2016年4月27日星期三

Tungsten Oxide Ceramic

Tungsten oxide (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.
Tungsten Oxide

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.

Tungsten Oxide Ceramic Varistor Property

The nonlinear tungsten oxide ceramic (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.
Tungsten Oxide
Undoped WO3 ceramics sintered has obvious varistor 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 Target

Tungsten oxide 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 density WO3 target is significant.
Tungsten Oxide Ceramic

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.

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, tungsten oxide 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.
WO3
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.