Activation energy of self-heating process Studied by DSC

In order to identify the kinetic process of self-heating in DSC experiment for Ti+3Al→TiAl₃ reaction, two approaches, linear-fitting approach developed from Semenov"s theory of spontaneous ignition and variation of Friedman method, were carried out with cylindrical Ti-75 at% Al samples. Following these approaches, two identical activation energies are obtained as 169±15 kJ mol^-1 and 170±5 kJ mol^-1, respectively. Compared with the activation energies of reactions and interdiffusions between Ti and Al, the possible rate-controlling process of self-heating in DSC experiment for Ti+3Al→TiAl₃ reaction is the interdiffusion between Ti and Al through TiAl₃-layer. This revised version was published online in August 2006 with corrections to the Cover Date.     

Reinforcement characterization of metal matrix composites by DSC technique

The metal matrix composites (MMC) being of very high interest for the aerospace industry, particularly to build up thermal-structural components, it is important to have available methods which are easy and simple to conduct for characterization of the mechanical strength of the material.This study shows that the differential scanning calorimetry (DSC) may enable us to quantify the energy of cohesion in the fiber/matrix interface area of an aluminium composite reinforced by SiC whiskers.The energy of cohesion is dependent on the state of reinforcement compared to the matrix strength and take into account both chemical and mechanical characteristics of the cohesion in the fiber/matrix interface area. However, this method would require a systematic calibration based on tests of mechanical characteristics.

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Zusammenfassung Metallmatrixverbundwerkstoffe (MMC) sind für die Raumfahrtindustrie von großem Interesse, insbesondere zum Bau thermisch-struktureller Komponenten ist es wichtig, Methoden zur Verfügung zu haben, die leicht und einfach zur Charakterisierung der mechanischen Festigkeit der Materialien benutzt werden können.Es wurde gezeigt, daß DSC es ermöglicht, die Kohäsionsenergie im Fiber/Matrix-Grenzflächengebiet eines mit SiC-Whiskern verstärkten Aluminiumverbundstoffes quantitativ zu bestimmen.Diese Kohäsionsenergie ist abgängig vom Verstärkungsgrad bezogen auf die Matrixfestigkeit und berücksichtigt sowohl die chemischen als auch mechanischen Eigenschaften der Kohäsion im Fiber/Matrix-Grenzflächengebiet.In jedem Falle bedarf diese Methode einer systematischen Kalibrierung anhand von Test der mechanischen Eigenschaften.

     

New applications of the thermodilatometry in the characterization of ceramics

Thermodilatometry is widely used at high temperatures for studying ceramics. The sintering process can be monitored but if the rate of shrinkage is too high, it may produce a structure damage, therefore an adapted software is available to work at a constant sintering rate.Structural changes can be studied as well as the expansion of ceramic fibres.

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Zusammenfassung Zur Untersuchung von keramischen Substanzen bei hohen Temperaturen ist Thermodilatometrie ein weitverbreitete Methode. Der Sinterproze\ kann verfolgt werden, ist jedoch die Schrumpfgeschwindigkeit zu hoch, kann es zu Strukturschäden kommen, deshalb ist eine angeglichene Software erhältlich, die das Arbeiten bei konstanter Sintergeschwindigkeit ermöglicht.Es können sowohl die strukturellen Veränderungen als auch die Ausdehnung von Keramikfasern untersucht werden.

     

High-pressure microwave dissolution of ceramics prior to trace metal determinations by microwave induced plasma atomic emission spectrometry

A commercial laboratory microwave acid digestion system was evaluated for the acid dissolution of ceramic powders (Al₂O₃, AlN, BN and Si₃N₄) prior to the determination of their trace element content by microwave induced plasma atomic emission spectrometry. Newly designed vessels, capable of withstanding internal pressures of over 110 bar, provide rapid and satisfactory results for sample dissolution. Sample preparation time was approximately 30 min (including the subsequent cooling time and preparation of the final solution). Results from conventional stainless-steel acid digestion vessel (Teflon bomb) dissolution are compared with the microwave bomb results of microwave plasma atomic emission spectrometry.     

Thermal analysis and the processing of electronic and magnetic ceramics

Control of processing parameters, in both conventional ceramic routes and novel gel processing routes, is vital in the production of electronic and magnetic ceramics. The role of thermal analysis in the provision of basic data for the understanding of both types of processing route is discussed with special emphasis being placed upon the production of 1–2–3 YBCO superconductors and MgO-based soft ferrite materials for TV deflection unit applications.     

基于差示扫描量热技术的生物质热解两步连续反应模型研究

采用差示扫描量热分析仪对我国的一种生物质试样在空气气氛中进行了实验, 发现试样从常温到923 K高温的低速升温过程中, 经历了两步明显的放热过程. 对放热机理的分析表明, 第一步主要是由半纤维素和纤维素的有氧热解过程控制, 第二步放热过程则受木质素热解和炭的氧化反应的共同作用. 采用等转化率方法和优化计算方法, 对热解过程的动力学模型进行了研究, 结果表明, 两步连续反应机理可用于描述生物质在空气气氛中热解的放热动力学.     

Modulated temperature DSC measurements relating to the cold crystallization process of poly(ethylene terephtalate)

The modulated temperature differential scanning calorimetric method (MT-DSC) yields three temperature dependent signals, an underlying heat capacity curve from the underlying heat flow rate (corresponding to the conventional DSC signal), and a complex heat capacity curve with a real part (storage heat capacity) and an imaginary part (loss heat capacity). These curves have been measured in the cold crystallization region for poly(ethylene terephtalate) with a modified Perkin-Elmer DSC-7. The underlying curve shows the well known large exothermic crystallization peak. The storage heat capacity shows a step change which reproduces the change in heat capacity during crystallization. This curve may be used as baseline, to separate the crystallization heat flow rate from the underlying heat flow rate curve. The loss heat capacity curve exhibits a small exothermic peak at the temperature of the step change of the storage curve. It could be caused by changes of the molecular mobility during crystallization.Dedicated to Professor Wunderlich on the occasion of his 65th birthday     

A combined characterization of zinc hot-dip galvanized wireswith DSC, XRD and SEM

The effect of Pb and Sn on the structure of zinc hot-dip galvanized coatings on steel wires is examined. The coating quality is often low because of the Sandelin effect. An improvement could be expected if 1.0 mass% Pb is added to the Zn bath. In this case the η phase formation is enhanced, while the coating thickness is reduced. The Sn addition at the same concentration promotes the formation of the η phase but it does not affect the thickness.     

DSC study and computer modelling of the melting process in ice slurry

In order to understand the non-isothermal melting kinetics in the ice slurry, a differential scanning calorimetry (DSC) was used. Experimental results were compared to those obtained by a numerical simulation in which a general enthalpy method was applied. In this work the ice slurry studied consists of ice particles uniformly dispersed within a water-antifreeze liquid mixture. The effects of the heating rate and the initial antifreeze mass fraction are discussed. It has been found that the temperature gradients inside the sample of the solution become important if either heating rate increases or initial antifreeze mass fraction decreases.     

Comparison of simulated and actual DSC measurements for first-order transitions

A system of differential equations modeling a heat flux DSC is solved and the results are compared with those obtained using a TA Instruments Q1000™ DSC.¹ It incorporates a new heat flow rate measurement technique that determines the heat flow rate between the sample and its pan. Two types of first-order transitions are investigated: melting of a pure substance and solidification of a pure substance including super-cooling. In both transitions, the peak shape obtained using the new heat flow rate measurement and predicted by the model is quite different from that measured using conventional DSC. It is shown that the differences are the result of simplifications implicit in the conventional heat flow rate measurement that is based solely on the difference between sample and reference calorimeter temperatures. Heat flow rates measured using the improved measurement agree very well with the model predictions for heat exchange between the sample and its pan.     

Thermal characterization of HCN polymers by TG-MS, TG, DTA and DSC methods

This paper presents a thermogravimetry (TG) study of hydrogen cyanide polymers, synthesized from the reaction of equimolar aqueous solutions of sodium cyanide and ammonium chloride. Differential thermal analysis (DTA) and differential scanning calorimetry (DSC) were also used to evaluate the thermal behaviour of these black polymers, which play an important role in prebiotic chemistry. A coupled TG-mass spectrometer (MS) system allowed us to analyze the principal volatile thermal decomposition and fragmentation products of the isolated HCN polymers under dynamic conditions and an inert atmosphere. After dehydration, a multi-step decomposition occurred in this particular polymeric system, due to the release of ammonia, hydrogen cyanide (depolymerization reaction), isocyanic acid (or cyanic acid) and formamide; these two latter species allow us identify bond connectivities. Finally, data collected from TG experiments in an oxidative atmosphere showed significant differences at higher temperatures, above 400 °C. According to these results, the different techniques of thermal analysis here applied have demonstrated to be an adequate methodology for the study and characterization of this complex macromolecular system, whose structure remains controversial even today.     

Application of DSC as a tool for honey floral species characterization and adulteration detection

The thermal behaviour of authentic honeys and sugar syrups (industrial and homemade) was investigated by DSC. To confirm the first previous results concerning the effect of adulteration on the thermal behaviour of authentic honeys, 30 honey samples (Robinia, Lavender, Chestnut and Fir) were analyzed by DSC and their T _g were measured following a suited experimental protocol. The results indicated that this parameter was useful to characterize and to distinguish significantly these varieties between them. Applied to honey samples artificially adulterated with different industrial syrups, DSC showed a detection level of 5–10% depending on the type of syrup. An endothermic phenomenon occurring between 40–90°C during the heating was studied by TMDSC and a new thermal transition similar to a glass-transition was highlighted.This revised version was published online in November 2005 with corrections to the Cover Date.     

Preparation and characterization of porous cordierite for potential use in cellular ceramics

Cordierite porous ceramics Z, X, and K were prepared using three mixtures of clay minerals: Z from kaolinite, talc, and aluminum hydroxide, X from kaolinite, talc, vermiculite, and aluminum hydroxide, and K from kaolinite, talc, and magnesium oxide. Ceramics were different in porosity, specific surface area, cordierite polymorphs, and secondary crystalline phases. Vermiculite influenced textural architecture of calcined cordierite ceramics X and predestinated crystallization of the high-temperature hexagonal α-cordierite with secondary minerals enstatite, spinel and corundum. Ceramics Z contained low-temperature orthorhombic β-cordierite, enstatite, and corundum, K was diphase of β-cordierite and forsterite. Total pore area (TPA) and specific surface area (SSA) of X, in spite of the higher porosity and the pore size distribution in the range of 300–1000 nm, were smaller in comparison with TPA and SSA of Z. Ceramics K retained high porosity, two maxima at 300–1000 nm and 50–200 nm in the pores size distribution, and the highest TPA and SSA compared to those observed in ceramics Z and X. Presented at the 8th Conference on Solid State Chemistry, 6-11 July 2008, Bratislava, Slovak Republic.     

DSC studies on thermally induced structural transformation of hydrogen-bonded amphoteric molecular assemblage

Summary Thermally induced structural transformation of fibrous hydrogen-bonded molecular assemblage formed from an amphoteric pyridinecarboxylic acid of 6-[2-propyl- 4-(4-pyridylazo)phenoxy]hexanoic acid (C5PR) was studied using differential scanning calorimetry (DSC), differential thermal analysis (DTA), and thermogravimetry (TG). The organized fibrous morphology formed in an aqueous solution was stable at temperatures below 150°C. The ordered crystalline solid phase (K1) of the original fibrous material altered to a disordered crystalline solid phase (K2) at 150°C and subsequently to an isotropic phase (I) at 172°C. In the isotropic state, the C5PR molecule was slowly decomposed by decarboxylation. Once the molecular assemblage was subjected to the mesophase by heating, another ordered crystalline solid phase (K3) appeared reversibly at 17°C. The heat budget analyses by DSC indicated that a conformational entropy change such as the side-chain propyl group and the main-chain pentamethylene unit in the hydrogen-bonded molecular assemblage took place between the two ordered crystalline solid phases K1 and K3.     

Thermal decomposition of silver cyano complex and characterization its decomposition products by ETA,DSC, DTA and TG

It was found by DTA and TG that [Phenyl₂I][Ag(CN)₂] in the solid state is chemically stable on heating in argon up to 160°C. During heating to higher temperatures it decomposes, forming volatile products such as [Phenyl]I, [Phenyl]NC and (CN)₂ [1]. After heating the sample to 500°C metallic silver resulted. The volatile and intermediate solid products were analysed by IR-spectroscopy.It was found by means of DTA and ETA that an isophase reversible transition takes place when the sample is heated and cooled, not higher than 100°C. At heating higher than 100°C the sample melts (melting pointT _m=135°C). The enthalpy melting was determined by means of DSC (H=–28 kJ·mol^–1).By means of ETA the disorder degree of the final decomposition product was estimated. The value of the activation energy of radon diffusion in the temperature range 720°–500°C equals 32.6 kJ·mol^–1.Dedicated to Prof. I. N. Bekman Moscow State University at the occasion of his 50th birthday     

Water effect on the rheologic behavior of PAN solution during thermal‐induced gelation process

The thermal‐induced gelation process in a polyacrylonitrile (PAN) solution containing different amounts of water was investigated through both dynamic and steady‐state rheologic measurements. During the cooling process, the storage modulus G′ was found to intersect with the loss modulus G″ at a temperature which can be considered as the incipient gel point T_ig. This crossover point increased with the water content in the PAN solution. On the basis of the power law $G'\tilde{G}\tilde{\omega} {}^{n} $ and the relation ${\rm tan}\ \delta = G(\omega )/G^{'}(\omega ) = {\rm tan}(n\pi /2) = {\rm const}$ , the loss tangent tan δ was traced with decreased temperature at various oscillation frequencies, and a crossover point was observed at the temperature a little higher than the T_ig above for all PAN solutions. The water content might affect the value of the relaxation exponent n via two different mechanisms, that is, hydration decoupling of the pendant nitrile groups in PAN at low water content (2 wt% water) and molecule aggregation at high water content (4 and 6 wt%). Nevertheless, the slight difference in the n values reflects the unique structure of the critical gels irrespective of the water level. The apparent viscosity η_a of either the PAN solution or the PAN gel was found to rise with increased water content and both showed pronounced shear‐thinning behavior. Compared with temperature, the water effect on the apparent viscosity η_a is more significant due to the water‐induced aggregation of the molecular chains. Copyright © 2010 John Wiley & Sons, Ltd.     

Birefringence in polycarbonate: Molecular orientation induced by cooling stresses. II. Effects of sectioning

Birefringence distributions in cross-sections of quenched polycarbonate disks are investigated. The effects of the introduction of new surfaces by sectioning are studied by using different cross-section widths, by quenching rod-shaped specimens, and by measuring the birefringence with oblique incident light. The experimental results are confirmed by numerical calculations of stress distributions in rods with varying aspect ratios. Stress release by appropriate sectioning is used to distinguish between stress and orientation contribution to birefringence. It is found that cutting cross-sections leads to a major stress redistribution in the width direction. This does not affect the birefringence measurement, but the decrease of the stress level in the long direction needs to be taken into account. The molecular orientation in the quenched samples cannot be explained only by the effect of transient thermal stresses above the glass transition. There is another orientation contribution, most likely related to stress relaxation below T_g. © 1993 John Wiley & Sons, Inc.     

Measuring the extent of phase separation during polymerization of composite latex particles using modulated temperature DSC

The morphological features of composite latex particles predominantly develop during the polymerization process and depend upon a significant number of variables. In this study, we have concentrated on the relative polarities of the two polymers in the particles and the rate at which we added the monomers during semibatch reactions containing the seed polymer latex. Our particular interest was to develop data that could reveal the extent of polymer phase separation as a function of the amount of monomer fed, and to characterize the morphology resulting from it. While TEM is the most common analytical technique employed, we show in this paper that modulated temperature DSC can generate data that allows us to follow the phase separation process as the monomer feed progresses. By considering the possibilities of having “phases” within the particles of pure polymer, homogeneously mixed (but nonequilibrium) polymers, gradient and interfacial polymer, we have been able to quite successfully simulate the DSC data. This results in quantitative estimates of the relative amounts of these “phases” and their polymer compositions. Combining these results with TEM photos showing the spatial characteristics of the morphology, we can achieve a much greater understanding of the physical structure of the composite latex particles. In many cases we find that phase separation is far from complete at the end of the reaction process. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 2790–2806, 2005     

Melting behavior of poly(butylene succinate) during heating scan by DSC

The melting behavior of isothermally crystallized poly(butylene succinate) (PBS) has been investigated using differential scanning calorimetry (DSC) and wide‐angle X‐ray analysis. The samples crystallized between 80°C to 100°C show middle endotherm at the position just before the high exotherm, while the others under 80°C show two endotherms (low and high). From the results of the melting peak vs. crystallization temperature plot, it was suggested that the middle endotherm corresponds to the melting process of the original crystallites and the high endotherms to the melting process of the recrystallized ones. As the DSC heating rate was increased, the peak temperature of the low and middle endotherms increased and that of the high endotherm decreased, indicating that the low endotherm was due to the original crystallites as well as the middle endotherm. Consequently, in the heating scan of PBS, the existence of two kinds of morphologically different crystallites as well as the process of melting and recrystallization becomes evident. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 1357–1366, 1999     

Calorimetric characterization of grass during its decomposition

Grass is cultivated for bioenergy purposes yet and expected to play a more prominent role as a source of renewable energy in the future. Understanding its burning characteristics is thus crucial to optimize the energetic efficiency. The aim of this study was to reveal the effect of i) microbial decomposition and ii) enzymatic depolymerisation of grass on its thermal behaviour. Thermal characteristics of grass during the course of aerobic decomposition and of fresh samples after treatment by enzymatic hydrolysis were measured by differential scanning calorimetry (DSC). Heat of reaction, 50% burnoff values and positions and areas of peaks were taken as indicators for the thermal behaviour. An increase in biochemical stability of the grass during 140 days of decomposition was indicated by an exponential mass loss and decreasing specific CO₂ respiration rates. Parameters representing the thermal stability of the materials coincided with this pattern. The relative contribution of the thermolabile fraction decreased, and 50% burnoff values increased almost steadily during decomposition from 366 to 407°C. These thermal stability parameters are thus indicators for biochemical degradation. However, peak temperatures of the individual peaks continuously decreased during decomposition. A comparison of composted and fresh, cellulase-treated samples without long-term microbial decomposition indicated that enzymatic depolymerisation rather than microbial consumption of the plant tissue led to the observed decrease in peak temperatures.