Kinetic parameters from thermogravimetric curves

A novel procedure is presented for the evaluation of kinetic parameters, activation energy (E) and reaction order (n), from thermogravimetric (TG) traces. From given values of conversion (α) and corresponding temperatures (T) are calculated values of E corresponding to various arbitrary values of n. Then, the resulting arbitrary E-values may be plotted against respective n-values and the region bounded by intersecting curves used to simultaneously estimate E and n. This procedure was tested against theoretical data, teflon, sodium bicarbonate, and magnesium hydroxide, and was found to be satisfactory. Some advantages of this method are: it is relatively simple to use; it allows the user to readily discern whether the data are homogeneous and, if not, at what conversions the data deviate. A less sensitive ancillary procedure is also described which can dispense with plotting in the estimation of E and n.     

Estimation of kinetic parameters from thermochemical data

A method is described by which kinetic parameters may be calculated from the measured temperature changes caused by the heat produced during a chemical reaction. An isoperibol titration calorimeter with an ampoule-breaking facility is used to obtain the temperature data. The temperature changes resulting from the reaction between tri-isopropyl phosphite and sulphur (S(8)) are used as an example to demonstrate the effectiveness of the method. The temperature changes are used to calculate an enthalpy of reaction. From the enthalpy of reaction and intermediate heats, instantaneous concentrations of the reactants may be calculated.     

Critical parameters from power series expansions

A simple and quite general method is developed for calculating critical para meters from power series expansions. The series coefficients of the problem function are introduced into a properly parametrized recurrence relationship. As a result, sequences are obtained that converge towards the critical parameters characterizing the closest singularity to the origin. The low-temperature series for the thermodynamic functions of spin-1/2 Ising models are discussed. Results are shown for the interfering, nonphysical singularities of the face-centred cubic and body-centred cubic lattices. The determination of these singularities would allow their factorization previous to the study of the physically important ones.     

Smectic order parameters from diffusion data

Microcanonical molecular dynamics simulations have been performed in the smectic A phase of an elementary liquid-crystal model. Smectic order parameters and diffusion coefficients along directions parallel and perpendicular to the director have been calculated during the same trajectory for a number of state points. This has permitted the satisfactory testing of a procedure, adopted in the analysis of experimental self-diffusion coefficients, leading to an estimate of the temperature dependence of the smectic order parameters. This methodology has been then confidently applied to two smectogenic thermotropic liquid crystals belonging to the 4,4(')-di-n-alkyl-azoxybenzene series. The derived smectic order parameters are larger for the homologue compound with the longest alkyl chains. This is consistent with the well-established increased tendency, for members of a homologue series, to form a smectic phase as their alkyl chains become longer.     

Thermal parameters of roofing slates from Czech Republic

Thermal performance of building materials is an important parameter from the point of view of energy consumption for heating buildings, which is obviously related to environmental protection standards. Thermal parameters of roofing slates were measured for samples from two different formations in the Czech Republic. These were rocks of lower Carboniferous Culm facies of Moravice Formation and Silesian Unit of Flysch Moravian-Silesian Carpathians. Thermal conductivity and thermal effusivity measurements were performed with use of TCi analyser. Thermal parameters were obtained in parallel and perpendicular direction to the bedding in rocks. Thermal conductivity of the Moravian slates in the direction perpendicular to the bedding ranges from 1.43 to 1.79 W m^?1 K^?1, while for samples from Carpathian region this parameter ranges from 1.99 to 3.15 W m^?1 K^?1. High values of thermal conductivity correlate to higher quartz content in the rocks. The measured thermal parameters (conductivity, effusivity, diffusivity) are strongly depending on the direction of measurement. Thermal conductivity of analysed rocks increases along with increase in temperature. The increase in thermal conductivity value is more significant in case of Moravian slates. In practice, the obtained results indicate that the traditional building material, such as roofing slates, shows better insulation properties at lower temperatures, while in conditions of strong sunlight the temperature conductivity increases. In case of roofing slates, which tend to be highly anisotropic, the essential information is the direction of thermal parameters measurement. Slates, due to their specific texture, are characterized by a very high thermal anisotropy coefficient.

     

由热重数据计算动力学参数

热重法(TG)是一种动态测量技术,可在程序升温或降温情况下对给定物质或体系提供一个连续的,以温度或时间为函数的重量变化曲线,所以由它跟踪反应能给出反应动力学的有关信息。这种非等温法与早期使用的等温法相比,有快速、连续和需要较少实验数据等优点,近年来在动力学研究方面得到广泛应用,但还只限于研究下列几种类型的简单     

Determination of order parameters from carbon-fluorine dipolar coupling

A convenient NMR method for the determination of the order parameters for liquid crystals containing a fluorinated phenyl ring is described. The technique consists of measuring the carbon-fluorine dipolar coupling constants in the one-dimensional C-13 spectrum of the molecule. The order parameters may then be calculated for the fluorine-containing ring with a high degree of precision because of the excellent resolution afforded by the 1-D C-13 spectra. The method is used to determine the core order parameters for 4-n-hexyloxybenzilidene-4'-fluoroaniline (FAB-OC6). The results of the carbon-fluorine dipolar coupling method are compared with two established methods for determining the core order parameters of phenyl rings, namely deuterium NMR spectroscopy and SLF/VAS, a two-dimensional C-13 NMR spectroscopy. Some comments about the orientational properties of the fluorinated liquid crystal FAB-OC6 are made.     

Mesoscale model parameters from molecular cluster calculations

We present an efficient, systematic, and universal method to estimate the interaction parameters used in mesoscale simulation methods such as dissipative particle dynamics and self-consistent field methods from molecular cluster calculations. The method is based on a generalized Flory-Huggins model in which molecules, or fragments thereof, are in contact with their van der Waals surface. We sample the density of states of molecular clusters in the space spanned by the coarse-grained degrees of freedom. From here, we calculate the sum over states and free energy of the cluster at a temperature of interest by histogram reweighting. The method allows to calculate the energy and entropy contributions to the cluster free energy explicitly. For two components, we then obtain the excess free energy of mixing and the Flory-Huggins chi-parameter, and their energetic and entropic contributions. We present two applications of the method: a simple liquid mixture of hexane and nitrobenzene, and a series of polymer blends. In the case of hexane/nitrobenzene, we compare to alternative simulation methods; here we find that the energy of mixing alone is too high to explain the critical point. By including the excess entropy of mixing, however, the predicted phase behavior is in reasonable agreement with experiment. The tendency of calculations based on average energy alone to overestimate the chi-parameter is also apparent in the polymer blend calculations.     

Exchange parameters from approximate self-interaction correction scheme

The approximate atomic self-interaction correction (ASIC) method to density functional theory is put to the test by calculating the exchange interaction for a number of prototypical materials, critical to local exchange and correlation functionals. ASIC total energy calculations are mapped onto a Heisenberg pairwise interaction and the exchange constants J are compared to those obtained with other methods. In general the ASIC scheme drastically improves the band structure, which for almost all the cases investigated resemble closely available photoemission data. In contrast the results for the exchange parameters are less satisfactory. Although ASIC performs reasonably well for systems where the magnetism originates from half-filled bands, it suffers from similar problems than those of local density approximation for other situations. In particular, the exchange constants are still overestimated. This reflects a subtle interplay between exchange and correlation energy, not captured by the ASIC.     

Possibility of deriving kinetic parameters from integrated DTA peaks

Three DTA peaks (two endo and one exothermic) were replotted in the form of peak area.a, or peak magnitude,T, as a function of temperature. The integral sigmoid curves were plotted in the form of logg() vs. 1/T or log logg()/T ² vs. 1/T. Both of the proposed summation methods gave satisfactory straight lines (F ₁ function), characterized by the same activation energies, correlation coefficients and standard deviations. Integration of the peak areas by Simpson's rule resulted in the same values as obtained by the summation procedure. Analysis by the suggested integral method resulted in activation energies that show a logarithmic divergence relative to the magnitude ofE _a estimated directly from the DTA peaks.     

Estimation of electron transfer parameters from AM1 calculations

The utility of AM1 calculations for estimation of the electron-transfer parameters lambda'(v) (the enthalpy part of the Marcus internal reorganization energy) and H(ab) (the electronic coupling between the charge-bearing units) is considered for some charge-localized intervalence bis(hydrazine) radical cations, for which these parameters have been experimentally determined from optical measurements. The Koopmans estimate of lambda'(v) that employs the orbital separation for the neutral compound at the radical cation geometry is far from that calculated from the enthalpies of the species involved (eq 1) and is not correct. The eq 1 lambda'(v) enthalpies estimated by AM1 are reasonably good for compounds with only alkyl substituents but are overestimated by 33-59% for aryl-substituted hydrazines. The Koopmans estimate of H(ab) as half the orbital separation for the neutral species at the transition state geometry requires adjustment for the twist angles to those of the relaxed ground state to produce useful H(ab) values. Symmetry breaking occurs for the electron-transfer transition states of the compounds with saturated bridges, and the Koopmans estimate predicts H(ab) values that are slightly less than half as large as the optical measurements.