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Nanostructuring has been proposed as an effective strategy for the reduction of the phonon contribution to the thermal conductivity, resulting in an increase in the figure of merit of thermoelectric materials. However, obtaining bulk samples presenting high relative density and nanometric grain size can be quite challenging, particularly in the case of ceramic phases. Only few examples have been reported and none in the case of Ca3Co4O9. In this work, we used a sol-gel synthesis coupled with ball milling to prepare powders of Ca3Co4O9 presenting a grain size as small as 4 nm. These nanopowders were then sintered at different temperature and pressures using the High-Pressure Field-Assisted Sintering Technique (HP-FAST). Relative densities up to 95 vol% where obtained while maintaining a nanometric grain size. The microstructural and electrical properties of the sintered samples have been characterized. The results show that in this oxide a reduction to the nanometric grain size produces a drastic reduction in the electrical conductivity, which cannot be compensated by the reduction in the thermal conductivity. The Seebeck effect, on the other hand, appears to be affected only marginally by the grain size and porosity.

Elemental Ge nanophases are obtained in silica starting from a sol-gel synthesis. The process comprises the preparation of silica xerogel doped by Ge(IV), substituting for Si(IV) in the oxide network, followed by reduction and phase separation during the sintering process by reaction with H-2 or CO. X-ray diffraction, Raman spectroscopy and high resolution transmission electron microscopy show that reactions with H-2 give rise in the porous silica network to nanometer-sized crystallization of elemental cubic germanium. (c) 2007 Elsevier Ltd. All rights reserved.

The preparation of Sn-doped SiO2 glasses with different Sn contents (Sn/(SiO2 + SnO2) ratio 0.01 to 10.0% w/w) was performed by two sol-gel routes. The first route (method A) used tin(IV) tert-butoxide as the tin precursor and in the presence of acetylacetone it resulted in transparent glasses with a maximum tin content of 0.1%. The precursor in the second route (method B) was dibutyltin diacetate; this was more efficient and transparent glasses of up to 1% tin content were obtained. As electron paramagnetic resonance can identify paramagnetic E' Sn centres, defects that form when silica glass is subjected to X-ray irradiation, we assessed the tin centres that substituted silicon in the silica network. At higher tin contents (Sn greater than or equal to 3%), method B gave crystalline nanosized particles of SnO2 (6-10 nm) dispersed throughout the silica matrix.

The changes in the absorption spectra induced by Co-60 irradiation and high temperature annealing was studied for selected PbWO4 crystals. Based on radiation and annealings induced absorption spectra, four color centers are proposed, which could be responsible for shaping the absorption spectrum of PbWO4 crystals below 3.6 eV, namely Pb3+, O-, F, and F+ centers. Thermoluminescence (TSL) glow curves above room temperature were also studied on these samples to determine further the trap levels resulting from irradiation. Correlation between the results obtained by TSL and Co-60 irradiation induced absorption changes is discussed. (C) 1997 American Institute of Physics.

The optical absorption band at 5.16 eV and the related photoluminescence were investigated in Ge-doped silica samples as a function of the Ge content to study the relation between impurities and oxygen deficient defects. The data, collected in a wide doping range, showed that the dependence of the intensity of the 5.16 eV absorption band on the Ge content depends on the thermochemical parameters of the preparation process. Further, two distinct ranges of Ge-doping were individuated showing different dependence of the density of optically active center on the impurity concentration. © 1997 Elsevier Science B.V.

Photoluminescence (PL) excitation spectra of the 4.2 eV alpha emission in Ge-doped silica were obtained at 290 and 9 K in the range 4.5-1- eV by synchrotron radiation excitation. The vacuum ultra-violet excitation pattern turns out to be different from that previously observed in pure silica. Four excitation bands were resolved at 4.9, 5.2, 6.4 and 7.2 eV. PL lifetime data were also analyzed to characterize the radiative relaxation channels. Data on differently prepared samples suggest to assign each excitation band to a distinct type of defect site. Nevertheless, the same results indicate the presence of only two types of PL sites, one of which possessing both intra- and inter-center excitation channels, possibly involving tunneling and defect conversion mechanisms.

Thermally Stimulated Luminescence (TSL) and Conductivity (TSC) induced by X-ray irradiation in SIMOX buried oxides have been studied from room temperature up to 400/spl deg/C. The characteristics of an X-ray induced TSL glow peak detected around 62/spl deg/C are presented: specifically, results on the emission wavelength and trap depth are shown. The X-ray induced TSC, observed at approximately 70/spl deg/C, is due to the same trapped species responsible of the TSL emission. The stability after irradiation and dose dependence of both TSL and TSC signals have also been investigated. The results have been compared with similar studies on high temperature annealed thermal SiO/sub 2/ films and bulk materials.

We present the effect of a rapid thermal treatment (RTT) at high temperature (1800degreesC) on the radio-luminescence properties of Ce-doped SiO2 glasses prepared by the sol-gel method and previously densified at 1050degreesC. Cerium concentrations ranging from 0.05 up to 1 mol % were considered. We found that, for all concentrations, the RTT induces a strong increase of the Ce3 radio-luminescence efficiency; the x-ray-induced luminescence intensity of the SiO2:0.1% Ce is about twice that of Bi3Ge4O12. The decay time of the scintillation response, evaluated as approximate to50 ns, is not affected by RTT. Infrared absorption measurements indicate that the luminescence increase cannot be related to significant release of OH groups during RTT. The conversion of Ce4 ions into Ce3 ions can also be ruled out since an increase of about 20% of the intensity of the 4.8 eV optical absorption band related to Ce4 was observed after RTT. The occurrence of dissolution of rare-earth aggregates is suggested. (C) 2002 American Institute of Physics.

Silica xerogels doped with Ge(IV), substituting for Si(IV) in the oxide network, are prepared from tetraethylorthosilicate and germanium-tetraethoxide. The sintering process is carried out in reducing atmosphere at 700 - 900° C by reaction with H 2 . Raman spectroscopy and high resolution transmission electron microscopy (TEM) show that reactions with H 2 give rise, in the porous silica network, to uncontrolled islands of crystallites of elemental cubic germanium with average size of 50 nm. Sintering process in reducing H 2 atmosphere at temperatures just below the phase separation, at about 610°C, gives materials where Ge atoms are dispersed in the matrix in conditions of incipient clustering. Evidences of segregation of germanium nanocrystals are observed with electron irradiation during TEM analysis. Furthermore, the electron beam induced precipitation leads to the formation of isolated quantum dots-like nanocrystals (5-6 nm in diameter) and with narrower size dispersion. The ranges of suitable temperature and germanium concentration are analysed, as well as the size dispersion of the resulting Ge nanophases.

Ti-Al co-doped erbium, tellurite glasses have been obtained by melting mixed Er2O3 TiO2 and TeO2 batches in AlO3 crucibles. By crucible dissolution Al2O3 amounts from 11.5 to 18.6 mol% were introduced in the synthesized glasses. Differential thermal analysis of glasses points to a strong dependence of glass transition temperature T-g with the substitution extent of TeO2 by the doping oxides. No crystallization features are observed up to 450 degrees C. The spectral features and decay kinetics of the infrared photolurninescence of erbium demonstrate the possibility to achieve more than 50% of quantum yield of light-emission at Er (3+) concentrations as large as 10(21) cm(-3), with about 2 ms of lifetime, 8 X 10(-21) cm(2) of Stimulated emission cross section, and no saturation at pump power densities higher than 10 kW cm(-2). The study of the kinetics of Er-Er energy transfer suggests to ascribe these features to a particularly homogeneous dispersion of Er 3+ ions in the modified tellurite network. Raman scattering measurements of the spectral distribution of vibrational modes evidence that the introduction of doping oxides leads to an increase of structural disorder without crystallization effects. (c) 2007 Published by Elsevier B.V.

Silica glass with SnO 2 nanocrystals, obtained from sol-gel synthesis and thermal densification at 1100 °C, was poled by means of a two-step process consisting of infrared 1064 nm laser irradiation followed by 532 nm laser exposure in high-voltage static electric field. Maker fringe experiments were then carried out at 1064 nm. The results show the formation of second-order nonlinearity with macroscopic nonlinear thickness (about 1 mm) and nonlinear susceptibility comparable with thermally poled silica (about 0.1 pm/V). Photoluminescence measurements suggest that mechanisms for this process should involve the activation and anisotropic ionization of defects at the interface between nanocrystals and glass.

The radioluminescence (RL) properties of Ce-doped silica glasses prepared by sol-gel method were investigated as a function of the dopant amount in a wide range of concentrations (Ce:Si molar ratios from 1 x 10(-5) to 5 x 10(-2)). The effects of xerogel densification temperature and of a postdensification thermal treatment were also considered. In order to understand the microstructural features governing Ce3 luminescence efficiency, optical absorption measurements in the ultraviolet, visible, and infrared regions and time-resolved photoluminescence experiments were performed. The complex dependence of RL intensity upon rare-earth (RE) concentration and thermal treatment was attributed to the role of OH vibrations as nonradiative recombination channels, as well as to the formation of RE aggregates in the silica matrix. Specifically, the segregation of CeO2 nanoparticles, in which Ce4 does not supply any radiative emission, has been revealed by infrared absorption measurements, in agreement with previous Raman, transmission electron microscopy, and X-ray diffraction data. Moreover, an optical absorption band, centered at 2.4 eV and whose intensity increases with the square of cerium concentration, was observed and tentatively assigned to an intervalence electron-transfer transition involving Ce3+Ce4 dimers. Postdensification thermal treatments markedly reduce the intensity of this band and increase RL intensity. The relationship between the 2.4 eV band and the RL properties will be outlined. In particular, the increase in RL intensity will be discussed, mainly as a consequence of microscopic modifications leading to an improvement of the charge-transfer efficiency toward emitting centers.

A photoinduced decrease of refractive index has been demonstrated in SiO2:SnO2 (15 mol%) optical-grade glass ceramic, exposed to a 780 nm wavelength, focused femtosecond laser radiation at 1 kHz repetition rate. Exposition have been performed translating the sample perpendicularly to the laser beam at a constant velocity of 17 μm/s, with energies ranging between 2 and 5 μJ per pulse. The samples were fabricated by the sol-gel method, from 85 mol% tetraetoxysilane and 15 mol% dibutyl tin-diacetate, adding water to a solution of the precursors. Xerogel was produced by slowly evaporating the solvent for 2 weeks at 35 °C. The final material was obtained heating in O2 the xerogel samples by a thermal ramp (4 °C/h) up to 1050 °C. Samples were then cut from the blocks with a diamond saw to 10 × 10 × 1 mm3 and all the faces polished. A negative variation of the refractive index has been demonstrated in the irradiated zone (-3 × 10-4) measured by the prism coupling technique. Multimode waveguides at 633 nm have been obtained exploiting a positive refractive index change at the boundaries of the irradiated zone. Raman spectra have been measured showing several broad bands centered at about 430, 800 and 1100 cm-1, and with two peaks at 490 and 610 cm-1, when exciting at 488 nm. Strong red luminescence has also been observed when exciting at 633 nm suggesting the creation of non-bridging oxygen defects in the silica-based glass. We proposed that the negative refractive index change in the irradiated zone is due to the amorphization of the nanocrystals, while further investigation is needed to fully understand the origin of the positive index change at the boundaries. © 2005 Elsevier B.V. All rights reserved.

Structural and morphological studies were carried out on cerium- and gadolinium-doped sol−gel silica glasses intended for scintillator applications, to deepen the understanding of rare earth ion incorporation into the glass matrix. Several compositions, ranging from 0 to 5 mol % Ce and from 0 to 8 mol % Gd, were studied by Raman spectroscopy. The vibrational response was compared to that of pure silica glasses: for cerium doping higher than 0.5 mol %, the F2g Raman mode, characteristic of CeO2, was observed. The presence of CeO2 nanocrystalline clusters, whose size depends on cerium concentration and thermal treatment, was confirmed also by X-ray powder diffraction (XRD) patterns and transmission electron microscopy (TEM) analyses. On the contrary, gadolinium-doped sol−gel silica glasses exhibited Raman spectra similar to those of pure silica glasses, at least for the investigated concentrations up to 8 mol %, and no crystalline particles were detected within the amorphous matrix.

Radio-luminescence (RL) and wavelength resolved thermally stimulated luminescence (TSL) measurements have been performed on sol-gel SiO2 glasses, undoped (with high OH- or low OH-content) and rare-earth (Ce, Gd and Tb) doped in the temperature range 10-310K. Moreover, a comparison has been performed between undoped sol-gel and commercial glasses. In undoped samples RL spectra are characterized by emissions at 1.9eV, 2.4 eV (only in high OH-content glasses) and 2.7 eV at 10K.; their nature is discussed and related to different intrinsic defects. Strong emission quenching is observed by temperature increasing. Besides similar intrinsic bands, the doped glasses RL emission spectra display typical emissions related to 5d-4f (Ce3+), 6P-8S (Gd3+) and 5D 3-7Fj, 5D4- 7Fj (Tb3+) transitions respectively, whose amplitudes increase by increasing the temperature above 150K. TSL measurements show the existence of glow peaks at ≈100 K and 220 K. The observed increase of RL rare-earth emission amplitudes above 150 K can be explained by considering (i) the progressive decrease of competitive host matrix recombination channels and (ii) the free carriers trapping at defect sites during irradiation (monitored by TSL), whose stability strongly depends upon temperature. © 2004 Elsevier B.V. All rights reserved.

A radioluminescent (RL) dosimetric system, based on a SiO2 optical fiber with the core doped by Ce3+ ions as luminescent activators has been investigated. Structural and optical properties of the luminescent fiber have been studied by Raman, refractive index, RL and scintillation time decay measurements, and compared to those obtained on bulk material. The RL response of a composite fiber made of a short portion of active Ce-doped fiber coupled to a long commercial one has been investigated by x-ray irradiation. A linear RL intensity response has been found in the dose rate interval 6x10(-3)-40 mGy/s together with a good radiation hardness, suggesting possible application in low-dose monitoring. (C) 2004 American Institute of Physics.

Nanoclusters of crystalline SnO2 have been grown in glass, starting from tin-doped silica prepared by a sol-gel method. Based upon a particular choice of molecular precursors, nanocrystallites with a mean radius of about 1 nm and a quite narrow size dispersion were obtained, resulting in wide-band-gap (>4 eV) quantum dots (QDs). In this system, differently from other semiconductor-doped glasses, both the glassy host and the nanophase are oxides of IV-group elements. Owing to their thermochemical compatibility, the two phases give stable optical-grade glass ceramics with potential applications in photonics, opening up the field to technological employments of wide-band-gap QDs. (C) 2002 American Institute of Physics.

The possibility of obtaining permanent photoinduced refractive index changes, up to $\ensuremath{-}{10}^{\ensuremath{-}3}$, in nanostructured silica-based composites has been demonstrated in ${\mathrm{S}\mathrm{i}\mathrm{O}}_{2}\ensuremath{\mathbin:}{\mathrm{S}\mathrm{n}\mathrm{O}}_{2}$ optical-grade glass ceramics exposed to ultraviolet radiation. Optical and electron paramagnetic resonance data suggest that the negative refractive index change involves a modification at the surface of the ${\mathrm{S}\mathrm{n}\mathrm{O}}_{2}$ nanoclusters, leading to a reduction of their crystalline size.

A new sol-gel route using tetraethoxysilane (TEOS) and dibutyltindiacetate (DBTDA) as precursors allowed the preparation of Sn-doped SiO2 glasses with a content of substitutional Sn atoms in regular Si sites up to 1.4 wt%, SnO2/(SnO2 + SiO2). Glasses with higher Sn content ( greater than or equal to 1.6 wt%.) showed a crystalline nanophase of SnO2 dispersed throughout the silica matrix. The hydrolysis and condensation reactions of TEOS and DBTDA were investigated by FTIR spectroscopy during sol-gel transition in order to understand the mechanism, at molecular level. which gives rise to substitutional Sn-doping or to SnO2 clustering. It was found that the hydrolysis of DBTDA was faster than TEOS. Hydrolyzed DBTDA acted as a crosslinker between molecules of hydrolyzed TEOS. Since tin atoms preferentially link to silicon atoms via bridging oxygens, no aggregation of Sn atoms occurred in sot and gel phases. Also in the Sti-doped SiO2 samples with Sn content greater than or equal to 1.6 wt% no aggregation of Sn atoms was observed in the xerogels. Segregation of SnO2 occurred during the thermal treatment to obtain glass. (C) 2001 Elsevier Science B.V. All rights reserved.

Silica glasses with substitutional Sn-doping or with dispersion of SnO2 nanoclusters were prepared by the sol–gel method. The occurrence of SnO2 clustering was investigated by optical absorption, Raman scattering and electron paramagnetic resonance spectroscopies, varying both the tin concentration and the densification temperature of Sn-doped silica xerogels. The 3.7 eV absorption edge of SnO2 and the A1g Raman mode at 630 cm−1 of the SnO2 phonon spectrum were used to assess the presence of SnO2 nanoclusters; the yield of X-ray induced paramagnetic E′ Sn centres was monitored to probe the variation in the amount of substitutional Sn centres. The results indicate that the clustering of SnO2 becomes effective for Sn concentrations ≥ 0.8 mol% and it is competitive with the substitutional doping in xerogels densified at T ≥ 900 °C. No evidence of pre-existent clustering was observed in xerogels densified at lower temperatures in the investigated doping range.

Sn centers are point defects easily formed by X-ray irradiation of Sn-doped silica. Point defects are a tool for assessing the substitutional character of the dopant in Sn-doped SiO2 glasses with different Sn content (SnO2/SiO2 ratio from 0.01 to 10.0 % w/w) and prepared by two sol-gel methods differing in tin precursors (tin tetraterbutoxide and tin dibutyldiacetate). The former method produced transparent crack-free monolithic glasses of Sn-doped silica with maximum tin content of 0.1 % (SnO2/SiO2 % w/w). The latter method is more efficient and produced transparent glasses with a tin content of 1%.

Thermally Stimulated Luminescence (TSL) properties of SIMOX oxides, induced by x-irradiation, have been studied in the temperature range 100-400 K. For the first time studied in SIMOX, a series of TSL peaks at 110, 150 and 200 K appear clearly. A continuous distribution of trap levels ranging from 0. 4 to 1. 1 eV has been found to be responsible for the broad and intense peak observed at 200 K; moreover, the wavelength distribution of the emitted light has revealed the presence of an emission centred at 2. 7 eV. A comparison with the TSL properties of synthetic silica is presented, and the results are discussed also in the light of previous characterizations of trap levels obtained by electrical techniques. This study gives new insight concerning the electron and hole traps structures in SIMOX compared to bulk silica. © 1998 IEEE.

A study of the thermally stimulated luminescence (TSL) and conductivity (TSC) properties of PbWO4 crystals was performed on a selected set of samples. Under X-ray irradiations at 90 K TSL peaks are observed in three temperature regions, (100–130 K, 160–200 K, 220–300 K) with corresponding TSC structures. Room temperature irradiations put in evidence a weak TSL peak around 360 K. The emission spectra of the TSL signals are also presented. On the basis of these measurements, the possible nature of the defects involved in the TSL processes is discussed.

Optical absorption, photoluminescence (PL), and electron paramagnetic resonance (EPR) data on unirradiated and UV-irradiated Ge-doped silica are reported. The analysis of the UV-induced effects shows the following facts: (i) both components of the 5-eV absorption band are partially bleached, as well as the PL excited within this absorption band; (ii) more consistent UV-induced changes arise at higher energies where bands at 5.8 and 6.4 eV grow during the treatment; (iii) Ge-E\ensuremath{'} and Si-E\ensuremath{'} EPR centers are UV generated, confirming their involvement in the photoconversion process but ruling out previous attribution of the 6.4 eV to Ge-E\ensuremath{'} centers; and (iv) the absorption band at 6.4 eV shows previously unobserved emission properties, probably due to excitation transfer processes. \textcopyright{} 1996 The American Physical Society.

Thermally stimulated luminescence (TSL) studies have been performed on x-irradiated Sawyer PQ quartz, both “as received” and “vacuum swept out”. The variation of the 100°C TSL peak after irradiation and subsequent thermal treatments (pre-dose effect) has been studied as a function of the irradiation temperature.