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  • Author or Editor: J. Dweck x
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Abstract  

The DSC curve obtained in conventional equipment usually only shows the resultant thermal effect due to simultaneous phenomena, which may occur during isothermal or dynamic analyses. This does not allow one to identify the processes properly and may cause an erroneous interpretation of the resulting curves. Modulated DSC equipment enhances the operating conditions and the analysis capacity of conventional DSC by superimposing a sinusoidal temperature modulation on the linear temperature control. Thus reversing and non-reversing heat flow curves are obtained, which are, respectively, the heat capacity and kinetic components of the DSC curve. Therefore, events that are related to these components can be separately analyzed. A method to obtain curves similar to the MDSC reversing and non-reversing components was developed using conventional DSC equipment in a non-conventional way. It was applied to analyze samples of poly(ethylene terephthalate) (PET) taken from bottles of mineral water. The second PET crystallization step that occurs during its melting was quantified and an apparent initial crystallinity was obtained from the resulting data.

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Abstract  

Differential thermal analysis (DTA) was the first thermal analysis technique used to qualitatively characterize natural clays and respective curves has been used since more than 60 years as their ‘fingerprint’. With the development of microprocessed equipments in the last decades, derivative thermogravimetric (DTG) curves also may be used for this purpose in some cases, which also may allow a quantitative characterization of clay components. TG and DTG curves are more indicated than DTA or DSC curves to identify and to better analyze the several decomposition steps of natural or synthetic organoclays. These questions are discussed in applications developed to characterize Brazilian kaolinitic clays, bentonites and organophilic clays.

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This work presents a study of the thermal decomposition of commercial vegetable oils and of some of their thermal properties by termogravimetry (TG), derivative termogravimetry (DTG) and by differential thermal analysis (DTA). Canola, sunflower, corn, olive and soybean oils were studied. A simultaneous SDT 2960 TG/DTA from TA Instruments was used, with a heating rate of 10 K min-1 from 30 to 700C. A flow of 100 mL min-1 of air as the purge gas was used in order to burnout the oils during analysis to estimate their heat of combustion. From the extrapolated decomposition onset temperatures obtained from TG curves, it can be seen that corn oil presents the highest thermal stability (306C), followed by the sunflower one (304C). Olive oil presents the lowest one (288C). The heat of combustion of each oil was estimated from DTA curves, showing the highest value for the olive oil. Except for corn oil, which presents a significantly different thermal decomposition behavior than the other oils, a perfect linear correlation is observed, with negative slope, between the heat of combustion of an oil and its respective extrapolated onset temperature of decomposition in air.

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Abstract  

Yttrium isopropoxide, an yttrium oxide precursor, is sometimes used as a sintering aid for producing aluminum nitride ceramics. In the present work, this sintering aid was used with isopropanol as the solvent and polyethyleneimine as the dispersing agent. After initial ball milling and drying steps, the burnout behaviour of samples taken from isostatically pressed pellets was studied by thermal analysis in nitrogen and air. In addition to the milled and pressed pellets, each component was also analyzed separately. Complete conversion to yttrium oxide, with no residual carbon, would be a desirable property of this system. However, during the preparation of the aluminum nitride pellet, there was only partial yttrium isopropoxide decomposition. The nitrogen burnout residue contained carbon formed from the yttrium isopropoxide and dispersant overlapping thermal decomposition, mostly from an intermediary decomposition stage of the former that occurs between 300 and 550°C. The residual carbon content and the previous yttrium isopropoxide decomposition were estimated by thermogravimetry.

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Abstract  

In the present work, a Portland cement blended with calcium carbonate is being used to study the solidification/stabilization (S/S) of a Brazilian tanning waste arising from leather production. Chromium is the element of greatest concern in this waste, but the waste also contains a residual organic material. Using thermogravimetry (TG) and derivative thermogravimetry (DTG) to identify and quantify the main hydrated phases present in the pastes, this paper presents a comparative study between the effects of Wyoming and Organophilic bentonites (B and OB) on cement hydration. Samples containing combinations of cement, B, OB and waste have been subjected to thermal analysis after different setting times during the first 28 days of the waste S/S process. Both bentonites affect the cement hydration, with no significant differences in hydration degree after 1 week. This work shows further examples of the great utility of thermal analysis techniques in the study of very complex systems containing both crystalline and amorphous mineral materials as well as organics.

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Abstract  

Fluidized catalytic cracking units of refineries normally use zeolite catalysts to treat heavy oil fractions. This catalyst is regenerated continuously, but due to the reduction of its activity during the process, it is partially substituted by a new catalyst make-up. The spent residue has a high content of silicon and aluminum oxides and usually presents pozzolanic properties. This paper presents the study of a Brazilian spent catalyst, which is being tested as a pozzolanic aggregate in partial substitution to cement. Pastes were prepared with 15, 20 and 25% in substitution to cement mass and analyzed after 28 days of hydration. Hydrated paste samples were analyzed by simultaneous thermogravimetry and differential thermal analysis, to quantify the calcium hydroxide consumption, as well as the content of other main hydrated cement phases. Compressive strength analysis was also performed after 28 days of hydration. Although, as spent catalyst content is increased, the pozzolanic activity is confirmed by the increase of calcium hydroxide consumption on cement mass basis, unlikely to other studied spent FCC catalysts, tested for the same purpose, the compressive strength of respective paste specimens decreases, due to the increase of other hydrated phases formation.

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Abstract

Gelcasting is a new method for forming advanced ceramics based on obtaining intermediary gels promoted by polimerization “in situ” within the ceramic slurry. Gelcast composites of an ALCOA A-1000 alumina and the copolymer obtained using acrylamide and N,N′-metylene-bisacrylamide as monomers were characterized by thermogravimetry. Polymer distribution was determined by thermogravimetric analysis of small size specimens that were taken from different parts of the alumina composites formed in different shapes. Derivative thermogravimetry was used to identify the resulting polymerization stage within the gelcast body, as well as to study the influence of mold materials and operating conditions. All the composites show an even distribution of the polymer in the bulk, which enables machining in the green state and obtaining high density alumina bodies after sintering.

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Abstract  

The intracellular water content of a microorganism is an important parameter which is a determinant factor of its physiological properties. It is usually measured by complex and time consuming procedures. Thermogravimetry using infrared balance has been used for this purpose, through the identification of different drying steps occurring during the analysis. This work employs the same method with much smaller samples, using conventional thermogravimetric equipment in a simpler and faster way than other conventional procedures. Commercial yeast (Saccharomyces cerevisiae ) washed samples are analyzed in isothermal procedures which are run in about 30 min. The drying rate curve, when plotted as a function of the residual mass of the cells, allows the identification of the step where the intracellular water is lost and the determination of its content. The obtained values, on extracellular water free basis, are in the range of 65 to 69% and agree with those measured by other techniques.

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Abstract  

This work is a study, by simultaneous thermogravimetry (TG) and differential thermal analysis (DTA), of the oxidation of a water resistant aluminum nitride powder which has a special protective coating, and an uncoated AlN powder which has become partially hydrated during its use. The activation energy for oxidation is estimated by the Kissinger and isoconversional methods. In the former method, the temperatures of the oxidation peaks were obtained from DTA and DTG curves. The activation energies for oxidation of the water resistant AlN, obtained by the Kissinger method, are 35710 kJ mol–1, 39212 kJ mol–1 using respectively DTG and DTA data. For the uncoated AlN, the values are 2437 and 2578 kJ mol–1, respectively. By the isoconversional method, the average values obtained for coated and uncoated samples are, respectively, 32310 and 2247 kJ mol–1. Therefore, the special coating, which protects the aluminum nitride from humidity action, also provides a higher resistance to oxidation.

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