32 research outputs found
The role of polymer phase on rigidity percolation of ABS−spherical nickel microparticles composites
No full textPercolation and rigidity transitions are very important phenomena in random heterogeneous microstructures, as various physical properties change abruptly at some critical volume fractions of one phase. The concepts of percolation and rigidity transitions are critical in various fields of sciences, which deal with the formation of networks in a structure. Significant efforts have been made to address the theory of percolation and rigidity transitions due to the universality of these transitions. Percolation and rigidity transitions are characterized by two parameters: exponents and the thresholds. Exponents are the measures of the sharpness of the transitions and thresholds are the critical volume fractions, where these transitions take place. Exponents are often regarded universal, as theoretically exponents are independent of geometrical details of a random microstructure. However, thresholds strongly depend on geometrical parameters.
Metal-polymer particulate composites are good systems to study percolation and rigidity thresholds, as metals and polymers have contrasting electrical and mechanical properties. The electrical resistivity of these composites is expected to decrease drastically at the percolation threshold, as a result of point contacts between the metal particles in a continuous network of metal particles. A rigidity threshold leads to a significant increment in mechanical strength and modulus, as a result of the networks of metal particles supporting load. The rigidity threshold is greater than percolation threshold, as connectivity is a necessary but not a sufficient criterion for rigidity. Rigidity transition in a particulate metal-polymer composite is the focus in the present study.
In the present study composites of ABS polymer filled with nickel particles of diameter 10 μm was prepared by hot compression moulding. Composites were prepared for various volume fractions, in 0.1 volume fraction increments till a maximum 0.6 volume fraction of nickel. Electrical resistivity and compression tests were carried out at 25 C for all composites, and the pure ABS. Percolation threshold was found to be ~ 0.4, as indicated by a drastic drop in electrical resistivity. Rigidity threshold was found to be ~ 0.6 as indicated by significant increment in Young's moduli and compressive strengths.
Rigidity transitions have been found in various systems in the past. The numerical value of the exponent is used to explain various load transferring mechanisms, although a clear picture is far from being achieved. The rigidity transition in a particulate polymer composite can happen by formation of load bearing network of rigid particles. Soft polymer phase plays important role in the rigidity transition. The polymer present adjacent to particles in the composites behaves differently compared to the bulk, depending on the chemical and physical interactions with the particles surface. Polymer in this region is called interphase. It was reported in past that presence of an immobilized interphase region at the polymer-particle interface can enhance the reinforcement effect. However, the role of interphase is expected to be insignificant for microparticles composites compared to the nanocomposites.
The present study primarily emphasizes on the composite of 0.6 volume fraction of nickel (ABS-0.6 nickel), which is beyond the rigidity threshold. It was demonstrated that under compressive loading, the soft polymer phase can play an important role, which does not come necessarily from an interphase region. The importance of the polymer phase is indicated by a significant difference in the compressive strength of ABS-0.6 nickel between room temperature and a temperature (120 C) above glass transition of the ABS. The glass transition temperature of ABS is 110 C. Differential scanning calorimetry (DSC) indicated that the glass transition temperature (Tg) of ABS-0.6 nickel is same as the pure ABS, suggesting an insignificant effect of immobilized polymer. The difference in the deformation mechanism is investigated with the help of digital image correlation (DIC).
In situ compression tests were carried out to study the state of stress in the nickel phase. In situ compression test of the ABS-0.6 nickel composite at 25 C by synchrotron x-ray diffraction shows that nickel particles are in a compressive state of stress. The strain distribution on the surface of the composite, as indicated in the DIC results, and the high compressive stress in the nickel phase shown in situ x-ray results suggest the presence of force chains, which carries compressive load through a network of nickel particles. The formation of force chain was supported by the results of confined compression test, which shows pressure sensitivity of the ABS-0.6 nickel composite. Formation of force chains is indicated by the ratio of radial to axial stress < 1 in confined compression test. Poisson's ratio measurements by DIC show that the ABS-0.6 nickel composite is under volume contraction in a compression test at the linear and strain hardening stage of the true stress-strain curve. The polymer phase present in the composite experiences volume contraction, as the volume of the composite reduces under uniaxial compression. The ABS behaves like a perfectly plastic material, as demonstrated by confined compression tests. Being a perfectly plastic material, the polymer phase exerts reaction stress in a response to the volume reduction of the polymer phase. It is suggested in this study that the reaction force exerted by the polymer phase prevents the force chains from buckling at 25 C.
Compression tests were carried out at 120 C to study the effect of polymer properties on the strength of the ABS-0.6 nickel composites. DIC results of the compression tests show marked difference in strain distribution on a surface of ABS-0.6 nickel at 25 C and 120 C. The DIC results show significant strain heterogeneity at 25 C. The strain heterogeneity is an indication of force chain formation. However, the strain distribution at 120 C shows homogeneous deformation till the peak stress is reached. Shear banding was observed beyond the peak stress, a result of possible buckling of the particle networks. ABS does not behave as a perfectly plastic material at 120 C, as it shows pressure sensitivity indicated by confined compression test. Thus, the reaction stress to the volume contraction is significantly lower. Therefore, the polymer does not prevent the particle network from buckling at 120 C, indicated by a substantial drop in overall compressive strength
A Pd–Pt-based bulk nanoporous alloy with continuous solubility for hydrogen
No full textMetal hydrides that enable reversible solute exchange with a reservoir often exhibit a miscibility gap at room temperature. Misfit strain during two-phase coexistence may then lead to degradation on repeated charging/discharging cycles. Furthermore, the miscibility gap impairs continuous and uniform composition tuning for functional applications. We explore electrochemical dealloying as a pathway to macroscopic monolithic samples of nanoporous Pd–Pt with continuous solubility for H at room temperature. The ligament size is tunable in the range of 4–40 nm, and sorption isotherms suggest a miscibility-gap critical point marginally below room temperature. With a maximum hydrogen fraction of 0.5, we demonstrate a reversible actuation strain of 3.3 % and a high cycle stability
The role of the soft phase in rigidity enhancements in a particulate composite
No full textFilling a non-conducting soft matrix with a hard conducting phase leads to two critical transitions: a percolation threshold with a drastic increase in conductivity and a rigidity threshold marking a substantial increase in the elastic modulus. A metal-particulate polymer composite has interpenetrating phases with substantial rigidity beyond the rigidity threshold. Experimental results show that the Young's modulus of the composite is enhanced significantly when an entrapped incompressible polymer phase exerts high hydrostatic stresses in uniaxial compression at the rigidity threshold. Additional experiments beyond the glass transition temperature of the polymer reveal a substantial reduction in the modulus of the composite at the rigidity threshold, as the polymer compressibility is increased significantly, confirming the role of the soft phase in rigidity enhancements. The present investigation provides a potential new designing criterion for enhancing the Young's modulus of an interpenetrating composite structure, by utilizing an incompressible second phase and negative volumetric strain
Metrics for the characteristic length scale in the random bicontinuous microstructure of nanoporous gold
No full textNanoporous gold (NPG) made by dealloying exemplifies materials with random bicontinuous microstructures that can be approximated by leveled-wave type models. As a distinguishing feature, the characteristic length scale – often quantified by the “ligament size” L – of NPG may be tuned over several orders of magnitude while the microstructural geometry retains a high degree of self-similarity. It is therefore essential to have at hand accurate procedures for determining the size by experiment and to match it to analogous size metrics of model scenarios. Working with a set of NPG samples of widely different size, we compare ligament size distributions determined by analysis of scanning electron micrographs to those of the leveled-wave model. The model is representative of various material types with random bicontinuous microstructures. The size distribution is remarkably uniform over the cross-section of experimental samples. Furthermore, the distribution evolves self-similarly upon coarsening, and the normalized distribution width agrees closely to that of the model. A measure for size determined by the electrochemical capacitance ratio method correlates well with L. This supports a protocol for converting between the two measures. As a dimensionless factor characteristic of the microstructural geometry of random dual phase microstructures, the product of L and the specific surface area is found consistent between experiment and model. The findings suggest conversion factors between the various metrics, and they advertise the combination of NPG and the leveled-wave model as a showcase for characterizing the characteristic length scale of random bicontinuous microstructures
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Growth Miracles and Growth Debacles: Exploring Root Causes
No full textIn this fascinating book, Sambit Bhattacharyya presents a detailed account of the socio-economic processes that create broad variations in living standards across the globe. The author examines the world's economic history over the last five centuries, replete with growth miracles and growth debacles: growth in Britain was steady, yet China lost her early advantage; North America settler colonies performed significantly better than those of Asia and Africa; Australia and Argentina were notably similar at the start of the twentieth century but delivered strikingly different growth outcomes. The book argues that these differences in growth rate are best explained by an interplay of factors, namely economic, political and geographical. In conclusion it presents long-run comparative growth narratives for Africa, China, India, the Americas, Russia and Western Europe. Presenting a unique and original analytical framework to explain economic growth and decline, and bridging empirical growth literature and economic history, this book will prove a stimulating read for both academic and professional economists, and scholars of economic history and economic growth. Other social scientists including sociologists, political scientists and economic historians will also find the book to be of great value
Penggabungan Sumber Internet Load Balancing Dua ISP Di Mikrotik Dengan Metode PCC Guna Memberikan Akses Internet Untuk Penggunaan Chrome Book (Studi Kasus Di SMP Negeri 1 Sambit)
No full textThe need for internet access is currently very high, both to find information, articles and the latest knowledge. Many schools have integrated the internet network into the teaching and learning process. It is hoped that students can easily find material and understand lessons, namely SMP Negeri 1 Sambit, an educational institution that has made it one of the main sources of internet access in the teaching and learning process, namely by using Chrome Books as learning media. SMP Negeri 1 Sambit wants a stable and reliable internet connection. Therefore a solution emerged to combine the two ISPs (Internet Service Provider) and make the proxy a network link. The author uses the PCC (Per Connection Classifier) method, which is a method that can be used in Load Balancing. With this PCC method, it can be used to group connection traffic that goes through or in and out of the router into several groups and divides the load on both internet connection lines so that overload does not occur.
Keywords: ISP (Internet Service Provider), Dual internet connection, Mikrotik, PCC (Per Connection Classifier), Chrome Book
A multi-scale deformation analysis for a particulate composite at the rigidity threshold
No full textThe rigidity threshold in a two-phase soft-rigid material is linked to the formation of networks of the rigid phase, which supports a substantial fraction of overall load. Metal filled particulate polymer composites were used to characterize rigidity transition experimentally, as metals and polymers have contrasting mechanical properties enabling easy detection of the rigidity transition. A detailed analysis of the load transferring mechanisms was performed using in-situ synchrotron X-ray diffraction to directly verify the compressive loading of the rigid metal particles at a microscopic scale. Elastic buckling of the force chains was attributed to the non-affine deformation at the rigidity threshold at room temperature, using digital image correlation (DIC) technique at a mesoscopic length scale. However, beyond the glass transition temperature of the polymer, the deformation was homogeneous due to the low viscosity and high compressibility of the polymer and the absence of buckling of force chains. Macroscopically, an increasing Poisson ratio was observed as a function of elastic strains, which is quite unique for these composites. The criteria for having increasing Poisson ratio is discussed based on elastic buckling of force chains and polymer incompressibility, and a link is proposed between deformation at microscopic, mesoscopic and macroscopic scale for the composite
Data Analytics in Web-based Education in the Higher-education Classroom
No full textAttention span of students in a classroom is very short. To overcome this, different active learning methodologies have been used in the past. Active learning keeps the students busy and engaged throughout the lecture. It breaks the lecture into certain time intervals by intermixing breaks, demonstrations and questions after each interval. For using active learning, clickers and laptops are commonly used in higher education classroom. Most experiments in higher education classroom studying different characteristics of students like learning performance and attention, use clickers and laptop. But, most of these experiments are in a controlled setting, not scalable and compromise the privacy of students. We overcome these problems in an active learning setup in the higher education classroom where we use a web-mediated teaching tool called ASQ. ASQ is a web application that helps to give presentation in a classroom where the presenter has control over the flow of the presentation. ASQ also allows the presenter to interleave the presentation with questions, videos and other interactive JavaScript components. Anyone can anonymously join a presentation in ASQ using a web browser. ASQ tracks the activity of every student interaction by generating event logs each second. In the previous work using ASQ, it has been shown that these logs could be used to infer the attention level of students in the classroom. The goal of this thesis is to gather insights about the fine-grained study behaviour of students in a higher education classroom by analyzing these event logs.We investigate (i) the effect of lecture elements (like the difficulty, relative positioning and spacing of questions; and duration of discussion in the slides) on study behaviour (like attention level, performance and reaction time while answering questions) of students; (ii) the relationship that might exist between attention percentage of students and their participation in the in-class questions; (iii) if students are taking external help when answering questions during the lecture and the relationship that might exist between their tendency to take external help with the difficulty of questions. We conduct our study in a classroom of around 300 students, for 15 lectures in the Web and Database Technology course at TU Delft taught by 2 instructors. We find significant effect of (i) spacing of questions on reaction time and instructor on performance; (ii) length of discussion time associated with a slide on the attention level of students which agrees with past studies; (iii) relative positioning of questions on the performance of students. However, we do not find significant effect of difficulty of questions on performance and reaction time of students while answering these questions. We also find significant effect that students with more attention percentage participate more in the in-class questions. Finally, we find that students take external help while answering questions but the tendency to take external help does not depend on the difficulty of questions
Root Causes of African Underdevelopment
No full textWhat are the root causes of Africa's current state of under-development? Is it the long history of slave trade, the legacy of extractive colonial institutions, or the fallout of malaria? We investigate the relative contributions of these factors using Atlantic distance, Indian Ocean distance, Saharan distance, Red Sea distance, log settler mortality and malaria ecology as instruments. The results show that malaria matters the most and all other factors are statistically insignificant. Malaria also negatively affects savings. The results are robust even when the malaria ecology instrument is replaced by frost, humidity and rainfall and when the latter are used as additional control variables. We find that frost alone is enough to knock off the effects of slave trade and institutions on long-term development in Africa. Copyright 2009 The author 2009. Published by Oxford University Press on behalf of the Centre for the Study of African Economies. All rights reserved. For permissions, please email: [email protected], Oxford University Press.
A framework for 3D x-ray CT iterative reconstruction using GPU-accelerated ray casting
No full textX-ray Computed Tomography (CT) is a powerful nondestructive evaluation (NDE) tool to characterize internal defects and flaws, regardless of surface conditions and sample materials. After data acquisition from a series of X-ray 2D projection imaging, reconstruction methods play a key role to convert raw data (2D radiography) to 3D models. For the past 50 years, standard reconstruction have been performed using analytical methods based on filtered back-projection (FBP) concepts. Numerous iterative methods that have been developed have shown some improvements on certain aspects of the reconstruction quality, but have not been widely adopted due to their high computational requirements. With modern high performance computing (HPC) and graphics processing unit (GPU) technologies, the computing power barrier for iterative methods have been reduced. Iterative methods have more potential to incorporate physical models and a priori knowledge to correct artifacts generated from analytical methods. In this work, we propose a generalized framework for iterative reconstruction with GPU acceleration, which can be adapted for different physical and statistical models in the inner iteration during reconstruction. The forward projection algorithm is an important part of the framework, and is analogous to the ray casting depth map algorithm that was implemented in an earlier work [I] and accelerated using the GPU. Within this framework, different sub-models could be developed in future to deal with different artifacts, such as beam hardening effect and limited angle data problem.This proceeding may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. This proceeding appeared in Zhang, Zhan, Sambit Ghadai, Onur Rauf Bingol, Adarsh Krishnamurthy, and Leonard J. Bond. "A framework for 3D x-ray CT iterative reconstruction using GPU-accelerated ray casting." AIP Conference Proceedings 2102, no. 1 (2019): 070002, and may be found at DOI: 10.1063/1.5099749. Posted with permission.</p
