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University of Twente Publications (58.020 recursos)

University of Twente Repository provides publications of UT scientific staff, in many cases full text and publicly available.

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Mostrando recursos 1 - 20 de 7.239

  1. The effectiveness of a math game: The impact of integrating conceptual clarification as support

    Vandercruysse, Sylke; Vrugte, Judith ter; Jong, Ton de; Wouters, Pieter; Oostendorp, Herre van; Verschaffel, Lieven; Moeyaert, Mariola; Elen, Jan
    This study investigates the impact of integrating conceptual clarifications as support in an educational math game, and explores the impact of adding this (internal vs. external) support on students’ game and mathematical performance, intrinsic motivation, and game perception. Three conditions are established: a condition in which internal support is offered, a condition in which (identical) external support is offered, and a control condition in which no support is added to the game. One hundred twenty-two vocational secondary education students participated in this study. The results of this study indicate that students benefit from playing with an educational game in order...

  2. Analysis of the driving force of hydrodynamics in Lake Naivasha, Kenya

    Ndungu, J.N.; Chen, W.; Augustijn, D.C.M.; Hulscher, S.J.M.H.
    Initial fixation of press-fit implants depends on interference fit, surface morphology, and bone material properties. To understand the biomechanical effect of each factor and their interactions, the pull-out strength of seven types of CoCrMo tapered implants, with four different interference fits, three different surface morphologies (low, medium and high roughness), and at two time points (0 and 30 min) were tested in trabecular bone with varying density. The effect of interference fit on pull-out strength depended on the surface morphology and time. In contrast with our expectations, samples with a higher roughness had a lower pull-out strength. We found a...

  3. CO Adsorption on Pt Nanoparticles in Low E-Fields Studied by ATR-IR Spectroscopy in a Microreactor

    Susarrey Arce, A.; Tiggelaar, R.M.; Gardeniers, J.G.E.; Houselt, A. van; Lefferts, L.
    The adsorption of carbon monoxide (CO) on platinum (Pt) nanoparticles under the influence of a low electrical field (E-field) is studied. The adsorption of CO is monitored by attenuated total reflection infrared (ATR-IR) spectroscopy, while an external electrical field is applied over the microreactor. Linearly (on top; COL), bridged (two-fold hollow; COB), and multiply (three-fold hollow; CO) adsorbed CO species were found on the Pt nanoparticles. In a sequence of adsorption experiments the integrated ATR-IR intensity of the absorption band of the linearly adsorbed COL was found to reach steady-state values after an initial decrease, which is attributed to a...

  4. Carbon nano-fiber based membrane reactor for selective nitrite hydrogenation

    Brunet Espinosa, Roger; Rafieian, Damon; Lammertink, Rob G.H.; Lefferts, Leon
    Catalytic hydrogenation of nitrite in drinking water demands control over the selectivity towards nitrogen, minimizing the formation of ammonia. This selectivity is strongly influenced by the H/N ratio of reaction intermediates at the catalyst surface. Therefore, we fabricated a membrane reactor that feeds separately hydrogen gas and a nitrite solution. This allows dosing low but homogeneous hydrogen concentrations along the axial direction of the catalyst bed. As a consequence, low H/N ratios can be achieved, favouring the formation of nitrogen without limiting the nitrite conversion. We demonstrate that this reactor concept offers better nitrogen selectivity than conventional reactor configurations where...

  5. Method and circuitry for CMOS transconductor linearization

    Kundur Subramaniyan, Harish; Klumperink, Eric; Srinivasan, Venkatesh; Kiaei, Ali; Nauta, Bram
    Third order distortion is reduced in a CMOS transconductor circuit that includes a first N-channel transistor and a first P-channel transistor, gates of the first N-channel transistor and the first P-channel transistor being coupled to receive an input signal. Drains of the first N-channel transistor and first P-channel transistor are coupled to an output conductor. A first degeneration resistor is coupled between a source of the first P-channel transistor and a first supply voltage and a second degeneration resistor is coupled between a source of the first N-channel transistor and a second supply voltage. A first low impedance bypass circuit...

  6. Transparency and performance communication: a case study of Dutch Railways

    Galetzka, Mirjam; Gelders, Dave; Verckens, Jan Pieter; Seydel, Erwin
    Purpose – In an attempt to gain insight into the effects of transparent communication on critical performances, this paper aims to explore performance measures and communication of a large‐scale service organization, i.e. the Dutch Railways (Nederlandse Spoorwegen, (NS)). Design/methodology/approach – Semi‐structured interviews with employees of NS and external stakeholders were analyzed to study which performances are measured by NS, why they are measured, and how they are communicated to the employees and stakeholders. Findings – An ambiguous view on transparency was observed. Although transparency is advocated by most employees and stakeholders, this study revealed the complexity of performance communication of a service organization. In this...

  7. A study on the quantities and potential use of produced water in southern Iraq

    Waisi, B.I.H.; Karim, U.F.A.; Augustijn, D.C.M.; Al-Furaiji, M.H.O.; Hulscher, S.J.M.H.
    This paper presents the results of an analysis of volumes and chemical composition of produced water (PW) accompanying oil production from five of the largest oilfields in the world situated in Basrah, Iraq. PW is potentially a valuable water resource particularly there where the ramp up of oil production puts further strains on water and the environment in an area already having severe water shortages. PW should therefore be seen as part of the country's strategic water reserves rather than as effluent. This study gives first estimates of anticipated PW volumes correlated to peak oil production and water consumption needs...

  8. Evaluation of water demand and supply in the south of Iraq

    Al-Furaiji, M.H.O.; Karim, U.F.A.; Augustijn, D.C.M.; Waisi, B.I.H.; Hulscher, S.J.M.H.
    This paper presents results from the first study that focuses on water resources availability and demand for different purposes in the four oil-rich provinces of southern Iraq. The region accounts for 23% of the surface area and 18% of the country's population, but holds 88% of its oil. A water shortage of 430 Mm3/year for 2010 is estimated for this region where irrigation accounts for 81% of the total water demand. Dhi Qar is the largest agricultural producer and water consumer while Al Basrah and Al Muthanna have the largest water shortages among the four provinces. The interrelationship of energy–water...

  9. Methods for analysing morphodynamics of tidal inlets

    Willemsen, P.W.J.M.; Hulscher, S.J.M.H.; Vermaas, T.

  10. Microfluidic organ-on-chip technology for blood-brain barrier research

    Helm, Marinke W. van der; Meer, Andries D. van der; Eijkel, Jan C.T.; Berg, Albert van den; Segerink, Loes I.
    Organs-on-chips are a new class of microengineered laboratory models that combine several of the advantages of current in vivo and in vitro models. In this review, we summarize the advances that have been made in the development of organ-on-chip models of the blood-brain barrier (BBBs-on-chips) and the challenges that are still ahead. The BBB is formed by specialized e3ndothelial cells and separates blood from brain tissue. It protects the brain from harmful compounds from the blood and provides homeostasis for optimal neuronal function. Studying BBB function and dysfunction is important for drug development and biomedical research. Microfluidic BBBs-on-chips enable real-time...

  11. Towards low power N-Path filters for flexible RF-Channel selection

    Klumperink, Eric A.M.; Soer, Michiel C.M.; Struiksma, Remko E.; Vliet, Frank E. van; Nauta, Bram
    N-path filters can offer high-linearity high-Q channel selection filtering at a flexibly programmable RF center frequency, which is highly wanted for Software Defined Radio. Relying on capacitors and MOSFET switches, driven by digital non-overlapping clocks, N-path filters fit well to CMOS and benefit from Moore’s law. The basis of this filtering is the linear periodically time variant (LPTV) behaviour of a switch-R-C series circuit, which realizes frequency translated filtering, where a baseband filter characteristic is shifted around the switching frequency. This paper reviews the basic concept of N-path filters and recent developments, with special attention to possibilities to reduce power...

  12. RF Self-Interference reduction techniques for compact full duplex radios

    Deballie, B.; Broek, D.J. van den; Lavin, C.; Liempd, B.; Klumperink, E.A.M.; Palacios, C.; Craninckx, J.; Nauta, B.
    This paper describes three RF self-interference reduction techniques for full-duplex wireless links, which specifically target integration in compact radios. Concretely, a self-interference cancelling front-end, a dual-polarized antenna, and an electrical balance duplexer are proposed. Each technique offers specific benefits in terms of implementation density, selfinterference rejection, bandwidth and flexibility. Depending on their characteristics, they can be adopted in different next generation full-duplex applications and standards. All concepts are prototyped, and achieve at least 45dB of self-interference reduction over more than 10MHz bandwidth. Index Terms—Full-duplex wireless, self-interference, polarized antenna, electrical balance duplexer, analog cancellation.

  13. An Ultra-Low-Power receiver using transmitted-reference and shifted limiters for in-band interference resilience

    Ye, Dawei; Zee, Ronan van der; Nauta, Bram
    The coexistence of more and more wireless standards in the ISM bands increases the design difficulty of interference-robust receivers (RX), especially for Wireless Sensor Nodes because of their Ultra-Low-Power (ULP) budget. Superheterodyne receivers [1-3] are difficult to design for low power due to the challenges of generating a clean oscillator signal with low power. The phase noise of the LO leads to reciprocal mixing, and non-linearities and limited low-Q RF filtering make them vulnerable to in-band interferers, even when external passive components are used [2]. To circumvent accurate frequency generation, uncertain IF has been proposed [4], together with envelope detection....

  14. Relationship between the Ca/P ratio of hydroxyapatite thin films and the spatial energy distribution of the ablation laser in pulsed laser deposition

    Nishikawa, H.; Hasegawa, T; Miyake, A.; Tashiro, Y.; Hashimoto, Y.; Blank, D.H.A.; Rijnders, A.J.H.M.
    Variation of the Ca/P ratio in hydroxyapatite (Ca10(PO4)6(OH)2) thin films was studied in relation to the spot size of the ablation laser for two different spatial energy distributions in pulsed laser deposition. One energy distribution is the defocus method with a raw distribution and the other is the projection method without the weak portion at the edges of the ablation laser spot. A Ca/P ratio closer to that of stoichiometry was obtained with the larger spot sizes for both methods, and with the projection method compared to the defocus method. It is considered that a more uniform spatial energy distribution...

  15. Characterization of the surface charge distribution on kaolinite particles using high resolution atomic force microscopy

    Kumar, N.; Zhao, C.; Klaassen, A.H.; Ende, H.T.M. van den; Mugele, F.; Siretanu, Igor
    Most solid surfaces, in particular clay minerals and rock surfaces, acquire a surface charge upon exposure to an aqueous environment due to adsorption and/or desorption of ionic species. Macroscopic techniques such as titration and electrokinetic measurements are commonly used to determine the surface charge and ζζ-potential of these surfaces. However, because of the macroscopic averaging character these techniques cannot do justice to the role of local heterogeneities on the surfaces. In this work, we use dynamic atomic force microscopy (AFM) to determine the distribution of surface charge on the two (gibbsite-like and silica-like) basal planes of kaolinite nanoparticles immersed in...

  16. Losing the roadmap: Renewable energy paralysis in Spain and its implications for the EU low carbon economy

    Martínez Alonso, Patricia; Hewitt, Richard; Diaz Pacheco, Jaime; Bermejo, Lara Roman; Hernández Jiménez, Verónica; Guillen, Jara Vicente.; Bressers, Hans; Boer, Cheryl de
    After many years at the forefront of renewable energy (RE) implementation both in Europe and worldwide, Spain experienced a sudden transformation in 2012 to its RE development model in which national government backing and financial incentives for renewables were removed, throwing the RE sector into a paralysis which continues to the present day. This is in marked contrast to the case of the other major European RE leader, Germany, where it has been argued that RE implementation has produced a “regime shift” that has transformed the energy generation model to a new resilient pathway. In this paper, key differences between...

  17. Convective Heat Transfer Enhancement Using Carbon Ninofibers (CNF's): Influence of Atmorphous CarbonLayer on Heat Transfer Performance

    Lefferts, L.; Meer, T.H. van der; Taha, T.J.

  18. Reversible Phase Transfer of (CdSe/ZnS) Quantum Dots between Organic and Aqueous Solutions

    Dorokhin, Denis; Tomczak, Nikodem; Han, Mingyong; Reinhoudt, David N.; Velders, Aldrik H.; Vancso, G. Julius
    Ttrioctylphosphine oxide (TOPO) stabilized CdSe/ZnS quantum dots (QD) were modified with 6-ferrocenyl-1-hexanethiol (FcHT) or 11-ferrocenyl-1-undecanethiol (FcUT) via ligand exchange. The presence of ferrocenyl thiol ligands on the surface of the QDs was proven by diffusion ordered NMR spectroscopy. Upon replacement of the initial TOPO ligand with ferrocene derivatives the emission of the QDs decreased. Phase transfer of ferrocene-modified QDs from organic solvents into water was achieved by complexation reactions with β-cyclodextrin (β-CD). The QDs coated with ferrocene thiols are soluble in nonpolar solvents and are transferred into the aqueous phase upon formation of host−guest complexes between the ferrocene units and...

  19. Profile: Albert van den Berg

    Berg, Albert van den

  20. Spontaneous Pattern Formation of Surface Nanodroplets from Competitive Growth

    Peng, Shuhua; Lohse, Detlef; Zhang, Xuehua
    Nanoscale droplets on a substrate are of great interest because of their relevance for droplet-based technologies for light manipulation, lab-on-chip devices, miniaturized reactors, encapsulation, and many others. In this work, we establish a basic principle for symmetrical arrangements of surface nanodroplets during their growth out of oversaturated solution established through solvent exchange, which takes place under simple and controlled flow conditions. In our model system, nanodroplets nucleate at the rim of spherical cap microstructures on a substrate, due to a pulse of oversaturation supplied by a solvent exchange process. We find that, while growing at the rim of the microcap, the nanodroplets self-organize into highly symmetric arrangements, with...

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