@article{KuehlBechtelerGirmscheidetal.2022, author = {K{\"u}hl, Hannes and Bechteler, Christian and Girmscheid, Ralf and R{\"u}bling, Achim}, title = {CNT/alumina-composites as electrically conductive ceramics}, series = {cfi ceramic forum international}, volume = {99}, journal = {cfi ceramic forum international}, number = {6}, publisher = {G{\"o}ller Verlag}, year = {2022}, abstract = {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.}, language = {en} } @article{BechtelerGirmscheidKuehl2021, author = {Bechteler, Christian and Girmscheid, Ralf and K{\"u}hl, Hannes}, title = {Influence of different alumina powders on thermal decomposition of CNTs during debinding of CNT/Al2O3-composite ceramics}, series = {Open Ceramics}, volume = {7}, journal = {Open Ceramics}, publisher = {Elsevier BV}, issn = {2666-5395}, doi = {10.1016/j.oceram.2021.100166}, pages = {11}, year = {2021}, language = {en} } @article{BechtelerKuehlGirmscheid2020, author = {Bechteler, Christian and K{\"u}hl, Hannes and Girmscheid, Ralf}, title = {Morphology and structure characterization of ceramic granules}, series = {Journal of the European Ceramic Society}, volume = {40}, journal = {Journal of the European Ceramic Society}, number = {12}, publisher = {Elsevier BV}, issn = {0955-2219}, doi = {10.1016/j.jeurceramsoc.2020.04.044}, pages = {4232 -- 4242}, year = {2020}, abstract = {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.}, language = {en} } @article{BechtelerKuehlTodd2024, author = {Bechteler, Christian and K{\"u}hl, Hannes and Todd, Richard I.}, title = {Boron induced abnormal grain growth in alumina}, series = {Open Ceramics}, volume = {19}, journal = {Open Ceramics}, publisher = {Elsevier BV}, issn = {2666-5395}, doi = {10.1016/j.oceram.2024.100636}, pages = {7}, year = {2024}, abstract = {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.}, language = {en} } @article{BechtelerRueblingGirmscheidetal.2021, author = {Bechteler, Christian and R{\"u}bling, Achim and Girmscheid, Ralf and K{\"u}hl, Hannes}, title = {Development of pressureless sintered and hot-pressed CNT/alumina composites including mechanical characterization}, series = {International Journal of Ceramic Engineering \& Science}, volume = {3}, journal = {International Journal of Ceramic Engineering \& Science}, number = {5}, publisher = {Wiley}, issn = {2578-3270}, doi = {10.1002/ces2.10103}, pages = {237 -- 248}, year = {2021}, abstract = {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.}, language = {en} } @article{BechtelerMachujHebendanzetal.2022, author = {Bechteler, Christian and Machuj, Lisa and Hebendanz, Kilian and R{\"u}bling, Achim and Girmscheid, Ralf and K{\"u}hl, Hannes}, title = {Electrical and thermal conductivity of CNT/alumina-nanocomposite ceramics}, series = {International Journal of Ceramic Engineering \& Science}, volume = {5}, journal = {International Journal of Ceramic Engineering \& Science}, number = {1}, publisher = {Wiley}, issn = {2578-3270}, doi = {https://doi.org/10.1002/ces2.10167}, pages = {13}, year = {2022}, abstract = {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.}, language = {en} }