Comparative investigations of structure and properties of BCN coatings deposited by thermal and plasma-enhanced CVD

Hard BCN films were deposited by chemical vapour deposition (CVD) on Si(100) substrates. TCVD (thermal activated) and PECVD (GD or RF microwave plasma-activated, respectively) were used. The films were analysed with respect to chemical state, composition, morphology and microstructure, oxidation behaviour and hardness. Wavelength dispersive X-ray spectroscopy (EPMA), infrared spectroscopy (IR), transmission electron microscopy (TEM), differential thermal analysis (DTA) and hardness evaluation were employed for film characterization. A correlation between deposition parameters and film composition, structure and hardness could be proved in every CVD process. Parallels between TCVD and PECVD films emerged in the case of chemical composition and the correlation between carbon content and hardness values. Considerable differences exist with regard to the microstructure, especially the texture of the films. Moreover in TCVD films the carbon is preferentially incorporated between the BN basal planes, whereas in PECVD films it is incorporated preferentially in as well as between the BN basal planes.     

分子动力学模拟无定形BCN体系结构特征

为研究无定形BCN材料的原子结构特征, 采用分子动力学方法模拟了高温下无定形BCN体系的动力学行为, 计算并分析了引入C原子对体系的径向分布函数(RDF)和配位数的影响. 模拟结果表明, C原子对BCN体系的RDF曲线的峰位置及形状影响很小, 但使其偏径向分布函数(PRDF)的一些峰向右偏移; 引入的C原子部分占据了原来N原子的位置, 使得B-N的配位数降低. 在模拟条件下, 未观察到BN相与C相分离.     

BCN化合物的合成与表征

以石墨和六方氮化硼(h-BN)粉为原料,利用高能机械球磨和高温高压技术对BCN化合物的形成、结构及相变进行了研究.经120h球磨制备出BCN非晶体.在1400cm^-1附近,BCN非晶有一宽化的强红外吸收峰,在740和1630cm^-1附近观察到弱的红外吸收峰;在1330cm^-1附近观察到一宽化的Raman散射峰.BCN非晶中B_1s的结合能为191.9eV,C_1s的结合能为284.9和286.8eV,N_1s的结合能为398.3和400.5eV.将BCN非晶在4GPa和1473K下退火45min后转化为六方结构的BCN晶体,其晶格常数为a=0.2505nm,c=0.6664nm.其红外光谱特征吸收峰分别出现在1398,1103,1024,925和802cm^-1.Raman散射峰分别出现在1328,1358,1582和1614cm^-1.并对非晶BCN的形成和相转变机制进行了研究.     

Specific features of synthesis,chemical composition,and thermal stability of anodic borosilicate coatings on silicon

Thickness and resistivity of anodic borosilicate coatings synthesized on n-type silicon were studied in relation to the conditions in which the final forming potentials are reached and to the content of boric acid in an ethylene glycol solution. A method for synthesis of the coatings for use in nanoelectronics was suggested on the basis of data on the thermal stability of their chemical composition.     

XPS investigations on boron nitride fibre coatings prepared by chemical vapour deposition in comparison to their hydrolytic rate

Investigations on CVD boron nitride films on fibres by means of photoelectron and X-ray spectroscopy resulted in B/N ratios above the stoichiometric value 1 and oxygen contents up to 25 at%. Compared to the hydrolytic rate of the films an apparent dependence was found on the deposition rate and some evidence of the oxygen concentration. CVD fibre coatings exhibit a hexagonal turbostratic structure with extremely small atomic layer plane dimensions, which was proved by transmission electron microscopy. Corresponding to oxygen concentrations in pyrolytic carbon films with similar structure a model is proposed, where the small atomic layers with dimensions of some nanometers cause a relatively high oxygen concentration in the boron nitride films. The oxygen atoms saturate the dangling bonds. Moreover the B/N ratio extents the expected stoichiometric ratio due to the oxygen atoms at nitrogen sites. Received: 15 July 1997 / Revised: 29 January 1998 / Accepted: 2 February 1998     

Influence of the composition of BCN films deposited by reactive magnetron sputtering on their properties

Compounds of the B--C--N system are very promising to produce superhard coatings with good tribological, chemical, and thermal properties. To investigate the influence of the composition of BCN films on their properties, films with five different compositions at nearly constant nitrogen content were deposited on silicon wafers by magnetron sputtering from hexagonal boron nitride and graphite targets operated in RF and DC mode, respectively. The compositions and binding states of the films were determined by XPS. The nitrogen content was found to be almost constant for all films at about a 40 at-%, whereas boron and carbon compositions ranged between 15-35 and 25-50 at-%, respectively. The electronic and bonding structure of the coatings were analyzed by REELS using three different electron beam energies to obtain information at different depths. An increase of the carbon content of the films resulted in a significant shift of the pi-pi* interband transition with respect to the energy loss corresponding to h-BN. The absence of the pi-pi* transition in the energy loss spectra acquired at a beam energy of 1900 eV indicates the existence of a very thin overlayer mostly sp(2) bonded and probably with a distorted hexagonal structure. The position of the bulk plasmon losses corresponded to the hexagonal phase for the overlayer and presented a shift of more than 1.5 eV to the higher energy loss direction for the spectra obtained at 1900 eV beam energy. This shift and the absence of the sp(2)-bond fingerprint induced the possibility of an underlying disordered structure with a majority of sp(3) bonds.     

Electrochemical behavior and composition of bismuth sulfide coatings

Cyclic potentiodynamic voltammetry and X-ray photoelectron spectroscopy were used to study the electrochemical behavior and composition of bismuth sulfide coatings deposited by successive ionic layer adsorption and reaction on glassy carbon in a colloid solution of bismuth(III) compounds, with Na₂S as a sulfidizing agent.     

Structural and thermal studies on sisal fibre

Mechanical characteristics of sisal fibre, such as UTS and Young's modulus, have been determined. The thermal degradation of sisal fibre has been observed by running its DSC and TG/DTG.

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Zusammenfassung Die mechanischen Kennwerte (UTS, Young's Modul) von Sisalfasern wurden bestimmt. Der thermische Abbau von Sisalfasern wurde mittels DSC und TG/DTG untersucht.

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The authors wish to thank Dr. R. K. Tiwari, Dr. S. V. Prasad and Dr. M. Patel for their useful suggestions.     

Structure, chemical composition, and reactivity correlations during the in situ oxidation of 2-propanol

Unraveling the complex interaction between catalysts and reactants under operando conditions is a key step toward gaining fundamental insight in catalysis. We report the evolution of the structure and chemical composition of size-selected micellar Pt nanoparticles (～1 nm) supported on nanocrystalline γ-Al(2)O(3) during the catalytic oxidation of 2-propanol using X-ray absorption fine-structure spectroscopy. Platinum oxides were found to be the active species for the partial oxidation of 2-propanol (<140 °C), while the complete oxidation (>140 °C) is initially catalyzed by oxygen-covered metallic Pt nanoparticles, which were found to regrow a thin surface oxide layer above 200 °C. The intermediate reaction regime, where the partial and complete oxidation pathways coexist, is characterized by the decomposition of the Pt oxide species due to the production of reducing intermediates and the blocking of O(2) adsorption sites on the nanoparticle surface. The high catalytic activity and low onset reaction temperature displayed by our small Pt particles for the oxidation of 2-propanol is attributed to the large amount of edge and corner sites available, which facilitate the formation of reactive surface oxides. Our findings highlight the decisive role of the nanoparticle structure and chemical state in oxidation catalytic reactions.     

Sol-gel coatings for chemical protection of stainless steel

Sol-gel thin coatings of ZrO₂, SiO₂, 70SiO₂-30TiO₂ and 88SiO₂-12Al₂O₃ compositions (mole %) have been prepared from sonocatalyzed sols and deposited by dip-coating technique on 316L stainless steel foils. The influence of the coatings on the chemical corrosion of the substrate has been measured through potentiodynamic polarization curves in aqueous 15% H₂SO₄ solution between 25 and 50°C. The values of the corrosion potential, polarization resistance and corrosion rate have been determined. Analysis of the data combined with scanning electron microscopy studies indicate that the films act as a geometric blocking against exposure to the corrosive media and increase the lifetime of the substrate up to a factor 8.5.     

Structure, stability, and dynamics of canonical and noncanonical base pairs: quantum chemical studies

The importance of non-Watson-Crick base pairs in the three-dimensional structure of RNA is now well established. The structure and stability of these noncanonical base pairs are, however, poorly understood. We have attempted to understand structural features of 33 frequently occurring base pairs using density functional theory. These are of three types, namely (i) those stabilized by two or more polar hydrogen bonds between the bases, (ii) those having one polar and another C-H...O/N type interactions, and (iii) those having one H-bond between the bases and another involving one of the sugars linked to the bases. We found that the base pairs having two polar H-bonds are very stable as compared to those having one C-H...O/N interaction. Our quantitatively analysis of structures of these optimized base pairs indicates that they possess a different amount of nonplanarity with large propeller or buckle values as also observed in the crystal structures. We further found that geometry optimization does not modify the hydrogen-bonding pattern, as values of shear and open angle of the base pairs remain conserved. The structures of initial crystal geometry and final optimized geometry of some base pairs having only one polar H-bond and a C-H...O/N interaction, however, are significantly different, indicating the weak nature of the nonpolar interaction. The base pair flexibility, as measured from normal-mode analysis, in terms of the intrinsic standard deviations of the base pair structural parameters are in conformity with those calculated from RNA crystal structures. We also noticed that deformation of a base pair along the stretch direction is impossible for all of the base pairs, and movements of the base pairs along shear and open are also quite restricted. The base pair opening mode through alteration of propeller or buckle is considerably less restricted for most of the base pairs.     

Optical fibre gratings as tools for chemical and biochemical sensing

Optical fibre gratings have recently been suggested as optical platforms for chemical and biochemical sensing. On the basis of the measurement of refractive index changes induced by a chemical and biochemical interaction in the transmission spectrum along the fibres, they are proposed as a possible alternative to the other label-free optical approaches, such as surface plasmon resonance and optical resonators. The combination of the use of optical fibres with the fact that the signal modulation is spectrally encoded offers multiplexing and remote measurement capabilities which the other technology platforms are not able to or can hardly offer. The fundamentals of the different types of optical fibre gratings are described and the performances of the chemical and biochemical sensors based on this approach are reviewed. Advantages and limitations of optical fibre gratings are considered, with a look at new perspectives for their utilization in the field.     

GC-MS studies of the chemical composition of two inedible mushrooms of the genus Agaricus

Background  

Mushrooms in the genus Agaricus have worldwide distribution and include the economically important species A. bisporus. Some Agaricus species are inedible, including A. placomyces and A. pseudopratensis, which are similar in appearance to certain edible species, yet are known to possess unpleasant odours and induce gastrointestinal problems if consumed. We have studied the chemical composition of these mushrooms using GC-MS.     

Characterisation of boron nitride fibre coatings with different crystalline order by TEM and XPS

Reinforcement effects in composites are widely influenced by fibre coatings. A detailed understanding of their microstructure and chemical composition is of great interest. Boron nitride films were deposited continuously on fibre rovings of various ceramics in CVD reactors of vertical as well as horizontal position. XPS depth profilings show that the film compositions are close to stoichiometric BN with carbon and oxygen impurities in the range of 10 at%. Cross-sections of separated fibres were investigated by HREM and TEM diffraction. All BN films are hexagonal turbostratic. The (002) layers with an increased distance (about 0.36 nm) showed a mean stacking sequence near to graphite and a characteristic orientation to the fibre in the interface region. We assume the gas flow type and hence the exchange rate of matter and energy determines the film structure in this region. With increasing film thickness the (002) layers fold randomly in all directions or form nanocrystals at elevated temperatures. Received: 7 September 1998 / Revised: 13 January 1999 / Accepted: 5 February 1999     

Structure and composition of polymer-bitumen composites

Features of the composition and structure of polymer-bitumen composites are examined, including the effects of the amount and quality of the basic components on the characteristics of the final product. The effect of butadiene-styrene thermoplastic elastomer on the properties of the cold applied polymerbitumen mastic is considered. Typical component mixing schemes are analyzed, and a flowsheet for production of polymer-bitumen mastic is presented.     

Microstructure and chemical composition of giant avian eggshells

The microstructure and composition of the layers of two giant avian eggshells were investigated using a combination of scanning electron microscopy, electron probe microanalyses, and X-ray absorption near-edge structure spectroscopy (XANES). The two species have some similarities and differences in their microstructure and composition; the composition is not homogeneous throughout the eggshell thickness. XANES studies show that sulfur is associated with amino acids in the inner organic membranes, whereas in the mineralised layers the sulfur is mainly associated with sulfated polysaccharides. These results are similar to those obtained on chicken eggshells, and confirm the active role of sulfated acidic polysaccharides in biomineralisation processes of carbonate skeletons.     

Solid-state NMR studies of coatings and interfaces in batteries

As the need for energy storage increases, battery technology continues to be developed and improved – aiming for rechargeable batteries that store more energy, charge faster, last longer, and are more sustainable. Solid-state NMR spectroscopy has emerged as a versatile technique for studying both the local structure and ion mobility of battery materials. Here, we explore the use of solid-state NMR to study coatings and interfaces within batteries. We focus on the study of the electrode–electrolyte interphases that form as a product of battery cycling, and artificial coatings that are used in batteries to improve their performance and longevity – both of which can have a crucial impact on battery performance. We also explore the experimental considerations that need to be taken into account, and how advances in NMR methodology have allowed thin coatings and interfaces to be studied.