Entwicklung eines Prozesses zur schlickerbasierten additiven Fertigung von Hochleistungskeramik

Die Herstellung dreidimensionaler keramischer Bauteile mit etablierten schlickerbasierten additiven Fertigungsmethoden für Keramik, wie der Stereolithographie, ist bislang sehr zeitaufwändig. Dies ist vor allem den verwendeten Suspensionen mit einem hohen Anteil an organischen Hilfsstoffen geschuldet. Der dadurch benötigte zeitaufwendige Entbinderungsschritt hat zur Folge, dass nur dünnwandige Bauteile ökonomisch hergestellt werden können. Das in dieser Arbeit untersuchte und optimierte neue schlickerbasierte additive Fertigungsverfahren Laser Induzierter Schlickerguss (LIS) ermöglicht hingegen einen Aufbau von komplexen Bauteilen unter Verwendung von konventionellen Schlicker mit einem geringen Organikanteil, die den aufwendigen Entbinderungsschritt überflüssig machen. Hierdurch kann dieses additive Fertigungsverfahren einfacher in die keramische Prozesskette eingebettet werden. Bei additiven Fertigungsverfahren werden computergenerierte Konstruktionsdaten des gewünschten Bauteils schichtweise durch ein abwechselndes Wiederholen von Schichtauftrag und Belichtungsschritt aufgebaut. In dieser Arbeit wird gezeigt, dass die gewünschte Geometrie im Belichtungsschritt durch ein lokales, selektives Trocknen einer Schlickerschicht mit einem Laser verfestigt werden kann und damit keramische Grünkörper hergestellt werden können. Anhand von Experimenten mit keramischen Schlickern, in diesem Fall Siliziumnitrid, wurden unterschiedliche Arten der Laser-Belichtung und des lokalen Trocknens untersucht. Es konnte gezeigt werden, dass einfache keramische Grünkörper durch das Trocknen mittels eines fokussierten Laserstrahls aufgebaut werden können. Es lassen sich auf der Schlickerschicht präzise beliebige Geometrien abbilden. Die in den Versuchen realisierten Strukturen verändern sich in ihrer Geometrie in z-Richtung nicht, das heißt es wurden nur 2,5D-Strukturen aufgebaut. Das Herauslösen der Strukturen aus dem sie umgebenden Schlicker war jedoch aufwendig und induzierte Fehler wie Risse. In einer Weiterentwicklung wurde ein defokussierter Laserstrahl zur lokalen schichtweisen Trocknung eingesetzt, der die Oberfläche der gewünschten Geometrie erhitzte. Durch Optimierung des Prozesses ist es möglich, aus typischen Gießschlickern durch einen lokalen laserunterstützten Trocknungsprozess Grünkörper herzustellen. Damit war es erstmals erfolgreich möglich verschiedenen Geometrien wie Dreiecke und auch Bauteile mit größerer Komplexität, wie zum Beispiel Strukturen mit Überhängen, aufzubauen. Im Vergleich zu anderen Verfahren ist die verwendete Schichtstärke von 400 µm recht hoch, was eine vergleichsweise hohe Aufbaurate ermöglicht. Die mechanischen Eigenschaften der ersten mit diesem Verfahren hergestellten gesinterten Si3N4- Bauteile erreichen mit einer Biegefestigkeit von 275 MPa nicht die Werte von Si3N4-Bauteilen fertig entwickelter Verfahren wie beispielsweise Stereolithographie, sind aber besser als kommerziell erhältliche additive gefertigte Bauteile anderer Keramiken. Das Materialportfolio des Laser Induzierten Schlickergusses wurde anschließend auf die Baustoffe erweitert und Versuche mit alternativen Bindemitteln, den alkali-aktivierten Materialien durchgeführt. Mit Lithiumaluminat wurde ein alkali-aktiviertes Material gefunden, mit dem komplexe Geometrien, aufgebaut wurden. Die mechanische Charakterisierung der Bauteile ergab eine Druckfestigkeit von 49 MPa und eine Biegefestigkeit von 12 MPa. Die erhaltenen mechanischen Eigenschaften sind vergleichbar mit konventionell hergestellten alkali-aktivierten Materialien. Durch diese Arbeit konnte gezeigt werden, dass der Laser Induzierte Schlickerguss ein vielversprechendes neues Verfahren für die additive Fertigung istThe production of three-dimensional ceramic components using established slurry-based additive manufacturing methods for ceramics, such as stereolithography, has so far been very time-consuming. This is mainly due to the suspensions used, which contain a high proportion of organic additives. Therefore, the time-consuming debinding step required, results in the fact that only thin-walled components can be produced economically. This work investigates and optimizes the new slurry-based additive manufacturing process Laser Induced Slip Casting (LIS), which makes it possible to build complex components using conventional slurries with a low organic content, thus eliminating the need for the time-consuming debinding step. As a result, this additive manufacturing process can be more easily embedded in the ceramic process chain. In additive manufacturing processes, computer-generated design data of the desired component is built up layer by layer by alternately repeating the layer deposition and illumination step. In this work, it is shown that the desired geometry can be solidified in the illumination step by a local selective drying of a slip layer with a laser and thereby ceramic green bodies can be fabricated. Based on experiments with ceramic slips, in this case silicon nitride, different types of laser exposure and local drying were investigated. It was shown that simple ceramic green bodies can be built by drying using a focused laser beam. On the slip layer, precisely arbitrary geometries can be illuminated. The structures realized in the experiments do not change in their geometry in the z-direction, i.e. only 2.5D structures were built. However, extracting the structures from the surrounding slip was time consuming and induced defects such as cracks. In a further development, a defocused laser beam was used for local layer-by-layer drying, which heated the surface of the desired geometry. By optimizing the process, it is possible to produce green bodies from typical casting slurries by a local laser-assisted drying process. This made it possible for the first time to successfully build up different geometries such as triangles and also components with greater complexity, such as structures with overhangs. Compared to other processes, the layer thickness of 400 µm used is quite high, which enables a comparatively high build-up rate. The mechanical properties of the first sintered Si3N4 components produced using this process, with a flexural strength of 275 MPa, do not reach the values of Si3N4 components of fully developed processes such as stereolithography, but are better than commercially available additively manufactured components of other ceramics. The Laser Induced Slip Casting material portfolio was then expanded to include construction materials and experiments were conducted with alternative binders, alkali-activated materials. With lithium aluminate, an alkali-activated material was found with which complex geometries could be built. The mechanical characterization of the components showed a compressive strength of 49 MPa and a flexural strength of 12 MPa. The mechanical properties obtained are comparable to conventionally produced alkali-activated materials. Through this work, it was demonstrated that Laser Induced Slip Casting is a promising new process for additive manufacturing

Robustness of target dose coverage to motion uncertainties for scanned carbon ion beam tracking therapy of moving tumors

Beam tracking with scanned carbon ion radiotherapy achieves highly conformal target dose by steering carbon pencil beams to follow moving tumors using real-time magnetic deflection and range modulation. The purpose of this study was to evaluate the robustness of target dose coverage from beam tracking in light of positional uncertainties of moving targets and beams. To accomplish this, we simulated beam tracking for moving targets in both water phantoms and a sample of lung cancer patients using a research treatment planning system. We modeled various deviations from perfect tracking that could arise due to uncertainty in organ motion and limited precision of a scanned ion beam tracking system. We also investigated the effects of interfractional changes in organ motion on target dose coverage by simulating a complete course of treatment using serial (weekly) 4DCTs from six lung cancer patients. For perfect tracking of moving targets, we found that target dose coverage was high (V¯95 was 94.8% for phantoms and 94.3% for lung cancer patients, respectively) but sensitive to changes in the phase of respiration at the start of treatment and to the respiratory period. Phase delays in tracking the moving targets led to large degradation of target dose coverage (up to 22% drop for a 15° delay). Sensitivity to technical uncertainties in beam tracking delivery was minimal for a lung cancer case. However, interfractional changes in anatomy and organ motion led to large decreases in target dose coverage (target coverage dropped approximately 8% due to anatomy and motion changes after 1 week). Our findings provide a better understand of the importance of each of these uncertainties for beam tracking with scanned carbon ion therapy and can be used to inform the design of future scanned ion beam tracking systems

Real-time dose compensation methods for scanned ion beam therapy of moving tumors [12.12.2011]

Scanned ion beam therapy provides highly tumor-conformal treatments. So far, only tumors showing no considerable motion during therapy have been treated as tumor motion and dynamic beam delivery interfere, causing dose deteriorations. One proposed technique to mitigate these deteriorations is beam tracking (BT), which adapts the beam position to the moving tumor. Despite application of BT, dose deviations can occur in the case of non-translational motion. In this work, real-time dose compensation combined with beam tracking (RDBT) has been implemented into the control system to compensate these dose changes by adaptation of nominal particle numbers during irradiation. Compared to BT, significantly reduced dose deviations were measured using RDBT. Treatment planning studies for lung cancer patients including the increased biological effectiveness of ions revealed a significantly reduced over-dose level (3/5 patients) as well as significantly improved dose homogeneity (4/5 patients) for RDBT. Based on these findings, real-time dose compensated re-scanning (RDRS) has been proposed that potentially supersedes the technically complex fast energy adaptation necessary for BT and RDBT. Significantly improved conformity compared to re-scanning, i.e., averaging of dose deviations by repeated irradiation, was measured in film irradiations. Simulations comparing RDRS to BT revealed reduced under- and overdoses of the former method. Strahlentherapie mit gescannten Teilchenstrahlen ermöglicht sehr tumorkonforme Dosisverteilungen. Bis jetzt sind jedoch nur Tumore, die sich während der Bestrahlung nicht merklich bewegen, behandelt worden. Tumorbewegung und dynamische Strahlapplikation verursachen Interferenzen, die die resultierende Dosisverteilung beeinträchtigen. Bei einer vorgeschlagenen Technik zur Abschwächung des Bewegungseinflusses, der bewegungskompensierten Bestrahlung (BKB), wird der Strahl der Tumorbewegung nachgeführt. Trotz Verwendung dieser Technik können bei nichttranslationaler Bewegung Dosisabweichungen auftreten. In dieser Arbeit wurde dosis- und bewegungskompensierte Bestrahlung (DBKB) implementiert, das diese Dosisabweichungen durch Anpassung der nominellen Teilchenzahlen in Echtzeit kompensiert. Im Vergleich zu BKB wurde eine signifikante Verringerung von Fehldosierungen gemessen. Bestrahlungsplanungsstudien für Lungentumore ergaben eine signifikante Verringerung der Überdosierungen (3/5 Patienten) und eine signifikante Verbesserung der Dosishomogenität (4/5 Patienten). Basierend auf dieser Methode wurde dosiskompensierte Mehrfachbestrahlung (DKMB) vorgeschlagen, das ohne die technisch komplexe schnelle Strahlenergieanpassung, die für BKB und DBKB benötigt wird, auskommt. Signifikant bessere Konformität von DKMB im Vergleich zu konventioneller Mehrfachbestrahlung wurde in Filmbestrahlungen gemessen. Außerdem zeigte DKMB in Simulationen geringere Unter- und Überdosierungen als BKB

Experimental verification of a real-time compensation functionality for dose changes due to target motion in scanned particle therapy

Implementation and experimental assessment of a real-time dose compensation system for beam tracking in scanned carbon beam therapy of intrafractionally moving targets.A real-time dose compensation functionality has been developed and implemented at the experimental branch of the beam tracking system at GSI Helmholtzzentrum für Schwerionenforschung (GSI). Treatment plans for different target geometries have been optimized. They have been delivered using scanned carbon ions with beam tracking (BT) and real-time dose compensation combined with beam tracking (RDBT), respectively. Target motion was introduced by a rotating table. Dose distributions were assessed by ionization chamber measurements and dose reconstructions. These distributions have been compared to stationary delivery for BT as well as RDBT. Additionally simulations have been performed to investigate the dependence of delivered dose distributions on varying motion starting phases for BT and RDBT, respectively.Average measured dose differences between static delivery and motion influenced delivery could be reduced from 27-68 mGy when BT was used to 12-37 mGy when RDBT was used. Nominal dose was 1000 mGy. Simulated dose deliveries showed improvements in dose delivery and robustness against varying starting motion phases when RDBT was used.A real-time dose compensation functionality extending the existing beam tracking functionality has been implemented and verified by measurements. Measurements and simulated dose deliveries show that real-time dose compensation can substantially improve delivered dose distributions for large rotational target motion compared to beam tracking alone

Multigating, a 4D Optimized Beam Tracking in Scanned Ion Beam Therapy

The treatment of moving tumors with a scanned ion beam is challenging due to interplay effects and changing beam range. We propose multigating, as a method for 4D-treatment optimization and delivery. In 3D beam tracking, tracking vectors are added during delivery to beam spot positions based on the detected motion phase. This has the disadvantage of dose errors in case of complex motion patterns and an uncertain out-of-target dose distribution. In multigating, the motion phase for each beam spot is predefined, which allows to add the tracking vector prior to beam weight optimization on all motion phases. The synchronization of delivery and target motion is assured by fast gating. The feasibility of the delivery was shown in a film experiment and required only minor software modification to the treatment planning system. In a treatment planning study in 4 lung cancer patients, target coverage could be restored to the level of a static reference plan by multigating (V95 > 99%) but not by standard beam tracking (V95 < 95%). The conformity of the multigating plans was only slightly lower than those of the static plan, with a conformity number of 72.0% (median, range 64.6–76.6%) compared to 75.8% (70.8–81.5%) in spite of target motion of up to 22 mm. In conclusion, we showed the technical feasibility of multigating, a 4D-optimization and delivery method using scanned beams that allows for conformal and homogeneous dose delivery to moving targets also in case of complex motion

4D optimization of scanned ion beam tracking therapy for moving tumors

Motion mitigation strategies are needed to fully realize the theoretical advantages of scanned ion beam therapy for patients with moving tumors. The purpose of this study was to determine whether a new four-dimensional (4D) optimization approach for scanned-ion-beam tracking could reduce dose to avoidance volumes near a moving target while maintaining target dose coverage, compared to an existing 3D-optimized beam tracking approach. We tested these approaches computationally using a simple 4D geometrical phantom and a complex anatomic phantom, that is, a 4D computed tomogram of the thorax of a lung cancer patient. We also validated our findings using measurements of carbon-ion beams with a motorized film phantom. Relative to 3D-optimized beam tracking, 4D-optimized beam tracking reduced the maximum predicted dose to avoidance volumes by 53% in the simple phantom and by 13% in the thorax phantom. 4D-optimized beam tracking provided similar target dose homogeneity in the simple phantom (standard deviation of target dose was 0.4% versus 0.3%) and dramatically superior homogeneity in the thorax phantom (D(5)-D(95) was 1.9% versus 38.7%). Measurements demonstrated that delivery of 4D-optimized beam tracking was technically feasible and confirmed a 42% decrease in maximum film exposure in the avoidance region compared with 3D-optimized beam tracking. In conclusion, we found that 4D-optimized beam tracking can reduce the maximum dose to avoidance volumes near a moving target while maintaining target dose coverage, compared with 3D-optimized beam tracking

Robustness of target dose coverage to motion uncertainties for scanned carbon ion beam tracking therapy of moving tumors

Beam tracking with scanned carbon ion radiotherapy achieves highly conformal target dose by steering carbon pencil beams to follow moving tumors using real-time magnetic deflection and range modulation. The purpose of this study was to evaluate the robustness of target dose coverage from beam tracking in light of positional uncertainties of moving targets and beams. To accomplish this, we simulated beam tracking for moving targets in both water phantoms and a sample of lung cancer patients using a research treatment planning system. We modeled various deviations from perfect tracking that could arise due to uncertainty in organ motion and limited precision of a scanned ion beam tracking system. We also investigated the effects of interfractional changes in organ motion on target dose coverage by simulating a complete course of treatment using serial (weekly) 4DCTs from 6 lung cancer patients. For perfect tracking of moving targets, we found that target dose coverage was high (V̄(95) was 94.8% for phantoms and 94.3% for lung cancer patients, respectively) but sensitive to changes in the phase of respiration at the start of treatment and to the respiratory period. Phase delays in tracking the moving targets led to large degradation of target dose coverage (up to 22% drop for a 15 degree delay). Sensitivity to technical uncertainties in beam tracking delivery was minimal for a lung cancer case. However, interfractional changes in anatomy and organ motion led to large decreases in target dose coverage (target coverage dropped approximately 8% due to anatomy and motion changes after 1 week). Our findings provide a better understand of the importance of each of these uncertainties for beam tracking with scanned carbon ion therapy and can be used to inform the design of future scanned ion beam tracking systems