Controllable synthesis of high aspect-ratio Mg2B2O5 nanowires and their applications in reinforced PHA composites【推荐论文】 .doc

精品论文Controllable synthesis of high aspect-ratio Mg2B2O5 nanowires and their applications in reinforced PHA composites5MO Zhaojun, FAN Ying, LIN Jing, XU Xuewen, CHEN Jinpeng, TANG Chengchun(School of Materials Science and Engineering, Hebei University of Technology, Tianjin 300130)Abstract: Highly pure magnesium borate (Mg2B2O5) nanowires with an average diameter of 30 nm,an average length of 15 m and a high aspect-ratio of 500 have been synthesized in a large scale via10a two-step method. MgBO2(OH) nanowires with high aspect-ratios were firstly prepared via aPVP-assisted hydrothermal technique. Then using the as-prepared MgBO2(OH) nanowires asprecursors, single crystalline Mg2B2O5 nanowires were synthesized by post-annealing treatment atrelatively low temperature of 700 °C. The important effect of conversion process on the morphology ofthe Mg2B2O5 nanowires was investigated, which indicated that the recrystallization process played an15important role in protection of 1D nanostructure. Moreover, the rigid and the toughness of theMg2B2O5 nanowires reinforced PHA composites had been tremendously improved compared withpure PHA. The results demonstrate the effectiveness of Mg2B2O5 nanowires for reinforcementapplications in polymer composites.Key words: Nanowire; Magnesium borate; Recrystallization; Reinforcement200IntroductionMetal borate is one of the most important technical ceramics owing to its interesting properties, such as high heat-resisting, corrosion-resisting, super-mechanical strength, super insulation and high coefficient of elasticity 1. Among the various kinds of metal borates, magnesium borate (Mg2B2O5) whisker has been considered as a promising reinforcement in25composite material applications due to its excellent mechanical properties, attractive thermal properties, thermo-luminescent properties2 and good antiwear behavior 3. In recent years, there has been a great deal of research in using Mg2B2O5 whiskers as reinforcement in Mg and Al matrix composites 4. The significant mechanical reinforcement of Mg and/or Al matrix composites has been successfully achieved. It should be noted that the excellent mechanical30properties of Mg2B2O5 whiskers also imply their promising applications as reinforcing additive for polymers. Polyhydroxyalkanoate (PHA) is a class of natural polyesters and has been currently under extensive investigations. This kind of polyester can be biologically synthesized from bio-renewable feedstocks and is generally biodegradable and biocompatible 5. However, the brittle nature of the PHA polymers has hindered their development in practical applications.35Reinforcing the PHA polymers by incorporation of Mg2B2O5 nanowires provides the possibility to obtain composite materials with high mechanical strength. However, the related study has not yet been initiated, and presents a challenge.Since reducing the size of whiskers into nanometer-scale can enhance their toughness and strength greatly 6, 7, one-dimensional (1D) Mg2B2O5 nanostructures are considered as more40excellent candidate as a reinforcing element in composites compared with their micron-sized counterpart. It should be noted that the novel properties of 1D nanostructure greatly depend on their morphologies and aspect ratios (length vs diameter). Nanowires with high aspect-ratio couldFoundations: RFDP (NO. 20091317110005), the Key Project of Chinese Ministry of Education (NO. 209011) Brief author introduction:Mo Zhaojun (1984-), Mail, Master. He is currently involved in the investigations of nanofibers and nano-compositesCorrespondance author: Tang Chengchun (1968-), Mail, Professor, Nanomaterials. E-mail: tangcchebut.edu.cn- 19 -be more beneficial to strengthen traditional materials. Therefore, the controlled synthesis of high aspect-ratio Mg2B2O5 nanostructures has become an important focus for their development in45composite applications. So far, there has been great progress in the synthesis of 1D Mg2B2O5 nanostructures 8-10 based on various synthetic routes, such as high-temperature sintering technique, chemical vapor deposition (CVD) method etc. For example, using MgCl2×6H2O and NaBH4 as starting materials, Li et al. 11 synthesized single crystalline Mg2B2O5 nanorods with diameters of 200-400 nm and lengths up to 5 mm by a two-step process involving the50mechanochemical technique and sintering method. Yi et al. 12 synthesized large quantities ofsingle crystalline Mg2B2O5 nanowires with typical diameters of 120-180 nm and lengths of 0.2mm through directly heating of the mixed tablet of Mg(BO2)2 and graphite in vacuum at 1200 °C for 1 h. Compared with the high-temperature sintering or CVD method, hydrothermal synthesis has shown some distinct advantages in preparing uniform nanocrystals with high crystallinity and55high purity, such as the moderate experimental conditions and the easy control of the process parameters 13, 14. Previous studies indicated that 1D Mg2B2O5 nanostructures could be prepared at a relatively low temperature based on the post-annealing of hydrothermally synthesized MgBO2(OH) nanostructures 15, 16. Zhu et al. 17, 18 reported the preparation of Mg2B2O5 nanowhiskers with diameters of 15.0-45.0 nm and lengths of 0.2-2.0 m by a low-temperature60annealing treatment at 650-700 °C using hydrothermally synthesized MgBO2(OH) nanowires as precursors. However, the breakage was unavoidable during the thermal conversion from MgBO2(OH) to Mg2B2O5 nanowires. Then the authors also 19 tried to directly synthesized Mg2B2O5 by co-precipitation and calcinations at 800-900 °C for 2 h. However, they failed in the synthesis of Mg2B2O5 nanowires. To date it is still a big challenge to develop a synthetic strategy65to prepare high aspect-ratio Mg2B2O5 nanowires at low temperature.In this paper, we report on the pioneering large-scale preparation of uniform Mg2B2O5nanowires with an aspect-ratio as high as 500 based on a two-step method. MgBO2(OH)nanowires with high aspect-ratio were firstly prepared via a PVP-assisted hydrothermal technique.Then using the as-prepared MgBO2(OH) nanowires as precursors, single crystalline Mg2B2O570nanowires were synthesized by post-annealing treatment at relatively low temperature of 700°C.Detailed thermal conversion analysis was performed that gave vital clues to understand therecrystallization process from MgBO2(OH) to Mg2B2O5. The slower heating rate was critical toget high aspect-ratio 1D Mg2B2O5 nanowires in this process. Then for the first time, theas-prepared Mg2B2O5 nanowires were used as additive in PHA polymers to form composites. A75remarkable increase of the rigid and the toughness of the composite had been obtained by theaddition of Mg2B2O5 nanowires into PHA. The results demonstrated the effectiveness of Mg2B2O5nanowires for reinforcement applications in polymer composites.1Experimental details1.1Synthesis of MgBO2(OH) nanowires80The starting materials of MgCl2, H3BO3, NH4OH and PVP were commercial grade and used without further purification. MgCl2 (1.904 g) and H3BO3 (1.855 g) were resolved in deionized water (25 ml), and PVP (0-1.5 g) was added into the solution and stirred for 1 h. Then NH4OH (10 ml) was carefully sprayed into the solution and stirred for another 1 h to obtain the slurry as a precursor. Subsequently, the slurry was put into a Teflon-lined stainless steel autoclave. The slurry85(precursor) was heated at 220 °C for 24 h and cooled down to room temperature. Finally the products were filtered and washed by deionized water for several times and dried at 120 °C for 129095100h.1.2Synthesis of Mg2B2O5 nanowires:The as-prepared MgBO2(OH) nanowires were placed in an alumina crucible and used as initial reaction constituent. The alumina crucible was situated into alumina tube and first heated at360 °C with different heating rate (1-10°C/min) for 4 h; then, heated (1-10°C/min) to 600 °C for another 4 h; finally, heated (1-10°C/min) to 700 °C for other 4 h to recrystallize. All the thermal conversions were carried in air conditions using a traditional resistance heating furnace. The resultant samples were then collected for characterization.1.3Synthesis of Mg2B2O5 nanowires reinforced PHA:2.4 mg Mg2B2O5 nanowires were dispraised into 20 ml chloroform by stirred and ultrasound treatment for 1 h respectively. Then 1.2 g PHA was put into the mixed solution and the stirring was continued until a translucent and homogeneous colloidal solution was obtained at 50 °C. Finally, films with thickness of 0.1mm and diameters of 10 cm were cast at room temperature for several hours.1051101151201251.4Characterization methods:The structure and morphology of the samples were identified by X-ray powder diffraction (XRD, BRUKER D8 FOCUS), a field emission scanning electron microscopy (SEM, HITACHI S-4800) and a high resolution transmission electron microscopy (HRTEM, JEM-2010). The thermal decomposition behavior of the sample was detected by differential thermal analysis (TG-DTA, SDT22960, TA Instruments, USA), carried out in dynamic air from room temperature to 900 °C at a heating rate 10 °C min-1. For the mechanical measurement, the as-prepared films were stretched on a Micro Bionix equipped with a 5 N load cell (MTS Systems Corporation) at a strain rate of 30mm min-1. Test Star IIV 4.0 (MTS Systems Corporation) was used as the data collection and equipment control software. Test samples were trimmed to 4 mm wide by 25 mm long and casting resulted in films that were approximately 0.1 mm thick. All the measurements were carried out at room temperature.2Results and discussion2.1The morphology and structure of MgBO2(OH) nanowiresFigure 1a shows the XRD pattern of the hydrothermally synthesized products. All diffraction peaks can be calculated as a monoclinic MgBO2(OH) with lattice parameters of a=1.2588, b=1.0400 and c=0.3125 nm. The result is in good accordance with those reported in (PDF No.33-0859) of bulk crystal. The diffraction peaks suggest that the samples are well crystallized. No XRD peaks arising from impurities have been detected, indicating the high purity of the product. Low-magnification SEM image (Figure 1b) shows a large scale of highly flexible nanowires with lengths of several tens of micrometers. Enlarged SEM images (Figure 1c) indicate that the nanowires have diameters of 20-60 nm. The aspect-ratio of the nanowires can be determined as high as 500. Therefore, we can conclude that high-aspect-ratio MgBO2(OH) nanowires have been fabricated at a high yield and high purity.Polyvinylpyrrolidone (PVP) has been considered as an important kind of surfactant in the hydrothermal synthesis of 1D nanostructure 20. Due to its strong ability of forming hydrogen bonding and complex, PVP is favorable for promoting 1D preferential growth and increasing the aspect-ratio of 1D nanostructure. The effect of PVP on the aspect-ratio of the MgBO2(OH)130135140145150nanowires were studied in Figure 2. A considerably obvious aspect-ratio dependence on the contents of PVP has been exhibited. Very short nanorods with an average diameter of 45 nm andFig. 1 Phase composition and microstructure of the as-prepared MgBO2(OH) nanowires. (a) XRD pattern of the final products. (b) Low-magnification SEM image of the as-prepared nanowires. (c) Enlarged SEM images of the MgBO2(OH) nanowires.Fig. 2 (a-e) SEM images of MgBO2(OH) nanowires synthesized in different contents of PVP: (a) 0 g, (b) 0.1 g, (c)0.5 g, (d) 1.0 g, (e) 1.5 g, (f) the aspect-ratio of MgBO2(OH) nanowires as a function of the content of PVP.length of 800 nm were obtained when PVP was not added during the synthesis process (Figure2a). With the increasing of the content of PVP to 0.1 g, the average length of the nanorodsincreased (Figure 2b). When the content of PVP increased to 0.5 g, tremendously longerMgBO2(OH) nanowires with lengths of several tens micrometers and an average diameter of 30nm were obtained (Figure 2c). However, the excessive PVP addition did not help to increase theaspect-ratio of the nanowires further. Figure 2d-e show SEM images of the MgBO2(OH)nanowires with PVP contents of 1.0-1.5g, respectively. Figure 2f shows the average aspect-ratio asa function of the content of PVP. The ratio rapidly increases with the content of PVP increasingfrom 0 to 0.5 g, and then it decreases as the content of PVP continuously increasing from 0.5 to1.5 g. Therefore, we can conclude that the usage of PVP is the key factor for the formation ofMgBO2(OH) nanowires with high aspect-ratio and the optimal synthetic condition should use PVPwith a content of 0.5 g.2.2The thermal conversion from MgBO2(OH) to Mg2B2O5 nanowiresFigure 3a shows the TG-DTG curves of the MgBO2(OH) nanowires, which were155160165Fig. 3 Phase transformation from MgBO2(OH) to Mg2B2O5, (a) TGDTG curves of the MgBO2(OH) nanowires; (b) XRD patterns of the products calcined at different temperatures (curve a) precursor, (curve b) 360 °C, (curve c) 600 °C, (curve d) 700 °C, (curve e) 850 °C.hydrothermally synthesized under the optimal synthetic condition (the morphology of the nanowires is shown in Figure 2c). The result clearly shows a weight loss in the sample occurred with a comparatively slow rate at temperatures below 360 °C and a fast rate in the temperature range of 360-600 °C. When the heating temperature increased above 600 °C, the sample weight remained almost constant with a very little variation. The sample weight loss during the heating process relates to the elimination of the physically absorbed water and the structural water, respectively. The weight loss between 360 °C and 600 °C is 10.67%, which is quite similar to the theoretical weight loss (10.7%) for the conversion of MgBO2(OH) to Mg2B2O5. It also indicates that the dehydration process basically completed at 600 °C. The conversion can be expressed as follow:1701751801851902 MgBO2(OH) (s) Mg2B2O5 (s) + H2O (g) (1) The decomposition temperature is lower than that of