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graphite nanosheet composites
Polymer Testing 30 (2011) 390–396
Contents lists available at ScienceDirect
Polymer Testing journal homepage: www.elsevier.com/locate/polytest
Material Properties
Preparation and characterization of phenol formaldehyde/Ag/graphite nanosheet composites Nailiang Liu*, Shuhua Qi, Shasha Li, Xinming Wu, Limin Wu Department of Applied Chemistry, School of Science, Northwestern Polytechnical University, Xi’an 710129, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 1 January 2011 Accepted 21 February 2011
A conductive phenol formaldehyde resin (PF/Ag/NanoG) composite with silver-plated graphite nanosheet filling was prepared. The graphite nanosheet (NanoG) was prepared by sonicating expanded graphite (EG) in an aqueous alcohol solution. The silver-plated graphite nanosheet (Ag-plated NanoG) was obtained by an electroless plating method. The PF/Ag/NanoG composite was synthesized through in situ polymerization. The microstructure of the Ag-plated NanoG was characterized by a scanning electron microscope (SEM). The thickness of the Ag-plated NanoG particles was w500 nm. Transmission electron microscopy (TEM) demonstrated that the Ag-plated NanoG was distributed homogeneously in the phenol formaldehyde (PF) resin. The electrical tests showed that the conductivity of the PF/Ag/NanoG composite significantly increased compared with that of the pure PF resin. From the thermogravimetric analysis (TGA), it was seen that the Ag/ NanoG filler had a beneficial effect on the thermal stability. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: Graphite nanosheet Silver plating Phenol formaldehyde resin Morphology Conductivity
1. Introduction As a new class of functional materials, conductive polymer composites consisting of inorganic conducting fillers and organic polymers, are increasingly attracting attention [1–3]. The advantages of conductive polymer composites are their low density, ease of preparation, chemical inertness and relatively low cost [4]. Metal powder, metal foil, metal fiber, carbon black, carbon fiber and graphite are commonly used as conductive fillers. Among these materials, natural flake graphite has good chemical stability and good electrical conductivity in the range of 104 S/m at room temperature [5]. However, the graphite particle size is on the mm or mm scale. To achieve satisfactory electrical properties, a substantial amount of graphite is added, which leads to graphite filler accumulation and deterioration of the composite mechanical properties. Therefore, preparation of finely divided natural flake graphite through other physical or chemical methods
* Corresponding author. Tel.: þ86 2988431638. E-mail address: [email protected] (N. Liu). 0142-9418/$ – see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymertesting.2011.02.008
is a focus of current research [6]. A small graphite sheet can be obtained by mechanical grinding in a ball mill but, unfortunately, crystalline graphite with a layered structure has high lubrication ability which makes it extremely difficult to reduce to nanometer-scale particles [7]. In addition, the grinding process requires a long time and a series of subsequent high energy operations such as dewatering, drying, regrinding and classification. Moreover, extensive plastic deformation induces structural transformation in graphite and generates different kinds of defects [8,9]. Detonating pure trinitrotoluene in a shielding gas can produce graphite nanosheets [10]. The same is true for detonating natural graphite in a liquid explosive [11]. However, the detonation method must be performed in an airtight container. The obvious risks involved in handling explosives preclude its use. Therefore, researchers have attempted to find other convenient methods to prepare graphite nanosheets. Fine-flake graphite was observed to pass through a 160 mm mesh to produce expanded graphite (EG) [12]. The EG layered structure is similar to that of natural flake graphite, which is composed of a large number of delaminated graphite sheets. Graphite nanosheets (NanoG), with thickness varying from 30 to 50 nm,
N. Liu et al. / Polymer Testing 30 (2011) 390–396
were prepared by an ultrasonic powdering technique [13]. The aspect ratio (diameter to thickness) is as high as 100– 300, leading to lower percolation thresholds in conductive polymer composites [14]. Many studies report the addition of NanoG to different matrices to prepare conductive polymers, such as polyaniline (PANI) [15], poly(arylene disulfide) [16], polymethylmethacrylate [17], Nylon 6 [18], Nylon 6,6 [5], polystyrene [19], polypropylene [20] and epoxy resin [21]. However, due to the limitations in the inherent conductivity of NanoG, further improvement of the conductivity of its composite is difficult. Therefore, metallization of the NanoG surfaces is considered to be an effective method to overcome this problem because it imparts high electrical conductivity and metallic thermal properties. Recently, silver, which has the highest electrical conductivity and excellent thermal properties, was used as the metal coating on the NanoG surface. The Ag-plated NanoG was used as the conducting filler to prepare highconductivity PANI/Ag/NanoG composites via an ultrasonic technique [22]. The flame retardation [23], frictional properties [24] and mechanical properties [25] of graphite-filled phenol formaldehyde resin composites (PF/NanoG) have been investigated by various groups. However, the conductive properties of the PF/NanoG composite are seldom studied. In this work, PF/Ag/NanoG composites were prepared via in situ polymerization. The particle size, morphology, electrical conductivity, mechanical properties and thermal properties of the composites were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared (FTIR) spectroscopy, transmission electron microscopy (TEM), ultrahigh electric resistancer tests and thermogravimetric analysis (TGA). 2. Experimental 2.1. Materials Phenol supplied by Tianjin Damao Chemical Reagent Plant (China) was distilled twice under reduced pressure before use. Paraformaldehyde was supplied by Linyi Jinyuan Formaldehyde Plant (China). Magnesium oxide (MgO) was purchased from Tianjin Fuchen Fine Chemical Reagent Plant (China). Graphite intercalation compounds (GICs) were supplied by Shangdong Qingdao Graphite Company (China). Silver nitrate (AgNO3), 36% hydrochloric acid (HCl), sodium hydroxide (NaOH), ammonia (NH3$H2O), stannous chloride (SnCl2), palladium dichloride (PdCl2), boric acid (H3BO3) and alcohol, all of analytical reagent grade, were provided by Shanghai Dafeng Chemical Industry (China). 2.2. Preparation of NanoG The GICs were heated at 1000 C for 15 s in a muffle furnace (SX2-10-12, Wuhan Jangyu Electric Furnace Manufacture Co., Ltd., China) to obtain EG. The NanoG was prepared according to a procedure in the literature [13]. A weighed amount (1 g) of EG was immersed in 400 mL of aqueous alcohol solution (70 vol.% alcohol and 30 vol.% distilled water), and then the mixture was subjected to 12 h
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powdering in an ultrasonic bath (KQ-300 DE, Kunshan Ultrasonic instruments Co., Ltd., China) with a power of 100 W. The resulting dispersion was then filtered, repeatedly washed with distilled water, and dried in a thermostatic vacuum oven at 110 C (ZKF030, Shanghai Experiment instrument Plant, China) to obtain the NanoG. The EG and NanoG treatment process is illustrated in Fig. 1. 2.3. Preparation of Ag-plated nanoG The preparation of the Ag-plated NanoG was according to a method reported in the literature [22,26], which included oxidation, sensitization, activation, rinsing and a subsequent reduction of AgNO3 in the presence of HCHO. A weighed amount (1 g) of NanoG powder was oxidized in a solution of NaOH (2 mol/L) at 40 C for 2 h. After rinsing with distilled water, the powder was treated with a solution consisting of PdCl2 (0.002 mol/L), HCl (0.5 mol/L) and H3BO3 (0.3 mol/L) at room temperature for 1 h. Afterwards, the powder was rinsed and mixed with SnCl2 (0.04 mol/L) and HCl (0.5 mol/L) aqueous solution at room temperature for 1 h. The treated NanoG powder was rinsed with distilled water until its pH was about 7 and then dried completely at 110 C. Subsequently, AgNO3 ammonia (0.1 mol/L) solution and HCHO (0.4 mol/L) were added simultaneously into the pretreated NanoG ethanol aqueous solution, with continuous stirring at room temperature. The Ag plating reaction was performed at 25 C for 25 min. Finally, the Ag-plated NanoG was obtained after being washed thoroughly with distilled water and dried in a vacuum at 100 C. The Ag plating process is illustrated in Fig. 2. 2.4. Preparation of PF/Ag/NanoG composites 94 g phenol, 45.2 g paraformaldehyde, and 1.3 g MgO were combined with the Ag-plated NanoG filler in a 250 mL three-necked round bottom flask equipped with a stirrer, a thermometer and a condenser. Various proportions of Ag-plated NanoG were used to produce different composites. The reaction mixture was kept at 90 C for 4 h. Afterwards, the mixture was quickly injected into a sealed glass mold coated with mold release agent. The mold was placed in a vacuum drying oven set at 0.08 MPa and 170 C for 2 h to cure the mixture. After cooling to room temperature, the resulting PF/Ag/NanoG composites were obtained. 2.5. Characterization The XRD was performed using a XRD-7000 instrument (SHIMADZU LIMITED, Japan) at a scan rate of 0.02 with CuKa1 radiation generated at a voltage of 40 kV and a current of 40 mA. The X-ray patterns for 2q from 10 to 80 were obtained. Morphological analysis was performed using a JSM6360 LV scanning electron microscope (HITACHI, Japan) and an H-800 transmission electron microscope (HITACHI, Japan). The FTIR spectra were obtained at 400–4000 cm1 using a WQF-310 FT-IR spectrophotometer (Ruili, China).
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Fig. 1. Schematic illustration of the preparation of NanoG.
The volume resistivity (rv) and surface resistivity (rs) were measured on a ZC-36 ultrahigh electric resistance meter (Shanghai, China) using the corresponding measurement standards of GB/T 1410-2006. TGA was performed using a TA Q600SDT (TA Instruments, USA) instrument from room temperature to 800 C at a heating rate of 10 C/min in air. 3. Results and discussion 3.1. Characterization of Ag-plated NanoG 3.1.1. XRD analysis The XRD patterns of the GICs, EG, NanoG, and Agplated NanoG are shown in Fig. 3. The same diffraction peaks at 2q ¼ 26.59 (d z 3.35 Å) and 55.11 (d z 1.67 Å)
were observed in the samples [Fig. 3(a)–(c)]. Despite the slight change in the location of the characteristic diffraction peaks, the intensity of the diffraction peak of the graphite sheets decreased significantly. This indicates that the graphite sheet structure was disturbed or partially delaminated during the heat and ultrasonic treatments. The diffraction peaks in Fig. 3(d) correspond to the (111), (200), (220), and (311) planes, which can be indexed to the face-centered cubic structure of Ag (JCPDS, File No. 40783) [27]. According to the Debye–Scherrer equation, the thickness of the Ag-plated NanoG calculated from the XRD data is 460 nm. The typical diffraction peaks of graphite and Ag can be observed in Fig. 3(d). However, the intensity of the typical diffraction peak of graphite has been greatly weakened because its surface was almost completely covered by Ag.
Fig. 2. Schematic illustration of the electroless coating procedure used to prepare the Ag-plated NanoG.
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and became almost transparent graphite nanosheets. The NanoG evidently is highly disordered, with a diameter of 5–10 mm and a thickness of 40–100 nm. The NanoG has a large radius-thickness ratio, i.e., it has a wider surface area than any other kind of graphite. From Fig. 4(g), the electroless-deposited silver particles, most of which are 190 nm, have been plated on the NanoG surface. The Ag particles are homogeneous and dense, and have merged to form a coat on the NanoG sheet. Even under high magnification, the Ag particles are packed tightly, with only a few points on the surface open [Fig. 4(h)]. The overall thickness of the Ag-plated nanoG is about 500 nm. The uniform and dense Ag-plated NanoG material facilitated the preparation of high electrical conductivity composites from the polymer at very low filler loading. 3.2. Characterization of PF/Ag/NanoG composites Fig. 3. XRD patterns of GICs, EG, NanoG, and Ag-plated NanoG.
3.1.2. Microscopy of GICs, EG, NanoG, and Ag-plated NanoG Microscopy results of GICs, EG, NanoG, and Ag-plated NanoG are shown in Fig. 4. From Fig. 4(a) and (b), it can be seen that the GIC sheets have irregular lamellar shapes, a diameter of w500 mm, and a thickness of 3–16 mm. Between the GIC sheets, many compounds can expand to stretch the GIC sheets during the heat treatment, thereby forming EG. The EG structure, which has a worm-like shape, is loose and porous due to the non-uniform distribution of compounds between the sheets. The EG volume is 800 times that of the GICs, which leads to the very low density of EG [28,29]. From Fig. 4(c), each worm structure is a GIC after expansion. One GIC has been split into thousands of smaller pieces without separating completely [Fig. 4(d)]. The ultrasonic wave could rapidly generate high energy, which induces high-velocity interparticle collisions and causes fragmentation of the NanoG. The SEM images of the NanoG are presented in Fig. 4(e) and (f). These small graphite pieces separated from the EG
3.2.1. FTIR analysis The FTIR spectra of pure PF and PF/Ag/NanoG composites are shown in Fig. 5. The broad band from 3500 to 3300 cm1 is the absorption peak corresponding to the stretching of eOH. The band at 1329 cm1 reflects its stretching vibration. The bands at 1598, 1511, and 753 cm1 correspond to the characteristic absorption bands of the benzene rings [30]. The bands at 1037 and 753 cm1 correspond to the plane bending and plane rocking vibrations of Ar–H, respectively. The absorption bands at 1212 and 1096 cm1 are due to the stretching of the CeO bonds [31]. The absorptions at 1475, 812, and 505 cm1 are related to the CeH outer bending vibrations. For the PF/Ag/NanoG composites [Fig. 5(b)], the absorption peaks are approximately similar to those of pure PF [Fig. 5(a)], except that the CeH stretching regions at 1380, 484, and 454 cm1 belong to the characteristic absorption bands of the C–O–C group. These indicate strong interactions between the PF and Ag-plated NanoG. 3.2.2. Structure of the PF/Ag/NanoG composites The TEM image of the PF/Ag/NanoG composites is shown in Fig. 6. The black flakes represent the Ag-plated
Fig. 4. SEM micrograph of GICs, EG, NanoG, and Ag-plated NanoG: (a) and (b) GICs; (c) and (d) EG; (e) and (f) NanoG; (g) and (h) Ag-plated NanoG.
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Fig. 5. FTIR absorption spectra of pure PF and PF/Ag/NanoG.
NanoG sheets, and the light colored areas are the PF resin. The Ag-plated NanoG are homogeneously dispersed in the PF matrix. Many of them are of nanometer dimensions and some of the sheets have overlapped, which facilitates the formation of the conductive bridge within the PF resin. The conductive bridge is considered to result in significantly improved electrical conductivity of the PF composites. 3.2.3. Conductivity characteristics According to literature reports [22], the conductivity of the Ag-plated NanoG is about 9.7 107 U cm and that of PF is about 1.6 1016 U cm. Therefore, the addition of Agplated NanoG to PF increases the conductivity of the PF resin. However, for the PF/Ag/NanoG composite, conductivity is determined by the synergistic conductive effect of the Ag-plated NanoG and PF. When the loading content of Ag-plated NanoG is low, the conductive sheets cannot touch each other even if the sheets are uniformly dispersed.
Fig. 6. TEM of PF/Ag/NanoG.
This slightly improves the conductivity of the composites. However, as more Ag-plated NanoG is added, the conductive sheets become close enough to form a net-like or chain-like conductive structure that significantly enhances the conductivity of the composite. The dependence of the electrical insulation property of the PF/Ag/NanoG composites on the weight percentage of the filler is presented in Fig. 7. The volume resistivity (rv) and surface resistivity (rs) continuously declined with increased filler concentration and decreased from 1.62 1016 U cm and 6.76 1015 U to 1.07 104 U cm and 2.63 104 U, respectively, at 3 wt.% Ag-plated NanoG content. This indicates that the graphite sheets could form a net-like conductive structure when the weight fraction of the Ag-plated NanoG reached 3 wt.%. As the amount of graphite sheets increased further, the electrical conductivity of the PF/Ag/NanoG increased only slightly. However, the composites still retain resistivity in the semi-conducting region because of the high insulating property of the PF. 3.2.4. Thermal degradation analysis The thermal stability of PF/Ag/NanoG was characterized by TGA (Fig. 8). With the increase of temperature, the PF/ Ag/NanoG composites decomposed in two stages. The first stage occurred at 200–350 C, and involved water condensation and volatilization of unreacted oligomers [32,33]. The second weight loss stage was above 350 C, at which the PF/Ag/NanoG composites disintegrated gradually, resulting in continuous weight loss due to the elimination of various volatiles. These volatiles include H2O, CO2, CO, CH4, C2H6, H2, phenol, and its methyl derivatives, as well as some condensed nuclear hydrocarbons [34–37]. Generally, the thermal stability of the composite is improved by an increase in the decomposition temperature of the polymer. From Fig. 8, the temperature of maximal degradation rate of pure PF was 532.8 C, but that of the PF/ Ag/NanoG composites with 1 wt.%, 3 wt.%, and 5 wt.% Agplated NanoG were 537.3, 538.7, and 553.3 C, respectively. Since Ag-plated graphite has high thermal stability, the residual quality increases with an increase in the Ag-plated graphite at the same temperature. The residual quality of
Fig. 7. Effect of the Ag-plated NanoG content on the electrical resistivity of the composites.
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References
Fig. 8. TGA curves of the PF/Ag/NanoG composites with different Ag-plated NanoG contents.
pure PF, 1 wt.%, 3 wt.%, and 5 wt.% PF/Ag/NanoG composites at 800 C are 52.65%, 54.15%, 58.52%, and 60.90%, respectively. The thermograms in Fig. 8 (b)–(d) differ from that in Fig. 8(a) at around 350 C. This is due to the Ag coating which hindered the combustion of the NanoG. That is, within the structure of the composites, a strong interaction between the surface Ag atoms, NanoG, and PF molecule chains exists. This interaction prevented the degradation of the PF matrix. 4. Conclusions In this study, PF/Ag/NanoG composites were prepared via in situ polymerization. NanoG was produced by sonicating the EG in an aqueous alcohol solution. The Ag-plated NanoG was obtained by Ag plating the NanoG surface using an electroless method. The thickness of the Ag-plated NanoG was about 500 nm, and that of the electrolessdeposited silver particles was about 190 nm. The TEM results of the PF/Ag/NanoG composite indicate a homogeneous dispersion of the Ag-plated NanoG in the PF resin. Interaction between Ag-plated NanoG and PF resin was apparent from the FTIR spectral analysis. According to the electrical resistivity tests, the volume and surface resistivities rapidly decreased with an increase in the Ag-plated NanoG due to the high conductivity of the Ag-plated NanoG. When the weight fraction of the Ag-plated NanoG reached 3 wt.%, a conducting bridge structure was formed in the PF matrix. As the graphite sheets further increased, the electrical conductivity of the composite improved only slightly. Based on TGA results the Ag-plated NanoG increases the thermal stability of the PF resin. Acknowledgements The authors gratefully acknowledge the financial support provided by the Shenyang Aircraft Design and Research Institute. The authors are also thankful to the companies and relatives who kindly offered the materials and help.
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Contents lists available at ScienceDirect
Polymer Testing journal homepage: www.elsevier.com/locate/polytest
Material Properties
Preparation and characterization of phenol formaldehyde/Ag/graphite nanosheet composites Nailiang Liu*, Shuhua Qi, Shasha Li, Xinming Wu, Limin Wu Department of Applied Chemistry, School of Science, Northwestern Polytechnical University, Xi’an 710129, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 1 January 2011 Accepted 21 February 2011
A conductive phenol formaldehyde resin (PF/Ag/NanoG) composite with silver-plated graphite nanosheet filling was prepared. The graphite nanosheet (NanoG) was prepared by sonicating expanded graphite (EG) in an aqueous alcohol solution. The silver-plated graphite nanosheet (Ag-plated NanoG) was obtained by an electroless plating method. The PF/Ag/NanoG composite was synthesized through in situ polymerization. The microstructure of the Ag-plated NanoG was characterized by a scanning electron microscope (SEM). The thickness of the Ag-plated NanoG particles was w500 nm. Transmission electron microscopy (TEM) demonstrated that the Ag-plated NanoG was distributed homogeneously in the phenol formaldehyde (PF) resin. The electrical tests showed that the conductivity of the PF/Ag/NanoG composite significantly increased compared with that of the pure PF resin. From the thermogravimetric analysis (TGA), it was seen that the Ag/ NanoG filler had a beneficial effect on the thermal stability. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: Graphite nanosheet Silver plating Phenol formaldehyde resin Morphology Conductivity
1. Introduction As a new class of functional materials, conductive polymer composites consisting of inorganic conducting fillers and organic polymers, are increasingly attracting attention [1–3]. The advantages of conductive polymer composites are their low density, ease of preparation, chemical inertness and relatively low cost [4]. Metal powder, metal foil, metal fiber, carbon black, carbon fiber and graphite are commonly used as conductive fillers. Among these materials, natural flake graphite has good chemical stability and good electrical conductivity in the range of 104 S/m at room temperature [5]. However, the graphite particle size is on the mm or mm scale. To achieve satisfactory electrical properties, a substantial amount of graphite is added, which leads to graphite filler accumulation and deterioration of the composite mechanical properties. Therefore, preparation of finely divided natural flake graphite through other physical or chemical methods
* Corresponding author. Tel.: þ86 2988431638. E-mail address: [email protected] (N. Liu). 0142-9418/$ – see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymertesting.2011.02.008
is a focus of current research [6]. A small graphite sheet can be obtained by mechanical grinding in a ball mill but, unfortunately, crystalline graphite with a layered structure has high lubrication ability which makes it extremely difficult to reduce to nanometer-scale particles [7]. In addition, the grinding process requires a long time and a series of subsequent high energy operations such as dewatering, drying, regrinding and classification. Moreover, extensive plastic deformation induces structural transformation in graphite and generates different kinds of defects [8,9]. Detonating pure trinitrotoluene in a shielding gas can produce graphite nanosheets [10]. The same is true for detonating natural graphite in a liquid explosive [11]. However, the detonation method must be performed in an airtight container. The obvious risks involved in handling explosives preclude its use. Therefore, researchers have attempted to find other convenient methods to prepare graphite nanosheets. Fine-flake graphite was observed to pass through a 160 mm mesh to produce expanded graphite (EG) [12]. The EG layered structure is similar to that of natural flake graphite, which is composed of a large number of delaminated graphite sheets. Graphite nanosheets (NanoG), with thickness varying from 30 to 50 nm,
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were prepared by an ultrasonic powdering technique [13]. The aspect ratio (diameter to thickness) is as high as 100– 300, leading to lower percolation thresholds in conductive polymer composites [14]. Many studies report the addition of NanoG to different matrices to prepare conductive polymers, such as polyaniline (PANI) [15], poly(arylene disulfide) [16], polymethylmethacrylate [17], Nylon 6 [18], Nylon 6,6 [5], polystyrene [19], polypropylene [20] and epoxy resin [21]. However, due to the limitations in the inherent conductivity of NanoG, further improvement of the conductivity of its composite is difficult. Therefore, metallization of the NanoG surfaces is considered to be an effective method to overcome this problem because it imparts high electrical conductivity and metallic thermal properties. Recently, silver, which has the highest electrical conductivity and excellent thermal properties, was used as the metal coating on the NanoG surface. The Ag-plated NanoG was used as the conducting filler to prepare highconductivity PANI/Ag/NanoG composites via an ultrasonic technique [22]. The flame retardation [23], frictional properties [24] and mechanical properties [25] of graphite-filled phenol formaldehyde resin composites (PF/NanoG) have been investigated by various groups. However, the conductive properties of the PF/NanoG composite are seldom studied. In this work, PF/Ag/NanoG composites were prepared via in situ polymerization. The particle size, morphology, electrical conductivity, mechanical properties and thermal properties of the composites were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared (FTIR) spectroscopy, transmission electron microscopy (TEM), ultrahigh electric resistancer tests and thermogravimetric analysis (TGA). 2. Experimental 2.1. Materials Phenol supplied by Tianjin Damao Chemical Reagent Plant (China) was distilled twice under reduced pressure before use. Paraformaldehyde was supplied by Linyi Jinyuan Formaldehyde Plant (China). Magnesium oxide (MgO) was purchased from Tianjin Fuchen Fine Chemical Reagent Plant (China). Graphite intercalation compounds (GICs) were supplied by Shangdong Qingdao Graphite Company (China). Silver nitrate (AgNO3), 36% hydrochloric acid (HCl), sodium hydroxide (NaOH), ammonia (NH3$H2O), stannous chloride (SnCl2), palladium dichloride (PdCl2), boric acid (H3BO3) and alcohol, all of analytical reagent grade, were provided by Shanghai Dafeng Chemical Industry (China). 2.2. Preparation of NanoG The GICs were heated at 1000 C for 15 s in a muffle furnace (SX2-10-12, Wuhan Jangyu Electric Furnace Manufacture Co., Ltd., China) to obtain EG. The NanoG was prepared according to a procedure in the literature [13]. A weighed amount (1 g) of EG was immersed in 400 mL of aqueous alcohol solution (70 vol.% alcohol and 30 vol.% distilled water), and then the mixture was subjected to 12 h
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powdering in an ultrasonic bath (KQ-300 DE, Kunshan Ultrasonic instruments Co., Ltd., China) with a power of 100 W. The resulting dispersion was then filtered, repeatedly washed with distilled water, and dried in a thermostatic vacuum oven at 110 C (ZKF030, Shanghai Experiment instrument Plant, China) to obtain the NanoG. The EG and NanoG treatment process is illustrated in Fig. 1. 2.3. Preparation of Ag-plated nanoG The preparation of the Ag-plated NanoG was according to a method reported in the literature [22,26], which included oxidation, sensitization, activation, rinsing and a subsequent reduction of AgNO3 in the presence of HCHO. A weighed amount (1 g) of NanoG powder was oxidized in a solution of NaOH (2 mol/L) at 40 C for 2 h. After rinsing with distilled water, the powder was treated with a solution consisting of PdCl2 (0.002 mol/L), HCl (0.5 mol/L) and H3BO3 (0.3 mol/L) at room temperature for 1 h. Afterwards, the powder was rinsed and mixed with SnCl2 (0.04 mol/L) and HCl (0.5 mol/L) aqueous solution at room temperature for 1 h. The treated NanoG powder was rinsed with distilled water until its pH was about 7 and then dried completely at 110 C. Subsequently, AgNO3 ammonia (0.1 mol/L) solution and HCHO (0.4 mol/L) were added simultaneously into the pretreated NanoG ethanol aqueous solution, with continuous stirring at room temperature. The Ag plating reaction was performed at 25 C for 25 min. Finally, the Ag-plated NanoG was obtained after being washed thoroughly with distilled water and dried in a vacuum at 100 C. The Ag plating process is illustrated in Fig. 2. 2.4. Preparation of PF/Ag/NanoG composites 94 g phenol, 45.2 g paraformaldehyde, and 1.3 g MgO were combined with the Ag-plated NanoG filler in a 250 mL three-necked round bottom flask equipped with a stirrer, a thermometer and a condenser. Various proportions of Ag-plated NanoG were used to produce different composites. The reaction mixture was kept at 90 C for 4 h. Afterwards, the mixture was quickly injected into a sealed glass mold coated with mold release agent. The mold was placed in a vacuum drying oven set at 0.08 MPa and 170 C for 2 h to cure the mixture. After cooling to room temperature, the resulting PF/Ag/NanoG composites were obtained. 2.5. Characterization The XRD was performed using a XRD-7000 instrument (SHIMADZU LIMITED, Japan) at a scan rate of 0.02 with CuKa1 radiation generated at a voltage of 40 kV and a current of 40 mA. The X-ray patterns for 2q from 10 to 80 were obtained. Morphological analysis was performed using a JSM6360 LV scanning electron microscope (HITACHI, Japan) and an H-800 transmission electron microscope (HITACHI, Japan). The FTIR spectra were obtained at 400–4000 cm1 using a WQF-310 FT-IR spectrophotometer (Ruili, China).
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Fig. 1. Schematic illustration of the preparation of NanoG.
The volume resistivity (rv) and surface resistivity (rs) were measured on a ZC-36 ultrahigh electric resistance meter (Shanghai, China) using the corresponding measurement standards of GB/T 1410-2006. TGA was performed using a TA Q600SDT (TA Instruments, USA) instrument from room temperature to 800 C at a heating rate of 10 C/min in air. 3. Results and discussion 3.1. Characterization of Ag-plated NanoG 3.1.1. XRD analysis The XRD patterns of the GICs, EG, NanoG, and Agplated NanoG are shown in Fig. 3. The same diffraction peaks at 2q ¼ 26.59 (d z 3.35 Å) and 55.11 (d z 1.67 Å)
were observed in the samples [Fig. 3(a)–(c)]. Despite the slight change in the location of the characteristic diffraction peaks, the intensity of the diffraction peak of the graphite sheets decreased significantly. This indicates that the graphite sheet structure was disturbed or partially delaminated during the heat and ultrasonic treatments. The diffraction peaks in Fig. 3(d) correspond to the (111), (200), (220), and (311) planes, which can be indexed to the face-centered cubic structure of Ag (JCPDS, File No. 40783) [27]. According to the Debye–Scherrer equation, the thickness of the Ag-plated NanoG calculated from the XRD data is 460 nm. The typical diffraction peaks of graphite and Ag can be observed in Fig. 3(d). However, the intensity of the typical diffraction peak of graphite has been greatly weakened because its surface was almost completely covered by Ag.
Fig. 2. Schematic illustration of the electroless coating procedure used to prepare the Ag-plated NanoG.
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and became almost transparent graphite nanosheets. The NanoG evidently is highly disordered, with a diameter of 5–10 mm and a thickness of 40–100 nm. The NanoG has a large radius-thickness ratio, i.e., it has a wider surface area than any other kind of graphite. From Fig. 4(g), the electroless-deposited silver particles, most of which are 190 nm, have been plated on the NanoG surface. The Ag particles are homogeneous and dense, and have merged to form a coat on the NanoG sheet. Even under high magnification, the Ag particles are packed tightly, with only a few points on the surface open [Fig. 4(h)]. The overall thickness of the Ag-plated nanoG is about 500 nm. The uniform and dense Ag-plated NanoG material facilitated the preparation of high electrical conductivity composites from the polymer at very low filler loading. 3.2. Characterization of PF/Ag/NanoG composites Fig. 3. XRD patterns of GICs, EG, NanoG, and Ag-plated NanoG.
3.1.2. Microscopy of GICs, EG, NanoG, and Ag-plated NanoG Microscopy results of GICs, EG, NanoG, and Ag-plated NanoG are shown in Fig. 4. From Fig. 4(a) and (b), it can be seen that the GIC sheets have irregular lamellar shapes, a diameter of w500 mm, and a thickness of 3–16 mm. Between the GIC sheets, many compounds can expand to stretch the GIC sheets during the heat treatment, thereby forming EG. The EG structure, which has a worm-like shape, is loose and porous due to the non-uniform distribution of compounds between the sheets. The EG volume is 800 times that of the GICs, which leads to the very low density of EG [28,29]. From Fig. 4(c), each worm structure is a GIC after expansion. One GIC has been split into thousands of smaller pieces without separating completely [Fig. 4(d)]. The ultrasonic wave could rapidly generate high energy, which induces high-velocity interparticle collisions and causes fragmentation of the NanoG. The SEM images of the NanoG are presented in Fig. 4(e) and (f). These small graphite pieces separated from the EG
3.2.1. FTIR analysis The FTIR spectra of pure PF and PF/Ag/NanoG composites are shown in Fig. 5. The broad band from 3500 to 3300 cm1 is the absorption peak corresponding to the stretching of eOH. The band at 1329 cm1 reflects its stretching vibration. The bands at 1598, 1511, and 753 cm1 correspond to the characteristic absorption bands of the benzene rings [30]. The bands at 1037 and 753 cm1 correspond to the plane bending and plane rocking vibrations of Ar–H, respectively. The absorption bands at 1212 and 1096 cm1 are due to the stretching of the CeO bonds [31]. The absorptions at 1475, 812, and 505 cm1 are related to the CeH outer bending vibrations. For the PF/Ag/NanoG composites [Fig. 5(b)], the absorption peaks are approximately similar to those of pure PF [Fig. 5(a)], except that the CeH stretching regions at 1380, 484, and 454 cm1 belong to the characteristic absorption bands of the C–O–C group. These indicate strong interactions between the PF and Ag-plated NanoG. 3.2.2. Structure of the PF/Ag/NanoG composites The TEM image of the PF/Ag/NanoG composites is shown in Fig. 6. The black flakes represent the Ag-plated
Fig. 4. SEM micrograph of GICs, EG, NanoG, and Ag-plated NanoG: (a) and (b) GICs; (c) and (d) EG; (e) and (f) NanoG; (g) and (h) Ag-plated NanoG.
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Fig. 5. FTIR absorption spectra of pure PF and PF/Ag/NanoG.
NanoG sheets, and the light colored areas are the PF resin. The Ag-plated NanoG are homogeneously dispersed in the PF matrix. Many of them are of nanometer dimensions and some of the sheets have overlapped, which facilitates the formation of the conductive bridge within the PF resin. The conductive bridge is considered to result in significantly improved electrical conductivity of the PF composites. 3.2.3. Conductivity characteristics According to literature reports [22], the conductivity of the Ag-plated NanoG is about 9.7 107 U cm and that of PF is about 1.6 1016 U cm. Therefore, the addition of Agplated NanoG to PF increases the conductivity of the PF resin. However, for the PF/Ag/NanoG composite, conductivity is determined by the synergistic conductive effect of the Ag-plated NanoG and PF. When the loading content of Ag-plated NanoG is low, the conductive sheets cannot touch each other even if the sheets are uniformly dispersed.
Fig. 6. TEM of PF/Ag/NanoG.
This slightly improves the conductivity of the composites. However, as more Ag-plated NanoG is added, the conductive sheets become close enough to form a net-like or chain-like conductive structure that significantly enhances the conductivity of the composite. The dependence of the electrical insulation property of the PF/Ag/NanoG composites on the weight percentage of the filler is presented in Fig. 7. The volume resistivity (rv) and surface resistivity (rs) continuously declined with increased filler concentration and decreased from 1.62 1016 U cm and 6.76 1015 U to 1.07 104 U cm and 2.63 104 U, respectively, at 3 wt.% Ag-plated NanoG content. This indicates that the graphite sheets could form a net-like conductive structure when the weight fraction of the Ag-plated NanoG reached 3 wt.%. As the amount of graphite sheets increased further, the electrical conductivity of the PF/Ag/NanoG increased only slightly. However, the composites still retain resistivity in the semi-conducting region because of the high insulating property of the PF. 3.2.4. Thermal degradation analysis The thermal stability of PF/Ag/NanoG was characterized by TGA (Fig. 8). With the increase of temperature, the PF/ Ag/NanoG composites decomposed in two stages. The first stage occurred at 200–350 C, and involved water condensation and volatilization of unreacted oligomers [32,33]. The second weight loss stage was above 350 C, at which the PF/Ag/NanoG composites disintegrated gradually, resulting in continuous weight loss due to the elimination of various volatiles. These volatiles include H2O, CO2, CO, CH4, C2H6, H2, phenol, and its methyl derivatives, as well as some condensed nuclear hydrocarbons [34–37]. Generally, the thermal stability of the composite is improved by an increase in the decomposition temperature of the polymer. From Fig. 8, the temperature of maximal degradation rate of pure PF was 532.8 C, but that of the PF/ Ag/NanoG composites with 1 wt.%, 3 wt.%, and 5 wt.% Agplated NanoG were 537.3, 538.7, and 553.3 C, respectively. Since Ag-plated graphite has high thermal stability, the residual quality increases with an increase in the Ag-plated graphite at the same temperature. The residual quality of
Fig. 7. Effect of the Ag-plated NanoG content on the electrical resistivity of the composites.
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References
Fig. 8. TGA curves of the PF/Ag/NanoG composites with different Ag-plated NanoG contents.
pure PF, 1 wt.%, 3 wt.%, and 5 wt.% PF/Ag/NanoG composites at 800 C are 52.65%, 54.15%, 58.52%, and 60.90%, respectively. The thermograms in Fig. 8 (b)–(d) differ from that in Fig. 8(a) at around 350 C. This is due to the Ag coating which hindered the combustion of the NanoG. That is, within the structure of the composites, a strong interaction between the surface Ag atoms, NanoG, and PF molecule chains exists. This interaction prevented the degradation of the PF matrix. 4. Conclusions In this study, PF/Ag/NanoG composites were prepared via in situ polymerization. NanoG was produced by sonicating the EG in an aqueous alcohol solution. The Ag-plated NanoG was obtained by Ag plating the NanoG surface using an electroless method. The thickness of the Ag-plated NanoG was about 500 nm, and that of the electrolessdeposited silver particles was about 190 nm. The TEM results of the PF/Ag/NanoG composite indicate a homogeneous dispersion of the Ag-plated NanoG in the PF resin. Interaction between Ag-plated NanoG and PF resin was apparent from the FTIR spectral analysis. According to the electrical resistivity tests, the volume and surface resistivities rapidly decreased with an increase in the Ag-plated NanoG due to the high conductivity of the Ag-plated NanoG. When the weight fraction of the Ag-plated NanoG reached 3 wt.%, a conducting bridge structure was formed in the PF matrix. As the graphite sheets further increased, the electrical conductivity of the composite improved only slightly. Based on TGA results the Ag-plated NanoG increases the thermal stability of the PF resin. Acknowledgements The authors gratefully acknowledge the financial support provided by the Shenyang Aircraft Design and Research Institute. The authors are also thankful to the companies and relatives who kindly offered the materials and help.
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