Terahertz (THz) technology has shown promise for several applications, but limitations in sources and detectors have prevented broader adoption. Existing THz detectors are rigid, planar, and fabricated using complex technology, making it difficult to integrate into systems. Here we demonstrate THz detectors fabricated by inkjet printing on submicrometer thick, ultraflexible substrates. By developing p- and n-type carbon nanotube inks, we achieve optically thick p–n junction and p-type devices, enabling antenna-free pixels for THz imaging. By further designing and folding the printed devices, we realize origami-inspired architectures with improved performance over single devices, achieving a noise-equivalent power of 12 nW/Hz1/2 at room temperature with no voltage bias. Our approach opens avenues for nonplanar, foldable, deployable, insertable, and retractable THz detectors for applications in nondestructive inspection.
We demonstrate chemical doping of a topological insulator Bi2Se3 using ion implantation. Ion beam-induced structural damage was characterized using grazing incidence X-ray diffraction and transmission electron microscopy. Ion damage was reversed using a simple thermal annealing step. Carrier-type conversion was achieved using ion implantation followed by an activation anneal in Bi2Se3 thin films. These two sets of experiments establish the feasibility of ion implantation for chemical modification of Bi2Se3, a prototypical topological insulator. Ion implantation can, in principle, be used for any topological insulator. The direct implantation of dopants should allow better control over carrier concentrations for the purposes of achieving low bulk conductivity. Ion implantation also enables the fabrication of inhomogeneously doped structures, which in turn should make possible new types of device designs.
Using galvanostatic pulse deposition, we studied the factors influencing the quality of electroformed Bi1-xSbxnanowires with respect to composition, crystallinity, and preferred orientation for high thermoelectric performance. Two nonaqueous baths with different Sb salts were investigated. The Sb salts used played a major role in both crystalline quality and preferred orientations. Nanowire arrays electroformed using an SbI3-based chemistry were polycrystalline with no preferred orientation, whereas arrays electroformed from an SbCl3-based chemistry were strongly crystallographically textured with the desired trigonal orientation for optimal thermoelectric performance. From the SbCl3 bath, the electroformed nanowire arrays were optimized to have nanocompositional uniformity, with a nearly constant composition along the nanowire length. Nanowires harvested from the center of the array had an average composition of Bi0.75Sb0.25. However, the nanowire compositions were slightly enriched in Sb in a small region near the edges of the array, with the composition approaching Bi0.700.30.
Nanostructuring of thermoelectric materials is expected to enhance thermoelectric properties by reducing the thermal conductivity and improving the power factor from that of homogeneous bulk materials. In multiphase, nanostructured thermoelectric materials, an understanding of precipitation mechanisms and phase stability is crucial for engineering systems with optimal thermoelectric performance. In this presentation we will discuss our investigations of the morphological evolution, orientation relationship, and composition of Ag{sub 2}Te precipitates in PbTe using transmission electron microscopy (TEM) and atom probe tomography (APT). Annealing in the region of two phase equilibrium between Ag{sub 2}Te and PbTe results in the formation of monoclinic {beta}-Ag{sub 2}Te precipitates as determined by x-ray and electron diffraction studies. These precipitates are aligned to the PbTe matrix with an orientation relationship that aligns the Te sub-lattices in the monoclinic and rock salt structures. This relationship is the same as we have reported earlier for {beta}-Ag{sub 2}Te precipitates in rocksalt AgSbTe{sub 2}. Observations using TEM and APT suggest that the Ag{sub 2}Te precipitates initially form as coherent spherical precipitates which upon coarsening evolve into flattened semi-coherent disks along the <100>PbTe directions which is consistent with theoretical predictions for elastically strained precipitates in a matrix. Our HRTEM observations show that sufficiently small precipitates are coherently embedded, while larger precipitates exhibit misfit dislocations and multiple monoclinic variants to relieve the elastic strain. Analysis of the composition of both precipitate groups using APT indicates that the larger precipitates exhibit compositions close to equilibrium while the smaller nanoscale precipitates exhibit enhanced Pb compositions. This detailed analysis of the orientation relationship, morphology, composition, and coarsening behavior of embedded Ag{sub 2}Te precipitates may be helpful in understanding the precipitation mechanisms and microstructure of related thermoelectric materials, such as LAST.
The precipitation of Ag{sub 2}Te in a PbTe matrix is investigated using electron microscopy and atom probe tomography. We observe the formation of oriented nanoscale Ag{sub 2}Te precipitates in PbTe. These precipitates initially form as coherent spherical nanoparticles and evolve into flattened semi-coherent disks during coarsening. This change in morphology is consistent with equilibrium shape theory for coherently strained precipitates. Upon annealing at elevated temperatures these precipitates eventually revert to an equiaxed morphology. We suggest this shape change occurs once the precipitates grow beyond a critical size, making it favorable to relieve the elastic coherency strains by forming interfacial misfit dislocations. These investigations of the shape and coherency of Ag{sub 2}Te precipitates in PbTe should prove useful in the design of nanostructured thermoelectric materials.
Power sources capable of supplying tens of watts are needed for a wide variety of applications including portable electronics, sensors, micro aerial vehicles, and mini-robotics systems. The utility of these devices is often limited by the energy and power density capabilities of batteries. A small combustion engine using liquid hydrocarbon fuel could potentially increase both power and energy density by an order of magnitude or more. This report describes initial development work on a meso-scale external combustion engine based on the Stirling cycle. Although other engine designs perform better at macro-scales, we believe the Stirling engine cycle is better suited to small-scale applications. The ideal Stirling cycle requires efficient heat transfer. Consequently, unlike other thermodynamic cycles, the high heat transfer rates that are inherent with miniature devices are an advantage for the Stirling cycle. Furthermore, since the Stirling engine uses external combustion, the combustor and engine can be scaled and optimized semi-independently. Continuous combustion minimizes issues with flame initiation and propagation. It also allows consideration of a variety of techniques to promote combustion that would be difficult in a miniature internal combustion engine. The project included design and fabrication of both the engine and the combustor. Two engine designs were developed. The first used a cylindrical piston design fabricated with conventional machining processes. The second design, based on the Wankel rotor geometry, was fabricated by through-mold electroforming of nickel in SU8 and LIGA micromolds. These technologies provided the requisite precision and tight tolerances needed for efficient micro-engine operation. Electroformed nickel is ideal for micro-engine applications because of its high strength and ductility. A rotary geometry was chosen because its planar geometry was more compatible with the fabrication process. SU8 lithography provided rapid prototypes to verify the design. A final high precision engine was created via LIGA. The micro-combustor was based on an excess enthalpy concept. Development of a micro-combustor included both modeling and experiments. We developed a suite of simulation tools both in support of the design of the prototype combustors, and to investigate more fundamental aspects of combustion at small scales. Issues of heat management and integration with the micro-scale Stirling engine were pursued using CFD simulations. We found that by choice of the operating conditions and channel dimensions energy conversion occurs by catalysis-dominated or catalysis-then-homogeneous phase combustion. The purpose of the experimental effort in micro-combustion was to study the feasibility and explore the design parameters of excess enthalpy combustors. The efforts were guided by the necessity for a practical device that could be implemented in a miniature power generator, or as a stand-alone device used for heat generation. Several devices were fabricated and successfully tested using methane as the fuel.
Resist substrates used in the LIGA process must provide high initial bond strength between the substrate and resist, little degradation of the bond strength during x-ray exposure, acceptable undercut rates during development, and a surface enabling good electrodeposition of metals. Additionally, they should produce little fluorescence radiation and give small secondary doses in bright regions of the resist at the substrate interface. To develop a new substrate satisfying all these requirements, we have investigated secondary resist doses due to electrons and fluorescence, resist adhesion before exposure, loss of fine features during extended development, and the nucleation and adhesion of electrodeposits for various substrate materials. The result of these studies is a new anodized aluminum substrate and accompanying methods for resist bonding and electrodeposition. We demonstrate successful use of this substrate through all process steps and establish its capabilities via the fabrication of isolated resist features down to 6 {micro}m, feature aspect ratios up to 280 and electroformed nickel structures at heights of 190 to 1400 {micro}m. The minimum mask absorber thickness required for this new substrate ranges from 7 to 15 {micro}m depending on the resist thickness.