A rapid,spatially dispersive frequency comb spectrograph aimed at gas phase chemical reaction kinetics

This New Views article will highlight some recent advances in high sensitivity gas detection using direct infrared absorption frequency comb laser spectroscopy, with a focus on frequency comb use in chemical reaction kinetics and our own contribution to this field. Our recently implemented detection technique uses a combination of a 12.9?GHz free spectral range virtually imaged phased array and diffraction grating to spatially disperse the mid-infrared frequency comb onto a camera. Individual frequencies or ‘comb teeth’ of a 250?MHz repetition-rate frequency comb are able to be resolved. High molecular sensitivity is achieved by increasing the interaction path length using a Herriott multipass cell. High spectral resolution, broadband spectral coverage, and high molecular sensitivity are all achieved on an adjustable 1–50 µs timescale, making this frequency comb apparatus ideal for measuring chemical reaction kinetics where multiple absorbing species can be monitored simultaneously. This New Views article will also discuss some of the challenges and decisions that chemists might face in implementing this advanced physics technology in their own laboratory.

Spatially dispersed 250 MHz mid-infrared frequency comb laser, with absorption of some frequencies by a dilute sample of methane.     

基于反应动力学的GaN LED参数退化模型的研究

加速实验中, 参数退化模型描述了参数的退化规律, 参数退化规律对应于器件退化机理, 而退化机理又对应于内部的物理化学反应. 因此, 本文基于反应动力学中物理化学反应的温度效应速率模型及反应量浓度随时间的变化规律, 研究并建立了GaN LED参数退化模型. 本模型尝试从物理机理上解释参数退化过程中的退化规律, 包括单调上升或单调下降退化规律、先上升后下降或先下降后上升等非单调退化规律, 解决了实验后拟合方法不能建立非单调退化模型的问题. 然后对GaN LED进行加速实验, 确定模型参数. 同时对GaN LED的退化规律进行分解, 并且量化了GaN LED两种退化规律的退化比例及时间常数. 关键词： 参数退化模型 反应动力学 加速实验 GaN LED     

锗硅/硅异质结材料的化学气相淀积生长动力学模型

基于化学气相淀积(CVD)的Grove理论和Fick第一定律,提出并建立了锗硅(SiGe)/硅(Si)异质结材料减压化学气相淀积(RPCVD)生长动力学模型.与以前锗硅/硅异质结材料生长动力学模型仅考虑表面反应控制不同,本模型同时考虑了表面反应和气相传输两种控制机理,并给出了两种控制机理极限情况下的模型.本模型不仅适用于低温锗硅/硅应变异质结材料生长的表征,也适用于表征高温锗硅/硅弛豫异质结材料生长的表征.将模型计算值与实验结果进行了对比,无论是625℃低温下的应变SiGe的生长,还是900℃高温下的弛豫 关键词： SiGe/Si异质结材料 化学气相淀积生长动力学模型 Grove理论 Fick第一定律     

A model of the chemical composition and pyrolysis kinetics of lignin

This study presents a novel approach for the chemical representation of lignin for modelling the reaction kinetics of lignin in lignocellulosic biomass. This methodology relies on the definition of dimeric pseudo-components containing phenolic functionalities, i.e., p-hydroxyphenyl, guaiacyl and syringyl groups, as measured in real biomass and native lignin through wet chemistry and spectroscopic techniques. The reactivities of the lignin pseudo-components are modelled through a series of lumped unidirectional reactions, whose product formation and reaction rate constants are optimised to replicate a comprehensive experimental dataset gathered from several works available in the literature. The new kinetic model contributes to the state-of-the-art by providing a more accurate depiction of the conversion rates, selectivity of char vs. volatiles, and aromatic composition in condensable products in line with the inherent reactivity of lignin functionalities and the empirical observations of lignin depolymerisation and thermal degradation at low (<1?K/s) and high heating rates (>50?K/s).     

Semi-gas kinetics model for performance modeling of flowing chemical oxygen-iodine lasers (COIL)

The gas pressure in a laser cavity of flowing chemical oxygen-iodine laser (COIL)is about 133.31333 Pa[1]. In this pressure range, effects of homogeneous and inhomo-geneous broadening competing with each other on the performances of a COIL are no-ticeable. Generally, a rate equation (RE) model is adopted to the performance modeling of a supersonic COIL, such as in refs. [1—4]. This model assumes the spectral line pro-file is homogeneously broadened. Homogeneous broadening assumption sim…     

Concerning the validity of the stochastic approach to chemical kinetics

A simple argument advanced recently in support of the legitimacy of the stochastic formulation of chemical kinetics has been criticized because it seems to require the imminent collision of widely separated molecules. It is argued here that this criticism is unwarranted because it is based on an incorrect use of probabilities. To illustrate the various probabilistic considerations involved, a detailed analysis is presented of a closely related but mathematically simpler problem: the calculation of the collision probability per unit time for a thermally equilibrized one-dimensional gas of point particles.     

化学反应与饱和模型对化学氧碘激光性能的影响

 将速率方程（RE）模型与化学动力学模型相结合，讨论了增益饱和模型与化学反应系统对COIL性能的影响。流动为预混的一维模型，考虑了10种成分和21个化学反应，分析计算了未分解碘分子，激发态氧产率，水含量以及温度等因素对COIL性能的影响。计算结果表明，碘流量过多，混合和反应过程中消耗大量能量；碘流量过低，导致粒子数反转和增益过低，对于能量的提取不利。     

Reduction of round-off errors in chemical kinetics

In this paper, a modification of chemical kinetics is introduced in order to reduce round-off errors in stiff source terms. Fast chemical processes close to partial equilibrium are often composed of different contributions that cancel each other out (for example forward and backward reactions). These contributions tend to be very large compared to their sum. Thus, computer evaluation of the chemical source term can lead to serious round-off errors due to cancellation. This can cause severe convergence problems within Newton's method when simulating the process with implicit integration methods and thus lead to additional step size reduction. Here, a method is presented that reduces the round-off error by making a slight change to the model. Furthermore, this change is smooth and does not change the dimension, so that it is also possible to apply the method locally when solving partial differential equations. Numerical examples presented in this paper verify the approach.     

材料化学分析的物理方法(Ⅰ)

材料的化学信息是理解科学、工程与技术领域各种过程、机制和材料行为的最基本要素，材料研究的第一步是要确定材料的化学，包括构成材料的原子的种类、分布以及具体的化学态等内容，任何具有元素特征的物理信息，包括原子量、电子的能级、原子核自旋，甚至局域的电子态密度等都可以用来做材料的化学分析，化学信息由来自材料本身的或用作探针的电子、光子、离子或中性原子携带，相应的分析技术包括X射线光电子能谱、俄歇电子谱、核磁共振、特征X射线分析、二次离子质谱、能量损失谱、溅射中性粒子质谱，各类离子散射谱以及扫描隧道显微学方法等等，文章对上述各种分析方法的物理原理、仪器以及应用等逐一做扼要的介绍。     

Analysis of RDX-TAGzT pseudo-propellant combustion with detailed chemical kinetics

A detailed model of steady-state combustion of a pseudo-propellant containing cyclotrimethylene trinitramine (RDX) and triaminoguanidinium azotetrazolate (TAGzT) is presented. The physicochemical processes occurring within the foam layer, comprised of a liquid and gas bubbles, and a gas-phase region above the burning surface are considered. The chemical kinetics is represented by a global thermal decomposition mechanism within the liquid by considering 18 species and eight chemical reactions. The reactions governing decomposition of TAGzT were deduced from separate confined rapid thermolysis experiments using Fourier transform infrared spectroscopy and time-of-flight mass spectrometry. Within the gas bubbles and gas-phase region, a detailed chemical kinetics mechanism was used by considering up to 93 species and 504 reactions. The pseudo-propellant burn rate was found to be highly sensitive to the global decomposition reactions of TAGzT. The predicted results of burn rate agree well with experimental burn-rate data. The increase in burn rate by inclusion of TAGzT is due in part from exothermic decomposition of the azotetrazolate within the foam layer, and from fast gas-phase reactions between triaminoguanidine decomposition products, such as hydrazine, and oxidiser products from the nitramine decomposition.     

Uncertainty propagation of chemical kinetics parameters and binary diffusion coefficients in predicting extinction limits of hydrogen/oxygen/nitrogen non-premixed flames

A comprehensive investigation of the uncertainties associated with the experimental and numerical evaluation of the extinction strain rate in hydrogen/oxygen/nitrogen non-premixed flames is presented in this work. The reported new experimental uncertainties of the extinction strain rate include several sources of uncertainties that typically affect the characterisation of velocity and boundary conditions of counterflow flames via particle image velocimetry. The uncertainties associated with the numerical determination of the extinction strain rate not only depend upon the selected chemical kinetics parameters but also on the binary diffusion coefficients. In order to identify the major sources of uncertainties in the chemical and diffusion models, a Monte Carlo based high-dimensional model representation analysis of the extinction curve was performed. Independent and simultaneous perturbations of relevant chemical kinetics and diffusion parameters have shown that the uncertainties associated with the binary diffusion coefficients are about a factor of 10 smaller than the uncertainty due to chemical kinetics parameters. Since the experimentally well known binary diffusion coefficient for hydrogen and nitrogen, , accounts for most of the propagated uncertainty of the diffusion model, it is shown here that only a reduction of the uncertainty of chemical kinetics parameters will have a significant impact in improving the accuracy of the extinction strain rate predictions.     

Minimal curvature trajectories: Riemannian geometry concepts for slow manifold computation in chemical kinetics

In dissipative ordinary differential equation systems different time scales cause anisotropic phase volume contraction along solution trajectories. Model reduction methods exploit this for simplifying chemical kinetics via a time scale separation into fast and slow modes. The aim is to approximate the system dynamics with a dimension-reduced model after eliminating the fast modes by enslaving them to the slow ones via computation of a slow attracting manifold. We present a novel method for computing approximations of such manifolds using trajectory-based optimization. We discuss Riemannian geometry concepts as a basis for suitable optimization criteria characterizing trajectories near slow attracting manifolds and thus provide insight into fundamental geometric properties of multiple time scale chemical kinetics. The optimization criteria correspond to a suitable mathematical formulation of “minimal relaxation” of chemical forces along reaction trajectories under given constraints. We present various geometrically motivated criteria and the results of their application to four test case reaction mechanisms serving as examples. We demonstrate that accurate numerical approximations of slow invariant manifolds can be obtained.     

Probabilistic parameter estimation in a 2-step chemical kinetics model for n-dodecane jet autoignition

This paper demonstrates the development of a simple chemical kinetics model designed for autoignition of n-dodecane in air using Bayesian inference with a model-error representation. The model error, i.e. intrinsic discrepancy from a high-fidelity benchmark model, is represented by allowing additional variability in selected parameters. Subsequently, we quantify predictive uncertainties in the results of autoignition simulations of homogeneous reactors at realistic diesel engine conditions. We demonstrate that these predictive error bars capture model error as well. The uncertainty propagation is performed using non-intrusive spectral projection that can also be used in principle with larger scale computations, such as large eddy simulation. While the present calibration is performed to match a skeletal mechanism, it can be done with equal success using experimental data only (e.g. shock-tube measurements). Since our method captures the error associated with structural model simplifications, we believe that the optimised model could then lead to better qualified predictions of autoignition delay time in high-fidelity large eddy simulations than the existing detailed mechanisms. This methodology provides a way to reduce the cost of reaction kinetics in simulations systematically, while quantifying the accuracy of predictions of important target quantities.     

Computational singular perturbation with non-parametric tabulation of slow manifolds for time integration of stiff chemical kinetics

This paper presents a novel tabulation strategy for the adaptive numerical integration of chemical kinetics using the computational singular perturbation (CSP) method. The strategy stores and reuses CSP quantities required to filter out fast dissipative processes, resulting in a non-stiff chemical source term. In particular, non-parametric regression on low-dimensional slow invariant manifolds (SIMs) in the chemical state space is used to approximate the CSP vectors spanning the fast chemical subspace and the associated fast chemical time-scales. The relevant manifold and its dimension varies depending on the local number of exhausted modes at every location in the chemical state space. Multiple manifolds are therefore tabulated, corresponding to different numbers of exhausted modes (dimensions) and associated radical species. Non-parametric representations are inherently adaptive, and rely on efficient approximate-nearest-neighbor queries. As the CSP information is only a function of the non-radical species in the system and has relatively small gradients in the chemical state space, tabulation occurs in a lower-dimensional state space and at a relatively coarse level, thereby improving scalability to larger chemical mechanisms. The approach is demonstrated on the simulation of homogeneous constant pressure H₂–air and CH₄–air ignition, over a range of initial conditions. For CH₄–air, results are shown that outperform direct implicit integration of the stiff chemical kinetics while maintaining good accuracy.     

Ultrasound-assisted theophylline polymorphic transformation: Selective polymorph nucleation,molecular mechanism and kinetics analysis

In this paper, the ultrasound-assisted solvent-mediated polymorphic transformation of theophylline was explored in detail. The induction time and reconstruction time were significantly decreased by ultrasound, thereby decreasing the total transformation time and promoting the transformation process. The ultrasound-promoted efficiency of nucleation was different in three alcoholic solvents, which was difficult to explain by traditional kinetic effects. To resolve the above confusion, binding energies calculated by Density Functional Theory were applied to explore the relationship between the ultrasound-promoted efficiency of nucleation and solute–solvent interactions. Then, a possible molecular self-assembly nucleation pathway affected by ultrasound was proposed: the ultrasound could change and magnify the crucial effect of the specific sites of solute–solvent interactions in the nucleation process. Finally, the transformation kinetics with different effective ultrasonic energies was quantitatively analyzed by Avrami-Erofeev model, indicating that the dissolution element in the rate-limiting step was gradually eliminated by higher ultrasonic energy. Fortunately, the elusive crystal form V could be easily obtained by the ultrasound-assisted polymorph transformation. This proved to be a robust method to produce high purity form V of theophylline. The outcome of this study demonstrated that the proper ultrasonic irradiation had the potential to produce specific polymorphs selectively.     

A LOI-RCCE methodology for reducing chemical kinetics, with application to laminar premixed flames

A framework for automated development of reduced mechanisms is presented by combining the methodologies of level of importance (LOI) and rate-controlled constrained equilibrium (RCCE). It is shown that these methods are complimentary, as they deal with different aspects of the overall reduction problem: LOI is a method of determining the species governed by fast/slow scales, while RCCE is a physical model that yields ODEs that describe the reduced model for a given selection. The potential for the synergy of the two methods is demonstrated by reducing a comprehensive mechanism for propane combustion (117 species, 665 reactions) and applying the reduced mechanism to the problem of 1-D laminar flame propagation. Results show that RCCE yields substantial CPU gainings, while predicting the major species as well as important minor species (such as radicals and C₂H₂) with sufficient accuracy.     

Multi-environment probability density function method for modelling turbulent combustion using realistic chemical kinetics

A computational fluid dynamics (CFD) tool for performing turbulent combustion simulations that require finite-rate chemistry is developed and tested by modelling a series of bluff-body stabilized flames that exhibit different levels of finite-rate chemistry effects ranging from near equilibrium to near global extinction. The new modelling tool is based on the multi-environment probability density function (MEPDF) methodology and combines the following: the direct quadrature method of moments (DQMOM); the interaction-by-exchange-with-the-mean (IEM) mixing model; and realistic combustion chemistry. Using DQMOM, the MEPDF model can be derived from the transport PDF equation by depicting the joint composition PDF as a weighted summation of a finite number of multi-dimensional Dirac delta functions in the composition space. The MEPDF method with multiple reactive scalars retains the unique property of the joint PDF method of treating chemical reactions exactly. However, unlike the joint PDF methods that typically must resort to particle-based Monte-Carlo solution schemes, the MEPDF equations (i.e. the transport equations of the weighted delta-peaks) can be solved by traditional Eulerian grid-based techniques. In the current study, a pseudo time-splitting scheme is adopted to solve the MEPDF equations; the reaction source terms are computed with a highly efficient and accurate in-situ adaptive tabulation (ISAT) algorithm. A 19-species reduced mechanism based on quasi-steady state assumptions is used in the simulations of the bluff-body flames. The modelling results are compared with the experimental data, including mixing, temperature, major species and important minor species such as CO and NO. Compared with simulations using a Monte-Carlo joint PDF method, the new approach shows comparable accuracy.     

Quasiclassical calculation of the chemical reaction Ba+CH3I

This paper reports the quasiclassical trajectory (QCT) calculations on extended London–Eyring–Polanyi–Sato (LEPS) potential energy surface (PES) for the reaction of Ba atom with CH₃I. In the calculations, the Sato parameters needed in construction of the reaction PES are obtained from simulation of experimental results of BaI product vibrational distributions. The parameters obtained are further tested by comparison of the calculated BaI vibrational distributions with another research group's experimental results at different collision energy. In addition, the reaction cross-section versus collision energy, product BaI angular distribution and alignment are obtained by QCT calculations on the PES. The possible reaction configurations are also discussed upon the calculations.     

Sr原子M-M跃迁激光的动力学模型

建立了一个反映高频脉冲放电激励的Sr原子M-M跃迁激光的动力学模型，阐明了各激光谱线上能级的主要抽运途径：其中301μm上能级是在放电脉冲早期通过更高能级的自发辐射和激光跃迁得到布居，而另3条谱线的上能级主要是通过余辉期一价Sr离子和电子的碰撞复合以及He和三重态Sr原子的混和碰撞实现布居.定量的计算结果与实验测量结果相一致，圆满解释了各种光脉冲的时间延迟关系. 关键词： 锶原子激光 M-M跃迁 动力学模型