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Bioglass 45S5 is widely known for its ability to regenerate bone. However, this glass cannot be fully densified by viscous flow due to its very high tendency for crystallization. In this work, we evaluate the sintering and bioactivity of Bioglass 45S5-based compositions in which Na2O is incrementally replaced by K2O. Our sintering tests demonstrated that the densification of Bioglass 45S5 powders can be significantly enhanced due to a decreased crystallization tendency. The composition in which half of the original Na2O content was substituted by K2O exhibits the highest densification rate, evidencing the mixed alkali effect (MAE) in the viscous-flow-driven sintering process. The replacement of Na2O by K2O significantly improved both the densification rate and the in vitro performance. These results are very relevant for this particular glass and also for the design of new bioactive glasses with improved sinterability.

Na2AlTi(PO4)3 (NATP) and Na1.8Al0.8Ge1.2(PO4)3 (NAGP) NASICON (Na-Superionic Conductor) glass-ceramics are obtained by applying different single (SHT) and double (DHT) heat treatments on the respective precursor glass to evaluate its effect on the microstructure and in turn, on the total ionic conductivity. The grain and grain boundary contributions are also analyzed in the NATP composition. SHT with longer crystallization times favors the development of well-defined grains in both compositions and also promotes the grain growth in NAGP samples. This behavior causes a decrease in the activation energy of the grain boundary, which enhances the total ionic conductivity. Regarding DHT samples, microstructure with larger grains and higher ionic conductivity were obtained with shorter nucleation times for NATP and NAGP compositions. Finally, the microstructural variation generated by the different thermal treatments causes the total conductivity to increase up to two times.

In this study, we synthesize glass-ceramics of the new Na1+xGe2(SiO4)x(PO4)3–x NASICON (Na super-ionic conductor) series to evaluate the effect of Si4+/P5+ substitution on the structural, microstru...

Eu3+ and CdS were singly doped and co-doped into chloroborosilicate glasses with composition 37.8SiO2–27B2O3–18BaO–3.6K2O–3.6Al2O3–10BaCl2 (mol%), by one-step melt quench technique. The samples were characterized by transmission electron microscopy (TEM), UV–Vis absorption, excitation, and emission spectroscopy. TEM images revealed the presence of CdS quantum dots (QDs) of size 5–20 nm in the CdS-doped glasses. The Eu-doped glass shows characteristic orange emission of Eu3+ at 393 nm excitation, while excitation at 250 and 317 nm revealed the existence of Eu2+ ions. There are strong evidences of energy transfer involving the charge transfer band (CTB) and different energy levels of Eu2+ and Eu3+ ions. The CdS-doped glass showed broad emission bands, originating from various electronic transitions involving defect and trap states in CdS crystalline structure. The emission band in the lower wavelength region, shows a red shift with increasing excitation wavelength, which proves the quantum confinement effect in CdS QDs. Eu/CdS co-doped glasses showed significant enhancements in Eu3+/Eu2+ emissions, up to 20 and 70 times, upon excitations at 393 and 317 nm, respectively, which is considerably higher compared to previously reported similar systems. This enhancement is attributed to a complex energy transfer occurring between Eu2+/Eu3+ and CdS QDs, reported for the first time in a glassy system, especially considering the in-situ growth of the QDs during melt. The co-doped glass also showed pure white emission at 6–7 nm slit width upon 387 nm excitation, which is a novel finding from an Eu/CdS co-doped glass.

The production of residual biomass, such as vine pruning, presents environmental problems since its elimination is usually carried out through the uncontrolled burning of the remaining materials and with the emission of greenhouse gases without any counterpart. The use of these residues to produce biochar presents several advantages. In addition to the more common energy recovery, other conversion ways allowing new uses, such as soil amendment and carbon sequestration, can be analyzed as options as well. In the present study, vine pruning biomasses are characterized to evaluate the behavior of the different constituents. Then, the different possible applications are discussed. It is concluded that materials resulting from the pruning of vineyards have excellent characteristics for energy recovery, with an increment of more than 50% in the heating value and almost 60% in the carbon content when carbonized. This recovery procedure contributes to creating new value chains for residual materials to promote sustainable practices in the wine sector.

In the search for new materials to be used as solid electrolytes, this paper discusses the substitution of phosphorus with silicon in the NaTi2(PO4)3 NASICON (Na-Super Ionic Conductor) compound, giving rise to the Na1+yTi2SiyP3-yO12 series. In fact, the substitution of P+5 for Si+4 enables the increase in the Na+ charger carrier concentration. The solid electrolytes are synthetized by the glass-ceramic route, which consists in the controlled crystallization of a precursor glass subjected to specific heat treatment. Experimental results indicate that precursor glasses are successfully crystallized in compositions containing y ≤ 1.2. X-ray diffraction patterns show the formation of NASICON phase in the Na1+yTi2SiyP3-yO12 glass-ceramics for y ≤ 0.8. However, with further addition of silicon, the major crystalline phase obtained is the Na(TiO)(PO4) phase. Surprisingly, the electrical characterization reveals that the y = 1.0 sample, whose main phase is the non-NASICON Na(TiO)(PO4), exhibits the lowest activation energy (0.31 eV) and the highest ionic conductivity of 1.0 × 10−4 S cm−1 at room temperature and 1.7 × 10−2 S cm−1 at 300 °C. Rietveld refinement and electrical conductivity results suggest that the increased ionic conductivity in the Na(TiO)(PO4) phase is due to the inclusion of some Si+4 ions in its structure, thus forming a new and highly Na(TiO)((Si)PO4) conductive phase.

Glass-ceramics from the Na1+xAlxGe2–x(PO4)3 system were synthesized for 0 ≤ x ≤ 1.0. The stability parameters of precursor glasses were analyzed indicating that systematic additions of aluminum in the NaGe2(PO4)3 composition improve the precursor glass forming ability. Considering the glass-ceramics, it is observed that an increase in the aluminum content, x, causes a decrease in the activation energy for conduction, and thus an increase in the ionic conductivity. As x increases, beyond the corresponding increase in the charge carrier (Na+) concentration, it is also observed an increase in the unit cell volume of the NASICON structure, which may also contribute to the enhancement of ionic conductivity. Concerning the microstructure, the addition of aluminum provokes an increase in the average grain-size of the obtained glass-ceramics. Ionic conductivity increases up to x = 0.8 but remains constant within the experimental error for the x = 1.0 sample, probably due to a limit in the solid solution range. Ionic conductivity of composition x = 0.8 is still improved by applying higher crystallization temperatures, which decreases the activation energy for ionic conduction.

The military ballistic protection market has reached approximately US$ 10 billion/year, and the civil market is also very significant and is steadily increasing, which drives research for new ballistic protection alternatives. Materials currently used and under development as transparent ballistic protection devices include glasses, glass–ceramics, single and polycrystalline ceramics, and polymer-composites. Glass–ceramics are a less expensive, versatile alternative to currently used transparent ceramics. Glass–ceramics are generally harder, stiffer, tougher, and stronger than glasses, and can be made transparent or opaque, according to the needs of the protective system. They can be easily shaped in simple or complex forms, are much less expensive than carbides and nitrades, and are less dense than transparent alumina or spinel. This positive combination of properties makes glass–ceramics suitable for a wide range of applications. However, for an effective development of new transparent glass–ceramics (TGC), it is vital to understand which key properties must be optimized. In this review, we compile information on the role of each material in multi-layer ballistic systems, give a brief description of ballistic impact, and the properties related to it, and discuss the development of TGC to be used as ballistic protection. Our aim is to describe the most important ballistic protection materials and to indicate the pathways that research on TGC for ballistic protection have taken. We finish by concluding that there are still several opportunities that warrant further research on this particular application of glass–ceramics. Keywords: Transparent glass–ceramic, Ballistic protection, Add-on armor

The structure of crystalline and amorphous materials in the sodium (Na) super-ionic conductor system Na1+xAlxGe2-x(PO4)3 with x = 0, 0.4, and 0.8 was investigated by combining (i) neutron and x-ray powder diffraction and pair-distribution function analysis with (ii) 27Al and 31P magic angle spinning (MAS) and 31P/23Na double-resonance nuclear magnetic resonance (NMR) spectroscopy. A Rietveld analysis of the powder diffraction patterns shows that the x = 0 and x = 0.4 compositions crystallize into space group-type R3¯, whereas the x = 0.8 composition crystallizes into space group-type R3¯c. For the as-prepared glass, the pair-distribution functions and 27Al MAS NMR spectra show the formation of sub-octahedral Ge and Al centered units, which leads to the creation of non-bridging oxygen (NBO) atoms. The influence of these atoms on the ion mobility is discussed. When the as-prepared glass is relaxed by thermal annealing, there is an increase in the Ge and Al coordination numbers that leads to a decrease in the fraction of NBO atoms. A model is proposed for the x = 0 glass in which super-structural units containing octahedral Ge(6) and tetrahedral P(3) motifs are embedded in a matrix of tetrahedral Ge(4) units, where superscripts denote the number of bridging oxygen atoms. The super-structural units can grow in size by a reaction in which NBO atoms on the P(3) motifs are used to convert Ge(4) to Ge(6) units. The resultant P(4) motifs thereby provide the nucleation sites for crystal growth via a homogeneous nucleation mechanism.

Network former mixing (NFM) effects on ionic conductivities have been characterized in ternary germanotellurite glasses of composition (A2O)0.3[(TeO2)x(GeO2)1-x]0.7 (A = Li, Na), revealing an interesting cation dependence. While the results demonstrate a weak positive NFM effect in the Li-containing system, the electrical conductivities of the Na-containing systems correspond to the weighted average values observed in their binary endmembers. The structural origins of these differing effects have been explored by Raman scattering and 125Te solid-state NMR spectroscopy, indicating no significant differences in the structural organization of these materials. Overall, the data suggest that the concentration of heteroatomic connectivities is relatively low. For the Li-containing system, an analysis of the motional narrowing effects observed in temperature-dependent static 7Li NMR spectra confirms the existence of a weak positive NFM effect at the local atomic level. Accordingly, the absence of an NFM effect for the Na-containing system is corroborated by low-temperature 23Na spin-echo decay spectroscopy, showing very similar network modifier distributions in those glasses.

Laser-induced defects in glasses are of considerable interest for many applications from optics to photonics. The importance of low-level impurities of polyvalent ions in aiding defect formation has been identified early on. (Fluoride-)phosphate glasses are used today as laser materials, waveguides, amplifiers and luminescing materials, all sensitive to a change of the materials transmission by the interaction with light during application. To better understand the processes of defect generation and recovery, a systematic comparison of defect formation in various glasses and for various radiation sources and dopants has been conducted over the last decades. Here we will focus on (fluoride-) phosphate glasses doped with 50 to 5000 ppm of the 4d and 5d ions Zr, Nb, Ta, Mo and W. Glasses were melted under air or under reducing conditions in order to shift the redox equilibrium of the dopants before irradiation with either the 193 nm or 248 nm excimer laser. Only for W, Mo and Nb reduced ion species were confirmed by optical and/or ESR spectroscopy in the pre-irradiated glasses. However, irradiation showed for all metaphosphates the presence of reduced dopant species (W5+, Mo5+, Ta4+, Nb4+, Zr3+), acting as extrinsic hole centers (HC) after being photo-oxidized by laser irradiation to the fully oxidized d0 ions (Mn+)+–HC. Only for Ta5+ with its (Ta5+)− -electron center (EC), photo-reduction to the tetravalent ion was observed. Defect recovery was followed up to 16 years after the irradiation experiments, showing that most (Mn+)+–HC were very stable, while intrinsic HC either recombined with EC or converted into extrinsic (Mn+)+–HC. Due to ubiquitous iron impurities, even these high purity glasses with iron levels of 5–10 ppm or less, showed the formation of (Fe2+)+-HC.

Doped CeGdO and codoped CeGdOSmO compositions were synthesized, giving rise to nanoparticulate powders. Ionic conductivities at bulk and grain boundaries of the sintered samples were determined, exhibiting increased conductivity in the samaria-codoped samples. Scanning electron microscopy (SEM) showed a significant reduction in the grain size of samaria-codoped electrolytes. This reduced grain size of the codoped samples caused a reduction in Schottky barrier height, increasing oxygen vacancy concentration in the space-charge layer of the grain boundary and culminating in greater ionic conductivity in the boundary region. For the gadolinium doped samples, high resolution transmission electron microscopy images at grains showed the presence of large cluster of defects (nanodomains), hindering the movement of charge carriers and reducing ionic conductivity. However, the samaria-codoped system displayed better homogeneity at atomic level, resulting in reduced oxygen vacancy ordering and, consequently, smaller nanodomains and higher bulk (grain) conductivity. The reduced grain sizes and smaller nanodomains caused by codoping favor the ionic conductivity of ceria-based ceramics, doped with gadolinia and codoped with samaria.

The glass-to-crystal transition of Na1+xAlxGe2−x(PO4)3 (NAGP), and Na1+xAlxTi2−x(PO4)3 (NATP), both crystallizing in variants of the Na-superionic conducting (NASICON) structure, has been investigated by solid-state NMR. The ceramic materials produced by annealing the precursor glasses above the glass transition temperature are candidate materials for solid-state separator membranes in sodium ion batteries. The different local structural environments involving both network former and network modifier species have been characterized by comprehensive 23Na, 27Al and 31P magic angle spinning nuclear magnetic resonance (MAS NMR) experiments. In crystalline Na1+xAlxGe2−x(PO4)3 samples multiple phosphate environments are observed, corresponding to n Al and 4-n Ge species in their second coordination spheres. In contrast, no site resolution is observed in the analogous Na1+xAlxTi2−x(PO4)3 (NATP) system. This can be understood on the basis of X-ray powder diffraction (XRD) data, which reveal a significant lattice expansion in the former, but no lattice expansion in the latter material. 27Al MAS-NMR data reveal that in the glassy state, Al occurs with coordination numbers four, five and six, with the fraction of four-coordinated Al being substantially higher in the NATP glasses than in the NAGP glasses. 23Na MAS-NMR and spin echo decay measurements reveal distinct differences between glassy and crystallized materials with regard to the local environments and the spatial distributions of the sodium ions.

Ternary lithium borotellurite glasses of composition(Li2O)y[(2TeO2)x-(B2O3)1 − x](1 − y) (y = 0.33 and 0.40; and 0 ≤ x ≤ 1) reveal a negative network former mixing effect, implying that their electrical conductivities and ionic mobilities are lower than those expected by linear interpolation between their binary endmembers. For the y = 0.33 system, the room temperature ionic conductivity decreases from ~ 10− 8 (Ω·cm)− 1 to 10− 11 (Ω·cm)−1, while the activation energy increases from ~ 0.7 to 1.0 eV with increasing tellurium content. The structural foundations of the network former mixing effect have been probed by multinuclear solid state NMR spectroscopy. Static 125Te NMR spectra can be modeled by considering inhomogeneous broadening due to the chemical shift anisotropy effects, subject to Gaussian distributions of principal tensor values. The lineshape parameters are only weakly dependent on glass composition. 11B magic angle spinning (MAS)-NMR studies reveal that the fraction of four-coordinate boron species, B(4), increases moderately with increasing tellurium content. Overall, the data suggest that the concentration of heteroatomic connectivities is relatively low, suggesting chemical segregation and/or incipient phase separation into binary lithium tellurite and lithium borate units at the atomic- or the nanoscale. Static 7Li NMR spectra at low temperatures are governed by the strength of homonuclear 7Li 7Li and heteronuclear 7Li 11B/7Li 10B magnetic dipole-dipole interactions. Considering the second moments for the dipolar magnetic interactions in the lithium-diborate model compound, the second moments of the 7Li NMR lineshapes could be successfully modeled by a random spatial distribution of the Li+ ions. Phenomenological modelling of the motional narrowing effects observed with increasing temperature results in a consistent description of the negative network former mixing effects in terms of activation energies and dynamic heterogeneities. The negative network former mixing effects is stronger in the system with lower lithium ion content (y = 0.33).

Solid electrolyte powders based on gadolium-doped and samaria-codoped ceria of compositions Ce 0.8 Gd 0.2−x Sm x O 1.9 (x = 0.00; 0.01; 0.03; 0.05) were sintered using the polymeric precursor method (Pechini). The X-ray diffractometry results confirmed the formation of a single crystalline phase, that is, a solid solution. Test specimens were compacted using uniaxial cold pressing followed by two-stage sintering. Relative densities were higher than 96% of theoretical density in all the samples. Ionic conductivity was determined using impedance spectroscopy, obtaining values of 10 −2 S cm −1 at 700 °C, with the best results for codoped electrolytes, primarily the Ce 0.8 Gd 0.15 Sm 0.05 O 1.9. system. This study aimed at correlating ionic conductivity variations of codoped electrolytes with microstructural alterations (on a nanometric scale) resulting from the addition of codopants. Energy dispersive spectroscopy (EDS) was used to analyze the grain boundary compositions of the electrolytes produced and assess possible dopant segregation in these regions. Interplanar spacings corresponding to the crystallographic planes of the cubic (fluorite-type) phase were identified using high-resolution transmission electron microscopy (HRTEM) images. Extra diffraction spots and diffuse stains were observed in the doped and codoped electrolytes, using selected area electron diffraction (SAED), confirming the presence of amorphous nanodomains at the atomic level. However, the codoped system exhibited diffraction patterns with no diffuse stains, indicating that codoping favored obtaining nanodomain-free regions. These regions displayed better structural homogeneity, which may explain the enhanced ionic conductivity in codoped systems.

Studies on fuel cell components have attracted interest due to the growing demand for sustainable energy sources. In the present study, synthesis of nanometric powders of the Ce 0.8 Sm 0.2−x Gd x O 1.9 system (x = 0; 0.05; 0.1) system was carried out using the polymeric precursor method (Pechini), followed by calcination at 600 °C for 1 h and pressureless sintering. Characterizations were carried out with differential thermal analysis (DTA), thermogravimetric analysis (TG), infrared spectroscopy (FTIR), and X-ray diffraction (XRD). The grain and grain boundary contributions to the ionic conductivity of the ceramic disks obtained were assessed by complex impedance spectroscopy (CIS). Microstructural characterization was conducted by SEM. Electrical characterization showed greater grain conductivity for samples that were codoped with increasing levels of gadolinium, likely due to less deformation in the crystalline lattice with the addition of an element that contains an ionic radius closer to that of the host matrix of ceria (Ce

Borosilicate glass samples singly- and co-doped with Er3+ and ZnSe, were obtained via the melt quenching technique. Structural and morphological characterization confirmed the formation of ZnSe nanoparticles (NPs) during glass preparation. The glassy samples present the characteristic absorption bands of the respective species. Optical band gap values indicate quantum confinement effect in ZnSe-doped glasses. Excitation spectra of the co-doped glasses also display the characteristic bands of both ZnSe and Er3+. Intensity depleted regions at 377 and 521 nm observed in the excitation band of ZnSe indicate absorption by Er3+, that is, energy transfer from the ZnSe NPs to Er3+ ion. Upon excitation at 367, 447 and 498 nm, ZnSe doped glasses give broad emission bands centered around 696 nm. This reveals a broad multiple-band structure due to the overlap of emissions from NPs (electron-hole recombination) and from defect to traps, formed owing to Se and Zn vacancies. The Er3+-doped glasses display green (526 and 550 nm) and NIR (1530 nm) emissions when excited at 377 nm. The co-doped glasses show emission bands of both Er3+ and ZnSe. When compared to ZnSe and Er3+ singly doped glasses, a decrease is observed in the overall red emission while the green and NIR emissions from Er3+ increase. The intensity enhancement of Er3+ emissions increase with increasing ZnSe content which is attributed to the energy transfer from ZnSe NPs to Er3+. It was also observed that the color of the emitted light from the co-doped glasses, changes with change in excitation slit width.

Glass and glass-ceramics (GCs) of the CaO-SrO-B 2 O 3 -TiO 2 -SiO 2 system are promising candidates as sealants for solid oxide fuel cells (SOFCs), mainly because of the possibility of the formation of Sr 2 SiO 4 and Sr(TiO 3 ) as crystalline phases, which minimizes the interfacial reaction between the interconnection elements and the sealant. Four glass compositions of this system, with different TiO 2 content, were obtained. The crystalline phases formed after crystallization were identified by X-ray diffraction (XRD). Combining Differential Scanning Calorimetry (DSC) and Optical Microscopy (OM), it was found that the overall activation energy for crystallization did not change with TiO 2 content. The predominant crystallization mechanism is controlled by the surface, and the growth of the crystalline layer is governed by diffusion. The sintering behavior was investigated by DSC and Hot Stage Microscopy (HSM). All glass compositions exhibited a single-stage shrinkage behavior, but those with a larger amount of TiO 2 showed high sinterability. The measured coefficient of thermal expansion (CTE) varied from 10.2 × 10 − 6 K − 1 (100–550 °C) in the glass to 13.9 × 10 − 6 K − 1 (100–550 °C) in the GCs, which is in good agreement with the typical CTE of SOFC components. These consistent values of the CTE and good sintering behavior allow us to propose that the glass composition with a higher percentage of titanium is a promising candidate for further experimentation and development as a sealant material.

The xAgI–(1 – x)AgPO3 glass system exhibits a wide and well-known range of variation in silver ion conductivity with the AgI molar ratio, x, while the total concentration of Ag+ ions does not change significantly. We propose an interpretation to explain this effect, based on the fact that only a fraction of Ag+ cations are effective charge carriers and that their concentration is determined by a dissociation equilibrium of AgI in the AgPO3 glass. In this case, the ionic conductivity stems from the thermodynamic activity of silver iodide, aAgI, dissolved in the AgPO3 glass. To verify this relationship experimentally, aAgI is determined from electromotive force measurements of solid-state cells in the temperature range of 25–90 °C. Our results reveal a linear dependence of silver ion conductivity on aAgI over 3 orders of magnitude. The substantial variations in aAgI observed with the increase in the AgI molar ratio x are justified assuming a regular solution model for the AgI–AgPO3 mixture.

Made available in DSpace on 2019-10-06T15:25:50Z (GMT). No. of bitstreams: 0 Previous issue date: 2019-02-01 Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) Oxyfluoride glasses with composition x WO3 - 30 NaPO3 - (70-x) NaF, with 30 ≤ x ≤ 70 were prepared by melt-quenching and characterized with respect to their bulk physical properties and ionic conductivities. As the NaF content increases, the glass transition temperature, Tg, decreases systematically. Impedance measurements reveal no clear effect of NaF content upon room temperature electrical conductivities and activation energies. There is, however, a significant decrease of the Arrhenius pre-exponential factor with increasing WO3 content, suggesting that the electrical conductivities measured in this glass series are increasingly influenced by an electronic contribution. This contribution may be related to a W5+ → W6+ electron hopping process detected in the optical absorption spectra. Detailed spectroscopic investigations by Raman scattering and multinuclear one- and two-dimensional solid-state NMR experiments indicate that NaF acts like a network modifier resulting in the formation of W-F bonds via breakage of W-O-W and W-O-P linkages, with the former process being the preferred one. The concomitant decrease in network connectivity caused by these processes accounts for the experimentally observed decrease in glass transition temperatures with increasing NaF contents. Chemistry Institute São Paulo State University – UNESP Chemistry Department Federal University of São Carlos -UFSCar São Carlos Institute of Physics University of São Carlos -USP Department of Materials Engineering Federal University of São Carlos -UFSCar Institut für Physikalische Chemie Westfälische Wilhelms-Universität Münster Chemistry Institute São Paulo State University – UNESP FAPESP: 2013/07793-6 FAPESP: 2013/23490-3

This work reports the microstructure, and structural and electrical properties of Na3.4AlxSc2-xSi0.4P2.6O12 (0.0 ≤ x ≤ 1.7) NASICON (Na+ Super-Ionic Conductor) compounds obtained from the crystallization of a precursor glass. The addition of aluminum in the Na3.4Sc2Si0.4P2.6O12 NASICON composition improves the glass-forming ability of the precursor glass, which is evaluated by thermal analysis. Moreover, the Al3+/Sc3+ isovalent substitution allows us to change the lattice parameters of the NASICON structure, without varying the number of charge carriers (Na+ ions) per unit formula. X-ray diffractograms reveal the formation of the Na3Sc2(PO4)3 NASICON-type phase after crystallization of the parent glasses. From the structural characterization, it is also noted that the inclusion of Al3+ in the NASICON structure causes a volume contraction, which in turn, increases the activation energy and decreases the ionic conductivity. Thus, we demonstrated that the substitution of Sc3+ ions in the Na3Sc2(PO4)3 NASICON-type structure by an ion of the same valence, but with a smaller atomic radius, decreases the ionic conductivity of the material.

In this paper, the structural changes and optical, dielectric and magnetic properties of Bi5Ti3FeO15-based compounds were investigated following the gradual replacement of bismuth by lanthanum. The powders were produced by a solid-state reaction method; the LaxBi5−xTi3FeO15 system with x varying from zero up to 2.5 was investigated. The phase formation, structure and particle morphology were evaluated for these materials, as well as the optical, dielectric and magnetic properties. The results revealed that lanthanum cations can be totally solubilized in the LaxBi5−xTi3FeO15 system up to x equals 1.5. Secondary phases begin to appear beyond this value. The study of optical properties revealed that the powders partially absorb the visible spectrum. The presence of iron cations reduced the bang gap energy of bismuth titanates, whereas Bi3+/La3+ substitution increased this parameter. The presence of lanthanum increased the electrical resistivity of the materials at room and higher temperatures up to the lanthanum-saturation limit. Simultaneously, the dielectric permittivity at low frequencies also increased. The investigation of magnetic properties demonstrated less defined tendencies depending on lanthanum content. However, the coercivity was gradually reduced by the presence of lanthanum in the materials structure, while the saturation magnetization enhanced for high lanthanum content. Therefore, this work presents promising properties of the LaxBi5−xTi3FeO15 system for several magneto-optoelectronic uses, and is also a guide to produce optimized materials and devices for these applications.

Precursor glasses with composition Na1+xAlxGe2-x(PO4)3 (NAGP) (0.6 ≤ x ≤ 1.0) are converted into Na-superionic conductor (NASICON) glass-ceramics by thermal treatments with varied duration and annealing temperature. Detailed X-ray powder diffraction with Rietveld refinement and 31P and 27Al solid-state nuclear magnetic resonance (NMR) spectroscopy show that extended annealing at the crystallization temperature leads to a progressive de-alumination, segregation of T-AlPO4 and other crystalline phases, accompanied by the formation of amorphous material. These results suggest that an earlier formed aluminum super-saturated structure equilibrates by losing aluminum upon extended annealing. However, the ionic conductivity of glass-ceramics is less affected than would be predicted by the Al loss encountered in the NASICON phase, suggesting that ionic conductivity in these samples is not only controlled by the composition of the NASICON phase but is further influenced by the other phases present, either by contributing directly to ion transport or by facilitating interparticle contacts.

Sodium-ion conducting glass-ceramics are synthesized by crystallization of Na1+xAlxTi2-x(PO4)3 (0 ≤ x ≤ 1.4) (NATP) parent glasses. The thermal characterization of glasses is performed by Differential Scanning Calorimetry (DSC) and allowed to determine the glass transition (Tg) and crystallization (Tx) temperatures. An increase in the Tx - Tg parameter is observed, indicating that the thermal stability of glasses against crystallization and also the glass-forming ability increase with the inclusion of Al2O3. The X-ray results show that the NaTi2(PO4)3 phase is the majority phase after the crystallization of the precursor glasses at Tx/ 30 min. X-ray results also confirm that the limit of solid solution is reached for x = 1.0. Electrical measurements indicate that the addition of aluminum ions promotes a decrease in activation energy resulting in the increment of the total ionic conductivity of glass-ceramics by up to five orders of magnitude when compared to the free-aluminum sample. The best sodium-conductive glass-ceramic is obtained for x = 1.0 with a total ionic conductivity of 3.2 × 10−3 S cm−1 at 300 °C and activation energy of 0.47 eV. Using the impedance data converted to the electric modulus formalism, the contribution of the grain and grain boundary in total ionic conductivity is analyzed. Finally, an increase in the average grain size of the glass-ceramics with the addition of aluminum is also observed, which contributes to the enhancement of overall ionic conductivity.

International audience; The electrochemical stability window of NASICON-structured glass-ceramics of the Li1+xCrx(GeyTi1-y)2-x(PO4)3 system is investigated by a combination of cyclic voltammetry, electrochemical impedance, and X-ray photoelectron spectroscopy techniques. Cyclic voltammetry analyses are performed using a three-electrode setup cell where Ag3SI/Ag is applied as a reference electrode. Cyclic voltammetry measurements are followed by in situ electrochemical impedance spectroscopy, enabling the effect of oxidation and reduction reactions on the electrical properties of the glass-ceramics in question to be determined. X-ray photoelectron spectroscopy, in turn, is applied to determine which chemical species undergo reduction/oxidation. Our findings reveal that the electrochemical stability of this material is limited by the reduction of Ti+4 cations in low potentials (around 2.1 V vs. Li+/Li) and by the oxidation of O−2 anions in high potentials (about 4.8 V vs. Li+/Li). After the first cycle, the electrolytes seem to be stable within a much broader electrochemical window than in the first cycle. However, the results indicate not only that the reduction at low potential is not deleterious to the electrical properties of the electrolytes but also that the oxidation reaction at high potentials is highly detrimental. These findings contradict the common perception about the outstanding stability of NASICON-structured electrolytes in oxidation potentials.

The fluorine motional dynamics in fluorosilicate glasses and glass-ceramics of the SiO2–PbF2–CdF2 system was investigated by complex impedance spectroscopy and 19F nuclear magnetic resonance (NMR) spectroscopy, and the coordination environment of a Cu2+ paramagnetic probe was examined by electron paramagnetic resonance (EPR) spectroscopy. Glass-ceramics were obtained by heat treatment of glass at a temperature between the glass-transition (Tg) and the onset of crystallization (Tx). Ionic conductivity of about 1.6 × 10–4 S/cm was obtained at 500 K for glass. The conductivity of the glass-ceramics was found to be lower than that obtained for the glass. The temperature dependence of the 19F NMR spin–lattice relaxation times was investigated between 300 and 770 K. The 19F NMR results of glass exhibit the qualitative features associated with fluorine mobility, namely, the presence of a relaxation rate maximum below Tg. The NMR relaxation data of the glass-ceramics were analyzed assuming two fluorine dynamics. ...

Lithium disilicate is a critical glass forming composition as it is the basis of some glass-ceramics and it is also a model glass for the study of homogeneous and heterogeneous crystal nucleation. Incorporation of nitrogen in this glass significantly changes its structure and affects different properties. In this paper, nitrided lithium disilicate glasses were prepared by partial substitution of oxygen up to 6 atm % N/(N + O). The modification of the silicate glass structure with nitrogen has been confirmed by Fourier transform infrared reflection (FTIR) spectroscopy. 29Si NMR and 7Li NMR have also been used to investigate the structural changes due to nitrogen incorporation. Substitution of oxygen by nitrogen with a higher coordination number (three) increases the connectivity of the glass. In fact, our NMR results indicate that nitrogen incorporation changes the Qn distribution (n = average number of bridging oxygens to silicon) of the silicate structural units: SiNO3, SiN2O2 and SiN3O, whereas there is no detectable change in the Li environment. We measured the ionic (Li+) conductivity by impedance spectroscopy and found that the incorporation of nitrogen leads to a decrease in the activation energy for conduction, resulting in an increase of up to four-fold in the ionic conductivity of the most nitrided glass. We explained this high conductivity by the Anderson and Stuart model. This work provides renewed interest in improving and understanding the ionic conductivity in oxynitride glasses.

This study examines the effect of substituting Ti by Cr and Ge on the glass stability of the precursor glass and on the electrical properties of the lithium ion-conducting glass-ceramics of the Li1+xCrx(GeyTi1-y)2-x(PO4)3 (LCGTP) system. A set of compositions of this system is synthesized by the melt-quenching method followed by crystallization. The main results indicate that the glass stability of the precursor glasses increases when Ti is replaced by Ge and Cr. After crystallization, all the glass-ceramics present NASICON-type phase, and their lattice parameters decrease with Ge and increase with Cr content, making it possible to adjust the unit cell volume of the NASICON-like structure. Furthermore, the ionic conductivity and activation energy for lithium conduction in the glass-ceramics are notably dependent on the unit cell volume of the NASICON-like structure. The Li1.6Cr0.6(Ge0.2Ti0.8)1.4(PO4)3 glass-ceramic composition shows the highest overall ionic conductivity (2.9 × 10−4 Ω−1 cm−1) at room temperature and reveals remarkably high ionic conductivity (1.2 × 10−3 Ω−1 cm−1) and low activation energy (0.259 eV) regarding grain contribution. The main findings suggest that the proposed system is promising to develop fast Li ion-conducting glass-ceramics, offering a compromise between the glass stability of the precursor glass and the electrical properties of the resulting glass-ceramic.

The crystal nucleation ( I ) and growth ( U ) rates of Na 2 O·2CaO·3SiO 2 glasses with increasing additions of Li 2 O were measured for the primary crystalline phase over wide and overlapping temperature intervals. A partial section of the phase diagram Na 2 O·2CaO·3SiO 2 –Li 2 O was constructed via DSC analysis and shows a narrow range of solid solution formation that is close to that of Na 2 O·2CaO·3SiO 2 . In the four-component system (Na 2 O–Li 2 O–CaO–SiO 2 ), a pseudo-binary eutectic system of Na 2 O·2CaO·3SiO 2 –Li 2 O·SiO 2 exists. The role of Li 2 O in the formation of the crystalline phases was investigated using DSC and X-ray analyses. The addition of Li 2 O results in decreases in the glass transition ( T g ) and liquidus ( T L ) temperatures. With increasing Li 2 O content, both the reduced glass transition temperature T gr ( T gr = T g / T L ) and the fragility index, m , pass through a minimum. These findings extend and confirm the known inverse correlation between T gr and I max (lower values of T gr correspond to higher values of I max ) and provide evidence for a similar correlation between m and I max . When using the low temperature data for the crystal growth rate ( U ) the viscosity (η) and diffusivity ( D ) were decoupled at T T g . Next, a parameter ( U ·η at T g ) that characterized the decoupling phenomenon was proposed.

In this study, lithium disilicate (LS2) glass samples with different particle sizes ranging from less than 105 to 850 μm were prepared. These specimens were inserted in a Pt-Rh DSC crucible and heated to 850°C at different rates (ϕ = 0.5–30 K/min) to identify their crystallization peaks. The activation energies for the overall crystallization (E) and the Avrami coefficient (n) were evaluated using different nonisothermal models. Specifically, n was evaluated using the Augis–Benett model and the Ozawa method, and E was evaluated using the Kissinger and Ligero methods. As expected, the coarse particles mainly crystallized in the volume, while surface crystallization was predominant in the samples with particle sizes of less than 350 μm. This result was confirmed through SEM analysis of the double stage heat-treated samples. In contrast with previous studies, our results demonstrated that the activation energy decreased as the particle size increased. In addition, no clear correlation between the peak intensity (δTp) and the particle size was observed.

In this work, mixed alkali metaphosphate glasses based on K–Na, Rb–Na, Rb–Li, Cs–Na and Cs–Li combinations were studied by differential scanning calorimetry (DSC), complex impedance spectroscopy, and Raman spectroscopy. DSC analyses show that both the glass transition (Tg) and melting temperatures (Tm) exhibit a clear mixed-ion effect. The ionic conductivity shows a strong mixed-ion effect and decreases by more than six orders of magnitude at room temperature for Rb–Na or Cs–Li alkali pairs. This study confirms that the mixed-ion effect may be explained as a natural consequence of random ion mixing because ion transport is favoured between well-matched energy sites and is impeded due to the structural mismatch between neighbouring sites for dissimilar ions.

This paper describes the preparation of a transparent glass-ceramic from the SiO2-K2O-ZnO-Al2O3-TiO2 system containing a single crystalline phase, gahnite (ZnAl2O4). TiO2 was used as a nucleating agent for the heat-induced precipitation of gahnite crystals of 5-10 nm. The evolution of the ZnAl2O4 spinel structure through the gradual formation of Al-O bonds was examined by infrared spectroscopy. The dark brown color of the transparent precursor glass and glass-ceramic was eliminated using CeO2. The increase in transparency of the CeO2-doped glass and glass-ceramics was demonstrated by UV-visible absorption spectroscopy. EPR measurements confirmed the presence of Ce3+ ions, indicating that CeO2 was effective in eliminating the brown color introduced by Ti3+ ions via oxidation to Ti+4. The hardness of the glass-ceramic was 30% higher than that of the as-prepared glasses. This work offers key guidelines to produce hard, transparent glass-ceramics which may be potential candidates for a variety of technological applications, such as armor and display panels.

International audience; Glass-ceramics (GC) generally possess enhanced mechanical properties compared to their parent glasses. The knowledge of how crystallization evolves and affects the mechanical properties with increasing temperature is essential to optimize the design of the crystallization cycle. In this study, we crystallized a glass of the MgO-Al2O3-SiO2 system with nucleating agents TiO2 and ZrO2. The crystallization cycle comprised a 48 hour nucleation treatment at the glass-transition temperature followed by a 10 hour growth step at a higher temperature. During this cycle, the evolution of crystalline phases was followed by high-temperature X-ray diffraction (HTXRD), which revealed the presence of karooite (MgO2TiO(2)), spinel (MgOAl2O3), rutile (TiO2), sillimanite (Al2O3SiO2), and sapphirine (4MgO5Al(2)O(3)2SiO(2)). The same heat treatment was applied for in situ measurement of elastic properties: elastic modulus, E, shear modulus, G, and Poisson's ratio, . The evolution of these parameters during the heating path from room temperature to the final crystallization temperature and during the nucleation and the crystallization plateaus is discussed. E and G evolve significantly in the first two hours of the growth step. At the end of the crystallization process, the elastic and shear moduli of the GC were approximately 20% larger than those of the parent glass.

Glass-ceramics have gained considerable importance for applications in high-energy technology. Li- and Na-superionic ion-conducting ceramics find widespread use in lithium- and sodium-ion batteries as separators, solid electrolytes, and cathode materials. The ionic conductivity of these materials is influenced by crystal chemical parameters and can be further optimized via microstructural control using glass-ceramic processing. This article summarizes the most promising glass-ceramic material systems currently in use, detailing recent progress in understanding their structure–property–performance relationships. We also highlight the power and potential of solid-state nuclear magnetic resonance techniques for providing quantitative knowledge about structure, phase composition, and ion dynamics in these materials.

Crystallization kinetics parameters of a stoichiometric glass with the composition Li1.5Al0.5Ge1.5(PO4)3 were investigated by subjecting parallelepipedonal samples (3 × 3 × 1.5 mm) to heat treatment in a differential scanning calorimeter at different heating rates (3, 5, 8 and 10 °C/min). The data were analyzed using Ligero's and Kissinger's methods to determine the activation energy (E) of crystallization, which yielded, respectively, E = 415 ± 37 kJ/mol and 378 ± 19 kJ/mol. Ligero's method was also employed to calculate the Avrami coefficient (n), which was found to be n = 3.0. A second set of samples were heat-treated in a tubular furnace at temperatures above the glass transition temperature, Tg, to induce crystallization. The X-ray diffraction analysis of these samples indicated the presence of LiGe2(PO4)3 which displays a NASICON-type structure. An analysis by optical microscopy revealed the presence of spheric crystals located primarily in the volume, in agreement with the crystallization mechanism predicted by the Avrami coefficient.

Highly ion conducting glass-ceramics, crystallizing in the Na-superionic conducting (NASICON) structure, have been prepared in the system Li1+xAlxSnyGe2-(x+y)(PO4)3 by crystallization of glassy precursor samples. For modest substitution levels (y = 0.25), these crystalline solid solutions show slightly higher electrical conductivity than corresponding samples without Sn, supporting the rationale that the lattice expansion associated with the substitution of Ge by its larger homologue Sn can enhance ionic conductivity. Higher Sn substitution levels (y = 0.45) do not result in any improvement. The glass-to-crystal transition has been characterized in detail by multinuclear single and double resonance NMR experiments. While substantial changes in the 31P and 27Al MAS NMR spectra indicate that the crystallization of the glasses is accompanied by significant modifications in the local environments of the phosphate and the aluminum species, the dipolar solid state NMR experiments indicate that the structures of...

Photo-thermo-refractive (PTR) glass is an optical Na–Al–Zn–K–O–F–Br silicate glass doped with Ag, Ce, Sb, and Sn that undergoes photo-thermo-induced volume crystallization of nanosized NaF responsible for localized refractive index changes. PTR glass has found numerous commercial applications, but the intricate mechanism of photo-thermo crystallization is far from being understood. In this paper, we demonstrate that, additional to crystalline phase precipitation, liquid–liquid phase separation (LLPS), i.e. amorphous droplets embedded in the matrix glass, appears concurrently over a wide range of temperatures. The immiscibility temperature is 925°C. The droplet phase is richer in SiO2, rendering the alkali-rich remaining matrix glass a lower glass transition temperature and a higher electrical conductivity than the original glass. The droplet's surface does not catalyze NaF nucleation. Although the effects of LLPS on optical properties of PTR glass are still to be explored, it could contribute to unwanted scattering losses and/or uncontrolled refractive index change. The substantial change in the original glass composition resulting from LLPS should play an important role on NaF crystallization kinetics, and therefore must be considered for an overall understanding of the crystallization mechanism underpinning the refractive index change in PTR glass.

Fully crystallized glass-ceramics of the Li 1 + x Al x Ti 2 − x (PO 4 ) 3 system were obtained by crystallization of a precursor glass, which shows a tendency for homogeneous nucleation. Different microstructures were obtained by single and double stage heat treatments. It is shown that, in the case of a single heat treatment, the ionic conductivity increases by three orders of magnitude when the temperature treatment increases from 700 to 1000 °C, reaching a maximum of 1.3 × 10 − 3 S⋅cm − 1 at room temperature. As deduced from X-ray diffraction, this enhancement in ionic conductivity is related to an increase of the sample's crystallinity. For samples obtained by double stage heat treatments, i.e., a heat treatment to induce nucleation followed by a second one for crystal growth, the ionic conductivity tends to increase when the duration of the nucleation treatment is reduced which leads to an increase in the average grain size of the glass-ceramics.

A Li1.5[Al0.5Ge1.5(PO4)3] glass composition was subjected to several crystallization treatments to obtain glass–ceramics with controlled microstructures. The glass transition (Tg), crystallization onset (Tx) and melting (Tm) temperatures of the parent glass were characterized by differential scanning calorimetry (DSC). The glass has a reduced glass transition temperature Tgr = Tg/Tm = 0.57 indicating the possibility of internal nucleation. This assumption was corroborated by the similar DSC crystallization peaks from monolithic and powder samples. The temperature of the maximum nucleation rate was estimated by DSC. Different microstructures were produced by double heat treatments, in which crystal nucleation was processed at the estimated temperature of maximum nucleation rate for different lengths of time. Crystals were subsequently grown at an intermediate temperature between Tg and Tx. Single phase glass–ceramics with Nasicon structures and grain sizes ranging from 220 nm to 8 μm were then synthesized and the influence of the microstructure on the electrical conductivity was analysed. The results showed that the larger the average grain size, the higher the electrical conductivity. Controlled glass crystallization allowed for the synthesis of glass–ceramics with fine microstructures and higher electrical conductivity than those of ceramics with the same composition obtained by the classical sintering route and reported in literature.

We investigated the kinetics of crystal nucleation, growth, and overall crystallization of a glass with composition close to the stoichiometric 1Na2O AE 2CaO AE 3SiO2. The nucleation and subsequent growth of sodium-rich crystals in this glass decreases the sodium content in the glassy matrix, drastically hindering further nucleation and growth. Compositional changes of the crystals and glassy matrix at different stages of the crystallization process were determined by EDS. These compositional variations were also monitored by electrical conductivity measurements, carried out by impedance spectroscopy, in glassy, partially, and fully crystallized samples. The electrical conductivity of both crystalline and glassy phases decreases with the increase of the crystallized volume fraction. Starting at a crystallized volume fraction of about 0.5, the crystalline phase dominates the electrical conductivity of the sample. This behavior was corroborated by an analysis of the activation energy for conduction. We show that electrical conductivity is highly sensitive and can indicate compositional shifts, changes in the spatial distribution of mobile ions in the glassy matrix. Conductivity measurements are thus a powerful tool for the investigation of complex heterogeneous systems, such as partially crystallized glasses and glass-ceramics. 2007 Elsevier B.V. All rights reserved.

Electrical conductivity of glasses from the system 0.40Li2O·0.60(xB2O3(1−x)Si2O4) (0≤x≤1) were measured by impedance spectroscopy. The influence of the substitution of the first glass former by the second in electrical conductivity and in the parameters of the Arrhenius expression, i.e., activation energy and pre-exponential factor, is discussed. The electrical conductivity increases from the silica to the boron oxide region, with an anomaly at x=0.3 but with no evidence of a mixed glass-former effect. The relaxation frequency, which is an intrinsic characteristic of each glass sample and is independent of geometrical parameters, was also deduced from impedance diagrams and are also presented for all compositions.

Electrical conductivity and dielectric relaxation frequency were measured along the X and Z crystallographic axes of a LiNbO3 single-crystal wafer, using the ac impedance spectroscopy technique. The measurements were carried out in the frequency range of 5 Hz–13 MHz and in the temperature range of 460–620 °C. Impedance data were plotted on Nyquist diagrams, which, in addition to the electrical resistance and relaxation frequency of samples, provide information about the charge carriers of the conduction process. Isothermal measurements were also taken as a function of time. Although no difference in the activation energy of conduction was found, a slight but unequivocal difference in electrical conductivity (about 0.16 on a log scale) was established between the two axes. In contrast, a more marked and substantial difference was found between the relaxation frequency of the axes, confirming that differences in the electrical behavior of the X and Z directions of LiNbO3 are more likely due to differences i...

International audience; The dielectric behavior of dense and porous NaNbO3 ceramic samples, synthesized by a suitable chemical route, was investigated by impedance spectroscopy between room temperature and 800°C in dry air. The dielectric behavior and thermal stability of the samples were evaluated as a function of several thermal cycles. The dielectric constant was calculated from the relaxation frequency, and from an alternative approach based on the variation of the opposite of the imaginary part of impedance as a function of reciprocal angular frequency. The values obtained using both relations were in a good agreement. After the porosity was corrected, the porous and dense samples presented the same dielectric constant. All samples evaluated displayed a broad dielectric anomaly between 300°–400°C. Neither the orthorhombic-tetragonal- nor the tetragonal-cubic-phase transitions were detected by dielectric measurements. The Curie-Weiss law was found to be valid above the transition temperature, whereas the corresponding phase transition presented a diffuse nature. The origin of the related thermal hysteresis is discussed herein.

International audience; The electrical behavior of NaNbO3 ceramic samples with different relative densities was investigated by ac impedance spectroscopy in a range of 13 MHz to 10−3 Hz between 400° and 800°C in dry air. Measurements were performed during heating and cooling cycles. The Nyquist impedance diagrams of dense sodium niobate exhibit only one semicircle representing the grain contribution with depression angles as small as 1°, indicating a high homogeneity of the specific electrical properties. In the case of porous samples, the data reveal an additional low-frequency semicircle related to microstructure. For all studied samples, the Arrhenius conductivity plots show a change in the activation energy around 640°C, attributed to the tetragonal-cubic phase transition. The electrical conductivity of porous samples appears to be higher than that of dense ones.