Thermal Qualification of the UHTCMCs Produced Using RF-CVI Technique with VMK Facility at DLR

Ultra high-temperature ceramic matrix composites (UHTCMCs) based on carbon fibre (Cf) have been shown to offer excellent temperature stability exceeding 2000 °C in highly corrosive environments, which are prime requirements for various aerospace applications. In C3Harme, a recent European Union-funded Horizon 2020 project, an experimental campaign has been carried out to assess and screen a range of UHTCMC materials for near-zero ablation rocket nozzle and thermal protection systems. Samples with ZrB2-impregnated pyrolytic carbon matrices and 2.5D woven continuous carbon fibre preforms, produced by slurry impregnation and radio frequency aided chemical vapour infiltration (RF-CVI), were tested using the vertical free jet facility at DLR, Cologne using solid propellants. When compared to standard CVI, RFCVI accelerates pyrolytic carbon densification, resulting in a much shorter manufacturing time. The samples survived the initial thermal shock and subsequent surface temperatures of >2000 °C with a minimal ablation rate. Post-test characterisation revealed a correlation between surface temperature and an accelerated catalytic activity, which lead to an understanding of the crucial role of preserving the bulk of the sample.

Understanding the Role of Carbon Fiber Skeletons in Silicone Rubber-Based Ablative Composites

At present, silicone rubber-based ablative composites are usually enhanced by carbon fibers (CFs) to protect the case of solid rocket motors (SRMs). However, the effect of the CFs’ length on the microstructure and ablation properties of the silicone rubber-based ablative composites has been ignored. In this work, different lengths of CFs were introduced into silicone rubber-based ablative composites to explore the effect of fiber length, and ceramic layers of various morphologies were constructed after ablation. It was found that a complete and continuous skeleton in ceramic layers was formed by CFs over 3 mm in length. In addition, the oxyacetylene ablation results showed that the linear ablation rate declined from 0.233 to 0.089 mm/s, and the maximum back-face temperature decreased from 117.7 to 107.9 °C as the length of the CFs increased from 0.5 to 3 mm. This can be attributed to the fact that successive skeletons concatenated and consolidated the ceramic fillers as well as residues to form an integrated, robust, and dense ceramic layer.

2450 MHz Microwave Ablation for Liver Metastases Under 3.0T Wide-bore Magnetic Resonance Guidance: a Pilot Study

Purpose: To investigate the feasibility and effectiveness of 3.0T wide-bore magnetic resonance (MR)-guided microwave ablation (MA) for liver metastases (LM).Patients and methods: From October 2018 to May 2020, 39 patients with 63 LM were treated with 3.0T wide-bore MR-guided 2450 MHz MA therapy. The procedure parameters, technical success, complications, biochemical indexes changes, local tumor response, local tumor progression (LTP) and overall survival were recorded and analyzed.Results: The mean tumor maximum diameter and total procedure time were 3.0 cm and 55.2 min, respectively. Technical success was 100% but 5 cases (12.8%) with grade-1 complications. Alanine transaminase, aspartate transaminase and total bilirubin showed slight a transient increase on day 3 (P<0.05) and returned to normal by day 30 (P>0.05). The complete ablation rate for ≦ 2.5 and >2.5 cm lesions were 100 % and 92.5 %, respectively, during the median followup of 12.0 months, LTP rate was 4.8 % (3/63), and the 6-, 12- and 18-month overall survival were 100%, 92.2%, 76.4% , respectively.Conclusion: 3.0T wide-bore MR-guided MA for LM is a safe and effective approach, especially for small LM.

Influence of Supraglacial Debris Thickness on Thermal Resistance of the Glaciers of Chandra Basin, Western Himalaya

A large number of glaciers in the Hindu-Kush Himalaya are covered with debris in the lower part of the ablation zone, which is continuously expanding due to enhanced glacier mass loss. The supraglacial debris transported over the melting glacier surface acts as an insulating barrier between the ice and atmospheric conditions and has a strong influence on the spatial distribution of surface ice melt. We conducted in-situ field measurements of point-wise ablation rate, supraglacial debris thickness, and debris temperature to examine the thermal resistivity of the debris pack and its influence on ablation over three glaciers (Bara Shigri, Batal, and Kunzam) in Chandra Basin of Western Himalaya during 2016–2017. Satellite-based supraglacial debris cover assessment shows an overall debris covered area of 15% for Chandra basin. The field data revealed that the debris thickness varied between 0.5 and 326 cm, following a spatially distributed pattern in the Chandra basin. The studied glaciers have up to 90% debris cover within the ablation area, and together represent ∼33.5% of the total debris-covered area in the basin. The supraglacial debris surface temperature and near-surface air temperature shows a significant correlation (r = &gt; 0.88, p = &lt; 0.05), which reflects the effective control of energy balance over the debris surface. The thermal resistivity measurements revealed low resistance (0.009 ± 0.01 m2°C W−1) under thin debris pack and high resistance (0.55 ± 0.09 m2°C W−1) under thick debris. Our study revealed that the increased thickness of supraglacial debris significantly retards the glacier ablation due to its high thermal resistivity.

Study on the Ablation Properties of Nano-graphite Modified EPDM Insulators

To improve the anti-ablative property of EPDM-based composites, nano-graphite powder as anti-ablation filler was introduced to optimize the EPDM insulation material formulas. Characterization of anti-ablation performance showed that the composite at the nano-graphite content of 10phr exhibited the best anti-ablation and mechanical performances, such as: a linear ablation rate of 0.062 mm/s, a mass ablation rate of 0.048 g/s, tensile strength of 5.69 MPa and Elongation at break of 391.2%. The nano-graphite was proven to be an effective material which is beneficial to improve the anti-ablation of the EPDM composites.

Ultrashort-pulsed laser processing with spatial and temporal beam shaping using a spatial light modulator and burst modes

We report on the effect of simultaneous spatial and temporal beam shaping on the ablation rate, ablation efficiency and the resulting surface characteristics of micromachined stainless steel using ultrashort-pulsed lasers. Beam shaping and the use of pulse bursts are promising methods to allocate the over the last decades increasing laser power of ultrashort-pulsed lasers in ablation processes. While the individual effects of beam shaping and pulse bursts on the ablation characteristics have recently been examined, the combination of both has not yet been adequately investigated. Using a spatial light modulator to generate different spot distributions with up to six spots and different separations it is possible to spatially distribute the available laser power. In combination with temporal beam shaping using a 200 kHz repetition rate and pulse bursts with a 40 MHz intra-burst rate, we investigate the influences in a scanning-based process and find an increasing ablation rate and efficiency for higher fluences. Subsequently using bursts in combination with a multi-spot beam profile, we found a distinctive emergence of cone like protrusions and a smoothing effect for fluences between 1.5 J/cm² and 3 J/cm² with six spot beam profile.

Sublimation-driven morphogenesis of Zen stones on ice surfaces

In this article, the formation of Zen stones on frozen lakes and the shape of the resulting pedestal are elucidated. Zen stones are natural structures in which a stone, initially resting on an ice surface, ends up balanced atop a narrow ice pedestal. We provide a physical explanation for their formation, sometimes believed to be caused by the melting of the ice. Instead, we show that slow surface sublimation is indeed the physical mechanism responsible for the differential ablation. Far from the stone, the sublimation rate is governed by the diffuse sunlight, while in its vicinity, the shade it creates inhibits the sublimation process. We reproduced the phenomenon in laboratory-scale experiments conducted in a lyophilizer and studied the dynamics of the morphogenesis. In this apparatus, which imposes controlled constant sublimation rate, a variety of model stones consisting of metal disks was used, which allows us to rule out the possible influence of the thermal conduction in the morphogenesis process. Instead, we show that the stone only acts as an umbrella whose shade hinders the sublimation, hence protecting the ice underneath, which leads to the formation of the pedestal. Numerical simulations, in which the local ablation rate of the surface depends solely on the visible portion of the sky, allow us to study the influence of the shape of the stone on the formation of the ice foot. Finally, we show that the far-infrared black-body irradiance of the stone itself leads to the formation of a depression surrounding the pedestal.

Effect of Organo Montmorillonite Nanoclay on Mechanical Properties Thermal Stability and Ablative Rate of Carbon fiber Polybenzoxazine Resin Composites

Organo-Montmorillonite (o-MMT) nanoclay added polybenzoxazine resin (type I composites) were prepared with varying amounts of clay (0, 1, 2, 4 and 6 wt %). Clay dispersion, changes in curing behaviour and thermal stability were assessed in type I composites. Findings from these studies of type I composites were used to understand thermal stability, mechanical, and mass ablation rate behaviour of nanoclay added carbon fiber reinforced polybenzoxazine composites (type II). Interlaminar shear strength and flexural strength of type II composites increase by 25% and 27%, respectively at 2 wt% addition of clay. An oxy-acetylene torch test with a constant heat flux of 125 w/cm2 was used to investigate mass ablation rate of type II composites. The ablation rate has increased as the weight percentage of clay has increased. This is contradicting to type I composites with up to 6 wt% clay and type II composites with up to 4 wt% clay, which have improved thermal stability. The microstructure of the ablated composites was examined using scanning electron microscopy. Increased ablation rates are due to the reaction of charred matrix with nanoclay, which exposes bare fibers to the ablation front, resulting in higher mechanical erosion losses.