have been investigated by using steady-state and time-resolved laser spectroscopy. Judd-Ofelt parameters were derived from the absorption spectra and used to calculate the 4 F 3/2 → 4 I 11/2 stimulated emission cross section and the 4 F 3/2 radiative lifetime. Site-selective spectroscopy and time resolved fluorescence line narrowing experiments were performed in the 4 F 3/2 → 4 I 9/2 , 4 I 11/2 transitions. For all samples studied a line narrowing of the 4 F 3/2 → 4 I 11/2 emission has been observed when exciting at the long wave side of the 4 I 9/2 → 4 F 3/2 transition. The lifetimes of the 4 F 3/2 state do not show a monotonic variation with the excitation wavelength indicating large site-to-site variations in the local crystal field. Spectral migration of excitation among the Nd 3ϩ ions has been studied from the time evolution of the 4 F 3/2 → 4 I 9/2 spectra under resonant excitation. The results can be interpreted in terms of a dipole-dipole energy transfer mechanism. Visible upconversion has been observed in these glasses under infrared laser excitation.

The visible luminescence of Pr3+-doped lead germanate glass of composition (in mol%) 60PbO-40GeO2 has been investigated for two Pr3+ concentrations at different temperatures by using steady-state and time-resolved laser spectroscopy. Concentration quenching of the 1D2 emission is observed for Pr3+ concentrations higher than x = 0.05 mol% even at 4.2 K and attributed to a cross relaxation process. Anti-Stokes emission from the 3P0 level following excitation of the 1D2 state has been observed for the sample doped with 0.5 mol% of Pr3+. The temporal behaviour of the anti-Stokes emission from the 3P0 level following excitation of the 1D2 state suggests that energy transfer is responsible for the upconversion process.

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Si 3 N 4 materials with distinct microstructures were prepared by hot-pressing, varying the holding time at the maximum temperature, and using different types and amounts of sintering additives. Materials with thermal conductivities of 15-82 W⅐(m⅐K) ؊1 were obtained by changing the processing variables. The highest conductivity was measured for the material with the coarsest microstructure. The effect of microstructural parameters, such as percentage of secondary phases, grain size, and texture on thermal properties of Si 3 N 4 ceramics, were studied. Hot-pressed Si 3 N 4 ceramics were modeled as a twophase composite made of large grains of high conductivity and a small-grained phase of low conductivity.

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