InformaApp - Publikationen

DOI: 10.1016/j.vacuum.2019.109146

Zeitschriftenartikel

W. Ni, L. Liu, Y. Zhang, C. Niu, H. Fan, G. Song, D. Liu, Günther Benstetter, G. Lei

Effect of intermittent He/D ion irradiations on W nano-fuzz growth over W targets

Vacuum, vol. 173, no. March

2020

The intermittent He/D ion irradiations of polycrystalline W have been performed at the ion energy of 50 eV by changing the time of the single irradiations and the irradiation temperature. All irradiated W specimens have been observed by scanning electron microscopy, and the effect of intermittent He/D ion irradiations on the W fuzz growth has been analyzed. The W fuzz growth over W targets does not show the clear dependence on the intermittent He/D ion irradiations, where the He/D ion fluence of the single irradiations typically varies from 5.0 × 1024 to 2.5 × 1025/m2. However, a slight change in the W surface temperature during the single He ion irradiations significantly affects the W fuzz growth rate. Analysis indicates that W fuzz growth is significantly affected by the total He ion fluence varying from 5.0 × 1024 to 5.0 × 1025/m2 and the irradiation temperature varying from 1100 to 1450 K. This current study will play a crucial role in understanding the W fuzz growth under the periodic He/D ion irradiations of W divertor in fusion reactors, such as ELMs.

DOI: 10.1088/1741-4326/ab2472

Zeitschriftenartikel

Z. Bi, D. Liu, Y. Zhang, L. Liu, Y. Xia, Y. Hong, H. Fan, Günther Benstetter, G. Lei, L. Yan

The evolution of He nanobubbles in tungsten under fusion-relevant He ion irradiation conditions

Nuclear Fusion, vol. 59, no. 8

2019

He-induced W nanofuzz growth over the W divertor target is one of the main limiting factors affecting the current design and development of fusion reactors. In this paper, based on He reaction rate model in W, we simulate the growth and evolution of He nanobubbles during W nanofuzz formation under fusion-relevant He+ irradiation conditions. Our modeling unveils the existence of He nanobubble-enriched W surface layer (<10 nm), formed due to the He diffusion in W crystal into defect sites. At an elevated temperature, the growth of He bubbles in the W surface layer prevents He atoms diffusing into the deep layer (>10 nm). The formation of W nanofuzz at the surface is attributed to surface bursting of high-density He bubbles with their radius of ~4 nm, and an increase in the surface area of irradiated W. Our findings have been well confirmed by the experimental measurements.

DOI: 10.1016/j.jnucmat.2019.151800

Zeitschriftenartikel

W. Ni, L. Liu, Y. Zhang, H. Fan, G. Song, D. Liu, Günther Benstetter, G. Lei

Mass loss of pure W, W-Re alloys, and oxide dispersed W under ITER-relevant He ion irradiations

Journal of Nuclear Materials, vol. 527

2019

Mobil Europan Campus Rottal-Inn

In this study, polycrystalline W, W-Re alloys, and La2O3 and Y2O3 dispersion-strengthened W have been irradiated by our large-power materials irradiation experimental system (LP-MIES) at the irradiation temperature of 1360–1460 K. Our measurements show that the W nano-fuzz layer which is < 5.2 μm thick has been formed over all the specimens exposed to the low-energy (50 or 100 eV) and high-flux (1.37 × 1022–1.62 × 1022 ions/m2⋅s) He+ irradiations. The mass loss of the fuzz layer almost linearly increases with the He+ fluence, which does not show any dependence on the thickness of fuzz layer varying from 1.1 to 5.2 μm La2O3 and Y2O3 dispersions into W significantly suppress the growth of W fuzz, indicating that He diffusion and the evolution of He nano-bubbles in the near-surface can be significantly influenced due to the dispersion. After He+ (100 eV) irradiation at He+ fluence of 5.83 × 1026/m2, the mass loss of 0.1 vol% - 1.0 vol% La2O3-dispersed W is about 20% lower than the one of the pure W, and the La2O3 dispersed W exhibits the best erosion resistance among various W material grades. Our analysis indicates that both the surface sputtering of W fuzz by energetic ions and surface bursting of He nano-bubbles can be responsible for the mass loss of W under ITER-relevant He+ irradiations.

Zeitschriftenartikel

Katerina Volchek, A. Liu, H. Song, D. Buhalis

Forecasting tourist arrivals at attractions: Search engine empowered methodologies

Tourism Economics, vol. 25, no. 3, pp. 425-447

2019

DOI: 10.1177/1354816618811558

Tourist decision to visit attractions is a complex process influenced by multiple factors of individual context. This study investigates how the accuracy of tourism demand forecasting can be improved at the micro level. The number of visits to five London museums is forecast and the predictive powers of Naïve I, seasonal Naïve, seasonal autoregressive moving average, seasonal autoregressive moving average with explanatory variables, SARMAX-mixed frequency data sampling and artificial neural network models are compared. The empirical findings extend understanding of different types of data and forecasting algorithms to the level of specific attractions. Introducing the Google Trends index on pure time-series models enhances the forecasts of the volume of arrivals to attractions. However, none of the applied models outperforms the others in all situations. Different models’ forecasting accuracy varies for short- and long-term demand predictions. The application of higher frequency search query data allows for the generation of weekly predictions, which are essential for attraction- and destination-level planning.

DOI: 10.1016/j.fusengdes.2019.01.003

Zeitschriftenartikel

L. Liu, S. Li, D. Liu, Günther Benstetter, O. Man, J. Michalicka, Y. Zhang, Y. Hong, H. Fan, W. Ni, Q. Yang, Y. Wu, Z. Bi

The effect of O2 impurity on surface morphology of polycrystalline W during low-energy and high-flux He+ irradiation

Fusion Engineering and Design, vol. 139, pp. 96-103

2019

The interaction between the impurities (such as carbon, nitrogen, oxygen) and the plasma-facing materials (PFMs) can profoundly influence the performance and service of the PFMs. In this paper, we investigated the influence of oxygen (O2) impurity in the helium radio frequency (RF) plasma on the surface morphology of polycrystalline tungsten (W) irradiated at the surface temperature of 1450 ± 50 K and the ion energy of 100 eV. The pressure ratio of O2 to He (R) in RF source varied from 4.0 × 10−6 to 9.0 × 10-2. The total irradiation flux and fluence were ˜1.2 × 1022 ions·m-2·s-1 and ˜1.0 × 1026 ions·m-2, respectively. After He+ irradiation, the specimen surface morphology was observed by scanning electron microscopy. It was found that with increasing R from 4.0 × 10−6 to 9.0 × 10-2 the thickness of nano-fuzz layer at the W surface was thinner and thinner, accompanied by the formation of rod-like structures. The erosion yield increased from 5.2 × 10-4 to 2.3 × 10-2 W/ion when R varied from 4.0 × 10-6 to 9.0 × 10-2. The X-ray diffraction analysis shows that tungsten oxides were formed at the near surface of specimens when R exceeded 1.8 × 10-2. The erosion yield measurements revealed that in addition to surface physical sputtering process, the chemical erosion process could occur due to the interaction between oxygen-containing species and W at the surface. The results indicated that the presence of O2 impurity in He plasma can obviously affect the surface microstructure of W. The study suggested that O2 impurity can effectively reduce the growth of nano-fuzz structures.

Vortrag

D. Liu, H. Fan, L. Liu, Y. You, Günther Benstetter, F. Zhang

Surface diffusion and growth of W self-interstitials during low-energy and large-flux H/He ion irridiations of polycrystalline W

International Conference on Plasma Surface Interactions in Controlled Fusion Devices, Princeton University, NJ, USA

2018

Zeitschriftenartikel

L. Liu, S. Li, D. Liu, Günther Benstetter, Y. Zhang, Y. Hong, H. Fan, W. Ni, Q. Yang, Y. Wu, Z. Bi

Surface damages of polycrystalline W and La2O3-doped W induced by high-flux He plasma irradiation

Journal of Nuclear Materials, vol. 501, no. April, pp. 275-281

2018

Nachhaltig Elektrotechnik und Medientechnik IQMA Maschinenbau und Mechatronik

In this study, polycrystalline tungsten (W) and three oxide dispersed strengthened W with 0.1 vol %, 1.0 vol % and 5.0 vol % lanthanum trioxide (La2O3) were irradiated with low-energy (200 eV) and high-flux (5.8 × 1021 or 1.4 × 1022 ions/m2⋅s) He+ ions at elevated temperature. After He+ irradiation at a fluence of 3.0 × 1025/m2, their surface damages were observed by scanning electron microscopy, energy dispersive spectroscopy, scanning electron microscopy-electron backscatter diffraction, and conductive atomic force microscopy. Micron-sized holes were formed on the surface of W alloys after He+ irradiation at 1100 K. Analysis shows that the La2O3 grains doped in W were sputtered preferentially by the high-flux He+ ions when compared with the W grains. For irradiation at 1550 K, W nano-fuzz was formed at the surfaces of both polycrystalline W and La2O3-doped W. The thickness of the fuzz layers formed at the surface of La2O3-doped W is 40% lower than the one of polycrystalline W. The presence of La2O3 could suppress the diffusion and coalescence of He atoms inside W, which plays an important role in the growth of nanostructures fuzz.

Beitrag (Sammelband oder Tagungsband)

Günther Benstetter, Alexander Hofer, D. Liu, Werner Frammelsberger, M. Lanza

Chapter 3: Fundamentals of CAFM Operation Modes

Conductive Atomic Force Microscopy: Applications in Nanomaterials, Weinheim

2017

ISBN: 978-3-527-34091-0

IQW Sonstige

Zeitschriftenartikel

A. Al Lily, J. Foland, D. Stoloff, A. Gogus, I. Erguvan, M. Awshar, J. Tondeur, M. Hammond, I. Venter, P. Jerry, A. Oni, Y. Liu, R. Badosek, López de la Madrid, M.C., E. Mazzoni, D. Vlachopoulos, H. Lee, K. Kinley, M. Kalz, U. Sambuu, T. Bushnaq, N. Pinkwart, N. Adedokun-Shittu, P.-O. Zander, K. Oliver, L. Teixeira Pombo, J. Balaban Sali, S. Gregory, S. Tobgay, M. Joy, J. Elen, Odeh Helal Jwaifell, M., M.N.H.M. Said, Y. Al-Saggaf, A. Naaji, J. White, K. Jordan, J. Gerstein, İ. Umit Yapici, C. Sanga, P. Nleya, B. Sbihi, M. Rocha Lucas, V. Mbarika, S. Schön, L. Sujo-Montes, M. Santally, P. Häkkinen, A. Al Saif, Andreas Gegenfurtner, S. Schatz, V. Padilla Vigil, C. Tannahill, S. Padilla Partida, Z. Zhang, K. Charalambous, A. Moreira, M. Coto, et al.

Academic domains as political battlegrounds

A global enquiry by 99 academics in the fields of education and technology

Information Development, vol. 33, no. 3, pp. 270-288

2017

DOI: 10.1177/0266666916646415

This article theorizes the functional relationship between the human components (i.e., scholars) and non-human components (i.e., structural configurations) of academic domains. It is organized around the following question: in what ways have scholars formed and been formed by the structural configurations of their academic domain? The article uses as a case study the academic domain of education and technology to examine this question. Its authorship approach is innovative, with a worldwide collection of academics (99 authors) collaborating to address the proposed question based on their reflections on daily social and academic practices. This collaboration followed a three-round process of contributions via email. Analysis of these scholars’ reflective accounts was carried out, and a theoretical proposition was established from this analysis. The proposition is of a mutual (yet not necessarily balanced) power (and therefore political) relationship between the human and non-human constituents of an academic realm, with the two shaping one another. One implication of this proposition is that these non-human elements exist as political ‘actors’, just like their human counterparts, having ‘agency’ – which they exercise over humans. This turns academic domains into political (functional or dysfunctional) ‘battlefields’ wherein both humans and non-humans engage in political activities and actions that form the identity of the academic domain.

Zeitschriftenartikel

H. Fan, Y. You, W. Ni, Q. Yang, L. Liu, Günther Benstetter, D. Liu, C. Liu

Surface degeneration of W crystal irradiated with low-energy hydrogen ions

Scientific Reports (Nature Publishing Group), vol. 6, no. Article number: 23738

2016

DOI: 10.1038/srep23738

The damage layer of a W (100) crystal irradiated with 120 eV hydrogen ions at a fluence of up to 1.5 × 1025/m2 was investigated by scanning electron microscopy and conductive atomic force microscopy (CAFM). The periodic surface degeneration of the W crystal at a surface temperature of 373 K was formed at increasing hydrogen fluence. Observations by CCD camera and CAFM indicate the existence of ultrathin surface layers due to low-energy H irradiation. The W surface layer can contain a high density of nanometer-sized defects, resulting in the thermal instability of W atoms in the surface layer. Our findings suggest that the periodic surface degeneration of the W crystal can be ascribed to the lateral erosion of W surface layers falling off during the low-energy hydrogen irradiation. Our density functional theory calculations confirm the thermal instability of W atoms in the top layer, especially if H atoms are adsorbed on the surface.

DOI: 10.1016/j.jnucmat.2016.01.001

Zeitschriftenartikel

L. Liu, D. Liu, Y. Hong, H. Fan, W. Ni, Q. Yang, Z. Bi, Günther Benstetter, S. Li

High-flux He+ irradiation effects on surface damages of tungsten under ITER relevant conditions

Journal of Nuclear Materials, vol. 471, no. April, pp. 1-7

2016

A large-power inductively coupled plasma source was designed to perform the continuous helium ions (He+) irradiations of polycrystalline tungsten (W) under International Thermonuclear Experimental Reactor (ITER) relevant conditions. He+ irradiations were performed at He+ fluxes of 2.3 × 1021–1.6 × 1022/m2 s and He+ energies of 12–220 eV. Surface damages and microstructures of irradiated W were observed by scanning electron microscopy. This study showed the growth of nano-fuzzes with their lengths of 1.3–2.0 μm at He+ energies of >70 eV or He+ fluxes of >1.3 × 1022/m2 s. Nanometer-sized defects or columnar microstructures were formed in W surface layer due to low-energy He+ irradiations at an elevated temperature (>1300 K). The diffusion and coalescence of He atoms in W surface layers led to the growth and structures of nano-fuzzes. This study indicated that a reduction of He+ energy below 12–30 eV may greatly decrease the surface damage of tungsten diverter in the fusion reactor.

Zeitschriftenartikel

Y. Ji, H. Fei, Y. Shi, V. Igelsias, D. Lewis, N. Jiebin, S. Long, M. Liu, Alexander Hofer, Werner Frammelsberger, Günther Benstetter, A. Scheuermann, P. McIntyre, M. Lanza

Characterization of the photocurrents generated by the laser of atomic force microscopes

Review of Scientific Instruments, vol. 87, no. 8

2016

DOI: 10.1063/1.4960597

The conductive atomic force microscope (CAFM) has become an essential tool for the nanoscale electronic characterization of many materials and devices. When studying photoactive samples, the laser used by the CAFM to detect the deflection of the cantilever can generate photocurrents that perturb the current signals collected, leading to unreliable characterization. In metal-coated semiconductor samples, this problem is further aggravated, and large currents above the nanometer range can be observed even without the application of any bias. Here we present the first characterization of the photocurrents introduced by the laser of the CAFM, and we quantify the amount of light arriving to the surface of the sample. The mechanisms for current collection when placing the CAFM tip on metal-coated photoactive samples are also analyzed in-depth. Finally, we successfully avoided the laser-induced perturbations using a two pass technique: the first scan collects the topography (laser ON) and the second collects the current (laser OFF). We also demonstrate that CAFMs without a laser (using a tuning fork for detecting the deflection of the tip) do not have this problem.

Zeitschriftenartikel

Q. Yang, Y.-W. You, L. Liu, H. Fan, W. Ni, D. Liu, C. Liu, Günther Benstetter, Y. Wang

Nanostructured fuzz growth on tungsten under low-energy and high-flux He irradiation

Scientific Reports (Nature Publishing Group), vol. 5, no. Article number: 10959, pp. 1-9

2015

DOI: 10.1038/srep10959

Zeitschriftenartikel

W. Ni, Q. Yang, H. Fan, L. Liu, Tobias Berthold, Günther Benstetter, D. Liu

Ordered arrangement of irradiation-induced defects of polycrystalline tungsten irradiated with low-energy hydrogen ions

Journal of Nuclear Materials, vol. 464, pp. 216-220

2015

Nachhaltig Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen

Zeitschriftenartikel

Q. Yang, H. Fan, W. Ni, L. Liu, Tobias Berthold, Günther Benstetter, D. Liu, Y. Wang

Observation of interstitial loops in He+ irradiated W by conductive atomic force microscopy

Acta Materialia, vol. 92, pp. 178-188

2015

Zeitschriftenartikel

S. Liu, S. Dai, C. Sang, J. Sun, Thomas Stirner, D. Wang

Molecular dynamics simulation of the formation, growth and bursting of bubbles in tungsten exposed to high fluxes of low energy deuterium

Journal of Nuclear Materials - PLASMA-SURFACE INTERACTIONS 21 — Proceedings of the 21st International Conference on Plasma-Surface Interactions in Controlled Fusion Devices Kanazawa, Japan, May 26-30, 2014, vol. 463, no. August, pp. 363-366

2015

DOI: 10.1016/j.jnucmat.2014.12.060

Molecular dynamics simulations are carried out to investigate the formation, growth and bursting of bubbles in tungsten exposed to the irradiation of an extremely high deuterium flux. It is found that the bubbles form in the region near the location of the implanted ion distribution peaks, and that the effect of the substrate temperature on the bubble formation depth is negligible; it is also found that the percentage of deuterium that is found in D2 molecules increases as the bubble grows, and that the evolution of the bubble’s internal pressure is strongly associated with the properties of its surrounding structure. The simulations display the development of a dome-shaped structure at the tungsten surface during the bubble growth. The merging of two deuterium bubbles is also observed. The present simulations also show that the bubble bursts by generating a partially opened lid, which has already been observed in previous independent experiments.

Zeitschriftenartikel

D. Liu, J. Gu, Z. Feng, D. Li, J. Niu, Günther Benstetter

Comparison of fluorocarbon film deposition by pulsed/continuous wave and downstream radio frequency plasmas

Vacuum, vol. 85, no. 2, pp. 253-262

2010

Fluorocarbon (FC) films have been deposited using pulsed and continuous wave (cw) radio frequency (rf) plasmas fed with hexafluoroethane (C2F6), octafluoropropane (C3F8), or octafluorocyclobutane (C4F8). The effects of feed gases used, discharge pressure, rf power, substrate positions and discharge modes (pulsed or cw) on the deposited films are examined. Film properties are determined using X-ray photoelectron spectroscopy, atomic force microscopy, and static contact angle measurements. The contact angles of FC films are well related to their compositions and structures. Feed gases used, discharge pressure, rf power, substrate positions and discharge modes strongly affect the morphology of the resulting film, as revealed by atomic force microscopy. Optical emission spectrometry measurements were performed to in-situ characterize the gas-phase compositions of the plasmas and radicals’ emission intensities during film deposition. Correlations between film properties, gas-phase plasma diagnostic data, and film growth processes were discussed. The film growth in pulsed or downstream plasmas was controlled by the surface migration of radicals, such as CF2 towards nucleation centers, which result in the deposition of FC films with less cross-linked nature and rougher surfaces. These results demonstrate that it is possible to control film compositions and surface structure by changing deposition parameters.

Zeitschriftenartikel

J. Niu, H.-X. Ding, Y. Cong, N. Yu, Günther Benstetter, D. Liu

Plasma-assisted chemical vapor deposition of titanium oxide films by dielectric barrier discharge

Submitted Article

Thin Solid Films

2010

Günther Benstetter, R. Biberger, D. Liu

Zeitschriftenartikel

A Review of Advanced Scanning Probe Microscope Analysis of Functional Films and Semiconductor Devices

Thin Solid Films, vol. 517, no. 17, pp. 5100-5105

2009

This paper gives an overview of established methods and new developments in the field of Scanning Probe Microscopy (SPM) of functional films and semiconductor devices. It focuses on both, SPM analyses of passive structures and devices in operation. The contribution includes techniques such as Scanning Capacitance Microscopy (SCM) and Scanning Spreading Resistance Microscopy (SSRM) for implant mapping, Conductive AFM (C-AFM) for thin dielectrics analysis and Kelvin Probe Force Microscopy (KPFM) to study the potential distribution across active electronic devices. Finally combinations of different SPM-based techniques are described and future challenges for SPM-based techniques are discussed.