Corrosive and mechanical resistance of MgO ceramics under metallizing and mild chlorination of spent nuclear fuel in molten salts

The present work is aimed at the study of the unirradiated and irradiated MgO ceramics corrosion and mechanical properties in the molten LiCl at 650–750 °C with addition of UCl₃ and Li₂O(LiCl + nLi₂O and LiCl + mUCl₃ molten salts with n = 1.0 and 2.0 mol. % and m = 0.25, 0.5 and 1.0 mol. %). MgO ceramics is suggested to be used as one of materials for pyrochemical technology for recycling of spent nuclear fuel.The gravimetric method with the exposure time during 100 h was the primary method of investigation. The investigation of surface and bulk corrosion of MgO samples by scanning electron microscopy (SEM) and X-ray spectroscopy (MRSA) was performed using scanning electron microscope equipped with a x-Act 6 energy-dispersive analytic system for X-rays characteristic (XRС). Determination of corrosion losses and average corrosion rates of MgO samples was based on the assumption that the ejection of the radionuclides ⁹⁵Zr, ¹⁷⁵Hf and ¹⁸¹Hf from the MgO samples.Incorporation of Li₂O and UCl₃ in molten LiCl result in increase in the rate of MgO ceramic corrosion both at 650 °C and 750 °C and acts on MgO compressive strength (σ_cs) and on the elemental composition of MgO surface layers. Besides the increasing of UCl₃ concentration led to the bulk corrosion of MgO sample grains.Short-term mechanical tests demonstrated the transition of MgO sample destruction pattern depending on the concentration of Li₂O and UCl₃ additions in LiCl melt.Doping of molten LiCl by 0.5 mol. % of UCl₃ at 650 °C and by 0.25 mol. % of UCl₃ at 750 °C had no influence on the ultimate compression strength of irradiated and unirradiated MgO samples. Increased UCl₃ concentration totaling 0.5 mol. % in the LiCl melts at 750 °C reduced the ultimate compression strength of irradiated MgO ceramic samples by ~15%.     

Composite materials with ZrO2–TiO2 matrix and ZrO2 containing ceramic fibres

Abstract

The aim of the work reported in the present paper was to obtain composites consisting of a ZrO₂–TiO₂ matrix reinforced with ceramic fibres containing 12 wt-% ZrO₂, which are resistant to temperatures higher than1500°C. The resulting ceramic matrix consisted of 95 wt-% ZrO₂, partially stabilised with CaO, and 5 wt-% rutile TiO₂. A ceramic fibre content of 0·82 vol.-% was used and for the matrix, several grades of ZrO₂ partially stabilised with CaO were explored, prepared by dry and wet grinding for various grinding periods. Composites were prepared by uniaxial die pressing at 350 MPa and sintered at two temperatures: 1360°C for 1 h and 1500°C for 3 h. The resulting composites showed the following range of properties: total drying and firing shrinkage 0·4–3·3%; apparent density 3·51–3·96 g cm^-3; porosity 25–34%; water absorption 6–10%; bend strength 12–43 MPa. The optimum ZrO₂ grades were determined based on physical and mechanical properties, and on structural determinations carried out by thermodifferential and thermogravimetric analyses, X-ray diffraction (XRD), and scanning electron microscopy (SEM). SEM evaluation illustrated the increase in average size of crystallites typical of ZrTiO₄ solid solution as a function of temperature, from 2 μm at 1360 up to 14 μm at 1600°C, and of their tendency to sinter.     

Phase transformation and microstructure of MgO–Al2O3–SiO2 system glass–ceramics under different heat treatment conditions

Abstract

Effects of heat treatment conditions on phase transformation, microstructure and thermal expansion coefficient (TEC) in MgO–Al₂O₃–SiO₂ system glass–ceramics were investigated by means of differential thermal analysis, X-ray diffraction and scanning electron microscopy. The magnesium aluminium titanate (MAT) precipitated firstly at 850°C and β-quartz solutions (β-QSS) formed at 950°C. Further increasing temperature to 1000°C, MAT disappeared and β-QSS became master phase, following little amount of α-cordierite, MgTi₂O₅, rutile and sapphirine. When glass was treated at 1050°C, β-QSS content decreased and α-cordierite became master phase. As temperature reached higher than 1100°C, β-QSS and sapphirine disappeared, and α-cordierite became master phase accompany with rutile and MgTi₂O₅ as secondary phase. The microstructure transformed gradually from particle shape crystallites to slat shape network with the increase in heat treatment temperature. By controlling heat treatment condition, an ideal glass–ceramics with proper TEC for matching sealing to 4J29 alloy has been obtained.     

Effect of Ni Doping on the Structural and Optical Properties of TiO<Subscript>2</Subscript> Nanoparticles at Various Concentration and Temperature

TiO₂ nanopowders doped by Ni were prepared by sol–gel method. The effects of Ni ion (transition metal ion) doping on the physical structural and optical properties of TiO₂ have been investigated by X-ray diffraction (XRD), scanning electron microscopy and UV–Vis absorption spectroscopy. XRD results suggest that adding impurities has a significant effect on anatase phase stability, crystallinity, and particle size of TiO₂. The phase transformation from anatase to rutile was inhibited by Ni ion doped TiO₂ at temperatures 675 °C. The lowest band gap value (2.83 eV) was obtained for TiO₂-4%Ni sample calcined at 675 °C.     

Effect of heat treatment on crystal phase and photocatalytic activity of Tm doped TiO2 nanoparticles

Abstract

Abstract

Tm doped TiO₂ nanoparticles have been synthesised by hydrolysis-precipitation method. The effect of heat treatment on the crystal phase and photocatalytic activity of Tm doped TiO₂ nanoparticles has been studied. The prepared samples were characterised by transmission electron microscopy, X-ray diffraction, Fourier transformed infrared and diffuse reflection spectrum analysis. The results show that Tm³⁺ doping can effectively inhibit the phase transformation from antase to rutile and decrease the crystallite size of nano-TiO₂ particles. There is an optimal Tm doping (1·4?mol.-%) after calcination at 550°C for the photocatalytic activity of methylene blue degradation.     

Experimental phase equilibria studies of the PbO–SiO2 system

Phase equilibria of the PbO–SiO₂ system have been established for a wide range of compositions: (i) liquid in equilibrium with silica polymorphs (quartz, tridymite, and cristobalite) between 740°C and 1580°C, at 60‐90 mol% SiO₂; (ii) with lead silicates (PbSiO₃, Pb₂SiO₄, and Pb₁₁Si₃O₁₇) and lead oxide (PbO) between 700°C and 810°C. A high‐temperature equilibration/quenching/electron probe X‐ray microanalysis (EPMA) technique has been used to accurately determine the compositions of the phases in equilibrium in the system. Significantly, no liquid immiscibility has been found in the high‐silica range, and the liquidus in this high‐silica region has been accurately measured. The phase equilibria information in the PbO–SiO₂ system is of practical importance for the improvement of the existing thermodynamic database of lead‐containing slag systems (Pb–Zn–Fe–Cu–Si–Ca–Al–Mg–O).     

Synthesis and characterization of functional copolymer of Linalool and vinyl acetate: A kinetic study

Linalool (LIN) and vinyl acetate (VA) were copolymerized by benzoyl peroxide (BPO) in p‐xylene at 60°C for 90 min. The system follows nonideal kinetics: R_pα[I]^0.6[LIN]^1.2[VA]^1.1. It results in the formation of alternating copolymer as evidenced from reactivity ratios as r₁ (VA) = 0.01, r₂ (LIN) = 0.0015, which have been calculated by Kelen–Tudos method. The overall activation energy is 82 kJ/mol. The FTIR spectrum of the copolymer shows the presence of the band at 3425 cm^?1 due to alcoholic group of LIN and at 1641 cm^?1 due to >C?O group of VA. The ¹H‐NMR spectrum shows peaks at 7.0–7.7 δ due to hydroxy proton of LIN and at 1.0–1.4 δ due to acetoxy protons of VA. ¹³C‐NMR spectrum of copolymer shows peaks at 167 ppm due to acetoxy group and at 75–77 ppm due to C? OH group. The Alfrey–Price Q–e parameters for LIN has been calculated as Q₂ = 1.24 and e₂ = 3.11. The copolymer is highly thermally stable and has a glass transition temperature (T_g) of 85°C, evaluated from DSC studies. The mechanism of copolymerization has been elucidated. This article also reports measurement of Mark–Houwink constants in THF at 25°C by means of GPC as α = 0.8 and K = 3.0 × 10^?4 dl/g. The thermal decompositions of copolymer are established with the help of TGA technique. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 1134–1143, 2004     

Thermophysical properties of softened seawater and salt solutions over a wide temperature and pressure range

Investigations on the thermophysical properties of treated seawater and NaCl, Na₂SO₄ and MgCl₂ solutions of various concentrations have been carried out. Density has been investigated at 20–200°C temperatures and up to 200 bar pressures. The heat conductivity of seawater and solutions of various concentrations was studied at 20–300°C accounting for the solubility and the results analyzed. The viscosity of NaCl solutions was studied by the capillary method at 20–350° C and to 300 bars. The absolute value of viscosity decreases with increase in temperature, but the relative value increases. Heat capacitance of NaCl solutions investigated at 20–200°C, was found to decrease with increase in concentration. The surface tension of NaCl solutions was studied by the method of two capillaries at20–200°C.     

Microstructure and EMW absorption properties of CVI Si3N4–SiCN ceramics with BN interface annealed in N2 atmosphere

A kind of chemical vapor infiltration (CVI) Si₃N₄–BN–SiCN composite ceramic with excellent electromagnetic wave (EMW) absorbing properties is obtained by CVI BN interface and SiCN matrix on porous Si₃N₄ ceramics, and then annealed at high temperatures (1200°C‐1500°C) in N₂ atmosphere. The crystallization behavior, EMW absorbing mechanism and mechanical properties of the composite ceramics have been investigated. Results showed CVI SiCN ceramics with BN interface were crystallized in the form of nanograins, and the crystallization temperature was lower. Moreover, both EMW absorbing properties and mechanical properties of CVI Si₃N₄–BN–SiCN composite ceramics firstly increased and then decreased with the increase in annealing temperature due to the influence of BN interface on the microstructure and phase composition of the composite ceramics. The minimum reflection coefficient (RC) and maximum effective absorption bandwidth (EAB) of the composite ceramics annealed at 1300°C were ?47.05 dB at the thickness of 4.05 mm and 3.70 GHz at the thickness of 3.65 mm, respectively. The flexural strength and fracture toughness of the composite ceramics annealed at 1300°C were 94 MPa and 1.78 MPa/m^1/2, respectively.     

Low–thermal–conductivity and high–toughness CeO2–Gd2O3 co–stabilized zirconia ceramic for potential thermal barrier coating applications

Yttria partially stabilized zirconia (～4.0?mol% Y₂O₃–ZrO₂, 4YSZ) has been widely employed as thermal barrier coatings (TBCs) to protect the high–temperature components of gas–turbine engines. The phase stability problem existing in the conventional 4YSZ has limited it to application below 1200?°C. Here we report an excellent zirconia system co–doped with 16?mol% CeO₂ and 4?mol% Gd₂O₃ (16Ce–4Gd) presenting nontransformable feature up to 1500?°C, in which no detrimental monoclinic (m) ZrO₂ phase formed on partitioning. It also exhibits a high fracture toughness of ～46?J m^?2 and shows high sintering resistance. Besides, the thermal conductivity and thermal expansion coefficient of 16Ce–4Gd are more competent for TBCs applications as compared to the 4YSZ. The combination of properties suggests that the 16Ce–4Gd system could be of potential use as a thermal barrier coating at 1500?°C.     

Crystallization Mechanism of CVD Si3N4–SiCN Composite Ceramics Annealed in N2 Atmosphere and Their Excellent EMW Absorption Properties

To tailor a new electromagnetic wave (EMW) absorbing material with lower reflection coefficient (RC) and larger operating frequency band, the CVD Si₃N₄–SiCN composite ceramics were prepared from SiCl₄–NH₃–C₃H₆–H₂–Ar system and then annealed at the temperatures of 1400–1700°C in N₂ atmosphere. Effect of the annealing temperatures on the microstructure, phase composition, permittivity, and microwave‐absorbing properties of the ceramic were investigated. Results showed that the CVD Si₃N₄–SiCN ceramics gradually crystallized into nanosized SiC grains, Si₃N₄ grains and graphite (T ≤ 1600°C), and then the grains grew up at T = 1700°C. The permittivity, dielectric loss, and electrical conductivity of as‐annealed CVD Si₃N₄–SiCN ceramics (T ≤ 1600°C) increased firstly due to the formation of conductivity and polarity network and the increase in nanograin boundary, and then decreased at 1700°C because of the growth of nanograins and the disappearance of nanograin boundary. The minimal RC and effective absorption bandwidth of the as‐annealed CVD Si₃N₄–SiCN ceramic at 1600°C was ?41.67 dB at the thickness of 2.55 mm and 3.95 GHz at the thickness of 3.05 mm, respectively, demonstrating that the totally crystallized CVD Si₃N₄–SiCN ceramic (T = 1600°C) had the superior microwave‐absorbing ability.     

Reactivity of SiO2 fume from ferrosilicon production with Ca(OH)2 under hydrothermal conditions

Reactivity has been studied of SiO₂ fume from ferrosilicon production with Ca(OH)₂ in suspension, with stirring, under hydrothermal conditions at 190°C and 200°C from 1 to 12 hours. Reaction product after a 4-hour synthesis at 200°C is calcium silicate hydrate xonotlite C₆S₆H, with some C-S-H(I) phase. When glass fibers were added to the slurry, which was subsequently filtered, pressed, and dried, the product thus obtained was lightweight thermal insulation material, temperature-resistant up to 800°C and usable up to 1000°C. The product has excellent insulation properties with thermal conductivity 0.042–0.081 W/mK at bulk densities 150–500 kg/m³.     

Preparation and compressive strength of calcium phosphate based cement dispersed with polycrystalline ceria doped tetragonal zirconia

Abstract

The preparation and strength of a calcium phosphate based cement with and without dispersed particles of polycrystalline 12 mol-%CeO₂ doped tetragonal zirconia (12Ce–TZP) were investigated. A cement mixture of α-tricalcium phosphate (α-Ca₃ (PO₄ )₂ ) and tetracalcium phosphate (Ca₄ (PO₄ )₂O) with a Ca/P ratio of 1·67 transformed completely to hydroxyapatite after curing under water for 7 d at 38°C. The compressive strength of a cement without dispersed 12Ce–TZP particles cured for 4 d at 38°C was ～11·1 MPa but increased to ～16 MPa by dispersing 3 wt-%12Ce–TZP particles, of average diameter 40 μm, which were treated in 10M NaOH and/or 0·01M Ca₅ (PO₄ )₂ gel solutions prior to dispersion.     

Absorption of Carbon Dioxide into Aqueous Xanthan Gum Solution Containing Monoethanolamine

Abstract

Carbon dioxide was absorbed into the aqueous xanthan gum (XG) solution in the range of 0–0.151 wt% containing monoethanolamine (MEA) of 0–2 kmol/m³ in a flat‐stirred vessel with the impeller of 0.05 m and agitation speed of 50 rpm at 25°C and 0.101 MPa to measure the absorption rate of CO₂. The volumetric liquid‐side mass transfer coefficient (k_La_L) of CO₂ decreased with increasing XG concentration, and was correlated with the empirical formula having the rheological behavior of XG solution. The chemical absorption rate of CO₂ was estimated by the film theory using the values of k_La_L and physicochemical properties of CO₂ and MEA. The aqueous XG solutions made the rate of absorption of CO₂ accelerated compared with the Newtonian liquid based on the same viscosity of the solution.     

Synthesis and characterization of novel polythiophenes bearing oligo(ethylene glycol) segments and azobenzene units

New copolymers of thiophenes containing azobenzene moieties in the side chain, 3-((((((4-phenyl)azo)phenoxy)ethyl)triethoxy)oxy)-4-methylthiophene/3-methyltetra (oxyethylene)oxy-4-methylthiophene (CP1) and 3-((((((4-phenyl)azo)phenoxy)ethyl) triethoxy) oxy)-4-methylthiophene/3-dodecylthiophene (CP2), were synthesized from 3-bromo-4-methylthiophene. These copolymers were characterized by ¹H-NMR spectroscopy, and their thermal, optical, and electrochemical properties were determined by thermogravimetric analysis, differential scanning calorimetry, absorption spectroscopy, and cyclic voltammetry. CP1 possesses higher degree of conjugation and azobenzene content than CP2, and its structure is more regio-regular. Besides, CP1 exhibited a higher glass transition temperature (T _g = 35 °C, T ₅ = 225 °C) and a lower thermal stability than CP2 (T _g = 13 °C, T ₅ = 307 °C). Both copolymers showed a similar solvatochromic behavior when they were dissolved in chloroform and mixtures chloroform:methanol.     

Ceramic matrix composite production by directed melt oxidation of pure aluminium metal using spinel dopants

Abstract

The production of Al₂O₃ matrix composites by directed melt oxidation of pure Al externally doped with spinel type dopants has been investigated. The presence of any one of MgAl₂O₄ , LiAlO₂, and ZnAl₂O₄ resulted in oxide growths in a similar fashion to the growths produced using elemental Mg, Li, and Zn. Rapid growth was achieved at 1180°C with MgAl₂O₄ and at 900°C with LiAlO₂ . The growth rates at 1180°C of the samples doped with ZnAl₂O₄ were less rapid than the growth rates of the samples doped with MgAl₂O₄ . The fact that growth is initiated by mixed oxide spinel dopants is further evidence in support of the cyclic reaction sequences that have been proposed for directed melt oxidation.     

One-pot synthesis of soluble wholly aromatic liquid crystalline copoly(ester amide)s with high thermal and dimensional stability

Abstract

Thermotropic liquid crystalline polymers (LCPs) have been of great interest for electronic packaging. Herein, we introduce a series of wholly aromatic, thermotropic LCPs from copoly(ester amide)s of 6-hydroxy-2-naphthalic acid, isophthalic acid, terephthalic acid, and 4-aminophenol, prepared by a convenient one-pot melt polycondensation. Almost synthesized copoly(ester amide)s exhibited good solubility in common organic solvents at room temperature. Furthermore, they possessed high thermal stability with 2% degradation temperatures (T_id) of 359–368?°C and the char yields (at 600?°C) of 50.3–55.6%. The synthesized copoly(ester amide)s had relatively low coefficient of thermal expansion (CTE) values, which were 35.85–41.21?ppm °C^?1 in the temperature range of 50–200?°C. Furthermore, an annealing process could be employed to improve the thermomechanical properties of synthesized polymers. For instance, the CTE of sample LCP3 in range temperature of 275–315?°C was reduced by more than 90% after annealing at 320?°C for 1?h, implying the feasibility for electronic packaging.     

Ozone absorption in alkaline solutions

Using a packed column, the rate of ozone absorption into KOH aqueous solutions has been measured. The pH range was 8.5–13.5 and the temperature varied from 18 to 27°C.Independent measurements of CO₂ absorption into buffer solutions containing KAsO₂ were used to determine the interfacial area.The results can be interpreted on the basis of a first order reaction in both O₃ and OH^?.     

Low temperature synthesis of a new yellowish brown ceramic pigment based on FeNbO4@ZrSiO4

FeNbO₄@ZrSiO₄ ceramic pigments have been successfully synthesized with MgO as mineralizer by an environment-friendly solid-state reaction method at low temperature. The formation of ZrSiO₄ has been investigated by X-ray diffraction, thermogravimetry and differential scanning calorimeter analysis, respectively. The reflectance spectrum and color properties of the FeNbO₄@ZrSiO₄ pigments have been measured and analyzed. Results illustrate that MgO together with Fe₂O₃–Nb₂O₅ can promote the formation of ZrSiO₄ and decrease the synthesis temperature to 1050?°C that is 400–500?°C lower than the characteristic temperature of ZrSiO₄ formation. A series of FeNbO₄@ZrSiO₄ pigments with high thermostability present color changing from bud yellow to earthy yellow as a function of Fe₂O₃–Nb₂O₅ content. The preparation method used in this work possesses many advantages of safety, simplicity of processing and is free of gas phases, indicating a more favorable method to synthesize ZrSiO₄-based pigments. The new yellowish brown FeNbO₄@ZrSiO₄ pigments are promising for high-temperature ceramic pigments applications.     

Studies on polycrystalline layered ceramic oxide: LiFeVO4

Abstract

A new polycrystalline layered ceramic oxide, LiFeVO₄, has been prepared by a standard solid state reaction technique. The preparation conditions were optimised using thermogravimmetric analysis (TGA) technique. Material formation under the reported conditions was confirmed by X-ray diffraction studies. A preliminary structural analysis indicated that the crystal structure was orthorhombic with lattice parameters: a=4·3368 Å, b=13·1119 Å and c=16·3426 Å. The phase morphology and surface property were studied by scanning electron microscopy. Complex impedance analysis of the sample indicated bulk contribution to electrical properties at T≤125°C, grain boundary effects at the temperatures ≥125°C, negative temperature coefficient of resistance (NTCR) effect and evidence of temperature dependent electrical relaxation phenomena in the sample. The dc conductivity σ_dc shows typical Arrhenius behaviour when observed as a function of temperature. The activation energy value was estimated to be 0·24 eV. The value of σ_dc, evaluated from complex impedance spectrum, shows a jump of nearly two orders of magnitude at higher temperature (～1·24 × 10^?5 S cm^?1 at 350°C) when compared with that of σ_dc (1·14 × 10^?6 S cm^?1 at 50°C). Alternating current conductivity spectrum obeys Jonscher's universal power law. The results of σ_ac v. temperature are also discussed.