Processing and characterization of syntactic carbon foams containing hollow carbon microspheres

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Processing and characterization of syntactic carbon foams containing hollow carbon microspheres Liying Zhang*, J. Ma School of Materials Science and Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singapore

A R T I C L E I N F O

A B S T R A C T

Article history:

The effects of heat-treatment on the properties of carbon foams were studied. The carbon

Received 26 June 2008

foam was first prepared by adding hollow carbon microspheres to phenolic resin, followed

Accepted 24 January 2009

by post-curing, pre-carbonization and carbonization. The mechanisms of failure behaviour

Available online 3 February 2009

and the increase of electrical and thermal conductivities showed that the properties of the foams were influenced by the heat-treatment temperature. Results showed that the introduction of more interval voids during carbonization resulting in a reduction of the mechanical properties. Carbon foams with electrical conductivity of 1.20 S/cm and thermal conductivity of 12.85 W/mK were obtained.  2009 Elsevier Ltd. All rights reserved.

1.

Introduction

Scientists and engineers have paid a lot of attention to carbon foam development due to their advantageous properties. Carbon foams were first reported by Walter Ford in the late 1960s [1]. In the initial preparation method, carbon foam was obtained by the pyrolysis of a thermosetting polymer, which was known as reticulated vitreous carbon (RVC) foam. Since then, carbon foam has been developed rapidly due to a few attractive properties, such as thermal insulation, impact absorption and gas filtration. In the late of last century, scientists focused on the production of carbon foams. Researchers from the US Air Force Materials Laboratory developed a pitchderived foam by applying a ‘blowing technique’ [2]. Coalbased foam with excellent thermal insulation and high strength properties was developed by a research group from West Virginia University [3]. More recently, the mechanical and functional properties of carbon foams have been studying. Bruneton et al. [4] investigated the mechanical behaviour of a carbon foam at very high temperature in relation with its microstructure. Bunning et al. [5] studied a coupling of thermal and mechanical properties of polyurethane-infiltrated carbon foam of various densities.

Wang et al. [6] reported that a novel carbon foam was prepared by thermal treating of coal tar based mesophase pitch and reinforced with clay-montmorillonite. The mechanical properties were improved and its thermal conductivity was markedly decreased from 2 W/mK to 0.25 W/mK. Due to their advantageous properties, the application of carbon foams was also attracting. Carbon foams prepared from mesophase pitch are reported to be light weight and adjustable to thermal and electrical conductivities [7–9]. The primary applications in relation to carbon foam properties are heat sink [7], power electronics cooling [10], aircraft brake pads [11]. Syntactic carbon foams are relatively new material systems in the family of carbon composites. These foams are largely derived from their controlled microstructure, where precise porosity and pore size distribution can be engineered. Fabrication of these foams adopts the pore forming approach, and the process is followed by carbonization in order to obtain the desired functional properties. The pore formation of porous materials is commonly achieved by the sacrifice route where the introduced pore formers are burnt away in the process. In this work, we will use hollow carbon microspheres as the additive to generate porosity. This method has two

* Corresponding author: Fax: +65 67909081. E-mail address: [email protected] (L. Zhang). 0008-6223/$ - see front matter  2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2009.01.037

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advantages. Firstly, the pores introduced are very stable and hence the resultant porosity and pore size distribution can be controlled. Secondly, the shell of the hollow spheres provides a strengthening effect during fracture. For carbon composites, the heat-treatment during carbonization is critical. In this paper, we report the effects of heat-treatment on foam properties during carbonization.

2.

Experiment

2.1.

Raw materials

Two basic materials, hollow carbon microsphere and a binder matrix material, were used to prepare the present carbon foam. A high carbon yield phenolic resin (International laboratory, USA) was chosen as the matrix precursor. First, hollow carbon microspheres were produced from hollow phenolic microspheres (BJO-093, Asia pacific/Eastech). The raw material was heated at a rate of 5 C per minute and dwelt at a 900 C for 3 h in an argon atmosphere. The specimens were then cooled to room temperature. The elemental composition for carbon microspheres formed is shown in Table 1.

2.2.

Preparation of carbon foam

The carbon microspheres were added to the phenolic resin while slowly stirring the mixture to minimize gas bubbles in the resin. In order to avoid agglomeration, the carbon microspheres were added in multiple steps to the phenolic resin. After dispersion, the syntactic foam was molded using an aluminum mold coated with a silicone release agent. The syntactic foam was then left under a constant pressure of 2.0 MPa for 24 h to cure at room temperature. Heat-treatment process plays a key role in forming the carbon foam. In this study, the syntactic foam was post-cured in a convection oven with circulated air (heated to a temperature of 230 C) for a period of 32 h. The specimens were then cooled to room temperature at 3 C/min. After post-curing, the samples were pre-carbonized in two steps. Firstly, the post-cured samples were heated to 400 C and dwelt for 3 h in an argon atmosphere and then cooled to room temperature at 3 C/min. Secondly, the samples were heated to 600 C and dwelt for 3 h in an argon atmosphere and cooled to room temperature at 3 C/min. After pre-carbonization, the specimens were heated at 900 C under a continuous purge of argon. The heating rate was maintained at 0.5 C /min in order to minimize the formation of shrinkage, cracks, and slit pores, which could be caused by thermal expansion mismatch between hollow carbon microspheres and phenolic resin matrix. Fig. 1 illustrates the process. The samples were heated at 25, 230, 400, 600, and 900 C in order to study the effects of various heating temperatures on them. These samples were labeled as C25, C230, C400, C600 and C900.

Hollow carbon microspheres

Phenolic resin

Curing at 25°C

Post-curing (230°C)

(dwelt for 32 hours)

Pre-carbonization 1 (400°C)

(dwelt for 3 hours)

Pre-carbonization 2 (600°C)

(dwelt for 3 hours)

Carbonization (900°C) Fig. 1 – Flowchart showing the processing of carbon foam.

2.3.

Mechanical tests

Two kinds of mechanical tests were performed. For the flexural tests, carbon foams were machined to specimens in dimensions of 127.0 · 12.7 · 3.0 mm3. The tests were performed by an Instron Tester (Model 5567). The strain rate was maintained at 0.01/min. The cross-head speed, z, was calculated by R  S2 ; ð1Þ 6d where R is the strain rate, S is the span of the support, which was chosen to be 48 mm, and d is the depth of the sample. All the results were calculated based on the average of five tests. The equation of the cross-head speed was recommended according to ASTM Standard D790-07. For the compression tests, the specimens were machined to blocks of 25 · 25 · 12 mm3 according to ASTM Standard C365/C 365M – 05. The tests were carried out at room temperature using an Instron Tester (Model 4206), which has a maximum capacity of 100 KN. The cross-head speed applied was 0.5 mm/min. The compressive yield strength rc was calculated by P ð2Þ rc ¼ ; A where rc is the compressive yield strength, P is the load at yield, and A is the cross-sectional area. All the results were calculated based on the average of five tests. The equation of compression modulus was based on ASTM Standard C365/C 365M – 05. z¼

2.4.

Microstructural characterization

The microstructure of the carbon foam was studied by a Jeol JSM 6360 scanning electron microscope (SEM). Table 1 – Carbon and hydrogen elemental analysis result. Element Weight percentage

C%

H%

81.21

1.256

2.5.

Electrical measurements

The electrical conductivity of specimens was measured using a four probe technique. The conductivity was measured on

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both sides of each specimen and the two measured values were then averaged. All the results were also calculated based on the average of five tests.

2.6.

Thermal conductivity measurements

The laser flash (Apparatus LFA 427) method was used to measure the thermal diffusivity of the carbon foam. Circular samples, with a nominal diameter of 12 mm, were used for testing. The disks were sprayed with graphite to obtain uniform heat distribution on the surface. The thermal conductivity (K) of each sample was calculated using the following standard equation: K ¼ d  Df  Cp ;

ð3Þ

where d is the density of carbon foam, Df is its thermal diffusivity and Cp is the specific heat capacity. The specific heat capacity of each sample was measured using the differential scanning calorimeter (Netzsch STA 449C).

3.

Results and discussion

3.1.

Shrinkage and weight loss

Fig. 2 shows the volume shrinkage and weight loss of a typical sample under different heat-treatment stages. The earlier heat-treatment of hollow carbon microspheres under different temperatures caused the properties of the composites to be dominated by phenolic resin. During pyrolysis from room temperature to 900 C, the composites were converted with both loss of weight and volume shrinkage into a black carbonaceous mass. The weight loss took place in two steps, while the volume shrinkage was achieved in one step. In the first step, the weight loss took place during the post-curing stage and was attributed to the phenolic resin. This process promoted the cross-linking and condensation reactions and led to the formation of long-chain, cross-linked polymeric structures in the matrix [12]. At the end of this stage, the matrix was still polymeric. In the first pre-carbonization stage between 230 and 400 C, the weight loss and volume shrinkage of the composites changed slightly. More weight loss and greater volume shrinkage occurred during the second pre-carbonization between 400 and 600 C. This was attributed to the

35

100

Weight (%) Shrinkage (%)

30

90

Weight (%)

20 15

70

10

Shrinkage (%)

25 80

60 5 50

0

C25

C230

C400

C600

C900

Fig. 2 – Typical sample volume shrinkage (%) and weight loss (%) during heating treatment.

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loss of volatile components and other organic compounds. In the second pre-carbonization stage, the matrix was converted to carbon. At the end of the pre-carbonization stage, the carbon to hydrogen ratio was 2:1. The remaining hydrogen was successively removed in the following carbonization stage, which was accompanied by the slight change of weight loss and volume shrinkage. After carbonization, the linearly conjugated carbon domains were interlinked resulting in a continuous turbostratic carbon structure [13]. The volume shrinkage of the composites was approximately 34% and was accompanied by a weight loss of 48%, which resulted in the low density (0.89 g/cm3) of carbon foam. The relationship between properties and heat-treatment stage will be discussed later.

3.2.

Microstructure of the carbon foam

Fig. 3 shows the microstructure of the syntactic carbon foam (C900). The foam contained three phases: the hollow microspheres, the carbon matrix, and the internal voids. It could be seen that hollow carbon microspheres remained substantially unbroken while internal voids were formed in the composites as a result of releasing some volatile components and other organic compounds during carbonization. Fig. 3 also shows that although the matrix contained a high carbon hollow microsphere content, very little hollow carbon microsphere connected with each other. The hollow microspheres were well dispersed in the carbonized phenolic resin binder and the matrix formed a good interconnected network in the composite.

3.3.

Effects of temperature on electrical conductivity

The electrical conductivity was given by the following classical expression, r¼

1 I ¼ ; R 2pS  V

ð4Þ

where r is the electrical conductivity (S/cm1), R is the resistivity (X cm), S is the probe spacing (cm), V is the voltage (V) and I is the current (A). Table 2 presents the electrical conductivity of different samples measured at room temperature. It was observed that the electrical conductivity remained constant from room temperature to 600 C. The electrical conductivity was determined by the electrical network within the matrix. Since phenolic resin is an insulator material, the electrical conductivity was observed to be low for the C25 to C600 samples. Low carbon content in the matrix could be the cause of the incomplete carbonization. After sufficient carbonization, an increase in electrical conductivity by approximately seven orders of magnitude was observed (sample C900). In the pre-carbonization stage, although the matrix could be partially converted to carbon, the carbon content did not form a good interconnected network. Chhowalla et al. [14] suggested that the sp2 carbon structures in the carbon materials predominantly promoted the electronic and transport properties. Fig. 4 shows a typical spectrum, characterized by two main peaks centred at 1350 and 1587 cm1, respectively. It was observed that both sp3 and sp2 scattering of C900 were much stronger than those of C600. In Fig. 4, the line (B), which

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Fig. 3 – Microstructure of the syntactic carbon foam (C900).

Table 2 – Electrical conductivity and resistivity at room temperature for different samples. Sample

Room temperature resistivity (X cm)

Room temperature conductivity (S cm1)

7.54 · 106 7.43 · 106 7.26 · 106 7.37 · 106 8.32 · 101

C25 C230 C400 C600 C900

1.33 · 107 1.35 · 107 1.38 · 107 1.36 · 107 1.20

1550 1587

1350

C600 C900

Raman Intensity

1500 1450 1400 1350

(A)

1300

(B)

1250 1200 1000

1250

1500

1750

2000

-1

Wavenumber (cm )

Fig. 4 – Typical Raman spectrum of C600 (B) and C900 (A).

corresponds to sp3-rich carbon, C600 was compared to the line (A), which corresponds to an increased sp2-banded carbon, C900; it was noted that the higher sp2 content caused the improvement of electrical conductivity. The electrical conductivity increased from 1.33 · 107 to 1.20, suggesting

that the growth of sp2 carbon structures allowed conduction between these regions.

3.4.

Effects of temperature on mechanical properties

Fig. 5 shows the flexure and compressive strengths of the carbon foam. It was observed that flexure strength increased after the post-curing stage. This could be due to the effect that long-chain, good cross-linking polymeric structures had formed in the matrix. In the pre-carbonization stage, the flexure strength decreased sharply between 230 and 400 C as a result of the decomposition of phenolic resin. Above 400 C, the flexure strength decreased slightly and kept constant between 600 and 900 C. Since the phenolic resin above 600 C was converted to glassy carbon, the corresponding composites showed matrix-dominated brittle fracture behaviour. Luxmoore and Owen [15] suggested that a crack will initiate from an oversized void when a composite is subjected to loading and the failure of the foam is attributed to the failure of the resin matrix, which in turn is attributed to the reduction in strength after carbonization. The introduction of more interval voids during carbonization formed more air spaces. These air spaces occupied a large volume of the composites, and this caused the overall strength of the whole structure to weaken, thus reducing their mechanical properties. Similar trends in compressive tests were also observed in Fig. 5. The typical stress–strain curve of compression tests of syntactic foam was divided into three distinct regions: elastic deformation, densification region and densification completed [16]. Interestingly, in this study, the stress–strain curves of C25 and C230 also showed three similar regions. Fig. 6 illustrates the compression stress–strain curve of C230 which is very similar to that of the typical compression curve. That means the polymeric matrix was still in the composites after the post-curing stage. Region 1 shows linear trend corresponds to the elastic behaviour of the foam. This region ends when

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70

Flexure Strength (MPa)

Flexure Strength Compressive Strength

80 70

50

60 40

50

30

40 30

20

20 10

10 0

0

Compressive Strength (MPa)

90 60

-10 C25

C230

C400

C600

C900

Fig. 5 – Flexure strength and compressive strength for C25, C230, C400, C600 and C900.

Compressive Strength (MPa)

120 100

3

80

2

60 40

1

20 0 0.0

0.1

0.2

0.3

0.4

0.5

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pression stress–strain curve of C600. Only the elastic deformation region (region 1) is exhibited in this curve as a result of the forming of the carbon matrix. At the end of this region, the material is considered to be failure as the matrix has been crushed.

3.5.

Effects of temperature on the thermal properties

Fig. 8A shows the specific heat capacity and thermal diffusitivity of different samples. It could be seen that both the specific heat capacity and thermal diffusitivity were low and only increased slightly between room temperature and 230 C. As discussed earlier, this was attributed to the insulating property of phenolic resin. A sharp increase in Cp occurred between 400 and 900 C because of the forming of the carbon matrix. For thermal diffusitivity, a sharp increase occurred between 400 and 600 C also as a result of significant conversion of the carbon matrix. However, beyond 600 C, the thermal diffusitivity kept relatively constant. Fig. 8B shows the thermal conductivity for different samples. The thermal conductivity of the composites was dominated by three factors including the density, specific heat capacity, and thermal diffusivity. As the density of the five specimens was similar, the thermal conductivity was mainly influenced by specific heat capacity and thermal diffusitivity. It could be seen that the thermal conductivity increased greatly after carbonization. Thus, the high thermal conductivity was attributed to the highly ordered carbon structure as commented earlier.

0.6

Engineering Strain (mm/mm)

Fig. 6 – Compression stress–strain curve of C230.

A Heat capacity Cp ( J/g.K)

0.030

Heat capacity Cp Thermal diffusitivity Df

5

0.025

4

0.020

3

0.015

2

0.010 0.005

1

C25

C230

C400

C600

C900

C25

C230

C400

C600

C900

2

0.000 0

B Thermal conductivity (w/(m.k))

3.5

Compressive Strength (MPa)

6

Thermal diffusitivity Df (cm /s)

the syntactic foam reaches its compressive yield strength. At the end of the region 1, yielding and a slight decrease in strength occur, which is the characteristic of region 2. This region corresponds to the implosion of the hollow carbon microsphere. When a large number of microspheres get crushed and compacted, further increase in the load results in the densification of the foam and is visible as the region 3 in the curve. However, above 230 C, the stress–strain curve of compressive test becomes different. Fig. 7 shows the com-

3.0 2.5 2.0

1

1.5 1.0 0.5 0.0

14 12 10 8 6 4 2 0

-0.5 0.000

0.003

0.006

0.009

0.012

Strain (mm/mm)

Fig. 7 – Compression stress–strain curve of C600.

Fig. 8 – (A) Specific heat capacity and thermal diffusitivity for C25, C230, C400, C600 and C900. (B) Thermal conductivity for C25, C230, C400, C600 and C900.

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4.

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Conclusions

This work reports the fabrication and characterization of carbon foam as a potential functional and structural material for advanced applications. The carbon foam is prepared by thermal treatment of hollow carbon microsphere composites. The process is followed by post-curing, pre-carbonization and carbonization. SEM shows the microstructure of the syntactic carbon foam (C900). The foam contains three phases: the hollow microspheres, the carbon matrix, and the internal voids. The introduction of more interval voids during carbonization will occupy a large volume of the composites and result in weakening the overall strength of the whole structure. As a result, the mechanical properties will be improved after the post-curing stage and decreased after further heat-treatment. The functional properties can be improved by carbonization. The electrical and thermal conductivities of the foam composites are increased to 1.20 S/cm and 12.85 W/mK, respectively.

Acknowledgements The authors would like to acknowledge the funding by Asian Office of Aerospace Research and Development (AOARD074058) and the support from Temasek Laboratory @ Nanyang Technological University on the present work.

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