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In the glass industry, nucleation agents are broadly used to obtain homogeneously dispersed and nanograined crystals during heat treatment leading to glass ceramics. In this paper, the role of zirconia and titania used as nucleation agents in a lithium alumosilicate glass is studied on the nanoscale using cutting edge analytical and imaging techniques performed in the transmission electron microscope. It is found that the formation of ZrTiO4 nanocrystals upon heat treatment is accompanied by the formation of a diffusion barrier surrounding these precipitates. At the interface between ZrTiO4 nanocrystals and the residual glass, an enrichment of alumina is observed, making the glass network more rigid and eventually preventing further growth or ripening of the nanosized crystals. Within the compositional gradient formed around the homogeneously distributed ZrTiO4 crystallites, nucleation of the secondary crystalline phase, a lithium alumosilicate high-quartz solid solution, is likely to be much facilitated rather than the never doubtlessly reported on epitaxial overgrowth assumed to act so far.

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The crystallisation of the nucleation agent ZrTiO 4 in a low thermal-expansion lithium aluminosilicate glass-ceramics is monitored as a function of time by combining transmission electron microscopy with Ti-L 2,3 X-ray absorption near-edge structure spectroscopy. The formation of liquid-liquid phaseseparation droplets is shown to precede ZrTiO 4 crystallisation within the latter nanosized droplets. Quantitative data on crystalline fractions enable conclusions on the self-limited growth of ZrTiO 4 nanocrystals in low thermal-expansion glass-ceramics and based on Avrami's equation, the growth is shown to be restricted by a barrier (the outer border of the phase-separation droplet). It is shown that liquid-liquid phase separation and crystallisation are temporally decoupled. The size of ZrTiO 4 crystallites is determined by the restricted volume of the phase-separation droplets they crystallise in. The volume of the droplets in turn is restricted by the formation of a diffusion barrier in the surrounding residual glass.

Measurements of the kinematic distributions of J /ψ mesons produced in p-C, p-Ti and p-W collisions at √ s = 41.6 GeV in the Feynman-x region −0.34 < x F < 0.14 and for transverse momentum up to p T = 5.4 GeV/c are presented. The x F and p T dependencies of the nuclear suppression parameter, α, are also given. The results are based on 2.4 × 10 5 J /ψ mesons reconstructed in both the e + e − and μ + μ − decay channels. The data have been collected by the HERA-B experiment at the HERA proton ring of the DESY laboratory. The measurement explores the negative region of x F for the first time. The average value of α in the measured x F region is 0.981 ± 0.015. The data suggest that the strong nuclear suppression of J /ψ production previously observed at high x F turns into an enhancement at negative x F .