Structure of NaF–TeO2 glasses and glass-ceramics

The structure of NaF–TeO₂ glasses and glass-ceramics has been studied by XRD, TEM, SEM, Raman and FTIR techniques. The results suggest that, for NaF ≤10 mol%, the entire NaF content enters the structure to convert TeO₄ units into TeO_3/2F and Na⁺[TeO₃₊₁]^– units. It has also been shown that NaF partially forms amorphous and/or crystalline phases for higher NaF content, where the relative concentration of each phase depends on the NaF content. SEM images show agglomerates of different sizes, which are discrete and spread within the structure. XRD revealed formation of crystalline Te₂O₃F₂ for NaF ≤50 mol%, and a dominant phase of crystalline NaF for NaF >50 mol%. Raman and FTIR spectra have been analyzed to calculate the concentrations of the various structural units in glasses and glass-ceramics.     

Phase and melting relationships of β, α and α′-Ca3(PO4)2 polymorphs in the Ca3(PO4)2-Zn3(PO4)2 system

In order to provide an exact knowledge of the phase transitions and melting relationships of Ca₃(PO₄)₂ (TCP) in the presence of zinc, a revisited version of the rich-Ca₃(PO₄)₂ region of the phase diagram of the system Ca₃(PO₄)₂-Zn₃(PO₄)₂ has been established in the present work. Experimental determination of this diagram was carried out by solid-state reactions of samples prepared from pure NH₄H₂PO₄, CaCO₃ and ZnO raw materials. X-ray Diffraction, Differential Thermal Analyses and Field Emission Scanning Electron Microscopy studies allowed to revise the α, β, α + β-TCP phase stability fields, delimitating for the first time the biphasic α + α′-TCP field and the melting relationships in the high temperature region of the system. The results allowed to determine two peritectic invariant points, at ≈1400 °C for 95 mol% Ca₃(PO₄)₂ and at ≈1490 °C for ≈99.5 mol% Ca₃(PO₄)₂.     

Structural and dielectrical properties of Mg3–Ca3(PO4)2 bioceramics obtained from hydroxyapatite by sol–gel method

Mg₃–Ca₃(PO₄)₂ bioceramics were prepared from hydroxyapatite (HAp) with high Mg contents using sol–gel method. The influence of magnesium on the phase composition, crystal structure, electrical properties, chemical structure and morphological characteristics of powder bioceramics was analyzed using X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy and scanning electron microscopy (SEM). Dielectrical properties of the bioceramics were investigated by a dielectric impedance spectroscopy method. It was observed that the crystallization degree for all the samples dramatically was decreased with the increasing Mg content. The average crystallite size of the samples was found to vary from 32 to 42 nm. The morphology, density and dielectric properties of the bioceramics were changed with the addition of the amount of Mg. The obtained results indicate that the Mg₃–Ca₃(PO₄)₂ bioceramics can be prepared by means of hydroxyapatite bioceramic.     

Effect of gallium addition on physical and structural properties of Ge–S chalcogenide glasses

GeS_2.5 chalcogenide glass was selected for studying effects of Ga addition on physical and structural properties. Glassy and partially crystallized samples of (100−x)GeS_2.5−xGa (5 mol% ≤ x ≤ 40 mol%) were prepared, and their thermal and optical properties were characterized. With increasing Ga content (x), values of T_g and optical band gap of glasses initially increased and then decreased, showing a maximal value at x = 25 mol%, that is, with stoichiometric composition of 85.7GeS₂·14.3Ga₂S₃. These changes were discussed and correlated to evolution of network structure, which was investigated by Raman spectra recorded in glassy matrices of (100−x)GeS_2.5−xGa (5 mol% ≤ x ≤ 40 mol%). This work contributes to understanding of composition–structure–property relationship of chalcogenide glasses.     

Microstructure–conductivity relationship of Na3Zr2(SiO4)2(PO4) ceramics

The ionic conductivity of solid electrolytes is dependent on synthesis and processing conditions, ie, powder properties, shaping parameters, sintering time (t_s), and sintering temperature (T_s). In this study, Na₃Zr₂(SiO₄)₂(PO₄) was sintered at 1200 and 1250°C for 0-10 hours and its microstructure and electrical performance were investigated by means of scanning electron microscopy and impedance spectroscopy. After sintering under all conditions, the sodium super-ionic conductor-type structure was formed along with ZrO₂ as a secondary phase. The microstructure investigation revealed a bimodal particle size distribution and grain growth at both T_s. The density of samples increased from 60% at 1200°C for 0 hours to 93% at 1250°C for 10 hours. The ionic conductivity of the samples increased with t_s due to densification and grain growth, ranging from 0.13 to 0.71 mS/cm, respectively. The corresponding equivalent circuit fitting for the impedance spectra revealed that grain boundary resistance is the prime factor contributing to the changing conductivity after sintering. The activation energy of the bulk conductivity (E_a,bulk) remained almost constant (0.26 eV) whereas the activation energy of the total conductivity (E_a) exhibited a decreasing trend from 0.37 to 0.30 eV for the samples with t_s = 0 and 10 hours, respectively—both sintered at 1250°C. In this study, the control of the grain boundaries improved the electrical conductivity by a factor of 6.     

Structure and properties of nanostructured Fe(AlCr)2O4–Cr–(AlCr)2O3–Fe composite coating prepared by plasma spraying

In-situ nanostructured Fe(AlCr)₂O₄-based composite coating (FACr_52.5 coating) was prepared by reactive plasma spraying with micro-sized Al–Fe₂O₃–Cr₂O₃ powders. The microstructure, toughness and Vickers hardness, and adhesive strength of the coating were investigated by X-ray diffraction, scanning electron microscopy, transmission electron microscopy and mechanical tests. The results indicated that the interlamellar spacing of the FACr_52.5 coating is only 1 μm. The coating exhibited nanostructured microstructure. The in-situ Cr (20 nm) and Fe (50–200 nm) particles were uniformly distributed in an Fe(AlCr)₂O₄ matrix, while the grain size of the Fe(AlCr)₂O₄ matrix is about 60 nm. The FACr_52.5 composite nano-coating exhibited much higher hardness, better wear resistance, stronger adhesive strength and toughness as compared to those of the composite nano-coating sprayed with Fe₂O₃–Al powders. Excellent mechanical properties of the FACr_52.5 coating were attributed to the uniform distribution of the in-situ nano-sized Cr particles in the coating matrix.     

Synthesis,physical, optical,mechanical, and radiation attenuation properties of TiO2–Na2O–Bi2O3–B2O3 glasses

A series of bismo-borate (50-x)B₂O₃-xTiO₂-15Na₂O–30Bi₂O₃ glass samples (where x = 0, 2.5, 5, 7.5, and 10 wt%) doped with TiO₂ were fabricated via the melt-quenching technique. The gamma and neutron shielding, physical, optical, and mechanical properties of the prepared samples were investigated. The experimental results were measured using an HPGe detector. ¹⁵²Eu, ¹³³Ba, ¹³⁷Cs, and ⁶⁰Co radioactive sources were used with energies in the range of 81–1408 keV. The experimental results were compared with both the FLUKA code and the XCOM database. The addition of TiO₂ increased the density of the glass samples and decreased their molar volume. The mass attenuation coefficient (MAC) decreased as photon energy decreased, while it increased as TiO₂ concentration increased. The half value layer (HVL) and mean free path (MFP) of the glass samples increased when the photon energy increased and decreased as the TiO2 concentration increased. The absorbance of the present samples is enhanced by using TiO₂, meaning they can be used to protect humans from UV light. Both direct and indirect band gaps decreased as TiO₂ content increased from 0 to 10 wt %. Moreover, the electronic transition between localized states is valid in the present samples. The radiation shielding, optical, physical, and mechanical properties of the fabricated glass samples demonstrate their utility for diagnostic gamma shielding.     

Elastic properties of quaternaryTeO2–ZnO–Nb2O5–Gd2O3 glasses

Tellurite glass systems in the form 75TeO₂–15ZnO–(10−x)Nb₂O₅–xGd₂O₃ (x=0.0, 0.5, 1.0, 1.5, 2.0, 2.5 mol%) have been prepared by the melt quenching technique. Both longitudinal and shear ultrasonic velocities were measured by using the pulse-echo method at 5 MHz frequency and at room temperature. Elastic moduli (longitudinal modulus, shear modulus, Young׳s modulus, Bulk modulus), Poisson׳s ratio, Debye temperature, micro-hardness and softening temperature have been calculated. Quantitative analysis of elastic moduli based on the number of bond per unit volume, average crosslink and number of vibrating atoms per unit volume has been achieved.     

Synthesis and electrochemical performance of LiVOPO4–Li3V2(PO4)3 cathode materials for lithium ion batteries

Lithium-ion batteries (LIBs) present the advantages of long cycle life, high voltage, and energy density and are widely made in the field of energy storage. LiVOPO₄ (LVOP), a cathode material used in LIBs, has a high conceptual capacity of 159 mAh g⁻¹ and high operating voltage of 3.9 V. However, its low electrical conductivity and cycle performance limit its commercial applications. According to the X-ray diffraction results, orthogonal crystal LVOP and monoclinic crystal Li₃V₂(PO₄)₃ (LVP) coexisted in the synthesised composite material. The transmission electron microscopy results also indicated that the LVOP and LVP phases coexist, which were coated by carbon layer of about 2.5 nm. The discharge of LVOP–LVP composite material initially was 143.2 mAh g⁻¹, and that after 120 cycles was 132.2 mAh g⁻¹ (at 0.1 C and 3–4.5 V). Thus, the electronic conductivity and first discharge specific capacity of the material enhanced due to the introduction of fast ion conductor LVP into LVOP. Electrochemical performance improved because the introduction of LVP led to an increase in Li⁺ pervasion channels in the original material and the acceleration of the Li⁺ transmission speed.     

Effects of TeO2/B2O3 substitution on synthesis,physical, optical and radiation shielding properties of ZnO–Li2O-GeO2-Bi2O3 glasses

Tellurite glass is a model material having superior features for several applications. It can be considered as a potential host matrix for different oxides, and this paper aims to study the effects of TeO₂/B₂O₃ substitution on synthesis, physical, optical and radiation shielding properties of ZnO–Li₂O-GeO₂-Bi₂O₃ glasses produced by melt quenching technique. The physical and optical features of the fabricated glasses were experimentally investigated by determining pivotal parameters such as density, XRD, tellurium ion concentration (N_i), linear refractive index (n_o), polaron radius (r_p) and inter nuclear distance (r_i). Moreover, the relative radiation deposition within the glasses was assessed via the attenuation coefficients (e.g. MAC), specific gamma ray constant (ᴦ), total stopping power (TSP), neutron cross sections, and dose rate (D). Our results suggest that both TeO₂ and B₂O₃ additives have a significant effect on the fundamental properties of the ZnO–Li₂O-GeO₂-Bi₂O₃ glasses. It also found that the lower thicknesses of the present glasses are required to provide the same level of shielding than ordinary, ilmenite, steel scrap, hematic-serpentine, ilmenite-limonite and basalt-magnetite concretes, RS253-G18 and RS360 glass shields. Therefore, presently investigated glasses are promising photon shields in different technological applications of gamma- and x-rays.     

Effect of Al(H2PO4)3/Zn/B4C doped resin on properties and microstructure of unfired Al2O3–C slide plate materials

The unfired Al₂O₃–C slide plates have the advantages of energy saving, environment friendly, efficiency and relatively low-cost. However, the decomposition and oxidation of the phenol-formaldehyde (PF) resin at elevated temperatures deteriorate the properties and decrease service life of the unfired slide plates. In order to improve the property of resin, the doped PF resin is prepared by incorporating Al(H₂PO₄)₃, Zn and B₄C powders. The effects of the doped resin on medium-high temperature properties and microstructure of the unfired slide plate materials have been investigated. The results show that Zn and B₄C doped resin contributes to notable increasing the density and strength properties at medium temperature, because Zn and B₄C easily oxide and thus protect resin from oxidation, leading to form a dense structure. Zn and B₄C doped resin can significantly improve hot modulus of rupture of the materials at 1400 °C, which is due to Zn and B₄C react with oxidative gases leading to increase in concentration of C(g), CO and N₂, Al and Si would react with C(g), CO(g) and N₂(g) to form AlN and SiC whiskers creating strengthening effect. Specimens with Zn and B₄C doped resin addition have good oxidation resistance at 1500 °C, because Zn and B₄C in the surface of the material react with O₂ to form ZnAl₂O₄ or mullite containing dense glass film, which would retard O₂ diffusion into the inner of the specimens.     

Microstructure and electrical properties of ZnO–ZrO2–Bi2O3–M3O4 (MCo,Mn) varistors

Phase evolution, microstructure and the electrical properties of ZrO₂-added pyrochlore-free ZnO–Bi₂O₃–M₃O₄ (MCo, Mn) varistors have been studied as functions of ZrO₂ content up to 10 vol% and the sintering temperature between 900 and 1300 °C. Zirconia remained as intergranular second phase particles up to 1100 °C, which retarded densification and inhibited the grain growth of ZnO. At higher temperatures, on the contrary, ZrO₂ particles began to be entrapped in ZnO grains and irreversibly transform from monoclinic to stable cubic phase dissolving transition metal ions. The grain size of ZnO decreased with increasing ZrO₂ content, and increased with the increase of the sintering temperature. Accordingly breakdown voltage changed with both ZrO₂ content and the sintering temperature as was expected. Nonlinear coefficient (α) depended primarily on the sintering temperature: it increased to >40 up to 1000 °C, and significantly decreased to <30 at higher temperatures probably due to the volatilization of Bi₂O₃. While the specimens sintered at 1200 °C or above had relatively high leakage current (I_L) and large clamping ratio (C_R), those with ZrO₂ content of 0.5–5.0 vol% and sintered below 1200 °C revealed low I_L of ⩽20 μA/cm² and C_R well below 2.0. In spite that varistor characteristics of ZrO₂-added system could not match those of commercial ZnO varistors, its low temperature sinterability and ease of breakdown voltage control via ZrO₂ content without a serious loss of its figures of merit are worth noticing, particularly for multi-layered chip varistor (MLV) application.     

Structure and microwave dielectric properties of La(Mg0.5Ti0.5)O3–CaTiO3 system

(1−x)La(Mg_0.5Ti_0.5)O₃ (LMT)–xCaTiO₃ (CT) [0<x<1] ceramics were prepared from powder obtained by a nonconventional chemical route based on the Pechini method. The crystal structure of the microwave dielectric ceramics has been refined by Rietveld method using X-ray powder diffraction data. LMT and CT were found to form a solid solution over the whole compositional range. The 0.9LMT–0.1CT composition was refined using P2₁/n space group, which allows taking into account B-site ordering. The compounds having x⩾0.3 were found to be disordered and were refined using Pbnm space group. Microstructure evolution was also analysed. Dielectric characterization at microwave frequencies was performed on the LMT–CT ceramics. The permittivity and the temperature coefficient of resonant frequency of the solid solutions showed a non-linear variation with composition. The quality factor demonstrates a considerable decrease with the increase of CT content.     

Corrosion of nickel ferrite refractory by Na3AlF6–AlF3–CaF2–Al2O3 bath

Based on research on cermet inert anodes for aluminium production, it has been suggested that nickel ferrite spinel might be suitable for use as a sidewall refractory in Hall-Héroult cells. A corrosion resistant sidewall would allow elimination of the frozen bath ledge, and has potentially huge benefits in terms of energy savings and increased productivity. However, little work has been done to assess nickel ferrite's suitability as a refractory.Dense nickel ferrite samples were prepared and characterized, and corrosion tests in cryolite based baths were conducted. Results confirm that the spinel does have good corrosion resistance. The corrosion mechanism is complex, involving grain boundary attack and formation of a Ni–Fe alloy. This alloy could pose a risk in terms of contamination of the aluminium. The use of additives to restrict penetration of grain boundaries may be the key to development of a successful spinel based refractory.     

Structure and microwave dielectric properties of Ba1−xSrxMg2V2O8 ceramics

The Ba_1−xSr_xMg₂V₂O₈ (0≤x≤0.4) microwave dielectric ceramics were fabricated by a standard solid-state reaction method. The formation of a continuous solid solution within the whole composition range was identified. The ceramic samples could be well densified in the temperature range of 885–975 °C in air for 4 h. The permittivity ε_r was found to increase with increasing ionic polarizabilities. The Q×f values were believed to be closely related with packing fraction and grain refinement. The Sr²⁺ substitution contributed to a monotonous increase of the A-site bond valence, such that the τ_f value experienced a considerable variation from negative to positive values. The optimum microwave dielectric properties of an ε_r of 13.3, a high Qxf of 86,640 GHz (9.6 Hz) and a near-zero τ_f of −6 ppm/°C could be yielded in the x=0.15 sample when sintered at 915 °C for 4 h.     

Characterization of chemosynthetic H3PO4–Al2O3–2SiO2 geopolymers

Pure chemosynthetic Al₂O₃–2SiO₂ powders fabricated by a sol–gel method exhibit high phosphoric acid-activated properties and high compressive strengths. The phosphoric acid-activated properties could be characterized by compressive strength. The phase structure evolution of synthetic powders and the resulting geopolymers were investigated by DTA-TG, XRD, FTIR and MAS NMR analysis. These results show that the phosphoric acid-activation region of the synthetic powders was in the range of 200–800 °C, which was much lower than the temperature at which kaolinite was converted into metakaolinite. ³¹P MAS NMR analysis revealed that [PO₄] tetrahedra were part of the geopolymer structure.     

Oxidative dehydrogenation of ethane and propane over magnesium–cobalt phosphates CoxMg3−x(PO4)2

The magnesium–cobalt phosphates Co_xMg_3–x(PO₄)₂ belonging to the olivine-type structure were synthesized by coprecipitation and then investigated in the oxidative dehydrogenation (ODH) of ethane and propane. The best yields, with the exception of Co_0.5Mg_2.5(PO₄)₂, were achieved with the compositions ranging between 1x2.5. Magnesium phosphate Mg₃(PO₄)₂ displayed no activity and pure cobalt phosphate Co₃(PO₄)₂ was found to be the less active component of the solid solution. Comparison of the catalysts performances showed that they all have similar activity in ethane and propane ODH, albeit, they are more selective in propylene than in ethylene production. The Co_xMg_3–x(PO₄)₂ solid solution was also studied, for characterization purposes, in butan-2-ol conversion. The samples presented acid–base properties due essentially to the (PO–H) groups but they do not bear conventional redox centers. All the catalysts were active at low temperatures in the alcohol dehydration. The dehydrogenation activity versus the phosphates composition displayed two maxima around x=1 and 2, respectively. Similar striking behavior was also observed in ethane and propane ODH. UV-visible investigations of Co_xMg_3–x(PO₄)₂ showed, in agreement with the XRD data, that the Co^{2 +} ions are distributed in the phosphate framework between six- and five-coordinated sites. The cobalt atoms in the five-coordinated sites Co(5) and their Co(5)–Co(5) interatomic distances were assumed to play the main role in the C–H bond activation and the appearance of maxima in the activity. Magnesium cations presumably intervene in acid–base properties of the samples and O₂ activation. Characterization of the samples showed that they do not undergo any noticeable transformation after the catalytic tests.     

Revisiting metastable immiscibility in SiO2–Al2O3: Structure and phase separation of supercooled liquids and glasses

Understanding and controlling liquid–liquid phase separation in aluminosilicates is crucial for optimizing glass properties. However, the metastable nature of aluminosilicates’ phase separation has made it difficult to study experimentally, and uncertainty persists regarding the compositional and temperature extents of the miscibility gap. Here, we present new experimental evidence that suggests a consolute temperature between 1440 and 1590°C and endmember compositions of 7 and 62 mol.% Al₂O₃ for the phase-separated glasses. Using containerless melt processing, deeply supercooled liquids over the 0–60 mol.% Al₂O₃ range are probed with in situ small- and wide-angle X-ray scattering, which simultaneously reveals changes in nanoscale density heterogeneity and atomic structure. Correlations between phase separation and atomic coordination environments are compared for liquids and glasses. Pair distribution function analysis shows mean O–(Si + Al) coordination increases with Al₂O₃ content and decreases with temperature.