Timothy D. Burchell

High-thermal-conductivity, mesophase-pitch-derived carbon foams: effect of precursor on structure and properties

Abstract Pitch-based carbon foams are not new, but the development of high thermal conductivity foams for thermal management applications has yet to be explored. The research reported here focused on a novel foaming technique and the evaluation of the foaming characteristics of two mesophase pitches (Mitsubishi ARA24 and Conoco Dry Mesophase). After graphitization to 2800°C, densities of the graphite foams ranged from 0.2 to 0.6 g/cm3, with average pore diameters ranging from 275 to 350 μm for the ARA24-derived foams, and from 60 to 90 μm for the Conoco-derived foams. Scanning electron microscopy and polarized light optical microscopy were performed to characterize the cell walls, revealing highly aligned graphitic-like structures along the axis of the ligaments. Analysis of X-ray diffraction results determined that the foams exhibited average interlayer (d002) spacings as low as 0.3355 nm, stack heights (Lc) up to 80 nm and crystallite sizes (La) up to 20 nm. Finally, thermal diffusivity measurements were performed revealing that the bulk thermal conductivity varied with density from 40 to 150 W/m K. The specific thermal conductivities of the graphitized foams were more than six times greater than solid copper.

A novel process and material for the separation of carbon dioxide and hydrogen sulfide gas mixtures

Abstract Carbon fiber composite molecular sieve (CFCMS) synthesis and characterization of the macro-, meso- and micropore structure are reported. CFCMS physical properties, including strength, thermal conductivity and electrical resistivity, are reported and the thermal conductivity of CFCMS compared with literature data for granular activated carbon (GAC) and packed beds of GAC. Adsorption studies, including isotherms for CO2 and CH4 at temperatures of 30, 60 and 100 °C on CFCMS samples activated to different burn-offs, are reported. High pressure adsorption data for CO2 and CH4 show that the CFCMS material has sufficient selectivity for CO2 over CH4 for a commercial separation. Breakthrough experiments were conducted for CO2/CH4 and H2S/H2 gas mixtures and the selective separation of CO2 and H2S was demonstrated. The electrical conductivity of our novel monolith was exploited to effect the rapid desorption of adsorbed gases. Desorption at low applied voltage was accompanied by a heating of the CFCMS to temperatures

A microstructurally based fracture model for polygranular graphites

The physical basis of, and assumptions behind, the Burchell fracture model for graphites are reported. The model combines a fracture mechanics failure criteria and a microstructurally based description of fracture. Microstructurally related inputs such as pore-size distribution, particle size, bulk density, particle fracture toughness, the number of pores per unit volume, and specimen geometry (size and stressed volume) are utilized in the model. Microstructural inputs to the model code, determined by quantitative image analysis of prepared specimens of the graphites, are reported. The Burchell fracture model was successfully applied to four graphites of widely different texture, ranging from a fine-grain, high-strength aerospace grade (POCO AXF-5Q) to a coarse-grain, low-strength electrode graphite (AGX). The excellent performance and versatility of the Burchell fracture model was attributed to its sound physical basis. The potential for applying the Burchell fracture model to irradiated graphites was explored and the model was shown to qualitatively predict the influence of neutron damage on the strength of a nuclear graphite.

Radiation Effects in Graphite and Carbon-Based Materials

Displacement damage in graphite and carbon-based materials can occur when energetic particles, such as neutrons, ions, or electrons impinge on the crystal lattice. The displacement of carbon atoms from their equilibrium positions results in lattice strain, bulk dimensional change, and profound changes in physical properties. This article will discuss the effects of displacement damage in graphites and carbon-based materials. The materials considered here are those whose bonding is sp 2 —that is, graphites, pyrolytic carbons and graphites, carbon fibers, and carbon-carbon (C/C) composites. Radiation damage in sp 3 (diamond) carbon forms is not discussed. Carbon-based materials and graphites are widely used in nuclear applications. For example, polygranular (manufactured) graphites have been employed as a moderator in nuclear reactors since the 1940s. More recently, pyrolytic graphites, artificial graphites, and C/C composites have been adopted as plasma-facing components in fusion devices. Engineering applications, such as those just cited, have necessitated a full understanding of the basic mechanisms of radiation damage, as well as the effects of radiation damage on the physical properties of carbon-based materials.

Low Pressure Storage of Natural Gas for Vehicular Applications

Natural gas is an attractive fuel for vehicles because it is a relatively clean-burning fuel compared with gasoline. Moreover, methane can be stored in the physically adsorbed state [at a pressure of 3.5 MPa (500 psi)] at energy densities comparable to methane compressed at 24.8 MPa (3600 psi). Here we report the development of natural gas storage monoliths [1]. The monolith manufacture and activation methods are reported along with pore structure characterization data. The storage capacities of these monoliths are measured gravimetrically at a pressure of 3.5 MPa (500 psi) and ambient temperature, and storage capacities of >150 V/V have been demonstrated and are reported.

Structure-related property changes in polycrystalline graphite under neutron irradiation

Abstract The properties of polycrystalline graphite are related to those of their component crystallites by structure-dependent factors. Neutron irradiation changes the properties of the crystals and indirectly the “structure factors” that relate the macroscopic to the microscopic properties. It is proposed and demonstrated that each of these structure factors depends upon a single parameter XT, defined as the difference in the crystal dimensional changes parallel and perpendicular to the hexagonal axis.

Thermal conductivity degradation of graphites due to nuetron irradiation at low temperature

Several graphites and carbon/carbon composites (C/Cs) have been irradiated with fission neutrons near 150°C and at fluences up to a displacement level of 0.24 dpa. The unirradiated room temperature thermal conductivity of these materials varied from 114 W/mK for H-451 isotropic graphite, to 670 W/mK for a unidirectional FMI-1D C/C composite. At the irradiation temperature a saturation reduction in thermal conductivity was seen to occur at displacement levels of approximately 0.1 dpa. All materials were seen to degrade to approximately 10 to 14% of their original thermal conductivity after irradiation. The significant recovery of thermal conductivity due to post-irradiation isochromal anneals is also presented.

The effects of radiation damage on the properties of GraphNOL N3M

GraphNOL N3M (N3M) is a bulk graphite developed at Oak Ridge National Laboratory (ORNL) for advanced structural applications in aerospace thermal protection systems. It is currently of interest to the United States fusion energy community for plasma facing components, such as the first wall armor tiles of the International Thermonuclear Experimental Reactor (ITER) because of its superior thermal shock resistance and irradiation lifetime. This paper reports the results of irradiation experiments on N3M graphite at two temperatures, 600 and 875 °C in the High Flux Isotope Reactor (HFIR) at ORNL. Maximum fluences of 4.2 × 1026 and 2 × 1026 n/m2 (E > 50 keV) or 28.4 and 13.5 dpa (graphite) were attained at 600 and 875° C, respectively. Data are presented on the dimensional stability, volume change, strength, Youngs modulus, critical stress intensity factor KIc, and coefficient of thermal expansion (CTE). The thermal shock resistance of GraphNOL N3M is discussed and comparisons made with other graphites. The influence of irradiation damage on thermal shock resistance is postulated.

The fracture of polygranular graphites

Abstract Six commonly used models of graphite failure are described and their performance critically reviewed against empirical data derived from a variety of tests to failure in tension and bending. The simpler models based on critical stress, critical strain and critical strain energy density criteria are shown to be remarkably unsuccessful in describing the experimental results. The Weibull model, though versatile in its application to geometry-related effects, is far less useful in treating the influence of microstructural variations. The final two treatments considered, the so-called Rose/Tucker and fracture mechanics models, are much wider ranging in their usefulness, and it is argued that a new theory which embodies the salient points of each should be developed. The microstructural processes which ought to be included in such a description are identified on the basis of observations on subcritical defect development prior to failure.

Passive CO2 removal using a carbon fiber composite molecular sieve

Abstract Manufacture and characterization of a carbon fiber composite molecular sieve (CFCMS), and its efficacy as a CO 2 gas adsorbent are reported. The CFCMS consists of an isotropic pitch derived carbon fiber and a phenolic resin derived carbon binder. Activation (selective gasification) of the CFCMS creates microporosity in the carbon fibers, yielding high micropore volumes (>0.5 cm 3 /g) and BET surface areas (>1000 m 2 /g). Moreover, the CFCMS material is a rigid, strong, monolith with an open structure that allows the free-flow of fluids through the material. This combination of properties provides an adsorbent material that has several distinct advantages over granular adsorbents in gas separation systems such as pressure swing adsorption (PSA) units. The results of our initial evaluations of the CO 2 adsorption capacity and kinetics of CFCMS are reported. The room temperature CO 2 adsorption capacity of CFCMS is >120 mg of CO 2 per g of CFCMS. A proposed project is described that targets the development, over a three-year period, of a demonstration separation system based on CFCMS for the removal of CO 2 from a flue gas slip stream at a coal-fired power plant. The proposed program would be conducted jointly with industrial and utility partners.