Design, fabrication and test of prototype furnace for continuous growth of wide silicon ribbon

A program having the overall objective of growing wide, thin silicon dendritic web crystals quasi-continuously from a semi-automated facility is discussed. The design considerations and fabrication of the facility as well as the test and operation phase are covered; detailed engineering drawings are included as an appendix. During the test and operation phase of the program, more than eighty growth runs and numerous thermal test runs were performed. At the conclusion of the program, 2.4 cm wide web was being grown at thicknesses of 100 to 300 micrometers. As expected, the thickness and growth rate are closely related. Solar cells made from this material were tested at NASA-Lewis and found to have conversion efficiencies comparable to devices fabricated from Czochralski material

Silicon ribbon study program

The feasibility is studied of growing wide, thin silicon dendritic web for solar cell fabrication and conceptual designs are developed for the apparatus required. An analysis of the mechanisms of dendritic web growth indicated that there were no apparent fundamental limitations to the process. The analysis yielded quantitative guidelines for the thermal conditions required for this mode of crystal growth. Crucible designs were then investigated: the usual quartz crucible configurations and configurations in which silicon itself is used for the crucible. The quartz crucible design is feasible and is incorporated into a conceptual design for a laboratory scale crystal growth facility capable of semi-automated quasi-continuous operation

Zero gravity crystal growth Final report

Experimental device for growing crystals under zero gravity condition

The Size and Nature of Lyman alpha Forest Clouds Probed by QSO Pairs and Groups

We describe a robust Bayesian statistical method for determining Lyman alpha forest cloud sizes in spherical and in thin disk geometries, using absorption in adjacent sightlines toward closely separated QSO pairs and groups, apply this method to the available data, and discuss implications of our results for models of Ly alpha clouds. Under the assumption of a population of uniform- size and unclustered clouds, the data from Q1343+2640A/B give a 99% confidence lower and upper bounds 61<R<533 kpc/h on the radius of spherical clouds at z about 1.8, with a median value of 149 kpc/h [$(\Omega_0, \Lambda_0) =(1,0)$]. The baryonic mass of such large clouds is comparable to that of dwarf irregular galaxies. Their cosmic overdensity is close to the turn-around density but generally below the virialization density, suggesting a population of gravi- tationally bound but unvirialized protogalactic objects at z about 2. Their comoving volume density is similar to that of the faint blue galaxies (FBGs) at the limiting magnitude B of 26-27. The dynamical collapsing timescale of over- densities like these clouds is also comparable with the cosmic time difference between z of 2 to 1. Both populations of objects show similar weak clustering in space. All this evidence suggests a possible identification of Ly alpha clouds as the collapsing progenitors of the FBGs at z about 1. We also investigate the other QSO pairs: Q0307-1931/1932, Q0107-0232/0235, and the triplet of Q1623+268. Imposing an uniform W_0 > 0.4 A threshold on all linelists, we find a trend of larger inferred cloud radius with larger proper separation of QSO pairs, significant at the 3.4 sigma level. This indicates that the idealization of unclustered, uniform-sized clouds does not accurately describe the Ly alpha cloud population.Comment: Astrophysical Journal accepted; 28 pages of uuencoded gzip compressed postscript file (including 8 figures). Also see the uncompressed postscript file at http://www.astro.columbia.edu/~fang

New Hope for the Working Poor: Effects After Eight Years for Families and Children

Implemented in 1994 in Milwaukee, New Hope provided full-time, low-wage workers with several benefits for three years: an earnings supplement, low-cost health insurance, and subsidized child care. A random assignment study shows positive effects for both adults and children, some of which persisted five years after the program ended

Large-area sheet task advanced dendritic web growth development

The thermal models used for analyzing dendritic web growth and calculating the thermal stress were reexamined to establish the validity limits imposed by the assumptions of the models. Also, the effects of thermal conduction through the gas phase were evaluated and found to be small. New growth designs, both static and dynamic, were generated using the modeling results. Residual stress effects in dendritic web were examined. In the laboratory, new techniques for the control of temperature distributions in three dimensions were developed. A new maximum undeformed web width of 5.8 cm was achieved. A 58% increase in growth velocity of 150 micrometers thickness was achieved with dynamic hardware. The area throughput goals for transient growth of 30 and 35 sq cm/min were exceeded

Large-area sheet task: Advanced dendritic-web-growth development

Thermally generated stresses in the growing web crystal were reduced. These stresses, which if too high cause the ribbon to degenerate, were reduced by a factor of three, resulting in the demonstrated growth of high-quality web crystals to widths of 5.4 cm. This progress was brought about chiefly by the application of thermal models to the development of low-stress growth configurations. A new temperature model was developed which can analyze the thermal effects of much more complex lid and top shield configurations than was possible with the old lumped shield model. Growth experiments which supplied input data such as actual shield temperature and melt levels were used to verify the modeling results. Desirable modifications in the melt level-sensing circuitry were made in the new experimental web growth furnace, and this furnace has been used to carry out growth experiments under steady-state conditions. New growth configurations were tested in long growth runs at Westinghouse AESD which produced wider, lower stress and higher quality web crystals than designs previously used