Experimental synthesis and density functional theory investigation of radiation tolerance of Zr3(Al1‐xSix)C2 MAX phases

Synthesis, characterization and density functional theory calculations have been combined to examine the formation of the Zr₃(Al_1–xSi_x)C₂ quaternary MAX phases and the intrinsic defect processes in Zr₃AlC₂ and Zr₃SiC₂. The MAX phase family is extended by demonstrating that Zr₃(Al_1–xSi_x)C₂, and particularly compositions with x≈0.1, can be formed leading here to a yield of 59 wt%. It has been found that Zr₃AlC₂ ‐ and by extension Zr₃(Al_1–xSi_x)C₂ ‐ formation rates benefit from the presence of traces of Si in the reactant mix, presumably through the in situ formation of Zr_ySi_z phase(s) acting as a nucleation substrate for the MAX phase. To investigate the radiation tolerance of Zr₃(Al_1–xSi_x)C₂, we have also considered the intrinsic defect properties of the end‐members. A‐element Frenkel reaction for both Zr₃AlC₂ (1.71 eV) and Zr₃SiC₂ (1.41 eV) phases are the lowest energy defect reactions. For comparison we consider the defect processes in Ti₃AlC₂ and Ti₃SiC₂ phases. It is concluded that Zr₃AlC₂ and Ti₃AlC₂ MAX phases are more radiation tolerant than Zr₃SiC₂ and Ti₃SiC₂, respectively. Their applicability as cladding materials for nuclear fuel is discussed.     

The influence of pentavalent Nb substitution for Zr on electrical property of oxide-ion conductor Gd2Zr2O7

Gd₂(Zr_1−xNb_x)₂O_7+x (0 ≤ x ≤ 0.2) ceramics are prepared via the solid state reaction process at 1973 K for 10 h in air. Gd₂(Zr_1−xNb_x)₂O_7+x (x = 0.1, 0.2) ceramics exhibit an ordered pyrochlore-type structure, whereas Gd₂Zr₂O₇ has a defective fluorite-type structure. The electrical property of Gd₂(Zr_1−xNb_x)₂O_7+x ceramics is investigated by electrochemical impedance spectroscopy over a frequency range of 10 Hz to 8 MHz from 623 to 923 K. The electrical conductivity obeys the Arrhenius equation. The grain conductivity of Gd₂(Zr_1−xNb_x)₂O_7+x ceramics varies with doping different Nb contents, and exhibits a maximum at the Nb content of x = 0.1 in the temperature range of 623-923 K. The conductivity in hydrogen atmosphere is a little bit higher than in air in the temperature range of 723-923 K, which indicates that the doping of Zr⁴⁺ by Nb⁵⁺ can increase the proton-type conduction and reduce the oxide-ionic conduction. The conduction of Gd₂(Zr_1−xNb_x)₂O_7+x is not a pure oxide-ionic conductor.     

Ablation behavior of high-entropy carbides ceramics (Hf0.2Zr0.2Ta0.2Nb0.2Ti0.2)C upon exposition to an oxyacetylene torch at 2000 °C

The ablation performance of a high-entropy ceramic carbide, (Hf_0.2Zr_0.2Ta_0.2Nb_0.2Ti_0.2)C, was performed by oxyacetylene ablation flame, simulating the extreme service environment at 2000 ºC. Phase and microstructure characterization at multi-length scales was carried out. During ablation, a compositionally and microstructurally complex oxidation layer formed on the ablation surface, which consisted of a combination of (Zr_xHf_1?x)₆(Nb_yTa_1?y)₂O₁₇, Ti(Nb_xTa_1?x)₂O₇, and Ti_x(Zr_aHf_bNb_cTa_1?a-b-c)_1?xO₂. Based on the microstructure information, the ablation mechanisms were proposed considering the oxidation thermodynamics and kinetics. Comparable rates of O inward diffusion and Ti outward diffusion are suggested, and a particular innermost dense layer composed of isolated (Zr_xHf_1?x)₆(Nb_yTa_1?y)₂O₁₇ grains embedded in a continuous Ti(Nb_xTa_1?x)₂O₇ matrix is considered to be beneficial for a better ablation resistance.     

Preparation and microstructural characterization of (Nd1−xGdx)2(Ce1−xZrx)2O7 solid solutions

(Nd_1−xGd_x)₂(Ce_1−xZr_x)₂O₇ (0 ≤ x ≤ 1.0) powders with an average particle size of 100 nm were synthesized with chemical-coprecipitation and calcination method, and were characterized by X-ray diffractometry and scanning electron microscopy. The sintering behaviour of (Nd_1−xGd_x)₂(Ce_1−xZr_x)₂O₇ powders was studied by pressureless sintering at 1600–1700 °C for 10 h in air. The relative densities of (Nd_1−xGd_x)₂(Ce_1−xZr_x)₂O₇ solid solutions increase with increasing the sintering temperature, and gradually decrease with increasing the content of neodymium and cerium at identical temperature levels. (Nd_1−xGd_x)₂(Ce_1−xZr_x)₂O₇ solid solutions have a single phase of defect fluorite-type structure among all the composition combinations studied. The lattice parameters of (Nd_1−xGd_x)₂(Ce_1−xZr_x)₂O₇ solid solutions agree well with the Vegard's rule.     

The effect of co-doping with MgO and Yb2O3 on the structure and electrical conductivity of the Sm2Zr2O7 pyrochlore

SmYb_1−xMg_xZr₂O_7−x/2 (0 ≤ x ≤ 0.15) ceramics are pressureless-sintered at 1973 K for 10 h in air. The structure and electrical conductivity of SmYb_1−xMg_xZr₂O_7−x/2 ceramics are investigated by the X-ray diffraction, scanning electron microscopy and impedance spectroscopy measurements. SmYb_1−xMg_xZr₂O_7−x/2 ceramics exhibit a defect fluorite-type structure. The measured electrical conductivities of SmYb_1−xMg_xZr₂O_7−x/2 ceramics obey the Arrhenius relation, and electrical conductivity of each composition increases with increasing temperature from 673 to 1173 K. At identical temperature levels, the electrical conductivity of SmYb_1−xMg_xZr₂O_7−x/2 ceramics gradually increases with increasing magnesia content. SmYb_1−xMg_xZr₂O_7−x/2 ceramics are oxide-ion conductors in the oxygen partial pressure range of 1.0 × 10⁻⁴ to 1.0 atm at all test temperature levels. The electrical conductivity obtained in SmYb_1−xMg_xZr₂O_7−x/2 ceramics reaches the highest value of 2.72 × 10⁻³ S cm⁻¹ at 1173 K for the SmYb_0.85Mg_0.15Zr₂O_6.925 ceramic.     

Effect of praseodymium oxide and cerium–praseodymium mixed oxide in the Pt electrocatalyst performance for the oxygen reduction reaction in PAFCs

The effect of praseodymium oxide and cerium–praseodymium mixed oxide in the Pt electrocatalyst performance for oxygen reduction reaction (ORR) in Phosphoric Acid Fuel Cells (PAFCs) has been studied. Three electrocatalysts (Pt/C, PtPrO _x /C and PtCe_0.9Pr_0.1O _y /C, where x and y are ≤2) have been prepared and tested by cyclic voltammetry (CV) and long term chronoamperometry (CA) experiments. The fresh and tested electrocatalysts have been characterized by X-ray diffraction (XRD) and Transmission Electron Microscopy–Energy Dispersion Spectroscopy (TEM–EDS). The Pr and Ce–Pr oxides improved Pt dispersion in the fresh electrocatalysts with regard to the Pt-only catalyst, and the PtPrO _x /C and PtCe_0.9Pr_0.1O _y /C electrocatalysts presented a slightly improved catalytic activity towards ORR in comparison to the reference Pt/C electrocatalyst. The activity decay during the long term CA tests was slower for PtPrO _x /C and PtCe_0.9Pr_0.1O _y /C than for Pt/C. Although the Pr and Ce–Pr oxides were dissolved during the CA measurements, the Pt sintering was prevented.     

The effects of the NaF flux on the oxidation state and localisation of praseodymium in Pr-doped zircon pigments

The role that NaF plays in the preparation of Pr-doped zircon pigments was studied through the analysis of the nature and localisation of the Pr cations into the zircon matrix in samples prepared in the absence and in the presence of NaF. As previously observed, the addition of NaF caused a decrease of the minimum temperature required for zircon formation from 1400 to 1100°C, and an increase of the yellow colour intensity. In the absence of NaF, the Pr cations mainly presented a threefold oxidation state, being located out of the zircon lattice as Pr₂Zr₂O₇, whereas in the presence of this flux, most of the Pr cations showed a fourfold valence and formed a solid solution with the zircon lattice, which was then the main responsible for the stronger yellow colour observed in this case. After heating this pigment at 1400°C, we detected an exsolution of the Pr (IV) cations as Pr₈Si₆O₂₄ which was accompanied by a decrease of the yellow colour intensity. Therefore, it was concluded that the main role of NaF in the preparation of yellow Pr-zircon pigments is to decrease the temperature of zircon formation to the range in which the chromophore responsible for the bright yellow colour, i.e. the Pr (IV)-zircon solid solution, is stable.     

Ternary phase relations in CeO2–DyO1.5–ZrO2 system

The present work explores the sub-solidus phase relations in the CeO₂–DyO_1.5–ZrO₂ ternary system. About 80 compositions in Zr_1−xDy_xO_2−x/2, Ce_1−xDy_xO_2−x/2, (Ce_0.8Zr_0.2)_1−xDy_xO_2−x/2, Zr_1−x(Ce_0.2Dy_0.8)_xO_2−0.4x, Ce_x(Dy_0.5Zr_0.5)_1−xO_1.75+x/4 systems, were synthesized and explored to investigate the phase fields in this ternary system. Detailed XRD analysis showed the existence of a variety of phase fields viz. Fluorite-type cubic, C-type cubic, biphasic fields containing both F-type and C-type phases as well as co-existence of two different fluorite type phases. A few compositions also showed the presence of monoclinic as well the tetragonal phases. The trends observed in cell parameter are found to be governed by the competing factors of average ionic radius and the repulsion between excess anions in the lattice due to the aliovalent substitution. This ternary system showed the existence of a very wide cubic phase field. This ternary phase relation has relevance to the inert matrix fuel concept.     

Patent Abstracts on Cellular Plastics

(US5019292): Granular detergent composition comprises: at least 1% detersive surfactant (I): 5–35% detergency builders (II); 1–25% naturally occurring hectorite clay ((Mg_3x Li _x )Si_4-y MeIII _y O₁₀(OH_2-z F _z )) - [(x+y) (x+y) Mn⁺]/n (III); and 1–10% additional fabric softener (IV) of formula (a) R₁R₂R₃N, (b) R₁OR₁NCOR₁₂: where MeIII is Al, Fe, or B, or y = 0; Mn⁺ is mono- or divalent metal; the clay (III) has a layer charge distribution (x+y) such that at least 50% of the layer charge is 0.23–0.31; R₁, = 6–20 C hydrocarbyl; R₂ = 1–20 C hydrocarbyl; R₃ = H or 1–20 C hydrocarbyl; R₁₀, R₁₁ = 1–22 C alk(en)yl, hydroxy alkyl, aryl, alkaryl; R₁₂ = H, 1–22 C alk(en)yl, aryl, alkaryl or OR₁₃; R₁₃ = 1–22 C alk(en)yl, aryl, alkaryl, or (c) a 1-(12–22 C alkyl) amide (1–4 C alkyl)-2-(12–22 C alkyl) imidazoline.     

Synthesis and characterization of PrxZr1-xSiO4 (x = 0–0.08) yellow pigments via non-hydrolytic sol-gel method

Yellow inorganic pigments Pr_xZr_1-xSiO₄ (x?=?0–0.08) have been prepared by a novel non-hydrolytic sol-gel (NHSG) method at 750?°C for 2?h. Replacing Pr⁴⁺ for Zr⁴⁺ in ZrSiO₄ increased the cell volume and changed the color from white to yellow gradually. The Si―O―Zr and Si―O―Pr bands were observed in the FT-IR spectra of xerogel, indicating it could reach homogeneous mixing at the atomic level. Therefore, it promoted the solid solution reaction between Pr and zircon at low temperature. The samples exhibit high doping limitation (x?=?0.08) and brilliant yellow hue (b^*?=?69.48) in contrast with the previously reported praseodymium zircon yellow pigments. The intense of yellow hue was increased with increasing the Pr doping content due to the increase of Pr⁴⁺/Pr³⁺ species. After applying on bisque ceramic tiles, the pigment exhibited excellent coloration, high thermal stability and low solubility in molten glazes, indicating its potential application in ceramic decoration.     

Preparation, structure and electrical conductivity of pyrochlore-type Gd1−xEu2xSm1−xZr2O7 ceramics with a constant lattice parameter

A novel series of Gd_1−xEu_2xSm_1−xZr₂O₇ (x = 0, 1/3, 1/2, 2/3, 1) ceramics with a constant lattice parameter are prepared by solid-state reaction, and are then evaluated as possible solid electrolytes. The microstructure and electrical properties of Gd_1−xEu_2xSm_1−xZr₂O₇ ceramics have been investigated by means of X-ray diffraction (XRD), scanning electron microscopy (SEM) and complex impedance analysis. Gd_1−xEu_2xSm_1−xZr₂O₇ ceramics exhibit a single phase of pyrochlore-type structure. The total conductivity of Gd_1−xEu_2xSm_1−xZr₂O₇ ceramics obeys the Arrhenius relation, and gradually increases with increasing temperature from 723 to 1173 K. At 973–1173 K, the composition has little effect on electrical conductivity of Gd_1−xEu_2xSm_1−xZr₂O₇ ceramics. Gd_1−xEu_2xSm_1−xZr₂O₇ ceramics are oxide-ion conductors in the oxygen partial pressure range of 1.0 × 10⁻⁴ to 1.0 atm at all test temperature levels. The maximum total conductivity of Gd_1−xEu_2xSm_1−xZr₂O₇ ceramics is about 1.01 × 10⁻² S cm⁻¹ at 1173 K in air.     

Electrochemical properties of Li1?z(Ni1?yFey)1+zO2 synthesized by the combustion method in an air atmosphere

For the syntheses of LiNi_1−y Fe _y O₂ (0.000 ≤ y ≤ 0.300), mixtures of the starting materials with the desired compositions were preheated in an air atmosphere at 400 °C for 30 min and calcined in air at 700 °C for 48 h. The phases appearing in the intermediate reaction steps for the formation of lithium nickel oxide are deduced from the DTA analysis. XRD analysis, FE-SEM observation, FTIR analysis and electrochemical measurement were performed for the synthesized Li_1−z (Ni_1−y Fe _y )_1+z O₂ (0.000 ≤ y ≤ 0.300) samples. The samples of Li_1−z (Ni_1−y Fe _y )_1+z O₂ with y = 0.025 and 0.050 have higher first discharge capacities than Li_1−z (Ni_1−y Fe _y )_1+z O₂ with y = 0.000 and better or similar cycling performance at the 0.1 C rate in the voltage range of 2.7–4.2 V. Similar results have not previously been reported except for Co-substituted LiNiO₂. The sample Li_1−z (Ni_0.975Fe_0.025)_1+z O₂ has the highest first discharge capacity (176.5 mAh g⁻¹). Rietveld refinement of the XRD patterns of LiNi_1−y Fe _y O₂ (0.000 < y ≤ 0.100) from a starting structure model [Li,Ni]_3b[Li,Ni,Fe]_3a[O₂]_6c showed that cation disordering occurred in the samples.     

Structure of the mixed oxides Me x (Ce0.5Zr0.5)1? x O y ( Me = Gd, Pr)

The structure of the mixed oxides Me _x (Ce_0.5Zr_0.5)_1−x O _y (x = 0, 0.1–0.30; y < 2) doped with trivalent Gd and Pr metals is investigated using the X-ray powder diffraction analysis, the full-profile analysis of the diffraction pattern, and EXAFS spectroscopy. The mixed oxides are solid solutions with a structure similar to the CeO₂ structure. An increase in the content of the introduced trivalent metal brings about an increase in the unit cell parameter and a disordering of the structure of the oxides under investigation. Original Russian Text ? V.P. Kol’ko, D.A. Zyuzin, V.A. Sadykov, V.V. Kriventsov, E.M. Moroz, 2007, published in Fizika i Khimiya Stekla.     

Y3Al5O12–SiO2 Glasses: Structure and Polyamorphism

Aerodynamic levitation and CO₂ laser melting have been used to synthesize the yttrium aluminosilicate glasses zY₂O₃–yAl₂O₃–xSiO₂ with z/y = 3/5 corresponding to the YAG (Y₃Al₅O₁₂) composition and x between ~5 and ~45 mol%. The low‐ and high‐density (LDA inclusion and HDA matrix) polyamorphic phases in glasses with less than ~14 mol% SiO₂ were identified with backscattering electron imaging. Polarized and depolarized Raman spectra show the formation of various Qⁿ SiO₄ species whose relative populations change smoothly as the SiO₂ content is altered. The AlOs (s = 4–6) and YOz (z = 6–9) polyhedra formed in the YAG glass are preserved upon silica additions while the terminal oxygens of the Q²AlO₄ tetrahedra are gradually bridged to the Qⁿ‐SiO₄ species. The low‐frequency Boson Peak overlaps with the vibrational spectrum and its maximum is redshifted with increasing silica content. Micro‐Raman spectra measured for the LDA and HDA amorphous phases are found to be similar to the spectra of the bulk glass indicating common structural characteristics. The stability of the LDA phase against crystallization appears to be lower than that of the HDA phase. The crystallinity on certain inclusions consisted of YAG microcrystals and a new unidentified microcrystalline phase within Y₄Al_2(1?x)Si_2xO_(9+x) solid solution.     

Preparation, structural and electrical characteristics of praseodymium modified lead titanate

Praseodymium modified lead titanate ceramics (Pb_1−xPr_xTi_1−2yMo_yFe_yO₃) with x = 0.02, 0.04, 0.06, 0.08 and 0.10 and y = 0.02 have been prepared by high temperature solid state reaction technique. Cold pressed pellets were sintered at 1100 °C for 2 h. Lattice parameters and crystal tetragonality was determined by X-ray diffraction analysis. All the synthesized samples show single phase with tetragonal structure. Tetragonality decreases and relative density increases with the increase in praseodymium substitution. Electrical characteristics which include dielectric properties and ac conductivity were studied as a function of temperature and frequency. Variation of dielectric constant with temperature shows a ferroelectric phase transition and transition temperature (T_C) decreases with increase in the praseodymium content. Nature of transition was studied and found to be diffused. Conductivity (σ_ac) was measured as a function of frequency in the range 10³–10⁶ Hz at different temperature suggesting hopping mechanism.     

Synthesis and characterization of praseodymium-containing ZrSiO4 solid solutions from gels

The preparation and characterization of a series of praseodymium-zircon solid solution (Pr_x-ZrSiO₄) materials with increasing nominal amounts of Pr is reported. Pr-doped zircon gels were prepared by gelling mixtures of zirconium n-propoxide, praseodymium acetylacetonate and tetraethylorthosilicate, and annealed over the range of temperature up to the formation of Pr-zircon solid solutions. The reaction sequence was followed by X-ray powder diffraction (XRD), ultraviolet-visible diffuse reflectance (DR) and infrared spectroscopy (IR). The first crystalline phase detected on annealing gels was a tetragonal praseodymium-containing ZrO₂ phase (t-Pr-ZrO₂). On further annealing, the subsequent transformation to the monoclinic form (m-Pr-ZrO₂) took place. The formation of final Pr-ZrSiO₄ solid solutions occurred by the reaction between m-Pr-ZrO₂ and amorphous silica phase. The mechanism of solid solution formation inferred from energy dispersive X-ray microanalysis (SEM/EDX) data, variation of lattice parameters and DR of final Pr-ZrSiO₄ solid solutions involved the replacement of Zr⁴⁺ by Pr⁴⁺ in dodecahedral sites of the zircon structure. DR revealed that a relatively small amount of Pr³⁺ was still present in final Pr-containing ZrSiO₄ products. The estimated solubility of praseodymium in the zircon was around 0.067 mol of praseodymium per mol of zircon (∼11.5 wt% as Pr₂O₃). This study opens new perspectives to the development of more ecological zircon-based ceramic pigmenting systems by using mineralizer-free sol-gel synthetic techniques.     

Amorphous LiCoO2 thin films on Li1+x+yAlxTi2−xSiyP3−yO12 prepared by radio frequency magnetron sputtering for all-solid-state Li-ion batteries

Amorphous LiCoO₂ thin films were deposited on the NASICON-type glass ceramics, Li_1+x+yAl_xTi_2−xSi_yP_3−yO₁₂ (LATSP), by radio frequency (RF) magnetron sputtering below 180 °C. The as-deposited LiCoO₂ thin films were characterized by X-ray diffraction, scanning electron microscopy and atomic force microscope. All-solid-state Li/PEO₁₈-Li (CF₃SO₂)₂N/LATSP/LiCoO₂/Au cells were fabricated using the amorphous film. The electrochemical performance of the cells was investigated by galvanostatic cycling, cyclic voltammetry, potentiostatic intermittent titration technique and electrochemical impedance spectroscopy. It was found that the amorphous LiCoO₂ thin film shows a promising electrochemical performance, making it a potential application in microbatteries for microelectronic devices.     

Preparation and electrochemical properties of Cr-doped Li9V3(P2O7)3(PO4)2 as cathode materials for lithium-ion batteries

Cr-doped Li₉V_3−xCr_x(P₂O₇)₃(PO₄)₂ (x = 0.0–0.5) compounds have been prepared using sol–gel method. The Rietveld refinement results indicate that single-phase Li₉V_3−xCr_x(P₂O₇)₃(PO₄)₂ (x = 0.0–0.5) with trigonal structure can be obtained. Although the initial specific capacity decreased with Cr content at a lower current rate, both cycle performance and rate capability have excited improvement with moderate Cr-doping content. Li₉V_2.8Cr_0.2(P₂O₇)₃(PO₄)₂ compound presents the good electrochemical rate and cyclic ability. The enhancement of rate and cyclic capability may be attributed to the optimizing particle size, morphologies, and structural stability during the proper amount of Cr-doping (x = 0.2) in V sites.     

Experimental study of the SnO2–ZrO2 phase diagram

The study of the SnO₂–ZrO₂ phase diagram in the 1230–1750 °C temperature range has shown the existence of an immiscibility gap, leading to two (Zr_1−xSn_x)O₂ and (Sn_1−yZr_y)O₂ limited solid solutions. Four compositions were synthesised for each solid solution, leading to pure phases, which were characterised by room-temperature and high-temperature X-ray diffraction. The unit-cell parameters of tetragonal (Sn_1−yZr_y)O₂, monoclinic (Zr_1−xSn_x)O₂ and tetragonal (Zr_1−xSn_x)O₂ were determined and correlated with the content of the substituted atom. The monoclinic to tetragonal and reverse reactions for the (Zr_1−xSn_x)O₂ series were also characterised (transition temperatures) when varying the tin mole fraction.