The effect of gelcasting parameters on microstructural optimization of porous Si3N4 ceramics

In this article we are reporting the optimization of gelcasting process which enabled us to manufacture silicon nitride bodies with around 30–37% porosity and 182–250?MPa flexural strength. Owing to their potential applications mainly in biomedical and aerospace, porous ${\text{Si}}_3{\text{N}}_4$ ceramics have gained ever increasing interests. However, the main challenge has been ensuring their high strength while maintaining high porosity. ${\text{Si}}_3{\text{N}}_4$ bodies were prepared via a controlled gelcasting followed by pressureless sintering in a coke bed. Monomers including acrylamide (AM) and $\text{N},{\text{N}}^{\text{'}}$ -methylenebisacrylamide (MBAM) were employed to formulate the primary slurry. Sub-micron ${\text{Si}}_3{\text{N}}_4$ powder of 35?vol% was used as solid loading where ammonium persulfate (APS) and $\text{N},\text{N},{\text{N}}^{\text{'}},{\text{N}}^{\text{'}}$ -tetramethylethylenediamine (TEMED) were added to initiate and catalyze polymerization reactions. Sintering was carried out at 1650?°C for 2–6?h and at 1750?°C for 2?h where 6?wt% mixture of ${\text{Al}}_2{\text{O}}_3?{\text{Y}}_2{\text{O}}_3$ was included as a sintering aid. Phase characterization and microstructural evolution were studied by X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively; while mechanical evaluation was based on bending test. It was found that by optimizing the viscosity of slurry and the idle time of gelation, a well distribution of high porosity structure is developed. The high strength of the sintered bodies is related to a high volume and unique microstructure of fine and elongated β- ${\text{Si}}_3{\text{N}}_4$ grains evolved during optimized sintering conditions. At temperatures above 1650?°C and sintering times of more than 4?h, the strength is decreased due to coarsening of β- ${\text{Si}}_3{\text{N}}_4$ grains, however.     

Molecular dynamics study of a macromolecular model of hydrous sodium aluminosilicate geopolymer with charge balancing by extra-structural aluminum for prediction of its properties

Geopolymer is a material with unique properties and has various uses. This substance is mainly amorphous, and its qualitative characteristics are related to its binder phase that is called the hydrous sodium aluminosilicate geopolymer. The molecular structural model of this geopolymer includes ${\text{Q}}^4\left(4\text{Al}\right),{\text{Q}}^4\left(3\text{Al}\right),{\text{Q}}^4\left(2\text{Al}\right)$, and ${\text{Q}}^4\left(1\text{Al}\right)\text{Si}$ units, which have been balanced in terms of electric charge by extra-framework $\text{Al}$ and ${\text{Na}}^{+}$ ions. In this study, we calculated the density, Young's modulus, and RDF curve of the geopolymer from the molecular dynamics simulation. The results of the simulation were in good agreement with the results of the laboratory obtained from several studies.     

La0.7−xLnxCa0.3MnO3(Ln=ProrSm) perovskites as electrode materials for SOFCs

In this work, the Pechini method was used to synthesize $L{a}_{0.7?x}L{n}_{x}C{a}_{0.3}Mn{O}_{3,}(Ln=ProrSm)$-${\text{La}}_{1?\text{x}}{\text{Ln}}_{\text{x}}{\text{Ca}}_{0.3}{\text{MnO}}_3$ type perovskites, evaluating the effect of the type of cation and composition on the structural, morphological and textural properties. The use of similar rare earth cations was studied to promote weak bonds between the reactive surface and adsorbed oxygen species that could facilitate the oxygen reduction reaction and thus improve electrochemical performance of SOFC devices. To achieve this goal, different compositions of Pr or Sm ions (x = 0.1, 0.3, 0.5 or 0.6) were used for the partial substitution of lanthanum at the A site. The results indicated that the substitution of La by Pr or Sm did not modify the original orthorhombic perovskite structure of Ca-doped lanthanum manganites. However, a shift in the main reflections can be obtained as the content of both cations increases due to cell distortion. Rietveld refinement confirms that the crystal structure belongs to the Pnma space group with a large distortion along the b-axis but no secondary phase formation. The compensated charge neutrality of the Mn³⁺/Mn⁴⁺ ratio influences the octahedral sites of MnO₆ and cell volume. Orthorhombic distortion ($\frac{c}{\sqrt{2}}<a<b$) occurs through Jahn-Teller mechanism promoting a hole-doped system for electrical conductivity. The adsorption-desorption isotherms reveal that in any composition of Pr, a mesoporous isotherm (type IV) is obtained. In contrast, the isotherms changed from micro- to meso-porous features depending on the amount of Sm substitution at the A-site. All the prepared samples showed a soft granular structure with agglomerates ranging between 200 and 300 nm with well-interconnected pores. Comparing the La substitution by Pr or Sm, it was found that Sm can form perovskites that can better promote oxygen vacancies and triple boundary phase formation, which is essential in SOFC devices.     

Dynamic response of Ti3SiC2 under shock compression up to 112 GPa

Ti₃SiC₂ is of interest due to its unique dual nature reminiscent of both brittle ceramics and ductile metals at ambient conditions. In this work, plate-impact experiments have been performed to study the dynamic behavior of Ti₃SiC₂ under shock compression up to 112 GPa by using laser velocity interferometer and electric pin techniques. Hugoniot elastic limits (HEL), spall strength, and Hugoniot equations of state have been obtained based on measured particle velocity profiles and shock wave velocities. The ratio of spall strength to HEL for Ti₃SiC₂ is larger than brittle ceramics but smaller than metals. This result indicates that the dual nature of Ti₃SiC₂ remains at least up to 10 GPa. On the other hand, the linearity of the Hugoniot equation of state, $D=6.9\text{0}1(22)+1.153(53)u_{\text{p}}$, suggests that the initial structure of Ti₃SiC₂ should be stable up to 112 GPa, in contrast to the result reported by Jordan et al. [J. Appl. Phys., 93 (2003) 9639].     

Correlation between structural and optical properties of Dy3+ doped BaGd2O4 phosphors intended for solid-state lighting applications

Highly crystalline and single phase BaGd${}_{2-x}$Dy${}_x$O${}_4$ (0.00 $\le$x $\le$ 0.16) phosphors, with an average crystallite size around 126 nm, have been synthesised using solid-state reaction technique. The structural and optical properties of these phosphors have been studied in detail to establish an unambiguous correlation between these properties. High-angle annular dark field (HAADF) images have confirmed that the constituent elements are homogeneously distributed in the particles, and their elemental composition has been established using X-ray photoelectron spectroscopy (XPS). The tuning of optical band gap with x has been achieved, which is a rare achievement in these phosphors. Also, the optimum concentration of Dy${}^{3+}$ ions has been found to be 0.8 mol%, which is the lowest among the Dy${}^{3+}$ doped BaGd${}_2$O${}_4$ phosphors reported so far. This concentration quenching effect has been discussed on the basis of a combination of decay curve analysis, calculation of average critical distance between the Dy${}^{3+}$ ions and integrated intensities of photoluminescence (PL) emission bands. The average crystallite size and optical band gap has also been found to decrease after x = 0.016, from which their correlation with concentration quenching effect has been investigated. The asymmetry ratio between the integrated intensities of yellow and blue PL emission bands has been observed to be greater than 2 throughout x, which confirmed the preferential lattice site for Dy${}^{3+}$ ions in these phosphors with present synthesis conditions. The variation of asymmetry ratio and Gd${}^{3+}$-dominated IR-active lattice vibrations with x, and Vegard’s law pertaining to the volume of a unit cell confirms that the local bonding environment in the lattice of these phosphors gets modified at x = 0.016. The photometric parameters for these phosphors reveal their suitability for fabrication of warm light orange LEDs on appropriate UV chips.     

Magnetoelectric coupling studies in lead-free multiferroic (Ba0.85Ca0.15)(Zr0.1Ti0.9)O3−(Ni0.7Zn0.3)Fe2O4 ceramic composites

Lead-free multiferroic 3–0 type particulate composites with a composition (1?x)(Ba_0.85Ca_0.15Zr_0.1Ti_0.9O₃) – x(Ni_0.7Zn_0.3Fe₂O₄) [(1?x)BCZT – xNZFO with 0 ≤ x ≤ 100 at%] were prepared using solid state reaction method. Structural and microstructural analysis using XRD, FESEM and Raman techniques confirmed the phase formation of the ferroelectric (BCZT) and magnetostrictive (NZFO) phases without any detectable presence of impurity phases. Rietveld refinement of the XRD data revealed a tetragonal (P4mm) and a cubic structure (Fd$\stackrel{￣}{3}$m) for the BCZT and NZFO phases, respectively. Elemental compositions of the constituent phases were assessed by EDS and XPS analyses. Electrical, magnetic, and magnetoelectric (ME) measurements were performed. The composites exhibit typical well-saturated magnetic hysteresis (M?H) loops at room temperature, having very low coercive field ($H_C$) values, indicating their soft ferromagnetic behavior. Various parameters extracted from the M?H curves including $H_C$, magneto-crystalline anisotropy, squareness, and magnetization were found to depend on x. Frequency dependence of capacitance and admittance exhibited a resonance behavior corresponding to the radial mode of the electromechanical resonance (EMR). ME coefficients were studied in both longitudinal (${\alpha }_{\text{E}33}$) and transverse (${\alpha }_{\text{E}31}$) modes. The highest coupling coefficients, ${\alpha }_{\text{E}31}$ ～14.5 mV/Oe.cm and ${\alpha }_{\text{E}33}$ ～13 mV/Oe.cm were obtained for composite with 50 at% NZF at off-resonance frequency of 1 kHz. At the EMR frequency of 314 kHz, the ${\alpha }_{\text{E}31}$ value in 0.5BCZT-0.5NZFO composite enhanced enormously to ～5.5 V/Oe.cm. The studies conclude that x = 0.5 is an optimum atomic fraction of NZFO in the particulate composite for maximum ME coupling.     

Comparison of theoretical and experimental microhardness of tetrahedral binary Zn1-xErxO semiconductor polycrystalline nanoparticles

${\mathrm{Zn}}_{1?\mathrm{x}}{\mathrm{Er}}_{\mathrm{x}}\mathrm{O}$ polycrystalline nanoparticles with various compositions $(x=0.01,0.02,0.03,0.04,0.05,$ and $0.10)$were prepared using sol–gel techniques, for which zinc acetate dihydrate and erbium 2–4 pentanedionate are used as precursors. Nanoparticles were pressed under a pressure of $4$?tons for $5$?min into disk-shaped compacts with 2?mm thicknesses and 10?mm diameters. The pressed samples were annealed at 400?°C for $30$?min. X-ray diffraction (XRD), scanning electron microscopy (SEM), and Vickers microhardness analyses of the produced Er-doped $\mathrm{ZnO}$ bulk nanomaterials were performed. Specifically, in this study we focused on the analysis of their mechanical properties. Undoped and Er-doped bulk samples were investigated according to Meyer's law; the proportional sample resistance (PSR), elastic/plastic deformation (EPD), and indentation-induced cracking (IIC) models; and the Hays–Kendal (HK) approach. As a result, the IIC model was more suitable to determine the micromechanical properties and the reverse indentation size effect ($\mathrm{R}\mathrm{ISE}$) behavior of Er-doped $\mathrm{ZnO}$ semiconductors.     

Effect of annealing on structure and properties of Ta–O–N films prepared by high power impulse magnetron sputtering

High power impulse magnetron sputtering of a Ta target in various $\mathrm{Ar}+{\mathrm{O}}_2+{\mathrm{N}}_2$ gas mixtures was utilized to prepare amorphous tantalum oxynitride (Ta–O–N) films with a finely controlled elemental composition in a wide range. We investigate the effect of film annealing at $900°\mathrm{C}$ in vacuum on structure and properties of the films. We show that the finely tuned elemental composition in combination with the annealing enables the preparation of crystalline Ta–O–N films exhibiting a single TaON phase with a monoclinic lattice structure, refractive index of $2.65$ and extinction coefficient of $2.0\times {10}^2$ (both at the wavelength of $550\mathrm{nm}$), optical band gap width of $2.45\mathrm{eV}$ (suitable for visible light absorption up to $505\mathrm{nm}$), low electrical resistivity of $0.4\text{Ω}\mathrm{cm}$ (indicating enhanced charge transport in the material as compared to the as-deposited counterpart), and appropriate alignment of the band gap with respect to the redox potentials for water splitting. These films are therefore promising candidates for application as visible-light-driven photocatalysts for water splitting.     

Fabrication of nanostructured ZnO thin film sensor for NO2 monitoring

Nanocrystalline zinc oxide (ZnO) thin films were deposited onto glass substrates by a spin coating method. These films were characterized for their structural and morphological properties by means of X-ray diffraction (XRD), scanning electron microscopy (SEM) and atomic force microscopy (AFM). The ZnO films are oriented along (1 0 1) plane with the hexagonal crystal structure. These films were utilized in NO₂ sensors. The dependence of the NO₂ response on the operating temperature, NO₂ concentration was investigated. The ZnO film showed selectivity for NO₂ over H₂S compared to NH₃ (${\text{S}}_{{\text{NO}}_2}/{\text{S}}_{{\text{H}}_2\text{S}}=3.32$ and ${\text{S}}_{{\text{NO}}_2}/{\text{S}}_{{\text{NH}}_3}=5.32$). The maximum gas response of 37.2% was achieved with 78% stability for ZnO films upon exposure of 100 ppm NO₂ at operating temperature 200 °C.     

Proton mobility in Ruddlesden–Popper phase H2La2Ti3O10 studied by 1H NMR

To study protons localization in H_1.83K_0.17La₂Ti₃O₁₀·0.17H₂O and their motional characteristics, complementary Nuclear Magnetic Resonance (NMR) techniques have been applied. ¹H Magic Angle Spinning NMR evidences the presence of different proton containing species. By analyzing the temperature dependence of the ¹H MAS NMR spectrum we attribute the observed lines to interlayer H⁺ in regular sites (isolated and in water rich environment), water protons and protons from various defects. The temperature behaviors of the spectral lines intensities and widths point out that intercalated water molecules are involved in translational motion that is confirmed by spin lattice relaxation rate ($R_1$) and spin-lattice relaxation rate in rotating frame ($R_{1\rho }$) measurements. It has been shown that for a correct determination of the proton motional parameters the Kohlrausch-Williams-Watts correlation function must be used. Its application results in the following parameters of proton motion in the interlayer space of H_1.83K_0.17La₂Ti₃O₁₀·0.17H₂O: $E_{\mathrm{a}}$?=?0.194(2) eV, $\mathrm{\beta }$?=?0.28(1), ${\tau }_0=6.2(1)\times {10}^{?10}$?s.