(737a) Scaling-Up the Gas-Phase Synthesis of Metallic Nanoparticles By Means of Electrical Discharges

Scientific journals and patent data banks are continuously filled with reports on new synthesis methods for nanoparticles. Although the therein-reported particle properties may be useful to test specific applications, it is unlikely that these synthesis methods are viable for scaling-up to industrial dimensions or that the desired particle properties found on a smaller scale would be conserved after scaling up. The preservation of the designed nanostructures and nanosystems with novel or predefined properties when the process is implemented at a commercially viable scale is unlikely without significant interdisciplinary cooperation between scientists, engineers and end-users.

 The BUONAPART-E project, a project funded by the EU, involving 21 partners from universities as well as industry, aims to demonstrate that a physical nanoparticle synthesis process can be economically scaled-up. BUONAPART-E investigates Better Upscaling and Optimization of Nanoparticle and Nanostructure Production by Means of Electrical Discharges. Experimental results as well as literature data indicate that an energy efficiency of 100 kWh/kg has been reached for a single unit. The challenge addressed in BUONAPART-E, which can only be met with new knowledge of the hitherto unknown fundamental mechanisms taking place, is to obtain an increase in the production rate, while retaining energy efficiency. The process allows for the synthesis of different materials using the same production platform. The basic evaporation unit (called hereafter the Optimal Single Unit or ‘OSU’) is a set of electrodes. A large number of these units can be placed in a single housing, contributing to the cost-effectiveness of the process. The use of many single production units in parallel, which can be thoroughly optimized and tested on a lab scale for a given material, ensures that a highly-effective scale-up of the synthesis process in terms of cost and energy consumption is possible. This strategy is analogous to supercomputers based on low-cost graphic cards from the consumer market. Further equipment, such as pumps, power supply to the OSUs and the particle collection unit, can be scaled-up as single units leading to additional cost benefits.

 The final decision about the implementation of a new industrial production line is taken on the basis of a thorough economic analysis. Initial decisions concerning raw materials/reactants for synthesis as well as choice of the unit operations with the necessary capital investment and power requirements are vital for the economic viability of a process. Taking into account the high price of chemical precursors for nanomaterials as well as the costs of solvents and related operations such as purification and disposal, simple cost estimations lead us to believe that our choice of physical synthesis based on an electrical discharge from solid elements in inert gases is economically well-founded.

 The process is designed in an environmentally friendly way as a closed loop system that avoids the use of hazardous precursors, solvents and stabilizers. The main input is electric power for the electric discharges and the pumps for gas recycling. The closed-loop cycle necessitates higher quality flanges than normally used in industry. This results in an inherently much lower possibility of workplace exposure to nanoparticles. The proposed process does not call for complicated and expensive safety measures, as quasi-atmospheric inert gas is used as transport medium and hazardous chemicals are avoided. All these factors make this process a clear example of green and sustainable synthesis process. The motivation behind the choice of spark discharge as a synthesis platform stems from the experience of BUONAPART-E consortium that this technology is cost-efficient to implement and easy to use, while still allowing for rapid process optimization. On the other hand, increased production rate of a single unit would lead, however, to engineering problems for this unit, such as an inabilities to dilute and rapidly cool to conserve small sizes, turbulence leading to broad size distributions, a lack of design and scaling-up rules, and scaling-up processes that are both lengthy and cost-intensive. As the basic unit is just a pair of electrodes and a large number of these pairs can be placed in a single housing, it is evident that a cost-effective scaling-up strategy include the use of multiple units in parallel.

 In BUONAPART-E will be demonstrated the feasibility of simultaneous operation of a large number of electrode pairs. The OSU is optimized with special emphasis given to energy efficiency. The resources of the project will be focused on the development of a technology driven by requirements for energy efficiency and allowing a no-risk, highly optimized scale-up, which fulfill industrial demands. The feasibility of a scale-up will be assessed in the project, along with criteria dealing with health, security and environment.

 In this talk, we will show the characteristics of the various discharge regimes (spark, glow discharge, thermal arc) as related to metal nanoparticle production. Of special interest are the production rates as well as specific electricity consumption.

This work was supported by the European Union´s Seventh Framework Program (EU FP7) under Grant Agreement No. 280765 (BUONAPART-E).