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Blacklight Sintering

Ceramics need to be fired. This slow process is known for its inflexibility and its energy intensity, accounting for ~2 % of global CO2 emissions. The sintering process is currently done by indirectly heating the ceramics through heating of furnaces following the same millennia old principle. This principle, however, is entangled with energy losses and an everlasting conflict between flexibility and efficiency. Revisiting the sintering step fundamentally, energy losses can be minimised if energy is only used exactly when and where needed. Direct power transmission into the ceramic can be achieved optically with blacklight, which also reduces the sintering time from hours to seconds.

Enchanted by the search for fast and efficient sintering methods scientists have explored direct heating by microwaves and electric fields. The high heating rates achievable have shown a particularly favourable impact on overall process time when transgressing the intermediate temperature regime as quickly as possible. With rapid sintering proven as a viable concept, facilitating energy transfer in an efficient and scalable manner is the key challenge for transgression of rapid heating into industrial processes. But how to supply the energy most effectively? Intense illumination! With the right wavelength for efficient absorption – blacklight with photon energy above the band gap – ceramic green bodies can be heated to beyond 1000 °C within seconds. Despite the high heating rates microstructural integrity can be assured by heating the whole body simultaneously and thermally de-coupling it with an insulative support. Process times in the seconds range allow hundreds of cycles per day promising unprecedented speed and flexibility in combination with outstanding energy efficiency. The principle Without the need for contact or a container, ceramic green bodies are heated directly by UV-light. This principle is illustrated in Fig. 1 a–e where key components of the setup and the central elements of the energy balance are illustrated. Incident light is shined onto the sample at high intensity which supplies the energy for heating. Thermal conduction to the surface is minimised by using an insulative support. This insulation turns out to also be highly favourable for reducing thermal gradients and achieving good sample quality. Some further heat is lost by convection, which is, however, a minor factor compared to infrared emission (black-body radiation) at high temperatures. Simply spoken, a chosen temperature can be held quite accurately by balancing the power density of the incident light with the temperature dependent IR-emission. This optical form of supplying energy allows for highly dynamic heating and cooling with several hundred Kelvin per seconds rate capacity, even for pieces with several millimetre thickness. In consequence, 1000-fold faster temperature profiles compared to conventional furnaces are feasible. Simultaneously, the digital control of the light allows a much higher fidelity and complexity of temperature profiles. Illumination times were found to be optimal in the second’s range. Such short processing times come with an additional advantage. If the illumination is short, there is simply no time to lose energy. This makes insulation and a housing altogether obsolete and allows operation on a desk in air (with safety shielding). Moreover, it allows energy efficiencies below 2 kWh/kg at 1600 °C firing temperature. This efficiency is equally attainable for one individual or one million pieces resolving the conflict between batch size and energy efficiency. With the capability to start and stop the process at will, adjusting production capacity to intermittently available green electricity supply also becomes feasible. Speed, flexibility and quick iteration The short process time has far-reaching consequences. In particular, because ceramic sample type and illumination parameter can be exchanged within seconds leading to a jump in flexibility. Quicker delivery times and much more versatile management of production flows are among the more obvious advantages, here. More intriguingly, the possibility to conduct several hundred experiments per day will change the way ceramic development is done. To demonstrate the fast development capacity, Fig. 2 a–i illustrates a number of key results which were all obtained within one day. For a detailed report, please see Porz, L.; et al.: Mater. Horiz. (2022) [9] 1717–1726. First of all, a broad applicability to a range of ceramics was demonstrated by sintering various material systems illustrated in Fig. 2 a–f. For some of the ceramics a dozen iterations executed within 20 min was sufficient to exceed 99,5 % density. Pieces with several centimetre lateral extension and several millimetres were also successfully fabricated. Moreover, also more complex parts, such as a multilayer capacitor (Fig. 2 g+h) were successfully sintered in one piece. Lastly, the use of light allows to manufacture intricate microstructures needed for energy applications such as fuel cells. One example is a bilayer of a dense and textured thin layer on top of a porous substrate (Fig. 2 i) manufactured from one homogeneous green body. In a more detailed study on the microstructure and conductivity, the functional properties of blacklight-sintered samples were found to be almost indistinguishable from conventionally sintered samples. Conductivity data, microstructures observed with ultra-high voltage electron microscopy and XRD-patterns of electron/hole conducting TiO2, oxygen conducting YSZ and Li-ion conducting Li0,33La0,57TiO3 can be found elsewhere (Porz, L.; et al.: J. Amer. Ceram Soc. 105 (2022) 7030–7035). Prospects for production capacity and energy efficiency For this novel technique coming freshly from the lab, a fundamental analysis of its industrial potential is due. A benchmark for energy efficiency was determined in an experiment of sintering TiO2 at 1600 °C within a 5-second illumination with a 450‑nm diode laser. The illumination required 0,83 kWh/ kg of optical energy which corresponds to an electricity consumption of <2 kWh/kg at an electric to optic conversion efficiency of 44 % of the 450-nm laser diodes. Such efficiency is competitive with industrial processes and attainable at any batch size. While, for the moment, processable pieces are limited in size, high production numbers are attainable by producing them one after another in quick succession. For this scenario, the production capacity for continuous operation can be estimated from the light source power and energy need per kg. With the example of a 1,5 kW light source about 1,8 kg can be produced per hour which corresponds to about 16 t per year. In summary, blacklight sintering holds potential for industrial application with batchsize- independent energy efficiency and process speed as key advantage. Moreover, its flexibility and the speed in which parameter optimization can be done further lower the entry barrier.

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