A number of sites have studied properties of nano-composites, such as epoxy resin with nano-particles or micro-particles. The studies have shown that the already very good electrical insulation properties of epoxy resin, used for example in protective coats of transformers, can be substantially improved by addition of nano-particles. Enrichment of epoxy resin with silica nano-particles increases inner resistance and significantly reduces the loss agent [1]. Improved properties of nano-composites are beneficial for all industries from military and cosmic to electric energy, electronics and cosmetics.

Due to the high carbon and hydrogen levels epoxy resin shows lower signal electron emission coefficient. The material electric conductivity is poor. When observing in the classical SEM the resin specimen must therefore be coated with a thick electrically conductive layer or observed under primary electron beam very low energy conditions, in the order of single keV units. Due to the high sensitivity of the sample to damage by radiation it is further necessary to significantly reduce the beam current and observe the specimen at higher scanning speeds. All of the abovementioned factors significantly complicate observation of these samples in the classical scanning electron microscope and due to the low detected signal to noise ratio make achievement of high resolution impossible. As a consequence of interactions of the signal electrons with the gas positive ions are generated in the environmental scanning electron microscope (ESEM), causing compensation of the emerging charge on the non-conductive samples. This allows for observation of non-conductive samples under conditions of higher energy of the electron beam without metal plating. The positive ions hitting the sample surface also remove contamination from the sample surface which facilitates escape of the low-energy secondary electrons from the sample and their getting to the detector. 

Although under the high gas pressure conditions in ESEM the charge is compensated with positive ions and the sample is in addition covered with a 5 nm thick layer of carbon, charging can be observed on the sample peaks and edges, see fig. 1A. The charging is caused by the higher beam current chosen to compensate for the insufficient intensity of signal of the secondary electrons penetrating through the carbon layer and in the effort to achieve higher resolution of the nano-particles. Despite all this the image shown in fig. 1 is blurred and does not show visible details. This may be addressed by the sample coating with a thicker conductive layer, which results in a substantially negative effect, as shown by the results of our experiments. The specimen surface is modified, the nano-structure is made invisible and artefacts are produced. This may be resolved by reduced beam current, optimisation of the detection ability of the detector by correct selection and setting of the microscope and especially by removal of the conductive layer form the sample surface. Figs. 1B and 1C show clearly visible nano-particles with a substantially higher resolution. In the case of figs. 1B and 1C the specimen is in its natural condition without use of any conductive layer, observed in the ESEM environment with 150 Pa vapour pressure. This work was supported by the project [4].

References:

[1] Hudec J, et al., Fine Mechanics and Optics 60 (9) (2015), p. 268.

[2] Neděla V, et al., Nucl. Instrum. Methods in Physics A, 645 (1) (2011), p. 79.

[3] Maxa J, et al., Advances in Military Technology 7 (2) (2012), p. 39.

[4] The European Commission (ALISI No. CZ.1.05/2.1.00/01.0017)

Figures:

Fig 1 Micro-structure of epoxy resin with SiO2 particles, GSED detector, beam energy 10 keV, water vapour pressure 150 Pa, WD 8 mm, ESEM FEI QUANTA 650 FEG; A) covered with carbon layer, probe current 140 pA; B) sputtered free sample, probe current 98 pA; C) high resolution image of sputtered free sample, probe current 98 pA.

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