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Application Background and Overview of Strontium Titanate
Strontium titanate has a high dielectric constant and is an important raw material for electronic ceramic powders. Its products have the advantages of low dielectric loss and good thermal stability, so they are widely used in the electronics industry. There are many reports on the preparation of strontium titanate in natural science magazines. The most typical method is the laboratory method provided by the "Research on New Methods for Synthesis of Strontium Titanate Powder" in the Inorganic Salt Industry in 2002. This method uses titanium tetrachloride and strontium chloride as raw materials, ammonium carbonate and ammonia water as precipitants, and uses chemical coprecipitation to synthesize strontium titanate powder. The influence of process conditions on product purity and strontium-titanium ratio is studied.
The reaction conditions are that the molar ratio of titanium tetrachloride to strontium chloride is 1.02, the molar ratio of ammonium carbonate to strontium chloride is 1.40, the reaction temperature is room temperature, and calcination at 900°C for 4 hours is required; the disadvantage of this method is that it requires a long time of calcination, and the particle size distribution may be uneven due to the growth of some strontium titanate grains; in addition, some studies have made a detailed description of the crystallization mechanism of strontium titanate, but its preparation method and the solution for synthesizing strontium titanate are less feasible in industrial production. The methods for preparing strontium titanate in the world include solid phase method, coprecipitation method, sol-gel method, organic complex precursor method and other processes.
The solid phase process is to calcine titanium oxide and strontium carbonate at high temperature. Since the particle size distribution of strontium titanate after calcination is difficult to control and the purity is low, it greatly affects the performance of the product. Coprecipitation is the earliest method used to synthesize ultrafine high-purity metal oxide particles by liquid phase chemical reaction. The precipitation method has low cost, but has the following problems: the precipitate is usually colloidal, which is difficult to wash and filter; the precipitant is easily mixed as an impurity, and various components may be segregated during the precipitation process, and some precipitates are dissolved during washing.
In addition, since a large number of metal ions are not easy to undergo precipitation reactions, the application of this method is also narrow. The sol-gel method generally uses organic metal alkoxide as a raw material, and obtains a solid precursor through hydrolysis, polymerization, drying and other processes, and finally obtains a nanomaterial through appropriate heat treatment; because metal alkoxide is used as a raw material, the cost of this method is relatively high, and the gelation process is slow, so the general synthesis cycle is relatively long.
In addition, some metal ions that are not easy to be hydrolyzed and polymerized are difficult to be firmly bound to the gel network, which limits the types of ultrafine high-purity composite oxides obtained by this method. Therefore, if strontium titanate is prepared by the current solid phase method, coprecipitation method, sol-gel method, organic complex precursor method and other processes, there are deficiencies and defects.
Strontium titanate is an important raw material in the electronics industry, used to automatically adjust heating elements and manufacture components with demagnetization. In the ceramic field, it is used to manufacture ceramic capacitors, piezoelectric ceramic materials, ceramic sensitive elements, and microwave ceramic elements. It can also be used as pigments, enamels, heat-resistant materials, and insulating materials. Strontium titanate single crystals can be used as optical materials and artificial gems.
Strontium titanate crystals have high refractive index and high dielectric constant, and its single crystals are used as optical materials and artificial gems. Low-temperature superconductors based on strontium titanate have also been made. Examples of its applications are as follows:
1. Prepare a strontium titanate transistor with simple process and good stability.
By using substitutional doping method, laser molecular beam epitaxy, pulsed laser deposition, magnetron sputtering, electron beam evaporation or molecular beam epitaxy and other film-making methods, n-type strontium titanate SrAxTi1-xO3 or Sr1-xLaxTiO3 thin film materials, where A is Nb or Sb, are prepared on single crystal substrates (such as SrTiO3, YSZ, LaAlO3, Nb:SrTiO3, etc.); p-type strontium titanate SrBxTi1-xO3 thin film materials, where B is In or Mn, are prepared. All x values range from 0.005 to 0.5.
When a layer of p-type strontium titanate and a layer of n-type strontium titanate are epitaxially grown together, these two layers of strontium titanate films with different conductivity types form a p-n junction at the interface, and this p-n junction constitutes a strontium titanate crystal diode; when a layer of p-type strontium titanate, a layer of n-type strontium titanate and another layer of p-type strontium titanate are epitaxially grown together, these three layers of strontium titanate films form a p-n-p junction, and this p-n-p junction constitutes a p-n-p strontium titanate triode; when a layer of n-type strontium titanate, a layer of p-type strontium titanate and another layer of n-type strontium titanate are epitaxially grown together, these three layers of strontium titanate films form an n-p-n junction, and this n-p-n junction constitutes an n-p-n strontium titanate triode.
The strontium titanate transistor adopts a full epitaxial process, so the thickness and carrier concentration of each layer are easier to control than the germanium silicon transistor, and the junction is sharper. Strontium titanate has a high melting point and good stability, so strontium titanate transistors will become a widely used electronic device and can also be developed into strontium titanate integrated circuits.
2. A method for preparing an amorphous strontium titanate thin film device,
including: spin coating a bottom electrode solution on the surface of a substrate and then annealing to form a bottom electrode; spin coating a strontium titanate solution on the surface of the bottom electrode and then baking to form an acid strontium film; annealing the strontium titanate film to form an amorphous film; electron beam evaporation sputtering on the surface of the amorphous film to form a top electrode to obtain an amorphous strontium titanate thin film device.
The biggest innovation of the amorphous strontium titanate thin film device is that the main oxide film layer of the device is an amorphous film, and the preparation method is simple, the temperature requirement during the preparation process is low, and mass production can be carried out. In addition, the prepared strontium titanate thin film device has good stability, fatigue resistance, and can be recycled. It has a large switching ratio. When annealed at 400°C, the switching ratio reaches 103, and it has a wide range of applications.
