Collaborators

• D. Bello, UMass-Lowell
• S. Buschhorn, Postdoc MIT AA
• H. Cebeci, ITU
• R.E. Cohen, MIT ChemE
• H.-M. Duong, NUS
• F. Fachin, MIT/Harvard/MGH HST
• E. J. Garcia, Gamesa
• K.E. Goodson, Stanford Univ.
• K. Gleason, MIT ChemE
• R. Guzman deVilloria, IMDEA
• A. J. Hart, Univ. of Michigan
• H. Thomas Hahn, UCLA
• S. Hofmann, Univ. of Cambridge
• S. Hong, Argonne Nat. Lab.
• S.S. Kessler, Metis Design Corp.
• S.-G. Kim, MIT ME
• S.-H. Kim, Brown Univ.
• N. Lachmann, Postdoc MIT AA
• S. Maruyama, Univ. of Tokyo
• G. McKinley, MIT ME
• A. Miravete, Univ. Zaragoza
• G. Nogueira, Postdoc MIT DMSE
• D. Papavassiliou, Univ. Oklahoma
• D. Plata, Duke
• M.F. Rubner, MIT DMSE
• L. Rocha, Univ. of Minho
• M. A. Schmidt, MIT EECS
• J.C. Seferis, Univ. Washington
• K. Schulte, TUHH
• J. Shiomi, Univ. of Tokyo
• A. H. Slocum, MIT ME
• S. Socrate, MIT ME
• K. Takahashi, UCLA
• C. Thompson, MIT DMSE
• M. Toner, MIT/Harvard/MGH HST
• D. Wagner, Weizmann Inst. of Sci.
• N. Yamamoto, MIT AA Postdoc
• Q. Zhang, Penn State Univ.

Graduate Students

• E. Colombini, Visiting AA
• D. Handlin, Res. Assistant, AA
• M. Kim, Res. Assistant, DMSE
• A. Kudo, Res. Assistant, DMSE
• R Li, Res. Assistant, AA
• A. Sepulveda, Visiting Student
• I. Stein, Res. Assistant, ME
• S.A. Steiner III, Res. Assistant, AA
• S. Wicks, Research Assistant, AA
• K. Takahashi , Visiting Student, AA

Undergraduate Students

• S. Chan, Visiting DMSE
• P. Florin, AA
• D. Lindston, AA
• H. Jethani, AA
• S. Kalamoun, Visiting AA
• H. Vincent, DMSE
• W. Wang, DMSE
• M. Williams, AA

Support Staff

• J. Kane, Research Specialist
• P.M. Lee, Financial Officer
• S. Chapman, Admin. Assist. II

Publications

Fachin, F., Chen, G.D., M. Toner, and B.L. Wardle, “Integration of Bulk Nanoporous Elements in Microfluidic Devices with Application to Biomedical Diagnostics,” J. of Microelectromechanical Systems (JMEMS), Vol. 20, No. 6, pp. 1428 – 1438, 2011.

Fachin, F., Nikles, S.A., Dugundji, J., and B.L. Wardle, “Analytical Extraction of Residual Stresses and Gradients in MEMS Structures with Application to CMOS Layered Materials,” J. of Micromechanics and Microengineering, 21 (2011) 095017 (12pp).

Kim, M., Hong, S., Miller, D.J., Dugundji, J., and B.L. Wardle, “Size Effect of Flexible Proof Mass on the Mechanical Behavior of Micron-scale Cantilevers for Energy Harvesting Applications,” Applied Physics Letters, 99, 243506 (2011).

Fachin, F., Nikles, S.A., and B.L. Wardle, “Mechanics of Out-of-Plane MEMS via Postbuckling: Model-Experiment Demonstration Using CMOS,” JMEMS, Vol. PP, Issue 99, 2012.

A.M. Marconnet, N. Yamamoto, M. Panzer, B.L. Wardle, and K.E. Goodson, “Thermal Conduction in Aligned Carbon Nanotube–Polymer Nanocomposites with High Packing Density,” NanoLetters, Vol. 5, No. 6, 2011, pp. 4818–4825.

Duong, H.M., Yamamoto, N., Papavassiliou, D.V., Maruyama, S., and B.L. Wardle, “Morphology Effects on Non-isotropic Thermal Conduction of Aligned Single- and Multi-walled Carbon Nanotubes in Polymer Nanocomposites,” J. Phys. Chem. C, (2011), 115 (10), 3872

Guzmán de Villoria, R., Hart, A.J., and B.L. Wardle, “Continuous High-Yield Production of Vertically Aligned Carbon Nanotubes on 2D and 3D Substrates,” ACSNano, Vol. 5, No. 6, pp. 4850–4857, 2011.

G.D. Chen,F. Fachin, M. Fernandez-Suarez, B. L. Wardle, M. Toner, “Nanoporous Elements in Microfluidics for Multiscale Manipulation of Bioparticles”, Small, Vol.7 (8),2011,pp. 1061–1067.

R. Guzman de Villoria, N. Yamamoto, A. Miravete, and B.L. Wardle, “Multi-Physics Damage Sensing in Nano-Engineered Structural Composites,” Nanotechnology, vol. 22, 2011, p. 185502 (7pp).

This work focuses on the development of non-metallic substances that can catalyze carbon nanotube (CNT) growth while remaining in a non-metallic state and the optimal conditions to obtain the best CNT growth yield utilizing such non-metallic substances. In our group’s previous work ^[1] , we successfully demonstrated that a non-metallic catalyst, nano-particulated zirconia, can catalyze CNT growth by thermal chemical vapor deposition (CVD) while remaining in an oxidized state. This novel class of catalyst for CNT growth will impact the potential to grow CNTs on historically challenging substrates, further expanding applications of CNTs.  Since this discovery, extensive studies have clarified the mechanisms through which these non-metallic nanoparticles work as catalysts, as well as the optimal conditions for them to obtain high yield growth.

In order to smoothly perform our parametric approach, we have built a new CVD reactor and mass-flow controller array system (Figure 1). MANGO-TANGO, the acronym for mass-flow array for nanotube growth optimization and table-top apparatus for nanotube growth optimization, has versatile capabilities such as handling twelve gas species at once, precise measurement of reaction spot temperature, and selective thermal treatment of gas species. Utilizing the new mass-flow controller array system (MANGO), we have confirmed that carbon xerogel–a type of nanoporous and amorphous carbon that is easy to fabricate–is as active a substrate as carbon aerogel for zirconia nanoparticles.

It has been understood that the precursor for non-metallic catalysts affects the morphology of resulting CNT growth. For example, both ready-made zirconia nanoparticles and zirconium oxychloride solutions work as good sources of catalysts, but the growth morphologies from them are substantially different (Figure 2).  We believe that our research employing more characterization techniques such as TEM, XPS, and Raman spectroscopy will provide important insights into the science behind CNT growth and the engineering applications of CNT.

1. S. A. Steiner III, T. F. Baumann, B. C. Bayer, R. Blume, M. A. Worsley, W. J. MoberlyChan, E. L. Shaw, R. Schloegl, A. John Hart, S. Hofmann, and B. L. Wardle, “Nanoscale zirconia as a nonmetallic catalyst for graphitization of carbon and growth of single- and multiwall carbon nanotubes,” Journal of the American Chemical Society, vol. 131, no. 34, pp. 12144-12154, 2009.

We integrated ultra-porous (99% porous) elements (nanoporous forests of vertically aligned carbon nanotubes (VACNTs)) in MEMS, showing their use in microfluidic applications for bioparticle isolation and health diagnostics.  Distinct from works where the effects of fluids on VACNT elements resulted in either structural deformation or catastrophic forest collapse ^[1] , our approach enables creation of high aspect ratio (~ 1-mm) nanoporous elements and preserves their shape under flow-through conditions. Figure 1 shows a device, consisting of a patterned and (wet) functionalized VACNT forest integrated into a PDMS microfluidic channel.

Compared to state-of-the-art designs that exploit solid materials (e.g., silicon, PDMS) for the structural features, our nanoporous elements enable flow around and through the VACNT elements, enhancing physical interaction between the particles in the flow and the functional elements.  Multiple device layouts demonstrated a ~7X increase in specific bioparticle capture when transitioning to VACNT porous designs ^[1] . The large surface-to-volume ratio of nanoporous materials yields a significant increase in the functional surface area (~250-500X for the layouts analyzed in our works ^{[2] [3]} ), with permeability comparable to that of macro-scale porous materials ^[4] , thus further promoting bioparticle capture.

Specific isolation of bioparticles ranging over 4 orders of magnitude in size (from cells to viruses) was experimentally demonstrated (Figure 1), including the ability to perform simultaneous multiphysics, multiscale isolation on a single chip ^[4] . Particles smaller than the average distance between single nanotubes in the VACNT elements (~80 nm) can penetrate the elements and can be isolated using chemical affinity; simultaneously, particles larger than 80 nm cannot enter the nanoporous elements and can be isolated on the elements’ outer surfaces using both mechanical filtration and biomolecular recognition. The nanoporous elements are versatile and could provide access to underexplored sub-micron particles (e.g., proteins, exosomes).

1. D. N. Futaba, K. Hata, T. Yamada, T. Hiraoka, Y. Hayamizu, Y. Kakudate, O. Tanaike, H. Hatori, M. Yumura, and S. Iijima. “Shape-engineerable and highly densely packed single-walled carbon nanotubes and their application as super-capacitor electrodes,” Nature Materials, vol. 5, pp. 987-994, 2006.
2. G. D. Chen, F. Fachin, M. Fernandez-Suarez, B. L. Wardle, and M. Toner, “Nanoporous elements in microfluidics for multiscale  manipulation of bioparticles,” Small, vol. 7, pp. 1061-1067, 2011.
3. F. Fachin, G. D. Chen, M. Toner, B. L. Wardle, “Integration of vertically-aligned carbon nanotube forests in microfluidic devices for multiscale isolation of bioparticles,” Proc. of IEEE Sensors, 47-51, 2010.
4. F. Fachin and G.D. Chen, M. Toner, and B. L Wardle, “Integration of bulk nanoporous elements in microfluidic devices with application to biomedical diagnostics,” Journal of Microelectromechanical Systems, vol. 20, no. 6, pp. 1428-1438, 2011.

Characterization of thin film layered materials is critical for many MEMS devices. Residual stresses from production determine both final shape and performance of microdevices and should therefore be accurately determined. Stresses are typically extracted using simple test structures (clamped beams and cantilevers, see Figures 1a-b) that allow for mean and gradient residual stress estimation [1]. However, current approaches to material characterization have two major limitations. First, their accuracy is directly proportional to their cost. This is especially true for mean compressive stress, where more accurate estimates require a larger number of different test structures. Second, they oversimplify test-structure boundary conditions by considering them to be ideal (e.g., perfectly clamped in the case of buckled beams for mean compressive stress determination ^[1] ). To overcome these issues, we have developed a new methodology for characterizing the complete stress state (effective mean and gradient stresses) in CMOS layered materials that also assesses non-ideality of clamped boundaries ^{[2] [3]} . The approach uses a closed-form solution of the postbuckling problem of micromachined beams including non-ideal boundaries (Figure 1). In Table 1 we show the results relative to the characterization of four different CMOS material combinations. The outcomes show mean compressive stresses ranging between -15 and -105MPa, thus demonstrating the method’s ability to characterize structures subjected to both large and small compressive stresses. This ability contrasts with traditional critical length methods that encounter difficulties in quantifying small compressive stresses due to their inability to distinguish between mean stress and gradient stress effects ^[2] . For the CMOS materials examined here, the accuracy was ± 2MPa for mean stresses and ±3MPa/µm for gradients. Boundary non-ideality is found to be 90% of perfectly clamped for the CMOS-released films, having such a significant effect on the extracted stresses that it must be considered. The analytical tool can also be extended to 3D MEMS design, where buckling is used to controllably place structural elements outside the wafer plane. Using this approach, we have demonstrated out-of-place architectures for applications from three-axis thermal sensing to 3D flow measurement ^[4] ).

1. M. J. Madou, Fundamentals of Microfabrication: The Science of Miniaturization, 2nd Edition, New York: CRC Press, 2002.
2. F. Fachin, M. Varghese, S. A. Nikles, and B. L. Wardle, “Characterization of the complete stress state in thin-film CMOS layered materials,” in Proc. Hilton Head, 2010.
3. F. Fachin, S.A. Nikles, J. Dugundji, and B. L. Wardle, “Analytical extraction of residual stresses and gradients in MEMS structures with application to CMOS-layered materials,” Journal of Micromechanics and Microengineering, vol. 21, pp. 095017095025 , 2011.
4. F. Fachin, S. A. Nikles, and B. L. Wardle, “Mechanics of out-of-plane MEMS via post-buckling: Model-experiment demonstration using CMOS,” Journal of Microelectromechanical Systems, to be published. (Available in Early Access Form at http://ieeexplore.ieee.org

Vertically-aligned carbon nanotube (CNT) arrays are grown on a moving substrate, demonstrating continuous growth of nanoscale materials with long-range order. A cold-wall chamber with an oscillating moving platform (see Figure 1) is used to locally heat a silicon growth substrate coated with a Fe/Al2O3 catalyst film for CNT growth via chemical vapor deposition.  The reactant gases are introduced over the substrate through a directed nozzle to attain high-yield CNT growth ^[1] .  Aligned multi-wall carbon nanotube (MWNT) arrays (or “forests”) with heights of ≈1 mm are achieved at substrate speeds up to 2.4 mm/s. Arrays grown on moving substrates at different velocities are studied to identify potential physical limitations of repeatable and fast growth on a continuous basis. No significant differences are noted between static and moving growth as characterized by SEM (as in Figure 2) and Raman spectroscopy, although overall growth height is marginally reduced at the highest substrate velocity. CNT arrays produced on moving substrates are also found to be comparable to those produced through well-characterized batch processes consistent with a base-growth mechanism. Growth parameters required for the moving furnace are found to differ only slightly from those used in a comparable batch process; thermal uniformity appears to be the critical parameter for achieving large-area uniform array growth.

Once the parameters have been optimized, a desktop continuous will growth apparatus has been designed and implemented to grow VACNTs on silicon wafers (Figure 2), flexible sheets, and alumina fibers continuously. We have demonstrated and reported the ability to manufacture VACNT arrays in a continuous manner, significantly reducing the time spent, energy consumed, and reaction products created as compared to batch processing of these technologicallyvaluable assemblies of nanoscale materials ^[2] .

1. R. Guzman de Villoria, S.L. Figueredo, A. J. Hart, S. A. Steiner III, A.H. Slocum, and B.L. Wardle, “High-yield growth of vertically aligned carbon nanotubes on a continuously moving substrate, “Nanotechnology, vol. 20, no. 40, pp. 405611-405618, 2009.
2. R. Guzman de Villoria, A. J. Hart, and B.L. Wardle, “Continuous high-yield production of vertically aligned carbon nanotubes on 2D and 3D substrates,” ACS Nano, vol.5, no.6, pp. 4850–4857, 2011.

Catastrophic structural failures cause many physical and personal losses, with prevention estimated at billions of dollars in savings each year. Non-destructive evaluation (NDE) techniques have been pursued and employed for damage detection of such structures to detect cracks and other damage at pre-critical levels for remediation ^{[1] [2]} . Here, a novel multi-physics approach is reported that addresses drawbacks in existing techniques by taking advantage of the effects that damage, such as a crack, has on the electric and thermal transport in a material containing a CNT network distributed in the bulk material. When a potential is applied to a nano-engineered structure(see Figure 1), electric field lines concentrate in the vicinity of cracks as electrons flow around damage, causing field concentrations and “hot spots” via Joule heating, an effect which is amplified because the heat flow is also impeded in areas of damage (e.g., across a crackface). These changes of temperature can be localized through a conventional infrared thermal camera. Low power operation (a 9V standard battery is exemplary, providing a 15C rise at 1 Watt as in Figure 2) and high spatial resolution are demonstrated that are beyond state-of-the-art levels in non-destructive evaluation.

Multiple applications have been identified using this technique such as crack detection in composite components that are joined by metallic fasteners, structures having internal nonvisible damage due to impact, and in situprogressive damage monitoring during a tensile strength test.  The thermal nano-engineered NDE technique demonstrated here can provide a new and effective inspection route for monitoring the nextgenerations of safer infrastructure ^{[3] [4]} .

1. D. Barber, S. Wicks, A. Raghavan, C.T.Dunn, S.S. Kessler, and B.L. Wardle, “Health monitoring of aligned carbon nanotube (CNT) enhanced composites,” presented at 2009 SAMPE Fall Technical Conference, Wichita, KS, 2009.
2. S. Wicks, A. Raghavan, R. Guzman de Villoria, S.S. Kessler, and B.L. Wardle, “Tomographic electrical resistance-based damage sensing in nano-engineered composite structures,” presented at 51st AIAA Structures, Structural Dynamics, and Materials Conference, Orlando, FL, 2010.
3. R. Guzman de Villoria, N. Yamamoto, A. Miravete, and B.L. Wardle, “Multi-physics damage sensing in nano-engineered structural Composites,” Nanotechnology, vol. 22, pp. 185502-185508, 2011.
4. R. Guzman de Villoria, A. Miravete, N. Yamamoto, and B.L. Wardle, “Enhanced thermographicdamage detection enabled by multifunctional nano-engineered composite laminates,” presented at 52^nd AIAA Structures, Structural Dynamics, and Materials Conference, Denver, CO, 2011.

Carbon nanotube (CNT) composites are promising new materials for structural applications thanks to their mechanical and multifunctional properties. We have undertaken a significant experimentally based program to understand both microstructures of aligned-CNT nanocomposites and nano-engineered advanced composite macrostructures hybridized with aligned CNTs.

Aligned nanocomposites are fabricated by mechanical densification and polymer wetting of aligned CNT forests ^[1] . Polymer wetting is driven by capillary forces that arise upon contact of the polymer with the nanostructured CNT forest ^{[2] [3]} , the rate of which depends on properties of the CNT forest (e.g., volume fraction) and the polymer (viscosity, contact angle, etc.). Here the polymer is unmodified aerospace-grade epoxy. CNT forests are grown to mm-heights on 1-cm² Si substrates using a modified chemical vapor deposition process. Following growth, the forests are released from the substrate and can be handled and infiltrated. The volume fraction of the as-grown CNT forests is about 1%; however, the distance between the CNTs (and thus the volume fraction of the forest) can be varied by applying a compressive force along the two axes of the plane of the forest to give volume fractions of CNTs exceeding 20% (see Figure 1a). Variable-volume fraction-aligned CNT nanocomposites were characterized using optical, scanning electron (SEM), and transmission electron (TEM) microscopy to analyze dispersion and alignment of CNTs as well as overall morphology. Extensive physical property testing is underway.

Nano-engineered composite macrostructures hybridized with aligned CNTs are prepared by placing long (>20 μm) aligned CNTs at the interface of advanced composite plies as reinforcement in the through-thickness axis of the laminate (see Figure 2). Three fabrication routes were developed: transplantation of CNT forests onto pre-impregnated plies ^[4] (the “nano-stitch” method), placement of detached CNT forests between two fabrics followed by subsequent infusion of matrix, and in-situ growth of aligned CNTs onto the surface of ceramic fibers followed by infusion or hand-layup ^{[5] [6] [7]} . Aligned CNTs are observed at the composite ply interfaces and give rise to significant improvement in interlaminar strength, toughness, and electrical properties. Interestingly, toughness improvement has demonstrated a favorable nano-scale size effect ^[7] . Analysis of the multifunctional properties and nanoscale interactions between the constituents in both the nanocomposites and hybrid macrostructures is underway. A new route to fabricate these materials in a continuous way has been developed.

1. B. L. Wardle, D. S. Saito, E. J. Garcia, A. J. Hart and R. Guzman de Villoria, “Fabrication and characterization of ultra-high volume fraction fligned carbon-nanotube-polymer composites,” Advanced Materials, vol. 20, pp. 2707-2714, 2008.
2. H. Cebeci, R. Guzman de Villoria, A. John Hart, and B. L. Wardle, “Multifunctional properties of high volume fraction aligned carbon nanotube polymer composites with controlled morphology,” Composites Science and Technology vol. 69, no 15-16, pp. 2649-2656, 2009.
3. E. J. García, A. J. Hart, B. L. Wardle, and A. H. Slocum, “Fabrication and nanocompression testing of aligned carbon-nanotube-polymer nanocomposites,” Advanced Materials, vol. 19, pp. 2151-2156, 2007.
4. E. J. Garcia, B. L. Wardle, and A. J. Hart, “Joining prepreg composite interfaces with aligned carbon nanotubes,” Composites Part A,vol. 39, pp. 1065-1070, 2008.
5. N. Yamamoto, A. J. Hart, S. S. Wicks, E. J. Garcia, B. L., and A. H. Slocum, “High-yield atmospheric-pressure growth of aligned carbon nanotubes on ceramic fibers for multifunctional enhancement of structural composites,” Carbon, vol. 47, pp. 551-560, 2009.
6. S. S. Wicks, R. Guzmán de Villoria, and B. L. Wardle, “Interlaminar and intralaminar reinforcement of composite laminates with aligned carbon nanotubes,” Composites Science and Technology, vol. 70, pp. 20–28, 2010.
7. J. Blanco, E. J. Garcia, R. Guzman de Villoria, and B. L. Wardle, “Limiting mechanisms in mode I interlaminar toughness of composites reinforced with aligned carbon nanotubes,” Journal of Composite Materials, vol. 43, no. 8, pp. 825-841, 2009.