Regeneration of articular cartilage remains challenging. The aim of this study was to increase the stability of pure bioactive glass (BG) scaffolds by means of solvent phase polymer infiltration and to maintain cell adherence on the glass struts. Therefore, BG scaffolds either pure or enhanced with three different amounts of poly(D-L-lactide-co-glycolide) (PLGA) were characterized in detail. Scaffolds were seeded with primary porcine articular chondrocytes (pACs) and human mesenchymal stem cells (hMSCs) in a dynamic long-term culture (35 days). Light microscopy evaluations showed that PLGA was detectable in every region of the scaffold. Porosity was greater than 70%. The biomechanical stability was increased by polymer infiltration. PLGA infiltration did not result in a decrease in viability of both cell types, but increased DNA and sulfated glycosaminoglycan (sGAG) contents of hMSCs-colonized scaffolds. Successful chondrogenesis of hMSC-colonized scaffolds was demonstrated by immunocytochemical staining of collagen type II, cartilage proteoglycans and the transcription factor SOX9. PLGA-infiltrated scaffolds showed a higher relative expression of cartilage related genes not only of pAC-, but also of hMSC-colonized scaffolds in comparison to the pure BG. Based on the novel data, our recommendation is BG scaffolds with single infiltrated PLGA for cartilage tissue engineering.
In diesem Vorhaben wurden verschiedene Bindemittel für die Herstellung von Hochleistungskeramiken analysiert. Ziel war es, biobasierte Binder zu identifizieren, die konventionelle auf fossilen Ressourcen basierende Systeme in Zukunft ersetzen können. Es wurden drei erdölbasierte Referenz-Bindersysteme und sieben biobasierte Alternativen bezüglich ihrer thermischen Stabilität unter Sauerstoffatmosphäre, der Viskosität ihrer wässrigen Lösungen oder Dispersionen sowie daraus hergestellter Schlicker und ihrer Haftung auf Aluminiumoxid-basierter Keramik untersucht. Im Rahmen des Vorhabens konnte ein biobasiertes Bindersystem mit Eigenschaften, vergleichbar zum besten Referenzmaterial gefunden werden. Unter Einsatz des biobasierten Binders sowie zweier ausgewählter Referenz-Bindersysteme
wurden Grünkörper hergestellt und deren Festigkeit ermittelt. Die Grünkörperfestigkeit ist in der keramischen Industrie ein entscheidender Faktor und stark vom gewählten Bindersystem abhängig. Die
erhaltenen Resultate deuten darauf hin, dass der Bio-Binder auch hinsichtlich der Festigkeitswerte den konventionellen Referenz-Bindersystemen als gleichwertig anzusehen ist. Die in diesem Vorlauf-
forschungsprojekt gewonnenen Erkenntnisse können als Ausgangslage für die Beantragung weiterer Fördermittel und die Akquise direkter Drittmittel aus der Industrie dienen.
CNT-reinforced alumina ceramics are an interesting option for the production of electrically conductive ceramics. It is shown that high electrical conductivity can be achieved with only 0.25 wt.-% of CNTs in the alumina matrix. These small amounts added enable the production of dense ceramics with very good mechanical properties, although CNTs massively inhibit densification of the ceramics during the sintering process, especially at higher concentrations. The article also describes the manufacturing process, the mechanical and thermal properties achieved and possible applications.
For the production of brittle materials like technical ceramics it is important to prohibit big defects in the material to achieve high mechanical strength. Due to that already the granule properties are important because they can cause defects in the final product. Characterization and quantification of the granule properties are important steps for improving the production process and properties of the granules for further processing. In this work a novel method for the characterization and quantification of granule morphology, especially sphericity, and internal structure like the ratio of macro pores is provided via digital microscopy. The new quantitative characterization method can be done in an academic and industrial field with appropriate effort. Differences between granules can be characterized, quantified and be used for further development.
In this work, hot-pressing of alumina in contact with hexagonal boron nitride or doped with boron carbide was conducted at 1500 °C for 30 min. After hot-pressing, abnormal grain growth induced by boron diffusion from these substances into alumina was detected, as clearly demonstrated with SEM, EDS, EBSD, and Raman spectroscopy. Grain boundary complexion transformations, solute drag, or another mechanism relating to interface-controlled grain boundary mobility are presumed to be the fundamental mechanism responsible for abnormal grain growth observed in this work.
AbstractA process for the production of carbon nanotube (CNT)/alumina composites on the basis of an aqueous suspension and without any extensive pretreatment was developed. Pressureless sintering and hot‐pressing of the nanocomposites were extensively researched and optimized. The influence of varying CNT contents, different alumina powders, sintering temperature and pressure on mechanical properties were investigated. Optimal hot‐pressing conditions are specified at 1550°C, 15 min dwell time, and 80 MPa applied pressure. Dense nanocomposites up to 3 wt.% and 0.5 wt.% CNT content were achieved by hot‐pressing and pressureless sintering, respectively. Furthermore, a 20% increase in hardness for CNT contents below 1.0 wt.% was detected, which is independent from the applied force and the alumina matrix. A highly anisotropic fracture toughness at increased CNT contents was detected by an indentation‐based method. The developed process provides a possibility to produce CNT/alumina composites with improved mechanical properties under reasonable effort, which could also be used for industrial production.
AbstractIn the present work, carbon nanotube (CNT)‐reinforced alumina nanocomposite ceramics were investigated about their electrical and, for the first time in such detail, thermal conductivity. Therefore, two different alumina powders with varying CNT‐contents were processed by pressureless sintering and hot pressing to achieve CNT/alumina composite ceramics with varying porosity and CNT‐content between 0 and 5 wt.% CNTs. A significant influence of the grain size on percolation threshold of the electrical conductivity was detected. The coarser CT 3000 SG‐based ceramic showed a threshold of <0.25 wt.%, which is the lowest reported threshold in literature. Pore orientation in the hot‐pressed materials shows a significant influence on the electrical and thermal conductivity of the composite, causing anisotropic properties. Both, electrical and thermal conductivity are higher parallel to the pore structure and perpendicular to the press‐direction, respectively, with electrical conductivity being up to three times and thermal conductivity up to 30% higher parallel to the pore structure. Unlike electrical conductivity, thermal conductivity decreases significantly with increasing CNT‐content. As two influences, CNT‐content and porosity, interact, each of them was analyzed separately in order to measure the isolated influence of CNT‐content on thermal conductivity at constant porosity. It was shown, that thermal conductivity decreases considerably with increasing CNT‐content even at constant porosity, because of a disturbed crystal structure due to a finer grain structure with more grain boundaries. This behavior is contrary to the expected, and sometimes reported, effect of CNTs. The combination of an increasing CNT‐content and the related increase in porosity causes a strongly decreasing thermal conductivity of the material from 35 W/m∙K for pure alumina to 10 W/m∙K for alumina with 5 wt.% CNTs. The presented results in this and other previously published investigations from the authors show that CNT/alumina‐nanocomposites have the potential of combining outstanding mechanical properties and electrical conductivity, which can be used as high performance electrically conductive ceramic material for a wide range of applications.
The fracture toughness of ceramic substrates made of alumina, ZTA and silicon nitride was determined using the single edge V-notched plate method. In order to make a statement about the measurement precision, not only 5, as required by the standard, but 30 specimens per material type were tested. In addition, a much more practical and less error-prone holding and testing device for the ceramic plates was used, which is presented in the paper. For all materials, including the comparatively fine-grained ZTA, the material-typical KIc values could be determined. However, the range of variation of the measured values is comparatively high with coefficients of variance up to 7.8 %. It is shown that a standard-compliant test on Al2O3 substrates can exhibit a fluctuation range of 0.5 MPa m0.5 without the material showing any differences. It is important to be aware of this when using the method. Especially for quality assurance, where deviations in the range of a few tenths of MPa∙m0.5 are considered problematic, the method should therefore be regarded as critical. An increase in the number of specimens should be considered, as well as the inclusion of a statement on the precision of the measurement method in the standard.