Celsian formation from barium-exchanged geopolymer precursor: Thermal evolution

In this paper, thermal evolution, including element & phase composition and microstructure, of Ba²⁺ exchanged K-based geopolymer precursor (BaGP) were systematically investigated during high-temperature treatment. The results proved that celsian precursor with lower residual alkaline cation content were obtained through amorphous geopolymer than traditional ion-exchanged celsian through crystallized zeolite. With the increase in temperature, weight loss of BaGP was due to evaporation of OH groups and decomposition of BaCO₃. Similar to K-based geopolymer, BaGP showed amorphous structure, and nanometer-sized celsian nucleuses first crystallized from the amorphous BaGP matrix after it was treated at 900 °C. In the treatment temperature range from 1000 to 1400 °C, hexagonal celsian became the main phase. After being treated at 1400 °C, hexagonal celsian grains were clearly noticeable with extra SiO₂ locating between celsian grains. It was therefore concluded that geopolymer precursor technique provides an alternative route for the preparation of celsian ceramics.     

Rapid preparation of Nd-doped zirconia ceramics for high-level radioactive waste immobilization

Rapid preparation of ceramic waste form at low temperature is of great engineering significance for immobilization of high-level radioactive waste. Herein, we demonstrated that Nd-doped zirconia ceramics can be rapidly prepared at low sintering temperature with the assistance of hydrothermal assisted sol-gel method. The effects of sintering temperature and Nd content on the phase and microstructure evolutions of the obtained ceramics were studied. With the increasing content of Nd, the phase of ZrO₂ transformed from monoclinic to cubic at first, then the pyrochlore Nd₂Zr₂O₇ phase appeared, and finally hexagonal neodymia solid solution structure formed. Moreover, the grain size and compactness of the obtained ceramics increased with the increment in sintering temperature. The successful accommodation of Nd within the ZrO₂ lattice was revealed by the detected increase of cell parameters. This work demonstrated that the hydrothermal assisted sol-gel process can be employed to effectively synthesize the Nd-doped zirconia ceramics as potential hosts for HLW immobilization.     

Design of a Eutectic Freeze Crystallization process for multicomponent waste water stream

Complex, hypersaline brines originating from the mining and extractive metallurgical industries have the potential to be treated using Eutectic Freeze Crystallization (EFC). Although EFC has been shown to be effective in separating a single salt and water, it has yet to be applied to the complex hypersaline brines that are typical of reverse osmosis retentates in South Africa. This paper focuses on the application of EFC for the purification of a typical brine containing high levels of sodium, chlorine, sulphate and ammonia that cannot be achieved with other separation techniques. The presence of ammonia prevents the application of membrane technology to treat the brine, leaving only cooling or evaporation as other possible options. Evaporation produces a mixed salt that requires further treatment. Modelling tools were applied to describe the phase behaviour of the complex saline systems under different process conditions and were experimentally validated. The results showed that Eutectic Freeze Crystallization could be used to selectively recover the sodium as a sodium sulphate salt. The simulation tools were especially useful in the design and optimisation of the process.     

Synthesis of pyrochlore-borosilicate glass-ceramics for immobilization of high-level nuclear waste

In this work, borosilicate based glass-ceramics with pyrochlore as crystalline phase for immobilization of high-level nuclear wastes (HLWs) were successfully synthesized by a one-step heat-treatment method. X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDXS) demonstrate that the obtained glass-ceramics show a regularly and uniformly distributed single pyrochlore (Ca,Na)(Nb,Ti)₂Nd_0.67O₆F crystalline phase. Moreover, the glass-ceramics prepared show LR_Na, LR_B, LR_Al, LR_Si, LR_Nd, LR_Ti and LR_Nb of about 6.8 × 10⁻³, 3.7 × 10⁻⁴, 1.5 × 10⁻², 2.2 × 10⁻³, 3.0 × 10⁻⁵, 5.1 × 10⁻⁵ and 5.5 × 10⁻⁶ g m⁻² d⁻¹ respectively after 28 leaching days, which are comparable to stable glass-ceramics for HLW immobilization. The results of this study are expected to provide an experimental reference for the engineering synthesis of glass-ceramics for the immobilization of certain HLWs.     

Properties and vibrational spectra of magnesium phosphate glasses for nuclear waste immobilization

The leaching behavior and structure of magnesium phosphate glasses containing 45–55 mol% MgO incorporated with simulated high level nuclear wastes (HLW) were studied. The leach rate of the waste glasses decrease with increasing of the simulated HLW content. The gross leach rate of the glass waste form containing 50 mol% MgO and 45 mass% simulated HLW is of the order of 10⁻⁶ g/cm² day at 90 °C, which is small enough as compared with the corresponding release from a currently used borosilicate glass waste form. The isolated ions such as dimeric (P₂O₇)⁴⁻ and monomeric (PO₄)³⁻ ions increase upon as increasing the incorporating amount of the simulated HLW. The changes in properties can be attributed to the structure changes owing to the incorporation of the simulated HLW.     

Crystallization and microstructure formation of glass with Y2Si2O7-mullite eutectic composition

The crystallization and microstructure formation of glass with a Y₂Si₂O₇-mullite eutectic composition were examined. A crystallization of Y₂Si₂O₇ and the mullite phase due to phase separation occurred at 1174 °C, and a well-developed Y₂Si₂O₇ facet crystal was formed. This sample was melted down and solidified via heat treatment at 1250 °C and 1300 °C, temperatures below the melting point of a Y₂Si₂O₇-mullite eutectic. As a result of this solidification, a directional eutectic microstructure was formed.     

Substituting alkali waste for feldspar in technical porcelain

Glass and Ceramics -     

Evaluation of solvent-mediated phase transformation of glycine using oleic acid: Morphology and characterization study

The polymorphic phase transformation of β-glycine to α-glycine was carried out both in the absence and presence of various concentrations of oleic acid used as additive at 25℃ in a water/ethanol medium. The effects of oleic acid and its concentration on phase transformation time were determined by continuously measuring the ultrasonic velocity. The crystals obtained by the completion of the phase transformation were characterized by XRD, SEM, and TG/DTG. The XRD and SEM results indicated that oleic acid significantly impacted phase transformation time and the morphological characteristics of the crystals. In addition to SEM analysis, detailed crystal shape analysis was performed and the circularity, elongation, and convexity parameters were determined quantitatively. TG/DTG analyses were performed to investigate thermal decomposition behavior and to calculate the activation energies based on different kinetic models such as FWO, KAS, Starink, and Tang kinetic models. With the addition of oleic acid to the medium, the calculated activation energy values increased from 89.63-90.63 to 153.8-155.4 kJ·mol^-1. The activation energy values showed that oleic acid was adsorbed on the crystal surface; this result was supported by FTIR, elemental, and Kjeldahl analyses.     

Evidence of liquid water formation during methane hydrates dissociation below the ice point

Dissociation of small methane hydrate samples formed from water droplets of size 0.25-2.5 mm has been investigated below the ice melting point in the temperature range of 240-273 K, where the self-preservation effect is observed for bulk hydrates. The experiments included optical microscopy observations combined with P-T measurements of the dissociation conditions for the methane hydrates. For the first time, the formation of supercooled liquid water during the hydrate dissociation was reliably detected in the temperature range of 253-273 K. The formation of the liquid phase was visually observed. The induction time of the ice nucleation for the metastable liquid water depended from the dissociation temperature and a size of water droplets formed during the hydrate dissociation. It was found that in the temperature range of 253-273 K values of the dissociation pressure for the small hydrate samples fall on the extension of the water-hydrate-gas equilibrium curve into the metastable region where supercooled water exist. The average molar enthalpy of 51.7 kJ/mol for the dissociation of the small methane hydrate samples in the temperature range of 253-273 K was calculated using Clausius-Clapeyron equation. This value agrees with the enthalpy of dissociation of bulk methane hydrates into water and gas at temperatures above 273 K.     

Mechanisms of immobilization of nuclear waste elements by cement minerals, cement and mortar

The mechanisms of immobilization of nuclear waste elements such as Cs, Sr, Ba, U, La and Nd (the latter two simulating Am and Cm) by three cement minerals, one cement and one mortar were investigated. Cement minerals did not immobilize Cs or Sr or Ba in the absence of CO₂ but immobilized 62 to 91% of the added Cs and all of the added Sr by forming carbonates when CO₂ was bubbled through the cement mineral suspensions. The elements La, Nd and U reacted significantly with various cement minerals, cement and mortar and precipitated as hydroxides. For example, C₃S mineral immobilized 92, 73 and 99.2 of the added La, Nd and U respectively. Reaction of cement with U resulted in the formation of basic calcium uranyl silicate hydrate or uranophane under simulated repository conditions. These results suggest that cements serve as not only physical barriers but also act as chemical barriers for the migration of especially U and transuranic elements of nuclear waste.     

改性膨润土在废水处理中的应用

概述了膨润土的主要特性，研究了膨润土的主要改性方法以及改性膨润土在废水处理中的应用，讨论了改性膨润士用于废水处理中存在的问题和今后的研究方向。     

Interplay of crystallization and liquid-liquid phase separation in polyolefin blends: A thermal history dependence study

The kinetic interplay between crystallization and liquid-liquid phase separation (LLPS) in random copolymer blends of poly(ethylene-ran-hexene) (PEH) and poly(ethylene-ran-butene) (PEB) has been studied using optical microscopy. Morphologies of blends gone through three different thermal histories are compared: (1) single-quench (SQ), a homogeneous melt quickly cooled to isothermal crystallization temperatures (T_cry), (2) double-quench (DQ), a homogeneous melt quickly cooled to an intermediate temperature (T_lps) between binodal and equilibrium melting temperature (T_m⁰) and stored for a period of time and then cooled to T_cry, and (3) cyclic-quench (CQ), a homogeneous melt quickly cooled to T_lps and stored for a period of time, then gone through four cycles of crystallization and remelting. Comparing DQ morphologies to SQ ones, both crystal growth rate and nucleation density in the former are affected by prior LLPS. A scaling argument has been provided to partially account for the observed phenomena. In CQ, characteristic lengths of secondary features induced by crystallization depend strongly on the overall PEH composition, whereas are insensitive to temperature cycling. The contrast of large domains becomes more prominent upon cyclic crystallization and remelting. On the other hand, primary LLPS domains coarsen with CQ while loosing the contrast.     

Structural and morphological development in poly(ethylene-co-hexene) and poly(ethylene-co-butylene) blends due to the competition between liquid-liquid phase separation and crystallization

Isothermal crystallization behavior of poly(ethylene-co-hexene) (PEH) and the 50/50 blend (H50) of PEH with amorphous poly(ethylene-co-butylene) (PEB) was studied by time-resolved synchrotron simultaneous small-angle X-ray scattering/wide-angle X-ray diffraction (SAXS/WAXD) techniques and optical microscopy (OM). The X-ray study revealed the changes of structural and morphological variables such as the scattering invariant, crystallinity and lamellar long period, et al. In H50, the lamellar morphology was found to be dependent on competition between liquid-liquid phase separation (LLPS) and crystallization. At high temperature, LLPS becomes dominating, resulted in crystallization of PEH with minimal influence of PEB. At low temperature, LLPS is suppressed, PEB component shows obvious influence on PEH crystallization, PEB is thought to be partially included into PEH lamellar stacks and PEH-PEB co-crystallization is unlikely, however, possible. Optical microscopy was used to monitor crystal nucleation and growth rates in PEH and H50, providing complementary information about the effect of temperature on LLPS and crystallization. Real-space lamellar morphologies in PEH and H50 were characterized by atomic force microscopy (AFM), PEH exhibited sheaf-like spherulites while H50 exhibited hedrites. Overall, the competition between LLPS and crystallization in H50 blend influences the structural and morphological development. Controlling the interplay between LLPS and crystallization of PEH/PEB blends, it is possible to control the structure and morphology as practically needed.     

Kinetics of phase separation and crystallization in poly(ethylene-ran-hexene) and poly(ethylene-ran-octene)

The kinetics of phase separation and crystallization in the blends of poly(ethylene-ran-hexene) (PEH) and poly(ethylene-ran-octene) (PEOC) at several compositions were studied using phase contrast optical microscopy and time-resolved simultaneous small-angle X-ray scattering and wide-angle X-ray diffraction. The phase contrast optical microscopy showed the interconnected bicontinuous structure during phase separation process, which is characteristic of a spinodal decomposition. During isothermal crystallization, the average lamellar spacing increases with time for blends at all concentrations. The crystallinity and crystal growth rate depend on the PEH concentration. At dilute PEH concentrations, crystallization of PEH chains is difficult because they are surrounded by many non-crystallizable PEOC chains. On the other hand, at higher PEH concentrations, crystallization processes are similar to pure PEH. For example, the spherulitic growth rates are similar for a PEH/PEOC=50/50 blend and pure PEH.     

Shear-induced phase separation and crystallization in semidilute solution of ultrahigh molecular weight polyethylene: Phase diagram in the parameter space of temperature and shear rate

In the previous papers, we elucidated enhancement of concentration fluctuations, phase separation, and crystallization induced by steady state or step-up shear flow, as observed by shear small-angle light scattering, optical microscopy, and birefringence, for a semidilute solution of ultrahigh molecular weight polyethylene in paraffin as an athermal solvent. However the studies were done only at a given temperature of 124 °C, which is higher than the nominal melting temperature of the quiescent solution T_nm (115–119 °C). It is crucial to extend the studies over a wider temperature range in order to generalize shear-induced phase behavior of the solution. Thus in this work we constructed a kind of phase diagram in the parameter space of temperature (T) and shear rate (). The temperature range covered was higher than T_nm, so that the phase diagram is strictly concerned with shear-induced phase behavior (i.e., without shear the solution is homogeneous and in a single-phase state). The diagram identified Regimes I–III in the T– space as will be detailed in the text. In constructing the phase diagram we found the following new points also. (i) The critical shear rate _cx which defines the boundary between Regimes I and II was independent of T. (ii) Regime III identified previously through the dependence of the integrated scattered intensity only at a particular temperature T = 124 °C was further separated into two regimes of III_a and III_b below and above a critical temperature (147 °C), respectively, through the observation of the dependence as a function of T: In Regime III_a, the sheared solution developed the optically anisotropic fibrous structures, indicative of the shear-induced crystallization triggered by the shear-induced concentration fluctuations in Regime II; In Regime III_b, the solution is so stable that it did not show a trend of the shear-induced crystallization even at the highest shear rates accessible in this experiment, but it only showed the shear-induced phase separation. (iii) The critical shear rates _c,streak and _cz, which define respectively the boundary between Regimes II and III_a and that between Regimes II and III_b, are sensitive to temperature.     

Bioactivity and biodegradability of high temperature sintered 58S ceramics

Bioactive glasses are often considered in bone tissue engineering applications where mechanical strength is essential. As such, bioactive glass scaffolds are often sintered to improve mechanical strength. However, sintering can lead to crystallization, which reduces bioactivity and biodegradability. It has generally been considered that amorphous biomaterials exhibit better bioactivity. However, the in-vitro bioactivity and biodegradability of the sintered 58S made from initial amorphous powder and partially crystalline powder with the same chemical compositions (60SiO₂-36CaO-4P₂O₅ (mol%)) have not been compared before.In this study, 58S bioactive glass (fully amorphous) and glass-ceramic (partially crystallized) powders were synthesized using the sol-gel process, followed by heat-treating at 600 °C for 3 h (calcination). The powders were mixed with carboxymethyl cellulose solution as a binder, shaped in a cylindrical mold, dried, and then sintered at 1100 °C for 5 h. The in-vitro bioactivity and biodegradability of the sintered samples were assessed in simulated body fluid (SBF) for times up to 28 days. The specimens were investigated before and after immersion in SBF using X-ray powder diffraction (XRD), Scanning Electron Microscopy (SEM), and Fourier transform infrared spectroscopy (FT-IR). The In-vitro bioactivity and biodegradability rate of the sintered 58S produced from the glass ceramic powder were higher than that from fully amorphous powder. This study shows that the initial structure after calcination is important and affects the subsequent crystallization during sintering. Therefore, crystallinity and formation of hydroxyapatite after calcination are important controlling mechanisms that can increase the bioactivity and biodegradability rate of sintered 58S.     

Morphology of polypropylene/poly(ethylene-co-propylene) in-reactor alloys prepared by multi-stage sequential polymerization and two-stage polymerization

The morphology of two polypropylene/poly(ethylene-co-propylene) (PP/EPR) in-reactor alloys prepared by multi-stage sequential polymerization (MSSP) and two-stage polymerization (TSP) processes, respectively, was investigated. It is observed that the alloy prepared by MSSP (sample 1) exhibits lower phase separation temperature than the alloy prepared by TSP (sample 2), probably due to the higher content of PP segments in the blocky copolymer fractions of sample 1. Two thermal treatments were applied to the samples: (1) The samples were directly quenched from 230 °C to 132 °C for isothermal crystallization; (2) The samples were firstly held at 160 °C for 60 min for phase separation and then cooled to 132 °C for crystallization. It is found that both microstructure and thermal treatment affect the morphology of the alloys, and the differences in morphology are interpreted in terms of phase diagram. For sample 1 and for the samples subjected to phase separation prior to crystallization, the EPR-rich phase contains more PP and thus is more viscous, which leads to more inclusion of the EPR-rich phase into the spherulites. A coarse spherulitic structure is formed due to crystallization of PP in the included EPR-rich phase. More included EPR-rich phase and its stronger crystallizability can further lead to the narrower boundaries and formation of connections between the adjacent spherulites.     

Relaxation of shear-enhanced crystallization in impact-resistant polypropylene copolymer: Insight from morphological evolution upon thermal treatment

Relaxation of shear-enhanced crystallization in an impact-resistant polypropylene copolymer (IPC) upon thermal treatment was systematically investigated, from the sight of phase structure and resultant morphological evolution. Shearing was capable of accelerating the crystallization of IPC, while upon melt annealing, a spontaneous morphological evolution can be observed accompanied by a gradual relaxation of shear-enhanced crystallization. It was found the underlying structural motion during relaxation is in essence a phase separation process towards a multi-layered phase structure, in which crystallizable polyethylene segments are spatially entangled with ethylene-propylene random copolymer chains. The shear-enhanced crystallization originates from the destruction of this multi-layered phase structure which releases crystallizable PE segments to contact the matrix serving as nuclei for the crystallization of polypropylene. Upon annealing, reconstruction of the multi-layered structure can be achieved leading to the relaxation of shear-enhanced crystallization. A conceptual model describing this relationship between phase structure and crystallization behavior in IPC has been proposed.     

Mechanisms of formation of iron precipitates from ferrous solutions at high and low pH

The oxidation and subsequent precipitation of iron from solutions are very important for most hydrometallurgical wastewater treatment practitioners. The particle formation process mechanisms are important for shaping the dewaterability and particle size distribution of the precipitates. In this study the objective is to elucidate the mechanisms of iron particles formation from ferrous solutions at high (pH 9.0) and low pH (pH 6.0). The results obtained showed that the precipitation process is dominated by nucleation at the initial stages and aggregation dominated at the final stages. The precipitates formed at higher pH transform faster and result in the formation of more stable precipitates. Precipitates formed at pH 6.0 are larger but less stable chemically. The results may mean higher throughputs and lower costs of post precipitation stabilisation for operations at higher pH. The benefits for operation at lower pH would be a better solid-liquid separation due to the bigger sized particles formed.