Additive manufacturing of C/C-SiC by fused filament fabrication

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Tracking the timely dissemination of clinical studies. Characteristics and impact of 10 tracking variables [version 1; peer review: 2 approved]

Background: Several meta-research studies and benchmarking activities have assessed how comprehensively and timely, academic institutions and private companies publish their clinical studies. These current “clinical trial tracking” activities differ substantially in how they sample relevant studies, and how they follow up on their publication. Methods: To allow informed policy and decision making on future publication assessment and benchmarking of institutions and companies, this paper outlines and discusses 10 variables that influence the tracking of timely publications. Tracking variables were initially selected by experts and by the authors through discussion. To validate the completeness of our set of variables, we conducted i) an explorative review of tracking studies and ii) an explorative tracking of registered clinical trials of three leading German university medical centres. Results: We identified the following 10 relevant variables impacting the tracking of clinical studies: 1) responsibility for clinical studies, 2) type and characteristics of clinical studies, 3) status of clinical studies, 4) source for sampling, 5) timing of registration, 6) determination of completion date, 7) timeliness of dissemination, 8) format of dissemination, 9) source for tracking, and 10) inter-rater reliability. Based on the description of these tracking variables and their influence, we discuss which variables could serve in what ways as a standard assessment of “timely publication”. Conclusions: To facilitate the tracking and consequent benchmarking of how often and how timely academic institutions and private companies publish clinical study results, we have two core recommendations. First, the improvement in the link between registration and publication, for example via institutional policies for academic institutions and private companies. Second, the comprehensive and transparent reporting of tracking studies according to the 10 variables presented in this paper

Phase Relations and Crystal Chemistry of Substituted Hexaferrites: Enhanced Hard Magnetic Materials

Basierend auf die hexagonale dauermagnetische Verbindung Sr-Hexaferrit SrFe12O19 (M-Typ) wurden mehrere polynäre Systeme und in dem Zusammenhang verschiedene Substitution der beteiligten Kationen untersucht. Grundlegend ist die Erforschung des pseudobinären Phasendiagramms SrO-FeO-Fe2O3. Keramische Proben wurden hierfür in Rohröfen gesintert (alle Temperaturen ± 5°C). Zur Analyse der Proben kam primär die Elektronenstrahlmikrosonde zum Einsatz. Drei Hexaphasen, darunter der an Luft noch nicht beobachtete X-Typ Sr2Fe2+2Fe3+28O46 konnten nachgewiesen werden. 1. SrFe12O19 = M-Typ: 810°C bis 1410°C 2. Sr2Fe2+2Fe28O46 = X-Typ: 1350°C bis 1420°C 3. SrFe2+2Fe16O27 = W-Typ: 1350°C bis 1440°C Da die Fe3+-Kationen die magnetischen Eigenschaften primär beeinflussen, steht deren Substitution im Vordergrund. Es werden einzeln oder gekoppelt Fe2+, Co2+, Zn2+, Mg2+ und Ti4+ in die M-Typ Struktur integriert. Zur Ladungskompensation ist manchmal der Einbau von La3+ statt Sr2+ nötig. Alle folgenden Versuche wurden an Luft bei 1200°C, 1300°C und 1380°C durchgeführt. Zuerst lag das System SrO-La2O3-FeO-Fe2O3 mit dem Substitutionsmechanismus Sr2+ + Fe3+ = La3+ + Fe2+ im Zentrum des Interesses. Bei 1380°C ist die Mischkristallreihe Sr1-xLaxFe2+xFe3+12-xO19 (0 Sr0,2La0,8Zn0,1Fe2+0,7Fe11,2O19). Der M-Typ Mischkristall spannt auch hier einen “2-dimensionalen Mischkristallbereich“ auf. Um die Konkurrenz mehrerer kleiner Kationen ging es in den beiden Systemen SrO-La2O3-ZnO-CoO-FeO-Fe2O3 und SrO-La2O3-MgO-CoO-FeO-Fe2O3. Generell gilt die folgende Substitutionsformel: Sr2+1-xLa3+xFe2+x-y-zCo2+y(Mg,Zn)2+zFe3+12-xO19. Es wird weniger FeO in den M-Typ integriert wird, wenn zwei Kationen in Konkurrenz zu Fe2+ stehen. Co2+ und Mg2+ werden vornehmlich auf Oktaederplätzen, Zn2+ eher auf Tetraederplätzen substituiert. Im Vergleich zu ZnO wird deutlich mehr MgO in den M-Typ eingebaut. Primärer Grund hierfür ist, dass MgO vor allem auf die energetisch günstigeren oktaedrischen Plätzen eingebaut wird, wie im Spinell MgFe2O4. Eine anderer Substitutionsmechanismus liegt im System SrO-TiO2-ZnO-FeO-Fe2O3 vor. Fe2O3 wird gekoppelt durch Ti4+ und Zn2+ sowie Fe2+ ersetzt (SrTixZnyFe2+x-yFe3+12-2xO19). Bei 1200°C ist ein M-Typ Mischkristall nachzuweisen, bei dem 30 % der Fe3+-Ionen ersetzt werden (SrTi1,8Zn1,4Fe2+0,4Fe8,4O19). Die maximale Ausdehnung mit einem Substitutionsgrad von rund 63 % wird bei 1300°C erreicht (SrTi3,8Zn3,0Fe2+0,9Fe4,4O19). TiO2-reiche Schmelzen verhindern bei 1380°C eine höhere Substitution (Endglied: SrTi1,9Zn1,04Fe2+0,86Fe8,2O19). Betrachtet man bei den Hexaphasen den Gehalt an FeO in Bezug zu ZnO, fällt auf, dass der Anteil von ZnO auch bei 1380°C überwiegt, anders als noch im System SrO-La2O3-ZnO-FeO-Fe2O3. Ursache dafür ist, dass mit TiO2 ein zusätzlicher Konkurrent für FeO und für die bevorzugten Oktaederplätze vorhanden ist. ZnO dagegen hat um die bevorzugten Tetraederpositionen keinen direkten Konkurrenten (nur Fe3+) und wird auch bei 1380°C überwiegend eingebaut.Based on the hexagonal compound Sr-hexaferrite SrFe12O19 (M-Type) phase relations in different polynary systems and cation substitutions were investigated. To examine the basic system Sr-Fe-O homogenized mixtures of oxide and carbonate powders were calcinated, pressed and sintered between 1200°C and 1550°C in air. Subsequently the resulting ceramic samples were quenched in water. The reported temperatures of sintering in vertical tube furnaces are within an accuracy of ± 5°C. For the quantitative measurement of the compounds electronprobe microanalysis was used. Three types of hexaphases could be discovered. Remarkable is the former undescribed X-type hexaferrite Sr2Fe2+2Fe3+28O46. 1. SrFe12O19 = M-type: 810°C to 1410°C 2. Sr2Fe2+2Fe28O46 = X-type: 1350°C to 1420°C 3. SrFe2+2Fe16O27 = W-type: 1350°C to 1440°C Substitution of Fe3+-ions promotes the possibility to enhance the magnetic properties of the compound. Fe3+ is replaced by ions like Fe2+, Co2+, Zn2+, Mg2+ and Ti4+ in the M-type hexaferrite. These substitutions occur one by one or by co-substitution, e. g. Sr2+ + Fe3+ = La3+ + Zn2+. At different temperatures (1200°C, 1300°C and 1380°C) the extent of solid solution of M-types and phase relations of seven different polynary systems were examined. In the first quaternary system SrO-La2O3-FeO-Fe2O3, Sr2+ and Fe3+ were replaced by La3+ and Fe2+. At 1380°C there is an complete solid solution series Sr1-xLaxFe2+xFe3+12-xO19 (0 Sr0,3La0,7Co0,5Fe2+0,2Fe11,3O19). There is a complete solid solution between SF6 and LF6 at 1300°C and 1380°C. The substitution model Sr2+ + Fe3+ = La3+ + (Co2+,Fe2+) yields to a “two dimensional range” of the solid solution, because of the different extent of incorporation of CoO and FeO. This is independent of Sr2+ substitution by La3+. Chemical compositions of the starting mixtures (supply of FeOx), temperature and coexisting phases change the ratio between CoO and FeO. Introducing ZnO instead of CoO to the system SrO-La2O3-ZnO-FeO-Fe2O3 shows similar results for the phase relations and the solid solution of the M-type hexaferrite. The substitution formula Sr1-xLaxZny2+Fe2+x-yFe3+12-xO19 could be confirmed and at lower temperatures more La3+ will be incorporated in the M-type hexaferrite, compared to the system including CoO (end members: 1200°C = Sr0,2La0,8Co0,6Fe2+0,2Fe11,3O19; 1300°C = LaZn0,4Fe2+0,6Fe11,3O19). At 1380°C only 80 % of the Sr2+-ions were substituted by La3+-ions together with the substitution of Fe3+ by Zn2+ and Fe2+ (La-rich end member: Sr0,2La0,8Zn0,1Fe2+0,7Fe11,2O19). At higher La-contents, electronprobe microanalysis shows the stability of spinelSS and La-perovskite. Nevertheless, at all chosen temperatures a “2-dimensional range” of solid solution of the M-type hexaferrite can be observed again. To examine the interaction of more small cations, the systems SrO-La2O3-ZnO-CoO-FeO-Fe2O3 and SrO-La2O3-MgO-CoO-FeO-Fe2O3 were considered. This was realized by two suitable mixtures. Fe3+-ions were replaced by Co2+, Fe2+ and Zn2+/Mg2+, which leads to the substitution formula Sr2+1-xLa3+xFe2+x-y-zCo2+y(Mg,Zn)2+zFe3+12-xO19. Because of the availability of the cations Co2+ and Zn2+/Mg2+, the FeO content in the M-type ferrite was lower than in the former systems. In the presence of Fe2+ and Co2+ the incorporation of Zn2+ is reduced especially at temperature above 1200°C. On the contrary Fe2+, Co2+ and Mg2+ share the octahedral positions. The FeO content increase with increasing temperature. Finally the system SrO-TiO2-ZnO-FeO-Fe2O3 was studied. The substitution formula of the M-type hexaferrite SrTixZnyFe2+x-yFe3+12-2xO19 shows, that only Fe3+-ions were replaced by Ti4+- and Zn2+/Fe2+-ions (2 Fe3+ = Ti4+ + Fe2+, Zn2+). At 1300°C up to 63 % of Fe2O3 was replaced by TiO2, FeO and ZnO (SrTi3,8Zn3,0Fe2+0,9Fe3+4,4O19). The degree of substitution at 1200°C and 1380°C is approximately 30 % (1200°C: x = 1,8; y = 1,4; 1300°C: x = 1,9; y = 1,0). The ratio of FeO to ZnO is always < 1, because the incorporation of Ti4+ in octahedral positions is more favourable than those of Fe2+. Besides the overall FeOx content decrease with increasing substitution of Fe3+-ions

CMC jackets for metallic pipes - a novel approach to prevent the creep deformation of thermo-mechanically loaded metals

Steel materials suffer extensive creep by the application at temperatures of about 700 °C and pressures about 350 bar in a power plant environment. The presented concept overwraps a steel pipe with a ceramic matrix composite (CMC) jacket in order to support the steel pipe and provide high temperature strength. Finite Element simulations show the influence of the wall thickness of CMC jacket and the coefficient of thermal expansion (CTE) on circumferential stresses within the hybrid metal ceramic pipe. Suitable fiber and matrix materials were studied, composites fabricated and mechanical properties determined. Finally, a prototype was designed in order to confirm the feasibility of the concept. The lifetime of a pure steel pipe was increased by more than four-fold by the additional CMC jacket

Frictional performance of C/C–SiC materials at high loads: The role of composition and third-body

Ceramic matrix composites like carbon fiber-reinforced silicon carbon (C/C–SiC) are brake materials for applications at high thermo-mechanical loads. This study investigates the influence of three C/C–SiC pad materials with different compositions on frictional performance. For this purpose, the ceramic pad materials were tested against a steel disk on a dynamometer at brake pressures from 20 to 60 MPa. A large effect due to the different Si and SiC contents of the three pad materials on the frictional behavior was expected. Although the wear rates differed from 40 to 140 mm3/MJ, only marginal differences were found for the coefficient of friction. Hence, additional tests to interrupt and sequence the brake procedure revealed the formation of an intermediate metallic transfer layer from the steel disk on the ceramic pads. The formation and disappearance of this third-body, which was not found in start-complete-stop testing, and the consequences for the frictional properties are discussed

Processing properties and pyrolysis behavior of novolak-hexamethylenetetramine mixtures

AbstractThis work investigates the influence of the hexamethylenetetramine (hexa) content on the rheological features, pyrolysis reactions, mass-loss kinetics and char yield for two phenolic resins, each with different novolak-hexa mixtures from 0 wt.-% to 15 wt.-%. Curing and viscosity of resins was studied using a rheometer. Pyrolysis, evolved gases and char yield were investigated with TG-FTIR-GC/MS. The obtained results show that the curing of resins shifts to lower temperatures whereas the viscosity and the residual carbon amount both increase with increasing hexa-contents. The presented data suggests a hexa-induced microgel formation for phenolic resins. Models for viscosity and char yield were developed and present a valuable tool to tailor the resin-hardener mixture for the desired application, enable a more efficient processing technology and contribute to a deeper understanding of the pyrolysis of novolak resins