2016年3月30日星期三

Functional Ceramics and Tungsten Oxide

Functional ceramics have many excellent properties in electricity, magnetism, sound, light, heat, etc. that make it difficult for other material to match, and some have more than one function. And these properties usually depends on the internal electronic state or structure of the atomic nucleus. It’s also known as electronic ceramics. It has been widely used in energy development, electronic technology, sensor technology, laser technology, photonics technology, infrared technology, biotechnology, environmental science and other aspects. In addition, the semiconductor ceramic, the insulating ceramic, the dielectric ceramic, luminescent ceramics, photosensitive ceramic, absorbing ceramics, ceramic laser, a ceramic nuclear fuel, propellants ceramics, ceramic converting solar light into energy storage and ceramics, ceramics solid battery, ceramic damping biotechnology ceramics, catalysis ceramics, film and other special functions, which also play important roles in automatic control, instrumentation, electronics, communications, energy, transportation, metallurgy, chemicals, precision machinery, aerospace, defense and other departments.

Tungsten Oxide


In recent years, nanotechnology has developed rapidly with many active researches. When entering the nanoscale, the particles has quantum size effect, surface effect and macroscopic quantum tunneling effect, making the material exhibit unusual physical and chemical properties. WO3 is an important functional material, widely applied in aspects of electrochromic, toxic gas detection and photocatalytic degradation. In 1994, Makarov and Trontelj found WO3 doped Na2CO3 and MnO2 has an obvious nonlinear I-V characteristic, which indicates WO3-based functional materials have the potential being applied to the varistor.

The electrical properties of tungsten oxide functional ceramics and WO3-based ceramics doped low metal elements and rare earth elements have been studied that there is a certain non-linear characteristic no matter WO3 ceramics doped or not, and the doping process can change its non-linear coefficient. But most researches on tungsten trioxide functional ceramics use micron WO3 as raw material, common electronic ceramics preparation process, focusing on changing the doping elements and proportions, observing the effects on the electrical behaviors, and there are few reports about Effect of changing the sintering process on tungsten trioxide-based functional ceramic electrical behaviors.

Ammonium Paratungstate Preparing Ultrafine Cesium-tungsten Oxide Powder

Cesium is a golden yellow metal, low melting point active metal, easily oxidized in air and can react violently with water to produce hydrogen and even explode. There is no elemental form of cesium in nature, cesium minimal distribution in the ocean in the form of cesium salt. Cesium tungsten bronze is widely used because of its low resistance, excellent visible light transmittance, near-infrared shielding properties. Also it is widely used in preparing conductive thin film, since being the glass transparent insulation coating as insulating agent, it has excellent properties like low resistance, excellent visible light transmittance and near-infrared shielding performance. The article provides a method that using ammonium paratungstate and cesium nitrate as raw materials to produce Cesium-tungsten oxide ultrafine powder, the specific steps are as follows:

Cesium

1. Weighing the cesium nitrate, ammonium paratungstate by the molar ratio of Cs/W being 1: (1.5 to 2.8), and adding chelating agent and alcohol reagent, reacting at 170°C for 3 hours;
2. Loading the mixture obtained in step 1 in a pressure vessel shells, raising the temperature to 260~270°C for reacting for 5 to 8 hours;
3. The reaction obtained in step 2 is carried out alcohol washing, centrifugation; then dried in the conditions of 80°C in vacuum, thus to generate complete crystalline cesium-tungsten powder.

Using this method to prepare cesium-tungsten oxide ultrafine powder has many advantages, such as: saving the materials, thus to save costs; shorten the preparation period, make it advantaged for industrial production; products prepared are ultrafine powders with a very low resistance.

Tungsten Oxide Crystalline Form Affects Tungsten Carbide Property

Under certain conditions the six-phase tungsten oxide(h-WO3)can exist in stable form. When annealing temperature exceeds 400℃, it can turns into monoclinic system(γ-WO3). Nano tungsten oxide applied in electric and optical field is different from tungsten oxide bulk. The main differences are phase transition temperature drops evidently, so sometimes orthorhombic tungsten oxide(β-WO3)appears and is stable under room temperature.

Chemical stoichiometric number of tungsten oxide seems simple, its phase transition and structure change is complex. The ideal tungsten oxide is octahedron, W6+  exists in the center part of it. Oas anion distribute in the other eight edges. Actually, W6+  often deviates from the center part, the whole structure turns into tetragonal crystal(α-WO3), temperature range is T>740℃. For orthorhombic system, 740℃>T>330℃; monoclinic system: 330℃>T>17℃ and triclinic system: 17℃>T>-43℃.
Below are SEM graphs of tungsten oxide, testing equipments are: ICP-AES Spectrograph, X-ray Powder Diffractometer, TSM-6360 LV scanning electron microscope, MICROPLUS laser diffraction particle size analyzer.
Tungsten Oxide SEM

In a word, tungsten oxide has various crystalline forms, its transition can affect its property, tungsten oxide crystalline form can largely affects tungsten carbide property. Due to this factor, tungsten oxide can be applied in many fields because of its multi-properties.

Tungsten Oxide Inorganic Ceramic Film

Tungsten oxide inorganic ceramic film has high efficiency, high temperature resistance, high performance and stable chemical property. It has wide application field. Compared to organic film, it has following characteristics:
Tungsten Oxide Inorganic Ceramic Film
1.Tungsten oxide inorganic ceramic film has small pore size distribution which shows as normal distribution, 80% of them has error within ±10%. For example, the 0.05um film, among which 80% of pores are 0.049um-0.051um, it insures the stability of the applied film. This has a big difference from organic film which the pore size distributes uniformly.
2.Porosity of tungsten oxide can reach 35%-40% which ensures film flux.
3.Tungsten oxide inorganic ceramic film departure structure is more reasonable, there are supporting layer and separating layer, pore size is 5-10、1.0、0.6、0.2um, it becomes really gradient films which improve pollutant-resistance of film. The separating layer is even thinner which is easier to clean and wash. On the contrary, organic film is symmetric film, entry of film needs strict pre-treatment.
4.Tungsten oxide inorganic ceramic film has higher strength, it can resist pressure to 16bar, the supporting part can resist pressure to 30bar. It can ensure the dealing effects and stability in quality.
5.Tungsten oxide inorganic ceramic film has high insulating property.
6.Service life of tungsten oxide inorganic ceramic film is more than 5 years, however the organic film is only 3~6 months.
7.Chemical stability and thermal stability of tungsten oxide inorganic ceramic film is better than organic film, the strong acid, alkali and oxidizing agent can be used as detergent, steam sterilization can be applied directly. However, organic film can not be operated under the above mentioned conditions.