Single-particle Compression Characteristics of Micron-sized Hollow Spheres

1. Background

With the continuous development of science and technology, research in the field of materials science is becoming increasingly in-depth. Among them, hollow sphere materials have shown broad application prospects in many fields due to their unique structure and performance. The mechanical properties of hollow sphere materials are closely related to their internal structure and composition. The SPFT (single-particle mechanical properties test) equipment of IEST can deeply understand the mechanical response of hollow sphere materials when compressed, including the crushing force, brittle characteristics, internal microstructure and other information of particles, providing a scientific basis for further optimization and application of materials.

2. Test Equipment and Methods

The SPFT (single-particle mechanical properties test) can control displacement and measure pressure with high precision, so as to accurately collect the pressure-displacement curve of the indenter loaded on a single particle. In addition, SPFT adopts the bottom optical system imaging method, and the entire picture of the particles being compressed, deformed and crushed can be synchronously recorded in the test software. This equipment is particularly suitable for evaluating the pressure resistance and crushing force of the material particle level, which is crucial to understanding the behavior of micron-sized hollow spheres during compression.

Figure 1. (a)Single Particle Force Properties Test System; (2) Bottom view of optical system

This paper selects two types of rare earth oxide hollow spheres (XT-1 and XT-2) and two types of fly ash hollow spheres (FM-1 and FM-2) for testing. The test process follows strict standardized steps: first, the sample is dispersed by ethanol to ensure the uniform distribution of particles; then, the single dispersed particles are located using an optical microscope; then, the experimental parameters are configured and the compression test is performed; during the test, the morphological changes of the particles before and after compression are observed and recorded. Each sample usually tests more than 5 particles to ensure the reliability and statistical significance of the data.

3. Results Analysis

3.1 Single particle compression curve and dynamic image display

Figure 2. Single particle compression curves of two different XT hollow spheres

Figure 3. Single particle compression curves of two different FM hollow spheres

Figure 4. Animation of the single particle compression process of XT hollow sphere (left) and FM hollow sphere (right) (excerpt)

3.2 Comparison of crushing force and compressive strength

From the results, it can be seen that the curve consistency of each sample tested 10-15 times in parallel is relatively good. The overall crushing force of the single particle of the two XT hollow spheres tested is much greater than that of the other two FM hollow spheres. Due to similar composition and particle size, there is little difference between different styles of XT hollow spheres/FM hollow spheres-1/-2.

According to the national standard GB/T 43091-2023 on the test method of powder compressive strength, the compressive strength of the particles can be calculated by the crushing force of a single particle and the particle size. The average results are shown in Table 1. The compressive strength of the two XT hollow spheres is about twice that of the two FM hollow spheres, and the compressive strength of XT-1 is slightly greater than that of XT-2. This may be because the rare earth material itself has a higher strength and hardness, which enables it to better resist deformation and damage when subjected to external forces. It can also be seen from the animated image in Figure 4 that the XT hollow sphere is relatively hard when broken, while the FM hollow sphere appears relatively soft.

Table 1. Single particle compressive strength of different XT hollow spheres and FM hollow spheres

3.3 Wave-shaped characteristics of force-displacement curve

On the single-particle compression curve, we can also see the special force-displacement curve characteristics of the hollow sphere after crushing. Their force-displacement curves all show a wavy shape after crushing, similar to the shape of a wave. This wavy shape may be caused by the complexity of the internal structure of the hollow sphere and the expansion and intersection of cracks during the crushing process. Specifically, in the early stage of compression, as the pressure head gradually presses down, the particles begin to deform elastically. When the stress reaches a certain level, cracks begin to appear in the particles and plastic deformation occurs. As the cracks expand and converge, the particles gradually break, and the force-displacement curve also fluctuates.

Causes of the wavy phenomenon:

1) Multi-crack expansion and intersection: Due to the different expansion speeds and directions of the cracks, different stress concentration areas will be generated, resulting in wavy force-displacement curves;

2) Interaction between fragments: When the hollow sphere is crushed into multiple fragments, these fragments will squeeze and rub against each other. This interaction will cause the force-displacement curve to fluctuate during the descent process;

3) Inhomogeneity inside the material: There may be inhomogeneities inside the hollow spherical particles, such as pores, inclusions, etc. These inhomogeneities will cause stress concentration and local damage when subjected to external forces, further exacerbating the fluctuations in the force-displacement curve. This undulating morphology not only reflects the microstructural characteristics inside the particles, but also reveals its mechanical response mechanism when subjected to external forces.

4. Summary

This paper evaluates the mechanical properties of rare earth oxide hollow spheres and fly ash hollow spheres by comparing their force-displacement curves and compressive strength data. The curves of the two tested rare earth oxide hollow spheres and two fly ash hollow spheres are consistent, and the overall crushing force and compressive strength of the rare earth oxide hollow spheres are higher.

In addition, the undulating morphology of the force-displacement curve also provides us with valuable information about the internal structure and mechanical response mechanism of the material. By analyzing these undulating characteristics, we can have a deeper understanding of the material’s damage mechanism and failure mode, thereby providing a theoretical basis for its performance optimization and improvement in practical applications.