First-principles lattice dynamics calculations of the phase boundary between beta-Si3N4 and gamma-Si3N4 at elevated temperatures and pressures

The phase boundary between beta-Si(3)N(4) and gamma-Si(3)N(4) is investigated at high pressure and high temperature using first-principles lattice dynamics calculations within the quasi-harmonic approximation. We find a positive slope of the phase boundary, hence, at higher temperatures it requires higher pressures to synthesize the high-pressure polymorph of silicon nitride. It turns out that the thermal expansion of the spinel-type gamma-phase is larger than that of the phenacite-type beta-phase. On the other side, pressure affects more the volume of beta-Si(3)N(4) than of gamma-Si(3)N(4), reflected in the higher bulk modulus of gamma-Si(3)N(4) up to about 40 GPa. The origin of the different temperature behavior of these phases, consequently, is rooted in a larger volume dependence of the zero point energy in gamma-Si(3)N(4) in comparison to beta-Si(3)N(4).     

Neue anorganische Ringsysteme. XXII. Neue Spirosilazane der Gerüsttypen Si5N4, Si5N5, Si5N4O und Si7N8

Novel Inorganic Ring Systems. XXII. Novel Spirosilazanes of the Si₅N₄, Si₅N₅, Si₅N₄O, and Si₇N₈ Skeleton We succeeded in preparing the four novel spirosilazane skeletons E , F , G , and H of the composition given in the title. The permethylated compounds of the mentioned systems have been characterized in their chemical and physical properties. The conformation of their structure was possible by elemental analysis and by n.m.r., mass, i.r., and Raman spectra.     

Characterization of the oxidized beta-Si3N4 whisker surface layer using XPS and TOF-SIMS.

The change in composition of the surface layer of beta-Si3N4 whiskers was examined after heat treatment in atmosphere. At 873 K, the beta-Si3N4 whisker was barely oxidized. At 1273 K, the oxidation of the surface layers of the whisker occurred easily. With the beta-Si3N4 oxidation, the Si-N bond gradually changed into the Si-N-O bond, and finally became the oxidized layer (amorphous layer) of the whisker surface. It was assumed that the whisker surface has a gradient interface structure which gradually changes from the oxide layer of the whisker's outer surface to the nitride crystal of the inside layer. It was confirmed that impurity elements such as Y and Ca existed mainly in the amorphous region near the interface between the amorphous layer and the crystal layer.     

Uniform micro-sized alpha- and beta-Si3N4 thin ribbons grown by a high-temperature thermal-decomposition/nitridation route

Uniform micro-sized alpha- and beta-Si(3)N(4) thin ribbons have been achieved by a high-temperature thermal-decomposition/nitridation route. As-grown ribbons were characterized by using powder X-ray diffraction, scanning electron microscopy, transmission electron microscopy, energy-dispersive X-ray spectroscopy, electron energy loss spectroscopy, and cathodoluminescence. These alpha- and beta-Si(3)N(4) ribbons are structurally uniform micro-sized single crystals, and have a width of approximately 2-3 microns, a thickness of approximately 20-60 nm, and a length, that ranges from several hundreds of microns to the order of millimeters. A room-temperature cathodoluminescence spectrum recorded from these ribbons shows one intensive blue emission peak at approximately 433 nm. The growth for the new ribbon form of this material is believed to be dominated by a vapor-solid process.     

XPS-investigations on laser-modified Si3N4 ceramics

For the ultimate use of micro machining in technology, the interaction of laser light and silicon nitride ceramic have been studied. Using X-ray photoelectron spectroscopy (XPS), different effects generated by radiation from Nd:YAG (free running mode) and excimer lasers have been observed. With the Nd:YAG laser (λ = 1.06 μm), reaction zone depths of several 100 nm, apparently caused by melting, were found. The excimer laser treatment with a lower penetration depth of the light (λ = 248 nm) led to very low chemical surface damage. Thus the excimer laser is favored for exact micro machining of nitride ceramics.     

Synthesis and microstructural analysis of Si3N4 nanorods.

Alpha-Si3N4 nanorods with 20-80 nm width were synthesized by carbothermal reduction of SiO with amorphous activated carbon (AAC) as a reductant. Microstructural characterization of the synthesized nanorods was carried out by high resolution transmission electron microscopy (HRTEM) and energy dispersive X-ray analysis. Many Si3N4 nanorods were found to be twisted. Each twisted nanorod contained several straight Si3N4 parts. The straight parts had the rod axes orientated along the (1010) direction, which is the closest packing direction of alpha-Si3N4. There were two kinds of joints between the two adjacent straight Si3N4 parts. Formation mechanism of the Si3N4 nanorods is discussed.     

Hierarchical assemblies of Si3N4 nanostructures

In the present work, for the first time, we report the growth of hierarchical assemblies of Si₃N₄ nanostructures via catalyst-assisted pyrolysis of a polymeric precursor on the Si substrates. The synthesized products were characterized by using field emission scanning electron microscopy, X-ray diffraction, and transmission electron microscopy. It is found that the size of the catalytic droplet plays a critical role on the formation of hierarchical assemblies of Si₃N₄ nanostructures rather than common single nanowire. A mechanism based on the Vapor–Liquid–Solid (VLS) process was proposed for the assembly of hierarchical Si₃N₄ nanostructures.     

Deep Oxidation of Methane on a Pt/Si3N4 Catalyst

We have studied platinum catalysts supported on silicon nitride Si₃N₄ in the process of deep oxidation of methane. We have used transmission electron microscopy and X-ray photoelectron spectroscopy to study the surface properties of the Pt/Si₃N₄ samples before and after the catalytic reaction. We have established that the metallic platinum particles in freshly prepared systems are characterized by average sizes of 1.7-5.3 nm, while after the catalytic reaction we observe formation of Pt crystallites up to 30-70 nm in size. We hypothesize that the observed deactivation of platinum catalysts in deep oxidation of methane is connected with crystallization of the metallic particles and their entrainment with the reaction products during catalysis.     

Acidity measurements at elevated temperatures. VII. Dissociation of water

The self-dissociation of water has been studied over the temperature range from 0 to 300°C and in KCl media from 0.02m to 2.7m. Also, isothermal pressure coefficients of the dissociation quotients have been obtained in these same media up to 250°C. A potentiometric method employing a hydrogen electrode concentration cell with flowing solutions was employed. The estimated accuracy of logQ _w values up to 250°C is 0.02 log units and at 300°C is 0.04 log units. Smoothing functions have been found which fit these data along with the precise potentiometric data of Harned and co-workers at low temperatures, the existing calorimetric data up to 55°C and the recent conductimetric measurements of pure water up to 271°C by Bignoldet al., within about 1.5 times the estimated errors. Thermodynamic quantities for the dissociation reaction have been tabulated for rounded values of temperature and ionic strength at the saturation pressure of water. The isothermal pressure coefficients of log Q_w varies approximately linearly with the square root of the ionic strength. This and the dependence of logK _w on the density of the water is consistent with the assumption that the molal volumes of aqueous ions vary linearly with the compressibility coefficient of water. The heat for the dissociation reaction at infinite dilution is also shown to be strongly dependent on density.     

SERS based monitoring of toluene vapors at ambient and elevated temperatures by using a ruffled silver nanolayer as a substrate

The authors describe a Surface enhanced Raman spectroscopy (SERS)-based method for the detection of gaseous toluene at different temperature regimes using 3D ruffled silver SERS substrates and a commercially available handheld Raman system equipped with a 785 nm laser. The 3D silver SERS substrates were synthesized via electroless deposition of silver on the ruffled sandpaper and HF-etched silicon wafers. The morphological characterization of the silver SERS substrates was carried out by atomic force microscopy and scanning electron microscopy. UV-Vis spectroscopy absorption spectra of the silver nanostructures showed plasmonic peaks at 522 nm and 731 nm. Toluene vapors were collected with a syringe at ambient temperature and at 100 °C, while SERS detection was always performed at room temperature. Toluene detection was based on the measurement of the Raman bands at 787 cm^?1 and 1003 cm^?1 (in the fingerprint region). The method allow gaseous toluene to be detected at its vapor concentrations of 522 ppm (mg/L), 261 ppm (mg/L) and 26 ppm (mg/L).

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Graphical abstract Schematic presentation of an original method for the detection of toluene vapors by SERS technique. The collection of toluene vapors was carried out at room and at high temperatures. The vapors were transferred to methanol by bubbling. The SERS measurements were carried out at room temperature.

     

Electrochemical performance of nano-sized ZnO-coated LiNi0.5Mn1.5O4 spinel as 5 V materials at elevated temperatures

ZnO-coated LiNi_0.5Mn_1.5O₄ powders with excellent electrochemical cyclability and structural stability have been synthesized. The electrochemical performance and structural stability of ZnO-coated LiNi_0.5Mn_1.5O₄ electrodes in the 5 V region at elevated temperature has been studied as function of the level of ZnO coating. The 1.5 wt% ZnO-coated LiNi_0.5Mn_1.5O₄ electrode delivers an initial discharge capacity of 137 mAh g⁻¹ with excellent cyclability at elevated temperature even at 55 °C. The reason for the excellent cycling performance of ZnO-coated LiNi_0.5Mn_1.5O₄ electrode is largely attributed to ZnO playing an important role of HF getting in the electrolyte.     

Thermodynamics of Silicon Nitride. Standard molar enthalpy of formation of amorphous Si3N4 at 298.15 K

The standard molar enthalpy of formation Δ_f H _m ⁰=–760±12 kJ for amorphous silicon nitride a-Si₃N₄ has been determined from fluorine combustion calorimetry measurements of the massic energy of the reaction: a-Si₃N₄(s)+6F₂(g)=3SiF₄(g)+2N₂(g). This value combined with Δ_f H _m ⁰= –828.9±3.4 kJ for a-Si₃N₄ indicates that determined for the first time molar enthalpy change for the transition from amorphous to α-crystalline form Δ_trs H _m ⁰=69±13 kJ is very evident, in spite of its large uncertainty range resulting from impurity corrections. This revised version was published online in July 2006 with corrections to the Cover Date.     

Formation mechanisms of Si3N4 and Si2N2O in silicon powder nitridation

Commercial silicon powders are nitrided at constant temperatures (1453 K; 1513 K; 1633 K; 1693 K). The X-ray diffraction results show that small amounts of Si₃N₄ and Si₂N₂O are formed as the nitridation products in the samples. Fibroid and short columnar Si₃N₄ are detected in the samples. The formation mechanisms of Si₃N₄ and Si₂N₂O are analyzed. During the initial stage of silicon powder nitridation, Si on the outside of sample captures slight amount of O₂ in N₂ atmosphere, forming a thin film of SiO₂ on the surface which seals the residual silicon inside. And the oxygen partial pressure between the SiO₂ film and free silicon is decreasing gradually, so passive oxidation transforms to active oxidation and metastable SiO(g) is produced. When the SiO(g) partial pressure is high enough, the SiO₂ film will crack, and N₂ is infiltrated into the central section of the sample through cracks, generating Si₂N₂O and short columnar Si₃N₄ in situ. At the same time, metastable SiO(g) reacts with N₂ and form fibroid Si₃N₄. In the regions where the oxygen partial pressure is high, Si₃N₄ is oxidized into Si₂N₂O.     

Si3N4陶瓷的连接研究进展

在系统介绍陶瓷与陶瓷、金属的连接方法基础上 ,重点评述了Ｓｉ3Ｎ4 陶瓷的直接钎焊法和Ｓｉ3Ｎ4 陶瓷的间接钎焊法的连接工艺进展及其存在的问题和解决的措施 ,同时对目前研究较少的Ｓｉ3Ｎ4 陶瓷的玻璃焊法也进行了评述 ,并简介了Ｓｉ3Ｎ3陶瓷固相压力扩散焊法。Ｉｎｔｈｉｓｐａｐｅｒ,ａｓｙｓｔｅｍａｔｉｃｄｅｓｃｒｉｐｔｉｏｎｏｆｊｏｉｎｉｎｇｔｅｃｈｎｉｑｕｅｓｏｆｃｅｒａｍｉｃｔｏｉｔｓｅｌｆａｎｄｗｉｔｈｍｅｔａｌ,ｅｓｐｅｃｉａｌｌｙｔｈｅｄｅｖｅｌｏｐｍｅｎｔｏｆＳｉ3Ｎ3ｃｅｒａｍｉｃｄｉｒｅｃｔｂｒａ…     

Topochemical 3D polymerization of C60 under high pressure at elevated temperatures

Fullerene C60 monomer crystals were compressed to a face-centered cubic (fcc) phase with a lattice parameter of a = 11.93(5) A and a micro-Vickers hardness of 4500 kg/mm2 using high-pressure and high-temperature conditions of 15 GPa at 500-600 degrees C. The hardness is compatible with that of cubic boron nitride (c-BN), suggesting the formation of a 3D C60 polymer. The single-crystal X-ray structural analysis revealed that each C60 molecule in the polymer was linked to the 12 nearest neighbors by [2+2] cycloaddition between the common pentagon-hexagon (56) edges. However, ab initio geometry optimization and molecular dynamics calculations suggested that the 3D polymer should have a rhombohedral structure with the space group of R containing [3+3] cycloaddition between the pentagons of C60 molecules within the plane perpendicular to the 3-fold axis. The higher apparent symmetry of fcc was observed as an averaged structure of different orientations of the rhombohedral structure. The R structure can be derived by only a slight rotation of each C60 unit in the (111) plane of the fcc structure. The band-structure calculation suggested that the 3D polymer (R) was a semiconductor; the activation energy for the electrical conductivity was experimentally determined to be 0.25 eV at 550 K.