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Polarized Raman spectra of lanthanum borogermanate (LBG) glasses of different compositions, as well as of LBG glasses obtained by the hot-extrusion technique, have been obtained. In addition, the crystallization behaviour of LBG glasses of different compositions has been studied by differential thermal analysis and X-ray diffraction. Both the Raman spectra profile and the crystallization behaviour depend on the lanthanum–boron ratio. Polarized Raman spectra of hot-extruded samples show differences from the unextruded samples. For the first time, structural changes in glasses under hot-extrusion treatment were observed by vibrational spectroscopy.

Both surface and bulk textures penetrating the whole volume of the glass may be formed in glasses of the La2O3-B 2O3-GeO2 system near the stoichiometry of LaBGeO5 stillwellite. These textures cause non-linear optic, ferroelectric and pyroelectric properties of crystallized glasses. The present work represents SEM study of surface and bulk grain-oriented crystallization of stillwellite glasses for different conditions of synthesis of both planar surface (non-linear optic) and bulk (ferro/pyroelectric) textures based on oriented needle- shaped crystals of LaBGeO5 stillwellite. © 2003 Elsevier Ltd. All rights reserved.

Solid state Si-29, P-31 and H-1 MAS-NMR were used to investigate structural aspects of gel-derived glasses of molar composition 10P(2)O(5).90SiO(2) (10P) and 30P(2)O(5) . 70SiO(2) (30P). P-31 MAS-NMR spectra of the 10P and 30P gels dried at 100degreesC show resonances characteristic of orthophosphoric and pyrophosphoric acid. No evidence for cross-linking with silicate matrix was found by 29Si NMR at this temperature while H-1 MAS-NMR experiments suggest that the chemical environment of OH groups is very similar in these samples. In spite of this initial similarity further heating causes markedly different structural rearrangements of the 10P and 30P gel matrix. (C) 2004 Elsevier B.V. All rights reserved.

Transparent glasses composition of which can be expressed by the formula: (100− x ) · (K 2 O · 2TiO 2 · P 2 O 5 ) · x (K 2 O · 2B 2 O 3 · 7SiO 2 ), where x =5, 10, 15 and 20 mol% (KTP– x KBS), were obtained by melt quenching technique. The structure and crystallization behavior of these glasses have been examined by Fourier transform infrared spectroscopy, differential thermal analysis and X-ray diffraction. In spite of their nominal composition, the studied glasses exhibit a similar oxygen polyhedra distribution. However, significant differences were found in the trigonal BO 3 units amount. During DTA runs all the examined glasses devitrify in two steps. In the former, very small crystals of an unknown crystalline phase are produced. In KTP–5KBS and KTP–10KBS glasses anatase phase was also detected. Attempts were made in order to identify the unknown phase (UTP) for which a AB 3 (XO 4 ) 2 (OH) 6 Crandallite-type structure was proposed where the A, B and X sites were occupied by K, Ti and/or Al, and P, respectively. In the second devitrification step the crystallization of the KTiOPO 4 phase occurs while the UTP phase previously formed disappears. Isothermal heat treatments performed at temperature just above T g have allowed one to obtain transparent crystal–glass nanocomposites, formed by crystalline nanostructure of the UTP phase uniformly dispersed in the amorphous matrix.

1H Magic Angle Spinning (MAS) NMR and 1H T1 and T1ρ relaxation times measurements were used to investigate structural aspects of gel derived glasses of composition 10P2O5·90SiO2 (10P) and 30P2O5·70SiO2 (30P), with the aim of understanding the different humidity sensitive behaviour observed for films of the same composition. The experimental evidence suggests that 10P and 30P gel samples heat treated at 100 °C have a similar POH environment while further heating causes a different structural evolution of POH units. In particular, the 30P sample heat treated at 400 °C shows a single 1H resonance while a number of resonances are seen in the 1H spectrum of the 10P sample heat treated at 400 °C. Further evidence of the different structure of 10P and 30P samples was given by the NMR relaxation times measurements, the gels were found to be heterogeneous on a nanometric scale. Domains, about 60 nm in size, related to 1H resonance in POH were evenly distributed in the matrix of both gels, only in the 10P gel also larger domains (about 700 nm) were found that can be related to 1H resonance in SiOH groups. Finally, MAS in combination with variable temperature static NMR spectra provided detailed information on the network structural features present in the 30P sample heat treated at 300 °C as well as in the 10P sample heat treated at 400 °C and allowed investigation of the dynamic processes taking place in these samples.

Solid state 29Si and 31P MAS NMR have been used to investigate the microstructural changes occurring in phosphosilicate gels during their conversion from a gel to the corresponding gel-derived glasses by heating. The studied gels have the molar compositions 10P2O5·90SiO2 and 30P2O5·70SiO2. It was found that the dried gels (100 °C) have very similar structures formed by a siloxane framework containing silanol groups and including trapped molecules of orthophosphoric acid along with a very small amount of pyrophosphoric acid. In spite of this initial similarity further heating causes markedly different structural rearrangements of their glassy matrices. Namely, the co-polymerisation of phosphate and silicate tetrahedra takes place at 300 °C for the gel with the higher phosphorus content whereas this occurs only after heat treatment for 30 minutes at 400 °C for the gel with the lower phosphorus content. Moreover, the presence of six-coordinated silicon in the glassy matrix of the 30P2O5·70SiO2 gel has been observed. The different evolution of the investigated gel microstructures mirrors their different crystallization behaviour: 10P2O5·90SiO2 keeps its amorphous nature up to 1000 °C while for 30P2O5·70SiO2 crystallization starts after heat treatment for 30 minutes at 400 °C.

This study was part of a research program in which the devitrification behaviour of lithium germanate glasses is investigated. The crystallizing phases and the kinetic parameters of crystal growth in a lithium germanate glass containing 5mol% lithium oxide were studied

Solid state B-11 MAS NMR has been used to investigate the structure of glasses from which the polar phase beta BaB2O4 crystallizes as the main phase. The compositions of the glasses studied are expressed by the general formula: (50 - x/2)BaO . (50 - x/2)B2O3 . xMO(2) with x = 4, 8, 16 and M = Si or Ti. The presence of both trigonal and tetrahedral boron in the amorphous matrix of all glasses studied has been ascertained. A monotonic increase in the BO4 fraction was seen with the silica content consistent with the silica acting as a glass former, converting BO3 groups into BO4 ones. A more complex variation in the BO4 fraction is seen with titania. At low content, TiO2 acts primarily as a glass former giving results similar to silica. However, at higher contents titanium enters the network in six-fold co-ordination and requires a balancing charge from the Ba2+, resulting in the conversion of BO4 groups to BO3 ones. As a consequence of the structural diversity, a different behavior with respect to the crystallization of beta BaB2O4 was found into the two glasses series. (C) 1999 Elsevier Science B.V. All rights reserved.

Structure and glass transition temperature of lanthanum aluminosilicate (LAS) glasses, containing: (15-25) mol% La2O3, (15-35) mol% Al2O3 and (40-70) mol% SiO2, were studied by FTIR spectroscopy and differential thermal analysis. The effects of La2O3 and Al2O3 content on the properties and structures of LAS glasses were evaluated. The Al+3 ion can assume both four-fold co-ordination, forming (AlO4/2) tetrahedra, and six-fold co-ordination forming (AlO6/2)(-3) octahedra. The amount of these depends on La+3 content; acting as network modifier, it can compensate the charge of three neighbouring (AlO4/2) units or one (AlO6/2)(-3) unit. (C) 1997 Elsevier Science S.A.

ZrO2Y2O3 (5%) gel was synthesized by the hydrolytic polycondensation reactions of zirconium propoxide and yttrium chloride. The transformation of the gel on heating, examined by differential thermal analysis, thermogravimetric analysis and X-ray diffraction, is reported and discussed. The studied gel crystallizes directly into metastable tetragonal phase at low temperatures. This phase keeps stable up to a temperature of 1200°C.

The gel synthesis and the crystn. behavior of glass powders in the Li2Ge4O9-Na2Ge4O9 compn. range were studied. The effects of replacing Li2O by Na2O on the glass transition temp. and devitrification behavior were examd. The gel prepn. involves hydrolysis and polycondensation of germanium ethoxide with lithium methoxide and/or sodium ethoxide. The gels thus prepd. are amorphous solids contg. water and org. residues that are lost on heating. Progressive substitution of sodium oxide for lithium oxide causes the glass transition temp. to go through a min. according to the mixed alkali effect. The crystg. phases and gels have similar structures.

The effects of the addition of Li2O to fly ash on the devitrification behaviour of the derived glass are investigated with the aid of differential thermal analysis (DTA), X-ray diffraction (XRD) and scanning electron microscopy (SEM). The phases crystallizing first for the as-quenched glass and the glass sample previously nucleated are different. Solid solutions of lithium-magnesium silicate (Li(2 + 2x)Mg(1 − x)SiO4) and a small quantity of β-eucryptite (LiAlSiO4) solid solution are formed from the as-quenched glass, while only solid solutions of lithium-magnesium silicate are formed from the sample previously nucleated. In both cases, these crystalline phases are then converted, at higher temperatures, into a large amount of β-eucryptite solid solution together with small quantities of diopside crystals (CaMgSi2O6). Kinetic parameters for nucleation and crystal growth were estimated from the DTA curves. The temperature of maximum nucleation rate was 620°C and the activation energy for crystal growth E = 316 kJ mol−1. The crystal morphology was investigated by SEM, and the crystal shape was consistent with the morphological index n calculated by DTA. The glass-ceramic obtained from a previously nucleated glass showed a fine-grained texture.

Li 2 O · 3GeO 2 gel was synthesized by the hydrolytic polycondensation of germanium ethoxide with lithium methoxide. The crystallization behaviour of the gel, examined by differential thermal analysis and X-ray diffraction, is reported and discussed. The phases crystallizing from the gel were identified as Li 2 GeO 3 , Li 2 Ge 4 O 9 and GeO 2 . The activation energy for the crystallization was found to be 562 kj mol −1 . The results were also compared with those obtained for the conventional glass of the same composition.

The non-isothermal devitrification of lithium germanate glasses, examined by DTA and XRD, is reported and discussed. The glass compositions are expressed by the general formula: xLi2O(1-x)GeO2 with x = 0.050, 0.125, 0.167, 0.200 and 0.250. All the glasses studied, unlike GeO2 glass, exhibit internal crystal nucleation without the addition of any nucleating agent. The devitrification processes occur in one or more steps. Phases which crystallized at each step are identified and crystallization mechanisms proposed. These crystallization mechanisms are related to structures of the crystallizing phases. Activation energy values as well as those for glass transition temperatures, do not vary linearly with increase in Li2O content but pass through a maximum at x = 0.200.

The non-isothermal devitrification of Li 2 O·7GeO 2 glass was studied using differential thermal analysis and X-ray diffraction. A crystallization mechanism in two steps is proposed. The kinetic parameters of each step were evaluated from the DTA curves.

The devitrification behaviour of Li2O · 4GeO2 glass examined by differential thermal analysis and X-ray diffraction is reported and discussed. Li2O · 4GeO2 glass was found to devitrify in two steps. In the primary transformation microcrystallites of Li2Ge4O9 are crystallised in an amorphous matrix. In the second transformation well shaped Li2Ge4O9 crystals are formed. The activation energies of the two stages are found to be 698 and 502 kJ/mol respectively.

On etudie l'effet du remplacement de Li 2 O par Na 2 O dans cinq verres de composition: (1-x) Li 2 O. x Na 2 O. SiO 2 ou x=0-0,25-0,5-0,75

A series of glasses in the lithium metasilicate-sodium metasilicate composition range was studied by differential thermal analysis and X-ray diffraction. The effect of replacing LiO2 by Na2O on the glass transition temperature and devitrification behaviour has been determined. The crystallizing phases have been identified and related to the devitrification mechanism. Kinetic parameters for nucleation and crystal growth were estimated from DTA curves.

Measurements of soil specific corrosivity were performed on bare steel in clay and sand at 15% moisture content (m.c). Polarization resistance (R p ) was measured by either the potentiodynamic linear polarization method or a.c. impedance techniques. The values obtained by the former method were corrected from IR ohmic drop contribution of the electrolyte resistance (R s ) deduced from a.c.impedance measurements, whereas, to avoid the effect of electrode processes with large relaxation time, proper sweep rates were chosen. A method to evaluate R p by a.c. impedance technique is proposed which generally gives results close to d.c. R p measurements. Impedance data allow also some interpretation of corrosion rate determining process.

Preparation, par le procede sol gel, d'un verre de composition: BaO•2SiO 2 . Etude des avantages a utiliser des gels comme materiaux de depart pour la preparation de vitroceramique

Wires of 2 high-C steels after H charging by cathodic polarization were tested in a programmable tensile machine at a const. load and various pH. At pH

The conditions leading to failure, under cathodic polarization, of high strength wires used to prestress concrete tubes, have been studied. A mechanism of nucleation and growth of microcracks in areas charged with hydrogen has been evidenced. Nucleation of cracks is rapid with a high current density and a lower pH, conditions that can be approached when excessive cathodic protection is applied to wires and the pH of the concrete decreases below the natural basic values by elution of OH−1 ions or by action of acidic solutions.

The effect on the glass transition temperature and the electrical conductivity wqas studied by (1) replacing P2O5 by Ga2O3 molar equivalent amount in an AgPO3 glass and by (2) increasing the amount of Ag2O in an Ag2O.0.1 Ga2O3.0.9P2O5 glass. Series 1 resulted in an increase in the glass transition temperature and the electrical conductivity. This was attributed to an increase in the polymerization degree of the glass structure and to a decrease in the number of non bridging oxygen ions which acted as trapping sites for the silver ion charged carriers. There was also a decrease in the electrical conduction activation energy. Series 2 resulted in a increase in the ionic conductivity, owing to the increase in the number Ag+ ions, and a decrease in the electrical conduction activation energy. The glass transition temperature remained constant through the series.

The effect of crystallization on the electrical conductivity of lithium metasilicate glass were studied. Conductivities were measured by a.c. complex impedance method. Lithium metasilicate glass devitrifies in two steps i.e. 1) crystallization of a first phase, Li 4 SiO 4 , with a composition different from the that of the initial glass and 2) at a higher temperature transformation of this phase into a second phase, Li 2 SiO 3 . After each crystallization step a decrease of the electrical conductivity and an increase of the conduction activation energy was observed.

The non-isothermal devitrification of lithium metasilicate glass was studied using differential thermal analysis and X-ray diffraction. A crystallization mechanism in two steps is proposed. The kinetic parameters of each step were evaluated from the DTA curves.

The sintering of zircon-based (ZrSiO 4 ) ceramics prepared from a batch containing ball clay and wollastonite as additives has been studied. Physical properties of extruded and dry pressed specimens fired at 1280 °C are compared with those of extruded test specimens of pure zircon fired at 1600 °C. The sintering aids of zircon determine the formation of a glassy phase in which the zircon is embedded. The partial crystallization of such a glassy phase, performed with a nucleation and crystal growth process, enhances the strength of the fired zircon-based bodies. These materials have been utilized as a grinding medium for industrial frits and glazes.

Investigations of new materials possessing both higher Raman gain and larger spectral bandwidth than fused silica are becoming mandatory in order to satisfy increasing telecommunications demands. Herein, silica-niobia based glasses have been prepared and characterised by ellipsometry and Raman spectroscopy. A fine glass structural characterization of bulk system has been performed and the Raman gain has been calculated. Our silica-based glass system (K2O-Nb2O5-SiO2 (KNS)) provides via thermal treatment a nanocomposite material of easy and cheap fabrication. In our glass composition a strong Raman gain (up to 30 times than fused silica) and a large bandwidth have been measured, and demonstrated to be related to nanostructured material domains.

The structure and devitrification behaviour of barium borotitanate glasses examined by Fourier transform infrared spectroscopy, differential thermal analysis and x-ray diffraction, are reported. Glass compositions are expressed by the formula: (50-x/2)BaO.(50-x/2)B2O3.xTiO(2) with x=4, 8 and 16. Titanium acting as glassformer gives a more polymerised structure increasing the glass stability with respect to devitrification. The nonisothermal devitrification of the studied glasses occurs in one or two steps depending on the TiO2 content in the glass composition and the specific surface area of the sample. The surface crystallisation was found to be dominant for all glasses. The investigated glasses devitrify forming beta-BaB2O4 microcrystals as the main crystalline phase.

The non-isothermal devitrification of LiNaSiO 3 glass was studied using differential thermal analysis and X-ray diffraction. A crystallisation mechanism in two steps was found. The temperature of maximum nucleation rate and the kinetic parameters of crystal growth were evaluated from the DTA curves.

Lithium tetragermanate gel was synthesized by hydrolitic polycondensation of germanium ethoxide with lithium hydroxide monohydrate in alcoholic medium. Crystallization behaviour of the gel, examined by differential thermal analysis and X-ray diffraction, is reported and discussed. Lithium tetragermanate gel crystallizes in two steps. In the primary transformation, at about 560-degrees-C, microcrystallites of Li2Ge4O9 and GeO2 are crystallized in an amorphous matrix. In the second transformation, at about 600-degrees-C, well shaped Li2Ge4O9 and Li2Ge7O15 are formed. The values of activation energy for the two stages are found to be 557 and 405 kJ mol-1, respectively. Activation energies are comparable with those reported with conventional melt glasses from oxides.

Stainless steels AISI 304 and 316 exposed to a Ca(OH)2 satd. soln., simulating concrete environment, behave much better than carbon steel. Even in the presence of chloride ions up to 125 mg l-1 they do not corrode spontaneously, and undergo some anodic polarization before pitting corrosion starts. The max. value of anodic polarization tolerated decreases as chloride content increases: it was 0.300 V at 125 mg I-1.

Li 2 O · 7GeO 2 gel was synthesized by the hydrolytic polycondensation reactions of germanium ethoxide with lithium methoxide. The crystallization behaviour of the gel, examined by differential thermal analysis and X-ray diffraction, is reported and discussed. Lithium heptagermanate gel was found to crystallize in two steps. In the primary transformation, microcrystallites of Li 2 Ge 7 O 15 are crystallized in an amorphous matrix. In the second transformation, well-shaped Li 2 Ge 7 O 15 crystals are formed. The activation energy values for the two stages are found to be 589 and 495 kJ mol −1 , respectively. The results are compared with those obtained for conventional lithium heptagermanate glass.

Results of conductivity (ac) measurements are reported for synthetic mixed metal hydroxides as a function of temperature, ranging from 323 to 353°K, and relative humidity, ranging from 50 to 80%. The polycrystalline compacts of layered mixed hydroxides show protonic conductivities arising from the interlayer region. The conductivity is strongly dependent on the temperature, relative humidity and composition of mixed metal hydroxides. The conductivities are anisotropic with the direction of forming pressure owing to the preferential orientation of lamellar crystals, particularly evident with powders characterized by high crystal size.

Glasses whose compositions are expressed by the general formula. (50-x/2)BaO. (50-x/2)B(2)O(3 .)xTiO(2) with x=4, 8 and 16 have been prepared and the structural modifications occurring in the glass network as consequence of the heat treatments performed near the glass transition temperature and their effect on the devitrification behaviour have been examined by Fourier transform infrared spectroscopy, differential thermal analysis and x-ray diffraction. The 16 h heat treatment produces a microphase separation in the glassy matrix of all investigated glasses with a typical 'droplet structure'. Furthermore, for the glass with x=16, during this hear treatment nucleation of BaB4O7 crystals occurs. These crystals, for long nucleation times, act as heterogeneous nuclei for the growth of beta-BaB2O4 crystals.

The corrosion of steel in clay soil at m.c. 15 % has been studied for a long exposure time by electrochemical methods, A.c. impedance measurements results show that at a short exposure time the corrosion process is controlled by the diffusion of H + coupled with a rate determining homogeneous reaction, whereas at a long exposure time the process is controlled by pure diffusion. We have hypothesized that the rate determining homogeneous reaction might be the clay particles cations exchange.

To understand concrete reinforcement behaviour from the corrosion point of view it is necessary to have a thorough knowledge of the parameters characterizing the reinforcement passivity state. These parameters, varying with the cement type, the w c ratio, the moisture content, etc., can be determined by the a.c. impedance method. In order to improve the knowledge of these parameters, tests were performed over extended time periods on steel/concrete electrodes for some commercial cements. Results of a.c. impedance measurements carried out both during the concrete setting and in the following period are presented and discussed.

Synthesis of ferroelectric materials by glass crystallisation is attractive to produce both high-quality textures and composites. In the present work, composites based on ferroelectric stillwellite-like LaBSiO5 and glasses having compositions near to compounds Pb5Ge3O11, PbLiPO4 and PbBPO5 were synthesised and their dielectric and pyroelectric properties examined. A pyroelectric coefficient of about 0.5 nC/cm(2) K may be obtained by liquid-phase sintering of pellets. Preliminary data on crystallisation processes of the lead-containing glasses as well as on the chemical interactions between stillwellite-like LaBSiO5 and glasses at the sintering temperature are reported. (C) 2000 Elsevier Science Ltd. All rights reserved.

The effect of dc poling on lanthanum boron germanate glasses has been investigated by means of Raman spectroscopy and birefringence experiments. The results show that the dc poling induces in the glass a breakage of its macroscopic centre of inversion and this effect extends to the whole bulk of the poled glasses.

The possibility to produce polar phases from glass has been examined for ternary oxide systems of the stillwellite family (trigonal LnBXO(5), where Ln=La,Ce,Pr,Sm; X=Si,Ge and RBPO5, where R=Ca,Sr,Ba,Pbl, tetragonal K-2(NbO)(2)Si4O12 and orthorhombic PbLiPO4. Both original and literature data on crystallization behaviour of glasses in the Ln(2)O(3)-B2O3-XO2, RO-B2O3-P2O5, K2O-Nb2O5-SiO2, and Li2O-PbO-P2O5 systems are reviewed. The anisotropy in properties and morphology of polar crystals is considered as the main reason of grain-oriented crystallisation of glasses. Three new types of ferroelectric glass ceramic textures in the systems mentioned are reported and their specific features are described.