What kind of material is “powder”?
Powders are ubiquitous in everyday life. When you enjoy a bowl of noodles for breakfast, the flour that makes it up is itself a powder; the ceramic bowl in your hand and the tiles under your feet are also made of powders. You apply makeup at breakfast—eyebrow powder and foundation both contain powdered ingredients. As you walk on a clean concrete road, its primary constituent is powder; even a dirt path is largely composed of fine natural powders. When you take a cold-relief pill, its active ingredients are powdered as well. And when the weather turns chilly, you stick on a warming patch whose active component is metallic iron powder.
In addition to these powders that clearly bear a “identity,” some powders are also embedded (or compounded) into other materials. For example, carbon black is added to automobile tires to enhance their strength or provide color; powder materials in coatings serve as fillers or pigments; and powders blended with plastics can improve strength, harden the material, or increase its volume. In short, powders are ubiquitous—so numerous that they are almost impossible to count—but there remains no clear definition of what exactly constitutes a “powder.” Where, then, do these powders originate?
Powder materials used in industry are generally not transported in their natural form; instead, they are typically prepared through physical or chemical methods. The choice of preparation method depends on the specific application and the properties of the material. The following sections will provide a brief overview of the various methods for preparing powder materials.
1. Physical methods:
Physical crushing methods: Powdered materials are obtained through mechanical crushing, electrical spark explosion, and other techniques. These methods are characterized by simple operation and low cost; however, the resulting products typically exhibit relatively low purity and non-uniform particle size distribution. Physical grinding can also be employed as a post-processing step for powder materials prepared via chemical routes, further reducing particle size. With appropriate process control, it is possible to produce powders with a narrow and uniform particle-size distribution and small particle dimensions.
Vacuum condensation: This method involves vaporizing or ionizing the raw materials through vacuum evaporation, heating, high-frequency induction, and other techniques, followed by quenching. Its advantages include high purity, well-defined crystal structure, and controllable particle size; however, it demands sophisticated equipment and technology.
2. Chemical Methods
Chemical vapor deposition: Nanomaterials are synthesized via the chemical reaction of metal compound vapors. This method is characterized by high product purity and a narrow particle size distribution.
Precipitation method: A precipitating agent is added to a salt solution to induce a reaction, followed by thermal treatment of the precipitate to obtain nanomaterials. This method is characterized by its simplicity and ease of implementation; however, it yields products with low purity and relatively large particle sizes, making it suitable for the preparation of oxides.
Hydrothermal synthesis: Nanoparticles are synthesized under high temperature and pressure in an aqueous solution or steam, followed by separation and heat treatment. This method is characterized by high purity, excellent dispersibility, and easy control of particle size.
Sol–gel method: Metal compounds are converted into sols and gels through solution processing, followed by low-temperature heat treatment to yield nanoparticles. This method is characterized by a wide variety of reactive species, uniform product particle size, and easy process control, making it suitable for the preparation of oxides and Group IV compounds. Microemulsion method: Two immiscible solvents form an emulsion in the presence of surfactants; within the microbubbles, nucleation, coalescence, agglomeration, and subsequent heat treatment produce nanoparticles. The resulting particles exhibit excellent monodispersity and favorable interfacial properties, and this approach is commonly used to synthesize Group IV semiconductor nanoparticles.
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