ABSTRACT
This chapter deals with additive manufacturing of thermoelectric energy conversion functional composite materials by integrating electrohydrodynamic manufacturing into 3D printing. The chapter resolves several major issues related to (i) how to design and make a new manufacturing machine consisting of an electrohydrodynamic manufacturing system and a 3D printer, (ii) how to use the machine to manufacture composite materials with functional thermoelectric nanoparticles uniformly distributed in polymer fibers, and (iii) how to heat-treat the composites to convert the polymer fibers into partially carbonized fibers. In addition, the composite materials with a significant thermoelectric response for application in thermal sensing and energy conversion are discussed. Finally, scalable manufacturing of composite material mats by this new technology is introduced. 11.1 IntroductionTraditional electrohydrodynamic (EHD) manufacturing machines can only produce randomly oriented nanofibers. To control the fiber orientation for practical applications in optoelectronics, energy conversion, sensing, and guided tissue regeneration, a new
manufacturing technology has to be researched. The existing 3D printing technology allows materials to be placed in controlled ways. However, the resolution of 3D printing is limited. Nanofibers can hardly be made directly through 3D printing. Integrating EHD casting into 3D printing results in a new additive manufacturing technology that allows the production of composite materials containing well-aligned 2D and/or 3D nanofibers for special applications. On the basis of our recent experimental research, a new manufacturing machine has been designed by assembling a self-built EHD processing unit in a 3D printer. This EHD processing unit can be driven by the 3D printer to generate preprogrammed x-y-z three-directional motions so that the nanofibers produced by the EHD processing unit can be placed through fully controlled preset programs. Fundamental studies has been carried out to understand the science underpinning the new manufacturing technology. The objective of the chapter is to introduce new knowledge of preparing 2D and 3D nanostructured composite materials through a layer-by-layer additive manufacturing process under the action of EHD forces. The nanostructured composite materials made by the new manufacturing technology have high surface areas and enhanced surface activities. Such properties could significantly increase the thermoelectric (TE) sensitivity of the materials to external signals and enhance the phonon-induced electron-hole pair generation at the surface of the materials. Manufacturing composite materials containing nanofibers and nanoparticles via an innovative process in which EHD casting is integrated into 3D printing has caught much attention. EHD casting works by exposing a small jet of the selected material to a relatively high voltage, usually in the range of 5 kV to 30 kV. This voltage causes the material to undergo stretching and bending to form nanofibers as it gets farther away from the jet. The traditional EHD manufacturing process can only produce randomly oriented nanofibers. To control the fiber orientation for practical applications in optoelectronics, energy conversion, sensing, and guided tissue regeneration, a new manufacturing technology of combining 3D printing and EHD has developed. The existing 3D printing technology allows materials to be placed in controlled ways. However, the resolution of 3D printing is limited. Nanofibers can hardly be made directly through 3D
printing. Integrating EHD casting into 3D printing results in a new additive manufacturing technology, which allows the production of composite materials containing well-aligned 2D and/or 3D nanofibers for special applications. This chapter presents the following aspects to readers: (i) how to design and make a new manufacturing machine consisting of an EHD casting system and a 3D printer, (ii) how to use the machine to manufacture composite materials with functional nanoparticles uniformly distributed in polymer fibers, (iii) how to perform heat treatment on the nanofiber composites to convert the polymer fibers into partially carbonized fibers, (iv) how to test the functions of the composite materials for sensing and energy conversions, and (v) how to characterize the micro-and nanostructures of composite materials to understand the structure-property relations. EHD forces include electric repulsion, fluid pump pressure, and body force (gravity). Such forces may facilitate a uniform distribution of nanoparticles in polymer nanofibers. The manufactured composite materials may have enhanced TE and photovoltaic properties due to the special structure formation in the combined EHD casting and 3D printing processes. It is possible to make high-performance sensors and energy converters with multiple components and 3D printed architectures. To show this technology, materials consisting of different dispersed nanoparticles in polyacrylonitrile (PAN) nanofibers are made. The nanoparticles are made from bismuth telluride and/or antimony telluride alloy compounds. Both are intrinsic narrow-band semiconductors. Therefore, these nanoparticles can generate TE and photovoltaic functions. The PAN polymer is mixed with the Bi-Te and Sb-Te alloy nanoparticles in an N,N-dimethylformamide (DMF) solvent. Uniform dispersion of nanoparticles in polymer nanofibers to form composites can be achieved via combined EHD casting and 3D printing. Then high-temperature heat treatment on the composites in hydrogen gas is conducted to convert the polymer nanofibers into partially carbonized fibers. The partially carbonized fibers are expected to have tuned electrical conductivities depending on the heat treatment temperatures. The higher the heat treatment temperature, the better is the conductivity of the carbon fibers. The carbon fibers will serve as electrical connectors to the Bi-Te or Sb-Te nanoparticles.
