Physical and structural properties of Ge-rich chalcogenide glass sandwiched by GeS crystalline layers

Ge-rich glass-ceramics sandwiched by GeS crystalline layers were fabricated through 10?h thermal treatments at different temperatures. Surface crystallization is evidenced by XRD investigation of glass-ceramic samples polished by different times. SEM observation shows that the thickness of the crystalline layer is about 100?µm for the sample thermally treated at 395?°C for 10?h. The physical properties, including transmission spectra, density, Vickers hardness, and thermal expansion coefficient, were characterized and discussed with the evolution of GeS crystalline layers. This work not only establishes the correlation between microstructure and physical properties of chalcogenide glass-ceramics sandwiched by GeS, but also provides important evidence of structural similarity for understanding the network structure of Ge-S chalcogenide glasses.     

Effect of different nucleating agent ratios on the crystallization and properties of MAS glass ceramics

The effects of different kinds of nucleating agents on crystallization, microstructure and performances of Magnesium Aluminosilicate (MgO-Al₂O₃-SiO₂, MAS) glass-ceramics which were fabricated by melting method in this study. Also, this paper systematically investigated the mechanism of glass stability, crystallization kinetics and element distribution of MAS glass-ceramics. Herein, we used three kinds of nucleating agents, which was TiO₂, ZrO₂ and composite nucleating agent (TiO₂/ZrO₂). The results showed after the doping of nucleating agent, the content of α-cordierite was increased, the stability and crystallization kinetics of glass was changed, the precipitated crystal phase was finer and more compact. Wherein, the sample with composite nucleating agents (TiO₂, ZrO₂) has the best performance due to the highest contents of α - cordierite, uniform distribution of elements without agglomeration in the crystal phase and the most compact structure, whose Vickers hardness and bending strength can reach 9.70 GPa and 312 MPa, respectively.     

Deepening our understanding of bioactive glass crystallization using TEM and 3D nano-CT

One key issue influencing a broader application of Bioglass 45S5 in tissue engineering is its inherent crystallization tendency, severely limiting the mechanical strength of 3D porous scaffolds. Despite numerous studies, Bioglass 45S5 crystallization is not yet fully understood with regard to the mechanisms involved or morphology of the crystal phases forming. Here we show how two cutting-edge imaging techniques, state-of-the-art transmission electron microscopy (TEM) with image correction including energy dispersive X-ray spectroscopy and X-ray nano-computed tomography (nano-CT), allowed us to visualize changes in microstructure from near-nucleation to almost full crystallization in bulk Bioglass 45S5. At early times of heat treatment at 660 °C the formation of phase-separated nano-droplets within the glassy matrix was observed. Later, besides surface crystallization, bulk crystallization of combeite spheres was predominant. The formation of the first combeite spheres, their coarsening with time and finally their merging at near full crystallization were recorded by in situ high-temperature optical microscopy videos. The 3D nature of these spheres was confirmed by nano-CT, while TEM showed that their internal structure was composed of sub-micron grains. X-ray diffraction analysis at early time points showed a much higher crystalline fraction in bulk samples compared to powder samples, highlighting the influence of processing and sample morphology. These results show the importance of using complementary techniques for gaining insight into the crystallization process in the volume. In addition, we show that TEM and nano-CT are suitable characterization techniques to visualize the crystallization even in fast crystallizing systems, such as bioactive glasses.     

Glass structure and crystallization via two distinct thermal histories: Melt crystallization and glass crystallization

This study focuses on the effect of structural inhomogeneity of 44.4CaO-33.2SiO₂-8.8Na₂O-6.2Al₂O₃-7.4B₂O₃ glass on its crystallization behavior. The crystallization behavior of the investigated lime-sodium aluminoborosilicate glass system is explored for the two thermal histories of melt crystallization and the heat treatment of quenched glass. The results show that sodium melilite (CaNaAlSi₂O₇) and larnite (β-Ca₂SiO₄) are the dominant crystalline phases during heating (glass crystallization) and cooling (melt crystallization), respectively. In the glass structure from the heat treatment at 610 °C, both Si₂O₇ and AlO₄ units (i.e., four-coordinated Al) are dominant. This may induce sodium melilite (CaNaAlSi₂O₇) crystallization, connecting Si₂O₇ and AlO₄ units with O and linking Ca and Na between interlayers. The melt structure, meanwhile, has more SiO₄ and AlO₆ units in its silicate and aluminate. Therefore, larnite (β-Ca₂SiO₄) crystallizes on cooling by combining SiO₄ units and Ca. The crystallization behavior is further discussed considering the inhomogeneity structure of supercooled liquids before nucleation.     

Unveiling crystallization mechanism for controlling nanocrystalline structure in glasses

Controlling nanocrystalline structure in glasses renders the exploration of new composite multiphase (glass-ceramic) materials with novel functionalities that determined by the precipitated nanocrystals and residual glassy matrix. Previous microstructural investigation of glass-ceramics focused only on one aspect of nanocrystalline structures, e.g., nano-polycrystalline or single nanocrystalline. The recognition of the microscopic mechanism of nanostructure formation in glasses is absent. Here, we use advanced microscopic techniques to show the formation of different nanocrystalline structures composed of nano-polycrystals and single nanocrystals in 80GeS₂·20In₂S₃ and 72.5GeS₂·14.5Sb₂S₃·13RbCl glasses, respectively. Crystallization mechanism for controlling the nanocrystalline structure in glasses was revealed to depend on whether the glass network former participates in crystallization process. The results may shed light not only on glass crystallization mechanism, but also on the fundamental nature of the network structure of chalcogenide glasses.     

Comprehensive study of tellurium based glass ceramics for thermoelectric application

Tellurium based glasses have interesting thermoelectric characteristics. However, their high electrical resistivity is still an obstacle to considering them for thermoelectric applications. In this work, the (Te₈₅Se₁₅)_60???0.6xAs_40???0.4xCu_x glass system was studied. This revealed that Cu can act as glass former and increase both glass thermal stability and electrical conductivity. The best candidate, (Te₈₅Se₁₅)₄₅As₃₀Cu₂₅, was chosen to prepare composites with Bi_0.5Sb_1.5Te₃ using spark plasma sintering. These glass ceramic samples exhibited a much better thermoelectric performance. Glass ceramics with 50?mol. % of Bi_0.5Sb_1.5Te₃ show a maximum ZT value equal to 0.37 at 413?K. Meanwhile, the advantages of glass including low sintering temperature and high formability are well maintained.     

Effects of Ag additive on structure and crystallization behaviors of As2(Se15Te85)3 glasses

In this work, glass samples of compositions of As_40-0.4x(Se₁₅Te₈₅)_60-0.6xAg_x (x = 0, 10, 16.7, 20, 25 at%) are prepared. The structural transformations of glasses are deduced from the variations of glass densities and Raman spectra with the addition of Ag. Differential scanning calorimetry is applied to determine the characteristic temperatures, evaluate the thermal stabilities against crystallization, and investigate the crystallization kinetics under non-isothermal conditions. Thermal treatment of the as-prepared glass samples is carried out at both low (190 °C) and high (260 °C) crystallization temperatures. X-ray diffraction patterns demonstrate that crystals precipitated from glass matrices are pure As-Te(Se) phases free of Ag. The results are consistent with the Raman spectra. The relevant mechanism can be understood based on the dual chemical role of the Ag addition on the variations of glass network.     

Effect of Yb doping concentration on crystallization kinetics in yttrium aluminum garnet nano-powders

Different ytterbium concentration–doped yttrium aluminum garnet (Yb:YAG) nanopowders were synthesized using coprecipitation with nitrate salts as the starting materials. The phase evolutions, morphologies, and microstructures of the powders synthesized from various ytterbium-doped precursors were investigated. Ytterbium doping concentration was discovered to have a crucial effect during the YAG phase formation from the precursor. Crystallized Yb:YAG powders were directly obtained at temperatures as low as 900 °C without the formation of any intermediate phase. The crystallization kinetics of the Yb:YAG precursors were analyzed using non-isothermal differential scanning calorimetry. Avarami parameters of 0.97, 1.00, 1.13, and 1.21 were obtained for Yb doping concentrations of 0, 3, 6, and 9 at% respectively, and crystallization activation energies of 1506±40, 1342±36, 1171±31, and 978.1±26 kJ/mol were calculated. The activation energy for YAG crystallization was lower when a high Yb doping concentration was used because the presence of Yb³⁺ prohibited the presence of anionic SO₄²⁻ in the precursors, thus enhancing the elemental diffusion between particles. Both the average grain size and particle size of Yb: YAG decreased when Yb doping concentration was increased, and at various calcination temperatures.     

PbWO4 formation during controlled crystallization of lead borate glasses

Lead borate glasses were prepared and then heat treated in order to obtain transparent glass-ceramics. Controlled crystallization of precursor lead borate glass at appropriate annealing temperature and time led to formation of the PbWO₄ crystallites. The observed broad blue emission band is related to the PbWO₄ crystallites. The influence of PbX₂ content (X=F, Cl, Br), PbF₂ concentration and lanthanide doping (Eu, Dy) on the excitation and emission spectra of lead borate glass-ceramics containing PbWO₄ phase was examined. The relationship between Pb–X bond and spectral line width of the blue emission can be successfully observed, when halogen X ions (X=F, Cl or Br) are also present in the distorted PbWO₄ crystallites.     

Effect of comonomer type on the crystallization kinetics and crystalline structure of random isotactic propylene 1-alkene copolymers

Isothermal crystallization kinetics and properties related to the crystalline structure of four series of random propylene 1-alkene copolymers have been comparatively studied in this work. Comonomers studied include ethylene, 1-butene, 1-hexene and 1-octene in a concentration range up to 21 mol%. All copolymers were synthesized with the same metallocene catalyst to provide an equivalent random distribution and a similar content of stereo and regio defects within the series. This has ensured that differences in crystallization kinetics and in crystalline properties of copolymers with matched compositions reflect the affinity of the comonomer type for co-crystallization with the propene units, and the effect of content and type of co-unit in the development of the crystalline structure. In the nucleation-driven crystallization range, that is for T_cs > T_{c max}, the values of the rate follow the sequence PB > PE > PH = PO for comonomer contents <13 mol%, and PB > PE > PH > PO for >13 mol% comonomer. These trends in overall crystallization are guided by differences in undercooling due to a similar progression of the degree of participation of the comonomer in the crystalline lattice. The variation of the rates at T_cs < T_{c max} follows the melt segmental dynamics driven by differences in T_g, especially at the highest co-unit contents, resulting in a reverse rate sequence for PHs and POs >15 mol%, i.e., PB > PE ∼ PO > PH. In addition to crystallization kinetics, a comparative polymorphic analysis and unit cell expansion, crystalline morphology, and melting behavior have been instrumental in resolving the partitioning of the four types of co-units between crystalline and non-crystalline regions. 1-Butene units participate at the highest level followed by the ethylene units, as demonstrated by solid-state NMR. However, both units are defects that hinder crystallization, as given by the decreasing rates, decreased levels of crystallinity and lowered melting temperatures with increasing co-unit content. All crystalline properties of PHs and POs conform to a rejection model of the 1-octene units from the crystals in the whole compositional range, and rejection of the 1-hexene units for PH <13 mol%, a conclusion also supported by NMR. The ability of PH >13 mol% to pack comonomer-rich sequences into a stable trigonal lattice leads at T_cs > T_{c max} to an increased number of crystallizable sequences, and to faster crystallization rates than for matched PO copolymers.     

New in situ method for the determination of the crystallization kinetics of glass-ceramics

An in situ method for the characterization of the crystallization behavior of glass-ceramics has been developed, which is based on an acoustic, nondestructive Young's modulus measurement technique according to ASTM E1876. The progression of the crystallization can be measured due to an increasing Young's modulus during the heat treatment, which is a result of the formation of solid precipitations in a viscous melt. The crystalline content can be deduced from the data using a superposition principle for elastic properties. The developed method was evaluated using bar-shaped bulk glass samples from a lithium-alumosilicate glass-ceramic system. The sample geometry, with its low surface-to-volume ratio, allowed a detailed analysis of the volume crystallization, due to a reduced influence of the surface crystallization on the measured signal. The development of the crystalline content was confirmed, and variations in the surface and bulk crystallization were examined using a reference method based on X-ray diffraction.     

Isothermal and non-isothermal crystallization kinetics of hydroxyl-functionalized polypropylene

Hydroxyl-modified polypropylenes (PPOH) with side chains containing OH groups were synthesized by copolymerization of the propylene and undecenyloxytrimethylsilane monomers. The isothermal and non-isothermal crystallization behavior of the modified polypropylenes (PPOH) with side chains containing up to 6.8 mol% OH groups were compared with that of polypropylene (PP). The introduction of the OH-comonomer decreased the overall rate of isothermal crystallization compared with PP due to steric effects of the hydroxyl-containing side-chains that hindered packing of the PP backbone chains into a lamellar structure. However, a maximum reduction in the rate of crystallization occurred at an intermediate hydroxyl concentration as a consequence of a competition between the effects of the comonomer on the nuclei density and the thermodynamic barrier to crystallization. Steric hindrance by the comonomer side-chains also reduced the radial growth rate of the crystals in PPOH and produced a coarser crystal morphology than that for PP. PP and PPOH exhibited an identical α-monoclinic crystal structure, but the introduction of only ∼6.8 mol% comonomer reduced the fold-surface free energy of the crystals by 42%. For non-isothermal crystallization, the crystallization peak temperature (T_p) decreased for low concentrations of OH, but above a critical OH concentration, T_p increased, a result similar to the isothermal crystallization rate.     

Effect of surface nucleation on isothermal crystallization kinetics: Theory, simulation and experiment

Surface nucleation of poly(l/d-lactide) at the interface with aluminum was studied by performing isothermal DSC analysis of amorphous samples of varying thickness between 100 °C and 130 °C. To ensure complete wetting of the aluminum surface, a hot melt laminating process was used to prepare the samples. Theoretical aspects of surface crystallization kinetics were explored and the resulting model was compared with the results of Monte-Carlo simulations. Three stages of surface crystallization were identified depending on the growth geometry: (1) impingement-free growth, (2) increasingly laterally-constrained transverse growth, and (3) interstitial growth. By fitting the Monte-Carlo simulation to the experimental half-times of crystallization the surface nucleation concentration and the bulk nucleation rate was estimated at 4 different temperatures. It was found that both surface nucleation concentration and the bulk nucleation concentration decrease with increasing crystallization temperature.     

Crystallization behaviors of glasses in the (Ge5Sb25Se70)1-xAgx system

In this work, the unique glass-ceramics containing thermoelectric AgSbSe₂ crystals were successfully prepared from glass matrices in the (Ge₅Sb₂₅Se₇₀)_1-xAg_x system (x = 0, 2.5, 5, 7.5, 10,15 mol %). Structure and crystallization behaviors of samples were investigated to have a fundamental understanding of the effect of silver doping on the glass matrices. Critical Ag content for precipitation of the pure TE AgSbSe₂ crystals in the present system are at least higher than 5 mol %. Raman spectra explained by the chemical bond approach imply that the continuous depolymerization of glass network with the increased Ag eventually induces the precipitation of AgSbSe₂ crystals from the glass matrix, which, in turn, plays the predominant role in increasing σ_RM of glass by around two orders of magnitude.     

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.     

Thermal evolution and crystallization kinetics of potassium-based geopolymer

In this paper crystallization kinetics of potassium-based geopolymer (Si/Al = 2.5, denoted as KG2.5) upon heating are investigated by differential thermal analysis (DTA) under non-isothermal conditions at various heating rates. From 35 to 700 °C KG2.5 shows a significant weight loss due to both evaporation of free water and condensation of hydroxyl groups. Crystallization of KG2.5 starts at a low temperature of ∼960 °C according to XRD and DTA analysis. KG2.5 also exhibits a low onset temperature of viscous sintering stage and on heating to 950 °C the sintering is completed. In the DTA graphs, the exothermic peaks which are caused by leucite crystallization shift to higher temperatures with increasing heating rate. The activation energy value of crystallization for leucite is found to be 455.9 kJ/mol and the corresponding Avarami constant is 3.89 indicating the three-dimensional crystal growth mechanism.     

Engineering the crystallization behavior of CsPbBr3 quantum dots in borosilicate glass through modulating the glass network modifiers for wide-color-gamut displays

The in-situ growth of CsPbBr₃ perovskite quantum dots (QDs) inside glass has been regarded as an alternative approach to improve their stability. Alkaline-earth metal oxides has multiple effects on the structure of the glass network. Herein, four types of alkaline-earth metal oxides are introduced into borosilicate glasses to modulate glass network structure, which has quite different effects on the crystallization behavior of CsPbBr₃ QDs. The reason can be ascribed to the different impacts of alkaline-earth metal on phase separation, nucleation, and growth procedure. Moreover, CsPbBr₃ QDs embedded in glass (CsPbBr₃ QD@glass) exhibit superior thermostability and photostability compared with CsPbBr₃ QDs powder. Finally, a white light-emitting diode achieving 124% of National Television System Committee (NTSC) gamut is fabricated using the CsPbBr₃ QD@glass, K₂SiF₆:Mn⁴⁺ phosphor film, and blue chip-on-board. This work provides a reference for modulating the glass network modifiers to regulate the crystallization behavior of perovskite QDs.     

Isothermal crystallization kinetics and effect of crystallinity on the optical properties of nanosized CeO2 powder

The isothermal crystallization kinetics and effect of crystallinity on the optical properties of cerium dioxide (CeO₂) nanopowders synthesized using a coprecipitation route at 293 K and pH 9 were investigated using X-ray diffraction, transmission electron microscopy, selected area electron diffraction, and ultraviolet–visible absorption spectrophotometry. The activation energy of CeO₂ crystallization from dried cerium dioxide precursor powders by isothermal method of 64.1±3.24 kJ/mol was obtained. The average value of the growth morphology parameter (n) is 1.94, meaning that two-dimensional growth with plate-like morphology was the primary mechanism of CeO₂ crystallization from cerium dioxide precursor powders. The indirect band gap energy (E_i) of CeO₂ decreased from 3.03 eV to 2.83 eV when the crystallinity increased from 18% to 82%, and the direct band gap energy (E_d) of CeO₂ also decreased from 3.76 eV to 3.64 eV.     

Glass transition and exclusion model in crystallization of polyether-polyester block copolymers with amide linkages

Polyether-polyester block copolymers having various polyetheramide contents were synthesized. Single glass transition intermediated in temperature between the glass transition temperatures of polyester and polyetheramide components was found for all of polyether-polyesters. The compositional variation of glass transition exhibited a similar trend to the predicted result of thermodynamic theory for compatible polymer blends. The incompatible pair of homopolyester and homopolyether was forced to be compatible after copolymerization. A modified theoretical prediction for the glass transition of copolymers based on the thermodynamic theory is proposed. Consistent results between theoretical prediction and experimental measurements were found. Unlike homopolyesters, the glass transition temperature of copolymer amorphous domains gradually decreases with crystallization time. An exclusion model for the crystallization of polyester segments in copolymers is proposed. The temperature width of the glass transition increases with crystallization time. The broadening towards the low temperature side in glass transition is interpreted as the evidence of crystallization-induced partial phase separation. Instead of forming macroscopic segregation, the excluded polyether segments resided in-between crystalline polyester lamellae and mix with amorphous polyesters to generate amorphous domains exhibiting concentration gradient along the lamellar basal surface normal. Further increasing the polyetheramide segment content brings the excluded polyetheramide segments to form domains among the crystallized polyester spherulites so as to inhibit the occurrence of spherulitic impingement.     

Influence of crystallization inside glass frit on seal stress in ceramic metal halide lamps

Ceramic metal halide lamps use polycrystalline aluminum oxide as an arc tube material; cracks inside the glass frit—used as the seal material—have been known to occur occasionally. This study measured the stress on the lamp seals caused by changes in the cooling rate during the sealing process by a 2D stress measurement method. Seal stress decreased with reducing cooling rate. Therefore, we discuss the influences of the glass frit's microstructure and the coefficients of thermal expansion (glass frit, capillary, and Nb wire) on the seal stress. The coefficient of thermal expansion of the annealed glass frit was essentially closer to those of the capillary and Nb wire, while that of the rapidly cooled glass frit differed greatly. Moreover, the glass frit of the rapidly cooled lamp seal contained only an amorphous phase (Dy, Si, Al, and O), while the glass frit of the annealed lamp seal contained both an amorphous phase (Dy, Si, Al, and O) and a crystalline one (Dy₂SiO₅ and Al₂O₃). Fracture toughness was found to be larger in the crystals than in the amorphous phase area. Moreover, it was larger in the area where crystalline Dy₂SiO₅ and Al₂O₃ were present compared to the area where only crystalline Dy₂SiO₅ was present.