0 0,1 0,2 0,3 0,4 0,5 0,6 1 2 3 4 5 6 wurtzite (g) batch 2a precipitazione ZnS Co-precipitation Synthesis Microstructure investigations • Hexagonal wurtzite ZnS nanopowder • Beige colour nanopowder • Wide band gap (3.77 eV) • High refractive index (n=2.75) • Nanoparticles size: 3 to 5 nm • Specific surface area: 38 m2 g-1 • Ethyl glycol & thiourea decompose at 250°C-290°C The two-step sintering method (TSS) is based on the heating of the sample to a high temperature T1 followed by a rapid cooling down to a lower temperature T2 and then holding it at T2 for a long period. The main characteristic of this method is that the grain boundary diffusion of the sample is maintained, while also avoiding grain boundary migration. Therefore, the grain growth associated with the final step of the sintering process is completely suppressed. Two-Step Sintering Process Commercially available ZnS powder (a mixture of 80% cubic and 20% hexagonal phase) was used as the starting powder, without any binder or sintering aid. Discs with a diameter of 8 mm and a thickness of 4 mm were prepared by uniaxial pressing at 100 MPa, followed by cold isostatic pressing at 150 MPa. Advantages  Finer microstructure  Sintering temperature reduction  Simple and pressureless method  Large-scale production of components with complex shapes  Binders not necessary  Lower costs The amount of solvent remained the same (60 ml of ethyl glycol) by re-using what remained of the solvent used in the previous reaction and topping up the quantity lost. Productivity yields increased over six successive reactions from 156 mg to 549 mg of ZnS nanopowder per batch. Self-alignament of ZnS nanopowder dispersed in ethanol, after being washed several times in acetone and ethanol. Take a picture to download the full video “Circular synthesis”: By recycling a used solvent obtained from previous reactions and topping up the quantity lost, the productivity yield increased 3.5 times. Method: The well-known reaction of ZnCl2, as the zinc source, with thiourea as the sulphur source, dissolved in ethyl-glycol at a carefully controlled, constant molar ratio (mMZn/mMS=1) under medium temperature conditions (140°C-150°C). The next step is using recycled ZnCl2, to make the synthesis fully circular. Astro Furnace Thermal technology Equipment and operating conditions  Graphite crucible  N2 flowing atmosphere  Pressureless solid-state thermal sintering SINGLE-STEP SINTERING (SSS) TWO-STEP SINTERING (TSS) 1250°C - 1h 1150°C t=0 1100°C - 5h 1250°C - 1 h 1250°C t=0 1250°C - 1 h 1250°C - 1 h MCM 2019 14th Multinational Congress on Microscopy September 15-20, 2019 in Belgrade, Serbia • The SSS processing of commercial ZnS powder at 1250°C gives dense ceramic samples with only the wurtzite phase present. • The TSS processing of the commercial powder at T1=1150°C and T2=1100°C gives ceramics of similar density but which contain a small percentage in the cubic phase (not fully transformed into the hexagonal phase). A possible solution: Increase the second step holding time. • The first step in the TSS processing of prepared ZnS nanopowder at T1= 1250°C did not prove successful. The next step is to improve the densification, using a higher compaction pressure to prepare the green body (200 MPa isostatic pressure) and heating it to a lower T1 (of around 1000°C). This project has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No.797951. Schematic representation of densification Green body COMMERCIAL ZnS POWDER ZnS NANOPOWDER Pyroelectric materials can harvest energy from naturally occurring ambient temperature changes, as well as artificial temperature changes due to exhaust gases or convection and solar energy. Pyroelectric energy harvesting is a highly promising technology for the future of autonomous and self-powered electronic devices (battery free) and could be the right methodology to harvest enormous am