Formation of Silica Nanoparticles by Hydrolysis of TEOS Using a Mixed Semi-Batch/Batch Method

A new method for preparing silica nanoparticles, which consists of a two-stage semi-batch/batch hydrolysis reaction of tetraethylorthosilicate (TEOS), is presented. A relatively slow rate of hydrolysis of the TEOS occurred during the semi-batch process, which resulted in larger silica particles with a narrower size distribution. This was in direct contrast to the batch process. An example of reduction in particle size for an initial semi-batch and subsequent batch reaction is shown. On completion of the initial semi-batch step, the silica particles had a diameter of 106 nm. As the subsequent batch reaction proceeded, the mean size of the particles decreased to 23 nm. In this work, it was found that the optimal conditions for the silica nanoparticles using this mixed method were as follows; (TEOS: 0.5 M, H₂O: 6.0 M, NH₄OH: 0.2 M, feed rate: 5.0 ml/min, temperature: 42.5°C). In conclusion, a mixedsemi-batch/batch system suggested a new probability for the synthesis of nanoparticles.     

Main chemical engineering parameters affecting radical polymerization processes in heterogeneous media

The main parameters which may affect both reactor performance and polymerization processes in heterogeneous media are described. Viscosity and its influence on heat and mass transfer, residence time distribution in tubular reactor, as well as, influence of agitation on coagulum formation and of monomer feed rate on polymerization rate in semi-batch processes are examined.     

The heat generation rate of nickel-metal hydride battery during charging/discharging

The heat generation rate of nickel-metal hydride battery is investigated during charging/discharging in this study. The heat capacity of 8 Ah cylindrical Ni-MH battery is measured using a large-scale calorimeter. An accelerating rate calorimeter is employed to provide an adiabatic environment for the battery. The generation rates of reaction heat, polarization heat, and combination heat are calculated through curve fitting. Results show that there exits a linear relationship between each generation rate of the three heat items and the charging/discharging currents. It is suggested that the ohm internal resistance of the battery needs to be as low as possible for reducing the ohm heat. In addition, it is better to avoid overcharging in the higher rate of 5 C for battery safety.     

Using the Transfer Function of an Isoperibolic Reaction Calorimeter for Thermo-kinetic Analysis

It is an aim of the present work to determine the chemical heat flow rate of a reaction without explicitly solving the heat balance equations. Therefore, it is necessary to calculate the heat flow rate directly from the temperature course of an experimentally determined reaction. For this transformation the transfer function of the calorimeter is needed 1 . An isoperibol reaction calorimeter was used for the experiments. With different calibrations and gained transfer functions, it is shown that the chemical heat flow rate can be determined from the temperature course of a reaction. The evaluation is fast and easy to use, which improves automation and prevents possible input errors.     

RD-I型热导式自动量热计的研制

在文献[10]、[12]的基础上,研制了一种能在常温到150°C的较大温度范围内工作的“RD-Ⅰ型热导式自动量热计”。本文报导了仪器结构和工作原理。通过电能测定,证明仪器设计合理,工作正常。其性能完全符合Tian氏方程的要求。而且重现性好(相对精度为0.5%-1%左右),灵敏度高(对1卡以上的热效应能够准确地进行测量)。量热计通过光笔记录仪实现了测量记录的自动化。     

A double twin isothermal microcalorimeter

The design and properties of a double twin heat conduction microcalorimeter are described. In this instrument two twin microcalorimeters are placed close together, one on top of the other. The size of the instrument is the same as that of a commercial single twin microcalorimeter and each of the twin parts has similar properties as one normal twin microcalorimeter. The cross-talk between the calorimeters can be made low; we measured <0.1% of the signal generated in one calorimeter in the other calorimeter. This figure is, however, dependent on how well the two sides of the instrument are thermally balanced. The paper also contains a general discussion of the use of a reference in reducing the effect of temperature changes in the heat sink.

The advantage with a double calorimeter is that one may easily perform two related calorimetric experiments at the same time and in close proximity to each other, e.g. both sorption isotherms and sorption enthalpies may be measured simultaneously, or the heat production rate of a biological process may be monitored at the same time as the CO₂ production is measured.     

Degradation of floor adhesives as a function of pH

Floor adhesives on cement-based substrates may degrade if the pH is high enough and this has in many cases led to emissions of odorous substances and deteriorated indoor air quality. We have used isothermal calorimetry to assess the degradation rate of two floor adhesives as a function of pH. The rate of heat production measured by the calorimeter is proportional to the reaction rate. The degradation rate was similar for a “standard” and a “low emitting” adhesive, but the low emitting adhesive did not release volatile reaction products. The results show that adhesive degradation is strongly pH dependent. A model of alkaline hydrolysis based on two reaction sites is discussed.     

Safety Assessment and Optimization of Semi-batch Reactions by Calorimetry

Semi-batch reactors are widely spread in the fine chemicals and specialties industry. The reason is that, compared to the pure batch operation, the feed of at least one of the reactants provides an additional way of controlling the reaction course, which represents a safety factor and increases the constancy of the product quality. Process temperature and feed rate can be optimized to satisfy safety constraints, i.e. cooling capacity and allowable accumulation. An economically better way of operating a semi-batch reactor is to adapt the feed rate to the allowed accumulation of reactants. An experimental method based on calorimetry will be presented and illustrated by an example. This revised version was published online in July 2006 with corrections to the Cover Date.     

Systematic errors in the measurement of heat by flow microcalorimetry

It is shown that a number of systematic errors must be considered when performing heat measurements by flow microcalorimetry because the nature of the flow technique is such that substantial heat loss can be incurred. The conventional procedure of electrical calibration is found to be an inadequate correction parameter. Equations to account for the effects of thermal disequilibrium are derived from the basic principles incorporated in the Tian equation. The predicted relationships are tested by simple experiments and shown to be correct. The various correction parameters are measured for a wide range of flow rate and heat input conditions. A composite equation is presented which allows for the correction of heat loss while deconvoluting electrical heat from a heat of reaction. The total heat output rate from a flow calorimeter can be calculated for most experimental conditions by reference to this equation and to the tabulated correction values.     

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.     

A Low-temperature Automated Adiabatic Calorimeter Heat Capacities of High-purity Graphite and Polystyrene

A computerized adiabatic calorimeter for heat capacity measurements in the temperature range 80–400 K has been constructed. The sample cell of the calorimeter, which is about 50 cm³ in internal volume, is equipped with a platinum resistance thermometer and surrounded by an adiabatic shield and a guard shield. Two sets of 6-junction chromel-copel thermocouples are mounted between the cell and the shields to indicate the temperature differences between them. The adiabatic conditions of the cell are automatically controlled by two sets of temperature controller. The reliability of the calorimeter was verified through heat capacity measurements on the standard reference material α-Al₂O₃. The results agreed well with those of the National Bureau of Standards (NBS): within ±0.2% throughout the whole temperature region. The heat capacities of high-purity graphite and polystyrene were precisely measured in the interval 260–370 K by using the above-mentioned calorimeter. The results were tabulated and plotted and the thermal behavior of the two materials was discussed in detail. Polynomial expressions for calculation of the heat capacities of the two substances are presented. This revised version was published online in August 2006 with corrections to the Cover Date.     

Study of burning behavior of small scale wood crib with cone calorimeter

Burning behavior of small-scale wood crib was studied by a serial of cone calorimeter tests. The heat release rate curves of these small wood cribs were different due to porosity factor and this shows that the control condition switches from one to another. The burning of some crib with small porosity factors was self-extinguished in fixed flow rate of air supply in cone calorimeter. These results were compared with Gross’s studies. The switch point of porosity-controlled and surface area controlled burning regime is different from Gross’s result.     

Emulsion copolymerization of tribromophenyl maleimide with styrene

Emulsion copolymerization of Tribromophenyl Maleimide (TBPMI) and styrene was conducted by semi-batch and batch processes. The effects of monomer composition and copolymerization method on copolymerization rate, molecular weight and molecular weight distribution, latex particle size and size distribution, glass transition temperature (T_g), thermal stability and mechanical properties were investigated. A kinetic study has shown that the rate of copolymerization in the batch process increased with increasing TBPMI content in the monomer feed. For the semi-batch polymerized samples, molecular weight decreased and molecular weight distribution increased with increasing TBPMI content in the monomer feed. For the batch polymerized samples, molecular weight also decreased but no obvious tendency was observed for the molecular weight distribution when TBPMI content increased. Compared with the batch copolymers, the semi-batch copolymers have a higher molecular weight at the same initial monomer mixture composition. Latex particle size decreased, while particle size distribution slightly increased with increasing TBPMI content in both semi-batch and batch latices. The semi-batch samples exhibit only a single T_g, the value of which increses linearly with increasing TBPMI content. For the batch copolymers, two T_gs were found, reflecting a mixture of styrene-rich and TBPMI-rich copolymer chains. TGA results indicate that the thermal stability of the semi-batch copolymers increased with increasing TBPMI concentration. Young's and flexural moduli increased, while tensile and flexural strengths decreased by increasing the TBPMI content for both the semi-batch and batch specimens. The semi-batch specimens have higher tensile and flexural strenghts than the batch ones.     

Heat flux calorimetry in intermetallic compound–H2(g) systems: heat measurements and modeling in the low pressures range

A heat conduction calorimeter has been used to determine enthalpies of solid–gas reactions in systems, such as intermetallic compounds–H₂(g). It is shown that the significance of the heat measurements must be carefully analyzed by controlling several parameters, which may influence the heat transfer conditions during the absorption and desorption reactions.

For this type of reaction, the measured heat flow is strongly dependent on the heat transfers through the gas phase in the low pressure range (Knudsen regime). It is demonstrated, experimentally and confirmed with simple models, that heat measurements can provide erroneous enthalpies due to our inability to account for the unavoidable modifications of the heat losses in the transition region. Experiments were carried out on the ZrNi–H₂ system.     

Calorimetric investigation of polymerization reactions. II. Copolymerization of diethyl fumarate with styrene

The bulk copolymerization of diethyl fumarate with styrene initiated by benzoyl peroxide or azobisisobutyronitrile was investigated with a differential scanning calorimeter (DSC) operated isothermally. The heat of copolymerization decreased almost linearly with the increase in diethyl fumarate content in the copolymer. The heat of homopolymerization for diethyl fumarate was equal to 15.5 ± 0.3 kcal/mole. Monomer reactivity ratios at 100°C were also determined. The rate of copolymerization was followed over the whole range of conversion. A gel effect was noticed in copolymerizations at lower but not at higher temperatures. The initial rate of copolymerization exhibited a maximum at an intermediate monomer feed composition, differing from the previously reported tendency. The difference was found to be due to the method used for determining the rate of copolymerization, and the superiority of the DSC method was proved. The mechanism of the radical-radical termination process was discussed. From the viewpoint of chemically controlled termination, the cross-termination factor ? was estimated to be 2.0, which is reasonable considering the steric hindrance; assumption of the predominance of a diffusion-controlled termination reaction was also consistent with the results.     

A new micro-fluid chip calorimeter for biochemical applications

Based on a new thermopile heat power sensor, which was developed in MEMS technology, a miniaturized flow-through calorimeter was constructed. The heat power sensor consists of a silicon chip with a thin film BiSb/Sb thermopile and a PMMA reaction chamber. To ensure high signal resolution the heat power sensor is mounted inside a high-precision thermostat, which has a temperature stability of less than 100 μK. The heat power sensitivity of the calorimeter is 4-7 V W⁻¹ depending on the thermal conductivity of the liquid, the height of the chosen reaction chamber and the volume flow rate. A limit of detection of less than 50 nW can be obtained for volume flows lower than 5 μl min⁻¹. An important advantage is the low sample consumption of the calorimeter. For special applications the sample need for one measurement pulse does not exceed 20 μl.     

Modelling of a high pressure calorimeter

The evaluation of the latent heat and enthalpy of fusion of food systems in the case of high pressure–low temperature processing is important for modelling purposes as well as for technical applications. A high pressure calorimeter has been designed for this purpose. The high pressure calorimeter was used to evaluate the latent heat during a pressure scan at constant temperature. It permits to measure the heat of phase transitions and to obtain the relationship between the initial freezing temperature T _ifp and the average pressure while the phase transition is going on. This work presents a modelling of results obtained from an experimental approach using a high pressure calorimeter and from a mathematical model developed from existing data on the phase change of pure water. The modelling work consisted in evaluating the latent heat measured in previous tests from computations taking into account the dependence of the latent heat of fusion of water on pressure. Models predicting the amount of frozen water in a food matrix under atmospheric conditions were used to determine the initial amount of frozen water in the sample. Then a stepwise procedure was operated in a program to reproduce the pressure rise occurring during a high pressure calorimeter test. The amount of melted ice at each pressure step was calculated using conventional ice fraction models, which were adapted to pressure dependence of the initial freezing temperature and the dependence of the latent heat pressure. The comparison was satisfactory, especially at low temperatures.     

Thermal decomposition kinetic of reactive solids based on isothermal calorimetry measurements

An isothermal method was applied to measure the thermal decomposition of reactive solids in a sensitive heat flux reaction calorimeter, C80. This technique experimentally clarified the decomposition mechanisms of unstable substances based on the shapes of the heat flow curves, from which autocatalysis, first-order reaction or pseudo-autocatalytic reaction could be recognized. Kinetic parameters were derived from the measured data.     

Advanced Calorimetric Techniques in Polymer Engineering

Summary: In polymer synthesis, reaction calorimetry (RC) is an appropriate technique for on-line process monitoring, since polymerization reactions are highly exothermic. Measurements are noninvasive, rapid, and straightforward. Nowadays RC is the technique recognized as the most powerful way to study such process in near-to-the- industrial conditions. Our approach was focused on temperature oscillation calorimetry (TOC). Two different reaction calorimeters were used, i.e. a isoperibolic calorimeter and a Calvet type high sensitivity differential calorimeter, respectively. A special attention was paid to the interpretation of the measured signals in order to obtain reliable calorimetric data. The evolution of heat transfer coefficient UA was followed by performing appropriate Joule effect calibrations, before and after the reaction. A convolution differential method of the measured heat flow by the generated one was used for determining the time constants and deconvoluting the measured heat flow.