2,375 research outputs found
A comparison of radiant and circulating space heaters in farm dwellings
This bulletin is based on a thesis submitted by Mr. Phillips to the Graduate Faculty of the University of Missouri in partial fulfillment of the requirements for the degree of Master of Science in Agricultural Engineering--P. [3].Digitized 2007 AES.Includes bibliographical references
Photophysics of azobenzene constrained in a UiO metalâorganic framework: effects of pressure, solvation and dynamic disorder
Photophysical studies of chromophoric linkers in metalâorganic frameworks (MOFs) are undertaken commonly in the context of sensing applications, in search of readily observable changes of optical properties in response to external stimuli. The advantages of the MOF construct as a platform for investigating fundamental photophysical behaviour have been somewhat overlooked. The linker framework offers a unique environment in which the chromophore is geometrically constrained and its structure can be determined crystallographically, but it exists in spatial isolation, unperturbed by interâchromophore interactions. Furthermore, highâpressure studies enable the photophysical consequences of controlled, incremental changes in local environment or conformation to be observed and correlated with structural data. This approach is demonstrated in the present study of the transâazobenzene chromophore, constrained in the form of the 4,4ââazobenzenedicarboxylate (abdc) linker, in a UiO topology framework. Previously unobserved effects of pressureâinduced solvation and conformational distortion on the lowest energy, nÏ* transition are reported, and interpreted the light of crystallographic data. It was found that transâazobenzene remains nonâfluorescent (with a quantum yield less than 10(â4)) despite the prevention of transâcis isomerization by the constraining MOF structure. We propose that efficient nonâradiative decay is mediated by the local, pedalâlike twisting of the azo group that is evident as dynamic disorder in the crystal structure
Spin correlations in Ca3Co2O6: A polarised-neutron diffraction and Monte Carlo study
We present polarised-neutron diffraction measurements of the Ising-like
spin-chain compound Ca3Co2O6 above and below the magnetic ordering temperature
TN. Below TN, a clear evolution from a single-phase spin-density wave (SDW)
structure to a mixture of SDW and commensurate antiferromagnet (CAFM)
structures is observed on cooling. For a rapidly-cooled sample, the majority
phase at low temperature is the SDW, while if the cooling is performed
sufficiently slowly, then the SDW and the CAFM structure coexist between 1.5
and 10 K. Above TN, we use Monte Carlo methods to analyse the magnetic diffuse
scattering data. We show that both intra- and inter-chain correlations persist
above TN, but are essentially decoupled. Intra-chain correlations resemble the
ferromagnetic Ising model, while inter-chain correlations resemble the
frustrated triangular-lattice antiferromagnet. Using previously-published bulk
property measurements and our neutron diffraction data, we obtain values of the
ferromagnetic and antiferromagnetic exchange interactions and the single-ion
anisotropy.Comment: 10 pages, 7 figure
Wideband TV white space transceiver design and implementation
For transceivers operating in television white space (TVWS), frequency agility and strict spectral mask fulfilments are vital. In the UK, TVWS covers a 320 MHz wide frequency band in the UHF range, and the aim of this paper is to present a wideband digital up- and down converter for this scenario. Sampling at radio frequency (RF), a two stage digital conversion is presented, which consists of a polyphase filter for implicit upsampling and decimation, and a filter bank-based multicarrier approach to resolve the 8MHz channels within the TVWS band. We demonstrate that the up- and down-conversion of 40 such channels is hardly more costly than that of a single channel. Appropriate filter design can satisfy the mandated spectral mask and control the reconstruction error. An FPGA implementation is discussed, capable of running the wideband transceiver on a single Virtex-7 device with sufficient word length to preserve the spectral mask requirements of the system
D-term Inflation in Superstring Theories
An inflationary stage dominated by a -term avoids the slow-roll problem of
inflation in supergravity and may emerge in theories with a non-anomalous or
anomalous U(1) gauge symmetry. The most intriguing and commonly invoked
possibility is that the Fayet-Iliopoulos -term triggering inflation is the
one emerging in superstring theories. We discuss the complications one has to
face when trying to build up a successful -term inflationary scenario in
superstring models. In particular, we show that the ``vacuum shifting''
phenomenon of string theories is usually very efficient even in the early
Universe, thus preventing inflation from taking place. On the other hand, when
-term inflation is free to occur, the presence of a plethora of fields and
several non-anomalous additional abelian symmetries in string theories may help
in reconciling the value of the Fayet-Iliopoulos -term required by the COBE
normalization with the value predicted by string theories. We also show that in
superstring -term inflation gravitinos are likely to pose no cosmological
problem.Comment: 19 pages, LaTeX fil
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