150 research outputs found
Solid/melt interface studies of high-speed silicon sheet growth
Radial growth-rate anisotropies and limiting growth forms of point nucleated, dislocation-free silicon sheets spreading horizontally on the free surface of a silicon melt have been measured for (100), (110), (111), and (112) sheet planes. Sixteen-millimeter movie photography was used to record the growth process. Analysis of the sheet edges has lead to predicted geometries for the tip shape of unidirectional, dislocation-free, horizontally growing sheets propagating in various directions within the above-mentioned planes. Similar techniques were used to study polycrystalline sheets and dendrite propagation. For dendrites, growth rates on the order of 2.5 m/min and growth rate anisotropies on the order of 25 were measured
Growth of silicon carbide crystals on a seed while pulling silicon crystals from a melt
A saturated solution of silicon and an element such as carbon having a segregation coefficient less than unity is formed by placing a solid piece of carbon in a body of molten silicon having a temperature differential decreasing toward the surface. A silicon carbide seed crystal is disposed on a holder beneath the surface of the molten silicon. As a rod or ribbon of silicon is slowly pulled from the melt, a supersaturated solution of carbon in silicon is formed in the vicinity of the seed crystal. Excess carbon is emitted from the solution in the form of silicon carbide which crystallizes on the seed crystal held in the cool region of the melt
High-purity silicon crystal growth
Crystal growth parameter effects on minority carrier lifetime and solar cell efficiencies were investigated using high purity techniques such as float zoning. Study objectives include the following: (1) optimize dopants and minority carrier lifetime in FZ material for high efficiency silicon solar cell applications; (2) improve the understanding of lifetime degradation mechanisms (point defects, impurities, thermal history, surface effects, etc.), and (3) crystallographic defect characterization of float zone and ribbon crystals via X-ray topography
Silicon ribbon growth by a capillary action shaping technique
The technique of silicon ribbon growth by the capillary action shaping is assessed for applicability to photovoltaic power device material. Ribbons 25 mm in width and up to 0.5 m in length have been grown from SiC dies, and some new characteristics of growth from such dies have been identified. Thermal modifiers have been studied, and systems were developed which reduce the frozen-in stress un silicon ribbons and improve the thickness uniformity of the ribbons. Preliminary spreading resistance measurements indicate that neither surface striations nor twin boundaries give rise to appreciable resistivity variations, but that large-angle grain boundaries cause local resistivity increases of up to 200%
Silicon ribbon growth by a capillary action shaping technique
The crystal growth method described is a capillary action shaping technique. Meniscus shaping for the desired ribbon geometry occurs at the vertex of a wettable die. As ribbon growth depletes the melt meniscus, capillary action supplies replacement material. A capillary die is so designed that the bounding edges of the die top are not parallel or concentric with the growing ribbon. The new dies allow a higher melt meniscus with concomitant improvements in surface smoothness and freedom from SiC surface particles, which can degrade perfection
The status of silicon ribbon growth technology for high-efficiency silicon solar cells
More than a dozen methods have been applied to the growth of silicon ribbons, beginning as early as 1963. The ribbon geometry has been particularly intriguing for photovoltaic applications, because it might provide large area, damage free, nearly continuous substrates without the material loss or cost of ingot wafering. In general, the efficiency of silicon ribbon solar cells has been lower than that of ingot cells. The status of some ribbon growth techniques that have achieved laboratory efficiencies greater than 13.5% are reviewed, i.e., edge-defined, film-fed growth (EFG), edge-supported pulling (ESP), ribbon against a drop (RAD), and dendritic web growth (web)
Silicon ribbon growth by a capillary action shaping technique
Substantial improvements in ribbon surface quality are achieved with a higher melt meniscus than that attainable with the film-fed (EFG) growth technique. A capillary action shaping method is described in which meniscus shaping for the desired ribbon geometry occurs at the vertex of a wettable die. As ribbon growth depletes the melt meniscus, capillary action supplies replacement material. Topics discussed cover experimental apparatus and growth procedures; die materials investigations, fabrication and evaluation; process development for 25 mm, 38 mm, 50 mm and 100 mm silicon ribbons; and long grain direct solidification of silicon. Methods for the structural and electrical characterization of cast silicon ribbons are assessed as well as silicon ribbon technology for the 1978 to 1986 period
High-purity silicon crystal growth investigations
The study of silicon sheet material requirements for high efficiency solar cells is reported. Research continued on obtaining long lifetime single crystal float zone silicon and on understanding and reducing the mechanisms that limit the achievement of long lifetimes. The mechanisms studied are impurities, thermal history, point defects, and surface effect. The lifetime related crystallographic defects are characterized by X-ray topography and electron beam induced current
Image-guided dissection of human white matter tracts as a new method of modern neuroanatomical training
Neuronavigation is a kind of image-guided surgery used during neurosurgical
procedures. Based on specific equipment which is compatible with the software
calculating and processing the patient’s data; this method allows the determination
of the location of anatomical structures and visualisation of surgical instruments
in the operative field. Although standard brain dissection is still the best
method of neuroanatomical training, some limitations occur. The most important
of these is the inability of conversion from three-dimensional (3D) view to
flat pictures of the brain structures, as viewed on computed tomography (CT)
and magnetic resonance imaging (MRI), being essential in neuroanatomical training
nowadays. The aim of the study was the implementation of a neuronavigating
system for brain anatomy training purposes. The study was performed on
10 human brain hemispheres, dissected due to classical methods (standard brain
anatomical sections, stepwise ventricular system opening and partial dissection
of white matter tracts using Klingler’s dissection technique). The material was
scanned in a 1.5 T magnetic resonance scanner using a modified neuronavigation
protocol. The brains were prepared before dissection as proposed by Klingler.
The subsequent steps of the dissection were documented with a digital
camera. The progress of the dissection was visualised using the neuronavigation
system (Medtronic Stealth Station Treon) with cranial application software. In
the course of the study, numerous 3D and 2D images were obtained. The images
were related to each other and linked anatomical structures in the specimen
with their appearance on CT and MRI scans. The implementation of a neuronavigation
system for brain structures dissection facilitates visualization and understanding
of their proper location. This new method offers a constant and precise
orientation and simplifies understanding of the relation of the 3D view of a specimen
to that of the 2D image
Radiological anatomy of the ambient cistern in children
Ambient cistern (AC) is a thin extension of the subarachnoid space surrounding
the brainstem at the level of the mesencephalon and pons. Despite various
definitions, it constitutes an important landmark in clinical assessment of intracranial
volume reserve. Although it is indisputably useful, there exists no
defined standard for radiological examination for the dimensions and ranges
in specific age groups. This paper aims to describe the ambient cistern anatomically
and give the ranges of dimensions for proper radiological interpretation.
The study was performed on 160 axial computed tomography (CT) examinations
of Polish children of both sexes, aged 1-18 years, admitted to the
hospital because of mild brain concussion. Pictures were made using a Siemens
8-row CT scanner, without contrast administration. We estimated distances
at the level of the pons and midbrain, based on axial cross-sections,
according to standard radiological protocol. The parameters included the width
of the AC in its anterior and posterior part, the width of the tentorial notch,
and the distance from the pons and sella. All measurements were analyzed
statistically with StatSoft Statistica 8.0 software. The average width of the AC
differs between age groups. It is greatest at 1-3 years (2.8 ± 0.6 mm) and
lowest at 4-10 years (2.4 ± 0.6 mm). AC is more likely to be greater in its
anterior part in boys. The distance from the sella to the pons is greatest in 1-3-year-old girls (6.9 ± 1.3 mm), and the tentorial notch is widest in the 15-18-year-old group (24.6 ± 2.4 mm). Dimensions of the AC correlate with intracranial
reserve volume. This is particularly visible in the youngest children. Thin
and narrow AC is not always a sign of raised intracranial pressure. It may be
specific for the child’s age. (Folia Morphol 2010; 69, 2: 78-83
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