Current hydrothermal method has been widely used in the synthesis of inorganic materials and material handling, compared to other methods have many advantages: a relatively simple equipment required for the reaction, and water and heat control has significant operational and adjustable degeneration; hydrothermal method particularly suitable for the synthesis of a special structure, a new specialty compounds and concentrations of condensed matter, rules-oriented and crystalline perfection of preparing a crystalline material balance defects; under hydrothermal conditions, the solution viscosity decreased, diffusion and mass transfer process more smoothly, active reactant has greatly improved, and thus can replace some of the hydrothermal synthesis temperature solid phase reaction, and promoting, the development of low-temperature synthetic chemistry.

In 1989, Sweden's Amberg, who for the first time by hydrothermal prepared a pyrochlore type tungsten oxide cubic system, the phase is a hydrothermal method in the study of pH = 3.0 synthetic magnesium tungstate in occasional crystals obtained using, parsed too known to be W2O6 • H2O (ie H2W2O7). Kenneth et al., 1992 reported a more detailed preparation conditions of the hydrothermal synthesis of crystalline tungsten oxide and its relationships, found in a pH range of 3.5 to 4.5 is obtained pyrochlore-type structure, at a pH of between 1.3 to 2.0 The resulting structure is hexagonal. Whereas at pH between 2.0 to 3.5 to obtain a pyrochlore type and hexagonal mixed phase, if the pH is greater than 5 are not precipitate the product. Guo et al., 1995 by hydrothermal synthesis using HCl to adjust the pH value of 6.0 was also prepared a pyrochlore-type oxide, tungsten, and the material has good electrochemical characteristics, indicating that factors pyrochlore type hydrothermal synthesis of tungsten oxide Many, wherein the pH of the environment is likely to be the most important factor.

Research on China Hydrothermal Method tungsten oxide pyrochlore type rarely, the method is slightly different: Within a pH range of 2.5 to 4.5 is obtained pyrochlore type superfine powder of pure tungsten oxide phase at pH get between 0.5 and 2.0 is hexagonal tungsten oxide. Studies have shown that while in the temperature range of 130 ~ 200 ℃ of the prepared powder grain size difference is not more than 5nm, the temperature can increase the grain size is slightly reduced, but little effect on the concentration of hydrochloric acid used in grain size. Powder aggregates mainly in the form of, EP-MA morphology shown in Figure 2, show the particle size analysis, the powder particle size 2 ~ 100μm bimodal distribution between the average particle diameter is large, and the powder for one month After the reunion phenomenon is more pronounced.

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Soft chemical synthesis under mild conditions, i.e. synthesis of inorganic functional material, or it is sought in the solution to a low temperature starting reactants are uniformly mixed on the molecular size, a controlled chemical reaction after generation precursor or intermediate, the last generation with the specified composition, structure and morphology of the material. 1988 French Coucou, who first synthesized by soft chemistry type of pyrochlore oxide, tungsten, they (NH4) 10-W12O41 • 5H2O as precursor glycol solution is heated in an acidic reflux get [(NH4) 2O] xW2O6 , repeatedly washed in acid solution, and the resulting powder is heated to remove water at 100 ℃, which obtained pyrochlore type tungsten oxide powder. Li et al., 1996, using the same method for preparing a series of (M2O) xWO3 • zH2O (M = H +, Li +, Na +, Ag +, and NH + 4), and to test the performance of the series powder humidity.

These methods often can only get there missing pyrochlore lattice type tungsten oxide, and water molecules which content is generally less than a full H2W2O7. Soft chemical method developed in recent years are generally presented Aurivil-lius Bi2W2O9 as precursor compounds obtained tungsten oxide pyrochlore type by acid treatment. RESchaak and ManabuKu-do, who devised the method detailed preparation process: Bi2O3 and WO3 will follow after 1:2 molar ratio of mixed 48h calcined at 800 ℃, resulting Bi2W2O9 into concentrated hydrochloric acid, and then vigorously stirred at room temperature for 72h to obtain undistorted H2W2O7 crystals. After H2W2O7 the vast majority of studies are based on this approach.

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Pyrochlore type oxide, tungsten mostly A bit deletion type of layered perovskite tissue, wherein a water molecule is formed directly in contact with the two-dimensional layer structure of tungsten by hydrogen bonding interactions between the layers. Chinese name is pyrochlore type oxide, tungsten foreign names Pyrochlore-typeH2W2O7, formula H2W2O7WO3 • 0.5H2O it for layered composites, sensing field, tungsten oxide calcined preparation.

Sodium ion exchange treatment pyrochlore type tungsten oxide were prepared containing H +, Ag +, NH + 4, Li + coke green
Stone type mixture, comparison found, [H-H2W2O7] has the lowest resistivity, and [Li-H2W2O7 relative humidity and resistivity between the number has the best linear law, and the sensor has a good durability and repeated sex. Focusing Greenstone type conducted research found that tungsten oxide Li insertion elements will cause changes in the lattice parameters, and could easily lead to a shift in the original amorphous crystal to generate a cavity structure and function with a certain sense.

In pyrochlore type tungsten oxide thin films as the cathode and lithium foil anode, 1mol / L of LiClO4 do electrolyte solution will power Li evenly embedded tungsten oxide pyrochlore type. The resulting products showed good electrochromic properties, and determines the chemical diffusion coefficient of lithium ions is 1.1 × 10-7cm2 / s, which is the highest value of the correlation coefficient can be achieved in the study.

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Different chain length alkyl amine embedded in the body of the layered inorganic microporous pillared material can be obtained, having a high specific surface area, porous structure with a pore size is adjustable, which research in the field of catalysis is important, but also in the adsorption, conduction, separation and catalysis and many other fields have broad application prospects. The several monohydric alkylamine (CnH2n + 1NH2, n = 4,8,12,16) embedded in a layered H2W2O7 behavior, they will H2W2O7 placed alkyl amine with a mixture of n-heptane, at room temperature 7d after reaction by washing, drying, to obtain a variety of layered composites. Alkylamine between the middle and the composite layer is formed by electrostatic attraction interaction, all-trans conformation bilayered arrangements, the layer spacing carbon atoms with alkylamines genus increases linearly. Bandgap semiconductor composite tungsten oxide is relatively much larger. Later they HTT as intermediates, but also indirectly by cation exchange mechanism of aniline cation H2W2O7 embedded between layers. Polyaniline molecules do not destroy the original laminate structure between the layers into a single vertical distribution. Comparative decomposition temperature H2W2O7 and alkylamine / H2W27 composite, polyaniline embedded greatly improved thermal stability of the material. Use H2W2O7 and n-octylamine was synthesized intercalation compound in a non-polar medium, heptane, after removal of the organic amine, the final calcination step to obtain a functional material, the material exhibits a very high sensitivity to alcohol. Also use H2W2O7 • xH2O with heptane / octane mixed solution reaction, between the release of the layer of water hydrolysis reaction with octylamine highly alkaline aqueous solution to form microdomains, [W2O7] bilayer is dissolved in an alkaline solution is high [WO4 ] monolayer, such as formula (7) below. Generation of H2-b (RNH3) WO4 quasi reverse micelles medium for highly ordered self-assembled inorganic - organic hybrid nanoribbons (C8NH2-HWO), the nano-dispersed in an aqueous solution with nitric acid, by magnetic stirring, the solid-liquid separation, washing, WO3 nanosheets after heat treatment, and the morphology, structure and formation mechanism of the product discussed. Later, after a series of studies, they will intercalation reaction of specific conditions were appropriate adjustments, the final establishment of the use H2W2O7 • xH2O preparation intercalation composites and nano-WO3 better process and noted that the method is suitable for large-scale WO3 nano production, low cost and meet the requirements of industrialization. H2W2O7 • xH2O + aRNH2H2-a (RNH3) W2O7 + xH2O (6) H2-a (RNH3) aW2O7 + (2b-a) RNH2 + H2O2H2-b (RNH3) bWO4 (7).

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Carbide coatings are produced in the form of cermets (ceramic and metal) they provide hardness and wear resistance coatings similar to solid sintered carbide components like carbide metal cutting tools. Metal oxides such as chromium oxide are also very hard and are very resistant to chemical attack so they can be an ideal solution when wear and corrosion are both present. Several metals and even some plastics are also used to provide wear resistant coatings.

Carbide coatings are produced in the form of cermets (ceramic and metal). Cermets provide hardness and wear resistance coatings similar to solid sintered carbide components like carbide metal working tools.

Tungsten carbide is an especially effective wear resistant coating, as it offers exceptionally high hardness levels (maximum 74 HRC hardness). Tungsten carbide is highly resistant to extreme temperatures below 650°C (1200°F) and corrosion.Metal oxides such as chromium oxide are also very hard and are very resistant to chemical attack so they can be an ideal solution when wear and corrosion are both present.


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The benefits related to tungsten carbide coating and hard face welding list as follow,
- Tools can be resurfaced not replaced
- Increased gripping power
- Carbide coating increases tool life
- Treatment cost a fraction of replacement tooling
- Decreases downtime
- Tungsten carbide coating surface hardness is up to 74 Rc
- Excellent uniformity
- No thermal distortion due to the low heat input process
- True metallurgical bond
- Carbide coating will not chip, peel or flake
- In most cases clamping pressure can be reduced
- Increase machining rates with additional gripping power
- Tools can be accurately masked off
- Increase or decrease the coefficient of friction
 

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Before a finished tool or component can be made, typically a blank needs to be made.  This is performed through a powder metallurgy and sintering process.
• Firstly a mould of the macro shape of the blank desired is made. This is made intentionally at a calculated larger size to accommodate the volumetric shrinkage (which can approach up to 50%) that will occur later in the sintering process.
• Powders are then prepared with an involved process of mixing and milling, paying attention to particle size and consistency, purity, quantity etc. This step has a significant effect on the “quality” of the overall blank produced.
• The prepared powders are then compacted into a solid “green” state blank with the use of high pressure press.  The green blank can then be handled and to some degree additional processes such as turning, drilling and milling can be performed on the item before it is sintered.
• The green parts are then put through a process of pre-sintering, sintering and Hot Isostatic Pressing (if specified) to remove the binder waxes, liquefy the binder and fully densify the cemented carbide. Shrinkage occurs during this process.
• The fully dense tungsten carbide blank may then be further finished by grinding, EDM wire cutting, or diamond polishing. For the most demanding high volume and hardest working production tools, tungsten carbide is a superior base material.
 

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Hard coatings of tungsten carbides have been obtained by the deposition of tungsten thin layers, on steel substrates (containing 0.7% wt. carbon), according to the cathodic magnetron sputtering held at temperature of 500°C. It is established by X-rays diffraction that, in the temperature range 500-800°C, no formation of tungsten carbides was observed. However, the annealing at a temperature greater than or equal to 900°C promotes the reaction between the constituents of the samples (W, Fe, C) and hence the formation of W2C carbide. No other compounds were detected. The micro-hardness measured by Vickers tests, increases with the rise in temperature, particularly from 900°C. The morphology of the surface samples depends on the temperature and duration of thermal annealing.


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Nano-Structure of CVD tungsten carbide coating is a sophisticated nano-structured material comprising a metallic tungsten matrix with dispersed tungsten carbide nano-particles with a typical size range of 1-10 nm.

This coating type increases abrasion resistance up to tweleve fold higher than that of hard chrome and improved hardness of more than 1100 Hv. The coating can be formed on titanium, low alloy and some tool steels, stainless steel, and Ni-, Co- and Cu-based alloys. This nano-structured material shown unprecedented impact resistance, crack resistance and toughness by tolerating 3000 microstrain deformations without any deterioration.
 

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CVD tungsten carbide coating is suitable for applications with many different components operating in extreme erosive and abrasive atmospheres, such as critical components of metal seated ball valves, downhole tools, and pumps handling abrasive fluids.

Ball valves coated with CVD tungsten carbide coating, demonstrate increased valve life thanks to the coating as it make them scratch-proof and able to resist erosion and abrasion.

CVD tungsten carbide coating extends the life and reduces downtime costs of downhole tools such as grippers for down-hole tractors, high loading bearings pins, and mud-driven hydraulic parts for directional drilling tools.

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