Structural characterization of (Sm,Tb)PO4 solid solutions and pressure-induced phase transitions

Sm_1-xTb_xPO₄ solid solutions were synthesized and extensively characterized by powder X-ray diffraction, vibrational spectroscopy, and X-ray absorption spectroscopy. At ambient conditions solid solutions up to x?=?0.75 crystallize in the monazite structure, whereas TbPO₄ is isostructural to xenotime. For x?=?0.8 a mixture of both polymorphs was obtained. Moreover, a phase with anhydrite structure was observed coexisting with xenotime, which was formed due to mechanical stress. Selected solid solutions were investigated at pressures up to ～40?GPa using in situ high pressure synchrotron X-ray diffraction and in situ high pressure Raman spectroscopy. SmPO₄ and Sm_0.5Tb_0.5PO₄ monazites are (meta)stable up to the highest pressures studied here. TbPO₄ xenotime was found to transform into the monazite structure at a pressure of about 10?GPa. The transformation of Sm_0.2Tb_0.8PO₄ xenotime into the monazite polymorph commences already at about 3?GPa. This study describes the reversibility of the pressure-induced (Sm,Tb)PO₄ xenotime-monazite transformation.     

XRD and TEM study of high pressure treated single-walled carbon nanotubes and C60-peapods

In situ synchrotron X-ray diffraction measurements of single-walled carbon nanotube and C₆₀-peapod samples under high pressures up to 13 GPa and at high temperature were carried out. Anisotropical shrinkages of their bundle two-dimensional lattices by compression at room temperature were observed. It was found that the lattices recover original forms reversibly upon pressure release. It was also found that irreversible phase transformations occur by raising temperature at the highest pressure. The high pressure and high temperature treated samples were examined by X-ray diffraction, transmission electron microscope, and Raman measurements. It was indicated by transmission electron microscope observation that hexagonal diamond is able to be synthesized by high pressure and high temperature treatment of C₆₀-peapods.     

Extension behavior and morphology in polypropylene extended under hydrostatic pressure

The effect of hydrostatic pressure on molecular deformation of polypropylene extended under hydrostatic pressure is investigated by using internal friction measurements, wide angle X-ray diffraction, and polarizing microscopy. A homogeneously transparent straight part is obtained by extending samples in the pressure range from 78 to 128 MPa. The overall mechanical properties of a sample extended under P = 102 MPa is significantly different from that of a sample extended under P = 128 MPa. The difference is related to the magnitude of hydrostatic pressure, the extension ratio, and the extent of transparency. The observed results on the β relaxation in the extended samples is found to be related to the morphological reorganization from a coarse spherulitic to a fine spherulitic structure. The cold-drawn sample at atmospheric pressure has the γ-peak at around °50°C, while the hydrostatically extruded samples and the extended samples have no γ-peak. Consequently, the hydrostatic pressure suppresses the formation of the molecular structure relevant to the γ relaxation.     

Transparent and semi-interpenetrating network P(vinyl alcohol)- P(Acrylic acid) hydrogels: pH responsive and electroactive application

ABSTRACT

In this paper, poly(vinyl alcohol)-poly(acrylic acid) based transparent semi-interpenetrating network (semi-IPN) hydrogels were synthesized by using a solvent mixture of dimethyl sulfoxide and deionized water via free radical polymerization and subsequent freeze-thaw technique. The formation of the semi-IPN hydrogels was conformed from FT-IR spectra. The acrylic acid concentration effect on the hydrogels was investigated in terms of transparency, crystalline structure and thermal stability by using UV-visible spectroscopy, X-ray diffraction and thermogravimetric analysis. Swelling behaviours of the semi-IPNPAP hydrogels were studied in deionized water and different pH solutions. The compression and electroactive behaviour was tested in fully hydrated stage by using compression test and by applying electrical voltage. The hydrogels showed displacements under the applying voltage and detailed experiment is illustrated.     

Effect of thickness on the structure,composition and properties of titanium nitride nano-coatings

Titanium nitride (TiN_x) coatings were grown by magnetron sputtering onto Si(100) substrates by varying time of deposition to produce coatings with variable thickness (d_TiN) in the range of 20–120 nm. TiN_x coatings were characterized by studying their structure, composition, and mechanical properties. Nuclear reaction analysis (NRA) combined with Rutherford backscattering spectrometry (RBS) analyses indicate that the grown coatings were stoichiometric TiN. Grazing incidence X-ray diffraction (GIXRD) measurements indicate that the texturing of TiN coatings changes as a function of d_TiN. The (111) and (002) peaks appear initially; (111) becomes intense while (002) disappears with increasing d_TiN. Dense, columnar grain structure was evident for all the coatings in electron microscopy analyses. The residual stress for TiN coatings with d_TiN~120 nm was 1.07 GPa in compression while thinner samples exhibit higher values of stress.     

High energy-storage density under low electric fields and improved optical transparency in novel sodium bismuth titanate-based lead-free ceramics

A novel (1-x)Na_0.5Bi_0.5TiO₃-xBaHfO₃ (abbreviated as (1-x)NBT-xBH) transparent ceramic was fabricated by the solid state reaction method. X-ray diffraction analysis showed that NBT-based transparent ceramics exhibit a cubic-like perovskite structure and the solid solubility of BH in NBT reached to 0.15. The Landau-Devonshire theory and I-E curves revealed that the transition between the antiferroelectric like phase and the ferroelectric phase deeply relies on the variation of composition and free energy. One sample (x = 0.15) was found to show a high dielectric constant (˜2418±10%) over the temperature range 57–400 °C. These ceramics also exhibited a high discharge energy density (W_d) of 2.1 J/cm³ and a high maximum polarization P_m of 34 μC/cm² under relatively low electric fields which were less than 175 kV/cm. There was also high transparency in the visible spectra (more than 0.5) when the sample thickness was 250 μm.     

Effects of different high-pressure syntheses on the magnetization reversal properties of Bi2FeMnO6 ceramics

Bi₂FeMnO₆ ceramics were prepared under atmospheric pressure and a high pressure of 1–6 GPa. The effects of different high-pressure syntheses on the microstructures were analyzed by X-ray diffraction and Rietveld refinement, scanning electron microscopy–energy-dispersive spectroscopy, X-ray photoelectron spectrometer, and Raman spectroscopy test. The temperature dependence of magnetization was systematically investigated for all samples. There was a transition from rhombohedral R3c structure to orthorhombic phase Pnam with increasing synthetic pressure. The samples synthesized under atmospheric pressure are rhombohedral R3c with antiferromagnetic (AFM) magnetic temperature (M–T curves). The samples synthesized under a high pressure of 5–6 GPa are orthorhombic phase Pnam with the magnetization reversal effect, whereas the lower high-pressure (1–3 GPa) synthesized samples might be in the transitional area between them. This structural transformation is accompanied by the transformation from normal AFM M–T curves to a typical magnetization reversal effect. The relationship between the lattice distortion and the internal magnetic competition mechanism of the material was discussed. The existence of magnetization reversal in these systems may ascribe to the competition between the single-ion magnetic anisotropy and the Dzyaloshinsky–Moriya interaction.     

Lithium disilicate glass produced at high pressure: Characterization of structural,thermal and mechanical properties

In this work, high-density lithium disilicate (LS₂) vitreous systems were produced by melting and quenching under high pressure (7.7 GPa) following two distinct experimental routes. In the first case, LS₂ glass was remelted at 7.7 GPa and 1600°C and, then, quenched. In the second case, a stoichiometric mixture of precursor oxides (Li₂O and SiO₂) was melted at 1600°C and 7.7 GPa before quenching. A reference LS₂ glass sample was produced at atmospheric pressure using conventional melting and quenching procedure. The samples were characterized by X-ray diffraction, differential thermal analysis, and instrumented ultramicro hardness measurements. X-ray diffraction confirmed that all samples were amorphous and thermal analysis suggests that different glassy structures were produced depending on the route of synthesis. Hardness and elastic modulus of the glasses produced under high pressure were higher than those of the reference glass, reflecting the irreversible densification effect induced by the high-pressure processing.     

A study on the preparation of oriented beta"-alumina ceramics using rod/flake-like boehmite as precursors and their properties

As a material for solid electrolytes and separators, beta"-Al₂O₃ is widely used in sodium beta alumina. In this study, a high fraction of beta"-phase for beta"-Al₂O₃ with preferred orientation was fabricated using the traditional solid-phase synthesis; and the effect of boehmite micro-structure, the aluminum source, on the phase, microstructure, mechanical properties, and conductivity properties of prepared ceramics was investigated. X-ray diffraction (XRD), and scanning electron microscopy (SEM) results illustrate that the C-axis of beta"-Al₂O₃ grains was along the direction of uniaxial compression. Above all, samples made of rod-shaped boehmite exhibited a better degree of orientation (0.21) and a higher fraction of beta" phase (96 %) than samples made of flake-shaped boehmite. Meanwhile, for samples fabricated with rod-shaped boehmite, the conductivity at 350 ℃ of the sample surface parallel to the uniaxial pressure was nearly 1.5 times (1.634E-1 S cm⁻¹) greater than that of surface perpendicular to the uniaxial pressure.     

Yield Strength of Transparent MgAl2O4 Nano-Ceramic at High Pressure and Temperature

We report here experimental results of yield strength and stress relaxation measurements of transparent MgAl₂O₄ nano-ceramics at high pressure and temperature. During compression at ambient temperature, the differential strain deduced from peak broadening increased significantly with pressure up to 2 GPa, with no clear indication of strain saturation. However, by then, warming the sample above 400°C under 4 GPa, stress relaxation was obviously observed, and all subsequent plastic deformation cycles are characterized again by peak broadening. Our results reveal a remarkable reduction in yield strength as the sintering temperature increases from 400 to 900°C. The low temperature for the onset of stress relaxation has attracted attention regarding the performance of transparent MgAl₂O₄ nano-ceramics as an engineering material.     

Effects of nucleation agents on the preparation of transparent glass–ceramics

Formation of transparent glass–ceramic in the system MgO–SiO₂–Al₂O₃–K₂O–B₂O₃–F with and without addition of LiF and NaF has been investigated. Crystallization of glass-sample was conducted by controlled thermal heat-treatment, at determined nucleation and crystallization temperatures. In this regard, the effects of addition of LiF and NaF were investigated on the crystallization behavior and transparency of the samples.Low transmission (less than 80% at 600 nm) was observed in the basic composition (K).The addition of NaF and LiF caused more intense phase separation in the system.The results indicated that the glass–ceramic can remain transparent if fine grains with nano size are precipitated but will turn into opaque when large grains appear, because of the difference in the refractive index between glass and precipitated crystals.     

Characteristics of CaZrO3 material prepared by in situ decomposition pore forming technology

CaZrO₃ samples were prepared by in situ decomposition pore forming technology in this paper. The influence of calcia/zirconia molar ratio and heating treatment on the microstructure and thermal conductivity of the sintered samples were investigated. The phase analysis of CaZrO₃ samples was detected by X-ray diffraction, and the microscopic morphology of CaZrO₃ was observed by scanning electron microscope combined with energy dispersive spectrometer for microzone composition analysis. The results showed that a satisfied CaZrO₃ sample could be obtained by in situ decomposition pore forming technology with calcium/zirconium molar ratio of 1:1.02. The CaZrO₃ grains were developed better when the samples were preheated at 200°C first and then heated at 1600°C for 3 h. The apparent porosity of CaZrO₃ samples were increased to 28.5%, and the thermal conductivity reduced to 1.219 W/(m·K) compared with the heavy CaZrO₃ samples, which could be used as CaZrO₃ refractory aggregates with the lower thermal conductivity.     

Fabrication and optical characterizations of Yb,Er codoped CaF2 transparent ceramic

Nanoparticles of Yb, Er codoped calcium fluoride were obtained by a co-precipitation method. Scanning electron microscope (SEM) and X-ray powder diffraction (XRD) analysis showed that the obtained nanoparticles were single fluorite phase with grains size around 30–50 nm. Yb, Er:CaF₂ transparent ceramics were fabricated by hot pressing (HP) the nanoparticles at a temperature of 800 °C in a vacuum environment. For a 2 mm thickness ceramic sample, the transmittance at 1200 nm reached about 83%. Microstructures were characterized using SEM analysis, and the average grain size was about 700 nm. Grain boundaries of the ceramic sample were clean and no impurities were detected. The absorption, upconversion and infrared emission spectra of transparent ceramic sample under 978 nm excitation were measured and discussed.     

Synthetic nanoclays with the structure of montmorillonite: Preparation, structure, and physico-chemical properties

Nanosized layered silicates with a montmorillonite structure of varying composition were synthesized under hydrothermal conditions (350°C, 700 atm). The hydrothermal treatment was applied to gels with the compositions calculated with the use of an ideal formula of the final product Na_2x (Al_{2(1 ? x)}, Mg_2x ) Si₄O₁₀(OH)₂ · nH₂O (0 ≤ x ≤ 1). The obtained samples were investigated by X-ray diffraction, IR and NMR spectroscopy, electron microscopy, and mercury porosimetry. The values of the cation-exchange capacity and specific surface are determined. The mean size of the particles of the synthesized samples is 40 ± 7 nm. A gradual substitution of aluminum for part of the magnesium in the initial gel made it possible to obtain samples with well-defined composition-dependent characteristics.     

Dependencies of microstructure and stress on the thickness of GdBa2Cu3O7 ? δ thin films fabricated by RF sputtering

GdBa₂Cu₃O_{7 − δ} (GdBCO) films with different thicknesses from 200 to 2,100 nm are deposited on CeO₂/yttria-stabilized zirconia (YSZ)/CeO₂-buffered Ni-W substrates by radio-frequency magnetron sputtering. Both the X-ray diffraction and scanning electron microscopy analyses reveal that the a-axis grains appear at the upper layers of the films when the thickness reaches to 1,030 nm. The X-ray photoelectron spectroscopy measurement implies that the oxygen content is insufficient in upper layers beyond 1,030 nm for a thicker film. The Williamson-Hall method is used to observe the variation of film stress with increasing thickness of our films. It is found that the highest residual stresses exist in the thinnest film, while the lowest residual stresses exist in the 1,030-nm-thick film. With further increasing film thickness, the film residual stresses increase again. However, the critical current (I_c) of the GdBCO film first shows a nearly linear increase and then shows a more slowly enhancing to a final stagnation as film thickness increases from 200 to 1,030 nm and then to 2,100 nm. It is concluded that the roughness and stress are not the main reasons which cause the slow or no increase in I_c. Also, the thickness dependency of GdBa₂Cu₃O_{7 − δ} films on the I_c is attributed to three main factors: a-axis grains, gaps between a-axis grains, and oxygen deficiency for the upper layers of a thick film.     

2D graphene-coupled ZnS:Mn2+ mechanodetectors for monitoring the pulse rate

In this work, we prepared a type of traditional mechanoluminescent (ML) material (i.e., ZnS:Mn²⁺) by using the sol-gel method with addition of K₂S to the Zinc Oleate and Mn(NO₃)₂ water solution. Then, 2D graphene-coupled ZnS:Mn²⁺ nanocomposites were achieved by coupling the ZnS:Mn²⁺ with the 2D graphene. All the samples were characterized using powder X-ray diffraction (XRD), transmission electron microscopy (TEM), photoluminescence (PL) and pressure-induced PL spectra, galvanostatic charge-discharge and cyclic voltammetry. The PL results indicated that the ZnS:Mn²⁺ and 2D graphene-coupled ZnS:Mn²⁺ samples exhibited a broad Mn²⁺ emission band at 585 nm. The pressure-induced PL spectra showed that the two samples had pressure-controlled luminescence. The pressure-induced PL positions were the same as the PL positions. Since the ML properties are related to the defects, here we added the Li⁺ ions into the ZnS:Mn²⁺ sample in order to study the influence of defects on the ML spectral variation, where the Li⁺ ions were used as the charge compensator. As a result, a mechanistic profile which could illustrate the ML reason was proposed. When the conductive polyoxometalate (POM) was dispersed into the ZnS:Mn²⁺ and graphene-coupled ZnS:Mn²⁺, we found further that the ML intensity and the used pressure value featured a linear relationship and the ML intensity of the two samples could recover after several cycles. Finally, we demonstrated that the graphene-coupled ZnS:Mn²⁺ could use for monitoring human health problem such as the pulse rate.     

Highly transparent Mg0.27Al2.58O3.73N0.27 ceramic fabricated by aqueous gelcasting,pressureless sintering,and post-HIP

Magnesium aluminum oxynitride (Mg_0.27Al_2.58O_3.73N_0.27, termed as MgAlON) ceramics with high transparency and complicated shape was prepared by aqueous gelcasting, pressureless sintering, and followed by hot isostatic pressing. No obvious hydration was found by the characterizations of X-ray diffraction, pH value, Fourier transform infrared and thermal analysis for the interaction between MgAlON spinel powders and water, leading to the stable MgAlON slurry with high solid loading (52 vol%) and low viscosity. This result may be due to different composition of MgAlON from that of MgAl₂O₄ and AlON. Besides, transparent MgAlON ceramic (1.95 mm in thickness) with a high in-line transmittance ~86.3% at 3.7 μm was fabricated. The refractive index ~1.7499 at 589.3 nm and absorption coefficient ~1.2 cm⁻¹ at 5 μm of MgAlON are between those of AlON and MgAl₂O₄ transparent ceramics, and Abbé number ~73.66 of MgAlON is the highest.     

Phase evolution and densification behavior of PBN ceramics

(Pb_1−xBa_x)Nb₂O₆ ceramics were prepared by conventional oxide mixture for studies on phase evolution and densification behavior. Formation mechanisms were examined by X-ray diffraction (XRD) for samples isothermally fired at several temperatures and by differential thermal analysis/thermogravimetric analysis (DTA/TGA). The analysis showed phase evolution distinct for each composition, although temperatures above 1200°C formed compositions with similar XRD patterns, which present tetragonal and orthorhombic forms of PBN phase. Dilatometric studies of ceramics bodies, from those high temperature calcined powders, revealed that substitution of Ba for Pb enhances the densification rate, mainly for the powders whose needle shape particles were broken by grinding. The densification of Pb-richest composition was more affected by the phase transformation rhombohedral-tetragonal (high temperature), only detected by shrinkage measurements.     

Ueber das Komprimierverhalten von NaCl,KCl und Hexamethylentetramin

In earlier publications it was established that the deformation hardness P of a powder compact increases under compression stress σ_c according to the following equation: P = P_max[1 ? exp(?γσ_cρ_r)], where P_max is the maximum possible hardness at σ_c → ∞, γ is the compression susceptibility parameter and ρ_r is the relative density with ρ_r ? 1 ? ?, ? = porosity of the compact. In the case of easily deformable powder material the deformation hardness P already approximates at medium pressures σ_c the maximum possible value P_max (e.g. Aspirin). In contrast to the above-mentioned material, sodium chloride, potassium chloride and hexamine form, under pressure, semitransparent or completely transparent compacts. Contrary to the above-mentioned equation, the deformation hardness P passes through a maximum value and decreases at higher pressures. This decrease can be partly related to the remaining solvent content of the powder compressed, which originates from the crystallization procedure. The view is put forward that the decrease in deformation hardness is due to a work-softening effect, i.e. to a decrease of lattice defects in the polycrystalline compact under the applied pressure.     

Influence of lattice defects on the high pressure properties of Ni0.66Mn2.34O4 NTC ceramics

The properties and hence the applications of negative temperature coefficient (NTC) oxides, Ni_xMn_3-xO₄, strongly depend on the Ni:Mn ratio, valence states, defects and cation distribution in the spinel structure. Here, the accuracy of determination of nickel and manganese cations distribution in the spinel structure of Ni_0.66Mn_2.34O₄ oxide, has been improved by taking into account the presence of tetrahedral vacancies. Additionally, the structural stability of cubic Ni_0.66Mn_2.34O₄ was investigated as a function of pressure up to 9.5 GPa using synchrotron radiation angle-dispersive X-ray powder diffraction and a diamond anvil cell. The bulk modulus and its first derivative were determined by fitting the Birch-Murnaghan equation of state (EoS) model to the experimental pressure-volume data. Rietveld refinement of the X-ray powder diffraction data reveals that the cubic spinel structure is stable upon compression to 9.5 GPa. The XRD data yielded a bulk modulus of K₀ = 126(7) GPa, with a pressure derivative K′ = 12(2). The obtained data were discussed in terms of defects in the cationic sublattice and compared with the elastic parameters of NiMn₂O₄.