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Processing and properties of boron carbide (B4C) reinforced LDPE composites for radiation shielding
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Processing and properties of boron carbide (B4C) reinforced LDPE composites for radiation shielding Suna Avcıoğlua,b, Merve Bulduc,d, Figen Kayaa, Cem Bülent Üstündağe, Erol Kamf, Yusuf Ziya Menceloğluc,d,g, Hayri Yılmaz Kaptanh, Cengiz Kayac,d,∗ a
Department of Metallurgical and Materials Engineering, Faculty of Chemistry and Metallurgy, Yıldız Technical University, Istanbul, 34349, Turkey Department of Metallurgical and Materials Engineering, Faculty of Engineering, Ondokuz Mayıs University, Samsun, 55220, Turkey c Materials Science and Nano Engineering, Faculty of Engineering and Natural Sciences, Sabancı University, Istanbul, 34956, Turkey d Nanotechnology Research and Application Center (SUNUM), Sabancı University, Istanbul, 34956, Turkey e Department of Bioengineering, Faculty of Chemistry and Metallurgy, Yıldız Technical University, Istanbul, 34349, Turkey f Department of Physics, Faculty of Arts and Science, Yıldız Technical University, Istanbul, 34349, Turkey g Integrated Manufacturing Technologies Research and Application Center & Composite Technologies Center of Excellence (SUIMC), Sabancı University, Istanbul, 34906, Turkey h Department of Energy Systems Engineering, Faculty of Engineering, Atılım University, Ankara, 06830, Turkey b
ARTICLE INFO
ABSTRACT
Keywords: Sol-gel Boron carbide LDPE Composite Neutron shielding Mechanical properties
In the present work, boron carbide (B4C) particles were synthesized with sol-gel technique following with heat treatment at 1500 °C in an argon atmosphere. 3-(Triethoxysilyl)-propylamine, a silane coupling agent, was doped on to the surface of synthesized B4C particles. The surface modified B4C particles were embedded in LDPE matrix in order to obtain flexible, lightweight and environmentally friendly shielding materials. The effect of surface functionalization and concentration of boron carbide on its distribution characteristics in the polymer matrix and its effects on the mechanical and neutron shielding properties of the composites are examined. The results showed that high purity-fully crystalline B4C powders with polyhedral-equiaxed morphology in the size range of 20 nm–500 nm were produced. It was found that even the very low amount (0.6–1.7 wt%) of incorporated nano/sub-micron B4C particles in LDPE matrix improved the neutron shielding (up to 39%), tensile strength (9.3%) and impact resistance (8%) of the composites.
1. Introduction Boron carbide (B4C) with a Mohs hardness between 9 and 10 is one of the hardest materials. The combination of high hardness with excellent mechanical strength, high melting point, and low specific gravity makes boron carbide (B4C) a suitable candidate for harsh industrial applications such as abrasives, wear-resistant products, high temperature nozzles and for the military applications like vehicle and body armors [1,2]. Boron has a high neutron absorption cross section for thermal neutrons (767 b) [3]. However, pure boron's relatively weak mechanical properties make boron carbide is a better alternative. Additionally, the atomic number density of boron carbide (0.11 Å−3) is close enough to pure boron (0.14 Å−3) [3]. Due to these reasons, in addition to conventional application areas, boron carbide also being used in nuclear reactors as radiation shield and control rods. Development of a low-temperature production route for boron carbide synthesis is in rising demand in order to reduce production cost. ∗
The sol-gel method is one of the most studied synthetic approaches for producing ceramic powders. This method is based on the formation of a network through polycondensation reactions of molecular precursors in a liquid. The well-homogenized mixture of raw materials at the molecular level, endorse the reduction of the heat treatment temperature which must be subsequently performed. Homogenization also provides better control over the purity, morphology and particle size of materials. Conversely, the main challenge on the production of boron carbide via a liquid state synthesis technique is achieving the final product with high purity. Many attempts have been made to remove residual carbon from boron carbide powder. However, the close oxidation temperature of carbon in the form of graphite and boron carbide makes the process difficult, particularly for the nano-sized boron carbide particles [4–7]. Therefore, recent studies have focused on the fabrication of boron carbide powder without free carbon and boron oxide/suboxides. It has been shown that synthesis of boron carbide particles from polymeric processors may aid to achieve high purity final product as
Corresponding author. Materials Science and Nano Engineering, Faculty of Engineering and Natural Sciences, Sabancı University, Istanbul, 34956, Turkey. E-mail address: [email protected] (C. Kaya).
https://doi.org/10.1016/j.ceramint.2019.08.268 Received 14 July 2019; Received in revised form 20 August 2019; Accepted 27 August 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Suna Avcıoğlu, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2019.08.268
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Fig. 1. Pictures of LDPE plate samples, (a–c) LDPE-Neat, (d–f) LDPE-B4C (1 wt%) composites.
Fig. 2. FT-IR analysis results of condensed gel.
well as lower the synthesis temperatures [8–12]. Electromagnetic and particle radiation from equipment's in medical diagnostic centers or nuclear reactors causes ionizing radiation including gamma rays, x-rays and neutron rays that are well known to be very harmful to human health. Due to the reason that neutrons do not interact with the electrons, they only lose very little energy while passing through heavy materials such as Pb. Using an effective neutron absorber material in a bulk form specifically for radiation shielding applications is neither cost-effective nor practical for production or use. Therefore, development of flexible, lightweight and environmentally friendly shielding materials, preferably the combination of hydrogenous materials and elements with a high absorption cross-section, such as boron are required. A common approach to solve this problem is developing polymer matrix composites filled with a material which is a high capability of neutron absorption. In the previous studies, the potential of high-strength polyimides, natural rubber, epoxy, polystyrene, poly(4-methyl-1-pentene) and low-density polyethylene have been
investigated as a polymeric matrix to produce radiation shielding composites [13]. Easy fabrication of polymers into sheets, extruded into profiles or injection-molded for structural parts is not the only advantage of polymer matrix composites. A polymer matrix contains a large amount of hydrogen atoms. Hydrogen is very effective in thermalization of high energy neutrons because of its light mass [3]. In the case of boron carbide filled polymer matrix composites, this phenomenon makes the absorption of a neutron by the boron nuclei easier. Abdel-Aziz et al. have fabricated boron carbide filled dual polymer matrix containing neutron shielding materials with ethylene-propylene diene rubber and low-density polyethylene. They reported that a high amount of boron carbide loading (57 wt%) had sharply reduced the neutron flux by approximately 85% [14]. Preparation of composites of high-density polyethylene containing three different amounts of modified boron carbide namely: 7 phr (parts per hundred parts of resin), 15 phr and 24 phr have been reported by T. Yasin et al. Their results indicate that the increased content of filler boron carbide enhances the
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Fig. 3. DTA/TG analysis results of condensed gel.
Fig. 4. XRD pattern of synthesized B4C powder.
shielding properties of composites. However, they also reported that mechanical properties such as; elongation at break, tensile strength, and modulus of composites inversely proportional with incorporated boron carbide amount [15]. M. Salimi et al. fabricated rubber based shielding composites. According to their report for every 5 wt% of additional boron carbide loading, the thermal neutron absorption coefficient of the composite material doubled [16]. Boron nitride and boron carbide filled polyethylene matrix composites for radiation shielding applications were reported by C. Harrison et al. [13,17]. In sharp contrast to previous studies, they reported that successful surface modification of filler particles enhances the mechanical properties of both boron nitride and boron carbide filled polyethylene matrix composites. However, the shielding performance of their composites was
slightly better than those polyethylene-neat samples [13,17]. Previous studies clearly show that an increased amount of boron filler content improves the shielding performance of polymer matrix composites. In our knowledge, the influence of low amount of nano to sub-micron sized boron carbide particle loading on neutron shielding properties of LDPE matrix composites has not been studied before. Therefore, in the present study, the main aim is to fabricate cost effective, lightweight and flexible polymer nano-composite plates, which could be used as radiation shielding panels in nuclear medicine and molecular imaging centers. Boron carbide which is well known for its excellent neutron absorption properties was used as filler in LDPE matrix. To improve the shielding properties of composite plates, nano/ sub-micron sized boron carbide particles were fabricated by using the
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chemical structure of gel. TG/DTA analysis (Netzsch STA 44C) was also carried out from room temperature to 1500 °C in an argon atmosphere with a heating rate of 5 °C min−1 to characterize the thermal behavior of gel. Condensed gel was calcined by using a muffle furnace in atmospheric conditions at 675 °C for 2 h. Then, calcined preceramic processors were ground to powder form by using an agate mortar. Final heat treatment of the preceramic precursors was carried out in alumina crucibles at 1500 °C for 5 h in an argon flow (200 ml/min) at a heating rate of 10 °C/min. The phase composition of synthesized powders was characterized by X-ray diffraction analysis (XRD, Bruker D2 Phaser) with Cu Kα radiation (λ = 1.540 Å, 30 kV and 10 mA) from 10° to 80° and a scanning speed of 1°/min. The morphology of the particles was inspected by scanning electron microscopy (SEM, Jeol JSM-6010LV, 15 kV) and high-resolution field emission scanning electron microscopy (FESEM, Zeiss Leo Supra 35 VP, Germany) techniques. The particle size distribution of synthesized powders was measured from FESEM images by means of Image-j and Fiji image analysis programs which are freely available in the public domain [18,19]. Transmission electron microscopy (TEM) studies were carried out to observe the primary particle formation. 2.2. Surface modification Surface modification of synthesized boron carbide particles was carried out by using analytical grade 3-(Triethoxysilyl)-propylamine (E. Merck). Firstly, 1 g of boron carbide powder was added in 50 ml distilled water. The mixture was sonicated by an ultrasonic probe for 20 min. Then 50 ml ethanol was added to the mixture and continuously stirred for 24 h at 40 °C. After this process another sonication step was applied for 10 min, then 15 ml 3-(Triethoxysilyl)-propylamine was added drop by drop to the mixture while stirring at 40 °C. This mixture was continuously stirred for 5h. After the treatment, the particles were washed with DI water and ethanol several times. Lastly, the particles were dried in an oven at 80 °C for 5 h. Fourier Transform Infrared (FTIR) analysis was performed to characterize functional groups of the surface modified boron carbide particles.
Fig. 5. (a) Scanning electron microscopy (SEM) and (b) high-resolution field emission scanning electron microscopy (FESEM) images of B4C filler powder.
sol-gel process. The effects of starting composition and production conditions on the phase composition, crystallinity, particle size and morphology of boron carbide powders were investigated. A simple route for surface modification of boron carbide particles is also presented. Boron carbide (B4C) loaded LDPE matrix nanocomposites were prepared by using low (0.6, 1, 1.7 wt%) amount of surface modified filler B4C powder. The influence of surface functionalization and concentration of boron carbide on the mechanical and neutron shielding properties of the composites have been investigated and discussed.
2.3. Composite preparation Low density polyethylene (LDPE) (Petkim Petrokimya A.Ş., Turkey) with a density of 0.920 g/cm3 and a melt flow rate of 2.5 g/10 min (according to ASTM D1238) was used as thermoplastic matrix. The synthesized and surface modified B4C powder was used as filler. To prepare flexible LDPE-B4C composites with different boron carbide loading (0.6, 1 and 1.7 wt% B4C), B4C powder was first compounded with LDPE matrix in a Gelimat high-shear thermokinetic mixer at 4080 rpm for 30 s and discharged at 210 °C. The blended mixture was cooled to room temperature and crushed before the next step. The crushed products were then hot pressed at 150 °C. LDPE-Neat (without B4C loading) and LDPE-B4C (1 and 1.7 wt% B4C loading) 10 × 10 cm plate samples were used for neutron shielding tests. The pictures of fabricated plates were shown in Fig. 1. For mechanical testing, a minimum of five tensile specimens (according to ISO 527–2) and five Charpy impact test specimens (according to ISO 179–1:2010) of LDPENeat (without B4C loading) and LDPE-B4C (0.6 wt% B4C loading) were injection molded using a Xplore IM12 laboratory scale injection molding machine. Mechanical properties of the prepared composites were measured by using Universal Testing Machine (UTM 5982, Instron) equipped with a 5 kN load cell at a crosshead speed of 2.0 mm/min in accordance with ISO 527-2 standard. The Charpy impact test (Ceast Resil Impactor 6967) was conducted using 5 unnotched samples according to ISO 179–1:2010 standard to measure the fracture toughness and energy
2. Materials and methods 2.1. Powder synthesis For the sol-gel synthesis of filler B4C powders, analytical grade boric acid (H3BO3), glycerin (C3H8O3) and tartaric acid (C4H6O6) were used as starting materials. All chemicals were purchased from E. Merck and used without further purification. Tartaric acid (0.15 mol) and boric acid (0.3 mol) were added directly in glycerin (0.3 mol) to fabricate condensed gel. The mixture was mixed continuously by a magnetic stirrer at 150 °C. After the dissolution of solid ingredients in glycerin, gelation has started to occur by dehydration and condensation reaction. Gel formation was completed in 30 min. At the end of this process, a condensed, amber colored solid gel was obtained. Fourier Transform Infrared (FT-IR) analysis was performed in the 400–4000 cm−1 wavenumber region using a Bruker Tensor 27 spectrometer to identify the
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Fig. 6. SEM EDX-point analysis results of synthesized B4C filler powder (a) scanning electron microscopy (SEM) image, (b) spectrum 29, (c) spectrum 30, and (d) spectrum 32.
Finally, neutron shielding capabilities of LDPE-Neat, LDPE-B4C (1 wt% B4C loading) and LDPE-B4C (1.7 wt% B4C loading) composite samples were investigated as described as in the literature by using an isotropic Am–Be neutron source with activity of 2 Ci [20,21]. 3. Results and discussion 3.1. Sol-gel synthesis of B4C filler particles Polymeric condensed gels were formed via dehydration and condensation reaction between boric acid (H3BO3), glycerin (C3H8O3) and tartaric acid (C4H6O6). It was observed that the degree of condensation and polymerization directly related to the reaction temperature. During the gel synthesis process, when the mixture temperature was raised to 150 °C gel viscosity started to increase rapidly. Fig. 2 shows the FT-IR spectra of condensed gel. The broad band observed between 3600 and 3100 cm−1 is the result of the C–OH and B–OH stretching modes. This band may also indicate the presence of some moisture in the sample. C–OH and B–OH bending modes are at around 1630 cm−1. The vibrational band shown at 2800–3000 cm−1 is the result of the C–H stretching [22,23]. The peaks between 1730 and 1680 cm−1 are related to the characteristic C]O stretching of carboxylic acid dimers [24]. The absorption peaks between 1300-1500 cm−1 and that at 1195 cm−1 were associated with the B–O and B–OH, respectively. The band of
Fig. 7. The particle size distribution of B4C filler powder.
absorption capacity of the prepared composites. Dog-bone specimens with dimensions of 75 × 5 × 2 mm and test bars 80 × 10 × 4 mm in dimensions were used for the tensile and impact tests, respectively. All tests were performed at room temperature (25 °C) and relative humidity.
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endothermic reaction and associated weight reduction. X-ray diffraction pattern (XRD) of the powder which is obtained by heat treatment at a temperature of 1500 °C for 5 h of pyrolyzed condensed gel was given in Fig. 4. All detected peaks indicate that high purity crystalline boron carbide (B4C) powder without any other crystalline phases such as graphite and boron oxide were fabricated. Scanning electron microscopy (SEM) and high-resolution field emission scanning electron microscopy (FESEM) images of B4C powders were shown in Fig. 5 a and b, respectively. Micrographs indicate that boron carbide particles formed with polyhedral-equiaxed morphology in a wide particle size distribution range. The boron carbide powders consist of sub-micron particles to nano-sized particles. EDX-point analysis results of synthesized B4C filler powders were shown in Fig. 6. The results indicate that powder contains mainly boron and carbon elements. No additional peaks indicating any impurity were found. The particle size distribution of boron carbide powders was measured by using Image-j and Fiji image analyses programs from FESEM images. Three different FESEM images were used to count individual particles. The resultant graph was given in Fig. 7. The particle size of the powders measured to be in the range of 50 nm–500 nm and the majority of particles are smaller than 300 nm. Transmission electron microscopy images were confirmed that B4C powder includes 20–30 nm sized primary particles (Fig. 8). Fig. 9 also shows the EDX-mapping analysis results of B4C primary particles. The homogenize distribution of boron and carbon in the particles were shown from Fig. 9 b and d. However, besides the boron and carbon, the presence of oxygen was also detected. The higher intensity of oxygen on the surface (particularly on the sharp regions with the relatively higher surface area) of particles indicates that the origin of the oxygen could be a thin oxide layer on the boron carbide (Fig. 9-c). 3.2. Surface modification of B4C powder Functional groups of silane modified boron carbide particles were examined by FT-IR analysis. The result was shown in Fig. 10. Characteristic bands of (C–B–C) and (B–C) stretching were detected at 1541 and 1070 cm−1, respectively [30,31]. A band located at 840 cm−1 indicates that the existence of free icosahedral B12 molecules [30]. The bands at 1400 and 701 cm−1 are belong to the B–O stretching and B–O–H deformation vibrations, respectively [32,33]. The existence of these bands shows that boron carbide is successfully hydrolyzed through the process and can provide attachment sites for the silane coupling agent (B–O–Si). The bands located at 1541, 1070 and 840 cm−1 may also indicate the existence of (Si–OH) and (Si–O–Si) functional groups in the sample. Most notably, the bands located at 949 and 870 cm−1 confirms the formation of the borosiloxane bond (B–O–Si) [34].
Fig. 8. Transmission electron microscopy (TEM) images of B4C filler powder.
O–B–O was also observed at 540 cm−1 [25]. Most considerably, the absorption peaks of borate ester bonds (B–O–C) which are formed by dehydration and condensation reaction were observed at 1080 and 1020 cm−1 [26,27]. The thermogravimetric and differential thermal analysis (TG/DTA) examinations were carried out to determine the decomposition temperature, the weight loss as well as the phase transformation of the condensed gel. Fig. 3 shows the resultant TG/DTA graph. Various peaks can be observed up to 400 °C. Endothermic peaks observed at lower (100–250 °C) temperatures are related to the transformation of boric acid to metaboric acid through dehydration and the melting of tartaric acid [28]. The sharp exothermic peak around 400 °C could be associated with the transformation of metaboric acid to boron trioxide. Additionally, the boron suboxides are also likely to be produced from the reduction of boron trioxides by the carbon [29]. At a temperature above 1400 °C, the boron oxide and carbon reacted to form boron carbide with a strong
3.3. Mechanical and shielding properties of composites The tensile strength of the LDPE-Neat and LDPE-B4C composites tested at room temperature are shown in Fig. 11. The addition of surface modified B4C particles (0.6 wt%) in LDPE matrix causes slight improvement on the tensile strength. The tensile strength of the LDPE-Neat and LDPE-B4C composite found to be 13.6 and 15 MPa, respectively. However, the measured modulus LDPE-B4C composite detected to be lower than that of LDPE-Neat which were 138.9 and 163.3 MPa, respectively. Increased tensile strength, as well as higher strain at breakpoint of LDPE-B4C composite, indicates that silane treatment of B4C particles may improve the bonding between LDPE matrix and boron carbide particles. However, previous studies report that silane treatment of boron carbide filler particles negatively affected the mechanical
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Fig. 9. TEM-EDX analysis results of B4C particles. (b–d) EDX-mapping results of the particles shown in (a).
Fig. 10. FT-IR analysis result of surface modified B4C powder.
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properties of polyethylene and high-density polyethylene matrix composites [13,15]. So, achieved improvement on mechanical properties of LDPE-B4C composite samples might because of using nano/sub-micron sized of B4C powder and lower filler amount, in addition to the silane treatment. Fracture surfaces of samples after tensile tests were inspected by scanning electron microscopy (SEM). The images are given in Fig. 12. On the surface, B4C particles and/or agglomerates could not be observed which may indicate that B4C particles were well dispersed into the polymer matrix. It is notable that the deformation on the fracture surface of LDPE-Neat sample (Fig. 12-a) relatively higher than LDPE-B4C composite (Fig. 12-b). This observation also confirms that the addition of silane treated nano/sub-micron sized of B4C powder enhance the mechanical properties of LDPE matrix composite materials. Fig. 13 shows the variation of the Charpy impact energy with the volume fraction of B4C particle reinforcing LDPE matrix composites. Even the low amount of B4C particle addition cause slight improvement on both impact resistance and energy. The calculated difference between LDPE-neat and LDPE- B4C (0.6 wt%) composite samples are 8%. Neutron shielding measurements were performed as a function of the sample thickness and B4C particle loading. The results were shown in Fig. 14. Detected neutron flux reduced by loading LDPE matrix with B4C particles. According to the results, 5.17% neutron flux was eliminated by increasing the plate thickness of the LDPE-Neat sample by 1 cm. Even though the neat sample doesn't contain any amount of B4C particles, it contains a considerable amount of hydrogen, and it's well known that hydrogen is very effective in thermalization of high energy neutrons [3]. On the other hand, further improvement on neutron shielding was observed by addition of nano/sub-micron sized B4C particles to LDPE matrix. 8.9% and 9.62% reduction of neutron flux were observed for each 1 cm increase of plate thickness of the LDPE1 wt% B4C and LDPE- 1.7 wt% B4C composite samples, respectively. Thus, the shielding capacity of the LDPE-B4C composite samples was improved by increasing the amount of filler B4C particles (1 wt% to 1.7 wt%) as well as increasing plate thickness. Overall 39% improvement on neutron shielding was observed in LDPE-B4C (1.7 wt%) composite in comparison with LDPE-Neat plate.
Fig. 11. Tensile stress-strain curves of LDPE-Neat and LDPE-B4C (with 0.6 wt% particle loading) composite plates.
4. Conclusions High purity and crystalline boron carbide (B4C) powders with polyhedral-equiaxed particle morphology were successfully fabricated via low-temperature sol-gel route. Heat treatment at 1500 °C for 5 h in argon was sufficient to achieve full reduction of boron oxide/sub-oxides to boron carbide (B4C). The synthesized powder has a wide particle size distribution, including nano-sized (20 nm) primary particles to submicron (500 nm) particles. The formation of borosiloxane bond (B–O–Si) was confirmed on surface treated boron carbide (B4C) powders. Surface modified boron carbide (B4C) particle reinforced LDPE matrix flexible composites were fabricated with a low amount of filler ratios (0.6, 1 and 1.7 wt%). The mechanical properties, tensile strength (9.3%) and impact resistance (8%) of the composites slightly increased. It is found that neutron shielding of the LDPE plates increased by the amount of loaded B4C particles and plate thickness. Even the low amount of B4C particle incorporation to the LDPE matrix improved the neutron shielding capacity dramatically (up to 39%).
Fig. 12. Scanning electron microscopy (SEM) images of failure surface of samples (a) LDPE-Neat, (b) LDPE-B4C after tensile test.
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Fig. 14. Shielding results of specimens having 0, 1 and 1.7 wt% B₄C.
Conflicts of interest None. Acknowledgments The authors are grateful for the financial support from The Scientific and Technological Research Council of Turkey (TUBITAK) and Yıldız Technical University under the contract numbers of 216M140, 216M145 and FDK-2017-3138, respectively. References [1] F. Thévenot, Boron carbide—a comprehensive review, J. Eur. Ceram. Soc. 6 (1990) 205–225 https://doi.org/10.1016/0955-2219(90)90048-K. [2] V. Domnich, Y. Gogotsi, M. Trenary, T. Tanaka, Nanoindentation and Raman spectroscopy studies of boron carbide single crystals, Appl. Phys. Lett. 81 (2002) 3783–3785 https://doi.org/10.1063/1.1521580. [3] M. Celli, F. Grazzi, M. Zoppi, A new ceramic material for shielding pulsed neutron scattering instruments, Nucl. Instrum. Methods Phys. Res. Sect. A Accel. Spectrom. Detect. Assoc. Equip. 565 (2006) 861–863 https://doi.org/10.1016/j.nima.2006. 05.234. [4] D. Annie, V. Chandramouli, S. Anthonysamy, Wet chemical method for determination of free carbon content in boron carbide, Int. J. Chem. Anal. Sci. 7 (2016). [5] V.Z. Shemet, A.P. Pomytkin, V.S. Neshpor, High-temperature oxidation behaviour of carbon materials in air, Carbon 31 (1993) 1–6 https://doi.org/10.1016/00086223(93)90148-4. [6] Y.L. Krut-Skii, G.V. Galevskii, A.A. Kornilov, Oxidation of ultra fine boron carbide, vanadium carbide, and chromium carbide powders, Sov. Powder Metall. Met. Ceram. 22 (1983). [7] H. Wei, C. Li, L. Zhao, Y. Lang, X. Deng, F. Wu, C.-a. Wang, Z. Xie, Preparation of submicron boron carbide powder through gas-solid reaction method, Ceram. Int. 45
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Processing and properties of boron carbide (B4C) reinforced LDPE composites for radiation shielding Suna Avcıoğlua,b, Merve Bulduc,d, Figen Kayaa, Cem Bülent Üstündağe, Erol Kamf, Yusuf Ziya Menceloğluc,d,g, Hayri Yılmaz Kaptanh, Cengiz Kayac,d,∗ a
Department of Metallurgical and Materials Engineering, Faculty of Chemistry and Metallurgy, Yıldız Technical University, Istanbul, 34349, Turkey Department of Metallurgical and Materials Engineering, Faculty of Engineering, Ondokuz Mayıs University, Samsun, 55220, Turkey c Materials Science and Nano Engineering, Faculty of Engineering and Natural Sciences, Sabancı University, Istanbul, 34956, Turkey d Nanotechnology Research and Application Center (SUNUM), Sabancı University, Istanbul, 34956, Turkey e Department of Bioengineering, Faculty of Chemistry and Metallurgy, Yıldız Technical University, Istanbul, 34349, Turkey f Department of Physics, Faculty of Arts and Science, Yıldız Technical University, Istanbul, 34349, Turkey g Integrated Manufacturing Technologies Research and Application Center & Composite Technologies Center of Excellence (SUIMC), Sabancı University, Istanbul, 34906, Turkey h Department of Energy Systems Engineering, Faculty of Engineering, Atılım University, Ankara, 06830, Turkey b
ARTICLE INFO
ABSTRACT
Keywords: Sol-gel Boron carbide LDPE Composite Neutron shielding Mechanical properties
In the present work, boron carbide (B4C) particles were synthesized with sol-gel technique following with heat treatment at 1500 °C in an argon atmosphere. 3-(Triethoxysilyl)-propylamine, a silane coupling agent, was doped on to the surface of synthesized B4C particles. The surface modified B4C particles were embedded in LDPE matrix in order to obtain flexible, lightweight and environmentally friendly shielding materials. The effect of surface functionalization and concentration of boron carbide on its distribution characteristics in the polymer matrix and its effects on the mechanical and neutron shielding properties of the composites are examined. The results showed that high purity-fully crystalline B4C powders with polyhedral-equiaxed morphology in the size range of 20 nm–500 nm were produced. It was found that even the very low amount (0.6–1.7 wt%) of incorporated nano/sub-micron B4C particles in LDPE matrix improved the neutron shielding (up to 39%), tensile strength (9.3%) and impact resistance (8%) of the composites.
1. Introduction Boron carbide (B4C) with a Mohs hardness between 9 and 10 is one of the hardest materials. The combination of high hardness with excellent mechanical strength, high melting point, and low specific gravity makes boron carbide (B4C) a suitable candidate for harsh industrial applications such as abrasives, wear-resistant products, high temperature nozzles and for the military applications like vehicle and body armors [1,2]. Boron has a high neutron absorption cross section for thermal neutrons (767 b) [3]. However, pure boron's relatively weak mechanical properties make boron carbide is a better alternative. Additionally, the atomic number density of boron carbide (0.11 Å−3) is close enough to pure boron (0.14 Å−3) [3]. Due to these reasons, in addition to conventional application areas, boron carbide also being used in nuclear reactors as radiation shield and control rods. Development of a low-temperature production route for boron carbide synthesis is in rising demand in order to reduce production cost. ∗
The sol-gel method is one of the most studied synthetic approaches for producing ceramic powders. This method is based on the formation of a network through polycondensation reactions of molecular precursors in a liquid. The well-homogenized mixture of raw materials at the molecular level, endorse the reduction of the heat treatment temperature which must be subsequently performed. Homogenization also provides better control over the purity, morphology and particle size of materials. Conversely, the main challenge on the production of boron carbide via a liquid state synthesis technique is achieving the final product with high purity. Many attempts have been made to remove residual carbon from boron carbide powder. However, the close oxidation temperature of carbon in the form of graphite and boron carbide makes the process difficult, particularly for the nano-sized boron carbide particles [4–7]. Therefore, recent studies have focused on the fabrication of boron carbide powder without free carbon and boron oxide/suboxides. It has been shown that synthesis of boron carbide particles from polymeric processors may aid to achieve high purity final product as
Corresponding author. Materials Science and Nano Engineering, Faculty of Engineering and Natural Sciences, Sabancı University, Istanbul, 34956, Turkey. E-mail address: [email protected] (C. Kaya).
https://doi.org/10.1016/j.ceramint.2019.08.268 Received 14 July 2019; Received in revised form 20 August 2019; Accepted 27 August 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Suna Avcıoğlu, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2019.08.268
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Fig. 1. Pictures of LDPE plate samples, (a–c) LDPE-Neat, (d–f) LDPE-B4C (1 wt%) composites.
Fig. 2. FT-IR analysis results of condensed gel.
well as lower the synthesis temperatures [8–12]. Electromagnetic and particle radiation from equipment's in medical diagnostic centers or nuclear reactors causes ionizing radiation including gamma rays, x-rays and neutron rays that are well known to be very harmful to human health. Due to the reason that neutrons do not interact with the electrons, they only lose very little energy while passing through heavy materials such as Pb. Using an effective neutron absorber material in a bulk form specifically for radiation shielding applications is neither cost-effective nor practical for production or use. Therefore, development of flexible, lightweight and environmentally friendly shielding materials, preferably the combination of hydrogenous materials and elements with a high absorption cross-section, such as boron are required. A common approach to solve this problem is developing polymer matrix composites filled with a material which is a high capability of neutron absorption. In the previous studies, the potential of high-strength polyimides, natural rubber, epoxy, polystyrene, poly(4-methyl-1-pentene) and low-density polyethylene have been
investigated as a polymeric matrix to produce radiation shielding composites [13]. Easy fabrication of polymers into sheets, extruded into profiles or injection-molded for structural parts is not the only advantage of polymer matrix composites. A polymer matrix contains a large amount of hydrogen atoms. Hydrogen is very effective in thermalization of high energy neutrons because of its light mass [3]. In the case of boron carbide filled polymer matrix composites, this phenomenon makes the absorption of a neutron by the boron nuclei easier. Abdel-Aziz et al. have fabricated boron carbide filled dual polymer matrix containing neutron shielding materials with ethylene-propylene diene rubber and low-density polyethylene. They reported that a high amount of boron carbide loading (57 wt%) had sharply reduced the neutron flux by approximately 85% [14]. Preparation of composites of high-density polyethylene containing three different amounts of modified boron carbide namely: 7 phr (parts per hundred parts of resin), 15 phr and 24 phr have been reported by T. Yasin et al. Their results indicate that the increased content of filler boron carbide enhances the
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Fig. 3. DTA/TG analysis results of condensed gel.
Fig. 4. XRD pattern of synthesized B4C powder.
shielding properties of composites. However, they also reported that mechanical properties such as; elongation at break, tensile strength, and modulus of composites inversely proportional with incorporated boron carbide amount [15]. M. Salimi et al. fabricated rubber based shielding composites. According to their report for every 5 wt% of additional boron carbide loading, the thermal neutron absorption coefficient of the composite material doubled [16]. Boron nitride and boron carbide filled polyethylene matrix composites for radiation shielding applications were reported by C. Harrison et al. [13,17]. In sharp contrast to previous studies, they reported that successful surface modification of filler particles enhances the mechanical properties of both boron nitride and boron carbide filled polyethylene matrix composites. However, the shielding performance of their composites was
slightly better than those polyethylene-neat samples [13,17]. Previous studies clearly show that an increased amount of boron filler content improves the shielding performance of polymer matrix composites. In our knowledge, the influence of low amount of nano to sub-micron sized boron carbide particle loading on neutron shielding properties of LDPE matrix composites has not been studied before. Therefore, in the present study, the main aim is to fabricate cost effective, lightweight and flexible polymer nano-composite plates, which could be used as radiation shielding panels in nuclear medicine and molecular imaging centers. Boron carbide which is well known for its excellent neutron absorption properties was used as filler in LDPE matrix. To improve the shielding properties of composite plates, nano/ sub-micron sized boron carbide particles were fabricated by using the
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chemical structure of gel. TG/DTA analysis (Netzsch STA 44C) was also carried out from room temperature to 1500 °C in an argon atmosphere with a heating rate of 5 °C min−1 to characterize the thermal behavior of gel. Condensed gel was calcined by using a muffle furnace in atmospheric conditions at 675 °C for 2 h. Then, calcined preceramic processors were ground to powder form by using an agate mortar. Final heat treatment of the preceramic precursors was carried out in alumina crucibles at 1500 °C for 5 h in an argon flow (200 ml/min) at a heating rate of 10 °C/min. The phase composition of synthesized powders was characterized by X-ray diffraction analysis (XRD, Bruker D2 Phaser) with Cu Kα radiation (λ = 1.540 Å, 30 kV and 10 mA) from 10° to 80° and a scanning speed of 1°/min. The morphology of the particles was inspected by scanning electron microscopy (SEM, Jeol JSM-6010LV, 15 kV) and high-resolution field emission scanning electron microscopy (FESEM, Zeiss Leo Supra 35 VP, Germany) techniques. The particle size distribution of synthesized powders was measured from FESEM images by means of Image-j and Fiji image analysis programs which are freely available in the public domain [18,19]. Transmission electron microscopy (TEM) studies were carried out to observe the primary particle formation. 2.2. Surface modification Surface modification of synthesized boron carbide particles was carried out by using analytical grade 3-(Triethoxysilyl)-propylamine (E. Merck). Firstly, 1 g of boron carbide powder was added in 50 ml distilled water. The mixture was sonicated by an ultrasonic probe for 20 min. Then 50 ml ethanol was added to the mixture and continuously stirred for 24 h at 40 °C. After this process another sonication step was applied for 10 min, then 15 ml 3-(Triethoxysilyl)-propylamine was added drop by drop to the mixture while stirring at 40 °C. This mixture was continuously stirred for 5h. After the treatment, the particles were washed with DI water and ethanol several times. Lastly, the particles were dried in an oven at 80 °C for 5 h. Fourier Transform Infrared (FTIR) analysis was performed to characterize functional groups of the surface modified boron carbide particles.
Fig. 5. (a) Scanning electron microscopy (SEM) and (b) high-resolution field emission scanning electron microscopy (FESEM) images of B4C filler powder.
sol-gel process. The effects of starting composition and production conditions on the phase composition, crystallinity, particle size and morphology of boron carbide powders were investigated. A simple route for surface modification of boron carbide particles is also presented. Boron carbide (B4C) loaded LDPE matrix nanocomposites were prepared by using low (0.6, 1, 1.7 wt%) amount of surface modified filler B4C powder. The influence of surface functionalization and concentration of boron carbide on the mechanical and neutron shielding properties of the composites have been investigated and discussed.
2.3. Composite preparation Low density polyethylene (LDPE) (Petkim Petrokimya A.Ş., Turkey) with a density of 0.920 g/cm3 and a melt flow rate of 2.5 g/10 min (according to ASTM D1238) was used as thermoplastic matrix. The synthesized and surface modified B4C powder was used as filler. To prepare flexible LDPE-B4C composites with different boron carbide loading (0.6, 1 and 1.7 wt% B4C), B4C powder was first compounded with LDPE matrix in a Gelimat high-shear thermokinetic mixer at 4080 rpm for 30 s and discharged at 210 °C. The blended mixture was cooled to room temperature and crushed before the next step. The crushed products were then hot pressed at 150 °C. LDPE-Neat (without B4C loading) and LDPE-B4C (1 and 1.7 wt% B4C loading) 10 × 10 cm plate samples were used for neutron shielding tests. The pictures of fabricated plates were shown in Fig. 1. For mechanical testing, a minimum of five tensile specimens (according to ISO 527–2) and five Charpy impact test specimens (according to ISO 179–1:2010) of LDPENeat (without B4C loading) and LDPE-B4C (0.6 wt% B4C loading) were injection molded using a Xplore IM12 laboratory scale injection molding machine. Mechanical properties of the prepared composites were measured by using Universal Testing Machine (UTM 5982, Instron) equipped with a 5 kN load cell at a crosshead speed of 2.0 mm/min in accordance with ISO 527-2 standard. The Charpy impact test (Ceast Resil Impactor 6967) was conducted using 5 unnotched samples according to ISO 179–1:2010 standard to measure the fracture toughness and energy
2. Materials and methods 2.1. Powder synthesis For the sol-gel synthesis of filler B4C powders, analytical grade boric acid (H3BO3), glycerin (C3H8O3) and tartaric acid (C4H6O6) were used as starting materials. All chemicals were purchased from E. Merck and used without further purification. Tartaric acid (0.15 mol) and boric acid (0.3 mol) were added directly in glycerin (0.3 mol) to fabricate condensed gel. The mixture was mixed continuously by a magnetic stirrer at 150 °C. After the dissolution of solid ingredients in glycerin, gelation has started to occur by dehydration and condensation reaction. Gel formation was completed in 30 min. At the end of this process, a condensed, amber colored solid gel was obtained. Fourier Transform Infrared (FT-IR) analysis was performed in the 400–4000 cm−1 wavenumber region using a Bruker Tensor 27 spectrometer to identify the
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Fig. 6. SEM EDX-point analysis results of synthesized B4C filler powder (a) scanning electron microscopy (SEM) image, (b) spectrum 29, (c) spectrum 30, and (d) spectrum 32.
Finally, neutron shielding capabilities of LDPE-Neat, LDPE-B4C (1 wt% B4C loading) and LDPE-B4C (1.7 wt% B4C loading) composite samples were investigated as described as in the literature by using an isotropic Am–Be neutron source with activity of 2 Ci [20,21]. 3. Results and discussion 3.1. Sol-gel synthesis of B4C filler particles Polymeric condensed gels were formed via dehydration and condensation reaction between boric acid (H3BO3), glycerin (C3H8O3) and tartaric acid (C4H6O6). It was observed that the degree of condensation and polymerization directly related to the reaction temperature. During the gel synthesis process, when the mixture temperature was raised to 150 °C gel viscosity started to increase rapidly. Fig. 2 shows the FT-IR spectra of condensed gel. The broad band observed between 3600 and 3100 cm−1 is the result of the C–OH and B–OH stretching modes. This band may also indicate the presence of some moisture in the sample. C–OH and B–OH bending modes are at around 1630 cm−1. The vibrational band shown at 2800–3000 cm−1 is the result of the C–H stretching [22,23]. The peaks between 1730 and 1680 cm−1 are related to the characteristic C]O stretching of carboxylic acid dimers [24]. The absorption peaks between 1300-1500 cm−1 and that at 1195 cm−1 were associated with the B–O and B–OH, respectively. The band of
Fig. 7. The particle size distribution of B4C filler powder.
absorption capacity of the prepared composites. Dog-bone specimens with dimensions of 75 × 5 × 2 mm and test bars 80 × 10 × 4 mm in dimensions were used for the tensile and impact tests, respectively. All tests were performed at room temperature (25 °C) and relative humidity.
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endothermic reaction and associated weight reduction. X-ray diffraction pattern (XRD) of the powder which is obtained by heat treatment at a temperature of 1500 °C for 5 h of pyrolyzed condensed gel was given in Fig. 4. All detected peaks indicate that high purity crystalline boron carbide (B4C) powder without any other crystalline phases such as graphite and boron oxide were fabricated. Scanning electron microscopy (SEM) and high-resolution field emission scanning electron microscopy (FESEM) images of B4C powders were shown in Fig. 5 a and b, respectively. Micrographs indicate that boron carbide particles formed with polyhedral-equiaxed morphology in a wide particle size distribution range. The boron carbide powders consist of sub-micron particles to nano-sized particles. EDX-point analysis results of synthesized B4C filler powders were shown in Fig. 6. The results indicate that powder contains mainly boron and carbon elements. No additional peaks indicating any impurity were found. The particle size distribution of boron carbide powders was measured by using Image-j and Fiji image analyses programs from FESEM images. Three different FESEM images were used to count individual particles. The resultant graph was given in Fig. 7. The particle size of the powders measured to be in the range of 50 nm–500 nm and the majority of particles are smaller than 300 nm. Transmission electron microscopy images were confirmed that B4C powder includes 20–30 nm sized primary particles (Fig. 8). Fig. 9 also shows the EDX-mapping analysis results of B4C primary particles. The homogenize distribution of boron and carbon in the particles were shown from Fig. 9 b and d. However, besides the boron and carbon, the presence of oxygen was also detected. The higher intensity of oxygen on the surface (particularly on the sharp regions with the relatively higher surface area) of particles indicates that the origin of the oxygen could be a thin oxide layer on the boron carbide (Fig. 9-c). 3.2. Surface modification of B4C powder Functional groups of silane modified boron carbide particles were examined by FT-IR analysis. The result was shown in Fig. 10. Characteristic bands of (C–B–C) and (B–C) stretching were detected at 1541 and 1070 cm−1, respectively [30,31]. A band located at 840 cm−1 indicates that the existence of free icosahedral B12 molecules [30]. The bands at 1400 and 701 cm−1 are belong to the B–O stretching and B–O–H deformation vibrations, respectively [32,33]. The existence of these bands shows that boron carbide is successfully hydrolyzed through the process and can provide attachment sites for the silane coupling agent (B–O–Si). The bands located at 1541, 1070 and 840 cm−1 may also indicate the existence of (Si–OH) and (Si–O–Si) functional groups in the sample. Most notably, the bands located at 949 and 870 cm−1 confirms the formation of the borosiloxane bond (B–O–Si) [34].
Fig. 8. Transmission electron microscopy (TEM) images of B4C filler powder.
O–B–O was also observed at 540 cm−1 [25]. Most considerably, the absorption peaks of borate ester bonds (B–O–C) which are formed by dehydration and condensation reaction were observed at 1080 and 1020 cm−1 [26,27]. The thermogravimetric and differential thermal analysis (TG/DTA) examinations were carried out to determine the decomposition temperature, the weight loss as well as the phase transformation of the condensed gel. Fig. 3 shows the resultant TG/DTA graph. Various peaks can be observed up to 400 °C. Endothermic peaks observed at lower (100–250 °C) temperatures are related to the transformation of boric acid to metaboric acid through dehydration and the melting of tartaric acid [28]. The sharp exothermic peak around 400 °C could be associated with the transformation of metaboric acid to boron trioxide. Additionally, the boron suboxides are also likely to be produced from the reduction of boron trioxides by the carbon [29]. At a temperature above 1400 °C, the boron oxide and carbon reacted to form boron carbide with a strong
3.3. Mechanical and shielding properties of composites The tensile strength of the LDPE-Neat and LDPE-B4C composites tested at room temperature are shown in Fig. 11. The addition of surface modified B4C particles (0.6 wt%) in LDPE matrix causes slight improvement on the tensile strength. The tensile strength of the LDPE-Neat and LDPE-B4C composite found to be 13.6 and 15 MPa, respectively. However, the measured modulus LDPE-B4C composite detected to be lower than that of LDPE-Neat which were 138.9 and 163.3 MPa, respectively. Increased tensile strength, as well as higher strain at breakpoint of LDPE-B4C composite, indicates that silane treatment of B4C particles may improve the bonding between LDPE matrix and boron carbide particles. However, previous studies report that silane treatment of boron carbide filler particles negatively affected the mechanical
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Fig. 9. TEM-EDX analysis results of B4C particles. (b–d) EDX-mapping results of the particles shown in (a).
Fig. 10. FT-IR analysis result of surface modified B4C powder.
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properties of polyethylene and high-density polyethylene matrix composites [13,15]. So, achieved improvement on mechanical properties of LDPE-B4C composite samples might because of using nano/sub-micron sized of B4C powder and lower filler amount, in addition to the silane treatment. Fracture surfaces of samples after tensile tests were inspected by scanning electron microscopy (SEM). The images are given in Fig. 12. On the surface, B4C particles and/or agglomerates could not be observed which may indicate that B4C particles were well dispersed into the polymer matrix. It is notable that the deformation on the fracture surface of LDPE-Neat sample (Fig. 12-a) relatively higher than LDPE-B4C composite (Fig. 12-b). This observation also confirms that the addition of silane treated nano/sub-micron sized of B4C powder enhance the mechanical properties of LDPE matrix composite materials. Fig. 13 shows the variation of the Charpy impact energy with the volume fraction of B4C particle reinforcing LDPE matrix composites. Even the low amount of B4C particle addition cause slight improvement on both impact resistance and energy. The calculated difference between LDPE-neat and LDPE- B4C (0.6 wt%) composite samples are 8%. Neutron shielding measurements were performed as a function of the sample thickness and B4C particle loading. The results were shown in Fig. 14. Detected neutron flux reduced by loading LDPE matrix with B4C particles. According to the results, 5.17% neutron flux was eliminated by increasing the plate thickness of the LDPE-Neat sample by 1 cm. Even though the neat sample doesn't contain any amount of B4C particles, it contains a considerable amount of hydrogen, and it's well known that hydrogen is very effective in thermalization of high energy neutrons [3]. On the other hand, further improvement on neutron shielding was observed by addition of nano/sub-micron sized B4C particles to LDPE matrix. 8.9% and 9.62% reduction of neutron flux were observed for each 1 cm increase of plate thickness of the LDPE1 wt% B4C and LDPE- 1.7 wt% B4C composite samples, respectively. Thus, the shielding capacity of the LDPE-B4C composite samples was improved by increasing the amount of filler B4C particles (1 wt% to 1.7 wt%) as well as increasing plate thickness. Overall 39% improvement on neutron shielding was observed in LDPE-B4C (1.7 wt%) composite in comparison with LDPE-Neat plate.
Fig. 11. Tensile stress-strain curves of LDPE-Neat and LDPE-B4C (with 0.6 wt% particle loading) composite plates.
4. Conclusions High purity and crystalline boron carbide (B4C) powders with polyhedral-equiaxed particle morphology were successfully fabricated via low-temperature sol-gel route. Heat treatment at 1500 °C for 5 h in argon was sufficient to achieve full reduction of boron oxide/sub-oxides to boron carbide (B4C). The synthesized powder has a wide particle size distribution, including nano-sized (20 nm) primary particles to submicron (500 nm) particles. The formation of borosiloxane bond (B–O–Si) was confirmed on surface treated boron carbide (B4C) powders. Surface modified boron carbide (B4C) particle reinforced LDPE matrix flexible composites were fabricated with a low amount of filler ratios (0.6, 1 and 1.7 wt%). The mechanical properties, tensile strength (9.3%) and impact resistance (8%) of the composites slightly increased. It is found that neutron shielding of the LDPE plates increased by the amount of loaded B4C particles and plate thickness. Even the low amount of B4C particle incorporation to the LDPE matrix improved the neutron shielding capacity dramatically (up to 39%).
Fig. 12. Scanning electron microscopy (SEM) images of failure surface of samples (a) LDPE-Neat, (b) LDPE-B4C after tensile test.
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Fig. 13. Charpy impact resistance test results of LDPE-Neat and LDPE-B4C (with 0.6 wt% particle loading) composite plates. (2019) 14749–14755 https://doi.org/10.1016/j.ceramint.2019.04.201. [8] S.K. Vijay, R. Krishnaprabhu, V. Chandramouli, S. Anthonysamy, Synthesis of nanocrystalline boron carbide by sucrose precursor method-optimization of process conditions, Ceram. Int. 44 (2018) 4676–4684 https://doi.org/10.1016/j.ceramint. 2017.12.047. [9] S. Mondal, A.K. Banthia, Low-temperature synthetic route for boron carbide, J. Eur. Ceram. Soc. 25 (2005) 287–291 https://doi.org/10.1016/j.jeurceramsoc.2004.08. 011. [10] M. Kakiage, N. Tahara, I. Yanase, H. Kobayashi, Low-temperature synthesis of boron carbide powder from condensed boric acid–glycerin product, Mater. Lett. 65 (2011) 1839–1841 https://doi.org/10.1016/j.matlet.2011.03.046. [11] S. Avcioglu, F. Kaya, C. Kaya, Synthesis of High Purity Boron Carbide Powders via Sol-Gel Method, 19th International Metallurgy and Materials Congress, (2018). [12] S. Avcioglu, F. Kaya, C. Kaya, Non-catalytic synthesis of boron carbide (B4C) nano structures with various morphologies by sol-gel process, Mater. Lett. 249 (2019) 201–205 https://doi.org/10.1016/j.matlet.2019.04.056. [13] C. Harrison, S. Weaver, C. Bertelsen, E. Burgett, N. Hertel, E. Grulke, Polyethylene/ boron nitride composites for space radiation shielding, J. Appl. Polym. Sci. 109 (2008) 2529–2538 https://doi.org/10.1002/app.27949. [14] M.M. Abdel-Aziz, S.E. Gwaily, A.S. Makarious, A. El-Sayed Abdo, Ethylene-propylene diene rubber/low density polyethylene/boron carbide composites as neutron shields, Polym. Degrad. Stab. 50 (1995) 235–240 https://doi.org/10.1016/01413910(95)00177-8. [15] T. Yasin, M.N. Khan, High density polyethylene/boron carbide composites for neutron shielding, E-Polymers 8 (2008), https://doi.org/10.1515/epoly.2008.8.1. 670. [16] M. Salimi, N. Ghal-Eh, E.A. Amirabadi, Characterization of a new shielding rubber for use in neutron–gamma mixed fields, Nucl. Sci. Tech. 29 (2018) 301 https://doi. org/10.1007/s41365-018-0371-7. [17] C. Harrison, E. Burgett, N. Hertel, E. Grulke, M.S. El-Genk, Polyethylene∕Boron composites for radiation shielding applications, AIP Conference Proceedings, Albuquerque (New Mexico), AIP, 10–14 February, 2008, pp. 484–491. [18] J. Schindelin, I. Arganda-Carreras, E. Frise, V. Kaynig, M. Longair, T. Pietzsch, S. Preibisch, C. Rueden, S. Saalfeld, B. Schmid, J.-Y. Tinevez, D.J. White, V. Hartenstein, K. Eliceiri, P. Tomancak, A. Cardona, Fiji: an open-source platform for biological-image analysis, Nat. Methods 9 (2012) 676–682 https://doi.org/10. 1038/nmeth.2019. [19] J. Schindelin, C.T. Rueden, M.C. Hiner, K.W. Eliceiri, The ImageJ ecosystem: an open platform for biomedical image analysis, Mol. Reprod. Dev. 82 (2015) 518–529 https://doi.org/10.1002/mrd.22489. [20] T. Korkut, O. Gencel, E. Kam, W. Brostow, X-ray, gamma, and neutron radiation tests on epoxy-ferrochromium slag composites by experiments and Monte Carlo simulations, Int. J. Polym. Anal. Charact. 18 (2013) 224–231 https://doi.org/10. 1080/1023666X.2013.755658. [21] O. Gencel, A. Bozkurt, E. Kam, T. Korkut, Determination and calculation of gamma and neutron shielding characteristics of concretes containing different hematite proportions, Ann. Nucl. Energy 38 (2011) 2719–2723 https://doi.org/10.1016/j. anucene.2011.08.010. [22] Y. Shigemasa, H. Matsuura, H. Sashiwa, H. Saimoto, Evaluation of different absorbance ratios from infrared spectroscopy for analyzing the degree of deacetylation in chitin, Int. J. Biol. Macromol. 18 (1996) 237–242 https://doi.org/10.1016/01418130(95)01079-3. [23] H.S. Mansur, C.M. Sadahira, A.N. Souza, A.A.P. Mansur, FTIR spectroscopy characterization of poly (vinyl alcohol) hydrogel with different hydrolysis degree and chemically crosslinked with glutaraldehyde, Mater. Sci. Eng. C 28 (2008) 539–548 https://doi.org/10.1016/j.msec.2007.10.088. [24] D. Li, Y. Xia, Electrospinning of nanofibers: reinventing the wheel? Adv. Mater. 16 (2004) 1151–1170 https://doi.org/10.1002/adma.200400719. [25] R.L. Siqueira, I.V.P. Yoshida, L.C. Pardini, M.A. Schiavon, Poly(borosiloxanes) as precursors for carbon fiber ceramic matrix composites, Mater. Res. 10 (2007) 147–151 https://doi.org/10.1590/S1516-14392007000200009. [26] J. Romanos, M. Beckner, D. Stalla, A. Tekeei, G. Suppes, S. Jalisatgi, M. Lee,
Fig. 14. Shielding results of specimens having 0, 1 and 1.7 wt% B₄C.
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