Enhancing the Strength, Electrical Properties and Versatility of Epoxy Polymer Composite Foam with Carbon Nanostructures

A polymer composite, in simple description, can be considered as a sophisticated multiphase material in which reinforcing fillers are intricately combined with a polymer matrix. This integration results in a synergistic enhancement of mechanical properties that cannot be attained by either component individually [1-3]. Such infusion of multiple component materials results in a final product with properties that surpass those of each individual constituent material. There are numerous compelling factors that can lead to the preference for new materials. Examples include materials that offer cost-efficiency, reduced weight, increased strength, enhanced durability [4-6] etc, compared to conventional materials. The distinct advantage of polymer matrix composites, in contrast to metals, lies in their manufacturing process that enables the production of intricately shaped components while offering lower density. This not only contributes to reduced fuel consumption in applications such as aviation and automotive industries but also facilitates higher speeds in competitive sports, longer ranges for missiles, and greater payload capacities in transportation [7,8].

In the majority of polymeric composite fabrication processes aimed at achieving desired material compositions, the primary challenges revolve around selecting the appropriate fibers, effectively distributing various phases, optimizing the aspect ratio of fibers, determining the spatial arrangements of continuous or short fibers, and devising an efficient process patterning strategy. In general, composite materials consist of one or more discontinuous phases dispersed within a continuous phase. In ‘hybrid’ composites, multiple discontinuous phases of varying natures are present [9,10]. The continuous phase is commonly referred to as the ‘matrix,’ while the discontinuous phase is termed as the ‘reinforcement’ material. By incorporating high-tensile strength reinforcements with exceptionally high modulus into a polymer matrix, it becomes feasible to enhance both the mechanical and thermal properties significantly. Additions of nano carbon materials [11,12] are trend to synthesize multifunctional composites. Also mechanical, electrical and thermal properties of these composites are fully customizable. Conducting fillers such as graphite-based nanostructures, carbon nanotubes, and carbon nanofibers are used as reinforcing agents for enabling the conducting behavior of epoxy resin.

Influencing properties of bulk materials by nano reinforcements are decade trend [13-16]. Hollow Glass Microspheres (HGMs) have low density and high strength with good thermal properties and thus can be employed in polymer matrix to form closed cell porous foams called syntactic foam. Unlike much other chemically synthesized polymeric foam, these syntactic foams have good mechanical, thermal and damping properties due to the presence of HGM into the polymer. These syntactic foams are used as buoyancy aid materials for marine applications [17]. These closed pore structures impart increased specific strength, reduced density, reduced coefficient of moisture absorptions and prevents thermal and electrical transports [18-22]. Syntactic foam properties are tailorable to good range and can be casted to any intricate shapes. Properties tailor ability is achieved either varying the volume fraction of HGM or by selecting HGM with suitable density (which depends on the shell thickness of HGM) [22,23]. Due to its reduced weight and improved specific properties attentions are given to use these composites for aerospace and marine structures as payload will be increased significantly [24,25]. The other applications of syntactic foams are discussed in [26,27]. Furthermore, previous works [28-30] have discussed the effects of properties on structural application of syntactic foams.

Nano filler reinforcement in the matrix enhances its cumulative properties which are attributed to commendable phase morphology and improved interfacial strength. Macro fillers forms finite interfaces with the matrix whereas, nano fillers have interfacial phases many folds increased [31-33]. Nano particulate syntactic foams are trending class of materials in which either HGM filler or epoxy matrix is modified by nano materials most commonly by nano carbon materials to achieve enhanced physical and engineering properties. When a small fraction of nano carbons is added to the composites it does not affect the density significantly and yields high performing multifunctional syntactic foams. Various nano carbon materials such as Carbon Nano Fibres (CNF), Carbon Nano Tubes (CNT) and Graphene (GP) evolve unique set of properties when reinforced with micro and bulk materials. High aspect ratio, smaller size and commendable mechanical properties makes carbon nano materials a highly preferable reinforcing material in syntactic foams [34-36]. Good dispersion of nano fillers in the matrix is crucial, as almost all the property enhancement in nano composites is attributed to increased interfacial phases, without which the adverse effect will be occurred in the composite. Entrapped voids during syntactic foam manufacturing are highly undesirable as it supports the initiation and propagation of the crack. Low density HGM floats on denser epoxy resin and higher viscosity of the resin- HGM are the major causes of voids entrapment. Reinforcing nano particles stabilizes voids in the system [37-39] which increases the risk of composite failure; hence effective processing methods are necessary.

Graphene platelets were proved to be potential two-dimensional filler in syntactic foams [40-44]. Large surface areas of graphene provide a good surface interaction with matrix and hence stress transfer [45-49]. Reinforcing graphene platelets to a maximum volume fraction of 0.5% in epoxy based syntactic foams shows insignificant increase in density [45]. However it’s important to note that poor dispersion of graphene platelets causes its entanglement and wrapping which leads to voids in the composites. Its dispersion can be increased by functionalizing its surface to bear hydroxyl and amine groups. These groups make van der-wall interactions with the polymer and also surface roughness of GP increases which leads to wrinkled topology of GP and hence mechanical interlocking of polymer chain with GP takes place [42-45].

In the HGM/epoxy composite foam [50-56], the introduction of Nano Carbons (NCs) is carried out through two distinct approaches: (a) coating NCs onto the HGM surface [34,57], and (b) incorporating them within the matrix [42-45], as depicted in Figure 1. Chemical Vapor Deposition (CVD) stands out as the most commonly employed method for growing Carbon Nanotubes (CNT) on the surface of HGM, typically with a catalyst coating, where catalyst materials like nickel, cobalt, or ferrous are used. The CNT growth process involves the decomposition of carbon containing gases at elevated temperatures. For instance, Ephraim et al. successfully grew CNT on HGM with a cobalt coating, maintaining temperatures between 650 °C and 850 °C, using methane as the carbon source [34]. P. Bhat and colleagues achieved CNT growth on HGM coated with nickel at 600 °C, utilizing acetylene as the source gas [57]. These NCs, whether in their pure form or as NC-coated HGM, can be integrated into the matrix through various techniques, such as solvent evaporation or melt mixing methods.

Figure 1:Schematic processing techniques of introducing carbon nanomaterials NC into epoxy polymer composite towards synthesis of efficient (NC/HGM/Polymer) composite foam.

Mechanical properties

Enhancements of mechanical properties are very serious challenge for syntactic foams as these foams serve as core in aerospace and marine structures and failure of which may lead to property and life loss. Several research have been reported mechanical properties enhancement by matrix modification with fillers such as short fibers, micro sized fibers and particulates and nano scaled fillers such as nano clay, CNT, graphene and PEEKMOH [58]. Stable structural defects in nanostructures add to further versatility [59]. Macro fibers show tensile and shear strength enhancement at low volume fraction and higher volume of micro fiber reinforcement softens the matrix and also possesses poor interaction with the matrix and hence brings down the structural reliability of syntactic core. Reinforcement of nano clay in syntactic foams has been studied and found limited enhancement in tensile properties with increase in density significantly [60-65]. It is reported in [62,63,66,67] that a density increases of 17-26% has occurred in syntactic foams with nano clay inclusion of 2-4 vol% and hence limits its application as structural cores.

A very low volume fraction of graphene platelets has enhanced the tensile and fracture properties of syntactic foams [45], as shown in Table 1. Capacity to deflect crack propagation, high surface area and excellent mechanical properties of graphene platelets contribute the mechanical property enhancement of syntactic foams [58]. Also GP proves to have stronger interaction with the matrix which restricts polymer chain mobility and hence delaying crack initiation and growth. GP shows improvement in mechanical properties of syntactic foams when added in lower volume fraction (<3%) and higher volume fraction of GP shows negative performance which is clearly attributed to the dispersion challenge of GP when its volume increases. Poor dispersion of GP causes its agglomeration, wrapping and entanglement as they are 2D nano particles and hence restrict the strain transfer which results in poor performance.

Table 1:Effect of NC reinforcement on tensile and compressive properties of HGM/polymer composite foam.

Syntactic foams show ductility under compression. Macro fiber reinforcement decreases the compression strength of syntactic foams. [62,63] reported that when 5 vol% of nano clay is added to epoxy based syntactic foams compressive strength increases with decrease in modulus significantly and with nano clay volume fraction less 5% the strength value decreases. Syntactic foams when loaded compressively it behaves elastically till the resilience level and then failure of microspheres occur corresponding to the plateau region in the stress strain curve and finally failure of matrix occurs. Syntactic foams are notably brittle when subjected to tensile loads. The tensile strength exhibits a decreasing trend with the rise in Hollow Glass Microspheres (HGM) volume fraction in syntactic foams, and failures occurring under tension are primarily influenced by the matrix. Tensile strength displays improvement when macro fibers are introduced, especially at volumes below 5%, but beyond that, it declines due to matrix softening with micro fiber reinforcements [66,67]. Interestingly, the addition of nano clay to epoxy-based syntactic foams, as reported in [45], increased tensile strength. However, these composites demonstrate poor damage tolerance and reduced load-bearing capacity. Furthermore, [45] reveals that the inclusion of graphene platelets in syntactic foams enhances both tensile modulus and tensile strength, particularly when the volume fraction of graphene platelets is at 0.3%, resulting in a remarkable 14.7% improvement compared to plain syntactic foams. However, when the volume of graphene platelets reinforcement exceeds 3%, adverse effects are observed [45]. Comparison of the resulting mechanical properties are in Table 1.

Electrical/dielectric properties

Conductivity of the composites increases with CNF reinforcement [68-71]. Electrical impedance increases with HGM volume in CNF/syntactic foams, which is because the porosity increases in the composites with HGM volume and hence the impedance. Denser HGM increases impedance of the composites. CNF decreases insulation and hence impedance decreases with CNF content in the composites [72]. Dielectric constant remains higher at low test frequencies, and it increases with the frequency. CNF volume in the syntactic foams increases dielectric constant and it reaches maximum at 10 wt% with 1 Hz test frequency. This increasing trend is because randomly connected CNF number increases with CNF volume in the composites and hence capacitance increases resulting higher dielectric constants [72-74]. Comparison of the resulting mechanical properties are in Table 2.

Table 2:Effect of NC reinforcement on electrical and dielectric properties of HGM/polymer composite foam.

In the materials industry, the imperative quest for weight reduction in designated applications takes center stage. This study underscores the remarkable accomplishments of Hollow Glass Microspheres (HGM) reinforcement, resulting in significant weight reduction and heightened structural rigidity. Furthermore, the paper delves into the documented contributions of nanocarbons, including graphene platelets, carbon nanotubes, and carbon nanofibers, in augmenting both mechanical and electrical characteristics. This concise review also underscores the ease with which nanocarbons can customize the electrical attributes of polymer composites, a versatile quality sought after in various applications.

MUK and AR acknowledge project No. 2019-2.1.13-TÉTIN- 2020-00059, which has been implemented with support provided by the National Research, Development and Innovation Fund of Hungary, and Department of Science and Technology DST, India, co-financed under the 20192.1.13-TÉT-IN funding scheme

1. Sajan S, Selvaraj DP (2021) A review on polymer matrix composite materials and their applications. Materials Today: Proceedings 47(15): 5493-5498.
2. Hsissou R, Seghiri R, Benzekri Z, Hilali M, Rafik M, et al. (2021) Polymer composite materials: A comprehensive review. Composite Structures 262: 113640.
3. Upadhyay Kahaly M, Schwingenschlögl U (2014) Enhanced solar light absorption of graphene by interaction with anisole. Carbon 77: 76-79.
4. Scott R (2022) 5 reasons to use fiber-reinforced polymer (FRP).
5. Kahaly MU, Ozdogan K, Schwingenschlögl U (2013) Half-metallic perovskite superlattices with colossal thermoelectric figure of merit. Journal of Materials Chemistry A 1(29): 8406-8410.
6. Upadhyay Kahaly M, Nazir S, Schwingenschlögl U (2011) Band structure engineering and vacancy induced metallicity at the GaAs-AlAs interface. Appl Phys Lett 99(12): 123501.
7. Imran M, Pal S, Upadhyay Kahaly M, Rahaman A (2021) Radially grown carbon nanomaterials on hollow glass microspheres and their application in composite foams with excellent electromagnetic interference shielding. Polymer Composites 42(8): 3786-3798.
8. Yuan S, Li S, Zhu J, Tang Y (2021) Additive manufacturing of polymeric composites from material processing to structural design. Composites Part B: Engineering 219: 108903.
9. Arabpour A, Shockravi A, Rezania H, Farahati R (2020) Investigation of anticorrosive properties of novel silane-functionalized polyamide/GO nanocomposite as steel coatings. Surf Interfaces 18: 100453.
10. Dong ZJ, Zhou T, Luan H, Christopher Williams R, Wang P (2019) Composite modification mechanism of blended bio-asphalt combining styrene-butadiene-styrene with crumb rubber: A sustainable and environmental-friendly solution for wastes. J Cleaner Prod 214: 593605.
11. Upadhyay Kahaly M, Waghmare UV (2008) Contrast in the electronic and magnetic properties of doped carbon and boron nitride nanotubes: A first-principles study. The Journal of Physical Chemistry C 112(10): 3464-3472.
12. Upadhyay Kahaly M, Waghmare UV (2007) Size dependence of thermal properties of armchair carbon nanotubes: A first-principles study. Applied Physics Letters 91(2):
13. Zhou Y, Wu P, Cheng Z, Ingram J, Jeelani S (2008) Improvement in electrical, thermal and mechanical properties of epoxy by filling carbon nanotube. Exp Polym Lett 2(1): 40-48.
14. Gojny F, Wichmann M, Fiedler B, Bauhofer W, Schulte K (2005) Multiwall carbon nanotube/epoxy composites produced by a masterbatch process. Compos A 42: 395-406.
15. Allaoui A, Bai S, Cheng H, Bai J (2002) Mechanical and electrical properties of a MWNT/epoxy composite. Compos Sci Technol 62(15): 1993-1998.
16. Bai J, Allaoui A (2003) Effect of the length and the aggregate size of MWNTs on the improvement efficiency of the mechanical and electrical properties of nanocomposites-experimental investigation. Compos A 34(8): 689-694.
17. Imran M, Rahaman A, Pal S (2021) Synthesis and characterization of carbon nanotubes over hollow glass microspheres and its nanocomposites. Materials Today: Proceedings 45(4): 4036-4039.
18. Yung KC, Zhu BL, Yue TM, Xie CS (2009) Preparation and properties of hollow glass microsphere-filled epoxy-matrix composites. Compos Sci Technol 69(2): 260-264.
19. Mutua FN, Lin P, Koech JK, Wang Y (2012) Surface modification of hollow glass microspheres. Mater Sci Appl 3(12): 856.
20. Kim HS, Khamis MA (2001) Fracture and impact behaviours of hollow micro-sphere/epoxy resin composites. Compos A 32(9): 1311-1317.
21. Wu ZX, Huang RJ, Chu XX, Huang CJ, Zhang JJ, et al. (2012) Cryogenic properties of hollow glass microsphere/epoxy composites. AIP Conference Proceedings 1435(1): 156-163.
22. Ferreira JAM, Capela C, Costa JD (2010) A study of the mechanical behaviour on fibre reinforced hollow microspheres hybrid composites. Compos A 41(3): 345-352.
23. Gupta N, Woldesenbet E (2003) Hygrothermal studies on syntactic foams and compressive strength determination. Compos Struct 61(4): 311-320.
24. Shunmugasamy V, Pinisetty D, Gupta N (2012) Thermal expansion behavior of hollow glass particle/vinyl ester composites. J Mater Sci 47: 5596-5604.
25. Shunmugasamy V, Pinisetty D, Gupta N (2013) Viscoelastic properties of hollow glass particle filled vinyl ester matrix syntactic foams: effect of temperature and loading frequency. J Mater Sci 48: 1685-1701.
26. Gupta N, Pinisetty D, Shunmugasamy VC (2013) Thermal expansion behavior of hollow glass particle/vinyl ester composites. Journal of Materials Science 47: 5596-5604.
27. Hodge AJ, Kaul RK, McMahon WM (2000) In Loud S et al (ed) Proceedings of the 45^th International SAMPE Symposium, SAMPE, p. 2293.
28. Gupta N, Ye R, Porfiri M (2010) Comparison of tensile and compressive characteristics of vinyl ester/glass microballoon syntactic foams. Compos B Eng 41(3): 236-245.
29. Choqueuse D, Davies P, Perreux D, Sohier L, Cognard JY (2010) Mechanical behavior of syntactic foams for deep sea thermally insulated pipeline. Applied Mech Mater 24-25: 97-102.
30. Porfiri M, Gupta N (2009) Effect of volume fraction and wall thickness on the elastic properties of hollow particle filled composites. Compos B Eng 40(2): 166-173.
31. Peter S, Woldesenbet E (2008) Nanoclay syntactic foam composites-High strain rate properties. Mater Sci Eng A 494(1-2): 179-187.
32. Shadlou S, Ahmadi-Moghadam B, Taheri F (2014) The effect of strain-rate on the tensile and compressive behavior of graphene reinforced epoxy/nanocomposites. Mater and Des 59: 439-447.
33. Wouterson EM, Boey FYC, Wong SC, Chen L, Hu X (2007) Nano-toughening versus micro-toughening of polymer syntactic foams. Compos Sci Technol 67(14): 2924-2933.
34. Zegeye E, Woldesenbet E (2012) Processing and mechanical characterization of carbon nanotube reinforced syntactic foams. J Reinf Plast Compos 31(15): 1045.
35. Dimchev M, Caeti R, Gupta N (2010) Effect of carbon nanofibers on tensile and compressive characteristics of hollow particle filled composites. Mater Des 31(3): 1332-1337.
36. Bortz DR, Heras EG, Martin-Gullon I (2012) Impressive fatigue life and fracture toughness improvements in graphene oxide/epoxy composites. Macromolecules 45(1): 238-245.
37. Wong JCH, Tervoort E, Busato S, Gauckler LJ, Ermanni P (2011) Controlling phase distributions in macroporous composite materials through particle-stabilized foams. Langmuir 27(7): 3254-3260.
38. Du Z, Bilbao-Montoya MP, Binks BP, Dickinson E, Ettelaie R, et al. (2003) Outstanding stability of particle-stabilized bubbles. Langmuir 19(8): 3106-3108.
39. Rodrigues JA, Rio E, Bobroff J, Langevin D, Drenckhan W (2011) Generation and manipulation of bubbles and foams stabilised by magnetic nanoparticles. Colloids Surf A 384(1-3): 408-416.
40. TP Kaloni, Upadhyay Kahaly M, Cheng YC, Schwingenschlögl U (2012) Ge-intercalated graphene: The origin of the p-type to n-type transition. EPL 99(5): 57002.
41. Obodo JT, Kahaly MU, Schwingenschlögl U(2015) Magnetoresistance of Mn-decorated topological line defects in graphene. Physical Review B 91(1): 014413.
42. McAllister MJ, Li JL, Adamson DH, Schniepp HC, Abdala AA, et al. (2007) Single sheet functionalized graphene by oxidation and thermal expansion of graphite. Chem Mater 19(18): 4396-4404.
43. Schniepp HC, Li JL, McAllister MJ, Sai H, Herrera-Alonso M, et al. (2006) Functionalized single graphene sheets derived from splitting graphite oxide. J Phys Chem B 110(17): 8535-8539.
44. Starr FW, Schrøder TB, Glotzer SC (2002) Molecular dynamics simulation of a polymer melt with a nanoscopic particle. ACS Macro Lett 35(11): 4481-4492.
45. Zegeye E, Ghamsari AK, Woldesenbet E (2014) Mechanical properties of graphene platelets reinforced syntactic foams. Composites: Part B 60: 268-273.
46. Maruyama B, Alam K (2002) Carbon nanotubes and nanofibers in composite materials. SAMPE J 38(3): 59-70.
47. Baughman R, Zakhidov A, Heer WD (2002) Carbon nanotubes-the route toward applications. Science 297(5582): 787-792.
48. Yu M, Lourie O, Dyer M, Moloni K, Kelly T, et al. (2000) Strength and breaking mechanism of multiwall carbon nanotubes under tensile load. Science 287(5453): 637-640.
49. Lee C, Wei XD, Li QY, Carpick R, Kysar JW (2009) Elastic and frictional properties of graphene. Basic Solid State Phys 246(11-12): 2562-2567.
50. Wong E, Sheehan P, Lieber C, (1997) Nanobeam mechanics: elasticity, strength, and toughness of nanorods and nanotubes. Science 277(5334): 1971-1975.
51. Hernandez E, Goze C, Bernier P, Rubio A (1998) Elastic properties of C and B_xC_yN_z composite nanotubes. Phys Rev Lett 80: 4502.
52. Lee C, Wei X, Kysar JW, Hone J (2008) Measurement of the elastic properties and intrinsic strength of monolayer graphene. Science 321(5887): 385-388.
53. Lawrence J, Berkan L, Nadarajah (2008) Elastic properties and morphology of individual carbon nanofibers. ACS Nano 2(6): 1230-1236.
54. Tsoukleri G, Parthenios J, Papagelis K, Jalil R, Ferrari AC, et al. (2009) Subjecting a graphene monolayer to tension and compression. Small 5(21): 2397-2402.
55. Che J, Agin T, Lii W (2000) Thermal conductivity of carbon nanotubes. Nanotechnology 11(2): 65.
56. Balandin AA, Ghosh S, Bao W, Calizo I, Teweldebrhan D, et al. (2008) Superior thermal conductivity of single-layer graphene. Nano Lett 8(3): 902-907.
57. Gupta N, Pinisetty D, Shunmugasamy VC (2013) Electrical properties of hollow glass particle filled vinyl ester matrix syntactic foams. Journal of Materials Science 49: 180-190.
58. Zegeye E, Ghamsari AK, Woldesenbet E (2014) Mechanical properties of graphene platelets reinforced syntactic foams. Composites Part B: Engineering 60: 268-273.
59. Kahaly MU (2009) Defect states in carbon nanotubes and related band structure engineering: A first-principles study. J Applied Physics 105: 024312.
60. John B, Nair CPR, Ninan KN (2010) Effect of nano clay on the mechanical, dynamic mechanical and thermal properties of cyanate ester syntactic foams. Mater Sci Eng A–Struct 527(21-22): 5435-5443.
61. Woldensenbet E, Sankella N (2009) Flexural properties of nano clay syntactic foam sandwich structures. J Sandwich Struct Mater 11(5): 425.
62. Maharsia R, Gupta N, Jerro HD (2006) Investigation of flexural strength properties of rubber and nano clay reinforced hybrid syntactic foams. Mater Sci Eng A–Struct 417(1-2): 249-258.
63. Maharsia RR, Jerro HD (2007) Enhancing tensile strength and toughness in syntactic foams through nano clay reinforcement. Mater Sci Eng A–Struct 454: 416-422.
64. Gupta N, Maharsia R (2005) Enhancement of energy absorption in syntactic foams by nano clay incorporation for sandwich core applications. Appl Compos Mater 12: 247-261.
65. Rafiee MA, Rafiee J, Srivastava I, Wang Z, Song H, et al. (2010) Fracture and fatigue in graphene nanocomposites. Small 6(2): 179-183.
66. Chawla S, Naraghi M, Davoudi A (2013) Effect of twist and porosity on the electrical conductivity of carbon nanofiber yarns. Nanotechnology 24: 255708.
67. Al-Saleh MH, Gelves GA, Sundararaj U (2013) Carbon nanofiber/polyethylene nanocomposite: Processing behavior, microstructure and electrical properties. Mater Des 52: 128-133.
68. Al-Saleh MH, Saadeh WH (2013) Hybrids of conductive polymer nanocomposites. Mater Des 52: 1071-1076.
69. Poveda RL, Gupta N (2014) Electrical properties of carbon nanofiber reinforced multiscale polymer composites. Mater and Des 56: 416-422.
70. Yang S, Benitez R, Fuentes A, Lozano K (2007) Dielectric analysis of VGCNF reinforced polyethylene composites. Compos Sci Technol 67(6): 1159-1166.
71. Liang GD, Tjong SC (2008) Electrical properties of percolative polystyrene/carbon nanofiber composites. IEEE Trans Dielectrics Elect Insul 15(1): 214-220.
72. Cardoso P, Silva J, Agostinho Moreira J, Klosterman D, Van Hattum FWJ, et al. (2012) Temperature dependence of the electrical conductivity of vapor grown carbon nanofiber/epoxy composites with different filler dispersion levels. Phys Lett A 376(45): 3290-3294.
73. Gimenez N, Sauvant MV, Satureau H (2008) Wet ageing of syntactic foams under high pressure/high temperature in deionized water. International Conference on Offshore Mechanics and Arctic Engineering, Halkidiki, Greece, pp. 205-210.
74. Sauvant MV, Gimenez N, Sautereau H (2006) Hydrolytic ageing of syntactic foams for thermal insulation in deep water: degradation mechanisms and water uptake model. J Mater Sci 41: 4047-4054.