Improved Crystal Quality by Detached Solidification in Microgravity

Directional solidification in microgravity has often led to ingots that grew with little or no contact with the ampoule wall. When this occurred, crystallographic perfection was usually greatly improved -- often by several orders of magnitude. Unfortunately, until recently the true mechanisms underlying detached solidification were unknown. As a consequence, flight experiments yielded erratic results. Within the past four years, we have developed a new theoretical model that explains many of the flight results. This model gives rise to predictions of the conditions required to yield detached solidification, both in microgravity and on earth. A discussion of models of detachment, the meniscus models and results of theoretical modeling, and future plans are presented

Modelling directional solidification

This grant, NAG8-831, was a continuation of a previous grant, NAG8-541. The long range goal of this program has been to develop an improved understanding of phenomena of importance to directional solidification, in order to enable explanation and prediction of differences in behavior between solidification on Earth and in space. Emphasis in the recently completed grant was on determining the influence of perturbations on directional solidification of InSb and InSb-GaSb alloys. In particular, the objective was to determine the influence of spin-up/spin-down (ACRT), electric current pulses and vibrations on compositional homogeneity and grain size

Influence of convection on microstructure

The primary motivation for this research was to determine the cause for space processing altering the microstructure of some eutectics, especially the MnBi-Bi eutectic. Four primary hypotheses were to be tested under this current grant: (1) A fibrous microstructure is much more sensitive to convection than a lamellar microstructure, which was assumed in our prior theoretical treatment. (2) An interface with one phase projecting out into the melt is much more sensitive to convection than a planar interface, which was assumed in our prior theoretical treatment. (3) The Soret effect is much more important in the absence of convection and has a sufficiently large influence on microstructure that its action can explain the flight results. (4) The microstructure is much more sensitive to convection when the composition of the bulk melt is off eutectic. These hypotheses were tested. It was concluded that none of these can explain the Grumman flight results. Experiments also were performed on the influence of current pulses on MnBi-Bi microstructure. A thorough review was made of all experimental results on the influence of convection on the fiber spacing in rod eutectics, including results from solidification in space or at high gravity, and use of mechanical stirring or a magnetic field. Contradictory results were noted. The predictions of models for convective influences were compared with the experimental results. Vigorous mechanical stirring appears to coarsen the microstructure by altering the concentration field in front of the freezing interface. Gentle convection is believed to alter the microstructure of a fibrous eutectic only when it causes a fluctuating freezing rate with a system for which the kinetics of fiber branching differs from that for fiber termination. These fluctuations may cause the microstructure to coarsen or to become finer, depending on the relative kinetics of these processes. The microstructure of lamellar eutectics is less sensitive to freezing rate fluctuations and to gentle convection

Use of Microgravity to Control the Microstructure of Eutectics

This grant began in June of 1996. Its long term goal is to be able to control the microstructure of directionally solidified eutectic alloys, through an improved understanding of the influence of convection. The primary objective of the projects in the present grant is to test hypotheses for the reported influence of microgravity on the microstructure of eutectics. The prior experimental results on the influence of microgravity on the microstructure of eutectics have been contradictory. With lamellar eutectics, microgravity had a negligible effect on the microstructure. Microgravity experiments with fibrous eutectics sometimes showed a finer microstructure and sometimes a coarser microstructure. Most research has been done on the MnBi/Bi rod-like eutectic. Larson and Pirich obtained a two-fold finer microstructure both from microgravity and by use of a magnetic field to quench buoyancy-driven convection. Smith, on the other hand, observed no change in microgravity. Prior theoretical work at Clarkson University showed that buoyancy-driven convection in the vertical Bridgman configuration is not vigorous enough to alter the concentration field in front of a growing eutectic sufficiently to cause a measurable change in microstructure. We assumed that the bulk melt was at the eutectic composition and that freezing occurred at the extremum, i.e. with minimum total undercooling at the freezing interface. There have been four hypotheses attempting to explain the observed changes in microstructure of fibrous eutectics caused by convection: I .A fluctuating freezing rate, combined with unequal kinetics for fiber termination and branching. 2. Off-eutectic composition, either in the bulk melt due to an off-eutectic feed or at the freezing interface because of departure from the extremum condition. 3. Presence of a strong habit modifying impurity whose concentration at the freezing interface would be altered by convection. At the beginning of the present grant, we favored the first of these hypotheses and set out to test it both experimentally and theoretically. We planned the following approaches: I .Pass electric current pulses through the MnBi/Bi eutectic during directional solidification in order to produce an oscillatory freezing rate. 2. Directionally solidify the MnBi/Bi eutectic on Mir using the QUELD II gradient freeze furnace developed by Professor Smith at Queen's University. 3. Select another fibrous eutectic system for investigation using the Accelerated Crucible Rotation Technique to introduce convection. 4. Develop theoretical models for eutectic solidification with an oscillatory freezing rate. Because of the problems with Mir, we substituted ground-based experiments at Queen's University with QUELD II vertical and horizontal, with and without vibration of the furnace. The Al-Si system was chosen for the ACRT experiments. Three related approaches were used to model eutectic solidification with an oscillatory freezing rate. A sharp interface model was used to calculate composition oscillations at the freezing interface in response to imposed freezing rate oscillations

Modeling of Detached Solidification

Our long term goal is to develop techniques to achieve detached solidification reliably and reproducibly, in order to produce crystals with fewer defects. To achieve this goal it is necessary to understand thoroughly the physics of detached solidification. It was the primary objective of the current project to make progress toward this complete understanding. 'Me products of this grant are attached. These include 4 papers and a preliminary survey of the observations of detached solidification in space. We have successfully modeled steady state detached solidification, examined the stability of detachment, and determined the influence of buoyancy-driven convection under different conditions. Directional solidification in microgravity has often led to ingots that grew with little or no contact with the ampoule wall. When this occurred, crystallographic perfection was usually greatly improved -- often by several orders of magnitude. Indeed, under the Soviet microgravity program the major objective was to achieve detached solidification with its resulting improvement in perfection and properties. Unfortunately, until recently the true mechanisms underlying detached solidification were unknown. As a consequence, flight experiments yielded erratic results. Within the past three years, we have developed a new theoretical model that explains many of the flight results. This model gives rise to predictions of the conditions required to yield detached solidification

Use of Microgravity to Control the Microstructure of Eutectics

The long term goal of this project is to be able to control the microstructure of directionally solidified eutectic alloys, through an improved understanding of the influence of convection. Prior experimental results on the influence of microgravity on the microstructure of fibrous eutectics have been contradictory. Theoretical work at Clarkson University showed that buoyancy-driven convection in the vertical Bridgman configuration is not vigorous enough to alter the concentration field in the melt sufficiently to cause a measurable change in microstructure when the eutectic grows at minimum supercooling. Currently, there are four other hypotheses that might explain the observed changes in microstructure of fibrous eutectics caused by convection: (1) Disturbance of the concentration boundary layer arising from an off-eutectic melt composition and growth at the extremum; (2) Disturbance of the concentration boundary layer of a habit-modifying impurity; (3) Disturbance of the concentration boundary layer arising from an off-eutectic interfacial composition due to non-extremum growth; and (4) A fluctuating freezing rate combined with differences in the kinetics of fiber termination and fiber formation. We favor the last of these hypotheses. Thus, the primary objective of the present grant is to determine experimentally and theoretically the influence of a periodically varying freezing rate on eutectic solidification. A secondary objective is to determine the influence of convection on the microstructure of at least one other eutectic alloy that might be suitable for flight experiments

A longitudinal investigation of the relationship between unconditional positive self-regard and posttraumatic growth

The present study investigated whether unconditional positive self-regard (UPSR) is associated with subsequent posttraumatic growth (PTG) following the experience of a traumatic life event. A total of 143 participants completed an online questionnaire to assess the experience of traumatic life events, posttraumatic stress, well-being and UPSR (Time 1). Three months later, 76 of the participants completed measures of well-being and perceived PTG (Time 2). Analyses were conducted to test for association between UPSR at Time 1 and perceptions of PTG at Time 2. Results showed that higher UPSR at T1 was associated with higher perceived PTG at Time 2. To measure actual growth, individual differences in well-being were computed between Time 1 and Time 2. Results showed that higher UPSR at T1 was associated with higher actual PTG. Implications of these findings are discussed and future directions for research in this area considered. Specifically, results are consistent with a person-centered understanding of therapeutic approaches to the facilitation of PT

Ячмени с гладкими остями

http://tartu.ester.ee/record=b1727444~S1*es

PERENCANAAN PENGEMBANGAN BANDAR UDARA KUABANG KAO KABUPATEN HALMAHERA UTARA PROVINSI MALUKU UTARA

Kabupaten Halmahera Utara dengan Ibukota Kabupaten Tobelo memiliki beberapa bandar udara antara lain Bandar Udara Gamarmalamo (Galela),Bandar Udara Kuabang (Kao) dan Bandar Udara Boso yang terletak di area operasional penggalian Mineral oleh PT. Nusa Halmahera Mineral, salah satu bandar udara yaitu Bandar Udara Kuabang Kao merupakan pintu masuk transportasi udara yang sangat penting di Kabupaten Halmahera Utara, namun sampai saat ini Bandar Udara Kuabang Kao masih memiliki kelemahan yang sangat signifikan baik disisi udara maupun sisi darat untuk itu dirasakan perlu untuk melakukan suatu pengembangan agar mampu menjawab kebutuhan transportasi udara di Kabupaten Halmahera Utara untuk masa yang akan datang. Penelitian yang dilakukan menggunakan data primer seperti data klimatologi, keadaan topografi dan existing bandara serta data sekunder seperti data arus lalu lintas udara selama lima tahun, data karakteristik pesawat dan data penduduk yang dijadikan acuan sebagai dasar perencanaan pengembangan Bandar Udara Kuabang Kao. Perencanaan pengembangan Bandar Udara Kuabang Kao meliputi Runway, Taxiway, Apron yang mengacu pada standar ICAO, sementara untuk perencanaan perkerasan mengacu pada standar PCA dan FAA, serta terminal area yang meliputi terminal penumpang, gudang dan area parkir kendaraan yang dianalisa berdasarkan hasil analisa perkembangan arus lalu lintas pesawat udara dan pergerakan pesawat serta penumpang pada jam sibuk di masa yang akan datang. Dari hasil analisis diperolehkan panjang Runway yang dibutuhkan adalah 1670 meter, yang berada pada orientasi 180–360, untuk perkerasan lentur Runway dan Taxiway didapat ketebalan adalah 34cm, luas Apron 165mx67m, tebal perkerasan Rigid pada Apron 18 cm, luas total terminal penumpang 6290m2, area parkir kendaraan 1001 m2 (50x20 m) serta luas gudang 22.03m2. Kata kunci: Bandar Udara Kuabang Kao, perencanaan, pengembangan, Runway, Taxiway, Apron,Terminal penumpang

Numerical Simulation of the Effect of Gravity on Weld Pool Shape A high gravitational field strongly affects the outward flow of the weld pool

ABSTRACT. Understanding the physical phenomena involved in the welding process is of substantial value to improving the weldability of materials. The intense heat and the arc inherent in fusion welding make direct experimental observation of the weld pool behavior rather difficult. Thus, numerical models that can predict the processes involved have become an invaluable tool for studying welding. One of the major factors affecting the motion within the molten weld pool is the gravity-driven buoyancy force. This force can act to oppose or enhance the Marangoni and/or electromagnetic driven convective flow within the weld pool. To study the effect of gravity on weld pool processes, a series of numerical simulations was performed. It was found that higher gravitational fields tend to enhance the convective flow within the weld pool and thus affect the heat transfer, the depth and width of the two phase region, and the pool depth-towidth ratio