Rate-independent dissipation and loading direction effects in compressed carbon nanotube arrays

Construction of Medusa-Like Adhesive Carbon Nanotube Array Induced by Deformation of Alumina Sheets

To change the binary structure of nanotube and nanotube array in vertically aligned carbon nanotube arrays, this work deposits regularly arranged amorphous alumina sheets on the classical array growth catalyst (10 nm-thick alumina and 2 nm-thick iron) and obtains an array similar to the Medusa head. Subsequent experiments revealed that these alumina sheets show both unstable and stable qualities during growth: unstable in that they thermally deform and change their newly discovered characteristics of blocking carbon source diffusion, which regulates the nanotube growth order in specific areas; stable in that they withstand the deformation caused by heat and sequential growth of nanotubes, serving as a substrate and buffer layer for Medusa's hair, i.e., nanotube bundles on the array surface. Their combination splits this binary structure into a tertiary architecture consisting of nanotubes, nanotube bundles, and the array spanning nano-, micro-, and milli-meter. Benefiting from this structure, this array exhibits a unique near-isotropic adhesion characteristic compared to existing reports and outperforms classical and patterned arrays with the same classical catalyst and growth conditions.

Determination of carbon nanotubes concentration by tabletop dynamic susceptometer

Carbon nanotubes (CNTs) can be incorporated in various materials to enhance their mechanical or electrical properties. Information on their precise concentration and local distribution is difficult to access non-invasively. For example, electron microscopy studies require cutting of samples. Another way to measure the concentration of CNTs is by the magnetic susceptibility of the ferrocene present in the CNTs by the synthesis process, which can be performed on sample coupons on a vibrating sample magnetometer (VSM); VSM is a bulky laboratory instrument, and the size of the samples studied is constrained. In order to provide a technique that is fast, easy, cheap, and adaptable to the size of the samples, we have developed a benchtop device that measures the CNT concentration through an original inductive dynamic measurement of the ferrocene magnetic susceptibility. We present the method for extracting CNT concentrations and show the results obtained on cement matrices with CNT concentrations of the order of a few percent.

Vertically-Aligned Multi-Walled Carbon Nano Tube Pillars with Various Diameters under Compression: Pristine and NbTiN Coated

In this paper, the compressive stress of pristine and coated vertically-aligned (VA) multi-walled (MW) carbon nanotube (CNT) pillars were investigated using flat-punch nano-indentation. VA-MWCNT pillars of various diameters (30-150 µm) grown by low-pressure chemical vapor deposition on silicon wafer. A conformal brittle coating of niobium-titanium-nitride with high superconductivity temperature was deposited on the VA-MWCNT pillars using atomic layer deposition. The coating together with the pillars could form a superconductive vertical interconnect. The indentation tests showed foam-like behavior of pristine CNTs and ceramic-like fracture of conformal coated CNTs. The compressive strength and the elastic modulus for pristine CNTs could be divided into three regimes of linear elastic, oscillatory plateau, and exponential densification. The elastic modulus of pristine CNTs increased for a smaller pillar diameter. The response of the coated VA-MWCNTs depended on the diffusion depth of the coating in the pillar and their elastic modulus increased with pillar diameter due to the higher sidewall area. Tuning the material properties by conformal coating on various diameter pillars enhanced the mechanical performance and the vertical interconnect access (via) reliability. The results could be useful for quantum computing applications that require high-density superconducting vertical interconnects and reliable operation at reduced temperatures.

Structure, mechanical properties, and modeling of cyclically compressed pulmonary emboli

Pulmonary embolism occurs when blood flow to a part of the lungs is blocked by a venous thrombus that has traveled from the lower limbs. Little is known about the mechanical behavior of emboli under compressive forces from the surrounding musculature and blood pressure. We measured the stress-strain responses of human pulmonary emboli under cyclic compression, and showed that emboli exhibit a hysteretic stress-strain curve. The fibrin fibers and red blood cells (RBCs) are damaged during the compression process, causing irreversible changes in the structure of the emboli. We showed using electron and confocal microscopy that bundling of fibrin fibers occurs due to compression, and damage is accumulated as more cycles are applied. The stress-strain curves depend on embolus structure, such that variations in composition give quantitatively different responses. Emboli with a high fibrin component demonstrate higher normal stress compared to emboli that have a high RBC component. We compared the compression response of emboli to that of whole blood clots containing various volume fractions of RBCs, and found that RBCs rupture at a certain critical stress. We describe the hysteretic response characteristic of foams, using a model of phase transitions in which the compressed foam is segregated into coexisting rarefied and densified phases whose fractions change during compression. Our model takes account of the rupture of RBCs in the compressed emboli and stresses due to fluid flow through their small pores. Our results can help in classifying emboli as rich in fibrin or rich in red blood cells, and can help in understanding what responses to expect when stresses are applied to thrombi in vivo.

Characterization of Vertically Aligned Carbon Nanotube Forests Grown on Stainless Steel Surfaces

Vertically aligned carbon nanotube (CNT) forests are a particularly interesting class of nanomaterials, because they combine multifunctional properties, such as high energy absorption, compressive strength, recoverability, and super-hydrophobicity with light weight. These characteristics make them suitable for application as coating, protective layers, and antifouling substrates for metallic pipelines and blades. Direct growth of CNT forests on metals offers the possibility of transferring the tunable CNT functionalities directly onto the desired substrates. Here, we focus on characterizing the structure and mechanical properties, as well as wettability and adhesion, of CNT forests grown on different types of stainless steel. We investigate the correlations between composition and morphology of the steel substrates with the micro-structure of the CNTs and reveal how the latter ultimately controls the mechanical and wetting properties of the CNT forest. Additionally, we study the influence of substrate morphology on the adhesion of CNTs to their substrate. We highlight that the same structure-property relationships govern the mechanical performance of CNT forests grown on steels and on Si.

Phase transitions during compression and decompression of clots from platelet-poor plasma, platelet-rich plasma and whole blood

Blood clots are required to stem bleeding and are subject to a variety of stresses, but they can also block blood vessels and cause heart attacks and ischemic strokes. We measured the compressive response of human platelet-poor plasma (PPP) clots, platelet-rich plasma (PRP) clots and whole blood clots and correlated these measurements with confocal and scanning electron microscopy to track changes in clot structure. Stress-strain curves revealed four characteristic regions, for compression-decompression: (1) linear elastic region; (2) upper plateau or softening region; (3) non-linear elastic region or re-stretching of the network; (4) lower plateau in which dissociation of some newly made connections occurs. Our experiments revealed that compression proceeds by the passage of a phase boundary through the clot separating rarefied and densified phases. This observation motivates a model of fibrin mechanics based on the continuum theory of phase transitions, which accounts for the pre-stress caused by platelets, the adhesion of fibrin fibers in the densified phase, the compression of red blood cells (RBCs), and the pumping of liquids through the clot during compression/decompression. Our experiments and theory provide insights into the mechanical behavior of blood clots that could have implications clinically and in the design of fibrin-based biomaterials.

STATEMENT OF SIGNIFICANCE

The objective of this paper is to measure and mathematically model the compression behavior of various human blood clots. We show by a combination of confocal and scanning electron microscopy that compression proceeds by the passage of a front through the sample that separates a densified region of the clot from a rarefied region, and that the compression/decompression response is reversible with hysteresis. These observations form the basis of a model for the compression response of clots based on the continuum theory of phase transitions. Our studies may reveal how clot rheology under large compression in vivo due to muscle contraction, platelet retraction and hydrodynamic flow varies under various pathophysiological conditions and could inform the design of fibrin based biomaterials.

Anomalous impact and strain responses in helical carbon nanotube foams

We describe the quasistatic and dynamic response of helical carbon nanotube (HCNT) foams in compression.

Tailoring viscoelastic response of carbon nanotubes cellular structure using electric field

Cellular structures of carbon nanotubes (CNT) are novel engineering materials, which are finding applications due to their remarkable structural and functional properties. Here, we report the effects of electric field, one of the most frequently used stimulants for harnessing the functional properties of CNT, on the viscoelastic response, an important design consideration for the structural applications of a cellular CNT sample. The application of an electric field results in electrostriction induced large actuation in freestanding CNT samples; however, if the CNT are prohibited to expand, an electric field dependent force is exerted by the sample on the constraining platens. In addition, the above force monotonically decreases with the pre-compressive strain imposed onto the sample. The viscoelastic recovery reveals a decrease in the stress relaxation with an increase in the pre-compressive strain in both the presence and absence of the electric field; however, the stress relaxation was significantly higher in the presence of the electric field. A model, based on a simple linear viscoelastic solid incorporating electric field, is developed to understand the experimental observations.