Hochschule für Technik und Umwelt FHNW
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Publikation Experimental investigations of size effects in thin copper foils(Springer Dordrecht, 2004) Simons, Gerd; Dual, Jürg; Weippert, Christina; Villain, Jürgen; Ahzi, S.; Cherkaoui, M.; Khaleel, M. A.; Zbib, H. M.; Zikry, M. A.; Lamatina, B.This work deals with the characterization of the deformation behavior of thin copper foils with the goal of investigating size effects. Tensile tests are performed with specimens, which possess a comparable microstructure, a constant thickness/width and width/length ratio whereas the thickness varies from 10 to 250 m. Results show a transition from ductile to a macroscopically “brittle” behavior in the range of about 20 m.04B - Beitrag KonferenzschriftPublikation Size effects in tensile testing of thin cold rolled and annealed Cu foils(Elsevier, 01/2006) Simons, Gerd; Weippert, Ch.; Dual, Jürg; Villain, Jürgen01A - Beitrag in wissenschaftlicher ZeitschriftPublikation Three-dimensional microstructure of thin copper foils revealed by ion beam cutting and electron backscatter diffraction (EBSD)(Trans Tech Publications, Ltd., 07/2005) Simons, Gerd; Kunze, Karsten; Hauffe, W.; Dual, Jürg; Esling, C.; Humbert, M.; Schwarzer, R.A.; Wagner, F.Tensile testing of thin rolled copper foils with thickness ranging from 10 to 250 µm shows a dependence of the fracture strain with respect to the thickness of the foils [G. Simons et al., in Solid Mechanics and its Applications, Vol. 114 (2004), pp. 89-96]. To understand the influence of the microstructure in the foils on this phenomenon the microtexture is investigated by orientation mapping through electron backscatter diffraction (EBSD). As the samples are rather small standard preparation techniques do not apply. Two methods are described which allow the investigation of different section cuts of the samples: Cross sections of the samples were produced by ion beam cutting with a wide beam of 7 keV Kr ions. Internal planes parallel to the specimen surface were made accessible by wet etching. The as-received material possesses a very strong texture consisting mostly of the cube component and some remnants of a previous rolling texture. Specimens tested in a tensile test do not show major microstructural changes compared to virgin samples. After a heat treatment at 300°C the cube texture has significantly weakened in favour of revived rolling components, and the fracture strain increased about ten times relative to the as-is material.04B - Beitrag KonferenzschriftPublikation Mechanical properties of MEMS structures(2005) Dual, Jürg; Simons, Gerd; Villain, Jürgen; Weippert, Chr.Mechanical characterization of MEMS (micro electromechanical systems) materials is increasingly important in view of improving reliability and assessing the life time of new miniaturized devices. In this paper first a number of testing methods are described. These methods include tensile, torsion and fatigue testing of specially designed microstructures. Difficulties arise from manufacturing and handling of small structures and the determination of its geometrical dimensions which directly affect the accuracy of material parameters extracted from the experiments. In addition, the measurement of mechanical parameters like small forces and torques or strains poses a challenge. This paper focuses on size effects in rolled copper foils of thickness between 10 and 250 microns as determined from tensile testing. Tensile testing was chosen as a testing method in order to minimize strain gradients. Depending on the size the copper foils are tested in a classical setup or in a special tensile apparatus which is adapted to the small size of the specimens. The special setup consists of a balance to measure the applied force. The specimens are strained with a lead screw driven translation stage. In order to take care of slip and elasticity in the fixations the strain is measured optically directly on the sample using a least square template matching algorithm. It was found that depending on the thickness of the foils the average fracture strain decreases from about 15% down to .5 % for the 250 and 10 micron specimens, respectively. In order to find a reason for this dramatic change many efforts have been undertaken in order to characterize the specimens more precisely. The microstructure of the samples was determined using various methods including conventional micrographs, hardness measurements and X-ray diffraction.04B - Beitrag KonferenzschriftPublikation Mechanical size effects in thin copper foils. An experimental study(ETH Zürich, 2004) Simons, Gerd; Dual, Jürg; Villain, JürgenThe goal of this work is the investigation of the effect of several size parameters on the mechanical behavior of thin copper foils in tensile testing, in particular the question is whether a smaller sample has a different mechanical behavior than a larger one. Attention is paid to the most relevant size parameter, the thickness, and the influence of the microstructure of the foils, a factor which has not been accounted for systematically in literature up to now. Copper foils with 10, 20 and 34 μm thickness are tensile tested in-house, thicker foils (50, 100 and 250 μm) by a project partner (Laboratory of Materials for Mecha- tronics and Electrical Engineering, University of Applied Sciences Augsburg, Prof. Villain). For the tensile tests, a new setup was built which extends a previous setup developed by [Mazza, 1997] and allows for an automatic testing of the samples at a controlled strain rate. The standard strain rate applied is ˙≤ = 10−4 1/s. Samples with a dogbone shape are tested, their geometry is scaled according to the thick- ness. The samples are produced by wet etching of rolled and electrodeposited copper foils (standard sample type, “as-received samples”). Some of the samples are heat treated after etching (“heat treated samples”). As the microstructure of a crystalline solid has a significant influence on its mechanical behavior it is characterized in detail. Different techniques such as met- allography, X-ray diffraction and electron backscatter diffraction are applied for this task. The rolled samples (10 and 20 μm thick) have a strong cube texture with elongated grains with an oblate cross-section (typical length 100 μm, small diameter 5 μm, long diameter 30 μm). The electrodeposited samples have a columnar grain structure with a weak fibre texture. Heat treatment changes the microstructure of the rolled foils considerably. The grains are equi-axed with an average diameter of 15 μm. Thus, the 10 and 20 μm heat treated foils have only 1-2 grains per thick- ness. Rolling texture components with 〈111〉 parallel to the rolling direction form the preferred orientations, some grains are still in cube orientation. The most important result of the tensile tests is that the thickness of the foils has an influence on the mechanical behavior in the size regime studied. When the thickness is reduced from 250 to 10 μm the fracture strain decreases for the as-received foils from approximately 20% to 0.2% and for the samples with heat treatment from 35% to 15%. The tensile strength increases with smaller thickness for the as-received samples if the surface roughness is taken into account for the stress calculations (the surface roughness of the thinner foils is a considerable fraction of the total thickness). The 10 μm as-received foils have the highest tensile strength which is 400 MPa. The heat treated samples do not show a pronounced size dependence of the tensile strength. xi To explain the effects observed, in particular the size dependence of the fracture strain and the tensile strength as well as the low fracture strain of the 10 and 20 μm as-received foils (in the order of 0.2%), the surfaces of the fractured samples and the microstructure of the samples are analyzed in detail. The analysis of the fracture surfaces shows for all samples a failure by necking in thickness direction. Meaning that samples which show macroscopically a low fracture strain, i.e. a behavior which is typical for brittle materials, display micro- scopically large plastic deformations, i.e. a ductile behavior. This discrepancy can be explained by a strongly localized deformation: a sample fails as soon as the stresses in a cross-section reach a critical value; there is hardly any redistribution of strain, which is typical for ductile material behavior. This is also reflected by an analysis of the microstructure after the tensile test. The as-received samples do not show large microstructural changes with respect to the unloaded state except at the location of breakage. In comparison to that, the heat treated foils show a moderate elongation of the grains and a strong increase in surface roughness after tensile testing. This increase can be explained by the formation of slip bands at the surface and by the rotation of grains out of their original plane. The rotation of grains is facilitated by the low number of grains per thickness in the heat treated samples, as grains, which are in contact with the surface, can deform more easily. The general trend that thinner samples have a smaller fracture strain is believed to be caused by a combination of various mechanisms. Firstly, local reductions in cross-section by an imperfect sample geometry and by statistically random, plastic deformations are more critical for thinner samples. Secondly, surface grains can deform more easily and hence the number of grains per thickness has an impact on the mechanical behavior. Thirdly, in thinner samples there are less grains which could result in a smaller number of activated gliding systems. Fourthly, dislocations cannot build up large plastic deformations in small grains. The influence of other parameters such as width and length of a sample, strain rate and orientation with respect to the rolling direction were studied as well. In comparison with the thickness, they only have a small influence on the mechanical behavior of the foils tested. It has to be stressed that the size dependence found in this work was measured in a tensile test, i.e. a test where no considerable strain gradients occur. Experi- mental verification of size effects in loading situations, where no strain gradients are present, is scarce (e.g. [Weiss et al., 2002] and [Espinosa et al., 2004]). This work also shows that, for the explanation of the effects observed, a thorough examination of the microstructure of the samples tested is mandatory. As the influence of many parameters has to be taken into account in detail, the experimental study of size effects turns out to be a complicated topic. Besides the experimental details, this work shows the characterization results for the microstructure of the copper foils before and after tensile testing as well as the tensile test results for various parameters. The influence of many factors on the mechanical behavior of thin foils is discussed thoroughly and the tensile test behavior is explained by means of a simple geometrical model.11 - Studentische ArbeitPublikation Growth and structural change of combustion aerosols at high relative humidity(American Chemical Society, 01.12.1995) Weingartner, Ernest; Baltensperger, Urs; Burtscher, Heinz01A - Beitrag in wissenschaftlicher ZeitschriftPublikation An analysis of accreted drop sizes and mass on rimed snow crystals(American Meteorological Society, 01.06.1994) Mosimann, Lukas; Weingartner, Ernest; Waldvogel, AlbertA classification of the degree of riming of snow crystals is established, based on a visual rating and expressed on a scale ranging from 0 (unrimed) to 5 (graupel). For 84 individual snow crystals collected from six cases over three winters, the size distribution of the accreted cloud droplets is studied. Relationships between the degree of riming and quantitative parameters, namely, the total number and mass of accreted cloud droplets and especially the rimed mass fraction of a snow crystal, are presented. These new relationships may be a powerful tool to estimate the aforementioned parameters by rating only the degree of riming of a snow crystal.01A - Beitrag in wissenschaftlicher ZeitschriftPublikation Hygroscopic properties of carbon and diesel soot particles(Elsevier, 08/1997) Weingartner, Ernest; Burtscher, Heinz; Baltensperger, UrsLaboratory experiments were performed in order to study the hygroscopic properties of freshly produced carbon and diesel soot particles at subsaturations (i.e. at relative humidity < 100%). Wetted carbon aggregates collapsed to a more compact structure. In contrast, the diesel combustion particles exhibited a much smaller restructuring combined with condensational growth. The hygroscopicity of the diesel particles was enhanced when the sulfur content of the fuel was increased or when the particles were subjected to an ozone and UV pre-treatment. By extrapolating the data with a Köhler model critical supersaturations were calculated. Freshly emitted combustion particles are unlikely to act as cloud condensation nuclei whereas an aging of the particles will enhance their nucleation ability.01A - Beitrag in wissenschaftlicher ZeitschriftPublikation Heterogeneous chemical processing of 13NO2 by monodisperse carbon aerosols at very low concentrations(American Chemical Society, 19.09.1996) Kalberer, Markus; Tabor, K.; Ammann, Markus; Parrat, Yves; Weingartner, Ernest; Piguet, D.; Rössler, E.; Jost, Dieter T.; Türler, Andreas; Gäggeler, Heinz W.; Baltensperger, UrsThe heterogeneous reaction of NO2 with different carbon aerosol particles was investigated in situ. The NO2 was labeled with the β+-emitter 13N (half-life 10.0 min) which allowed application of NO2 at very low concentrations. The carbon aerosol was either produced by a spark discharge generator using graphite electrodes or by a brush generator resuspending commercial soot material. Monodisperse size cuts between 50- and 490-nm diameter were selected and mixed with the 13NO2. After a defined reaction time, the different reaction products were separated by means of selective traps and detected on-line by γ-spectrometry. A sticking coefficient for chemisorption of NO2 between 0.3 × 10ˉ⁴ and 4.0 × 10ˉ⁴ and a rate constant for the reduction of adsorbed NO2 to NO(g) between 4.0 × 10 ˉ⁴ and 9.4 × 10 ˉ⁴ /s were determined for both aerosols. The sticking coefficient obtained in this study in situ with aerosol particles is 2 orders of magnitudes smaller than the uptake coefficient recently reported with bulk carbon material.01A - Beitrag in wissenschaftlicher ZeitschriftPublikation Seasonal and diurnal variation of aerosol size distributions (10(Wiley, 01.11.1999) Weingartner, Ernest; Nyeki, Stephan; Baltensperger, UrsDuring an extended field campaign at a high-alpine site (Jungfraujoch; 3580 m asl, Switzerland) from March 1997 to May 1998, the particle number size distribution (diameter D = 18–750 nm) and number concentration N (D>10 nm) were measured on a continuous basis. The number size distribution was dominated by particles with D<100 nm, while most of the surface area was in the accumulation mode size range (0.1–1 μm). Average size distributions exhibited a distinct bimodal shape which is generated and maintained by cloud processes and is less distinct in the free troposphere. Fitted modal diameters and standard deviations σ of the Aitken (20–100 nm) and accumulation modes were surprisingly constant throughout the year (DAit = 43±3 nm; DAcc = 140±6 nm; σAit = 2.13±0.11; σAcc = 1.61±0.03). The relative number concentrations in both modes are responsible for the seasonally observed in the shape of the size distribution. The high seasonality of NAcc with summer and winter values of ∼260 and ∼40 cmˉ³, respectively, is mainly due to transport of planetary boundary layer air to the station. In contrast, new particle formation is responsible for the high concentrations of nucleation mode particles (D<20 nm) which exhibited a maximum during the winter months. The relatively low seasonality found for NAit (summer, winter values: ∼530, ∼310 cmˉ³) is due to gas-to-particle conversion as well as transport processes. An analysis showed that a significant fraction of nucleation mode particles were formed by photochemical reactions.01A - Beitrag in wissenschaftlicher Zeitschrift