Yong Hoon Jang

Conductive Fiber‐Based Ultrasensitive Textile Pressure Sensor for Wearable Electronics

A flexible and sensitive textile-based pressure sensor is developed using highly conductive fibers coated with dielectric rubber materials. The pressure sensor exhibits superior sensitivity, very fast response time, and high stability, compared with previous textile-based pressure sensors. By using a weaving method, the pressure sensor can be applied to make smart gloves and clothes that can control machines wirelessly as human-machine interfaces.

Linear elastic contact of the Weierstrass profile

A contact problem is considered in which an elastic half–plane is pressed against a rigid fractally rough surface, whose profile is defined by a Weierstrass series. It is shown that no applied mean pressure is sufficiently large to ensure full contact and indeed there are not even any contact areas of finite dimension — the contact area consists of a set of fractal character for all values of the geometric and loading parameters. A solution for the partial contact of a sinusoidal surface is used to develop a relation between the contact pressure distribution at scale n − 1 and that at scale n. Recursive numerical integration of this relation yields the contact area as a function of scale. An analytical solution to the same problem appropriate at large n is constructed following a technique due to Archard. This is found to give a very good approximation to the numerical results even at small n, except for cases where the dimensionless applied load is large. The contact area is found to decrease continuously with n, tending to a power–law behaviour at large n which corresponds to a limiting fractal dimension of (2 − D), where D is the fractal dimension of the surface profile. However, it is not a ‘simple’ fractal, in the sense that it deviates from the power–law form at low n, at which there is also a dependence on the applied load. Contact segment lengths become smaller at small scales, but an appropriately normalized size distribution tends to a limiting function at large n.† The authors dedicate this paper to the memory of Dr J. F. Archard, 1918–1989.

Effect of contact statistics on electrical contact resistance

The flow of electrical current through a microscopic actual contact spot between two conductors is influenced by the flow through adjacent contact spots. A smoothed version of this interaction effect is developed and used to predict the contact resistance when the statistical size and spatial distribution of contact spots is known. To illustrate the use of the method, an idealized fractal rough surface is defined using the random midpoint displacement algorithm, and the size distribution of contact spots is assumed to be given by the intersection of this surface with a constant height plane. With these assumptions, it is shown that including finer scale detail in the fractal surface, equivalent to reducing the sampling length in the measurement of the surface, causes the predicted resistance to approach the perfect contact limit.

Analytical model development for the prediction of the frictional resistance of a capsule endoscope inside an intestine

Abstract For the purpose of optimizing the design of the locomotion mechanism as well as the body shape of a self-propelled capsule endoscope, an analytical model for the prediction of frictional resistance of the capsule moving inside the small intestine was first developed. The model was developed by considering the contact geometry and viscoelasticity of the intestine, based on the experimental investigations on the material properties of the intestine and the friction of the capsule inside the small intestine. In order to verify the model and to investigate the distributions of various stress components applied to the capsule, finite element (FE) analyses were carried out. The comparison of the frictional resistance between the predicted and the experimental values suggested that the proposed model could predict the frictional force of the capsule with reasonable accuracy. Also, the FE analysis results of various stress components revealed the stress relaxation of the intestine and explained that such stress relaxation characteristics of the intestine resulted in lower frictional force as the speed of the capsule decreased. These results suggested that the frontal shape of the capsule was critical to the design of the capsule with desired frictional performance. It was shown that the proposed model can provide quantitative estimation of the frictional resistance of the capsule under various moving conditions inside the intestine. The model is expected to be useful in the design optimization of the capsule locomotion inside the intestine.

Transient thermoelastic contact problems for an elastic foundation

Abstract The paper presents a numerical solution of the problem of a hot rigid indenter sliding over a thermoelastic Winkler foundation at constant speed. It is shown analytically that no steady-state solution can exist for sufficiently high temperature or sufficiently small normal load or speed. The numerical solution shows that the steady-state solution, when it exists, is the final condition regardless of the initial conditions imposed. This suggests that the steady-state is also stable. When there is no steady-state, the predicted transient behavior involves regions of transient stationary contact interspersed with regions of separation. Initially, the system typically exhibits a small number of relatively large contact and separation regions, but as time progresses, larger and larger numbers of small contact areas are established, until eventually the accuracy of the algorithm is limited by the discretization used.

Multiscale electrical contact resistance in clustered contact distribution

For contact between rough surfaces of conductors in which a clustered contact spot distribution is dominant through a multiscale process, electrical contact resistance (ECR) is analysed using a smoothed version of Greenwoods model (Jang and Barber 2003 J. Appl. Phys. 94 7215), which is extended to estimate the statistical distribution of contact spots considering the size and the location simultaneously. The application of this statistical method to a contact spot distribution, generated by the finite element method using a fractal surface defined by the random midpoint displacement algorithm, identifies the effect of the clustered contact distribution on ECR, showing that including a finer scale in the fractal contact surface causes the predicted resistance to approach a finite limit. It is also confirmed that the results are close to that of Barbers analogy (Barber 2003 Proc. R. Soc. Lond. A 459 53) regarding incremental stiffness and conductance for elastic contact.

Effects of nanosized contact spots on thermal contact resistance

We investigate the effects of nanosized contact spots on the thermal contact resistance (TCR) in multiscale contacts. As the contact size decreases below the phonon mean free path, the thermal conductivity varies with the size of the contact and is not the same as its bulk counterpart. We take this into account in our model and we calculate the TCR of silicon contacted with other silicon. The TCR increases as the number of nanosized contact spots increases. However, if we do not consider the thermal conductivity reduction as the contact size decreases below the size of the phonon mean free path, there is a finite limit of the TCR. A parametric study on the effects of distance and size of the contact spots is also presented.

Electrical conductance between conductors with dissimilar temperature-dependent material properties

The method of Greenwood and Williamson is extended to give a general solution for the coupled nonlinear problem of steady-state electrical and thermal conduction across an interface between two conductors of dissimilar materials, for both of which the electrical resistivity and thermal conductivity are functions of temperature. The method presented is sufficiently general to cover all combinations of conductor geometry, material properties and boundary values provided that (i) the current enters and leaves the conductor through two equipotential isothermal surfaces, (ii) the remaining boundaries of the conductor are thermally and electrically insulated and (iii) the interface(s) between different materials would be equipotential surfaces in the corresponding linear problem. Under these restrictions, the problem can be decomposed into the solution of a pair of nonlinear algebraic equations involving the boundary values and the material properties, followed by a linear mapping of the resulting one-dimensional solution into the actual conductor geometry. Examples are given involving single and multiple contact areas between dissimilar half spaces.

Multiscale Electrical Contact Resistance Between Gas Diffusion Layer and Bipolar Plate in Proton Exchange Membrane Fuel Cells

The contact resistance between gas diffusion layer and bipolar plate in a fuel cell stack is calculated through multiscale contact analysis, which deals with rough surfaces dependent on scales. The rough surface according to scale shows that the surface parameters vary with scale, leading to inaccurate contact resistance. A numerical model is established to reflect the contact interaction of carbon graphite fiber in the contact interface. Two separate analyses are performed, static analysis to determine the contact area and electrical conduction analysis to calculate the electrical contact resistance. Results show that the contact area decreases and the corresponding contact resistance increases as the scale decreases. To accurately estimate the contact resistance, an asymptotic contact resistance according to scale variation is predicted using error analysis. The computed contact resistance is validated via comparison with previously reported values. [DOI: 10.1115/1.4006050]

Effect of unsymmetrical spiro dianhydride structure on properties of fully aliphatic polyimides

Structure-property relationship was studied for fully aliphatic polyimides containing alicyclic dianhydride and diamine units. Rel-(1′R,3S,5′S)-spiro[furan-3(2H),6′-[3]oxabicyclo[3.2.1]octane]-2,2′,4′,5(4H)-tetrone (DAn) was used as an unsymmetrical spiro dianhydride, and 1,2,3,4-cyclopentanetetracarboxylic dianhydride (CPDA) and bicyclo[2.2.2]oct-7-ene-2,3,5,6-tetracarboxylic dianhydride (BOCA) were used as symmetrical non-spiro dianhydrides. 4,4′-Methylenebis(2-methylcyclohexylamine) (MMCA) and 5-amino-1,3,3-trimethylcyclohexanemethylamine (AMCH) were N-silylated, and reacted with the dianhydrides to prepare the fully alicyclic polyimides. The formation of the polyimides was confirmed by Fourier transform infrared spectroscopy. DAn-based polyimides showed higher solubility than the polyimides derived from CPDA or BOCA. This was explained based on the results of wide-angle X-ray diffraction (WAXD) analysis of the polyimides. The WAXD study showed that DAn-based polyimides have greater full width at half maximum (FWHM) and d-spacing values than the other polyimides. This indicates that DAn-containing polyimides have lower intermolecular order, decreased intermolecular interaction, and less close chain packing compared to the other polyimides. It is considered that the morphology of DAn polyimides is attributable to the unsymmetrical spiro structure of DAn that leads to bulkiness, irregularity, and non-linearity of the polyimide chains. Decomposition temperatures of the polyimides were investigated by thermogravimetric analysis (TGA) and UV-visible spectroscopy was performed to evaluate the optical transparency of the polyimides.