W.C. Forsman

Chemistry of graphite intercalation by nitric acid

Abstract Gas-phase intercalation of graphite by nitric acid is accompanied by evolution of a brown gas which has been identified as nitrogen dioxide by chemiluminescence measurements. The reaction is inhibited by the presence of oxygen or water vapor, but not by nitrogen. These results, and the existence of induction times of 5–20 min before intercalation begins, is interpreted as evidence that intercalation is effected by oxidation of graphite by adsorbed nitronium ions with the release of NO 2 . The graphite lattice then intercalates NO 3 − ions along with neutral HNO 3 molecules.

Graphite intercalation chemistry: An interpretive review

It was but five years ago that Forsman, and Ebert and Selig suggested independently that redox chemistry might be intrinsic to the formation of so-called acceptor compounds. Since then, much of the work in chemical aspects of intercalation has been specifically directed at verifying or disproving this hypothesis. The wealth of new data compiled in the process, coupled with a meticulous review of past evidence, confirms the essential role of oxidation in this genre of intercalation reactions. This review addresses two particular issues in the chemistry of acceptor type intercalants. First, how the oxidative or coordinating capability of a chemical species relates to its behavior as an intercalant. There has been considerable emphasis on the broad spectrum of interactions which exist, from weak charge transfer (Br2 compounds), to oxidative intercalation (e.g., HNO3), to reactions so vigorous that intercalation is only achieved with accompanying fluorination of the lattice and partial destruction of the sp2 character of the carbon network (certain inorganic fluoride systems). Ebert and Selig have pointed out that detrimental oxidations of this sort are always an alternate reaction pathway in oxidative intercalation; the particular intercalant and reaction conditions determine which products will dominate. Secondly, what is the mechanism by which the intercalant interacts with the π-electron system and what are the reactive intermediates? In contrast to the previous topic—which relies on concrete evidence such as final product composition and properties for its resolution—here the approach has been largely hypothetical, relying on analogy to inorganic coordination compounds and organic π-systems which participate in charge transfer reactions. Here also lies, perhaps, the greatest future challenge to the intercalation chemist. For while the devising of possible reaction mechanisms is accomplished easily enough, experimental confirmation is always difficult. However, the past record of cooperation between chemist and physicist, experimenter and theoretician in intercalation science is an impressive one. If this question of mechanism is approached with the same gusto that has characterized past efforts, we should have some answers before long.

Intercalation of graphite by nitronium ion attack

Graphite is attacked by nitronium ions in tetramethylene sulfone solutions of nitronium tetrafluoroborate and hexafluoroantimonate giving highly electrically conducting intercalation compounds; evidence also indicates that solvent molecules are inserted along with BF4− or SbF6− ions and, possibly, NO2BF4 or NO2SbF6 molecules.

The system graphite-MnCl2-AlCl3 Kinetics, structure and mechanism of formation

Abstract Graphite-MnCl2-AlCl3 intercalation compounds have been prepared via MnCl2-AlCl3 complexes. First stage compounds are eventually formed at both 325 and 500°C; products formed at the lower reaction temperature are richer in Mn (C5.95MnCl2(AlCl3)0.21) than those at 500° (C7.15MnCl2(AlCl2.5)0.3). At 325° the mechanism is a quasi-selective intercalation of AlCl3 to the second stage followed by insertion of MnCl2 to a mixed stage I compound, and finally an isostage Mn enrichment with Al depletion. At 500°, insertion proceeds differently. Rate limiting reactions at each temperature are proposed. Powder X-ray diffractograms give I c = 9.51 A for Mn-rich first stage compounds. Comparison of calculated and observed (00l) structure factors gives an Mn-Cl spacing of 1.46 A as in the pure dichloride. Analysis of (hkl) positions and the (10l) and (11l) intensities of the inserted MnCl2 indicate an intercalant superlattice with a 0 = 3.69 A and c 0 = 3 × I c = 28.53 A . First stage compounds prepared at 500° are considerably less ordered. Results obtained at 325° are especially significant in that 1. (1) this represents the lowest temperature at which dichlorides have been intercalated to rich stages, and 2. (2) long-range (i.e. 3-dimensional) ordering has been documented in a dichloride intercalation compound for the first time, and is sensitive to reaction temperature.

Mobility of spacer molecules in graphite nitrate

Abstract Neutral nitric acid (HNO3) molecules in graphite nitrate prepared from highly oriented pyrolytic graphite at 24 °C with anhydrous HNO3 are labile and can be moved in and out of the lattice by adjusting the partial pressure of HNO3. Both the initial intercalation and the subsequent diffusion of HNO3 into the lattice followed a simplified first-order rate model to a reasonable approximation. The rate constant, however, was somewhat higher for simple diffusion of HNO3 into the lattice than for the case when the graphite was being oxidized by HNO3. A compound, intercalated to stage 2 as determined by X-ray analysis and weight gain, demonstrated an electrical resistivity of 3.0 × 10−6 ω cm. When sufficient HNO3 was removed under vacuum to give the final weight associated with a stage 4 compound, the product demonstrated the correct X-ray pattern for such a compound and the electrical resistivity remained essentially unchanged. These results suggest that the electrical conductivity is related to the ionic content of the intercalation compound and is not substantially affected by the presence of neutral spacers.

Statistical Mechanics of Ion-Pair Association in Ionomers

When ion-pairs in bulk ionomer or its solutions associate to form clusters there is a change in the free energy of the system. Cluster formation must, of course, lead to a minimum in the free energy at the equilibrium state. One can think of the free energy change with cluster formation as consisting of three additive components: an energy of cluster formation, an entropy loss due to localization of the ion-pairs within the clusters, and an entropy loss associated with perturbation of the conformational statistics of the chain backbone. This paper discusses these three contributions to the free energy of cluster formation, along with related implications.

Synthesis and stability of Br2, ICI and IBr intercalated pitch-based graphite fibers

Abstract This work presents a further study of the intercalation of halogens in pitch-based fiber and the stability of the resultant intercalation compounds. P-100 fibers were intercalated with purified IBr at 50 °C to produce high electrical conductivity graphite intercalation compounds (GICs). After intercalation and subsequent equilibration in ambient atmosphere, the fibers average a five-fold conductivity,enhancement over the pristine fiber, and after nine weeks in air, the conductivity, σ, degrades by only 1%. The intercalation of ICl in P-100 was performed in vacuum-sealed vessels at 50 °C, 20° and 0 °C. Both the lowest equilibrium resistivity and its smallest ambient gain were observed for fibers reacted at 20 °C. P-100 brominated at 68 °C in vacuum-sealed vessels showed no loss in electrical and stability properties over those reacted at 20 °C in the presence of air. Energy dispersive spectroscopy (EDS) results confirm the existance of excess bromine and chlorine in the iodine interhalide GICs, which is predicted by the oxidation mechanism proposed for this class of intercalation reactions.

Nonreductive, spontaneous deintercalation of graphite nitrate

Abstract The stability under static vacuum of graphite nitrate produced from Grade 2135 and Micro 250 graphite has been monitored gravimetrically and via mass spectrometry of the evolved products. The gasses contain CO and CO 2 , indicating that the graphite has been oxidized. The percentages of CO x and H 2 O found in the gaseous products imply that HNO 3 , and not merely the ionic intercalant, NO 3 − , participates in the oxidation reactions. Removal of gasses under static vacuum from even low stage compounds gave little HNO 3 yet did release carbon oxides. Possible reactions and their effects on the intercalation compound are discussed. Deintercalation under vacuum at room temperature did not return the compound to its pristine graphite weight and chemical analysis of the final product revealed traces of N, O, and H. The gravimetric data demonstrates the effect of particle size on intercalate uptake and removal.

Conversion of carbon-graphite fibers to fibers of graphite oxide

Abstract Graphite fibers were chemically oxidized in the liquid phase to fibers of graphite oxide. Initial resistivity increases as high as 10 4 were obtained but, when the oxidized fibers were exposed to the atmosphere, resistivities decreased again and reached an apparently steady state value of about 10 3 times the resistivity of the starting fibers. In one experiment the initial increase in resistivity amounted to a factor of 10 6 . The best results were obtained on the most highly graphitized fibers. Electrochemical oxidation yielded a lower resistivity increase, about ten times, but provided a controllable method of synthesis and an insight into the mechanism of reaction. The oxidized fibers retained 84% of the tensile strength and 81% of the modulus of the pristine fibers.

Statistics and Dynamics of Polymer Molecules in Solution

This chapter is intended to serve two purposes. First, it offers a short commentary on theoretical material that can be used as a background for much of the experimental and theoretical discussion given in subsequent chapters. It is aimed at scholars with a serious interest in polymer theory, but not particularly familiar with its development. The coverage is not exhaustive, nor is this chapter intended to be a full-scale technical review. Rather, the intent is to offer a view of the mathematical and physical underpinnings upon which current theoretical and experimental methods are built. Much of the material is thus found in various treatises, the earliest now being almost a classic.1–3 Little is said here about the newer theoretical developments. These subjects will be introduced at the appropriate places in subsequent chapters, and have been recently reviewed.4,5