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The mysterious "black box" of the hydrothermal method is opened

2020-04-17 11:25:57  Knowledge

For the first time, the University of Science and Technology of China used graphene oxide's liquid crystal behavior and gelation ability to obtain a gel with a ring-shaped polar structure. It revealed the fluid expression in hydrothermal synthesis according to the microstructure of the gel.

 

With the development of nanomaterials, nanotechnology has gradually matured and can be roughly divided into nanomaterial technology and nano processing technology, nanometer measurement technology, nanotechnology, and other aspects. Among them, nanomaterial technology focuses on the production of nano-functional materials, (such as ultrafine powder, coating, nano-modified materials, etc.) performance detection technology (such as chemical composition, microstructure, surface morphology, physical, chemical, electrical, magnetic, thermal And optical properties). Due to the recognition of nanomaterials in the market and the rapid advancement of nanomaterial technology, nanomaterials have been widely used in every corner of the production field.

 

For example, nano-magnetic materials have unique magnetic properties, the size of nanoparticles is small, and the single magnetic domain structure and coercivity are high. The magnetic recording material made of it not only has excellent sound quality, image, and signal-to-noise ratio but also has a recording density ratio. γ-Fe2O3 is dozens of times higher. Superparamagnetic robust magnetic nanoparticles can also be made into magnetic liquids, which are used in the fields of electroacoustic devices, damping devices, rotary sealing, lubrication, and beneficiation.

 

Some of the nanomaterials are very sensitive to temperature changes, infrared rays, and automobile exhaust. Therefore, they can be used to make temperature sensors, infrared detectors, and car exhaust detectors, and the detection sensitivity is much higher than conventional similar ceramic sensors. Also, making semiconductor materials such as silicon and gallium arsenide into nanomaterials will be given better performance and play an essential role in the fields of large-scale integrated circuit devices and optoelectronic devices.

 

With the use of semiconductor nanoparticles, a new type of solar cell with high photoelectric conversion efficiency and regular operation in rainy and rainy days can also be prepared. Because the electrons and holes generated by the nano-semiconductor particles are irradiated with light, they have substantial reducing and oxidizing capabilities, so they can oxidize toxic inorganic substances, degrade most organic materials, and finally generate non-toxic and tasteless carbon dioxide and water. Therefore, the use of semiconductor nanoparticles can also use solar energy to catalyze the decomposition of inorganic and organic matter. Nanomaterials also play a massive role in the fields of medical treatment, computers, home appliances, environmental protection, textiles, and machinery.

 

Nanomaterials are so "magic," so where did it come from? The utilization rate of natural nanomaterials is meager, and most of the nanomaterials used in practice are artificially manufactured. The hydrothermal method is commonly used in the synthesis of nanomaterials and has many synthetic advantages. Hydrothermal synthesis can complete the formation and crystallization of products in a single step. The process is simple, the cost is relatively low, and it is easy to obtain well-oriented and perfect crystals; in the growing crystals, it can be uniformly doped and regulate the crystal generation—environmental atmosphere.

 

Due to the many advantages of hydrothermal synthesis, this method has been widely used and developed in the past 100 years. At present, it has become a standard method for synthesizing various materials such as single crystals, metal oxides, ceramics, zeolites, and nanocomposites. However, the information available in hydrothermal synthesis is limited to input raw materials, output products, and reaction conditions, and people do not know how the processes in closed system reactions occur. However, to effectively control the quality of hydrothermal synthesis products, it is particularly important to understand and understand the heat and mass transfer process.

 

All along, in the hydrothermal process, because the hydrothermal reactor is in a sealed state and high temperature and high-pressure conditions, it is challenging to achieve in-situ observation. Therefore, how to open this "black box" has become a challenge in the field of hydrothermal synthesis research. For the first time, the University of Science and Technology of China used graphene oxide's liquid crystal behavior and gelation ability to obtain a gel with a circular polar structure. The researchers revealed the fluid expression in hydrothermal synthesis based on the microstructure of the gel.

 

The results of the study show that, regardless of the size and shape of the polytetrafluoroethylene lining in the reactor, convection always exists in hydrothermal synthesis. For a specific reaction, temperature difference and reactor liner size are the most critical factors affecting convection. The effect of enhancing convection is the same as that of mechanical disturbances, and the uniformity of the product becomes poor, especially for the large-scale synthesis of nanowires, nanosheets, or bulk gel materials using hydrothermal methods. Will produce more impurities or cause the internal structure of the three-dimensional block to be uneven.

 

The technological breakthroughs and important discoveries of the University of Science and Technology of China have further enhanced the understanding of the process and mechanism of preparing micro-nano materials of different sizes and shapes by hydrothermal synthesis. They have extraordinary significance for the future development of hydrothermal synthesis nanomaterial technology Guiding significance. It is believed that the new construction and utilization of hydrothermal synthesis will bring better market opportunities for nanomaterials.

 

In the past 100 years, hydrothermal synthesis has been widely used and developed. It has become a standard method for synthesizing various materials such as single crystals, metal oxides, ceramics, and nanocomposites. However, the information available in hydrothermal synthesis is limited to input raw materials, output products, and reaction conditions. It is unclear how the processes in closed system reactions occur. To effectively control the quality of hydrothermal synthesis products, it is particularly important to understand and understand the heat and mass transfer process. Therefore, how to open this "black box" has become a challenge in the field of hydrothermal synthesis research.

 

The researchers found that under hydrothermal conditions, graphene oxide nanosheets can be aligned along the direction of the flow field under the action of fluid shear. Besides, graphene oxide nanosheets can be fixed in situ by cross-linking with phenolic resin to form an axisymmetric gel with a ring structure. Researchers can infer the fluid behavior in hydrothermal synthesis through direct observation and analysis of gel morphology and composition. Based on this, the researchers carried out some factors such as heating temperature, solution viscosity, and reactor size/morphology.

 

The results show that for a specific reaction, the temperature difference and the size of the reactor lining are the most critical factors affecting convection. The larger the volume of the reactor, the more uneven the heat transfer of the reaction liquid, and the higher the temperature difference, the stronger the convection. The effect of enhancing convection is the same as that of mechanical disturbance, and the uniformity of the product becomes poor, especially it will have a non-negligible impact on the large-scale synthesis of nanowires, nanosheets or bulk gel materials using hydrothermal methods. More impurities are generated, or the internal structure of the three-dimensional block is uneven.

 

This research further enhances the understanding of the process and mechanism of preparing micro-nano materials of different sizes and shapes by hydrothermal synthesis and has important guiding significance for the future development of hydrothermal synthesis nanomaterial technology.

 

With the advancement of science and technology, people's research tends to be refined, elegant, and excellent. Especially in the manufacturing industry, the application of materials is becoming more and more particular. Materials with superior performance and small size are more popular. Among them, nanomaterials have continued to gain popularity in the market for many years based on their insignificant volume and excellent performance. Under the development of people, the variety of nanomaterials has become very rich. Since the successful development of nanometal materials, new nanomaterials such as nano semiconductor films, nanoceramics, nanoceramic materials, and nano biomedical materials have been launched.


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