Hubert Langbein

Investigation on the formation of manganese-zinc ferrites by thermal decomposition of solid solution oxalates

Abstract Homogeneous solid solution oxalates of Mn(II), Zn(II) and Fe(II) were prepared by copredpitation from metal acetate solution with oxalic arid, with the partial replacement of water by acetone as solvent. During the thermal decomposition of solid solution oxalates in a nitrogen atmosphere (10 ppm O 2 ) the oxygen partial pressure is considerably lowered by the mixture of CO-CO 2 produced. Under this condition a single phase ferrite of spinel structure is formed by thermal decomposition in the range 400–600°C with a short annealing time of 3 h. With annealing for a longer time in a nitrogen atmosphere (10 ppm O 2 ), α-Fe 2 O 3 is formed as well as the spinel according to an extrapolated p (O 2 )-θ diagram for manganese ferrite.

Investigations into phase formation of LiFe5o8 from decomposed freeze-dried Li-Fe-formates

Lithium ferrite (LiFe5O8) was prepared by decomposition of a freeze-dried Li-Fe-formate precursor at unusually low temperatures of 250 °C–700 °C. The phase formation processes are studied by means of X-ray powder diffraction, IR-spectroscopy, Mo¨ssbauer spectroscopy and magnetic measurements. The decomposition of the Li-Fe-formate at 250 °C produces a spinel phase similar to γ-Fe2O3. Insertion of Li+ ions into the γ-Fe2O3 lattice and cation ordering processes take place on thermal treatment in the temperature range from 300 °C–700 °C without an intermediate formation of α-Fe2O3. At 700 °C a pure crystalline lithium ferrite with the expected magnetic properties is obtained.

Thermal decomposition of freeze-dried μ-oxo-carboxylates of manganese and iron

Abstract The decomposition of freeze-dried mixed carboxylates of manganese and iron was investigated by means of DTA, TG, mass spectroscopy and X-ray powder diffractometry. The three main steps of decomposition are characterized as release of (a) H 2 O, (b) carboxylic acid and CO 2 /CO, and (c) the corresponding carbonyl compound and CO 2 . In particular, the course of process (b) strongly depends on the stability of the metal–carboxylate link in the three investigated carboxylates. Well-crystallized single-phase manganese ferrites can be obtained on decomposition of formates of appropriate composition and thermal treatment of decomposition products at 600°C while maintaining a p (O 2 ) within the coexistence field of manganese ferrite.

Investigation of the formation of nickel-zinc ferrite from coprecipitated oxalates

Abstract Homogeneous solid-solution oxalates of Ni 2+ , Fe 2+ and Zn 2+ ions were synthesized by coprecipitation from acetate solutions using a special double-inlet technique and freeze drying of the precipitates. The mixed oxalates decomposed on heating to form ferrites with a spinel structure. The conditions of precipitation, the characteristics of the precipitates and the decomposition conditions affect phase formation and product characteristics. When preparing the oxalate precursor by dropwise addition of ammonium oxalate solution to a metal sulphate solution, as well as the ferrite, simple oxides are partially formed on decomposition. On decomposition of the substances prepared according to the method described here, however, the appropriate ferrite was formed as the sole phase. The grain size distribution of the oxalate precursors can be widely influenced by the precipitation conditions. For finely dispersed initial powders, the grain size distribution and grain shape are extensively maintained during the decomposition process.

Preparation of copper niobates by thermal decomposition of freeze-dried complex oxalate solutions

Abstract Freeze-dried complex oxalates of copper and niobium are reactive precursors for ternary copper niobium oxides. The thermal decomposition of the amorphous precursors has been investigated by DTA, TG and mass spectroscopy. The gaseous decomposition products are assigned to steps in the TG graph. Above 500°C, crystallization of complex oxides takes place. In the temperature range up to 900°C, only the ternary phases CuNb 2 O 6 and Cu 3 Nb 2 O 8 can be synthesized. The thermal decomposition of the oxalate precursor is advantageous with regard to the preparation of pure monoclinic CuNb 2 O 6 . The transformation between monoclinic and orthorhombic CuNb 2 O 6 is reversible with a high activation energy of transformation.

On the thermal decomposition of [Fe2NiO(CH3COO)6(H2O)3]·2H2O

The heteronuclearΜ-oxoacetate with the composition [Fe2NiO(CH2COO)6(H2O)3]·2H2O decomposed on heating, forming nickel ferrite NiFe2O4 and (depending on the decomposition conditions) in part other solid phases. H2O, CH3COOH, acetone and CO2 were also formed in the decomposition. A reaction scheme is given for the decomposition. The products were porous powders with grain diameters between 3 and 10Μm. On increase of the temperature of decomposition from 300 to 800 ‡C, the BET surface area and the surface area of the pores decreased, but only a small alteration in grain size was observed. As a result of thermal treatment in the temperature region abone 800‡C, larger aggregates of grains were formed in sintering processes.ZusammenfassungHeteronukleareΜ-Oxoazetate der Zusammensetzung [Fe2NiO(CH3COO)6(H2O)3]·2H2O werden durch Erhitzen zersetzt, wobei Nickelferrite NiFe2O4 und — in AbhÄngigkeit von den Bedingungen der Zersetzung — mit einem Teil anderer fester Phasen gebildet wird. In der Zersetzungsreaktion werden auch H2O, CH3COOH, Azeton und CO2 gebildet. Es wird ein Reaktionsschema für die Zersetzung angegeben. Die Produkte sind poröse Pulver mit einem Korndurchmesser zwischen 3 und 10 m. Wird die Zersetzungstemperatur von 300 auf 800‡C erhöht, nimmt die BET-OberflÄche und die PorenoberflÄche ab, wobei sich die Korngrö\e aber nur wenig verÄndert. Im Ergebnis der WÄrmebehandlung im Temperaturbereich oberhalb 800‡C werden durch Sinterprozesse grö\ere Partikelaggregate gebildet.

The thermal decomposition of crystalline and freeze-dried μ-oxo-acetato-complexes of manganese and iron

Abstract Mixed nuclear μ-oxo-acetato-complexes of Mn2+ and Fe3+ have been synthesized by means of two different synthetic paths (precipitation or freeze drying). These complexes are interesting because after the decomposition, a manganese ferrite with spinel structure is produced. That is why the decomposition process of both substances has been investigated by means of DTA, TG and mass spectroscopy and the results compared. Water, acetic acid, carbon dioxide and acetone are identified as decomposition products and assigned to the steps of the TG graph. Quantification of the decomposition products using wet analysis methods enables a complete decomposition mechanism to be worked out.

Zur Hydrolyse von Trisalkoxyvanadylverbindungen / On the Hydrolysis of Trisalkoxyvanadyl Compounds

Trisalkoxyvanadyl compounds and products of their hydrolysis were characterized by 51V NMR spectroscopy. There are characteristic connections between interactions of VO(OR)3 compounds in solution and the hydrolysis behaviour. On hydrolysis, a fast condensation process leads to intermediate decavanadate species. In a following slow reaction depending on the composition of the solution different equilibrium species are formed.

Zur Kenntnis rhomboedrischer Perowskite (La,Sr)(Mn,Fe)O3/Contributions to the Chemistry of Rhombohedral Perovskites (La,Sr)(Mn,Fe)O3

Abstract Perovskites La1−xSrxMn1−xFexO3+δ (x = 0.1 - 0.5) were prepared by a freeze-drying method and for comparison by conventional solid state reaction. Freeze-dried precursors are more reactive, forming the final perovskites at lower temperatures and within shorter reaction times. Under the reaction conditions employed (air atmosphere, 800 - 1350 °C), the perovskite La0.9Sr0.1Mn0.9Fe0.1O3 was obtained in an orthorhombic modification. By annealing under oxygen at 800 °C, however, a rhombohedral modification La0.9Sr0.1Mn0.9Fe0.1O3.1 with an excess oxygen content can be prepared. The structures of the other perovskites of the series (x = 0.2 - 0.5) were refiṉed on the assumption of a rhombohedral distortion of the ideal perovskite structure (space group R3c). There are characteristic correlations between composition of samples, volume and distortion of the AO12- and BO6-polyhedra in the ABO3 structure.

Preparation of YBa2Cu3O7−x by thermal decomposition of coprecipitated oxalates

Yttrium, barium and copper oxalates are coprecipitated quantitatively from metal acetate solutions with oxalic acid in water-acetone mixtures. The particle-size distribution of the precipitate can be controlled by the precipitation conditions. Thermal decomposition of the coprecipitate leads to single-phase YBa2Cu3O7-x. Despite the intermediate formation of BaCO3, the minimal reaction temperature (770 °C) is clearly lower than that for the solid-state reaction. Results of differential thermal analysis and isothermal thermogravimetry are discussed. The products are characterized by X-ray diffraction and infrared spectroscopy.