HAWK University of Applied Sciences and Arts

Gottingen, Germany
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Gerhard C.,HAWK University of Applied Sciences and Arts | Gerhard C.,Fraunhofer Institute for Surface Engineering and Thin Films | Tasche D.,HAWK University of Applied Sciences and Arts | Munser N.,IMPEX HighTech GmbH | Dyck H.,IMPEX HighTech GmbH
Optics Letters | Year: 2017

In this Letter we present a novel approach for increasing the nanosecond laser-induced damage threshold (LIDT) of sapphire windows. It is shown that after direct dielectric barrier discharge plasma treatment at atmospheric pressure for 90 s the LIDT is increased by a factor of 1.5 with respect to untreated samples. Several possible underlying mechanisms are introduced. For instance, organic contaminants and residues from polishing agents were removed by the plasma as ascertained by XPS measurements. © 2016 Optical Society of America.

Schwerdtfeger M.,University of Marburg | Krugener K.,HAWK University of Applied Sciences and Arts | Viol W.,HAWK University of Applied Sciences and Arts | Koch M.,University of Marburg
Applied Optics | Year: 2013

We use reflection terahertz spectroscopy to locate and produce three-dimensional images of air gaps between stones that resemble fractures, even of subwavelength thicknesses. This technique is found to be promising tool for sculpture and building damage evaluation as well as structural quality control in other dielectric materials. © 2013 Optical Society of America.

Einfeld D.,Lund University | Plesko M.,COSYLAB | Schaper J.,HAWK University of Applied Sciences and Arts
Journal of Synchrotron Radiation | Year: 2014

By the beginning of 1990, three third-generation synchrotron light sources had been successfully commissioned in Grenoble, Berkeley and Trieste (ESRF, ALS and ELETTRA). Each of these new machines reached their target specifications without any significant problems. In parallel, already at that time discussions were underway regarding the next generation, the 'diffraction-limited light source (DLSR)', which featured sub-nmrad electron beam emittance, photon beam brilliance exceeding 10 22 and the potential to emit coherent radiation. Also, at about that time, a first design for a 3GeV DLSR was developed, based on a modified multiple-bend achromat (MBA) design leading to a lattice with normalized emittance of x = 0.5nmrad. The novel feature of the MBA lattice was the use of seven vertically focusing bend magnets with different bending angles throughout the achromat cell to keep the radiation integrals and resulting beam emittance low. The baseline design called for a 400m ring circumference with 12 straight sections of 6m length. The dynamic aperture behaviour of the DLSR lattice was estimated to produce >5h beam lifetime at 100mA stored beam current. © 2014 International Union of Crystallography.

Hahn H.,Fraunhofer Institute for Wind Energy and Energy System Technology | Ganagin W.,HAWK University of Applied Sciences and Arts | Hartmann K.,University of Applied Sciences Aschaffenburg | Wachendorf M.,University of Kassel
Bioresource Technology | Year: 2014

With the share of intermittent renewable energies within the electricity system rising, balancing services from dispatchable power plants are of increasing importance. Highlighting the importance of the need to keeping fuel costs for flexible power generation to a minimum, the study aims to identify favourable biogas plant configurations, supplying biogas on demand. A cost analysis of five configurations based on biogas storing and flexible biogas production concepts has been carried out. Results show that additional flexibility costs for a biogas supply of 8h per day range between 2€ and 11€MWh-1 and for a 72h period without biogas demand from 9€ to 19€MWh-1. While biogas storage concepts were identified as favourable short term supply configurations, flexible biogas production concepts profit from reduced storage requirements at plants with large biogas production capacities or for periods of several hours without biogas demand. © 2014 Elsevier Ltd.

Schafer A.,HAWK University of Applied Sciences and Arts | Schafer A.,Curtin University Australia | Hall T.,Curtin University Australia | Muller G.,Ruckenzentrum am Michel | Briffa K.,Curtin University Australia
European Spine Journal | Year: 2011

The objective is to determine if pain and disability outcomes of patients treated with neural mobilisation differ for sub-classifications of low back and leg pain (LB&LP). Radiating leg pain is a poor prognostic factor for recovery in patients with LBP. To improve outcome, a new pathomechanism-based classification system was proposed: neuropathic sensitization (NS), denervation (D), peripheral nerve sensitization (PNS) and musculoskeletal (M). Seventy-seven patients with unilateral LB&LP were recruited. Following classification, all subjects were treated seven times with neural mobilisation techniques. A successful outcome was defined as achieving a minimal clinically important change in pain intensity (11-point numerical rating scale), physical function (Roland Morris disability questionnaire) and global perceived change (7-point Likert scale: from 1 = "completely recovered" to 7 = "worse than ever"). The proportion of responders was significantly greater in PNS (55.6%) than the other three groups (NS 10%; D 14.3% and M10%). After adjusting for baseline differences, mean magnitude of improvement of the outcome measures were significantly greater in PNS compared to the other groups. Patients classified as PNS have a more favourable prognosis following neural mobilisation compared to the other groups. © Springer-Verlag 2010.

Meiners A.,HAWK University of Applied Sciences and Arts | Leck M.,HAWK University of Applied Sciences and Arts | Abel B.,University of Leipzig
Review of Scientific Instruments | Year: 2010

The characteristic feature of a dielectric barrier discharge (DBD) is the dielectric barrier placed between the electrodes. In the present work, the influence of the dielectric barrier to the properties of a DBD in air was investigated. Spectroscopic characterization of the DBD and electrical measurements were carried out. It was shown that the efficiency of a DBD can be considerably improved by optimizing the dielectric barrier. The dielectric material should possess an appropriate relative permittivity and thickness. For thin dielectric barriers, a high secondary emission coefficient becomes important. Additionally, the use of only one dielectric barrier is advantageous. © 2010 American Institute of Physics.

Gerhard C.,Clausthal University of Technology | Gerhard C.,HAWK University of Applied Sciences and Arts | Tasche D.,HAWK University of Applied Sciences and Arts | Bruckner S.,Clausthal University of Technology | And 3 more authors.
Optics Letters | Year: 2012

In this Letter, we report on the near-surface modification of fused silica by applying a hydrogenous atmospheric pressure plasma jet at ambient temperature. A significant decrease in UV-transmission due to this plasma treatment was observed. By the use of secondary ion mass spectroscopy, the composition of the plasma-modified glass surface was investigated. It was found that the plasma treatment led to a reduction of a 100 nm thick SiO2 layer to SiOx of gradual depth-dependent composition. For this plasma-induced layer, depth-resolved characteristic optical parameters, such as index of refraction and dispersion, were determined. Further, a significant plasma-induced increase of the concentration of hydrogen in the bulk material was measured. The decrease in transmission is explained by the plasma-induced near-surface formation of SiOx on the one hand and the diffusion of hydrogen into the bulk material on the other hand. © 2012 Optical Society of America.

Bieker D.,HAWK University of Applied Sciences and Arts | Rust S.,HAWK University of Applied Sciences and Arts
Silva Fennica | Year: 2010

Accurate estimates of the water conducting sapwood area are necessary to scale sapflow measurements to tree and stand level transpiration. We tested a non-destructive method, electric resistivity tomography (ERT), to estimate the area of conductive sapwood in 9 Pinus sylvestris L. trees in lower Saxony, Germany. Tomograms were compared to cross-sections stained with benzidine after harvesting. All tomograms displayed a distinct pattern of low resistivity at the stem perimeter and high resistivity in the stem centre with a steep increase in resistivity in between, assumed to indicate the transition from sapwood to heartwood. The tomograms showed a sapwood width 2 cm smaller than the staining method. This indicates that staining methods overestimate the amount of active sapwood because when heartwood is formed, moisture content decreases before extractive contents reach levels visible by staining. The ERT method is a new powerful method for the non-destructive estimation of sapwood and heartwood width.

Klarhofer L.,Clausthal University of Technology | Viol W.,Laser Laboratorium Gottingen e.V. | Viol W.,HAWK University of Applied Sciences and Arts | Maus-Friedrichs W.,Clausthal University of Technology
Holzforschung | Year: 2010

Cellobiose and lignin were plasma treated in synthetic air and argon using a dielectric barrier discharge at atmospheric pressure. Changes due to the plasma modification of the surfaces were studied by the techniques of X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy, and metastable impact electron spectroscopy. The combination of these techniques makes it possible to distinguish between hydroxyl and carbonyl groups at the surface, on the one hand, and conjugated and non-conjugated carbon bonds, on the other hand. This type of differentiation would not be easily possible based on XPS alone, even at very high resolution. The plasma treatment in oxygen containing atmospheres oxidizes the lignin surface by the generation of hydroxyl, carbonyl, and carboxyl groups and reduces cellulose surfaces by the degradation of hydroxyl groups and the formation of double bonds between carbon and oxygen. The plasma treatment in argon leads to the reduction of both lignin and cellulose by the formation of double bonds under degradation of hydroxyl groups. Copyright © by Walter de Gruyter.

Rust S.,HAWK University of Applied Sciences and Arts
Urban Forestry and Urban Greening | Year: 2014

Inventory data of six urban tree species from seven cities across Germany were used to model regional variation of height growth, allometric scaling, and slenderness over a wide range of size and age using a quantile regression approach. Variation within and between species and cities was large. Height did not reach an asymptote but declined at higher ages, presumably because of reduction cuts. Allometric scaling and slenderness varied with wind climate, modulus of elasticity, and coefficient of drag. Our data do not support the use of a threshold value of slenderness in tree risk assessment. © 2013 Elsevier GmbH.

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