J. Chem. Chem. Eng. 6 (2012) 1056-1060
干冰怎么制作Change in the Pore Structure of Carbon-Carbon Composites during Successive Stages of High-Pressure Impregnation and Heat Treatment
Ekaterina V. Kogan1*, Yury M. Volfkovich2, Artem P. Malakho1, Valery V. Kulakov3, Anatoly M. Kenigfest3 and Valentin E. Sonkin2
1. Chemistry Department, Moscow State University, Moscow 119991, Russia
2. Frumkin Institute of Physical Chemistry and Electrochemistry, Russian Academy of Science, Moscow 119071, Russia
3. OAO Rubin Aircraft Corporation, Balashikha 143900, Russia
Received: November 01, 2012 / Accepted: November 20, 2012 / Published: December 25, 2012.
Abstract: Pore structure of C/C (Carbon-Carbon) composite after veral stages of pitch impregnation under the high pressure and heat treatment was investigated by means of low temperature nitrogen adsorption and the standard contact porosimetry. Total pore volume, pore size distribution and specific
surface area were calculated for samples of composite after veral successive stages of treatment. The radius of pores prented in the material changes from 1 nm to 90 µm. Total pore volume and specific surface area both decrea after successive stages of pitch impregnation under the pressure, whereas heat treatment up to 1,750 °C and 2,000 °C leads to creation of some porous space and pore volume expansion. The bulk porosity of C/C composite comes down from 33.7% to 13.7% after the rial stages of treatment and the specific surface area is reduced by half compared to the initial material.
Key words: Carbon-carbon composite, pore structure, total pore volume, specific surface area, high-pressure impregnation, heat treatment.
1. Introduction
C/C (carbon-carbon) composites are widely ud as brake materials in the aerospace industry. Compared to traditional metal-bad friction materials C/C composites have many advantages, such as low density and large heat capacity combined with strength retention at elevated temperature and low coefficient of thermal expansion [1].
C/C composite consists of carbon matrix reinforced with carbon fibers having random or oriented arr
angement. The manufacture of the composite usually starts with formation of fibrous reinforcement and subquent impregnation of it by matrix material.
*Corresponding author: Ekaterina V. Kogan, Ph.D., Rearch Scientist, rearch fields: adsorption, adsorbents, carbon materials, pore structure measurement. E-mail: *****************.msu.ru.The impregnation can be carried out with u of CVI (chemical vapor infiltration) method or liquid impregnation with subquent heat treatment (HIPIC). Liquid impregnation by pitches or organic resins usually performed in veral stages, aiming to obtain maximum density and minimum porosity of the material.
Porosity is important characteristic of C/C composite for brakes. Cavities of various sizes are located both in a carbon matrix and at the interface fiber-matrix. Obviously, significant volumes of voids adverly affect the strength and wear resistance of friction material and deteriorate heat capacity and thermal conductivity. There are some studies focud on the problem of friction material porosity. Wang et al.
[2] investigated influence of the pore structure of C/C
composite on the Si infiltration and mechanical
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properties of C/C-SiC friction material.
什么是住家保姆In previous work, the authors have shown the prence of meso- and macropores with radius from 1 nm to 100 µm in C/C friction composite [3]. The prent rearch is devoted to the changes in C/C composite pore structure during the successive stages of pitch impregnation under the high pressure and subquent heat treatment. The variations of total pore volume, pore size distribution and specifi
c surface area under the process were investigated and the influence of the particular production process on the characteristics was discusd.
2. Materials and Methods
C/C composite perform with pitch-bad matrix containing of 40% of carbon fiber was annealed at 800-1,000 °C (carbonization) and subquently compacted by means of veral stages of liquid pitch impregnation with subquent heat treatment under the pressure of 40 MPa. After the first stage of HIPIC process, the sample of composite was heated up to 1,750 °C. Then two other stages of pitch impregnation under the pressure were carried out. After the third stage of pitch impregnation, the composite was subjected to a final heat treatment at 2,000 °C. The pressure and temperature conditions for samples treatment were similar with conditions in the industrial process of C/C composite production.
Samples of composite after each stage of pitch impregnation (HIPIC) and HT (heat treatment) were investigated with low temperature nitrogen adsorption and the standard contact porosimetry [4]. The isotherms of nitrogen adsorption at 77 K were measured using standard volumetric method on the Sorptomatic 1990 (Thermo Electron Corporation). On the ba of the isotherms, specific surface area (BET) was calculated for the samples of composite after each stage of treatment.
Integral and differential pore size distribution curves were calculated by means of the standard contact porosimetry for all the samples of composite. For measurements, octane was ud as an impregnated substance becau this compound provides good wettability for most of the surfaces. The total pore volume was also obtained with the standard contact porosimetry.
3. Results and Discussion
Values of surface area and total pore volume for samples taken after successive stages of pitch impregnation and heat treatment are shown in Fig. 1. Pitch impregnation under the pressure results in reduction of total porosity and decrea of specific surface area of C/C composite (stage 2, 4 and 5 in Fig.
1). The largest decrea of the total pore volume was obrved after the first stage of pitch impregnation. It was more than 50% compared to initial material. The cond and third stages of pitch impregnation under the pressure result in a smaller reduction in total porosity compared to the previous stage of impregnation. The value of specific surface area also shows reduction after each stage of pitch impregnation under the pressure.
The heat treatment after HIPIC process has an opposite effect on the characteristics. After the tre
atment at 1,750 °C (stage 3 in Fig. 1) next to the first impregnation stage as well as after the final heat treatment at 2,000 °C (stage 6 in Fig. 1), the total pore volume noticeable increas. Specific surface area shows the similar behavior. On the whole specific surface area is reduced by half from 1.8 m2/g for the initial composite to 0.9 m2/g after the final heat treatment.
Heat treatment is ud to obtain more den, strength and resistant C/C material. However, this process is accompanied by relea of volatile components of the pitch leading to the development of the porous structure and creation of additional porosity.This is confirmed by the growth of total pore volume and specific surface area in the samples of composite. Using the method of standard contact porosimetry with octane as an impregnation substance integral and
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Fig. 1 Specific surface area and total pore volume for C/C composite after veral stages of treatment. Number of stages: 1: initial material after carbonization; 2: 1st HIPIC; 3: HT at 1,750 °C; 4: 2nd HIPIC; 5: 3rd HIPIC; 6: inal HT at 2,000 °C.
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Fig. 2 Pore size distribution for initial carbon-carbon composite (1: integral curve, 2: differential curve).
differential PSD (pore size distribution) curves for the samples of composite after each stage of treatment were obtained. Fig. 2 shows the PSD curves for the sample of the initial composite after carbonization at
800-1,000 °C. This material has low bulk density (nearly 1.29 g/cm 3) and the total pore volume is about 0.34 cm 3/cm 3. The most of the pore space accounts for pore size from 100 nm to 90 µm.
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明朝那些事儿是正史吗
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Fig. 3 Volume of different groups of pore in C/C composite after successive stages of treatment.
Histogram with analysis results of integral pore size distribution curves for samples under study is prented in Fig. 3. The modification of the volume for different groups of pore in carbon-carbon composite after successive impregnation/heat treatment cycles is illustrated.
The decrea of volume for large pores from 10 µm to 90 µm after each stage of treatment is obrved, including the thermal treatment. For the smaller pores with radius from 20 nm to 10 µm heat treatment leads to the significant increa of volume. Large pores allow easier delivery of the pitch during the high-pressure impregnation so their volume gradually decreas under the process. The remove of volatile components during the heat treatment results in creation of porous space, but the size or the pores is less than 10 µm as one may e in Fig. 3.
The total volume of the smallest pores (in the range from 1 nm to 20 nm) is less than 0.02 cm 3/cm 3 for the samples after all stages of treatment. Pores with radius from 1 nm to 25 nm are mesopores according to the IUPAC classification [5]. As the primary size of pores
in the C/C composite is more than 20 nm, it relates in the main to the macropore range. The first and the cond stages of high-pressure pitch impregnation are accompanied by increa of mesopores volume. This may be explained by the filling of larger pores with appropriate decrea of their size. The heat treatment at 1,750 °C in contrary leads to the reduction of mesopores volume becau of the removal of some volatile materials and the growth of pore size. According to measurements, the C/C composite after the final heat treatment at 2,000 °C has total pore volume nearly 0.14 cm 3/cm 3 and bulk density of about 1.82 g/cm 3.
4. Conclusions
Serial stages of high-pressure pitch impregnation and heat treatment result in substantial changes in pore structure of carbon-carbon composite. The decrea of total pore volume and specific surface area is obrved after each impregnation stage, whereas the heat treatment at 1,750 °C and 2,000 °C leads to the growth of the characteristics. The primary pores in C/C
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composite are macropores with the radius up to 90 microns. The increa of the total pore volume under the heat treatment is due to the pores with radius from 20 nm to 10 µm, whereas the volume of pores more than 10 µm decreas also after the heat treatment. During all successive stages of treatment, total volume of pore in the carbon-carbon composite decread from 0.34 cm3/cm3 to 0.14 cm3/cm3 and specific surface area reduced by half from 1.8 m2/g to 0.9 m2/g.
Acknowledgments
The work was supported by the Government of the Russian Federation Ministry of Education and Science under an integrated project implementation agreement in conformity with the RF Government Decree No. 218 (April 9, 2010): On Measures for State Support to the Development of Cooperation between RF Higher Education Institutions and Organizations That Implement High-Technology Development Integrated Projects. Contract No. 13.G25.31.0072. References
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