Temperature measurement and control

Several factors in temperature measurement that can affect the precision of melting points and phase-change phenomena are discussed. In many cases, critical errors may arise in the measurement and control of temperatures due to incorrect placement and/or interpretation of the output of temperature sensors in the various system types that are in current use. Advantages can be obtained by using one temperature sensor only for temperature measurement and temperature control in a low mass infrared gold image fumace for the analytical studies in both the constant rate and stepwise isothermal thermoanalytical heating and cooling modes. Illustrations of the use of this instrumentation for measurements in both modes are given.     

The Use of Modulated Temperature Programs in Thermal Methods

Temperature modulation has long been used in various aspects of thermal methods. Historically, the principle areas of application have been the determination of kinetic parameters using variants of the temperature jump method and the measurement of heat capacity by AC calorimetry. More recently the introduction of temperature modulation in a variety of techniques has been used in combination with deconvolution algorithms to separate sample responses that are dependent on rate of change of temperature from those principally dependent on temperature. Finally, temperature modulation is important in the new field of micro-thermal analysis. This revised version was published online in July 2006 with corrections to the Cover Date.     

Bias in isothermal time-to-event studies due to approach to test temperature

Oxidative induction time (OIT), constant temperature stability (CTS) and isothermal crystallization are examples of isothermal time-to-event (TTE) measurements obtained using differential scanning calorimetry. In TTE experiments, a test specimen is heated/cooled at a constant rate from the setup temperature to an isothermal test temperature. Once the test temperature is achieved, a clock is started and the time to the thermal event (e.g., onset to oxidation, thermal decomposition or crystallization exotherm peak) is measured. Such TTE values may be used to rank stability of the material at the test temperature. Some portion of the reaction of interest, however, takes place during the pre-isothermal period as the test specimen approaches the test temperature. This amount of reaction is unmeasured and represents a bias in the resultant TTE value. An equation has been derived and numerically integrated to estimate this bias. This approach shows that the bias is dependent upon the activation energy of the test reaction, the heating/cooling rate used and the temperature range between the melting temperature and the test temperature. For commonly used heating rates, the bias for OIT and CTS tests is small. Further, the myth that isothermal crystallization kinetics determinations required high cooling rates is dispelled with the bias of less than 0.9 min resulting from heating rates as low as 10°C min^–1. Knowledge of magnitude of this bias permits the selection of experimental conditions without the expense of high heating/cooling rate apparatus or extra cost cooling accessories.     

DTA中升温速率及样品热导率对T_g测量值影响的模拟研究

就不同升温速率和实际样品的不同热导率对差热分析 (DTA)中高分子材料的玻璃化转变曲线的影响进行了MonteCarlo模拟研究 ,发现当所有样品刚完成玻璃化转变时 ,在Tg 曲线中该特征点要低于Tg 的转变中点。转变中点所对应的样品温度肯定要高于实际的玻璃化转变温度。如果以玻璃化转变曲线的转变中点所对应的样品温度作为该材料的玻璃化转变温度 ,那么 ,升温速率越快、样品的热导率越小 ,所测得的玻璃化转变温度就越大 ,反之亦然。DTA测得的玻璃化转变温度与升温速率间有很好的线性依赖关系 ,但与样品热导率间的关系是非线性的     

Gas concentration programming – a new approach to sample controlled thermal analysis

Sample controlled thermal analysis (SCTA) is the generic name used to describe a family of techniques where the heating rate is not pre-determined as in conventional thermal analysis, but altered as some function of a property of the sample (i.e. mass loss, rate of gas evolution, etc.). We demonstrate here a new form of SCTA, where the reaction rate for gas–solid reactions can be controlled by programming the concentration of the reactive gas whilst keeping the temperature constant.     

Mechanistic Analysis of Solution Reactions by Non-Isothermal Calorimetry

For the rapid kinetic and energetic analysis of reactions in solution (τ_20°C > 10^-4s) by differential thermal analysis (DTA), the following thermogram parameters are defined as characterizing the start of the reaction: 1. initial temperature, 2. activation energy of the initiation reaction. In conjunction with the shape index (asymmetry of the DTA curve) and the half width of the DTA curve referred to standard physical conditions (cell constant, heating rate, and temperature difference), these quantities allow a simple distinction between one-step reactions of first and second order and composite reactions. It is possible to recognize whether a process involves parallel, successive, or equilibrium reactions, or combinations of these. The reaction mechanism can be clarified in many cases by measurements at various concentrations and heating rates and by discussion of the enthalpy values. The shortened method described in this report for the evaluation of the thermograms was derived with the aid of an analog computer and checked experimentally.     

Thermal stability of Ag-exchanged clinoptilolite rich mineral

Thermal stability of clinoptilolite rich mineral from Western Anatolia, Turkey and its Ag-exchange forms was investigated. Parent mineral of different sizes were heated up to 1000°C with heating rate of 2 and 10°C min^-1 using differential thermal analyzer (DTA) and thermogravimetric analyzer (TG). Ag exchange was conducted both in conventional constant temperature waterbath and microwave at 40, 60 and 80°C. The exchanged minerals were then characteized by scanning electron microscopy (SEM), X-ray diffractometry (XRD), Fourier transform infrared spectroscopy (FTIR), DTA and TG. The particle size and heating rate do not have significant effect on the thermal behavior of the parent mineral and no structural changes were observed with Ag exchange, only decomposition temperature was lowered. It was finally concluded that, Ag-exchanged clinoptilolite rich minerals were less thermally stable compared to parent mineral that does not affect their use for possible applications.     

Temperature Modulated DSC (TMDSC) Applications and Limits of Phase Information, cp Determination and Effect Separation

The ADSC (Alternating DSC [1]) technique superimposes upon the conventional constant heating rate a periodically varying modulation [2–8]. The modulation creates high instantaneous heating rates which increases sensitivity. The low underlying constant heating rate is used to get better resolution. With ADSC it is possible to separate overlapping thermal effects without loss of sensitivity and to determine heat capacities under quasi-isothermal conditions. It has been reported that there are also some limitations for the use of the modulation techniques, i.e. that the accuracy of c_p determination is reduced at higher modulation frequencies due also to thermal diffusivity within the sample itself [9, 10]. In this contribution, the limitations given by the measuring system itself will be discussed. A key value is the limit frequency of the sensor arrangement. In the Mettler Toledo DSC821^c this frequency is approximately 1/3 Hz. From these findings the following recommendations amongst others can be given: for light mass crucibles, 30 s periods are reasonable with amplitudes not exceeding the heating/cooling rates possible. A blank and a calibration measurement will eliminate cell asymmetry and will enhance the accuracy of c_p measurements even at higher modulation frequencies. This revised version was published online in August 2006 with corrections to the Cover Date.     

Melting of indium by temperature-modulated differential scanning calorimetry

The melting and crystallization of a sharply melting standard has been explored for the calibration of temperature-modulated differential scanning calorimetry, TMDSC. Modulated temperature and heat flow have been followed during melting and crystallization of indium. It is observed that indium does not supercool as long as crystal nuclei remain in the sample when analyzing quasi-isothermally with a small modulation amplitude. For standard differential scanning calorimetry, DSC, the melting and crystallization temperatures of indium are sufficiently different not to permit its use for calibration on cooling, unless special analysis modes are applied. For TMDSC with an underlying heating rate of 0.2 K min^–1 and a modulation amplitude of 0.5–1.5 K at periods of 30–90 s, the extrapolated onsets of melting and freezing were within 0.1 K of the known melting temperature of indium. Further work is needed to separate the effects originating from loss of steady state between sample and sensor on the one hand and from supercooling on the other.On leave from Toray Research Center, Inc., Otsu, Shiga 520, Japan.This work was supported by the Division of Materials Research, National Science Foundation, Polymers Program, Grant # DMR 90-00520 and the Division of Materials Sciences, Office of Basic Energy Sciences, U.S. Department of Energy at Oak Ridge National Laboratory, managed by Lockheed Martin Energy Research Corp. for the U.S. Department of Energy, under contract number DEACOS-960R22464. Support for instrumentation came from TA Instruments, Inc. and Mettler-Toledo Inc. Research support was also given by ICI Paints.     

Calibration of a quenching and deformation differential dilatometer upon heating and cooling: Thermal expansion of Fe and Fe-Ni alloys

Dilatometry is a technique for precise measurement of thermal dilatation of materials during heating or cooling. A procedure has been presented for calibration of a differential dilatometer operating with electromagnetic heating for metallic specimens both upon heating and cooling as well as under uniaxial compressive and tensile loading. The dilation signal has been calibrated for both heating and cooling and for uniaxial loading (compressive and tensile) using platinum or iron reference specimens, for which recommended dilational data are available. The ferro- to paramagnetic transition (characterised by the Curie temperature) of pure iron or iron-based alloys has been adopted to calibrate the temperature in the dilatometric measurement under different loading modes during heating and cooling. On this basis calibrated data for the thermal expansion coefficients of Fe and Fe-Ni alloys have been obtained.     

Glass transition temperature and thermal decomposition of cellulose powder

Cellulose powder and cellulose pellets obtained by pressing the microcrystalline powder were studied using differential scanning calorimetry (DSC), differential thermal analysis (DTA), and thermal gravimetry (TG). The TG method enabled the assessment of water content in the investigated samples. The glass phase transition in cellulose was studied using the DSC method, both in heating and cooling runs, in a wide temperature range from −100 to 180 °C. It is shown that the DSC cooling runs are more suitable for the glass phase transition visualisation than the heating runs. The discrepancy between glass phase transition temperature T _g found using DSC and predictions by Kaelbe’s approach are observed for “dry” (7 and 5.3% water content) cellulose. This could be explained by strong interactions between cellulose chains appearing when the water concentration decreases. The T _g measurements vs. moisture content may be used for cellulose crystallinity index determination.     

示差扫描量热法进展及其在高分子表征中的应用

示差扫描量热法(DSC)是表征材料热性能和热反应的一种高效研究工具,具有操作简便、应用广泛、测量值物理意义明确等优点.近年来DSC技术的发展大大拓展了高分子材料表征的测试范围,促进了对高分子物理转变的热力学和动力学的深入研究.温度调制示差扫描量热法(TMDSC)是DSC在20世纪90年代的标志性进展,它在传统DSC的线性升温速率的基础之上引入了调制速率,从而可将总热流信号分解为可逆信号和不可逆信号两部分,并能测量准等温过程的可逆热容.闪速示差扫描量热法(FSC)是DSC技术近年来的创新性发展,它采用体积微小的氮化硅薄膜芯片传感器替代传统DSC的坩埚作为试样容器和控温系统,实现了超快速的升降温扫描速率以及微米尺度上的样品测试,使得对于高分子在扫描过程中的结构重组机制的分析以及对实际的生产加工条件的直接模拟成为可能.本文从热分析基础出发,依次对传统DSC、TMDSC和FSC进行了介绍,内容覆盖其发展历史、方法原理、操作技巧及其在高分子表征中的应用举例,最后对DSC未来的发展和应用进行了展望.本文希望通过综述DSC原理、实验技巧和应用进展,帮助读者加深对DSC这一常用表征技术的理解,进一步拓展DSC表征高分子材料的应用.     

Combustion characteristics of lignite and oil shale samples by thermal analysis techniques

In this research thermal analysis and kinetics of ten lignite's and two oil shale samples of different origin were performed using a TA 2960 thermal analysis system with thermogravimetry (TG/DTG) and differential al analysis (DTA) modules. Experiments were performed with a sample size of ~10 mg, heating rate of 10°C min^-1. Flow rate was kept constant (10 L h^-1) in the temperature range of 20-900°C. Mainly three different reaction regions were observed in most of the samples studied. The first region was due to the evaporation of moisture in the sample. The second region was due to the release of volatile matter and burning of carbon and called as primary reaction region. Third region was due to the decomposition of mineral matter in samples studied. In kinetic calculations, oxidation of lignite and oil shale is described by first-order kinetics. Depending on the characteristics of the samples, the activation energy values are varied and the results are discussed. This revised version was published online in August 2006 with corrections to the Cover Date.     

Molecular Thermal Hysteresis in Helix‐Dimer Formation of Sulfonamidohelicene Oligomers in Solution

Sulfonamidohelicene oligomers up to the nonamer level were synthesized by the repeated coupling reactions of a building block. A tetramer formed a helix dimer in 1,3‐difluorobenzene, which unfolded to a random coil with heating. This structural change exhibited thermal hysteresis in which different thermal responses were observed in the course of temperature increase and decrease. The feature of the hysteresis was examined under different heating/cooling modes, and the mechanisms are discussed on the basis of the population change and the presence of an induction period. A proposal regarding the use of thermal hysteresis for sensing a temperature increase/decrease is also given.     

生物质废弃物的热解研究

生物质能是可再生能源,在生长过程中通过光合作用将碳和能量固定下来,利用生物质能CO2排放很少.为实现可持续能源生产和减少温室气体排放的目的,中国已于2006年1月开始实施《中华人民共和国可再生能源法》.     

两种量热模式下物质热分解的动力学补偿效应

热分析量热仪主要包括动态、等温、恒温及绝热四种操作模式。很多学者基于动态及等温模式的测试结果,采用Arrhenius速率常数进行动力学计算,进而发现了所谓的“动力学补偿效应”。为了解绝热模式下是否也存在动力学补偿效应,分别采用绝热加速量热法(ARC)及动态差示扫描量热法(DSC)研究了过氧化二异丙苯(DCP)、40%(质量分数,下同)DCP溶液、葡萄糖、45%葡萄糖溶液的热分解特性,在此基础上基于Arrhenius公式计算了对应的表观活化能E和指前因子A,并对计算结果进行了分析。结果表明:绝热模式下,不同质量的同种样品及其溶液的最佳动力学参数,或者同一组数据采用不同的反应级数获得的lnA和E之间均存在明显的线性关系。此外,尽管由动态DSC数据计算获得的E和lnA普遍小于绝热模式的结果,但两种模式下获得的lnA和E之间仍然存在动力学补偿效应。由此可以推断,具有相同或类似反应机理的反应,虽然实验模式不同,但其E和lnA之间存在明显的动力学补偿效应。     

Application of heat pipe technology in thermal analysis of metals

Summary Thermal analysis has been used in foundry applications to assess the quality of the melt before casting. The high-end thermal analysis techniques such as DSC or DTA are expensive and not suitable for foundry applications. The Computer-Aided Cooling Curve Analysis (CA-CCA) method based on one thermocouple has been widely used as a batch process with poor control over heat extraction and cooling rates during solidification. A heat pipe apparatus has been developed as a thermal analysis tool. The apparatus can assess the melt quality more accurately, as well as, allow for better control of heating and cooling rates. Moreover, the solidification process can be modeled more accurately, and thus the casting parameters affecting the casting quality can be closely simulated and consequently controlled. In this paper the principles of a heat-pipe assisted thermal analysis system are highlighted. The advantages of the new system are described and the possibility of its adoption in melt assessments is discussed.     

氯化天然橡胶的等速升温热降解动力学

天然胶乳;氯化天然橡胶的等速升温热降解动力学     

The heat capacity of polymers

The long path to an understanding of heat capacity is traced from isothermal and adiabatic calorimetries to the most recent three methods of isoperibol, scanning, and temperature-modulated calorimetry (TMDSC). These latter three methods are: the traditional method of scanning thermal analysis; the quasi-isothermal method of finding the maximum amplitude of the periodic heat flow in response to a temperature modulation at a constant base temperature; and the pseudo-isothermal analysis of a temperature-modulated scanning experiment by subtracting the effect due to the underlying constant heating rate. In parallel, the development of the knowledge about phases and molecules is traced from its beginning to present-day nanophases and macromolecules.