Expandable graphite modification by boric acid
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Electrochemical oxidation of graphite in mixed solutions of H2SO4–H3BO3 with various mass ratios was investigated. The potential correction to concentration in the formation of graphite intercalation compound II stage in the system graphite–H2SO4–H3BO3 was determined and compared with other systems. Boric acid was shown not to be co-intercalated with sulfuric acid into graphite matrix, but to be distributed on the surface of expandable graphite (EXP). The amount of boric acid on EXP depends on concentration of H3BO3 in electrolyte and it ranges from 3.7 to 11.0 wt%. Content of boric oxide formed after thermoshocking is equal to 3–9 wt% in exfoliated graphite (EG). Modification resulted in reducing specific surface area of EG. As the pores in modified EG were blocked by boric oxide, the temperature of oxidation of the EG and graphite foil increased by 200 °C.
I. INTRODUCTION
For many years, graphite has remained in the center of scientists’ attention in various research directions. Bright prospects opened up with the synthesis of graphite intercalation compounds (GIC). The results of fundamental studies in this field formed the basis for the development of materials possessing new exploitation properties. GICs with strong acids (H2SO4, HNO3) are of great practical importance as the precursors of synthesizing a whole number of carbon materials, namely, expandable graphite (EXP), exfoliated graphite (EG), and graphite foil (GF). Traditional production technique of these materials consists of several steps: intercalation of strong acids into graphite, further hydrolysis for synthesizing EXP, production of EG via thermal shocking of EXP, and pressing EG without binder to obtain GF. Recently, preparation of EG by another method—ultrasound irradiation—was reported.1 EXP foams, when heated, increase its volume hundreds of times; therefore, it is used in fire protection materials.2 EG is mainly applied to prepare GF. EG’s unique properties, such as low bulk density and high specific surface area, make it possible to use EG as a highly effective sorbent.3 Flexible GF is widely used to produce different types of sealing materials,4 shields for electromagnetic interference,5 heating elements,6 battery anodes, and current collectors.7 Although all these materials have excellent properties, their application at high temperature is restricted to vacuum or to an inert atmosphere. In connection with this, an important problem is the increase a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2012.39 1054
J. Mater. Res., Vol. 27, No. 7, Apr 14, 2012
http://journals.cambridge.org
Downloaded: 01 Oct 2014
in thermal stability of carbon materials mentioned above. Recently, the addition of some oxidation inhibitors, such as SiC,8 ZrO2,9 and BN,10 into the bulk carbon materials by coating technique was investigated, and modification by boric compounds was found to be one of the most effective solutions in reducing oxidation loss. B4C-mixed C–C composites were reported to have excellent oxidation resis
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