Patrick K. Gallagher

Kinetic analyses using simultaneous TG/DSC measurements: Part I: decomposition of calcium carbonate in argon

Abstract Dynamic kinetic analyses are performed on CaCO3 using simultaneous thermogravimetry (TG) and differential scanning calorimetry (DSC) measurements in both wet and dry flowing Ar. Differences in the results are discussed in terms of the property measured and the possible influence of the atmosphere on the structural nature of the solid product, CaO. Although the quality of the DSC results was inferior compared with those based on TG, it is clear that they yield different results in subsequent kinetic analyses using a wide range of numerical methods. The presence of moisture in the atmosphere produced slightly lower values of the Arrhenius parameters with less variation during the course of the reaction. Reservations are described concerning the general interpretation of such results.

Kinetics of the oxidation of magnetite using simultaneous TG/DSC

Kinetics of the oxidation of magnetite (Fe3O4) to hematite (a-Fe2O3) are studied in air using simultaneous TG/DSC. The mechanism is complex and the differences between the kinetic conclusions and Arrhenius parameters based on either TG or DSC are discussed. As in our previous work on CaCO3 [1], the determination of a satisfactory baseline for the DSC results adds considerable uncertainty to those kinetic results. Consequently the calculations based on the TG data are considered superior. Solid state reactivity varies from one source of material to another and the results are compared for two different commercial samples of magnetite, both presumably prepared by wet chemical methods. These materials are much more reactive than the material studied previously [2], which had been coarsened and refined at high temperatures. In that earlier study, the metastable spinel, g-Fe2O3, was formed as an intermediate in the oxidation to the final stable form, a-Fe2O3. The exothermic reaction of the gamma to alpha form of the product during the oxidation process destroys the direct comparison between the TG and DSC results, since the former only detects the change in mass of the sample and not the crystallographic transformation. The TG results, however, represent the true oxidation process without superposition of the structural aspects.

Characterization of hydroxyapatite: Before and after plasma spraying

Hydroxyapatite (HA) powder was characterized before and after plasma spraying using infrared spectroscopy and thermoanalytical techniques. Structural changes were observed from the infrared spectra for the plasma-sprayed powder. These changes involved dehydroxylation and misoriented hydroxyl ions with different configurations. Dehydroxylation was accompanied by decreased intensities for the O-H stretching mode at 3571 cm-1 and the O-H librational mode at 633 cm-1. A broad infrared band near 3400 cm-1 that was observed after spraying was attributed to the misoriented hydroxyl ions. The combined results from thermoanalytical techniques indicated that adsorbed water evolved in three stages below 500 °C, and that the dehydroxylation of HA started as low as 700 °C. A weight gain of the plasma sprayed HA was observed above 500 °C in inert atmospheres. This result indicates a strong tendency of dehydroxylated HA to restore hydroxyls. The decrease in the decomposition temperature of HA after spraying was attributed to largely perturbed structures.

Some applications of thermal analysis to fullerenes

Thermal analysis is a convenient means of characterizing the soot used as a source of the carbon clusters, the extracted mixture of fullerenes, and the individual clusters themselves. TG in an inert atmosphere will rapidly assay the volatile fractions, presumably the lower molecular weight clusters. TG in oxygen indicates — a slight weight gain for the soots and clusters prior to their combustion.DSC in oxygen is used to determine the heat of combustion for the separated C60 and C70 materials. The combustion occurs around 300°C for the clusters. There is a small exothermic peak before this which is attributed to the oxidation associated with the slight weight gain. The heats of combustion measured are −18.7 and −21.0 kJ·mol−1 for the C60 and C70 respectively.MS-EGA indicates the loss of small amounts of water and argon at temperatures around 250°C in vacuum and of solvent at about 350°C prior to sublimation.ZusammenfassungThermoanalyse ist ein praktisches Hilfsmittel zur Charakterisierung von Ruß als Quelle für Kohlenstoffcluster, extrahierte Gemische von Fullerenen und die individuellen Cluster an sich. TG in einer inerten Atmosphäre ergibt schnell die flüchtigen Fraktionen, wahrscheinlich die niedermolekularen Cluster. TG in Sauerstoff zeigt einen leichten Gewichtszuwachs für Ruß und Cluster vor ihrer Zersetzung.DSC in Sauerstoff wurde angewendet, um die Verbrennungswärme der getrennten C60 und C70 Materialien zu ermitteln. Die Verbrennung erfolgt für die Cluster bei etwa 300°C. Davor kann ein kleiner exothermer Peak beobachtet werden, welcher der Oxidation in Verbindung mit dem geringen Gewichtszuwachs zugeschrieben wird. Die gemessenen Verbrennungswärmen betragen −18.7 und −21.0 kJ·mol−1 für C60 bzw. C70.MS-EGA zeigt den Verlust von geringen Mengen an Wasser und Argon bei Temperaturen von etwa 250°C in Vakuum und von Lösungsmittel bei etwa 350°C, bevor die Sublimation eintritt.

Some observations on the use of strontium carbonate as a temperature standard for DTA

Abstract The simultaneous decomposition of strontium carbonate that occurs during the solid-to-solid phase transition is a detriment to the use of this material as a high temperature standard for DTA. An atmosphere of carbon dioxide is shown to deter the reversible decomposition to much higher temperatures where it is no longer a problem. The phase transition temperature was observed to shift during the initial heating through the transition in carbon dioxide but remain constant thereafter. Changes were also observed in the X-ray diffraction pattern during the initial heating. It is proposed that the use of this material as a DTA standard be restricted to an atmosphere of carbon dioxide using a sample that has undergone at least one temperature cycle in that atmosphere.

Influence of water vapor on the thermal dehydration of Li2SO4·H2O

Abstract Kinetics of thermal dehydration of lithium sulfate monohydrate have been studied by using TG and DSC for Li 2 SO 4 ·H 2 O samples with different characteristics, i.e., powdered sample, pressed pellet, plate crystal and cubic crystal. The TG measurements in N 2 saturated with water and dry N 2 showed that the dehydration of Li 2 SO 4 ·H 2 O was affected by the presence of water, particularly for the powdered sample at the early stage of dehydration. This phenomenon is explained by consideration of H 2 O diffusion. The apparent activation energies were calculated based on the Ozawa method from TG data obtained at different heating rates. No significant difference was found in the apparent activation energy due to the presence of external water vapor. DSC measurements on the dehydration of all four samples were also performed, and the apparent activation energies, obtained from analyzing partial areas by the Ozawa method, are comparable to those obtained from TG.

A study of calcium oxalate monohydrate using dynamic differential scanning calorimetry and other thermoanalytical techniques

Abstract The focus of this work is the characterization of the order-disorder phase transition of calcium oxalate monohydrate. MDSC™, power-compensated DSC, EGA/MS, XRD, TG, and TMA were utilized to investigate the reproducibility, reversibility, and heating rate dependence of this transition. Flowing atmospheres saturated with water vapor were used to shift the dehydration equilibrium to a higher temperature so as to better resolve the order-disorder phase transition from the dehydration. Differences in thermal behavior of the sample due to the use of open versus semi-closed sample pans were also investigated. Correlations were made with spectroscopic (Raman, DRIFTS/MS, IR emission) and XRD results from the literature. This article reports the first MDSC™, DSC and TMA results for the order-disorder phase transition of CaC2O4·H2O.

The thermal decomposition of coprecipitates and physical mixtures of magnesium-iron oxalates

Abstract Physical mixtures and coprecipitates of magnesium and divalent iron oxalates having molar ratios of 1:1, 1:2 and 1:4 were prepared. Comparisons were made between the thermal decomposition of the physically mixed oxalate and the coprecipitated oxalate having the same composition. Samples were studied by thermogravimetry (TG), thermomagnetometry (TM), differential thermal analysis (DTA), mass spectroscopic evolved gas analysis (EGA), and powder X-ray diffraction from 25 to 1400°C in both oxidizing and inert atmospheres. Coprecipitates were expected to be solid solutions, showing only single weight losses for the dehydration and decomposition steps. This is true for the coprecipitates having more iron present than magnesium. In contrast, the physical mixtures were expected to behave as separate materials, showing two separate weight losses for dehydration and decomposition. This held true except for the dehydration step, which takes place as one weight loss. TM and X-ray diffraction results indicate the coprecipitate forms magnesium ferrite (MgFe 2 O 4 ) more readily than the equivalent physically mixed material. An oxidizing atmosphere was necessary to facilitate the production of magnesium ferrite.

Preliminary round-robin studies on the ICTAC certified reference materials for DTA

The ICTAC Committee on Standardisation has formed a Task Group to investigate the suitability of the ICTAC Certified Reference Materials for DTA, covering the temperature range 450°–1100°C, for accurate temperature calibration purposes and to evaluate their potential as enthalpy calibrants for DTA and DSC equipment. This paper reports the results of preliminary round-robin studies on barium carbonate and strontium carbonate, using a dual-point calibration method based on the melting points of aluminium and gold. In addition the fusion of ICTAC potassium sulphate has been investigated as a possible calibration transition.ZusammenfassungDer ICTAC Committee on Standardisation bildete eine Arbeitsgruppe zur Untersuchung der Eignung ICTAC-zertifizierter DTA-Referenzsubstanzen im Temperaturbereich 450°C–1100°C für präzise Temperaturkalibrierung und zur Auswertung deren Einsatzmöglichkeit als Enthalpiekalibriermaterialien für DTA- und DSC-Apparaturen. Vorliegend wird über die Ergebnisse von Vorstudien an Bariumcarbonat und Strontiumcarbonat berichtet, wobei eine Zweikalibrierpunktmethode verwendet wurde, die auf den Schmelzpunkten von Aluminium und Gold beruht. Zusätzlich wurde das Schmelzen vonICTAC Kaliumsulfat als mögliche Kalbrierungsumwandlung untersucht.

Applications of thermoanalytical methods to the study of thin films

Studies of the formation and properties of thin films is a growing area in both science and technology. Characterization of these films and the processes by which they are prepared is a major factor in understanding, controlling, and optimizing their synthesis and usefulness.Thermal methods can play an important role in such studies, provided that they can overcome the difficulties imposed by the massive amount of associated substrate. Unless one is studying the film — substrate interaction, the substrates presence only serves to decrease the sensitivity by diluting or reducing the thermal effect being measured.The three basic strategies that have evolved are discussed and examples described. One approach is to remove the film from the substrate, when this is appropriate and feasible, and then study the film separately. A second method is to use larger samples and careful measurements in an effort to overcome the reduced sensitivity. The final means is to develop very sensitive and/or selective techniques to investigate the film and its potential interactions with the substrate.ZusammenfassungSowohl in Wissenschaft als auch in Technologie gewinnt die Untersuchung von Bildung und Eigenschaften dünner Schichten immer mehr an Bedeutung. Die Beschreibung dieser Schichten und ihres Herstellungsverfahrens ist ein wichtiger Gesichtspunkt zum Verständnis, zur Kontrolle und Optimierung ihrer Herstellung und Nutzbarkeit.In derartigen Untersuchungen können thermische Methoden eine wichtige Rolle spielen, vorausgesetzt es gelingt diejenigen Schwierigkeiten zu überwinden, die durch die beträchtliche Menge assoziierten Substrates verursacht werden. Untersucht man nicht gerade die Wechselwirkung Schicht-Substrat, dann verursacht die Gegenwart des Substrates durch Abschwächung oder Verringerung des gemessenen thermischen Effektes lediglich eine geringere Empfindlichkeit.Es werden die drei bestehenden Grundstrategien diskutiert und Beispiele beschrieben. Eine Lösung ist es, die Schicht, wenn es geeignet erscheint und möglich ist, vom Substrat zu entfernen und dann gesondert zu untersuchen. Eine zweite Methode ist die Verwendung grö\erer Proben und sorgfältiger Messungen, um die verringerte Empfindlichkeit zu überwinden. Die letzte Möglichkeit besteht in der Entwicklung empfindlicher und/oder selektiver Verfahren, um die Schicht und ihre potentiellen Wechselwirkungen mit dem Substrat zu untersuchen.