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phenolic nanocomposite by introducing ZrB2 nanoparticles
Accepted Manuscript Enhanced Thermal Resistance of GO/C/Phenolic Nanocomposite by Introducing ZrB2 Nanoparticles Zahra Amirsardari, Masoud Salavati-Niasari, Rouhollah Mehdinavaz Aghdam, Saeed Shakhesi PII:
S1359-8368(15)00027-X
DOI:
10.1016/j.compositesb.2015.01.011
Reference:
JCOMB 3359
To appear in:
Composites Part B
Received Date: 25 October 2014 Revised Date:
30 November 2014
Accepted Date: 13 January 2015
Please cite this article as: Amirsardari Z, Salavati-Niasari M, Aghdam RM, Shakhesi S, Enhanced Thermal Resistance of GO/C/Phenolic Nanocomposite by Introducing ZrB2 Nanoparticles, Composites Part B (2015), doi: 10.1016/j.compositesb.2015.01.011. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Enhanced Thermal Resistance of GO/C/Phenolic Nanocomposite by Introducing ZrB2
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Nanoparticles
Zahra Amirsardaria, Masoud Salavati-Niasaria*, Rouhollah Mehdinavaz Aghdamb, Saeed Shakhesib Institute of Nano Science and Nano Technology, University of Kashan, Kashan, P.O. Box 87317-51167, I. R. Iran b Department of NanoTechnology, Space Transportation Research Institute, Tehran, I. R. Iran ∗
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Corresponding author. Tel.: +98-361-591 23 83, Fax: +98-361-555 29 35.
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E-mail address: [email protected] (M. Salavati-Niasari). Abstract
The effectiveness of different additives on improving the thermal stabilities of phenolic composite was investigated by incorporating of graphene oxide sheets (GO) into the carbon/phenolic (PR), and then the
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ZrB2 nanoparticles into the GO/PR composite. The GOs dissipate heat throughout the sample thereby reducing thermal gradients and the intensity of heating at the surface exposed to flame. Also, at higher exposure time, the resistance to oxidation of the nanocomposite begins taking advantage of the ongoing
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formation of an oxide coating layer (ZrO2) on the exposed face. This protected the underlying unoxidized
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material from the structural damage caused by thermal shocks and high shear forces. Keywords: A. Ceramic-matrix composites (CMCs); A. Nano-structures; B. High-temperature properties; D. Electron microscopy; E. Thermal analysis.
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1.
Introduction
Organic resins can be used as ablative systems by absorbing heat and rejecting it into the boundary layer gas and re-radiation [1]. For thermal protection systems two very different classes of materials have been
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considered: carbon fiber composites, and ultra-high temperature ceramics [2]. Carbon-fiber-reinforced composites containing a polymer resin matrix can be used for an ablation-resistant material that requires moderate environments, low cost and relatively low performance. The matrix with carbon fibers is less
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oxidized, and almost no change of the morphology is observed [3]. To satisfy the developing demands of new materials, their ablation properties need to be further elevated. Inorganic fillers, such as carbon
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nanotubes, carbon nanofiber, graphene oxide, and graphene have high potentials in both thermal insulation and char structural integrity. Recent publications on investigations to improve ablation resistant phenolic composites have been reported on the effect of the addition of CNTs [4]. Based on the facts that an improvement in flammability properties of polymers has been obtained with graphene nanofillers that
structure [5].
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provided another candidate as a synergist agent due to its unique two-dimensional (2D) atomic carbon sheet
Graphene oxide (GO), a two-dimensional single-layered carbon material, has recently attracted extensive
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attention owing to its unique structure, large specific surface area, superior electronic conductivity, high
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thermal conductivity, excellent mechanical properties, and low cost [6, 7]. Graphene nanofillers act as cementing agents and form network structure in the char layer and improve the char layer resistance against stripping during high flow rate of hot gases [8]. Because of their layered structures, graphene and graphene oxide can be applied as barriers reducing the heat released and insulating against the transfer of combustion gases into the polymer matrix. Therefore, it has attracted considerable attention for the preparation of nanocomposites to enhance the thermal stabilities and flame retardancy of matrices upon addition of a small amount [9]. Until now, few studies have focused on the thermal properties of GO/phenolic composites which 2
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evaluated for potential usage in flame retardant and other heat resistant applications [10]. Also, the oxidation onset temperature of organic resins can be increased by the ceramic particles such as zirconium diboride (ZrB2) which is an ideal candidate for surviving the extreme conditions [11]. ZrB2 is an ultra-high
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temperature ceramic (UHTC) with potential applications in the industry because of its electrical and thermal conductivities, good oxidation and chemical attack resistances [12]. In the oxidative conditions, researchers reported the formation of an oxide scale containing zirconia (ZrO2) and boria (B2O3) to protect of the surface
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at high temperatures which directly cuts the diffusion channel for oxygen to attack the substrate. The formation of boria product of ZrB2 can form a liquid layer on the oxidized surface, which can hinder the
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oxygen transportation [13]. ZrO2 is known as thermal insulator which forms a layer to protect the degradation of underlying nanocomposite. This can lead to an increase in temperature gradient at high heating times because of the difficult dissipation of heat from the surface, which accelerated high temperature oxidation and fracture [14]. However, the enhanced thermal property is necessary in the extreme
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heat conditions. For this fact is that the common applications of the two components offer the possibility to observe synergetic effects, improve in thermal conductivity and prevent the diffusion of oxygen. Different thermal radiation and conduction abilities of ZrB2 and GO dominate various surface temperatures. The
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ablation of GO in the matrix is influenced by ZrB2 in the composites, resulting in the slower increase of ablation rates. Previous studies have demonstrated that the thermal conductivity of the composites can be
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improved by the simply dispersed CNT loadings [15]. The several studies have been conducted on ZrB2–SiC composites, but ablation of SiC would be accelerated by ZrO2 from decomposition of ZrB2 with the defects at the interface between carbon fiber and matrix [11, 16, 17]. Using GO and ZrB2 as reinforcements to control hot corrosion of carbon/phenolic composite at 2000oC is a novel concept, which is not explored so far by any research group. This study is aimed (i) To develop GO and ZrB2 in phenolic matrix, and to study the ablative properties of composites made out of this 3
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combination, and (ii) To study the enhanced thermal resistance of phenolic composites due to ZrO2 in reduction of ablation rate. Experimental Section
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2.
2.1. Materials
Zirconium n-propoxide (Zr(OPr)4, Sigma-Aldrich) and Phenolic resin (PR, Resitan Co.) based on a phenolic resol type resin were used in the present process. All of reagents and solvents were obtained from
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Merck. The salen ligand and GO were prepared according to previous work [18, 19].
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2.2. Preparation of ZrB2 nanoparticles
To a solution of the zirconium (IV) n-propoxide (6.3 ml), H2salen (1.3 g), methanol (25 ml) and dionized water (4 ml) was added boric acid in acetic acid (30 ml) followed by citric acid at room temperature. The resulting mixture was stirred for 3 h at 60oC, and then the mixture dried at 120oC. Afterward, it was placed
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into a tubular furnace at 1500oC for 2h under argon atmosphere. Finally, ZrB2 nanopowder was obtained. 2.3. Fabrication process of phenolic nanocomposites The graphene oxide/resol composites which contained 0 and 7 wt% of ZrB2 nanoparticles were referred to
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as GO/PR and Zr/GO/PR, respectively. The GO/PR composites were prepared by dispersing 1 wt% of GO in the resol solution (50 wt% in ethanol), and then dispersed by ultrasound for 1 h at room temperature to
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prepare dispersion of GO/PR. The GO/PR solution was added into carbon fibers (CF, an average diameter of about 7 µm). CFs were added at a weight ratio to the phenolic of 1:1. The mixture was dried at 70oC. The composites were cured at 150oC for 2 h in hot press. The pressure of 9 MPa was necessary to seal the mould frame. Three types composites were fabricated by this procedure: carbon- phenolic resin (PR), GO/carbonphenolic (GO/PR) and ZrB2/GO/carbon- phenolic (Zr/GO/PR). 2.4. Characterization 4
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FTIR measurements were made over the range 4000–400 cm-1 using a Bruner FT-IR Spectrometer using the KBr pellet method. The TEM observation was made at 150 kV using a Philips CM30. Field emission scanning electron microscopy (FESEM, Mira 2 Tescan) equipped with energy dispersion spectroscopy
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(EDS) was applied to study the microstructure of the samples. The thermal stability of the produced materials was investigated using a TGA instrument (Linseys STA 1600). Measurements conducted in nitrogen at dynamic scans of 10oC/min from 30oC to 1000oC. The ablation performance was operated at an
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oxyacetylene test bed. The flame was generated by the primary gas, oxygen, and the secondary gas, acetylene, making it an excellent choice for simulating the extreme condition. The gun was set at a stand-off
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distance of 50 mm relative to the samples. This stand-off distance is sufficient that it ensures the sample will be fully exposed to high flow. The samples were cut into a size of 25 mm in diameter and 20 mm in height for ablation test, and held in position using a stainless fixture and exposed for 160 s. 3.
Results and discussion
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In order to obtain ZrB2 nanopowder with better properties, it was adopted sol–gel method, which is high intimacy at atomic-scale or molecular-scale. To synthesize uniform and pure ZrB2 nanoparticles, salen was used as a Schiff base ligand to form a compound of Zr+4 stable against hydrolysis and could coordinate with
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metal to form stable complexes. The XRD pattern of the as-synthesized ZrB2 nanocrystals is shown in Fig.
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1. The pattern of the ZrB2 was indexed as a pure hexagonal structure with the literature value. In order to explore the ZrB2 nanoparticle size and morphological characteristics in more detail, the TEM (Fig. 2a) and FESEM (Fig. 2b) analyses were performed. These images showed small spherical and uniform nanoparticles with the size of about 20–30 nm in diameter. In the FTIR spectra (Fig. 3), GO/PR and Zr/GO/PR presented the typical spectra of functional groups: –OH at 3429 cm-1, C=O at 1638 cm-1 and –CH2 bending at 1420 cm-1 [20]. The absorption band around 1540 cm-1 was more than likely from the C=C stretching mode of GO and phenolic rings. The methylol stretching 5
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appeared at around 1124 cm-1 for the functionalization of GO/PR. Also, the weakened peak of the O–H deformation vibration of PR existed at 1371 cm-1, and the new peak of C–O stretching vibration for the nucleophilic substitution reaction between O-groups of PR and oxygen containing groups of GO was at 1095
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cm-1 [21]. The bands at 800 and 875 cm-1 were the characteristics of the benzene ring. Also, a twin peak at 2856 near 2923 cm−1 was responsible for the C–H symmetric and anti-symmetric stretching vibration in Fig. 3a.
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Figure 4 presents the TGA and derivative TGA (DTGA) curves (a and b, respectively) of PR, GO/PR and Zr/GO/PR composite samples under nitrogen atmosphere. The maximum temperature of decomposition
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(Tmax) of the phenolic composite occurred at about 460oC, and the maximum weight loss percent was 23%. Though the addition of these particles had no desired effect on residual mass of PR at 1000oC in TGA curves, it can even lead to a reduction in it [22]. Graphene oxide exfoliates and decomposes when rapidly heated to ~1050°C through the large pressures generated during gasification of oxygenated functional groups
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intercalated between graphite oxide layers and formation of finely dispersed amorphous carbon which exhibits a distinct weight loss [23, 24]. The DTGA graph of PR sample showed that, after the initial evaporation of residual water, the degradation of methylene linkages in phenolic resins and subsequently the
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decomposition of aromatic phenol rings took place in two steps. Incorporating these particles could further enhance the thermal stability of PR due to the incomplete oxidation of the charred material and composition
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of ZrO2 at higher temperature than 1000oC. However, the TGA patterns of the two composites (GO/PR and Zr/GO/PR) were nearly the same, indicating that at the nanofiller load, the ZrB2 had no effect on the thermal stability of the GO/PR composite [25]. Carbon fibers have been utilized as a representative reinforcement for the ablation purpose owing to their high carbon content, excellent thermal stability, low thermal expansion, low density and non- flammability. In addition, the physical–chemical interaction at the fiber–matrix interface plays an important role in 6
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improving the properties of a fiber-reinforced composite at a high content of 50 wt % [3]. On the other hand, the good distribution of a small amount of GO sheets in resin guarantees the efficient thermal resistance properties of composite. There are a large number of oxygen-containing functional groups attached on the
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basal planes and edges of GO sheets such as hydroxyl, epoxide, carboxyl, and carbonyl groups, which could allow homogeneous dispersion of GO nanosheets in water or different solvents and effectively improve interfacial interaction and compatibility between GO and polymeric matrix [21, 26]. The existence of this
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type of covalent bonds will increase the density of the filler/polymer interactions, thus improving the filler dispersion into the polymer matrix [27]. The functional groups present in the GO are very useful in making
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covalent bond linkages with phenolic backbone [28]. The results of the thermal investigation from previous studies obtained with different GO contents do not show clear differences in the thermal behavior. This is attributed to the presence of well dispersed GO sheets within the matrix which generate an increase of its thermal stability by suppressing the polymer chain mobility [22]. The composites are exposed to a high
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temperature flow in order to evaluate the effects of GO and ZrB2 on the oxidation behavior of PR. It was found that the presence of both graphene oxide sheets and ZrB2 nanoparticles influences the surface properties [29]. The front side of the samples was directly exposed to heat flux at 2000oC for 160 s. When
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the flame impacted on the sample, heat flux and oxidizing species can be attacked on the sample surface. Meanwhile, GO can reflect a large amount of heat to reduce the surface temperature of the sample and
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weaken oxyacetylene ablation. The boria product of ZrB2 overflows from the micro-cracks distributed on the surface. The overflowed boria gas decreases the partial pressure of oxidizing species such as CO2 produced by oxyacetylene torch. Afterward, B2O3 gas covers some parts of the sample surface and disperses in the gas layer between flame and the sample surface [30]. The thickness and mass loss measurements were obtained by taking the specimen height and weight difference before and after ablation test. These data were used as a performance variable to analyze the effect 7
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of processing parameters in the fabrication of the ablative materials. All samples showed that in terms of the recession rate, the PR composite exhibited higher erosion than the GO/PR and Zr/GO/PR nanocomposites. The addition of GO in PR is believed to enhance the oxidation resistance due to an enhanced thermal
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property, and the formation of char layer. Since the carbon matrix is oxygen sensitive, the composite will be largely consumed if the surface has not a good oxidation protective ability, which will lead to the high weight loss of the sample [31]. The formation of char layer can be an oxidation protective coating which
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could provide an effective protection to stop the diffusion of oxygen into the sample [32]. When char layer is not formed, or detaches from the surface of composite, there will be a significant mass loss of the sample.
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Therefore, the uniform GO network provided a protective layer, and as a consequence, the loading of GO to phenolic resin reduced the mass loss which could effectively prevent oxygen diffusing into the substrate. The 1 wt% GO in PR exhibited a lower ablation rate (61%) comparing with neat phenolic resin, which was attributed, in part, to the good thermal insulation of the carbonized GO network. The ablation rate ratio of 7
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wt% ZrB2 in GO/PR to the neat specimen was a significantly lower (90%) which should be due to the fact that reinforcing ZrB2 could convert to ZrO2 as a heat dissipater and provide an insulated surface for transferring heat [30]. The GO sheets were demonstrated to be the key factor for the one- dimensional
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growth of ZrO2 (from decomposition of ZrB2). The obtained zirconia–GO has dispersion ability in the surface due to interactions between the zirconia and GO; this composite will provide a more fire retardant
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material. Zirconium cation is a strong Lewis acid, and might be physically-chemically adsorbed by the functional groups of graphene oxide, and then zirconium cations transform into ZrO2 on the GO sheet as the temperature increases [33]. Then, the surface was completely covered by incorporation/formation of small, closely spaced, and uniformly distributed oxide. Accordingly, in the transition zone, the mechanical denudation became weakened, this coating was not consumed out, and the oxides remained at the surface which prevented the composite from ablation further. The oxidation channels could be sealed by the oxide. 8
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The decrease in the percentage mass loss was an outcome of improved thermal stability with ZrB2 and GO additives. The mass loss of GO/PR composite at 7 weight percent of ZrB2 was about 72% lower than that of PR. The ablative properties of composites at 2000oC after 160 s are presented in Table 1.
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The time-temperature plots of the front side temperatures of the samples were presented in Fig. 5a, and the backside temperatures were measured in situ during exposure to the flow in Fig. 5b. The GO/PR possessed a higher surface temperature than Zr/GO/PR in the beginning whereas they reached the same temperature after
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40 s. When the temperature reached more than 1200°C, B2O3 quickly evaporated which can be caused to decrease of temperature from the surface [34]. As illustrated in Fig. 5b, by effectively transferring heat away
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from the front of the sample, the backside temperatures were greatly reduced. The back face temperature reached 600°C after 160s for PR, but the temperature of Zr/GO/PR composite reached 90°C. In the lower temperatures, GO could still decrease the surface temperature, but in the higher temperature, fast oxidation of ZrB2 and evaporation of B2O3 made the ZrB2 addition to be a useful role for modification of ablation. It
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showed that the decrease in the back side temperature after 160 s with GO (150oC) could be due to a high interface thermal resistance and re-radiation of thermal energy from the outer surface into the atmosphere. The ZrO2 based system exhibited a lower thermal diffusivity than the GO/PR composite, it can be concluded
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that this difference was directly related to the thermal conductivity [35]. In order to understand the ablation mechanism, we compared the microstructures of the oxidized
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composites. Fig. 6a shows the surface of samples after oxidation. It can be seen that PR has visible structural damage with pitch carbon fibers, but GO/PR and Zr/GO/PR are fully covered by a char layer. Fig. 6b and c shows how the FESEM micrographs of the surface vary dramatically between GO/PR and ZrB2 reinforced GO/PR before and after ablation test, respectively. It is found that the carbon fibers were randomly and uniformly dispersed in the matrix without the clumps of fibers in PR sample. The inset in Fig. 6c, shows that the area inside PR composite is heavily oxidized. In contrast, the GO/PR and Zr/GO/PR show no signs of 9
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cracks throughout the sample. A lot of gray phase coating on the GO/PR surface can be seen, arising from GO that has appeared in the form of char which is vastly different from the Zr/GO/PR. From Fig. 6c for Zr/GO/PR, it is clearly observed that the surface has been covered by the char and ZrO2 with white color.
of ZrB2. It indicated that the oxide layer contained no boron.
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Electron dispersive spectroscopy (EDS) was performed on the oxide layer in order to examine the amounts
As a result, the ablation rate of UHTCs modified GO/PR composite was slower than GO/PR composite. In
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another word, ablation sensitivity versus heat flux of GO/PR with low ceramic nanoparticle content was lower than pure composite and GO/PR. The GO became a physical shield decreasing external heat flux
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changing by the transmission of heat from thermal conduction to radiation transfer. Surprisingly, incorporation of both GO and ZrB2 nanoparticles strongly influenced on the reduction of linear ablation rate. 4.
Conclusions
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Carbon-phenolic composites fabricated with zirconium diboride and graphene oxide can produce a protective layer on the surface, leading to reduced back face temperature when exposed to high temperature. The high heat dissipation of the GO/phenolic composite decreased the intensity of heat near the front
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surface. In addition, the presence of ZrB2 with formation ZrO2 grains in phenolic composite has decreased the ablation rate, and thus delayed the degradation process of GO/PR composite. The linear and mass
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ablation rates of Zr/GO/PR composites compared with PR decreased by 90% and 72%, respectively. Acknowledgments
The authors are grateful to the Space Transportation Research Institute for financial support.
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Figure captions: Fig. 1. XRD pattern of the ZrB2 nanoparticles pyrolyzed at 1500oC.
Fig. 3. FT-IR spectra of the phenolic composites after curing in 150°C. Fig. 4. (a) TGA and (b) DTG curves of the phenolic samples.
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Fig. 2. (a) TEM (b) FESEM images of ZrB2 prepared with using Schiff base.
Fig. 5. The temperatures of (a) surface center and (b) back surface as a function of ablation time during test.
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Fig. 6. FESEM images of the composites: (a) macro-morphologies after oxyacetylene torch test, (b)
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microstructure of composites before ablation test and (c) the ablated surfaces.
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Table 1. Ablative properties of three composites at 2000oC after 160 s.
Composite
Ablation depth
Linear rate (mm/s)
Mass loss (wt%)
1.55
0.00970
GO/PR
0.61
0.00380
Zr/GO/PR
0.15
0.00094
31.70
15.68 8.84
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PR
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(mm)
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Fig. 1. XRD pattern of the ZrB2 nanoparticles pyrolyzed at 1500oC.
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Fig. 2. (a) TEM (b) FESEM images of ZrB2 prepared with using Schiff base.
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Fig. 3. FT-IR spectra of the phenolic composites after curing in 150°C.
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Fig. 4. (a) TGA and (b) DTG curves of the phenolic samples.
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Fig. 5. The temperatures of (a) surface center and (b) back surface as a function of ablation time during test.
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Fig. 6. FESEM images of the composites: (a) macro-morphologies after oxyacetylene torch test, (b) microstructure of composites before ablation test and (c) the ablated surfaces.
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S1359-8368(15)00027-X
DOI:
10.1016/j.compositesb.2015.01.011
Reference:
JCOMB 3359
To appear in:
Composites Part B
Received Date: 25 October 2014 Revised Date:
30 November 2014
Accepted Date: 13 January 2015
Please cite this article as: Amirsardari Z, Salavati-Niasari M, Aghdam RM, Shakhesi S, Enhanced Thermal Resistance of GO/C/Phenolic Nanocomposite by Introducing ZrB2 Nanoparticles, Composites Part B (2015), doi: 10.1016/j.compositesb.2015.01.011. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Enhanced Thermal Resistance of GO/C/Phenolic Nanocomposite by Introducing ZrB2
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Nanoparticles
Zahra Amirsardaria, Masoud Salavati-Niasaria*, Rouhollah Mehdinavaz Aghdamb, Saeed Shakhesib Institute of Nano Science and Nano Technology, University of Kashan, Kashan, P.O. Box 87317-51167, I. R. Iran b Department of NanoTechnology, Space Transportation Research Institute, Tehran, I. R. Iran ∗
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Corresponding author. Tel.: +98-361-591 23 83, Fax: +98-361-555 29 35.
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E-mail address: [email protected] (M. Salavati-Niasari). Abstract
The effectiveness of different additives on improving the thermal stabilities of phenolic composite was investigated by incorporating of graphene oxide sheets (GO) into the carbon/phenolic (PR), and then the
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ZrB2 nanoparticles into the GO/PR composite. The GOs dissipate heat throughout the sample thereby reducing thermal gradients and the intensity of heating at the surface exposed to flame. Also, at higher exposure time, the resistance to oxidation of the nanocomposite begins taking advantage of the ongoing
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formation of an oxide coating layer (ZrO2) on the exposed face. This protected the underlying unoxidized
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material from the structural damage caused by thermal shocks and high shear forces. Keywords: A. Ceramic-matrix composites (CMCs); A. Nano-structures; B. High-temperature properties; D. Electron microscopy; E. Thermal analysis.
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1.
Introduction
Organic resins can be used as ablative systems by absorbing heat and rejecting it into the boundary layer gas and re-radiation [1]. For thermal protection systems two very different classes of materials have been
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considered: carbon fiber composites, and ultra-high temperature ceramics [2]. Carbon-fiber-reinforced composites containing a polymer resin matrix can be used for an ablation-resistant material that requires moderate environments, low cost and relatively low performance. The matrix with carbon fibers is less
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oxidized, and almost no change of the morphology is observed [3]. To satisfy the developing demands of new materials, their ablation properties need to be further elevated. Inorganic fillers, such as carbon
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nanotubes, carbon nanofiber, graphene oxide, and graphene have high potentials in both thermal insulation and char structural integrity. Recent publications on investigations to improve ablation resistant phenolic composites have been reported on the effect of the addition of CNTs [4]. Based on the facts that an improvement in flammability properties of polymers has been obtained with graphene nanofillers that
structure [5].
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provided another candidate as a synergist agent due to its unique two-dimensional (2D) atomic carbon sheet
Graphene oxide (GO), a two-dimensional single-layered carbon material, has recently attracted extensive
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attention owing to its unique structure, large specific surface area, superior electronic conductivity, high
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thermal conductivity, excellent mechanical properties, and low cost [6, 7]. Graphene nanofillers act as cementing agents and form network structure in the char layer and improve the char layer resistance against stripping during high flow rate of hot gases [8]. Because of their layered structures, graphene and graphene oxide can be applied as barriers reducing the heat released and insulating against the transfer of combustion gases into the polymer matrix. Therefore, it has attracted considerable attention for the preparation of nanocomposites to enhance the thermal stabilities and flame retardancy of matrices upon addition of a small amount [9]. Until now, few studies have focused on the thermal properties of GO/phenolic composites which 2
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evaluated for potential usage in flame retardant and other heat resistant applications [10]. Also, the oxidation onset temperature of organic resins can be increased by the ceramic particles such as zirconium diboride (ZrB2) which is an ideal candidate for surviving the extreme conditions [11]. ZrB2 is an ultra-high
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temperature ceramic (UHTC) with potential applications in the industry because of its electrical and thermal conductivities, good oxidation and chemical attack resistances [12]. In the oxidative conditions, researchers reported the formation of an oxide scale containing zirconia (ZrO2) and boria (B2O3) to protect of the surface
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at high temperatures which directly cuts the diffusion channel for oxygen to attack the substrate. The formation of boria product of ZrB2 can form a liquid layer on the oxidized surface, which can hinder the
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oxygen transportation [13]. ZrO2 is known as thermal insulator which forms a layer to protect the degradation of underlying nanocomposite. This can lead to an increase in temperature gradient at high heating times because of the difficult dissipation of heat from the surface, which accelerated high temperature oxidation and fracture [14]. However, the enhanced thermal property is necessary in the extreme
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heat conditions. For this fact is that the common applications of the two components offer the possibility to observe synergetic effects, improve in thermal conductivity and prevent the diffusion of oxygen. Different thermal radiation and conduction abilities of ZrB2 and GO dominate various surface temperatures. The
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ablation of GO in the matrix is influenced by ZrB2 in the composites, resulting in the slower increase of ablation rates. Previous studies have demonstrated that the thermal conductivity of the composites can be
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improved by the simply dispersed CNT loadings [15]. The several studies have been conducted on ZrB2–SiC composites, but ablation of SiC would be accelerated by ZrO2 from decomposition of ZrB2 with the defects at the interface between carbon fiber and matrix [11, 16, 17]. Using GO and ZrB2 as reinforcements to control hot corrosion of carbon/phenolic composite at 2000oC is a novel concept, which is not explored so far by any research group. This study is aimed (i) To develop GO and ZrB2 in phenolic matrix, and to study the ablative properties of composites made out of this 3
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combination, and (ii) To study the enhanced thermal resistance of phenolic composites due to ZrO2 in reduction of ablation rate. Experimental Section
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2.
2.1. Materials
Zirconium n-propoxide (Zr(OPr)4, Sigma-Aldrich) and Phenolic resin (PR, Resitan Co.) based on a phenolic resol type resin were used in the present process. All of reagents and solvents were obtained from
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Merck. The salen ligand and GO were prepared according to previous work [18, 19].
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2.2. Preparation of ZrB2 nanoparticles
To a solution of the zirconium (IV) n-propoxide (6.3 ml), H2salen (1.3 g), methanol (25 ml) and dionized water (4 ml) was added boric acid in acetic acid (30 ml) followed by citric acid at room temperature. The resulting mixture was stirred for 3 h at 60oC, and then the mixture dried at 120oC. Afterward, it was placed
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into a tubular furnace at 1500oC for 2h under argon atmosphere. Finally, ZrB2 nanopowder was obtained. 2.3. Fabrication process of phenolic nanocomposites The graphene oxide/resol composites which contained 0 and 7 wt% of ZrB2 nanoparticles were referred to
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as GO/PR and Zr/GO/PR, respectively. The GO/PR composites were prepared by dispersing 1 wt% of GO in the resol solution (50 wt% in ethanol), and then dispersed by ultrasound for 1 h at room temperature to
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prepare dispersion of GO/PR. The GO/PR solution was added into carbon fibers (CF, an average diameter of about 7 µm). CFs were added at a weight ratio to the phenolic of 1:1. The mixture was dried at 70oC. The composites were cured at 150oC for 2 h in hot press. The pressure of 9 MPa was necessary to seal the mould frame. Three types composites were fabricated by this procedure: carbon- phenolic resin (PR), GO/carbonphenolic (GO/PR) and ZrB2/GO/carbon- phenolic (Zr/GO/PR). 2.4. Characterization 4
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FTIR measurements were made over the range 4000–400 cm-1 using a Bruner FT-IR Spectrometer using the KBr pellet method. The TEM observation was made at 150 kV using a Philips CM30. Field emission scanning electron microscopy (FESEM, Mira 2 Tescan) equipped with energy dispersion spectroscopy
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(EDS) was applied to study the microstructure of the samples. The thermal stability of the produced materials was investigated using a TGA instrument (Linseys STA 1600). Measurements conducted in nitrogen at dynamic scans of 10oC/min from 30oC to 1000oC. The ablation performance was operated at an
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oxyacetylene test bed. The flame was generated by the primary gas, oxygen, and the secondary gas, acetylene, making it an excellent choice for simulating the extreme condition. The gun was set at a stand-off
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distance of 50 mm relative to the samples. This stand-off distance is sufficient that it ensures the sample will be fully exposed to high flow. The samples were cut into a size of 25 mm in diameter and 20 mm in height for ablation test, and held in position using a stainless fixture and exposed for 160 s. 3.
Results and discussion
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In order to obtain ZrB2 nanopowder with better properties, it was adopted sol–gel method, which is high intimacy at atomic-scale or molecular-scale. To synthesize uniform and pure ZrB2 nanoparticles, salen was used as a Schiff base ligand to form a compound of Zr+4 stable against hydrolysis and could coordinate with
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metal to form stable complexes. The XRD pattern of the as-synthesized ZrB2 nanocrystals is shown in Fig.
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1. The pattern of the ZrB2 was indexed as a pure hexagonal structure with the literature value. In order to explore the ZrB2 nanoparticle size and morphological characteristics in more detail, the TEM (Fig. 2a) and FESEM (Fig. 2b) analyses were performed. These images showed small spherical and uniform nanoparticles with the size of about 20–30 nm in diameter. In the FTIR spectra (Fig. 3), GO/PR and Zr/GO/PR presented the typical spectra of functional groups: –OH at 3429 cm-1, C=O at 1638 cm-1 and –CH2 bending at 1420 cm-1 [20]. The absorption band around 1540 cm-1 was more than likely from the C=C stretching mode of GO and phenolic rings. The methylol stretching 5
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appeared at around 1124 cm-1 for the functionalization of GO/PR. Also, the weakened peak of the O–H deformation vibration of PR existed at 1371 cm-1, and the new peak of C–O stretching vibration for the nucleophilic substitution reaction between O-groups of PR and oxygen containing groups of GO was at 1095
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cm-1 [21]. The bands at 800 and 875 cm-1 were the characteristics of the benzene ring. Also, a twin peak at 2856 near 2923 cm−1 was responsible for the C–H symmetric and anti-symmetric stretching vibration in Fig. 3a.
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Figure 4 presents the TGA and derivative TGA (DTGA) curves (a and b, respectively) of PR, GO/PR and Zr/GO/PR composite samples under nitrogen atmosphere. The maximum temperature of decomposition
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(Tmax) of the phenolic composite occurred at about 460oC, and the maximum weight loss percent was 23%. Though the addition of these particles had no desired effect on residual mass of PR at 1000oC in TGA curves, it can even lead to a reduction in it [22]. Graphene oxide exfoliates and decomposes when rapidly heated to ~1050°C through the large pressures generated during gasification of oxygenated functional groups
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intercalated between graphite oxide layers and formation of finely dispersed amorphous carbon which exhibits a distinct weight loss [23, 24]. The DTGA graph of PR sample showed that, after the initial evaporation of residual water, the degradation of methylene linkages in phenolic resins and subsequently the
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decomposition of aromatic phenol rings took place in two steps. Incorporating these particles could further enhance the thermal stability of PR due to the incomplete oxidation of the charred material and composition
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of ZrO2 at higher temperature than 1000oC. However, the TGA patterns of the two composites (GO/PR and Zr/GO/PR) were nearly the same, indicating that at the nanofiller load, the ZrB2 had no effect on the thermal stability of the GO/PR composite [25]. Carbon fibers have been utilized as a representative reinforcement for the ablation purpose owing to their high carbon content, excellent thermal stability, low thermal expansion, low density and non- flammability. In addition, the physical–chemical interaction at the fiber–matrix interface plays an important role in 6
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improving the properties of a fiber-reinforced composite at a high content of 50 wt % [3]. On the other hand, the good distribution of a small amount of GO sheets in resin guarantees the efficient thermal resistance properties of composite. There are a large number of oxygen-containing functional groups attached on the
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basal planes and edges of GO sheets such as hydroxyl, epoxide, carboxyl, and carbonyl groups, which could allow homogeneous dispersion of GO nanosheets in water or different solvents and effectively improve interfacial interaction and compatibility between GO and polymeric matrix [21, 26]. The existence of this
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type of covalent bonds will increase the density of the filler/polymer interactions, thus improving the filler dispersion into the polymer matrix [27]. The functional groups present in the GO are very useful in making
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covalent bond linkages with phenolic backbone [28]. The results of the thermal investigation from previous studies obtained with different GO contents do not show clear differences in the thermal behavior. This is attributed to the presence of well dispersed GO sheets within the matrix which generate an increase of its thermal stability by suppressing the polymer chain mobility [22]. The composites are exposed to a high
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temperature flow in order to evaluate the effects of GO and ZrB2 on the oxidation behavior of PR. It was found that the presence of both graphene oxide sheets and ZrB2 nanoparticles influences the surface properties [29]. The front side of the samples was directly exposed to heat flux at 2000oC for 160 s. When
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the flame impacted on the sample, heat flux and oxidizing species can be attacked on the sample surface. Meanwhile, GO can reflect a large amount of heat to reduce the surface temperature of the sample and
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weaken oxyacetylene ablation. The boria product of ZrB2 overflows from the micro-cracks distributed on the surface. The overflowed boria gas decreases the partial pressure of oxidizing species such as CO2 produced by oxyacetylene torch. Afterward, B2O3 gas covers some parts of the sample surface and disperses in the gas layer between flame and the sample surface [30]. The thickness and mass loss measurements were obtained by taking the specimen height and weight difference before and after ablation test. These data were used as a performance variable to analyze the effect 7
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of processing parameters in the fabrication of the ablative materials. All samples showed that in terms of the recession rate, the PR composite exhibited higher erosion than the GO/PR and Zr/GO/PR nanocomposites. The addition of GO in PR is believed to enhance the oxidation resistance due to an enhanced thermal
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property, and the formation of char layer. Since the carbon matrix is oxygen sensitive, the composite will be largely consumed if the surface has not a good oxidation protective ability, which will lead to the high weight loss of the sample [31]. The formation of char layer can be an oxidation protective coating which
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could provide an effective protection to stop the diffusion of oxygen into the sample [32]. When char layer is not formed, or detaches from the surface of composite, there will be a significant mass loss of the sample.
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Therefore, the uniform GO network provided a protective layer, and as a consequence, the loading of GO to phenolic resin reduced the mass loss which could effectively prevent oxygen diffusing into the substrate. The 1 wt% GO in PR exhibited a lower ablation rate (61%) comparing with neat phenolic resin, which was attributed, in part, to the good thermal insulation of the carbonized GO network. The ablation rate ratio of 7
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wt% ZrB2 in GO/PR to the neat specimen was a significantly lower (90%) which should be due to the fact that reinforcing ZrB2 could convert to ZrO2 as a heat dissipater and provide an insulated surface for transferring heat [30]. The GO sheets were demonstrated to be the key factor for the one- dimensional
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growth of ZrO2 (from decomposition of ZrB2). The obtained zirconia–GO has dispersion ability in the surface due to interactions between the zirconia and GO; this composite will provide a more fire retardant
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material. Zirconium cation is a strong Lewis acid, and might be physically-chemically adsorbed by the functional groups of graphene oxide, and then zirconium cations transform into ZrO2 on the GO sheet as the temperature increases [33]. Then, the surface was completely covered by incorporation/formation of small, closely spaced, and uniformly distributed oxide. Accordingly, in the transition zone, the mechanical denudation became weakened, this coating was not consumed out, and the oxides remained at the surface which prevented the composite from ablation further. The oxidation channels could be sealed by the oxide. 8
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The decrease in the percentage mass loss was an outcome of improved thermal stability with ZrB2 and GO additives. The mass loss of GO/PR composite at 7 weight percent of ZrB2 was about 72% lower than that of PR. The ablative properties of composites at 2000oC after 160 s are presented in Table 1.
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The time-temperature plots of the front side temperatures of the samples were presented in Fig. 5a, and the backside temperatures were measured in situ during exposure to the flow in Fig. 5b. The GO/PR possessed a higher surface temperature than Zr/GO/PR in the beginning whereas they reached the same temperature after
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40 s. When the temperature reached more than 1200°C, B2O3 quickly evaporated which can be caused to decrease of temperature from the surface [34]. As illustrated in Fig. 5b, by effectively transferring heat away
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from the front of the sample, the backside temperatures were greatly reduced. The back face temperature reached 600°C after 160s for PR, but the temperature of Zr/GO/PR composite reached 90°C. In the lower temperatures, GO could still decrease the surface temperature, but in the higher temperature, fast oxidation of ZrB2 and evaporation of B2O3 made the ZrB2 addition to be a useful role for modification of ablation. It
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showed that the decrease in the back side temperature after 160 s with GO (150oC) could be due to a high interface thermal resistance and re-radiation of thermal energy from the outer surface into the atmosphere. The ZrO2 based system exhibited a lower thermal diffusivity than the GO/PR composite, it can be concluded
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that this difference was directly related to the thermal conductivity [35]. In order to understand the ablation mechanism, we compared the microstructures of the oxidized
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composites. Fig. 6a shows the surface of samples after oxidation. It can be seen that PR has visible structural damage with pitch carbon fibers, but GO/PR and Zr/GO/PR are fully covered by a char layer. Fig. 6b and c shows how the FESEM micrographs of the surface vary dramatically between GO/PR and ZrB2 reinforced GO/PR before and after ablation test, respectively. It is found that the carbon fibers were randomly and uniformly dispersed in the matrix without the clumps of fibers in PR sample. The inset in Fig. 6c, shows that the area inside PR composite is heavily oxidized. In contrast, the GO/PR and Zr/GO/PR show no signs of 9
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cracks throughout the sample. A lot of gray phase coating on the GO/PR surface can be seen, arising from GO that has appeared in the form of char which is vastly different from the Zr/GO/PR. From Fig. 6c for Zr/GO/PR, it is clearly observed that the surface has been covered by the char and ZrO2 with white color.
of ZrB2. It indicated that the oxide layer contained no boron.
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Electron dispersive spectroscopy (EDS) was performed on the oxide layer in order to examine the amounts
As a result, the ablation rate of UHTCs modified GO/PR composite was slower than GO/PR composite. In
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another word, ablation sensitivity versus heat flux of GO/PR with low ceramic nanoparticle content was lower than pure composite and GO/PR. The GO became a physical shield decreasing external heat flux
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changing by the transmission of heat from thermal conduction to radiation transfer. Surprisingly, incorporation of both GO and ZrB2 nanoparticles strongly influenced on the reduction of linear ablation rate. 4.
Conclusions
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Carbon-phenolic composites fabricated with zirconium diboride and graphene oxide can produce a protective layer on the surface, leading to reduced back face temperature when exposed to high temperature. The high heat dissipation of the GO/phenolic composite decreased the intensity of heat near the front
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surface. In addition, the presence of ZrB2 with formation ZrO2 grains in phenolic composite has decreased the ablation rate, and thus delayed the degradation process of GO/PR composite. The linear and mass
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ablation rates of Zr/GO/PR composites compared with PR decreased by 90% and 72%, respectively. Acknowledgments
The authors are grateful to the Space Transportation Research Institute for financial support.
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Figure captions: Fig. 1. XRD pattern of the ZrB2 nanoparticles pyrolyzed at 1500oC.
Fig. 3. FT-IR spectra of the phenolic composites after curing in 150°C. Fig. 4. (a) TGA and (b) DTG curves of the phenolic samples.
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Fig. 2. (a) TEM (b) FESEM images of ZrB2 prepared with using Schiff base.
Fig. 5. The temperatures of (a) surface center and (b) back surface as a function of ablation time during test.
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Fig. 6. FESEM images of the composites: (a) macro-morphologies after oxyacetylene torch test, (b)
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microstructure of composites before ablation test and (c) the ablated surfaces.
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Table 1. Ablative properties of three composites at 2000oC after 160 s.
Composite
Ablation depth
Linear rate (mm/s)
Mass loss (wt%)
1.55
0.00970
GO/PR
0.61
0.00380
Zr/GO/PR
0.15
0.00094
31.70
15.68 8.84
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Fig. 1. XRD pattern of the ZrB2 nanoparticles pyrolyzed at 1500oC.
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Fig. 2. (a) TEM (b) FESEM images of ZrB2 prepared with using Schiff base.
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Fig. 3. FT-IR spectra of the phenolic composites after curing in 150°C.
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Fig. 4. (a) TGA and (b) DTG curves of the phenolic samples.
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Fig. 5. The temperatures of (a) surface center and (b) back surface as a function of ablation time during test.
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Fig. 6. FESEM images of the composites: (a) macro-morphologies after oxyacetylene torch test, (b) microstructure of composites before ablation test and (c) the ablated surfaces.
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