H2O self-diffusion restricted by clay platelets with immobilized bound H2O layers: PGSE NMR study of water-rich saponite gels

The role of bound or less-mobile H₂O layers near negatively charged clay platelets in water-rich saponite gels as obstacles to the diffusion of unbound H₂O molecules in the pore-space was examined. Self-diffusion coefficients of H₂O molecules, D, in gels of synthetic Na-rich saponite were measured by pulsed-gradient spin-echo proton nuclear magnetic resonance (PGSE proton NMR) to evaluate these obstruction effects. The clay weight fraction, w, of the gel samples ranged from 0.0 to 27.9 wt.% and the sample temperature was 20.0 to 60.6 °C. The NMR results showed that D normalized by D₀ was independent of sample temperature and decreased with increasing w: ln(D/D₀)=1.33[exp(−0.0290w)−1] where D₀ was D in bulk water. This diffusion behavior was interpreted through random-walk computer simulations of a gel-structure model originally developed for montmorillonite, hectorite and stevensite. In the model, unbound H₂O diffuses in the porous gel structure by avoiding randomly distributed obstacles representing clay platelets sandwiched between completely immobilized bound H₂O layers. A quantitative dependence of the platelet volume fraction and the bound H₂O layers thickness on D/D₀ was obtained by the simulations. The ratio, χ=(volume of clay platelets and immobilized H₂O layers)/(volume of clay platelets), was introduced as a measure of the thickness of the immobilized H₂O layers. χ was estimated to be 5.7 for water-rich saponite gels (w≤7.99 wt.%) by fitting the results of the random-walk simulation to the PGSE NMR diffusion data. This value corresponds to an immobilized bound H₂O layer thickness of 2.4 nm assuming that each clay particle in the gels consists of a single 1-nm-thick platelet. The thickness of immobilized H₂O layers in water-rich gels of six smectite species was identified on this basis to decrease in the following order: montmorillonite (χ=18, 13)>saponite (χ=5.7)>hectorite (χ=4.2, 4.0)>stevensite (χ=3.6). This order correlates well with the order of the cation exchange capacity (CEC), suggesting that the thickness of the bound H₂O layers increases with increasing layer charge. The present study confirmed that the gel-structure model developed previously for montmorillonite, hectorite and stevensite was also applicable to saponite.     

Variability in water sorption and in particle thickness of standard smectites

Variability in sorbed water content was studied on several standard smectites, as dependent on the relative humidity (RH=0.5, 0.95 and 1.0), on temperature (constant 30 °C and variable room temperature) and on sorption time (t_s=1, 12 and 32 days). The particle thickness, δ was estimated, assuming a given number of molecular layers of sorbed water at the given RH. δ decreased with the increase in the factors mentioned above.The thickest particles in dry state were found in hectorite and in synthetic mica/smectite (SHCa-1 and Syn-1, δ=22 and 20 unit layers [u.l.], respectively), the thinnest ones-in Ca-smectite (SAz-1) and in nontronite (SWa-1, δ=5–6 u.l.) and the intermediate both in Na-smectite (SWy-1) and in Ca-smectite (STx-1), δ=12 and 8 u.l., respectively.On prolonged storage at RH=1.0, t_s=32 days, the smectite particles disintegrated gradually into separate crystallites, δ=3–5 u.l., thus close to the thickness measured by XRD. An exception was hectorite, SHCa-1, with δ=6–8 u.l. possibly due to “gluing” by calcium carbonate present.The decrease in δ (delamination) observed on increasing RH was partly reversible. The hysteresis between water sorption on its increasing (WS) and water retention on its lowering to RH=0.5 (WR) was especially pronounced in SHCa-1, SWy-1 and Syn-1 of initially thick particles. This was due either to a different number of the sorbed water layers, higher in WR, or to incomplete collapse in WR.     

Theoretical evaluation of pillared clay adsorbents: Part III: The total porosity and the macrostructure of Al-pillared montmorillonite and hectorite

As a new class of porous materials, pillared clays have been more and more investigated as a potential substituent or complement to zeolite materials. Despite of intensive research, real industrial applications are still lacking. Somehow, the porosity of these materials is lower than what was expected. To evaluate the potential of the pillared clays as adsorbent, theoretical calculations were carried out to obtain a better insight of the influence of the PILC's macrostructure on the micro- and mesoporosity.Assuming the layers to stack in a 3-dimensional brick-like fashion, the micro- and mesoporosity and interpillar free distance were calculated taking into account: the clay layer dimensions, interlayer free spacing, lateral distance between adjacent layers, partial pillaring/collapse and pillar symmetry. The calculations were performed for Al-pillared montmorillonite and hectorite and validated with experimental results.Nomenclature a height of the mesopore - Al-pilM aluminium pillared montmorillonite - Al-pilH aluminium pillared hectorite - b lateral distance between two stacks - CEC cation exchange capacity meq/g - d _pp interpillar free distance - H height of a clay layer Å - IFS interlayer free spacing Å - L length and width of a clay layer Å - P pillar height Å - pil/g number of pillars in 1 g clay - (pil/g)_in number of pillars, in 1 g clay, intercalated between the layers - (pil/g)_out number of pillars, in 1 g clay, not intercalated - (pil/g)_tot total number of pillars in 1 g clay - pl/g number of clay layers in 1 g clay - s number of layers in a stack - SA^av layer surface area available for pillaring m²/g - SA_BET BET surface area m²/g - SA_max maximum surface area of the clay layers m²/g - SA _meso ^av available surface area in the mesopores m²/g - Ustr unit structure - Ustr/g number of unit structures in 1 g clay - V_free free volume in the unit structure - V _meso ^av calculated available mesopore volume cc/g - V _meso ^ult calculated ultimate mesopore volume cc/g - V _micro ^av calculated available micropore volume cc/g - V _micro ^ult calculated ultimate micropore volume cc/g - V_1pil volume of one pillar cc/g - V_pil(in) total volume of the pillars between the layers cc/g - V_pil(out) total volume of the pillars outside the layers cc/g - V_tot total pore volume atp/p ₀ = 0.98 cc/g - W_pil width of a pillar Å - Wt%_pil weight percentage Al₂O₃ present as pillars     

Factors influencing the hydration of layered double hydroxides (LDHs) and the appearance of an intermediate second staging phase

The hydration of layered double hydroxides (LDHs) was investigated by changing the interlayer anion species, the Mg/Al ratio of the LDH hosts and the relative humidity (RH). The anions were CO₃²⁻, Cl⁻, Br⁻, NO₃⁻, I⁻, SO₄²⁻, and ClO₄⁻ (listed in the order of ion size, small to large) and LDHs with Mg/Al = 1.90 (LDH2) and 2.91 (LDH3) were used. Their XRD profiles were measured by an XRD diffractometer while controlling the RH in the range 0–95% at 25 °C. Only I⁻, SO₄²⁻, and ClO₄⁻ LDH2s and SO₄²⁻ LDH3 showed a large step-wise basal-spacing expansion, 0.24–0.28 nm, under high RH conditions (> ca. 60%) probably due to the insertion of one water layer into the interlayer space. Such hydration occurred more favorably for the LDHs with larger anions and those with a higher layer charge (LDH2). Among them, I⁻ and ClO₄⁻ LDH2s exhibited the second staging – alternate stacking of hydrated (H) and non-hydrated (NH) interlayers – in the intermediate RH region.     

Dissolution characteristics of hectorite in inorganic acids

The effect of acid type (HCl, HNO₃ and H₂SO₄) and concentration on the dissolution rate of hectorite were monitored through chemical analyses and XRD. The rate of dissolution increased with increasing acid concentration during the first 2–4 h of contact. After that time, the correlation between acid concentration and the amounts of dissolved elements strongly decreased and often higher concentrations were found in 0.25 M solutions than in 1 M solutions. The monitoring of the amount of Si is somewhat more complex since it is the result of two processes: release from the mineral and reprecipitation as an amorphous end product. In the case of HCl, the behavior of Li, Mg and Fe differed from their behavior in the other acids, but further research is necessary to characterize the reactions that may occur. From the half times of dissolution, the dissolution curves and the XRD data, it could be concluded that the dissolution rate of hectorite decreased in the order H₂SO₄>HNO₃≥HCl at the same molar concentration, which is the reverse of what was found by other investigators. After 8 h, for example, the 1 M H₂SO₄ treatment dissolved more than 70% of all Li present in the hectorite, whereas equal molar HNO₃ and HCl dissolved 58% and 53%, respectively. Oriented XRD patterns only showed a background scatter after 6 h of contact with 0.25 M H₂SO₄ and after 4 h using a concentration of 1 M H₂SO₄. Treatments with 0.25 M HNO₃ and 0.25 M HCl still gave reflections after 6 h and even after 8 h, the d(001) XRD peak could still be observed. In 1 M HNO₃ and 1 M HCl, no more reflections could be seen after 4–6 h. At that time, XRD powder patterns showed that the crystal structure was still partly preserved in the a- and b-direction of the mineral. It must be stressed that absence or a decrease in intensity of the d(001) peaks may not be fully assigned to the breakdown of the mineral structure since the Si, extracted from the lattice reprecipitates as amorphous silica, which may disrupt the layer stacking and decrease the coherence of the reflections. For the comparison of the three acids on a normal basis, the quarter times of dissolution were found to be a more appropriate tool than the half times of dissolution. The results showed that the differences among the three acids were rather small and that the concentration of the acids was the main parameter affecting the dissolution rate of the hectorite, in particular, in the beginning of the treatment.     

Evaluation of RuxWySez Catalyst as a Cathode Electrode in a Polymer Electrolyte Membrane Fuel Cell

The oxygen reduction reaction (ORR) on Ru_xW_ySe_z is of great importance in the development of a novel cathode electrode in a polymer electrolyte membrane fuel cell (PEMFC) technology. The Ru_xW_ySe_z electrocatalyst was synthesised in an organic solvent for 3 h. The powder was characterised by transmission electron microscopy (TEM), and powder X‐ray diffraction (XRD). The electrocatalyst consisted of agglomerates of nanometric size (∼50–150 nm) particles. In the electrochemical studies, rotating disc electrode (RDE) and rotating ring‐disc electrode (RRDE) techniques were used to determine the oxygen reduction kinetics in 0.5 M H₂SO₄. The kinetic studies include the determination of Tafel slope (112 mV dec^–1), exchange current density at 25 °C (1.48 × 10^–4 mA cm^–2) and the apparent activation energy of the oxygen reaction (52.1 � 0.4 kJ mol^–1). Analysis of the data shows a multi‐electron charge transfer process to water formation, with 2% H₂O₂ production. A single PEMFC with the Ru_xW_ySe_z cathode catalysts generated a power density of 180 mW cm^–2. Performance achieved with a loading of 1.4 mg cm^–2 of a 40 wt% Ru_xW_ySe_z and 60 wt% carbon Vulcan (i.e. 0.56 mg cm^–2 of pure Ru_xW_ySe_z). Single PEMFC working was obtained with hydrogen and oxygen at 80 °C with 30 psi.     

Experimental Investigation of Air Relative Humidity (RH) Cycling Tests on MEA/Cell Aging in PEMFC Part I: Study of High RH Cycling Test With air RH at 62%/100%

The effect of high air relative humidity (RH) cycling (RH_C 62%/100%) on the degradation mechanisms of a single (5 × 5 cm²) proton exchange membrane fuel cells was investigated. The cell performance was compared to a cell operated at constant humidification (RH_C = 62%). Runs were conducted over approximately 1,500 h at 0.3 A cm^–2. The overall loss in cell performance for the high RH cycling test was 12 μV h^–1 whereas it was at 3 μV h^–1 under constant humidification. Impedance spectroscopy reveals that the ohmic and charge transfer resistances were little modified in both runs. H₂ crossover measurement indicated that both high RH cycling and constant RH test did not promote serious effect on gas permeability. The electroactive surface loss for anode and cathode during high air RH cycling was more significant than at constant RH operation. The water uptake determined by ¹H nuclear magnetic resonance within the membrane electrode assembly (MEA) after high RH cycling was reduced by 12% in comparison with a fresh MEA. Transmission electron microscopy showed bubbles and pinholes formation in the membrane, catalyst particles agglomeration (also observed by X‐ray diffraction), catalyst particles migration in the membrane and thickness reduction of the catalytic layers. Scanning electron microscopy was conducted to observe the changes in morphology of gas diffusion layers after the runs.     

Preparation and characterization of novel organo-clay minerals for Hg(II) ions adsorption from aqueous solution

Novel organo-clay minerals for adsorption of Hg(II) ions were prepared. The clay minerals were grafted with 2-(3-(2-aminoethylthio)propylthio)ethanamine (AEPE). AEPE-montmorillonite and AEPE-hectorite were characterized by X-ray diffraction (XRD), infrared spectroscopy (IR), thermogravimetric analysis (TGA), CHN element analysis (EA) and surface area analysis. The XRD patterns indicated that the chelating agents (AEPE) were mainly grafted on the external surface of montmorillonite while AEPE was grafted on both the external and interlayer surfaces of hectorite. The results from IR, TGA and EA showed a difference in chemical composition of the unmodified and modified clay minerals. The results confirmed that montmorillonite and hectorite were modified with the desired organic groups. The extent of ligand loading depended on the nature of the clay mineral. The AEPE-modified clay minerals were good chelating materials for Hg(II) ions, compared to the unmodified clay minerals. The adsorption capacity for Hg(II) of AEPE-montmorillonite and AEPE-hectorite was 46.1 and 54.7 mg g^− 1, respectively, for solution containing 140 mg L^− 1 Hg(II) ions (pH 4).     

Surface immobilisation of transition metal substituted Krebs type polyoxometalates, [X2W20M2O70(H2O)6] (X = Bi or Sb, M = Co or Cu), by the layer by layer technique

A series of transition metal (i.e. Cu²⁺ and Co²⁺) substituted Krebs type polyoxometalates (POMs), of the general formula [X₂W₂₀M₂O₇₀(H₂O)₆]ⁿ⁻, X = Sb or Bi, M = Co(II) or Cu(II), have been successfully immobilised onto carbon electrode surfaces through the employment of the layer-by-layer (LBL) technique. This involved the construction of alternating anionic POM, [X₂W₂₀M₂O₇₀(H₂O)₆]ⁿ⁻, layers and the cationic metallodendrimer, Ru(II)-metallodendrimer as the cationic layers, in addition to a [poly(diallyldimethylammonium chloride)] PDDA base layer. Stable multielectron redox couples associated with the W–O framework, for the Krebs type POMs, and the Ru(III/II) for the metallodendrimer, were clearly observed upon layer construction and redox switching within the pH domain of 2–6.5. The constructed multilayer assemblies exhibited pH dependent redox activity and thin layer behaviour up to 100 mV s⁻¹. The porosity and permeability of the individual multilayer assemblies towards an anionic probe were determined by AC impedance and cyclic voltammetry. The surface morphology of each multilayer was also determined by Atomic Force Microscopy (AFM).     

Synthesis and Characterization of Pillared High Layer Charged Synthetic Saponite

Alumina pillared clays were prepared using synthetic saponites with different layer charges, and their cracking activities were characterized by cumene conversion reaction. It was found that the pillared saponite with super high layer charges (2.0) exhibited stable cracking activity in the thermal treatment temperature range from 500 to 700°C. The BET surface areas of the pillared saponites (calcined at 500°C for 2 h) with layer charges of 1.0, 1.5 and 2.0 were 360, 280 and 310 m²/g respectively, according to the N₂ adsorption-desorption determinations at 77 K. A significant layer charge effect was found on the catalytic cracking activity of the pillared clays according to the model reaction, indicating that substitutions in the tetrahedral sheets played an important role in the development of surface Brönsted acidic sites of pillared clay. Because of strong Brönsted sites of pillared saponites, peaks centered at 432°C were found in the spectra of the temperature programmed desorption of ammonia. The structure of pillared saponite with 2.0 layer charges, being the best solid acid catalyst in this study, was also characterized by ²⁷Al MAS NMR. A new peak centered at 1.9 ppm after pillaring was observed, which was ascribed mainly to the octahedral aluminum in the pillar.     

Etching Induced Stepped Nanostructure on Pb(Mg(1–x/2)Mn(x/2)W1/2)O3 Ceramics

A new solid solution system, Pb(Mg_(1–x/2)Mn_(x/2)W_1/2)O₃, is formed by replacing Mg²⁺ with Mn²⁺ in the Pb(Mg_1/2W_1/2)O₃ complex perovskite. The solid solution remains in the orthorhombic Pmcn symmetry up to x = 0.1. It is observed that for such a chemical modification, the surface of the grain changes significantly. A stepped surface nanostructure appears in the Mn‐substituted perovskite. Detailed surface morphology of the stepped patterns was revealed by atomic force microscopy (AFM), which determined that the interlayer step height varies from 2.3 to 8.3 nm. Analyse by XRD, SEM, EDX, and AFM suggest that the nature of the stepped surface structure may be the result of etching rather than spiral growth or two‐dimensional nucleation as it was previously believed.     

A comparative study of the physicochemical and electrochemical properties of Cr and Ni-W-P amorphous electrocoatings

A comparative study of the physicochemical and electrochemical properties of Cr and amorphous Ni-W-P electrocoatings is presented here. Amorphous Ni-W-P alloys were successfully produced by electrodeposition at 70 °C on copper substrate under galvanostatic control in the range of 50-400 mA cm⁻² and constant loads of 500 and 1600 C, using a solution containing 0.20 mol L⁻¹ Na₂WO₄.2H₂O; 0.02 mol L⁻¹ NiSO₄·6H₂O; 0.02 mol L⁻¹ NaPH₂O₂; 0.02 mol L⁻¹ H₃BO₃; 0.07 mol L⁻¹ (NH₄)₂SO₄; 0.20 mol L⁻¹ Na₃C₆H₅O₇·2H₂O; 0.0001 mol L⁻¹ CH₃(CH₂)₁₀·CH₂OSO₃Na. Cr electrocoatings were obtained from an industrial plating solution. The physicochemical characterization of the as-electrodeposited and as-annealed samples was carried out by scanning electron microscopy (SEM), X-ray diffraction (XRD), and energy dispersive X-ray analysis (EDX) techniques. Corrosion tests were carried out at room temperature in 10⁻¹ mol L⁻¹ NaCl aqueous solutions, using potentiodynamic linear polarization (PLP). Among the various Ni-W-P electrocoatings studied here, the Ni₆₅W₂₀P₁₅ layer presented the best corrosion behavior and a slightly superior corrosion potential than the Cr electrocoating. Heat treatments gave rise to a cracked surface morphology in the Cr layers, while the surface morphology of the Ni₆₅W₂₀P₁₅ layers remained homogeneous and devoid of cracks. Heat treatments at 400 and 600 °C led to crystallization of the Ni-W-P layer, with precipitation of the Ni₃P, Ni and Ni-W phases and increasing hardness of the Ni-W-P layer as the heat treatment temperature rose. All the annealed Cr layers showed cracked surfaces and their hardness diminished as the annealing temperature increased. The presence of cracks impairs the mechanical and corrosion resistance properties of Cr layers. Ni₆₅W₂₀P₁₅ layer is a potential candidate to replace Cr in industrial applications, mainly at operational temperatures that exceed room temperature.     

Solvothermal preparation of TiO2/saponite nanocomposites and photocatalytic activity

A saponite-rich clay sample from western Macedonia, Greece was used for the preparation of TiO₂–saponite nanocomposites by solvothermal reaction of a mixture of water and ethanol as the solvent, hexamethylene tetramine as the precipitant and titanium trichloride as the TiO₂ precursor. Phase composition was determined by X-ray diffraction (XRD) and particle morphology and elemental content was characterized by scanning electron microscopy and energy dispersive spectroscopy (SEM–EDS). These samples were also characterized by attenuated total reflection using Fourier transform infrared spectroscopy (ATR–FTIR). Particle-size analysis was carried out using dynamic light scattering method (DLS) and specific surface area and pore-size distribution analyses using BET N₂ adsorption–desorption isotherms. The composite photocatalysts showed a mesoporous structure with the distribution of pore diameters centered at 3.8 and at 5.7–9.8 nm, with high specific surface areas. The photocatalytic activities of titania–saponite nanocomposites in decomposing NOx gas were measured as a function of two different TiCl₃–saponite ratios. TiO₂:saponite with a mass ratio of 0.2:1 (Ti-Sap1) showed the highest photocatalytic activity because of its relatively large specific surface area and higher TiO₂ content. Both the TiO₂–saponite nanocomposites showed higher photocatalytic activity than the standard titania (Degussa P25) based on TiO₂ content because the titania was well dispersed on saponite.     

Hydration of ethene over W–P mixed metal oxide catalysts

W–P mixed metal oxide catalysts are active and selective for the gas-phase hydration of ethene to ethanol. The activity and selectivity of this catalytic reaction depend on the W/P atomic ratio. However, ethene conversion slightly decreases at higher W/(W + P) atomic ratio. The selectivity for ethanol increases with the W/P atomic ratio and reaches the highest value (92%) at W_0.81P_0.19O_x. The W_0.81P_0.19O_x catalyst is less active than the conventional H₃PO₄/SiO₂ catalyst, but the activity is maintained for a long time without the supply of any catalyst components. The reaction temperature does not affect substantially the rate of ethene hydration over the W_0.81P_0.19O_x catalyst. The H₂O/ethene molar ratio of 0.4 is the most appropriate for both reaction rate and selectivity. The active species of W–P mixed metal oxide are amorphous. But there is Keggin structure of W–P oxide species (PW₁₂O₄₀ ³⁻) in the presence of steam. And the species are the active sites for the hydration of ethene, confirmed by in situ Raman spectroscopy. This revised version was published online in July 2006 with corrections to the Cover Date.     

Theoretical evaluation of pillared clay adsorbents: Part I. The microporosity of Al- and Ti-pillared montmorillonite

Fillared clays have become a well known class of porous materials. Due to the versatile porosity they seem to be complementary to zeolites and can be very useful for adsorption and catalytic processes. Despite intensive research, real industrial applications are still lacking. Somehow, the porosity of these pillared clays is not what was expected. The present study reports theoretical calculations, based on simple geometrical models in combination with experimental data, in order to obtain a better insight in the PILC structure and its limiting factors. Parameters such as interpillar free distance and maximum micropore volume were calculated and compared to experimental or literature data. The final goal is to find out the potential of pillared clays as adsorbents to be used as substituents or complements for zeolites. This part reports calculations, in terms of microporosity, performed for two types of pillared clays having completely different porosity properties: Al- and Ti-PILC.Nomenclature Al-pilM aluminium pillared montmorillonite - c number of collapsed interlayer volumes in a stack - CEC cation exchange capacity (meq/g) - d _pp interpillar free distance (Å) - IFS interlayer free spacing (Å) - L length of a clay layer (Å) - MW molecular weight (g/mol) - N _av Avogadro constant (molecules/mol) - P pillar height (Å) - pil/g number of pillars in 1 g clay - (pil/g)_in number of pillars, in 1 g clay, intercalated between the layers - (pil/g)_out number of pillars, in 1 g clay, not intercalated - pl/g number of clay layers in 1 g clay - quasi cryst/g number of stacks or quasi crystals in 1 g clay - s number of layers in a stack or quasi crystal - SA^av layer surface area available for pillaring (m²/g) - SA_BET BET surface area (m²/g) - SA_max maximum surface area of the clay layers (m²/g) - SA^unav layer surface area not contributing to the micropore volume (m²/g) - SA/pillar available surface area for a pillar on a clay layer (Ų) - SA_1pl surface area of a single clay layer (Ų) - Ti-pi1M titanium pillared montmorillonite - V _micro calculated micropore volume (cc/g) - V _micro ^exp experimental micropore volume (cc/g) - V _micro ^ideal calculated micropore volume for an idealised situation (cc/g) - V _pil total volume of the pillars (cc/g) - V _pil (inside) total volume of the pillars between the layers (cc/g) - V _tot total pore volume atp/p ₀ = 0.98 (cc/g) - W width of a clay layer (Å) - W _pil width of a pillar (Å) - Wt%_pil weight percentage Al₂O₃ or TiO₂ present as pillars     

High-temperature resistant polyimide-based sandwich-structured dielectric nanocomposite films with enhanced energy density and efficiency

Development of advanced dielectric materials with both high-electric energy density and high-temperature resistant attributes is highly desirable in modern electronics and electrical systems. Herein, a series of polyimide (PI)-based sandwich-structured dielectric nanocomposite films have been attempted to develop the advanced high-temperature resistant capacitor films, wherein the boron nitride nanosheets/PI nanocomposite acts as the outer layers and the zinc oxide (ZnO)/PI as the middle layer. Benefitting from the merits of both fillers and the unique structure, the resulting nanocomposite films can simultaneously achieve both high-dielectric constant and high-breakdown strength, as well as low-electrical conduction loss, thus leading to improved discharged energy densities (U_e) and charge/discharge efficiency (η) at elevated temperatures. It is found that the sandwich-structured nanocomposite film with 0.4 vol% ZnO (0.4ZnO/PI-S) can deliver a maximum U_e of 5.29 J cm⁻³ at 400 MV m⁻¹ and 150°C, which is about 1.9 times that of the pristine PI film. Moreover, outstanding dielectric stability over 10,000 charge/discharge cycles has been demonstrated in such PI-based sandwich-structured nanocomposite films at 150°C and 200 MV m⁻¹. This research may provide a new paradigm to explore polymer nanocomposites having excellent energy storage and efficiency at elevated temperatures.     

Hydrothermal synthesis and characterization of dioctahedral smectites: A montmorillonites series

The aim of this study is to synthesize and finely characterize montmorillonite samples, dioctahedral smectites without tetrahedral charges (structural formulae Na_x(Al_(2 − x)Mg_x)Si₄O₁₀(OH)₂), to allow their use as reference samples in clay science. The montmorillonites synthesis under hydrothermal conditions at different pressures and with various layer charge deficit has been attempted. The temperature was fixed at 320 °C, the pressure parameter values were 20 MPa, 80 MPa, 120 MPa and 200 MPa. The Mg content varied from 0.25 to 0.60 per half unit cell. The reaction products have been characterized with multi-technique analyses (ICP-AES, EMP, CEC, XRD, FTIR, NMR and TGA).Montmorillonite phase was only produced at 120 and 200 MPa.At 20 and 80 MPa, the results suggest that a 0.33 and 0.16-tetrahedral charge deficit exist in the formed samples. Moreover, the octahedral occupancies are higher than two (2.15 and 2.07 at 20 and 80 MPa respectively). In these experimental conditions, the synthetic smectites are mixtures between montmorillonite, beidellite and saponite.At 120 MPa and for a Mg content of 0.25 or higher than 0.33, the synthetic products were also mixtures of smectites. Tetrahedral charge deficits of 0.11, 0.11 and 0.15 were found for Mg contents of 0.25, 0.50 and 0.60 respectively. The octahedral occupancy was also higher than 2.00.A montmorillonite phase with only octahedral charges and an octahedral occupancy near 2.00 was synthesized for a Mg content of 0.33 and at pressures equal to or higher than 120 MPa. This low charge reference smectite shows a very low amount of accessory minerals and an octahedral charge deficit only created by the presence of magnesium in the structure. This montmorillonite can be compared structurally to the most studied natural one: the montmorillonite SWy-2 from Wyoming.     

Modification of layer charge in smectites by microwaves

The layer charge reduction of two Li-saturated montmorillonites is referred to application of microwave radiation at 2.45 GHz for dispersions and 30.0 GHz for the solid powders. Efficiency of these treatments was compared to the same conditions applying conventional heating. The samples were heated in the temperature and time windows corresponding to 190–270 °C and 30–120 min, respectively. Changes in the mean layer charge were monitored by the determination of cation exchange capacity values using exchange of triethylene tetraamino copper ions. The charge reduction of the montmorillonites in aqueous dispersions was rather low (< 30%) despite the fact, that high Li⁺ concentration dissolved in solution was selected (1 mol L^− 1). This behaviour was attributed to the very high water content in used dispersions and high hydration enthalpy of Li⁺ cations. Nevertheless, the microwave heated dispersions (2.45 GHz) showed detectable layer charge reduction as compared to conventionally heated dispersions, where no significant changes were found.Solid powders with different content of exchangeable lithium ions were prepared with solutions having different molar fraction of Li⁺ and Na⁺ cations (0%, 20%, 40%, 60%, 80%, 100% of Li⁺). Final composition of interlayer cations was analysed with ICP-OES. In contrary to dispersions, the microwave treatment of the prepared powders revealed high layer charge reduction, which was much higher than for conventionally heated powders. The efficiency was enhanced with increasing interlayer lithium content but reached a limiting value depending on the montmorillonite used. Migration of the lithium into the structure of the montmorillonite supported with microwaves was extremely fast, finished probably within the few minutes of the treatments. The exchangeable Li⁺ cations are accelerated through alternating electric field of microwaves and highly efficient layer charge neutralisation occurs. Infrared spectroscopy showed that the products obtained with microwave treatments correspond to the materials heated conventionally for much longer periods of time. Hence, applying microwaves the time and energy requirement can be significantly reduced. The X-ray diffraction showed that montmorillonite layers were able to swell in ethyleneglycol upon charge reduction, if the cation exchange capacity was not reduced more than 20–40%.     

Hierarchically structured Bi2MoxW1-xO6 solid solutions with enhanced piezocatalytic activities

Hierarchical structure Bi₂Mo_xW_1-xO₆ (x = 0, 0.1, 0.2, 0.3, 0.4 and 0.5) solid solutions with high surface area are prepared by hydrothermal method without employing any surfactant. All the as-prepared products are hierarchical microspheres self-assembled by nanosheets. The piezocatalytic performance of Bi₂Mo_xW_1-xO₆ solid solutions are investigated by the degradation of RhB under ultrasonic vibration. The experimental results show that the x value of Bi₂Mo_xW_1-xO₆ has great influence on the piezocatalytic efficiency. Among which, the Bi₂Mo_0.4W_0.6O₆ presents a high value of the rate constant k of 0.119 min^-1 by degrading 97.5% of RhB dye solution in 30 min. It is about 43.4% higher than that of pure Bi₂WO₆ sample, for which k is 0.083 min^-1. Radical trapping test indicates that holes (h⁺) are the dominating active species in the degradation process. The improved piezocatalytic performance for Bi₂Mo_0.4W_0.6O₆ sample ascribes to the larger piezoelectric potential generated under a strained state, which facilitates the transfer of charge carriers and accelerate the piezocatalytic process. Our work presents a new design strategy of piezocatalysts for remediation of water pollution.     

Influence of Sr2+ on Calcium‐Deficient Hydroxyapatite Formation Kinetics and Morphology in Partially Amorphized α‐TCP

Hydration of partially amorphized α‐TCP powders with Sr²⁺ concentrations ranging from 0 to 10 mol% substitution for Ca²⁺ was analyzed by isothermal calorimetry and quantitative in‐situ XRD. Hydration of both crystalline α‐TCP and amorphous TCP (ATCP) forming CDHA was retarded to an increasing extent with increasing Sr²⁺ content. Sr²⁺ slightly reduced the crystallite size (XRD coherent scattering domains) of the CDHA formed during hydration, while the size of crystals visible under SEM was not noticeably affected. Reaction enthalpies of ΔH_{R(Sr‐α‐TCP→Sr‐CDHA)} = 122 ± 8 J/g_TCP and ΔH_{R(Sr‐ATCP→Sr‐CDHA)} = 257 ± 8 J/g_TCP were determined for the hydration of crystalline α‐TCP and ATCP containing 5 mol% Sr²⁺ substitution for Ca²⁺. This is comparable with the corresponding reaction enthalpies previously obtained for undoped samples, which are 106 ± 7 J/g_TCP for α‐TCP and 250 ± 7 J/g_TCP for ATCP.