A structure model for phase separated fluoroaluminosilicate glass system by molecular dynamic simulations

Oxyfluoride glass and glass ceramics have been widely used in optical devices with Ln³⁺ doping. Especially, fluoride phase separation is considered as the initial stage of nanocrystallization that can help to realize the controllable crystallization and also provide the low phonon energy environment in oxyfluoride glass. Therefore, a structure model to describe this phase separated glass system is in great help for designing optical materials with high luminescence. In this study, large-scale molecular dynamics (MD) simulations were performed to understand the structural heterogeneity and make attempt to propose a structure model for fluoroaluminosilicate glass. The simulation results reveal immiscible behaviors of the fluoroaluminosilicate glass that separate into fluoride-rich phase and aluminosilicate-rich residual matrix, which have structural features analogous to fluoride and aluminosilicate glass, respectively. Besides, Al³⁺ plays an essential role to inter-connect oxygen and fluorine ions on the oxide-fluoride phase interface, maintaining the stability of the glass system.     

Design,simulation, elaboration and luminescence of Tb3+-doped Ba0.84Gd0.16F2.16 fluoroaluminosilicate scintillating glass ceramics

Extensive researches on scintillators have been executed to satisfy the excellent radiation detection materials in broad applications. However, practical application of conventional scintillators is limited due to the limitations of high cost, time-consuming fabrication process and insufficient radioluminescence. Herein, high density precursor glass doped with Tb³⁺ was designed to absorb X-ray efficiently and produce green emission. Molecular dynamics simulation was used to simulate the phase separation process in melting process. Then, Tb³⁺-doped Ba_0.84Gd_0.16F_2.16 glass ceramics (GCs) with excellent structural and optical properties were elaborated by melt quenching technic and further heat treating. Their structural properties, photoluminescence (PL) and X-ray excited luminescence (XEL) were explored detailedly. The internal quantum efficiency of PL is 64 % in GCs. The XEL intensity is 192 % of that of Bi₄Ge₃O₁₂ (BGO) commercial scintillator. Our results suggest that Ba_0.84Gd_0.16F_2.16:Tb³⁺ GCs might have potential application in X-ray detection.     

Structural analysis of oxyfluoride borate glass and BaF2 crystallization from phase separation

Oxyfluoride glass and glass-ceramics are being studied due to the possibility of their utilization in many applications, such as thermoluminescent dosimeters, optical fibers, fluorescent concentrators, and temperature sensors. In order to improve the properties of glass, they are often crystallized. For this crystallization to be effective, knowledge and control of the structure of the starting material is required. Much work was done on silicate glass; however, a great interest exists in obtaining these kinds of materials in other matrices such as germanates, phosphates, and borates. Here, we present a structural study of some oxyfluoride borate glass with different concentrations of PbF₂ by means of Nuclear Magnetic Resonance (NMR), Raman, and Fourier Transform Infrared (FTIR) spectroscopies. We also analyzed glass-crystallization using Differential Scanning Calorimetry (DSC). The crystallization study was complemented with the use of Transmission Electron Microscopy (TEM) and Selected Area Electron Diffraction (SAED), for the purpose of identifying possible mechanisms of crystallization. Our results indicate, from the structural point of view, that the glass present groups with one or two BO₄ species, such as diborate or pentaborate. The small variation in the BO₄ fraction with the increase in PbF₂ concentration would be an indication of the formation of F⁻BO_3/2 species. Oxyfluoride borate glass containing BaO as a cation modifier and 30% PbF₂ represents a good starting point for obtaining glass-ceramics with a narrow size distribution of BaF₂ nanocrystals. We also propose that the possible mechanism for oxyfluoride borate glass crystallization started with a separation of phases.     

A survey of the rheological properties,phase morphology,and crystallization behavior of PP-POSS materials with weak phase separation

Polypropylene (PP) compounds with varying amounts (0.4-8.2 vol%) of tailored allyl-isobutyl polyhedral oligomeric silsesquioxane (POSS) were prepared by melt-blending. The dependence of the crystallization behavior and crystalline structure of PP on the melt-state phase morphology of PP-POSS materials is addressed. PP-POSS systems were predicted to exhibit weak phase separation in both the molten and solid states based on Bagley plot using Hansen solubility parameters. Small-amplitude shear rheometry analysis suggested that the highest loaded (8.2 vol%) PP-POSS system behaves as a two-liquid emulsion, whereas the low-content POSS (0.4-4.0 vol%) systems deviate from this model, resembling a polymer nanocomposite. Based on these findings we hypothesized the formation of a heterogeneous phase morphology in the molten state comprised of nano-size “pseudo-solid” POSS clusters and micrometer-size “pseudo-liquid” POSS droplets dispersed within the PP matrix, depending on the POSS content. Upon cooling, POSS droplets comingle, forming cube-like micrometer-sized crystal domains. The nonisothermal crystallization of PP is enhanced by the presence of POSS clusters. Small-angle X-ray scattering analysis revealed the formation of thinner and more heterogeneous folded chain PP lamellae in the PP-POSS systems.     

Precipitation of cesium lead halide perovskite nanocrystals in glasses based on liquid phase separation

Incorporation of cesium lead halide perovskite nanocrystals (CsPbX₃ PNCs, X = Cl, Br, I) into glasses can significantly improve their stabilities and extend their application areas. Precipitation of CsPbX₃ PNCs in glasses is mainly based on thermal treatment; however, the formation of mechanism is not clarified. Here, several in situ methods are employed to illustrate the precipitation mechanism of CsPbX₃ PNCs. It is found that precipitation of CsPbX₃ PNCs in glasses is based on the liquid phase separation in solid amorphous matrix, followed by crystallization in the supercooled state. Liquid phase separation process determines the composition of CsPbX₃ PNCs, and cooling process has strong effect on the crystallinity and quality of CsPbX₃ PNCs. These results clarify the precipitation mechanism of CsPbX₃ PNCs in glasses and provide important guideline for the development of CsPbX₃ PNCs embedded glasses for opto-electronic applications.     

Effect of Sr and Ca substitution of Ba on the photoluminescence properties of the Eu2+ activated Ba2MgSi2O7 phosphor

The effects of Sr and Ca substitution of Ba on the Ba_1.98-xSr_x(Ca_x)MgSi₂O₇:Eu²⁺ photoluminescence properties have been investigated. The physical mechanisms for the photoluminescence variations are discussed. With Rietveld refinement method, the crystal structure of Ba_1.98MgSi₂O₇:0.02Eu²⁺ and the lattice parameters of Sr and Ca substituted phosphors were refined. The emission band shift, the photoluminescence intensity variation, the phosphor chromaticity evolution, the Eu²⁺ lifetime distribution and the thermal stability elevation were investigated. With Sr and Ca substitution, the cell is shrinks. The cell shrinkage is resulting in the increase of the Eu²⁺ 5d electron crystal field splitting intensity, which is the reason for the emission band shift towards the long wavelength band. The photoluminescence intensity is increased firstly and then decreased. The intensity variation is the competitive result between the increase of the crystal structure rigidity and the rise of the lattice defect. The correlated color temperature can be cut down and the color purity can be adjusted. The photoluminescence life time of Eu²⁺ is raised firstly and then decreased. For Sr and Ca substitution, the thermal stability can be elevated. With the forbidden band gap calculation, the reason for the thermal stability elevation was investigated that for the substituted phosphors the forbidden band gap is enlarged and then limits the Eu²⁺ 5d self-ionization from the splitting levels to the conduction band. This work reveals that the Sr and Ca substitution of Ba can elevate the Ba_1.98-xSr_x(Ca_x)MgSi₂O₇:Eu²⁺ photoluminescence properties and improve the applications for the White Light Emitting Diode.     

Unveiling the evolution of early phase separation induced by P2O5 for controlling crystallization in lithium disilicate glass system

The nucleation and crystallization of glass-ceramics are typically influenced by early phase separation, which can impact glass properties. However, it has been challenging to characterize the nanoscale phase separation and understand the nucleation mechanism of lithium disilicate (L₂S) glass-ceramics, which has resulted in some controversy. Here, we raised the direct evidence of nanoscale clustering in the glassy phase prior to formal nucleation and crystallization by element distribution. Firstly, the amorphous Li₃PO₄ phase formed on the boundary between the phase separation area and residual glass matrix, and then nucleation tended to start on this phase boundary. Furthermore, the effect of phase-separation on nucleation and final crystallize products was illustrated. By sufficient phase-separation, the formation of desired Li₂Si₂O₅ and LiAlSi₄O₁₀ microcrystals was effectively motivated, which is prerequisite for high mechanical properties and transparency. We hope this work provides guidance to rationally understand the early phase separation in glass for subsequent controlling crystallization.     

Mixed alkali effects in Er3+-doped borate glasses: Influence on physical,mechanical, and photoluminescence properties

Engineering borate glass structure by simple compositional modification is essential to meet the particular demands from both industrial and photonic research communities. In this article, we demonstrate how the mixing of Li₂O with Na₂O, K₂O, or Cs₂O influences the relative abundance of 3- and 4-coordinated borons in the glass network, as exhibited by nuclear magnetic resonance (NMR) and Raman spectra. A systematic study is given of the alkali mixing effect on the glass properties including: glass transition temperature, microhardness, density, refractive index, and particularly the near-infrared photoluminescence properties (eg, bandwidth and lifetime) of Er³⁺ which has been barely studied in mixed alkali borate glasses. It is interesting to note that the glass transition temperature, refractive index and emission lifetime of Er³⁺ manifest mixed alkali effect, in sharp contrast with microhardness, density, and emission bandwidth which vary monotonically upon the alkali mixing. The possible causes for the differences are discussed in the light of the compositional dependence of boron species and nonbridging oxygen upon alkali mixing.     

Relation between liquid-liquid phase separation and crystallization in isotactic and syndiotactic polypropylene solutions

The effects of the stereospecificity of a polymer chain and of the interaction in polypropylene (PP) solutions on the relation between liquid-liquid phase separation and crystallization were investigated by using an isotactic PP (i-PP) and a syndiotactic PP (s-PP) of high stereoregularity and of similar molar mass. Dialkyl phthalate was used as a solvent. A series of dialkyl phthalates with a different number of carbon atoms in the alkyl chain was employed to control the interaction between polymer and solvent. Phase transition temperatures were measured by optical microscopy with a hot stage. Liquid-liquid phase separation temperature (T_L-L) in the system of i-PP and dihexyl phthalate was located below its melting temperature (T_m). However, T_L-L for the s-PP system in the same solvent was elevated much above its T_m due to a decreased T_m and increased T_L-L. The reduced solubility of s-PP is primarily attributed to enhanced hydrophobicity arising from alternate positioning of the methyl groups along the polymer chain. As the length of the alkyl chain in the phthalate increases, T_L-L decreases significantly and T_m decreases slightly, resulting in the value of T_L-L shifting below that of T_m for the solution of s-PP and dinonyl phthalate. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 67: 159–163, 1998     

Improved photoluminescence and multi-mode optical thermometry of Er3+/Yb3+ co-doped (Ba,Sr)3Lu4O9 phosphors

Contactless optical thermometers have attracted extensive attentions for applications in scientific research and technological fields due to their apparent advantages. Herein, a novel sequence of Ba_3-xSr_xLu₄O₉ (B_3-xS_xLO):Er³⁺/Yb³⁺ phosphors were successfully prepared to investigate the temperature sensing property. By establishing energy transfer from Yb³⁺ to Er³⁺ and regulating the local lattice environment, up-conversion luminescence of Er³⁺ is dramatically improved when excited by 980 nm laser. This can effectively promote signal-noise ratio and reduce the errors in temperature detection. Furthermore, a multi-mode optical thermometry, which includes the fluorescence intensity ratio (FIR) from two thermally coupled levels of ²H_11/2/⁴S_3/2, FIR based on non-thermally coupled system of ²H_11/2/⁴F_9/2 and fluorescence lifetime of ⁴S_3/2 state of Er³⁺, was explored systematically. The fabricated samples exhibit the superior temperature measurement performances containing wide temperature-sensing range, superior signal discriminability, high sensitivity and favorable repeatability, indicative of the enormous utilization prospects of B_3-xS_xLO:Er³⁺/Yb³⁺ for thermometry.     

Effect of small additions of Li2O on phase separation and crystallization of barium-silicate glasses

It is known that the addition of Li₂O to 33.3BaO-66.7SiO₂ glass, whose composition is the same as BaSi₂O₅, promotes crystallization of BaSi₂O₅. In this study, in order to clarify the effect of a smaller amount of Li₂O, xLi₂O-(30-x)BaO-70SiO₂[mol%] (x = 0, 0.2, 0.5) glasses were prepared. The main crystalline phases in the heat treatments near the maximum crystallization peak temperature, were high-BaSi₂O₅ and low-BaSi₂O₅ which transformed from high-BaSi₂O₅. It is found that the introduction of only 0.2 mol% and 0.5 mol% Li₂O significantly changes the crystallization behavior. In the composition without Li₂O, only high-BaSi₂O₅ was formed after heat treatment even for 24 h. For compositions containing Li₂O, low-BaSi₂O₅ was formed within 1 h of heat treatment. In these compositions, it is found that the addition of Li₂O enhances phase separation in the early stage of heat treatment, resulting in the formation of Si-rich droplet phases and Ba-rich phases. The composition of the Ba rich glass phase would be close to the stoichiometric composition of BaSi₂O₅, suggesting a significant change in crystallization behavior.     

Influence of the process on phase separation induced by radical copolymerization of styrene and divinylbenzene in the presence of reactive polyurethane network precursors

Interpenetrating polymer systems based on crosslinked polyurethane (PU) and polystyrene (PS) were prepared at room temperature by a one‐shot (in situ) method, starting from an initial homogeneous mixture of reagents via noninterfering mechanisms. Both polymerizations were performed either simultaneously or one after the other. True simultaneity at every stages of the process is not possible. Despite the difference in refractive index of the components, hazy or optically clear films were obtained, thus indicating various levels of phase separation, also confirmed by glass transition temperature (T_g) measurements. The results suggest that controlling the chemistry and process (crosslink density, composition, and time sequence of events) of in situ interpenetrating network formation will give various morphologies, and hence properties, ranging from microphase separated materials to larger macrophase separated materials. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci, 2006     

Preparation,characterization and in vitro biological response of simultaneous co-substitution of Zr+4/Sr+2 58S bioactive glass powder

In the present study, structure of zirconium-containing bioactive glass (58S-BG (Zr-BG)) with optimal fixed Zr content (5 mol.%) was modified by incorporation of strontium (Sr). These Zr and Sr-containing BGs (ZS-BGs) were synthesized by sol-gel method and substitution of Ca with modifier ions (Sr content = 0, 3, 6, 9, and 12 mol.%). The results obtained from characterization by X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier-transform infrared (FTIR), and energy-dispersive X-ray spectroscopy (EDS) techniques from surface of all the ZS-BGs revealed formation of hydroxyapatite (HA) after 7 days of immersion in the simulated body fluid (SBF) solution. Evaluation of changes in the SBF solution, by monitoring pH variations and ions? concentration, was in agreement with the results of morphological and structural investigations. The in-vitro biological function of synthesized BGs was studied through (MTT) assay and alkaline phosphatase (ALP) activity analysis. The results showed that all the specimens significantly stimulated proliferation and viability of MC3T3 osteoblast-like cells. Furthermore, antibacterial studies confirmed less resistance of methicillin-resistant Staphylococcus aureus (MRSA) bacteria against ZS-BGs. Eventually, the results of in-vitro bio-analysis were clarified and confirmed by two cell staining techniques of Live/Dead and Dapi/Actin. This confirmation was achieved by observing the increased quantity of live cells and their nuclei as well as the decreased number of dead cells after co-culturing with all ZS-BGs.     

First-principles study of phase-transition temperature and optical properties of alkaline earth metal (Be,Mg, Ca,Sr or Ba)-doped VO2

Vanadium dioxide (VO₂) is an attractive material for energy-saving smart windows due to its metal-to-insulator reversible phase transition near ambient temperature, accompanied by large changes in its optical properties. We conducted first-principles calculations to study the phase-transition temperature and optical properties of alkaline earth metal (Be, Mg, Ca, Sr or Ba)-doped VO₂. The results show that the Be atom prefers to locate at the octahedral interstitial site, while Mg, Ca, Sr and Ba atoms prefer to substitute for the V atom in VO₂. Be, Mg, Ca, Sr and Ba doping reduces the phase-transition temperature of VO₂ 0by 51.4, 59.7, 61.5, 58.4 and 58.3?K, respectively, when the doping concentration is set at one atomic percentage. In addition, the introduction of alkaline earth metal scales the band structures of VO₂, which enhances the ability to block the infrared light (in the order of Be > Mg > Ca > Sr > Ba) and promotes the transmission of visible light (in the order of Be > Mg ≈ Ca > Sr > Ba).     

Effect of B2O3 on the structural and in vitro biological assessment of mesoporous bioactive glass nanospheres

In this paper, the boron-containing mesoporous bioactive glass (MBG) nanospheres have been successfully synthesized by modified sol-gel method assisted by surfactant, and the effect of boron substitution on structure and bioactivity was evaluated by combining experiments and ab initio molecular dynamics (AIMD) simulations. All of the samples exhibit regularly uniform mesoporous spherical microstructure with an average size of about 60 nm, and the boron-containing MBGs show higher specific surface area with the value up to 416.20 m²/g. The simulated body fluid (SBF) immersion test confirms that the deposited hydroxyapatite (HA) evidently increases with the increasing of boron content, indicating that the biological behavior has been significantly improved resulting from incorporation of boron. Additionally, our results also reveal that B₂O₃ substitution has positive impact on cell proliferation of human periodontal ligament cells (hPDLCs) at lower extracted concentration. Furthermore, AIMD simulation is employed to understand the relationship between structural changes and in vitro bioactivity in terms of structural information, especially the boron coordination number. The results illustrate that the boron-containing MBG nanospheres with excellent bioactivity are great potential for biomedical applications.     

Cation Deficiency in Bi2(Sr,Ca)3Cu2O8+d

X-ray lattice parameter measurements of the Bi_{2- x} (Sr,Ca)_{3- y} Cu_{2- z} O_{8+ d} system show that a continuously varying solid-solution cation nonstoichiometry exists within the overall composition ranges x < 0.32, -0.2 < y < 0.32, and z < 0.25. A modulated structure along the b axis remains present over these composition ranges. Defects introduced by cation deficiency have been clarified to be vacancies by density measurements. The oxygen content determined by titration decreases proportionately with increasing cation vacancy concentrations. In Bi-deficient compositions, the interstitial oxygen ion concentration decreased and the oxygen vacancy concentration increased with increasing concentration of Bi vacancies. The critical temperature, T _c , shows a maximum value at the stoichiometric cation composition and decreases to a slightly lower (5 to 10 K) constant value across the nonstoichiometric composition region with both increasing alkaline-earth and Cu deficiency.     

Distribution of Tm3+ and Ni2+ in chalcogenide glass ceramics containing Ga2S3 nanocrystals: Influence on photoluminescence properties

The distribution of Tm³⁺ and Ni²⁺ ions is unambiguously exhibited in 80GeS₂-20Ga₂S₃ chalcogenide glass ceramics (GCs) containing Ga₂S₃ nanocrystals (NCs) by using advanced analytical transmission electron microscopy. Distinctively different distribution patterns of Tm³⁺ and Ni²⁺ ions are observed in the GCs obtained by controlled crystallization. The distribution of the dopants imposes strong influence on their optical properties which are revealed by absorption and photoluminescence (PL) spectra. Detailed discussions are given of the mechanisms of the crystallization-induced PL enhancement and quenching of the Tm³⁺ mid-infrared and Ni²⁺ near-infrared emissions, respectively.     

Effect of singly,doubly and triply ionized ions on downconversion photoluminescence in Eu3+ doped Na2Sr2Al2PO4Cl9 phosphor: A comparative study

We report a change in the red photoluminescence of the Eu³⁺ doped Na₂Sr₂Al₂PO₄Cl₉ phosphor via doping of singly, doubly and triply ionized ions. The synthesized phosphors show good crystalline nature. The EDS analysis confirms the presence of desired elements in the phosphor samples. The vibrational feature of the phosphor was confirmed by FTIR analysis. The photoluminescence excitation spectra of the phosphor show three peaks at 317, 395 and 467 nm. The Eu³⁺ doped Na₂Sr₂Al₂PO₄Cl₉ phosphor emits intense red color on excitations with 395 and 467 nm wavelengths. However, the photoluminescence intensity of the phosphor is larger for 395 nm excitation. When the singly, doubly and triply ionized ions are co-doped in the Eu³⁺ doped Na₂Sr₂Al₂PO₄Cl₉ phosphor (i.e. F⁻, WO₄²⁻, MoO₄²⁻, VO₄³⁻, La³⁺, and Y³⁺) the photoluminescence intensity of the phosphor is decreased significantly. The decrease in photoluminescence intensity is due to change in local crystal structure created by these ions. Interestingly, the photoluminescence intensity of phosphor increases many times when the (Y³⁺) ion incorporated phosphor is excited with 317 nm wavelength. The CIE diagram shows color emitted in the red region of visible spectrum and the color purity is larger for triply ionized (Y³⁺) ion. Thus, the singly, doubly and triply ionized ions activated Na₂Sr₂Al₂PO₄Cl₉: Eu³⁺ phosphor may be used in displays devices, photonic devices, solid state lighting and white LEDs.