylindrical device used in grinding (or mixing) materials like ores, chemicals, ceramic raw materials and paints. Ball mills rotate around a horizontal axis, partially filled with the material to be ground plus the grinding medium. Different materials are used as media, including ceramic balls, flint pebbles and stainless steel balls. An internal cascading effect reduces the material to a fine powder. Industrial ball mills can operate continuously, fed at one end and discharged at the other end. Large to medium-sized ball mills are mechanically rotated on their axis, but small ones normally consist of a cylindrical capped container that sits on two drive shafts (pulleys and belts are used to transmit rotary motion). A rock tumbler functions on the same principle. Ball mills are also used in pyrotechnics and the manufacture of black powder, but cannot be used in the preparation of some pyrotechnic mixtures such as flash powder because of their sensitivity to impact. High-quality ball mills are potentially expensive and can grind mixture particles to as small as 5 nm, enormously increasing surface area and reaction rates. The grinding works on the principle of critical speed. The critical speed can be understood as that speed after which the steel balls (which are responsible for the grinding of particles) start rotating along the direction of the cylindrical device; thus causing no further grinding.

The main factors that affect the efficiency of the ball mill are: 

(1) ball mill speed. Ball mill speed directly affects the state of motion in the barrel, ball mill attached to the cylinder wall if over-speed, losing crushing effect; if the speed is too slow, that is much lower than the critical speed, ball mill barrel rise a little then came down, crushing effect is little; when the speed is appropriate, ball is close to the tube wall, after some distance, ball falls away from the tube wall to give powder the greatest impact and abrasive action, with the highest crushing efficiency. 

(2) ball. The more ball was added during ball milling, the higher crushing efficiency, but too much balls will occupy the available space, resulting in lower overall efficiency. 

(3) addition amount of water and electrolytes. 

(4) loading. 

The WO3 powder directly weighed, adding some deionized water and wet grinding carbide ball in a plastic drum, dried and sieved to obtain a powder-like nano.

Tungsten Oxide Ceramic

Tungsten trioxide (WO3) is an extremely important high-tech materials, this material is non-linear, high-dielectric constant, electrochromic, with gas detection, chemical catalysis and other features. For the features of WO3, the nano-powder of WO3 can be fired into varistor ceramics, ceramic capacitors, light (electricity) color ceramic film, gas-sensing ceramics, photocatalytic degradation ceramic membrane, battery electrode ceramic materials, microwave absorbing ceramic film, new high-temperature thermoelectric ceramics and functional ceramics and ceramic films, which has great potential in many chemical, energy, electricity and other fields.

There are two preparation methods of ceramic raw materials, that mechanical disruption and synthesis. The former one uses mechanical principle crushing coarse particles to obtain fine powder, which has the advantages of large amount of production and low cost, but there are problems of impurities mixed in the crushing process, and it is difficult to obtain submicron particle size. And the powder produced by synthesis has high purity, small particle size, component uniformity, suitable for high performance requirements, low production needs of advanced ceramic materials.

Nanometer WO3 powders prepared by mechanical milling as a raw material of sintering tungsten trioxide ceramics, the systematic study of influence of milling parameters on particle size, the grinding can achieve the best results.