Electron microscopy investigations of the morphology of a polyester-polyurethane

In the current paper the morphology of a slightly crosslinked polyester-polyurethane (hard-segment content ca. 33 wt.-%) is studied by means of transmission electron microscopy (TEM). A new staining technique based on ruthenium tetroxide was applied. This new technique allows the unambiguous visualization of the typical microdomain structure of the polyurethane and also of a wide size spectrum of globular hard-segment-rich domains which result from phase separation in the early stage of preparation. The hard-segment-rich microdomains have a size between 1.5 and 10 nm while the globular domains span a size spectrum from 10 to 2000 nm.     

Thermal decomposition of aluminium hydroxides

The thermal decompositions of the crystalline aluminium hydroxides hydrargillite, bayerite and nordstrandite were investigated by thermogravimetry, differential thermal analysis, X-ray diffraction and infrared spectrophotometry. It was found that these aluminium hydroxides undergo thermal decomposition in the following sequences: hydrargillite-1

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Zusammenfassung Die thermische Zersetzung von kristallinen Aluminiumhydroxiden, wie Hydrargillit, Bayerit und Nordstrandit, wurde thermogravimetrisch, differentialthermoanalytisch, röntgendiffraktometrisch und IR-spektrophotometrisch untersucht. Es wurde festgestellt, da\ diese Aluminiumhydroxide in der folgenden Reihenfolge zersetzt werden: Hydrargillit-I

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Studies on the reactions of aluminium oxides and hydroxides

The hydrothermal and decomposition reactions of differently ground gibbsite and its reaction products were studied. The samples were treated isothermally under hydrothermal conditions or in air, and the products were characterized by thermogravimetry and IR spectroscopy. It was found that the method and duration of grinding and the particle size of the starting gibbsite influenced the reactivity of the gibbsite itself and that of the primary reaction products. In a later step of the reaction sequence (hydrothermal rehydration of-alumina), the effects of the differences in the properties of the starting gibbsites were insignificant.

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Zusammenfassung Es wurden die hydrothermischen und Zersetzungsreaktionen verschiedener Gibbsitproben und deren Reaktionsprodukten untersucht. Die Proben wurden unter hydrothermischen Bedingungen oder in Luft isothermisch behandelt, die Produkte mittels TG und IR beschrieben. Es wurde festgestellt, da\ Art und Weise der Zerkleinerung und Korngrö\e des Gibbsit-Ausgangsmateriales die ReaktivitÄt von Gibbsit selbst, aber auch die der primÄren Reaktionsprodukte beeinflu\t. In spÄteren Schritten des Reaktionsverlaufes (hydrothermische Rehydratation von-Aluminiumoxid) besitzen Unterschiede in den Eigenschaften des Gibbsit-Ausgangsmateriales keinen Einflu\.

     

Influence of aluminium precursor on physico-chemical properties of aluminium hydroxides and oxides

An attempt to obtain aluminium hydroxide that could give aluminium oxides of increased thermal stability was made. Aluminium hydroxide was precipitated during a hydrolysis of aluminium chloride in ammonia medium. The influence of preparative conditions, such as a dosing rate of aluminium precursor, pH, duration of the precipitate refluxing and temperature of calcination, on the properties of obtained hydroxides and oxides was investigated. The materials were studied with the following methods: thermal analysis, IR spectroscopy, low-temperature nitrogen adsorption and adsorption–desorption of benzene vapours. Precipitated boehmites had high values of S _BET determined from nitrogen adsorption (220–300 m²g^–1), good sorption capacity for benzene vapours, developed mesoporous structure and hydrophilic character. It has been proved that a high pH value during the precipitation of aluminium hydroxide favoured better crystallisation of boehmite structure, higher temperature of its dehydroxylation into γ-Al₂O₃, and delayed transformation of γ phase into α-Al₂O₃. Aluminium oxides derived from the hydroxides precipitated at a high pH were the most stable at high temperatures, and were characterised with the best surface properties. The online version of the original article can be found at     

Influence of aluminium precursor on physico-chemical properties of aluminium hydroxides and oxides

The process of hydrolysis of aqueous aluminium sulfate was carried on in ammonia medium at 100°C and for different time intervals (0, 20, 39 or 59 h). The products thus obtained were calcined at 550, 900 or 1200°C for 2 h with the aim to obtain aluminium oxides. The materials were studied with the following methods: thermal analysis, IR spectroscopy, X-ray diffraction, low-temperature nitrogen adsorption, adsorption–desorption of benzene vapours and scanning electron microscopy. Freshly precipitated material was an amorphous basic aluminium sulfate which after prolonged refluxing at elevated temperature in a mother liquor underwent a phase transformation into highly crystalline NH₄Al₁₃(SO₄)₂(OH)₆ containing tridecameric unit Al₁₃. It was accompanied by a decrease of specific surface area and the formation of a porous structure less accessible for benzene molecules. Regardless of the duration of the hydrolysis process, all products were characterised with poorly developed porous structure and hydrophilic character. Their calcination at the temperature up to 1200°C resulted in the formation of α-Al₂O₃ via transition forms of γ/η- and δ-Al₂O₃. The samples of aluminium oxides obtained after calcination at 550 and 900°C had higher values of specific surface area than starting materials due to processes of dehydroxylation and desulfurization. The process of calcination up to 900°C was reflected in developing of not only porous structure but also hydrophobic character of prepared materials. The S _BET values calculated for the oxide samples obtained from aged products of hydrolysis at 1200°C were lower than for the analogous sample prepared without the ageing step. It was concluded that prolonged refluxing at elevated temperature of the products of hydrolysis of aluminium sulfate decreased thermal stability of final aluminium oxides.     

Influence of aluminium precursor on physico-chemical properties of aluminium hydroxides and oxides

The paper concerns aluminium hydroxides precipitated during hydrolysis of aluminium acetate in ammonia medium, as well as aluminium oxides obtained through their calcination at 550, 900 or 1200°C for 2 h. The following techniques were used for analysing of obtained materials: thermal analysis, IR spectroscopy, X-ray diffraction, low-temperature nitrogen adsorption, adsorption-desorption of benzene vapours and scanning electron microscopy. Freshly precipitated boehmite/pseudoboehmite had high value of S _BET, very good sorption capacity for benzene vapours, developed mesoporous structure and hydrophilic character. After prolonged refluxing at elevated temperature its crystallinity increased which was accompanied by a decrease of specific surface determined from nitrogen adsorption, decrease of sorption capacity for benzene vapours and weakening of the hydrophilic character. Calcination of all hydroxides at the temperature up to 1200°C resulted in the formation of α-Al₂O₃ via transition forms of γ-, δ-and θ-Al₂O₃. The samples of aluminium oxides obtained after calcination at 550 and 900°C were characterised with high values of specific surface area and displayed quite high heat resistance, probably due to a specific morphology of starting hydroxides. The process of ageing at elevated temperature developed thermal stability of aluminium oxides.     

Studies on the thermal reactions of aluminium oxides and hydroxides

The gibbsite -alumina decomposition (in air) and the-alumina boehmite transformation (under hydrothermal conditions) were investigated isothermally. Reaction products were characterized by TG and X-ray diffraction.The rate of the gibbsite -alumina reaction below 250 °C appears to be nucleation and growth controlled. That of the hydrothermal transformation of-alumina to boehmite depends on the time of grinding and the particle size of the gibbsite from which the-oxide has been prepared. During this reaction, partial re-formation of gibbsite was observed at lower temperatures, but the final product was always boehmite.

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Zusammenfassung Die Zersetzung von Hydrargillit zu-Aluminiumoxid in Luft und die Umwandlung von-Aluminiumoxid zu Böhmit unter hydrothermalen Bedingungen bei isothermer Versuchsführung wurden untersucht. Die Reaktionsprodukte wurden mittels Thermogravimetrie und Röntgenbeugung charakterisiert. Die Geschwindigkeit der Umsetzung von Hydrargillit zu-Aluminiumoxid unterhalb 250 °C wird durch Keimbildung und Wachstum bestimmt. Die Geschwindigkeit der hydrothermalen Umsetzung-Al₂O₃Böhmit hängt von der Mahldauer und der Teilchengrössen des Hydrargillits ab, aus dem das-Al₂O₃ präpariert worden war. Während letztere Reaktion wird bei tieferer Temperatur partiell Hydrargillit gebildet, das Endprodukt war stets Böhmit.

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Formation of YAG from coprecipitated yttrium aluminium hydroxides

Thermal decomposition of the amorphous coprecipitate of yttrium and aluminium hydroxides forming yttrium aluminium garnet has been investigated employing thermal analyses, X-ray diffraction and IR spectroscopy. On heating, the coprecipitate progressively loses water forming a stable but highly disordered hydroxy garnet which crystallizes at 1180 K and decomposes to YAG at 1290 K. Nucleation of the crystalline phase appears to begin at 800 K.The authors thank Dr. P. V. Ravindran of the Analytical Chemistry and Shri N. K. Kulkarni of the Fuel Chemistry Divisions of BARC for their assistance in the thermoanalytical experiments.     

Investigation of thermal transformations in aluminium hydroxides subjected to mechanical treatment

The influence of the mechanical activation of hydrargillite Al(OH)₃ on the mechanism of its' thermal dehydration and crystallization in response to treatment with steam was studied. The effects of grinding on the texture, structure and thermal behavior were examined by means of XRD, DTA, TG, DSC, IR spectroscopy and SEM. The mechanical activation of hydrargillite leads to formation of the amorphous state, and also results in a change in the thermal dehydration mechanism in the dynamic mode in the air, with a decrease in the H value of dehydration. The transition of hydrargillite into corundum on treatment with steam is facilitated after mechanical activation. The activated state of hydrargillite was found to be unstable and its reactivity decreases as time elapses.     

Nanostructure investigations with the analytical transmission electron microscope

Concerning the conventional TEM-imaging as well as the analytical procedures the capabilities are pointed out: electron diffraction, energy dispersive X-ray spectroscopy (EDXS) and electron energy loss spectroscopy (EELS). The possibilities of investigation of both nanocrystalline materials and multilayers are discussed, accompanied by examples of current investigations: At alloys, produced by intense milling, at single nanocrystals the imaging by diffraction contrast was successful, the analysis has failed because of the sample thickness. By means of energy spectroscopic imaging multilayers from Fe-SiB/NbCu and Fe/Cr as well as Al₂O₃/TiN have been characterized. Received: 15 July 1997 / Revised: 16 February 1998 / Accepted: 21 February 1998     

Electron microprobe investigations on phase boundaries steel-enamel

Summary This report describes investigations carried out on a new type of enamel (Liberty Coat Enamel) for steel sheets from continuously cast material. Subsequent to technological tests, EPMA investigations were conducted to study the mechanism of enamel adherence. For this purpose, we employed our Concentration Mapping (CM) technique. In order to obtain representative data, a special evaluation method was used. The results, which are partly in contradiction with statements in the literature, suggest that the concept of enamel adherence reactions should be given further critical consideration.The paper has been presented at 12th Colloquium on Materials Analysis, Institute for Analytical Chemistry, Technical University in Vienna, May 13–15, 1985.     

Thermal decomposition of nickel aluminium mixed hydroxides and formation of nickel aluminate spinel

Various nickel aluminium mixed hydroxide samples of different compositions were prepared by co-precipitation from their nitrate solutions using dilute NH₄OH. Additional samples were prepared by impregnation of hydrated Al₂O₃, preheated at 600 and 900°C, with nickel nitrate solution in an equimolar ratio. The thermal decomposition of different mixed solids was studied using DTA. The X-ray investigation of thermal products of the mixed solids was also studied.The results obtained revealed that the presence of NiO up to 33.3 mole % with aluminium oxide much enhanced the degree of crystallinity of the γ-Al₂O₃ phase. In contrast, the presence of Al₂O₃ much retarded the crystallization process of the NiO phase. With the exception of samples containing 20 mole% NiO, all the mixed hydroxide samples, when heated in air at 900°C, led to the formation of well-crystalline Ni Al₂O₄ spinel, alone, or together with either NiO or γ-Al₂O₃, depending on the composition of the mixed oxide samples. The solid containing 20% NiO and heated at 900°C was constituted of amorphous NiO dispersed in γ-Al₂O₃. Heating the nickel nitrate-impregnated Al₂O₃ in air at 800–1000°C led to the formation of Ni Al₂O₄ together with non-reacted NiO and γ-Al₂O₃. The degree of crystallinity of the spinel was found to increase by increasing the calcination temperature of the impregnated solids from 800 to 1000°C and by increasing the preheating temperature of the hydrated Al₂O₃ employed from 600 to 900°C.     

The dehydroxylation of aluminium hydroxides and the kinetics ofα-Al2O3 formation

Studies were made of the dehydroxylation of several aluminium hydroxide modifications and the kinetics of-Al₂O₃ formation. The investigated samples differed in both mineral composition and the level of alkali admixtures. It was found that the rate of formation and the quantity of-Al₂O₃ depend mainly on the purity of the aluminium hydroxides, while the transition forms of alumina depend on the initial type of the aluminium hydroxide.

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Zusammenfassung Es wurde die Dehydroxylierung verschiedener Aluminiumhydroxidmodifizierungen sowie die kinetische Untersuchung der Bildung von-Al₂O₃ durchgeführt. Die untersuchten Proben unterschieden sich sowohl in ihrer Mineralienzusammensetzung als auch im Grad der Verschmutzung mit Alkalien. Man stellte fest, daß sowohl Geschwindigkeit als auch Menge von-Al₂O₃ hauptsächlich von der Reinheit des Aluminiumhydroxides abhängen und daß die Übergangsformen von Aluminiumoxid von ihrem ursprünglichen Typ von Aluminiumhydroxid abhängen.

     

A vibrational study of the nature of hydroxyl groups chemical bonding in two aluminium hydroxides

Lengths, strengths and valences of OH bonds in the two aluminium hydroxides gibbsite and bayerite were determined on the basis of vibrational spectral data. The uncoupled OD stretching modes in the range 2400–2800 cm⁻¹ were recorded by means of infrared diffuse reflectance, thereby avoiding effects of surface, vibrational coupling or particle shape. The assignment of the corresponding Raman spectra resulted in the determination of harmonic wavenumbers, force constants and anharmonicity coefficients of bulk OH groups in the two minerals. OH bond lengths deduced from these data varied from 0.964 Å to 0.975 Å in gibbsite and 0.962 Å to 0.973 Å in bayerite. These lengths appear to correspond to weak H-bonds contrary to previously recognized data from X-ray diffraction and neutron diffraction studies. Finally, bond valences were calculated on the basis of these new bond lengths and discussed as a function of crystallographic structures and the nature of hydrogen bonding in these two structures.     

Electron microscope studies on a quenched FeMo alloy

An alloy of FeMo (50 atom%) quenched from the melt was studied using a high-resolution electron microscope. Three phases were frequently identified, viz, the σ-phase, the μ-phase, and the P-phase. The latter two were found to be rich in planar defects of intergrowth and twin type. The resolution obtained was better than 4 Å, as shown with calculated images. It was possible to derive the detailed atomic structure of the defects observed.     

Electron microscope study of viscoelastic cationic surfactant systems

Some cationic surfactants such as cetyltrimethylammonium bromide (CTAB) show a very spectacular viscoelasticity in solution, in even very diluted states, with some aromatic substances added, such as salicylic acid. Formerly, the authors established that the remarkable solution behaviour was the result of the entanglement of such enormously elongated giant micelles, based on recognition of the existence of large aggregates, through direct observation of them as substantial images under the electron microscope.In this report, we further confirm the relation between micelle length and solution viscoelasticity after arbitrarily obtaining different size aggregates by altering the media pH, or by admixing shorter chain length surfactants than CTAB, which alone did not induce viscoelasticity.     

Computational studies of the cationic aluminium(chloro) hydroxides by quantum chemical ab initio methods

Cationic aluminium(chloro) hydroxide complexes with two to four aluminium atoms were studied using quantum chemical methods. Complexes were studied in both gas and liquid phase. The liquid environment was modeled by using a conductor-like screening model (COSMO). COSMO calculations were carried out as a single point calculation at the optimized gas phase structures. Water (epsilon = 78.54) was used as the solvent. The minimum energy structures obtained from the gas phase studies were mostly compact cyclic structures. Aluminium preferred to be five-coordinated in oxygen rich clusters. Core oxygen preferred three-fold coordination but in the largest clusters the four-coordinated oxygen was observed. Water reacted dissociatively with hydrogen poor clusters. The COSMO calculations showed that the optimal structures of cationic aluminium(chloro) hydroxides tend to be more open in the liquid than in the gas phase.