SHORT COMMUNICATION Polymer derived nitride matrix composites reinforced by 25-dimensional silica fibre Y. G. Jiang, C R Zhang F. Cao, S.Q. Wang, G.J. Qi and Y. B. Cao Polyborosilazane, a low viscosity preceramic polymer with good infiltration efficiency and high char yield, was used to prepare amorphous composites with polymer derived nitride matrix einforced by 2. 5-dimentional silica fibre, and the mechanical properties and microstructures were investigated. The composites without fibre coating showed typical brittle fracture behaviour with a smooth fracture surface and a longitude flexural strength of just 46.3 MPa whereas the composites with fibre coating exhibited non-brittle fracture behaviour with distinct fibre pull-out on the fracture surface and a high longitude flexural strength of 129. 5 MPa. It was the controlled fibre/matrix interface by precoating treatment that contributed to the high mechanical property of the composites Keywords: Polymer derived, Silica fibre, Precoating, Mechanical rties. Microstructures Introduction polysilazane(PHPS) and the composites have mechanical and dielectric properties. In the Continuous fibre reinforced ceramic matrix composites previous work, the silica fibre reinforced have received considerable attention for structural Ix con posites(2. 5D SiO2dSi3 N4-B applications because of their excellent thermal stability, prepared by the PIP method through repeated light weight and damage tolerance. Yet, a few com- tion of hybrid precursor polyborosilazane osites are appropriate for electromagnetic wave com- Results showed that the composites had m munication and thermal protection of spacecraft. mechanical, dielectric and ablation properties Silica fibres. with excellent ablation resistance. ther mal shock resistance, dielectric properties and flexibility, sites were fabricated by the Pip method and the effects are widely used to fabricate high temperature radome or of silica fibre coating on the mechanical properties and antenna window materials. Nevertheless, the disadvan- microstructures were mainly investigated tage of the silica fibre is that the high temperature processing will lead to serious degradation. For exam- ple,", an inorganic SiO2/Sio2 material based Experimental procedure on repetitive pyrolysis and reimpregnation of the ADL- Raw materials 10 material, showed too tight bonding at the fibre/matrix interface and low strength; the same is true of other 2.5- Glass Corporation( China), have a density of 2.2 g cm,a dimensional (2. 5D)SiO2 SiO2 materials made by the acuum assisted liquid phase infiltration#or electro- room temperature tensile strength of 1700 MPa and an phoretic infiltration method. Therefore, it is difficult to elastic modulus of 78 GPa. To form a 2.5D silica fibre fabric(2. 5D SiO), with 46 vol-% of the fibre. those fabricate braided silica fibre reinforced ceramic matrix fabrics were woven by Beijing Fibreglass Research and composites with high strength The process of preceramic polymer infiltration and five plain woven clothes(2D) with certain interlocking pyrolysis(PIP) maybe a good method to fabricate between them, to prevent the material from delamination, braided fibre reinforced ceramic composites because of leading to the so called 2. 5D architecture. The hybrid its effectiveness at low temperatures. However, because precursor of the ceramic matrix, PBSZ, was synthesised by materials containing carbon will decrease microwave a modification of the procedure developed by Su and co- precursor for infiltration and pyrolysis into a ceramic matrix, PHPS, was synthesised by the ammonolysis of 4 ceramIc composi co-workers at low temperatures with precursor by thermal condensation of HsNBH, according to aboratory of New Ceramic Fibers and Composites, College Composites preparation Aerospace and Materials Engineering, National University of Defens Technology, Changsha 410073, China The preparation of 2. 5D SiO2Si3NA-BN composites Correspondingauthoremailiygemail@yahoo.com.cn included four stages. First, the 2. 5D SiO2r fabric was 0 2007 Institute of Materials, Minerals and Mining ry 2007: accepted 8 March 2007 880D0110.179/174328407X185875 Materials Science and Technology 2007 VOL 23 No 7
SHORT COMMUNICATION Polymer derived nitride matrix composites reinforced by 2?5-dimensional silica fibre Y. G. Jiang, C. R. Zhang, F. Cao, S. Q. Wang, G. J. Qi and Y. B. Cao Polyborosilazane, a low viscosity preceramic polymer with good infiltration efficiency and high char yield, was used to prepare amorphous composites with polymer derived nitride matrix reinforced by 2?5-dimentional silica fibre, and the mechanical properties and microstructures were investigated. The composites without fibre coating showed typical brittle fracture behaviour with a smooth fracture surface and a longitude flexural strength of just 46?3 MPa whereas the composites with fibre coating exhibited non-brittle fracture behaviour with distinct fibre pull-out on the fracture surface and a high longitude flexural strength of 129?5 MPa. It was the controlled fibre/matrix interface by precoating treatment that contributed to the high mechanical property of the composites. Keywords: Polymer derived, Silica fibre, Precoating, Mechanical properties, Microstructures Introduction Continuous fibre reinforced ceramic matrix composites have received considerable attention for structural applications because of their excellent thermal stability, light weight and damage tolerance.1,2 Yet, a few composites are appropriate for electromagnetic wave communication and thermal protection of spacecraft. Silica fibres, with excellent ablation resistance, thermal shock resistance, dielectric properties and flexibility, are widely used to fabricate high temperature radome or antenna window materials. Nevertheless, the disadvantage of the silica fibre is that the high temperature processing will lead to serious degradation. For example, ‘Markite’, an inorganic SiO2f/SiO2 material based on repetitive pyrolysis and reimpregnation of the ADL- 10 material, showed too tight bonding at the fibre/matrix interface and low strength;3 the same is true of other 2?5- dimensional (2?5D) SiO2f/SiO2 materials made by the vacuum assisted liquid phase infiltration4 or electrophoretic infiltration method.5 Therefore, it is difficult to fabricate braided silica fibre reinforced ceramic matrix composites with high strength. The process of preceramic polymer infiltration and pyrolysis (PIP) maybe a good method to fabricate braided fibre reinforced ceramic composites because of its effectiveness at low temperatures.2 However, because materials containing carbon will decrease microwave transmission efficiency, it is not easy to find a no-carbon precursor for infiltration and pyrolysis into a ceramic matrix. Recently, three-dimensional silica fibre reinforced Si3N4 ceramic composites were prepared by Qi and co-workers6,7 at low temperatures with precursor perhydropolysilazane (PHPS) and the composites have good mechanical and dielectric properties. In the authors’ previous work,8 the silica fibre reinforced nitride matrix composites (2?5D SiO2f/Si3N4–BN) were prepared by the PIP method through repeated infiltration of hybrid precursor polyborosilazane (PBSZ). Results showed that the composites had excellent mechanical, dielectric and ablation properties. In the present paper, 2?5D SiO2f/Si3N4–BN composites were fabricated by the PIP method and the effects of silica fibre coating on the mechanical properties and microstructures were mainly investigated. Experimental procedure Raw materials The silica fibres, produced by JingZhou Feilihua Silica Glass Corporation (China), have a density of 2?2 g cm23 , a room temperature tensile strength of 1700 MPa and an elastic modulus of 78 GPa. To form a 2?5D silica fibre fabric (2?5D SiO2f), with 46 vol.-% of the fibre, those fabrics were woven by Beijing Fibreglass Research and Design Institute (China). The preform is made of a stack of five plain woven clothes (2D) with certain interlocking between them, to prevent the material from delamination, leading to the so called 2?5D architecture.9 The hybrid precursor of the ceramic matrix, PBSZ, was synthesised by a modification of the procedure developed by Su and coworkers.10 The precursor for silicon nitride (Si3N4) ceramic matrix, PHPS, was synthesised by the ammonolysis of dichlorosilane–pyridine adducts.11 Borazine, a precursor for boron nitride (BN) ceramic matrix, was prepared by thermal condensation of H3NBH3 according to literature.12 Composites preparation The preparation of 2?5D SiO2f/Si3N4–BN composites included four stages. First, the 2?5D SiO2f fabric was National Key Laboratory of New Ceramic Fibers and Composites, College of Aerospace and Materials Engineering, National University of Defense Technology, Changsha 410073, China Corresponding author, email jygemail@yahoo.com.cn 880 2007 Institute of Materials, Minerals and Mining Published by Maney on behalf of the Institute Received 6 January 2007; accepted 8 March 2007 DOI 10.1179/174328407X185875 Materials Science and Technology 2007 VOL 23 NO 7
Jiang et al. Polymer derived nitride matrix composites 105 0.14 100 0.12 0.10 95 0.08 90 0.04 0.02 904006008001000 0.00 0.00.20.40.60.81.01.214 Temperature(℃) Displacement(mm) 1 Thermogravimetric analysis curve of cured PBSz in 2 Load-displacement curves 2. 5D SiO2/Si3N/-BN NH3 atmosphere pretreated to prevent the reaction between the fibre and of the residual solvent or low molecular weight silazane the matrix according to literature , Second, the fabric and borazine 2 compounds before 400C.The was infiltrated with hybrid precursor in vacuum. Third, gain from 400 to 600 C was due to the nitridation effect the fabric filled with precursor cured at 100-200'C for 3- by reactive ammonia atmosphere. However, in the 6 h in an inert atmosphere. Finally, the cured fabrics hybrid precursor, the Si-H bond decreased for the were pyrolysed at 800C in flowing anhydrous ammonia, reaction between PHPS and PBZ, so the nitridation which helped reducing the residual carbon in the final effect and weight gain are not obvious. The weight loss ceramic matrix, hence obtaining the 2: 5D SiO2Si3N after 800 C was small and the char yield was very high (~86wt-% were repeated for four times to densify the composites During the pretreatment before polymer infiltration, Properties of composites ome fibre fabrics were precoated and the corresponding The properties of the 2 5D Sio2/Si3N4-BN composites composites were denoted B, whereas the composites are shown in Table 1 and the deviations have been without fibre precoating were denoted A attached to the mean values of the density, longitude Composites characterisation flexural strength and modulus. The density of the specimen a was a little larger than that of the specimen The density of the composites after four infiltration- B after the same infiltration-cure-pyrolysis cycles cure-pyrolysis cycles was measured according to However, the sI len b had a higher longitude flexural Archimedes'principle. The pyrolysis of the matrix was strength and modulus. The load-displacement curves of studied by thermogravimetric analysis (TGA)under an the composites are illustrated in Fig. 2. The load of the perature to 1000 C. The longitude flexural strength was until a catastrophic failur r with the displacement a low energy measured using a three point bending test machine absorbing ability, whereas the (WDW-100) with specimen dimensions of 36x 4 brittle failure behaviour with hi acture load and 3 mm, a bending span of 30 mm and a crosshead speed displacement, showing an improved deformation resis- of 0.5 mm min. For mechanical properties tests, at tant abilit least three specimens were measured for each composite. A scanning electron microscope (JSM-5600LV) was Microstructure of composites used to observe the morphology of the fracture surfaces. Figure 3 illustrates the SEM images of the fracture surface of the 2. 5D SiO2Si3Na-BN. For specimen A Results the interface between the fibres and the matrix was Properties of precursor undistinguishable as a result of the strong adhesion and no fibre pull-out was observed on the fracture surface The as received precursor was a pale yellow liquid with whereas specimen B showed relatively weak interface osity of 10-35 mPa s and a density of bonding with distinct fibre pull-out and fracture, 1.07 g cm. After some heat treatment under nitro- exhibiting the effectiveness of fibre reinforcement gen atmosphere, the precursor could be changed into ighly cross-linked PBSZ. Figure I shows the TGA curve of PBSZ, which exhibits a weight loss in the Dis scussion beginning followed by a little weight gain and a weight Figure 3 revealed that the precursor efficiently infiltrated loss again. The weight loss was due to the volatilisation into the interyarn and intrayarn porosity of the silica Table 1 Properties of 2 5D SiO2wSi3Na-BN composites Specimen Pyrolysis cycle ensity, g cm Longitude flexural strength, MPa Elastic modulus, GPa A 1·70+002 463+20 173+10 1-68+002 1295+70 254+15 Materials Science and Technology 2007 VOL 23 No 7 881
pretreated to prevent the reaction between the fibre and the matrix according to literature.6,7 Second, the fabric was infiltrated with hybrid precursor in vacuum. Third, the fabric filled with precursor cured at 100–200uC for 3– 6 h in an inert atmosphere. Finally, the cured fabrics were pyrolysed at 800uC in flowing anhydrous ammonia, which helped reducing the residual carbon in the final ceramic matrix, hence obtaining the 2?5D SiO2f/Si3N4– BN composites. The infiltration–cure–pyrolysis cycles were repeated for four times to densify the composites. During the pretreatment before polymer infiltration, some fibre fabrics were precoated and the corresponding composites were denoted B, whereas the composites without fibre precoating were denoted A. Composites characterisation The density of the composites after four infiltration– cure–pyrolysis cycles was measured according to Archimedes’ principle. The pyrolysis of the matrix was studied by thermogravimetric analysis (TGA) under an ammonia atmosphere (10uC min21 ) from room temperature to 1000uC. The longitude flexural strength was measured using a three point bending test machine (WDW-100) with specimen dimensions of 36646 3 mm, a bending span of 30 mm and a crosshead speed of 0?5 mm min21 . For mechanical properties tests, at least three specimens were measured for each composite. A scanning electron microscope (JSM-5600LV) was used to observe the morphology of the fracture surfaces. Results Properties of precursor The as received precursor was a pale yellow liquid with a viscosity of 10–35 mPa s and a density of y1?07 g cm23 . After some heat treatment under nitrogen atmosphere, the precursor could be changed into highly cross-linked PBSZ. Figure 1 shows the TGA curve of PBSZ, which exhibits a weight loss in the beginning followed by a little weight gain and a weight loss again. The weight loss was due to the volatilisation of the residual solvent or low molecular weight silazane and borazine12 compounds before 400uC. The weight gain from 400 to 600uC was due to the nitridation effect by reactive ammonia atmosphere.11 However, in the hybrid precursor, the Si–H bond decreased for the reaction between PHPS and PBZ,10 so the nitridation effect and weight gain are not obvious. The weight loss after 800uC was small and the char yield was very high (y86 wt-%). Properties of composites The properties of the 2?5D SiO2f/Si3N4–BN composites are shown in Table 1 and the deviations have been attached to the mean values of the density, longitude flexural strength and modulus. The density of the specimen A was a little larger than that of the specimen B after the same infiltration–cure–pyrolysis cycles. However, the specimen B had a higher longitude flexural strength and modulus. The load–displacement curves of the composites are illustrated in Fig. 2. The load of the specimen A was almost linear with the displacement until a catastrophic failure, showing a low energy absorbing ability, whereas the composite B shows nonbrittle failure behaviour with higher fracture load and displacement, showing an improved deformation resistant ability. Microstructure of composites Figure 3 illustrates the SEM images of the fracture surface of the 2?5D SiO2f/Si3N4–BN. For specimen A, the interface between the fibres and the matrix was undistinguishable as a result of the strong adhesion and no fibre pull-out was observed on the fracture surface whereas specimen B showed relatively weak interface bonding with distinct fibre pull-out and fracture, exhibiting the effectiveness of fibre reinforcement. Discussion Figure 3 revealed that the precursor efficiently infiltrated into the interyarn and intrayarn porosity of the silica 1 Thermogravimetric analysis curve of cured PBSZ in NH3 atmosphere Table 1 Properties of 2?5D SiO2f/Si3N4–BN composites Specimen Pyrolysis cycles Density, g cm23 Longitude flexural strength, MPa Elastic modulus, GPa A4 1.70¡0. 02 46. 3¡2.0 17.3¡1. 0 B4 1. 68¡0. 02 129. 5¡7. 0 25.4¡1. 5 2 Load–displacement curves of 2?5D SiO2f/Si3N4–BN composites A and B Jiang et al. Polymer derived nitride matrix composites Materials Science and Technology 2007 VOL 23 NO 7 881
Jiang et aL. Polymer derived nitride matrix composites b Holes 26k 3 Scanning electron microscopy images of fracture surface of 2. 5D SiO2Si3Na-BN composites fabric because of the low viscosity and good wettal the pip method. which is a fast route to fabricate dense Together with the high ceramic yield, all contribl composites due to the good wettability and high char the fast weight pick-ups of 2 5D SiO2 si3N yield of the precursor. composites. The density of the composites reached The precoating process plays an important role in 1-68 or 1. 70 g cm3 after only four PIP cycles. controlling the microstructures and mechanical proper Therefore, PBSZ is a good precursor for preparing ties of the composites. The composites without fibre silica fibre preform reinforced nitride matrix composites. coating showed strong fibre/matrix interfacial bonding Generally after the maximum value of the load is and low mechanical properties, whereas the precoated reached, the subsequent extension degree of a ceramic composites exhibited controlled interfacial adhesion and matrix composite is strongly dependent on the nature of higher mechanical properties composite is consistent with its mechanical properties. 3 It is also well known that the interfacial bonding Acknowledgements strength can be evaluated by the morphology of fracture surfaces. Extensive fibre pull-out indicates relatively The project was supported by State Key Laboratory of weak fibre/matrix interfacial bonding, whereas little fibre Advanced Ceramic Fibres and Composites Foundation pull-out and short pull-out length indicates strong fibre/ under Contact No. 2004js51488-0101kg01-3 and matrix interfacial bonding. In the present study, strong Innovation Foundation of National University of interfacial bonding was observed for specimen A, thus Defense Technology for Graduate Student (No 0603) decreasing the mechanical properties, whereas for speci- The authors are also grateful to Mr J. F. Tian and X. z men B, the relatively weak interfacial bonding and the Zhao for their help in SEM examination energy absorbing ability by fibre pull-out contributed to gh mechanical properties, including the longitude References flexural strength and modulus. Considering the same reinforcing fibre fabric, precursor and preparation 1. Y. Matsuda. N. Akikawa and T. Satoh: Ceran Eng. Soc. Pre temperature, the difference in the mechanical property 2. K Jian, Z. H. Chen, Q.S. Ma and w.w. Zheng: Mater. Sci. Eng between specimens A and B is due to the different fibre/ A,2005,A390,154-157 matrix interfacial bonding state resulting from the fibre 3. J. P Brazel and R. Fenton: Proc. 13th Symp. on'Electromagnetic coating process. The fibre coating probably prevented windows, Atlanta, GA, USA, September 1976. Georgia Institute the chemical reactions between the fibres and the matrix of Technology, 9. under high temperature, which is interesting and 4. H Chen, L M. Zhang, G.Y. Jia, W.H. Luo and S. Yu: Key eng Mater,2003,249,159-162 currently in progress 5. L M. Manocha C.N. Panchal and S Manocha: Ceram. Eng. Sci. According to the literature, silica fibres and PbSZ Proc.,2002,23.655-661 derived Si3 N4-BN still remain amorphous at 800C; 6. G J Qi, C.R. Zhang, H. F Hu and F Cao: Mater. Sci. Eng. 4, 2006,A416,317-320 should be amorphous. What is more, a longitude 7. G. 1. O', C R. Zhang and H F. Hu: J. Non-crystz Solids, 2006,352 flexural strength of 129.5 MPa for specimen B is much 8 Y.G. Jiang. C.R. Zhang F Cao, S.Q.Wang,HFHu and GJ higher than that for the conventional continuous silica Qh:Adh. Eng. Mater.,2007,(1-2),114116 nforced ceramic matrix composites, which 9. GBoitier, J. Vcens and J. L. Chermant: Mater. Sci. Eng. A, 2000 proves that the PIP method is a promising route to A279.7380. prepare continuous silica fibre reinforcement nitrid 10. K. Su. E. E. Remsen. G.A. Zank and L. G. Sneddon: chem Mater,1993,5,547-5 matrix composites with excellent mechanical properties. 11. 0. Funayama, Y. Tashiro,AKamo,MOkumura and TIsoda: J. Mater. Sci. 1994. 29. 48 Conclusions 12. W. V. Hough, C. R. Guibert and G. T. Hefferan: United States Patent4150097,17 PBSZ was used to fabricate amorphous composites of 13. Z F Chen, L. T. Zhang L F Cheng and Y D. xur: Ceram. Int 2·5-dim nal silica fibre reinforced nitride matrix by 005.31573-580 882 Materials Science and Technology 2007 VOL 23 No 7
fabric because of the low viscosity and good wettability. Together with the high ceramic yield, all contribute to the fast weight pick-ups of 2?5D SiO2f/Si3N4–BN composites. The density of the composites reached 1?68 or 1?70 g cm23 after only four PIP cycles. Therefore, PBSZ is a good precursor for preparing silica fibre preform reinforced nitride matrix composites. Generally after the maximum value of the load is reached, the subsequent extension degree of a ceramic matrix composite is strongly dependent on the nature of the fibre/matrix interface and the microstructure of the composite is consistent with its mechanical properties.13 It is also well known that the interfacial bonding strength can be evaluated by the morphology of fracture surfaces. Extensive fibre pull-out indicates relatively weak fibre/matrix interfacial bonding, whereas little fibre pull-out and short pull-out length indicates strong fibre/ matrix interfacial bonding.2 In the present study, strong interfacial bonding was observed for specimen A, thus decreasing the mechanical properties, whereas for specimen B, the relatively weak interfacial bonding and the energy absorbing ability by fibre pull-out contributed to high mechanical properties, including the longitude flexural strength and modulus. Considering the same reinforcing fibre fabric, precursor and preparation temperature, the difference in the mechanical property between specimens A and B is due to the different fibre/ matrix interfacial bonding state resulting from the fibre coating process. The fibre coating probably prevented the chemical reactions between the fibres and the matrix under high temperature, which is interesting and currently in progress. According to the literature,10 silica fibres and PBSZ derived Si3N4–BN still remain amorphous at 800uC; therefore, the present 2?5D SiO2f/Si3N4–BN composite should be amorphous. What is more, a longitude flexural strength of 129?5 MPa for specimen B is much higher than that for the conventional continuous silica fibre reinforced ceramic matrix composites,4 which proves that the PIP method is a promising route to prepare continuous silica fibre reinforcement nitride matrix composites with excellent mechanical properties. Conclusions PBSZ was used to fabricate amorphous composites of 2?5-dimensional silica fibre reinforced nitride matrix by the PIP method, which is a fast route to fabricate dense composites due to the good wettability and high char yield of the precursor. The precoating process plays an important role in controlling the microstructures and mechanical properties of the composites. The composites without fibre coating showed strong fibre/matrix interfacial bonding and low mechanical properties, whereas the precoated composites exhibited controlled interfacial adhesion and higher mechanical properties. Acknowledgements The project was supported by State Key Laboratory of Advanced Ceramic Fibres and Composites Foundation, under Contact No. 2004js51488?0101.kg01?3 and Innovation Foundation of National University of Defense Technology for Graduate Student (No. 0603). The authors are also grateful to Mr J. F. Tian and X. Z. Zhao for their help in SEM examination. References 1. Y. Matsuda, N. Akikawa and T. Satoh: Ceram. Eng. Soc. Proc., 2001, 22, 463–470. 2. K. Jian, Z. H. Chen, Q. S. Ma and W. W. Zheng: Mater. Sci. Eng. A, 2005, A390, 154–157. 3. J. P. Brazel and R. Fenton: Proc. 13th Symp. on ‘Electromagnetic windows’, Atlanta, GA, USA, September 1976, Georgia Institute of Technology, 9. 4. H. Chen, L. M. Zhang, G. Y. Jia, W. H. Luo and S. Yu: Key Eng. Mater., 2003, 249, 159–162. 5. L. M. Manocha, C. N. Panchal and S. Manocha: Ceram. Eng. Sci. Proc., 2002, 23, 655–661. 6. G. J. Qi, C. R. Zhang, H. F. Hu and F. Cao: Mater. Sci. Eng. A, 2006, A416, 317–320. 7. G. J. Qi, C. R. Zhang and H. F. Hu: J. Non-cryst. Solids, 2006, 352, 3794–3798. 8. Y. G. Jiang, C. R. Zhang, F. Cao, S. Q. Wang, H. F. Hu and G. J. Qi: Adv. Eng. Mater., 2007, (1–2), 114–116. 9. G. Boitier, J. Vcens and J. L. Chermant: Mater. Sci. Eng. A, 2000, A279, 73–80. 10. K. Su, E. E. Remsen, G. A. Zank and L. G. Sneddon: Chem. Mater., 1993, 5, 547–556. 11. O. Funayama, Y. Tashiro, A. Kamo, M. Okumura and T. Isoda: J. Mater. Sci., 1994, 29, 4883. 12. W. V. Hough, C. R. Guibert and G. T. Hefferan: United States Patent 4150097, 17 April 1979. 13. Z. F. Chen, L. T. Zhang, L. F. Cheng and Y. D. Xu: Ceram. Int., 2005, 31, 573–580. a specimen A; b specimen B 3 Scanning electron microscopy images of fracture surface of 2?5D SiO2f/Si3N4–BN composites Jiang et al. Polymer derived nitride matrix composites 882 Materials Science and Technology 2007 VOL 23 NO 7
Copyright of Materials Science Technology is the property of Maney Publishing and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use