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.

Tungsten Trioxide Photocatalysts:

Hexagonal (h-) and monoclinic (m-) tungsten trioxide (WO₃₎ nano particles with controlled composition  were prepared through annealing (NH₄)_xWO_3−y. The formation, structure, composition, morphology, and optical properties of the samples were analyzed by powder X-ray diffraction, scanning and transmission electron microscopy combined with electron diffraction, and Raman, X-ray photoelectron, H magic angle spinning nuclear magnetic resonance, diffuse reluctance ultraviolet–visual, and photoluminescence spectroscopy. Oxidized m-WO₃ (m-WO₃ ox) was the most active photocatalyst both in the aqueous and in the gas phase, followed by the oxidized h-WO₃ (h-WO₃ ox) sample. Reduced h-WO₃ (h-WO₃ red) and m-WO₃ (m-WO₃ red) exhibited much lower activity. Thus, in contrast to TiO₂, where crystalline structure (rutile or anatase) plays a key effect in photocatalysis, for WO₃, it is the composition that is of greatest importance: the more oxidized the WO₃ sample, the better a photocatalyst it is. The crystal structure of WO₃ has only an indirect effect, in that it influences the composition of WO₃ samples. Consequently, an oxidized monoclinic WO₃ material will always provide better photocatalytic activity than an oxidized hexagonal one.

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