Improved compaction of ZnO nano-powder triggered by the presence of acetate and its effect on sintering

The retention of nanocrystallinity in dense ceramic materials is still a challenge, even with the application of external pressure during sintering. The compaction behavior of high purity and acetate enriched zinc oxide (ZnO) nano-powders was investigated. It was found that acetate in combination with water plays a key role during the compaction into green bodies at moderate temperatures. Application of constant pressure resulted in a homogeneous green body with superior packing density (86% of theoretical value) at moderate temperature (85 °C) in the presence of water. In contrast, no improvement in density could be achieved if pure ZnO powder was used. This compaction behavior offers superior packing of the particles, resulting in a high relative density of the consolidated compact with negligible coarsening. Dissolution accompanying creep diffusion based matter transport is suggested to strongly support reorientation of ZnO particles towards densities beyond the theoretical limit for packing of ideal monosized spheres. Finally, the sintering trajectory reveals that grain growth is retarded compared to conventional processing up to 90% of theoretical density. Moreover, nearly no radial shrinkage was observed after sinter-forging for bodies performed with this advanced processing method

Synthese von nanostrukturiertem Zinkoxid mittels der elektrischen Feld-aktivierten Sintertechnologie : Effekt von adsorbiertem Wasser auf Defektstöchiometrie und Verdichtungsmechanismus

Es wurde der Effekt von gebundenem Oberflächenwasser auf das Verdichtungsverhalten, die Defekt Stöchiometrie und die Gefügeentwicklung während des Sinterns von Zinkoxid untersucht. Dazu wurde nanokristallines Zinkoxid Pulver getrocknet oder befeuchtet und anschließend unter hohen Heizraten (100 K/min) mittels Feld-aktivierter Sintertechnologie/ Spark Plasma Sintern (FAST/SPS) erhitzt. Eine ausgeprägte Verstärkung der Verdichtung von Zinkoxid in Gegenwart von adsorbiertem Wasser ermöglicht die Reduzierung der Sintertemperatur von 800 °C auf 400 °C. Daraus folgt eine Verringerung des Kornwachstums um eine Größenordnung, während die vollständige Verdichtung des Formkörpers erzielt wird. Die dabei entstandene kristalline Textur entwickelt sich unabhängig von der Gegenwart gebundenen Wassers. Ein anisotropes Gefügewachstum von langgestreckten Körnern wurde ausschließlich beim Sintern in der Gegenwart von gebundenem Wasser beobachtet, welche sich senkrecht zum uniaxial aufgebrachten Druck ausbilden. Diese begünstigte Wachstumsrichtung wurde stimmt mit der kristallographischen c-Achse von Zinkoxid überein und wird die anisotrope Oberflächenenergie von Zinkoxid zurückgeführt. Dies hat eine bevorzugte Adsorption von Wasser und einer Verstärkung der Diffusivität zur Folge. Die Verdichtung in Gegenwart von adsorbiertem Wasser wird durch die Diffusion von Hydroxid-Ionen verstärkt, welche einen geringeren Ionenradius und eine geringere Valenz als Sauerstoff Ionen aufweisen. Beim Sintern zeigt der elektrische Stromfluss keinen Einfluss auf die Verstärkung des Sinterverhaltens oder auf die verringerte Aktivierungsenergie zur Verdichtung. Die Stöchiometrie und Art der Defekte von gesintertem Zinkoxid wurde anhand spektroskopischer Analysemethoden untersucht, welche eine Dotierung von 0,5 at% Wasserstoff unter der Bildung von Hydroxid-Ionen beim Sintern in der Gegenwart von Wasser nahelegen. Dagegen konnte beim Sintern von trockenem Zinkoxid Pulver kein Wasserstoff detektiert werden

Anomalous coarsening of nanocrystalline zinc oxide particles in humid air

Zinc acetate in combination with water plays a key role during the coarsening of zinc oxide (ZnO) nanocrystals at moderate temperature (85 °C) in air. The growth of ZnO nanocrystals is well known in liquid phase systems, but this work shows that this process is strongly enhanced in powder form by the presence of residual acetate. The growth of the ZnO nanocrystals was documented by X-ray diffraction (XRD) and high resolution transmission electron microscopy (HRTEM), showing a preferred growth along [0001] crystallographic direction. An increase of more than 400% in crystal size was observed, which could be related to coarsening but not due to precipitation from solution. In contrast, particle size stayed almost constant if pure zinc oxide powder was used. This growth is expected to slowly occur during storage even under ambient conditions. The limited stability of nanopowders limits their applicability as well as pressing into bulk materials

Improved compaction of ZnO nano-powder triggered by the presence of acetate and its effect on sintering

The retention of nanocrystallinity in dense ceramic materials is still a challenge, even with the application of external pressure during sintering. The compaction behavior of high purity and acetate enriched zinc oxide (ZnO) nano-powders was investigated. It was found that acetate in combination with water plays a key role during the compaction into green bodies at moderate temperatures. Application of constant pressure resulted in a homogeneous green body with superior packing density (86% of theoretical value) at moderate temperature (85 °C) in the presence of water. In contrast, no improvement in density could be achieved if pure ZnO powder was used. This compaction behavior offers superior packing of the particles, resulting in a high relative density of the consolidated compact with negligible coarsening. Dissolution accompanying creep diffusion based matter transport is suggested to strongly support reorientation of ZnO particles towards densities beyond the theoretical limit for packing of ideal monosized spheres. Finally, the sintering trajectory reveals that grain growth is retarded compared to conventional processing up to 90% of theoretical density. Moreover, nearly no radial shrinkage was observed after sinter-forging for bodies performed with this advanced processing method

FAST/SPS sintering of nanocrystalline zinc oxide—Part II: Abnormal grain growth, texture and grain anisotropy

This second part describes the retention of nanocrystallinity during sintering of ZnO by means of Field-assisted Sintering Technique/Spark-Plasma-Sintering (FAST/SPS), whereas the first part [doi: 10.1016/j.jeurceramsoc.2015.12.009] concentrated on hydroxide-ion-enhanced densification and defect stoichiometry. Interface design by surface bound water on zinc oxide offers a novel method to control in a new way diffusion in nanocrystalline polycrystals. Therefore, zinc oxide powder was humidified or dried and afterwards heated quickly (100 K/min) by FAST/SPS. Interestingly, the densification is strongly promoted in presence of water reducing the sintering temperature to 400 °C. Thus, grain growth is decreased by one order of magnitude while achieving full densification. The crystalline texture developed irrespective of temperature or presence of water. Moreover, the formation of hydroxide complexion at grain boundaries is discussed as it might modify grain boundary mobility and lead to pronounced grain anisotropy perpendicular to the uniaxial applied load

FAST/SPS sintering of nanocrystalline zinc oxide—Part I: Enhanced densification and formation of hydrogen-related defects in presence of adsorbed water

This part is focused on the effect of surface bound water on the densification behavior and defect stoichiometry of zinc oxide. The second part [doi: 10.1016/j.jeurceramsoc.2015.12.008] concentrates on the effect of hydroxide complexions on the microstructural development, texture formation and anisotropic grain morphology. Nanocrystalline zinc oxide powder was humidified or dried followed by quick heating (100 K/min) with field-assisted sintering technique/spark plasma sintering (FAST/SPS). Densification is strongly enhanced due to hydroxide-ion-diffusion mechanism, which shows species with lower valence and ionic radius in comparison to oxygen ions. The lowered activation energy for densification exhibits no impact of the sintering electric current on this enhanced densification behavior. The defect stoichiometry and structure of sintered zinc oxide was analyzed by several spectroscopic methods, indicating the formation of hydrogen-related defects for sintering in presence of bound water, while no hydrogen was detected for sintering of dried powder

S2k guideline: Rosacea

This updated and upgraded S2k guideline deals with the diagnosis and treatment of rosacea, which is a common, chronic inflammatory skin disease mostly affecting the face. Initially, rosacea is characterized by recurrent erythema, telangiectasia and flushing. Later, the inflammatory component predominates, with persistent erythema with follicular papules, papulopustules and pustules. The development of phyma, which usually occurs on the acral localizations, is the most severe manifestation. For the treatment of rosacea, the interdisciplinary guideline committee, with representatives of the German Dermatological Society (DDG), the Professional Association of German Dermatologists (BVDD), the German Opthalmological Society (DOG), the Society for Dermopharmacy (GD), the Swiss Society for Dermatology and Venereology (SGDV) and the German Rosacea Aid e. V., recommends the avoidance of trigger factors and topical applications of metronidazole, azelaic acid or ivermectin. For symptomatic treatment of persistent centrofacial erythema, the topical vasoconstrictors brimonidine or oxymetazoline can also be used. Systemic therapy is recommended for therapy-resistant and severe forms of rosacea papulopustulosa. The drug of choice is low-dose doxycycline. Alternatively, low-dose isotretinoin can be recommended. Ocular rosacea should be treated with lid margin hygiene. For topical treatment, ciclosporin eye drops, azithromycin, ivermectin or metronidazole are suggested