Structure development in the early stages of crystallization during melt spinning

The earliest stage of crystallization during melt spinning was examined for four polymers: HDPE, PVDF, nylon 6 and poly(oxymethylene). The four polymers have very similar melt viscosities. Of particular interest is the dependence of the time for the onset of detectable crystallization on the take-up speed. The results for all four polymers lie on the same onset time versus take-up speed curve, indicating that this condition depends chiefly upon chain orientation and not appreciably on chain chemistry or specific undercooling. The result is consistent with a condition of critical strain level. A similar, but less stringent, result is found for further crystallization in the spinline.     

Microstructural and phase transformations during sintering of a phillipsite rich zeolitic tuff

Microstructural and phase transformations during sintering of a Phillipsite rich zeolitic tuff, from Tenerife, Canary Islands, was investigated in order to their utilization in ceramic manufacturing industries. Green samples were obtained from powders and pressed at 150 MPa and heat-treated for 1 hour in the temperature range between 900-1080 °C. The zeolitic tuffs show endothermic peaks at about 100 °C and 200 °C, corresponding to dehydration of zeolitic water observed in the thermo- gravimetric curves and at ∼700 °C a small endothermic peak was identified corresponding to the structural breakdown of Phillipsite. According to the dilatometric traces, at about 870 °C, sintering of solid particles starts and a linear shrinkage of about 6% is reached at about 1050 °C. The maximum absolute and apparent densities were obtained after sintering at 1040 °C (absolute density = 2.59 g/cm³; apparent density = 2.33 g/cm³). Over this temperature, the sample heat-treated at 1060 °C, density results show a decreasing trend. The chemical composition of studied zeolitic tuffs make possible the liquid phase formation during heat-treatment, through the supplement of alkaline oxides. SEM image of the sample obtained at 1040 °C shows a zone with micro-crystallinity around the boundary of Sanidine grain highlighting the beginning of the phase transition from Sanidine, to Microcline. Heat-treatment effect of zeolitic tuff leads to the decomposition of Phillipsite and Sanidine and the formation of a new crystal phase of Hematite.     

Gd-doping effect on crystallization behaviour and valence state of Ce-bearing zirconolite glass-ceramic

Zirconolite-based glass-ceramic is a waste matrix used for incorporating high-level waste (HLW) which is rich in minor actinides. In this study, Ce-bearing zirconolite glass-ceramics were prepared via a traditional two-step thermal treatment method, and gadolinium was selected as the trivalent actinide residual to investigate co-doping effects. Co-doped samples were then characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy, and X-ray photoelectron spectroscopy (XPS). The XRD results indicated that zirconolite was the sole crystalline phase synthesized from a SiO₂–B₂O₃–Na₂O–CaO–ZrO₂–TiO₂–Al₂O₃–CeO₂–Gd₂O₃ system. A slight lattice expansion in the Ce-bearing zirconolite was also found after Gd doping. However, the average size of zirconolite particles obtained empirically via SEM remained constant (~1.8 μm) and was within the error range. A progressive decrease of the volume fraction of zirconolite was also observed after doping. In addition, XPS results confirmed that the Ce³⁺ fraction incorporated into zirconolite decreased with the increase in Gd³⁺, while it still dominated the Ce species, even for a Gd-doping concentration of as much as 1.0 wt%. This study aimed to advance the current understanding of the immobilization of actinide residuals in zirconolite glass-ceramics as well as provide some basic technical data for the deep geological disposal of HLW.     

Greatly enhanced energy storage density of alkali-free glass-ceramics after dual optimizations by thickness and crystallization temperature

Glass-ceramics show a great application potential in sustainable development, environmental protection, high temperature, high voltage resistance, and so on. Given the breakdown strength has a great contribution to the energy storage density, alkali-free niobate-based glass-ceramics have emerged as a prominent energy storage material. In this study, the 13.64BaCO₃-13.64SrCO₃-32.72Nb₂O₅-40SiO₂ alkali-free glass-ceramics were optimized in thickness and crystallization temperature. The thinning of thickness improves the breakdown strength. At the same time, the dielectric constant gets a maximum value by adjusting the crystallization temperature. Therefore, an ultra-high theoretical energy storage density of 27.47 J·cm⁻³ is obtained. In addition, the finite element software simulates the electric field distribution and electric potential evolution during the development of electric branches, which illustrates the role of glass phase in hindering the development of electric branches and partaking the high electric field. Finally, the effective energy storage density obtained by using P-E loops is 1.49 J·cm⁻³ under 850 kV/cm.     

Morphology development in immiscible polymer blends during crystallization of the dispersed phase under shear flow

Crystallization under shear of dispersed polybutylene terephthalate (PBT) fibers in copolymer polyethylene-methyl acrylate matrix (EMA) was investigated using a hot optical shear device. Crystallization during isotherm and cooling process was studied. Static crystallization experiments were carried out for comprehension purpose. Differential scanning calorimetry (DSC) analysis was performed in order to predict the crystallization behavior of PBT. Shear enhancement of its crystallization was thus demonstrated from rheological experiments. Interfacial tension of EMA/PBT blend was experimentally measured using the hot optical shear device. Theoretical break-up times of PBT fibers were also calculated. Control of the morphology through shear rate and crystallization time balance was demonstrated. Static crystallization experiments show that decreasing crystallization time favor fibrillar morphology. Breaking up of fibers was brought to the fore during dynamic crystallization experiments due to heterogeneous development of the crystallization along the fiber. During the dynamic crystallization, rapid quenching enables fibrillar morphology. Long crystallization times associated with low shear rates allow nodular morphology.     

Kinetics and in situ observation of nonisothermal crystallization in Bayan Obo tailing-based nanocrystalline glass-ceramic

In recent years, the preparation of CMAS nanocrystalline glass-ceramics has shown potential as an application of secondary resourcing technology in utilizing Bayan Obo iron ore tailings containing rare earth elements. The crystallization mechanism for nanodiopside-type glass-ceramics was studied via an investigation of the nonisothermal crystallization kinetics of the glass system, combined with the in situ observation of softening and crystallization of the basic glass using a high-temperature laser confocal microscope. The results show that the activation energy of nucleation in the glass system is higher than that of crystal growth by using the Ozawa model. The crystallization mechanism changes as the crystallization fraction increases, that is, from the three-dimensional growth in which the nucleation rate increases with time in an interface-controlled manner (a > 1, b = 1, m = 3) at the initial stage of crystallization to a decreased nucleation rate in a diffusion-controlled growth (a = 0.5, b = 0.5, m = 3) at the middle and later stages. This process involves both surface crystallization and volume crystallization. The crystallization was observed in situ, and it was further confirmed that there exists a critical nucleation temperature between T_g and T_x, which is related to the interface free energy and critical Gibbs free energy difference. When the temperature exceeds the critical value of T_g + 55 K, the system begins to exhibit visible crystallization. With an increase in temperature, the basic glass softened considerably, while the crystal grew significantly. In addition, the surface roughness can be used to characterize the crystallization process, providing a new research method for crystal growth.     

Morphology development and exclusion of noncrystalline polymer during crystallization in PVDF/PMMA blends

Morphology development during the crystallization of poly(vinylidene fluoride) (PVDF)/poly(methyl methacrylate) (PMMA) blends was investigated at various crystallization temperatures (T_C) by means of time-resolved light scattering measurements and atomic force microscopy (AFM). A coarse spherulite obtained at a high T_C of 162 °C was found to be developed with a two-step crystallization process. The ordering in the spherulites (Pr) increased with time at the early stages and then decreased at the later stages. The rate of spherulite growth started to decrease when Pr started to decrease. In contrast, in the compact spherulite obtained at a low T_C of 148 °C, Pr decreased monotonously with time while the growth rate was constant. AFM observation revealed that such characteristic crystallization behavior is attributed to the exclusion of PMMA from the crystal growth during the crystallization; i.e., the amount of excluded PMMA becomes larger as the distance from the spherulite center increases and the crystallization temperature rises.     

水泥熟料烧成技术的研究进展

围绕优质高产、节能降耗这一主线,简要说明了水泥熟料烧成技术的发展进程.详细介绍了高效节能烧成技术的发展和技术现状;同时,对比了流态化烧成技术与预分解技术,指明了前者的优点,展望了熟料煅烧设备今后的发展方向.     

Stage-sensitive microstructural evolution of nanostructured TBCs during thermal exposure

Nanostructured thermal barrier coatings (TBCs) often exhibit bimodal structure comprised of both nanozones and lamellar zones, and therefore, their sintering behaviour can be different from that of conventional coatings. In this study, changes in the microstructure and properties of nanostructured TBCs were investigated. The results show that their microstructural evolution is highly time-sensitive during long thermal exposure at 1150?°C. In stage I (0–20?h), changes in mechanical properties were significant. The dominant microstructural change was faster healing of flat pores, whereas the macroscopic structure seemed less affected. In stage II (20–500?h), the changes in properties were much slighter and some large macroscopic voids appeared. In brief, the microscopic healing of pores in lamellar zones leads to a significant change in mechanical properties in stage I, whereas sintering of the nanozones leads to macroscopic voids in stage II.     

The observation of rapid surface growth during the crystallization of polyhydroxybutyrate

A two stage growth during isothermal cold crystallization of PHB has been observed in the temperature range 5-40 °C when a free surface is present. This growth has been investigated with optical and atomic force microscopy both in-situ and ex-situ. Initially, crystal growth is observed to be composed of lamellae oriented approximately flat-on relative to the free surface. At later stages of growth there can be a change to a distinctly different form of crystal growth that is composed of edge-on lamellae which grow at a substantially higher crystallization rate. This change in growth rate at constant temperature gives rise to curved interfaces between the slower growing flat-on growth and the faster growing edge-on growth. Several possible explanations for this change in growth rate are put forward.     

Molecular dynamics simulation of orientation and crystallization of polyethylene during uniaxial extension

Molecular dynamics simulations of realistic, united atom models of polyethylene undergoing uniaxial extension are described. Systems composed of chains ranging from 25 to 400 carbons have been studied, under a variety of processing histories, including isothermal deformation at constant applied stress below the melt temperature T_m, isothermal deformation below T_m followed by annealing, isothermal deformation above T_m followed by crystallization at a quench temperature below T_m, and non-isothermal crystallization during simultaneous deformation and cooling through T_m. Extension and orientation of large segments of flexible chains by uniaxial deformation accelerates the primary nucleation rate to a time scale accessible by molecular dynamics simulation. Entanglements operative during active deformation promote extension and orientation without nucleation of a crystal phase, while relaxation of stress at constant strain is sufficient to allow slippage of chains past pinning points and rapid nucleation and growth of crystallites as neighboring oriented chains come into registry. Isothermal crystallization of pre-oriented systems shows an apparent increase in nucleation density at lower temperatures; the resulting ordered regions are smaller and more closely aligned in the direction of orientation. During non-isothermal deformation, where stretching and cooling occur simultaneously, a first order transition is observed, with discontinuities in the volume and global order parameter, when the system crystallizes.     

Measurement of hysteresis in crystallization with a quartz crystal sensor

Monitoring of the cycle of crystal formation and dissolution gives important information for manipulating solute concentration and solution temperature for the preparation of mother solution in crystallization processes. In this study a direct monitoring technique using a quartz crystal sensor is applied to the crystallization of potassium bromide solution, and the monitoring performance is investigated. The experimental results indicate that the monitoring shows the detailed process of crystal formation and dissolution, and there is a hysteresis of 0.98 °C between the initiation temperature of crystal formation and the completion temperature of dissolution in the first measurement. In addition, it is demonstrated that the application of the technique is simple and efficient for the control of crystallization processes.     

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.     

Direct ink writing of wollastonite-diopside glass-ceramic scaffolds from a silicone resin and engineered fillers

Wollastonite-diopside scaffolds have been successfully developed by direct ink writing of an ink made of silicone polymer and inorganic fillers. The main reason for using a silicone in the ink formulation consisted in its double effect, in controlling the ink rheology and in developing of wollastonite and diopside crystalline phases upon heat treatment. The obtained 3D wollastonite-diopside scaffolds featured regular geometries, and a high compressive strength (3.9–4.9 MPa) when considering the large amount of porosity (68–76 vol.%). A glass with the same oxide composition as the silicone-based ink and crystallizing into wollastonite and diopside, was produced and used as additional filler. This addition enabled the fabrication of even stronger 3D printed scaffolds (∼8 MPa for a porosity of 67 vol%), owing to the enhanced viscous flow upon firing which reduced the micro-cracks in the scaffold struts generated by the preceramic polymer decomposition. The obtained highly porous wollastonite-diopside glass-ceramic scaffolds are suitable candidates for bone tissue engineering.     

Doping of tantalum,niobium, and hafnium in a translucent ZrO2-SiO2 nanocrystalline glass-ceramic

The addition of dopant(s) is an effective strategy to regulate the microstructure and properties of ZrO₂-based ceramics. In this study, we investigated the effects of ternary element alloying, namely tantalum (Ta), niobium (Nb), and hafnium (Hf) elements, on the microstructure and transformability of ZrO₂ nanocrystallites in a ZrO₂-SiO₂ nanocrystalline glass-ceramic (NCGC) during sintering and thermal treatments. The ternary dopants enhanced the transformability of tetragonal ZrO₂ (t-ZrO₂) nanocrystallites during sintering, i.e., the dopants acted as t-ZrO₂ destabilizer. The Ta, Nb and Hf elements dissolved in ZrO₂ nanocrystallites, forming ZrO₂ solid solution. Meanwhile, lamella nanotwins were formed within many ZrO₂ nanocrystallites. No obvious segregation of dopants was detected at ZrO₂ grain boundaries. t-ZrO₂ and monoclinic (m) ZrO₂ nanocrystallites were metastable in thermal treatments process, with “t” to “m” and the reverse “m” to “t” polymorphic transformation occurred simultaneously. Meanwhile, t-ZrO₂ and m-ZrO₂ nanocrystallites had a great tendency to grow larger during thermal treatments.     

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.     

Structural evolution at short and medium range distances during crystallization of a P2O5-Li2O-Al2O3-SiO2 glass

Li₂O-Al₂O₃-SiO₂ (LAS) glass-ceramics have important industrial applications and bulk nucleation is usually achieved by using nucleating agents. In particular, P₂O₅ is an efficient agent in glasses containing a low level of Al₂O₃ but its role in the first stages of nucleation is not well established. In this study, we combine structural investigations from local to mesoscales to describe the structural evolution during crystallization of LAS glass-ceramics. Local environment is probed using ²⁹Si and ³¹P MAS-NMR, indicating organization of P in poorly crystallized Li₃PO₄ species prior to any crystallization. To better understand the detailed nanoscale changes of the glass structure, ³¹P-³¹P DQ-DRENAR homonuclear correlation experiments have been carried out, revealing the gradual segregation of P atoms associated with the formation of disordered Li₃PO₄. Small-angle neutron scattering data also show the apparition of nanoscale heterogeneities associated with Li₃PO₄ species upon heating treatments and allow the determination of their average sizes. These new structural information enhance our understanding of the role of P in nucleation mechanisms. Nucleation is initiated by gradual change in P environment implying P segregation upon heating treatments, forming disordered Li₃PO₄ heterogeneities. The segregation of P atoms enables the precipitation of meta- and disilicate phases.     

Molecular dynamics investigation of the fracture mechanism of a glass-ceramic containing cleavable crystals

In this study on a novel glass-ceramic containing hexagonal CaAl₂Si₂O₈ crystals embedded in a SiO₂–Al₂O₃–CaO glass, we used molecular dynamics simulations to unravel the toughening mechanism of the partially crystallized composite material. The crystalline phase is composed of alternate layers of SiO₄/AlO₄ tetrahedra and calcium ions. After careful modeling of crystals embedded in the glass matrix, we conducted crack propagation simulations using single-notched models. We found that: (a) when a crack propagates parallel to the cleavable calcium layer, the glass-ceramic breaks in a brittle way since the crack passes through the fragile interlayer promptly, (b) the stiffer SiO₄/AlO₄ oxide layer can inhibit crack propagation, and the crack is thus deflected to the interface between the crystal and the glass matrix; and (c) a calcium layer present between the glass matrix and the edge of the CaAl₂Si₂O₈ crystal is more fragile than those inside the crystal, indicating that cracks prefer to travel along the glass-crystal interface. These theoretical simulations successfully demonstrated that the anisotropy and the fragile feature of the crystals lead to microcrack toughening of the glass-ceramic. In addition, we discuss deformation anisotropy in the microscale by constructing a larger model that includes randomly orientated multiple CaAl₂Si₂O₈ crystals.     

New insights into the characteristics of early stage crystallization of a polyethylene

Small angle light scattering has been used to probe structure formation during isothermal crystallization of an ethylene-1-hexene copolymer (EH064, M_w = 70,000 g/mol, ρ = 0.900 g/cm³, M_w/M_n ∼ 2, 6.4 mol% hexene). It is shown that clear structural information on size scales ranging from hundreds of nanometers to several micrometers during early stage crystallization can be obtained by this method when crystallizing the polyethylenes at the high temperatures (above the peak melting temperature of a rapidly crystallized polymer sample) required for resolving early stage crystallization without the influence of the crystal growth. The results show that the early stage crystallization is characterized by large scale orientation fluctuations that precede the formation of local crystalline order manifest in X-ray scattering and the initial collapse of these large scale anisotropic/ordered domains. The scattering intensity increases exponentially with time initially, and the wave vector dependence of the growth rate of fluctuations is consistent with predictions for initial stages of a phase transformation process. However, the detailed mechanism cannot be described by existing models. The implications of our results are discussed within the context of proposed models for early stage crystallization.