No abstract

Although silicosis has been studied extensively, the mechanism is still not fully understood. Experiments do provide evidence that the actions of unique properties of silica surface on the cell membrane are the starting point of silicotic processes. This paper summarizes literature on chemical properties of silica surface, and the effect of particle size on silica toxicity. This paper also discusses the ways in which silica dusts are thought to interact with the cell membrane, with emphasis on freshness, hydrogen bonding, and free-radical interactions.

Using electron-nuclear double-resonance measurements on the As04 radical as a spin probe, we have measured correlation times (r values) for the intersite hopping motions of 'H, "P, and ' K nuclei in KH2PO4 above its T, (123 K). The r values are found to be essentially equal for all of these nuclei; ranging from about 10 ' s at 150 K to longer than 10 ' s as T T"with an average Arrhenius barrier of 0.16~0.04 eV. These results suggest that not only the H2PO4 groups but alamo the cations are disordered over two or possibly more sites above T"ordering into one of the two ferroelectric sites below T,.Recently McMahon et aI. ' reported on a high-resolution neutron-diAraction investigation of the possible positional disorder of the P atom in KDqPO4 (DKDP) in the paraelectric phase. The authors concluded that at 5' above T, (229 K), the P atom has a~40% probability of occupying each of the two sites that differ by 0.08 A along the z axis. This observation was not in accord with the standard model of the phase transitions of the KHqPO4 (KDP) type of compounds. The standard model assumes that the transition involves an ordering of the H atoms in one of two possible sites along the 0 -H . 0 bonds and a concomitant displacive transition of the heavy-atom framework.The neutron-diff'r action results supported earlier Raman-scattering studies which have indicated that the site symmetry of the P04 group remains C2"" rather than becoming S4, on going through T, into the paraelectric phase. The Raman studies thus suggest that at temperature T & T, the entire H2P04 groups, and not just H atoms, may be disordered over two sites. However, NMR and nuclear quadrupole resonance studies seem to favor the S4 symmetry at T& T,. With a view of obtaining additional experimental data to help resolve this controversy, we have carried out electronnuclear double-resonance (ENDOR) measurements of the protons as well as the P and the K nuclei in KDP in the vicinity of T, in the paraelectric phase. Since KDP is diamagnetic, for making ENDOR measurements, KDP crystals were doped with about 0.01% KH2As04, and then y irradiated to form the As04 radical. The As04 radical is formed via the capture of a y-ray-released photo-electron by the As04 ion in the unirradiated lattice. The unpaired electron of the As04 center is known to be delocalized over the whole KH2P(As)04 framework and should serve as a sensitive probe of ionic displacements around the As04 center. In fact, earlier electron-spin-resonance ' (ESR) and ENDOR (Refs. 11-13)studies have provided direct evidence for the hopping of protons between their two possible equilibrium sites in the 0 -H . 0 bonds. However, only recently the availability of high rf power and a special ENDOR cavity design made it possible to detect ENDOR signals from 'H, 'P, and K nuclei in the close vicinity of the As04 center in KDP. ' This recent development ' enabled detection of the motion of protons around T, which was not possible by ESR or low-power ENDOR " ' techniques. In the present work we have used the new EN-...

At a quantum critical point (QCP)--a zero-temperature singularity in which a line of continuous phase transition terminates--quantum fluctuations diverge in space and time, leading to exotic phenomena that can be observed at nonzero temperatures. Using a quantum antiferromagnet, we present calorimetric evidence that nuclear spins frozen in a high-temperature nonequilibrium state by temperature quenching are annealed by quantum fluctuations near the QCP. This phenomenon, with readily detectable heat release from the nuclear spins as they are annealed, serves as an excellent marker of a quantum critical region around the QCP and provides a probe of the dynamics of the divergent quantum fluctuations.