Combined Quantitative X-ray Diffraction, Scanning Electron Microscopy, and Transmission Electron Microscopy Investigations of Crystal Evolution in CaO–Al2O3–SiO2–TiO2–ZrO2–Nd2O3–Na2O System

Glass-ceramics, with a specific crystalline phase assembly, can combine the advantages of glass and ceramic and avoid their disadvantages. In this study, both cubic-zirconia and zirconolite-based glass-ceramics were obtained by the crystallization of SiO2-CaO-Al2O3-TiO2-ZrO2-Nd2O3-Na2O glass. Results show that all samples underwent a phase transformation from cubic-zirconia to zirconolite when crystallized at 900, 950, and 1000 °C. The size of the cubic-zirconia crystal could be controlled by temperature and dwelling time. Both cubic-zirconia and zirconolite crystals/particles show dendrite shapes, but with different dendrite branching. The dendrite cubic-zirconia showed highly oriented growth. Scanning electron microscopy images show that the branches of the cubic-zirconia crystal had a snowflake-like appearance, while those in zirconolite were composed of many individual crystals. Rietveld quantitative analysis revealed that the maximum amount of zirconolite was ∼19 wt %. A two-stage crystallization method was used to obtain different microstructures of zirconolite-based glass-ceramic. The amount of zirconolite remained approximately 19 wt %, but the individual crystals were smaller and more homogeneously dispersed in the dendrite structure than those obtained from one-stage crystallization. This process-control feature can result in different sizes and morphologies of cubic-zirconia and zirconolite crystals to facilitate the design of glass-ceramic waste forms for nuclear wastes

Stabilization Mechanisms and Reaction Sequences for Sintering Simulated Copper-Laden Sludge with Alumina

To stabilize copper-laden sludge using alumina-based ceramic raw materials, this study quantifies the copper transformation behavior during the sintering process. Results indicate crystallochemical incorporation of hazardous copper through the formation of copper aluminate spinel (CuAl₂O₄) and cuprous aluminate (CuAlO₂). To quantify the copper transformation and reveal reaction sequences, CuO was mixed with γ-Al₂O₃ and α-Al₂O₃ precursors and fired at 1050–1150 °C for 15–180 min. The sintered products were examined using X-ray diffraction (XRD), and copper transformations into both aluminate phases were quantitatively determined through Rietveld refinement analysis of XRD data. When γ-Al₂O₃ was used, CuAlO₂ was predominately generated from CuAl₂O₄ decomposition. However, CuAlO₂ was largely generated by the interaction between CuO and α-Al₂O₃. This study also compared the sintering behavior of both precursor systems and observed the relatively slower decomposition of CuAl₂O₄ in the γ-Al₂O₃ system. The reoxidation of CuAlO₂ into CuAl₂O₄ with an extended sintering time was detected in the α-Al₂O₃ system. The sample leachability analysis reveals that both CuAl₂O₄ and CuAlO₂ structures were superior in copper stabilization compared to the oxide forms. Such results suggest reliable mechanisms of incorporating hazardous copper into a ceramic matrix and demonstrate the potential of using waste materials as part of ceramic raw materials to produce detoxified products

Analysis of the new ternary phase with C6Cr23 structure in Mg-Co-B system by Rietveld method and physical properties of its Ni-substituting effect

A new ternary Mg1.4Co21.6B6 compound in the Mg-Co-B system was synthesized via a conventional solid-state reaction method and the effect of Ni-substitution on its crystal structure, thermal stability, solid solubility and physical properties were systematically investigated. The crystal structure of the Mg1.4Co21.6B6 compound was fully determined by the X-ray diffraction technique with Rietveld refinement method. It is found that Mg1.4Co21.6B6 crystallizes in the form of C6Cr23 structure type (space group: Fm-3m (No.225), a = 10.5617(2)Å, Z = 4). The results showed that the 4a sites have been occupied completely by Co atoms in present compound which with M2-xNi21+xB6 form belonging to the W2Cr21C6-type. When Mg1.4Co21.6B6 is repeatedly sintered at elevated temperatures, it becomes unstable and decomposes into Co3B and Mg. The lattice parameters of the Mg1.4Co21.6B6 solid solution alters dramtically with increasing Ni substitution, with no regular trend being observed. The electrical and magnetic performances of the 3.6Mg:3Co:17Ni:6B and 3.6Mg:3Co:18Ni:6B (nominal compositions) samples suggest that both samples are typical ferromagnetic materials. The temperature in the maximum drop of the ρ(T) curve decreases as a function of the Ni content. Base on the correlation between the critical temperature and Ni content, a linear fitting equation is obtained and the critical temperature of Mg1.4Co21.6B6 calculated utilizing the linear fitting equation. The findings in this work may provide certain reference values for material science on electrical magnetic properties and other references for researching the material further

Combined Fe₂O₃ and CaCO₃ Additives To Enhance the Immobilization of Pb in Cathode Ray Tube Funnel Glass

Cathode ray tube (CRT) funnel glass has posed an increasing threat to the environment due to its rapid replacement by new technology in recent years. In this study, a well-control thermal scheme was applied for synthesizing a specific crystalline phase, PbFe₁₂O₁₉, for Pb immobilization when reusing CRT funnel glass as raw materials for the ceramics industry. The Fourier transform infrared spectroscopy results show that introduction of CaCO₃ facilitated the breakage of strongly connected bonds between O–Si–O and Pb–O, which were firmly linked in the glass network. The X-ray diffraction results demonstrate that 30 wt % CaCO₃ loading effectively facilitated the transformation of Pb in CRT funnel glass to the stable-phase PbFe₁₂O₁₉. A higher sintering temperature increased Pb transformation efficiency while a longer dwelling time only showed a slight increase in PbFe₁₂O₁₉ formation. The prolonged toxic characteristic leaching procedure results show a substantial improvement in the acid resistance (approximately 2 mg/L) of the thermally treated product with 30 wt % CaCO₃ loading and sintering under 1000 °C for 5 h compared to the original CRT funnel glass (500 mg/L). The results of this study demonstrate that incorporation of CaCO₃ and Fe₂O₃ into CRT funnel glass can effectively promote Pb immobilization and provide a new strategy for stabilizing waste CRT funnel glass

Copper Sludge from Printed Circuit Board Production/Recycling for Ceramic Materials: A Quantitative Analysis of Copper Transformation and Immobilization

The fast development of electronic industries and stringent requirement of recycling waste electronics have produced a large amount of metal-containing waste sludge. This study developed a waste-to-resource strategy to beneficially use such metal-containing sludge from the production and recycling processes of printed circuit board (PCBs). To observe the metal incorporation mechanisms and phase transformation processes, mixtures of copper industrial waste sludge and kaolinite-based materials (kaolinite and mullite) were fired between 650 and 1250 °C for 3 h. The different copper-hosting phases were identified by powder X-ray diffraction (XRD) in the sintered products, and CuAl₂O₄ was found to be the predominant hosting phase throughout the reactions, regardless of the strong reduction potential of copper expected at high temperatures. The experimental results indicated that CuAl₂O₄ was generated more easily and in larger quantities at low-temperature processing when using the kaolinite precursor. Maximum copper transformations reached 86% and 97% for kaolinite and mullite systems, respectively, when sintering at 1000 °C. To monitor the stabilization effect after thermal process, prolonged leaching tests were carried out using acetic acid with an initial pH value of 2.9 to leach the sintered products for 20 days. The results demonstrated the decrease of copper leachability with the formation of CuAl₂O₄, despite different sintering behavior in kaolinite and mullite systems. This study clearly indicates spinel formation as the most crucial metal stabilization mechanism when sintering copper sludge with aluminosilicate materials, and suggests a promising and reliable technique for reusing metal-containing sludge as ceramic materials

Crystal Structures of Al–Nd Codoped Zirconolite Derived from Glass Matrix and Powder Sintering

Zirconolite is a candidate host for immobilizing long-lived radionuclides. Zirconolite-based glass-ceramics in the CaO-SiO₂-Al₂O₃-TiO₂-ZrO₂-Nd₂O₃-Na₂O matrix are a potential waste form for immobilizing actinide radionuclides and can offer double barriers to immobilize radioactive elements. However, the X-ray diffraction patterns of the zirconolite derived from the glass matrix (glass ceramic, GC) are significantly different from those prepared by powder sintering (PS). In this Article, the crystal structures of Al–Nd codoped zirconolite grown via the glass matrix route and the powder sintering route are investigated in detail. Two samples of Al–Nd codoped zirconolite were prepared: one was grown from a CaO-SiO₂-Al₂O₃-TiO₂-ZrO₂-Nd₂O₃-Na₂O glass matrix, and the other was prepared with a Ca_0.75Nd_0.25Zr­Ti_1.75Al_0.25O₇ composition by powder sintering. The samples were then characterized using powder X-ray diffraction (PXRD), scanning electron microscopy (SEM), transmission electron microscopy–energy dispersive X-ray spectroscopy (TEM-EDX), and selected area electron diffraction (SAED). The chemical composition of the 100–500 nm zirconolite crystals grown from a glass matrix was determined by TEM-EDX to be Ca_0.83Nd_0.25Zr_0.85­Ti_1.95Al_0.11O₇. PXRD and SAED results showed that these two Al–Nd codoped zirconolite phases were crystallized in space group <i>C</i>12/<i>c</i>1. The HRTEM images and SAED results showed that there were heavy stacking faults in the zirconolite crystals grown from the glass matrix. In contrast, far fewer defects were found in the zirconolite crystals prepared by powder sintering. The split-atom model was adopted for the first time to construct the Al–Nd codoped zirconolite structure grown from glass during the Rietveld refinement. The isostructural method assisted by Rietveld refinement was used to resolve the Al–Nd codoped zirconolite structures prepared by different methods. The occupancies of the cation sites were identified, and the distribution behavior of Nd³⁺ was further investigated. The results indicate that the heavy stacking faults may lead to substantial differences in the Al–Nd codoped zirconolite structures prepared by these two fabrication routes

Cadmium Detoxification by Sintering with Ceramic Matrices

Su, M, Shih, K and Chen, D (2020). Cadmium detoxification by sintering with ceramic matrices. In: "Evolutionary Progress in Science, Technology, Engineering, Arts, and Mathematics (STEAM)", Wang, Lawrence K. and Tsao, Hung-ping (editors). Volume 2, Number 3, March 2020; 38 pages. Lenox Institute Press, Newtonville, NY, 12128-0405, USA. No. STEAM-VOL2-NUM3-MAR2020; ISBN 978-0-9890870-3-2. --------------- ABSTRACT: The use of various low-cost and attainable ceramic matrices (amorphous SiO2, γ-Al2O3, α-Fe2O3 and Fe3O4) to interact with Cd-containing waste is a promising method of Cd stabilization. Heating mixtures of cadmium oxide (CdO) and ceramic matrices at various molar ratios and temperatures (600-1000 degree C) for 3 h could achieve the goal of Cd incorporation. Phase transformation was assessed using X-ray diffraction (XRD), and the efficiency of Cd incorporation was quantified through Rietveld analysis of the obtained XRD patterns. The XRD results show that Cd can be crystallochemically incorporated into CdSiO3, Cd2SiO4, Cd3SiO5, CdAl4O7 and CdFe2O4 phases. The treatment temperature greatly affected the Cd incorporation reactions. The Cd incorporation efficiency was quantified and expressed as a transformation ratio according to the weight fractions of crystalline phases in the sintering products. To evaluate the metal stabilization effect of the Cd detoxification process, a series of constant-pH leaching tests was conducted. A remarkable reduction in Cd leachability was achieved by forming different Cd-hosting crystalline products, particularly spinel phase CdFe2O4. Overall, the efforts to stabilize Cd by sintering with ceramic matrices suggest a promising strategy for the detoxification of Cd in wastes