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This publication concerns the design of advanced nanostructured inorganic materials using supercritical fluids. A brief overview of the different experimental and numerical tools, which are now available for the scientific community and engineers, is proposed giving access to a better understanding but also a better control on material synthesis. The versatility of the supercritical fluids route for the preparation of different natures of inorganic materials is emphasized based on the access to numerous solvents, precursors and their associated chemistries. It is possible to produce materials with physicochemical properties, which can not be obtained with other routes and at large scale. All these chemistries give access to a wide range of nanobricks opening a new area for the preparation of advanced materials by design through the development of one pot multistep processes.

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Supercritical fluid synthesis offers an attractive and unique method to produce metal oxide nanoparticles such as barium strontium titanate (Ba 1−x Sr x TiO 3 with 0 ≤ x ≤ 1). This synthesis pathway has the advantage of producing high quality, highly crystalline nanoparticles in a narrow size range, with accurate control of the elemental composition not easily afforded by other methods of production. Coupled with moderate reaction temperatures and short reaction times, this method could be an environmentally preferred synthesis route compared to conventional synthesis pathways. This paper examines the potential environmental impacts leading from the lab-scale supercritical synthesis of 1 kg of Ba 0.6 Sr 0.4 TiO 3 nanoparticles of an average size 16 nm using the approach of an anticipatory life-cycle assessment. A cradle-to-gate assessment was completed, estimating the impacts across resource extraction, material processing, and production of the nanoparticles, while excluding any associated use-phase or end-of-life considerations.The life-cycle assessment highlights a number of ways by which the environmental profile of the supercritical synthesis of barium strontium titanate nanoparticles could be improved. Lab-scale synthesis was bound by physical constraints of the reactor, whereby the precursor concentration was kept artificially low. Being able to synthesize nanoparticles with precursor concentrations of 0.1 and 1.0 molar would reduce the average life-cycle impacts by nearly 81% and 95%, respectively. Additionally, the recovery and reuse of solvents at high recycling rates (e.g. 90%) could reduce average life-cycle impacts by 56%. At high precursor concentrations and solvent recycling rates, the environmental performance was further limited by the precursors, namely barium and titanium alkoxides, which have high upstream life-cycle demands (i.e. isopropanol production and metal processing). General impact reductions were seen as the ratio of strontium : barium increased. Further reductions could be achieved by replacing the barium, strontium and titanium alkoxides with precursors having better life-cycle profiles such as barium and strontium acetates or barium and strontium hydroxides.

The production of BaTi1-yZryO3 (0 ≤ y ≤ 1, BTZ) nanocrystals is known to be challenging due the low reactivity of zirconium precursors. Here we have successfully studied the zirconium impact on the BTZ particles formation in suband supercritical fluid conditions along the entire solid solution. In situ synchrotron wide angle X-ray scattering (WAXS) analyses were conducted in batch at 150°C and 400°C to follow in real time the BTZ crystallites synthesis. This revealed the complexity underneath the nucleation and growth mechanisms of ABO3 nanocrystals, especially going towards high zirconium content (more than 50 atomic %). This type of substitution induces, among other things, microstrain within the structure. Moreover, for BaTi0.4Zr0.6O3 and BaTi0.2Zr0.8O3 cases, the experiments showed the apparition of two crystallite size populations. In the BaTi0.4Zr0.6O3 case, at 400°C, these two size populations merged in a single one after at least eight minutes but not in the BaTi0.2Zr0.8O3 one. The zirconium content being higher, the particles become more refractory, thus, the temperature is not high enough to enable their ripening. It is important to note that this behavior was not observed for particles produced at 400°C but in flow, with a residence time of only 50 s. There, the particles presented a single size population, close to the one obtained after eight minutes in batch. Meaning that, for batch syntheses, a longer time is required to achieve a similar product quality to the one obtained with a flow process.

Supercritical fluid flow synthesis exploits unique properties of solvents in order to achieve reactions that proceed quickly to produce high-quality nanocrystals. Supercritical fluids are often referred to “green” solvents because they can proceed at moderate temperatures. Therefore, this study sought to compare the supercritical fluid flow synthesis of TiO2 to that of a conventional precipitation method from an environmental and human health perspective. A life-cycle assessment was conducted to determine the impacts of producing 1 kg of dry TiO2 nanoparticles using either the supercritical or precipitation route. While the results suggest that supercritical fluid flow synthesis may indeed be a preferable synthesis route compared with a conventional route such as precipitation, the inherent uncertainty underlying this emerging technology indicates that there are a number of trade-offs in switching from one technology to another. Supercritical fluid flow synthesis was likely a better technology option from a cumulative energy demand and climate change perspective; however, there was less evidence for this from a human health and ecotoxicological perspective, for example. In particular, occupational exposure to emissions of TiO2 nanoparticles could be an issue for this emerging industry if the proper protective controls are not put in place.

Accurate control of residual defect density is required for reliable investigation and use of ferroelectric materials. After reviewing the long term endeavor to decrease defect contributions in bulk materials, which reached mass production decades ago, recent challenges are underlined. These mostly result from the continuous trend towards integration which has reached the nanometre range. The contribution of solid state chemistry is of key relevance for improving the present processing routes and suggesting alternative ones, for example by controlling a large density of charged defects to reach unprecedented functionalities. Some of these breakthroughs are reviewed.