Introduction

The radiation environment in space consists of trapped radiation belts, cosmic rays, and solar energetic particles which is extremely hazardous to the health of astronauts and the electronics components in the spacecraft. Currently, most spaceflight missions are still around the Earth, such as the low Earth orbit (LEO), and the shielding provided by the Earth’s magnetic field is capable of deflecting the major high-energy electron radiation that causes biomedical effects. However, future human space missions will extend to Moon base, habitations on Mars and even outer space. During such missions, electron radiation is one of the hazardous cosmic radiations, with energies up to tens of MeV, and capable of inducing carcinogenic effects in manned missions and/or serious damage to the spacecraft structural material and electronics components [1,2,3,4,5].

In order to maintain the functions of spacecraft electronics and the health of astronauts from the hazard radiations in manned missions, relatively high atomic number (Z) material, such as tantalum (Ta), through lower-Z material, aluminum (Al), have been applied [6,7,8]. Owing to high mechanical strength, low-cost and reliable fabrication process, Al is typically used for radiation shielding; however, its shielding effectiveness against high-energy electron radiation is limited by shielding effectiveness of the secondary bremsstrahlung photons that are produced by inelastic collisions of the electron radiation with the atomic nucleus of Al [9, 10]. Thus, to ensure the safety of personnel, more heavy material will be required for shielding, resulting in heavier payload and increasing mission cost and adversely affecting the mission duration due to more fuel consumption.

Alternatively, polymers which are usually composed of low-Z elements (such as hydrogen) with a weight advantage compared to metallic materials but capable of attenuating cosmic radiations without the massive production of secondary radiations are potential materials for space radiation shielding applications [11]. Pure polymers may not be effective shielding materials against high-energy electron radiation due to their low atomic number compounds when compared to the metal counterparts. However, addition of high-Z nanofillers in polymer matrix will gradually attenuate electron irradiation. Due to the low electron density of nanocomposite compared to metallic materials, high-Z nanoparticles added nanocomposite will produce relatively low bremsstrahlung compared to metallic counterparts [11, 12]. Thus, the reinforcement of polymers with nanomaterials can result in improved electron attenuation characteristics of nanocomposites compared to Al [13,14,15].

The utilization of nanomaterials is one of the current trends in the aerospace industry. In recent studies, some groups have reported advantages of carbon-based nanomaterials (such as carbon nanotubes) as shielding materials against ionizing radiations [16,17,18,19,20,21]. It is reported that negligible deformations of MWCNT including radiation-induced formation of interstitial clusters and marginal shrinkage of inner carbon walls [18, 20]. By adding MWCNT into polymer matrix, compared to pure polymer, tensile strength of nanocomposite is able to be enhanced along with thermal stability due to formation of cross-linking sites and hindering effect under electron irradiation [22]. It indicates that MWCNT is able to stabilize thermal and mechanical properties of nanocomposite under electron-beam irradiation. In addition, studies investigating the potential of polymer composites for photon radiation shielding have shown that bismuth oxide (Bi2O3) has promising ability as nanomaterial reinforcement of polymers [23,24,25] and has also been demonstrated as safe alternative to lead (Pb) which is extremely toxic [23, 24]. Regarding shielding gamma radiation with energies lower than 2 MeV, Bi2O3 provides predominate function in polyester resin composites with uniform distribution [25]. However, as of yet, it has not been reported to use Bi2O3 as high-energy radiation absorber and/or shielding material for space applications.

Our group had previously reported that poly (methyl methacrylate) (PMMA) with three weight percentage (3 wt%) MWCNT nanocomposite (PMMA/MWCNT), compared to Al, was lighter, cost-effective, and a promising material to shield high-energy proton radiations in outer space [21]. In this study, we hypothesize that a hybrid nanocomposite including MWCNTs and high-Z nanoparticles dispersed in PMMA (PMMA/MWCNT/Bi2O3) can effectively attenuate electron beams with energies of 9–20 MeV similar to those reported for outer space [5, 26]. The PMMA/MWCNT/Bi2O3 nanocomposite has been characterized using thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), Fourier-transform infrared (FTIR) spectroscopy and scanning electron microscope (SEM) and the attenuation properties under electron beams have been investigated. Advantages of PMMA/MWCNT/Bi2O3 nanocomposite have been discussed and compared with Al, pure PMMA, and our previously developed PMMA/MWCNT nanocomposite.

Methods

PMMA with MWCNT and Bi2O3 nanocomposites (PMMA/MWCNT/Bi2O3) were fabricated, and the composition is 3 wt% of MWCNT, 30 wt% of Bi2O3, and 67 wt% of PMMA. Increasing weight percentage of Bi2O3 in composites has proven to be capable of achieving improved radiation shielding performance [27,28,29]. However, radiation shielding effectiveness or weight advantage threshold may be reached when increasing weight percentage of Bi2O3 above 30%. To adopt the best radiation shielding property and maintain the uniform dispersion quality in nanocomposite, 30 wt% of Bi2O3 was applied in our nanocomposite. In this study, its material properties such as thermal decomposition, glass transition temperature, chemical structure, and dispersion were characterized. Pure PMMA, PMMA/MWCNT, and aluminum were used as reference materials.

Fabrication

Research grade PMMA powders (Sigma-Aldrich, #182230), MWCNT (Nanostructured & Amorphous Materials, –COOH, 95+%, OD 10–20 nm), and Bi2O3 nanoparticles (Sigma-Aldrich, #202827) were used for the fabrication and tests. Melt mixer (Haake Rheocord 90) and hot press instrument were used for fabrication of pure PMMA, PMMA/MWCNT, and PMMA/MWCNT/Bi2O3 nanocomposite. PMMA powder (micron-sized particles) were heated and melted at 200 °C in the mixer followed by addition of 3 wt% of MWCNT. Subsequently, 30 wt% of Bi2O3 nanoparticles was added to the molten mixture. PMMA samples were then loaded into the hot press at 180 °C and pressure of 1.6 MPa; the press is used for molding thermoplastics into solid squares 30 × 30 cm2. As shown in Fig. 1, pure PMMA, PMMA/MWCNT, and PMMA/MWCNT/Bi2O3 were cut into 3.5 × 3.5 cm plates by laser cutter (VLS 2.30, Universal laser systems).

Figure 1
figure 1

a Pure PMMA, b PMMA/3 wt% MWCNT, c PMMA/3 wt% MWCNT/30 wt% Bi2O3

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Characterization studies

Pure PMMA, PMMA/MWCNT, and PMMA/MWCNT/Bi2O3 samples were characterized with tensile strength tester, TGA, DSC, FTIR, and SEM. All those samples were measured before and after electron-beam irradiation in order to determine radiation effects on the thermal properties, chemical structure, and/or any visible morphological changes in the materials.

In terms of mechanical property of materials, the tensile strengths of both the irradiated and non-irradiated samples were measured with an Instron 5548 Micro Tester following the ASTM standard of D638-10. Samples were prepared with the required shape and dimensions with the laser engraving system.

TGA apparatus was used to identify components and thermal degradation process of materials with programmed temperature. The test temperature was set as increasing from room temperature to 800 °C with ramp of 20 °C/min under nitrogen environment in TA instrument Q500. Weight of materials was monitored in real time to compare the thermal degradation as a function of increase in temperature.

DSC is an important method to measure the glass transition temperatures (Tg) of polymer materials. Samples were tested in a sealed chamber in TA Q2000 under 25–180 °C at ramp rate of 10 °C/min followed by decreasing rate of 20 °C/min from 180 °C to room temperature. Heat transition through samples over raising temperature can be plotted to analyze modified Tg in composite compared to pure polymer.

FTIR spectroscopy can be used to represent a molecular ‘fingerprint’ of the sample and observe some chemical structure change in composites. Tested samples were grinded into micro-sized powders used to form pellets with potassium bromide (KBr) under pressure. All materials were prepared at the same concentration and dried in vacuum oven overnight before testing with IR radiation.

Moreover, SEM images were acquired using Zeiss Ultra plus SEM. Samples were coated with a thin layer of gold (20 nm) to improve image quality. Surface microstructure of nanocomposites was evaluated at a magnitude of 12kX.

Electron-beam attenuation test

Figure 2 shows the setup of the electron-beam irradiation of the sample materials. All measurements were done using a Clinac® 21EX System machine at the Grand River Regional Cancer Centre (Kitchener, ON, Canada). The dose delivered to each sample was 100 monitor unit (MU) (100 cGy) at a dose rate of 1000 MU/min for this work. A 6 × 6 cm2 electron cone applicator with 3 cm diameter lead cutout insert was used for the electron-beam irradiation. Pure PMMA (reference material), PMMA/MWCNT (nanocomposite), PMMA/MWCNT/Bi2O3 (nanocomposite), and Al (reference material) were used to study the electron-beam attenuation characteristics for electron-beam energies of 9, 12, 16, and 20 MeV. The samples were placed at the end of the electron cone with the aid of a holder. The distance between the electron source/target and the irradiated samples holder was at 95 cm from the source as shown in Fig. 2. Various samples with different thicknesses were placed on the sample holder to investigate the attenuation properties. An ionization chamber (Farmer chamber), positioned at a distance of 110 cm from the electron target, was used to measure the transmitted electrons beam through the sample. An ion chamber reading without the sample in place was first taken as a baseline reading. For purposes of this study, the ion chamber reading can be denoted as Ct when a sample is placed between the source and the chamber reading, C0 when there is no sample in place. Then the relative electron attenuation (%) of the samples can be calculated as: η = ((C0 − Ct)/C0) × 100%. The attenuation results are plotted as a function of areal density (A) derived from the product of mass density (ρ) and thickness (t) of the material (A = ρ × t). In this study, areal density is indicative of the weight required to stop programmed electron-beam energy for each material. Thus, it is used to compare weight advantages between nanocomposites and reference materials such as Al. Transmission characteristics for five different areal densities (0.58 g/cm2, 1.00 g/cm2, 1.44 g/cm2, 3.60 g/cm2, and 6.43 g/cm2) of each sample irradiated at four different electron-beam energies (9, 12, 16, and 20 MeV) were measured. At the same electron attenuation percentage (η), weight advantages of samples over Al can be calculated as: D = ((Aaluminum − Asample)/Aaluminum) × 100%.

Figure 2
figure 2

Electron irradiation test setup

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Results and discussion

Material characterization

The tensile strength results are given in Fig. 3. Efficiently stress transfer at the particle/polymer interface is the most important factor to improve mechanical strength of nanocomposite [30]. The addition of homogeneously dispersed nanomaterials has shown improved mechanical property in PMMA matrix. Compared to pure PMMA, PMMA/MWCNT and PMMA/MWCNT/Bi2O3 are showing about 35% and 37.7% increase in tensile strength, respectively.

Figure 3
figure 3

Tensile strength data of the fabricated samples

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The TGA results are shown in Fig. 4. The first 2% weight loss of all materials can be considered as removal of contaminants during the degassing process followed by a plateau and subsequently starts to drop around 210 °C with increasing temperature. Based on the TGA data, 98% of total weight of PMMA/MWCNT is left at 237 °C, the starting temperature for material decomposition (initial decomposition temperature) which is 27 °C higher than that of pure PMMA (210 °C). Moreover, the nanocomposite with Bi2O3 nanoparticles showed significant thermal stability improvement such that the initial decomposition temperature (278 °C) is about 68 °C (about 32.4%) higher than that of the pure PMMA. This is caused by the high thermal stability of the homogenously distributed Bi2O3 nanoparticles, and also the MWCNT in polymer matrix restrained the degradation of PMMA chains during pyrolysis [31,32,33,34,35].

Figure 4
figure 4

TGA data of samples before and after irradiation of electron beam

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Compared to the non-irradiated samples, the initial decomposition temperature of electron-beam irradiated samples (PMMA: 211 °C, PMMA/MWCNT: 235 °C, PMMA/MWCNT/Bi2O3: 276 °C) differed by only less than 1%, which suggests that irradiated samples (PMMA: 210 °C, PMMA/MWCNT: 237 °C, PMMA/MWCNT/Bi2O3: 278 °C) maintain their thermal properties.

On the other hand, about 33 wt% and 3 wt% of residual material of PMMA/MWCNT/Bi2O3 and PMMA/MWCNT, respectively, remained when temperature increased above 425 °C. For pure PMMA, almost no residual was found at temperatures > 425 °C; the polymer was totally decomposed. Since the nanomaterials (Bi2O3 and MWCNT) are highly heat resistant, the residual weight percentages (33 wt% and 3 wt%) precisely indicate the original weight percentages of MWCNT and Bi2O3 in PMMA/MWCNT/Bi2O3 and MWCNTs in PMMA/MWCNT.

DSC data of both electron-beam irradiated and non-irradiated materials are shown in Fig. 5. Both nanomaterials, MWCNT and Bi2O3, largely improve the Tg of pure PMMA. PMMA/MWCNT/Bi2O3 nanocomposite has considerably higher Tg: 13.3% and 3.5% than pure PMMA and PMMA/MWCNT, respectively. It is evident that the nanofillers can potentially enhance the working temperature range of the nanocomposites making them ideal candidates for space applications. For example, the radiation shielding components, used in space missions in the low Earth orbit, require a wide range of working temperatures (up to 100 °C) in order to avoid potential risks of thermal degradation due to the harsh environment [36]. Post radiation, Tg of PMMA and nanocomposites dropped less than 1 °C. This minor change may be caused by minimal/low interaction of the high-energy electrons with the samples such that the thermal properties remained unaltered [37].

Figure 5
figure 5

DSC data of samples before and after electron radiation

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FTIR was used to investigate interactions between nanoparticles and PMMA polymer chain in nanocomposites before and after electron irradiation. As shown in Fig. 6, both nanocomposites (PMMA/MWCNT and PMMA/MWCNT/Bi2O3) have shown intensity changes in the range of 600–1100 cm−1 results in the formation the linkages between carbon and PMMA. Around 987 and 750 cm−1, increased density with respect to both nanocomposites has shown additional C atoms in PMMA polymer chain [38, 39]. However, addition of Bi2O3 has shown less intensity changes compared to PMMA/MWCNT due to less PMMA chains in the nanocomposite. Besides that, all materials have not indicated significant chemical structure changes after high-energy electron radiation tests.

Figure 6
figure 6

FTIR spectrum of the samples before (solid lines) and after (dash lines) electron irradiation (e represents electron irradiated in the graph)

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SEM images of PMMA/MWCNT/Bi2O3 before and after electron-beam irradiation are shown in Fig. 7. The uniformly distributed spherical particles represent Bi2O3 nanoparticles surrounded by bright web-like structure of the MWCNTs. No sign of aggregation of MWCNT is observed in all images, demonstrating uniform dispersion of nanomaterials in the PMMA matrix. The SEM images of PMMA/MWCNT/Bi2O3 cross sections before and after electron-beam irradiation show no obvious deformations including radiation-induced polymer matrix degradation that can lead to exposure and aggregation of MWCNT [40, 41].

Figure 7
figure 7

SEM images of PMMA/MWCNT/Bi2O3 nanocomposites before (a, c) and after (b, d) electron-beam irradiation

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Several studies have been reported on radiation-induced chain scission and/or cross-linking effects in polymers, such as PMMA [42,43,44]. However, compared to 20 Gy in this study, those drastically changed mechanical and thermal properties of polymers can be induced by higher radiation dose (several kGy). Thus, the chain scission and cross-linking effects were less likely to have occurred in our irradiated samples.

Electron-beam attenuation test

Besides the radiation resistance property, electron attenuation characteristics of developed materials are discussed. The percentage attenuation characteristics for each of the four energies (9, 12, 16, and 20 MeV) are shown in Fig. 8. As the electron-beam energy increases from 9 to 20 MeV, more materials for each sample were required to attenuate the electron beam. As shown in Fig. 8, the attenuation for all materials increases with increase in areal density and then saturate at higher areal densities. The percentage attenuation characteristic of PMMA/MWCNT nanocomposite is similar to the control sample (PMMA), and the overall trend of materials was found to be similar for all energies. However, the PMMA/MWCNT/Bi2O3 nanocomposite shows better shielding properties than the other materials for any given areal density. To achieve equivalent electron shielding performance, the nanocomposite shielded better and required less amount of material in comparison to Al. For example, at 94% attenuation under 9 MeV electron beam, the PMMA/MWCNT/Bi2O3 nanocomposite was about 33.33% lighter than Al. Of all the materials, the PMMA/MWCNT/Bi2O3 nanocomposite was found to be the lightest for given exposure.

Figure 8
figure 8

Percentage attenuation of all materials at a 9 MeV, b 12 MeV, c 16 MeV, d 20 MeV

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Besides that, under electron radiation with energy of 9 MeV, maximum electron attenuation of 98% for all shielding materials is infinitely approached at areal densities above ~ 3.3 g/cm2 (shown in Fig. 8). The areal density of 3.3 g/cm2 is denoted as saturated point of areal density for 9 MeV electron radiation. As a result, increasing thickness of shielding materials over saturated point will no longer improve electron attenuation effectiveness. This phenomenon is observed in other electron energies as well (12, 16, and 20 MeV). One reason is that continuous slowing down approximation (CSDA) ranges of Al and PMMA (5.33 and 4.67 g/cm2 at 9 MeV electron beam, respectively) suggest, beyond areal density of 3.3 g/cm2, few existing primary electrons still contribute the 2% unshielded radiations [45]. Secondly, generated by the interactions between primary electrons and shielding materials as well as other components in the experiment setup, secondary radiations such as bremsstrahlung photons and Auger electrons were counted [15]. At each electron shielding saturated point, a noticeable phenomenon was found that the maximum attenuation percentages at saturated points decrease accordingly when primary electron-beam energy increases from 9 to 20 MeV. In other words, greater quantity of transmitted primary electrons and secondary radiations can be detected when more shielding materials are used at higher electron energies.

Besides that, as illustrated in Fig. 9, areal densities of saturated points as a function of all four electron energies were plotted to investigate saturated points for the all electron energies. The linear relationship between them is presented, but electron energy of 9 MeV is showing slightly drifting from the other points. Potentially, this plot is able to predict saturated areal densities for electron radiation shielding at other electron energies than the four measured in this study.

Figure 9
figure 9

Linear fit of saturated points for all energies

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Moreover, according to the plots in Fig. 8, the maximum weight advantages of PMMA/MWCNT/Bi2O3 at various electron irradiation energies are found and listed in Table 1. The attenuation characteristics of all the materials as a function of different areal densities and energies show that the PMMA/MWCNT/Bi2O3 nanocomposite would be much lighter material than Al for electron radiation shielding. For instance, at electron energy of 9 MeV, the nanocomposite is found to have maximum 37.49% weight advantage in comparison with Al at electron-beam attenuation of 90%, which is found to be the best shielding characteristics of the nanocomposite (PMMA/MWCNT/Bi2O3) for all measured energies. However, around 37% weight advantage compared to Al is observed regardless of electron energy. Moreover, it is adverted that electron attenuation percentages at the greatest weight advantages of nanocomposite decrease accordingly when the primary electron-beam energy goes up, which has a similar trend as the maximum attenuation percentages at the saturated points.

Table 1 Weight advantage with respect to Al for all energies
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For each of the areal densities, the percentage attenuation behavior was plotted as a function of increasing energy (Fig. 10). For all areal densities, the PMMA/MWCNT/Bi2O3 nanocomposite showed advantages over Al. However, the PMMA/MWCNT/Bi2O3 nanocomposite shows higher attenuation of electron beam compared to Al at lower areal densities. At areal densities < 3.6 g/cm2 (Fig. 10a, b, c), compared to areal density of 3.6 and 6.43 g/cm2, PMMA/MWCNT/Bi2O3 showed higher attenuation advantage (≥ 5%) over Al at all the energies used in this study. Regarding areal density of 6.43 g/cm2, PMMA/MWCNT/Bi2O3 nanocomposite shows attenuation advantages at higher electron-beam energies (16 and 20 MeV). Nevertheless, at lower energies (9 and 12 MeV), all materials reach the same attenuation characteristics. As discussed above, this is because that areal density of 6.43 g/cm2 has reached and exceeded electron shielding saturated points for radiation energies of 9 and 12 MeV, where all materials show minor or the same electron attenuation percentage.

Figure 10
figure 10

Percentage attenuation as a function of energy at areal density of a 0.58 g/cm2, b 1.00 g/cm2, c 1.44 g/cm2, d 3.60 g/cm2, and e 6.43 g/cm2

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Conclusion

In space applications, Al constructed units used for protecting radiation-sensitive components from ionizing space radiations, such as electrons, will open the way for innovative material with light-weight advantage. The present research has demonstrated that Bi2O3 added hybrid nanocomposite possesses greater than 70% electron attenuation at electron-beam energies ranging from 9 to 20 MeV when maximum weight advantage (around 37%) compared to Al is achieved. The weight advantage of nanocomposite is showing slightly higher value (37.49%) at lower electron-beam energies (9 MeV); however, it is found that nanocomposite has no weight advantage if areal density of shielding materials reaches saturated point. For instance, all materials with areal density greater than 3.3 g/cm2 can achieve the same 98% electron attenuation under 9 MeV electron-beam radiation. Besides that, saturated points of materials are linear to all electron-beam energies. Based on the characterization results, tensile strength and thermal stability (TGA and DSC) of PMMA are considerably improved by addition of homogenously dispersed MWCNT and Bi2O3 nanoparticles. Moreover, deformations of nanocomposites after electron irradiation are barely found from thermal property tests, FTIR, or SEM images. This work has indicated that PMMA/MWCNT/Bi2O3 nanocomposite is promising in aerospace applications. Incorporating with our previous study, a multilayer design with the two nanocomposites (PMMA/MWCNT and PMMA/MWCNT/Bi2O3) can potentially be developed to shield both high-energy proton and electron radiations in future research.