Hermann Schmalzried

Internal solid state reactions

Abstract Internal solid state reactions in nonmetallic systems (in particular in oxide systems) are reviewed. Firstly, internal oxidations are discussed, both in respect of theory and experiment. Secondly, internal reductions are treated. Thirdly, reactions of type A + B = AB which occur in a solid matrix (γ) are introduced and some pertinent examples are given. Finally, electrochemical internal reactions are analyzed, the characteristics of which are junctions of the type electronic-ionic.

Reaktionsmechanismen der Spinellbildung im festen Zustand

Es wird der Mechanismus von Festkörperreaktionen zwischen CoO auf der einen und Ti02, Al2Oa und Gr203 auf der andern Seite untersucht. Zu diesem Zweck wird die WAGNERsche Theorie für Festkörperreaktionen auf Anionenwanderung erweitert und die wesentlichen Formeln zur Berechnung von Reaktionsgeschwindigkeiten zusammengestellt. Markierungsversuche und Messungen der Reaktionsgeschwindigkeit für die Spinellbüdungsreaktion in sauerstoffhaltiger Atmosphäre und der Vergleich mit den aus Selbstdiffusionskoeffizienten und freien Bildungsenthalpien für die Spinellbildungsreaktion berechneten Reaktionskonstanten können nur gedeutet werden, falls man Transport von Sauerstoff über die Gasphase annimmt. Dagegen ist in sauerstofffreier Atmosphäre im Falle von Co2Ti04 und CoOr2Ot die Spinellbildungsreaktion durch entgegengesetzte Kationendiffusion zu deuten, während die Ergebnisse für CoAl2Oi nicht eindeutig sind.

Experimental evidence of the interference between ionic and electronic flows in an oxide with prevailing electronic conduction

Abstract Linear transport theory prescribes that for, e.g. a binary oxide Co1−δO, a flow of cations (J1) may be induced not only by its direct cause which is a gradient of the cationic electrochemical potential (▿η1), but also indirectly by a gradient of the electronic electrochemical potential (▿η2), orJ1=−L11▿η1−L12▿η2,Lik being the transport coefficient. Routine practice, however, has been neglecting the cross coefficient, i.e. L12=0. We have determined experimentally the charge of transport of the cation α∗1 (≡ L 12 L 11 ) for Co1-δO with three different and independent techniques, and concluded that the cross coefficient is not negligible by any means. All the numerical data on α∗1 up to date are compiled. Consequences of the non-zero cross coefficient and the physico-chemical nature of the charge of transport are discussed.

Cross Effect Between Electronic and Ionic Flows in Semiconducting Transition Metal Oxides

In semiconducting transition metal oxides, e.g. Coj _¿0, an electronic flux may be induced indirectly by an ion flux even in the absence of its own direct cause, a gradient of the electronic electrochemical potential. The magnitude of this cross effect is determined by the charge of transport, aCo, of the migrating ion, the absolute value of which may be interpreted as the number of electrons dragged by a Co2 + or the number of holes dragged by a Vc„. The formal and exhaustive analysis of this phenomenon is made on the basis of a proper distinction between building units and structure elements of the crystal. A preliminary result of the experimental determination of the cross effect, and the reanalysis of the literature data reveal that, for Coi -áO with VCo prevailing, ac„ is by no means negligible. The implications and consequences of this non-zero cross effect are discussed.

Diffusion und Fehlordnung in Spinellen

Zusammenfassung Es werden mit Hilfe von radioaktiven Isotopen die Selbstdiffusionskoeffizienten der Kationen Oo2+ und Cr3+ in den Spinellphasen Co22TO4, CoAliLOi, CoCrfli und NiCr¿Ot bestimmt. Um die thermodynamischen Variabein eindeutig festzulegen, werden die Spinelle ins Gleichgewicht mit ihren Einzeloxyden gebracht. Aus dem Verhältnis der Selbstdiffusionskoeffizienten, falls der Spinell mit dem einen oder dem andern der Einzeloxyde im Gleichgewicht steht, lassen sich unter bestimmten Voraussetzungen Aussagen über den vorherrschenden Fehlordnungstypus gewinnen. Es werden die Zuverlässigkeit der experimentellen Methode und die Grenzen der Theorie für die untersuchten Spinellphasen diskutiert.

Tracer diffusion in chemical potential gradients

Abstract Simultaneous cation tracer diffusion and drift has been measured in cobalt oxide, which was exposed to an oxygen potential gradient. The drift of the tracer cations is due to the cation vacancy flux. The cobalt oxide crystal as a whole is therefore shifted in the oxygen potential gradient, towards the higher oxygen potential. The motion of the cation tracers in the oxygen potential gradient is analyzed theoretically, and an average effective charge of the vacancies (0.6±20%) is found by comparison of experimental and theoretical tracer concertration profiles. No correlated vacancy motion could be detected in the applied oxygen potential gradient.

Reaktionen zwischen festen Halogeniden

Zusammenfassung Es wird der Mechanismus von Festkörperreaktionen zwischen KCl und SrOl2 bzw. BaCl2 (KBr und SrBr2 bzw. BaBr2) untersucht. Die Ergebnisse können gedeutet werden unter der Annahme, daß die Ionen durch die Reaktionsprodukte diffundieren. Die WAONERsche Theorie, auf Anionenwanderung erweitert, gestattet eine quantitative Beschreibung der Zusammenhänge. Als geschwindigkeitsbestimmender Schritt wird die Diffusion von Kaliumionen durch die Reaktionsschicht gefunden.

On the morphological changes on CoO-surfaces during vacancy relaxation processes (is DIGM involved?)

Etude sur des monocristaux et des polycristaux de CoO par microscopie optique a haute temperature et microscopie electronique a balayage

The effect of an applied electric field on a heterogeneous solid-state reaction

Abstract The effect of an external electric field applied, at elevated temperatures, to a thin-film diffusion couple has been studied. Thin films of epitactic Fe-oxide were deposited on {001} MgO by means of pulsed-laser deposition. The diffusion couple was then reacted at 1150 °C in air for 2 h in an applied field of 5 kV cm −1 . The distribution of phases after the reaction was determined using backscatter low-voltage scanning electron microscopy. In performing the analysis, images taken from the field-applied diffusion couple were compared to those of a standard diffusion couple of the same geometry with no applied field. The results showed that the application of an electric field affects both the diffusion of the cations and the interface morphology.

Interdiffusion in oxides (A,B)O (II): Experimental investigation of point-defect relaxation on concentration profiles for MnO-FeO

Abstract Vacancy relaxation phenomena during interdiffusion of FeO and MnO have been studied on the basis of very accurate concentration-distance profiles of the cations, measured by X-ray microanalysis. The profiles clearly depended on annealing time and form of the samples. This observation directly demonstrates that local thermodynamic equilibrium of defects is not established in the reacting system. In addition, the deviation from local equilibrium is most pronounced at the early stage of the reaction in a thick sample. This can be explained by the long relaxation time the cation vacancies need to reach a free surface surrounded by oxygen gas as a vacancy sink.