Size, Shape, and Material Effects in Ferroelectric Octahedral Nanoparticles

Composite materials composed of multiferroelectric nanoparticles in dielectric matrixes have attracted enormous attention for their potential applications in developing future functional devices. However, the functionalities of ferroelectric nanoparticles depend on shapes, sizes, and materials. In this paper, a time-dependent Landau-Ginzburg method has been used and combined with a method as the coupled-physics finite-element-method-based simulations are used to illustrate the polarization behavior in isolated BaTiO3 or PbTiO3 octahedral nanoparticles embedded in a dielectric medium, like SrTiO3 (ST, high dielectric permittivity) and amorphous silica (a-SiO2, low dielectric permittivity). The equilibrium polarization topology of the octahedral nanoparticle is strongly affected by the choice of inclusion and the size of matrix materials. Also, there are three equilibrium polarization patterns, i.e., monodomain, vortex-like, and multidomain, because of the various sizes and material parameters combination. There is a critical particle size below which ferroelectricity vanishes in our calculations. This size of the PbTiO3 octahedral nanoparticle is 2.5 and 3.6 nm for high- and low-permittivity matrix materials, respectively. However, this size of the BaTiO3 octahedral nanoparticle is 3.6 nm regardless of the matrix materials.

Tailoring Adsorption Induced Switchability of a Pillared Layer MOF by Crystal Size Engineering

The pillared layer framework DUT-8(Zn) (Zn₂(2,6-ndc)₂(dabco), 2,6-ndc = 2,6-naphthalenedicarboxylate, dabco = 1,4-diazabicyclo-[2.2.2]-octane, DUT = Dresden University of Technology) is a prototypical switchable MOF, showing characteristic adsorption and desorption induced open phase (<i>op</i>) to closed phase (<i>cp</i>) transformation associated with huge changes in cell volume. We demonstrate switchability strongly depends on a framework-specific critical particle size (d<i>_crit</i>). The solvent removal process (pore desolvation stress contracting the framework) significantly controls the <i>cp</i>/<i>op</i> ratio after desolvation and, subsequently, the adsorption induced switchability characteristics of the system. After desolvation, the dense <i>cp</i> phase of DUT-8(Zn) shows no adsorption-induced reopening and therefore is non-porous for N₂ at 77 K and CO₂ at 195 K. However, polar molecules with a higher adsorption enthalpy, such as the polar molecules such as chloromethane at 249 K and dichloromethane (DCM) at 298 K can reopen the macro-sized crystals upon adsorption. For macro-sized particles, the outer surface energy is negligible and only the type of metal (Zn, Co, Ni) controls the DCM-induced gate opening pressure. The framework stiffness increases from Zn to Ni as confirmed by DFT calculations, X-ray crystal structural analyses, and low frequency Raman spectroscopy. The partial disintegration of the Zn based node hinges produces an overall increased stabilization of<i> cp </i>vs. <i>op</i> phase shifts the critical particle size at which switchability starts to become suppressed to even lower values (d<i>_crit</i> < 200 nm) as compared to the Ni-based system (<i>d_crit</i> ≈ 500 nm). Hence, the three factors affecting switchability (energetics of the empty host, (<i>E_op-E_cp</i>) (I), particle size (II), and desolvation stress (III)) appear to be of the same order of magnitude and should be considered collectively, not individually.

Tailoring Adsorption Induced Switchability of a Pillared Layer MOF by Crystal Size Engineering

The pillared layer framework DUT-8(Zn) (Zn₂(2,6-ndc)₂(dabco), 2,6-ndc = 2,6-naphthalenedicarboxylate, dabco = 1,4-diazabicyclo-[2.2.2]-octane, DUT = Dresden University of Technology) is a prototypical switchable MOF, showing characteristic adsorption and desorption induced open phase (<i>op</i>) to closed phase (<i>cp</i>) transformation associated with huge changes in cell volume. We demonstrate switchability strongly depends on a framework-specific critical particle size (d<i>_crit</i>). The solvent removal process (pore desolvation stress contracting the framework) significantly controls the <i>cp</i>/<i>op</i> ratio after desolvation and, subsequently, the adsorption induced switchability characteristics of the system. After desolvation, the dense <i>cp</i> phase of DUT-8(Zn) shows no adsorption-induced reopening and therefore is non-porous for N₂ at 77 K and CO₂ at 195 K. However, polar molecules with a higher adsorption enthalpy, such as the polar molecules such as chloromethane at 249 K and dichloromethane (DCM) at 298 K can reopen the macro-sized crystals upon adsorption. For macro-sized particles, the outer surface energy is negligible and only the type of metal (Zn, Co, Ni) controls the DCM-induced gate opening pressure. The framework stiffness increases from Zn to Ni as confirmed by DFT calculations, X-ray crystal structural analyses, and low frequency Raman spectroscopy. The partial disintegration of the Zn based node hinges produces an overall increased stabilization of<i> cp </i>vs. <i>op</i> phase shifts the critical particle size at which switchability starts to become suppressed to even lower values (d<i>_crit</i> < 200 nm) as compared to the Ni-based system (<i>d_crit</i> ≈ 500 nm). Hence, the three factors affecting switchability (energetics of the empty host, (<i>E_op-E_cp</i>) (I), particle size (II), and desolvation stress (III)) appear to be of the same order of magnitude and should be considered collectively, not individually.

Sealing Performance and Mechanism of a High-Temperature Near-Neutral Sealing Adhesive in the Ternary System of ZrO2-Al2O3-SiO2

A high-temperature sealing adhesive was prepared using kyanite, andalusite, and magnesia fully stabilized zirconia (Mg-FSZ) as raw materials and nanozirconia (ZrO2) sol as binder. The adhesive is chemically near-neutral and suitable for bonding acid, neutral, or alkaline materials at 1560 °C. The mineral composition, volume expansion, and air permeability of multiple thermal cycles were analyzed. Results showed that the volume expansion caused by the mullitization of kyanite and andalusite can compensate for the sintering shrinkage high temperature. MgO could gradually desolvatize from the Mg-FSZ particles and react with SiO2 and Al2O3 to produce a stable forsterite and magnesium aluminate spinel. Hence, Mg-FSZ became unstable, and cubic ZrO2 transformed into monoclinic ZrO2 when cooled, leading to a volume expansion of the adhesive, which ensured the sealing effect. A larger critical particle size of Mg-FSZ can provide the adhesive with a more persistent volume expansion during thermal cycles due to the more durable desolvatization of MgO. The nanoZrO2 sol binder can improve the sintering of the adhesive and bonding of the joint, resulting in a low gas permeability.

The Origins of Enhanced and Retarded Crystallization in Nanocomposite Polymers

Controlling the crystallinity of hybrid polymeric systems has an important impact on their properties and is essential for developing novel functional materials. The crystallization of nanocomposite polymers with gold nanoparticles is shown to be determined by free space between nanoparticles. Results of large-scale molecular dynamics simulations reveal while crystallinity is affected by the nanoparticle size and its volume fraction, their combined effects can only be measured by interparticle free space and characteristic size of the crystals. When interparticle free space becomes smaller than the characteristic extended length of the polymer molecule, nanoparticles impede the crystallization because of the confinement effects. Based on the findings from this work, equations for critical particle size or volume fraction that lead to this confinement-induced retardation of crystallization are proposed. The findings based on these equations are demonstrated to agree with the results reported in experiments for nanocomposite systems. The results of simulations also explain the origin of a two-tier crystallization regime observed in some of the hybrid polymeric systems with planar surfaces where the crystallization is initially enhanced and then retarded by the presence of nanoparticles.

Effects of Antioxidants Particle Size on Oxidation Resistance of MgO-C Refractory

In this study, Si and SiC powder with a critical particle size ranged from 0.1 to 0.4mm was added into MgO-C refractory as antioxidants. At 1200°C in air atmosphere, oxidation weight losses of cylindrical specimens with additives were measured and the effective diffusion coefficient of oxygen in refractories was calculated from the results. Thus, the effects of antioxidants particle size on the oxidation resistance were researched. The result shows that the particle size of antioxidant has a considerable influence on oxidation resistance of material. The oxidation resistance of MgO-C refractories increased at first as the critical particle size of Si powder increased from 0.1 to 0.2mm and then decreased as the critical particle size increased up to 0.4mm, while the oxidation resistance of MgO-C refractories decreased as the critical particle size of SiC additives increased from 0.1 to 0.4mm. The minimum effective diffusion coefficients of oxygen in MgO-C refractories added by Si and SiC were 10.90 and 14.09cm2/min, individually.

The local and average structure of Ba(Ti, Ce)O3 perovskite solid solution: effect of cerium concentration and particle size

The amazing properties of ferroelectric perovskite BaTiO3 (BT) and its solid solutions make them indispensable for many technological applications (e.g. multilayer capacitors). Unfortunately, the so-called `size effect' limits their use. Indeed, under a certain critical particle size, these materials show a suppression of the spontaneous polarization and thus of the ferroelectric properties. In pure nanometric BaTiO3, this is related to a certain local structural disorder. However, only a few studies have explored BT solid solutions, where the doping effect, coupled to the reduced particle size, can play an important role. Therefore, in this work, the structure of BaCe x Ti1–x O3 (x = 0.02–0.20) was explored by traditional Rietveld method and Pair Distribution Function. Samples present a particle size from 80–160 nm to 400–1000 nm depending on increasing x. The carbox approach was applied, investigating the evolution of the local structure, its modifications and the structural coherent correlation length, as a function of cerium amount. Results demonstrate a cooperative effect of composition and reduced size in the ferroelectricity loss. The two, in fact, contribute to intensify the local structural disorder, decreasing the structural coherent correlation length. The local structural disorder is thus confirmed to be a relevant factor in the ferroelectric properties degradation.

Study on Permeability and Blocking Resistance of Composite Specimen with Double-Layer Permeable Asphalt Mixture

In order to study the permeability and antiblocking performance of composite specimens with double-layer permeable asphalt mixture, three types of PAC-10 asphalt mixture with different target porosity (20%, 22%, and 24%) and PAC-16 asphalt mixture with a target porosity of 22% were designed, and the double-layer Marshall specimen was fabricated through “hot + hot” method. Their orthogonal vertical sections were scanned with X-ray CT. The pore distribution and its characteristics of the specimens were analyzed by digital image analysis technology. The permeability of composite specimens was studied through penetration test, and their blocking resistances were studied through using different particle sizes of fine machine-made sands as blocking materials. The results show that the permeable capacity increases linearly with the increase of porosity. The permeable capacity of PAC-16 with 22% porosity is greater than PAC-10 (20%, 22%, and 24%) porosity. The porosity of the upper layer increases, and the permeability of the double-layer composite samples increases linearly. The critical particle size causing blocking is 0.15 mm, followed by 0.3 mm. The vertical permeability coefficient decreases exponentially with the increasing of blocking times.