B. Rand

Particle interactions in aqueous kaolinite suspensions: I. Effect of pH and electrolyte upon the mode of particle interaction in homoionic sodium kaolinite suspensions

Abstract Flow curves of dilute homoionic sodium kaolinite suspensions have been measured as a function of pH and electrolyte concentration. The changes in Bingham yield stress have been shown to be entirely in accord with the Schofield-van Olphen model of particle interactions, and it has been possible to elucidate the conditions under which edge-face, edge-edge, and face-face coagulated structures exist. The results have also enabled the isoelectric point of the edge surface of this kaolinite sample to be located at pH 7.3 ± 0.2. The electrolyte concentration required to develop the edge-edge structure fully increases as the edge surface potential increases on either side of the edge isoelectric point; the results are in accord with the DLVO theory of colloidal stability. Comparison of the effects of 1:1, 1:2, 2:2, and 2:1 electrolytes on the Bingham yield stress suggests that the anionic species may be the more important in converting edge-face structures into the edge-edge form at low pH values. The effect of pH and electrolyte concentration on the plastic viscosity is also discussed in terms of the electrical double layers at the kaolinite/water interfaces.

The properties of aqueous phase suspensions of barium titanate

Abstract Recent literature is reviewed concerning the influence of suspension pH and ion concentrations on the zeta-potential of near-stoichiometric BaTiO3 powders. New work is reported concerning the surface chemistry and rheology of BaTiO3 powder in aqueous phase suspensions prepared without the use of surfactants or polyelectrolytes.

Mechanical properties of magnesia-spinel composites

Abstract A set of dense magnesia-magnesium aluminate spinel composites has been prepared by hot-pressing magnesia powder using 0 to 30 wt.% of spinel powder of mean particle size 3, 11 and 22 μm. Bend stength, modulus, and fracture toughness, have been measured. Strength and modulus decrease with increasing spinel content, and for a given loading, spinel particle size, as a result of microcracking caused by thermal expansion mismatch between the magnesia matrix grains and the spinel particles, the effects of which may be intensified by recrystallization of spinel. Fracture is predominantly transgranular for the pure magnesia, and intergranular for the composite materials. Possible reasons for the differences in behaviour between this system and the SiC–Al 2 O 3 system with a similar thermal expansion mismatch are examined.

On the empirical nature of the Dubinin—Radushkevich equation of adsorption

Abstract The Dubinin-Radushkevich equation, when applied to the adsorption of vapors on microporous carbons, frequently shows one of three general types of deviation from linearity. These three types of behavior are discussed and it is shown that the data can usually be linearized by using the more generalized Dubinin-Astakhov equation, i.e., W = W 0 exp [− k( ∈ E ) n ]. The significance of this equation is briefly discussed and it is shown, for the limited number of systems reported so far, (CO 2 and N 2 adsorbed onto microporous carbons) that the constants E and k are independent of both the physical nature of the adsorbent and temperature. Hence, the energetic heterogeneity of the surface is described entirely by one parameter, n .

A role of charge-transfer complex with iodine in the modification of coal tar pitch

Abstract Iodine introduced into coal tar pitch (CTP) can drastically alter its reology and carbonization behavior. Here, the mechanism of interaction between iodine and CTP has been investigated by using various spectroscopic methods such as 127I-NMR, EPR and FT–IR. It is shown that some iodine molecules infiltrated into the CTP and form charge transfer complexes with the relative large aromatic components of the CTP. Hyperfine sublevel correlation spectroscopy (HYSCORE) revealed the molecular size of cation radicals, which contain more than ten benzene rings, and the location of iodine anion that is incorporated at ca. 0.3 nm from the aromatic cation radicals. The elemental H/C ratio decreased and the viscosity of CTP increased with the density of the charge-transfer complexes, which was also increased by the iodine treatment. These results strongly suggest that dehydrogenative polymerization of CTP occurs during the iodine treatment. The cation radicals in the charge-transfer complexes accelerate the dehydrogenative polymerization and result in a high carbon yield.

Surface characteristics of carbon fibres from PAN

Abstract Surface characteristics of the three types of commercial carbon fibres from PAN, in both treated and untreated forms, have been investigated along with a series of high modulus (HM) fibres oxidised in HNO 3 . N 2 adsorption isotherms have been used to elucidate physical characteristics and O 2 chemisorption measurements to determine the area on which surface complexes can form, the so called “active surface area”. Comparative plots show that there is no microporosity developed during surface treatment of fibres, but high temperature degassing does bring about a degree of microporosity, as suggested by Mimeault and McKee[5]. The external area of HM fibres is increased by HNO 3 oxidation but there is little difference between the areas of untreated fibres and those surface treated by the supplier. Both types of treatment, however, increase the active surface areas by factors of 3–30 times, and also increase the fraction of the external surface which is in the form of edge, or active, sites.

Investigating carbonization and graphitization using electron energy loss spectroscopy (EELS) in the transmission electron microscope (TEM)

Electron energy loss spectroscopy (EELS) in the transmission electron microscope (TEM) is explored as a useful characterization technique in the study of carbonization and graphitization of organic precursors. A model series of carbon materials was prepared from highly graphitizable petroleum pitch heat treated in the range 200–2730°C. Initial characterization was performed using the established techniques of X-ray diffraction (XRD), He pycnometry, TEM, electron diffraction and high-resolution lattice imaging (HREM). EELS in the TEM was then examined. Two routes are presented to quantify the change in the proportion of sp 2 type hybridization accompanying the heat treatment as the material transforms to the graphitic state. Both routes suggest an initial relative sp 2 content of ∼70%, rapidly increasing to ∼90% during mesophase development and carbonization, and then slowly increasing to 100% during graphitization. The peak position of the bulk valence plasmon (π + σ) is shown to be an excellent measure of the degree of graphitic character, and its fundamental dependence upon sample density (ρ) is confirmed. The appearance and definition of features within the core loss region representing the density of unoccupied σ* states are demonstrated to be an excellent measure of the extent of order. Finally, a method is established by which to extract the C–C bond length from core loss EELS spectra with an accuracy of ±0.1 pm. This method suggests an average bond length of 1.44 Å in samples with low heat treatment temperatures, decreasing to the theoretical length of 1.42 Å as both the heteroatom content and proportion of non-sp 2-type hybridized carbon atoms decrease.

Quantitative valence plasmon mapping in the TEM: viewing physical properties at the nanoscale

Using a series of graphitising carbons heat treated at different temperatures, the peak position of the bulk (pi+sigma) plasmon was measured using electron energy loss spectroscopy and observed to shift between 22 and 27eV. Experimental data is presented and discussed showing the effects of the collection conditions and sample orientation upon the observed spectra. We present an empirical technique by which quantitative energy filtered transmission electron microscopy (EFTEM) maps with two energy windows selected in the plasmon region can be readily acquired and processed, the results of which may be interpreted as graphitisation maps and subsequently physical property maps. An experimentally established resolution of approximately 1.6nm makes this technique a very useful tool with which to examine nanoscale properties in microstructural regions of interest in TEM specimens such as fibre/matrix interfaces within carbon-carbon composites, multi-walled carbon nanotubes and graphitic inclusions in carbon steels. Also presented is data demonstrating the unsuitability of pi(*)-related chemical EFTEM maps in both the low-loss region and at the carbon K ionisation edge for mapping bonding in such highly anisotropic media due to the strong orientation dependence of the intensity of the transitions involved. This is followed by suggestions for wider application of the plasmon mapping technique within systems other than those based upon carbon.

Thermogravimetric investigation of the pyrolysis of pitch materials. A compensation effect and variation in kinetic parameters with heating rate

Abstract Thermogravimetric measurements of weight loss accompanying the pyrolysis of four pitches have been made over a range of linear heating rates. For three of the samples, the data at each heating rate could be described by an integral and a differential method of analysis, assuming a simple order function for f(α), with the result that the apparent activation energy increased with heating rate. The data for all four samples could also be satisfactorily described by the Ozawa or Friedman multiple heating rate methods, and these resulted in apparent activation energies (Ea) which increased with the value of β at which they were determined. It is suggested that this tendency for the apparent activation energy to increase, as the temperature is raised, is due to a change in the relative importance of the different reactions which lead to weight loss in this system. The apparent kinetic parameters all fall on a common compensation plot which is used to explain the relative magnitude of Ea values from Ozawa and Doyle methods of analysis. The higher values of Ea from Friedman than from Ozawa analyses are also explained.

Rheological investigation of coal-tar pitch during its transformation to mesophase☆

Abstract Rheological characteristics of two coal-tar pitches, during their transformation to mesophase, have been followed by rotational viscometry. In certain instances samples were also taken, enabling the mesophase microstructure to be correlated with the flow behaviour. Flow curves established that the pitches are Newtonian liquids at low temperatures, but non-Newtonian character appears at temperatures above 380 °C. Scanning the viscosity-temperature curve of a binder pitch showed that the viscosity began to change, as a result of polymerization processes, at about 240 °C. In both pitches the apparent viscosity, at all rates of shear, increased very rapidly at temperatures above 420 °C when the mesophase content was in excess of 25%. Shear-thinning behaviour was apparent and a peak in the apparent viscosity-temperature curves appeared at all the rates of shear investigated. It is suggested that the systems can be regarded as emulsions. Initially an emulsion of mesophase in isotropic liquid exists but this transforms, at higher temperatures, to one of isotropic liquid in mesophase. The maximum in the apparent viscosity appears to correspond to the phase inversion point.