This grant provides funding for a multi-institution, multi-disciplinary project to understand the processing-structure-property relationships of composite nanoheater-joining-material structures. The focus is on joining applications at the microscale level where spatial and temporal control of temperature profiles is important in complex geometries and heterogeneous devices with temperature sensitive parts. This research aims to understand (1) the fabrication of nanoheater composites of nanoheaters and joining materials, including the effect of mixing on proper distribution of heat output; (2) the deposition of the nanoheater-joining material composite onto flexible substrates; (3) the controlled, non-contact ignition of the nanoheaters; and (4) the functionality and reliability of the joining/interconnects. Both metal-based structures and polymer adhesives will be investigated. Fabrication and deposition of the composite joining system will be done by both ultrasonic powder consolidation and printing or direct electrospinning/electrospraying. Ignition experiments and modeling of self-ignition will be conducted. Finally, joint quality and robustness will be characterized.
If successful, this research will enable new ways of joining materials in conventional applications that increase productivity, reduce energy and material usage, lower costs, and broaden the range of products. This research is anticipated to lead to new ways to build microscale devices such as Lab-On-Chips, flexible electronics, micro-optical devices, sensors, medical devices, and energy and information storage devices. The project will also contribute to human resource development (especially women and minorities) and increased public understanding of STEM through collaborations with the Urban Massachusetts Louis Stokes Alliance for Minority Participation (UMLSAMP), local K-12 schools, the Museum of Science, and international partners (University of Cyprus).
This research project was a collaboration among Tufts University, UMass Lowell, and Northeastern Univ. to study the use of nanoheater structures for microjoining applications. This research builds from previous NSF funded research on the fabrication of Nanoheaters - metallic based materials that have high exothermic reactions. Microjoining applications are important for electronics, biomedical, and telecommunication industries. The Tufts research focused on developing computer simulations and physical experiments in a range of geometries and materials relevant to microjoining applications. The computer simulations were based on a thermophysical computer model developed in COMSOL software - a commercially available modeling program. The physical experiments used the new nanoheater materials being developed by Northeastern Univ (a composite) and UMass Lowell (particle format) and commercially available nanofoil product - thin layer form of nanoheaters. The physical tests help to understand the range of critical factors in microjoining - heat transfer, ignition, contact resistance, stresses, and delamination. These tests also were used to verify the computer model which can provide more specific temperature and stress data than can observed experimentally. There were many challenges in both the experimental and computational tests. In the physical tests, holding the un-joined materials in place was a practical issue that required clamps, folding materials, and other techniques to maintain a repeatable test. In the computer model, the microscale nature of the geometries required careful selection of aspect ratios, boundary conditions, and spacing. The material properties needed to take into account the large temperature range (several hundred degrees Celsius) that the materials are experiencing from the nanoheater energy. In fact, for one of the materials, a custom liquid crystal polymer, needed to be tested at Tufts (Physics Dept) to determine the thermal properties. The softening and melting of the materials was modeled using a variable specific heat - an approximation of melting and solidification. The physical tests were carefully chosen for geometries, materials, and nanoheater energy so that three important tasks could be achieved: (1) the range of tests are relevant to microelectronics (in this case a sealed battery), (2) the computer model can simulate the same test range so that it can be verified, and (3) the results could also be examined from a dimensional analysis (a way of generalizing the results so that patterns can be found). The outcomes of the physical tests and computer simulations included test data (i.e., joint strength and weld area), thermophysical distributions (i.e., temperature profiles and stress maps), and a set of dimensional parameters (i.e., generalized results to apply to other situations). Another outcome was a prototype battery that had the chemical components sealed within a liquid crystal polymer and that was able to provide partial power to light a LED. This research supported one graduate student (Mechanical Engineering) for two years and two undergraduate students (one Mechanical Engineering and one Chemical Engineering) during a summer each. These students collaborated with the other researchers at UMass Lowell and Northeastern Univ. The PI is working with the other collaborators on writing technical papers for publication to disseminate the research findings. This research also has broader impacts in K-12 education as the PI explored developing activities and videos for middle grade students to see exothermic reactions similar to nanoheaters. This work is ongoing and will support recent Next Generation Science Standards related to physical science and the practice of engineering. In summary, this collaborative research project has made technical progress in developing nanoheater fabrication approaches, computer models for microjoining with nanoheaters, a broader understanding of the thermophysical applications of nanoheaters, and the education of several students in engineering.
