1. Definition
A particle refers to the smallest independent and discrete unit formed by the nucleation and growth of substances in a specific reaction system (such as combustion, precipitation, gas-phase synthesis, etc.), with regular or irregular geometric shapes. It can be understood as the most fundamental individual that is "innate" in the process of material formation.
Secondary aggregates refer to more complex composite particles formed by the aggregation of multiple primary particles through a certain force. It is not "born", but formed "after birth".
2、 Difference
There are significant differences between the two in terms of structure and composition, formation method, bonding force, stability, and performance impact, as shown in the following figure:
| Iem |
Primary particles |
secondary aggregates |
|
Structure and Composition |
A single, independent solid unit |
Composite formed by focusing multiple primary particles |
|
Formation method |
Nucleation occurs naturally through chemical reactions |
Focusing is formed through physical forces |
|
binding force |
Strong chemical bonds (covalent bonds, ionic bonds, metal bonds) require high energy to break |
Weak physical forces (van der Waals forces, electrostatic attraction, etc.) are relatively easy to break |
|
stability |
Structural stability is a thermodynamically stable state. |
The structure is metastable, indicating a tendency towards further densification in the dynamic transient state |
|
Performance impact |
Determine the intrinsic properties of the material, such as bandgap, magnetism, and catalytic active sites. |
Affects the macroscopic process performance of materials, such as dispersibility, flowability, settling rate, and actual specific surface area. |
3、 Differentiation method
1) Electron microscopy method
Scanning electron microscope (SEM): can provide information on the morphology, size, and distribution of particles. At high magnification, it can be observed that aggregates are composed of many smaller, well-defined primary particles. Particles typically exhibit regular geometric shapes (such as spheres or cubes), while aggregates have irregular shapes.
Transmission Electron Microscopy (TEM): It has higher resolution than SEM and can observe the lattice patterns and internal structure of particles more clearly, as well as accurately measure their particle size. It is the gold standard for distinguishing nanoscale primary particles from their aggregates.
2) Particle size analysis technology
Laser particle size analyzer: This method measures the hydration kinetic diameter of particles in a medium (usually a liquid) by scattering light from them. It measures the apparent size of aggregates in a dispersed state. If the result measured by the laser particle size analyzer is much larger than the primary particle size observed by the electron microscope, it indicates that the sample has undergone significant secondary agglomeration in water or solvent.
3) Specific surface area analysis (BET method)
Determine the specific surface area of particles by measuring the gas adsorption capacity. For spherical particles, there is a formula: particle size ≈ 6/(density x specific surface area). The calculated particle size is based on the theoretical primary particle size assuming that all particles are independent spheres.
4) Dispersion and Ultrasonic Experiment
Disperse the powder sample in a suitable solvent and let it stand for observation. If it rapidly settles and forms a hard precipitate, it usually means strong agglomeration. Subsequently, the suspension was subjected to ultrasonic treatment.
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