Dennis C. Nagle

Carbonization of wood for advanced materials applications

Abstract A unique process for conversion of monolithic wood structures to carbons that retain the cellular structure of the wood without the formation of cracks and other defects associated with charcoal materials is described. A variety of wood species are carbonized to produce the materials which are characterized using TGA, density, dimensional changes, acoustic velocity, SEM and mechanical testing. We demonstrate that through controlled pyrolysis monolithic carbonized wood can be produced without the macro-cracks normally associated with charcoal. A linear relationship is established between the bulk densities of wood and carbonized wood which spans the entire range of species. For the conditions presented, the carbonized wood had 82% of the bulk density of the precursor wood. Carbonized wood acoustic velocity ranged from 4.7 to 1.3 mm/μs for Tilia americana and Ochroma pyramidale , respectively. Carbonization resulted in decreased acoustic velocity in the axial principal direction and increases in the radial and tangential directions. Acoustic anisotropy was retained through carbonization, but reduced in magnitude. Mechanical testing showed the carbonized wood to be 28% stronger than the precursor. The cellular morphology of the porous carbonized wood is described and compared to carbon foams.

Carbonized wood monoliths—Characterization

By using a combination of very accurately cut wood specimens and highly controlled pyrolysis conditions, we have for the first time obtained precise volumetric and linear shrinkage measurements for wood carbonization which we use to formulate a new theory of cellulose microfibril dominance. Characterization of these crack-free carbonized wood monoliths was performed by measuring density, dimensions, acoustic velocity, yield and crystallinity for heat treatment temperatures ranging from 400 to 2500 °C. Axial dimensions increased at the highest temperatures while radial and tangential dimensions continued to decrease. Acoustic velocity in the axial direction remained constant at 5 mm/μs above 1200 °C. Radial and tangential velocities went through a maximum before diminishing at the highest temperatures. Helium density of powdered carbonized monoliths was found to go through a maximum of 1.9 g/cm3 at 1000 °C before decreasing at the highest temperatures to 1.4 g/cm3, the value obtained for 400 °C char. The experimental results are used to support a model for the relationship between the cellulose molecular arrangement in the wood and the microstructure of the resulting carbon monoliths. It is proposed that cellulose microfibrils dominate the mechanism of dimensional change during pyrolysis and result in a preferred orientation of crystallites in the carbonized wood.

Enhanced proliferation and osteocalcin production by human osteoblast-like MG63 cells on silicon nitride ceramic discs

The biocompatibility of silicon nitride (Si3N4) was assessed in an in vitro model using the human osteoblast-like MG-63 cell line. Cells were propagated on the surface of: reaction-bonded silicon nitride discs, sintered after reaction-bonded silicon nitride discs or control polystyrene surface for 48 h. Compared to cells propagated on polystyrene surface, cells grown on the surface of unpolished silicon nitride discs had significantly lower cell yield and decreased osteocalcin production. In contrast, cells on the surface of polished silicon nitride discs showed similar proliferative capacity to control cells propagated on polystyrene surface. Cells propagated on polished discs also produced higher levels of osteocalcin than cells on unpolished discs. SEM analysis showed cells with well-delineated morphology and cytoplasmic extensions when propagated on polished sintered after reaction-bonded discs. Cells appeared more spherical, when grown on polished reaction-bonded discs. The results of this study suggest that silicon nitride is a non-toxic, biocompatible ceramic surface for the propagation of functional human bone cells in vitro. Its high wear resistance and ability to support bone cell growth and metabolism make silicone nitride an attractive candidate for clinical application. Further studies are needed to explore the feasibility of using silicon nitride clinically as an orthopedic biomaterial.

Evaluation of carbonized medium-density fiberboard for electrical applications

Abstract The conversion of wood-based fiberboard materials into crack-free, monolithic, porous hard carbons is of significant interest due to their ability to perform in a multifunctional capacity. Three varieties of carbonized medium-density fiberboard (c-MDF) were studied for electrical, mechanical, and structural properties. X-ray diffraction data suggested that the volume fraction of large turbostratic crystallites increased with carbonization temperature ( T carb ). The volume fraction of large turbostratic crystallites had a positive correlation with elastic modulus and electrical conductivity. The c-MDF materials were approximately isotropic with respect to elastic modulus and exhibited increasing stiffness with increasing T carb (up to 4.5 GPa). Between 600 and 1400 °C, the electrical resistivity of c-MDF varied by seven orders of magnitude. The electrical resistivity of the hard carbon material in c-MDF 1400 °C was found to be within about an order of magnitude of polycrystalline graphite.

Characterization of carbons derived from cellulose and lignin and their oxidative behavior.

In this study the oxidative behavior of carbons derived from cellulose and lignin were compared using thermogravimetric analysis (TGA). Specific surface area and chemical composition of the two types of carbon were analyzed using nitrogen adsorption at 77K and infrared spectroscopy respectively. The results demonstrate that cellulose carbon has a higher reaction order and lower activation energy than lignin carbon under identical experimental conditions when they were prepared at temperatures lower than 500 degrees C. However, such differences were considerably reduced for the carbon samples prepared at temperatures greater than 700 degrees C. It was verified that lignin carbon is more stable than cellulose carbon due to its higher content of aromatic structures when they are prepared at lower temperature. The specific surface area and porosity have a more limited contribution to the differential oxidative behaviors of the two types of carbon. This research has significance related to the formation of carbon nanotubes from plant materials during low temperature carbonization.

Proinflammatory cytokine expression of IL‐1β and TNF‐α by human osteoblast‐like MG‐63 cells upon exposure to silicon nitride in vitro

This study was designed to determine the effect of Si(3)N(4) disks and particulates on human osteoblast-like MG-63 cells in vitro. The MG-63 (10(5)/mL) cells were plated onto 24-well polystyrene plates fitted with either sintered reaction-bonded (SRBSN) or reaction-bonded (RBSN) 15-mm disks. Controls consisted of wells without Si(3)N(4) disks. Cells propagated at 37 degrees C, 5% CO(2) for 48 h on Si(3)N(4) disks and control polystyrene surfaces exhibited similar proliferative capacities (7000 and 4000 cpm/10(5) cells, respectively, p > 0.05). Cells incubated with 1, 10, or 100 microgram/ml of Si(3)N(4) particles (<1.00 to 5.00 micrometer) for 24 h did not exhibit a decrease in DNA synthetic activity: 12 +/- 1.3 x 10(4), 10.5 +/- 1.5 x 10(4), and 11.0 +/- 1.7 x 10(4) cpm, respectively, compared to 11.6 +/- 2.6 x 10(4) cpm/10(5) for the control cells, as indicated by (3)H-thymidine uptake. Cells propagated on RBSN displayed increased expression of cytokines IL-1beta and TNF-alpha compared to the control cells, as shown by reverse transcriptase-polymerase chain reaction (RT-PCR). In contrast, cells propagated on SRBSN surfaces expressed the same level of IL-1beta and TNF-alpha as that of control cells. Incubation of MG-63 cells with 1-10 microgram/mL of particles did not increase IL-1beta expression. However, at 100 microgram/mL, TNF-alpha expression was greater than that of the control cells. Silicon nitride, evaluated here as disks or as particulates (1-10 microgram/mL), is biocompatible and does not hinder the proliferation or induce proinflammatory cytokine expression of human osteoblast-like MG-63 cells in vitro.

Monolithic activated carbon sheets from carbonized medium-density fiberboard

Structural monolithic activated carbon sheets were made from carbonized medium-density fiberboard (c-MDF) using a carbon dioxide physical activation process. The activated c-MDF exhibited surface area as high as 1044 m2/g, and appeared to have the potential for even higher surface areas. Large activated c-MDF sheets were cut for evaluating surface area uniformity and mechanical properties (four-point bending). Surface area variation was significant in-plane, but negligible through-thickness. Variations on the activation method demonstrated improvement in in-plane uniformity of surface area. Bending modulus changed little from activation, but peak stress decreased for activations yielding high surface area.

Formation of dense silicon carbide by liquid silicon infiltration of carbon with engineered structure

Fully dense and net-shaped silicon carbide monoliths were produced by liquid silicon infiltration of carbon preforms with engineered bulk density, median pore diameter, and chemical reactivity derived from carbonization of crystalline cellulose and phenolic resin blends. The ideal carbon bulk density and minimum median pore diameter for successful formation of fully dense silicon carbide by liquid silicon infiltration are 0.964 g cm −3 and approximately 1 μm. By blending crystalline cellulose and phenolic resin in various mass ratios as carbon precursors, we were able to adjust the bulk density, median pore diameter, and overall chemical reactivity of the carbon preforms produced. The liquid silicon infiltration reactions were performed in a graphite element furnace at temperatures between 1414 and 1900 °C and under argon pressures of 1550, 760, and 0.5 Torr for periods of 10, 15, 30, 60, 120, and 300 min. Examination of the results indicated that the ideal carbon preform was produced from the crystalline cellulose and phenolic resin blend of 6:4 mass ratio. This carbon preform has a bulk density of 0.7910 g cm −3 , an actual density of 2.1911 g cm −3 , median pore diameter of 1.45 μm, and specific surface area of 644.75 m 2 g −1 . The ideal liquid silicon infiltration reaction conditions were identified as 1800 °C, 0.5 Torr, and 120 min. The optimum reaction product has a bulk density of 2.9566 g cm −3 , greater than 91% of that of pure β–SiC, with a β–SiC volume fraction of approximately 82.5%.

Development of a High-Temperature Optical Coating for Thermal Management on Solar Probe Plus

NASA’s Solar Probe Plus (SPP) is approaching within 9.5 solar radii from the center of the sun. The SPP thermal protection system (TPS) is a 2.7 meter heat shield. The heat shield reaches temperatures of 1400uC on its front surface, its worst thermal case, and is subjected to launch loads, its worst mechanical case. The front surface of the thermal protection system is coated with an optically white coating in order to reduce the front surface temperature of the TPS and reduce the resulting heat flow into the spacecraft. At the temperatures experienced by the TPS, the optical properties are influenced by temperature more than in standard thermal control surfaces. Being able to accurate predict the optical performance of the coating at the temperature extremes of the mission is critical to understanding the thermal capabilities of the spacecraft and thermal protection system. A coating has been developed that can meet the requirements of the SPP TPS and it has been engineered to improve its optical properties at high temperature.

Passive Optical-Based Thermal Management Approach for Spacecraft Operating in the Near Solar Environment

[Abstract] The Johns Hopkins University Applied Physics Laboratory (JHU/APL) and NASA’s Goddard Space Flight Center are currently evaluating optical coatings made from ceramics as a means of passive thermal management for spacecraft operating in the solar environment. Ceramics were selected based on chemical stability; and inertness to radiation damage and hydrogen degradation. Investigations have focused on “white” ceramics such as aluminum oxide, pyrolytic boron nitride, and barium zirconium phosphate which have been shown to also possess desirable optical characteristics making them ideal candidates.