Curie temperature in Fe(Ni)Nb based mechanically alloyed materials

Summary A conventional thermogravimeter has been adapted with a small magnet to detect the Curie temperature, T_C. The measurements were performed in several Fe(Ni)NbB alloys developed in a nanocrystalline form by mechanical alloying. The B addition favors a slight diminution (10-20°C), and the Ni addition the existence of three transitions related with bccFe, fcc(Fe-Ni) abd fccNi-rich environments. Furthermore, complementary analysis were performed by means of differential thermal analysis, scanning electron microscopy with energy dispersive X-ray microanalysis and by induced coupled plasma. Small contamination was found. A mass increase (about 1 mass%) was detected by thermogravimetry related to oxidation. Analysis allows us to state the inhomogeneity of the alloys obtained after 80 h of milling.     

Mullite Formation of Colloidal Matrix Hybrid Aluminosilicate Gel

A hybrid aluminosilicate gel composed of a relatively small amount of single-phase gel in a diphasic matrix forms when a diphasic gel of AlOOH and SiO₂ is heated at 700°C for 7 h. The formation of mullite from this hybrid gel is a sequential conversion process, but not of two entirely independent processes of single-phase gel and diphasic gel. The high commencement temperature (>1250°C) of mullite formation from this hybrid gel indicates that the dominant component (diphasic component) strongly affects or controls the mullite transformation behavior of the hybrid gel. The apparent activation energy for mullitization from this hybrid gel is 846 kJ/mol, which is somewhat lower than that of pure diphasic gel but much higher than that of pure single-phase gel.     

Thermoanalytical study of acetylacetonate-modified titanium(IV) isopropoxide as a precursor for TiO<Subscript>2</Subscript> films

Thermal decomposition of dried TiO₂ gel, obtained by hydrolysing acetylacetonate-modified titanium(IV) isopropoxide, was monitored by simultaneous TG/DTA/EGA-FTIR measurements in dynamic air up to 900&deg;C. XRD and FTIR were employed to identify the solid reaction products. Thermal degradation of the TiO₂ gel consists of five distinct mass loss steps, the total mass loss being 43.8&percnt;. EGA by FTIR revealed the release of H₂O below 120&deg;C; followed by acetone, isopropyl acetate and 1-propanol around 200-300&deg;C, and finally CO and CO₂ up to 550&deg;C. Highly exothermic reaction at 410-550&deg;C is caused by the combustion of carbon residues. Crystalline TiO₂-anatase is formed around 500&deg;C and TiO₂-rutile close to 800&deg;C.     

Effect of nickel and cobalt ions on low temperature synthesis of mullite by sol–gel technique

Synthesis of mullite has been achieved at a low temperature of 600 °C by sol–gel technique in presence of nickel and cobalt ions. Samples were characterized by DTA, XRD, FESEM and FTIR spectroscopy. Mullite formation was found to depend on the concentration of the ions to a certain extent. Highly crystalline spherical mullite particles of dimension 35 nm were obtained at 0.02 M nickel or cobalt concentration.     

Use of thermoanalytical methods in quantification of drug load in mesoporous silicon microparticles

Summary Thermally carbonised mesoporous silicon microparticles were produced and loaded with two active pharmaceutical ingredients, ibuprofen and antipyrine. By combining the results measured with TG and DSC, reliable estimations for the degrees of the drug loads were obtained. To distinguish the drug adsorbed on the surfaces of the microparticles from that absorbed into the pores, the principle of thermoporometry on the DSC measurements was employed. According to the principle, the drug held in the capillaries of porous material has a depressed melting temperature because of the higher pressure of the drug in cavities with a curved interface. On the other hand, the drug located on the external surface of the microparticles exhibits the normal melting of bulk drug. The loading degrees obtained with the thermoanalytical methods (31 and 26 mass&percnt; for ibuprofen and antipyrine, respectively) were comparable with the results obtained with helium pycnometry (the corresponding values were 33 and 28 mass&percnt;). Nitrogen sorption studies were not reliable for quantitative determinations due to the inability of nitrogen to penetrate in all pores, which might be blocked by the drug on the surface of the microparticles.     

Highly fluorinated graphite prepared from graphite fluoride formed using BF3 catalyst

Fluorinated graphites (CF_0.47) were obtained by reaction at room temperature of fluorine gas with graphite in the presence of boron trifluoride and hydrogen fluoride as catalysts. Their thermal treatments under fluorine at temperatures up to 600 °C lead to a progressive increase of the fluorine level resulting in an highly fluorinated graphite (CF_1.02). Whatever the fluorination level, a stage one fluorine-graphite intercalation compound is obtained. The sp² carbon hybridization is maintained for treatment temperature below 300 °C and two types of structure coexist for T_T in the range 350-550 °C. Finally, above 550 °C, carbon hybridization is sp³.The resulting materials were studied by ¹¹B, ¹H, and ¹⁹F NMR and EPR at different experimental temperatures giving informations about the intercalated fluoride species, the temperature of their removal from the host fluorocarbon matrix, as well as their mobility.     

New data on the thermal analysis of diphasic mullite gel

A diphasic mullite gel has been synthesized by using Ludox and Al(NO₃)₃-9H₂O in ammoniacal solution. Both DTA and DDTA studies have been performed with a sensitive differential thermal analyzer system. Besides usual exotherm due to mullite formation, a pronounced exotherm between 800–1300°C has been noted. Considering previous XRD data, it is explained as due to formation of Si-Al spinel phase, which is subsequently transformed exothermally at 1320°C to mullite.     

Phase Transformation During the Heat Treatment of Sol-Gel Mullite

The mechanism of mullite formation depends upon the method of combining the silica and alumina-containing reactants. In this work, the effects of the processing variables (dilution, pH, gelling temperature and water content) on mullite formation were investigated. Monophasic gel was synthesized from aluminium nitrate nanohydrate dissolved in an absolute ethyl alcohol and silica sol mixture. The gelling was performed at room temperature and at 60°C. It was found experimentally that the temperature of mullite formation depends on the microstructure of the gel. The lowest temperature of mullite formation (970°C) was observed when the mullite gel was prepared at low pH (pH=1), low water content and high gelling temperature. This revised version was published online in July 2006 with corrections to the Cover Date.     

Thermal behavior of the heart of SHR and wistar rats

Summary Hypertension is a major and growing public health problem. It is responsible for the mortality of millions of people around world, and is increasing each year. Nevertheless, the understanding of the relationship between the composition of the heart&apos;s cells, hypertension and health diseases is still very incomplete. The present study focuses on the evaluation of the attributes of some hearts of Spontaneous Hypertension Rats (SHR), comparing with SHR which received additional amounts of polysaccharide (SHR+P) and with wistar rats (normal blood pressure) by Thermal Analysis. Some differences could be seen between groups, as the residue content after 800&deg;C was different for rats from different groups, and of wistar rats hearts samples showed 5.3&plusmn;0.3&percnt; of residues vs. 8.3&plusmn;1.5&percnt; of the SHR. DSC profiles for wistar rats showed one intense endothermic event at 160&deg;C, with enthalpy transition of 450 J g^-1 and more three small events. Thermal analyses curves also showed some differences between freezing and no freezing samples, probably associated to the denaturation of proteins and degradation of organic materials.     

Tracer diffusion of Co2+ ions in multielectrolyte systems: Activation energy and obstruction effect

The activation energy for the tracer diffusion of Co²⁺ ions in multielectrolyte systems containing alkali bromides has been determined in agar gel medium over the temperature range of 25–45°C. The decrease in the value of the Arrhenius parameters, E and D_o, with gel percentage is explained on the basis of the transition state theory. Further, studies of the influence of electrolyte concentration on activation energy and obstruction effect reveal that both parameters decrease with the former. The decrease in activation energy is explained by considering the changes in physical properties of the solution with concentration at microscopic level, while the decrease in the extent of obstruction effect is attributed to competitive hydration between ions and agar molecules in a diffusion system.     

Sintering of kaolinite with ammonium fluoride

The present work represents a study of the influence of ammonium fluoride on the thermal behaviour of kaolinite by using a derivatograph. The sintering of kaolinite with ammonium fluoride was found to be complicated. Different products of sintering are obtained, depending on the temperature and the amount of ammonium fluoride. They were identified microscopically and by using a Siemens Crystalloflex diffratometer. These include an ammonium aluminium fluoride complex, cryptohalite, aluminium fluoride, mullite, topaz and corundum. The DTA curves (using kaolinite and ammonium fluoride mixes of ratio 1:1) indicate the formation of the ammonium aluminum fluoride complex and cryptohalite at 120–280°C and the appearance of aluminium fluoride, topaz and mullite at 640°C. The intensive formation of topaz takes place at 750°C and its subsequent dissociation at 940°C with corundum formation. The very small endothermic peak at 1010°C represents the formation of mullite. In experiments using mixes of kaolinite—ammonium fluoride in the ratios 1:1 and 1:1.3 the end-product of sintering consists of corundum and mullite. When using mixes of the ratio 1:1.7, aluminium fluoride constitutes the main composition of the end-product.     

Crystallization of Gels in the Apatite-Mullite System

Crystallization processes in gels of the apatite-mullite system were studied to obtain information for the synthesis of bioglass-ceramics and composite materials. SiO2-sol, Al(NO3)3·9H2O, Ca(NO3)2·4H2O, (NH4)3PO4·3H2O and CaF2 were used as precursors. CaF2 was added before and after gelation. Mixtures of mullite gel-glass and fluorapatite in the range 10 to 90 mol% were investigated for synthesis of composites. All the samples were heat treated at different temperatures in the range 950–1250°C and the structural changes were established using X-ray diffraction and IR-spectroscopy. When the gels were treated at 1050°C and at 1150°C, the main crystalline phases found were fluorapatite and mullite independent of the CaF2 content and the manner of its addition. At 1250°C the relative amounts of fluorapatite and mullite decrease and gehlenite appears. Composite materials containing fluorapatite and mullite as main crystalline phases can be obtained only when the content of mullite gel-glass in the initial mixture is more than 60 mol%.     

The Effect of Precursor Chemistry on the Crystallisation and Densification of Sol-Gel Derived Mullite Gels and Powders

Stoichiometric and silica-rich mullite gels and powders were prepared using four different sol-gel methods. Thermal analysis, X-ray powder diffraction and dilatometry techniques were used to investigate the thermal decomposition, crystallisation and sintering of these mullite precursor gels. The method of preparation, by controlled hydrolysis of various mixtures of tetraethylorthosilicate, aluminium sec-butoxide and aluminium nitrate, affected the texture of the gels, producing single-phase or diphasic samples.The crystallisation sequence of the gels depended on the composition and method of preparation. Single phase mullite crystallised from homogeneous gels at 980°C, while diphasic gels initially formed of a mixture of -Al₂O₃ spinel and mullite, or simple -Al₂O₃ spinel, which subsequently transformed to mullite at 1260°C.Dilatometry and density measurement were used to investigate the sintering of compacts formed by pressing powders prepared from gels precalcined at 500°C. Varying the heating rates from 2 to 10°C min^-1 had little effect on the densification to 1500°C. However, the densification rate was sensitive to the degree of crystallinity and the amount and type of phases present at the sintering temperature. The presence of -Al₂O₃ spinel in the structure initially promoted densification, but the sintering rate was reduced considerably after mullite crystallised. Diphasic materials, especially those with an excess amount of silica in the original gel, sintered to higher densities due to the presence of excess silica promoting densification by viscous phase sintering.     

Resolving the different dynamics of the fluorine sublattices in the anionic conductor BaSnF(4) by using high-resolution MAS NMR techniques

19F and (119)Sn MAS NMR spectroscopy have been used to investigate the fluoride ion conductor, BaSnF(4), a member of the MSnF(4) family of fluorite-related anionic conductors containing double layers of Sn(2+) and M(2+) cations. Two fluorine sublattices were observed by (19)F MAS NMR, which could be assigned to specific sites in the lattice. The first sublattice is due to fluorine atoms located in Ba(2+) double layers and is rigid on the MAS NMR time scale at room temperature. The second sublattice comprises the fluoride ions between the Ba(2+) and Sn(2+) layers, and the few fluorine atoms that inhabit the Sn(2+)-Sn(2+) double layers. These ions are in rapid exchange with each other, and an extremely short correlation time tau(C) for the motion of these ions of <3 x 10(-)(5) s is obtained at -100 degrees C. T(1) measurements indicate that tau(C) approaches 10(-)(8) s at room temperature. (19)F-to-(119)Sn cross-polarization (CP) experiments confirmed the assignments of the resonances, and that the fluorine atoms located next to the tin atoms are extremely mobile at room temperature (and thus do not contribute to the CP process). Two-dimensional (19)F exchange experiments showed that exchange between the rigid and mobile lattice does occur, but at a much slower rate (tau(C) approximately 10 ms at 250 degrees C). Low-temperature (19)F MAS and (19)F-to-(119)Sn CP NMR spectra demonstrate that the motion of the fluoride ions has almost completely frozen out by -150 degrees C. The results are consistent with rapid two-dimensional (anisotropic) conductivity involving the fluoride ions between the Ba and Sn layers. Conductivity in three dimensions requires hops between the ions in the BaF(2)-like layers and the mobile ions. This process does occur, but with exchange rates that are at least 6-7 orders of magnitude slower.     

Novel non-hydrolytic sol-gel route to metal oxides

Monolithic alumina and aluminosilicate gels have been prepared using a novel sol-gel process based on the non-hydrolytic condensation reaction between a metal halide and a metal alkoxide. XRD indicated that the alumina gel remained amorphous at 750°C; solid state ²⁷Al NMR indicated the presence in the dried gel and in the amorphous calcined sample of a large amount of pentacoordinated aluminum atoms. A study of the sol formation using liquid state ²⁷Al NMR suggested that the gel structure was reminiscent of the oligomeric structure of the chloroisopropoxide precursors. Differential thermal analysis and XRD indicated that the aluminosilicate gels were converted to mullite below 1000°C, suggesting a high degree of homogeneity in these precursors.     

Mullite-Alumina Composites Prepared by Sol-Gel

Ceramic composite gels in the mullite-alumina system (with mullite phase contents ranging from 20 to 100 wt%) were prepared by sol-gel in acid media and using a water/alkoxide molar ratio of 4. Powders were uniaxially pressed to obtain discs.The effect of heat treatment on the materials was followed by thermal analysis and X-ray diffraction (XRD). The crystalline phases formed from the gels were dependent upon the sample composition and the heat treatment temperature. For samples with composition close to the stoichiometric mullite composition the crystallization of mullite was observed after heat treatment at temperatures 900°C, and corundum appeared at higher temperatures. For samples with higher alumina contents mullite formation together with the spinel phase was observed at the lower heat treatment temperatures, while mullite, silimanite, corundum, and other transient phases of alumina were also detected at higher temperatures. The densities and microstructures of materials sintered at 1400°C were studied. Densification of the samples was low due to the agglomeration of the powders, as confirmed by scanning electron microscopy.     

Activation Energy of Tracer-Diffusion of Manganese Ions (Mn2+) in Alkali Metal Chloride Solutions

The activation energy of the tracer diffusion of Mn²⁺ ions in alkali chlorid solutions (0.1M) has been determined in agar gel medium (1–2.5%) over the temperature range of 25–45 °C. The decrease in the value of the Arrhenius parameters, E and D ₀, with gel percentage is explained on the basis of the transition state theory. Further, the activation energy as a function of electrolyte concentration is also investigated using 1% agar gel in the temperature range of 25–45 °C. In both the cases, the activation energies are determined by the least square fitting of the diffusion coefficient data obtained at various temperatures through the Arrhenius plots.