Yoshiaki Ohkawara

Quantitative Analysis of Hydrogen in Amorphous Films of Hydrogenated Carbon Nitride

The amorphous phase of hydrogenated carbon nitride, a-CNx:H (0 x 1), films may have clusters consisting of a mixture of sp2- and sp3-hybridized materials with cluster sizes of 0.2–2 nm. The hydrogen termination limits the size of the carbon and carbon nitride clusters. It also influences the mechanical properties of the sample. In this experiment, the relationship between the hydrogen content and the mechanical properties of carbon and related materials was investigated using elastic recoil detection analysis (ERDA), nanoindentation techniques and Raman spectroscopy. The samples were classified into three categories of hardness: mechanically soft a-CNx:H (hardness: 1–8 GPa), mechanically hard a-CNx:H (8–30 GPa) and hard hydrogenated amorphous carbon (a-C:H) (more than 30 GPa). The hydrogen contents of the sample were 10–50 at.%, 5–40 at.%, and less than 3 at.% for soft a-CNx:H, hard a-CNx:H and hard a-C:H, respectively.

Dehydrogenation of Nitrogen-Containing Carbon Films by High-Energy He2+ Irradiation

It is well known that amorphous carbon and related films consist of nano-sized carbon clusters. With the incorporation of hydrogen into these films, the hydrogen termination limits the cluster size and decreases the bond strength among clusters. In this study, dehydrogenation from amorphous hydrogenated carbon nitride films was accomplished using 3.75 MeV-He2+ irradiation. The hydrogen atoms forming methyl and ethylene groups were mainly removed from the film during the irradiation procedure. With the progress of dehydrogenation from methyl and ethylene groups, the degree of order of the atomic configuration became large, resulting in an increase of the cluster size. The mechanical properties of amorphous films were improved due to cluster growth.

Field emission property of Al:ZnO whiskers modified by amorphous carbon and related films

A new type of ceramic field emitter was designed using the chemical vapor deposition process. There are two major features of the materials used to construct the emitter: an electrically conductive Al:ZnO whisker with an extremely sharp tip and amorphous carbon and related coating materials with a relatively low electron affinity at the tip region. Some aggregations of whiskers were synthesized as a function of the radius of curvature at the whisker tip. In addition, four types of amorphous coatings were provided for this experiment: C:H, CNx:H with C–H and N–H terminations, CNx:H with N–H termination and CNx films. The field emission property was influenced not only by the radius of curvature at the tip but also by the termination structure of the amorphous carbon and related films.

Hardness and Structure of a-CNx Films Synthesized by Chemical Vapor Deposition

Mechanically hard a-CNx films were synthesized using a combination of ion bombardment and the chemical vapor deposition process using the dissociative excitation reaction of BrCN with Ar metastable atoms. Nanoindentation tests disclosed that the indentation hardness, Youngs modulus and elastic recovery increased with increasing ion-accelerating voltage. Moreover, the degree of flow among clusters decreased in the ion-bombarded sample. The D (disordered)-band absorption on an infrared absorption spectrum was replaced by a C–N absorption assigned to the tertiary aromatic amine. These results suggest that the internal and external structures of the carbon nitride cluster change from the two-dimensional order to the three-dimensional order of C–N. The structure of hard a-CNx is clearly distinguishable from nitrogen-containing diamond-like carbon.

Work Function of Amorphous Carbon Nitride with Various Functional Groups

The work function of the amorphous phase of hydrogenated carbon nitride (a-CNx:H) was calculated using Fowler–Nordheim field emission. First, metal films were deposited on conductive ZnO:Al whiskers to determine the geometric field enhancement factor at the tip of the field emitter. The radius of curvature at the whisker tip after the deposition of metal films was measured. Next, the geometric field enhancement factor of ceramic whiskers was calculated using the work function of the metals and the radius of curvature of the emitter tip. Finally, the work function of a-CNx:H was introduced using the Fowler–Nordheim equation with the geometric field enhancement factor. The work function of a-CNx:H is dependent upon the functional group.

Ion-Induced Processes in the Dissociative Excitation Reaction of BrCN to Synthesize Mechanically Hard Amorphous Carbon Nitride Films in the Microwave Plasma Chemical Vapor Deposition System

High-resolution emission spectra have been observed for the dissociative excitation reaction of BrCN with the microwave-discharge flow of Ar to synthesize mechanically hard a-CNx films. From the analysis of intensity of the CN(B2Σ+–X2Σ+) emission under the conditions of trapping charged species and the addition of SF6, BrCN is found to be excited via the charge transfer from Ar+ followed by the recombination of BrCN+ with thermal electrons as well as the energy transfer from Ar(3P0,2). The relative intensity of the resonance lines of Ar+ and Ar, IAr+ /IAr, has been measured under the RF-biased condition. The dependence of IAr+ /IAr on the RF-bias voltage is found to be strongly correlated with that of the reported hardness of films [Tanaka et al.: Jpn. J. Appl. Phys. 39 (2000) 4148], confirming the proposed mechanism of film hardening by the bombardment of the film surface by Ar+.

X-ray Source with Cold Emitter Fabricated Using ZnO Conductive Whiskers

A new structure for the surface illuminant X-ray source system using a low-cost aluminum-doped zinc oxide (ZnO:Al) whisker cold emitter is proposed and a prototype setup is introduced. The X-ray source was fabricated using a cold cathode that consists of an aggregation of conductive ZnO:Al whiskers. The ZnO:Al whiskers were deposited on an n-type single crystalline wafer of silicon by an atmospheric chemical-vapor-deposition method. The electron emission current from the ZnO:Al whiskers varied with the Al concentration of ZnO. The diode and triode X-ray sources generated X-rays from the tungsten target used as an anode. X-ray images of a leaf and a small sardine were taken on medical X-ray films. Clear X-ray images were obtained.

Synthesis of graphite using laser decomposition of SiC

The surface of polycrystalline silicon carbide with and without binder was modified by one pulse of 5 ms irradiation using an Nd:YAG laser in argon, carbon dioxide and air atmospheres. At the irradiated area on the surface of the sample, the color of the surface was completely different from that of the nonirradiated area, with the occurrence of surface ablation. In addition, Raman spectra indicated a very sharp and intense peak at 1580 cm−1, suggesting the presence of crystallized graphite. X-ray diffractometry revealed the existence of the (002) diffraction line that is attributable to graphite.

Hydrogen Storage in Amorphous Phase of Hydrogenated Carbon Nitride

A hydrogen storage characteristic of amorphous phase of hydrogenated carbon nitride (a-CNx:H) was evaluated at room temperature under high-pressure hydrogen. The hydrogen content in the sample was directly measured using a volumetric analysis established for measuring the hydrogen adsorption of metal alloys. The content of stored hydrogen in a-CNx:H was twice as much as those of multiwalled nanotubes, charcoal activated powder and carbon fiber.

Hydrogen storage phenomenon in amorphous phase of hydrogenated carbon nitride

A hydrogen storage characteristic of the amorphous phase of hydrogenated carbon nitride (a-CNx:H) deposited using a gas mixture of CH4 and N2 by a chemical-vapor-deposition method enhanced by electron-cyclotron-resonance plasma was evaluated under high-pressure hydrogen atmosphere. The hydrogen content in the sample at 300 K and 77 K was directly measured using a volumetric analysis established for evaluation of hydrogen absorption of metal alloys. The contents of stored hydrogen in the sample under 12 MPa were determined to be 1.1 wt.% and 2.2 wt.% at 300 K and 77 K, respectively. The contents of stored hydrogen in the sample were larger than those of a-CNx:H deposited from the dissociated reaction of CH3CN and BrCN. The results of the storage behavior of hydrogen and infrared absorption suggest that the mechanism of hydrogen storage might be explained as chemical and physical absorption.