The availability of suitable joining technologies is paramount to the further advancement of ceramic components and their use in HTRs. Among other joining technologies, a modified brazing technology using a laser beam for heating the components to be joined has been developed at TU Dresden.The laser-induced heating behavior of the ceramic material is determined by the interactions between the material and the laser beam. This was shown in two different silicon carbide materials (SSiC and SiCf-reinforced ceramic material) using a diode laser with wavelengths 808 nm and 940 nm.The laser-based technique was illustrated by three different examples: sealing of monolithic SiC with a pin configuration for fuel claddings, sealing of SiC heat pipes with a length of 1 m, and demonstration of the transferability of the laser technique to fiber-reinforced components by means of a SiCf/SiCN material.Because the covalent bonding of SiC does not allow conventional welding, much research has been devoted to developing alternative filler systems. Glass or glass-ceramic fillers enable the tailoring of properties such as crE and viscosity. Glasses are thermally stable up to their glass transition temperatures. It was shown that the crystallization of the yttrium aluminosilicate glass composition of the present work allows it to be used at 1050 degrees C without any significant changes occurring in braze tightness. For the SiC heat pipes with sodium as the working fluid, a sodium-resistant metal braze consisting of Ni-Ti-Si was formed. The long-term resistance of this filler to sodium at 800 degrees C was proven. The results demonstrate the possibility of using the laser-based joiningtechnique for the joining of different SiC materials as well as for different brazing materials. (C) 2016 Elsevier B.V. All rights reserved.

Heat exchangers based on ceramic heat pipes were designed for use under highly abrasive and corrosive atmospheres at temperatures in the range of 800-1200 degrees C for high-temperature power-engineering applications. The presented heat pipes are gravity assisted and based on a multi-layer concept comprising a ceramic cladding and an inner metal tube that contains sodium as the working fluid. Hermetical encapsulation of the working fluid was achieved by electron-beam welding of the inner metal tube. Subsequently, closure of the surrounding ceramic tube was performed by laser brazing technology using a glass solder. Temperature resistance and functionality of the manufactured ceramic thermosyphons could be confirmed experimentally in a hot combustion gas atmosphere at temperatures up to 1100 degrees C. The ceramic tubes used had an outer diameter of 22 mm and a total length of 770 mm. The measured axial heat transfer of the ceramic gravity assisted heat pipes at the stationary operating point with cold/hot gas temperature of 100 degrees C/900 degrees C was 400 W. The result of the calculation using the created mathematical model amounted to 459 W. (C) 2014 Elsevier Ltd. All rights reserved.

Heat exchange applications at high temperatures of greater than 800 °C under corrosive or abrasive conditions require heat exchangers based on ceramic materials instead of conventionally used metals. Heat exchangers based on heat pipes are exceptionally suitable since temperature gradients and correspondent thermal stresses are inherently low for this design. At high temperatures greater than 800 °C, the structural material SSiC and working fluids sodium or zinc appear to be the most promising options. Encapsulating the working fluid in ceramic heat pipes with a sealing joint ensuring long term stability and high temperature resistance is particularly challenging. A nickel‐based alloy has been identified as solder material for SSiC heat pipes using sodium as working fluid and a glass solder was used in case of zinc filled heat pipes. Manufactured heat pipes were tested in a hot‐gas test rig at temperatures up to 1000 °C.

Ceramic heat pipes and heat pipe based heat exchangers are tailored for automatically heat removal and heat distribution in thermally, chemically and abrasive high stressed systems. The manufacture of silicon carbide heat pipes was carried out. These were filled with sodium or zinc and sealed by laser brazing using metallic and glassy solder materials. High-temperature performance tests revealed a stable operating regime for both ceramic heat pipes with sodium and zinc as working fluid, respectively. Specifically the heat transferred by a zinc filled heat pipe of 22 mm in diameter and 750 mm in length accounted for 600 W at a temperature difference of 400 K. Notably the internal heat transfer capacity of the working fluid was even higher however, the total heat transfer was limited by the external active heat transfer area of the heat pipe. In order to evaluate the long-term stability of the heat pipes, particularly with respect to the joining seam, manufactured heat pipes are currently being tested in long-term annealing experiments at a temperature of 1000 degrees C under a variety of corrosive atmospheres.