CNR Roadshow 2014

Gecko Mimetic Adhesives for space applications

Elettra – Sincrotrone Trieste, Italy Luca Gregoratti, Andrea Goldoni, Olexander Trygub, Marco Marazzi

Massimo Tormen, Gianluca Grenci, Simone Dalzilio CNR – IOM, Trieste, Italy

Alessandro Gasparetto, Renato Vidoni, Sander De Ridder Università degli studi di Udine, Udine, Italy

A1-6313/09/NL/SFe

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Presentation of Elettra - Sincrotrone Trieste

Elettra Synchrotron

Research Center

Trieste

Elettra is an international multisciplinary laboratory specialized in synchrotron radiation and its use in the science of matter. It hosts more than 35 laboratories.

The mission of Elettra - Sincrotrone Trieste through its Elettra Research Center is to promote cultural, social and economic growth through basic and applied research in

relevant fields, technical and scientific training, and technology transfer.

Elettra is the coordinator of

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Local partnership & collaboration

Elettra - Sincrotrone Trieste is part of

• Multi Sectoral Tech Park • Technology Transfer • 65 tenant companies • 21 research centres

Elettra - Sincrotrone Trieste is associate Laboratory of

Sincrotrone Trieste has permanent collaborations with

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CNT wide research

CNT

X-ray beam spot

C

Au 1.5 µm

CNT diameter: 60-70 nm

From high density to low density MCNT arrays

Confined patches on single nanostructure

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Outline

•  Aim of the project

•  State of the art

•  Approaches

-  model systems

-  all metal carbon

-  polymer/metal/carbon

•  Mechanical and other tests

•  Conclusions

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Aim of the project

The project aims at developing the gecko mimetic tape concept from its current laboratory origins

Current status

•  Production limited to small areas •  Laboratory scale technologies •  High production costs •  Rigid substrates •  Reversibility (?)

Future technologies

•  Low cost technologies •  Industrial scale •  High performance

Why a dry adhesive? •  For space business they should overcome the main limitations of conventional

adhesives: operating temperature range, outgassing performance in vacuum applications.

•  Further applications also in daylife applications

Literature review - state of the art

•  Van der Waals Forces •  Capillary Forces •  Anisotropic adhesion •  Anti-bunching conditions •  Self-Cleaning properties •  A low-to-no adhesion state in

absence of shear

Picture from: Yu et al., Adv. Funct. Mater. 2011, 21, 3010–3018

Picture from web..See: Kesel et al., J Exp Biol206, 2733-2738, 2003

•  Relatively soft elastomer fibers with mushroom-shaped tips

•  Stiff, very high aspect ratio fibers, which exhibit good friction forces, but either low or zero normal adhesion strength

Natural adhesion mechanisms

Mimicking materials/methods

Fabrication methods

•  Micro/nano casting •  Gas phase fabrication methods

Manufacturing and processing parameters

• high-density (i.e. pattern and periodicity of the synthetic setae)

to achieve a high adhesion force • high aspect-ratio (i.e. length, diameter and stiffness of the shafts)

to achieve a high adhesion force and an anti-bunching condition • mushroom-shaped structures (i.e. size, shape and stiffness of the spatulae ends)

to increase the contact area and optimize adhesion • angled/inclined fibers

to produce directionality, make the detaching more convenient and tune the mechanical

properties • hierachical and branched structure

to improve the surface adaptability

The existing results show that necessary conditions and key parameters for creating an effective dry-adhesive are:

Hierarchical level structures

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State of the art

•  The already developed dry adhesives are typically produced as very small samples of few squared millimetres of active area. Technical solution for a large area samples are missing.

•  In most of the cases the proposed technologies are lab-sized and cannot be easily extended to a large scale production. Due to it the available technologies are also expensive.

•  Data available in the literature rarely include reversibility and repetition rate of the adhesion process with the same sample. This feature must be investigated more deeply.

•  Most of the technical solutions reported in literature use rigid substrates which are fine for proof of principle experiments but have large limitations when used on real systems.

The technical solutions developed in this project will hopefully contribute to overcome these limitations and as will be described later have been selected and studied having in mind the above mentioned scenario.

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Approaches

Development of “dry” reversible adhesives main routes

All metal/carbon technology

Compliant substrates:

Polymer-plastic/carbon technologies

Nanostructures: VA-CNTs

polymer-plastic

Nanostructures:

•  thin metal foils (e.g. Ni) •  metal cantilevers •  CNT bushes •  VA-CNTs

•  polymer-plastic nanostructures

For the simplest architectures no need for lithographic steps

Use of lithographic approaches

Compliant substrates:

Need to trasfer CNTs from growing substrate to

polymer/plastic

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Model systems

Growth of the Vertically Aligned Multiwall Carbon Nanotubes (VA-CNTs): CVD

•  650-700 °C •  2 feeding gases (H2, C2H2) •  30 mbar of C2H2

VA-MWCNTs Diameter: < 10 – 30 nm

Model system: Vertically Aligned Multiwall Carbon Nanotubes on Si substrates

Major limitations: •  Rigid substrate •  No compliance

•  Expensive

HV environment (2 vaccum stages) / near ambient pressure environment

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Deposition of the catalyst

Selected catalyst for the growth of VA-CNTs: Fe

Ways of deposition investigated •  HV evaporation of pure metallic Fe •  Evaporation of pentacene •  Spin coating of Fe nanoparticles

HV required

Ambient pressure

Spin coating: not very homogeneous coverage: need better tuning of the parameters

Need for post-evaporation thermal treatments

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Deposition of the catalyst - summary

Results achieved •  The best results have been obtained by evaporating the Fe directly from the

metallic source. •  Use alternative lower cost procedures such as ferrocene evaporation and spin

coating of a solution containing Fe nanoparticles is possible. No uniform distribution catalyst nanoparticles.

Results not achieved and drawbacks •  Need more experiments (e.g. to define the temperature of the evaporation of

ferrocene or the solvent used to disperse the Fe nanoparticles and their density in the solution) for trimming ferrocene evaporation and spin coating of a solution containing Fe nanoparticles.

What’s new with respect to the state of the art technology •  The evaporation of Ferrocene molecules and the spin coating of Fe

nanoparticles represent two valid low cost alternatives for the deposition of the catalyst needed for the growth of the CNTs. Both solutions are not sufficiently documented in the literature and have never been used for this application.

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All metal / Carbon

VA-MWCNTs/Ni thin foil Self compliant CNTs

CNTs bushes + VA-MWCNTs

Ni thin foils (0.05-0.1 mm thk) are: Flexible and cheap

1st hierarchical level

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All metal / Carbon

VA-MWCNTs/Ni thin foil Self compliant CNTs

Problems found •  Low adhesion of CNTs on substrate •  Need for buffer layer: Si and Al2O3 tested •  Bad results for Si not easy to make a

uniform layer of Al2O3

Compression test of the CNT bushes

Growth of the second VA-CNTs hierarchical level under electrical field

Al2O3 deposition: -  Sputtering -  Al dep in O2

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CNTs on Si/Ni substrates, self compliant CNTs - summary

Results achieved •  The CVD procedure used for the growth of the CNTs is largely used in the scientific community.

Conditions to grow VA-MCNTs on Ni foils have been found •  Large region of samples with the two layers of CNTs were grown. Some regions are covered by

vertically aligned CNTs while other regions look smoother and at a closer view show randomly distributed CNTs.

Results not achieved and drawbacks •  The improvement of growth of the second layer of CNTs grown on top of the bushes by

trimming the parameters can in principle be possible, but required more resources and efforts. The resulting adhesion of these samples was not significant.

What’s new with respect to the state of the art technology •  This approach is really innovative; to our knowledge no similar experiment has been published

in peer review journals so far. It needs to be investigated and trimmed to show all its potentialities but we consider it a valid solution for the generation of a compliant substrate covered by Van der Waals force generators.

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Lithographic growth of Ni cantilevers

Hot embossing

Ni cantilever before mould

release

Ni cantilever after mould

release

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Lithographic growth of Ni cantilevers

Array of ‘’clean’’ cantilevers After the growth of VA-CNTs

High temperature!!

After adhesion the ‘’weaker’’ cantilevers tend

to bend: necessity for better design

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Lithographic growth of Ni cantilevers - summary

Results achieved •  Samples of Ni cantilevers have been realized; the microstructures have a good

precision and reproducibility. •  VA-MCNTs were grown on top of the Ni cantilevers. The CNTs forms bundles

which are vertically aligned. •  The approach based on micron sized flexible levers for the compliance is in our

opinion valid; other solution are possible. •  Low mechanical adhesion. Results not achieved and drawbacks •  The lithographic procedure to obtain good Ni cantilevers is very long and

complicated. •  During the adhesion tests performed with these samples we encountered

several problems due to the balance that is required between the adhesion of the CNTs to the foreign material and that of the Ni cantilevers to the substrate. No significant results were obtained.

What’s new with respect to the state of the art technology •  Combination of mesoscale compliant substrate with nanoscale structures for

VdW generation. No reference in literature.

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Mechanical tests

Loading tests Sample holder for pull-off and shear tests

Surfaces used for adhesion tests:

Polystyrene (rigid) Al foil

•  Results obtained by using patterned/not patterned

surfaces •  Sample stored and used

in normal athmosphere

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Mechanical tests

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Mechanical tests

Pull off avg adhesion loads

CNTs / Si CNTs / Ni foil Small area samples

A < 1cm2 0.5 – 2 N/cm2

Good repetition rate -

Large area samples A > 1cm2

< 0.5 N/cm2

Good repetition rate < 1.5 N/cm2

Bad repetition rate

Shear avg adhesion load

CNTs / Si Small area samples

A < 1cm2 1.2 – 9 N/cm2

Large area samples A > 1cm2 < 1 N/cm2

•  Percentage of samples which crossed the adhesion threshold (0.1-0.2 N/cm2): 15-20% •  No major differences were noticed while using polystyrene or aluminum as testing substrates. •  No major differences were recorded between patterned and not patterned samples – maybe

already ideal density.

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Mechanical tests - failures

•  Rigid substrates are very sensible to dust/defect particles

•  Damage of CNTs

•  Detachment of CNTs from substrate

Ni foil – buffer layers

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Mechanical tests

Results achieved •  Absolute load values found for the CNTs samples grown on Si and Ni substrates are close to

what typically found in the literature. •  Realization of samples with large active area •  15-20% of the samples produced for adhesion showed an adhesion load higher than 0.1 N/

cm2. Average loads for Si substrates pull-off 0.5-2 N/cm2. •  The higher adhesion values/cm2 have been achieved for small samples with active areas < 1

cm2 but measurable values have been recorded also for large active area samples (area > 1 cm2).

•  No major differences were noticed while using polystyrene or aluminum as testing substrates. •  The use of Ni foils as substrates for the CNTs did not enhanced the adhesion loads. Poor

adhesion of CNTs with the Ni foil. This problem could be fixed in principle by adding a proper buffer layer.

•  As expected adhesion tests of the shear mode produced higher adhesion loads than what achieved during pull off.

•  Samples manipulated and tested in normal environments rather than clean rooms typically available in the laboratories.

•  Good repetition of the adhesion tests that can be performed on the sample with reasonable load values. The reversibility of the adhesion is a parameter typically not reported in the literature where only the best adhesion result is provided.

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Mechanical tests

Results not achieved and drawbacks •  The adhesion loads achieved with the Ni foils did not produce the expected

results. The reason was the poor adhesion of the CNTs to the Ni foil; we tried to introduce a buffer layer between the CNTs and the Ni in order to improve the adhesion; among the materials tested the Si did not improved significantly the adhesion and we did not have enough time to trim the growth/deposition of Al2O3 buffer layer.

•  As already discussed the use of cantilevers to perform a compliant substrate suffered from the complexity of the realization and cannot be considered as a valid solution.

What’s new with respect to the state of the art technology •  The active area of the sample produced is generally larger than what reported in

the literature. •  We reported the data for the repetition rate of the adhesion that is generally not

provided in literature.

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Other tests (following the ESA technical requirements)

q  Functional & Performance Requirements

v  Storage and working conditions of the material

-80 °C

q  Environmental Requirements

q  Human Factors Requirements

q  Degassing UHV mass spectrometry & quarz microbalance, no

organic pollutants

q  Fine chemical characterization: photoemission

spectromicroscopy

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Conclusions

Main results (also partially achieved)

Ø  Growth of CNTs on flexible/compliant substrates Ø  Use of larger substrates Ø  Investigation of low cost technical solutions

Main drawbacks Ø  Low reproducibility of the CNTs parameters Ø  Low reproducibility of the adhesion values Ø  Best adhesion values still far from wet adhesives

Possible next steps

§  Micro-punching §  Self compliant CNT §  Transfer of CNTs

Obtaining compliant micro cantilevers with micro-punching

masters

§  High level surface finishing §  Fine alignment for large areas

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Conclusions / other results

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Conclusions

•  We produced and successfully measured the adhesion properties of large samples (2.20 cm2 of active area). Results reported in literature are usually referred to much smaller samples.

•  We reported results about reversibility/reproducibility of the adhesion of several samples showing how samples can be attached and removed from substrates several times. Again results reported in literature never describe this property which is very important in case of a spread use of the adhesive.

•  We got some good results using as substrate thin Ni foils, which exhibit a first level of compliance with respect to the Si substrates commonly used. Ni foils do not represent the ideal substrate like plastic or polymers but the technical solutions developed represent a step forward with respect to the state of the art technology mainly based on Silicon. The results obtained demonstrate how the growth of CNTs can be performed directly on metallic substrates.

•  Other technologies such as the growth of vertically aligned CNTs on CNTs bushes have been explored but need further development to demonstrate their potentialities.