Etching of High Purity Zinc

A method of etching high purity zinc to reveal various etch figures on {101¯0} planes is presented in this paper. Etch figures are formed by polishing in a dichromic acid solution after the introduction of mercury to the crystal surface. No measurable aging time is required to form etch figures at newly formed dislocation sites when mercury is on the surface prior to deformation. The mercury concentrates at the sites where etch figures form and may be removed by vacuum distillation and chemical polishing before it appreciably affects the purity of the bulk of the crystal

Dislocations and etch figures in high purity zinc

A method of etching high purity zinc single crystals to reveal various etch figures on {1010} planes is presented in the preceding paper. The procedure involves the introduction of mercury to the crystal surface prior to a chemical polish with dichromic acid. The mercury was found to be concentrated at the etch figures. This paper presents the results of several experiments which support the conclusion that there exists a one-to-one correspondence between etch figures and dislocations. Some observations of slip on (0001) basal planes and {1212} pyramidal planes, and of twinning in zinc are also presented

Orientation Dependence of a Dislocation Etch for Zinc

The dislocation etch for (101-[bar]0] surfaces of zinc reported by Brandt, Adams, and Vreeland have been further explored. Additional surface orientations have been found where dislocation etching takes place. These orientations cover an area located between 3 degrees and 12.2 degrees to the [0001], and the area is symmetric about that axis. Attempts to produce dislocation etching on within 2 degrees of (0001) were generally unsuccessful. This is in contrast to etching of many crystals which takes place only within a few degrees of a low index plane

Deconstructing bioluminescence: from molecular detail to in vivo imaging.

Bioluminescence is the chemical production of light that results when a luciferase enzyme catalyzes the luminogenic oxidation of a small-molecule luciferin substrate. The numerous luciferases and luciferins nature has evolved can be used to illuminate biological processes, from in vitro assays to imaging processes in live animals. However, we can improve the utility of bioluminescence through modification of these enzymes and substrates. My thesis work focuses on developing reporters that expand the bioluminescent toolkit and improving our understanding of how bioluminescence works on a molecular level. The first part of my thesis focuses on characterizing luciferases and luciferins that improve bioluminescence imaging in vivo. Some of our luciferins can outperform the natural D-luciferin substrate in live mouse imaging, while others are selectively utilized by mutant luciferases in live mouse brain. We also engineered luciferins that can selectively report on endogenous enzymatic activity in live mice. The second part of my thesis focuses on determining the molecular details of how enzymes related to firefly luciferase, long-chain fatty acyl-CoA synthetases (ACSLs), can function as latent luciferases. I have determined the structure for one of these enzymes and improved its bioluminescent activity with synthetic luciferins enough to image in live mouse brain. I also characterized the selectivity in chimerized enzymes that combine firefly luciferase and ACSLs. In summary, my work improves the utility of bioluminescence for in vivo use and informs us about how evolutionarily-related enzymes function as luciferases on a molecular level

Radar mapping, archaeology, and ancient land use in the Maya lowlands

Data from the use of synthetic aperture radar in aerial survey of the southern Maya lowlands suggest the presence of very large areas drained by ancient canals for the purpose of intensive cultivation. Preliminary ground checks in several very limited areas confirm the existence of canals and raised fields. Excavations and ground surveys by several scholars provide valuable comparative information. Taken together, the new data suggest that Late Classic period Maya civilization was firmly grounded in large-scale and intensive cultivation of swampy zones

Damage Due to Electric Spark Discharge Machining of Zinc

Electric spark discharge machining has been observed to produce severe damage in the form of kinking and slip in single crystals of high-purity zinc. A mechanism by which conditions of plastic anisotropy, relatively high machining temperatures and loading from spark discharge machining interact to produce this damage is proposed. A method for preventing the damage is explained

Tachyons in Compact Spaces

We discuss condensations of closed string tachyons localized in compact spaces. Time evolution of an on-shell condensation is naturally related to the worldsheet RG flow. Some explicit tachyonic compactifications of Type II string theory is considered, and some of them are shown to decay into supersymmetric theories known as the little string theories.Comment: 14 page

Second Order Pyramidal Slip in Zinc

Measurements of strain, dislocation density, and dislocation velocity have been made in 99.999% Zn stressed in compression along the hexagonal axis, [0001]. Slip bands on the second order pyramid,〈1213〉{1212}, system were observed. The average dislocation density increases linearly with strain. The velocity of edge dislocations in slip bands obeys the relation v = (Γ/ Γ_0)^n with v in in. / sec, n = 8.7, Γ_0 = 870 lb/in.^2, and Γ the resolved shear stress in lb/in^2. These observations together with measurements of the strain rate sensitivity of the flow stress show that the stress dependence of the density of moving dislocations is more important than the stress dependence of the dislocation velocity as they affect the strain rate