Particle Size Classification

Unit Cell Size

A unit cell is the most basic geometric unit that constitutes a crystal, with its shape and size identical to the parallelepiped unit of the space lattice, retaining all the characteristics of the entire lattice. The size and shape of the unit cell can be described by the lattice constants a, b, c, and the angles between the edges α, β, γ. The unit cell size is typically represented by the lattice constants α, b, c, which determine the arrangement of atoms or ions in the crystal.

Figure 1: Unit cell size

Grain Size

A grain refers to a complete part of a single crystal, with a certain shape and size. The size of the grain is usually obtained through microscopic observation and has a significant impact on the properties of the material. The grain size can be influenced by sintering temperature, holding time, and other process conditions, so it can be controlled by adjusting these parameters.

Figure 2: A grain refers to a complete part of a single crystal,

Primary Particle Size

Primary particle size, also known as the original particle size, refers to the size of a single small crystal. The primary particle size is the smallest particle size in the material that has not undergone agglomeration, reflecting the initial state of the material. For example, in the particle size test of carbon black, the primary particle size is the size of a single carbon black particle.

Figure 3: In the particle size test of carbon black, the primary particle size is the size of a single carbon black particle

Secondary Particle Size

Secondary particle size refers to the size of larger particles formed due to the interaction forces between primary particle sizes. When small crystals aggregate due to surface energy, secondary particle sizes are formed. For example, when crystals with a primary particle size of 0.33 μm aggregate into larger particles, the size of these larger particles is the secondary particle size.

Figure 4: When crystals with a primary particle size of 0.33 μm aggregate into larger particles, the size of these larger particles is the secondary particle size

Agglomerates

Agglomerates refer to the collective body formed by primary particles or aggregates connected by edges or corners. The process of forming agglomerates is called agglomeration, and its specific surface area is not significantly different from the sum of the specific surface areas of the particles or aggregates that make it up. Agglomerates can be soft or hard, with soft agglomerates mainly caused by electrostatic forces and van der Waals forces, while hard agglomerates may contain chemical bonds.

Figure 5: Agglomerates

Connections and Differences

• Unit cells and grains: Unit cells are the basic units that make up crystals, while grains are larger structural units composed of multiple unit cells. The size and shape of the grains depend on the arrangement and size of the internal unit cells.

• Primary particle size and secondary particle size: The primary particle size is the size of a single crystal that has not undergone agglomeration, while the secondary particle size is the size of larger particles formed by the agglomeration of multiple primary particles. Changes in secondary particle size directly affect the physical and chemical properties of the material.

• Unit cells and grains: Unit cells are theoretical geometric units used to describe the arrangement of atoms or ions within a crystal; grains are actual physical entities with a certain shape and size.

• Primary particle size and secondary particle size: The primary particle size focuses on the smallest particle size of a single crystal that has not undergone agglomeration; the secondary particle size focuses on the larger particle size formed by the agglomeration of multiple primary particles. Secondary particle sizes are usually much larger than primary particle sizes, and their changes have a significant impact on the performance of the material.

• Unit cells and grains: Unit cells are theoretical geometric units used to describe the arrangement of atoms or ions within a crystal; grains are actual physical entities with a certain shape and size.

• Primary particle size and secondary particle size: The primary particle size focuses on the smallest particle size of a single crystal that has not undergone agglomeration; the secondary particle size focuses on the larger particle size formed by the agglomeration of multiple primary particles. Secondary particle sizes are usually much larger than primary particle sizes, and their changes have a significant impact on the performance of the material.

Understanding the above concepts and their interrelationships is crucial for studying and applying various materials in materials science. By controlling the primary and secondary particle sizes and studying agglomerates, materials can be optimized to play an important role in industrial and technical fields.

Specific Impact of Unit Cell Size on Crystal Properties

The specific impact of unit cell size on crystal properties is mainly reflected in the following aspects:

• Mechanical properties: The grain size has a significant impact on the mechanical properties of nanocrystalline polycrystalline iron. For example, the introduction of helium atoms at grain boundaries significantly reduces the peak stress and promotes the initiation and growth of intergranular cracks. In addition, when the grain size is small, the impact of triple junctions and grain boundaries on the mechanical properties of the crystal is significant; when the grain size is large, the volume fraction of the grains plays a dominant role, and the impact of triple junctions and grain boundaries is smaller.

• Thermoelectric properties: The grain size has an important impact on the thermoelectric properties of compounds. Studies have shown that as the grain size decreases, both lattice thermal conductivity and carrier thermal conductivity decrease, especially when the grain size decreases from the micrometer scale to the nanometer scale, the lattice thermal conductivity decreases significantly.

• Charge order and magnetism: In perovskite manganites, the reduction of grain size inhibits the charge order transition, and the system always appears in a spin glass state at a certain temperature. This indicates that changes in grain size can significantly affect the charge order and magnetic behavior of materials.

• Piezoelectric and ferroelectric properties: Changes in grain size cause changes in the crystal structure, dielectric, piezoelectric, and ferroelectric properties of bismuth titanate-based lead-free piezoelectric ceramics. This shows that grain size is one of the key factors determining the performance of these materials.

• Optical and electronic properties: The electronic and optical properties of nano-semiconductors are closely related to their own particle size, and their special photoelectric properties have great application value. Therefore, controlling the grain size can adjust the optical and electronic properties of nanomaterials.

The impact of unit cell size on crystal properties is multifaceted, including mechanical properties, thermoelectric properties, charge order and magnetism, piezoelectric and ferroelectric properties, and optical and electronic properties.

How to Optimize Material Mechanical Properties by Controlling Grain Size

Controlling grain size can significantly optimize the mechanical properties of materials, as follows:

• Refining grain size: Refining the grain size to the submicron or nanocrystalline range can significantly improve the mechanical properties of metal materials. This is because small grains increase the number of grain boundaries, which are important factors that hinder the movement of dislocations, thereby improving the strength and hardness of the material.

• Dynamic recrystallization: In the production process of polycrystalline nickel-based superalloys, uniform and fine grain sizes can be achieved by controlling deformation temperature, strain rate, and strain amount, thereby obtaining ideal mechanical properties. This method is particularly suitable for hot processing processes that require superplastic deformation.

• Mechanical processing methods: Grain size can be reduced by mechanical processing methods such as steel ball grinding or rolling. These methods can directly change the microstructure of the material through physical means, thereby improving its mechanical properties.

• Adding grain refiners: In casting alloys, adding specific grain refiners can effectively control grain size and morphology, thereby improving the strength, toughness, and corrosion resistance of the alloy.

• Optimizing heat treatment processes: Adjusting heat treatment parameters, such as temperature and time, can further refine grain size, thereby optimizing the mechanical properties of materials.

Conversion Mechanism Between Primary and Secondary Particle Sizes

The conversion mechanism between primary and secondary particle sizes mainly involves the agglomeration process of particles. In this process, individual particle crystals (i.e., primary particle sizes) aggregate through physical or chemical actions to form larger particles (i.e., secondary particle sizes). Specifically, high molecular weight dispersants can form a double electric layer on the surface of nano-particles through ionization, thereby stabilizing these particles and preventing them from further agglomeration.

In addition, emulsification dispersion technology can also be used to achieve this conversion. For example, using cholesterol and sodium dodecyl sulfate as mixed emulsifiers can effectively disperse primary particle size particles and re-aggregate them into larger secondary particles under appropriate conditions.

Therefore, the conversion mechanism between primary and secondary particle sizes is mainly achieved by using physical or chemical methods to make individual particles aggregate, thereby forming larger secondary particles.

The Role of Agglomerates in Different Types of Alloys and Their Impact on Alloy Performance

Agglomerates play a significant role in different types of alloys and have an important impact on the performance of alloys. Here are the specific roles and impacts of agglomerates in several different alloys:

• In SiCp/reinforced aluminum matrix composites, the agglomeration behavior of the reinforcement significantly affects the performance of the alloy. Therefore, choosing SiCp with a smooth shape and uniform size can improve its flowability in the aluminum matrix, thereby maximizing the reinforcement effect of the whiskers.

• In Mo alloys, the agglomeration problem of nano-TiC affects the tensile strength of the alloy. By adding organic substances as dispersants, the TiC agglomerates in the Mo matrix can be transformed from spherical to a more dispersed state, thereby significantly improving the tensile strength of the alloy.

• In this type of multiphase material, local stress concentration leads to a decrease in the toughness of the intergranular region. When the distance between particles is less than a certain value, a very high triaxial stress is generated, causing the material to continue to break until fracture.

• Ti-Al-V alloys with different α/β cluster ratios have differences in cast structure and mechanical properties. When the α/β cluster ratio is 1:4, the alloy has the best cast structure and mechanical properties; when the ratio is too large or too small, the cast structure and mechanical properties of the alloy are negatively affected.

• In HgCdTe alloys, the atomic agglomeration effect significantly affects the short-range order of the alloy. When Cd or Hg atoms aggregate within the lattice constant range, they preferentially choose the same cation as the nearest neighbor. This tendency can be represented by the agglomeration parameter β. The larger the β value, the more severe the atomic agglomeration, and the stronger the short-range order of the alloy.

• During solid solution treatment, the γ matrix around the agglomerate first undergoes initial melting. Although borides are not the nucleation points of initial melting, they play a key role in the formation of initial melting. A higher B content makes the alloy have better performance.

• In nickel-coated tungsten carbide hard alloys, agglomeration phenomena have a detrimental effect on alloy performance. To seek better preparation methods, it is necessary to control the precipitation rate to avoid agglomeration phenomena on the surface and inside.

• With the increase of particle content, the number of particle agglomeration and defects at the grain boundaries increases. When the particle content is 3%, there are un-reacted residual salts in the Al matrix, and the number of particle agglomerates increases, the size increases, and the number of defects such as pores in the alloy increases.

• Adding binders such as PVA can improve the compressive strength of briquettes, but as the PVA addition ratio increases from 0 to 1.0%, the compressive strength of ICA actually decreases.

• In TA15 titanium alloy, dynamic agglomeration kinetics are sensitively related to deformation parameters. With the increase of strain and temperature, the dynamic agglomeration fraction increases; while with the decrease of strain rate, the dynamic agglomeration fraction decreases. The rate of dynamic agglomeration kinetics increases with the increase of strain and then decreases.

Agglomerates significantly change the mechanical properties, organizational morphology, and overall performance of alloys by affecting the microstructure and stress distribution of materials.

How to Effectively Control Primary and Secondary Particle Sizes to Improve Material Processability and Performance in Practical Applications?

In practical applications, controlling primary and secondary particle sizes to improve the processability and performance of materials can refer to the following aspects:

• Choose the appropriate particle size range: Different materials and applications have different requirements for particle size. For example, in SiC/Al composites, as the SiC particle size increases from 7 microns to 250 microns, the thermal conductivity significantly increases, but the bending strength decreases significantly. Therefore, it is necessary to choose the appropriate particle size range according to specific application requirements.

• Optimize particle size distribution: The particle size distribution has a significant impact on the mechanical properties and crack resistance of materials. For example, the fracture toughness of aluminum-polyester composites will change with the volume fraction of different particle sizes. By optimizing the particle size distribution, the comprehensive performance of materials can be improved.

• Control sintering temperature and time: Sintering temperature and time also have an important impact on the performance of composite materials. Studies have shown that when the average particle size of alumina is 3, 12, and 48 microns, sintering at a temperature and time in the range of 500-600°C for 30-90 minutes can effectively control the microstructure and mechanical properties of composite materials.

• Utilize advanced preparation technology: For example, photocuring 3D printing technology can prepare diamond-resin composites containing different particle sizes of diamond powder. Increasing the particle size of the powder is beneficial for improving the bending strength and elastic modulus of the composite material.

• Consider the interaction between particles: The interaction forces and fluidity between particles also affect the performance of materials. Larger carbon black particles have higher filling efficiency and lower fluidity, while smaller particles help improve the mechanical properties of materials.