Needle Growth Research Articles

Silicide formation and silicide-mediated crystallization of nickel-implanted amorphous silicon thin films

The nucleation and growth of isolated nickel disilicide precipitates in Ni-implanted amorphous Si thin films and the subsequent low-temperature silicide-mediated crystallization of Si was studied using in situ transmission electron microscopy. Analysis of the spatial distribution of the NiSi2 precipitates strongly suggested the occurrence of site saturation during nucleation. NiSi2 precipitates were observed in situ to migrate through the amorphous Si thin films leaving a trail of crystalline Si at temperatures as low as ∼484 °C. Initially, a thin region of epitaxial Si formed on {111} faces of the octahedral NiSi2 precipitates with a coherent interface which was shown by high-resolution electron microscopy to be Type A. Migration of the NiSi2 precipitates led to the growth of needles of Si which were parallel to 〈111〉 directions. The growth rate of the crystalline Si was limited by diffusion through the NiSi2 precipitates, and an effective diffusivity was determined at 507 and 660 °C. A mechanism for the enhanced growth rate of crystalline Si is proposed.

E-beam-induced fabrication of microstructures

E-beam-induced deposition of tungsten, W, from a metal-organic gas has been used for the formation of microstructures of various geometries. Three examples have been selected for demonstration: line deposition, needle growth, and bridge formation. Important parameters of the W deposition process from W(CO)/sub 6/ gas are given, among them minimum feature size, positional accuracy, and growth rate.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Monoterpene emissions from Scots pine and Norwegian spruce

Rates of monoterpene emissions from Scots pine (Pinus sylvestris) and Norwegian spruce (Picea abies) have been measured at four sites in Sweden with a dynamic flow chamber technique. Forest floor emissions have been made in the pine forest with the static chamber technique. Sampling was done with Tenax TA and analysis and detection by GC and ion trap detection. The compounds Δ3‐carene and α‐pinene were the predominant terpenes emitted from the crown and floor of the Scots pine forest. Alpha‐pinene was the main terpene emitted from Norwegian spruce at the sites in southern and central Sweden, while Δ3‐carene was predominant at the northern site. The relative composition of the emission of both species underwent changes in early spring and fall. Emission rates, normalized to temperature, were seen to vary diurnally with a maximum at midday, and seasonally with maxima in early May and October, and a summer maximum in June–July. The possible dependence of the emission rate on needle growth rate and other plant‐physiological processes is discussed. A higher emission rate and different relative composition of the emission was seen to occur when the vegetation was wet, as compared to dry vegetation. The emission from the pine forest floor was seen to have a composition different from that of the crown and a seasonality of the rate similar to that of the crown. The ground emission could not be explained by sources in the litter or ground vegetation alone, and it is suggested that the root system of the trees is also an emission source. The emission rate from the pine forest floor was of the order of 30% of the crown emission. The July rate of emission from the crown of Scots pine, normalized to 20°C and averaged over four sites in Sweden, was 0.8 ± 0.4 μg (gdw (grams dry weight) h)−1, and for Norwegian spruce, 0.5 ± 0.7 μg(gdw h)−1. It would seem that previous regional and global estimates of hydrocarbon fluxes to the atmosphere have used emission factors which are too high for boreal coniferous forests.

Growth responses of ponderosa pine to long-term exposure to ozone, wet and dry acidic deposition, and drought

A 3-year field study of the cumulative effects of ozone (O3), wet and dry acidic deposition, and soil water availability was conducted on ponderosa pine (Pinusponderosa Laws.) in the Sierra Nevada of California from 1988 to 1990. Thirty-six 2-year-old potted seedlings were placed in each of 30 chambers and exposed from May through October to three levels of O3 (charcoal-filtered (CF), nonfiltered (NF), and NF plus 1.5 times ambient O3 (NF150)); three levels of acidity in simulated rain (pH 3.5, 4.4, 5.3); two levels of dry deposition (60 or 90% filtration), and two levels of soil water availability (well watered (WW) or drought stressed (DS)). An additional six plots served as ambient air (AA) controls. One-third (432) of the trees were harvested at the end of each exposure season. Low soil water availability was the only stress factor to significantly affect growth following the first exposure season. After the second season, O3 significantly reduced foliar biomass in WW–NF150 trees, but DS seedlings did not respond to O3. After 3 years of exposure, WW–NF150 trees averaged 70% loss of 1988 needles and 48% loss of 1989 foliage. Ozone-injured seedlings compensated for these losses by increased growth of current-year needles and stems and also increased growth of fine feeder roots. Radial stem growth and coarse-root growth were significantly reduced in O3-injured trees. DS trees in NF150 chambers averaged half the needle loss of WW trees and showed no reduction in radial growth in response to O3. Rain pH and dry deposition had no direct effects on growth of ponderosa pine. These cumulative responses to interacting stresses indicate the importance of multifactorial, long-term studies to evaluate forest tree responses to atmospheric deposition.

Stability of two-dimensional needle growth.

The stability of two-dimensional well-defined needle growth is considered beyond the perturbative approach, until singularity formation, when surface tension may no longer be neglected. The needle growth is shown to be represented by the development of a single dominant Fourier mode, and the conditions under which all other modes are negligibly small, and not influenced by this dominant one, are derived. The dominant mode divides the Fourier spectrum into two parts with different criteria for stability. It is shown that for modes higher than the dominant one the initial smoothness (i.e., the decay of the Fourier spectrum with wave numbers) should be at least exponential for needle stability, whereas for modes lower than the dominant one the condition for stability is more restrictive in that the initial law of decay should be faster than Gaussian.

Dynamics of Pinus radiata foliage in relation to water and nitrogen stress: I. Needle production and properties

Measurements of the following parameters were made over a 4 year period in 10- to 14-year-old stands of Pinus radiata subjected to markedly different degrees of water and nitrogen (N) stress: needle length, weight and specific leaf area (every 2 weeks), foliage biomass production (annually), pre-dawn needle water potential (every 2 weeks) and needle litter N concentration (monthly). Increments in needle length were a useful estimate of increments in needle weight for any given forest treatment and year because there was no consistent variation in weight per unit length of needles as they developed during the growing season. However, for well-illuminated needles, the ratio of weight per unit of needle length showed a large (approximately two-fold) variation attributable to treatment and year of foliage elongation. The ratio was loosely positively correlated with needle length (or the favourability of growing conditions) on non-irrigated plots, and appeared to result largely from increases in needle thickness rather than density. Final needle length, ranging between 40 and 160 mm, depended mainly on the amount of water and N stress experienced by trees during the growing season. The majority (greater than 90%) of needle extension occurred during a 4 month period (October–January) in spring and summer and the pattern of needle growth was affected only by water availability. Needle extension rates were negatively linearly correlated with the water stress integral ( S ψ , a temporal integration of the effects of both water and N availability on needle water potentials) for monthly periods during the growing season. Needle extension, was most sensitive to the S ψ in mid-spring (October/November) when needles were elongating rapidly and still less than one-half grown. About 80% of the variation in annual foliage production (3.2–8.5 t ha −1) could be explained in terms of both (a) S ψ during the previous summer (when primordia were initiated), and (b) the water and N status of trees concurrent with needle extension. Final needle length and total foliage biomass production in the same year were poorly correlated. The specific leaf area (SLA, all sides) ranged from 10 to 17 m 2 kg −1 and was greater for needles formed under low light. Irrigation or fertilisation had only an indirect effect on SLA by hastening canopy closure.

Simplified diffusion-limited aggregation models

Computer simulations have been used to explore two-dimensional diffusion-limited aggregation models in which growth is allowed only at the tips of needle shaped arms that are directed away from the origin. For the case of two adsorbing needles with growth velocities equal to the rate at which the needle tips capture random walkers the critical angle, θ c, (between the two needles) for stable growth is about 30°–33° (in good agreement with the theoretical value of π[1 − ( 2 3 ) 1 2 ] or about 33.03° obtained by Derrida and Hakim (preprint, 1992). For the growth of n symmetrically directed arms with the same boundary conditions all n arms grow in a stable pattern for n ⩽ 6. For n ⩾ 7 the growth of n needles is not stable. Quite different results are reported for other needle growth models with different boundary conditions.

Carbon and phosphorus partitioning in Pinus serotina seedlings growing under hypoxic and low-phosphorus conditions.

Ten-week-old pond pine (Pinus serotina Michx.) seedlings were grown in solution culture at 5 or 100 microM P and under aerobic or hypoxic solution conditions. After 6 and 10 weeks in the treatments, changes in relative growth rate (RGR), P acquisition and allocation, and carbohydrate partitioning were determined by analyzing tissue for total P, soluble sugars and starch. Six weeks of low-P growth conditions decreased seedling dry weight and the ratio of shoot dry weight to root dry weight (S/R) by 39 and 51%, respectively, in comparison to seedlings from the aerobic, high-P (control) treatment. Mean RGRs of shoots in the low-P treatment were reduced by 33%, whereas root growth was unaffected. After 10 weeks of low-P growth conditions, however, both shoot and root RGRs were significantly reduced, and plants had lower S/R ratios than in any other treatment. Slowed shoot growth was accompanied by starch and nonstructural carbohydrate accumulation in needles, indicating that needle growth was not limited by carbohydrate supply. Six weeks of low-P growth conditions decreased total seedling P by 75%, reflecting a 97% reduction in the net uptake rate (NUR). Shoot NUR as a fraction of seedling NUR was also greatly reduced in the low-P treatment, indicating that low-P growth conditions affected P translocation to the shoot more than P accumulation by roots. In contrast, 6 weeks of hypoxic growth conditions decreased total dry weight of seedlings in the high-P treatment by 41% relative to their aerobic counterparts. Root growth was affected more than shoot growth, however, and S/R ratios increased. After 10 weeks, S/R ratios doubled, primarily because of the reduction in root RGR. Nevertheless, roots of hypoxic seedlings contained a higher percentage of total seedling P than their aerobic counterparts. Net P acquisition per seedling decreased by more than 50% under hypoxic growth conditions, as a result of reductions in both root RGR and seedling NUR. Starch accumulation in shoots of hypoxic seedlings reflected reductions both in root growth and in transport of carbohydrates to nonwoody roots. Carbohydrate availability did not appear to be limiting growth of hypoxic woody roots, which are well-aerated internally, but it may have limited metabolic processes in nonwoody roots of seedlings from the high-P treatment.

Hydrogen bond connectivity patterns and hydrophobic interactions in crystal structures of small, acyclic peptides.

Crystal structures of all available unblocked linear peptides with two to five residues were retrieved from the Cambridge Structural Database and their intermolecular contacts and packing modes studied using molecular graphics. This survey reveals that interactions between hydrophobic portions of the molecules are critically important in determining the overall features of their crystal packing patterns. Distinct hydrophobic columns or layers are observed in almost all crystal structures. Analyses of the relationships between these interactions and crystal growth properties of small peptides are given. It is suggested that needle growth is promoted by hydrophobic packing, usually along a short crystallographic axis (4.6-6.0 angstroms). Also contributing to these morphologic characteristics are entropic factors associated with hydrophobic aggregation as well as tightly bound water molecules on hydrophobic faces. The paper also provides a comprehensive overview of hydrogen bond patterns in acyclic peptide crystals. It is demonstrated that one of their primary roles is to provide a scaffolding within which hydrophobic groups can aggregate. Even though there is a high density of hydrogen bonds in the crystals, often with complex patterns and networks, certain motifs are found to recur in a number of structures indicating specific hydrogen bond preferences. Water, for example, is an integral part of the hydrogen bond networks in these crystals, usually acting as the primary donor for main-chain carboxylate groups in peptide hydrates.

The laboratory generation of test aerosols with well defined morphologies

There is a need for particles with well defined characteristics for the calibration and assessment of aerosol instrumentation. Currently the only well characterised particles that are readily available are spheres. The responses of instruments to non-spherical particles are usually unquantified. A number of techniques for the production of particles with a variety of shapes have been investigated. This paper outlines some of these techniques and in particular examines the growth of caffeine needles and a number of inorganic salt crystals with specific shapes.

Growth and micro-indentation studies of ?-In2Te3 whiskers

Growth of hollow needles using the physical vapour deposition method. X-ray analysis of these crystals was done to determine the structure and lattice constant. Microhardness studies on the (111) prism planes of these crystals were also reported

Growth and microindentation analysis of ∝-In 2Te 3 whiskers

Needle type whiskers of indium telluride in ∝-modification have been grown by the physical vapour deposition (PVD) method. X-ray topographic investigation established that the grown crystals had an fcc structure with lattice constant a = 18.471 A ̊ . The possible mechanism for the growth of hollow needles has been suggested on the basis of morphological studies. Microindentation studies were also carried out on the prism faces of the needles to understand their mechanical behaviour.

A new approach to corona needle growth based on geometrical and structural analyses

A radial dense array of metallic microneedles is known to form on a field cathode interacting with a low-pressure corona plasma in ambient metal carbonyl. These so-called corona needles have been subjected to geometrical and structural analyses, with the aid of detailed electron microscope observations. The highly disordered geometry that the needles exhibit was found to be interpretable in terms of the concept of fractal growth. It was also revealed that the needles possess a “dual” structure, composed of a core and crystallites grown on the core. The core is atomically thin, whereas the crystallites lack symmetry, suggesting that irreversible processes of non-equilibrium are involved in the growth of the corona needles. The needles lengthen while emitting field electrons, so their growth geometry may represent the potential distribution in the proximity of the growth sites.

Bifurcation in singular needle growth.

Bifurcation in singular needle growth.

Productivity and vitality of spruce stands: dynamic feedback simulation for responses to different annual and seasonal levels of magnesium supply from soil

Responses of Norway spruce stands to gradually decreased annual magnesium supply from soil, with two seasonal courses, are evaluated by computer simulation. At low supply, magnesium concentrations in the needles are reduced to levels inducing chlorosis, but annual growth of new needles remains at the same level as stands with adequate magnesium. Above-ground permanent biomass shows a decreased annual production with lower supply, mainly because of shorter net carbon gain caused by decreased assimilation which is due to magnesium deficiency in needles. An additional but less important reason is the magnesium deficiency in stem and branch cambium, which is most severe during summer at low magnesium supply rates. The interaction of magnesium and carbon balance is emphasized. Modeling and simulation are not used for prediction but to test the consistency of scientific hypotheses.

Laser-induced electrodeposition of transition metals on silicon

Laser-induced structured deposition of narrow transition metal traces was studied on cathodically biased p-type single crystalline silicon in an aqueous Schottky-junction configuration. The influence of the potential, the laser parameters, and the status of the interfacial oxidic layers on the electron transfer, and the MIS-junction characteristics were studied. Nucleation and growth were observed by potentiodynamic current measurements in combination with scanning electron microscopy. The metal deposition is characterized by instantaneous 2-D or needle growth, respectively, and subsequent 3-D growth followed by coalescence of hemispheres. The illuminated and metallized areas exhibited various charging effects of interfacial electronic states. This, and their implication for structured laser-induced deposition, is discussed. Metal patterns generated in a computer-controlled scanning laser spot apparatus matched closely the beam trace of the scanning focused cw krypton ion laser beam.

Crystallographic and mechanistic aspects of growth by shear and by diffusional processes

Growth by shear and by diffusional processes, both taking place predominantly by means of ledge mechanisms, are reviewed for the purpose of distinguishing critically between them at the atomic, microscopic, and macroscopic levels. At the atomic level, diffusional growth is described as individual, poorly coordinated, thermally activated jumps occurring in the manner of biased random walk, whereas growth by shear is taken to be tightly coordinated “glide” of atoms to sites in the product phase which are “predestined” to within the radius of a shuffle. Obedience to the invariant plane strain (IPS) surface relief effect and the transformation crystallography prescribed by the phenomenological theory of martensite is shown to be an unsatisfactory means of distinguishing between these two fundamentally different atomic growth mechanisms. In substitutional alloys, continuous differences in compositionand in long-range order (LRO) from the earliest stages of growth onward are concluded to be the most useful phenomenological approach to achieving differentiation. At a more fundamental level, however, the details of interphase boundary structure are the primary determinant of the operative mechanism (when the driving force for growth is sufficient to permit either to occur). In the presence of a stacking sequence change across the boundary, terraces of ledges are immobile irrespective of their structural details during diffusional growth. Kinks on the risers of superledges are probably the primary sites for diffusional transfer of atoms across interphase boundaries. In martensitic transformations, on the other hand, terraces containing edge dislocations in glide orientation or pure screw dislocations are mobile and accomplish the lattice invariant deformation (LID), though probably only after being overrun by a transformation dislocation. Risers associated with transformation dislocations are also mobile and cause the crystal structure change during growth by shear. The successes achieved by the invariant line (IL) component of the phenomenological theory of martensite in predicting precipitate needle growth directions and precipitate plate habit planes (Dahmen and co-workers) are here ascribed to the rate of ledge formation usually being a minimum at an interface containing an IL, primarily because nuclei formed sympathetically at this boundary orientation are likely to have the highest edge energies. Since martensite plate broad faces also contain the IL, the ability of the phenomenological theory to predict the habit plane and the orientation relationships of both precipitate and martensite plates is no longer surprising. The IPS relief effect at a free surface can be generated by precipitate plates when growth ledges are generated predominantly on only one broad face and only one of several crystallographically equivalent Burgers vectors of growth ledges is operative. Both pReferences probably result from larger reductions in transformation strain energy for the particular geometry with which a given plate intercepts the free surface. Precipitate morphology often differs significantly from that of martensite even if precipitates are plate-shaped and can readily differ very greatly. Whereas martensite morphology is determined by the need to minimize shear strain energy, that of precipitates derives from the more flexible base of the interphase boundary orientation-dependence of the reciprocal of the average intergrowth ledge spacing, as modified by both the orientation-dependence of interkink spacing on growth ledge risers and the spacing/ height ratio dependence of diffusion field overlap upon growth kinetics.

Seasonal course of height and needle growth in Pinus nigra grown in summer-dry Central Anatolia

Terminal buds of 8–10-year-old Pinus nigra Arnold, planted near Ankara in Central Anatolia, started to break in late March and early April. Leader growth accelerated as the season proceeded, reaching its peak in late May. Trees completed 80–85% of their shoot elongation by early June, and ceased growth by late July. Extra elongation in July was mainly the result of bud maturation. The average terminal shoot growth was 18–20 cm/year. Leader length in P. nigra in year n + 1 showed significant positive correlation with April rainfall of year n, and with bud length formed in year n. These relationships suggest that the amount of shoot growth is largely predetermined by the environmental conditions prevailing during bud formation in the previous year. Needle growth began 6–7 weeks later than leader growth and appeared to be more sensitive than shoot elongation to changes in the physical environment within the current growing-seasons. Needles stopped growing about 9 weeks later than shoots if temperature and other environmental conditions were favorable. The average needle length was 65–85 mm at the time when needle growth ceased.

Seasonal variation in the monoterpenes in needles of Picea abies (L.) Karst.

This investigation was conducted to obtain information about the fluctuations in composition and amount of needle monoterpenes during the development of spruce needles. Studies conducted with two Norway spruce clones clearly revealed the existence of fluctuations. In juvenile needles, the amounts of the oxygenated terpenes increase constantly with age during the first 2 months of needle growth. The hydrocarbon terpenes dominate within the first weeks, some of them even showing a very distinct first maximum within the first 3 weeks after bud burst. All terpenes, including the oxygenated ones, have a maximum in June/July, which favours the hypothesis of a substitution of the hydrocarbons later on. There are significant changes even in mature needles of Norway spruce. The terpene level of 1-year-old needles of the clonal trees increased from spring to early summer and then dropped again towards winter. In addition, fluctuations in mature needles were shown for a set of ten wild trees. Needles of the same age class, which emerged in 1986, were sampled 4 times from 1986 to 1988. The needle terpene concentrations of the 1 -year-old needles were considerably lower in spring at the time of bud burst than in autumn. The terpene level of older needles thus seems to be influenced by biosynthetic and catabolic activities.

Growth of single crystals in Bi 2(Sr, Ca) n + 1Cu nO x system

Flux growth of crystals in the general system Bi 2(Sr, Ca) n + 1 Cu n O x with n = 1 and n = 2 is described. The solute concentration in solution has been found to govern the phase, morphology and quality of the grown crystals. A controlled excess of CuO and CaO when added to the solute of stoichiometry 2212 yielded crystals of superconducting phase free from secondary growth of CuO needles.