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Radiation-protective polymer-matrix nanostructured composites
Journal of Alloys and Compounds 536S (2012) S522–S526
Contents lists available at SciVerse ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Radiation-protective polymer-matrix nanostructured composites S.D. Kaloshkin a , V.V. Tcherdyntsev a , M.V. Gorshenkov a,∗ , V.N. Gulbin a , S.A. Kuznetsov b a b
College of Advanced Materials and Nanotechnologies, National University of Science and Technology “MISiS”, Leninsky Prospect, 4 Moscow, Russia Russian State Technological University “MATI”, Orshanskaya 3, Moscow, Russia
a r t i c l e
i n f o
Article history: Received 11 July 2011 Received in revised form 29 December 2011 Accepted 11 January 2012 Available online 28 January 2012
a b s t r a c t UHMWPE-based nanostructured composites containing B4 C and W nanopowders were fabricated and studied. The mechanical and ␥-radiation protective properties of the polymer-matrix nanocomposites were determined experimentally. For selected composites the mechanical properties were studied prior to and after the irradiation. © 2012 Elsevier B.V. All rights reserved.
Keywords: Polymer-matrix nanocomposites Boron carbide Tungsten Radiation-protective properties Nanopowder
1. Introduction Growing demand on the use of radioactive materials as well as production of radioactive waste leads to more severe requirements for radiation safety. New methods of protection against harmful radiation should be developed and implemented. One of the approaches is the development of new radiation protective composite materials containing no toxic lead that requires a special disposal procedure. It is known that prolonged irradiation of polymer composite materials in some cases leads to destruction of the material; other composites, on the contrary, develop three-dimension crosslinked structures with advanced physicomechanical properties [1]. Ultrahigh molecular weight polyethylene (UHMWPE) is one of the polymers resistant to irradiation. Irradiation-induced crosslinking can be used to enhance properties of UHMWPE-based composites, in particular wear resistance [2–6]. The influence of irradiation on UHMWPE has been extensively studied [7–11]. Radiation-resistant UHMWPE matrix can be used to develop composite materials filled with powders capable of absorbing ␥ and neutron radiation. Such materials can serve as radiation-protective materials for personnel and electronic equipment. It was shown [12] that the application of materials containing ultra-dispersed powders provides high protective characteristics against X-rays and thermal neutrons. In addition, it was found [13]
∗ Corresponding author. E-mail address: [email protected] (M.V. Gorshenkov). 0925-8388/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2012.01.061
that the application of nanoparticles of radiation-absorbing materials (BN, B4 C, Pb and W) leads to an increase in the absorption coefficient of neutrons by 50% and the X-ray scattering coefficient by 30–40%, so the use of radiation-absorbing nanoparticles is a promising way for the development of radiation-protective materials. This work is devoted to the production and study of new radiation-protective UHMWPE-based composites filled with B4 C and W nanopowders.
2. Experimental To prepare the radiation-protective polymer-matrix composites (PMC), UHMWPE with an average molecular weight of 4 × 106 g/mol and a granule size of 60 m was used. Commercial boron carbide (B4 C) with an average particle size of 100 m and tungsten nanopowder (Wn ) with an average particle size of 60–80 nm were used as fillers. Tungsten nanopowders were synthesized by precipitation of metal hydroxides from solutions of the salts with the subsequent hydrogen reduction of the intermediate product. The starting material for the synthesis of the intermediate product was tungstic acid (H2 WO4 ). The method of nanopowder production was described in detail in [14]. Prior to preparing PMC, B4 C was ground to the dispersed state with the particle size from 0.1 to 10 m in a Fritsch Pulverisette 5 planetary ball mill with chromium steel vials containing steel balls. The components of PMC were mixed in the ball mills of two types with different energies supplied to the material. The air-cooled planetary ball mill Fritsch Pulverisette 5 with agate vials (500 ml) with agate milling balls was used for mixing components of the composites as the first low energy type of mill. Ball milling was conducted during 1 h with breaks for cooling (3 min milling–5 min cooling) at a rotation speed of 350 rpm, the ball-powder mass ratio was 10:1. A more powerful water-cooled planetary ball mill APF-3 (produced by “Novosibirskiy test center”) with steel vials of 1000 ml and steel balls of 5–9 mm was used
S.D. Kaloshkin et al. / Journal of Alloys and Compounds 536S (2012) S522–S526
S523
Fig. 1. Photographs of disk specimens of PMC: (a) UHMWPE modified with Pb-contain fillers, (b) pure UHMWPE and (c) UHMWPE modified with B4 C and Wn .
as the second type of mill. Milling parameters were: rotation speed of 450 rpm, the ball-powder mass ratio - 10:1, duration - 1 h (5 min milling–5 min cooling). The following compositions were studied: (1) 8 wt.% B4 C + 10 wt.% Wn + UHMWPE (at balance), (2) 12 wt.% B4 C + 18 wt.% Wn + UHMWPE (at balance), (3) 30 wt.% B4 C + 20 wt.% Wn + UHMWPE (at balance), and (4) 60 wt.% Wn + UHMWPE (at balance). For the comparative tests, the specimens of pure UHMWPE were produced. The mixed compositions after ball milling were compressed in molds on a heated press APVM-904 with an applied pressure of 50 MPa at a temperature of 180 ◦ C. After sintering the shaped composite samples were gradually cooled to room temperature under compression. For the tests on radiation-protective properties, disc specimens of 50 mm in diameter and 1, 2, and 3 mm thick were used (Fig. 1). As it is seen, the disc samples with different fillers have various colors. The first two samples contain such conventional for the radiation-protective materials fillers as Pb(BO2 )2 powder (white-pink sample) and PbO powder (pale yellow sample). The third sample is pure UHMWPE. All other black samples contain B4 C and Wn as fillers. For the tensile tests, the specimens (Fig. 2) of each composition with 3 mm thickness were used. Tensile tests were performed on a universal testing machine (Instron 5500) with a 5 kN load cell. To ensure the accurate measurement of elastic modulus, the extension was recorded by a video extensometer MXR 5021 (Instron Co., USA). The tensile tests were conducted at a low cross-beam speed (2.5 mm/min). A beryllium converter of (p,n), (p,␥), which was irradiated with protons with an energy of 32 MeV, the currents up to 30 A, and the integral doses of protons on the converter up to 5 × 1018 particles, was used as the source of neutrons and the accompanying gamma quanta. This provided a neutron dose of 1016 neutron/cm2 with the energies of 0.3–32 MeV on the specimens during 10 h.
3. Microstructure Fig. 3 gives the micrographs of the cross-sections of the composite material. The micrographs were obtained with a scanning electron microscope in the back-scattered electron mode, after the mechanical actions of various intensities. The presence of heavy metal W in the light polymeric matrix enabled us to identify adequately the domains that are enriched with or deprived of the heavy filler.
Fig. 3a shows a section of the composite material, which was prepared in the low-energy ball mill Fritsch Pulverisette 5. It is seen that the material contains the regions that differ significantly in the content of W. Dark areas on the micrograph are the regions of almost pure polymer material free of the fillers. Light areas are the regions enriched with W. The dimension of dark areas coincides with the size of the initial UHMWPE granules that were slightly strained as a result of mechanical action and thermo pressing. From this micrograph, we can conclude that highly heterogeneous material is obtained as a result of low-power treatment. The structural heterogeneity can have a detrimental effect on the mechanical and radiation-protective properties of the composites. Micrograph of Fig. 3b shows the composite material that was prepared in the high-energy ball mill APF-3. It is seen that the homogeneity of material is much higher than in the sample in micrograph in Fig. 3a. The mechanical treatment of UHMWPE with the nanofillers is characterized by the absence of destruction processes of the polymeric particles. Therefore, in the course of mechanical action, the polymeric granule surface is only packed with the nanofiller. Increase of time and intensity of the treatment results in a better intermixing of the components, deeper penetration of the filling agents into the polymer particles and a decrease of structural heterogeneity of the specimens (Fig. 3b). From the viewpoint of radiation protection, more uniform distribution of particles over the polymeric matrix is preferable, because it hampers the generation of extremely undesirable effect – channeling of neutrons or other radiation in the material. A nonuniform distribution of particles can lead to the formation of areas containing considerably smaller amount of filler and, correspondingly, exhibiting significantly lower absorption of radiation. The radiation background behind this area of material can be higher, which can have a detrimental effect on the protected equipment or stuff.
4. Mechanical properties
Fig. 2. The specimens for the tensile tests.
Table 1 lists the experimental data on the mechanical properties of several selected composites. The mechanical properties were studied after the high-energy treatment in the planetary ball mill APF-3, corresponding composite structure is shown in Fig. 3b. Fig. 4 gives experimental curves of tensile tests for the composites. The results of mechanical tests of PMC show that after filling UHMWPE with nanoparticles of W and B4 C the elastic modulus, the tensile stress and tensile strength increase. Concurrently, the specific elongation decreases considerably. Such behavior of mechanical properties of polymeric composites is conventional, reinforcing of polymer matrix with hard particles (including
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Table 1 Mechanical properties of PMC of various compositions. Specimen
Elastic modulus (E-modulus) (GPa)
Tensile yield point (0.2%) (MPa)
Ultimate tensile strength (MPa)
Specific elongation (%)
UHMWPE GUR 4120 pure UHMWPE + 18% W + 12% B4 C UHMWPE + 18% W + 12% B4 C irradiated with neutrons (E > 0.3 MeV) up to a dose of 1016 neutron/cm2 UHMWPE + 30% W + 20% B4 C UHMWPE + 60% Wn
0.8 1.4 1.7
17 19 20
21 26 25
400 90 12
0.4 0.4
18 13
27 26
2.00 100
nanosized powders) results in an increase of elasticity modules and yield stress accompanied by simultaneous decrease of elongation [15]. Very often this gain in elasticity modulus and yield stress is desirable, because in most cases these properties define the stress limit of material under loading and it does not undergo plastic deformation. This is most pronounced for the specimen containing 20% B4 C and 30% Wn . In this case, the specimen starts to break with almost zero elongation. An addition of a large amount of B4 C leads to the embrittlement of composite material. Probably, this is associated with rather large dimensions of B4 C particles as compared to Wn and poor adhesion of B4 C to the polymeric matrix. Actually, B4 C particles act as the defects in the polymeric matrix. The effect of irradiation with fast neutrons on the mechanical properties of the composite material was studied for the PMC containing 18% W + 12% B4 C. The specimens of the composite material, which were prepared for the tensile tests, were irradiated with fast neutrons. Only the working part of the specimen was irradiated. After the irradiation, the specimens had an induced radioactive background and, prior to the tests, were kept up to reaching a safe value of induced background.
The results of mechanical tests of composite material prior to and after the irradiation are presented in Table 1. It is seen that after the irradiation, the plasticity of the composite material abruptly decreases, the fracture becomes virtually brittle. However, the tensile stress and tensile strength do not change; they remain at the same level as before the irradiation. 5. The ␥-ray protection The ␥-radiation protective properties of polymer-matrix nanocomposites were determined by the method of the flux attenuation coefficient of monoenergetic 57 Co ␥-ray with an energy of 122 KeV. In the course of the experiment, the ␥-ray flux attenuation was measured prior to and after the introduction to the polymer target. The linear flux attenuation coefficient was calculated using the equation: = 1/xLn(N0 /N), where x is the thickness of polymeric specimen, N0 is the ␥-ray flux without polymer target, and N is the ␥-ray flux after passing through the polymer target. Table 2 presents the experimental results of linear ␥-ray attenuation coefficient. It is seen that, as expected, the linear ␥-ray attenuation coefficient increases with increasing tungsten content
Fig. 3. Micrographs of the cross-sections of the composite material: (a) prepared in a low-energy ball mill Fritsch Pulverisette 5 and (b) prepared in a high-energy ball mill APF-3.
S.D. Kaloshkin et al. / Journal of Alloys and Compounds 536S (2012) S522–S526
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Table 2 The measured linear ␥-ray attenuation coefficient. Matrix
W (wt.%)
B4 C (wt.%)
Linear flux attenuation coefficient, (cm−1 )
UHMWPE UHMWPE UHMWPE UHMWPE
10 18 30 60
8 12 20 –
0.43 0.73 1.36 3.43
Fig. 4. Experimental curves of tensile tests.
in the composite. The highest ␥-ray attenuation coefficient was obtained for the composite containing 60% Wn . As expected, an increase of the content of tungsten in the composite leads to an increase in the linear absorption coefficient due to a high absorbability of tungsten with respect to X-ray radiation [16]. This PMC provides a linear attenuation coefficient of 3.43 cm−1 for the monoenergetic 57 Co ␥-ray flux with an energy of 122 KeV. The PMC retains high mechanical properties and remains sufficiently plastic after the irradiation. Interactions of photons with material have three energy loss mechanisms: photoelectric effect, Compton scattering and pair production. If a photon enters material with an energy under 1.022 MeV, it may interact by both processes of photoelectric effect and Compton scattering. The photoelectric absorption probability strongly depends on the high atomic number Z [17]. Thus, in this work, the existence of high-Z elements, W, plays an important role in radiation protection property of the composite. Additionally, the presence and symmetrical distribution of nanosized W particles enhance the interaction probability between photon and the composite. As it was shown in [13], on the filler nanoparticles, the linear absorption coefficient of gamma radiation increases due to an increase in the coherent scattering angles. This results in an increase of the optical distance of radiation in the material and, correspondingly, its additional absorption. This is true for the energy of incident radiation close to the X-ray range (0.1–10 KeV). A decrease
of the length for gamma-radiation leads to a decrease of the coherent scattering angles, a considerable increase of optical distance, and the coherent scattering does not contribute to the absorption processes in the material. Thus, at an irradiation energy of 122 KeV, the photoelectric absorption and inelastic scattering (Compton effect) are the main absorption processes in these composites. 6. Conclusions In this work, the radiation-protective polymer-matrix nanostructured composites based on UHMWPE with B4 C and W nanopowders were fabricated by solid state intermixing and thermal pressing. A study of mechanical properties showed that a higher content of the fillers in the polymer-matrix nanocomposites results in an increase of the modulus of elasticity, the tensile stress, and tensile strength. However, the specific elongation decreases considerably. The irradiation of material with a flux of fast neutrons induces a residual radioactivity in the material. The mechanical properties of irradiated composite exhibit a decrease of the plasticity and elongation, whereas the ultimate strength and the yield strength remain constant. The ␥-ray protection properties increase with an increase in the content of tungsten in the composite. The highest attenuation coefficient was observed at a W content of 60% and reached 3.43 cm−1 . The photoelectric absorption and inelastic scattering
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(Compton effect) are the main absorption processes in the PMC. The application of nanosized fillers does not raise the linear absorption coefficient. Acknowledgments This work is supported in the frame of federal target program “Research and development on the priority directions of the development of scientific and technological industry of Russia for 2007-2013 years”, state contract 16.516.11.6074. References [1] A. Charlesby, Atomic Radiation and Polymers, Pergamon Press, Oxford/London/New York/Paris, 1960, p. 522. [2] D.S. Xiong, R.Y. Ma, J.M. Lin, N. Wang, Z.M. Jin, Proc. Inst. Mech. Eng. Part J: J. Eng. Tribol. 221 (2007) 315–320. [3] B.R. Burroughs, T.A. Blanchet, Tribol. Trans. 44 (2001) 215–223.
[4] L. Xiong, D. Xiong, J. Jin, J. Bionic Eng. 6 (2009) 7–13. [5] E. Oral, O.K. Muratoglu, Nucl. Instrum. Methods Phys. Res. B 265 (2007) 18– 22. [6] L.A. Pruitt, Biomaterials 26 (2005) 905–915. [7] S.M. Kurtz, M.L. Villarraga, M.P. Herr, J.S. Bergstrom, C.M. Rimnac, A.A. Edidin, Biomaterials 23 (2002) 3681–3697. [8] M.J. Martinez-Morlanes, F.J. Medel, M.D. Mariscal, J.A. Puertolas, Polym. Test 29 (4) (2010) 425–432. [9] A. Turos, A.M. Abdul-Kader, R. Ratajczak, A. Stonert, Vacuum 83 (2009) S54–S56. [10] P. O’Neil, et al., Polym. Degrad. Stabil. 63 (1999) 31–39. [11] S.A. Atwood, D.W. Van Citters, E.W. Patten, F. Jevan, M.D. Ries, L.A. Pruitt, J. Mech. Behav. Biomed. Mater. 4 (2011) 1033–1045. [12] M.I. Alymov, S.I. Averin, S.A. Tikhomirov, V.A. Zelenskii, Russ. Metall. (Metally) 5 (2005) 427–430. [13] V.A. Artem’ev, Tech. Phys. Lett. 23 (3) (1997) 212–213. [14] M.I. Alymov, I.V. Tregubova, K.B. Povarova, A.B. Ankudinov, E.V. Evstratov, Russ. Metall. (Metally) 3 (2006) 217–220. [15] S.-Y. Fu, X.-Q. Feng, B. Lauke, Y.-W. Mai, Composites: Part B 39 (2008) 933–961. [16] G.H. Fan, L. Geng, G.S. Wang, Z.Z. Zheng, Mater. Des. 30 (2009) 862–866. [17] S. Naveer, J.S. Kanwar, S. Kulwant, Radiat. Meas. 41 (2006) 84–88.
Contents lists available at SciVerse ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Radiation-protective polymer-matrix nanostructured composites S.D. Kaloshkin a , V.V. Tcherdyntsev a , M.V. Gorshenkov a,∗ , V.N. Gulbin a , S.A. Kuznetsov b a b
College of Advanced Materials and Nanotechnologies, National University of Science and Technology “MISiS”, Leninsky Prospect, 4 Moscow, Russia Russian State Technological University “MATI”, Orshanskaya 3, Moscow, Russia
a r t i c l e
i n f o
Article history: Received 11 July 2011 Received in revised form 29 December 2011 Accepted 11 January 2012 Available online 28 January 2012
a b s t r a c t UHMWPE-based nanostructured composites containing B4 C and W nanopowders were fabricated and studied. The mechanical and ␥-radiation protective properties of the polymer-matrix nanocomposites were determined experimentally. For selected composites the mechanical properties were studied prior to and after the irradiation. © 2012 Elsevier B.V. All rights reserved.
Keywords: Polymer-matrix nanocomposites Boron carbide Tungsten Radiation-protective properties Nanopowder
1. Introduction Growing demand on the use of radioactive materials as well as production of radioactive waste leads to more severe requirements for radiation safety. New methods of protection against harmful radiation should be developed and implemented. One of the approaches is the development of new radiation protective composite materials containing no toxic lead that requires a special disposal procedure. It is known that prolonged irradiation of polymer composite materials in some cases leads to destruction of the material; other composites, on the contrary, develop three-dimension crosslinked structures with advanced physicomechanical properties [1]. Ultrahigh molecular weight polyethylene (UHMWPE) is one of the polymers resistant to irradiation. Irradiation-induced crosslinking can be used to enhance properties of UHMWPE-based composites, in particular wear resistance [2–6]. The influence of irradiation on UHMWPE has been extensively studied [7–11]. Radiation-resistant UHMWPE matrix can be used to develop composite materials filled with powders capable of absorbing ␥ and neutron radiation. Such materials can serve as radiation-protective materials for personnel and electronic equipment. It was shown [12] that the application of materials containing ultra-dispersed powders provides high protective characteristics against X-rays and thermal neutrons. In addition, it was found [13]
∗ Corresponding author. E-mail address: [email protected] (M.V. Gorshenkov). 0925-8388/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2012.01.061
that the application of nanoparticles of radiation-absorbing materials (BN, B4 C, Pb and W) leads to an increase in the absorption coefficient of neutrons by 50% and the X-ray scattering coefficient by 30–40%, so the use of radiation-absorbing nanoparticles is a promising way for the development of radiation-protective materials. This work is devoted to the production and study of new radiation-protective UHMWPE-based composites filled with B4 C and W nanopowders.
2. Experimental To prepare the radiation-protective polymer-matrix composites (PMC), UHMWPE with an average molecular weight of 4 × 106 g/mol and a granule size of 60 m was used. Commercial boron carbide (B4 C) with an average particle size of 100 m and tungsten nanopowder (Wn ) with an average particle size of 60–80 nm were used as fillers. Tungsten nanopowders were synthesized by precipitation of metal hydroxides from solutions of the salts with the subsequent hydrogen reduction of the intermediate product. The starting material for the synthesis of the intermediate product was tungstic acid (H2 WO4 ). The method of nanopowder production was described in detail in [14]. Prior to preparing PMC, B4 C was ground to the dispersed state with the particle size from 0.1 to 10 m in a Fritsch Pulverisette 5 planetary ball mill with chromium steel vials containing steel balls. The components of PMC were mixed in the ball mills of two types with different energies supplied to the material. The air-cooled planetary ball mill Fritsch Pulverisette 5 with agate vials (500 ml) with agate milling balls was used for mixing components of the composites as the first low energy type of mill. Ball milling was conducted during 1 h with breaks for cooling (3 min milling–5 min cooling) at a rotation speed of 350 rpm, the ball-powder mass ratio was 10:1. A more powerful water-cooled planetary ball mill APF-3 (produced by “Novosibirskiy test center”) with steel vials of 1000 ml and steel balls of 5–9 mm was used
S.D. Kaloshkin et al. / Journal of Alloys and Compounds 536S (2012) S522–S526
S523
Fig. 1. Photographs of disk specimens of PMC: (a) UHMWPE modified with Pb-contain fillers, (b) pure UHMWPE and (c) UHMWPE modified with B4 C and Wn .
as the second type of mill. Milling parameters were: rotation speed of 450 rpm, the ball-powder mass ratio - 10:1, duration - 1 h (5 min milling–5 min cooling). The following compositions were studied: (1) 8 wt.% B4 C + 10 wt.% Wn + UHMWPE (at balance), (2) 12 wt.% B4 C + 18 wt.% Wn + UHMWPE (at balance), (3) 30 wt.% B4 C + 20 wt.% Wn + UHMWPE (at balance), and (4) 60 wt.% Wn + UHMWPE (at balance). For the comparative tests, the specimens of pure UHMWPE were produced. The mixed compositions after ball milling were compressed in molds on a heated press APVM-904 with an applied pressure of 50 MPa at a temperature of 180 ◦ C. After sintering the shaped composite samples were gradually cooled to room temperature under compression. For the tests on radiation-protective properties, disc specimens of 50 mm in diameter and 1, 2, and 3 mm thick were used (Fig. 1). As it is seen, the disc samples with different fillers have various colors. The first two samples contain such conventional for the radiation-protective materials fillers as Pb(BO2 )2 powder (white-pink sample) and PbO powder (pale yellow sample). The third sample is pure UHMWPE. All other black samples contain B4 C and Wn as fillers. For the tensile tests, the specimens (Fig. 2) of each composition with 3 mm thickness were used. Tensile tests were performed on a universal testing machine (Instron 5500) with a 5 kN load cell. To ensure the accurate measurement of elastic modulus, the extension was recorded by a video extensometer MXR 5021 (Instron Co., USA). The tensile tests were conducted at a low cross-beam speed (2.5 mm/min). A beryllium converter of (p,n), (p,␥), which was irradiated with protons with an energy of 32 MeV, the currents up to 30 A, and the integral doses of protons on the converter up to 5 × 1018 particles, was used as the source of neutrons and the accompanying gamma quanta. This provided a neutron dose of 1016 neutron/cm2 with the energies of 0.3–32 MeV on the specimens during 10 h.
3. Microstructure Fig. 3 gives the micrographs of the cross-sections of the composite material. The micrographs were obtained with a scanning electron microscope in the back-scattered electron mode, after the mechanical actions of various intensities. The presence of heavy metal W in the light polymeric matrix enabled us to identify adequately the domains that are enriched with or deprived of the heavy filler.
Fig. 3a shows a section of the composite material, which was prepared in the low-energy ball mill Fritsch Pulverisette 5. It is seen that the material contains the regions that differ significantly in the content of W. Dark areas on the micrograph are the regions of almost pure polymer material free of the fillers. Light areas are the regions enriched with W. The dimension of dark areas coincides with the size of the initial UHMWPE granules that were slightly strained as a result of mechanical action and thermo pressing. From this micrograph, we can conclude that highly heterogeneous material is obtained as a result of low-power treatment. The structural heterogeneity can have a detrimental effect on the mechanical and radiation-protective properties of the composites. Micrograph of Fig. 3b shows the composite material that was prepared in the high-energy ball mill APF-3. It is seen that the homogeneity of material is much higher than in the sample in micrograph in Fig. 3a. The mechanical treatment of UHMWPE with the nanofillers is characterized by the absence of destruction processes of the polymeric particles. Therefore, in the course of mechanical action, the polymeric granule surface is only packed with the nanofiller. Increase of time and intensity of the treatment results in a better intermixing of the components, deeper penetration of the filling agents into the polymer particles and a decrease of structural heterogeneity of the specimens (Fig. 3b). From the viewpoint of radiation protection, more uniform distribution of particles over the polymeric matrix is preferable, because it hampers the generation of extremely undesirable effect – channeling of neutrons or other radiation in the material. A nonuniform distribution of particles can lead to the formation of areas containing considerably smaller amount of filler and, correspondingly, exhibiting significantly lower absorption of radiation. The radiation background behind this area of material can be higher, which can have a detrimental effect on the protected equipment or stuff.
4. Mechanical properties
Fig. 2. The specimens for the tensile tests.
Table 1 lists the experimental data on the mechanical properties of several selected composites. The mechanical properties were studied after the high-energy treatment in the planetary ball mill APF-3, corresponding composite structure is shown in Fig. 3b. Fig. 4 gives experimental curves of tensile tests for the composites. The results of mechanical tests of PMC show that after filling UHMWPE with nanoparticles of W and B4 C the elastic modulus, the tensile stress and tensile strength increase. Concurrently, the specific elongation decreases considerably. Such behavior of mechanical properties of polymeric composites is conventional, reinforcing of polymer matrix with hard particles (including
S524
S.D. Kaloshkin et al. / Journal of Alloys and Compounds 536S (2012) S522–S526
Table 1 Mechanical properties of PMC of various compositions. Specimen
Elastic modulus (E-modulus) (GPa)
Tensile yield point (0.2%) (MPa)
Ultimate tensile strength (MPa)
Specific elongation (%)
UHMWPE GUR 4120 pure UHMWPE + 18% W + 12% B4 C UHMWPE + 18% W + 12% B4 C irradiated with neutrons (E > 0.3 MeV) up to a dose of 1016 neutron/cm2 UHMWPE + 30% W + 20% B4 C UHMWPE + 60% Wn
0.8 1.4 1.7
17 19 20
21 26 25
400 90 12
0.4 0.4
18 13
27 26
2.00 100
nanosized powders) results in an increase of elasticity modules and yield stress accompanied by simultaneous decrease of elongation [15]. Very often this gain in elasticity modulus and yield stress is desirable, because in most cases these properties define the stress limit of material under loading and it does not undergo plastic deformation. This is most pronounced for the specimen containing 20% B4 C and 30% Wn . In this case, the specimen starts to break with almost zero elongation. An addition of a large amount of B4 C leads to the embrittlement of composite material. Probably, this is associated with rather large dimensions of B4 C particles as compared to Wn and poor adhesion of B4 C to the polymeric matrix. Actually, B4 C particles act as the defects in the polymeric matrix. The effect of irradiation with fast neutrons on the mechanical properties of the composite material was studied for the PMC containing 18% W + 12% B4 C. The specimens of the composite material, which were prepared for the tensile tests, were irradiated with fast neutrons. Only the working part of the specimen was irradiated. After the irradiation, the specimens had an induced radioactive background and, prior to the tests, were kept up to reaching a safe value of induced background.
The results of mechanical tests of composite material prior to and after the irradiation are presented in Table 1. It is seen that after the irradiation, the plasticity of the composite material abruptly decreases, the fracture becomes virtually brittle. However, the tensile stress and tensile strength do not change; they remain at the same level as before the irradiation. 5. The ␥-ray protection The ␥-radiation protective properties of polymer-matrix nanocomposites were determined by the method of the flux attenuation coefficient of monoenergetic 57 Co ␥-ray with an energy of 122 KeV. In the course of the experiment, the ␥-ray flux attenuation was measured prior to and after the introduction to the polymer target. The linear flux attenuation coefficient was calculated using the equation: = 1/xLn(N0 /N), where x is the thickness of polymeric specimen, N0 is the ␥-ray flux without polymer target, and N is the ␥-ray flux after passing through the polymer target. Table 2 presents the experimental results of linear ␥-ray attenuation coefficient. It is seen that, as expected, the linear ␥-ray attenuation coefficient increases with increasing tungsten content
Fig. 3. Micrographs of the cross-sections of the composite material: (a) prepared in a low-energy ball mill Fritsch Pulverisette 5 and (b) prepared in a high-energy ball mill APF-3.
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Table 2 The measured linear ␥-ray attenuation coefficient. Matrix
W (wt.%)
B4 C (wt.%)
Linear flux attenuation coefficient, (cm−1 )
UHMWPE UHMWPE UHMWPE UHMWPE
10 18 30 60
8 12 20 –
0.43 0.73 1.36 3.43
Fig. 4. Experimental curves of tensile tests.
in the composite. The highest ␥-ray attenuation coefficient was obtained for the composite containing 60% Wn . As expected, an increase of the content of tungsten in the composite leads to an increase in the linear absorption coefficient due to a high absorbability of tungsten with respect to X-ray radiation [16]. This PMC provides a linear attenuation coefficient of 3.43 cm−1 for the monoenergetic 57 Co ␥-ray flux with an energy of 122 KeV. The PMC retains high mechanical properties and remains sufficiently plastic after the irradiation. Interactions of photons with material have three energy loss mechanisms: photoelectric effect, Compton scattering and pair production. If a photon enters material with an energy under 1.022 MeV, it may interact by both processes of photoelectric effect and Compton scattering. The photoelectric absorption probability strongly depends on the high atomic number Z [17]. Thus, in this work, the existence of high-Z elements, W, plays an important role in radiation protection property of the composite. Additionally, the presence and symmetrical distribution of nanosized W particles enhance the interaction probability between photon and the composite. As it was shown in [13], on the filler nanoparticles, the linear absorption coefficient of gamma radiation increases due to an increase in the coherent scattering angles. This results in an increase of the optical distance of radiation in the material and, correspondingly, its additional absorption. This is true for the energy of incident radiation close to the X-ray range (0.1–10 KeV). A decrease
of the length for gamma-radiation leads to a decrease of the coherent scattering angles, a considerable increase of optical distance, and the coherent scattering does not contribute to the absorption processes in the material. Thus, at an irradiation energy of 122 KeV, the photoelectric absorption and inelastic scattering (Compton effect) are the main absorption processes in these composites. 6. Conclusions In this work, the radiation-protective polymer-matrix nanostructured composites based on UHMWPE with B4 C and W nanopowders were fabricated by solid state intermixing and thermal pressing. A study of mechanical properties showed that a higher content of the fillers in the polymer-matrix nanocomposites results in an increase of the modulus of elasticity, the tensile stress, and tensile strength. However, the specific elongation decreases considerably. The irradiation of material with a flux of fast neutrons induces a residual radioactivity in the material. The mechanical properties of irradiated composite exhibit a decrease of the plasticity and elongation, whereas the ultimate strength and the yield strength remain constant. The ␥-ray protection properties increase with an increase in the content of tungsten in the composite. The highest attenuation coefficient was observed at a W content of 60% and reached 3.43 cm−1 . The photoelectric absorption and inelastic scattering
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(Compton effect) are the main absorption processes in the PMC. The application of nanosized fillers does not raise the linear absorption coefficient. Acknowledgments This work is supported in the frame of federal target program “Research and development on the priority directions of the development of scientific and technological industry of Russia for 2007-2013 years”, state contract 16.516.11.6074. References [1] A. Charlesby, Atomic Radiation and Polymers, Pergamon Press, Oxford/London/New York/Paris, 1960, p. 522. [2] D.S. Xiong, R.Y. Ma, J.M. Lin, N. Wang, Z.M. Jin, Proc. Inst. Mech. Eng. Part J: J. Eng. Tribol. 221 (2007) 315–320. [3] B.R. Burroughs, T.A. Blanchet, Tribol. Trans. 44 (2001) 215–223.
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