Glass-like Carbon Research Articles

Two-step activation of glass-like carbon spheres–An approach to high yield activation

A series of activated carbons was prepared using a two-step activation process. The starting glass-like carbon spheres were heated at 500°C for 3 h (first step activation), and then at a low temperature, below 415°C, for different times (second step activation), in a flow of dry air. Their pore structures were evaluated from N 2 gas adsorption/desorption isotherms at 77K by BET, α s-plot and BJH methods. The first step of activation generated a few large pores (mesopores and even macropores), on the surface of the spheres, which formed the entrance for the activating reagent gas (air), to increase micropores at a low temperature in the second step of activation. This two-step activation process was found to be an efficient approach to get a comparatively high surface area and a volume of micropores with a high activation yield.

RB-SiC Ceramics Derived from the Phenol Resin Added with Starch

Reaction bonded silicon carbide (RB-SiC) was fabricated by phenol resin, starch, solidified agent and silicon powder through the following steps: first, carbonizing at high temperature for 7-9h, infiltrating silicon at 1450-1600oC for 0.5-2h, and then removing excessive silicon at 1700oC for 0.5h. Scanning electron microscopy and X-ray diffraction were employed to characterize and analyze the microstructures and phase composition of the preforms and the final RB-SiC products. In addition, the effect of carbonization temperature, the amount of starch and solidified agent on strength and apparent porosity of final RB-SiC were also discussed. The results showed that the carbonization process of phenol resin can be divided into three steps: at temperatures from 400oC to 600oC, the structure of polymer changes less; at temperatures from 600oC to 1000oC, the fundamental chain of polymer is destroyed, and inverts to glass-like carbon; at temperatures from 1000oC to 1200oC, with the increasing of carbonization temperature, the structure of glass-like carbon changes into the structure of disorder graphite. And the increased micro-pores could be obtained by adding starch.

Control of cluster synthesis in nano-glassy carbon films

Carbon films were prepared by pulsed laser deposition (PLD), changing buffer gas nature and pressure and laser power density. Nanometer-sized cluster assembled (CA) films, resulting from direct aggregation of carbon clusters in the ablation plume, were obtained. Visible Raman spectroscopy shows that all films are trigonally co-ordinated and structurally disordered, with a dependence of the degree of disorder on the deposition parameters. The microstructure and morphology of the films were studied in a complementary way by scanning electron microscopy (SEM) both in plane and in cross-section, and by atomic force microscopy (AFM). Different growth modes are found in the deposited CA films, depending on the interplay of laser fluence and nature-pressure of the buffer gas. Threshold fluences of increasing value separate dense columnar growth from sponge like morphology, from an open dendritic structure. AFM pictures show that our glass-like carbon films consist of agglomerates of nanometer-sized clusters. Cluster formation in the plume is modeled, allowing to estimate the average number of carbon atoms per cluster. The calculated size of the clusters depends mainly on ambient gas pressure. Cluster sizes obtained by model predictions agree with those directly observed by transmission electron microscopy (TEM) imaging and with the deduced film coherence length from Raman spectroscopy conducted herein.

Glass-like carbon

Glass-like carbon

Changes in electrochemical insertion of lithium into glass-like carbon affected by catalytic graphitization at 1000 °C

Glass-like carbon spheres produced by activated carbonization of phenol resin in CO 2 atmosphere demonstrated an XRD pattern typical for disordered carbons. This material was entirely useless as anode for lithium-ion cell because of both its extremely low discharge capacity (27 mAh/g) and cyclic reversibility for the first cycle (39%). Heat treatment of the original carbon at temperature 2700 °C appeared to be ineffective for obtaining graphite material. The product of such a treatment yielded discharge capacity of 74 mAh/g, which remained at an unacceptably low level. A simple heat treatment of disordered carbon with iron powder at relatively low temperature of 1000 °C allowed us to improve electrochemical behaviour of the carbon. Due to a partial transformation of glass-like carbon into graphite, discharge capacity increased to about 250 mA/g.

Preparation and characterization of glass-like carbon/silica composites from rice hull and phenolic resin

Glass-like carbon/silica composites were prepared from rice hull and phenolic resin. Monolithic composites were prepared by carbonizing green bodies made from phenolic resin and rice hull in nitrogen atmosphere at temperatures raging from 1173 K to 1773 K. The resultant composites consisted of approximately 70 mass% of glass-like carbon and 30 mass% of silica, the latter being originated from rice hull. The highest compressive strength, bending strength, and fracture toughness were 227 MPa, 50 MPa, and 0.80 MPam1/2, respectively. The composites did not show any detectable hygroscopic expansion upon immersing in water. The mechanical properties were not deteriorated up to 1573 K, as long as the silica maintained its amorphousness.

Structure Evolution of Woodceramics by X-ray Diffraction and Raman Spectroscopy

The laser Raman spectroscopy and X-ray diffraction (XRD) techniques were applied to investigate the structure of woodceramics (WCS) from carbonizing tobacco stems and phenolic resin at different carbonization temperatures up to 1773K. Experimental results show WCS are non-graphitic carbon consisting of tobacco stems-originated amorphous carbon reinforced by glass-like carbon generated from phenolic resin. With the increase of carbonization temperature, the number of diffraction peaks is increasing, but the interlayer spacing of graphene sheets, d002, is decreasing. As carbonization temperature increased above 973K, the turbostratic crystallites grew very little, but the graphene sheets grew substantially from 2.7 to 5.6nm by Scherrer equation. The Raman spectra of WCS show the known D band which is related to disorder carbon and G band which corresponds to graphite with varying characteristics that are similar to other graphitoidal materials. The integrated intensity ratio R = ID /IG(R-value) which is the degree of disorder of WCS is decreasing with the increase of carbonization temperature above 973K. And the trend of La calculated by using the Tuinstra-Koening empirical relation is opposite but is consistent by Scherrer equation of XRD technique. The obtained results also indicate that 973K is the turning point temperature for preparation of WCS and the Raman spectroscopy will become a quick method for the structure analysis of WCS.

1414 集束レーザ光を用いた三次元微細形状の加工(S80-2 特殊加工プロセス(2),S80 特殊加工プロセス)

Development of a high-speed microfabrication technique using focused laser beam was conducted. Conventional CAD/CAM system was used and the beam was scanned mechanically by attaching both optical mirror arrays and a focal lens to the column of an ultraprecision machining center. Measured groove width and pocket depth obtained by grooving and pocket machining were used as a tool diameter and an axial depth of cut. A glass-like carbon having over 80% laser beam absorptivity and 650MHv hardness was employed as a workpiece. It was verified through the machining test that pseudo 3 dimensional concave prisms could be fabricated by digging square pockets with different square sizes and the same pocket depth to negative Z direction. Shifting laser focal point from the work surface could control a depth of the square pocket, so that a high-speed roughing by adjusting focal point on the work surface could be conducted. In addition, the laser micromachining system could be adapted to a high-speed finishing by shifting focal point from the work surface.

FIB Etching of Glass-Like Carbon Die and Mold Press of Micro Glass Lens

To produce micro glass lens by mold-press, 3D micro lens die was fabricated on glass-like carbon (GC) by focused ion beam (FIB) etching. The dimensional accuracy and surface roughness, as well as transcriptional performance, were investigated. A precise and smooth die was completed by optimizing dwell time, current and step number of 2D bit map data, which agreed well with theoretical estimation. Micro glass lenses with different diameter and gradient angle were mold-pressed using the GC die. The diameter and sag were confirmed to be nearly equivalent to or just smaller than those of GC die.

Ion Beam Modification of Carbon Materials

A study has been made of surface properties of carbon materials modified by ion beams. Substrates used were natural diamonds, glass-like carbon plates and polymer sheets. Ion species were chemically-active elements such as C, N and O, inert gas elements such as He, Ne and Ar, and metallic elements such as Cr and Ti. It was found that diamond becomes electrically conductive in ion implanted layers, which are amorphous or graphite-like structures. Electrical conductivity depends on implanted species, doses and target temperatures. It was found that glass-like carbon consisting of graphite and disordered graphite becomes amorphous due to ion beam bombardment. Amorphization causes the wear resistance to improve. The electrochemical properties changes depending on implanted species. The wear resistance and electrochemical properties depended on the target temperature during ion implantation. Ion beam bombardment to polymers has been carried out to control the electrical conductivity, cell adhesion and bio-compatibility. The electrical conductivity of polyimide films increases as the dose increases. The saturated sheet resistivity of implanted layers depends on ion species, dose and dose rate. It was found that the cell adhesion can be controlled by ion beam bombardment. The results were used in the fields of clinical examinations. In summary, ion beam bombardment to carbon materials is useful to control the carbon structures and surface properties depending on ion implantation conditions.

Thin graphite skin on glass-like carbon fiber prepared at high temperature from cellulose fiber

as precursor fibers. The fibers were produced by Lenzing Co., Australia and supplied by Moririn Co. Ltd., and the trade name is Lyocell. The diameters of the fibers were around 3–4 lm. These fibers were heated to 900 C with a heating rate of 2 C/min in nitrogen atmosphere using an infrared image furnace and kept at this temperature for 2 h. The 900 C-treated fibers were further heated to 3200 C with a heating rate of 20 C/min in argon atmosphere using a graphite resistance furnace and kept at this temperature for 10 min. Scanning electron microscope (SEM) observations were carried out for the surfaces and cross sections of the 3200 C-treated carbon fibers. Transmission electron microscope (TEM) observations were made for the cross sections of the carbon fibers. A 0 0 2 X-ray diffraction (XRD) measurement was made for the pulverized sample of the carbon fibers using CuKa radiation and silicon powder as an internal standard. Maximum transverse magnetoresistance Dq/q was measured for a single filament of the carbon fiber at 77 K in fields up to 1 T. In order to examine the surface structure of the carbon fiber, the first order Raman spectra were measured for some of single filaments of the carbon fibers at room temperature in air using a 514.5 nm line of an argon ion laser. The SEM images of a surface of the 3200 C-treated carbon fiber and cross section perpendicular to the fiber axis are shown in Fig. 1. The diameters of the fibers were around 2–3 lm. Even after the heat treatment at 3200 C, a granular texture being characteristic of glass-like carbon [8] was observed on the cross section.

Structure Analysis of Oxidized Glass-like Carbon at Surface Region by XPS

Structure Analysis of Oxidized Glass-like Carbon at Surface Region by XPS

Radiation damages and bubble formation of ion implanted furan-resin-derived carbon

Furan-resin-derived carbon generally produces a glass-like carbon (GC) having entangled graphene layers after high temperature heat treatments. However, furan-resin-derived carbon produced well-graphitized thin skin on surfaces after heat-treatment at 3000 °C. Since ion implantation is a useful technique for surface modification, we used them to investigate the mechanism of well-graphitized thin skin on surfaces of furan-resin-derived carbon after heat-treatment at 3000 °C. In this investigation, we observed morphological changes on Ar + or Xe + implanted GC surfaces using SEM&TEM and Raman spectroscopy. Ar or Xe ions were implanted into the furan-resin-derived carbon heat-treated at 3000 °C at energy of 60–160 keV at a fluence ranging from 1×10 15 to 1×10 17 ions/cm 2. Microstructural changes were examined by Raman spectroscopy. The implanted surfaces of the specimens were changed to an amorphous structure by the ion implantation. Moreover, morphological SEM&TEM observations revealed that clear bubbles with diameter of 10–80 nm were present in the Ar +-implanted specimen at a fluence of 1×10 17 ions/cm 2. However, bubbles did not appear in the Ar +-implanted specimen at a fluence of 1×10 16 ions/cm 2. On the other hand, bubbles with diameter of 15–30 nm were present in the Xe +-implanted specimens at a fluence of 1×10 16 ions/cm 2. The Xe +-implanted specimens were subjected to annealing of re-heat-treatment at 3000 °C. After the annealing, the bubbles were lost in the interior of specimens by cross-sectional SEM observations. Moreover, on the implanted surfaces appeared big bubbles, which were not presented on the implanted surface before annealing. The sizes of these big bubbles were about 2–10 μm. Using Raman spectroscopy, we observed that on the surface of the big bubbles the structure was re-well-graphitized. On the surface of non-bubbled areas it was less graphitized. Thus annealing provided a restoration of the structural, but much effective only on the bubble surfaces.

A study on the reaction between chlorine trifluoride gas and glass-like carbon

The reaction between glass-like carbon (GC) and chlorine trifluoride (ClF 3) gas was investigated with weight measurements, surface analysis, and gas desorption measurements, where the ClF 3 gas is used for the in situ cleaning of tubes in silicon-related fabrication equipment. From Auger electron spectroscopy and X-ray photoelectron spectroscopy measurements, a carbon mono-fluoride, –(CF) n –, film near the surface of GC is considered to be grown onto the GC surface above 400 °C by the chemical reaction with ClF 3, and this thickness of the fluoride film depends on the temperature. The grown fluoride film desorbs by annealing in a vacuum up to 600 °C. Although GC is apparently etched by ClF 3 over 600 °C, the etch rate of GC is much lower than that of SiC and quartz.

Structural and property changes in glass-like carbons formed by heat treatment and addition of filler

Glass-like carbon precursors shrink significantly during curing and carbonization, which leads to crack formation and bending. Cured furan resin powder and ethanol were added to furan resin to diminish the weight loss, to suppress the shrinkage and bending, and to readily release the gases evolved during polymerization and curing. Curing and carbonization were controlled by pressure and slow heating to avoid damage to the samples. The effect of the filler and ethanol on the fabrication process was examined by measuring the properties of the glass-like carbon, such as the specific gravity, bending strength, electrical resistivity, and microstructural change. The specific gravities of the filler-added glass-like carbons were higher than those of the ethanol-added samples because of the formation of macropores from the vaporization of ethanol during the curing and polymerization processes. Although the ethanol-added glass-like carbons exhibited lower bending strengths after carbonization than did the filler-added samples, the opposite result was observed after aging at 2,600 oC. We found that the macropores created from ethanol were contracted and removed upon heat treatment. The electrical resistivity of the glass-like carbon aged at 2,600°C was lower than those of the samples carbonized at 1,000 oC. We attribute this phenomenon to the fact that aging at high temperature led to well-developed microstructures, the removal of macropores, and the reduction of the surface area.

Observation of electron field emission patterns from B-doped diamond films

To develop electron beam sources of carbon materials, field emission patterns were observed in three different setups. The first was a diode-type, in which a carbon specimen was facing to a positively biased fluorescence plate. The second was a triode-type, in which a positively biased grid was placed between them. In the third setup, a commercial electron gun was modified so that it could accommodate a carbon specimen and a grid. A fluorescence plate was placed in a vacuum chamber outside the gun. As the carbon specimen for electron emission source, B-doped diamond films, a single crystal diamond with a B-doped layer, an undoped diamond film and a glass-like carbon both with a fibrous structure at the surface, and a sponge carbon were used. It was found that electron emission from edges was dominant for 1×1 cm diamond films and carbon specimens in the diode-type setup. In the triode-type setup, the edges of the specimens were masked with a Kapton® tape. The electron emission occurred only from some spots on the specimen. In the electron gun setup, it was confirmed that an electron beam was generated, and a fairly uniform circle was seen on the fluorescence plate under defocused situation, while the circle became smaller by adjusting the current of the focusing lens. Although more uniform emission from the electron source materials seemed to be necessary for practical applications, it was demonstrated that an electron beam could be generated even in such a simple setup.

Wettability between Glass-Like Carbon and Optical Glasses

Wettability between Glass-Like Carbon and Optical Glasses

Structural correlation between micro-texture of furan resin and its derived glass-like carbon

Structural correlation between micro-texture of furan resin and its derived glass-like carbon

Effects of filler and ethanol on mechanical properties of glass-like carbon

The effects of filler and ethanol on the mechanical properties of glass-like carbon were studied. The specific gravities of glass-like carbons with the filler were higher than those of alcohol added glass-like carbons due to the formation of pores and bubbles as the alcohol was vaporized in the curing and polymerization stages. The ethanol added glass-like carbon samples also had lower bending strength than the filler added glass-like carbon samples after carbonization; a contrary result was observed after heat treatment at 2600°C. This was due to the pores that were made with ethanol which plays the role of pore former during carbonization. The fabricated glass-like carbons are quite brittle material. The shore hardness of the glass-like carbon was observed not to be affected by the addition of filler or ethanol, but increased a lot after carbonization.

Chemical and micro-structural changes in glass-like carbon during high temperature heat treatment

A glass-like carbon was fabricated using furan resin. The influence of heat treatment temperature during fabrication process on the chemical and micro-structural changes was studied by various analytical and spectroscopic methods including TGA, FT-IR, CHN, TEM and XRD. The chemical resistance properties of the fabricated glasslike carbon were also investigated. It has been found that the heat-treated samples at higher temperature up to 2600 °C in N2 atmosphere had little weight loss, small amounts of functional groups, and high carbon content. The fabricated glass-like carbons upon heat treatment at 2600 °C showed an amorphous stage without any grain growth and/or reconstruction of structure. The glass-like carbon had much better chemical resistance than the artificial graphite, and exhibited a high chemical resistance due to its low surface areas, minimum impurities, and low graphite crystallites.