Metatungstate is one of many types of tungstate, the only compound which the structures have been identified as colorless crystals. It has high solubility in water, and can be derived from metatungstate. Since the metatungstate itself is extremely unstable and easily degradable, therefore, it has a large difficulty in preparation. In the traditional way, sodium tungstate often used to prepare metatungstate crystals, but the complex process, high production costs, low productivity, poor product quality and the environmental pollution and other defects make the way no to be better. Nowadays, harmonious environment, cost savings, simplified procedures are promoted, so its necessary to develop a process which has the simplified process, low cost and environmental pollution, and also conducive to industrial producing metatungstate.

Studies have pointed out that ammonium paratungstate (APT) can be used to prepare metatungstate solution. The steps as following:
1. APT powder heated to 270℃~350℃ for 30-60 minutes to dissociate, thus to generate ammonium metatungstate;
2. Add ammonium metatungstate powder into deionized water to leaching with stirring to get an emulsion;
3. Carry out filtration to remove insoluble, and the filtrate was placed in a water bath of 60~90℃ aging for 4~24 hours;
4. Filtered again, and exchanged by Cation Resin, then we get metatungstate solution.

As we can see from the above, the method APT used as raw material to prepare metatungstate solution has the advantages like: simplify process, easy to control, and low cost; In addition, since the reaction process doesn’t introduce any impurity element, so the purity and quality of metatungstate can be ensured; finally, the production process does not use ether and other volatile organic compounds, with minimal impact on the environment, so it is suitable for industrial mass production.

Preparation Process:
Preparing various samples at different sintering temperatures after the samples were prepared, than measuring different sets of samples according to voltammeters, impedance spectroscopy and dielectric spectroscopy curves. We can obtain micron tungsten trioxide nonlinear coefficient polyethylene composite samples by calculating the nonlinear coefficients of main sample data. In the composite sample, the tungsten trioxide particles in the sample content has polyethylene nonlinear coefficients, and the nonlinear coefficient of the sintered samples, sintering the other samples at 250 ℃, up to 98%. When the organic matter content of polyethylene is 80%, the sample becomes an insulator; the prepared tungsten trioxide polyethylene material is more stable.

Sample Analysis
The experimental methods commonly is based on varistor methods, and calculating barrier trends, comparing barrier with nonlinear coefficient, typical sample is made by six-phase morphology in the structural analysis. Comparing samples with nonlinear in the function of the body as well as micro particles and nanoparticles composite sample, which shows the trends of barrier are consistent with nonlinear coefficients.

Test results of morphology and electrical properties show that polyethylene resistance and nonlinear crystal polyethylene material are applied in the electric field of tungsten trioxide samples, which resulting shift toward low-resistance crystalline phase, and it has a close relationship. The results of nonlinear of typical sample with impedance spectroscopy show the bias under the grain boundary resistance are not changed significantly. Resistance is low because their low resistances phase structures that are mainly monoclinic.

Tungsten trioxide prepared by nanometer tungsten trioxide by the hydrothermal method and calcination treatment, which is characterized by higher non-symmetry axis and the second symmetry axis, the symmetry plane is more complicated. Monoclinic tungsten trioxide has a strong heterogeneity, which has three principal refractive indexes.

Taking Na₂WO₄·2H₂O as raw materials, DL- malice acid as supplement aid, in this context, which can be synthesized in a three-dimensional spherical shape containing precursor (WO_3·xH₂O). It shows that the addition of malice acid dose of WO_3·xH₂O morphology and crystalline phase having a significant impact. Thus gas test shows that the optimum operating temperature of three samples reaches to 250 ℃, and square-shaped monoclinic tungsten trioxide shows the highest sensitivity, which mainly due to its unique and innovative three-dimensional porous structure. At 300 ℃, the optimum operating temperature is the lowest detectable concentration of material, which can reach 0.1ppm, material optimum calcination temperature is determined at500 ℃,the sensitivity of 5ppm acetone sample gas reaches to 7.9; in the relative humidity conditions , the sensitivity of 5ppm acetone resistance remains 5.72.

In tests of different gases (such as acetone, methanol, ethanol, ammonia, etc.) in the gas sensing of different working conditions, monoclinic tungsten trioxide gas shows high selectivity and sensitivity. Taking Na₂WO₄·2H₂O as raw materials, which are used by Na₂C₂O₄, Na₂SO₄ and H₂C₂O₄ supplement. The hydrothermal conditions keep 24 hours at 180 ℃. Experimental results show that the solution plays the role in Na⁺、SO₄^-、C₂O₂^-、HC₂O₄^-  ions and H₂C₂O₄ nano WO₃ crystal nucleation and growth process. The results show that three different adjuvant does not effect on crystal of product system. Subsequent tests show Na₂WO₄ assisting synthesis of WO₃ nimrods at 320 ℃ for certain have concentrations to gas sensing, which mainly due to its high degree of dispersion and crystalline.