Low temperature degradation behaviour of 10Ce-TZP/Al2O3 bioceramics obtained by microwave sintering technology

Zirconia is one of the most used ceramics, especially for biomedical applications, due to its exceptional mechanical properties. However, it is commonly known that its properties can be diminished owing to a low temperature degradation (LTD). This phenomenon consists on a spontaneous phase transformation, from tetragonal to monoclinic, under certain conditions, which is accelerated when the samples are exposed under high levels of humidity at a temperature range between 20-300 ºC. In addition to the fact that the monoclinic phase presents worse mechanical properties than the tetragonal one, there is a volume change of 4% between phases that gives rise to defects in the material as microcracks. Due to this reason, zirconia prostheses failed catastrophically inside the human body between 1999 and 2001 (1). Previous researches reveal that Al2O3 addition suppress the propagation of phase transformation (2). Thus, the aim of the present work is to study the hydrothermal ageing of zirconia doped with ceria and toughened with alumina (10Ce-TZP/Al2O3) composite, which has been sintered by microwave employing two different frequencies: 2.45 and 5.8 GHz. Microwave heating technology is based on the absorption of electromagnetic radiation by the material, which allows the sample to be heated. So far, most microwave heating equipments use 2.45 GHz; accordingly, the novelty of this study is to employ a frequency of 5.8 GHz and to investigate its effect on LTD. LTD is carried out in an autoclaved in steam at 120 ºC and 1.2 bar, because these conditions accelerate the hydrothermal aging process (3). In order to characterize the degraded samples, micro-Raman spectroscopy, AFM, nanoindentation technique and electronic microscopy have been performed. References 1. Norton, M. R., Yarlagadda, R., Anderson, G. H. J. Bone Joint Surg. Br., 2002, 84–B, 631–635. 2. Fabbri, P., Piconi, C., Burresi, E., Magnani, G., Mazzanti, F., Mingazzini, C. Dent. Mater., 2014. 3. Presenda, Á., Salvador, M. D., Moreno, R., Borrell, A. J. Am. Ceram. Soc., 2015, 98, 3680–3689

Temperature Assessment Of Microwave-Enhanced Heating Processes

[EN] In this study, real-time and in-situ permittivity measurements under intense microwave electromagnetic fields are proposed as a powerful technique for the study of microwave-enhanced thermal processes in materials. In order to draw reliable conclusions about the temperatures at which transformations occur, we address how to accurately measure the bulk temperature of the samples under microwave irradiation. A new temperature calibration method merging data from four independent techniques is developed to obtain the bulk temperature as a function of the surface temperature in thermal processes under microwave conditions. Additionally, other analysis techniques such as Differential Thermal Analysis (DTA) or Raman spectroscopy are correlated to dielectric permittivity measurements and the temperatures of thermal transitions observed using each technique are compared. Our findings reveal that the combination of all these procedures could help prove the existence of specific non-thermal microwave effects in a scientifically meaningful way.The authors wish to thank the project MAT2017-86450-C4-1-R.García-Baños, B.; Jimenez-Reinosa, J.; Penaranda-Foix, FL.; Fernandez, JF.; Catalá Civera, JM. (2019). Temperature Assessment Of Microwave-Enhanced Heating Processes. Scientific Reports. 9:1-10. https://doi.org/10.1038/s41598-019-47296-0S1109Zhou, J. et al. A new type of power energy for accelerating chemical reactions: the nature of a microwave-driving force for accelerating chemical reactions. Sci. Rep. 6, 25149 (2016).Clark, D. E., Folz, D. C. & West, J. K. Processing materials with microwave energy. Mater. Sci. Eng. A287, 153–158 (2000).Thostenson, E. T. & Chou, T. W. Microwave processing: fundamentals and applications. Composites A30(9), 1055–1071 (1999).Çengel, Y. A. Green thermodynamics. Int. J. Energy Res. 31, 1088–1104 (2007).Adam, D. 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Characterization method of dielectric properties of free falling drops in a microwave processing cavity and its application in microwave internal gelation

[EN] Microwave internal gelation (MIG) is a chemical process proposed for the production of nuclear particle fuel. The internal gelation reaction is triggered by a temperature increase of aqueous droplets falling by gravity by means of non-contact microwave heating. Due to the short residence time of a solution droplet in a microwave heating cavity, a detailed knowledge of the interaction between microwaves and chemical solution (shaped in small drops) is required. This paper describes a procedure that enables the measurement of the dielectric properties of aqueous droplets that freely fall through a microwave cavity. These measurements provide the information to determine the optimal values of the parameters (such as frequency and power) that dictate the heating of such a material under microwaves.This work is a part of the PINE (Platform for Innovative Nuclear FuEls) project which targets the development of an advanced production method for Sphere-Pac fuel and is financed by the Swiss Competence Center for Energy and Mobility. The work has been also financed by the European Commission through contract no 295664 regarding the FP7 PELGRIMM Project, as well as contract no 295825 regarding the FP7-ASGARD Project. MC-S would like to thank the ITACA research team (UPV Valencia, Spain) and the EMPA Thun (Switzerland) for their support in the measurements and Carl Beard (PSI, Switzerland) for the help provided in respect with CST simulations. The work of FLP-F was supported by the Conselleria d'Educacio of the Generalitat Valenciana for economic support (BEST/2012/010).Cabanes Sempere, M.; Catalá Civera, JM.; Penaranda-Foix, FL.; Cozzo, C.; Vaucher, S.; Pouchon, MA. (2013). Characterization method of dielectric properties of free falling drops in a microwave processing cavity and its application in microwave internal gelation. Measurement Science and Technology. 24(9). https://doi.org/10.1088/0957-0233/24/9/095009S24

Effect of frequency on MW assisted sintering: 2.45 GHz versus 5.8 GHz

Innovative non-conventional approaches, such as microwave sintering, are being developed as a method for sintering a variety of materials which shown advantages over conventional sintering procedures. This work involves an investigation of the microwave sintering of an ATZ composite with two different microwave applicators and frequency generators: 2.45 GHz and 5.8 GHz. Zirconia doped with ceria and toughened with alumina (10Ce-TZP/Al2O3) is the composite used in this study. The samples were sintered by microwave in air at 1200 and 1300 \ub0C with 10 min of dwell time at 2.45 and 5.8 GHz in order to evaluate their effects on sintering, using an optimized experimental configuration. In addition, the mechanical properties of MW-sintered samples were compared with those obtained for the same composites sintered by the conventional method (1500 \ub0C/120 min), such as relative density, hardness and fracture toughness

Modulating redox properties of solid-state ion-conducting materials using microwave irradiation

[EN] The industrial adoption of low-carbon technologies and renewable electricity requires novel tools for electrifying unitary steps and efficient energy storage, such as the catalytic synthesis of valuable chemical carriers. The recently-discovered use of microwaves as an effective reducing agent of solid materials provides a novel framework to improve this chemical-conversion route, thanks to promoting oxygen-vacancy formation and O-2-surface exchange at low temperatures. However, many efforts are still required to boost the redox properties and process efficiency. Here, we scrutinise the dynamics and the physicochemical dependencies governing microwave-induced redox transformations on solid-state ion-conducting materials. The reduction is triggered upon a material-dependent induction temperature, leading to a characteristically abrupt rise in electric conductivity. This work reveals that the released O-2 yield strongly depends on the material's composition and can be tuned by controlling the gas-environment composition and the intensity of the microwave power. The reduction effect prevails at the grain surface level and, thus, amplifies for fine-grained materials, and this is ascribed to limitations in oxygen-vacancy diffusion across the grain compared to a microwave-enhanced surface evacuation. The precise cyclability and stability of the redox process will enable multiple applications like gas depuration, energy storage, or hydrogen generation in several industrial applications.This study forms part of the MFA programme and was supported by MCIN with funding from European Union NextGenerationEU (PRTR-C17.I1) and by Generalitat Valenciana. Financial support by the Spanish Ministry of Science and Innovation (PID2022-139663OB-100 and CEX2021-001230-S grants funded by MCIN/AEI/10.13039/501100011033, and "Ramon y Cajal" Fellowship RYC2021-033889-I), and the Universitat Politecnica de Valencia (UPV) are gratefully acknowledged. Also, we acknowledge the support of the Servicio de Microscopia Electronica of the UPV.Serra Alfaro, JM.; Balaguer Ramirez, M.; Santos-Blasco, J.; Borrás-Morell, JF.; García-Baños, B.; Plaza González, PJ.; Catalán-Martínez, D.... (2023). Modulating redox properties of solid-state ion-conducting materials using microwave irradiation. Materials Horizons. 10(12):5796-5804. https://doi.org/10.1039/d3mh01339a57965804101