DTA Peak Research Articles

Hydrophobic and physical properties of the ambient pressure dried silica aerogels with sodium silicate precursor using various surface modification agents

The experimental results on the synthesis and physical properties of the ambient pressure dried hydrophobic silica aerogels in the presence of various surface modification (silylating) agents are presented. The silica aerogels were prepared with 1.12 specific gravity ion exchanged sodium silicate solution, 1N ammonium hydroxide, solvent exchanged with ethanol and hexane, and surface modification with 20% silylating agent in hexane followed by drying the modified gel up to 200 °C. The molar ratio of sodium silicate, water, ammonium hydroxide and silylating agent was kept at 1:45:4.3 × 10 −2:5, respectively. The physical properties of the aerogels such as density, % of porosity, pore volume, thermal conductivity and contact angle measurements were studied by using various mono, di and tri alkyl or aryl silylating agents (SAs). The tri alkyl silylating agents produced low % of volume shrinkage (2%), low density (0.06 g/cm 3), low refractive index (1.011), more pore volume (16.15cm 3/g), high percentage of porosity (96.9%) and hydrophobic (contact angle >150°) silica aerogels. It was found from the Fourier transform infrared spectroscopic (FTIR) studies of the aerogels that the intensity of the bands related to the Si C and C H are more and the Si OH and OH are less with the tri than mono and di alkyl SAs. It was found from the TGA-DTA studies of the aerogels with increase in temperature above 325 °C, the % of weight decrease in TGA and exothermic peak in DTA are more with tri than the mono and di alkyl SAs. The SEM studies of the aerogels showed the large pore and particle sizes in the silica network with the tri alkyl SAs. The % of optical transmission of the aerogels is less with the tri alkyl SAs than the mono and di alkyl SAs. It was found from the contact angle and water adsorption studies that the hydrophobicity of the silica aerogel is more with tri alkyl than the di and mono alkyl silylating agents.

Thermal analysis of tributylammonium montmorillonite and laponite

Montmorillonite and Laponite loaded with different amounts of tributylammonium cations (TBAH+), up to 40 and 30 mmol, respectively, per 100 g clay, were studied by thermo-XRD-analysis. TBAH-smectites heated at 300 and 420°C exhibited basal spacings of 1.30 and 1.24 nm, attributed to smectite tactoids with low- and high-temperature-stable monolayer charcoals, respectively in the interlayers. DTA-EGA and TG of the TBAH-smectites showed four stages of mass loss labeled A, B, C and D. Stage A below 250°C, accompanied by an endothermic DTA peak, resulted from the dehydration of the clay. Mass loss stages B, C and D, at 250–380, 380–605°C and above 605°C, respectively, accompanied by exothermic DTA peaks, were due to three oxidation steps of the organic matter. In mass loss stage B (first oxidation step) mainly organic hydrogen was oxidized to H2O whereas carbon and nitrogen formed low- and high-temperature-stable charcoals. In stages C and D (second and third oxidation steps) low- and high-temperature- stable charcoals were oxidized, respectively. Dehydroxylation of the smectites occurred together with the second and third oxidation steps. Thermal mass loss at each step was calculated from the TG curves showing that in montmorillonite the percentage of high-temperature-stable charcoal from total charcoal decreased with higher TBAH+ loadings of the clay whereas in Laponite this percentage increased with higher loadings of the clay.

Thermal decomposition kinetics of strontium oxalate

AbstractThe thermal decomposition behavior in air of SrC2O4 · 1.25H2O was studied up to the formation of SrO using DTA-TG-DTG techniques. The decomposition proceeds through four well-defined steps. The first two steps are attributed to the dehydration of the salt, while the third and fourth ones are assigned to the decomposition of the anhydrous strontium oxalate into SrCO3 and the decomposition of SrCO3 to SrO, respectively. The exothermic DTA peak found at around 300°C is ascribed to the recrystallization of the anhydrous strontium oxalate. On the other hand, the endothermic DTA peak observed at 910°C can be attributed to the transition of orthorhombic-hexagonal phase of SrCO3. The kinetics of the thermal decomposition of anhydrous strontium oxalate and strontium carbonate, which are formed as stable intermediates, have been studied using non-isothermal TG technique. Analysis of kinetic data was carried out assuming various solid-state reaction models and applying three different computational methods. The data analysis according to the composite method showed that the anhydrous oxalate decomposition is best described by the two-dimensional diffusion-controlled mechanism (D2), while the decomposition of strontium carbonate is best fitted by means of the three-dimensional phase boundary-controlled mechanism (R3). The values of activation parameters obtained using different methods were compared and discussed.

Growth, structure and thermal properties of CuAlxGa1‐xSe2 alloys

AbstractIngots of the CuAlxGa1‐xSe2 (0 ≤ x ≤ 1) alloys system were prepared by direct fusion of the stoichiometric mixture of the elements. The analysis of X‐ray Powder Diffraction data showed the presence of one single phase with chalcopyrite tetragonal structure at room temperature for all the studied compositions. The lattice parameters, a and c, and the bond lengths were calculated. The phase transition temperatures were obtained by the onset method from Differential Thermal Analysis measurements performed on samples sealed in evacuated quartz ampoules. Fusion or transition enthalpies were determined from the area of the corresponding DTA peak. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Thermal and X-ray studies on mixed alkali uranates of Na–K–U–O system

Solid state reactions of Na 2U 2O 7 and K 2U 2O 7 in air at 1273 K, formed mixed uranates of type (K x Na 1− x )U 2O 7. Reduction of these mixed uranates formed (K x Na 1− x )UO 3 perovskite type of phase, having cubic structure for 0.5 ≤ x ≤ 1.0 and orthorhombic structure for 0 ≤ x < 0.5. The kinetics of oxidation of (K x Na 1− x )UO 3 in air, studied by thermogravimetry showed that the reaction is diffusion controlled with an activation energy in the range 135–202 kJ/mol. The enthalpy of oxidation of these perovskite phases was determined from DTA peak area measurement and the values are in the range 40–90 kJ/mol. High temperature X-ray diffraction studies showed that the % thermal expansion increases with increase in sodium concentration in KUO 3 phase.

Thermal decomposition of iron(VI) oxides, K 2FeO 4 and BaFeO 4, in an inert atmosphere

The thermal decomposition of solid samples of iron(VI) oxides, K 2FeO 4·0.088 H 2O ( 1) and BaFeO 4·0.25H 2O ( 2) in inert atmosphere has been examined using simultaneous thermogravimetry and differential thermal analysis (TG/DTA), in combination with in situ analysis of the evolved gases by online coupled mass spectrometer (EGA–MS). The final decomposition products were characterized by 57Fe Mössbauer spectroscopy. Water molecules were released first, followed by a distinct decomposition step with endothermic DTA peak of 1 and 2 at 273 and 248 °C, respectively, corresponding to the evolution of molecular oxygen as confirmed by EGA–MS. The released amounts of O 2 were determined as 0.42 and 0.52 mol pro formula of 1 and 2, respectively. The decomposition product of K 2FeO 4 at 250 °C was determined as Fe(III) species in the form of KFeO 2. Formation of an amorphous mixture of superoxide, peroxide, and oxide of potassium may be other products of the thermal conversion of iron(VI) oxide 1 to account for less than expected released oxygen. The thermogravimetric and Mössbauer data suggest that barium iron perovskite with the intermediate valence state of iron (between III and IV) was the product of thermal decomposition of iron(VI) oxide 2.

Thermal dehydration of lithium metaborate dihydrate and phase transitions of anhydrous product

Reaction steps and mechanisms of the thermal dehydration of lithium metaborate dihydrate were investigated by means of thermoanalytical measurements, high temperature powder X-ray diffractometry, FT-IR spectroscopy, and microscopic observations. The first half of thermal dehydration was characterized by the melting of the sample producing viscous surface layer, the formation of bubbles on the particle surfaces, and the sudden mass-loss taking place by an opportunity of cracking and/or bursting of the bubble surface layer. The second half of the dehydration with a long-tailed mass-loss process in a wide temperature region was divided further into three distinguished reaction steps by the measurements of controlled rate thermal analysis. During the course of the thermal dehydration, four different poorly crystalline phases of intermediate hydrates were observed, in addition to an amorphous phase produced by an isothermal annealing. Just after completing the thermal dehydration, an exothermic DTA peak of the crystallization of β-LiBO 2 was appeared at around 750 K. The phase transition from β-LiBO 2 to α-LiBO 2 was observed in the temperature range of 800–900 K, which subsequently melted by indicating a sharp endothermic DTA peak with the onset temperature at 1101.4 ± 0.6 K.

Formation and Thermal Stability of Hydrate-Like Compounds of Ba2 ( In1 − xMIIIx ) 2O5 ∙ nH2O (MIII = Ga , Sc, Lu, and Y)

The thermal stability of the hydrate-like , in which denotes the trivalent elements Ga, Sc, Lu, and Y, was studied. When brownmillerite and its related type were cooled from to room temperature in a thermogravimetry-differential thermal analysis (TG-DTA) apparatus under a wet atmosphere, the exothermal peak in DTA accompanying an abrupt mass increase attributable to the formation of hydrate-like was observed; the mass increase occurred in the temperature range of . For , Lu, and Y, the value of equaled 1, and the structure was the same as that of . For , the hydrate-like compound of appeared. The phase transformation temperature from that of into that of for , Lu, and Y was higher than that for the unsubstituted , while it was lower for than for . The increase in thermal stability by of with , Lu, and Y was discussed in terms of the higher affinity for the hydroxyl group of the basic Sc, Lu, and Y than that of amphoteric In and Ga.

Effect of nucleating agents on the crystallization of Li2O-Al2O3-SiO2 system glass

The processes of nucleation of Li2O-Al2O3-SiO2 glasses with TiO2 and TiO2+ZrO2 as nucleating agents were discussed. The DTA peak temperature and DTA peak height shown a strong dependence on the nucleation temperature in the glass with TiO2, while in the glass with TiO2+ZrO2 this tendency was small. The optimum nucleation temperatures were 745 and 760°C for two glasses. It suggested that with TiO2+ZrO2 as nucleating agents, the crystallization had lower sensitivity for nucleation temperature, and the glass had higher nucleation efficiency than with TiO2.

Reactivities of Blair Athol and Nang Tong coals in relation to their behavior in PFBC boiler

The combustion reactivities of Blair Athol (BA) and Nang Tong (NT) coals were measured by thermogravimetry to understand their different behaviors in the PFBC boiler. The reactivity of BA was much the same as that of NT coal but their chars showed different characteristics. BA char of higher surface area (25 m 2/g) showed slightly higher reactivity than that of NT char of smaller surface area (7 m 2/g). BA coal showed heterogeneous ignition even at its particle size of as large as +355 μm while NT coal showed homogeneous ignition at the average particle size over 75 μm heterogenous one occurring with finer particle size (−75 μm). Higher calorific value of BA volatile matter and higher reactivity of its char than those of NT coal causes of its heterogeneous ignition with an intense DTA peak, which may lead to local heating at its combustion and to yield reactive CaO from limestones causing of bed materials agglomeration in the PFBC.

Synthesis and Microstructural Properties of Fe-TiO2Nanocrystalline Particles Obtained by a Modified Sol-Gel Method

A series of iron/titanium oxide nanocrystalline particles with Fe/Ti molar ratios up to 0.15 were synthesized by a modified sol-gel technique using Ti(IV)-isopropoxide and anhydrous Fe(II)-acetate. The precursors were mixed and subsequently hydrolyzed with water molecules generated in situ by an esterification reaction between acetic acid and ethanol. As-synthesized samples were amorphous for XRD, independently of the relative amount of doped iron. The undoped samples and samples with the molar ratio Fe/Ti = 0.01, treated at up to 500°C, contained anatase as the dominant phase and rutile as the minor phase. The samples with the Fe/Ti molar ratio of 0.15, treated at the same temperature, contained anatase (major phase), rutile (minor phase) and a very small amount of an unidentified phase. The crystallite size of the dominant phase in the samples was estimated from the XRD line broadening using the Scherrer formula. Thermogravimetric analysis showed that weight loss was accelerated and completed at lower temperatures as the relative concentration of iron in the Fe-TiO2 samples increased. The strong exothermic peak in the DTA curve between 300 and 450°C in the undoped TiO2 sample shifted to the lower temperatures and became much more asymmetrical with increased iron doping. This DTA peak corresponded to the amorphous-to-anatase-transition and it included several steps such as (i) the thermal degradation of strongly bound organic molecules, (ii) the condensation of unhydrolyzed –OR groups, (iii) the sintering and growth of particles and (iv) the rearrangement of newly formed chemical bonds. The center of the most intense Raman band of the Eg mode at 143.8 cm−1 in the undoped TiO2 sample continually shifted to higher wave numbers and the full-width at half maximum increased with iron doping. Transmission electron microscopy revealed decrease of the mean particle size from 16.3 nm in undoped sample to 9.7 nm in the highest iron doped sample. The particle size distribution becomes narrower with iron doping. The narrowest particle size distribution was found in sample with the Fe/Ti molar ratio of 0.05, calcined at 500°C. Scanning electron microscopy of undoped samples calcined at 580°C showed irregular aggregates having a relatively flat surface. On the contrary, the samples doped with 15 mol% of iron and treated at the same temperature exhibited a non-uniform sponge-like surface with distributed micrometer holes.

Sulphuric Acid Activation of a Calcium Bentonite

The objective of this work was to determine the changes of thermal and surface properties of a bentonite from Çankιrι after activation using different concentrations of H2SO4 solutions (0.5–4 M). XRD, FTIR, DTA, and TG analyses of the samples were performed to examine the structural changes of the bentonite before and after acid activation. DTA peaks showed an exothermic change in the temperature region 900–1000°C. Changes at low acid concentrations resulted from cation exchange (exchangeable cations with H ions) and removal of impurities from the bentonite. Differences of surface properties and thermal properties at higher acid concentrations (2–4 M H2SO4) were caused by structural changes and partial decomposition of the samples. Surface area measurements showed that with the increase of concentration of H2SO4 solution the total BET surface area increased, while the maximum value of specific surface area (240.9 m2/g) was attained by the sample activated with 2 M H2SO4. Activation with higher concentrations (3–4 M) caused a decrease in the surface area. The particle size distributions of the original and acid-activated samples showed that the acid activation strongly affected the particle sizes of the particles. While 97.8% of the original bentonite sample is less than 10 µm in size, 97.8% of the activated samples with 0.5 M acid and 92% of the activated sample with 4 M acid are less than 28.25 µm in size.

DTA peak shift studies of primary crystallization in glasses

Following the recognition during the last decade that knowledge of the shift in the exothermic crystallization DTA peak as a function of pre-DTA isothermal heat-treatment times and temperatures, can provide quantitative information about the crystallization kinetics, there has been renewed interest in DTA investigations of crystallization of glasses. Most studies to date, however, have focussed on the kinetics of polymorphic crystallization (where the compositions of the crystal and the parent glass are the same). These studies have established that the DTA peak shifts to lower temperatures with increased pre-DTA heat-treatment times and JMAK-based formalisms have been developed to extract the steady state nucleation rate from the DTA peak shift data. In this paper, we report new results on the DTA peak shift in systems undergoing primary crystallization (where the compositions of the crystal and glass are different). The DTA results show that the exothermic peak temperature decreases initially but increases later on, becoming significantly larger than the initial value, with increase in the pre-DTA heat-treatment time at a fixed temperature. This increase at long times has not been reported previously and is qualitatively different than the monotonic decrease reported for polymorphic crystallization. To rationalize these new results, a model of primary crystallization has been developed which includes homogeneous nucleation, diffusion-controlled growth, Gibbs–Thomson effect, and a mean field soft-impingement correction during growth. Based on this model and experimental results, it is concluded that the initial shift to lower temperatures is due to an increase in the number of nuclei (as concluded previously by others for the case of polymorphic crystallization) and the later shift towards high temperatures in our experiments is due to diffusion-controlled growth during the pre-DTA heat treatment.

Thermal evolution of Fe62.5Co6Ni7.5Zr6Nb2Cu1B15 metallic glass

Thermal evolution of Fe62.5Co6Ni7.5Zr6Nb2Cu1B15 amorphous alloy prepared by one-roll melt-spinning technique was studied by XRD and DTA. The crystallisation process, occurring in several steps, can be summarised as follows: a → a′ + α-Fe → a″ + α-Fe + γ-Fe → α-Fe + γ-Fe + ZrB12, where a and a″ are amorphous phases, and a′ can be indexed as a γ-Fe (fcc) structure, with a crystalline order on an average distance of 8 A. The metallic glass demixed on quenching, but component phases tended to mix by exchanging Fe atoms in a temperature range overlapped with the first crystallisation, which yields α-Fe nanocrystals (≈27 A). Higher temperature exo-peaks correspond mainly to a recrystallisation of the phases formed at lower temperature. It was found that this alloy has nanocrystalline structure also after heating at a well higher temperature than first crystallization. Even after the last exo-peak, the average crystallite size (D) was considerably smaller than that found in the literature for analogous metallic glasses; D values for our alloy were comparable to those of nanocrystalline phases of other systems heat treated below the temperature of exothermal DTA peaks. Extensive oxidation above 600°C, even at a low oxygen content (co2 ≤ 2 ppm), led to a marked modification of the surface layer: two zirconia polymorphs were identified on the surface of the ribbons, and the ratio of γ-Fe to α-Fe content increased with respect to the bulk. Differences in thermal evolution between outer layer and bulk are attributed to a different phase composition and non-uniform distribution since the as-quenched stage.

In situ generation of metal clusters in interlamellar spacing of montmorillonite clay and their thermal behaviour

Intercalated Ni–Ni(CH 3COO) 2–montmorillonite ( I), Ni 0–montmorillonite ( II), Zn–Zn(CH 3COO) 2–montmorillonite ( III) and Zn 0–montmorillonite ( IV) composites have been synthesised and their thermal behaviour is studied by TG, DTG and DTA substantiated by XRD and TEM. The endothermic DTA peaks of the composites I– IV due to dehydration and dehydroxylation, in general, occur at lower temperature compared to those of untreated Na–montmorillonite. The phase change of the clay matrix for the composites I and II takes place at slightly higher temperature compared to Na–montmorillonite, while for the composites III and IV it occurs at lower temperature. XRD data show that the basal spacing ( d 0 0 1 value 13.7 Å) of the intercalated metal cluster composite II is higher than that of the composite IV ( d 0 0 1 value 12.8 Å) at about 300 °C revealing intercalated metal cluster of size 4.1 and 3.2 Å, respectively. TEM study shows that the particle size of the metals for the composite II is in the range of about 0–80 nm while for the composite IV, the size range is about 0–30 nm. Therefore, bigger sized metal nanoparticles (>0.41 nm) are not present in the interlamellar spacing of the clay.

Thermoanalytical contribution to the study on paper degradation : Characterisation of oxidised paper

In this paper we studied the effect of oxidation on paper stability by means of simultaneous differential thermal analysis and thermogravimetry. In our laboratory we oxidised Whatman n. 1 chromatography grade paper and performed thermal analyses both on non-oxidised and on 6, 21 and 96 h oxidised samples. The results showed characteristic thermal effects, particularly performing the experiments under oxygen flow: it was noticed that the temperature of the main cellulose degradation DTA peak gradually decreases with the paper oxidation degree. An application of this method was extended to the study of differently oxidised parts of a 1948's book.

Crystallization of the K 2O·Nb 2O 5·2SiO 2 glass: evidences for existence of bulk nanocrystalline structure

The crystallization of the K 2O · Nb 2O 5 · 2SiO 2 (KNS-50) glass has been studied by DTA, XRD, SEM and FTIR. The as-quenched glass crystallizes in two steps during a DTA run. In the first step only potassium niobates crystallize while at the higher temperature step KNbSi 2O 7 ferroelectric crystallizes. Heat treatments of the bulk glass at temperatures not far from T g produce a nanocrystalline structure penetrating the whole volume of the glass and clearly observed by high resolution SEM as isometric nuclei. The observed nanostructure is the result of two processes occurring during the heat treatment: amorphous phase separation followed by the crystallization of the high alkali and niobium content glassy phase. The heat-treated glass, in which the nanostructure is grown, shows an additional DTA peak at about 900°C related to the crystallization of an unknown crystalline phase, not obtained in the as-quenched glass. The existence of the nanostructure clarifies the origin of SHG discovered recently in potassium niobium silicate glasses.

Processing and crystallisation of rapidly solidified Al2O3–Y2O3fibres

Amorphous fibres of the Al2O3–Y2O3 system were prepared by a melt extraction technique, and subjected to crystallisation. The quality of the melt extracted fibres is controlled by the wheel edge and rotational speed, with both having a significant effect on fibre diameter and avoidance of irregularities and instabilities along the fibre length. Tensile strength in the glassy state varied from 0·6 to 1·0 GPa. Crystallisation activation energies calculated from scan-rate dependence of DTA peaks are 741 and 1374 kJ mol-1 for E1 (Al2O3–yttrium aluminium garnet (YAG) eutectic), 390 kJ mol-1 for YAG, and 438 kJ mol-1 for E2 (YAG–yttrium aluminium perovskite (YAP) eutectic) by the Kissinger method; and 698 and 1346 kJ mol-1 for E1, 352 kJ mol-1 for YAG, and 399 kJ mol-1 for E2 by the Augis–Bennett method.

Determination of evolved gas transfer time from a thermoanalyser to a coupled gas detection system using pyrite oxidation

A method is described in which the time taken for molecules evolved during simultaneous TG–DTA analysis to travel from the sample crucible to an evolved gas analysis (EGA) detector, defined as the transfer time, τ, may be determined. It involves heating a <45 μm fraction of pyrite to about 550°C at a rate of 2.5°C min −1 in air flowing at rates ranging from 8 to 91 cm 3 min −1 in a thermal analyser and passing the SO 2 evolved into a Fourier transform infra-red (FTIR) or mass spectrometer (MS). Two sharp exothermic events occurred in the region 455–475°C which corresponded to equally sharp peaks in the FTIR and MS signals. The time shift between the DTA peak and the corresponding EGA peak was taken to be the transfer time and it was found that this increased with reduction in gas flow rate. For the equipment used in this experiment transfer times of SO 2 produced during the two events agreed within experimental error for gas flows down to 20 cm 3 min −1 but significant differences were measured for lower flow rates. This method should be useful in determining the transfer times of any combination of thermal analyser and FTIR or MS, provided the thermal analyser has a DTA detector.

Nucleation and crystallization in calcium aluminate glasses

The crystallization and nucleation mechanisms of a eutectic composition (46.5 wt% CaO, 53.5 wt% Al 2O 3) calcium aluminate glass were investigated by DTA, XRD and SEM. The crystallization temperature was found to vary from 940°C to 1040°C, and two phases appeared in the crystallized samples: Ca 12Al 14O 33 (major phase) and CaAl 2O 4 (minor phase). Two kinds of crystal structures, dendritic and spherulitic, were observed by optical microscopy. Heating rate and particle size effects on crystal growth were studied by DTA. The average activation energies of crystal growth in two different particle sizes were determined to be 739 ± 4 kJ/mol by the Kissinger method and 694 ± 4 kJ/mol by the Augis–Bennett method. The average Avrami constant for two different sized particles was determined to be 1.7 ± 0.2 by the Ozawa equation. The DTA peak heights decreased with increasing particle size, which shows that the crystallization mechanism includes surface crystallization. A nucleation-rate type curve was obtained by investigating the relation between nucleation temperature and DTA peak height, which suggests that there are two independent nucleation rate maxima. Two, distinct crystallization peaks were observed in monolithic (non-powdered) samples by DTA, and were further studied by in situ XRD and SEM. It was determined that the two peaks correspond to surface and internal crystallization, respectively.