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Tatsumi, Kazuyoshi; Tanaka, Isao; Adachi, Hirohiko; Oba, Fumiyasu; Sekine, Toshimori
doi: 10.1111/j.1151-2916.2002.tb00029.xpmid: N/A
New phases of Si3N4 that may be stable at higher pressure than spinel have been searched using a first‐principles plane‐wave pseudopotential method. The CaTi2O4‐type phase is found to be the prime candidate for the post‐spinel phase among six phases selected on the analogy to high‐pressure oxides. The phase transformation from the spinel is predicted to occur at 210 GPa. All silicon atoms of the new phase are coordinated by six anions, similar to the case of the high‐pressure forms of SiO2 and SiC. Because of its high energy at zero pressure, this new phase may be difficult to quench. The bandgap increases with an increase of pressure when compared in the same polymorph. However, the bandgap and the net charge decrease in the order of β, spinel, and CaTi2O4‐type phases at zero pressure. The theoretical bulk modulus of the CaTi2O4‐type phase is comparable with that of spinel.
Ching, Wai‐Yim; Mo, Shang‐Di; Chen, Yu
doi: 10.1111/j.1151-2916.2002.tb00030.xpmid: N/A
Using a recently developed first‐principles supercell method that includes the electron and core‐hole interaction, the XANES/ELNES spectra of Si‐L2,3, Si‐K, and N‐K edges in α‐Si3N4, β‐Si3N4, spinel c‐Si3N4, and Si2N2O were calculated and compared. The difference in total energies between the initial ground state and the final core‐hole state provides the transition energy. The calculated spectra are found to be in good agreement with the experimental measurements on β‐Si3N4 and c‐Si3N4. The differences in the XANES/ELNES spectra for the same element in different crystals are explained in terms of differences in local bonding. The use of orbital‐decomposed local density of states to explain the measured spectra is shown to be inadequate. These results reaffirm the importance of including the core‐hole effect in any XANES/ELNES spectral calculation.
doi: 10.1111/j.1151-2916.2002.tb00031.xpmid: N/A
Important hard phases are included in the quaternary compositional system Si‐N‐C‐B. This paper reviews ternary amorphous and crystalline phases in the system Si‐N‐C and deliberates on the issue of stability of the binary C3N4, a hypothetical phase harder than diamond, and instability of nitrides in general. There is a tendency for nitrogen atoms to agglomerate and be released as nitrogen molecules. Stabilization of CN radicals can be achieved through ternary phases: carbonitrides metal‐C‐N. Ternary Si‐N‐C phases have been synthesized by pyrolysis of polyorganosilazanes, physical vapor deposition, and chemical vapor deposition. The crystalline α‐Si3N4:C phase can incorporate about 6 at.% C and yields enhancement of hardness and wear resistance. Other crystalline phases contain more carbon, for example, Si2CN4.
Aizawa, Tatsuhiko; Akhadejdamrong, Thananan; Iwamoto, Chihiro; Ikuhara, Yuichi; Mitsuo, Atsushi
doi: 10.1111/j.1151-2916.2002.tb00032.xpmid: N/A
Chlorine implantation into TiN coatings decreases the wear loss and the friction coefficient. Even by low‐dose chlorine implantation, the wear volume is decreased by three orders of magnitude or more, and the friction coefficient becomes <0.1. This self‐lubrication mechanism is related to the presence and mobility of implanted chlorine atoms inside the columnar TiN microstructure. According to observations of chlorine‐implanted TiN coatings using high‐resolution transmission electron microscopy, the chlorine atoms are present in the damaged region, where TiN is composed of nanosized grains. From these data, a self‐lubrication mechanism is proposed with chlorine catalyzing the oxidation of titanium and leading to the formation of some tribological reaction product.
Gu, Hui; Cannon, Rowland M.; Seifert, Hans J.; Hoffmann, Michael J.; Tanaka, Isao
doi: 10.1111/j.1151-2916.2002.tb00033.xpmid: N/A
The nitrogen solubility in the SiO2‐rich liquid in the metastable binary SiO2‐Si3N4 system has been determined by analytical TEM to be 1%–4% of N/(O + N) at 1973–2223 K. Analysis of the near edge structure of the electron energy loss peak indicates that nitrogen is incorporated into the silicate network rather than being present as molecular N2. A regular solution model with a positive enthalpy of mixing for the liquid was used to match the data for the metastable solubility of N in the presence of crystalline Si3N4 and to adjust the computed phase diagram. The solubility of Si3N4 in fused SiO2 is far less than reported in liquid silicates also containing Al, Mg, and/or Y. Apparently, these cations act as modifiers that break anion bridges in the silicate network and, thereby, allow further incorporation of Si3N4 without prohibitive amounts of network cross‐linking. Finally, indications emerged regarding the diffuse nature of the Si3N4‐SiO2 interface that leads to amorphous regions of higher N content.
doi: 10.1111/j.1151-2916.2002.tb00034.xpmid: N/A
Contrary to the widely accepted observation that grain‐boundary amorphous films for a given Si3N4 composition have common (equilibrium) widths and compositions, a significant variation for both parameters from film to film was observed in an undoped high‐purity Si3N4 prepared using a hot isostatic pressing method. This material previously has been reported to have an equilibrium film width of 0.6 nm, as measured using a high‐resolution electron microscopy (HREM) method; this value is significantly different from that which is typical for other high‐purity Si3N4 ceramics (1.0 nm). A total of four boundaries were analyzed, using spatially resolved electron energy‐loss spectroscopy methods, which can give the chemical width and composition for the film. Widths of these grain‐boundary films were substantially different from each other; only the thinnest matches the previous HREM observations. The nitrogen content in the film decreased concurrently as the film thickened. This material had many cavities and complicated configurations at triple pockets, because of the very low total‐SiO2 content (0.55 vol%). They created locally different equilibrium conditions for grain‐boundary films, in comparison with other fully densified Si3N4, causing such strong variation in both film structure and chemistry. This observation reveals the importance of triple pockets in equilibrium film structures, providing new insight in evaluating the absorption and wetting models. The thinnest film may correspond to the amorphous structure that is required to bind two randomly oriented Si3N4 grains under greater local stress.
Lee, Soo Min; Kim, Young Hoon; Chung, Su Jin
doi: 10.1111/j.1151-2916.2002.tb00035.xpmid: N/A
GaN buffer and main layers were grown by the conventional hydride vapor phase epitaxy technique using GaCl3 consecutively. The deposited buffer layers were investigated by atomic force microscopy and X‐ray analysis. To examine the behavior of the buffer layers at main layer growth temperature, heat treatment was conducted at 900°C. Based on the results of the buffer layer study, GaN thick films were grown at 1050°C. Optimum deposition conditions of buffer layer from the buffer and main layer studies generally coincided. On the φ scanning pattern, the GaN films grown on (0001) Al2O3 were single‐crystalline. Band‐edge emission dominated photoluminescence was observed at room temperature.
doi: 10.1111/j.1151-2916.2002.tb00036.xpmid: N/A
Two different non‐oxide ceramics, Si3N4 and SiC, were characterized with respect to their grain‐boundary structure employing both scanning and transmission electron microscopy. The latter method, which enables one to gain direct insight of the atomistic interface structure, was utilized to verify whether grain‐boundary wetting occurred. SEM imaging of plasma‐etched surfaces revealed a characteristic bright contrast along interfaces for both ceramics, Si3N4 as well as SiC, suggesting the presence of an intergranular glass film. High‐resolution TEM studies of the Si3N4 sample confirmed that these fine bright lines along grain boundaries represent intergranular glass films separating Si3N4 matrix grains. However, when high‐resolution TEM was employed on SiC samples, which showed a similar contrast variation across SiC grain boundaries in the SEM, the presence of residual glass films was not detected. The SiC materials showed clean grain boundaries with no indication of residual glass even at triple pockets. Chemical analysis monitored yttrium and aluminum segregation at interfaces, which creates a potential barrier (space charges) and therefore affects both the inner mean potential at the interface (Fresnel fringes) and the plasma‐etching response. Although SEM imaging showed a similar interface contrast for both Si3N4 and SiC ceramics, HRTEM studies clearly revealed grain‐boundary wetting in the former and clean interfaces in the latter material, respectively. Hence, SEM imaging and Fresnel fringe TEM imaging alone are not conclusive when characterizing interface wetting in ceramic polycrystals.
Tavernier, Philip R.; Clarke, David R.
doi: 10.1111/j.1151-2916.2002.tb00037.xpmid: N/A
The essential steps required to create GaN seed crystals for bulk crystal growth from thick films are described. These steps include the growth of low‐dislocation‐density GaN films using a hydride vapor‐phase epitaxy route, the separation of the films from the sapphire substrate on which they are grown, and the subsequent growth of GaN on both sides of the free‐standing films. These steps are illustrated with examples that emphasize the growth of material with low threading dislocations.
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