Local Chemical Changes Research Articles

Tendency and Scale of Chemistry and Bonding Changes at SrTiO3 Grain Boundaries by Fe Segregation

ABSTRACTUsing a new EELS analysis method the local chemical and structural changes induced by Fe segregation to two types of grain boundaries in SrTiO3 were studied. At Σ5 boundary in bicrystals Fe segregation lowers O/Ti ratio but increases substantially Ti and O concentrations in boundary region. Sr-O bond has been severely changed by the segregation as revealed by ELNES. Grain boundaries in a polycrystalline sample were covered with titania-based amorphous films where Fe content is higher but Ti and O concentrations are both lower than the bulk levels, which are strikingly different from the bicrtysal. More dopants segregated to glass pockets at triple junctions. SiO2 was detected in one of the large pocket but not at the grain boundary films within the detection limit. These observations suggest the equilibrium defect chemistry and the related space charge theory may not be the only explanation for the grain boundary segregation in SrTiO3 Local structure modification at gram boundaries can trigger dramatical change of the chemistry to a degree higher than the segregation level. The existence of titania-based amorphous films at general grain boundaries makes it better to understand Fe segregation from the two phase (SrTiO3-TiO2) equilibrium.

Local communication within dendritic spines: Models of second messenger diffusion in granule cell spines of the mammalian olfactory bulb

Dendritic spines are generally believed to play a role in modulating synaptically induced electrical events. In addition, they may also confine second messengers and thus topologically limit the distance over which second messenger cascades may be functionally significant. In order to address this possibility, computer simulations of transient second messenger concentration changes were performed. The results show the importance of spine morphology and binding and extrusion mechanisms in controlling second messenger transients. In the presence of intrinsic cytoplasmic binding sites and kinetic rates similar to that expected for calcium, second messengers were confined to the spine head. In the absence of binding/extrusion mechanisms, the size and time course of the input transient to the spine head influenced the second messenger transients that might be seen at the base of the spine neck and in other spines. Large and/or sustained increases in second messenger concentration in the spine head were communicated to the spine base and to other spine heads. The results emphasize the importance of a knowledge of breakdown pathways, concentrations and kinetics of binding sites, and extrusion mechanisms for understanding the dynamics of local chemical changes for dendritic spine function.

Thin Glass Film between Ultrafine Conductor Particles in Thick‐Film Resistors

Thick‐film resistors arc electrical composites containing ultrafine particles of ruthenate conductor (Pb2Ru2O7 in the present materials) distributed in a highly modified silicate glass. We show that conductor particles remain flocced in the absence of any applied or capillary pressures, but are separated at equilibrium by a nanometer‐thick film of glass. Microstructures show evidence for liquid‐phase sintering, i.e., contact flattening of particles, under van der Waals attraction alone. Titania addition, which in dilute concentrations markedly increases the resistivity, decreases the temperature coefficient of resistance, and improves voltage stability and noise, is found to increase the equilibrium film thickness between particles by a few angstroms. STEM analyses show that the added titania preferentially concentrates in the silicate‐rich grain boundary film, as well as at particle–glass interfaces. The roles of interparticle forces and adsorption on the glass film thickness with and without titania are discussed. The large increase in resistivity caused by titania additions is attributed to the increase in Film thickness as well as to local chemical changes of two possible types. Titania enrichment within the glass film itself is expected to decrease the local ruthenium ion solubility, and this along with the possible formation of a more insulating titania‐substituted surface layer on ruthenate grains will decrease the tunneling conductivity between conductor grains.

The dynamics of local chemical changes underlying white-cell chemotaxis measured with digital imaging microscopy

The ability of cells to polarize and move towards or away from a chemical stimulus is a fundamental property of virtually all cell types at some stage in their development. This process is important for biological processes ranging from morphogenesis, to wound healing, to the attack by leukocytes of invading organisms in our body. Despite the importance of this phenomenon, surprisingly little is known regarding the mechanism whereby an external chemical gradient is converted into an intracellular gradient of 2° messengers, how the local chemistry of the cell is compared and how the result of that comparison gives rise to the polarized organization of the cell and the resulting directional migration of the cells. In order to obtain insights into this fundamental cellular process, we have been investigating the local chemical changes underlying the response of newt eosinophils to chemoattractants. This cell system is particularly well suited for asking such questions as the cells are large (30 microns wide, 70 microns long), thereby facilitating micro-injection of various fluorescent compounds as well as the imaging of local differences in the concentration of important species involved in responses to chemoattractants. Furthermore, these cells respond in seconds to gradients of serum chemoattractants, thereby facilitating an analysis of the sequence of events linking the gradient of chemoattractant to the generation of local chemical signals ultimately leading to the polarization and directed migration of these cells.

Hydration-induced climb dissociation of dislocations in naturally deformed mantle olivine

Widely dissociated dislocations have been observed in mantle olivine from the Erro-Tobbio peridotite in N.W. Italy. Analysis of diffraction contrast in transmission electron microscopy (TEM) indicates that the dissociation reaction involves the climb dissociation on (001) and {021} planes of b=[001] unit dislocations into partial dislocations with Burgers vectors approximately equal to 1/x 〈011〉. In the most extreme case a unit dislocation dissociates into four partials which bound three planar defects. The unusually wide dissociation and the greater extent of dissociation in olivine from amphibole-bearing rocks suggests that the dissociation is related to hydration. The occurrence of fluid inclusions along the dislocations confirms that the partials and planar defects are saturated with volatiles. Analysis of possible planar defect structures in Fo90 shows that; (i) the most likely partial Burgers vectors are b=〈0 3/11 1/4〉; (ii) two of the planar defects are cation-deficient and can be stabilised by segregation of H+ to produce (Mg, Fe) (OH)2 layers which are iso-structural with the OH-rich interlayer of the humite group minerals; (iii) the central planar defect is formed by removing a stoichiometric olivine (002) layer so does not produce any local chemical changes. The climb dissociation provides a possible mechanism for the transformation of olivine to a humite group mineral. OH-rich interlayers may nucleate on dislocations and extend into the crystal by climb resulting in a gradual increase of (Mg, Fe) (OH)2 content. The only addition of material required is hydrogen which can rapidly diffuse into olivine. If the dissociation is stable and occurs at high temperatures and pressures it may significantly influence the nature and kinetics of deformation mechanisms and the olivine — spinel shear transformation mechanism in hydrated olivine.

Indirect Interactions Between Temporally Separated Herbivores Mediated by the Host Plant

I tested the hypothesis that early season herbivory by leaf—chewing insects affects distribution, densities, and survivorship of late—feeding, leaf—mining insects; early chewing may physically and chemically alter quality of leaf miners on the shared host plant, Quercus emoryi. Proportions of intact leaves and leaves damaged by native leaf chewers on six control and six experimental trees were determined over two growing seasons. I manually damaged °50% of leaves on experimental trees to increase the total fraction of damaged leaves to °75%. Leaf—miner densities, distribution, survivorship, and mortality were monitored on control and experimental trees and in intact and damaged leaves within trees. Leaf miners occurred more frequently than expected by chance on intact than damaged leaves for both control and experimental trees. Leaf—miner densities did not differ between control and experimental trees. This result suggests ovipositing leaf miners did not discriminate between damaged and undamaged trees, but selected leaves within trees. More leaf miners survived intact than damaged leaves in both growing seasons. Survivorship was less in damaged leaves because of significantly increased rates of parasitism on these leaves. However, death from other causes (including bacterial and fungal death) was significantly less for leaf miners on damaged leaves. This positive effect did not compensate for overall lower survivorship of leaf miners in damaged leaves due to increased parasitism. Survivorship and mortality did not differ between control and experiment trees when damaged and intact leaves were pooled within treatments. However, in 1982—1983, survival was lower and parasitism greater for leaf miners in control—damaged leaves than experimental—damaged leaves. Survival and parasitism were not different between control—intact and experimental—intact leaves. These result suggest that (1) the effect of early season herbivory on leaf miners was inversely density dependent and, (2) occurrence of leaf miners on intact leaves may be caused by avoidance of damaged leaves rather than preference for intact leaves. Early season herbivory caused localized changes in phytochemistry within trees, and these chemical alterations were consistent with observed distributions and survivorship of leaf miners. Both early season herbivory and experimental damage resulted in higher levels of condensed tannins and lower protein content in damaged leaves within trees, but herbivory had no effect on between—tree chemistry differences. Leaf miner distributions correspond to these localized chemical changes: leaf miners avoided damaged leaves within trees but showed no between—tree preferences. Parasitism of miners on damaged leaves was higher, possibly because parasitoids used physical and chemical changes as cues to locate leaf—miner hosts, or because exposure of leaf miners to parasitoids was prolonged. However, when many leaves were damaged (experimental trees), the negative effect of parasites was mitigated because physical and chemical cues associated with damaged leaves were less effective. Lower leaf—miner mortality from other causes may be related to the bactericidal of fungicidal properties of increased tannins in damaged leaves. This study demonstrates that temporally separated guilds can interact subtly at low levels of herbivory through changes in the host plant. Current theories of within— and between—guild organization of phytophagous insects may need to be re—evaluated if such interactions are common.

SEM/microprobe study of some symplectic intergrowths replacing cordierite

Scanning electron microscopy (SEM), involving images produced by back-scattered electrons, reveals the microstructures of symplectic intergrowths replacing cordierite, which generally are too fine-grained and/or intricately intergrown for investigation using an optical microscope. Microprobe analysis, combined with the SEM technique, enables accurate identification of the intergrown minerals, and permits inference of equations for the local chemical changes involved in the formation of the intergrowths. Generally, the local replacement appears to have involved partial reactions taking place in a larger reactive system. Intergrowths forming partial pseudomorphs of cordierite in three metamorphic complexes consists of up to three minerals, only two of which comprise the intergrowth in any one domain. The following intergrown pairs of minerals have been identified: quartz-Al 2SiO 3, quartz-biotite, biotite-oligoclase, biotite-K-feldspar, muscovite-oligoclase, oligoclase-andalusite, K-feldspar-andalusite, aluminous orthopyroxene-Al 2SiO 3, and aluminous anthophylite-Al 2SiO 3.

Reflex inhibition of intestinal motility.

SINCE the classical study of intestinal motility by Bayliss and Starling1 it has been generally assumed that the parasympathetic and the sympathetic components of the autonomic nervous system exert a central, reciprocal control of the activity of intestinal smooth muscle. Thus, sympathetic fibres, running in the splanchnic outflow, should convey centrally induced inhibitory effects. Exactly how this inhibitory influence is brought about in reflex excitations of the sympatho-adrenal system is, however, not known in detail. At least four principally different modes of action may be considered: (1) Specific, inhibitory sympathetic fibres in direct contact with the intestinal smooth muscles, according to the classical conception. (2) A local ‘overflow’ of the adrenergic transmitter, released at the intestinal vasoconstrictor nerve endings. (3) Local chemical changes induced by the neurogenic reduction of the blood flow to the intestine. (4) Hormones from the suprarenal medulla, released by splanchnic nerve activation.

Influence of local chemical change on ventilatory stimulus from the legs during exercise.

Experiments on normal humans are presented which were designed to study the effect of graded local chemical change in the legs on the ventilatory response to exercise. The ventilatory stimulus from proprioceptive ventilatory receptors is augmented by local chemical change which occurs in the extremities during exercise. The strength of the stimulus is proportional to the degree of physicochemical change. Submitted on May 28, 1956

Evidence against the existence of specific ventilatory chemoreceptors in the legs.

Experiments on normal human subjects are reported which were designed to demonstrate, if present, specific ventilatory chemoreceptors in the legs. Local chemical changes were produced in the absenc...

OSSIFICATION

Ossification is a local transformation of fibrous tissue into solid bone, the constituents of which are derived from the blood. The mechanism of this local chemical change is not understood, and so the problems of normal and pathologic calcification have been approached essentially from the systemic standpoint. Obviously such a procedure limits the understanding of the chemical pathology of bone. We have seen cases of obvious rickets as well as of delayed union of bone resist systemic treatment, and so we have initiated some studies on the local chemical processes involved in calcification. This paper is concerned with the relation of callus formation to calcification. The systemic mechanism of calcification is presented graphically. Bone is a living tissue ever in equilibrium, with blood as its liquid phase. The reversible chemical system may be shifted in the direction of calcification by an increase in the bloodpHfavoring the attainment