Advanced pressurelessly prepared adhesive composites based on modified preceramic polymer for the joining of amorphous SiBON ceramics

A series of polysilazane (PSZ)-based adhesives, which could be employed for high temperature environment in aerospace industry, were prepared to bind amorphous SiBON ceramics. It was found that the addition of dopants had a significant influence on the binding strength of samples, and physicochemical properties of adhesives were also closely correlated to their kinds and quantities. The fracture mechanism and phase transformation in joints were characterized. New phases including C₃N₄, Si₃N₄, SiC, BN, Si, ZrB₂ and amorphous SiCN, were formed with the increase of treatment temperature, influencing the binding strengths of samples. The maximum binding strength of joints could reach 16 MPa after heating at 1100 °C in N₂. The macroscopic and microscopic images of the fractured surface of joints and finite element analysis of the joint indicated that the failure of the joint occurred not only at the binding layer but also at the ceramic substrate. This work can serve as a guide for binding amorphous ceramic for high temperature environment.     

Preparation of Cu(II)‐chelated poly(vinyl alcohol) nanofibrous membranes for catalase immobilization

Poly(vinyl alcohol) (PVA) nanofibers were formed by electrospinning. Metal chelated nanofibrous membranes were prepared by reaction between Cu(II) solution and nanofibers, and which were used as the matrix for catalases immobilization. The constants of Cu(II) adsorption and properties of immobilized catalases were studied in this work. The Cu(II) concentration was determined by atomic absorption spectrophotometer (AAS), the immobilized enzymes were confirmed by the Fourier transform infrared spectroscopy (FTIR), and the amounts of immobilized enzymes were determined by the method of Bradford on an ultraviolet spectrophotometer (UV). Adsorption of Cu(II) onto PVA nanofibers was studied by the Langmuir isothermal adsorption model. The maximum amount of coordinated Cu(II) (q_m) was 2.1 mmol g⁻¹ (dry fiber), and the binding constant (K_l) was 0.1166 L mmol⁻¹. The immobilized catalases showed better resistance to pH and temperature inactivation than that of free form, and the thermal and storage stabilities of immobilized catalases were higher than that of free catalases. Kinetic parameters were analyzed for both immobilized and free catalases. The value of V_max (8425.8 μmol mg⁻¹) for the immobilized catalases was smaller than that of the free catalases (10153.6 μmol mg⁻¹), while the K_m for the immobilized catalases were larger. It was also found that the immobilized catalases had a high affinity with substrate, which demonstrated that the potential of PVA‐Cu(II) chelated nanofibrous membranes applied to enzyme immobilization and biosensors. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Kr ion irradiation study of polymer-derived SiFeOC–C–SiC nanocomposite

This study focuses on the pyrolysis and ion irradiation behaviors of polymer-derived SiFeOC–C–SiC ceramic. The pyrolyzed material is composed of SiO₂ and SiOC (amorphous), carbon (amorphous and turbostratic), and Fe₃Si and β-SiC (nanocrystalline). Irradiation was carried out at both room temperature and 600°C using 400 keV Kr ions with fluences of 4 × 10¹⁵ and 1 × 10¹⁶ ions cm⁻², respectively. The Fe₃Si and SiC nanocrystals are stable against irradiation up to 3 displacement per atom (dpa) at room temperature and up to 12 dpa at 600°C. The SiOC tetrahedrals show phase separation and minor carbothermal reduction. The high irradiation resistance and the dense, defect-free amorphous microstructure of SiFeOC–C–SiC after prolonged irradiation demonstrate its great potential for advanced nuclear reactor applications.     

Synthesis,Thermal Properties and Electrical Conductivity of Na-Sialate Geopolymer

This work aims to study the thermal behavior of basic-geopolymers derived from metakaolin (clay). The geopolymers were characterized by different techniques: thermal analysis (DTA, TGA), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and impedance spectroscopy. Some physicochemical properties of the products were also determined: the phases obtained after geopolymer heat treatment and their electrical properties. The results obtained after drying and heat treatment showed that the products kept their initial shapes, but revealed variable colors depending on the temperatures at which they were treated. The products obtained are amorphous between 300 up to 600 °C with peaks relating to the presence of nanocrystallites of muscovites and zeolite, thus at 900 °C it is quite amorphous but only contains nanocrystallites of muscovites. From the temperature of 950 °C, we notice that the geopolymer has been transformed into a crystalline compound predominated by the Nepheline (NaAlSiO₄) with the presence of a crystalline phase by minor peaks of Muscovite, this crystalline character has been increased at 1100 °C to obtain a whole phase crystalline of a Nepheline. The treatment of this geopolymer for one hour at 1200 °C shows an amorphous phase again corresponding to corundum (α-Al₂O₃). This indicates that the dissolution of the grains by the liquid phase induces the conversion of the material structure from sialate [–Si–O–Al–O] to sialate siloxo [–Si–O–Al–O–Si–O–] and the formation of a new crystalline phase (α-Al₂O₃). This development of sialate to sialate-siloxo was confirmed by IR spectroscopy. As mentioned above, from 300 to 900 °C, Na-sialate geopolymer exhibits the same disorder structure of nepheline. The crystal structure of nepheline is characterized by layers of six-membered tetrahedral rings of exclusively oval conformation. The rings are built by Regularly alternating tetrahedral AlO₄ and SiO₄. Stacking the layer’s parallel to the c axis gives a three-dimensional network containing channels occupied by Na cations. This topology favors easy movement of Na⁺ ions throughout the structure. For this reason, ionic migration in nepheline is widely reported. The refinement of Na-Sialate geopolymer at room temperature gives bulk high ionic conductivity of about 5 × 10^?5 S cm^?1 and this is due to the probable joint contribution of H⁺ and Na⁺ ions. Above 200 °C, Na⁺ seems to remain the only charge carrier with a low activation energy of about Ea?=?0.26 eV. At higher temperatures, the characteristic frequencies become so close that it is impossible to distinguish the contributions. A total resistance comprising both grain and grain boundaries contribution is then determined.

     

Rapid synthesis of Gd2Zr2O7 glass-ceramics using spark plasma sintering

Glass-ceramics are possible host matrix for high level waste immobilization. The Gd₂Zr₂O₇ glass-ceramic matrix was successfully synthesized using spark plasma sintering (SPS) method in 5 minutes. The phase transition with sintering temperature was studied using X-ray diffraction, Raman and transmission electron microscopy. It revealed that samples kept a main defected fluorite phase as being sintered below 1800°C. Glass phase increased rapidly beyond 1850°C. The amorphous structure became the main body at 1900°C, with nanoscale crystal scattered in the bulk. With the increase of glass phase, the grain boundary became almost indistinguishable. The relationship between the final phase of Gd₂Zr₂O₇ with its synthetic temperature range and corresponding technology was reviewed. Gd₂Zr₂O₇ glass-ceramics could be acquired by extending the sintering temperature beyond 1850°C using SPS method.     

Highly dispersed zinc phosphate microdomains in mesoporous silica

In situ immobilization of crystalline zinc phosphate nuclei in the mesoporous silica material resulted in a highly ordered 2D-hexagonal mesoporous material with evenly dispersed crystalline NaZnPO₄ microdomains in its matrix using the self-assembly of cationic surfactant under hydrothermal synthesis condition. Four samples with different Si:Zn:P:Na mole ratios have been prepared. X-ray diffraction (XRD) patterns of the as-synthesized as well as template-free samples indicated the presence of mesophase in each case. N₂ adsorption data indicated mesoporosity in the samples together with the existence of crystalline NaZnPO₄ phase for the materials synthesized with Si/Zn mole ratio 5-12. ²⁹Si MAS NMR results showed high value of the Q⁴/Q³ ratio in these materials suggesting highly crosslinked structure.     

Performance of electrochemically generated Li21Si5 phase for lithium-ion batteries

The nature of Li-Si alloy phases that are generated in electrochemical lithiation is examined as a function of temperature. The electrochemical lithiation is performed at 0.0 V (vs. Li/Li⁺) by short-circuiting an amorphous Si thin-film electrode with a Li metal counter electrode. At 25-85 °C, the well-known Li₁₅Si₄ phase (theoretical specific capacity = 3580 mA h g⁻¹) forms. At 100-120 °C, however, Li₂₁Si₅ (4008 mA h g⁻¹) that is known to be the most Li-rich phase in Li-Si system is generated. The crystallization into Li₂₁Si₅ is, however, so kinetically slow that it does not appear in the transient cycling experiment. The Li₂₁Si₅ phase is converted to amorphous Si upon de-lithiation, but the restoration back to the initial phase is only observed at 100-120 °C after a prolonged lithiation at 0.0 V. The cycleability of this phase is poor due to a successive Li trapping inside the Si matrix, which is caused by the formation of electrically isolated Si islands.     

Thermal evolution of lithium ion substituted cesium-based geopolymer under high temperature treatment,Part I: Effects of holding temperature

In this paper, a high temperature treatment procedure was designed to evaluate the effect of holding temperature on thermal evolution process of Li⁺ substituted Cs-based geopolymer (Cs_0.7Li_0.3GP), including the thermal analysis, phase composition and microstructure evolution. With rising of holding temperature, amorphous unheated Cs_0.7Li_0.3GP gradually transformed into a multiphase system during the high temperature treatment process, which consisted of pollucite (CsAlSi₂O₆), spodumene (LiAlSi₂O₆) and amorphous glass phase. In the multiphase system, Cs⁺ ions were in the form of pollucite grains, while Li⁺ ions were in the form of spodumene nanocrystallines distributed in amorphous matrix. The pollucite grains gradually coarsened with rise in holding temperature, and the densification of the resulting products were also improved synchronously, which were related to the presence of amorphous glass phases. The amorphous glass phase would be in a molten state when holding temperature over 800?°C. And the presence of molten amorphous phase would make the mass transfer process easier, which could contribute to the growth of the crystal grains and the elimination of the pores.     

Al2O3 films with Ni-Based buffer layer prepared by plasma-ion assisted deposition on Cu substrate

In this study, Al₂O₃ films with an Ni-based buffer layer were prepared on a Cu substrate by plasma-ion assisted deposition (PIAD). The main purpose of this study is to develop a novel electrical insulating film to be used at high temperature. X-ray diffraction (XRD) spectra show the Al₂O₃ films prepared by this method are amorphous. The results of atomic force microscopy (AFM), scanning electron microscopy (SEM), and auger electron spectroscopy (AES) analyses reveal that the Al₂O₃ films perfectly adhere to the substrate through the buffer layer, no visible defects were observed, and no impurity from Ni or Cu was detected. The diffusion of Cu into the Al₂O₃ film at high temperature is suppressed. Al₂O₃ films with an Ni-based buffer layer exhibit excellent resistivity (>10¹⁰Ω·cm) even after experiencing a high temperature environment as high as 600°C 10 times.     

Evaluation of novel composition (SiO2–CaO–Na2O–P2O5, SrO–CuO) degradable amorphous silicate glasses properties and comprehensive characterization of the thermal features

The current work presents and discusses the findings of a comprehensive study on the structural, chemical and thermal properties of SrO and CuO incorporated SiO₂–CaO–Na₂O–P₂O₅ amorphous silicate glass with a novel composition. Here, fundamental features (experimental density, oxygen density, and hardness) of all glasses were determined and chemical as well as phase composition of the glasses was verified with XRF and XRD, respectively. Moreover, the thermal behavior (viscos flow and crystallization kinetics) of amorphous silicate glass was investigated by non-isothermal methods using DTA analysis. The activation energies of glass transition (E_g) were calculated in the range of 546–1115 kJ/mol by Kissinger method, whereas the activation energies of crystallization (E_c) were calculated in the range of 164–270 kJ/mol by three different methods (Kissinger, Ozawa, Yinnon and Uhlmann). Avrami exponent (n) values ranged from 1.17 to 3.28 demonstrated that amorphous silicate glasses have different crystallization mechanism. Working temperature, which is one of the parameters indicating glass stability, increased with the incorporation of Sr and Cu from 187 °C to 245 °C. The initial dissolution measurement has been applied to study the degradability behavior of Sr and Cu incorporated amorphous glasses in vitro. Quantitative evaluation of Si⁴⁺ (0.156–0.373 kV), Ca²⁺ (0.043–0.332 kV), Na⁺ (0.044–0.329 kV), P⁵⁺ (0.057–0.289 kV), Sr²⁺ (0.134–0.385 kV), and Cu²⁺ (0.090–0.203 kV) depending on the ion activation energy (E_a-ion) and ion concentration at different temperature values (24, 37 and 55 °C) was performed in contact with Tris-HCl solution by ICP-OES analysis. The results revealed that investigated glasses were degradable and incorporation of Sr and Cu affected the glass initial dissolution. Overall, investigated glasses are suitable for various application such as hot-working production, glass-ceramic manufacturing, and glass or glass-ceramic scaffolds fabrication, due to wide working temperature ranges and high crystallization tendencies of the developed glasses.     

Carbonation of C–S–H and C–A–S–H samples studied by 13C, 27Al and 29Si MAS NMR spectroscopy

Synthesized calcium silicate hydrate (C–S–H) samples with Ca/Si ratios of 0.66, 1.0, and 1.5 have been exposed to atmospheric CO₂ at room temperature and high relative humidity and studied after one to 12 weeks. ²⁹Si NMR reveals that the decomposition of C–S–H caused by carbonation involves two steps and that the decomposition rate decreases with increasing Ca/Si ratio. The first step is a gradual decalcification of the C–S–H where calcium is removed from the interlayer and defect sites in the silicate chains until Ca/Si = 0.67 is reached, ideally corresponding to infinite silicate chains. In the seconds step, calcium from the principal layers is consumed, resulting in the final decomposition of the C–S–H and the formation of an amorphous silica phase composed of Q³ and Q⁴ silicate tetrahedra. The amount of solid carbonates and of carbonate ions in a hydrous environment increases with increasing Ca/Si ratio for the C–S–H, as shown by ¹³C NMR. For C–A–S–H samples with Ca/Si = 1.0 and 1.5, ²⁷Al NMR demonstrates that all aluminium sites associated with the C–S–H are consumed during the carbonation reactions and incorporated mainly as tetrahedral Al(–OSi)₄ units in the amorphous silica phase. A small amount of penta-coordinated Al sites has also been identified in the silica phase.     

Energetics and Structure of Polymer‐Derived Si–(B–)O–C Glasses: Effect of the Boron Content and Pyrolysis Temperature

The structure and properties of polymer‐derived Si–(B–)O–C glasses have been shown to be significantly influenced by the boron content and pyrolysis temperature. This work determined the impact of these two parameters on the thermodynamic stability of these glasses. High‐temperature oxide melt solution calorimetry was performed on a series of amorphous samples, with varying boron contents (0–7.7 at.%), obtained by pyrolysis of precursors made by a sol–gel technique. Thermodynamic analysis of the calorimetric results demonstrated that at a constant pyrolysis temperature, adding boron makes the materials energetically less stable. While the B‐containing glasses pyrolyzed at 1000°C were energetically less stable than the competitive crystalline components, increasing the pyrolysis temperature to 1200°C led to their enthalpic stability. ²⁹Si and ¹¹B MAS nuclear magnetic resonance (NMR) spectroscopy measurements on selected samples confirmed a decrease in the concentrations of mixed Si‐centered SO_iC_4?i and B‐centered BO_jC_3?j bonds at the expense of formation of SiO₄ and B(OSi)₃ species (indicating a tendency toward phase separation) when the boron content and pyrolysis temperature increased. In light of the findings from calorimetry and NMR spectroscopy, we propose a structure–energetic relationship in Si–(B–)O–C glasses.     

An amorphous Si thin film anode with high capacity and long cycling life for lithium ion batteries

A Si thin film of thickness 275 nm was deposited on rough Cu foil by magnetron sputtering for use as lithium ion battery anode material. X-ray diffraction (XRD) and TEM analysis revealed that the Si thin film was completely of amorphous structure. The electrochemical performance of the Si thin film was investigated by cyclic voltammetry and constant current charge/discharge test. The film exhibited a high capacity of 3,134 mAh g⁻¹ at 0.025 C rate. The capacity retention was 61.3% at 0.5 C rate for 500 cycles. An island structure formed on the Cu foil substrate after cycling adhered to the substrate firmly and provided electrical connection. This is the possible reason for the long cycling life of Si thin film anode. Moreover, the cycling performance was further improved by annealing at 300 °C. The Li⁺ diffusion coefficients (D ₀) of Si thin film, measured by cyclic voltammetry, are 1.47 × 10⁻⁹ cm² s⁻¹ and 2.16 × 10⁻⁹ cm² s⁻¹ for different reduced peaks.     

Growth of nanolaminate structure of tetragonal zirconia by pulsed laser deposition

Alumina/zirconia (Al₂O₃/ZrO₂) multilayer thin films were deposited on Si (100) substrates at an optimized oxygen partial pressure of 3 Pa at room temperature by pulsed laser deposition. The Al₂O₃/ZrO₂ multilayers of 10:10, 5:10, 5:5, and 4:4 nm with 40 bilayers were deposited alternately in order to stabilize a high-temperature phase of zirconia at room temperature. All these films were characterized by X-ray diffraction (XRD), cross-sectional transmission electron microscopy (XTEM), and atomic force microscopy. The XRD studies of all the multilayer films showed only a tetragonal structure of zirconia and amorphous alumina. The high-temperature XRD studies of a typical 5:5-nm film indicated the formation of tetragonal zirconia at room temperature and high thermal stability. It was found that the critical layer thickness of zirconia is ≤10 nm, below which tetragonal zirconia is formed at room temperature. The XTEM studies on the as-deposited (Al₂O₃/ZrO₂) 5:10-nm multilayer film showed distinct formation of multilayers with sharp interface and consists of mainly tetragonal phase and amorphous alumina, whereas the annealed film (5:10 nm) showed the inter-diffusion of layers at the interface.     

High-temperature microstructural evolution of SiCN fibers derived from polycarbosilane with different C/N ratios

Research into the high-temperature microstructural evolution of SiCN ceramic fibers is important for the aerospace application of advanced ceramic matrix composites in harsh environments. In this work, we studied the microstructural evolution of SiCN fibers with different C/N ratios that derived from polycarbosilane fibers at the annealing temperature range of 1400∼1600 °C. These results showed that the phase separation of SiC_xN_y phase and the two-dimension grain growth process of free carbon nanoclusters could be processed at the researched temperature range. As the annealing temperature increased to 1600 °C, the crystallization of amorphous SiC and Si₃N₄ could be detected. SEM and Raman analysis showed that the decomposition and carbothermal reduction of the Si₃N₄ phase at high temperatures played primary roles in contributing to the fiber strength degradation. Thus, a higher C/N ratio, which is beneficial for inhibiting the decomposition of amorphous Si₃N₄, helps SiCN fibers retain high tensile strength at high temperatures.     

Abrasion resistance of ceramic thin films of ZrO2/SiO2 on stretched grade polymethyl methacrylate substrates

Abrasion resistance of stretched grade polymethyl methacrylate (PMMA) was increased by using the sol–gel method to have it coated with a ZrO₂/SiO₂ thin film. Different molar ratios of Zr(OPr)₄/Si(OPr)₄ sols were prepared as precursors with propanol. These sols were used for dip-coating the stretched PMMA surfaces to establish very smooth thin films of amorphous Zr–O–Si. Fourier Transform Infrared spectroscopy (FT-IR) was employed to study vibrations of Zr–O–Si bonds within the thin film. The phase analysis was undertaken via X-ray Diffraction (XRD) method. The morphology and thickness of coatings on PMMA were investigated by means of Scanning Electron Microscopy (SEM). The results showed that coating had an amorphous structure with its thickness within the range of 80–100 nm. The water contact angle of PMMA substrates altered from 73° before coating to less than 64° after coating. Once coated, the PMMA substrate had its transparency characteristic (within the UV–vis region) increased. Furthermore, the influences of thermal treatment temperature and molar ratio of precursors (Zr(OPr)₄/Si(OPr)₄) on abrasion resistance of the coatings were studied.     

Synthesis and thermal evolution of polysilazane-derived SiCN(O) aerogels with variable C content stable at 1600 °C

Ceramic aerogels possess intriguing thermophysical properties which make them excellent candidates for high temperature thermal insulators. However, their properties can degrade at high temperature because of crystallization phenomena or because of densification (causing a sensible reduction of their specific surface area and porosity).The polymer derived ceramic (PDC) route is a relatively new way of developing ceramic aerogels. Several aspects influence the properties of the final product when dealing with preceramic polymers, among them their chemical composition and molecular architecture.In this work, we investigated the possibility of producing aerogels belonging to the SiCN system from polysilazanes mixtures, namely perhydropolysilazane (PHPS) and a methyl/vinyl-containing polysilazane, namely Durazane 1800®, thus changing the C/Si ratio of the amorphous pyrolyzed products. It is shown that the chemical composition of the ceramic aerogel affects the main properties of the porous materials, such as thermal stability and specific surface area (SSA). Results show that the presence of carbon in the aerogels inhibits crystallization of Si₃N₄ up to 1600 °C in N₂ and allows to maintain a SSA of ~90 m²/g up to this temperature.     

High-capacity isomorphic immobilization and leaching mechanism of Gd2Zr2O7-based ceramic waste form

Gd₂Zr₂O₇(GZO)-based ceramic waste form has attracted broad interest due to its potential applications in the long-term disposal of high-level nuclear waste. However, its high-capacity isomorphic immobilization and leaching mechanism are not well understood yet. Herein, GZO-based waste form with a single pyrochlore phase and homogeneous element distribution, achieving an isomorphic waste load as high as 50 wt%, was fabricated successfully. Isomorphism failure of the waste form is due to the Ln₆MoO₁₂ (Ln is lanthanides) formation and direct precipitation of PdO from the oversaturated solid solution. Chemical durability tests showed that the 42-day leaching rates of nearly all elements in the waste form with the maximum waste load were only between 10⁻⁶ and 10⁻⁵ gm⁻²d⁻¹. The leaching of 0.5SW·0.5GZO over a period of 42 days is primarily controlled by dissolution, leading to a gradual restructuring of the surface in two distinct stages: the formation of an amorphous passivation film and the agglomeration of amorphous precipitates. Notably, the formation of the amorphous passivation film stands as the primary factor influencing the leaching rate of the waste form.     

Formation of hydroxyapatite on CaSiO3 powders in simulated body fluid

CaSiO₃ powders were prepared from ethanol solutions of Ca(NO₃)₂·4H₂O and Si(OC₂H₅)₄ using NaOH as a precipitant. The resultant powders were heated at three different temperature regimes, (1) 500°C, (2) 500 and 1000^oC and (3) 500 and 1400°C, to obtain the amorphous phase (amorphous-CS), low temperature phase (β-CS), and high temperature phase (α-CS) of CaSiO₃, respectively. The different amorphous and crystalline phases exhibited different microtextures and specific surface areas of the powders. The rough, porous particles of amorphous-CS and β-CS have higher specific surface areas than the smooth, dense particles of α-CS. These CaSiO₃ powders were soaked in a simulated body fluid (SBF) at 36.5°C for 2 h to 30 days. Formation of hydroxyapatite (HAp) was observed on the surfaces of all samples, but the formation behavior and microstructures were different, resulting the differences in microstructure and crystal structure of the starting powders as well as particle size and specific surface area. The HAp formed on the amorphous-CS was a loose porous layer consisting of uniformly-sized tiny ball-like agglomerated particles, while that formed on the β-CS and α-CS was a dense layer consisting of larger ball-like agglomerated particles.     

Highly Dense Amorphous Si2BC3N Monoliths with Excellent Mechanical Properties Prepared by High Pressure Sintering

The fabrication of dense amorphous Si–B–C–N monoliths is a processing challenge given that it is hard to avoid crystallization at the sintering temperatures needed to attain full density up to 1900°C for conventional hot pressing and SPS methods. We report here successful densification of amorphous Si₂BC₃N monoliths achieved by heating at 1100°C and 5 GPa. The relationships between microstructure, types of chemical bonding, and mechanical properties were investigated. The strong amorphous 3‐D networks of Si–C, C–B, C‐N (sp³), N‐B (sp³), and C–B–N bonds provide high densities at high applied pressure and thus amorphous Si₂BC₃N monoliths show high hardness of 29.4 GPa and elastic modulus of 291 GPa. The amorphous structure is lost with crystallization of β‐SiC and BN(C) reducing contributions from Si–C, C‐N (sp³), and C–B–N bond networks thereby decreasing mechanical properties.