Relaxor‐ferroelectric materials find application in a broad range of technological devices, including ultrasonic imaging transducers, nanopositioning, and high‐performance capacitors. They generally ...exhibit occupationally disordered structures creating local polar fluctuations that are highly sensitive to applied electric or stress fields. The sensitivity of the material structure to external field and stress conditions also makes them likely to develop skin or surface phases that are unique from the bulk. Surface layers can adjust the material response and also lead to ambiguity in structural characterization. Here, using a combination of X‐ray diffraction methods, it is shown that a ≈20 µm skin structure commonly exists in the lead‐free relaxor‐ferroelectric ceramic (Na1/2Bi1/2)TiO3–BaTiO3. Using experiments and density functional theory calculations, it is shown that the combined action of oxygen vacancies providing internal chemical pressure and the surface plane stress state dictates the stability and structure of the skin layer. This work can be extended to all perovskite relaxor ferroelectrics and provides new insights into the origin of skin layers in these materials. The opportunity exists to further enhance the functionality of these materials through engineering of surface structures using the methods outlined here.
Surface structural distortions in relaxor ferroelectrics are demonstrated and their origin is linked to defect chemistry. The combined action of internal chemical pressure from the oxygen vacancies and a plane stress state at the surface results in a different structure than the bulk. This provides new insights into the origin of surface structures in perovskite relaxor ferroelectrics.
The need for high efficiency energy production, conversion, storage and transport is serving as a robust guide for the development of new materials. Materials with physical-chemical properties ...matching specific functions in devices are produced by suitably tuning the crystallographic- defect- and micro-structure of the involved phases. In this review, we discuss the case of Rare Earth doped Ceria. Due to their high oxygen diffusion coefficient at temperatures higher than ~500°C, they are very promising materials for several applications such as electrolytes for Solid Oxide Fuel and Electrolytic Cells (SOFC and SOEC, respectively). Defects are integral part of the conduction process, hence of the final application. As the fluorite structure of ceria is capable of accommodating a high concentration of lattice defects, the characterization and comprehension of such complex and highly defective materials involve expertise spanning from computational chemistry, physical chemistry, catalysis, electrochemistry, microscopy, spectroscopy, and crystallography. Results coming from different experimental and computational techniques will be reviewed, showing that structure determination (at different scale length) plays a pivotal role bridging theoretical calculation and physical properties of these complex materials.
ID15A is a newly refurbished beamline at the ESRF devoted to operando and time‐resolved diffraction and imaging, total scattering and diffraction computed tomography. The beamline is optimized for ...rapid alternation between the different techniques during a single operando experiment in order to collect complementary data on working systems. The high available energy (up to 120 keV) means that even bulky and highly absorbing systems may be studied. The beamline is equipped with optimized focusing optics and a photon‐counting CdTe pixel detector, allowing for both unprecedented data quality at high energy and for very rapid triggered experiments. A large choice of imaging detectors and ancillary probes and sample environments is also available.
ID15A is a newly refurbished beamline at the ESRF devoted to operando and time‐resolved diffraction and imaging, total scattering and diffraction computed tomography.
The direct conversion of methane to methanol using oxygen is a challenging but potentially rewarding pathway towards utilizing methane. By using a stepwise chemical looping approach, copper‐exchanged ...zeolites can convert methane to methanol, but productivity is still too low for viable implementation. However, if the nature of the active site could be elucidated, that information could be used to design more effective catalysts. By employing anomalous X‐ray powder diffraction with support from theory and other X‐ray techniques, we have derived a quantitative and spatial description of the highly selective, active copper sites in zeolite omega (Cu‐omega). This is the first comprehensive description of the structure of non‐copper‐oxo active species and will provide a pivotal model for future development for materials for methane to methanol conversion.
By employing anomalous X‐ray powder diffraction along with support from theory and other X‐ray techniques, the active copper site for methane‐to‐methanol conversion in zeolite omega has been determined spatially and quantitatively.
Cellulose nanocrystals (CNCs) exhibit outstanding gas barrier properties, which supports their use as a biobased and biodegradable barrier coating on flexible food packaging materials. As highly ...hydrophilic biopolymers, however, CNCs have a strong sensitivity to water that can be detrimental to applications with fresh foods and in moist conditions due to the loss of barrier properties. In this work, the oxygen and water vapor permeability of polyethylene terephthalate (PET) films coated with CNCs obtained from cotton linters were measured at varying levels of relative humidity, both in adsorption and desorption, and from these data, the diffusion and solubility coefficients were estimated. Therefore, the characterization of CNCs was aimed at understanding the fundamentals of the water-CNCs interaction and proposing counteractions. The CNCs' moisture absorption and desorption isotherms at 25 °C were collected in the range of relative humidity 0⁻97% using different techniques and analyzed through GAB (Guggenheim-Anderson-de Boer) and Oswin models. The effects of moisture on the water status, following the freezable water index, and on the crystal structure of CNCs were investigated by Differential Scanning Calorimetry and by X-ray Powder Diffraction, respectively. These findings point to the opportunity of coupling CNCs with hydrophobic layers in order to boost their capabilities as barrier packaging materials.
The electric field response of the lead-free solid solution (1-
)Bi
Na
TiO
-
BaTiO
(BNT-BT) in the higher BT composition range with
= 0.12 was investigated using in situ synchrotron X-ray powder ...diffraction. An introduced Bi-excess non-stoichiometry caused an extended morphotropic phase boundary, leading to an unexpected fully reversible relaxor to ferroelectric (R-FE) phase transformation behavior. By varying the field frequency in a broad range from 10
up to 10
Hz, BNT-12BT showed a frequency-dependent gradual suppression of the field induced ferroelectric phase transformation in favor of the relaxor state. A frequency triggered self-heating within the sample was found and the temperature increase exponentially correlated with the field frequency. The effects of a lowered phase transformation temperature T
, caused by the non-stoichiometric composition, were observed in the experimental setup of the freestanding sample. This frequency-dependent investigation of an R-FE phase transformation is unlike previous macroscopic studies, in which heat dissipating metal contacts are used.
Here we report on the impact of reducing the crystalline size on the structural and magnetic properties of γ-Fe2O3 maghemite nanoparticles. A set of polycrystalline specimens with crystallite size ...ranging from ~2 to ~50 nm was obtained combining microwave plasma synthesis and commercial samples. Crystallite size was derived by electron microscopy and synchrotron powder diffraction, which was used also to investigate the crystallographic structure. The local atomic structure was inquired combining pair distribution function (PDF) and X-ray absorption spectroscopy (XAS). PDF revealed that reducing the crystal dimension induces the depletion of the amount of Fe tetrahedral sites. XAS confirmed significant bond distance expansion and a loose Fe-Fe connectivity between octahedral and tetrahedral sites. Molecular dynamics revealed important surface effects, whose implementation in PDF reproduces the first shells of experimental curves. The structural disorder affects the magnetic properties more and more with decreasing the nanoparticle size. In particular, the saturation magnetization reduces, revealing a spin canting effect. Moreover, a large effective magnetic anisotropy is measured at low temperature together with an exchange bias effect, a behavior that we related to the existence of a highly disordered glassy magnetic phase.
Supported, bimetallic catalysts have shown great promise for the selective hydrogenation of CO2 to methanol. In this study, we decipher the catalytically active structure of Ni–Ga-based catalysts. To ...this end, model Ni–Ga-based catalysts, with varying Ni:Ga ratios, were prepared by a surface organometallic chemistry approach. In situ differential pair distribution function (d-PDF) analysis revealed that catalyst activation in H2 leads to the formation of nanoparticles based on a Ni–Ga face-centered cubic (fcc) alloy along with a small quantity of GaO x . Structure refinements of the d-PDF data enabled us to determine the amount of both alloyed Ga and GaO x species. In situ X-ray absorption spectroscopy experiments confirmed the presence of alloyed Ga and GaO x and indicated that alloying with Ga affects the electronic structure of metallic Ni (viz., Niδ−). Both the Ni:Ga ratio in the alloy and the quantity of GaO x are found to minimize methanation and to determine the methanol formation rate and the resulting methanol selectivity. The highest formation rate and methanol selectivity are found for a Ni–Ga alloy having a Ni:Ga ratio of ∼75:25 along with a small quantity of oxidized Ga species (0.14 mol GaO x molNi –1). Furthermore, operando infrared spectroscopy experiments indicate that GaO x species play a role in the stabilization of formate surface intermediates, which are subsequently further hydrogenated to methoxy species and ultimately to methanol. Notably, operando XAS shows that alloying between Ni and Ga is maintained under reaction conditions and is key to attaining a high methanol selectivity (by minimizing CO and CH4 formation), while oxidized Ga species enhance the methanol formation rate.
In the present paper, the wettability of polyarylethersulfones (PESs) was promoted through the controlled introduction of sulfonic groups in PES polymeric chain. Homogeneous synthesis using a ...sulfonated comonomer introduced sulfonic groups while exerting a tight control over the degree of sulfonation (DS) and avoiding the undesired side reactions brought about by heterogeneous sulfonation reactions. A series of sulfonated polyarylethersulfones (SPESs) with very low amounts of sulfonic groups - 0.5, 0.75, and 1 meq SO
3
-*
g
-1
of polymer - was synthesized via polycondensation using 4,4’-difluorodiphenylsulfone, 4,4’-dihydroxydiphenyl and a sulfonated comonomer, 2,5-dihydroxybenzene-1-sulfonate potassium salt. The presence of sulfonic groups was confirmed by Fourier transform infrared (FTIR) spectra; the macromolecular structure and the real DS of SPESs were determined by
1
H NMR; the molecular weights were investigated by size exclusion chromatography (SEC); ion exchange capacity (IEC) values, measured by potentiometric titration, are in good agreement with the experimental DS. The polyelectrolyte effect was studied on intrinsic viscosity (IV) measurements. The effect of DS on the thermal properties of SPES membranes was studied by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC); wetting properties were characterized by static water contact angle (SWCA) measurements on membranes obtained
via
solution casting. The results showed that an increased DS results in higher glass transition temperatures (
T
g
) and lower water contact angles, the latter dropping from
91
¯
∘
to
43
¯
∘
as DS was raised from 0 to 1.0 meq SO
3
-*
g
-1
. This work demonstrates that homogeneous synthesis of SPES is an efficient way to prepare SPES membranes with tightly controlled DS and enhanced hydrophilic properties, in particular, an excellent hydrophilic SWCA even at very low amounts of sulfonic moieties.
PDF
