NEW CARBON MATERIALS Volume 27, Issue 1, Feb 2012 Online English edition of the Chinese language journal Cite this article as: New Carbon Materials, 2012, 27(1):12–18.
RESEARCH PAPER
Improvement of the oxidation stability and the mechanical properties of flexible graphite foil by boron oxide impregnation D.V. Savchenko1,*, A.A. Serdan1, V.A. Morozov1, G. Van Tendeloo2, S.G. Ionov1 1
Department of Chemistry, Moscow State University, Moscow, 119991, Russia;
2
EMAT, University of Antwerp, Groenenborgerlaan 171, B-2020 Antwerp, Belgium
Abstract: Flexible graphite foil produced by rolling expanded graphite impregnated with boron oxide was analyzed by laser mass spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy and thermogravimetry. It was shown that the modification of the graphite foil by boron oxide increases the onset temperature of oxidation by ~ 150 °C. Impregnation of less than 2 mass% boron oxide also increased the tensile strength of the materials. The observed improvement was attributed to the blocking of active sites by boron oxide, which is probably chemically bonded to the edges of graphene sheets in expanded graphite particles. Key Words: Expanded graphite; Graphite foil; Boron oxide; Oxidation protection; Tensile strength
1
Introduction
Because of its unique physical and chemical characteristics, flexible graphite foil (GF), a low-density carbon material, obtained by rolling expanded graphite (EG) without a binder, is widely used in the production of different types of sealing materials, shields for electromagnetic and thermal radiation, resistance elements in electrical heaters[1], different parts of hydrogen-air fuel-cell elements[2-3] and so on. Graphite foil is stable up to ~3 000 °C in inert atmosphere or in vacuum, but appreciable oxidation starts at 450 °C in air. Many studies are devoted to the protection of different carbon materials from oxidation. Most of them are based on two approaches[4]. One is to cover the surface of carbon materials by compounds that are more stable to oxidation and the other is to block active sites by compound, where oxidation starts. The first approach can prevent oxygen from accessing the material. But the difference in thermal expansion between the carbon material and the protective coating usually leads to crack formation on the surface of the carbon material and as a result, the protective coating is destroyed during the first heating-cooling cycle. In the second approach, there is the problem of non-uniform distribution of antioxidant over the whole material. Boron oxide is widely used for oxidation protection of different carbon materials[4-10]. B2O3 has high volatility and oxygen permeability above 800 °C, so even in case of complete surface covering, it is effective only up to this temperature[8]. To distribute it uniformly over the surface and inside all
the pores of a sample, the carbon material is usually exposed to molten B2O3 in inert atmosphere at high pressure for a long time[10]. This method is unsuitable for expanded graphite (EG), because it may lead to significant densification and change in its physical and mechanical properties. Another method is impregnation of carbon materials with a boric acid solution followed by heat treatment at 600-800 °C [6]. To produce EG, heat treatment is also needed[1], so these two processes can be integrated into one single process. Therefore, expandable graphite is first impregnated with a boric acid solution and then subjected to heat treatment to produce EG. To find the optimal concentration of B2O3 for oxidation protection of flexible graphite foil without any significant change in its mechanical properties, EG and flexible graphite foil with different contents of boron oxide were obtained and their physical and chemical properties were investigated.
2
Experimental
2.1 Sample preparation Natural graphite has an average flake size of 0.3-0.4 mm (di = 0.335 nm) and an ash content of about 4.5%. Nitric acid (~98%), sulphuric (98%) acid, solid boric acid and potassium dichromate were used as received. Natural graphite with different ash contents was obtained by chemical purification with different extents by first heating with a 45% sodium hydroxide solution at 350 °C for 3 h with different weight ratios, then neutralizing with a 10% nitric acid solution, and finally
Received date: 23 August 2011; Revised date: 31 January 2012 *Corresponding author. E-mail:
[email protected] Copyright©2012, Institute of Coal Chemistry, Chinese Academy of Sciences. Published by Elsevier Limited. All rights reserved. DOI: 10.1016/S1872-5805(12)60001-8
D.V. Savchenko et al. / New Carbon Materials, 2012, 27(1): 12–18
washing with distilled water and drying to constant weight at 100 °C. The ash contents in natural graphite and in all the samples of graphite foil were determined by burning them at 900 °C in air. The stage-2 (N = 2; the stage number (N) is equal to the number of graphene layers between two nearest intercalated layers) graphite intercalation compound (GIC) with nitric acid (graphite nitrate) was synthesized by reaction of natural graphite with ~98% nitric acid (liquid phase technique)[11-12]. Expandable graphite was obtained by hydrolyzing the GIC with distilled water and drying at room temperature. The stage-1 (N = 1) GIC with sulphuric acid (graphite bisulphate) was obtained by reaction of natural graphite with 98% sulphuric acid and a stoichiometric amount of potassium dichromate[13-14]. Expandable graphite was obtained from as-synthesized GIC by hydrolyzing with distilled water and drying at 100 °C. Expandable graphite obtained from graphite nitrate was treated with different quantities of 5% boric acid water solution. Heat treatment and decomposition of H3BO3 were carried out to obtain expanded graphite (EG1-EG5) with different contents of B2O3. All the samples were dried at room temperature to constant weight. Flexible graphite foil (GF1-GF5) with a density of 1 g/cm and a thickness of 0.3 mm was prepared from expanded graphite (EG1-EG5) by rolling method with a set of laboratory devices (Fig.1). These devices allow of production of EG and GF in large quantities under dynamic conditions (20-100 g of expandable graphite). GF samples were obtained by thermal shock of expandable graphite at 950 °C followed by a further cold rolling without a binder. 3
2.2
Sample characterization
Powder X-ray diffraction (XRD) was carried out using a thermo ARL X’TRA diffractometer with CuKα radiation (λ=0.154 18 nm, Bragg-Brentano geometry) and a Si(Li) Peltier-cooled detector. Data were collected over the range 5°-60°in 2θ° with a step of 0.02° (2θ). XRD can be used to check if the intercalation reaction is complete because the introduction of intercalate species expands the interlayer spacing in graphite. The XRD pattern of nitrate GIC exhibits a characteristic peak at 7.9° corresponding to the identity period (Ic) of 1.122 nm while the XRD pattern of bisulfate GIC shows a characteristic peak at 11.1° corresponding to the Ic of 0.797 nm. The obtained Ic values of 1.122 nm and 0.797 nm correspond to stage-2 nitrate GIC and stage-1 bisulfate GIC, respectively[11-15]. Elemental analysis was performed with a laser energy mass analyzer EMAL-2. This is a mass spectrometer designed for solid sample analysis. The samples were ionized by a focused laser beam with an energy of 108-1010 W/cm2. The process was carried out in the scanning mode at 5 mm×5 mm areas to obtain the average element content throughout the sample.
X-ray photoelectron spectroscopy (XPS) analysis was performed on a Kratos AXIS Ultra DLD spectrometer. A monochromatic emitter (AlKα, 1 486.9 eV) was used. The analyzed area was 300 µm×700 µm. Spectra were recorded in the range 5-160 eV with an increment of 0.05-0.5 eV; the number of XPS scans were varied for each region to obtain the best signal/noise ratio. Mathematical processing of the spectra was carried out using the Unifit 2003 software. The binding energy values for different peaks were compared with the existing reference data[16-18]. The microstructure of the flexible graphite foil sample with 4.2 mass% B2O3 was studied using a Jeol JEM-5510 scanning electron microscope equipped with an EDX spectrometer (INCA Energy+, energy resolution below 1 eV) at a 20 kV acceleration voltage. Thermogravimetry (TG) measurements were carried out on a STA 449 Jupiter® NETZSCH at a heating rate of 10 °C /min in air. The samples were examined at an air flow rate of 40 mL/min from room temperature to 950 °C. The specific surface areas of the EG samples (SBET, m2/g) were determined by nitrogen adsorption measurements using the Brunauer-Emmett-Teller (BET) equation via an Qsurf Surface Area Analyzer 9600. To remove the adsorbed water, the samples were preheated in a nitrogen flow at 250 °C for 3 h. Nitrogen adsorption was carried out at 77 K and desorption was performed by heating to room temperature. The tensile strength was measured with a Hounsfield H100K-S universal testing machine. Samples of rectangular cross section of 150 mm×25 mm were stretched with a constant speed of 7 mm/min till destruction. The tensile strength (σ) was calculated by the formula σ = Fmax/(b·δ) (МPa), where Fmax is maximal load at destruction, b and δ (mm) are sample width and thickness, respectively.
Fig.1 Laboratory device for preparation of expanded graphite and graphite foil samples (1-sampler,2-heating furnace, 3-rolling mill, 4-air compressor, 5, 6-process controller)
D.V. Savchenko et al. / New Carbon Materials, 2012, 27(1): 12–18
3
Results and discussion
Elemental analysis of the flexible graphite foil samples obtained from hydrolyzed stage-2 GIC with nitric acid confirmed the presence of nitrogen, oxygen and boron, besides carbon (Table 1). The B:O mass ratio does not correspond exactly to the stoichiometry in B2O3 (B:O = 1:2.22), because of the presence of oxygen (up to 0.3%) in the unmodified graphite foil GF1 (Table 1). According to XRD data, the ash obtained after burning the modified GF samples consists mainly of glassy boron oxide. The ash content is close to the weight concentration of B2O3 calculated from elemental analysis. Only O1s, C1s and B1s lines are present in the XPS spectra of all the flexible graphite foil (GF:B2O3) samples. The B1s line has a low intensity and appears only after a long exposure time because of the low photoionization cross section and the low element concentration (≤4.2 mass%). Fig.2 shows the XPS spectrum of a GF sample with the maximal B2O3 content (4.2 mass%). Only one component at 193 eV is observed. This corresponds to the O–B binding energy[16-18] and significantly differs from the C–B binding energy (188-189 eV)[17-20]. It is therefore concluded that boron in the GF samples exists as B2O3 oxide, not as C–B bonds. Fig.3 shows the results of a SEM investigation of the in side part of the flexible graphite foil sample with 4.2 mass% B2O3. In Fig.3a, b, 2-5-µm solid-phase particles containing oxygen (Fig. 3c-e) are observed. Only CKα1 (0.241 keV) and OKα1 (0.486 keV) lines are found in the EDX spectrum (Fig.3c). The high intensity of the CKα1 line does not allow us to resolve the BKα1 line (0.148 keV) in the EDX spectrum. The absence of lines corresponding to elements other than carbon and oxygen allows us to make a conclusion that the observed solid phase is boron oxide and its distribution in the sample can be obtained on the base of the OKα1 map (Fig.3e). Thus XPS, EDX and elemental analysis data allow us to conclude that the particles observed on the SEM images are indeed B2O3 particles, which are likely located on the edges of graphene sheets.
30%) in the tensile strength of the flexible graphite foil. However, a further increase in B2O3 (4-5 mass%) dramatically decreases the tensile strength below the initial value. Such degradation is probably associated with the formation of large B2O3 agglomerates (Fig.3b) during the heat treatment of expandable graphite. These agglomerates act as foreign inclusions and are stress concentrators like impurities in the initial graphite. Information about the influence of foreign inclusions on the mechanical properties of graphite foil is practically absent in the literature. Dowell et al.[27] found that the tensile strength of graphite foil decreases with increase in the ash. However, the authors studied a type of industrial flexible graphite foil with unknown history, but the impurity characteristics were not investigated. To investigate the influence of impurities on the tensile strength the graphite foil, GF samples with different ash contents were prepared. It was shown that the tensile strength decreases linearly with increase in the ash content (Fig.4с), and the slope does not depend on the nature of the GIC used for obtaining GF samples. The particle size of the impurities (Fig.4d) on the graphite foil surface is close the size of the B2O3 particles (Fig.3a, 3b), i.e., approximately 2-4 µm. Thus, our assumption about the negative influence of large amounts of B2O3 on the tensile strength of graphite foil was validated. According to several studies[21-24], oxidation of any graphite-like material starts in the most susceptible areas, so-called active sites, such as graphene edges, point defects in graphene plane, dislocations or functional groups on the surface. Natural graphite contains such active sites, but expanded Table 1 Elemental content (mass%) of graphite foil (GF1-5) samples (ρ=1.0 g/cm3) Element
GF1
GF2
GF3
GF4
GF5
C
99.5
99.2
98.1
98.0
95.1
B
