Electrorheological effect of carbonaceous materials with hierarchical porous structures

https://doi.org/10.1016/j.colsurfa.2009.02.036Get rights and content

Abstract

Carbonaceous materials with different hierarchical porous structures for electrorheological (ER) dispersed phase have been synthesized by carbonization of as-prepared starch/silica precurser at different temperatures. The N2 adsorption isotherms show that Cmeso-700 and Cmeso-500 particles have the BET surface areas of 1028 and 603 m2 g−1, respectively. They both have the mesoporous pores with size of about 4.6 nm and the microporous pores (1.1 and 1.5 nm, respectively). The BET surface areas and C/O atomic ratio of porous carbon materials can be increased with the carbonization temperatures. The rheological measurements indicate that the Cmeso-700 and Cmeso-500 ERF have the better ER effect resulted from their hierarchical porous structures. The shear stress of Cmeso-700 ERF is 900 Pa at 1000 s−1 under 3 kV mm−1, which is almost 4.5 times larger than that of Cmicro-350 ERF. The mesoporous carbon ERFs also show the better sedimentation stability than microporous carbon ERFs. The different ER effect of carbonaceous particles may derive from their different dielectric polarization property induced by the hierarchical porous structures.

Introduction

Electrorheological fluid (ERF) is a kind of novel intelligent soft matter as a two-phase suspension system formed by solid particles dispersing in the insulating medium oil. It can transform between a liquid-like state and a solid-like state under the external electric fields within several milliseconds and can be used in various industries [1], [2], [3]. Many kinds of organic or inorganic particles and their composites [4], [5], [6] have been used as the ER materials. In recent years, more attention have been paid on particles possessing of some specific structures, for the surface and interface polarization associated to the ER effects have been promoted by these structures. Zhao and Yin [7] developed the rare-earth-doped titania ER materials according to the dielectric design and found the distinct enhancement in their yield stress due to defect and impurities of TiO2 doping with different valent Cr ions. Furthermore, the mesoporous rare-earth-doped TiO2 particle [8] have been proofed to have high ER activity in which both the active internal structure and interface or surface structure merit the pursuit of suitable dielectric and conduction properties of the particles. To improve the dielectric response of particles, new kind particles with core-shell structures were introduced and obtained a lot of success [9], [10]. The nano-microscale complex structures [11] were also developed to improve the wettability and dielectric property of nanopapilla particles which was important to their ER effects. It is found that the mesoporous structures within the TiO2 particles can improve the surface polarization ability and the anti-sedimentation capability, which can lead to a higher ER effect [8]. In addition, carbonaceous materials have shown some improvements in ER fields [12], [13], [14], [15], but these conventional materials only have the microporous structures regardless of their originals and preparation. The correlations between ER activities and the hierarchical porous structures, such as micro/meso or meso/macro-hierarchical porous structures are few or no reported. It needs further understanding to realize the effect of the various porosity. By dint of carbonization of as-prepared starch/silica composites, the micro/meso-hierarchical porous structures carbonaceous materials were synthesized and used to reveal the relationship between ER effect and the intrinsic structures of the particles.

Porous carbon materials have received a great deal of attention due to their many applications on gas separation, water purification, catalyst supports, and electrodes for electrochemical double layer capacitors and fuel cells, etc. [16]. Many kinds of traditional rigid and designed inorganic templates were applied to prepare the porous carbon materials possess of different intrinsic structures [17], [18], [19], [20], [21], [22]. Recently, more attention has been taken on the carbonaceous materials with controllable pore size and hierarchical porous structures, and it is found that the different porous texture exhibit the specific functions and thus the performance of materials are improved [23], [24].

Starch is the only significant crystalline intracellular biopolymer in the living cell. It is also a dominating carbohydrate polymer on earth and its abundance and quality makes starch interesting for a number of food and material applications [25], [26]. The major polysaccharides of starch granules are amylopectin and amylose linked by two types of bonds: α-1, 4 and α-1, 6 glucosidic linkages. Amylopectin (typically ∼75%) is a semi-crystalline highly branched polysaccharide with an α-1, 4 glucosidic backbone and 4–5% α-1, 6 branch points while amylose (typically ∼25%) is amorphous in the native starch granule and is composed of essentially linear chains of α-1, 4 linked glucose units [27], [28]. When heated in the boiling water, the amorphous section of the starch granule swells and expands due to absorbing water. The highly expanded starch form a semi-transparent paste and the original structures in the granule are opened up. After that, the low temperature makes the paste gelatization and recrystalization (retrogradation) to form new structures different from original.

Starch is also a traditional ER material mainly due to its hydroxy groups and adsorped water which introduced some limitations in applications. Zhao et al. [29] developed a new type of organic/inorganic hybrid colloid to overcome the shortage, which were made of modified carboxylmethyl starch/titanium oxide and/or surfactant modified polysaccharide/titanium oxide, and they can effectively combine the advantages of the two components and obtained better ER effects.

Herein, the carbonaceous particles with hierarchical porous structures possess of both mesoporous and microporous pore for ER materials were prepared with the template assistance. The environment-friendly and easily decomposing native starch was used as the carbon source and the porous carbonaceous materials were obtained by carbonizing under nitrogen atmosphere. The carbonaceous materials exhibit the difference in their specific surface areas (SBET), intrinsic porous structures (mesoporous and microporous) and C/O atomic ratio resulted from their different treatment temperatures, and these can affect the ER property of the particles.

Section snippets

Preparation of the porous carbon materials

The experiment process used the native starch (purchased as eatable food grade) as the carbon source, the silica sol as the template, the deionized water as the dispersing phase, the anhydrous ethanol as the dehydrate reagent and the KOH as the template remover. All chemical reagents used were analytical purity as reagents which were received without further purification. In this experiment, the silica sol is used as the template which can provide silica particles to form the mesoporous

Porous characterization

Fig. 2 and Table 1 show the pore property of carbonaceous particles. The isotherms of Cmeso-700 and Cmeso-500 show the featured curves suggesting a mesopore size distribution at the mean value of about 4.6 nm (DBJH, Fig. 2(a)). The mesoporous structures are introduced by the silica and have the similar value for two materials treated under different temperatures (500 and 700 °C). However, the BET surface areas (SBET) of the two materials show the increasing trend with the temperatures, which are

Conclusions

In this paper, the carbonaceous particles with hierarchical porous structures for ER materials have been synthesized by carbonization of template assistant starch/silica particles at different temperatures. The self-assembling of the amylopectin and amylase structures within the starch granule and the rigid template (silica) assist the formation of hierarchical porous structures in the carbonaceous particles.

  • (1)

    The N2 adsorption isotherms show that Cmeso-700 and Cmeso-500 have both the mesoporous

Acknowledgements

This work is supported by the National Natural Science Foundation of China (60778042) and NPU Foundation for Fundamental Research (No. WO18101).

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