Please note, due to essential maintenance online transactions will not be possible between 02:30 and 04:00 BST, on Tuesday 17th September 2019 (22:30-00:00 EDT, 17 Sep, 2019). We apologise for any inconvenience.
To send content items to your account,
please confirm that you agree to abide by our usage policies.
If this is the first time you use this feature, you will be asked to authorise Cambridge Core to connect with your account.
Find out more about sending content to .
To send content items to your Kindle, first ensure email@example.com
is added to your Approved Personal Document E-mail List under your Personal Document Settings
on the Manage Your Content and Devices page of your Amazon account. Then enter the ‘name’ part
of your Kindle email address below.
Find out more about sending to your Kindle.
Note you can select to send to either the @free.kindle.com or @kindle.com variations.
‘@free.kindle.com’ emails are free but can only be sent to your device when it is connected to wi-fi.
‘@kindle.com’ emails can be delivered even when you are not connected to wi-fi, but note that service fees apply.
The recent discovery of superconducting oxide ceramics with critical temperatures (Tc) near 100°K has stimulated research at an unprecedented pace. Single phase YBa2 Cu30x (x = 6.9), the subject of most of the interest, is an oxygen deficient 1:1:3 stacked perovskite derivative, (The structure of this materials has been described by a number of authors, many of whom are referenced by Jorgensen et al.) At room temperture it has an orthorhombic structure with lattice parameters near: a = 3.82Å, b = 3.88Å, and c - 11.68 Å. At lower oxygen values (x≤6.5) the structure is tetragonal with lattice parameters near: a = 3.86 and c = 11.80. It has been suggested that tetragonal YBa2 Cu30x is not superconducting. YBa2 Cu30x, regardless of oxygen content, is referred to here as 123.
Small-angle x-ray scattering measurements on partially hydrolyzed silicon tetraethoxide solutions indicate the formation of colloidal particles which have fractal structures or fractally rough surfaces. The structures and growth kinetics are consistent with chemically limited nucleation and growth of the particles from slowly generated reactive silanol species. Two dimensional computer simulations of nucleation and random growth of clusters from partially hydrolyzed monomers generate the same range of non-fractal, fractally rough and fractal clusters observed in the experiment.
In situ FTIR, NMR and SAXS were used to investigate the synthesis and molecular structure of xLi2O-(1−x)B2O3 gels derived from tri-n-butyl borate (TBB) and lithium methoxide. In solution the fraction of tetrahedrally coordinated borons (N4) increases linearly with x, and a critical value of N4 must be exceeded in order to form gels. The primary criterion affecting gel formation is the kinetic stability of borate bonds toward molecules which are able to undergo dissociative chemisorption.
Small angle scattering experiments have demonstrated that the structure of the silicate species produced by the hydrolysis of silicon alkoxides in non-aqueous solvents ranges from extended, weakly cross-linked polymers to highly condensed, colloidal particles. In contrast, inorganic, aqueous silicate solutions yield primarily colloidal particles because the silicate species have a number of different silanol sites available and the preferred condensation reaction is that of weakly condensed species with highly cross-linked branch sites, such as those on an amorphous silica surface. It is proposed that in the alkoxide systems, however, the hydrolysis reaction may control the number and type of silanol sites available for condensation. In acid catalyzed reactions, the rate of hydrolysis of a silicate tetrahedron tends to decrease as alkoxide groups are removed. This favors the production of silanol sites on the end of chains, thus generating linear polymers. In base catalyzed reactions, it is argued that each subsequent hydrolysis of a tetrahedron should proceed more rapidly than the previous one, producing numerous branch points which are the preferred sites for condensation.
Quebrada Tacahuay, located on the south coast of Peru, is one of the oldest expressions of maritime adaptations in the Western Hemisphere. Excavations conducted in 1997 and 1998 indicate that humans focused their activities on the collection and butchering of marine birds, particularly cormorants and boobies, and other marine resources more than 10,290 years ago (uncalibrated radiocarbon years B. P.). In addition to abundant zooarchaeological remains, cultural material includes unifacial lithic tools and one worked marine mammal rib. We report on the use of marine resources at the site in conjunction with the taphonomic history of site formation. Geological data indicate that El Niño flood events initially occurred during the Pleistocene and at various times during the Holocence. The abundant use of seafood indicates that Quebrada Tacahuay represents a specialized coastal extraction station used by Late Paleo-Indian populations with a well-developed littoral adaptation.
Email your librarian or administrator to recommend adding this to your organisation's collection.