B. Kauerauf’s scientific contributions

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Publications (9)


Analysis of Diffusive Cellular Patterns in Directional Solidification of Bulk Samples
  • Article

January 2004

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20 Reads

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14 Citations

Metallurgical and Materials Transactions A

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Stephan Rex

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Bernd Kauerauf

The onset and evolution of cellular patterns at the solid-liquid interface were investigated in directional solidification. A bulk sample of the diluted transparent alloy succinonitrile-acetone was used and the experiments were carried out under diffusive conditions onboard a Space Shuttle mission. The experimental parameters and material properties were selected to cause a supercritical planar-to-cellular transition to small-amplitude cells. Distributions of cell spacings and cell neighbors were determined from CCD images as a function of time. In the steady-state regime, a hexagonal arrangement on the interface with approximately 50 pct defects was found. The average cell spacing of 210 µm was compared to different models, of which the Lu/Hunt model concurred best. A minimal-spanning-tree analysis was carried out to examine the disorder of the cellular patterns as a function of time. The resulting order parameters were compared to those of reference patterns generated from Monte-Carlo simulations. In these simulations cellular organization is often modeled by planar lattices disturbed with Gaussian noise. We have extended this model by the constraint of a minimum distance between the lattice points in analogue to the experimentally found distinct minimum spacings. With this new inhibition model, enhanced quantitative agreement with the experimental cellular patterns was found.


Directional solidification of cellular arrays in a transparent organic alloy

February 2002

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20 Reads

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12 Citations

Advances in Space Research

A new experimental database for supercritical cellular solidification close to the planar-cellular transition in bulk samples is presented. Using the transparent alloy Succinotrile-0.075 wt % Acetone a series of directional solidification experiments were performed under free-falling conditions of space to provide purely diffusive growth. The cellular patterns observed in top-view were analysed concerning their main quantitative parameter primary spacing. It is found that no sharp wavelength selection occurs. For the primary spacing as well as for the number of neighbours of each individual cell the patterns reveal wide distributions which remain statistically unchanged in the final state of the experiments. The theoretical evaluation of the cellular patterns focused on a comparison of linear stability analysis with experimental results. While cellular patterns in a subcritical regime are controlled by higher harmonics the linear mode is sufficient to explain the observed database in the supercritical regime.


Directional cellular growth of succinonitrile–0.075 wt% acetone bulk samples: Part 1: Results of space experiments

February 2001

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20 Reads

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24 Citations

Journal of Crystal Growth

Interface pattern evolution during directional solidification has been investigated in the microgravity environment of a space shuttle mission providing diffusive conditions for cellular pattern evolution. A model transparent alloy of succinonitrile–acetone was used in these studies for in situ observation of the dynamic process of pattern evolution. In addition to the microgravity environment, alloy composition and growth condition experiment parameters were selected in such a way that the planar-to-cellular transition was supercritical, and pattern evolution was examined close to the transition velocity that gave rise to very small amplitude cells. Since the supercritical transition shows continuous increase in amplitude with velocity, non-linear effects in wavelength selection are small. This means that linear theory could be used to interpret the results. The evolution of primary spacing and the order of cellular arrangements were characterized as a function of time for three different velocities. No sharp wavelength selection in primary spacing was observed in the experimental results, and the distribution of steady-state spacing was characterized for each velocity. Wavelength selection for supercritical transition near the transition point can also be explained by stability analysis.


Directional cellular growth of succinonitrile–0.075wt% acetone bulk samples Part 2: Analysis of cellular pattern

February 2001

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90 Reads

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18 Citations

Journal of Crystal Growth

Cellular pattern formation in the system succinonitrile–0.075 wt% acetone in a supercritical regime is analyzed on the basis of a set of purely diffusive space experiments in bulk samples. Order–disorder investigations using the minimal-spanning-tree approach yield steady-state patterns for the microgravity experiments of an order which suggests a honeycomb structure disturbed by Gaussian noise. The most important morphology parameter, primary spacing of the cellular array, is critically discussed in comparison to the established theoretical models. It is found that analytical models fail to describe cell sizes in supercritical cellular growth. In contrast, excellent agreement is demonstrated using the numerical model of Hunt. This implies that modelling supercritical cellular growth requires consideration of interacting solute diffusion fields ahead of the cells and free development of cell shapes.


Pattern Formation during Directional Solidification of Succinonitrile-Acetone under Microgravity Conditions
  • Article
  • Full-text available

January 2001

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32 Reads

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Cellular Growth in Transparent Model Substances under Microgravity and Ground Based Conditions

January 2000

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7 Reads

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2 Citations

The influence of convection on the formation of cellular patterns is demonstrated by comparison of purely diffusive space experiments with ground-based reference experiments. The solid-liquid interfaces of the space experiments are covered by regular cellular patterns. Patterns obtained on ground reveal strong irregularities. Despite of the differences in the regularity of the patterns only slight deviations of the quantitative parameter primary spacing between ground and space experiments are found. Linear stability analysis in combination with a stagnant film model is used to include convective effects into quantitative stability curves as well as to predictions of the primary spacing.



Planar to cellular transition in the system succinonitrile–acetone during directional solidification of a bulk sample

October 1998

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26 Reads

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37 Citations

Journal of Crystal Growth

Directional solidification with a cellular solid–liquid interface morphology is characterized by two typical length scales – the size of the cells and the thickness of the solutal boundary layer ahead of the growing interface. The interaction of the solutal field with the morphology is investigated by using the model alloy succinonitrile–acetone which allows by its transparency the in situ observation of the interface structure. To avoid wall effects and decouple the solidification parameters temperature gradient and solidification velocity a rod-like bulk sample and a Bridgman Stockbarger-type gradient furnace are used. In the experiments the development of the interface morphology is affected by buoyancy driven melt convection which stabilizes significantly the planar morphology. At the planar to cellular transition quasi-hexagonal and elongated cells exist simultaneously, both at grain boundaries as well as inside the grains. With increasing solidification length a quasi-hexagonal pattern covers the interface which shows a random rearrangement of the cells. The size of the cells varies in a wide range and no wavelength selection is found.


Optical image formation of a transparent solid-liquid interface during the MOMO experiment on board STS-84

January 1997

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9 Reads

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3 Citations

Microgravity Science and Technology

Transparent substances as model for metallic alloys are a common tool for real time studies of solidification processes. Within the experiment MOMO the transparent alloy succinonitrile-acetone was directionally solidified using a Bridgman furnace. The goal of the experiment is to investigate quantitatively and in situ the growing cellular pattern of an extended solidification front (bulk sample) under reduced gravity conditions. The required generation of images of the solidification morphology in a top view is subjected to several constraining conditions: The cellular pattern to be observed is a phase object, thus dark field illumination has to be used. The available power is strongly limited and the image sensor has to accept varying illumination intensities possibly caused by scattering structures in the solid. The realized optical setup uses LED illumination which couples light into the glass wall of the experiment cell. The imaging is achieved by using a rod lense endoscope and a CCD camera with 12bit ADC. Ground test and flight images confirm the high performance of the optical system. Cellular solidification structure even with an inhomogeneous background illumination can be imaged with high resolution.

Citations (6)


... Directional solidification experiments under low gravity conditions provide a unique framework for investigating microstructure development in spatially extended sample geometries, with negligible convection effects and under wellcontrolled conditions of growth rate, temperature gradient, and alloy composition [7][8][9] . The Directional Solidification Insert (DSI) is dedicated to in situ and real-time characterization of the dynamical selection of the solid-liquid interface morphology in bulk samples of transparent materials. ...

Reference:

Solidification furnace for in situ observation of bulk transparent systems and image analysis methods
Directional cellular growth of succinonitrile–0.075 wt% acetone bulk samples: Part 1: Results of space experiments
  • Citing Article
  • February 2001

Journal of Crystal Growth

... Model alloys with low melting temperatures are particularly suitable for the implementation of automated solidification instruments intended for being installed onboard microgravity facilities [27][28][29] . There is a fundamental motivation for carrying out solidification studies in reduced gravity. ...

Cellular Growth in Transparent Model Substances under Microgravity and Ground Based Conditions

... Experimental values of cellular spacings have been published by several authors. Rex et al. [344] studied primary cell spacings close to the planar-cellular transition in directional growth of a dilute succinonitrileacetone alloy. The experiments were performed under purely diffusive growth conditions and the interface morphology was observed in top-view. ...

Directional solidification of cellular arrays in a transparent organic alloy
  • Citing Article
  • February 2002

Advances in Space Research

... The electromagnetic force periodically compresses the melt, which leads to its spreading. With directional crystallization, the melt flow can contribute to the stability of the S/L interface and delay the transition from the planar growth regime to the cellular one [93,94] and from the cellular to the dendritic growth regime [94,95]. Consequently, the facilitation of the passage of the solution by the melt flow also cannot explain the reason for the instability of the interface caused by the application of a pulsed magnetic field. ...

Planar to cellular transition in the system succinonitrile–acetone during directional solidification of a bulk sample
  • Citing Article
  • October 1998

Journal of Crystal Growth

... [4][5][6] Directional solidification experiments under low gravity conditions provide a unique framework for investigating microstructure development in spatially extended sample geometries, with negligible convection effects and under well-controlled conditions of growth rate, temperature gradient, and alloy composition. [7][8][9] The Directional Solidification Insert (DSI) is dedicated to in situ and real-time characterization of the dynamical selection of the solid-liquid interface morphology in bulk samples of transparent materials. It was developed by the French Space Agency (CNES) in the frame of the DEvice for the study of Critical LIquids and Crystallization (DECLIC) project. ...

Analysis of Diffusive Cellular Patterns in Directional Solidification of Bulk Samples
  • Citing Article
  • January 2004

Metallurgical and Materials Transactions A

... Directional solidification experiments under low gravity conditions provide a unique framework for investigating microstructure development in spatially extended sample geometries, with negligible convection effects and under wellcontrolled conditions of growth rate, temperature gradient, and alloy composition [7][8][9] . The Directional Solidification Insert (DSI) is dedicated to in situ and real-time characterization of the dynamical selection of the solid-liquid interface morphology in bulk samples of transparent materials. ...

Directional cellular growth of succinonitrile–0.075wt% acetone bulk samples Part 2: Analysis of cellular pattern
  • Citing Article
  • February 2001

Journal of Crystal Growth