Reversible Size Change Research Articles

Reversible grain size changes in ball-milled nanocrystalline Fe–Cu alloys

Nanocrystalline FexCu100−x solid solutions (x < 60) with single-phase fcc structure have been prepared by mechanical alloying. The average grain size of the powders (8–20 nm) depends on the composition of the material. Varying the composition changes the grain size reversibly. This can be explained by the underlying mechanism of plastic deformation and solution hardening during mechanical alloying coupled with the recovery behavior of the material.

Minimum Grain Size in Nanocrystalline Metal Powders

ABSTRACTNanocrystalline metal powders can be synthesized by mechanical attrition in a highenergy ball mill. A general relation determining the grain size of these materials is inferred. The ultimate grain size of nanocrystalline metals (typically 6 − 22 nm) is governed by the competition between the severe plastic deformation introduced during ball milling and the recovery behavior of the material. The lower bound grain size achievable by mechanical attrition is given by the minimum distance between two dislocations in a pile-up within a grain for all pure metals. Foar binary alloys the ultimate grain size depends on the composition of the material. Varying the composition changes the grain size reversibly. This can be explained by introducing solid solution hardening effects in the general relation for the lower bound grain size in pure metals. Thus, the proposed model for the ultimate grain size achievable by ball milling seems to be. applicable to all metals and alloys subjected to heavy mechanical deformation. However, reversible grain size changes are not restricted to mechanical attrition, but have also been observed for nanocrystalline Pd-H solid solutions produced by hydriding at constant pressure. Solid solutions prepared at different compositions, i.e. samples with different compositions, exhibit different grain sizes. Cycling between different temperatures/compositions changes the grain size reversibly. This cannot be explained by a model based on plastic deformation as in the case of ball-milled metal powders. The results are compared with data for ball-milled powders and samples prepared by inert gas condensation. The grain size changes are discussed with respect to the compositional changes and the grain boundary energy of the material.

Gaba-antagonists alter spatial summation in receptive field centres of rod- but not cone-drive cat retinal ganglion Y-cells.

1. When Y-type cat retinal ganglion cells were driven by cones, the i.v. administration of the GABA-antagonist picrotoxin failed to alter receptive field centre size. 2. This result is in marked contrast to our previous finding that when Y-cells were driven by rods, GABA-antagonists led to specific and reversible changes in centre size. 3. These results taken together suggest that for centre signals of Y-cells, the rod and cone pathways are pharmacologically distinct.

GABA-antagonists and spatial summation in Y-type cat retinal ganglion cells.

1. The effect of GABA-antagonists on centre size of Y-type retinal ganglion cells was measured under conditions which ensured that cells were rod driven. 2. Both bicuculline and picrotoxin, administered intravenously, led to reliable and reversible changes in centre size as determined by area-sensitivity measurements. 3. The administration of GABA-antagonists produced opposite results in on- and off-centre cells; the centre summing area decreased in on-centre cells and increased in off-centre cells.

Shape and volume changes in erythrocyte ghosts and spectrin-actin networks.

In response to changes in electrolyte concentration and pH, erythrocyte ghosts can exhibit some of the characteristic shapes seen in the intact erythrocyte. These shape changes are accompanied by volume changes; both are reversible, not energy dependent, and not inhibited by sulfhydryl reagents. The volume reduction can also be seen in isolated Triton-free spectrin-actin lattices, showing that this network is capable of reversible contraction. The results suggest that reversible changes in size of the underlying cytoskeleton of the erythrocyte membrane can control cell shape.

Massive diurnally modulated photoreceptor membrane turnover in crab light and dark adaptation

1. Effects of light and dark adaptation on the photoreceptor membranes of the rock crabGrapsus have been quantitatively studied using light and electron microscopy to document changes in rhabdom size and their fine structural basis. Some comparative data were also obtained for the ghost crabOcypode. Animals in the laboratory were maintained on a daily light (L)-dark (D)cycle 12L∶12D. Periods of adaptation ranged from 3–28 h (Table 1) and light adaptation was effected at one moderate intensity equal to the light period in the daily sequence. 2. Massive rapid reversible changes in rhabdom size (Fig. 1) comprising mainly diameter modulation (Figs. 2, 3) and some alteration in length, normally resulted in maximum increases in receptor organelle volume of about 20× between a thinner, shorter fully light adapted condition (2×290 μm) to the fully dark adapted state (7.9× 336 μm). 3. Rhabdom growth during dark adaptation (Table 2) was due mainly (Fig. 8) to microvillus elongation (+154%) as well as substantial increase (+117%) in the number of microvilli present in cross sections (Figs. 2, 3, 10); microvillus diameter (Figs. 4, 5, 9) also increased (+14%) but accounts for only a minor part of the overall change commensurate with observed differences in rhabdom length (+16%). Calculation shows that from fully light adapted to fully dark adapted state the area of photoreceptor membrane increased nearly 20× in a few hours. 4. Although duration of adaptation affected its amplitude, six hours of either light or darkness evoked the maximum changes documented; shorter and longer periods had less effect (Figs. 6, 7). 5. Time of day had a strong periodic influence on the amplitude of membrane adaptation produced by a given exposure to dark or light. Thus maximum dark adaptation occurred with 6 h of darkness terminating at midnight and maximum light adaptation with 6 h of light ending at noon of the diurnal light cycle (Fig. 11). 6. Direct effects of light and dark on synthesis and degradation of photoreceptor membrane must therefore be superimposed on indirect or direct cyclic neuroendocrine control presumably coupled to an entrained circadian oscillator.

Ultrastructural changes in the mitochondria of brown adipose cells during the hibernation cycle of Citellus lateralis.

The mitochondrial structure in the brown adipose cells of the golden mantled squirrel, Citellus lateralis, was examined throughout the year in biopsy samples. The mitochondria showed remarkable and apparently reversible changes in size and internal structure related to the physiologic activity of the animal. In the active animal the size of the largest mitochondria was 2.4 micrometer X 1.5 micrometer; during hibernation it increased to 7 micrometer X 2.5 micrometer; and during arousal it reached 11.2 micrometer X 5.3 micrometer. The cristae of the mitochondria in the brown adipose cells of the animals in hibernation phase formed loops, whorls and mesh-like interconnections. During the arousal phase they underwent further configurational changes. The most remarkable structure was associated with mitochondria of most unusual proportions which by dissolution gave rise to a new generation. This was a common finding during arousal but did not occur in any other phase of the hibernation cycle. The new mitochondria were virtually indistinguishable from those of brown adipose cells of any active animal.

Thermal Contraction in Twigs

CHANGES in linear dimensions or volume of plant parts often are assumed to represent changes in growth. However, plants routinely undergo reversible changes in size resulting from hydration and thermal effects. Sometimes such reversible changes are not very large and do not complicate growth studies. At other times, however, they may actually exceed changes resulting from accretion through meristematic activity. For this reason more attention should be given to distinguish between total dimensional changes of plants and those caused by tissue growth alone. Hydration effects on plants are considered elsewhere1,2. The I present discussion deals primarily with thermal influences on dimensional changes in woody plants.