Plasma Processes in Activated Thin Film Deposition

This paper deals with plasma aspects in physical and chemical vapour deposition processes. A systematic representation of deposition methods is given and possibilities of plasma diagnostic are summerized. Characteristic parameters of electron kinetics (mean free paths, collision frequencies, energy distribution functions) and rate equations for calculation of particle densities are discussed for a plasma in plasma activated physical vapour deposition device. Theoretical results are compared with measurements (T_e, N_e, E_z) in the plasma of a TiN_x-deposition device with hollow cathode arc evaporator. Mechanism of plasma polymerisation is discussed. Results of experimental investigations of the plasma in an Ar-hexamethyldisiloxane D.C. low pressure glow discharge are presented and compared with results of modelling of this plasma taking into account direct-, stepwise- and Penning-ionization. The necessivity of knowledge of reactive cross sections for understanding of plasma assisted thin film formation processes is accentuated.     

非零色散位移光纤的制造新工艺研究

介绍了一种新的改进化学气相沉积法(MCVD)+气相轴向沉积法(VAD)预制棒制备工艺,该工艺按照归一化波导结构进行工艺设计和参数控制,即采用MCVD法制备归一化结构参数轴向一致的芯棒,然后采用VAD法轴向沉积相应的外包层,从而得到波导结构轴向均匀一致的预制棒.采用新工艺实验研究了非零色散位移光纤的制造过程,成功制造了波...     

Highly efficient Yb-doped silica fibers prepared by powder sinter technology

We report on the characteristics of an active fiber with core material made by sintering of Yb-doped silica powders as an alternative to a conventional modified chemical vapor deposition (MCVD) technique. This material provides the possibility to design very large and homogenously rare-earth doped active fiber cores. We have determined a fiber background attenuation of 20 dB/km and measured a slope efficiency of 80%. These values are comparable to established fibers made by MCVD technology.     

Characterization of LPCVD and PECVD silicon oxynitride films

The chemical composition and structure of silicon oxynitrides, deposited in low pressure and plasma enhanced chemical vapour deposition processes are discussed. From an extrapolation of the characteristics of plasma grown oxynitrides a model for the deposition of LPCVD material is derived. A main conclusion of this model is that Si-Si bonds have a larger tendency to occur in this material than Si dangling bonds.     

非零色散位移光纤的改进设计及制造

介绍了一种新颖的非零色散位移光纤结构设计方法及其MCVD+OVD制造工艺,所制备的光纤有效面积达到71 μm2以上.采用关键结构区域精确微扰方法,改进了光纤的色散特性.1550 nm处色散斜率由0.0715 ps/(nm2·km),分别减小至0.0605 ps/(nm2·km),0.0466 ps/(nm2·km),零色散波长由1500 nm附近移至1450 nm以下.测量表明,所得光纤具有优越的光学传输特性、抗弯曲性能和熔接性能,适用于C+L和S+C+L工作波长的大容量高速率长距离密集波分复用系统.光纤关键结构区域精确微扰是改进光纤性能的一种有效方法,该方法不限于MCVD工艺和非零色散位移光纤,对新型光纤的设计和生产具有积极的指导意义.     

Simultaneous Compensation of Dispersion and Losses using Er-Doped Double-Core Optical Fiber

The Er-doped double-core dispersion compensating fiber (EDDCF) has been fabricated using modified chemical vapor deposition (MCVD) technique. We have obtained 14 dB gain at 1550 nm (using a diode laser of 980 nm wavelength which provides 100 mW of pump power) with dispersion of about −165 ps/km nm. It is useful for the optical fiber network where amplification as well as negative dispersion is necessary. We are the first to report the experimental realization and characterization of the EDDCF.     

Analysis of model dimensionality,particle shrinkage,boundary layer reactions on particle-scale modelling of biomass char conversion under pulverized fuel combustion conditions

In this work, the effects of model dimensionality, particle shrinkage, and boundary layer reactions on particle-scale modelling of biomass char conversion under pulverized fuel combustion conditions have been analysed by using six models: zero-dimensional models with constant particle size (0D_Cons) or shrinking particle size (0D_SPM), one-dimensional models with/without considering particle shrinkage (1D_Cons/1D_SPM), and 1D_Cons and 1D_SPM with considering boundary layer reactions (1D_Cons_BH and 1D_SPM_BH). A comparison with existing experimental data shows that the 1D_SPM_BH model with consideration of intra-particle heat and mass transfer, particle shrinkage, and boundary layer reactions is an appropriate model to describe biomass char conversion over a wide range of conditions. The 0D_Cons model is a good approximation for the conditions of small particle size (< 1 mm) at 1273–1473 K, but overestimates the char conversion rate for larger biomass char particle or at high temperatures (regime III). The 0D_SPM model gives a reasonable prediction on char conversion time but predicts a larger contribution of reaction between char and O₂ as compared to the 1D_SPM_BH model. The consideration of intra-particle heat and mass transfer in particle-scale modelling (1D_Cons and 1D_SPM) is beneficial to improving the model prediction of char conversion time and the contributions of char oxidation and gasification reactions. The boundary layer reactions have a significant effect on the prediction of char conversion for large particles (> 1 mm) and high temperatures (> 1473 K). An implication for the selection of a particle-scale model in CFD modelling is also given.     

Fluorescence Emission Centres and the Corresponding Infrared Fluorescence Saturation in a Bismuth-Doped Silica Fibre

We investigate the fluorescence characteristics of bismuth doped silica fibres with and without A1 co-dopant which are fabricated by means of modified chemical vapour deposition （MCVD） technique, and find that the fluorescences in the red region （centred around 750nm） and in the infrared region （centred around llOOnm） may originate from different emission sites in the fibre. Strong upconversion phenomena are observed in both Al-codoped and non A1 codoped bismuth fibres when the fibres are excited by an acoustic-optic Q-switched Nd：YVO4 laser. Both the aspects indicate that the upper energy level absorption reported in the work of the bismuth doped silica fibre lasers may result from the fluorescence emission sites that are not responsible for the infrared emission. It is thus expected that optimizing the compositions and the fabrication conditions of the fibre and then transferring more fluorescence emission centres are helpful for the infrared emission.     

Solid freeform fabrication of alumina using laser-assisted ESAVD

Electrostatic spray assisted vapour deposition (ESAVD) has been used successfully to produce films and coatings. A combination of laser-assisted (LA) deposition and ESAVD has been developed for the direct fabrication of alumina ceramics. This novel laser-assisted ESAVD technique involves spraying a liquid precursor onto a substrate whilst a laser beam is simultaneously used to heat the precursor and cause the decomposition and chemical reactions of the precursor in order to produce a solid deposit. A desired three-dimensional (3-D) object can be formed through layer-by-layer deposition using a computer-controlled platform. Dense or porous alumina ceramics can be deposited on the substrate by optimizing the processing parameters. This paper reports the fabrication of alumina parts by the laser-assisted ESAVD method. The effect of processing parameters including laser scanning speed, laser beam size and laser power on the microstructure will be presented. Scanning electron microscope (SEM) and X-ray diffraction (XRD) have been used to characterise the phase and structure of the deposited alumina parts.     

Limitations of mathematical modelling and numerical simulation of industrial and laboratory high-pressure processes

High pressures up to several hundreds of MPa are utilised in a wide range of applications in chemical engineering, bioengineering, and food engineering, aiming at selective control of (bio-)chemical reactions. Non-uniformity of process conditions may threaten the safety and quality of the resulting products as the process conditions such as pressure, temperature, and treatment history are crucial for the course of (bio-)chemical reactions. Therefore, thermofluid dynamical phenomena during the high-pressure process have to be examined, and tools to predict process uniformity and to optimise the processes have to be developed. Recently, mathematical models and numerical simulations of laboratory and industrial scale high-pressure processes have been set up and validated by experimental results. This contribution deals with the assumption of the modelling that relevant (bio-)chemical compounds are ideally dissolved or diluted particles in a continuum flow. By considering the definition of the continuum hypothesis regarding the minimum particle population in a distinct volume, limitations of this modelling and simulation are addressed.     

Homogeneous nucleation and growth in iron-platinum vapour investigated by molecular dynamics simulation

Homogeneous nucleation and growth from binary metal vapour is investigated by molecular dynamics simulation. It is focused here mainly on the iron-platinum system with a mole fraction of 0.5. The simulations are started in the highly supersaturated vapour phase. Argon is added as carrier gas removing the heat of condensation from the forming clusters. The embedded atom method is employed for modelling of the force field of iron and platinum. The simulation runs are evaluated with respect to the nucleation rate, monomer temperature, monomer amount, and with respect to the size of the largest cluster in the system including possible pure metal clusters. It turns out that depending on the composition of the complete system pure platinum clusters with sizes up to 10 to 15 atoms are formed in addition to binary clusters. Due to the high temperature of these clusters iron atoms less likely condense at the beginning of the particle formation simulation. This leads to temporary difference in the temperatures of the platinum and the iron subsystems, which eventually approach each other when only binary clusters are present. In summary, the results obtained from the cluster statistics show that pure platinum nucleation and growth can take place to some extent within the binary system.     

Heat and momentum transport to particulates injected intolow-pressure (~80 mbar) nonequilibrium plasmas

An analytical model for plasma flow around a spherical particle has been developed using a two-temperature chemical nonequilibrium approach that includes rarefaction and particle-charging effects. The contribution of each component of the plasma to the heat and momentum transfer to injected particulates is studied by simulating the plasma flow around a sphere for conditions typical of thermal spray processes at pressures from 80 mbar to 1 bar. A parametric study of the 80-mbar case is also presented for free-stream velocities in the 0-3000 m/s range and free-stream temperatures in the 8000-12000 K range for both ionization equilibrium and nonequilibrium. The results demonstrate that there are departures from kinetic and ionization equilibrium in the plasma around a particle, and that heat transfer to a particle can be enhanced by the increased ion flux to the particle surface when the surrounding plasma is in ionization nonequilibrium     

Thin-film polycrystalline Si solar cells on foreign substrates: film formation at intermediate temperatures (700–1300 °C)

We give an overview and analysis of research on thin-film polycrystalline Si solar cells on foreign substrates, with layers formed at intermediate temperatures (700–1300 °C), covering substrates, deposition techniques and solar cell processing. The main deposition techniques that have been investigated are solution growth (SG) and chemical vapour deposition (CVD). Insufficient nucleation on foreign substrates is an important problem with SG, which could be solved with appropriate surface preparation techniques and growth conditions. With CVD, continuous layers are achieved routinely, but the electronic quality of the material is usually very low. Solar cell performance appears to be limited by a very large recombination activity of grain boundaries. Improvement can be achieved reducing the grain boundary density and recombination activity, and experimental examples are given. Devices have been demonstrated with efficiencies up to 5.5%. PACS 73.40.Lq; 73.50.Gr; 84.60.It     

Formation and photoluminescence properties of boron nanocones

This paper reports that a simple chemical vapour deposition method has been adopted to fabricate large scale, high density boron nanocones with thermal evaporation of B/B2O3 powders precursors in an Ar/H2 gas mixture at the synthesis temperature of 1000-1200℃. The lengths of boron nanocones are several micrometres, and the diameters of nanocone tops are in a range of 50-100 nm. transmission electron microscopy and selected area electron diffraction indicate that the nanocones are single crystalline α-tetragonal boron. The vapour liquid solid mechanism is the main formation mechanism of boron nanocones. One broad photolumineseence emission peak at the central wavelength of about 650 nm is observed under the 532 nm light excitation. Boron nanocones with good photoluminescence properties are promising candidates for applications in optical emitting devices.     

Laser-assisted deposition of thin films from photoexcited vapour phases

Laser-assisted chemical vapour deposition (LCVD) has been extensively studied in the last two decades. A vast range of applications encompass various areas such as microelectronics, micromechanics, microelectromechanics and integrated optics, and a variety of metals, semiconductors and insulators have been grown by LCVD. In this article, we review briefly the LCVD process and present two case studies of thin film deposition related to laser thermal excitation (e.g., boron carbide) and non-thermal excitation (e.g., CrO₂) of the gas phase. PACS 81.15.Fg; 81.15.Kk     

Initial steps in the photochemical vapour deposition of amorphous silicon

The overall process of chemical vapour deposition (CVD) is divided into reaction steps in the gas phase and on the solid-state surface. Energy input and primary reaction of the gas phase educt molecules constitute the initial steps considered in this paper. Special emphasis is placed on the photochemical vapour deposition (photo-CVD) of amorphous silicon from Sill4 and Si2H6 including uv single photon and ir multiple-photon activation. Experimentally convenient excitation processes and energetically possible primary decomposition pathways are summarized, compared, and discussed with respect to the selectivity of the chemical reactions. Finally the current status of photo-CVD is briefly presented.     

Feasibility and design studies for heat recovery from a refrigeration system with a canopus heat exchanger

This paper presents an investigation of the feasibility of heat recovery from the condenser of a vapour compression refrigeration (VCR) system through a Canopus heat exchanger (CHE) between the compressor and condenser components. The presence of the CHE makes it possible to recover the superheat of the discharged vapour and utilize it for increasing the temperature of the external fluid (water) removing heat from the condenser. The effects of the operating temperatures in the condenser and evaporator for different inlet water temperatures and mass flow rates on the heat recovery output and its distribution over the condenser and CHE (the fraction of the condenser heat available through the CHE), available outlet water temperature and heat recovery factor have all been studied and optimum operating parameters for feasible heat recovery have been ascertained. The parametric results obtained for different working fluids, such as R-22, R-12, R-717 and R-500, have been presented. It is found that, in general, a heat recovery factor of the order of 2.0 and 40% of condenser heat can be recovered through the Canopus heat exchanger for a typical set of operating conditions.     

硒化锌单晶薄膜近带边发射的荧光寿命

在2K下,对常压MOCVD方法生长的ZnSe单晶薄膜进行了光致荧光近带边发射谱的测量和辨认.并采用激光超短脉冲技术和时间相关单光子计数技术相结合的方法,对其在2K下近带边发射的荧光寿命进行了测量和分析,并讨论了自由激子弛豫过程.     

Detection of explosive vapour using surface-enhanced Raman spectroscopy

A commercially available nano-structured gold substrate was used for activating surface-enhanced Raman scattering (SERS). Raman spectra of the vapour of explosive material, triacetonetriperoxide (TATP), at trace concentrations produced from adsorbed molecules on such surfaces have been studied. Prominent Raman lines of the explosive molecular species were recorded at a sample temperature of ∼35°C, which is near to human body temperature. For this study, the concentration of the adsorbed TATP molecules on the nano-structured surface was varied by heating the sample to different temperatures and exposing the substrate to the sample vapour for different lengths of time. The intensities of the Raman lines have been found to increase with the increase in temperature and also with the increase in the duration of exposure for a fixed temperature. However, as expected, the Raman intensities have been found to saturate at higher temperatures and longer exposures. These saturation effects of the strengths of the Raman lines in the SERS of TATP vapour have been investigated in this paper. The results indicate that the optimisation for vapour deposition on the surface could be a crucial factor for any quantitative estimate of the concentration of the molecular species adsorbed on the nano-structured substrates.     

Very-large-mode-area photonic bandgap Bragg fiber polarizing in a wide spectral range

A design of a polarizing all-glass Bragg fiber with a microstructure core has been proposed for the first time. This design provides suppression of high-order modes and of one of the polarization states of the fundamental mode. The polarizing fiber was fabricated by a new, simple method based on a combination of the modified chemical vapor deposition (MCVD) process and the rod-in-tube technique. The mode field area has been found to be about 870 μm2 near λ=1064 nm. The polarization extinction ratio better than 13 dB has been observed over a 33% wavelength range (from 1 to 1.4 μm) after propagation in a 1.7 m fiber piece bent to a radius of 70 cm.