Nanopowders

Nanopowders refer to solid micro-particles in the transition area of atoms, molecules and macroscopic objects. After a substance becomes a nanopowder, its surface area per unit mass is much larger than the original bulk solid, so its function is completely different from that of a solid. Nanopowders have two basic characteristics: surface effect and volume effect.

Figure 1. Application and microtopography of nano-ZrO₂

Applications:

• Solid fuel: As the specific surface area of nanometal powder is larger than that of ordinary metal powder, it has strong chemical reaction and metallurgical reaction capabilities, so the functional characteristics of metal particles are greatly enhanced. Compared with ordinary aluminum powder, nanometer aluminum powder has the characteristics of fast combustion speed and large heat release, and it has become a kind of low-cost fuel.
• Electronic devices: As the particle size of the powder decreases sharply, its physical properties will change greatly. The normal melting point of silver is 670℃, while the melting point of ultrafine silver particles can be less than 100℃. Therefore, the conductive paste made of nanometer powder can significantly reduce the sintering temperature of ceramics, improve the reliability and yield of chips, and reduce the production cost. And nanometer conductive paste can be widely used in wiring, packaging, and connection in the microelectronics industry, and it plays an important role in the miniaturization of microelectronic devices.
• Magnetic materials: Nanometal powders are widely used in the manufacture of nano-magnetic recording materials, magnetic liquids, nano-magnetic particle film materials, etc. For example, magnetic liquids prepared with nanometer Co, Fe, Ni and other magnetic metal powders can be used in rotary seals, damping devices, magnetic medicine, magnetic liquid brake, etc., and can also be used for magnetic liquid printing, mineral separation, precision grinding and polishing.
• Other fields: In addition to the above applications, nanopowders can also be used as high-efficiency catalysts for petroleum cracking, automobile exhaust gas treatment, photocatalysis, water treatment, etc. In addition, it can also be used for medicine manufacturing, environmental pollution evaluation, nano-sensing, bonding between metal and metal or between metal and ceramic, etc.

Production Processes:

The preparation method of nanometer powder must be able to control the particle size of nanometer powder, and also make the product yield high, easy to collect, stable, and easy to preserve.

• Mechanical method: This method refers to the use of grinding, airflow, liquid flow, or ultrasonic method to break large solids, so it is also called crushing. The advantage of the mechanical method is that the process is simple, it can prepare high-melting-point metal or alloy nanomaterials that are difficult to obtain by conventional methods, and prepare powders that do not participate in chemical changes.
• Physical method: This method is called physical vapor deposition method, that is, under certain pressure, various forms of thermal energy conversion are used to evaporate the bulk material to form fine particles of gaseous particles, which are condensed on the collector. This method is very suitable for the production of high-purity metal powder.
• Chemical method: This method is to use reactants to produce products under certain conditions, and obtain nanopowder through certain technology. Commonly used chemical methods include liquid phase chemical reduction method, vapor deposition method and sol-gel method.

References:

1. Djamila Bahloul-Hourlier, Benoıˆt Doucey, Etienne Laborde, Paul Goursat. Investigations on thermal reactivity of Si/C/N nanopowders produced by laser aerosol or gas interactions [J]. J. Mater. Chem., 2001, 11, 2028–2034.
2. Kunihiro Ichimura, Ken'ichi Aoki, Haruhisa Akiyama, Shin Horiuchi, Shusaku Naganod, Shinji Horie. Properties of core–shell structured nanopowders of molecular crystals fabricated by dry grinding [J]. J. Mater. Chem., 2010, 20, 4312–4320.
3. Celine Rousse, Jerome Josse, Valerie Mancier, Samuel Levi, Sophie C. Gangloff, Patrick Fricoteaux. Synthesis of copper–silver bimetallic nanopowders for a biomedical approach; study of their antibacterial properties [J]. RSC Adv., 2016, 6, 50933–50940.