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Carbon nanotube dry adhesives with temperature-enhanced adhesion over a large temperature range

Introduction

Adhesion between different solids permeates all aspects of our daily lives. Depending on the nature of applications, adhesives are subjected to diverse environments, ranging from the polymer-based adhesives (for example, Scotch tape, Scotch super glue, 3M Super Sticky Post-it Notes) used at ambient temperature for daily essentials, through specially designed rubbery sealants for automobile and aerospace vehicles in sub-freezing space or polar regions, to ceramic or metallic permanent glues for some specific operation processes at high temperatures (for example, solar cell panels, space shuttle launching and landing). However, conventional adhesives often show structural or performance deterioration at extreme temperatures, which could cause catastrophes1. For example, for high-temperature applications (for example, >500 °C), ceramic adhesives and/or metal welding are normally considered since they can stand up to temperatures even over 1,000 °C (refs 2, 3). Nevertheless, interfacial debonding cannot be prevented due to the differential thermal expansions between the adhesive layer and target surfaces, especially over thermal transitions with a wide temperature range.

Results

Figure 1a–d schematically shows the procedure for preparing our CNT dry adhesive (CNT length: 300–500 μm, diameter: 7–10 nm, Supplementary Figs 1 and 2) while details for the material preparation can be found in the Methods. Briefly, the VA-DWNT array was synthesized according to the published procedure12, followed by well-controlled oxygen plasma etching13,14 and argon annealing (Supplementary Figs 3 and 4) to remove the nonaligned nanotube segments, leading to the nanotube bundling and the top node formation (Fig. 1e,f). In previous studies, nonaligned nanotube top segments have been used to enhance the shear adhesion force (up to 100 N cm−2) for VA-MWNT arrays against smooth surfaces (for example, glass plates, Si wafers)4 whereas similar plasma-etched VA-MWNT arrays with bundled tops were found to show weak adhesions (<12 N cm−2) on the smooth surfaces8. However, plasma etching has been previously demonstrated to provide the additional advantage of facilitating the hydrogen-bonding mediated interaction between nanotubes and hydrophilic surfaces. Unlike previous studies, the plasma-induced bundled top nodes were designed in this study to facilitate a node-guided penetration into the surface cavities (Fig. 1c,d), initiating and ensuring an intimate contact of the VA-DWNT adhesive with temperature-induced rough surfaces.

By sandwiching a free-standing film of the CNT dry adhesive (4 mm × 4 mm, Fig. 1g) between two copper surfaces (Alfa Aesar, A Johnson Matthey Company) with the constituent CNTs normal to the target surface by finger pressing (7 N cm−2), we used a torch for heating to make a quick demonstration of the wide operation temperature range and measured the shear adhesion force using a digital spring balance (AWS H-110) (Fig. 1g–j, Methods, Supplementary Figs 5 and 6). The high melting temperature (1,083 °C), together with its excellent thermal and electrical properties for thermal and electrical managements15, makes the copper foil a model target surface for demonstrating CNT adhesion at high temperatures. Nevertheless, the methodology developed in this study is applicable to many other metallic and non-metallic substrates, including aluminum foil, glass plate, silicon wafer and polymer films (for example, a fluorinated ethylene propylene (FEP) film).

As seen in Fig. 1g, our newly developed CNT adhesive showed a room-temperature (24.2 °C) adhesion strength of 37 N cm−2, a value which is similar to that of a 3M double-side sticky tape (Methods). Upon continuous heating by a butane torch (Master Appliance MT-30 Table Top Self Igniting Microtorch with the flame temperature up to 1,970 °C), we found with surprise that the adhesion strength increased with increasing temperature from 37 N cm−2 (24.2 °C) through 60 N cm−2 (418.7 °C) to 124 N cm−2 (1,033 °C) (Fig. 1g–i). As far as we are aware, such a temperature-induced adhesion enhancement has never been reported for any existing adhesive materials, and the value of 124 N cm−2 is among the highest adhesion strength for all known pure CNT dry adhesives (Supplementary Table 1). On the other hand, it was found that the room temperature adhesion strength remained almost unchanged (34 N cm−2) even when our CNT dry adhesive was cooled down near to the liquid nitrogen temperature (−190.7 °C, Fig. 1j)—still outperforming all conventional viscoelastic tapes (for example, 3M double-sided sticky tape), which would have become glassy and detached from the target surface in such a cold environment.

Figure 1k shows the retained vertical alignment of the nanotube trunks for a VA-DWNT dry adhesive even after being tested at 1,033 °C. The corresponding Raman spectra (Fig. 1l) and X-ray photoelectron spectroscopic (XPS) results (Fig. 1m) show an improvement of the nanotube structure after the high temperature testing due to a thermally induced increase in the graphitization degree with a concomitant loss of physically adsorbed oxygen molecules16, as reflected by the increase in the Raman G band relative to the D band and decrease in the XPS O 1s peak with respect to the XPS C 1s peak. This, together with the absence of catalyst residue from our nanotube materials (Fig. 1m; Supplementary Fig. 1), ensured the superior thermal stability even in air for our VA-DWNTs, consistent with the previous report17.

To more quantitatively study the interlock adhesion mechanism and the associated thermally induced adhesion enhancement, we replot the data from the ex situ measurements shown in Fig. 1n, along with the corresponding plot of the enhancement factor (defined as the normalization of adhesion strength at a specific temperature to the corresponding adhesion strength at room temperature), as a function of temperature in Fig. 3. As seen in the inset of Fig. 3a, the CNT adhesion increases by about six times (6.2 × ) with temperatures over −196 to 1,000 °C. However, either the vdW force dominated CNT adhesion4 or the viscoelastic interactions between the CNTs should be insensitive to the temperature variation, as demonstrated previously17. Clearly, therefore, an unknown yet decisive factor, in addition to the vdW and viscoelastic interactions, is regulating the observed temperature influence on the adhesion performance of the CNT adhesives.

Our SEM images taken from the copper foil under annealing at different temperatures show that surface segregation occurred upon heating the relatively smooth copper foil above 200 °C, leading to an increase in the surface roughness with increasing temperature (Supplementary Figs 9 and 10). It could guide the collapsed CNT segments around the node (cf. Fig. 3b) to penetrate further into the temperature-induced irregular profiles to mechanically lock them into the surface, which explained the enhanced adhesion under higher preloading by pushing more CNT (bundle) screws'' into deeper surface cavities to produce higher vdW forces for stronger surface fastening (Fig. 3b; Supplementary Fig. 11).

To verify the enhanced adhesion through this CNT nano-interlocking mechanism, we estimated the extra work to pull the CNT bundles out of pits. The VA-DWNT bundle was modelled as an elastic strand to be embedded in a surface asperity, which was assumed as a cylindrical rigid hole with depth of l and radius of a for simplicity, and subjected to pull-out force (F) (inset in Fig. 3c, cf. Supplementary Fig. 12). While the detailed derivation is given in Supplementary Equations 1–9 (Supplementary Discussions), equation (1) gives the enhancement factor at a specific temperature (T) as a function of the pull-out work associated with the asperities at T defined by l(T) and the fractional area of holes with CNT strand embedded in per unit area of surface, ϕ (where, ϕ=nπa2, ranging from 0 to 1, and there are n holes per unit area of the surface):18

The above equation illustrates that the screw-like enhancement mechanism at high temperatures produces deeper holes, leading to a deeper penetration of CNTs into the holes for stronger surface fastening. Equation (1) reveals that if once the depth l(T) is increased to greater than one-fourth (1/4) of the radius of the hole (a), the screw-like enhancement makes the contribution to the extra energy for detachment. The embedded length l(T) was approximated by using the roughness values (Rq) from the atomic force microscopic (AFM) investigation on the target copper surface at different temperatures (cf. Supplementary Fig. 10; Supplementary Table 2) and the radius of the hole was60 nm from SEM observation. We estimated the enhancement factors at different temperatures from equation (1) and found that it agreed well with the experimental data while ϕ was taken as 0.30 (red dots in Fig. 3c, along with the model calculation by using ϕ=0.05 and 0.50 for comparison). This result means that about 30% of the surface area contributes to the nano-interlocking extra energy in addition to the vdW contact.

Owing to the generic nature of the thermally induced surface roughness, the concept of the node-guided CNT nano-interlocking reported here could be regarded as a general strategy for the development of thermally enhanced CNT dry adhesives for a variety of target surfaces, ranging from polymer films to metal foils, over a wide range of operation temperatures (Supplementary Fig. 13). Indeed, Fig. 3d shows similar thermally induced adhesion enhancements for our CNT dry adhesives against a FEP film (American Durafilm, MOT=204 °C (ref. 19)) and aluminum foil (Al, Fisher Scientific, MOT=600 °C), but not against a silicon wafer (Silicon Quest International, MOT=1,300 °C). As expected, the temperature-invariant adhesion seen for the silicon plate in Fig. 3d suggests that the temperature-insensitive, hard and smooth silicon surface could not support the formation of the thermally induced interlocking structure with a very small value of 0.005 for ϕ. For the FEP and Al surfaces, a good agreement between the model fit with equation (1) and experimental data was obtained when ϕ is 0.46 and 0.23, respectively, indicating that 46% and 23% of the surface areas contribute to the nano-interlocking extra work in addition to the vdW force with the FEP and Al surface.

Electrical and thermal managements

In addition to the thermally induced nano-interlocking interactions described above, similar adhesion behaviours were observed for our newly developed CNT dry adhesives against various naturally rough surfaces, including rough metal foils, plastic films, wood pieces, paper sheets, and even painted walls, at ambient temperature (Supplementary Figs 17 and 18 and associated Supplementary Movies 1 and 2, see also Supplementary Table 3). These results clearly indicate the potential of our CNT dry adhesives for room temperature applications with a variety of rough surfaces, apart from the low- and high-temperature applications described above. Thus, the newly discovered node-guided nano-interlocking adhesion mechanism can be applied for the development of high-performance CNT dry adhesives for a large variety of applications, ranging from adhesion to electrical and thermal management, over wide operation temperature ranges from −196 to 1,000 °C.

Methods

Double-walled CNT array synthesis and fabrication

The vertically-aligned carbon nanotube (VA-CNT) arrays were synthesized by low pressure chemical vapor deposition (CVD) on 4 × 4 mm2 SiO2 (400 nm)/Si wafers12. To start with, a 3-nm thick Fe layer was sputter coated on the wafers after the deposition of a 10-nm Al film. The catalyst coated substrate was then firstly inserted into a quartz tube furnace and heated up to 650 °C in air. This was followed by pumping the furnace chamber to a pressure less than 30 m torr. The aligned CNT arrays were grown by flowing a mixture gas of 40v/v% Ar, 30v/v% H2, 30v/v% C2H2 at 750 °C under 200 torr for 10–20 min. The resulting VA-CNT arrays were examined on scanning electron microscope (SEM, Hitachi S-4500), and VA-CNT arrays with length around 300–500 μm long were selected for dry adhesive testing in this experiment. Then, a piece of glass slide was used to gently push the VA-CNT array from its side in the direction parallel to the substrate. By so doing, the VA-CNT was easily removed whilst the structural integrity was maintained.

SEM observation

For structure observation, SEM images were taken using a Hitachi S-4500 instrument to observe the surface morphologies of the CNT arrays and target surfaces.

TEM observation

For nanotube structural characterization, transmission electron microscopy images were taken using a JEOL JEM-2000FX instrument.

Raman

Raman spectroscopy was performed on a Thermo-Electron Raman spectrometer with 532-nm excitation wavelength.

X-ray photoelectron spectroscopic

XPS measurements were carried out on a VG Microtech ESCA 2000 using a monochromic Al X-ray source (97.9 W, 93.9 eV).

Electrical and thermal property measurements

Electrical conductivity was measured on Signatone S-1160 Probe Station while thermal diffusivity was measured on LFA 457, Netzsch Laser flash.

Atomic force microscopy

AFM investigation was carried out on a Model of 5420 from Agilent Technologies.

Surface roughness and morphology measurements

Surface roughness and morphology were measured by AFM in a non-contact mode. To remove the contamination, the samples were cleaned by ethanol and dried before measurements 1 × 1 μm area AFM images were used for analysis. The root mean squared roughness (Rq) as was adopted to distinguish the roughness features of various target surfaces. Five tests were conducted on different areas to get average values.

Data availability

All the data that support the findings of this study are available from the corresponding author upon reasonable request.

How to cite this article: Xu, M. et al. Carbon nanotube dry adhesives with temperature-enhanced adhesion over a large temperature range. Nat. Commun. 7, 13450 doi: 10.1038/ncomms13450 (2016).

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Acknowledgements

This work was support by AFOSR (FA9550-12-1-0037) and NSF (CMMI-1400274). This work was jointly supported by the National Thousand Talents Plan of China, the National Natural Science Foundation of China (51402117, 51572095), State Key Laboratory of Materials Processing and Die & Mould Technology, Fundamental Research Funds for the Central Universities, and Shenzhen Basic Research Project (JCYJ20140903171444756).

Author information

Authors

Contributions

M.X. and L.D. initiated the idea and designed the experiments; M.X., F.D. and S.G. performed the experiments. M.X., L.D. and A.R. analysed the data and co-wrote the paper.

Corresponding author

Correspondence to Liming Dai.

Ethics declarations

Competing interests

The authors declare no competing financial interests.

Supplementary information

Supplementary Information

Supplementary Figures 1-18, Supplementary Tables 1-3, Supplementary Discussion and Supplementary References. (PDF 1883 kb)

Supplementary Movie 1

A carbon nanotube dry adhesive on a rough surface. (WMV 1008 kb)

Supplementary Movie 2

A demonstration of a carbon nanotube dry adhesive hanging a human on a painted wall. (WMV 606 kb)

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Xu, M., Du, F., Ganguli, S. et al. Carbon nanotube dry adhesives with temperature-enhanced adhesion over a large temperature range. Nat Commun 7, 13450 (2016). https://doi.org/10.1038/ncomms13450

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