Scanning electron microscopy as a useful tool for the analysis of non-conductive materials

Scanning electron microscopy (SEM) in the analysis of non-conductive samples became one of the most important methods for the investigation of material properties. In this work, we used SEM microstructure analysis for the investigation of the origin of cracks in granite composites and also, we tested the porosity inside the regenerated carbon biowaste, potentially used as a clean source of carbon for the future applications in materials production. Additionally, the morphology and the chemical composition of small particles used for the moulding processes of plastics were also tested. The importance of the microstructure investigation was supported by Energy-Dispersive X-ray Spectroscopy (EDS) often used for the chemical composition evaluation of these non-conductive materials.


1
Introduction Scanning electron microscopy (SEM) represents a powerful technique for the microstructure and morphology analysis of a wide range of materials [1,2]. Considering the advanced material analysis techniques nowadays, SEM method can be used not only for the conductive materials but also for the non-conductive samples and even for the biological tissues [3,4]. However, this application strongly depends on the technique of SEM and also on the preparation of tested materials [5]. On the other hand, the investigation of such samples contributes to the problem solutions in the material industry and other applications [1]. For instance, SEM microstructure analysis can easily reveal the origin of the cracks in materials or the porosity of small particles. In combination with Energy-Dispersive X-ray Spectroscopy (EDS) analysis, this creates the powerful tool for detail analysis of various materials [6]. Thus, in this work, we focused on three different types of non-conductive samples which were analysed by SEM and EDS. The chosen samples were industry-oriented, for instance the origin of cracks in granite composites or the porosity measurements in the regenerated biowaste materials and the analysis of plastic granules.

Materials
Various non-conductive materials were tested in our laboratory, including the analysis of crack initiation and defects, and also new material testing which is the most typical analysis of metallographic laboratory. Nevertheless, in this work we focused only on three, industry-oriented types of materials. More specifically, the first materials for analysis were granites composites, mainly used in house-hold equipment. The granite composites were tested in the form of the angular and round house-hold sinks which cracked during the service and were further cut into small samples for the analysis (Figure 1). Subsequently, the porosity of the carbon biowaste was tested in the chosen samples, specifically in the regenerated coffee mixtures (Figure 2a), processed with different inorganic compounds for the activation (such as Na2CO3 or H3PO4). Finally, as-delivered plastic TECHNYL® cylinder-like shape particles were tested and the origin of the small fibres on the surface of these particles was investigated ( Figure 2b).

SEM analysis of granite composites cracks
Granite composites became a good alternative for the stainless-steel materials, especially in a household equipment such as sinks [8]. Nevertheless, the widespread usage of these composites is hindered with the insufficient identification of the defects and morphology analyses [9]. Thus, it is necessary to investigate the cracks and other defects and their origins should be identified to understand and avoid the mistakes during the production processes. Due to high fragmentation of the angular sink (Figure 3a), it was quite hard to determine the area of the fracture origin. Nevertheless, we chose the part from the bottom edge which is the probable area of the fraction origin (or one of the areas of fracture origin) and also its surface was appropriate for investigation by SEM. On the other hand, the round sink (Figure 3b) was investigated at the bottom part, other parts were intact. At the bottom part, all the cracks have radial orientation towards the centre hole of the sink. Initially, we investigated the metallographic cross-sections (in the plane perpendicular to the fracture surface) of the angular sink to understand the microstructure and morphology of the material (Figure 4). This microstructure contains the mixture of relatively large particles homogenously distributed in the matrix. The EDS analysis revealed that the Al-fine particles were distributed through the large, Si-based particles ( Figure 5). However, the EDS analysis can detect only the elements with local content more than 0.1 weight % [10]. Subsequently, the fracture initiation area was found in the bottom edge of the angular sink, from where a large secondary crack also propagated through the bottom of the sink (red circle on the left, Figure 6). The layer of the melted material was observed at fracture plane, just below the surface of the sink (red arrow in the middle and on the right, Figure 6). This unique feature was not found in any other areas of the fracture surface and it was also not observed at metallographic section. Overall, this material defect could be assumed to cause fracture initiation. The chemical composition was also investigated by point and map EDS analysis. On the other hand, the character of damage in the round sink was completely different. As discussed previously, several cracks were observed only at the bottom of the otherwise intact sink, covering slightly more than one fourth of the bottom area. Crack initiation area showed brittle defragmentation of Si-based particles which resulted in the crack initiation at a particle-matrix interface (Figure 7). Crack propagation was further supported by thermo-mechanical fatigue. In comparison to the angular sink, the round sink showed more areas with locally increased Ti and occasionally also Fe contents. However, the average Ti and Fe contents were still below 0.1 weight % and these elements were therefore not detected by plane EDS analysis. Figure 7. The crack initiation of the round sink and EDS chemical analysis.

SEM analysis of a porosity of regenerated coffee grounds
The importance of regenerated carbon biowaste, such as that from the coffee or tea grounds has increased recently, as the renewable source of porous carbon plays an important role in a wide range of applications such as bioplastics, biofuels and other productions [11], [12]. However, the activation of such regenerated particles of carbon deals with the problems in cleaning and the size of the pores. Thus, these pores in the grounds after different activation processes (in acidic or base conditions) were evaluated and compared ( Figure 8). SEM images showed that the basic conditions (activation with Na2CO3 in Figure 8a) led to creation of larger pores of better quality than the activation under acidic conditions which resulted in smaller and poor-quality pores in coffee grounds (activation with H3PO4 in Figure 8b). The parameters of both activations are included in [13].

SEM analysis of plastic granules and fibres
Homogeneity of plastic particles presents the key property for the nonproblematic production process of the various plastic items [14]. However, sometimes the storage of such granules introduces the problem of secondary particles or fibres which is necessary to investigate [15]. Thus, the analysis of the origin of the heterogeneous objects created during the manipulation with the plastic granules revealed that the small fibres were confirmed in the whole volume of storage package. Specifically, the fibres were observed on the edge of the granules (Figure 9). Furthermore, EDS analysis revealed that these fibres had the same chemical composition as the primary granules. a) b) Figure 9. SEM and EDS analyses of the plastic granules and fibers (a-overall view of the granule, b-detail of the part with the fiber).

Conclusion
Without any doubt, SEM and EDS techniques represent the fast and precise investigation of nonconductive materials. They are used not only for the analysis of the origins of cracks but also to determine the microstructure and chemical analysis of plastic granules or biowaste-carbon particles, etc. Specifically, in this work, the results obtained from SEM images clearly confirmed that the cracks in the tested granite composites have different origins such as the brittle defragmentation Si-based particles on one side or the thermo-mechanical fatigue on the other side. Further, SEM images also revealed that the porosity of regenerated coffee grounds strongly depends on the chemical conditions (acidic or basic environments) of the activation process. Finally, SEM analysis of plastic particles and fibres on theses particle surfaces confirmed the same chemical conditions for both parts.